Stress- and mitogen-induced phosphorylation of the synapse-associated protein SAP90/PSD-95 by activation of SAPK3/p38gamma and ERK1/ERK2

Hepatic kynurenic acid mediates phosphorylation of Nogo-A in the medial prefrontal cortex to regulate chronic stress-induced anxiety-like behaviors in mice

Exercise training effectively relieves anxiety disorders via modulating specific brain networks. The role of post-translational modification of proteins in this process, however, has been underappreciated. Here we performed a mouse study in which chronic restraint stress-induced anxiety-like behaviors can be attenuated by 14-day persistent treadmill exercise, in association with dramatic changes of protein phosphorylation patterns in the medial prefrontal cortex (mPFC). In particular, exercise was proposed to modulate the phosphorylation of Nogo-A protein, which drives the ras homolog family member A (RhoA)/ Rho-associated coiled-coil-containing protein kinases 1(ROCK1) signaling cascade. Further mechanistic studies found that liver-derived kynurenic acid (KYNA) can affect the kynurenine metabolism within the mPFC, to modulate this RhoA/ROCK1 pathway for conferring stress resilience. In sum, we proposed that circulating KYNA might mediate stress-induced anxiety-like behaviors via protein phosphorylation modification within the mPFC, and these findings shed more insights for the liver-brain communications in responding to both stress and physical exercise.

P38 kinase in gastrointestinal cancers

Gastrointestinal cancers are a leading cause of cancer morbidity and mortality worldwide with 4.2 million new cases and 3.2 million deaths estimated in 2020. Despite the advances in primary and adjuvant therapies, patients still develop distant metastases and require novel therapies. Mitogen‑activated protein kinase (MAPK) cascades are crucial signaling pathways that regulate many cellular processes, including proliferation, differentiation, apoptosis, stress responses and cancer development. p38 Mitogen Activated Protein Kinases (p38 MAPKs) includes four isoforms: p38α (MAPK14), p38β (MAPK11), p38γ (MAPK12), and p38δ (MAPK13). p38 MAPK was first identified as a stress response protein kinase that phosphorylates different transcriptional factors. Dysregulation of p38 pathways, in particular p38γ, are associated with cancer development, metastasis, autophagy and tumor microenvironment. In this article, we provide an overview of p38 and p38γ with respect to gastrointestinal cancers. Furthermore, targeting p38γ is also discussed as a potential therapy for gastrointestinal cancers.

p38γ MAPK Inflammatory and Metabolic Signaling in Physiology and Disease

p38γ MAPK (also called ERK6 or SAPK3) is a family member of stress-activated MAPKs and has common and specific roles as compared to other p38 proteins in signal transduction. Recent studies showed that, in addition to inflammation, p38γ metabolic signaling is involved in physiological exercise and in pathogenesis of cancer, diabetes, and Alzheimer's disease, indicating its potential as a therapeutic target. p38γphosphorylates at least 19 substrates through which p38γ activity is further modified to regulate life-important cellular processes such as proliferation, differentiation, cell death, and transformation, thereby impacting biological outcomes of p38γ-driven pathogenesis. P38γ signaling is characterized by its unique reciprocal regulation with its specific phosphatase PTPH1 and by its direct binding to promoter DNAs, leading to transcriptional activation of targets including cancer-like stem cell drivers. This paper will review recent findings about p38γ inflammation and metabolic signaling in physiology and diseases. Moreover, we will discuss the progress in the development of p38γ-specific pharmacological inhibitors for therapeutic intervention in disease prevention and treatment by targeting the p38γ signaling network.

Inhibition of p38α MAPK restores neuronal p38γ MAPK and ameliorates synaptic degeneration in a mouse model of DLB/PD

Alterations in the p38 mitogen-activated protein kinases (MAPKs) play an important role in the pathogenesis of dementia with Lewy bodies (DLB) and Parkinson's disease (PD). Activation of the p38α MAPK isoform and mislocalization of the p38γ MAPK isoform are associated with neuroinflammation and synaptic degeneration in DLB and PD. Therefore, we hypothesized that p38α might be associated with neuronal p38γ distribution and synaptic dysfunction in these diseases. To test this hypothesis, we treated in vitro cellular and in vivo mouse models of DLB/PD with SKF-86002, a compound that attenuates inflammation by inhibiting p38α/β, and then investigated the effects of this compound on p38γ and neurodegenerative pathology. We found that inhibition of p38α reduced neuroinflammation and ameliorated synaptic, neurodegenerative, and motor behavioral deficits in transgenic mice overexpressing human α-synuclein. Moreover, treatment with SKF-86002 promoted the redistribution of p38γ to synapses and reduced the accumulation of α-synuclein in mice overexpressing human α-synuclein. Supporting the potential value of targeting p38 in DLB/PD, we found that SKF-86002 promoted the redistribution of p38γ in neurons differentiated from iPS cells derived from patients with familial PD (carrying the A53T α-synuclein mutation) and healthy controls. Treatment with SKF-86002 ameliorated α-synuclein-induced neurodegeneration in these neurons only when microglia were pretreated with this compound. However, direct treatment of neurons with SKF-86002 did not affect α-synuclein-induced neurotoxicity, suggesting that SKF-86002 treatment inhibits α-synuclein-induced neurotoxicity mediated by microglia. These findings provide a mechanistic connection between p38α and p38γ as well as a rationale for targeting this pathway in DLB/PD.

miR164-NAC21/22 module regulates the resistance of Malus hupehensis against Alternaria alternata by controlling jasmonic acid signaling

Apple leaf spot disease caused by Alternaria alternata apple pathotype (A. alternata AP) is one of the most severe fungal diseases affecting apple cultivation. Transcription factors are involved in various disease-resistance responses, and many of them are regulated by miRNAs. Here, we performed RNA-Seq to investigate gene expression changes during the defense response of Malus hupehensis against A. alternata AP. NAC21/22 was induced upon A. alternata AP infection and silenced by miR164 via direct mRNA cleavage. Contrasting expression patterns were noted between mature miR164 and NAC21/22 during infection. Contrary to NAC21/22 silencing, transiently overexpressing NAC21/22 in M. hupehensis alleviated disease symptoms on 'gala' leaves, impeded A. alternata AP growth, and promoted jasmonic acid (JA) signaling-related gene expression. Importantly, transient miR164f overexpression in 'gala' leaves enhanced A. alternata AP sensitivity, due perhaps to NAC21/22 downregulation, whereas miR164 suppression produced an opposite effect. In summary, the miR164-NAC21/22 module plays a pivotal role in apple resistance against A. alternata AP by regulating JA signaling.

Evolutionary analysis of p38 stress-activated kinases in unicellular relatives of animals suggests an ancestral function in osmotic stress

p38 kinases are key elements of the cellular stress response in animals. They mediate the cell response to a multitude of stress stimuli, from osmotic shock to inflammation and oncogenes. However, it is unknown how such diversity of function in stress evolved in this kinase subfamily. Here, we show that the p38 kinase was already present in a common ancestor of animals and fungi. Later, in animals, it diversified into three JNK kinases and four p38 kinases. Moreover, we identified a fifth p38 paralog in fishes and amphibians. Our analysis shows that each p38 paralog has specific amino acid substitutions around the hinge point, a region between the N-terminal and C-terminal protein domains. We showed that this region can be used to distinguish between individual paralogs and predict their specificity. Finally, we showed that the response to hyperosmotic stress in Capsaspora owczarzaki, a close unicellular relative of animals, follows a phosphorylation-dephosphorylation pattern typical of p38 kinases. At the same time, Capsaspora's cells upregulate the expression of GPD1 protein resembling an osmotic stress response in yeasts. Overall, our results show that the ancestral p38 stress pathway originated in the root of opisthokonts, most likely as a cell's reaction to salinity change in the environment. In animals, the pathway became more complex and incorporated more stimuli and downstream targets due to the p38 sequence evolution in the docking and substrate binding sites around the hinge region. This study improves our understanding of p38 evolution and opens new perspectives for p38 research.

