DdNek2, the first non-vertebrate homologue of human Nek2, is involved in the formation of microtubule-organizing centers

Dynamic Mitotic Localization of the Centrosomal Kinases CDK1, Plk, AurK, and Nek2 in Dictyostelium amoebae

The centrosome of the amoebozoan model Dictyostelium discoideum provides the best-established model for an acentriolar centrosome outside the Opisthokonta. Dictyostelium exhibits an unusual centrosome cycle, in which duplication is initiated only at the G2/M transition and occurs entirely during the M phase. Little is known about the role of conserved centrosomal kinases in this process. Therefore, we have generated knock-in strains for Aurora (AurK), CDK1, cyclin B, Nek2, and Plk, replacing the endogenous genes with constructs expressing the respective green fluorescent Neon fusion proteins, driven by the endogenous promoters, and studied their behavior in living cells. Our results show that CDK1 and cyclin B arrive at the centrosome first, already during G2, followed by Plk, Nek2, and AurK. Furthermore, CDK1/cyclin B and AurK were dynamically localized at kinetochores, and AurK in addition at nucleoli. The putative roles of all four kinases in centrosome duplication, mitosis, cytokinesis, and nucleolar dynamics are discussed.

Nek2 Kinase Signaling in Malaria, Bone, Immune and Kidney Disorders to Metastatic Cancers and Drug Resistance: Progress on Nek2 Inhibitor Development

Cell cycle kinases represent an important component of the cell machinery that controls signal transduction involved in cell proliferation, growth, and differentiation. Nek2 is a mitotic Ser/Thr kinase that localizes predominantly to centrosomes and kinetochores and orchestrates centrosome disjunction and faithful chromosomal segregation. Its activity is tightly regulated during the cell cycle with the help of other kinases and phosphatases and via proteasomal degradation. Increased levels of Nek2 kinase can promote centrosome amplification (CA), mitotic defects, chromosome instability (CIN), tumor growth, and cancer metastasis. While it remains a highly attractive target for the development of anti-cancer therapeutics, several new roles of the Nek2 enzyme have recently emerged: these include drug resistance, bone, ciliopathies, immune and kidney diseases, and parasitic diseases such as malaria. Therefore, Nek2 is at the interface of multiple cellular processes and can influence numerous cellular signaling networks. Herein, we provide a critical overview of Nek2 kinase biology and discuss the signaling roles it plays in both normal and diseased human physiology. While the majority of research efforts over the last two decades have focused on the roles of Nek2 kinase in tumor development and cancer metastasis, the signaling mechanisms involving the key players associated with several other notable human diseases are highlighted here. We summarize the efforts made so far to develop Nek2 inhibitory small molecules, illustrate their action modalities, and provide our opinion on the future of Nek2-targeted therapeutics. It is anticipated that the functional inhibition of Nek2 kinase will be a key strategy going forward in drug development, with applications across multiple human diseases.

The Dictyostelium Centrosome

The centrosome of Dictyostelium amoebae contains no centrioles and consists of a cylindrical layered core structure surrounded by a corona harboring microtubule-nucleating γ-tubulin complexes. It is the major centrosomal model beyond animals and yeasts. Proteomics, protein interaction studies by BioID and superresolution microscopy methods led to considerable progress in our understanding of the composition, structure and function of this centrosome type. We discuss all currently known components of the Dictyostelium centrosome in comparison to other centrosomes of animals and yeasts.

Cep192, a Novel Missing Link between the Centrosomal Core and Corona in Dictyostelium Amoebae

The Dictyostelium centrosome is a nucleus-associated body with a diameter of approx. 500 nm. It contains no centrioles but consists of a cylindrical layered core structure surrounded by a microtubule-nucleating corona. At the onset of mitosis, the corona disassembles and the core structure duplicates through growth, splitting, and reorganization of the outer core layers. During the last decades our research group has characterized the majority of the 42 known centrosomal proteins. In this work we focus on the conserved, previously uncharacterized Cep192 protein. We use superresolution expansion microscopy (ExM) to show that Cep192 is a component of the outer core layers. Furthermore, ExM with centrosomal marker proteins nicely mirrored all ultrastructurally known centrosomal substructures. Furthermore, we improved the proximity-dependent biotin identification assay (BioID) by adapting the biotinylase BioID2 for expression in Dictyostelium and applying a knock-in strategy for the expression of BioID2-tagged centrosomal fusion proteins. Thus, we were able to identify various centrosomal Cep192 interaction partners, including CDK5RAP2, which was previously allocated to the inner corona structure, and several core components. Studies employing overexpression of GFP-Cep192 as well as depletion of endogenous Cep192 revealed that Cep192 is a key protein for the recruitment of corona components during centrosome biogenesis and is required to maintain a stable corona structure.

