Integration of Fgf and Shh signaling during limb development

Abstract

During limb development, Sonic Hedgehog (Shh) is a key signal, produced in the posterior domain known as the Zone of Polarizing Activity (ZPA) regulating anterior‐posterior. Fibroblast growth factor (FGF) is another important inductive agent, made by the Apical Ectodermal Ridge (AER) at the tip of the limb bud, which maintains the underlying mesenchyme and allows outgrowth and proximodistal patterning to take place. Cross‐talk between the Shh and Fgf pathways is also critical for proper patterning and growth of the developing limb bud. Several genes involved in regional specification of the limb, such as members of the Hoxd cluster, require both Fgf and Shh signaling for their induction. In addition, the expression of each factor requires the activity of the other. For example Shh expression is lost once Fgf signaling is disrupted. We have examined the FGF dependent activation of Etv4/5 ETS transcription factors in the distal limb, and their contribution to proximal‐distal limb outgrowth. Surprisingly, blockage of Etv4/5 activity in early distal mesenchyme results in ectopic anterior expansion of Shh (rather than its loss as expected for perturbation of the pathway downstream of Fgf activity.) This leads to a polydactylous phenotype, indicating a novel function for these molecules in anterior‐posterior patterning. Together, out data suggest FGF activity in the limb is not only responsible for maintaining Shh expression, but also required for precise posterior restriction of Shh in an Etv4/5 dependent manner, providing another level of interaction between the orthogonal axes during limb development.