Cholesterol-Dependent and -Independent CD40 Internalization and Signaling Activation in Cardiovascular Endothelial Cells

Objective— It remains elusive how CD40 endocytosis or clustering on the cell surface is induced by different forms of CD40 agonist. This study aims to investigate whether lipid rafts differentially regulate CD40 traffic and signaling in proinflammatory activation of cardiovascular endothelial cells (ECs).

Methods and Results— Using fluorescent microscopy and flow cytometry, we demonstrated that soluble CD40L and agonistic antibody G28.5 induced CD40 internalization via clathrin-independent pathway. Furthermore, depletion of cholesterol by methyl-β-cyclodextrin (MCD) or siRNA knockdown of caveolin-1 efficiently blocked CD40 internalization, suggesting that caveolae-rafts pathway regulates CD40 internalization. In contrast, a membrane-bound CD40L mimic (megamer) triggered aggregation of CD40 rafts outside of the conventional cholera toxin B subunit-positive lipid rafts resistant to cholesterol depletion. Finally, both G28.5 and megamer induced CD40 translocation to Brij58-insoluble, low buoyant density rafts, a movement insensitive to cholesterol depletion. However, MCD effectively inhibited G28.5 but not megamer-induced CD40 activation, and such inhibition could be alleviated by cholesterol reconstitution, suggesting that 2 different raft structures of CD40 induced by G28.5 or megamer possess differential sensitivity to cellular cholesterol levels in downstream signaling.

Conclusions— Depending on different forms of agonist, CD40 uses either a cholesterol-dependent or -independent mode for trafficking and signaling in ECs.

Oxidative stress, lipid rafts, and macrophage reprogramming

Oxidant stress, induced under a variety of conditions, is known to lead to the molecular reprogramming of the tissue-fixed macrophage. This reprogramming is associated with an altered response to subsequent inflammatory stimuli, such as lipopolysaccharide (LPS), leading to enhanced liberation of proinflammatory chemokines and cytokines. Due to this altered response, dysregulated immunity ensues, leading to the development of clinical syndromes such as multiple organ dysfunction syndrome (MODS). Although the mechanisms responsible for this altered macrophage activity by oxidant stress remains complex and poorly elucidated, it appears, based on recent research, that early and direct alterations within lipid rafts are responsible. This early and direct interaction with lipid rafts by oxidants leads to the mobilization of annexin VI from lipid raft constructs, leading to the release of calcium. This increased cytosolic concentration of this secondary messenger, in turn, results in the activation of calcium-dependent kinases, leading to further alterations in lipid raft lipids and eventually lipid raft proteins. Due to these lipid raft compositional changes, preassembly of receptor complexes occur, leading to enhanced proinflammatory activation. Within this review, the complexity of oxidant-induced reprogramming within the tissue fixed macrophage as currently understood is explained.

Monocyte cholesterol homeostasis correlates with the presence of detergent resistant membrane microdomains

BACKGROUND

Lipid membrane microdomains are involved in the regulation of biological functions of monocyte membrane proteins. These microdomains show a relative resistance to non-ionic detergents providing an easy analytical tool to study them.

METHODS

Here, we applied a rapid detergent-based flow cytometric assay to investigate microdomain association of proteins on monocytes from whole blood samples. The association of known surface antigens with detergent resistant fraction of membranes (DRMs) was compared using monocytes from healthy blood donors, patients with genetic disorders affecting cellular cholesterol traffic and patients with systemic inflammatory response.

RESULTS

All investigated surface antigens of Niemann-Pick type C (NPC)-mutant monocytes with impaired cholesterol influx and defective late endosome cholesterol trafficking, presented a strongly increased DRM-association. Though, membrane antigens of ATP binding cassette transporter A1 (ABCA1)-mutant monocytes with impaired cholesterol efflux did not show alterations in DRM-association. Differential CD14-dependent receptor clustering within microdomains was also investigated in response to in vivo lipopolysaccharide (LPS) and/or atherogenic lipoprotein activation. Increased DRM-association of the GPI-anchored proteins CD14, CD55, the Fcgamma receptor CD64, the scavenger receptors CD36, CD91 and CD163, the integrin CD11a, and complement receptor 3 complex CD11b/CD18 were observed from patients with systemic inflammatory response syndrome (SIRS)/sepsis or coronary artery disease (CAD)/myocardial infarction. Interestingly, the tetraspanin CD81 presented increased DRM-association in SIRS/sepsis patients, but not in CAD patients. Moreover, the pentaspanin CD47 and the Fcgamma RIII CD16 showed an increased DRM partition in CAD patients but disassembled from DRMs in SIRS/sepsis patients.

CONCLUSIONS

Our results demonstrate that flow cytometric analysis of short time in situ detergent extraction provides a powerful tool for rapid screening of blood monocyte DRMs to preselect patients with potential raft/microdomain abnormalities for more detailed analysis.

Evidence for a role of caveolin-1 in neurokinin-1 receptor plasma-membrane localization, efficient signaling, and interaction with beta-arrestin 2

This study was focused on the relationship between the plasma-membrane localization of neurokinin-1 receptor (NK1-R) and its endocytic and signaling properties. First, we employed electron paramagnetic resonance (EPR) to study the domain structure of HEK-293 cells and NK1-R microlocalization. EPR spectra and the GHOST condensation routine demonstrated that NK1-R was distributed in a well-ordered domain of HEK-293 cells possibly representing lipid raft/caveolae microdomains, whereas the impairment of caveolae changed the NK1-R plasma-membrane distribution. Internalization and second messenger assays combined with bioluminescence resonance energy transfer were employed subsequently to evaluate the functional importance of the NK1-R microlocalization in lipid raft/caveolae microdomains. The internalization pattern was delineated through the use of dominant-negative mutants (DNM) of caveolin-1 S80E (Cav1 S80E), dynamin-1 K44A (Dyn K44A), and beta-arrestin (beta-arr 319-418) and by means of cell lines that expressed various endogenous levels of beta-arrestins. NK1-R displayed rapid internalization that was substantially reduced by DNMs of dynamin-1 and beta-arrestin and even more profoundly in cells lacking both beta-arrestin1 and beta-arrestin2. These internalization data were highly suggestive of the predominant use of the clathrin-mediated pathway by NK1-R, even though NK1-R tended to reside constitutively in lipid raft/caveolae microdomains. Evidence was also obtained that the proper clustering of the receptor in these microdomains was important for effective agonist-induced NK1-R signaling and for its interaction with beta-arrestin2.

Intracellular trafficking of raft/caveolae domains: insights from integrin signaling

Cells have a complex system for delivering and compartmentalizing proteins and lipids in order to achieve spatio-temporal coordination of signaling. Rafts/caveolae are plasma membrane microdomains that regulate signaling pathways and processes such as cell migration, polarization and proliferation. Regulation of raft/caveolae trafficking involves multiple steps regulated by different proteins to ensure coordination of signaling cascades. The best studied raft-mediated endocytic route is controlled by caveolins. Recent data suggest integrin-mediated cell adhesion is a key regulator of caveolar endocytosis. In this review we examine the regulation of caveolar trafficking and the interplay between integrins, cell adhesion and caveolae internalization.

