Role of denervation in modulating IIb MHC gene expression in response to T₃ plus unloading state

Previously, we have reported that the combination of hindlimb suspension (HS) and thyroid hormone [triiodothyronine (T3)] treatment induces the de novo expression of the fast IIb myosin heavy chain (MHC) gene in the soleus. Thus we tested the hypotheses that the induction of IIb MHC expression with HS + T3 is prevented with denervation and that this IIb induction is regulated by transcriptional processes. Adult female rats were subjected to 2 wk of combined HS + T3 in which one side of the lower leg was simultaneously denervated. HS + T3 caused decreased expression of the slow type I MHC and concomitant increases in both the fast type IIx and IIb MHC isoforms in the intact soleus muscle. Denervation prevented the endogenous expression of the IIb MHC gene at both the protein and mRNA levels. Although HS + T3intervention was able to markedly increase the expression of the 2.6-kb IIb MHC promoter-reporter construct using direct gene transfer, this induction, however, was not inhibited by denervation. These findings collectively suggest that normal innervation is essential for inducing the unique expression of the IIb MHC in a slow muscle in response to HS + T3; however, in the denervated muscle, there is a discordance between the regulation of the endogenous IIb MHC gene relative to the exogenous IIb MHC promoter-reporter construct.