A bifurcated signaling cascade of NIMA-related kinases controls distinct kinesins in anaphase

Loss of function of the ALS-associated NEK1 kinase disrupts microtubule homeostasis and nuclear import

Loss-of-function variants in NIMA-related kinase 1 (NEK1) constitute a major genetic cause of amyotrophic lateral sclerosis (ALS), accounting for 2 to 3% of all cases. However, how NEK1 mutations cause motor neuron (MN) dysfunction is unknown. Using mass spectrometry analyses for NEK1 interactors and NEK1-dependent expression changes, we find functional enrichment for proteins involved in the microtubule cytoskeleton and nucleocytoplasmic transport. We show that α-tubulin and importin-β1, two key proteins involved in these processes, are phosphorylated by NEK1 in vitro. NEK1 is essential for motor control and survival in Drosophila models in vivo, while using several induced pluripotent stem cell (iPSC)-MN models, including NEK1 knockdown, kinase inhibition, and a patient mutation, we find evidence for disruptions in microtubule homeostasis and nuclear import. Notably, stabilizing microtubules with two distinct classes of drugs restored NEK1-dependent deficits in both pathways. The capacity of NEK1 to modulate these processes that are critically involved in ALS pathophysiology renders this kinase a formidable therapeutic candidate.

ALK-JNK signaling promotes NLRP3 inflammasome activation and pyroptosis via NEK7 during Streptococcus pneumoniae infection

Streptococcus pneumoniae (S. pneumoniae), a clinically important pathogen worldwide, causes serious invasive diseases, such as pneumonia, otitis media, and meningitis. The NLR family pyrin domain-containing 3 (NLRP3) inflammasome, an important component of the innate immune system, plays a key role in defense against pathogen infection; however the specific activation mechanism induced by S. pneumoniae infection is not fully understood. Here, primary mouse macrophages were selected as the in vitro cell model, and the effect of kinases on S. pneumoniae infection-induced NLRP3 inflammasome activation was investigated in vivo and in vitro using the western blot/RT-PCR/Co-IP/immunofluorescence staining/ELISA with or without kinase inhibitor or siRNA pretreatment. In this study, we found that the formation of the NEK7-NLRP3 complex significantly increased during S. pneumoniae infection and that anaplastic lymphoma kinase (ALK) and Jun N-terminal kinase (JNK) were phosphorylated rapidly. ALK and JNK inhibitors significantly reduced the ability of bacterial killing, the gene expression of NLRP3 inflammasome, the formation of apoptosis-associated speck-like protein containing caspase-recruitment domain (ASC) specks and the NEK7-NLRP3 complex, which in turn decreased the activation level of NLRP3 inflammasome-associated molecules and the maturation of interleukin-1β (IL-1β). In addition, ALK regulated the phosphorylation of JNK. Interestingly, the ALK/JNK/NEK7-NLRP3 signaling pathway is also involved in regulating pyroptosis and IL-1β secretion triggered by S. pneumoniae infection. In conclusion, our data suggest, for the first time, that the ALK/JNK/NEK7-NLRP3 signaling pathway may play an important role in NLRP3 inflammasome activation and pyroptosis and consequently regulate the host immune response upon S. pneumoniae infection.

Novel variants of NEK9 associated with neonatal arthrogryposis: Two case reports and a literature review

Objective: Pathogenic variants in NEK9 (MIM: 609798) have been identified in patients with lethal congenital contracture syndrome 10 (OMIM: 617022) and arthrogryposis, Perthes disease, and upward gaze palsy (APUG and OMIM: 614262). The shared core phenotype is multiple joint contractures or arthrogryposis. In the present study, three novel variants of NEK9 associated with neonatal arthrogryposis were reported. Methods: The clinical data of two premature infants and their parents were collected. The genomic DNA was extracted from their peripheral blood samples and subjected to trio-whole-exome sequencing (trio-WES) and copy number variation analysis. Results: Using trio-WES, a total of three novel pathogenic variants of NEK9 were detected in the two families. Patient 1 carried compound heterozygous variations of c.717C > A (p. C239*741) and c.2824delA (p.M942Cfs*21), which were inherited from his father and mother, respectively. Patient 2 also carried compound heterozygous variations of c.61G > T (p. E21*959) and c. 2824delA (p. M942Cfs*21), which were inherited from his father and mother, respectively. These variants have not been previously reported in the ClinVar, HGMD, or gnomAD databases. Conclusion: This is the first report about NEK9-related arthrogryposis in neonatal patients. The findings from this study suggest that different types of mutations in NEK9 lead to different phenotypes. Our study expanded the clinical phenotype spectrum and gene spectrum of NEK9-associated arthrogryposis.

