Two-site interaction of nuclear factor of activated T cells with activated calcineurin

The mechanism of the NFAT transcription factor family involved in oxidative stress response

As a transcriptional activator widely expressed in various tissues, nuclear factor of activated T cells (NFAT) is involved in the regulation of the immune system, the development of the heart and brain systems, and classically mediating pathological processes such as cardiac hypertrophy. Oxidative stress is an imbalance of intracellular redox status, characterized by excessive generation of reactive oxygen species, accompanied by mitochondrial dysfunction, calcium overload, and subsequent lipid peroxidation, inflammation, and apoptosis. Oxidative stress occurs during various pathological processes, such as chronic hypoxia, vascular smooth muscle cell phenotype switching, ischemia-reperfusion, and cardiac remodeling. Calcium overload leads to an increase in intracellular calcium concentration, while NFAT can be activated through calcium-calcineurin, which is also the main regulatory mode of NFAT factors. This review focuses on the effects of NFAT transcription factors on reactive oxygen species production, calcium overload, mitochondrial dysfunction, redox reactions, lipid peroxidation, inflammation, and apoptosis in response to oxidative stress. We hope to provide a reference for the functions and characteristics of NFAT involved in various stages of oxidative stress as well as related potential targets.

Hidden Multivalency in Phosphatase Recruitment by a Disordered AKAP Scaffold

Disordered scaffold proteins provide multivalent landing pads that, via a series of embedded Short Linear Motifs (SLiMs), bring together the components of a complex to orchestrate precise spatial and temporal regulation of cellular processes. One such protein is AKAP5 (previously AKAP79), which contains SLiMs that anchor PKA and Calcineurin, and recruit substrate (the TRPV1 receptor). Calcineurin is anchored to AKAP5 by a well-characterised PxIxIT SLiM. Here we show, using a combination of biochemical and biophysical approaches, that the Calcineurin PxIxIT-binding groove also recognises several hitherto unknown lower-affinity SLiMs in addition to the PxIxIT motif. We demonstrate that the assembly is in reality a complex system with conserved SLiMs spanning a wide affinity range. The capture is analogous to that seen for many DNA-binding proteins that have a weak non-specific affinity for DNA outside the canonical binding site, but different in that it involves (i) two proteins, and (ii) hydrophobic rather than electrostatic interactions. It is also compatible with the requirement for both stable anchoring of the enzyme and responsive downstream signalling. We conclude that the AKAP5 C-terminus is enriched in lower-affinity/mini-SLiMs that, together with the canonical SLiM, maintain a structurally disordered but tightly regulated signalosome.

Microscale thermophoresis and fluorescence polarization assays of calcineurin-peptide interactions

Calcineurin is a Ca²⁺/calmodulin-dependent phosphatase. It is very important to study the affinity between calcineurin and its substrate or other interacting proteins. Two conserved motifs have been reported on the interactive proteins of calcineurin, namely, the PxIxIT motif and the LxVP motif. Here, we used 5(6)-carboxyfluorescein to fluorescently label the N-terminus of the short peptides derived from the two motifs and then determined the affinity between the protein and polypeptides. Microscale thermophoresis (MST) is very suitable for determining calcineurin with peptides containing the LxVP motif. The K_d values of the binding of calcineurin with NFATc1-YLAVP, NHE1-YLTVP, and A238L-FLCVK peptides were 6.72 ± 0.19 μM, 17.14 ± 0.35 μM, and 15.57 ± 0.10 μM, respectively. The GST pull-down results further confirmed the binding trend of the three peptides to calcineurin. However, fluorescently labeled PxIxIT polypeptides are not suitable for MST due to their own aggregation. We determined the binding affinity of the RCAN1-PSVVVH polypeptide to calcineurin by the fluorescence polarization (FP) method. MST and FP assays are fast and accurate in determining the affinity between protein-peptide interactions. Our research laid the foundation for screening the molecules that affect the binding between calcineurin and its substrates in the future.

Calcineurin inactivation inhibits pyruvate dehydrogenase complex activity and induces the Warburg effect

Calcineurin is a calcium- and calmodulin-dependent serine/threonine protein phosphatase that connects the Ca²⁺-dependent signalling to multiple cellular responses. Calcineurin inhibitors (CNIs) have been widely used to suppress immune response in allograft patients. However, CNIs significantly increase cancer incidence in transplant recipients compared with the general population. Accumulating evidence suggests that CNIs may promote the malignant transformation of cancer cells in addition to its role in immunosuppression, but the underlying mechanisms remain poorly understood. Here, we show that calcineurin interacts with pyruvate dehydrogenase complex (PDC), a mitochondrial gatekeeper enzyme that connects two key metabolic pathways of cells, glycolysis and the tricarboxylic acid cycle. Mitochondrial-localized calcineurin dephosphorylates PDHA1 at Ser232, Ser293 and Ser300, and thus enhances PDC enzymatic activity, remodels cellular glycolysis and oxidative phosphorylation, and suppresses cancer cell proliferation. Hypoxia attenuates mitochondrial translocation of calcineurin to promote PDC inactivation. Moreover, CNIs promote metabolic remodelling and the Warburg effect by blocking calcineurin-mediated PDC activation in cancer cells. Our findings indicate that calcineurin is a critical regulator of mitochondrial metabolism and suggest that CNIs may promote tumorigenesis through inhibition of the calcineurin-PDC pathway.

Calcineurin in the heart: New horizons for an old friend

Calcineurin, also known as PP2B or PPP3, is a member of the PPP family of protein phosphatases that also includes PP1 and PP2A. Together these three phosphatases carryout the majority of dephosphorylation events in the heart. Calcineurin is distinct in that it is activated by the binding of calcium/calmodulin (Ca2+/CaM) and therefore acts as a node for integrating Ca2+ signals with changes in phosphorylation, two fundamental intracellular signaling cascades. In the heart, calcineurin is primarily thought of in the context of pathological cardiac remodeling, acting through the Nuclear Factor of Activated T-cell (NFAT) family of transcription factors. However, calcineurin activity is also essential for normal heart development and homeostasis in the adult heart. Furthermore, it is clear that NFAT-driven changes in transcription are not the only relevant processes initiated by calcineurin in the setting of pathological remodeling. There is a growing appreciation for the diversity of calcineurin substrates that can impact cardiac function as well as the diversity of mechanisms for targeting calcineurin to specific sub-cellular domains in cardiomyocytes and other cardiac cell types. Here, we will review the basics of calcineurin structure, regulation, and function in the context of cardiac biology. Particular attention will be given to: the development of improved tools to identify and validate new calcineurin substrates; recent studies identifying new calcineurin isoforms with unique properties and targeting mechanisms; and the role of calcineurin in cardiac development and regeneration.

