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Proviral Protein Provides Placental Function

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Proviral Protein Provides Placental Function COMMENTARY Proviral protein provides placental function Jonathan P. Stoye1 Division of Virology, Medical Research Council, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, United Kingdom uring the course of evolution, all vertebrates have been ex- posed to multiple waves of cross-species infection by ret- Droviruses. Some of these viruses have succeeded in infecting germ cells and are now inherited in integrated, proviral form (1). These so-called endogenous retroviruses have undergone further am- plification and now make up a greater fraction of our DNA than do normal coding sequences (2). For a consider- able while it was an open question whether these proviruses provided sig- nificant benefits to their hosts or simply represented junk or selfish DNA. It is now clear that several such elements contribute to protection against retrovi- ral infection (3). Moreover, a number of lines of indirect evidence have pointed to a possible role in placental develop- ment (4). Now comes compelling evi- dence that one or more proviruses are ‘‘paying their way’’ with a specific con- tribution to normal physiology. In this issue of PNAS, Dupressoir et al. (5) show that mutant mice in which the syn- cytin-A gene has been knocked out die in utero, apparently as a result of the failure of trophoblast cells to fuse and form one of the two syncytiotrophoblast layers present in the placenta of mice. These structures are present on the ex- terior of the placenta, at the fetal–maternal interface, and are thought to play a key role in exchanging nutrients and waste between mother and fetus. Syncytin-A is derived from a ret- roviral env gene (6). The receptor binding and membrane fusion activities that play key roles in Fig. 1. Consequences of Env–receptor interactions. (A) Retrovirus infection. After receptor binding, the retrovirus life cycle (7) are provided Env-mediated fusion between virus and cell membranes occurs, resulting in the uptake of viral cores into by the two protein products of the env the cell cytoplasm. Precise details of the triggering of fusion, other factors involved, and the site of gene, SU (surface) and TM (transmem- membrane fusion vary from virus to virus. (B) Restriction of virus entry. The product of a retrovirus-derived brane). Synthesized as one polypeptide env gene interacts with receptor protein leading to the absence of free receptor on the cell surface thereby precursor that is proteolytically cleaved preventing infection. Where in the cell the receptor–Env interaction takes place and the mechanism of during transport to the cell surface, they resistance, e,g. blocking or down-regulation from the cell surface, is likely to vary on a case-by-case basis. (C) Cell fusion. Env-mediated membrane fusion between two cells results in the formation of multinu- remain physically linked even after virus cleate cells. production. Binding of SU to its cognate receptor can have a number of conse- quences (Fig. 1). In the viral life cycle, be found in a number of species, includ- membrane fusion activity allows viruses mains to be determined. In any event, to enter target cells (8) (Fig. 1A). Alter- ing chickens, mice, and cats (3); they Env-mediated membrane fusion is a key natively, Env produced by infected cells represent some of the first examples of feature of the retroviral life cycle, and it can down-regulate or saturate free re- ‘‘useful’’ endogenous retroviruses. Fi- is this activity that now appears to have ceptor on the cell surface (Fig. 1B), re- nally, there are multiple examples of sulting in resistance to infection (3). If retrovirus infection leading to cell mem- encoded by an endogenous retrovirus, brane fusion and the formation of Author contributions: J.P.S. wrote the paper. such resistance will be inherited in Men- multinucleated cells (9, 10) (Fig. 1C); The author declares no conflict of interest. delian fashion. Examples of resistance to the importance of this phenomenon for See companion article on page 12127. retroviruses arising in this manner can retrovirus-induced pathogenicity re- 1E-mail: [email protected]. www.pnas.org͞cgi͞doi͞10.1073͞pnas.0906295106 PNAS ͉ July 21, 2009 ͉ vol. 106 ͉ no. 29 ͉ 11827–11828 Downloaded by guest on September 29, 2021 been appropriated for use in placental for env genes with ORFs (only 16 were virally derived syncytin-A plays a key formation. identified) followed by functional testing role in placentation. A description of Most endogenous proviruses have (16). Detailed SNP analysis yielded no the properties of the syncytin-B knock- been present in the germ line for many data inconsistent with a functional role out is not yet available but will be of millions of years and have suffered the for syncytin-1 or syncytin-2 (17). considerable interest given observed