In Mitosis You Are Not: The NIMA Family of Kinases in Aspergillus, Yeast, and Mammals

The Never in mitosis gene A (NIMA) family of serine/threonine kinases is a diverse group of protein kinases implicated in a wide variety of cellular processes, including cilia regulation, microtubule dynamics, mitotic processes, cell growth, and DNA damage response. The founding member of this family was initially identified in Aspergillus and was found to play important roles in mitosis and cell division. The yeast family has one member each, Fin1p in fission yeast and Kin3p in budding yeast, also with functions in mitotic processes, but, overall, these are poorly studied kinases. The mammalian family, the main focus of this review, consists of 11 members named Nek1 to Nek11. With the exception of a few members, the functions of the mammalian Neks are poorly understood but appear to be quite diverse. Like the prototypical NIMA, many members appear to play important roles in mitosis and meiosis, but their functions in the cell go well beyond these well-established activities. In this review, we explore the roles of fungal and mammalian NIMA kinases and highlight the most recent findings in the field.

Integrative Analysis of the Expression Levels and Prognostic Values for NEK Family Members in Breast Cancer

Background: In the latest rankings, breast cancer ranks first in incidence and fifth in mortality among female malignancies worldwide. Early diagnosis and treatment can improve the prognosis and prolong the survival of breast cancer (BC) patients. The NIMA-related kinase (NEK), a group of serine/threonine kinase, is a large and conserved gene family that includes NEK1–NEK11. The NEK plays a pivotal role in the cell cycle and microtubule formation. However, an integrative analysis of the effect and prognosis value of NEK family members on BC patients is still lacking.

Methods: In this study, the expression profiles of NEK family members in BC and its subgroups were analyzed using UALCAN, GEPIA2, and Human Protein Atlas datasets. The prognostic values of NEK family members in BC were evaluated using the Kaplan–Meier plotter. Co-expression profiles and genetic alterations of NEK family members were analyzed using the cBioPortal database. The function and pathway enrichment analysis of the NEK family were performed using the WebGestalt database. The correlation analysis of the NEK family and immune cell infiltration in BC was conducted using the TIMER 2.0 database.

Results: In this study, we compared and analyzed the prognosis values of the NEKs. We found that NEK9 was highly expressed in normal breast tissues than BC, and NEK2, NEK6, and NEK11 were significantly highly expressed in BC than adjacent normal tissues. Interestingly, the expression levels of NEK2, NEK6, and NEK10 were not only remarkably correlated with the tumor stage but also with the molecular subtype. Through multilevel research, we found that high expression levels of NEK1, NEK3, NEK8, NEK9, NEK10, and NEK11 suggested a better prognosis value in BC, while high expression levels of NEK2 and NEK6 suggested a poor prognosis value in BC.

Conclusion: Our studies show the prognosis values of the NEKs in BC. Thus, we suggest that NEKs may be regarded as novel biomarkers for predicting potential prognosis values and potential therapeutic targets of BC patients.

Role of kinesin family member 14 in disease monitoring and prognosis in patients with gastrointestinal cancer

Kinesin family member 14 (KIF14) is not only involved in numerous essential biological activities, such as cytokinesis and myelination, but also regulates several malignant behaviors and progression of cancer. However, its role in gastrointestinal cancer is rarely reported. Therefore, the present study aimed to investigate the association of KIF14 expression with disease-free survival (DFS) and overall survival (OS) times in patients with gastrointestinal cancer. A total of 101 patients with gastrointestinal cancer (36 patients with gastric cancer and 65 patients with colorectal cancer) were retrospectively reviewed, and their cancer samples were collected to detect the protein and mRNA expression levels of KIF14 using immunohistochemistry and reverse transcription-quantitative PCR, respectively. KIF14 protein expression was increased in cancer tissues compared with adjacent tissues (all P<0.001). The protein expression levels of KIF14 were positively associated with T stage (P<0.001), distant metastases (P=0.007) and TNM stage (P<0.001), while KIF14 mRNA expression was positively associated with T stage (P<0.001), lymph node metastasis (P=0.004), distant metastases (P=0.001) and TNM stage (P<0.001). High protein and mRNA expression levels of KIF14 were associated with worse DFS (P<0.001) and OS (P=0.016) times. In addition, high KIF14 protein expression independently predicted unfavorable DFS times (P=0.007). Subgroup analysis revealed that in patients with gastric cancer, KIF14 expression was associated with DFS and OS times, while in patients with colorectal cancer, KIF14 expression was only associated with DFS time, but not with OS time. In conclusion, KIF14 expression was not only associated with advanced pathological differentiation and TNM stage but was also associated with poor survival time in patients with gastrointestinal cancer. These results indicate the potential of KIF14 as a biomarker for gastrointestinal cancer prognosis.

