A NIMA-related kinase, Fa2p, localizes to a novel site in the proximal cilia of Chlamydomonas and mouse kidney cells

NEK8, a NIMA-family protein kinase at the core of the ciliary INV complex

Here we describe the current knowledge about the ciliary kinase NEK8, highlighting what we know and what we don't know about its regulation, substrates and potential functions. We also review the literature about the pathological consequences of different NEK8 variants in patients of nephronophthisis, renal-hepatic-pancreatic dysplasia and autosomal dominant polycystic kidney disease, three different types of ciliopathies. NEK8 belongs to the NIMA family of serine/threonine protein kinases. Like its closest relative, NEK9, it contains a protein kinase and an RCC1 domain, but lacks the C-terminal region that is key for NEK9's regulation as a G2/M kinase. Importantly, NEK8 localizes to cilia as part of a multimeric protein complex that assembles in a fibrillar fashion at the proximal half of this signaling organelle, defining what is known as the INV compartment. NEK8 and its INV compartment partners inversin, ANKS6 and NPHP3 are necessary for left-right determination and the correct development of different organs such as the kidney, the heart and the liver. But the kinase substrates, regulatory mechanism and activating cues and thus the molecular basis of NEK8 important physiological roles remain elusive. We present the current findings regarding NEK8 and also highlight what we miss in order to progress towards the understanding of the kinase and the function of the INV complex at the cilia.

Assembly of FAP93 at the proximal axoneme in Chlamydomonas cilia

To identify proteins specific to the proximal ciliary axoneme, we used iTRAQ to compare short (~2 μm) and full-length (~11 μm) axonemes of Chlamydomonas. Known components of the proximal axoneme such as minor dynein heavy chains and LF5 kinase as well as the ciliary tip proteins FAP256 (CEP104) and EB1 were enriched in short axonemes whereas proteins present along the length of the axoneme were of similar abundance in both samples. The iTRAQ analysis revealed that FAP93, a protein of unknown function, and protein phosphatase 2A (PP2A) are enriched in the short axonemes. Consistently, immunoblots show enrichment of FAP93 and PP2A in short axonemes and immunofluorescence confirms the localization of FAP93 and enrichment of PP2A at the proximal axoneme. Ciliary regeneration reveals that FAP93 assembles continuously but more slowly than other axonemal structures and terminates at 1.03 μm in steady-state axonemes. The length of FAP93 assembly correlates with ciliary length, demonstrating ciliary length-dependent assembly of FAP93. Dikaryon rescue experiments show that FAP93 can assemble independently of IFT transport. In addition, FRAP analysis of GFP-tagged FAP93 demonstrates that FAP93 is stably anchored in the axoneme. FAP93 may function as a scaffold for assembly of other specific proteins at the proximal axoneme.

Plasmodium NEK1 coordinates MTOC organisation and kinetochore attachment during rapid mitosis in male gamete formation

Mitosis is an important process in the cell cycle required for cells to divide. Never in mitosis (NIMA)-like kinases (NEKs) are regulators of mitotic functions in diverse organisms. Plasmodium spp., the causative agent of malaria is a divergent unicellular haploid eukaryote with some unusual features in terms of its mitotic and nuclear division cycle that presumably facilitate proliferation in varied environments. For example, during the sexual stage of male gametogenesis that occurs within the mosquito host, an atypical rapid closed endomitosis is observed. Three rounds of genome replication from 1N to 8N and successive cycles of multiple spindle formation and chromosome segregation occur within 8 min followed by karyokinesis to generate haploid gametes. Our previous Plasmodium berghei kinome screen identified 4 Nek genes, of which 2, NEK2 and NEK4, are required for meiosis. NEK1 is likely to be essential for mitosis in asexual blood stage schizogony in the vertebrate host, but its function during male gametogenesis is unknown. Here, we study NEK1 location and function, using live cell imaging, ultrastructure expansion microscopy (U-ExM), and electron microscopy, together with conditional gene knockdown and proteomic approaches. We report spatiotemporal NEK1 location in real-time, coordinated with microtubule organising centre (MTOC) dynamics during the unusual mitoses at various stages of the Plasmodium spp. life cycle. Knockdown studies reveal NEK1 to be an essential component of the MTOC in male cell differentiation, associated with rapid mitosis, spindle formation, and kinetochore attachment. These data suggest that P. berghei NEK1 kinase is an important component of MTOC organisation and essential regulator of chromosome segregation during male gamete formation.

Chlamydomonas CHT7 is involved in repressing DNA replication and mitotic genes during synchronous growth

In the green alga Chlamydomonas reinhardtii, regulation of the cell cycle in response to external cues is critical for survival in a changing environment. The loss of the nuclear COMPROMISED HYDROLYSIS OF TRIACYLGLYCEROLS 7 (CHT7) protein affects the expression of many genes especially in response to nitrogen availability. Cells lacking CHT7 exhibit abnormal cell morphology following nitrogen deprivation and fail to resume normal cell division after N resupply. To investigate the function of CHT7 in the regulation of cell cycle-related pathways, cells were synchronized, and RNA-seq analysis was performed during various stages of the cell cycle. In the cht7 mutant following nitrogen deprivation, the cells were not dividing, but a subset of cell cycle genes involved in DNA replication and mitosis were found to be derepressed, suggesting that the CHT7 protein plays a role in cell cycle regulation that is opposite to that of the mitotic cyclin-dependent kinases. Furthermore, genes for cell wall synthesis and remodeling were found to be abnormally induced in nondividing cht7 cells; this misregulation may deplete cellular resources and thus contribute to cell death following nitrogen deprivation. Lastly, 43 minimally characterized kinases were found to be highly misregulated in cht7. Further analysis suggested that some of these CHT7-regulated kinases may be related to the MAP3K and Aurora-like kinases, while others are unique. Together, these results suggest a role of CHT7 in transcriptional regulation of the cell cycle and reveal several pathways and genes whose expression appears to be subject to a CHT7-mediated regulatory network.

Anterior-posterior pattern formation in ciliates

As single cells, ciliates build, duplicate, and even regenerate complex cortical patterns by largely unknown mechanisms that precisely position organelles along two cell‐wide axes: anterior–posterior and circumferential (left–right). We review our current understanding of intracellular patterning along the anterior–posterior axis in ciliates, with emphasis on how the new pattern emerges during cell division. We focus on the recent progress at the molecular level that has been driven by the discovery of genes whose mutations cause organelle positioning defects in the model ciliate Tetrahymena thermophila. These investigations have revealed a network of highly conserved kinases that are confined to either anterior or posterior domains in the cell cortex. These pattern‐regulating kinases create zones of cortical inhibition that by exclusion determine the precise placement of organelles. We discuss observations and models derived from classical microsurgical experiments in large ciliates (including Stentor) and interpret them in light of recent molecular findings in Tetrahymena. In particular, we address the involvement of intracellular gradients as vehicles for positioning organelles along the anterior‐posterior axis.