Modelling and simulation techniques for membrane biology

One of the most important aspects of Computational Cell Biology is the understanding of the complicated dynamical processes that take place on plasma membranes. These processes are often so complicated that purely temporal models cannot always adequately capture the dynamics. On the other hand, spatial models can have large computational overheads. In this article, we review some of these issues with respect to chemistry, membrane microdomains and anomalous diffusion and discuss how to select appropriate modelling and simulation paradigms based on some or all the following aspects: discrete, continuous, stochastic, delayed and complex spatial processes.

Identification of a novel lipid raft-targeting motif in Src homology 2-containing phosphatase 1

The tyrosine phosphatase Src homology 2-containing phosphatase 1 (SHP-1) is a key negative regulator of TCR-mediated signaling. Previous studies have shown that in T cells a fraction of SHP-1 constitutively localizes to membrane microdomains, commonly referred to as lipid rafts. Although this localization of SHP-1 is required for its functional regulation of T cell activation events, how SHP-1 is targeted to the lipid rafts was unclear. In this study, we identify a novel, six-amino acid, lipid raft-targeting motif within the C terminus of SHP-1 based on several biochemical and functional observations. First, mutations of this motif in the context of full-length SHP-1 result in the loss of lipid raft localization of SHP-1. Second, this motif alone restores raft localization when fused to a mutant of SHP-1 (SHP-1 DeltaC) that fails to localize to rafts. Third, a peptide encompassing the 6-mer motif directly binds to phospholipids whereas a mutation of this motif abolishes lipid binding. Fourth, whereas full-length SHP-1 potently inhibits TCR-induced tyrosine phosphorylation of specific proteins, expression of a SHP-1-carrying mutation within the 6-mer motif does not. Additionally, although SHP-1 DeltaC was functionally inactive, the addition of the 6-mer motif restored its functionality in inhibiting TCR-induced tyrosine phosphorylation. Finally, this 6-mer mediated targeting of SHP-1 lipid rafts was essential for the function of this phosphatase in regulating IL-2 production downstream of TCR. Taken together, these data define a novel 6-mer motif within SHP-1 that is necessary and sufficient for lipid raft localization and for the function of SHP-1 as a negative regulator of TCR signaling.

Age-related impairment of GM-CSF-induced signalling in neutrophils: role of SHP-1 and SOCS proteins

Functional activities of mature human neutrophils are strongly influenced by the pro-inflammatory cytokine granulocyte macrophage-colony stimulating factor (GM-CSF). Accordingly, a defective response to GM-CSF might have dramatic consequences for neutrophil functions and the host defence against infections. Such an event is most likely to occur in senescence. A number of studies have, in fact, reported an impairment of the GM-CSF capacity to prime and/or to activate respiratory burst, as well as to delay apoptotic events, in neutrophils from elderly individuals. In the last 2 decades many efforts have been made to explore at molecular levels the mechanism underlying these defects. Recent studies let us depict a scenario in which an increased activity of inhibitory molecules, such as Src homology domain-containing protein tyrosine phosphatase-1 (SHP-1) and suppressors of cytokine signalling (SOCS), is responsible for the age-related failure of GM-CSF to stimulate neutrophil functions via inhibition of Lyn-, phosphoinositide 3-kinase (PI3-K)/extracellular signal-regulated kinase (ERK)- and signal transducers and activators of transcription (STAT)-dependent pathways. The control of SHP-1 and/or SOCS activity might therefore be an important therapeutic target for the restoration of normal immune responses during senescence.

Membrane regulation of the stress response from prokaryotic models to mammalian cells

"Membrane regulation" of stress responses in various systems is widely studied. In poikilotherms, membrane rigidification could be the first reaction to cold perception: reducing membrane fluidity of membranes at physiological temperatures is coupled with enhanced cold inducibility of a number of genes, including desaturases (see J.L. Harwood's article in this Proceedings volume). A similar role of changes in membrane physical state in heat (oxidative stress, etc.) sensing- and signaling gained support recently from prokaryotes to mammalian cells. Stress-induced remodeling of membrane lipids could influence generation, transduction, and deactivation of stress signals, either through global effects on the fluidity of the membrane matrix, or by specific interactions of boundary (or raft) lipids with receptor proteins, lipases, ion channels, etc. Our data point to membranes not only as targets of stress, but also as sensors in activating a stress response.

Lipid rafts, fluid/fluid phase separation, and their relevance to plasma membrane structure and function

Novel biophysical approaches combined with modeling and new biochemical data have helped to recharge the lipid raft field and have contributed to the generation of a refined model of plasma membrane organization. In this review, we summarize new information in the context of previous literature to provide new insights into the spatial organization and dynamics of lipids and proteins in the plasma membrane of live cells. Recent findings of large-scale separation of liquid-ordered and liquid-disordered phases in plasma membrane vesicles demonstrate this capacity within the complex milieu of plasma membrane proteins and lipids. Roles for membrane heterogeneity and reorganization in immune cell activation are discussed in light of this new information.

Cellular internalization of green fluorescent protein fused with herpes simplex virus protein VP22 via a lipid raft-mediated endocytic pathway independent of caveolae and Rho family GTPases but dependent on dynamin and Arf6

VP22 is a structural protein of the herpes simplex virus and has been reported to possess unusual trafficking properties. Here we examined the mechanism of cellular uptake of VP22 using a fusion protein between the C-terminal half of VP22 and green fluorescent protein (GFP). Adsorption of VP22-GFP onto a cell surface required heparan sulfate proteoglycans and basic amino acids, in particular, Arg-164 of VP22. Inhibitor treatment, RNA interference, expression of dominant-negative mutant genes, and confocal microscopy all indicated that VP22-GFP enters cells through an endocytic pathway independent of clathrin and caveolae but dependent on dynamin and Arf6 activity. As with CD59 (a lipid raft marker), cell-surface VP22-GFP signals were resistant to Triton X-100 treatment but only partially overlapped cell-surface CD59 signals. Furthermore, unlike other lipid raft-mediated endocytic pathways, no Rho family GTPase was required for VP22-GFP internalization. Internalized VP22 initially entered early endosomes and then moved to lysosomes and possibly recycling endosomes.

Neuronal conduction of excitation without action potentials based on ceramide production

Background

Action potentials are the classic mechanism by which neurons convey a state of excitation throughout their length, leading, after synaptic transmission, to the activation of other neurons and consequently to network functioning. Using an in vitro integrated model, we found previously that peripheral networks in the autonomic nervous system can organise an unconventional regulatory reflex of the digestive tract motility without action potentials.

Methodology/Principal Findings

In this report, we used combined neuropharmacological and biochemical approaches to elucidate some steps of the mechanism that conveys excitation along the nerves fibres without action potentials. This mechanism requires the production of ceramide in membrane lipid rafts, which triggers in the cytoplasm an increase in intracellular calcium concentration, followed by activation of a neuronal nitric oxide synthase leading to local production of nitric oxide, and then to guanosine cyclic monophosphate. This sequence of second messengers is activated in cascade from rafts to rafts to ensure conduction of the excitation along the nerve fibres.

