The in vivo expression pattern of mouse Nek2, a NIMA-related kinase, indicates a role in both mitosis and meiosis

A Single-Cell Landscape of Spermioteleosis in Mice and Pigs

(1) Background: Spermatozoa acquired motility and matured in epididymis after production in the testis. However, there is still limited understanding of the specific characteristics of sperm development across different species. In this study, we employed a comprehensive approach to analyze cell compositions in both testicular and epididymal tissues, providing valuable insights into the changes occurring during meiosis and spermiogenesis in mouse and pig models. Additionally, we identified distinct gene expression signatures associated with various spermatogenic cell types. (2) Methods: To investigate the differences in spermatogenesis between mice and pigs, we constructed a single-cell RNA dataset. (3) Results: Our findings revealed notable differences in testicular cell clusters between these two species. Furthermore, distinct gene expression patterns were observed among epithelial cells from different regions of the epididymis. Interestingly, regional gene expression patterns were also identified within principal cell clusters of the mouse epididymis. Moreover, through analysing differentially expressed genes related to the epididymis in both mouse and pig models, we successfully identified potential marker genes associated with sperm development and maturation for each species studied. (4) Conclusions: This research presented a comprehensive single-cell landscape analysis of both testicular and epididymal tissues, shedding light on the intricate processes involved in spermatogenesis and sperm maturation, specifically within mouse and pig models.

Differential Expression of NEK Kinase Family Members in Esophageal Adenocarcinoma and Barrett's Esophagus

The incidence of esophageal adenocarcinoma (EAC) has risen rapidly during the past four decades, making it the most common type of esophageal cancer in the USA and Western countries. The NEK (Never in mitosis A (NIMA) related kinase) gene family is a group of serine/threonine kinases with 11 members. Aberrant expression of NEKs has been recently found in a variety of human cancers and plays important roles in tumorigenesis, progression, and drug-resistance. However, the expression of the NEKs in EAC and its precancerous condition (Barrett's esophagus, BE) has not been investigated. In the present study, we first analyzed the TCGA and 9 GEO databases (a total of 10 databases in which 8 contain EAC and 6 contain BE) using bioinformatic approaches for NEKs expression in EAC and BE. We identified that several NEK members, such as NEK2 (7/8), NEK3 (6/8), and NEK6 (6/8), were significantly upregulated in EAC as compared to normal esophagus samples. Alternatively, NEK1 was downregulated in EAC as compared to the normal esophagus. On the contrary, genomic alterations of these NEKs are not frequent in EAC. We validated the above findings using qRT-PCR and the protein expression of NEKs in EAC cell lines using Western blotting and in primary EAC tissues using immunohistochemistry and immunofluorescence. Our data suggest that frequent upregulation of NEK2, NEK3, and NEK7 may be important in EAC.

Never in Mitosis Kinase 2 regulation of metabolism is required for neural differentiation

Wnt and Hh are known signalling pathways involved in neural differentiation and recent work has shown the cell cycle regulator, Never in Mitosis Kinase 2 (Nek2) is able to regulate both pathways. Despite its known function in pathway regulation, few studies have explored Nek2 within embryonic development. The P19 embryonal carcinoma cell model was used to investigate Nek2 and neural differentiation through CRISPR knockout and overexpression studies. Loss of Nek2 reduced cell proliferation in the undifferentiated state and during directed differentiation, while overexpression increased cell proliferation. Despite these changes in proliferation rates, Nek2 deficient cells maintained pluripotency markers after neural induction while Nek2 overexpressing cells lost these markers in the undifferentiated state. Nek2 deficient cells lost the ability to differentiate into both neurons and astrocytes, although Nek2 overexpressing cells enhanced neuron differentiation at the expense of astrocytes. Hh and Wnt signalling were explored, however there was no clear connection between Nek2 and these pathways causing the observed changes to differentiation phenotypes. Mass spectrometry was also used during wildtype and Nek2 knockout cell differentiation and we identified reduced electron transport chain components in the knockout population. Immunoblotting confirmed the loss of these components and additional studies showed cells lacking Nek2 were exclusively glycolytic. Interestingly, hypoxia inducible factor 1α was stabilized in these Nek2 knockout cells despite culturing them under normoxic conditions. Since neural differentiation requires a metabolic switch from glycolysis to oxidative phosphorylation, we propose a mechanism where Nek2 prevents HIF1α stabilization, thereby allowing cells to use oxidative phosphorylation to facilitate neuron and astrocyte differentiation.

