Cain, a novel physiologic protein inhibitor of calcineurin

Comparative in situ hybridization protocols in zebrafish

In situ hybridization is a commonly used technique in molecular biology to assess the temporal and spatial expression of a given gene. As a long and labor-intensive protocol, double in situ hybridization, which detects two genes in series, is challenging and can require a lot of troubleshooting. Optional additives, polyvinyl alcohol and dextran sulfate, were tested in a standard in situ hybridization protocol and several colorimetric stain pairings using double in situ hybridization in zebrafish embryos. Optional additives can improve staining time and reduce nonspecific background. Nitro-blue tetrazolium chloride/5-bromo-4-chloro-3-indolyl phosphate (BCIP) + Fast Red/BCIP was the most effective stain pairing. As a proof-of-concept, this work shows that Cabin1 and atoh1b are expressed in distinct regions of the developing zebrafish brain.

A comparison of colorimetric stains and protocols in double in situ hybridization in whole-mount zebrafish embryos.

Cabin1 domain-containing gene picd-1 interacts with pry-1/Axin to regulate multiple processes in Caenorhabditis elegans

The Axin family of scaffolding proteins control diverse processes, such as facilitating the interactions between cellular components and providing specificity to signaling pathways. While several Axin family members have been discovered in metazoans and shown to play crucial roles, their mechanism of action are not well understood. The Caenorhabditis elegans Axin homolog, pry-1, is a powerful tool for identifying interacting genes and downstream effectors that function in a conserved manner to regulate Axin-mediated signaling. Our lab and others have established pry-1's essential role in developmental processes that affect the reproductive system, seam cells, and a posterior P lineage cell, P11.p. Additionally, pry-1 is crucial for lipid metabolism, stress responses, and aging. In this study, we expanded on our previous work on pry-1 by reporting a novel interacting gene named picd-1 (pry-1-interacting and Cabin1 domain-containing). PICD-1 protein shares sequence conservation with CABIN1, a component of the HUCA complex. Our findings have revealed that PICD-1 is involved in several pry-1-mediated processes, including stress response and lifespan maintenance. picd-1's expression overlapped with that of pry-1 in multiple tissues throughout the lifespan. Furthermore, PRY-1 and PICD-1 inhibited CREB-regulated transcriptional coactivator homolog CRTC-1, which promotes longevity in a calcineurin-dependent manner. Overall, our study has demonstrated that picd-1 is necessary for mediating pry-1 function and provides the basis to investigate whether Cabin-1 domain-containing protein plays a similar role in Axin signaling in other systems.

Decoding the Phosphatase Code: Regulation of Cell Proliferation by Calcineurin

Calcineurin, a calcium-dependent serine/threonine phosphatase, integrates the alterations in intracellular calcium levels into downstream signaling pathways by regulating the phosphorylation states of several targets. Intracellular Ca²⁺ is essential for normal cellular physiology and cell cycle progression at certain critical stages of the cell cycle. Recently, it was reported that calcineurin is activated in a variety of cancers. Given that abnormalities in calcineurin signaling can lead to malignant growth and cancer, the calcineurin signaling pathway could be a potential target for cancer treatment. For example, NFAT, a typical substrate of calcineurin, activates the genes that promote cell proliferation. Furthermore, cyclin D1 and estrogen receptors are dephosphorylated and stabilized by calcineurin, leading to cell proliferation. In this review, we focus on the cell proliferative functions and regulatory mechanisms of calcineurin and summarize the various substrates of calcineurin. We also describe recent advances regarding dysregulation of the calcineurin activity in cancer cells. We hope that this review will provide new insights into the potential role of calcineurin in cancer development.

A Review of Calcineurin Biophysics with Implications for Cardiac Physiology

Calcineurin, also known as protein phosphatase 2B, is a heterodimeric serine threonine phosphatase involved in numerous signaling pathways. During the past 50 years, calcineurin has been the subject of extensive investigation. Many of its cellular and physiological functions have been described, and the underlying biophysical mechanisms are the subject of active investigation. With the abundance of techniques and experimental designs utilized to study calcineurin and its numerous substrates, it is difficult to reconcile the available information. There have been a plethora of reports describing the role of calcineurin in cardiac disease. However, a physiological role of calcineurin in healthy cardiomyocyte function requires clarification. Here, we review the seminal biophysical and structural details that are responsible for the molecular function and inhibition of calcineurin. We then focus on literature describing the roles of calcineurin in cardiomyocyte physiology and disease.

Tacrolimus ameliorates podocyte injury by restoring FK506 binding protein 12 (FKBP12) at actin cytoskeleton

FKBP12 was identified as a binding protein of tacrolimus (Tac). Tac binds to FKBP12 and exhibits immunosuppressive effects in T cells. Although it is reported that Tac treatment directly ameliorates the dysfunction of the podocyte in nephrotic syndrome, the precise pharmacological mechanism of Tac is not well understood yet. It is also known that FKBP12 functions independently of Tac. However, the localization and the physiological function of FKBP12 are not well elucidated. In this study, we observed that FKBP12 is highly expressed in glomeruli, and the FKBP12 in glomeruli is restricted in podocytes. FKBP12 in cultured podocytes was expressed along the actin cytoskeleton and associated with filamentous actin (F-actin). FKBP12 interacted with the actin-associated proteins 14-3-3 and synaptopodin. RNA silencing for FKBP12 reduced 14-3-3 expression, F-actin staining, and process formation in cultured podocytes. FKBP12 expression was decreased in the nephrotic model caused by adriamycin (ADR) and the cultured podocyte treated with ADR. The process formation was deteriorated in the podocytes treated with ADR. Tac treatment ameliorated these decreases. Tac treatment to the normal cells increased the expression of FKBP12 at F-actin in processes and enhanced process formation. Tac enhanced the interaction of FKBP12 with synaptopodin. These observations suggested that FKBP12 at actin cytoskeleton participates in the maintenance of processes, and Tac treatment ameliorates podocyte injury by restoring FKBP12 at actin cytoskeleton.

Calcineurin Participation in Hebbian and Homeostatic Plasticity Associated With Extinction

In nature, animals need to adapt to constant changes in their environment. Learning and memory are cognitive capabilities that allow this to happen. Extinction, the reduction of a certain behavior or learning previously established, refers to a very particular and interesting type of learning that has been the basis of a series of therapies to diminish non-adaptive behaviors. In recent years, the exploration of the cellular and molecular mechanisms underlying this type of learning has received increasing attention. Hebbian plasticity (the activity-dependent modification of the strength or efficacy of synaptic transmission), and homeostatic plasticity (the homeostatic regulation of plasticity) constitute processes intimately associated with memory formation and maintenance. Particularly, long-term depression (LTD) has been proposed as the underlying mechanism of extinction, while the protein phosphatase calcineurin (CaN) has been widely related to both the extinction process and LTD. In this review, we focus on the available evidence that sustains CaN modulation of LTD and its association with extinction. Beyond the classic view, we also examine the interconnection among extinction, Hebbian and homeostatic plasticity, as well as emergent evidence of the participation of kinases and long-term potentiation (LTP) on extinction learning, highlighting the importance of the balance between kinases and phosphatases in the expression of extinction. Finally, we also integrate data that shows the association between extinction and less-studied phenomena, such as synaptic silencing and engram formation that open new perspectives in the field.

