Establishment and maintenance of cell polarity for asymmetric meiotic cell divisions in mouse oocytes
Professor Rong Li, Stowers Institute for Medical Research, US
Mammalian oocytes undergo two rounds of asymmetric cell divisions during meiotic maturation. In in-vitro cultured oocytes, symmetry breaking occurs concurrent with migration of meiosis I spindle to a subcortical localization, whereby the meiotic chromatin induces the assembly of a polar actomyosin domain critical for the 1st polar body extrusion. Subsequently, meiosis II spindle forms at a nearby subcortical location and maintains oocyte cortical polarity in preparation for the second polar body extrusion upon fertilization. Recent studies have shown that, whereas the chromatin provides the inductive signal for oocyte cortical polarity, dynamic actin assembly at different cellular locations powers chromosome migration in meiosis I and the maintenance of spindle positioning in meiosis II. Recent progress on the mechanisms of actin-based force production and a mechanical model of cellular symmetry breaking will be presented.
Modelling the roles of small GTPases in cell polarization, and cell shape
Professor Leah Edelstein-Keshet, University of British Columbia, US
The small GTPAses Cdc42, Rac, and Rho are signalling proteins that coordinate the assembly, disassembly, and dynamics of the actin cytoskeleton. In this way, they regulate both cell polarization, cell shape, and motility in eukaryotic cells such as neutrophils. In my talk, I will describe spatio-temporal models of small GTPases, showing how their mutual interactions and feedback can affect the ability of the cell to respond to stimuli, polarize robustly, as well as its sensitivity to new stimuli. Both detailed and simplified (abstract) models have contributed to our understanding of these phenomena. Furthermore, in studying such models, we have utilized some recently developed mathematical methods that are of wider applicability in analysis of pattern formation. I will briefly describe these methods and our results.
Positive feedback and mutual antagonism in cell polarity
Dr Barry Thompson, Francis Crick Institute, UK
Epithelial tissues are composed of polarised cells with distinct apical and basolateral membrane domains. In the Drosophila ovarian follicle cell epithelium, apical membranes are specified by several apical determinants: the transmembrane protein Crumbs (Crb), its binding partner Stardust (Sdt), and the aPKC-Par6-cdc42 complex. Basolateral membranes are specified by the determinants Lgl, Dlg and Scrib. Apical and basolateral determinants are known to act in a mutually antagonistic fashion, but it remains unclear how this interaction generates polarity. We have built a computer model of apico-basal polarity which suggests that the combination of positive feedback among apical determinants plus mutual antagonism between apical and basal determinants is essential for polarisation. In agreement with this model, in vivo experiments define a positive feedback loop in which Crb self-recruits via Crb-Crb extracellular domain interactions, recruitment of Sdt and aPKC-Par6-cdc42 to the plasma membrane, aPKC phosphorylation, and recruitment of Expanded and Kibra to prevent endocytic removal of Crb from the plasma membrane. Ectopic activation of components of this loop can generate runaway positive feedback and ectopic spreading of apical determinants. Lgl antagonises the operation of this feedback loop, explaining why apical determinants do not normally spread into the basolateral domain. Once Crb is removed from the plasma membrane, it undergoes recycling via Rab11 endosomes. Our results provide a dynamic model for understanding how epithelial polarity is maintained in Drosophila follicle cells.
Cell shape changes: quantitative role of membrane, cytoskeleton, and how they are attached
Professor Cécile Sykes, Institut Curie, France
In order to unveil generic mechanisms of cell movements and shape changes, we design stripped-down experimental systems that reproduce cellular behaviours in simplified conditions, using liposome membranes on which the cytoskeleton is attached. Such stripped-down systems allow for a controlled study of the physical mechanisms that underlie cell movements and cell shape changes. Moreover, these experimental systems are used to address biological issues within a controlled, simplified environment.
We have reconstituted the actin cortex of cells inside liposomes, and used it as a simplified system to study endocytosis. We will present our work on reconstituted actin cortices at the membrane of liposomes, and a characterization of their mechanical properties measured by tube pulling, and liposome spreading, as done previously in cells. We will show how these cortices contract in the presence of myosin motors, and how such experiments shed light of the mechanisms of cell shape changes.