Functional hierarchy of two L domains in porcine endogenous retrovirus (PERV) that influence release and infectivity

ESCRT machinery and virus infection

The endosomal sorting complex required for transport (ESCRT) machinery plays a significant role in the spread of human viruses. However, our understanding of how the host ESCRT machinery responds to viral infection remains limited. Emerging evidence suggests that the ESCRT machinery can be hijacked by viruses of different families to enhance their replication. Throughout their life cycle, these viruses can interfere with or exploit ESCRT-mediated physiological processes to increase their chances of infecting the host. In contrast, to counteract virus infection, the interferon-stimulated gene 15 (ISG15) or the E3 ISG15-protein ligase (HERC5) system within the infected cells is activated to degrade the ESCRT proteins. Many retroviral and RNA viral proteins have evolved "late (L) domain" motifs, which enable them to recruit host ESCRT subunit proteins to facilitate virus transport, replication, budding, mature, and even endocytosis, Therefore, the L domain motifs and ESCRT subunit proteins could serve as promising drug targets for antiviral therapy. This review investigated the composition and essential functions of the ESCRT, shedding light on the impact of ESCRT subunits and viral L domain motifs on the replication of viruses. Furthermore, the antiviral effects facilitated by the ESCRT machinery have been investigated, aiming to provide valuable insights to guide the development and utilization of antiviral drugs.

Importance of Viral Late Domains in Budding and Release of Enveloped RNA Viruses

Late assembly (L) domains are conserved sequences that are necessary for the late steps of viral replication, acting like cellular adaptors to engage the ESCRT membrane fission machinery that promote virion release. These short sequences, whose mutation or deletion produce the accumulation of immature virions at the plasma membrane, were firstly identified within retroviral Gag precursors, and in a further step, also in structural proteins of many other enveloped RNA viruses including arenaviruses, filoviruses, rhabdoviruses, reoviruses, and paramyxoviruses. Three classes of L domains have been identified thus far (PT/SAP, YPXnL/LXXLF, and PPxY), even if it has recently been suggested that other motifs could act as L domains. Here, we summarize the current state of knowledge of the different types of L domains and their cellular partners in the budding events of RNA viruses, with a particular focus on retroviruses.

Porcine Endogenous Retrovirus (PERV) - Molecular Structure and Replication Strategy in the Context of Retroviral Infection Risk of Human Cells

The xenotransplantation of porcine tissues may help overcome the shortage of human organs for transplantation. However, there are some concerns about recipient safety because the risk of porcine endogenous retrovirus (PERV) transmission to human cells remains unknown. Although, to date, no PERV infections have been noted in vivo, the possibility of such infections has been confirmed in vitro. Better understanding of the structure and replication cycle of PERVs is a prerequisite for determining the risk of infection and planning PERV-detection strategies. This review presents the current state of knowledge about the structure and replication cycle of PERVs in the context of retroviral infection risk.

Koala Retroviruses: Evolution and Disease Dynamics

A retroviral etiology for malignant neoplasias in koalas has long been suspected. Evidence for retroviral involvement was bolstered in 2000 by the isolation of a koala retrovirus (KoRV), now termed KoRV-A. KoRV-A is an endogenous retrovirus-a retrovirus that infects germ cells-a feature that makes it a permanent resident of the koala genome. KoRV-A lacks the genetic diversity of an exogenous retrovirus, a quality associated with the ability of a retrovirus to cause neoplasias. In 2013, a second KoRV isolate, KoRV-B, was obtained from koalas with lymphomas in the Los Angeles Zoo. Unlike KoRV-A, which is present in the genomes of all koalas in the United States, KoRV-B is restricted in its distribution and is associated with host pathology (neoplastic disease). Here, our current understanding of the evolution of endogenous and exogenous KoRVs, and the relationship between them, is reviewed to build a perspective on the future impact of these viruses on koala sustainability.

Infection barriers to successful xenotransplantation focusing on porcine endogenous retroviruses

Xenotransplantation may be a solution to overcome the shortage of organs for the treatment of patients with organ failure, but it may be associated with the transmission of porcine microorganisms and the development of xenozoonoses. Whereas most microorganisms may be eliminated by pathogen-free breeding of the donor animals, porcine endogenous retroviruses (PERVs) cannot be eliminated, since these are integrated into the genomes of all pigs. Human-tropic PERV-A and -B are present in all pigs and are able to infect human cells. Infection of ecotropic PERV-C is limited to pig cells. PERVs may adapt to host cells by varying the number of LTR-binding transcription factor binding sites. Like all retroviruses, they may induce tumors and/or immunodeficiencies. To date, all experimental, preclinical, and clinical xenotransplantations using pig cells, tissues, and organs have not shown transmission of PERV. Highly sensitive and specific methods have been developed to analyze the PERV status of donor pigs and to monitor recipients for PERV infection. Strategies have been developed to prevent PERV transmission, including selection of PERV-C-negative, low-producer pigs, generation of an effective vaccine, selection of effective antiretrovirals, and generation of animals transgenic for a PERV-specific short hairpin RNA inhibiting PERV expression by RNA interference.