Conclusions/Significance

Our results indicate that second messengers are involved in neuronal conduction of excitation without action potentials. This mechanism represents the first evidence—to our knowledge—that excitation is carried along nerves independently of electrical signals. This unexpected ceramide-based conduction of excitation without action potentials along the autonomic nerve fibres opens up new prospects in our understanding of neuronal functioning.

Can the stress protein response be controlled by 'membrane-lipid therapy'?

In addition to high temperature, other stresses and clinical conditions such as cancer and diabetes can lead to the alteration of heat-shock protein (HSP) levels in cells. Moreover, HSPs can associate with either specific lipids or with areas of special membrane topology (such as lipid rafts), and changes in the physical state of cellular membranes can alter hsp gene expression. We propose that membrane microheterogeneity is important for regulating the HSP response. In support of this hypothesis, when particular membrane intercalating compounds are used to alter membrane properties, the simultaneous normalization of dysregulated expression of HSPs causes beneficial responses to disease states. Therefore, these compounds (such as hydroxylamine derivatives) have the potential to become a new class of pharmaceuticals for use in 'membrane-lipid therapy'.

Shedding of collagen XXIII is mediated by furin and depends on the plasma membrane microenvironment

Collagen XXIII belongs to the class of type II orientated transmembrane collagens. A common feature of these proteins is the presence of two forms of the molecule: a membrane-bound form and a shed form. Here we demonstrate that, in mouse lung, collagen XXIII is found predominantly as the full-length form, whereas in brain, it is present mostly as the shed form, suggesting that shedding is tissue-specific and tissue-regulated. To analyze the shedding process of collagen XXIII, a cell culture model was established. Mutations introduced into two putative proprotein convertase cleavage sites showed that altering the second cleavage site inactivated much of the shedding. This supports the idea that furin, a major physiological protease, is predominantly responsible for shedding. Furthermore, our studies indicate that collagen XXIII is localized in lipid rafts in the plasma membrane and that ectodomain shedding is altered by a cholesterol-dependent mechanism. Moreover, newly synthesized collagen XXIII either is cleaved inside the Golgi/trans-Golgi network or reaches the cell surface, where it becomes protected from processing by being localized in lipid rafts. These mechanisms allow the cell to regulate the amounts of cell surface-bound and secreted collagen XXIII.

The involvement of lipid rafts in epidermal growth factor-induced chemotaxis of breast cancer cells

Metastasis is the major cause of morbidity and mortality in cancer. Recent studies reveal a role of chemotaxis in cancer cell metastasis. Epidermal growth factor receptors (EGFR) have potent chemotactic effects on human breast cancer cells. Lipid rafts, organized microdomain on plasma membranes, regulate the activation of many membrane receptors. In the current study, we investigated the role of lipid rafts in EGFR-mediated cancer cell chemotaxis. Our confocal microscopy results suggested that EGFR co-localized with GM1-positive rafts. Disrupting rafts with methyl-beta-cyclodextrin (mbetaCD) inhibited EGF-induced chemotaxis of human breast cancer cells. Supplementation with cholesterol reversed the inhibitory effects. Pretreatment with mbetaCD also impaired directional migration of cells in an in vitro "wound healing" assay, EGF-induced cell adhesion, actin polymerization, Akt phosphorylation and protein kinase Czeta (PKCzeta) translocation. Taken together, our study indicated that integrity of lipid rafts was critical in EGF-induced chemotaxis of human breast cancer cells.

Insights into the uterus

A better understanding of the mechanisms that generate and modulate uterine contractility is needed if progress is to be made in the prevention or treatment of problems in labour. Dysfunctional labour describes the condition when uterine contractility is too poor to dilate the cervix, and it is the leading cause of emergency Caesarean sections. Recently, insight has been gained into a possible causal mechanism for dysfunctional labour. Study of the physiological mechanisms that produce excitation in the uterus, the subsequent Ca(2)(+) signals and biochemical pathway leading to contraction has underpinned this progress. In this review, I give an account of excitation-contraction signalling in the myometrium and explore the implications of recent findings concerning lipid rafts for these processes. I also discuss how changes of pH are fundamentally enmeshed in uterine activity and biochemistry and explore the effect that pH changes will have on human myometrium. Finally, I present the evidence that acidification of the myometrium is correlated with dysfunctional labour and suggest the processes by which it is occurring. It is only by gaining a better understanding of uterine physiology and pathophysiology that progress will be made and research findings translated into clinical benefit for women and their families.

Cholesterol level of lipid raft microdomains regulates apoptotic cell death in prostate cancer cells through EGFR-mediated Akt and ERK signal transduction

BACKGROUND

Lipid rafts are cholesterol-enriched microdomains in cell membranes that have been shown to regulate signal transduction. We investigated whether membrane cholesterol could regulate apoptosis and attempted to elucidate the mechanism by which apoptosis is induced in prostate cancer cells.

METHODS

LNCaP cells were exposed to 2-hydroxyprophyl-beta-cyclodextrin (HPCD) to deplete membrane cholesterol. Cell viability and apoptosis were evaluated by Celltiter Bluetrade mark Cell Viability assay and ethidium bromide/acridine orange staining. Signal transduction was investigated by immunoblot analysis of cell lysates.

RESULTS

Cell viability was dose dependent inhibited by HPCD and restored by replenishment of cholesterol. HPCD induced apoptotic cell death through down-regulation of Bcl-xL and up-regulation of caspase-3 and PARP cleavages. HPCD inhibited both EGFR/Akt and EGFR/ERK signal transduction.

CONCLUSIONS

Lipid raft cholesterol regulates apoptotic cell death in prostate cancer cells through EGFR-mediated Akt and ERK pathways.

Lubrol-RAFTs in Melanoma Cells: A Molecular Platform for Tumor-Promoting Ephrin-B2–Integrin-β1 Interaction

Ephrins control cell motility and matrix adhesion. These functions play a pivotal role in cancer progression, for example, in malignant melanomas. We have previously shown that the ephrin-B2-tumor-promoting action is partly mediated by integrin-beta1 interaction. However, the subcellular prerequisites for molecular interaction like molecular proximity and co-compartmentalization have not been elucidated yet. Specific cholesterol-rich microdomains, termed lipid rafts (RAFTs), are known to be essential for functional ephrin-B2 signalling and integrin-mediated effects. Therefore, we addressed the question whether RAFT co-compartmentalization of both molecules could provide the molecular platform for their tumor-promoting interaction. In this study, we show that overexpressed ephrin-B2 is not only compartmentalized to classical Triton X-100 RAFTs in B16 melanoma cells, but also to the recently defined Lubrol-RAFTs. Interestingly, in the melanoma cells investigated, integrin-beta1 is also preferentially detected in such Lubrol-RAFTs. Accordingly, the presence of ephrin-B2 and integrin-beta1 in RAFTs and their function in cell migration and matrix attachment are highly sensitive to RAFT disruption by cholesterol depletion. Confocal fluorescence microscopy analyses also support the concept of a close molecular proximity and functional interplay of ephrin-B2 and integrin-beta1 in the plasma membrane. We conclude that Lubrol-RAFTs probably represent the platform for tumor-promoting ephrin-B2-integrin-beta1 interaction, which could become an interesting target for future antitumoral therapies.

