Sphingolipids: modulators of HIV-1 infection and pathogenesis

Normal and Dysregulated Sphingolipid Metabolism: Contributions to Podocyte Injury and Beyond

Podocyte health is vital for maintaining proper glomerular filtration in the kidney. Interdigitating foot processes from podocytes form slit diaphragms which regulate the filtration of molecules through size and charge selectivity. The abundance of lipid rafts, which are ordered membrane domains rich in cholesterol and sphingolipids, near the slit diaphragm highlights the importance of lipid metabolism in podocyte health. Emerging research shows the importance of sphingolipid metabolism to podocyte health through structural and signaling roles. Dysregulation in sphingolipid metabolism has been shown to cause podocyte injury and drive glomerular disease progression. In this review, we discuss the structure and metabolism of sphingolipids, as well as their role in proper podocyte function and how alterations in sphingolipid metabolism contributes to podocyte injury and drives glomerular disease progression.

Convergent Evolution Dynamics of SARS-CoV-2 and HIV Surface Envelope Glycoproteins Driven by Host Cell Surface Receptors and Lipid Rafts: Lessons for the Future

Although very different, in terms of their genomic organization, their enzymatic proteins, and their structural proteins, HIV and SARS-CoV-2 have an extraordinary evolutionary potential in common. Faced with various selection pressures that may be generated by treatments or immune responses, these RNA viruses demonstrate very high adaptive capacities, which result in the continuous emergence of variants and quasi-species. In this retrospective analysis of viral proteins, ensuring the adhesion of these viruses to the plasma membrane of host cells, we highlight many common points that suggest the convergent mechanisms of evolution. HIV and SARS-CoV-2 first recognize a lipid raft microdomain that acts as a landing strip for viral particles on the host cell surface. In the case of mucosal cells, which are the primary targets of both viruses, these microdomains are enriched in anionic glycolipids (gangliosides) forming a global electronegative field. Both viruses use lipid rafts to surf on the cell surface in search of a protein receptor able to trigger the fusion process. This implies that viral envelope proteins are both geometrically and electrically compatible to the biomolecules they select to invade host cells. In the present study, we identify the surface electrostatic potential as a critical parameter controlling the convergent evolution dynamics of HIV-1 and SARS-CoV-2 surface envelope proteins, and we discuss the impact of this parameter on the phenotypic properties of both viruses. The virological data accumulated since the emergence of HIV in the early 1980s should help us to face present and future virus pandemics.

Microbial lectome versus host glycolipidome: How pathogens exploit glycosphingolipids to invade, dupe or kill

Glycosphingolipids (GSLs) are ubiquitous components of the cell membranes, found across several kingdoms of life, from bacteria to mammals, including humans. GSLs are a subclass of major glycolipids occurring in animal lipid membranes in clusters named “lipid rafts.” The most crucial functions of GSLs include signal transduction and regulation as well as participation in cell proliferation. Despite the mainstream view that pathogens rely on protein–protein interactions to survive and thrive in their hosts, many also target the host lipids. In particular, multiple pathogens produce adhesion molecules or toxins that bind GSLs. Attachment of pathogens to cell surface receptors is the initial step in infections. Many mammalian pathogens have evolved to recognize GSL-derived receptors. Animal glycosphingolipidomes consist of multiple types of GSLs differing in terminal glycan and ceramide structures in a cell or tissue-specific manner. Interspecies differences in GSLs dictate host specificity as well as cell and tissue tropisms. Evolutionary pressure exerted by pathogens on their hosts drives changes in cell surface glycoconjugates, including GSLs, and has produced a vast number of molecules and interaction mechanisms. Despite that abundance, the role of GSLs as pathogen receptors has been largely overlooked or only cursorily discussed. In this review, we take a closer look at GSLs and their role in the recognition, cellular entry, and toxicity of multiple bacterial, viral and fungal pathogens.