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

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

Nek8445, a protein kinase required for microtubule regulation and cytokinesis in Giardia lamblia

Giardia has 198 Nek kinases whereas humans have only 11. Giardia has a complex microtubule cytoskeleton that includes eight flagella and several unique microtubule arrays that are utilized for parasite attachment and facilitation of rapid mitosis and cytokinesis. The need to regulate these structures may explain the parallel expansion of the number of Nek family kinases. Here we use live and fixed cell imaging to uncover the role of Nek8445 in regulating Giardia cell division. We demonstrate that Nek8445 localization is cell cycle regulated and this kinase has a role in regulating overall microtubule organization. Nek8445 depletion results in short flagella, aberrant ventral disk organization, loss of the funis, defective axoneme exit, and altered cell shape. The axoneme exit defect is specific to the caudal axonemes, which exit from the posterior of the cell, and this defect correlates with rounding of the cell posterior and loss of the funis. Our findings implicate a role for the funis in establishing Giardia's cell shape and guiding axoneme docking. On a broader scale our results support the emerging view that Nek family kinases have a general role in regulating microtubule organization.

The primary cilium: Its role as a tumor suppressor organelle

The primary cilium is an organelle that nearly all cells within the body contain. Its function is to sense the extracellular environment through its abundance of receptors and linked signaling pathways, working as an antenna. Ciliary defects lead to different pathologies. In particular, many tumors lose primary cilia, and this is linked with negative implications for the cell such as an increase in malignancy. In this work we will go through the knowledge of the role of primary cilia in normal conditions, how it is involved in diverse signaling pathways, and in disease, particularly in cancer, highlighting its tumor suppressor properties.

Primary cilium loss in mammalian cells occurs predominantly by whole-cilium shedding

The primary cilium is a central signaling hub in cell proliferation and differentiation and is built and disassembled every cell cycle in many animal cells. Disassembly is critically important, as misregulation or delay of cilia loss leads to cell cycle defects. The physical means by which cilia are lost are poorly understood but are thought to involve resorption of ciliary components into the cell body. To investigate cilium loss in mammalian cells, we used live-cell imaging to comprehensively characterize individual events. The predominant mode of cilium loss was rapid deciliation, in which the membrane and axoneme of the cilium was shed from the cell. Gradual resorption was also observed, as well as events in which a period of gradual resorption was followed by rapid deciliation. Deciliation resulted in intact shed cilia that could be recovered from culture medium and contained both membrane and axoneme proteins. We modulated levels of katanin and intracellular calcium, two putative regulators of deciliation, and found that excess katanin promotes cilia loss by deciliation, independently of calcium. Together, these results suggest that mammalian ciliary loss involves a tunable decision between deciliation and resorption.

Mitotic Regulation by NEK Kinase Networks

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

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

A signaling module of NIMA-related kinases (Neks) regulates two kinesins, Mklp2 and Kif14, to spatiotemporally coordinate their subcellular localizations and activities. This is important for faithful completion of cytokinesis and reveals novel mechanisms by which Neks regulate late mitosis.

In mitosis, cells undergo a precisely orchestrated series of spatiotemporal changes in cytoskeletal structure to divide their genetic material. These changes are coordinated by a sophisticated network of protein–protein interactions and posttranslational modifications. In this study, we report a bifurcation in a signaling cascade of the NIMA-related kinases (Neks) Nek6, Nek7, and Nek9 that is required for the localization and function of two kinesins essential for cytokinesis, Mklp2 and Kif14. We demonstrate that a Nek9, Nek6, and Mklp2 signaling module controls the timely localization and bundling activity of Mklp2 at the anaphase central spindle. We further show that a separate Nek9, Nek7, and Kif14 signaling module is required for the recruitment of the Rho-interacting kinase citron to the anaphase midzone. Our findings uncover an anaphase-specific function for these effector kinesins that is controlled by specific Nek kinase signaling modules to properly coordinate cytokinesis.

A Forward Genetic Screen and Whole Genome Sequencing Identify Deflagellation Defective Mutants in Chlamydomonas, Including Assignment of ADF1 as a TRP Channel

With rare exception, ciliated cells entering mitosis lose their cilia, thereby freeing basal bodies to serve as centrosomes in the formation of high-fidelity mitotic spindles. Cilia can be lost by shedding or disassembly, but either way, it appears that the final release may be via a coordinated severing of the nine axonemal outer doublet microtubules linking the basal body to the ciliary transition zone. Little is known about the mechanism or regulation of this important process. The stress-induced deflagellation response of Chlamydomonas provides a basis to identifying key players in axonemal severing. In an earlier screen we uncovered multiple alleles for each of three deflagellation genes, ADF1, FA1, and FA2 Products of the two FA genes localize to the site of axonemal severing and encode a scaffolding protein and a member of the NIMA-related family of ciliary-cell cycle kinases. The identity of the ADF1 gene remained elusive. Here, we report a new screen using a mutagenesis that yields point mutations in Chlamydomonas, an enhanced screening methodology, and whole genome sequencing. We isolated numerous new alleles of the three known genes, and one or two alleles each of at least four new genes. We identify ADF1 as a TRP ion channel, which we suggest may reside at the flagellar transition zone.

DNAH11 Localization in the Proximal Region of Respiratory Cilia Defines Distinct Outer Dynein Arm Complexes

Primary ciliary dyskinesia (PCD) is a recessively inherited disease that leads to chronic respiratory disorders owing to impaired mucociliary clearance. Conventional transmission electron microscopy (TEM) is a diagnostic standard to identify ultrastructural defects in respiratory cilia but is not useful in approximately 30% of PCD cases, which have normal ciliary ultrastructure. DNAH11 mutations are a common cause of PCD with normal ciliary ultrastructure and hyperkinetic ciliary beating, but its pathophysiology remains poorly understood. We therefore characterized DNAH11 in human respiratory cilia by immunofluorescence microscopy (IFM) in the context of PCD. We used whole-exome and targeted next-generation sequence analysis as well as Sanger sequencing to identify and confirm eight novel loss-of-function DNAH11 mutations. We designed and validated a monoclonal antibody specific to DNAH11 and performed high-resolution IFM of both control and PCD-affected human respiratory cells, as well as samples from green fluorescent protein (GFP)-left-right dynein mice, to determine the ciliary localization of DNAH11. IFM analysis demonstrated native DNAH11 localization in only the proximal region of wild-type human respiratory cilia and loss of DNAH11 in individuals with PCD with certain loss-of-function DNAH11 mutations. GFP-left-right dynein mice confirmed proximal DNAH11 localization in tracheal cilia. DNAH11 retained proximal localization in respiratory cilia of individuals with PCD with distinct ultrastructural defects, such as the absence of outer dynein arms (ODAs). TEM tomography detected a partial reduction of ODAs in DNAH11-deficient cilia. DNAH11 mutations result in a subtle ODA defect in only the proximal region of respiratory cilia, which is detectable by IFM and TEM tomography.

