Membranous Web Formation Research Articles

LC3B is not recruited along with the autophagy elongation complex (ATG5-12/16L1) at HCV replication site and is dispensable for viral replication.

Hepatitis C virus (HCV) infection is known to induce autophagosome accumulation as observed by the typical punctate cytoplasmic distribution of LC3B-II in infected cells. Previously, we showed that viral RNA-dependent RNA polymerase (NS5B) interacts with ATG5, a major component of the autophagy elongation complex that is involved in the formation of double-membrane vesicles (DMV), and demonstrated that the autophagy elongation complex (ATG5-12/16L1) but not LC3B is required for proper membranous web formation. In this study, the colocalization and in situ interaction of all HCV replicase components with the constituent of the autophagy elongation complex and LC3B were analyzed. The results clearly show the recruitment of the elongation complex to the site of viral replication. Using in situ proximity ligation assay, we show that ATG5, but not ATG16L1, interacts with several HCV replicase components suggesting that the recruitment is directed via the ATG5-12 conjugate. Interestingly, no E3-like conjugation activity of ATG5-12/16L1 can be detected at the at HCV replication site since LC3B-II is not found along with the elongation complex at the site of viral replication. In agreement with this result, no sign of in situ interaction of LC3B with the replicase components is observed. Finally, using dominant negative forms of ATG proteins, we demonstrate that ATG5-12 conjugate, but not LC3-II formation, is critical for viral replication. Altogether, these findings suggest that although HCV needs the elongation complex for its replication, it has developed a mechanism to avoid canonical LC3-II accumulation at viral replication site.

Interferon-inducible cholesterol-25-hydroxylase restricts hepatitis C virus replication through blockage of membranous web formation.

Hepatitis C virus (HCV) is a positive-strand RNA virus that primarily infects human hepatocytes. Infections with HCV constitute a global health problem, with 180 million people currently chronically infected. Recent studies have reported that cholesterol 25-hydroxylase (CH25H) is expressed as an interferon-stimulated gene and mediates antiviral activities against different enveloped viruses through the production of 25-hydroxycholesterol (25HC). However, the intrinsic regulation of human CH25H (hCH25H) expression within the liver as well as its mechanistic effects on HCV infectivity remain elusive. In this study, we characterized the expression of hCH25H using liver biopsies and primary human hepatocytes. In addition, the antiviral properties of this protein and its enzymatic product, 25HC, were further characterized against HCV in tissue culture. Levels of hCH25H messenger RNA were significantly up-regulated both in HCV-positive liver biopsies and in HCV-infected primary human hepatocytes. The expression of hCH25H in primary human hepatocytes was primarily and transiently induced by type I interferon. Transient expression of hCH25H in human hepatoma cells restricted HCV infection in a genotype-independent manner. This inhibition required the enzymatic activity of CH25H. We observed an inhibition of viral membrane fusion during the entry process by 25HC, which was not due to a virucidal effect. Yet the primary effect by 25HC on HCV was at the level of RNA replication, which was observed using subgenomic replicons of two different genotypes. Further analysis using electron microscopy revealed that 25HC inhibited formation of the membranous web, the HCV replication factory, independent of RNA replication. Infection with HCV causes up-regulation of interferon-inducible CH25H in vivo, and its product, 25HC, restricts HCV primarily at the level of RNA replication by preventing formation of the viral replication factory.

Soraphen A: A broad-spectrum antiviral natural product with potent anti-hepatitis C virus activity

