Our aim is to study the organization of neurotransmitter receptors on nerve and muscle cells in relationship to the development and function of synapses. Our recent work has focused upon the factors, extrinsic and intrinsic to the developing skeletal muscle fiber, which regulate the distribution of nicotinic acetylcholine receptors. Acetylcholine receptor aggregation is induced on cultured myotubes by neuronal factors, and this system is used to study the mechanisms of receptor aggregation, as well as the stabilization or elimination of aggregates which occur in developing neuromuscular junctions. Our major findings in the past year are as follows: 1) A 43 kilodalton nonactin protein is precisely colocalized with newly formed receptor aggregates. This protein may play a role in receptor immobilization. 2) 60% of the receptor aggregating activity of embryonic brain extract is absorbed by antiserum against a receptor aggregating fraction from the extracellular matrix of Torpedo electric organ, suggesting the presence of immunologically related aggregation molecules in the two preparations. 3) The formation of muscle cell basal lamina induced by embryonic brain extract and ciliary ganglion explants is blocked by ascorbate oxidase, suggesting that nerve induced formation of basal lamina is mediated by an ascorbate-like factor.
