Moesin orchestrates the reorganisation of the actin cortex and shape changes during mitotic progression in an epithelium

• Main Author: Lacerda Rodrigues, N. T.
• Published: University College London (University of London) 2015
• Subjects: 571.6
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.668441

Animal cells endure dramatic actin-dependent changes in shape as they progress through mitosis – they round up at mitotic entry, elongate at anaphase and split into two at cytokinesis. In this thesis I explore the role of Moesin, an actin-membrane crosslinker and the sole ERM protein expressed in Drosophila, in orchestrating rearrangements of the actin cortex and morphological changes in epithelial cells undergoing mitosis. To perform my studies I used the fly notum and sensory organ precursor (SOP) cells therein as a model system. In this thesis I show that Moesin is required for the stabilisation of the actomyosin cortex at metaphase. This mechanism is dependent upon phosphorylation of Moesin by the Slik kinase, which activates the ERM protein. Reduced levels of Moesin or Slik lead to myosin-II-driven cortical instabilities. Cortical stabilisation in mitotic SOP cells ensures the efficient accumulation of fate determinants at the plasma membrane. At mitotic exit, a pool of active, phosphorylated Moesin is lost from the cell poles, thereby triggering polar relaxation and initiating anaphase cell elongation. These two events precede furrow formation, are independent of centrosome or astral microtubules-derived signals, and are induced by proximity of the segregating chromosomes to the cell poles. I show that a pool of kinetochore-localised PP1-87B phosphatase and its regulatory subunit Sds22 inactivate cortical Moesin and elicit the dismantling of the actomyosin cortex at mid-anaphase. Cells with reduced amounts of PP1-87B or Sds22 fail to clear Moesin and actin from the anaphase poles. Importantly, these defects in polar relaxation are mimicked by the expression of a constitutively active form of Moesin in fly tissues. Finally, I demonstrate that delocalisation of PP1/Sds22 from the kinetochores via KNL1 depletion abolishes polar blebbing at anaphase and impairs cell elongation. My work shows how the dynamic regulation of Moesin activation and localisation controls shape changes in cells undergoing mitosis. Moreover, it sheds light on a novel mechanism of polar relaxation at anaphase, in which a kinetochore-derived signal instructs the cell cortex to become polarised, thereby initiating cytokinesis.