Based on the Article below,\" Organization and functions of lipid rafts.\" Quest

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Based on the Article below," Organization and functions of lipid rafts."

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Article:
Only a year after the seminal paper of Singer and Nicolson in which the fluid mosaic model for biomembrane organization was proposed1, the first observations that cell membranes can be separated into detergent-labile and detergent-resistant fractions2 sparked the idea that distinct membrane subcompartments are present in biological membranes (for a brief history of biomembrane models, see REF. 3). This finding was followed by a number of observations that suggested that cellular membranes are laterally heterogeneous at the submicrometre scale4–9. The membrane raft (or lipid raft) hypothesis emerged as a way of explaining this lateral membrane inhomogeneity: it proposed that the interactions between specific lipids (for example, cholesterol, relatively saturated lipids and glycosylated lipids) in the plane of the membrane drive the formation of functionally important, relatively ordered membrane regions that recruit other lipids and proteins10. This concept was supported by observations of biomimetic model membranes, which provide clear evidence that certain lipids interact preferentially with one another, engage in collective behaviour and generate large-scale lateral domains as a consequence of liquid–liquid phase separation11.

However, the presence and relevance of such ordered membrane domains in vivo were unclear, owing in part to the lack of direct observations of these domains and uncertain definitions of the lipid raft concept. To address this uncertainty, a consensus operational definition of lipid rafts was formulated in 2006, with the available evidence suggesting that rafts are heterogeneous, dynamic (in terms of both lateral mobility and association–dissociation), cholesterol-and sphingolipid-enriched membrane nanodomains (10–200 nm) that have the potential to form micro­scopic domains (>300 nm) upon clustering induced by protein–protein and protein–lipid interactions12 (FIG. 1). These domains are present in both the inner and the outer leaflets of an asymmetric cell membrane, are pre­sumably coupled across leaflets13,14 and form functional platforms for the regulation of cellular processes15. Recently, several emerging biochemical and biophysical techniques have provided support for the presence of these domains in cells and suggested key roles for mem­brane heterogeneity in various cellular functions. The conservation of lipid rafts throughout the tree of life has also been demonstrated (Supplementary information S1 (box)), which has provided further support for their biological significance. However, lipid rafts continue to elude direct microscopic detection; thus, the presence and exact nature of rafts in live cells remain the subject of debate, particularly as different methodologies can often yield seemingly contradictory results16.

Here, we define rafts as transient, relatively ordered membrane domains, the formation of which is driven by lipid–lipid and lipid–protein interactions, and we discuss the technological advances that have reignited excitement around this concept and its in vivo relevance. In particular, we focus on the current understanding of the mechanisms of raft formation and maintenance, and conclude with a discussion of the challenges that remain Studying lipid rafts

The definition of rafts has been influenced, in large part, by the development of methodologies available for their investigation. The term ‘lipid rafts’ has been applied generically to many distinct, although potentially related, types of membrane assemblies (FIG. 2a). The techniques and tools used to visualize and study membrane hetero­geneity have evolved considerably since the introduction of the concept (FIG. 2b–d), and with the recent advent of super-resolution optical microscopy (Supplementary information S2 (box)) we may now have a key tool for resolving the continuing controversy.

Biochemical tools. The first evidence for a laterally heterogeneous cell membrane came from the obser­vation of differential solubilization of membrane lipids and proteins by detergents in the 1970s2. The basis of the assay is that cellular membranes can be separated into distinct fractions — containing detergent-soluble membranes (DSMs) or detergent-resistant membranes (DRMs) — following extraction with non-ionic deter­gents under specific conditions (most notably, cold temperatures) (FIG. 2b). These fractions have clearly distinct compositions, with DRMs enriched in choles­terol, sphingolipids17,18 and glycosylphosphatidylinositol (GPI)-anchored proteins5. Although extraction of DRMs became the method of choice for probing membrane raft composition, it quickly became clear that DRMs do not reflect the native composition and organization of lipid rafts in living cells. For example, the protein com­position of DRMs varies widely depending on the choice of detergent used for isolation19. Similarly, subtle vari­ations in temperature or detergent concentration yield different results and considerably modify the organiza­tion of membrane proteins20, which has led to contra­dictory reports about the protein composition of rafts. Thus, although DRM assays may provide information about the propensity of some molecules to associate with specialized membrane regions21,22, they do not faithfully reflect the native molecular or biophysical composi­tion and organization of rafts23; therefore, the findings from these assays require confirmation by more robust and consistent methods such as those discussed below (for an excellent recent example, see REF. 22).

