Distinct characteristics of the DNA damage response in mammalian oocytes.

DNA damage is a critical threat that poses significant challenges to all cells. To address this issue, cells have evolved a sophisticated molecular and cellular process known as the DNA damage response (DDR). Among the various cell types, mammalian oocytes, which remain dormant in the ovary for extended periods, are particularly susceptible to DNA damage. The occurrence of DNA damage in oocytes can result in genetic abnormalities, potentially leading to infertility, birth defects, and even abortion. Therefore, understanding how oocytes detect and repair DNA damage is of paramount importance in maintaining oocyte quality and preserving fertility. Although the fundamental concept of the DDR is conserved across various cell types, an emerging body of evidence reveals striking distinctions in the DDR between mammalian oocytes and somatic cells. In this review, we highlight the distinctive characteristics of the DDR in oocytes and discuss the clinical implications of DNA damage in oocytes.

Oocytes can repair DNA damage during meiosis via a microtubule-dependent recruitment of CIP2A-MDC1-TOPBP1 complex from spindle pole to chromosomes.

Because DNA double-strand breaks (DSBs) greatly threaten genomic integrity, effective DNA damage sensing and repair are essential for cellular survival in all organisms. However, DSB repair mainly occurs during interphase and is repressed during mitosis. Here, we show that, unlike mitotic cells, oocytes can repair DSBs during meiosis I through microtubule-dependent chromosomal recruitment of the CIP2A-MDC1-TOPBP1 complex from spindle poles. After DSB induction, we observed spindle shrinkage and stabilization, as well as BRCA1 and 53BP1 recruitment to chromosomes and subsequent DSB repair during meiosis I. Moreover, p-MDC1 and p-TOPBP1 were recruited from spindle poles to chromosomes in a CIP2A-dependent manner. This pole-to-chromosome relocation of the CIP2A-MDC1-TOPBP1 complex was impaired not only by depolymerizing microtubules but also by depleting CENP-A or HEC1, indicating that the kinetochore/centromere serves as a structural hub for microtubule-dependent transport of the CIP2A-MDC1-TOPBP1 complex. Mechanistically, DSB-induced CIP2A-MDC1-TOPBP1 relocation is regulated by PLK1 but not by ATM activity. Our data provide new insights into the critical crosstalk between chromosomes and spindle microtubules in response to DNA damage to maintain genomic stability during oocyte meiosis.

Selective utilization of non-homologous end-joining and homologous recombination for DNA repair during meiotic maturation in mouse oocytes.

DNA double-strand breaks (DSBs) are highly toxic lesions that can cause genomic instability and can be repaired by non-homologous end-joining (NHEJ) and homologous recombination (HR) pathways. Despite extensive studies about DSB repair pathways, the roles of each pathway during meiotic maturation in oocytes are not well understood. Here we show that oocytes selectively utilize NHEJ and HR to repair DSBs during meiotic maturation. Inhibition of NHEJ impaired the meiotic maturation of oocytes with DNA damage by activating the spindle assembly checkpoint (SAC) with a concomitant increase in metaphase I (MI) arrest and DNA damage levels. In contrast, oocytes with DNA damage bypassed SAC-mediated MI arrest despite the presence of fragmented DNA when HR was inhibited. Notably, this bypass of SAC arrest by HR inhibition was associated with a loss of centromere integrity and subsequent impairment of chromosome architecture. Our results demonstrate that, while NHEJ is critical for the meiotic maturation of oocytes with DNA damage, HR is essential to maintain centromere integrity against DNA damage during meiotic maturation, revealing distinct roles of NHEJ and HR during meiotic maturation in mouse oocytes.

ROS-independent cytotoxicity of 9,10-phenanthrenequinone inhibits cell cycle progression and spindle assembly during meiotic maturation in mouse oocytes.

Diesel exhaust particles (DEPs) are major components of ambient particulate matter and are associated with various adverse health effects. Typically, DEPs contain a vast number of organic compounds, among which 9,10-phenanthrenequinone (9,10-PQ), the quinone derivative of the polycyclic aromatic hydrocarbon phenanthrene, is one of the most abundant and toxic. 9,10-PQ can produce excessive reactive oxygen species (ROS) via redox cycling and exhibit cytotoxicity in various cells. However, the underlying mechanisms involved in cytotoxicity of 9,10-PQ remain elusive. In this study, we investigated the effects of exposure to 9,10-PQ using mouse oocytes as a model system. We found that 9,10-PQ compromised meiotic maturation by impairing acentriolar microtubule organizing center (MTOC) assembly and subsequent spindle formation during meiotic maturation. Moreover, 9,10-PQ exposure prevented cell cycle progression by inhibiting Cdk1 activation via disturbance of cyclin B1 accumulation. Importantly, meiotic defects induced by 9,10-PQ exposure were not rescued by decreasing ROS levels, revealing that 9,10-PQ has ROS-independent activity that regulates cell cycle progression and spindle assembly. Therefore, our findings reveal that 9,10-PQ has novel activity that regulates cell-cycle progression and spindle formation in an ROS-independent manner during meiotic maturation in mouse oocytes.

