Establishing and maintaining cortical asymmetry in Drosophila neural stem cells

• Main Author: Hannaford, Matthew
• Format: Dissertation
• Language: English
• Published: University of Dundee 2017
• Subjects: Asymmetric cell division; Drosophila; Polarity

The asymmetric segregation of fate determinants is a conserved process by which differential cell fate can be acquired upon cell division. In this thesis we investigate how the asymmetric localisation of fate determinants is achieved in Drosophila neuroblasts (NBs, Neural Stem Cells). In particular we focus on the localisation of the fate determinant Miranda, which is segregated to the basal pole of the NB cell cortex in mitosis and carries a series of signalling molecules into one of the two daughter cells, promoting differentiation. The most widely accepted model for how Miranda becomes polarised at mitosis is based on its phosphorylation by the apically localised kinase, aPKC (atypical protein kinase C). This model proposes that aPKC localises to the apical cortex and phosphorylates Miranda, excluding it from the apical domain by phosphorylation of Miranda’s membrane binding motif. However, earlier work demonstrated that the acto-myosin cell cortex is essential for asymmetric Miranda localisation. Thus far these two models have not been successfully integrated. In this thesis we generated flies carrying fluorescent reporters for apical and basal polarity proteins and imaged their localisation live. We reveal that localisation appears to happen in two stages. Firstly, Miranda is localised uniformly to the plasma membrane, from where it is cleared by aPKC at the onset of prophase in an actin independent manner. After NEB, Miranda returns to the cell cortex, localising to a basal crescent in an acto-myosin dependent manner. Furthermore, the size of the basal domain to which Miranda localises appears to be under the control of Rho kinase, and linked to cell size asymmetry. Together these data suggest that in mitosis, Miranda localisation is under structural control. Therefore, we reveal that aPKC and Actin-myosin activity contribute to Miranda localisation at distinct time points in the cell cycle.