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Neural stem cells: balancing self-renewal with differentiation

Neural stem cells: balancing self-renewal with differentiation REVIEW 1575 Development 135, 1575-1587 (2008) doi:10.1242/dev.014977 Chris Q. Doe Stem cells are captivating because they have the potential to of proneural gene expression. High levels of the proneural genes make multiple cell types yet maintain their undifferentiated achaete, scute or lethal of scute repress Notch activity and state. Recent studies of Drosophila and mammalian neural stem promote NB formation; low levels of proneural gene expression cells have shed light on how stem cells regulate self-renewal allow high Notch activity, which maintains neuroectodermal fate versus differentiation and have revealed the proteins, processes and ultimately leads to epidermal differentiation (Artavanis- and pathways that all converge to regulate neural progenitor Tsakonas et al., 1991). Thus, proneural genes promote self-renewal. If we can better understand how stem cells neurogenesis (i.e. NB formation), whereas Notch signaling balance self-renewal versus differentiation, we will significantly inhibits neurogenesis. In this review, I briefly discuss embryonic advance our knowledge of embryogenesis, cancer biology and NBs and focus instead on the central brain NBs, where most is brain evolution, as well as the use of stem cells for therapeutic known about the mechanisms that regulate self-renewal. purposes. Larval NBs, which have many attributes of self-renewing stem cells, lie in a specialized cellular niche; they are undifferentiated, do Introduction not express any known neuron- or glial-specific markers; are highly A defining feature of stem cells is their ability to continuously proliferative yet never form tumors; can undergo mitotic quiescence maintain a stem cell population (self-renew) while generating without differentiating; and, most importantly, can generate differentiated progeny. Thus, stem cells are faced with a uniquely hundreds of neuronal progeny without losing their position, size, difficult task: to avoid cell cycle exit and differentiation, and to avoid identity or mitotic potential. These features make larval NBs an ideal uncontrolled proliferation and tumor formation. How stem cells system in which to study the basic biology of stem cell self-renewal walk this developmental tightrope is an extremely interesting (see Box 1 for NB-based self-renewal assays). However, there is a question that is of relevance to our understanding of the processes of potential limitation of larval NBs as a stem cell model: as they cell differentiation and cancer, and of the developmental diseases divide, they might gradually lose the ability to make early-born cell that result from the premature loss of stem cell pools. types within their lineage (termed a ‘progressive restriction in Here I review recent insights from studies of neural stem cells competence’), similar to the situation for embryonic NBs (Isshiki et (NSCs) in Drosophila and mice. There are surprising similarities in al., 2001; Pearson and Doe, 2003). If true, it would mean that the NB the transcription factor profiles of NSCs in flies and mice, although is not precisely self-renewing with every division. Nevertheless, many have not been functionally tested in both organisms. Both fly mammalian NSCs of the cortex and retina also undergo progressive and mammalian NSCs have unique cellular contacts, but the role of restriction (Desai and McConnell, 2000; Livesey and Cepko, 2001), these contacts (their ‘niche’) has only recently begun to be explored. and the study of Drosophila NBs might help us understand this Much more progress has been made on the role of cell polarity process. proteins in regulating self-renewal in Drosophila neuroblasts, and In the mammalian embryonic CNS, particularly in the ventral their conservation in mammalian cortical stem cells should lead to telencephalon during mid-neurogenesis and, to a lesser extent, in rapid progress in this system. Finally, I discuss the role of spindle the dorsal telencephalon, neuroepithelial cells give rise to radial orientation in regulating NSC self-renewal; recent identification of glia, which differentiate into basal progenitors that each form two mutants that disrupt spindle orientation without affecting cell postmitotic neurons (see Fig. 1B). Both radial glia and polarity in both flies and vertebrates now permits, for the first time, neuroectodermal cells can directly generate neurons (Gotz and time-lapse imaging studies to correlate spindle orientation, cell Huttner, 2005), and both neuroepithelial cells and radial glia can polarity components and sibling cell fate. The goal of this review is self-renew while producing basal progenitors, neurons or glia. to summarize recent research, to untangle conflicting results and to These self-renewing cell types share a similar epithelial highlight areas for future exploration. morphology (they span the neuroepithelium), both express the intermediate filament Nestin and have an apically located mitotic Neurogenesis in Drosophila and mammals spindle, and both can be distinguished by an array of molecular During Drosophila neurogenesis, neuroepithelial cells markers (Gotz and Huttner, 2005). By contrast, most basal differentiate into neuroblasts (NBs), which divide to form a NB progenitors lack self-renewal potential and typically generate two and a ganglion mother cell (GMC). GMCs are intermediate postmitotic neurons (Haubensak et al., 2004; Miyata et al., 2004; progenitors that have a limited mitotic potential and typically Noctor et al., 2001). They do not span the neuroepithelium and divide just once to generate a pair of postmitotic neurons (as undergo mitosis in a basal region termed the subventricular zone summarized in Fig. 1A). Embryonic neuroepithelial cells are bi- (SVZ) (Haubensak et al., 2004; Miyata et al., 2004; Noctor et potent cells that can form either NBs (stem-cell-like neural al., 2001; Noctor et al., 2004). Thus, only neuroectodermal progenitors) or epidermis. This choice is determined by the level and radial glial cells can self-renew, and as such are a focus of this review. [Excellent reviews have recently been published on neural progenitors of the mammalian spinal cord, retina, Howard Hughes Medical Institute, Institute of Neuroscience, Institute of Molecular adult hippocampus and dentate gyrus (see Chapouton et al., Biology, University of Oregon, Eugene, OR, USA. 2007; Gould, 2007; Ninkovic and Gotz, 2007; Sutter et al., e-mail: cdoe@uoregon.edu 2007).] DEVELOPMENT 1576 REVIEW Development 135 (9) Fig. 1. Neural stem cell formation and neuronal differentiation. (A) Drosophila neurogenesis. Neuroectodermal cells (NE; yellow, the apical surface is uppermost) give rise to neuroblasts (NBs; green) by delamination, and each NB divides in a stem cell mode to bud off a chain of ganglion mother cells (GMCs; orange) from its basal surface. GMCs are intermediate precursors that typically generate two postmitotic neurons (n; red). Larval NBs are closely associated with glia (blue). Thoracic and brain NBs become mitotically quiescent in late embryos and resume proliferation during larval stages. Approximate cell cycle times or quiescence times are given in hours (h). epi, embryonic epidermis. (B) Mammalian embryonic cortical neurogenesis. Initially the cortex has only neuroepithelial cells (NE; yellow, the apical surface is uppermost), which mature into radial glia (RG; green). Radial glia and neuroectodermal cells generate basal progenitors (BP; orange), which are intermediate progenitors that generate a limited number of neurons (n; red). NE and RG can also generate neurons. VZ, ventricular zone, adjacent to the lumen; SVZ, subventricular zone; MZ, marginal zone. The neural stem cell niche cells per clone but maintain the NB (Zhu et al., 2008). Thus, Activin Both Drosophila NBs and vertebrate NSCs lie in a unique cellular signaling regulates NB proliferation or neuronal survival, but not NB microenvironment compared with their differentiating progeny. self-renewal. Similarly, decreased expression of the FGF receptor Here I discuss the evidence for the role of niche-derived cues in (Branchless), Hh, or the Hh- and FGF-binding protein Perlecan (Trol regulating stem cell proliferation and self-renewal. – FlyBase), reduces the number of proliferating NBs. Adding exogenous human FGF2 or increasing Cyclin E levels after this The Drosophila NSC niche phenotype has become apparent rescues and returns to normal the Larval NBs contact cortex glial cells on their apical and lateral sides number of proliferating NBs (Park et al., 2003), indicating that the (Dumstrei et al., 2003), while the basal side forms E-cadherin-rich affected NBs were mitotically quiescent rather than dead or contacts with new-born GMCs (Fig. 2A). Larval glia secrete the differentiated. Thus, the mitogens FGF and Hh are necessary for Anachronism (Ana) protein, which keeps NBs quiescent during maintaining NB proliferation but not for NB self-renewal or survival. early larval stages (Ebens et al., 1993). The possibility that glial- Finally, there is evidence that Notch signaling regulates NB self- derived signals also promote larval NB proliferation is supported by renewal. Notch signaling is robust in larval NBs, based on the the glial-specific expression of a dominant-negative E-cadherin strong, specific expression of a Notch reporter gene (Almeida and protein, which results in fewer proliferating NBs (Dumstrei et al., Bray, 2005). Reducing Notch activity decreases central brain NB 2003). Although the cellular basis for this phenotype is unknown, it numbers (Wang et al., 2007), but has no effect on thoracic NB is consistent with the loss of a glial-neuroblast contact and failure to numbers (Almeida and Bray, 2005); conversely, increasing Notch transmit a glial-derived proliferation cue. Alternatively, non-specific activity by expressing a constitutively active Notch intracellular effects, such as loss of the glial brain/hemolymph barrier, could domain or by removing the Notch inhibitor Numb increases brain generate this phenotype. NB numbers (Lee et al., 2006a; Wang, H. et al., 2006). The identity Is there any evidence that the Transforming growth factor β and cellular source of the Notch ligand have not been determined, (TGFβ), Activin, Notch, Wnt, Hedgehog (Hh) or Fibroblast growth but this pathway is clearly implicated in supporting NB self-renewal, factor (FGF) signaling pathways have a role in maintaining similar to its role in mammals (see below). Drosophila NB self-renewal or proliferation, as described below for mammalian NSCs? Recent work suggests that Activin, Hh and FGF The mammalian NSC niche promote NB proliferation, whereas Notch signaling promotes NB Neuroepithelial and radial glial cells have a columnar epithelial self-renewal; the role of the Wnt pathway has not yet been addressed. morphology. Their apical process is exposed to the ventricular fluid, Activin and the redundant Activin-related Daw ligands are expressed their basal (pial) process contacts the extracellular matrix (ECM), in larval brain glia. NBs that lack the Activin receptor contain fewer and they have lateral contacts with each other, including at the region DEVELOPMENT Development 135 (9) REVIEW 1577 Box 1. Neural stem cell self-renewal assays in flies and mice Neural stem cell (NSC) self-renewal assays in Drosophila include scoring the total number of NBs in the larval brain. Normally, ~200 central brain NBs exist per larval brain; mutants with increased NB self-renewal are expected to have increased NB numbers. Conversely, mutants in which NB self-renewal has failed should have fewer NBs. Confirmation of a protein’s role in NSC self-renewal in Drosophila is typically achieved by generating genetically marked single NB mutant clones. In wild-type flies, marked clones consist of a single NB and a family of marked GMC/neuronal progeny. If the mutated gene normally promotes differentiation, a homozygous mutant clone will generate multiple NBs. If the mutated gene normally promotes NB self-renewal, NBs will often be lost from the clone. In mammals, self-renewal assays include a similar clonal analysis following the viral delivery of a marker gene (such as GFP) co- expressed with a gene-overexpression construct or a gene- knockdown construct [such as a short-hairpin RNA (shRNA)]. The in vitro ‘neurosphere assay’ is also widely used (Breunig et al., 2007), in which single cells derived from dissociated cortical tissue are cultured in vitro to determine the percentage that can generate a multi- lineage primary clone that contains single cells competent to generate a secondary multi-lineage neurosphere (reviewed by Jensen and Parmar, 2006). Fig. 2. Neural stem cell niche. (A) Drosophila larval neuroblast (NB) niche. (Left) Schematic showing NB/glial contact (light blue bars) and cadherin-rich NB/ganglion mother cell (GMC) contacts (red bars). (Right) Confocal image showing glial membrane staining relative to of subapical adherens junctions (Fig. 2B). Thus, cues from apical, neuroblast and GMC membranes. Glial membrane was detected with basal or lateral directions could modulate neuroepithelial/radial glial repo-gal4 UAS-cd8:GFP (blue, or as a single channel in the right-hand self-renewal. Here I focus on the well-characterized roles of the image). NB, GMC and neuronal membranes were labeled with Wnt, Notch and sonic hedgehog (Shh) pathways in regulating Scribbled (red). Image courtesy of Jason Q. Boone (University of mammalian NSC self-renewal. Evidence for the role of the Oregon). (B) Mammalian neuroepithelia/radial glia niche. (Left) JAK/STAT, FGF, TGFβ and Toll-related pathways in regulating Neuroepithelial/radial glia cells contact the ventricular fluid at their apical domain (green dots), neighboring cells via cadherin-rich adherens NSC proliferation and possibly self-renewal is summarized junctions (AJs; red bars), and the basal lamina at their basal domain elsewhere (Rolls et al., 2007; Shi et al., 2008). (dashed lines). Basal progenitors (BP; orange) and neurons (n; red) lack The canonical Wnt pathway promotes neuroepithelial/radial access to apical and basal cues. (Right) E10 mouse neuroepithelial cells glial identity. The reduction of Wnt ligand levels or the removal showing cadherin enrichment at the subapical AJs (red) and prominin 1 of the canonical pathway component β-catenin results in fewer localization to the apical domain (green) that contacts the ventricular neuroepithelial/radial glial stem cells and in precocious neuronal fluid [modified and reproduced with permission from Kosodo et al. differentiation (Machon et al., 2003; Zechner et al., 2003). By (Kosodo et al., 2004)]. contrast, increased Wnt signaling expands the stem cell pool (Chenn and Walsh, 2002; Machon et al., 2007; Viti et al., 2003; Woodhead et al., 2006; Zechner et al., 2003). Wnt signaling also Thus, further study is needed to identify the context-dependent promotes NSC self-renewal during postnatal