MicroRNA-323a-3p Negatively Regulates NEK6 in Colon Adenocarcinoma Cells

Objective. The activity of NEK6 is enhanced in several cancer cells, including colon adenocarcinoma (COAD) cells. However, there are few reports on the microRNA (miRNA/miR) regulation of NEK6. In this study, we aimed to investigate the effects of miRNAs targeting NEK6 in COAD cells. Methods. Public data and online analysis sites were used to analyze the expression levels of NEK6 and miR-323a-3p in COAD tissues as well as the relationship between NEK6 or miR-323a-3p levels and survival in patients with COAD and to predict miRNAs targeting NEK6. Real-time polymerase chain reaction and western blotting were performed to determine the levels of NEK6 and miR-323a-3p in COAD cells. The targeting of NEK6 by miR-323a-3p was verified using a dual-luciferase reporter assay. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, 5-ethynyl-2′-deoxyuridine assay, propidium iodide (PI) staining, annexin V-fluorescein isothiocyanate/PI staining, and transwell assay were employed to test the proliferation, apoptosis, migration ability, and invasiveness of COAD cells. Results. In COAD cells, NEK6 was highly expressed, whereas miR-323a-3p was expressed at low levels and negatively regulated NEK6. Upregulating the level of miR-323a-3p impaired the proliferation, migration, and invasion of COAD cells and promoted apoptosis, whereas supplementing NEK6 alleviated the damage of the proliferation, migration, and invasion of COAD cells caused by miR-323a-3p and inhibited miR-323a-3p-induced apoptosis. These findings indicate that miR-323a-3p regulates the proliferation, migration, invasion, and apoptosis of COAD cells by targeting NEK6. Conclusion. miR-323a-3p downregulates NEK6 in COAD cells; this provides a novel basis for further understanding the occurrence and development of COAD.

NEK7 promotes gastric cancer progression as a cell proliferation regulator

Background

Gastric cancer is one of the most common malignant tumors of the digestive system. However, its targeted therapy develops at a slow pace. Thus, exploring the mechanisms of the malignant behavior of gastric cancer cells is crucial to exploit its treatment. Mammalian never-in-mitosis A (NIMA)-related kinases (NEKs) are considered to play a significant role in cancer cell proliferation. However, no study has reported on NIMA family proteins in gastric cancer.

Methods

Bioinformatics analysis was employed to clarify the expression patterns of NEK1–NEK11 and their effects on prognosis. The effects of NEK7 on immune infiltration and NEK7 related pathways were also analyzed. At the cell level, 5-ethynyl-2-deoxyuridine, cell cycle, and Cell Counting Kit-8 assays were utilized to clarify the effect of NEK7 on gastric cancer cell proliferation. A mouse subcutaneous model revealed the regulating effect of NEK7 on gastric cancer cell proliferation in vivo.

Results

Bioinformatics analysis revealed that NEK7 is upregulated in gastric cancer and is related to poor prognosis. NEK7 is also related to T-stage, which is closely associated with cell proliferation. Further analysis showed that NEK7 was correlated with infiltration of multiple immune cells as well as gastric cancer-related pathways. Cell experiments indicated the promoting effect of NEK7 on cell proliferation, while the absence of NEK7 could lead to inhibition of gastric cancer proliferation and G1/S arrest.

Conclusion

NEK7 exerts a regulatory effect on cell proliferation and is closely related to tumor immune infiltration.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12935-021-02148-8.

