Function and regulation mechanism of Chk1 during meiotic maturation in porcine oocytes

Claspin haploinsufficiency leads to defects in fertility, hyperplasia and an increased oncogenic potential

Claspin is an adaptor protein required for ATR-dependent phosphorylation of CHK1 during S-phase following DNA replication stress. Claspin expression is highly variable in cancer, with low levels frequently correlating with poor patient survival. To learn more about the biological consequences of reduced Claspin expression and its effects on tumorigenesis, we investigated mice with a heterozygous knockout of the Clspn gene. Claspin haploinsufficiency resulted in reduced female fertility and a maternally inherited defect in oocyte meiosis I cell cycle progression. Furthermore, aged Clspn+/- mice developed spontaneous lymphoid hyperplasia and increased susceptibility to non-alcoholic fatty liver disease. Importantly, we demonstrate a tumour suppressor role for Claspin. Reduced Claspin levels result in increased liver damage and tumourigenesis in the DEN model of hepatocellular carcinoma. These data reveal that Clspn haploinsufficiency has widespread unanticipated biological effects and establishes the importance of Claspin as a regulatory node controlling tumorigenesis and multiple disease aetiologies.

Knockdown of Chk1 inhibits proliferation and promotes apoptosis in mouse granulosa cells and its regulation mechanism by miR-15a and miR-16

Checkpoint kinase 1 (Chk1) is a protein kinase which preserves the genome integrity, and works as an evolutionally conserved DNA damage response and cell cycle checkpoint. However, the functional roles and regulatory mechanism of Chk1 in mouse granulosa cells (GCs) have not been fully elucidated. In this study, by RNA interfering, Chk1 gene was knocked down in GCs. Knockdown of Chk1 inhibited proliferation and increased apoptosis of GCs (p < 0.05), respectively; in addition, cell cycle of GCs was arrested at S and G2/M phases. Further qRT-PCR results showed that cell cycle factors (Cyclin B1 and Cyclin D 1) and a marker gene of proliferation (PCNA) were downregulated (p < 0.001), while apoptotic factors (p53b, p21, caspase-3, and Bax) were upregulated (p < 0.01), which suggested that knockdown of Chk1 may inhibit proliferation, regulate cell cycle, and promote apoptosis at the transcriptional level in GCs. In vitro studies showed a negative correlation between Chk1 mRNA and miR-16 expression during follicular development. To elucidate the relationship between Chk1 and miR-15a/16, luciferase reporter plasmids were constructed and luciferase assays revealed that both miR-15a and miR-16 could bind to the 3' UTR of Chk1 mRNA, and significantly downregulate the protein level of Chk1 (p < 0.01), while miR-16, not miR-15a, could significantly decrease the mRNA level of Chk1 (p < 0.05). This result indicated that miR-16 directly induced Chk1 mRNA destabilization, while miR-15a regulated Chk1 expression through translational repression. Taken together, this study uncovered the roles of Chk1 in mouse granulosa cells and its regulation by miR-15a and miR-16 through different mechanisms.

Molecular determinants of the meiotic arrests in mammalian oocytes at different stages of maturation

Mammalian oocytes undergo two rounds of developmental arrest during maturation: at the diplotene of the first meiotic prophase and metaphase of the second meiosis. These arrests are strictly regulated by follicular cells temporally producing the secondary messengers, cAMP and cGMP, and other factors to regulate maturation promoting factor (composed of cyclin B1 and cyclin-dependent kinase 1) levels in the oocytes. Out of these normally appearing developmental arrests, permanent arrests may occur in the oocytes at germinal vesicle (GV), metaphase I (MI), or metaphase II (MII) stage. This issue may arise from absence or altered expression of the oocyte-related genes playing key roles in nuclear and cytoplasmic maturation. Additionally, the assisted reproductive technology (ART) applications such as ovarian stimulation and in vitro culture conditions both of which harbor various types of chemical agents may contribute to forming the permanent arrests. In this review, the molecular determinants of developmental and permanent arrests occurring in the mammalian oocytes are comprehensively evaluated in the light of current knowledge. As number of permanently arrested oocytes at different stages is increasing in ART centers, potential approaches for inducing permanent arrests to obtain competent oocytes are discussed.

