Cell cycle regulation in Aspergillus by two protein kinases

Fungal Extracellular Vesicles Are Involved in Intraspecies Intracellular Communication

Fungal infections are associated with high mortality rates in humans. The risk of fungal diseases creates the urgent need to broaden the knowledge base regarding their pathophysiology. In this sense, the role of extracellular vesicles (EVs) has been described to convey biological information and participate in the fungus-host interaction process. We hypothesized that fungal EVs work as an additional element in the communication routes regulating fungal responses in intraspecies interaction systems. In this respect, the aim of this study was to address the gene regulation profiles prompted by fungal EVs in intraspecies recipient cells. Our data demonstrated the intraspecies uptake of EVs in pathogenic fungi, such as Candida albicans, Aspergillus fumigatus, and Paracoccidioides brasiliensis, and the effects triggered by EVs in fungal cells. In C. albicans, we evaluated the involvement of EVs in the yeast-to-hypha transition, while in P. brasiliensis and A. fumigatus the function of EVs as stress transducers was investigated. P. brasiliensis and A. fumigatus were exposed to an inhibitor of glycosylation or UV light, respectively. The results demonstrated the role of EVs in regulating the expression of target genes and triggering phenotypic changes. The EVs treatment induced cellular proliferation and boosted the yeast to hyphal transition in C. albicans, while they enhanced stress responsiveness in A. fumigatus and P. brasiliensis, establishing a role for EVs in fungal intraspecies communication. Thus, EVs regulate fungal behavior, acting as potent message effectors, and understanding their effects and mechanism(s) of action could be exploited in antifungal therapies. IMPORTANCE Here, we report a study about extracellular vesicles (EVs) as communication mediators in fungi. Our results demonstrated the role of EVs from Candida albicans, Aspergillus fumigatus, and Paracoccidioides brasiliensis regulating the expression of target genes and phenotypic features. We asked whether fungal EVs play a role as message effectors. We show that fungal EVs are involved in fungal interaction systems as potent message effectors, and understanding their effects and mechanisms of action could be exploited in antifungal therapies.

Fungal Cell Cycle: A Unicellular versus Multicellular Comparison

All cells must accurately replicate DNA and partition it to daughter cells. The basic cell cycle machinery is highly conserved among eukaryotes. Most of the mechanisms that control the cell cycle were worked out in fungal cells, taking advantage of their powerful genetics and rapid duplication times. Here we describe the cell cycles of the unicellular budding yeast Saccharomyces cerevisiae and the multicellular filamentous fungus Aspergillus nidulans. We compare and contrast morphological landmarks of G1, S, G2, and M phases, molecular mechanisms that drive cell cycle progression, and checkpoints in these model unicellular and multicellular fungal systems.

Nek7 is overexpressed in hepatocellular carcinoma and promotes hepatocellular carcinoma cell proliferation in vitro and in vivo

NIMA-related kinase-7 (Nek7) is a serine/threonine kinase involved in cell-cycle progression via mitotic spindle formation and cytokinesis. In this study, we investigated whether Nek7 involves in hepatocellular carcinoma (HCC). Interestingly, we found that Nek7 was significantly overexpressed in HCC than in liver tissues. In HCC patients, high Nek7 expression was significantly correlated with tumor numbers, tumor diameter, adjacent organs invasion, tumor grade and TNM stage. Furthermore, Nek7 expression pattern showed close relationship with that of Ki-67, a well-stablished cell proliferation marker. More importantly, patients with higher expression levels of Nek7 had significantly lower 5-years survival rate. Likewise, Nek7 expression was significantly higher in HCC cell lines than normal hepatic cell line. By Nek7 silencing using lentivirus-mediated Nek7 interference approach, the growth of HCC cell lines was inhibited and the tumor growth in xenograft mouse model was also suppressed. Mechanistic studies showed that silencing of Nek7 resulted in decreasing cyclinB1 level both in vitro and in vivo. In conclusion, this study highlights for the first time the possible role of Nek7 in HCC progression. Nek7 would be a useful biomarker that early predicts HCC patients at higher risk of poor prognosis. Also, Nek7 could be a novel HCC therapeutic target.

Expression of Nek1 during kidney development and cyst formation in multiple nephron segments in the Nek1-deficient kat2J mouse model of polycystic kidney disease

BACKGROUND

Neks, mammalian orthologs of the fungal protein kinase never-in-mitosis A, have been implicated in the pathogenesis of polycystic kidney disease. Among them, Nek1 is the primary protein inactivated in kat2J mouse models of PKD.

RESULT

We report the expression pattern of Nek1 and characterize the renal cysts that develop in kat2J mice. Nek1 is detectable in all murine tissues but its expression in wild type and kat2J heterozygous kidneys decrease as the kidneys mature, especially in tubular epithelial cells. In the embryonic kidney, Nek1 expression is most prominent in cells that will become podocytes and proximal tubules. Kidney development in kat2J homozygous mice is aberrant early, before the appearance of gross cysts: developing cortical zones are thin, populated by immature glomeruli, and characterized by excessive apoptosis of several cell types. Cysts in kat2J homozygous mice form postnatally in Bowman's space as well as different tubular subtypes. Late in life, kat2J heterozygous mice form renal cysts and the cells lining these cysts lack staining for Nek1. The primary cilia of cells lining cysts in kat2J homozygous mice are morphologically diverse: in some cells they are unusually long and in others there are multiple cilia of varying lengths.

CONCLUSION

Our studies indicate that Nek1 deficiency leads to disordered kidney maturation, and cysts throughout the nephron.

