Complexities of Microtubule Population Dynamics within the Mitotic Spindle
AbstractThe mitotic spindle functions to move chromosomes to alignment at metaphase, then segregate sister chromatids during anaphase. Analysis of spindle microtubule kinetics utilizing fluorescence dissipation after photoactivation described two main populations, a slow and a fast turnover population, historically taken to reflect kinetochore versus non-kinetochore microtubules respectively. This two component demarcation seems likely oversimplified. Microtubule turnover may vary among different spindle microtubules, regulated by spatial distribution and interactions with other microtubules and with organelles such as kinetochores, chromosome arms, and the cell cortex. How turnover among various spindle microtubules is differentially regulated and its significance remains unclear. We tested the concept of kinetochore versus non-kinetochore microtubules by disrupting kinetochores through depletion of the Ndc80 complex. In the absence of functional kinetochores, microtubule dynamics still exhibited slow and fast turnover populations, though proportions and timings of turnover were altered. Importantly, the data obtained following Hec1/Ndc80 depletion suggests other sub-populations, in addition to kinetochore microtubules, contribute to the slow turnover population. Further manipulation of spindle microtubules revealed a complex landscape. Dissection of the dynamics of microtubule populations will provide a greater understanding of mitotic spindle kinetics and insight into roles in facilitating chromosome attachment, movement, and segregation during mitosis.