Programmed Cell Death 5 Provides Negative Feedback on Cardiac Hypertrophy Through the Stabilization of Sarco/Endoplasmic Reticulum Ca2+-ATPase 2a Protein

PDCD5 (programmed cell death 5) is ubiquitously expressed in tissues, including the heart; however, the mechanism underlying the cardiac function of PDCD5 has not been understood. We investigated the mechanisms of PDCD5 in the pathogenesis of cardiac hypertrophy. Cardiac-specific PDCD5 knockout mice developed severe cardiac hypertrophy and impaired cardiac function, whereas PDCD5 protein was significantly increased in transverse aortic constriction mouse hearts and phenylephrine-stimulated cardiomyocytes. Overexpression of PDCD5 inhibited phenylephrine-induced cardiomyocyte hypertrophy, and knockdown of PDCD5 induced cardiomyocyte hypertrophy and aggravated phenylephrine-induced hypertrophy. The expression of PDCD5 protein was regulated by NFATc2 (nuclear factor of activated T cells c2) during hypertrophy. SERCA2a (sarco/endoplasmic reticulum Ca²⁺-ATPase 2a) expression was decreased in PDCD5-deficient mouse hearts because of increased ubiquitination. PDCD5-deficient cardiomyocytes displayed decreased calcium uptake rate, slowed decay of Ca²⁺ transients, decreased calcium stores, and diastolic dysfunction. Moreover, reintroduction of PDCD5 in PDCD5-deficient mouse hearts reserved SERCA2a protein, suppressed NFATc2 protein, and rescued the hypertrophy and cardiac dysfunction. Our results revealed that PDCD5 is a novel target of NFATc2 in the hypertrophic heart and provides negative feedback to protect the heart against excessive hypertrophy via the stabilization of SERCA2a protein.

Revisiting the Concept of Targeting NFAT to Control T Cell Immunity and Autoimmune Diseases

The nuclear factor of activated T cells (NFAT) family of transcription factors, which includes NFAT1, NFAT2, and NFAT4, are well-known to play important roles in T cell activation. Most of NFAT proteins are controlled by calcium influx upon T cell receptor and costimulatory signaling results increase of IL-2 and IL-2 receptor. NFAT3 however is not shown to be expressed in T cells and NFAT5 has not much highlighted in T cell functions yet. Recent studies demonstrate that the NFAT family proteins involve in function of lineage-specific transcription factors during differentiation of T helper 1 (Th1), Th2, Th17, regulatory T (Treg), and follicular helper T cells (Tfh). They have been studied to make physical interaction with the other transcription factors like GATA3 or Foxp3 and they also regulate Th cell signature gene expressions by direct binding on promotor region of target genes. From last decades, NFAT functions in T cells have been targeted to develop immune modulatory drugs for controlling T cell immunity in autoimmune diseases like cyclosporine A, FK506, etc. Due to their undesirable side defects, only limited application is available in human diseases. This review focuses on the recent advances in development of NFAT targeting drug as well as our understanding of each NFAT family protein in T cell biology. We also discuss updated detail molecular mechanism of NFAT functions in T cells, which would lead us to suggest an idea for developing specific NFAT inhibitors as a therapeutic drug for autoimmune diseases.

Calcineurin and Its Role in Synaptic Transmission

Calcineurin (CaN) is a serine/threonine phosphatase widely expressed in different cell types and structures including neurons and synapses. The most studied role of CaN is its involvement in the functioning of postsynaptic structures of central synapses. The role of CaN in the presynaptic structures of central and peripheral synapses is less understood, although it has generated a considerable interest and is a subject of a growing number of studies. The regulatory role of CaN in synaptic vesicle endocytosis in the synapse terminals is actively studied. In recent years, new targets of CaN have been identified and its role in the regulation of enzymes and neurotransmitter secretion in peripheral neuromuscular junctions has been revealed. CaN is the only phosphatase that requires calcium and calmodulin for activation. In this review, we present details of CaN molecular structure and give a detailed description of possible mechanisms of CaN activation involving calcium, enzymes, and endogenous and exogenous inhibitors. Known and newly discovered CaN targets at pre- and postsynaptic levels are described. CaN activity in synaptic structures is discussed in terms of functional involvement of this phosphatase in synaptic transmission and neurotransmitter release.

Reduced expression of Na+/Ca2+ exchangers is associated with cognitive deficits seen in Alzheimer's disease model mice

Na⁺/Ca²⁺ exchangers (NCXs) are expressed primarily in the plasma membrane of most cell types, where they mediate electrogenic exchange of one Ca²⁺ for three Na⁺ ions, depending on Ca²⁺ and Na⁺ electrochemical gradients across the membrane. Three mammalian NCX isoforms (NCX1, NCX2, and NCX3) are each encoded by a distinct gene. Here, we report that NCX2 and NCX3 protein and mRNA levels are relatively reduced in hippocampal CA1 of APP23 and APP-KI mice. Likewise, NCX2^+/- or NCX3^+/- mice exhibited impaired hippocampal LTP and memory-related behaviors. Moreover, relative to controls, calcium/calmodulin-dependent protein kinase II (CaMKII) autophosphorylation significantly decreased in NCX2^+/- mouse hippocampus but increased in hippocampus of NCX3^+/- mice. NCX2 or NCX3 heterozygotes displayed impaired maintenance of hippocampal LTP, a phenotype that in NCX2^+/- mice was correlated with elevated calcineurin activity and rescued by treatment with the calcineurin (CaN) inhibitor FK506. Likewise, FK506 treatment significantly restored impaired hippocampal LTP in APP-KI mice. Moreover, Ca²⁺ clearance after depolarization following high frequency stimulation was slightly delayed in hippocampal CA1 regions of NCX2^+/- mice. Electron microscopy revealed relatively decreased synaptic density in CA1 of NCX2^+/- mice, while the number of spines with perforated synapses in CA1 significantly increased in NCX3^+/- mice. We conclude that memory impairment seen in NCX2^+/- and NCX3^+/- mice reflect dysregulated hippocampal CaMKII activity, which alters dendritic spine morphology, findings with implications for memory deficits seen in Alzheimer's disease model mice.

Putting a brake on synaptic vesicle endocytosis

In chemical synapses, action potentials evoke synaptic vesicle fusion with the presynaptic membrane at the active zone to release neurotransmitter. Synaptic vesicle endocytosis (SVE) then follows exocytosis to recapture vesicle proteins and lipid components for recycling and the maintenance of membrane homeostasis. Therefore, SVE plays an essential role during neurotransmission and is one of the most precisely regulated biological processes. Four modes of SVE have been characterized and both positive and negative regulators have been identified. However, our understanding of SVE regulation remains unclear, especially the identity of negative regulators and their mechanisms of action. Here, we review the current knowledge of proteins that function as inhibitors of SVE and their modes of action in different forms of endocytosis. We also propose possible physiological roles of such negative regulation. We believe that a better understanding of SVE regulation, especially the inhibitory mechanisms, will shed light on neurotransmission in health and disease.

Regulation of the phosphatase PP2B by protein-protein interactions

Protein dephosphorylation is important for regulating cellular signaling in a variety of contexts. Protein phosphatase-2B (PP2B), or calcineurin, is a widely expressed serine/threonine phosphatase that acts on a large cross section of potential protein substrates when activated by increased levels of intracellular calcium in concert with calmodulin. PxIxIT and LxVP targeting motifs are important for maintaining specificity in response to elevated calcium. In the present study, we describe the mechanism of PP2B activation, discuss its targeting by conserved binding motifs and review recent advances in the understanding of an A-kinase anchoring protein 79/PP2B/protein kinase A complex's role in synaptic long-term depression. Finally, we discuss potential for targeting PP2B anchoring motifs for therapeutic benefit.