Comparative expression profiling of testis-enriched genes regulated during the development of spermatogonial cells

The testis has been identified as the organ in which a large number of tissue-enriched genes are present. However, a large portion of transcripts related to each stage or cell type in the testis still remains unknown. In this study, databases combined with confirmatory measurements were used to investigate testis-enriched genes, localization in the testis, developmental regulation, gene expression profiles of testicular disease, and signaling pathways. Our comparative analysis of GEO DataSets showed that 24 genes are predominantly expressed in testis. Cellular locations of 15 testis-enriched proteins in human testis have been identified and most of them were located in spermatocytes and round spermatids. Real-time PCR revealed that expressions of these 15 genes are significantly increased during testis development. Also, an analysis of GEO DataSets indicated that expressions of these 15 genes were significantly decreased in teratozoospermic patients and polyubiquitin knockout mice, suggesting their involvement in normal testis development. Pathway analysis revealed that most of those 15 genes are implicated in various sperm-related cell processes and disease conditions. This approach provides effective strategies for discovering novel testis-enriched genes and their expression patterns, paving the way for future characterization of their functions regarding infertility and providing new biomarkers for specific stages of spematogenesis.

Molecular cloning of Plk1 and Nek2 and their expression in mature gonads of the teleost fish Nile tilapia (Oreochromis niloticus)

Polo-like kinase 1 (Plk1) and NIMA-related kinase 2 (Nek2) are serine/threonine kinases that are involved in the G2/M phase transition of the cell cycle in vertebrates. Although they also play critical roles in the regulation of spermatogenesis and oogenesis, their characterization in meiosis has not been elucidated fully, particularly in teleost fish. To investigate the evolutionary significance of serine/threonine kinases in the reproductive system of fish, we cloned the cDNAs of Plk1 and Nek2 from a Nile tilapia (Oreochromis niloticus) testicular cDNA library. Tilapia Plk1 encodes a protein of 582 amino acids that shares 75% homology with human Plk1, while tilapia Nek2 encodes a putative protein of 446 amino acids that shares 70% homology with human Nek2. Analyses of tissue distribution by RT-PCR and Southern blotting revealed that Plk1 and Nek2 are strongly expressed in the ovary and testis. Northern blot analysis revealed two Nek2 transcripts in the ovary and testis with different expression patterns, which indicates the presence of two structural variants for tilapia Nek2. Moreover, the localization of Plk1 and Nek2 mRNAs in tilapia gonads was determined by in situ hybridization analysis. In the ovary, Plk1 and Nek2 were expressed predominantly in oocytes. In the testis, on the other hand, Plk1 was expressed in primary spermatocytes, while Nek2 was generally expressed in primary and secondary spermatocytes. These results suggest that Plk1 and Nek2 are key factors in the progression of meiosis in fish.

Alternative splicing controls nuclear translocation of the cell cycle-regulated Nek2 kinase

Nek2 is a cell cycle-regulated serine/threonine protein kinase that is up-regulated in human cancers. Functionally, it is implicated in control of centrosome separation and bipolar spindle formation in mitotic cells and chromatin condensation in meiotic cells. Two major splice variants have been described in vertebrates, Nek2A and Nek2B, that differ in their non-catalytic C termini. Recently, a third splice variant, Nek2C, was identified that lacks an eight-amino acid internal sequence within the C-terminal domain of Nek2A. This excision occurs at the same position as the Nek2A/Nek2B splice point. As predicted from their high degree of similarity, we show here that Nek2C shares many properties with Nek2A including kinase activity, dimerization, protein phosphatase 1 interaction, mitotic degradation, microtubule binding, and centrosome localization. Unexpectedly, though, the non-centrosomal pool of protein exhibits a marked difference in distribution for the three splice variants. Nek2C is mainly nuclear, Nek2B is mainly cytoplasmic, and Nek2A is evenly distributed within nuclei and cytoplasm. Mutagenesis experiments revealed a functional bipartite nuclear localization sequence (NLS) that spans the splice site leading to Nek2C having a strong NLS, Nek2A having a weak NLS, and Nek2B having no NLS. Finally, we identified a 28-kDa protein in nuclear extracts as a potential novel substrate of Nek2. Thus, alternative splicing provides an unusual mechanism for modulating Nek2 localization, enabling it to have both nuclear and cytoplasmic functions.

Cell cycle in mouse development

Mice likely represent the most-studied mammalian organism, except for humans. Genetic engineering in embryonic stem cells has allowed derivation of mouse strains lacking particular cell cycle proteins. Analyses of these mutant mice, and cells derived from them, facilitated the studies of the functions of cell cycle apparatus at the organismal and cellular levels. In this review, we give some background about the cell cycle progression during mouse development. We next discuss some insights about in vivo functions of the cell cycle proteins, gleaned from mouse knockout experiments. Our text is meant to provide examples of the recent experiments, rather than to supply an extensive and complete list.