Biophysical tools. In parallel with studies of DRMs iso­lated from cells, artificial model membranes have been developed and used to study the liquid–liquid phase separation that is believed to underlie the physical principle behind lipid raft formation24 (FIG. 2b). Across various experimental set-ups, membranes that consist of relatively saturated lipids with a high melting tem­perature, unsaturated phospholipid species with a low melting temperature and cholesterol can separate into two distinct liquid phases: a relatively packed, ordered phase enriched in saturated lipid species and choles­terol25 (termed the liquid-ordered (Lo) phase), and a more fluid, disordered phase comprising mainly the unsaturated lipids26,27 (termed the liquid-disordered (Ld) phase). Owing to its tight molecular packing and enrichment of sterol and saturated lipids, the Lo phase is considered to be the model for lipid rafts. Biomimetic monolayers28, supported lipid bilayers29, nanoscopic bilayer vesicles30 and giant unilamellar vesicles (GUVs)26 have all been used to elucidate the molecular details of this phase separation31,32; however, despite their impor­tant role in revealing the physical principles of Lo domain formation, a number of caveats and limitations prevent direct translation of findings from these model mem­branes to biological ones. First, most of these experi­ments are performed in lipid-only systems, and although there are methods for incorporating integral membrane proteins into artificial systems33,34, they are complex, inefficient and very rarely result in high protein/lipid ratios. This is in contrast to biological membranes, in which proteins are estimated to constitute up to 25% of the cross-sectional area of the membrane35. Second, perhaps because of the scarcity (or even a complete lack). of proteins, some features of domains established in syn­thetic membranes may not be representative of in vivo domains. For example, ordered domains in synthetic membranes have extremely high molecular order and tight packing, whereas the other extreme is observed in the disordered domains27,36. These caveats can be avoided by studying more natural systems such as giant plasma membrane vesicles (GPMVs)37,38. GPMVs are cell-derived, intact plasma membrane vesicles that maintain the lipid39 and protein40 diversity of cellular membranes, although it is notable that they lack an assembled cortical actin cytoskeleton, phosphorylated lipids41 and strict lipid asymmetry between separate leaflets of the membrane bilayer (see REF. 42 for a detailed discussion of the advantages, caveats and applications of GPMVs). In these biological membranes, the disparity in molecular order between coexisting ordered and disordered domains is much smaller than in synthetic GUVs, for example. These differences in molecular order between the two phases may account for the fact that the ordered phase of GUVs excludes almost all trans­membrane proteins and most fluorescent lipid probes (see below), whereas the same molecules are sometimes enriched in the ordered phase in GPMVs (as would be expected for lipid rafts in vivo)36,43 (BOX 1).

Analytical tools. In cells, rafts are believed to be nano­scopic domains (<200 nm)7,8, and therefore they cannot be resolved by conventional optical microscopy, which has an approximately 250 nm resolution limit that is set by diffraction (see Supplementary information S2 (box)). Although the colocalization of certain molecules with putative lipid raft markers (such as the multivalent cholera toxin) detected by confocal microscopy has been used as evidence of their association with rafts44, in gen­eral the resolution of confocal microscopy is insufficient to directly assay raft domain structure and composition. To overcome this limitation, several optical tools have been developed recently45,46 and have been applied to investigate nanoscale structures and dynamics in cells. For example, super-resolution optical microscopy approaches such as photoactivated localization microscopy (PALM), stimulated emission depletion (STED) microscopy (Supplementary information S2 (box)) and near-field scanning optical microscopy (NSOM) have been used to visualize lipid-mediated protein clustering47–50.

For more dynamic measurements, single-molecule-based techniques such as single-particle tracking (SPT) have been used to evaluate the diffusion of membrane molecules and relate it to models of heterogeneous organization of the membrane51. Such studies can reveal oligomerization52, transient arrest, domain incorporation, and/or confined diffusion and hop diffusion (also known as compartmentalized diffusion)53 of tracked molecules (FIG. 2b). Recently, interferometric scattering microscopy (iSCAT) has further increased the sensitiv­ity of SPT54 and has shown great potential for assess­ing membrane heterogeneity. For example, iSCAT was used to show that lipids can transiently stall and become incorporated into sub20 nm domains within model membranes55,56. A technique complementary to SPT, fluorescence correlation spectroscopy (FCS), has been applied in combination with spot variation (svFCS57) or a STED microscope (STED-FCS58) to probe the lateral diffusion of membrane components over vari­ous length scales. Particularly in STED-FCS, the size of the observation spot can be reduced to approximately 20–40 nm, which reveals underlying nanoscopic features of the plasma membrane58,59. Finally, Förster resonance energy transfer (FRET; FIG. 2b) is a key tool for investigat­ing membrane raft structure and composition60,61. The spatial regime probed by this technique makes it ideal for studying nanoscopic domains, and it has been applied to both model membranes62 and live cells63, not only to probe the existence of domains but also to define their size62,64 by using fluorescent probes with different FRET efficiencies. For a detailed review of these techniques and their caveats, see REF. 45.

Most of the aforementioned methodologies rely on fluorescent labels. This is a particular issue in the investigation of membranes because the behaviour of lipids is inherently dependent on their amphiphilic properties and molecular packing, both of which are potentially affected by tags such as fluorophores, which are often almost the size of the lipid molecules. Thus, the native behaviour of lipids is often altered considerably by the reporter36. To address this concern, a number of label-free tech­niques have been developed. Mass spectroscopy (FIG. 2b), for example, is one of the most accurate tools for probing the lipid and protein composition of membranes with­out the necessity of external labelling65, and it has been used for label-free determination of membrane domain composition in model membranes and cell-derived membranes66–71. iSCAT has also facilitated label-free observation of the dynamics of ordered domains in model membranes72. Raman spectroscopy is another label-free technique that has been applied successfully to monitor membrane domain composition73. Likewise, small-angle neutron scattering has also been used to detect raft-like domains74 and determine their size75. Finally, electron microscopy has the necessary resolution to obtain a snapshot of molecular arrangements at the cell surface, and a number of studies of outer and inner leaflet lipid-tethered proteins (including GPI-anchored proteins, glycolipids and RAS proteins) have revealed the nanoscopic organization of proteins in rafts76. One potential caveat of these methods is that they usually require cell fixation and staining, which are notoriously problematic for visualizing lipid molecules. Therefore, fluorescence microscopy remains the preferred tech­nique for direct live imaging of putative lipid raft compo­nents, and this necessitates the continued optimization of fluorescent labels for membrane components.