Calcineurin signaling in the heart: The importance of time and place

The calcium-activated protein phosphatase, calcineurin, lies at the intersection of protein phosphorylation and calcium signaling cascades, where it provides an essential nodal point for coordination between these two fundamental modes of intracellular communication. In excitatory cells, such as neurons and cardiomyocytes, that experience rapid and frequent changes in cytoplasmic calcium, calcineurin protein levels are exceptionally high, suggesting that these cells require high levels of calcineurin activity. Yet, it is widely recognized that excessive activation of calcineurin in the heart contributes to pathological hypertrophic remodeling and the progression to failure. How does a calcium activated enzyme function in the calcium-rich environment of the continuously contracting heart without pathological consequences? This review will discuss the wide range of calcineurin substrates relevant to cardiovascular health and the mechanisms calcineurin uses to find and act on appropriate substrates in the appropriate location while potentially avoiding others. Fundamental differences in calcineurin signaling in neonatal verses adult cardiomyocytes will be addressed as well as the importance of maintaining heterogeneity in calcineurin activity across the myocardium. Finally, we will discuss how circadian oscillations in calcineurin activity may facilitate integration with other essential but conflicting processes, allowing a healthy heart to reap the benefits of calcineurin signaling while avoiding the detrimental consequences of sustained calcineurin activity that can culminate in heart failure.

Investigating the human Calcineurin Interaction Network using the πɸLxVP SLiM

Ser/thr phosphorylation is the primary reversible covalent modification of proteins in eukaryotes. As a consequence, it is the reciprocal actions of kinases and phosphatases that act as key molecular switches to fine tune cellular events. It has been well documented that ~400 human ser/thr kinases engage substrates via consensus phosphosite sequences. Strikingly, we know comparatively little about the mechanism by which ~40 human protein ser/thr phosphatases (PSPs) dephosphorylate ~15000 different substrates with high specificity. The identification of substrates of the essential PSP calcineurin (CN) has been exceptionally challenging and only a small fraction has been biochemically confirmed. It is now emerging that CN binds regulators and substrates via two short linear motifs (SLiMs), the well-studied PxIxIT SLiM and the LxVP SLiM, which remains controversial at the molecular level. Here we describe the crystal structure of CN in complex with its substrate NFATc1 and show that the LxVP SLiM is correctly defined as πɸLxVP. Bioinformatics studies using the πɸLxVP SLiM resulted in the identification of 567 potential CN substrates; a small subset was experimentally confirmed. This combined structural-bioinformatics approach provides a powerful method for dissecting the CN interaction network and for elucidating the role of CN in human health and disease.

Calcineurin Links Mitochondrial Elongation with Energy Metabolism

Canonical protein phosphatase 3/calcineurin signaling is central to numerous physiological processes. Here we provide evidence that calcineurin plays a pivotal role in controlling systemic energy and body weight homeostasis. Knockdown of calcineurin in Drosophila melanogaster led to a decrease in body weight and energy stores, and increased energy expenditure. In mice, global deficiency of catalytic subunit Ppp3cb, and tissue-specific ablation of regulatory subunit Ppp3r1 from skeletal muscle, but not adipose tissue or liver, led to protection from high-fat-diet-induced obesity and comorbid sequelæ. Ser637 hyperphosphorylation of dynamin-related protein 1 (Drp1) in skeletal muscle of calcineurin-deficient mice was associated with mitochondrial elongation into power-cable-shaped filaments and increased mitochondrial respiration, but also with attenuated exercise performance. Our data suggest that calcineurin acts as highly conserved pivot for the adaptive metabolic responses to environmental changes such as high-fat, high-sugar diets or exercise.

Shifting towards a model of mGluR5 dysregulation in schizophrenia: Consequences for future schizophrenia treatment

Metabotropic glutamate receptor subtype 5 (mGluR5), encoded by the GRM5 gene, represents a compelling novel drug target for the treatment of schizophrenia. mGluR5 is a postsynaptic G-protein coupled glutamate receptor strongly linked with several critical cellular processes that are reported to be disrupted in schizophrenia. Accordingly, mGluR5 positive allosteric modulators show encouraging therapeutic potential in preclinical schizophrenia models, particularly for the treatment of cognitive dysfunctions against which currently available therapeutics are largely ineffective. More work is required to support the progression of mGluR5-targeting drugs into the clinic for schizophrenia treatment, although some obstacles may be overcome by comprehensively understanding how mGluR5 itself is involved in the neurobiology of the disorder. Several processes that are necessary for the regulation of mGluR5 activity have been identified, but not examined, in the context of schizophrenia. These processes include protein-protein interactions, dimerisation, subcellular trafficking, the impact of genetic variability or mutations on protein function, as well as epigenetic, post-transcriptional and post-translational processes. It is essential to understand these aspects of mGluR5 to determine whether they are affected in schizophrenia pathology, and to assess the consequences of mGluR5 dysfunction for the future use of mGluR5-based drugs. Here, we summarise the known processes that regulate mGluR5 and those that have already been studied in schizophrenia, and discuss the consequences of this dysregulation for current mGluR5 pharmacological strategies. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

Opposing calcium-dependent signalling pathways control skeletal muscle differentiation by regulating a chromatin remodelling enzyme

Calcium signaling is important for differentiation-dependent gene expression, but is also involved in other cellular functions. Therefore mechanisms must exist to distinguish calcium signaling relevant to differentiation. Calcineurin is a calcium-regulated phosphatase that is required for myogenic gene expression and skeletal muscle differentiation. Here, we demonstrate that inhibition of calcineurin blocks chromatin remodeling and that the Brg1 ATPase of the SWI/SNF chromatin remodeling enzyme, which is required for the activation of myogenic gene expression, is a calcineurin substrate. Furthermore, we identify the calcium-regulated classical protein kinase C beta (PKCβ) as a repressor of myogenesis and as the enzyme that opposes calcineurin function. Replacement of endogenous Brg1 with a phosphomimetic mutant in primary myoblasts inhibits myogenesis, while replacement with a non-phosphorylatable mutant allows myogenesis despite inhibition of calcineurin signaling, demonstrating the functionality of calcineurin/PKC modified residues. Thus the Brg1 chromatin remodeling enzyme integrates two antagonistic calcium-dependent signaling pathways that control myogenic differentiation.

Lysosomal calcium signalling regulates autophagy through calcineurin and ​TFEB

The view of the lysosome as the terminal end of cellular catabolic pathways has been challenged by recent studies showing a central role of this organelle in the control of cell function. Here we show that a lysosomal Ca²⁺ signaling mechanism controls the activities of the phosphatase calcineurin and of its substrate TFEB, a master transcriptional regulator of lysosomal biogenesis and autophagy. Lysosomal Ca²⁺ release via mucolipin 1 (MCOLN1) activates calcineurin, which binds and de-phosphorylates TFEB, thus promoting its nuclear translocation. Genetic and pharmacological inhibition of calcineurin suppressed TFEB activity during starvation and physical exercise, while calcineurin overexpression and constitutive activation had the opposite effect. Induction of autophagy and lysosomal biogenesis via TFEB required MCOLN1-mediated calcineurin activation, linking lysosomal calcium signaling to both calcineurin regulation and autophagy induction. Thus, the lysosome reveals itself as a hub for the signaling pathways that regulate cellular homeostasis.

