Mechanisms of Stability of the Metaphase Spindle and Associated Proteins Regulating Microtubule Flux

Abstract How the metaphase spindle in higher eukaryotes maintains stability in the presence of poleward microtubule (MT) flux is a confusing issue. Here, we present a model for the spindle by incorporating augmin-mediated MT connections and MT crosslinking by NuMA proteins. On the basis of the model, we study computationally the dynamics of the spindle for the wild-type case and for the case with depletion, inhibition, or overexpression of various associated proteins such as kinesin-8 KIF18A, CLASP, NuMA, kinesin-4 KIF4A, augmin complex, kinesin-13 MCAK, kinesin-13 KIF2A, and kinesin-5, as well as with addition of the MT-targeting agent BAL27862. The numerical results are consistent with the available experimental data. We explain the mechanism of the stability of the spindle, namely, the mechanism of how each MT maintains a constant length, how antiparallel MT overlaps maintain their constant lengths, how each MT maintains on average a fixed position relative to another one despite having different flux rates, and how the spindle length is kept constant. The mechanisms of different associated proteins regulating differentially the MT flux rate, antiparallel MT overlap length, spindle length, and interkinetochore distance are explained. Moreover, our model explains well various other puzzling experimental results such as those showing that the normal MT flux was still present despite MT minus ends being detached from the spindle poles and the MT depolymerization at spindle poles being inhibited.