Skeletal muscle BMAL1 is necessary for transcriptional adaptation of local and peripheral tissues in response to endurance exercise training

Abstract

Objective: The skeletal muscle circadian clock plays a pivotal role in muscle homeostasis and metabolic flexibility. Recently, this clock mechanism has been linked to both transcriptional and metabolic responses to acute exercise. However, the contribution of the circadian clock mechanism to the molecular and phenotypic adaptations to exercise training have not been defined. Methods: Inducible skeletal muscle-specific Bmal1-floxed mice were treated with tamoxifen to induce skeletal muscle specific deletion of Bmal1 (iMSBmal1KO) or given a vehicle. Mice were assigned to normal cage conditions, or 6-weeks of progressive treadmill training. Exercise performance, body composition, and tissue/serum indices of metabolic health were assessed over the timecourse of training. Gastrocnemius muscles were collected 48-hours after their last exercise bout for histological, biochemical, and molecular analyses including RNA-sequencing and untargeted metabolomics. Results: Improvements in exercise workload and maximal performance were comparable between iMSBmal1KO mice and vehicle treated controls after 6-weeks of exercise training. However, exercise training in the absence of Bmal1 was not able to rescue the metabolic phenotype and hyperinsulinemia of the iMSBmal1KO mice, attributed to the continued dysregulation of core clock components and gene expression relating to glucose metabolism. Importantly, a much larger and divergent transcriptional reprogramming occurred in the muscle of iMSBmal1KO mice in comparison to their vehicle treated counterparts. This response included a large compensatory upregulation of genes associated with fatty acid β-oxidation, pyruvate metabolism, citric acid cycle components and oxidative phosphorylation components, including mitochondrial subunits and mitoribosome units. Conclusions: Collectively, we propose that endurance training requires muscle Bmal1, and the core clock network, to elicit well recognized molecular adaptations. In the absence of Bmal1, exercise training results in a much larger and divergent re-networking of the basal skeletal muscle transcriptome and metabolome. We also demonstrate that skeletal muscle Bmal1 is indispensable for the transcriptional regulation of glucose homeostasis, even after a 6-weeks exercise training programme.