The Set1/COMPASS histone H3 methyltransferase helps regulate mitosis with the CDK1 and NIMA mitotic kinases in Aspergillus nidulans

Mitosis is promoted and regulated by reversible protein phosphorylation catalyzed by the essential NIMA and CDK1 kinases in the model filamentous fungus Aspergillus nidulans. Protein methylation mediated by the Set1/COMPASS methyltransferase complex has also been shown to regulate mitosis in budding yeast with the Aurora mitotic kinase. We uncover a genetic interaction between An-swd1, which encodes a subunit of the Set1 protein methyltransferase complex, with NIMA as partial inactivation of nimA is poorly tolerated in the absence of swd1. This genetic interaction is additionally seen without the Set1 methyltransferase catalytic subunit. Importantly partial inactivation of NIMT, a mitotic activator of the CDK1 kinase, also causes lethality in the absence of Set1 function, revealing a functional relationship between the Set1 complex and two pivotal mitotic kinases. The main target for Set1-mediated methylation is histone H3K4. Mutational analysis of histone H3 revealed that modifying the H3K4 target residue of Set1 methyltransferase activity phenocopied the lethality seen when either NIMA or CDK1 are partially functional. We probed the mechanistic basis of these genetic interactions and find that the Set1 complex performs functions with CDK1 for initiating mitosis and with NIMA during progression through mitosis. The studies uncover a joint requirement for the Set1 methyltransferase complex with the CDK1 and NIMA kinases for successful mitosis. The findings extend the roles of the Set1 complex to include the initiation of mitosis with CDK1 and mitotic progression with NIMA in addition to its previously identified interactions with Aurora and type 1 phosphatase in budding yeast.

Identification of interphase functions for the NIMA kinase involving microtubules and the ESCRT pathway

The Never in Mitosis A (NIMA) kinase (the founding member of the Nek family of kinases) has been considered a mitotic specific kinase with nuclear restricted roles in the model fungus Aspergillus nidulans. By extending to A. nidulans the results of a synthetic lethal screen performed in Saccharomyces cerevisiae using the NIMA ortholog KIN3, we identified a conserved genetic interaction between nimA and genes encoding proteins of the Endosomal Sorting Complex Required for Transport (ESCRT) pathway. Absence of ESCRT pathway functions in combination with partial NIMA function causes enhanced cell growth defects, including an inability to maintain a single polarized dominant cell tip. These genetic insights suggest NIMA potentially has interphase functions in addition to its established mitotic functions at nuclei. We therefore generated endogenously GFP-tagged NIMA (NIMA-GFP) which was fully functional to follow its interphase locations using live cell spinning disc 4D confocal microscopy. During interphase some NIMA-GFP locates to the tips of rapidly growing cells and, when expressed ectopically, also locates to the tips of cytoplasmic microtubules, suggestive of non-nuclear interphase functions. In support of this, perturbation of NIMA function either by ectopic overexpression or through partial inactivation results in marked cell tip growth defects with excess NIMA-GFP promoting multiple growing cell tips. Ectopic NIMA-GFP was found to locate to the plus ends of microtubules in an EB1 dependent manner, while impairing NIMA function altered the dynamic localization of EB1 and the cytoplasmic microtubule network. Together, our genetic and cell biological analyses reveal novel non-nuclear interphase functions for NIMA involving microtubules and the ESCRT pathway for normal polarized fungal cell tip growth. These insights extend the roles of NIMA both spatially and temporally and indicate that this conserved protein kinase could help integrate cell cycle progression with polarized cell growth.

The NIMA kinase is required to execute stage-specific mitotic functions after initiation of mitosis

The G2-M transition in Aspergillus nidulans requires the NIMA kinase, the founding member of the Nek kinase family. Inactivation of NIMA results in a late G2 arrest, while overexpression of NIMA is sufficient to promote mitotic events independently of cell cycle phase. Endogenously tagged NIMA-GFP has dynamic mitotic localizations appearing first at the spindle pole body and then at nuclear pore complexes before transitioning to within nuclei and the mitotic spindle and back at the spindle pole bodies at mitotic exit, suggesting that it functions sequentially at these locations. Since NIMA is indispensable for mitotic entry, it has been difficult to determine the requirement of NIMA for subaspects of mitosis. We show here that when NIMA is partially inactivated, although mitosis can be initiated, a proportion of cells fail to successfully generate two daughter nuclei. We further define the mitotic defects to show that normal NIMA function is required for the formation of a bipolar spindle, nuclear pore complex disassembly, completion of chromatin segregation, and the normal structural rearrangements of the nuclear envelope required to generate two nuclei from one. In the remaining population of cells that enter mitosis with inadequate NIMA, two daughter nuclei are generated in a manner dependent on the spindle assembly checkpoint, indicating highly penetrant defects in mitotic progression without sufficient NIMA activity. This study shows that NIMA is required not only for mitotic entry but also sequentially for successful completion of stage-specific mitotic events.

Regulation of mitosis by the NIMA kinase involves TINA and its newly discovered partner, An-WDR8, at spindle pole bodies

The NIMA kinase is required for mitotic nuclear pore complex disassembly and potentially controls other mitotic-specific events. To investigate this possibility, we imaged NIMA-green fluorescent protein (GFP) using four-dimensional spinning disk confocal microscopy. At mitosis NIMA-GFP locates to spindle pole bodies (SPBs), which contain Cdk1/cyclin B, followed by Aurora, TINA, and the BimC kinesin. NIMA promotes NPC disassembly in a spatially regulated manner starting near SPBs. NIMA is also required for TINA, a NIMA-interacting protein, to locate to SPBs during initiation of mitosis, and TINA is then necessary for locating NIMA back to SPBs during mitotic progression. To help expand the NIMA-TINA pathway, we affinity purified TINA and found it to uniquely copurify with An-WDR8, a WD40-domain protein conserved from humans to plants. Like TINA, An-WDR8 accumulates within nuclei during G2 but disperses from nuclei before locating to mitotic SPBs. Without An-WDR8, TINA levels are greatly reduced, whereas TINA is necessary for mitotic targeting of An-WDR8. Finally, we show that TINA is required to anchor mitotic microtubules to SPBs and, in combination with An-WDR8, for successful mitosis. The findings provide new insights into SPB targeting and indicate that the mitotic microtubule-anchoring system at SPBs involves WDR8 in complex with TINA.

NEK9 depletion induces catastrophic mitosis by impairment of mitotic checkpoint control and spindle dynamics

NEK9 is known to play a role in spindle assembly and in the control of centrosome separation, but the consequences of NEK9 targeting in cancer cells remain to be elucidated. In this study, we used siRNA to investigate the consequences of targeting NEK9 in glioblastoma and kidney cancer cells as a first step in assessing its potential as an anti-cancer therapeutic target. Live cell imaging revealed that NEK9 depletion of U1242 glioblastoma and Caki2 kidney carcinoma cells resulted in failure of cytokinesis. Interestingly, NEK9-depleted Caki2 cells overrode mitosis under incorrect chromosome alignment and were converted to a micronucleated phenotype, leading to cell death. Whereas, the RPE1 normal epithelium cell line was refractory to abnormal mitosis upon NEK9 knockdown. Nocodazole-induced mitotic arrest was compromised after NEK9 depletion, indicating that NEK9 has an important role in mitotic checkpoint system. Taken together, we propose that NEK9 inhibition represents a novel anti-cancer strategy by induction of mitotic catastrophe via impairment of spindle dynamics, cytokinesis and mitotic checkpoint control.

