SPC3, a V3 loop-derived synthetic peptide inhibitor of HIV-1 infection, binds to cell surface glycosphingolipids

HIV and SIV Envelope Glycoproteins Interact with Glycolipids and Lipids

The present study demonstrates that, in addition to interacting with galactosylceramide (GalCer), HIV-1, HIV-2, and SIV envelope glycoproteins are able to interact with glucosylceramide (GlcCer), lactosylceramide (LacCer), and ceramide. These interactions were characterized by using three complementary approaches based on molecular binding and physicochemical assays. The binding assays showed that iodinated radiolabeled HIV-1 and HIV-2 glycoproteins (¹²⁵I-gp) interact physically with GalCer, GlcCer, LacCer, and ceramide previously separated by thin layer chromatography (TLC) or directly coated on a flexible 96-well plate. These interactions are specific as demonstrated, on the one hand, by the dose-dependent inhibition in the presence of various dilutions of immune, but not non-immune, sera, and, on the other hand, by the absence of interaction of these glycolipids/lipids with ¹²⁵I-IgG used as an unrelated control protein. These interactions were further confirmed in a physicochemical assay, based on the capacity of these glycolipids for insertion in a pre-established monomolecular film, as a model of the cell membrane, with each glycolipid/lipid. The addition of HIV envelope glycoproteins, but not ovomucoid protein used as a negative control, resulted in a rapid increase in surface pressure of the glycolipid/lipid films, thus indirectly confirming their interactions with GalCer, GlcCer, LacCer, and ceramide. In summary, we show that HIV and SIV envelope glycoproteins bind to GalCer, GlcCer, LacCer, and ceramide in a dose-dependent, saturable, and specific manner. These interactions may function as receptors of attachment in order to facilitate infection of CD4 low or negative cells or promote interactions with other receptors leading to the activation of signaling pathways or pathogenesis.

Electrostatic Surface Potential as a Key Parameter in Virus Transmission and Evolution: How to Manage Future Virus Pandemics in the Post-COVID-19 Era

Virus-cell interactions involve fundamental parameters that need to be considered in strategies implemented to control viral outbreaks. Among these, the surface electrostatic potential can give valuable information to deal with new epidemics. In this article, we describe the role of this key parameter in the hemagglutination of red blood cells and in the co-evolution of synaptic receptors and neurotransmitters. We then establish the functional link between lipid rafts and the electrostatic potential of viruses, with special emphasis on gangliosides, which are sialic-acid-containing, electronegatively charged plasma membrane components. We describe the common features of ganglioside binding domains, which include a wide variety of structures with little sequence homology but that possess key amino acids controlling ganglioside recognition. We analyze the role of the electrostatic potential in the transmission and intra-individual evolution of HIV-1 infections, including gatekeeper and co-receptor switch mechanisms. We show how to organize the epidemic surveillance of influenza viruses by focusing on mutations affecting the hemagglutinin surface potential. We demonstrate that the electrostatic surface potential, by modulating spike-ganglioside interactions, controls the hemagglutination properties of coronaviruses (SARS-CoV-1, MERS-CoV, and SARS-CoV-2) as well as the structural dynamics of SARS-CoV-2 evolution. We relate the broad-spectrum antiviral activity of repositioned molecules to their ability to disrupt virus-raft interactions, challenging the old concept that an antibiotic or anti-parasitic cannot also be an antiviral. We propose a new concept based on the analysis of the electrostatic surface potential to develop, in real time, therapeutic and vaccine strategies adapted to each new viral epidemic.

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.

A Vaccine Strategy Based on the Identification of an Annular Ganglioside Binding Motif in Monkeypox Virus Protein E8L

The recent outbreak of Monkeypox virus requires the development of a vaccine specifically directed against this virus as quickly as possible. We propose here a new strategy based on a two-step analysis combining (i) the search for binding domains of viral proteins to gangliosides present in lipid rafts of host cells, and (ii) B epitope predictions. Based on previous studies of HIV and SARS-CoV-2 proteins, we show that the Monkeypox virus cell surface-binding protein E8L possesses a ganglioside-binding motif consisting of several subsites forming a ring structure. The binding of the E8L protein to a cluster of gangliosides GM1 mimicking a lipid raft domain is driven by both shape and electrostatic surface potential complementarities. An induced-fit mechanism unmasks selected amino acid side chains of the motif without significantly affecting the secondary structure of the protein. The ganglioside-binding motif overlaps three potential linear B epitopes that are well exposed on the unbound E8L surface that faces the host cell membrane. This situation is ideal for generating neutralizing antibodies. We thus suggest using these three sequences derived from the E8L protein as immunogens in a vaccine formulation (recombinant protein, synthetic peptides or genetically based) specific for Monkeypox virus. This lipid raft/ganglioside-based strategy could be used for developing therapeutic and vaccine responses to future virus outbreaks, in parallel to existing solutions.

