Palmitoylation mediates membrane association of hepatitis E virus ORF3 protein and is required for infectious particle secretion

Virus-Host Cell Interplay during Hepatitis E Virus Infection

The molecular interplay between cellular host factors and viral proteins is a continuous process throughout the viral life cycle determining virus host range and pathogenesis. The hepatitis E virus (HEV) is a long-neglected RNA virus and the major causative agent of acute viral hepatitis in humans worldwide. However, the mechanisms of liver pathology and clinical disease remain poorly understood for HEV infection. This review summarizes our current understanding of HEV-host cell interactions and highlights experimental strategies and techniques to identify novel host components required for the viral life cycle as well as restriction factors. Understanding these interactions will provide insight into the viral life cycle of HEV and might further help to devise novel therapeutic strategies and antiviral targets.

Hepatitis E Virus: How It Escapes Host Innate Immunity

Hepatitis E virus (HEV) is a leading cause of viral hepatitis in the world. It is usually responsible for acute hepatitis, but can lead to a chronic infection in immunocompromised patients. The host's innate immune response is the first line of defense against a virus infection; there is growing evidence that HEV RNA is recognized by toll-like receptors (TLRs) and retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs), leading to interferon (IFN) production. The IFNs activate interferon-stimulated genes (ISGs) to limit HEV replication and spread. HEV has developed strategies to counteract this antiviral response, by limiting IFN induction and signaling. This review summarizes the advances in our knowledge of intracellular pathogen recognition, interferon and inflammatory response, and the role of virus protein in immune evasion.

On the Host Side of the Hepatitis E Virus Life Cycle

Hepatitis E virus (HEV) infection is one of the most common causes of acute hepatitis in the world. HEV is an enterically transmitted positive-strand RNA virus found as a non-enveloped particle in bile as well as stool and as a quasi-enveloped particle in blood. Current understanding of the molecular mechanisms and host factors involved in productive HEV infection is incomplete, but recently developed model systems have facilitated rapid progress in this area. Here, we provide an overview of the HEV life cycle with a focus on the host factors required for viral entry, RNA replication, assembly and release. Further developments of HEV model systems and novel technologies should yield a broader picture in the future.

Hepatitis E virus replication in human intestinal cells

OBJECTIVE

Hepatitis E virus (HEV), one of the most common agent of acute hepatitis worldwide, is mainly transmitted enterically, via contaminated water for HEV genotypes 1 (HEV1) and HEV2, or by eating raw or undercooked infected meat for HEV genotype 3 (HEV3) and HEV4. However, little is known about how the ingested HEV reaches the liver or its ability to replicate in intestinal cells.

DESIGN

We developed human primary cultures of small intestine epithelial cells and intestinal explants obtained from small bowel resections. The epithelial cells were also polarised on transwells. Cells were infected with Kernow-p6 strain or clinically derived virions.

RESULTS

Primary intestinal cells supported the growth of Kernow-p6 strain and HEV1 and HEV3 clinically derived virions. Polarised enterocytes infected with HEV1 and HEV3 strains released HEV particles vectorially: mostly into the apical compartment with a little basally. Iodixanol density gradient centrifugation of enterocyte-derived HEV virions gave bands at a density of 1.06-1.08 g/cm³, corresponding to that of quasi-enveloped HEV particles. Ribavirin therapy inhibited HEV excretion from the basal surface but not from the apical side of infected human enterocytes. HEV virions also infected intestinal tissue explants. Lastly, HEV RNA and antigen were detected in the intestinal crypts of a chronically infected patient.

CONCLUSION

HEV can replicate in intestinal cells and reaches the liver as quasi-enveloped virions.

Optimized Hepatitis E Virus (HEV) Culture and its Application to Measurements of HEV Infectivity

Hepatitis E virus (HEV) is a major concern in public health worldwide. Infections with HEV genotypes 3, 4, or 7 can lead to chronic hepatitis while genotype 1 infections can trigger severe hepatitis in pregnant women. Infections with all genotypes can worsen chronic liver diseases. As virions are lipid-associated in blood and naked in feces, efficient methods of propagating HEV clinical strains in vitro and evaluating the infectivity of both HEV forms are needed. We evaluated the spread of clinical strains of HEV genotypes 1 (HEV1) and 3 (HEV3) by quantifying viral RNA in culture supernatants and cell lysates. Infectivity was determined by endpoint dilution and calculation of the tissue culture infectious dose 50 (TCID50). An enhanced HEV production could be obtained varying the composition of the medium, including fetal bovine serum (FBS) and dimethylsulfoxide (DMSO) content. This increased TCID50 from 10 to 100-fold and allowed us to quantify HEV1 infectivity. These optimized methods for propagating and measuring HEV infectivity could be applied to health safety processes and will be useful for testing new antiviral drugs.

