Producing cells retain and recycle wingless in Drosophila embryos

There is considerable interest in the mechanisms that drive and control the spread of morphogens in developing animals. Although much attention is given to events occurring after release from expressing cells, release itself could be an important modulator of range. Indeed, a dedicated protein, Dispatched, is needed to release Hedgehog from the surface of expressing cells. We find that, in Drosophila embryos, much Wingless (as well as a GFP-Wingless fusion protein) remains tightly associated with secreting cells. Retention occurs both within the secretory pathway and at the cell surface and requires functional heparan sulfate proteoglycans. As a further means of retention, secreting cells readily endocytose Wingless protein that does reach the cell surface. Such endocytosed Wingless can in turn be sent back to the cell surface (the first direct observation of ligand recycling in live embryos). Recycling may serve to sustain high-level signaling in this region of the epidermis.