Isoform-Selective NFAT Inhibitor: Potential Usefulness and Development

Nuclear factor of activated T cells (NFAT), which is the pharmacological target of immunosuppressants cyclosporine and tacrolimus, has been shown to play an important role not only in T cells (immune system), from which their name is derived, but also in many biological events. Therefore, functional and/or structural abnormalities of NFAT are linked to the pathogenesis of diseases in various organs. The NFAT protein family consists of five isoforms, and each isoform performs diverse functions and has unique expression patterns in the target tissues. This diversity has made it difficult to obtain ideal pharmacological output for immunosuppressants that inhibit the activity of almost all NFAT family members, causing serious and wide-ranging side effects. Moreover, it remains unclear whether isoform-selective NFAT regulation can be achieved by targeting the structural differences among NFAT isoforms and whether this strategy can lead to the development of better drugs than the existing ones. This review summarizes the role of the NFAT family members in biological events, including the development of various diseases, as well as the usefulness of and problems associated with NFAT-targeting therapies, including those dependent on current immunosuppressants. Finally, we propose a novel therapeutic strategy based on the molecular mechanisms that enable selective regulation of specific NFAT isoforms.

Functional analyses of STIM1 mutations reveal a common pathomechanism for tubular aggregate myopathy and Stormorken syndrome

Tubular aggregate myopathy (TAM) is a progressive disorder characterized by muscle weakness, cramps, and myalgia. TAM clinically overlaps with Stormorken syndrome (STRMK), combining TAM with miosis, thrombocytopenia, hyposplenism, ichthyosis, short stature, and dyslexia. TAM and STRMK arise from gain-of-function mutations in STIM1 (stromal interaction molecule 1) or ORAI1, both encoding key regulators of Ca²⁺ homeostasis, and mutations in either gene result in excessive extracellular Ca²⁺ entry. The pathomechanistic similarities and differences between TAM and STRMK are only partially understood. Here we provide functional in vitro experiments demonstrating that STIM1 harboring the TAM D84G or the STRMK R304W mutation similarly cluster and exert a dominant effect on the wild-type protein. Both mutants recruit ORAI1 to the clusters, increase cytosolic Ca²⁺ levels, promote major nuclear import of the Ca²⁺ -dependent transcription factor NFAT (nuclear factor of activated T cells), and trigger the formation of circular membrane stacks. In conclusion, the analyzed TAM and STRMK mutations have a comparable impact on STIM1 protein function and downstream effects of excessive Ca²⁺ entry, highlighting that TAM and STRMK involve a common pathomechanism.

Ca2+ as a therapeutic target in cancer

Ca2+ is a ubiquitous and dynamic second messenger molecule that is induced by many factors including receptor activation, environmental factors, and voltage, leading to pleiotropic effects on cell function including changes in migration, metabolism and transcription. As such, it is not surprising that aberrant regulation of Ca2+ signals can lead to pathological phenotypes, including cancer progression. However, given the highly context-specific nature of Ca2+-dependent changes in cell function, delineation of its role in cancer has been a challenge. Herein, we discuss the distinct roles of Ca2+ signaling within and between each type of cancer, including consideration of the potential of therapeutic strategies targeting these signaling pathways.

Canales de calcio como blanco de interés farmacológico

Los canales de calcio son proteínas de membrana que constituyen la vía más importante para el ingreso del ion calcio (Ca2+) a la célula. Al abrirse, permiten el ingreso selectivo del ion, iniciando una variedad de procesos como contracción muscular, secreción endocrina y liberación de neurotransmisores, entre otros. Estas proteínas se agrupan en tres categorías de acuerdo con sus propiedades estructurales y funcionales: (i) Canales de Ca2+ operados por interacción receptor-ligando (ROCC), (ii) Canales activados por parámetros físicos (Transient Receptor Potencial, TRP) y (iii) Canales de Calcio dependientes de voltaje (VDCCs), siendo estos últimos los más estudiados debido a su presencia en células excitables. Dada la importancia de Ca2+ en la fisiología celular, los canales de Ca2+ constituyen un punto de acción farmacológica importante para múltiples tratamientos y, por tanto, son objeto de estudio para el desarrollo de nuevos fármacos. El objetivo de esta revisión es explicar la importancia de los canales de Ca2+ desde una proyección farmacológica, a partir de la exploración documental de artículos publicados hasta la fecha teniendo en cuenta temas relacionados con la estructura de los canales Ca2+, sus propiedades biofísicas, localización celular, funcionamiento y su interacción farmacológica.

A novel peptide exerts potent immunosuppression by blocking the two-site interaction of NFAT with calcineurin

The calcineurin/nuclear factor of activated T cell (CN/NFAT) signaling pathway plays a critical role in the immune response. Therefore, inhibition of the CN/NFAT pathway is an important target for inflammatory disease. The conserved PXIXIT and LXVP motifs of CN substrates and targeting proteins have been recognized. Based on the affinity ability and inhibitory effect of these docking sequences on CN, we designed a bioactive peptide (named pep3) against the CN/NFAT interaction, which has two binding sites derived from the RCAN1-PXIXIT motif and the NFATc1-LXVP motif. The shortest linker between the two binding sites in pep3 is derived from A238L, a physiological binding partner of CN. Microscale thermophoresis revealed that pep3 has two docking sites on CN. Pep3 also has the most potent inhibitory effect on CN. It is suggested that pep3 contains an NFATc1-LXVP-substrate recognition motif and RCAN1-PXIXIT-mediated anchoring to CN. Expression of this peptide significantly suppresses CN/NFAT signaling. Cell-permeable 11-arginine-modified pep3 (11R-pep3) blocks the NFAT downstream signaling pathway. Intranasal administration of the 11R-pep3 peptide inhibits airway inflammation in an ovalbumin-induced asthma model. Our results suggest that pep3 is promising as an immunosuppressive agent and can be used in topical remedies.

Identification of novel interacting regions involving calcineurin and nuclear factor of activated T cells

Nuclear factor of activated T cells (NFAT) leads to the transcription of diverse inducible genes involved in many biological processes; therefore, aberrant NFAT expression is responsible for the development and exacerbation of various disorders. Since five isoforms of NFAT (NFATc1-c4, NFAT5) exhibit distinct and overlapping functions, selective control of a part, but not all, of NFAT family members is desirable. By comparing the binding activity of each NFATc1-c4 with its regulatory enzyme, calcineurin (CN), using a quantitative immunoprecipitation assay, we found a new CN-binding region (CNBR) selectively functioning in NFATc1 and NFATc4. This region, termed CNBR3, is located between two preexisting CNBR1 and CNBR2, within the Ca²⁺ regulatory domain. The nuclear translocation of NFATc1 but not NFATc2 in T cells was suppressed by ectopic expression of CNBR3 and, accordingly, NFATc1-dependent cytokine expression was downregulated. Through competition assays using NFATc1-derived partial peptides and mass spectrometry with photoaffinity technology, we identified 18 amino acids in NFATc1 (Arg²⁵⁸ to Pro²⁷⁵ ) and 13 amino acids in CN catalytic subunit (CNA) (Asn⁷⁷ to Gly⁸⁹ ) responsible for CNA/CNBR3 binding in which Cys²⁶³ and Asp⁸² , respectively, played crucial roles. The possible selective regulation of NFAT-mediated biological processes by targeting this new CN/NFAT-binding region is suggested.

