Impaired signaling for neuromuscular synaptic maintenance is a feature of Motor Neuron Disease

A central event in the pathogenesis of motor neuron disease (MND) is the loss of neuromuscular junctions (NMJs), yet the mechanisms that lead to this event in MND remain to be fully elucidated. Maintenance of the NMJ relies upon neural agrin (n-agrin) which, when released from the nerve terminal, activates the postsynaptic Muscle Specific Kinase (MuSK) signaling complex to stabilize clusters of acetylcholine receptors. Here, we report that muscle from MND patients has an increased proportion of slow fibers and muscle fibers with smaller diameter. Muscle cells cultured from MND biopsies failed to form large clusters of acetylcholine receptors in response to either non-MND human motor axons or n-agrin. Furthermore, levels of expression of MuSK, and MuSK-complex components: LRP4, Caveolin-3, and Dok7 differed between muscle cells cultured from MND patients compared to those from non-MND controls. To our knowledge, this is the first time a fault in the n-agrin-LRP4-MuSK signaling pathway has been identified in muscle from MND patients. Our results highlight the n-agrin-LRP4-MuSK signaling pathway as a potential therapeutic target to prolong muscle function in MND.

Cep55 regulation of PI3K/Akt signaling is required for neocortical development and ciliogenesis

Homozygous nonsense mutations in CEP55 are associated with several congenital malformations that lead to perinatal lethality suggesting that it plays a critical role in regulation of embryonic development. CEP55 has previously been studied as a crucial regulator of cytokinesis, predominantly in transformed cells, and its dysregulation is linked to carcinogenesis. However, its molecular functions during embryonic development in mammals require further investigation. We have generated a Cep55 knockout (Cep55-/-) mouse model which demonstrated preweaning lethality associated with a wide range of neural defects. Focusing our analysis on the neocortex, we show that Cep55-/- embryos exhibited depleted neural stem/progenitor cells in the ventricular zone as a result of significantly increased cellular apoptosis. Mechanistically, we demonstrated that Cep55-loss downregulates the pGsk3β/β-Catenin/Myc axis in an Akt-dependent manner. The elevated apoptosis of neural stem/progenitors was recapitulated using Cep55-deficient human cerebral organoids and we could rescue the phenotype by inhibiting active Gsk3β. Additionally, we show that Cep55-loss leads to a significant reduction of ciliated cells, highlighting a novel role in regulating ciliogenesis. Collectively, our findings demonstrate a critical role of Cep55 during brain development and provide mechanistic insights that may have important implications for genetic syndromes associated with Cep55-loss.

Cilia, Centrosomes and Skeletal Muscle

Primary cilia are non-motile, cell cycle-associated organelles that can be found on most vertebrate cell types. Comprised of microtubule bundles organised into an axoneme and anchored by a mature centriole or basal body, primary cilia are dynamic signalling platforms that are intimately involved in cellular responses to their extracellular milieu. Defects in ciliogenesis or dysfunction in cilia signalling underlie a host of developmental disorders collectively referred to as ciliopathies, reinforcing important roles for cilia in human health. Whilst primary cilia have long been recognised to be present in striated muscle, their role in muscle is not well understood. However, recent studies indicate important contributions, particularly in skeletal muscle, that have to date remained underappreciated. Here, we explore recent revelations that the sensory and signalling functions of cilia on muscle progenitors regulate cell cycle progression, trigger differentiation and maintain a commitment to myogenesis. Cilia disassembly is initiated during myoblast fusion. However, the remnants of primary cilia persist in multi-nucleated myotubes, and we discuss their potential role in late-stage differentiation and myofiber formation. Reciprocal interactions between cilia and the extracellular matrix (ECM) microenvironment described for other tissues may also inform on parallel interactions in skeletal muscle. We also discuss emerging evidence that cilia on fibroblasts/fibro-adipogenic progenitors and myofibroblasts may influence cell fate in both a cell autonomous and non-autonomous manner with critical consequences for skeletal muscle ageing and repair in response to injury and disease. This review addresses the enigmatic but emerging role of primary cilia in satellite cells in myoblasts and myofibers during myogenesis, as well as the wider tissue microenvironment required for skeletal muscle formation and homeostasis.

TDP-43 Mutation Affects Stress Granule Dynamics in Differentiated NSC-34 Motoneuron-Like Cells

Amyotrophic Lateral Sclerosis (ALS) is characterized by degeneration of motor neurons in the brain and spinal cord. Cytoplasmic inclusions of TDP-43 are frequently reported in motor neurons of ALS patients. TDP-43 has also been shown to associate with stress granules (SGs), a complex of proteins and mRNAs formed in response to stress stimuli that temporarily sequester mRNA translation. The effect of pathogenic TDP-43 mutations within glycine-rich regions (where the majority of ALS-causing TDP-43 mutations occur) on SG dynamics in motor neurons is poorly understood. To address this issue, we generated murine NSC-34 cell lines that stably over-express wild type TDP-43 (TDP-43^WT) or mutant forms (ALS-causing TDP-43 mutations TDP-43^A315T or TDP-43^M337V). We then differentiated these NSC-34 lines into motoneuron-like cells and evaluated SG formation and disassembly kinetics in response to oxidative or osmotic stress treatment. Wild type and mutant TDP-43 appeared to be largely retained in the nucleus following exposure to arsenite-induced oxidative stress. Upon arsenite removal, mutant TDP-43 clearly accumulated within HuR positive SGs in the cytoplasm, whereas TDP-43^WT remained mostly within the nucleus. 24 h following arsenite removal, all SGs were disassembled in both wild type and mutant TDP-43 expressing cells. By contrast, we observed significant differences in the dynamics of mutant TDP-43 association with SGs in response to hyperosmotic stress. Specifically, in response to sorbitol treatment, TDP-43^WT remained in the nucleus, whereas mutant TDP-43 relocalized to HuR positive SGs in the cytoplasm following exposure to sorbitol stress, resulting in a significant increase in TDP-43 SG numbers. These SGs remained assembled for 24 h following removal of sorbitol. Our data reveal that under certain stress conditions the rates of SG formation and disassembly is modulated by TDP-43 mutations associated with ALS, and suggest that this may be an early event in the seeding of insoluble cytoplasmic inclusions observed in ALS.

