Alternative splicing of a neural-specific Src mRNA (Src+) is a rapid and protein synthesis-independent response to neural induction in Xenopus laevis.

Alternative splicing is used by the src protooncogene to derive two gene products, pp60c-src and pp60+. pp60+ is expressed exclusively in neurons, whereas pp60c-src is expressed in many tissues. The specific role of the src+ gene product in neurons is not yet understood. Expression of src+ mRNA begins early in neural development and is restricted to the neural plate in Xenopus embryos. Using a reverse transcription/polymerase chain reaction assay, we demonstrate that expression of src+ mRNA is dependent on neural induction. Src+ mRNA is expressed in animal cap ectoderm which is cocultured with mesoderm tissue, but not in ectoderm cultured alone. The protein kinase C (PKC) activator 12-O-tetradecanoylphorbol-13-acetate (TPA) induces src+ mRNA expression in cultured stage 10+ animal cap ectoderm. This induction is rapid, with src+ mRNA being detected within 1 hr of TPA addition. Only dorsal ectoderm tissue is competent to express src+ mRNA upon TPA treatment, and stage 10+ ectoderm tissue becomes unresponsive to TPA after 4 hr in culture. TPA-dependent induction of src+ mRNA in ectoderm tissue is not blocked by cycloheximide and therefore is not dependent on protein synthesis.

Identification and developmental expression of Src+ mRNAs in Xenopus laevis.

Alternative splicing of src mRNA has been demonstrated in several vertebrate species to yield a neuron-specific form of src protein termed pp60+. The function of pp60+ is unknown. The early developmental expression pattern of src+ RNA has not been previously examined. We have identified and characterized src+ transcripts corresponding to the two src genes in Xenopus laevis using a reverse transcription/polymerase chain reaction (RT/PCR) method. Both Xenopus pp60+ proteins have a 5-amino-acid insert in contrast to the 6-amino-acid insert in fish, birds, and mammals. Src+ mRNA first appears in neural plate stage Xenopus embryos, after neural induction signaling events but prior to neural differentiation. Analysis of dissected neural plate stage embryos showed that src+ mRNAs are localized to the neural plate. These findings suggest that pp60+ may play a role in elaboration of neuron structure.

The yes proto-oncogene is present in amphibians and contributes to the maternal RNA pool in the oocyte.

Low stringency screening of a cDNA library from the frog Xenopus laevis with a viral yes oncogene probe resulted in the isolation of clones with restriction maps which were distinct from previously identified X. laevis src cDNA clones. The polypeptide produced by in vitro translation of RNA synthesized from one of the clones was slightly larger in size than X. laevis pp60src, but gave protease cleavage patterns which were nearly identical to those yielded by pp60src. Determination of the DNA sequence of this clone revealed that it is derived from a X. laevis yes gene homologue. Transcripts from the yes gene were found in the ovary and consisted of a major 3.4 kb species and additional minor species. These findings indicate that the duplication event separating the src and yes genes occurred prior to the appearance of amphibians and that the yes gene product is likely to play an important role in oogenesis or early development.