Inactivation of the intrinsic muscle clock does not cause sarcopenia

Effect of Aerobic Exercise and Time-Restricted Feeding on Metabolic Markers and Circadian Rhythm in Mice Fed with the High-Fat Diet

SCOPE

Diet and exercise are significant players in obesity and metabolic diseases. Time-restricted feeding (tRF) has been shown to improve metabolic responses by regulating circadian clocks but whether it acts synergically with exercise remains unknown. It is hypothesized that forced exercise alone or combined with tRF alleviates obesity and its metabolic complications.

METHODS AND RESULTS

Male C57bl6 mice are fed with high-fat or a control diet for 12 weeks either ad libitum or tRF for 10 h during their active period. High-fat diet (HFD)-fed mice are divided into exercise (treadmill for 1 h at 12 m min-1 alternate days for 9 weeks and 16 m min-1 daily for the following 3 weeks) and non-exercise groups. tRF and tRF-Ex significantly decreased body weight, food intake, and plasma lipids, and improved glucose tolerance. However, exercise reduced only body weight and plasma lipids. tRF and tRF-Ex significantly downregulated Fasn, Hmgcr, and Srebp1c, while exercise only Hmgcr. HFD feeding disrupted clock genes, but exercise, tRF, and tRF-Ex coordinated the circadian clock genes Bmal1, Per2, and Rev-Erbα in the liver, adipose tissue, and skeletal muscles.

CONCLUSION

HFD feeding disrupted clock genes in the peripheral organs while exercise, tRF, and their combination restored clock genes and improved metabolic consequences induced by high-fat diet feeding.

Running skeletal muscle clocks on time- the determining factors

Circadian rhythms generate 24 h-long oscillations, which are key regulators of many aspects of behavior and physiology. Recent circadian transcriptome studies have discovered rhythmicity at the transcriptional level of hundreds of skeletal muscle genes, with roles in skeletal muscle growth, maintenance, and metabolic functions. These rhythms allow this tissue to perform molecular functions at the appropriate time of the day in order to anticipate environmental changes. However, while the last decade of research has characterized several aspects of the skeletal muscle molecular clock, many still are unexplored, including its functions, regulatory mechanisms, and interactions with other tissues. The central clock is believed to be located in the suprachiasmatic nucleus (SCN) of the brain hypothalamus, providing entrainment to peripheral organs through humoral and neuronal signals. However, these mechanisms of action are still unknown. Conversely, muscle tissue can be entrained through extrinsic, SCN-independent factors, such as feeding and physical activity. In this review, we provide an overview of the recent research about the extrinsic and intrinsic factors required for skeletal muscle clock regulation. Furthermore, we discuss the need for future studies to elucidate the mechanisms behind this regulation, which will in turn help dissect the role of circadian disruption at the onset of aging and diseases.

Circadian regulation of muscle growth independent of locomotor activity

A long-standing question, particularly in physiotherapy and sports medicine, is whether time of day affects muscle metabolism and hence growth, either intrinsically or in response to exercise or nutrition. Answers would help to identify the best time of day to exercise, build muscle, and prevent aging- or disease-related sarcopenia. Here, we address this question in live zebrafish myotome in vivo, without interference from other circadian oscillations such as locomotor activity and food intake. We show that active muscle anabolizes more in the day and grows faster, while catabolizing more at night and growing slower. Such day/night differences remain in inactive muscle but disappear after clock disruption. We conclude that muscles display circadian differences in growth independent of activity and feeding.

Muscle tissue shows diurnal variations in function, physiology, and metabolism. Whether such variations are dependent on the circadian clock per se or are secondary to circadian differences in physical activity and feeding pattern is unclear. By measuring muscle growth over 12-h periods in live prefeeding larval zebrafish, we show that muscle grows more during day than night. Expression of dominant negative CLOCK (ΔCLK), which inhibits molecular clock function, ablates circadian differences and reduces muscle growth. Inhibition of muscle contraction reduces growth in both day and night, but does not ablate the day/night difference. The circadian clock and physical activity are both required to promote higher muscle protein synthesis during the day compared to night, whereas markers of protein degradation, murf messenger RNAs, are higher at night. Proteasomal inhibitors increase muscle growth at night, irrespective of physical activity, but have no effect during the day. Although physical activity enhances TORC1 activity, and the TORC1 inhibitor rapamycin inhibits clock-driven daytime growth, no effect on muscle growth at night was detected. Importantly, day/night differences in 1) muscle growth, 2) protein synthesis, and 3) murf expression all persist in entrained larvae under free-running constant conditions, indicating circadian drive. Removal of circadian input by exposure to either permanent darkness or light leads to suboptimal muscle growth. We conclude that diurnal variations in muscle growth and metabolism are a circadian property that is independent of, but augmented by, physical activity, at least during development.

The functional significance of the skeletal muscle clock: lessons from Bmal1 knockout models

The circadian oscillations of muscle genes are controlled either directly by the intrinsic muscle clock or by extrinsic factors, such as feeding, hormonal signals, or neural influences, which are in turn regulated by the central pacemaker, the suprachiasmatic nucleus of the hypothalamus. A unique feature of circadian rhythms in skeletal muscle is motor neuron-dependent contractile activity, which can affect the oscillation of a number of muscle genes independently of the muscle clock. The role of the intrinsic muscle clock has been investigated using different Bmal1 knockout (KO) models. A comparative analysis of these models reveals that the dramatic muscle wasting and premature aging caused by global conventional KO are not present in muscle-specific Bmal1 KO or in global Bmal1 KO induced in the adult, therefore must reflect the loss of Bmal1 function during development in non-muscle tissues. On the other hand, muscle-specific Bmal1 knockout causes impaired muscle glucose uptake and metabolism, supporting a major role of the muscle clock in anticipating the sleep-to-wake transition, when glucose becomes the predominant fuel for the skeletal muscle.