Nek2B stimulates zygotic centrosome assembly in Xenopus laevis in a kinase-independent manner

Pronuclear migration and formation of the first mitotic spindle depend upon assembly of a functional zygotic centrosome. For most animals, this involves both paternal and maternal contributions as sperm basal bodies are converted into centrosomes competent for microtubule nucleation through recruitment of egg proteins. Nek2B is a vertebrate NIMA-related protein kinase required for centrosome assembly, as its depletion from egg extracts delays microtubule aster formation from sperm basal bodies. Using Xenopus as a model system, we now show that protein expression of Nek2B begins during mid-oogenesis and increases further upon oocyte maturation. This is regulated, at least in part, at the level of protein translation. Nek2B protein is weakly phosphorylated in mitotic egg extracts but its recruitment to the sperm basal body, which occurs independently of its kinase activity, stimulates its phosphorylation, possibly through sequestration from a phosphatase present in mitotic egg cytoplasm. Importantly, although Nek2B is not required to organize acentrosomal microtubule asters, we show that addition of either active or kinase-dead recombinant Nek2B can restore centrosome assembly in a dose-dependent manner to a depleted extract. These results support a model in which maternal Nek2B acts to promote assembly of a functional zygotic centrosome in a kinase-independent manner.

Phosphorylation of high-mobility group protein A2 by Nek2 kinase during the first meiotic division in mouse spermatocytes

The mitogen-activated protein kinase (MAPK) pathway is required for maintaining the chromatin condensed during the two meiotic divisions and to avoid a second round of DNA duplication. However, molecular targets of the MAPK pathway on chromatin have not yet been identified. Here, we show that the architectural chromatin protein HMGA2 is highly expressed in male meiotic cells. Furthermore, Nek2, a serine-threonine kinase activated by the MAPK pathway in mouse pachytene spermatocytes, directly interacts with HMGA2 in vitro and in mouse spermatocytes. The interaction does not depend on the activity of Nek2 and seems constitutive. On progression from pachytene to metaphase, Nek2 is activated and HMGA2 is phosphorylated in an MAPK-dependent manner. We also show that Nek2 phosphorylates in vitro HMGA2 and that this phosphorylation decreases the affinity of HMGA2 for DNA and might favor its release from the chromatin. Indeed, we find that most HMGA2 associates with chromatin in mouse pachytene spermatocytes, whereas it is excluded from the chromatin upon the G2/M progression. Because hmga2-/- mice are sterile and show a dramatic impairment of spermatogenesis, it is possible that the functional interaction between HMGA2 and Nek2 plays a crucial role in the correct process of chromatin condensation in meiosis.

Differential control of the NIMA-related kinases, Nek6 and Nek7, by serum stimulation

Neks (NIMA-related kinases) are mammalian serine/threonine (Ser/Thr) protein kinases structurally related to Aspergillus NIMA (Never in Mitosis, gene A), which plays essential roles in mitotic signaling. Among these kinases, Nek6 and Nek7 are structurally related and constitute a subfamily in the NIMA/Nek family, although their functions still remain almost elusive. In this report, we studied the enzymatic regulation of Nek6 and Nek7 to gain an insight into their cellular functions. Recombinant Nek7 produced in bacteria was active comparably to Nek6; however, the Nek7 activity in mammalian cells was found to be significantly lower than Nek6. Since Nek6 previously has been reported to in vitro phosphorylate p70 ribosomal S6 kinase at Thr412, we examined if Nek6 and Nek7 activities were controlled by the amino acid supplement, which is known to affect the phosphorylation at Thr412, and did not observe any significant effect. However, we unexpectedly found that Nek7 kinase activity was rapidly and efficiently increased by serum deprivation, while Nek6 activity was decreased. This is well consistent with the lower activity of Nek7 in cells under normal growth conditions. In addition, it was suggested that Nek7 activity would be regulated in a cell cycle-dependent manner, although Nek6 was not. These clear differences in enzymatic control between the highly similar kinases, Nek6 and Nek7, suggest their distinct signaling functions in mammalian cells.

Nek11, a new member of the NIMA family of kinases, involved in DNA replication and genotoxic stress responses

DNA replication and genotoxic stresses activate various checkpoint-associated protein kinases, and checkpoint dysfunction often leads to cell lethality. Here, we have identified new members of the mammalian NIMA family of kinases, termed Nek11L and Nek11S (NIMA-related kinase 11 Long and Short isoform) as novel DNA replication/damage stresses-responsive kinases. Molecular cloning and biochemical studies showed that the catalytic domain of Nek11 is most similar to Nek4 and Nek3, and substrate specificity of Nek11L is distinguishable from those of NIMA and Nek2. The expression of nek11L mRNA increased through S to G(2)/M phase, and subcellular localization of Nek11 protein altered between interphase and prometaphase, suggesting multiple roles of Nek11. We found an activation of Nek11 kinase activity when cells were treated with various DNA-damaging agents and replication inhibitors, and this activation of Nek11 was suppressed by caffeine in HeLaS3 cells. The transient expression of wild-type Nek11L enhanced the aphidicolin-induced S-phase arrest, whereas the aphidicolin-induced S-phase arrest was reduced in the U2OS cell lines expressing kinase-negative Nek11L (K61R), and these cells were more sensitive to aphidicolin-induced cell lethality. Collectively, these results suggest that Nek11 has a role in the S-phase checkpoint downstream of the caffeine-sensitive pathway.