CDK5RAP2 Is an Essential Scaffolding Protein of the Corona of the Dictyostelium Centrosome

Dictyostelium centrosomes consist of a nucleus-associated cylindrical, three-layered core structure surrounded by a corona consisting of microtubule-nucleation complexes embedded in a scaffold of large coiled-coil proteins. One of them is the conserved CDK5RAP2 protein. Here we focus on the role of Dictyostelium CDK5RAP2 for maintenance of centrosome integrity, its interaction partners and its dynamic behavior during interphase and mitosis. GFP-CDK5RAP2 is present at the centrosome during the entire cell cycle except from a short period during prophase, correlating with the normal dissociation of the corona at this stage. RNAi depletion of CDK5RAP2 results in complete disorganization of centrosomes and microtubules suggesting that CDK5RAP2 is required for organization of the corona and its association to the core structure. This is in line with the observation that overexpressed GFP-CDK5RAP2 elicited supernumerary cytosolic MTOCs. The phenotype of CDK5RAP2 depletion was very reminiscent of that observed upon depletion of CP148, another scaffolding protein of the corona. BioID interaction assays revealed an interaction of CDK5RAP2 not only with the corona markers CP148, γ-tubulin, and CP248, but also with the core components Cep192, CP75, and CP91. Furthermore, protein localization studies in both depletion strains revealed that CP148 and CDK5RAP2 cooperate in corona organization.

Overexpression of NIMA-related kinase 2 is associated with poor prognoses in malignant glioma

Eleated expression of NIMA-related kinase 2 (NEK2) was frequently observed in a variety of malignant cancers, and it appears to be involved in the initiation, maintenance, progression, metastasis of cancer and is positively associated with poor prognosis. We sought to investigate NEK2 expression and its predictive roles in malignant gliomas, and study the correlation of NEK2 protein expression with proliferation, clinical parameters, overall survival and some other parameters. We investigate NEK2 protein expression in 99 samples of malignant gliomas, including 35 WHO grade II, 22 grade III, and 42 grade IV gliomas, by immunohistochemistry and western blot (n = 50). We then made correlative analysis of protein overexpression using the Kaplan-Meier method, Log rank test, and Cox proportional-hazards model analysis. NEK2 protein was overexpressed in malignant gliomas, but not in normal brain tissues. Overexpression of NEK2 correlated with malignancy, proliferation and adverse overall survival in gliomas. Moreover, chemotherapy, resection extent and WHO grade also correlate with overall survival in gliomas. However, within WHO grade II glioma subgroup, NEK2 overexpression showed no impact on overall survival. The present study firstly reveals that NEK2 protein is widely overexpressed in gliomas. NEK2 overexpression correlates significantly with malignancy (WHO grades), proliferation (Ki-67) and prognosis in malignant gliomas. NEK2 is a potential gene therapy target and prognostic indicator.

CP39, CP75 and CP91 are major structural components of the Dictyostelium centrosome's core structure

The acentriolar Dictyostelium centrosome is a nucleus-associated body consisting of a core structure with three plaque-like layers, which are surrounded by a microtubule-nucleating corona. The core duplicates once per cell cycle at the G2/M transition, whereby its central layer disappears and the two outer layers form the mitotic spindle poles. Through proteomic analysis of isolated centrosomes, we have identified CP39 and CP75, two essential components of the core structure. Both proteins can be assigned to the central core layer as their centrosomal presence is correlated to the disappearance and reappearance of the central core layer in the course of centrosome duplication. Both proteins contain domains with centrosome-binding activity in their N- and C-terminal halves, whereby the respective N-terminal half is required for cell cycle-dependent regulation. CP39 is capable of self-interaction and GFP-CP39 overexpression elicited supernumerary microtubule-organizing centers and pre-centrosomal cytosolic clusters. Underexpression stopped cell growth and reversed the MTOC amplification phenotype. In contrast, in case of CP75 underexpression of the protein by RNAi treatment elicited supernumerary MTOCs. In addition, CP75RNAi affects correct chromosome segregation and causes co-depletion of CP39 and CP91, another central core layer component. CP39 and CP75 interact with each other directly in a yeast two-hybrid assay. Furthermore, CP39, CP75 and CP91 mutually interact in a proximity-dependent biotin identification (BioID) assay. Our data indicate that these three proteins are all required for proper centrosome biogenesis and make up the major structural components of core structure's central layer.

