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In continuously beating cells like cardiac myocytes, there are rapid alterations of cytosolic Ca2+ levels. We therefore hypothesize that decoding Ca2+ signals for hypertrophic signaling requires intracellular Ca2+ microdomains that are partly independent from cytosolic Ca2+. Furthermore, there is a need for a Ca2+ sensor within these microdomains that translates Ca2+ signals into hypertrophic signaling. Recent evidence suggested that the nucleus of cardiac myocytes might be a Ca2+ microdomain and that calcineurin, once translocated into the nucleus, could act as a nuclear Ca2+ sensor. We demonstrate that nuclear calcineurin was able to act as a nuclear Ca2+ sensor detecting local Ca2+ release from the nuclear envelope via IP3R. Nuclear calcineurin mutants defective for Ca2+ binding failed to activate NFAT-dependent transcription. Under hypertrophic conditions Ca2+ transients in the nuclear microdomain were significantly higher than in the cytosol providing a basis for sustained calcineurin/NFAT-mediated signaling uncoupled from cytosolic Ca2+. Measurements of nuclear and cytosolic Ca2+ transients in IP3 sponge mice showed no increase of Ca2+ levels during diastole as we detected in wild-type mice. Nuclei, isolated from ventricular myocytes of mice after chronic Ang II treatment, showed an elevation of IP3R2 expression which was dependent on calcineurin/NFAT signaling and persisted for 3 weeks after removal of the Ang II stimulus. These data provide an explanation how Ca2+ and calcineurin might regulate transcription in cardiomyocytes in response to neurohumoral signals independently from their role in cardiac contraction control. • Calcineurin acts as an intranuclear Ca2+ sensor to promote NFAT activity. • Nuclear Ca2+ in cardiac myocytes increases via IP3R2 upon Ang II stimulation. • IP3R2 expression is directly dependent on calcineurin/NFAT.

Aims Elevated levels of inositol 1,4,5-trisphosphate (IP3) in adult cardiac myocytes are typically associated with the development of cardiac hypertrophy, arrhythmias, and heart failure. IP3 enhances intracellular Ca2+ release via IP3 receptors (IP3Rs) located at the sarcoplasmic reticulum (SR). We aimed to determine whether IP3-induced Ca2+ release affects mitochondrial function and determine the underlying mechanisms. Methods and results We compared the effects of IP3Rs- and ryanodine receptors (RyRs)-mediated cytosolic Ca2+ elevation achieved by endothelin-1 (ET-1) and isoproterenol (ISO) stimulation, respectively, on mitochondrial Ca2+ uptake and adenosine triphosphate (ATP) generation. Both ET-1 and isoproterenol induced an increase in mitochondrial Ca2+ (Ca2 +m) but only ET-1 led to an increase in ATP concentration. ET-1-induced effects were prevented by cell treatment with the IP3 antagonist 2-aminoethoxydiphenyl borate and absent in myocytes from transgenic mice expressing an IP3 chelating protein (IP3 sponge). Furthermore, ET-1-induced mitochondrial Ca2+ uptake was insensitive to the mitochondrial Ca2+ uniporter inhibitor Ru360, however was attenuated by RyRs type 1 inhibitor dantrolene. Using real-time polymerase chain reaction, we detected the presence of all three isoforms of IP3Rs and RyRs in murine ventricular myocytes with a dominant presence of type 2 isoform for both receptors. Conclusions Stimulation of IP3Rs with ET-1 induces Ca2+ release from the SR which is tunnelled to mitochondria via mitochondrial RyR leading to stimulation of mitochondrial ATP production.

Background— E193, a heterozygous truncating mutation in the human transcription cofactor Eyes absent 4 (Eya4), causes hearing impairment followed by dilative cardiomyopathy. Methods and Results— In this study, we first show Eya4 and E193 alter the expression of p27 kip1 in vitro, suggesting Eya4 is a negative regulator of p27. Next, we generated transgenic mice with cardiac-specific overexpression of Eya4 or E193. Luciferase and chromatin immunoprecipitation assays confirmed Eya4 and E193 bind and regulate p27 expression in a contradictory manner. Activity and phosphorylation status of the downstream molecules casein kinase-2α and histone deacetylase 2 were significantly elevated in Eya4- but significantly reduced in E193-overexpressing animals compared with wild-type littermates. Magnetic resonance imaging and hemodynamic analysis indicate Eya4-overexpression results in an age-dependent development of hypertrophy already under baseline conditions with no obvious functional effects, whereas E193 animals develop onset of dilative cardiomyopathy as seen in human E193 patients. Both cardiac phenotypes were aggravated on pressure overload. Finally, we identified a new heterozygous truncating Eya4 mutation, E215, which leads to similar clinical features of disease and a stable myocardial expression of the mutant protein as seen with E193. Conclusions— Our results implicate Eya4/Six1 regulates normal cardiac function via p27/casein kinase-2α/histone deacetylase 2 and indicate that mutations within this transcriptional complex and signaling cascade lead to the development of cardiomyopathy.

