Entangled in a membranous web: ER and lipid droplet reorganization during hepatitis C virus infection

Abstract

Hepatitis C virus (HCV) is a major cause of liver disease worldwide. To establish and maintain chronic infection, HCV extensively rearranges cellular organelles to generate distinct compartments for viral RNA replication and virion assembly. Here, we review our current knowledge of how HCV proliferates and remodels ER-derived membranes while preserving and expanding associated lipid droplets during viral infection. Unraveling the molecular mechanisms responsible for HCV-induced membrane reorganization will enhance our understanding of the HCV life-cycle, the associated liver pathology, and the biology of the ER:lipid droplet interface in general.

FIGURE 1. HCV replication and assembly are coordinated via intracellular organelles

Left, HCV infection stimulates production of DMVs and LDs. 1) After viral entry and uncoating, HCV RNA is released into the cytoplasm. 2) The HCV RNA genome is translated at the rough ER into a single large polyprotein, which is cleaved into structural and nonstructural proteins. 3) Viral proteins NS4B and NS5A, along with host factors, induce changes in the ER membrane to produce DMVs. DMVs remain attached to the ER or bud off into the cytosol and form a membranous web hosting viral RNA replicase complexes. 4) LD production increases during infection to serve as a scaffold for assembly. NS5A and core proteins are loaded onto LDs and promote HCV RNA translocation from DMVs. Upper right, DMV formation from ER membranes. Following translation and processing at ER membranes, NS5A activates PI4KIIIα, locally enriching the membrane in PI4P. FAPP2 is recruited to these sites via interaction with PI4P recruiting associated glycosphingolipids. Membrane curvature is induced by these lipid changes, along with the coordinated actions of NS5A and NS4B. Lower right, HCV assembly at LDs. The core-NS5A complex recruits replication complexes to ER membranes close to LDs. Eventually, LDs connect with ER regions containing the viral glycoproteins. Assembly begins when core and viral RNA, mediated by NS2 and NS3/4A, are transferred back to the cytosolic membrane of the ER.

FIGURE 2. The modulation of lipogenesis and lipolysis by HCV

Left, Lipogenesis is induced during HCV infection. Interaction of the HCV genome 3′-UTR with DEAD box polypeptide 3, X-linked activates IκB kinase-α, which results in SREBP-mediated expression of lipogenic genes, such as fatty acid synthase (FASN). Right, Lipolysis is inhibited during HCV infection. HCV infection has been linked to several lipases, including ATGL, HSL, AADAC and PNPL3. Core strengthens the interaction between ATGL and its activator CGI-58, and increases the recruitment of the complex to LDs. Importantly, this interaction results in suppression of ATGL activity. Core expression and HCV infection modulate HSL phosphorylation to potentially impair its activity. HCV infection also regulates the temporal expression of the putative lipase AADAC. The I148M variant of PNPLA3 causes liver steatosis in mice, which is accompanied by accumulation of the enzyme on LDs, similar to what is observed with ATGL.