Mechanisms of late restriction induced by an endogenous retrovirus

Marburg virus exploits the Rab11-mediated endocytic pathway in viral-particle production

Filoviruses produce viral particles with characteristic filamentous morphology. The major viral matrix protein, VP40, is trafficked to the plasma membrane and promotes viral particle formation and subsequent viral egress. In the present study, we assessed the role of the small GTPase Rab11-mediated endocytic pathway in Marburg virus (MARV) particle formation and budding. Although Rab11 was predominantly localized in the perinuclear region, it exhibited a more diffuse distribution in the cytoplasm of cells transiently expressing MARV VP40. Rab11 was incorporated into MARV-like particles. Expression of the dominant-negative form of Rab11 and knockdown of Rab11 decreased the amount of VP40 fractions in the cell periphery. Moreover, downregulation of Rab11 moderately reduced the release of MARV-like particles and authentic MARV. We further demonstrated that VP40 induces the distribution of the microtubule network toward the cell periphery, which was partly associated with Rab11. Depolymerization of microtubules reduced the accumulation of VP40 in the cell periphery along with viral particle formation. VP40 physically interacted with α-tubulin, a major component of microtubules, but not with Rab11. Taken together, these results suggested that VP40 partly interacts with microtubules and facilitates their distribution toward the cell periphery, leading to the trafficking of transiently tethering Rab11-positive vesicles toward the cell surface. As we previously demonstrated the role of Rab11 in the formation of Ebola virus particles, the results here suggest that filoviruses in general exploit the vesicle-trafficking machinery for proper virus-particle formation and subsequent egress. These pathways may be a potential target for the development of pan-filovirus therapeutics.IMPORTANCEFiloviruses, including Marburg and Ebola viruses, produce distinct filamentous viral particles. Although it is well known that the major viral matrix protein of these viruses, VP40, is trafficked to the cell surface and promotes viral particle production, details regarding the associated molecular mechanisms remain unclear. To address this knowledge gap, we investigated the role of the small GTPase Rab11-mediated endocytic pathway in this process. Our findings revealed that Marburg virus exploits the Rab11-mediated vesicle-trafficking pathway for the release of virus-like particles and authentic virions in a microtubule network-dependent manner. Previous findings demonstrated that Rab11 is also involved in Ebola virus-particle production. Taken together, these data suggest that filoviruses, in general, may hijack the microtubule-dependent vesicle-trafficking machinery for productive replication. Therefore, this pathway presents as a potential target for the development of pan-filovirus therapeutics.

Evolutionary conservation of an ancient retroviral gagpol gene in Artiodactyla

The genomes of mammals contain fingerprints of past infections by ancient retroviruses that invaded the germline of their ancestors. Most of these endogenous retroviruses (ERVs) contain only remnants of the original retrovirus; however, on rare occasions, ERV genes can be co-opted for a beneficial host function. While most studies of co-opted ERVs have focused on envelope genes, including the syncytins that function in placentation, there are examples of co-opted gag genes including one we recently discovered in simian primates. Here, we searched for other intact gag genes in non-primate mammalian lineages. We began by examining the genomes of extant camel species, which represent a basal lineage in the order Artiodactyla. This identified a gagpol gene with a large open reading frame (ORF) (>3,500 bp) in the same orthologous location in Artiodactyla species but that is absent in other mammals. Thus, this ERV was fixed in the common ancestor of all Artiodactyla at least 64 million years ago. The amino acid sequence of this gene, termed ARTgagpol, contains recognizable matrix, capsid, nucleocapsid, and reverse transcriptase domains in ruminants, with an RNase H domain in camels and pigs. Phylogenetic analysis and structural prediction of its reverse transcriptase and RNase H domains groups ARTgagpol with gammaretroviruses. Transcriptomic analysis shows ARTgagpol expression in multiple tissues suggestive of a co-opted host function. These findings identify the oldest and largest ERV-derived gagpol gene with an intact ORF in mammals, an intriguing milestone in the co-evolution of mammals and retroviruses. IMPORTANCE Retroviruses are unique among viruses that infect animals as they integrate their reverse-transcribed double-stranded DNA into host chromosomes. When this happens in a germline cell, such as sperm, egg, or their precursors, the integrated retroviral copies can be passed on to the next generation as endogenous retroviruses (ERVs). On rare occasions, the genes of these ERVs can be domesticated by the host. In this study we used computational similarity searches to identify an ancient ERV with an intact viral gagpol gene in the genomes of camels that is also found in the same genomic location in other even-toed ungulates suggesting that it is at least 64 million years old. Broad tissue expression and predicted preservation of the reverse transcriptase fold of this protein suggest that it may be domesticated for a host function. This is the oldest known intact gagpol gene of an ancient retrovirus in mammals.

Understanding Retroviral Life Cycle and its Genomic RNA Packaging

Members of the family Retroviridae are important animal and human pathogens. Being obligate parasites, their replication involves a series of steps during which the virus hijacks the cellular machinery. Additionally, many of the steps of retrovirus replication are unique among viruses, including reverse transcription, integration, and specific packaging of their genomic RNA (gRNA) as a dimer. Progress in retrovirology has helped identify several molecular mechanisms involved in each of these steps, but many are still unknown or remain controversial. This review summarizes our present understanding of the molecular mechanisms involved in various stages of retrovirus replication. Furthermore, it provides a comprehensive analysis of our current understanding of how different retroviruses package their gRNA into the assembling virions. RNA packaging in retroviruses holds a special interest because of the uniqueness of packaging a dimeric genome. Dimerization and packaging are highly regulated and interlinked events, critical for the virus to decide whether its unspliced RNA will be packaged as a "genome" or translated into proteins. Finally, some of the outstanding areas of exploration in the field of RNA packaging are highlighted, such as the role of epitranscriptomics, heterogeneity of transcript start sites, and the necessity of functional polyA sequences. An in-depth knowledge of mechanisms that interplay between viral and cellular factors during virus replication is critical in understanding not only the virus life cycle, but also its pathogenesis, and development of new antiretroviral compounds, vaccines, as well as retroviral-based vectors for human gene therapy.

The Oldest Co-opted gag Gene of a Human Endogenous Retrovirus Shows Placenta-Specific Expression and Is Upregulated in Diffuse Large B-Cell Lymphomas

Vertebrate genomes contain endogenous retroviruses (ERVs) that represent remnants of past germline infections by ancient retroviruses. Despite comprising 8% of the human genome, the human ERVs (HERVs) do not encode a replication competent retrovirus. However, some HERV genes have been co-opted to serve host functions, most notably the viral envelope-derived syncytins involved in placentation. Here, we identify the oldest HERV intact gag gene with an open reading frame, gagV1. Its provirus contains an intact env, envV1, and the first open reading frame found in an HERV gag leader, pre-gagV1, which encodes a novel protein. This HERV is linked to a related gag gene, gagV3, and these three genes all show patterns of evolutionary conservation in primates. gagV1 and pre-gagV1 orthologs are present in all simian primate lineages indicating that this HERV entered the germline of the common simian primate ancestor at least 43 Ma, whereas gagV3 is found in Old and New World monkeys. gagV1 and gagV3 have undergone recurrent gene conversion events and positive selection. Expression of gagV1, gagV3, and pre-gagV1 is restricted to the placenta in humans and macaques suggesting co-option for placenta-specific host functions. Transcriptomic analysis of human tumors also found upregulated levels of gagV1 transcripts in diffuse large B-cell lymphomas. These findings suggest that these HERV-V genes may be useful markers for the most common type of non-Hodgkin's lymphoma and that they may have contributed to the successive domestications of env and gag genes in eutherians involved in the ongoing ERV-driven evolution of the placenta.