Identification and mutational analyses of phosphorylation sites of the calcineurin-binding protein CbpA and the identification of domains required for calcineurin binding in Aspergillus fumigatus

Calcineurin is a key protein phosphatase required for hyphal growth and virulence in Aspergillus fumigatus, making it an attractive antifungal target. However, currently available calcineurin inhibitors, FK506 and cyclosporine A, are immunosuppressive, limiting usage in the treatment of patients with invasive aspergillosis. Therefore, the identification of endogenous inhibitors of calcineurin belonging to the calcipressin family is an important parallel strategy. We previously identified the gene cbpA as the A. fumigatus calcipressin member and showed its involvement in hyphal growth and calcium homeostasis. However, the mechanism of its activation/inhibition through phosphorylation and its interaction with calcineurin remains unknown. Here we show that A. fumigatus CbpA is phosphorylated at three distinct domains, including the conserved SP repeat motif (phosphorylated domain-I; PD-I), a filamentous fungal-specific domain (PD-II), and the C-terminal CIC motif (Calcipressin Inhibitor of Calcineurin; PD-III). While mutation of three phosphorylated residues (Ser208, Ser217, Ser223) in the PD-II did not affect CbpA function in vivo, mutation of the two phosphorylated serines (Ser156, Ser160) in the SP repeat motif caused reduced hyphal growth and sensitivity to oxidative stress. Mutational analysis in the key domains in calcineurin A (CnaA) and proteomic interaction studies confirmed the requirement of PxIxIT motif-binding residues (352-NIR-354) and the calcineurin B (CnaB)-binding helix residue (V371) for the binding of CbpA to CnaA. Additionally, while the calmodulin-binding residues (442-RVF-444) did not affect CbpA binding to CnaA, three mutations (T359P, H361L, and L365S) clustered between the CnaA catalytic and the CnaB-binding helix were also required for CbpA binding. This is the first study to analyze the phosphorylation status of calcipressin in filamentous fungi and identify the domains required for binding to calcineurin.

Aberrant behavioral sensitization by methamphetamine in junctophilin-deficient mice

Junctophilins (JPs) expressed in the endoplasmic/sarcoplasmic reticulum (ER/SR) interact with the plasma membrane, thereby constructing junctional membrane complexes (JMC). We here reported that double-knockout mice lacking both JP3 and JP4 (JP-DKO mice) exhibit aberrant synaptic plasticity in the corticostriatal circuits and irregular methamphetamine (METH)-induced behavioral sensitization when METH (1.0 mg/kg) was administrated six consecutive days and assessed the striatal glutamatergic population spike (PS) by stimulation of cortical white matter. When we assessed the striatal PS by stimulation of cortical white matter, the long-term depression (LTD) was observed in JP-DKO mouse striatum similar to that in control (JP-double hetero mice (JP-DHE mice)). Importantly, LTD converted to long-term potentiation (LTP) following chronic METH treatment concomitant with behavioral sensitization in JP-DHE mice. LTD in JP-DKO mice, however failed to convert to LTP with lacks of behavioral sensitization. LTP impairment in JP-DKO mice was restored by pretreatment with FK506, calcineurin (CaN) inhibitor, but not with apamin, SK channel inhibitor. In immunoblotting analyses, calcium/calmodulin-dependent protein kinase II (CaMKII) autophosphorylation was significantly increased following METH treatment in the striatum of JP-DHE mice. However, CaMKII autophosphorylation did not changed by METH treatment in the striatum of JP-DKO mouse. The increased CaMKII autophosphorylation was closely associated with elevated CaN activity in JP-DKO mice. The lack of increased CaMKII activity in JP-DKO mice was correlated with the impaired METH-induced behavioral sensitization. Thus, elevated CaN and aberrant CaMKII activities in the striatum of JP-DKO mice likely accounts for lack of METH-induced behavioral sensitization.

MEF2 is restricted to the male gonad and regulates expression of the orphan nuclear receptor NR4A1

Leydig cell steroidogenesis is controlled by the pituitary gonadotropin LH that activates several signaling pathways, including the Ca(2+)/calmodulin kinase I (CAMKI) pathway. In other tissues, CAMKI regulates the activity of the myocyte enhancer factor 2 (MEF2) transcription factors. MEF2 factors are essential regulators of cell differentiation and organogenesis in numerous tissues but their expression and role in the mammalian gonad had not been explored. Here we show that MEF2 factors are expressed in a sexually dimorphic pattern in the mouse gonad. MEF2 factors are present in the testis throughout development and into adulthood but absent from the ovary. In the testis, MEF2 was localized mainly in the nucleus of both somatic lineages, the supporting Sertoli cells and the steroidogenic Leydig cells. In Leydig cells, MEF2 was found to activate the expression of Nr4a1, a nuclear receptor important for hormone-induced steroidogenesis. In these cells MEF2 also cooperates with forskolin and CAMKI to enhance Nr4a1 promoter activity via two MEF2 elements (-318 and -284 bp). EMSA confirmed direct binding of MEF2 to these elements whereas chromatin immunoprecipitation revealed that MEF2 recruitment to the proximal Nr4a1 promoter was increased following hormonal stimulation. Modulation of endogenous MEF2 protein level (small interfering RNA-mediated knockdown) or MEF2 activity (MEF2-Engrailed active dominant negative) led to a significant decrease in Nr4a1 mRNA levels in Leydig cells. All together, our results identify MEF2 as a novel testis-specific transcription factor, supporting a role for this factor in male sex differentiation and function. MEF2 was also positioned upstream of NR4A1 in a regulatory cascade controlling Leydig cell gene expression.

Direct association of the unique C-terminal tail of transmembrane AMPA receptor regulatory protein γ-8 with calcineurin

Transmembrane α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor regulatory proteins (TARPs) are auxiliary subunits that regulate AMPA receptor trafficking to the plasma membrane and localization to postsynaptic sites. The classical TARP family consists of four members: stargazin/γ-2, γ-3, γ-4 and γ-8. The TARP γ-8 isoform, which is highly expressed in the hippocampus, has a unique, long C-terminal domain with five distinct regions: two glycine-rich regions, a serine/arginine-rich region, a proline/alanine (P/A) rich region, and a PSD-95/Dlg/ZO-1 (PDZ) binding motif. We performed mass spectrometry and immunoprecipitation assays to identify specific binding partners for the γ-8 C-terminal tail and found that γ-8, but not stargazin/γ-2, co-immunoprecipitated with calcineurin/PP2B, a Ca(2+) /calmodulin-dependent Ser/Thr phosphatase. Co-immunoprecipitation and immunoblot analyses of lysates from COS-7 cells co-transfected with calcineurin and either wild type or chimeric γ-8 revealed that a section of the C-terminal tail (residues 356-421) can bind calcineurin. Futhermore, γ-8 lacking the P/A-rich region (residues 383-399) did not bind to calcineurin. In addition, the GST-γ-8 C-terminal tail (residues 353-414) fusion protein containing the P/A-rich region bound to purified calcineurin in a Ca(2+) /calmodulin-dependent manner, whereas GST-γ-8 with a deletion of the P/A-rich region did not. Peptide competition assays demonstrated that γ-8 may interact with the hydrophobic pocket defined by β-sheet 14 and/or adjacent regions of the catalytic A subunit of calcineurin. These results indicate that the γ-8 P/A-rich region is essential for binding calcineurin, suggesting that the γ-8/calcineurin complex may regulate AMPA receptor phosphorylation and trafficking.

EBV-encoded LMP-1 sensitizes nasopharyngeal carcinoma cells to genotoxic drugs by down-regulating Cabin1 expression

The oncogenic latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV) is involved in the pathogenesis of human nasopharyngeal carcinoma (NPC) and lymphoma. We and other authors have shown earlier that LMP1 induces apoptosis and inhibits xenograft tumor growth in mice, but the mechanism underlying these processes has not been investigated so far. In the present study, we show that knockdown of LMP1 renders the EBV-positive NPC cell line CG-1 resistant to various genotoxic drugs (cisplatin, etoposide, and adriamycin). LMP1 inhibits the expression of Cabin1, a Ca(2+) regulated protein shown earlier to inhibit calcineurin. Knockdown of calcineurin binding protein (Cabin1) with small hairpin RNA sensitizes CG-1 cells to genotoxic drugs. In contrast, LMP1 overexpression reduces Cabin1 level and renders both CG-1 cells and EBV-negative NPC cell lines sensitive to cisplatin. The c-Jun-N-terminal kinase (JNK) and ERK pathways are required for LMP1-induced suppression of Cabin1 at the transcriptional level. Chromatin immunoprecipitation assays further confirm that the JNK-activated transcription factor AP-1 mediates the LMP1-induced down-regulation of Cabin1 gene expression. LMP1 knockdown also increases the resistance of xenograph tumors to cisplatin in mice, therefore confirming the relevance of our findings in vivo. This study reveals the molecular mechanism underlying the pro-apoptotic activity of LMP1 during cisplatin-based NPC chemotherapy.