Soraphen A (SorA) is a myxobacterial metabolite that inhibits the acetyl-CoA carboxylase, a key enzyme in lipid biosynthesis. We have previously identified SorA to efficiently inhibit the human immunodeficiency virus (HIV). The aim of the present study was to evaluate the capacity of SorA and analogues to inhibit hepatitis C virus (HCV) infection. SorA inhibition capacity was evaluated in vitro using cell culture derived HCV, HCV pseudoparticles and subgenomic replicons. Infection studies were performed in the hepatoma cell line HuH7/Scr and in primary human hepatocytes. The effects of SorA on membranous web formation were analysed by electron microscopy. SorA potently inhibits HCV infection at nanomolar concentrations. Obtained EC50 values were 0.70 nM with a HCV reporter genome, 2.30 nM with wild-type HCV and 2.52 nM with subgenomic HCV replicons. SorA neither inhibited HCV RNA translation nor HCV entry, as demonstrated with subgenomic HCV replicons and HCV pseudoparticles, suggesting an effect on HCV replication. Consistent with this, evidence was obtained that SorA interferes with formation of the membranous web, the site of HCV replication. Finally, a series of natural and synthetic SorA analogues helped to establish a first structure-activity relationship. SorA has a very potent anti-HCV activity. Since it also interferes with the membranous web formation, SorA is an excellent tool to unravel the mechanism of HCV replication.

P096: Interferon‐inducible cholesterol‐25‐hydroxylase restricts hepatitis C virus replication via blockage of membranous web formation

Journal of Viral HepatitisVolume 22, Issue S2 p. 69-69 Poster PresentationFree Access P096: Interferon-inducible cholesterol-25-hydroxylase restricts hepatitis C virus replication via blockage of membranous web formation First published: 25 June 2015 https://doi.org/10.1111/jvh.91_12425AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume22, IssueS2Special Issue: 15th International Symposium on Viral Hepatitis and Liver Diseases (ISVHLD) held from June 26th to 28th, 2015, in Berlin, GermanyJune 2015Pages 69-69 RelatedInformation

Analysis of different HCV NS4B domains for the development of global consensus sequence.

The non-structural 4B (NS4B) protein of hepatitis C virus (HCV) is ahydrophobic protein implicated recently in the formation of membranous web, aplatform for the formation of replication complex and thus is potential target for antivirals. The CLC main workbench was used to generate genotype-specific consensus sequence, global consensus sequence and arepresentative phylogenetic tree from non-structural 4 B (NS4B) protein sequences of seven different HCV genotypes reported from all over the world. The C-terminal domain (CTD) of NS4B protein especially the residues involved in interaction with ER membrane were found to be highly conserved. Other residues found to be highly conserved across all HCV genotypes included; 5 aromatic residues of N-terminal domain (NTD) (F49, W50, W55, F57, and Y63), 3 hydrophobic leucine residues (L237, L240, L245), and 2 positively charged residues of CTD (R248 and H250), dimerization motif of transmembrane domain 3 (TMD3) (G143YGAG147) and its surrounding residues (F118 and F155) and TMD1 Ser/Thr cluster residues (T87, S88 and T95) involved in the hydrogen (H) bond interactions. In short, amino acids of NTD, TMD and CTD domains involved in the membrane association/anchoring of NS4B and formation of membranous web are highly conserved and can serve as potential targets for antivirals and peptide vaccines.These conserved residues formed the basis for the development of five short peptides proposed to serve as potential therapeutic target. The phylogenetic analysis was particularly interesting for NS4B sequences of 3a Pakistani isolates. The high degree of variability prevented the clustering of Pakistani isolates with other sequences in phylogenetic tree, revealing geographical disparity.

Aminoterminal amphipathic α-helix AH1 of hepatitis C virus nonstructural protein 4B possesses a dual role in RNA replication and virus production.

Nonstructural protein 4B (NS4B) is a key organizer of hepatitis C virus (HCV) replication complex formation. In concert with other nonstructural proteins, it induces a specific membrane rearrangement, designated as membranous web, which serves as a scaffold for the HCV replicase. The N-terminal part of NS4B comprises a predicted and a structurally resolved amphipathic α-helix, designated as AH1 and AH2, respectively. Here, we report a detailed structure-function analysis of NS4B AH1. Circular dichroism and nuclear magnetic resonance structural analyses revealed that AH1 folds into an amphipathic α-helix extending from NS4B amino acid 4 to 32, with positively charged residues flanking the helix. These residues are conserved among hepaciviruses. Mutagenesis and selection of pseudorevertants revealed an important role of these residues in RNA replication by affecting the biogenesis of double-membrane vesicles making up the membranous web. Moreover, alanine substitution of conserved acidic residues on the hydrophilic side of the helix reduced infectivity without significantly affecting RNA replication, indicating that AH1 is also involved in virus production. Selective membrane permeabilization and immunofluorescence microscopy analyses of a functional replicon harboring an epitope tag between NS4B AH1 and AH2 revealed a dual membrane topology of the N-terminal part of NS4B during HCV RNA replication. Luminal translocation was unaffected by the mutations introduced into AH1, but was abrogated by mutations introduced into AH2. In conclusion, our study reports the three-dimensional structure of AH1 from HCV NS4B, and highlights the importance of positively charged amino acid residues flanking this amphipathic α-helix in membranous web formation and RNA replication. In addition, we demonstrate that AH1 possesses a dual role in RNA replication and virus production, potentially governed by different topologies of the N-terminal part of NS4B.