MKK6 deficiency promotes cardiac dysfunction through MKK3-p38γ/δ-mTOR hyperactivation

Stress-activated p38 kinases control a plethora of functions, and their dysregulation has been linked to the development of steatosis, obesity, immune disorders, and cancer. Therefore, they have been identified as potential targets for novel therapeutic strategies. There are four p38 family members (p38α, p38β, p38γ, and p38δ) that are activated by MKK3 and MKK6. Here, we demonstrate that lack of MKK6 reduces the lifespan in mice. Longitudinal study of cardiac function in MKK6 KO mice showed that young mice develop cardiac hypertrophy which progresses to cardiac dilatation and fibrosis with age. Mechanistically, lack of MKK6 blunts p38α activation while causing MKK3-p38γ/δ hyperphosphorylation and increased mammalian target of rapamycin (mTOR) signaling, resulting in cardiac hypertrophy. Cardiac hypertrophy in MKK6 KO mice is reverted by knocking out either p38γ or p38δ or by inhibiting the mTOR pathway with rapamycin. In conclusion, we have identified a key role for the MKK3/6-p38γ/δ pathway in the development of cardiac hypertrophy, which has important implications for the clinical use of p38α inhibitors in the long-term treatment since they might result in cardiotoxicity.

High Level Forebrain Expression of Active Tau Kinase p38γ Exacerbates Cognitive Dysfunction in Aged APP-transgenic Alzheimer's Mice

Persistent improvement of cognitive deficits in Alzheimer's disease (AD), a common form of dementia, is an unattained therapeutic objective. Gene therapy holds promise for treatment of familial and sporadic forms of AD. p38γ, a member of the p38 mitogen-activated protein (MAP) kinase family, inhibits amyloid-β toxicity through regulation of tau phosphorylation. We recently showed that a gene delivery approach increasing p38γ resulted in markedly better learning and memory performance in mouse models of AD at advanced stages of amyloid-β- and tau-mediated cognitive impairment. Notably, low-to-moderate expression of p38γ had beneficial outcomes on cognition. The impact of high levels of p38γ on neuronal function remain unclear. Therefore, we addressed the outcomes of high levels of active p38γ on brain function, by direct injection of p38γ-encoding adeno-associated virus (AAV) into the forebrain of aged mice of an APP transgenic AD mouse model. While motor function in p38γ-expressing APP transgenic mice 2 months post-injection was comparable to control treated APP mice, their activity was markedly reduced in the open field test and included frequent bouts of immobility. Moreover, their learning and memory function was markedly impaired compared to control-treated aged APP mice. These results suggest that high neuronal levels of active p38γ emphasize a stress kinase role of p38γ, perturbing circuit function in motivation, navigation, and spatial learning. Overall, this work shows excessive neuronal p38γ levels can aggravate circuit dysfunction and advises adjustable expression systems will be required for sustainable AD gene therapy based on p38γ activity.

p38γ and p38δ regulate postnatal cardiac metabolism through glycogen synthase 1

During the first weeks of postnatal heart development, cardiomyocytes undergo a major adaptive metabolic shift from glycolytic energy production to fatty acid oxidation. This metabolic change is contemporaneous to the up-regulation and activation of the p38γ and p38δ stress-activated protein kinases in the heart. We demonstrate that p38γ/δ contribute to the early postnatal cardiac metabolic switch through inhibitory phosphorylation of glycogen synthase 1 (GYS1) and glycogen metabolism inactivation. Premature induction of p38γ/δ activation in cardiomyocytes of newborn mice results in an early GYS1 phosphorylation and inhibition of cardiac glycogen production, triggering an early metabolic shift that induces a deficit in cardiomyocyte fuel supply, leading to whole-body metabolic deregulation and maladaptive cardiac pathogenesis. Notably, the adverse effects of forced premature cardiac p38γ/δ activation in neonate mice are prevented by maternal diet supplementation of fatty acids during pregnancy and lactation. These results suggest that diet interventions have a potential for treating human cardiac genetic diseases that affect heart metabolism.

Interaction between the guanylate kinase domain of PSD-95 and the proline-rich region and microtubule binding repeats 2 and 3 of tau

The microtubule-associated protein tau is a key factor in neurodegenerative proteinopathies and is predominantly found in the neuronal axon. However, somatodendritic localization of tau occurs for a subset of pathological and physiologic tau. Dendritic tau can localize to post-synapses where it interacts with proteins of the post-synaptic density (PSD) protein PSD-95, a membrane-associated guanylate kinase (MAGUK) scaffold factor for organization of protein complexes within the PSD, to mediate downstream signals. The sub-molecular details of this interaction, however, remain unclear. Here, we use interaction mapping in cultured cells to demonstrate that tau interacts with the guanylate kinase (GUK) domain in the C-terminal region of PSD-95. The PSD-95 GUK domain is required and sufficient for a complex with full-length human tau. Mapping the interaction of the MAGUK core on tau revealed the microtubule binding repeats 2 and 3 and the proline-rich region contribute to this interaction, while the N- and C-terminal regions of tau inhibit interaction. These results reveal intramolecular determinants of the protein complex of tau and PSD-95 and increase our understanding of tau interactions regulating neurotoxic signaling at the molecular level.

The role of stress kinases in metabolic disease

Obesity is a health condition that has reached pandemic levels and is implicated in the development and progression of type 2 diabetes mellitus, cancer and heart failure. A key characteristic of obesity is the activation of stress-activated protein kinases (SAPKs), such as the p38 and JNK stress kinases, in several organs, including adipose tissue, liver, skeletal muscle, immune organs and the central nervous system. The correct timing, intensity and duration of SAPK activation contributes to cellular metabolic adaptation. By contrast, uncontrolled SAPK activation has been proposed to contribute to the complications of obesity. The stress kinase signalling pathways have therefore been identified as potential targets for the development of novel therapeutic approaches for metabolic syndrome. The past few decades have seen intense research efforts to determine how these kinases are regulated in a cell-specific manner and to define their contribution to the development of obesity and insulin resistance. Several studies have uncovered new and unexpected functions of the non-classical members of both pathways. Here, we provide an overview of the role of SAPKs in metabolic control and highlight important discoveries in the field.

The palmitoyl acyltransferase ZDHHC14 controls Kv1-family potassium channel clustering at the axon initial segment

The palmitoyl acyltransferase (PAT) ZDHHC14 is highly expressed in the hippocampus and is the only PAT predicted to bind Type-I PDZ domain-containing proteins. However, ZDHHC14’s neuronal roles are unknown. Here, we identify the PDZ domain-containing Membrane-associated Guanylate Kinase (MaGUK) PSD93 as a direct ZDHHC14 interactor and substrate. PSD93, but not other MaGUKs, localizes to the axon initial segment (AIS). Using lentiviral-mediated shRNA knockdown in rat hippocampal neurons, we find that ZDHHC14 controls palmitoylation and AIS clustering of PSD93 and also of Kv1 potassium channels, which directly bind PSD93. Neurodevelopmental expression of ZDHHC14 mirrors that of PSD93 and Kv1 channels and, consistent with ZDHHC14’s importance for Kv1 channel clustering, loss of ZDHHC14 decreases outward currents and increases action potential firing in hippocampal neurons. To our knowledge, these findings identify the first neuronal roles and substrates for ZDHHC14 and reveal a previously unappreciated role for palmitoylation in control of neuronal excitability.

p38 MAPK Pathway in the Heart: New Insights in Health and Disease

The p38 mitogen-activated kinase (MAPK) family controls cell adaptation to stress stimuli. p38 function has been studied in depth in relation to cardiac development and function. The first isoform demonstrated to play an important role in cardiac development was p38α; however, all p38 family members are now known to collaborate in different aspects of cardiomyocyte differentiation and growth. p38 family members have been proposed to have protective and deleterious actions in the stressed myocardium, with the outcome of their action in part dependent on the model system under study and the identity of the activated p38 family member. Most studies to date have been performed with inhibitors that are not isoform-specific, and, consequently, knowledge remains very limited about how the different p38s control cardiac physiology and respond to cardiac stress. In this review, we summarize the current understanding of the role of the p38 pathway in cardiac physiology and discuss recent advances in the field.

