The VT+ and VT- isoforms of the fibroblast growth factor receptor type 1 are differentially expressed in the presumptive mesoderm of Xenopus embryos and differ in their ability to mediate mesoderm formation

Previously, we cloned a variant form of the type 1 fibroblast growth factor receptor (FGFR1), FGFR-VT-, from Xenopus embryos (Gillespie, L. L., Chen, G., and Paterno, G. D. (1995) J. Biol. Chem. 270, 22758-22763). This isofor m differed from the reported FGFR1 sequence (FGFR-VT+) by a a-amino acid de letion, Val(423)-Thr(424), in the juxtamembrane region. This deletion arise s from the use of an alternate 5' splice donor site, and the activity of th e VT+ and VT- forms of the FGFR1 was regulated by phosphorylation at this s ite. We have now investigated the expression pattern and function of these two isoforms in mesoderm formation in Xenopus embryos. Cells within the mar ginal zone are induced to form mesoderm during blastula stages. RNase prote ction analysis of blastula stage embryos revealed that the VT+ isoform was expressed throughout the embryo but that the VT- isoform was expressed almo st exclusively in the marginal zone. The ratio of VT+:VT- transcripts in th e marginal zone indicated that the VT+ form was predominant throughout blas tula stages except for a brief interval, coinciding with the start of zygot ic transcription, when a dramatic increase in VT- expression levels was det ected. This increase could be mimicked in part by treatment of animal cap e xplants with FGF-2, Overexpression of the VT+ isoform in Xenopus embryos re sulted in development of tadpoles with severe reductions in trunk and tail structures, while embryos overexpressing the VT- isoform developed normally . A standard mesoderm induction assay revealed that a 10-fold higher concen tration of FGF-2 was required to reach 50% induction in VT+-overexpressing animal cap explants compared with those overexpressing the VT-isoform, Furt hermore, little or no expression of the pan-mesodermal marker Bratchyury (X bra) was detected in VT+-overexpressing embryos, while VT- overexpressing e mbryos showed normal staining. This demonstrates that VT+ overexpression ha d a negative effect on mesoderm formation in vivo. These data are consisten t with a model in which mesoderm formation in vivo is regulated, at least i n part, by the relative expression levels of the VT+ and VT- isoforms.