PTPA is required for MIRA complex localization during drosophila neuroblasts asymmetric divisions

Abstract

How a cell divides is one of the most important issues in developmental and cell biology. Generally, there are two kinds of cell division. One is symmetric division, where one cell divides into two identical daughter cells; the other is asymmetric division, where the two daughter cells are different in size, protein contents and developmental fate. Asymmetric cell division not only provides a fundamental process to generate cell diversity in multicellular organisms, but also maintains a balance between stem cells and differentiated cells. Drosophila neural stem cells, neuroblasts, have been used as an excellent model system in studying asymmetric cell division for more than two decades. Drosophila neuroblast divides asymmetrically to generate a larger daughter cell, which remains as a stem cell, and a smaller differentiating daughter cell known as Ganglia mother cell (GMC), which will divide terminally to generate two neurons or glial cells. The different cell fates of two daughter cells are achieved by asymmetric inheritance of cell fate determinants. In order to do so, cell fate determinants are asymmetrically localized to one side of cell cortex before cytokinesis; and the mitotic spindle must also be properly orientated so that cell fate determinants are segregated preferentially in the differentiating GMC. In this thesis, I describe Phosphotyrosyl phosphatase activator (PTPA) as a novel player in the asymmetric cell division of Drosophila neuroblasts. PTPA mediates localization of Miranda (Mira) and its associated cell fate determinants during neuroblast asymmetric division in both interphase and metaphase. Mira is an obligatory adaptor protein of two cell fate determinants, Prospero (Pros) and Brain tumor (Brat), for their asymmetric localization. In ptpa mutant neuroblasts, Mira and Pros, which normally form crescents on basal cortex, are mislocalized into cytoplasm in metaphase. In interphase, ptpa mutant neuroblasts show ectopic nuclear Mira and Pros. In addition, ptpa mutant larval brain exhibits premature neuroblast loss. Previous studies have shown that atypical Protein Kinase C (aPKC) phosphorylation on Mira is necessary and sufficient to displace Mira from cell cortex. However, attenuating aPKC activity in ptpa mutant neuroblasts cannot restore Mira cortical association in metaphase. Therefore, PTPA acts downstream from or in parallel to aPKC pathway in asymmetric localization of Mira in metaphase. Genetics and biochemistry results suggest that loss of specific activity of Protein phosphatase 2A (PP2A) in ptpa mutant is, at least partially, the cause of Mira mislocalization. Together, PTPA mediates Mira localization downstream from or in parallel to aPKC phosphorylation by activating specific activity of PP2A during the asymmetric division of Drosophila neuroblasts.