Characterization of fibroblast growth factor receptor type I isoforms in Xenopus laevis embryonic development

Fibroblast growth factor receptors are encoded by four genes, FGFR1-4, which are alternatively spliced to produce a large number of variant isoforms. This project was designed to investigate the molecular mechanism of the FGFR-VT+ isoform compared to its counterpart FGFR-VT- and the expression of four additional isoforms (FGFR-PS+, FGFR-PS-, α-FGFR1 and β-FGFR1). FGFR-VT- and FGFR-VT+ differ only by a dipeptide (Val⁴²³-Thr⁴²⁴) deletion. FGFR-PS- and FGFR-PS+ differ only by a dipeptide (Pro⁴⁴²-Ser⁴⁴³) deletion. α-FGFR and β-FGFR differ by the inclusion or exclusion of the first of the three immunoglobulin-like (Ig-like) loops. -- Previous work has shown that overexpression of the VT+ form in Xenopus embryos resulted in posterior truncations, whereas embryos overexpressing the VT- form developed normally. In an effort to elucidate the molecular basis of these deformities, expression patterns of Xenopus molecular markers known to be important for the development of the anterior-posterior axis were investigated. Of the markers studied (BMP-4, Xenopus forkhead, Goosecoid, Mix-1, Noggin, Xenopus brachyury, Xwnt-8, Xenopus posterior), no difference in expression pattern was observed, as determined by RT-PCR. -- Expression during early embryonic development of the FGFR variants PS+/PS- and α-FGFR1/β-FGFR1 were also examined by RT-PCR. Results suggested that PS- is more abundant than PS+ (1.3-1.8X higher) during early Xenopus development, however by stage 11.5 the ratio of PS-/PS+ approaches 1.0. Analysis of the Ig variants indicated that the α-FGFR1 form is the predominant transcript (2.5-4.1X higher) in early development. As development proceeds into tadpole stages, β-FGFR1 shows an increase in expression levels approaching that of α-FGFR1 at the same stage of development, with the ratio of α-FGFR1/β-FGFR1 approaching 1.0