The molecular forms of BMP15 in a range of mammalian species

BMP15 is an oocyte-secreted growth factor that is critical for ovarian follicular development and fertility in mammals. To improve our understanding of the species-specific effects of BMP15 on fertility, the aim of this study was to elucidate the molecular forms of BMP15 in mammals with high (rat and pigs) and low (sheep and deer) litter sizes. Western blotting experiments were undertaken using a monoclonal antibody directed against a semi-conserved area on the mature region of E.coli-expressed ovine BMP15.  Within the oocyte lysate of all species tested, the predominant form was promature BMP15, although mature BMP15 was also present. Within oocyte-conditioned media of sheep, pigs and deer, the promature:mature BMP15 ratio dropped significantly from what was seen inside the oocyte, with promature BMP15 only slightly predominant in deer, and relatively equal amounts of each protein were present in pigs and sheep. No BMP15 was detected in the oocyte-conditioned media of rats. Cross-linking studies did not provide any evidence of BMP15/BMP15 homodimers or BMP15/GDF9 heterodimers in any species tested.  Intra-oocyte levels of both promature and mature BMP15, from highest to lowest, were detected in deer, followed by sheep and pigs (similar), and then rats (significantly lower). In the oocyte-conditioned media, with the exception of the rat where BMP15 was absent, there was a similar pattern of promature BMP15 levels observed between species, whereas mature BMP15 levels showed no inter-species variation.  In summary, the molecular forms and relative amounts of BMP15 protein differ across species. Generally, high BMP15 levels were associated with low litter size; however the pig is an exception. Furthermore, the molecular forms observed in this study differed from those reported in studies using recombinant BMP15 suggesting that recombinantly-expressed protein may not provide a fair representation of native BMP15.