Cholesterol depletion induces large scale domain segregation in living cell membranes

Detergent-resistant membranes and the protein composition of lipid rafts

The plasma membrane of eukaryotic cells contains lipid rafts with protein and lipid compositions differing from the bulk plasma membrane. Several recent proteomic studies have addressed the composition of lipid rafts, but the different definitions used for lipid rafts need scrutinizing before results can be evaluated.

Cellular cholesterol depletion triggers shedding of the human interleukin-6 receptor by ADAM10 and ADAM17 (TACE)

Interleukin-6 (IL-6) activates cells by binding to the membrane-bound IL-6 receptor (IL-6R) and subsequent formation of a glycoprotein 130 homodimer. Cells that express glycoprotein 130, but not the IL-6R, can be activated by IL-6 and the soluble IL-6R which is generated by shedding from the cell surface or by alternative splicing. Here we show that cholesterol depletion of cells with methyl-beta-cyclodextrin increases IL-6R shedding independent of protein kinase C activation and thus differs from phorbol ester-induced shedding. Contrary to cholesterol depletion, cholesterol enrichment did not increase IL-6R shedding. Shedding of the IL-6R because of cholesterol depletion is highly dependent on the metalloproteinase ADAM17 (tumor necrosis factor-alpha-converting enzyme), and the related ADAM10, which is identified here for the first time as an enzyme involved in constitutive and induced shedding of the human IL-6R. When combined with protein kinase C inhibition by staurosporine or rottlerin, breakdown of plasma membrane sphingomyelin or enrichment of the plasma membrane with ceramide also increased IL-6R shedding. The effect of cholesterol depletion was confirmed in human THP-1 and Hep3B cells and in primary human peripheral blood monocytes, which naturally express the IL-6R. For decades, high cholesterol levels have been considered harmful. This study indicates that low cholesterol levels may play a role in shedding of the membrane-bound IL-6R and thereby in the immunopathogenesis of human diseases.

Sphingomyelinase activates GLUT4 translocation via a cholesterol-dependent mechanism

A basis for the insulin mimetic effect of sphingomyelinase on glucose transporter isoform GLUT4 translocation remains unclear. Because sphingomyelin serves as a major determinant of plasma membrane cholesterol and a relationship between plasma membrane cholesterol and GLUT4 levels has recently become apparent, we assessed whether GLUT4 translocation induced by sphingomyelinase resulted from changes in membrane cholesterol content. Exposure of 3T3-L1 adipocytes to sphingomyelinase resulted in a time-dependent loss of sphingomyelin from the plasma membrane and a concomitant time-dependent accumulation of plasma membrane GLUT4. Degradation products of sphingomyelin did not mimic this stimulatory action. Plasma membrane cholesterol amount was diminished in cells exposed to sphingomyelinase. Restoration of membrane cholesterol blocked the stimulatory effect of sphingomyelinase. Increasing concentrations of methyl-beta-cyclodextrin, which resulted in a dose-dependent reversible decrease in membrane cholesterol, led to a dose-dependent reversible increase in GLUT4 incorporation into the plasma membrane. Although increased plasma membrane GLUT4 content by cholesterol extraction with concentrations of methyl-beta-cyclodextrin above 5 mM most likely reflected decreased GLUT4 endocytosis, translocation stimulated by sphingomyelinase or concentrations of methyl-beta-cyclodextrin below 2.5 mM occurred without any visible changes in the endocytic retrieval of GLUT4. Furthermore, moderate loss of cholesterol induced by sphingomyelinase or low concentrations of methyl-beta-cyclodextrin did not alter membrane integrity or increase the abundance of other plasma membrane proteins such as the GLUT1 glucose transporter or the transferrin receptor. Regulation of GLUT4 translocation by moderate cholesterol loss did not involve known insulin-signaling proteins. These data reveal that sphingomyelinase enhances GLUT4 exocytosis via a novel cholesterol-dependent mechanism.

