TrkA and TrkC neurotrophin receptor-like proteins in the lizard gut

The tyrosine kinase proteins (Trk), encoded by the trk family of proto-onco genes, mediate, in mammals, the action of neurotrophins, a family of growth factors acting on the development and maintenance of the nervous system. N eurotrophins and their specific receptors, TrkA, TrkB and TrkC, seem to be phylogenetically well preserved but, in reptiles, data regarding the occurr ence of Trk-like proteins are very scarce, especially in nonnervous organs. Western blot analysis demonstrated that the lizard gut contains TrkA- and TrkC-like, but not TrkB-like, proteins. Consistently, TrkA- and TrkC-like i mmunoreactivity were both observed in neurons of the anterior intestine, wh ereas endocrine cells of the stomach and anterior intestine only displayed TrkA-like immunoreactivity. These results demonstrate for the first time th e occurrence of Trk-like proteins in non-neuronal tissues of reptilians and provide further evidence for the evolutionary preservation of the molecula r mass and cell distribution of Trk neurotrophin receptor-like proteins in the gut of vertebrates.