NEK9 regulates primary cilia formation by acting as a selective autophagy adaptor for MYH9/myosin IIA

Autophagy regulates primary cilia formation, but the underlying mechanism is not fully understood. In this study, we identify NIMA-related kinase 9 (NEK9) as a GABARAPs-interacting protein and find that NEK9 and its LC3-interacting region (LIR) are required for primary cilia formation. Mutation in the LIR of NEK9 in mice also impairs in vivo cilia formation in the kidneys. Mechanistically, NEK9 interacts with MYH9 (also known as myosin IIA), which has been implicated in inhibiting ciliogenesis through stabilization of the actin network. MYH9 accumulates in NEK9 LIR mutant cells and mice, and depletion of MYH9 restores ciliogenesis in NEK9 LIR mutant cells. These results suggest that NEK9 regulates ciliogenesis by acting as an autophagy adaptor for MYH9. Given that the LIR in NEK9 is conserved only in land vertebrates, the acquisition of the autophagic regulation of the NEK9–MYH9 axis in ciliogenesis may have possible adaptive implications for terrestrial life.

Ciliogenesis is a tightly regulated process, although the role of selective autophagy is unclear. Here, the authors show NIMA-related kinase 9 controls actin network stabilization and subsequently ciliogenesis by targeting myosin MYH9 for autophagic degradation via GABARAP interaction.

Role of NLRP3 Inflammasome Activation in Obesity-Mediated Metabolic Disorders

NLRP3 inflammasome is one of the multimeric protein complexes of the nucleotide-binding domain, leucine-rich repeat (NLR)-containing pyrin and HIN domain family (PYHIN). When activated, NLRP3 inflammasome triggers the release of pro-inflammatory interleukins (IL)-1β and IL-18, an essential step in innate immune response; however, defective checkpoints in inflammasome activation may lead to autoimmune, autoinflammatory, and metabolic disorders. Among the consequences of NLRP3 inflammasome activation is systemic chronic low-grade inflammation, a cardinal feature of obesity and insulin resistance. Understanding the mechanisms involved in the regulation of NLRP3 inflammasome in adipose tissue may help in the development of specific inhibitors for the treatment and prevention of obesity-mediated metabolic diseases. In this narrative review, the current understanding of NLRP3 inflammasome activation and regulation is highlighted, including its putative roles in adipose tissue dysfunction and insulin resistance. Specific inhibitors of NLRP3 inflammasome activation which can potentially be used to treat metabolic disorders are also discussed.

Physiological and Pathological Roles of Mammalian NEK7

NEK7 is the smallest NIMA-related kinase (NEK) in mammals. The pathological and physiological roles of NEK7 have been widely reported in many studies. To date, the major function of NEK7 has been well documented in mitosis and NLRP3 inflammasome activation, but the detailed mechanisms of its regulation remain unclear. This review summarizes current advances in NEK7 research involving mitotic regulation, NLRP3 inflammasome activation, related diseases and potential inhibitors, which may provide new insights into the understanding and therapy of the diseases associated with NEK7, as well as the subsequent studies in the future.

EML4-ALK V3 oncogenic fusion proteins promote microtubule stabilization and accelerated migration through NEK9 and NEK7

EML4-ALK is an oncogenic fusion present in ∼5% of non-small cell lung cancers. However, alternative breakpoints in the EML4 gene lead to distinct variants of EML4-ALK with different patient outcomes. Here, we show that, in cell models, EML4-ALK variant 3 (V3), which is linked to accelerated metastatic spread, causes microtubule stabilization, formation of extended cytoplasmic protrusions and increased cell migration. EML4-ALK V3 also recruits the NEK9 and NEK7 kinases to microtubules via the N-terminal EML4 microtubule-binding region. Overexpression of wild-type EML4, as well as constitutive activation of NEK9, also perturbs cell morphology and accelerates migration in a microtubule-dependent manner that requires the downstream kinase NEK7 but does not require ALK activity. Strikingly, elevated NEK9 expression is associated with reduced progression-free survival in EML4-ALK patients. Hence, we propose that EML4-ALK V3 promotes microtubule stabilization through NEK9 and NEK7, leading to increased cell migration. This represents a novel actionable pathway that could drive metastatic disease progression in EML4-ALK lung cancer.

