The effects of oocytes on Smad signaling pathways in granulosa cells
It is well established that two oocyte-secreted growth factors (OSF), namely growth differentiation factor 9 (GDF9) and bone morphogenetic protein 15 (BMP15) regulate the growth of ovarian follicles and ovulation rate. Moreover, it has been reported that the gene expression ratio of Gdf9:Bmp15 varies between species and may, at least in part, influence litter size in mammals. Generally, poly-ovulatory and mono-ovulatory species express a high and low ratio of Gdf9:Bmp15, respectively. Intriguingly pigs, a poly-ovulatory species, have a low GDF9: BMP15 ratio and are the exception to this rule. To further our understanding of species differences in oocyte-secreted growth factors (OSF), and how this impacts follicular growth, oocytes and follicular cells (granulosa cells; GC) from a range of species that differ in litter size were cultured together. The aims of this study were to perform same- and cross-species co-incubations of oocytes with GCs to elucidate species-specific effects of OSF on (i) GC proliferation rate, (ii) expression levels of GC type I and type II receptors and (ii) activation of Smad signaling pathways in rats, sheep, pigs and red deer. The methodologies utilised were tritiated thymidine incorporation assays, qPCR and Western blotting, respectively. The co-incubation studies revealed in general, that rat GC are more proliferative than GCs from any other species tested, especially with time in culture. Moreover deer oocytes, which express negligible GDF9 mRNA, are unable to stimulate proliferation in rat GCs to the same extent as oocytes from the other species. This supports previous studies that suggest that rat GC only require GDF9 to proliferate, whilst both BMP15 and GDF9 are able to stimulate the GCs of sheep. This may illustrate that these two OSF play divergent roles in folliculogenesis dependant on the species. Interestingly, deer oocytes decreased Bmpr1b mRNA expression in rat GC but increased BMPR1B mRNA expression in pig GC. This may suggest that whilst the rat and pig are both poly-ovulatory species, their GCs respond in different ways to negligible amounts of GDF9, compared to BMP15. Although deer oocytes differentially affected receptor expression levels in GCs of various species, they were able to increase Smad 1/5/8 levels in both rat and deer GCs, illustrating that the BMP15 secreted by deer oocytes utilise this signaling pathway. The cross-species incubations of rat GCs with either rat, sheep or pig oocytes stimulated proliferation to a high extent. However, sheep oocytes concomitantly increased Tgfβr1 expression, and increased Smad 2/3 in rat GCs, whilst rat and pig oocytes did not. This suggests that OSF from different species may elicit proliferation by utilising different type I receptors and signaling pathways. In summary, this study illustrated that GCs of rat were different to that of the other species tested due to their suspected inability to respond to the proliferative actions of BMP15. Moreover, species-specific differences exist in the ability of OSFs to stimulate GC proliferation and activate downstream signaling pathways.