Centrosome cohesion is regulated by a balance of kinase and phosphatase activities

Insights into NEK2 inhibitors as antitumor agents: From mechanisms to potential therapeutics

NEK2, a serine/threonine protein kinase, is integral to mitotic events such as centrosome duplication and separation, microtubule stabilization, spindle assembly checkpoint, and kinetochore attachment. However, NEK2 overexpression leads to centrosome amplification and chromosomal instability, which are significantly associated with various malignancies, including liver, breast, and non-small cell lung cancer. This overexpression could facilitate tumor development and confer resistance to therapy by promoting aberrant cell division and centrosome amplification. Consequently, inhibiting NEK2 is considered as a promising strategy for oncological therapy. To date, no small molecule NEK2-specific inhibitors have advanced into clinical trials, highlighting the necessity for optimized design and the deployment of innovative technologies. In this review, we will provide a comprehensive summary of the chemical structure, biological functions, and disease associations of NEK2, focusing on the existing NEK2 small molecule inhibitors, especially their structure-activity relationships, limitations, and research strategies. Our objective is to provide valuable insights for the future development of NEK2 inhibitors and analysis of challenges faced in translating these findings into clinical applications.

A cryo-electron tomography study of ciliary rootlet organization

Ciliary rootlets are striated bundles of filaments that connect the base of cilia to internal cellular structures. Rootlets are critical for the sensory and motile functions of cilia. However, the mechanisms underlying these functions remain unknown, in part due to a lack of structural information of rootlet organization. In this study, we obtain 3D reconstructions of membrane-associated and purified rootlets from mouse retina using cryo-electron tomography. We show that flexible protrusions on the rootlet surface, which emanate from the cross-striations, connect to intracellular membranes. In purified rootlets, the striations were classified into amorphous (A)-bands, associated with accumulations on the rootlet surface, and discrete (D)-bands corresponding to punctate lines of density that run through the rootlet. These striations connect a flexible network of longitudinal filaments. Subtomogram averaging suggests the filaments consist of two intertwined coiled coils. The rootlet's filamentous architecture, with frequent membrane-connecting cross-striations, lends itself well for anchoring large membranes in the cell.

Dual roles of CCDC102A in governing centrosome duplication and cohesion

In animal cells, the dysregulation of centrosome duplication and cohesion maintenance leads to abnormal spindle assembly and chromosomal instability, contributing to developmental disorders and tumorigenesis. However, the molecular mechanisms involved in maintaining accurate centrosome number control and tethering are not fully understood. Here, we identified coiled-coil domain-containing 102A (CCDC102A) as a centrosomal protein exhibiting a barrel-like structure in the proximal regions of parent centrioles, where it prevents centrosome overduplication by restricting interactions between Cep192 and Cep152 on centrosomes, thereby ensuring bipolar spindle formation. Additionally, CCDC102A regulates the centrosome linker by recruiting and binding C-Nap1; it is removed from the centrosome after Nek2A-mediated phosphorylation at the onset of mitosis. Overall, our results indicate that CCDC102A participates in controlling centrosome number and maintaining centrosome cohesion, suggesting that a well-tuned system regulates centrosome structure and function throughout the cell cycle.

The Impact of Centrosome Pathologies on Ovarian Cancer Development and Progression with a Focus on Centrosomes as Therapeutic Target

The impact of centrosome abnormalities on cancer cell proliferation has been recognized as early as 1914 (Boveri, Zur Frage der Entstehung maligner Tumoren. Jena: G. Fisher, 1914), but vigorous research on molecular levels has only recently started when it became fully apparent that centrosomes can be targeted for new cancer therapies. While best known for their microtubule-organizing capabilities as MTOC (microtubule organizing center) in interphase and mitosis, centrosomes are now further well known for a variety of different functions, some of which are related to microtubule organization and consequential activities such as cell division, migration, maintenance of cell shape, and vesicle transport powered by motor proteins, while other functions include essential roles in cell cycle regulation, metabolic activities, signal transduction, proteolytic activity, and several others that are now heavily being investigated for their role in diseases and disorders (reviewed in Schatten and Sun, Histochem Cell Biol 150:303-325, 2018; Schatten, Adv Anat Embryol Cell Biol 235:43-50, 2022a; Schatten, Adv Anat Embryol Cell Biol 235:17-35, 2022b).Cancer cell centrosomes differ from centrosomes in noncancer cells in displaying specific abnormalities that include phosphorylation abnormalities, overexpression of specific centrosomal proteins, abnormalities in centriole and centrosome duplication, formation of multipolar spindles that play a role in aneuploidy and genomic instability, and several others that are highlighted in the present review on ovarian cancer. Ovarian cancer cell centrosomes, like those in other cancers, display complex abnormalities that in part are based on the heterogeneity of cells in the cancer tissues resulting from different etiologies of individual cancer cells that will be discussed in more detail in this chapter.Because of the critical role of centrosomes in cancer cell proliferation, several lines of research are being pursued to target centrosomes for therapeutic intervention to inhibit abnormal cancer cell proliferation and control tumor progression. Specific centrosome abnormalities observed in ovarian cancer will be addressed in this chapter with a focus on targeting such aberrations for ovarian cancer-specific therapies.

Signaling pathways and regulation of gene expression in hematopoietic cells

Cellular functions are regulated by signal transduction pathway networks consisting of protein-modifying enzymes that control the activity of many downstream proteins. Protein kinases and phosphatases regulate gene expression by reversible phosphorylation of transcriptional factors, which are their direct substrates. Casein kinase II (CK2) is a serine/threonine kinase that phosphorylates a large number of proteins that have critical roles in cellular proliferation, metabolism and survival. Altered function of CK2 has been associated with malignant transformation, immunological disorders and other types of diseases. Protein phosphatase 1 (PP1) is a serine/threonine phosphatase, which regulates the phosphorylation status of many proteins that are essential for cellular functions. IKAROS is a DNA-binding protein, which functions as a regulator of gene transcription in hematopoietic cells. CK2 directly phosphorylates IKAROS at multiple phosphosites which determines IKAROS activity as a regulator of gene expression. PP1 binds to IKAROS via the PP1-consensus recognition site and dephosphorylates serine/threonine residues that are phosphorylated by CK2. Thus, the interplay between CK2 and PP1 signaling pathways have opposing effects on the phosphorylation status of their mutual substrate - IKAROS. This review summarizes the effects of CK2 and PP1 on IKAROS role in regulation of gene expression and its function as a tumor suppressor in leukemia.

