Agrin induces, whereas acetylcholine (ACh) disperses,
ACh receptor (AChR) clusters during
neuromuscular synaptogenesis. Such counteractive
interaction leads to eventual dispersal
of nonsynaptic AChR-rich sites and formation
of receptor clusters at the postjunctional membrane.
However, the underlying mechanisms
are not well understood. Here we show that
calpain, a calcium-dependent protease, is activated
by the cholinergic stimulation and is
required for induced dispersion of AChR clusters.
Interestingly, the AChR-associated protein
rapsyn interacted with calpain in an agrin-dependent
manner, and this interaction inhibited
the protease activity of calpain. Disrupting
the endogenous rapsyn/calpain interaction
enhanced CCh-induced dispersion of AChR
clusters. Moreover, the loss of AChR clusters
in agrin mutant mice was partially rescued by
the inhibition of calpain via overexpressing
calpastatin, an endogenous calpain inhibitor,
or injecting calpeptin, a cell-permeable calpain
inhibitor. These results demonstrate that calpain
participates in ACh-induced dispersion of
AChR clusters, and rapsyn stabilizes AChR
clusters by suppressing calpain activity.