WDR62 is required for centriole duplication in spermatogenesis and manchette removal in spermiogenesis

WDR62 is a scaffold protein involved in centriole duplication and spindle assembly during mitosis. Mutations in WDR62 can cause primary microcephaly and premature ovarian insufficiency. We have generated a genetrap mouse model deficient in WDR62 and characterised the developmental effects of WDR62 deficiency during meiosis in the testis. We have found that WDR62 deficiency leads to centriole underduplication in the spermatocytes due to reduced or delayed CEP63 accumulation in the pericentriolar matrix. This resulted in prolonged metaphase that led to apoptosis. Round spermatids that inherited a pair of centrioles progressed through spermiogenesis, however, manchette removal was delayed in WDR62 deficient spermatids due to delayed Katanin p80 accumulation in the manchette, thus producing misshapen spermatid heads with elongated manchettes. In mice, WDR62 deficiency resembles oligoasthenoteratospermia, a common form of subfertility in men that is characterised by low sperm counts, poor motility and abnormal morphology. Therefore, proper WDR62 function is necessary for timely spermatogenesis and spermiogenesis during male reproduction.

Uda Ho et al find that loss of centriolar scaffold protein WDR62 in mouse testis leads to defects in spermatogenesis. They find that WDR62 deficiency leads to centriole underduplication in spermatocytes and delayed manchette removal in spermatids due to delayed Katanin p80 accumulation.

Pathophysiological Significance of WDR62 and JNK Signaling in Human Diseases

The c-Jun N-terminal kinase (JNK) is highly evolutionarily conserved and plays important roles in a broad range of physiological and pathological processes. The WD40-repeat protein 62 (WDR62) is a scaffold protein that recruits different components of the JNK signaling pathway to regulate several human diseases including neurological disorders, infertility, and tumorigenesis. Recent studies revealed that WDR62 regulates the process of neural stem cell mitosis and germ cell meiosis through JNK signaling. In this review we summarize the roles of WDR62 and JNK signaling in neuronal and non-neuronal contexts and discuss how JNK-dependent signaling regulates both processes. WDR62 is involved in various human disorders via JNK signaling regulation, and may represent a promising therapeutic strategy for the treatment of related diseases.

Centrosome Reduction Promotes Terminal Differentiation of Human Cardiomyocytes

Centrosome reduction and redistribution of pericentriolar material (PCM) coincides with cardiomyocyte transitions to a post-mitotic and matured state. However, it is unclear whether centrosome changes are a cause or consequence of terminal differentiation. We validated that centrosomes were intact and functional in proliferative human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs), consistent with their immature phenotype. We generated acentrosomal hPSC-CMs, through pharmacological inhibition of centriole duplication, and showed that centrosome loss was sufficient to promote post-mitotic transitions and aspects of cardiomyocyte maturation. As Hippo kinases are activated during post-natal cardiac maturation, we pharmacologically activated the Hippo pathway using C19, which was sufficient to trigger centrosome disassembly and relocalization of PCM components to perinuclear membranes. This was due to specific activation of Hippo kinases, as direct inhibition of YAP-TEAD interactions with verteporfin had no effect on centrosome organization. This suggests that Hippo kinase-centrosome remodeling may play a direct role in cardiac maturation.

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Centrosomes are intact and functional in immature human cardiomyocytes

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Centrosome loss promotes maturation of human cardiomyocytes

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Centrosomes are returned with cell cycle reinitiation in post-natal cardiomyocytes

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Hippo kinases promote disassembly and redistribution of centrosomal proteins

The Spindle-Associated Microcephaly Protein, WDR62, Is Required for Neurogenesis and Development of the Hippocampus

Primary microcephaly genes (MCPH) are required for the embryonic expansion of the mammalian cerebral cortex. However, MCPH mutations may spare growth in other regions of the developing forebrain which reinforces context-dependent functions for distinct MCPH genes in neurodevelopment. Mutations in the MCPH2 gene, WD40-repeat protein 62 (WDR62), are causative of primary microcephaly and cortical malformations in humans. WDR62 is a spindle microtubule-associated phosphoprotein that is required for timely and oriented cell divisions. Recent studies in rodent models confirm that WDR62 loss or mutation causes thinning of the neocortex and disrupted proliferation of apical progenitors reinforcing critical requirements in the maintenance of radial glia. However, potential contributions for WDR62 in hippocampal development had not been previously defined. Using CRISPR/Cas9 gene editing, we generated mouse models with patient-derived non-synonymous missense mutations (WDR62^V66M and WDR62^R439H) and a null mutation (herein referred to as WDR62^Stop) for comparison. We find that WDR62 deletion or mutation resulted in a significant reduction in the thickness of the hippocampal ventricular zone and the area of the dentate gyrus (DG). This was associated with the mitotic arrest and depletion of radial glia and intermediate progenitors in the ammonic neuroepithelium. As a consequence, we find that the number of mitotic dentate precursors in the migratory stream and granule neurons in the DG was reduced with WDR62 mutation. These findings reveal that WDR62 is required for neurogenesis and the growth of the hippocampus during embryonic development.

Elevated levels of Drosophila Wdr62 promote glial cell growth and proliferation through AURKA signalling to AKT and MYC

WD40-Repeat Protein 62 (WDR62) is required to maintain neural and glial cell populations during embryonic brain growth. Although elevated expression of WDR62 is frequently associated with several tumour types, potential effects of excess WDR62 on proliferative growth remain undefined. Here, we demonstrate that glia specific overexpression of WDR62 in Drosophila larval brains resulted in increased cell size, over-proliferation and increased brain volume, without overt disruption of tissue organization. We further demonstrate WDR62 promoted over-proliferation and brain overgrowth by activating AURKA and pAKT signalling to increase MYC function in glial cells. Together these data suggest WDR62 normally functions in the glial lineage to activate oncogenic signalling networks, promoting proliferation and brain overgrowth.

