Mammalian NDR protein kinases: from regulation to a role in centrosome duplication

Chronically activated microglia in ALS gradually lose their immune functions and develop unconventional proteome

Neuroinflammation and chronic activation of microglial cells are the prominent features of amyotrophic lateral sclerosis (ALS) pathology. While alterations in the mRNA profile of diseased microglia have been well documented, the actual microglia proteome remains poorly characterized. Here we performed a functional characterization together with proteome analyses of microglial cells at different stages of disease in the SOD1-G93A model of ALS. Functional analyses of microglia derived from the lumbar spinal cord of symptomatic mice revealed: (i) remarkably high mitotic index (close to 100% cells are Ki67+) (ii) significant decrease in phagocytic capacity when compared to age-matched control microglia, and (iii) diminished response to innate immune challenges in vitro and in vivo. Proteome analysis revealed a development of two distinct molecular signatures at early and advanced stages of disease. While at early stages of disease, we identified several proteins implicated in microglia immune functions such as GPNMB, HMBOX1, at advanced stages of disease microglia signature at protein level was characterized with a robust upregulation of several unconventional proteins including rootletin, major vaults proteins and STK38. Upregulation of GPNMB and rootletin has been also found in the spinal cord samples of sporadic ALS. Remarkably, the top biological functions of microglia, in particular in the advanced disease, were not related to immunity/immune response, but were highly enriched in terms linked to RNA metabolism. Together, our results suggest that, over the course of disease, chronically activated microglia develop unconventional protein signatures and gradually lose their immune identity ultimately turning into functionally inefficient immune cells.

Hippo pathway in intestinal diseases: focusing on ferroptosis

The incidence of intestinal diseases, such as inflammatory bowel disease, gastric cancer, and colorectal cancer, has steadily increased over the past decades. The Hippo pathway is involved in cell proliferation, tissue and organ damage, energy metabolism, tumor formation, and other physiologic processes. Ferroptosis is a form of programmed cell death characterized by the accumulation of iron and lipid peroxides. The Hippo pathway and ferroptosis are associated with various intestinal diseases; however, the crosstalk between them is unclear. This review elaborates on the current research on the Hippo pathway and ferroptosis in the context of intestinal diseases. We summarized the connection between the Hippo pathway and ferroptosis to elucidate the underlying mechanism by which these pathways influence intestinal diseases. We speculate that a mutual regulatory mechanism exists between the Hippo pathway and ferroptosis and these two pathways interact in several ways to regulate intestinal diseases.

The XPA Protein-Life under Precise Control

Nucleotide excision repair (NER) is a central DNA repair pathway responsible for removing a wide variety of DNA-distorting lesions from the genome. The highly choreographed cascade of core NER reactions requires more than 30 polypeptides. The xeroderma pigmentosum group A (XPA) protein plays an essential role in the NER process. XPA interacts with almost all NER participants and organizes the correct NER repair complex. In the absence of XPA's scaffolding function, no repair process occurs. In this review, we briefly summarize our current knowledge about the XPA protein structure and analyze the formation of contact with its protein partners during NER complex assembling. We focus on different ways of regulation of the XPA protein's activity and expression and pay special attention to the network of post-translational modifications. We also discuss the data that is not in line with the currently accepted hypothesis about the functioning of the XPA protein.

Prognostic and Immunological Role of STK38 across Cancers: Friend or Foe?

Although STK38 (serine-threonine kinase 38) has been proven to play an important role in cancer initiation and progression based on a series of cell and animal experiments, no systemic assessment of STK38 across human cancers is available. We firstly performed a pan-cancer analysis of STK38 in this study. The expression level of STK38 was significantly different between tumor and normal tissues in 15 types of cancers. Meanwhile, a prognosis analysis showed that a distinct relationship existed between STK38 expression and the clinical prognosis of cancer patients. Furthermore, the expression of STK38 was related to the infiltration of immune cells, such as NK cells, memory CD4+ T cells, mast cells and cancer-associated fibroblasts in a few cancers. There were three immune-associated signaling pathways involved in KEGG analysis of STK38. In general, STK38 shows a significant prognostic value in different cancers and is closely associated with cancer immunity.

The Dictyostelium Centrosome

The centrosome of Dictyostelium amoebae contains no centrioles and consists of a cylindrical layered core structure surrounded by a corona harboring microtubule-nucleating γ-tubulin complexes. It is the major centrosomal model beyond animals and yeasts. Proteomics, protein interaction studies by BioID and superresolution microscopy methods led to considerable progress in our understanding of the composition, structure and function of this centrosome type. We discuss all currently known components of the Dictyostelium centrosome in comparison to other centrosomes of animals and yeasts.

The STK38-XPO1 axis, a new actor in physiology and cancer

The Hippo signal transduction pathway is an essential regulator of organ size during developmental growth by controlling multiple cellular processes such as cell proliferation, cell death, differentiation, and stemness. Dysfunctional Hippo signaling pathway leads to dramatic tissue overgrowth. Here, we will briefly introduce the Hippo tumor suppressor pathway before focusing on one of its members and the unexpected twists that followed our quest of its functions in its multifarious actions beside the Hippo pathway: the STK38 kinase. In this review, we will precisely discuss the newly identified role of STK38 on regulating the nuclear export machinery by phosphorylating and activating, the major nuclear export receptor XPO1. Finally, we will phrase STK38's role on regulating the subcellular distribution of crucial cellular regulators such as Beclin1 and YAP1 with its implication in cancer.

XPA: DNA Repair Protein of Significant Clinical Importance

The nucleotide excision repair (NER) pathway is activated in response to a broad spectrum of DNA lesions, including bulky lesions induced by platinum-based chemotherapeutic agents. Expression levels of NER factors and resistance to chemotherapy has been examined with some suggestion that NER plays a role in tumour resistance; however, there is a great degree of variability in these studies. Nevertheless, recent clinical studies have suggested Xeroderma Pigmentosum group A (XPA) protein, a key regulator of the NER pathway that is essential for the repair of DNA damage induced by platinum-based chemotherapeutics, as a potential prognostic and predictive biomarker for response to treatment. XPA functions in damage verification step in NER, as well as a molecular scaffold to assemble other NER core factors around the DNA damage site, mediated by protein–protein interactions. In this review, we focus on the interacting partners and mechanisms of regulation of the XPA protein. We summarize clinical oncology data related to this DNA repair factor, particularly its relationship with treatment outcome, and examine the potential of XPA as a target for small molecule inhibitors.

Downregulation of NDR1 contributes to metastasis of prostate cancer cells via activating epithelial-mesenchymal transition

The 5‐year survival rate decreases rapidly once the prostate cancer has invaded distant organs, although patients with localized prostate cancer have a good prognosis. In recent years, increasing numbers of reports showed that circulating tumor cells (CTCs) may play an important role in tumor metastasis and they have stronger potential of invasion and migration compared with their parental cells. In our previous investigation, we isolated CTCs from prostate cancer cell lines PC3. In this study, we found a novel antimetastasis gene NDR1 by analyzing different gene expression between CTCs and PC3. Lower NDR1 gene and protein expression were found in both prostate cancer cell lines and clinical specimens. Besides, NDR1 function acting as metastasis inhibitor was discovered both in vitro and in vivo. Further, we also discovered that several epithelial‐mesenchymal transition (EMT)‐related genes were upregulated when decreased NDR1 in PC3 cell lines. Therefore, our results revealed a role of NDR1 in the suppression of prostate cancer cell metastasis and provided a potential mechanism of action, thus offering new therapeutic strategies against prostate cancer metastasis.