Fingolimod inhibits multiple stages of the HIV-1 life cycle

Antiretroviral drugs that target various stages of the Human Immunodeficiency Virus (HIV) life cycle have been effective in curbing the AIDS epidemic. However, drug resistance, off-target effects of antiretroviral therapy (ART), and varying efficacy in prevention underscore the need to develop novel and alternative therapeutics. In this study, we investigated whether targeting the signaling molecule Sphingosine-1-phosphate (S1P) would inhibit HIV-1 infection and generation of the latent reservoir in primary CD4 T cells. We show that FTY720 (Fingolimod), an FDA-approved functional antagonist of S1P receptors, blocks cell-free and cell-to-cell transmission of HIV and consequently reduces detectable latent virus. Mechanistically, FTY720 impacts the HIV-1 life cycle at two levels. Firstly, FTY720 reduces the surface density of CD4, thereby inhibiting viral binding and fusion. Secondly, FTY720 decreases the phosphorylation of the innate HIV restriction factor SAMHD1 which is associated with reduced levels of total and integrated HIV, while reducing the expression of Cyclin D3. In conclusion, targeting the S1P pathway with FTY720 could be a novel strategy to inhibit HIV replication and reduce the seeding of the latent reservoir.

Glycosphingolipid-Protein Interaction in Signal Transduction

Glycosphingolipids (GSLs) are a class of ceramide-based glycolipids essential for embryo development in mammals. The synthesis of specific GSLs depends on the expression of distinctive sets of GSL synthesizing enzymes that is tightly regulated during development. Several reports have described how cell surface receptors can be kept in a resting state or activate alternative signalling events as a consequence of their interaction with GSLs. Specific GSLs, indeed, interface with specific protein domains that are found in signalling molecules and which act as GSL sensors to modify signalling responses. The regulation exerted by GSLs on signal transduction is orthogonal to the ligand–receptor axis, as it usually does not directly interfere with the ligand binding to receptors. Due to their properties of adjustable production and orthogonal action on receptors, GSLs add a new dimension to the control of the signalling in development. GSLs can, indeed, dynamically influence progenitor cell response to morphogenetic stimuli, resulting in alternative differentiation fates. Here, we review the available literature on GSL–protein interactions and their effects on cell signalling and development.

C-glycosphingolipid precursors via iodocyclization of homoallyic trichloroacetimidates

The iodocyclization of homoallylic trichloroacetimidates derived from α-C-allyl galactoside were investigated. In line with the stereochemical trend observed for less substituted non-glycosylated frameworks, E and Z substrates delivered stereoselectively the 1,3-anti and 1,3-syn amino alcohol motifs, respectively. These products are advanced precursors to C-glycosides of the potent immunostimulatory glycolipid KRN7000.

Membrane organization of virus and target cell plays a role in HIV entry

The initial steps of the Human Immunodeficiency Virus (HIV) replication cycle play a crucial role that arbitrates viral tropism and infection efficiency. Before the release of its genome into the host cell cytoplasm, viruses operate a complex sequence of events that take place at the plasma membrane of the target cell. The first step is the binding of the HIV protein envelope (Env) to the cellular receptor CD4. This triggers conformational changes of the gp120 viral protein that allow its interaction with a co-receptor that can be either CCR5 or CXCR4, defining the tropism of the virus entering the cell. This sequential interaction finally drives the fusion of the viral and host cell membrane or to the endocytosis of the viruses. Here, we discuss how the membrane composition and organization of both the virus and the target cell can affect these steps and thus influence the capability of the viruses to infect cells.

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An overview of lipid role in HIV infection is proposed.

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We discuss the influence of lipid composition on HIV early steps of infection.

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We discuss the role of membrane organization an dynamics in HIV entry.