Encephalomyocarditis virus 2A protein is required for viral pathogenesis and inhibition of apoptosis

The encephalomyocarditis virus (EMCV), a Picornaviridae virus, has a wide host spectrum and can cause various diseases. EMCV virulence factors, however, are as yet ill defined. Here, we demonstrate that the EMCV 2A protein is essential for the pathogenesis of EMCV. Infection of mice with the B279/95 strain of EMCV resulted in acute fatal disease, while the clone C9, derived by serial in vitro passage of the B279/95 strain, was avirulent. C9 harbored a large deletion in the gene encoding the 2A protein. This deletion was incorporated into the cDNA of a pathogenic EMCV1.26 strain. The new virus, EMCV1.26Δ2A, was capable of replicating in vitro, albeit more slowly than EMCV1.26. Only mice inoculated with EMCV1.26 triggered death within a few days. Mice infected with EMCV1.26Δ2A did not exhibit clinical signs, and histopathological analyses showed no damage in the central nervous system, unlike EMCV1.26-infected mice. In vitro, EMCV1.26Δ2A presented a defect in viral particle release correlating with prolonged cell viability. Unlike EMCV1.26, which induced cytopathic cell death, EMCV1.26Δ2A induced apoptosis via caspase 3 activation. This strongly suggests that the 2A protein is required for inhibition of apoptosis during EMCV infection. All together, our data indicate that the EMCV 2A protein is important for the virus in counteracting host defenses, since Δ2A viruses were no longer pathogenic and were unable to inhibit apoptosis in vitro.

An LYPSL late domain in the gag protein contributes to the efficient release and replication of Rous sarcoma virus

The efficient release of newly assembled retrovirus particles from the plasma membrane requires the recruitment of a network of cellular proteins (ESCRT machinery) normally involved in the biogenesis of multivesicular bodies and in cytokinesis. Retroviruses and other enveloped viruses recruit the ESCRT machinery through three classes of short amino acid consensus sequences termed late domains: PT/SAP, PPXY, and LYPX(n)L. The major late domain of Rous sarcoma virus (RSV) has been mapped to a PPPY motif in Gag that binds members of the Nedd4 family of ubiquitin ligases. RSV Gag also contains a second putative late domain motif, LYPSL, positioned 5 amino acids downstream of PPPY. LYPX(n)L motifs have been shown to support budding in other retroviruses by binding the ESCRT adaptor protein Alix. To investigate a possible role of the LYPSL motif in RSV budding, we constructed PPPY and LYPSL mutants in the context of an infectious virus and then analyzed the budding rates, spreading profiles, and budding morphology. The data imply that the LYPSL motif acts as a secondary late domain and that its role in budding is amplified in the absence of a fully functional PPPY motif. The LYPXL motif proved to be a stronger late domain when an aspartic acid was substituted for the native serine, recapitulating the properties of the LYPDL late domain of equine infectious anemia virus. The overexpression of human Alix in the absence of a fully functional PPPY late domain partially rescued both the viral budding rate and viral replication, supporting a model in which the RSV LYPSL motif mediates budding through an interaction with the ESCRT adaptor protein Alix.

Porcine endogenous retrovirus released by a bioartificial liver infects primary human cells

BACKGROUND

Porcine endogenous retrovirus (PERV) remains a safety risk in pig-to-human xenotransplantation. There is no evidence of in vivo productive infection in humans because PERV is inactivated by human serum. However, PERV can infect human cell lines and human primary cells in vitro and inhibit human immune functions.

AIMS

We investigated the potential of primary porcine liver cells to transmit PERV to primary human cells in a bioreactor-based bioartificial liver (BAL).

METHODS

Primary human hepatocytes, endothelial cells and the human cell line HEK 293 were exposed to supernatants from BAL or from the porcine cell line PK-15. PERV polymerase-specific reverse-transcriptase polymerase chain reaction (RT-PCR) and PCR were used to investigate PERV transmission to human cells. An assay of RT activity was used to detect the presence of retrovirus in the supernatants of BAL, primary human hepatocytes and endothelial cells.

RESULTS

Primary human hepatocytes (hHep), endothelial cells and HEK 293 cells were reproducibly infected by PERV, originating from primary porcine liver cells within the BAL and from PK-15 cells. Infected cells were positive for PERV-specific DNA and RNA after 8-10 days on an average, and RT activity was detectable in the supernatants of infected hHep and HEK 293 cells.

CONCLUSION

A risk of PERV infection in human cells is documented in this study, indicating that short-term contact of primary porcine liver cell supernatants with primary human cells could result in PERV transmission.

Basal budding and replication of the murine leukemia virus are independent of the gag L domains

Moloney murine leukemia virus (MMuLV) Gag protein contains three identified late (L) domains, PPPY, YPAL, and PSAP, which are thought to interact with the endosomal sorting machinery to assist budding. We created single and combined L-domain mutants in all permutations and tested the resulting clones for budding and replication. Budding and replication of all viruses with mutated PPPY were greatly reduced; however, the basal replication level was retained, demonstrated by the slow spread of the viruses in culture. Mutations in PSAP or YPAL did not affect budding or spreading, demonstrating that these two motifs are dispensable for efficient MMuLV replication. Furthermore, the basal budding level was maintained following inhibition of endosomal sorting machinery, emphasizing that the basal budding of MMuLV is independent of this machinery.