Blocking gap junctional intercellular communication in myoblasts inhibits myogenin and MRF4 expression

Cells rely heavily on cues from their extracellular environment and other cells to coordinate normal physiological processes, and the exchange of molecules via gap junctions has been suggested as on important avenue for cell-cell communication. Gap junctions are found in virtually all mammalian tissues with the notable exception of adult skeletal muscle. However, since functional gap junctions have been detected during the early stages of muscle development, gap junctional intercellular communication (GJIC) may play on important role in myogenesis. In this study, GJIC in normal 16 myoblasts was inhibited using the known blockers l-octanol and beta-glycyrrhetinic acid (beta-GA). Under differentiation promoting conditions, 16 cells fused to form multinucleated myotubes, but when treated with either octanol or beta-GA, no fusion was observed. The expression of two muscle regulatory factors (MRFs), myogenin and MRF4, was examined in both the blocked and control cells. As expected, the activation of both the myogenin and MRF4 genes coincided with the onset of differentiation in the control 16 cells. Neither of these genes were turned on in the blocked cells, even when grown under low serum conditions. This inhibition of differentiation by octanol and beta-GA was reversible, since the activation of both MRF genes as well as myoblast fusion were observed when the blocking medium was replaced with normal differentiating medium. These results suggest that intercellular communication via gap junctions plays an important role in skeletal muscle development and perhaps in the cell signaling events that trigger the activation of muscle-specific MRF genes.