Do membrane rafts contribute to human immunosenescence?

Aging is associated with an alteration of the immune response called immunosenescence. It is now well accepted that all parts of the immune system, the adaptive as well as the innate, undergo immunosenescence. However, the adaptive immune response and especially T cell functions are the most affected by aging. Aging is associated with profound changes in lymphocytes subpopulations, however, the functional changes within these subsets are more important to elucidate. Indeed, T cells present functional modifications resulting in a decreased clonal expansion and interleukin-2 (IL-2) production. So there should be an alteration in the activation process of T cells with aging involving the T cell receptor (TCR) and CD28 receptor signaling cascades. The alterations in membrane rafts composition and function can underline this altered activation of T cells with aging and then contribute to human immunosenescence. The experimental data in favor of this hypothesis will be reviewed.

Membrane cholesterol modulates Kv1.5 potassium channel distribution and function in rat cardiomyocytes

Membrane lipid composition is a major determinant of cell excitability. In this study, we assessed the role of membrane cholesterol composition in the distribution and function of Kv1.5-based channels in rat cardiac membranes. In isolated rat atrial myocytes, the application of methyl-beta-cyclodextrin (MCD), an agent that depletes membrane cholesterol, caused a delayed increase in the Kv1.5-based sustained component, I(kur), which reached steady state in approximately 7 min. This effect was prevented by preloading the MCD with cholesterol. MCD-increased current was inhibited by low 4-aminopyridine concentration. Neonatal rat cardiomyocytes transfected with Green Fluorescent Protein (GFP)-tagged Kv1.5 channels showed a large ultrarapid delayed-rectifier current (I(Kur)), which was also stimulated by MCD. In atrial cryosections, Kv1.5 channels were mainly located at the intercalated disc, whereas caveolin-3 predominated at the cell periphery. A small portion of Kv1.5 floated in the low-density fractions of step sucrose-gradient preparations. In live neonatal cardiomyocytes, GFP-tagged Kv1.5 channels were predominantly organized in clusters at the basal plasma membrane. MCD caused reorganization of Kv1.5 subunits into larger clusters that redistributed throughout the plasma membrane. The MCD effect on clusters was sizable 7 min after its application. We conclude that Kv1.5 subunits are concentrated in cholesterol-enriched membrane microdomains distinct from caveolae, and that redistribution of Kv1.5 subunits by depletion of membrane cholesterol increases their current-carrying capacity.

Hyperfluidization-coupled membrane microdomain reorganization is linked to activation of the heat shock response in a murine melanoma cell line

Targeting of the Hsp function in tumor cells is currently being assessed as potential anticancer therapy. An improved understanding of the molecular signals that trigger or attenuate the stress protein response is essential for advances to be made in this field. The present study provides evidence that the membrane fluidizer benzyl alcohol (BA), a documented nondenaturant, acts as a chaperone inducer in B16(F10) melanoma cells. It is demonstrated that this effect relies basically on heat shock transcription factor 1 (HSF1) activation. Under the conditions tested, the BA-induced Hsp response involves the up-regulation of a subset of hsp genes. It is shown that the same level of membrane fluidization (estimated in the core membrane region) attained with the closely analogous phenethyl alcohol (PhA) does not generate a stress protein signal. BA, at a concentration that activates heat shock genes, exerts a profound effect on the melting of raft-like cholesterol-sphingomyelin domains in vitro, whereas PhA, at a concentration equipotent with BA in membrane fluidization, has no such effect. Furthermore, through the in vivo labeling of melanoma cells with a fluorescein labeled probe that inserts into the cholesterol-rich membrane domains [fluorescein ester of polyethylene glycol-derivatized cholesterol (fPEG-Chol)], we found that, similarly to heat stress per se, BA, but not PhA, initiates profound alterations in the plasma membrane microdomain structure. We suggest that, apart from membrane hyperfluidization in the deep hydrophobic region, a distinct reorganization of cholesterol-rich microdomains may also be required for the generation and transmission of stress signals to activate hsp genes.

SNARE proteins and 'membrane rafts'

The original 'lipid raft' hypothesis proposed that lipid-platforms/rafts form in the exoplasmic plasmalemmal leaflet by tight clustering of sphingolipids and cholesterol. Their physical state, presumably similar to liquid-ordered phases in model membranes, would confer detergent resistance to rafts and enriched proteins therein. Based on this concept, detergent resistant membranes (DRMs) from solubilized cells were considered to reflect pre-existing 'lipid rafts' in live cells. To date, more than 200 proteins were found in DRMs including also members of the SNARE superfamily, which are small membrane proteins involved in intracellular fusion steps. Their raft association indicates that they are not uniformly distributed, and, indeed, microscopic studies revealed that SNAREs concentrate in submicrometre-sized, cholesterol-dependent clusters at which vesicles fuse. However, the idea that SNARE clusters are 'lipid rafts' was challenged, as they do not colocalize with raft markers, and SNAREs are excluded from liquid-ordered phases in model membranes. Independent from this disagreement, in recent years the solubilization criterion has been criticized for several reasons, calling for a more exact definition of rafts. At a recent consensus on a revised raft model, the term 'lipid rafts' was replaced by 'membrane rafts' that were defined as 'small (10-200 nm), heterogeneous, highly dynamic, sterol- and sphingolipid-enriched domains that compartmentalize cellular processes'. As a result, after dismissing the terms 'detergent resistant' and 'liquid-ordered', it now appears that SNARE clusters are bona fide 'membrane rafts'.

Functional expression and microdomain localization of prestin in cultured cells

INTRODUCTION

Prestin is an essential component of the molecular motor of cochlear outer hair cells that contribute to frequency selectivity and sensitivity of mammalian hearing. A model system to study prestin employs its transfection into cultured HEK 293 cells. Our goal was to characterize prestin's trafficking pathway and localization in the plasma membrane.

METHODS

We used immuno-colocalization of prestin with intracellular and plasma membrane markers and sucrose density fractionation to analyze prestin in membrane compartments. Voltage clamping was used to measure nonlinear capacitance (NLC), prestin's electrical signature.

RESULTS & DISCUSSION

Prestin targets to the membrane by 24 hours post-transfection when NLC is measurable. Prestin then concentrates into membrane foci that colocalize and fractionate with membrane microdomains. Depleting membrane cholesterol content altered prestin localization and NLC.

CONCLUSION

Prestin activity in HEK 293 cells results from expression in the plasma membrane and altering membrane lipid content affects prestin localization and activity.

Shedding of the p75NTRneurotrophin receptor is modulated by lipid rafts

Protein ectodomain shedding is the proteolytic release of the extracellular domain of membrane-bound proteins. Neurotrophin receptor p75(NTR) is known to be affected by shedding. The present work provides evidence, in rat brain synaptosomes, that p75(NTR) is present in detergent-resistant membranes (DRM), also known as lipid rafts, only in its full-length form. Disrupting the integrity of lipid rafts causes solubilization of p75(NTR) after detergent treatment and enhancement of the shedding. Analyses of the enzymes described as being responsible for p75(NTR) shedding, i.e. tumor necrosis factor alpha convertase (TACE) and presenilin-1 (PS1), revealed that TACE is absent in DRM, while variable proportions of the C-terminal and N-terminal fragments of PS1 are found. In summary, our results point to a role of lipid rafts in the modulation of the shedding of the p75(NTR) receptor.

