The Role of Cytosolic Lipid Droplets in Hepatitis C Virus Replication, Assembly, and Release

Abstract

The hepatitis C virus (HCV) causes chronic hepatitis by establishing a persistent infection. Patients with chronic hepatitis frequently develop hepatic cirrhosis, which can lead to liver cancer-the progressive liver damage results from the host's immune response to the unresolved infection. The HCV replication process, including the entry, replication, assembly, and release stages, while the virus circulates in the bloodstream, it is intricately linked to the host's lipid metabolism, including the dynamic of the cytosolic lipid droplets (cLDs). This review article depicts how this interaction regulates viral cell tropism and aids immune evasion by coining viral particle characteristics. cLDs are intracellular organelles that store most of the cytoplasmic components of neutral lipids and are assumed to play an increasingly important role in the pathophysiology of lipid metabolism and host-virus interactions. cLDs are involved in the replication of several clinically significant viruses, where viruses alter the lipidomic profiles of host cells to improve viral life cycles. cLDs are involved in almost every phase of the HCV life cycle. Indeed, pharmacological modulators of cholesterol synthesis and intracellular trafficking, lipoprotein maturation, and lipid signaling molecules inhibit the assembly of HCV virions. Likewise, small-molecule inhibitors of cLD-regulating proteins inhibit HCV replication. Thus, addressing the molecular architecture of HCV replication will aid in elucidating its pathogenesis and devising preventive interventions that impede persistent infection and prevent disease progression. This is possible via repurposing the available therapeutic agents that alter cLDs metabolism. This review highlights the role of cLD in HCV replication.

Figure 1

Structure of the HCV genome and translation. The HCV genome comprises a single-stranded positive RNA genome with a single open reading frame that codes for structural and nonstructural proteins and the 5′ untranslated region (UTR) with four highly organized domains, such as I, II, III, and IV, and the IRES. The 3′ UTR consists of stable stem-loop structures and an internal poly(U)/polypyrimidine tract. Depending on the HCV genotype, the single open reading frame codes for a polyprotein with over 3000 amino acids. The 10 HCV proteins' polyprotein enzymatic processing is depicted. The diamond shape (colored purple) represents cleavage sites for host-cell signal peptidase, the triangle shape (colored green) represents autocatalytic cleavage by the NS2-NS3 zinc-dependent metalloproteinase of the NS2-NS3 junction, and the triangle shape (colored grey) represents cleavage sites for the NS3-NS4 serine proteinase complex.

Figure 2

Basic morphology of cLDs. The hydrophobic neutral lipid core of cLDs is encircled by a monolayer phospholipid membrane, which separates hydrophobic neutral lipids from the aqueous cytoplasmic environment. Neutral lipids, primarily TG and CE, are stored in varying ratios in the hydrophobic core of cLDs. Besides membrane phospholipid compositions, the surface protein comprises perilipin (PLIN) proteins.

Figure 3

Replication and assembly of viral particles. The replication complex (RC) brings together the NS3 protease and its cofactors, such as NS4A, NS4B, NS5A, and NS5B, to form the “membranous web” (1). NS4B protein mediates intracellular membrane rearrangement, particularly in partially closed double membrane structures where replication occurs (2). Following the assembly of the replication complex, the core accumulates on cLDs which is required to initiate the assembly phase (3). Several endogenous mechanisms coordinate the translocation of NS5A to the LD (4), presumably transferring the nascent HCV RNA to the budding nucleocapsid (NC) (5). Through apoE-E1E2 contact, the immature viral particle merges with or adheres to a luminal lipid droplet for maturation and secretion resembling that of VLDL (6).

Figure 4

Lifecycle of HCV. (a) Binding of virus along with its internalization. (b) Release in cytoplasm followed by uncoating. (c) IRES-mediated translation and processing of polyprotein. (d) RNA replication. (e) Assembly of virions. (f) Maturation of virions and their subsequent release.

Figure 5

Coupling of cLD metabolism and HCV replication. (a) HCV replication induces extensive ER membrane remodeling to establish a structural complex for replication and morphogenesis. (b) cLD-associated proteins (LDAPs), particularly TIP47 and ADRP, are essential in initiating the morphogenesis of cLDs, initially by coordinating the accumulation of neutral lipids at the ER leaflets and eventually the excision of the cLD from the ER. (c) LDAPs, including TIP47 and ADRP, regulate the metabolic dynamic of cLDs in response to the metabolic status of the cell as to whether they stimulate lipogenesis (cLD enlargement) or lipolysis (cLD diminution). (d) The HCV assembly and release are coupled with the metabolic and structural dynamics of cLDs and apolipoproteins. LSAPs like ADRB and TIP47 mutually coordinate the metabolism and morphogenesis of cLDs and directly interact with HCV proteins like core and NS5A to facilitate the assembly and release of HCV progeny particles.