Quantitation of two pathways for cholesterol excretion from the brain in normal mice and mice with neurodegeneration

Although the pool of cholesterol in the adult central nervous system (CNS) is large and of constant size, little is known of the process(es) involved in regulation of sterol turnover in this pool. In 7-week-old mice, net excretion of cholesterol from the brain equaled 1.4 mg/day/kg body weight, and from the whole animal was 179 mg/day/kg. Deletion of cholesterol 24-hydroxylase, an enzyme highly expressed in the CNS, did not alter brain growth or myelination, but reduced sterol excretion from the CNS 64% to 0.5 mg/day/kg. In mice with a mutation in the Niemann-Pick C gene that had ongoing neurodegeneration, sterol excretion from the CNS was increased to 2.3 mg/day/kg. Deletion of cholesterol 24-hydroxylase activity in these animals reduced net excretion only 22% to 1.8 mg/day/kg. Thus, at least two different pathways promote net sterol excretion from the CNS. One uses cholesterol 24-hydroxylase and may reflect sterol turnover in large neurons in the brain. The other probably involves the movement of cholesterol or one of its metabolites across the blood-brain barrier and may more closely mirror sterol turnover in pools such as glial cell membranes and myelin.

A lipid raft environment enhances Lyn kinase activity by protecting the active site tyrosine from dephosphorylation

The plasma membrane contains ordered lipid domains, commonly called lipid rafts, enriched in cholesterol, sphingolipids, and certain signaling proteins. Lipid rafts play a structural role in signal initiation by the high affinity receptor for IgE. Cross-linking of IgE-receptor complexes by antigen causes their coalescence with lipid rafts, where they are phosphorylated by the Src family tyrosine kinase, Lyn. To understand how lipid rafts participate in functional coupling between Lyn and FcepsilonRI, we investigated whether the lipid raft environment influences the specific activity of Lyn. We used differential detergent solubility and sucrose gradient fractionation to isolate Lyn from raft and nonraft regions of the plasma membrane in the presence or absence of tyrosine phosphatase inhibitors. We show that Lyn recovered from lipid rafts has a substantially higher specific activity than Lyn from nonraft environments. Furthermore, this higher specific activity correlates with increased tyrosine phosphorylation at the active site loop of the kinase domain. Based on these results, we propose that lipid rafts exclude a phosphatase that negatively regulates Lyn kinase activity by constitutive dephosphorylation of the kinase domain tyrosine residue of Lyn. In this model, cross-linking of FcepsilonRI promotes its proximity to active Lyn in a lipid raft environment.

Lymphocyte lipid rafts: structure and function

Evidence has accumulated over the past few years to suggest that specialized plasma membrane regions enriched in cholesterol and glycolipids, called 'lipid rafts', are primarily involved in the initiation and propagation of the signal transduction cascade associated with lymphocyte activation. Considering the multitude of recent and often contradictory data, however, it appears that a critical reconsideration of the role of lipid rafts in lymphocyte activation is necessary and timely, particularly in light of a series of new experimental results that challenge the traditional view of the role of lipid rafts in lymphocyte activation.

Formation of functional cell membrane domains: the interplay of lipid- and protein-mediated interactions

Numerous cell membrane associated processes, including signal transduction, membrane sorting, protein processing and virus trafficking take place in membrane subdomains. Protein-protein interactions provide the frameworks necessary to generate biologically functional membrane domains. For example, coat proteins define membrane areas destined for sorting processes, viral proteins self-assemble to generate a budding virus, and adapter molecules organize multimolecular signalling assemblies, which catalyse downstream reactions. The concept of raft lipid-based membrane domains provides a different principle for compartmentalization and segregation of membrane constituents. Accordingly, rafts are defined by the physical properties of the lipid bilayer and function by selective partitioning of membrane lipids and proteins into membrane domains of specific phase behaviour and lipid packing. Here, I will discuss the interplay of these independent principles of protein scaffolds and raft lipid microdomains leading to the generation of biologically functional membrane domains.

