Hepatitis C virus is primed by CD81 protein for low pH-dependent fusion

Hepatitis C virus (HCV) entry into permissive cells is a complex process that involves interactions with at least four co-factors followed by endocytosis and low pH-dependent fusion with endosomes. The precise sequence of receptor engagement and their roles in promoting HCV E1E2 glycoprotein-mediated fusion are poorly characterized. Because cell-free HCV tolerates an acidic environment, we hypothesized that binding to one or more receptors on the cell surface renders E1E2 competent to undergo low pH-induced conformational changes and promote fusion with endosomes. To test this hypothesis, we examined the effects of low pH and of the second extracellular loop (ECL2) of CD81, one of the four entry factors, on HCV infectivity. Pretreatment with an acidic buffer or with ECL2 enhanced infection through changing the E1E2 conformation, as evidenced by the altered reactivity of these proteins with conformation-specific antibodies and stable association with liposomes. However, neither of the two treatments alone permitted direct fusion with the cell plasma membrane. Sequential HCV preincubation with ECL2 and acidic buffer in the absence of target cells resulted in a marked loss of infectivity, implying that the receptor-bound HCV is primed for low pH-dependent conformational changes. Indeed, soluble receptor-pretreated HCV fused with the cell plasma membrane at low pH under conditions blocking an endocytic entry pathway. These findings suggest that CD81 primes HCV for low pH-dependent fusion early in the entry process. The simple triggering paradigm and intermediate conformations of E1E2 identified in this study could help guide future vaccine and therapeutic efforts to block HCV infection.     

Kinetics of pH-dependent fusion between influenza virus and liposomes

The pH-dependent fusion between influenza virus and liposomes (large unilamellar vesicles) has been investigated as a model for the fusion step in the infectious entry of the virus into cells. Fusion was monitored continuously, with a fluorescence assay based on resonance energy transfer (RET) [Struck, D. K., Hoekstra, D., & Pagano, R. E. (1981) Biochemistry 20, 4093-4099], which allows an accurate quantitation of the fusion process. Evidence is presented indicating that the dilution of the RET probes from the liposomal bilayer into the viral membrane is not due to transfer of individual lipid molecules. The initial rate and final extent of the fusion reaction increase dramatically with decreasing pH, fusion being virtually complete within 1 min at pH 4.5-5.0. From experiments in which the ratio of virus to liposomes was varied, it is concluded that virus-liposome fusion products continue to fuse with liposomes, but not with virus. Fusion is most efficient with liposomes consisting of negatively charged phospholipids, while phosphatidylcholine and sphingomyelin are inhibitory. The reaction is completely blocked by an antiserum against the virus and inhibited by pretreatment of the virus with trypsin. The effect of proteolytic pretreatment at pH 7.4 is enhanced after preincubation of the virus at pH 5.0, consistent with the occurrence of a low pH induced, irreversible, conformational change in the viral fusion protein, the hemagglutinin (HA), exposing trypsin cleavage sites. When, after initiation of the fusion reaction at pH 5.0, the pH is readjusted to neutral, the process is arrested instantaneously, indicating that the low pH induced conformational change in the HA protein, in itself, is not sufficient to trigger fusion activity.     

Hepatitis C virus glycoproteins interact with DC-SIGN and DC-SIGNR

DC-SIGN and DC-SIGNR are two closely related membrane-associated C-type lectins that bind human immunodeficiency virus (HIV) envelope glycoprotein with high affinity. Binding of HIV to cells expressing DC-SIGN or DC-SIGNR can enhance the efficiency of infection of cells coexpressing the specific HIV receptors. DC-SIGN is expressed on some dendritic cells, while DC-SIGNR is localized to certain endothelial cell populations, including hepatic sinusoidal endothelial cells. We found that soluble versions of the hepatitis C virus (HCV) E2 glycoprotein and retrovirus pseudotypes expressing chimeric forms of both HCV E1 and E2 glycoproteins bound efficiently to DC-SIGN and DC-SIGNR expressed on cell lines and primary human endothelial cells but not to other C-type lectins tested. Soluble E2 bound to immature and mature human monocyte-derived dendritic cells (MDDCs). Binding of E2 to immature MDDCs was dependent on DC-SIGN interactions, while binding to mature MDDCs was partly independent of DC-SIGN, suggesting that other cell surface molecules may mediate HCV glycoprotein interactions. HCV interactions with DC-SIGN and DC-SIGNR may contribute to the establishment or persistence of infection both by the capture and delivery of virus to the liver and by modulating dendritic cell function.     