Modulation of hepatitis C virus genome replication by glycosphingolipids and four-phosphate adaptor protein 2

Hepatitis C virus (HCV) assembles its replication complex on cytosolic membrane vesicles often clustered in a membranous web (MW). During infection, HCV NS5A protein activates PI4KIIIα enzyme, causing massive production and redistribution of phosphatidylinositol 4-phosphate (PI4P) lipid to the replication complex. However, the role of PI4P in the HCV life cycle is not well understood. We postulated that PI4P recruits host effectors to modulate HCV genome replication or virus particle production. To test this hypothesis, we generated cell lines for doxycycline-inducible expression of short hairpin RNAs (shRNAs) targeting the PI4P effector, four-phosphate adaptor protein 2 (FAPP2). FAPP2 depletion attenuated HCV infectivity and impeded HCV RNA synthesis. Indeed, FAPP2 has two functional lipid-binding domains specific for PI4P and glycosphingolipids. While expression of the PI4P-binding mutant protein was expected to inhibit HCV replication, a marked drop in replication efficiency was observed unexpectedly with the glycosphingolipid-binding mutant protein. These data suggest that both domains are crucial for the role of FAPP2 in HCV genome replication. We also found that HCV significantly increases the level of some glycosphingolipids, whereas adding these lipids to FAPP2-depleted cells partially rescued replication, further arguing for the importance of glycosphingolipids in HCV RNA synthesis. Interestingly, FAPP2 is redistributed to the replication complex (RC) characterized by HCV NS5A, NS4B, or double-stranded RNA (dsRNA) foci. Additionally, FAPP2 depletion disrupts the RC and alters the colocalization of HCV replicase proteins. Altogether, our study implies that HCV coopts FAPP2 for virus genome replication via PI4P binding and glycosphingolipid transport to the HCV RC.

IMPORTANCE

Like most viruses with a positive-sense RNA genome, HCV replicates its RNA on remodeled host membranes composed of lipids hijacked from various internal membrane compartments. During infection, HCV induces massive production and retargeting of the PI4P lipid to its replication complex. However, the role of PI4P in HCV replication is not well understood. In this study, we have shown that FAPP2, a PI4P effector and glycosphingolipid-binding protein, is recruited to the HCV replication complex and is required for HCV genome replication and replication complex formation. More importantly, this study demonstrates, for the first time, the crucial role of glycosphingolipids in the HCV life cycle and suggests a link between PI4P and glycosphingolipids in HCV genome replication.

Intramolecular nitrogen delivery for the synthesis of C-glycosphingolipids. Application to the C-glycoside of the immunostimulant KRN7000

The key reaction in this approach to C-glycosphingolipids is the stereoselective iodocyclization of a sugar-linked homoallylic carbonimidothioate. E and Z reaction substrates were assembled in a convergent fashion via an alkene metathesis strategy and exhibited the same alkene facial selectivity in the iodocyclization irrespective of alkene geometry, although the E alkene was found to be less reactive.

The productive entry pathway of HIV-1 in macrophages is dependent on endocytosis through lipid rafts containing CD4

Macrophages constitute an important reservoir of HIV-1 infection, yet HIV-1 entry into these cells is poorly understood due to the difficulty in genetically manipulating primary macrophages. We developed an effective genetic approach to manipulate the sub-cellular distribution of CD4 in macrophages, and investigated how this affects the HIV-1 entry pathway. Pluripotent Stem Cells (PSC) were transduced with lentiviral vectors designed to manipulate CD4 location and were then differentiated into genetically modified macrophages. HIV-1 infection of these cells was assessed by performing assays that measure critical steps of the HIV-1 lifecycle (fusion, reverse transcription, and expression from HIV-1 integrants). Expression of LCK (which tethers CD4 to the surface of T cells, but is not normally expressed in macrophages) in PSC-macrophages effectively tethered CD4 at the cell surface, reducing its normal endocytic recycling route, and increasing surface CD4 expression 3-fold. This led to a significant increase in HIV-1 fusion and reverse transcription, but productive HIV-1 infection efficiency (as determined by reporter expression from DNA integrants) was unaffected. This implies that surface-tethering of CD4 sequesters HIV-1 into a pathway that is unproductive in macrophages. Secondly, to investigate the importance of lipid rafts (as detergent resistant membranes - DRM) in HIV-1 infection, we generated genetically modified PSC-macrophages that express CD4 mutants known to be excluded from DRM. These macrophages were significantly less able to support HIV-1 fusion, reverse-transcription and integration than engineered controls. Overall, these results support a model in which productive infection by HIV-1 in macrophages occurs via a CD4-raft-dependent endocytic uptake pathway.

