Changes in Titin Isoform Composition Following Knockout of Bmal1 , a Core Molecular Clock Factor, in Skeletal Muscle

Recent work from our lab has shown that disruption of the endogenous molecular clock mechanism in skeletal muscle is sufficient to induce muscle weakness with changes in fiber type and fibrosis. To begin to discern the molecular mechanisms that link the changes in the molecular clock with changes in muscle function, we tested whether the muscle of inducible, skeletal muscle specific Bmal1 knockout (iMS Bmal1 −/− ) mice would also exhibit changes in sarcomeric protein expression. One of the proteins we focused on was titin, the giant filamentous protein that maintains sarcomeric structure, underlies passive tension, and serves as a scaffold for many proteins and signaling molecules. In our studies, we determined that the tibialis anterior muscle of the iMS Bmal1 −/− mice exhibits a significant increase in the longer, more compliant isoform of titin (38% long isoform/total titin in iMS Bmal1 −/− mice vs. 19% in iMS Bmal1 +/+ mice). While there is no change in the total amount of titin expressed, comparisons of results from a microarray data set between the two treatment groups indicate that probe sets covering the PEVK region were significantly up‐regulated in the gastrocnemius muscle of these mice. This region of titin pre‐mRNA is known to be spliced out in the presence of the RNA binding protein RBM20. Using this logic, we noted that expression of Rbm20 mRNA is decreased approximately 24% in iMS Bmal1 −/− gastrocnemius muscle compared to iMS Bmal1 +/+ controls suggesting that the molecular clock could be acting through this protein to modify titin alternative splicing. Ongoing studies are focusing on the connection between the molecular clock and RBM20 and if the change in RBM20 expression is responsible for the difference in titin spliceform expression. Support or Funding Information NIH AR066082