Xenopus polo-like kinase Plx1 regulates XErp1, a novel inhibitor of APC/C activity

Egg Overactivation-An Overlooked Phenomenon of Gamete Physiology

In many vertebrates, mature ovulated eggs are arrested at metaphase II prior to fertilization. The eggs exit meiotic arrest after fertilization-induced or parthenogenetic activation, followed by embryo development or egg degradation, respectively. Calcium-dependent activation of meiotically-arrested eggs has been thoroughly investigated in various species. In addition, several recent studies have detailed the excessive activation of ovulated frog eggs, so-called overactivation. This overview highlights the major events of overactivation observed in mature ovulated eggs of the African clawed frog Xenopus laevis with a focus on similarities and differences between spontaneous, oxidative stress-induced, and mechanical stress-induced overactivation. The dramatically different cell death scenarios that unfold in activated and overactivated eggs are also exposed in the article.

A phosphate-binding pocket in cyclin B3 is essential for XErp1/Emi2 degradation in meiosis I

To ensure the correct euploid state of embryos, it is essential that vertebrate oocytes await fertilization arrested at metaphase of meiosis II. This MII arrest is mediated by XErp1/Emi2, which inhibits the ubiquitin ligase APC/C (anaphase-promoting complex/cyclosome). Cyclin B3 in complex with Cdk1 (cyclin-dependent kinase 1) is essential to prevent an untimely arrest of vertebrate oocytes in meiosis I by targeting XErp1/Emi2 for degradation. Yet, the molecular mechanism of XErp1/Emi2 degradation in MI is not well understood. Here, by combining TRIM-Away in oocytes with egg extract and in vitro studies, we demonstrate that a hitherto unknown phosphate-binding pocket in cyclin B3 is essential for efficient XErp1/Emi2 degradation in meiosis I. This pocket enables Cdk1/cyclin B3 to bind pre-phosphorylated XErp1/Emi2 facilitating further phosphorylation events, which ultimately target XErp1/Emi2 for degradation in a Plk1- (Polo-like kinase 1) dependent manner. Key elements of this degradative mechanism are conserved in frog and mouse. Our studies identify a novel, evolutionarily conserved determinant of Cdk/cyclin substrate specificity essential to prevent an untimely oocyte arrest at meiosis I with catastrophic consequences upon fertilization.

High-sensitivity whole-mount in situ Hybridization of Mouse Oocytes and Embryos Visualizes the Super-resolution Structures and Distributions of mRNA Molecules

Mammalian oocytes accumulate more than ten thousand mRNAs, of which three to four thousand mRNAs are translationally repressed. The timings and sites of translational activation of these dormant mRNAs are crucial for promoting oocyte maturation and embryonic development. How these mRNAs are accumulated and distributed in oocytes is therefore a fundamental issue to be explored. A method that enables visualization of mRNA molecules with high resolution in a simple manner would be valuable for understanding how oocytes accumulate and regulate the dormant mRNAs. We have developed a highly sensitive whole-mount in situ hybridization method using in vitro-synthesized RNA probes and the tyramide signal amplification (TSA) system optimized for mouse oocytes and embryos. By using this method, Pou5f1/Oct4, Emi2, and cyclin B1 mRNAs were detected in immature oocytes and 2-cell stage embryos. Confocal microscopy showed that these mRNAs formed granular structures in the oocyte cytoplasm. The structures of Pou5f1/Oct4 and cyclin B1 mRNAs persisted in 2-cell stage embryos. Pou5f1/Oct4 RNA granules exhibited a solid-like property in immature oocytes and became liquid-like droplets in 2-cell stage embryos. Double-staining of cyclin B1 mRNA with Emi2 or Pou5f1/Oct4 mRNA revealed that these mRNAs were distributed as different RNA granules without overlapping each other and that the size of cyclin B1 RNA granules tended to be larger than that of Emi2 RNA granules. The structures and distribution patterns of these mRNAs were further analyzed by N-SIM super-resolution microscopy. This analysis revealed that the large-sized RNA granules consist of many small-sized granules, suggesting the accumulation and regulation of dormant mRNAs as basal-sized RNA granules. The method established in this study can easily visualize the structure and distribution of mRNAs accumulated in mammalian oocytes and embryos with high sensitivity and super-resolution. This method is useful for investigating the cellular and molecular mechanisms of translational control of mRNAs by which maturation and early developmental processes are promoted.

FBXO43 promotes cell cycle progression in cancer cells through stabilizing SKP2

FBXO43 is a member of the FBXO subfamily of F-box proteins, known to be a regulatory hub during meiosis. A body of data showed that FBXO43 is overexpressed in a number of human cancers. However, whether and how FBXO43 affects cell cycle progression and growth of cancer cells remain elusive. In this study, we provide first piece of evidence, showing a pivotal role of FBXO43 in cell cycle progression and growth of cancer cells. Specifically, FBXO43 acts as a positive cell cycle regulator with an oncogenic activity in variety types of human cancer, including non-small cell lung cancer, hepatocellular carcinoma and sarcoma. Mechanistically, FBXO43 interacts with phosphorylated SKP2 induced by AKT1, leading to reduced SKP2 auto-ubiquitylation and subsequent proteasome degradation. Taken together, our study demonstrates that FBXO43 promotes cell cycle progression by stabilizing SKP2, and FBXO43 could serve as a potential anti-cancer target.

Single-cell analysis identifies PLK1 as a driver of immunosuppressive tumor microenvironment in LUAD

PLK1 (Polo-like kinase 1) plays a critical role in the progression of lung adenocarcinoma (LUAD). Recent studies have unveiled that targeting PLK1 improves the efficacy of immunotherapy, highlighting its important role in the regulation of tumor immunity. Nevertheless, our understanding of the intricate interplay between PLK1 and the tumor microenvironment (TME) remains incomplete. Here, using genetically engineered mouse model and single-cell RNA-seq analysis, we report that PLK1 promotes an immunosuppressive TME in LUAD, characterized with enhanced M2 polarization of tumor associated macrophages (TAM) and dampened antigen presentation process. Mechanistically, elevated PLK1 coincides with increased secretion of CXCL2 cytokine, which promotes M2 polarization of TAM and diminishes expression of class II major histocompatibility complex (MHC-II) in professional antigen-presenting cells. Furthermore, PLK1 negatively regulates MHC-II expression in cancer cells, which has been shown to be associated with compromised tumor immunity and unfavorable patient outcomes. Taken together, our results reveal PLK1 as a novel modulator of TME in LUAD and provide possible therapeutic interventions.

CDK1-PP2A-B55 interplay ensures cell cycle oscillation via Apc1-loop300

Cell cycle control relies on a delicate balance of phosphorylation with CDK1 and phosphatases like PP1 and PP2A-B55. Yet, identifying the primary substrate responsible for cell cycle oscillations remains a challenge. We uncover the pivotal role of phospho-regulation in the anaphase-promoting complex/cyclosome (APC/C), particularly through the Apc1-loop300 domain (Apc1-300L), orchestrated by CDK1 and PP2A-B55. Premature activation of PP2A-B55 during mitosis, induced by Greatwall kinase depletion, leads to Apc1-300L dephosphorylation, stalling APC/C activity and delaying Cyclin B degradation. This effect can be counteracted using the B55-specific inhibitor pEnsa or by removing Apc1-300L. We also show Cdc20's dynamic APC/C interaction across cell cycle stages, but dephosphorylation of Apc1-300L specifically inhibits further Cdc20 recruitment. Our study underscores APC/C's central role in cell cycle oscillation, identifying it as a primary substrate regulated by the CDK-PP2A partnership.