pH-dependent fusion of liposomes using titratable polycations

Polylysine promoted extensive membrane mixing of liposomes only if the buffer pH was below the pKa of the lysyl residues. This observation suggested that fusion could be regulated in a physiological pH range if the homopolymer of L-histidine was substituted as fusogen. Microgram quantities of polyhistidine were added to liposomes composed of soybean phospholipids, or to defined phospholipid-cholesterol mixtures which simulate the lipid composition of plasma membranes. A quantitative resonance energy transfer assay determined the extent of lipid phase mixing related to fusion. No fusion was detected at pH 7.4, but when the pH was lowered to 6.5 or below, fusion was rapid and substantial. The extent of membrane mixing increased with progressive acidification of the vesicle-fusogen suspension. The charge density of each polyhistidine molecule, not the total cationic charge per vesicle, influenced the extent of fusion. The kinetics of the fusion reaction were rapid, as membrane mixing was completed within 1 min. If the vesicle suspension was acidified before fusogen addition, the rate of membrane mixing slowed 4-fold. This, as well as a slight increase in light scattering noted whenever polyhistidine was added at pH 7.4, suggests an enhancement of fusion kinetics by preaggregation of vesicles at neutral pH. The lipid composition, regulation of membrane mixing by pH in a physiological range, and rapid kinetics suggest that this model of liposome fusion may be pertinent to understanding some biological fusion events.     

Baculovirus gp64 envelope glycoprotein is sufficient to mediate pH-dependent membrane fusion.

The baculovirus gp64 envelope glycoprotein is a major component of the envelope of the budded virus (BV) and is involved in BV entry into the host cell by endocytosis. To determine whether gp64 alone was sufficient to mediate membrane fusion, the Orgyia pseudotsugata multicapsid nuclear polyhedrosis virus gp64 protein was transiently expressed in uninfected insect cells. Cells expressing the baculovirus gp64 protein were examined for membrane fusion activity by using a syncytium formation assay under various conditions of exposure to low pH. Cells expressing the gp64 protein mediated membrane fusion and syncytium formation in a pH-dependent manner. A pH of 5.5 or lower was required to induce membrane fusion. In addition, exposure of gp64-expressing cells to low pH for as little as 5 s was sufficient to induce gp64-mediated syncytium formation. These studies provide direct evidence that gp64 is a pH-dependent membrane fusion protein and suggest that gp64 is the protein responsible for fusion of the virion envelope with the endosome membrane during BV entry into the host cell by endocytosis.     

pH-dependent stability and fusion of liposomes combining protonatable double-chain amphiphiles with phosphatidylethanolamine

We have prepared a series of novel double-chain amphiphiles with protonatable head groups, including acylated derivatives of various 2-substituted palmitic acids, amino acid conjugates of these species, and 1,2-dioleoyl-3-succinylglycerol. These species can be combined with phosphatidylethanolamine (PE) to prepare reverse-phase evaporation vesicles that are stable and trap hydrophilic solutes at pH 7. At weakly acidic pH values (as high as 6.5, depending on the titratable amphiphilic component), these pH-sensitive vesicles exhibit fusion, with a limited extent of contents mixing and extensive mixing of lipids, accompanied by leakage of aqueous contents. Protons and divalent cations show strong synergistic effects in promoting mixing of both lipids and aqueous contents between pH-sensitive vesicles prepared with any of a variety of double-chain titratable amphiphiles. Calorimetric results indicate that the relative stabilities of different types of pH-sensitive liposomes at low pH cannot be simply correlated with the propensity of the lipids to form a hexagonal II phase under these conditions. Fluorescence measurements demonstrate that single-chain fatty acids, but not double-chain titratable amphiphiles such as N-acyl-2-aminopalmitic acids, are rapidly removed from pH-sensitive vesicles in the presence of other lipid vesicles, serum albumin, or serum. Additionally, pH-sensitive liposomes containing double-chain titratable amphiphiles retain their aqueous contents better than do those containing single-chain amphiphiles in the presence of lipid membranes or albumin. Surprisingly, however, pH-sensitive vesicles of either type show retention of contents in the presence of serum that is comparable to that observed with vesicles composed purely of phospholipids. A model is proposed to explain these latter findings.     

