FGF-8 stimulates neuronal differentiation through FGFR-4a and interferes with mesoderm induction in Xenopus embryos.
The role of fibroblast growth factors (FGFs) in neural induction is controversial [1,2]. Although FGF signalling has been implicated in early neural induction [3-5], a late role for FGFs in neural development is not well established. Indeed, it is thought that FGFs induce a precursor cell fate but are not able to induce neuronal differentiation or late neural markers [6-8]. It is also not known whether the same or distinct FGFs and FGF receptors (FGFRs) mediate the effects on mesoderm and neural development. We report that Xenopus embryos expressing ectopic FGF-8 develop an abundance of ectopic neurons that extend to the ventral, non-neural, ectoderm, but show no ectopic or enhanced notochord or somitic markers. FGF-8 inhibited the expression of an early mesoderm marker, Xbra, in contrast to eFGF, which induced ectopic Xbra robustly and neuronal differentiation weakly. The effect of FGF-8 on neurogenesis was blocked by dominant-negative FGFR-4a (DeltaXFGFR-4a). Endogenous neurogenesis was also blocked by DeltaXFGFR-4a and less efficiently by dominant-negative FGFR-1 (XFD), suggesting that it depends preferentially on signalling through FGFR-4a. The results suggest that FGF-8 and FGFR-4a signalling promotes neurogenesis and, unlike other FGFs, FGF-8 interferes with mesoderm induction. Thus, different FGFs show specificity for mesoderm induction versus neurogenesis and this may be mediated, at least in part, by the use of distinct receptors.
PubMed ID: 11114518
Article link: Curr Biol.
Genes referenced: fgf4 fgf8 fgfr1 fgfr4 neurog2 nog t tubb2b
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|Fig. 1. (a) Embryos injected with FGF-8/LacZ RNA show abundant ectopic neurogenesis. (b) Compared with FGF-8, eFGF RNA had a very weak effect on neurogenesis. Although some ectopic N-tubulin formed, the percentage of the affected embryos was lower (see text) and the phenotype less severe. The posterior elongation seen in the second eFGF panel was observed in approximately 50% of the high dose eFGF RNA injected embryos (n = 25) and was not seen in FGF-8 RNA-injected embryos. (c) Comparison of FGF-8 activity to that of noggin, RA, and X-ngnr1 (see text). Black arrows point to the interstripe region. All panels show expression of N-tubulin with the exception of the inset (XSox3). In all panels anterior is to the left and most panels show dorsal views, lateral views are indicated.|
|Fig. 2. (a) FGF-8 induces ectopic N-tubulin without inducing ectopic collagen type II. Control and FGF-8 RNA-injected embryos were hybridised with probes to collagen type II and N-tubulin (doubly or singly, as indicated) and sectioned. In all embryos, ectopic N-tubulin (black arrow) was not underlain by ectopic collagen type II. (b) Qualitatively different effect of eFGF and FGF-8 on Xbra expression. Injection of eFGF/lacZ RNA expanded Xbra whereas injection of FGF-8/lacZ RNA suppressed Xbra. Control lacZ injections had no effect on Xbra (inset). (c) Suppression of Xbra by FGF-8 and induction of Xbra by eFGF is dose dependent. A representative experiment is shown. The concentration difference between the low and high dose was threefold for both FGF-8 and eFGF. Expansion of Xbra in response to FGF-8 differed from that obtained with eFGF in that it was weaker in intensity and did not extend to the animal pole. In some FGF-8 RNA-injected embryos, both suppression and expansion of Xbra was observed in the same embryo. Low FGF-8, n=14; high FGF-8, n=15; low eFGF, n=16; high eFGF, n=24.|
|Fig. 3. Blocking signalling through FGFR-4a with ΔXFGFR-4a rescues the effect of FGF-8 on neurogenesis. Embryos were injected unilaterally at the two-cell stage with FGF-8 or FGF-8 plus ΔXFGFR-4a RNA. All were coinjected with lacZ and hybridised with N-tubulin. Top panels show side views of the same embryo; lower panels show a dorsal view of two embryos. When FGF-8 was coinjected with ΔXFGFR-4a, ectopic N-tubulin was abolished and endogenous N-tubulin was reduced. Note that abundant ectopic neurogenesis was still observed on the uninjected side, suggesting that it responds directly to FGF-8.|
|Fig. 4. Both ΔXFGFR-4a (90pg) and XFD (140pg) were very effective in blocking Xbra expression (100%, n=51 for XFD; n=41 for ΔXFGFR-4a). Using the same concentrations, ΔXFGFR-4a was more effective in blocking N-tubulin expression. Measurements of N-tubulin suppression represent the average percentage of seven experiments. XFD, n=193; ΔXFGFR-4a, n=136.|