Maintenance of neurite contacts at junctional regions of cultured individual muscle fibers from aged rats is correlated with the presence of a synapse-associated protein, gelasmin.

When adult skeletal muscle is denervated as a result of injury or disease, it can usually be reinnervated. Throughout the life of an animal, some skeletal muscles are thought to undergo cycles of denervation and reinnervation with concomitant remodelling of the neuromuscular junction, even in the absence of injury or disease. In old animals, this reinnervation process may be faulty and may not occur at all in some aged muscle fibers, leaving them permanently denervated. In general, the former endplate is the preferred site of reinnervation, which has led to the speculation that specific molecular cues persist, particularly in the basal lamina of this region, that may mediate endplate reinnervation. Although these molecular cues are as yet unidentified, one candidate is gelasmin, a 93 kD glycoprotein we have isolated and characterized from preparations of rat synaptic extracellular matrix. Because studies of reinnervation of aged muscle in vivo are extremely difficult to perform, we have devised a tissue culture model system of muscle reinnervation composed of isolated individual aged (17-26 months old) or young adult (3-5 months old; control) rat skeletal muscle fibers and embryonic (day 13 in utero) ventral spinal cord explants. We found that (1) gelasmin was present at all sites of nerve-muscle contact on muscle fibers from both young adult and aged animals over a 10-day culture period, that (2) twice as many aged (88%) as young adult fibers (41%) had neurite contacts in the former junctional region at 10 days and (3) gelasmin was found on significantly more aged (95%) than young adult fibers (60%) grown without nerve explants. Furthermore, although no extrajunctional contacts were found on young adult fibers by the end of the 10-day culture period, substantial numbers of extrajunctional contacts were seen on aged fibers; gelasmin was present at all of these contact sites. These results are consistent with the idea that gelasmin, which is made by muscle fibers, may act to mediate or stabilize the contacts made by reinnervating nerve and that aged muscle fibers may regulate gelasmin or similar molecules differently from young adult muscle fibers.