Altering Neurospora crassa MOB2A exposes its functions in development and affects its interaction with the NDR kinase COT1

The Neurospora crassa Mps One Binder (MOB) proteins MOB2A and MOB2B physically interact with the Nuclear Dbf2 Related (NDR) kinase COT1 and have been shown to have overlapping functions in various aspects of asexual development. Here, we identified two N. crassa MOB2A residues, Tyr117 and Tyr119, which are potentially phosphorylated. Using phosphomimetic mob-2a mutants we have been able to establish that apart from their previously described roles, MOB2A/B are involved in additional developmental processes. Enhanced conidial germination, accompanied by conidial agglutination, in the phosphomimetic mutants indicated that MOB2A is a negative regulator of germination. Thick-section imaging of perithecia revealed slow maturation and a lack of asci alignment in the mutant strains demonstrating a role for MOB2A in sexual development. We demonstrate that even though MOB2A and MOB2B have some overlapping functions, MOB2B cannot compensate for the roles MOB2A has in conidiation and germination. Altering Tyr residues 117 and 119 impaired the physical interactions between MOB2A and COT1, most likely contributing to some of the observed effects. As cot-1 and the phosphomimetic mutants share an extragenic suppressor (gul-1), we concluded that at least some of the effects imposed by altering Tyr117 and Tyr119 are mediated by the NDR kinase.

Stk38 Modulates Rbm24 Protein Stability to Regulate Sarcomere Assembly in Cardiomyocytes

RNA-binding protein Rbm24 is a key regulator of heart development and required for sarcomere assembly and heart contractility. Yet, its underlying mechanism remains unclear. Here, we link serine/threonine kinase 38 (Stk38) signaling to the regulation of Rbm24 by showing that Rbm24 phosphorylation and its function could be modulated by Stk38. Using co-immunoprecipitation coupled with mass spectrometry technique, we identified Stk38 as an endogenous binding partner of Rbm24. Stk38 knockdown resulted in decreased Rbm24 protein level in cardiomyocytes. Further studies using Stk38 kinase inhibitor or activator showed that Rbm24 protein stability was regulated in a kinase activity-dependent manner. Deficiency of Stk38 caused reduction of sarcomere proteins and disarrangement of sarcomere, suggesting that Stk38 is essential for Rbm24 to regulate sarcomere assembly. Our results revealed that Stk38 kinase catalyzes the phosphorylation of Rbm24 during sarcomerogensis and this orchestrates accurate sarcomere alignment. This furthers our understanding of the regulatory mechanism of cardiac sarcomere assembly in both physiologic and pathologic contexts, and uncovers a potential novel pathway to cardiomyopathy through modulating the Stk38/Rbm24 protein activity.

Tricornered Kinase Regulates Synapse Development by Regulating the Levels of Wiskott-Aldrich Syndrome Protein

Precise regulation of synapses during development is essential to ensure accurate neural connectivity and function of nervous system. Many signaling pathways, including the mTOR (mechanical Target of Rapamycin) pathway operate in neurons to maintain genetically determined number of synapses during development. mTOR, a kinase, is shared between two functionally distinct multi-protein complexes- mTORC1 and mTORC2, that act downstream of Tuberous Sclerosis Complex (TSC). We and others have suggested an important role for TSC in synapse development at the Drosophila neuromuscular junction (NMJ) synapses. In addition, our data suggested that the regulation of the NMJ synapse numbers in Drosophila largely depends on signaling via mTORC2. In the present study, we further this observation by identifying Tricornered (Trc) kinase, a serine/threonine kinase as a likely mediator of TSC signaling. trc genetically interacts with Tsc2 to regulate the number of synapses. In addition, Tsc2 and trc mutants exhibit a dramatic reduction in synaptic levels of WASP, an important regulator of actin polymerization. We show that Trc regulates the WASP levels largely, by regulating the transcription of WASP. Finally, we show that overexpression of WASP (Wiskott-Aldrich Syndrome Protein) in trc mutants can suppress the increase in the number of synapses observed in trc mutants, suggesting that WASP regulates synapses downstream of Trc. Thus, our data provide a novel insight into how Trc may regulate the genetic program that controls the number of synapses during development.

Phosphorylation-dependent inhibition of Cdc42 GEF Gef1 by 14-3-3 protein Rad24 spatially regulates Cdc42 GTPase activity and oscillatory dynamics during cell morphogenesis

The 14-3-3 protein Rad24 modulates the availability of Cdc42 GEF Gef1, spatially regulating Cdc42 activity during cell morphogenesis. Gef1 is sequestered in the cytoplasm upon 14-3-3 interaction, mediated by Orb6 kinase. The resulting competition for Gef1 promotes anticorrelated Cdc42 oscillations at cell tips.

Active Cdc42 GTPase, a key regulator of cell polarity, displays oscillatory dynamics that are anticorrelated at the two cell tips in fission yeast. Anticorrelation suggests competition for active Cdc42 or for its effectors. Here we show how 14-3-3 protein Rad24 associates with Cdc42 guanine exchange factor (GEF) Gef1, limiting Gef1 availability to promote Cdc42 activation. Phosphorylation of Gef1 by conserved NDR kinase Orb6 promotes Gef1 binding to Rad24. Loss of Rad24–Gef1 interaction increases Gef1 protein localization and Cdc42 activation at the cell tips and reduces the anticorrelation of active Cdc42 oscillations. Increased Cdc42 activation promotes precocious bipolar growth activation, bypassing the normal requirement for an intact microtubule cytoskeleton and for microtubule-dependent polarity landmark Tea4-PP1. Further, increased Cdc42 activation by Gef1 widens cell diameter and alters tip curvature, countering the effects of Cdc42 GTPase-activating protein Rga4. The respective levels of Gef1 and Rga4 proteins at the membrane define dynamically the growing area at each cell tip. Our findings show how the 14-3-3 protein Rad24 modulates the availability of Cdc42 GEF Gef1, a homologue of mammalian Cdc42 GEF DNMBP/TUBA, to spatially control Cdc42 GTPase activity and promote cell polarization and cell shape emergence.

Lower Levels of Human MOB3B Are Associated with Prostate Cancer Susceptibility and Aggressive Clinicopathological Characteristics

Mps one binder (MOB) proteins are integral components of signaling pathways that control important cellular processes, such as mitotic exit, centrosome duplication, apoptosis, and cell proliferation. However, the biochemical and cellular functions of the human MOB (hMOB) protein family remain largely unknown. The present study investigated the association between hMOB3B expression and clinicopathological characteristics of prostate cancer (PCa).Study subjects included 137 PCa patients and 137 age-matched benign prostatic hyperplasia (BPH) patients. hMOB3B expression was estimated using real-time PCR and compared with clinicopathological parameters of PCa. hMOB3B mRNA expression was significantly lower in PCa tissues than in BPH control tissues (P<0.001). According to receiver operating characteristics curve analysis, the sensitivity of hMOB3B expression for PCa diagnosis was 84.7%, with a specificity of 86% (AUC=0.910; 95% CI=0.869-0.941; P<0.001). hMOB3B expression was significantly lower in patients with elevated prostate specific antigen (PSA) levels (≥10 ng/mL), a Gleason score≥8, and metastatic disease (any T, N+/M+) than in those with low PSA levels, a low Gleason score, and non-metastatic disease (each P<0.05). In conclusion, low levels of hMOB3B are closely associated with aggressive clinicopathologic features in patients with PCa. Our results suggest that hMOB3B may act as a tumor suppressor in human PCa.