Structure of a Ty1 restriction factor reveals the molecular basis of transposition copy number control

Excessive replication of Saccharomyces cerevisiae Ty1 retrotransposons is regulated by Copy Number Control, a process requiring the p22/p18 protein produced from a sub-genomic transcript initiated within Ty1 GAG. In retrotransposition, Gag performs the capsid functions required for replication and re-integration. To minimize genomic damage, p22/p18 interrupts virus-like particle function by interaction with Gag. Here, we present structural, biophysical and genetic analyses of p18m, a minimal fragment of Gag that restricts transposition. The 2.8 Å crystal structure of p18m reveals an all α-helical protein related to mammalian and insect ARC proteins. p18m retains the capacity to dimerise in solution and the crystal structures reveal two exclusive dimer interfaces. We probe our findings through biophysical analysis of interface mutants as well as Ty1 transposition and p18m restriction in vivo. Our data provide insight into Ty1 Gag structure and suggest how p22/p18 might function in restriction through a blocking-of-assembly mechanism.

In Saccharomyces cerevisiae, unchecked proliferation of Ty1 retrotransposons is controlled by the process of copy number control (CNC), which requires the p22/p18 protein, translated from an internal transcript within the Ty1 GAG gene. Here, the authors present the 2.8 Å crystal structure of a minimal p18 from Ty1-Gag that is able to restrict Ty1 transposition and identify two dimer interfaces in p18, whose roles were probed by mutagenesis both in vitro and in vivo. As p22/p18 contains only one of two conserved domains required for retroelement Gag assembly, they propose that p22/p18-Gag interactions block the Ty1 virus-like particle assembly pathway, resulting in defective particles incapable of supporting retrotransposition.

Origins and evolutionary consequences of ancient endogenous retroviruses

Retroviruses infect a broad range of vertebrate hosts that includes amphibians, reptiles, fish, birds and mammals. In addition, a typical vertebrate genome contains thousands of loci composed of ancient retroviral sequences known as endogenous retroviruses (ERVs). ERVs are molecular remnants of ancient retroviruses and proof that the ongoing relationship between retroviruses and their vertebrate hosts began hundreds of millions of years ago. The long-term impact of retroviruses on vertebrate evolution is twofold: first, as with other viruses, retroviruses act as agents of selection, driving the evolution of host genes that block viral infection or that mitigate pathogenesis, and second, through the phenomenon of endogenization, retroviruses contribute an abundance of genetic novelty to host genomes, including unique protein-coding genes and cis-acting regulatory elements. This Review describes ERV origins, their diversity and their relationships to retroviruses and discusses the potential for ERVs to reveal virus-host interactions on evolutionary timescales. It also describes some of the many examples of cellular functions, including protein-coding genes and regulatory elements, that have evolved from ERVs.

Budding of Ebola Virus Particles Requires the Rab11-Dependent Endocytic Recycling Pathway

The Ebola virus-encoded major matrix protein VP40 traffics to the plasma membrane, which leads to the formation of filamentous viral particles and subsequent viral egress. However, the cellular machineries underlying this process are not fully understood. In the present study, we have assessed the role of host endocytic recycling in Ebola virus particle formation. We found that a small GTPase Rab11, which regulates recycling of molecules among the trans-Golgi network, recycling endosomes, and the plasma membrane, was incorporated in Ebola virus-like particles. Although Rab11 predominantly localized in the perinuclear region, it distributed diffusely in the cytoplasm and partly localized in the periphery of the cells transiently expressing VP40. In contrast, Rab11 exhibited a perinuclear distribution when 2 VP40 derivatives that lack ability to traffic to the plasma membrane were expressed. Finally, expression of a dominant-negative form of Rab11 or knockdown of Rab11 inhibited both VP40-induced clusters at the plasma membrane and release of viral-like particles. Taken together, our findings demonstrate that Ebola virus exploits host endocytic recycling machinery to facilitate the trafficking of VP40 to the cell surface and the subsequent release of viral-like particles for its establishment of efficient viral egress.

The evolution of endogenous retroviral envelope genes in bats and their potential contribution to host biology

Bats are the primary reservoirs and carriers of a wide range of viruses of unknown infectivity and pathogenic potential. Some of those if transmitted to other species can cause enormous economic losses in agriculture, and mortality in humans. Bats can be persistently infected with viruses while not showing any symptoms of disease, despite having high virus titre levels in their tissues and shedding virions for months or years after primary infection. It has been suggested that the lack of symptoms of viral infections and low mortality rate in bats might be due to immune adaptations that result from their long-term co-evolution with viruses. In this study, we screened all publicly available bat genomes from six bat families within which we have identified several envelope sequences of retroviral origin (gammaretroviruses). We analysed the identified sequences with Bayesian methods and maximum-likelihood inference to generate a phylogenetic tree with additional reference sequences of known endogenous and exogenous viral envelope genes. We also identified groups of orthologous viral envelopes and analysed them to determine if any of them might be an EVE (endogenous virus element) with an EDI (EVE- derived immunity) function or a candidate for a bat syncytin gene, which is an endogenized viral envelope, mostly known from its function in placentation in animals. Our study shows that bat genomes contain a substantial number of large, intact envelopes with open reading frames, which were found clustering closely on a phylogenetic tree reconstruction with syncytin sequences of other species. That might indicate that such sequences are good candidates for further bat-syncytin/EDI search.

Ovine Pulmonary Adenocarcinoma: A Unique Model to Improve Lung Cancer Research

Lung cancer represents a major worldwide health concern; although advances in patient management have improved outcomes for some patients, overall 5-year survival rates are only around 15%. In vitro studies and mouse models are commonly used to study lung cancer and their use has increased the molecular understanding of the disease. Unfortunately, mouse models are poor predictors of clinical outcome and seldom mimic advanced stages of the human disease. Animal models that more accurately reflect human disease are required for progress to be made in improving treatment outcomes and prognosis. Similarities in pulmonary anatomy and physiology potentially make sheep better models for studying human lung function and disease. Ovine pulmonary adenocarcinoma (OPA) is a naturally occurring lung cancer that is caused by the jaagsiekte sheep retrovirus. The disease is endemic in many countries throughout the world and has several features in common with human lung adenocarcinomas, including histological classification and activation of common cellular signaling pathways. Here we discuss the in vivo and in vitro OPA models that are currently available and describe the advantages of using pre-clinical naturally occurring OPA cases as a translational animal model for human lung adenocarcinoma. The challenges and options for obtaining these OPA cases for research purposes, along with their use in developing novel techniques for the evaluation of chemotherapeutic agents or for monitoring the tumor microenvironment in response to treatment, are also discussed.

