Cooperation of Mad and Akt signaling in a Drosophila model of epithelial plasticity. Courtney Onodera^4,8, Björn Gärtner^5,8, Samantha Aguinaldo-Wetterholm^2,9, David Casso^2,9, J. Alex Rondon^2,6,9, Samuel Meier⁵, Aiguo Tian^2,7, Brandy Alexander², Rik Derynck^1,2,3, Jun S. Song^1,4, Julia Zeitlinger⁵, Katja Brückner^1,2,3. 1) Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research; 2) Department of Cell and Tissue Biology; 3) Department of Anatomy; 4) Institute for Human Genetics, University of California San Francisco, San Francisco, CA; 5) Stowers Institute for Medical Research, Kansas City, MO; 6) present address: Genentech; 7) present address: UT Southwestern; 8) equal contribution; 9) equal contribution.

   Transforming Growth Factor- (TGF-) and Bone Morphogenetic Protein (BMP) cooperate with Akt signaling in many systems of epithelial plasticity during development and in fibrosis and tumor metastasis. However, the molecular basis of this cooperation remains incompletely understood. Drosophila has been an excellent model to study epithelial architecture and epithelial plasticity in vivo, yet no cell-based system has been available to take advantage of Drosophila in the molecular dissection of epithelial plasticity. We now introduce KaBrü1D, a Drosophila epithelial cell line closely related to wing imaginal disc cells, that undergoes BMP/decapentaplegic (dpp) induced epithelial plasticity, similar to the elongation of wing imaginal cells during thorax closure of the developing fly. Based on an RNAi screen comprising all Drosophila kinases and phosphatases, expression profiling, and ChIP analyses, we identified Mad targets and genes functionally involved in Dpp/BMP-induced epithelial plasticity. Akt/Tor signaling is essential in this process, and activity of this pathway is enhanced over the course of several days of BMP stimulation, consistent with a secondary transcriptional wave leading to elevated receptor tyrosine kinase signaling. Focusing on the mechanism of cooperation between the BMP and Akt pathways, we identified differential binding of Mad to transcriptional targets, and we dissect this regulation in cell culture and during thorax closure in vivo.