Reduction of advanced tau-mediated memory deficits by the MAP kinase p38γ

Hyperphosphorylation of the neuronal tau protein contributes to Alzheimer's disease (AD) by promoting tau pathology and neuronal and cognitive deficits. In contrast, we have previously shown that site-specific tau phosphorylation can inhibit toxic signals induced by amyloid-β (Aβ) in mouse models. The post-synaptic mitogen-activated protein (MAP) kinase p38γ mediates this site-specific phosphorylation on tau at Threonine-205 (T205). Using a gene therapeutic approach, we draw on this neuroprotective mechanism to improve memory in two Aβ-dependent mouse models of AD at stages when advanced memory deficits are present. Increasing activity of post-synaptic kinase p38γ that targets T205 in tau reduced memory deficits in symptomatic Aβ-induced AD models. Reconstitution experiments with wildtype human tau or phosphorylation-deficient tauT205A showed that T205 modification is critical for downstream effects of p38γ that prevent memory impairment in APP-transgenic mice. Furthermore, genome editing of the T205 codon in the murine Mapt gene showed that this single side chain in endogenous tau critically modulates memory deficits in APP-transgenic Alzheimer's mice. Ablating the protective effect of p38γ activity by genetic p38γ deletion in a tau transgenic mouse model that expresses non-pathogenic tau rendered tau toxic and resulted in impaired memory function in the absence of human Aβ. Thus, we propose that modulating neuronal p38γ activity serves as an intrinsic tau-dependent therapeutic approach to augment compromised cognition in advanced dementia.

Involvement of p38 MAPK in Synaptic Function and Dysfunction

Many studies have revealed a central role of p38 MAPK in neuronal plasticity and the regulation of long-term changes in synaptic efficacy, such as long-term potentiation (LTP) and long-term depression (LTD). However, p38 MAPK is classically known as a responsive element to stress stimuli, including neuroinflammation. Specific to the pathophysiology of Alzheimer’s disease (AD), several studies have shown that the p38 MAPK cascade is activated either in response to the Aβ peptide or in the presence of tauopathies. Here, we describe the role of p38 MAPK in the regulation of synaptic plasticity and its implication in an animal model of neurodegeneration. In particular, recent evidence suggests the p38 MAPK α isoform as a potential neurotherapeutic target, and specific inhibitors have been developed and have proven to be effective in ameliorating synaptic and memory deficits in AD mouse models.

Memory and the circadian system: Identifying candidate mechanisms by which local clocks in the brain may regulate synaptic plasticity

The circadian system is an endogenous biological network responsible for coordinating near-24-h cycles in behavior and physiology with daily timing cues from the external environment. In this review, we explore how the circadian system regulates memory formation, retention, and recall. Circadian rhythms in these memory processes may arise through several endogenous pathways, and recent work highlights the importance of genetic timekeepers found locally within tissues, called local clocks. We evaluate the circadian memory literature for evidence of local clock involvement in memory, identifying potential nodes for direct interactions between local clock components and mechanisms of synaptic plasticity. Our discussion illustrates how local clocks may pervasively modulate neuronal plastic capacity, a phenomenon that we designate here as circadian metaplasticity. We suggest that this function of local clocks supports the temporal optimization of memory processes, illuminating the potential for circadian therapeutic strategies in the prevention and treatment of memory impairment.

Differential expression of DLG1 as a common trait in different human diseases: an encouraging issue in molecular pathology

Human disc large (DLG1) is a scaffolding protein that through the interaction with diverse cell partners participates in the control of key cellular processes such as polarity, proliferation and migration. Experimental data have mainly identified DLG1 as a tumor suppressor. An outstanding point for DLG1 protein is that altered DLG1 expression and DLG1 gene mutations were observed in different pathologies, including cancer and neurological and immunological disorders. Evident changes in DLG1 abundance and/or cell localization were identified in a number of studies suggesting its participation in molecular mechanisms responsible for the development of such illnesses. In this review, we focus on some of the latest findings regarding DLG1 alterations in different diseases as well as its potential use as a biomarker for pathological progression. We further address the current knowledge on the molecular mechanisms regulating DLG1 expression and the posttranslational modifications that may affect DLG1 cell localization and functions. Despite the advances in this field, there are still open questions about the precise molecular link between alterations in DLG1 expression and the development of each specific pathology. The complete understanding of this concern will give us new scenarios for the design of promising diagnosis and therapeutic tools.

Evidence that a defined population of neurons in lateral amygdala is directly involved in auditory fear learning and memory

Memory is thought to be encoded within networks of neurons within the brain, but the identity of the neurons involved and circuits they form have not been described for any memory. Previously, we used fos-tau-lacZ (FTL) transgenic mice to identify discrete populations of neurons in different regions of the brain which were specifically activated following fear conditioning. This suggested that these populations of neurons form nodes in a network that encodes fear memory. In particular, one population of learning activated neurons was found within a discrete region of the lateral amygdala (LA), a key nucleus required for fear conditioning. In order to provide evidence that this population is directly involved in fear conditioning, we have analysed the expression of a key molecular requirement for fear conditioning in LA, phosphorylated Extracellular Signal Regulated Kinase 1 and 2 (pERK1/2). The only neurons in LA that specifically expressed pERK1/2 following auditory fear conditioning were in the ventrolateral nucleus of the LA (LAvl), in the same discrete region where we found learning specific FTL⁺ neurons. Double labelling experiments in FTL mice showed that a substantial proportion of the learning activated neurons expressed both pERK1/2 and FTL. These experiments provide clear evidence that the learning specific neurons we identified within LAvl are directly involved in auditory fear conditioning. In addition, learning specific expression of pERK1/2 was found in a dense network of dendrites contained within the border region of the LAvl. This network of dendrites may represent an activated dendritic field involved in fear conditioning in LA.

p38γ MAPK contributes to left ventricular remodeling after pathologic stress and disinhibits calpain through phosphorylation of calpastatin

Despite the high and preferential expression of p38γ MAPK in the myocardium, little is known about its function in the heart. The aim of the current study was to elucidate the physiologic and biochemical roles of p38γ in the heart. Expression and subcellular localization of p38 isoforms was determined in mouse hearts. Comparisons of the cardiac function and structure of wild-type and p38γ knockout (KO) mice at baseline and after abdominal aortic banding demonstrated that KO mice developed less ventricular hypertrophy and that contractile function is better preserved. To identify potential substrates of p38γ, we generated an analog-sensitive mutant to affinity tag endogenous myocardial proteins. Among other proteins, this technique identified calpastatin as a direct p38γ substrate. Moreover, phosphorylation of calpastatin by p38γ impaired its ability to inhibit the protease, calpain. We have identified p38γ as an important determinant of the progression of pathologic cardiac hypertrophy after aortic banding in mice. In addition, we have identified calpastatin, among other substrates, as a novel direct target of p38γ that may contribute to the protection observed in p38γKO mice.-Loonat, A. A., Martin, E. D., Sarafraz-Shekary, N., Tilgner, K., Hertz, N. T., Levin, R., Shokat, K. M., Burlingame, A. L., Arabacilar, P., Uddin, S., Thomas, M., Marber, M. S., Clark, J. E. p38γ MAPK contributes to left ventricular remodeling after pathologic stress and disinhibits calpain through phosphorylation of calpastatin.