Calcineurin-AKAP interactions: therapeutic targeting of a pleiotropic enzyme with a little help from its friends

The ubiquitous Ca²⁺ /calmodulin-dependent phosphatase calcineurin is a key regulator of pathological cardiac hypertrophy whose therapeutic targeting in heart disease has been elusive due to its role in other essential biological processes. Calcineurin is targeted to diverse intracellular compartments by association with scaffold proteins, including by multivalent A-kinase anchoring proteins (AKAPs) that bind protein kinase A and other important signalling enzymes determining cardiac myocyte function and phenotype. Calcineurin anchoring by AKAPs confers specificity to calcineurin function in the cardiac myocyte. Targeting of calcineurin 'signalosomes' may provide a rationale for inhibiting the phosphatase in disease.

Screening for Selective Protein Inhibitors by Using the IANUS Peptide Array

Finding new road blacks: A peptidic inhibitor of calcineurin (CaN)-mediated nuclear factor of activated T cells (NFAT) dephosphorylation, which is developed through a template-assisted IANUS (Induced orgANisation of strUcture by matrix-assisted togethernesS) peptide array, is cell permeable and able to block the translocation of green fluorescent protein-NFAT fusion protein (GFP-NFAT) into the nucleus after stimulation.

Rho GDP dissociation inhibitor-β in renal cell carcinoma

Rho GDP dissociation inhibitor-β (ARHGDIB) is an important mediator of cell signaling. The expression of ARHGDIB is associated with tumor growth and metastasis in a variety of non-genitourinary cancers; however, the role of ARHGDIB in renal cell carcinoma (RCC) has not yet been evaluated. In the present study, tissue samples from 105 patients undergoing surgery for RCC were obtained. The expression levels of ARHGDIB mRNA in normal kidney tissues and in corresponding cancer tissues were analyzed by reverse transcription-quantitative polymerase chain reaction. Differences in relative mRNA expression levels were assessed using paired two-sample t-tests. Expression levels were analyzed with respect to various clinical parameters, and associations were tested using a bivariate logistic regression model. Relative mRNA expression levels in healthy renal tissues compared with cancerous tissues from the same kidney were assessed using paired t-tests. Expression data were compared with respect to survival data by the Kaplan-Meier method/Cox regression analysis. The results revealed that the relative mRNA expression level of ARHGDIB was significantly higher in the lysates of RCC tumor tissues (P<0.001) when compared with healthy renal tissues in a paired analysis of 74 samples; this finding was consistent with the analysis of ARHGDIB mRNA expression levels in all RCC samples, as well as in the subset of clear cell RCC (ccRCC) samples. The relative mRNA expression level of ARHGDIB was also increased in ccRCC tissues compared with papillary RCC tissues (P<0.001). On univariate Cox regression analysis, recurrence-free survival (RFS) was significantly associated with metastasis, locally advanced disease and tumor grade (P=0.018, P=0.002 and P<0.001, respectively). Furthermore, in the subgroup of patients with ccRCC, increased ARHGDIB mRNA expression was significantly associated with a longer RFS time (P=0.001). In summary, the results indicate that ARHGDIB mRNA is highly expressed in RCC tissues in general and is positively associated with RFS in ccRCC. As ARHGDIB has a known effect on angiogenesis and immune modulation, the present study suggests that the functional analysis of ARHGDIB should be performed in the future.

Peptides derived from transcription factor EB bind to calcineurin at a similar region as the NFAT-type motif

Calcineurin (CN) is involved in many physiological processes and interacts with multiple substrates. Most of the substrates contain similar motifs recognized by CN. Recent studies revealed a new CN substrate, transcription factor EB (TFEB), which is involved in autophagy. We showed that a 15-mer QSYLENPTSYHLQQS peptide from TFEB (TFEB-YLENP) bound to CN. When the TFEB-YLENP peptide was changed to YLAVP, its affinity for CN increased and it had stronger CN inhibitory activity. Molecular dynamics simulations revealed that the TFEB-YLENP peptide has the same docking sites in CN as the 15-mer DQYLAVPQHPYQWAK motif of the nuclear factor of activated T cells, cytoplasmic 1 (NFATc1-YLAVP). Moreover expression of the NFATc1-YLAVP peptide suppressed the TFEB activation in starved Hela cells. Our studies first identified a CN binding site in TFEB and compared the inhibitory capability of various peptides derived from CN substrates. The data uncovered a diversity in recognition sequences that underlies the CN signaling within the cell. Studies of CN-substrate interactions should lay the groundwork for developing selective CN peptide inhibitors that target CN-substrate interaction in vitro experiments.

RCAN-11R peptide provides immunosuppression for fully mismatched islet allografts in mice

Calcineurin inhibitors have been used for transplant therapy. However, the inhibition of calcineurin outside the immune system has a number of side effects. We previously developed a cell-permeable inhibitor of NFAT (nuclear factor of activated T cells) using the polyarginine peptide delivery system. This peptide (11R-VIVIT) selectively interferes with calcineurin-NFAT interaction without affecting the activity of calcineurin phosphatase and provides immunosuppression for fully mismatched islet allografts in mice. However, our recent study showed that 11R-VIVIT affected cell viability in vitro when it was used at higher concentration because of the VIVIT sequence. The aim of this study is to develop a safer NFAT inhibitor (RCAN-11R) that does not affect cell viability, and which is less toxic than calcineurin inhibitors. The minimal sequence of the protein family of regulators of calcineurin (RCAN) that is responsible for the inhibition of calcineurin-NFAT signaling was recently characterized. The peptide could selectively interfere with the calcineurin-NFAT interaction without affecting the activity of calcineurin phosphatase, similar to 11R-VIVIT. RCAN-11R did not affect cell viability when it was used at a higher concentration than the toxic concentration of 11R-VIVIT. RCAN-11R could therefore be useful as a therapeutic agent that is less toxic than current drugs or 11R-VIVIT.

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.

