We carry on basic research into the cellular and molecular mechanisms involved in the development of the mammalian neuromuscular junction. We use a novel nerve-muscle culture system which is chracterized by extensive early development of postsynaptic structure at sites of nerve- muscle contact followed later by maturation of the synaptic cleft and the presynaptic apparatus. We previously showed that the capacity to induce acetylcholine receptor aggregation is specific to the developing axons of individual ventral spinal cord neurons. This suggests that one or more of the signals for the induction of postsynaptic differentiation has a polarized distribution in the innervating neurons. We are currently exploring the possibility that adhesive interactions with muscle are specific to axons as opposed to dendrites, and that they may be important for the initiation of postsynaptic differentiation. This is being done by scanning and transmission electron microscopy as well as immunocytochemical localization of specific cell-surface adhesion molecules. Our results indicate that the closest adhesive interactions between nerve and muscle cells occur at sites of contact between myotubes and axons that induce acetylcholine receptor aggregation. Moreover, the myotube surface adjacent to these contacts exhibits ruffles and microvilli indicating a motile response to axonal contact. This motility, together with adhesion, increases the area of cell contact and thus may serve to facilitate critical surface interactions between the axon and myotube. We now plan to explore the relationship between the signal transduction process triggering this response and that involved in acetylcholine receptor aggregation. Agrin is a 400kDa proteoglycan that is believed to be a primary signal for the induction of postsynaptic differentiation in muscle. We are exploring the expression of alternatively-spliced isoforms of agrin mRNA in the neurons that induce acetylcholine receptor aggregation and the possible effect of contact with myotubes on that expression.
