Lats2/Kpm is required for embryonic development, proliferation control and genomic integrity

The Drosophila melanogaster warts/lats tumour suppressor has two mammalian counterparts LATS1/Warts-1 and LATS2/Kpm. Here, we show that mammalian Lats orthologues exhibit distinct expression profiles according to germ cell layer origin. Lats2(-/-) embryos show overgrowth in restricted tissues of mesodermal lineage; however, lethality ultimately ensues on or before embryonic day 12.5 preceded by defective proliferation. Lats2(-/-) mouse embryonic fibroblasts (MEFs) acquire growth advantages and display a profound defect in contact inhibition of growth, yet exhibit defective cytokinesis. Lats2(-/-) embryos and MEFs display centrosome amplification and genomic instability. Lats2 localizes to centrosomes and overexpression of Lats2 suppresses centrosome overduplication induced in wild-type MEFs and reverses centrosome amplification inherent in Lats2(-/-) MEFs. These findings indicate an essential role of Lats2 in the integrity of processes that govern centrosome duplication, maintenance of mitotic fidelity and genomic stability.     

The tumor suppressor Lats2 is pivotal in Aurora A and Aurora B signaling during mitosis

Accurate coordination between chromosome segregation and cytokinesis by various mitotic kinases, such as Aurora, prevent tetraploidization and subsequent tumorigensis. The tumor suppressors Lats1 and Lats2 are serine/threonine kinases that localize to the centrosome and regulate cell cycle progression and apoptosis. In the present study, Aurora A was demonstrated to phosphorylate Lats2 on serine 380 (S380) during mitosis. Immunocytochemical observations revealed that the subcellular localization of Lats2 was distinct during the cell cycle and depended on which site was phosphorylated. Interestingly, the S380-phosphorylated Lats2 protein (pS380) colocalized at the central spindle with Aurora B. Physical interactions were observed between Aurora A, Lats2, Lats1 and Aurora B. The Lats1 kinase was shown to phosphorylate Aurora B. Cells expressing a nonphosphorylated mutant (S380A) of Lats2 caused chromosome missegregation and cytokinesis failure, similar to cells with aberrantly expressed Aurora B. Together, the results suggest that the Aurora A-Lats1/2-Aurora B axis might be a novel pathway that regulates accurate mitotic progression by ensuring the proper mitotic localization of Lats2.     

Live cell imaging reveals distinct roles in cell cycle regulation for Nek2A and Nek2B

Two splice variants of Nek2 kinase, a member of the NIMA-related family, have been identified as Nek2A and Nek2B. Nek2A regulates centrosome disjunction, spindle formation checkpoint signaling, and faithful chromosome segregation. A specific role for Nek2B has not yet been identified. Here, we have examined the distinct roles of Nek2A and Nek2B using timelapse video microscopy to follow the fate of cells progressing through the cell cycle in the absence of either Nek2A or Nek2B. We show that the down-regulation of Nek2B leads to a mitotic delay in the majority of cells. Upon exiting mitosis, cells exhibit mitotic defects such as the formation of multinucleated cells. Such phenotypes are not observed in cells that exit mitosis in the absence of Nek2A. These observations suggest that Nek2B may be required for the execution of mitotic exit.     

RASSF1A is part of a complex similar to the Drosophila Hippo/Salvador/Lats tumor-suppressor network

The Ras Association Domain Family 1A (RASSF1A) gene is one of the most frequently silenced genes in human cancer. RASSF1A has been shown to interact with the proapoptotic kinase MST1. Recent work in Drosophila has led to the discovery of a new tumor-suppressor pathway involving the Drosophila MST1 and MST2 ortholog, Hippo, as well as the Lats/Warts serine/threonine kinase and a protein named Salvador (Sav). Little is known about this pathway in mammalian cells. We report that complexes consisting of RASSF1A, MST2, WW45 (the human ortholog of Sav), and LATS1 exist in human cells. MST2 enhances the RASSF1A-WW45 interaction, which requires the C-terminal SARAH domain of both proteins. Components of this complex are localized at centrosomes and spindle poles from interphase to telophase and at the midbody during cytokinesis. Both RASSF1A and WW45 activate MST2 by promoting MST2 autophosphorylation and LATS1 phosphorylation. Mitosis is delayed in Rassf1a(-/-) mouse embryo fibroblasts and frequently results in cytokinesis failure, similar to what has been observed for LATS1-deficient cells. RASSF1A, MST2, or WW45 can rescue this defect. The complex of RASSF1A, MST2, WW45, and LATS1 consists of several tumor suppressors, is conserved in mammalian cells, and appears to be involved in controlling mitotic exit.     

