Here, we showed that DYNLL1 transcript abundance decreases during oocyte and follicular growth, and during oocyte meiotic maturation in mice. DYNLL1 is known to promote error-prone non-homologous end joining (NHEJ) repair and in contrast to limit error-free homologous recombination (HR) repair; thus, it participates in DSB repair choice (He et al., 2018; West et al., 2019; Becker et al., 2018; Berkel and Cacan, 2021). Therefore, its decreased cellular levels during oocyte and follicular growth might be associated with increased genomic stability due to relatively increased HR repair and decreased NHEJ repair events (He et al., 2018; West et al., 2019; Becker et al., 2018). Since the oocytes in the non-growing primordial follicles is arrested in the last stage of prophase and does not undergo further mitotic division until sexual maturity, these oocytes might have increased NHEJ repair events and decreased HR repair events due to their higher DYNLL1 levels (Clarke, 2017). However, during reproductive life, these non-growing or dormant primordial follicles begin to proliferate mitotically and to be recruited into the growing pool; thus, they may gradually decrease their DYNLL1 levels in the course of follicular growth to promote error-free HR repair over error-prone NHEJ repair (Clarke, 2017; Hirshfield, 1991; Da Silva-Buttkus et al., 2008; Lee and Chang, 2019;Wright et al., 2020). This is in line with the observation that the choice of DSB repair pathway is dependent on the cell cycle phases, and that mitotic cells can repair DSBs via a homology-directed pathway, a major pathway used for HR repair (Sakamoto et al., 2021). In contrast, Stringer et al. showed that around 90 % of prophase-arrested oocytes in primordial follicles exposed to γ-irradiation utilized HR repair pathway and the remaining 10 % utilized the NHEJ repair pathway (Newman et al., 2022). However, somatic granulosa cells surrounding oocytes in these follicles primarily utilized NHEJ repair pathway (Newman et al., 2022). Therefore, it might be stated that oocytes in primordial follicles utilizes mostly HR repair pathway even though they have higher DYNLL1 levels. Supporting our findings, authors reported that most of the non-growing prophase-arrested oocytes in primordial follicles exposed to even low doses of γ-irradiation died; however, growing follicles remained largely unaffected, pointing out the unique sensitivity of non-growing prophase-arrested oocytes in primordial follicles compared to oocytes in growing follicles (Newman et al., 2022). Considering that the gamma radiation leads to dose-dependent increases in DNA damage, this differences in sensitivity between growing follicles and primordial follicles might be due to, at least to a certain extent, lower DYNLL1 levels in oocytes in growing follicles (thus even higher HR repair events) compared to oocytes in primordial follicles, although primary DSB repair choice in oocytes in primordial follicles is HR repair. However, these differences in sensitivity might also be attributed to the differences in the expression of key regulators of cell death.
Although they did not study DYNLL1, Horta et al. showed that transcript abundance of most other DNA repair genes is lower in MII oocytes compared to GV oocytes in young mice, in parallel to our findings showing that DYNLL1 transcript abundance decreases during oocyte meiotic maturation from GV stage to MII stage in mice (Horta et al., 2021). However, they observed that in old mice, transcript abundance of some of the DNA repair genes increases during the GV-MII transition, showing the effect of age on relative transcript abundance of DNA repair genes in GV to MII transition in mice. However, we observed that DYNLL1 mRNA transcript levels decrease in both young and aged mice from immature GV stage to mature MII stage. Therefore, considering that the age is an important parameter in terms of women reproductive health and has a major impact in oocyte quality and reserve, changes in the cellular levels and activities of DNA repair proteins should be studied in more detail to better understand age-dependent changes in genomic stability in oocytes. These studies will ultimately have the potential to improve clinical success rates of assisted reproductive technologies in older women by helping to develop novel strategies to increase genomic stability in their oocytes (Newman et al., 2022; Horta et al., 2020).
We also found that ovaries from high-fertility mice with increased ovulation numbers have decreased DYNLL1 transcript abundance compared to that of control mice, and the induction of ovulation with human chorionic gonadotropin (hCG) results in a decrease in DYNLL1 mRNA transcript levels in cumulus oocyte complexes isolated from mouse ovaries. In other words, DYNLL1 levels might be inversely associated with fertility and ovulation in mice. This might be due to increased number of error-free HR repair events in ovaries and oocytes with lower DYNLL1 levels as detailed above. As it is shown that DNA repair is sufficient to restore functional fertility, and DNA repair may be an important mechanism for ensuring female fertility, higher HR repair preference due to lower DYNLL1 levels might be a contributing factor to the higher fertility observed in mice with decreased DYNLL1 levels (Stringer et al., 2020; Horta et al., 2021; Stringer et al., 2018; Winshipt et al., 2018).
Furthermore, a decrease in DYNLL1 transcript abundance during in vitro oocyte maturation from GV stage to MII stage was also reported in bovine oocytes, showing that these changes might not be species-specific and may point to some universal roles of DYNLL1 in oocyte development in diverse taxa (Racedo et al., 2008). Further research is needed to make stronger inferences.