F-box protein 43 promoter methylation as a novel biomarker for hepatitis B virus-associated hepatocellular carcinoma

BACKGROUND

Hepatocellular carcinoma (HCC) has a high prevalence and poor prognosis worldwide. Therefore, it is urgent to find effective and timely diagnostic markers. The objective of this study was to evaluate the diagnostic value of F-box protein 43 promoter methylation in peripheral blood mononuclear cells (PBMCs) for HCC.

METHOD

A total of 247 participants were included in this study, comprising individuals with 123 hepatitis B virus-associated HCC, 79 chronic hepatitis B, and 45 healthy controls. F-box protein 43 methylation and mRNA levels in PBMCs were detected by MethyLight and quantitative real-time PCR.

RESULT

F-box protein 43 promoter methylation levels were significantly lower in HCC PBMCs than the chronic hepatitis B (P < 0.001) and healthy control PBMCs (P < 0.001). Relative mRNA expression levels of F-box protein 43 in HCC PBMCs were significantly higher than those in chronic hepatitis B (P < 0.001) and healthy control PBMCs (P < 0.001). Receiver operating characteristic analysis of F-box protein 43 promoter methylation levels yielded an area under curve (AUC) of 0.793 with 76.42% sensitivity and 68.35% specificity when differentiating HCC from chronic hepatitis. These values for the F-box protein 43 promoter methylation level were superior to those of the alpha-fetoprotein serum (AFP) level (AUC: 0.780, sensitivity: 47.97%, and specificity: 96.20%), with increments in values for the combination of F-box protein 43 promoter methylation AFP levels (AUC: 0.888, sensitivity: 76.42%, and specificity: 86.08%).

CONCLUSION

Hypomethylation of the F-box protein 43 promoter in PBMCs is a promising biochemical marker for HBV-associated HCC.

Phosphorylation of AHR by PLK1 promotes metastasis of LUAD via DIO2-TH signaling

Metastasis of lung adenocarcinoma (LUAD) is a major cause of death in patients. Aryl hydrocarbon receptor (AHR), an important transcription factor, is involved in the initiation and progression of lung cancer. Polo-like kinase 1 (PLK1), a serine/threonine kinase, acts as an oncogene promoting the malignancy of multiple cancer types. However, the interaction between these two factors and their significance in lung cancer remain to be determined. In this study, we demonstrate that PLK1 phosphorylates AHR at S489 in LUAD, leading to epithelial-mesenchymal transition (EMT) and metastatic events. RNA-seq analyses reveal that type 2 deiodinase (DIO2) is responsible for EMT and enhanced metastatic potential. DIO2 converts tetraiodothyronine (T4) to triiodothyronine (T3), activating thyroid hormone (TH) signaling. In vitro and in vivo experiments demonstrate that treatment with T3 or T4 promotes the metastasis of LUAD, whereas depletion of DIO2 or a deiodinase inhibitor disrupts this property. Taking together, our results identify the AHR phosphorylation by PLK1 and subsequent activation of DIO2-TH signaling as mechanisms leading to LUAD metastasis. These findings can inform possible therapeutic interventions for this event.

Single-cell analysis characterizes PLK1 as a catalyst of an immunosuppressive tumor microenvironment in LUAD

PLK1 (Polo-like kinase 1) plays a critical role in the progression of lung adenocarcinoma (LUAD). Recent studies have unveiled that targeting PLK1 improves the efficacy of immunotherapy, highlighting its important role in the regulation of tumor immunity. Nevertheless, our understanding of the intricate interplay between PLK1 and the tumor microenvironment (TME) remains incomplete. Here, using genetically engineered mouse model and single-cell RNA-seq analysis, we report that PLK1 promotes an immunosuppressive TME in LUAD, characterized with enhanced M2 polarization of tumor associated macrophages (TAM) and dampened antigen presentation process. Mechanistically, elevated PLK1 coincides with increased secretion of CXCL2 cytokine, which promotes M2 polarization of TAM and diminishes expression of class II major histocompatibility complex (MHC-II) in professional antigen-presenting cells. Furthermore, PLK1 negatively regulates MHC-II expression in cancer cells, which has been shown to be associated with compromised tumor immunity and unfavorable patient outcomes. Taken together, our results reveal PLK1 as a novel modulator of TME in LUAD and provide possible therapeutic interventions.

Phosphorylation of AHR by PLK1 promotes metastasis of LUAD via DIO2-TH signaling

Metastasis of Lung adenocarcinoma (LUAD) is a major cause of death in patients. Aryl hydrocarbon receptor (AHR) is an important transcription factor involved in the initiation and progression of lung cancer. Polo-like kinase 1 (PLK1), a serine/threonine kinase, is an oncogene that promotes the malignancy of multiple cancer types. Nonetheless, the interaction between these two factors and significance in lung cancer remains to be determined. Here, we demonstrate that PLK1 phosphorylates AHR at S489 in LUAD, which leads to epithelial-mesenchymal transition (EMT) and metastatic events. RNA-seq analyses show that type 2 deiodinase (DIO2) is responsible for EMT and enhanced metastatic potential. DIO2 converts tetraiodothyronine (T4) to triiodothyronine (T3), which then activates thyroid hormone signaling. In vitro and in vivo experiments demonstrate that treatment with T3 or T4 promotes the metastasis of LUAD, whereas depletion of DIO2 or deiodinase inhibitor disrupts this property. Taken together, our results identify the phosphorylation of AHR by PLK1 as a mechanism leading to the progression of LUAD and provide possible therapeutic interventions for this event.

Clinical significance of FBXO43 in hepatocellular carcinoma and its impact on tumor cell proliferation, migration and invasion

Background

The effects of FBXO43 on hepatocellular carcinoma (HCC) and its clinical significance have not yet been determined. This study aims to determine the clinical significance of FBXO43 in HCC and its impact on the biological functions of HCC cells.

Methods

Data from TCGA database were downloaded to investigate the expression of FBXO43 in HCC and its correlation with prognosis and immune infiltration. Immunohistochemical staining images of FBXO43 in HCC were acquired from the HPA website. HCC cells (BEL-7404 and SMMC-7721) were transfected with the lentivirus targeting FBXO43 to decrease FBXO43 expression in HCC cells. Western blotting assay was conducted to evaluate the expression level of FBXO43 protein. MTT assay was used to detect the proliferation of HCC cells. The migration and invasion of HCC cells were investigated by performing scratch wound-healing and Transwell invasion assays, respectively.

Results

In comparison to normal tissues, FBXO43 is overexpressed in HCC tissue, and high FBXO43 expression is linked to late T stage, TNM stage and tumor grade. Elevated FBXO43 expression is a risk factor for HCC. In patients with high FBXO43 expression, the overall survival, disease-specific survival, progression-free survival and disease-free survival are poorer. The proliferation, migration and invasion of HCC cells are significantly attenuated in FBXO43 knockdown cells. Also, TCGA data analysis reveals that FBXO43 exhibits a positive correlation with immunosuppression of HCC.

Conclusion

FBXO43 is overexpressed in HCC, and is linked to late tumor stage, worse prognosis and tumor immunosuppression. FBXO43 knockdown restrains the proliferation, migration and invasion of HCC.

Association between F-box-only protein 43 overexpression and hepatocellular carcinoma pathogenesis and prognosis

BACKGROUND

Despite great advances in the prevention, diagnosis, treatment, and management regarding hepatocellular carcinoma (HCC), the overall prognosis of HCC remains unfavorable. The expression profile, prognostic role, and biological functions of F-box-only protein 43 (FBXO43) in HCC remain unclear. Here, we determine the expression profile and prognostic value of FBXO43 in patients with HCC.

