MEF2 transcriptional activity maintains mitochondrial adaptation in cardiac pressure overload

AIMS: The transcription factor MEF2 is a downstream target for several hypertrophic signalling pathways in the heart, suggesting that MEF2 may act as a valuable therapeutic target in the treatment of heart failure. METHODS AND RESULTS: In this study, we investigated the potential benefits of overall MEF2 inhibition in a mouse model of chronic pressure overloading, by subjecting transgenic mice expressing a dominant negative form of MEF2 (DN-MEF2 Tg) in the heart, to transverse aortic constriction (TAC). Histological analysis revealed no major differences in cardiac remodelling between DN-MEF2 Tg and control mice after TAC. Surprisingly, echocardiographic analysis revealed that DN-MEF2 Tg mice had a decrease in cardiac function compared with control animals. Analysis of the mitochondrial respiratory chain showed that DN-MEF2 Tg mice displayed lower expression of NADH dehydrogenase subunit 6 (ND6), part of mitochondrial Complex I. The reduced expression of ND6 in DN-MEF2 Tg mice after pressure overload correlated with an increase in cell death secondary to overproduction of reactive oxygen species (ROS). CONCLUSION: Our data suggest that MEF2 transcriptional activity is required for mitochondrial function and its inhibition predisposes the heart to impaired mitochondrial function, overproduction of ROS, enhanced cell death, and cardiac dysfunction, following pressure overloa

Beat-to-beat variability in preload unmasks latent risk of Torsade de Pointes in anesthetized chronic atrioventricular block dogs

Background:Beat-to-beat variability in ventricular repolarization (BVR) associates with increased arrhythmic risk. Proarrhythmic remodeling in the dog with chronic AV-block (CAVB) compromises repolarization reserve and associates with increased BVR, which further increases upon dofetilide infusion and correlates with Torsade de Pointes (TdP) arrhythmias. It was hypothesized that these pro-arrhythmia-associated increases in BVR are induced by beat-to-beat variability in preload.Methods and Results:Left ventricular monophasic action potential duration (LVMAPD) was recorded in acute (AAVB) and CAVB dogs, before and after dofetilide infusion. BVR was quantified as short-term variability of LVMAPD. The PQ-interval was controlled by pacing: either a constant or an alternating preload pattern was established, verified by PV-loop. The effect of the stretch-activated channel blocker, streptomycin, on BVR was evaluated in a second CAVB group. At alternating preload only, BVR was increased after proarrhythmic remodeling (0.45±0.14 ms AAVB vs. 2.2±1.1 ms CAVB,

MEF2 activates a genetic program promoting chamber dilation and contractile dysfunction in calcineurin-induced heart failure

BACKGROUND: Hypertrophic growth, a risk factor for mortality in heart disease, is driven by reprogramming of cardiac gene expression. Although the transcription factor myocyte enhancer factor-2 (MEF2) is a common end point for several hypertrophic pathways, its precise cardiac gene targets and function in cardiac remodeling remain to be elucidated. METHODS AND RESULTS: We report the existence of synergistic interactions between the nuclear factor of activated T cells and MEF2 transcription factors triggered by calcineurin signaling. To circumvent the embryonic lethality and mitochondrial deficiency associated with germ-line null mutations for MEF2C and MEF2A respectively, we used conditional transgenesis to express a dominant-negative form of MEF2 in the murine postnatal heart and combined this with magnetic resonance imaging to assess MEF2 transcriptional function in Ca2+/calcineurin-induced cardiac remodeling. Surprisingly, end-diastolic and end-systolic ventricular dimensions and contractility were normalized in the presence of severely hypertrophied left ventricular walls on MEF2 inhibition in calcineurin transgenic mice. In line, we generated lines of transgenic mice expressing MEF2A in the heart, which displayed primarily chamber dilation. Microarray profiling indicated that MEF2 promotes a gene profile functioning primarily to or at the nucleus, cytoskeletal and microtubular networks, and mitochondria. CONCLUSIONS: These findings assign a novel function to MEF2 transcription factors in the postnatal heart, where they activate a genetic program that minimally affects cardiac growth yet promotes chamber dilation, mechanical dysfunction, and dilated cardiomyopath

Plakophilin-2 Haploinsufficiency Causes Calcium Handling Deficits and Modulates the Cardiac Response Towards Stress.

