Hepatitis C virus infection and tight junction proteins: The ties that bind

Abstract

The hepatitis C virus (HCV) is a major cause of liver diseases ranging from liver inflammation to advanced liver diseases like cirrhosis and hepatocellular carcinoma (HCC). HCV infection is restricted to the liver, and more specifically to hepatocytes, which represent around 80% of liver cells. The mechanism of HCV entry in human hepatocytes has been extensively investigated since the discovery of the virus 30 years ago. The entry mechanism is a multi-step process relying on several host factors including heparan sulfate proteoglycan (HSPG), low density lipoprotein receptor (LDLR), tetraspanin CD81, Scavenger Receptor class B type I (SR-BI), Epidermal Growth Factor Receptor (EGFR) and Niemann-Pick C1-like 1 (NPC1L1). Moreover, in order to establish a persistent infection, HCV entry is dependent on the presence of tight junction (TJ) proteins Claudin-1 (CLDN1) and Occludin (OCLN). In the liver, tight junction proteins play a role in architecture and homeostasis including sealing the apical pole of adjacent cells to form bile canaliculi and separating the basolateral domain drained by sinusoidal blood flow. In this review, we will highlight the role of liver tight junction proteins in HCV infection, and we will discuss the potential targeted therapeutic approaches to improve virus eradication.

Keywords: Claudin-1; Hepatitis C virus; Occludin; Tight junctions; Viral entry and spreading.

Figure 1. Overview of the HCV entry process in human hepatocytes and highlight of strategies targeting TJ proteins.

Schematic representation of the HCV entry process from cell binding to genome cytoplasmic release. After attachment to the surface of hepatocytes through interaction with HSPG, LDLR and SR-BI (1), HCV binds to CD81 and activates EGFR (2). EGFR activation, as well as H-Ras and other Rho GTPAses activation, leads to the lateral diffusion of the HCV/SR-BI/CD81/EGFR (HSCE) complex towards TJs (3). During its migration the HSCE complex encounters and associates with CLDN1 and OCLN probably not involved in TJs formation (4). The virus is then internalized through clathrin-mediated endocytosis (5), before delivering its genome inside the cell via a pH-dependant membrane fusion mechanism (6). The blue boxes indicate the different strategies developed aiming at inhibiting HCV entry through interfering with TJ proteins. CLDN1-specifc mAbs are described in [, , –147], CLDN1-derived recombinant proteins are described in [151, 152], CLDN1-derived peptide is described in [ 149] and CLDN1-trafficking modulators are described in [75, 153]. OCLN-specific mAbs are described in [–157]. Figure modified from [121].

Figure 2. Model of human claudin-1 protein structure.

The TJ protein CLDN1 is composed of 4 transmembrane domains shown here as helix. Each residue of intracellular parts and extracellular loops are individually shown as a filled circle. Red-filled circles (W₃₀, I₃₂, D₃₈, EGLW₅₁) and yellow-filled circles, circled with red (C₅₄, C₆₄), depict residues involved in HCV entry, as demonstrated [12, 49, 52, 61, 62]. The yellow-filled circles (C₅₄, C₆₄) correspond to the 2 cystein residues forming a disulfide bridge in the first extracellular loop (EL1) [63]. The purple-filled circles (Y₂₁₀, V₂₁₁) at the C-terminal extremity belong to the PDZ binding domain (K₂₀₈, D₂₀₉, Y₂₁₀, V₂₁₁) suggested to be involved in CLDN1 plasma membrane localization and HCV entry [74]. The orange line indicates the peptide sequence (S53-T80) described as being involved in CLDN1-CLDN1 and CLDN1-OCLN interaction [104]. The red line shows the position of the CLDN1-derived peptide CL58 (M₁-W₁₈) described as being able to inhibit HCV infection [149]. The residues marked with a black star (W₃₀, Y₃₃, Y₃₅, D₃₉, GLW₅₁, C₅₄, C₆₄) correspond to the epitope recognized by the OM-7D3-B3 mAb [142]. The residues marked with a green star (S₇₄, M₁₅₂) belong to the epitope bound by the mAbs described by Fukasawa et al. [146].

Figure 3. Model of human occludin structure.

The TJ protein OCLN is composed of 4 transmembrane domains shown here as helix. Each residue of intracellular parts and extracellular loops are individually depicted as a filled circle. Red-filled circles (A₂₂₃, A₂₂₄, I₂₇₉, W₂₈₁) and yellow-filled circles, circled with red (C₂₁₆, C₂₃₇) were shown to be involved in HCV entry [99, 101, 103]. The yellow-filled circles (C₂₁₆, C₂₃₇) correspond to the 2 cystein residues forming a putative disulfide bridge in the second extracellular loop (EL2) [100]. The orange line correspond to the region (C₂₁₆-P₂₂₂) encompassing the epitope bond by the mAbs described by Okai et al. [157]. Residues marked with a black stars in EL1 (Y₁₀₄, P₁₀₅, G₁₀₇, G₁₀₈, G₁₁₀, F₁₁₁) and residues marked with a green star in EL2 (C₂₁₆, P₂₂₂, A₂₂₃, C₂₃₇) indicate the epitopes bond by the different mAbs described by Shimizu et al.[155].