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.

Identification and characterization of Nek6 protein kinase, a potential human homolog of NIMA histone H3 kinase

In Aspergillus nidulans, the kinase activity of NIMA (never in mitosis, gene A) is critical for the initiation of mitosis. NIMA regulates mitotic chromatin condensation through phosphorylation of histone H3 at serine 10. In the present study, we identified human Nek6 (hNek6), a member of the mammalian NIMA-related kinases. The predicted hNek6 protein is comprised of 338 amino acids. Northern blot analysis revealed that hNek6 transcripts are ubiquitously expressed with the highest expression found in the heart and skeletal muscle. Lower cell cycle-dependent expression of hNek6 transcripts was observed in the early G1 phase. GFP-fused hNek6 protein showed both nuclear and cytoplasmic localizations in HeLa cells. Fluorescence in situ hybridization using full-length hNek6 cDNA as a probe showed that the hNek6 gene is localized to human chromosome 9q33-34, a region at which the loss of heterozygosity is associated with transitional cell carcinomas. Importantly, recombinant hNek6 protein produced in insect cells effectively phosphorylated histones H1 and H3, but not casein. Thus, these results suggest that, unlike other mammalian NIMA-related kinases, Nek6 is a mitotic histone kinase which regulates chromatin condensation in mammalian cells.

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

Dictyostelium Nek2 (DdNek2) is the first structural and functional non-vertebrate homologue of human Nek2, a NIMA-related serine/threonine kinase required for centrosome splitting in early mitosis. DdNek2 shares 43% overall amino-acid identity with its human counterpart and 54% identity within the catalytic domain. Both proteins can be subdivided in an N-terminal catalytic domain, a leucine zipper and a C-terminal domain. Kinase assays with bacterially expressed DdNek2 and C-terminal deletion mutants revealed that catalytic activity requires the presence of the leucine zipper and that autophosphorylation occurs at the C-terminus. Microscopic analyses with DdNek2 antibodies and expression of a GFP-DdNek2 fusion protein in Dictyostelium showed that DdNek2 is a permanent centrosomal resident and suggested that it is a component of the centrosomal core. The GFP-DdNek2-overexpressing mutants frequently exhibit supernumerary microtubule-organizing centers (MTOCs). This phenotype did not require catalytic activity because it was also observed in cells expressing inactive GFP-K33R. However, it was shown to be caused by overexpression of the C-terminal domain since it also occurred in GFP-mutants expressing only the C-terminus or a leucine zipper/C-terminus construct but not in those mutants expressing only the catalytic domain or a catalytic domain/leucine zipper construct. These results suggest that DdNek2 is involved in the formation of MTOCs. Furthermore, the localization of the GFP-fusion proteins revealed two independent centrosomal targeting domains of DdNek2, one within the catalytic or leucine zipper domain and one in the C-terminal domain.

The MAPK pathway triggers activation of Nek2 during chromosome condensation in mouse spermatocytes

Chromosome condensation during the G2/M progression of mouse pachytene spermatocytes induced by the phosphatase inhibitor okadaic acid (OA) requires the activation of the MAPK Erk1. In many cell systems, p90Rsks are the main effectors of Erk1/2 function. We have identified p90Rsk2 as the isoform that is specifically expressed in mouse spermatocytes and have shown that it is activated during the OA-triggered meiotic G2/M progression. By using the MEK inhibitor U0126, we have demonstrated that activation of p90Rsk2 during meiotic progression requires activation of the MAPK pathway. Immunofluorescence analysis indicates that activated Erks and p90Rsk2 are tightly associated with condensed chromosomes during the G2/M transition in meiotic cells. We also found that active p90Rsk2 was able to phosphorylate histone H3 at Ser10 in vitro, but that the activation of the Erk1/p90Rsk2 pathway was not necessary for phosphorylation of H3 in vivo. Furthermore, phosphorylation of H3 was not sufficient to cause condensation of meiotic chromosomes in mouse spermatocytes. Other proteins known to associate with chromatin may represent effectors of Erk1 and p90Rsk2 during chromosome condensation. Nek2 (NIMA-related kinase 2), which associates with chromosomes, plays an active role in chromatin condensation and is stimulated by treatment of pachytene spermatocytes with okadaic acid. We show that inhibition of the MAPK pathway by preincubation of spermatocytes with U0126 suppresses Nek2 activation, and that incubation of spermatocyte cell extracts with activated p90Rsk2 causes stimulation of Nek2 kinase activity. Furthermore, we show that the Nek2 kinase domain is a substrate for p90Rsk2 phosphorylation in vitro. These data establish a connection between the Erk1/p90Rsk2 pathway, Nek2 activation and chromosome condensation during the G2/M transition of the first meiotic prophase.