A NIMA-related kinase, CNK4, regulates ciliary stability and length

NIMA-related kinases (Nrks or Neks) have emerged as key regulators of ciliogenesis. In human, mutations in Nek1 and Nek8 cause cilia-related disorders. The ciliary functions of Nrks are mostly revealed by genetic studies; however, the underlying mechanisms are not well understood. Here we show that a Chlamydomonas Nrk, CNK4, regulates ciliary stability and length. CNK4 is localized to the basal body region and the flagella. The cnk4-null mutant exhibited long flagella, with formation of flagellar bulges. The flagella gradually became curled at the bulge formation site, leading to flagellar loss. Electron microscopy shows that the curled flagella involved curling and degeneration of axonemal microtubules. cnk4 mutation resulted in flagellar increases of IFT trains, as well as its accumulation at the flagellar bulges. IFT speeds were not affected, however, IFT trains frequently stalled, leading to reduced IFT frequencies. These data are consistent with a model in which CNK4 regulates microtubule dynamics and IFT to control flagellar stability and length.

The NIMA-like kinase Nek2 is a key switch balancing cilia biogenesis and resorption in the development of left-right asymmetry

Vertebrate left-right (LR) asymmetry originates at a transient left-right organizer (LRO), a ciliated structure where cilia play a crucial role in breaking symmetry. However, much remains unknown about the choreography of cilia biogenesis and resorption at this organ. We recently identified a mutation affecting NEK2, a member of the NIMA-like serine-threonine kinase family, in a patient with congenital heart disease associated with abnormal LR development. Here, we report how Nek2 acts through cilia to influence LR patterning. Both overexpression and knockdown of nek2 in Xenopus result in abnormal LR development and reduction of LRO cilia count and motility, phenotypes that are modified by interaction with the Hippo signaling pathway. nek2 knockdown leads to a centriole defect at the LRO, consistent with the known role of Nek2 in centriole separation. Nek2 overexpression results in premature ciliary resorption in cultured cells dependent on function of the tubulin deacetylase Hdac6. Finally, we provide evidence that the known interaction between Nek2 and Nup98, a nucleoporin that localizes to the ciliary base, is important for regulating cilium resorption. Together, these data show that Nek2 is a switch balancing ciliogenesis and resorption in the development of LR asymmetry.

A NIMA-Related Kinase Suppresses the Flagellar Instability Associated with the Loss of Multiple Axonemal Structures

CCDC39 and CCDC40 were first identified as causative mutations in primary ciliary dyskinesia patients; cilia from patients show disorganized microtubules, and they are missing both N-DRC and inner dynein arms proteins. In Chlamydomonas, we used immunoblots and microtubule sliding assays to show that mutants in CCDC40 (PF7) and CCDC39 (PF8) fail to assemble N-DRC, several inner dynein arms, tektin, and CCDC39. Enrichment screens for suppression of pf7; pf8 cells led to the isolation of five independent extragenic suppressors defined by four different mutations in a NIMA-related kinase, CNK11. These alleles partially rescue the flagellar length defect, but not the motility defect. The suppressor does not restore the missing N-DRC and inner dynein arm proteins. In addition, the cnk11 mutations partially suppress the short flagella phenotype of N-DRC and axonemal dynein mutants, but do not suppress the motility defects. The tpg1 mutation in TTLL9, a tubulin polyglutamylase, partially suppresses the length phenotype in the same axonemal dynein mutants. In contrast to cnk11, tpg1 does not suppress the short flagella phenotype of pf7. The polyglutamylated tubulin in the proximal region that remains in the tpg1 mutant is reduced further in the pf7; tpg1 double mutant by immunofluorescence. CCDC40, which is needed for docking multiple other axonemal complexes, is needed for tubulin polyglutamylation in the proximal end of the flagella. The CCDC39 and CCDC40 proteins are likely to be involved in recruiting another tubulin glutamylase(s) to the flagella. Another difference between cnk11-1 and tpg1 mutants is that cnk11-1 cells show a faster turnover rate of tubulin at the flagellar tip than in wild-type flagella and tpg1 flagella show a slower rate. The double mutant shows a turnover rate similar to tpg1, which suggests the faster turnover rate in cnk11-1 flagella requires polyglutamylation. Thus, we hypothesize that many short flagella mutants in Chlamydomonas have increased instability of axonemal microtubules. Both CNK11 and tubulin polyglutamylation play roles in regulating the stability of axonemal microtubules.

C. elegans NIMA-related kinases NEKL-2 and NEKL-3 are required for the completion of molting

Caenorhabditis elegans molting is a process during which the apical extracellular matrix of the epidermis, the cuticle, is remodeled through a process of degradation and re-synthesis. Using a genetic approach, we identified nekl-3 as essential for the completion of molting. NEKL-3 is highly similar to the mammalian NEK kinase family members NEK6 and NEK7. Animals homozygous for a hypomorphic mutation in nekl-3, sv3, had a novel molting defect in which the central body region, but not the head or tail, was unable to shed the old cuticle. In contrast, a null mutation in nekl-3, gk506, led to complete enclosure within the old cuticle. nekl-2, which is most similar to mammalian NEK8, was also essential for molting. Mosaic analyses demonstrated that NEKL-2 and NEKL-3 were specifically required within the large epidermal syncytium, hyp7, to facilitate molting. Consistent with this, NEKL-2 and NEKL-3 were expressed at the apical surface of hyp7 where they localized to small spheres or tubular structures. Inhibition of nekl-2, but not nekl-3, led to the mislocalization of LRP-1/megalin, a cell surface receptor for low-density lipoprotein (LDL)-binding proteins. In addition, nekl-2 inhibition led to the mislocalization of several other endosome-associated proteins. Notably, LRP-1 acts within hyp7 to facilitate completion of molting, suggesting at least one mechanism by which NEKL-2 may influence molting. Notably, our studies failed to reveal a requirement for NEKL-2 or NEKL-3 in cell division, a function reported for several mammalian NEKs including NEK6 and NEK7. Our findings provide the first genetic and in vivo evidence for a role of NEK family members in endocytosis, which may be evolutionarily conserved.