AurkA nuclear localization is promoted by TPX2 and counteracted by protein degradation

The AurkA kinase is a well-known mitotic regulator, frequently overexpressed in tumors. The microtubule-binding protein TPX2 controls AurkA activity, localization, and stability in mitosis. Non-mitotic roles of AurkA are emerging, and increased nuclear localization in interphase has been correlated with AurkA oncogenic potential. Still, the mechanisms leading to AurkA nuclear accumulation are poorly explored. Here, we investigated these mechanisms under physiological or overexpression conditions. We observed that AurkA nuclear localization is influenced by the cell cycle phase and nuclear export, but not by its kinase activity. Importantly, AURKA overexpression is not sufficient to determine its accumulation in interphase nuclei, which is instead obtained when AURKA and TPX2 are co-overexpressed or, to a higher extent, when proteasome activity is impaired. Expression analyses show that AURKA, TPX2, and the import regulator CSE1L are co-overexpressed in tumors. Finally, using MCF10A mammospheres we show that TPX2 co-overexpression drives protumorigenic processes downstream of nuclear AurkA. We propose that AURKA/TPX2 co-overexpression in cancer represents a key determinant of AurkA nuclear oncogenic functions.

BICD2 phosphorylation regulates dynein function and centrosome separation in G2 and M

The activity of dynein is regulated by a number of adaptors that mediate its interaction with dynactin, effectively activating the motor complex while also connecting it to different cargos. The regulation of adaptors is consequently central to dynein physiology but remains largely unexplored. We now describe that one of the best-known dynein adaptors, BICD2, is effectively activated through phosphorylation. In G2, phosphorylation of BICD2 by CDK1 promotes its interaction with PLK1. In turn, PLK1 phosphorylation of a single residue in the N-terminus of BICD2 results in a structural change that facilitates the interaction with dynein and dynactin, allowing the formation of active motor complexes. Moreover, modified BICD2 preferentially interacts with the nucleoporin RanBP2 once RanBP2 has been phosphorylated by CDK1. BICD2 phosphorylation is central for dynein recruitment to the nuclear envelope, centrosome tethering to the nucleus and centrosome separation in the G2 and M phases of the cell cycle. This work reveals adaptor activation through phosphorylation as crucial for the spatiotemporal regulation of dynein activity.

Protocol to measure centrosome cohesion deficits mediated by pathogenic LRRK2 in cultured cells using confocal microscopy

The present protocol allows for quantification of inter-centrosome distances in G2 phase cells by confocal fluorescence microscopy to determine centrosome cohesion deficits. We describe transfection and immunofluorescence approaches followed by image acquisition and analysis of inter-centrosome distances. This protocol is for adherent A549 cells transiently overexpressing pathogenic LRRK2 and for immortalized murine embryonic fibroblasts endogenously expressing LRRK2 but is amenable to any other cultured cell type as well. For complete details on the use and execution of this protocol, please refer to Fdez et al.1 and Lara Ordóñez et al.2.

The LRRK2 signaling network converges on a centriolar phospho-Rab10/RILPL1 complex to cause deficits in centrosome cohesion and cell polarization

The Parkinson's-disease-associated LRRK2 kinase phosphorylates multiple Rab GTPases including Rab8 and Rab10, which enhances their binding to RILPL1 and RILPL2. The nascent interaction between phospho-Rab10 and RILPL1 blocks ciliogenesis in vitro and in the intact brain, and interferes with the cohesion of duplicated centrosomes in dividing cells. We show here that regulators of the LRRK2 signaling pathway including vps35 and PPM1H converge upon causing centrosomal deficits. The cohesion alterations do not require the presence of other LRRK2 kinase substrates including Rab12, Rab35 and Rab43 or the presence of RILPL2. Rather, they depend on the RILPL1-mediated centrosomal accumulation of phosphorylated Rab10. RILPL1 localizes to the subdistal appendage of the mother centriole, followed by recruitment of the LRRK2-phosphorylated Rab proteins to cause the centrosomal defects. The centrosomal alterations impair cell polarization as monitored by scratch wound assays which is reverted by LRRK2 kinase inhibition. These data reveal a common molecular pathway by which enhanced LRRK2 kinase activity impacts upon centrosome-related events to alter the normal biology of a cell.

Summary: The Parkinson's disease LRRK2 signaling pathway converges upon the formation of a complex at the subdistal appendage of the mother centriole which causes centrosomal deficits and impairs appropriate cell polarization.

Pathogenic LRRK2 regulates centrosome cohesion via Rab10/RILPL1-mediated CDK5RAP2 displacement

Mutations in LRRK2 increase its kinase activity and cause Parkinson's disease. LRRK2 phosphorylates a subset of Rab proteins which allows for their binding to RILPL1. The phospho-Rab/RILPL1 interaction causes deficits in ciliogenesis and interferes with the cohesion of duplicated centrosomes. We show here that centrosomal deficits mediated by pathogenic LRRK2 can also be observed in patient-derived iPS cells, and we have used transiently transfected cell lines to identify the underlying mechanism. The LRRK2-mediated centrosomal cohesion deficits are dependent on both the GTP conformation and phosphorylation status of the Rab proteins. Pathogenic LRRK2 does not displace proteinaceous linker proteins which hold duplicated centrosomes together, but causes the centrosomal displacement of CDK5RAP2, a protein critical for centrosome cohesion. The LRRK2-mediated centrosomal displacement of CDK5RAP2 requires RILPL1 and phospho-Rab proteins, which stably associate with centrosomes. These data provide fundamental information as to how pathogenic LRRK2 alters the normal physiology of a cell.

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.

Protein phosphatase 1 in association with Bud14 inhibits mitotic exit in Saccharomyces cerevisiae

Mitotic exit in budding yeast is dependent on correct orientation of the mitotic spindle along the cell polarity axis. When accurate positioning of the spindle fails, a surveillance mechanism named the spindle position checkpoint (SPOC) prevents cells from exiting mitosis. Mutants with a defective SPOC become multinucleated and lose their genomic integrity. Yet, a comprehensive understanding of the SPOC mechanism is missing. In this study, we identified the type 1 protein phosphatase, Glc7, in association with its regulatory protein Bud14 as a novel checkpoint component. We further showed that Glc7-Bud14 promotes dephosphorylation of the SPOC effector protein Bfa1. Our results suggest a model in which two mechanisms act in parallel for a robust checkpoint response: first, the SPOC kinase Kin4 isolates Bfa1 away from the inhibitory kinase Cdc5, and second, Glc7-Bud14 dephosphorylates Bfa1 to fully activate the checkpoint effector.