Membrane dynamics and cell polarity: the role of sphingolipids

In recent years, glycosphingolipids (GSLs) have attracted widespread attention due to the appreciation that this class of lipids has a major impact on biological life. Inhibition of the synthesis of glucosylceramide, which serves as a precursor for the generation of complex glycosphinglipids, is embryonic lethal. GSLs play a major role in growth and development. Metabolites of sphingolipids, such as ceramide, sphinganine, and sphingosine, may function as second messengers or regulators of signal transduction that affect events ranging from apoptosis to the (co)regulation of the cell cycle. In addition, GSLs can provide a molecular platform for clustering of signal transducers. The ability of sphingolipids, with or without cholesterol, to form microdomains or rafts is critical in sorting and membrane transport that underlies the biogenesis of polarized membrane domains. Here, a brief summary is presented of some recent developments in this field, with a particular emphasis on raft assembly and membrane transport in the establishment of membrane polarity.

Imaging metabolism of phosphatidylinositol 4,5-bisphosphate in T-cell GM1-enriched domains containing Ras proteins

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) and Ras proteins are involved in signalling pathways originating at the plasma membrane. The localisation and metabolism of PI(4,5)P(2) was studied in Jurkat T cells using fluorescence microscopic imaging with EGFP-tagged and antibody probes. Software was developed to objectively quantitate colocalisation and was used to show that plasma membrane PI(4,5)P(2) was enriched in lipid raft-containing patches of GM1 ganglioside, formed by crosslinking cholera toxin B-subunit (CT-B). The PI(4,5)P(2) metabolites phosphatidylinositol 3,4,5-trisphosphate and diacylglycerol appeared in plasma membrane CT-B-GM1 patches upon induction of signalling. Transferrin receptor and the CD45 tyrosine phosphatase did not colocalise with CT-B-GM1 patches, whereas the tyrosine kinase Lck, the scaffolding protein LAT, and endogenous Ras proteins did partially colocalise with CT-B-GM1 patches as did transfected EGFP-K-Ras(4B) and EGFP-H-Ras. The results demonstrate that T-cell PI(4,5)P(2) metabolism is occurring in GM1-enriched domains and that Ras proteins are present in these domains in vivo.

Lipid rafts: bringing order to chaos

Lipid rafts are subdomains of the plasma membrane that contain high concentrations of cholesterol and glycosphingolipids. They exist as distinct liquid-ordered regions of the membrane that are resistant to extraction with nonionic detergents. Rafts appear to be small in size, but may constitute a relatively large fraction of the plasma membrane. While rafts have a distinctive protein and lipid composition, all rafts do not appear to be identical in terms of either the proteins or the lipids that they contain. A variety of proteins, especially those involved in cell signaling, have been shown to partition into lipid rafts. As a result, lipid rafts are thought to be involved in the regulation of signal transduction. Experimental evidence suggests that there are probably several different mechanisms through which rafts control cell signaling. For example, rafts may contain incomplete signaling pathways that are activated when a receptor or other required molecule is recruited into the raft. Rafts may also be important in limiting signaling, either by physical sequestration of signaling components to block nonspecific interactions, or by suppressing the intrinsic activity of signaling proteins present within rafts. This review provides an overview of the physical characteristics of lipid rafts and summarizes studies that have helped to elucidate the role of lipid rafts in signaling via receptor tyrosine kinases and G protein-coupled receptors.

Human immunodeficiency virus type 1 assembly and lipid rafts: Pr55(gag) associates with membrane domains that are largely resistant to Brij98 but sensitive to Triton X-100

