The quantification and regulation of microtubule dynamics in the mitotic spindle
Introduction
Microtubules (MTs) are highly dynamic polymers composed of tubulin dimers that follow an assembly/disassembly cycle known as dynamic instability [1]. Dynamic instability can be described as the co-existence of growing and shrinking microtubules in the same solution conditions. A great deal has been learned about the structural and energetic properties of tubulin that drive the spontaneous assembly and disassembly of individual MTs [2]. Furthermore, a large number of accessory proteins have been identified that can alter the rate constants associated with dynamic instability and, accordingly, influence cellular MT turnover and dynamics. However, because MT turnover occurs rapidly and requires specialized, high resolution imaging equipment to measure, the details of this process in living cells are not fully fleshed out.
Section snippets
Microtubule dynamics in the spindle and chromosome instability
Turnover of MT polymer in the mitotic spindle is quite rapid, taking place with a half-life of just a few minutes [3]. The extent to which the behavior of microtubules is either individually stochastic, or coordinated within the spindle, is dependent upon a variety of factors including the proximity of chromatin, the association of MT end-binding complexes, the spatial distribution of regulators and kinetochore capture. For example, individual MTs may be captured or released from kinetochores
New players regulating MT assembly in mitosis
There are many proteins that are able to regulate MT dynamics in the cell, either in interphase or during mitosis. Examples of some well-studied proteins that influence MT polymerization are the plus end binding proteins EB and ch-TOG [11]. Another example would be the microtubule nucleation factor TPX2. This protein is required for mitotic spindle formation and is able to suppress tubulin subunit off-rate during MT assembly and disassembly [12,13]. However, there are also a growing number of
Measurement of microtubule assembly in mitotic spindles
The metaphase mitotic spindle is a remarkable structure in static form (Figure 1c) but the complexity of the behavior of individual MTs in the spindle renders microtubule dynamics within the spindle difficult to parameterize. Even in interphase cells it can be difficult to impossible to distinguish one MT from its neighbor. Live fluorescently labeled MTs are below the level of resolution of most super resolution microscopes making it difficult to characterize individual MT behavior in densely
Alternative read-outs for microtubule assembly changes in mitotic spindles
Subtle increases in MT assembly rates are correlated with increased levels of CIN [7•]. The mechanism by which CIN is increased is presently unknown; however, the observation is interesting because there may be many unexpected pathways that subtly impact MTs in this manner [33]. As it turns out there exists another read-out that has been shown to correlate with increased MT assembly rates and that does not rely on live imaging or rate measurements. For reasons that are imperfectly understood,
Conclusions
Recent studies suggest that MT dynamics is fine-tuned such that even small disruptions to MT assembly rates in mitotic spindles can impact cell division and the development of cancer. For this reason, it is important to develop methods to measure MT assembly rates in a rapid and unbiased manner and to further characterize what these changes in MT assembly rates mean in the cell. Often changes in MT dynamics will subtly change the distribution of MTs within the cell in accordance with a change
Conflict of interest statement
Nothing declared.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
We apologize to those researchers whose work could not be discussed as well as to all the authors whose original work could not be cited due to space limitations. This work was supported by the National Institutes of Health [grant number GM069429].
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