Ganglioside binding domains in proteins: Physiological and pathological mechanisms

Gangliosides are anionic lipids that form condensed membrane clusters (lipid rafts) and exert major regulatory functions on a wide range of proteins. In this review, we propose a new view of the structural features of gangliosides with special emphasis on emerging properties associated with protein binding modes. We analyze the different possibilities of molecular associations of gangliosides in lipid rafts and the role of cholesterol in this organization. We are particularly interested in amide groups of N-acetylated sugars which make it possible to neutralize the negative charge of the carboxylate group of sialic acids. We refer to this effect as "NH trick" and we demonstrate that it is operative in GM1, GD1a, GD1b and GT1b gangliosides. The NH trick is key to understand the different topologies adopted by gangliosides (chalice-like at the edge of lipid rafts, condensed clusters in central areas) and their impact on protein binding. We define three major types of ganglioside-binding domains (GBDs): α-helical, loop shaped, and large flat surface. We describe the mode of interaction of each GBD with typical reference proteins: synaptotagmin, 5HT1A receptor, cholera and botulinum toxins, HIV-1 surface envelope glycoprotein gp120, SARS-CoV-2 spike protein, cellular prion protein, Alzheimer's β-amyloid peptide and Parkinson's disease associated α-synuclein. We discuss the common mechanisms and peculiarities of protein binding to gangliosides in the light of physiological and pathological conditions. We anticipate that innovative ganglioside-based therapies will soon show an exponential growth for the treatment of cancer, microbial infections, and neurodegenerative diseases.

Structural, in silico, and functional analysis of a Disabled-2-derived peptide for recognition of sulfatides

Disabled-2 (Dab2) is an adaptor protein that regulates the extent of platelet aggregation by two mechanisms. In the first mechanism, Dab2 intracellularly downregulates the integrin α_IIbβ₃ receptor, converting it to a low affinity state for adhesion and aggregation processes. In the second mechanism, Dab2 is released extracellularly and interacts with the pro-aggregatory mediators, the integrin α_IIbβ₃ receptor and sulfatides, blocking their association to fibrinogen and P-selectin, respectively. Our previous research indicated that a 35-amino acid region within Dab2, which we refer to as the sulfatide-binding peptide (SBP), contains two potential sulfatide-binding motifs represented by two consecutive polybasic regions. Using molecular docking, nuclear magnetic resonance, lipid-binding assays, and surface plasmon resonance, this work identifies the critical Dab2 residues within SBP that are responsible for sulfatide binding. Molecular docking suggested that a hydrophilic region, primarily mediated by R42, is responsible for interaction with the sulfatide headgroup, whereas the C-terminal polybasic region contributes to interactions with acyl chains. Furthermore, we demonstrated that, in Dab2 SBP, R42 significantly contributes to the inhibition of platelet P-selectin surface expression. The Dab2 SBP residues that interact with sulfatides resemble those described for sphingolipid-binding in other proteins, suggesting that sulfatide-binding proteins share common binding mechanisms.

Sperm can act as vectors for HIV-1 transmission into vaginal and cervical epithelial cells

PROBLEM

Sperm are the major cells in semen. Human sperm possess a number of HIV-1 gp120 binding ligands including sulfogalactosylglycerolipid (SGG). However, the mechanisms of how sperm capture HIV-1 onto their surface are unclear. Furthermore, the ability of sperm to deliver HIV-1 to vaginal/cervical epithelial cells lining the lower female reproductive tract, as a first step in HIV-1 transmission, needs to be determined.

METHOD OF STUDY

Sperm from healthy donors were incubated with dual-tropic HIV-1_CS204 (clinical isolate), and virus capture was determined by p24 antigen ELISA. The involvement of SGG in HIV-1 capture was assessed by determining K_d values of HIV-1 gp120-SGG binding as well as computational docking of SGG to the gp120 V3 loop. The ability of sperm-associated HIV-1 to infect peripheral blood mononuclear cells (PBMCs) and TZM-bl indicator cells was determined. Lastly, infection of vaginal (Vk2/E6E7), ectocervical (Ect1/E6E7), and endocervical (End1/E6E7) epithelial cells mediated by HIV-1-associated sperm was evaluated.

RESULTS

Sperm were able to capture HIV-1 in a dose-dependent manner, and the capture reached a maximum within 5 minutes. Captured HIV-1, however, could be removed from sperm by Percoll-gradient centrifugation. Affinity of gp120 for SGG was substantial, implicating sperm SGG in HIV-1 capture. Sperm-associated HIV-1 could productively infect PBMCs and TZM-bl cells, and was capable of being transmitted into vaginal/cervical epithelial cells.

CONCLUSION

Sperm are able to capture HIV-1, which remains infectious and is able to be transmitted into vaginal/cervical epithelial cells, a result indicating the importance of sperm in HIV transmission.

Mapping out the intricate relationship of the HIV envelope protein and the membrane environment

The HIV gp160 envelope fusion protein is situated in the viral membrane and mediates virus entry into its host cell. Increasing evidence suggests that virtually all parts of the HIV envelope are structurally and functionally dependent on membranes. Protein-lipid interactions and membrane properties influence the dynamics of a manifold of gp160 biological activities such as membrane fusion, immune suppression and gp160 incorporation into virions during HIV budding and assembly. In the following we will summarize our current understanding of this interdependence between membrane interaction, structural conformation and functionality of the different gp160 domains. This article is part of a Special Issue entitled: Lipid order/lipid defects and lipid-control of protein activity edited by Dirk Schneider.

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.