The assembly and budding of human immunodeficiency virus type 1 (HIV-1) at the plasma membrane are directed by the viral core protein Pr55(gag). We have analyzed whether Pr55(gag) has intrinsic affinity for sphingolipid- and cholesterol-enriched raft microdomains at the plasma membrane. Pr55(gag) has previously been reported to associate with Triton X-100-resistant rafts, since both intracellular membranes and virus-like Pr55(gag) particles (VLPs) yield buoyant Pr55(gag) complexes upon Triton X-100 extraction at cold temperatures, a phenotype that is usually considered to indicate association of a protein with rafts. However, we show here that the buoyant density of Triton X-100-treated Pr55(gag) complexes cannot be taken as a proof for raft association of Pr55(gag), since lipid analyses of Triton X-100-treated VLPs demonstrated that the detergent readily solubilizes the bulk of membrane lipids from Pr55(gag). However, Pr55(gag) might nevertheless be a raft-associated protein, since confocal fluorescence microscopy indicated that coalescence of GM1-positive rafts at the cell surface led to copatching of membrane-bound Pr55(gag). Furthermore, extraction of intracellular membranes or VLPs with Brij98 yielded buoyant Pr55(gag) complexes of low density. Lipid analyses of Brij98-treated VLPs suggested that a large fraction of the envelope cholesterol and phospholipids was resistant to Brij98. Collectively, these results suggest that Pr55(gag) localizes to membrane microdomains that are largely resistant to Brij98 but sensitive to Triton X-100, and these membrane domains provide the platform for assembly and budding of Pr55(gag) VLPs.

Membrane lipid organization is critical for human neutrophil polarization

In response to chemoattractants neutrophils extend an actin-rich pseudopod, which imparts morphological polarity and is required for migration. Even when stimulated by an isotropic bath of chemoattractant, neutrophils exhibit persistent polarization and continued lamellipod formation at the front, suggesting that the cells establish an internal polarity. In this report, we show that perturbing lipid organization by depleting plasma membrane cholesterol levels reversibly inhibits cell polarization and migration. Among other receptor-mediated responses, beta(2) integrin up-regulation was unaffected, and initial calcium mobilization was only partially reduced by cholesterol depletion, indicating that this treatment did not abrogate initial receptor-mediated signal transduction. Interestingly, cholesterol depletion did not prevent initial activation of the GTPase Rac or an initial burst of actin polymerization, but rather it inhibited prolonged activation of Rac and sustained actin polymerization. Collectively, these findings support a model in which the plasma membrane is organized into domains that aid in amplifying the chemoattractant gradient and maintaining cell polarization.

Role of cholesterol in lipid raft formation: lessons from lipid model systems

Biochemical and cell-biological experiments have identified cholesterol as an important component of lipid 'rafts' and related structures (e.g., caveolae) in mammalian cell membranes, and membrane cholesterol levels as a key factor in determining raft stability and organization. Studies using cholesterol-containing bilayers as model systems have provided important insights into the roles that cholesterol plays in determining lipid raft behavior. This review will discuss recent progress in understanding two aspects of lipid-cholesterol interactions that are particularly relevant to understanding the formation and properties of lipid rafts. First, we will consider evidence that cholesterol interacts differentially with different membrane lipids, associating particularly strongly with saturated, high-melting phospho- and sphingolipids and particularly weakly with highly unsaturated lipid species. Second, we will review recent progress in reconstituting and directly observing segregated raft-like (liquid-ordered) domains in model membranes that mimic the lipid compositions of natural membranes incorporating raft domains.

Statin effects on cholesterol micro-domains in brain plasma membranes

Recent epidemiological studies revealed inhibitors of the hydroxymethylglutaryl-coenzyme A reductase, so-called statins, to be effective in lowering the prevalence of Alzheimer's disease (AD). In vitro, statins strongly reduced the cellular amyloid beta-protein load by modulating the processing of the amyloid beta precursor protein. Both observations are probably linked to cellular cholesterol homeostasis in brain. So far, little is known about brain effects of statins. Recently, we could demonstrate that treatment of mice with the lipophilic compound lovastatin resulted in a discrete reduction of brain membrane cholesterol levels. To follow up these findings, we subsequently carried out a further in vivo study including lovastatin and simvastatin as lipophilic agents, as well as pravastatin as a hydrophilic compound, focussing on their efficiency to affect subcellular membrane cholesterol pools in synaptosomal plasma membranes of mice. In contrast to the hydrophilic pravastatin, the lipophilic lovastatin and simvastatin strongly reduced the levels of free cholesterol in SPM. Interestingly, lovastatin and pravastatin but not simvastatin significantly reduced cholesterol levels in the exofacial membrane leaflet. These changes were accompanied by modified membrane bulk fluidity. All three statins reduced the expression of the raft marker protein flotillin. Alterations in transbilayer cholesterol distribution have been suggested as the underlying mechanism that forces amyloidogenic processing of APP in AD. Thus, our data give some first insight in the mode of action of statins to reduce the prevalence of AD in clinical trials.