Identification of small-molecule inhibitors of calcineurin-NFATc signaling that mimic the PxIxIT motif of calcineurin binding partners

Calcineurin (CN), a serine and threonine protein phosphatase that depends on Ca(2+) and calmodulin for its activity, is the target of the immunosuppressant drugs cyclosporin A (CsA) and tacrolimus (FK506). CN dephosphorylates and activates members of the NFATc (nuclear factor of activated T cells) family of transcription factors in T cells by binding to their conserved PxIxIT motif. Upon dephosphorylation, NFATc proteins translocate to the nucleus, where they stimulate the expression of genes encoding cytokines and chemokines that are required for T cell proliferation and the immune response. We performed a pharmacophore-based virtual screening of ~5.5 million commercially available, "drug-like" compounds to identify nonpeptidic compounds that inhibited the CN-dependent activation of NFATc signaling and that could serve as potential drug candidates for immunosuppressive therapy. Of 32 compounds that mimicked the PxIxIT motif, 7 competed with NFATc for binding to CN in vitro without interfering with the phosphatase activity of CN. Furthermore, in activated human CD4(+) T cells, four of the seven compounds inhibited the expression of NFATc-dependent genes, cytokine production, and cell proliferation, suggesting that these may have therapeutic potential as immunosuppressive agents.

The LQLP calcineurin docking site is a major determinant of the calcium-dependent activation of human TRESK background K+ channel

Calcium-dependent activation of human TRESK (TWIK-related spinal cord K(+) channel, K2P18.1) depends on direct targeting of calcineurin to the PQIIIS motif. In the present study we demonstrate that TRESK also contains another functionally relevant docking site for the phosphatase, the LQLP amino acid sequence. Combined mutations of the PQIIIS and LQLP motifs were required to eliminate the calcium-dependent regulation of the channel. In contrast to the alanine substitutions of PQIIIS, the mutation of LQLP to AQAP alone did not significantly change the amplitude of TRESK activation evoked by the substantial elevation of cytoplasmic calcium concentration. However, the AQAP mutation slowed down the response to high calcium. In addition, modest elevation of [Ca(2+)], which effectively regulated the wild type channel, failed to activate TRESK-AQAP. This indicates that the AQAP mutation diminished the sensitivity of TRESK to calcium. Even if PQIIIS was replaced by the PVIVIT sequence of high calcineurin binding affinity, the effect of the AQAP mutation was clearly detected in this TRESK-PVIVIT context. Substitution of the LQLP region with the corresponding fragment of NFAT transcription factor, perfectly matching the previously described LXVP calcineurin-binding consensus sequence, increased the calcium-sensitivity of TRESK-PVIVIT. Thus the enhancement of the affinity of TRESK for calcineurin by the incorporation of PVIVIT could not compensate for or prevent the effects of LQLP sequence modifications, suggesting that the two calcineurin-binding regions play distinct roles in the regulation. Our results indicate that the LQLP site is a fundamental determinant of the calcium-sensitivity of human TRESK.

Inhibition of interleukin-2 gene expression by human herpesvirus 6B U54 tegument protein

Human herpesvirus 6B (HHV-6B) is a ubiquitous pathogen causing lifelong infections in approximately 95% of humans worldwide. To persist within its host, HHV-6B has developed several immune evasion mechanisms, such as latency, during which minimal proteins are expressed, and the ability to disturb innate and adaptive immune responses. The primary cellular targets of HHV-6B are CD4(+) T cells. Previous studies by Flamand et al. (L. Flamand, J. Gosselin, I. Stefanescu, D. Ablashi, and J. Menezes, Blood 85:1263-1271, 1995) reported on the capacity of HHV-6A as well as UV-irradiated HHV-6A to inhibit interleukin-2 (IL-2) synthesis in CD4(+) lymphocytes, suggesting that viral structural components could be responsible for this effect. In the present study, we identified the HHV-6B U54 tegument protein (U54) as being capable of inhibiting IL-2 expression. U54 binds the calcineurin (CaN) phosphatase enzyme, causing improper dephosphorylation and nuclear translocation of NFAT (nuclear factor of activated T cells) proteins, resulting in suboptimal IL-2 gene transcription. The U54 GISIT motif (amino acids 293 to 297), analogous to the NFAT PXIXIT motif, contributed to the inhibition of NFAT activation. IMPORTANCE Human herpesvirus 6A (HHV-6A) and HHV-6B are associated with an increasing number of pathologies. These viruses have developed strategies to avoid the immune response allowing them to persist in the host. Several studies have illustrated mechanisms by which HHV-6A and HHV-6B are able to disrupt host defenses (reviewed in L. Dagna, J. C. Pritchett, and P. Lusso, Future Virol. 8:273-287, 2013, doi:10.2217/fvl.13.7). Previous work informed us that HHV-6A is able to suppress synthesis of interleukin-2 (IL-2), a key immune growth factor essential for adequate T lymphocyte proliferation and expansion. We obtained evidence that HHV-6B also inhibits IL-2 gene expression and identified the mechanisms by which it does so. Our work led us to the identification of U54, a virion-associated tegument protein, as being responsible for suppression of IL-2. Consequently, we have identified HHV-6B U54 protein as playing a role in immune evasion. These results further contribute to our understanding of HHV-6 interactions with its human host and the efforts deployed to ensure its long-term persistence.

Inhibition of breast cancer cell proliferation through disturbance of the calcineurin/NFAT pathway by human herpesvirus 6B U54 tegument protein

Nuclear factor of activated T cell (NFAT) proteins are key regulators involved in multiple physiological mechanisms, such as immune response and cell growth. The capacity of selective calcineurin/NFAT inhibitors to decrease NFAT-dependent cancer cell progression, particularly in breast cancer, has already been demonstrated. In this study, we report a role for the human herpesvirus 6B (HHV-6B) U54 tegument protein in inhibiting MCF-7 breast cancer cell proliferation by inhibiting NFAT activation.

Calcineurin-mediated YB-1 dephosphorylation regulates CCL5 expression during monocyte differentiation

Y-box (YB) protein-1 serves as a master regulator in gene transcription and mRNA translation. YB-1 itself is regulated at various levels, e.g. through post-translational modifications. In our previous work, we identified RANTES/CCL5 as a transcriptional target of YB-1. We previously demonstrated that YB-1 protein is transiently up-regulated during monocyte/macrophage differentiation evidenced in monocytic cells (THP-1 cells) that were differentiated using phorbol myristate acetate (PMA). Here we provide evidence that YB-1 phosphorylation, specifically at its serine residue 102 (Ser-102), increases early on in THP-1 cells following PMA treatment as well as in differentiated primary human monocytes. This process is mediated through the Akt signaling pathway. Ser-102-phosphorylated YB-1 displays stronger binding affinity and trans-activating capacity at the CCL5 gene promoter. Notably, Ser-102-phosphorylated YB-1 disappears at later stages of the monocyte/macrophage differentiation process. We demonstrate that serine-threonine phosphatase calcineurin (CN) dephosphorylates YB-1 preventing it from binding to and trans-activating the CCL5 promoter. Co-immunoprecipitation assays prove a direct YB-1/CN interaction. Furthermore, analyses in kidney tissues from mice that were treated with the CN inhibitor cyclosporine A revealed an in vivo effect of CN on the YB-1 phosphorylation status. We conclude that YB-1 phosphorylation at Ser-102 is an important prerequisite for CCL5 promoter activation during macrophage differentiation. Our findings point to a critical role of YB-1 in the resolution of inflammatory processes which may largely be due to CN-mediated dephosphorylation.

