Continuous exposure to isoprenaline reduced myotube size by delaying myoblast differentiation and fusion through the NFAT-MEF2C signaling pathway

We aimed to explore whether superfluous sympathetic activity affects myoblast differentiation, fusion, and myofiber types using a continuous single-dose isoprenaline exposure model in vitro and to further confirm the role of distinct NFATs in ISO-mediated effects. Compared with delivery of single and interval single, continuous single-dose ISO most obviously diminished myotube size while postponing myoblast differentiation/fusion in a time- and dose-dependent pattern, accompanied by an apparent decrease in nuclear NFATc1/c2 levels and a slight increase in nuclear NFATc3/c4 levels. Overexpression of NFATc1 or NFATc2, particularly NFATc1, markedly abolished the inhibitory effects of ISO on myoblast differentiation/fusion, myotube size and Myh7 expression, which was attributed to a remarkable increase in the nuclear NFATc1/c2 levels and a reduction in the nuclear NFATc4 levels and the associated increase in the numbers of MyoG and MEF2C positive nuclei within more than 3 nuclei myotubes, especially in MEF2C. Moreover, knockdown of NFATc3 by shRNA did not alter the inhibitory effect of ISO on myoblast differentiation/fusion or myotube size but partially recovered the expression of Myh7, which was related to the slightly increased nuclear levels of NFATc1/c2, MyoG and MEF2C. Knockdown of NFATc4 by shRNA prominently increased the number of MyHC +, MyoG or MEF2C + myoblast cells with 1 ~ 2 nuclei, causing fewer numbers and smaller myotube sizes. However, NFATc4 knockdown further deteriorated the effects of ISO on myoblast fusion and myotube size, with more than 5 nuclei and Myh1/2/4 expression, which was associated with a decrease in nuclear NFATc2/c3 levels. Therefore, ISO inhibited myoblast differentiation/fusion and myotube size through the NFAT-MyoG-MEF2C signaling pathway.