Cellular signalling and activation of oocyte secreted human growth and differentiation factors BMP15 and GDF9

Bone morphogenetic protein 15 (BMP15) and growth differentiation factor 9 (GDF9) are growth factors expressed by oocytes and are among the key regulators transmitting the signaling between the oocyte and the surrounding granulosa cells. Genetic studies have shown that these factors are essential regulators of normal fertility in mammals. BMP15 gene deficient mice exhibit a mild reduction in fertility, whereas sheep homozygous for a mutant form of BMP15 are sterile. In the case of GDF9, knockout mice and homozygous mutant sheep are also sterile the ovaries being non functional due to an early block in folliculogenesis. In humans, aberrant expression of these factors has been involved in premature ovarian failure (POF), polycystic ovary syndrome (PCOS) or dizygotic twinning. ----- This thesis was aimed to define the signaling and activation of human BMP15 (hBMP15) and GDF9 (hGDF9) proteins. The lack of bioactive hGDF9 had restricted the in vitro studies previously and the first aim of this thesis was to produce and purify bioactive hGDF9 protein. We found that hGDF9 is secreted in a latent form. Although mouse GDF9 (mGDF9) and hBMP15 are produced in active form their bioactivity is lost when a C-terminal affinity purification tag is added. We found also, that an N-terminal epitope tag does not have a deleterious effect on the bioactivity of mGDF9 or hBMP15 and they can be purified using immobilized metal affinity (IMAC) and high performance liquid chromatography approaches. A purified wild type hGDF9 (hGDF9wt) mature region became available to us during this thesis project and we found that it is a highly bioactive and stable protein activating the Smad3/4 signaling pathway. Subsequently we produced and purified a bioactive wild type hBMP15 (hBMP15wt) form, which was highly bioactive and activated the Smad1/5/8 signaling pathway in human granulosa luteal (hGL) cells. Physico-chemical characterization of BMP15 revealed that it consists of P16 and P17 forms, where P16 seems to be phosphorylated and P17 glycosylated. ------ It has been shown previously that a recombinant BMP type II receptor ectodomain Fc fusion protein (BMPR2ecd Fc) is able to inhibit the actions of BMP15 and GDF9 in vitro. For studying the effect of BMPR2ecd Fc on mouse folliculogenesis, we produced and purified this protein. These in vivo studies demonstrated that by administering the BMPR2ecd-Fc protein, we were able to dose-dependently modulate ovarian folliculogenesis in mice. -------- In order to determine the cell surface receptor binding components for human BMP15, we engineered and produced a covalent dimer of hBMP15 and labeled it radioactively. We found that it behaves biologically in a similar manner as the hBMP15wt. Human BMP15 strongly bound to BMPR1B and BMPR2 on the COV434 cell surface, an association which was further verified in COS-7 cells by overexpression of these receptors. In conclusion, we have for the first time produced and characterized recombinant human GDF9 and BMP15 protein in native form and thus developed valuable tools for studies aiming at understanding their role in human reproductive medicine. The production and purification methods developed during this thesis are widely applicable for the isolation and characterization of comparable oocyte growth factors in other species and thus have wide applicability in reproductive biology studies in the future.