Molecular components of the synaptic basal lamina that direct differentiation of regenerating neuromuscular junctions.

Results of experiments outlined here provide evidence that components of the myofiber basal lamina sheath direct the formation of active zones in regenerating motor nerve terminals and the development of infoldings and the aggregation of AChRs in the plasma membrane of regenerating myofibers. As a step toward identifying the basal lamina molecules that aggregate AChRs, we are now studying an ECM fraction from the Torpedo electric organ that causes AChRs to aggregate on cultured myotubes. We have solubilized and purified the electric organ AChR-aggregating molecules over 1000-fold. Only nanogram amounts of the most purified extracts are required to cause detectable AChR aggregation. We have also shown that similar activity can be extracted in relatively small amounts from muscle. Antiserum raised against the partially purified electric organ material completely blocked and immunoprecipitated the AChR-aggregating activity in extracts of the electric organ and muscle and bound to components of the basal lamina of frog muscle fibers. Although several polypeptides are present in our most purified extracts, an antiserum against polypeptides in the range of 80 kD completely blocked AChR aggregation by soluble extracts of the electric organ. These findings demonstrate the feasibility of isolating molecules from the synapse-rich electric organ that cause AChR aggregation and comparing them by immunological techniques with those in basal lamina at the neuromuscular junction.