The role of Schwann cell-derived agrin and neuregulin at the neuromuscular junction

To explore novel roles of glial cells in synaptic function and formation, we examined the expression of agrin and neuregulin-1 in frog Schwann cells and tested their role in the aggregation and synthesis of acetylcholine receptors (AChRs). We also investigated the role of a Schwann cell-derived factor in promoting synaptogenesis at the neuromuscular junction (NMJ). Schwann cells along nerve fibers in tadpoles expressed only the inactive agrin isoform B0, but began to also express active agrin isoforms B11 and B19 around metamorphosis. During nerve regeneration in the adult, the expression of these active agrin isoforms in Schwann cells was upregulated, including the appearance of the most potent isoform in aggregating AChRs, B8. This upregulation was induced by regenerating axons, but not by nerve injury per se. In muscle cultures, the presence of adult Schwann cells enhanced the AChR aggregation. The active agrin expression in Schwann cells and the ability of Schwann cells to aggregate AChRs were further enhanced with pre-exposure to neurons. By selectively ablating perisynaptic Schwann cells (PSCs) at the NMJ, we found that PSCs also expressed active agrin isoforms B11 and B19, and these active isoforms were upregulated, including the appearance of B8, during reinnervation. Observation in vivo showed that extrajunctional AChR aggregates were associated with PSC sprouts following nerve injury and subsequent reinnervation. These results suggest that, contrary to the prevailing view that only neurons express active agrin, glial cells also express active agrin and play a role in the aggregation of AChRs both in vitro and in vivo. In addition to the aggregation of AChRs on muscle, Schwann cell-conditioned medium was able to upregulate AChR synthesis in vitro , mediated by neuregulin-1. Schwann cells were not only able to cause aggregation on muscle themselves; Schwann cell-conditioned medium also restored neuronal agrin expression and promoted synapse formation at the NMJ in vitro. Schwann cell-derived factor was able to switch the growth state of cultured spinal neurons to the synaptogenic state. Taken together, Schwann cell-derived factors play active roles at the NNJ: promoting the formation of postsynaptic specialization and inducing presynaptic gene expression crucial for the synaptogenesis.