Bone morphogenetic protein-6 enhances gonadotrophin-dependent progesterone and inhibin secretion and expression of mRNA transcripts encoding gonadotrophin receptors and inhibin/activin subunits in chicken granulosa cells

Abstract

The aims were to examine ovarian expression of bone morphogenetic protein (BMP) ligands/receptor mRNAs in the chicken and to test the hypothesis that theca-derived BMP(s) modulates granulosa cell function in a paracrine manner. RT-PCR revealed expression of multiple BMPs in granulosa and theca cells from prehierarchical and preovulatory follicles with greater expression in theca cells; both cell types expressed BMP receptors-IA, -IB and -II consistent with tissue responsiveness. Preovulatory granulosa cells (F1, F2 and F3/4) were cultured with BMP-6 (expressed by theca but not granulosa) in the presence/absence of LH, FSH or 8-Br-cAMP. BMP-6 increased 'basal' and gonadotrophin-induced inhibin-A and progesterone secretion by each cell type but did not enhance the effect of 8-Br-cAMP. This indicates that the observed synergism between BMP-6 and gonadotrophin might involve BMP-induced up-regulation of gonadotrophin receptors. In support of this, BMP-6 alone increased LH-receptor (LHR) mRNA in F1 cells and FSH-receptor (FSHR) mRNA in F1, F2 and F3/4 cells. BMP-6 also enhanced LH/FSH-induced LHR transcript amount in each cell type but did not raise FSHR transcript amounts above those induced by BMP-6 alone. To further explore BMP-6 action on inhibin-A secretion, we quantified inhibin/ activin subunits (α, βA, βB) mRNAs. Consistent with its effect on inhibin-A secretion, BMP-6 enhanced 'basal' expression of α- and ΒA-subunit mRNA in F1, F2 and F3/4 cells, and βB-subunit mRNA in F3/4 cells. BMP-6 markedly enhanced FSH/LH-induced expression of α-subunit in all follicles and FSH-induced β A-Subunit in F2 and F3/4 follicles but not in F1 follicles. Neither BMP-6 alone, nor FSH/AH alone, affected 'basal' βB mRNA abundance. However, co-treatment with gonadotrophin and BMP-6 greatly increased βB-subunit expression, the response being lowest in F1 follicles and greatest in F3/ 4 follicles. Collectively, these results support the hypothesis that intra-ovarian BMPs of thecal origin have a paracrine role in modulating granulosa cell function in the chicken in a preovulatory stage-dependent manner. © 2007 Society for Reproduction and Fertility.