Vascular signaling through cholesterol-rich domains: implications in hypertension

PURPOSE OF REVIEW

Lipid rafts are emerging as key players in the integration of cellular responses. Alterations in these highly regulated signaling cascades are important in structural, mechanical and functional abnormalities that underlie vascular pathological processes. The present review focuses on recent advances in signal transduction through caveolae/lipid rafts, implicated in hypertensive processes.

RECENT FINDINGS

Caveolae/lipid rafts function as sites of dynamic regulatory events in receptor-induced signal transduction. Mediators of vascular function, including G-protein coupled receptors, Src family tyrosine kinases, receptor tyrosine kinases, protein phosphatases and nitric oxide synthase, are concentrated within these microdomains. The assembly of functionally active nicotinamide adenine dinucleotide phosphate oxidase and subsequent reactive oxygen species production are also dependent on interactions within the caveolae/lipid rafts. Recent findings have also demonstrated the importance of actin-cytoskeleton and focal adhesion sites for protein interactions with caveolae/lipid raft.

SUMMARY

Many vascular signaling processes are altered in hypertension. Whether these events involve lipid rafts/caveolae remains unclear. A better understanding of how signaling molecules compartmentalize in lipid rafts/caveolae will provide further insights into molecular mechanisms underlying vascular damage in cardiovascular disease.

Shed gangliosides provide detergent-independent evidence for Type-3 glycosynapses

Membrane microdomains, or rafts, at the plasma membrane have been invoked to explain many cellular processes. Protein-protein interactions within such microdomains including, for example, the tetraspanin web are reported to provide a scaffold for signal transduction. However, the nature of such protein-protein interactions is not fully elucidated. Hakomori [S.I. Hakomori, The glycosynapse, Proc. Natl. Acad. Sci. USA 99 (2002) 225-232] has advanced the concept that glycosphingolipids, particularly gangliosides, provide the intermediary link between transmembrane receptors and signal transducers and has redefined membrane rafts as Type-1, -2 or -3 glycosynapses. Here, using simple immunofluorescent analysis of the ganglioside complexes naturally released from cellular microprocesses (termed "footprints") we show that the ganglioside can determine the nature of protein-protein associations. Specifically, we demonstrate that CD36 and the tetraspanin CD151, both of which interact with beta1 integrins, associate together only in the presence of the gangliosides GD2/GD3. These results substantiate the glycosynapse hypothesis and suggest that the nature of the tetraspanin web may be determined by gangliosides.

EXPRESSION OF FLOTILLIN-1 ON EIMERIA TENELLA SPOROZOITES AND ITS ROLE IN HOST CELL INVASION

Lipid rafts are detergent-resistant, liquid-ordered microdomains in plasma membranes that are enriched in cholesterol and sphingolipids and involved in intracellular signal transduction, membrane trafficking, and molecular sorting. In this study, we investigated the possibility that lipid rafts on Eimeria tenella sporozoites may act as platforms for host cell invasion. Flotillin-1, a resident protein of lipid rafts, was identified on E. tenella sporozoites and was prominently expressed at the apex of the cells, a region mediating host cell invasion. Pretreatment of sporozoites with antibody against flotillin-1 blocked parasite invasion. Furthermore, the anticoccidial drug, monensin, disrupted the localization of flotillin-1 within raft structures resulting in loss of invasion. We conclude that Eimeria sporozoites utilize lipid rafts containing flotillin-1 for internalization into host cells.

Insulin Resistance as a Membrane Microdomain Disorder

Membrane microdomains (lipid rafts) are now recognized as critical for proper compartmentalization of insulin signaling, but their role in the pathogenesis of insulin resistance has not been investigated. Detergent-resistant membrane microdomains (DRMs), isolated in the low density fractions, are highly enriched in cholesterol, glycosphingolipids and various signaling molecules. TNFalpha induces insulin resistance in type 2 diabetes, but its mechanism of action is not fully understood. We have found a selective increase in the acidic glycosphingolipid ganglioside GM3 in 3T3-L1 adipocytes treated with TNFalpha, suggesting a specific function for GM3. We were able to extend these in vitro observations to living animals using obese Zucker fa/fa rats and ob/ob mice, in which the GM3 synthase mRNA levels in the white adipose tissues are significantly higher than in their lean controls. In the DRMs from TNFalpha-treated 3T3-L1 adipocytes, GM3 levels were doubled, compared to results in normal adipocytes. Additionally, insulin receptor (IR) accumulations in the DRMs were diminished, while caveolin and flotillin levels were unchanged. GM3 depletion was able to counteract the TNFalpha-induced inhibition of IR accumulation into DRMs. Together, these findings provide compelling evidence that in insulin resistance the insulin metabolic signaling defect can be attributed to a loss of IRs in the microdomains due to an accumulation of GM3.

[Role of lipid rafts in trimeric G protein-mediated signal transduction]

Lipid rafts and caveolae are microdomains in the cell membranes, which contain cholesterol, glycolipids, and sphingomyelin. While caveolae are relatively stable because caveolin, an integral protein, supports the structure, lipid rafts are considered to be unstable, being dynamically produced and degraded. Recent studies have reported that lipid rafts contain many signaling molecules, such as glycosylphosphatidylinositol-anchored proteins, acylated proteins, G-protein-coupled receptors (GPCRs), trimeric and small G-proteins and their effectors, suggesting that the lipid rafts have an important role in receptor-mediated signal transduction. Therefore drugs that modify the composition of lipid rafts might influence the efficacy of cellular signal transduction. In this review, we demonstrate the role of lipid rafts in GPCR-G-protein signaling and also present our recent results showing that the wasp toxin mastoparan modifies G(q/11)-mediated phospholipase C activation through the interaction with gangliosides in lipid rafts.

Smac-mediated sensitization of human B-lymphoma cells to staurosporine- and lactacystin-triggered apoptosis is apoptosome-dependent

Second mitochondrial activator of caspase (Smac)-derived peptides have previously been shown to facilitate apoptosis of various types of cancer cells. However, it remains unclear whether the effects of such Smac agonists are dependent on apoptotic protease-activating factor-1 (Apaf-1), a key component of the apoptosome. Here, we explored the role of Apaf-1 through overexpression of this protein in the B-lymphoma cell line Raji that is defective for cytosolic Apaf-1 expression. Enforced expression of Apaf-1 rendered Raji cells sensitive to staurosporine as well as to the proteasome inhibitor, lactacystin. Importantly, co-treatment with Smac peptides resulted in a threefold higher degree of apoptosis in Apaf-1-expressing Raji cells, but not in mock-transfected cells. Smac peptides also potentiated apoptosis of the DG-75 cell line following liberation of endogenous Apaf-1 from the plasma membrane, but were ineffective when added alone. Furthermore, we observed high levels of expression in several B-lymphoma cell lines of cellular inhibitor of apoptosis protein-2 (cIAP2), and immunodepletion of cIAP2 (a target of Smac) was found to sensitize Apaf-1-overexpressing Raji cells to cytochrome c-dependent caspase activation. Collectively, these results demonstrate the importance of Apaf-1 in Smac-mediated potentiation of apoptosis of B-lymphoma-derived cells.