neurogenesis factors that switch Wnt signaling between promoting and (Machon et al., 2007; Machon et al., 2003; Wexler et al., 2008; inhibiting NSC self-renewal. Zhou et al., 2004), where it can also promote the proliferation of Notch signaling components are expressed in embryonic committed neuronal progenitors (Lie et al., 2005). Wnt signaling neuroepithelial/radial glial stem cells, as well as in adult NSCs directly activates cyclin D and the NSC factors Sox2 and Rest (Mizutani et al., 2007; Stump et al., 2002). Mutations in the genes (RE1-silencing transcription factor) (Megason and McMahon, encoding Dll1 (a Notch ligand), Notch1 (a Notch receptor), RBPJk 2002; Nishihara et al., 2003; Takemoto et al., 2006), which may (Rbpj – Mouse Genome Informatics; a Notch transcriptional contribute to NSC maintenance. Later in cortical development, effector), Hes1, Hes3 or Hes5 (RBPJk-induced transcription factors) Wnt signaling is a potent inducer of neuronal differentiation, in all lead to the depletion of radial glia stem cells and to precocious part by activating the proneural gene neurogenin 1 (Ngn1; neuronal differentiation in the mouse embryo (de la Pompa et al., Neurog1) (Hirabayashi et al., 2004; Israsena et al., 2004; 1997; Handler et al., 2000; Hatakeyama et al., 2004; Mizutani et al., Muroyama et al., 2004; Viti et al., 2003). The difference in early 2007; Yoshimatsu et al., 2006), and to NSC loss in the adult (Gaiano versus late Wnt function is highlighted by the observation that the et al., 2000). Conversely, misexpression of Hes1, Hes3 or of expression of stabilized β-catenin at embryonic day (E) 10 activated Notch in the embryonic cortex blocks neuronal promotes neuroepithelial proliferation and self-renewal (Chenn differentiation (Chambers et al., 2001; Ishibashi et al., 1994). Radial and Walsh, 2002), whereas at E14 it promotes neuronal glia stem cells from Dll1, Notch1, Rbpj, Hes1 and Hes5 mouse differentiation (Hirabayashi and Gotoh, 2005). It has been mutants all have a reduced neurosphere-forming ability (see Box 1), proposed that Wnt alone stimulates neuronal differentiation, indicating that they have a reduced ability to self-renew (Hitoshi et whereas Wnt plus the mitogen Fgf2 inhibits neural differentiation al., 2002; Ohtsuka et al., 2001; Yoon et al., 2004). Furthermore, (Israsena et al., 2004; Viti et al., 2003), although evidence against radial glial cells that express a Notch-induced GFP reporter can be this model has also been presented (Hirabayashi et al., 2004). sorted by flow cytometry into Notch-high (GFP+) and Notch-low DEVELOPMENT 1578 REVIEW Development 135 (9) (GFP–) populations; the Notch-high cells are more potent at Transcriptional regulation and NSC self-renewal in generating primary and secondary neurospheres, and can be Drosophila transplanted in vivo to generate all three neural lineages – neurons, Genes transiently expressed in newly formed NBs include the astrocytes and oligodendrocytes (Mizutani et al., 2007). Thus, Notch proneural genes achaete, scute and lethal of scute. These encode signaling correlates with, and is required for, the maintenance of basic helix-loop-helix (bHLH) TFs that promote the transition of a embryonic and postnatal NSCs. neuroectodermal cell to a NB, and thus are responsible for triggering The Shh pathway includes the Shh ligand, the transmembrane NB delamination and NB-specific gene expression (epithelial genes smoothened (Smo) protein, and the nuclear effectors Gli2/3, as well off, NB-specific genes on). This is partly accomplished by the as many other proteins. Shh is expressed in the embryonic transient suppression of Notch signaling, as Notch signaling is neuroepithelium (Lai et al., 2003), and in regions of adult necessary and sufficient to maintain neuroectodermal cell fate neurogenesis – the hippocampus and dentate gyrus (Ahn and Joyner, (reviewed by Artavanis-Tsakonas et al., 1991). Several other TFs are 2005; Machold et al., 2003; Palma et al., 2005). When conditional expressed in subsets of neuroectoderm and delaminating NBs, Smo mutants are used to block Hh signaling in the postnatal where they collaborate with the proneural proteins to promote NB hippocampus and dentate gyrus, these tissues produce fewer primary formation. These include the SoxB group genes SoxN and Dichaete, neurospheres when cultured in vitro (Machold et al., 2003). which encode high mobility group (HMG) transcriptional activators However, this effect could be due to a reduced stem cell population (Cremazy et al., 2000; Nambu and Nambu, 1996; Russell et al., prior to explant; the ability to form multi-lineage secondary 1996). The first function of the proneural and SoxB genes is to neurospheres was not assayed, which would have tested for stem cell induce neurogenesis within the ventral ectoderm, which otherwise self-renewal during neurosphere passage (see Box 1). Subsequent would produce only epidermis. experiments showed that E18.5 cortical tissue from Gli2 or Gli3 A second class of NB TFs are permanently expressed in NBs but mutant mice were deficient in both primary and secondary are not maintained in their GMC/neuronal progeny. These TFs are neurosphere formation, providing evidence that Shh promotes stem the best candidates for promoting NB self-renewal, and include the cell self-renewal (Palma and Ruiz i Altaba, 2004). Furthermore, zinc-finger protein Worniu, the bHLH proteins Deadpan and there is compelling evidence that Shh both promotes proliferation Asense, and the SoxB family member SoxN (Ashraf and Ip, 2001; and inhibits differentiation in postnatal cerebellar granule cell Bier et al., 1992; Brand et al., 1993; Cai et al., 2001; Cremazy et al., precursors (Argenti et al., 2005; Wechsler-Reya and Scott, 1999). 2000). Surprisingly, very little is known about the function of these Finally, it has recently been proposed that Shh might promote the genes in regulating NB self-renewal. deadpan and asense single transition of stem cells to more-rapidly dividing, committed mutants have only mild post-embryonic CNS defects (Bier et al., progenitors (Agathocleous et al., 2007), rather than maintaining 1992; Brand et al., 1993), although Deadpan can repress expression stem cell identity per se. Thus, the role of Shh in promoting NSC of the cell cycle inhibitor dacapo (Wallace et al., 2000), consistent self-renewal needs further investigation. with a role in promoting NB proliferation. Similarly, worniu mutants Integrins are a family of cell-surface adhesion and signaling have mild defects in larval CNS axial shortening (Ashraf et al., proteins that bind ECM proteins, such as laminin. β1-integrin 2004), and the Sox gene mutants have reduced embryonic NB (Itgβ1) is enriched at regions that contain embryonic and adult numbers, but this is probably due to a failure in NB formation not NSCs, and at the periphery of neurospheres where NSCs reside self-renewal (Cremazy et al., 2000; Nambu and Nambu, 1996; (Campos et al., 2004). When forebrain tissue from postnatal day 1 Russell et al., 1996; Zhao et al., 2007). It is tempting to speculate that mutant mice that carry floxed Itgb1 alleles is depleted of β1-integrin the Sox TFs act in NBs to prevent neuronal differentiation initiated over a 10-day period, nestin stem cells from this tissue show a by the proneural genes, similar to the proposed role of SoxB1 family reduced neurosphere-forming ability and increased cell death TFs in vertebrates (see below). However, the function of Sox TFs in (Leone et al., 2005), indicating that integrin signaling might also self-renewal has not yet been tested. promote NSC survival. The flip side of NB self-renewal is neuronal differentiation. NBs Overall, findings to date show that in both mammals and flies, rapidly lose the expression of the proneural genes, so what might Notch signaling promotes NSC self-renewal. Wnt and Shh promote neuronal differentiation in their lineage? The divergent pathways might also regulate NSC self-renewal in mammals, but homeodomain TF Prospero is crucial for initiating neuronal this role has yet to be tested in Drosophila. Less, however, is known differentiation. prospero is transcribed and translated in all NBs, but about the cellular nature of the niche. In the mammalian cortex, it is exported from the nucleus (Demidenko et al., 2001); the mRNA is not clear whether self-renewal cues come from ventricular fluid, and protein are segregated into the GMC during NB asymmetric cell the basal ECM, the neuroepithelial/radial glial cells themselves, or division (Broadus et al., 1998; Knoblich et al., 1995; Spana and Doe, none of the above. In Drosophila, existing data suggest that glial 1995), where the protein enters the nucleus to repress cell cycle cells are required for larval NB proliferation, but whether they serve genes and activate neural differentiation genes (Choksi et al., 2006; as a local NB niche needs to be directly tested by glial ablation Li and Vaessin, 2000). When prospero mutant clones are induced in experiments. single larval NBs, many GMCs fail to differentiate and instead form NB tumors (Bello et al., 2006; Betschinger et al., 2006; Choksi et Nuclear control of self-renewal al., 2006; Lee et al., 2006c). The recent identification of transcription factors (TFs) that are What about chromatin remodeling genes? In mammals, the sufficient to reprogram human differentiated cells into cells that Polycomb group chromatin remodeling factor Bmi1 is required for resemble embryonic stem (ES) cells (Takahashi et al., 2007; Yu et postnatal NSC renewal (Molofsky et al., 2005; Molofsky et al., al., 2007) indicates that there also might be TFs or chromatin factors 2003), raising the possibility that Drosophila NBs might also require that specify the identity of tissue-specific stem cells. Numerous TFs Polycomb for self-renewal. A recent paper tests this hypothesis by are also known to be expressed in NSCs (see Table 1). In this generating mutant clones null for several Polycomb group genes section, I discuss TF/chromatin factor expression and function in within single larval NBs (Bello et al., 2007). All Polycomb group NSCs. mutant clones had fewer neurons and lacked the NB, consistent with DEVELOPMENT Development 135 (9) REVIEW 1579 Table 1. Regulators of gene expression involved in neural stem cell self-renewal Drosophila Mammals † ‡ § † ‡ § Protein Expression Function References Protein Expression Function References SoxB NE, NB FSR (Cremazy et al., 2000; SoxB1 NE, RG FSR (Bylund et al., 2003; Nambu and Nambu, 1996; Ferri et al., 2004; Russell et al., 1996) Graham et al., 2003; Wang, T. W. et al., 2006) E(spl)m NB ? (Almeida and Bray, 2005) Hes1,3,5 NE, RG FSR (Hatakeyama et al., 2004; Ishibashi et al., 1994) Deadpan NB ? (Bier et al., 1992) Hes related* Worniu NB ? (Ashraf and Ip, 2001) Slug/Snail related* Asense NB ? (Brand et al., 1993) Mash3 related* Musashi NB ? (Nakamura et al., 1994) Musashi NE, RG ? (Kaneko et al., 2000; Siddall et al., 2006) – – – Rest NE, RG FSR (Ballas and Mandel, 2005) Proneural NE, NB FDIFF (Skeath and Carroll, 1994) Proneural NE, N FDIFF (Guillemot, 2007) Prospero GMC FDIFF (Bello et al., 2006; Choksi Prox1 BP, N FDIFF (Dyer, 2003; Lavado et al., 2006; Knoblich et and Oliver, 2007; Torii al., 1995; Lee et al., 2006c; et al., 1999) Spana and Doe, 1995) Brain tumor GMC FDIFF (Bello et al., 2006; Lee et Trim3 ? ? al., 2006c; Betschinger et al., 2006) p53 ? ? p53 N FDIFF (Meletis et al., 2006) Protein orthologs or homologs are shown on the same line; –, gene ortholog has not been identified; *, groups of related proteins. Expression in Drosophila neuroectoderm (NE), neuroblasts (NB), ganglion mother cells (GMC), neurons (N); or in mammalian telencephalon neuroepithelium (NE), radial glia (RG), basal progenitors (BP), neurons (N). Function in promoting self-renewal (FSR) or in promoting differentiation (FDIFF); ?, functional studies have not been reported. a failure in NB self-renewal. However, the co-expression of the cell induced by the proneural proteins Mash1 and the Ngns (Bertrand et death inhibitor p35 rescued NB survival and normal clone size. al., 2002; Bylund et al., 2003; Ge et al., 2006), and proneural Thus, the Polycomb group proteins are required to maintain NB proteins can directly bind and inhibit SoxB1 protein function. Thus, survival, but are dispensable for larval NB self-renewal (Bello et al., the balance of SoxB1 and proneural activity determines the tempo 2007). of neurogenesis. How this balance is regulated over time is In conclusion, proneural genes promote NB expression of unknown. One additional factor that promotes NSC self-renewal is Worniu, Deadpan, Asense and Prospero. The first three TFs are good the Rest transcriptional repressor, which is expressed in NSCs and candidates for maintaining NB self-renewal, whereas Prospero is in most non-neuronal cells, where it induces a repressive chromatin asymmetrically localized into the GMC where it promotes neuronal state that blocks the expression of neuronal differentiation genes differentiation. This is an elegant mechanism for ensuring NB (Ballas et al., 2005). Neurons express a small modulatory double- homeostasis while producing a constant stream of neurons. stranded (ds) RNA that induces differentiation by blocking Rest activity at the protein level (not the RNA level, surprisingly) Transcriptional regulation and NSC self-renewal in (Kuwabara et al., 2004). Lastly, the RNA-binding protein musashi mammals is expressed in both germline and NSCs (Kaneko et al., 2000; As in Drosophila, the bHLH proneural proteins Mash1 (Ascl1), Siddall et al., 2006); it promotes germline stem cell self-renewal Ngn1 and Ngn2 (Neurog2) are expressed in mammals in partially (Siddall et al., 2006), but its function in NSC self-renewal is yet to overlapping populations of neuroepithelial cells, where they are be determined. required for the acquisition of NSC properties. Subsequently, they An important stem cell attribute is the ability to proliferate. are maintained in newly differentiating neurons, where they induce Maintenance of postnatal NSC proliferation is partly regulated by neuronal differentiation (Guillemot, 2007). How do neuroepithelial the Polycomb group transcriptional repressor Bmi1. Loss of Bmi1 cells and radial glia express these proneural genes without results in an increase of the cell cycle inhibitor p16Ink4a (Cdkn2a) differentiating? This is the role of the SoxB1 family members (Sox1, and in postnatal stem cell depletion, without affecting embryonic 2, 3). The SoxB1 proteins are expressed in embryonic and adult NSCs (Molofsky et al., 2005; Molofsky et al., 2003). One important NSCs, as well as in a few postmitotic neurons (Graham et al., 2003; negative regulator of proliferation might be Prox1, which is related Wang, T. W. et al., 2006). A reduction in SoxB1 levels leads to to the Drosophila transcriptional repressor Prospero. Mash1 induces precocious neural differentiation and to the depletion of the Prox1 expression in newly differentiating neurons (Torii et al., progenitor pool, whereas misexpression of SoxB1 family members 1999), and Prox1 inhibits proliferation in the mammalian retina can block neuronal differentiation and maintain the progenitor (Dyer, 2003; Li and Vaessin, 2000), and might have a similar population (Bylund et al., 2003; Ferri et al., 2004; Graham et al., function in the cortex. Experimentally lengthening the cell cycle also 2003), although without maintaining proliferation (Bylund et al., increases progenitor differentiation (Calegari and Huttner, 2003). 