LATS1 tumour suppressor affects cytokinesis by inhibiting LIMK1

LATS (large tumour suppressor) is a family of conserved tumour suppressors identified in Drosophila and mammals. Here we show that human LATS1 binds to LIMK1 in vitro and in vivo and colocalizes with LIMK1 at the actomyosin contractile ring during cytokinesis. LATS1 inhibits both the phosphorylation of cofilin by LIMK1 and LIMK1-induced cytokinesis defects. Inactivation of LATS1 by antibody microinjection or RNA-mediated interference in cells, or gene knockout in mice, abrogates cytokinesis and increases the percentage of multinucleate cells. Our findings indicate that LATS1 is a novel cytoskeleton regulator that affects cytokinesis by regulating actin polymerization through negative modulation of LIMK1.     

KIBRA Regulates Aurora Kinase Activity and Is Required for Precise Chromosome Alignment During Mitosis

The Hippo pathway controls organ size and tumorigenesis by inhibiting cell proliferation and promoting apoptosis. KIBRA was recently identified as a novel regulator of the Hippo pathway. Several of the components of the Hippo pathway are important regulators of mitosis-related cell cycle events. We recently reported that KIBRA is phosphorylated by the mitotic kinases Aurora-A and -B. However, the role KIBRA plays in mitosis has not been established. Here, we show that KIBRA activates the Aurora kinases and is required for full activation of Aurora kinases during mitosis. KIBRA also promotes the phosphorylation of large tumor suppressor 2 (Lats2) on Ser⁸³ by activating Aurora-A, which controls Lats2 centrosome localization. However, Aurora-A is not required for KIBRA to associate with Lats2. We also found that Lats2 inhibits the Aurora-mediated phosphorylation of KIBRA on Ser⁵³⁹, probably via regulating protein phosphatase 1. Consistent with playing a role in mitosis, siRNA-mediated knockdown of KIBRA causes mitotic abnormalities, including defects of spindle and centrosome formation and chromosome misalignment. We propose that the KIBRA-Aurora-Lats2 protein complexes form a novel axis that regulates precise mitosis.     

The mob as tumor suppressor gene is essential for early development and regulates tissue growth in Drosophila

Studies in Drosophila have defined a new growth inhibitory pathway mediated by Fat (Ft), Merlin (Mer), Expanded (Ex), Hippo (Hpo), Salvador (Sav)/Shar-pei, Warts (Wts)/Large tumor suppressor (Lats), and Mob as tumor suppressor (Mats), which are all evolutionarily conserved in vertebrate animals. We previously found that the Mob family protein Mats functions as a coactivator of Wts kinase. Here we show that mats is essential for early development and is required for proper chromosomal segregation in developing embryos. Mats is expressed at low levels ubiquitously, which is consistent with the role of Mats as a general growth regulator. Like mammalian Mats, Drosophila Mats colocalizes with Wts/Lats kinase and cyclin E proteins at the centrosome. This raises the possibility that Mats may function together with Wts/Lats to regulate cyclin E activity in the centrosome for mitotic control. While Hpo/Wts signaling has been implicated in the control of cyclin E and diap1 expression, we found that it also modulates the expression of cyclin A and cyclin B. Although mats depletion leads to aberrant mitoses, this does not seem to be due to compromised mitotic spindle checkpoint function.     

