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Hepatitis C virus (HCV) assembly remains a poorly understood process. Lipid droplets (LDs) are thought to act as platforms for the assembly of viral components. The JFH1 HCV strain replicates and assembles in association with LD-associated membranes, around which viral core protein is predominantly detected. In contrast, despite its intrinsic capacity to localize to LDs when expressed individually, we found that the core protein of the high-titer Jc1 recombinant virus was hardly detected on LDs of cell culture-grown HCV (HCVcc)-infected cells, but was mainly localized at endoplasmic reticulum (ER) membranes where it colocalized with the HCV envelope glycoproteins. Furthermore, high-titer cell culture-adapted JFH1 virus, obtained after long-term culture in Huh7.5 cells, exhibited an ER-localized core in contrast to non-adapted JFH1 virus, strengthening the hypothesis that ER localization of core is required for efficient HCV assembly. Our results further indicate that p7 and NS2 are HCV strain-specific factors that govern the recruitment of core protein from LDs to ER assembly sites. Indeed, using expression constructs and HCVcc recombinant genomes, we found that p7 is sufficient to induce core localization at the ER, independently of its ion-channel activity. Importantly, the combined expression of JFH1 or Jc1 p7 and NS2 induced the same differential core subcellular localization detected in JFH1- vs. Jc1-infected cells. Finally, results obtained by expressing p7-NS2 chimeras between either virus type indicated that compatibilities between the p7 and the first NS2 trans-membrane domains is required to induce core-ER localization and assembly of extra- and intra-cellular infectious viral particles. In conclusion, we identified p7 and NS2 as key determinants governing the subcellular localization of HCV core to LDs vs. ER and required for initiation of the early steps of virus assembly., Author Summary Hepatitis C virus (HCV), an enveloped virus that causes chronic liver infection, encodes a polyprotein that is translated and undergoes maturation by cleavage at the endoplasmic reticulum (ER). The assembly of the viral structural components, including core, the capsid protein, the E1/E2 envelope glycoproteins, and the vRNA is believed to occur at the ER, requiring a coordinated integration of cellular and viral pathways in which the HCV non-structural proteins play a major role. The cytosolic lipid droplets (LDs) induce concentration of core close to the ER-located assembly site and may provide a physical link with the vRNA replication site, also localized in specialized, ER-derived structures. Here, we analyzed the subcellular localization pattern of core protein in HCV-infected cells with a particular focus on core colocalization with E2 in the ER or with specific markers of the LDs. We show that the p7 and NS2 proteins are key viral determinants governing the cellular localization of HCV core to LDs vs. ER and are required for virus assembly. Our results also underscore a requirement for compatibilities between the p7 trans-membranes and the NS2 amino-terminus that dictates core-E2 colocalization in the ER, leading to initiation of virion assembly.

Several conserved domains critical for E1E2 assembly and hepatitis C virus entry have been identified in E1 and E2 envelope glycoproteins. However, the role of less conserved domains involved in cross-talk between either glycoprotein must be defined to fully understand how E1E2 undergoes conformational changes during cell entry. To characterize such domains and to identify their functional partners, we analyzed a set of intergenotypic E1E2 heterodimers derived from E1 and E2 of different genotypes. The infectivity of virions indicated that Con1 E1 did not form functional heterodimers when associated with E2 from H77. Biochemical analyses demonstrated that the reduced infectivity was not related to alteration of conformation and incorporation of Con1 E1/H77 E2 heterodimers but rather to cell entry defects. Thus, we generated chimeric E1E2 glycoproteins by exchanging different domains of each protein in order to restore functional heterodimers. We found that both the ectodomain and transmembrane domain of E1 influenced infectivity. Site-directed mutagenesis highlighted the role of amino acids 359, 373, and 375 in transmembrane domain in entry. In addition, we identified one domain involved in entry within the N-terminal part of E1, and we isolated a motif at position 219 that is critical for H77 function. Interestingly, using additional chimeric E1E2 complexes harboring substitutions in this motif, we found that the transmembrane domain of E1 acts as a partner of this motif. Therefore, we characterized domains of E1 and E2 that have co-evolved inside a given genotype to optimize their interactions and allow efficient entry.

