An analysis of N-Src splice variants in Xenopus development

The ubiquitously expressed cellular-Src (C-Src) protein kinase has a number of neural isoforms known as neural-Src (N-Src) kinases. One such isoform, N1-Src has an insert of five or six amino acids in its SH3 domain, changing target protein specificity. Previous data have shown that N1-Src expression levels correlate positively with good prognosis childhood neuroblastoma cancer; which develop from neural crest-derived tissues. In this project it was shown using RT- PCR that peaks in N1-Src expression correspond to stages of primary and secondary neurogenesis in the Xenopus tropicalis and zebrafish model organisms. An additional Xenopus-specific isoform, termed N3-Src, which previously had no ascribed function, possesses 22 additional amino acids in its SH3 domain. Here it was shown that N3-Src expression levels also peak during primary and secondary neurogenesis and that both isoforms are somewhat upregulated in noggin mRNA-injected animal cap explants. In situ hybridisation using locked nucleic acid (LNA) probes showed that the expression of these isoforms is restricted to neural and neural crest structures during early Xenopus tropicalis development. Antisense morpholino oligo-mediated knockdown of Xenopus tropicalis N1-Src and N3-Src resulted in locomotive defects, the expansion of the proliferative neural plate marker sox3 and the reduction of the neural differentiation marker n. tubulin in neurula stage Xenopus tropicalis embryos and a subtle reduction in phox2a expression at tailbud stages; a marker of noradrenergic cells from which neuroblastoma cancers derive. Whilst N1-Src overexpression had no detectable effect on Xenopus tropicalis embryo phenotype or the expression of the neural markers analysed, N3-Src overexpression caused a reduction in both eye pigmentation and the expression of phox2a. These data support functions of neural Src isoforms in the regulation of vertebrate neural development.