Disruption of centrosome structure,chromosome segregation,and cytokinesis by misexpression of human Cdc14A phosphatase

In budding yeast, the Cdc14p phosphatase activates mitotic exit by dephosphorylation of specific cyclin-dependent kinase (Cdk) substrates and seems to be regulated by sequestration in the nucleolus until its release in mitosis. Herein, we have analyzed the two human homologs of Cdc14p, hCdc14A and hCdc14B. We demonstrate that the human Cdc14A phosphatase is selective for Cdk substrates in vitro and that although the protein abundance and intrinsic phosphatase activity of hCdc14A and B vary modestly during the cell cycle, their localization is cell cycle regulated. hCdc14A dynamically localizes to interphase but not mitotic centrosomes, and hCdc14B localizes to the interphase nucleolus. These distinct patterns of localization suggest that each isoform of human Cdc14 likely regulates separate cell cycle events. In addition, hCdc14A overexpression induces the loss of the pericentriolar markers pericentrin and gamma-tubulin from centrosomes. Overproduction of hCdc14A also causes mitotic spindle and chromosome segregation defects, defective karyokinesis, and a failure to complete cytokinesis. Thus, the hCdc14A phosphatase appears to play a role in the regulation of the centrosome cycle, mitosis, and cytokinesis, thereby influencing chromosome partitioning and genomic stability in human cells.     

Zyxin, a regulator of actin filament assembly, targets the mitotic apparatus by interacting with h-warts/LATS1 tumor suppressor

The mitotic apparatus plays a pivotal role in dividing cells to ensure each daughter cell receives a full set of chromosomes and complement of cytoplasm during mitosis. A human homologue of the Drosophila warts tumor suppressor, h-warts/LATS1, is an evolutionarily conserved serine/threonine kinase and a dynamic component of the mitotic apparatus. We have identified an interaction of h-warts/LATS1 with zyxin, a regulator of actin filament assembly. Zyxin is a component of focal adhesion, however, during mitosis a fraction of cytoplasmic-dispersed zyxin becomes associated with h-warts/LATS1 on the mitotic apparatus. We found that zyxin is phosphorylated specifically during mitosis, most likely by Cdc2 kinase, and that the phosphorylation regulates association with h-warts/LATS1. Furthermore, microinjection of truncated h-warts/LATS1 protein, including the zyxin-binding portion, interfered with localization of zyxin to mitotic apparatus, and the duration of mitosis of these injected cells was significantly longer than that of control cells. These findings suggest that h-warts/LATS1 and zyxin play a crucial role in controlling mitosis progression by forming a regulatory complex on mitotic apparatus.     

The endocytic adaptor protein ARH associates with motor and centrosomal proteins and is involved in centrosome assembly and cytokinesis

Numerous proteins involved in endocytosis at the plasma membrane have been shown to be present at novel intracellular locations and to have previously unrecognized functions. ARH (autosomal recessive hypercholesterolemia) is an endocytic clathrin-associated adaptor protein that sorts members of the LDL receptor superfamily (LDLR, megalin, LRP). We report here that ARH also associates with centrosomes in several cell types. ARH interacts with centrosomal (gamma-tubulin and GPC2 and GPC3) and motor (dynein heavy and intermediate chains) proteins. ARH cofractionates with gamma-tubulin on isolated centrosomes, and gamma-tubulin and ARH interact on isolated membrane vesicles. During mitosis, ARH sequentially localizes to the nuclear membrane, kinetochores, spindle poles and the midbody. Arh(-/-) embryonic fibroblasts (MEFs) show smaller or absent centrosomes suggesting ARH plays a role in centrosome assembly. Rat-1 fibroblasts depleted of ARH by siRNA and Arh(-/-) MEFs exhibit a slower rate of growth and prolonged cytokinesis. Taken together the data suggest that the defects in centrosome assembly in ARH depleted cells may give rise to cell cycle and mitotic/cytokinesis defects. We propose that ARH participates in centrosomal and mitotic dynamics by interacting with centrosomal proteins. Whether the centrosomal and mitotic functions of ARH are related to its endocytic role remains to be established.     