LAMMER Kinase LkhA plays multiple roles in the vegetative growth and asexual and sexual development of Aspergillus nidulans

LAMMER kinase plays pivotal roles in various physiological processes in eukaryotes; however, its function in filamentous fungi is not known. We performed molecular studies on the function of the Aspergillus nidulans LAMMER kinase, LkhA, and report its involvement in multiple developmental processes. The gene for LkhA was highly expressed during reproductive organ development, such as that of conidiophores and cleistothecia. During vegetative growth, the patterns of germ tube emergence and hyphal polarity were changed and septation was increased by lkhA deletion. Northern analyses showed that lkhA regulated the transcription of brlA, csnD, and ppoA, which supported the detrimental effect of lkhA-deletion on asexual and sexual differentiation. LkhA also affected expression of cyclin-dependent kinase NimX^cdc2, a multiple cell cycle regulator, and StuA, an APSES family of fungal transcription factors that play pivotal roles in multiple differentiation processes. Here, for the first time, we present molecular evidence showing that LAMMER kinase is involved in A. nidulans development by modulating the expression of key regulators of developmental processes.

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

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

Cyclin dependent protein kinases and stress responses in plants

Plants have to adjust, grow and establish themselves in various changing environmental conditions. Additionally, the sessile life-style of plants requires the development of response mechanisms for their adaptation in such environmental cues. Under biotic and abiotic stress, plant growth is negatively affected mainly as a result of cell cycle inhibition. The perception of stress involves the activation of signaling cascades that result in a prolonged S-phase and delayed entry into mitosis. Although the molecular interactions that link the cell cycle machinery to perception of stress are not fully understood, recent studies indicated the involvement of Cyclin Dependent Kinases (CDKs) in the plant response machinery. CDKs are core cell cycle regulators but their activity has been implicated in additional diverse cellular processes. Here we review the impact of different types of abiotic stress on plant cell cycle progression and CDK activity, and discuss the contribution of CDK function in the signaling control of stress tolerance.

Conserved components, but distinct mechanisms for the placement and assembly of the cell division machinery in unicellular and filamentous ascomycetes

Cytokinesis is essential for cell proliferation, yet its molecular description is challenging, because >100 conserved proteins must be spatially and temporally co-ordinated. Despite the high importance of a tight co-ordination of cytokinesis with chromosome and organelle segregation, the mechanism for determining the cell division plane is one of the least conserved aspects of cytokinesis in eukaryotic cells. Budding and fission yeast have developed fundamentally distinct mechanisms to ensure proper nuclear segregation. The extent to which these pathways are conserved in multicellular fungi remains unknown. Recent progress indicates common components, but different mechanisms that are required for proper selection of the septation site in the different groups of Ascomycota. Cortical cues are used in yeast- and filament-forming species of the Saccharomycotina clade that are established at the incipient bud site or the hyphal tip respectively. In contrast, septum formation in the filament-forming Pezizomycotina species Aspergillus nidulans and Neurospora crassa seems more closely related to the fission yeast programme in that they may combine mitotic signals with a cell end-based marker system and Rho GTPase signalling. Thus, significant differences in the use and connection of conserved signalling modules become apparent that reflect the phylogenetic relationship of the analysed models.

Cdk1 activity is required for mitotic activation of aurora A during G2/M transition of human cells

In mammalian cells entry into and progression through mitosis are regulated by multiple mitotic kinases. How mitotic kinases interact with each other and coordinately regulate mitosis remains to be fully understood. Here we employed a chemical biology approach using selective small molecule kinase inhibitors to dissect the relationship between Cdk1 and Aurora A kinases during G(2)/M transition. We find that activation of Aurora A first occurs at centrosomes at late G(2) and is required for centrosome separation independently of Cdk1 activity. Upon entry into mitosis, Aurora A then becomes fully activated downstream of Cdk1 activation. Inactivation of Aurora A or Plk1 individually during a synchronized cell cycle shows no significant effect on Cdk1 activation and entry into mitosis. However, simultaneous inactivation of both Aurora A and Plk1 markedly delays Cdk1 activation and entry into mitosis, suggesting that Aurora A and Plk1 have redundant functions in the feedback activation of Cdk1. Together, our data suggest that Cdk1, Aurora A, and Plk1 mitotic kinases participate in a feedback activation loop and that activation of Cdk1 initiates the feedback loop activity, leading to rapid and timely entry into mitosis in human cells. In addition, live cell imaging reveals that the nuclear cycle of cells becomes uncoupled from cytokinesis upon inactivation of both Aurora A and Aurora B kinases and continues to oscillate in a Cdk1-dependent manner in the absence of cytokinesis, resulting in multinucleated, polyploidy cells.

AtNEK6 interacts with ARIA and is involved in ABA response during seed germination

ARIA is an ARM repeat protein that is a positive regulator of ABA response. To identify ARIA-interacting proteins, we conducted yeast two-hybrid screening. One of the positive clones obtained from the screen encoded a protein kinase, AtNEK6, which belongs to the NIMA (Never In Mitosis, gene A)-related kinase family. We analyzed AtNEK6 over-expression (OX) and knockout (KO) lines to investigate its in vivo function. The AtNEK6 OX lines grew slowly, whereas the KO line germinated and grew faster than wild type plants. AtNEK6 also affected ABA and stress responses. During seed germination, AtNEK6 OX lines were hypersensitive to ABA and high osmolarity, whereas its KO line was partially insensitive to ABA and high osmolarity. Previously, AtNEK6 was shown to be involved in epidermal cell morphogenesis. Our results indicate that AtNEK6 is also involved in plant growth regulation and responses to ABA and high osmolarity during the seed germination stage.

