Atypical protein kinase C regulates Drosophila neuroblast polarity and cell-fate specification
Atwood, Scott X.
Cellular polarity is a biological mechanism that is conserved across metazoa and is used in many different biological processes, one of which is stem cell self-renewal and differentiation. Stem cells generate cellular diversity during development by polarizing molecular determinants responsible for directing one daughter cell to maintain stem cell-like qualities and the other daughter cell to initiate a specific cell fate. The stem cell self-renewal versus differentiation choice is critical to avoid overproliferation of stem cells and tumor formation or underdevelopment of tissues and early animal death. Drosophila neural stem cells (neuroblasts) undergo asymmetric cell division (ACD) to populate the fly central nervous system and provide an excellent model system to study processes involving cellular polarity, ACD, stem cell self-renewal, and differentiation. Neuroblasts divide unequally to produce a large, apical self-renewing neuroblast and a small, basal ganglion mother cell that goes on to divide and form two neurons or glia. In this way, a small population of neuroblasts can give rise to thousands of neurons and glia to generate a functional central nervous system. Atypical Protein Kinase C (aPKC) is critical to establish and maintain neuroblast polarity, ACD, stem cell self-renewal, and differentiation. aPKC is part of the evolutionarily conserved Par complex, whose other members include Bazooka and Par-6, and they localize to the neuroblast apical cortex and function to restrict cell-fate determinants into one daughter cell. How aPKC is asymmetrically localized and how its activity translates into cell-fate specification are of incredible importance as apkc mutants where localization is disrupted no longer segregate cell-fate determinants. This work will show that Cdc42 recruits the Par-6/aPKC complex to the neuroblast apical cortex independent of Bazooka. Once there, aPKC phosphorylates the cell-fate determinant Miranda to exclude it from the apical cortex and restrict it basally. Par-6 and Cdc42 regulate aPKC kinase activity though inter- and intramolecular interactions that allow high aPKC kinase activity at the apical cortex and suppressed activity elsewhere. Cdc42 also functions to keep aPKC asymmetrically localized by recruiting the PAK kinase Mushroom bodies tiny to regulate cortical actin and provide binding sites for cortical polarity determinants. This dissertation includes previously published co-authored material.