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.

The molecular mechanism of substrate engagement and immunosuppressant inhibition of calcineurin

Ser/thr phosphatases dephosphorylate their targets with high specificity, yet the structural and sequence determinants of phosphosite recognition are poorly understood. Calcineurin (CN) is a conserved Ca(2+)/calmodulin-dependent ser/thr phosphatase and the target of immunosuppressants, FK506 and cyclosporin A (CSA). To investigate CN substrate recognition we used X-ray crystallography, biochemistry, modeling, and in vivo experiments to study A238L, a viral protein inhibitor of CN. We show that A238L competitively inhibits CN by occupying a critical substrate recognition site, while leaving the catalytic center fully accessible. Critically, the 1.7 Å structure of the A238L-CN complex reveals how CN recognizes residues in A238L that are analogous to a substrate motif, "LxVP." The structure enabled modeling of a peptide substrate bound to CN, which predicts substrate interactions beyond the catalytic center. Finally, this study establishes that "LxVP" sequences and immunosuppressants bind to the identical site on CN. Thus, FK506, CSA, and A238L all prevent "LxVP"-mediated substrate recognition by CN, highlighting the importance of this interaction for substrate dephosphorylation. Collectively, this work presents the first integrated structural model for substrate selection and dephosphorylation by CN and lays the groundwork for structure-based development of new CN inhibitors.

Novel interactions between NFATc1 (Nuclear Factor of Activated T cells c1) and STAT-3 (Signal Transducer and Activator of Transcription-3) mediate G protein-coupled receptor agonist, thrombin-induced biphasic expression of cyclin D1, with first phase influencing cell migration and second phase directing cell proliferation

Thrombin, a G protein-coupled receptor agonist, induced a biphasic expression of cyclin D1 in primary vascular smooth muscle cells. Although both phases of cyclin D1 expression require binding of the newly identified cooperative complex, NFATc1·STAT-3, to its promoter, the second phase, which is more robust, depends on NFATc1-mediated recruitment of p300 onto the complex and the subsequent acetylation of STAT-3. In addition, STAT-3 is tyrosine-phosphorylated in a biphasic manner, and the late phase requires NFATc1-mediated p300-dependent acetylation. Furthermore, interference with acetylation of STAT-3 by overexpression of acetylation null STAT-3 mutant led to the loss of the late phase of cyclin D1 expression. EMSA analysis and reporter gene assays revealed that NFATc1·STAT-3 complex binding to the cyclin D1 promoter led to an enhanceosome formation and facilitated cyclin D1 expression. In the early phase of its expression, cyclin D1 is localized mostly in the cytoplasm and influenced cell migration. However, during the late and robust phase of its expression, cyclin D1 is translocated to the nucleus and directed cell proliferation. Together, these results demonstrate for the first time that the dual function of cyclin D1 in cell migration and proliferation is temperospatially separated by its biphasic expression, which is mediated by cooperative interactions between NFATc1 and STAT-3.

CNP-1 (ARRD-17), a novel substrate of calcineurin, is critical for modulation of egg-laying and locomotion in response to food and lysine sensation in Caenorhabditis elegans

Calcineurin is a Ca(2+)/calmodulin-dependent protein phosphatase involved in calcium signaling pathways. In Caenorhabditis elegans, the loss of calcineurin activity causes pleiotropic defects including hyperadaptation of sensory neurons, hypersensation to thermal difference and hyper-egg-laying when worms are refed after starvation. In this study, we report on arrd-17 as calcineurin-interacting protein-1 (cnp-1), which is a novel molecular target of calcineurin. CNP-1 interacts with the catalytic domain of the C. elegans calcineurin A subunit, TAX-6, in a yeast two-hybrid assay and is dephosphorylated by TAX-6 in vitro. cnp-1 is expressed in ASK, ADL, ASH and ASJ sensory neurons as TAX-6. It acts downstream of tax-6 in regulation of locomotion and egg-laying after starvation, ASH sensory neuron adaptation and lysine chemotaxis, that is known to be mediated by ASK neurons. Altogether, our biochemical and genetic evidence indicates that CNP-1 is a direct target of calcineurin and required in stimulated egg-laying and locomotion after starvation, adaptation to hyperosmolarity and attraction to lysine, which is modulated by calcineurin. We suggest that the phosphorylation status of CNP-1 plays an important role in regulation of refed stimulating behaviors after starvation and attraction to amino acid, which provides valuable nutritious information.

Genome-wide microarray analyses identify the protein C receptor as a novel calcineurin/nuclear factor of activated T cells-dependent gene in vascular smooth muscle cell phenotypic modulation

OBJECTIVE

Calcineurin (Cn) and the nuclear factor of activated T cells (NFAT) family of transcription factors are critical in vascular smooth muscle cell (SMC) development and pathology. Here, we used a genomics approach to identify and validate NFAT gene targets activated during platelet-derived growth factor-BB (PDGF-BB)-induced SMC phenotypic modulation.

METHODS AND RESULTS

Genome-wide expression arrays were used to identify genes both (1) differentially activated in response to PDGF-BB and (2) whose differential expression was reduced by both the Cn inhibitor cyclosporin A and the NFAT inhibitor A-285222. The 20 most pharmacologically sensitive genes were validated by quantitative reverse transcription-polymerase chain reaction analysis of PDGF-BB-stimulated SMCs in the presence of Cn/NFAT inhibitors, including the VIVIT peptide. In all experiments, protein C receptor (PROCR) gene activation was reduced. We showed that PROCR expression was virtually absent in untreated, quiescent SMCs. PDGF-BB stimulation, however, induced significant PROCR promoter activation and downstream protein expression in a Cn/NFAT-dependent manner. Mutation of a species-conserved, NFAT binding motif significantly attenuated PDGF-BB-induced PROCR promoter activity, thereby distinguishing NFAT as the first PROCR transcriptional activator to date. Moreover, SMC PROCR expression was upregulated in the neointima as early as 7 days following acute vascular injury in rat carotid arteries.

