Myosin 2-Induced Mitotic Rounding Enables Columnar Epithelial Cells to Interpret Cortical Spindle Positioning Cues

During epithelial cell proliferation, planar alignment of the mitotic spindle allows the daughter cells to stay within the epithelium. Previous work has identified cortical cues that regulate spindle orientation and the division axis [1, 2]. One such cue is cortical Pins (LGN in vertebrates) [3-6], which recruits the conserved Mud/NuMA protein and the dynein/dynactin complex to the cortex. The dynein/dynactin motor complex pulls astral microtubules to orient the spindle. Cortical Pins can therefore dictate the division axis. In addition to cortical cues, cell shape can also serve as a division orientation cue [7-9]. Here, we investigated the interplay between cortical cues and cell shape in a proliferating tissue. We analyzed division orientation in the first mitotic divisions of the early Drosophila embryo, where groups of epithelial cells synchronously divide. Using chemical inhibitors, knockdowns, and mutants with known deficits in motor activity, we showed that the myosin 2 motor is required to orient cell division in the plane of a columnar epithelium. Disrupting myosin activity caused the division axis to orient perpendicular to the epithelial plane. This effect was independent of Pins cortical localization, which became uncoupled from spindle orientation. Instead, myosin motor activity was required for the formation of the actomyosin cortex and for cell rounding upon mitotic entry. We propose that mitotic cell rounding in columnar epithelia allows cells to properly interpret cortical cues that orient the spindle. In the absence of mitotic rounding, geometric cues imposed by tight cell packing prevail and cells divide along their long apical-basal axis.