Rationale: Mitochondrial dysfunction facilitates heart failure development forming a therapeutic target, but the mechanism involved remains unclear. We studied whether the Hippo signaling pathway mediates mitochondrial abnormalities that results in onset of dilated cardiomyopathy (DCM). Methods: Mice with DCM due to overexpression of Hippo pathway kinase Mst1 were studied. DCM phenotype was evident in adult animals but contractile dysfunction was identified as an early sign of DCM at 3 weeks postnatal. Electron microscopy, multi-omics and biochemical assays were employed. Results: In 3-week and adult DCM mouse hearts, cardiomyocyte mitochondria exhibited overt structural abnormalities, smaller size and greater number. RNA sequencing revealed comprehensive suppression of nuclear-DNA (nDNA) encoded gene-sets involved in mitochondria turnover and all aspects of metabolism. Changes in cardiotranscriptome were confirmed by lower protein levels of multiple mitochondrial proteins in DCM heart of both ages. Mitochondrial DNA-encoded genes were also downregulated; due apparently to repression of nDNA-encoded transcriptional factors. Lipidomics identified remodeling in cardiolipin acyl-chains, increased acylcarnitine content but lower coenzyme Q10 level. Mitochondrial dysfunction was featured by lower ATP content and elevated levels of lactate, branched-chain amino acids and reactive oxidative species. Mechanistically, inhibitory YAP-phosphorylation was enhanced, which was associated with attenuated binding of transcription factor TEAD1. Numerous suppressed mitochondrial genes were identified as YAP-targets. Conclusion: Hippo signaling activation mediates mitochondrial damage by repressing mitochondrial genes, which causally promotes the development of DCM. The Hippo pathway therefore represents a therapeutic target against mitochondrial dysfunction in cardiomyopathy.

Background Cardiac fibrosis is a core pathological process associated with heart failure. The recruitment and differentiation of primitive fibroblast precursor cells of bone marrow origin play a critical role in pathological interstitial cardiac fibrosis. The K C a 3.1 channels are expressed in both ventricular fibroblasts and circulating mononuclear cells in rats and are upregulated by angiotensin II . We hypothesized that K C a 3.1 channels mediate the inflammatory microenvironment in the heart, promoting the infiltrated bone marrow–derived circulating mononuclear cells to differentiate into myofibroblasts, leading to myocardial fibrosis. Methods and Results We established a cardiac fibrosis model in rats by infusing angiotensin II to evaluate the impact of the specific K C a 3.1 channel blocker TRAM ‐34 on cardiac fibrosis. At the same time, mouse CD 4 + T cells and rat circulating mononuclear cells were separated to investigate the underlying mechanism of the TRAM ‐34 anti–cardiac fibrosis effect. TRAM ‐34 significantly attenuated cardiac fibrosis and the inflammatory reaction and reduced the number of fibroblast precursor cells and myofibroblasts. Inhibition of K C a 3.1 channels suppressed angiotensin II –stimulated expression and secretion of interleukin‐4 and interleukin‐13 in CD 4 + T cells and interleukin‐4– or interleukin‐13–induced differentiation of monocytes into fibrocytes. Conclusions K C a 3.1 channels facilitate myocardial inflammation and the differentiation of bone marrow‐derived monocytes into myofibroblasts in cardiac fibrosis caused by angiotensin II infusion.

Activation of the sympatho‐β‐adrenergic receptors (β‐ARs) system is a hallmark of heart failure, leading to fibrosis and arrhythmias. Connexin 43 (Cx43) is the most abundant gap junctional protein in the myocardium. Current knowledge is limited regarding Cx43 remodelling in diverse cell types in the diseased myocardium and the underlying mechanism. We studied cell type‐dependent changes in Cx43 remodelling due to β‐AR overactivation and molecular mechanisms involved. Mouse models of isoproterenol stimulation or transgenic cardiomyocyte overexpression of β 2 ‐AR were used, which exhibited cardiac fibrosis and up‐regulated total Cx43 abundance. In both models, whereas Cx43 expression in cardiomyocytes was reduced and more laterally distributed, fibroblasts exhibited elevated Cx43 expression and enhanced gap junction communication. Mechanistically, activation of β 2 ‐AR in fibroblasts in vitro elevated Cx43 expression, which was abolished by the β 2 ‐antagonist ICI‐118551 or protein kinase A inhibitor H‐89, but simulated by the adenylyl cyclase activator forskolin. Our in vitro and in vivo data showed that β‐AR activation‐induced production of IL‐18 sequentially stimulated Cx43 expression in fibroblasts in a paracrine fashion. In summary, our findings demonstrate a pivotal role of β‐AR in mediating distinct and cell type‐dependent changes in the expression and distribution of Cx43, leading to pathological gap junction remodelling in the myocardium.

Heart failure is associated with sympatho-βAR (β-adrenoceptor) activation and cardiac fibrosis. Gal-3 (galectin-3) and K Ca 3.1 channels that are upregulated in diverse cells of diseased heart are implicated in mediating myocardial inflammation and fibrosis. It remains unclear whether Gal-3 interacts with K Ca 3.1 leading to cardiac fibrosis in the setting of βAR activation. We tested the effect of K Ca 3.1 blocker TRAM-34 on cardiac fibrosis and inflammation in cardiac-restricted β 2 -TG (β 2 AR overexpressed transgenic) mice and determined K Ca 3.1 expression in β 2 -TG×Gal-3 −/− mouse hearts. Mechanisms of K Ca 3.1 in mediating Gal-3 induced fibroblast activation were studied ex vivo. Expression of Gal-3 and K Ca 3.1 was elevated in β 2 -TG hearts. Gal-3 gene deletion in β 2 -TG mice decreased K Ca 3.1 expression in inflammatory cells but not in fibroblasts. Treatment of β 2 -TG mice with TRAM-34 for 1 or 2 months significantly ameliorated cardiac inflammation and fibrosis and reduced Gal-3 level. In cultured fibroblasts, Gal-3 upregulated K Ca 3.1 expression and channel currents with enhanced membrane potential and Ca 2+ entry through TRPV4 (transient receptor potential V4) and TRPC6 (transient receptor potential C6) channels leading to fibroblast activation. In conclusion, βAR stimulation promotes Gal-3 production that upregulates K Ca 3.1 channels in noncardiomyocyte cells and activates K Ca 3.1 channels in fibroblasts leading to hyperpolarization of membrane potential and Ca 2+ entry via TRP channels. Gal-3–K Ca 3.1 signaling mobilizes diverse cells facilitating regional inflammation and fibroblast activation and hence myocardial fibrosis.

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