Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

Location, Location, Location: Is Membrane Partitioning Everything When It Comes to Innate Immune Activation?

Location, Location, Location: Is Membrane Partitioning Everything When It Comes to Innate Immune... Hindawi Publishing Corporation Mediators of Inflammation Volume 2011, Article ID 186093, 10 pages doi:10.1155/2011/186093 Review Article Location, Location, Location: Is Membrane Partitioning Everything When It Comes to Innate Immune Activation? 1, 2 3, 4 1 Martha Triantafilou, Philipp M. Lepper, Robin Olden, 2 1, 2 Ivo de Seabra Rodrigues Dias, and Kathy Triantafilou Department of Child Health, School of Medicine, University Hospital of Wales, Cardiff University, Cardiff CF14 4XN, UK Infection and Immunity Group, School of Life Sciences,University of Sussex, Falmer, Brighton BN1 9QG, UK Department of Pneumology, Bern University Hospital (Inselspital), University of Bern, 3010 Bern, Switzerland Department of Internal Medicine V-Pneumology, Allergology and Respiratory Critical Care Medicine, University Hospital of Saarland, 66424 Homburg, Germany Correspondence should be addressed to Kathy Triantafilou, ktrian@hotmail.com Received 22 November 2010; Accepted 27 March 2011 Academic Editor: Giamila Fantuzzi Copyright © 2011 Martha Triantafilou et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In the last twenty years, the general view of the plasma membrane has changed from a homogeneous arrangement of lipids to a mosaic of microdomains. It is currently thought that islands of highly ordered saturated lipids and cholesterol, which are laterally mobile, exist in the plane of the plasma membrane. Lipid rafts are thought to provide a means to explain the spatial segregation of certain signalling pathways emanating from the cell surface. They seem to provide the necessary microenvironment in order for certain specialised signalling events to take place, such as the innate immune recognition. The innate immune system seems to employ germ-lined encoded receptors, called pattern recognition receptors (PRRs), in order to detect pathogens. One family of such receptors are the Toll-like receptors (TLRs), which are the central “sensing” apparatus of the innate immune system. In recent years, it has become apparent that TLRs are recruited into membrane microdomains in response to ligands. These nanoscale assemblies of sphingolipid, cholesterol, and TLRs stabilize and coalesce, forming signalling platforms, which transduce signals that lead to innate immune activation. In the current paper, we will investigate all past and current literature concerning recruitment of extracellular and intracellular TLRs into lipid rafts and how this membrane organization modulates innate immune responses. 1. Introduction are the spontaneous partitioning of lipids and proteins in discrete membrane domains, a behaviour based on their The general view of the cellular plasma membrane has physicochemical characteristics and the possibility to recover evolved over the last twenty years from that of a homo- these microdomains and their associated protein machinery geneous arrangement of lipids with embedded proteins as detergent-resistant entities using biochemical flotation towards that of a mosaic of microdomains, each having a experiments. Microdomains appear as small dynamic struc- specific protein and lipid composition [1]. Over the last tures that can aggregate into larger platforms in response to couple of decades, evidence has accumulated for organisa- various stimuli [5]. tion of the plasma membrane into lipid-based microdomains Currently, lipid rafts are thought to allow different or lipid rafts. A new model of membrane architecture has protein-lipid and protein-protein interactions that tem- been suggested [2] in which the membrane is patchy with porarily compartmentalise the plasma membrane. Lipid segregated cholesterol-rich portions, called lipid rafts. Lipid rafts are thought to provide a means to explain the spatial rafts are envisaged as islands of highly ordered saturated segregation of certain signalling pathways emanating from lipids and cholesterol that are laterally mobile in the plane the cell surface. They seem to provide the necessary microen- of a more fluid disordered bilayer of largely unsaturated vironment in order for certain specialised signalling events lipids [3, 4]. The hallmark of the lipid raft hypothesis to take place. Recent studies have shown the importance 2 Mediators of Inflammation of lipid raft formation in the acquired immune response. factor kappa B (NF-κB) [12, 13], which in turn leads to the Major Histocompatibility Complex- (MHC-) restricted T- secretion of proinflammatory cytokines such as TNF-α,IL-6, cell activation seems to be facilitated by lipid raft formation and IL-8. [6]. Furthermore, we have recently found that mediators TLR4 was found to recognise bacterial lipopolysaccha- of the innate immune response also concentrate in lipid ride (LPS) or endotoxin [14, 15]; TLR2 was found to rafts in order to facilitate signal transduction [7, 8], thus recognise lipoteichoic acid (LTA) and peptidoglycan [16]; suggesting that both the acquired and innate immune TLR3 was able to sense double-stranded viral RNA [17]; systems utilise membrane partitioning as means of activation TLR5 was found to recognise bacterial flagellin [18], TLR7 against invading pathogens. Crucial receptors for both innate [19]and TLR8 [20] to sense single stranded viral RNA, and acquired immunity seem to oligomerize in nonrandom whereas TLR9 to recognise bacterial CpG DNA [21]. In membrane structures, bringing together their signalling addition, TLR2 was found to recognise different motifs machinery. Thus accumulation of receptors within these including several components of Gram-positive bacteria “floating islands” on the cell membrane seems to bring such as peptidoglycan [22], lipoteichoic acid (LTA) [23], together intracellularly all the adaptor molecules that are lipoarabinomannan [24], lipoproteins [25], and different necessary for signalling. In this paper, we will investigate LPS from certain Gram-negative bacteria [26], yeast [27], further the mechanisms of innate immune recognition and spirochete, and fungi [28, 29] through its unique ability review past and current literature that leads us to believe that to heterodimerize with TLRs 1 and 6 [30]. Studies using membrane partitioning and lipid rafts play a central role in diacylated and triacylated lipoproteins have revealed that innate immune activation. diacylated lipoproteins require TLR2/6 heterodimers for activation, whereas triacylated lipoproteins induce activation of the innate immune system independently of TLR6 and 2. The Innate Immune System mainly through TLR2/TLR1 heterodimers [25, 31–35]. The innate immune system constitutes the most archaic part The membrane distribution of TLRs as well as their intra- of our immune defences and has survived through years cellular trafficking has only now begun to be investigated. of evolution. Its function is thought to be the recognition Most TLRs (TLR1, TLR2, TLR4, TLR5, and TLR6) seem to of invading pathogens, the activation of inflammation to activate cells by engaging their ligands on the cell surface, controlthe pathogen,and the subsequent activation of whereas TLR3, TLR7, TLR8, and TLR9 seem to trigger the acquired immune response. As part of its mechanism signalling intracellularly. These TLRs have been shown to of activation, the innate immune system employs germ- reside in the ER and to recognise their ligands once they have lined encoded receptors, called pattern recognition receptors been endocytosed [36, 37]. (PRRs) in order to “sense” pathogens. These PRRs recognise a restricted collection of microbial signatures, able to sense 2.2. Innate Immune Recognition of Bacterial Endotoxin or different types of microbial pathogens ranging from bacteria Lipopolysaccharide. Investigations into the innate immune and viruses to fungi and spirochetes. Lipid rafts seem to be a recognition of bacterial endotoxin led to the discovery of the key feature of the innate immune response, playing a crucial TLR family. TLR4 is the most studied TLR, mainly because role in phagocytosis, receptor-receptor as well as receptor- of its involvement with sepsis and septic shock. Sepsis is pathogen associations as well as signal transduction. Families a paradoxical and complex disorder that results from an of PRRs, such as the Toll-like receptor family (TLR) as well overreaction of our innate immune system to bacterial as the C-type lectin family seem to localise in lipid rafts infections. The mechanisms that are designed to protect for their function thus demonstrating the importance of the host against infection by bacterial pathogens, either this membrane partitioning for the function of the innate Gram-negative or Gram-positive, can lead to oversecretion immune response. of cytokines and fatal sepsis syndrome. It is now widely accepted that the overreaction of the host occurs at the 2.1. The Toll-Like Receptor Family. The TLR receptor family level of the innate immune system and is directly linked to were the first pattern recognition receptors to be identified the recognition of bacterial cell wall components, such as [9, 10]. This family of at least ten germ-line encoded recep- lipopolysaccharide (LPS) from Gram-negative bacteria or tors is able to “sense” microbial signatures and trigger activa- lipoteichoic acid (LTA) from Gram-positive bacteria. Thus tion leading to proinflammatory cytokine secretion. TLRs are the recognition of bacterial products by the innate immune expressed on immune cells and are able to distinguish a great system under certain conditions seems to be detrimental for variety of microbial ligands, such as cell wall components the host. like lipopolysaccharide (LPS) from Gram-negative bacteria In the last twenty-five years, great leaps forward in our and lipoteichoic acid from Gram-positive bacteria, bacterial understanding of the molecular events that lead to the innate flagellin, CpG DNA, and viral DNA or single stranded RNA recognition of pathogens have occurred. One of the seminal [11]. discoveries has been the identification of a serum protein, All identified TLRs are type I transmembrane proteins, lipopolysaccharide-binding protein (LBP), which binds LPS whose intracellular domains contain regions homologous or LTA and delivers it to its cellular targets [38]. Probably to the intracellular domains of IL-1R and are referred to as the most important discovery has been that the main family TIR domains [11]. These intracellular domains are able to of receptors employed by the innate immune system are the trigger signalling pathways known to activate the nuclear Toll-like receptors (TLRs). Mediators of Inflammation 3 As far as sepsis and bacterial recognition is concerned, TLR1 and TLR6. These heterodimers preexist and are not TLR4 seems to be the central sensor of Gram-negative induced by the ligand (Figure 1(a)). TLR2/6 heterodimers bacterial products [15, 39], whereas TLR2 seems to be are recruited within lipid rafts and associate with lipid raft- the key receptor in activating the immune system against resident proteins CD14 and CD36 upon ligand engagement. Gram-positive bacteria [23]. In addition to the involvement Binding of appropriate microbial substances leads to energy- of TLRs, other accessory molecules seem to be involved. dependent clustering of heterotypic receptors and activation CD14 is believed to act as a transfer molecule for both of intracellular signaling cascades that lead, for example, Gram-negative and Gram-positive bacteria [40, 41]. In the via NF-κB to production and secretion of proinflammatory case of LPS recognition, it has been further shown that a cytokines (Figure 1(b)). soluble molecule, MD-2 [42], as well as activation clusters Functional associations of TLRs with non-TLR mole- involving several other receptors [43–45]. In the case of cules have also been demonstrated, for example, TLR2 LTA recognition, TLR2 seems to form receptor clusters as association with dectin-1 is required for macrophage and well, comprising of at least CD14, TLR2, TLR6, and CD36 dendritic cell activation by β-glucan-containing particles. [46]. Thus we are moving away from the single-receptor More recently, functional interactions of TLR2 and CD36 model of activation, and a more complex picture is emerging. have been shown to be involved in the recognition of The mechanism that leads to activation seems to involve diacylglycerides [46]. TLR4 seems to be the best example of the careful interplay of several receptor molecules as well TLRs associating with non-TLR molecules. As it has already as serum proteins. Therefore such a complex orchestration been mentioned, TLR4 has been shown to form at least of events requires a nonrandom membrane architecture a trimolecular complex with CD14 and MD2 in order to specifically geared to bring receptor molecules together and recognise bacterial LPS [39]. The possibility that additional trigger activation within the lipid bilayer and lipid rafts or receptor components such as heat shock proteins [43, 50], membrane microdomains seem to provide this platform. CXCR4 [43], or CD55 [45] have been suggested to be part of this activation cluster, possibly acting as additional LPS 2.3. Protein-Protein Interactions in Innate Immunity: PRRs Are transfer molecules. Furthermore, it has been demonstrated that different “shapes” of LPS