Role of glycosphingolipids in dendritic cell-mediated HIV-1 trans-infection

Glycosphingolipids (GSLs) are components of the cell membrane that comprise a membrane bound lipid, ceramide, coupled to an extracellular carbohydrate. GSLs impact numerous aspects of membrane biology, including membrane fluidity, curvature, and organization. The role of these molecules in both chronic inflammation and infectious disease and underlying pathogenic mechanisms are just starting to be recognized. As a component of the cell membrane, GSLs are also incorporated into lipid bilayers of diverse enveloped viruses as they bud out from the host cell and can go on to have a significant influence on viral pathogenesis. Dendritic cell (DC) subsets located in the peripheral mucosal tissues are proposed to be one of the earliest cell types that encounter transmitted viruses and help initiate adaptive immune responses against the invading pathogen by interacting with T cells. In turn, viruses, as obligatory intracellular parasites, rely on host cells for completing their replication cycle, and not surprisingly, HIV has evolved to exploit DC biology for the initial transmission event as well as for its dissemination and propagation within the infected host. In this review, we describe the mechanisms by which GSLs impact DC-mediated HIV trans-infection by either modulating virus infectivity, serving as a direct virus particle-associated host-derived ligand for specific interactions with DCs, or modulating the T cell membrane in such a way as to impact viral entry and thereby productive infection of CD4(+) T cells.

Sphingopeptides: dihydrosphingosine-based fusion inhibitors against wild-type and enfuvirtide-resistant HIV-1

Understanding the structural organization of lipids in the cell and viral membranes is essential for elucidating mechanisms of viral fusion that lead to entry of enveloped viruses into their host cells. The HIV lipidome shows a remarkable enrichment in dihydrosphingomyelin, an unusual sphingolipid formed by a dihydrosphingosine backbone. Here we investigated the ability of dihydrosphingosine to incorporate into the site of membrane fusion mediated by the HIV envelope (Env) protein. Dihydrosphingosine as well as cholesterol, fatty acid, and tocopherol was conjugated to highly conserved, short HIV‐1 Env‐derived peptides with no antiviral activity otherwise. We showed that dihydrosphingosine exclusively endowed nanomolar antiviral activity to the peptides (IC₅₀ as low as 120 nM) in HIV‐1 infection on TZM‐bl cells and on Jurkat T cells, as well as in the cell‐cell fusion assay. These sphingopeptides were active against enfuvirtide‐resistant and wild‐type CXCR4 and CCR5 tropic HIV strains. The anti‐HIV activity was determined by both the peptides and their dihydrosphingosine conjugate. Moreover, their mode of action involved accumulation in the cells and viruses and binding to membranes enriched in sphingomyelin and cholesterol. The data suggest that sphingopeptides are recruited to the HIV membrane fusion site and provide a general concept in developing inhibitors of sphingolipid‐mediated biological systems.—Ashkenazi, A., Viard, M., Unger, L., Blumenthal, R., Shai, Y. Sphingopeptides: dihydrosphingosine‐based fusion inhibitors against wild‐type and enfuvirtide‐resistant HIV‐1. FASEB J. 26, 4628–4636 (2012). http://www.fasebj.org

Selected amino acid changes in HIV-1 subtype-C gp41 are associated with specific gp120(V3) signatures in the regulation of co-receptor usage