Cytotoxicity of 9,10-Phenanthrenequinone Impairs Mitotic Progression and Spindle Assembly Independent of ROS Production in HeLa Cells.

The polycyclic aromatic hydrocarbon quinone derivative 9,10-phenanthrenequinone (9,10-PQ) is one of the most abundant and toxic components found in diesel exhaust particles (DEPs). These DEPs are created during diesel fuel combustion and are considered the main source of urban air pollution. As 9,10-PQ can produce excessive reactive oxygen species (ROS) through redox cycling, it has been shown to exert potent cytotoxic effects against various cell types. However, the mechanisms underlying this cytotoxicity remain unclear. In this study, we showed that 9,10-PQ exerts cytotoxicity by impairing mitotic progression and spindle assembly in HeLa cells. Exposure to 9,10-PQ impaired spindle assembly and chromosome alignment, resulting in delayed mitotic entry and progression in HeLa cells. Furthermore, 9,10-PQ exposure decreased the CEP192 and p-Aurora A levels at the spindle poles. Notably, these mitotic defects induced by 9,10-PQ were not rescued by scavenging ROS, implying the ROS-independent activity of 9,10-PQ. Therefore, our results provide the first evidence that 9,10-PQ exerts its cytotoxicity through specific inhibition of mitotic progression and spindle assembly, independent of ROS.

The Capacity to Repair Sperm DNA Damage in Zygotes is Enhanced by Inhibiting WIP1 Activity.

Maintaining genome integrity in germ cells is essential not only for successful fertilization and embryo development, but also to ensure proper transmission of genetic information across generations. However, unlike oocytes, sperm are incapable of repairing DNA damage. Therefore, sperm DNA damage is repaired after fertilization in zygotes using maternal DNA repair factors. In this study, we found that zygotic repair of paternal DNA damage is enhanced by inhibiting WIP1 activity. Oxidative stress induced DNA damage in sperm and severely impaired motility. Although DNA damage in sperm did not compromise fertilization, it increased DNA damage in the paternal pronucleus of zygotes. However, WIP1 inhibition during fertilization reduced DNA damage in the paternal pronucleus, improving the rate of two-cell development, and subsequent zygotic genome activation. Therefore, our results suggest that WIP1 inhibition could enhance maternal DNA repair capacity and thereby decrease paternal DNA damage in zygotes.

MDC1 is essential for G2/M transition and spindle assembly in mouse oocytes.

Mammalian oocytes are particularly susceptible to accumulating DNA damage. However, unlike mitotic cells in which DNA damage induces G2 arrest by activating the ATM-Chk1/2-Cdc25 pathway, oocytes readily enter M-phase immediately following DNA damage. This implies a lack of a robust canonical G2/M DNA damage checkpoint in oocytes. Here we show that MDC1 plays a non-canonical role in controlling G2/M transition by regulating APC/C-Cdh1-mediated cyclin B1 degradation in response to DNA damage in mouse oocytes. Depletion of MDC1 impaired M-phase entry by decreasing cyclin B1 levels via the APC/C-Cdh1 pathway. Notably, the APC/C-Cdh1 regulation mediated by MDC1 was achieved by a direct interaction between MDC1 and APC/C-Cdh1. This interaction was transiently disrupted after DNA damage with a concomitant increase in Cdh1 levels, which, in turn, decreased cyclin B1 levels and delayed M-phase entry. Moreover, MDC1 depletion impaired spindle assembly by decreasing the integrity of microtubule organizing centers (MTOCs). Therefore, our results demonstrate that MDC1 is an essential molecule in regulating G2/M transition in response to DNA damage and in regulating spindle assembly in mouse oocytes. These results provide new insights into the regulation of the G2/M DNA damage checkpoint and cell cycle control in oocytes.

Discovery and Characterization of a Novel MASTL Inhibitor MKI-2 Targeting MASTL-PP2A in Breast Cancer Cells and Oocytes.

Although microtubule-associated serine/threonine kinase-like (MASTL) is a promising target for selective anticancer treatment, MASTL inhibitors with nano range potency and antitumor efficacy have not been reported. Here, we report a novel potent and selective MASTL inhibitor MASTL kinase inhibitor-2 (MKI-2) identified in silico through a drug discovery program. Our data showed that MKI-2 inhibited recombinant MASTL activity and cellular MASTL activity with IC50 values of 37.44 nM and 142.7 nM, respectively, in breast cancer cells. In addition, MKI-2 inhibited MASTL kinase rather than other AGC kinases, such as ROCK1, AKT1, PKACα, and p70S6K. Furthermore, MKI-2 exerted various antitumor activities by inducing mitotic catastrophe resulting from the modulation of the MASTL-PP2A axis in breast cancer cells. The MKI-2 treatment showed phenocopies with MASTL-null oocyte in mouse oocytes, which were used as a model to validate MKI-2 activity. Therefore, our study provided a new potent and selective MASTL inhibitor MKI-2 targeting the oncogenic MAST-PP2A axis in breast cancer cells.