2003). SoxB1 TFs antagonize the neuronal differentiation that is Thus, slowing or stopping the cell cycle can induce neuronal DEVELOPMENT 1580 REVIEW Development 135 (9) differentiation, and prolonging cell cycle progression can prevent stem cell depletion, although quiescent stem cells clearly have a mechanism to prevent differentiation. How self-renewal and cell cycle pathways intersect will be an important and challenging area of future research. In conclusion, data from flies and mice are consistent with a common model for neurogenesis, in which SoxB1 proteins confer progenitors (neuroepithelial cells in vertebrates, neuroectodermal cells and NBs in flies) with the potential to self-renew. Proneural proteins then induce progenitor delamination and neural differentiation, the latter being blocked by SoxB1 proteins. Finally, nuclear Prospero/Prox1 initiates cell cycle exit and neural differentiation. Several aspects of this model remain to be tested, including the role of the Drosophila SoxB proteins in antagonizing proneural activity and in promoting self-renewal, and the role of vertebrate Prox1 in promoting neuronal differentiation. Cell polarity and self-renewal Recent data suggest that cell polarity plays a key role in regulating self-renewal versus differentiation in both fly and mammalian NSCs, and that several of the proteins involved have evolutionarily conserved functions. But there are some surprising differences, and many proteins have only been tested in one animal to date. Fig. 3. Neural stem cell polarity. (A) Drosophila neuroblast cell polarity. (Left) Proteins that asymmetrically localize in a dividing NB. Cell polarity and NSC self-renewal in Drosophila Green, apical proteins; red, basal proteins, including those associated Drosophila NBs divide asymmetrically to self-renew a NB while with GMC contact site. Lines between proteins indicate physical budding off a small, differentiating GMC. A growing number of interactions. Baz binds Par6 and aPKC (not shown). Arrow indicates proteins are known to be segregated into the NB or GMC during this that Baz is required for Cdc42 localization. T-bar indicates that aPKC asymmetric cell division. Proteins segregated into the NB include excludes Lgl, Numb and Mira from the cortex, and Lgl excludes aPKC. Bazooka (Baz/Par3), Cdc42, Par6, atypical protein kinase C (aPKC) (Right) Schematic of a dividing NB, showing the apical (green) and basal (which may all form a single protein complex), Inscuteable (Insc), (red) cortical domains; spindle and centrosomes, brown; DNA, yellow. Partner of Inscuteable (Pins; Rapsynoid – FlyBase) and Gαi (B) Mammalian neuroepithelial (NE) cell polarity. (Left) Proteins that asymmetrically localize in a dividing NE cell. Green, apical proteins; red, (G-iα65A – FlyBase) (which may form a distinct protein complex AJ-enriched proteins; blue basolateral proteins. Lines between protein that links to Baz via Insc). Proteins partitioned into the GMC include names indicate physical interactions. Par3 binds Par6 and aPKC (not the scaffolding protein Miranda and its cargo proteins Staufen, shown). T-bar indicates that aPKC excludes Lgl and Numb from the Prospero and Brain tumor (Brat), as well as Numb and Partner of cortex. (Right) Schematic of a dividing NE (center). Spindle and Numb (Caussinus and Hirth, 2007; Gonzalez, 2007) (see Fig. 3A). centrosomes, brown; DNA, yellow. Baz, Bazooka (fly Par3); Insc, The first protein identified to positively regulate NB self-renewal Inscuteable; Pins, Partner of Insc; aPKC, Atypical protein kinase C; Lgl, was aPKC. aPKC mutants have fewer NBs per larval brain lobe, and Lethal giant larvae; Pon, Partner of Numb; Mira, Miranda; Pros, overexpression of a membrane-tethered aPKC in NBs dramatically Prospero; Brat, Brain tumor; Stau, Staufen; Ecad, E-cadherin; Apc2, increases brain NB numbers (Lee et al., 2006b). Similarly, lethal (2) Adenomatosis polyposis coli 2; Prom1, prominin 1; αcat, α-catenin; giant larvae (lgl) mutants have ectopic cortical aPKC in NBs and a βcat, β-catenin; LGN (Gpsm2), a homolog of Pins. corresponding increase in brain NB numbers that can be fully suppressed in lgl aPKC double mutants (Lee et al., 2006b). Taken together, these data show that aPKC is sufficient to turn GMCs into data showing that increasing cell cycle length triggers the NBs (ectopic NB self-renewal), but it is not absolutely required for differentiation of vertebrate neural progenitors (Calegari and NB self-renewal as aPKC mutants maintain a subset of their brain Huttner, 2003). Consistent with this model, aPKC mutant NBs NBs. aPKC probably acts redundantly with a second pathway to prematurely stop dividing (Rolls and Doe, 2004), although whether promote NB self-renewal, most likely the Notch pathway, which is the NB becomes quiescent, dies or differentiates is unknown. It also sufficient but not necessary for NB self-renewal (see above). would be interesting to determine if the overexpression of aPKC can This model needs to be tested by assaying Notch aPKC double speed up the GMC cell cycle, and whether this is the cause of the mutants for a complete loss of NB self-renewal. extra NB phenotype; conversely, does lengthening the NB cell cycle How does aPKC promote self-renewal? One attractive model is induce precocious differentiation and reduced NB numbers? that aPKC phosphorylates and inactivates neuronal differentiation A second protein required for NB self-renewal is Pins, a factors – such as Lgl, Numb or the Miranda-Prospero-Brat complex scaffolding protein that binds to Gαi, Insc and many other proteins – to keep these proteins out of the self-renewing NB. For example, (reviewed by Wodarz and Nathke, 2007). pins mutants initially show aPKC is known to phosphorylate and inhibit the cortical localization normal NB numbers in early larval development but have of Numb (Smith et al., 2007) and Lgl (Betschinger et al., 2003), as dramatically fewer NBs in late larval stages (Lee et al., 2006b). In well as to inhibit the cortical localization of Miranda by an unknown addition, whereas wild-type NB clones always contain one NB and mechanism (Rolls et al., 2003). A more speculative model is that a family of GMC/neuronal progeny, pins mutant NB clones contain aPKC positively regulates cell cycle progression, and a speedy cell fewer total cells and often lack a NB (Lee et al., 2006b). pins mutants cycle promotes stem cell self-renewal. This model is inspired by fail to localize aPKC to the apical cortex of larval NBs (Lee et al., DEVELOPMENT Development 135 (9) REVIEW 1581 2006b), which may contribute to the defect in self-renewal, but it is Box 2. The complexity of being Numb unknown whether forced expression of membrane-tethered aPKC In Drosophila, Numb is required to promote neuronal differentiation can rescue the pins mutant phenotype. Surprisingly, pins mutant and to inhibit NB self-renewal (Lee et al., 2006c; Wang, H. et al., brain tissue can form tumors when transplanted into adult hosts 2006). In mammals, the situation is more complex. The conditional (Caussinus and Gonzalez, 2005). The reason for this discrepancy is deletion of Numb/Nbl early or late in neurogenesis (at E8.5 or E14, unknown, but a possible explanation is that transplanted cells are respectively) results in loss of neuroepithelial/radial glial progenitors prone to genomic instability (Caussinus and Gonzalez, 2005), and (Petersen et al., 2002; Petersen et al., 2004). Conversely, the removal any pins mutant cell that loses the lgl gene from the tip of of Numb/Nbl with Emx1-Cre at E9.5 results in neural progenitor chromosome 2 would generate lgl pins double-mutant cells that are hyperproliferation, delayed cell cycle exit, and depletion of late-born known to form massive brain tumors (Lee et al., 2006b). neurons (Li et al., 2003). Most recently, the same Numb/Nbl Emx1- Cre conditional mutant was shown to have a loss of adherens Proteins that negatively regulate NB self-renewal (i.e. that junctions and defective apical/basal polarity owing to reduced E- promote neuronal differentiation) usually segregate into the cadherin-positive vesicle targeting to the junctional domain (Rasin et differentiating GMC during NB asymmetric cell division, and al., 2007). This might deplete NSC numbers, as seen with the early include the Miranda coiled-coil scaffolding protein, its cargo loss of apical/basal polarity following Par3 or Par6 depletion, but in proteins Prospero and Brat, Lgl and Numb. Loss of any of these fact the authors report no effect on progenitor maintenance or proteins transforms GMCs into NBs and produces a stem cell neuronal differentiation (Rasin et al., 2007). This ‘Numb paradox’ overgrowth phenotype (Bello et al., 2006; Betschinger et al., 2006; could be resolved by using the neurosphere stem cell self-renewal Choksi et al., 2006; Lee et al., 2006b; Lee et al., 2006c; Li and assay with Numb/Nbl mutant tissue, which, surprisingly, has never Vaessin, 2000; Wang, H. et al., 2006). Transplantation of larval brain been reported. An even better experiment would be to perform tissue from these mutants into adult Drosophila hosts also leads to clonal analysis of Numb/Nbl mutant cells in a wild-type background to determine whether the mutant cells leave the apical domain and metastatic tumor formation (Caussinus and Gonzalez, 2005). Each differentiate, or remain in the apical domain and form progenitor of these proteins probably has a slightly different mode of action. tumors or rosettes. Prospero is a transcriptional repressor that downregulates cell cycle A final complexity when studying Numb is to identify the relevant genes (Choksi et al., 2006; Li and Vaessin, 2000), whereas Brat is a effector(s). Numb can block Notch signaling (Yoon and Gaiano, translational repressor that is required to restrain cell growth, in part 2005), but it can also inhibit Shh signaling by promoting the by blocking myc (dm – FlyBase) translation (Betschinger et al., ubiquitylation of Gli proteins (Di Marcotullio et al., 2006), and it can 2006), as well as having a poorly understood role in maintaining elevate levels of the p53 tumor suppressor by blocking its Prospero levels (Bello et al., 2006; Betschinger et al., 2006; Lee et degradation (Colaluca et al., 2008). This latter function might be al., 2006c). Numb is a multi-functional protein that antagonizes highly relevant to NSC self-renewal, as a reduction of p53 leads to Notch signaling (Yoon and Gaiano, 2005), which is one mechanism increased NSC self-renewal at the expense of neuronal differentiation (Meletis et al., 2006; Piltti et al., 2006; Vanderluit et al., 2007). Thus, it uses to inhibit NSC self-renewal in Drosophila. However, both Numb and p53 may be required for timely neuronal mammalian Numb also regulates Hh signaling and levels of the differentiation. This model has yet to be tested in Drosophila. tumor suppressor p53 (Trp53) (see Box 2), which have yet to be tested for a role in Drosophila NB self-renewal. Two regulators of cortical polarity also act as tumor suppressors in Drosophila larval brain development: the Polo and Aurora A (Aurora Cell polarity and NSC self-renewal in mammals – FlyBase) kinases. Both are evolutionarily conserved centrosomal Neuroepithelial cells and radial glia both have an epithelial and cytoplasmic kinases that regulate cell cycle progression (Taylor morphology and apical/basal cell polarity (Fig. 3B); by contrast, and Peters, 2008). polo mutants have supernumerary larval NBs at the basal progenitors lack epithelial morphology and localization of expense of neurons, both in homozygous mutant larval brains and in apical/basal polarity markers has not yet been analyzed in these homozygous mutant single NB clones (Wang et al., 2007). This cells. Neuroepithelial cells localize the Par-complex proteins Par3 phenotype is partly due to the failure of polo mutants to phosphorylate (Pard3), Par6 (Pard6α), aPKC (aPKCλ; Prkcι) and Cdc42 to Partner of Numb, and the corresponding loss of the basal localization the apical cortex early in mouse cortical neurogenesis when of Numb. In addition, polo mutant NBs show uniform cortical aPKC. neuroepithelial/radial glial self-renewal is maximal, with levels Reduced Numb and ectopic aPKC in GMCs would both favor the declining at later stages concurrent with the loss of self-renewal transformation of GMCs into NBs. aurora A mutants show a similar potential (Cappello et al., 2006; Costa et al., 2008; Imai et al., phenotype: ectopic aPKC localization and reduced basal Numb 2006). Consistent with these findings, the reduction of Par3 or localization leading to an increase in NB numbers at the expense of Cdc42 levels in neuroepithelial cells (at E9.5 using Emx1-Cre for neurons (Lee et al., 2006a; Wang, H. et al., 2006). Whether these two Cdc42; at E10 using shRNA-expressing lentiviral vectors for Par3) kinases act in a common pathway (e.g. Aurora A activating Polo, or leads to loss of Pax6 neuroepithelial/radial glial cells, smaller vice versa) remains to be determined. clone sizes, and to precocious neuronal differentiation (Cappello et Rapid progress has been made in the last two years on the role of al., 2006; Costa et al., 2008). Conversely, the overexpression of cortical polarity in regulating NB self-renewal versus differentiation, Par3 or Par6 results in larger clone sizes that contain additional but many questions remain unsolved. How are apical and basal Pax6 NSCs (Costa et al., 2008). The removal of one of the two polarity proteins delivered and tethered to their respective aPKC isoforms (at E15.5 using nestin-Cre to remove aPKCλ) or of membrane domains? What are the targets of aPKC and the Notch Cdc42 (at E14 using GFAP-Cre) from radial glial cells led to a signaling pathway that promote NB self-renewal? Might it be similar but milder phenotype (Cappello et al., 2006; Imai et al., sufficient to merely prevent exposure of the NB to the differentiation 2006). Thus, Cdc42 and the Par complex are apical proteins that are factors Prospero and Brat? Do aPKC and Notch act in the same or necessary and sufficient to maintain NSC identity in the embryonic parallel pathways? Teasing out the relationship between cell cycle, cortex. These proteins have not yet been tested for a role in adult cell polarity and self-renewal will be a key task for the next few NSC self-renewal, in which apical/basal polarity is not as well years. defined. DEVELOPMENT 1582 REVIEW Development 135 (9) Mice mutant for the adherens junction (AJ) component α-catenin supernumerary stem cells, and shorter cell cycle times (Groszer lack AJs and have a faster neuroepithelial/radial glia cell cycle et al., 