p38α MAPK proximity assay reveals a regulatory mechanism of alternative splicing in cardiomyocytes

The p38 mitogen-activated protein kinase (MAPK) signaling pathway is essential for normal heart function. However, p38 also contributes to heart failure pathogenesis by affecting cardiomyocytes contractility and survival. To unravel part of the complex role of p38 in cardiac function, we performed an APEX2-based proximity assay in cultured neonatal rat ventricular myocytes and identified the protein interaction networks (interactomes) of two highly expressed p38 isoforms in the heart. We found that p38α and p38γ have distinct interactomes in cardiomyocytes under both basal and osmotic stress-activated states. Interestingly, the activated p38α interactome contains many RNA-binding proteins implicated in splicing, including the serine/arginine-rich splicing factor 3 (SRSF3). Its interaction with the activated p38α was validated by co-immunoprecipitation. The cytoplasmic abundance and alternative splicing function of SRSF3 are also both modulated by the p38 signaling pathway. Our findings reveal a new function for p38 as a specific regulator of SRSF3 in cardiomyocytes.

Dendritic Tau in Alzheimer's Disease

The microtubule-associated protein tau and amyloid-β (Aβ) are key players in Alzheimer's disease (AD). Aβ and tau are linked in a molecular pathway at the post-synapse with tau-dependent synaptic dysfunction being a major pathomechanism in AD. Recent work on site-specific modification of dendritic and more specifically post-synaptic tau has revealed new endogenous functions of tau that limits synaptic Aβ toxicity. Thus, molecular studies opened a new perspective on tau, placing it at the center of neurotoxic and neuroprotective signaling at the post-synapse. Here, we review recent advances on tau in the dendritic compartments, with implications for understanding and treatment of AD and related neurological conditions.

Cell Type-Specific p38δ Targeting Reveals a Context-, Stage-, and Sex-Dependent Regulation of Skin Carcinogenesis

Activation and/or upregulated expression of p38δ are demonstrated in human skin malignancies including cutaneous squamous cell carcinoma, suggesting a role for p38δ in skin carcinogenesis. We previously reported that mice with germline deletion of the p38δ gene are significantly protected from chemical skin carcinogenesis. Here, we investigated the effects of cell-selective targeted ablation of p38δ in keratinocytes and in immune (myeloid) cells on skin tumor development in a two-stage 7,12-dimethylbenz(a)anthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA) chemical mouse skin carcinogenesis model. Conditional keratinocyte-specific p38δ ablation (p38δ-cKO^∆K) did not influence the latency, incidence, or multiplicity of chemically-induced skin tumors, but led to increased tumor volume in females during the TPA promotion stage, and reduced malignant progression in males and females relative to their wild-type counterparts. In contrast, conditional myeloid cell-specific p38δ deletion (p38δ-cKO^∆M) inhibited DMBA/TPA-induced skin tumorigenesis in male but not female mice. Thus, tumor onset was delayed, and tumor incidence, multiplicity, and volume were reduced in p38δ-cKO^∆M males compared with control wild-type males. Moreover, the percentage of male mice with malignant tumors was decreased in the p38δ-cKO^∆M group relative to their wild-type counterparts. Collectively, these results reveal that cell-specific p38δ targeting modifies susceptibility to chemical skin carcinogenesis in a context-, stage-, and sex-specific manner.

The Role of Extracellular Signal-Regulated Kinases (ERK) in the Regulation of mGlu5 Receptors in Neurons

The metabotropic glutamate (mGlu) receptor 5 is a G protein-coupled receptor and is densely expressed in the mammalian brain. Like other glutamate receptors, mGlu5 receptors are tightly regulated by posttranslational modifications such as phosphorylation, although underlying mechanisms are incompletely investigated. In this study, we investigated the role of a prime kinase, extracellular signal-regulated kinase 1 (ERK1), in the phosphorylation and regulation of mGlu5 receptors in vitro and in striatal neurons. We found that recombinant ERK1 proteins directly bound to the C-terminal tail (CT) of mGlu5 receptors in vitro. Endogenous ERK1 also interacted with mGlu5 receptor proteins in adult rat striatal neurons in vivo. The kinase showed the ability to phosphorylate mGlu5 receptors. A serine residue in the distal region of mGlu5 CT was found to be a primary phosphorylation site sensitive to ERK1. In functional studies, we found that pharmacological inhibition of ERK with an inhibitor U0126 reduced the efficacy of mGlu5 receptors in stimulating production of cytoplasmic inositol-1,4,5-triphosphate, a major downstream conventional signaling event, in striatal neurons under normal conditions. These results identify mGlu5 as a new biochemical substrate of ERK1. The kinase can interact with and phosphorylate an intracellular domain of mGlu5 receptors in striatal neurons and thereby control its signaling efficacy.

Targeting an oncogenic kinase/phosphatase signaling network for cancer therapy

Protein kinases and phosphatases signal by phosphorylation and dephosphorylation to precisely control the activities of their individual and common substrates for a coordinated cellular outcome. In many situations, a kinase/phosphatase complex signals dynamically in time and space through their reciprocal regulations and their cooperative actions on a substrate. This complex may be essential for malignant transformation and progression and can therefore be considered as a target for therapeutic intervention. p38γ is a unique MAPK family member that contains a PDZ motif at its C-terminus and interacts with a PDZ domain-containing protein tyrosine phosphatase PTPH1. This PDZ-coupled binding is required for both PTPH1 dephosphorylation and inactivation of p38γ and for p38γ phosphorylation and activation of PTPH1. Moreover, the p38γ/PTPH1 complex can further regulate their substrates phosphorylation and dephosphorylation, which impacts Ras transformation, malignant growth and progression, and therapeutic response. This review will use the p38γ/PTPH1 signaling network as an example to discuss the potential of targeting the kinase/phosphatase signaling complex for development of novel targeted cancer therapy.

The role of p38 MAPK pathway in p53 compromised state and telomere mediated DNA damage response

There is an intricate balance of DNA damage response and repair which determines the homeostasis of human genome function. p53 protein is widely known for its role in cell cycle regulation and tumor suppressor activity. In case of several cancers where function of p53 gene gets compromised either by mutation or partial inactivation, the role of p53 in response to DNA damage needs to be supplemented by another molecule or pathway. Due to sedentary lifestyle and exposure to genotoxic agents, genome is predisposed to chronic stress, which ultimately leads to unrepaired or background DNA damage. p38 MAPK signaling pathway is strongly activated in response to various environmental and cellular stresses. DNA damage response and the repair options have crucial links with chromosomal integrity. Telomere that regulates integrity of genome is protected by a six member shielding unit called shelterin complex which communicates with other pathways for functionality of telomeres. There are evidences that p38 gets activated through ATM in response to DNA damage. Dysfunctional telomere leads to activation of ATM which subsequently activates p38 suggesting a crosstalk between p38, ATM and shelterin complex. This review focuses on activation of p38 in response to genotoxic stress induced DNA damage in p53 mutated or compromised state and its possible cross talk with telomere shelterin proteins. Thus p38 may act as an important target to treat various diseases and in majority of cancers in p53 mutated state.

p38γ and p38δ: From Spectators to Key Physiological Players

Although the physiological roles of p38γ and p38δ signalling pathways are largely unknown, new genetic and pharmacological tools are providing groundbreaking information on the function of these two stress-activated protein kinases. Recent studies show the importance of p38γ and p38δ in the regulation of processes as diverse as cytokine production, protein synthesis, exocytosis, cell migration, gene expression, and neuron activity, which have an acute impact on the development of pathologies related to inflammation, diabetes, neurodegeneration, and cancer. These recent breakthroughs are resolving some of the questions that have long been asked regarding the function of p38γ and p38δ in biology and pathology.