Old origin of a protective endogenous retrovirus (enJSRV) in the Ovis genus

Sheep, the Jaagsiekte sheep retrovirus (JSRV) and its endogenous forms (enJSRVs) are a good model to study long-time relationships between retroviruses and their hosts. Taking advantage of 76 whole genome resequencing data of wild and domestic Ovis, we investigated the evolution of this relationship. An innovative analysis of re-sequencing data allowed characterizing 462 enJSRVs insertion sites (including 435 newly described insertions) in the Ovis genus. We focused our study on endogenous copies inserted in the q13 locus of chromosome 6 (6q13). Those copies are known to confer resistance against exogenous JSRV thanks to alleles bearing a mutation in the gag gene. We characterized (i) the distribution of protective and non-protective alleles across Ovis species and (ii) the copy number variation of the 6q13 locus. Our results challenged the previous hypothesis of fixation and amplification of the protective copies in relation with domestication, and allowed building a new model for the evolution of the 6q13 locus. JSRV would have integrated the 6q13 locus after the Ovis-Capra divergence (5-11 MYA) and before the Ovis diversification (2.4-5 MYA). The protective mutation in the enJSRV 6q13 copy appeared shortly after its insertion and was followed by genomic amplifications, after the divergence between Pachyform lineage on one side and the Argaliform and moufloniform lineages on the other (2.4-5 MYA). Considering the potential selective advantage of the protective mutation, its fixation in both sheep and its closest wild relative Ovis orientalis may be due to natural selection before domestication from O. orientalis populations.

Retroviruses and microtubule-associated motor proteins

Retroviruses are obligate intracellular parasites of eukaryotic cells. After reverse transcription, the viral DNA contained in the preintegration complex is delivered to the nucleus of the host cell, where it integrates. Before reaching the nucleus, the incoming particle and the preintegration complex must travel throughout the cytoplasm. Likewise, the newly synthesized viral proteins and viral particles must transit the cytoplasm during exit. The cytoplasm is a crowded environment, and simple diffusion is difficult. Therefore, viruses have evolved to utilize the cellular mechanisms of movement through the cytoplasm, where microtubules are the roads, and the ATP-dependent motors dynein and kinesin are the vehicles for retrograde and anterograde trafficking. This review will focus on how different retroviruses (Mazon-Pfizer monkey virus, prototype foamy virus, bovine immunodeficiency virus, human immunodeficiency virus type 1, and murine leukemia virus) have subjugated the microtubule-associated motor proteins for viral replication. Although there have been advances in our understanding of how retroviruses move along microtubules, the strategies are different among them. Thus, a better understanding of the mechanisms used by each retrovirus to functionally subvert microtubule motor proteins will provide important clues in the design of new antiretroviral drugs that can specifically disrupt intracellular viral trafficking.

Lack of association between polymorphic copies of endogenous Jaagsiekte sheep retrovirus (enJSRVs) and Ovine Pulmonary Adenocarcinoma

Ovine Pulmonary Adenocarcinoma (OPA) is a retrovirus-induced lung tumor of sheep, goat and mouflon, and its etiologic agent, Jaagsiekte sheep retrovirus (JSRV) is the only virus known to cause a naturally occurred lung adenocarcinoma. The oncogenic JSRV has several endogenous counterparts termed enJSRVs, some of which have been shown to interfere with JSRV replication at early and late stages of the retroviral cycle inhibiting JSRV exit from the cell, and thus, protecting sheep against the infection. In this work, Latxa sheep breed animals were classified depending on the presence/absence of OPA-characteristic clinical lesions in the lung. Using a PCR genotyping method and a logistic regression-based association study, five polymorphic enJSRV copies were analyzed in 49 OPA positive sheep and 124 control individuals. Our results showed that the frequency of the provirus enJSRV-16 is much higher in Latxa sheep breed than in other breeds, suggesting a recent proliferation of this provirus in the studied breed. However, no polymorphic enJSRV was found to be statistically associated with the susceptibility/resistance to OPA development.

Mammalian Endogenous Retroviruses

Over 40% of mammalian genomes comprise the products of reverse transcription. Among such retrotransposed sequences are those characterized by the presence of long terminal repeats (LTRs), including the endogenous retroviruses (ERVs), which are inherited genetic elements closely resembling the proviruses formed following exogenous retrovirus infection. Sequences derived from ERVs make up at least 8 to 10% of the human and mouse genomes and range from ancient sequences that predate mammalian divergence to elements that are currently still active. In this chapter we describe the discovery, classification and origins of ERVs in mammals and consider cellular mechanisms that have evolved to control their expression. We also discuss the negative effects of ERVs as agents of genetic disease and cancer and review examples of ERV protein domestication to serve host functions, as in placental development. Finally, we address growing evidence that the gene regulatory potential of ERV LTRs has been exploited multiple times during evolution to regulate genes and gene networks. Thus, although recently endogenized retroviral elements are often pathogenic, those that survive the forces of negative selection become neutral components of the host genome or can be harnessed to serve beneficial roles.

Recycling Endosomes and Viral Infection

Many viruses exploit specific arms of the endomembrane system. The unique composition of each arm prompts the development of remarkably specific interactions between viruses and sub-organelles. This review focuses on the viral–host interactions occurring on the endocytic recycling compartment (ERC), and mediated by its regulatory Ras-related in brain (Rab) GTPase Rab11. This protein regulates trafficking from the ERC and the trans-Golgi network to the plasma membrane. Such transport comprises intricate networks of proteins/lipids operating sequentially from the membrane of origin up to the cell surface. Rab11 is also emerging as a critical factor in an increasing number of infections by major animal viruses, including pathogens that provoke human disease. Understanding the interplay between the ERC and viruses is a milestone in human health. Rab11 has been associated with several steps of the viral lifecycles by unclear processes that use sophisticated diversified host machinery. For this reason, we first explore the state-of-the-art on processes regulating membrane composition and trafficking. Subsequently, this review outlines viral interactions with the ERC, highlighting current knowledge on viral-host binding partners. Finally, using examples from the few mechanistic studies available we emphasize how ERC functions are adjusted during infection to remodel cytoskeleton dynamics, innate immunity and membrane composition.