Modulation of lipid rafts by Omega-3 fatty acids in inflammation and cancer: implications for use of lipids during nutrition support

Current understanding of biologic membrane structure and function is largely based on the concept of lipid rafts. Lipid rafts are composed primarily of tightly packed, liquid-ordered sphingolipids/cholesterol/saturated phospholipids that float in a sea of more unsaturated and loosely packed, liquid-disordered lipids. Lipid rafts have important clinical implications because many important membrane-signaling proteins are located within the raft regions of the membrane, and alterations in raft structure can alter activity of these signaling proteins. Because rafts are lipid-based, their composition, structure, and function are susceptible to manipulation by dietary components such as omega-3 polyunsaturated fatty acids and by cholesterol depletion. We review how alteration of raft lipids affects the raft/nonraft localization and hence the function of several proteins involved in cell signaling. We focus our discussion of raft-signaling proteins on inflammation and cancer.

Redistribution of P-selectin glycoprotein ligand-1 (PSGL-1) in chemokine-treated neutrophils: a role of lipid microdomains

P-selectin glycoprotein ligand-1 (PSGL-1) is a mucin-like cell adhesion molecule expressed on leukocyte plasma membranes and involved in platelet-leukocyte and endothelium-leukocyte interactions. The treatment of neutrophils with a low concentration of IL-8 induced the redistribution of PSGL-1 to one end of the cell to form a cap-like structure. We investigated the role of lipid microdomains in the redistribution of PSGL-1 and its effect on the adhesive characteristics of IL-8-treated neutrophils. The redistribution of PSGL-1 induced by IL-8 was inhibited by cholesterol-perturbing agents such as methyl-beta-cyclodextrin and filipin. Sucrose density gradient centrifugation analysis revealed that PSGL-1 was enriched in a low-density fraction together with the GM1 ganglioside after solubilization of the cell membranes with a nonionic detergent, Brij 58. However, when Triton X-100 was used for the solubilization, PSGL-1 was no longer recovered in the low-density fraction, although GM1 ganglioside remained in the low-density fraction. Furthermore, immunofluorescence microscopic observation demonstrated that the localization of PSGL-1 differed from that of GM1 ganglioside, suggesting that PSGL-1 is associated with a microdomain distinct from that containing the GM1 ganglioside. Treatment of neutrophils with IL-8 increased the formation of microaggregates composed of neutrophils and activated platelets, and this treatment also enhanced reactive oxygen species production in neutrophils induced by the cross-linking of PSGL-1 with antibodies. These results suggest that the association of PSGL-1 with lipid microdomains is essential for its redistribution induced by IL-8 stimulation and that the redistribution modulates neutrophil functions mediated by interactions with P-selectin.

Differential detergent resistance of the apical and basolateral NPPases: relationship with polarized targeting

Targeting of glycosylphosphatidylinositol-anchored proteins to the apical surface of epithelial cells involves clustering in Triton X-100-resistant membrane microdomains or rafts. The role of these microdomains in sorting transmembrane proteins is more questionable because, unlike glycosylphosphatidylinositol-anchored proteins, apical transmembrane proteins are rather soluble in Triton X-100. They are, however, resistant to milder detergents such as Lubrol WX or Tween 20. It has been proposed that specific membrane microdomains, defined by resistance to these detergents, would carry transmembrane proteins to the apical surface. We have used MDCK cells stably transfected with the apical and basolateral pyrophosphatases/phosphodiesterases, NPP3 and NPP1, to examine the relationship between detergent resistance and apical targeting. The apically expressed wild-type NPP3 was insoluble in Lubrol WX whereas wild-type NPP1, which is expressed basolaterally, was essentially soluble. By using tail mutants and chimeric constructs that combine the cytoplasmic, transmembrane and extracellular domains of NPP1 and NPP3, we show that there is not a strict correlation between detergent resistance and apical targeting. Lubrol resistance is an intrinsic property of NPP3, which is acquired early during the biosynthetic process irrespective of its final destination, and depends on positively charged residues in its cytoplasmic tail.

Sterol and pH interdependence in the binding, oligomerization, and pore formation of Listeriolysin O

Listeriolysin O (LLO) is the most important virulence factor of the intracellular pathogen Listeria monocytogenes. Its main task is to enable escape of bacteria from the phagosomal vacuole into the cytoplasm. LLO belongs to the cholesterol-dependent cytolysin (CDC) family but differs from other members, as it exhibits optimal activity at low pH. Its pore forming ability at higher pH values has been largely disregarded in Listeria pathogenesis. Here we show that high cholesterol concentrations in the membrane restore the low activity of LLO at high pH values. LLO binds to lipid membranes, at physiological or even slightly basic pH values, in a cholesterol-dependent fashion. Binding, insertion into lipid monolayers, and permeabilization of calcein-loaded liposomes are maximal above approximately 35 mol % cholesterol, a concentration range typically found in lipid rafts. The narrow transition region of cholesterol concentration separating low and high activity indicates that cholesterol not only allows the binding of LLO to membranes but also affects other steps in pore formation. We were able to detect some of these by surface plasmon resonance-based assays. In particular, we show that LLO recognition of cholesterol is determined by the most exposed 3beta-hydroxy group of cholesterol. In addition, LLO binds and permeabilizes J774 cells and human erythrocytes in a cholesterol-dependent fashion at physiological or slightly basic pH values. The results clearly show that LLO activity at physiological pH cannot be neglected and that its action at sites distal to cell entry may have important physiological consequences for Listeria pathogenesis.

Cleavage fragments of the third complement component (C3) enhance stromal derived factor-1 (SDF-1)-mediated platelet production during reactive postbleeding thrombocytosis

We hypothesized that the third complement component (C3) cleavage fragments (C3a and (des-Arg)C3a) are involved in stress/inflammation-related thrombocytosis, and investigated their potential role in reactive thrombocytosis induced by bleeding. We found that platelet counts are lower in C3-deficient mice in response to excessive bleeding as compared to normal littermates and that C3a and (des-Arg)C3a enhance stromal-derived factor-1 (SDF-1)-dependent megakaryocyte (Megs) migration, adhesion and platelet shedding. At the molecular level, C3a stimulates in Megs MAPKp42/44 phosphorylation, and enhances incorporation of CXCR4 into membrane lipid rafts increasing the responsiveness of Megs to SDF-1. We found that perturbation of lipid raft formation by statins decreases SDF-1/C3a-dependent platelet production in vitro and in an in vivo model statins ameliorated post-bleeding thrombocytosis. Thus, inhibition of lipid raft formation could find potential clinical application as a means of ameliorating some forms of thrombocytosis.

