Thrombin action decreases acetylcholine receptor aggregate number and stability in cultured mouse myotubes

Neurons develop and make very stable, long-term synaptic connections with other nerve cells and with muscle. Synaptic stability at the neuromuscular junction changes over development in that a proliferation of synaptic input are made to individual myotubes and synapses from all but one neuron are lost during development. In an established co-culture paradigm in which spinal motoneurons synaptically contact myotubes, thrombin and associated protease inhibitors have been shown to affect the loss of functional synaptic contacts [6]. Evidence has not been provided which clearly demonstrate whether protease/protease inhibitors affect either the pre- or postsynaptic terminal, or both. In an effort to determine whether these reagents directly affect postsynaptic receptors on myotubes, myotubes were cultured in the absence of neurons and the spontaneous presence and stability of aggregates of acetylcholine receptors (AChR) in control and thrombin-containing media were evaluated. In dishes fixed after treatment and in dishes in which individual aggregates were observed live, thrombin action appeared to increase loss of AChR aggregates over time. Hirudin, a specific inhibitor of the thrombin protease, diminished this loss. Neither reagent affected the overall incorporation or degradation of AChR; therefore, it appears these protease/protease inhibitors affect the state of AChR aggregation.