121. EXTRACELLULAR ROLES FOR PROREGIONS OF MOUSE BMP15 AND GDF9 IN VIVO

Abstract

Bone morphogenetic protein 15 (BMP15) and growth differentiation factor 9 (GDF9) are essential for normal follicular development and ovulation in mammalian species. As TGF-beta family proteins, BMP15 and GDF9 are expressed as pre-pro-mature proteins, with a signal region prompting extracellular secretion, a proregion that is post-translationally cleaved, and a mature protein that is known to be biologically active as a non-covalently interacting dimer. Until recently, the proregion was thought primarily to facilitate correct folding and dimerization of the mature proteins within the oocyte prior to processing and cleavage. However, our in vitro work with recombinant mouse BMP15 and GDF9 has shown that each of the proregion, mature protein and the non-processed promature protein are secreted from transfected 293H cells. We showed non-covalent interactions existing between the proregion and mature protein of each of BMP15 and GDF9, and between the BMP15 proregion and GDF9 mature protein. Importantly, a mouse BMP15 proregion antibody was able to abolish cooperative BMP15 and GDF9 bioactivity measured using a granulosa cell thymidine incorporation bioassay, providing strong evidence for an extracellular role for the mouse BMP15 proregion. Currently, to find out whether BMP15 and GDF9 proregions have extracellular roles in vivo, our investigation has utilised knock-down of BMP15 and GDF9 proregion proteins by mouse immunisation. Ovaries of mice immunised with the GDF9 proregion had significantly increased numbers of corpora lutea (p<0.005), while ovaries from mice immunised with BMP15 proregion peptides had significantly fewer corpora lutea (p<0.005). These findings provide the first evidence that the proregions of mouse GDF9 and BMP15 have different physiological roles outside the oocyte. Our future aim is to elucidate the function of these proregions, how these may differ between BMP15 and GDF9, and whether these aspects differ between species with different ovulation quota.