A microbial avenue to cell cycle control in the plant superkingdom

Research in yeast and animals has resulted in a well-supported consensus model for eukaryotic cell cycle control. The fit of this model to early diverging eukaryotes, such as the plant kingdom, remains unclear. Using the green alga Chlamydomonas reinhardtii, we developed an efficient pipeline, incorporating robotics, semiautomated image analysis, and deep sequencing, to molecularly identify >50 genes, mostly conserved in higher plants, specifically required for cell division but not cell growth. Mutated genes include the cyclin-dependent kinases CDKA (resembling yeast and animal Cdk1) and the plant-specific CDKB. The Chlamydomonas cell cycle consists of a long G1 during which cells can grow >10-fold, followed by multiple rapid cycles of DNA replication and segregation. CDKA and CDKB execute nonoverlapping functions: CDKA promotes transition between G1 and entry into the division cycle, while CDKB is essential specifically for spindle formation and nuclear division, but not for DNA replication, once CDKA-dependent initiation has occurred. The anaphase-promoting complex is required for similar steps in the Chlamydomonas cell cycle as in Opisthokonts; however, the spindle assembly checkpoint, which targets the APC in Opisthokonts, appears severely attenuated in Chlamydomonas, based on analysis of mutants affecting microtubule function. This approach allows unbiased integration of the consensus cell cycle control model with innovations specific to the plant lineage.

Primary cilium: an elaborate structure that blocks cell division?

A primary cilium is a microtubule-based membranous protrusion found in almost all cell types. A primary cilium has a "9+0" axoneme that distinguishes this ancient organelle from the canonical motile "9+2" cilium. A primary cilium is the sensory center of the cell that regulates cell proliferation and embryonic development. The primary ciliary pocket is a specialized endocytic membrane domain in the basal region. The basal body of a primary cilium exists as a form of the centriole during interphase of the cell cycle. Although conventional thinking suggests that the cell cycle regulates centrosomal changes, recent studies suggest the opposite, that is, centrosomal changes regulate the cell cycle. In this regard, centrosomal kinase Aurora kinase A (AurA), Polo-like kinase 1 (Plk1), and NIMA related Kinase (Nek or Nrk) propel cell cycle progression by promoting primary cilia disassembly which indicates a non-mitotic function. However, the persistence of primary cilia during spermatocyte division challenges the dominate idea of the incompatibility of primary cilia and cell division. In this review, we demonstrate the detailed structure of primary cilia and discuss the relationship between primary cilia disassembly and cell cycle progression on the background of various mitotic kinases.

“Stop Ne(c)king around”: How interactomics contributes to functionally characterize Nek family kinases

Aside from Polo and Aurora, a third but less studied kinase family involved in mitosis regulation is the never in mitosis-gene A (NIMA)-related kinases (Neks). The founding member of this family is the sole member NIMA of Aspergillus nidulans, which is crucial for the initiation of mitosis in that organism. All 11 human Neks have been functionally assigned to one of the three core functions established for this family in mammals: (1) centrioles/mitosis; (2) primary ciliary function/ciliopathies; and (3) DNA damage response (DDR). Recent findings, especially on Nek 1 and 8, showed however, that several Neks participate in parallel in at least two of these contexts: primary ciliary function and DDR. In the core section of this in-depth review, we report the current detailed functional knowledge on each of the 11 Neks. In the discussion, we return to the cross-connections among Neks and point out how our and other groups’ functional and interactomics studies revealed that most Neks interact with protein partners associated with two if not all three of the functional contexts. We then raise the hypothesis that Neks may be the connecting regulatory elements that allow the cell to fine tune and synchronize the cellular events associated with these three core functions. The new and exciting findings on the Nek family open new perspectives and should allow the Neks to finally claim the attention they deserve in the field of kinases and cell cycle biology.

From the cytoplasm into the cilium: bon voyage

The primary cilium compartmentalizes a tiny fraction of the cell surface and volume, yet many proteins are highly enriched in this area and so efficient mechanisms are necessary to concentrate them in the ciliary compartment. Here we review mechanisms that are thought to deliver protein cargo to the base of cilia and are likely to interact with ciliary gating mechanisms. Given the immense variety of ciliary cytosolic and transmembrane proteins, it is almost certain that multiple, albeit frequently interconnected, pathways mediate this process. It is also clear that none of these pathways is fully understood at the present time. Mechanisms that are discussed below facilitate ciliary localization of structural and signaling molecules, which include receptors, G-proteins, ion channels, and enzymes. These mechanisms form a basis for every aspect of cilia function in early embryonic patterning, organ morphogenesis, sensory perception and elsewhere.

The kinases LF4 and CNK2 control ciliary length by feedback regulation of assembly and disassembly rates

BACKGROUND

Many of the diverse functions of cilia depend upon tight control of their length. Steady-state length reflects a balance between rates of ciliary assembly and disassembly, two parameters likely controlled by a length sensor of unknown identity or mechanism.

RESULTS

A null mutation in Chlamydomonas CNK2, a member of the evolutionarily conserved family of NIMA-related kinases, reveals feedback regulation of assembly and disassembly rates. cnk2-1 mutant cells have a mild long-flagella (lf) phenotype as a consequence of reduced rates of flagellar disassembly. This is in contrast to the strong lf mutant lf4-7, which exhibits an aberrantly high rate of assembly. Cells carrying both mutations have even longer flagella than lf4-7 single mutants do. In addition to their high rate of assembly, lf4-7 mutants have a CNK2-dependent increase in disassembly rate. Finally, cnk2-1 cells have a decreased rate of turnover of flagellar subunits at the tip of the flagellum, demonstrating that the effects on disassembly are compensated by a reduced rate of assembly.

CONCLUSIONS

We propose a model wherein CNK2 and LF4 modulate rates of disassembly and assembly respectively in a feedback loop that is activated when flagella exceed optimal length.

The N-DRC forms a conserved biochemical complex that maintains outer doublet alignment and limits microtubule sliding in motile axonemes

The nexin–dynein regulatory complex (N-DRC) is implicated in the control of dynein activity as a structural component of the nexin link. This study identifies several new subunits of the N-DRC and demonstrates for the first time that it forms a discrete biochemical complex that maintains outer doublet integrity and regulates microtubule sliding.