induce the formation of Part of Multicomponent Sensor Apparatuses. PRRs employed by the innate immune system have been shown to have different activation clusters, involving the association of the ability to bind and recognise conserved products of TLR4 with a variety of molecules mentioned above, which seems to determine LPS responses [51]. pathogens that are unique to the invading microorganisms but not to the host, it is becoming increasingly apparent Recent structural studies have shed some light onto TLR that the model of a single PRR recognising foreign antigen associations, supporting the hypothesis of cluster formation, is an oversimplified one. With the discovery of the Toll-like since all TLRs that have been crystallised have been found to receptors as the main signal transducing molecules of the be in a dimer formation, thus the hypothesis has been put innate immune system, an onslaught of research has shown forward that dimerisation or clustering might be a common that PRRs are part of multicomponent sensor apparatuses. feature of the TLRs and might be essential for signalling. TLRs have been shown to function as homo- or het- Structural studies of TLRs have been an attractive area of research since structural information is crucial erodimers and to even form functional interactions with non-TLR molecules. Many of these interactions are highly in understanding receptor function. In 2005, the crystal stable, whereas others are transient, forming dynamic associ- structureof TLR3 was thefirstone to be revealed [52]. It was surprising, that although the structure did not have a ations in response to specific stimuli. Whether homotypic, heterotypic, stable, or transient, these different protein ligand, TLR3 was crystallised as a dimer. In 2007 and 2008, combinations generate considerable functional diversity for three structures of TLR-ligand complexes were revealed, the innate immune system by triggering distinct signalling TLR1-TLR2-lipoprotein, TLR4-MD-2-Eritoran, and TLR3- cascades leading to cellular activation. There are a number dsRNA [53–55]. The ectodomains were found to form dimers, which were strikingly similar in shape. Prior to the of examples that suggest that TLR associations are required for cellular activation. TLR4 seems to form a complex with publication of the crystal structures, Gayand Gangloff [56] at least two other molecules, CD14 and MD2, in order to suggested a possible model of activation, where dimerization was ligand induced. These observations have suggested the recognise bacterial LPS [47]. In addition,itseems toassociate with a Toll-like receptor homologue RP105, which acts as a hypothesis that dimerization of the ectodomains forces the negative regulator of TLR4 responses [48]. TLR2 has been intracellular TIR domains to dimerize, and this initiates signalling by recruiting the intracellular adaptor molecules, found to heterodimerize with TLR1 or TLR6 for recognition of yeast components [30] and to associate with TLR1 for the such as MyD88, MAL, TRIF, and TRAM in order to initiate recognition of bacterial lipoproteins. In addition, TLR2 has signalling. The structures of the TIR domains of TLR1, been shown to also interact with scavenger receptors in order TLR2, and TLR10 have been revealed [57]. Interestingly, to recognise lipoproteins [46] and most recently it was shown the TIR domain of TLR10 was shown to be involved in a homodimeric interaction. However, it is not certain that TLR2 associates with CXCR4, which acts as a negative regulator of TLR2 responses [49](Figure 1). Figure 1 depicts whether the structure seen in the crystal corresponds to the possible model of TLR activation, and how TLRs and a physiologically relevant dimer of TLR10 TIR domains because they have been found to exist as monomers in other receptors are organized in lipid rafts on the cell surface before and after stimulation. TLR2 forms heterodimers with solution. Interestingly it has been recently suggested [58]that 4 Mediators of Inflammation TLR2/6 CXCR4 CD36 CD14 TLR2/6 CD14 CD36 CXCR4 Lipid raft P Lipid raft Conformational change Receptor complex Binding bacterial products Activation of signalling cascades Golgi apparatus Secretion of proinflammatory mediators, for example, TNF-α (a) (b) = bacterial products = energy dependent/phosphorylation step = gene activation/transcription/translation Figure 1: Activation of TLRs and adjuvant receptors on cell surface before and after stimulation by bacterial products. (a) TLR2 forms heterodimers with TLR1 and TLR6 on the cell surface and these heterodimer preexist and are not induced by the ligand. These heterodimers do not reside in lipid rafts before stimulation but are recruited to lipid rafts upon stimulation. This process is independent of signaling and facilitates the trafficking of TLRs from the cell surface to the Golgi. (b) TLR2/6 heterodimers are recruited within lipid rafts and associate with lipid raft-resident proteins CD14 and CD36 upon ligand stimulation. Binding of an appropriate microbial substance leads to energy- dependent clustering of heterotypic receptors and activation of intracellular signaling cascades that lead via NF-κB to the production and secretion of proinflammatory cytokines. MyD88 interacts with IRAK4 in an 8 : 4 ratio in solution, apparatus [63]. This intracellular targeting was shown to be suggesting that maybe there is higher oligomer formation. independent of signalling, thus suggesting that accumulation In order for such higher oligomers to be formed and in in lipid rafts only facilitated ligand recognition and signalling order to have such a well-orchestrated accumulation of re- that was initiated at the cell surface and not in the intracellu- ceptors and signalling machinery membrane partitioning lar compartments where TLR4 was targeted to [63]. seems to be crucial for the formation of these “TLR multi- More recently it had been proposed that the molecular component sensor apparatuses”. mechanism for signalling by the TLRs must involve a series of protein conformational changes initiated by dimerization 2.4. TLR4 Recruitment to Membrane Microdomains upon of their extracellular domains [64]. It was suggested that this Ligand Engagement. TLR4 wasthe first oneto beshown to be receptor-receptor association of the extracellular domains recruited to lipid rafts upon stimulation by bacterial LPS [7]. forced the association of the cytoplasmic domains as well. Within these membrane microdomains it was shown that Motshwene et al. [58] recently proved this experimentally, TLR4 formed clusters with non-TLR molecules that tailored demonstrating that the death domains of human MyD88, the immune response against the particular pathogen [43, one of the adaptor proteins used by all but one of the 59–62]. TLRs, and IRAK4 assemble into closed complexes with It was subsequently shown that this accumulation in lipid stoichiometries of 7 : 4 and 8 : 4, which they called the Myd- rafts also influenced its internalization and targeting. TLR4 dosome. The ability to form 7 : 4 or even 8 : 4 stoichiometries was found to accumulate in lipid rafts, to internalize in a suggests a mechanism by which clusters of activated receptors lipid-raft-dependent manner and to be targeted to the Golgi concentrate in lipid rafts and their intracellular machinery Mediators of Inflammation 5 clusters as well, forming a signalling platform that seems to the raft-associated molecules, such as TLR4, targeted to? And be crucial for TLR activation. most importantly why? This intracellular targeting seems to be independent of 2.5. Does Membrane Partitioning Play a Major Role in Protein signalling. TLR2 has also been found to reside in lipid rafts Uptake and Intracellular Routing? Cell membranes display after stimulation by Gram-positive bacterial products and to be similarly targeted to the Golgi apparatus [72](Figure 1). a tremendous complexity of lipid and proteins designed to perform the functions cells require. To coordinate these The question that remains is whether lipid raft association is functions, the membrane is able to laterally segregate its common for all TLRs expressed at the cell surface? If this is the case, do they all follow the same intracellular route? Do constituents. Lipid rafts were originally proposed as an expla- nation for a nonrandom membrane architecture and their different signalling cascades require differential targeting of function was originally thought to be linked with membrane TLRs and their ligands? trafficking. However, rafts proved to be able to influence In the case of the ER-resident TLRs, very little evidence of organization of membrane receptors and bioactivity as well their trafficking upon stimulation exists. To date only TLR9 as membrane trafficking. has been found to translocate from the ER to lysosomes in It is now emerging that this membrane partitioning response to its ligand, CpG DNA [73]. Based on the findings might play a major role in protein uptake and intracellular for TLR9, a hypothesis has been put forward that ER-resident TLRs might become accessible to endosomal and lysosomal routing. It is becoming more apparent that this differential sorting on the cell surface might pre-dispose the intra- compartments after the ER fuses with sites of microbial entry. cellular fate of a given molecule. Since the discovery of If this is the case, then it would seem that ER membrane clathrin-coated pits by Roth and Porter in 1964 [65], as fusion might be critical for microbial recognition by ER- specialised sites for the selective recruitment of specialised resident TLRs. molecules that are internalised into eukaryotic cells, clathrin- independent endocytic pathways have now emerged. Endo- 2.6. Do Lipid Rafts Control Endosomal Innate Immune Dy- cytic pathways that do not rely on the formation of clathrin namics? The regulation of endosome dynamics is crucial coated pits include the earliest identified pathways such for fundamental cellular functions, such as nutrient in as phagocytosis, macropinosis, and caveolae. Lipid rafts take/digestion, membrane receptor recycling/degradation, might involved in all of these pathways. In particular for antigen presentation, cell migration, and intracellular signal- phagocytosis, it has been shown that lipid rafts play a ing [74–76]. The system is also utilised by various pathogens crucial role in the phagocytic uptake of latex microspheres to bud in and from the cell [77]. [66], suggesting that these specialised microdomains on In addition to the function asadistribution centre, it the plasma membrane are necessary for endocytosis and has been proposed that the endosome system serves as an phagocytosis. intracellular signalling station [78]. In the case of innate Furthermore, caveolae which is defined as small, un- immunity this is certainly the case, since TLR3, TLR7, TLR8, coated invaginations in the plasma membrane containing and TLR9 all reside within the endosome waiting to capture the plasma protein caveolin 1 has been shown to be incoming PAMPs and trigger signalling. Endosomes are able to bind cholesterol and to be resistant to detergent pleiomorphic organelles composed of tubular elements as extraction [67], and this has led to the suggestion that well as vesicular regions with a characteristic multivesicular caveolae might constitute a type of lipid raft [68]. Lipid rafts appearance. The question that remains is whether in addition are increasingly becoming linked with clathrin-independent to these morphologically distinguishable regions, endosomal endocytosis, since nearly all molecules that are known membranes are further subcompartmentalized into mem- to be internalised independently of clathrin are found in brane lipid rafts or microdomains. biochemically defined rafts [69]. It has been suggested that Lipid rafts have mostly been studied at the plasma raft components might be taken up preferentially by clathrin- membrane, mainly due to accessibility for microscopy and independent endocytosis. There are likely to be several types biophysical methods [79]. Characterisation of lipid rafts has of clathrin-independent endocytosis. The extent to which also been extensively based on their resistance to detergent these different pathways require lipid rafts to operate or are solubilisation, although this has inherent limitations [80], somehow selective for lipid rafts is currently the subject of as well as fluorescent microscopy [79]. Although most intensive investigation. Recently, Nichols et al. have described studies have focused on the existence of lipid rafts on plasma a rapid lipid-raft-dependent targeting from the cell surface membranes, many intracellular organelles appear to contain to the Golgi apparatus [70]. In addition, a new clathrin- raft-like domains [81–84]. Due to its low cholesterol content, independent mechanism has been described that can lead to the endoplasmic reticulum was originally thought not to delivery of receptor molecules from the plasma membrane contain cholesterol-dependent microdomains. However, to caveolin-1-containing endosomes, termed “caveosomes” recently several studies have reported their existence [85, 86]. [71]. With the emergence of these new clathrin-independent Raft-like domains have been described in the Golgi and uptake mechanisms the idea that different types of endocyto- trans-Golgi network [87, 88], along the endocytic pathway sis have markedly different functions is beginning to become [84]aswell asinthe endosomes [89–91]. Although potential