Deregulated human Cdc14A phosphatase disrupts centrosome separation and chromosome segregation

We show that human Cdc14A phosphatase interacts with interphase centrosomes, and that this interaction is independent of microtubules and Cdc14A phosphatase activity, but requires active nuclear export. Disrupting the nuclear export signal (NES) led to Cdc14A being localized in nucleoli, which in unperturbed cells selectively contain Cdc14B (ref. 1). Conditional overproduction of Cdc14A, but not its phosphatase-dead or NES-deficient mutants, or Cdc14B, resulted in premature centrosome splitting and formation of supernumerary mitotic spindles. In contrast, downregulation of endogenous Cdc14A by short inhibitory RNA duplexes (siRNA) induced mitotic defects including impaired centrosome separation and failure to undergo productive cytokinesis. Consequently, both overexpression and downregulation of Cdc14A caused aberrant chromosome partitioning into daughter cells. These results indicate that Cdc14A is a physiological regulator of the centrosome duplication cycle, which, when disrupted, can lead to genomic instability in mammalian cells.     

Fission yeast Clp1p phosphatase regulates G2/M transition and coordination of cytokinesis with cell cycle progression.

Background: In Saccharomyces cerevisiae the mitotic-exit network (MEN) functions in anaphase to promote the release of the Cdc14p phosphatase from the nucleolus. This release causes mitotic exit via inactivation of the cyclin-dependent kinase (Cdk). Cdc14p-like proteins are highly conserved; however, it is unclear if these proteins regulate mitotic exit as in S. cerevisiae. In Schizosaccharomyces pombe a signaling pathway homologous to the MEN and termed the septation initiation network (SIN) is required not for mitotic exit, but for initiation of cytokinesis and for a cytokinesis checkpoint that inhibits further cell cycle progression until cytokinesis is complete.Results: We have identified the S. pombe Cdc14p homolog, Clp1p, and show that it is not required for mitotic exit but rather functions together with the SIN in coordinating cytokinesis with the nuclear-division cycle. As cells enter mitosis, Clp1p relocalizes from the nucleolus to the spindle and site of cell division. Clp1p exit from the nucleolus does not depend on the SIN, but the SIN is required for keeping Clp1p out of the nucleolus until completion of cytokinesis. Clp1p, in turn, may promote the activation of the SIN by antagonizing Cdk activity until cytokinesis is complete and thus ensuring that cytokinesis is completed prior to the initiation of the next cell cycle. In addition to its roles in anaphase, Clp1p regulates the G2/M transition since cells deleted for clp1 enter mitosis precociously and cells overexpressing Clp1p delay mitotic entry. Unlike Cdc14p, Clp1p appears to antagonize Cdk activity by preventing dephosphorylation of Cdc2p on tyrosine.Conclusions: S. pombe Clp1p affects cell cycle progression in a markedly different manner than its S. cerevisiae homolog, Cdc14p. This finding raises the possibility that related phosphatases in animal cells will prove to have important roles in coordinating the onset of cytokinesis with the events of mitosis.     

Budding Yeast DNA Damage Adaptation Mutants Exhibit Defects in Mitotic Exit

In the presence of double strand breaks, DNA damage checkpoint halts cell cycle progression. However, cells ultimately escape the checkpoint arrest and re-enter cell cycle in the presence of irreparable DNA damage. cdc5-ad was identified as a mutant that fails to adapt to the cell cycle arrest induced by DNA damage checkpoint. In budding yeast, Cdc5 protein kinase is a component of both MEN and FEAR pathways that are required for mitotic exit. It remains unclear whether the adaptation defect of cdc5-ad mutant cells is related to the function of Cdc5 in mitotic exit. Here we present evidence indicating that cdc5-ad mutant cells exhibit defects in mitotic exit. cdc5-ad mutant cells are sensitive to high dosage of Amn1, a negative regulator of MEN. It also shows synthetic growth defects with mutants in MEN pathway. Moreover, mutants in FEAR pathway exhibit defects in DNA damage adaptation. Thus, we conclude that the compromised mitotic exit pathway contributes to DNA damage adaptation defects in cdc5-ad mutant cells.     

