Local control of acetylcholinesterase gene expression in multinucleated skeletal muscle fibers: Individual nuclei respond to signals from the overlying plasma membrane

Nuclei in multinucleated skeletal muscle fibers are capable of expressing d ifferent sets of muscle-specific genes depending on their locations within the fiber. Here we test the hypothesis that each nucleus can behave autonom ously and responds to signals generated locally on the plasma membrane. We used acetylcholinesterase (AChE) as a marker because its transcripts and pr otein are concentrated at the neuromuscular and myotendenous junctions. Fir st, we show that tetrodotoxin (TTX) reversibly suppresses accumulation of c ell surface AChE clusters, whereas veratridine or scorpion venom (ScVn) inc rease them. AChE mRNA levels are also regulated by membrane depolarization. We then designed chambered cultures that allow application of sodium chann el agonists or antagonists to restricted regions of the myotube surface. Wh en a segment of myotube is exposed to TTX, AChE cluster formation is suppre ssed only on that region. Conversely, ScVn increases AChE cluster formation only where in contact with the muscle surface. Likewise, both the synthesi s and secretion of AChE are shown to be locally regulated. Moreover, using in situ hybridization, we show that the perinuclear accumulation of AChE tr anscripts also depends on signals that each nucleus receives locally. Thus AChE can be up- and downregulated in adjacent regions of the same myotubes. These results indicate that individual nuclei are responding to locally ge nerated signals for cues regulating gene expression.