apparent. Ultimately we have to speculate that sorting at the roles of lipid rafts for the outer membrane have been demon- plasma membrane might predispose the intracellular route strated, including endocytosis, exocytosis, vesicle formation, that a molecule might take. If that is the case, then where are and signalling, the functions of lipid rafts in the processes of 6 Mediators of Inflammation TLR9 Receptor complex Binding viral products Endosome ssRNA TLR7 and 8 Transport (UNC93B1) Activation of signalling -cleavage in endosome cascades TLR9 ER Mitochondria Secretion of pro-inflammatory mediators Lipid raft conformationa (a) (b) = bacterial products = energy dependent (phosphorylation) step = gene activation/transcription/translation Figure 2: Activation of TLRs in endosomes. Various TLRs recognize microbial patterns within the endosome. TLR7 and 8 recognize ssRNA, whereas TLR9 recognises CpG DNA. As nucleic acid recognition bears a potential source for the induction of autoimmunity, TLR7 and 9 exist in a full-length and truncated version, where the ectodomain is cleaved. Only the endosomally processed forms are capable to recruit MyD88 and to induce signaling. Transport from the endoplasmatic reticulum (ER) is facilitated by UNC93B1 (a). Ligation of TLR7 by ssRNA leads to clustering within a lipid raft at the endosomal membrane and activation of intracellular signaling cascades that lead via NF-κBto production and secretion of proinflammatory cytokines (b). endosomal membrane dynamics are currently unknown. signaling cascades that lead via NF-κB to production and We can only speculate that they are contributing to similar secretion of proinflammatory cytokines (Figure 2(b)). functions. It has been suggested that protein and lipid sorting into and out of the endosomes may be controlled 2.7. Existence of Other PPRs in Lipid Rafts. C-type lectin by endosomal membrane partitioning [89], but whether receptors (CLRs), such as Dectin 1, are a family of pattern these microdomains control signalling and in particular recognition receptors which bind β-glucan found in the cell TLR signalling has not been investigated. Since most walls of pathogenic fungi such as Candida albicans .In partic- extracellular TLRs have been shown to be recruited to ular Dectin 1 has been shown to mediate the phagocytosis of lipid rafts upon ligand activation, it is safe to assume that yeast and yeast-derived particles, such as zymosan, activating endosomal TLRs act in a similar manner, especially since the production of inflammatory cytokines [92–94]. Inter- the existence of cholesterol-dependent microdomains on estingly, Dectin-1 possesses an immunotyrosine-activated the endosomal membranes has been proven (Figure 2). motif (ITAM) in its cytoplasmic tail, suggesting that it is Thus it is safe to assume that membrane partitioning capable of mediating signalling analogous to the BCR and control both extracellular and intracellular TLR-dependent TCR. Since both the BCR and TCR have been found to reside signalling. We are proposing a model for endosomal TLR in lipid rafts it was suggested that Dectin-1 might also be activation, where ligation of endosomal TLRs by their recruited there upon activation. A recent study has revealed respective ligands can lead to clustering within lipid rafts that Dectin-1 and possibly other CLRs are recruited to lipid at the endosomal membrane and activation of intracellular rafts upon activation and raft integrity is important for Mediators of Inflammation 7 signalling [95]. Thus suggesting that recruitment to lipid [2] L. J. Pike, “Lipid rafts: bringing order to chaos,” Journal of Lipid Research, vol. 44, no. 4, pp. 655–667, 2003. rafts is a common feature for most PRRs, including TLRs [3] K. Simons and E. Ikonen, “Functional rafts in cell mem- and CLRs. branes,” Nature, vol. 387, no. 6633, pp. 569–572, 1997. [4] A.Pralle,P. Keller, E. L. Florin, K.Simons, and J. K.H.Horb ¨ er, 2.8. Concluding Remarks. Cell membranes are complicated “Sphingolipid-cholesterol rafts diffuse as small entities in in composition but precise in purpose: to selectively com- the plasma membrane of mammalian cells,” Journal of Cell partmentalize its constituents in order to coordinate cellular Biology, vol. 148, no. 5, pp. 997–1007, 2000. functions. In this way, the membrane is able to compart- [5] T. Harder, P. Scheiffele, P. Verkade, and K. Simons, “Lipid mentalize, segregate receptors as well as their signalling domain structure of the plasma membrane revealed by machinery and create oligomeric signalling platforms in patching of membrane components,” Journalof CellBiology, order to transduce signals. Once the required function has vol. 141, no. 4, pp. 929–942, 1998. [6] H. A. Anderson, E. M. Hiltbold, and P. A. Roche, “Concentra- subsided, these segregated islands are involved in internal- tion of MHC class II molecules in lipid rafts facilitates antigen ization and membrane trafficking, thus bringing the whole presentation,” Nature Immunology, vol. 1, no. 2, pp. 156–162, function to a close. The innate and acquired immune systems seem to utilise this membrane partitioning for [7] M. Triantafilou, K. Miyake, D. T. Golenbock, and K. Tri- their functions. In this paper, we have extensively looked antafilou, “Mediators of innate immune recognition of bacte- at the use of this membrane partitioning by the innate ria concentrate in lipid rafts and facilitate lipopolysaccharide- immune system and most particular by the TLRs. The induced cell activation,” JournalofCellScience, vol. 115, no. molecular mechanism involved in LPS recognition and TLR 12, pp. 2603–2611, 2002. signalling in general, utilises a series of protein lipid as [8] M. Triantafilou, S. Morath, A. Mackie, T. Hartung, and K. well as protein-protein interactions. The plasma membrane Triantafilou, “Lateral diffusion of Toll-like receptors reveals seems to be heterogeneous and to coalesce to more stable that they are transiently confined within lipid rafts on the membrane-ordered assemblies upon activation by ligands. plasma membrane,” JournalofCellScience, vol. 117, no. 17, pp. 4007–4014, 2004. This partitioning of the membrane and the assembly of [9] R.Medzhitov and C.A.Janeway, “Decoding the patterns of self more stable raft platforms in the functionalized state must and nonself by the innate immune system,” Science, vol. 296, be initiated by raft-resident proteins, which form protein- no. 5566, pp. 298–300, 2002. lipid as well as protein-protein interactions. The TLRs and [10] S. Akira, “Toll-like receptors and innate immunity,” Advances other PRRs associate with the raft-resident proteins and in Immunology, vol. 78, pp. 1–56, 2001. are recruited to these “floating islands” forming higher [11] K. Takeda, T. Kaisho, and S. Akira, “Toll-like receptors,” oligomers, both extracellularly as well as inside the cell, Annual Review of Immunology, vol. 21, pp. 335–376, 2003. concentrating their signalling machinery which finally leads [12] R. Medzhitov, P. Preston-Hurlburt, E. Kopp et al., “MyD88 is to a functional, focused, and coordinated activation of the an adaptor protein in the hToll/IL-1 receptor family signaling innate immune system. pathways,” Molecular Cell, vol. 2, no. 2, pp. 253–258, 1998. The lifetime of these domains and the length of the [13] L. O’Neill, “The Toll/interleukin-1 receptor domain: a molec- ular switch for inflammation and host defence,” Biochemical response will depend on their size and factors that may sta- Society Transactions, vol. 28, no. 5, pp. 557–563, 2000. bilise or destabilise them. These factors will include not only [14] A. Poltorak, X. L. He, I. Smirnova et al., “Defective LPS lipid-lipid, lipid-protein and protein-protein interactions signaling in C3H/Hej and C57BL/10ScCr mice: mutations in both in the plane of the membrane but also elements of the TLR4 gene,” Science, vol. 282, no. 5396, pp. 2085–2088, 1998. cytoskeleton, pericellular matrix adjacent to the membrane [15] S. T. Qureshi, L. Larivier ` e, G. Leveque et al., “Endotoxin- as well as transmembrane and cytoplasmic domains of the tolerant mice have mutations in Toll-like receptor 4 (TlR4),” receptors involved. In thecaseof TLRs, theassociation of the Journal of Experimental Medicine, vol. 189, no. 4, pp. 615–625, TIR domains intracellularly would stabilise the ectodomains extracellularly and provide the molecular scaffold that will [16] O. Takeuchi, K. Hoshino, T. Kawai et al., “Differential roles of recruit the adaptor molecules that contribute to signalling. TLR2 and TLR4 in recognition of gram-negative and gram- The challenge for the future will be to visualise the assembly positive bacterial cell wall components,” Immunity,vol.11, no. 4, pp. 443–451, 1999. and stoichiometry of these large and transient oligomeric [17] L. Alexopoulou, A. C. Holt, R. Medzhitov, and R. A. Flavell, complexes in vivo. Thus refining existing biophysical meth- “Recognition of double-stranded RNA and activation of NF- ods, such as single particle tracking (SPT), fluorescence κB by Toll-like receptor 3,” Nature, vol. 413, no. 6857, pp. 732– correlation spectroscopy (FCS), and fluorescence resonance 738, 2001. energy transfer (FRET) will help us reveal these dynamic [18] F. Hayashi, K. D. Smith, A. Ozinsky et al., “The innate immune nanoassemblies of sterol, sphingolipid, and proteins in living response to bacterial flagellin is mediated by Toll-like receptor cell and provide us with the first dynamic picture of the 5,” Nature, vol. 410, no. 6832, pp. 1099–1103, 2001. innate immune response. [19] J. M. Lund,L. Alexopoulou,A.Sato et al., “Recognition of single-stranded RNA viruses by Toll-like receptor 7,” Proceedings of the National Academy of Sciences of the United References States of America, vol. 101, no. 15, pp. 5598–5603, 2004. [1] J.F. Hancock,“Lipid rafts:contentious only from simplistic [20] F. Heil, H. Hemmi, H. Hochrein et al., “Species-specific recognition of single-stranded RNA via Toll-like receptor 7 standpoints,” Nature Reviews Molecular Cell Biology,vol. 7,no. 6, pp. 456–462, 2006. and 8,” Science, vol. 303, no. 5663, pp. 1526–1529, 2004. 8 Mediators of Inflammation [21] H. Hemmi, O. Takeuchi, T. Kawai et al., “A Toll-like receptor [36] F. Heil, P. Ahmad-Nejad, H. Hemmi et al., “The Toll-like recognizes bacterial DNA,” Nature, vol. 408, no. 6813, pp. 740– receptor 7 (TLR7)-specific stimulus loxoribine uncovers a 745, 2000. strong relationship within the TLR7, 8 and 9 subfamily,” Euro- pean Journal of Immunology, vol. 33, no. 11, pp. 2987–2997, [22] A. Yoshimura, E. Lien,R.R. Ingalls,E.Tuomanen, R. Dziarski, and D. Golenbock, “Cutting edge: recognition of Gram- positive bacterial cell wall components by the innate immune [37] T. Nishiya and A. L. DeFranco, “Ligand-regulated chimeric system occurs via Toll-like receptor 2,” Journal of Immunology, receptor approach reveals distinctive subcellular localization vol. 163, no. 1, pp. 1–5, 1999. and signaling properties of the Toll-like receptors,” Journal of Biological Chemistry, vol. 279, no. 18, pp. 19008–19017, 2004. [23] R. Schwandner, R. Dziarski, H. Wesche,M.Rothe,and C. J. Kirschning, “Peptidoglycan- and lipoteichoic acid-induced [38] P. S. Tobias, K. Soldau, and R. J. Ulevitch, “Isolation of a cell activation is mediated by Toll-like receptor 2,” Journal of lipopolysaccharide-binding acute phase reactant from rabbit Biological Chemistry, vol. 274, no. 25, pp. 17406–17409, 1999. serum,” Journal of Experimental Medicine, vol. 164, no. 3, pp. 777–793, 1986. [24] T. K. Means, E. Lien,A.Yoshimura, S.Wang, D. T. Golenbock, and M. J. Fenton, “The CD14 ligands lipoarabinomannan [39] A. Poltorak, P. Ricciardi-Castagnoli, S. Citterio, and B. Beutler, and lipopolysaccharide differ in their requirement for Toll-like “Physical contact between lipopolysaccharide and Toll-like receptors,” Journal of Immunology, vol. 163, no. 12, pp. 6748– receptor 4 revealed by genetic complementation,” Proceedings 6755, 1999. of the National Academy of Sciences of the United States of America, vol. 97, no. 5, pp. 2163–2167, 2000. [25] O. Takeuchi, S. Sato, T. Horiuchi et al., “Cutting edge: role of Toll-like receptor 1 in mediating immune response to [40] S. D. Wright,R. A. Ramos,P.S.Tobias, R. J. Ulevitch, microbial lipoproteins,” Journal of Immunology, vol. 169, no. and J. C. Mathison, “CD14, a receptor for complexes of 1, pp. 10–14, 2002. lipopolysaccharide (LPS) and LPS binding protein,” Science, vol. 249, no. 4975, pp. 1431–1433, 1990. [26] C. Werts, R. I. Tapping, J. C. Mathison et al., “Leptospiral lipopolysaccharide activates cells through a TLR2-dependent [41] D. Gupta, T. N. Kirkland, S. Viriyakosol, and R. Dziarski, mechanism,” Nature Immunology, vol. 2, no. 4, pp. 346–352, “CD14 is a cell-activating receptor for bacterial peptidogly- 2001. can,” Journalof BiologicalChemistry, vol. 271, no. 38, pp. 23310–23316, 1996. [27] D. M. Underhill, A. Ozinsky, A. M. Hajjar et al., “The Toll- like receptor 2 is recruited to macrophage phagosomes and [42] Y. Nagai, S. Akashi, M. Nagafuku et al., “Essential role of discriminates between pathogens,” Nature, vol. 401, no. 6755, MD-2 in LPS responsiveness and TLR4 distribution,” Nature pp. 811–815, 1999. Immunology, vol. 3, no. 7, pp. 667–672, 2002. [28] B. N. Gantner, R. M. Simmons, S. J. Canavera, S. Akira, and [43] K. Triantafilou, M. Triantafilou, and R. L. Dedrick, “A CD14- D. M. Underhill, “Collaborative induction of inflammatory independent LPS receptor cluster,” Nature Immunology,vol. 2, responses by dectin-1 and