The association of CD81 with tetraspanin-enriched microdomains is not essential for Hepatitis C virus entry

<p>Abstract</p> <p>Background</p> <p>Three percent of the world's population is chronically infected with hepatitis C virus (HCV) and thus at risk of developing liver cancer. Although precise mechanisms regulating HCV entry into hepatic cells are still unknown, several cell surface proteins have been identified as entry factors for this virus. Among these molecules, the tetraspanin CD81 is essential for HCV entry. Interestingly, CD81 is also required for <it>Plasmodium </it>infection. A major characteristic of tetraspanins is their ability to interact with each other and other transmembrane proteins to build tetraspanin-enriched microdomains (TEM).</p> <p>Results</p> <p>In our study, we describe a human hepatoma Huh-7 cell clone (Huh-7w7) which has lost CD81 expression and can be infected by HCV when human CD81 (hCD81) or mouse CD81 (mCD81) is ectopically expressed. We took advantage of these permissive cells expressing mCD81 and the previously described MT81/MT81w mAbs to analyze the role of TEM-associated CD81 in HCV infection. Importantly, MT81w antibody, which only recognizes TEM-associated mCD81, did not strongly affect HCV infection. Furthermore, cholesterol depletion, which inhibits HCV infection and reduces total cell surface expression of CD81, did not affect TEM-associated CD81 levels. In addition, sphingomyelinase treatment, which also reduces HCV infection and cell surface expression of total CD81, raised TEM-associated CD81 levels.</p> <p>Conclusion</p> <p>In contrast to <it>Plasmodium </it>infection, our data show that association of CD81 with TEM is not essential for the early steps of HCV life cycle, indicating that these two pathogens, while using the same molecules, invade their host by different mechanisms.</p

The CD81 Partner EWI-2wint Inhibits Hepatitis C Virus Entry

Two to three percent of the world's population is chronically infected with hepatitis C virus (HCV) and thus at risk of developing liver cancer. Although precise mechanisms regulating HCV entry into hepatic cells are still unknown, several cell surface proteins have been identified as entry factors for this virus. Among these molecules, the tetraspanin CD81 is essential for HCV entry. Here, we have identified a partner of CD81, EWI-2wint, which is expressed in several cell lines but not in hepatocytes. Ectopic expression of EWI-2wint in a hepatoma cell line susceptible to HCV infection blocked viral entry by inhibiting the interaction between the HCV envelope glycoproteins and CD81. This finding suggests that, in addition to the presence of specific entry factors in the hepatocytes, the lack of a specific inhibitor can contribute to the hepatotropism of HCV. This is the first example of a pathogen gaining entry into host cells that lack a specific inhibitory factor

Formation of native hepatitis C virus glycoprotein complexes

The hepatitis C virus (HCV) glycoproteins (E1 and E2) interact to form a heterodimeric complex, which has been proposed as a functional subunit of the HCV virion envelope. As examined in cell culture transient-expression assays, the formation of properly folded, noncovalently associated E1E2 complexes is a slow and inefficient process. Due to lack of appropriate immunological reagents, it has been difficult to distinguish between glycoprotein molecules that undergo productive folding and assembly from those which follow a nonproductive pathway leading to misfolding and aggregation. Here we report the isolation and characterization of a conformation-sensitive E2-reactive monoclonal antibody (H2). The H2 monoclonal antibody selectively recognizes slowly maturing E1E2 heterodimers which are noncovalently linked, protease resistant, and no longer associated with the endoplasmic reticulum chaperone calnexin. This complex probably represents the native prebudding form of the HCV glycoprotein heterodimer. Besides providing a novel reagent for basic studies on HCV virion assembly and entry, this monoclonal antibody should be useful for optimizing production and isolation of native HCV glycoprotein complexes for serodiagnostic and vaccine applications.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Topological changes in the transmembrane domains of hepatitis C virus envelope glycoproteins

Hepatitis C virus proteins are synthesized as a polyprotein cleaved by a signal peptidase and viral proteases. The behaviour of internal signal sequences at the C-terminus of the transmembrane domains of hepatitis C virus envelope proteins E1 and E2 is essential for the topology of downstream polypeptides. We determined the topology of these transmembrane domains before and after signal sequence cleavage by tagging E1 and E2 with epitopes and by analysing their accessibility in selectively permeabilized cells. We showed that, after cleavage by signal peptidase in the endoplasmic reticulum, the C-terminal orientation of these transmembrane domains changed from luminal to cytosolic. The dynamic behaviour of these transmembrane domains is unique and it is linked to their multifunctionality. By reorienting their C-terminus toward the cytosol and being part of a transmembrane domain, the signal sequences at the C-terminus of E1 and E2 contribute to new functions: (i) membrane anchoring; (ii) E1E2 heterodimerization; and (iii) endoplasmic reticulum retention

The interaction between HCV glycoproteins and CD81 is blocked when CD81 interacts with partner protein(s).

