Studies on the Role of Fibroblast Growth Factor Signaling in Neurogenesis Using Conjugated/Aged Animal Caps and Dorsal Ectoderm-Grafted Embryos

Abstract

Basic fibroblast growth factor (bFGF) has been shown to induce neural fate in dissociated animal cap (AC) cells or in AC explants cultured in low calcium and magnesium concentrations. However, long-term disclosure of the cap may cause diffusion of the secreted molecule bone morphogenetic protein 4 (BMP-4), a neural inhibitor present in the AC. This may contribute to the subsequent neurogenesis induced by bFGF. Here we used conjugated and aged blastula AC to avoid diffusion of endogenous molecules from the AC. Unlike noggin, bFGF failed to induce neural tissue in this system. However, it enhanced neuralization elicited by a dominant negative BMP receptor (DN-BR) that inhibits the BMP-4 signaling. Posterior neural markers were turned on by bFGF in AC expressing DN-BR or chordin. Blocking the endogenous FGF signal with a dominant negative FGF receptor (XFD) mainly inhibited development of posterior neural tissue in neuralized ACs. These in vitro studies were confirmed in vivo in embryos grafted with XFD-expressing ACs in the place of neuroectoderm. Expression of some regional neural markers was inhibited, although markers for muscle and posterior notochord were still detectable in the grafted embryos, suggesting that XFD specifically affected neurogenesis but not the dorsal mesoderm. The use of these in vitro and in vivo model systems provides new evidence that FGF, although unable to initiate neurogenesis on its own, is required for neural induction as well as for posteriorization.

Neural induction: old problem, new findings, yet more questions

“…However, it has been proposed that neural induction may involve FGFR4 rather than FGFR1 (Hongo et al, 1999;Hardcastle et al, 2000;Umbhauer et al, 2000), and a more recent study using a general inhibitor of FGFRs has uncovered a clear requirement for FGF signalling in neural induction in Xenopus (Delaune et al, 2005). Overall, it is now generally accepted that FGF signalling is required for neural induction both in amphibians (Launay et al, 1996;Sasai et al, 1996;Xu et al, 1997;Hardcastle et al, 2000;Strong et al, 2000;Pera et al, 2003;Linker and Stern, 2004;Delaune et al, 2005) and in chick (Streit et al, 2000;Wilson et al, 2000;Linker and Stern, 2004).…”

Neural induction requires BMP inhibition only as a late step, and involves signals other than FGF and Wnt antagonists

“…In Xenopus, however, the evidence for FGF in neural induction has been controversial (Kengaku and Okamoto, 1995;Lamb and Harland, 1995;Launay et al, 1996;Sasai et al, 1996;Xu et al, 1997;Holowacz and Sokol, 1999;Hongo et al, 1999;Curran and Grainger, 2000;Ribisi et al, 2000;Umbhauer et al, 2000). One of the reasons for this may be that, in Xenopus, FGF signalling has mainly been inhibited using a dominantnegative version of the FGF receptor 1 (FGFR1), which may not inhibit all FGF signalling as FGF signals in neural induction appear to be transmitted at least in part by FGFR4 (Hardcastle et al, 2000;Umbhauer et al, 2000).…”

Morphogen gradients, positional information, and Xenopus: Interplay of theory and experiment

“…FGF does seem to be able to diffuse over several cell diameters (Christen and Slack, 1999), and it has been proposed to be a morphogen in early posterior patterning of the nervous system in Xenopus: embryonic FGF (eFGF) may act during gastrulation stages to induce expression of posterior Hox genes and posterior neural markers (Xu et al, 1997;Isaacs et al, 1998;Pownall et al, 1998;Holowacz and Sokol, 1999). However, more detailed analysis of FGF's potential morphogen activity is difficult because the rapid increase in anatomic complexity makes quantitation problematic.…”