Alternative splice variants of the human centrosome kinase Nek2 exhibit distinct patterns of expression in mitosis

Nek2 is a cell-cycle-regulated protein kinase that localizes to the centrosome and is likely to be involved in regulating centrosome structure at the G(2)/M transition. Here, we localize the functional human Nek2 gene to chromosome 1 and show that alternative polyadenylation signals provide a mechanism for generating two distinct isoforms. Sequencing of products generated by reverse transcriptase PCR, immunoblotting of cell extracts and transfection of antisense oligonucleotides together demonstrate that human Nek2 is expressed as two splice variants. These isoforms, designated Nek2A and Nek2B, are detected in primary blood lymphocytes as well as adult transformed cells. Nek2A and Nek2B, which can form homo- and hetero-dimers, both localize to the centrosome, although only Nek2A can induce centrosome splitting upon overexpression. Importantly, Nek2A and Nek2B exhibit distinct patterns of cell-cycle-dependent expression. Both are present in low amounts in the G(1) phase and exhibit increased abundance in the S and G(2) phases. However, Nek2A disappears in prometaphase-arrested cells, whereas Nek2B remains elevated. These results demonstrate that two alternative splice variants of the human centrosomal kinase Nek2 exist that differ in their expression patterns during mitosis. This has important implications for our understanding of both Nek2 protein kinase regulation and the control of centrosome structure during mitosis.

Changes in gene expression profiles in developing B cells of murine bone marrow

Gene expression profiles of five consecutive stages of mouse B cell development were generated with high-density oligonucleotide arrays from as few as 2 x 10(4) ex vivo isolated and flow-cytometrically purified cells. Between 2.8% and 6.8% of all genes change on differentiation from one cellular stage to the next by at least twofold. The entire pathway involves differential expression of 10.7% of all genes. Previously known expression patterns of 15 genes (like surrogate light chain, RAG-1/2, MHC class II, mel-14 antigen) are confirmed. The gene expression patterns of the proliferating pre-BI and large pre-BII cells on the one hand, and the resting immature and mature B cells on the other hand, are most similar to each other. Small pre-BII cells display a pattern that is transitional between these two groups. Most of the genes expressed in early precursors are involved in general processes, like protein folding or cell cycle regulation, whereas more mature precursors express genes involved in more specific molecular programs (cell surface receptors, secreted factors, and adhesion molecules, among others). Between 19 and 139 genes share a given expression pattern. Combining knowledge about gene function and expression pattern allows identification of novel candidate genes potentially involved in self-maintenance of pre-BI cells, allelic exclusion and pre-B cell receptor signaling in large pre BII cells, cell-cycle arrest of small pre-BII cells, propensity toward apoptosis or anergization in immature B cells, propensity toward cell division and activation in mature B cells, and stage-specific interactions with stromal cells in the bone marrow.

Nek2 expression and localization in porcine oocyte during maturation

We studied the role of the Ser/Thr protein kinase Nek2 on meiosis progression by using in vitro porcine oocyte maturation system. Nek2 is a candidate of a mammalian homologue of NIMA, which was found in Aspergillus nidulans as an essential molecule for mitosis progression. We cloned porcine Nek2 cDNA, and examined the mRNA and protein expression levels during meiosis progression. The expression levels did not change through the oocyte maturation, but fluorescence microscopy observation of Nek2 in the oocytes at various maturation steps showed that Nek2 was detected only at metaphase II, but not before metaphase II. At metaphase II, Nek2 was found exclusively on the chromosomes. These results suggest that Nek2 changes the cellular localization during maturation and accumulates on the chromosomes to play a role on the entry and progression of metaphase II.

Isolation and characterization of two evolutionarily conserved murine kinases (Nek6 and nek7) related to the fungal mitotic regulator, NIMA

Entrance and exit from mitosis in Aspergillus nidulans require activation and proteolysis, respectively, of the NIMA (never in mitosis, gene A) serine/threonine kinase. Four different NIMA-related kinases were reported in mammals (Nek1-4), but none of them has been shown to perform mitotic functions related to those demonstrated for NIMA. We describe here the isolation of two novel murine protein kinase genes, designated nek6 and nek7, which are highly similar to each other (87% amino acid identity in the predicted kinase domain). Interestingly, Nek6 and Nek7 are also highly similar to the F19H6.1 protein kinase of Caenorhabditis elegans (76 and 73% amino acid identity in the kinase domain, respectively), and phylogenetic analysis suggests that these three proteins constitute a novel subfamily within the NIMA family of serine/threonine kinases. In contrast to the other documented NIMA-related kinases, Nek6/7 and F19H6.1 harbor their catalytic domain in the C-terminus of the protein. Immunofluorescence suggests that Nek6 and Nek7 are cytoplasmic. Linkage analysis, using the murine BXD recombinant inbred strain panel, localized nek6 to chromosome 2 at 28 cM. Using a mouse/hamster radiation hybrid panel, we assigned the nek7 gene to chromosome 1 at approximately 73 cM.