Toll-like receptor 2,” Journal of no. 4, pp. 338–345, 2001. Experimental Medicine, vol. 197, no. 9, pp. 1107–1117, 2003. [44] A. Pfeiffer, A. Bottcher, E. Orso et al., “Lipopolysaccharide and [29] H. Heine and E. Lien, “Toll-like receptors and their function ceramide docking to CD14 provokes ligand-specific receptor in innate and adaptive immunity,” International Archives of clustering in rafts,” European Journal of Immunology,vol.31, Allergy and Immunology, vol. 130, no. 3, pp. 180–192, 2003. no. 11, pp. 3153–3164, 2001. [30] A. Ozinsky, D. M. Underhill, J. D. Fontenot et al., “The [45] H. Heine, V. El-Samalouti, C. Notse ¨ l et al., “CD55/decay repertoire for pattern recognition of pathogens by the innate accelerating factor is part of the lipopolysaccharide-induced immune system is defined by cooperation between Toll-like receptor complex,” European Journal of Immunology,vol.33, receptors,” Proceedings of the National Academy of Sciences of no. 5, pp. 1399–1408, 2003. the United States of America, vol. 97, no. 25, pp. 13766–13771, [46] K. Hoebe, P. Georgel, S. Rutschmann et al., “CD36 is a sensor of diacylglycerides,” Nature, vol. 433, no. 7025, pp. 523–527, [31] L. Alexopoulou, V. Thomas, M. Schnare et al., “Hypore- 2005. sponsiveness to vaccination with Borrelia burgdorferi OspA [47] A. Visintin, E. Latz, B. G. Monks, T. Espevik, and D. T. in humans and in TLR1- and TLR2-deficient mice,” Nature Golenbock, “Lysines 128 and 132 enable lipopolysaccharide Medicine, vol. 8, no. 8, pp. 878–884, 2002. binding to MD-2, leading to Toll-like receptor-4 aggregation [32] M. Morr, O. Takeuchi, S. Akira, M. M. Simon, and P. F. and signal transduction,” Journal of Biological Chemistry,vol. Mu ¨hlradt, “Differential recognition of structural details of 278, no. 48, pp. 48313–48320, 2003. bacterial lipopeptides by Toll-like receptors,” European Journal [48] S. Divanovic, A. Trompette, S. F. Atabani et al., “Negative of Immunology, vol. 32, no. 12, pp. 3337–3347, 2002. regulation of Toll-like receptor 4 signaling by the Toll-like [33] O. Takeuchi,T.Kawai, P.F. Mu ¨hlradt et al., “Discrimination receptor homolog RP105,” Nature Immunology,vol. 6,no. 6, of bacterial lipoproteins by Toll-like recepttor 6,” International pp. 571–578, 2005. Immunology, vol. 13, no. 7, pp. 933–940, 2001. [49] G. Hajishengallis, M. Wang, S.Liang,M.Triantafilou,and [34] U. Buwitt-Beckmann, H. Heine, K. H. Wiesmu ¨ller et al., K. Triantafilou, “Pathogen induction of CXCR4/TLR2 cross- “Toll-like receptor 6-independent signaling by diacylated talk impairs host defense function,” Proceedings of the National lipopeptides,” European Journal of Immunology,vol.35, no. 1, Academy of Sciences of the United States of America, vol. 105, pp. 282–289, 2005. no. 36, pp. 13532–13537, 2008. [35] M. Triantafilou, F. G. J. Gamper, R. M. Haston et al., “Mem- [50] C. A. Byrd, W. Bornmann, H. Erdjument-Bromage et al., brane sorting of Toll-like receptor (TLR)-2/6 and TLR2/1 “Heat shock protein 90 mediates macrophage activation by heterodimers at the cell surface determines heterotypic asso- Taxol and bacterial lipopolysaccharide,” Proceedings of the ciations with CD36 and intracellular targeting,” Journal of National Academy of Sciences of the United States of America, Biological Chemistry, vol. 281, no. 41, pp. 31002–31011, 2006. vol. 96, no. 10, pp. 5645–5650, 1999. Mediators of Inflammation 9 [51] M. Triantafilou, K. Brandenburg, S. Kusumoto et al., “Com- [68] T. Harder and K. Simons, “Caveolae, DIGs, and the dynamics binational clustering of receptors following stimulation by of sphingolipid-cholesterol microdomains,” Current Opinion bacterial products determines lipopolysaccharide responses,” in Cell Biology, vol. 9, no. 4, pp. 534–542, 1997. Biochemical Journal, vol. 381, no. 2, pp. 527–536, 2004. [69] B. J. Nichols and J. Lippincott-Schwartz, “Endocytosis with- [52] J. Choe,M.S.Kelker, and I.A. Wilson, “Crystal structure of out clathrin coats,” Trends in Cell Biology, vol. 11, no. 10, pp. human Toll-like receptor 3 (TLR3) ectodomain,” Science,vol. 406–412, 2001. 309, no. 5734, pp. 581–585, 2005. [70] B. J. Nichols, A. K. Kenworthy, R. S. Polishchuk et al., “Rapid [53] M. S. Jin, S. E. Kim, J. Y. Heo et al., “Crystal structure of the cycling of lipid raft markers between the cell surface and golgi TLR1-TLR2 heterodimer induced by binding of a tri-acylated complex,” Journalof CellBiology, vol. 152, no. 3, pp. 529–541, lipopeptide,” Cell, vol. 130, no. 6, pp. 1071–1082, 2007. [54] H. M. Kim, B. S. Park, J. I. Kim et al., “Crystal structure of [71] L. Pelkmans and A. Helenius, “Endocytosis via caveolae,” the TLR4-MD-2 complex with bound endotoxin antagonist Traffic, vol. 3, no. 5, pp. 311–320, 2002. eritoran,” Cell, vol. 130, no. 5, pp. 906–917, 2007. [72] M. Trianiafilou, M. Manukyan, A. Mackie et al., “Lipoteichoic [55] L. Liu, I. Botos, Y. Wang et al., “Structural basis of Toll-like acid and Toll-like receptor 2 internalization and targeting receptor 3 signaling with double-stranded RNA,” Science,vol. to theGolgi arelipid raft-dependent,” Journal of Biological 320, no. 5874, pp. 379–381, 2008. Chemistry, vol. 279, no. 39, pp. 40882–40889, 2004. [56] N. J. Gay and M. Gangloff, “Structure of Toll-like receptors,” [73] E. Latz, A. Schoenemeyer, A. Visintin et al., “TLR9 signals Handbook of Experimental Pharmacology, no. 183, pp. 181– after translocating from the ER to CpG DNA in the lysosome,” 200, 2008. Nature Immunology, vol. 5, no. 2, pp. 190–198, 2004. [57] T. Nyman, P. Stenmark, S. Flodin, I. Johansson, M. Ham- [74] V. Dudu, P. Pantazis, and M. Gonzale ´ z-Gaitan, ´ “Membrane marstrom, ¨ and P. R. Nordlund, “The crystal structure of the traffic during embryonic development: epithelial formation, human Toll-like receptor 10 cytoplasmic domain reveals a cell fate decisions and differentiation,” Current Opinion in Cell putative signaling dimer,” Journalof BiologicalChemistry,vol. Biology, vol. 16, no. 4, pp. 407–414, 2004. 283, no. 18, pp. 11861–11865, 2008. [75] M. C. Jones, P. T. Caswell, and J. C. Norman, “Endocytic recy- [58] P. G. Motshwene, M. C. Moncrieffe, J. G. Grossmann et al., “An cling pathways: emerging regulators of cell migration,” Current oligomeric signaling platform formed by the Toll-like receptor Opinion in Cell Biology, vol. 18, no. 5, pp. 549–557, 2006. signal transducers MyD88 and IRAK-4,” Journal of Biological [76] G. Emery and J. A. Knoblich, “Endosome dynamics during Chemistry, vol. 284, no. 37, pp. 25404–25411, 2009. development,” Current Opinion in Cell Biology, vol. 18, no. 4, [59] M. Triantafilou, K. Brandenburg, S. Kusumoto et al., “Com- pp. 407–415, 2006. binational clustering of receptors following stimulation by [77] J. Gruenberg and F. G. van der Goot, “Mechanisms of bacterial products determines lipopolysaccharide responses,” pathogen entry through the endosomal compartments,” Biochemical Journal, vol. 381, no. 2, pp. 527–536, 2004. Nature Reviews Molecular Cell Biology,vol.7,no. 7, pp. [60] M. Triantafilou and K. Triantafilou, “The dynamics of LPS 495–504, 2006. recognition: complex orchestration of multiple receptors,” [78] M. Miaczynska, L. Pelkmans, and M. Zerial, “Not just a Journal of Endotoxin Research, vol. 11, no. 1, pp. 5–11, 2005. sink: endosomes in control of signal transduction,” Current [61] M. Triantafilou and K. Triantafilou, “Receptor cluster for- Opinion in Cell Biology, vol. 16, no. 4, pp. 400–406, 2004. mation during activation by bacterial products,” Journal of [79] K. Jacobson, O. G. Mouritsen, and R. G. W. Anderson, “Lipid Endotoxin Research, vol. 9, no. 5, pp. 331–335, 2003. rafts: at a crossroad between cell biology and physics,” Nature [62] H. E. Humphries, M. Triantafilou, B. L. Makepeace, J. E. Cell Biology, vol. 9, no. 1, pp. 7–14, 2007. Heckels, K. Triantafilou, and M. Christodoulides, “Activation [80] H. Heerklotz, “Triton promotes domain formation in lipid of human meningeal cells is modulated by lipopolysaccharide raft mixtures,” Biophysical Journal, vol. 83, no. 5, pp. 2693– (LPS) and non-LPS components of Neisseria meningitidis and 2701, 2002. is independent of Toll-like receptor (TLR)4 and TLR2 sig- [81] D. A. Brown and J. K. Rose, “Sorting of GPI-anchored proteins nalling,” Cellular Microbiology, vol. 7, no. 3, pp. 415–430, 2005. to glycolipid-enriched membrane subdomains during trans- [63] E. Latz, A. Visintin, E. Lien et al., “Lipopolysaccharide port to the apical cell surface,” Cell, vol. 68, no. 3, pp. 533–544, rapidly traffics to and from the golgi apparatus with the Toll-like receptor 4-MD-2-CD14 complex in a process that is [82] R. Gagescu, N. Demaurex, R. G. Parton, W. Hunziker, L. A. distinct from the initiation of signal transduction,” Journal of Huber, and J. Gruenberg, “The recycling endosome of Madin- Biological Chemistry, vol. 277, no. 49, pp. 47834–47843, 2002. Darby canine kidney cells is a mildly acidic compartment rich [64] N. J. Gay, M. Gangloff, and A. N. R. Weber, “Toll-like receptors in raft components,” Molecular Biology of the Cell, vol. 11, no. as molecular switches,” Nature Reviews Immunology,vol.6, 8, pp. 2775–2791, 2000. no. 9, pp. 693–698, 2006. [83] J. F. Dermine, S. Duclos, J. Garin et al., “Flotillin-1-enriched [65] T. F. Roth and K. R. Porter, “Yolk protein uptake in the oocyte lipid raft domains accumulate on maturing phagosomes,” of the mosquito Aedes aegypti,” The Journal of Cell Biology, Journal of Biological Chemistry, vol. 276, no. 21, pp. 18507– vol. 20, no. 2, pp. 313–330, 1964. 18512, 2001. [66] G. Nagao, K. Ishii, K. Hirota, K. Makino, and H. Terada, [84] M. Fivaz, F. Vilbois, S. Thurnheer et al., “Differential sorting “Role of lipid rafts in phagocytic uptake of polystyrene latex and fate of endocytosed GPI-anchored proteins,” The EMBO microspheres by macrophages,” Anticancer Research,vol.30, Journal, vol. 21, no. 15, pp. 3989–4000, 2002. no. 8, pp. 3167–3176, 2010. [67] M. Sargiacomo, M.Sudol,Z. Tang, and M.P.Lisanti,“Signal [85] D. T. Browman, M. E. Resek, L. D. Zajchowski, and S. M. Robbins, “Erlin-1 and erlin-2 are novel members of transducing molecules and glycosyl-phosphatidylinositol- linked proteins form a caveolin-rich insoluble complex in the prohibitin family of proteins that define lipid-raft-like domains of the ER,” Journal of Cell Science, vol. 119, no. 15, MDCK cells,” JournalofCellBiology, vol. 122, no. 4, pp. 789–807, 1993. pp. 3149–3160, 2006. 10 Mediators of Inflammation [86] L. K. Pielsticker, K. J. Mann, W. L. Lin, and D. Sevlever, “Raft- like membrane domains contain enzymatic activities involved in the synthesis of mammalian glycosylphosphatidylinositol anchor intermediates,” Biochemical and Biophysical Research Communications, vol. 330, no. 1, pp. 163–171, 2005. [87] J. Fu ¨llekrug and K. Simons, “Lipid rafts and apical membrane traffic,” Annals of the New York Academy of Sciences, vol. 1014, pp. 164–169, 2004. [88] H. B. Eberle, R. L.Serrano,J.Ful ¨ lekrug et al., “Identification and characterization of a novel human plant pathogenesis- related protein that localizes to lipid-enriched microdomains in the Golgi complex,” Journal of Cell Science, vol. 115, no. 4, pp. 827–838, 2002. [89] K. Sobo,J.Chevallier,R. G.Parton, J. Gruenberg, and F.G. van der Goot, “Diversity of raft-like domains in late endosomes,” PLoS One, vol. 2, no. 4, article e391, 2007. [90] S. Ignoul, J. Simaels, D. Hermans, W. Annaert, and J. Eggermont, “Human CIC-6 is a late endosomal glycoprotein that associates with detergent-resistant lipid domains,” PLoS One, vol. 2, no. 5, article e474, 2007. [91] S. Nada,A.Hondo, A.Kasai et al., “The novellipid raft adaptor p18 controls endosome dynamics by anchoring the MEK-ERK pathway to late endosomes,” The EMBO Journal, vol. 28, no. 5, pp. 477–489, 2009. [92] G. D. Brown and S. Gordon, “Immune recognition. A new receptor for beta-glucans,” Nature, vol. 413, pp. 36–37, 2001. [93] G. D. Brown, P. R. Taylor,D.M.Reid et al., “Dectin-1 is amajor β-glucan receptor on macrophages,” Journal of Experimental Medicine, vol. 196, no. 3, pp. 407–412, 2002. [94] P. R. Taylor, S.V.Tsoni,J.A. Willmentet al.,“Dectin-1 is required for β-glucan recognition and control of fungal infection,” Nature Immunology, vol. 8, no. 1, pp. 31–38, 2007. [95] S. Xu, J.Huo, M.Gunawan,I. H. Su, and K. P. Lam,“Activated dectin-1 localizes to lipid raft microdomains for signaling and activation of phagocytosis and cytokine production in dendritic cells,” JournalofBiologicalChemistry, vol. 284, no. 33, pp. 22005–22011, 2009. MEDIATORS of INFLAMMATION The Scientific Gastroenterology Journal of World Journal Research and Practice Diabetes Research Disease Markers Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 International Journal of Journal of Immunology Research Endocrinology Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 Submit your manuscripts at http://www.hindawi.com BioMed PPAR Research Research International Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 Journal of Obesity Evidence-Based Journal of Journal of Stem Cells Complementary and Ophthalmology International Alternative Medicine Oncology Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 Parkinson’s Disease Computational and Behavioural Mathematical Methods AIDS Oxidative Medicine and in Medicine Research and Treatment Cellular Longevity Neurology Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Mediators of Inflammation Hindawi Publishing Corporation

Location, Location, Location: Is Membrane Partitioning Everything When It Comes to Innate Immune Activation?

Loading next page...