A self-encoded capsid derivative restricts Ty1 retrotransposition in Saccharomyces

Retrotransposons and retroviral insertions have molded the genomes of many eukaryotes. Since retroelements transpose via an RNA intermediate, the additive nature of the replication cycle can result in massive increases in copy number if left unchecked. Host organisms have countered with several defense systems, including domestication of retroelement genes that now act as restriction factors to minimize propagation. We discovered a novel truncated form of the Saccharomyces Ty1 retrotransposon capsid protein, dubbed p22 that inhibits virus-like particle (VLP) assembly and function. The p22 restriction factor expands the repertoire of defense proteins targeting the capsid and highlights a novel host-parasite strategy. Instead of inhibiting all transposition by domesticating the restriction gene as a distinct locus, Ty1 and budding yeast may have coevolved a relationship that allows high levels of transposition when Ty1 copy numbers are low and progressively less transposition as copy numbers rise. Here, we offer a perspective on p22 restriction, including its mode of expression, effect on VLP functions, interactions with its target, properties as a nucleic acid chaperone, similarities to other restriction factors, and future directions.

Advances in the study of transmissible respiratory tumours in small ruminants

Sheep and goats are widely infected by oncogenic retroviruses, namely Jaagsiekte Sheep RetroVirus (JSRV) and Enzootic Nasal Tumour Virus (ENTV). Under field conditions, these viruses induce transformation of differentiated epithelial cells in the lungs for Jaagsiekte Sheep RetroVirus or the nasal cavities for Enzootic Nasal Tumour Virus. As in other vertebrates, a family of endogenous retroviruses named endogenous Jaagsiekte Sheep RetroVirus (enJSRV) and closely related to exogenous Jaagsiekte Sheep RetroVirus is present in domestic and wild small ruminants. Interestingly, Jaagsiekte Sheep RetroVirus and Enzootic Nasal Tumour Virus are able to promote cell transformation, leading to cancer through their envelope glycoproteins. In vitro, it has been demonstrated that the envelope is able to deregulate some of the important signaling pathways that control cell proliferation. The role of the retroviral envelope in cell transformation has attracted considerable attention in the past years, but it appears to be highly dependent of the nature and origin of the cells used. Aside from its health impact in animals, it has been reported for many years that the Jaagsiekte Sheep RetroVirus-induced lung cancer is analogous to a rare, peculiar form of lung adenocarcinoma in humans, namely lepidic pulmonary adenocarcinoma. The implication of a retrovirus related to Jaagsiekte Sheep RetroVirus is still controversial and under investigation, but the identification of an infectious agent associated with the development of lepidic pulmonary adenocarcinomas might help us to understand cancer development. This review explores the mechanisms of induction of respiratory cancers in small ruminants and the possible link between retrovirus and lepidic pulmonary adenocarcinomas in humans.

Turnover Rate of NS3 Proteins Modulates Bluetongue Virus Replication Kinetics in a Host-Specific Manner

Bluetongue virus (BTV) is an arbovirus transmitted to livestock by midges of the Culicoides family and is the etiological agent of a hemorrhagic disease in sheep and other ruminants. In mammalian cells, BTV particles are released primarily by virus-induced cell lysis, while in insect cells they bud from the plasma membrane and establish a persistent infection. BTV possesses a ten-segmented double-stranded RNA genome, and NS3 proteins are encoded by segment 10 (Seg-10). The viral nonstructural protein 3 (NS3) plays a key role in mediating BTV egress as well as in impeding the in vitro synthesis of type I interferon in mammalian cells. In this study, we asked whether genetically distant NS3 proteins can alter BTV-host interactions. Using a reverse genetics approach, we showed that, depending on the NS3 considered, BTV replication kinetics varied in mammals but not in insects. In particular, one of the NS3 proteins analyzed harbored a proline at position 24 that leads to its rapid intracellular decay in ovine but not in Culicoides cells and to the attenuation of BTV virulence in a mouse model of disease. Overall, our data reveal that the genetic variability of Seg-10/NS3 differentially modulates BTV replication kinetics in a host-specific manner and highlight the role of the host-specific variation in NS3 protein turnover rate.

IMPORTANCE

BTV is the causative agent of a severe disease transmitted between ruminants by biting midges of Culicoides species. NS3, encoded by Seg-10 of the BTV genome, fulfills key roles in BTV infection. As Seg-10 sequences from various BTV strains display genetic variability, we assessed the impact of different Seg-10 and NS3 proteins on BTV infection and host interactions. In this study, we revealed that various Seg-10/NS3 proteins alter BTV replication kinetics in mammals but not in insects. Notably, we found that NS3 protein turnover may vary in ovine but not in Culicoides cells due to a single amino acid residue that, most likely, leads to rapid and host-dependent protein degradation. Overall, this study highlights that genetically distant BTV Seg-10/NS3 influence BTV biological properties in a host-specific manner and increases our understanding of how NS3 proteins contribute to the outcome of BTV infection.

Induction and characterization of a replication competent cervid endogenous gammaretrovirus (CrERV) from mule deer cells

Endogenous retroviruses (ERVs) were acquired during evolution of their host organisms after infection and mendelian inheritance in the germline by their exogenous counterparts. The ERVs can spread in the host genome and in some cases they affect the host phenotype. The cervid endogenous gammaretrovirus (CrERV) is one of only a few well-defined examples of evolutionarily recent invasion of mammalian genome by retroviruses. Thousands of insertionally polymorphic CrERV integration sites have been detected in wild ranging mule deer (Odocoileus hemionus) host populations. Here, we describe for the first time induction of replication competent CrERV by cocultivation of deer and human cells. We characterize the physical properties and tropism of the induced virus. The genomic sequence of the induced virus is phylogenetically related to the evolutionarily young endogenous CrERVs described so far. We also describe the level of replication block of CrERV on deer cells and its capacity to establish superinfection interference.

Pericentriolar Targeting of the Mouse Mammary Tumor Virus GAG Protein

The Gag protein of the mouse mammary tumor virus (MMTV) is the chief determinant of subcellular targeting. Electron microscopy studies show that MMTV Gag forms capsids within the cytoplasm and assembles as immature particles with MMTV RNA and the Y box binding protein-1, required for centrosome maturation. Other betaretroviruses, such as Mason-Pfizer monkey retrovirus (M-PMV), assemble adjacent to the pericentriolar region because of a cytoplasmic targeting and retention signal in the Matrix protein. Previous studies suggest that the MMTV Matrix protein may also harbor a similar cytoplasmic targeting and retention signal. Herein, we show that a substantial fraction of MMTV Gag localizes to the pericentriolar region. This was observed in HEK293T, HeLa human cell lines and the mouse derived NMuMG mammary gland cells. Moreover, MMTV capsids were observed adjacent to centrioles when expressed from plasmids encoding either MMTV Gag alone, Gag-Pro-Pol or full-length virus. We found that the cytoplasmic targeting and retention signal in the MMTV Matrix protein was sufficient for pericentriolar targeting, whereas mutation of the glutamine to alanine at position 56 (D56/A) resulted in plasma membrane localization, similar to previous observations from mutational studies of M-PMV Gag. Furthermore, transmission electron microscopy studies showed that MMTV capsids accumulate around centrioles suggesting that, similar to M-PMV, the pericentriolar region may be a site for MMTV assembly. Together, the data imply that MMTV Gag targets the pericentriolar region as a result of the MMTV cytoplasmic targeting and retention signal, possibly aided by the Y box protein-1 required for the assembly of centrosomal microtubules.