Glycosphingolipids internalized via caveolar-related endocytosis rapidly merge with the clathrin pathway in early endosomes and form microdomains for recycling

We have previously demonstrated that glycosphingolipids are internalized from the plasma membrane of human skin fibroblasts by a clathrin-independent, caveolar-related mechanism and are subsequently transported to the Golgi apparatus by a process that is dependent on microtubules, phosphatidylinositol 3-kinase, Rab7, and Rab9. Here we characterized the early steps of intracellular transport of a fluorescent glycosphingolipid analog, BODIPY-lactosylceramide (LacCer), and compared this to fluorescent transferrin (Tfn), a well established marker for the clathrin pathway. Although these two markers were initially internalized into separate vesicles by distinct mechanisms, they became co-localized in early endosomes within 5 min. These results demonstrate that glycosphingolipid-containing vesicles derived from caveolar-related endocytosis fuse with the classical endosomal system. However, in contrast to Tfn, internalization and trafficking of LacCer was independent of Rab5a, a key regulator of transport to early endosomes. By taking advantage of the monomer/excimer properties of the fluorescent lipid analog, we were also able to visualize LacCer segregation into distinct microdomains of high (red emission) and low (green emission) concentrations in the early endosomes of living cells. Interestingly, the high concentration "red" microdomains co-localized with fluorescent Tfn upon exit from early endosomes and passed through Rab11-positive "recycling endosomes" prior to being transported back to the plasma membrane. These results together with our previous studies suggest that glycosphingolipids internalized by caveolar endocytosis are rapidly delivered to early endosomes where they are fractionated into two major pools, one that is transported via late endosomes to the Golgi apparatus and the other that is returned to the plasma membrane via the recycling compartment.

Stearoyl-CoA desaturase inhibits ATP-binding cassette transporter A1-mediated cholesterol efflux and modulates membrane domain structure

Liver X receptor/retinoid X receptor (LXR/RXR) transcription factors have been found to induce a number of genes involved in the regulation of cellular cholesterol efflux, including the ATP-binding cassette transporter A1 (ABCA1), which mediates the active efflux of cellular cholesterol and phospholipids to extracellular acceptors, such as apolipoprotein A-I (apoA-I). In a screen for macrophage LXR/RXR target genes, we identified stearoyl-CoA desaturases 1 and 2 (Scd1 and Scd2), and subsequently tested the hypothesis that SCD activity might modulate cellular cholesterol efflux. In HEK 293 cells co-transfection of ABCA1 with either SCD1 or SCD2 inhibited ABCA1-mediated cholesterol efflux but not phospholipid efflux. In Chinese hamster ovary (CHO) cells with moderate stable overexpression of SCD1, cholesterol efflux to apoA-I was inhibited by 73%, whereas phospholipid efflux and ABCA1 protein levels were unchanged. In contrast, cholesterol efflux to HDL(2), which is not dependent on ABCA1, was increased 2-fold in CHO-SCD1 cells. The effect of SCD on cholesterol efflux to apoA-I was independent of acyl-CoA:cholesterol acyltransferase (ACAT) activity. SCD activity led to an increased content of plasma membrane monounsaturated fatty acids (18:1) at the expense of saturated fatty acids (18:0). As shown by confocal microscopy, SCD overexpression led to a decrease of Triton X-100-resistant domains in the plasma membrane, indicating a decrease in membrane-ordered regions. The data suggest that SCD changes membrane organization and depletes a specific pool of membrane cholesterol supporting ABCA1-mediated efflux, whereas increasing availability of cholesterol for passive efflux by HDL(2). ABCA1-mediated cholesterol and phospholipid efflux may be uncoupled in pathological states associated with high SCD activity, as in hyperinsulinemic obese mice, or in animals treated with LXR activators.

