Mapping load-bearing in the mammalian spindle reveals local kinetochore-fiber anchorage that provides mechanical isolation and redundancy

Summary Active forces generated at kinetochores move chromosomes, and the dynamic spindle must robustly anchor kinetochore-fibers (k-fibers) to bear this load. We know that the mammalian spindle body can bear the load of chromosome movement far from poles, but do not know where and how – physically and molecularly – this load is distributed across the spindle. In part, this is because perturbing and reading out spindle mechanics in live cells is difficult. Yet, answering this question is key to understanding how the spindle generates and responds to force, and performs its diverse mechanical functions. Here, we map load-bearing across the mammalian spindle in space-time, and dissect local anchorage mechanics and mechanism. To do so, we laser ablate single k-fibers at different spindle locations, and in different molecular backgrounds, and quantify at high time resolution the immediate relaxation of chromosomes, k-fibers, and microtubule speckles. We find that load redistribution is locally confined in all directions: along the first 3-4 μm from kinetochores, scaling with k-fiber length, and laterally within ~2 μm of k-fiber sides, without neighboring k-fibers sharing load. A phenomenological model constrains the mechanistic underpinnings of these data: it suggests that dense, transient crosslinks to the spindle along k-fibers bear the load of chromosome movement, but that these connections do not limit the timescale of spindle reorganization. The microtubule crosslinker NuMA is needed for the local load-bearing observed, while Eg5 and PRC1 are not, suggesting specialization in mechanical function and a novel function for NuMA throughout the spindle body. Together, the data and model suggest that widespread NuMA-mediated crosslinks locally bear load, providing mechanical isolation and redundancy while allowing spindle fluidity. These features are well-suited to support robust chromosome segregation.