Microtubules stabilize intercellular contractile force transmission during tissue folding

During development, forces transmitted between cells are critical for sculpting epithelial tissues. Actomyosin contractility in the middle of the cell apex (medioapical) can change cell shape (e.g., apical constriction), but can also result in force transmission between cells via attachments to adherens junctions. How actomyosin networks maintain attachments to adherens junctions under tension is poorly understood. Here, we discovered that microtubules stabilize actomyosin intercellular attachments in epithelia during Drosophila mesoderm invagination. First, we used live imaging to show a novel arrangement of the microtubule cytoskeleton during apical constriction: medioapical, non-centrosomal Patronin (CAMSAP) foci formed by actomyosin contraction organizes an apical microtubule network. Microtubules were required for mesoderm invagination but were not necessary for apical contractility or adherens junction assembly. Instead, microtubules promoted the stable connection between medioapical actomyosin and adherens junctions. These results define a role for coordination between actin and microtubule cytoskeletal systems in intercellular force transmission and tissue morphogenesis.