The association of microcephaly protein WDR62 with CPAP/IFT88 is required for cilia formation and neocortical development

WDR62 mutations that result in protein loss, truncation or single amino-acid substitutions are causative for human microcephaly, indicating critical roles in cell expansion required for brain development. WDR62 missense mutations that retain protein expression represent partial loss-of-function mutants that may therefore provide specific insights into radial glial cell processes critical for brain growth. Here we utilized CRISPR/Cas9 approaches to generate three strains of WDR62 mutant mice; WDR62 V66M/V66M and WDR62R439H/R439H mice recapitulate conserved missense mutations found in humans with microcephaly, with the third strain being a null allele (WDR62stop/stop). Each of these mutations resulted in embryonic lethality to varying degrees and gross morphological defects consistent with ciliopathies (dwarfism, anophthalmia and microcephaly). We find that WDR62 mutant proteins (V66M and R439H) localize to the basal body but fail to recruit CPAP. As a consequence, we observe deficient recruitment of IFT88, a protein that is required for cilia formation. This underpins the maintenance of radial glia as WDR62 mutations caused premature differentiation of radial glia resulting in reduced generation of neurons and cortical thinning. These findings highlight the important role of the primary cilium in neocortical expansion and implicate ciliary dysfunction as underlying the pathology of MCPH2 patients.

Quantitative proteomic analyses of dynamic signalling events in cortical neurons undergoing excitotoxic cell death

Excitotoxicity, caused by overstimulation or dysregulation of ionotropic glutamate receptors (iGluRs), is a pathological process directing neuronal death in many neurological disorders. The aberrantly stimulated iGluRs direct massive influx of calcium ions into the affected neurons, leading to changes in expression and phosphorylation of specific proteins to modulate their functions and direct their participation in the signalling pathways that induce excitotoxic neuronal death. To define these pathways, we used quantitative proteomic approaches to identify these neuronal proteins (referred to as the changed proteins) and determine how their expression and/or phosphorylation dynamically changed in association with excitotoxic cell death. Our data, available in ProteomeXchange with identifier PXD008353, identified over 100 changed proteins exhibiting significant alterations in abundance and/or phosphorylation levels at different time points (5–240 min) in neurons after glutamate overstimulation. Bioinformatic analyses predicted that many of them are components of signalling networks directing defective neuronal morphology and functions. Among them, the well-known neuronal survival regulators including mitogen-activated protein kinases Erk1/2, glycogen synthase kinase 3 (GSK3) and microtubule-associated protein (Tau), were selected for validation by biochemical approaches, which confirmed the findings of the proteomic analysis. Bioinformatic analysis predicted Protein Kinase B (Akt), c-Jun kinase (JNK), cyclin-dependent protein kinase 5 (Cdk5), MAP kinase kinase (MEK), Casein kinase 2 (CK2), Rho-activated protein kinase (Rock) and Serum/glucocorticoid-regulated kinase 1 (SGK1) as the potential upstream kinases phosphorylating some of the changed proteins. Further biochemical investigation confirmed the predictions of sustained changes of the activation states of neuronal Akt and CK2 in excitotoxicity. Thus, future investigation to define the signalling pathways directing the dynamic alterations in abundance and phosphorylation of the identified changed neuronal proteins will help elucidate the molecular mechanism of neuronal death in excitotoxicity.

Doublecortin X (DCX) serine 28 phosphorylation is a regulatory switch, modulating association of DCX with microtubules and actin filaments

Doublecortin X (DCX) plays essential roles in neuronal development via its regulation of cytoskeleton dynamics. This is mediated through direct interactions between its doublecortin (DC) domains (DC1 and DC2) with microtubules (MTs) and indirect association with actin filaments (F-ACT). While the regulatory role of the DCX C-terminus following DC2 (i.e. DCX residues 275-366) has been established, less is known of the possible contributions made by the DCX N-terminus preceding DC1 (i.e. DCX residues 1-44). Here, we assessed the influence of DCX Ser28 within the DCX N-terminus, on the association of DCX with MTs and F-ACT. We compared the cytoskeletal interactions of the DCX S28E phosphomimetic and DCX S28A phospho-resistant mutants and wild-type DCX. Immunoprecipitation and colocalisation analyses indicated increased association of DCX S28E with F-ACT but decreased interaction with MTs, and conversely enhanced DCX S28A association with MTs but decreased association with F-ACT. To evaluate the impact of DCX mutants on cytoskeletal filaments we performed fluorescence recovery after photobleaching (FRAP) studies on SiR-tubulin and β-actin-mCherry and observed comparable tubulin and actin exchange rates in the presence of DCX WT and DCX S28A. However, we observed faster tubulin exchange rates but slower actin exchange rates in the presence of DCX S28E. Moreover, DCX S28E enhanced the association with the actin-binding protein spinophilin (Spn) suggesting the shift to favour association with both F-ACT and Spn in the presence of DCX S28E. Taken together, our results highlight a new role for DCX S28 as a regulatory switch for cytoskeletal organisation.

MEKK3 coordinates with FBW7 to regulate WDR62 stability and neurogenesis

Mutations of WD repeat domain 62 (WDR62) lead to autosomal recessive primary microcephaly (MCPH), and down-regulation of WDR62 expression causes the loss of neural progenitor cells (NPCs). However, how WDR62 is regulated and hence controls neurogenesis and brain size remains elusive. Here, we demonstrate that mitogen-activated protein kinase kinase kinase 3 (MEKK3) forms a complex with WDR62 to promote c-Jun N-terminal kinase (JNK) signaling synergistically in the control of neurogenesis. The deletion of Mekk3, Wdr62, or Jnk1 resulted in phenocopied defects, including premature NPC differentiation. We further showed that WDR62 protein is positively regulated by MEKK3 and JNK1 in the developing brain and that the defects of wdr62 deficiency can be rescued by the transgenic expression of JNK1. Meanwhile, WDR62 is also negatively regulated by T1053 phosphorylation, leading to the recruitment of F-box and WD repeat domain-containing protein 7 (FBW7) and proteasomal degradation. Our findings demonstrate that the coordinated reciprocal and bidirectional regulation among MEKK3, FBW7, WDR62, and JNK1, is required for fine-tuned JNK signaling for the control of balanced NPC self-renewal and differentiation during cortical development.

