Shaping 3D geometry in tubulogenesis: a PDZ domain-containing protein Arc regulates Crumbs to determine salivary gland morphology in Drosophila embryogenesis
Authors: Ji Hoon Kim; Kwon Kim; Devin Vertrees; Rika Maruyama; Deborah Andrew
Affiliation: Johns Hopkins University School of Medicine, Baltimore, MD
Keywords: q. epithelial sheets; a. cytoskeleton
The proper architecture of an organ is inseparable from its optimal functionality and physiology. During development, three dimensional tubular organs arise from two dimensional primordia through dynamic changes in cell shape and arrangement. By using embryonic salivary gland (SG) development as a model system to study tubulogenesis, we have discovered that the FoxA transcription factor Fork head (Fkh) is essential for tube formation and internalization. Thus, a subset of Fkh target genes should have roles in SG morphogenesis. arc, an early-expressed SG gene whose expression in the SG and other tubular epithelia requires Fkh, encodes a large cytoplasmic protein containing two PDZ domains. We have discovered that Arc contributes to overall SG dimensions; loss of arc results in shorter, stubbier SG tubes with more cells in circumference and Arc overexpression results in highly elongated SG tubes with fewer cells in circumference. Similar SG phenotypes were induced by perturbing function of non-muscle myosin II (MyoII) and its antagonistic transmembrane protein Crumbs (Crb) in SG. Both the hyperactivation of MyoII and the suppression of Crb activity resulted in shorter SGs with more cells in circumference, as seen in arc loss-of-function mutants. Moreover, Crb levels were significantly reduced in arc null mutant SGs and Arc overexpression resulted in the mis-localization of Crb protein. Importantly, Arc co-localized with Crb at the cell-cell junctions of SG placode cells, suggesting that the two proteins could interact directly, likely through one of the Arc PDZ domains and the cytoplasmic PDZ-binding motif found at the C-terminus of Crb. Indeed, we have discovered that the first PDZ domain of Arc is required for its co-localization with Crb. Based on these observations, we propose that Arc functions as a newly discovered player in SG morphogenesis. We hypothesize that Arc modulates Crb dynamics by affecting its membrane stability and/or localization through direct physical interactions. We further propose that the inhibitory actions of Crb on MyoII serve to limit MyoII activity, consequently limiting the number of SG cells that internalize at any given time and ultimately shaping the final dimensions of the mature SG tube.