Skeletal muscle, autophagy, and physical activity: the ménage à trois of metabolic regulation in health and disease.

Metabolic homeostasis is essential for cellular survival and proper tissue function. Multi-systemic metabolic regulation is therefore vital for good health. A number of tissues have the task of maintaining appropriate metabolism, and skeletal muscle is the most abundant of them. Muscle possesses a remarkable plasticity and is able to rapidly adapt to changes in energetic demands by fine-tuning the balance between catabolic and anabolic processes. Autophagy is a catabolic process responsible for the degradation of protein aggregates and damaged organelles, through the autophagosome-lysosome system. Proper regulation of autophagy flux is fundamental for organism homeostasis under physiological conditions and even more in response to metabolic stress, such as during physical activity and nutritional deficits. Both deficient and excessive autophagy are harmful for health and have devastating consequences in a myriad of pathologies. The regulation of autophagy flux in various tissues, and in particular in skeletal muscle, is of great importance for health and tissue homeostasis and represents a feasible mechanism by which physical exercise exerts its beneficial effects on muscle and whole body metabolism. This review is focused on the key molecular mechanisms regulating macromolecule and organelle turnover in muscle during alterations in nutrient availability and energetic demands, as well as their involvement in disease pathogenesis.