Nek1 silencing slows down DNA repair and blocks DNA damage-induced cell cycle arrest

Never in mitosis A (NIMA)-related kinases (Nek) are evolutionarily conserved proteins structurally related to the Aspergillus nidulans mitotic regulator NIMA. Nek1 is one of the 11 isoforms of the Neks identified in mammals. Different lines of evidence suggest the participation of Nek1 in response to DNA damage, which is also supported by the interaction of this kinase with proteins involved in DNA repair pathways and cell cycle regulation. In this report, we show that cells with Nek1 knockdown (KD) through stable RNA interference present a delay in DNA repair when treated with methyl-methanesulfonate (MMS), hydrogen peroxide (H(2)O(2)) and cisplatin (CPT). In particular, interstrand cross links induced by CPT take much longer to be resolved in Nek1 KD cells when compared to wild-type (WT) cells. In KD cells, phosphorylation of Chk1 in response to CPT was strongly reduced. While WT cells accumulate in G(2)/M after DNA damage with MMS and H(2)O(2), Nek1 KD cells do not arrest, suggesting that G(2)/M arrest induced by the DNA damage requires Nek1. Surprisingly, CPT-treated Nek1 KD cells arrest with a 4N DNA content similar to WT cells. This deregulation in cell cycle control in Nek1 KD cells leads to an increased sensitivity to genotoxic agents when compared to WT cells. These results suggest that Nek1 is involved in the beginning of the cellular response to genotoxic stress and plays an important role in preventing cell death induced by DNA damage.

Cytoplasmic male sterility-related protein kinase, OsNek3, is regulated downstream of mitochondrial protein phosphatase 2C, DCW11

OsNek3 (Oryza sativa L. NIMA-related kinase) and DCW11 encoding a mitochondrial putative protein phosphatase 2C were found in our previous microarray study as down-regulated genes in the rice CW-CMS line, which lacked pollen germination ability. Further analysis of DCW11 revealed that DCW11 is strongly correlated with CW-CMS occurrence. Here we show the relationship between OsNek3 and DCW11. OsNek3 was preferentially expressed in mature pollen. A knockout mutant with Tos17 inserted into OsNek3 did not show any pollen-defective phenotype. On the other hand, plants overexpressing OsNek3 occasionally produced a peculiar pollen structure in which the outer cell wall of four pollen grains fused together even at the mature pollen stages, which resembled that of quartet mutants in Arabidopsis. OsNek3 was shown to interact with a LIM domain-containing protein, OsPLIM2b, whose expression was strongly specific in mature pollen, suggesting that OsNek3 might play a role in pollen germination. OsNek3 was shown to be down-regulated in DCW11-knockdown lines, whereas osnek3 mutation did not result in DCW11 down-regulation. These results suggest that OsNek3 is downstream of DCW11 in retrograde signaling from the mitochondria to the nucleus and is involved in CW-CMS.

Copy number suppressors of the Aspergillus nidulans nimA1 mitotic kinase display distinctive and highly dynamic cell cycle-regulated locations

The Aspergillus nidulans NIMA kinase is essential for mitosis and is the founding member of the conserved NIMA-related kinase (Nek) family of protein kinases. To gain insight into NIMA function, a copy number suppression screen has been completed that defines three proteins termed MCNA, MCNB, and MCNC (multi-copy-number suppressor of nimA1 A, B, and C). All display a distinctive and dynamic cell cycle-specific distribution. MCNC has weak similarity to Saccharomyces cerevisiae Def1 within a shared CUE-like domain. MCNC, like Def1, is a cytoplasmic protein with slow mobility during sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and its deletion causes polarization defects and a small colony phenotype. MCNC enters nuclei during mitosis. In contrast, MCNB is a nuclear protein displaying increased nuclear levels as cells progress through interphase but is lost from nuclei at mitosis. MCNB is highly related to the Schizosaccharomyces pombe forkhead transcription factor Sep1 and is likely a transcriptional activator of nimA. Most surprisingly, MCNA, a protein restricted to the aspergilli and pathogenic systemic dimorphic fungi (the Eurotiomycetes), defines a nuclear body located near nucleoli at the nuclear periphery of G(2) nuclei. During progression through mitosis, the MCNA body is excluded from nuclei. Cytoplasmic MCNA bodies then diminish during early stages of interphase, and single MCNA bodies are formed within nuclei as interphase progresses. Three sites of MCNA phosphorylation were mapped and mutated to implicate proline-directed phosphorylation in the equal segregation of MCNA during the cell cycle. The data indicate all three MCN proteins likely have cell cycle functions.

Members of the plant NIMA-related kinases are involved in organ development and vascularization in poplar, Arabidopsis and rice

NIMA-related kinases (Neks) are a family of serine/threonine kinases that have been linked to cell-cycle regulation in fungi and mammals. Information regarding the function of Neks in plants is very limited. We screened the three plant species that have had their genomes sequenced in an attempt to improve our understanding of their role in plants. We retrieved seven members in Arabidopsis thaliana, nine in Populus trichocarpa and six in Oryza sativa. Phylogenetic analysis showed that plant Neks are closely related to each other and contain paralogous genes. Moreover, their chromosome distribution and their exon-intron structure revealed that the actual plant Nek family was derived from a single representative followed by large segmental duplication events. Functional expression analyses in the three species relied on RTqPCR in poplar and publicly available microarray data for Arabidopsis and rice. Although plant Neks are present in every organ analyzed, their expression profiles suggest their involvement in plant development processes. Furthermore, we showed that PNek1, a member of the poplar family, is expressed at sites of free auxin synthesis and is specifically involved during the vascularization process.

A point mutation in the Aspergillus nidulans sonBNup98 nuclear pore complex gene causes conditional DNA damage sensitivity

The nuclear pore complex (NPC) is embedded in the nuclear envelope where it mediates transport between the cytoplasm and nucleus and helps to organize nuclear architecture. We previously isolated sonB1, a mutation encoding a single amino acid substitution within the Aspergillus nidulans SONBnNup98 NPC protein (nucleoporin). Here we demonstrate that this mutation causes marked DNA damage sensitivity at 42 degrees . Although SONBnNup98 has roles in the G2 transition, we demonstrate that the G2 DNA damage checkpoint is functional in the sonB1 mutant at 42 degrees . The MRN complex is composed of MRE11, RAD50, and NBS1 and functions in checkpoint signaling, DNA repair, and telomere maintenance. At 42 degrees we find that the DNA damage response defect of sonB1 mutants causes synthetic lethality when combined with mutations in scaANBS1, the A. nidulans homolog of NBS1. We provide evidence that this synthetic lethality is independent of MRN cell cycle checkpoint functions or MREAMRE11-mediated DNA repair functions. We also demonstrate that the single A. nidulans histone H2A gene contains the C-terminal SQE motif of histone H2AX isoforms and that this motif is required for the DNA damage response. We propose that the sonB1 nucleoporin mutation causes a defect in a novel part of the DNA damage response.

