Class III PI3K is essential for Wingless secretion and Evi/Wls recycling in Drosophila

Wnt/Wingless (Wg) signalling is a key regulator of tissue patterning and morphogenesis in Drosophila, coordinating cell fate decisions and long-range morphogen signalling. In wing imaginal discs, Wg proteins rely on specialized trafficking machinery, including the cargo receptor Evi/Wls, and are secreted via multiple routes, comprising a glypican-dependent Wg pool and an endocytosis-dependent Wg pool. However, the cellular mechanisms controlling Wg secretion and post-endocytic trafficking in Drosophila remain incompletely understood. We performed an in vivo kinome- and phosphatome-wide CRISPR-Cas9 screen in Drosophila wing imaginal discs using endogenous fluorescently tagged Wg as a readout. Genetic perturbations were combined with super-resolution microscopy and ex vivo pharmacological treatments to resolve Wg and Evi/Wls secretion dynamics. We identified Vps15, a core subunit of the class III phosphatidylinositol 3-kinase (PI3K (III)) complex, as a critical factor of Wg secretion. Loss of Vps15 caused apical accumulation of Wg in Wg-secreting cells, selectively impairing an endocytosis-dependent Wg pool while leaving a glypican-mediated Wg pool intact. In contrast to Wg, Evi/Wls did not accumulate in PI3K (III) mutant cells, but instead was subject to proteasome-dependent degradation. Super-resolution imaging further revealed frequent spatial separation of Wg and Evi/Wls in Wg-secreting cells prior to the uptake of the endocytosis-dependent Wg pool. Our study establishes PI3K (III) perturbation as a powerful approach to dissect distinct Wg secretion routes in Drosophila wing imaginal discs. We uncovered divergent post-endocytic fates of Wg and Evi/Wls upon perturbation and provided new mechanistic insight into Wg-Evi/Wls dynamics.