Large hepatitis delta antigen is phosphorylated at multiple sites and phosphorylation is associated with protein conformational change

Cellular Factors Involved in the Hepatitis D Virus Life Cycle

Hepatitis D virus (HDV) is a defective RNA virus with a negative-strand RNA genome encompassing less than 1700 nucleotides. The HDV genome encodes only for one protein, the hepatitis delta antigen (HDAg), which exists in two forms acting as nucleoproteins. HDV depends on the envelope proteins of the hepatitis B virus as a helper virus for packaging its ribonucleoprotein complex (RNP). HDV is considered the causative agent for the most severe form of viral hepatitis leading to liver fibrosis/cirrhosis and hepatocellular carcinoma. Many steps of the life cycle of HDV are still enigmatic. This review gives an overview of the complete life cycle of HDV and identifies gaps in knowledge. The focus is on the description of cellular factors being involved in the life cycle of HDV and the deregulation of cellular pathways by HDV with respect to their relevance for viral replication, morphogenesis and HDV-associated pathogenesis. Moreover, recent progress in antiviral strategies targeting cellular structures is summarized in this article.

Life cycle and pathogenesis of hepatitis D virus: A review

Hepatitis D virus (HDV) is a defective RNA virus which requires the help of hepatitis B virus (HBV) virus for its replication and assembly of new virions. HDV genome contains only one actively transcribed open reading frame which encodes for two isoforms of hepatitis delta antigen. Post-translational modifications of small and large delta antigens (S-HDAg and L-HDAg) involving phosphorylation and isoprenylation respectively confer these antigens their specific properties. S-HDAg is required for the initiation of the viral genome replication, whereas L-HDAg serves as a principal inhibitor of replication and is essential for the assembly of new virion particles. Immune mediation has usually been implicated in HDV-associated liver damage. The pathogenesis of HDV mainly involves interferon-α signaling inhibition, HDV-specific T-lymphocyte activation and cytokine responses, and tumor necrosis factor-alpha and nuclear factor kappa B signaling. Due to limited protein coding capacity, HDV makes use of host cellular proteins to accomplish their life cycle processes, including transcription, replication, post-transcriptional and translational modifications. This intimate host-pathogen interaction significantly alters cell proteome and is associated with an augmented expression of pro-inflammatory, growth and anti-apoptotic factors which explains severe necroinflammation and increased cell survival and an early progression to hepatocellular carcinoma in HDV patients. The understanding of the process of viral replication, HBV-HDV interactions, and etio-pathogenesis of the severe course of HDV infection is helpful in identifying the potential therapeutic targets in the virus life cycle for the prophylaxis and treatment of HDV infection and complications.

Molecular and clinical aspects of hepatitis D virus infections

Hepatitis D virus (HDV) is a defective virus with circular, single-stranded genomic RNA which needs hepatitis B virus (HBV) as a helper virus for virion assembly and infectivity. HDV virions are composed of a circular shape HDV RNA and two types of viral proteins, small and large HDAgs, surrounded by HBV surface antigen (HBsAg). The RNA polymerase II from infected hepatocytes is responsible for synthesizing RNAs with positive and negative polarities for HDV, as the virus does not code any enzyme to replicate its genome. HDV occurs as co-infection or super-infection in up to 5% of HBsAg carriers. A recent multi-center study highlighted that pegylated interferon α-2a (PEG-IFN) is currently the only treatment option for delta hepatitis. Nucleotide/nucleoside analogues, which are effective against HBV, have no relevant effects on HDV. However, additional clinical trials combining PEG-IFN and tenofovir are currently ongoing. The molecular interactions between HDV and HBV are incompletely understood. Despite fluctuating patterns of HBV viral load in the presence of HDV in patients, several observations indicate that HDV has suppressive effects on HBV replication, and even in triple infections with HDV, HBV and HCV, replication of both concomitant viruses can be reduced. Additional molecular virology studies are warranted to clarify how HDV interacts with the helper virus and which key cellular pathways are used by both viruses. Further clinical trials are underway to optimize treatment strategies for delta hepatitis.

Modification of small hepatitis delta virus antigen by SUMO protein

Hepatitis delta antigen (HDAg) is a nuclear protein that is intimately involved in hepatitis delta virus (HDV) RNA replication. HDAg consists of two protein species, the small form (S-HDAg) and the large form (L-HDAg). Previous studies have shown that posttranslational modifications of S-HDAg, such as phosphorylation, acetylation, and methylation, can modulate HDV RNA replication. In this study, we show that S-HDAg is a small ubiquitin-like modifier 1 (SUMO1) target protein. Mapping data showed that multiple lysine residues are SUMO1 acceptors within S-HDAg. Using a genetic fusion strategy, we found that conjugation of SUMO1 to S-HDAg selectively enhanced HDV genomic RNA and mRNA synthesis but not antigenomic RNA synthesis. This result supports our previous proposition that the cellular machinery involved in the synthesis of HDV antigenomic RNA is different from that for genomic RNA synthesis and mRNA transcription, requiring different modified forms of S-HDAg. Sumoylation represents a new type of modification for HDAg.

Transcription of subgenomic mRNA of hepatitis delta virus requires a modified hepatitis delta antigen that is distinct from antigenomic RNA synthesis

Hepatitis delta virus (HDV) contains a viroid-like, 1.7-kb circular RNA genome, which replicates via a double-rolling-circle model. However, the exact mechanism involved in HDV genome RNA replication and subgenomic mRNA transcription is still unclear. Our previous studies have shown that the replications of genomic and antigenomic HDV RNA strands have different sensitivities to alpha-amanitin and are associated with different nuclear bodies, suggesting that these two strands are synthesized in different transcription machineries in the cells. In this study, we developed a unique quantitative reverse transcription-PCR (qRT-PCR) procedure for detection of various HDV RNA species from an RNA transfection system. Using this qRT-PCR procedure and a series of HDV mutants, we demonstrated that Arg-13 methylation, Lys-72 acetylation, and Ser-177 phosphorylation of small hepatitis delta antigen (S-HDAg) are important for HDV mRNA transcription. In addition, these three S-HDAg modifications are dispensable for antigenomic RNA synthesis but are required for genomic RNA synthesis. Furthermore, the three RNA species had different sensitivities to acetylation and deacetylation inhibitors, showing that the metabolic requirements for the synthesis of HDV antigenomic RNA are different from those for the synthesis of genomic RNA and mRNA. In sum, our data support the hypothesis that the cellular machinery involved in the synthesis of HDV antigenomic RNA is different from that of genomic RNA synthesis and mRNA transcription, even though the antigenomic RNA and the mRNA are made from the same RNA template. We propose that acetylation and deacetylation of HDAg may provide a molecular switch for the synthesis of the different HDV RNA species.

Post-translational modification of delta antigen of hepatitis D virus

The hepatitis delta virus (HDV) genome has only one open reading frame, which encodes the viral small delta antigen. After RNA editing, the same open reading frame is extended 19 amino acids at the carboxyl terminus and encodes the large delta antigen. These two viral proteins escort the HDV genome through different cellular compartments for the complicated phases of replication, transcription and, eventually, the formation of progeny virions. To orchestrate these events, the delta antigens have to take distinct cues to traffic to the right compartments and make correct molecular contacts. In eukaryotes, post-translational modification (PTM) is a major mechanism of dictating the multiple functions of a single protein. Multiple PTMs, including phosphorylation, isoprenylation, acetylation, and methylation, have been identified on hepatitis delta antigens. In this chapter we review these PTMs and discuss their functions in regulating and coordinating the life cycle of HDV.