Microcephaly is a neural developmental disorder characterized by significantly reduced brain size and variable intellectual disability. WD repeat domain 62 (WDR62) was identified as the second most common gene for autosomal recessive primary microcephaly (MCPH) in human. Here, we studied the underlying regulatory mechanism of WDR62 and the impact on generation of new neurons. We show that mitogen-activated protein kinase kinase kinase 3 (Mekk3), Wdr62, and c-Jun N-terminal kinase 1 (Jnk1) knockout (KO) mice have defects in the generation and maturation of neurons. We demonstrate that WDR62 stability is positively regulated by a mitogen-activated protein kinase kinase kinase (MAPKKK), MEKK3, but negatively regulated by the E3 ligase, F-box and WD repeat domain-containing protein 7 (FBW7). These positive and negative factors calibrate the strength of the activity of the JNK signaling pathway, which controls self-renewal and differentiation of neural progenitor cells (NPCs) during brain development. This finding improves our understanding of the molecular pathogenesis of MCPH.

Glial-Specific Functions of Microcephaly Protein WDR62 and Interaction with the Mitotic Kinase AURKA Are Essential for Drosophila Brain Growth

The second most commonly mutated gene in primary microcephaly (MCPH) patients is wd40-repeat protein 62 (wdr62), but the relative contribution of WDR62 function to the growth of major brain lineages is unknown. Here, we use Drosophila models to dissect lineage-specific WDR62 function(s). Interestingly, although neural stem cell (neuroblast)-specific depletion of WDR62 significantly decreased neuroblast number, brain size was unchanged. In contrast, glial lineage-specific WDR62 depletion significantly decreased brain volume. Moreover, loss of function in glia not only decreased the glial population but also non-autonomously caused neuroblast loss. We further demonstrated that WDR62 controls brain growth through lineage-specific interactions with master mitotic signaling kinase, AURKA. Depletion of AURKA in neuroblasts drives brain overgrowth, which was suppressed by WDR62 co-depletion. In contrast, glial-specific depletion of AURKA significantly decreased brain volume, which was further decreased by WDR62 co-depletion. Thus, dissecting relative contributions of MCPH factors to individual neural lineages will be critical for understanding complex diseases such as microcephaly.

Aurora A phosphorylation of WD40-repeat protein 62 in mitotic spindle regulation

Mitotic spindle organization is regulated by centrosomal kinases that potentiate recruitment of spindle-associated proteins required for normal mitotic progress including the microcephaly protein WD40-repeat protein 62 (WDR62). WDR62 functions underlie normal brain development as autosomal recessive mutations and wdr62 loss cause microcephaly. Here we investigate the signaling interactions between WDR62 and the mitotic kinase Aurora A (AURKA) that has been recently shown to cooperate to control brain size in mice. The spindle recruitment of WDR62 is closely correlated with increased levels of AURKA following mitotic entry. We showed that depletion of TPX2 attenuated WDR62 localization at spindle poles indicating that TPX2 co-activation of AURKA is required to recruit WDR62 to the spindle. We demonstrated that AURKA activity contributed to the mitotic phosphorylation of WDR62 residues Ser49 and Thr50 and phosphorylation of WDR62 N-terminal residues was required for spindle organization and metaphase chromosome alignment. Our analysis of several MCPH-associated WDR62 mutants (V65M, R438H and V1314RfsX18) that are mislocalized in mitosis revealed that their interactions and phosphorylation by AURKA was substantially reduced consistent with the notion that AURKA is a key determinant of WDR62 spindle recruitment. Thus, our study highlights the role of AURKA signaling in the spatiotemporal control of WDR62 at spindle poles where it maintains spindle organization.

Opposing roles for JNK and Aurora A in regulating the association of WDR62 with spindle microtubules

WD40-repeat protein 62 (WDR62) is a spindle pole protein required for normal cell division and neuroprogenitor differentiation during brain development. Microcephaly-associated mutations in WDR62 lead to mitotic mislocalization, highlighting a crucial requirement for precise WDR62 spatiotemporal distribution, although the regulatory mechanisms are unknown. Here, we demonstrate that the WD40-repeat region of WDR62 is required for microtubule association, whereas the disordered C-terminal region regulates cell-cycle-dependent compartmentalization. In agreement with a functional requirement for the WDR62–JNK1 complex during neurogenesis, WDR62 specifically recruits JNK1 (also known as MAPK8), but not JNK2 (also known as MAPK9), to the spindle pole. However, JNK-mediated phosphorylation of WDR62 T1053 negatively regulated microtubule association, and loss of JNK signaling resulted in constitutive WDR62 localization to microtubules irrespective of cell cycle stage. In contrast, we identified that Aurora A kinase (AURKA) and WDR62 were in complex and that AURKA-mediated phosphorylation was required for the spindle localization of WDR62 during mitosis. Our studies highlight complex regulation of WDR62 localization, with opposing roles for JNK and AURKA in determining its spindle association.

A beacon of hope in stroke therapy-Blockade of pathologically activated cellular events in excitotoxic neuronal death as potential neuroprotective strategies

Excitotoxicity, a pathological process caused by over-stimulation of ionotropic glutamate receptors, is a major cause of neuronal loss in acute and chronic neurological conditions such as ischaemic stroke, Alzheimer's and Huntington's diseases. Effective neuroprotective drugs to reduce excitotoxic neuronal loss in patients suffering from these neurological conditions are urgently needed. One avenue to achieve this goal is to clearly define the intracellular events mediating the neurotoxic signals originating from the over-stimulated glutamate receptors in neurons. In this review, we first focus on the key cellular events directing neuronal death but not involved in normal physiological processes in the neurotoxic signalling pathways. These events, referred to as pathologically activated events, are potential targets for the development of neuroprotectant therapeutics. Inhibitors blocking some of the known pathologically activated cellular events have been proven to be effective in reducing stroke-induced brain damage in animal models. Notable examples are inhibitors suppressing the ion channel activity of neurotoxic glutamate receptors and those disrupting interactions of specific cellular proteins occurring only in neurons undergoing excitotoxic cell death. Among them, Tat-NR2B9c and memantine are clinically effective in reducing brain damage caused by some acute and chronic neurological conditions. Our second focus is evaluation of the suitability of the other inhibitors for use as neuroprotective therapeutics. We also discuss the experimental approaches suitable for bridging our knowledge gap in our current understanding of the excitotoxic signalling mechanism in neurons and discovery of new pathologically activated cellular events as potential targets for neuroprotection.