 
/lp/hindawi-publishing-corporation/location-location-location-is-membrane-partitioning-everything-when-it-oDcP0iGsin

References (98)

Publisher
Hindawi Publishing Corporation
Copyright
Copyright © 2011 Martha Triantafilou et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ISSN
0962-9351
eISSN
1466-1861
DOI
10.1155/2011/186093
Publisher site
See Article on Publisher Site

Abstract

Hindawi Publishing Corporation Mediators of Inflammation Volume 2011, Article ID 186093, 10 pages doi:10.1155/2011/186093 Review Article Location, Location, Location: Is Membrane Partitioning Everything When It Comes to Innate Immune Activation? 1, 2 3, 4 1 Martha Triantafilou, Philipp M. Lepper, Robin Olden, 2 1, 2 Ivo de Seabra Rodrigues Dias, and Kathy Triantafilou Department of Child Health, School of Medicine, University Hospital of Wales, Cardiff University, Cardiff CF14 4XN, UK Infection and Immunity Group, School of Life Sciences,University of Sussex, Falmer, Brighton BN1 9QG, UK Department of Pneumology, Bern University Hospital (Inselspital), University of Bern, 3010 Bern, Switzerland Department of Internal Medicine V-Pneumology, Allergology and Respiratory Critical Care Medicine, University Hospital of Saarland, 66424 Homburg, Germany Correspondence should be addressed to Kathy Triantafilou, ktrian@hotmail.com Received 22 November 2010; Accepted 27 March 2011 Academic Editor: Giamila Fantuzzi Copyright © 2011 Martha Triantafilou et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In the last twenty years, the general view of the plasma membrane has changed from a homogeneous arrangement of lipids to a mosaic of microdomains. It is currently thought that islands of highly ordered saturated lipids and cholesterol, which are laterally mobile, exist in the plane of the plasma membrane. Lipid rafts are thought to provide a means to explain the spatial segregation of certain signalling pathways emanating from the cell surface. They seem to provide the necessary microenvironment in order for certain specialised signalling events to take place, such as the innate immune recognition. The innate immune system seems to employ germ-lined encoded receptors, called pattern recognition receptors (PRRs), in order to detect pathogens. One family of such receptors are the Toll-like receptors (TLRs), which are the central “sensing” apparatus of the innate immune system. In recent years, it has become apparent that TLRs are recruited into membrane microdomains in response to ligands. These nanoscale assemblies of sphingolipid, cholesterol, and TLRs stabilize and coalesce, forming signalling platforms, which transduce signals that lead to innate immune activation. In the current paper, we will investigate all past and current literature concerning recruitment of extracellular and intracellular TLRs into lipid rafts and how this membrane organization modulates innate immune responses. 1. Introduction are the spontaneous partitioning of lipids and proteins in discrete membrane domains, a behaviour based on their The general view of the cellular plasma membrane has physicochemical characteristics and the possibility to recover evolved over the last twenty years from that of a homo- these microdomains and their associated protein machinery geneous arrangement of lipids with embedded proteins as detergent-resistant entities using biochemical flotation towards that of a mosaic of microdomains, each having a experiments. Microdomains appear as small dynamic struc- specific protein and lipid composition [1]. Over the last tures that can aggregate into larger platforms in response to couple of decades, evidence has accumulated for organisa- various stimuli [5]. tion of the plasma membrane into lipid-based microdomains Currently, lipid rafts are thought to allow different or lipid rafts. A new model of membrane architecture has protein-lipid and protein-protein interactions that tem- been suggested [2] in which the membrane is patchy with porarily compartmentalise the plasma membrane. Lipid segregated cholesterol-rich portions, called lipid rafts. Lipid rafts are thought to provide a means to explain the spatial rafts are envisaged as islands of highly ordered saturated segregation of certain signalling pathways emanating from lipids and cholesterol that are laterally mobile in the plane the cell surface. They seem to provide the necessary microen- of a more fluid disordered bilayer of largely unsaturated vironment in order for certain specialised signalling events lipids [3, 4]. The hallmark of the lipid raft hypothesis to take place. Recent studies have shown the importance 2 Mediators of Inflammation of lipid raft formation in the acquired immune response. factor kappa B (NF-κB) [12, 13], which in turn leads to the Major Histocompatibility Complex- (MHC-) restricted T- secretion of proinflammatory cytokines such as TNF-α,IL-6, cell activation seems to be facilitated by lipid raft formation and IL-8. [6]. Furthermore, we have recently found that mediators TLR4 was found to recognise bacterial lipopolysaccha- of the innate immune response also concentrate in lipid ride (LPS) or endotoxin [14, 15]; TLR2 was found to rafts in order to facilitate signal transduction [7, 8], thus recognise lipoteichoic acid (LTA) and peptidoglycan [16]; suggesting that both the acquired and innate immune TLR3 was able to sense double-stranded viral RNA [17]; systems utilise membrane partitioning as means of activation TLR5 was found to recognise bacterial flagellin [18], TLR7 against invading pathogens. Crucial receptors for both innate [19]and TLR8 [20] to sense single stranded viral RNA, and acquired immunity seem to oligomerize in nonrandom whereas TLR9 to recognise bacterial CpG DNA [21]. In membrane structures, bringing together their signalling addition, TLR2 was found to recognise different motifs machinery. Thus accumulation of receptors within these including several components of Gram-positive bacteria “floating islands” on the cell membrane seems to bring such as peptidoglycan [22], lipoteichoic acid (LTA) [23], together intracellularly all the adaptor molecules that are lipoarabinomannan [24], lipoproteins [25], and different necessary for signalling. In this paper, we will investigate LPS from certain Gram-negative bacteria [26], yeast [27], further the mechanisms of innate immune recognition and spirochete, and fungi [28, 29] through its unique ability review past and current literature that leads us to believe that to heterodimerize with TLRs 1 and 6 [30]. Studies using membrane partitioning and lipid rafts play a central role in diacylated and triacylated lipoproteins have revealed that innate immune activation. diacylated lipoproteins require TLR2/6 heterodimers for activation, whereas triacylated lipoproteins induce activation of the innate immune system independently of TLR6 and 2. The Innate Immune System mainly through TLR2/TLR1 heterodimers [25, 31–35]. The innate immune system constitutes the most archaic part The membrane distribution of TLRs as well as their intra- of our immune defences and has survived through years cellular trafficking has only now begun to be investigated. of evolution. Its function is thought to be the recognition Most TLRs (TLR1, TLR2, TLR4, TLR5, and TLR6) seem to of invading pathogens, the activation of inflammation to activate cells by engaging their ligands on the cell surface, controlthe pathogen,and the subsequent activation of whereas TLR3, TLR7, TLR8, and TLR9 seem to trigger the acquired immune response. As part of its mechanism signalling intracellularly. These TLRs have been shown to of activation, the innate immune system employs germ- reside in the ER and to recognise their ligands once they have lined encoded receptors, called pattern recognition receptors been endocytosed [36, 37]. (PRRs) in order to “sense” pathogens. These PRRs recognise a restricted collection of microbial signatures, able to sense 2.2. Innate Immune Recognition of Bacterial Endotoxin or different types of microbial pathogens ranging from bacteria Lipopolysaccharide. Investigations into the innate immune and viruses to fungi and spirochetes. Lipid rafts seem to be a recognition of bacterial endotoxin led to the discovery of the key feature of the innate immune response, playing a crucial TLR family. TLR4 is the most studied TLR, mainly because role in phagocytosis, receptor-receptor as well as receptor- of its involvement with sepsis and septic shock. Sepsis is pathogen associations as well as signal transduction. Families a paradoxical and complex disorder that results from an of PRRs, such as the Toll-like receptor family (TLR) as well overreaction of our innate immune system to bacterial as the C-type lectin family seem to localise in lipid rafts infections. The mechanisms that are designed to protect for their function thus demonstrating the importance of the host against infection by bacterial pathogens, either this membrane partitioning for the function of the innate Gram-negative or Gram-positive, can lead to oversecretion immune response. of cytokines and fatal sepsis syndrome. It is now widely accepted that the overreaction of the host occurs at the 2.1. The Toll-Like Receptor Family. The TLR receptor family level of the innate immune system and is directly linked to were the first pattern recognition receptors to be identified the recognition of bacterial cell wall components, such as [9, 10]. This family of at least ten germ-line encoded recep- lipopolysaccharide (LPS) from Gram-negative bacteria or tors is able to “sense” microbial signatures and trigger activa- lipoteichoic acid (LTA) from Gram-positive bacteria. Thus tion leading to proinflammatory cytokine secretion. TLRs are the recognition of bacterial products by the innate immune expressed on immune cells and are able to distinguish a great system under certain conditions seems to be detrimental for variety of microbial ligands, such as cell wall components the host. like lipopolysaccharide (LPS) from Gram-negative bacteria In the last twenty-five years, great leaps forward in our and lipoteichoic acid from Gram-positive bacteria, bacterial understanding of the molecular events that lead to the innate flagellin, CpG DNA, and viral DNA or single stranded RNA recognition of pathogens have occurred. One of the seminal [11]. discoveries has been the identification of a serum protein, All identified TLRs are type I transmembrane proteins, lipopolysaccharide-binding protein (LBP), which binds LPS whose intracellular domains contain regions homologous or LTA and delivers it to its cellular targets [38]. Probably to the intracellular domains of IL-1R and are referred to as the most important discovery has been that the main family TIR domains [11]. These intracellular domains are able to of receptors employed by the innate immune system are the trigger signalling pathways known to activate the nuclear Toll-like receptors (TLRs). Mediators of Inflammation 3 As far as sepsis and bacterial recognition is concerned, TLR1 and TLR6. These heterodimers preexist and are not TLR4 seems to be the central sensor of Gram-negative induced by the ligand (Figure 1(a)). TLR2/6 heterodimers bacterial products [15, 39], whereas TLR2 seems to be are recruited within lipid rafts and associate with lipid raft- the key receptor in activating the immune system against resident proteins CD14 and CD36 upon ligand engagement. Gram-positive bacteria [23]. In addition to the involvement Binding of appropriate microbial substances leads to energy- of TLRs, other accessory molecules seem to be involved. dependent clustering of heterotypic receptors and activation CD14 is believed to act as a transfer molecule for both of intracellular signaling cascades that lead, for example, Gram-negative and Gram-positive bacteria [40, 41]. In the via NF-κB to production and secretion of proinflammatory case of LPS recognition, it has been further shown that a cytokines (Figure 1(b)). soluble molecule, MD-2 [42], as well as activation clusters Functional associations of TLRs with non-TLR mole- involving several other receptors [43–45]. In the case of cules have also been demonstrated, for example, TLR2 LTA recognition, TLR2 seems to form receptor clusters as association with dectin-1 is required for macrophage and well, comprising of at least CD14, TLR2, TLR6, and CD36 dendritic cell activation by β-glucan-containing particles. [46]. Thus we are moving away from the single-receptor More recently, functional interactions of TLR2 and CD36 model of activation, and a more complex picture is emerging. have been shown to be involved in the recognition of The mechanism that leads to activation seems to involve diacylglycerides [46]. TLR4 seems to be the best example of the careful interplay of several receptor molecules as well TLRs associating with non-TLR molecules. As it has already as serum proteins. Therefore such a complex orchestration been mentioned, TLR4 has been shown to form at least of events requires a nonrandom membrane architecture a trimolecular complex with CD14 and MD2 in order to specifically geared to bring receptor molecules together and recognise bacterial LPS [39]. The possibility that additional trigger activation within the lipid bilayer and lipid rafts or receptor components such as heat shock proteins [43, 50], membrane microdomains seem to provide this platform. CXCR4 [43], or CD55 [45] have been suggested to be part of this activation cluster, possibly acting as additional LPS 2.3. Protein-Protein Interactions in Innate