Increased WIP1 Expression With Aging Suppresses the Capacity of Oocytes to Respond to and Repair DNA Damage.

If fertilization does not occur for a prolonged time after ovulation, oocytes undergo a time-dependent deterioration in quality in vivo and in vitro, referred to as postovulatory aging. The DNA damage response is thought to decline with aging, but little is known about how mammalian oocytes respond to the DNA damage during in vitro postovulatory aging. Here we show that increased WIP1 during in vitro postovulatory aging suppresses the capacity of oocytes to respond to and repair DNA damage. During in vitro aging, oocytes progressively lost their capacity to respond to DNA double-strand breaks, which corresponded with an increase in WIP1 expression. Increased WIP1 impaired the amplification of γ-H2AX signaling, which reduced the DNA repair capacity. WIP1 inhibition restored the DNA repair capacity, which prevented deterioration in oocyte quality and improved the fertilization and developmental competence of aged oocytes. Importantly, WIP1 was also found to be high in maternally aged oocytes, and WIP1 inhibition enhanced the DNA repair capacity of maternally aged oocytes. Therefore, our results demonstrate that increased WIP1 is responsible for the age-related decline in DNA repair capacity in oocytes, and WIP1 inhibition could restore DNA repair capacity in aged oocytes.

MKI-1, a Novel Small-Molecule Inhibitor of MASTL, Exerts Antitumor and Radiosensitizer Activities Through PP2A Activation in Breast Cancer.

Although MASTL (microtubule-associated serine/threonine kinase-like) is an attractive target for anticancer treatment, MASTL inhibitors with antitumor activity have not yet been reported. In this study, we have presented a novel MASTL inhibitor, MKI-1, identified through in silico screening and in vitro analysis. Our data revealed that MKI-1 exerted antitumor and radiosensitizer activities in in vitro and in vivo models of breast cancer. The mechanism of action of MKI-1 occurred through an increase in PP2A activity, which subsequently decreased the c-Myc protein content in breast cancer cells. Moreover, the activity of MKI-1 in the regulation of MASTL-PP2A was validated in a mouse oocyte model. Our results have demonstrated a new small-molecule inhibitor of MASTL, MKI-1, which exerts antitumor and radiosensitizer activities through PP2A activation in breast cancer in vitro and in vivo.

Melatonin protects mouse oocytes from DNA damage by enhancing nonhomologous end-joining repair.

Mammalian oocytes remain arrested at the first prophase of meiosis in ovarian follicles for an extended period. During this protracted arrest, oocytes are remarkably susceptible to the accumulation of DNA damage. Melatonin (N-acetyl-5-methoxytryptamine), a hormone secreted by the pineal gland, has diverse effects on various physiological processes. However, the effect of melatonin on DNA damage response in mammalian oocytes has not been explored. Here, we showed that melatonin protected mouse oocytes from DNA damage induced by double-strand breaks (DSBs) during prophase arrest and subsequently improved oocyte quality. We found that DNA damage during prophase arrest impaired subsequent meiotic maturation and deteriorated oocyte quality, increasing chromosome fragmentation, spindle abnormality, mitochondrial aggregation, and oxidative stress. However, melatonin treatment during DNA damage accumulation at prophase improved meiotic maturation and relieved the quality decline of oocytes. In addition, melatonin inhibited the accumulation of DNA damage during prophase arrest by reducing the γ-H2AX levels. Although activated ATM levels were decreased by melatonin treatment, the effect of melatonin on DNA damage response was not a direct consequence of ATM inhibition. Instead, melatonin enhanced DNA repair via nonhomologous end-joining (NHEJ) pathway. Interestingly, these actions of melatonin on DNA damage response are receptor-independent in mouse oocytes. Therefore, our results demonstrated that melatonin protects oocytes from DNA damage during prophase arrest by enhancing DNA repair via NHEJ and subsequently prevents the deterioration of oocyte quality during meiotic maturation.

WIP1 phosphatase suppresses the DNA damage response during G2/prophase arrest in mouse oocytes.

Maternal DNA damage during meiosis causes genetic abnormalities that can lead to infertility, birth defects, and abortion. While DNA damage can rapidly halt cell cycle progression and promote DNA repair in somatic cells, mammalian oocytes are unable to mount a robust G2/prophase arrest in response to DNA damage unless damage levels are severe. Here, we show that inhibition of WIP1 phosphatase enhances the ability of oocytes to respond to DNA damage. We found that WIP1 was expressed constantly during meiotic maturation, and that inhibition of WIP1 activity did not impair meiotic maturation. However, oocytes in G2/prophase were sensitized to DNA damage following WIP1 inhibition, not only increasing γ-H2AX level and ATM phosphorylation, but also decreasing entry into meiosis. Moreover, WIP1 inhibition significantly promoted the repair of damaged DNA during G2/prophase arrest, suggesting that WIP1 suppresses DNA repair in oocytes. Therefore, our results suggest that WIP1 is a key suppressor of the DNA damage response during G2/prophase arrest in mouse oocytes.