Nek1 shares structural and functional similarities with NIMA kinase

The Aspergillus NIMA serine/threonine kinase plays a pivotal role in controlling entrance into mitosis. A major function attributed to NIMA is the induction of chromatin condensation. We show here that the founder murine NIMA-related kinase, Nek1, is larger than previously reported, and that the full-length protein conserves the structural hallmarks of NIMA. Even though Nek1 bears two classical nuclear localization signals (NLS), the endogenous protein localizes to the cytoplasm. Ectopic overexpression of various Nek1 constructs suggests that the C-terminus of Nek1 bears cytoplasmic localization signal(s). Overexpression of nuclear constructs of Nek1 resulted in abnormal chromatin condensation, with the DNA mainly confined to the periphery of the nucleus. Advanced condensation phenotype was associated with nuclear pore complex dispersal. The condensation was not accompanied by up-regulation of mitotic or apoptotic markers. A similar phenotype has been described following NIMA overexpression, strengthening the notion that the mammalian Nek1 kinase has functional similarity to NIMA.

The csnD/csnE signalosome genes are involved in the Aspergillus nidulans DNA damage response

The signalosome (CSN) is a conserved multiprotein complex involved in regulation of eukaryotic development and is also required to activate ribonucleotide reductase for DNA synthesis. In Aspergillus nidulans, csnD/csnE are key regulators of sexual development. Here, we investigated whether the csnD/csnE genes are involved in the DNA damage response in this fungus. The growth of the csnD/csnE deletion mutants was reduced by subinhibitory concentrations of hydroxyurea, camptothecin, 4-nitroquinoline oxide, and methyl methanesulfonate. A. nidulans increases csnD/csnE mRNA levels when it is challenged by different DNA-damaging agents. There is no significant transcriptional induction of the csnE promoter fused with lacZ gene in the presence of DNA-damaging agents, suggesting that increased mRNA accumulation is due to increased mRNA stability. Septation was not inhibited in the csnD/csnE deletion mutants while DeltauvsB DeltacsnE presented an increase in septation upon DNA damage caused by methyl methanesulfonate, suggesting that uvsB(ATR) and csnE genetically interact during checkpoint-dependent inhibition of septum formation. The double DeltacsnD/DeltacsnE DeltanpkA mutants were more sensitive to DNA-damaging agents than were the respective single mutants. Our results suggest that csnD/csnE genes are involved in the DNA damage response and that NpkA and UvsB(ATR) genetically interact with the signalosome.

CaSm-Mediated Cellular Transformation Is Associated with Altered Gene Expression and Messenger RNA Stability

CaSm (cancer-associated Sm-like) was originally identified based on elevated expression in pancreatic cancer and in several cancer-derived cell lines. CaSm encodes a 133 amino acid protein that contains two Sm motifs found in the common small nuclear RNA proteins and the LSm (like-Sm) family of proteins. Compared with normal human prostate tissue and primary prostate epithelial cells, some primary prostate tumors and prostate cancer-derived cell lines have elevated CaSm expression. Expression of antisense CaSm RNA in DU145 cells results in reduced CaSm protein levels and less transformed phenotype, measured by anchorage-independent growth in vitro and tumor formation in severe combined immunodeficient mice in vivo. Additional data shows that adenoviral delivery of antisense CaSm inhibits the growth of prostate cancer cell lines by altering cell cycle progression, and is associated with reduced expression of cyclin B1 and CDK1 proteins. Consistent with failure of antisense-treated cells to enter mitosis, microarray analysis identified altered expression of NEK2 and nucleophosmin/B23. Although the mechanisms by which CaSm contributes to neoplastic transformation and cellular proliferation are unknown, it has been shown that the yeast homologue (spb8/LSm1) of CaSm is required for 5' to 3' degradation of specific mRNAs. We provide data consistent with a similar role for CaSm in human cells, supporting the hypothesis that elevated CaSm expression observed in cancer leads to destabilization of multiple gene transcripts, contributing to the mutator phenotype of cancer cells.

The Aspergillus nidulans npkA gene encodes a Cdc2-related kinase that genetically interacts with the UvsBATR kinase

The DNA damage response is a protective mechanism that ensures the maintenance of genomic integrity. We have used Aspergillus nidulans as a model system to characterize the DNA damage response caused by the antitopoisomerase I drug, camptothecin. We report the molecular characterization of a p34Cdc2-related gene, npkA, from A. nidulans. The npkA gene is transcriptionally induced by camptothecin and other DNA-damaging agents, and its induction in the presence of camptothecin is dependent on the uvsBATR gene. There were no growth defects, changes in developmental patterns, increased sensitivity to DNA-damaging agents, or effects on septation or growth rate in the A. nidulans npkA deletion strain. However, the DeltanpkA mutation can partially suppress HU sensitivity caused by the DeltauvsBATR and uvsD153ATRIP checkpoint mutations. We demonstrated that the A. nidulans uvsBATR gene is involved in DNA replication and the intra-S-phase checkpoints and that the DeltanpkA mutation can suppress its intra-S-phase checkpoint deficiency. There is a defect in both the intra-S-phase and DNA replication checkpoints due to the npkA inactivation when DNA replication is slowed at 6 mm HU. Our results suggest that the npkA gene plays a role in cell cycle progression during S-phase as well as in a DNA damage signal transduction pathway in A. nidulans.

