Mechanism of Ca2+-mediated regulation of NDR protein kinase through autophosphorylation and phosphorylation by an upstream kinase

A review of nuclear Dbf2-related kinase 1 (NDR1) protein interaction as promising new target for cancer therapy

Nuclear Dbf2-related kinase 1 (NDR1) is a nuclear Dbf2-related (NDR) protein kinase family member, which regulates cell functions and participates in cell proliferation and differentiation through kinase activity. NDR1 regulates physiological functions by interacting with different proteins. Protein-protein interactions (PPIs) are crucial for regulating biological processes and controlling cell fate, and as a result, it is beneficial to study the actions of PPIs to elucidate the pathological mechanism of diseases. The previous studies also show that the expression of NDR1 is deregulated in numerous human cancer samples and it needs the context-specific targeting strategies for NDR1. Thus, a comprehensive understanding of the direct interaction between NDR1 and varieties of proteins may provide new insights into cancer therapies. In this review, we summarize recent studies of NDR1 in solid tumors, such as prostate cancer and breast cancer, and explore the mechanism of action of PPIs of NDR1 in tumors.

Microautophagy regulated by STK38 and GABARAPs is essential to repair lysosomes and prevent aging

Lysosomes are degradative organelles and signaling hubs that maintain cell and tissue homeostasis, and lysosomal dysfunction is implicated in aging and reduced longevity. Lysosomes are frequently damaged, but their repair mechanisms remain unclear. Here, we demonstrate that damaged lysosomal membranes are repaired by microautophagy (a process termed "microlysophagy") and identify key regulators of the first and last steps. We reveal the AGC kinase STK38 as a novel microlysophagy regulator. Through phosphorylation of the scaffold protein DOK1, STK38 is specifically required for the lysosomal recruitment of the AAA+ ATPase VPS4, which terminates microlysophagy by promoting the disassembly of ESCRT components. By contrast, microlysophagy initiation involves non-canonical lipidation of ATG8s, especially the GABARAP subfamily, which is required for ESCRT assembly through interaction with ALIX. Depletion of STK38 and GABARAPs accelerates DNA damage-induced cellular senescence in human cells and curtails lifespan in C. elegans, respectively. Thus, microlysophagy is regulated by STK38 and GABARAPs and could be essential for maintaining lysosomal integrity and preventing aging.

A cryptic microdeletion del(12)(p11.21p11.23) within an unbalanced translocation t(7;12)(q21.13;q23.1) implicates new candidate loci for intellectual disability and Kallmann syndrome

In a patient diagnosed with both Kallmann syndrome (KS) and intellectual disability (ID), who carried an apparently balanced translocation t(7;12)(q22;q24)dn, array comparative genomic hybridization (aCGH) disclosed a cryptic heterozygous 4.7 Mb deletion del(12)(p11.21p11.23), unrelated to the translocation breakpoint. This novel discovery prompted us to consider the possibility that the combination of KS and neurological disorder in this patient could be attributed to gene(s) within this specific deletion at 12p11.21-12p11.23, rather than disrupted or dysregulated genes at the translocation breakpoints. To further support this hypothesis, we expanded our study by screening five candidate genes at both breakpoints of the chromosomal translocation in a cohort of 48 KS patients. However, no mutations were found, thus reinforcing our supposition. In order to delve deeper into the characterization of the 12p11.21-12p11.23 region, we enlisted six additional patients with small copy number variations (CNVs) and analyzed eight individuals carrying small CNVs in this region from the DECIPHER database. Our investigation utilized a combination of complementary approaches. Firstly, we conducted a comprehensive phenotypic-genotypic comparison of reported CNV cases. Additionally, we reviewed knockout animal models that exhibit phenotypic similarities to human conditions. Moreover, we analyzed reported variants in candidate genes and explored their association with corresponding phenotypes. Lastly, we examined the interacting genes associated with these phenotypes to gain further insights. As a result, we identified a dozen candidate genes: TSPAN11 as a potential KS candidate gene, TM7SF3, STK38L, ARNTL2, ERGIC2, TMTC1, DENND5B, and ETFBKMT as candidate genes for the neurodevelopmental disorder, and INTS13, REP15, PPFIBP1, and FAR2 as candidate genes for KS with ID. Notably, the high-level expression pattern of these genes in relevant human tissues further supported their candidacy. Based on our findings, we propose that dosage alterations of these candidate genes may contribute to sexual and/or cognitive impairments observed in patients with KS and/or ID. However, the confirmation of their causal roles necessitates further identification of point mutations in these candidate genes through next-generation sequencing.

The Role of Mammalian STK38 in DNA Damage Response and Targeting for Radio-Sensitization

Protein kinases, found in the nucleus and cytoplasm, play essential roles in a multitude of cellular processes, including cell division, proliferation, apoptosis, and signal transduction. STK38 is a member of the protein kinase A (PKA)/PKG/PKC family implicated in regulating cell division and morphogenesis in yeast and C. elegans. However, its function remained largely unknown in mammals. In recent years, advances in research on STK38 and the identification of its substrates has led to a better understanding of its function and role in mammals. This review discusses the structure, expression, and regulation of activity as a kinase, its role in the DNA damage response, cross-talk with other signaling pathways, and its application for radio-sensitization.

A microdeletion del(12)(p11.21p11.23) with a cryptic unbalanced translocation t(7;12)(q21.13;q23.1) implicates new candidate loci for intellectual disability and Kallmann syndrome

In an apparently balanced translocation t(7;12)(q22;q24)dn exhibiting both Kallmann syndrome (KS) and intellectual disability (ID), we detected a cryptic heterozygous 4.7 Mb del(12)(p11.21p11.23) unrelated to the translocation breakpoint. This new finding raised the possibility that KS combined with neurological disorder in this patient could be caused by gene(s) within this deletion at 12p11.21-12p11.23 instead of disrupted or dysregulated genes at the genomic breakpoints. Screening of five candidate genes at both breakpoints in 48 KS patients we recruited found no mutation, corroborating our supposition. To substantiate this hypothesis further, we recruited six additional subjects with small CNVs and analyzed eight individuals carrying small CNVs in this region from DECIPHER to dissect 12p11.21-12p11.23. We used multiple complementary approaches including a phenotypic-genotypic comparison of reported cases, a review of knockout animal models recapitulating the human phenotypes, and analyses of reported variants in the interacting genes with corresponding phenotypes. The results identified one potential KS candidate gene ( TSPAN11 ), seven candidate genes for the neurodevelopmental disorder ( TM7SF3 , STK38L , ARNTL2 , ERGIC2 , TMTC1 , DENND5B , and ETFBKMT ), and four candidate genes for KS with ID ( INTS13 , REP15 , PPFIBP1 , and FAR2 ). The high-level expression pattern in the relevant human tissues further suggested the candidacy of these genes. We propose that the dosage alterations of the candidate genes may contribute to sexual and/or cognitive impairment in patients with KS and/or ID. Further identification of point mutations through next generation sequencing will be necessary to confirm their causal roles.

