ROLE OF THE ADENOSINE MONOPHOSPHATE-ACTIVATED PROTEIN KINASE IN MAINTAINING MUSCLE INTEGRITY AND INSULIN SENSITIVITY THROUGHOUT THE LIFESPAN

Abstract

Introduction: Aging is associated with a progressive decline in skeletal muscle health. In both rodents and humans, aging-associated reductions in skeletal muscle AMP-activated protein kinase (AMPK) activity and mitochondrial function have been linked to the development of muscle insulin resistance, but whether these are precipitating factors causing insulin resistance is unknown. Autophagy is a cellular quality control process that is essential for tissue health and regulated by AMPK. Aging is also associated with reduced basal autophagy, but it has yet to be investigated if AMPK is required for autophagy in skeletal muscle. Methods: Insulin sensitivity was measured using glucose and insulin tolerance tests, the hyperinsulinemic-euglycaemic clamp and 2-DG uptake experiments. Autophagy was measured in the basal and fasted state by electroporating RFP-LC3 into skeletal muscle, inhibiting autophagic flux with colchicine, western blotting and qRT-PCR experiments. Results: We found that fasting mice lacking skeletal muscle AMPK (AMPK-MKO) results in hypoglycemia and hyperketosis. This is not due to defective fatty acid oxidation, but instead is related to a block in muscle proteolysis that leads to reduced circulating levels of alanine, an essential amino acid required for gluconeogenesis. Markers of muscle autophagy including phosphorylation of Ulk1 Ser555, Ser757 and aggregation of RFP-LC3 puncta are impaired. Consistent with impaired autophagy, aged AMPK-MKO mice possess a significant myopathy characterized by reduced muscle function, mitochondrial disease and accumulation of the autophagy/mitophagy proteins p62 and Parkin. Suprisingly, despite the considerable myopathy and mitochondrial disease observed in aged AMPK-MKO mice, skeletal muscle insulin sensitivity was preserved. Conclusions and significance: These data demonstrate that reductions in AMPK and mitochondrial activity do not cause age-induced insulin resistance, but do establish an essential requirement for skeletal muscle AMPK-mediated autophagy in preserving blood glucose levels during prolonged fasting as well as maintaining muscle integrity and mitochondrial function during aging.