Synaptic ERK2 Phosphorylates and Regulates Metabotropic Glutamate Receptor 1 In Vitro and in Neurons

A synaptic pool of extracellular signal-regulated kinases (ERK) controls synaptic transmission, although little is known about its underlying signaling mechanisms. Here, we found that synaptic ERK2 directly binds to postsynaptic metabotropic glutamate receptor 1a (mGluR1a). This binding is direct and the ERK-binding site is located in the intracellular C-terminus (CT) of mGluR1a. Parallel with this binding, ERK2 phosphorylates mGluR1a at a cluster of serine residues in the distal part of mGluR1a-CT. In rat cerebellar neurons, ERK2 interacts with mGluR1a at synaptic sites, and active ERK constitutively phosphorylates mGluR1a under normal conditions. This basal phosphorylation is critical for maintaining adequate surface expression of mGluR1a. ERK is also essential for controlling mGluR1a signaling in triggering distinct postreceptor signaling transduction pathways. In summary, we have demonstrated that mGluR1a is a sufficient substrate of ERK2. ERK that interacts with and phosphorylates mGluR1a is involved in the regulation of the trafficking and signaling of mGluR1.

Keratinocyte p38δ loss inhibits Ras-induced tumor formation, while systemic p38δ loss enhances skin inflammation in the early phase of chemical carcinogenesis in mouse skin

p38δ expression and/or activity are increased in human cutaneous malignancies, including invasive squamous cell carcinoma (SCC) and head and neck SCC, but the role of p38δ in cutaneous carcinogenesis has not been well-defined. We have reported that mice with germline loss of p38δ exhibited a reduced susceptibility to skin tumor development compared with wild-type mice in the two-stage 7,12-dimethylbenz(a)anthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA) chemical skin carcinogenesis model. Here, we report that p38δ gene ablation inhibited the growth of tumors generated from v-ras(Ha) -transformed keratinocytes in skin orthografts to nude mice, indicating that keratinocyte-intrinsic p38δ is required for Ras-induced tumorigenesis. Gene expression profiling of v-ras(Ha) -transformed p38δ-null keratinocytes revealed transcriptional changes associated with cellular responses linked to tumor suppression, such as reduced proliferation and increased differentiation, cell adhesion, and cell communications. Notably, a short-term DMBA/TPA challenge, modeling the initial stages of chemical skin carcinogenesis treatment, elicited an enhanced inflammation in p38δ-null skin compared with skin of wild-type mice, as assessed by measuring the expression of pro-inflammatory cytokines, including IL-1β, IL-6, IL-17, and TNFα. Additionally, p38δ-null skin and p38δ-null keratinocytes exhibited increased p38α activation and signaling in response to acute inflammatory challenges, suggesting a role for p38α in stimulating the elevated inflammatory response in p38δ-null skin during the initial phases of the DMBA/TPA treatment compared with similarly treated p38δ(+/+) skin. Altogether, our results indicate that p38δ signaling regulates skin carcinogenesis not only by keratinocyte cell-autonomous mechanisms, but also by influencing the interaction between between the epithelial compartment of the developing skin tumor and its stromal microenvironment.

p38γ and p38δ Mitogen Activated Protein Kinases (MAPKs), New Stars in the MAPK Galaxy

The protein kinases p38γ and p38δ belong to the p38 mitogen-activated protein kinase (MAPK) family. p38MAPK signaling controls many cellular processes and is one of the most conserved mechanisms in eukaryotes for the cellular response to environmental stress and inflammation. Although p38γ and p38δ are widely expressed, it is likely that they perform specific functions in different tissues. Their involvement in human pathologies such as inflammation-related diseases or cancer is starting to be uncovered. In this article we give a general overview and highlight recent advances made in defining the functions of p38γ and p38δ, focusing in innate immunity and inflammation. We consider the potential of the pharmacological targeting of MAPK pathways to treat autoimmune and inflammatory diseases and cancer.

PSD-95 regulates CRFR1 localization, trafficking and β-arrestin2 recruitment

Corticotropin-releasing factor (CRF) is a neuropeptide commonly associated with the hypothalamic-pituitary adrenal axis stress response. Upon release, CRF activates two G protein-coupled receptors (GPCRs): CRF receptor 1 (CRFR1) and CRF receptor 2 (CRFR2). Although both receptors contribute to mood regulation, CRFR1 antagonists have demonstrated anxiolytic and antidepressant-like properties that may be exploited in the generation of new pharmacological interventions for mental illnesses. Previous studies have demonstrated CRFR1 capable of heterologously sensitizing serotonin 2A receptor (5-HT2AR) signaling: another GPCR implicated in psychiatric disease. Interestingly, this phenomenon was dependent on Postsynaptic density 95 (PSD-95)/Disc Large/Zona Occludens (PDZ) interactions on the distal carboxyl termini of both receptors. In the current study, we demonstrate that endogenous PSD-95 can be co-immunoprecipitated with CRFR1 from cortical brain homogenate, and this interaction appears to be primarily via the PDZ-binding motif. Additionally, PSD-95 colocalizes with CRFR1 within the dendritic projections of cultured mouse neurons in a PDZ-binding motif-dependent manner. In HEK 293 cells, PSD-95 overexpression inhibited CRFR1 endocytosis, whereas PSD-95 shRNA knockdown enhanced CRFR1 endocytosis. Although PSD-95 does not appear to play a significant role in CRF-mediated cAMP or ERK1/2 signaling, PSD-95 was demonstrated to suppress β-arrestin2 recruitment: providing a potential mechanism for PSD-95's inhibition of endocytosis. In revisiting previously documented heterologous sensitization, PSD-95 shRNA knockdown did not prevent CRFR1-mediated enhancement of 5-HT2AR signaling. In conclusion, we have identified and characterized a novel functional relationship between CRFR1 and PSD-95 that may have implications in the design of new treatment strategies for mental illness.