Human variants in plakophilin-2 (PKP2) associate with most cases of familial arrhythmogenic cardiomyopathy (ACM). Recent studies show that PKP2 not only maintains intercellular coupling, but also regulates transcription of genes involved in Ca2+ cycling and cardiac rhythm. ACM penetrance is low and it remains uncertain, which genetic and environmental modifiers are crucial for developing the cardiomyopathy. In this study, heterozygous PKP2 knock-out mice (PKP2-Hz) were used to investigate the influence of exercise, pressure overload, and inflammation on a PKP2-related disease progression. In PKP2-Hz mice, protein levels of Ca2+-handling proteins were reduced compared to wildtype (WT). PKP2-Hz hearts exposed to voluntary exercise training showed right ventricular lateral connexin43 expression, right ventricular conduction slowing, and a higher susceptibility towards arrhythmias. Pressure overload increased levels of fibrosis in PKP2-Hz hearts, without affecting the susceptibility towards arrhythmias. Experimental autoimmune myocarditis caused more severe subepicardial fibrosis, cell death, and inflammatory infiltrates in PKP2-Hz hearts than in WT. To conclude, PKP2 haploinsufficiency in the murine heart modulates the cardiac response to environmental modifiers via different mechanisms. Exercise upon PKP2 deficiency induces a pro-arrhythmic cardiac remodeling, likely based on impaired Ca2+ cycling and electrical conduction, versus structural remodeling. Pathophysiological stimuli mainly exaggerate the fibrotic and inflammatory response

Loss of muscle-specific RING-finger 3 predisposes the heart to cardiac rupture after myocardial infarction

RING-finger proteins commonly function as ubiquitin ligases that mediate protein degradation by the ubiquitin-proteasome pathway. Muscle-specific RING-finger (MuRF) proteins are striated muscle-restricted components of the sarcomere that are thought to possess ubiquitin ligase activity. We show that mice lacking MuRF3 display normal cardiac function but are prone to cardiac rupture after acute myocardial infarction. Cardiac rupture is preceded by left ventricular dilation and a severe decrease in cardiac contractility accompanied by myocyte degeneration. Yeast two-hybrid assays revealed four-and-a-half LIM domain (FHL2) and γ-filamin proteins as MuRF3 interaction partners, and biochemical analyses showed these proteins to be targets for degradation by MuRF3. Accordingly, FHL2 and γ-filamin accumulated to abnormal levels in the hearts of mice lacking MuRF3. These findings reveal an important role of MuRF3 in maintaining cardiac integrity and function after acute myocardial infarction and suggest that turnover of FHL2 and γ-filamin contributes to this cardioprotective function of MuRF3

Reduced Cx43 expression triggers increased fibrosis due to enhanced fibroblast activity

Arrhythmogenic ventricular remodeling is hallmarked by both reduced gap junction expression and increased collagen deposition. We hypothesized that reduced connexin43 (Cx43) expression is responsible for enhanced fibrosis in the remodeled heart, resulting in an arrhythmogenic substrate. Therefore, we investigated the effect of normal or reduced Cx43 expression on the formation of fibrosis in a physiological (aging) and pathophysiological (transverse aortic constriction [TAC]) mouse model. The Cx43(fl/fl) and Cx43(CreER(T)/fl) mice were aged 18 to 21 months or, at the age of 3 months, either TAC or sham operated and euthanized after 16 weeks. Epicardial activation mapping of the right and left ventricles was performed on Langendorff perfused hearts. Sustained ventricular arrhythmias were induced in 0 of 11 aged Cx43(fl/fl) and 10 of 15 Cx43(Cre-ER(T)/fl) mice (P <0.01). Cx43 expression was reduced by half in aged Cx43(CreER(T)/fl) compared with aged Cx43(fl/fl) mice, whereas collagen deposition was significantly increased from 1.1±0.2% to 7.4±1.3%. Aged Cx43(CreER(T)/fl) mice with arrhythmias had significantly higher levels of fibrosis and conduction heterogeneity than aged Cx43(CreER(T)/fl) mice without arrhythmias. The TAC operation significantly increased fibrosis in control compared with sham (4.0±1.2% versus 0.4±0.06%), but this increase was significantly higher in Cx43(CreER(T)/fl) mice (10.8±1.4%). Discoidin domain receptor 2 expression was unchanged, but procollagen peptide I and III expression and collagen type 1α2 mRNA levels were higher in TAC-operated Cx43HZ mice. Reduced cellular coupling results in more excessive collagen deposition during aging or pressure overload in mice due to enhanced fibroblast activity, leading to increased conduction in homogeneity and proarrhythmi