Loss of miR-223 and JNK Signaling Contribute to Elevated Stathmin in Malignant Pleural Mesothelioma

Malignant pleural mesothelioma (MPM) is often fatal, and studies have revealed that aberrant miRNAs contribute to MPM development and aggressiveness. Here, a screen of miRNAs identified reduced levels of miR-223 in MPM patient specimens. Interestingly, miR-223 targets Stathmin (STMN1), a microtubule regulator that has been associated with MPM. However, whether miR-223 regulates STMN1 in MPM and the functions of miR-223 and STMN1 in this disease are yet to be determined. STMN1 is also regulated by c-Jun N-terminal kinase (JNK) signaling, but whether this occurs in MPM and whether miR-223 plays a role are unknown. The relationship between STMN1, miR-223, and JNK was assessed using MPM cell lines, cells from pleural effusions, and MPM tissue. Evidence indicates that miR-223 is decreased in all MPM tissue compared with normal/healthy tissue. Conversely, STMN1 expression was higher in MPM cell lines when compared with primary mesothelial cell controls. Following overexpression of miR-223 in MPM cell lines, STMN1 levels were reduced, cell motility was inhibited, and tubulin acetylation induced. Knockdown of STMN1 using siRNAs led to inhibition of MPM cell proliferation and motility. Finally, miR-223 levels increased while STMN1 was reduced following the re-expression of the JNK isoforms in JNK-null murine embryonic fibroblasts, and STMN1 was reduced in MPM cell lines following the activation of JNK signaling.

IMPLICATIONS

miR-223 regulates STMN1 in MPM, and both are in turn regulated by the JNK signaling pathway. As such, miR-223 and STMN1 play an important role in regulating MPM cell motility and may be therapeutic targets.

Evidence that the MEK/ERK but not the PI3K/Akt pathway is required for protection from myocardial ischemia-reperfusion injury by 3',4'-dihydroxyflavonol

The novel pro-drug of 3'4'-dihydroxyflavonol, NP202, potently reduces myocardial infarct size resulting from ischemia-reperfusion (I/R) through mechanisms that remain to be fully defined. In this study, we investigated whether cardioprotection induced by NP202 depended on activation of the reperfusion injury survival kinase (RISK) pathways. We therefore examined the effects of PD98059 and LY294002, specific inhibitors of the MEK/ERK1/2 and PI3K/Akt pathways, respectively. In isolated cardiomyocytes, H2O2induced oxidative stress activated ERK1/2 and this was further enhanced by DiOHF, the active parent compound of NP202. Although oxidative stress did not stimulate Akt in cardiomyocytes, co-treatment with DiOHF substantially increased Akt phosphorylation. This suggests that DiOHF is a potent modulator of RISK pathways specifically in the context of stress stimulation. In anesthetised sheep, following 1h ischemia and 3h reperfusion, the contribution of the RISK pathways to NP202-mediated cardioprotection was determined by treating the animals with PD98059, LY294002 or vehicle prior to NP202 administration and reperfusion. Infarct size, as a percentage of the area-at-risk, was substantially reduced by NP202 (from 78±6 to 46±4%, P<0.05). Inhibition of MEK/ERK1/2 abolished the cardioprotective effects of NP202 (infarct size 81±4%), whereas inhibition of PI3K/Akt had no effect (infarct size 53±4%). Our combined cellular and animal studies indicate that NP202 potently protects against myocardial I/R injury through complex mechanisms that involved augmentation of MEK/ERK1/2 signaling, but not PI3K/Akt signaling.

Dual role of Src kinase in governing neuronal survival

BACKGROUND

Src-family kinases (SFKs) are involved in neuronal survival and their aberrant regulation contributes to neuronal death. However, how they control neuronal survival and death remains unclear.

OBJECTIVE

To define the effect of inhibition of Src activity and expression on neuronal survival.

RESULTS

In agreement with our previous findings, we demonstrated that Src was cleaved by calpain to form a 52-kDa truncated fragment in neurons undergoing excitotoxic cell death, and expression of the recombinant truncated Src fragment induced neuronal death. The data confirm that the neurotoxic signaling pathways are intact in the neurons we used for our study. To define the functional role of neuronal SFKs, we treated these neurons with SFK inhibitors and discovered that the treatment induced cell death, suggesting that the catalytic activity of one or more of the neuronal SFKs is critical to neuronal survival. Using small hairpin RNAs that suppress Src expression, we demonstrated that Src is indispensable to neuronal survival. Additionally, we found that neuronal death induced by expression of the neurotoxic truncated Src mutant, treatment of SFK inhibitors or knock-down of Src expression caused inhibition of the neuroprotective protein kinases Erk1/2, or Akt.

CONCLUSIONS

Src is critical to both neuronal survival and death. Intact Src sustains neuronal survival. However, in the excitotoxic condition, calpain cleavage of Src generates a neurotoxic truncated Src fragment. Both intact Src and the neurotoxic truncated Src fragment exert their biological actions by controlling the activities of neuroprotective protein kinases.

cAMP-dependent protein kinase and c-Jun N-terminal kinase mediate stathmin phosphorylation for the maintenance of interphase microtubules during osmotic stress

Dynamic microtubule changes after a cell stress challenge are required for cell survival and adaptation. Stathmin (STMN), a cytoplasmic microtubule-destabilizing phosphoprotein, regulates interphase microtubules during cell stress, but the signaling mechanisms involved are poorly defined. In this study ectopic expression of single alanine-substituted phospho-resistant mutants demonstrated that STMN Ser-38 and Ser-63 phosphorylation were specifically required to maintain interphase microtubules during hyperosmotic stress. STMN was phosphorylated on Ser-38 and Ser-63 in response to hyperosmolarity, heat shock, and arsenite treatment but rapidly dephosphorylated after oxidative stress treatment. Two-dimensional PAGE and Phos-tag gel analysis of stress-stimulated STMN phospho-isoforms revealed rapid STMN Ser-38 phosphorylation followed by subsequent Ser-25 and Ser-63 phosphorylation. Previously, we delineated stress-stimulated JNK targeting of STMN. Here, we identified cAMP-dependent protein kinase (PKA) signaling as responsible for stress-induced STMN Ser-63 phosphorylation. Increased cAMP levels induced by cholera toxin triggered potent STMN Ser-63 phosphorylation. Osmotic stress stimulated an increase in PKA activity and elevated STMN Ser-63 and CREB (cAMP-response element-binding protein) Ser-133 phosphorylation that was substantially attenuated by pretreatment with H-89, a PKA inhibitor. Interestingly, PKA activity and subsequent phosphorylation of STMN were augmented in the absence of JNK activation, indicating JNK and PKA pathway cross-talk during stress regulation of STMN. Taken together our study indicates that JNK- and PKA-mediated STMN Ser-38 and Ser-63 phosphorylation are required to preserve interphase microtubules in response to hyperosmotic stress.