Posttranslational Modifications Regulate the Postsynaptic Localization of PSD-95

The postsynaptic density (PSD) consists of a lattice-like array of interacting proteins that organizes and stabilizes synaptic receptors, ion channels, structural proteins, and signaling molecules required for normal synaptic transmission and synaptic function. The scaffolding and hub protein postsynaptic density protein-95 (PSD-95) is a major element of central chemical synapses and interacts with glutamate receptors, cell adhesion molecules, and cytoskeletal elements. In fact, PSD-95 can regulate basal synaptic stability as well as the activity-dependent structural plasticity of the PSD and, therefore, of the excitatory chemical synapse. Several studies have shown that PSD-95 is highly enriched at excitatory synapses and have identified multiple protein structural domains and protein-protein interactions that mediate PSD-95 function and trafficking to the postsynaptic region. PSD-95 is also a target of several signaling pathways that induce posttranslational modifications, including palmitoylation, phosphorylation, ubiquitination, nitrosylation, and neddylation; these modifications determine the synaptic stability and function of PSD-95 and thus regulate the fates of individual dendritic spines in the nervous system. In the present work, we review the posttranslational modifications that regulate the synaptic localization of PSD-95 and describe their functional consequences. We also explore the signaling pathways that induce such changes.

p38γ and δ promote heart hypertrophy by targeting the mTOR-inhibitory protein DEPTOR for degradation

Disrupted organ growth leads to disease development. Hypertrophy underlies postnatal heart growth and is triggered after stress, but the molecular mechanisms involved in these processes are largely unknown. Here we show that cardiac activation of p38γ and p38δ increases during postnatal development and by hypertrophy-inducing stimuli. p38γ/δ promote cardiac hypertrophy by phosphorylating the mTORC1 and mTORC2 inhibitor DEPTOR, which leads to its degradation and mTOR activation. Hearts from mice lacking one or both kinases are below normal size, have high levels of DEPTOR, low activity of the mTOR pathway and reduced protein synthesis. The phenotype of p38γ/δ^−/− mice is reverted by overactivation of mTOR with amino acids, shRNA-mediated knockdown of Deptor, or cardiomyocyte overexpression of active p38γ and p38δ. Moreover, in WT mice, heart weight is reduced by cardiac overexpression of DEPTOR. Our results demonstrate that p38γ/δ control heart growth by modulating mTOR pathway through DEPTOR phosphorylation and subsequent degradation.

mTOR signalling pathway is a critical regulator of cardiac hypertrophy. Here the authors show that two kinases, p38γ and p38δ, control heart growth by promoting mTOR activity via phosphorylation and consequent proteasome degradation of mTOR inhibitor DEPTOR, extending our knowledge of cardiac hypertrophy regulation.

Regulation of Group I Metabotropic Glutamate Receptors by MAPK/ERK in Neurons

Group I metabotropic glutamate receptors (mGluR1 and mGluR5 subtypes) are regulated by protein kinases. A recent focus is mitogen-activated protein kinases (MAPK). A prototypic subclass of MAPKs, extracellular signal-regulated kinases (ERK), is densely expressed in adult brain postmitotic neurons. This kinase resides in not only the cytoplasm around the nucleus, also the neuronal peripheral structures such as synapses. Recombinant ERK2 binds to C terminal tails of mGluR1a in vitro and native ERK1/2 forms complexes with mGluR1/5 in neurons in vivo. Association of ERK with mGluR1/5 enables the kinase to phosphorylate mGluR1/5 at a cluster of serine sites in the distal C terminus, including a serine residue within the Homer binding site. The ERK-mediated phosphorylation of mGluR1/5 promotes surface expression of mGluR1a in cerebellar neurons. ERK also regulates mGluR1/5 signaling and functions. Among different functional outputs surveyed, ERK exerts an output-specific role in either potentiating or inhibiting their activities. In sum, synaptic group I mGluRs are sufficient substrates of MAPK/ERK. Phosphorylation of mGluR1/5 by ERK has a significant impact on subcellular expression and function of phospho-modified receptors.

Synaptically Localized Mitogen-Activated Protein Kinases: Local Substrates and Regulation

Mitogen-activated protein kinases (MAPKs) are expressed in postmitotic neurons and act as important regulators in intracellular signaling. In addition to their nuclear distribution and roles in regulating gene expression, MAPKs, especially the extracellular signal-regulated kinase (ERK) subclass, reside in peripheral dendritic spines and synapses, including the postsynaptic density (PSD) microdomain. This peripheral pool of MAPKs/ERKs is either constitutively active or sensitive to changing synaptic input. Active MAPKs directly interact with and phosphorylate local substrates to alter their trafficking and subcellular/subsynaptic distributions, through which MAPKs regulate function of substrates and contribute to long-lasting synaptic plasticity. A number of physiologically relevant substrates of MAPKs have been identified at synaptic sites. Central among them are key synaptic scaffold proteins (PSD-95 and PSD-93), cadherin-associated proteins (δ-catenin), Kv4.2 K⁺ channels, and metabotropic glutamate receptors. Through a reversible phosphorylation event, MAPKs rapidly and efficiently modulate the function of these substrates and thus determine the strength of synaptic transmission. This review summarizes the recent progress in cell biology of synaptic MAPKs and analyzes roles of this specific pool of MAPKs in regulating local substrates and synaptic plasticity.

Regulation of synaptic MAPK/ERK phosphorylation in the rat striatum and medial prefrontal cortex by dopamine and muscarinic acetylcholine receptors

Dopamine and acetylcholine are two principal transmitters in the striatum and are usually balanced to modulate local neural activity and to maintain striatal homeostasis. This study investigates the role of dopamine and muscarinic acetylcholine receptors in the regulation of a central signaling protein, i.e., the mitogen-activated protein kinase (MAPK). We focus on the synaptic pool of MAPKs because of the fact that these kinases reside in peripheral synaptic structures in addition to their somatic locations. We show that a systemic injection of dopamine D1 receptor (D1R) agonist SKF81297 enhances phosphorylation of extracellular signal-regulated kinases (ERKs), a prototypic subclass of MAPKs, in the adult rat striatum. Similar results were observed in another dopamine-responsive region, the medial prefrontal cortex (mPFC). The dopamine D2 receptor agonist quinpirole had no such effects. Pretreatment with a positive allosteric modulator (PAM) of muscarinic acetylcholine M4 receptors (M4Rs), VU0152100, attenuated the D1R agonist-stimulated ERK phosphorylation in the two regions, whereas the PAM itself did not alter basal ERK phosphorylation. All drug treatments had no effect on phosphorylation of c-Jun N-terminal kinases (JNKs), another MAPK subclass, in the striatum and mPFC. These results demonstrate that dopamine and acetylcholine are integrated to control synaptic ERK but not JNK activation in striatal and mPFC neurons in vivo. Activation of M4Rs exerts an inhibitory effect on the D1R-mediated upregulation of synaptic ERK phosphorylation.

p38 MAPK in cardioprotection - are we there yet?

PKs transfer a phosphate from ATP to the side-chain hydroxyl group of a serine, threonine or tyrosine residue of a substrate protein. This in turn can alter that protein's function; modulating fundamental cellular processes including, metabolism, transcription, growth, division, differentiation, motility and survival. PKs are subdivided into families based on homology. One such group are the stress-activated kinases, which as the name suggests, are activated in response to cellular stresses such as toxins, cytokines, mechanical deformation and osmotic stress. Members include the p38 MAPK family, which is composed of α, β, γ and δ, isoforms which are encoded by separate genes. These kinases transduce extracellular signals and coordinate the cellular responses needed for adaptation and survival. However, in cardiovascular and other disease states, these same systems can trigger maladaptive responses that aggravate, rather than alleviate, the disease. This situation is analogous to adrenergic, angiotensin and aldosterone signalling in heart failure, where inhibition is beneficial despite the importance of these hormones to homeostasis. The question is whether similar benefits could accrue from p38 inhibition? In this review, we will discuss the structure and function of p38, the history of p38 inhibitors and their use in preclinical studies. Finally, we will summarize the results of recent cardiovascular clinical trials with p38 inhibitors.

Activation of pro-survival CaMK4β/CREB and pro-death MST1 signaling at early and late times during a mouse model of prion disease

Background

The signaling pathways most critical to prion disease pathogenesis are as yet incompletely characterized. We have developed a kinomics approach to identify signaling pathways that are dysregulated during prion pathogenesis. The approach is sensitive and specific enough to detect signaling pathways dysregulated in a simple in vitro model of prion pathogenesis. Here, we used this approach to identify signaling pathways dysregulated during prion pathogenesis in vivo.

