Mitochondria, contractility and Ca2+ handling : cardiac and skeletal muscle adaptations in health and disease

Sammanfattning: Contractility is a fundamental feature of skeletal and cardiac muscles. An indispensable step in the cellular signal for contraction is a transient elevation in cytoplasmic free [Ca2+] ([Ca2+]i). Both the production of contractile force and Ca2+ handling processes are highly energy demanding. Mitochondrial ATP production from the respiratory chain is thus of pivotal importance for muscle cell function. Moreover, the mitochondria are also directly involved in Ca2+ signaling and are the foremost source of reactive oxygen species (ROS). Mutations in the genomes encoding for the mitochondrial respiratory chain, either the nuclear or the mitochondrial (mtDNA) genome, can give rise to primary mitochondrial diseases. Mitochondrial dysfunction is also implicated in other diseases, such as heart failure, obesity and diabetes. Paper I and II investigate two mouse models of primary mitochondrial myopathy (Tfam KO) and cardiomyopathy (Mterf3 KO). Furthermore, the obese, pre-diabetic ob/ob mouse was studied in papers III and IV. Skeletal and cardiac muscle cells were studied primarily with respect to contractility, [Ca2+]i and mitochondrial ROS production. Skeletal muscle fibers of myopathy Tfam KO mice produce less force compared to control mice. This was explained by reduced tetanic [Ca2+]i, decreased SR Ca2+ release and reduced SR Ca2+ storage via calsequestrin 2. Moreover, Tfam KO but not control fibers, displayed a markedly increased mitochondrial [Ca2+] during fatigue, partly through a cyclosporin A-sensitive mechanism. Elevated [Ca2+] in the mitochondria can trigger cellular damage. Thus, reducing mitochondrial Ca2+ with cyclosporin A may provide one way for treatment of mitochondrial myopathy. Mterf3 KO mice develop hypertrophic cardiomyopathy and die suddenly. Cardiomyocytes from these mice display elevated SR Ca2+ cycling, increased SR Ca2+ load and aberrant pro-arrhythmic Ca2+ releases compatible with that seen in sympathetic stimulation. In support of this, electrocardiography revealed signs of elevated catecholaminergic drive. Moreover, in the moribund stage Mterf3 KO mice develop terminal AV-block and bradycardia. Acutely exposing WT cardiomyocytes to an excess of the saturated fatty acid palmitate caused dissipation of the mitochondrial membrane potential and a large increase in mitochondrial ROS production. In turn, this ROS increase impaired the cellular Ca2+ cycling and contractility. However in ob/ob cardiomyocytes, palmitate did not cause increased ROS production and the function of ob/ob cardiomyocytes was in fact improved by palmitate. This suggests that the ob/ob heart has adapted to a high fat environment and metabolizes fatty acids without the producing large amounts of mitochondrial ROS. In WT cardiomyocytes, application of the beta-adrenergic agonist isoproterenol (ISO) stimulated mitochondrial ROS production. Concomitant application of the ROS scavenger N-acetylcysteine (NAC) diminished the inotropic effect of ISO on cardiomyocyte [Ca2+]i transients and contractility. On the other hand, ob/ob cardiomyocytes failed to increase ROS production when exposed to ISO and NAC did not alter the effect of ISO on [Ca2+]i transients. Hence, mitochondrial ROS is integrated in, but not essential to, the inotropic mechanism of beta-adrenergic stimulation. In all the studied disease models, neither an increase in mitochondrial ROS production nor signs of oxidative damage were found. In conclusion, dysfunctional mitochondria cause long-term adaptive/maladaptive changes in Ca2+ handling as seen in the Tfam and Mterf3 KO mice. Mitochondrial functions can influence cellular Ca2+ handling also in the short term. This is evident by the effects of palmitate and ISO-stimulated mitochondrial ROS production.