<p>A, HCV E1E2 heterodimers immobilized onto anti-E2 coated beads interacted with CD81 in all cells lysed in Triton X-100 (TX). However, the interaction of HCV-E1E2 with CD81 was blocked in Daudi and Ramos cells lysed in Brij (Bj). Maintenance of CD81 with other tetraspanins (T) and partners (P) is disrupted by the indicated lysis conditions, as diagrammed. Anti-CD81 and irrelevant (Cont) mAbs were used in immunoprecipitations as controls. Precipitation of CD81 was revealed by western blotting with the anti-CD81 5A6 mAb. CD81-LEL corresponds to the large extracellular loop of CD81 fused to the glutathione-S transferase. B, After cell surface biotinylation, the indicated cell lines were lysed with Bj/EDTA, immunoprecipitated with 5A6 mAb and the proteins revealed by Western blotting with HRP-conjugated streptavidin. The values on the right are molecular sizes in kilodaltons. C, EWI-2wint production is directly connected to EWI-2 expression. Daudi cells interfered with negative siRNA or EWI-2 siRNA were biotinylated, lysed in Bj/EDTA, immunoprecipitated with the anti-CD81 5A6 mAb or an anti-EWI-2 mAb (8A12) and blotted sequentially with HRP-conjugated streptavidin and 5A6 mAb.</p

EWI-2wint is a cleavage product of EWI-2.

<p>CHO cells were (co)-transfected with pcDNA3.1/CD81 and pcDNA3.1/EWI-2FLAG (A and D), with pcDNA3.1/wint-FLAG (B and E), or with pcDNA3.1/CD81 and pcDNA3.1/EWI-2FLAG^fur (C and F). EWI-2FLAG^fur corresponds to EWI-2FLAG in which a furin cleavage site has been engineered. After cell surface biotinylation, cells were lysed with Bj/EDTA (A–C) or TX/EDTA (D–F) and proteins were immunoprecipitated with the indicated mAbs. Proteins were revealed by Western blotting with HRP-conjugated streptavidin. Asterisks indicate additional cleavage products of EWI-2.</p

Hepatitis E virus lifecycle and identification of 3 forms of the ORF2 capsid protein

BACKGROUND & AIMS: Hepatitis E virus (HEV) infection is a major cause of acute hepatitis worldwide. Approximately 2 billion people live in areas endemic for HEV and are at risk of infection. The HEV genome encodes 3 proteins, including the ORF2 capsid protein. Detailed analyses of the HEV life cycle has been hampered by the lack of an efficient viral culture system. METHODS: We performed studies with gt3 HEV cell culture-produced particles and patient blood and stool samples. Samples were fractionated on iodixanol gradients and cushions. Infectivity assays were performed in vitro and in human liver chimeric mice. Proteins were analyzed by biochemical and proteomic approaches. Infectious particles were analyzed by transmission electron microscopy. HEV antigen levels were measured with the Wantai enzyme-linked immunosorbent assay. RESULTS: We developed an efficient cell culture system and isolated HEV particles that were infectious in vitro and in vivo. Using transmission electron microscopy, we defined the ultrastructure of HEV cell culture-produced particles and particles from patient sera and stool samples. We also identified the precise sequence of the infectious particle-associated ORF2 capsid protein. In cultured cells and in samples from patients, HEV produced 3 forms of the ORF2 capsid protein: infectious/intracellular ORF2 (ORF2i), glycosylated ORF2 (ORF2g), and cleaved ORF2 (ORF2c). The ORF2i protein associated with infectious particles, whereas the ORF2g and ORF2c proteins were massively secreted glycoproteins not associated with infectious particles. ORF2g and ORF2c were the most abundant antigens detected in sera from patients. CONCLUSIONS: We developed a cell culture system and characterized HEV particles; we identified 3 ORF2 capsid proteins (ORF2i, ORF2g, and ORFc). These findings will advance our understanding of the HEV life cycle and improve diagnosis

EWI-2wint expression in Huh-7 cells.

<p>After cell surface biotinylation, Huh-7 cells (A) stably expressing pcDNA3.1 (B), HAEWI-2FLAG (C) or EWI-2FLAG^fur (D) were lysed with Bj/EDTA and analyzed by immunoprecipitation with indicated mAbs. Proteins were revealed by Western blotting with HRP-conjugated streptavidin. The molecular weights of the prestained molecular ladder are indicated in KDa. The asterisk indicates an additional cleavage product of EWI-2. The band indicated by a light arrow likely corresponds to a dimer of CD81, and the band indicated by a full arrow to an unidentified EWI-2 associated protein.</p