Toward the identification of ZDHHC enzymes required for palmitoylation of viral protein as potential drug targets

Introduction: S-acylation is the attachment of fatty acids not only to cysteines of cellular, but also of viral proteins. The modification is often crucial for the protein´s function and hence for virus replication. Transfer of fatty acids is mediated by one or several of the 23 members of the ZDHHC family of proteins. Since their genes are linked to various human diseases, they represent drug targets.Areas covered: The authors explore whether targeting acylation of viral proteins might be a strategy to combat viral diseases. Many human pathogens contain S-acylated proteins; the ZDHHCs involved in their acylation are currently identified. Based on the 3D structure of two ZDHHCs, the regulation and the biochemistry of the palmitolyation reaction and the lipid and protein substrate specificities are discussed. The authors then speculate how ZDHHCs might recognize S-acylated membrane proteins of Influenza virus.Expert opinion: Although many viral diseases can now be treated, the available drugs bind to viral proteins that rapidly mutate and become resistant. To develop inhibitors for the genetically more stable cellular ZDHHCs, their binding sites for viral substrates need to be identified. If only a few cellular proteins are recognized by the same binding site, development of specific inhibitors may have therapeutic potential.

Recombinant Hepatitis E Viruses Harboring Tags in the ORF1 Protein

Hepatitis E virus (HEV) infection is an important cause of acute hepatitis and may lead to chronic infection in immunocompromised patients. Knowledge of the viral life cycle is incomplete due to the limited availability of functional tools. In particular, low levels of expression of the ORF1 protein or limited sensitivity of currently available antibodies or both limit our understanding of the viral replicase. Here, we report the successful establishment of subgenomic HEV replicons and full-length genomes harboring an epitope tag or a functional reporter in the ORF1 protein. These novel tools should allow further characterization of the HEV replication complex and to improve our understanding of the viral life cycle.

Hepatitis E virus (HEV) is one of the most common causes of acute hepatitis and jaundice in the world. Current understanding of the molecular virology and pathogenesis of hepatitis E is incomplete, due particularly to the limited availability of functional tools. Here, we report the development of tagged HEV genomes as a novel tool to investigate the viral life cycle. A selectable subgenomic HEV replicon was subjected to random 15-nucleotide sequence insertion using transposon-based technology. Viable insertions in the open reading frame 1 (ORF1) protein were selected in a hepatoblastoma cell line. Functional insertion sites were identified downstream of the methyltransferase domain, in the hypervariable region (HVR), and between the helicase and RNA-dependent RNA polymerase domains. HEV genomes harboring a hemagglutinin (HA) epitope tag or a small luciferase (NanoLuc) in the HVR were found to be fully functional and to allow the production of infectious virus. NanoLuc allowed quantitative monitoring of HEV infection and replication by luciferase assay. The use of HA-tagged replicons and full-length genomes allowed localization of putative sites of HEV RNA replication by the simultaneous detection of viral RNA by fluorescence in situ hybridization and of ORF1 protein by immunofluorescence. Candidate HEV replication complexes were found in cytoplasmic dot-like structures which partially overlapped ORF2 and ORF3 proteins as well as exosomal markers. Hence, tagged HEV genomes yield new insights into the viral life cycle and should allow further investigation of the structure and composition of the viral replication complex.

IMPORTANCE Hepatitis E virus (HEV) infection is an important cause of acute hepatitis and may lead to chronic infection in immunocompromised patients. Knowledge of the viral life cycle is incomplete due to the limited availability of functional tools. In particular, low levels of expression of the ORF1 protein or limited sensitivity of currently available antibodies or both limit our understanding of the viral replicase. Here, we report the successful establishment of subgenomic HEV replicons and full-length genomes harboring an epitope tag or a functional reporter in the ORF1 protein. These novel tools should allow further characterization of the HEV replication complex and to improve our understanding of the viral life cycle.

