Understanding the Mechanisms that Give Rise to the Mammalian Spindle's Response to Force using Theory and Microneedle Manipulation

Abstract

The spindle is the force-generating structure that drives chromosome segregation at cell division. In mammalian spindles, bundles of spindle microtubules called kinetochore-fibers (k-fibers) pull on chromosomes to move them. While we know nearly all components necessary for spindle function, how k-fibers respond to force and maintain themselves under force remains poorly understood. Our recent ability to exert local force on the mammalian spindle with microneedles provides key information to answering this question. Here, we use a modeling approach based on Euler-Bernoulli beam theory to identify the minimal mechanical features of the spindle necessary to recapitulate how k-fibers deform under external load. First, we find that force and moment generation at spindle poles are needed to recapitulate observed k-fiber shapes, both with and without external load. Then, we find that crosslinking near kinetochores, which has been experimentally observed, is necessary and sufficient to recapitulate observed k-fiber shapes, assuming no moment generation at kinetochores. By probing the limits of our model under large k-fiber deformations, we infer conditions under high external load beyond which the mechanical integrity of the k-fiber appears compromised, suggesting that structural changes occur under such forces. Finally, we assess the possibility of using our modeling formalism to learn about the applied loads based purely on k-fiber shape analysis. The modeling framework we developed not only helps us understand the mechanisms underlying the spindle's response to force, but will serve as a quantitative framework for probing how the architecture and dynamics of the k-fiber and its surrounding network give rise to mechanics and function.

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