COMPARTMENTALIZATION OF ACETYLCHOLINESTERASE MESSENGER-RNA AND ENZYMEAT THE VERTEBRATE NEUROMUSCULAR-JUNCTION

Acetylcholinesterase (AChE) is concentrated at the vertebrate neuromus cular synapse. To determine whether increased transcript levels could underlie this selective accumulation, we employed a quantitative rever se transcription polymerase chain reaction-based assay to determine mR NA copy number in samples as small as single neuromuscular junctions ( NMJs) and a microassay to measure AChE enzyme activity at single synap ses. Our results show that AChE mRNA is an intermediate transcript at NMJs, whereas in noninnervated regions of muscle fibers, AChE transcri pts are either undetectable or rare. In contrast, alpha-actin transcri pt levels in the same samples are similar in junctional and extrajunct ional regions. However, compared with AChE enzyme activity and alpha-a ctin mRNA levels, the levels of AChE transcripts at NM)s are highly va riable. These results indicate that AChE mRNA and protein expression a re compartmentalized at the vertebrate NMJ and provide a direct approa ch toward dissecting the molecular events leading from synaptic activa tion to plastic changes in gene expression at single vertebrate synaps es.