Loss of NDR1/2 kinases impairs endomembrane trafficking and autophagy leading to neurodegeneration

Autophagy is essential for neuronal development and its deregulation contributes to neurodegenerative diseases. NDR1 and NDR2 are highly conserved kinases, implicated in neuronal development, mitochondrial health and autophagy, but how they affect mammalian brain development in vivo is not known. Using single and double Ndr1/2 knockout mouse models, we show that only dual loss of Ndr1/2 in neurons causes neurodegeneration. This phenotype was present when NDR kinases were deleted both during embryonic development, as well as in adult mice. Proteomic and phosphoproteomic comparisons between Ndr1/2 knockout and control brains revealed novel kinase substrates and indicated that endocytosis is significantly affected in the absence of NDR1/2. We validated the endocytic protein Raph1/Lpd1, as a novel NDR1/2 substrate, and showed that both NDR1/2 and Raph1 are critical for endocytosis and membrane recycling. In NDR1/2 knockout brains, we observed prominent accumulation of transferrin receptor, p62 and ubiquitinated proteins, indicative of a major impairment of protein homeostasis. Furthermore, the levels of LC3-positive autophagosomes were reduced in knockout neurons, implying that reduced autophagy efficiency mediates p62 accumulation and neurotoxicity. Mechanistically, pronounced mislocalisation of the transmembrane autophagy protein ATG9A at the neuronal periphery, impaired axonal ATG9A trafficking and increased ATG9A surface levels further confirm defects in membrane trafficking, and could underlie the impairment in autophagy. We provide novel insight into the roles of NDR1/2 kinases in maintaining neuronal health.

Calmodulin activates the Hippo signaling pathway by promoting LATS1 kinase-mediated inhibitory phosphorylation of the transcriptional coactivator YAP

The Hippo signaling pathway regulates tissue growth and cell fate, and its dysregulation can induce tumorigenesis. When Hippo is activated by cell–cell contact, extracellular signals, or cell polarity among others, the large tumor suppressor 1 (LATS1) kinase catalyzes inhibitory phosphorylation of the transcriptional coactivator Yes-associated protein (YAP) to maintain YAP in the cytoplasm or promote its degradation. Separately, calmodulin is a Ca²⁺-dependent protein that modulates the activity of target proteins and regulates several signaling cascades; however, its potential role in the Hippo pathway has not been identified. Here, using diverse experimental approaches, including in vitro binding analyses, kinase assays, RT–PCR, and confocal microscopy, we reveal that calmodulin promotes Hippo signaling. We show that purified YAP and LATS1 bind directly to calmodulin and form a Ca²⁺-dependent ternary complex in vitro. Importantly, Ca²⁺/calmodulin directly stimulated the activity of LATS1 kinase. In cultured mammalian cells, we demonstrated that endogenous YAP and LATS1 coimmunoprecipitate with endogenous calmodulin. In cells with activated Hippo signaling, we show that calmodulin antagonism significantly (i) decreases YAP phosphorylation, (ii) increases expression of two Hippo target genes (connective tissue growth factor [CTGF] and cysteine-rich angiogenic inducer 61 [CYR61]) that regulate cell proliferation and tumor progression, and (iii) enhances the interaction of YAP with its major transcription factor, thereby facilitating transcription of target genes. Collectively, our data demonstrate that calmodulin activates the Hippo kinase cascade and inhibits YAP activity via a direct interaction with LATS1 and YAP, thereby uncovering previously unidentified crosstalk between the Ca²⁺/calmodulin and Hippo signaling pathways.

The NDR kinase-MOB complex FgCot1-Mob2 regulates polarity and lipid metabolism in Fusarium graminearum

Members of the NDR (nuclear Dbf2-related) protein-kinase family are essential for cell differentiation and polarized morphogenesis. However, their functions in plant pathogenic fungi are not well understood. Here, we characterized the NDR kinase FgCot1 and its activator FgMob2 in Fusarium graminearum, a major pathogen causing Fusarium head blight (FHB) in wheat. FgCot1 and FgMob2 formed a NDR kinase-MOB protein complex. Localization assays using FgCot1-GFP or FgMob2-RFP constructs showed diverse subcellular localizations, including cytoplasm, septum, nucleus and hyphal tip. ΔFgcot1 and ΔFgmob2 exhibited serious defects in hyphal growth, polarity, fungal development and cell wall integrity as well as reduced virulence in planta. In contrast, lipid droplet accumulation was significantly increased in these two mutants. Phosphorylation of FgCot1 at two highly conserved residues (S462 and T630) as well as five new sites synergistically contributed its role in various cellular processes. In addition, non-synonymous mutations in two MAPK (mitogen-activated protein kinase) proteins, FgSte11 and FgGpmk1, partially rescued the growth defect of ΔFgmob2, indicating a functional link between the FgCot1-Mob2 complex and the FgGpmk1 signalling pathway in regulating filamentous fungal growth. These results indicated that the FgCot1-Mob2 complex is critical for polarity, fungal development, cell wall organization, lipid metabolism and virulence in F. graminearum.

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.

The hippo kinase STK38 ensures functionality of XPO1

The proper segregation of basic elements such as the compartmentalization of the genome and the shuttling of macromolecules between the nucleus and the cytoplasm is a crucial mechanism for homeostasis maintenance in eukaryotic cells. XPO1 (Exportin 1) is the major nuclear export receptor and is required for the export of proteins and RNAs out of the nucleus. STK38 (also known as NDR1) is a Hippo pathway serine/threonine kinase with multifarious functions in normal and cancer cells. In this review, we summarize the history of the discovery of the nucleo/cytoplasmic shuttling of proteins and focus on the major actor of nuclear export: XPO1. After describing the molecular events required for XPO1-mediated nuclear export of proteins, we introduce the Hippo pathway STK38 kinase, synthetize its regulation mechanisms as well as its biological functions in both normal and cancer cells, and finally its intersection with XPO1 biology. We discuss the recently identified mechanism of XPO1 activation by phosphorylation of XPO1_S1055 by STK38 and contextualize this finding according to the biological functions previously reported for both XPO1 and STK38, including the second identity of STK38 as an autophagy regulator. Finally, we phrase this newly identified activation mechanism into the general nuclear export machinery and examine the possible outcomes of nuclear export inhibition in cancer treatment.

Kinome Profiling Reveals Abnormal Activity of Kinases in Skeletal Muscle From Adults With Obesity and Insulin Resistance

CONTEXT

Obesity-related insulin resistance (OIR) is one of the main contributors to type 2 diabetes and other metabolic diseases. Protein kinases are implicated in insulin signaling and glucose metabolism. Molecular mechanisms underlying OIR involving global kinase activities remain incompletely understood.

OBJECTIVE

To investigate abnormal kinase activity associated with OIR in human skeletal muscle.

DESIGN

Utilization of stable isotopic labeling-based quantitative proteomics combined with affinity-based active enzyme probes to profile in vivo kinase activity in skeletal muscle from lean control (Lean) and OIR participants.

PARTICIPANTS

A total of 16 nondiabetic adults, 8 Lean and 8 with OIR, underwent hyperinsulinemic-euglycemic clamp with muscle biopsy.

RESULTS

We identified the first active kinome, comprising 54 active protein kinases, in human skeletal muscle. The activities of 23 kinases were different in OIR muscle compared with Lean muscle (11 hyper- and 12 hypo-active), while their protein abundance was the same between the 2 groups. The activities of multiple kinases involved in adenosine monophosphate-activated protein kinase (AMPK) and p38 signaling were lower in OIR compared with Lean. On the contrary, multiple kinases in the c-Jun N-terminal kinase (JNK) signaling pathway exhibited higher activity in OIR vs Lean. The kinase-substrate-prediction based on experimental data further confirmed a potential downregulation of insulin signaling (eg, inhibited phosphorylation of insulin receptor substrate-1 and AKT1/2).