The polo-like kinase PLKA is required for initiation and progression through mitosis in the filamentous fungus Aspergillus nidulans

Polo-like kinases (PLK) function during multiple stages of mitotic progression and in cytokinesis. We identified and cloned a PLK homologue in Aspergillus nidulans, plkA, which is the first PLK reported in a filamentous fungus and the largest member of the PLK family to date. As plkA was essential, the effects of overexpression and localization of protein in living cells were explored to determine PLKA function. Overexpression of PLKA permitted hyphal formation, but blocked nuclear division in interphase. In NIMA or NIMT temperature-sensitive backgrounds, overexpression of PLKA impaired normal entry into mitosis upon release from restrictive temperature, supporting a role for PLKA during G2/M. In the few mitotic cells present, spindles were monopolar or disorganized, and chromatin condensation and segregation were impaired, suggesting additional roles for PLKA in spindle formation and in chromosome dynamics. Consistent with this, green fluorescent protein (GFP)-tagged PLKA could localize to the spb during interphase, and to the spb and nucleus throughout mitosis. Intriguingly, PLKA remained on the spb during telophase and into G1, in contrast to other PLK. In addition, spb localization was independent of NIMA function, unlike that demonstrated in Schizosaccharomyces pombe where PLK localization to the spb required the NIMA homologue Fin1. PLKA was not detected at cortical, septation-associated sites, and overexpression did not drive septum formation, also in contrast to that observed with other PLK. Therefore, PLKA is important for multiple events during mitosis, similar to PLK in higher organisms, but exhibits differences in size, localization and influence on septation/cytokinesis, suggesting additional novel regulatory features.

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.

Nek8, a NIMA family kinase member, is overexpressed in primary human breast tumors

The family of human Nek (NIMA Related Kinase) kinases currently contains 11 members. We have identified Nek8 as a new member of the Nek kinase family. For many of the Nek family members, primary tumor expression data and function have been limited. However, all of the Nek family proteins share considerable homology with the Never In Mitosis, gene A (NIMA) kinase from the filamentous fungus Aspergillus nidulans. NIMA, as well as its most closely related human ortholog, Nek2, are required for G(2)/M progression and promote centrosome maturation during mitosis. We isolated Nek8 from a primary human colon cDNA library, and found it to be highly homologous to murine Nek8. Recently, a previously named Nek8 sequence was renamed Nek9/Nercc1 in Genbank due to its lack of homology to murine Nek8 and its high homology to murine Nek9. Interestingly, in our study, phylogenetic analysis suggests that human Nek8 and Nek9 form a subfamily within the Nek family. Nek8 has high homology to the Nek family kinase domain as well as to a regulator of chromosome condensation domain (RCC1), which is also present in Nek9. The open reading frame of human Nek8 encodes a 692 amino-acid protein with a calculated molecular weight of 75 kDa. Nek8 is differently expressed between normal human breast tissue and breast tumors. Overexpression of a mutated kinase domain Nek8 in U2-0S cells led to a decrease in actin protein, and a small increase in the level of cdk1/cyclinB1. Our data demonstrate for the first time that Nek8 is a novel tumor associated gene, and shares considerable sequence homology with the Nek family of protein kinases and may be involved in G(2)/M progression.

Aspergillus nidulans as a model system to characterize the DNA damage response in eukaryotes

Interest in DNA repair in Aspergillus nidulans had mainly grown out of studies of three different biological processes, namely mitotic recombination, inducible responses to detrimental environmental changes, and genetic control of the cell cycle. Ron Morris started the investigation of the genetic control of the cell cycle by screening hundreds of cell cycle temperature sensitive Aspergillus mutants. The sequencing and innovative analysis of these genes revealed not only several components of the cell cycle machinery that are directly involved in checkpoint response, but also components required for DNA replication and DNA damage response machinery. Here, we will provide an overview about currently known aspects of the DNA damage response in A. nidulans. Emphasis is put on analyzed mutants that are available and review epistatic relationships and other interactions among them. Furthermore, a comprehensive list of A. nidulans genes involved in different processes of the DNA damage response, as identified by homology of genome sequences with well-characterized human and yeast DNA repair genes, is shown.

The early impact of genetics on our understanding of cell cycle regulation in Aspergillus nidulans

The application of genetic analysis was crucial to the rapid progress that has been made in cell cycle research. Ron Morris, one of the first to apply genetics to cell cycle research, developed Aspergillus nidulans into an important model system for the analysis of many aspects of cell biology. Within the area of cell cycle research, Ron's laboratory is noted for development of novel cell biological and molecular genetic approaches as well as seminal insights regarding the regulation of mitosis, checkpoint regulation of the cell cycle, and the role of microtubule-based motors in chromosome segregation. In this special edition of FGB dedicated to Ron Morris, and in light of the recent progress in fungal genomics, we review the outstanding contributions his work made to our understanding of mitotic regulation. Indeed, his efforts have provided many mutants and experimental tools along with the conceptual framework for current and future studies of mitosis in A. nidulans.

The pot1+ homologue in Aspergillus nidulans is required for ordering mitotic events

Orderly progression through mitosis is essential to reduce segregation errors in the cell's genetic material. We have used a cytological screen to identify a mutant that progresses through mitosis aberrantly and have cloned the complementing gene, nimU, which encodes a protein related to Pot1 and other telomere end-binding proteins. We show that loss of nimU function leads to premature mitotic spindle elongation, premature mitotic exit, errors in chromosome segregation, and failure to delay mitotic exit under conditions that normally evoke the mitotic spindle checkpoint response. Whereas premature mitotic exit is dependent upon anaphase promoting complex function, premature spindle elongation is not. We conclude that nimU is constitutively required for orderly mitotic progression under normal growth conditions and also required for the conditional mitotic spindle checkpoint response.

Septation and cytokinesis in fungi

Cytokinesis is the ultimate step of a cell cycle resulting in the generation of two progeny. Failure of correct cell division may be lethal for both, mother and daughter cells, and thus such a process must be tightly regulated with other events of the cell cycle. Differing solutions to the same problem have been developed in bacteria and plants while cytokinesis in animal and fungal cells is highly similar and requires a contractile ring containing actomyosin. Cytokinesis in fungi can be viewed as a three-stage process: (i) selection of a division site, (ii) orderly assembly of protein complexes, and finally (iii) dynamic events that lead to a constriction of the contractile ring and septum construction. Elaborate mechanisms known as the Mitotic Exit Network (MEN) and the Septation Initiation Network (SIN) have evolved to link these events, particularly the final steps of cytokinesis, with nuclear division. The purpose of this review was to discuss the latest developments in the fungal field and to describe the central known players required for key steps on the road to cell division. Differences in the cytokinesis of yeast-like fungi that result in complete cell separation in contrast to septation which leads to the compartmentalization of fungal hyphae are highlighted.