NEK2 serves as a prognostic biomarker for hepatocellular carcinoma

Never in mitosis gene A (NIMA)-related kinase 2 (NEK2) is a microtubule-associated protein that regulates spindle assembly in human cells and is overexpressed in various malignancies. However, the role of NEK2 in hepatocellular carcinoma (HCC) remains undetermined. We performed RNA-seq of the HCC cell line SMMC-7721 and the normal liver cell line HL-7702 using the Ion Proton System. NEK2 expression was detected using quantitative reverse transcription polymerase chain reaction in two cell lines and 5 matched HCC and adjacent non-tumorous liver tissues. The correlation between survival and NEK2 expression was analyzed in 359 patients with HCC using RNASeqV2 data available from The Cancer Genome Atlas (TCGA) website (https://tcga-data.nci.nih.gov/tcga/). The expression of NEK2, phospho-AKT and MMP-2 was evaluated by immunohistochemistry in 63 cases of HCC and matched adjacent non-tumorous liver tissues. Relationships between protein expression and clinicopathological parameters were assessed, and the correlations between NEK2 with phospho-AKT and MMP-2 expressions were evaluated. A total of 610 differentially expressed genes (DEGs) were revealed in the transcriptome comparison, 297 of which were upregulated and 313 were downregulated in HCC. NEK2, as the most obviously different DEG in cells and tissues from the RNA-seq data, was listed as an HCC candidate biomarker for further verification. NEK2 was overexpressed in HCC cells and tissues (P=0.002, P=0.013) and HCC patients with a high expression of NEK2 had a poor prognosis (P=0.0145). Clinical analysis indicated that the overexpression of NEK2 in HCC was significantly correlated with diolame complete (P<0.001), tumor nodule number (P=0.012) and recurrence (P=0.004). NEK2 expression was positively correlated with the expression of phospho-AKT (r=0.883, P<0.01) and MMP-2 (r=0.781, P<0.01). Overexpression of NEK2 was associated with clinicopathological characteristics and poor patient outcomes, suggesting that NEK2 serves as a prognostic biomarker for HCC. Alteration of NEK2 protein levels may contribute to invasion and metastasis of HCC, which may occur through activation of AKT signaling and promotion of MMP-2 expression.

CP91 is a component of the Dictyostelium centrosome involved in centrosome biogenesis

The Dictyostelium centrosome is a model for acentriolar centrosomes and it consists of a three-layered core structure surrounded by a corona harboring microtubule nucleation complexes. Its core structure duplicates once per cell cycle at the G2/M transition. Through proteomic analysis of isolated centrosomes we have identified CP91, a 91-kDa coiled coil protein that was localized at the centrosomal core structure. While GFP-CP91 showed almost no mobility in FRAP experiments during interphase, both GFP-CP91 and endogenous CP91 dissociated during mitosis and were absent from spindle poles from late prophase to anaphase. Since this behavior correlates with the disappearance of the central layer upon centrosome duplication, CP91 is a putative component of this layer. When expressed as GFP-fusions, CP91 fragments corresponding to the central coiled coil domain and the preceding N-terminal part (GFP-CP91cc and GFP-CP91N, respectively) also localized to the centrosome but did not show the mitotic redistribution of the full length protein suggesting a regulatory role of the C-terminal domain. Expression of all GFP-fusion proteins suppressed expression of endogenous CP91 and elicited supernumerary centrosomes. This was also very prominent upon depletion of CP91 by RNAi. Additionally, CP91-RNAi cells exhibited heavily increased ploidy due to severe defects in chromosome segregation along with increased cell size and defects in the abscission process during cytokinesis. Our results indicate that CP91 is a central centrosomal core component required for centrosomal integrity, proper centrosome biogenesis and, independently, for abscission during cytokinesis.

Epigallocatechin gallate hinders human hepatoma and colon cancer sphere formation

BACKGROUND AND AIM

The long-term survival of patients with hepatocellular carcinoma remains unsatisfactory because of the presence of cancer stem cells (CSCs), which are responsible for tumor recurrence and chemoresistance after hepatectomy. Drugs that selectively target CSCs thus offer great promise for cancer treatment. Although the antitumor effects of epigallocatechin gallate (EGCG) have been reported in some cancer cells, its effects on CSCs remain poorly studied. In this study, we investigated the effects of EGCG on human hepatoma and colon CSCs.