Inhibition of HCV Replication by Cyclophilin Antagonists Is Linked to Replication Fitness and Occurs by Inhibition of Membranous Web Formation

Replication of hepatitis C virus (HCV) requires host cell factors, such as cyclophilin A (CypA). CypA binds to HCV's nonstructural protein (NS)5A to promote replication of viral RNA. CypA antagonists, such as cyclosporines, are potent inhibitors of HCV replication. NS2 modulates sensitivity of HCV to cyclosporines. We investigated why cyclosporines require NS2 to increase their inhibitory effect and how they block HCV replication. We determined replication fitness and sensitivity of various HCV replicons, containing or lacking NS2, to cyclosporine and other direct-acting antiviral agents. We also analyzed the effects of cyclosporine on membranous web formation by electron microscopy. NS2-5B replicons of genotype 2a (JFH1), but not genotype 1b, had increased sensitivity to cyclosporine. This difference was lost with replication-attenuated NS3-5B JFH1 RNAs, showing that cyclosporine sensitivity is linked to reduced replication fitness of NS2-containing HCV RNAs. Fitness also determined sensitivity to a nucleoside analogue and an NS5A inhibitor, but not to telaprevir. Cyclosporine blocked de novo formation of the membranous web, but had little effect on established membranous replication factories. This block was prevented by cyclosporine resistance mutations in NS5A. Cleavage at the NS2/3 junction is a rate-limiting step in replication of particular HCV isolates and determines their sensitivity to CypA inhibitors. These drugs target de novo formation of the membranous web and RNA replication.

The Role of the Phosphatidylinositol 4-Kinase PI4KA in Hepatitis C Virus-Induced Host Membrane Rearrangement

BackgroundHepatitis C virus (HCV), like other positive-sense RNA viruses, replicates on an altered host membrane compartment that has been called the “membranous web.” The mechanisms by which the membranous web are formed from cellular membranes are poorly understood. Several recent RNA interference screens have demonstrated a critical role for the host phosphatidylinositol 4-kinase PI4KA in HCV replication. We have sought to define the function of PI4KA in viral replication.Methodology/Principal FindingsUsing a nonreplicative model of membranous web formation, we show that PI4KA silencing leads to aberrant web morphology. Furthermore, we find that PI4KA and its product, phosphatidylinositol 4-phosphate, are enriched on membranous webs and that PI4KA is found in association with NS5A in HCV-infected cells. While the related lipid kinase PI4KB also appears to support HCV replication, it does not interact with NS5A. Silencing of PI4KB does not overtly impair membranous web morphology or phosphatidylinositol 4-phosphate enrichment at webs, suggesting that it acts at a different point in viral replication. Finally, we demonstrate that the aberrant webs induced by PI4KA silencing require the activity of the viral NS3-4A serine protease but not integrity of the host secretory pathway.Conclusions/SignificancePI4KA is necessary for the local enrichment of PI 4-phosphate at the HCV membranous web and for the generation of morphologically normal webs. We also show that nonreplicative systems of web formation can be used to order molecular events that drive web assembly.

