The Skeletal Muscle Molecular Clock Regulates Titin Isoform Expression

Abstract

The circadian clock transcription factors, BMAL1 and CLOCK, are fundamental transcriptional regulators of cell time keeping and critical cell specific genes important for homeostasis. To determine the specific role of the molecular clock in adult skeletal muscle, our lab developed the inducible, skeletal muscle specific Bmal1 knockout (iMSBmal1-/-) mouse. Notably, skeletal muscles from these mice exhibit decreased specific tension and reduced unstimulated tension developed during a fatigability test. To begin to discern the molecular mechanisms that link the changes in the molecular clock with changes in muscle function, we tested whether iMSBmal1-/- muscle would also exhibit changes in sarcomeric protein expression. We determined that the tibialis anterior muscle of the iMSBmal1-/- mice exhibits a significant increase (38% in iMSBmal1-/- mice vs. 19% in iMSBmal1+/+ mice) in expression of a longer isoform of titin. It is established that titin, the giant filamentous protein that maintains sarcomeric structure, underlies passive tension, and contributes to active tension development. To determine if this increased heterogeneity of titin isoforms within the TA had an effect on sarcomere length, we performed immunohistochemistry. An antibody to α-actinin was used to demarcate Z-lines in longitudinal sections of the tibialis anterior muscle and sarcomere length was measured as the distance between Z-lines. While average sarcomere length was not different between iMSBmal1+/+ and iMSBmal1-/- muscle, variation in sarcomere length was increased following Bmal1 knockout in skeletal muscle. Further studies with our model are currently ongoing and will lead to increased knowledge of the importance of titin isoform maintenance in a variety of pathological conditions.