Field-induced reorganization of the neural membrane lipid bilayer: a proposed role in the regulation of ion-channel dynamics

We present a computational model demonstrating that an electric field propagating in the plane of the neural membrane during transmembrane ion movement creates lateral concentration gradients of the lipids. Due to this field-induced reorganization, ethenes of the lipid chains become aligned and polarized. This finding is interpreted within the context of molecular studies of protein folding in biological membranes. We propose that electrostatic interactions between membrane dipoles and charged amino acid residues of the unfolded ion-channel protein regulate protein-folding kinetics (channel closing). These electrostatic interactions thus regulate electrical signaling in neurons.

Cholesterol depletion from the plasma membrane triggers ligand-independent activation of the epidermal growth factor receptor

We recently demonstrated that depletion of plasma membrane cholesterol with methyl-beta-cyclodextrin (MbetaCD) caused activation of MAPK (Chen, X., and Resh, M. D. (2001) J. Biol. Chem. 276, 34617-34623). MAPK activation was phosphatidylinositol 3-kinase (PI3K)-dependent and involved increased tyrosine phosphorylation of the p85 subunit of PI3K. We next determined whether MbetaCD treatment induced tyrosine phosphorylation of other cellular proteins. Here we report that cholesterol depletion of serum-starved COS-1 cells with MbetaCD or filipin caused an increase in Tyr(P) levels of a 180-kDa protein that was identified as the epidermal growth factor receptor (EGFR). Cross-linking experiments showed that MbetaCD induced dimerization of EGFR, indicative of receptor activation. Reagents that block release of membrane-bound EGFR ligands did not affect MbetaCD-induced tyrosine phosphorylation of EGFR, indicating that MbetaCD activation of EGFR is ligand-independent. Moreover, MbetaCD treatment resulted in increased tyrosine phosphorylation of EGFR downstream targets and Ras activation. Incubation of cells with the specific EGFR inhibitor AG4178 blocked MbetaCD-induced phosphorylation of EGFR, SHC, phospholipase C-gamma, and Gab-1 as well as MAPK activation. We conclude that cholesterol depletion from the plasma membrane by MbetaCD causes ligand-independent activation of EGFR, resulting in MAPK activation by PI3K and Ras-dependent mechanisms. Moreover, these studies reveal a novel mode of action of MbetaCD, in addition to its ability to disrupt membrane rafts.

Sphingolipid trafficking and protein sorting in epithelial cells

Sphingolipids represent a minor, but highly dynamic subclass of lipids in all eukaryotic cells. They are involved in functions that range from structural protection to signal transduction and protein sorting, and participate in lipid raft assembly. In polarized epithelial cells, which display an asymmetric apical and basolateral membrane surface, rafts have been proposed as a sorting principle for apical resident proteins, following their biosynthesis. However, raft-mediated trafficking is ubiquitous in cells. Also, sphingolipids per se, which are strongly enriched in the apical domain, are subject to sorting in polarity development. Next to the trans Golgi network, a subapical compartment called SAC or common endosome appears instrumental in regulating these sorting events.

Lipid rafts and assembly of enveloped viruses

The plasma membrane, late secretory pathway and endosomal compartments contain detergent-insoluble raft microdomains that are enriched in sphingolipids and cholesterol. Rafts are currently an intensively studied topic of cell biology, and raft involvement has been implicated in numerous cellular processes. A number of recent reports have localized structural proteins of several enveloped viruses to rafts, thus raising the possibility that rafts also play a role in the assembly and budding of viruses, but what exactly that role might be is still unknown.

Myristoylation as a target for inhibiting HIV assembly: unsaturated fatty acids block viral budding

Modification of HIV-1 Gag with myristic acid, a saturated 14-carbon fatty acid (14:0), is essential for HIV-1 assembly. We recently showed that exogenous treatment of cells with unsaturated 14-carbon fatty acids, 5-cis-tetradecenoic acid (14:1n-9) and 5-cis,8-cis-tetradecadienoic acid (14:2n-6), reduces the affinity of some myristoylated proteins for plasma membrane rafts, membrane subdomains that have been shown to be required for efficient assembly of HIV. Here we demonstrate that treatment of cells with 14:1n-9 and 14:2n-6 fatty acids reduced the affinity of Gag for rafts but not membranes in general. Furthermore, treatment of cells with 14-carbon unsaturated fatty acids inhibited Gag-driven particle assembly. These effects most likely reflect covalent modification of Gag with unsaturated fatty acids. Treatment with 14:1n-9 and 14:2n-6 fatty acids did not alter intracellular protein trafficking, nor did it reduce cell viability. These studies suggest a strategy to attack HIV assembly by selectively altering the patterns of fatty acid modification.