MATERIALS AND METHODS

A total of 467 HCC patients and their clinicopathological data were collected from the Second Affiliated Hospital of Jiaxing University, the Cancer Genome Atlas (TCGA), and Genotype-Tissue Expression (GTEx) databases. The expression profile, prognostic value, biological functions, and underlying mechanism of its involvement of FBXO43 were explored based on TCGA, Gene Expression Omnibus (GEO), LinkedOmics, and Cancer Dependency Map (DepMap). The expression of FBXO43 in 93 paired liver tissues was investigated via immunohistochemical staining, tissue microarray analysis, and Western blot. The prognostic value was assessed using survival analysis.

RESULTS

FBXO43 RNA was upregulated in HCC liver tissues and was associated with an unfavorable prognosis (p < 0.05). Furthermore, FBXO43 protein was overexpressed in HCC liver tissues compared with that in paired normal liver tissues. Overexpression of FBXO43 protein was significantly associated with advanced TNM stage, large tumor size, lymphatic invasion, distant metastasis, earlier cancer recurrence, and decreased overall survival after radical surgery (p < 0.05). Cox regression analysis showed that FBXO43 had significant prognostic value in HCC. Importantly, FBXO43 and its co-expressed genes were mainly involved in cell cycle regulation, DNA replication, metabolic regulation, and so on. FBXO43 knockdown could significantly affect the HCC cell lines growth and proliferation.

CONCLUSIONS

We first revealed that FBXO43 was overexpressed in liver HCC tissues at the RNA and protein levels and served as an independent prognostic factor for HCC patients. Therefore, FBXO43 is worth investigating as a potential HCC treatment target.

Egg Development After In Vitro Insemination in Japanese Quail (Coturnix japonica)

In vitro fertilization has been widely used to produce offspring in several mammalian species. We previously successfully produced Japanese quail chicks using intracytoplasmic sperm injection (ICSI), whereas in vitro insemination was not successful. This may be due to the difficulties associated with mimicking the sperm-egg fusion process and subsequent events in physiological polyspermic fertilization in vitro. In the present study, we observed egg development after in vitro insemination and investigated the inactivation of metaphase-promoting factor (MPF) and cytostatic factor (CSF), which are downstream of the Ca²⁺ signaling pathway in the egg, due to fertilizing sperm. We found a sperm number-dependent increase in hole formation caused by sperm penetration of the perivitelline membrane, the extracellular coat surrounding the egg. Egg development was observed following in vitro insemination; however, the developmental rate and stages after 24-h culture were inferior to those of ICSI eggs, even when insemination was performed with a high number of sperm (2 × 10⁴). We also noted the downregulation of inositol 1,4,5-trisphosphate receptor-1, ryanodine receptor-3, cyclin B1, and c-MOS, which are important regulatory components of MPF and CSF in the egg, which was dependent on the number of sperm used for insemination. However, the decreases observed in these components did not reach the levels observed in the ICSI eggs. Collectively, the present results suggest that a sperm number higher than 2 × 10⁴ is required for the progression of the Ca²⁺ signaling pathway, which initiates subsequent egg development in Japanese quail.

The translation regulator Zar1l controls timing of meiosis in Xenopus oocytes

Oocyte maturation and early embryo development occur in vertebrates in the near absence of transcription. Thus, sexual reproduction of vertebrates critically depends on the timely translation of mRNAs already stockpiled in the oocyte. Yet how translational activation of specific mRNAs is temporally coordinated is still incompletely understood. Here, we elucidate the function of Zar1l, a yet uncharacterized member of the Zar RNA-binding protein family, in Xenopus oocytes. Employing TRIM-Away, we demonstrate that loss of Zar1l accelerates hormone-induced meiotic resumption of Xenopus oocytes due to premature accumulation of the M-phase-promoting kinase cMos. We show that Zar1l is a constituent of a large ribonucleoparticle containing the translation repressor 4E-T and the central polyadenylation regulator CPEB1, and that it binds directly to the cMos mRNA. Partial, hormone-induced degradation of Zar1l liberates 4E-T from CPEB1, which weakens translational repression of mRNAs encoding cMos and likely additional M-phase-promoting factors. Thus, our study provides fundamental insights into the mechanisms that ensure temporally regulated translation of key cell cycle regulators during oocyte maturation, which is essential for sexual reproductivity.

The kinase PLK1 promotes the development of <i>Kras</i>/<i>Tp53</i>-mutant lung adenocarcinoma through transcriptional activation of the receptor RET

Increased abundance of polo-like kinase 1 (PLK1) is observed in various tumor types, particularly in lung adenocarcinoma (LUAD). Here, we found that PLK1 accelerated the progression of LUAD through a mechanism that was independent of its role in mediating mitotic cell division. Analysis of human tumor databases revealed that increased PLK1 abundance in LUAD correlated with mutations in KRAS and p53, with tumor stage, and with reduced survival in patients. In a mouse model of KRAS^G12D-driven, p53-deficient LUAD, PLK1 overexpression increased tumor burden, decreased tumor cell differentiation, and reduced animal survival. PLK1 overexpression in cultured cells and mice indirectly increased the expression of the gene encoding the receptor tyrosine kinase RET by phosphorylating the transcription factor TTF-1. Signaling by RET and mutant KRAS in these tumors converged to activate the mitogen-activated protein kinase (MAPK) pathway. Pharmacological inhibition of the MAPK pathway kinase MEK combined with inhibition of either RET or PLK1 markedly suppressed tumor growth. Our findings show that PLK1 can amplify MAPK signaling and reveal a potential target for stemming progression in lung cancers with high PLK1 abundance.

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.

Bi-allelic mutations in MOS cause female infertility characterized by preimplantation embryonic arrest

STUDY QUESTION

Are mutations in MOS (MOS proto-oncogene, serine/threonine kinase) involved in early embryonic arrest in infertile women?

SUMMARY ANSWER

We identified mutations in MOS that may cause human female infertility characterized by preimplantation embryonic arrest (PREMBA), and the effects of the mutations in human embryonic kidney 293T (HEK293T cells) and mouse oocytes provided evidence for a causal relation between MOS and female infertility.

WHAT IS KNOWN ALREADY

MOS, an activator of mitogen-activated protein kinase, mediates germinal vesicle breakdown and metaphase II arrest. Female MOS knockout mice are viable but sterile. Thus, MOS seems to be an important part of the mammalian cell cycle mechanism that regulates female meiosis.

STUDY DESIGN, SIZE, DURATION

Whole-exome sequencing, bioinformatics filtering analysis and genetic analysis were performed to identify two different biallelic mutations in MOS in two independent families. The infertile patients presenting with early embryonic arrest were recruited from October 2018 to June 2020.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The female patients diagnosed with primary infertility were recruited from the reproduction centres of local hospitals. Genomic DNA from the affected individuals, their family members and healthy controls was extracted from peripheral blood. We performed whole-exome sequencing in patients diagnosed with PREMBA. Functional effects of the mutations were investigated in HEK293T cells by western blotting and in mouse oocytes by microinjection and immunofluorescence.

MAIN RESULTS AND THE ROLE OF CHANCE

We identified the homozygous missense mutation c.285C>A (p.(Asn95Lys)) and the compound heterozygous mutations c.467delG (p.(Gly156Alafs*18)) and c.956G>A (p.(Arg319His)) in MOS in two independent patients. The mutations c.285C>A (p.(Asn95Lys)) and c.467delG (p.(Gly156Alafs*18)) reduced the protein level of MOS, and all mutations reduced the ability of MOS to phosphorylate its downstream target, extracellular signal-regulated kinase1/2. In addition, the identified mutations reduced the capacity of exogenous human MOS to rescue the metaphase II exit phenotype, and the F-actin cytoskeleton of mouse oocytes was affected by the patient-derived mutations.