Methods

Mice intraperitoneally infected with scrapie (strain RML) were euthanized at 70, 90, 110, 130 days post-infection (dpi) or at terminal stages of disease (155–190 dpi). The levels of 139 protein kinases in brainstem-cerebellum homogenates were analyzed by multiplex Western blots, followed by hierarchical clustering and analyses of activation states.

Results

Hierarchical and functional clustering identified CaMK4β and MST1 signaling pathways as potentially dysregulated. Targeted analyses revealed that CaMK4β and its downstream substrate CREB, which promotes neuronal survival, were activated at 70 and 90 dpi in cortical, subcortical and brainstem-cerebellum homogenates from scrapie-infected mice. The activation levels of CaMK4β/CREB signaling returned to those in mock-infected mice at 110 dpi, whereas MST1, which promotes neuronal death, became activated at 130 dpi.

Conclusion

Pro-survival CaMK4β/CREB signaling is activated in mouse scrapie at earlier times and later inhibited, whereas pro-death MST1 signaling is activated at these later times.

Electronic supplementary material

The online version of this article (doi:10.1186/1743-422X-11-160) contains supplementary material, which is available to authorized users.

Role of SAP97 protein in the regulation of corticotropin-releasing factor receptor 1 endocytosis and extracellular signal-regulated kinase 1/2 signaling

The corticotropin-releasing factor (CRF) receptor 1 (CRFR1) is a target for the treatment of psychiatric diseases such as depression, schizophrenia, anxiety disorder, and bipolar disorder. The carboxyl-terminal tail of the CRFR1 terminates in a PDZ-binding motif that provides a potential site for the interaction of PSD-95/Discs Large/Zona Occludens 1 (PDZ) domain-containing proteins. In this study, we found that CRFR1 interacts with synapse-associated protein 97 (SAP97; also known as DLG1) by co-immunoprecipitation in human embryonic 293 (HEK 293) cells and cortical brain lysates and that this interaction is dependent upon an intact PDZ-binding motif at the end of the CRFR1 carboxyl-terminal tail. Similarly, we demonstrated that SAP97 is recruited to the plasma membrane in HEK 293 cells expressing CRFR1 and that mutation of the CRFR1 PDZ-binding motif results in the redistribution of SAP97 into the cytoplasm. Overexpression of SAP97 antagonized agonist-stimulated CRFR1 internalization, whereas single hairpin (shRNA) knockdown of endogenous SAP97 in HEK 293 cells resulted in increased agonist-stimulated CRFR1 endocytosis. CRFR1 was internalized as a complex with SAP97 resulting in the redistribution of SAP97 to endocytic vesicles. Overexpression or shRNA knockdown of SAP97 did not significantly affect CRFR1-mediated cAMP formation, but SAP97 knockdown did significantly attenuate CRFR1-stimulated ERK1/2 phosphorylation in a PDZ interaction-independent manner. Taken together, our studies show that SAP97 interactions with CRFR1 attenuate CRFR1 endocytosis and that SAP97 is involved in coupling G protein-coupled receptors to the activation of the ERK1/2 signaling pathway.

Polarity protein complex Scribble/Lgl/Dlg and epithelial cell barriers

The Scribble polarity complex or module is one of the three polarity modules that regulate cell polarity in multiple epithelia including blood-tissue barriers. This protein complex is composed of Scribble, Lethal giant larvae (Lgl) and Discs large (Dlg), which are well conserved across species from fruitflies and worms to mammals. Originally identified in Drosophila and C. elegans where the Scribble complex was found to work with the Par-based and Crumbs-based polarity modules to regulate apicobasal polarity and asymmetry in cells and tissues during embryogenesis, their mammalian homologs have all been identified in recent years. Components of the Scribble complex are known to regulate multiple cellular functions besides cell polarity, which include cell proliferation, assembly and maintenance of adherens junction (AJ) and tight junction (TJ), and they are also tumor suppressors. Herein, we provide an update on the Scribble polarity complex and how this protein complex modulates cell adhesion with some emphasis on its role in Sertoli cell blood-testis barrier (BTB) function. It should be noted that this is a rapidly developing field, in particular the role of this protein module in blood-tissue barriers, and this short chapter attempts to provide the information necessary for investigators studying reproductive biology and blood-tissue barriers to design future studies. We also include results of recent studies from flies and worms since this information will be helpful in planning experiments for future functional studies in the testis to understand how Scribble-based proteins regulate BTB dynamics and spermatogenesis.

Eukaryotic elongation factor 2 controls TNF-α translation in LPS-induced hepatitis

Bacterial LPS (endotoxin) has been implicated in the pathogenesis of acute liver disease through its induction of the proinflammatory cytokine TNF-α. TNF-α is a key determinant of the outcome in a well-established mouse model of acute liver failure during septic shock. One possible mechanism for regulating TNF-α expression is through the control of protein elongation during translation, which would allow rapid cell adaptation to physiological changes. However, the regulation of translational elongation is poorly understood. We found that expression of p38γ/δ MAPK proteins is required for the elongation of nascent TNF-α protein in macrophages. The MKK3/6-p38γ/δ pathway mediated an inhibitory phosphorylation of eukaryotic elongation factor 2 (eEF2) kinase, which in turn promoted eEF2 activation (dephosphorylation) and subsequent TNF-α elongation. These results identify a new signaling pathway that regulates TNF-α production in LPS-induced liver damage and suggest potential cell-specific therapeutic targets for liver diseases in which TNF-α production is involved.

Amphetamine increases phosphorylation of MAPK/ERK at synaptic sites in the rat striatum and medial prefrontal cortex

Mitogen-activated protein kinases (MAPKs) play a central role in cell signaling. Extracellular signal-regulated kinase (ERK) is a prototypic subclass of MAPKs and is densely expressed in postmitotic neurons of adult mammalian brains. Active ERK translocates into the nucleus to regulate gene expression. Additionally, ERK is visualized in neuronal peripheries, such as distal synaptic structures. While nuclear ERK is a known sensitive target of psychostimulants, little is known about the responsiveness of synaptic ERK to stimulants. In this study, we focused on ERK at synaptic versus extrasynaptic sites and investigated its responses to the psychostimulant amphetamine in the adult rat striatum and medial prefrontal cortex (mPFC) in vivo. We used a pre-validated biochemical fractionation procedure to isolate synapse- and extrasynapse-enriched membranes. We found that two common ERK isoforms (ERK1 and ERK2) were concentrated more in extrasynaptic fractions than in synaptic fractions in striatal and cortical neurons under normal conditions. At synaptic sites, ERK2 was noticeably more abundant than ERK1. Acute injection of amphetamine induced an increase in ERK2 phosphorylation in the synaptic fraction of striatal neurons, while the drug did not alter extrasynaptic ERK2 phosphorylation. Similar results were observed in the mPFC. In both synaptic and extrasynaptic compartments, total ERK1/2 proteins remained stable in response to amphetamine. Our data establish the subsynaptic distribution pattern of MAPK/ERK in striatal and cortical neurons. Moreover, the synaptic pool of ERK2 in these neurons can be selectively activated by amphetamine.

p38γ Mitogen-activated protein kinase signals through phosphorylating its phosphatase PTPH1 in regulating ras protein oncogenesis and stress response

Phosphatase plays a crucial role in determining cellular fate by inactivating its substrate kinase, but it is not known whether a kinase can vice versa phosphorylate its phosphatase to execute this function. Protein-tyrosine phosphatase H1 (PTPH1) is a specific phosphatase of p38γ mitogen-activated protein kinase (MAPK) through PDZ binding, and here, we show that p38γ is also a PTPH1 kinase through which it executes its oncogenic activity and regulates stress response. PTPH1 was identified as a substrate of p38γ by unbiased proteomic analysis, and its resultant phosphorylation at Ser-459 occurs in vitro and in vivo through their complex formation. Genetic and pharmacological analyses showed further that Ser-459 phosphorylation is directly regulated by Ras signaling and is important for Ras, p38γ, and PTPH1 oncogenic activity. Moreover, experiments with physiological stimuli revealed a novel stress pathway from p38γ to PTPH1/Ser-459 phosphorylation in regulating cell growth and cell death by a mechanism dependent on cellular environments but independent of canonical MAPK activities. These results thus reveal a new mechanism by which a MAPK regulates Ras oncogenesis and stress response through directly phosphorylating its phosphatase.