CONCLUSIONS

These findings provide a global view of the kinome activity in OIR and Lean muscle, pinpoint novel specific impairment in kinase activities in signaling pathways important for skeletal muscle insulin resistance, and may provide potential drug targets (ie, abnormal kinase activities) to prevent and/or reverse skeletal muscle insulin resistance in humans.

The Hippo network kinase STK38 contributes to protein homeostasis by inhibiting BAG3-mediated autophagy

Chaperone-assisted selective autophagy (CASA) initiated by the cochaperone Bcl2-associated athanogene 3 (BAG3) represents an important mechanism for the disposal of misfolded and damaged proteins in mammalian cells. Under mechanical stress, the cochaperone cooperates with the small heat shock protein HSPB8 and the cytoskeleton-associated protein SYNPO2 to degrade force-unfolded forms of the actin-crosslinking protein filamin. This is essential for muscle maintenance in flies, fish, mice and men. Here, we identify the serine/threonine protein kinase 38 (STK38), which is part of the Hippo signaling network, as a novel interactor of BAG3. STK38 was previously shown to facilitate cytoskeleton assembly and to promote mitophagy as well as starvation and detachment induced autophagy. Significantly, our study reveals that STK38 exerts an inhibitory activity on BAG3-mediated autophagy. Inhibition relies on a disruption of the functional interplay of BAG3 with HSPB8 and SYNPO2 upon binding of STK38 to the cochaperone. Of note, STK38 attenuates CASA independently of its kinase activity, whereas previously established regulatory functions of STK38 involve target phosphorylation. The ability to exert different modes of regulation on central protein homeostasis (proteostasis) machineries apparently allows STK38 to coordinate the execution of diverse macroautophagy pathways and to balance cytoskeleton assembly and degradation.

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The Hippo network kinase STK38 interacts with the cochaperone BAG3.

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STK38 inhibits BAG3-dependent chaperone-assisted selective autophagy (CASA).

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CASA inhibition does not require the kinase activity of STK38.

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Hippo network kinases that act upstream of STK38 contribute to CASA regulation.

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STK38 disrupts interactions of BAG3 with the CASA components HSPB8 and SYNPO2.

Identification of the kinase STK25 as an upstream activator of LATS signaling

The Hippo pathway maintains tissue homeostasis by negatively regulating the oncogenic transcriptional co-activators YAP and TAZ. Though functional inactivation of the Hippo pathway is common in tumors, mutations in core pathway components are rare. Thus, understanding how tumor cells inactivate Hippo signaling remains a key unresolved question. Here, we identify the kinase STK25 as an activator of Hippo signaling. We demonstrate that loss of STK25 promotes YAP/TAZ activation and enhanced cellular proliferation, even under normally growth-suppressive conditions both in vitro and in vivo. Notably, STK25 activates LATS by promoting LATS activation loop phosphorylation independent of a preceding phosphorylation event at the hydrophobic motif, which represents a form of Hippo activation distinct from other kinase activators of LATS. STK25 is significantly focally deleted across a wide spectrum of human cancers, suggesting STK25 loss may represent a common mechanism by which tumor cells functionally impair the Hippo tumor suppressor pathway.

Two Antagonistic Hippo Signaling Circuits Set the Division Plane at the Medial Position in the Ciliate Tetrahymena

In a single cell, ciliates maintain a complex pattern of cortical organelles that are arranged along the anteroposterior and circumferential axes. The underlying molecular mechanisms of intracellular pattern formation in ciliates are largely unknown. Ciliates divide by tandem duplication, a process that remodels the parental cell into two daughters aligned head-to-tail. In the elo1-1 mutant of Tetrahymena thermophila, the segmentation boundary/division plane forms too close to the posterior end of the parental cell, producing a large anterior and a small posterior daughter cell, respectively. We show that ELO1 encodes a Lats/NDR kinase that marks the posterior segment of the cell cortex, where the division plane does not form in the wild-type. Elo1 acts independently of CdaI, a Hippo/Mst kinase that marks the anterior half of the parental cell, and whose loss shifts the division plane anteriorly. We propose that, in Tetrahymena, two antagonistic Hippo circuits focus the segmentation boundary/division plane at the equatorial position, by excluding divisional morphogenesis from the cortical areas that are too close to cell ends.

A Hippo-like Signaling Pathway Controls Tracheal Morphogenesis in Drosophila melanogaster

Hippo-like pathways are ancient signaling modules first identified in yeasts. The best-defined metazoan module forms the core of the Hippo pathway, which regulates organ size and cell fate. Hippo-like kinase modules consist of a Sterile 20-like kinase, an NDR kinase, and non-catalytic protein scaffolds. In the Hippo pathway, the upstream kinase Hippo can be activated by another kinase, Tao-1. Here, we delineate a related Hippo-like signaling module that Tao-1 regulates to control tracheal morphogenesis in Drosophila melanogaster. Tao-1 activates the Sterile 20-like kinase GckIII by phosphorylating its activation loop, a mode of regulation that is conserved in humans. Tao-1 and GckIII act upstream of the NDR kinase Tricornered to ensure proper tube formation in trachea. Our study reveals that Tao-1 activates two related kinase modules to control both growth and morphogenesis. The Hippo-like signaling pathway we have delineated has a potential role in the human vascular disease cerebral cavernous malformation.

Structural Basis for Auto-Inhibition of the NDR1 Kinase Domain by an Atypically Long Activation Segment

The human NDR family kinases control diverse aspects of cell growth, and are regulated through phosphorylation and association with scaffolds such as MOB1. Here, we report the crystal structure of the human NDR1 kinase domain in its non-phosphorylated state, revealing a fully resolved atypically long activation segment that blocks substrate binding and stabilizes a non-productive position of helix αC. Consistent with an auto-inhibitory function, mutations within the activation segment of NDR1 dramatically enhance in vitro kinase activity. Interestingly, NDR1 catalytic activity is further potentiated by MOB1 binding, suggesting that regulation through modulation of the activation segment and by MOB1 binding are mechanistically distinct. Lastly, deleting the auto-inhibitory activation segment of NDR1 causes a marked increase in the association with upstream Hippo pathway components and the Furry scaffold. These findings provide a point of departure for future efforts to explore the cellular functions and the mechanism of NDR1.

Downregulated NDR1 protein kinase inhibits innate immune response by initiating an miR146a-STAT1 feedback loop

Interferon (IFN)-stimulated genes (ISGs) play crucial roles in the antiviral immune response; however, IFNs also induce negative regulators that attenuate the antiviral response. Here, we show that both viral and bacterial invasion downregulate Nuclear Dbf2-related kinase 1 (NDR1) expression via the type I IFN signaling pathway. NDR1 promotes the virus-induced production of type I IFN, proinflammatory cytokines and ISGs in a kinase-independent manner. NDR1 deficiency also renders mice more susceptible to viral and bacterial infections. Mechanistically, NDR1 enhances STAT1 translation by directly binding to the intergenic region of miR146a, thereby inhibiting miR146a expression and liberating STAT1 from miR146a-mediated translational inhibition. Furthermore, STAT1 binds to the miR146a promoter, thus decreasing its expression. Together, our results suggest that NDR1 promotion of STAT1 translation is an important event for IFN-dependent antiviral immune response, and suggest that NDR1 has an important role in controlling viral infections.

The authors show that NDR1 promotion of STAT1 translation is an important event for IFN-dependent antiviral immune response. These data suggest that NDR1 has an important role in controlling viral infections.

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.