Immunity: PRRs Are transfer molecules. Furthermore, it has been demonstrated that different “shapes” of LPS induce the formation of Part of Multicomponent Sensor Apparatuses. PRRs employed by the innate immune system have been shown to have different activation clusters, involving the association of the ability to bind and recognise conserved products of TLR4 with a variety of molecules mentioned above, which seems to determine LPS responses [51]. pathogens that are unique to the invading microorganisms but not to the host, it is becoming increasingly apparent Recent structural studies have shed some light onto TLR that the model of a single PRR recognising foreign antigen associations, supporting the hypothesis of cluster formation, is an oversimplified one. With the discovery of the Toll-like since all TLRs that have been crystallised have been found to receptors as the main signal transducing molecules of the be in a dimer formation, thus the hypothesis has been put innate immune system, an onslaught of research has shown forward that dimerisation or clustering might be a common that PRRs are part of multicomponent sensor apparatuses. feature of the TLRs and might be essential for signalling. TLRs have been shown to function as homo- or het- Structural studies of TLRs have been an attractive area of research since structural information is crucial erodimers and to even form functional interactions with non-TLR molecules. Many of these interactions are highly in understanding receptor function. In 2005, the crystal stable, whereas others are transient, forming dynamic associ- structureof TLR3 was thefirstone to be revealed [52]. It was surprising, that although the structure did not have a ations in response to specific stimuli. Whether homotypic, heterotypic, stable, or transient, these different protein ligand, TLR3 was crystallised as a dimer. In 2007 and 2008, combinations generate considerable functional diversity for three structures of TLR-ligand complexes were revealed, the innate immune system by triggering distinct signalling TLR1-TLR2-lipoprotein, TLR4-MD-2-Eritoran, and TLR3- cascades leading to cellular activation. There are a number dsRNA [53–55]. The ectodomains were found to form dimers, which were strikingly similar in shape. Prior to the of examples that suggest that TLR associations are required for cellular activation. TLR4 seems to form a complex with publication of the crystal structures, Gayand Gangloff [56] at least two other molecules, CD14 and MD2, in order to suggested a possible model of activation, where dimerization was ligand induced. These observations have suggested the recognise bacterial LPS [47]. In addition,itseems toassociate with a Toll-like receptor homologue RP105, which acts as a hypothesis that dimerization of the ectodomains forces the negative regulator of TLR4 responses [48]. TLR2 has been intracellular TIR domains to dimerize, and this initiates signalling by recruiting the intracellular adaptor molecules, found to heterodimerize with TLR1 or TLR6 for recognition of yeast components [30] and to associate with TLR1 for the such as MyD88, MAL, TRIF, and TRAM in order to initiate recognition of bacterial lipoproteins. In addition, TLR2 has signalling. The structures of the TIR domains of TLR1, been shown to also interact with scavenger receptors in order TLR2, and TLR10 have been revealed [57]. Interestingly, to recognise lipoproteins [46] and most recently it was shown the TIR domain of TLR10 was shown to be involved in a homodimeric interaction. However, it is not certain that TLR2 associates with CXCR4, which acts as a negative regulator of TLR2 responses [49](Figure 1). Figure 1 depicts whether the structure seen in the crystal corresponds to the possible model of TLR activation, and how TLRs and a physiologically relevant dimer of TLR10 TIR domains because they have been found to exist as monomers in other receptors are organized in lipid rafts on the cell surface before and after stimulation. TLR2 forms heterodimers with solution. Interestingly it has been recently suggested [58]that 4 Mediators of Inflammation TLR2/6 CXCR4 CD36 CD14 TLR2/6 CD14 CD36 CXCR4 Lipid raft P Lipid raft Conformational change Receptor complex Binding bacterial products Activation of signalling cascades Golgi apparatus Secretion of proinflammatory mediators, for example, TNF-α (a) (b) = bacterial products = energy dependent/phosphorylation step = gene activation/transcription/translation Figure 1: Activation of TLRs and adjuvant receptors on cell surface before and after stimulation by bacterial products. (a) TLR2 forms heterodimers with TLR1 and TLR6 on the cell surface and these heterodimer preexist and are not induced by the ligand. These heterodimers do not reside in lipid rafts before stimulation but are recruited to lipid rafts upon stimulation. This process is independent of signaling and facilitates the trafficking of TLRs from the cell surface to the Golgi. (b) TLR2/6 heterodimers are recruited within lipid rafts and associate with lipid raft-resident proteins CD14 and CD36 upon ligand stimulation. Binding of an appropriate microbial substance leads to energy- dependent clustering of heterotypic receptors and activation of intracellular signaling cascades that lead via NF-κB to the production and secretion of proinflammatory cytokines. MyD88 interacts with IRAK4 in an 8 : 4 ratio in solution, apparatus [63]. This intracellular targeting was shown to be suggesting that maybe there is higher oligomer formation. independent of signalling, thus suggesting that accumulation In order for such higher oligomers to be formed and in in lipid rafts only facilitated ligand recognition and signalling order to have such a well-orchestrated accumulation of re- that was initiated at the cell surface and not in the intracellu- ceptors and signalling machinery membrane partitioning lar compartments where TLR4 was targeted to [63]. seems to be crucial for the formation of these “TLR multi- More recently it had been proposed that the molecular component sensor apparatuses”. mechanism for signalling by the TLRs must involve a series of protein conformational changes initiated by dimerization 2.4. TLR4 Recruitment to Membrane Microdomains upon of their extracellular domains [64]. It was suggested that this Ligand Engagement. TLR4 wasthe first oneto beshown to be receptor-receptor association of the extracellular domains recruited to lipid rafts upon stimulation by bacterial LPS [7]. forced the association of the cytoplasmic domains as well. Within these membrane microdomains it was shown that Motshwene et al. [58] recently proved this experimentally, TLR4 formed clusters with non-TLR molecules that tailored demonstrating that the death domains of human MyD88, the immune response against the particular pathogen [43, one of the adaptor proteins used by all but one of the 59–62]. TLRs, and IRAK4 assemble into closed complexes with It was subsequently shown that this accumulation in lipid stoichiometries of 7 : 4 and 8 : 4, which they called the Myd- rafts also influenced its internalization and targeting. TLR4 dosome. The ability to form 7 : 4 or even 8 : 4 stoichiometries was found to accumulate in lipid rafts, to internalize in a suggests a mechanism by which clusters of activated receptors lipid-raft-dependent manner and to be targeted to the Golgi concentrate in lipid rafts and their intracellular machinery Mediators of Inflammation 5 clusters as well, forming a signalling platform that seems to the raft-associated molecules, such as TLR4, targeted to? And be crucial for TLR activation. most importantly why? This intracellular targeting seems to be independent of 2.5. Does Membrane Partitioning Play a Major Role in Protein signalling. TLR2 has also been found to reside in lipid rafts Uptake and Intracellular Routing? Cell membranes display after stimulation by Gram-positive bacterial products and to be similarly targeted to the Golgi apparatus [72](Figure 1). a tremendous complexity of lipid and proteins designed to perform the functions cells require. To coordinate these The question that remains is whether lipid raft association is functions, the membrane is able to laterally segregate its common for all TLRs expressed at the cell surface? If this is the case, do they all follow the same intracellular route? Do constituents. Lipid rafts were originally proposed as an expla- nation for a nonrandom membrane architecture and their different signalling cascades require differential targeting of function was originally thought to be linked with membrane TLRs and their ligands? trafficking. However, rafts proved to be able to influence In the case of the ER-resident TLRs, very little evidence of organization of membrane receptors and bioactivity as well their trafficking upon stimulation exists. To date only TLR9 as membrane trafficking. has been found to translocate from the ER to lysosomes in It is now emerging that this membrane partitioning response to its ligand, CpG DNA [73]. Based on the findings might play a major role in protein uptake and intracellular for TLR9, a hypothesis has been put forward that ER-resident TLRs might become accessible to endosomal and lysosomal routing. It is becoming more apparent that this differential sorting on the cell surface might pre-dispose the intra- compartments after the ER fuses with sites of microbial entry. cellular fate of a given molecule. Since the discovery of If this is the case, then it would seem that ER membrane clathrin-coated pits by Roth and Porter in 1964 [65], as fusion might be critical for microbial recognition by ER- specialised sites for the selective recruitment of specialised resident TLRs. molecules that are internalised into eukaryotic cells, clathrin- independent endocytic pathways have now emerged. Endo- 2.6. Do Lipid Rafts Control Endosomal Innate Immune Dy- cytic pathways that do not rely on the formation of clathrin namics? The regulation of endosome dynamics is crucial coated pits include the earliest identified pathways such for fundamental cellular functions, such as nutrient in as phagocytosis, macropinosis, and caveolae. Lipid rafts take/digestion, membrane receptor recycling/degradation, might involved in all of these pathways. In particular for antigen presentation, cell migration, and intracellular signal- phagocytosis, it has been shown that lipid rafts play a ing [74–76]. The system is also utilised by various pathogens crucial role in the phagocytic uptake of latex microspheres to bud in and from the cell [77]. [66], suggesting that these specialised microdomains on In addition to the function asadistribution centre, it the plasma membrane are necessary for endocytosis and has been proposed that the endosome system serves as an phagocytosis. intracellular signalling station [78]. In the case of innate Furthermore, caveolae which is defined as small, un- immunity this is certainly the case, since TLR3, TLR7, TLR8, coated invaginations in the plasma membrane containing and TLR9 all reside within the endosome waiting to capture the plasma protein caveolin 1 has been shown to be incoming PAMPs and trigger signalling. Endosomes are able to bind cholesterol and to be resistant to detergent pleiomorphic organelles composed of tubular elements as extraction [67], and this has led to the suggestion that well as vesicular regions with a characteristic multivesicular caveolae might constitute a type of lipid raft [68]. Lipid rafts appearance. The question that remains is whether in addition are increasingly becoming linked with clathrin-independent to these morphologically distinguishable regions, endosomal endocytosis, since nearly all molecules that are known membranes are further subcompartmentalized into mem- to be internalised independently of clathrin are found in brane lipid rafts or microdomains. biochemically defined rafts [69]. It has been suggested that Lipid rafts have mostly been studied at the plasma raft components might be taken up preferentially by clathrin- membrane, mainly due to accessibility for microscopy and independent endocytosis. There are likely to be several types biophysical methods [79]. Characterisation of lipid rafts has of clathrin-independent endocytosis. The extent to which also been extensively based on their resistance to detergent these different pathways require lipid rafts to operate or are solubilisation, although this has inherent limitations [80], somehow selective for lipid rafts is currently the subject of as well as fluorescent microscopy [79]. Although most intensive investigation. Recently, Nichols et al. have described studies have focused on the existence of lipid rafts on plasma a rapid lipid-raft-dependent targeting from the cell surface membranes, many intracellular organelles appear to contain to the Golgi apparatus [70]. In addition, a new clathrin- raft-like domains [81–84]. Due to its low cholesterol content, independent mechanism has been described that can lead to the endoplasmic reticulum was originally thought not to delivery of receptor molecules from the plasma membrane contain cholesterol-dependent microdomains. However, to caveolin-1-containing endosomes, termed “caveosomes” recently several studies have reported their existence [85, 86]. [71]. With the emergence of these new clathrin-independent Raft-like domains have been described in the Golgi and uptake mechanisms the idea that different types of endocyto- trans-Golgi network [87, 88], along the endocytic pathway sis have markedly different