The majority of studies have characterized the tropism of HIV-1 subtype-B isolates, but little is known about the determinants of tropism in other subtypes. So, the goal of the present study was to genetically characterize the envelope of viral proteins in terms of co-receptor usage by analyzing 356 full-length env sequences derived from HIV-1 subtype-C infected individuals. The co-receptor usage of V3 sequences was inferred by using the Geno2Pheno and PSSM algorithms, and also analyzed to the "11/25 rule". All reported env sequences were also analyzed with regard to N-linked glycosylation sites, net charge and hydrophilicity, as well as the binomial correlation phi coefficient to assess covariation among gp120(V3) and gp41 signatures and the average linkage hierarchical agglomerative clustering were also performed. Among env sequences present in Los Alamos Database, 255 and 101 sequences predicted as CCR5 and CXCR4 were selected, respectively. The classical V3 signatures at positions 11 and 25, and other specific V3 and gp41 amino acid changes were found statistically associated with different co-receptor usage. Furthermore, several statistically significant associations between V3 and gp41 signatures were also observed. The dendrogram topology showed a cluster associated with CCR5-usage composed by five gp41 mutated positions, A22V, R133M, E136G, N140L, and N166Q that clustered with T2V(V3) and G24T(V3) (bootstrap=1). Conversely, a heterogeneous cluster with CXCR4-usage, involving S11GR(V3), 13-14insIG/LG(V3), P16RQ(V3), Q18KR(V3), F20ILV(V3), D25KRQ(V3), Q32KR(V3) along with A30T(gp41), S107N(gp41), D148E(gp41), A189S(gp41) was identified (bootstrap=0.86). Our results show that as observed for HIV-1 subtype-B, also in subtype-C specific and different gp41 and gp120V3 amino acid changes are associated individually or together with CXCR4 and/or CCR5 usage. These findings strengthen previous observations that determinants of tropism may also reside in the gp41 protein.

Synthesis, gp120 binding and anti-HIV activity of fatty acid esters of 1,1-linked disaccharides

Inspired by the anti-human immunodeficiency virus (HIV) activity of analogues of β-galactosylceramide (GalCer), a set of mono- and di-saccharide fatty acid esters were designed as GalCer mimetics and their binding to the V3 loop peptide of HIV-1 and anti-HIV activity evaluated. 1,1-linked Gal-Man and Glu-Man disaccharides with an ester on the Man subunit bound the V3 loop peptide and inhibited HIV infectivity in single round infection assays with the TZM-bl cell line. IC(50)'s were in the 50 μM range with no toxicity to the cells at concentrations up to 200 μM. These compounds appear to inhibit virus entry at early steps in viral infection since they were inactive if added post viral entry. Although these compounds were found to bind to the V3 loop peptide of gp120, it is not clear that this interaction is responsible for their anti-HIV activity because the relative binding affinity of closely related analogues did not correlate with their antiviral behavior. The low cytotoxicity of these 1,1-linked disaccharide fatty acid esters, combined with the easy accessibility to structurally diverse analogues make these molecules attractive leads for new topical anti-viral agents.

Roles for biological membranes in regulating human immunodeficiency virus replication and progress in the development of HIV therapeutics that target lipid metabolism

Infection by the human immunodeficiency virus (HIV) involves a number of important interactions with lipid components in host membranes that regulate binding, fusion, internalization, and viral assembly. Available data suggests that HIV actively modifies the sphingolipid content of cellular membranes to create focal environments that are favorable for infection. In this review, we summarize the roles that membrane lipids play in HIV infection and discuss the current status of therapeutics that attempt to modify biological membranes to inhibit HIV.

The Role of Lipids in Retrovirus Replication

Retroviruses undergo several critical steps to complete a replication cycle. These include the complex processes of virus entry, assembly, and budding that often take place at the plasma membrane of the host cell. Both virus entry and release involve membrane fusion/fission reactions between the viral envelopes and host cell membranes. Accumulating evidence indicates important roles for lipids and lipid microdomains in virus entry and egress. In this review, we outline the current understanding of the role of lipids and membrane microdomains in retroviral replication.

