Abstract

Transforming growth factor (TGF-β) is involved in several cellular processes such as cell proliferation, differentiation, and apoptosis. At the cell surface, TGF-β binds to serine-threonine kinase transmembrane receptors (type II and type I) to initiate Smad-dependent intracellular signaling cascades. During the early stages of skeletal muscle differentiation, myotubes start to evoke spontaneous electrical activity in association with contractions that arise following the maturation of the excitation-contraction apparatus. In this work, we report that TGF-β-dependent signaling is regulated by electrical activity in developing rat primary myotubes, as determined by Smad2 phosphorylation, Smad4 nuclear translocation, and p3TPLux reporter activity. This electrical activity-dependent regulation is associated with changes in TGF-β type I receptor (TβRI) levels, correlated with changes in transducing receptors at the cell membrane (measured through radiolabeling binding assays). The inhibition of electrical activity with tetrodotoxin, a voltage-dependent sodium channel blocker, increases TβRI levels via a transcription-dependent mechanism. In contrast, the promotion of electrical activity in myotube cultures, induced by the upregulation of voltage-dependent sodium channels or by direct stimulation with extracellular electrodes, causes TβRI levels to decrease. Similar results were obtained in denervated adult muscles, suggesting that electrical activity-dependent regulation of TβRI also occurs in vivo. Additional results suggest that this activity dependent regulation is mediated by myogenin. Altogether, these findings support the possibility for a novel regulatory mechanism acting on TGF-β signaling cascade in skeletal muscle cells. © 2006 by The American Society for Biochemistry and Molecular Biology, Inc.