Taking advantage of the wide tropism of baculoviruses (BVs), we constructed a recombinant BV (BVCAR) pseudotyped with human coxsackie B-adenovirus receptor (CAR), the high-affinity attachment receptor for adenovirus type 5 (Ad5), and used the strategy of piggybacking Ad5-green fluorescent protein (Ad5GFP) vector on BVCARto transduce various cells refractory to Ad5 infection. We found that transduction of all cells tested, including human primary cells and cancer cell lines, was significantly improved using the BVCAR-Ad5GFP biviral complex compared to that obtained with Ad5GFP or BVCARGFP alone. We determined the optimal conditions for the formation of the complex and found that a high level of BVCAR-Ad5GFP-mediated transduction occurred at relatively low adenovirus vector doses, compared with transduction by Ad5GFP alone. The increase in transduction was dependent on the direct coupling of BVCARto Ad5GFP via CAR-fiber knob interaction, and the cell attachment of the BVCAR-Ad5GFP complex was mediated by the baculoviral envelope glycoprotein gp64. Analysis of the virus-cell binding reaction indicated that the presence of BVCARin the complex provided kinetic benefits to Ad5GFP compared to the effects with Ad5GFP alone. The endocytic pathway of BVCAR-Ad5GFP did not require Ad5 penton base RGD-integrin interaction. Biodistribution of BVCAR-Ad5Luc complex in vivo was studied by intravenous administration to nude BALB/c mice and compared to Ad5Luc injected alone. No significant difference in viscerotropism was found between the two inocula, and the liver remained the preferred localization. In vitro, coagulation factor X drastically increased the Ad5GFP-mediated transduction of CAR-negative cells but had no effect on the efficiency of transduction by the BVCAR-Ad5GFP complex. Various situations in vitro or ex vivo in which our BVCAR-Ad5 duo could be advantageously used as gene transfer biviral vector are discussed.

(IF : 4,608); International audience; Cystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, and the cellular trafficking of the CFTR protein is an essential factor that determines its function in cells. The aim of our study was to develop an Ad vector expressing a biologically active green fluorescent protein (GFP)- CFTR chimera that can be tracked by both its localization and chloride channel function. No study thus far has demonstrated a GFP-CFTR construct that displayed both of these functions in the airway epithelia. Tracheal glandular cells, MM39 (CFTRwt) and CFKM4 (CFTRDF508), as well as human airway epithelial cells from a patient with cystic fibrosis (CF-HAE) and from a healthy donor (HAE) were used for the functional analysis of our Ad vectors, Ad5/ GFP-CFTRwt and Ad5/GFP-CFTRDF508. The GFP-CFTRwt protein expressed was efficiently addressed to the plasma membrane of tracheal cells and to the apical surface of polarized CF-HAE cells, while GFP-CFTRDF508 mutant was sequestered intracellularly. The functionality of the GFP-CFTRwt protein was demonstrated by its capacity to correct the chloride channel activity both in CF-KM4 and CF-HAE cells after Ad transduction. A correlation between the proportion of Ad5-transduced CF-KM4 cells and correction of CFTR function showed that 55 to 70% transduction resulted in 70% correction of the Cl2 channel function. In reconstituted CF-HAE, GFP-CFTRwt appeared as active as the nontagged CFTRwt protein in correcting the transepithelial Cl2 transport. We show for the first time a GFP-CFTR chimera that localized to the apical surface of human airway epithelia and restored epithelial chloride transport to similar levels as nontagged CFTR.

Most human carcinoma cell lines lack the high-affinity receptors for adenovirus serotype 5 (Ad5) at their surface and are nonpermissive to Ad5. We therefore tested the efficiency of retargeting Ad5 to alternative cellular receptors via immunoglobulin (Ig)-binding domains inserted at the extremity of short-shafted, knobless fibers. The two recombinant Ad5's constructed, Ad5/R7-Z(wt)-Z(wt) and Ad5/R7-C2-C2, carried tandem Ig-binding domains from Staphylococcal protein A (abbreviated Z(wt)) and from Streptococcal protein G (C2), respectively. Both viruses bound their specific Ig isotypes with the expected affinity. They transduced human carcinoma cells independently of the CAR pathway, via cell surface receptors targeted by specific monoclonal antibodies, that is, EGF-R on A549, HT29 and SW1116, HER-2/neu on SK-OV-3 and SK-BR-3, CA242 (epitope recognized by the monoclonal antibody C242) antigen on HT29 and SW1116, and PSMA (prostate-specific membrane antigen) expressed on HEK-293 cells, respectively. However, Colo201 and Colo205 cells were neither transduced by targeting CA242 or EGF-R nor were LNCaP cells transduced by targeting PSMA. Our results suggested that one given surface receptor could mediate transduction of certain cells but not others, indicating that factors and steps other than cell surface expression and virus-receptor interaction are additional determinants of Ad5-mediated transduction of tumor cells. Using penton base RGD mutants, we found that one of these limiting steps was virus endocytosis.