LIMITATIONS, REASONS FOR CAUTION

Owing to the lack of in vivo data from patient oocytes, the exact molecular mechanism affected by MOS mutations and leading to PREMBA is still unknown and should be further investigated using knock-out or knock-in mice.

WIDER IMPLICATIONS OF THE FINDINGS

We identified recessive mutations in MOS in two independent patients with the PREMBA phenotype. Our findings reveal the important role of MOS during human oocyte meiosis and embryonic development and suggest that mutations in MOS may be precise diagnostic markers for clinical genetic counselling.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by the National Natural Science Foundation of China (81725006, 81822019, 81771581, 81971450, 81971382,82001538 and 82071642), the project supported by the Shanghai Municipal Science and Technology Major Project (2017SHZDZX01), the Project of the Shanghai Municipal Science and Technology Commission (19JC1411001), the Natural Science Foundation of Shanghai (19ZR1444500 and 21ZR1404800), the Shuguang Program of the Shanghai Education Development Foundation and the Shanghai Municipal Education Commission (18SG03), the Foundation of the Shanghai Health and Family Planning Commission (20154Y0162), the Capacity Building Planning Program for Shanghai Women and Children's Health Service and the collaborative innovation centre project construction for Shanghai Women and Children's Health. The authors have no conflicts of interest to declare.

TRIAL REGISTRATION NUMBER

N/A.

Design of novel oocyte activation methods: The role of zinc

Oocyte activation occurs at the time of fertilization and is a series of cellular events initiated by intracellular Ca2+ increases. Consequently, oocytes are alleviated from their arrested state in meiotic metaphase II (MII), allowing for the completion of meiosis. Oocyte activation is also an essential step for somatic cell nuclear transfer (SCNT) and an important tool to overcome clinical infertility. Traditional artificial activation methods aim to mimic the intracellular Ca2+ changes which occur during fertilization. Recent studies emphasize the importance of cytoplasmic Zn2+ on oocyte maturation and the completion of meiosis, thus suggesting artificial oocyte activation approaches that are centered around the concentration of available Zn2+in oocytes. Depletion of intracellular Zn2+ in oocytes with heavy metal chelators leads to successful oocyte activation in the absence of cellular Ca2+ changes, indicating that successful oocyte activation does not always depends on intracellular Ca2+ increases. Current findings lead to new approaches to artificially activate mammalian oocytes by reducing available Zn2+ contents, and the approaches improve the outcome of oocyte activation when combined with existing Ca2+ based oocyte activation methods. Here, we review the important role of Ca2+ and Zn2+ in mammalian oocyte activation and development of novel oocyte activation approaches based on Zn2+ availability.

Downregulation of the FBXO43 gene inhibits tumor growth in human breast cancer by limiting its interaction with PCNA

BACKGROUND

The function and regulatory mechanism of FBXO43 in breast cancer (BC) are still unclear. Here, we intended to determine the role and mechanism of FBXO43 in BC.

METHODS

FBXO43 expression in BC was evaluated by analysis of The Cancer Genome Atlas (TCGA). RT-qPCR and western blotting were utilized to detect FBXO43 expression in BC cell lines. Lentivirus was applied to downregulate FBXO43 in human BC cells. Proliferation assays were performed to evaluate the proliferative ability of BC cells. The apoptosis and cell cycle analysis of BC cells were analyzed by flow cytometry. Cell migration and invasion were investigated via Transwell assays. The function of FBXO43 in vivo was evaluated by constructing a xenograft mouse model. The proteins that might interact with FBXO43 in BC were identified by mass spectrometry, bioinformatics analysis, and co-immunoprecipitation (Co-IP) assays. Finally, rescue experiments were conducted to validate the recovery effects of the proteins interacting with FBXO43.

RESULTS

FBXO43 was highly expressed in BC and was significantly downregulated after FBXO43 knockdown. The proliferation, migration, and invasion of BC cells were inhibited, and cell apoptosis was induced by FBXO43 knockdown. In addition, an in vivo experiment indicated that FBXO43 knockdown could inhibit the cell growth of BC. The results of the Co-IP assay showed that FBXO43 interacted with PCNA. Further rescue experiments confirmed that overexpression of PCNA significantly reversed the effects of FBXO43 knockdown on BC cells.

CONCLUSION

Downregulation of FBXO43 inhibits the tumor growth of BC by limiting its interaction with PCNA. FBXO43 might be a new potential oncogene and a therapeutic target for BC.

FBXO43 variants in patients with female infertility characterized by early embryonic arrest

STUDY QUESTION

Can any new genetic factors responsible for early embryonic arrest in infertile patients be identified, together with the mechanism of pathogenic variants?

SUMMARY ANSWER

We identified three homozygous variants in the F-box protein 43 gene (FBXO43) in infertile patients and studies on the effects of the variants in HEK293T cells and mouse oocytes provided evidence for a causal relation between FBXO43 and female infertility.

WHAT IS KNOWN ALREADY

FBXO43, an inhibitor of the anaphase-promoting complex/cyclosome, mediates Metaphase II arrest as a component of the cytostatic factor in oocytes. Both male and female Fbxo43 knockout mice are viable but sterile. FBXO43, therefore, appears to be an essential component of the mammalian cell-cycle machinery that regulates both male and female meiosis. Until now, only one article has reported a homozygous FBXO43 variant associated with teratozoospermia, but the causal relationship was not established with functional evidence.

STUDY DESIGN, SIZE, DURATION

Whole-exome sequencing (WES) and homozygosity mapping were performed in 24 probands from consanguineous families who suffered from early embryonic arrest, and two different homozygous variants in FBXO43 were identified in two independent families. WES data from a further 950 infertile women with early embryonic arrest were screened for homozygous and compound heterozygous variants in FBXO43, and a third individual with an additional homozygous variant in FBXO43 was identified. The infertile patients presenting with early embryonic arrest were recruited from August 2016 to May 2020.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The women diagnosed with primary infertility were recruited from the reproduction centers of local hospitals. Genomic DNA samples from the affected individuals, their family members, and healthy controls were extracted from peripheral blood. The FBXO43 variants were identified using WES, homozygosity mapping, in silico analysis, and variant screening. All of the variants were confirmed by Sanger sequencing, and the effects of the variants were investigated in human embryonic kidney (HEK) 293T cells by western blotting and in mouse oocytes by complementary RNA injection.

MAIN RESULTS AND THE ROLE OF CHANCE

We identified three homozygous variants in FBXO43 (NM_001029860.4)-namely, c.1490_1497dup (p.(Glu500Serfs*2)), c.1747C>T (p.(Gln583*)), and c.154delG (p.(Asp52Thrfs*30))-in three independent families. All of the homozygous variants reduced the protein level of FBXO43 and reduced the level of its downstream target Cyclin B1 in HEK293T cells. In addition, the variants reduced the ability of exogenous human FBXO43 to rescue the parthenogenetic activation phenotype in Fbxo43 knockdown mouse oocytes.

LIMITATIONS, REASONS FOR CAUTION

Owing to the lack of in vivo data from the oocytes of patients, the exact molecular mechanism remains unknown and should be further investigated using knock out or knock in mice.

WIDER IMPLICATIONS OF THE FINDINGS

Our study has identified three pathogenic variants in FBXO43 that are involved in human early embryonic arrest. These findings contribute to our understanding of the role of FBXO43 in human early embryonic development and provide a new genetic marker for female infertility.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by the National Key Research and Development Program of China (2018YFC1003800, 2017YFC1001500, and 2016YFC1000600), the National Natural Science Foundation of China (81725006, 81822019, 81771581, 81971450, 81971382, and 82001552), the project supported by the Shanghai Municipal Science and Technology Major Project (2017SHZDZX01), the Project of the Shanghai Municipal Science and Technology Commission (19JC1411001), the Natural Science Foundation of Shanghai (19ZR1444500), the Shuguang Program of the Shanghai Education Development Foundation and the Shanghai Municipal Education Commission (18SG03), the Foundation of the Shanghai Health and Family Planning Commission (20154Y0162), the Capacity Building Planning Program for Shanghai Women and Children's Health Service, and the collaborative innovation center project construction for Shanghai Women and Children's Health. None of the authors have any competing interests.