The nexin–dynein regulatory complex (N-DRC) is proposed to coordinate dynein arm activity and interconnect doublet microtubules. Here we identify a conserved region in DRC4 critical for assembly of the N-DRC into the axoneme. At least 10 subunits associate with DRC4 to form a discrete complex distinct from other axonemal substructures. Transformation of drc4 mutants with epitope-tagged DRC4 rescues the motility defects and restores assembly of missing DRC subunits and associated inner-arm dyneins. Four new DRC subunits contain calcium-signaling motifs and/or AAA domains and are nearly ubiquitous in species with motile cilia. However, drc mutants are motile and maintain the 9 + 2 organization of the axoneme. To evaluate the function of the N-DRC, we analyzed ATP-induced reactivation of isolated axonemes. Rather than the reactivated bending observed with wild-type axonemes, ATP addition to drc-mutant axonemes resulted in splaying of doublets in the distal region, followed by oscillatory bending between pairs of doublets. Thus the N-DRC provides some but not all of the resistance to microtubule sliding and helps to maintain optimal alignment of doublets for productive flagellar motility. These findings provide new insights into the mechanisms that regulate motility and further highlight the importance of the proximal region of the axoneme in generating flagellar bending.

CDKL5 regulates flagellar length and localizes to the base of the flagella in Chlamydomonas

Two mutations in LF5, which encodes a protein kinase orthologous to human CDKL5, cause abnormally long flagella in Chlamydomonas. The localization of LF5p to the very proximal region of flagella in WT cells is regulated by three other LF gene products, which make up the cytoplasmic length regulatory complex.

The length of Chlamydomonas flagella is tightly regulated. Mutations in four genes—LF1, LF2, LF3, and LF4—cause cells to assemble flagella up to three times wild-type length. LF2 and LF4 encode protein kinases. Here we describe a new gene, LF5, in which null mutations cause cells to assemble flagella of excess length. The LF5 gene encodes a protein kinase very similar in sequence to the protein kinase CDKL5. In humans, mutations in this kinase cause a severe form of juvenile epilepsy. The LF5 protein localizes to a unique location: the proximal 1 μm of the flagella. The proximal localization of the LF5 protein is lost when genes that make up the proteins in the cytoplasmic length regulatory complex (LRC)—LF1, LF2, and LF3—are mutated. In these mutants LF5p becomes localized either at the distal tip of the flagella or along the flagellar length, indicating that length regulation involves, at least in part, control of LF5p localization by the LRC.

Nek2 localises to the distal portion of the mother centriole/basal body and is required for timely cilium disassembly at the G2/M transition

The NIMA-related kinase Nek2 promotes centrosome separation at the G2/M transition and, consistent with this role, is known to be concentrated at the proximal ends of centrioles. Here, we show by immunofluorescence microscopy that Nek2 also localises to the distal portion of the mother centriole. Its accumulation at this site is cell cycle-dependent and appears to peak in late G2. These findings are consistent with previous data implicating Nek2 in promoting reorganisation of centrosome-anchored microtubules at the G2/M transition, given that microtubules are anchored at the subdistal appendages of the mother centriole in interphase. In addition, we report that siRNA-mediated depletion of Nek2 compromises the ability of cells to resorb primary cilia before the onset of mitosis, while overexpression of catalytically active Nek2A reduces ciliation and cilium length in serum-starved cells. Based on these findings, we propose that Nek2 has a role in promoting cilium disassembly at the onset of mitosis. We also present evidence that recruitment of Nek2 to the proximal ends of centrioles is dependent on one of its substrates, the centrosome cohesion protein C-Nap1.

Giardia lamblia Nek1 and Nek2 kinases affect mitosis and excystation

The NIMA-related serine/threonine kinases (Neks) function in the cell cycle and regulate ciliary and flagellar length. The Giardia lamblia genome encodes 198 Neks, of which 56 are predicted to be active. Here we believe that we report the first functional analysis of two G. lamblia Neks. The GlNek1 and GlNek2 kinase domains share 57% and 43% identity to the kinase domains of human Nek1 and Nek2, respectively. Both GlNeks are active in vitro, have dynamic relocalisation during the cell cycle, and are expressed throughout the life cycle, with GlNek1 being upregulated in cysts. Over-expression of inactive GlNek1 delays disassembly of the parental attachment disc and cytokinesis, whilst over-expression of either wild type GlNek1 or inactive mutant GlNek2 inhibits excystation.

The secret life of kinases: functions beyond catalysis

Protein phosphorylation participates in the regulation of all fundamental biological processes, and protein kinases have been intensively studied. However, while the focus was on catalytic activities, accumulating evidence suggests that non-catalytic properties of protein kinases are essential, and in some cases even sufficient for their functions. These non-catalytic functions include the scaffolding of protein complexes, the competition for protein interactions, allosteric effects on other enzymes, subcellular targeting, and DNA binding. This rich repertoire often is used to coordinate phosphorylation events and enhance the specificity of substrate phosphorylation, but also can adopt functions that do not rely on kinase activity. Here, we discuss such kinase independent functions of protein and lipid kinases focussing on kinases that play a role in the regulation of cell proliferation, differentiation, apoptosis, and motility.

The minimal kinome of Giardia lamblia illuminates early kinase evolution and unique parasite biology

Background

The major human intestinal pathogen Giardia lamblia is a very early branching eukaryote with a minimal genome of broad evolutionary and biological interest.

Results

To explore early kinase evolution and regulation of Giardia biology, we cataloged the kinomes of three sequenced strains. Comparison with published kinomes and those of the excavates Trichomonas vaginalis and Leishmania major shows that Giardia's 80 core kinases constitute the smallest known core kinome of any eukaryote that can be grown in pure culture, reflecting both its early origin and secondary gene loss. Kinase losses in DNA repair, mitochondrial function, transcription, splicing, and stress response reflect this reduced genome, while the presence of other kinases helps define the kinome of the last common eukaryotic ancestor. Immunofluorescence analysis shows abundant phospho-staining in trophozoites, with phosphotyrosine abundant in the nuclei and phosphothreonine and phosphoserine in distinct cytoskeletal organelles. The Nek kinase family has been massively expanded, accounting for 198 of the 278 protein kinases in Giardia. Most Neks are catalytically inactive, have very divergent sequences and undergo extensive duplication and loss between strains. Many Neks are highly induced during development. We localized four catalytically active Neks to distinct parts of the cytoskeleton and one inactive Nek to the cytoplasm.

Conclusions

The reduced kinome of Giardia sheds new light on early kinase evolution, and its highly divergent sequences add to the definition of individual kinase families as well as offering specific drug targets. Giardia's massive Nek expansion may reflect its distinctive lifestyle, biphasic life cycle and complex cytoskeleton.

Regulating the transition from centriole to basal body

The role of centrioles changes as a function of the cell cycle. Centrioles promote formation of spindle poles in mitosis and act as basal bodies to assemble primary cilia in interphase. Stringent regulations govern conversion between these two states. Although the molecular mechanisms have not been fully elucidated, recent findings have begun to shed light on pathways that regulate the conversion of centrioles to basal bodies and vice versa. Emerging studies also provide insights into how defects in the balance between centrosome and cilia function could promote ciliopathies and cancer.

