Long-term effects of innervation at the neuromuscular junction: Induction of localized transcription of adult-type nAChRS.

The mechanisms which define the molecular components, and thus the functional characteristics, of synapses are not known but, the interaction of pre- and post-synaptic elements can induce morphological and functional changes. At the developing neuromuscular junction (NMJ), innervation causes the nicotinic acetylcholine receptor (nAChR) to become concentrated at the NMJ. This ligand-gated ion channel is composed of five protein subunits $(\alpha2\beta\gamma\delta)$ in developing muscle. During development, and coincident with innervation, the $\gamma$-subunit is replaced by an $\varepsilon$-subunit, causing a change in nAChR ion channel properties. Denervation results in the reappearance of $\gamma$-subunit-containing nAChRs in extrajunctional regions of the muscle. Thus, innervation affects synaptic properties by altering the levels, localization and subunit composition of the nAChR. The general aim of the experiments presented in this thesis was to elucidate mechanisms by which the molecular components, and thus the functional characteristics, of a model synapse, the neuromuscular junction, are regulated. I have characterized the role muscle innervation plays in nAChR RNA expression. The main results of this work are: (1) Innervation has a long-lasting effect on localization of nAChR transcripts in skeletal muscle; (2) Epsilon RNA is expressed at times which coincide with innervation and localized at the NMJ following innervation, yet, does not require the nerve to maintain this localized expression; and (3) Increased cAMP does not mediate this local endplate-specific induction but may participate in decreased epsilon gene expression in extrajunctional regions of developing muscle fibers. These results, taken together, show that innervation causes long-term changes in localization of specific transcripts at the NMJ, of which the $\varepsilon$-subunit mRNA is an example, and that this effect is not mediated by cAMP in the rat as had been proposed in previous studies.PhDNeuroscienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/105030/1/9624734.pdfDescription of 9624734.pdf : Restricted to UM users only