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J Biol Chem, Vol. 275, Issue 13, 9581-9586, March 31, 2000

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^*

Gary D. Paterno, Paula J. Ryan, Kenneth R. Kao, and Laura L. Gillespie

From the Terry Fox Cancer Research Laboratories, Division of Basic Medical Sciences, Faculty of Medicine, Memorial University of Newfoundland, St. John's, Newfoundland A1B 3V6, Canada

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 isoform differed from the reported FGFR1 sequence (FGFR-VT+) by a 2-amino acid deletion, Val⁴²³-Thr⁴²⁴, in the juxtamembrane region. This deletion arises from the use of an alternate 5' splice donor site, and the activity of the VT+ and VT forms of the FGFR1 was regulated by phosphorylation at this site. We have now investigated the expression pattern and function of these two isoforms in mesoderm formation in Xenopus embryos. Cells within the marginal zone are induced to form mesoderm during blastula stages. RNase protection analysis of blastula stage embryos revealed that the VT+ isoform was expressed throughout the embryo but that the VT isoform was expressed almost exclusively in the marginal zone. The ratio of VT+:VT transcripts in the marginal zone indicated that the VT+ form was predominant throughout blastula stages except for a brief interval, coinciding with the start of zygotic transcription, when a dramatic increase in VT expression levels was detected. This increase could be mimicked in part by treatment of animal cap explants with FGF-2. Overexpression of the VT+ isoform in Xenopus embryos resulted 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 concentration of FGF-2 was required to reach 50% induction in VT+-overexpressing animal cap explants compared with those overexpressing the VT isoform. Furthermore, little or no expression of the panmesodermal marker Brachyury (Xbra) was detected in VT+-overexpressing embryos, while VT-overexpressing embryos showed normal staining. This demonstrates that VT+ overexpression had a negative effect on mesoderm formation in vivo. These data are consistent with a model in which mesoderm formation in vivo is regulated, at least in part, by the relative expression levels of the VT+ and VT isoforms.

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