Amphipathic α-Helix AH2 Is a Major Determinant for the Oligomerization of Hepatitis C Virus Nonstructural Protein 4B

Nonstructural protein 4B (NS4B) is a key organizer of hepatitis C virus (HCV) replication complex formation. It induces a specific membrane rearrangement, designated membranous web, that serves as a scaffold for the HCV replication complex. However, the mechanisms underlying membranous web formation are poorly understood. Based on fluorescence resonance energy transfer (FRET) and confirmatory coimmunoprecipitation analyses, we provide evidence for an oligomerization of NS4B in the membrane environment of intact cells. Several conserved determinants were found to be involved in NS4B oligomerization, through homotypic and heterotypic interactions. N-terminal amphipathic α-helix AH2, comprising amino acids 42 to 66, was identified as a major determinant for NS4B oligomerization. Mutations that affected the oligomerization of NS4B disrupted membranous web formation and HCV RNA replication, implying that oligomerization of NS4B is required for the creation of a functional replication complex. These findings enhance our understanding of the functional architecture of the HCV replication complex and may provide new angles for therapeutic intervention. At the same time, they expand the list of positive-strand RNA virus replicase components acting as oligomers.

A Dynamic View of Hepatitis C Virus Replication Complexes

Hepatitis C virus (HCV) replicates its genome in a membrane-associated replication complex (RC). Specific membrane alterations, designated membranous webs, represent predominant sites of HCV RNA replication. The principles governing HCV RC and membranous web formation are poorly understood. Here, we used replicons harboring a green fluorescent protein (GFP) insertion in nonstructural protein 5A (NS5A) to study HCV RCs in live cells. Two distinct patterns of NS5A-GFP were observed. (i) Large structures, representing membranous webs, showed restricted motility, were stable over many hours, were partitioned among daughter cells during cell division, and displayed a static internal architecture without detectable exchange of NS5A-GFP. (ii) In contrast, small structures, presumably representing small RCs, showed fast, saltatory movements over long distances. Both populations were associated with endoplasmic reticulum (ER) tubules, but only small RCs showed ER-independent, microtubule (MT)-dependent transport. We suggest that this MT-dependent transport sustains two distinct RC populations, which are both required during the HCV life cycle.

Computational Analysis of the Hepatitis C Virus Non-Structural Protein 4B and Functional Implications

Introduction: NS4B is a multifunctional protein, involved in membranous web formation and regulation of signal transduction cascades. The protein is located with the endoplasmic reticulum membrane. Four to five transmembrane domains (TMDs) were discussed for the membrane topology. N- and C-termini are most probably oriented towards the cytoplasm. Although the flaviviridae family has a similar genetic organisation, a comparison of HCV NS4B to other members of the family is still missing. Methods: We used bioinformatic approaches to analyse structure and function relationships in HCV NS4B. Using the BLAST suite of programs (NCBI-NR and UniProt), we extracted a set of candidate sequences. Representative sequences were selected for each clade of the phylogeny. Subsequently, multiple alignments were generated and hand-curated with CLUSTAL W. To get reliable overall results, we used secondary structure and transmembrane prediction methods, as well as consensus procedures. Results: We could confirm a central non-globular membrane portion with four to five TMDs from a N- and C-terminal part with non-transmembrane helices. The central part showed new functionally-relevant sequence motif. The N-terminal part of NS4B was the most heterogeneous in phylogenetic and sequence analysis of flavi-, pesti-, GB- and hepaciviruses, with GB-viruses more closely related to hepaci- than to other flaviviruses. A heptad repeat within a leucine-rich amphipathic helix has been characterized, suspected as functionally important interaction site. The C-terminal part shows significant similarities in sequence and secondary structure throughout flaviviridae. Conclusions: NS4B N- and C-terminal parts seem to be functionally important, whereas the central protein portion serves as membrane anchor. A N-terminal leucine zipper motif is unique in hepaciviruses and probably important to interact with the basic leucine zipper (bZIP) domain of CREB-RP/ATF6β, a human activating transcription factor, stimulated upon ER-stress. That way HCV could be able to interfere and modulate ER-stress induced gene expression patterns. Site-directed mutational studies are needed to identify crucial positions within the NS4B bZIP motif and study effects on viral replication and cellular transformation.