Description: Facioscapulohumeral muscular dystrophy (FSHD), a common hereditary myopathy, is characterized by atrophy and weakness of selective muscle groups. FSHD is considered an autosomal dominant disease with incomplete penetrance and unpredictable variability of clinical expression within families. Mice overexpressing FRG1 (FSHD region gene 1), a candidate gene for this disease, develop a progressive myopathy with features of the human disorder. Here, we show that in FRG1- overexpressing mice, fast muscles, which are the most affected by the dystrophic process, display anomalous fast skeletal troponin T (fTnT) isoform, resulting from the aberrant splicing of the Tnnt3 mRNA that precedes the appearance of dystrophic signs. We determine that muscles of FRG1 mice develop less strength due to impaired contractile properties of fast-twitch fibers associated with an anomalous MyHC-actin ratio and a reduced sensitivity to Ca 2+ . We demonstrate that the decrease of Ca 2+ sensitivity of fast-twitch fibers depends on the anomalous troponin complex and can be rescued by the substitution with the wild-type proteins. Finally, we find that the presence of aberrant splicing isoforms of TNNT3 characterizes dystrophic muscles in FSHD patients. Collectively, our results suggest that anomalous TNNT3 profile correlates with the muscle impairment in both humans and mice. On the basis of these results, we propose that aberrant fTnT represents a biological marker of muscle phenotype severity and disease progression.

Description: The role of 3,5,3'-triiodo- l -thyronine (T3) and its metabolite 3,5-diiodo- l -thyronine (T2) in modulating the intracellular Ca 2+ concentration ([Ca 2+ ] i ) and endogenous nitric oxide (NO) synthesis was evaluated in pituitary GH 3 cells in the absence or presence of extracellular Ca 2+ . When applied in Ca 2+ -free solution, T2 and T3 increased [Ca 2+ ] i , in a dose-dependent way, and NO levels. Inhibition of neuronal NO synthase by N G -nitro- l -arginine methyl ester and l- n 5 -(1- iminoethyl)ornithine hydrochloride significantly reduced the [Ca 2+ ] i increase induced by T2 and T3. However, while depletion of inositol trisphosphate-dependent Ca 2+ stores did not interfere with the T2- and T3-induced [Ca 2+ ] i increases, the inhibition of phosphatidylinositol 3-kinase by LY-294002 and the dominant negative form of Akt mutated at the ATP binding site prevented these effects. Furthermore, the mitochondrial protonophore carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone prevented the increases in both [Ca 2+ ] i and NO elicited by T2 or T3. Interestingly, rotenone blocked the early [Ca 2+ ] i increases elicited by T2 and T3, while antimycin prevented only that elicited by T3. Inhibition of mitochondrial Na + /Ca 2+ exchanger by CGP37157 significantly reduced the [Ca 2+ ] i increases induced by T2 and T3. In the presence of extracellular calcium (1.2 mM), under carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone, T2 and T3 increased both [Ca 2+ ] i and intracellular Na + concentration; nimodipine reduced the [Ca 2+ ] i increases elicited by T2 and T3, but inhibition of NO synthase and blockade of the Na + /H + pump by 5-( N -ethyl- N -isopropyl)amiloride prevented only that elicited by T3; and CB-DMB, bisindolylmaleimide, and LY-294002 (inhibitors of the Na + /Ca 2+ exchanger, PKC, and phosphatidylinositol 3-kinase, respectively) failed to modify the T2- and T3-induced effects. Collectively, the present results suggest that T2 and T3 exert short-term nongenomic effects on intracellular calcium and NO by modulating plasma membrane and mitochondrial pathways that differ between these iodothyronines.