CONCLUSION

We hereby report PROCR as a novel, NFAT-dependent gene that may be implicated in vascular restenosis and consequent inward remodeling.

Smyd1b_tv1, a key regulator of sarcomere assembly, is localized on the M-line of skeletal muscle fibers

Background

Smyd1b is a member of the Smyd family that plays a key role in sarcomere assembly during myofibrillogenesis. Smyd1b encodes two alternatively spliced isoforms, smyd1b_tv1 and smyd1b_tv2, that are expressed in skeletal and cardiac muscles and play a vital role in myofibrillogenesis in skeletal muscles of zebrafish embryos.

Methodology/Principal Findings

To better understand Smyd1b function in myofibrillogenesis, we analyzed the subcellular localization of Smyd1b_tv1 and Smyd1b_tv2 in transgenic zebrafish expressing a myc-tagged Smyd1b_tv1 or Smyd1b_tv2. The results showed a dynamic change of their subcellular localization during muscle cell differentiation. Smyd1b_tv1 and Smyd1b_tv2 were primarily localized in the cytosol of myoblasts and myotubes at early stage zebrafish embryos. However, in mature myofibers, Smyd1b_tv1, and to a small degree of Smyd1b_tv2, exhibited a sarcomeric localization. Double staining with sarcomeric markers revealed that Smyd1b_tv1was localized on the M-lines. The sarcomeric localization was confirmed in zebrafish embryos expressing the Smyd1b_tv1-GFP or Smyd1b_tv2-GFP fusion proteins. Compared with Smyd1b_tv1, Smyd1b_tv2, however, showed a weak sarcomeric localization. Smyd1b_tv1 differs from Smyd1b_tv2 by a 13 amino acid insertion encoded by exon 5, suggesting that some residues within the 13 aa insertion may be critical for the strong sarcomeric localization of Smyd1b_tv1. Sequence comparison with Smyd1b_tv1 orthologs from other vertebrates revealed several highly conserved residues (Phe223, His224 and Gln226) and two potential phosphorylation sites (Thr221 and Ser225) within the 13 aa insertion. To determine whether these residues are involved in the increased sarcomeric localization of Smyd1b_tv1, we mutated these residues into alanine. Substitution of Phe223 or Ser225 with alanine significantly reduced the sarcomeric localization of Smyd1b_tv1. In contrast, other substitutions had no effect. Moreover, replacing Ser225 with threonine (S225T) retained the strong sarcomeric localization of Smyd1b_tv1.

Conclusion/Significance

Together, these data indicate that Phe223 and Ser225 are required for the M-line localization of Smyd1b_tv1.

Calcineurin selectively docks with the dynamin Ixb splice variant to regulate activity-dependent bulk endocytosis

Depolarization of nerve terminals stimulates rapid dephosphorylation of two isoforms of dynamin I (dynI), mediated by the calcium-dependent phosphatase calcineurin (CaN). Dephosphorylation at the major phosphorylation sites Ser-774/778 promotes a dynI-syndapin I interaction for a specific mode of synaptic vesicle endocytosis called activity-dependent bulk endocytosis (ADBE). DynI has two main splice variants at its extreme C terminus, long or short (dynIxa and dynIxb) varying only by 20 (xa) or 7 (xb) residues. Recombinant GST fusion proteins of dynIxa and dynIxb proline-rich domains (PRDs) were used to pull down interacting proteins from rat brain nerve terminals. Both bound equally to syndapin, but dynIxb PRD exclusively bound to the catalytic subunit of CaNA, which recruited CaNB. Binding of CaN was increased in the presence of calcium and was accompanied by further recruitment of calmodulin. Point mutations showed that the entire C terminus of dynIxb is a CaN docking site related to a conserved CaN docking motif (PXIXI(T/S)). This sequence is unique to dynIxb among all other dynamin variants or genes. Peptide mimetics of the dynIxb tail blocked CaN binding in vitro and selectively inhibited depolarization-evoked dynI dephosphorylation in nerve terminals but not of other dephosphins. Therefore, docking to dynIxb is required for the regulation of both dynI splice variants, yet it does not regulate the phosphorylation cycle of other dephosphins. The peptide blocked ADBE, but not clathrin-mediated endocytosis of synaptic vesicles. Our results indicate that Ca(2+) influx regulates assembly of a fully active CaN-calmodulin complex selectively on the tail of dynIxb and that the complex is recruited to sites of ADBE in nerve terminals.

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.

Calcineurin regulates coelomocyte endocytosis via DYN-1 and CUP-4 in Caenorhabditis elegans

C. elegans coelomocytes are macrophage-like scavenger cells that provide an excellent in vivo system for the study of clathrin-mediated endocytosis. Using this in vivo system, several genes involved in coelomocyte endocytosis have been identified previously. However, the detailed mechanism of endocytic pathway is still unknown. Here, we report a new function of calcineurin, an evolutionarily conserved Ca(2+)/calmodulin-dependent Ser/Thr protein phosphatase, in coelomocyte endocytosis. We found that calcineurin mutants show defective coelomocyte endocytosis. Genetic analysis suggests that calcineurin and a GTPase, dynamin (DYN-1), may function upstream of an orphan receptor, CUP-4, to regulate endocytosis. Therefore, we propose a model in which calcineurin may regulate coelomocyte endocytosis via DYN-1 and CUP-4 in C. elegans.

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.

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.

A conserved docking surface on calcineurin mediates interaction with substrates and immunosuppressants

The phosphatase calcineurin, a target of the immunosuppressants cyclosporin A and FK506, dephosphorylates NFAT transcription factors to promote immune activation and development of the vascular and nervous systems. NFAT interacts with calcineurin through distinct binding motifs: the PxIxIT and LxVP sites. Although many calcineurin substrates contain PxIxIT motifs, the generality of LxVP-mediated interactions is unclear. We define critical residues in the LxVP motif, and we demonstrate its binding to a hydrophobic pocket at the interface of the two calcineurin subunits. Mutations in this region disrupt binding of mammalian calcineurin to NFATC1 and the interaction of yeast calcineurin with substrates including Rcn1, which contains an LxVP motif. These mutations also interfere with calcineurin-immunosuppressant binding, and an LxVP-based peptide competes with immunosuppressant-immunophilin complexes for binding to calcineurin. These studies suggest that LxVP-type sites are a common feature of calcineurin substrates, and that immunosuppressant-immunophilin complexes inhibit calcineurin by interfering with this mode of substrate recognition.

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.

