A cysteine-rich form of Xenopus neuregulin induces the expression of acetylcholine receptors in cultured myotubes.

Neuregulin-1 (NRG-1) has diverse functions in neural development, and one of them is to up regulate the expression of acetylcholine receptors (AChRs) at muscle fibers during the formation of neuromuscular junctions. NRG-1 has two prominent alternative splicing sites at the N-terminus; it could be an immunoglobulin (Ig)-like domain named Ig-NRG-1 or an apolar cysteine-rich domain (CRD) named CRD-NRG-1. cDNAs encoding Xenopus CRD-NRG-1 were isolated by cross-hybridization with Xenopus Ig-NRG-1 cDNA fragment. The amino acid sequence of Xenopus CRD-NRG-1 is 45 to 70% identical to the human, rat, and chick homologs. Similar to Ig-NRG-1, two variation sites within CRD-NRG-1 were identified at the spacer domain with 0 or 43 amino acids inserted and at the C-terminus of the EGF-like domain to derive either alpha or beta isoform. Two transcripts encoding CRD-NRG-1, approximately 7.5 and approximately 9.0 kb, were revealed in adult brain and spinal cord, but the expression in muscle was below the detectable level. The recombinant Xenopus CRD-NRG-1 when applied onto cultured myotubes was able to induce the tyrosine phosphorylation of ErbB receptors and the expression of AChR. The AChR-inducing activity of CRD-NRG-1 was precipitated by anti-NRG-1 antibody but not by heparin. In situ hybridization showed a strong expression of CRD-NRG-1 mRNA in developing brain, spinal cord, and myotomal muscles of Xenopus embryo. Similar to the results in other species, both CRD-NRG-1 and Ig-NRG-1 may play a role in the developing Xenopus neuromuscular junctions.