CD38/CD19: a lipid raft-dependent signaling complex in human B cells

The present work deals with the mechanisms of signal transduction mediated via CD38 in normal and neoplastic human B lymphocytes. The results indicate that CD38 is a receptor and that CD38-mediated signals are tightly regulated at 3 distinct levels. The first concerns the structural organization of CD38, which is clearly divided into monomeric and dimeric forms. The second level of regulation is based on the dynamic localization of CD38 molecules in lipid microdomains within the plasma membrane. Lateral associations with other proteins, namely with the CD19/CD81 complex, determine the third level of control. Raft localization and association with the CD19 complex are prerequisites for CD38-mediated signals in tonsillar B cells and in continuous lines. Lastly, the results indicate that lipid microdomain disruption and silencing of CD19 directly impacts on CD38's ability to mediate Ca(2+) fluxes, while leaving its surface expression unchanged. CD38 is also an enzyme capable of producing several calcium-mobilizing metabolites including cyclic adenosine diphosphate ribose (cADPR). Our inability to identify a correlation between the production of cADPR and the receptorial functions support the hypothesis that CD38 is a pleiotropic molecule whose behavior as a receptor is independent from its enzymatic activity.

Rafts are required for acetylcholine receptor clustering

Cholesterol-sphingolipid microdomains, or lipid rafts, are major regulators of molecular interactions in membrane organization. Because lipid rafts can move laterally and cluster into larger patches, they have been proposed to play a role in the redistribution of specific molecules to specialized cellular structures. Rafts have been shown to favor formation and maintenance of synaptic receptor clusters in neurons of the central nervous system. However, little is known about their role in formation of the neuromuscular junction (NMJ). To determine whether lipid rafts are involved in acetylcholine receptor (AChR) cluster formation and stabilization in myogenic cells, two standard tools were employed: (1) Perturbation of lipid rafts by drugs that deplete membrane cholesterol was carried out to verify that cholesterol is required for AChR clustering in agrin-treated C2C12 myotubes; and (2) detergent resistance of lipid-ordered domains was also used to demonstrate that AChRs, as well as key components of the postsynaptic membrane of the NMJ, are associated with rafts.

Effect of lipid rafts on Cb2 receptor signaling and 2-arachidonoyl-glycerol metabolism in human immune cells

Recently, we have shown that treatment of rat C6 glioma cells with the raft disruptor methyl-beta-cyclodextrin (MCD) doubles the binding of anandamide (AEA) to type-1 cannabinoid receptors (CB1R), followed by CB1R-dependent signaling via adenylate cyclase and p42/p44 MAPK activity. In the present study, we investigated whether type-2 cannabinoid receptors (CB2R), widely expressed in immune cells, also are modulated by MCD. We show that treatment of human DAUDI leukemia cells with MCD does not affect AEA binding to CB2R, and that receptor activation triggers similar [35S]guanosine-5'-O-(3-thiotriphosphate) binding in MCD-treated and control cells, similar adenylate cyclase and MAPK activity, and similar MAPK-dependent protection against apoptosis. The other AEA-binding receptor transient receptor potential channel vanilloid receptor subunit 1, the AEA synthetase N-acyl-phosphatidylethanolamine-phospholipase D, and the AEA hydrolase fatty acid amide hydrolase were not affected by MCD, whereas the AEA membrane transporter was inhibited (approximately 55%) compared with controls. Furthermore, neither diacylglycerol lipase nor monoacylglycerol lipase, which respectively synthesize and degrade 2-arachidonoylglycerol, were affected by MCD in DAUDI or C6 cells, whereas the transport of 2-arachidonoylglycerol was reduced to approximately 50%. Instead, membrane cholesterol enrichment almost doubled the uptake of AEA and 2-arachidonoylglycerol in both cell types. Finally, transfection experiments with human U937 immune cells, and the use of primary cells expressing CB1R or CB2R, ruled out that the cellular environment could account per se for the different modulation of CB receptor subtypes by MCD. In conclusion, the present data demonstrate that lipid rafts control CB1R, but not CB2R, and endocannabinoid transport in immune and neuronal cells.

Glycoprotein VI agonists have distinct dependences on the lipid raft environment

BACKGROUND

It has been reported that the association of glycoprotein VI (GPVI) with lipid rafts regulates GPVI signaling in platelets.

OBJECTIVE

Secreted adenosine 5'-diphosphate (ADP) potentiates GPVI-induced platelet aggregation at particular agonist concentrations. We have investigated whether the decrease in GPVI signaling, previously reported in platelets with disrupted rafts, is a result of the loss of agonist potentiation by ADP.

METHODS

We disrupted platelet lipid rafts with methyl-beta-cyclodextrin and measured signaling events downstream of GPVI activation.

RESULTS

Lipid raft disruption decreases aggregation induced by low concentrations of convulxin, but this decrease is almost eliminated in the presence of ADP antagonists. Signaling indicators, such as protein phosphorylation and calcium mobilization, were not affected by raft disruption in collagen or convulxin stimulated platelets. Interestingly, however, raft disruption directly reduced GPVI signaling induced by collagen-related peptide.

CONCLUSIONS

Lipid rafts do not directly contribute to signaling by the physiologic agonist collagen. The effects of disruption of lipid rafts in in vitro assays can be attributed to inhibition of ADP feedback that potentiates GPVI signaling.

Do caveolins regulate cells by actions outside of caveolae?

Caveolae (caveolin-containing lipid rafts) are plasma membrane domains that scaffold and organize a variety of important proteins in eukaryotic cells. Recent work shows that caveolins can act independently of caveolae, both in cells that lack caveolae (e.g. neurons and leukocytes) and in non-caveolar regions of cells that have caveolae (e.g. cardiac myocytes and fibroblasts). Phosphorylation of caveolins can influence the scaffolding of protein partners, and caveolins appear to participate in the protection and trafficking of proteins to and from the plasma membrane. Together, these results suggest that, despite their name, caveolins should now be thought of as proteins that scaffold signaling and other proteins in both caveolar and non-caveolar regions.

Transit of hormonal and EGF receptor-dependent signals through cholesterol-rich membranes

The functional consequences of changes in membrane lipid composition that coincide with malignant growth are poorly understood. Sufficient data have been acquired from studies of lipid binding proteins, post-translational modifications of signaling proteins, and biochemical inhibition of lipidogenic pathways to indicate that growth and survival pathways might be substantially re-directed by alterations in the lipid content of membranes. Cholesterol and glycosphingolipids segregate into membrane patches that exhibit a liquid-ordered state in comparison to membrane domains containing relatively lower amounts of these classes of lipids. These "lipid raft" structures, which may vary in size and stability in different cell types, both accumulate and exclude signaling proteins and have been implicated in signal transduction through a number of cancer-relevant pathways. In prostate cancer cells, signaling from epidermal growth factor receptor (EGFR) to the serine-threonine kinase Akt1, as well as from IL-6 to STAT3, have been demonstrated to be influenced by experimental interventions that target cholesterol homeostasis. The recent finding that classical steroid hormone receptors also reside in these microdomains, and thus may function within these structures in a signaling capacity independent of their role as nuclear factors, suggests a novel means of cross-talk between receptor tyrosine kinase-derived and steroidogenic signals. Potential points of intersection between components of the EGFR family of receptor tyrosine kinases and androgen receptor signaling pathways, which may be sensitive to disruptions in cholesterol metabolism, are discussed. Understanding the manner in which these pathways converge within cholesterol-rich membranes may present new avenues for therapeutic intervention in hormone-dependent cancers.