NFAT gene family in inflammation and cancer

Calcineurin-NFAT signaling is critical for numerous aspects of vertebrate function during and after embryonic development. Initially discovered in T cells, the NFAT gene family, consisting of five members, regulates immune system, inflammatory response, angiogenesis, cardiac valve formation, myocardial development, axonal guidance, skeletal muscle development, bone homeostasis, development and metastasis of cancer, and many other biological processes. In this review we will focus on the NFAT literature relevant to the two closely related pathological systems: inflammation and cancer.

Regulation of MEF2 transcriptional activity by calcineurin/mAKAP complexes

The calcium/calmodulin-dependent protein phosphatase calcineurin is required for the induction of transcriptional events that initiate and promote myogenic differentiation. An important effector for calcineurin in striated muscle is the transcription factor myocyte enhancer factor 2 (MEF2). The targeting of the enzyme and substrate to specific intracellular compartments by scaffold proteins often confers specificity in phosphatase activity. We now show that the scaffolding protein mAKAP organizes a calcineurin/MEF2 signaling complex in myocytes, regulating gene transcription. A calcineurin/mAKAP/MEF2 complex can be isolated from C2C12 cells and cardiac myocytes, and the calcineurin/MEF2 association is dependent on mAKAP expression. We have identified a peptide comprising the calcineurin binding domain in mAKAP that can disrupt the binding of the phosphatase to the scaffold in vivo. Dominant interference of calcineurin/mAKAP binding blunts the increase in MEF2 transcriptional activity seen during myoblast differentiation, as well as the expression of endogenous MEF2-target genes. Furthermore, disruption of calcineurin binding to mAKAP in cardiac myocytes inhibits adrenergic-induced cellular hypertrophy. Together these data illustrate the importance of calcineurin anchoring by the mAKAP scaffold for MEF2 regulation.

FHL2 binds calcineurin and represses pathological cardiac growth

Stress-induced hypertrophic growth of the heart predisposes the heart to arrhythmia, contractile dysfunction, and clinical heart failure. FHL2 (four-and-a-half LIM domain protein 2) is expressed predominantly in the heart, and inactivation of the gene coding for FHL2 leads to exaggerated responsiveness to adrenergic stress. Activation of calcineurin occurs downstream of β-adrenergic signaling and is required for isoproterenol-induced myocardial hypertrophy. Based on these facts, we hypothesized that FHL2 suppresses stress-induced activation of calcineurin. FHL2 is upregulated in mouse hearts exposed to isoproterenol, a β-adrenergic agonist, and isoproterenol-induced increases in the NFAT target genes RCAN1.4 and BNP were amplified significantly in FHL2 knockout (FHL2(-/-)) mice compared with levels in wild-type (WT) mice. To determine whether the effect of FHL2 on NFAT target gene transcript levels occurred at the level of transcription, HEK 293 cells and neonatal rat ventricular myocytes (NRVMs) were transfected with a luciferase reporter construct harboring the NFAT-dependent promoters of either RCAN1 or interleukin 2 (IL-2). Consistent with the in vivo data, small interfering RNA (siRNA) knockdown of FHL2 led to increased activation of these promoters by constitutively active calcineurin or the calcium ionophore ionomycin. Importantly, activation of the RCAN1 promoter by ionomycin, in control and FHL2 knockdown cells, was abolished by the calcineurin inhibitor cyclosporine, confirming the calcineurin dependence of the response. Overexpression of FHL2 inhibited activation of both NFAT reporter constructs. Furthermore, NRVMs overexpressing FHL2 exhibited reduced hypertrophic growth in response to constitutively active calcineurin, as measured by cell cross-sectional area and fetal gene expression. Finally, immunostaining in isolated adult cardiomyocytes revealed colocalization of FHL2 and calcineurin predominantly at the sarcomere and activation of calcineurin by endothelin-1-facilitated interaction between FHL2 and calcineurin. FHL2 is an endogenous, agonist-dependent suppressor of calcineurin.

Calcium-dependent dephosphorylation of the histone chaperone DAXX regulates H3.3 loading and transcription upon neuronal activation

Activity-dependent modifications of chromatin are believed to contribute to dramatic changes in neuronal circuitry. The mechanisms underlying these modifications are not fully understood. The histone variant H3.3 is incorporated in a replication-independent manner into different regions of the genome, including gene regulatory elements. It is presently unknown whether H3.3 deposition is involved in neuronal activity-dependent events. Here, we analyze the role of the histone chaperone DAXX in the regulation of H3.3 incorporation at activity-dependent gene loci. DAXX is found to be associated with regulatory regions of selected activity-regulated genes, where it promotes H3.3 loading upon membrane depolarization. DAXX loss not only affects H3.3 deposition but also impairs transcriptional induction of these genes. Calcineurin-mediated dephosphorylation of DAXX is a key molecular switch controlling its function upon neuronal activation. Overall, these findings implicate the H3.3 chaperone DAXX in the regulation of activity-dependent events, thus revealing a new mechanism underlying epigenetic modifications in neurons.

Multiple transcription factor families regulate axon growth and regeneration

Understanding axon regenerative failure remains a major goal in neuroscience, and reversing this failure remains a major goal for clinical neurology. Although an inhibitory central nervous system environment clearly plays a role, focus on molecular pathways within neurons has begun to yield fruitful insights. Initial steps forward investigated the receptors and signaling pathways immediately downstream of environmental cues, but recent work has also shed light on transcriptional control mechanisms that regulate intrinsic axon growth ability, presumably through whole cassettes of gene target regulation. Here we will discuss transcription factors that regulate neurite growth in vitro and in vivo, including p53, SnoN, E47, cAMP-responsive element binding protein (CREB), signal transducer and activator of transcription 3 (STAT3), nuclear factor of activated T cell (NFAT), c-Jun activating transcription factor 3 (ATF3), sex determining region Ybox containing gene 11 (Sox11), nuclear factor κ-light chain enhancer of activated B cells (NFκB), and Krüppel-like factors (KLFs). Revealing the similarities and differences among the functions of these transcription factors may further our understanding of the mechanisms of transcriptional regulation in axon growth and regeneration.

