Polo-like kinase 1, on the rise from cell cycle regulation to prostate cancer development

PLK1 Mitigates Intervertebral Disc Degeneration by Delaying Senescence of Nucleus Pulposus Cells

Intervertebral disc degeneration (IVDD) is the primary cause of low back pain; however, the molecular mechanisms involved in the pathogenesis of IVDD are not fully understood. Polo-like kinase 1 (PLK1) plays numerous roles in the cell cycle, including in cell proliferation and senescence. To investigate the involvement of PLK1 in IVDD, we used patient tissues and an animal model of IVDD. Samples were analyzed via immunoblotting, quantitative real-time polymerase chain reaction (qPCR), immunofluorescence, and immunohistochemistry. Our results demonstrated that PLK1 expression was decreased in nucleus pulposus cells (NPCs) of degenerative IVDs. The inhibition of PLK1 kinase activity in normal NPCs increased the expression of p53 protein, inhibited cell proliferation, and induced senescence. Our results suggest that PLK1 regulates the degeneration of the IVD through p53, revealing the function and mechanism of PLK1 in IVDD and providing a theoretical basis and experimental evidence for the potential treatment of low back pain.

Effect of PLK1 inhibition on cisplatin-resistant gastric cancer cells

OBJECTIVE

This study aims to investigate the effect of polo-like kinase 1 (PLK1) inhibition on cisplatin (DDP)-resistant gastric cancer (GC) cells.

METHODS

The transcriptional level of PLK1 was measured by quantitative reverse-transcription polymerase chain reaction. Expressions of PLK1 and its downstream mediators as well as autophagy-related protein LC3 I/LC3 II were detected by western blot. An 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and 5-ethynyl-2'-deoxyuridine immunofluorescent staining were conducted to evaluate the cell viability and replication activity separately. Flow cytometry was carried out to determine the cell cycle status. The GFP-LC3 vector contributed toward tracking the formation and aggregation of autophagosomes.

RESULTS

Drug-resistant SGC-7901/DDP cells showed insignificant changes in all phases after DDP treatment, including DNA replication, cell proliferation, cell cycle, and apoptosis, whereas DDP could significantly improve the autophagy level of SGC-7901/DDP as well as PLK1expression. By downregulating the expression of PLK1, both BI2536 andsi-PLK1 enhanced SGC-7901/DDP sensitivity to DDP, suppressing the proliferation and autophagy as well as improving the apoptosis rate. PLK1 inhibition also resulted in the repression of cell division regulators CDC25C and cyclin B1.

CONCLUSION

Together, our experimental results illustrated that the DDP resistance of GC cells might be associated with the aberrant overexpression of PLK1. PLK1 inhibition, including si-PLK1 and BI2536 treatment, could restore the chemosensitivity of drug-resistant SGC-7901/DDP cells and enhance the efficacy of DDP, revealing the potential value of PLK1 inhibition in GC chemotherapy.

Plk1 promotes the migration of human lung adenocarcinoma epithelial cells via STAT3 signaling

Polo-like kinase (Plk)1 contributes to the development of human cancer via multiple mechanisms, such as promoting the migration of cancer cells. However, the mechanistic basis for the regulation of cell migration by Plk1 remains unknown. To address this question, the present study investigated the effect of Plk1 inhibition on the migration of human lung adenocarcinoma epithelial A549 cells and the molecular factors involved. A549 cells were treated with the Plk1 inhibitor, BI2536, and cell migration was evaluated with the wound-healing assay. The expression of matrix metallopeptidase (MMP)2, vascular endothelial growth factor (VEGF)A, total and phosphorylated signal transducer and activator of transcription (STAT)3 was assessed by western blotting and reverse transcription-polymerase chain reaction following Plk1 knockdown and/or STAT3 overexpression. The interaction between Plk1 and STAT3 was evaluated by co-immunoprecipitation. The levels of MMP2 and VEGFA were decreased by treatment with Plk1 inhibitor. The phosphorylation of STAT3, which acts upstream of MMP2 and VEGFA, was also decreased by Plk1 knockdown, an effect that was abrogated by STAT3 overexpression. In addition, Plk1 was detected to bind with STAT3 either directly or as part of a complex by co-immunoprecipitation experiments. These results indicated that Plk1 may promote the migration of A549 cells via regulation of STAT3 signaling.