Ciliary transition zone activation of phosphorylated Tctex-1 controls ciliary resorption, S-phase entry and fate of neural progenitors

Primary cilia are displayed during the G₀/G₁ phase of many cell types. Cilia are reabsorbed as cells prepare to re-enter the cell cycle, but the causal and molecular link between these two cellular events remains unclear. We show that phospho(T94)Tctex-1 is recruited to ciliary transition zones prior to S-phase entry and plays a pivotal role in both ciliary disassembly and cell cycle progression. Tctex-1’s role in S-phase entry, however, is dispensable in non-ciliated cells. Exogenously added phosphomimic Tctex-1 T94E accelerates cilium disassembly and S-phase entry. These results support a model in which the cilia act as a brake to prevent cell cycle progression. Mechanistic studies show the involvement of actin dynamics in Tctex-1 regulated cilium resorption. Phospho(T94)Tctex-1 is also selectively enriched at the ciliary transition zones of cortical neural progenitors, and plays a key role in controlling G₁ length, cell cycle entry, and fate determination of these cells during corticogenesis.

A role for central spindle proteins in cilia structure and function

Cytokinesis and ciliogenesis are fundamental cellular processes that require strict coordination of microtubule organization and directed membrane trafficking. These processes have been intensely studied, but there has been little indication that regulatory machinery might be extensively shared between them. Here, we show that several central spindle/midbody proteins (PRC1, MKLP-1, INCENP, centriolin) also localize in specific patterns at the basal body complex in vertebrate ciliated epithelial cells. Moreover, bioinformatic comparisons of midbody and cilia proteomes reveal a highly significant degree of overlap. Finally, we used temperature-sensitive alleles of PRC1/spd-1 and MKLP-1/zen-4 in C. elegans to assess ciliary functions while bypassing these proteins' early role in cell division. These mutants displayed defects in both cilia function and cilia morphology. Together, these data suggest the conserved reuse of a surprisingly large number of proteins in the cytokinetic apparatus and in cilia.

Posttranslational protein modifications in cilia and flagella

Tubulin and other flagellar and ciliary proteins are the substrates for a host of posttranslational modifications (PTMs), many of which have been highly conserved over evolutionary time. In addition to the binding of MAPs (microtubule-associated proteins) that provide a specific functionality, or the use of different tubulin isotypes to convey a specific function, most cells rely on an array of PTMs. These include phosphorylation, acetylation, glycylation, glutamylation, and methylation. The first and the last of this list are not unique to the tubulin in cilia and flagella, while the others are. This chapter will review briefly these varying modifications and will conclude with detailed methods for their detection and localization at the limit of resolution provided by electron microscopy.

Primary cilia and the cell cycle

Cilia are microtubule-based structures that protrude from the cell surface and function as sensors for mechanical and chemical environmental cues that regulate cellular differentiation or division. In metazoans, ciliary signaling is important during organismal development and in the homeostasis controls of adult tissues, with receptors for the Hedgehog, platelet derived growth factor (PDGF), Wnt, and other signaling cascades arrayed and active along the ciliary membrane. In normal cells, cilia are dynamically regulated during cell cycle progression: present in G0 and G1 cells, and usually in S/G2 cells, but almost invariably resorbed before mitotic entry, to reappear post-cytokinesis. This periodic resorption and reassembly of cilia, specified by the intrinsic cell cycle the intrinsic cell cycle machinery, influences the susceptibility of cells to the influence of extrinsic signals with cilia-associated receptors. Pathogenic conditions of mammals associated with loss of or defects in ciliary integrity include a number of developmental disorders, cystic syndromes in adults, and some cancers. With the continuing expansion of the list of human diseases associated with ciliary abnormalities, the identification of the cellular mechanisms regulating ciliary growth and disassembly has become a topic of intense research interest. Although these mechanisms are far from being understood, a number of recent studies have begun to identify key regulatory factors that may begin to offer insight into disease pathogenesis and treatment. In this chapter we will discuss the current state of knowledge regarding cell cycle control of ciliary dynamics, and provide general methods that can be applied to investigate cell cycle-dependent ciliary growth and disassembly.

Regulation of cilia assembly, disassembly, and length by protein phosphorylation

The exact mechanism by which cells are able to assemble, regulate, and disassemble cilia or flagella is not yet completely understood. Recent studies in several model systems, including Chlamydomonas, Tetrahymena, Leishmania, Caenorhabditis elegans, and mammals, provide increasing biochemical and genetic evidence that phosphorylation of multiple protein kinases plays a key role in cilia assembly, disassembly, and length regulation. Members of several protein kinase families--including aurora kinases, never in mitosis A (NIMA)-related protein kinases, mitogen-activated protein (MAP) kinases, and a novel cyclin-dependent protein kinase--are involved in the ciliary regulation process. Among the newly identified protein kinase substrates are Chlamydomonas kinesin-13 (CrKinesin13), a microtubule depolymerizer, and histone deacetylase 6 (HDAC6), a microtubule deacetylase. Chlamydomonas aurora/Ipl1p-like protein kinase (CALK) and CrKinesin13 are two proteins that undergo phosphorylation changes correlated with flagellar assembly or disassembly. CALK becomes phosphorylated when flagella are lost, whereas CrKinesin13 is phosphorylated when new flagella are assembled. Conversely, suppressing CrKinesin13 expression results in cells with shorter flagella.

The NIMA-related kinase NEK1 cycles through the nucleus

Mutations in NEK1 in mice are causal for cystic kidneys, and model the ciliopathy polycystic kidney disease caused by abnormal ciliary structure or signaling. NEK1 has previously been shown to localize near centrosomes and to play a role in centrosomal stability and ciliogenesis. Recent data suggest that the etiology of kidney cysts involves aberrant signaling from the primary cilium to the nucleus. Here we demonstrate that NEK1 contains functional nuclear localization signals, is exported from the nucleus via a nuclear export signal-dependent pathway and that the protein cycles through the nucleus. Our data suggest that NEK1 is a candidate to transduce messages from the ciliary-basal body region to the regulation of nuclear gene expression.

The NIMA-family kinase Nek3 regulates microtubule acetylation in neurons

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

Identification of dyneins that localize exclusively to the proximal portion of Chlamydomonas flagella

The movements of cilia and flagella are driven by multiple species of dynein heavy chains (DHCs), which constitute inner- and outer-dynein arms. In Chlamydomonas, 11 DHC proteins have been identified in the axoneme, but 14 genes encoding axonemal DHCs are present in the genome. Here, we assigned each previously unassigned DHC gene to a particular DHC protein and found that DHC3, DHC4 and DHC11 encode novel, relatively low abundance DHCs. Immunofluorescence microcopy revealed that DHC11 is localized exclusively to the proximal ∼2 μm region of the ∼12 μm long flagellum. Analyses of growing flagella suggested that DHC3 and DHC4 are also localized to the proximal region. By contrast, the DHC of a previously identified inner-arm dynein, dynein b, displayed an inverse distribution pattern. Thus, the proximal portion of the flagellar axoneme apparently differs in dynein composition from the remaining portion; this difference might be relevant to the special function performed by the flagellar base.