Role of sulfatide in influenza A virus replication

Sulfatide is a 3-O-sulfated galactosylceramide that is abundantly expressed in the gastrointestinal tract, kidney, trachea, and particularly the central nervous system. Cellular sulfatide is mainly localized in the Golgi apparatus, cellular membrane, and lysosomes in cytosol. Since our earlier report showed that the influenza A virus specifically binds to sulfatide, we have investigated the roles of sulfatide in the influenza A virus lifecycle. The viral binding is independent of sialic acids, which function as virus receptors in virus attachment to the host cell surface. Sulfatide is recognized by the ectodomain of the viral envelope glycoprotein hemagglutinin (HA). Nascent HA is transported on the surface membrane of infected cells. The binding of HA with sulfatide on the cell surface induces apoptosis through potential loss of the mitochondrial membrane and nuclear translocation of apoptosis-inducing factor in mitochondria, where PB1-F2 peptide from the viral gene is accumulated. In the nucleus of infected cells, viral ribonucleoprotein (vRNP) complexes are formed from viral RNA genomes, viral nucleoprotein, and viral RNA polymerase subunits, and these complexes are selectively exported into cytosol through the nuclear membrane. The apoptosis significantly enhances the nuclear export of vRNP complexes, resulting in efficient formation of progeny viruses and facilitation of virus replication. At that time, activation of the Raf/mitogen-activated protein extracellular kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway through sulfatide is associated with virus replication. Our studies have demonstrated that sulfatide is not a viral receptor for virus infection, and that the binding of HA with sulfatide functions as an initiation switch for the formation of progeny viruses.

Aβ1-25-Derived Sphingolipid-Domain Tracer Peptide SBD Interacts with Membrane Ganglioside Clusters via a Coil-Helix-Coil Motif

The Amyloid-β (Aβ)-derived, sphingolipid binding domain (SBD) peptide is a fluorescently tagged probe used to trace the diffusion behavior of sphingolipid-containing microdomains in cell membranes through binding to a constellation of glycosphingolipids, sphingomyelin, and cholesterol. However, the molecular details of the binding mechanism between SBD and plasma membrane domains remain unclear. Here, to investigate how the peptide recognizes the lipid surface at an atomically detailed level, SBD peptides in the environment of raft-like bilayers were examined in micro-seconds-long molecular dynamics simulations. We found that SBD adopted a coil-helix-coil structural motif, which binds to multiple GT1b gangliosides via salt bridges and CH–π interactions. Our simulation results demonstrate that the CH–π and electrostatic forces between SBD monomers and GT1b gangliosides clusters are the main driving forces in the binding process. The presence of the fluorescent dye and linker molecules do not change the binding mechanism of SBD probes with gangliosides, which involves the helix-turn-helix structural motif that was suggested to constitute a glycolipid binding domain common to some sphingolipid interacting proteins, including HIV gp120, prion, and Aβ.

Blood Groups in Infection and Host Susceptibility

Blood group antigens represent polymorphic traits inherited among individuals and populations. At present, there are 34 recognized human blood groups and hundreds of individual blood group antigens and alleles. Differences in blood group antigen expression can increase or decrease host susceptibility to many infections. Blood groups can play a direct role in infection by serving as receptors and/or coreceptors for microorganisms, parasites, and viruses. In addition, many blood group antigens facilitate intracellular uptake, signal transduction, or adhesion through the organization of membrane microdomains. Several blood groups can modify the innate immune response to infection. Several distinct phenotypes associated with increased host resistance to malaria are overrepresented in populations living in areas where malaria is endemic, as a result of evolutionary pressures. Microorganisms can also stimulate antibodies against blood group antigens, including ABO, T, and Kell. Finally, there is a symbiotic relationship between blood group expression and maturation of the gastrointestinal microbiome.

α-Aminoisobutyric acid incorporation induces cell permeability and antiviral activity of HIV-1 major homology region fragments

A cell-penetrating peptide derived from HIV-1 major homology region, incorporating the non-coded α-aminoisobutyric acid, inhibits viral assembly and infectivity.

Role of sulfatide in normal and pathological cells and tissues

Sulfatide is 3-O-sulfogalactosylceramide that is synthesized by two transferases (ceramide galactosyltransferase and cerebroside sulfotransferase) from ceramide and is specifically degraded by a sulfatase (arylsulfatase A). Sulfatide is a multifunctional molecule for various biological fields including the nervous system, insulin secretion, immune system, hemostasis/thrombosis, bacterial infection, and virus infection. Therefore, abnormal metabolism or expression change of sulfatide could cause various diseases. Here, we discuss the important biological roles of sulfatide in the nervous system, insulin secretion, immune system, hemostasis/thrombosis, cancer, and microbial infections including human immunodeficiency virus and influenza A virus. Our review will be helpful to achieve a comprehensive understanding of sulfatide, which serves as a fundamental target of prevention of and therapy for nervous disorders, diabetes mellitus, immunological diseases, cancer, and infectious diseases.

A novel VSV/HIV pseudotype approach for the study of HIV microbicides without requirement for level 3 biocontainment

Studies of potential HIV mucosal microbicides are difficult to undertake due to the requirement for a suitable animal model and the use of biosafety level 3 containment, which are not always available to researchers. Here we show the use of a mouse model of vaginal and rectal transmission of an HIV chimeric virus that does not require level 3 biosafety containment, to test the ex vivo efficacy of soluble Gb3 analogs for the prevention of mucosal HIV infection. The model uses a pseudoenvelope-typed vesicular stomatitis virus (VSV)/HIV recombinant virus that can infect all murine cell types. We demonstrate that the envelope glycoproteins VSV-G of VSV and gp-120 of HIV both bind Gb3. We show that soluble Gb3 analogs inhibit in vitro infection of cervical and vaginal-derived cell lines by both intact HIV and the VSV/HIV recombinant virus. Soluble Gb3 analogs incorporated into gel or used alone and applied directly to the vaginal and rectal mucosal tissue of mice were able to resist viral infection as monitored by PCR and quantitative real-time PCR copy number of HIV cDNA extracted from mouse tissue. Only a trend towards significant efficacy for prevention of mucosal transmission through lower copy number in the treatment groups was evident from these studies; however, this finding warrants further evaluation. In addition, we illustrate a methodology to evaluate inflammatory responses in either vagina or rectum after administration of soluble microbicidal compounds. These studies provide a potential new ex vivo methodology suitable for animal facilities in general, to screen microbicide drug candidates, including drug candidates that target viral proteins, for efficacy and safety, in order to accelerate development and discovery of prophylactic and therapeutic agents for HIV/AIDS.

