In the ovary, connexin-coupled gap junctions in granulosa cells play crucial roles in follicular and oocyte development as well as in corpus luteum formation. Our previous work has shown that theca cell-derived bone morphogenetic protein (BMP)4 and BMP7 decrease gap junction intercellular communication (GJIC) activity via the down-regulation of connexin43 (Cx43) expression in immortalized human granulosa cells. However, the effects of oocyte-derived growth factors on Cx43 expression remain to be elucidated. The present study was designed to investigate the effects of oocyte-derived growth differentiation factor (GDF)9 and BMP15 on the expression of Cx43 in a human granulosa cell line, SVOG. We also examined the effect relative to GJIC activity and investigated the potential mechanisms of action. In SVOG cells, treatment with BMP15 but not GDF9 significantly decreased Cx43 mRNA and protein levels and GJIC activity. These suppressive effects, along with the induction of Smad1/5/8 phosphorylation, were attenuated by co-treatment with a BMP type I receptor inhibitor, dorsomorphin. Furthermore, knockdown of the central component of the transforming growth factor-β superfamily signaling pathway, Smad4, using small interfering RNA reversed the suppressive effects of BMP15 on Cx43 expression and GJIC activity. The suppressive effects of BMP15 on Cx43 expression were further confirmed in primary human granulosa-lutein cells obtained from infertile patients undergoing an in vitro fertilization procedure. These findings suggest that oocyte-derived BMP15 decreases GJIC activity between human granulosa cells by down-regulating Cx43 expression, most likely via a Smad-dependent signaling pathway.
Keywords: BMP15; GDF9; connexin43; human granulosa cell; smad.