The DNA Damage Response in Fully Grown Mammalian Oocytes

DNA damage in cells can occur physiologically or may be induced by exogenous factors. Genotoxic damage may cause cancer, ageing, serious developmental diseases and anomalies. If the damage occurs in the germline, it can potentially lead to infertility or chromosomal and genetic aberrations in the developing embryo. Mammalian oocytes, the female germ cells, are produced before birth, remaining arrested at the prophase stage of meiosis over a long period of time. During this extensive state of arrest the oocyte may be exposed to different DNA-damaging insults for months, years or even decades. Therefore, it is of great importance to understand how these cells respond to DNA damage. In this review, we summarize the most recent developments in the understanding of the DNA damage response mechanisms that function in fully grown mammalian oocytes.

A Hypothesis: Linking Phase Separation to Meiotic Sex Chromosome Inactivation and Sex-Body Formation

During meiotic prophase I, X and Y chromosomes in mammalian spermatocytes only stably pair at a small homologous region called the pseudoautosomal region (PAR). However, the rest of the sex chromosomes remain largely unsynapsed. The extensive asynapsis triggers transcriptional silencing - meiotic sex chromosome inactivation (MSCI). Along with MSCI, a special nuclear territory, sex body or XY body, forms. In the early steps of MSCI, DNA damage response (DDR) factors, such as BRCA1, ATR, and γH2AX, function as sensors and effectors of the silencing signals. Downstream canonical repressive histone modifications, including methylation, acetylation, ubiquitylation, and SUMOylation, are responsible for the transcriptional repression of the sex chromosomes. Nevertheless, mechanisms of the sex-body formation remain unclear. Liquid-liquid phase separation (LLPS) may drive the formation of several chromatin subcompartments, such as pericentric heterochromatin, nucleoli, inactive X chromosomes. Although several proteins involved in phase separation are found in the sex bodies, when and whether these proteins exert functions in the sex-body formation and MSCI is still unknown. Here, we reviewed recent publications on the mechanisms of MSCI and LLPS, pointed out the potential link between LLPS and the formation of sex bodies, and discussed its implications for future research.

Checkpoint kinases are required for oocyte meiotic progression by the maintenance of normal spindle structure and chromosome condensation

Checkpoint kinases (Chk) 1/2 are known for DNA damage checkpoint and cell cycle control in somatic cells. According to recent findings, the involvement of Chk1 in oocyte meiotic resumption and Chk2 is regarded as an essential regulator for progression at the post metaphase I stage (MI). In this study, AZD7762 (Chk1/2 inhibitor) and SB218078 (Chk1 inhibitor) were used to uncover the joint roles of Chk1/2 and differentiate the importance of Chk1 and Chk2 during oocyte meiotic maturation. Inhibition of Chk1/2 or Chk1 alone had no significant effect on germinal vesicle breakdown (GVBD) but significantly inhibited the first polar body (PB1). Interestingly, inhibition of Chk1 alone could not increase or completely block the extrusion of PB1 like Chk1/2 inhibition. Also, Chk1/2 inhibition resulted in defective meiotic spindle organization and chromosome condensation both in MI and metaphase II (MII) stages of oocytes. The location of γ-tubulin and Securin were abnormal or missing, while P38 MAPK was activated by Chk1/2 inhibition. Meanwhile, Chk1/2 inhibition reduced the percentage of the second polar body extrusion and pronuclear formation. In conclusion, our results further understand the functions and regulatory mechanism of Chk1/2 during oocyte meiotic maturation.

CHK1 monitors spindle assembly checkpoint and DNA damage repair during the first cleavage of mouse early embryos

OBJECTIVES

DNA damage and errors of accurate chromosome segregation lead to aneuploidy and foetal defects. DNA repair and the spindle assembly checkpoint (SAC) are the mechanisms developed to protect from these defects. Checkpoint kinase 1 (CHK1) is reported to be an important DNA damage response protein in multiple models, but its functions remain unclear in early mouse embryos.

MATERIALS AND METHODS

Immunofluorescence staining, immunoblotting and real-time reverse transcription polymerase chain reaction were used to perform the analyses. Reactive oxygen species levels and Annexin-V were also detected.

RESULTS

Loss of CHK1 activity accelerated progress of the cell cycle at the first cleavage; however, it disturbed the development of early embryos to the morula/blastocyst stages. Further analysis indicated that CHK1 participated in spindle assembly and chromosome alignment, possibly due to its regulation of kinetochore-microtubule attachment and recruitment of BubR1 and p-Aurora B to the kinetochores, indicating its role in SAC activity. Loss of CHK1 activity led to embryonic DNA damage and oxidative stress, which further induced early apoptosis and autophagy, indicating that CHK1 is responsible for interphase DNA damage repair.