A mitotic cascade of NIMA family kinases. Nercc1/Nek9 activates the Nek6 and Nek7 kinases

The Nek family of protein kinases in humans is composed of 11 members that share an amino-terminal catalytic domain related to NIMA, an Aspergillus kinase involved in the control of several aspects of mitosis, and divergent carboxyl-terminal tails of varying length. Nek6 (314AA) and Nek7 (303AA), 76% identical, have little noncatalytic sequence but bind to the carboxyl-terminal noncatalytic tail of Nercc1/Nek9, a NIMA family protein kinase that is activated in mitosis. Microinjection of anti-Nercc1 antibodies leads to spindle abnormalities and prometaphase arrest or chromosome missegregation. Herein we show that Nek6 is increased in abundance and activity during mitosis; activation requires the phosphorylation of Ser206 on the Nek6 activation loop. This phosphorylation and the activity of recombinant Nek6 is stimulated by coexpression with an activated mutant of Nercc1. Moreover, Nercc1 catalyzes the direct phosphorylation of prokaryotic recombinant Nek6 at Ser206 in vitro concomitant with 20-25-fold activation of Nek6 activity; Nercc1 activates Nek7 in vitro in a similar manner. Nercc1/Nek9 is likely to be responsible for the activation of Nek6 during mitosis and probably participates in the regulation of Nek7 as well. These findings support the conclusion that Nercc1/Nek9 and Nek6 represent a novel cascade of mitotic NIMA family protein kinases whose combined function is important for mitotic progression.

TINA interacts with the NIMA kinase in Aspergillus nidulans and negatively regulates astral microtubules during metaphase arrest

The tinA gene of Aspergillus nidulans encodes a protein that interacts with the NIMA mitotic protein kinase in a cell cycle-specific manner. Highly similar proteins are encoded in Neurospora crassa and Aspergillus fumigatus. TINA and NIMA preferentially interact in interphase and larger forms of TINA are generated during mitosis. Localization studies indicate that TINA is specifically localized to the spindle pole bodies only during mitosis in a microtubule-dependent manner. Deletion of tinA alone is not lethal but displays synthetic lethality in combination with the anaphase-promoting complex/cyclosome mutation bimE7. At the bimE7 metaphase arrest point, lack of TINA enhanced the nucleation of bundles of cytoplasmic microtubules from the spindle pole bodies. These microtubules interacted to form spindles joined in series via astral microtubules as revealed by live cell imaging. Because TINA is modified and localizes to the spindle pole bodies at mitosis, and lack of TINA causes enhanced production of cytoplasmic microtubules at metaphase arrest, we suggest TINA is involved in negative regulation of the astral microtubule organizing capacity of the spindle pole bodies during metaphase.

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.

Induction of apoptosis by sphingoid long-chain bases in Aspergillus nidulans

Sphingolipid metabolism is implicated to play an important role in apoptosis. Here we show that dihydrosphingosine (DHS) and phytosphingosine (PHS), two major sphingoid bases of fungi, have potent fungicidal activity with remarkably high structural and stereochemical specificity against Aspergillus nidulans. In fact, only naturally occurring DHS and PHS are active. Further analysis revealed that DHS and PHS induce rapid DNA condensation independent of mitosis, large-scale DNA fragmentation, and exposure of phosphatidylserine, all common morphological features characteristic of apoptosis, suggesting that DHS and PHS induce apoptosis in A. nidulans. The finding that DNA fragmentation requires protein synthesis, which implies that an active process is involved, further supports this proposition. The induction of apoptosis by DHS and PHS is associated with the rapid accumulation of reactive oxygen species (ROS). However, ROS are not required for apoptosis induced by DHS and PHS, as scavenging of ROS by a free radical spin trap has no effect. We further demonstrate that apoptosis induced by DHS and PHS is independent of metacaspase function but requires mitochondrial function. Together, the results suggest that DHS and PHS induce a type of apoptosis in A. nidulans most similar to the caspase-independent apoptosis observed in mammalian systems. As A. nidulans is genetically tractable, this organism should be an ideal model system for dissecting sphingolipid signaling in apoptosis and, importantly, for further elucidating the molecular basis of caspase-independent apoptosis.

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.

Dynamic distribution of BIMG(PP1) in living hyphae of Aspergillus indicates a novel role in septum formation

Mutation of bimG, the major protein phosphatase 1 gene in Aspergillus nidulans, causes multiple cell cycle and hyphal growth defects that are associated with overphosphorylation of subcellular components. We have used functional translational fusions with the green fluorescent protein (GFP) to show that BIMG has at least four discrete locations within growing hyphae. Three of these locations, the hyphal tip, the spindle pole body and the nucleus, correlate with previously known requirements for bimG(PP1) in mitosis and hyphal growth and are highly dynamic. BIMG-GFP in the hyphal tip seemed to be associated with the plasma membrane and formed a collar of fluorescence within the apical dome. The distribution of nuclear BIMG-GFP varied depending on nutritional conditions; on poor medium, it concentrated more in the nucleolus than in the nucleoplasm, whereas on rich medium, it was more evenly distributed between the two nuclear regions. The association of BIMG-GFP with developing septa was transient, and we present evidence that BIMG phosphatase plays a direct role in septum formation, distinct from its role in mitosis. We conclude that, by being physically present at several sites, the BIMG phosphatase has roles in multiple cellular processes.

Nercc1, a mammalian NIMA-family kinase, binds the Ran GTPase and regulates mitotic progression

The protein kinase NIMA is an indispensable pleiotropic regulator of mitotic progression in Aspergillus. Although several mammalian NIMA-like kinases (Neks) are known, none appears to have the broad importance for mitotic regulation attributed to NIMA. Nercc1 is a new NIMA-like kinase that regulates chromosome alignment and segregation in mitosis. Its NIMA-like catalytic domain is followed by a noncatalytic tail containing seven repeats homologous to those of the Ran GEF, RCC1, a Ser/Thr/Pro-rich segment, and a coiled-coil domain. Nercc1 binds to another NIMA-like kinase, Nek6, and also binds specifically to the Ran GTPase through both its catalytic and its RCC1-like domains, preferring RanGDP in vivo. Nercc1 exists as a homooligomer and can autoactivate in vitro by autophosphorylation. Nercc1 is a cytoplasmic protein that is activated during mitosis and is avidly phosphorylated by active p34(Cdc2). Microinjection of anti-Nercc1 antibodies in prophase results in spindle abnormalities and/or chromosomal misalignment. In Ptk2 cells the outcome is prometaphase arrest or aberrant chromosome segregation and aneuploidy, whereas in CFPAC-1 cells prolonged arrest in prometaphase is the usual response. Nercc1 and its partner Nek6 represent a new signaling pathway that regulates mitotic progression.

