Differential requirements of the fused kinase for hedgehog signalling in the Drosophila embryo.

The two signalling proteins, Wingless and Hedgehog, play fundamental roles in patterning cells within each metamere of the Drosophila embryo. Within the ventral ectoderm, Hedgehog signals both to the anterior and posterior directions: anterior flanking cells express the wingless and patched Hedgehog target genes whereas posterior flanking cells express only patched. Furthermore, Hedgehog acts as a morphogen to pattern the dorsal cuticle, on the posterior side of cells where it is produced. Thus responsive embryonic cells appear to react according to their position relative to the Hedgehog source. The molecular basis of these differences is still largely unknown. In this paper we show that one component of the Hedgehog pathway, the Fused kinase accumulates preferentially in cells that could respond to Hedgehog but that Fused concentration is not a limiting step in the Hedgehog signalling. We present direct evidence that Fused is required autonomously in anterior cells neighbouring Hedgehog in order to maintain patched and wingless expression while Wingless is in turn maintaining engrailed and hedgehog expression. By expressing different components of the Hedgehog pathway only in anterior, wingless-expressing cells we could show that the Hedgehog signalling components Smoothened and Cubitus interruptus are required in cells posterior to Hedgehog domain to maintain patched expression whereas Fused is not necessary in these cells. This result suggests that Hedgehog responsive ventral cells in embryos can be divided into two distinct types depending on their requirement for Fused activity. In addition, we show that the morphogen Hedgehog can pattern the dorsal cuticle independently of Fused. In order to account for these differences in Fused requirements, we propose the existence of position-specific modulators of the Hedgehog response.