Basal spot junctions of epithelial tissues respond to morphogenetic forces and regulate Hippo signalling

Organ size is controlled by numerous factors including mechanical forces, which are mediated in part by the Hippo pathway. In growing Drosophila melanogaster epithelial tissues, cytoskeletal tension influences Hippo signalling by modulating the subcellular localisation of key pathway proteins in different apical domains, namely adherens junctions, the sub-apical region and the medial apical cortex. Here, using both electron and light microscopy, we have discovered the existence of basal spot junctions in D. melanogaster epithelial tissues, and that they respond to morphogenetic forces and also influence Hippo signalling. Like adherens junctions, the Warts kinase is recruited to basal spot junctions via Ajuba and E-cadherin, which prevent Warts activation by segregating it from upstream Hippo pathway proteins. Basal spot junctions are prominent when tissues undergo morphogenesis and are highly sensitive to fluctuations in cytoskeletal tension. Basal spot junctions are distinct from focal adhesions, but the latter profoundly influences the abundance of spot junctions by modulating the basal-medial actomyosin network and tension experienced by spot junctions. Thus, basal spot junctions potentially couple morphogenetic forces to Hippo pathway activity and organ growth.