NDR1 modulates the UV-induced DNA-damage checkpoint and nucleotide excision repair

Nucleotide excision repair (NER) is the sole mechanism of UV-induced DNA lesion repair in mammals. A single round of NER requires multiple components including seven core NER factors, xeroderma pigmentosum A-G (XPA-XPG), and many auxiliary effector proteins including ATR serine/threonine kinase. The XPA protein helps to verify DNA damage and thus plays a rate-limiting role in NER. Hence, the regulation of XPA is important for the entire NER kinetic. We found that NDR1, a novel XPA-interacting protein, modulates NER by modulating the UV-induced DNA-damage checkpoint. In quiescent cells, NDR1 localized mainly in the cytoplasm. After UV irradiation, NDR1 accumulated in the nucleus. The siRNA knockdown of NDR1 delayed the repair of UV-induced cyclobutane pyrimidine dimers in both normal cells and cancer cells. It did not, however, alter the expression levels or the chromatin association levels of the core NER factors following UV irradiation. Instead, the NDR1-depleted cells displayed reduced activity of ATR for some set of its substrates including CHK1 and p53, suggesting that NDR1 modulates NER indirectly via the ATR pathway.

Separate to operate: control of centrosome positioning and separation

The centrosome is the main microtubule (MT)-organizing centre of animal cells. It consists of two centrioles and a multi-layered proteinaceous structure that surrounds the centrioles, the so-called pericentriolar material. Centrosomes promote de novo assembly of MTs and thus play important roles in Golgi organization, cell polarity, cell motility and the organization of the mitotic spindle. To execute these functions, centrosomes have to adopt particular cellular positions. Actin and MT networks and the association of the centrosomes to the nuclear envelope define the correct positioning of the centrosomes. Another important feature of centrosomes is the centrosomal linker that connects the two centrosomes. The centrosome linker assembles in late mitosis/G1 simultaneously with centriole disengagement and is dissolved before or at the beginning of mitosis. Linker dissolution is important for mitotic spindle formation, and its cell cycle timing has profound influences on the execution of mitosis and proficiency of chromosome segregation. In this review, we will focus on the mechanisms of centrosome positioning and separation, and describe their functions and mechanisms in the light of recent findings.

Hippo-YAP signaling pathway: A new paradigm for cancer therapy

In the past decades, the Hippo signaling pathway has been delineated and shown to play multiple roles in the control of organ size in both Drosophila and mammals. In mammals, the Hippo pathway is a kinase cascade leading from Mst1/2 to YAP and its paralog TAZ. Several studies have demonstrated that YAP/TAZ is a candidate oncogene and that other members of the Hippo pathway are tumor suppressive genes. The dysregulation of the Hippo pathway has been observed in a variety of cancers. This review chronicles the recent progress in elucidating the function of Hippo signaling in tumorigenesis and provide a rich source of potential targets for cancer therapy.

Regulation of a LATS-homolog by Ras GTPases is important for the control of cell division

Background

Nuclear Dbf-related/large tumor suppressor (NDR/LATS) kinases have been shown recently to control pathways that regulate mitotic exit, cytokinesis, cell growth, morphological changes and apoptosis. LATS kinases are core components of the Hippo signaling cascade and important tumor suppressors controlling cell proliferation and organ size in flies and mammals, and homologs are also present in yeast and Dictyostelium discoideum. Ras proto-oncogens regulate many biological functions, including differentiation, proliferation and apoptosis. Dysfunctions of LATS kinases or Ras GTPases have been implicated in the development of a variety of cancers in humans.

Results

In this study we used the model organism Dictyostelium discoideum to analyze the functions of NdrC, a homolog of the mammalian LATS2 protein, and present a novel regulatory mechanism for this kinase. Deletion of the ndrC gene caused impaired cell division and loss of centrosome integrity. A yeast two-hybrid analysis, using activated Ras proteins as bait, revealed NdrC as an interactor and identified its Ras-binding domain. Further in vitro pull-down assays showed that NdrC binds RasG and RasB, and to a lesser extent RasC and Rap1. In cells lacking NdrC, the levels of activated RasB and RasG are up-regulated, suggesting a functional connection between RasB, RasG, and NdrC.

Conclusions

Dictyostelium discoideum NdrC is a LATS2-homologous kinase that is important for the regulation of cell division. NdrC contains a Ras-binding domain and interacts preferentially with RasB and RasG. Changed levels of both, RasB or RasG, have been shown previously to interfere with cell division. Since a defect in cell division is exhibited by NdrC-null cells, RasG-null cells, and cells overexpressing activated RasB, we propose a model for the regulation of cytokinesis by NdrC that involves the antagonistic control by RasB and RasG.

Proper actin ring formation and septum constriction requires coordinated regulation of SIN and MOR pathways through the germinal centre kinase MST-1

Nuclear DBF2p-related (NDR) kinases constitute a functionally conserved protein family of eukaryotic regulators that control cell division and polarity. In fungi, they function as effector kinases of the morphogenesis (MOR) and septation initiation (SIN) networks and are activated by pathway-specific germinal centre (GC) kinases. We characterized a third GC kinase, MST-1, that connects both kinase cascades. Genetic and biochemical interactions with SIN components and life cell imaging identify MST-1 as SIN-associated kinase that functions in parallel with the GC kinase SID-1 to activate the SIN-effector kinase DBF-2. SID-1 and MST-1 are both regulated by the upstream SIN kinase CDC-7, yet in an opposite manner. Aberrant cortical actomyosin rings are formed in Δmst-1, which resulted in mis-positioned septa and irregular spirals, indicating that MST-1-dependent regulation of the SIN is required for proper formation and constriction of the septal actomyosin ring. However, MST-1 also interacts with several components of the MOR network and modulates MOR activity at multiple levels. MST-1 functions as promiscuous enzyme and also activates the MOR effector kinase COT-1 through hydrophobic motif phosphorylation. In addition, MST-1 physically interacts with the MOR kinase POD-6, and dimerization of both proteins inactivates the GC kinase hetero-complex. These data specify an antagonistic relationship between the SIN and MOR during septum formation in the filamentous ascomycete model Neurospora crassa that is, at least in part, coordinated through the GC kinase MST-1. The similarity of the SIN and MOR pathways to the animal Hippo and Ndr pathways, respectively, suggests that intensive cross-communication between distinct NDR kinase modules may also be relevant for the homologous NDR kinases of higher eukaryotes.

Cytokinesis is a fundamental cellular process essential for cell proliferation of uni- and multicellular organisms. The molecular pathways that regulate cytokinesis are highly complex and involve a large number of components that form elaborate interactive networks. The fungal septation initiation network (SIN) functions as tripartite kinase cascade that connects cell cycle progression with the control of cell division. Mis-regulation of the homologous Hippo pathway in animals results in excessive proliferation and formation of tumors, underscoring the conservation and importance of these kinase networks. A second morphogenesis (MOR) pathway involves homologous components and is controlling cell polarity in fungi and higher eukaryotes. Here we show that the promiscuous functioning Ste20-related kinase MST-1 has a dual role in regulating SIN and MOR network function. Moreover, SIN and MOR coordination through MST-1 can be achieved in an enzyme-independent manner through hetero-dimerization of germinal centre kinases, providing an additional level for activity regulation of signaling networks that is not dependent on phosphate transfer. Given the functional conservation of NDR kinase signaling modules and their regulation, our work may define general mechanisms by which NDR kinase pathway are coordinated in fungi and higher eukaryotes.

