Oocyte Competence of Prepubertal Sheep and Goat Oocytes: An Assessment of Large-Scale Chromatin Configuration and Epidermal Growth Factor Receptor Expression in Oocytes and Cumulus Cells

The oocyte competence of prepubertal females is lower compared to that of adults, mainly because they originate from small follicles. In adult females, the germinal vesicle (GV) and epidermal growth factor receptor (EGFR) have been associated with oocyte competence. This study aimed to analyze GV chromatin configuration and EGFR expression in prepubertal goat and sheep oocytes obtained from small (<3 mm) and large (≥3 mm) follicles and compare them with those from adults. GV chromatin was classified from diffuse to condensed as GV1, GVn, and GVc for goats and NSN, SN, and SNE for sheep. EGFR was quantified in cumulus cells (CCs) by Western blotting and in oocytes by immunofluorescence. Oocytes from prepubertal large follicles and adults exhibited highly condensed chromatin in goats (71% and 69% in GVc, respectively) and sheep (59% and 75% in SNE, respectively). In both species, EGFR expression in CCs and oocytes was higher in prepubertal large follicles than in small ones. In adult females, EGFR expression in oocytes was higher than in prepubertal large follicles. In conclusion, GV configuration and EGFR expression in CCs and oocytes were higher in the large than small follicles of prepubertal females.

Genetic insights into biological mechanisms governing human ovarian ageing

Reproductive longevity is essential for fertility and influences healthy ageing in women1,2, but insights into its underlying biological mechanisms and treatments to preserve it are limited. Here we identify 290 genetic determinants of ovarian ageing, assessed using normal variation in age at natural menopause (ANM) in about 200,000 women of European ancestry. These common alleles were associated with clinical extremes of ANM; women in the top 1% of genetic susceptibility have an equivalent risk of premature ovarian insufficiency to those carrying monogenic FMR1 premutations3. The identified loci implicate a broad range of DNA damage response (DDR) processes and include loss-of-function variants in key DDR-associated genes. Integration with experimental models demonstrates that these DDR processes act across the life-course to shape the ovarian reserve and its rate of depletion. Furthermore, we demonstrate that experimental manipulation of DDR pathways highlighted by human genetics increases fertility and extends reproductive life in mice. Causal inference analyses using the identified genetic variants indicate that extending reproductive life in women improves bone health and reduces risk of type 2 diabetes, but increases the risk of hormone-sensitive cancers. These findings provide insight into the mechanisms that govern ovarian ageing, when they act, and how they might be targeted by therapeutic approaches to extend fertility and prevent disease.

The DNA damage response is required for oocyte cyst breakdown and follicle formation in mice

Mammalian oogonia proliferate without completing cytokinesis, forming cysts. Within these, oocytes differentiate and initiate meiosis, promoting double-strand break (DSBs) formation, which are repaired by homologous recombination (HR) causing the pairing and synapsis of the homologs. Errors in these processes activate checkpoint mechanisms, leading to apoptosis. At the end of prophase I, in contrast with what is observed in spermatocytes, oocytes accumulate unrepaired DSBs. Simultaneously to the cyst breakdown, there is a massive oocyte death, which has been proposed to be necessary to enable the individualization of the oocytes to form follicles. Based upon all the above-mentioned information, we hypothesize that the apparently inefficient HR occurring in the oocytes may be a requirement to first eliminate most of the oocytes and enable cyst breakdown and follicle formation. To test this idea, we compared perinatal ovaries from control and mutant mice for the effector kinase of the DNA Damage Response (DDR), CHK2. We found that CHK2 is required to eliminate ~50% of the fetal oocyte population. Nevertheless, the number of oocytes and follicles found in Chk2-mutant ovaries three days after birth was equivalent to that of the controls. These data revealed the existence of another mechanism capable of eliminating oocytes. In vitro inhibition of CHK1 rescued the oocyte number in Chk2-/- mice, implying that CHK1 regulates postnatal oocyte death. Moreover, we found that CHK1 and CHK2 functions are required for the timely breakdown of the cyst and to form follicles. Thus, we uncovered a novel CHK1 function in regulating the oocyte population in mice. Based upon these data, we propose that the CHK1- and CHK2-dependent DDR controls the number of oocytes and is required to properly break down oocyte cysts and form follicles in mammals.