The NIMA-related kinase X-Nek2B is required for efficient assembly of the zygotic centrosome in Xenopus laevis

Nek2 is a mammalian cell cycle-regulated serine/threonine kinase that belongs to the family of proteins related to NIMA of Aspergillus nidulans. Functional studies in diverse species have implicated NIMA-related kinases in G(2)/M progression, chromatin condensation and centrosome regulation. To directly address the requirements for vertebrate Nek2 kinases in these cell cycle processes, we have turned to the biochemically-tractable system provided by Xenopus laevis egg extracts. Following isolation of a Xenopus homologue of Nek2, called X-Nek2B, we found that X-Nek2B abundance and activity remained constant through the first mitotic cycle implying a fundamental difference in Nek2 regulation between embryonic and somatic cell cycles. Removal of X-Nek2B from extracts did not disturb either entry into mitosis or the accompanying condensation of chromosomes providing no support for a requirement for Nek2 in these processes at least in embryonic cells. In contrast, X-Nek2B localized to centrosomes of adult Xenopus cells and was rapidly recruited to the basal body of Xenopus sperm following incubation in egg extracts. Recruitment led to phosphorylation of the X-Nek2B kinase. Most importantly, depletion of X-Nek2B from extracts significantly delayed both the assembly of microtubule asters and the recruitment of gamma-tubulin to the basal body. Hence, these studies demonstrate that X-Nek2B is required for efficient assembly of a functional zygotic centrosome and highlight the possibility of multiple roles for vertebrate Nek2 kinases in the centrosome cycle.

Nek2B, a novel maternal form of Nek2 kinase, is essential for the assembly or maintenance of centrosomes in early Xenopus embryos

Nek2, a NIMA-related kinase, has been postulated to play a role in both the meiotic and mitotic cell cycles in vertebrates. Xenopus has two Nek2 splice variants, Nek2A and Nek2B, which are zygotic and maternal forms, respectively. Here we have examined the role of Nek2B in oocyte meiosis and early embryonic mitosis. Specific inhibition of Nek2B function does not interfere with the oscillation of Cdc2 activity in either the meiotic or mitotic cell cycles; however, it does cause abortive cleavage of early embryos, in which bipolar spindle formation is severely impaired due to fragmentation or dispersal of the centrosomes, to which endogenous Nek2B protein localizes. In contrast, inhibition of Nek2B function does not affect meiotic spindle formation in oocytes, in which functional centrosomes are absent. Thus, strikingly, Nek2B is specifically required for centrosome assembly and/or maintenance (and hence for normal bipolar spindle formation and cleavage) in early Xenopus embryos. Finally, (ectopic) Nek2A but not Nek2B is very labile in cleaving embryos, suggesting that Nek2A cannot replace the centrosomal function of Nek2B in early embryos.

Activity and substrate specificity of the murine STK2 Serine/Threonine kinase that is structurally related to the mitotic regulator protein NIMA of Aspergillus nidulans

We isolated a murine STK2 (mSTK2) cDNA that is homologous to murine Nek1 serine/threonine kinase, a family member related to the cell cycle regulator kinase NIMA of Aspergillus nidulans. Structural comparison demonstrated that the kinase domain of mSTK2 is highly similar to NIMA/Nek family but the C-terminal region is not similar to any proteins except for human STK2 (hSTK2). Similarly to Nek1, mSTK2 is expressed ubiquitously among various organs and is upregulated in the testis. The expression and localization of mSTK2 are not associated with the cell cycle progression of mitogen-activated lymphocyte and DNA-transfected fibroblast. The substrate specificity of mSTK2 is similar to NIMA, but the phosphorylation is observed exclusively upon threonine residues rather than serine. The mSTK2 is shown to be a new member of the NIMA/Nek family with similar substrate specificity, which might participate in a different role from NIMA kinase involved in the cell cycle regulation.

