Mechanics of multi-centrosomal clustering in bipolar mitotic spindles

Abstract

To segregate chromosomes in mitosis, cells assemble mitotic spindle, a molecular machine with centrosomes at two opposing cell poles and chromosomes at the equator. Microtubules and molecular motors connect the poles to kinetochores, specialized protein assemblies on the centromere regions of the chromosomes. Bipolarity of the spindle is crucial for the proper cell division, and two centrosomes in animal cells naturally become two spindle poles. Cancer cells are often multi-centrosomal, yet they are able to assemble bipolar spindles by clustering centrosomes into two spindle poles. Mechanisms of this clustering are debated. In this study, we computationally screen effective forces between a) centrosomes, b) centrosomes and kineto-chores, c) centrosomes and chromosome arms, d) centrosomes and cell cortex, to understand mechanics that determines three-dimensional spindle architecture. To do this, we use stochastic Monte Carlo search for stable mechanical equilibria in effective energy landscape of the spindle. We find that the following conditions have to be met to robustly assemble the bipolar spindle in a multi-centrosomal cell: 1) strengths of centrosomes attraction to each other and to the cell cortex have to be proportional to each other; 2) strengths of centrosomes attraction to kinetochores and repulsion from the chromosome arms have to be proportional to each other. We also find that three other spindle configurations emerge if these conditions are not met: a) collapsed, b) monopolar, c) multipolar spindles, and the computational screen reveal mechanical conditions for these abnormal spindles. Significance statementTo segregate chromosomes, cells assemble bipolar mitotic spindle. Multiple mechanical forces generated by microtubules and molecular motors in the spindle govern the spindle architecture, but it is unclear what force balances support the bipolarity of the spindle. This problem is especially difficult and important in cancer cells, which often have multiple centrosomes that somehow are able to cluster into two spindle poles. By using stochastic energy minimization in an effective energy landscape of the spindle and computationally screening forces, we find mechanical conditions for mono-, multi- and bi-polar spindles. We predict how microtubule and motor parameters have to be regulated in mitosis in multi-centrosomal cells.