TRIAL REGISTRATION NUMBER

N/A.

Meiotic Instability Generates a Pathological Condition in Mammalian Ovum

Maintenance of metaphase-II (M-II) arrest in ovum is required to present itself as a right gamete for successful fertilization in mammals. Surprisingly, instability of meiotic cell cycle results in spontaneous exit from M-II arrest, chromosomal scattering and incomplete extrusion of second polar body (PB-II) without forming pronuclei so called abortive spontaneous ovum activation (SOA). It remains unclear what causes meiotic instability in freshly ovulated ovum that results in abortive SOA. We propose the involvement of various signal molecules such as reactive oxygen species (ROS), cyclic 3',5' adenosine monophosphate (cAMP) and calcium (Ca²⁺) in the induction of meiotic instability and thereby abortive SOA. These signal molecules through their downstream pathways modulate phosphorylation status and activity of cyclin dependent kinase (cdk1) as well as cyclin B1 level. Changes in phosphorylation status of cdk1 and its activity, dissociation and degradation of cyclin B1 destabilize maturation promoting factor (MPF). The premature MPF destabilization and defects in other cell cycle regulators possibly cause meiotic instability in ovum soon after ovulation. The meiotic instability results in a pathological condition of abortive SOA and deteriorates ovum quality. These ova are unfit for fertilization and limit reproductive outcome in several mammalian species including human. Therefore, global attention is required to identify the underlying causes in greater details in order to address the problem of meiotic instability in ova of several mammalian species icluding human. Moreover, these activated ova may be used to create parthenogenetic embryonic stem cell lines in vitro for the use in regenerative medicine.Graphical abstract.

EMI2 expression as a poor prognostic factor in patients with breast cancer

Early mitotic inhibitor 2 (EMI2, gene symbol FBXO43), an APC/C inhibitor regulated by Plx1, is essential for cytostatic factor (CSF) activity. It belongs to subclass FBXO of the F-box proteins family. The aim of this study is to examine the clinicopathological significance of EMI2 in breast cancer. In this study, immunohistochemistry analysis was used to evaluate EMI2 expression in breast cancer tissues and then the association between EMI2 expression and clinicopathological factors was examined. Correlation of EMI2 with patient survival was analyzed by Kaplan-Meier survival curves. Among 192 patients analyzed, 105 (54.7%) had high expression of EMI2, and this was significantly associated with shortened disease free survival and overall survival in breast cancer patients. EMI2 expression was significantly associated with tumor grade (P = .006), tumor size (P < .001), and lymph node metastasis (P = .008). However, there was no significant correlation between EMI2 status and other biomarkers including ER, PR and Her2 status. Our results revealed that elevated EMI2 expression is a risk factor (hazard ratio = 3.93) for breast cancer and overexpression of EMI2 in breast cancer predicts higher risk of metastasis and worse survival. Therefore, EMI2 may be a potential therapeutic target for breast cancer.

The G2-to-M Transition Is Ensured by a Dual Mechanism that Protects Cyclin B from Degradation by Cdc20-Activated APC/C

In the eukaryotic cell cycle, a threshold level of cyclin B accumulation triggers the G2-to-M transition, and subsequent cyclin B destruction triggers mitotic exit. The anaphase-promoting complex/cyclosome (APC/C) is the E3 ubiquitin ligase that, together with its co-activator Cdc20, targets cyclin B for destruction during mitotic exit. Here, we show that two pathways act in concert to protect cyclin B from Cdc20-activated APC/C in G2, in order to enable cyclin B accumulation and the G2-to-M transition. The first pathway involves the Mad1-Mad2 spindle checkpoint complex, acting in a distinct manner from checkpoint signaling after mitotic entry but employing a common molecular mechanism-the promotion of Mad2-Cdc20 complex formation. The second pathway involves cyclin-dependent kinase phosphorylation of Cdc20, which is known to reduce Cdc20's affinity for the APC/C. Cooperation of these two mechanisms, which target distinct APC/C binding interfaces of Cdc20, enables cyclin B accumulation and the G2-to-M transition.

APC/C: current understanding and future perspectives

The separation of sister chromatids at anaphase, which is regulated by an E3 ubiquitin ligase called the anaphase-promoting complex/cyclosome (APC/C), is arguably the most important irrevocable event during the cell cycle. The APC/C and cyclin-dependent kinase 1 (Cdk1) are just two of the many significant cell cycle regulators and exert control through ubiquitylation and phosphorylation, respectively. The temporal and spatial regulation of the APC/C is achieved by multiple mechanisms, including phosphorylation, interaction with the structurally related co-activators Cdc20 and Cdh1, loading of distinct E2 ubiquitin-conjugating enzymes, binding with inhibitors and differential affinities for various substrates. Since the discovery of APC/C 25 years ago, intensive studies have uncovered many aspects of APC/C regulation, but we are still far from a full understanding of this important cellular machinery. Recent high-resolution cryogenic electron microscopy analysis and reconstitution of the APC/C have greatly advanced our understanding of molecular mechanisms underpinning the enzymatic properties of APC/C. In this review, we will examine the historical background and current understanding of APC/C regulation.

Calcineurin promotes APC/C activation at meiotic exit by acting on both XErp1 and Cdc20

Vertebrate oocytes await fertilization arrested at metaphase of the second meiotic division. Fertilization triggers a transient calcium wave, which induces the activation of the anaphase-promoting complex/cyclosome (APC/C) and its co-activator Cdc20 resulting in the destruction of cyclin B and hence meiotic exit. Two calcium-dependent enzymes are implicated in fertilization-induced APC/C^{C dc20} activation: calcium-/calmodulin-dependent kinase type II (CaMKII) and calcineurin (CaN). While the role of CaMKII in targeting the APC/C inhibitor XErp1/Emi2 for destruction is well-established, it remained elusive how CaN affects APC/C^{C dc20} activation. Here, we discover that CaN contributes to APC/C^{C dc20} activation in Xenopus laevis oocytes by two independent but interrelated mechanisms. First, it facilitates the degradation of XErp1 by dephosphorylating it at a site that is part of a phosphorylation-dependent recruiting motif for PP2A-B'56, which antagonizes inhibitory phosphorylation of XErp1. Second, it dephosphorylates Cdc20 at an inhibitory site, thereby supporting its APC/C-activating function. Thus, our comprehensive analysis reveals that CaN contributes to timely APC/C activation at fertilization by both negatively regulating the APC/C inhibitory activity of XErp1 and positively regulating the APC/C-activating function of Cdc20.

Genome-wide differential expression of long noncoding RNAs and mRNAs in ovarian follicles of two different chicken breeds

Bashang long-tail chickens are an indigenous breed with dual purpose in China (meat and eggs) but have low egg laying performance. To improve the low egg laying performance, a genome-wide analysis of mRNAs and long noncoding RNAs (lncRNAs) from Bashang long-tail chickens and Hy-Line brown layers was performed. A total of 16,354 mRNAs and 8691 lncRNAs were obtained from ovarian follicles. Between the breeds, 160 mRNAs and 550 lncRNAs were found to be significantly differentially expressed. Integrated network analysis suggested some differentially expressed genes were involved in ovarian follicular development through oocyte meiosis, progesterone-mediated oocyte maturation, and cell cycle. The impact of lncRNAs on cis and trans target genes, indicating some lncRNAs may play important roles in ovarian follicular development. The current results provided a catalog of chicken ovarian follicular lncRNAs and genes for further study to understand their roles in regulation of egg laying performance.

