An envelope glycoprotein of the human endogenous retrovirus HERV-W is expressed in the human placenta and fuses cells expressing the type D mammalian retrovirus receptor

Abstract

A new human endogenous retrovirus (HERV) family, termed HERV-W, was recently described (J.-L. Blond, F. Besème, L. Duret, O. Bouton, F. Bedin, H. Perron, B. Mandrand, and F. Mallet, J. Virol. 73:1175-1185, 1999). HERV-W mRNAs were found to be specifically expressed in placenta cells, and an env cDNA containing a complete open reading frame was recovered. In cell-cell fusion assays, we demonstrate here that the product of the HERV-W env gene is a highly fusogenic membrane glycoprotein. Transfection of an HERV-W Env expression vector in a panel of cell lines derived from different species resulted in formation of syncytia in primate and pig cells upon interaction with the type D mammalian retrovirus receptor. Moreover, envelope glycoproteins encoded by HERV-W were specifically detected in placenta cells, suggesting that they may play a physiological role during pregnancy and placenta formation.

FIG. 1

Detection of envelope glycoprotein in HERV-W Env-transfected cells. TELCeB6 cells were transfected with plasmids expressing either the HERV-W env cDNA in the antisense (Control) or positive (HERV-W) orientation or a hyperfusogenic mutant amphotropic MLV envelope glycoprotein (A-Rless). Two days later, the Env-transfected cells were lysed and their supernatants were harvested, filtered, and ultracentrifuged at 150,000 × g to pellet the viral particles. Immunoblots of cell lysates (A) and viral pellets (B) were probed with antibodies (Ab) against HERV-W Env, MLV SU, or MLV CA protein, as indicated. The positions of molecular mass markers are shown to the left of the gels (in kilodaltons).

FIG. 2

Formation of syncytia by HERV-W envelope glycoprotein. TELac2 cells, derived from TE671 human rhabdomyosarcoma cells and constitutively expressing a nuclear β-galactosidase, were transfected with plasmids expressing either the HERV-W env cDNA in the positive (HERV-W) or antisense (Control) orientation or a hyperfusogenic mutant amphotropic MLV envelope glycoprotein (A-Rless). Transfected cells were overlaid with HeLa indicator cells. The determination of the fusion activity of the transfected envelope glycoproteins was performed after 36 h of coculture. (A) Results are expressed as percentages of the fusion indices (means ± standard deviations; n = 5). (B) Cocultures were stained with X-Gal substrate to reveal β-galactosidase activity and to visualize the nuclei of the producer cells (arrows) and then with May-Grünwald and Giemsa solutions. Magnification, ×250.

FIG. 2

Formation of syncytia by HERV-W envelope glycoprotein. TELac2 cells, derived from TE671 human rhabdomyosarcoma cells and constitutively expressing a nuclear β-galactosidase, were transfected with plasmids expressing either the HERV-W env cDNA in the positive (HERV-W) or antisense (Control) orientation or a hyperfusogenic mutant amphotropic MLV envelope glycoprotein (A-Rless). Transfected cells were overlaid with HeLa indicator cells. The determination of the fusion activity of the transfected envelope glycoproteins was performed after 36 h of coculture. (A) Results are expressed as percentages of the fusion indices (means ± standard deviations; n = 5). (B) Cocultures were stained with X-Gal substrate to reveal β-galactosidase activity and to visualize the nuclei of the producer cells (arrows) and then with May-Grünwald and Giemsa solutions. Magnification, ×250.

FIG. 3

Cell-cell fusion assays in RDR-transfected XC cells. A plasmid encoding RDR, the human type D mammalian retrovirus receptor, was expressed in XC rat cells. The A-Rless or HERV-W fusogenic envelope glycoprotein was transiently expressed in RDR-transfected XC cells (hatched bars) as well as in parental XC cells (black bars). The results are expressed as percentages of the fusion indices (means ± standard deviations n = 3).

FIG. 4

In situ detection of HERV-W envelope glycoprotein in tissue sections. The tissue sections originated from human tissue donors' 13-week placenta, uterus, and small intestine. Slides were labeled (A) or not (B) with the 6A2B2 monoclonal antibody and revealed by immunoperoxidase staining and hematoxylin counterstaining. Magnification, ×100.