Nucleocytoplasmic protein translocation during mitosis in the social amoebozoan Dictyostelium discoideum

Mitosis is a fundamental and essential life process. It underlies the duplication and survival of all cells and, as a result, all eukaryotic organisms. Since uncontrolled mitosis is a dreaded component of many cancers, a full understanding of the process is critical. Evolution has led to the existence of three types of mitosis: closed, open, and semi-open. The significance of these different mitotic species, how they can lead to a full understanding of the critical events that underlie the asexual duplication of all cells, and how they may generate new insights into controlling unregulated cell division remains to be determined. The eukaryotic microbe Dictyostelium discoideum has proved to be a valuable biomedical model organism. While it appears to utilize closed mitosis, a review of the literature suggests that it possesses a form of mitosis that lies in the middle between truly open and fully closed mitosis-it utilizes a form of semi-open mitosis. Here, the nucleocytoplasmic translocation patterns of the proteins that have been studied during mitosis in the social amoebozoan D. discoideum are detailed followed by a discussion of how some of them provide support for the hypothesis of semi-open mitosis.

Lre1 directly inhibits the NDR/Lats kinase Cbk1 at the cell division site in a phosphorylation-dependent manner

BACKGROUND

The nuclear Dbf2 related (NDR) family of protein kinases play important roles in cell-cycle regulation, apoptosis, cell morphogenesis, and development in a variety of organisms. In budding yeast, the NDR kinase complex composed of Cbk1 and its regulatory subunit, Mob2, have an established role in the control of cell separation/abscission that follows cytokinesis. Whereas the activators of Cbk1-Mob2 have been more extensively described, the mechanisms that restrict or inhibit Cbk1-Mob2 catalytic activity remain largely unknown.

RESULTS

We identified the protein Lre1 as a direct inhibitor of Cbk1-Mob2 catalytic activity. We show that Lre1 accumulates at the cell division site in late anaphase and associates with both Mob2 and Cbk1 in vivo and in vitro. Biochemical and functional analysis established that the ability of Lre1 to associate with Cbk1-Mob2 was reduced by mitotic Cdk1 activity and promoted by Cdc14 phosphatase at the end of mitosis. The inhibition of Cbk1-Mob2 by Lre1 was critical to promote the survival of cells lacking the actomyosin driven pathway of cytokinesis.

CONCLUSIONS

We established Lre1 as a direct inhibitor of the NDR kinase Cbk1-Mob2, which is regulated in a cell-cycle-dependent manner. We propose that similar inhibitory proteins may also provide fine tuning for the activity of NDR kinases in other organisms.

Identification of a Novel Link between the Protein Kinase NDR1 and TGFβ Signaling in Epithelial Cells

Transforming growth factor-beta (TGFβ) is a secreted polypeptide that plays essential roles in cellular development and homeostasis. Although mechanisms of TGFβ-induced responses have been characterized, our understanding of TGFβ signaling remains incomplete. Here, we uncover a novel function for the protein kinase NDR1 (nuclear Dbf2-related 1) in TGFβ responses. Using an immunopurification approach, we find that NDR1 associates with SnoN, a key component of TGFβ signaling. Knockdown of NDR1 by RNA interference promotes the ability of TGFβ to induce transcription and cell cycle arrest in NMuMG mammary epithelial cells. Conversely, expression of NDR1 represses TGFβ-induced transcription and inhibits the ability of TGFβ to induce cell cycle arrest in NMuMG cells. Mechanistically, we find that NDR1 acts in a kinase-dependent manner to suppress the ability of TGFβ to induce the phosphorylation and consequent nuclear accumulation of Smad2, which is critical for TGFβ-induced transcription and responses. Strikingly, we also find that TGFβ reciprocally regulates NDR1, whereby TGFβ triggers the degradation of NDR1 protein. Collectively, our findings define a novel and intimate link between the protein kinase NDR1 and TGFβ signaling. NDR1 suppresses TGFβ-induced transcription and cell cycle arrest, and counteracting NDR1's negative regulation, TGFβ signaling induces the downregulation of NDR1 protein. These findings advance our understanding of TGFβ signaling, with important implications in development and tumorigenesis.

S100B protein in tissue development, repair and regeneration

The Ca²⁺-binding protein of the EF-hand type, S100B, exerts both intracellular and extracellular regulatory activities. As an intracellular regulator, S100B is involved in the regulation of energy metabolism, transcription, protein phosphorylation, cell proliferation, survival, differentiation and motility, and Ca²⁺ homeostasis, by interacting with a wide array of proteins (i.e., enzymes, enzyme substrates, cytoskeletal subunits, scaffold/adaptor proteins, transcription factors, ubiquitin E3 ligases, ion channels) in a restricted number of cell types. As an extracellular signal, S100B engages the pattern recognition receptor, receptor for advanced glycation end-products (RAGE), on immune cells as well as on neuronal, astrocytic and microglial cells, vascular smooth muscle cells, skeletal myoblasts and cardiomyocytes. However, RAGE may not be the sole receptor activated by S100B, the protein being able to enhance bFGF-FGFR1 signaling by interacting with FGFR1-bound bFGF in particular cell types. Moreover, extracellular effects of S100B vary depending on its local concentration. Increasing evidence suggests that at the concentration found in extracellular fluids in normal physiological conditions and locally upon acute tissue injury, which is up to a few nM levels, S100B exerts trophic effects in the central and peripheral nervous system and in skeletal muscle tissue thus participating in tissue homeostasis. The present commentary summarizes results implicating intracellular and extracellular S100B in tissue development, repair and regeneration.

Linkers of cell polarity and cell cycle regulation in the fission yeast protein interaction network

The study of gene and protein interaction networks has improved our understanding of the multiple, systemic levels of regulation found in eukaryotic and prokaryotic organisms. Here we carry out a large-scale analysis of the protein-protein interaction (PPI) network of fission yeast (Schizosaccharomyces pombe) and establish a method to identify ‘linker’ proteins that bridge diverse cellular processes - integrating Gene Ontology and PPI data with network theory measures. We test the method on a highly characterized subset of the genome consisting of proteins controlling the cell cycle, cell polarity and cytokinesis and identify proteins likely to play a key role in controlling the temporal changes in the localization of the polarity machinery. Experimental inspection of one such factor, the polarity-regulating RNB protein Sts5, confirms the prediction that it has a cell cycle dependent regulation. Detailed bibliographic inspection of other predicted ‘linkers’ also confirms the predictive power of the method. As the method is robust to network perturbations and can successfully predict linker proteins, it provides a powerful tool to study the interplay between different cellular processes.

Analysis of protein interaction networks has been of use as a means to grapple with the complexity of the interactome of biological organisms. So far, network based approaches have only been used in a limited number of organisms due to the lack of high-throughput experiments. In this study, we investigate by graph theoretical network analysis approaches the protein-protein interaction network of fission yeast, and present a new network measure, linkerity, that predicts the ability of certain proteins to function as bridges between diverse cellular processes. We apply this linkerity measure to a highly conserved and coupled subset of the fission yeast network, consisting of the proteins that regulate cell cycle, polarized cell growth, and cell division. In depth literature analysis confirms that several proteins identified as linkers of cell polarity regulation are indeed also associated with cell cycle and/or cell division control. Similarly, experimental testing confirms that a mostly uncharacterized polarity regulator identified by the method as an important linker is regulated by the cell cycle, as predicted.