Vesicle shapes from molecular dynamics simulations

Lipid bilayer membranes are known to form various structures such as large sheets or vesicles. When the two leaflets of the bilayer have an equal composition, the membrane preferentially forms a flat sheet or a spherical vesicle. However, a difference in the composition of the two leaflets may result in a curved bilayer or in a wide variety of vesicle shapes. Vesicles with different shapes have already been shown in experiments and diverse vesicle shapes have been predicted theoretically from energy minimization of continuous curves. Here we present a molecular dynamics study of the effect of small changes in the phospholipid headgroups on the spontaneous curvature of the bilayer and on the resulting vesicle shape transformations. Small asymmetries in the bilayers already result in high spontaneous curvature and large vesicle deformations. Vesicle shapes that are formed include ellipsoids, discoids, pear-shaped vesicles, cup-shaped vesicles, as well as budded vesicles. Comparison of these vesicles with theoretically derived vesicle shapes shows both resemblances and differences.

Chirality-induced budding: a raft-mediated mechanism for endocytosis and morphology of caveolae?

The formation of transport carriers (spherical vesicles and tubules) involves membrane budding, growth, and ultimately fission. We propose a mechanism of membrane budding, wherein the tilt and chirality of constituent molecules, confined to a patch of area A, induces buds of approximately 50-100 nm that are comparable to vesicles involved in endocytosis. Because such chiral and tilted lipid molecules are likely to exist in "rafts", we suggest the involvement of this mechanism in generating membrane buds in the clathrin and dynamin-independent, raft-component mediated endocytosis of glycosylphosphatidylinositol-anchored proteins. We argue that caveolae, permanent cell surface structures with characteristic morphology and enriched in raft constituents, are also likely to be formed by this mechanism. Thus, molecular chirality and tilt, and its expression over large spatial scales may be a common organizing principle in membrane budding of transport carriers.

Ganglioside GD1a is an essential coreceptor for Toll-like receptor 2 signaling in response to the B subunit of type IIb enterotoxin

Innate recognition and signaling by Toll-like receptors (TLRs) is facilitated by functionally associated coreceptors, although the cooperativity mechanisms involved are poorly understood. As a model we investigated TLR2 interactions with the GD1a ganglioside binding subunit of type IIb Escherichia coli enterotoxin (LT-IIb-B(5)). Both LT-IIb-B(5) and a GD1a binding-defective mutant (LT-IIb-B(5)(T13I)) could modestly bind to TLR2, but only the wild-type molecule displayed a dramatic increase in TLR2 binding activity in the presence of GD1a (although not in the presence of irrelevant gangliosides). Moreover, fluorescence resonance energy transfer experiments indicated that LT-IIb-B(5) induces lipid raft recruitment of TLR2 and TLR1 and their clustering with GD1a, in contrast to the GD1a binding-defective mutant, which moreover fails to activate TLR2 signaling. LT-IIb-B(5)-induced cell activation was critically dependent upon the Toll/IL-1 receptor domain-containing adaptor protein, which was induced to colocalize with TLR2 and GD1a, as shown by confocal imaging. Therefore, GD1a provides TLR2 coreceptor function by enabling the ligand to recruit, bind, and activate TLR2. These findings establish a model of TLR2 coreceptor function and, moreover, suggest novel mechanisms of adjuvanticity by non-toxic derivatives of type II enterotoxins dependent upon GD1a/TLR2 cooperative activity.

Atorvastatin restores Lck expression and lipid raft-associated signaling in T cells from patients with systemic lupus erythematosus

Loss of tolerance to self-Ags in patients with systemic lupus erythematosus (SLE), a prototypic autoimmune disease, is associated with dysregulation of T cell signaling, including the depletion of total levels of lymphocyte-specific protein kinase (Lck) from sphingolipid-cholesterol-enriched membrane microdomains (lipid rafts). Inhibitors of 3-hyroxy-3-methylgluteryl CoA reductase (statins) can modify the composition of lipid rafts, resulting in alteration of T cell signaling. In this study, we show that atorvastatin targets the distribution of signaling molecules in T cells from SLE patients, by disrupting the colocalization of total Lck and CD45 within lipid rafts, leading to a reduction in the active form of Lck. Upon T cell activation using anti-CD3/anti-CD28 in vitro, the rapid recruitment of total Lck to the immunological synapse was inhibited by atorvastatin, whereas ERK phosphorylation, which is decreased in SLE T cells, was reconstituted. Furthermore, atorvastatin reduced the production of IL-10 and IL-6 by T cells, implicated in the pathogenesis of SLE. Thus, atorvastatin reversed many of the signaling defects characteristic of SLE T cells. These findings demonstrate the potential for atorvastatin to target lipid raft-associated signaling abnormalities in autoreactive T cells and provide a rationale for its use in therapy of autoimmune disease.

The local phospholipid environment modulates the activation of blood clotting

Examples abound of membrane-bound enzymes for which the local membrane environment plays an important role, including the ectoenzyme that triggers blood clotting, the plasma serine protease, factor VIIa, bound to the integral membrane protein, tissue factor. The activity of this enzyme complex is markedly influenced by lipid bilayer composition and further by tissue factor partitioning into membrane microdomains on some cell surfaces. Unfortunately, little is known about how membrane microdomain composition controls factor VIIa-tissue factor activity, as reactions catalyzed by membrane-tethered enzymes are typically studied under conditions in which the experimenter cannot control the composition of the membrane in the immediate vicinity of the enzyme. To overcome this problem, we used a nanoscale approach that afforded complete control over the membrane environment surrounding tissue factor by assembling the factor VIIa.tissue factor complex on stable bilayers containing 67 +/- 1 phospholipid molecules/leaflet (Nanodiscs). We investigated how local changes in phospholipid bilayer composition modulate the activity of the factor VIIa.tissue factor complex. We also addressed whether this enzyme requires a pool of membrane-bound protein substrate (factor X) for efficient catalysis, or alternatively if it could efficiently activate factor X, which binds directly to the membrane nanodomain adjacent to tissue factor. We have shown that full proteolytic activity of the factor VIIa.tissue factor complex requires extremely high local concentrations of anionic phospholipids and further that a large pool of membrane-bound factor X is not required to support sustained catalysis.

Lipid raft proteome of the human neutrophil azurophil granule

Detergent-resistant membrane domains (DRMs) are present in the membranes of azurophil granules in human neutrophils (Feuk-Lagerstedt et al., J. Leukoc. Biol. 2002, 72, 970). Using a proteomic approach, we have now identified 106 proteins in a DRM preparation from these granule membranes. Among these proteins were the lipid raft structural proteins flotillin-1 and -2, cytoskeletal proteins such as actin, vimentin and tubulin, and membrane fusion promoting proteins like annexins and dysferlin. Our results suggest that the azurophil granule membrane, in similarity to the plasma membrane, is an elaborate structure that takes part in intracellular signaling and functions other than the mere delivery of bactericidal effector molecules to the phagosome.