The Roles of Syncytin-Like Proteins in Ruminant Placentation

Recent developments in genome sequencing techniques have led to the identification of huge numbers of endogenous retroviruses (ERV) in various mammals. ERVs, which occupy 8%–13% of mammalian genomes, are believed to affect mammalian evolution and biological diversity. Although the functional significance of most ERVs remains to be elucidated, several ERVs are thought to have pivotal roles in host physiology. We and other groups recently identified ERV envelope proteins (e.g., Fematrin-1, Syncytin-Rum1, endogenous Jaagsiekte sheep retrovirus Env) that may determine the morphogenesis of the unique fused trophoblast cells, termed trinucleate cells and syncytial plaques, found in ruminant placentas; however, there are still a number of outstanding issues with regard to the role of ERVs that remain to be resolved. Here, we review what is known about how these ERVs have contributed to the development of ruminant-specific trophoblast cells.

A trans-dominant form of Gag restricts Ty1 retrotransposition and mediates copy number control

Saccharomyces cerevisiae and Saccharomyces paradoxus lack the conserved RNA interference pathway and utilize a novel form of copy number control (CNC) to inhibit Ty1 retrotransposition. Although noncoding transcripts have been implicated in CNC, here we present evidence that a truncated form of the Gag capsid protein (p22) or its processed form (p18) is necessary and sufficient for CNC and likely encoded by Ty1 internal transcripts. Coexpression of p22/p18 and Ty1 decreases mobility more than 30,000-fold. p22/p18 cofractionates with Ty1 virus-like particles (VLPs) and affects VLP yield, protein composition, and morphology. Although p22/p18 and Gag colocalize in the cytoplasm, p22/p18 disrupts sites used for VLP assembly. Glutathione S-transferase (GST) affinity pulldowns also suggest that p18 and Gag interact. Therefore, this intrinsic Gag-like restriction factor confers CNC by interfering with VLP assembly and function and expands the strategies used to limit retroelement propagation.

IMPORTANCE

Retrotransposons dominate the chromosomal landscape in many eukaryotes, can cause mutations by insertion or genome rearrangement, and are evolutionarily related to retroviruses such as HIV. Thus, understanding factors that limit transposition and retroviral replication is fundamentally important. The present work describes a retrotransposon-encoded restriction protein derived from the capsid gene of the yeast Ty1 element that disrupts virus-like particle assembly in a dose-dependent manner. This form of copy number control acts as a molecular rheostat, allowing high levels of retrotransposition when few Ty1 elements are present and inhibiting transposition as copy number increases. Thus, yeast and Ty1 have coevolved a form of copy number control that is beneficial to both "host and parasite." To our knowledge, this is the first Gag-like retrotransposon restriction factor described in the literature and expands the ways in which restriction proteins modulate retroelement replication.

"Ménage à Trois": the evolutionary interplay between JSRV, enJSRVs and domestic sheep

Sheep betaretroviruses represent a fascinating model to study the complex evolutionary interplay between host and pathogen in natural settings. In infected sheep, the exogenous and pathogenic Jaagsiekte sheep retrovirus (JSRV) coexists with a variety of highly related endogenous JSRVs, referred to as enJSRVs. During evolution, some of them were co-opted by the host as they fulfilled important biological functions, including placental development and protection against related exogenous retroviruses. In particular, two enJSRV loci, enJS56A1 and enJSRV-20, were positively selected during sheep domestication due to their ability to interfere with the replication of related competent retroviruses. Interestingly, viruses escaping these transdominant enJSRVs have recently emerged, probably less than 200 years ago. Overall, these findings suggest that in sheep the process of endogenization is still ongoing and, therefore, the evolutionary interplay between endogenous and exogenous sheep betaretroviruses and their host has not yet reached an equilibrium.

The sheep tetherin paralog oBST2B blocks envelope glycoprotein incorporation into nascent retroviral virions

Bone marrow stromal cell antigen 2 (BST2) is a cellular restriction factor with a broad antiviral activity. In sheep, the BST2 gene is duplicated into two paralogs termed oBST2A and oBST2B. oBST2A impedes viral exit of the Jaagsiekte sheep retroviruses (JSRV), most probably by retaining virions at the cell membrane, similar to the "tethering" mechanism exerted by human BST2. In this study, we provide evidence that unlike oBST2A, oBST2B is limited to the Golgi apparatus and disrupts JSRV envelope (Env) trafficking by sequestering it. In turn, oBST2B leads to a reduction in Env incorporation into viral particles, which ultimately results in the release of virions that are less infectious. Furthermore, the activity of oBST2B does not seem to be restricted to retroviruses, as it also acts on vesicular stomatitis virus glycoproteins. Therefore, we suggest that oBST2B exerts antiviral activity using a mechanism distinct from the classical tethering restriction observed for oBST2A.

IMPORTANCE

BST2 is a powerful cellular restriction factor against a wide range of enveloped viruses. Sheep possess two paralogs of the BST2 gene called oBST2A and oBST2B. JSRV, the causative agent of a transmissible lung cancer of sheep, is known to be restricted by oBST2A. In this study, we show that unlike oBST2A, oBST2B impairs the normal cellular trafficking of JSRV envelope glycoproteins by sequestering them within the Golgi apparatus. We also show that oBST2B decreases the incorporation of envelope glycoprotein into JSRV viral particles, which in turn reduces virion infectivity. In conclusion, oBST2B exerts a novel antiviral activity that is distinct from those of BST2 proteins of other species.

Inhibition of Borna disease virus replication by an endogenous bornavirus-like element in the ground squirrel genome

Animal genomes contain endogenous viral sequences, such as endogenous retroviruses and retrotransposons. Recently, we and others discovered that nonretroviral viruses also have been endogenized in many vertebrate genomes. Bornaviruses belong to the Mononegavirales and have left endogenous fragments, called "endogenous bornavirus-like elements" (EBLs), in the genomes of many mammals. The striking features of EBLs are that they contain relatively long ORFs which have high sequence homology to the extant bornavirus proteins. Furthermore, some EBLs derived from bornavirus nucleoprotein (EBLNs) have been shown to be transcribed as mRNA and probably are translated into proteins. These features lead us to speculate that EBLs may function as cellular coopted genes. An EBLN element in the genome of the thirteen-lined ground squirrel (Ictidomys tridecemlineatus), itEBLN, encodes an ORF with 77% amino acid sequence identity to the current bornavirus nucleoprotein. In this study, we cloned itEBLN from the ground squirrel genome and investigated its involvement in Borna disease virus (BDV) replication. Interestingly, itEBLN, but not a human EBLN, colocalized with the viral factory in the nucleus and appeared to affect BDV polymerase activity by being incorporated into the viral ribonucleoprotein. Our data show that, as do certain endogenous retroviruses, itEBLN potentially may inhibit infection by related exogenous viruses in vivo.

