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1.  The Mouse IAPE Endogenous Retrovirus Can Infect Cells through Any of the Five GPI-Anchored EphrinA Proteins 
PLoS Pathogens  2011;7(10):e1002309.
The IAPE (Intracisternal A-type Particles elements with an Envelope) family of murine endogenous retroelements is present at more than 200 copies in the mouse genome. We had previously identified a single copy that proved to be fully functional, i.e. which can generate viral particles budding out of the cell and infectious on a series of cells, including human cells. We also showed that IAPE are the progenitors of the highly reiterated IAP elements. The latter are now strictly intracellular retrotransposons, due to the loss of the envelope gene and re-localisation of the associated particles in the course of evolution. In the present study we searched for the cellular receptor of the IAPE elements, by using a lentiviral human cDNA library and a pseudotype assay on transduced cells. We identified Ephrin A4, a GPI-anchored molecule involved in several developmental processes, as a receptor for the IAPE pseudotypes. We also found that the other 4 members of the Ephrin A family –but not those of the closely related Ephrin B family- were also able to mediate IAPE cell entry, thus significantly increasing the amount of possible cell types susceptible to IAPE infection. We show that these include mouse germline cells, as illustrated by immunohistochemistry experiments, consistent with IAPE genomic amplification by successive re-infection. We propose that the uncovered properties of the identified receptors played a role in the accumulation of IAPE elements in the mouse genome, and in the survival of a functional copy.
Author Summary
In mammals, nearly half the genome is composed of reiterated scattered sequences. Some of them, called endogenous retroviruses, have a structure similar to that observed for the integrated form of infectious retroviruses. The current theory to account for their presence is that an infectious retrovirus once infected the germline of its host. This viral genome was then transmitted to the progeny and expressed from there, producing new infectious particles, which could re-infect new germline cells and thus increase the viral genomic copy number. However no evidence has yet been provided to support this model. In this study, we identify a family of five cellular proteins, the Ephrin As, as receptors for a model mouse family of endogenous retroviruses, the IAPE elements. We analyse their expression pattern and show that both the oocytes and some male germline cells express Ephrin A proteins and can thus be infected by IAPE particles. This finding strongly supports the current model of ERVs amplification. In addition, the IAPE envelope ability to use five different cellular receptors suggests that it might be impossible for the host to evolve a resistance against this viral element, and provides a clue on how the IAPE family survived so long in the mouse genome.
PMCID: PMC3197615  PMID: 22028653
5.  Spontaneous Heteromerization of Gammaretrovirus Envelope Proteins: a Possible Novel Mechanism of Retrovirus Restriction▿  
Journal of Virology  2008;82(19):9789-9794.
The env gene of gammaretroviruses encodes a glycoprotein conserved among diverse retroviruses, except for the domains involved in receptor binding. Here we show that pairs of gammaretrovirus envelope proteins (from Friend virus and GALV or xenotropic viruses) assemble into heteromers when coexpressed. This assembly results in a strong inhibition of infectivity. An unrelated envelope protein does not assemble in heteromers with the gammaretrovirus glycoproteins tested and does not affect their infectivity, demonstrating the specificity of the mechanism we describe. We propose that the numerous copies of endogenous retroviral env genes conserved within mammalian genomes act as restriction factors against infectious retroviruses.
PMCID: PMC2546955  PMID: 18667519
6.  Murine Endogenous Retrovirus MuERV-L Is the Progenitor of the “Orphan” Epsilon Viruslike Particles of the Early Mouse Embryo▿  
Journal of Virology  2007;82(3):1622-1625.
Viruslike particles which displayed a peculiar wheellike appearance that distinguished them from A-, B- or C-type particles had previously been described in the early mouse embryo. The maximum expression of these so-called epsilon particles was observed in two-cell-stage embryos, followed by their rapid decline at later stages of development and no particles detected at the zygote one-cell stage. Here, we show that these particles are in fact produced by a newly discovered murine endogenous retrovirus (ERV) belonging to the widespread family of mammalian ERV-L elements and named MuERV-L. Using antibodies that we raised against the Gag protein of these elements, Western blot analysis and in toto immunofluorescence studies of the embryos at various stages disclosed the same developmental expression profile as that observed for epsilon particles. Using expression vectors for cloned, full-length, entirely coding MuERV-L copies and cell transfection, direct identification of the epsilon particles was finally achieved by high-resolution electron microscopy.
PMCID: PMC2224431  PMID: 18045933
7.  Immunization with a Lentivector That Targets Tumor Antigen Expression to Dendritic Cells Induces Potent CD8+ and CD4+ T-Cell Responses▿  
Journal of Virology  2007;82(1):86-95.