dif- inevitable mutational decay of genomic These findings are highly suggestive of ferences in the fusion specificity of syn- passengers not subject to positive selec- a role for one or both syncytins in pla- cytin-A and syncytin-B (6). Further, the tion (11). Thus, of the tens of thousands cental morphogenesis but do not repre- syncytins may prove valuable tools for of proviral elements in the human ge- sent definitive proof. Because HERV-W studying placental development. nome, few, if any, can replicate as vi- and HERV-FRD proviruses are present It should be stressed that the pair of ruses although many are transcribed in a only in primates, gene knockout ap- murine syncytins are unrelated in se- differentiation-specific manner. Searches proaches to test the functions of syncy- quence or chromosomal location to the for functional proviral genes have there- tin-1 and syncytin-2 are not feasible. primate genes and were acquired inde- fore focused on viral sequences that (i) However, the discovery in mice of an pendently from one another. The hu- are conserved in evolution and encode additional pair of env genes, syncytin-A man loci resemble intact proviruses ORFs and/or (ii) show interesting pat- much more closely than do the murine terns of expression. One such gene is genes that retain little more than the the env gene encoded by the single-copy Syncytins may prove env genes of the integrating viruses ERV-3 locus (12), a provirus that is (6, 15, 16). Nevertheless the results with present in all apes and Old World mon- the syncytin-A knockout would appear valuable tools for to strengthen the case for the involve- keys and expressed primarily in the pla- ment of syncytins in human placenta- centa. However, a detailed survey of studying placental tion. Furthermore evidence using an in SNPs in the human population revealed vivo morpholino loss of function ap- the presence of a homozygous stop development. proach also argues for a role of one or codon within the env gene of ERV-3 in more endogenous retrovirus-encoded 1% of the population (13), a finding env genes, clearly different from the syn- that would be inconsistent with an es- and syncytin-B, at chromosomal loca- cytins, in the regulation of periimplanta- sential role in placenta formation. tions 5qG2 and 14qD1, with properties tion placental growth and differentiation Attention turned to a gene dubbed syn- analogous to the human genes, opens up in sheep (18). It therefore appears that cytin (14). It corresponds to the env this possibility (6). Syncytin-A and retroviral env genes have been co-opted gene of one particular member of the syncytin-B are present in all Muridae to play a part in placental development HERV-W family of proviruses (15). It is and show placenta-specific expression on a number of independent occasions. conserved in evolution and highly ex- and in vitro fusogenic activity. Dupres- Comparative studies reveal significant pressed in placenta, specifically in the soir et al. (5) describe the properties of structural diversity among the placentae syncytiotrophoblast. When introduced mice carrying a null mutation in the of different orders of mammals (19, 20); into cultured cells, it can mediate fusion, syncytin-A gene. Homozygous embryos one might therefore speculate that the an activity that can be blocked by addi- die in utero between 11.5 and 13.5 days specific roles played by these viral genes tion of antisera directed against syncytin post coitum. Cellular and subcellular may differ, perhaps depending on the to the media. Subsequently, a second examination of placentae from the null cellular expression patterns of Envs and conserved env gene (syncytin-2), highly mice revealed disruption of the syncy- their specific receptors. It is possible conserved in evolution and encoded by tiotrophoblast-containing regions associ- that proviral inheritance will provide a an endogenous provirus of the HERV- ated with a surplus of trophoblasts, sug- better predictor of the diversities of syn- FRD group, also with fusion activity, gesting their failure to fuse and cytial morphologies than taxonomy was identified by a genomewide screen providing strong evidence that the retro- alone. 1. Boeke JD, Stoye JP (1997) Retrotransposons, endoge- JM, Hughes SH, Varmus HE (Cold Spring Harbor Lab 15. Blond JL, et al. (2000) An envelope glycoprotein of the nous retroviruses, and the evolution of retroelements. Press, Cold Spring Harbor, NY), pp 263–334. human endogenous retrovirus HERV-W is expressed in Retroviruses, eds Coffin JM, Hughes SH, Varmus HE 8. Melikyan GB (2008) Common principles and interme- the human placenta and fuses cells expressing the type (Cold Spring Harbor Lab Press, Cold Spring Harbor, NY), diates of viral protein-mediated fusion: The HIV-1 par- D mammalian retrovirus receptor. J Virol 74:3321– pp 343–435. adigm. Retrovirology 5:111. 3329. 2. Jern P, Coffin JM (2008) Effects of retroviruses on host 9.
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