Diagnosis and phylogenetic analysis of ovine pulmonary adenocarcinoma in China

Ovine pulmonary adenocarcinoma (OPA) is a lung tumor of sheep caused by jaagsiekte sheep retrovirus (JSRV). OPA is common in sheep, and it is most commonly observed in China. Without preventative vaccines and serological diagnostic tools for assay of OPA, identification of JSRV based on reverse transcription polymerase chain reaction (RT-PCR) is very important for prevention and control measures for OPA in practice management. In this study, the diagnosis of OPA was made from analysis of clinical signs, pathological observations, JSRV-like particle discovery, and RT-PCR of the target env gene. The phylogenetic analysis showed that the China Shandong (SD) strain studied in this article belonged to exogenous JSRV, and it was very similar to 92k3, which was isolated from sheep in the Kenya (Y18305). The current study reported a severe outbreak of OPA in Shandong Province, China. The observations could offer a comparative view of the env gene of JSRV.

Host species barriers to Jaagsiekte sheep retrovirus replication and carcinogenesis

Understanding the factors governing host species barriers to virus transmission has added significantly to our appreciation of virus pathogenesis. Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine pulmonary adenocarcinoma (OPA), a transmissible lung cancer of sheep that has rarely been found in goats. In this study, in order to further clarify the pathogenesis of OPA, we investigated whether goats are resistant to JSRV replication and carcinogenesis. We found that JSRV induces lung tumors in goats with macroscopic and histopathological features that dramatically differ from those in sheep. However, the origins of the tumor cells in the two species are identical. Interestingly, in experimentally infected lambs and goat kids, we revealed major differences in the number of virus-infected cells at early stages of infection. These differences were not related to the number of available target cells for virus infection and cell transformation or the presence of a host-specific immune response toward JSRV. Indeed, we also found that goats possess transcriptionally active endogenous retroviruses (enJSRVs) that likely influence the host immune response toward the exogenous JSRV. Overall, these results suggest that goat cells, or at least those cells targeted for viral carcinogenesis, are not permissive to virus replication but can be transformed by JSRV.

Experimental transmission of enzootic nasal adenocarcinoma in sheep

Enzootic nasal adenocarcinoma (ENA) is a contagious neoplasm of the secretory epithelial cells of the nasal mucosa of sheep and goats. It is associated with the betaretrovirus, enzootic nasal tumor virus (ENTV), but a causative relationship has yet to be demonstrated. In this study, 14-day-old lambs were experimentally infected via nebulization with cell-free tumor filtrates derived from naturally occurring cases of ENA. At 12 weeks post-infection (wpi), one of the five infected lambs developed clinical signs, including continuous nasal discharge and open mouth breathing, and was euthanized. Necropsy revealed the presence of a large bilateral tumor occupying the nasal cavity. At 45 wpi, when the study was terminated, none of the remaining infected sheep showed evidence of tumors either by computed tomography or post-mortem examination. ENTV-1 proviral DNA was detected in the nose, lung, spleen, liver and kidney of the animal with experimentally induced ENA, however there was no evidence of viral protein expression in tissues other than the nose. Density gradient analysis of virus particles purified from the experimentally induced nasal tumor revealed a peak reverse transcriptase (RT) activity at a buoyant density of 1.22 g/mL which was higher than the 1.18 g/mL density of peak RT activity of virus purified from naturally induced ENA. While the 1.22 g/mL fraction contained primarily immature unprocessed virus particles, mature virus particles with a similar morphology to naturally occurring ENA could be identified by electron microscopy. Full-length sequence analysis of the ENTV-1 genome from the experimentally induced tumor revealed very few nucleotide changes relative to the original inoculum with only one conservative amino acid change. Taken together, these results demonstrate that ENTV-1 is associated with transmissible ENA in sheep and that under experimental conditions, lethal tumors are capable of developing in as little as 12 wpi demonstrating the acutely oncogenic nature of this ovine betaretrovirus.

Application of next generation sequencing in mammalian embryogenomics: lessons learned from endogenous betaretroviruses of sheep

Endogenous retroviruses (ERVs) are present in the genome of all vertebrates and are remnants of ancient exogenous retroviral infections of the host germline transmitted vertically from generation to generation. The sheep genome contains 27 JSRV-related endogenous betaretroviruses (enJSRVs) related to the pathogenic Jaagsiekte sheep retrovirus (JSRV) that have been integrating in the host genome for the last 5-7 million years. The exogenous JSRV is a causative agent of a transmissible lung cancer in sheep, and enJSRVs are able to protect the host against JSRV infection. In sheep, the enJSRVs are most abundantly expressed in the uterine epithelia as well as in the conceptus (embryo and associated extraembryonic membranes) trophectoderm. Sixteen of the 27 enJSRV loci contain an envelope (env) gene with an intact open reading frame, and in utero loss-of-function experiments found the enJSRVs Env to be essential for trophoblast outgrowth and conceptus elongation. Collectively, available evidence supports the ideas that genes captured from ancestral retroviruses were pivotal in the acquisition of new, important functions in mammalian evolution and were positively selected for biological roles in genome plasticity, protection of the host against infection of related pathogenic and exogenous retroviruses, and a convergent physiological role in placental morphogenesis and thus mammalian reproduction. The discovery of ERVs in mammals was initially based on molecular cloning discovery techniques and will be boosted forward by next generation sequencing technologies and in silico discovery techniques.

Endogenous retroviruses of sheep: a model system for understanding physiological adaptation to an evolving ruminant genome

Endogenous retroviruses (ERVs) are present in the genome of all vertebrates and are remnants of ancient exogenous retroviral infections of the host germline transmitted vertically from generation to generation. Sheep betaretroviruses offer a unique model system to study the complex interaction between retroviruses and their host. The sheep genome contains 27 endogenous betaretroviruses (enJSRVs) related to the exogenous and pathogenic Jaagsiekte sheep retrovirus (JSRV), the causative agent of a transmissible lung cancer in sheep. The enJSRVs can protect their host against JSRV infection by blocking early and late steps of the JSRV replication cycle. In the female reproductive tract, enJSRVs are specifically expressed in the uterine luminal and glandular epithelia as well as in the conceptus (embryo and associated extraembryonic membranes) trophectoderm and in utero loss-of-function experiments found the enJSRVs envelope (env) to be essential for conceptus elongation and trophectoderm growth and development. Collectively, available evidence in sheep and other mammals indicate that ERVs coevolved with their hosts for millions of years and were positively selected for biological roles in genome plasticity and evolution, protection of the host against infection of related pathogenic and exogenous retroviruses, and placental development.

