The Exchangeable Apolipoprotein ApoC-I Promotes Membrane Fusion of Hepatitis C Virus

Abstract

Cell entry of hepatitis C virus (HCV) is strikingly linked to lipoproteins and their receptors. Particularly, high density lipoprotein (HDL) enhances infectivity of HCV by involving the lipid-transfer function of the scavenger receptor BI, a receptor for both HDL and HCV. Here, we demonstrate that apoC-I, an exchangeable apolipoprotein that predominantly resides in HDL, specifically enhances HCVcc and HCVpp infectivity and increases the fusion rates between viral and target membranes via a direct interaction with the HCV surface. We identify the hypervariable region 1, located at the N terminus of the HCV E2 glycoprotein, as an essential viral component that modulates apoC-I-mediated enhancement of HCV fusion properties. The affinity of apoC-I for the HCV membrane may predispose it for fusion with a target membrane via alterations of its outer phospholipid layer. Conversely, we found that excess apoC-I provided as lipoprotein-free protein induces the disruption of the HCV membrane and subsequent loss of infectivity. Furthermore, our data indicate that HDL neither interacts nor spontaneously exchanges apoC-I with HCV virions. Because apoC-I is not present in serum as a lipoprotein-free form, our results suggest that HDL-embedded apoC-I could be released from the lipoprotein particle through a fine-tuned mechanism regulated via a triple interplay between hypervariable region 1, HDL, and scavenger receptor BI, resulting in optimal apoC-I recruitment on the viral membrane. These results provide the first description of a host serum factor helping the fusion process of an enveloped virus. With an estimated 170 million infected individuals, hepatitis C virus (HCV)4 has a major impact on public health (2).HCV is an enveloped, positive-stranded RNA virus of the Flaviviridae family. Its genome encodes a single polyprotein processed by viral and cellular proteases into three structural (core, E1 and E2 glycoproteins) and seven non-structural proteins (3, 4). For a long time the study of HCV cell entry has remained limited because the ex vivo characterization of HCV derived from plasma has proven extremely difficult. This is due in large part to the low infectivity of the virus in cultures of primary hepatocytes, to its high genetic heterogeneity, and to its association through different forms with lipoproteins. Thus, to overcome these severe limitations toward the molecular characterization of HCV infection, several surrogate assays have been developed. Two relevant and complementary in vitro cell entry assays consist of cell culture-grown genuine HCV (HCVcc) derived from a fulminant hepatitis C, JFH-1 (5-7), and of HCV pseudo-particles (HCVpp) harboring authentic E1E2 glycoproteins, which are particularly amenable to mutagenesis analysis (8 -10). Although HCVcc further permit investigation of the late infection steps, HCVpp, which can be produced in nonhepatic cells that can be readily complemented with hepatic factors, offers a particularly flexible plate form to study the structure/function relationship of HCV glycopro...

Cellular models for the screening and development of anti-hepatitis C virus agents

“…Indeed, siRNA (small interfering RNA) directed to clathrin heavy chain and molecules that neutralise the acidic pH of endosomes produce a potent inhibition of the entry of HCVpp and HCVcc in various cell lines. ApoC-I, an exchangeable apolipoprotein that predominantly resides in HDL, was shown to increase HCV infectivity and the rate of fusion between viral and target membranes via a direct interaction with the virion surface (Dreux et al, 2007). Consistent with this, recent studies demonstrated that the C-terminal region of ApoC-1 associates with surface components of virions during morphogenesis (Meunier et al, 2008).…”

Identification and Characterization of Broadly Neutralizing Human Monoclonal Antibodies Directed against the E2 Envelope Glycoprotein of Hepatitis C Virus

“…However, it remains unclear if this system is truly indicative of HCV neutralization in vivo. Confounding factors such as enhancement of HCV infection by apolipoprotein (7,21) and epitope masking by high-density lipoprotein found in serum (37) are not present in this in vitro system. Unfortunately, the currently employed in vivo models have many drawbacks as well.…”

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“…Likewise, apoB-containing lipoproteins (27) or high density lipoprotein (HDL) (28) appear to facilitate the interaction of HCV pseudoparticles or cell culture-grown HCV with scavenger receptor class B type I (SR-BI) (29). The exchangeable apolipoprotein apoCI, which is predominantly present in HDL, seems to interact with HCV particles via hypervariable region 1 (HVR1) in the N terminus of E2 thus promoting interaction and fusion of the HCV envelope with cellular membranes (30,31).…”