METHODS

HepG2 and HCT-116 cell lines were enriched by sphere formation, and their gene-expression profiles were analyzed by quantitative real-time polymerase chain reaction. EGCG-induced growth inhibition in the parental cells was determined by WST-8 assay, and protein expression was assessed by western blotting. Cell cycle profile and apoptosis analysis was performed using flow cytometer.

RESULTS

Sphere-derived cells grown in serum-free, nonadherent cultures showed increased expression of stem cell markers, Nek2, and ATP-binding cassette transporter genes, compared with parental cells grown in conventional culture. EGCG induced growth inhibition in the parental cells in a dose-dependent manner. EGCG also inhibited self-renewal in hepatoma and colon CSCs, attenuated the expression of stem cell markers and ATP-binding cassette transporter genes, which are putative molecules associated with treatment resistance in CSCs, and decreased the transcription of Nek2 and p-Akt, resulting in the inhibition of Akt signaling. EGCG also altered cell cycle profile and apoptosis, which may in part play an important role in EGCG-induced cancer cell death.

CONCLUSIONS

Overall, these results suggest that EGCG could be a useful chemopreventive agent for targeting hepatocellular carcinoma and colon CSCs, in combination with standard chemotherapies.

Role of NEK2A in human cancer and its therapeutic potentials

Chromosome instability (CIN) has been identified as a common feature of most human cancers. A number of centrosomal kinases are thought to cause CIN in cancer cells. Part of those centrosomal kinases exhibit elevated expression in a wide variety of tumours and cancer cell lines. Additionally, critical roles in many aspects of cancer cell growth, proliferation, metastasis, and drug resistance have been assigned to some of these centrosomal kinases, such as polo-like kinase 1 (PLk1) and Aurora-A kinase. Recent studies from our group and others revealed that a centrosomal kinase, Never in Mitosis (NIMA) Related Kinase 2A (NEK2A), is frequently upregulated in multiple types of human cancers. Uncontrolled activity of NEK2A activates several oncogenic pathways and ABC transporters, thereby leading to CIN, cancer cell proliferation, metastasis, and enhanced drug resistance. In this paper, we highlight recent findings on the aberrant expression and functional significance of NEK2A in human cancers and emphasize their significance for therapeutic potentials.

Nucleocytoplasmic protein translocation during mitosis in the social amoebozoan Dictyostelium discoideum

Mitosis is a fundamental and essential life process. It underlies the duplication and survival of all cells and, as a result, all eukaryotic organisms. Since uncontrolled mitosis is a dreaded component of many cancers, a full understanding of the process is critical. Evolution has led to the existence of three types of mitosis: closed, open, and semi-open. The significance of these different mitotic species, how they can lead to a full understanding of the critical events that underlie the asexual duplication of all cells, and how they may generate new insights into controlling unregulated cell division remains to be determined. The eukaryotic microbe Dictyostelium discoideum has proved to be a valuable biomedical model organism. While it appears to utilize closed mitosis, a review of the literature suggests that it possesses a form of mitosis that lies in the middle between truly open and fully closed mitosis-it utilizes a form of semi-open mitosis. Here, the nucleocytoplasmic translocation patterns of the proteins that have been studied during mitosis in the social amoebozoan D. discoideum are detailed followed by a discussion of how some of them provide support for the hypothesis of semi-open mitosis.

CP55, a novel key component of centrosomal organization in Dictyostelium

Dictyostelium centrosomes consist of a layered core structure surrounded by a microtubule-nucleating corona. At the G2/M transition, the corona dissociates and the core structure duplicates, yielding two spindle pole bodies. Finally, in telophase, the spindle poles mature into two new, complete centrosomes. CP55 was identified in a centrosomal proteome analysis. It is a component of the centrosomal core structure, and persists at the centrosome throughout the entire cell cycle. FRAP experiments revealed that during interphase the majority of centrosomal GFP-CP55 is immobile, which indicates a structural task of CP55 at the centrosome. The CP55null mutant is characterized by increased ploidy, a less structured, slightly enlarged corona, and by supernumerary, cytosolic MTOCs, containing only corona proteins and lacking a core structure. Live cell imaging showed that supernumerary MTOCs arise in telophase. Lack of CP55 also caused premature recruitment of the corona organizer CP148 to mitotic spindle poles, already in metaphase instead of telophase. Forces transmitted through astral microtubules may expel prematurely acquired or loosely attached corona fragments into the cytosol, where they act as independent MTOCs. CP55null cells were also impaired in growth, most probably due to difficulties in centrosome splitting during prophase. Furthermore, although they were still capable of phagocytosis, they appeared unable to utilize phagocytosed nutrients. This inability may be attributed to their partially disorganized Golgi apparatus.