Probes selective for membrane domains. Non-perturbing, specific labelling of raft or non-raft domains in cells has been, and remains, one of the foremost challenges in the field. Several fluorescent markers — including cyanine dyes (for example, DiO, DiI and DiD)77, polycyclic aromatic hydrocarbons (for example, naphthopyrene)78 and fluorescently labelled lipids36,79 — have been used to distinguish between different mem­brane compartments (FIG. 2c). As mentioned above, the reliability of these fluorescent lipid analogues depends strongly on the choice of both the native lipid and the fluorescent moiety36. The fluorescent lipids that are free from artefacts linked to fluorescent labelling are intrinsi­cally fluorescent cholesterol analogues such as dehydroergosterol80 and cholestatrienol81; however, their poor photophysical characteristics compared with artificially tagged lipids have prevented their widespread applica­tion. In the case of phospholipids, it is often challenging to preserve the natural physicochemical behaviour of the lipid after attaching a fluorophore82,83. In general, the strategy that causes least disruption to lipid behaviour is to label the head group instead of the acyl chain and to add a hydrophilic linker to ensure that the fluorophores do not affect the head groups of the surrounding lipids84.

In addition to lipid analogues that can reveal the gen­eral organization of the membrane into subdomains of variable composition, reporters that selectively bind to core raft components can potentially be used to visualize domains. These include cholesterol-binding agents such as filipin85 and perfringolysin O86, sphingolipid report­ers such as ostreolysin A87, lysenin88 and pleurotolysin89, as well as ganglioside lipid ligands such as cholera toxin90. The major caveats for these probes are: first, their poten­tial perturbation of native membrane organization by, for example, inducing the clustering of their binding partners, as is the case for cholera toxin; and, second, their reduced specificity in the cellular context, in which they can potentially exhibit off-target binding that thereby lowers their specificity for raft domains. lateral diffusion of membrane components over vari­ous length scales. Particularly in STED-FCS, the size of the observation spot can be reduced to approximately 20–40 nm, which reveals underlying nanoscopic features of the plasma membrane58,59. Finally, Förster resonance energy transfer (FRET; FIG. 2b) is a key tool for investigat­ing membrane raft structure and composition60,61. The spatial regime probed by this technique makes it ideal for studying nanoscopic domains, and it has been applied to both model membranes62 and live cells63, not only to probe the existence of domains but also to define their size62,64 by using fluorescent probes with different FRET efficiencies. For a detailed review of these techniques and their caveats, see REF. 45.

Most of the aforementioned methodologies rely on fluorescent labels. This is a particular issue in the investigation of membranes because the behaviour of lipids is inherently dependent on their amphiphilic properties and molecular packing, both of which are potentially affected by tags such as fluorophores, which are often almost the size of the lipid molecules. Thus, the native behaviour of lipids is often altered considerably by the reporter36. To address this concern, a number of label-free tech­niques have been developed. Mass spectroscopy (FIG. 2b), for example, is one of the most accurate tools for probing the lipid and protein composition of membranes with­out the necessity of external labelling65, and it has been used for label-free determination of membrane domain composition in model membranes and cell-derived membranes66–71. iSCAT has also facilitated label-free observation of the dynamics of ordered domains in model membranes72. Raman spectroscopy is another label-free technique that has been applied successfully to monitor membrane domain composition73. Likewise, small-angle neutron scattering has also been used to detect raft-like domains74 and determine their size75. Finally, electron microscopy has the necessary resolution to obtain a snapshot of molecular arrangements at the cell surface, and a number of studies of outer and inner leaflet lipid-tethered proteins (including GPI-anchored proteins, glycolipids and RAS proteins) have revealed the nanoscopic organization of proteins in rafts76. One potential caveat of these methods is that they usually require cell fixation and staining, which are notoriously problematic for visualizing lipid molecules. Therefore, fluorescence microscopy remains the preferred tech­nique for direct live imaging of putative lipid raft compo­nents, and this necessitates the continued optimization of fluorescent labels for membrane components.

Probes selective for membrane domains. Non-perturbing, specific labelling of raft or non-raft domains in cells has been, and remains, one of the foremost challenges in the field. Several fluorescent markers — including cyanine dyes (for example, DiO, DiI and DiD)77, polycyclic aromatic hydrocarbons (for example, naphthopyrene)78 and fluorescently labelled lipids36,79 — have been used to distinguish between different mem­brane compartments (FIG. 2c). As mentioned above, the reliability of these fluorescent lipid analogues depends strongly on the choice of both the native lipid and the fluorescent moiety36. The fluorescent lipids that are free from artefacts linked to fluorescent labelling are intrinsi­cally fluorescent cholesterol analogues such as dehydroergosterol80 and cholestatrienol81; however, their poor photophysical characteristics compared with artificially tagged lipids have prevented their widespread applica­tion. In the case of phospholipids, it is often challenging to preserve the natural physicochemical behaviour of the lipid after attaching a fluorophore82,83. In general, the strategy that causes least disruption to lipid behaviour is to label the head group instead of the acyl chain and to add a hydrophilic linker to ensure that the fluorophores do not affect the head groups of the surrounding lipids84.