The NDR kinase scaffold HYM1/MO25 is essential for MAK2 map kinase signaling in Neurospora crassa

Cell communication is essential for eukaryotic development, but our knowledge of molecules and mechanisms required for intercellular communication is fragmentary. In particular, the connection between signal sensing and regulation of cell polarity is poorly understood. In the filamentous ascomycete Neurospora crassa, germinating spores mutually attract each other and subsequently fuse. During these tropic interactions, the two communicating cells rapidly alternate between two different physiological states, probably associated with signal delivery and response. The MAK2 MAP kinase cascade mediates cell–cell signaling. Here, we show that the conserved scaffolding protein HYM1/MO25 controls the cell shape-regulating NDR kinase module as well as the signal-receiving MAP kinase cascade. HYM1 functions as an integral part of the COT1 NDR kinase complex to regulate the interaction with its upstream kinase POD6 and thereby COT1 activity. In addition, HYM1 interacts with NRC1, MEK2, and MAK2, the three kinases of the MAK2 MAP kinase cascade, and co-localizes with MAK2 at the apex of growing cells. During cell fusion, the three kinases of the MAP kinase module as well as HYM1 are recruited to the point of cell–cell contact. hym-1 mutants phenocopy all defects observed for MAK2 pathway mutants by abolishing MAK2 activity. An NRC1-MEK2 fusion protein reconstitutes MAK2 signaling in hym-1, while constitutive activation of NRC1 and MEK2 does not. These data identify HYM1 as a novel regulator of the NRC1-MEK2-MAK2 pathway, which may coordinate NDR and MAP kinase signaling during cell polarity and intercellular communication.

Intercellular communication and cellular morphogenesis are essential for eukaryotic development. Our knowledge of molecules and mechanisms associated with these processes is, however, fragmentary. In particular, the molecular connection between signal sensing and regulation of cell polarity is poorly understood. Fungal hyphae share with neurons and pollen tubes the distinction of being amongst the most highly polarized cells in biology. The robust genetic tractability of filamentous fungi provides an unparalleled opportunity to determine common principles that underlie polarized growth and its regulation through cell communication. In Neurospora crassa, germinating spores mutually attract each other, establish physical contact through polarized tropic growth, and fuse. During this process, the cells rapidly alternate between two different physiological states, probably associated with signal delivery and response. Here, we show that the conserved scaffolding protein HYM1/MO25 interacts with the polarity and cell shape-regulating NDR kinase complex as well as a MAP kinase module, which is essential for cell communication during the tropic interaction. We propose that this dual use of a common regulator in both molecular complexes may represent an intriguing mechanism of linking the perception of external cues with the polarization machinery to coordinate communication and tropic growth of interacting cells.

Protein kinases of the Hippo pathway: regulation and substrates

The "Hippo" signaling pathway has emerged as a major regulator of cell proliferation and survival in metazoans. The pathway, as delineated by genetic and biochemical studies in Drosophila, consists of a kinase cascade regulated by cell-cell contact and cell polarity that inhibits the transcriptional coactivator Yorkie and its proliferative, anti-differentiation, antiapoptotic transcriptional program. The core pathway components are the GC kinase Hippo, which phosphorylates the noncatalytic polypeptide Mats/Mob1 and, with the assistance of the scaffold protein Salvador, phosphorylates the ndr-family kinase Lats. In turn phospho-Lats, after binding to phospho-Mats, autoactivates and phosphorylates Yorkie, resulting in its nuclear exit. Hippo also uses the scaffold protein Furry and a different Mob protein to control another ndr-like kinase, the morphogenetic regulator Tricornered. Architecturally homologous kinase cascades consisting of a GC kinase, a Mob protein, a scaffolding polypeptide and an ndr-like kinase are well described in yeast; in Saccharomyces cerevisiae, e.g., the MEN pathway promotes mitotic exit whereas the RAM network, using a different GC kinase, Mob protein, scaffold and ndr-like kinase, regulates cell polarity and morphogenesis. In mammals, the Hippo orthologs Mst1 and Mst2 utilize the Salvador ortholog WW45/Sav1 and other scaffolds to regulate the kinases Lats1/Lats2 and ndr1/ndr2. As in Drosophila, murine Mst1/Mst2, in a redundant manner, negatively regulate the Yorkie ortholog YAP in the epithelial cells of the liver and gut; loss of both Mst1 and Mst2 results in hyperproliferation and tumorigenesis that can be largely negated by reduction or elimination of YAP. Despite this conservation, considerable diversification in pathway composition and regulation is already evident; in skin, e.g., YAP phosphorylation is independent of Mst1Mst2 and Lats1Lats2. Moreover, in lymphoid cells, Mst1/Mst2, under the control of the Rap1 GTPase and independent of YAP, promotes integrin clustering, actin remodeling and motility while restraining the proliferation of naïve T cells. This review will summarize current knowledge of the structure and regulation of the kinases Hippo/Mst1&2, their noncatalytic binding partners, Salvador and the Rassf polypeptides, and their major substrates Warts/Lats1&2, Trc/ndr1&2, Mats/Mob1 and FOXO.

PKL01, an Ndr kinase homologue in plant, shows tyrosine kinase activity

Protein phosphorylation by protein tyrosine (Tyr) kinases plays important roles in a variety of signalling pathways in cell growth, differentiation and oncogenesis in animals. Despite the absence of classical Tyr kinases in plants, a similar ratio of phosphotyrosine residues in phosphorylated proteins was found in Arabidopsis thaliana as in human. However, protein kinases responsible for tyrosine phosphorylation in plants except some dedicated dual-specificity kinases still remain unclear. In this study, we found that PKL01, a nuclear Dbf2-related (Ndr) kinase homologue in Lotus japonicus, was autophosphorylated at a tyrosine residue when it was expressed in Escherichia coli, but kinase-dead mutant of PKL01 was not. Tyrosine phophorylation site in PKL01 was identified as Tyr-56 by LC-MS/MS analysis. Recombinant PKL01, which had been dephosphorylated by an alkaline phosphatase, could be phosphorylated again at the Tyr residue when it was incubated with ATP. Furthermore, other Ndr kinases in plants and PKL01 phosphorylated on Tyr residues in the exogenous substrates such as poly(Glu, Tyr)(4:1) and casein. Therefore, the Ndr kinases in plants, which had been assumed as protein serine (Ser)/threonine (Thr) kinases, turned out to be dual-specificity kinases responsible for phosphorylation of Tyr residues and Ser/Thr residues in plant proteins.

The RAM network in pathogenic fungi

The regulation of Ace2 and morphogenesis (RAM) network is a protein kinase signaling pathway conserved among eukaryotes from yeasts to humans. Among fungi, the RAM network has been most extensively studied in the model yeast Saccharomyces cerevisiae and has been shown to regulate a range of cellular processes, including daughter cell-specific gene expression, cell cycle regulation, cell separation, mating, polarized growth, maintenance of cell wall integrity, and stress signaling. Increasing numbers of recent studies on the role of the RAM network in pathogenic fungal species have revealed that this network also plays an important role in the biology and pathogenesis of these organisms. In addition to providing a brief overview of the RAM network in S. cerevisiae, we summarize recent developments in the understanding of RAM network function in the human fungal pathogens Candida albicans, Candida glabrata, Cryptococcus neoformans, Aspergillus fumigatus, and Pneumocystis spp.

