Denervated single myofibers: neurite interactions and synaptic molecules.

As a first step in defining the molecular cues that may be important for reinnervation of long-term denervated muscle, single adult rat muscle fibers that had been denervated from 2 to 24 months in vivo were maintained in culture for 5 days. Embryonic ventral spinal cord explants were added to some of these cultures. Interactions of neurites with individual short-term (up to 5 months) and long-term (17-24 months) denervated muscle fibers were compared with neurite interactions in cultures of young adult muscle fibers (from 3 to 5-month-old rats) or aged muscle fibers (from 17 to 26-month-old rats). We found the following. (1) Three molecules that are found at the neuromuscular junction (NMJ)--acetylcholinesterase (AChE), acetylcholine receptors (AChRs), and gelasmin (an acetylcholine receptor clustering factor that is found enriched at NMJ of adult muscle)--were reduced with increasing periods of denervation but not with aging. (2) The number of neurite contacts at junctional regions of muscle fibers that were formed and maintained on cultured muscle fibers depended on denervation time of the muscle in vivo; very few contacts were made or maintained on long-term denervated fibers. (3) Gelasmin, but not AChE or AChRs, was found at points of neurite contact on all muscle fibers examined, raising the possibility that it may serve as a cue for reinnervation and that its loss from long-term denervated muscle may be, at least in part, involved in the failure of neurite contacts to be made or maintained in culture and possibly in vivo.