Glycosphingolipid functions

The combination of carbohydrate and lipid generates unusual molecules in which the two distinctive halves of the glycoconjugate influence the function of each other. Membrane glycolipids can act as primary receptors for carbohydrate binding proteins to mediate transmembrane signaling despite restriction to the outer bilayer leaflet. The extensive heterogeneity of the lipid moiety plays a significant, but still largely unknown, role in glycosphingolipid function. Potential interplay between glycolipids and their fatty acid isoforms, together with their preferential interaction with cholesterol, generates a complex mechanism for the regulation of their function in cellular physiology.

Multivalent dendrimeric compounds containing carbohydrates expressed on immune cells inhibit infection by primary isolates of HIV-1

Specific glycosphingolipids (GSL), found on the surface of target immune cells, are recognized as alternate cell surface receptors by the human immunodeficiency virus type 1 (HIV-1) external envelope glycoprotein. In this study, the globotriose and 3'-sialyllactose carbohydrate head groups found on two GSL were covalently attached to a dendrimer core to produce two types of unique multivalent carbohydrates (MVC). These MVC inhibited HIV-1 infection of T cell lines and primary peripheral blood mononuclear cells (PBMC) by T cell line-adapted viruses or primary isolates, with IC(50)s ranging from 0.1 to 7.4 μg/ml. Inhibition of Env-mediated membrane fusion by MVC was also observed using a dye-transfer assay. These carbohydrate compounds warrant further investigation as a potential new class of HIV-1 entry inhibitors. The data presented also shed light on the role of carbohydrate moieties in HIV-1 virus-host cell interactions.

Molecular insights into amyloid regulation by membrane cholesterol and sphingolipids: common mechanisms in neurodegenerative diseases

Alzheimer, Parkinson and other neurodegenerative diseases involve a series of brain proteins, referred to as ‘amyloidogenic proteins’, with exceptional conformational plasticity and a high propensity for self-aggregation. Although the mechanisms by which amyloidogenic proteins kill neural cells are not fully understood, a common feature is the concentration of unstructured amyloidogenic monomers on bidimensional membrane lattices. Membrane-bound monomers undergo a series of lipid-dependent conformational changes, leading to the formation of oligomers of varying toxicity rich in β-sheet structures (annular pores, amyloid fibrils) or in α-helix structures (transmembrane channels). Condensed membrane nano- or microdomains formed by sphingolipids and cholesterol are privileged sites for the binding and oligomerisation of amyloidogenic proteins. By controlling the balance between unstructured monomers and α or β conformers (the chaperone effect), sphingolipids can either inhibit or stimulate the oligomerisation of amyloidogenic proteins. Cholesterol has a dual role: regulation of protein–sphingolipid interactions through a fine tuning of sphingolipid conformation (indirect effect), and facilitation of pore (or channel) formation through direct binding to amyloidogenic proteins. Deciphering this complex network of molecular interactions in the context of age- and disease-related evolution of brain lipid expression will help understanding of how amyloidogenic proteins induce neural toxicity and will stimulate the development of innovative therapies for neurodegenerative diseases.

The human Pk histo-blood group antigen provides protection against HIV-1 infection

Several human histo-blood groups are glycosphingolipids, including P/P1/Pk. Glycosphingolipids are implicated in HIV-host-cell-fusion and some bind to HIV-gp120 in vitro. Based on our previous studies on Fabry disease, where Pk accumulates and reduces infection, and a soluble Pk analog that inhibits infection, we investigated cell surface–expressed Pk in HIV infection. HIV-1 infection of peripheral blood–derived mononuclear cells (PBMCs) from otherwise healthy persons, with blood group P1k, where Pk is overexpressed, or blood group p, that completely lacks Pk, were compared with draw date–matched controls. Fluorescence-activated cell sorter analysis and/or thin layer chromatography were used to verify Pk levels. P1k PBMCs were highly resistant to R5 and X4 HIV-1 infection. In contrast, p PBMCs showed 10- to 1000-fold increased susceptibility to HIV-1 infection. Surface and total cell expression of Pk, but not CD4 or chemokine coreceptor expression, correlated with infection. Pk liposome–fused cells and CD4+ HeLa cells manipulated to express high or low Pk levels confirmed a protective effect of Pk. We conclude that Pk expression strongly influences susceptibility to HIV-1 infection, which implicates Pk as a new endogenous cell-surface factor that may provide protection against HIV-1 infection.

A fluorescent glycolipid-binding peptide probe traces cholesterol dependent microdomain-derived trafficking pathways

BACKGROUND

The uptake and intracellular trafficking of sphingolipids, which self-associate into plasma membrane microdomains, is associated with many pathological conditions, including viral and toxin infection, lipid storage disease, and neurodegenerative disease. However, the means available to label the trafficking pathways of sphingolipids in live cells are extremely limited. In order to address this problem, we have developed an exogenous, non-toxic probe consisting of a 25-amino acid sphingolipid binding domain, the SBD, derived from the amyloid peptide Abeta, and conjugated by a neutral linker with an organic fluorophore. The current work presents the characterization of the sphingolipid binding and live cell trafficking of this novel probe, the SBD peptide. SBD was the name given to a motif originally recognized by Fantini et al in a number of glycolipid-associated proteins, and was proposed to interact with sphingolipids in membrane microdomains.