Ion Channel Function During Oocyte Maturation and Fertilization

The proper maturation of both male and female gametes is essential for supporting fertilization and the early embryonic divisions. In the ovary, immature fully-grown oocytes that are arrested in prophase I of meiosis I are not able to support fertilization. Acquiring fertilization competence requires resumption of meiosis which encompasses the remodeling of multiple signaling pathways and the reorganization of cellular organelles. Collectively, this differentiation endows the egg with the ability to activate at fertilization and to promote the egg-to-embryo transition. Oocyte maturation is associated with changes in the electrical properties of the plasma membrane and alterations in the function and distribution of ion channels. Therefore, variations on the pattern of expression, distribution, and function of ion channels and transporters during oocyte maturation are fundamental to reproductive success. Ion channels and transporters are important in regulating membrane potential, but also in the case of calcium (Ca2+), they play a critical role in modulating intracellular signaling pathways. In the context of fertilization, Ca2+ has been shown to be the universal activator of development at fertilization, playing a central role in early events associated with egg activation and the egg-to-embryo transition. These early events include the block of polyspermy, the completion of meiosis and the transition to the embryonic mitotic divisions. In this review, we discuss the role of ion channels during oocyte maturation, fertilization and early embryonic development. We will describe how ion channel studies in Xenopus oocytes, an extensively studied model of oocyte maturation, translate into a greater understanding of the role of ion channels in mammalian oocyte physiology.

Post-translational regulation of the maternal-to-zygotic transition

The maternal-to-zygotic transition (MZT) is essential for the developmental control handed from maternal products to newly synthesized zygotic genome in the earliest stages of embryogenesis, including maternal component (mRNAs and proteins) degradation and zygotic genome activation (ZGA). Various protein post-translational modifications have been identified during the MZT, such as phosphorylation, methylation and ubiquitination. Precise post-translational regulation mechanisms are essential for the timely transition of early embryonic development. In this review, we summarize recent progress regarding the molecular mechanisms underlying post-translational regulation of maternal component degradation and ZGA during the MZT and discuss some important issues in the field.

Emi2 Is Essential for Mouse Spermatogenesis

The meiotic functions of Emi2, an inhibitor of the APC/C complex, have been best characterized in oocytes where it mediates metaphase II arrest as a component of the cytostatic factor. We generated knockout mice to determine the in vivo functions of Emi2-in particular, its functions in the testis, where Emi2 is expressed at high levels. Male and female Emi2 knockout mice are viable but sterile, indicating that Emi2 is essential for meiosis but dispensable for embryonic development and mitotic cell divisions. We found that, besides regulating cell-cycle arrest in mouse eggs, Emi2 is essential for meiosis I progression in spermatocytes. In the absence of Emi2, spermatocytes arrest in early diplotene of prophase I. This arrest is associated with decreased Cdk1 activity and was partially rescued by a knockin mouse model of elevated Cdk1 activity. Additionally, we detected expression of Emi2 in spermatids and sperm, suggesting potential post-meiotic functions for Emi2.

PLK1 has tumor-suppressive potential in APC-truncated colon cancer cells

The spindle assembly checkpoint (SAC) acts as a molecular safeguard in ensuring faithful chromosome transmission during mitosis, which is regulated by a complex interplay between phosphatases and kinases including PLK1. Adenomatous polyposis coli (APC) germline mutations cause aneuploidy and are responsible for familial adenomatous polyposis (FAP). Here we study the role of PLK1 in colon cancer cells with chromosomal instability promoted by APC truncation (APC-ΔC). The expression of APC-ΔC in colon cells reduces the accumulation of mitotic cells upon PLK1 inhibition, accelerates mitotic exit and increases the survival of cells with enhanced chromosomal abnormalities. The inhibition of PLK1 in mitotic, APC-∆C-expressing cells reduces the kinetochore levels of Aurora B and hampers the recruitment of SAC component suggesting a compromised mitotic checkpoint. Furthermore, Plk1 inhibition (RNAi, pharmacological compounds) promotes the development of adenomatous polyps in two independent Apc Min/+ mouse models. High PLK1 expression increases the survival of colon cancer patients expressing a truncated APC significantly.

MPF-based meiotic cell cycle control: Half a century of lessons from starfish oocytes

In metazoans that undergo sexual reproduction, genomic inheritance is ensured by two distinct types of cell cycle, mitosis and meiosis. Mitosis maintains the genomic ploidy in somatic cells reproducing within a generation, whereas meiosis reduces by half the ploidy in germ cells to prepare for successive generations. The meiotic cell cycle is believed to be a derived form of the mitotic cell cycle; however, the molecular mechanisms underlying both of these processes remain elusive. My laboratory has long studied the meiotic cell cycle in starfish oocytes, particularly the control of meiotic M-phase by maturation- or M phase-promoting factor (MPF) and the kinase cyclin B-associated Cdk1 (cyclin B-Cdk1). Using this system, we have unraveled the molecular principles conserved in metazoans that modify M-phase progression from the mitotic type to the meiotic type needed to produce a haploid genome. Furthermore, we have solved a long-standing enigma concerning the molecular identity of MPF, a universal inducer of M-phase both in mitosis and meiosis of eukaryotic cells.

Taming the Beast: Control of APC/CCdc20-Dependent Destruction

The anaphase-promoting complex/cyclosome (APC/C) is a large multisubunit ubiquitin ligase that triggers the metaphase-to-anaphase transition in the cell cycle by targeting the substrates cyclin B and securin for destruction. APC/C activity toward these two key substrates requires the coactivator Cdc20. To ensure that cells enter mitosis and partition their duplicated genome with high accuracy, APC/C^Cdc20 activity must be tightly controlled. Here, we discuss the mechanisms that regulate APC/C^Cdc20 activity both before and during mitosis. We focus our discussion primarily on the chromosomal pathways that both accelerate and delay APC/C activation by targeting Cdc20 to opposing fates. The findings discussed provide an overview of how cells control the activation of this major cell cycle regulator to ensure both accurate and timely cell division.

Delayed APC/C activation extends the first mitosis of mouse embryos

The correct temporal regulation of mitosis underpins genomic stability because it ensures the alignment of chromosomes on the mitotic spindle that is required for their proper segregation to the two daughter cells. Crucially, sister chromatid separation must be delayed until all the chromosomes have attached to the spindle; this is achieved by the Spindle Assembly Checkpoint (SAC) that inhibits the Anaphase Promoting Complex/Cyclosome (APC/C) ubiquitin ligase. In many species the first embryonic M-phase is significantly prolonged compared to the subsequent divisions, but the reason behind this has remained unclear. Here, we show that the first M-phase in the mouse embryo is significantly extended due to a delay in APC/C activation. Unlike in somatic cells, where the APC/C first targets cyclin A2 for degradation at nuclear envelope breakdown (NEBD), we find that in zygotes cyclin A2 remains stable for a significant period of time after NEBD. Our findings that the SAC prevents cyclin A2 degradation, whereas over-expressed Plk1 stimulates it, support our conclusion that the delay in cyclin A2 degradation is caused by low APC/C activity. As a consequence of delayed APC/C activation cyclin B1 stability in the first mitosis is also prolonged, leading to the unusual length of the first M-phase.

