Finalist EPFL doctorate Award 2017 – Alessandro De Simone

The two centrosomes present at the onset of mitosis must be separated properly along the nuclear envelope to ensure proper bipolar spindle assembly. The microtubule-associated minus-end directed motor dynein plays a pivotal role in centrosome separation, but the underlying mechanisms remained elusive, particularly regarding how dynein coordinates this process in space and time. We addressed these questions by combining time-lapse microscopy, image processing and computational modeling to dissect centrosome separation in the one-cell C. elegans embryo. By comparing wild-type and mutant/RNAi conditions, we revealed that centrosome separation is powered by the combined action of dynein at the nuclear envelope and the cell cortex. Strikingly, cortical dynein acts by harnessing polarized actomyosin cortical flows initiated by the centrosomes earlier in the cell cycle. These flows always move away from centrosomes, thus leading to their separation. Furthermore, computational simulations demonstrate that nuclear dynein exerts forces that are organized intrinsically to favor centrosome separation, owing to centrosomes being positioned between the nuclear envelope and the cortex, which results in asymmetric microtubule asters. In conclusion, we uncovered a novel organizing principle in which dynein, coupled with cell geometry and flow pattern, robustly separates centrosomes and thus ensure genome stability.