In addition to lipid analogues that can reveal the gen­eral organization of the membrane into subdomains of variable composition, reporters that selectively bind to core raft components can potentially be used to visualize domains. These include cholesterol-binding agents such as filipin85 and perfringolysin O86, sphingolipid report­ers such as ostreolysin A87, lysenin88 and pleurotolysin89, as well as ganglioside lipid ligands such as cholera toxin90. The major caveats for these probes are: first, their poten­tial perturbation of native membrane organization by, for example, inducing the clustering of their binding partners, as is the case for cholera toxin; and, second, their reduced specificity in the cellular context, in which they can potentially exhibit off-target binding that thereby lowers their specificity for raft domains. Probes sensitive to membrane environments. Coexisting lipid domains inherently have different physicochemical properties. A defining property of lipid rafts is their tight lipid packing, which is due to the condensing interactions between relatively saturated lipids and cholesterol91. Of note, there is no specific, unique type of molecular packing that is common to the plasma membrane and its domains in different cells and contexts92. The diver­sity of membrane compositions and physical properties across cell types, and within cell types during physio­logical events such as secretory granule release92 or cell cycle progression93, implies that a wide range of different packing states exists in living cells. This lipid packing can be quantified using probes, such as laurdan, that sense the level of hydration in the bilayer94 in combination with two-photon32 or conventional confocal95 microscopy. The emission spectra of these probes shift depending on the polarity (that is, the aqueous content or hydra­tion) of the environment96 (FIG. 2c). This shift provides a ratiometric, concentration-independent quantification of the local environment, which, for membranes, is deter­mined largely by lipid packing97 (that is, more tightly packed membranes exclude water more efficiently). Imaging of membrane packing using these probes has been applied to investigate membrane heterogeneity in live cells47,98. More recently, in addition to spectral shift, the lifetime99 and energy transfer100 properties of similar probes have been used to further investigate lipid pack­ing in living membranes, which has expanded the scope and sensitivity of their potential applications. Enabling the efficient use of these probes in super-resolution microscopy will be an important future development. Raft-targeting drugs. A common paradigm in the study of the physiological roles of lipid rafts has been the use of drugs or enzymes to impair the structure and function of these domains (FIG. 2d). As cholesterol is thought to be enriched in rafts, the most common raft-disrupting agent in use is methyl--cyclodextrin (MCD), which selectively and efficiently extracts cho­lesterol from membranes101. However, it is important to consider that MCD-mediated cholesterol removal has broad pleiotropic effects that extend beyond raft disruption. For example, it increases membrane perme­ability to ions and thereby disrupts membrane poten­tial102, and it is potentially cytotoxic103. Moreover, this reagent appears to preferentially deplete cholesterol from Ld (non-raft) domains in model membranes104, which can produce unexpected and inconsistent21,67 effects on lipid packing in more complex membranes. Drugs that target cholesterol synthesis (statins105 and zaragozic acid106), or cholesterol-modifying enzymes (for example, cholesterol oxidase107), have the potential to replace the use of MCD to disrupt rafts, but their specificity and effectiveness remain to be demonstrated conclusively. Sphingolipids are another core component of rafts in cells, and a number of reagents can inter­fere with their synthesis (for example, fumonisin B1 (REF. 108) and myriocin109) or stability (for example, sphingomyelinases110). However, these reagents suffer from potential off-target effects on processes such as general sphingolipid metabolism and the generation of ceramides, which can then alter membrane properties in other ways.

Molecular dynamics simulations. One of the biggest challenges remaining in our understand­ing of biomembranes is how the myriad of inter­actions between membrane molecules determines membrane organization. Overcoming this challenge requires a combination of complementary experimen­tal approaches as well as in silico techniques that inte­grate experimental observations (for example, data about the structure and energetics of the system) into a simulation framework that ideally can reconstitute the natural behaviour in silico solely on the basis of physical interactions111. An inherent advantage of such in silico approaches is that they simultaneously model a multi­tude of molecules at a high spatial (atomic level) and temporal (nanosecond–microsecond) resolution with­out relying on exogenous probes or labels. Thus, in silico molecular dynamics simulations can be regarded as a ‘computational microscope’ (REF. 112) that is capable of visualizing molecular behaviour with unprecedented precision. Currently, such computational microscopes have the opposite limitations to optical microscopes, in that they reveal only short processes (over microseconds) at a nanoscopic scale (thousands of molecules), as opposed to processes that occur over longer timescales and at lower resolution that are accessible by optical microscopy112,113. To close the gap between computational and experimental approaches, methods such as coarse-grained simulations have been developed to extend the spatiotemporal scale of molecular dynamics simulations without sacrificing the molecular details114. Such simulations have been used successfully to study lipid–lipid and lipid–protein interactions115,116 and lipid domains in complex membranes14,117,118. It is impor­tant to note that such in silico observations are inher­ently model-driven and must ultimately be verified by experiments. Unfortunately, in the case of membrane domains, the spatiotemporal gap between the simu­lated and experimental observables is still too large to allow direct comparisons111. However, efforts to bridge this divide will ensure progress towards a molecular understanding of how complex membrane components self-organize into functional substructures.

