Nuclear progesterone receptor expression, localization and function during bovine oocyte growth

Bovine oocytes and follicle cells express the nPR throughout their growth; moreover, nPR expression is colocalised with mitochondria in secondary follicle stage oocytes. Potential nPR target genes which gain expression during oocyte growth populate pathways associated with nuclear and mitochondrial function.

Previous work from our group demonstrated that blocking nuclear progesterone receptor (nPR) signalling during bovine in vitro oocyte maturation decreases embryo development following subsequent in vitro fertilization and embryo culture, suggesting a critical role for nPR activity in oocyte developmental competence. The objective of this study was to characterize the expression, localization, and function role of the nPR during bovine oocyte growth. Bovine cumulus-oocyte complexes (COCs) were recovered via ovarian slicing. Oocyte diameter was measured and oocytes were allocated to groups according to their size for protein expression and immunofluorescence analysis. In addition, ovarian cortex slides were prepared for immunohistochemistry. To determine a putative functional role of nPR during oocyte growth, an in-silico analysis of genes with increased expression during bovine oocyte growth was conducted to identify those containing the progesterone response element (PRE) core sequence in their promoter regions. The results demonstrated continuous nPR expression during follicle growth in both oocytes and follicular cells. The in-silico analysis revealed that 20% of genes with increased expression during oocyte growth contained a PRE in their promoter regions and were enriched in nuclear and mitochondrial pathways. MitoTracker labelling revealed extensive nPR colocalisation with mitochondria in small bovine oocytes (40-100 μm), which exhibited the highest mitochondrial activity. This study provides new insights into nPR expression in bovine oocytes, COCs, and follicle cells during folliculogenesis and oocyte growth, and suggests an association between nPR and mitochondria in the latter process.