Understanding the positioning and orientation mechanisms of mammalian mitotic spindles

Abstract

During mitosis, a parent cell splits into two daughter cells, each containing an identical copy of the original genetic material packaged as chromosomes. Chromosome segregation is mediated by the mitotic spindle, a self-assembled macromolecular structure that divides chromosomes and whose midline ultimately sets the division plane of the cell. Thus, mitotic fidelity relies on precise spatiotemporal regulation of forces on the spindle. Although much work has uncovered which proteins may be involved in spindle positioning and orientation, the physical aspect of cell division remains largely unstudied. Here, we use a magnetic tweezer system to perform the first direct force measurements of the positioning and orientation mechanism in mammalian cells. We found that the spindle’s rotation response is not regulated by cytoplasmic viscosity, but rather by molecular interactions between cortical molecules and microtubules that exert pulling forces on the spindle. In addition, using molecular perturbations combined with force measurements, we found that the interactions between cortical proteins is more complex than previously thought. Further, we apply our method of combining physical and molecular understanding to anaphase chromosome segregation and ER mechanics.