Nature and composition of lipid rafts

Dissecting the physical properties — the lifetime, size, and coverage area — of lipid rafts in the cellular environment remains one of most vexing issues in the field. Computational models have confirmed the intuitive assumption that both the temporal and spatial compart­mentalization of membrane molecules into domains is crucial for membrane function119. Unfortunately, both the small size and short lifetime of putative raft domains in vivo complicate direct measurement of their prop­erties in living cells. Furthermore, the complexity of plasma membranes suggests that a range of raft-like domains with varying sizes and lifetimes can be estab­lished in vivo92,98,120, further complicating interpretations of experimental measurements. The original model of lipid rafts suggested the existence of a Ld (non-raft) membrane punctuated by more-ordered (raft) domains with minimal coverage121. However, recent data indi­cate a much greater extent of ordered raft-like regions in membranes (which suggests that ordered membrane domains might in fact predominate and possibly cover the majority of the plasma membrane) with interspersed less-ordered (non-raft) domains47,59 (FIG. 3a). The rela­tive area as well as the size and lifetime of membrane domains may be further tuned by cellular processes such as signalling and membrane trafficking (FIG. 3b), which makes it even more challenging to draw conclusions regarding these membrane domains.

In the original formulation of the lipid raft model, raft formation was based on preferential interactions between sphingolipids and cholesterol10. Consistent with this notion, sphingomyelin has been identified as a core component of DRMs2 and ordered lipid phases122, owing in part to strong hydrogen bonding of lipids with cholesterol123,124 (FIG. 4a). However, the partitioning of cholesterol between more-and less-ordered domains is less clear: experimental30 and computational125 stud­ies suggest that it is abundant in both ordered (raft-like) and disordered (non-raft) phases, although it is enriched in more-ordered domains. Ganglioside lipids were also found to interact with cholesterol, which results in the formation of cholesterol-rich domains in model membranes70, and these lipids have been detected consistently in the ordered domains of model membranes90. In addition, other lipids such as relatively saturated phospholipids have often been associated with raft-like environments, especially in model membranes. Whereas the biophysical basis for the lipid composi­tion of rafts can be explained by these simple principles, the basis for the selective incorporation of proteins into more-ordered (raft-like) domains largely remains a mys­tery. In general, proteins that interact with the membrane via lipid anchors follow the rules set by the lipids: saturated lipid anchors such as GPI or palmitoyl moieties gen­erally favour ordered membrane environments, whereas branched or unsaturated anchors such as prenyl groups prefer disordered (non-raft) regions126. In fact, GPI-anchored proteins were some of the first proteins to be identified in DRMs5 and later in the ordered domains of model membranes33,127. Lateral GPI-anchored protein domains have been extensively characterized by single-molecule approaches128,129. Although their relationship to membrane rafts remains unresolved130, the interactions between these lipid-anchored proteins and lipids almost certainly regulate membrane structure and function14,22,69.

However, lipidated proteins are certainly not the only protein species that associate with raft-like domains. In fact, in a recent experiment, 35% of all plasma membrane proteins were found in ordered domains in GPMVs43. These ‘raftophilic’ proteins included GPI-anchored proteins and palmitoylated proteins, as expected (each constituting approximately one-third of the identified proteins)43. However, the remaining one-third of raftophilic proteins contained neither a GPI nor a pal­mitoyl anchor, and the mechanism of association of many of these proteins to raft-like domains is currently unclear. Some proteins are known to become more raftophilic upon oligomerization, which may modulate their activ­ity131. Recently, a database of putative raftophilic proteins identified in mass spectrometry studies of isolated DRMs has been established (RaftProt)132, although it is impor­tant to emphasize that these studies may be subject to the DRM-associated artefacts described above. As the actual protein content of membrane domains is uncertain, few generalizable insights into the structural determinants of raftophilic behaviour of transmembrane proteins are available133. Interestingly, a recent study demonstrated that the length of the transmembrane domain (TMD) appears to be a key feature determining the raftophilic properties of a protein — longer TMDs preferentially target the protein to the thicker, ordered domains134.

Mechanisms of domain regulation

Although the raft concept and its in vivo relevance have been controversial, the principle of lateral membrane compartmentalization by lipids is intuitive: there are clear differences in the interaction affinities between various lipids, and these differences may be sufficient to produce a heterogeneous lipid distribution. For systems in thermodynamic equilibrium (including synthetic and biological model membranes42), the manifestation of these phenomena is macroscopic phase separation, which can be regulated by temperature135, lipid com­position21,26,67 or specific interactions that enhance the inherent connectivity of particular components and thus lead to enhanced clustering136. However, cell membranes in vivo are not closed systems in chemical and thermodynamic equilibrium, and many potential modes of regulation contribute to the ultimate output of the inherent self-organizing capacity of biological lipids and their separation into distinct domains (FIG. 4).