Cell Culture Models for Hepatitis E Virus

Despite a growing awareness, hepatitis E virus (HEV) remains understudied and investigations have been historically hampered by the absence of efficient cell culture systems. As a result, the pathogenesis of HEV infection and basic steps of the HEV life cycle are poorly understood. Major efforts have recently been made through the development of HEV infectious clones and cellular systems that significantly advanced HEV research. Here, we summarize these systems, discussing their advantages and disadvantages for HEV studies. We further capitalize on the need for HEV-permissive polarized cell models to better recapitulate the entire HEV life cycle and transmission.

Hepatitis E Virus Assembly and Release

Hepatitis E is an underestimated threat to public health, caused by the hepatitis E virus (HEV). HEV is the most common cause of acute viral hepatitis in the world, with no available direct-acting antiviral treatment. According to a recent WHO report, 20 million people become infected with HEV annually, resulting in 44,000 deaths. However, due to the scarcity of efficient in vitro cell culture systems for HEV, our knowledge of the life cycle of HEV is incomplete. Recently, significant progress has been made towards gaining a more comprehensive view of the HEV life cycle, as several in vitro culturing systems have been developed in recent years. Here, we review current knowledge and recent advances with regard to the HEV life cycle, with a particular focus on the assembly and release of viral particles. We also discuss the knowledge gaps in HEV assembly and release. Meanwhile, we highlight experimental platforms that could potentially be utilized to fill these gaps. Lastly, we offer perspectives on the future of research into HEV virology and its interaction with host cells.

Isolation of Subtype 3c, 3e and 3f-Like Hepatitis E Virus Strains Stably Replicating to High Viral Loads in an Optimized Cell Culture System

The hepatitis E virus (HEV) is transmitted via the faecal-oral route in developing countries (genotypes 1 and 2) or through contaminated food and blood products worldwide (genotypes 3 and 4). In Europe, HEV subtypes 3c, 3e and 3f are predominant. HEV is the leading cause of acute hepatitis globally and immunocompromised patients are particularly at risk. Because of a lack of cell culture systems efficiently propagating wild-type viruses, research on HEV is mostly based on cell culture-adapted isolates carrying uncommon insertions in the hypervariable region (HVR). While optimizing the cell culture system using the cell culture-adapted HEV strain 47832c, we isolated three wild-type strains derived from clinical specimens representing the predominant spectrum of HEV in Europe. The novel isolates 14-16753 (3c), 14-22707 (3e) and 15-22016 (3f-like) replicate to high viral loads of 10⁸, 10⁹ and 10^6.5 HEV RNA copies/mL at 14 days post-inoculation, respectively. In addition, they could be kept as persistently infected cell cultures with constant high viral loads (~10⁹ copies/mL) for more than a year. In contrast to the latest isolates 47832c, LBPR-0379 and Kernow-C1, the new isolates do not carry genome insertions in the HVR. Optimization of HEV cell culture identified amphotericin B, distinct salts and fetal calf serum (FCS) as important medium supplements. Overconfluent cell layers increased infectivity and virus production. PLC/PRF/5, HuH-7-Lunet BLR, A549 and HepG2/C3A supported replication with different efficiencies. The novel strains and optimized cell culture system may be useful for studies on the HEV life cycle, inactivation, specific drug and vaccine development.

Genetic Variability and Evolution of Hepatitis E Virus

Hepatitis E virus (HEV) is a single-stranded positive-sense RNA virus. HEV can cause both acute and chronic hepatitis, with the latter usually occurring in immunocompromised patients. Modes of transmission range from the classic fecal-oral route or zoonotic route, to relatively recently recognized but increasingly common routes, such as via the transfusion of blood products or organ transplantation. Extrahepatic manifestations, such as neurological, kidney and hematological abnormalities, have been documented in some limited cases, typically in patients with immune suppression. HEV has demonstrated extensive genomic diversity and a variety of HEV strains have been identified worldwide from human populations as well as growing numbers of animal species. The genetic variability and constant evolution of HEV contribute to its physiopathogenesis and adaptation to new hosts. This review describes the recent classification of the Hepeviridae family, global genotype distribution, clinical significance of HEV genotype and genomic variability and evolution of HEV.