Mdm2-Mediated Downmodulation of GRK2 Restricts Centrosome Separation for Proper Chromosome Congression

The timing of centrosome separation and the distance moved apart influence the formation of the bipolar spindle, affecting chromosome stability. Epidermal growth factor receptor (EGFR) signaling induces early centrosome separation through downstream G protein-coupled receptor kinase GRK2, which phosphorylates the Hippo pathway component MST2 (Mammalian STE20-like protein kinase 2), in turn allowing NIMA kinase Nek2A activation for centrosomal linker disassembly. However, the mechanisms that counterbalance centrosome disjunction and separation remain poorly understood. We unveil that timely degradation of GRK2 by the E3 ligase Mdm2 limits centrosome separation in the G2. Both knockout expression and catalytic inhibition of Mdm2 result in GRK2 accumulation and enhanced centrosome separation before mitosis onset. Phosphorylation of GRK2 on residue S670 enables a complex pattern of non-K48-linked polyubiquitin chains assembled by Mdm2, which correlate with kinase protein degradation. Remarkably, GRK2-S670A protein fails to phosphorylate MST2 despite overcoming Mdm2-dependent degradation, which results in defective centrosome separation, shorter spindles, and abnormal chromosome congression. Conversely, extra levels of wild-type kinase in the G2 cause increased inter-centrosome distances with longer spindles, also converging in congression issues. Our findings show that the signals enabling activity of the GRK2/MST2/Nek2A axis for separation also switches on Mdm2 degradation of GRK2 to ensure accurate centrosome dynamics and proper mitotic spindle functionality.

On Broken Ne(c)ks and Broken DNA: The Role of Human NEKs in the DNA Damage Response

NIMA-related kinases, or NEKs, are a family of Ser/Thr protein kinases involved in cell cycle and mitosis, centrosome disjunction, primary cilia functions, and DNA damage responses among other biological functional contexts in vertebrate cells. In human cells, there are 11 members, termed NEK1 to 11, and the research has mainly focused on exploring the more predominant roles of NEKs in mitosis regulation and cell cycle. A possible important role of NEKs in DNA damage response (DDR) first emerged for NEK1, but recent studies for most NEKs showed participation in DDR. A detailed analysis of the protein interactions, phosphorylation events, and studies of functional aspects of NEKs from the literature led us to propose a more general role of NEKs in DDR. In this review, we express that NEK1 is an activator of ataxia telangiectasia and Rad3-related (ATR), and its activation results in cell cycle arrest, guaranteeing DNA repair while activating specific repair pathways such as homology repair (HR) and DNA double-strand break (DSB) repair. For NEK2, 6, 8, 9, and 11, we found a role downstream of ATR and ataxia telangiectasia mutated (ATM) that results in cell cycle arrest, but details of possible activated repair pathways are still being investigated. NEK4 shows a connection to the regulation of the nonhomologous end-joining (NHEJ) repair of DNA DSBs, through recruitment of DNA-PK to DNA damage foci. NEK5 interacts with topoisomerase IIβ, and its knockdown results in the accumulation of damaged DNA. NEK7 has a regulatory role in the detection of oxidative damage to telomeric DNA. Finally, NEK10 has recently been shown to phosphorylate p53 at Y327, promoting cell cycle arrest after exposure to DNA damaging agents. In summary, this review highlights important discoveries of the ever-growing involvement of NEK kinases in the DDR pathways. A better understanding of these roles may open new diagnostic possibilities or pharmaceutical interventions regarding the chemo-sensitizing inhibition of NEKs in various forms of cancer and other diseases.

Phospho-regulation of mitotic spindle assembly

The assembly of the bipolar mitotic spindle requires the careful orchestration of a myriad of enzyme activities like protein posttranslational modifications. Among these, phosphorylation has arisen as the principle mode for spatially and temporally activating the proteins involved in early mitotic spindle assembly processes. Here, we review key kinases, phosphatases, and phosphorylation events that regulate critical aspects of these processes. We highlight key phosphorylation substrates that are important for ensuring the fidelity of centriole duplication, centrosome maturation, and the establishment of the bipolar spindle. We also highlight techniques used to understand kinase-substrate relationships and to study phosphorylation events. We conclude with perspectives on the field of posttranslational modifications in early mitotic spindle assembly.

Involvement of NEK2 and its interaction with NDC80 and CEP250 in hepatocellular carcinoma

BACKGROUND

NEK2 has an established involvement in hepatocellular carcinoma (HCC) but the roles of NEK2 and its interacting proteins in HCC have not been systematically explored.

METHODS

This study examined NEK2 and its interacting proteins in HCC based on multiple databases.

RESULTS

NEK2 mRNA was highly expressed in HCC tissues compared with normal liver tissues. The survival of HCC patients with high NEK2 mRNA expression was shorter than those with low expression. MAD1L1, CEP250, MAPK1, NDC80, PPP1CA, PPP1R2 and NEK11 were the interacting proteins of NEK2. Among them, NDC80 and CEP250 were the key interacting proteins of NEK2. Mitotic prometaphase may be the key pathway that NEK2 and its interacting proteins contributed to HCC pathogenesis. NEK2, NDC80 and CEP250 mRNAs were highly expressed in HCC tissues compared with normal liver tissues. The mRNA levels of NEK2 were positively correlated with those of NDC80 or CEP250. Univariate regression showed that NEK2, NDC80 and CEP250 mRNA expressions were significantly associated with HCC patients' survival. Multivariate regression showed that NDC80 mRNA expression was an independent predictor for HCC patients' survival. Methylations and genetic alterations of NEK2, NDC80 and CEP250 were observed in HCC samples. The alterations of NEK2, NDC80 and CEP250 genes were co-occurrence. Patients with high mRNA expression and genetic alterations of NEK2, NDC80 and CEP250 had poor prognosis.

CONCLUSIONS

NEK2 and its interacting proteins NDC80 and CEP250 play important roles in HCC development and progression and thus may be potentially used as biomarkers and therapeutic targets of HCC.

PROTAC-mediated degradation reveals a non-catalytic function of AURORA-A kinase

The mitotic kinase AURORA-A is essential for cell cycle progression and is considered a priority cancer target. While the catalytic activity of AURORA-A is essential for its mitotic function, recent reports indicate an additional non-catalytic function, which is difficult to target by conventional small molecules. We therefore developed a series of chemical degraders (PROTACs) by connecting a clinical kinase inhibitor of AURORA-A to E3 ligase-binding molecules (e.g. thalidomide). One degrader induced rapid, durable and highly specific degradation of AURORA-A. In addition ,we found that the degrader complex was stabilized by cooperative binding between AURORA-A and CEREBLON. Degrader-mediated AURORA-A depletion caused an S-phase defect, which is not the cell cycle effect observed upon kinase inhibition, supporting an important non-catalytic function of AURORA-A during DNA replication. AURORA-A degradation induced rampant apoptosis in cancer cell lines, and thus represents a versatile starting point for developing new therapeutics to counter AURORA-A function in cancer.