A truncated fragment of Src protein kinase generated by calpain-mediated cleavage is a mediator of neuronal death in excitotoxicity

Excitotoxicity resulting from overstimulation of glutamate receptors is a major cause of neuronal death in cerebral ischemic stroke. The overstimulated ionotropic glutamate receptors exert their neurotoxic effects in part by overactivation of calpains, which induce neuronal death by catalyzing limited proteolysis of specific cellular proteins. Here, we report that in cultured cortical neurons and in vivo in a rat model of focal ischemic stroke, the tyrosine kinase Src is cleaved by calpains at a site in the N-terminal unique domain. This generates a truncated Src fragment of ~52 kDa, which we localized predominantly to the cytosol. A cell membrane-permeable fusion peptide derived from the unique domain of Src prevents calpain from cleaving Src in neurons and protects against excitotoxic neuronal death. To explore the role of the truncated Src fragment in neuronal death, we expressed a recombinant truncated Src fragment in cultured neurons and examined how it affects neuronal survival. Expression of this fragment, which lacks the myristoylation motif and unique domain, was sufficient to induce neuronal death. Furthermore, inactivation of the prosurvival kinase Akt is a key step in its neurotoxic signaling pathway. Because Src maintains neuronal survival, our results implicate calpain cleavage as a molecular switch converting Src from a promoter of cell survival to a mediator of neuronal death in excitotoxicity. Besides unveiling a new pathological action of Src, our discovery of the neurotoxic action of the truncated Src fragment suggests new therapeutic strategies with the potential to minimize brain damage in ischemic stroke.

Identification and characterization of bi-thiazole-2,2'-diamines as kinase inhibitory scaffolds

Based on bioinformatics interrogation of the genome, >500 mammalian protein kinases can be clustered within seven different groups. Of these kinases, the mitogen-activated protein kinase (MAPK) family forms part of the CMGC group of serine/threonine kinases that includes extracellular signal regulated kinases (ERKs), cJun N-terminal kinases (JNKs), and p38 MAPKs. With the JNKs considered attractive targets in the treatment of pathologies including diabetes and stroke, efforts have been directed to the discovery of new JNK inhibitory molecules that can be further developed as new therapeutics. Capitalizing on our biochemical understanding of JNK, we performed in silico screens of commercially available chemical databases to identify JNK1-interacting compounds and tested their in vitro JNK inhibitory activity. With in vitro and cell culture studies, we showed that the compound, 4'-methyl-N(2)-3-pyridinyl-4,5'-bi-1,3-thiazole-2,2'-diamine (JNK Docking (JD) compound 123, but not the related compound (4'-methyl-N~2~-(6-methyl-2-pyridinyl)-4,5'-bi-1,3-thiazole-2,2'-diamine (JD124), inhibited JNK1 activity towards a range of substrates. Molecular docking, saturation transfer difference NMR experiments and enzyme kinetic analyses revealed both ATP- and substrate-competitive inhibition of JNK by JD123. In characterizing JD123 further, we noted its ATP-competitive inhibition of the related p38-γ MAPK, but not ERK1, ERK2, or p38-α, p38-β or p38-δ. Further screening of a broad panel of kinases using 10μM JD123, identified inhibition of kinases including protein kinase Bβ (PKBβ/Aktβ). Appropriately modified thiazole diamines, as typified by JD123, thus provide a new chemical scaffold for development of inhibitors for the JNK and p38-γ MAPKs as well as other kinases that are also potential therapeutic targets such as PKBβ/Aktβ.

WD40-repeat protein 62 is a JNK-phosphorylated spindle pole protein required for spindle maintenance and timely mitotic progression

The impact of aberrant centrosomes and/or spindles on asymmetric cell division in embryonic development indicates the tight regulation of bipolar spindle formation and positioning that is required for mitotic progression and cell fate determination. WD40-repeat protein 62 (WDR62) was recently identified as a spindle pole protein linked to the neurodevelopmental defect of microcephaly but its roles in mitosis have not been defined. We report here that the in utero electroporation of neuroprogenitor cells with WDR62 siRNAs induced their cell cycle exit and reduced their proliferative capacity. In cultured cells, we demonstrated cell-cycle-dependent accumulation of WDR62 at the spindle pole during mitotic entry that persisted until metaphase-anaphase transition. Utilizing siRNA depletion, we revealed WDR62 function in stabilizing the mitotic spindle specifically during metaphase. WDR62 loss resulted in spindle orientation defects, decreased the integrity of centrosomes displaced from the spindle pole and delayed mitotic progression. Additionally, we revealed JNK phosphorylation of WDR62 is required for maintaining metaphase spindle organization during mitosis. Our study provides the first functional characterization of WDR62 and has revealed requirements for JNK/WDR62 signaling in mitotic spindle regulation that may be involved in coordinating neurogenesis.

Phosphoinositide 3-kinase (PI3K(p110alpha)) directly regulates key components of the Z-disc and cardiac structure

Maintenance of cardiac structure and Z-disc signaling are key factors responsible for protecting the heart in a setting of stress, but how these processes are regulated is not well defined. We recently demonstrated that PI3K(p110α) protects the heart against myocardial infarction. The aim of this study was to determine whether PI3K(p110α) directly regulates components of the Z-disc and cardiac structure. To address this question, a unique three-dimensional virtual muscle model was applied to gene expression data from transgenic mice with increased or decreased PI3K(p110α) activity under basal conditions (sham) and in a setting of myocardial infarction to display the location of structural proteins. Key findings from this analysis were then validated experimentally. The three-dimensional virtual muscle model visually highlighted reciprocally regulated transcripts associated with PI3K activation that encoded key components of the Z-disc and costamere, including melusin. Studies were performed to assess whether PI3K and melusin interact in the heart. Here, we identify a novel melusin-PI3K interaction that generates lipid kinase activity. The direct impact of PI3K(p110α) on myocyte structure was assessed by treating neonatal rat ventricular myocytes with PI3K(p110α) inhibitors and examining the myofiber morphology of hearts from PI3K transgenic mice. Results demonstrate that PI3K is critical for myofiber maturation and Z-disc alignment. In summary, PI3K regulates the expression of genes essential for cardiac structure and Z-disc signaling, interacts with melusin, and is critical for Z-disc alignment.