Monopolar spindle-one-binder protein 2 regulates the activity of large tumor suppressor/yes-associated protein to inhibit the motility of SMMC-7721 hepatocellular carcinoma cells

Accumulating evidence implicates monopolar spindle-one-binder protein (MOB)2 as an inhibitor of nuclear-Dbf2-related kinase (NDR) by competing with MOB1 for interaction with NDR1/2. NDR/large tumor suppressor (LATS) kinases may function similarly to yes-associated protein (YAP) kinases and be considered as members of the Hippo core cassette. MOB2 appears to serve roles in cell survival, cell cycle progression, responses to DNA damage and cell motility. However, the underlying mechanisms involved remain unclarified. In the present study, it was demonstrated that the knockout of MOB2 by clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR associated protein 9 promoted migration and invasion, induced phosphorylation of NDR1/2 and decreased phosphorylation of YAP in SMMC-7721 cells when compared with the blank vector-transduced cells. By contrast, the overexpression of MOB2 resulted in the opposite results. Mechanistically, MOB2 regulated the alternative interaction of MOB1 with NDR1/2 and LATS1, which resulted in increased phosphorylation of LATS1 and MOB1 and thereby led to the inactivation of YAP and consequently inhibition of cell motility. The results of the present study provide evidence of MOB2 serving a positive role in LATS/YAP activation by activating the Hippo signaling pathway.

The Neurospora crassa PP2A Regulatory Subunits RGB1 and B56 Are Required for Proper Growth and Development and Interact with the NDR Kinase COT1

COT1 is the founding member of the highly conserved nuclear Dbf2-related (NDR) Ser/Thr kinase family and plays a role in the regulation of polar growth and development in Neurospora crassa and other fungi. Changes in COT1 phosphorylation state have been shown to affect hyphal elongation, branching, and conidiation. The function of NDR protein kinases has been shown to be regulated by type 2A protein phosphatases (PP2As). PP2As are heterotrimers comprised of a catalytic and scaffolding protein along with an interchangeable regulatory subunit involved in determining substrate specificity. Inactivation of the N. crassa PP2A regulatory subunits rgb-1 and b56 conferred severe hyphal growth defects. Partial suppression of defects observed in the rgb-1^RIP strain (but not in the Δb56 mutant) was observed in cot-1 phosphomimetic mutants, demonstrating that altering COT1 phosphorylation state can bypass, at least in part, the requirement of a functional RGB1 subunit. The functional fusion proteins RGB1::GFP and B56::GFP predominantly localized to hyphal tips and septa, respectively, indicating that their primary activity is in different cellular locations. COT1 protein forms exhibited a hyperphosphorylated gel migration pattern in an rgb-1^RIP mutant background, similar to that observed when the fungus was cultured in the presence of the PP2A inhibitor cantharidin. COT1 was hypophosphorylated in a Δb56 mutant background, suggesting that this regulatory subunit may be involved in determining COT1 phosphorylation state, yet in an indirect manner. Reciprocal co-immunoprecipitation analyses, using tagged COT1, PPH1, RGB1, and B56 subunits established that these proteins physically interact. Taken together, our data determine the presence of a functional and physical link between PP2A and COT1 and show that two of the PP2A regulatory subunits interact with the kinase and determine COT1 phosphorylation state.

Regulation of Protein Interactions by Mps One Binder (MOB1) Phosphorylation

MOB1 is a multifunctional protein best characterized for its integrative role in regulating Hippo and NDR pathway signaling in metazoans and the Mitotic Exit Network in yeast. Human MOB1 binds both the upstream kinases MST1 and MST2 and the downstream AGC group kinases LATS1, LATS2, NDR1, and NDR2. Binding of MOB1 to MST1 and MST2 is mediated by its phosphopeptide-binding infrastructure, the specificity of which matches the phosphorylation consensus of MST1 and MST2. On the other hand, binding of MOB1 to the LATS and NDR kinases is mediated by a distinct interaction surface on MOB1. By assembling both upstream and downstream kinases into a single complex, MOB1 facilitates the activation of the latter by the former through a trans-phosphorylation event. Binding of MOB1 to its upstream partners also renders MOB1 a substrate, which serves to differentially regulate its two protein interaction activities (at least in vitro). Our previous interaction proteomics analysis revealed that beyond associating with MST1 (and MST2), MOB1A and MOB1B can associate in a phosphorylation-dependent manner with at least two other signaling complexes, one containing the Rho guanine exchange factors (DOCK6-8) and the other containing the serine/threonine phosphatase PP6. Whether these complexes are recruited through the same mode of interaction as MST1 and MST2 remains unknown. Here, through a comprehensive set of biochemical, biophysical, mutational and structural studies, we quantitatively assess how phosphorylation of MOB1A regulates its interaction with both MST kinases and LATS/NDR family kinases in vitro Using interaction proteomics, we validate the significance of our in vitro studies and also discover that the phosphorylation-dependent recruitment of PP6 phosphatase and Rho guanine exchange factor protein complexes differ in key respects from that elucidated for MST1 and MST2. Together our studies confirm and extend previous work to delineate the intricate regulatory steps in key signaling pathways.

NDR1-Dependent Regulation of Kindlin-3 Controls High-Affinity LFA-1 Binding and Immune Synapse Organization

Antigen-specific adhesion between T cells and antigen-presenting cells (APC) during the formation of the immunological synapse (IS) is mediated by LFA-1 and ICAM-1. Here, LFA-1–ICAM-1 interactions were measured at the single-molecule level on supported lipid bilayers. High-affinity binding was detected at low frequencies in the inner peripheral supramolecular activation cluster (SMAC) zone that contained high levels of activated Rap1 and kindlin-3. Rap1 was essential for T cell attachment, whereas deficiencies of ste20-like kinases, Mst1/Mst2, diminished high-affinity binding and abrogated central SMAC (cSMAC) formation with mislocalized kindlin-3 and vesicle transport regulators involved in T cell receptor recycling/releasing machineries, resulting in impaired T cell-APC interactions. We found that NDR1 kinase, activated by the Rap1 signaling cascade through RAPL and Mst1/Mst2, associated with and recruited kindlin-3 to the IS, which was required for high-affinity LFA-1/ICAM-1 binding and cSMAC formation. Our findings reveal crucial roles for Rap1 signaling via NDR1 for recruitment of kindlin-3 and IS organization.

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.

Structural basis for autoinhibition and its relief of MOB1 in the Hippo pathway

MOB1 protein is a key regulator of large tumor suppressor 1/2 (LATS1/2) kinases in the Hippo pathway. MOB1 is present in an autoinhibited form and is activated by MST1/2-mediated phosphorylation, although the precise mechanisms responsible for autoinhibition and activation are unknown due to lack of an autoinhibited MOB1 structure. Here, we report on the crystal structure of full-length MOB1B in the autoinhibited form and a complex between the MOB1B core domain and the N-terminal regulation (NTR) domain of LATS1. The structure of full-length MOB1B shows that the N-terminal extension forms a short β-strand, the SN strand, followed by a long conformationally flexible positively-charged linker and α-helix, the Switch helix, which blocks the LATS1 binding surface of MOB1B. The Switch helix is stabilized by β-sheet formation of the SN strand with the S2 strand of the MOB1 core domain. Phosphorylation of Thr12 and Thr35 residues structurally accelerates dissociation of the Switch helix from the LATS1-binding surface by the "pull-the-string" mechanism, thereby enabling LATS1 binding.