2001). Compared with the wild type, Pten mutant mice progression, which results in additional neuroepithelial/radial glia generate neurospheres that can be maintained for longer in serial stem cells and neurons being formed, without a change in their ratio. culture assays while maintaining their multi-lineage potential This results in enlarged brains (Lien et al., 2006). Transcriptional (Groszer et al., 2006). This indicates that Pten mutant stem cells profiling has shown that Hh-response genes are upregulated in α- have an increased self-renewal capability. Consistent with a role catenin mutant brains; indeed, virtually all aspects of the α-catenin for wild-type PTEN in promoting neuronal differentiation, human mutant phenotype can be suppressed by a Hh pathway inhibitor PTEN mutations are associated with brain tumors and (Lien et al., 2006). Do AJs act via a contact-based inhibition of a macrocephaly, and mouse Pten mutations with germline proliferation mechanism that keeps Hh levels low? If so, then why teratomas (reviewed by Stiles et al., 2004). In Drosophila, Pten is there no striking increase in stem cell proliferation following AJ co-localizes with the self-renewal-promoting factor aPKC (von disruption in Cdc42 or Par-complex mutant mice? One possibility Stein et al., 2005), so it is tempting to speculate that aPKC and is that the Cdc42-Par complex is required for both junctional Pten act antagonistically on common targets to regulate self- integrity and rapid cell cycle progression. renewal. Another apical protein that promotes NSC self-renewal in the embryonic cortex is the AJ protein β-catenin. Forced expression of Spindle orientation and self-renewal a stabilized β-catenin results in a large brain owing to increased Spindle orientation can impact stem cell self-renewal by positioning numbers of proliferative progenitors and a corresponding decrease daughter cells relative to extrinsic or intrinsic self-renewal cues. It in differentiated neurons (Chenn and Walsh, 2002). Because β- is thus important to monitor both extrinsic and intrinsic asymmetry catenin has a dual role, as a junctional protein and in canonical Wnt relative to spindle orientation, to determine which correlates with signaling, the phenotype could be due to increased Wnt signaling self-renewal. For example, a change in spindle orientation relative (which is linked to NSC self-renewal, see above) or to increased to extrinsic landmarks might be meaningless if there is no change in junctional stability, which might decrease the formation of basal the relationship of the spindle to functionally important intrinsic progenitors (owing to a failure to dissolve apical junctions). It would determinants. In the section below, I describe the progress, and be informative to distinguish these two pathways by specifically limitations, in our understanding of spindle orientation relative to reducing Wnt signaling (e.g. in Lef1/TCF1α mutants) or AJs (e.g. intrinsic and extrinsic cues and how it relates to NSC self-renewal. in Cdc42 mutants) to see which is required for the stabilized β- catenin phenotype. Spindle orientation in Drosophila neuroblasts If apical proteins promote NSC self-renewal, are basolateral Drosophila NBs invariably align their mitotic spindle along the proteins required for differentiation? The vertebrate Lgl1/2 (Llgl1/2 apical/basal cell polarity axis (Fig. 4), resulting in the NB inheriting in mouse) proteins are located basolaterally in Xenopus and in the apical proteins, while the differentiating GMC inherits the mammalian epithelia, as is the related Drosophila Lgl protein basally localized proteins (see Fig. 3). Spindle orientation is fixed at (reviewed by Lien et al., 2006). Drosophila lgl mutants have prophase, when one centrosome becomes anchored at the future increased NB numbers and decreased neuronal differentiation (Lee apical cortex, while the other migrates throughout the cytoplasm et al., 2006b); similarly, Lgl1-knockout mice have neuroepithelial before settling down at the basal cortex (Rebollo et al., 2007; Rusan cells with fewer AJs, increased proliferation, decreased neuronal and Peifer, 2007). By tightly linking spindle orientation with proven differentiation, and a neural rosette morphology that resembles that intrinsic determinants and with potential extrinsic cues, every NB of primitive neuroepithelial tumors (Klezovitch et al., 2004). The division results in a self-renewed NB and a differentiating daughter Lgl1 mutant phenotype might be partly due to reduced Numb cell. This precisely maintains brain NB numbers while constantly function, as Numb protein is delocalized in these mutants and increasing neuron numbers. expression of the Notch reporter Hes5 is increased (Klezovitch et Although it is commonly assumed that aligning the mitotic al., 2004). Thus, the basolateral Lgl1 protein is required for Numb spindle with the intrinsic cortical polarity axis is essential for localization and neuronal differentiation, paralleling its function in generating NB/GMC siblings, this has never been rigorously tested. the Drosophila CNS. For example, if the two spindle poles are functionally asymmetric, The role of the related Numb and numb-like proteins (henceforth as suggested by recent studies (Rebollo et al., 2007; Rusan and referred to as Numb/Nbl) in mammalian neurogenesis is Peifer, 2007), and this asymmetry helps specify NB versus GMC controversial (see Box 2). Recent microscopy studies clearly show identity, then any spindle axis may reliably generate NB and GMC that Numb localizes to AJs and to the basolateral membranes in siblings, irrespective of spindle/cortical polarity alignment. A embryonic neuroepithelial/radial glial cells and to the postnatal prerequisite for studying the role of spindle orientation in self- ependymal cells of the SVZ (Kuo et al., 2006; Rasin et al., 2007), renewal is to identify mutations that alter spindle orientation without consistent with previous reports of Numb having a basolateral disrupting cortical polarity; this has only been shown for one, localization in many animals, from fly to chick (see Rasin et al., perhaps two, genes so far. One is aurora A, which encodes a 2007). Thus, Numb is an evolutionarily conserved basolateral centrosomal and cytoplasmic kinase. aurora A homozygous mutants protein that is excluded from the apical membrane domain. assayed at an early larval stage, when some maternal Aurora A Identifying its precise role in NSC self-renewal, and the pathways protein was still present, showed defects in spindle alignment that it regulates, await more-detailed future studies. relative to apical/basal cortical polarity, and a slight increase in brain The kinase Akt (Akt1) and the phosphatase Pten have opposing NB numbers (Lee et al., 2006a; Wang, H. et al., 2006). However, functions in the Akt/Pten pathway (Narbonne and Roy, 2008), and neither study directly showed that the NBs with misaligned spindles have opposing NSC self-renewal phenotypes. Reduced Akt levels always or ever produced two sibling NBs. Furthermore, NBs from lead to loss of neuroepithelial/radial glia self-renewal in older mutants had ectopic cortical aPKC and delocalized Numb sequential neurosphere assays (Sinor and Lillien, 2004), whereas proteins, raising the concern that the younger mutants might have mice lacking Pten in the embryonic CNS have a larger brain, mild defects in aPKC or Numb that cause the increase in NB DEVELOPMENT Development 135 (9) REVIEW 1583 Fig. 4. Relationship between spindle orientation and sibling cell fate in neural stem cells. (A) Drosophila neuroblasts. Wild-type neuroblasts (left) invariably align their spindle along the apical/basal polarity axis, resulting in apical neuroblast (NB) and basal GMC cell fates. The cell fates that are acquired when the spindle is misoriented (e.g. in aurora A or mud mutants; right) have not been established. Spindle, blue lines; cleavage furrow plane, dotted line; apical domain, green; basal domain, red. (B) Vertebrate neuroepithelial cells. Conclusions from three different studies are shown. AJs, red balls; apical domain, green; basolateral domain, orange. SC, neuroepithelial cell; BP, basal progenitor; N, neuron. Apical is uppermost. (a) In Kosodo et al. (Kosodo et al., 2004), spindle orientation was concluded to regulate sibling cell fates in mouse embryonic neuroepithelial cells: if both siblings receive apical components (green), they both self-renew as progenitors (SCs; left); but if one cell lacks apical components, it differentiates into a neuron (N, middle and right). This type of asymmetric division can occur when the spindle is positioned in a near-planar orientation (middle) or in an apical/basal orientation (right). Thus, spindle orientation alone is insufficient to predict cell fate outcome. (b) Konno et al. (Konno et al., 2008) concluded that spindle orientation regulates sibling cell fates in mouse embryonic neuroepithelial cells: only siblings that inherit both apical and basal components will self-renew as progenitors (left); cells containing only apical domain become basal progenitors (BP; middle), whereas cells containing only the basal process become neurons (N; middle and right). (c) Morin et al. (Morin et al., 2007) concluded that spindle orientation does not affect sibling cell fates in the chick spinal cord. Normally, all divisions during early neurogenesis have a planar spindle orientation and form two progenitors (left). However, when the basolateral protein LGN (Gpsm2) was reduced by siRNA, the spindle could align with the apical/basal axis, yet both siblings still maintained progenitor identity by molecular marker expression (right), although the non-apical sibling was displaced out of the ventricular zone. Apical membrane and junction markers were not used in this study and thus are not shown. number. Stronger evidence that spindle orientation defects can lead siblings maintaining neuroepithelial/radial glial identity; by contrast, to expansion of the NB population comes from mushroom body vertical spindle alignment (along the apical/basal axis; apical/basal defective (mud) mutants. Mud shares domain organization and cell division) results in only the apical cell inheriting the apical limited sequence similarity with vertebrate NuMA (Numa1); both membrane domain and remaining a progenitor, with the most-basal are primarily localized to the centrosome, and Mud can also be sibling taking a neuronal fate (Cayouette and Raff, 2003; Chenn and detected at the apical cortex during prophase (Bowman et al., 2006; McConnell, 1995). More recently, it has been reported that planar Izumi et al., 2006; Siller et al., 2006), when spindle orientation is divisions might actually be asymmetric apical/basal cell divisions established (Rebollo et al., 2007; Rusan and Peifer, 2007; Siller and because the tiny apical domain is partitioned into only one sibling Doe, 2008). Similar to early aurora A mutants, mud mutants have (Kosodo et al., 2004) (see Fig. 4Ba). Furthermore, the long basal normal metaphase cortical polarity but fail to align the mitotic process may only be partitioned into one sibling in planar and spindle with the cortical polarity axis (see Fig. 4A), and have too apical/basal divisions (Das et al., 2003; Miyata et al., 2001; Miyata many brain NBs (Bowman et al., 2006; Izumi et al., 2006). It is et al., 2004). This raises an extremely important point: what is the important to note that the NBs with misaligned spindles were not structure that is associated with neuroepithelial/radial glial self- directly shown to produce two sibling NBs in these experiments. renewal – the apical domain, the AJs, the basal process, or none of The best experiment would be to perform in vivo live imaging of these? Two groups have reported that the apical cortical domain is a mutant NBs that express vital spindle, polarity and cell fate markers. good predictor of neuroepithelial progenitor fate (Kosodo et al., This would reveal whether spindle alignment defects always 2004; Sanada and Tsai, 2005) (see Fig. 4Ba). By contrast, another produce two NBs, whether they occasionally produce two GMCs, group has shown that only cells that inherit both the apical domain or whether spindle alignment is completely unrelated to the and the basal process will remain as neuroepithelial progenitors expansion in NB number in these mutants. (Konno et al., 2008) (Fig. 4Bb). Finally, another group reports that spindle orientation is unrelated to progenitor fate, but instead Spindle orientation in mammalian neural progenitors regulates daughter cell position (Morin et al., 2007) (Fig. 4Bc). The relationship between spindle orientation and cell fate has been A large group of centrosomal proteins are required to maintain studied in apical neuroepithelial cells and radial glia, but not in basal planar spindle orientation during the early phase of neuroepithelial progenitors. Neuroepithelial cells have a small prominin 1 apical expansion prior to E11.5 (Feng and Walsh, 2004; Fish et al., 2006; membrane domain that contacts the ventricular surface, as well as Konno et al., 2008; Morin et al., 2007; Xie et al., 2007). As predicted an adjacent ring of AJs and a long basal membrane domain that by the results of Konno et al., most of these mutants have a depleted contacts the pial surface. In early studies of neuroepithelial cells, a apical neuroepithelial pool and have ectopic proliferating cells in horizontal mitotic spindle alignment (perpendicular to the more-basal regions of the CNS (Konno et al., 2008). At least some apical/basal axis; planar cell division) was reported to result in both of these ectopic cells express neuroepithelial progenitor markers but DEVELOPMENT 1584 REVIEW Development 135 (9) not basal progenitor markers (Feng and Walsh, 2004; Fish et al., regulates neural progenitor cell proliferation in the larval central brain of Drosophila. Development 133, 2639-2648. 2006; Konno et al., 2008; Morin et al., 2007; Xie et al., 2007). Taken Bello, B., Holbro, N. and Reichert, H. (2007). Polycomb group genes are together, it appears that spindle orientation plays an important role required for neural stem cell survival in postembryonic neurogenesis of in maintaining neuroepithelial/radial glial progenitors within the Drosophila. Development 134, 1091-1099. Bertrand, N., Castro, D. S. and Guillemot, F. (2002). Proneural genes and the neuroepithelium, but the role of spindle orientation in regulating specification of neural cell types. Nat. Rev. Neurosci. 3, 517-530. sibling cell fate remains an open question. Betschinger, J., Mechtler, K. and Knoblich, J. A. (2003). The Par complex directs asymmetric cell division by phosphorylating the cytoskeletal protein Lgl. Nature Conclusions 422, 326-330. Betschinger, J., Mechtler, K. and Knoblich, J. A. (2006). 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Neural stem cells: balancing self-renewal with differentiation

Development , Volume 135 (9) – May 1, 2008

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© 2021 The Company of Biologists. All rights reserved.