NEK7: a potential therapy target for NLRP3-related diseases

NLRP3 inflammasome plays an essential role in innate immunity, yet the activation mechanism of NLRP3 inflammasome is not clear. In human or animal models, inappropriate NLRP3 inflammasome activation is implicated in many NLRP3-related diseases, such as tumors, inflammatory diseases and autoimmune diseases. Until now, a great number of inhibitors have been used to disturb the related signaling pathways, such as IL-1β blockade, IL-18 blockade and caspase-1 inhibitors. Unfortunately, most of these inhibitors just disturb the signaling pathways after the activation of NLRP3 inflammasome. Inhibitors that directly regulate NLRP3 to abolish the inflammation response may be more effective. NEK7 is a multifunctional kinase affecting centrosome duplication, mitochondrial regulation, intracellular protein transport, DNA repair and mitotic spindle assembly. Researchers have made significant observations on the regulation of gene transcription or protein expression of the NLRP3 inflammasome signaling pathway by NEK7. Those signaling pathways include ROS signaling, potassium efflux, lysosomal destabilization, and NF-κB signaling. Furthermore, NEK7 has been proved to be involved in many NLRP3-related diseases in humans or in animal models. Inhibitors focused on NEK7 may regulate NLRP3 to abolish the inflammation response and NEK7 may be a potential therapeutic target for NLRP3-related diseases.

Regulation of lung endothelial permeability by NEK kinases

Dysregulation of lung endothelial barrier function may lead to lethal outcomes, as demonstrated in the case of the acute respiratory distress syndrome (ARDS). p53 participates in the regulation of the lung endothelial barrier, and it has been associated both in vivo and in vitro with protective effects against the LPS-induced hyperpermeability. Family members of the never in mitosis A-related kinases (NEKs) are crucial mediators of fundamental cellular processes, including mitosis, and have been shown to posttranslationally modify p53. Since such modifications affect p53 stability and activity, it is highly probable that NEK kinases are also regulators of lung endothelial permeability. Thus, they may serve as possible therapeutic targets for treatment of pathologies associated with endothelial barrier dysfunction.

Getting out of mitosis: spatial and temporal control of mitotic exit and cytokinesis by PP1 and PP2A

Here, we will review the evidence showing that mitotic exit is initiated by regulated proteolysis and then driven by the PPP family of phosphoserine/threonine phosphatases. Rapid APC/CCDC20 and ubiquitin-dependent proteolysis of cyclin B and securin initiates sister chromatid separation, the first step of mitotic exit. Because proteolysis of Aurora and Polo family kinases dependent on APC/CCDH1 is relatively slow, this creates a new regulatory state, anaphase, different to G2 and M-phase. We will discuss how the CDK1-counteracting phosphatases PP1 and PP2A-B55, together with Aurora and Polo kinases, contribute to the temporal regulation and order of events in the different stages of mitotic exit from anaphase to cytokinesis. For PP2A-B55, these timing properties are created by the ENSA-dependent inhibitory pathway and differential recognition of phosphoserine and phosphothreonine. Finally, we will discuss how Aurora B and PP2A-B56 are needed for the spatial regulation of anaphase spindle formation and how APC/C-dependent destruction of PLK1 acts as a timer for abscission, the final event of cytokinesis.

Comprehensive substrate specificity profiling of the human Nek kinome reveals unexpected signaling outputs

Human NimA-related kinases (Neks) have multiple mitotic and non-mitotic functions, but few substrates are known. We systematically determined the phosphorylation-site motifs for the entire Nek kinase family, except for Nek11. While all Nek kinases strongly select for hydrophobic residues in the −3 position, the family separates into four distinct groups based on specificity for a serine versus threonine phospho-acceptor, and preference for basic or acidic residues in other positions. Unlike Nek1-Nek9, Nek10 is a dual-specificity kinase that efficiently phosphorylates itself and peptide substrates on serine and tyrosine, and its activity is enhanced by tyrosine auto-phosphorylation. Nek10 dual-specificity depends on residues in the HRD+2 and APE-4 positions that are uncommon in either serine/threonine or tyrosine kinases. Finally, we show that the phosphorylation-site motifs for the mitotic kinases Nek6, Nek7 and Nek9 are essentially identical to that of their upstream activator Plk1, suggesting that Nek6/7/9 function as phospho-motif amplifiers of Plk1 signaling.