METHODOLOGY/PRINCIPAL FINDINGS

In accordance with Fantini's model, optimal SBD binding to membranes depends on the presence of sphingolipids and cholesterol. In synthetic membrane binding assays, SBD interacts preferentially with raft-like lipid mixtures containing sphingomyelin, cholesterol, and complex gangliosides in a pH-dependent manner, but is less glycolipid-specific than Cholera toxin B (CtxB). Using quantitative time-course colocalization in live cells, we show that the uptake and intracellular trafficking route of SBD is unlike that of either the non-raft marker Transferrin or the raft markers CtxB and Flotillin2-GFP. However, SBD traverses an endolysosomal route that partially intersects with raft-associated pathways, with a major portion being diverted at a late time point to rab11-positive recycling endosomes. Trafficking of SBD to acidified compartments is strongly disrupted by cholesterol perturbations, consistent with the regulation of sphingolipid trafficking by cholesterol.

CONCLUSIONS/SIGNIFICANCE

The current work presents the characterization and trafficking behavior of a novel sphingolipid-binding fluorescent probe, the SBD peptide. We show that SBD binding to membranes is dependent on the presence of cholesterol, sphingomyelin, and complex glycolipids. In addition, SBD targeting through the endolysosomal pathway in neurons is highly sensitive to cholesterol perturbations, making it a potentially useful tool for the analysis of sphingolipid trafficking in disease models that involve changes in cholesterol metabolism and storage.

Sphingolipids: modulators of HIV-1 infection and pathogenesis

HIV-1 infects host cells by sequential interactions of its fusion protein (gp120-gp41) with receptors CD4, CXCR4 and/or CCR5 followed by fusion of viral and host membranes. Studies indicate that additional factors such as receptor density and composition of viral and cellular lipids can dramatically modulate the fusion reaction. Lipid rafts, which primarily consist of sphingolipids and cholesterol, have been implicated for infectious route of HIV-1 entry. Plasma membrane Glycosphingolipids (GSLs) have been proposed to support HIV-1 infection in multiple ways: (a) as alternate receptor(s) for CD4-independent entry in neuronal and other cell types, (b) viral transmission, and (c) gp120-gp41-mediated membrane fusion. However, the exact mechanism(s) by which GSLs support fusion is still elusive. This article will focus on the contribution of target membrane sphingolipids and their metabolites in modulating viral entry. We will discuss the current working hypotheses underlying the mechanisms by which these lipids promote and/or block HIV-1 entry. Recent approaches in the design and development of novel glycosyl derivatives, as anti-HIV agents will be summarized.

Preventing the binding of pathogens to the host by controlling sphingolipid metabolism

The binding of many pathogens and toxins to human cells can be inhibited by (1) depleting host cells of their surface glycosphingolipids; (2) coating the binding sites on pathogens (adhesins) with glycosphingolipid-like substances (decoys); (3) coating the host's glycosphingolipids with substances that compete with the pathogen for binding. Details of using these methods are described.

The role of glycosphingolipids in HIV signaling, entry and pathogenesis

Although HIV uses CD4 and coreceptors (CCR5 and CXCR4) for productive infection of T cells, glycosphingolipids (GSL) may play ancillary roles in lymphoid and non-lymphoid cells. Interactions of the HIV Envelope Glycoprotein (Env) with GSL may help HIV in various steps of its pathogenesis. Physical-chemical aspects of the interactions between HIV Env and GSL leading to CD4-dependent entry into lymphocytes, the role of GSL in HIV transcytosis, and CD4-independent entry into non-lymphoid cells are reviewed. An overview of signaling properties of HIV receptors is provided with some speculation on how GSL may play a role in these events by virtue of being in membrane rafts. Finally, we summarize how interactions between HIV and coreceptors leading to signaling and/or fusion can be analyzed by the use of various tyrosine kinase and cytoskeletal inhibitors.

A novel soluble analog of the HIV-1 fusion cofactor, globotriaosylceramide (Gb(3)), eliminates the cholesterol requirement for high affinity gp120/Gb(3) interaction

We have analyzed the interaction of adamantyl Gb(3) (adaGb(3)), a semi-synthetic soluble analog of Gb(3), with HIV-1 surface envelope glycoprotein gp120. In this analog, which was orginally designed to inhibit verotoxin binding to its glycolipid receptor, Gb(3), the fatty acid chain is replaced with a rigid globular hydrocarbon frame (adamantane). Despite its solubility, adaGb(3) forms monolayers at an air-water interface. Compression isotherms of such monolayers demonstrated that the adamantane substitution resulted in a larger minimum molecular area and a more rigid, less compressible film than Gb(3). Insertion of gp120 into adaGb(3) monolayers was exponential whereas the gp120/Gb(3) interaction curve was sigmoidal with a lag phase of 40 min. Adding cholesterol into authentic Gb(3) monolayers abrogated the lag phase and increased the initial rate of interaction with gp120. This effect of cholesterol was not observed with phosphatidylcholine or sphingomyelin. In addition, verotoxin-bound adaGb(3) or Gb(3) plus cholesterol was recovered in fractions of comparable low density after ultracentrifugation through sucrose-density gradients in the presence of Triton X-100. The unique biological and physico-chemical properties of adaGb(3) suggest that this analog may be a potent soluble mimic of Gb(3), providing a novel concept for developing GSL-derived viral fusion inhibitors.

