ISOLATION AND DEVELOPMENTAL EXPRESSION OF THE AMPHIBIAN HOMOLOG OF THE FIBROBLAST GROWTH-FACTOR RECEPTOR-3

Recent observations suggest that fibroblast growth factors (FGFs) and their receptors are involved in the control of embryogenesis. Several FGF receptor genes have been identified so far and their expression is differentially regulated. As part of a continuing effort to analyse t he differential expression of FGF receptors and their potential role d uring amphibian development, we have isolated a Pleurodeles homolog of FGF receptor 3 (FGFR-3), which we designated PFR-3 because of its hig hest homology to human FGFR-3 (75% overall identity). PFR-3 is a mater nally derived mRNA. While a low level of expression persists during th e cleavage and gastrula stages, a significant increase in the mRNA was observed at the end of the gastrula stage. RNase protection analysis on dissected tissues showed that PFR-3 mRNA was mainly localized to th e ectoderm at the early gastrula stage and then shifted to the embryon ic neural tissues, whereas adult brain had decreased levels of PFR-3 m RNA expression. Consistent with the loss of FGF receptors during skele tal muscle terminal differentiation, PFR-3 as well as other FGF recept or mRNAs were undetectable in the adult skeletal muscle. However, high est levels of PFR-3 mRNA expression were found in the testis. In situ hybridization revealed strong expression in the germinal epithelium of the embryonic brain (especially the diencephalon and rhombencephalon) and neural tube, in the lens and the cranial ganglia. The epithelium of the developing gut, like the pharynx and esophagus, also prominentl y expressed PFR-3 mRNA. Other sites of expression were found in the li ver and in the mesenchymal condensation sites of branchial arches. The se observations further suggest that individual FGF receptor genes are differentially regulated during amphibian embryogenesis, and that PFR -3 may play a role in mesoderm induction and in other FGF-mediated dev elopmental processes.