The Role of GSK-3 beta Phosphorylation in the Regulation of Slow Myosin Expression in Soleus Muscle during Functional Unloading

The myosin phenotype of a skeletal muscle (i.e., prevailing myosin isoform MyHC or isoforms in the muscle) determines contractile and fatigue characteristics of the muscle. The present study was aimed to verify the hypothetical functional link between the reduced NO content, decreased GSK-3 beta phosphorylation (which enhances the GSK-3 beta activation), reduced NFATcl accumulation in myonuclei, and a decline of the expression of slow MyHC I(13) during gravitational unloading of rat soleus muscle. Male Wistar rats were divided into five groups: (1) vivarium control; (2) animals with a 7-day hindlimb suspension by the Yliyn-Novikov method, modified by Morey-Holton; (3) animals with 7-day hindlimb suspension combined with administration of the NO donor (L-arginine); (4) animals with 7-day hindlimb suspension combined with administration of L-arginine and NO-synthase inhibitor (L-NAME), and (5) animals with 7-day hindlimb suspension combined with administration of GSK-3 beta inhibitor. We showed that the 7-day unloading of the muscle causes a decrease of the NO content in soleus muscle and this decrease can be prevented by L-arginine, and L-NAME blocks this effect of L-arginine. L-arginine also prevents unloading-induced GSK-3 beta dephosphorylation, as well as export of NFATcl from myonuclei and a decrease of the MyHC I(beta) expression; all these effects can be blocked by the NO-synthase inhibitor. Administration of the GSK-3 beta inhibitor blocks unloading-induced export of NFATcl from myonuclei and a decrease of the MyHC I(beta) expression as well. The prevention of the MyHC I(beta) expression decrease and of the NFATcl export from myonuclei by the selective inhibition of GSK-3 beta confirms the hypothesis on the NO influence on the MyHC I((beta) expression and NFATcl myonuclei export by the GSK-3 beta phosphorylation decrease. Thus, the NO deficiency in soleus muscle induced by gravitational unloading increases the GSK-3 beta activity, which favors the NFATcl export from myonuclei and stabilization of the fast myosin phenotype.