Inhibiting the calcineurin-NFAT (nuclear factor of activated T cells) signaling pathway with a regulator of calcineurin-derived peptide without affecting general calcineurin phosphatase activity

Calcineurin phosphatase plays a crucial role in T cell activation. Dephosphorylation of the nuclear factors of activated T cells (NFATs) by calcineurin is essential for activating cytokine gene expression and, consequently, the immune response. Current immunosuppressive protocols are based mainly on calcineurin inhibitors, cyclosporine A and FK506. Unfortunately, these drugs are associated with severe side effects. Therefore, immunosuppressive agents with higher selectivity and lower toxicity must be identified. The immunosuppressive role of the family of proteins regulators of calcineurin (RCAN, formerly known as DSCR1) which regulate the calcineurin-NFAT signaling pathway, has been described recently. Here, we identify and characterize the minimal RCAN sequence responsible for the inhibition of calcineurin-NFAT signaling in vivo. The RCAN-derived peptide spanning this sequence binds to calcineurin with high affinity. This interaction is competed by a peptide spanning the NFAT PXIXIT sequence, which binds to calcineurin and facilitates NFAT dephosphorylation and activation. Interestingly, the RCAN-derived peptide does not inhibit general calcineurin phosphatase activity, which suggests that it may have a specific immunosuppressive effect on the calcineurin-NFAT signaling pathway. As such, the RCAN-derived peptide could either be considered a highly selective immunosuppressive compound by itself or be used as a new tool for identifying innovative immunosuppressive agents. We developed a low throughput assay, based on the RCAN1-calcineurin interaction, which identifies dipyridamole as an efficient in vivo inhibitor of the calcineurin-NFAT pathway that does not affect calcineurin phosphatase activity.

Mechanism of nuclear factor of activated T-cells mediated FasL expression in corticosterone -treated mouse Leydig tumor cells

BACKGROUND

Fas and FasL is important mediators of apoptosis. We have previously reported that the stress levels of corticosterone (CORT, glucocorticoid in rat) increase expression of Fas/FasL and activate Fas/FasL signal pathway in rat Leydig cells, which consequently leads to apoptosis. Moreover, our another study showed that nuclear factor of activated T-cells (NFAT) may play a potential role in up-regulation of FasL during CORT-treated rat Leydig cell. It is not clear yet how NFAT is involved in CORT-induced up-regulation of FasL. The aim of the present study is to investigate the molecular mechanisms of NFAT-mediated FasL expression in CORT-treated Leydig cells.

RESULTS

Western blot analysis showed that NFAT2 expression is present in mouse Leydig tumor cell (mLTC-1). CORT-induced increase in FasL expression in mLTC-1 was ascertained by Western Blot analysis and CORT-induced increase in apoptotic frequency of mLTC-1 cells was detected by FACS with annexin-V labeling. Confocal imaging of NFAT2-GFP in mLTC-1 showed that high level of CORT stimulated NFAT translocation from the cytoplasm to the nucleus. RNA interference-mediated knockdown of NFAT2 significantly attenuated CORT-induced up-regulation of FasL expression in mLTC. These results corroborated our previous finding that NFAT2 is involved in CORT-induced FasL expression in rat Leydig cells and showed that mLTC-1 is a suitable model for investigating the mechanism of CORT-induced FasL expression. The analysis of reporter constructs revealed that the sequence between -201 and +71 of mouse FasL gene is essential for CORT-induced FasL expression. The mutation analysis demonstrated that CORT-induced FasL expression is mediated via an NFAT binding element located in the -201 to +71 region. Co-transfection studies with an NFAT2 expression vector and reporter construct containing -201 to +71 region of FasL gene showed that NFAT2 confer a strong inducible activity to the FasL promoter at its regulatory region. In addition, chromatin immunoprecipitation assay further confirmed the results of reporter gene studies by showing the specific binding of NFAT2 to the -201 to +71 region.

CONCLUSION

In the present study, we demonstrated that NFAT2 directly stimulates transcription of FasL in high level CORT-treated mLTC-1. In conclusion, the present study provides further evidence for our finding that CORT-induced FasL expression in Leydig cells is mediated by NFAT.

Domains involved in calcineurin phosphatase inhibition and nuclear localisation in the African swine fever virus A238L protein

The African swine fever virus A238L protein inhibits calcineurin phosphatase activity and activation of NF-kappaB and p300 co-activator. An 82 amino acid domain containing residues 157 to 238 at the C-terminus of A238L was expressed in E. coli and purified. This purified A238L fragment acted as a potent inhibitor of calcineurin phosphatase in vitro with an IC50 of approximately 70 nM. Two putative nuclear localisation signals were identified between residues 80 to 86 (NLS-1) and between residues 203 to 207 overlapping with the N-terminus of the calcineurin docking motif (NLS-2). Mutation of these motifs independently did not reduce nuclear localisation compared to the wild type A238L protein, whereas mutation of both motifs significantly reduced nuclear localisation of A238L. Mutation of the calcineurin docking motif resulted in a dramatic increase in the nuclear localisation of A238L provided an intact NLS was present. We propose that binding of calcineurin to A238L masks NLS-2 contributing to the cytoplasmic retention of A238L.

T-type Ca2+ channel blockers prevent cardiac cell hypertrophy through an inhibition of calcineurin-NFAT3 activation as well as L-type Ca2+ channel blockers

T-type Ca2+ channels (TCCs) are involved in cardiac cell growth and proliferation in cultured cardiomyocytes. Underlying molecular mechanisms are not well understood. In this study, we investigated the role of TCCs in signal transduction in cardiac hypertrophy compared with L-type Ca2+ channels (LCCs). Cardiomyocytes dissociated from neonatal mouse ventricles were cultured until stabilization. Cell hypertrophy was induced by reapplication of 1% fatal bovine serum (FBS) following a period (24 h) of FBS depletion. Cell surface area increased from 862+/-73 microm2 to 2153+/-131 microm2 by FBS stimulation in control (250+/-1.8%). T-type Ca2+ current (I(CaT)) was inhibited dose-dependently by kurtoxin (KT) and efonidipine (ED) with IC50 0.07 microM and 3.2 microM, respectively in whole-cell voltage clamp. On the other hand, 1 microM KT which inhibits I(CaT) over 90% did not effect on L-type Ca2+ current (I(CaL)). 10 microM ED had the ability of I(CaL) blockade as well as that of I(CaT) blockade. 3 microM nisoldipine (ND) suppressed I(CaL) by over 80%. The increase in cell surface area following reapplication of FBS as observed in control (250+/-1.8%) was significantly reduced in the presence of 1 microM KT (216+/-1.2%) and virtually abolished in the presence of 10 microM ED (97+/-0.8%) and 3 microM ND (80+/-1.1%). Hypertrophy was associated with an increase in BNP mRNA of 316+/-3.6% in control and this increase was reduced as well in the presence of 1 microM KT (254+/-1.8%) and almost abolished in the presence of 10 microM ED (116+/-1.1%) and 3 muM ND (93+/-0.8%). Immunolabeling showed that translocation of nuclear factor of activated T cells (NFAT3) into the nucleus in response to FBS stimulation was markedly inhibited by either KT or ED as well as ND. Calcineurin phosphatase activity was upregulated 2.2-fold by FBS, but KT, ED and ND decreased this upregulation (1.7-fold, 0.8-fold, and 0.7-fold with KT, ED and ND respectively). These results suggest that blockade of Ca2+ entry into cardiomyocytes via TCCs may block pathophysiological signaling pathways leading to hypertrophy as well as via LCCs. The mechanism may be the inhibition of calcineurin-mediated NFAT3 activation resulting in prevention of its translocation into the nucleus.