Xenopus laevis Kif18A is a highly processive kinesin required for meiotic spindle integrity

The assembly and functionality of the mitotic spindle depends on the coordinated activities of microtubule-associated motor proteins of the dynein and kinesin superfamily. Our current understanding of the function of motor proteins is significantly shaped by studies using Xenopus laevis egg extract as its open structure allows complex experimental manipulations hardly feasible in other model systems. Yet, the Kinesin-8 orthologue of human Kif18A has not been described in Xenopus laevis so far. Here, we report the cloning and characterization of Xenopus laevis (Xl) Kif18A. Xenopus Kif18A is expressed during oocyte maturation and its depletion from meiotic egg extract results in severe spindle defects. These defects can be rescued by wild-type Kif18A, but not Kif18A lacking motor activity or the C-terminus. Single-molecule microscopy assays revealed that Xl_Kif18A possesses high processivity, which depends on an additional C-terminal microtubule-binding site. Human tissue culture cells depleted of endogenous Kif18A display mitotic defects, which can be rescued by wild-type, but not tail-less Xl_Kif18A. Thus, Xl_Kif18A is the functional orthologue of human Kif18A whose activity is essential for the correct function of meiotic spindles in Xenopus oocytes.

Summary: The highly processive kinesin Kif18A, which is expressed during oocyte maturation in Xenopus laevis, is required for correct spindle formation in meiotic egg extracts and can functionally complement human Kif18A in tissue culture cells.

Regulation of Cell Division

The challenging task of mitotic cell divisions is to generate two genetically identical daughter cells from a single precursor cell. To accomplish this task, a complex regulatory network evolved, which ensures that all events critical for the duplication of cellular contents and their subsequent segregation occur in the correct order, at specific intervals and with the highest possible fidelity. Transitions between cell cycle stages are triggered by changes in the phosphorylation state and levels of components of the cell cycle machinery. Entry into S-phase and M-phase are mediated by cyclin-dependent kinases (Cdks), serine-threonine kinases that require a regulatory cyclin subunit for their activity. Resetting the system to the interphase state is mediated by protein phosphatases (PPs) that counteract Cdks by dephosphorylating their substrates. To avoid futile cycles of phosphorylation and dephosphorylation, Cdks and PPs must be regulated in a manner such that their activities are mutually exclusive.

Analyses of EMI functions on meiotic maturation of porcine oocytes

Cyclin B (CCNB) accumulation is essential for regulating maturation/M-phase promoting factor activity during vertebrate oocyte maturation. Anaphase-promoting-complex/cyclosome (APC/C) degrades CCNB, allowing the cell cycle to progress; this complex is inhibited by Early mitotic inhibitors 1 and 2 (EMI1 and EMI2). The involvement of both EMI proteins in meiotic maturation has been reported in Xenopus and mouse oocytes, although a recent study described a marked difference in their respective function during meiotic resumption. Mouse is currently the only mammal in which the contribution of EMI to the oocyte maturation has been analyzed, so we used RNA injection methods to overexpress and knock down EMI1 and EMI2 to investigate their roles during porcine oocyte maturation. Up-regulation of either porcine EMI promoted precocious germinal vesicle breakdown (GVBD) with early CCNB1 accumulation in oocytes-which is consistent with their activities in mouse but not Xenopus oocytes. Knockdown of EMI1, but not EMI2, delayed GVBD and meiotic progression of oocytes from GVBD to meiotic metaphase I (MI). In contrast, knockdown of EMI2, but not EMI1, released oocytes from meiotic metaphase II (MII) arrest to produce a pronucleus. When injected oocytes were parthenogenetically activated, the up-regulation of EMI2, but not EMI1, prevented pronucleus formation. These results point to the similarities and differences of porcine EMI function with those of mouse versus Xenopus EMI, and generally contribute to our understanding of EMI function during mammalian oocyte maturation. Mol. Reprod. Dev. 83: 983-992, 2016 © 2016 Wiley Periodicals, Inc.

The anaphase-promoting complex initiates zygote division in Arabidopsis through degradation of cyclin B1

As the start of a new life cycle, activation of the first division of the zygote is a critical event in both plants and animals. Because the zygote in plants is difficult to access, our understanding of how this process is achieved remains poor. Here we report genetic and cell biological analyses of the zygote-arrest 1 (zyg1) mutant in Arabidopsis, which showed zygote-lethal and over-accumulation of cyclin B1 D-box-GUS in ovules. Map-based cloning showed that ZYG1 encodes the anaphase-promoting complex/cyclosome (APC/C) subunit 11 (APC11). Live-cell imaging studies showed that APC11 is expressed in both egg and sperm cells, in zygotes and during early embryogenesis. Using a GFP-APC11 fusion construct that fully complements zyg1, we showed that GFP-APC11 expression persisted throughout the mitotic cell cycle, and localized to cell plates during cytokinesis. Expression of non-degradable cyclin B1 in the zygote, or mutations of either APC1 or APC4, also led to a zyg1-like phenotype. Biochemical studies showed that APC11 has self-ubiquitination activity and is able to ubiquitinate cyclin B1 and promote degradation of cyclin B1. These results together suggest that APC/C-mediated degradation of cyclin B1 in Arabidopsis is critical for initiating the first division of the zygote.

Involvement of Cyclin-Dependent Kinase 1 during Postovulatory Aging-Mediated Abortive Spontaneous Egg Activation in Rat Eggs Cultured In Vitro

Freshly ovulated rat eggs do not remain arrested at metaphase II (MII) and undergo exit from MII arrest with initiation of extrusion of the second polar body (PBII), a characteristic feature of abortive spontaneous egg activation (SEA). The biochemical and molecular changes during postovulatory aging-mediated abortive SEA remain poorly understood. We investigated the morphological, cellular, and molecular changes during postovulatory aging-mediated abortive SEA in eggs cultured in vitro. Our results suggest that postovulatory egg aging in vitro induced initiation of PBII extrusion in a time-dependent manner. Postovulatory aging increased Wee1 kinase and Thr-14/Tyr-15 phosphorylated cyclin-dependent kinase 1 (Cdk1) levels, whereas Thr-161 phosphorylated Cdk1 and cyclin B1 levels were significantly decreased in eggs cultured in vitro. The early mitotic inhibitor 2 (Emi2) level was significantly reduced, but anaphase promoting complex/cyclosome (APC/C) and mitotic arrest deficient protein (MAD2) levels were increased initially and then reduced during a later period of in vitro culture. These results suggest that an increased Wee1 kinase level modulated the specific phosphorylation status of Cdk1, increased Cdk1 activity, and decreased the cyclin B1 level. Furthermore, the decreased Emi2 level was associated with an increased level of APC/C and decreased level of cyclin B1, which resulted in maturation promoting factor (MPF) destabilization and finally led to postovulatory aging-mediated abortive SEA in rat eggs cultured in vitro.

Calcium signaling and cell proliferation

Cell proliferation is orchestrated through diverse proteins related to calcium (Ca(2+)) signaling inside the cell. Cellular Ca(2+) influx that occurs first by various mechanisms at the plasma membrane, is then followed by absorption of Ca(2+) ions by mitochondria and endoplasmic reticulum, and, finally, there is a connection of calcium stores to the nucleus. Experimental evidence indicates that the fluctuation of Ca(2+) from the endoplasmic reticulum provides a pivotal and physiological role for cell proliferation. Ca(2+) depletion in the endoplasmatic reticulum triggers Ca(2+) influx across the plasma membrane in an phenomenon called store-operated calcium entries (SOCEs). SOCE is activated through a complex interplay between a Ca(2+) sensor, denominated STIM, localized in the endoplasmic reticulum and a Ca(2+) channel at the cell membrane, denominated Orai. The interplay between STIM and Orai proteins with cell membrane receptors and their role in cell proliferation is discussed in this review.