Palmitoylation by DHHC5/8 targets GRIP1 to dendritic endosomes to regulate AMPA-R trafficking

Palmitoylation, a key regulatory mechanism controlling protein targeting, is catalyzed by DHHC-family palmitoyl acyltransferases (PATs). Impaired PAT activity is linked to neurodevelopmental and neuropsychiatric disorders, suggesting critical roles for palmitoylation in neuronal function. However, few substrates for specific PATs are known, and functional consequences of palmitoylation events are frequently uncharacterized. Here, we identify the closely related PATs DHHC5 and DHHC8 as specific regulators of the PDZ domain protein GRIP1b. Binding, palmitoylation, and dendritic targeting of GRIP1b require a PDZ ligand unique to DHHC5/8. Palmitoylated GRIP1b is targeted to trafficking endosomes and may link endosomes to kinesin motors. Consistent with this trafficking role, GRIP1b's palmitoylation turnover rate approaches the highest of all reported proteins, and palmitoylation increases GRIP1b's ability to accelerate AMPA-R recycling. To our knowledge, these findings identify the first neuronal DHHC5/8 substrate, define novel mechanisms controlling palmitoylation specificity, and suggest further links between dysregulated palmitoylation and neuropathological conditions.

New Insights into the p38γ and p38δ MAPK Pathways

The mammalian p38 mitogen-activated protein kinases (MAPKs) family is composed of four members (p38α, p38β, p38γ, and p38δ), which are very similar in amino acid sequence but differ in their expression patterns. This suggests that they may have specific functions in different organs. In the last years most of the effort has been centred on the study of the function of the p38α isoform, which is widely referred to as p38 in the literature. However, the role that other p38 isoforms play in cellular functions and their implication in some of the pathological conditions have not been precisely defined so far. In this paper we highlight recent advances made in defining the functions of the two less studied alternative p38MAPKs, p38γ and p38δ. We describe that these p38MAPKs show similarities to the classical p38α isoform, although they may play central and distinct role in certain physiological and pathological processes.

Electroacupuncture at PC6 (Neiguan) Improves Extracellular Signal-Regulated Kinase Signaling Pathways Through the Regulation of Neuroendocrine Cytokines in Myocardial Hypertrophic Rats

Electroacupuncture (EA) therapy has been widely accepted as a useful therapeutic technique with low or no risk in the clinical prevention of cardiac hypertrophy. However, the signaling transduction mechanism underlying this effect remains unclear. The current study investigates the effects of EA on the signaling pathways of myocardial hypertrophy (MH) in rats. Up to 40 3-month-old Sprague-Dawley (SD) rats were randomly divided into normal, model, PC6 (Neiguan), and LI4 (Hegu) groups, with ten rats in each group. All the rats except for the normal group received 3 mg/kg·d of isoprinosine hydrochloride (ISO) injection into the back skin. The rats in the PC6 and LI4 groups received EA for 14 days. On the 15th day, electrocardiograms were recorded, and the ultrastructure of the myocardial cells was observed. The myocardial hypertrophy indices (MHIs), electrocardiograph (ECG), ultrastructure observation, levels of plasma angiotensin II (Ang II) and endothelin (ET), as well as protein expression of extracellular signal-regulated kinase (ERK), and phosphorylation extracellular signal regulating kinase (p-ERK) in the left ventricular myocardial tissue were measured. The results indicated that EA can improve cardiac function in MH rats by modulating upstream neuroendocrine cytokines that regulate the ERK signaling pathways.

Identification of AKAP79 as a protein phosphatase 1 catalytic binding protein

The ubiquitously expressed and highly promiscuous protein phosphatase 1 (PP1) regulates many cellular processes. Targeting PP1 to specific locations within the cell allows for the regulation of PP1 by conferring substrate specificity. In the present study, we identified AKAP79 as a novel PP1 regulatory subunit. Immunoprecipitaiton of the AKAP from rat brain extract found that the PP1 catalytic subunit copurified with the anchoring protein. This is a direct interaction, demonstrated by pulldown experiments using purified proteins. Interestingly, the addition of AKAP79 to purified PP1 catalytic subunit decreased phosphatase activity with an IC(50) of 811 ± 0.56 nM of the anchoring protein. Analysis of AKAP79 identified a PP1 binding site that conformed to a consensus PP1 binding motif (FxxR/KxR/K) in the first 44 amino acids of the anchoring protein. This was confirmed when a peptide mimicking this region of AKAP79 was able to bind PP1 by both pulldown assay and surface plasmon resonance. However, PP1 was still able to bind to AKAP79 upon deletion of this region, suggesting additional sites of contact between the anchoring protein and the phosphatase. Importantly, this consensus PP1 binding motif was found not to be responsible for PP1 inhibition, but rather enhanced phosphatase activity, as deletion of this domain resulted in an increased inhibition of PP1 activity. Instead, a second interaction domain localized to residues 150-250 of AKAP79 was required for the inhibition of PP1. However, the inhibitory actions of AKAP79 on PP1 are substrate dependent, as the anchoring protein did not inhibit PP1 dephosphorylation of phospho-PSD-95, a substrate found in AKAP79 complexes in the brain. These combined observations suggest that AKAP79 acts as a PP1 regulatory subunit that can direct PP1 activity toward specific targets in the AKAP79 complex.

Scaffolds are 'active' regulators of signaling modules

Signaling cascades, in addition to proteins with obvious signaling-relevant activities (e.g. protein kinases or receptors), also employ dedicated 'inactive' proteins whose functions appear to be the organization of the former components into higher order complexes through protein-protein interactions. The core function of signaling adaptors, anchors and scaffolds is the recruitment of proteins into one macromolecular complex. Several recent studies have demonstrated that the recruiter and the recruited molecules mutually influence each other in a scaffolded complex. This yields fundamentally novel properties for the signaling complex as a whole. Because these are not merely additive to the properties of the individual components, scaffolded signaling complexes may behave as functionally distinct modules.

Signaling endosomes trigger synapse assembly

In this issue of Neuron, Sharma et al. report that target-derived NGF exerts long-distance control of postsynaptic assembly. A major advance is the discovery that TrkA signaling endosomes are retrogradely transported to neuronal dendrites and directly trigger the clustering of postsynaptic density proteins.

ERK5 pathway regulates the phosphorylation of tumour suppressor hDlg during mitosis

Human disc-large (hDlg) is a scaffold protein critical for the maintenance of cell polarity and adhesion. hDlg is thought to be a tumour suppressor that regulates the cell cycle and proliferation. However, the mechanism and pathways involved in hDlg regulation during these processes is still unclear. Here we report that hDlg is phosphorylated during mitosis, and we establish the identity of at least three residues phosphorylated in hDlg; some are previously unreported. Phosphorylation affects hDlg localisation excluding it from the contact point between the two daughter cells. Our results reveal a previously unreported pathway for hDlg phosphorylation in mitosis and show that ERK5 pathway mediates hDlg cell cycle dependent phosphorylation. This is likely to have important implications in the correct timely mitotic entry and mitosis progression.