Bone Morphogenetic Protein-15 (BMP-15), a member of the TGF-beta superfamily, is an oocyte-specific growth factor. Recent studies of a naturally occurring mutation in sheep [Inverdale (FecX')] have suggested an essential role for BMP-15 in folliculogenesis and fertility. In the homozygous FecX' female, follicular development arrests at the primary stage, resulting in infertility. In contrast, the heterozygous FecX1 females exhibit increases in ovulation rate and in twin and triplet births. It has now been shown that a substitution of valine by aspartic acid at residue 31 (V31D) of the BMP-15 gene is present in FecX' carriers. Therefore, BMP-15 mutation appears to be associated with infertility and super fertility in a dosage-sensitive manner. Our laboratory has recently produced recombinant BMP-15 and identified granulosa cells (GCs) as target cells for this factor. Using the recombinant protein, we have found that BMP-15 (i) stimulates rat GC proliferation, (ii) inhibits FSH receptor expression, and (iii) stimulates kit ligand expression in the GCs. These findings establish the new concept that oocyte-derived BMP-15 is functionally involved in regulating three major aspects of follicle development, namely GC proliferation, GC cytodifferentiation, and oogenesis. Here, we propose to elucidate the molecular basis of BMP-15 action from the receptors to the target genes by characterizing BMP-15 receptor and signaling molecules (Aim 1) and the molecular mechanisms by which BMP-15 regulates FSH receptor and kit ligand gene expression (Aim 2). We have also found that, despite lacking the inter-subunit disulfide bonds seen in most of the dimeric TGF-beta superfamily members, BMP-15 and the closely related GDF-9 are homodimers. Furthermore, we have obtained evidence that BMP-15 and GDF-9 can form heterodimers, and thus we propose to explore the biological roles of BMP-15/GDF-9 heterodimers (Aim 3). Although the V31D mutation in the BMP15 gene was found in Inverdale sheep, the role of BMP-15-V31D mutation in Inverdale sheep is yet unclear. We will explore the biological activities of the BMP-15-V31D mutation and test our hypothesis that BMP-15-V31D could influence the biosynthesis of GDF-9 (Aim 4). Finally we will generate the same mutation in mice and determine the in vivo effect of the BMP-15-V31D mutation on folliculogenesis and ovulation (Aim 5). These studies should provide significant advances in understanding regulation of the events central to female fertility and ultimately improve approaches for birth control and for treating gonadal abnormalities and infertility.
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