Asymmetric cell division generates cell diversity during development and regulates stem-cell self-renewal in Drosophila and mammals. In Drosophila, neuroblasts align their spindle with a cortical Partner of Inscuteable (Pins)-G alpha i crescent to divide asymmetrically, but the link between cortical polarity and the mitotic spindle is poorly understood. Here, we show that Pins directly binds, and coimmunoprecipitates with, the NuMA-related Mushroom body defect (Mud) protein. Pins recruits Mud to the neuroblast apical cortex, and Mud is also strongly localized to centrosome/spindle poles, in a similar way to NuMA. In mud mutants, cortical polarity is normal, but the metaphase spindle frequently fails to align with the cortical polarity axis. When spindle orientation is orthogonal to cell polarity, symmetric division occurs. We propose that Mud is a functional orthologue of mammalian NuMA and Caenorhabditis elegans Lin-5, and that Mud coordinates spindle orientation with cortical polarity to promote asymmetric cell division.

Regulation of stem cell self-renewal versus differentiation is critical for embryonic development and adult tissue homeostasis. Drosophila larval neuroblasts divide asymmetrically to self-renew, and are a model system for studying stem cell self-renewal. Here we identify three mutations showing increased brain neuroblast numbers that map to the aurora-A gene, which encodes a conserved kinase implicated in human cancer. Clonal analysis and time-lapse imaging in aurora-A mutants show single neuroblasts generate multiple neuroblasts (ectopic self-renewal). This phenotype is due to two independent neuroblast defects: abnormal atypical protein kinase C (aPKC)/Numb cortical polarity and failure to align the mitotic spindle with the cortical polarity axis. numb mutant clones have ectopic neuroblasts, and Numb overexpression partially suppresses aurora-A neuroblast overgrowth (but not spindle misalignment). Conversely, mutations that disrupt spindle alignment but not cortical polarity have increased neuroblasts. We conclude that Aurora-A and Numb are novel inhibitors of neuroblast self-renewal and that spindle orientation regulates neuroblast self-renewal.[Keywords: Aurora-A; Drosophila; neuroblast; numb; self-renewal; spindle orientation] Supplemental material is available at http://www.genesdev.org.

The mitotic spindle determines the cleavage furrow site during metazoan cell division1,2, but whether other mechanisms exist remains unknown. Here we identify a spindle-independent mechanism for cleavage furrow positioning in Drosophila neuroblasts. We show that early and late furrow proteins (Pavarotti, Anillin, and Myosin) are localized to the neuroblast basal cortex at anaphase onset by a Pins cortical polarity pathway, and can induce a basally-displaced furrow even in the complete absence of a mitotic spindle. Rotation or displacement of the spindle results in two furrows: an early polarity-induced basal furrow and a later spindle-induced furrow. This spindle-independent cleavage furrow mechanism may be relevant to other highly polarized mitotic cells, such as mammalian neural progenitors.