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

Abstract

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.

Phospho-Regulation of Meiotic Prophase

“…whereas the combined activity of CHK1 and CHK2 is required to reduce oocyte number and promote folliculogenesis in mice (Martínez-Marchal et al, 2020).…”

“…Intriguingly, the pachytene checkpoint can also affect subsequent germline development. In Drosophila , this pathway reduces the levels of the developmental regulator Gurken, leading to impaired egg chamber polarization ( Abdu et al, 2002 ), whereas the combined activity of CHK1 and CHK2 is required to reduce oocyte number and promote folliculogenesis in mice ( Martínez-Marchal et al, 2020 ).…”

Chek2 Signaling Is the Key Regulator of Oocyte Survival After Chemotherapy

“…Recent GWAS studies identified CHEK2 as a major factor associated with ovarian aging and the onset of natural menopause in women (154,155). Animal model studies showed that this is due to a larger PMF reserve found in CHEK2-deficient mice (26,155,156). Furthermore, we showed that CHEK2deficient primordial oocytes are more resistant to DNA damage induced by radiation (26) and chemotherapy drugs (this study), and we expect that they will be resistant to other types of DNA damaging exposures.…”

Regulation of Meiotic Prophase One in Mammalian Oocytes

“…In females, ATR has been implicated in detecting oocytes with DSBs and activating TAp63α ( Kim et al, 2019 ). Besides CHK2, another checkpoint kinase, CHK1 has also been found to mediate removal of damaged oocytes ( Martinez-Marchal et al, 2020 ; Rinaldi et al, 2020 ).…”