Mammalian Hippo signalling: a kinase network regulated by protein-protein interactions

The Hippo signal transduction cascade controls cell growth, proliferation and death, all of which are frequently deregulated in tumour cells. Since initial studies in Drosophila melanogaster were instrumental in defining Hippo signalling, the machinery was named after the central Ste20-like kinase Hippo. Moreover, given that loss of Hippo signalling components Hippo, Warts, and Mats resulted in uncontrolled tissue overgrowth, Hippo signalling was defined as a tumour-suppressor cascade. Significantly, all of the core factors of Hippo signalling have mammalian orthologues that functionally compensate for loss of their counterparts in Drosophila. Furthermore, studies in Drosophila and mammalian cell systems showed that Hippo signalling represents a kinase cascade that is tightly regulated by PPIs (protein-protein interactions). Several Hippo signalling molecules contain SARAH (Salvador/RASSF1A/Hippo) domains that mediate specific PPIs, thereby influencing the activities of MST1/2 (mammalian Ste20-like serine/threonine kinase 1/2) kinases, the human Hippo orthologues. Moreover, WW domains are present in several Hippo factors, and these domains also serve as interaction surfaces for regulatory PPIs in Hippo signalling. Finally, the kinase activities of LATS1/2 (large tumour-suppressor kinase 1/2), the human counterparts of Warts, are controlled by binding to hMOB1 (human Mps one binder protein 1), the human Mats. Therefore Hippo signalling is regulated by PPIs on several levels. In the present paper, I review the current understanding of how these regulatory PPIs are regulated and contribute to the functionality of Hippo signalling.

Crosstalk between NDR kinase pathways coordinates cell cycle dependent actin rearrangements

Regulation of cytoskeletal remodeling is essential for cell cycle transitions. In fission yeast two NDR kinase signaling cascades, MOR and SIN, regulate the actin cytoskeleton to promote polarized growth during interphase and cytokinesis respectively. Our understanding of how these signaling pathways are coordinated to assist transition between the two cell-cycle stages is limited. Here, we review work from our laboratory, which reveals that cross talk between the SIN and MOR pathways is required for inhibition of interphase polarity programs during cytokinesis. Given the conservation of NDR kinase signaling pathways, our results may define general mechanisms by which these pathways are coordinated in higher organisms.

Cellular kinases incorporated into HIV-1 particles: passive or active passengers?

Phosphorylation is one of the major mechanisms by which the activities of protein factors can be regulated. Such regulation impacts multiple key-functions of mammalian cells, including signal transduction, nucleo-cytoplasmic shuttling, macromolecular complexes assembly, DNA binding and regulation of enzymatic activities to name a few. To ensure their capacities to replicate and propagate efficiently in their hosts, viruses may rely on the phosphorylation of viral proteins to assist diverse steps of their life cycle. It has been known for several decades that particles from diverse virus families contain some protein kinase activity. While large DNA viruses generally encode for viral kinases, RNA viruses and more precisely retroviruses have acquired the capacity to hijack the signaling machinery of the host cell and to embark cellular kinases when budding. Such property was demonstrated for HIV-1 more than a decade ago. This review summarizes the knowledge acquired in the field of HIV-1-associated kinases and discusses their possible function in the retroviral life cycle.

The yeast Cbk1 kinase regulates mRNA localization via the mRNA-binding protein Ssd1

In the absence of Cbk1 phosphorylation Ssd1-associated mRNAs are redirected from sites of polarized cell growth to stress granules and P-bodies.

The mRNA-binding protein Ssd1 is a substrate for the Saccharomyces cerevisiae LATS/NDR orthologue Cbk1, which controls polarized growth, cell separation, and cell integrity. We discovered that most Ssd1 localizes diffusely within the cytoplasm, but some transiently accumulates at sites of polarized growth. Cbk1 inhibition and cellular stress cause Ssd1 to redistribute to mRNA processing bodies (P-bodies) and stress granules, which are known to repress translation. Ssd1 recruitment to P-bodies is independent of mRNA binding and is promoted by the removal of Cbk1 phosphorylation sites. SSD1 deletion severely impairs the asymmetric localization of the Ssd1-associated mRNA, SRL1. Expression of phosphomimetic Ssd1 promotes polarized localization of SRL1 mRNA, whereas phosphorylation-deficient Ssd1 causes constitutive localization of SRL1 mRNA to P-bodies and causes cellular lysis. These data support the model that Cbk1-mediated phosphorylation of Ssd1 promotes the cortical localization of Ssd1–mRNA complexes, whereas Cbk1 inhibition, cellular stress, and Ssd1 dephosphorylation promote Ssd1–mRNA interactions with P-bodies and stress granules, leading to translational repression.

Mammalian Hippo pathway: from development to cancer and beyond

The Hippo pathway was discovered as a signal transduction pathway that regulates organ size in Drosophila melanogaster. It is composed of three components: cell surface upstream regulators including cell adhesion molecules and cell polarity complexes; a kinase cascade comprising two serine-threonine kinases with regulators and adaptors; and a downstream target, a transcription coactivator. The coactivator mediates the transcription of cell proliferation-promoting and anti-apoptotic genes. The pathway negatively regulates the coactivator to restrict cell proliferation and to promote cell death. Thus, the pathway prevents tissue overgrowth and tumourigenesis. The framework of the pathway is conserved in mammals. A dysfunction of the pathway is frequently detected in human cancers and correlates with a poor prognosis. Recent works indicated that the Hippo pathway plays an important role in tissue homoeostasis through the regulation of stem cells, cell differentiation and tissue regeneration.

Human NDR kinases control G(1)/S cell cycle transition by directly regulating p21 stability

The G(1) phase of the cell cycle is an important integrator of internal and external cues, allowing a cell to decide whether to proliferate, differentiate, or die. Multiple protein kinases, among them the cyclin-dependent kinases (Cdks), control G(1)-phase progression and S-phase entry. With the regulation of apoptosis, centrosome duplication, and mitotic chromosome alignment downstream of the HIPPO pathway components MST1 and MST2, mammalian NDR kinases have been implicated to function in cell cycle-dependent processes. Although they are well characterized in terms of biochemical regulation and upstream signaling pathways, signaling mechanisms downstream of mammalian NDR kinases remain largely unknown. We identify here a role for human NDR in regulating the G(1)/S transition. In G(1) phase, NDR kinases are activated by a third MST kinase (MST3). Significantly, interfering with NDR and MST3 kinase expression results in G(1) arrest and subsequent proliferation defects. Furthermore, we describe the first downstream signaling mechanisms by which NDR kinases regulate cell cycle progression. Our findings suggest that NDR kinases control protein stability of the cyclin-Cdk inhibitor protein p21 by direct phosphorylation. These findings establish a novel MST3-NDR-p21 axis as an important regulator of G(1)/S progression of mammalian cells.

In vivo-derived horse blastocysts show transcriptional upregulation of developmentally important genes compared with in vitro-produced horse blastocysts

In vitro-produced (IVP) equine blastocysts can give rise to successful pregnancies, but their morphology and developmental rate differ from those of in vivo-derived equine blastocysts. The aim of the present study was to evaluate this difference at the genetic level. Suppression subtractive hybridisation (SSH) was used to construct a cDNA library enriched for transcripts preferentially expressed in in vivo-derived equine blastocysts compared with IVP blastocysts. Of the 62 different genes identified in this way, six genes involved in embryonic development (BEX2, FABP3, HSP90AA1, MOBKL3, MCM7 and ODC) were selected to confirm this differential expression by reverse transcription–quantitative real-time polymerase chain reaction (RT-qPCR). Using RT-qPCR, five genes were confirmed to be significantly upregulated in in vivo-derived blastocysts (i.e. FABP3, HSP90AA1 (both P < 0.05), ODC, MOBKL3 and BEX2 (P < 0.005 for all three)), confirming the results of the SSH. There was no significant difference in MCM7 expression between IVP and in vivo-derived blastocysts. In conclusion, five genes that are transcriptionally upregulated in in vivo-derived equine blastocysts compared with IVP blastocysts have been identified. Because of their possible importance in embryonic development, the expression of these genes can be used as a marker to evaluate in vitro embryo production systems in the horse.