BI2536 induces mitotic catastrophe and radiosensitization in human oral cancer cells

BI2536 has been developed as a potential therapeutic agent for various cancers but not in oral cancer cells. Since BI2536 exhibits mitosis-regulating activity which are the most radiosensitive, we hypothesized that BI2536 might modulate the radiosensitivity of oral cancer cells. Human normal fibroblasts, oral cancer SAS, and OECM1 cells were treated with BI2536 (0-50 nM) and/or radiation (0-4 Gy). MTT assay, Liu's staining, flow cytometry, clonogenic assay, Annexin V/propidium iodide (PI) staining, western blot analysis, and small interfering RNA knockdown experiments were used to assess cell viability, morphology, cell cycle progression, radiation survival, and expression of regulatory proteins in vitro. Male BALB/c nude mice implanted with SAS cells were used to examine the effects of BI2536 in vivo. Treatment with BI2536 preferentially inhibited the viability of SAS and OECM1 cells, but not the normal fibroblasts. Morphological examination and Annexin V/PI staining of BI2536-treated oral cancer cells showed mitotic catastrophe and apoptosis. A DNA histogram revealed BI2536 induced G2/M and upregulation of phosphorylated H3 indicating accumulation in the M phase. BI2536 modulated the expression of PLK1, cell division control protein (Cdc)2, Cdc20, Cdc25c, adenomatous polyposis coli 3, and cyclin B1. At 10 nM, BI2536 exhibited low cytotoxicity, effectively induced mitotic catastrophe, and more importantly, sensitized oral cancer cells to radiotherapy. The animal study showed that BI2536 (10 mg/kg) + radiation (2 Gy) resulted in stronger tumor inhibition than that associated with radiation alone. Our findings showed that BI2536 could be an effective radiosensitizer both in vitro and in vivo.

The Emerging Role of Polo-Like Kinase 1 in Epithelial-Mesenchymal Transition and Tumor Metastasis

Polo-like kinase 1 (PLK1) is a serine/threonine kinase that plays a key role in the regulation of the cell cycle. PLK1 is overexpressed in a variety of human tumors, and its expression level often correlates with increased cellular proliferation and poor prognosis in cancer patients. It has been suggested that PLK1 controls cancer development through multiple mechanisms that include canonical regulation of mitosis and cytokinesis, modulation of DNA replication, and cell survival. However, emerging evidence suggests novel and previously unanticipated roles for PLK1 during tumor development. In this review, we will summarize the recent advancements in our understanding of the oncogenic functions of PLK1, with a focus on its role in epithelial-mesenchymal transition and tumor invasion. We will further discuss the therapeutic potential of these functions.

Evaluation of Polo-like kinase 1 as a potential therapeutic target in Merkel cell carcinoma

BACKGROUND

Merkel cell carcinoma (MCC) is a rare and aggressive malignancy of the skin. Treatment options for MCC include surgery, radiotherapy, and chemotherapy. The purpose of this study was to assess the expression of Polo-like kinase 1 (PLK1) in MCC and the role of the inhibitor, BI2536, as a potential therapeutic option in MCC.

METHODS

PLK1 expression was assessed in tissue samples from 28 patients with MCC and 5 healthy skin samples via immunohistochemistry and furthermore in the 2 MCC cell lines, MCC13 and MCC26, via immunoblotting. The impact of increasing doses of BI2536 alone and in combination with cisplatin or irradiation on cell viability was measured using the CCK-8 assay. Colony forming assays were performed to evaluate long-term effects of combination treatments. Additionally, the induction of apoptotic cell death was measured via flow cytometry.

RESULTS

PLK1 is moderately to strongly expressed in 75% of the patients with MCC. The PLK1 inhibitor, BI2536, demonstrated marked inhibition of cell proliferation with IC50 in the low nM range (from 10.07-12.39 nM). Furthermore, BI2536 induces apoptosis in MCC cell lines and acts synergistically in combination with irradiation and cisplatin.

CONCLUSION

Because of the marked upregulation of PLK1 in MCC tumor samples and potent inhibition of cell proliferation using a specific clinically available inhibitor, targeting of PLK1 qualifies as a potential novel therapeutic strategy in MCC. © 2015 Wiley Periodicals, Inc. Head Neck 38: E1918-E1925, 2016.