Drosophila melanogaster as a model organism for bluetongue virus replication and tropism

Bluetongue virus (BTV) is the etiological agent of bluetongue (BT), a hemorrhagic disease of ruminants that can cause high levels of morbidity and mortality. BTV is an arbovirus transmitted between its ruminant hosts by Culicoides biting midges (Diptera: Ceratopogonidae). Recently, Europe has experienced some of the largest BT outbreaks ever recorded, including areas with no known history of the disease, leading to unprecedented economic and animal welfare issues. The current lack of genomic resources and genetic tools for Culicoides restricts any detailed study of the mechanisms involved in the virus-insect interactions. In contrast, the genome of the fruit fly (Drosophila melanogaster) has been successfully sequenced, and it is used extensively as a model of molecular pathways due to the existence of powerful genetic technology. In this study, D. melanogaster is investigated as a model for the replication and tropism of BTV. Using reverse genetics, a modified BTV-1 that expresses the fluorescent mCherry protein fused to the viral nonstructural protein NS3 (BTV-1/NS3mCherry) was generated. We demonstrate that BTV-1/NS3mCherry is not only replication competent as it retains many characteristics of the wild-type virus but also replicates efficiently in D. melanogaster after removal of the bacterial endosymbiont Wolbachia pipientis by antibiotic treatment. Furthermore, confocal microscopy shows that the tissue tropism of BTV-1/NS3mCherry in D. melanogaster resembles that described previously for BTV in Culicoides. Overall, the data presented in this study demonstrate the feasibility of using D. melanogaster as a genetic model to investigate BTV-insect interactions that cannot be otherwise addressed in vector species.

Widespread horizontal gene transfer from circular single-stranded DNA viruses to eukaryotic genomes

Background

In addition to vertical transmission, organisms can also acquire genes from other distantly related species or from their extra-chromosomal elements (plasmids and viruses) via horizontal gene transfer (HGT). It has been suggested that phages represent substantial forces in prokaryotic evolution. In eukaryotes, retroviruses, which can integrate into host genome as an obligate step in their replication strategy, comprise approximately 8% of the human genome. Unlike retroviruses, few members of other virus families are known to transfer genes to host genomes.

Results

Here we performed a systematic search for sequences related to circular single-stranded DNA (ssDNA) viruses in publicly available eukaryotic genome databases followed by comprehensive phylogenetic analysis. We conclude that the replication initiation protein (Rep)-related sequences of geminiviruses, nanoviruses and circoviruses have been frequently transferred to a broad range of eukaryotic species, including plants, fungi, animals and protists. Some of the transferred viral genes were conserved and expressed, suggesting that these genes have been coopted to assume cellular functions in the host genomes. We also identified geminivirus-like and parvovirus-like transposable elements in genomes of fungi and lower animals, respectively, and thereby provide direct evidence that eukaryotic transposons could derive from ssDNA viruses.

Conclusions

Our discovery extends the host range of circular ssDNA viruses and sheds light on the origin and evolution of these viruses. It also suggests that ssDNA viruses act as an unforeseen source of genetic innovation in their hosts.

The signal peptide of a recently integrated endogenous sheep betaretrovirus envelope plays a major role in eluding gag-mediated late restriction

The exogenous and pathogenic Jaagsiekte sheep retrovirus (JSRV) coexists with highly related and biologically active endogenous retroviruses (enJSRVs). The endogenous enJS56A1 locus possesses a defective Gag polyprotein which blocks the late replication steps of related exogenous and endogenous retroviruses by a mechanism known as JSRV late restriction (JLR). Conversely, enJSRV-26, which most likely integrated into the sheep genome less than 200 years ago, is able to escape JLR. In this study, we demonstrate that the ability of enJSRV-26 to escape JLR is due to a single-amino-acid substitution in the signal peptide (SP) of its envelope glycoprotein. We show that enJSRV-26 SP does not localize to the nucleolus, unlike the functional SPs of related exogenous and endogenous sheep betaretroviruses. In addition, enJSRV-26 SP function as a posttranscriptional regulator of viral gene expression is impaired. enJSRV-26 JLR escape relies on the presence of the functional enJS56A1 SP. Moreover, we show that the ratio between enJSRV-26 and enJS56A1 Gag is critical to elude JLR. Interestingly, we found that the domestic sheep has acquired, by genome amplification, several copies of the enJS56A1 provirus. These data further reinforce the notion that transdominant enJSRV proviruses have been positively selected in domestic sheep, and that the coevolution between endogenous and exogenous sheep betaretroviruses and their host is still occurring.

Recruitment of the oncoprotein v-ErbA to aggresomes

Aggresome formation, a cellular response to misfolded protein aggregates, is linked to cancer and neurodegenerative disorders. Previously we showed that Gag-v-ErbA (v-ErbA), a retroviral variant of the thyroid hormone receptor (TRα1), accumulates in and sequesters TRα1 into cytoplasmic foci. Here, we show that foci represent v-ErbA targeting to aggresomes. v-ErbA colocalizes with aggresomal markers, proteasomes, hsp70, HDAC6, and mitochondria. Foci have hallmark characteristics of aggresomes: formation is microtubule-dependent, accelerated by proteasome inhibitors, and they disrupt intermediate filaments. Proteasome-mediated degradation is critical for clearance of v-ErbA and T(3)-dependent TRα1 clearance. Our studies highlight v-ErbA's complex mode of action: the oncoprotein is highly mobile and trafficks between the nucleus, cytoplasm, and aggresome, carrying out distinct activities within each compartment. Dynamic trafficking to aggresomes contributes to the dominant negative activity of v-ErbA and may be enhanced by the viral Gag sequence. These studies provide insight into novel modes of oncogenesis across multiple cellular compartments.

Jaagsiekte sheep retrovirus biology and oncogenesis

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of a lung cancer in sheep known as ovine pulmonary adenocarcinoma (OPA). The disease has been identified around the world in several breeds of sheep and goats, and JSRV infection typically has a serious impact on affected flocks. In addition, studies on OPA are an excellent model for human lung carcinogenesis. A unique feature of JSRV is that its envelope (Env) protein functions as an oncogene. The JSRV Env-induced transformation or oncogenesis has been studied in a variety of cell systems and in animal models. Moreover, JSRV studies have provided insights into retroviral genomic RNA export/expression mechanisms. JSRV encodes a trans-acting factor (Rej) within the env gene necessary for the synthesis of Gag protein from unspliced viral RNA. This review summarizes research pertaining to JSRV-induced pathogenesis, Env transformation, and other aspects of JSRV biology.

Interplay between ovine bone marrow stromal cell antigen 2/tetherin and endogenous retroviruses

Endogenous betaretroviruses (enJSRVs) of sheep are expressed abundantly in the female reproductive tract and play a crucial role in conceptus development and placental morphogenesis. Interestingly, the colonization of the sheep genome by enJSRVs is likely still ongoing. During early pregnancy, enJSRV expression correlates with the production of tau interferon (IFNT), a type I IFN, by the developing conceptus. IFNT is the pregnancy recognition signal in ruminants and possesses potent antiviral activity. In this study, we show that IFNT induces the expression of bone marrow stromal cell antigen 2 (BST2) (also termed CD317/tetherin) both in vitro and in vivo. The BST2 gene is duplicated in ruminants. Transfection assays found that ovine BST2 proteins (oBST2A and oBST2B) block release of viral particles produced by intact enJSRV loci and of related exogenous and pathogenic jaagsiekte sheep retrovirus (JSRV). Ovine BST2A appears to restrict enJSRVs more efficiently than oBST2B. In vivo, the expression of BST2A/B and enJSRVs in the endometrium increases after day 12 and remains high between days 14 and 20 of pregnancy. In situ hybridization analyses found that oBST2A is expressed mainly in the endometrial stromal cells but not in the luminal and glandular epithelial cells, in which enJSRVs are highly expressed. In conclusion, enJSRVs may have coevolved in the presence of oBST2A/B by being expressed in different cellular compartments of the same organ. Viral expression in cells unable to express BST2 may be one of the mechanisms used by retroviruses to escape restriction.