Heat shock protein 90 inhibition depletes LATS1 and LATS2, two regulators of the mammalian hippo tumor suppressor pathway

Heat shock protein 90 (HSP90), which regulates the functions of multiple oncogenic signaling pathways, has emerged as a novel anticancer therapeutic target, and multiple small-molecule HSP90 inhibitors are now in clinical trials. Although the effects of HSP90 inhibitors on oncogenic signaling pathways have been extensively studied, the effects of these agents on tumor suppressor signaling pathways are currently unknown. Here, we have examined how HSP90 inhibitors affect LATS1 and the related protein LATS2, two kinases that relay antiproliferative signals in the Hippo tumor suppressor pathway. Both LATS1 and LATS2 were depleted from cells treated with the HSP90 inhibitors 17-allylamino-17-demethoxygeldanamycin (17-AAG), radicicol, and PU-H71. Moreover, these kinases interacted with HSP90, and LATS1 isolated from 17-AAG-treated cells had reduced catalytic activity, thus showing that the kinase is a bona fide HSP90 client. Importantly, LATS1 signaling was disrupted by 17-AAG in tumor cell lines in vitro and clinical ovarian cancers in vivo as shown by reduced levels of LATS1 and decreased phosphorylation of the LATS substrate YAP, an oncoprotein transcriptional coactivator that regulates genes involved in cell and tissue growth, including the CTGF gene. Consistent with the reduced YAP phosphorylation, there were increased levels of CTGF, a secreted protein that is implicated in tumor proliferation, metastasis, and angiogenesis. Taken together, these results identify LATS1 and LATS2 as novel HSP90 clients and show that HSP90 inhibitors can disrupt the LATS tumor suppressor pathway in human cancer cells.

The mitosis-to-interphase transition is coordinated by cross talk between the SIN and MOR pathways in Schizosaccharomyces pombe

The SIN pathway blocks inappropriate actin rearrangements during cytokinesis by preventing activation of the MOR pathway component Orb6.

The mechanisms that regulate cytoskeletal remodeling during the transition between mitosis and interphase are poorly understood. In fission yeast the MOR pathway promotes actin polarization to cell tips in interphase, whereas the SIN signaling pathway drives actomyosin ring assembly and cytokinesis. We show that the SIN inhibits MOR signaling in mitosis by interfering with Nak1 kinase-mediated activation of the most downstream MOR component, the NDR family kinase Orb6. Inactivation of the MOR may be a key function of the SIN because attenuation of MOR signaling rescued the cytokinetic defects of SIN mutants and allowed weak SIN signaling to trigger ectopic cytokinesis. Furthermore, failure to inhibit the MOR is toxic when the cell division apparatus is compromised. Together, our results reveal a mutually antagonistic relationship between the SIN and MOR pathways, which is important for completion of cytokinesis and coordination of cytoskeletal remodeling at the mitosis-to-interphase transition.

The T-loop extension of the tomato protein kinase AvrPto-dependent Pto-interacting protein 3 (Adi3) directs nuclear localization for suppression of plant cell death

In tomato (Solanum lycopersicum), resistance to Pseudomonas syringae pv. tomato is elicited by the interaction of the host Pto kinase with the pathogen effector protein AvrPto, which leads to various immune responses including localized cell death termed the hypersensitive response. The AGC kinase Adi3 functions to suppress host cell death and interacts with Pto only in the presence of AvrPto. The cell death suppression (CDS) activity of Adi3 requires phosphorylation by 3-phosphoinositide-dependent protein kinase 1 (Pdk1) and loss of Adi3 function is associated with the hypersensitive response cell death initiated by the Pto/AvrPto interaction. Here we studied the relationship between Adi3 cellular localization and its CDS activity. Adi3 is a nuclear-localized protein, and this localization is dictated by a nuclear localization signal found in the Adi3 T-loop extension, an approximately 80 amino acid insertion into the T-loop, or activation loop, which is phosphorylated for kinase activation. Nuclear localization of Adi3 is required for its CDS activity and loss of nuclear localization causes elimination of Adi3 CDS activity and induction of cell death. This nuclear localization of Adi3 is dependent on Ser-539 phosphorylation by Pdk1 and non-nuclear Adi3 is found in punctate structures throughout the cell. Our data support a model in which Pdk1 phosphorylation of Adi3 directs nuclear localization for CDS and that disruption of Adi3 nuclear localization may be a mechanism for induction of cell death such as that during the Pto/AvrPto interaction.

Molecular characterization of human homologs of yeast MOB1

MOB (Mps one binder) was originally identified in yeast as a regulator of mitotic exit and cytokinesis, and was later identified as a tumor suppressor and a component of an emerging Hippo-LATS tumor suppressor pathway in Drosophila (D). So far, 7 human homologs of yeast MOB (hMOB1A, 1B, 2A, 2B, 2C, 3, 4) have been identified. Although hMOB1A/B has been extensively studied, the biological features of other hMOBs are largely unknown. In addition, while hMOB1 has been reported to interact with and activate LATS (Large tumor suppressor)/Warts tumor suppressor, the functional significance of this is unknown. In this study, we have characterized, for the first time, the cellular and biochemical function of all human MOBs. By examining hMOB mRNAs expression in various human tissues, we found that hMOBs demonstrated different expression patterns. Further biochemical characterization of hMOBs showed that only hMOB1A and hMOB1B interact with both LATS1 and LATS2 in vitro and in vivo. Significantly, we have discovered that overexpression of hMOB1 in human cancer cells activated LATS activity and inhibited cell proliferation or caused apoptosis while hMOB1, targeting the plasma membrane, led to a more significant phenotype. Reciprocally, short-hairpin (sh) RNA-mediated suppression of hMOB1 causes increased cell proliferation. Our findings provided evidence that hMOB1A and hMOB1B are 2 LATS-binding proteins that may function as tumor suppressors in human cancer cells.

The MST1 and hMOB1 tumor suppressors control human centrosome duplication by regulating NDR kinase phosphorylation

BACKGROUND

Human MST/hSAV/LATS/hMOB tumor suppressor cascades are regulators of cell death and proliferation; however, little is known about other functions of MST/hMOB signaling. Mob1p, one of two MOB proteins in yeast, appears to play a role in spindle pole body duplication (the equivalent of mammalian centrosome duplication). We therefore investigated the role of human MOB proteins in centrosome duplication. We also addressed the regulation of human centrosome duplication by mammalian serine/threonine Ste20-like (MST) kinases, considering that MOB proteins can function together with Ste20-like kinases in eukaryotes.

RESULTS

By studying the six human MOB proteins and five MST kinases, we found that MST1/hMOB1 signaling controls centrosome duplication. Overexpression of hMOB1 caused centrosome overduplication, whereas RNAi depletion of hMOB1 or MST1 impaired centriole duplication. Significantly, we delineated an hMOB1/MST1/NDR1 signaling pathway regulating centrosome duplication. More specifically, analysis of shRNA-resistant hMOB1 and NDR1 mutants revealed that a functional NDR/hMOB1 complex is critical for MST1 to phosphorylate NDR on the hydrophobic motif that in turn is required for human centrosome duplication. Furthermore, shRNA-resistant MST1 variants revealed that MST1 kinase activity is crucial for centrosome duplication whereas MST1 binding to the hSAV and RASSF1A tumor suppressor proteins is dispensable. Finally, by studying the PLK4/HsSAS-6/CP110 centriole assembly machinery, we also observed that normal daughter centriole formation depends on intact MST1/hMOB1/NDR signaling, although HsSAS-6 centriolar localization is not affected.