Lipids and membrane microdomains in HIV-1 replication

Several critical steps in the replication cycle of human immunodeficiency virus type 1 (HIV-1) - entry, assembly and budding - are complex processes that take place at the plasma membrane of the host cell. A growing body of data indicates that these early and late steps in HIV-1 replication take place in specialized plasma membrane microdomains, and that many of the viral and cellular components required for entry, assembly, and budding are concentrated in these microdomains. In particular, a number of studies have shown that cholesterol- and sphingolipid-enriched microdomains known as lipid rafts play important roles in multiple steps in the virus replication cycle. In this review, we provide an overview of what is currently known about the involvement of lipids and membrane microdomains in HIV-1 replication.

Glycoside analogs of beta-galactosylceramide, a novel class of small molecule antiviral agents that inhibit HIV-1 entry

The interaction between HIV gp120 and galactose-containing cell surface glycolipids such as GalCer or Gb3 is known to facilitate HIV binding to both CD4+ as well as CD4- cells. In an effort to develop small molecule HIV-1 entry inhibitors with improved solubility and efficacy, we have synthesized a series of C-glycoside analogs of GalCer and tested their anti HIV-1 activity. The analogs were tested for gp120 binding using a HIV-1 (IIIB) V3-loop specific peptide. Two of the six analogs that interfered with gp120 binding also inhibited HIV Env-mediated cell-to-cell fusion and viral entry in the absence of any significant cytotoxicity. Analogs with two side chains did not show inhibition of fusion and/or infection under identical conditions. The inhibition of virus infection seen by these compounds was not coreceptor dependent, as they inhibited CXCR4, CCR5 as well as dual tropic viruses. These compounds showed inhibition of HIV entry at early steps in viral infection since the compounds were inactive if added post viral entry. Temperature-arrested state experiments showed that the compounds act at the level of virus attachment to the cells likely at a pre-CD4 engagement step. These compounds also showed inhibition of VSV glycoprotein-pseudotyped virus. The results presented here show that the glycoside derivatives of GalCer with simple side chains may serve as a novel class of small molecule HIV-1 entry inhibitors that would be active against a number of HIV isolates as well as other enveloped viruses.

Restricted lateral mobility of plasma membrane CD4 impairs HIV-1 envelope glycoprotein mediated fusion

We investigated the effect of receptor mobility on HIV-1 envelope glycoprotein (Env)-triggered fusion using B16 mouse melanoma cells that are engineered to express CD4 and CXCR4 or CCR5. These engineered cells are resistant to fusion mediated CD4-dependent HIV-1 envelope glycoprotein. Receptor mobility was measured by fluorescence recovery after photobleaching (FRAP) using either fluorescently-labeled antibodies or transient expression of GFP-tagged receptors in the cells. No significant differences between B16 and NIH3T3 (fusion-permissive) cells were seen in lateral mobility of CCR5 or lipid probes. By contrast CD4 mobility in B16 cells was about seven-fold reduced compared to its mobility in fusion-permissive NIH3T3 cells. However, a CD4 mutant (RA5) that localizes to non-raft membrane microdomains exhibited a three-fold increased mobility in B16 cells as compared with WT-CD4. Interestingly, the B16 cells expressing the RA5 mutant (but not the wild type CD4) and coreceptors supported HIV-1 Env-mediated fusion. Our data demonstrate that the lateral mobility of CD4 is an important determinant of HIV-1 fusion/entry.

HIV-1 envelope glycoprotein-mediated fusion and pathogenesis: implications for therapy and vaccine development