Polo-like kinase 1 induces epithelial-to-mesenchymal transition and promotes epithelial cell motility by activating CRAF/ERK signaling

Polo-like kinase 1 (PLK1) is a key cell cycle regulator implicated in the development of various cancers, including prostate cancer. However, the functions of PLK1 beyond cell cycle regulation remain poorly characterized. Here, we report that PLK1 overexpression in prostate epithelial cells triggers oncogenic transformation. It also results in dramatic transcriptional reprogramming of the cells, leading to epithelial-to-mesenchymal transition (EMT) and stimulation of cell migration and invasion. Consistently, PLK1 downregulation in metastatic prostate cancer cells enhances epithelial characteristics and inhibits cell motility. The signaling mechanisms underlying the observed cellular effects of PLK1 involve direct PLK1-dependent phosphorylation of CRAF with subsequent stimulation of the MEK1/2-ERK1/2-Fra1-ZEB1/2 signaling pathway. Our findings highlight novel non-canonical functions of PLK1 as a key regulator of EMT and cell motility in normal prostate epithelium and prostate cancer. This study also uncovers a previously unanticipated role of PLK1 as a potent activator of MAPK signaling.

Increased Expression of Phosphorylated Polo-Like Kinase 1 and Histone in Bypass Vein Graft and Coronary Arteries following Angioplasty

Interventional procedures, including percutaneous transluminal coronary angioplasty (PTCA) and coronary artery bypass surgery (CABG) to re-vascularize occluded coronary arteries, injure the vascular wall and cause endothelial denudation and medial vascular smooth muscle cell (VSMCs) metaplasia. Proliferation of the phenotypically altered SMCs is the key event in the pathogenesis of intimal hyperplasia (IH). Several kinases and phosphatases regulate cell cycle in SMC proliferation. It is our hypothesis that increased expression and activity of polo-like kinase-1 (PLK1) in SMCs, following PTCA and CABG, contributes to greater SMC proliferation in the injured than uninjured blood vessels. Using immunofluorescence (IF), we assessed the expression of PLK1 and phosphorylated-PLK1 (pPLK1) in post-PTCA coronary arteries, and superficial epigastric vein grafts (SEV) and compared it with those in the corresponding uninjured vessels. We also compared the expressions of mitotic marker phospho-histone, synthetic-SMC marker, contractile SMC marker, IFN-γ and phosphorylated STAT-3 in the post-PTCA arteries, SEV-grafts, and the uninjured vessels. Immunostaining demonstrated an increase in the number of cells expressing PLK1 and pPLK1 in the neointima of post PTCA-coronary arteries and SEV-grafts compared to their uninjured counterparts. VSMCs in the neointima showed an increased expression of phospho-histone, synthetic and contractile SMC markers, IFN-γ and phosphorylated STAT-3. However, VSMCs of uninjured coronaries and SEV had no significant expression of the aforementioned proteins. These data suggest that PLK1 might play a critical role in VSMC mitosis in hyperplastic intima of the injured vessels. Thus, novel therapies to inhibit PLK1 could be developed to inhibit the mitogenesis of VSMCs and control neointimal hyperplasia.

PLK1 and HOTAIR Accelerate Proteasomal Degradation of SUZ12 and ZNF198 during Hepatitis B Virus-Induced Liver Carcinogenesis

Elucidating mechanisms of hepatitis B virus (HBV)-mediated hepatocarcinogenesis is needed to gain insights into the etiology and treatment of liver cancer. Cells where HBV is replicating exhibit increased expression of Plk1 kinase and reduced levels of two transcription repression factors, SUZ12 and ZNF198. SUZ12 is an essential subunit of the transcription repressive complex PRC2. ZNF198 stabilizes the transcription repressive complex composed of LSD1, Co-REST, and HDAC1. These two transcription repressive complexes are held together by binding the long noncoding RNA HOTAIR. In this study, we linked these regulatory events mechanistically by showing that Plk1 induces proteasomal degradation of SUZ12 and ZNF198 by site-specific phosphorylation. Plk1-dependent ubiquitination of SUZ12 and ZNF198 was enhanced by expression of HOTAIR, significantly reducing SUZ12 and ZNF198 stability. In cells expressing the HBV X protein (HBx), downregulation of SUZ12 and ZNF198 mediated global changes in histone modifications. In turn, HBx-expressing cells propagated an altered chromatin landscape after cell division, as exemplified by changes in histone modifications of the EpCAM promoter, a target of PRC2 and LSD1/Co-REST/HDAC1 complexes. Notably, liver tumors from X/c-myc bitransgenic mice exhibited downregulation of SUZ12 and ZNF198 along with elevated expression of Plk1, HOTAIR, and EpCAM. Clinically, similar effects were documented in a set of HBV-related liver tumors consistent with the likelihood that downregulation of SUZ12 and ZNF198 leads to epigenetic reprogramming of infected hepatocytes. Because both Plk1 and HOTAIR are elevated in many human cancers, we propose that their combined effects are involved in epigenetic reprogramming associated broadly with oncogenic transformation.

