Role of the nuclear progesterone receptor during bovine oocyte growth and maturation

The oocyte is the foundation of life, and its development is a meticulously coordinated process involving both nuclear and cytoplasmic maturation, ultimately leading to the acquisition of competence. A competent oocyte, often referred to as a high-quality oocyte, possesses the inherent potential to progress beyond the blastocyst stage and develop into an independent organism. Evidence suggests that increasing circulating progesterone levels during the growth of the dominant follicle can enhance pregnancy rates in sub-fertile cows. Progesterone receptors (PR) have been identified in fully-grown and mature bovine oocytes, implicating a functional role during oocyte maturation and competence acquisition. Despite these findings, the specific molecular and cellular pathways through which progesterone influences oocyte development and maturation remain poorly understood. Therefore, the aim of this thesis is to investigate the expression and functionality of the nuclear progesterone receptor (nPR) during bovine oocyte growth. This research provides crucial insights into nPR timing and functionality, revealing a novel association between nPR and mitochondria in growing bovine oocytes. A comprehensive descriptive analysis demonstrated continuous expression of nPR from the early to late stages of folliculogenesis in both follicle cells and oocytes, with an increase in nPR associated with oocyte size. Functional studies on growing bovine oocytes revealed that nPR negatively affects cumulus-oocyte complex survival and reduces the oocyte's ability to resume meiosis. Further in silico analysis highlighted the enrichment of nuclear and mitochondrial pathways regulated by nPR during oocyte growth. Notably, this research presents the first evidence of nPR and mitochondrial colocalization in smaller oocytes. The findings of this thesis significantly enhance our understanding of nPR role in oocyte development, emphasizing its importance in determining oocyte quality. These insights could inform future strategies aimed at improving in vitro embryo production and fertility treatments.