Quantitative high-throughput assays for flagella-based motility in chlamydomonas using plate-well image analysis and transmission correlation spectroscopy

Cilia are motile and sensory organelles with important roles in human development, physiology, and disease. Genetic defects in cilia produce a host of disease symptoms, including polycystic kidney disease, hydrocephalus, retinal degeneration, chronic bronchiectasis, infertility, and polydactyly. Currently, there are no known drugs for pharmacological remediation of ciliary defects. Small-molecule modulators of ciliary assembly or function would provide potential lead compounds for drug discovery efforts and would immediately be invaluable tools for a chemical biology approach to studying cilia. Here the author describes 2 assays for ciliary motility that are quantitative, automatable, cost-effective, and simple to implement. Both assays exploit cell-based strategies using the model organism Chlamydomonas reinhardtii. The first assay scores cilia-dependent gravitaxis by analyzing the cell distribution in wells of U-bottom microplates, using a simple and robust image analysis algorithm. The second assay measures motility directly by estimating the time required for cells to swim across a small illuminated aperture using a method equivalent to fluorescence correlation spectroscopy adapted to transmitted-light microscopy. The 2 assays have different advantages in terms of speed and sensitivity to small reductions in motility and may be most efficiently used in combination.

Phosphatase inhibitor 2 promotes acetylation of tubulin in the primary cilium of human retinal epithelial cells

BACKGROUND

Primary cilia are flagella-like projections from the centriole of mammalian cells that have a key role in cell signaling. Human diseases are linked to defects in primary cilia. Microtubules make up the axoneme of cilia and are selectively acetylated and this is thought to contribute to the stability of the structure. However, mechanisms to regulate tubulin acetylation in cilia are poorly understood.

RESULTS

Endogenous phosphatase inhibitor-2 (I-2) was found concentrated in cilia of human epithelial cells, and was localized to cilia early in the process of formation, prior to the full acetylation of microtubules. Knockdown of I-2 by siRNA significantly reduced the acetylation of microtubules in cilia, without a net decrease in whole cell tubulin acetylation. There was a reduction in the percentage of I-2 knockdown cells with a primary cilium, but no apparent alteration in the cilium length, suggesting no change in microtubule-based transport processes. Inhibition of either histone deacetylases with trichostatin A, or protein phosphatase-1 with calyculin A in I-2 knockdown cells partially rescued the acetylation of microtubules in cilia and the percentage of cells with a primary cilium.

CONCLUSION

The regulatory protein I-2 localizes to the primary cilium where it affects both Ser/Thr phosphorylation and is required for full tubulin acetylation. Rescue of tubulin acetylation in I-2 knockdown cells by different chemical inhibitors shows that deacetylases and phosphatases are functionally interconnected to regulate microtubules. As a multifunctional protein, I-2 may link cell cycle progression to structure and stability of the primary cilium.

Katanin knockdown supports a role for microtubule severing in release of basal bodies before mitosis in Chlamydomonas

Katanin is a microtubule-severing protein that participates in the regulation of cell cycle progression and in ciliary disassembly, but its precise role is not known for either activity. Our data suggest that in Chlamydomonas, katanin severs doublet microtubules at the proximal end of the flagellar transition zone, allowing disengagement of the basal body from the flagellum before mitosis. Using an RNA interference approach we have discovered that severe knockdown of the p60 subunit of katanin, KAT1, is achieved only in cells that also carry secondary mutations that disrupt ciliogenesis. Importantly, we observed that cells in the process of cell cycle-induced flagellar resorption sever the flagella from the basal bodies before resorption is complete, and we find that this process is defective in KAT1 knockdown cells.

The NIMA-family kinase, Nek1 affects the stability of centrosomes and ciliogenesis

BACKGROUND

Mutations in Nek1 (NIMA-Related Kinase 1) are causal in the murine models of polycystic kidney disease kat and kat2J. The Neks are known as cell cycle kinases, but recent work in protists has revealed that in addition to roles in the regulation of cell cycle progression, some Neks also regulate cilia. In most cells, cilia are disassembled prior to mitosis and are regenerated after cytokinesis. We propose that Neks participate in the coordination of ciliogenesis with cell cycle progression. Mammalian Nek1 is a candidate for this activity because renal cysts form in response to dysfunctional ciliary signalling.

RESULTS

Here we report that over-expression of full-length mNek1 inhibited ciliogenesis without disrupting centrosomes in the murine renal epithelial cell line IMCD3. In contrast, over-expression of the kinase domain with its associated basic region, but without the acidic domain, caused loss of centrosomes. As expected, these cells also failed to grow cilia. Both defective ciliogenesis in response to too much mNek1 and disassembly of centrosomes in response to expression of the kinase lacking the presumptive regulatory domain was abrogated by kinase-inactivating mutations or by removal of the coiled-coil domain. We observed that kinase-inactive, C-terminal truncations of mNek1 retaining the coiled-coil domain localized to the cilium, and we define a ciliary targeting region within the coiled-coil domain.

CONCLUSION

Based on our data, we propose that Nek1 plays a role in centrosome integrity, affecting both ciliogenesis and centrosome stability.

Cell cycle-dependent ciliogenesis and cancer

In mammals, most cell types have primary cilia, protruding structures involved in sensing mechanical and chemical signals from the extracellular environment that act as major communication hubs for signaling controlling cell differentiation and polarity. The list of clinical disorders associated with ciliary dysfunction has expanded from polycystic kidney disease to include many others. Transformed cells commonly lack cilia, but whether this lack is cause or consequence of transformation is not well understood. Here we discuss work addressing recently identified actions of the cancer-promoting proteins Aurora A and HEF1/NEDD9/CAS-L at cilia. Together with older studies, this work suggests that loss of cilia in cancer may contribute to the insensitivity of cancer cells to environmental repressive signals, based in part on derangement of cell cycle checkpoints governed by cilia and centrosomes.

