Circadian genes are necessary for skeletal muscle function and phenotype

An endogenous central clock in the brain, coupled to the environmental light cycle, drives circadian rhythms of behavior and physiology. Circadian rhythm of core clock gene expression occur widely in peripheral tissues such as liver, heart and skeletal muscle. One of the ongoing questions in circadian biology is how these core clock genes regulate tissue specific processes. Much of the research has focused on liver, heart, and kidney with skeletal muscle receiving no attention. We show here that in both Clock mutant and Bmal1-/- mice there was and a significant deficit in skeletal muscle force production, reduction in mitochondrial volume density, and alteration in mitochondrial ultrastructure. Thus it appears that an intact molecular clock is necessary for maintenance of both skeletal muscle structure and function. Expression profiling of skeletal muscle mRNA was performed with the aim of identifying genes that may link altered core clock function with force deficits and mitochondrial impairments in skeletal muscle. Analysis of gene array results identified MyoD1 and Pgc-1beta as being expressed in a circadian fashion in skeletal muscle of wildtype mice while remaining non-cyclic and suppressed in the skeletal muscle of Clock mutant mice. Furthermore, measurements of maximal isometric force production in the EDL of MyoD1-/- mice revealed a 30% force deficit, similar to the deficit observed in the Clock mutant and Bmal1-/- mice. These results suggest that a disruption in the core clock mechanism is responsible for the lost and/or mis-expression of MyoD1 and Pgc-1beta, potentially contributing to the observed deficit in force production and loss in mitochondrial volume, respectively. To determine the necessity of Bmal1 expression in regulating mitochondrial function, respiration was measured in the mitochondria isolated from the diaphragm and gastrocnemius of 12 week old Bmal1-/- and wildtype mice. A significant reduction in mitochondrial efficiency and increase in oxidant activity, in the skeletal muscle of Bmal1-/- mice, suggests that Bmal1 is necessary for the maintenance of mitochondrial function and oxidant balance.