CP250, a novel acidic coiled-coil protein of the Dictyostelium centrosome, affects growth, chemotaxis, and the nuclear envelope

The Dictyostelium centrosome is a nucleus associated body consisting of a box-shaped core surrounded by the corona, an amorphous matrix functionally equivalent to the pericentriolar material of animal centrosomes which is responsible for the nucleation and anchoring of microtubules. Here we describe CP250 a component of the corona, an acidic coiled coil protein that is present at the centrosome throughout interphase while disappearing during prophase and reappearing at the end of late telophase. Amino acids 756-1148 of the 2110 amino acids are sufficient for centrosomal targeting and cell cycle-dependent centrosome association. Mutant cells lacking CP250 are smaller in size, growth on bacteria is delayed, chemotaxis is altered, and development is affected, which, in general, are defects observed in cytoskeletal mutants. Furthermore, loss of CP250 affected the nuclear envelope and led to reduced amounts and altered distribution of Sun-1, a conserved nuclear envelope protein that connects the centrosome to chromatin.

The NIMA-family kinase Nek3 regulates microtubule acetylation in neurons

NIMA-related kinases (Neks) belong to a large family of Ser/Thr kinases that have critical roles in coordinating microtubule dynamics during ciliogenesis and mitotic progression. The Nek kinases are also expressed in neurons, whose axonal projections are, similarly to cilia, microtubule-abundant structures that extend from the cell body. We therefore investigated whether Nek kinases have additional, non-mitotic roles in neurons. We found that Nek3 influences neuronal morphogenesis and polarity through effects on microtubules. Nek3 is expressed in the cytoplasm and axons of neurons and is phosphorylated at Thr475 located in the C-terminal PEST domain, which regulates its catalytic activity. Although exogenous expression of wild-type or phosphomimic (T475D) Nek3 in cultured neurons has no discernible impact, expression of a phospho-defective mutant (T475A) or PEST-truncated Nek3 leads to distorted neuronal morphology with disturbed polarity and deacetylation of microtubules via HDAC6 in its kinase-dependent manner. Thus, the phosphorylation at Thr475 serves as a regulatory switch that alters Nek3 function. The deacetylation of microtubules in neurons by unphosphorylated Nek3 raises the possibility that it could have a role in disorders where axonal degeneration is an important component.

Proteome analysis of Legionella vacuoles purified by magnetic immunoseparation reveals secretory and endosomal GTPases

Legionella pneumophila, the causative agent of Legionnaires' disease, replicates in macrophages and amoebae within 'Legionella-containing vacuoles' (LCVs), which communicate with the early secretory pathway and the endoplasmic reticulum. Formation of LCVs requires the bacterial Icm/Dot type IV secretion system. The Icm/Dot-translocated effector protein SidC selectively anchors to LCVs by binding the host lipid phosphatidylinositol-4-phosphate (PtdIns(4)P). Here, we describe a novel and simple approach to purify intact vacuoles formed by L. pneumophila within Dictyostelium discoideum by using magnetic immunoseparation with an antibody against SidC, followed by density gradient centrifugation. To monitor LCV purification by fluorescence microscopy, we used Dictyostelium producing the LCV marker calnexin-GFP and L. pneumophila labeled with the red fluorescent protein DsRed. A proteome analysis of purified LCVs by liquid chromatography coupled to tandem mass spectrometry revealed 566 host proteins, including known LCV components, such as the small GTPases Arf1, Rab1 and Rab7. Rab8, an endosomal regulator of the late secretory pathway originating from the trans Golgi network, and the endosomal GTPase Rab14 were identified as novel LCV components, which were found to be present on vacuoles harboring wild-type but not Icm/Dot-deficient L. pneumophila. Thus, LCVs also communicate with the late secretory and endosomal pathways. Depletion of Rab8 or Arf1 by RNA interference reduced the amount of SidC on LCVs, indicating that the GTPases promote the recruitment of Legionella effectors by regulating the level of PtdIns(4)P.

