Receptor complementation and mutagenesis reveal SR-BI as an essential HCV entry factor and functionally imply its intra- and extra-cellular domains.
PLoS Pathog 2009;
5:e1000310. [PMID:
19229312 PMCID:
PMC2636890 DOI:
10.1371/journal.ppat.1000310]
[Citation(s) in RCA: 102] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/10/2008] [Accepted: 01/23/2009] [Indexed: 12/11/2022] Open
Abstract
HCV entry into cells is a multi-step and slow process. It is believed that the
initial capture of HCV particles by glycosaminoglycans and/or lipoprotein
receptors is followed by coordinated interactions with the scavenger receptor
class B type I (SR-BI), a major receptor of high-density lipoprotein (HDL), the
CD81 tetraspanin, and the tight junction protein Claudin-1, ultimately leading
to uptake and cellular penetration of HCV via low-pH endosomes.
Several reports have indicated that HDL promotes HCV entry through interaction
with SR-BI. This pathway remains largely elusive, although it was shown that HDL
neither associates with HCV particles nor modulates HCV binding to SR-BI. In
contrast to CD81 and Claudin-1, the importance of SR-BI has only been addressed
indirectly because of lack of cells in which functional complementation assays
with mutant receptors could be performed. Here we identified for the first time
two cell types that supported HCVpp and HCVcc entry upon ectopic SR-BI
expression. Remarkably, the undetectable expression of SR-BI in rat hepatoma
cells allowed unambiguous investigation of human SR-BI functions during HCV
entry. By expressing different SR-BI mutants in either cell line, our results
revealed features of SR-BI intracellular domains that influence HCV infectivity
without affecting receptor binding and stimulation of HCV entry induced by
HDL/SR-BI interaction. Conversely, we identified positions of SR-BI ectodomain
that, by altering HCV binding, inhibit entry. Finally, we characterized
alternative ectodomain determinants that, by reducing SR-BI cholesterol uptake
and efflux functions, abolish HDL-mediated infection-enhancement. Altogether, we
demonstrate that SR-BI is an essential HCV entry factor. Moreover, our results
highlight specific SR-BI determinants required during HCV entry and
physiological lipid transfer functions hijacked by HCV to favor infection.
More than 180 million people are chronically infected by hepatitis C virus (HCV),
a leading cause of liver failure and cancer, stimulating the need to fully
define the biology of HCV infection for developing novel and effective
therapeutics. During the first steps of infection, the virus is taken up and
penetrates hepatocytes. HCV entry is thought to be a coordinated multi-step
process mediated by specific factors, including CD81, Claudin-1, and the
scavenger receptor BI (SR-BI). Whereas the involvement of CD81 and Claudin-1 was
demonstrated by rendering susceptible cells that are otherwise refractory, SR-BI
complementation assays were lacking, raising questions as to its functions
during HCV entry. Here, we identify one hepatoma rat cell line, in which SR-BI
complementation assay and targeted mutagenesis could be performed. We therefore
demonstrate that SR-BI is an essential HCV entry factor. Our results shed light
on SR-BI intracellular domain functions in HCV entry, and, further, emphasize
the remarkable capacity of HCV to hijack the lipid transfer function of SR-BI,
hence favoring infection.
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