Syncytin-A and -B, envelope genes of retroviral origin, have been shown to contribute independently to the formation of the two syncytiotrophoblast layers during mouse placenta formation. In addition to this important role in placenta formation, syncytin-A and -B expression has been described in other tissues, and their highly fusogenic properties suggested that they might be involved in the fusion of other cell types. In order to elucidate the potential role of the syncytin-A and -B in osteoclast fusion and in overall bone metabolism/homeostasis, we analysed the bone phenotype of the syncytin-B knock out mice both ex vivo and in vivo. We first demonstrated by qPCR that both syncytin-A and -B are expressed during mouse splenocytes differentiation into osteoclasts. These genes are expressed through 14 days of culture, but the expression level decreases with differentiation. In the splenocyte and marrow macrophage cultures from syncytin-B KO mice, a reduced index of fusion and number of multinucleated osteoclasts are observed without impacting the osteoclastic resorption. Syncytin-B was expressed in TRAP+ cells of the periosteum at E16.5. The bone phenotype of the syncytin-B KO mice revealed no difference between the WT and KO littermates in term of BMD (at 6, 12, and 24 weeks of age), BV/TV (6 and 24 weeks of age) and D-pyridinolin/creatinuria (at 6 weeks). These results are in accordance with the ex vivo results showing a decrease of the fusion index without effect on the osteoclast function. Altogether the results suggest that although essential for placenta formation, syncytin-B is not essential to the maintenance of the bone homeostasis while it is expressed in osteoclast precursors and plays an early role in osteoclast fusion.
Disclosure: The authors declared no competing interests. The work was funded by the Agence Nationale de la Recherche (ANR)-07-e-RARE-010-01 OSTEOPETR and by a grant from Fondation pour la Recherche Médicale to MCdeV.