In continuously beating cells like cardiac myocytes, there are rapid alterations of cytosolic Ca

Introduction: Eyes absent 4 (Eya4) is a transcription cofactor involved in a number of cellular and developmental processes. We have previously shown that a mutation in Eya4 (E193) leads to late-onset familial dilated cardiomyopathy and heart failure. A precise role for Eya4 in the myocardium has not yet been identified. It appears to regulate the cyclin dependent kinase inhibitor p27 kip1 (p27), a protein shown to regulate hypertrophic responses in the adult cardiomyocyte. This study was aimed to explore the role of Eya4 in induced cardiac hypertrophy. Methods and results: We generated transgenic mice with cardiac-specific overexpression of HA-tagged Eya4 or E193 to elucidate the function of these proteins in the development of heart failure in vivo . These and wildtype littermates were challenged with angiotensin II (ATII) or subjected to transaortic constriction (TAC). Magnetic resonance imaging to visualize cardiac structures in detail showed that in response to sustained ATII stimulation and TAC, Eya4 mice exhibited a phenotype with significantly increased parameters of hypertrophy compared to WT and E193 overexpressing animals as judged by increases in heart weight and LV free wall diameter, cross-sectional cell surface areas and fibrosis. MRI also showed mild cardiac hypertrophy in Eya4 transgenic mice already under baseline conditions in an age dependent fashion. Moreover, Eya4 overexpression induced a significant suppression of p27 protein expression and resulted in increased levels of phosphorylated histone deacetylase 2 (HDAC2). E193 overexpression induced age dependent wall thinning and ventricular dilation under baseline conditions with no obvious structural or functional defects. ATII or TAC induced significant changes in HW/BW ratio, IVS, fibrosis, hemodynamic and cell size measurements, albeit to a lesser extent than seen with Eya4 mice. p27 expression and pHDAC2 levels were only slightly altered. Conclusion: In summary, we have demonstrated a mutation in Eya4 to disturb cardiac physiology. We now provide evidence that Eya4 is also involved in forms of acquired heart disease. It seems to suppress p27, which leads to phosphorylation and activation of HDAC2 and results in the development of cardiac hypertrophy.

Background: Recent evidence demonstrates that not only NFAT, but also calcineurin is translocated into the nucleus upon hypertrophic stimulation. Previously it was also demonstrated that calpain-mediated degradation caused a constitutive active calcineurin. We hypothesised that nuclear calcineurin is an intranuclear Ca 2+ sensor hypertrophied myocardium and that inhibition of nuclear translocation of calcineurin is a therapeutic strategy to prevent hypertrophy. Methods: Employing a transgene mouse model with conditional calpastatin overexpression (”tet-off”, resulting in calpain inhibition), different adenoviral calcineurin mutants and confocal microscopy in isolated adult cardiac myocytes we investigated calcineurin translocation and nuclear Ca 2+ transients. Assessment of cardiac function if transgenic animals was performed by 7T MRI. Results: We could demonstrate that chronic Ang II stimulation of mice caused calpain-mediated degradation of calcineurin resulting in a constitutive active calcineurin with nuclear translocation. The constitutive active calcineurin in the nucleus escaped further degradation by the UPS and sustained an ongoing hypertrophic response, even after removal of Ang II. Inhibition of nuclear translocation of activated calcineurin by a small inhibitory peptide prevented myocardial hypertrophy in vivo. Transgenic inhibition of calpain activity by calpastatin overexpression prevented proteolysis of calcineurin and allowed for relocation of calcineurin from the nucleus back to the cytosol and regression hypertrophy after removal of Ang II. We were also able to demonstrate that Ang II increases nuclear Ca 2+ transients via InsP3 receptors and that calcineurin is able to act as nuclear Ca 2+ sensor detecting local Ca 2+ release from the nuclear envelope via InsP3R. Nuclear calcineurin mutants that are defective for Ca 2+ activation failed to activate NFAT dependent transcription. Conclusion: This provides an explanation how Ca 2+ and calcineurin can regulate transcription in cardiomyocytes in response to neurohumoral signals apart from Ca 2+ changes in contraction regulation.