L_i_p_i_d_–l_i_p_i_d_ _a_n_d_ _l_i_p_i_d_–p_r_o_t_e_i_n_ _i_n_t_e_r_a_c_t_i_o_n_s_._ _In the traditional raft model, the formation of raft domains is driven mainly by the preferential binding of choles­terol to sphingolipids124 and possibly other lipids such as gangliosides69 (FIG. 4a). However, an inherent limitation of studying the factors that regulate raft domain proper­ties is the difficulty of measuring these properties in situ. To address this limitation, several recent studies93,137 have focused on factors that regulate the temperature at which macroscopic raft-like domains form in GPMVs, with the underlying inference that higher phase separation tem­peratures suggest more stable domains. This paradigm is based on observations of a specific type of phase separation in GPMVs that occurs near a compositional ‘critical point’ and involves large-scale fluctuations at temperatures close to the phase transition135. Such ‘critical fluc­tuations’ are present in all systems that exhibit critical behaviour, which suggests that there are scaling laws that allow extrapolation of domain size and stability to living cells135,139. It is important to note that this hypothesis has yet to be formally evaluated; however, if validated, it will provide an important methodological tool for assaying raft properties. For example, it was recently demonstrated that the stability of more-ordered domains in GPMVs is affected by dietary fatty acids. In particular, exogenously supplied polyunsaturated fatty acids such as the fish oil component docosahexaenoic acid are robustly incorporated into cellular membranes, in which they induce extensive changes in lipid composition and bio­physical properties, including increasing the stability of raft-like domains67. A study relating these effects to cell behaviour showed that incorporation of docosahexaenoic acid into membranes, and the concomitant increase in the stability of raft-like domains, can repress the stem cell properties and motility of breast cancer cells by interfering with the plasma membrane remodelling that is necessary for the epithelial–mesenchymal transition140.

Although variations in lipid composition are cer­tainly key drivers of lipid membrane heterogeneity, protein–lipid interactions also have important roles in raft regulation. For example, some proteins, including the HIV glycoprotein gp41 (REF. 141), have cholesterol-binding motifs that regulate their membrane distribution (FIG. 4b). Other proteins specifically bind glycosphingo­lipids138 or sphingomyelin142, which potentially medi­ates their recruitment to raft-like membrane domains. Furthermore, a variation on the role of palmitoylation in postsynaptic density protein 95 (PSD95; also known as DLG4). Comprehensive lipidomic analysis of neuronal synapses suggested that raft-like domains are specifically recruited to synaptic sites, which was proposed to be mediated by the integration of palmitoylated PSD95 into the postsynaptic density protein scaffold143. In this scenario, instead of raft-like domains recruiting palmitoylated proteins, it is the immobilized palmitoylated proteins that recruit saturated lipids and thus nucleate ordered domains at specific cellular sites143 (FIG. 4c). This hypo­thetical mechanism and its applicability to other cellular contexts remain to be confirmed. However, the evidence that another palmitoylated protein, membrane palmi­toylated protein 1 (MPP1; also known as p55), nucleates raft-like environments in erythroid cells144,145 suggests the existence of a more general mechanism whereby proteins dictate, or at least considerably influence, the localization and stability of organized domains.

Hydrophobic match or mismatch. Mammalian mem­brane lipids can contain hydrocarbon acyl chains that are 12–24 carbons in length; thus, there is the potential to yield drastically different hydrophobic tail lengths for individual lipids. To minimize the unfavourable exposure of hydrophobic tails to the aqueous environment, lipids segregate according to their acyl chain length, which can potentially introduce lateral heterogeneity. In phase-separated model membranes, this thickness mismatch between longer saturated (raft) and shorter unsaturated (non-raft) lipids appears to regulate the size of the coexisting domains, such that large mismatches give rise to large domains, and vice versa146. Similarly, the TMDs of nearly all eukaryotic integral membrane proteins consist of -helices with hydrophobic amino acid side chains, which are buried inside the hydrophobic core of the membrane. Hydrophobic matching between these TMDs and the sur­rounding membrane lipids minimizes the energetically unfavourable exposure of hydrophobic residues to aque­ous environments147 (FIG. 4d). In the case of a significant length mismatch between TMDs and the surrounding lipids, lateral protein-rich aggregates can potentially be induced148. However, the role of hydrophobic mismatch in membrane domain dynamics in the plasma membrane of living cells needs to be demonstrated unambiguously.

