Oocyte-derived bone morphogenetic protein-15 (BMP15) is a regulator of folliculogenesis, via actions on granulosa cell growth and differentiation. Studies in sheep have also shown that BMP15 is a primary determinant of ovulation rate within species. What is less clear is whether BMP15 could contribute to the marked differences in ovulation rate between species?
Like other TGF-β superfamily ligands, BMP15 is synthesised as a precursor with the N-terminal prodomain mediating the folding and dimerisation of the C-terminal mature domain. Dimeric precursors are cleaved by proprotein convertases and BMP15 is secreted from the oocyte non-covalently associated with its prodomain.
In this study, we found that hBMP15 was expressed at much higher levels than oBMP15, whereas mBMP15 was not expressed. Sequence analysis of the region of the prodomain (α1-helix) that mediates synthesis of TGF-β ligands identified significant differences between species. Substituting non-conserved hBMP15 residues across this region into m- or oBMP15 resulted in a significant increase in growth factor expression. Thus, specific adaptions in the m- and oBMP15 prodomains limit mature BMP15 expression.
Following purification, hBMP15 potently stimulates a luciferase response in COV-434 granulosa cells. BMP15 activity is dependent upon its ability to assemble a signalling complex of type I (ALK6) and type II (BMPRII) receptors. Within the putative type I receptor binding site of BMP15, significant species differences are observed (Arg329/Asp330 in human; Pro329/Tyr330 in mouse; His329/Tyr330 in sheep). Substituting Arg329/Asp330 of hBMP15 for the corresponding m- or oBMP15 residues reduced bioactivity 5- and 40-fold respectively, identifying these residues as mediators of high affinity interactions with ALK6.
Collectively, we have identified specific residues in the pro- and mature domains of hBMP15 that enhance growth factor expression and activity. These adaptations may contribute to the variation observed in ovulation rate and fecundity between mammals.