Overview of Retrovirology

In the 100 years since their discovery, retroviruses have played a special role in virology and in molecular biology. These agents have been at the center of cancer research and shaped our understanding of cell growth, differentiation and survival in ways that stretch far beyond investigations using these viruses. The discovery of retroviral oncogenes established the central paradigm that altered cellular genes can provide a dominant signal initiating cancer development. Their unique replication mechanism and their integration into cellular DNA allow these viruses to alter the properties of their hosts beyond the life span of the infected individual and contribute to the evolution of species. This same property has made retroviral vectors an important tool for gene therapy. Indeed, the impact of retrovirus research has been far-reaching and despite the amazing progress that has been made, retroviruses continue to reveal new insights into the host – pathogen interaction.

The binding of NCAM to FGFR1 induces a specific cellular response mediated by receptor trafficking

Neural cell adhesion molecule (NCAM) associates with fibroblast growth factor (FGF) receptor-1 (FGFR1). However, the biological significance of this interaction remains largely elusive. In this study, we show that NCAM induces a specific, FGFR1-mediated cellular response that is remarkably different from that elicited by FGF-2. In contrast to FGF-induced degradation of endocytic FGFR1, NCAM promotes the stabilization of the receptor, which is recycled to the cell surface in a Rab11- and Src-dependent manner. In turn, FGFR1 recycling is required for NCAM-induced sustained activation of various effectors. Furthermore, NCAM, but not FGF-2, promotes cell migration, and this response depends on FGFR1 recycling and sustained Src activation. Our results implicate NCAM as a nonconventional ligand for FGFR1 that exerts a peculiar control on the intracellular trafficking of the receptor, resulting in a specific cellular response. Besides introducing a further level of complexity in the regulation of FGFR1 function, our findings highlight the link of FGFR recycling with sustained signaling and cell migration and the critical role of these events in dictating the cellular response evoked by receptor activation.

Friendly viruses: the special relationship between endogenous retroviruses and their host

Endogenous retroviruses (ERVs) are present in the genome of all vertebrates and have coevolved with their hosts for millions of years. Some ERVs play a critical role in placental development, contribute to genome plasticity, and protect the host against infection of related pathogenic and exogenous retroviruses, thus some ERVs have been positively selected and maintained in the host genome. The sheep genome contains 27 endogenous retroviruses (enJSRVs) related to the pathogenic Jaagsiekte sheep retrovirus (JSRV), the causative agent of a transmissible lung cancer in sheep. enJSRVs are able to protect their host against JSRV infection by blocking different steps of the viral replication cycle. In addition, enJSRVs are absolutely required for sheep placental development. Thus, enJSRVs-JSRV provides a unique and interesting model to study the symbiotic relationship and interplay between host ERVs and evolution. This review will provide some examples of the biological functions of ERVs. In particular, the role of ERVs in reproductive biology and in protecting the host against pathogenic retrovirus infections will be emphasized using enJSRVs/JSRV and the sheep as a model.

The signal peptide of a simple retrovirus envelope functions as a posttranscriptional regulator of viral gene expression

Retroviruses use different strategies to regulate transcription and translation and exploit the cellular machinery involved in these processes. This study shows that the signal peptide of the envelope glycoprotein (Env) of Jaagsiekte sheep retrovirus (JSRV) plays a major role in posttranscriptional viral gene expression. Expression of the JSRV Env in trans increases viral particle production by mechanisms dependent on (i) its leader sequence, (ii) an intact signal peptide cleavage site, (iii) a cis-acting RNA-responsive element located in the viral genome, (iv) Crm1, and (v) B23. The signal peptide of the JSRV Env (JSE-SP) is 80 amino acid residues in length and contains putative nuclear localization and export signals, in addition to an arginine-rich RNA binding motif. JSE-SP localizes both in the endoplasmic reticulum and in the nucleus, where it colocalizes with nucleolar markers. JSE-SP is a multifunctional protein, as it moderately enhances nuclear export of unspliced viral mRNA and considerably increases viral particle release by favoring a posttranslational step of the replication cycle.

A single amino acid substitution in a segment of the CA protein within Gag that has similarity to human immunodeficiency virus type 1 blocks infectivity of a human endogenous retrovirus K provirus in the human genome

Human endogenous retrovirus K (HERV-K) is the most intact retrovirus in the human genome. However, no single HERV-K provirus in the human genome today appears to be infectious. Since the Gag protein is the central component for the production of retrovirus particles, we investigated the abilities of Gag from two HERV-K proviruses to support production of virus-like particles and viral infectivity. HERV-K113 has full-length open reading frames for all viral proteins, while HERV-K101 has a full-length gag open reading frame and is expressed in human male germ cell tumors. The Gag of HERV-K101 allowed production of viral particles and infectivity, although at lower levels than observed with a consensus sequence Gag. Thus, including HERV-K109, at least two HERV-K proviruses in human genome today have functional Gag proteins. In contrast, HERV-K113 Gag supported only very low levels of particle production, and no infectivity was detectable due to a single amino acid substitution (I516M) near the extreme C terminus of the CA protein within Gag. The sequence of this portion of HERV-K CA showed similarities to that of human immunodeficiency virus type 1 and other primate immunodeficiency viruses. The extreme C terminus of CA may be a general determinant of retrovirus particle production. In addition, precise mapping of the defects in HERV-K proviruses as was done here identifies the key polymorphisms that need to be analyzed to assess the possible existence of infectious HERV-K alleles within the human population.

Structure of the capsid amino-terminal domain from the betaretrovirus, Jaagsiekte sheep retrovirus

Jaagsiekte sheep retrovirus is a betaretrovirus and the causative agent of pulmonary adenocarcinoma, a transmissible lung tumour of sheep. Here we report the crystal structure of the capsid amino-terminal domain and examine the self-association properties of Jaagsiekte sheep retrovirus capsid. We find that the structure is remarkably similar to the amino-terminal domain of the alpharetrovirus, avian leukosis virus, revealing a previously undetected evolutionary similarity. Examination of capsid self-association suggests a mode of assembly not driven by the strong capsid carboxy-terminal domain interactions that characterise capsid assembly in the lentiviruses. Based on these data, we propose this structure provides a model for the capsid of betaretroviruses including the HML-2 family of endogenous human betaretroviruses.