Molecular dynamics of PLK1 during mitosis

Polo-like kinase 1 (PLK1) is a key regulator of eukaryotic cell division. During mitosis, dynamic regulation of PLK1 is crucial for its roles in centrosome maturation, spindle assembly, microtubule–kinetochore attachment, and cytokinesis. Similar to other members of the PLK family, the molecular architecture of PLK1 protein is characterized by 2 domains—the kinase domain and the regulatory substrate-binding domain (polo-box domain)—that cooperate and control PLK1 function during mitosis. Mitotic cells employ many layers of regulation to activate and target PLK1 to different cellular structures in a timely manner. During the last decade, numerous studies have shed light on the precise molecular mechanisms orchestrating the mitotic activity of PLK1 in time and space. This review aims to discuss available data and concepts related to regulation of the molecular dynamics of human PLK1 during mitotic progression.

Identification of candidate miRNA biomarkers from miRNA regulatory network with application to prostate cancer

Background

MicroRNAs (miRNAs) are a class of non-coding regulatory RNAs approximately 22 nucleotides in length that play a role in a wide range of biological processes. Abnormal miRNA function has been implicated in various human cancers including prostate cancer (PCa). Altered miRNA expression may serve as a biomarker for cancer diagnosis and treatment. However, limited data are available on the role of cancer-specific miRNAs. Integrative computational bioinformatics approaches are effective for the detection of potential outlier miRNAs in cancer.

Methods

The human miRNA-mRNA target network was reconstructed by integrating multiple miRNA-mRNA interaction datasets. Paired miRNA and mRNA expression profiling data in PCa versus benign prostate tissue samples were used as another source of information. These datasets were analyzed with an integrated bioinformatics framework to identify potential PCa miRNA signatures. In vitro q-PCR experiments and further systematic analysis were used to validate these prediction results.

Results

Using this bioinformatics framework, we identified 39 miRNAs as potential PCa miRNA signatures. Among these miRNAs, 20 had previously been identified as PCa aberrant miRNAs by low-throughput methods, and 16 were shown to be deregulated in other cancers. In vitro q-PCR experiments verified the accuracy of these predictions. miR-648 was identified as a novel candidate PCa miRNA biomarker. Further functional and pathway enrichment analysis confirmed the association of the identified miRNAs with PCa progression.

Conclusions

Our analysis revealed the scale-free features of the human miRNA-mRNA interaction network and showed the distinctive topological features of existing cancer miRNA biomarkers from previously published studies. A novel cancer miRNA biomarker prediction framework was designed based on these observations and applied to prostate cancer study. This method could be applied for miRNA biomarker prediction in other cancers.

Identification of pyrroloformamide as a cytokinesis modulator

Discovered in the late 1940s, the pyrrolinonodithioles represent a family of potent disulfide-containing natural products. Although they are understood in a synthetic and biosynthetic context, the biological role of these materials remains unresolved. To date, their activity has been suggested to arise through regulating RNA metabolism, and more recently they have been suggested to function as backup thiols for detoxification. Using materials identified through a natural products program, we now identify the biological function of one member of this family, pyrroloformamide, as an antimitotic agent acting, in part, by disrupting cytokinesis.