A guiding torch at the poles: the multiple roles of spindle microtubule-organizing centers during cell division

The spindle constitutes the cellular machinery that enables the segregation of the chromosomes during eukaryotic cell division. The microtubules that form this fascinating and complex genome distribution system emanate from specialized structures located at both its poles and known as microtubule-organizing centers (MTOCs). Beyond their structural function, the spindle MTOCs play fundamental roles in cell cycle control, the activation and functionality of the mitotic checkpoints and during cellular aging. This review highlights the pivotal importance of spindle-associated MTOCs in multiple cellular processes and their central role as key regulatory hubs where diverse intracellular signals are integrated and coordinated to ensure the successful completion of cell division and the maintenance of the replicative lifespan.

Phosphatases in Mitosis: Roles and Regulation

Mitosis requires extensive rearrangement of cellular architecture and of subcellular structures so that replicated chromosomes can bind correctly to spindle microtubules and segregate towards opposite poles. This process originates two new daughter nuclei with equal genetic content and relies on highly-dynamic and tightly regulated phosphorylation of numerous cell cycle proteins. A burst in protein phosphorylation orchestrated by several conserved kinases occurs as cells go into and progress through mitosis. The opposing dephosphorylation events are catalyzed by a small set of protein phosphatases, whose importance for the accuracy of mitosis is becoming increasingly appreciated. This review will focus on the established and emerging roles of mitotic phosphatases, describe their structural and biochemical properties, and discuss recent advances in understanding the regulation of phosphatase activity and function.

Cep85 Relays Plk1 Activity to Phosphorylated Nek2A for Its Timely Activation in Centrosome Disjunction

Timely centrosome separation is critical for accurate chromosome separation. It is initiated by Nek2A at the onset of mitosis, but the mechanism for the strict requirement of phosphorylated Nek2A for its own activation remains unclear. In this study, we have found that Plk1 interacts with Cep85 and forms a ternary complex with Cep85-Nek2A. Nek2A binding, but not its kinase activity, is pre-required for Cep85 to be phosphorylated by Plk1. Nek2A-dependent Cep85 phosphorylation, in turn, leads to the dissociation of phosphorylated Cep85 exclusively from phospho-Nek2A, thereby increasing the freed phospho-Nek2A activity. Both kinases are also required for phosphorylating endogenous Cep85 in cells, and timely phosphorylation of Cep85 and Nek2A is crucial for initiating centrosome disjunction at G2/M. Overall, our study has uncovered a previously unrecognized role of Plk1 and Nek2A and identified Cep85 as a missing piece directly relaying Plk1 activity to Nek2A for its activation in centrosome disjunction.

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Cep85 prevents centrosome separation by binding to and inhibiting Nek2A in interphase

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Plk1 binds to Cep85 and forms a ternary Plk1-Cep85-Nek2A complex in late G2

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Nek2A-assisting Cep85 phosphorylation by Plk1 releases phospho-Nek2A from Cep85

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Freed phospho-Nek2A initiates centrosome separation in G2/M

CCDC102B functions in centrosome linker assembly and centrosome cohesion

The proteinaceous centrosome linker is an important structure that allows the centrosome to function as a single microtubule-organizing center (MTOC) in interphase cells. However, the assembly mechanism of the centrosome linker components remains largely unknown. In this study, we identify CCDC102B as a new centrosome linker protein that is required for maintaining centrosome cohesion. CCDC102B is recruited to the centrosome by C-Nap1 (also known as CEP250) and interacts with the centrosome linker components rootletin and LRRC45. CCDC102B decorates and facilitates the formation of rootletin filaments. Furthermore, CCDC102B is phosphorylated by Nek2A (an isoform encoded by NEK2) and is disassociated from the centrosome at the onset of mitosis. Together, our findings reveal a molecular role for CCDC102B in centrosome cohesion and centrosome linker assembly.This article has an associated First Person interview with the first authors of the paper.

PPP1R35 is a novel centrosomal protein that regulates centriole length in concert with the microcephaly protein RTTN

Centrosome structure, function, and number are finely regulated at the cellular level to ensure normal mammalian development. Here, we characterize PPP1R35 as a novel bona fide centrosomal protein and demonstrate that it is critical for centriole elongation. Using quantitative super-resolution microscopy mapping and live-cell imaging we show that PPP1R35 is a resident centrosomal protein located in the proximal lumen above the cartwheel, a region of the centriole that has eluded detailed characterization. Loss of PPP1R35 function results in decreased centrosome number and shortened centrioles that lack centriolar distal and microtubule wall associated proteins required for centriole elongation. We further demonstrate that PPP1R35 acts downstream of, and forms a complex with, RTTN, a microcephaly protein required for distal centriole elongation. Altogether, our study identifies a novel step in the centriole elongation pathway centered on PPP1R35 and elucidates downstream partners of the microcephaly protein RTTN.

Once and only once: mechanisms of centriole duplication and their deregulation in disease

Centrioles are conserved microtubule-based organelles that form the core of the centrosome and act as templates for the formation of cilia and flagella. Centrioles have important roles in most microtubule-related processes, including motility, cell division and cell signalling. To coordinate these diverse cellular processes, centriole number must be tightly controlled. In cycling cells, one new centriole is formed next to each pre-existing centriole in every cell cycle. Advances in imaging, proteomics, structural biology and genome editing have revealed new insights into centriole biogenesis, how centriole numbers are controlled and how alterations in these processes contribute to diseases such as cancer and neurodevelopmental disorders. Moreover, recent work has uncovered the existence of surveillance pathways that limit the proliferation of cells with numerical centriole aberrations. Owing to this progress, we now have a better understanding of the molecular mechanisms governing centriole biogenesis, opening up new possibilities for targeting these pathways in the context of human disease.