Cutting edge: quantitative imaging of raft accumulation in the immunological synapse

Although the accumulation of lipid rafts at the immunological synapse is now well accepted, the degree of the accumulation, the localization within the fine structure of the immunological synapse, and the region from which lipid rafts are recruited have not been defined. In this work we show that lipid rafts preferentially accumulate in the central zone of the immunological synapse, the central supramolecular activation complex (C-SMAC). However, quantitative analyses indicate that the level of recruitment of lipid rafts to the C-SMAC is relatively small and suggests that rearrangement of lipid rafts from the peripheral zone of the synapse into the C-SMAC can account for this accumulation. We also assessed the effects of CD28 deficiency on lipid raft recruitment to the immunological synapse. The accumulation of lipid occurred independently of the CD28/B7 system and was not measurably altered by CD28.

Peering inside lipid rafts and caveolae

Clustering of proteins into membrane microdomains, such as lipid rafts and caveolae, could act as a mechanism for regulating cell signaling and other cellular functions. Certain lipid modifications are hypothesized to target proteins to these domains on the cytoplasmic leaflet of the plasma membrane. This concept has now been tested in living cells using an assay sensitive to the lateral distribution of proteins in membranes over sub-micron distances.

Plasma membrane microdomains

Several lines of evidence indicate that the lipids in the plasma membrane of animal cells are inhomogeneously distributed, and that various types of specialized lipid domains play an important role in many biological processes. The characteristics of these domains, such as size, composition and dynamics, are currently under active investigation. It appears that there are many different types of membrane domains in the plasma membrane, and perhaps the entire membrane should be viewed as a mosaic of microdomains.

Endocytosis of lipid rafts: an identity crisis

Membrane rafts enriched in cholesterol and sphingolipids have been hypothesized to be key mediators of sorting and signaling functions of associated molecules. Apart from a limited number of biophysical studies in living cell membranes, raft-association has been defined by a simple biochemical criterion, namely the ability to partition with detergent-resistant membranes (DRMs). Here we examine the evidence for the specification of internalization mechanisms and endocytic pathways by rafts as defined by this criterion. We have surveyed the endocytic trafficking of a variety of molecules such as lipids, toxins, glycosylphosphatidylinositol (GPI)-anchored proteins, and DRM-associated transmembrane proteins.

Cytoskeleton-dependent membrane domain segregation during neutrophil polarization

On treatment with chemoattractant, the neutrophil plasma membrane becomes organized into detergent-resistant membrane domains (DRMs), the distribution of which is intimately correlated with cell polarization. Plasma membrane at the front of polarized cells is susceptible to extraction by cold Triton X-100, whereas membrane at the rear is resistant to extraction. After cold Triton X-100 extraction, DRM components, including the transmembrane proteins CD44 and CD43, the GPI-linked CD16, and the lipid analog, DiIC(16), are retained within uropods and cell bodies. Furthermore, CD44 and CD43 interact concomitantly with DRMs and with the F-actin cytoskeleton, suggesting a mechanism for the formation and stabilization of DRMs. By tracking the distribution of DRMs during polarization, we demonstrate that DRMs progress from a uniform distribution in unstimulated cells to small, discrete patches immediately after activation. Within 1 min, DRMs form a large cap comprising the cell body and uropod. This process is dependent on myosin in that an inhibitor of myosin light chain kinase can arrest DRM reorganization and cell polarization. Colabeling DRMs and F-actin revealed a correlation between DRM distribution and F-actin remodeling, suggesting that plasma membrane organization may orient signaling events that control cytoskeletal rearrangements and, consequently, cell polarity.