Synergistic roles of bone morphogenetic protein 15 and growth differentiation factor 9 in ovarian function

Knockout mouse technology has been used over the last decade to define the essential roles of ovarian-expressed genes and uncover genetic interactions . In particular, we have used this technology to study the function of mult iple members of the transforming growth factor-beta superfamily including i nhibins, activins, and growth differentiation factor 9 (GDF-9 or Gdf9). Kno ckout mice lacking GDF-9 are infertile due to a block in folliculogenesis a t the primary follicle stage. In addition, recombinant GDF-9 regulates mult iple cumulus granulosa cell functions in the periovulatory period including hyaluronic acid synthesis and cumulus expansion. We have also cloned an oo cyte-specific homolog of GDF-9 from mice and humans, which is termed bone m orphogenetic protein 15 (BMP-15 or Bmp15). To define the function of BMP-15 in mice, we generated embryonic stem cells and knockout mice, which have a null mutation in this X-linked gene. Male chimeric and Bmp15 null mice are normal and fertile. In contrast to Bmp15 null males and Gdf9 knockout fema les, Bmp15 null females (Bmp15(-/-)) are subfertile and usually have minima l ovarian histopathological defects, but demonstrate decreased ovulation an d fertilization rates. To further decipher possible direct or indirect gene tic interactions between GDF-9 and BMP-15, we have generated double mutant mice lacking one or both alleles of these related homologs. Double homozygo te females (Bmp15(-/-)Gdf9(-/-)) display oocyte loss and cysts and resemble Gdf9(-/-) mutants. In contrast, Bmp15(-/-)Gdf9(+/-) female mice have more severe fertility defects than Bmp15(-/-) females, which appear to be due to abnormalities in ovarian folliculogenesis, cumulus cell physiology, and fe rtilization. Thus, the dosage of intact Bmp15 and Gdf9 alleles directly inf luences the destiny of the oocyte during folliculogenesis and in the periov ulatory period. These studies have important implications for human fertili ty control and the maintenance of fertility and normal ovarian physiology.