The Roles of NDR Protein Kinases in Hippo Signalling

The Hippo tumour suppressor pathway has emerged as a critical regulator of tissue growth through controlling cellular processes such as cell proliferation, death, differentiation and stemness. Traditionally, the core cassette of the Hippo pathway includes the MST1/2 protein kinases, the LATS1/2 protein kinases, and the MOB1 scaffold signal transducer, which together regulate the transcriptional co-activator functions of the proto-oncoproteins YAP and TAZ through LATS1/2-mediated phosphorylation of YAP/TAZ. Recent research has identified additional kinases, such as NDR1/2 (also known as STK38/STK38L) and MAP4Ks, which should be considered as novel members of the Hippo core cassette. While these efforts helped to expand our understanding of Hippo core signalling, they also began to provide insights into the complexity and redundancy of the Hippo signalling network. Here, we focus on summarising our current knowledge of the regulation and functions of mammalian NDR kinases, discussing parallels between the NDR pathways in Drosophila and mammals. Initially, we provide a general overview of the cellular functions of NDR kinases in cell cycle progression, centrosome biology, apoptosis, autophagy, DNA damage signalling, immunology and neurobiology. Finally, we put particular emphasis on discussing NDR1/2 as YAP kinases downstream of MST1/2 and MOB1 signalling in Hippo signalling.

Mechanisms of Hippo pathway regulation

In this review, Meng et al. focus on recent developments in our understanding of the molecular actions of the core Hippo kinase cascade and discuss key open questions in Hippo pathway regulation and function.

The Hippo pathway was initially identified in Drosophila melanogaster screens for tissue growth two decades ago and has been a subject extensively studied in both Drosophila and mammals in the last several years. The core of the Hippo pathway consists of a kinase cascade, transcription coactivators, and DNA-binding partners. Recent studies have expanded the Hippo pathway as a complex signaling network with >30 components. This pathway is regulated by intrinsic cell machineries, such as cell–cell contact, cell polarity, and actin cytoskeleton, as well as a wide range of signals, including cellular energy status, mechanical cues, and hormonal signals that act through G-protein-coupled receptors. The major functions of the Hippo pathway have been defined to restrict tissue growth in adults and modulate cell proliferation, differentiation, and migration in developing organs. Furthermore, dysregulation of the Hippo pathway leads to aberrant cell growth and neoplasia. In this review, we focus on recent developments in our understanding of the molecular actions of the core Hippo kinase cascade and discuss key open questions in the regulation and function of the Hippo pathway.

Epidermis-Derived Semaphorin Promotes Dendrite Self-Avoidance by Regulating Dendrite-Substrate Adhesion in Drosophila Sensory Neurons

Precise patterning of dendritic arbors is critical for the wiring and function of neural circuits. Dendrite-extracellular matrix (ECM) adhesion ensures that the dendrites of Drosophila dendritic arborization (da) sensory neurons are properly restricted in a 2D space, and thereby facilitates contact-mediated dendritic self-avoidance and tiling. However, the mechanisms regulating dendrite-ECM adhesion in vivo are poorly understood. Here, we show that mutations in the semaphorin ligand sema-2b lead to a dramatic increase in self-crossing of dendrites due to defects in dendrite-ECM adhesion, resulting in a failure to confine dendrites to a 2D plane. Furthermore, we find that Sema-2b is secreted from the epidermis and signals through the Plexin B receptor in neighboring neurons. Importantly, we find that Sema-2b/PlexB genetically and physically interacts with TORC2 complex, Tricornered (Trc) kinase, and integrins. These results reveal a novel role for semaphorins in dendrite patterning and illustrate how epidermal-derived cues regulate neural circuit assembly.

The kinases NDR1/2 act downstream of the Hippo homolog MST1 to mediate both egress of thymocytes from the thymus and lymphocyte motility

The serine and threonine kinase MST1 is the mammalian homolog of Hippo. MST1 is a critical mediator of the migration, adhesion, and survival of T cells; however, these functions of MST1 are independent of signaling by its typical effectors, the kinase LATS and the transcriptional coactivator YAP. The kinase NDR1, a member of the same family of kinases as LATS, functions as a tumor suppressor by preventing T cell lymphomagenesis, which suggests that it may play a role in T cell homeostasis. We generated and characterized mice with a T cell-specific double knockout of Ndr1 and Ndr2 (Ndr DKO). Compared with control mice, Ndr DKO mice exhibited a substantial reduction in the number of naïve T cells in their secondary lymphoid organs. Mature single-positive thymocytes accumulated in the thymus in Ndr DKO mice. We also found that NDRs acted downstream of MST1 to mediate the egress of mature thymocytes from the thymus, as well as the interstitial migration of naïve T cells within popliteal lymph nodes. Together, our findings indicate that the kinases NDR1 and NDR2 function as downstream effectors of MST1 to mediate thymocyte egress and T cell migration.

Regulation of NDR1 activity by PLK1 ensures proper spindle orientation in mitosis

Accurate chromosome segregation during mitosis requires the physical separation of sister chromatids which depends on correct position of mitotic spindle relative to membrane cortex. Although recent work has identified the role of PLK1 in spindle orientation, the mechanisms underlying PLK1 signaling in spindle positioning and orientation have not been fully illustrated. Here, we identified a conserved signaling axis in which NDR1 kinase activity is regulated by PLK1 in mitosis. PLK1 phosphorylates NDR1 at three putative threonine residues (T7, T183 and T407) at mitotic entry, which elicits PLK1-dependent suppression of NDR1 activity and ensures correct spindle orientation in mitosis. Importantly, persistent expression of non-phosphorylatable NDR1 mutant perturbs spindle orientation. Mechanistically, PLK1-mediated phosphorylation protects the binding of Mob1 to NDR1 and subsequent NDR1 activation. These findings define a conserved signaling axis that integrates dynamic kinetochore-microtubule interaction and spindle orientation control to genomic stability maintenance.

Coordinate control of terminal dendrite patterning and dynamics by the membrane protein Raw

The directional flow of information in neurons depends on compartmentalization: dendrites receive inputs whereas axons transmit them. Axons and dendrites likewise contain structurally and functionally distinct subcompartments. Axon/dendrite compartmentalization can be attributed to neuronal polarization, but the developmental origin of subcompartments in axons and dendrites is less well understood. To identify the developmental bases for compartment-specific patterning in dendrites, we screened for mutations that affect discrete dendritic domains in Drosophila sensory neurons. From this screen, we identified mutations that affected distinct aspects of terminal dendrite development with little or no effect on major dendrite patterning. Mutation of one gene, raw, affected multiple aspects of terminal dendrite patterning, suggesting that Raw might coordinate multiple signaling pathways to shape terminal dendrite growth. Consistent with this notion, Raw localizes to branch-points and promotes dendrite stabilization together with the Tricornered (Trc) kinase via effects on cell adhesion. Raw independently influences terminal dendrite elongation through a mechanism that involves modulation of the cytoskeleton, and this pathway is likely to involve the RNA-binding protein Argonaute 1 (AGO1), as raw and AGO1 genetically interact to promote terminal dendrite growth but not adhesion. Thus, Raw defines a potential point of convergence in distinct pathways shaping terminal dendrite patterning.