Cortical actin cytoskeleton. The cortical actin cytoskeleton is undoubtedly one of the most important factors that influence membrane organization149 and mechanics150. The actin scaffold has been shown to determine molecular diffusion dynamics (for example, trapped and hop diffusion) and supramolecular arrange­ments in the membrane129,151–154. In in vitro cholesterol-containing membrane systems that are capable of large-scale phase separation, actin can directly stabilize or abrogate this separation depending on the nature of the lipid species that are coupled to actin153,155–157. If actin filaments are coupled to, for example, saturated acyl chain-containing lipid species, they tend to stabilize Lo domains, but prevent large-scale phase separation153. In a living cell, it is likely that the dynamics of actin filaments will influence the organization of the mem­brane components that are associated with actin. In fact, a theoretical framework for understanding the interplay between the organization of the cortical actin cytoskeleton and living asymmetric membranes has emerged from studies of the actomyosin-dependent clustering behav­iour of GPI-anchored proteins in the outer leaflet of the plasma membrane. It was proposed that such clustering is the result of dynamic self-organization of acto­myosin into nanoscopic contractile assemblies termed asters158,159. These assemblies bind to and transiently immobilize the charged lipid phosphatidylserine in the inner membrane leaflet, possibly via specific interactions between actin and membrane adaptor proteins. This lipid species contains long saturated acyl chains that engage in cholesterol-mediated transbilayer interactions with long acyl chain-containing GPI-anchored proteins located in the outer leaflet, which results in the formation of local raft-like domains14 (FIG. 4e). Thus, an actin-driven cluster­ing mechanism may be responsible for the formation of ordered domains in membranes of living cells, even under conditions (for example, temperature and/or lipid com­position) that are not normally conducive for phase separation. A proofofprinciple for this mechanism has been demonstrated recently in vitro160 by showing that dynam­ically remodelling actomyosin networks can organize and segregate associated lipids in a synthetic supported mem­brane bilayer system. In addition, recent live-cell work has shown that self-organizing cortical actin patterns such as asters generate more-ordered membrane environments in the immediate plasma membrane areas159. As an addition to the chemical principles of lipid–lipid interactions, this actin-driven mechanism of membrane ordering provides a consistent explanation for the dynamic properties and non-equilibrium distribution of nanoclusters that are formed by several lipid or protein species. These include GPI-anchored proteins, glycolipids in the outer leaflet and RAS proteins in the inner leaflet of live-cell membranes161. The molecular machinery that generates these actin-based nanoclusters has not been identified, and further work is necessary to understand how these small actin-based nanoclusters may give rise to larger-scale ordered membrane domains with functional significance161.

Physiological functions of rafts

The most apparent function of raft-like domains (or heterogeneity in membrane lipid order in general) is to segregate specific elements in order to regulate their interactions with other membrane components and hence their activity. In addition, interactions with raftophilic lipids (cholesterol or glycosphingolipids), or with the distinct biophysical environment of rafts, may change the conformation of a raft-resident protein and thus its activity162,163 (FIG. 5a). These general modes of regulation may be broadly employed in cellular physiology, and a few examples are described here. However, it should be emphasized that the direct mechanistic effects of lipid rafts on cell function and dysfunction are unclear owing to the inherent difficulties in defining raft composition and properties and in achieving specificity when perturbing their function. Immune signalling. Compartmentalization of cellular signalling in membrane domains may be used to concen­trate positive regulatory components (such as kinases164), together with excluding negative regulatory elements (such as phosphatases165) (FIG. 5b). Immunoglobulin E (IgE)-mediated signalling was the first signalling path­way that was shown to be associated with lipid rafts166. Since then, several studies have implicated these domains in various innate and adaptive immune responses167. In these contexts, the key immune receptors, including the high-affinity IgE receptor (FcRI), the T cell recep­tor168 and the B cell receptor44, were found in DSMs in resting or immature cells, but these shifted to DRMs following receptor activation, which suggests that the translocation to membrane rafts is associated with active signalling through these receptors169–171. This notion is supported by the coenrichment in DRMs of the proxi­mal signal transduction machinery that lies downstream of the immune receptors, which includes lymphocyte cell-specific protein tyrosine kinase (LCK) and a proto-oncoprotein, the tyrosine kinase FYN164, as well as the signalling adaptor protein linker for activation of T cells (LAT)43. Furthermore, several other immune-associated proteins are GPI-anchored (suggesting that they are preferentially targeted to rafts) and have been found in DRMs172; these include CD14, the receptor for bacterial lipopolysaccharides, and THY1 (also known as CD90), which is crucial for T cell activation173.

Host–pathogen interactions. Interest in lipid rafts as modulators of host–pathogen interactions has been boosted by the recent discovery of a high level of saturated lipids (in particular, sphingolipids) and choles­terol in the viral envelope (of HIV174, for example) and by finding ordered membrane domains in pathogenic microorganisms175. There is now substantial evidence that viruses and bacterial products such as toxins bind preferentially to detergent-resistant highly ordered plasma membrane regions to penetrate the cell. This could be due to the enrichment of their receptors in rafts, as is the case for glycolipids176 (which function as receptors for cholera toxin90, for example) or virus recep­tors177. Furthermore, binding of HIV Gag protein (which is necessary for virus budding and release from host cells) has been shown to occur preferentially in mem­brane domains with high cholesterol content178, which suggests that rafts might be the preferred sites for virus budding. Cancer. A large number of proteins that are associated with malignancies have been found in DRMs: these include mucin 1 (MUC1), the overexpression of which leads to several cancer forms179; urokinase plasmino­gen activator surface receptor (UPAR), which plays a part in tumour invasion, migration and angiogenesis in breast cancer180; and RAS proteins, which show raft-dependent oncogenic activity in breast cancer181. The localization of oncogenic proteins to raft-like domains, together with the fact that mitogenic signalling is initi­ated from various cell surface receptors, suggests that rafts are potentially involved in cancer development and progression. Consistent with this idea, drugs that modulate membrane organization — including the raft-associated alkyl-phospholipids edelfosine, miltefosine and perifosine, which disrupt the raft localization of proton pumps182 — have been shown to exhibit anticancer activity183.

