Final Report Summary - SPINDLEDESIGN (Design Principles of Microtubule Cytoskeleton Architectures during Cell Division)
The microtubule cytoskeleton is crucial for a multitude of essential intracellular processes in eukaryotic cells. The mitotic/meiotic spindle segregates the genetic material during cell division. During the cell cycle, the cytoskeleton adopts characteristically different architectures. Although most of the key players involved in the organisation of the cytoskeleton and their functions have been identified, we are still far from a mechanistic understanding of the reorganizations of the cytoskeleton. Which minimal sets of activities are required for distinct morphological features of the cytoskeleton is an open question. A major goal of this project was to use biochemical reconstitution approaches combined with fluorescence microscopy imaging to better understand the design principles of different microtubule cytoskeleton architectures as observed in human cells. We succeeded in the reconstitution of an important regulated microtubule nucleation module that controls the locality of microtubule nucleation during cell division (e.g. Roostalu et al., NCB, 2015) and made significant progress in understanding the kinetics of microtubule nucleation by the key microtubule-templating nucleator gammaTuRC. Furthermore, we could show how geometrical confinement influences the self-organisation of purified motors and microtubules in cell-sized droplets (e.g. Juniper et al., Soft Matter, 2018). We were also able to reconstitute the recruitment of the main minus end directed motor dynein to dynamic microtubule plus ends (Duellberg et al., NCB, 2014) and provide insight into the regulation controlling the balance between passive plus end tracking and active minus end motility of this motor (Jha et al., EMBO J., 2017). We made considerable progress in understanding self-organizing microtubule/motor networks mimicking mitotic behaviour. We were able to explain the conditions promoting nematic versus radially polar active microtubule network formation, providing insight into design principles of metaphase spindles (Roostalu et al., Cell, 2018). Moreover, we succeeded in reconstituting the self-organization of minimal anaphase spindle-like microtubule bundles and could explain the mechanism (Hannabuss et al., Curr Biol., 2019). Taken together, this project provided insight into molecular mechanisms underlying microtubule cytoskeleton reorganizations as they happen during the cell cycle, covering the interplay between fundamental activities like microtubule nucleation, control of microtubule dynamics and spatial organization by motors in confined and unconfined space.