CONCLUSIONS

Our results indicate that CHK1 is a key regulator of the SAC and DNA damage repair during early embryonic development in mice.

Maternal Cytokines CXCL12, VEGFA, and WNT5A Promote Porcine Oocyte Maturation via MAPK Activation and Canonical WNT Inhibition

Maternal regulatory factors endow the oocyte with developmental competence in vivo, which might be absent in current in vitro maturation (IVM) systems, thereby compromising oocyte quality. In the present study, by employing RNA sequencing data analysis, we expect to identify potential contributing factors to support porcine oocyte maturation through binding to their receptors on the oolemma. Here, C-X-C motif chemokine ligand 12 (CXCL12), vascular endothelial growth factor A (VEGFA), and Wingless-type MMTV integration site family member 5A (WNT5A), termed CVW, are selected and confirmed to be important maternal cytokines for porcine oocyte maturation. Combined supplementation of CVW promotes the nuclear maturation percentage from 57.2% in controls to 75.9%. More importantly, these maternal cytokines improve the developmental potential of matured oocytes by parthenogenesis, fertilization, and cloning, as their blastocyst formation efficiencies and total cell numbers are increased. CVW supplementation also enlarges perivitelline space and promotes cumulus expansion, which results in a more complete transzonal projection retraction on the zona pellucida, and a reduced incidence of polyspermy in fertilized oocytes. Meanwhile, inhibiting the CVW receptor-mediated signaling pathways severely impairs oocyte meiotic resumption and cumulus expansion during IVM. We further determine that maturation improvement by CVW is achieved through activating the MAPK pathway in advance and inhibiting the canonical WNT pathway at the end of the IVM period. These findings provide a new combination of three cytokines to promote the porcine IVM process, which also holds potential to be used in human assisted reproduction technologies as well as in other species.

Survivin regulates chromosome segregation by modulating the phosphorylation of Aurora B during porcine oocyte meiosis

SURVIVIN is an essential chromosomal passenger complex (CPC) subunit and participates in cell division. In this study, we used porcine oocyte as a model to investigate the roles of Survivin during porcine oocyte maturation. Survivin was highly expressed in germinal vesicle (GV) and germinal vesicle breakdown (GVBD) stages oocytes, mainly localized in the GV at GV stage and on the chromosomes after GVBD. We have used RNA interference to specifically deplete Survivin in oocytes during in vitro maturation (IVM). Immunofluorescence assay showed that Survivin-depleted oocytes failed to produce polar body in meiosisⅠ (failed to complete cytokinesis), and they were arrested in metaphaseⅠwith misaligned chromosomes. The homologous chromosomes in Survivin-depleted oocytes could not be separated normally. Moreover, both the phosphorylation levels of Aurora B and the mRNA level of Mad2L1 related to spindle assembly checkpoint (SAC) was decreased in Survivin-depleted oocytes, which thus inhibited the degradation of Cyclin B1 (CCNB1) to complete meiosis. Taken together, we conclude that Survivin is an important mediator of centromere and midbody docking of Aurora-B as well as its activity and regulates SAC and MPF activity during meiosis in porcine oocytes.

Genomic structure, expression, and functional characterization of checkpoint kinase 1 from Penaeus monodon

Chk1 is a cell-cycle regulator. Chk1 has been identified in organisms ranging from yeast to humans, but few researchers have studied Chk1 in shrimps. We cloned Chk1 from the black tiger shrimp (Penaeus monodon). The full-length cDNA sequence of PmChk1 had 3,334 base pairs (bp), with an open reading frame of 1,455 bp. The complete genomic sequence of PmChk1 (11,081 bp) contained 10 exons separated by nine introns. qRT-PCR showed that PmChk1 was highly expressed in the ovaries and gills of P. monodon. The lowest PmChk1 expression was noted in stage III of ovarian development in P. monodon. PmChk1 expression decreased significantly after injection of 5-hydroxytryptamine and eyestalk ablation in P. monodon ovaries. RNA interference experiments were undertaken to examine the expression of PmChk1, PmCDC2, and PmCyclin B. PmChk1 knockdown in the ovaries and hepatopancreas by dsRNA-Chk1 was successful. The localization and level of PmChk1 expression in the hepatopancreas was studied using in situ hybridization, which showed that data were in accordance with those of qRT-PCR. The Gonadosomatic Index of P. monodon after dsRNA-Chk1 injection was significantly higher than that after injection of dsRNA-GFP or phosphate-buffered saline. These data suggest that PmChk1 may have important roles in the ovarian maturation of P. monodon.