Title: Mechanisms of polarized trafficking induced by cytoskeleton asymmetries
Funder: Medical Research Council

Abstract: Asymmetric cell division, the process by which one cell divides into two daughter cells that have different fates, gives rise to the plethora of cell types found in an organism. Differential cell fate assignation after mitosis relies on the biased dispatch of fate determinants between the two daughter cells. This biased dispatch uses specific spatial cues, which targets fate determinants to opposite poles of the cortex during division. In recent years, novel mechanisms of asymmetric dispatch of cell fate determinants have been found to operate in the cytoplasm, away from the cell cortex. In particular, we recently discovered that during the asymmetric division of Drosophila Sensory Organ Precursors, the central spindle, the antiparallel array of microtubules characteristic of late mitosis, is asymmetric. This central spindle asymmetry is in turn “read” by molecular motors to carry Sara endosomes, a subpopulation of endosomes containing cell fate determinants, into only one of the two daughter cells. Our overall goal is to establish the molecular mechanisms that generate cytoskeletal asymmetries during asymmetric cell division and to determine how, in turn, these cytoskeletal asymmetries generate polarized trafficking/signaling during development. Specifically, our projects aim at addressing three key questions: i) How universal is the Sara endosome mechanism? Indeed, the cell contains many other types of compartments. Are they all asymmetrically segregated? To address this question, we developed several imaging techniques to follow quantitatively both cytoskeleton dynamics and membrane trafficking in the developing fly. In particular, we developed algorithms to average movies in space and time in order to take into account cell-to-cell variability and stochasticity. ii) How does symmetry breaking of the central spindle occur? Symmetry breaking of the central spindle occurs downstream of cortical polarity signals, but the molecular mechanisms involved remain elusive. In particular, symmetry breaking of a cytosolic structure is not trivial, since the cytosol tends to be homogenous due to diffusion. To understand this process, we have established methods to reshape the landscape of cytoskeleton asymmetries in space and time during cell division using nanobodies fused to polarity cues coupled to optogenetics. iii) How do molecular motors “read” the asymmetry of the spindle? We combine in vitro reconstitution of a model central spindle together with Total Internal Reflexion Microscopy to study the emergent behavior of purified motors, both at the single molecule level or collectively on liposomes. Coupled with physical theory and structural studies of motors, this approach will provide a quantitative molecular understanding of polarized trafficking and signaling. Our ultimate goal is to develop a theory that applies beyond asymmetric division to understand polarized traffic in other antiparallel microtubule arrays, such as vertebrate dendrites. Although we currently focus on microtubule asymmetries, in further years we will include in our approach other cytoskeleton elements, such as actin or intermediate filaments, since they have been shown to cooperate with microtubules for trafficking. Similarly, while we currently focus on Drosophila as a simple model system, our ultimate endeavor is to expand the potential of our work towards mammalian systems, in particular neuronal stem cells.


Research activity codes: 2.1 BIOLOGICAL AND ENDOGENOUS FACTORS
Health categories: GENERIC HEALTH RELEVANCE