OVEREXPRESSED MONOMERIC HUMAN ACETYLCHOLINESTERASE INDUCES SUBTLE ULTRASTRUCTURAL MODIFICATIONS IN DEVELOPING NEUROMUSCULAR-JUNCTIONS OF XENOPUS-LAEVIS EMBRYOS

Formation of a functional neuromuscular junction (NMJ) involves the bi osynthesis and transport of numerous muscle-specific proteins, among t hem the acetylcholine-hydrolyzing enzyme acetylcholinesterase (AChE). To study the mechanisms underlying this process, we have expressed DNA encoding human AChE downstream of the cytomegalovirus promoter in ooc ytes and developing embryos of Xenopus laevis. Recombinant human AChE (rHAChE) produced in Xenopus was biochemically and immunochemically in distinguishable from native human AChE but clearly distinguished from the endogenous frog enzyme. In microinjected embryos, high levels of c atalytically active rHAChE induced a transient state of overexpression that persisted for at least 4 days postfertilization. rHAChE appeared exclusively as nonassembled monomers in embryos at times when endogen ous Xenopus AChE displayed complex oligomeric assembly. Nonetheless, c ell-associated rHAChE accumulated in myotomes of 2- and 3-day-old embr yos within the same subcellular compartments as native Xenopus AChE. N MJs from 3-day-old DNA-injected embryos displayed fourfold or greater overexpression of AChE, a 30% increase in postsynaptic membrane length , and increased folding of the postsynaptic membrane. These findings i ndicate that an evolutionarily conserved property directs the intracel lular trafficking and synaptic targeting of AChE in muscle and support a role for AChE in vertebrate synaptogenesis.