functions is beginning to become [84]aswell asinthe endosomes [89–91]. Although potential apparent. Ultimately we have to speculate that sorting at the roles of lipid rafts for the outer membrane have been demon- plasma membrane might predispose the intracellular route strated, including endocytosis, exocytosis, vesicle formation, that a molecule might take. If that is the case, then where are and signalling, the functions of lipid rafts in the processes of 6 Mediators of Inflammation TLR9 Receptor complex Binding viral products Endosome ssRNA TLR7 and 8 Transport (UNC93B1) Activation of signalling -cleavage in endosome cascades TLR9 ER Mitochondria Secretion of pro-inflammatory mediators Lipid raft conformationa (a) (b) = bacterial products = energy dependent (phosphorylation) step = gene activation/transcription/translation Figure 2: Activation of TLRs in endosomes. Various TLRs recognize microbial patterns within the endosome. TLR7 and 8 recognize ssRNA, whereas TLR9 recognises CpG DNA. As nucleic acid recognition bears a potential source for the induction of autoimmunity, TLR7 and 9 exist in a full-length and truncated version, where the ectodomain is cleaved. Only the endosomally processed forms are capable to recruit MyD88 and to induce signaling. Transport from the endoplasmatic reticulum (ER) is facilitated by UNC93B1 (a). Ligation of TLR7 by ssRNA leads to clustering within a lipid raft at the endosomal membrane and activation of intracellular signaling cascades that lead via NF-κBto production and secretion of proinflammatory cytokines (b). endosomal membrane dynamics are currently unknown. signaling cascades that lead via NF-κB to production and We can only speculate that they are contributing to similar secretion of proinflammatory cytokines (Figure 2(b)). functions. It has been suggested that protein and lipid sorting into and out of the endosomes may be controlled 2.7. Existence of Other PPRs in Lipid Rafts. C-type lectin by endosomal membrane partitioning [89], but whether receptors (CLRs), such as Dectin 1, are a family of pattern these microdomains control signalling and in particular recognition receptors which bind β-glucan found in the cell TLR signalling has not been investigated. Since most walls of pathogenic fungi such as Candida albicans .In partic- extracellular TLRs have been shown to be recruited to ular Dectin 1 has been shown to mediate the phagocytosis of lipid rafts upon ligand activation, it is safe to assume that yeast and yeast-derived particles, such as zymosan, activating endosomal TLRs act in a similar manner, especially since the production of inflammatory cytokines [92–94]. Inter- the existence of cholesterol-dependent microdomains on estingly, Dectin-1 possesses an immunotyrosine-activated the endosomal membranes has been proven (Figure 2). motif (ITAM) in its cytoplasmic tail, suggesting that it is Thus it is safe to assume that membrane partitioning capable of mediating signalling analogous to the BCR and control both extracellular and intracellular TLR-dependent TCR. Since both the BCR and TCR have been found to reside signalling. We are proposing a model for endosomal TLR in lipid rafts it was suggested that Dectin-1 might also be activation, where ligation of endosomal TLRs by their recruited there upon activation. A recent study has revealed respective ligands can lead to clustering within lipid rafts that Dectin-1 and possibly other CLRs are recruited to lipid at the endosomal membrane and activation of intracellular rafts upon activation and raft integrity is important for Mediators of Inflammation 7 signalling [95]. Thus suggesting that recruitment to lipid [2] L. J. Pike, “Lipid rafts: bringing order to chaos,” Journal of Lipid Research, vol. 44, no. 4, pp. 655–667, 2003. rafts is a common feature for most PRRs, including TLRs [3] K. Simons and E. Ikonen, “Functional rafts in cell mem- and CLRs. branes,” Nature, vol. 387, no. 6633, pp. 569–572, 1997. [4] A.Pralle,P. Keller, E. L. Florin, K.Simons, and J. K.H.Horb ¨ er, 2.8. Concluding Remarks. Cell membranes are complicated “Sphingolipid-cholesterol rafts diffuse as small entities in in composition but precise in purpose: to selectively com- the plasma membrane of mammalian cells,” Journal of Cell partmentalize its constituents in order to coordinate cellular Biology, vol. 148, no. 5, pp. 997–1007, 2000. functions. In this way, the membrane is able to compart- [5] T. Harder, P. Scheiffele, P. Verkade, and K. Simons, “Lipid mentalize, segregate receptors as well as their signalling domain structure of the plasma membrane revealed by machinery and create oligomeric signalling platforms in patching of membrane components,” Journalof CellBiology, order to transduce signals. Once the required function has vol. 141, no. 4, pp. 929–942, 1998. [6] H. A. Anderson, E. M. Hiltbold, and P. A. Roche, “Concentra- subsided, these segregated islands are involved in internal- tion of MHC class II molecules in lipid rafts facilitates antigen ization and membrane trafficking, thus bringing the whole presentation,” Nature Immunology, vol. 1, no. 2, pp. 156–162, function to a close. The innate and acquired immune systems seem to utilise this membrane partitioning for [7] M. Triantafilou, K. Miyake, D. T. Golenbock, and K. Tri- their functions. In this paper, we have extensively looked antafilou, “Mediators of innate immune recognition of bacte- at the use of this membrane partitioning by the innate ria concentrate in lipid rafts and facilitate lipopolysaccharide- immune system and most particular by the TLRs. The induced cell activation,” JournalofCellScience, vol. 115, no. molecular mechanism involved in LPS recognition and TLR 12, pp. 2603–2611, 2002. signalling in general, utilises a series of protein lipid as [8] M. Triantafilou, S. Morath, A. Mackie, T. Hartung, and K. well as protein-protein interactions. The plasma membrane Triantafilou, “Lateral diffusion of Toll-like receptors reveals seems to be heterogeneous and to coalesce to more stable that they are transiently confined within lipid rafts on the membrane-ordered assemblies upon activation by ligands. plasma membrane,” JournalofCellScience, vol. 117, no. 17, pp. 4007–4014, 2004. This partitioning of the membrane and the assembly of [9] R.Medzhitov and C.A.Janeway, “Decoding the patterns of self more stable raft platforms in the functionalized state must and nonself by the innate immune system,” Science, vol. 296, be initiated by raft-resident proteins, which form protein- no. 5566, pp. 298–300, 2002. lipid as well as protein-protein interactions. The TLRs and [10] S. Akira, “Toll-like receptors and innate immunity,” Advances other PRRs associate with the raft-resident proteins and in Immunology, vol. 78, pp. 1–56, 2001. are recruited to these “floating islands” forming higher [11] K. Takeda, T. Kaisho, and S. Akira, “Toll-like receptors,” oligomers, both extracellularly as well as inside the cell, Annual Review of Immunology, vol. 21, pp. 335–376, 2003. concentrating their signalling machinery which finally leads [12] R. Medzhitov, P. Preston-Hurlburt, E. Kopp et al., “MyD88 is to a functional, focused, and coordinated activation of the an adaptor protein in the hToll/IL-1 receptor family signaling innate immune system. pathways,” Molecular Cell, vol. 2, no. 2, pp. 253–258, 1998. The lifetime of these domains and the length of the [13] L. O’Neill, “The Toll/interleukin-1 receptor domain: a molec- ular switch for inflammation and host defence,” Biochemical response will depend on their size and factors that may sta- Society Transactions, vol. 28, no. 5, pp. 557–563, 2000. bilise or destabilise them. These factors will include not only [14] A. Poltorak, X. L. He, I. Smirnova et al., “Defective LPS lipid-lipid, lipid-protein and protein-protein interactions signaling in C3H/Hej and C57BL/10ScCr mice: mutations in both in the plane of the membrane but also elements of the TLR4 gene,” Science, vol. 282, no. 5396, pp. 2085–2088, 1998. cytoskeleton, pericellular matrix adjacent to the membrane [15] S. T. Qureshi, L. Larivier ` e, G. Leveque et al., “Endotoxin- as well as transmembrane and cytoplasmic domains of the tolerant mice have mutations in Toll-like receptor 4 (TlR4),” receptors involved. In thecaseof TLRs, theassociation of the Journal of Experimental Medicine, vol. 189, no. 4, pp. 615–625, TIR domains intracellularly would stabilise the ectodomains extracellularly and provide the molecular scaffold that will [16] O. Takeuchi, K. Hoshino, T. Kawai et al., “Differential roles of recruit the adaptor molecules that contribute to signalling. TLR2 and TLR4 in recognition of gram-negative and gram- The challenge for the future will be to visualise the assembly positive bacterial cell wall components,” Immunity,vol.11, no. 4, pp. 443–451, 1999. and stoichiometry of these large and transient oligomeric [17] L. Alexopoulou, A. C. Holt, R. Medzhitov, and R. A. Flavell, complexes in vivo. Thus refining existing biophysical meth- “Recognition of double-stranded RNA and activation of NF- ods, such as single particle tracking (SPT), fluorescence κB by Toll-like receptor 3,” Nature, vol. 413, no. 6857, pp. 732– correlation spectroscopy (FCS), and fluorescence resonance 738, 2001. energy transfer (FRET) will help us reveal these dynamic [18] F. Hayashi, K. D. Smith, A. Ozinsky et al., “The innate immune nanoassemblies of sterol, sphingolipid, and proteins in living response to bacterial flagellin is mediated by Toll-like receptor cell and provide us with the first dynamic picture of the 5,” Nature, vol. 410, no. 6832, pp. 1099–1103, 2001. innate immune response. [19] J. M. Lund,L. Alexopoulou,A.Sato et al., “Recognition of single-stranded RNA viruses by Toll-like receptor 7,” Proceedings of the National Academy of Sciences of the United References States of America, vol. 101, no. 15, pp. 5598–5603, 2004. [1] J.F. Hancock,“Lipid rafts:contentious only from simplistic [20] F. Heil, H. Hemmi, H. Hochrein et al., “Species-specific recognition of single-stranded RNA via Toll-like receptor 7 standpoints,” Nature Reviews Molecular Cell Biology,vol. 7,no. 6, pp. 456–462, 2006. and 8,” Science, vol. 303, no. 5663, pp. 1526–1529, 2004. 8 Mediators of Inflammation [21] H. Hemmi, O. Takeuchi, T. Kawai et al., “A Toll-like receptor [36] F. Heil, P. Ahmad-Nejad, H. Hemmi et al., “The Toll-like recognizes bacterial DNA,” Nature, vol. 408, no. 6813, pp. 740– receptor 7 (TLR7)-specific stimulus loxoribine uncovers a 745, 2000. strong relationship within the TLR7, 8 and 9 subfamily,” Euro- pean Journal of Immunology, vol. 33, no. 11, pp. 2987–2997, [22] A. Yoshimura, E. Lien,R.R. Ingalls,E.Tuomanen, R. Dziarski, and D. Golenbock, “Cutting edge: recognition of Gram- positive bacterial cell wall components by the innate immune [37] T. Nishiya and A. L. DeFranco, “Ligand-regulated chimeric system occurs via Toll-like receptor 2,” Journal of Immunology, receptor approach reveals distinctive subcellular localization vol. 163, no. 1, pp. 1–5, 1999. and signaling properties of the Toll-like receptors,” Journal of Biological Chemistry, vol. 279, no. 18, pp. 19008–19017, 2004. [23] R. Schwandner, R. Dziarski, H. Wesche,M.Rothe,and C. J. Kirschning, “Peptidoglycan- and lipoteichoic acid-induced [38] P. S. Tobias, K. Soldau, and R. J. Ulevitch, “Isolation of a cell activation is mediated by Toll-like receptor 2,” Journal of lipopolysaccharide-binding acute phase reactant from rabbit Biological Chemistry, vol. 274, no. 25, pp. 17406–17409, 1999. serum,” Journal of Experimental Medicine, vol. 164, no. 3, pp. 777–793, 1986. [24] T. K. Means, E. Lien,A.Yoshimura, S.Wang, D. T. Golenbock, and M. J. Fenton, “The CD14 ligands lipoarabinomannan [39] A. Poltorak, P. Ricciardi-Castagnoli, S. Citterio, and B. Beutler, and lipopolysaccharide differ in their requirement for Toll-like “Physical contact between lipopolysaccharide and Toll-like receptors,” Journal of Immunology, vol. 163, no. 12, pp. 6748– receptor 4 revealed by genetic complementation,” Proceedings 6755, 1999. of the National Academy of Sciences of the United States of America, vol. 97, no. 5, pp. 2163–2167, 2000. [25] O. Takeuchi, S. Sato, T. Horiuchi et al., “Cutting edge: role of Toll-like receptor 1 in mediating immune response to [40] S. D. Wright,R. A. Ramos,P.S.Tobias, R. J. Ulevitch, microbial lipoproteins,” Journal of Immunology, vol. 169, no. and J. C. Mathison, “CD14, a receptor for complexes of 1, pp. 10–14, 2002. lipopolysaccharide (LPS) and LPS binding protein,” Science, vol. 249, no. 4975, pp. 1431–1433, 1990. [26] C. Werts, R. I. Tapping, J. C. Mathison et al., “Leptospiral lipopolysaccharide activates cells through a TLR2-dependent [41] D. Gupta, T. N. Kirkland, S. Viriyakosol, and R. Dziarski, mechanism,” Nature Immunology, vol. 2, no. 4, pp. 346–352, “CD14 is a cell-activating receptor for bacterial peptidogly- 2001. can,” Journalof BiologicalChemistry, vol. 271, no. 38, pp. 23310–23316, 1996. [27] D. M. Underhill, A. Ozinsky, A. M. Hajjar et al., “The Toll- like receptor 2 is recruited to macrophage phagosomes and [42] Y. Nagai, S. Akashi, M. Nagafuku et al., “Essential role of discriminates between pathogens,” Nature, vol. 401, no. 6755, MD-2 in LPS responsiveness and TLR4 distribution,” Nature pp. 811–815, 1999. Immunology, vol. 3, no. 7, pp. 667–672, 2002. [28] B. N. Gantner, R. M. Simmons, S. J. Canavera, S. Akira, and [43] K. Triantafilou, M. Triantafilou, and R. L. Dedrick, “A CD14- D. M. Underhill, “Collaborative induction of inflammatory independent LPS