The intraovarian cellular origins of GDF9 and BMP15 in the mouse and aspects of their biological properties
Bone morphogenetic protein 15 (BMP15) and growth differentiation factor 9 (GDF9) are both members of the TGF-ß protein superfamily and are known to be essential for normal follicular development in mammals. Several studies have highlighted the species-specific effects of BMP15 and GDF9, which could be attributed, at least in part to the differences in the follicular expression patterns and to different forms of the secreted proteins. In the mouse, GDF9 is required for follicular development, whereas BMP15 appears to be only required near ovulation with contradictory reports as to the timing of BMP15 expression. However, mouse BMP15 and GDF9 are known to have the capability of acting together synergistically. The aims of this thesis were to characterise in the mouse ovary, the expression patterns (localisation and levels) of Bmp15 and Gdf9 mRNA throughout follicular development, and to determine the peri-ovulatory expression of the corresponding proteins. In situ hybridisation and quantitative PCR analyses of ovarian samples and follicular cells collected from control and superovulated mice confirmed that Gdf9 and Bmp15 mRNA are expressed exclusively in oocytes from primary and early secondary stage follicles respectively. qPCR analysis of denuded oocytes (DO) revealed a tight correlation, and therefore co-regulation, between the expression levels of Bmp15 and Gdf9 irrespective of follicular developmental stage, with steady expression until the preovulatory LH surge when down-regulation of Bmp15 and Gdf9 occurred. Throughout the follicular developmental stages examined, Gdf9 was expressed in greater abundance relative to Bmp15, with a Bmp15:Gdf9 mRNA ratio of 1:4.12. [...] In conclusion, oocyte-derived Bmp15 and Gdf9 mRNA expression is co-regulated throughout follicular development in mice, with Gdf9 being more abundant than Bmp15, which might be an important factor in determining high ovulation quota. The expression of the target genes is down-regulated as the oocyte reaches developmental competence following the preovulatory LH surge. Protein expression data provided evidence that in vivo the immature mouse oocyte is capable of secreting all BMP15 protein forms previously detected in vitro. After the preovulatory LH surge, all visible protein forms are associated with the somatic follicular cells, in particular with the expanded cumulus mass. Of particular interest is the presence of the large protein complexes in the cumulus cell lysates, which suggests a storage and activation process involving ECM proteins, similar to the mechanism reported for other TGF-ß superfamily members, such as TGF-ß1 and myostatin. The finding that the BMP15 precursor protein is biologically active with a different activity to that of the processed mature protein form suggests that the full-length precursor protein may regulate or provide at least a portion of the biological activity of BMP15 in mice.