Dissecting stem cell self-renewal: The roles of mitotic kinases in Drosophila neuroblast 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 two proteins involved in distinct steps of the cell cycle that regulate neuroblast self-renewal. We first describe 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). We conclude that Aurora-A and Numb are novel inhibitors of neuroblast self-renewal and that spindle orientation regulates neuroblast self-renewal.;We next identified an sgt1 (suppressor-of-G2-allele-of-skp1 ) mutant that had fewer neuroblasts. We found that sgt1 neuroblasts have two polarity phenotypes: failure to establish apical cortical polarity at prophase and lack of cortical Scribble localization throughout the cell cycle. Apical cortical polarity was partially restored at metaphase by a microtubule-induced cortical polarity pathway. Double mutants lacking Sgt1 and Pins (a microtubule-induced polarity pathway component) resulted in neuroblasts without detectable cortical polarity and formation of "neuroblast tumors." Mutants in hsp83 (encoding the predicted Sgt1-binding protein Hsp90), LKB1, or AMPKalpha all show a similar apical cortical phenotype (but no Scribble phenotype), and activated AMPKalpha rescued the sgt1 mutant phenotype. We propose that an Sgt1/Hsp90-LKB1-AMPK pathway acts redundantly with a microtubule-induced polarity pathway to generate neuroblast cortical polarity, and the absence of neuroblast cortical polarity can produce neuroblast tumors.;This dissertation includes published and unpublished co-authored material.