Cloning of cDNAs encoding xenopus neuregulin: expression in myotomal muscle during embryo development.

Neuregulin has diverse functions in neural development, and one of them is the up regulation of acetylcholine receptors (AChRs) at the muscle fiber during the formation of neuromuscular junctions. Although the primary source of neuregulin is derived from motor neuron, the expression in muscle has also been demonstrated. The precise role of neuron-derived and muscle-derived neuregulin during the early stages of development is not known. In order to study the role of neuregulin during early embryo development, we isolated the cDNAs encoding Xenopus neuregulin by cross-hybridization with its chick homologue. The amino acid sequence of Xenopus protein is 50 to 70% identical to members of the neuregulin family. The cDNAs encoding different isoforms of Xenopus neuregulin were identified, and these isoforms have two variation sites: (i) the spacer domain with either 0 or 43 amino acid insertion; and (ii) the C-terminus of EGF-like domain to derive either alpha or beta isoform. When the EGF-like domain of Xenopus neuregulin was expressed in mammalian cells, the recombinant protein was able to induce the expression of AChR and the tyrosine phosphorylation of erbB receptors in cultured myotubes. An approximately 6.5 kb transcript corresponding to neuregulin was detected in RNA isolated from brain and muscle. Various splicing variants were expressed in different Xenopus tissues. In situ hybridization showed a strong expression of neuregulin in developing brain and spinal cord of Xenopus embryo. In addition, it was also prominently expressed in the myotomal muscle. These data suggest that in addition to motor neurons, the postsynaptic muscle cells can also contribute neuregulin for synaptogenesis.