Can Threshold Choices Influence Observed Microtubule Aging?

Microtubules (MTs), built from tubulin subunits, are biological polymers intimately involved in a number of key cellular processes. One major aspect of MT activity that is still poorly understood is the mechanism of dynamic instability (DI), where in vivo and in vitro MT polymers undergo periods of growth and shrinkage, with transitions between these phases occurring stochastically through events known as rescue and catastrophe. In vitro, it has been observed that the rate of catastrophe increases over the lifetime of the MT. This phenomenon of MT “aging” has been observed by a number of labs, and simulations have both confirmed this effect and allowed for the development of hypotheses to explain aging in molecular detail. While the idea that MTs age is gaining a consensus, there are a few features of MT aging that make it worthy of closer examination. For example, MT aging has been observed to occur at only one end of the MT (the “plus” end) and not the other (“minus” end), which is unusual given that the processes at the two ends may be expected to be similar. The two ends have different dynamic properties, which may lead to masking of minus end aging or cause a bias towards identifying catastrophe events of longer, “aged”, MTs. Additionally, experimental measurements of catastrophe are impeded by unavoidable detection thresholds caused by the diffraction limit of light, which prevents the measurement of shorter and newly formed (i.e. “young”) MTs and could influence the perception of aging. We are using a simplified MT simulation system that does not have microscopy-induced measurement limitations to determine if discrepancies in observed aging can be explained, in part, by measurement thresholds.