The hr1 and fusion peptide regions of the subgroup B avian sarcoma and leukosis virus envelope glycoprotein influence low pH-dependent membrane fusion

The avian sarcoma and leukosis virus (ASLV) envelope glycoprotein (Env) is activated to trigger fusion by a two-step mechanism involving receptor-priming and low pH fusion activation. In order to identify regions of ASLV Env that can regulate this process, a genetic selection method was used to identify subgroup B (ASLV-B) virus-infected cells resistant to low pH-triggered fusion when incubated with cells expressing the cognate TVB receptor. The subgroup B viral Env (envB) genes were then isolated from these cells and characterized by DNA sequencing. This led to identification of two frequent EnvB alterations which allowed TVB receptor-binding but altered the pH-threshold of membrane fusion activation: a 13 amino acid deletion in the host range 1 (hr1) region of the surface (SU) EnvB subunit, and the A32V amino acid change within the fusion peptide of the transmembrane (TM) EnvB subunit. These data indicate that these two regions of EnvB can influence the pH threshold of fusion activation.     

Envelope glycoproteins of hantavirus can mediate cell-cell fusion independently

Hantaviruses (HTVs) are enveloped viruses and can induce low PH-dependent cell fusion. In this report we molecularly cloned viral glycoproteins (GPs) cDNA and nucleocapsid (NP) cDNA of two strains of Hantaan virus and one strain of Seoul virus and expressed in Vero E6 cells under control of a CMV promoter. The examinations of viral gene expressions were carried out by IFA and immune-precipitation. After treatment with low PH (PH 5.8) medium the syncytium were observed in the cells transfected with the GPs clones while in the cells transfected with the NP clones we did not find this phenomenon. Furthermore cotransfection of the NP and GPs did not enhance fusion activity. Treatment with anti-GP monoclonal antibodies could inhibit fusion activity whereas the antibodies against NP could not. These results indicated that GPs can mediate cell-cell fusion independently.     

Parameters affecting low-pH-mediated fusion of liposomes with the plasma membrane of cells infected with influenza virus

Unilamellar liposomes can be fused at low pH with the plasma membrane of cells that express the hemagglutinin glycoprotein of influenza virus on their surface [van Meer, G., & Simons, K. (1983) J. Cell Biol. 97, 1365-1374]. Here, we have resolved this fusion process into two kinetically distinct steps. The first and more rapid step converts the bound liposome to a form that can no longer be released by neuraminidase. The second step is the actual membrane fusion as measured by the loss of resonance energy transfer between two liposomal fluorescent phospholipids, N-(7-nitro-2,1,3-benzoxadiazol-4-yl)dioleoylphosphatidylethanolami ne (N-NBD-PE) and N-(lissamine rhodamine B sulfonyl)dioleoylphosphatidylethanolamine (N-Rh-PE). In contrast to the first step, the rate of the second one was highly dependent on the liposomal lipid composition and the cell type used. The replacement of 50% of the phosphatidylcholine (PC) in egg PC-cholesterol liposomes by unsaturated phosphatidylethanolamine (PE) species increased the rate of fusion at least 2-fold. Of the PE-containing liposomes that were associated with Madin-Darby canine kidney (MDCK) cells after 30 s of fusion, 80% had actually fused with the plasma membrane. Fringe pattern fluorescence photobleaching experiments showed that after fusion a fraction of the cell-associated N-Rh-PE diffused laterally in the plasma membrane. Without fusion, the N-Rh-PE was completely immobile. Under optimal conditions, the mobile fractions were 65% on MDCK cells and 78% on baby hamster kidney cells. The mobility was acquired simultaneously with the dilution of the fluorescent phospholipids as measured from the loss of resonance energy transfer. The mobile fraction of N-Rh-PE on the cell surface can therefore be used as a second independent measure of actual membrane fusion. Finally, we observed that upon fusion up to 80% of the nonexchangeable liposome markers cholesterol [14C]oleate and glycerol tri[14C]oleate became accessible to cellular hydrolases. The results showed that this hydrolysis assay can also be used to monitor the second step of the fusion process.     