Lentivectors stimulate potent immune responses to antigen transgenes and are being developed as novel genetic vaccines. To improve safety while retaining efficacy, we constructed a lentivector in which transgene expression was restricted to antigen-presenting cells using the mouse dectin-2 gene promoter. This lentivector expressed a green fluorescent protein (GFP) transgene in mouse bone marrow-derived dendritic cell cultures and in human skin-derived Langerhans and dermal dendritic cells. In mice GFP expression was detected in splenic dectin-2+ cells after intravenous injection and in CD11c+ dendritic cells in the draining lymph node after subcutaneous injection. A dectin-2 lentivector encoding the human melanoma antigen NY-ESO-1 primed an NY-ESO-1-specific CD8+ T-cell response in HLA-A2 transgenic mice and stimulated a CD4+ T-cell response to a newly identified NY-ESO-1 epitope presented by H2 I-Ab. As immunization with the optimal dose of the dectin-2 lentivector was similar to that stimulated by a lentivector containing a strong constitutive viral promoter, targeting antigen expression to dendritic cells can provide a safe and effective vaccine.
PMCID: PMC2224357  PMID: 17959670
8.  Murine MusD Retrotransposon: Structure and Molecular Evolution of an “Intracellularized” Retrovirus▿  
Journal of Virology  2006;81(4):1888-1898.
We had previously identified active autonomous copies of the MusD long terminal repeat-retrotransposon family, which have retained transpositional activity. These elements are closely related to betaretroviruses but lack an envelope (env) gene. Here we show that these elements encode strictly intracellular virus-like particles that can unambiguously be identified by electron microscopy. We demonstrate intracellular maturation of the particles, with a significant proportion of densely packed cores for wild-type MusD but not for a protease mutant. We show that the molecular origin of this unexpected intracellular localization is solely dependent on the N-terminal part of the Gag protein, which lacks a functional sequence for myristoylation and plasma membrane targeting: replacement of the N-terminal domain of the MusD matrix protein by that of its closest relative—the Mason-Pfizer monkey virus—led to targeting of the MusD Gag to the plasma membrane, with viral particles budding and being released into the cell supernatant. These particles can further be pseudotyped with a heterologous envelope protein and become infectious, thus “reconstituting” a functional retrovirus prone to proviral insertions. Consistent with its retroviral origin, a sequence with a constitutive transport element-like activity can further be identified at the MusD 3′ untranslated region. A molecular scenario is proposed that accounts for the transition, during evolution, from an ancestral infectious betaretrovirus to the strictly intracellular MusD retrotransposon, involving not only the loss of the env gene but also an inability to escape the cell—via altered targeting of the Gag protein—resulting de facto in the generation of a very successful “intracellularized” insertional mutagen.
PMCID: PMC1797557  PMID: 17151128
9.  Identification of a Functional Envelope Protein from the HERV-K Family of Human Endogenous Retroviruses 
Journal of Virology  2005;79(24):15573-15577.
Genome-wide screening of sequence databases for human endogenous retroviruses (HERVs) has led to the identification of 18 coding env genes, among which two—the syncytin genes—encode fusogenic ENV proteins possibly involved in placenta physiology. Here we show that a third ENV, originating from the most “recent” HERV-K(HML2) family, is functional. Immunofluorescence analysis of env-transduced cells demonstrates expression of the protein at the cell surface, and we show that the protein confers infectivity to simian immunodeficiency virus pseudotypes. Western blot analysis of the pseudotyped virions further discloses the expected specific cleavage of the ENV precursor protein. This functional ENV could play a role in the amplification—via infection of the germ line—of the HERV-K genomic copies, all the more as coding HERV-K gag and pol genes can similarly be found in the human genome, which could therefore generate infectious virions of a fully endogenous origin.
PMCID: PMC1315997  PMID: 16306628
10.  Identification of an Envelope Protein from the FRD Family of Human Endogenous Retroviruses (HERV-FRD) Conferring Infectivity and Functional Conservation among Simians 
Journal of Virology  2004;78(2):1050-1054.
A member of the HERV-W family of human endogenous retroviruses (HERV) had previously been demonstrated to encode a functional envelope which can form pseudotypes with human immunodeficiency virus type 1 virions and confer infectivity on the resulting retrovirus particles. Here we show that a second envelope protein sorted out by a systematic search for fusogenic proteins that we made among all the HERV coding envelope genes and belonging to the HERV-FRD family can also make pseudotypes and confer infectivity. We further show that the orthologous envelope genes that were isolated from simians—from New World monkeys to humans—are also functional in the infectivity assay, with one singular exception for the gibbon HERV-FRD gene, which is found to be fusogenic in a cell-cell fusion assay, as observed for the other simian envelopes, but which is not infectious. Sequence comparison of the FRD envelopes revealed a limited number of mutations among simians, and one point mutation—located in the TM subunit—was shown to be responsible for the loss of infectivity of the gibbon envelope. The functional characterization of the identified envelopes is strongly indicative of an ancestral retrovirus infection and endogenization, with some of the envelope functions subsequently retained in evolution.
PMCID: PMC368808  PMID: 14694139

Results 1-10 (10)