RE-PROGRAMMING OF EXPRESSION OF THE KGFR AND BEK VARIANTS OF FIBROBLAST GROWTH-FACTOR RECEPTOR-2 DURING LIMB REGENERATION IN NEWTS (NOTOPHTHALMUS-VIRIDESCENS)

We have previously shown, by in situ hybridization, that fibroblast gr owth factor receptor 2 (FGFR2) is present in the basal layer of wound epithelium during limb regeneration in newts (Notophthalmus viridescen s). In contrast, FGFR1 expression is observed throughout the blastema mesenchyme but is distinctly absent from the wound epithelium (Poulin et al. [1993] Development 119:353-361). Sequence analysis revealed tha t we have isolated both the KGFR and bek variants of FGFR2. These two variants differ only in the second half of the last of their three (or two) Ig-like domains. In this report, we show the expression patterns of FGFR2 variants during limb regeneration by in situ hybridization. During the pre-blastema stages of regeneration, FGFR2 expression was o bserved in the basal layer of the wound epithelium and in the cells of the periosteum. The wound epithelial hybridization was observed when the KGFR-specific probe was used while the bek-specific probe hybridiz ed to mRNA in the cells of the periosteum. As regeneration progresses to the blastema stages, KGFR expression continued to be observed in th e basal layer of the wound epithelium with additional hybridization se en in the blastema mesenchyme closely associated with the bisected bon es. The bek-specific hybridization pattern observed at this stage corr esponds specifically to the mesenchymal hybridization. In the differen tiation stages of regeneration, the mesenchymal expression of FGFR2 be comes restricted to the cells of the condensing cartilage and later to the perichondrium. Interestingly, there appears to be a dorsoventral gradient of the expression of both KGFR and bek variants of FGFR2, whi ch are opposite each other at the later stages of regeneration. Thus, re-programming of expression of the two FGFR2 variants is required dur ing the initial wound closure of limb regeneration. Remarkably, the ex pression patterns of KGFR and bek mimic those observed in the mouse li mb bud during early embryonic development (Orr-Urtreger et al. [1993] Dev. Biol. 158:475-486). Moreover, our results suggest that the two FG FR2 variants have distinct roles in limb regeneration. Further investi gation regarding the potential sources of the FGF ligands will help es tablish the roles that FGFs and FGFRs play in limb regeneration. (C) 1 995 Wiley-Liss, Inc.