Syndecan-4 is essential for development of concentric myocardial hypertrophy via stretch-induced activation of the calcineurin-NFAT pathway

Sustained pressure overload leads to compensatory myocardial hypertrophy and subsequent heart failure, a leading cause of morbidity and mortality. Further unraveling of the cellular processes involved is essential for development of new treatment strategies. We have investigated the hypothesis that the transmembrane Z-disc proteoglycan syndecan-4, a co-receptor for integrins, connecting extracellular matrix proteins to the cytoskeleton, is an important signal transducer in cardiomyocytes during development of concentric myocardial hypertrophy following pressure overload. Echocardiographic, histochemical and cardiomyocyte size measurements showed that syndecan-4^−/− mice did not develop concentric myocardial hypertrophy as found in wild-type mice, but rather left ventricular dilatation and dysfunction following pressure overload. Protein and gene expression analyses revealed diminished activation of the central, pro-hypertrophic calcineurin-nuclear factor of activated T-cell (NFAT) signaling pathway. Cardiomyocytes from syndecan-4^−/−-NFAT-luciferase reporter mice subjected to cyclic mechanical stretch, a hypertrophic stimulus, showed minimal activation of NFAT (1.6-fold) compared to 5.8-fold increase in NFAT-luciferase control cardiomyocytes. Accordingly, overexpression of syndecan-4 or introducing a cell-permeable membrane-targeted syndecan-4 polypeptide (gain of function) activated NFATc4 in vitro. Pull-down experiments demonstrated a direct intracellular syndecan-4-calcineurin interaction. This interaction and activation of NFAT were increased by dephosphorylation of serine 179 (pS179) in syndecan-4. During pressure overload, phosphorylation of syndecan-4 was decreased, and association between syndecan-4, calcineurin and its co-activator calmodulin increased. Moreover, calcineurin dephosphorylated pS179, indicating that calcineurin regulates its own binding and activation. Finally, patients with hypertrophic myocardium due to aortic stenosis had increased syndecan-4 levels with decreased pS179 which was associated with increased NFAT activation. In conclusion, our data show that syndecan-4 is essential for compensatory hypertrophy in the pressure overloaded heart. Specifically, syndecan-4 regulates stretch-induced activation of the calcineurin-NFAT pathway in cardiomyocytes. Thus, our data suggest that manipulation of syndecan-4 may provide an option for therapeutic modulation of calcineurin-NFAT signaling.

Specific glucose-induced control of insulin receptor substrate-2 expression is mediated via Ca2+-dependent calcineurin/NFAT signaling in primary pancreatic islet β-cells

OBJECTIVE

Insulin receptor substrate-2 (IRS-2) plays an essential role in pancreatic islet β-cells by promoting growth and survival. IRS-2 turnover is rapid in primary β-cells, but its expression is highly regulated at the transcriptional level, especially by glucose. The aim was to investigate the molecular mechanism on how glucose regulates IRS-2 gene expression in β-cells.

RESEARCH DESIGN AND METHODS

Rat islets were exposed to inhibitors or subjected to adenoviral vector-mediated gene manipulations and then to glucose-induced IRS-2 expression analyzed by real-time PCR and immunoblotting. Transcription factor nuclear factor of activated T cells (NFAT) interaction with IRS-2 promoter was analyzed by chromatin immunoprecipitation assay and glucose-induced NFAT translocation by immunohistochemistry.

RESULTS

Glucose-induced IRS-2 expression occurred in pancreatic islet β-cells in vivo but not in liver. Modulating rat islet β-cell Ca(2+) influx with nifedipine or depolarization demonstrated that glucose-induced IRS-2 gene expression was dependent on a rise in intracellular calcium concentration derived from extracellular sources. Calcineurin inhibitors (FK506, cyclosporin A, and a peptide calcineurin inhibitor [CAIN]) abolished glucose-induced IRS-2 mRNA and protein levels, whereas expression of a constitutively active calcineurin increased them. Specific inhibition of NFAT with the peptide inhibitor VIVIT prevented a glucose-induced IRS-2 transcription. NFATc1 translocation to the nucleus in response to glucose and association of NFATc1 to conserved NFAT binding sites in the IRS-2 promoter were demonstrated.

CONCLUSIONS

The mechanism behind glucose-induced transcriptional control of IRS-2 gene expression specific to the islet β-cell is mediated by the Ca(2+)/calcineurin/NFAT pathway. This insight into the IRS-2 regulation could provide novel therapeutic means in type 2 diabetes to maintain an adequate functional mass.

Heterologous down-regulation of angiotensin type 1 receptors by purinergic P2Y2 receptor stimulation through S-nitrosylation of NF-kappaB

Cross-talk between G protein-coupled receptor (GPCR) signaling pathways serves to fine tune cellular responsiveness by neurohumoral factors. Accumulating evidence has implicated nitric oxide (NO)-based signaling downstream of GPCRs, but the molecular details are unknown. Here, we show that adenosine triphosphate (ATP) decreases angiotensin type 1 receptor (AT(1)R) density through NO-mediated S-nitrosylation of nuclear factor κB (NF-κB) in rat cardiac fibroblasts. Stimulation of purinergic P2Y(2) receptor by ATP increased expression of inducible NO synthase (iNOS) through activation of nuclear factor of activated T cells, NFATc1 and NFATc3. The ATP-induced iNOS interacted with p65 subunit of NF-κB in the cytosol through flavin-binding domain, which was indispensable for the locally generated NO-mediated S-nitrosylation of p65 at Cys38. β-Arrestins anchored the formation of p65/IκBα/β-arrestins/iNOS quaternary complex. The S-nitrosylated p65 resulted in decreases in NF-κB transcriptional activity and AT(1)R density. In pressure-overloaded mouse hearts, ATP released from cardiomyocytes led to decrease in AT(1)R density through iNOS-mediated S-nitrosylation of p65. These results show a unique regulatory mechanism of heterologous regulation of GPCRs in which cysteine modification of transcriptional factor rather than protein phosphorylation plays essential roles.

Interaction of calcineurin with substrates and targeting proteins

Calcineurin is a calcium activated protein phosphatase with a major role in calcium signaling in diverse cells and organs and clinical importance as the target of the immunosuppressive drugs cyclosporin A and tacrolimus (FK506). Cell biology studies have focused mainly on the role of calcineurin in transcriptional signaling. Calcium entry in response to extracellular stimuli results in calcineurin activation, and signal transmission from the cytosol into the nucleus through dephosphorylation and nuclear translocation of the transcription factor nuclear factor of activated T cells (NFAT). This initiates a cascade of transcriptional events involved in physiological and developmental processes. Molecular analyses of the calcineurin-NFAT interaction have been extended recently to encompass the interaction of calcineurin with other substrates, targeting proteins and regulators of calcineurin activity. These studies have increased our understanding of how this essential calcium activated enzyme orchestrates intracellular events in cooperation with other signaling pathways, and have suggested a link between altered calcineurin signaling and the developmental anomalies of Down syndrome.

Cabin1 expression suggests roles in neuronal development

Nervous system assembly and function depends on precise regulation of developmental gene expression. Cabin1, an essential gene in developing mice, is enriched in regions of the developing zebrafish central nervous system (CNS). Cabin1 is a repressor of MEF2- (myocyte enhancer factor 2) and calcineurin-mediated transcription in the immune system, but its function in the CNS during development is unknown. We identified Cabin1 from a library of genes enriched in developing neurons and determined the temporal and spatial expression of Cabin1 mRNA during CNS development. We found Cabin1 mRNA expression in the developing brain at times correlated with later aspects of neuronal differentiation. In some regions of the CNS Cabin1 expression overlaps with regions that also express proteins known to interact with Cabin1: MEF2 and/or calcineurin. We suggest that Cabin1 could act as a regulator of MEF2 and calcineurin activity in the developing nervous system, given their roles in neuronal differentiation and synaptic refinement.

Structure of p300 bound to MEF2 on DNA reveals a mechanism of enhanceosome assembly

Transcription co-activators CBP and p300 are recruited by sequence-specific transcription factors to specific genomic loci to control gene expression. A highly conserved domain in CBP/p300, the TAZ2 domain, mediates direct interaction with a variety of transcription factors including the myocyte enhancer factor 2 (MEF2). Here we report the crystal structure of a ternary complex of the p300 TAZ2 domain bound to MEF2 on DNA at 2.2Å resolution. The structure reveals three MEF2:DNA complexes binding to different sites of the TAZ2 domain. Using structure-guided mutations and a mammalian two-hybrid assay, we show that all three interfaces contribute to the binding of MEF2 to p300, suggesting that p300 may use one of the three interfaces to interact with MEF2 in different cellular contexts and that one p300 can bind three MEF2:DNA complexes simultaneously. These studies, together with previously characterized TAZ2 complexes bound to different transcription factors, demonstrate the potency and versatility of TAZ2 in protein–protein interactions. Our results also support a model wherein p300 promotes the assembly of a higher-order enhanceosome by simultaneous interactions with multiple DNA-bound transcription factors.