Mitotic regulation by NIMA-related kinases

The NIMA-related kinases represent a family of serine/threonine kinases implicated in cell cycle control. The founding member of this family, the NIMA kinase of Aspergillus nidulans, as well as the fission yeast homologue Fin1, contribute to multiple aspects of mitotic progression including the timing of mitotic entry, chromatin condensation, spindle organization and cytokinesis. Mammals contain a large family of eleven NIMA-related kinases, named Nek1 to Nek11. Of these, there is now substantial evidence that Nek2, Nek6, Nek7 and Nek9 also regulate mitotic events. At least three of these kinases, as well as NIMA and Fin1, have been localized to the microtubule organizing centre of their respective species, namely the centrosome or spindle pole body. Here, they have important functions in microtubule organization and mitotic spindle assembly. Other Nek kinases have been proposed to play microtubule-dependent roles in non-dividing cells, most notably in regulating the axonemal microtubules of cilia and flagella. In this review, we discuss the evidence that NIMA-related kinases make a significant contribution to the orchestration of mitotic progression and thereby protect cells from chromosome instability. Furthermore, we highlight their potential as novel chemotherapeutic targets.

Structure and regulation of the human Nek2 centrosomal kinase

The dimeric Ser/Thr kinase Nek2 regulates centrosome cohesion and separation through phosphorylation of structural components of the centrosome, and aberrant regulation of Nek2 activity can lead to aneuploid defects characteristic of cancer cells. Mutational analysis of autophosphorylation sites within the kinase domain identified by mass spectrometry shows a complex pattern of positive and negative regulatory effects on kinase activity that are correlated with effects on centrosomal splitting efficiency in vivo. The 2.2-A resolution x-ray structure of the Nek2 kinase domain in complex with a pyrrole-indolinone inhibitor reveals an inhibitory helical motif within the activation loop. This helix presents a steric barrier to formation of the active enzyme and generates a surface that may be exploitable in the design of specific inhibitors that selectively target the inactive state. Comparison of this "auto-inhibitory" conformation with similar arrangements in cyclin-dependent kinase 2 and epidermal growth factor receptor kinase suggests a role for dimerization-dependent allosteric regulation that combines with autophosphorylation and protein phosphatase 1c phosphatase activity to generate the precise spatial and temporal control required for Nek2 function in centrosomal maturation.

Towards a molecular understanding of human diseases using Dictyostelium discoideum

The social amoeba Dictyostelium discoideum is increasingly being used as a simple model for the investigation of problems that are relevant to human health. This article focuses on several recent examples of Dictyostelium-based biomedical research, including the analysis of immune-cell disease and chemotaxis, centrosomal abnormalities and lissencephaly, bacterial intracellular pathogenesis, and mechanisms of neuroprotective and anti-cancer drug action. The combination of cellular, genetic and molecular biology techniques that are available in Dictyostelium often makes the analysis of these problems more amenable to study in this system than in mammalian cell culture. Findings that have been made in these areas using Dictyostelium have driven research in mammalian systems and have established Dictyostelium as a powerful model for human-disease analysis.

NIMA-related kinase TbNRKC is involved in basal body separation in Trypanosoma brucei

The NIMA-related kinase 2 (NEK 2) has important cell cycle functions related to centriole integrity and splitting. Trypanosoma brucei does not possess centrioles, however, cytokinesis is coupled to basal body separation events. Here we report the first functional characterisation of a T. brucei basal body-cytoskeletal NIMA-related kinase (NRK) protein, TbNRKC. The TbNRKC kinase domain has high amino acid identity with the human NEK1 kinase domain (50%) but also shares 42% identity with human NEK2. TbNRKC is expressed in bloodstream and procyclic cells and functions as a bona fide kinase in vitro. Remarkably, RNAi knockdown of TbNRKC and overexpression of kinase-dead TbNRKC in procyclic forms induces the accumulation of cells with four basal bodies, whereas overexpression of active protein produces supernumary basal bodies and blocks cytokinesis. TbNRKC is located on mature and immature basal bodies and is the first T. brucei NRK to be found associated with the basal body cytokinesis pathway.

