PKAc-directed interaction and phosphorylation of Ptc is required for Hh signaling inhibition in Drosophila

Phosphatidic acid binding to Patched contributes to the inhibition of Smoothened and Hedgehog signaling in Drosophila wing development

Hedgehog (Hh) signaling controls growth and patterning during embryonic development and homeostasis in adult tissues. Hh binding to the receptor Patched (Ptc) elicits intracellular signaling by relieving Ptc-mediated inhibition of the transmembrane protein Smoothened (Smo). We uncovered a role for the lipid phosphatidic acid (PA) in the regulation of the Hh pathway in Drosophila melanogaster. Deleting the Ptc C-terminal tail or mutating the predicted PA-binding sites within it prevented Ptc from inhibiting Smo in wing discs and in cultured cells. The C-terminal tail of Ptc directly interacted with PA in vitro, an association that was reduced by Hh, and increased the amount of PA at the plasma membrane in cultured cells. Smo also interacted with PA in vitro through a binding pocket located in the transmembrane region, and mutating residues in this pocket reduced Smo activity in vivo and in cells. By genetically manipulating PA amounts in vivo or treating cultured cells with PA, we demonstrated that PA promoted Smo activation. Our findings suggest that Ptc may sequester PA in the absence of Hh and release it in the presence of Hh, thereby increasing the amount of PA that is locally available to promote Smo activation.

Phenotypical and genetical characterization of the Mad1-2 allele during Drosophila wing development

Growth and patterning of Drosophila wing depends upon the sequential organizing activities of Hedgehog (Hh) and Decapentaplegic (Dpp) signaling pathways. The Hh signaling directly activates the expression of dpp through the transcription factor cubitus interruptus (Ci). Dpp itself functions as a long-range morphogen to promote cell proliferation and differentiation through an essential transcription factor encoded by Mad. Here we report that the Mad^1-2 allele exhibits phenotypes distinct from classical Dpp pathway mutants in the developing wing. The activity of Dpp signaling is attenuated in Mad^1-2 mutant cells. However, activation of Dpp signaling is found in a subset of cells surrounding homozygous Mad^1-2 clones when the clones are located at the anterior compartment of wing disc. Further analysis reveals that Mad^1-2 mutant cells display high level of Hh signaling activity and accumulate significant amount of Ci. Unexpectedly, whole genome resequencing identifies multiple mutations in the 3'UTR region of Pka-C1 genomic loci in the Mad^1-2 stock. We provide genetic and molecular evidence that the Pka-C1 mutations carried by Mad^1-2 likely underlies the observed Hh signaling defects. Therefore, the contribution of Pka-C1 mutation should be taken in consideration when analyzing Mad^1-2 phenotypes. The isolation of independent Mad and Pka-C1 alleles from the Mad^1-2 stock further supports our conclusions.