Differential expression and regulation of two distinct fibroblast growth factor receptors during early development of the urodele amphibian Pleurodeles waltl.

Fibroblast growth factor (FGF) has been shown to be involved in mesoderm induction during amphibian development. Its presence in the embryo suggests that FGF is an endogenous inducer. By polymerase chain reaction (PCR) methods and by screening a Pleurodeles waltl tail-bud cDNA library with a cDNA probe for human FGF receptor, we have isolated two full-length cDNA clones, which we designate PFR1 and PFR4 based on their homology to the human FGF receptors FGFR-1 and FGFR-4. Both cDNA clones encode Pleurodeles FGF receptors that share characteristics common to members of the FGF receptor superfamily. The deduced amino acid sequence of PFR1 is 85% identical overall with the human fms-like-gene (FLG). PFR4 is most closely related to the human FGFR-4 (66% overall identity). The tyrosine kinase catalytic domains of both receptors are remarkably conserved. The two receptors show distinct patterns of regulation during early development. PFR1 first appears as a maternally derived mRNA and mRNA levels remain constant during early developmental stages. However, PFR4 mRNA is first expressed at the late blastula stage, which suggests that its expression is a result of zygotic transcription. Furthermore, northern blot analysis indicates that PFR1 mRNA is distributed evenly in the early gastrula while PFR4 mRNA is predominantly localized to the presumptive ectoderm. At tail-bud stage, PFR1 transcripts are localized primarily to the neural and mesodermal tissues, PFR4 transcripts are most abundantly expressed in neural tissue, and more transcripts are detected in lateral plate mesoderm than in the somites. When animal cap explants of blastulae are cultured in the presence of mesoderm-inducing factors, PFR1 mRNA levels are maintained by bFGF and activin A. In contrast, PFR4 mRNA levels are significantly down-regulated. These observations suggest a differential expression and regulation of FGF receptors in early amphibian development.