Maternal Wnt/STOP signaling promotes cell division during early Xenopus embryogenesis

During Xenopus development, Wnt signaling is thought to function first after midblastula transition to regulate axial patterning via β-catenin-mediated transcription. Here, we report that Wnt/glycogen synthase kinase 3 (GSK3) signaling functions posttranscriptionally already in mature oocytes via Wnt/stabilization of proteins (STOP) signaling. Wnt signaling is induced in oocytes after their entry into meiotic metaphase II and declines again upon exit into interphase. Wnt signaling inhibits Gsk3 and thereby protects proteins from polyubiquitination and degradation in mature oocytes. In a protein array screen, we identify a cluster of mitotic effector proteins that are polyubiquitinated in a Gsk3-dependent manner in Xenopus. Consequently inhibition of maternal Wnt/STOP signaling, but not β-catenin signaling, leads to early cleavage arrest after fertilization. The results support a novel role for Wnt signaling in cell cycle progression independent of β-catenin.

Calcium signaling and meiotic exit at fertilization in Xenopus egg

Calcium is a universal messenger that mediates egg activation at fertilization in all sexually reproducing species studied. However, signaling pathways leading to calcium generation and the mechanisms of calcium-induced exit from meiotic arrest vary substantially among species. Here, we review the pathways of calcium signaling and the mechanisms of meiotic exit at fertilization in the eggs of the established developmental model, African clawed frog, Xenopus laevis. We also discuss calcium involvement in the early fertilization-induced events in Xenopus egg, such as membrane depolarization, the increase in intracellular pH, cortical granule exocytosis, cortical contraction, contraction wave, cortical rotation, reformation of the nuclear envelope, sperm chromatin decondensation and sister chromatid segregation.

The zinc-binding region (ZBR) fragment of Emi2 can inhibit APC/C by targeting its association with the coactivator Cdc20 and UBE2C-mediated ubiquitylation

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Overexpression of the ZBR fragment of Emi2, but not of Emi1, induces abnormal cell division.

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The Emi2 ZBR fragment impairs the association of the coactivator Cdc20 with APC/C.

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The Emi2 ZBR fragment inhibits ubiquitylation by the cullin-RING of APC/C and E2C.

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The Emi2 ZBR-specific residues for APC/C inhibitory activity have been identified.

Anaphase-promoting complex or cyclosome (APC/C) is a multisubunit ubiquitin ligase E3 that targets cell-cycle regulators. Cdc20 is required for full activation of APC/C in M phase, and mediates substrate recognition. In vertebrates, Emi2/Erp1/FBXO43 inhibits APC/C-Cdc20, and functions as a cytostatic factor that causes long-term M phase arrest of mature oocytes. In this study, we found that a fragment corresponding to the zinc-binding region (ZBR) domain of Emi2 inhibits cell-cycle progression, and impairs the association of Cdc20 with the APC/C core complex in HEK293T cells. Furthermore, we revealed that the ZBR fragment of Emi2 inhibits in vitro ubiquitin chain elongation catalyzed by the APC/C cullin-RING ligase module, the ANAPC2–ANAPC11 subcomplex, in combination with the ubiquitin chain-initiating E2, E2C/UBE2C/UbcH10. Structural analyses revealed that the Emi2 ZBR domain uses different faces for the two mechanisms. Thus, the double-faced ZBR domain of Emi2 antagonizes the APC/C function by inhibiting both the binding with the coactivator Cdc20 and ubiquitylation mediated by the cullin-RING ligase module and E2C. In addition, the tail region between the ZBR domain and the C-terminal RL residues [the post-ZBR (PZ) region] interacts with the cullin subunit, ANAPC2. In the case of the ZBR fragment of the somatic paralogue of Emi2, Emi1/FBXO5, these inhibitory activities against cell division and ubiquitylation were not observed. Finally, we identified two sets of key residues in the Emi2 ZBR domain that selectively exert each of the dual Emi2-specific modes of APC/C inhibition, by their mutation in the Emi2 ZBR domain and their transplantation into the Emi1 ZBR domain.

Possible role of p38 MAPK-MNK1-EMI2 cascade in metaphase-II arrest of mouse oocytes

The Mos-MAPK signaling pathway involving the Mos-MEK1/2-ERK1/2-RSK1/2/3 or MSK1-EMI2 cascade is directly linked to metaphase-II arrest of vertebrate oocytes. In this study, we examined whether p38, a member of the MAPK subfamily, is regulated under the control of Mos and contributes to metaphase-II arrest in the mouse oocyte. Morpholino oligonucleotide-mediated depletion of Mos revealed a remarkable decrease in phosphorylation of p38. Simultaneous treatment of oocytes with two chemical inhibitors of p38 and MEK1/2 induced both release from metaphase II and degradation of cyclin B1, whereas the treatment with each of these two inhibitors had little effect. Moreover, phosphorylation of EMI2 was dramatically abolished by addition of the two inhibitors. Indeed, MNK1, a kinase downstream of p38, exhibited the ability to phosphorylate EMI2. These results suggest that in addition to the Mos-MEK1/2 pathway, the Mos-mediated p38 pathway may be implicated in metaphase-II arrest.

Emi2 mediates meiotic MII arrest by competitively inhibiting the binding of Ube2S to the APC/C

In vertebrates, unfertilized eggs are arrested at metaphase of meiosis II by Emi2, a direct inhibitor of the APC/C ubiquitin ligase. Two different ubiquitin-conjugating enzymes, UbcH10 and Ube2S, work with the APC/C to target APC/C substrates for degradation. However, their possible roles and regulations in unfertilized/fertilized eggs are not known. Here we use Xenopus egg extracts to show that both UbcH10 and Ube2S are required for rapid cyclin B degradation at fertilization, when APC/C binding of Ube2S, but not of UbcH10, increases several fold, coincidently with (SCF(β-TrCP)-dependent) Emi2 degradation. Interestingly, before fertilization, Emi2 directly inhibits APC/C-Ube2S binding via the C-terminal tail, but on fertilization, its degradation allows the binding mediated by the Ube2S C-terminal tail. Significantly, Emi2 and Ube2S bind commonly to the APC/C catalytic subunit APC10 via their similar C-terminal tails. Thus, Emi2 competitively inhibits APC/C-Ube2S binding before fertilization, while its degradation on fertilization relieves the inhibition for APC/C activation.

Daucosterol promotes the proliferation of neural stem cells

Neural stem cells (NSCs) are self-regenerating cells, but their regenerative capacity is limited. The present study was conducted to investigate the effect of daucosterol (a sterolin) on the promotion of NSC proliferation and determine the corresponding molecular mechanism. Results of cell counting kit-8 (CCK-8) assay showed that daucosterol significantly increased the quantity of viable cells and the effectiveness of daucosterol was similar to that of basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF). Flow cytometry detection of CFSE-labeled (CFSE, carboxyfluorescein diacetate succinimidyl ester) NSCs showed that Div Index (or the average number of cell divisions) and % Divided (or the percentage of cells that divided at least once) of the cells were increased, indicating that daucosterol increased the percentage of NSCs re-entering the cell cycle. mRNA microarray analysis showed that 333 genes that are mostly involved in the mitotic cell cycle were up-regulated. By contrast, 627 genes that are mostly involved in differentiation were down-regulated. In particular, insulin-like growth factor I (IGF1) was considered as an important regulatory gene that functionally promoted NSC proliferation, and the increased expression of IGF1 protein was validated by ELISA. In addition, the phosphorylation of AKT was increased, indicating that the proliferation-enhancing activity of daucosterol may be involved in IGF1-AKT pathway. Our study provided information about daucosterol as an efficient and inexpensive growth factor alternative that could be used in clinical medicine and research applications.