Pyrene phospholipid as a biological fluorescent probe for studying fusion of virus membrane with liposomes

We are using fluorescent endogenous phospholipids in virus membranes to study the factors that promote fusion on interaction with receptor membranes. To this end, vesicular stomatitis virus (VSV) grown in baby hamster kidney (BHK-21) cells was biologically labeled with fluorescent lipids, primarily phosphatidylcholine and phosphatidylethanolamine, derived from pyrene fatty acids. The pyrene lipids present in the virions showed a fluorescence spectrum typical of pyrene with an intense monomer and a broad excimer. Interaction of pyrene lipid labeled VSV with serum lipoproteins led to a spontaneous fast transfer of the small amount of pyrene fatty acids present in the envelope (t1/2 less than or equal to 7 min), followed by a considerably slower transfer of pyrene phospholipids from the membrane of the virions (t1/2 greater than or equal to 12 h). Incubation of pyrene phospholipid labeled VSV with phosphatidylserine small unilamellar vesicles resulted in fusion at low pH (pH 5.0) as measured by the change in the excimer/monomer fluorescence intensity ratio. Fusion kinetics was rapid, reaching a plateau after 4 min at pH 5.0 and 37 degrees C. Only negligible fusion was noted at neutral pH or at 4 degrees C. Fully infectious virions labeled biologically with fluorescent lipids provide a useful tool for studying mechanisms of cell-virus interactions and neutralization of viral infectivity by specific monoclonal antibodies reactive with viral membrane glycoprotein.     

Diverse hepatitis C virus glycoproteins mediate viral infection in a CD81-dependent manner

We recently reported that retroviral pseudotypes bearing the hepatitis C virus (HCV) strain H and Con1 glycoproteins, genotype 1a and 1b, respectively, require CD81 as a coreceptor for virus-cell entry and infection. Soluble truncated E2 cloned from a number of diverse HCV genotypes fail to interact with CD81, suggesting that viruses of diverse origin may utilize different receptors and display altered cell tropism. We have used the pseudotyping system to study the tropism of viruses bearing diverse HCV glycoproteins. Viruses bearing these glycoproteins showed a 150-fold range in infectivity for hepatoma cells and failed to infect lymphoid cells. The level of glycoprotein incorporation into particles varied considerably between strains, generally reflecting the E2 expression level within transfected cells. However, differences in glycoprotein incorporation were not associated with virus infectivity, suggesting that infectivity is not limited by the absolute level of glycoprotein. All HCV pseudotypes failed to infect HepG2 cells and yet infected the same cells after transduction to express human CD81, confirming the critical role of CD81 in HCV infection. Interestingly, these HCV pseudotypes differed in their ability to infect HepG2 cells expressing a panel of CD81 variants, suggesting subtle differences in the interaction of CD81 residues with diverse viral glycoproteins. Our current model of HCV infection suggests that CD81, together with additional unknown liver specific receptor(s), mediate the virus-cell entry process.     

Reconstitution of hepatitis C virus envelope glycoproteins into liposomes as a surrogate model to study virus attachment

The envelope glycoproteins, E1 and E2, of hepatitis C virus (HCV) assemble intracellularly to form a noncovalent heterodimer that is expected to be essential for viral assembly and entry. However, due to the lack of a cell culture system supporting efficient HCV replication, it is very difficult to obtain relevant information on the functions of this glycoprotein oligomer. To get better insights into its biological and biochemical properties, HCV envelope glycoprotein heterodimer expressed by a vaccinia virus recombinant was purified by immunoaffinity. Purified E1E2 heterodimer was recognized by conformation-dependent monoclonal antibodies, showing that the proteins were properly folded. In addition, it interacted with human CD81, a putative HCV receptor, as well as with human low and very low density lipoproteins, which have been shown to be associated with infectious HCV particles isolated from patients. Purified E1E2 heterodimer was also reconstituted into liposomes. E1E2-liposomes were recognized by a conformation-dependent monoclonal antibody as well as by human CD81. Together, these data indicate that E1E2-liposomes are a valuable tool to study the molecular requirements for HCV binding to target cells.     