The conserved plant sterility gene HAP2 functions after attachment of fusogenic membranes in Chlamydomonas and Plasmodium gametes

The cellular and molecular mechanisms that underlie species-specific membrane fusion between male and female gametes remain largely unknown. Here, by use of gene discovery methods in the green alga Chlamydomonas, gene disruption in the rodent malaria parasite Plasmodium berghei, and distinctive features of fertilization in both organisms, we report discovery of a mechanism that accounts for a conserved protein required for gamete fusion. A screen for fusion mutants in Chlamydomonas identified a homolog of HAP2, an Arabidopsis sterility gene. Moreover, HAP2 disruption in Plasmodium blocked fertilization and thereby mosquito transmission of malaria. HAP2 localizes at the fusion site of Chlamydomonas minus gametes, yet Chlamydomonas minus and Plasmodium hap2 male gametes retain the ability, using other, species-limited proteins, to form tight prefusion membrane attachments with their respective gamete partners. Membrane dye experiments show that HAP2 is essential for membrane merger. Thus, in two distantly related eukaryotes, species-limited proteins govern access to a conserved protein essential for membrane fusion.

Defects in ciliary localization of Nek8 is associated with cystogenesis

Mutations in the human NIMA (Never in Mitosis gene A)-related kinase 8 (Nek8) are associated with a rare form of the juvenile renal cystic disease, nephronophthisis type 9, and mutations in murine Nek8 cause renal cysts in jck mice. Cystogenesis involves dysfunctional ciliary signaling, and we have previously reported that Nek8 localizes to the primary cilium in mouse kidney epithelial cells. We now report that in developing mouse kidney, Nek8 is detected in the cilia of a subset of ureteric-bud-derived tubules at embryonic day (E)15.5. An increasing proportion of ureteric-bud-derived tubules express ciliary Nek8 until E18.5. Postnatal day 1 and 7 Nek8 is observed with equal frequency in both ureteric-bud and non-ureteric-bud-derived tubules. To investigate the cell biological consequences of kinase-deficient and jck mutant forms of Nek8, we transiently expressed green fluorescent protein (GFP)-tagged constructs in vitro. Mutations in the kinase and C-terminal domains of Nek8 adversely affected ciliary targeting but did not affect ciliogenesis or ciliary length. Consistent with these in vitro observations, kidneys from homozygous jck mice revealed reduced ciliary expression of Nek8 compared with kidneys from heterozygous (unaffected) mice. These data indicate that the ciliary localization of Nek8 in a subset of ureteric-bud-derived kidney tubules is essential for maintaining the integrity of those tubules in the mammalian kidney.

Rapid spatiotemporal patterning of cytosolic Ca2+ underlies flagellar excision in Chlamydomonas reinhardtii

Ca(2+)-dependent signalling processes are implicated in many aspects of flagella function in the green alga, Chlamydomonas. In this study, we examine the spatiotemporal dynamics of cytosolic Ca2+ ([Ca2+](cyt)) in single Chlamydomonas cells during the process of flagellar excision, using biolistically loaded calcium-responsive dyes. Acid-induced deflagellation occurred in parallel with a single transient elevation in whole-cell [Ca2+](cyt), which was absent in the acid deflagellation-deficient adf1 mutant. Deflagellation could also be induced by elevated external Ca2+ ([Ca2+](ext)), which promoted very rapid spiking of [Ca2+](cyt) across the whole cell and in the flagella. We also detected very rapid apically localised Ca2+ signalling events with an approximate duration of 500 msec. Ninety-seven per cent of deflagellation events coincided with a rapid elevation in [Ca2+](cyt) in the apical region of the cell, either in the form of a whole cell or an apically localised increase, indicating that [Ca2+](cyt) elevations in the apical region play an underlying role in deflagellation. Our data indicate that elevated [Ca2+](ext) acts to disrupt Ca2+ homeostasis which induces deflagellation by both Adf1-dependent and Adf1-independent mechanisms. Elevated [Ca2+](ext) also results in further [Ca2+](cyt) elevations after the main period of whole cell spiking which are very strongly associated with deflagellation, exhibit a high degree of apical localisation and are largely absent in the adf1 mutant. We propose that these later elevations may act as specific signals for deflagellation.

Nek8 regulates the expression and localization of polycystin-1 and polycystin-2

Nek8 is a serine/threonine kinase that is mutated in the jck (juvenile cystic kidneys) mouse, a model of autosomal recessive juvenile polycystic kidney disease, but its function is poorly understood. We used the jck mouse to study the functional relationship between Nek8 and other proteins that have been implicated in polycystic kidney diseases. In the collecting tubules and collecting ducts of wild-type mice, we found that Nek8 was localized to the proximal portion of primary cilia and was weakly detected in the cytosol. In the jck mutant, however, Nek8 was found along the entire length of cilia. Coimmunoprecipitation experiments demonstrated that Nek8 interacted with polycystin-2, but not with polycystin-1, and that the jck mutation did not affect this interaction. Western blot analysis and real-time reverse transcriptase PCR revealed that the protein and mRNA expression of polycystin-1 (PC1) and polycystin-2 (PC2) were increased in jck mouse kidneys. The jck mutation also led to abnormal phosphorylatin of PC2, and this was associated with longer cilia and ciliary accumulation of PC1 and PC2. Our data suggests that Nek8 interacts with the signal transduction pathways of the polycystins and may control the targeting of these ciliary proteins. Dysfunction Nek8 may lead to cystogenesis by altering the structure and function of cilia in the distal nephron.

NEK8 mutations affect ciliary and centrosomal localization and may cause nephronophthisis

Nephronophthisis, an autosomal recessive kidney disease, is the most frequent genetic cause of chronic renal failure in the first 3 decades of life. Causative mutations in 8 genes (NPHP1-8) have been identified, and homologous mouse models for NPHP2/INVS and NPHP3 have been described. The jck mouse is another model of recessive cystic kidney disease, and this mouse harbors a missense mutation, G448V, in the highly conserved RCC1 domain of Nek8. We hypothesized that mutations in NEK8 might cause nephronophthisis in humans, so we performed mutational analysis in a worldwide cohort of 588 patients. We identified 3 different amino acid changes that were conserved through evolution (L330F, H425Y, and A497P) and that were absent from at least 80 ethnically matched controls. All 3 mutations were within RCC1 domains, and the mutation H425Y was positioned within the same RCC1 repeat as the mouse jck mutation. To test the functional significance of these mutations, we introduced them into full-length mouse Nek8 GFP-tagged cDNA constructs. We transiently overexpressed the constructs in inner medullary collecting duct cells (IMCD-3 cell line) and compared the subcellular localization of mutant Nek8 to wild-type Nek8. All mutant forms of Nek8 showed defects in ciliary localization to varying degrees; the H431Y mutant (human H425Y) was completely absent from cilia and the amount localized to centrosomes was decreased. Overexpression of these mutants did not affect overall ciliogenesis, mitosis, or centriole number. Our genetic and functional data support the assumption that mutations in NEK8 cause nephronophthisis (NPHP9), adding another link between proteins mutated in cystic kidney disease and their localization to cilia and centrosomes.