New targets for inhibitors of HIV-1 replication

Despite the success of protease and reverse transcriptase inhibitors, new drugs to suppress HIV-1 replication are still needed. Several other early events in the viral life cycle (stages before the viral genome is inserted into host cell DNA) are susceptible to drugs, including virus attachment to target cells, membrane fusion and post-entry events such as integration, accessory-gene function and assembly of viral particles. Among these, inhibitors of virus-cell fusion and integration are the most promising candidates.

Synthesis of single- and double-chain fluorocarbon and hydrocarbon galactosyl amphiphiles and their anti-HIV-1 activity

Galactosylceramide (GalCer) is an alternative receptor allowing HIV-1 entry into CD4(-)/GalCer(+) cells. This glycosphingolipid recognizes the V3 loop of HIV gp120, which plays a key role in the fusion of the HIV envelope and cellular membrane. To inhibit HIV uptake and infection, we designed and synthesized analogs of GalCer. These amphiphiles and bolaamphiphiles consist of single and double hydrocarbon and/or fluorocarbon chain beta-linked to galactose and galactosamine. They derive from serine (GalSer), cysteine (GalCys), and ethanolamine (GalAE). The anti-HIV activity and cytotoxicity of these galactolipids were evaluated in vitro on CEM-SS (a CD4(+) cell line), HT-29, a CD4(-) cell line expressing high levels of GalCer receptor, and/or HT29 genetically modified to express CD4. GalSer and GalAE derivatives, tested in aqueous medium or as part of liposome preparation, showed moderate anti-HIV-1 activities (IC50 in the 20-220 microM range), whereas none of the GalCys derivatives was found to be active. Moreover, only some of these anti-HIV active analogs inhibited the binding of [3H]suramin (a polysulfonyl compound which displays a high affinity for the V3 loop) to SPC3, a synthetic peptide which contains the conserved GPGRAF region of the V3 loop. Our results most likely indicate that the neutralization of the virion through masking of this conserved V3 loop region is not the only mechanism involved in the HIV-1 antiviral activity of our GalCer analogs.

Role of glycosphingolipid microdomains in CD4-dependent HIV-1 fusion

The fusion of HIV-1 with the plasma membrane of CD4+ cells is triggered by the interaction of HIV-1 surface envelope glycoprotein gp120 with the CD4 receptor, and requires coreceptors (CCR5 and CXCR4). Recent advances in the study of HIV-1 entry into CD4+ cells suggest that glycosphingolipids (GSL) may also participate in the fusion process. GSL are organized in functional microdomains which are associated with specific membrane proteins such as CD4. GSL-enriched microdomains were purified from human lymphocytes and reconstituted as a monomolecular film at the air-water interface of a Langmuir film balance. Surface pressure measurements allowed to characterize the sequential interaction of GSL with CD4 and with gp120. Using this approach, we identified globotriaosylceramide (Gb3) and ganglioside GM3 as the main lymphocyte GSL recognized by gp120. In both cases, the interaction was saturable and dramatically increased by CD4. We propose that GSL microdomains behave as moving platforms allowing the recruitment of HIV-1 coreceptors after the initial interaction between the viral particle and CD4. According to this model, the GSL microdomain may: i) stabilize the attachment of the virus with the cell surface through multiple low affinity interactions between the V3 domain of gp120 and the carbohydrate moiety of GSL, and ii) convey the virus to an appropriate coreceptor by moving freely in the outer leaflet of the plasma membrane. This model can be extrapolated to all envelope viruses (e.g. influenza virus) that use cell surface GSL of the host cells as receptors or coreceptors.

Role of the N-terminal peptides of viral envelope proteins in membrane fusion

Membrane fusion is an important biological process that is observed in a wide variety of intra and intercellular events. In this review, work done in the last few years on the molecular mechanism of viral membrane fusion is highlighted, focusing in particular on the role of the fusion peptide and the modification of the lipid bilayer structure. While the Influenza hemagglutinin is currently the best understand fusion protein, there is still much to be learned about the key events in enveloped virus fusion reactions. This review compares our current understanding of the membrane fusion activity of Influenza and retrovirus viruses. We shall be concerned especially with the studies that lead to interpretations at the molecular level, so we shall concentrate on model membrane systems where the molecular components of the membrane and the environment are strictly controlled.

A convenient oxidation of natural glycosphingolipids to their "ceramide acids" for neoglycoconjugation. Bovine serum albumin-glycosylceramide acid conjugates as investigative probes for HIV gp120 coat protein-glycosphingolipid interactions