Constitutively active NDR1-PIF kinase functions independent of MST1 and hMOB1 signalling

The human MST1/hMOB1/NDR1 tumour suppressor cascade regulates important cellular processes, such as centrosome duplication. hMOB1/NDR1 complex formation appears to be essential for NDR1 activation by autophosphorylation on Ser281 and hydrophobic motif (HM) phosphorylation at Thr444 by MST1. To dissect these mechanistic relationships in MST1/hMOB1/NDR signalling, we designed NDR1 variants carrying modifications that mimic HM phosphorylation and/or abolish hMOB1/NDR1 interactions. Significantly, the analyses of these variants revealed that NDR1-PIF, an NDR1 variant containing the PRK2 hydrophobic motif, remains hyperactive independent of hMOB1/NDR1-PIF complex formation. In contrast, as reported for the T444A phospho-acceptor mutant, NDR1 versions carrying single phospho-mimicking mutations at the HM phosphorylation site, namely T444D or T444E, do not display increased kinase activities. Collectively, these observations suggest that in cells Thr444 phosphorylation by MST1 depends on the hMOB1/NDR1 association, while Ser281 autophosphorylation of NDR1 can occur independently. By testing centrosome-targeted NDR1 variants in NDR1- or MST1-depleted cells, we further observed that centrosome-enriched NDR1-PIF requires neither hMOB1 binding nor MST1 signalling to function in centrosome overduplication. Taken together, our biochemical and cell biological characterisation of NDR1 versions provides novel unexpected insights into the regulatory mechanisms of NDR1 and NDR1's role in centrosome duplication.

Cell cycle regulated interaction of a yeast Hippo kinase and its activator MO25/Hym1

Hippo pathways are ancient signaling systems that contribute to cell growth and proliferation in a wide diversity of eukaryotes, and have emerged as a conserved regulator of organ size control in metazoans. In budding yeast, a Hippo signaling pathway called the Regulation of Ace2 and Morphogenesis (RAM) network promotes polarized cell growth and the final event in the separation of mother and daughter cells. A crucial regulatory input for RAM network control of cell separation is phosphorylation of a conserved hydrophobic motif (HM) site on the NDR/LATS family kinase Cbk1. Here we provide the first direct evidence that the Hippo-like kinase Kic1 in fact phosphorylates the HM site of Cbk1, and show that Kic1 is allosterically activated by Hym1, a highly conserved protein related to mammalian MO25. Using the structure of mammalian MO25 in complex with the Kic1-related pseudokinase STRAD, we identified conserved residues on Kic1 that are required for interaction with Hym1. We find that Kic1 and Hym1 protein levels remain constant throughout the cell cycle but the proteins’ association is regulated, with maximal interaction coinciding with peak Cbk1 HM site phosphorylation. We show that this association is necessary but not sufficient for this phosphorylation, suggesting another level of regulation is required to promote the complex to act upon its substrates. This work presents a previously undiscovered cell cycle regulated interaction between a Hippo kinase and a broadly conserved allosteric activator. Because of the conserved nature of this pathway in higher eukaryotes, this work may also provide insight into the modularity of Hippo signaling pathways.

Regulation and functions of mammalian LATS/NDR kinases: looking beyond canonical Hippo signalling

The metazoan Hippo pathway is an essential tumour suppressor signalling cascade that ensures normal tissue growth by co-ordinating cell proliferation, cell death and cell differentiation. Over the past years, various genetic and biochemical studies in Drosophila and mammals have defined a conserved core Hippo signalling module, composed of members of the Ste20-like kinase, the MOB co-activator and the AGC kinase families. In Drosophila, stimulated Hippo kinase phosphorylates and thereby activates the Mats/Warts complex, which consequently phosphorylates and inactivates the transcriptional co-activator Yorkie. In mammals, the counterparts of the Hippo/Mats/Warts/Yorkie cascade, namely MST1/2, MOB1A/B, LATS1/2 and YAP/TAZ, function in a similar fashion. These canonical Hippo pathways are so highly conserved that human MST2, hMOB1A and LATS1 can compensate for the loss of Hippo, Mats and Warts in flies. However, recent reports have shown that Hippo signalling is more diverse and complex, in particular in mammals. In this review, we summarize our current understanding of mammalian LATS1/2 kinases together with their closest relatives, the NDR1/2 kinases. The regulation of the LATS/NDR family of kinases will be discussed, followed by a summary of all currently known LATS/NDR substrates. Last, but not least, the biological roles of LATS/NDR kinases will be reviewed with specific emphasis on recent discoveries of canonical and non-canonical LATS/NDR functions in the extended Hippo pathway.

The N-terminal region of the Neurospora NDR kinase COT1 regulates morphology via its interactions with MOB2A/B

Nuclear Dbf2p-related (NDR) protein kinases are important for cell differentiation and polar morphogenesis in various organisms, yet some of their functions are still elusive. Dysfunction of the Neurospora crassa NDR kinase COT1 leads to cessation of tip extension and hyperbranching. NDR kinases require the physical interaction between the kinase's N-terminal region (NTR) and the MPS1-binding (MOB) proteins for their activity and functions. To study the interactions between COT1 and MOB2 proteins, we mutated several conserved residues and a novel phosphorylation site within the COT1 NTR. The phenotypes of these mutants suggest that the NTR is required for COT1 functions in regulating hyphal elongation and branching, asexual conidiation and germination. Interestingly, while both MOB2A and MOB2B promote proper hyphal growth, they have distinct COT1-dependent roles in regulation of macroconidiation. Immunoprecipitation experiments indicate physical association of COT1 with both MOB2A and MOB2B, simultaneously. Furthermore, the binding of the two MOB2 proteins to COT1 is mediated by different residues at the COT1 NTR, suggesting a hetero-trimer is formed. Thus, although MOB2A/B may have some overlapping functions in regulating hyphal tip extension, their function is not redundant and they are both required for proper fungal development.

Cyclin D1 promotes cell cycle progression through enhancing NDR1/2 kinase activity independent of cyclin-dependent kinase 4

Cyclin/cyclin-dependent kinases (Cdks) are critical protein kinases in regulating cell cycle progression. Among them, cyclin D1/Cdk4 exerts its function mainly in the G1 phase. By using the tandem affinity purification tag approach, we identified a set of proteins interacting with Cdk4, including NDR1/2. Interestingly, confirming the interactions between NDR1/2 and cyclin D1/Cdk4, we observed that NDR1/2 interacted with cyclin D1 independent of Cdk4, but NDR1/2 and cyclin D1/Cdk4 did not phosphorylate each other. In addition, we found that NDR1/2 did not affect the kinase activity of cyclin D1/Cdk4 upon phosphorylation of GST-Rb. However, cyclin D1 but not Cdk4 promoted the kinase activity of NDR1/2. We also demonstrated that cyclin D1 K112E, which could not bind Cdk4, enhanced the kinase activity of NDR1/2. To test whether cyclin D1 promotes G1/S transition though enhancing NDR1/2 kinase activity, we performed flow cytometry analysis using cyclin D1 and cyclin D1 K112E Tet-On inducible cell lines. The data show that both cyclin D1 and cyclin D1 K112E promoted G1/S transition. Importantly, knockdown of NDR1/2 almost completely abolished the function of cyclin D1 K112E in promoting G1/S transition. Consistently, we found that the protein level of p21 was reduced in cells overexpressing cyclin D1 K112E but not when NDR1/2 was knocked down. Taken together, these results reveal a novel function of cyclin D1 in promoting cell cycle progression by enhancing NDR kinase activity independent of Cdk4.