The Sak polo-box comprises a structural domain sufficient for mitotic subcellular localization

The small family of polo-like kinases (Plks) includes Cdc5 from Saccharomyces cerevisiae, Plo1 from Schizosaccharomyces pombe, Polo from Drosophila melanogaster and the four mammalian genes Plk1, Prk/Fnk, Snk and Sak. These kinases control cell cycle progression through the regulation of centrosome maturation and separation, mitotic entry, metaphase to anaphase transition, mitotic exit and cytokinesis. Plks are characterized by an N-terminal Ser/Thr protein kinase domain and the presence of one or two C-terminal regions of similarity, termed the polo box motifs. These motifs have been demonstrated for Cdc5 and Plk1 to be required for mitotic progression and for subcellular localization to mitotic structures. Here we report the 2.0 A crystal structure of a novel domain composed of the polo box motif of murine Sak. The structure consists of a dimeric fold with a deep interfacial cleft and pocket, suggestive of a ligand-binding site. We show that this domain forms homodimers both in vitro and in vivo, and localizes to centrosomes and the cleavage furrow during cytokinesis. The requirement of the polo domain for Plk family function and the unique physical properties of the domain identify it as an attractive target for inhibitor design.     

Drosophila Nek2 protein kinase knockdown leads to centrosome maturation defects while overexpression causes centrosome fragmentation and cytokinesis failure

The Nima-related kinase 2 (Nek2) has been implicated in the regulation of centrosome integrity and separation in several species and is a candidate for cell transformation. We now show that reduction of levels of the Drosophila Nek2 by RNAi in cultured cells leads to both dispersal of centrosomal antigens and formation of ectopic bodies of centrosomal antigens. Overexpression of the active DmNek2 kinase resulted in an increase in the number of mitotic cells with fragmented centrosomes. The DmNek2 protein kinase is associated with punctuate bodies within the centrosome consistent with its presence on centrioles. In addition, it is present at lower levels on the midbody during cytokinesis. Midbody association was enhanced following overexpression, whereupon the DmNek2 protein kinase also localised to the cell cortex becoming concentrated in the region of the cleavage furrow in late telophase. Many of such cells showed abnormalities in the organisation of anillin and actin in the cleavage furrow that was associated with formation of ectopic membrane protrusions between each daughter cell. We discuss potential roles for DmNek2 in maintaining centrosome integrity in mitosis, during cytokinesis, and consequently for the fidelity of chromosome segregation.     

LATS2-Ajuba complex regulates gamma-tubulin recruitment to centrosomes and spindle organization during mitosis

LATS2 is a human homolog of Drosophila tumor suppressor lats/warts, and encodes a mitotic kinase whose physiological roles remain to be elucidated. We performed yeast two-hybrid screening and identified a LIM protein Ajuba, as a binding partner of LATS2. LATS2 was localized to the centrosomes throughout the cell cycle and was associated with Ajuba during mitosis, contributing to latter's mitotic phosphorylation. Depletion of LATS2 or Ajuba impaired centrosomal accumulation of gamma-tubulin and spindle formation at the onset of mitosis, suggesting that the LATS2-Ajuba complex regulates organization of the spindle apparatus through recruitment of gamma-tubulin to the centrosome.     

Polo样激酶1功能及其在肿瘤靶向治疗中的应用

Polo样激酶1(polo-like kinase1,PLK1)是一种广泛存在于真核细胞中的丝/苏氨酸激酶,在细胞周期调控中发挥关键的作用。其主要功能包括参与激活cyclin B/CDK1复合体,协助中心体的功能成熟,活化细胞分裂后期促进复合物(anaphase promoting complex,APC),促进染色体正常分离、分配和调控胞质分裂等。现已发现PLK1在多种肿瘤中表达增高并与某些肿瘤的预后密切相关。利用反义寡核苷酸、RNA干扰技术和化学合成PLK1小分子抑制剂等方法阻断PLK1的表达或降低其激酶活性,能够有效抑制肿瘤细胞的增殖并介导肿瘤细胞的凋亡,但对正常细胞没有明显影响,因此PLK1在肿瘤靶向治疗中具有重要的应用前景。     

Inactivation of mitotic kinase triggers translocation of MEN components to mother-daughter neck in yeast