Nek2 localizes to multiple sites in mitotic cells, suggesting its involvement in multiple cellular functions during the cell cycle

Nek2 is a mammalian protein kinase that is structurally homologous to NIMA, a mitotic regulator in Aspergillus nidulans. To understand the possible cellular processes in which Nek2 participates during the cell cycle, we investigated the expression and subcellular localization of Nek2 in mitotic cells. The Nek2 protein levels were observed to be regulated in a cell cycle stage-specific manner in cultured cells. The cell cycle stage specificity of Nek2 expression was also confirmed in cells undergoing mitosis in vivo. Nek2 proteins were localized in both the nucleus and cytoplasm throughout the cell cycle, but exhibited dynamic changes in distribution, depending on the cell cycle stage. Nek2 was associated with chromosomes from prophase to metaphase and then was dissociated upon entering into anaphase. Nek2 then appeared at the midbody of the cytoplasmic bridge at telophase. Nek2 was also associated with the centrosome throughout the cell cycle as observed previously by others. Additionally, the nuclear localization of Nek2 was increased during S phase. Such dynamic behavior of Nek2 suggests that Nek2 may be a mitotic regulator that is involved in diverse cell cycle events.

Comparison of morphogenetic networks of filamentous fungi and yeast

Fungi generally display either of two growth modes, yeast-like or filamentous, whereas dimorphic fungi, upon environmental stimuli, are able to switch between the yeast-like and the filamentous growth mode. Signal transduction pathways have been elucidated in the budding yeast Saccharomyces cerevisiae, establishing a morphogenetic network that links cell-cycle events with cellular morphogenesis. Recent molecular genetic studies in several filamentous fungal model systems revealed key components required for distinct steps from fungal spore germination to the maintenance of polar hyphal growth, mycelium formation, and nuclear division. This allows a mechanistic comparison of yeast-like and hyphal growth and the establishment of a core model morphogenetic network for filamentous growth including signaling via the cAMP pathway, Rho modules, and cell cycle kinases. Appreciating similarities between morphogenetic networks of the unicellular yeasts and the multicellular filamentous fungi will open new research directions, help in isolating the central network components, and ultimately pave the way to elucidate the central differences (of many) that distinguish, e.g., the growth mode of filamentous fungi from that of their yeast-like relatives, the role of cAMP signaling, and nuclear division.

Mitotic kinases as regulators of cell division and its checkpoints

Mitosis and cytokinesis are undoubtedly the most spectacular parts of the cell cycle. Errors in the choreography of these processes can lead to aneuploidy or genetic instability, fostering cell death or disease. Here, I give an overview of the many mitotic kinases that regulate cell division and the fidelity of chromosome transmission.

Extragenic suppressors of the nimX2(cdc2) mutation of Aspergillus nidulans affect nuclear division, septation and conidiation

The Aspergillus nidulans NIMX(CDC2) protein kinase has been shown to be required for both the G(2)/M and G(1)/S transitions, and recent evidence has implicated a role for NIMX(CDC2) in septation and conidiation. While much is understood of its G(2)/M function, little is known about the functions of NIMX(CDC2) during G(1)/S, septation, and conidiophore development. In an attempt to better understand how NIMX(CDC2) is involved in these processes, we have isolated four extragenic suppressors of the A. nidulans nimX2(cdc2) temperature-sensitive mutation. Mutation of these suppressor genes, designated snxA-snxD for suppressor of nimX, affects nuclear division, septation, and conidiation. The cold-sensitive snxA1 mutation leads to arrest of nuclear division during G(1) or early S. snxB1 causes hyperseptation in the hyphae and sensitivity to hydroxyurea, while snxC1 causes septation in the conidiophore stalk and aberrant conidiophore structure. snxD1 leads to slight septation defects and hydroxyurea sensitivity. The additional phenotypes that result from the suppressor mutations provide genetic evidence that NIMX(CDC2) affects septation and conidiation in addition to nuclear division, and cloning and biochemical analysis of these will allow a better understanding of the role of NIMX(CDC2) in these processes.

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.

Mitotic histone H3 phosphorylation by the NIMA kinase in Aspergillus nidulans

Phosphorylation of histone H3 serine 10 correlates with chromosome condensation and is required for normal chromosome segregation in Tetrahymena. This phosphorylation is dependent upon activation of the NIMA kinase in Aspergillus nidulans. NIMA expression also induces Ser-10 phosphorylation inappropriately in S phase-arrested cells and in the absence of NIMX(cdc2) activity. At mitosis, NIMA becomes enriched on chromatin and subsequently localizes to the mitotic spindle and spindle pole bodies. The chromatin-like localization of NIMA early in mitosis is tightly correlated with histone H3 phosphorylation. Finally, NIMA can phosphorylate histone H3 Ser-10 in vitro, suggesting that NIMA is a mitotic histone H3 kinase, perhaps helping to explain how NIMA promotes chromatin condensation in A. nidulans and when expressed in other eukaryotes.

NIMA-related kinase 2 (Nek2), a cell-cycle-regulated protein kinase localized to centrosomes, is complexed to protein phosphatase 1

The cell cycle-regulated protein serine/threonine NIMA-related kinase 2 (Nek2), which shows a predominant localization at centrosomes, is identified as a protein which interacts with protein phosphatase 1 (PP1) using the yeast two-hybrid system. Complex formation between Nek2 and PP1 is supported by co-precipitation of the two proteins using transfected expression constructs of Nek2 and the endogenous Nek2/PP1 proteins. The sequence KVHF in the C-terminal region of Nek2, which conforms to the consensus PP1-binding motif, is shown to be essential for the interaction of Nek2 with PP1. Nek2 activity increases with autophosphorylation and addition of phosphatase inhibitors and decreases in the presence of PP1. PP1 is a substrate for Nek2 and phosphorylation of PP1gamma(1) on two C-terminal sites reduces its phosphatase activity. The presence of a ternary complex containing centrosomal Nek2-associated protein (C-Nap1), Nek2 and PP1 has also been demonstrated, and C-Nap1 is shown to be a substrate for both Nek2 and PP1 in vitro and in cell extracts. The implications of kinase-phosphatase complex formation involving Nek2 and PP1 are discussed in terms of the coordination of centrosome separation with cell cycle progression.