A new method to cleave the double bond of sphingolipids has been developed. Using limited concentrations of KMnO4 and an excess of NaIO4, in a neutral aqueous tert-butanol solvent system gave nearly quantitative yields of the oxidized product. A variety of natural glycosphingolipids (GSLs): GlcC, GalC, SGC, LC, Gb3C, Gb4C, Gg4C, Gb5C, and GM1C, gave the corresponding acids: 2-hydroxy-3-(N-acyl)-4-(O-glycosyl)-oxybutyric acids, i.e. "glycosyl ceramide acids" (GSL.CCOOH) in excellent yields (80-90%). Deacyl GSLs (dGSLs) were oxidized to acids containing the oligosaccharides devoid of hydrocarbon chains, i.e. "ceramide oligosaccharides" (dGSL. NRR1CCOOH, where R = R1 = H; R = H, R1 = CH3CO; or R = R1 = Me). The efficacy of this method was demonstrated by transforming natural GSLs: GlcC, GalC, GalS, SGC, LC, Gb3C, and Gb4C into neoglycoproteins via coupling glycosyl ceramide acids (except GalS, which was coupled directly) to bovine serum albumin (BSA). Mass spectroscopic analysis of GalC-BSA conjugates, (GalC.CONH)nBSA and (GalS.NHCO)nBSA gave a value of 9 +/- 1 and 16 +/- 2 for n. Neoglycoconjugates derived from GlcC, GalC (type I and II and the behenic analog), SGC, LC, and Gb3C were recognized by the recombinant human immunodeficiency virus coat protein gp120 (rgp120). The GalS conjugate showed significantly reduced binding, and the Gb4C conjugate showed no binding. Thus, rgp120/GSL-BSA interaction requires a terminal galactose and/or glucose residue. Terminal N-acetylgalactosamine containing GSLs are not bound. The ceramide acid conjugates provide a more effective scaffold for presentation of glycone for rgp120 binding than those derived from dGSLs. The retention of receptor specificity of the glycoconjugates was validated by retention of the expected binding specificity of VT1 and VT2e for Gb3C and Gb4C conjugates, respectively. These studies open a new vista in the generation of glycoconjugates from GSLs and further emphasize the role of aglycone in glycolipid recognition.

V3 loop-derived peptide SPC3 inhibits infection of CD4- and galactosylceramide- cells by LAV-2/B

SPC3, a synthetic multibranched peptide including the GPGRAF consensus motif of the human immunodeficiency virus type 1 (HIV-1) gp120 V3-loop is a potent inhibitor of HIV infection of human CD4+ lymphocytes, macrophages and CD4-/galactosylceramide+ human colon epithelial cells and is currently tested in phase II clinical trials (FDA protocol 257 A). The antiviral property of SPC3 was further investigated for its ability to inhibit LAV-2/B, an HIV-2 clone with a CD4-independent tropism. SPC3 inhibited the LAV-2/B-mediated infection of B-cell line which does not express the CD4 and the galactosylceramide molecules on their cell surface, suggesting an SPC3-sensitive CD4/galactosylceramide-independent pathway of viral infection in HIV susceptible cells. The molecular mechanism of the peptide inhibition was also investigated. The data suggested that the SPC3-mediated inhibition does not result from a direct competition between SPC3 and gp120 binding to the cell surface of the target cell.

Human erythrocyte glycosphingolipids as alternative cofactors for human immunodeficiency virus type 1 (HIV-1) entry: evidence for CD4-induced interactions between HIV-1 gp120 and reconstituted membrane microdomains of glycosphingolipids (Gb3 and GM3)

Glycosphingolipids from human erythrocytes mediate CD4-dependent fusion with cells expressing human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins. To identify the glycosphingolipid(s) which participates in the fusion process, we have analyzed the interaction of HIV-1 gp120 (X4 and R5X4 isolates) with reconstituted membrane microdomains of human erythrocyte glycosphingolipids. We identified globotriaosylceramide (Gb3) and ganglioside GM3 as the main glycosphingolipids recognized by gp120. In the presence of CD4, Gb3 interacted preferentially with the X4 gp120, whereas GM3 interacted exclusively with the R5X4 gp120. These data suggest that glycosphingolipid microdomains are required in CD4-dependent fusion and that Gb3 and/or GM3 may function as alternative entry cofactors for selected HIV-1 isolates.

Glycolipids as potential binding sites for HIV: topology in the sperm plasma membrane in relation to the regulation of membrane fusion

Although human sperm cells can bind human immunodeficiency virus (HIV-1), they lack CD4, galactoceramides (GalCer) and sulfogalactoceramides (SGalCer) as gp120 receptors. However, sperm specific glycolipids (sulfogalactosylalkylacylglycerol (SGalAAG) and galactosylalkylacylglycerol (GalAAG)) are structurally closely related to SGalCer and GalCer as predicted by computer simulated molecular modelling. SGalAAG and GalAAG are exclusively localized in the outer leaflet of the human sperm plasma membrane, and therefore we tested whether they could serve as alternative receptors for the gp120. Purified SGalAAG and GalAAG had similar affinities to recombinant gp120 as the hydroxy fatty acid (HFA) SGalCer and HFA-GalCer respectively. However, nonhydroxy fatty acid forms of (S)GalCer, galactosyldiacylglycerol and the deacylated (sulfo)galactosyllipids did not recognize recombinant gp120. Data obtained by surface pressure experiments revealed that the lipid monolayers that contained HFA-GalCer or GalAAG resulted in a similar significant penetration of recombinant gp120 in the monolayer. The penetration was a factor of two lower in monolayers with HFA-SGalCer or SGalAAG. The binding of recombinant gp120 to human sperm cells colocalized with GalAAG and could be blocked with monoclonal antibodies against galactolipids. The possible relevance of gp120 binding to glycolipids for HIV entry in sperm cells is discussed.