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0950-1991
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10.1242/dev.014977
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Abstract

REVIEW 1575 Development 135, 1575-1587 (2008) doi:10.1242/dev.014977 Chris Q. Doe Stem cells are captivating because they have the potential to of proneural gene expression. High levels of the proneural genes make multiple cell types yet maintain their undifferentiated achaete, scute or lethal of scute repress Notch activity and state. Recent studies of Drosophila and mammalian neural stem promote NB formation; low levels of proneural gene expression cells have shed light on how stem cells regulate self-renewal allow high Notch activity, which maintains neuroectodermal fate versus differentiation and have revealed the proteins, processes and ultimately leads to epidermal differentiation (Artavanis- and pathways that all converge to regulate neural progenitor Tsakonas et al., 1991). Thus, proneural genes promote self-renewal. If we can better understand how stem cells neurogenesis (i.e. NB formation), whereas Notch signaling balance self-renewal versus differentiation, we will significantly inhibits neurogenesis. In this review, I briefly discuss embryonic advance our knowledge of embryogenesis, cancer biology and NBs and focus instead on the central brain NBs, where most is brain evolution, as well as the use of stem cells for therapeutic known about the mechanisms that regulate self-renewal. purposes. Larval NBs, which have many attributes of self-renewing stem cells, lie in a specialized cellular niche; they are undifferentiated, do Introduction not express any known neuron- or glial-specific markers; are highly A defining feature of stem cells is their ability to continuously proliferative yet never form tumors; can undergo mitotic quiescence maintain a stem cell population (self-renew) while generating without differentiating; and, most importantly, can generate differentiated progeny. Thus, stem cells are faced with a uniquely hundreds of neuronal progeny without losing their position, size, difficult task: to avoid cell cycle exit and differentiation, and to avoid identity or mitotic potential. These features make larval NBs an ideal uncontrolled proliferation and tumor formation. How stem cells system in which to study the basic biology of stem cell self-renewal walk this developmental tightrope is an extremely interesting (see Box 1 for NB-based self-renewal assays). However, there is a question that is of relevance to our understanding of the processes of potential limitation of larval NBs as a stem cell model: as they cell differentiation and cancer, and of the developmental diseases divide, they might gradually lose the ability to make early-born cell that result from the premature loss of stem cell pools. types within their lineage (termed a ‘progressive restriction in Here I review recent insights from studies of neural stem cells competence’), similar to the situation for embryonic NBs (Isshiki et (NSCs) in Drosophila and mice. There are surprising similarities in al., 2001; Pearson and Doe, 2003). If true, it would mean that the NB the transcription factor profiles of NSCs in flies and mice, although is not precisely self-renewing with every division. Nevertheless, many have not been functionally tested in both organisms. Both fly mammalian NSCs of the cortex and retina also undergo progressive and mammalian NSCs have unique cellular contacts, but the role of restriction (Desai and McConnell, 2000; Livesey and Cepko, 2001), these contacts (their ‘niche’) has only recently begun to be explored. and the study of Drosophila NBs might help us understand this Much more progress has been made on the role of cell polarity process. proteins in regulating self-renewal in Drosophila neuroblasts, and In the mammalian embryonic CNS, particularly in the ventral their conservation in mammalian cortical stem cells should lead to telencephalon during mid-neurogenesis and, to a lesser extent, in rapid progress in this system. Finally, I discuss the role of spindle the dorsal telencephalon, neuroepithelial cells give rise to radial orientation in regulating NSC self-renewal; recent identification of glia, which differentiate into basal progenitors that each form two mutants that disrupt spindle orientation without affecting cell postmitotic neurons (see Fig. 1B). Both radial glia and polarity in both flies and vertebrates now permits, for the first time, neuroectodermal cells can directly generate neurons (Gotz and time-lapse imaging studies to correlate spindle orientation, cell Huttner, 2005), and both neuroepithelial cells and radial glia can polarity components and sibling cell fate. The goal of this review is self-renew while producing basal progenitors, neurons or glia. to summarize recent research, to untangle conflicting results and to These self-renewing cell types share a similar epithelial highlight areas for future exploration. morphology (they span the neuroepithelium), both express the intermediate filament Nestin and have an apically located mitotic Neurogenesis in Drosophila and mammals spindle, and both can be distinguished by an array of molecular During Drosophila neurogenesis, neuroepithelial cells markers (Gotz and Huttner, 2005). By contrast, most basal differentiate into neuroblasts (NBs), which divide to form a NB progenitors lack self-renewal potential and typically generate two and a ganglion mother cell (GMC). GMCs are intermediate postmitotic neurons (Haubensak et al., 2004; Miyata et al., 2004; progenitors that have a limited mitotic potential and typically Noctor et al., 2001). They do not span the neuroepithelium and divide just once to generate a pair of postmitotic neurons (as undergo mitosis in a basal region termed the subventricular zone summarized in Fig. 1A). Embryonic neuroepithelial cells are bi- (SVZ) (Haubensak et al., 2004; Miyata et al., 2004; Noctor et potent cells that can form either NBs (stem-cell-like neural al., 2001; Noctor et al., 2004). Thus, only neuroectodermal progenitors) or epidermis. This choice is determined by the level and radial glial cells can self-renew, and as such are a focus of this review. [Excellent reviews have recently been published on neural progenitors of the mammalian spinal cord, retina, Howard Hughes Medical Institute, Institute of Neuroscience, Institute of Molecular adult hippocampus and dentate gyrus (see Chapouton et al., Biology, University of Oregon, Eugene, OR, USA. 2007; Gould, 2007; Ninkovic and Gotz, 2007; Sutter et al., e-mail: cdoe@uoregon.edu 2007).] DEVELOPMENT 1576 REVIEW Development 135 (9) Fig. 1. Neural stem cell formation and neuronal differentiation. (A) Drosophila neurogenesis. Neuroectodermal cells (NE; yellow, the apical surface is uppermost) give rise to neuroblasts (NBs; green) by delamination, and each NB divides in a stem cell mode to bud off a chain of ganglion mother cells (GMCs; orange) from its basal surface. GMCs are intermediate precursors that typically generate two postmitotic neurons (n; red). Larval NBs are closely associated with glia (blue). Thoracic and brain NBs become mitotically quiescent in late embryos and resume proliferation during larval stages. Approximate cell cycle times or quiescence times are given in hours (h). epi, embryonic epidermis. (B) Mammalian embryonic cortical neurogenesis. Initially the cortex has only neuroepithelial cells (NE; yellow, the apical surface is uppermost), which mature into radial glia (RG; green). Radial glia and neuroectodermal cells generate basal progenitors (BP; orange), which are intermediate progenitors that generate a limited number of neurons (n; red). NE and RG can also generate neurons. VZ, ventricular zone, adjacent to the lumen; SVZ, subventricular zone; MZ, marginal zone. The neural stem cell niche cells per clone but maintain the NB (Zhu et al., 2008). Thus, Activin Both Drosophila NBs and vertebrate NSCs lie in a unique cellular signaling regulates NB proliferation or neuronal survival, but not NB microenvironment compared with their differentiating progeny. self-renewal. Similarly, decreased expression of the FGF receptor Here I discuss the evidence for the role of niche-derived cues in (Branchless), Hh, or the Hh- and FGF-binding protein Perlecan (Trol regulating stem cell proliferation and self-renewal. – FlyBase), reduces the number of proliferating NBs. Adding exogenous human FGF2 or increasing Cyclin E levels after this The Drosophila NSC niche phenotype has become apparent rescues and returns to normal the Larval NBs contact cortex glial cells on their apical and lateral sides number of proliferating NBs (Park et al., 2003), indicating that the (Dumstrei et al., 2003), while the basal side forms E-cadherin-rich affected NBs were mitotically quiescent rather than dead or contacts with new-born GMCs (Fig. 2A). Larval glia secrete the differentiated. Thus, the mitogens FGF and Hh are necessary for Anachronism (Ana) protein, which keeps NBs quiescent during maintaining NB proliferation but not for NB self-renewal or survival. early larval stages (Ebens et al., 1993). The possibility that glial- Finally, there is evidence that Notch signaling regulates NB self- derived signals also promote larval NB proliferation is supported by renewal. Notch signaling is robust in larval NBs, based on the the glial-specific expression of a dominant-negative E-cadherin strong, specific expression of a Notch reporter gene (Almeida and protein, which results in fewer proliferating NBs (Dumstrei et al., Bray, 2005). Reducing Notch activity decreases central brain NB 2003). Although the cellular basis for this phenotype is unknown, it numbers (Wang et al., 2007), but has no effect on thoracic NB is consistent with the loss of a glial-neuroblast contact and failure to numbers (Almeida and Bray, 2005); conversely, increasing Notch transmit a glial-derived proliferation cue. Alternatively, non-specific activity by expressing a constitutively active Notch intracellular effects, such as loss of the glial brain/hemolymph barrier, could domain or by removing the Notch inhibitor Numb increases brain generate this phenotype. NB numbers (Lee et al., 2006a; Wang, H. et al., 2006). The identity Is there any evidence that the Transforming growth factor β and cellular source of the Notch ligand have not been determined, (TGFβ), Activin, Notch, Wnt, Hedgehog (Hh) or Fibroblast growth but this pathway is clearly implicated in supporting NB self-renewal, factor (FGF) signaling pathways have a role in maintaining similar to its role in mammals (see below). Drosophila NB self-renewal or proliferation, as described below for mammalian NSCs? Recent work suggests that Activin, Hh and FGF The mammalian NSC niche promote NB proliferation, whereas Notch signaling promotes NB Neuroepithelial and radial glial cells have a columnar epithelial self-renewal; the role of the Wnt pathway has not yet been addressed. morphology. Their apical process is exposed to the ventricular fluid, Activin and the redundant Activin-related Daw ligands are expressed their basal (pial) process contacts the extracellular matrix (ECM), in larval brain glia. NBs that lack the Activin receptor contain fewer and they have lateral contacts with each other, including at the region DEVELOPMENT Development 135 (9) REVIEW 1577 Box 1. Neural stem cell self-renewal assays in flies and mice Neural stem cell (NSC) self-renewal assays in Drosophila include scoring the total number of NBs in the larval brain. Normally, ~200 central brain NBs exist per larval brain; mutants with increased NB self-renewal are expected to have increased NB numbers. Conversely, mutants in which NB self-renewal has failed should have fewer NBs. Confirmation of a protein’s role in NSC self-renewal in Drosophila is typically achieved by generating genetically marked single NB mutant clones. In wild-type flies, marked clones consist of a single NB and a family of marked GMC/neuronal progeny. If the mutated gene normally promotes differentiation, a homozygous mutant clone will generate multiple NBs. If the mutated gene normally promotes NB self-renewal, NBs will often be lost from the clone. In mammals, self-renewal assays include a similar clonal analysis following the viral delivery of a marker gene (such as GFP) co- expressed with a gene-overexpression construct or a gene- knockdown construct [such as a short-hairpin RNA (shRNA)]. The in vitro ‘neurosphere assay’ is also widely used (Breunig et al., 2007), in which single cells derived from dissociated cortical tissue are cultured in vitro to determine the percentage that can generate a multi- lineage primary clone that contains single cells competent to generate a secondary multi-lineage neurosphere (reviewed by Jensen and Parmar, 2006). Fig. 2. Neural stem cell niche. (A) Drosophila larval neuroblast (NB) niche. (Left) Schematic showing NB/glial contact (light blue bars) and cadherin-rich NB/ganglion mother cell (GMC) contacts (red bars). (Right) Confocal image showing glial membrane staining relative to of subapical adherens junctions (Fig. 2B). Thus, cues from apical, neuroblast and GMC membranes. Glial membrane was detected with basal or lateral directions could modulate neuroepithelial/radial glial repo-gal4 UAS-cd8:GFP (blue, or as a single channel in the right-hand self-renewal. Here I focus on the well-characterized roles of the image). NB, GMC and neuronal membranes were labeled with Wnt, Notch and sonic hedgehog (Shh) pathways in regulating Scribbled (red). Image courtesy of Jason Q. Boone (University of mammalian NSC self-renewal. Evidence for the role of the Oregon). (B) Mammalian neuroepithelia/radial glia niche. (Left) JAK/STAT, FGF, TGFβ and Toll-related pathways in regulating Neuroepithelial/radial glia cells contact the ventricular fluid at their apical domain (green dots), neighboring cells via cadherin-rich adherens NSC proliferation and possibly self-renewal is summarized junctions (AJs; red bars), and the basal lamina at their basal domain elsewhere (Rolls et al., 2007; Shi et al., 2008). (dashed lines). Basal progenitors (BP; orange) and neurons (n; red) lack The canonical Wnt pathway promotes neuroepithelial/radial access to apical and basal cues. (Right) E10 mouse neuroepithelial cells glial identity. The reduction of Wnt ligand levels or the removal showing cadherin enrichment at the subapical AJs (red) and prominin 1 of the canonical pathway component β-catenin results in fewer localization to the apical domain (green) that contacts the ventricular neuroepithelial/radial glial stem cells and in precocious neuronal fluid [modified and reproduced with permission from Kosodo et al. differentiation (Machon et al., 2003; Zechner et al., 2003). By (Kosodo et al., 2004)]. contrast, increased Wnt signaling expands the stem cell pool (Chenn and Walsh, 2002; Machon et al., 2007; Viti et al., 2003; Woodhead et al., 2006; Zechner et al., 2003). Wnt signaling also Thus, further study is needed to identify the context-dependent promotes NSC self-renewal during postnatal neurogenesis factors that switch Wnt signaling between promoting and (Machon et al., 2007; Machon et al., 2003; Wexler et al., 2008; inhibiting NSC self-renewal. Zhou et al., 2004), where it can also promote the