Caught Nek-ing: cilia and centrioles

The Nek family of cell-cycle kinases is widely represented in eukaryotes and includes numerous proteins that were described only recently and remain poorly characterized. Comparing Neks in the context of clades allows us to examine the question of whether microbial eukaryotic Neks, although not strictly orthologs of their vertebrate counterparts, can provide clues to ancestral functions that might be retained in the vertebrate Neks. Relatives of the Nek2/NIMA proteins play important roles at the G2-M transition in nuclear envelope breakdown and centromere separation. Nek6, Nek7 and Nek9 also seem to regulate mitosis. By contrast, Nek1 and Nek8 have been linked with polycystic kidney disease. Results of statistical analysis indicate that the family coevolved with centrioles that function as both microtubule-organizing centers and the basal bodies of cilia. This evolutionary perspective, taken together with functional studies of microbial Neks, provides new insights into the cellular roles of the proteins and disease with which some of them have been linked.

Characterization of a poplar NIMA-related kinase PNek1 and its potential role in meristematic activity

Meristems are sites of undifferentiated cell division, which carry on developing into functional organs. Using the two-hybrid system with a poplar 14-3-3, we uncovered poplar NIMA-related kinase 1 (PNek1) as an interacting protein. PNek1 shows high homology to the mammalian NIMA-related kinases, which are thought to be involved in cell cycle progression. Using a synchronized poplar cell suspension, we observed an accumulation of PNek1 mRNA at the G1/S transition and throughout the G2-to-M progression. Moreover, PNek1-GFP fusion protein localized in the cytoplasm and in both the nuclear and nucleolar regions. Overexpression of PNek1-GFP in Arabidopsis caused morphological abnormalities in flower and siliques. Overall, these results suggest that PNek1 is involved in plant development.

Nek2 kinase in chromosome instability and cancer

Aneuploidy and chromosome instability are two of the most common abnormalities in cancer cells. They arise through defects in cell division and, specifically, in the unequal segregation of chromosomes between daughter cells during mitosis. A number of cell cycle dependent protein kinases have been identified that control mitotic progression and chromosome segregation. Some of these localize to the centrosome and regulate mitotic spindle formation. One such protein is Nek2, a member of the NIMA-related serine/threonine kinase family. Data are emerging that Nek2 is abnormally expressed in a wide variety of human cancers. In this review, we summarize current knowledge on the expression, regulation and function of Nek2, consider how Nek2 may contribute to chromosome instability, and ask whether it might make an attractive target for chemotherapeutic intervention in human cancer.

Identification and sequence analysis of six new members of the NIMA-related kinase family in Chlamydomonas

The NIMA kinases are an evolutionarily conserved protein family with enigmatic roles in the regulation of mitosis. We report six new members of this family in Chlamydomonas, in addition to the previously identified NIMA-related kinase, Fa2p. Chlamydomonas NIMA-related kinases (CNKs) 1-6 were sequenced from subclones generated by RT-PCR using information from EST libraries and the recently sequenced Chlamydomonas genome. Phylogenetic and bioinformatic approaches were used to determine the relationships of the six new members with known members of the NIMA-related kinase family. Although humans express at least eleven NIMA-related kinases, the eukaryotic microbes that have been studied to date express only one or two members of the family. Thus, the discovery that Chlamydomonas expresses a total of at least seven NIMA-related kinases is intriguing. Our analyses suggest that members of this family may play roles in the assembly and function of cilia.

Nucleolar Nek11 is a novel target of Nek2A in G1/S-arrested cells

We previously reported that Nek11, a member of the NIMA (never-in-mitosis A) family of kinases, is activated in G(1)/S-arrested cells. We provide herein several lines of evidence for a novel interaction between Nek11 and Nek2A. Both Nek11 and Nek2A, but not Nek2B, were detected at nucleoli, and the Nek2A-specific C-terminal end (amino acids 399-445) was responsible for nucleolar localization. Endogenous Nek11 coimmunoprecipitated with endogenous Nek2A, and non-catalytic regions of each kinase were involved in the complex formation. Nek11L interacted with phosphorylated Nek2A but barely with the kinase-inactive Nek2A (K37R) mutant. In addition, both Nek2A autophosphorylation activity and the Nek11L-Nek2A complex formation increased in G(1)/S-arrested cells. These results indicate that autophosphorylation of Nek2A could stimulate its interaction with Nek11L at the nucleolus. Moreover, Nek2 directly phosphorylated Nek11 in the C-terminal non-catalytic region and elevated Nek11 kinase activity. The non-catalytic region of Nek11 showed autoinhibitory activity through intramolecular interaction with its N-terminal catalytic domain. Nek2 dissociated this autoinhibitory interaction. Altogether, our studies demonstrate a unique mechanism of Nek11 activation by Nek2A in G(1)/S-arrested cells and suggest a novel possibility for nucleolar function of the NIMA family.

