Hedgehog (Hh) is secreted from polarized epithelial cells in the posterior (P) region of the imaginal disc of the developing Drosophila wing, crossing the border to the anterior (A) compartment to form a morphogenetic gradient. The precise mechanism whereby Hh spreads, and whether the gradient forms at the apical or the basolateral surface of the epithelium, have been controversial. Callejo et al. found that Hh was abundant at both the apical and basolateral membranes of P cells in wild-type disks, suggesting that it was secreted from both surfaces. Using discs with a dynamin mutation that “froze” endocytosis at the originating membrane, they determined that Hh accumulated basolaterally in recipient A cells, whereas its receptor Patched (with which it is endocytosed in recipient cells) accumulated both apically and basolaterally suggesting that the long-range Hh gradient forms on the basolateral surface. Intriguingly, however, Hh accumulated at the apical surface of dynamin mutant P cells, suggesting that it was internalized by the same cells that secrete it. Indeed, Hh appeared in P cell puncta, together with internalized dextran. Mutation of various endocytic proteins indicated that Hh was endocytosed at the apical surface of P cells by means of dynamin and Rab5 and then recycled to the basolateral membrane for secretion (rather than targeted for lysosomal degradation as in A cells). Dispatched, a protein previously implicated in the release of functional Hh, colocalized with Hh in endocytic vesicles and coimmunoprecipitated with it. Observation of its trafficking, together with analyses of the effects of its functional disruption, implicated Dispatched in Hh vesicular trafficking to the basolateral cell surface and indicated that it was crucial for long-range Hh signaling. Two proteins involved in Hh reception in A cells, Dally-like and Interference Hh, were also implicated in its basolateral release. Thus, the authors conclude that Hh secreted at the apical membrane is internalized and recycled to the basolateral membrane for secretion and establishment of the long-range morphogenetic gradient. Couso provides thoughtful commentary.
A. Callejo, A. Bilioni, E. Mollica, N. Gorfinkiel, G. Andrés, C. Ibáñez, C. Torroja, L. Doglio, J. Sierra, I. Guerrero, Dispatched mediates Hedgehog basolateral release to form the long-range morphogenetic gradient in the Drosophila wing disk epithelium. Proc. Natl. Acad. Sci. U.S.A. 108, 12591–12598 (2011). [Abstract] [Full Text]
J. P. Couso, Hedgehog, Patched and Dispatched near the basement. Proc. Natl. Acad. Sci. U.S.A. 108, 12565–12566 (2011). [Full Text]