A spatial and temporal map of FGF/Erk1/2 activity and response repertoires in the early chick embryo

During early vertebrate development Fibroblast Growth Factor (FGF) signalling is required for multiple activities including specification of mesodermal, neural and heart tissue, as well as gastrulation movements and regulation of differentiation and pattern onset in the extending body axis. A current challenge is to understand how FGF signalling generates such diverse outcomes. A key FGF downstream pathway is the Ras-MAPK/Erk1/2 cascade, which culminates in the phosphorylation of target proteins, such as the Ets family of transcription factors. To begin to assess specificity downstream of FGF in the chick embryo we have characterised the patterns of Fgfr1-4 expression and Erk1/2 activation, as well as expression of the Erk1/2 specific phosphatase, Mkp3 and of three Ets factor genes (Erm, Pea3 and Er81) from early blastula to the 10 somite stage. We identify new sites of Fgfr expression and show that nearly all regions of Erk1/2 activity are within Fgfr expression domains and require FGF signalling. Differences in intensity, duration, distribution and sub-cellular localisation of activated Erk1/2 are observed in distinct cell populations within the embryo and during wound healing. With few exceptions, a tight correspondence between Erk1/2 activation and Mkp3 expression is found, while specific combinations of Ets factors are associated with distinct regions of Erk1/2 activation. These findings provide a comprehensive spatial and temporal map of FGF/Erk1/2 activity during early chick development and identify region and tissue specific differences in expression of Fgfrs as well as Erk1/2 phosphorylation and transcriptional targets which help to define response specificity.