Protein kinase A, Ca2+/calmodulin-dependent kinase II, and calcineurin regulate the intracellular trafficking of myopodin between the Z-disc and the nucleus of cardiac myocytes

Spatial and temporal resolution of intracellular signaling can be achieved by compartmentalizing transduction units. Myopodin is a dual-compartment, actin-bundling protein that shuttles between the nucleus and the Z-disc of myocytes in a differentiation- and stress-dependent fashion. Importin alpha binding and nuclear import of myopodin are regulated by serine/threonine phosphorylation-dependent binding of myopodin to 14-3-3. Here we show that in the heart myopodin forms a Z-disc signaling complex with alpha-actinin, calcineurin, Ca2+/calmodulin-dependent kinase II (CaMKII), muscle-specific A-kinase anchoring protein, and myomegalin. Phosphorylation of myopodin by protein kinase A (PKA) or CaMKII mediates 14-3-3 binding and nuclear import in myoblasts. Dephosphorylation of myopodin by calcineurin abrogates 14-3-3beta binding. Activation of PKA or inhibition of calcineurin in adult cardiac myocytes releases myopodin from the Z-disc and induces its nuclear import. The identification of myopodin as a direct target of PKA, CaMKII, and calcineurin defines a novel intracellular signaling pathway whereby changes in Z-disc dynamics may translate into compartmentalized signal transduction in the heart.

Structure of the calcineurin-NFAT complex: defining a T cell activation switch using solution NMR and crystal coordinates

Calcineurin (Cn) is a serine/threonine protein phosphatase that plays pivotal roles in many physiological processes, including cell proliferation, development, and apoptosis. Most prominently, Cn targets the nuclear factors of activated T cell (NFATs), transcription factors that activate cytokine genes. Calcium-activated Cn dephosphorylates multiple residues within the regulatory domain of NFAT, triggering joint nuclear translocation. This relies crucially on the interaction between the catalytic domain of Cn (CnCat) and the conserved PxIxIT motif located in a region distinct from the dephosphorylation sites of NFAT. Here, we present the structure of the complex between the 39 kDa CnCat and a 14 residue peptide containing a PVIVIT segment that was derived from affinity-driven peptide selection based on the conserved PxIxIT motif of NFATs. The structure of the complex was determined by using NMR assignments and structural constraints and the coordinates of the CnCat crystal structure. The NMR analysis relied on recently developed labeling and spectroscopic techniques. The VIVIT peptide is accommodated in a hydrophobic cleft formed by beta strands 11 and 14, and the loop between beta strands 11 and 12, forming a short parallel beta sheet with the exposed beta strand 14 in Cn. The side chains of conserved residues in the PxIxIT sequences make extensive interactions with conserved residues in Cn, while those of nonconserved residues are solvent exposed. The architecture of the interface explains the diversity of recognition sequences compatible with NFAT function and uncovers a potential targeting site for immune-suppressive agents. The structure reveals that the orientation of the bound PxIxIT directs the phosphorylation sites in NFAT's regulatory domain toward the Cn catalytic site.

Nuclear translocation of calcineurin Aβ but not calcineurin Aα by platelet-derived growth factor in rat aortic smooth muscle

Calcineurin regulates the proliferation of many cell types through activation of the nuclear factor of activated T cells (NFAT). Two main isoforms of the calcineurin catalytic subunit [calcineurin A (CnA)α and CnAβ] have been identified, although their expression and function are largely unknown in smooth muscle. Western blot analysis and confocal imaging were performed in freshly isolated and cultured rat aortic myocytes to identify these CnA isoforms and elucidate the effect of PDGF on their cellular distribution and interaction with NFAT isoforms. CnAα and CnAβ isoforms displayed differential cellular distribution, with CnAα being evenly distributed between the nucleus and cytosol and CnAβ being restricted to the cytosol. In contrast with the rat brain, we found no evidence for particulate/membrane localization of calcineurin. PDGF caused significant nuclear translocation of CnAβ and induced smooth muscle cell proliferation, with both effects being abrogated by the calcineurin inhibitor cyclosporin A, the novel NFAT inhibitors A-285222 and inhibitor of NFAT-calcineurin association-6, and the adenylyl cyclase activator forskolin. PDGF also caused cyclosporin A-sensitive translocation of NFATc3, with no apparent effect on either CnAα or NFATc1 distribution. Moreover, ∼87% of nuclear CnAβ was found to colocalize with NFATc3, consistent with the finding that CnAβ bound more avidly than CnAα to a glutathione S-transferase-NFATc3 fusion protein. Based on their differential distribution in aortic muscle, our results suggest that CnAα and CnAβ are likely to have different cellular functions. However, CnAβ appears to be specifically activated by PDGF, and we postulate that calcineurin-dependent nuclear translocation of NFATc3 is involved in smooth muscle proliferation induced by this mitogen.

Structure of Calcineurin in Complex with PVIVIT Peptide: Portrait of a Low-affinity Signalling Interaction

The protein phosphatase calcineurin recognizes a wide assortment of substrates and controls diverse developmental and physiological pathways in eukaryotic cells. Dephosphorylation of the transcription factor NFAT and certain other calcineurin substrates depends on docking of calcineurin at a PxIxIT consensus site. We describe here the structural basis for recognition of the PxIxIT sequence by calcineurin. We demonstrate that the high-affinity peptide ligand PVIVIT adds as a beta-strand to the edge of a beta-sheet of calcineurin; that short peptide segments containing the PxIxIT consensus sequence suffice for calcineurin-substrate docking; and that sequence variations within the PxIxIT core modulate the K(d) of the interaction within the physiological range 1 microM to 1 mM. Calcineurin can adapt to a wide variety of substrates, because recognition requires only a PxIxIT sequence and because variation within the core PxIxIT sequence can fine-tune the affinity to match the physiological signalling requirements of individual substrates.