Opposing actions of extracellular signal-regulated kinase (ERK) and signal transducer and activator of transcription 3 (STAT3) in regulating microtubule stabilization during cardiac hypertrophy

Excessive proliferation and stabilization of the microtubule (MT) array in cardiac myocytes can accompany pathological cardiac hypertrophy, but the molecular control of these changes remains poorly characterized. In this study, we examined MT stabilization in two independent murine models of heart failure and revealed increases in the levels of post-translationally modified stable MTs, which were closely associated with STAT3 activation. To explore the molecular signaling events contributing to control of the cardiac MT network, we stimulated cardiac myocytes with an α-adrenergic agonist phenylephrine (PE), and observed increased tubulin content without changes in detyrosinated (glu-tubulin) stable MTs. In contrast, the hypertrophic interleukin-6 (IL6) family cytokines increased both the glu-tubulin content and glu-MT density. When we examined a role for ERK in regulating cardiac MTs, we showed that the MEK/ERK-inhibitor U0126 increased glu-MT density in either control cardiac myocytes or following exposure to hypertrophic agents. Conversely, expression of an activated MEK1 mutant reduced glu-tubulin levels. Thus, ERK signaling antagonizes stabilization of the cardiac MT array. In contrast, inhibiting either JAK2 with AG490, or STAT3 signaling with Stattic or siRNA knockdown, blocked cytokine-stimulated increases in glu-MT density. Furthermore, the expression of a constitutively active STAT3 mutant triggered increased glu-MT density in the absence of hypertrophic stimulation. Thus, STAT3 activation contributes substantially to cytokine-stimulated glu-MT changes. Taken together, our results highlight the opposing actions of STAT3 and ERK pathways in the regulation of MT changes associated with cardiac myocyte hypertrophy.

c-Jun N-terminal kinase phosphorylation of stathmin confers protection against cellular stress

The cell stress response encompasses the range of intracellular events required for adaptation to stimuli detrimental to cell survival. Although the c-Jun N-terminal kinase (JNK) is a stress-activated kinase that can promote either cell survival or death in response to detrimental stimuli, the JNK-regulated mechanisms involved in survival are not fully characterized. Here we show that in response to hyperosmotic stress, JNK phosphorylates a key cytoplasmic microtubule regulatory protein, stathmin (STMN), on conserved Ser-25 and Ser-38 residues. In in vitro biochemical studies, we identified STMN Ser-38 as the critical residue required for efficient phosphorylation by JNK and identified a novel kinase interaction domain in STMN required for recognition by JNK. We revealed that JNK was required for microtubule stabilization in response to hyperosmotic stress. Importantly, we also demonstrated a novel cytoprotective function for STMN, as the knockdown of STMN levels by siRNA was sufficient to augment viability in response to hyperosmotic stress. Our findings show that JNK targeting of STMN represents a novel stress-activated cytoprotective mechanism involving microtubule network changes.

Severe heart failure and early mortality in a double-mutation mouse model of familial hypertrophic cardiomyopathy

BACKGROUND

Familial hypertrophic cardiomyopathy (FHC) is characterized by genetic and clinical heterogeneity. Five percent of FHC families have 2 FHC-causing mutations, which results in earlier disease onset, increased cardiac dysfunction, and a higher incidence of sudden death events. These observations suggest a relationship between the number of gene mutations and phenotype severity in FHC.

METHODS AND RESULTS

We sought to develop, characterize, and investigate the pathogenic mechanisms in a double-mutant murine model of FHC. This model (designated TnI-203/MHC-403) was generated by crossbreeding mice with the Gly203Ser cardiac troponin I (TnI-203) and Arg403Gln alpha-myosin heavy chain (MHC-403) FHC-causing mutations. The mortality rate in TnI-203/MHC-403 mice was 100% by age 21 days. At age 14 days, TnI-203/MHC-403 mice developed a significantly increased ratio of heart weight to body weight, marked interstitial myocardial fibrosis, and increased expression of atrial natriuretic factor and brain natriuretic peptide compared with nontransgenic, TnI-203, and MHC-403 littermates. By age 16 to 18 days, TnI-203/MHC-403 mice rapidly developed a severe dilated cardiomyopathy and heart failure, with inducibility of ventricular arrhythmias, which led to death by 21 days. Downregulation of mRNA levels of key regulators of Ca(2+) homeostasis in TnI-203/MHC-403 mice was observed. Increased levels of phosphorylated STAT3 were observed in TnI-203/MHC-403 mice and corresponded with the onset of disease, which suggests a possible cardioprotective response.

CONCLUSIONS

TnI-203/MHC-403 double-mutant mice develop a severe cardiac phenotype characterized by heart failure and early death. The presence of 2 disease-causing mutations may predispose individuals to a greater risk of developing severe heart failure than human FHC caused by a single gene mutation.

Myoseverin disrupts sarcomeric organization in myocytes: an effect independent of microtubule assembly inhibition

Although disruption of the microtubule (MT) array inhibits myogenesis in myocytes, the relationship between the assembly of microtubules (MT) and the organization of the contractile filaments is not clearly defined. We now report that the assembly of mature myofibrils in hypertrophic cardiac myocytes is disrupted by myoseverin, a compound previously shown to perturb the MT array in skeletal muscle cells. Myoseverin treated cardiac myocytes showed disruptions of the striated Z-bands containing alpha-actinin and desmin and the localization of tropomyosin, titin and myosin on mature sarcomeric filaments. In contrast, MT depolymerization by nocodazole did not perturb sarcomeric filaments. Similarly, expression of constitutively active stathmin as a non-chemical molecular method of MT depolymerization did not prevent sarcomere assembly. The extent of MT destabilization by myoseverin and nocodazole were comparable. Thus, the effect of myoseverin on sarcomere assembly was independent of its capacity for MT inhibition. Furthermore, we found that upon removal of myoseverin, sarcomeres reformed in the absence of an intact MT network. Sarcomere formation in cardiac myocytes therefore, does not appear to require an intact MT network and thus we conclude that a functional MT array appears to be dispensable for myofibrillogenesis.

