Cardiac sympathetic neurons provide trophic signal to the heart via beta2-adrenoceptor-dependent regulation of proteolysis

Aims: 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. Methods and results 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 b2-adrenoceptor (AR), as atrophy was prevented by treatment of denervated mice with the b2-AR agonist clenbuterol. Consistent with these data, we also observed that b2-AR KO mice showed cardiac atrophy at rest. Conclusion Cardiac SNs are strong regulators of the cardiomyocyte size via b2-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 b-AR in cardiovascular diseases and their modulation in therapy.