In vivo phosphoproteomics analysis reveals the cardiac targets of β-adrenergic receptor signaling

β-Blockers are widely used to prevent cardiac arrhythmias and to treat hypertension by inhibiting β-adrenergic receptors (βARs) and thus decreasing contractility and heart rate. βARs initiate phosphorylation-dependent signaling cascades, but only a small number of the target proteins are known. We used quantitative in vivo phosphoproteomics to identify 670 site-specific phosphorylation changes in murine hearts in response to acute treatment with specific βAR agonists. The residues adjacent to the regulated phosphorylation sites exhibited a sequence-specific preference (R-X-X-pS/T), and integrative analysis of sequence motifs and interaction networks suggested that the kinases AMPK (adenosine 5'-monophosphate-activated protein kinase), Akt, and mTOR (mammalian target of rapamycin) mediate βAR signaling, in addition to the well-established pathways mediated by PKA (cyclic adenosine monophosphate-dependent protein kinase) and CaMKII (calcium/calmodulin-dependent protein kinase type II). We found specific regulation of phosphorylation sites on six ion channels and transporters that mediate increased ion fluxes at higher heart rates, and we showed that phosphorylation of one of these, Ser(92) of the potassium channel KV7.1, increased current amplitude. Our data set represents a quantitative analysis of phosphorylated proteins regulated in vivo upon stimulation of seven-transmembrane receptors, and our findings reveal previously unknown phosphorylation sites that regulate myocardial contractility, suggesting new potential targets for the treatment of heart disease and hypertension.

Mitotic exit and separation of mother and daughter cells

Productive cell proliferation involves efficient and accurate splitting of the dividing cell into two separate entities. This orderly process reflects coordination of diverse cytological events by regulatory systems that drive the cell from mitosis into G1. In the budding yeast Saccharomyces cerevisiae, separation of mother and daughter cells involves coordinated actomyosin ring contraction and septum synthesis, followed by septum destruction. These events occur in precise and rapid sequence once chromosomes are segregated and are linked with spindle organization and mitotic progress by intricate cell cycle control machinery. Additionally, critical paarts of the mother/daughter separation process are asymmetric, reflecting a form of fate specification that occurs in every cell division. This chapter describes central events of budding yeast cell separation, as well as the control pathways that integrate them and link them with the cell cycle.

STK38 is a critical upstream regulator of MYC's oncogenic activity in human B-cell lymphoma

The MYC protooncogene is associated with the pathogenesis of most human neoplasia. Conversely, its experimental inactivation elicits oncogene addiction. Besides constituting a formidable therapeutic target, MYC also has an essential function in normal physiology, thus creating the need for context-specific targeting strategies. The analysis of post-translational MYC activity modulation yields novel targets for MYC inactivation. Specifically, following regulatory network analysis in human B-cells, we identify a novel role of the STK38 kinase as a regulator of MYC activity and a candidate target for abrogating tumorigenesis in MYC-addicted lymphoma. We found that STK38 regulates MYC protein stability and turnover in a kinase activity-dependent manner. STK38 kinase inactivation abrogates apoptosis following B-cell receptor activation, whereas its silencing significantly decreases MYC levels and increases apoptosis. Moreover, STK38 knockdown suppresses growth of MYC-addicted tumors in vivo, thus providing a novel viable target for treating these malignancies.

Thermodynamic and kinetic analysis of peptides derived from CapZ, NDR, p53, HDM2, and HDM4 binding to human S100B

S100B is a member of the S100 subfamily of EF-hand proteins that has been implicated in malignant melanoma and neurodegenerative conditions such as Alzheimer's disease and Parkinson's disease. Calcium-induced conformational changes expose a hydrophobic binding cleft, facilitating interactions with a wide variety of nuclear, cytoplasmic, and extracellular target proteins. Previously, peptides derived from CapZ, p53, NDR, HDM2, and HDM4 have been shown to interact with S100B in a calcium-dependent manner. However, the thermodynamic and kinetic basis of these interactions remains largely unknown. To gain further insight, we screened these peptides against the S100B protein using isothermal titration calorimetry and nuclear magnetic resonance. All peptides were found to have binding affinities in the low micromolar to nanomolar range. Binding-induced changes in the line shapes of S100B backbone (1)H and (15)N resonances were monitored to obtain the dissociation constants and the kinetic binding parameters. The large microscopic K(on) rate constants observed in this study (≥1 × 10(7) M(-1) s(-1)) suggest that S100B utilizes a "fly casting mechanism" in the recognition of these peptide targets.

Conserved Orb6 phosphorylation sites are essential for polarized cell growth in Schizosaccharomyces pombe

The Ndr-related Orb6 kinase is a key regulator of polarized cell growth in fission yeast, however the mechanism of Orb6 activation is unclear. Activation of other Ndr kinases involves both autophosphorylation and phosphorylation by an upstream kinase. Previous reports suggest that the Nak1 kinase functions upstream from Orb6. Supporting this model, we show that HA-Orb6 overexpression partially restored cell polarity in nak1 ts cells. We also demonstrated by coimmunoprecipitation and in vitro binding assays that Nak1 and Orb6 physically interact, and that the Nak1 C-terminal region is required for Nak1/Orb6 complex formation in vivo. However, results from in vitro kinase assays did not show phosphorylation of recombinant Orb6 by HA-Nak1, suggesting that Orb6 activation may not involve direct phosphorylation by Nak1. To investigate the role of Orb6 phosphorylation and activity, we substituted Ala at the ATP-binding and conserved phosphorylation sites. Overexpression of kinase-dead HA-Orb6(K122A) in wild-type cells resulted in a loss of cell polarity, suggesting that it has a dominant-negative effect, and it failed to rescue the polarity defect of nak1 or orb6 ts mutants. Recombinant GST-Orb6(S291A) did not autophosphorylate in vitro suggesting that Ser291 is the primary autophosphorylation site. HA-Orb6(S291A) overexpression only partially rescued the orb6 polarity defect and failed to rescue the nak1 defect, suggesting that autophosphorylation is important for Orb6 function. GST-Orb6(T456A) autophosphorylated in vitro, indicating that the conserved phosphorylation site at Thr456 is not essential for kinase activity. However, HA-Orb6(T456A) overexpression had similar effects as overexpressing kinase-dead HA-Orb6(K122A), suggesting that Thr456 is essential for Orb6 function in vivo. Also, we found that both phosphorylation site mutations impaired the ability of Myc-Nak1 to coimmunoprecipitate with HA-Orb6. Together, our results suggest a model whereby autophosphorylation of Ser291 and phosphorylation of Thr456 by an upstream kinase promote Nak1/Orb6 complex formation and Orb6 activation.

Chemical genetic identification of NDR1/2 kinase substrates AAK1 and Rabin8 Uncovers their roles in dendrite arborization and spine development

Dendrite arborization and synapse formation are essential for wiring the neural circuitry. The evolutionarily conserved NDR1/2 kinase pathway, important for polarized growth from yeast to mammals, controls dendrite growth and morphology in the worm and fly. The function of NDR1/2 in mammalian neurons and their downstream effectors were not known. Here we show that the expression of dominant negative (kinase-dead) NDR1/2 mutants or siRNA increase dendrite length and proximal branching of mammalian pyramidal neurons in cultures and in vivo, whereas the expression of constitutively active NDR1/2 has the opposite effect. Moreover, NDR1/2 contributes to dendritic spine development and excitatory synaptic function. We further employed chemical genetics and identified NDR1/2 substrates in the brain, including two proteins involved in intracellular vesicle trafficking: AAK1 (AP-2 associated kinase) and Rabin8, a GDP/GTP exchange factor (GEF) of Rab8 GTPase. We finally show that AAK1 contributes to dendrite growth regulation, and Rabin8 regulates spine development.