Molecular and functional analysis of the dictyostelium centrosome

The centrosome is a nonmembranous, nucleus-associated organelle that functions not only as the main microtubule-organizing center but also as a cell cycle control unit. How the approximately 100 different proteins that make up a centrosome contribute to centrosome function is still largely unknown. Considerable progress in the understanding of centrosomal functions can be expected from comparative cell biology of morphologically different centrosomal structures fulfilling conserved functions. Dictyostelium is an alternative model organism for centrosome research in addition to yeast and animal cells. With the elucidation of morphological changes and dynamics of centrosome duplication, the establishment of a centrosome isolation protocol, and the identification of many centrosomal components, there is a solid basis for understanding the biogenesis and function of this fascinating organelle. Here we give an overview of the prospective protein inventory of the Dictyostelium centrosome based on database searches. Moreover, we focus on the comparative cell biology of known components of the Dictyostelium centrosome including the gamma-tubulin complex and the homologues of centrin, Nek2, XMAP215, and EB1.

Never say never. The NIMA-related protein kinases in mitotic control

Mitosis sees a massive reorganization of cellular architecture. The microtubule cytoskeleton is reorganized to form a bipolar spindle between duplicated microtubule organizing centers, the chromosomes are condensed, attached to the spindle at their kinetochores, and, through the action of multiple molecular motors, the chromosomes are segregated into two daughter cells. Mitosis also sees a substantial wave of protein phosphorylation, controlling signaling events that coordinate mitotic processes and ensure accurate chromosome segregation. The key switch for the onset of mitosis is the archetypal cyclin-dependent kinase, Cdc2. Under the direction of Cdc2 is an executive of protein serine/threonine kinases that fall into three families: the Polo kinases, Aurora kinases and the NIMA-related kinases (Nrk). The latter family has proven the most enigmatic in function, although recent advances from several sources are beginning to reveal a common functional theme.

Identification of CfNek, a novel member of the NIMA family of cell cycle regulators, as a polypeptide copurifying with tubulin polyglutamylation activity in Crithidia

Post-translational glutamylation of tubulin plays an important role in regulating the interaction between microtubules and associated proteins, but so far the enzymes involved in this process have not been cloned from any cellular source. Using a modified purification scheme that employs a hydroxyapaptite chromatography as the final step we identified a 54 kDa band as the major polypeptide copurifying with tubulin polyglutamylation activity from the trypanosomatid Crithidia fasciculata. Based on peptide sequence information we have cloned the corresponding cDNA and identify Crithidia p54 as a novel member (termed CfNek) of the NIMA family of putative cell cycle regulators. CfNek is a protein of 479 amino acids that contains an unusual protein kinase domain that lacks the glycine-rich loop in subdomain I. The protein also harbours a PEST sequence and a pleckstrin homology domain. The tubulin polyglutamylase preparation displays the beta-casein phosphorylation activity typical for NIMA related kinases. Recombinant His-tagged CfNek expressed in Crithidia localises to the flagellar attachment zone/basal body of the parasite. After purification on a Ni(2+)-column the recombinant enzyme preparation displays ATP-dependent tubulin polyglutamylation activity as well as casein-phosphorylation activity.

The Nek2 protein kinase: a novel regulator of centrosome structure

Regulation of the centrosome, the major microtubule organizing centre in an animal cell, is in large part controlled by cell cycle-dependent protein phosphorylation. Along with cyclin dependent kinases, polo kinases and Aurora kinases, NIMA-related kinases are emerging as critical regulators of centrosome structure and function. Nek2 is the most closely related vertebrate protein by sequence to the essential mitotic regulator NIMA of Aspergillus nidulans. Nek2 is highly enriched at the centrosome and functional studies in human and Xenopus systems support a role for Nek2 in both maintenance and modulation of centrosome architecture. In particular, current evidence supports a model in which one function of Nek2 kinase activity is to promote the splitting of duplicated centrosomes at the onset of mitosis through phosphorylation of core centriolar proteins. Recent studies in lower organisms have raised the possibility that kinases related to Nek2 may have conserved functions in MTOC organization, as well as in other aspects of mitotic progression.