Nek9 Phosphorylation Defines a New Role for TPX2 in Eg5-Dependent Centrosome Separation before Nuclear Envelope Breakdown

Centrosomes [1, 2] play a central role during spindle assembly in most animal cells [3]. In early mitosis, they organize two symmetrical microtubule arrays that upon separation define the two poles of the forming spindle. Centrosome separation is tightly regulated [4, 5], occurring through partially redundant mechanisms that rely on the action of microtubule-based dynein and kinesin motors and the actomyosin system [6]. While centrosomes can separate in prophase or in prometaphase after nuclear envelope breakdown (NEBD), prophase centrosome separation optimizes spindle assembly and minimizes the occurrence of abnormal chromosome attachments that could end in aneuploidy [7, 8]. Prophase centrosome separation relies on the activity of Eg5/KIF11, a mitotic kinesin [9] that accumulates around centrosomes in early mitosis under the control of CDK1 and the Nek9/Nek6/7 kinase module [10-17]. Here, we show that Eg5 localization and centrosome separation in prophase depend on the nuclear microtubule-associated protein TPX2 [18], a pool of which localizes to the centrosomes before NEBD. This localization involves RHAMM/HMMR [19] and the kinase Nek9 [20], which phosphorylates TPX2 nuclear localization signal (NLS) preventing its interaction with importin and nuclear import. The pool of centrosomal TPX2 in prophase has a critical role for both microtubule aster organization and Eg5 localization, and thereby for centrosome separation. Our results uncover an unsuspected role for TPX2 before NEBD and define a novel regulatory mechanism for centrosome separation in prophase. They furthermore suggest NLS phosphorylation as a novel regulatory mechanism for spindle assembly factors controlled by the importin/Ran system.

A novel interaction between kinase activities in regulation of cilia formation

Background

The primary cilium is an extension of the cell membrane that encloses a microtubule-based axoneme. Primary cilia are essential for transmission of environmental cues that determine cell fate. Disruption of primary cilia function is the molecular basis of numerous developmental disorders. Despite their biological importance, the mechanisms governing their assembly and disassembly are just beginning to be understood. Cilia growth and disassembly are essential events when cells exit and reenter into the cell cycle. The kinases never in mitosis-kinase 2 (Nek2) and Aurora A (AurA) act to depolymerize cilia when cells reenter the cell cycle from G₀.

Results

Coexpression of either kinase with its kinase dead companion [AurA with kinase dead Nek2 (Nek2 KD) or Nek2 with kinase dead AurA (AurA KD)] had different effects on cilia depending on whether cilia are growing or shortening. AurA and Nek2 are individually able to shorten cilia when cilia are growing but both are required when cilia are being absorbed. The depolymerizing activity of each kinase is increased when coexpressed with the kinase dead version of the other kinase but only when cilia are assembling. Additionally, the two kinases act additively when cilia are assembling but not disassembling. Inhibition of AurA increases cilia number while inhibition of Nek2 significantly stimulates cilia length. The complex functional relationship between the two kinases reflects their physical interaction. Further, we identify a role for a PP1 binding protein, PPP1R42, in inhibiting Nek2 and increasing ciliation of ARPE-19 cells.

Conclusion

We have uncovered a novel functional interaction between Nek2 and AurA that is dependent on the growth state of cilia. This differential interdependence reflects opposing regulation when cilia are growing or shortening. In addition to interaction between the kinases to regulate ciliation, the PP1 binding protein PPP1R42 directly inhibits Nek2 independent of PP1 indicating another level of regulation of this kinase. In summary, we demonstrate a complex interplay between Nek2 and AurA kinases in regulation of ciliation in ARPE-19 cells.

Electronic supplementary material

The online version of this article (10.1186/s12860-017-0149-5) contains supplementary material, which is available to authorized users.

Paxillin regulates cell polarization and anterograde vesicle trafficking during cell migration

Cell polarization and directed migration play pivotal roles in diverse physiological and pathological processes. Herein, we identify new roles for paxillin-mediated HDAC6 inhibition in regulating key aspects of cell polarization in both two-dimensional and one-dimensional matrix environments. Paxillin, by modulating microtubule acetylation through HDAC6 regulation, was shown to control centrosome and Golgi reorientation toward the leading edge, a hallmark of cell polarization to ensure directed trafficking of promigratory factors. Paxillin was also required for pericentrosomal Golgi localization and centrosome cohesion, independent of its localization to, and role in, focal adhesion signaling. In addition, we provide evidence of an accumulation of paxillin at the centrosome that is dependent on focal adhesion kinase (FAK) and identify an important collaboration between paxillin and FAK signaling in the modulation of microtubule acetylation, as well as centrosome and Golgi organization and polarization. Finally, paxillin was also shown to be required for optimal anterograde vesicular trafficking to the plasma membrane.

Protein Phosphatase 1 Down Regulates ZYG-1 Levels to Limit Centriole Duplication

In humans perturbations of centriole number are associated with tumorigenesis and microcephaly, therefore appropriate regulation of centriole duplication is critical. The C. elegans homolog of Plk4, ZYG-1, is required for centriole duplication, but our understanding of how ZYG-1 levels are regulated remains incomplete. We have identified the two PP1 orthologs, GSP-1 and GSP-2, and their regulators I-2^SZY-2 and SDS-22 as key regulators of ZYG-1 protein levels. We find that down-regulation of PP1 activity either directly, or by mutation of szy-2 or sds-22 can rescue the loss of centriole duplication associated with a zyg-1 hypomorphic allele. Suppression is achieved through an increase in ZYG-1 levels, and our data indicate that PP1 normally regulates ZYG-1 through a post-translational mechanism. While moderate inhibition of PP1 activity can restore centriole duplication to a zyg-1 mutant, strong inhibition of PP1 in a wild-type background leads to centriole amplification via the production of more than one daughter centriole. Our results thus define a new pathway that limits the number of daughter centrioles produced each cycle.

The centrosomes are responsible for organizing the mitotic spindle a microtubule-based structure that centers, then segregates, the chromosomes during cell division. When a cell divides it normally possesses two centrosomes, allowing it to build a bipolar spindle and accurately segregate the chromosomes to two daughter cells. Appropriate control of centrosome number is therefore crucial to maintaining genome stability. Centrosome number is largely controlled by their regulated duplication. In particular, the protein Plk4, which is essential for duplication, must be strictly limited as an overabundance leads to excess centrosome duplication. We have identified protein phosphatase 1 as a critical regulator of the C. elegans Plk4 homolog (known as ZYG-1). When protein phosphatase 1 is down-regulated, ZYG-1 levels increase leading to centrosome amplification. Thus our work identifies a novel mechanism that limits centrosome duplication.