Chromosome segregation, mitotic exit, and cytokinesis are executed in this order during mitosis. Although a scheme coordinating sister chromatid separation and initiation of mitotic exit has been proposed, the mechanism that temporally links the onset of cytokinesis to mitotic exit is not known. Exit from mitosis is regulated by the mitotic exit network (MEN), which includes a GTPase (Tem1) and various kinases (Cdc15, Cdc5, Dbf2, and Dbf20). Here, we show that Dbf2 and Dbf20 functions are necessary for the execution of cytokinesis. Relocalization of these proteins from spindle pole bodies to mother daughter neck seems to be necessary for this role because cdc15-2 mutant cells, though capable of exiting mitosis at semipermissive temperature, are unable to localize Dbf2 (and Dbf20) to the "neck" and fail to undergo cytokinesis. These cells can assemble and constrict the actomyosin ring normally but are incapable of forming a septum, suggesting that MEN components are critical for the initiation of septum formation. Interestingly, the spindle pole body to neck translocation of Dbf2 and Dbf20 is triggered by the inactivation of mitotic kinase. The requirement of kinase inactivation for translocation of MEN components to the division site thus provides a mechanism that renders mitotic exit a prerequisite for cytokinesis.     

Phosphorylation of CHO1 by Lats1/2 regulates the centrosomal activation of LIMK1 during cytokinesis

Large tumor suppressor 1 and 2 (Lats1/2) regulate centrosomal integrity, chromosome segregation and cytokinesis. As components of the centralspindlin complex, the kinesin-like protein CHO1 and its splicing variant MKLP1 colocalize with chromosome passenger proteins and GTPases and regulate the formation of the contractile ring and cytokinesis; however, the regulatory mechanisms of CHO1/MKLP1 remain elusive. Here, we show that Lats1/2 phosphorylate Ser716 in the F-actin-interacting region of CHO1, which is absent in MKLP1. Phosphorylated CHO1 localized to the centrosomes and midbody, and the actin polymerization factor LIM-kinase 1 (LIMK1) was identified as its binding partner. Overexpression of constitutively phosphorylated and non-phosphorylated CHO1 altered the mitotic localization and activation of LIMK1 at the centrosomes in HeLa cells, leading to the inhibition of cytokinesis through excessive phosphorylation of Cofilin and mislocalization of Ect2. These results suggest that Lats1/2 stringently control cytokinesis by regulating CHO1 phosphorylation and the mitotic activation of LIMK1 on centrosomes.     

Phosphorylation of dynamin II at serine-764 is associated with cytokinesis

Calcineurin is a phosphatase that is activated at the last known stage of mitosis, abscission. Among its many substrates, it dephosphorylates dynamin II during cytokinesis at the midbody of dividing cells. However, dynamin II has several cellular roles including clathrin-mediated endocytosis, centrosome cohesion and cytokinesis. It is not known whether dynamin II phosphorylation plays a role in any of these functions nor have the phosphosites involved in cytokinesis been directly identified. We now report that dynamin II from rat lung is phosphorylated to a low stoichiometry on a single major site, Ser-764, in the proline-rich domain. Phosphorylation on Ser-764 also occurred in asynchronously growing HeLa cells and was greatly increased upon mitotic entry. Tryptic phospho-peptides isolated by TiO₂ chromatography revealed only a single phosphosite in mitotic cells. Mitotic phosphorylation was abolished by roscovitine, suggesting the mitotic kinase is cyclin-dependent kinase 1. Cyclin-dependent kinase 1 phosphorylated full length dynamin II and Glutathione-S-Transferase-tagged-dynamin II-proline-rich domain in vitro, and mutation of Ser-764 to alanine reduced proline-rich domain phosphorylation by 80%, supporting that there is only a single major phosphosite. Ser-764 phosphorylation did not affect clathrin-mediated endocytosis or bulk endocytosis using penetratin-based phospho-deficient or phospho-mimetic peptides or following siRNA depletion/rescue experiments. Phospho-dynamin II was enriched at the mitotic centrosome, but this targeting was unaffected by the phospho-deficient or phospho-mimetic peptides. In contrast, the phospho-mimetic peptide displaced endogenous dynamin II, but not calcineurin, from the midbody and induced cytokinesis failure. Therefore, phosphorylation of dynamin II primarily occurs on a single site that regulates cytokinesis downstream of calcineurin, rather than regulating endocytosis or centrosome function.