CONCLUSIONS

Our observations propose a novel pathway in control of human centriole duplication after recruitment of HsSAS-6 to centrioles.

Roles of mammalian sterile 20-like kinase 2-dependent phosphorylations of Mps one binder 1B in the activation of nuclear Dbf2-related kinases

Mammalian nuclear Dbf2-related (NDR) kinases (LATS1, LATS2, NDR1 and NDR2) play a role in cell proliferation, apoptosis and morphological changes. Mammalian sterile 20-like (MST) kinases and Mps one binder (MOB) proteins are important in the activation of NDR kinases. MOB1 is phosphorylated by MST1 and MST2 and this phosphorylation enhances the ability of MOB1 to activate NDR kinases. The phosphorylated MOB1 can be more effective as a scaffold protein to facilitate the MST-dependent phosphorylation of NDR kinases and/or as a direct activator of NDR kinases. We previously reported that Thr74 of MOB1B is phosphorylated by MST2. Thr12 and Thr35 have also been identified as phosphorylation sites. In this study, we quantified the phosphorylation of Thr74 using the phosphorylated Thr74-specific antibody. Thr74 is indeed phosphorylated by MST2, but the efficiency is low, suggesting that MOB1B can activate NDR kinases without the phosphorylation of Thr74. We also showed that the phosphorylated MOB1B activates NDR1 T444D and LATS2 T1041D, in which threonine residues phosphorylated by MST kinases are replaced with phosphorylation-mimicking aspartic acid, more efficiently than the unphosphorylated MOB1B does. This finding supports that the phosphorylation of MOB1B enhances its ability as a direct activator of NDR kinases.

Cell elongation and branching are regulated by differential phosphorylation states of the nuclear Dbf2-related kinase COT1 in Neurospora crassa

Dysfunction of the Neurospora crassa nuclear Dbf2-related kinase COT1 leads to cessation of tip extension and massive induction of new sites of growth. To determine the role phosphorylation plays in COT1 function, we mutated COT1 residues corresponding to positions of highly conserved nuclear Dbf2-related phosphorylation sites. Analyses of the point-mutation cot-1 strains (mimicking non- and constitutively phosphorylated states) indicate the involvement of COT1 phosphorylation in the regulation of hyphal elongation and branching as well as asexual development by altering cell wall integrity and actin organization. Phosphorylation of COT1's activation segment (at Ser417) is required for proper in vitro kinase activity, but has only a limited effect on hyphal growth. In marked contrast, even though phosphorylation of the C-terminal hydrophobic motif (at Thr589) is crucial for all COT1 functions in vivo, the lack of Thr589 phosphorylation did not significantly affect in vitro COT1 kinase activity. Nevertheless, its regulatory role has been made evident by the significant increase observed in COT1 kinase activity when this residue was substituted in a manner mimicking constitutive phosphorylation. We conclude that COT1 regulates elongation and branching in an independent manner, which is determined by its phosphorylation state.

Hippo pathway-dependent and -independent roles of RASSF6

The Hippo pathway restricts cell growth and proliferation and promotes apoptosis to control organ size. The Drosophila melanogaster isoform of RASSF (Ras association domain family; dRASSF) antagonizes proapoptotic Hippo signaling by inhibiting the binding of the adaptor protein Salvador to the kinase Hippo. Paradoxically, however, dRASSF also functions as a tumor suppressor. In mammals, RASSF1A induces apoptosis by stimulating the mammalian Ste20-like kinases (MSTs) 1 and 2, which are Hippo homologs. Here, we characterize the interaction between MST2 and another mammalian RASSF isoform, RASSF6. When bound to MST2, RASSF6 inhibited MST2 activity to antagonize Hippo signaling. However, RASSF6 caused apoptosis when released from activated MST2 in a manner dependent on WW45, the mammalian Salvador homolog. Thus, RASSF6 antagonizes Hippo signaling and mediates apoptosis through a pathway that is parallel to the canonical Hippo pathway. Our findings suggest that activation of MST2 causes apoptosis through the Hippo pathway, as well as through a RASSF6-mediated pathway.

Mammalian NDR/LATS protein kinases in hippo tumor suppressor signaling

The NDR/LATS family of kinases is a subgroup of the AGC group of protein kinases and is conserved from lower eukaryotes to humans. Like other AGC kinases, NDR/LATS kinases require phosphorylation of conserved Ser/Thr residues for activation. On the one hand, binding of the coactivator MOB to NDR/LATS allows autophosphorylation. On the other hand, MST kinases directly phosphorylate NDR/LATS kinases. In addition to our understanding of the molecular activation mechanisms, recent studies have shown that LATS kinases play a central role in Hippo/SWH (Salvador/Warts/Hippo) tumor suppressor pathways, which coordinate cell proliferation and apoptosis by regulating proto-oncogenes, such as YAP and TAZ. In this review, we summarize current knowledge of Merlin/MST/SAV/MOB/LATS/NDR/YAP/TAZ networks (also termed mammalian Hippo signaling) and their roles in mammalian cellular transformation.

Dbf2 is essential for cytokinesis and correct mitotic spindle formation in Candida albicans

We have characterized the DBF2 gene, encoding a protein kinase of the NDR family in Candida albicans, and demonstrate that this gene is essential for cell viability. Conditional mutants were constructed by using the MET3 promoter to analyse the phenotype of cells lacking this kinase. The absence of Dbf2 resulted in cells arrested as large-budded pairs that failed to contract the actomyosin ring, a function similar to that described for its Saccharomyces cerevisiae orthologue. In addition to its role in cytokinesis, Dbf2 regulates mitotic spindle organization and nuclear segregation as Dbf2-depleted cells have abnormal microtubules and severe defects in nuclear migration to the daughter cell, which results in a cell cycle block during mitosis. Taken together, these results imply that Dbf2 performs several functions during exit from mitosis and cytokinesis. Consistent with a role in spindle organization, the protein localizes to the mitotic spindle during anaphase, and it interacts physically with tubulin, as indicated by immunoprecipitation experiments. Finally, DBF2 depletion also resulted in impaired true hyphal growth.

Budding yeast centrosome duplication requires stabilization of Spc29 via Mps1-mediated phosphorylation

Protein phosphorylation plays an important role in the regulation of centrosome duplication. In budding yeast, numerous lines of evidence suggest a requirement for multiple phosphorylation events on individual components of the centrosome to ensure their proper assembly and function. Here, we report the first example of a single phosphorylation event on a component of the yeast centrosome, or spindle pole body (SPB), that is required for SPB duplication and cell viability. This phosphorylation event is on the essential SPB component Spc29 at a conserved Thr residue, Thr(240). Mutation of Thr(240) to Ala is lethal at normal gene dosage, but an increased copy number of this mutant allele results in a conditional phenotype. Phosphorylation of Thr(240) was found to promote the stability of the protein in vivo and is catalyzed in vitro by the Mps1 kinase. Furthermore, the stability of newly synthesized Spc29 is reduced in a mutant strain with reduced Mps1 kinase activity. These results demonstrate the first evidence for a single phosphorylation event on an SPB component that is absolutely required for SPB duplication and suggest that the Mps1 kinase is responsible for this protein-stabilizing phosphorylation.