Cardiomyocyte proteostasis is mediated by the ubiquitin/proteasome system (UPS) and autophagy/lysosome system and is fundamental for cardiac adaptation to both physiologic (e.g., exercise) and pathologic (e.g., pressure overload) stresses. Both the UPS and autophagy/lysosome system exhibit reduced efficiency as a consequence of aging, and dysfunction in these systems is associated with cardiomyopathies. The muscle-specific ubiquitin ligase atrogin-1 targets signaling proteins involved in cardiac hypertrophy for degradation. Here, using atrogin-1 KO mice in combination with in vivo pulsed stable isotope labeling of amino acids in cell culture proteomics and biochemical and cellular analyses, we identified charged multivesicular body protein 2B (CHMP2B), which is part of an endosomal sorting complex (ESCRT) required for autophagy, as a target of atrogin-1–mediated degradation. Mice lacking atrogin-1 failed to degrade CHMP2B, resulting in autophagy impairment, intracellular protein aggregate accumulation, unfolded protein response activation, and subsequent cardiomyocyte apoptosis, all of which increased progressively with age. Cellular proteostasis alterations resulted in cardiomyopathy characterized by myocardial remodeling with interstitial fibrosis, with reduced diastolic function and arrhythmias. CHMP2B downregulation in atrogin-1 KO mice restored autophagy and decreased proteotoxicity, thereby preventing cell death. These data indicate that atrogin-1 promotes cardiomyocyte health through mediating the interplay between UPS and autophagy/lysosome system and its alteration promotes development of cardiomyopathies.

Significance: Arrhythmias are potentially life-threatening electrical heart diseases that are difficult to predict and are mostly treated by empirical interventions. The mechanisms of arrhythmia initiation (triggers) are still poorly understood, mostly due to the technical limitations of conventional experimental methods in cardiac electrophysiology. Here, we use optogenetics, a technique based on the expression of photoactivated proteins such as ChannelRhodopsin-2 (ChR2), which allows for noninvasive control of the cell membrane potential through illumination with blue light. By developing mice with cell-specific expression of ChR2, we investigated the role of the different cardiomyocyte types present in the heart and determined the factors triggering arrhythmic beats in the normal heart and during myocardial ischemia, a condition frequently associated with lethal arrhythmias causing sudden cardiac death.

Significance: Mitochondrial calcium uniporter (MCU) is the core channel subunit of the mitochondrial Ca ²⁺ uniporter complex (MCUC) and contributes to the regulation of ATP production. Here, we identify that the adaptive and maladaptive remodeling occurring in rodent and human cardiomyocytes is associated with changes in MCU content, which are inversely correlated with those of its regulator microRNA-1 (miR-1). The present study thus defines the molecular mechanism by which MCU content and, consequently, mitochondrial Ca ²⁺ uptake are directly modified by alteration of miR-1, a key regulator affecting physiological and pathological heart growth. These findings provide evidence for the miR-1/MCU axis as a potential target for the development of novel therapeutic approaches.

Increased cardiac sympathetic neuron (SN) activity has been associated with pathologies such as heart failure and hypertrophy, suggesting that cardiac innervation regulates cardiomyocyte trophism. Whether continuous input from the SNs is required for the maintenance of the cardiomyocyte size has not been determined thus far. To address the role of cardiac innervation in cardiomyocyte size regulation, we monitored the effect of pharmacological sympathetic denervation in mice on cardiac structure, function, and signalling from 24 h to 30 days in the absence of other pathological stimuli. SN ablation caused an immediate reduction in the cardiomyocyte size with minimal consequences on the resting contractile function. Atrophic remodelling was mediated by the ubiquitin–proteasome system through FOXO-dependent early induction of the muscle-specific E3 ubiquitin ligases Atrogin-1/MAFbx and MuRF1, which was followed by activation of the autophagy–lysosome system. MuRF1 was found to be determinant in denervation atrophy as remodelling did not develop in denervated MuRF1 knock-out (KO) hearts. These effects were caused by decreased basal stimulation of cardiomyocyte β2-adrenoceptor (AR), as atrophy was prevented by treatment of denervated mice with the β2-AR agonist clenbuterol. Consistent with these data, we also observed that β2-AR KO mice showed cardiac atrophy at rest. Cardiac SNs are strong regulators of the cardiomyocyte size via β2-AR-dependent repression of proteolysis, demonstrating that the neuro-cardiac axis operates constitutively for the determination of the physiological cardiomyocyte size. These results are of great clinical relevance given the role of β-AR in cardiovascular diseases and their modulation in therapy.