Our overall goal is to understand how viral envelope proteins mediate membrane fusion and pathogenesis. Membrane fusion is a crucial step in the delivery of the viral genome into the cell resulting in infection. On the other hand, fusion activity of viral envelope glycoproteins expressed in infected cells may cause the demise of uninfected bystander cells by apoptosis. Our general approach is to kinetically resolve steps in the pathway of viral envelope glycoprotein-mediated membrane fusion and to uncover physical parameters underlying those steps using a variety of biochemical, biophysical, virological, and molecular and cell biological techniques. Since HIV fusion involves a complex cascade of interactions of the envelope glycoprotein with two receptors, membrane organization plays an important role and interfering with it may modulate entry. To study this phenomenon, we have either examined cell lines with differential expression of sphingolipids (such as GM3), or altered membrane organization by modifying levels of cholesterol, ceramides, or glycosphingolipids. We show that the localized plasma membrane lipid microenvironment (and not the specific membrane lipids) in the vicinity of CD4 controls receptor mobility and HIV-1 fusion. The complex cascade of conformational changes that must occur to allow virus entry is also a very important target for therapy and vaccine development. We have recently designed and tested peptide analogs composed of chemical spacers and reactive moieties positioned strategically to promote permanent attachment. Using a temperature-arrested state in vitro assay we show evidence for the trapping of a pre-six-helix bundle fusion intermediate by a covalent reaction with the inhibitory reactive peptide. Also, using photo-reactive hydrophobic probes we have found ways to inactivate viral envelope glycoproteins while leaving their overall structures intact. Finally, in order to study the envelope glycoprotein effects on pathogenesis, we have used an in vitro model of co-culture of envelope-expressing cells as effectors and CD4+ T cells as targets. We delineated that apoptosis mediated by envelope glycoprotein in bystander cells correlates with transmembrane subunit (gp41)-induced hemifusion. The apoptotic pathway initiated by this interaction involves caspase-3-dependent mitochondrial depolarization and reactive oxygen species production, which depends on the phenotype of the envelope glycoprotein associated with the virus. Taken as a whole, our studies have many different important implications for antiviral therapies and vaccine development.

Lipids as modulators of membrane fusion mediated by viral fusion proteins

Enveloped viruses infect host cells by fusion of viral and target membranes. This fusion event is triggered by specific glycoproteins in the viral envelope. Fusion glycoproteins belong to either class I, class II or the newly described third class, depending upon their arrangement at the surface of the virion, their tri-dimensional structure and the location within the protein of a short stretch of hydrophobic amino acids called the fusion peptide, which is able to induce the initial lipid destabilization at the onset of fusion. Viral fusion occurs either with the plasma membrane for pH-independent viruses, or with the endosomal membranes for pH-dependent viruses. Although, viral fusion proteins are parted in three classes and the subcellular localization of fusion might vary, these proteins have to act, in common, on lipid assemblies. Lipids contribute to fusion through their physical, mechanical and/or chemical properties. Lipids can thus play a role as chemically defined entities, or through their preferential partitioning into membrane microdomains called “rafts”, or by modulating the curvature of the membranes involved in the fusion process. The purpose of this review is to make a state of the art on recent findings on the contribution of cholesterol, sphingolipids and glycolipids in cell entry and membrane fusion of a number of viral families, whose members bear either class I or class II fusion proteins, or fusion proteins of the recently discovered third class.

Recent advances in the glycosylation of sphingosines and ceramides

Glycosphingolipids (GSLs) are ubiquitous components of eukaryotic cell membranes. They are highly bioactive and are involved in many aspects of cell signalling like cell-cell interaction, cell-substratum interaction and cell-pathogen interaction. GSLs also are involved in the modulation of signal transduction, resulting in regulation of cell proliferation and differentiation. The biological importance and complexity of these compounds afford many opportunities to prepare synthetic analogues for studies of their metabolism in intra- and intercellular processes. This review focuses on recent contributions in the synthesis of GSLs, highlighting improvements in glycosylation reactions leading to alpha and beta glycosyl sphingosines and ceramides and related compounds. Literature from 2000 to the present is covered. The glycosylation reactions leading to the synthesis of GSLs are classified in function of the configuration of the created glycosidic bond (alpha or beta) and of the acceptor used, either azido-sphingosine or ceramide.