The neutral glycosphingolipid globotriaosylceramide promotes fusion mediated by a CD4-dependent CXCR4-utilizing HIV type 1 envelope glycoprotein

Previously, we showed that the addition of human erythrocyte glycosphingolipids (GSLs) to nonhuman CD4(+) or GSL-depleted human CD4(+) cells rendered those cells susceptible to HIV-1 envelope glycoprotein-mediated cell fusion. Individual components in the GSL mixture were isolated by fractionation on a silica-gel column and incorporated into the membranes of CD4(+) cells. GSL-supplemented target cells were then examined for their ability to fuse with TF228 cells expressing HIV-1LAI envelope glycoprotein. We found that one GSL fraction, fraction 3, exhibited the highest recovery of fusion after incorporation into CD4(+) nonhuman and GSL-depleted HeLa-CD4 cells and that fraction 3 contained a single GSL fraction. Fraction 3 was characterized by MS, NMR spectroscopy, enzymatic analysis, and immunostaining with an antiglobotriaosylceramide (Gb3) antibody and was found to be Gal(alpha1-->4)Gal(beta1-->4)Glc-Cer (Gb3). The addition of fraction 3 or Gb3 to GSL-depleted HeLa-CD4 cells recovered fusion, but the addition of galactosylceramide, glucosylceramide, the monosialoganglioside, GM3, lactosylceramide, globoside, the disialoganglioside, GD3, or alpha-galactosidase A-digested fraction 3 had no effect. Our findings show that the neutral GSL, Gb3, is required for CD4/CXCR4-dependent HIV-1 fusion.

Sequential interaction of CD4 and HIV-1 gp120 with a reconstituted membrane patch of ganglioside GM3: implications for the role of glycolipids as potential HIV-1 fusion cofactors

The fusion of HIV-1 with CD4+ cells involves, in addition to CD4, specific cell surface molecules acting as fusion cofactors. Recently, we reported that the V3 loop of HIV-1 gp120 binds to GM3, a ganglioside abundantly expressed on CD4+ lymphocytes and macrophages. In the present study, we show that CD4 interacts with a reconstituted patch of GM3 by measuring the surface pressure with a Langmuir film balance. A biphasic increase in surface pressure is observed after the sequential addition of CD4 and gp120 under the GM3 monolayer, indicating the formation of the trimolecular complex GM3-CD4-gp120. Neutralization of gp120 with an anti-V3 antibody inhibits the secondary interaction with GM3, suggesting that the CD4-induced conformational change in gp120 allows the V3 loop to interact with GM3. In conclusion, this study supports the concept that glycolipids can function as HIV-1 fusion cofactors.

Specific interaction of HIV-1 and HIV-2 surface envelope glycoproteins with monolayers of galactosylceramide and ganglioside GM3

Cellular glycosphingolipids mediate the fusion between some viruses and the plasma membrane of target cells. In the present study, we have analyzed the interaction of human immunodeficiency virus (HIV)-1 and HIV-2 surface envelope glycoproteins from distinct viral isolates with monolayers of various glycosphingolipids at the air-water interface. The penetration of the viral glycoproteins into glycosphingolipid monolayers was detected as an increase in the surface pressure. We found that HIV-1 recombinant gp120 (IIIB isolate) could penetrate into a monomolecular film of alpha-hydroxylated galactosylceramide (GalCer-HFA), while ceramides, GluCer, and nonhydroxylated GalCer were totally inactive. The glycoproteins isolated from HIV-1 isolates LAI and NDK and from HIV-2(ROD) could also interact with a GalCer-HFA monolayer, whereas gp120 from HIV-1(SEN) and HIV-1(89.6) did not react. These data correlated with the ability of the corresponding viruses to gain entry into the CD4(-)/GalCer+ cell line HT-29, demonstrating the determinant role of GalCer-HFA in this CD4-independent pathway of HIV-1 and HIV-2 infection. In contrast, all HIV-1 and HIV-2 glycoproteins tested were found to interact with a monolayer of GM3, a ganglioside abundantly expressed in the plasma membrane of CD4(+) lymphocytes and macrophages. A V3 loop-derived synthetic peptide inhibitor of HIV-1 and HIV-2 infection in both CD4(-) and CD4(+) cells could penetrate into various glycosphingolipid monolayers, including GalCer-HFA and GM3. Taken together, these data suggest that the adsorption of human immunodeficiency viruses to the surface of target cells involves an interaction between the V3 domain of the surface envelope glycoprotein and specific glycosphingolipids, i.e. GalCer-HFA for CD4(-) cells and GM3 for CD4(+) cells.

Human erythrocyte glycolipids promote HIV-1 envelope glycoprotein-mediated fusion of CD4+ cells

We examined the role of target membrane glycolipids in CD4-mediated HIV-1 fusion by altering the glycolipid levels in CD4+ cells. CD4+ human cells exhibited 50% reduction in extent of fusion with gp120-gp41 expressing cells (TF228) when grown in the presence of a glycolipid synthesis inhibitor PPMP. We added erythrocyte glycolipids (GL) to fusion-incompetent CD4+ non-human cells by influenza-hemagglutinin-mediated fusion between GL-containing liposomes and target cells. Human erythrocyte GL (HuGL)-modified CD4+ non-human cells became susceptible to fusion with TF228 cells. Transfer of bovine erythrocyte glycolipids (BoGL) to CD4+ non-human cells under similar conditions did not complement HIV-1 fusion. Furthermore, addition of HuGL, but not BoGL, to PPMP-inhibited cells rescued fusion to the original levels. Our observations demonstrate that human erythrocyte glycolipids promote CD4-mediated HIV-1 fusion and certain glycolipid(s) from human erythrocytes may serve as alternative and/or additional cofactors in HIV-1 entry.