proliferation of Notch signaling components are expressed in embryonic committed neuronal progenitors (Lie et al., 2005). Wnt signaling neuroepithelial/radial glial stem cells, as well as in adult NSCs directly activates cyclin D and the NSC factors Sox2 and Rest (Mizutani et al., 2007; Stump et al., 2002). Mutations in the genes (RE1-silencing transcription factor) (Megason and McMahon, encoding Dll1 (a Notch ligand), Notch1 (a Notch receptor), RBPJk 2002; Nishihara et al., 2003; Takemoto et al., 2006), which may (Rbpj – Mouse Genome Informatics; a Notch transcriptional contribute to NSC maintenance. Later in cortical development, effector), Hes1, Hes3 or Hes5 (RBPJk-induced transcription factors) Wnt signaling is a potent inducer of neuronal differentiation, in all lead to the depletion of radial glia stem cells and to precocious part by activating the proneural gene neurogenin 1 (Ngn1; neuronal differentiation in the mouse embryo (de la Pompa et al., Neurog1) (Hirabayashi et al., 2004; Israsena et al., 2004; 1997; Handler et al., 2000; Hatakeyama et al., 2004; Mizutani et al., Muroyama et al., 2004; Viti et al., 2003). The difference in early 2007; Yoshimatsu et al., 2006), and to NSC loss in the adult (Gaiano versus late Wnt function is highlighted by the observation that the et al., 2000). Conversely, misexpression of Hes1, Hes3 or of expression of stabilized β-catenin at embryonic day (E) 10 activated Notch in the embryonic cortex blocks neuronal promotes neuroepithelial proliferation and self-renewal (Chenn differentiation (Chambers et al., 2001; Ishibashi et al., 1994). Radial and Walsh, 2002), whereas at E14 it promotes neuronal glia stem cells from Dll1, Notch1, Rbpj, Hes1 and Hes5 mouse differentiation (Hirabayashi and Gotoh, 2005). It has been mutants all have a reduced neurosphere-forming ability (see Box 1), proposed that Wnt alone stimulates neuronal differentiation, indicating that they have a reduced ability to self-renew (Hitoshi et whereas Wnt plus the mitogen Fgf2 inhibits neural differentiation al., 2002; Ohtsuka et al., 2001; Yoon et al., 2004). Furthermore, (Israsena et al., 2004; Viti et al., 2003), although evidence against radial glial cells that express a Notch-induced GFP reporter can be this model has also been presented (Hirabayashi et al., 2004). sorted by flow cytometry into Notch-high (GFP+) and Notch-low DEVELOPMENT 1578 REVIEW Development 135 (9) (GFP–) populations; the Notch-high cells are more potent at Transcriptional regulation and NSC self-renewal in generating primary and secondary neurospheres, and can be Drosophila transplanted in vivo to generate all three neural lineages – neurons, Genes transiently expressed in newly formed NBs include the astrocytes and oligodendrocytes (Mizutani et al., 2007). Thus, Notch proneural genes achaete, scute and lethal of scute. These encode signaling correlates with, and is required for, the maintenance of basic helix-loop-helix (bHLH) TFs that promote the transition of a embryonic and postnatal NSCs. neuroectodermal cell to a NB, and thus are responsible for triggering The Shh pathway includes the Shh ligand, the transmembrane NB delamination and NB-specific gene expression (epithelial genes smoothened (Smo) protein, and the nuclear effectors Gli2/3, as well off, NB-specific genes on). This is partly accomplished by the as many other proteins. Shh is expressed in the embryonic transient suppression of Notch signaling, as Notch signaling is neuroepithelium (Lai et al., 2003), and in regions of adult necessary and sufficient to maintain neuroectodermal cell fate neurogenesis – the hippocampus and dentate gyrus (Ahn and Joyner, (reviewed by Artavanis-Tsakonas et al., 1991). Several other TFs are 2005; Machold et al., 2003; Palma et al., 2005). When conditional expressed in subsets of neuroectoderm and delaminating NBs, Smo mutants are used to block Hh signaling in the postnatal where they collaborate with the proneural proteins to promote NB hippocampus and dentate gyrus, these tissues produce fewer primary formation. These include the SoxB group genes SoxN and Dichaete, neurospheres when cultured in vitro (Machold et al., 2003). which encode high mobility group (HMG) transcriptional activators However, this effect could be due to a reduced stem cell population (Cremazy et al., 2000; Nambu and Nambu, 1996; Russell et al., prior to explant; the ability to form multi-lineage secondary 1996). The first function of the proneural and SoxB genes is to neurospheres was not assayed, which would have tested for stem cell induce neurogenesis within the ventral ectoderm, which otherwise self-renewal during neurosphere passage (see Box 1). Subsequent would produce only epidermis. experiments showed that E18.5 cortical tissue from Gli2 or Gli3 A second class of NB TFs are permanently expressed in NBs but mutant mice were deficient in both primary and secondary are not maintained in their GMC/neuronal progeny. These TFs are neurosphere formation, providing evidence that Shh promotes stem the best candidates for promoting NB self-renewal, and include the cell self-renewal (Palma and Ruiz i Altaba, 2004). Furthermore, zinc-finger protein Worniu, the bHLH proteins Deadpan and there is compelling evidence that Shh both promotes proliferation Asense, and the SoxB family member SoxN (Ashraf and Ip, 2001; and inhibits differentiation in postnatal cerebellar granule cell Bier et al., 1992; Brand et al., 1993; Cai et al., 2001; Cremazy et al., precursors (Argenti et al., 2005; Wechsler-Reya and Scott, 1999). 2000). Surprisingly, very little is known about the function of these Finally, it has recently been proposed that Shh might promote the genes in regulating NB self-renewal. deadpan and asense single transition of stem cells to more-rapidly dividing, committed mutants have only mild post-embryonic CNS defects (Bier et al., progenitors (Agathocleous et al., 2007), rather than maintaining 1992; Brand et al., 1993), although Deadpan can repress expression stem cell identity per se. Thus, the role of Shh in promoting NSC of the cell cycle inhibitor dacapo (Wallace et al., 2000), consistent self-renewal needs further investigation. with a role in promoting NB proliferation. Similarly, worniu mutants Integrins are a family of cell-surface adhesion and signaling have mild defects in larval CNS axial shortening (Ashraf et al., proteins that bind ECM proteins, such as laminin. β1-integrin 2004), and the Sox gene mutants have reduced embryonic NB (Itgβ1) is enriched at regions that contain embryonic and adult numbers, but this is probably due to a failure in NB formation not NSCs, and at the periphery of neurospheres where NSCs reside self-renewal (Cremazy et al., 2000; Nambu and Nambu, 1996; (Campos et al., 2004). When forebrain tissue from postnatal day 1 Russell et al., 1996; Zhao et al., 2007). It is tempting to speculate that mutant mice that carry floxed Itgb1 alleles is depleted of β1-integrin the Sox TFs act in NBs to prevent neuronal differentiation initiated over a 10-day period, nestin stem cells from this tissue show a by the proneural genes, similar to the proposed role of SoxB1 family reduced neurosphere-forming ability and increased cell death TFs in vertebrates (see below). However, the function of Sox TFs in (Leone et al., 2005), indicating that integrin signaling might also self-renewal has not yet been tested. promote NSC survival. The flip side of NB self-renewal is neuronal differentiation. NBs Overall, findings to date show that in both mammals and flies, rapidly lose the expression of the proneural genes, so what might Notch signaling promotes NSC self-renewal. Wnt and Shh promote neuronal differentiation in their lineage? The divergent pathways might also regulate NSC self-renewal in mammals, but homeodomain TF Prospero is crucial for initiating neuronal this role has yet to be tested in Drosophila. Less, however, is known differentiation. prospero is transcribed and translated in all NBs, but about the cellular nature of the niche. In the mammalian cortex, it is exported from the nucleus (Demidenko et al., 2001); the mRNA is not clear whether self-renewal cues come from ventricular fluid, and protein are segregated into the GMC during NB asymmetric cell the basal ECM, the neuroepithelial/radial glial cells themselves, or division (Broadus et al., 1998; Knoblich et al., 1995; Spana and Doe, none of the above. In Drosophila, existing data suggest that glial 1995), where the protein enters the nucleus to repress cell cycle cells are required for larval NB proliferation, but whether they serve genes and activate neural differentiation genes (Choksi et al., 2006; as a local NB niche needs to be directly tested by glial ablation Li and Vaessin, 2000). When prospero mutant clones are induced in experiments. single larval NBs, many GMCs fail to differentiate and instead form NB tumors (Bello et al., 2006; Betschinger et al., 2006; Choksi et Nuclear control of self-renewal al., 2006; Lee et al., 2006c). The recent identification of transcription factors (TFs) that are What about chromatin remodeling genes? In mammals, the sufficient to reprogram human differentiated cells into cells that Polycomb group chromatin remodeling factor Bmi1 is required for resemble embryonic stem (ES) cells (Takahashi et al., 2007; Yu et postnatal NSC renewal (Molofsky et al., 2005; Molofsky et al., al., 2007) indicates that there also might be TFs or chromatin factors 2003), raising the possibility that Drosophila NBs might also require that specify the identity of tissue-specific stem cells. Numerous TFs Polycomb for self-renewal. A recent paper tests this hypothesis by are also known to be expressed in NSCs (see Table 1). In this generating mutant clones null for several Polycomb group genes section, I discuss TF/chromatin factor expression and function in within single larval NBs (Bello et al., 2007). All Polycomb group NSCs. mutant clones had fewer neurons and lacked the NB, consistent with DEVELOPMENT Development 135 (9) REVIEW 1579 Table 1. Regulators of gene expression involved in neural stem cell self-renewal Drosophila Mammals † ‡ § † ‡ § Protein Expression Function References Protein Expression Function References SoxB NE, NB FSR (Cremazy et al., 2000; SoxB1 NE, RG FSR (Bylund et al., 2003; Nambu and Nambu, 1996; Ferri et al., 2004; Russell et al., 1996) Graham et al., 2003; Wang, T. W. et al., 2006) E(spl)m NB ? (Almeida and Bray, 2005) Hes1,3,5 NE, RG FSR (Hatakeyama et al., 2004; Ishibashi et al., 1994) Deadpan NB ? (Bier et al., 1992) Hes related* Worniu NB ? (Ashraf and Ip, 2001) Slug/Snail related* Asense NB ? (Brand et al., 1993) Mash3 related* Musashi NB ? (Nakamura et al., 1994) Musashi NE, RG ? (Kaneko et al., 2000; Siddall et al., 2006) – – – Rest NE, RG FSR (Ballas and Mandel, 2005) Proneural NE, NB FDIFF (Skeath and Carroll, 1994) Proneural NE, N FDIFF (Guillemot, 2007) Prospero GMC FDIFF (Bello et al., 2006; Choksi Prox1 BP, N FDIFF (Dyer, 2003; Lavado et al., 2006; Knoblich et and Oliver, 2007; Torii al., 1995; Lee et al., 2006c; et al., 1999) Spana and Doe, 1995) Brain tumor GMC FDIFF (Bello et al., 2006; Lee et Trim3 ? ? al., 2006c; Betschinger et al., 2006) p53 ? ? p53 N FDIFF (Meletis et al., 2006) Protein orthologs or homologs are shown on the same line; –, gene ortholog has not been identified; *, groups of related proteins. Expression in Drosophila neuroectoderm (NE), neuroblasts (NB), ganglion mother cells (GMC), neurons (N); or in mammalian telencephalon neuroepithelium (NE), radial glia (RG), basal progenitors (BP), neurons (N). Function in promoting self-renewal (FSR) or in promoting differentiation (FDIFF); ?, functional studies have not been reported. a failure in NB self-renewal. However, the co-expression of the cell induced by the proneural proteins Mash1 and the Ngns (Bertrand et death inhibitor p35 rescued NB survival and normal clone size. al., 2002; Bylund et al., 2003; Ge et al., 2006), and proneural Thus, the Polycomb group proteins are required to maintain NB proteins can directly bind and inhibit SoxB1 protein function. Thus, survival, but are dispensable for larval NB self-renewal (Bello et al., the balance of SoxB1 and proneural activity determines the tempo 2007). of neurogenesis. How this balance is regulated over time is In conclusion, proneural genes promote NB expression of unknown. One additional factor that promotes NSC self-renewal is Worniu, Deadpan, Asense and Prospero. The first three TFs are good the Rest transcriptional repressor, which is expressed in NSCs and candidates for maintaining NB self-renewal, whereas Prospero is in most non-neuronal cells, where it induces a repressive chromatin asymmetrically localized into the GMC where it promotes neuronal state that blocks the expression of neuronal differentiation genes differentiation. This is an elegant mechanism for ensuring NB (Ballas et al., 2005). Neurons express a small modulatory double- homeostasis while producing a constant stream of neurons. stranded (ds) RNA that induces differentiation by blocking Rest activity at the protein level (not the RNA level, surprisingly) Transcriptional regulation and NSC self-renewal in (Kuwabara et al., 2004). Lastly, the RNA-binding protein musashi mammals is expressed in both germline and NSCs (Kaneko et al., 2000; As in Drosophila, the bHLH proneural proteins Mash1 (Ascl1), Siddall et al., 2006); it promotes germline stem cell self-renewal Ngn1 and Ngn2 (Neurog2) are expressed in mammals in partially (Siddall et al., 2006), but its function in NSC self-renewal is yet to overlapping populations of neuroepithelial cells, where they are be determined. required for the acquisition of NSC properties. Subsequently, they An important stem cell attribute is the ability to proliferate. are maintained in newly differentiating neurons, where they induce Maintenance of postnatal NSC proliferation is partly regulated by neuronal differentiation (Guillemot, 2007). How do neuroepithelial the Polycomb group transcriptional repressor Bmi1. Loss of Bmi1 cells and radial glia express these proneural genes without results in an increase of the cell cycle inhibitor p16Ink4a (Cdkn2a) differentiating? This is the role of the SoxB1 family members (Sox1, and in postnatal stem cell depletion, without affecting embryonic 2, 3). The SoxB1 proteins are expressed in embryonic and adult NSCs (Molofsky et al., 2005; Molofsky et al., 2003). One important NSCs, as well as in a few postmitotic neurons (Graham et al., 2003; negative regulator of proliferation might be Prox1, which is related Wang, T. W. et al., 2006). A reduction in SoxB1 levels leads to to the Drosophila transcriptional repressor Prospero. Mash1 induces precocious neural differentiation and to the depletion of the Prox1 expression in newly differentiating neurons (Torii et al., progenitor pool, whereas misexpression of SoxB1 family members 1999), and Prox1 inhibits proliferation in the mammalian retina can block neuronal differentiation and maintain the progenitor (Dyer, 2003; Li and Vaessin, 2000), and might have a similar population (Bylund et al., 2003; Ferri et al., 2004; Graham et al., function in the cortex. Experimentally lengthening the cell cycle also 2003), although without maintaining proliferation (Bylund et al., increases progenitor differentiation (Calegari and Huttner, 2003). 