Cyclosporin A inhibits the growth of neonatal MHC-expressing myotubes independent of NFATc1 and NFATc3 in the mechanically overloaded soleus muscle of mice

The molecular signaling pathway linked to hypertrophy of the anti-gravity/postural soleus muscle after mechanical overloading has not been identified. Using Western blot and immunohistochemical analyses, we investigated whether the amounts of NFATc3, GSK-3, NFATc1, and neonatal MHC change in the mechanically overloaded soleus muscle after cyclosporin A (CsA) treatment. Adult male ICR mice were subjected to a surgical ablation of the gastrocnemius muscle and treated with either CsA (25 mg/Kg) or vehicle once daily. They were sacrificed at 2, 4, 7, 10, and 14 days post-injury. Mechanical overloading resulted in a significant increase in the wet weight and the cross-sectional area of slow and fast fibers of the soleus muscle in placebo-treated mice but not CsA-treated mice. After 4 days of mechanical overloading, we observed a similar co-localization of neonatal MHC and NFATc3 in several myotubes of both mice. The placebo-treated mice possessed larger myotubes with neonatal MHC than CsA-treated mice. At 7 days, mechanical overloading induced marked expression of neonatal MHC in myotubes and/or myofibers. Such neonatal MHC-positive fibers emerged less often in the hypertrophied soleus muscle subjected to treatment with CsA. CsA treatment did not significantly change the amount of GSK-3 protein in the soleus muscle. The modulation of growth in neonatal MHC-positive myofibers by CsA treatment may inhibit the hypertrophic process in the soleus muscle after mechanical overloading.