Oocyte-specific overexpression of mouse bone morphogenetic protein-15 leads to accelerated folliculogenesis and an early onset of acyclicity in transgenic mice

Whereas mutations in the bmp15 gene cause infertility in ewes and women due to defects in folliculogenesis, most defects in female mice lacking bone morphogenetic protein (BMP)-15 are confined to the ovulation process, supportive of the observation that functional mouse BMP-15 is barely detected in oocytes in vivo until after the LH surge. In addition, the mouse BMP-15 proprotein is not processed into the functional mature protein in transfected cells. However, a chimeric protein consisting of the human proregion, human cleavage site, and mouse mature region ( termed hhmBMP-15) is processed and the mature protein secreted. To study the role of BMP-15 in folliculogenesis, we generated transgenic mice overexpressing hhmBMP-15, exclusively in oocytes during folliculogenesis and confirmed the overexpression of mouse BMP-15 mature protein. Immature transgenic mice exhibited accelerated follicle growth with decreased primary follicles and an increase in secondary follicles. Granulosa cells of immature mice displayed an increased mitotic index and decreased FSH receptor mRNA expression. Adult mice had normal litter sizes but an increased number of atretic antral follicles. Interestingly, aging mice exhibited an early onset of acyclicity marked by increased diestrus length and early occurrence of constant diestrus. These findings indicate the role of BMP-15 in vivo in promoting follicle growth and preventing follicle maturation, resulting in an early decline in the ovarian reserve of transgenic mice. Therefore, the lack of mouse BMP-15 during early folliculogenesis in the wild-type mice may be relevant to their polyovulatory nature as well as the preservation of ovarian function as the mice age.