Identification and characterization of TESK2, a novel member of the LIMK/TESK family of protein kinases, predominantly expressed in testis

In this study we present the cDNA sequence of a novel putative protein kinase, denoted TESK2. The open reading frame of TESK2 encodes a putative 555-amino-acid protein, including a protein kinase consensus sequence in the N-terminal half. The protein kinase domain of TESK2 is structurally similar to the kinase domain of the protein serine/threonine kinase TESK1 (64% identity) and to those of the LIMK1 and LIMK2 kinases (42 and 39% identity, respectively). TESK2, together with TESK1, constitutes a second subgroup of the LIMK/TESK family of protein kinases, as revealed by phylogenetic analysis of the protein kinase domains. Chromosomal localization of human TESK2 was assigned to 1p32. Expression analysis of human TESK2 revealed a single mRNA species of 3.0 kb predominantly expressed in testis and prostate and low expression in most other tissues examined. Rat testicles expressed a single species of TESK2 mRNA of approximately 3.5 kb. However, the transcript was first detectable in rat testis after day 30 of postnatal development and was predominantly expressed in round spermatids. These observations suggest that TESK2 plays an important role in spermatogenesis.

NIMA-related kinases: isolation and characterization of murine nek3 and nek4 cDNAs, and chromosomal localization of nek1, nek2 and nek3

The Aspergillus NIMA kinase plays a key role in controlling entrance into mitosis, and recent evidence suggests that mammalian NIMA-related kinases perform similar functions. We report here the cloning of the mouse nek3 and nek4 genes. Mouse nek3 is probably the ortholog of the partially sequenced, human nek3, whereas murine nek4 cDNA is probably the ortholog of human STK2. Nek4 is highly conserved between mouse and human, whereas Nek3 is somewhat less conserved (96.5 and 88% identity in the kinase domains, respectively). Northern analysis shows preferential expression of nek3 in mitotically active tissue, whereas nek4 is highly abundant in the testis. Within the developing testicular germ cells, in-situ analysis demonstrated that nek1, 2 and 4 exhibit differential patterns of expression, suggesting overlapping, but non-identical functions. Linkage analysis, using the mouse recombinant inbred strain panel (BXD), was used to localize nek1, 2 and 3. nek1 was mapped between Cpe and D8Mit8 on chromosome 8 at around 32cM, nek2 was mapped to the distal region of chromosome 1, and nek3 was mapped to the most centromeric region of chromosome 8.

Activity of the human centrosomal kinase, Nek2, depends on an unusual leucine zipper dimerization motif

Nek2 is a human cell cycle-regulated kinase that is structurally related to the mitotic regulator, NIMA, of Aspergillus nidulans. Localization studies have shown that Nek2 is a core component of the centrosome, the microtubule organizing center of the cell, and functional approaches suggest a possible role for Nek2 in centrosome separation at the G2/M transition. Here, we have investigated the importance of an unusual leucine zipper coiled-coil motif present in the C-terminal noncatalytic domain of the Nek2 kinase. Glycerol gradient centrifugation indicated that endogenous Nek2 is present in HeLa cells as a salt-resistant 6 S complex, the predicted size of a Nek2 homodimer. Recombinant Nek2 overexpressed in insect cells also formed a 6 S complex, whereas a Nek2 mutant specifically lacking the leucine zipper motif was monomeric. Using yeast two-hybrid interaction analyses and coprecipitation assays, we found that Nek2 can indeed form homodimers both in vivo and in vitro and that this dimerization specifically required the leucine zipper motif. Moreover, deletion of the leucine zipper prevented a trans-autophosphorylation reaction on the C-terminal domain of Nek2 and strongly reduced Nek2 kinase activity on exogenous substrates. Finally, we emphasize that the Nek2 leucine zipper described here differs from classical leucine zippers in that it exhibits a radically different arrangement of hydrophobic and charged amino acids. Thus, this study reveals not only an important mechanism for the regulation of the Nek2 kinase but, furthermore, highlights an unusual organization of a leucine zipper dimerization motif.

Cloning and characterization of the murine Nek3 protein kinase, a novel member of the NIMA family of putative cell cycle regulators

We have cloned and characterized murine Nek3 (NIMA-related kinase 3), a novel mammalian gene product structurally related to the cell cycle-regulatory kinase NIMA of Aspergillus nidulans. By RNase protection, low levels of Nek3 expression could be detected in all organs examined, regardless of proliferative index. In contrast to Nek1 and Nek2, Nek3 levels were not particularly elevated in either the male or the female germ line. Nek3 levels showed at most marginal variations through the cell cycle, but they were elevated in G0-arrested, quiescent fibroblasts. Furthermore, no cell cycle-dependent changes in Nek3 activity could be detected, and no effects upon cell cycle progression could be observed upon antibody microinjection or overexpression of either wild-type or catalytically inactive Nek3. Finally, Nek3 was found to be a predominantly cytoplasmic enzyme. These data indicate that Nek3 differs from previously characterized Neks with regard to all parameters investigated, including organ specificity of expression, cell cycle dependence of expression and activity, and subcellular localization. Hence, the structural similarity between mammalian Neks may not necessarily be indicative of a common function, and it is possible that some members of this kinase family may perform functions that are not directly related to cell cycle control.