Nuclear factor kappaB downregulates the transient outward potassium current I(to,f) through control of KChIP2 expression

RATIONALE

The fast transient outward K(+) current (I(to,f)) plays a critical role in early repolarization of the heart. I(to,f) is consistently downregulated in cardiac disease. Despite its importance, the regulation of I(to,f) in disease remains poorly understood.

OBJECTIVE

Because the transcription factor nuclear factor (NF)-κB is activated in cardiac hypertrophy and disease, we studied the role of NF-κB in mediating I(to,f) reductions induced by hypertrophy.

METHODS AND RESULTS

Culturing neonatal rat ventricular myocytes in the presence of phenylephrine (PE) plus propranolol (Pro), to selectively activate α(1)-adrenergic receptors, caused reductions in I(to,f), as well as KChIP2 and Kv4.3 expression, while increasing Kv4.2 expression. Inhibition of NF-κB, via overexpression of a phosphorylation-deficient mutant of IκBα (IκBαSA) prevented PE/Pro-induced reductions in I(to,f) and KChIP2 mRNA, without affecting Kv4.2 or Kv4.3 expression, suggesting NF-κB mediates the I(to,f) reductions by repressing KChIP2. Indeed, overexpression of the NF-κB activator IκB kinase-β also decreased KChIP2 expression and I(to,f) (despite increasing Kv4.2), whereas IκBαSA overexpression elevated KChIP2 and decreased Kv4.2 levels. In addition, the classic NF-κB activator tumor necrosis factor α also induced NF-κB-dependent reductions of KChIP2 and I(to,f). Finally, inhibition of calcineurin did not prevent PE/Pro-induced reductions in KChIP2.

CONCLUSIONS

NF-κB regulates KChIP2 and Kv4.2 expression. The reductions in I(to,f) observed following α-adrenergic receptor stimulation or tumor necrosis factor α application require NF-κB-dependent decreases in KChIP2 expression.

Primers on molecular pathways--the NFAT transcription pathway in pancreatic cancer

The calcineurin-responsive nuclear factor of activated T cells (NFAT) family of transcription factors was originally identified as a group of inducible nuclear proteins, which regulate transcription during T lymphocyte activation. However, following their initial discovery, a multitude of studies quickly established that NFAT proteins are also expressed in cells outside the immune system, where they participate in the regulation of the expression of genes influencing cell growth and differentiation. Ectopic activation of individual NFAT members is now recognized as an important aspect for oncogenic transformation in several human malignancies, most notably in pancreatic cancer. Sustained activation of the Ca(2+)/calcineurin/NFAT signaling pathway has emerged as a powerful regulatory principle governing pancreatic cancer cell growth. Activated NFAT proteins form complexes with key oncogenic proteins to regulate the transcription of master cell cycle regulators and proteins with functions in cell survival, migration and angiogenesis. This review pays particular attention to recent advances in our understanding of how the NFAT transcription pathway controls gene expression during development and progression of pancreatic cancer. and IAP.

Cracking the phosphatase code: docking interactions determine substrate specificity

Phosphoserine- and phosphothreonine-directed phosphatases display remarkable substrate specificity, yet the sites that they dephosphorylate show little similarity in amino acid sequence. Studies reveal that docking interactions are key for the recognition of substrates and regulators by two conserved phosphatases, protein phosphatase 1 (PP1) and the Ca2+-calmodulin-dependent phosphatase calcineurin. In each case, a small degenerate sequence motif in the interacting protein directs low-affinity binding to a docking surface on the phosphatase that is distinct from the active site; several such interactions combine to confer overall binding specificity. Some docking surfaces are conserved, such as a hydrophobic groove on a face opposite the active site that serves as a major recognition surface for the "RVxF" motif of proteins that interact with PP1 and the "PxIxIT" motif of substrates of calcineurin. Secondary motifs combine with this primary targeting sequence to specify phosphatase binding. A comprehensive interactome for mammalian PP1 was described, analysis of which defines several PP1-binding motifs. Studies of "LxVP," a secondary calcineurin-binding sequence, establish that this motif is a conserved feature of calcineurin substrates and that the immunosuppressants FK506 and cyclosporin A inhibit the phosphatase by interfering with LxVP-mediated docking.

The novel calcineurin inhibitor CN585 has potent immunosuppressive properties in stimulated human T cells

The Ca2+/calmodulin-dependent protein phosphatase calcineurin is a key mediator in antigen-specific T cell activation. Thus, inhibitors of calcineurin, such as cyclosporin A or FK506, can block T cell activation and are used as immunosuppressive drugs to prevent graft-versus-host reactions and autoimmune diseases. In this study we describe the identification of 2,6- diaryl-substituted pyrimidine derivatives as a new class of calcineurin inhibitors, obtained by screening of a substance library. By rational design of the parent compound we have attained the derivative 6-(3,4-dichloro-phenyl)-4-(N,N-dimethylaminoethylthio)-2-phenyl-pyrimidine (CN585) that noncompetitively and reversibly inhibits calcineurin activity with a K(i) value of 3.8 mum. This derivative specifically inhibits calcineurin without affecting other Ser/Thr protein phosphatases or peptidyl prolyl cis/trans isomerases. CN585 shows potent immunosuppressive effects by inhibiting NFAT nuclear translocation and transactivation, cytokine production, and T cell proliferation. Moreover, the calcineurin inhibitor exhibits no cytotoxicity in the effective concentration range. Therefore, calcineurin inhibition by CN585 may represent a novel promising strategy for immune intervention.

Novel inhibitors of the calcineurin/NFATc hub - alternatives to CsA and FK506?

The drugs cyclosporine A (CsA) and tacrolimus (FK506) revolutionized organ transplantation. Both compounds are still widely used in the clinic as well as for basic research, even though they have dramatic side effects and modulate other pathways than calcineurin-NFATc, too. To answer the major open question - whether the adverse side effects are secondary to the actions of the drugs on the calcineurin-NFATc pathway - alternative inhibitors were developed. Ideal inhibitors should discriminate between the inhibition of (i) calcineurin and peptidyl-prolyl cis-trans isomerases (PPIases; the matchmaker proteins of CsA and FK506), (ii) calcineurin and the other Ser/Thr protein phosphatases, and (iii) NFATc and other transcription factors. In this review we summarize the current knowledge about novel inhibitors, synthesized or identified in the last decades, and focus on their mode of action, specificity, and biological effects.

Calcineurin inhibitor protein (CAIN) attenuates Group I metabotropic glutamate receptor endocytosis and signaling

Group I metabotropic glutamate receptors (mGluRs) are coupled via phospholipase Cbeta to the hydrolysis of phosphoinositides and function to modulate neuronal excitability and synaptic transmission at glutamatergic synapses. The desensitization of Group I mGluR signaling is thought to be mediated primarily via second messenger-dependent protein kinases and G protein-coupled receptor kinases. We show here that both mGluR1 and mGluR5 interact with the calcineurin inhibitor protein (CAIN). CAIN is co-immunoprecipitated in a complex with Group I mGluRs from both HEK 293 cells and mouse cortical brain lysates. Purified CAIN and its C-terminal domain specifically interact with glutathione S-transferase fusion proteins corresponding to the second intracellular loop and the distal C-terminal tail domains of mGluR1. The interaction of CAIN with mGluR1 could also be blocked using a Tat-tagged peptide corresponding to the mGluR1 second intracellular loop domain. Overexpression of full-length CAIN attenuates the agonist-stimulated endocytosis of both mGluR1a and mGluR5a in HEK 293 cells, but expression of the CAIN C-terminal domain does not alter mGluR5a internalization. In contrast, overexpression of either full-length CAIN or the CAIN C-terminal domain impairs agonist-stimulated inositol phosphate formation in HEK 293 cells expressing mGluR1a. This CAIN-mediated antagonism of mGluR1a signaling appears to involve the disruption of receptor-Galpha(q/11) complexes. Taken together, these observations suggest that the association of CAIN with intracellular domains involved in mGluR/G protein coupling provides an additional mechanism by which Group I mGluR endocytosis and signaling are regulated.