2003). SoxB1 TFs antagonize the neuronal differentiation that is Thus, slowing or stopping the cell cycle can induce neuronal DEVELOPMENT 1580 REVIEW Development 135 (9) differentiation, and prolonging cell cycle progression can prevent stem cell depletion, although quiescent stem cells clearly have a mechanism to prevent differentiation. How self-renewal and cell cycle pathways intersect will be an important and challenging area of future research. In conclusion, data from flies and mice are consistent with a common model for neurogenesis, in which SoxB1 proteins confer progenitors (neuroepithelial cells in vertebrates, neuroectodermal cells and NBs in flies) with the potential to self-renew. Proneural proteins then induce progenitor delamination and neural differentiation, the latter being blocked by SoxB1 proteins. Finally, nuclear Prospero/Prox1 initiates cell cycle exit and neural differentiation. Several aspects of this model remain to be tested, including the role of the Drosophila SoxB proteins in antagonizing proneural activity and in promoting self-renewal, and the role of vertebrate Prox1 in promoting neuronal differentiation. Cell polarity and self-renewal Recent data suggest that cell polarity plays a key role in regulating self-renewal versus differentiation in both fly and mammalian NSCs, and that several of the proteins involved have evolutionarily conserved functions. But there are some surprising differences, and many proteins have only been tested in one animal to date. Fig. 3. Neural stem cell polarity. (A) Drosophila neuroblast cell polarity. (Left) Proteins that asymmetrically localize in a dividing NB. Cell polarity and NSC self-renewal in Drosophila Green, apical proteins; red, basal proteins, including those associated Drosophila NBs divide asymmetrically to self-renew a NB while with GMC contact site. Lines between proteins indicate physical budding off a small, differentiating GMC. A growing number of interactions. Baz binds Par6 and aPKC (not shown). Arrow indicates proteins are known to be segregated into the NB or GMC during this that Baz is required for Cdc42 localization. T-bar indicates that aPKC asymmetric cell division. Proteins segregated into the NB include excludes Lgl, Numb and Mira from the cortex, and Lgl excludes aPKC. Bazooka (Baz/Par3), Cdc42, Par6, atypical protein kinase C (aPKC) (Right) Schematic of a dividing NB, showing the apical (green) and basal (which may all form a single protein complex), Inscuteable (Insc), (red) cortical domains; spindle and centrosomes, brown; DNA, yellow. Partner of Inscuteable (Pins; Rapsynoid – FlyBase) and Gαi (B) Mammalian neuroepithelial (NE) cell polarity. (Left) Proteins that asymmetrically localize in a dividing NE cell. Green, apical proteins; red, (G-iα65A – FlyBase) (which may form a distinct protein complex AJ-enriched proteins; blue basolateral proteins. Lines between protein that links to Baz via Insc). Proteins partitioned into the GMC include names indicate physical interactions. Par3 binds Par6 and aPKC (not the scaffolding protein Miranda and its cargo proteins Staufen, shown). T-bar indicates that aPKC excludes Lgl and Numb from the Prospero and Brain tumor (Brat), as well as Numb and Partner of cortex. (Right) Schematic of a dividing NE (center). Spindle and Numb (Caussinus and Hirth, 2007; Gonzalez, 2007) (see Fig. 3A). centrosomes, brown; DNA, yellow. Baz, Bazooka (fly Par3); Insc, The first protein identified to positively regulate NB self-renewal Inscuteable; Pins, Partner of Insc; aPKC, Atypical protein kinase C; Lgl, was aPKC. aPKC mutants have fewer NBs per larval brain lobe, and Lethal giant larvae; Pon, Partner of Numb; Mira, Miranda; Pros, overexpression of a membrane-tethered aPKC in NBs dramatically Prospero; Brat, Brain tumor; Stau, Staufen; Ecad, E-cadherin; Apc2, increases brain NB numbers (Lee et al., 2006b). Similarly, lethal (2) Adenomatosis polyposis coli 2; Prom1, prominin 1; αcat, α-catenin; giant larvae (lgl) mutants have ectopic cortical aPKC in NBs and a βcat, β-catenin; LGN (Gpsm2), a homolog of Pins. corresponding increase in brain NB numbers that can be fully suppressed in lgl aPKC double mutants (Lee et al., 2006b). Taken together, these data show that aPKC is sufficient to turn GMCs into data showing that increasing cell cycle length triggers the NBs (ectopic NB self-renewal), but it is not absolutely required for differentiation of vertebrate neural progenitors (Calegari and NB self-renewal as aPKC mutants maintain a subset of their brain Huttner, 2003). Consistent with this model, aPKC mutant NBs NBs. aPKC probably acts redundantly with a second pathway to prematurely stop dividing (Rolls and Doe, 2004), although whether promote NB self-renewal, most likely the Notch pathway, which is the NB becomes quiescent, dies or differentiates is unknown. It also sufficient but not necessary for NB self-renewal (see above). would be interesting to determine if the overexpression of aPKC can This model needs to be tested by assaying Notch aPKC double speed up the GMC cell cycle, and whether this is the cause of the mutants for a complete loss of NB self-renewal. extra NB phenotype; conversely, does lengthening the NB cell cycle How does aPKC promote self-renewal? One attractive model is induce precocious differentiation and reduced NB numbers? that aPKC phosphorylates and inactivates neuronal differentiation A second protein required for NB self-renewal is Pins, a factors – such as Lgl, Numb or the Miranda-Prospero-Brat complex scaffolding protein that binds to Gαi, Insc and many other proteins – to keep these proteins out of the self-renewing NB. For example, (reviewed by Wodarz and Nathke, 2007). pins mutants initially show aPKC is known to phosphorylate and inhibit the cortical localization normal NB numbers in early larval development but have of Numb (Smith et al., 2007) and Lgl (Betschinger et al., 2003), as dramatically fewer NBs in late larval stages (Lee et al., 2006b). In well as to inhibit the cortical localization of Miranda by an unknown addition, whereas wild-type NB clones always contain one NB and mechanism (Rolls et al., 2003). A more speculative model is that a family of GMC/neuronal progeny, pins mutant NB clones contain aPKC positively regulates cell cycle progression, and a speedy cell fewer total cells and often lack a NB (Lee et al., 2006b). pins mutants cycle promotes stem cell self-renewal. This model is inspired by fail to localize aPKC to the apical cortex of larval NBs (Lee et al., DEVELOPMENT Development 135 (9) REVIEW 1581 2006b), which may contribute to the defect in self-renewal, but it is Box 2. The complexity of being Numb unknown whether forced expression of membrane-tethered aPKC In Drosophila, Numb is required to promote neuronal differentiation can rescue the pins mutant phenotype. Surprisingly, pins mutant and to inhibit NB self-renewal (Lee et al., 2006c; Wang, H. et al., brain tissue can form tumors when transplanted into adult hosts 2006). In mammals, the situation is more complex. The conditional (Caussinus and Gonzalez, 2005). The reason for this discrepancy is deletion of Numb/Nbl early or late in neurogenesis (at E8.5 or E14, unknown, but a possible explanation is that transplanted cells are respectively) results in loss of neuroepithelial/radial glial progenitors prone to genomic instability (Caussinus and Gonzalez, 2005), and (Petersen et al., 2002; Petersen et al., 2004). Conversely, the removal any pins mutant cell that loses the lgl gene from the tip of of Numb/Nbl with Emx1-Cre at E9.5 results in neural progenitor chromosome 2 would generate lgl pins double-mutant cells that are hyperproliferation, delayed cell cycle exit, and depletion of late-born known to form massive brain tumors (Lee et al., 2006b). neurons (Li et al., 2003). Most recently, the same Numb/Nbl Emx1- Cre conditional mutant was shown to have a loss of adherens Proteins that negatively regulate NB self-renewal (i.e. that junctions and defective apical/basal polarity owing to reduced E- promote neuronal differentiation) usually segregate into the cadherin-positive vesicle targeting to the junctional domain (Rasin et differentiating GMC during NB asymmetric cell division, and al., 2007). This might deplete NSC numbers, as seen with the early include the Miranda coiled-coil scaffolding protein, its cargo loss of apical/basal polarity following Par3 or Par6 depletion, but in proteins Prospero and Brat, Lgl and Numb. Loss of any of these fact the authors report no effect on progenitor maintenance or proteins transforms GMCs into NBs and produces a stem cell neuronal differentiation (Rasin et al., 2007). This ‘Numb paradox’ overgrowth phenotype (Bello et al., 2006; Betschinger et al., 2006; could be resolved by using the neurosphere stem cell self-renewal Choksi et al., 2006; Lee et al., 2006b; Lee et al., 2006c; Li and assay with Numb/Nbl mutant tissue, which, surprisingly, has never Vaessin, 2000; Wang, H. et al., 2006). Transplantation of larval brain been reported. An even better experiment would be to perform tissue from these mutants into adult Drosophila hosts also leads to clonal analysis of Numb/Nbl mutant cells in a wild-type background to determine whether the mutant cells leave the apical domain and metastatic tumor formation (Caussinus and Gonzalez, 2005). Each differentiate, or remain in the apical domain and form progenitor of these proteins probably has a slightly different mode of action. tumors or rosettes. Prospero is a transcriptional repressor that downregulates cell cycle A final complexity when studying Numb is to identify the relevant genes (Choksi et al., 2006; Li and Vaessin, 2000), whereas Brat is a effector(s). Numb can block Notch signaling (Yoon and Gaiano, translational repressor that is required to restrain cell growth, in part 2005), but it can also inhibit Shh signaling by promoting the by blocking myc (dm – FlyBase) translation (Betschinger et al., ubiquitylation of Gli proteins (Di Marcotullio et al., 2006), and it can 2006), as well as having a poorly understood role in maintaining elevate levels of the p53 tumor suppressor by blocking its Prospero levels (Bello et al., 2006; Betschinger et al., 2006; Lee et degradation (Colaluca et al., 2008). This latter function might be al., 2006c). Numb is a multi-functional protein that antagonizes highly relevant to NSC self-renewal, as a reduction of p53 leads to Notch signaling (Yoon and Gaiano, 2005), which is one mechanism increased NSC self-renewal at the expense of neuronal differentiation (Meletis et al., 2006; Piltti et al., 2006; Vanderluit et al., 2007). Thus, it uses to inhibit NSC self-renewal in Drosophila. However, both Numb and p53 may be required for timely neuronal mammalian Numb also regulates Hh signaling and levels of the differentiation. This model has yet to be tested in Drosophila. tumor suppressor p53 (Trp53) (see Box 2), which have yet to be tested for a role in Drosophila NB self-renewal. Two regulators of cortical polarity also act as tumor suppressors in Drosophila larval brain development: the Polo and Aurora A (Aurora Cell polarity and NSC self-renewal in mammals – FlyBase) kinases. Both are evolutionarily conserved centrosomal Neuroepithelial cells and radial glia both have an epithelial and cytoplasmic kinases that regulate cell cycle progression (Taylor morphology and apical/basal cell polarity (Fig. 3B); by contrast, and Peters, 2008). polo mutants have supernumerary larval NBs at the basal progenitors lack epithelial morphology and localization of expense of neurons, both in homozygous mutant larval brains and in apical/basal polarity markers has not yet been analyzed in these homozygous mutant single NB clones (Wang et al., 2007). This cells. Neuroepithelial cells localize the Par-complex proteins Par3 phenotype is partly due to the failure of polo mutants to phosphorylate (Pard3), Par6 (Pard6α), aPKC (aPKCλ; Prkcι) and Cdc42 to Partner of Numb, and the corresponding loss of the basal localization the apical cortex early in mouse cortical neurogenesis when of Numb. In addition, polo mutant NBs show uniform cortical aPKC. neuroepithelial/radial glial self-renewal is maximal, with levels Reduced Numb and ectopic aPKC in GMCs would both favor the declining at later stages concurrent with the loss of self-renewal transformation of GMCs into NBs. aurora A mutants show a similar potential (Cappello et al., 2006; Costa et al., 2008; Imai et al., phenotype: ectopic aPKC localization and reduced basal Numb 2006). Consistent with these findings, the reduction of Par3 or localization leading to an increase in NB numbers at the expense of Cdc42 levels in neuroepithelial cells (at E9.5 using Emx1-Cre for neurons (Lee et al., 2006a; Wang, H. et al., 2006). Whether these two Cdc42; at E10 using shRNA-expressing lentiviral vectors for Par3) kinases act in a common pathway (e.g. Aurora A activating Polo, or leads to loss of Pax6 neuroepithelial/radial glial cells, smaller vice versa) remains to be determined. clone sizes, and to precocious neuronal differentiation (Cappello et Rapid progress has been made in the last two years on the role of al., 2006; Costa et al., 2008). Conversely, the overexpression of cortical polarity in regulating NB self-renewal versus differentiation, Par3 or Par6 results in larger clone sizes that contain additional but many questions remain unsolved. How are apical and basal Pax6 NSCs (Costa et al., 2008). The removal of one of the two polarity proteins delivered and tethered to their respective aPKC isoforms (at E15.5 using nestin-Cre to remove aPKCλ) or of membrane domains? What are the targets of aPKC and the Notch Cdc42 (at E14 using GFAP-Cre) from radial glial cells led to a signaling pathway that promote NB self-renewal? Might it be similar but milder phenotype (Cappello et al., 2006; Imai et al., sufficient to merely prevent exposure of the NB to the differentiation 2006). Thus, Cdc42 and the Par complex are apical proteins that are factors Prospero and Brat? Do aPKC and Notch act in the same or necessary and sufficient to maintain NSC identity in the embryonic parallel pathways? Teasing out the relationship between cell cycle, cortex. These proteins have not yet been tested for a role in adult cell polarity and self-renewal will be a key task for the next few NSC self-renewal, in which apical/basal polarity is not as well years. defined. DEVELOPMENT 1582 REVIEW Development 135 (9) Mice mutant for the adherens junction (AJ) component α-catenin supernumerary stem cells, and shorter cell cycle times (Groszer lack AJs and have a faster neuroepithelial/radial glia cell cycle et al., 2001). Compared with the wild type, Pten mutant mice progression, which results in additional neuroepithelial/radial glia generate neurospheres that can be maintained for longer in serial stem cells and neurons being formed, without a change in their ratio. culture assays while maintaining their multi-lineage potential This results in enlarged brains (Lien et al., 2006). Transcriptional (Groszer et al., 2006). This indicates that Pten mutant stem cells profiling has shown that Hh-response genes are upregulated in α- have an increased self-renewal capability. Consistent with a role catenin mutant brains; indeed, virtually all aspects of the α-catenin for wild-type