Role of EST2/ETV5 as transcription factors in the regulation of SHH by FGF (541.3)

During limb development, fibroblast growth factors (FGFs) from the apical ectodermal ridge (AER) regulate the expression of Sonic hedgehog (SHH) in the underlying posterior distal mesoderm. The molecules or pathways involved in the regulation of SHH by FGF are unknown. A recent study suggests that ETS and ETV, members of the ETS transcription factor family, are intermediate regulators in the Fgf to Shh cascade and directly interact with the limb-specific Shh regulatory region in the murine limb model. To determine whether this was a conserved function of ETS/ETV transcription factors, we used the chick model and examined  ETS2/ETV5  expression by  in situ  hybridization following ectopic application of FGF in the posterior limb bud, an assay known to induce  SHH .  ETV5  is expressed in the distal mesoderm sub-adjacent to the AER and overlaps the  SHH  expression domain.  ETS2  expression is most prominent in the developing chick zeugopod and appears to correlate with the condensing radius and ulna. The posterior expression of  ETS2  is more robust than its anterior expression. Both factors show increased expression around ectopically applied FGF2-laden beads at 24 hours after bead implantation. These data demonstrate expression patterns that spatially complement and overlap the  SHH  expression domain. Furthermore, both factors are upregulated by ectopic FGF prior to or coincident with  SHH  upregulation. These data support a role for ETS transcription factors in the regulation of SHH by FGF.