Microsomal protein mediates a pH-dependent fusion of liposomes to rat brain microsomes

The fusion between rat brain microsomes and liposomes is investigated by measuring the release of octadecylrhodamine B (R18) fluorescence self-quenching. In the experimental conditions used in this work, the method allows a rapid and quantitative evaluation of the mixing of microsome and liposome lipid phases. The decrease of pH below 7 produces an extensive fusion between microsomes and acidic phospholipid liposomes. Microsomal protein is necessary for fusion, which is inactivated by exposure of microsomes to pronase. Therefore, H(+)-induced fusion differs from Ca(2+)-induced fusion since the latter does not require microsomal protein. The pretreatment of microsomes with trinitrobenzenesulfonic acid (TNBS) in nonpenetrating conditions does not affect the extent of fusion. On the other hand, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), a reagent able to react with carboxyl groups, causes an extensive inactivation of fusion. Therefore, the H(+)-induced fusion described here depends on some microsomal protein and may have physiological significance because it occurs at pH values present in the living cell. H(+)-dependent fusion can be also considered as a means to enrich membranes in some selected lipid.     

Stapled peptide-based membrane fusion inhibitors of hepatitis C virus

The strategy of peptide stapling was used to develop new molecules to inhibit the hepatitis C virus infection via disrupting the binding of HCV envelope glycoprotein E2 with human cell surface protein CD81. The peptide sequence was designed based on the large extra-cellular loop of CD81 with known importance in the HCV E2 binding interaction. Our results showed that the stapled peptides exhibited significantly higher α-helicity and proteolytic stability as compared to their linear peptide counterpart. The optimal compound was found to have an EC₅₀ value of ca. 17–39 μM against different HCV subtypes and represented a new HCV membrane fusion inhibitor.     

pH-dependent membrane fusion is promoted by various colicins.

The ability of colicin A, a bacteriocin produced by some Enterobacteriaceae, to fuse phospholipid vesicles at acidic pH, was demonstrated by electron microscopy and resonance energy transfer. The fusion depends on protein concentration and on the nature of the phospholipids. Vesicles, prepared from Escherichia coli phospholipids, fused one or more rounds at pH 4.5 upon addition of stoichiometric amounts of colicin A. Fusion was not only induced by pore-forming colicins (E1, K) but also by colicins that contain nuclease activities (E2, E3). By recombinant DNA technology it is shown that the first glycine-rich 70 NH2-terminal amino acids and, most probably, the extreme COOH-terminal end of colicin A are involved in the fusion activity of the protein. The physiological relevance of this property of colicins is discussed.     

Incorporation of adenylate cyclase into membranes of giant liposomes using membrane fusion with recombinant baculovirus-budded virus particles

Recombinant transmembrane adenylate cyclase (AC) was incorporated into membranes of giant liposomes using membrane fusion between liposomes and baculovirus-budded virus (BV). AC genes were constructed into transfer vectors in a form fused with fluorescent protein or polyhistidine at the C-terminus. The recombinant BVs were collected by ultracentrifugation and AC expression was verified using western blotting. The BVs and giant liposomes generated using gentle hydration were fused under acidic conditions; the incorporation of AC into giant liposomes was demonstrated by confocal laser scanning microscopy through the emission of fluorescence from their membranes. The AC-expressing BVs were also fused with liposomes containing the substrate (ATP) with/without a specific inhibitor (SQ 22536). An enzyme immunoassay on extracts of the sample demonstrated that cAMP was produced inside the liposomes. This procedure facilitates direct introduction of large transmembrane proteins into artificial membranes without solubilization.     

Cell fusion activities of Hantaan virus envelope glycoproteins

Hantaan virus (HTNV)-infected Vero E6 cells undergo cell fusion with both infected and uninfected cells under low-pH conditions. Flow cytometry and fluorescence microscopy of HTNV-infected Vero E6 cells showed that envelope glycoproteins (GPs) were located both on the cell surface and in the cytoplasm. Neutralizing monoclonal antibodies (MAbs) against the G1 and G2 envelope GPs inhibited cell fusion, whereas nonneutralizing MAbs against G1 or G2 and MAbs against the nucleocapsid protein (NP) did not. Transfected Vero E6 cells that expressed GPs but not those that expressed NP fused and formed syncytia. These results indicate that HTNV GPs act as fusogens at the cell surface. No fusion activity was observed either in infected Vero cells that were passaged more than 150 times or in BHK-21 cells, although GPs appeared to localize to the cell surface. This variability in fusion induction suggests the involvement of host cell factors in the process of cell membrane fusion.