The skeletal muscle circadian clock regulates sarcomere length homeostasis through titin splicing

Circadian rhythms in skeletal muscle are maintained by a transcriptional-translational feedback loop known as the molecular clock. While previous research suggested a role for the molecular clock in regulating skeletal muscle structure and function, no mechanisms have connected the molecular clock to sarcomeric proteins. Utilizing inducible, skeletal muscle specific, Bmal1 knockout (iMS Bmal1 -/-) mice, we show that deletion of the skeletal muscle molecular clock alters titin isoform and skeletal muscle sarcomere length. We then use U7 snRNPs in myotubes to directly alter titin splicing in vitro . Truncating titin’s proximal Ig domain results in altered sarcomere length. Finally, we identify a mechanism whereby the skeletal muscle molecular clock regulates titin isoform expression through RBM20, a potent splicing regulator of the titin transcript. Our findings demonstrate the importance of the skeletal muscle molecular clock in maintaining sarcomere length homogeneity through its regulation of RBM20 expression. Because circadian rhythm disruption is a feature of many diseases, our results highlight a pathway that could be targeted to maintain skeletal muscle structure and function in a range of pathologies. Significance Statement Circadian rhythms regulate many aspects of skeletal muscle physiology; however, the exact mechanisms connecting the molecular underpinnings of these rhythms to skeletal muscle structure and function are poorly understood. Here we describe how the skeletal muscle molecular clock regulates titin, the protein ruler regulating sarcomere length and muscle strength. Since circadian rhythms and skeletal muscle weakness underly a number of diseases, these results highlight a potential target for future therapeutic strategies. ### Competing Interest Statement The authors have declared no competing interest.