Mitotic kinase cascades orchestrating timely disjunction and movement of centrosomes maintain chromosomal stability and prevent cancer

Centrosomes are microtubule-organizing centers that duplicate in S phase to form bipolar spindles that separate duplicated chromosomes faithfully into two daughter cells during cell division. Recent studies show that proper timing of centrosome dynamics, the disjunction and movement of centrosomes, is tightly linked to spindle symmetry, correct microtubule-kinetochore attachment, and chromosome segregation. Here, we review mechanisms that regulate centrosome dynamics, with emphasis on the roles of key mitotic kinases in the proper timing of centrosome dynamics and how aberrancies in these processes may cause chromosomal instability and cancer.

Protein Phosphatases Involved in Regulating Mitosis: Facts and Hypotheses

Almost all eukaryotic proteins are subject to post-translational modifications during mitosis and cell cycle, and in particular, reversible phosphorylation being a key event. The recent use of high-throughput experimental analyses has revealed that more than 70% of all eukaryotic proteins are regulated by phosphorylation; however, the mechanism of dephosphorylation, counteracting phosphorylation, is relatively unknown. Recent discoveries have shown that many of the protein phosphatases are involved in the temporal and spatial control of mitotic events, such as mitotic entry, mitotic spindle assembly, chromosome architecture changes and cohesion, and mitotic exit. This implies that certain phosphatases are tightly regulated for timely dephosphorylation of key mitotic phosphoproteins and are essential for control of various mitotic processes. This review describes the physiological and pathological roles of mitotic phosphatases, as well as the versatile role of various protein phosphatases in several mitotic events.

Dishevelled is a NEK2 kinase substrate controlling dynamics of centrosomal linker proteins

Dishevelled (DVL) is a key scaffolding protein and a branching point in Wnt signaling pathways. Here, we present conclusive evidence that DVL regulates the centrosomal cycle. We demonstrate that DVL dishevelled and axin (DIX) domain, but not DIX domain-mediated multimerization, is essential for DVL's centrosomal localization. DVL accumulates during the cell cycle and associates with NIMA-related kinase 2 (NEK2), which is able to phosphorylate DVL at a multitude of residues, as detected by a set of novel phospho-specific antibodies. This creates interfaces for efficient binding to CDK5 regulatory subunit-associated protein 2 (CDK5RAP2) and centrosomal Nek2-associated protein 1 (C-NAP1), two proteins of the centrosomal linker. Displacement of DVL from the centrosome and its release into the cytoplasm on NEK2 phosphorylation is coupled to the removal of linker proteins, an event necessary for centrosomal separation and proper formation of the mitotic spindle. Lack of DVL prevents NEK2-controlled dissolution of loose centrosomal linker and subsequent centrosomal separation. Increased DVL levels, in contrast, sequester centrosomal NEK2 and mimic monopolar spindle defects induced by a dominant negative version of this kinase. Our study thus uncovers molecular crosstalk between centrosome and Wnt signaling.

Chronic Exposure to Particulate Chromate Induces Premature Centrosome Separation and Centriole Disengagement in Human Lung Cells

Particulate hexavalent chromium (Cr(VI)) is a well-established human lung carcinogen. Lung tumors are characterized by structural and numerical chromosome instability. Centrosome amplification is a phenotype commonly found in solid tumors, including lung tumors, which strongly correlates with chromosome instability. Human lung cells exposed to Cr(VI) exhibit centrosome amplification but the underlying phenotypes and mechanisms remain unknown. In this study, we further characterize the phenotypes of Cr(VI)-induced centrosome abnormalities. We show that Cr(VI)-induced centrosome amplification correlates with numerical chromosome instability. We also show chronic exposure to particulate Cr(VI) induces centrosomes with supernumerary centrioles and acentriolar centrosomes in human lung cells. Moreover, chronic exposure to particulate Cr(VI) affects the timing of important centriolar events. Specifically, chronic exposure to particulate Cr(VI) causes premature centriole disengagement in S and G2 phase cells. It also induces premature centrosome separation in interphase. Altogether, our data suggest that chronic exposure to particulate Cr(VI) targets the protein linkers that hold centrioles together. These centriolar linkers are important for key events of the centrosome cycle and their premature disruption might underlie Cr(VI)-induced centrosome amplification.

Characterization of Cep85 - a new antagonist of Nek2A that is involved in the regulation of centrosome disjunction

Nek2 has been implicated in centrosome disjunction at the onset of mitosis to promote bipolar spindle formation, and hyperactivation of Nek2 leads to the premature centrosome separation. Its activity, therefore, needs to be strictly regulated. In this study, we report that Cep85, an uncharacterized centrosomal protein, acts as a binding partner of Nek2A. It colocalizes with isoform A of Nek2 (Nek2A) at centrosomes and forms a granule meshwork enveloping the proximal ends of centrioles. Opposite to the effects of Nek2A, overexpression of Cep85 in conjunction with inhibition of the motor protein Eg5 (also known as KIF11) leads to the failure of centrosome disjunction. By contrast, depletion of Cep85 results in the precocious centrosome separation. We also define the Nek2A binding and centrosome localization domains within Cep85. Although the Nek2A-binding domain alone is sufficient to inhibit Nek2A kinase activity in vitro, both domains are indispensable for full suppression of centrosome disjunction in cells. Thus, we propose that Cep85 is a bona fide Nek2A-binding partner that surrounds the proximal ends of centrioles where it cooperates with PP1γ (also known as PPP1CC) to antagonize Nek2A activity in order to maintain the centrosome integrity in interphase in mammalian cells.

Summary: Cep85 acts as a binding partner of Nek2A to prevent premature centrosome separation in interphase by inhibiting Nek2A activity.

The centrosomal linker and microtubules provide dual levels of spatial coordination of centrosomes

The centrosome is the principal microtubule organizing center in most animal cells. It consists of a pair of centrioles surrounded by pericentriolar material. The centrosome, like DNA, duplicates exactly once per cell cycle. During interphase duplicated centrosomes remain closely linked by a proteinaceous linker. This centrosomal linker is composed of rootletin filaments that are anchored to the centrioles via the protein C-Nap1. At the onset of mitosis the linker is dissolved by Nek2A kinase to support the formation of the bipolar mitotic spindle. The importance of the centrosomal linker for cell function during interphase awaits characterization. Here we assessed the phenotype of human RPE1 C-Nap1 knockout (KO) cells. The absence of the linker led to a modest increase in the average centrosome separation from 1 to 2.5 μm. This small impact on the degree of separation is indicative of a second level of spatial organization of centrosomes. Microtubule depolymerisation or stabilization in C-Nap1 KO cells dramatically increased the inter-centrosomal separation (> 8 μm). Thus, microtubules position centrosomes relatively close to one another in the absence of linker function. C-Nap1 KO cells had a Golgi organization defect with a two-fold expansion of the area occupied by the Golgi. When the centrosomes of C-Nap1 KO cells showed considerable separation, two spatially distinct Golgi stacks could be observed. Furthermore, migration of C-Nap1 KO cells was slower than their wild type RPE1 counterparts. These data show that the spatial organization of centrosomes is modulated by a combination of centrosomal cohesion and microtubule forces. Furthermore a modest increase in centrosome separation has major impact on Golgi organization and cell migration.