The co-chaperone UNC45A is essential for the expression of mitotic kinase NEK7 and tumorigenesis

Cumulative evidence suggests that the heat shock protein 90 (Hsp90) co-chaperone UNC-45 myosin chaperone A (UNC45A) contributes to tumorigenesis and that its expression in cancer cells correlates with proliferation and metastasis of solid tumors. However, the molecular mechanism by which UNC45A regulates cancer cell proliferation remains largely unknown. Here, using siRNA-mediated gene silencing and various human cells, we report that UNC45A is essential for breast cancer cell growth, but is dispensable for normal cell proliferation. Immunofluorescence microscopy, along with gene microarray and RT-quantitative PCR analyses, revealed that UNC45A localizes to the cancer cell nucleus, where it up-regulates the transcriptional activity of the glucocorticoid receptor and thereby promotes expression of the mitotic kinase NIMA-related kinase 7 (NEK7). We observed that UNC45A-deficient cancer cells exhibit extensive pericentrosomal material disorganization, as well as defects in centrosomal separation and mitotic chromosome alignment. Consequently, these cells stalled in metaphase and cytokinesis and ultimately underwent mitotic catastrophe, phenotypes that were rescued by heterologous NEK7 expression. Our results identify a key role for the co-chaperone UNC45A in cell proliferation and provide insight into the regulatory mechanism. We propose that UNC45A represents a promising new therapeutic target to inhibit cancer cell growth in solid tumor types.

Observations on spontaneous tumor formation in mice overexpressing mitotic kinesin Kif14

The KIF14 locus is gained and overexpressed in various malignancies, with prognostic relevance. Its protein product, a mitotic kinesin, accelerates growth of normal mammary epithelial cells in vitro and retinoblastoma tumours in a mouse model, while KIF14 knockdown blocks growth of brain, liver, ovarian, breast, prostate, and other tumour cells and xenografts. However, the tumour-initiating effects of Kif14 overexpression have not been studied. We aged a cohort of Kif14-overexpressing transgenic mice and wild-type littermates and documented survival, cause of death, and tumour burden. The Kif14 transgene was expressed in all tissues examined, and was associated with increased proliferation marker expression. Neither mouse weights nor overall survival differed between genotypes. However, Kif14 transgenic mice showed a higher incidence of fatal lymphomas (73 vs. 50%, p = 0.03, Fisher’s exact test), primarily follicular and diffuse B-cell lymphomas. Non-tumour findings included a bilateral ballooning degeneration of lens in 12% of Kif14 transgenic mice but no wild-type mice (p = 0.02). Overall, this work reveals a novel association of Kif14 overexpression with lymphoma but suggests that Kif14 does not have as prominent a role in initiating cancer in other cell types as it does in accelerating tumour development in response to other oncogenic insults.

NEK7 regulates dendrite morphogenesis in neurons via Eg5-dependent microtubule stabilization

Organization of microtubules into ordered arrays is best understood in mitotic systems, but remains poorly characterized in postmitotic cells such as neurons. By analyzing the cycling cell microtubule cytoskeleton proteome through expression profiling and targeted RNAi screening for candidates with roles in neurons, we have identified the mitotic kinase NEK7. We show that NEK7 regulates dendrite morphogenesis in vitro and in vivo. NEK7 kinase activity is required for dendrite growth and branching, as well as spine formation and morphology. NEK7 regulates these processes in part through phosphorylation of the kinesin Eg5/KIF11, promoting its accumulation on microtubules in distal dendrites. Here, Eg5 limits retrograde microtubule polymerization, which is inhibitory to dendrite growth and branching. Eg5 exerts this effect through microtubule stabilization, independent of its motor activity. This work establishes NEK7 as a general regulator of the microtubule cytoskeleton, controlling essential processes in both mitotic cells and postmitotic neurons.

NEK7 is a kinase known for its role in mitotic spindle assembly, driving centrosome separation in prophase through regulation of the kinesin Eg5. Here, the authors show that NEK7 and Eg5 also control dendrite morphogenesis in postmitotic neurons.