Down syndrome critical region 1 enhances the proteolytic cleavage of calcineurin

Down syndrome critical region 1 (DSCR1), an oxidative stress-response gene, interacts with calcineurin and represses its phosphatase activity. Recently it was shown that hydrogen peroxide inactivates calcineurin by proteolytic cleavage. Based on these facts, we investigated whether oxidative stress affects DSCR1- mediated inactivation of calcineurin. We determined that overexpression of DSCR1 leads to increased proteolytic cleavage of calcineurin. Convertsely, knockdown of DSCR1 abolished calcineurin cleavage upon treatment with hydrogen peroxide. The PXIIXT motif in the COOH-terminus of DSCR1 is responsible for both binding and cleavage of calcineurin. The knockdown of overexpressed DSCR1 in DS fibroblast cells also abrogated calcineurin proteolysis by hydrogen peroxide. These results suggest that DSCR1 has the ability to inactivate calcineurin by inducing proteolytic cleavage of calcineurin upon oxidative stress.

Multiple signalling pathways redundantly control glucose transporter GLUT4 gene transcription in skeletal muscle

Increased glucose transporter GLUT4 expression in skeletal muscle is an important benefit of regular exercise, resulting in improved insulin sensitivity and glucose tolerance. The Ca(2+)-calmodulin-dependent kinase II (CaMKII), calcineurin and AMPK pathways have been implicated in GLUT4 gene regulation based on pharmacological evidence. Here, we have used a more specific genetic approach to establish the relative role of the three pathways in fast and slow muscles. Plasmids coding for protein inhibitors of CaMKII or calcineurin were co-transfected in vivo with a GLUT4 enhancer-reporter construct either in normal mice or in mice expressing a kinase dead (KD) AMPK mutant. GLUT4 reporter activity was not inhibited in the slow soleus muscle by blocking either CaMKII or calcineurin alone, but was inhibited by blocking both pathways. GLUT4 reporter activity was likewise unchanged in the soleus of KD-AMPK mice, but was significantly reduced by incapacitation of either CaMKII or calcineurin in these mice. On the other hand, in the fast tibialis anterior (TA) muscle, calcineurin appears to exert a prominent role in the control of GLUT4 reporter activity, independent of CaMKII and AMPK. The results point to a muscle type-specific and redundant regulation of GLUT4 enhancer based on the interplay of multiple signalling pathways, all of which are known to affect myocyte enhancing factor 2 (MEF2) transcriptional activity, a point of convergence of different pathways on muscle gene regulation.

miR-23a functions downstream of NFATc3 to regulate cardiac hypertrophy

Cardiac hypertrophy is accompanied by maladaptive cardiac remodeling, which leads to heart failure or sudden death. MicroRNAs (miRNAs) are a class of small, noncoding RNAs that mediate posttranscriptional gene silencing. Recent studies show that miRNAs are involved in the pathogenesis of hypertrophy, but their signaling regulations remain to be understood. Here, we report that miR-23a is a pro-hypertrophic miRNA, and its expression is regulated by the transcription factor, nuclear factor of activated T cells (NFATc3). The results showed that miR-23a expression was up-regulated upon treatment with the hypertrophic stimuli including isoproterenol and aldosterone. Knockdown of miR-23a could attenuate hypertrophy, suggesting that miR-23a is able to convey the hypertrophic signal. In exploring the molecular mechanism by which miR-23a is up-regulated, we identified that NFATc3 could directly activate miR-23a expression through the transcriptional machinery. The muscle specific ring finger protein 1, an anti-hypertrophic protein, was identified to be a target of miR-23a. Its translation could be suppressed by miR-23a. Our data provide a model in which the miRNA expression is regulated by the hypertrophic transcriptional factor.

Calcineurin a-binding protein, a novel modulator of the calcineurin-nuclear factor of activated T-cell signaling pathway, is overexpressed in wilms' tumors and promotes cell migration

Current therapeutic strategies against Wilms' tumor (WT) reach 80% to 85% success rate. In spite of this, a remaining 15% to 20% of tumors relapse and are associated with increased metastasis and poor prognosis. To identify new regulators of WT progression, we screened for developmental target genes of Pax2, a key regulator of kidney development and a WT signature gene. We show that one of these target genes, calcineurin A-binding protein (CnABP), is coexpressed with Pax2 during kidney development and is overexpressed in >70% of WT samples analyzed. The CnABP gene encodes a novel protein product conserved in higher vertebrates. We show that CnABP promotes cell proliferation and migration in cell culture experiments. Biochemical analyses additionally identified an interaction between CnABP and calcineurin Abeta, the catalytic subunit of the calcium-responsive serine/threonine phosphatase calcineurin. We show that this interaction leads to the inhibition of calcineurin phosphatase activity and prevents nuclear factor of activated T-cell (NFAT) nuclear translocation. Inhibition of NFAT nuclear localization results in decreased NFAT transcriptional response. Together, these data identify a new modulator of calcineurin signaling up-regulated in WTs.

Homer and the ryanodine receptor

Homer proteins have recently been identified as novel high-affinity ligands that modulate ryanodine receptor (RyR) Ca(2+) release channels in heart and skeletal muscle, through an EVH1 domain which binds to proline-rich regions in target proteins. Many Homer proteins can also self-associate through a coiled-coil domain that allows their multimerisation. In other tissues, especially neurons, Homer anchors proteins embedded in the surface membrane to the Ca(2+) release channel in the endoplasmic reticulum and can anchor membrane or cytosolic proteins to the cytoskeleton. Although this anchoring aspect of Homer function has not been extensively investigated in muscle, there are consensus sequences for Homer binding in the RyR and on many of the proteins that it interacts with in the massive RyR ion channel complex. In this review we explore the potential of Homer to contribute to a variety of cell processes in muscle and neurons that also involve RyR channels.

Calmodulin-dependent phosphatase, kinases, and transcriptional corepressors involved in T-cell activation

The second messenger calcium plays an essential role in mediating the T-cell receptor (TCR) signaling pathway leading to cytokine production and T-cell clonal expansion. The immunosuppressive drugs cyclosporine A and FK506 have served both as therapeutic agents and as molecular probes for unraveling the protein phosphatase calcineurin as a rate-limiting enzyme involved in the transmission of calcium signal from the cytosol into the nucleus to reprogram gene expression. The use of mouse knockout models has helped to verify and further elucidate the functions of different isoforms of calcineurin in both helper T-cell activation and thymocyte development. In addition to calcineurin, three other classes of calmodulin-binding proteins have also been shown to play important roles in calcium signaling in T cells. Thus, Cabin1 and class II histone deacetylases have been found to constitute a novel calcium-signaling module in conjunction with the transcription factor myocyte enhance factor family and the transcriptional coactivator p300 to suppress and activate cytokine gene transcription in a calcium-dependent manner. The calmodulin-dependent protein kinases II and IV were also shown to play negative and positive regulatory functions, respectively, in TCR-mediated cytokine production. The crosstalks among these and other signal transducers in T cells form an extensive nonlinear signaling network that dictates the final outcome of the TCR signaling pathway.