A GFP Complementation-based Dual-expression System for Assessing Cell-Cell Contact Mediated by Cytonemes in Live <em>Drosophila</em> Wing Imaginal Discs

Embryonic tissue growth and patterning are largely controlled by signals exchanged locally between cell populations within the tissues themselves. Cytonemes are a type of signaling filopodia first identified in Drosophila that connect and mediate exchange between signal-producing and signal-receiving cells. In the developing Drosophila wing imaginal disc, cytonemes are involved in signal exchange between distinct populations of cells within the disc proper (DP) epithelium, which will form the adult wing, as well as between DP cells and cells in adjacent disc-associated tissues. Cytonemes synapse with target cells to form intimate membrane contacts. Here, we present a protocol for quantifying cytoneme-mediated contact between DP cells and cells of the adjacent peripodial membrane (PerM) epithelium, which is separated from the DP cells by the disc lumen, using a GFP reconstitution approach in live wing discs. Using the GAL4-UAS and LexA-LexAop systems, complementary fragments of split-GFP (spGFP1-10, spGFP11), each fused to the transmembrane domain of CD4, are expressed on either side of the disc lumen. Imaging of reconstituted GFP fluorescence in live wing disc preparations by confocal microscopy is then used to generate image stacks from which reconstituted GFP fluorescence can be localized and quantified. Using this system it is possible to co-express protein-coding or RNA interference transgenes in either cytoneme-producing or target cells to gauge their effect on DP-PerM cell contacts. This system, easily adaptable to other tissues, thus enables the identification of factors important for cytoneme formation or function.