C-Nap1 mutation affects centriole cohesion and is associated with a Seckel-like syndrome in cattle

Caprine-like Generalized Hypoplasia Syndrome (SHGC) is an autosomal-recessive disorder in Montbéliarde cattle. Affected animals present a wide range of clinical features that include the following: delayed development with low birth weight, hind limb muscular hypoplasia, caprine-like thin head and partial coat depigmentation. Here we show that SHGC is caused by a truncating mutation in the CEP250 gene that encodes the centrosomal protein C-Nap1. This mutation results in centrosome splitting, which neither affects centriole ultrastructure and duplication in dividing cells nor centriole function in cilium assembly and mitotic spindle organization. Loss of C-Nap1-mediated centriole cohesion leads to an altered cell migration phenotype. This discovery extends the range of loci that constitute the spectrum of autosomal primary recessive microcephaly (MCPH) and Seckel-like syndromes.

The tumor suppressor proteins ASPP1 and ASPP2 interact with C-Nap1 and regulate centrosome linker reassembly

Centrosome linker tethers interphase centrosomes together allowing them to function as a single microtubule organization center. The centrosome linker is disrupted at the onset of mitosis to ensure timely centrosome disjunction and bipolar spindle formation and is reassembled at the end of mitosis. While the mechanism controlling centrosome linker disassembly at early mitosis has been well explored, little is known about how the linker is subsequently reassembled before mitotic exit. Here we report that ASPP1 and ASPP2, two members of the apoptosis stimulating proteins of p53 (ASPP) family, are involved in centrosome linker reassembly. We showed that ASPP1/2 interacted with centrosome linker protein C-Nap1. Co-depletion of ASPP1 and ASPP2 inhibited re-association of C-Nap1 with centrosome at the end of mitosis. Moreover, ASPP1/2 facilitated the interaction between C-Nap1 and PP1α, and this interaction was significantly reduced by co-depletion of ASPP1/2. ASPP1/2 antagonized the NEK2A-mediated C-Nap1 Ser2417/2421 phosphorylation in a PP1-dependent manner. Co-depletion of ASPP1 and ASPP2 inhibited dephosphorylation of C-Nap1 (Ser2417/2421) at the end of mitosis. Based on these findings, we propose that ASPP1/2 act as PP1-targeting subunits to facilitate C-Nap1 dephosphorylation and centrosome linker reassembly at the end of mitosis.

Cep68 can be regulated by Nek2 and SCF complex

Centrosome cohesion maintains centrosomes in close proximity until mitosis, when cell cycle-dependent regulatory signaling events dissolve cohesion and promote centrosome separation in preparation for bipolar spindle assembly at mitosis. Cohesion is regulated by the antagonistic activities of the mitotic NIMA-related kinase 2 (Nek2), protein phosphatase 1, the cohesion fiber components rootletin, centrosomal Nek2-associated protein 1 (C-Nap1) and Cep68. The centrosomal protein Cep68 is essential for centrosome cohesion and dissociates from centrosomes at the onset of mitosis. Here, our cell line studies show the C-terminal 300-400 amino acids of Cep68 are necessary to localize Cep68 to interphase centrosomes while C-terminal 400-500 amino acids might regulate Cep68 dissociation from centrosomes at mitotic onset. In addition, Nek2 was demonstrated to phosphorylate Cep68 in vivo and this phosphorylation appears to promote Cep68 degradation in mitosis. We further show that the SCF complex destroys Cep68 at mitosis through recognition by the beta-Trcp F box component of SCF. Together, the findings provide a new insight into the control of centrosome separation by Cep68 during mitosis.

Overexpression of the Nek2 kinase in colorectal cancer correlates with beta-catenin relocalization and shortened cancer-specific survival

The serine/threonine kinase Nek2 (NIMA-related kinase 2) regulates centrosome separation and mitotic progression, with overexpression causing induction of aneuploidy in vitro. Overexpression may also enable tumour progression through effects upon Akt signalling, cell adhesion markers and the Wnt pathway. The objective of this study was to examine Nek2 protein expression in colorectal cancer (CRC). Nek2 protein expression was examined in a panel of CRC cell lines using Western blotting and immunofluorescence microscopy. Nek2 and beta-catenin expression were examined by immunohistochemistry in a series of resected CRC, as well as their matched lymph node and liver metastases, and correlated with clinicopathological characteristics. Nek2 protein expression in all CRC lines examined was higher than in the immortalised colonocyte line HCEC. Nek2 overexpression was present in 86.4% of resected CRC and was significantly associated with advancing AJCC tumour stage and shortened cancer-specific survival. Elevated Nek2 expression was maintained within all matched metastases from overexpressing primary tumours. Nek2 overexpression was significantly associated with lower tumour membranous beta-catenin expression and higher cytoplasmic and nuclear beta-catenin accumulation. These data support a role for Nek2 in CRC progression and confirm potential for Nek2 inhibition as a therapeutic avenue in CRC.

Protein phosphatase 1 catalytic isoforms: specificity toward interacting proteins

The coordinated and reciprocal action of serine-threonine protein kinases and protein phosphatases produces transitory phosphorylation, a fundamental regulatory mechanism for many biological processes. Phosphoprotein phosphatase 1 (PPP1), a major serine-threonine phosphatase, in particular, is ubiquitously distributed and regulates a broad range of cellular functions, including glycogen metabolism, cell cycle progression, and muscle relaxation. PPP1 has evolved effective catalytic machinery but in vitro lacks substrate specificity. In vivo, its specificity is achieved not only by the existence of different PPP1 catalytic isoforms, but also by binding of the catalytic moiety to a large number of regulatory or targeting subunits. Here, we will address exhaustively the existence of diverse PPP1 catalytic isoforms and the relevance of their specific partners and consequent functions.

