A non-canonical role for Yorkie and the Salvador/Warts/Hippo pathway in tracheal tube-size regulation. Renée M. Robbins, Samantha C. Gbur, Greg J. Beitel. Molecular Biosciences, Northwestern Univ, Evanston, IL.

   The size of epithelial tubes is critical for organ function, yet the mechanisms of size control are poorly understood. In the Drosophila trachea, our group and others have demonstrated that cell junctions, apical extracellular matrix (aECM) and cell polarity proteins regulate tube size. Here, we show that the Salvador/Warts/Hippo (SWH) pathway also regulates tracheal tube size, but oppositely of what was expected from the previously characterized role of the SWH pathway. The SWH pathway regulates cell growth and proliferation by negatively regulating the transcription factor Yorkie (Yki), the fly homolog of YAP. Yki activity upregulates expression of genes that promote cell cycle progression and cell growth, so yki mutations typically reduce growth and were expected to decrease tracheal length. Surprisingly, yki mutant embryos have dramatically over-elongated trachea despite normal cell junctions and aECM. Similarly, mutations in hippo, salvador and warts cause shorter rather than longer trachea. Tracheal cell number is not affected in yki mutants, and quantification shows that cell volume and length changes are not correlated in yki mutants. Thus, Yki does not appear to control tracheal tube-size via cell growth or cell division. Consistent with these results, during WT development, tracheal cell volume can decrease by 28% despite a 8% increase in length and a 294% increase in lumenal diameter between stages 14 and 16. Thus, embryonic tracheal tube size increases are not driven by cell size increases. Mutations in the Yki target bantam (a miRNA) did not change tracheal size, but mutations in th/DIAP (Drosophila inhibitor of apoptosis protein) strongly increased tracheal length. Since yki mutations do not alter tracheal cell number or growth, we conclude that Yki controls tracheal tube size through a novel, non-apoptotic function of DIAP that regulates the amount of apical membrane surface. We are currently using a DIAP-lacZ reporter to quantify Yki activity in tracheal cells and determine whether other tracheal pathways act through Yki to control tracheal tube-size.