Plant sterol feeding induces tumor formation and alters sterol metabolism in the intestine of Apc(Min) mice

Dietary plant sterols reduce the absorption of cholesterol and therefore increase intraluminal cholesterol concentration. We examined how plant sterol esters from functional foods affect intestinal tumorigenesis in tumor-prone adenomatous polyposis coli (Apc)(Min) mice. Feeding plant sterols at 0.8% increased the number of intestinal adenomas, and the effect was significant in female mice. The concentration of mucosal free sitosterol increased by eightfold in plant sterol males and by threefold in plant sterol females when compared with respective controls. The concentration of mucosal free cholesterol was significantly lower in plant sterol males than in control males, and the decrease in free cholesterol was accompanied with a significant increase in nuclear sterol regulatory element binding protein-2. No difference was found in the levels of β-catenin, cyclin D1, epidermal growth factor receptor, extracellular signal-regulated kinase 1/2, or caveolin-1 in either gender after plant sterol feeding. Among all measured parameters, higher levels of estrogen receptor β and free cholesterol in the mucosa were among the strongest predictors of increased intestinal tumorigenesis. In addition, gene expression data showed significant enrichment of up-regulated genes of cell cycle control and cholesterol biosynthesis in plant sterol females. The results indicate that high intake of plant sterols accelerates intestinal tumorigenesis in female Apc (Min)mice; however, the mechanism behind the adverse effect remains to be discovered.

Abortive cell cycle events in the brains of scrapie-infected hamsters with remarkable decreases of PLK3/Cdc25C and increases of PLK1/cyclin B1

Polo-like kinases (PLKs) consist of a family of kinases which play critical roles during multiple stages of cell cycle progression. Increase of PLK1 and decrease of PLK3 are associated with the developments and metastases of many types of human malignant tumors; however, the situations of PLKs in prion diseases are less understood. Using Western blots and immunohistochemical and immunofluorescent assays, marked increase of PLK1 and decrease of PLK3 were observed in the brains of scrapie strain 263K-infected hamsters, presenting obviously a time-dependent phenomenon along with disease progression. Similar alterations of PLKs were also detected in a scrapie infectious cell line SMB-S15. Both PLK1 and PLK3 were observed in neurons by confocal microscopy. Accompanying with the changes of PLKs in the brains of 263K-infected hamsters, Cdc25C and its phosphorylated forms (p-Cdc25C-Ser198 and p-Cdc25C-Ser216) were significantly down-regulated, whereas Cyclin B1 and PCNA were obviously up-regulated, while phospho-histone H3 remained almost unchanged. Moreover, exposure of the cytotoxic peptide PrP106-126 on the primary cultured cortical neuron cells induced similar changes of cellular PLKs and some cell cycle-related proteins, such as Cdc25C and its phosphorylated forms, phospho-histone H3. Those results illustrate obviously aberrant expressions of cell cycle regulatory proteins in the prion-infected neurons, which may lead to the cell cycle arrest at M phase. Possibly due to the ill-regulation of some key cell cycle events during prion infection, together with the fact that neurons are unable to complete mitosis, the cell cycle reentry in prion-infected neurons is definitely abortive, which may lead to neuron apoptosis and neuron degeneration.

Scytonemin inhibits cell proliferation and arrests cell cycle through downregulating Plk1 activity in multiple myeloma cells

Multiple myeloma is the second most common hematologic malignancy. During the pursuit for novel and more selective anticancer drugs, different approaches have pointed to polo-like kinase 1 (Plk1) as a promising target. So we used a novel agent, scytonemin, to inhibit the activity of Plk1 to investigate the effect of Plk1 in multiple myeloma cells. MTT (3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay was used to examine the effect of scytonemin on the cell proliferation of three multiple myeloma cell lines with different concentration and different time. Flow cytometry was used to examine the effect of scytonemin on the cell cycle of multiple myeloma U266 cells with different concentration and different time. Moreover, the expression of Plk1 was analyzed by Western blot and real-time PCR in myeloma U266 cells with the treatment of scytonemin. Statistical analysis was used to analyze the effect of scytonemin on the cell proliferation and cell cycle with different concentration and different time and the association between Plk1 expression and activity with the treatment of scytonemin. Scytonemin was able to inhibit the proliferation of three myeloma cells in a dose-dependent manner, while U266 was the most sensitive one to scytonemin. Treatment with 3 and 4 μM scytonemin gradually increased the percentage of cells in the G2-M phase in U266 cells upon 48- and 72-h treatment. Scytonemin (at 3 and 4 μM concentration) inhibited multiple myeloma cell growth associated with downregulation of the activity of Plk1 but no effect on the expression of Plk1. Scytonemin, representing a novel Plk1 inhibitor, induced the inhibition of cell growth and cell cycle arrest in multiple myeloma cells by specifically decreasing Plk1 activity. Taken together, scytonemin is a promising novel agent for the treatment of multiple myeloma.