Introduction: We previously identified a mutation in the human transcriptional cofactor Eya4 (E193) as cause of familial dilated cardiomyopathy (DCM) and heart failure. Upon interaction with real transcription factors such as Six family members, Eya4 is recruited to and interacts with target genes. One of the few known Eya-Six targets expressed in the heart is the cyclin-dependent kinase inhibitor p27kip1, which has been shown to inhibit hypertrophic growth in adult cardiomyocytes, is. We therefore hypothesize that Eya4/Six1 regulates targets relevant in normal cardiac function. Methods and results: We examined p27 expression in response to Eya4 in permanent mammalian cell lines. Western blot analysis demonstrated that an overexpression of Eya4 led to a significant downregulation of p27, whereas the overexpression of E193 had no effect on p27 levels. Studies using a p27 promoter fragment including Six1 consensus sites cloned in front of a Luciferase reporter gene proved Eya4 acts as a suppressor of p27 already at the transcriptional level, whereas E193 could not sufficiently inhibit p27 expression. Further transfection and knockdown experiments revealed that under basal conditions an Eya4 overexpression decreased protein synthesis in primary cardiac myocytes. We constructed transgenic mouse models with a constitutive myocardial overexpression of E193 and Eya4 to study the precise role of Eya4 in the heart. First analysis of these animals using magnetic resonance imaging to visualize cardiac structures in detail show that an overexpression of E193 leads to an age related onset of DCM as seen in an increase in LVEDV, whereas Eya4 overexpressing mice show no signs of heart disease. Conclusion: In summary, we identified a mutation in Eya4 to cause DCM. We now provide evidence that the Eya4/Six1 signalling cascade is also relevant in more common forms of acquired heart disease. Eya4/Six1 seems to regulate the expression of p27kip1, an important inhibitor of the development of hypertrophy in postmitotic cardiomyocytes. Studies using E193 overexpressing mice support our hypothesis whereas tempering the Eya4/Six1 signalling cascade disturbs cardiac physiology.

Background. Inhibition of calcineurin (CnA) activity by cyclosporine A (CsA) is the mainstay in immunosuppressive therapy. CsA inhibits the phosphatase activity of the cytosolic phosphatase CnA and, therefore, prevents the dephosphorylation and subsequently nuclear translocation of the transcription factor nuclear factor of activated T cells (NFAT). However, CsA has multiple other targets within the cell and is, therefore, not specific. We developed a new approach to inhibit CnA/NFAT signaling. This synthetic peptide prevented CnA nuclear translocation in vitro. The purpose of this study was to demonstrate that this novel approach could potentially inhibit T-cell function in vitro and in vivo. Methods. T-cell activation (Jurkat T cells, naive rat T cells, and peripheral human T cells) was assessed by protein synthesis, interleukin (IL)-2 promoter activity, and IL-2 levels after T-cell activation. Immunohistological stainings for CnA were performed to investigate nuclear localization of CnA. The immunosuppressive effects in vivo of the synthetic peptide were investigated in rats with heterotopic transplanted hearts. Results. The nuclear localization signal peptide significantly decreased alloantigen-specific T-lymphocyte proliferation, IL-2 promoter activity, and IL-2 production (338%±27% vs. 149%±11%, n=8, P

Background: The calcineurin (Cn)/NF-AT signaling cascade plays a crucial role during T-cell activation and development of myocardial hypertrophy. We previously demonstrated that, in addition to NF-AT, Cn is translocated to the nucleus. We also developed a synthetic peptide (IBP) which inhibited the nuclear import of Cn without affecting calcineurin phosphatase activity. The antihypertrophic effect of IBP on the rat myocardium was shown in vivo and in vitro . Here we extended our studies on a potential use of IBP in the prevention of transplant rejection. Methods and results: Coimmunoprecipitation experiments revealed that the synthetic IBP peptide, which is identical to the nuclear localization sequence of calcineurin, disrupts importin/calcineurin interaction. We further demonstrated that inhibition of the importin/calcineurin interaction by the small competitive peptide (IBP) is sufficient to inactivate the calcineurin/NF-AT signaling cascade. Inhibitory effects of IBP on T-cell activation were analyzed by [H 3 ]-thymidine incorporation (7135 ± 2503 vs. 2957 ± 1161 [%]) in activated T-cells in vitro (dentritic cells were used as antigen stimulation). To verify immunosuppressive effect of IBP in vitro , heterotopic heart transplantations were performed on rats. Following transplantation rats were treated with IBP (i.p. 130mg/kg/d) or peanut oil as control (i.p. 1 ml peanut oil/d) for 14 days. Time until transplant rejection was significantly prolonged in IBP treated animals compared to control group (15.2 ± 1.5 day vs. 4 ± 2.8 day; n = 8; p < 0.05). Immunohistochemical studies from transplanted hearts demonstrated that the synthetic calcineurin inhibitory peptide IBP prevented calcineurin nuclear translocation in infiltrating T cells in the donor hearts. Conclusions: The synthetic inhibitory peptide IBP disrupts the calcineurin/NF-AT cascade by inhibition of Cn nuclear import. This suppresses T-cell activation in vitro and in vivo, thereby revealing a potential use of IBP as novel agent to protect donor organs from rejection.