Agrin and acetylcholine receptors are removed from abandoned synaptic sites at reinnervated frog neuromuscular junctions

Abstract

Changes in the distribution of agrin and acetylcholine receptors (AChRs) were examined during reinnervation and following permanent denervation as a means of understanding mechanisms controlling the distribution of these molecules. Following nerve damage in the peripheral nervous system, regenerating nerve terminals preferentially return to previous synaptic sites leading to the restoration of synaptic activity. However, not all portions of original synaptic sites are reoccupied: Some of the synaptic sites are abandoned by both the nerve terminal and the Schwann cell. Abandoned synaptic sites contain agrin, AChRs, and acetylcholinesterase (AChE) without an overlying nerve terminal or Schwann cell providing a unique location to observe changes in the distribution of these synapse-specific molecules. The distribution of anti-agrin and AChR staining at abandoned synaptic sites was altered during the process of reinnervation, changing from a dense, wide distribution to a punctate, pale pattern, and finally becoming entirely absent. Agrin and AChRs were removed from abandoned synaptic sites in reinnervated frog neuromuscular junctions, while in contralateral muscles which were permanently denervated, anti-agrin and AChR staining remained at abandoned synaptic sites. Decreasing synaptic activity during reinnervation delayed the removal of agrin and AChRs from abandoned synaptic sites. Altogether, these results support the hypothesis that synaptic activity controls a cellular mechanism that directs the removal of agrin from synaptic basal lamina and the loss of agrin leads to the dispersal of AChRs.

Development of the Vertebrate Neuromuscular Junction

“…Consistent with this idea is the otherwise puzzling observation that AChRs are lost from denervated portions of incompletely reoccupied NMJs following reinnervation (Aframian & Grinnell 1988, Werle & Sojka 1996, Stanco & Werle 1997, Astrow et al 1996, whereas AChRs at completely denervated junctions are relatively stable (Frank et al 1976, Moss & Schuetze 1987. Apparently, inactive parts of a postsynaptic apparatus are stable in isolation but disappear when they are adjacent to active regions.…”