Molecular classification of prostate cancer progression: foundation for marker-driven treatment of prostate cancer

Recently, many therapeutic agents for prostate cancer have been approved that target the androgen receptor and/or the prostate tumor microenvironment. Each of these therapies has modestly increased patient survival. A better understanding of when in the course of prostate cancer progression specific therapies should be applied, and of what biomarkers would indicate when resistance arises, would almost certainly improve survival due to these therapies. Thus, applying the armamentarium of therapeutic agents in the right sequences in the right combination at the right time is a major goal in prostate cancer treatment. For this to occur, an understanding of prostate cancer evolution during progression is required. In this review, we discuss the current understanding of prostate cancer progression, but challenge the prevailing view by proposing a new model of prostate cancer progression, with the goal of improving biologic classification and treatment strategies. We use this model to discuss how integrating clinical and basic understanding of prostate cancer will lead to better implementation of molecularly targeted therapeutics and improve patient survival.

Polo-like kinase 1 is overexpressed in renal cancer and participates in the proliferation and invasion of renal cancer cells

Polo-like kinase 1 (Plk1) is an interesting molecule both as a biomarker and as a target for highly specific cancer therapy for several reasons. However, the functional significance of Plk1 in renal cell carcinoma (RCC) has not been reported. To explore whether Plk1 plays a general role in renal carcinoma, we examined the expression of Plk1 protein in renal urothelial carcinoma and cell lines, and analyzed the relationship between Plk1 protein expression and development, proliferation, and invasion of renal carcinoma. Immunohistochemisty was used to detect the expression of Plk1 in 100 renal carcinoma tissues. Moreover, the expression of Plk1 was analyzed by western blot and real-time polymerase chain reaction (PCR) in 80 renal carcinoma tissues and 20 normal renal tissues. CCK-8 assay, colony formation assay, and Transwell assay were used to examine proliferation and invasion ability of renal cancer cells with treatment of scytonemin (the specific inhibitor of Plk1). Statistical analysis was used to discuss the association between Plk1 expression and clinicopathologic parameters, and proliferation and invasion ability of renal cancer cells. Plk1 expressions were greater in cancerous tissues than in normal tissues (P<0.05). With an increase in tumor grade and stage, tumor metastasis, and recurrence, the level of Plk1 increased significantly in renal cancerous tissues. Moreover, there was a significantly higher expression of Plk1 in higher degree of malignant renal adenocarcinoma cell ACHN than that in renal adenocarcinoma cell 769-P. With increasing concentration of scytonemin, we found that cell proliferation and invasion activity decreased significantly. Plk1 expression status was closely correlated with important histopathologic characteristics (grades, stages, metastasis, and recurrence) of renal carcinomas. Furthermore, Plk1 played an important function on renal cancer cells' proliferation and invasion.

Nek9 regulates spindle organization and cell cycle progression during mouse oocyte meiosis and its location in early embryo mitosis

Nek9 (also known as Nercc1), a member of the NIMA (never in mitosis A) family of protein kinases, regulates spindle formation, chromosome alignment and segregation in mitosis. Here, we showed that Nek9 protein was expressed from germinal vesicle (GV) to metaphase II (MII) stages in mouse oocytes with no detectable changes. Confocal microscopy identified that Nek9 was localized to the spindle poles at the metaphase stages and associated with the midbody at anaphase or telophase stage in both meiotic oocytes and the first mitotic embyros. Depletion of Nek9 by specific morpholino injection resulted in severely defective spindles and misaligned chromosomes with significant pro-MI/MI arrest and failure of first polar body (PB1) extrusion. Knockdown of Nek9 also impaired the spindle-pole localization of γ-tubulin and resulted in retention of the spindle assembly checkpoint protein Bub3 at the kinetochores even after 10 h of culture. Live-cell imaging analysis also confirmed that knockdown of Nek9 resulted in oocyte arrest at the pro-MI/MI stage with abnormal spindles, misaligned chromosomes and failed polar body emission. Taken together, our results suggest that Nek9 may act as a MTOC-associated protein regulating microtubule nucleation, spindle organization and, thus, cell cycle progression during mouse oocyte meiotic maturation, fertilization and early embryo cleavage.