Sphingomyelinase restricts the lateral diffusion of CD4 and inhibits human immunodeficiency virus fusion

Previously, we reported that treatment of cells with sphingomyelinase inhibits human immunodeficiency virus type 1 (HIV-1) entry. Here, we determined by measuring fluorescence recovery after photobleaching that the lateral diffusion of CD4 decreased 4-fold following sphingomyelinase treatment, while the effective diffusion rate of CCR5 remained unchanged. Notably, sphingomyelinase treatment of cells did not influence gp120 binding, HIV-1 attachment, or fluid-phase and receptor-mediated endocytosis. Furthermore, sphingomyelinase treatment did not affect the membrane disposition of the HIV receptor proteins CD4, CXCR4, and CCR5, as determined by Triton X-100 extraction. Restriction of CD4 diffusion by antibody cross-linking also inhibited HIV infection. We therefore interpret the decrease in CD4 lateral mobility following sphingomyelinase treatment in terms of clustering of CD4 molecules. Examination of fusion intermediates indicated that sphingomyelinase treatment inhibited HIV at a step in the fusion process after CD4 engagement. Maximal inhibition of fusion was observed following short coculture times and with target cells that express low levels of CD4. As HIV entry into cells requires the sequential engagement of viral envelope protein with CD4 and coreceptor, we propose that sphingomyelinase inhibits HIV infection by inducing CD4 clustering that prevents coreceptor engagement and HIV fusion.

Synchrotron SAX and WAX diffraction study of a hydrated very long-chain, dendritic amphiphile+DPPC mixture

The tri-headed anionic dendritic amphiphile, 4-(2-carboxyethyl)-4-[(icosyloxycarbonyl)amino]heptanedioic acid (3CCb20), forms mixed aggregates with dipalmitoylphosphatidylcholine (DPPC) in excess water at 3CCb20:DPPC = 0.91:1 molar ratio. On heating, these mixed aggregates transform into fluid bilayers stacked in the liquid crystalline lamellar L(alpha) phase at about 40 degrees C. This phase transition and the microstructure of 3CCb20 + DPPC aggregates were studied with small- and wide-angle synchrotron X-ray diffraction. The ability of 3CCb20 to solubilize solidlike lipid bilayers could contribute to the antimicrobial activities of 3CCb20, including its anti-HIV activity.

Sphingolipids in apoptosis, survival and regeneration in the nervous system

Simple sphingolipids such as ceramide, sphingosine and sphingosine 1-phosphate are key regulators of diverse cellular functions. Their roles in the nervous system are supported by extensive evidence derived primarily from studies in cultured cells. More recently animal studies and studies with human samples have revealed the importance of ceramide and its metabolites in the development and progression of neurodegenerative disorders. The roles of sphingolipids in neurons and glial cells are complex, cell dependent, and many times contradictory. In this review I will summarize the effects elicited by ceramide and ceramide metabolites in cells of the nervous system, in particular those effects related to cell survival and death, emphasizing the molecular mechanisms involved. I also discuss recent evidence for the implication of sphingolipids in the development and progression of certain dementias.

Yeast sphingolipids: recent developments in understanding biosynthesis, regulation, and function

Sphingolipids function as required membrane components of virtually all eukaryotic cells. Data indicate that members of the sphingolipid family of lipids, including sphingoid bases, sphingoid base phosphates, ceramides, and complex sphingolipids, serve vital functions in cell biology by both direct mechanisms (e.g., binding to G-protein coupled receptors to transduce an extracellular signal) and indirect mechanisms (e.g., facilitating correct intracellular protein transport). Because of the diverse roles these lipids play in cell biology, it is important to understand not only their biosynthetic pathways and regulation of sphingolipid synthesis, but also the mechanisms by which some sphingolipid species with specific functions are modified or converted to other sphingolipid species with alternate functions. Due to many factors including ease of culture and genetic modification, and conservation of major sphingolipid metabolic pathways, Saccharomyces cerevisiae has served as an ideal model system with which to identify enzymes of sphingolipid biosynthesis and to dissect sphingolipid function. Recent exciting developments in sphingolipid synthesis, transport, signaling, and overall biology continue to fuel vigorous investigation and inspire investigations in mammalian sphingolipid biology.