Modeling and Analysis of Chromosome Attachment Error Correction and Congression During Mitosis

Abstract

Mitosis is the part of the cell cycle in which replicated chromosomes segregate on the mitotic spindle. Improper segregation results in aneuploidy, abnormal chromosome number, which is a hallmark of both cancer and birth defects. Chromosomes attach through kinetochores, built on centromeric DNA, to microtubules that make up the mitotic spindle. Proper attachment requires that kinetochores “biorient,” or make attachments to microtubules from opposite spindle poles, which generates tension in the kinetochore-microtubule attachment. By contrast, initial and improper attachments lack tension and are eliminated by Aurora B/Ipl1 kinase-mediated, tension-sensitive phosphorylation (human: Aurora B, yeast: Ipl1). Thus, how initial tension-lacking attachments transition to stable tension-generating attachments, the “the initiation problem of biorientation (IPOB),” remains an enigma. A kinetochore- microtubule error correction model of budding yeast mitosis was developed using a well-established model of yeast metaphase microtubule dynamics and integrating kinase-mediated error correction. The model illuminates how improper kinetochore-microtubule attachments are destabilized and replaced by correct kinetochore-microtubule attachments within twenty minutes, solving the IPOB. Following biorientation, chromosomes align or “congress” at the mitotic spindle equator. In the fungi S. cerevisiae and C. albicans, chromosome congression is facilitated by the motor protein kinesin-5, which promotes length-dependent depolymerization of kinetochore microtubules (microtubules within the spindle attached to kinetochores). Spindle length is controlled by the motor protein kinesin-8, which promotes length-dependent depolymerization of interpolar microtubules (microtubules within the spindle not attached to kinetochores). However in human cells, congression has been linked to kinesin-8. Kinesin-5 and kinesin-8 were tested in the fly Drosophila melanogaster S2 cells using RNAi-induced gene silencing. As with fungi, kinetochore microtubules were longer, and kinetochores less congressed in kinesin-5 knockdown spindles compared to control. Knocking down kinesin-8 levels resulted in longer spindles overall. These results are consistent with those from fungi, indicating that kinesin-5, and not kinesin-8, has a major role in promoting chromosome congression. These studies are potentially relevant medically in light of the fact that both Aurora B and kinesin-5 are targets for anticancer drugs. Altogether, these studies use in silico modeling in yeast and experimentation in animal cells to obtain a better understanding of how cells elegantly segregate chromosomes in mitosis in order to maintain proper ploidy.