Sensors at centrosomes reveal determinants of local separase activity

Separase is best known for its function in sister chromatid separation at the metaphase-anaphase transition. It also has a role in centriole disengagement in late mitosis/G1. To gain insight into the activity of separase at centrosomes, we developed two separase activity sensors: mCherry-Scc1(142-467)-ΔNLS-eGFP-PACT and mCherry-kendrin(2059-2398)-eGFP-PACT. Both localize to the centrosomes and enabled us to monitor local separase activity at the centrosome in real time. Both centrosomal sensors were cleaved by separase before anaphase onset, earlier than the corresponding H2B-mCherry-Scc1(142-467)-eGFP sensor at chromosomes. This indicates that substrate cleavage by separase is not synchronous in the cells. Depletion of the proteins astrin or Aki1, which have been described as inhibitors of centrosomal separase, did not led to a significant activation of separase at centrosomes, emphasizing the importance of direct separase activity measurements at the centrosomes. Inhibition of polo-like kinase Plk1, on the other hand, decreased the separase activity towards the Scc1 but not the kendrin reporter. Together these findings indicate that Plk1 regulates separase activity at the level of substrate affinity at centrosomes and may explain in part the role of Plk1 in centriole disengagement.

CDK2-dependent phosphorylation of Suv39H1 is involved in control of heterochromatin replication during cell cycle progression

Although several studies have suggested that the functions of heterochromatin regulators may be regulated by post-translational modifications during cell cycle progression, regulation of the histone methyltransferase Suv39H1 is not fully understood. Here, we demonstrate a direct link between Suv39H1 phosphorylation and cell cycle progression. We show that CDK2 phosphorylates Suv39H1 at Ser391 and these phosphorylation levels oscillate during the cell cycle, peaking at S phase and maintained during S-G2-M phase. The CDK2-mediated phosphorylation of Suv39H1 at Ser391 results in preferential dissociation from chromatin. Furthermore, phosphorylation-mediated dissociation of Suv39H1 from chromatin causes an enhanced occupancy of JMJD2A histone demethylase on heterochromatin and alterations in inactive histone marks. Overexpression of phospho-mimic Suv39H1 induces early replication of heterochromatin, suggesting the importance of Suv39H1 phosphorylation in the replication of heterochromatin. Moreover, overexpression of phospho-defective Suv39H1 caused altered replication timing of heterochromatin and increases sensitivity to replication stress. Collectively, our data suggest that phosphorylation-mediated modulation of Suv39H1-chromatin association may be an initial step in heterochromatin replication.

Multisite phosphorylation of C-Nap1 releases it from Cep135 to trigger centrosome disjunction

During mitotic entry, centrosomes separate to establish the bipolar spindle. Delays in centrosome separation can perturb chromosome segregation and promote genetic instability. However, interphase centrosomes are physically tethered by a proteinaceous linker composed of C-Nap1 (also known as CEP250) and the filamentous protein rootletin. Linker disassembly occurs at the onset of mitosis in a process known as centrosome disjunction and is triggered by the Nek2-dependent phosphorylation of C-Nap1. However, the mechanistic consequences of C-Nap1 phosphorylation are unknown. Here, we demonstrate that Nek2 phosphorylates multiple residues within the C-terminal domain of C-Nap1 and, collectively, these phosphorylation events lead to loss of oligomerization and centrosome association. Mutations in non-phosphorylatable residues that make the domain more acidic are sufficient to release C-Nap1 from the centrosome, suggesting that it is an increase in overall negative charge that is required for this process. Importantly, phosphorylation of C-Nap1 also perturbs interaction with the core centriolar protein, Cep135, and interaction of endogenous C-Nap1 and Cep135 proteins is specifically lost in mitosis. We therefore propose that multisite phosphorylation of C-Nap1 by Nek2 perturbs both oligomerization and Cep135 interaction, and this precipitates centrosome disjunction at the onset of mitosis.

TEIF associated centrosome activity is regulated by EGF/PI3K/Akt signaling

Centrosome amplification, which is a characteristic of cancer cells, has been understood as a driving force of genetic instability in the development of cancer. In previous work, we demonstrated that TEIF (transcriptional element-interacting factor) distributes in the centrosomes and regulates centrosome status under both physiologic and pathologic conditions. Here we identify TEIF as a downstream effector in EGF/PI3K/Akt signaling. The addition of EGF or transfection of active Akt stimulates centrosome TEIF distribution, resulting in an increase of centrosome splitting and amplification, while inhibitors of either PI3K or Akt attenuate these changes in TEIF and the associated centrosome status. A consensus motif for Akt phosphorylation (RHRVLT) proved to be involved in centrosomal TEIF localization, and the 469-threonine of this motif may be phosphorylated by Akt both in vitro and in vivo. Elimination of this phosphorylated site on TEIF caused reduced centrosome distribution and centrosome splitting or amplification. Moreover, TEIF closely co-localized with C-NAP1 at the proximal ends of centrioles, and centriolar loading of TEIF stimulated by EGF/Akt could displace C-NAP1, resulting in centrosome splitting. These findings reveal linkage of the EGF/PI3K/Akt signaling pathway to regulation of centrosome status which may act as an oncogenic pathway and induce genetic instability in carcinogenesis.

KIBRA: In the brain and beyond

In mammals, the KIBRA locus has been associated with memory performance and cognition by genome-wide single nucleotide polymorphism screening. Genetic studies in Drosophila and human cells have identified KIBRA as a novel regulator of the Hippo signaling pathway, which plays a critical role in human tumorigenesis. Recent studies also indicated that KIBRA is involved in other physiological processes including cell polarity, membrane/vesicular trafficking, mitosis and cell migration. At the biochemical level, KIBRA protein is highly phosphorylated by various kinases in epithelial cells. Here, we discuss the updates concerning the function and regulation of KIBRA in the brain and beyond.

Therapeutic melting pot of never in mitosis gene a related kinase 2 (Nek2): a perspective on Nek2 as an oncology target and recent advancements in Nek2 small molecule inhibition

The global incidence of cancer is on the rise, and within the next decade, the disease is expected to become the leading cause of death worldwide. Forthcoming strategies used to treat cancers focus on the design and implementation of multidrug therapies to target complementary cancer specific pathways. A more direct means by which this multitargeted approach can be achieved is by identifying and targeting interpathway regulatory factors. Recent advances in understanding Nek2 (NIMA related kinase 2) biology suggest that the kinase potentially represents a multifaceted therapeutic target. In this regard, pharmacologic modulation of Nek2 with a single agent may effect several mechanisms important for tumor growth, survival, progression, and metastasis. We herein review the development of Nek2 as an oncology target and provide a succinct chronology of drug discovery campaigns focused on targeting Nek2.