Cardiovascular diseases. Atherosclerosis is a leading cause of cardiovascular disease, and it develops as a result of the uptake by macrophages of cholesterol that accumulates in the artery walls as oxidized low-density lipoprotein (oxLDL). This uptake causes a transforma­tion of macrophages into foam cells, which accumulate in necrotic lesions in the arterial wall and can thereby clog blood vessels and lead to strokes, heart attacks and peripheral vascular diseases184. Of note, this tran­sition of macrophages into foam cells appears to be raft-dependent, as oxLDL receptors localize to raft-like domains following stimulation by oxLDL185. In addition, caveolae, the formation of which has often been associ­ated with lipid rafts, are also essential for normal cardiac functions, as various cardiac ion channels have been shown to localize to these membrane pits186.

Conclusions and perspective

Accumulating evidence suggests that cellular mem­branes are laterally heterogeneous, forming distinct, highly ordered lipid raft domains alongside less organ­ized and more fluid regions. This heterogeneity is poten­tially important for various cellular functions, owing to the potential of membrane domains to regulate inter­actions between membrane-associated components. However, the mechanisms driving and regulating lateral membrane heterogeneity remain poorly understood. For this reason, the concept of lipid rafts has received a dis­proportionate share of both popularity and controversy. At its peak, hundreds of papers on membrane rafts were published every year; at its nadir, many refrained from using the word ‘raft’ to avoid the inevitable semantic quicksand that it conjured. The major predicament in membrane raft research has been, and continues to be, a lack of direct visualization of these domains in unper­turbed living cells. However, the remarkable advances in microscopy technology over the past decade now allow direct observation of processes occurring with the spatial (nanometres) and temporal (milliseconds) regimes that are believed to be relevant for raft domains in living cells. These advances, together with improvements in in silico membrane modelling, suggest that direct detection of these elusive domains in cell membranes, although still challenging, may be within reach187. Direct imaging of phase separation in isolated plasma membranes such as GPMVs37,38 has already provided evidence that the isolated plasma membrane bilayer is capable of gener­ating coexisting Lo and Ld domains. Moreover, domains remarkably similar to these ordered and disordered phase-separated domains in GPMVs have been visualized directly in the subcellular organelles of budding yeast188, which suggests that an investigation of internal membranes may also be a fruitful direction.

Much of the controversy about the properties of lipid rafts (such as size, lifetime and abundance) stems from attempts to make general statements about the organization of a number of different membrane com­ponents (including glycolipids, sphingomyelin, choles­terol, GPI-anchored proteins and minimal palmitoylated motifs) by using a common raft paradigm. First, it is important to note that a very specific set of physical and compositional features should not be expected for lipid rafts. Living membranes are extremely complex and varied, and thus their organization will be inher­ently context-dependent, and they may potentially con­tain many different types of coexisting domains. Such varied assemblies may have distinct organizational principles and cellular functions, which may only be apparent at specific spatial and temporal scales. Second, it is important to consider that most molecules that typically associate with rafts are not simply domain probes, but also possess distinct bioactivities that may affect domain organization and dynamics. Furthermore, these bioactivities may be affected by the specific conditions of an experiment; for example, the cell type or the cell cycle phase. Altogether, to obtain reproducible results that pertain to raft formation and their biophysical properties, it may be necessary to introduce fully syn­thetic probes (instead of semi-native labels) that exhibit validated affinities for ordered membrane domains84, and thus allow careful associations to be made between ordered domain affinity and other experimental read­outs22. The application of label-free methods for the detection of domains is another approach that would minimize experimental differences72.

Ultimately, the controversies about the organization and dynamics of membrane domains will be resolved by direct observation of well-validated probes with high spatial and temporal resolution over extended timescales and large areas. Such data could be complemented by detailed lipidomic and proteomic analysis of nanometric regions of the cell surface70 as well as in silico membrane modelling. The next step will be to integrate these obser­vations into the framework of cellular dynamics to link membrane heterogeneity to cell biological processes. To achieve this, it will be necessary to simultaneously observe the organization, dynamics and bioactivity of specific raft components to dissect the key principles of how domain localization modulates molecular func­tion. Clearly, such advances will require the parallel application and development of a variety of techniques, which suggests that this field has an exciting future of interdisciplinary investigation.

Explanation / Answer

The article describes the advancement in the structure and function of plasma membrane (PM). The anger and Nicolson model of PM is further modified in recent years which suggest the presence of organized structures within the plasma membrane which was considered as ocean of lipid with free floating proteins in it. However, further research in this field indicated that there are ordered structures in PM that can be separated with rest of the membranes which then led to idea of the sub-compartment in the PM later called “Lipid Rafts”. Studied in the structure of these micro-domains showed them to be rich in specific type of lipids i.e. Cholesterol and sphingolipid. The cholesterol and sphingolipid interact with each other and thus restrict the lateral movement of PM lipids creating the ordered micro-domains. The protein content of the lipid rafts is also suggested to be specific, for example GPI-anchored proteins and doubly acylated proteins are exclusively present in lipid rafts. However some proteins are reported to move in and out of the rafts depending upon the cellular requirements.

The existence of these micro-domains in natural conditions is still considered debatable as there are still techniques lacking the potential to directly visualize rafts. But consistent effort have shown much improvements in studying the rafts. The different methods and their pricnciple to study rafts are described below:

The methods described above are used based to understand rafts. Several studies report the involvement of lipid rafts in various disease making them one of the most interesting cellular domains to study. In addition to rafts are reported to be involved in various cellular functions which include:

Further research in this field could transform the understanding of PM and associated functions leading to better methods to prevent and cure the disease.

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