Coordination of Protein Kinase and Phosphoprotein Phosphatase Activities in Mitosis

Dynamic changes in protein phosphorylation govern the transitions between different phases of the cell division cycle. A "tug of war" between highly conserved protein kinases and the family of phosphoprotein phosphatases (PPP) establishes the phosphorylation state of proteins, which controls their function. More than three-quarters of all proteins are phosphorylated at one or more sites in human cells, with the highest occupancy of phosphorylation sites seen in mitosis. Spatial and temporal regulation of opposing kinase and PPP activities is crucial for accurate execution of the mitotic program. The role of mitotic kinases has been the focus of many studies, while the contribution of PPPs was for a long time underappreciated and is just emerging. Misconceptions regarding the specificity and activity of protein phosphatases led to the belief that protein kinases are the primary determinants of mitotic regulation, leaving PPPs out of the limelight. Recent studies have shown that protein phosphatases are specific and selective enzymes, and that their activity is tightly regulated. In this review, we discuss the emerging roles of PPPs in mitosis and their regulation of and by mitotic kinases, as well as mechanisms that determine PPP substrate recognition and specificity.

Mitotic Regulation by NEK Kinase Networks

Genetic studies in yeast and Drosophila led to identification of cyclin-dependent kinases (CDKs), Polo-like kinases (PLKs) and Aurora kinases as essential regulators of mitosis. These enzymes have since been found in the majority of eukaryotes and their cell cycle-related functions characterized in great detail. However, genetic studies in another fungal species, Aspergillus nidulans, identified a distinct family of protein kinases, the NEKs, that are also widely conserved and have key roles in the cell cycle, but which remain less well studied. Nevertheless, it is now clear that multiple NEK family members act in networks to regulate specific events of mitosis, including centrosome separation, spindle assembly and cytokinesis. Here, we describe our current understanding of how the NEK kinases contribute to these processes, particularly through targeted phosphorylation of proteins associated with the microtubule cytoskeleton. We also present the latest findings on molecular events that control the activation state of the NEKs and how these are revealing novel modes of enzymatic regulation relevant not only to other kinases but also to pathological mechanisms of disease.

Of rings and spines: The multiple facets of Citron proteins in neural development

The Citron protein was originally identified for its capability to specifically bind the active form of RhoA small GTPase, leading to the simplistic hypothesis that it may work as a RhoA downstream effector in actin remodeling. More than two decades later, a much more complex picture has emerged. In particular, it has become clear that in animals, and especially in mammals, the functions of the Citron gene (CIT) are intimately linked to many aspects of central nervous system (CNS) development and function, although the gene is broadly expressed. More specifically, CIT encodes two main isoforms, Citron-kinase (CIT-K) and Citron-N (CIT-N), characterized by complementary expression pattern and different functions. Moreover, in many of their activities, CIT proteins act more as upstream regulators than as downstream effectors of RhoA. Finally it has been found that, besides working through actin, CIT proteins have many crucial functional interactions with the microtubule cytoskeleton and may directly affect genome stability. In this review, we will summarize these advances and illustrate their actual or potential relevance for CNS diseases, including microcephaly and psychiatric disorders.

Two mechanisms coordinate the recruitment of the chromosomal passenger complex to the plane of cell division

Proper positioning of the chromosomal passenger complex (CPC) at the cell division plane is required for cytokinesis. We show here that CPC targeting to the equatorial cortex depends on both the kinesin MKlp2 and a direct interaction with actin. These recruitment mechanisms converge to promote successful cleavage furrow ingression.

During cytokinesis, the chromosomal passenger complex (CPC) promotes midzone organization, specifies the cleavage plane, and regulates furrow contractility. The localizations of the CPC are coupled to its cytokinetic functions. At the metaphase-to-anaphase transition, the CPC dissociates from centromeres and localizes to midzone microtubules and the equatorial cortex. CPC relocalization to the cell middle is thought to depend on MKlp2-driven, plus end–directed transport. In support of this idea, MKlp2 depletion impairs cytokinesis; however, cytokinesis failure stems from furrow regression rather than failed initiation of furrowing. This suggests that an alternative mechanism(s) may concentrate the CPC at the division plane. We show here that direct actin binding, via the inner centromere protein (INCENP), enhances CPC enrichment at the equatorial cortex, thus acting in tandem with MKlp2. INCENP overexpression rescues furrowing in MKlp2-depleted cells in an INCENP-actin binding–dependent manner. Using live-cell imaging, we also find that MKlp2-dependent targeting of the CPC is biphasic. MKlp2 targets the CPC to the anti-parallel microtubule overlap of the midzone, after which the MKlp2-CPC complex moves in a nondirected manner. Collectively, our work suggests that both actin binding and MKlp2-dependent midzone targeting cooperate to precisely position the CPC during mitotic exit, and that these pathways converge to ensure successful cleavage furrow ingression.