Serine-threonine kinase 38 is regulated by glycogen synthase kinase-3 and modulates oxidative stress-induced cell death

Serine-threonine kinase 38 (STK38) is a member of the protein kinase A (PKA)/PKG/PKC-like family. In the present study, we investigated the regulatory mechanism of STK38 and assessed its role in the cellular stress response. Among various environmental stresses, STK38 was specifically activated by H(2)O(2), and the phosphatidylinositol 3-kinase inhibitor wortmannin or AKT inhibitor IV suppressed this activation. STK38 was also activated by a constitutively active AKT1 or by GSK-3β inhibitor VII. The phosphorylation level of GSK-3β was correlated with the STK38 activity, in response to various stimuli and in different cell lines. Co-immunoprecipitation analysis revealed that GSK-3β physically interacted with STK38 in cells. GSK-3β overexpression inhibited the H(2)O(2)-stimulated STK38 activity. GSK-3β phosphorylated STK38 on residues S6 and T7 in vitro, depending largely on a PKA-mediated priming phosphorylation of STK38 on residues S10 and S11, respectively. STK38's H(2)O(2)-stimulated activity was enhanced by alanine substitution at its priming sites and/or at S6 and T7, and it was partially reduced by a phosphomimetic mutation at S6 or T7. STK38 knockdown enhanced the H(2)O(2)-induced JNK phosphorylation and cell death. Our results indicate that that GSK-3β inhibits STK38's full activation, and suggest that STK38 activation is required to prevent cell death in response to oxidative stress.

MICAL-1 is a negative regulator of MST-NDR kinase signaling and apoptosis

MICALs (molecules interacting with CasL) are atypical multidomain flavoenzymes with diverse cellular functions. The molecular pathways employed by MICAL proteins to exert their cellular effects remain largely uncharacterized. Via an unbiased proteomics approach, we identify MICAL-1 as a binding partner of NDR (nuclear Dbf2-related) kinases. NDR1/2 kinases are known to mediate apoptosis downstream of the mammalian Ste-20-like kinase MST1, and ablation of NDR1 in mice predisposes the mice to cancer as a result of compromised apoptosis. MST1 phosphorylates NDR1/2 kinases at their hydrophobic motif, thereby facilitating full NDR kinase activity and function. However, if and how this key phosphorylation event is regulated are unknown. Here we show that MICAL-1 interacts with the hydrophobic motif of NDR1/2 and that overexpression or knockdown of MICAL-1 reduces or augments NDR kinase activation or activity, respectively. Surprisingly, MICAL-1 is a phosphoprotein but not an NDR or MST1 substrate. Rather, MICAL-1 competes with MST1 for NDR binding and thereby antagonizes MST1-induced NDR activation. In line with this inhibitory effect, overexpression or knockdown of MICAL-1 inhibits or enhances, respectively, NDR-dependent proapoptotic signaling induced by extrinsic stimuli. Our findings unveil a previously unknown biological role for MICAL-1 in apoptosis and define a novel negative regulatory mechanism of MST-NDR signaling.

The growing role of the Hippo--NDR kinase signalling in neuronal development and disease

The nuclear Dbf2-realted (NDR) family members are highly conserved serine/threonine protein kinases that function in concert with the Hippo signalling pathway to play crucial roles in regulation of cell proliferation and survival in non-neuronal cells. Recent studies employing a range of animal models have implicated NDR kinases as regulators of multiple aspects of development in post-mitotic neurons including progenitor proliferation, fate specification and circuit formation, all of which are crucial for neuronal functions. This review summarizes the recent advances in our understanding of the neuronal functions of NDR kinases and discusses their association with neuronal diseases.

Exonic SINE insertion in STK38L causes canine early retinal degeneration (erd)

Fine mapping followed by candidate gene analysis of erd - a canine hereditary retinal degeneration characterized by aberrant photoreceptor development - established that the disease cosegregates with a SINE insertion in exon 4 of the canine STK38L/NDR2 gene. The mutation removes exon 4 from STK38L transcripts and is predicted to remove much of the N terminus from the translated protein, including binding sites for S100B and Mob proteins, part of the protein kinase domain, and a Thr-75 residue critical for autophosphorylation. Although known to have roles in neuronal cell function, the STK38L pathway has not previously been implicated in normal or abnormal photoreceptor development. Loss of STK38L function in erd provides novel potential insights into the role of the STK38L pathway in neuronal and photoreceptor cell function, and suggests that genes in this pathway need to be considered as candidate genes for hereditary retinal degenerations.

Differential NDR/LATS interactions with the human MOB family reveal a negative role for human MOB2 in the regulation of human NDR kinases

MOB proteins are integral components of signaling pathways controlling important cellular processes, such as mitotic exit, centrosome duplication, apoptosis, and cell proliferation in eukaryotes. The human MOB protein family consists of six distinct members (human MOB1A [hMOB1A], -1B, -2, -3A, -3B, and -3C), with hMOB1A/B the best studied due to their putative tumor-suppressive functions through the regulation of NDR/LATS kinases. The roles of the other MOB proteins are less well defined. Accordingly, we characterized all six human MOB proteins in the context of NDR/LATS binding and their abilities to activate NDR/LATS kinases. hMOB3A/B/C proteins neither bind nor activate any of the four human NDR/LATS kinases. We found that both hMOB2 and hMOB1A bound to the N-terminal region of NDR1. However, our data suggest that the binding modes differ significantly. Our work revealed that hMOB2 competes with hMOB1A for NDR binding. hMOB2, in contrast to hMOB1A/B, is bound to unphosphorylated NDR. Moreover, RNA interference (RNAi) depletion of hMOB2 resulted in increased NDR kinase activity. Consistent with these findings, hMOB2 overexpression interfered with the functional roles of NDR in death receptor signaling and centrosome overduplication. In summary, our data indicate that hMOB2 is a negative regulator of human NDR kinases in biochemical and biological settings.

Ablation of the kinase NDR1 predisposes mice to the development of T cell lymphoma

Defective apoptosis contributes to the development of various human malignancies. The kinases nuclear Dbf2-related 1 (NDR1) and NDR2 mediate apoptosis downstream of the tumor suppressor proteins RASSF1A (Ras association domain family member 1A) and MST1 (mammalian Ste20-like kinase 1). To further analyze the role of NDR1 in apoptosis, we generated NDR1-deficient mice. Although NDR1 is activated by both intrinsic and extrinsic proapoptotic stimuli, which indicates a role for NDR1 in regulating apoptosis, NDR1-deficient T cells underwent apoptosis in a manner similar to that of wild-type cells in response to different proapoptotic stimuli. Analysis of the abundances of NDR1 and NDR2 proteins revealed that loss of NDR1 was functionally compensated for by an increase in the abundance of NDR2 protein. Despite this compensation, NDR1(-/-) and NDR1(+/-) mice were more prone to the development of T cell lymphomas than were wild-type mice. Tumor development in mice and humans was accompanied by a decrease in the overall amounts of NDR proteins in T cell lymphoma samples. Thus, reduction in the abundance of NDR1 triggered a decrease in the total amount of both isoforms. Together, our data suggest that a reduction in the abundances of the NDR proteins results in defective responses to proapoptotic stimuli, thereby facilitating the development of tumors.