Small G-protein Rho is involved in the maintenance of cardiac myocyte morphology

The use of small membrane-permeable sequences or protein transduction domains (PTDs) can facilitate the transport of proteins into many cell types. In preliminary studies with the application of three PTDs (penetratin, modified penetratin, and the HIV TAT transduction domains) to cardiac myocytes, we found that the TAT and penetratin sequences showed high efficiency of uptake and low toxicity. Rho has been previously shown to be an important regulator of cytoskeletal organization and morphology in other non-cardiac cell types. To evaluate a role for Rho in cardiac myocyte morphology, we used the TAT-PTD to deliver a RhoA-specific inhibitor, the C3 exoenzyme, to cultured cardiac myocytes. We showed that this incubation with TAT-C3 abolished the basal levels of RhoA activity, demonstrating the efficacy of this treatment. Incubation with TAT-C3 also altered cardiac myocyte morphology so that TAT-C3-treated cells produced multiple projections from the major cell body. This was accompanied by a statistically significant increase in cell size, albeit to a lesser extent than the changes accompanying exposure to the hypertrophic agent, endothelin-1. Furthermore, the change in size of TAT-C3-treated cells was not accompanied by the induction of atrial natriuretic factor (ANF) expression that accompanies the hypertrophy of cardiac myocytes. These results reveal a role for RhoA in the maintenance of normal myocyte morphology.

GRIM-19, a cell death regulatory protein, is essential for assembly and function of mitochondrial complex I

Mitochondria play essential roles in cellular energy production via the oxidative phosphorylation system (OXPHOS) consisting of five multiprotein complexes and also in the initiation of apoptosis. NADH:ubiquinone oxidoreductase (complex I) is the largest complex that catalyzes the first step of electron transfer in the OXPHOS system. GRIM-19 was originally identified as a nuclear protein with apoptotic nature in interferon (IFN)- and all-trans-retinoic acid (RA)-induced tumor cells. To reveal its biological role, we generated mice deficient in GRIM-19 by gene targeting. Homologous deletion of GRIM-19 causes embryonic lethality at embryonic day 9.5. GRIM-19(-/-) blastocysts show retarded growth in vitro and, strikingly, display abnormal mitochondrial structure, morphology, and cellular distribution. We reexamined the cellular localization of GRIM-19 in various cell types and found its primary localization in the mitochondria. Furthermore, GRIM-19 is detected in the native form of mitochondrial complex I. Finally, we show that elimination of GRIM-19 destroys the assembly and electron transfer activity of complex I and also influences the other complexes in the mitochondrial respiratory chain. Our result demonstrates that GRIM-19, a gene product with a specific role in IFN-RA-induced cell death, is a functional component of mitochondrial complex I and is essential for early embryonic development.

Activation of signal transducer and activator of transcription (STAT) pathways in failing human hearts

OBJECTIVES

The signal transduction pathways mediating the progression to failure have been intensively studied in a variety of in vitro and in vivo animal models. Recently, acute activation of the Janus kinases (JAKs) and signal transducers and activators of transcription (STATs) has been observed in the heart, but whether this is sustained in ischemic heart disease (IHD) or dilated cardiomyopathy (DCM) has not been previously addressed.

METHODS

We assessed the tyrosine phosphorylation of STAT1, 3, 5 and 6 in ventricular samples of explanted human hearts with IHD (n=11) and DCM (n=9) as an indication of STAT activation. Samples from normal donor hearts (n=9) acted as controls. In parallel, we also assessed protein expression and phosphorylation of three major families of mitogen-activated protein kinases (MAPKs); ERK, p38 MAPK and c-Jun NH(2)-terminal kinase (JNK).

RESULTS

All STAT isoforms were significantly phosphorylated in DCM. In contrast, only the phosphorylation of STATs 1 and 5 were significantly enhanced in IHD. Expression of total STAT protein remained unchanged. For the MAPKs, significant phosphorylation of p38(MAPK) was only observed in IHD. In contrast, there was no change in ERK or JNK activation despite abundant protein expression.

CONCLUSIONS

We have shown that different members of the STAT transcription factor family are chronically phosphorylated in the failing heart as a result of IHD (STAT1 and 5) or DCM (STAT1, 3, 5 and 6). In contrast, IHD but not DCM showed significant p38(MAPK) phosphorylation. Whilst the differences noted between IHD and DCM may reflect different initiating events, the common activation of STATs 1 and 5 suggests that these transcription factors may play a common role regulating the progression of heart failure.

Taking the cell by stealth or storm? Protein transduction domains (PTDs) as versatile vectors for delivery

A cell delivery system is increasing in use in many areas of cell and molecular biology and bio-medicine. This system is based on a number of naturally occurring protein motifs and/or sequences which show the remarkable ability to rapidly cross the mammalian cell membrane without compromising its structure or function. These so-called Protein Transduction Domains (PTDs) offer unprecedented advantages for intracellular delivery. These advantages include, but are not limited to, their applicability to all cell types (no cell type has yet been described which is not transduced by these PTDs), and the range of cargoes that can be transduced (including peptides, small proteins, full-length enzymes, DNA oligomers, peptide-nucleic acid oligomers, liposomes, and magnetic nanoparticles). Furthermore, the PTDs have been demonstrated to be suitable for in vivo delivery including delivery across the blood brain barrier, and have been shown to cross the plasma membrane rapidly and enter the cytoplasm and nuclear regions of the cell. In this review, the general properties of the most commonly used PTDs are described. The strategies currently being undertaken also highlight that improvements in membrane transduction are possible despite our lack of understanding of the exact biochemical and/or physical mechanisms of transduction. Recent examples of the range of potential applications are also discussed.