PTEN in promoting neuronal differentiation, human mutant phenotype can be suppressed by a Hh pathway inhibitor PTEN mutations are associated with brain tumors and (Lien et al., 2006). Do AJs act via a contact-based inhibition of a macrocephaly, and mouse Pten mutations with germline proliferation mechanism that keeps Hh levels low? If so, then why teratomas (reviewed by Stiles et al., 2004). In Drosophila, Pten is there no striking increase in stem cell proliferation following AJ co-localizes with the self-renewal-promoting factor aPKC (von disruption in Cdc42 or Par-complex mutant mice? One possibility Stein et al., 2005), so it is tempting to speculate that aPKC and is that the Cdc42-Par complex is required for both junctional Pten act antagonistically on common targets to regulate self- integrity and rapid cell cycle progression. renewal. Another apical protein that promotes NSC self-renewal in the embryonic cortex is the AJ protein β-catenin. Forced expression of Spindle orientation and self-renewal a stabilized β-catenin results in a large brain owing to increased Spindle orientation can impact stem cell self-renewal by positioning numbers of proliferative progenitors and a corresponding decrease daughter cells relative to extrinsic or intrinsic self-renewal cues. It in differentiated neurons (Chenn and Walsh, 2002). Because β- is thus important to monitor both extrinsic and intrinsic asymmetry catenin has a dual role, as a junctional protein and in canonical Wnt relative to spindle orientation, to determine which correlates with signaling, the phenotype could be due to increased Wnt signaling self-renewal. For example, a change in spindle orientation relative (which is linked to NSC self-renewal, see above) or to increased to extrinsic landmarks might be meaningless if there is no change in junctional stability, which might decrease the formation of basal the relationship of the spindle to functionally important intrinsic progenitors (owing to a failure to dissolve apical junctions). It would determinants. In the section below, I describe the progress, and be informative to distinguish these two pathways by specifically limitations, in our understanding of spindle orientation relative to reducing Wnt signaling (e.g. in Lef1/TCF1α mutants) or AJs (e.g. intrinsic and extrinsic cues and how it relates to NSC self-renewal. in Cdc42 mutants) to see which is required for the stabilized β- catenin phenotype. Spindle orientation in Drosophila neuroblasts If apical proteins promote NSC self-renewal, are basolateral Drosophila NBs invariably align their mitotic spindle along the proteins required for differentiation? The vertebrate Lgl1/2 (Llgl1/2 apical/basal cell polarity axis (Fig. 4), resulting in the NB inheriting in mouse) proteins are located basolaterally in Xenopus and in the apical proteins, while the differentiating GMC inherits the mammalian epithelia, as is the related Drosophila Lgl protein basally localized proteins (see Fig. 3). Spindle orientation is fixed at (reviewed by Lien et al., 2006). Drosophila lgl mutants have prophase, when one centrosome becomes anchored at the future increased NB numbers and decreased neuronal differentiation (Lee apical cortex, while the other migrates throughout the cytoplasm et al., 2006b); similarly, Lgl1-knockout mice have neuroepithelial before settling down at the basal cortex (Rebollo et al., 2007; Rusan cells with fewer AJs, increased proliferation, decreased neuronal and Peifer, 2007). By tightly linking spindle orientation with proven differentiation, and a neural rosette morphology that resembles that intrinsic determinants and with potential extrinsic cues, every NB of primitive neuroepithelial tumors (Klezovitch et al., 2004). The division results in a self-renewed NB and a differentiating daughter Lgl1 mutant phenotype might be partly due to reduced Numb cell. This precisely maintains brain NB numbers while constantly function, as Numb protein is delocalized in these mutants and increasing neuron numbers. expression of the Notch reporter Hes5 is increased (Klezovitch et Although it is commonly assumed that aligning the mitotic al., 2004). Thus, the basolateral Lgl1 protein is required for Numb spindle with the intrinsic cortical polarity axis is essential for localization and neuronal differentiation, paralleling its function in generating NB/GMC siblings, this has never been rigorously tested. the Drosophila CNS. For example, if the two spindle poles are functionally asymmetric, The role of the related Numb and numb-like proteins (henceforth as suggested by recent studies (Rebollo et al., 2007; Rusan and referred to as Numb/Nbl) in mammalian neurogenesis is Peifer, 2007), and this asymmetry helps specify NB versus GMC controversial (see Box 2). Recent microscopy studies clearly show identity, then any spindle axis may reliably generate NB and GMC that Numb localizes to AJs and to the basolateral membranes in siblings, irrespective of spindle/cortical polarity alignment. A embryonic neuroepithelial/radial glial cells and to the postnatal prerequisite for studying the role of spindle orientation in self- ependymal cells of the SVZ (Kuo et al., 2006; Rasin et al., 2007), renewal is to identify mutations that alter spindle orientation without consistent with previous reports of Numb having a basolateral disrupting cortical polarity; this has only been shown for one, localization in many animals, from fly to chick (see Rasin et al., perhaps two, genes so far. One is aurora A, which encodes a 2007). Thus, Numb is an evolutionarily conserved basolateral centrosomal and cytoplasmic kinase. aurora A homozygous mutants protein that is excluded from the apical membrane domain. assayed at an early larval stage, when some maternal Aurora A Identifying its precise role in NSC self-renewal, and the pathways protein was still present, showed defects in spindle alignment that it regulates, await more-detailed future studies. relative to apical/basal cortical polarity, and a slight increase in brain The kinase Akt (Akt1) and the phosphatase Pten have opposing NB numbers (Lee et al., 2006a; Wang, H. et al., 2006). However, functions in the Akt/Pten pathway (Narbonne and Roy, 2008), and neither study directly showed that the NBs with misaligned spindles have opposing NSC self-renewal phenotypes. Reduced Akt levels always or ever produced two sibling NBs. Furthermore, NBs from lead to loss of neuroepithelial/radial glia self-renewal in older mutants had ectopic cortical aPKC and delocalized Numb sequential neurosphere assays (Sinor and Lillien, 2004), whereas proteins, raising the concern that the younger mutants might have mice lacking Pten in the embryonic CNS have a larger brain, mild defects in aPKC or Numb that cause the increase in NB DEVELOPMENT Development 135 (9) REVIEW 1583 Fig. 4. Relationship between spindle orientation and sibling cell fate in neural stem cells. (A) Drosophila neuroblasts. Wild-type neuroblasts (left) invariably align their spindle along the apical/basal polarity axis, resulting in apical neuroblast (NB) and basal GMC cell fates. The cell fates that are acquired when the spindle is misoriented (e.g. in aurora A or mud mutants; right) have not been established. Spindle, blue lines; cleavage furrow plane, dotted line; apical domain, green; basal domain, red. (B) Vertebrate neuroepithelial cells. Conclusions from three different studies are shown. AJs, red balls; apical domain, green; basolateral domain, orange. SC, neuroepithelial cell; BP, basal progenitor; N, neuron. Apical is uppermost. (a) In Kosodo et al. (Kosodo et al., 2004), spindle orientation was concluded to regulate sibling cell fates in mouse embryonic neuroepithelial cells: if both siblings receive apical components (green), they both self-renew as progenitors (SCs; left); but if one cell lacks apical components, it differentiates into a neuron (N, middle and right). This type of asymmetric division can occur when the spindle is positioned in a near-planar orientation (middle) or in an apical/basal orientation (right). Thus, spindle orientation alone is insufficient to predict cell fate outcome. (b) Konno et al. (Konno et al., 2008) concluded that spindle orientation regulates sibling cell fates in mouse embryonic neuroepithelial cells: only siblings that inherit both apical and basal components will self-renew as progenitors (left); cells containing only apical domain become basal progenitors (BP; middle), whereas cells containing only the basal process become neurons (N; middle and right). (c) Morin et al. (Morin et al., 2007) concluded that spindle orientation does not affect sibling cell fates in the chick spinal cord. Normally, all divisions during early neurogenesis have a planar spindle orientation and form two progenitors (left). However, when the basolateral protein LGN (Gpsm2) was reduced by siRNA, the spindle could align with the apical/basal axis, yet both siblings still maintained progenitor identity by molecular marker expression (right), although the non-apical sibling was displaced out of the ventricular zone. Apical membrane and junction markers were not used in this study and thus are not shown. number. Stronger evidence that spindle orientation defects can lead siblings maintaining neuroepithelial/radial glial identity; by contrast, to expansion of the NB population comes from mushroom body vertical spindle alignment (along the apical/basal axis; apical/basal defective (mud) mutants. Mud shares domain organization and cell division) results in only the apical cell inheriting the apical limited sequence similarity with vertebrate NuMA (Numa1); both membrane domain and remaining a progenitor, with the most-basal are primarily localized to the centrosome, and Mud can also be sibling taking a neuronal fate (Cayouette and Raff, 2003; Chenn and detected at the apical cortex during prophase (Bowman et al., 2006; McConnell, 1995). More recently, it has been reported that planar Izumi et al., 2006; Siller et al., 2006), when spindle orientation is divisions might actually be asymmetric apical/basal cell divisions established (Rebollo et al., 2007; Rusan and Peifer, 2007; Siller and because the tiny apical domain is partitioned into only one sibling Doe, 2008). Similar to early aurora A mutants, mud mutants have (Kosodo et al., 2004) (see Fig. 4Ba). Furthermore, the long basal normal metaphase cortical polarity but fail to align the mitotic process may only be partitioned into one sibling in planar and spindle with the cortical polarity axis (see Fig. 4A), and have too apical/basal divisions (Das et al., 2003; Miyata et al., 2001; Miyata many brain NBs (Bowman et al., 2006; Izumi et al., 2006). It is et al., 2004). This raises an extremely important point: what is the important to note that the NBs with misaligned spindles were not structure that is associated with neuroepithelial/radial glial self- directly shown to produce two sibling NBs in these experiments. renewal – the apical domain, the AJs, the basal process, or none of The best experiment would be to perform in vivo live imaging of these? Two groups have reported that the apical cortical domain is a mutant NBs that express vital spindle, polarity and cell fate markers. good predictor of neuroepithelial progenitor fate (Kosodo et al., This would reveal whether spindle alignment defects always 2004; Sanada and Tsai, 2005) (see Fig. 4Ba). By contrast, another produce two NBs, whether they occasionally produce two GMCs, group has shown that only cells that inherit both the apical domain or whether spindle alignment is completely unrelated to the and the basal process will remain as neuroepithelial progenitors expansion in NB number in these mutants. (Konno et al., 2008) (Fig. 4Bb). Finally, another group reports that spindle orientation is unrelated to progenitor fate, but instead Spindle orientation in mammalian neural progenitors regulates daughter cell position (Morin et al., 2007) (Fig. 4Bc). The relationship between spindle orientation and cell fate has been A large group of centrosomal proteins are required to maintain studied in apical neuroepithelial cells and radial glia, but not in basal planar spindle orientation during the early phase of neuroepithelial progenitors. Neuroepithelial cells have a small prominin 1 apical expansion prior to E11.5 (Feng and Walsh, 2004; Fish et al., 2006; membrane domain that contacts the ventricular surface, as well as Konno et al., 2008; Morin et al., 2007; Xie et al., 2007). As predicted an adjacent ring of AJs and a long basal membrane domain that by the results of Konno et al., most of these mutants have a depleted contacts the pial surface. In early studies of neuroepithelial cells, a apical neuroepithelial pool and have ectopic proliferating cells in horizontal mitotic spindle alignment (perpendicular to the more-basal regions of the CNS (Konno et al., 2008). At least some apical/basal axis; planar cell division) was reported to result in both of these ectopic cells express neuroepithelial progenitor markers but DEVELOPMENT 1584 REVIEW Development 135 (9) not basal progenitor markers (Feng and Walsh, 2004; Fish et al., regulates neural progenitor cell proliferation in the larval central brain of Drosophila. Development 133, 2639-2648. 2006; Konno et al., 2008; Morin et al., 2007; Xie et al., 2007). Taken Bello, B., Holbro, N. and Reichert, H. (2007). Polycomb group genes are together, it appears that spindle orientation plays an important role required for neural stem cell survival in postembryonic neurogenesis of in maintaining neuroepithelial/radial glial progenitors within the Drosophila. Development 134, 1091-1099. Bertrand, N., Castro, D. S. and Guillemot, F. (2002). Proneural genes and the neuroepithelium, but the role of spindle orientation in regulating specification of neural cell types. Nat. Rev. Neurosci. 3, 517-530. sibling cell fate remains an open question. Betschinger, J., Mechtler, K. and Knoblich, J. A. (2003). The Par complex directs asymmetric cell division by phosphorylating the cytoskeletal protein Lgl. Nature Conclusions 422, 326-330. Betschinger, J., Mechtler, K. and Knoblich, J. A. (2006). Asymmetric The last few years have seen phenomenal progress in our segregation of the tumor suppressor brat regulates self-renewal in Drosophila understanding of NSC self-renewal in Drosophila and mammals, neural stem cells. Cell 124, 1241-1253. based in part on new methods. Marked mutant clones (MARCM) Bier, E., Vaessin, H., Younger-Shepherd, S., Jan, L. Y. and Jan, Y. N. (1992). deadpan, an essential pan-neural gene in Drosophila, encodes a helix-loop-helix technology has made it easier to generate single NB clones in protein similar to the hairy gene product. Genes Dev. 6, 2137-2151. Drosophila that lack a particular gene and to determine whether NB Bowman, S. K., Neumuller, R. A., Novatchkova, M., Du, Q. and Knoblich, J. numbers increase or decrease in response to a specific gene A. (2006). The Drosophila NuMA Homolog Mud regulates spindle orientation in asymmetric cell division. Dev. Cell 10, 731-742. mutation. In mammals, mosaic analysis with double markers Brand, M., Jarman, A. 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DevelopmentThe Company of Biologists

Published: May 1, 2008

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