Molecular adaptations of skeletal muscle in health and disease

Abstract: Appropriate function of skeletal muscle is essential for locomotion, everyday activities and athletic performance. In addition to its mechanic tasks, skeletal muscle communicates with other organs via metabolic pathways and regulatory processes. Skeletal muscle is a plastic tissue that adapts to external stimuli, including hormonal signalling, exercise, physical inactivity and prolonged disuse. Molecular adaptations at the muscle fibre level have local effects on force-production and fatigue-resistance. In addition, they can alter metabolic pathways and regulatory processes with effects on whole-body physiology. The aim of this thesis was to study local and systemic effects of molecular adaptations of skeletal muscle in physiologic and pathologic conditions. In paper I, we investigated the consequences of muscular adaptations to sprint interval training (SIT) on contractile force. We demonstrate that a single session of SIT induces modifications of the ryanodine receptor (RyR1) in untrained humans and that repeated exposure to SIT provides some protection from SIT-induced RyR1 modifications. We moreover show that a three-week SIT program improves exercise performance but does not accelerate recovery of neuromuscular function after SIT. In the second paper, we studied molecular adaptations of skeletal muscle in a mouse model of amyotrophic lateral sclerosis (ALS), a neuromuscular disease that causes denervation and muscle weakness. The aim of this study was to determine whether muscle weakness in ALS is caused by the degeneration of motor neurons and subsequent atrophy or whether muscle fibre intrinsic defects (ie, altered Ca2+ handling or altered contractile properties) contribute to the loss of contractile force. Muscles of symptomatic ALS mice exhibited motor neuron loss, atrophy and reduced absolute force. However, at the single fibre level, Ca2+ handling was preserved, force-generating capacity intact and fibres displayed endurance training-like adaptations with increased fatigue-resistance and signs of mitochondrial biogenesis. Hence, surviving muscle fibres of ALS mice were strong and adaptable and muscle weakness was caused by muscle atrophy and not by muscle fibre intrinsic defects. Papers III and IV looked at molecular adaptations of skeletal muscle that have systemic effects on regulatory and metabolic pathways. In paper III, we studied skeletal muscles of humans lacking the structural protein α-actinin-3 (ACTN3) due to a common null polymorphism in the ACTN3 gene. The lack of ACTN3 has undergone positive selection in recent evolution and seems to provide a survival advantage in cold areas potentially linked to increased skeletal muscle Ca2+ cycling. In our study, humans with ACTN3 deficiency showed improved cold-resilience when exposed to an acute cold-challenge in conjunction with a shift in the expression of the Ca2+ handling proteins SERCA and calsequestrin. In addition, we observed altered muscle fibre distribution in ACTN3 deficient subjects with an increased proportion of type I and a decreased proportion of type IIx fibres. In summary, ACTN3 deficient subjects are more efficient at maintaining their body temperature during acute cold exposure potentially linked to an increased proportion of type I muscle fibres. In paper IV, we looked at molecular adaptations of skeletal muscle in response to endurance exercise that affect peripheral kynurenine (KYN) metabolism and cross-talk between muscle and brain. The degradation of KYN to NAD+ produces neurotoxic metabolites, which have been associated with depression. In an alternative pathway, KYN is converted to the neuroprotective kynurenic acid (KYNA) and this process is catalysed by kynurenine aminotransferases (KATs). Recent animal studies have shown that endurance exercise increases the expression of KATs in skeletal muscle resulting in a shift of the peripheral KYN metabolism towards the neuroprotective branch, thereby protecting from stress-induced depression. In our study, we show that healthy humans who participate in regular endurance training have increased expression of skeletal muscle KATs. We moreover demonstrate that acute endurance exercise increases the flux through the neuroprotective branch of the KYN pathway resulting in a transient increase of circulating KYNA. In contrast, high-intensity eccentric exercise did not affect circulating KYN metabolites. In summary, our study shows that prolonged, metabolically demanding exercise alters peripheral KYN metabolism which may have bearing on training recommendations for patients with depression. Taken together, the four studies presented in this thesis underline the multifacetedness of skeletal muscle as a tissue and the implications of molecular adaptations for athletic performance and for chronic disease.