Two structural variants of Nek2 kinase, termed Nek2A and Nek2B, are differentially expressed in Xenopus tissues and development

Nek2 kinase, a NIMA-related kinase, has been suggested to play both meiotic and mitotic roles in mammals, but its function(s) during development is poorly understood. We have isolated here cDNAs encoding a Xenopus homolog of mammalian Nek2 and have shown that Xenopus Nek2 has two structural variants, termed Nek2A and Nek2B. Nek2A, most likely a C-terminally spliced form, corresponds to the previously described human and mouse Nek2, while Nek2B is most probably a novel, C-terminally unspliced form of Nek2. As a consequence of this (probable) alternative splicing, Nek2B lacks the C-terminal 70-amino-acid sequence of Nek2A, which contains a PEST sequence (or a motif for rapid degradation). Western blot analysis reveals that Nek2A is expressed predominantly in the testis (presumably in spermatocytes) and very weakly in the stomach and, during development, only after the neurula stage. By contrast, Nek2B is expressed mainly in the ovary and in both primary and secondary oocytes and early embryos up to the neurula stage. These results suggest that Nek2A and Nek2B may play both meiotic and mitotic roles, but in a spatially and temporally complementary manner during Xenopus development, and that Nek2B, rather than Nek2A (or the conventional form of Nek2), may play an important role in early development. We discuss the possibility that a counterpart of Xenopus Nek2B might also exist and function in early mammalian development.

Stage-specific expression of testis-specific protein kinase 1 (TESK1) in rat spermatogenic cells

TESK1 (testis-specific protein kinase 1) is a serine/threonine kinase, with a unique structure composed of an N-terminal protein kinase domain and a C-terminal proline-rich domain. Northern blot analysis revealed that TESK1 mRNA is predominantly expressed in testicular germ cells. We present here evidence that expression of TESK1 mRNA and protein in the rat testes is developmentally regulated and increases after 20-22 postnatal days. To identify cells which express TESK1 mRNA and protein during male germ cell differentiation, in situ hybridization and immunohistochemistry were done using frozen sections of adult rat testes. Prominent expression of TESK1 mRNA and protein was detected in testicular germ cells at stages of late pachytene spermatocytes to round spermatids, but not in somatic cells such as Sertoli and Leydig cells. Expression of TESK1 mRNA and protein at specific stages of testicular germ cells suggests a role for this kinase in spermatogenesis, particularly at stages of meiosis and/or early spermiogenesis.

Cdc2-independent induction of premature mitosis by okadaic acid in HeLa cells

The induction of premature mitosis by okadaic acid (OA) in HeLa cells in S-phase or in G2-phase has been studied using light microscopy, immunofluorescence, and immunochemical techniques. The observations indicate an involvement of a cdc2-independent pathway in these cells. It has been claimed that inhibition of an OA-sensitive phosphatase, possibly of PP1, induces activation of a kinase which is sensitive to staurosporine and Zn2+. This kinase brings about mitosis-specific cytoskeletal rearrangements, chromosome condensation, and nuclear envelope breakdown, inducing a mitosis-like state. However, other mitotic events do not follow. The possibility that this kinase may be a NIMA-like Nek2 kinase is discussed.

C-Nap1, a novel centrosomal coiled-coil protein and candidate substrate of the cell cycle-regulated protein kinase Nek2

Nek2 (for NIMA-related kinase 2) is a mammalian cell cycle-regulated kinase structurally related to the mitotic regulator NIMA of Aspergillus nidulans. In human cells, Nek2 associates with centrosomes, and overexpression of active Nek2 has drastic consequences for centrosome structure. Here, we describe the molecular characterization of a novel human centrosomal protein, C-Nap1 (for centrosomal Nek2-associated protein 1), first identified as a Nek2-interacting protein in a yeast two-hybrid screen. Antibodies raised against recombinant C-Nap1 produced strong labeling of centrosomes by immunofluorescence, and immunoelectron microscopy revealed that C-Nap1 is associated specifically with the proximal ends of both mother and daughter centrioles. On Western blots, anti-C-Nap1 antibodies recognized a large protein (>250 kD) that was highly enriched in centrosome preparations. Sequencing of overlapping cDNAs showed that C-Nap1 has a calculated molecular mass of 281 kD and comprises extended domains of predicted coiled-coil structure. Whereas C-Nap1 was concentrated at centrosomes in all interphase cells, immunoreactivity at mitotic spindle poles was strongly diminished. Finally, the COOH-terminal domain of C-Nap1 could readily be phosphorylated by Nek2 in vitro, as well as after coexpression of the two proteins in vivo. Based on these findings, we propose a model implicating both Nek2 and C-Nap1 in the regulation of centriole-centriole cohesion during the cell cycle.