Journal of Neuroscience, Vol 13, 1315-1325, Copyright © 1993 by Society for Neuroscience
Calcium influx and protein phosphorylation mediate the metabolic stabilization of synaptic acetylcholine receptors in muscle
P Caroni, S Rotzler, JC Britt and HR Brenner
Friedrich Miescher Institute, Basel, Switzerland.
During neuromuscular synapse development, the degradation rate of ACh
receptors (AChRs) accumulated in the synaptic portion of the muscle
membrane is drastically reduced under neural control, their half-life t1/2
increasing from 1 d to about 12 d. Recent evidence suggests that the
metabolic stability of synaptic AChRs is mediated by the muscle activity
induced by the nerve. We have now investigated the pathway linking muscle
activity and metabolic stabilization of synaptic AChRs in organ cultured
rat muscle. Soleus and diaphragm muscles were denervated for 14-40 d, a
procedure leading to the destabilization of synaptic AChRs, and conditions
required to restabilize synaptic AChRs in the denervated muscle were
analyzed. The activity-dependent stabilization of synaptic AChRs in
chronically denervated endplates required calcium entry through
dihydropyridine-sensitive Ca2+ channels activated by high-frequency
stimulation for approximately 6 hr and was specific for synaptic AChRs. As
in vivo, extrasynaptic AChRs were not stabilized, and their t1/2 remained 1
d. The stabilization process was not dependent on de novo protein
synthesis, and it could also be brought about by elevated cAMP levels.
Furthermore, it required shorter stimulation periods in the presence of the
phosphatase inhibitors okadaic acid and calyculin A, whereas blockade of
protein kinases with high doses of staurosporine blocked the stabilization.
Activity- dependent, dihydropyridine-sensitive as well as cAMP-dependent
phosphorylation of myosin light chain was observed. These findings are
consistent with the notion that muscle activity initiates AChR
stabilization via the activation of calcium-dependent protein
phosphorylation reactions.