Fgf signalling through MAPK cascade is required for development of the subpallial telencephalon in zebrafish embryos

The telencephalon is formed in the most anterior part of the central nervous system (CNS) and is organised into ventral subpallial and dorsal pallial domains. In mice, it has been demonstrated that Fgf signalling has an important role in induction and patterning of the telencephalon. However, the precise role of Fgf signalling is still unclear, owing to overlapping functions of Fgf family genes. To address this, we have examined, in zebrafish embryos, the activation of Ras/mitogen-activated protein kinase (MAPK), one of the major downstream targets of Fgf signalling. Immunohistochemical analysis reveals that an extracellular signal-regulated kinase (ERK), a vertebrate MAPK is activated in the anterior neural boundary (ANB) of the developing CNS at early segmentation stages. Experiments with Fgf inhibitors reveal that ERK activation at this stage is totally dependent on Fgf signalling. Interestingly, a substantial amount of ERK activation is observed in ace mutants in which fgf8 gene is mutated. We then examine the function of Fgf signalling in telencephalic development by use of several inhibitors to Fgf signalling cascade, including dominant-negative forms of Ras (Ras(N17)) and the Fgf receptor (Fgfr), and a chemical inhibitor of Fgfr, SU5402. In treated embryos, the induction of telencephalic territory normally proceeded but the development of the subpallial telencephalon was suppressed, indicating that Fgf signalling is required for the regionalisation within the telencephalon. Finally, antisense experiments with morpholino-modified oligonucleotides suggest that zebrafish fgf3, which is also expressed in the ANB, co-operates with fgf8 in subpallial development.