receptor cluster,” Nature Immunology,vol. 2, responses by dectin-1 and Toll-like receptor 2,” Journal of no. 4, pp. 338–345, 2001. Experimental Medicine, vol. 197, no. 9, pp. 1107–1117, 2003. [44] A. Pfeiffer, A. Bottcher, E. Orso et al., “Lipopolysaccharide and [29] H. Heine and E. Lien, “Toll-like receptors and their function ceramide docking to CD14 provokes ligand-specific receptor in innate and adaptive immunity,” International Archives of clustering in rafts,” European Journal of Immunology,vol.31, Allergy and Immunology, vol. 130, no. 3, pp. 180–192, 2003. no. 11, pp. 3153–3164, 2001. [30] A. Ozinsky, D. M. Underhill, J. D. Fontenot et al., “The [45] H. Heine, V. El-Samalouti, C. Notse ¨ l et al., “CD55/decay repertoire for pattern recognition of pathogens by the innate accelerating factor is part of the lipopolysaccharide-induced immune system is defined by cooperation between Toll-like receptor complex,” European Journal of Immunology,vol.33, receptors,” Proceedings of the National Academy of Sciences of no. 5, pp. 1399–1408, 2003. the United States of America, vol. 97, no. 25, pp. 13766–13771, [46] K. Hoebe, P. Georgel, S. Rutschmann et al., “CD36 is a sensor of diacylglycerides,” Nature, vol. 433, no. 7025, pp. 523–527, [31] L. Alexopoulou, V. Thomas, M. Schnare et al., “Hypore- 2005. sponsiveness to vaccination with Borrelia burgdorferi OspA [47] A. Visintin, E. Latz, B. G. Monks, T. Espevik, and D. T. in humans and in TLR1- and TLR2-deficient mice,” Nature Golenbock, “Lysines 128 and 132 enable lipopolysaccharide Medicine, vol. 8, no. 8, pp. 878–884, 2002. binding to MD-2, leading to Toll-like receptor-4 aggregation [32] M. Morr, O. Takeuchi, S. Akira, M. M. Simon, and P. F. and signal transduction,” Journal of Biological Chemistry,vol. Mu ¨hlradt, “Differential recognition of structural details of 278, no. 48, pp. 48313–48320, 2003. bacterial lipopeptides by Toll-like receptors,” European Journal [48] S. Divanovic, A. Trompette, S. F. Atabani et al., “Negative of Immunology, vol. 32, no. 12, pp. 3337–3347, 2002. regulation of Toll-like receptor 4 signaling by the Toll-like [33] O. Takeuchi,T.Kawai, P.F. Mu ¨hlradt et al., “Discrimination receptor homolog RP105,” Nature Immunology,vol. 6,no. 6, of bacterial lipoproteins by Toll-like recepttor 6,” International pp. 571–578, 2005. Immunology, vol. 13, no. 7, pp. 933–940, 2001. [49] G. Hajishengallis, M. Wang, S.Liang,M.Triantafilou,and [34] U. Buwitt-Beckmann, H. Heine, K. H. Wiesmu ¨ller et al., K. Triantafilou, “Pathogen induction of CXCR4/TLR2 cross- “Toll-like receptor 6-independent signaling by diacylated talk impairs host defense function,” Proceedings of the National lipopeptides,” European Journal of Immunology,vol.35, no. 1, Academy of Sciences of the United States of America, vol. 105, pp. 282–289, 2005. no. 36, pp. 13532–13537, 2008. [35] M. Triantafilou, F. G. J. Gamper, R. M. Haston et al., “Mem- [50] C. A. Byrd, W. Bornmann, H. Erdjument-Bromage et al., brane sorting of Toll-like receptor (TLR)-2/6 and TLR2/1 “Heat shock protein 90 mediates macrophage activation by heterodimers at the cell surface determines heterotypic asso- Taxol and bacterial lipopolysaccharide,” Proceedings of the ciations with CD36 and intracellular targeting,” Journal of National Academy of Sciences of the United States of America, Biological Chemistry, vol. 281, no. 41, pp. 31002–31011, 2006. vol. 96, no. 10, pp. 5645–5650, 1999. Mediators of Inflammation 9 [51] M. Triantafilou, K. Brandenburg, S. Kusumoto et al., “Com- [68] T. Harder and K. Simons, “Caveolae, DIGs, and the dynamics binational clustering of receptors following stimulation by of sphingolipid-cholesterol microdomains,” Current Opinion bacterial products determines lipopolysaccharide responses,” in Cell Biology, vol. 9, no. 4, pp. 534–542, 1997. Biochemical Journal, vol. 381, no. 2, pp. 527–536, 2004. [69] B. J. Nichols and J. Lippincott-Schwartz, “Endocytosis with- [52] J. Choe,M.S.Kelker, and I.A. Wilson, “Crystal structure of out clathrin coats,” Trends in Cell Biology, vol. 11, no. 10, pp. human Toll-like receptor 3 (TLR3) ectodomain,” Science,vol. 406–412, 2001. 309, no. 5734, pp. 581–585, 2005. [70] B. J. Nichols, A. K. Kenworthy, R. S. Polishchuk et al., “Rapid [53] M. S. Jin, S. E. Kim, J. Y. Heo et al., “Crystal structure of the cycling of lipid raft markers between the cell surface and golgi TLR1-TLR2 heterodimer induced by binding of a tri-acylated complex,” Journalof CellBiology, vol. 152, no. 3, pp. 529–541, lipopeptide,” Cell, vol. 130, no. 6, pp. 1071–1082, 2007. [54] H. M. Kim, B. S. Park, J. I. Kim et al., “Crystal structure of [71] L. Pelkmans and A. Helenius, “Endocytosis via caveolae,” the TLR4-MD-2 complex with bound endotoxin antagonist Traffic, vol. 3, no. 5, pp. 311–320, 2002. eritoran,” Cell, vol. 130, no. 5, pp. 906–917, 2007. [72] M. Trianiafilou, M. Manukyan, A. Mackie et al., “Lipoteichoic [55] L. Liu, I. Botos, Y. Wang et al., “Structural basis of Toll-like acid and Toll-like receptor 2 internalization and targeting receptor 3 signaling with double-stranded RNA,” Science,vol. to theGolgi arelipid raft-dependent,” Journal of Biological 320, no. 5874, pp. 379–381, 2008. Chemistry, vol. 279, no. 39, pp. 40882–40889, 2004. [56] N. J. Gay and M. Gangloff, “Structure of Toll-like receptors,” [73] E. Latz, A. Schoenemeyer, A. Visintin et al., “TLR9 signals Handbook of Experimental Pharmacology, no. 183, pp. 181– after translocating from the ER to CpG DNA in the lysosome,” 200, 2008. Nature Immunology, vol. 5, no. 2, pp. 190–198, 2004. [57] T. Nyman, P. Stenmark, S. Flodin, I. Johansson, M. Ham- [74] V. Dudu, P. Pantazis, and M. Gonzale ´ z-Gaitan, ´ “Membrane marstrom, ¨ and P. R. Nordlund, “The crystal structure of the traffic during embryonic development: epithelial formation, human Toll-like receptor 10 cytoplasmic domain reveals a cell fate decisions and differentiation,” Current Opinion in Cell putative signaling dimer,” Journalof BiologicalChemistry,vol. Biology, vol. 16, no. 4, pp. 407–414, 2004. 283, no. 18, pp. 11861–11865, 2008. [75] M. C. Jones, P. T. Caswell, and J. C. Norman, “Endocytic recy- [58] P. G. Motshwene, M. C. Moncrieffe, J. G. Grossmann et al., “An cling pathways: emerging regulators of cell migration,” Current oligomeric signaling platform formed by the Toll-like receptor Opinion in Cell Biology, vol. 18, no. 5, pp. 549–557, 2006. signal transducers MyD88 and IRAK-4,” Journal of Biological [76] G. Emery and J. A. Knoblich, “Endosome dynamics during Chemistry, vol. 284, no. 37, pp. 25404–25411, 2009. development,” Current Opinion in Cell Biology, vol. 18, no. 4, [59] M. Triantafilou, K. Brandenburg, S. Kusumoto et al., “Com- pp. 407–415, 2006. binational clustering of receptors following stimulation by [77] J. Gruenberg and F. G. van der Goot, “Mechanisms of bacterial products determines lipopolysaccharide responses,” pathogen entry through the endosomal compartments,” Biochemical Journal, vol. 381, no. 2, pp. 527–536, 2004. Nature Reviews Molecular Cell Biology,vol.7,no. 7, pp. [60] M. Triantafilou and K. Triantafilou, “The dynamics of LPS 495–504, 2006. recognition: complex orchestration of multiple receptors,” [78] M. Miaczynska, L. Pelkmans, and M. Zerial, “Not just a Journal of Endotoxin Research, vol. 11, no. 1, pp. 5–11, 2005. sink: endosomes in control of signal transduction,” Current [61] M. Triantafilou and K. Triantafilou, “Receptor cluster for- Opinion in Cell Biology, vol. 16, no. 4, pp. 400–406, 2004. mation during activation by bacterial products,” Journal of [79] K. Jacobson, O. G. Mouritsen, and R. G. W. Anderson, “Lipid Endotoxin Research, vol. 9, no. 5, pp. 331–335, 2003. rafts: at a crossroad between cell biology and physics,” Nature [62] H. E. Humphries, M. Triantafilou, B. L. Makepeace, J. E. Cell Biology, vol. 9, no. 1, pp. 7–14, 2007. Heckels, K. Triantafilou, and M. Christodoulides, “Activation [80] H. Heerklotz, “Triton promotes domain formation in lipid of human meningeal cells is modulated by lipopolysaccharide raft mixtures,” Biophysical Journal, vol. 83, no. 5, pp. 2693– (LPS) and non-LPS components of Neisseria meningitidis and 2701, 2002. is independent of Toll-like receptor (TLR)4 and TLR2 sig- [81] D. A. Brown and J. K. Rose, “Sorting of GPI-anchored proteins nalling,” Cellular Microbiology, vol. 7, no. 3, pp. 415–430, 2005. to glycolipid-enriched membrane subdomains during trans- [63] E. Latz, A. Visintin, E. Lien et al., “Lipopolysaccharide port to the apical cell surface,” Cell, vol. 68, no. 3, pp. 533–544, rapidly traffics to and from the golgi apparatus with the Toll-like receptor 4-MD-2-CD14 complex in a process that is [82] R. Gagescu, N. Demaurex, R. G. Parton, W. Hunziker, L. A. distinct from the initiation of signal transduction,” Journal of Huber, and J. Gruenberg, “The recycling endosome of Madin- Biological Chemistry, vol. 277, no. 49, pp. 47834–47843, 2002. Darby canine kidney cells is a mildly acidic compartment rich [64] N. J. Gay, M. Gangloff, and A. N. R. Weber, “Toll-like receptors in raft components,” Molecular Biology of the Cell, vol. 11, no. as molecular switches,” Nature Reviews Immunology,vol.6, 8, pp. 2775–2791, 2000. no. 9, pp. 693–698, 2006. [83] J. F. Dermine, S. Duclos, J. Garin et al., “Flotillin-1-enriched [65] T. F. Roth and K. R. Porter, “Yolk protein uptake in the oocyte lipid raft domains accumulate on maturing phagosomes,” of the mosquito Aedes aegypti,” The Journal of Cell Biology, Journal of Biological Chemistry, vol. 276, no. 21, pp. 18507– vol. 20, no. 2, pp. 313–330, 1964. 18512, 2001. [66] G. Nagao, K. Ishii, K. Hirota, K. Makino, and H. Terada, [84] M. Fivaz, F. Vilbois, S. Thurnheer et al., “Differential sorting “Role of lipid rafts in phagocytic uptake of polystyrene latex and fate of endocytosed GPI-anchored proteins,” The EMBO microspheres by macrophages,” Anticancer Research,vol.30, Journal, vol. 21, no. 15, pp. 3989–4000, 2002. no. 8, pp. 3167–3176, 2010. [67] M. Sargiacomo, M.Sudol,Z. Tang, and M.P.Lisanti,“Signal [85] D. T. Browman, M. E. Resek, L. D. Zajchowski, and S. M. Robbins, “Erlin-1 and erlin-2 are novel members of transducing molecules and glycosyl-phosphatidylinositol- linked proteins form a caveolin-rich insoluble complex in the prohibitin family of proteins that define lipid-raft-like domains of the ER,” Journal of Cell Science, vol. 119, no. 15, MDCK cells,” JournalofCellBiology, vol. 122, no. 4, pp. 789–807, 1993. pp. 3149–3160, 2006. 10 Mediators of Inflammation [86] L. K. Pielsticker, K. J. Mann, W. L. Lin, and D. Sevlever, “Raft- like membrane domains contain enzymatic activities involved in the synthesis of mammalian glycosylphosphatidylinositol anchor intermediates,” Biochemical and Biophysical Research Communications, vol. 330, no. 1, pp. 163–171, 2005. [87] J. Fu ¨llekrug and K. Simons, “Lipid rafts and apical membrane traffic,” Annals of the New York Academy of Sciences, vol. 1014, pp. 164–169, 2004. [88] H. B. Eberle, R. L.Serrano,J.Ful ¨ lekrug et al., “Identification and characterization of a novel human plant pathogenesis- related protein that localizes to lipid-enriched microdomains in the Golgi complex,” Journal of Cell Science, vol. 115, no. 4, pp. 827–838, 2002. [89] K. Sobo,J.Chevallier,R. G.Parton, J. Gruenberg, and F.G. van der Goot, “Diversity of raft-like domains in late endosomes,” PLoS One, vol. 2, no. 4, article e391, 2007. [90] S. Ignoul, J. Simaels, D. Hermans, W. Annaert, and J. Eggermont, “Human CIC-6 is a late endosomal glycoprotein that associates with detergent-resistant lipid domains,” PLoS One, vol. 2, no. 5, article e474, 2007. [91] S. Nada,A.Hondo, A.Kasai et al., “The novellipid raft adaptor p18 controls endosome dynamics by anchoring the MEK-ERK pathway to late endosomes,” The EMBO Journal, vol. 28, no. 5, pp. 477–489, 2009. [92] G. D. Brown and S. Gordon, “Immune recognition. A new receptor for beta-glucans,” Nature, vol. 413, pp. 36–37, 2001. [93] G. D. Brown, P. R. Taylor,D.M.Reid et al., “Dectin-1 is amajor β-glucan receptor on macrophages,” Journal of Experimental Medicine, vol. 196, no. 3, pp. 407–412, 2002. [94] P. R. Taylor, S.V.Tsoni,J.A. Willmentet al.,“Dectin-1 is required for β-glucan recognition and control of fungal infection,” Nature Immunology, vol. 8, no. 1, pp. 31–38, 2007. [95] S. Xu, J.Huo, M.Gunawan,I. H. Su, and K. P. Lam,“Activated dectin-1 localizes to lipid raft microdomains for signaling and activation of phagocytosis and cytokine production in dendritic cells,” JournalofBiologicalChemistry, vol. 284, no. 33, pp. 22005–22011, 2009. MEDIATORS of INFLAMMATION The Scientific Gastroenterology Journal of World Journal Research and Practice Diabetes Research Disease Markers Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 International Journal of Journal of Immunology Research Endocrinology Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 Submit your manuscripts at http://www.hindawi.com BioMed PPAR Research Research International Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 Journal of Obesity Evidence-Based Journal of Journal of Stem Cells Complementary and Ophthalmology International Alternative Medicine Oncology Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 Parkinson’s Disease Computational and Behavioural Mathematical Methods AIDS Oxidative Medicine and in Medicine Research and Treatment Cellular Longevity Neurology Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014

Journal

Mediators of InflammationHindawi Publishing Corporation

Published: Jun 21, 2011

There are no references for this article.