Modeling Microtubule Time to Catastrophe

Megan Wang, Elaine Lin, Mohamed Soufi

Abstract

Microtubules are fundamental for many processes in eukaryotic cells. For example, they form the cytoskeleton which provides structure and shape to the cells, allow intracellular transport, facilitate cell division by pulling chromosomes apart, form cilia and flagella, and much more. They are assembled through a process known as nucleation by microtubule-organizing centers (MTOC), where tubulin monomers are added on successively which lengths the microtubule. The microtubule dynamically switches between growing and shrinking. If the tubulin monomer is bound to GTP, the tubulin is considered stable and will stay on the microtubule, promoting further growth. If the tubulin is bound to GDP, the tubulin monomer is unstable, promoting rapid depolymerization of the microtubule. This switch between the growing and shrinking state is known as a catastrophe event. The kinetics of microtubule catastrophe are still largely unknown as there are likely to be multiple factors that modulate the mechanism and duration of this event. We contribute to elucidating these unknowns by attempting to understand how microtubule catastrophe time may be modeled. The data was obtained from Jonathon Howard’s Lab at the Max Planck Institute of Molecular Cell Biology and Genetics. The microtubules were exposed to fluorescently labeled tubulin as well as various concentrations of tubulin in vitro, and we analyzed the time it takes for catastrophe to occur after growth begins. We propose a model that we think may be appropriate to model microtubule catastrophe and compare it to a Gamma Distribution which other groups have proposed as the model. We ultimately observe that our proposed model is not more appropriate for microtubule catastrophe, disproving at least one theory of the mechanism of microtubule catastrophe.

Experimental Methods

The Howard Lab studied the dynamics of microtubule catastrophe by imaging microtubules with TIRF microscopy and then recorded the times it took for the catastrophe event to be triggered after growth began. TIRF microscopy is effective as it allows for selectivity of which molecules will fluoresce, and the Howard Lab leveraged this by studying microtubules exposed to Alexa 488 labeled tubulin. Decreasing fluorescence intensity will indicate catastrophe has occurred, and the time it took for this event to occur was measured. To ensure this method of imaging did not alter microtubule dynamics, they compared time to catastrophe for microtubules with labeled tubulin and unlabeled tubulin (imaged using DIC microscopy). Then, after concluding fluorescent labeling indeed does not affect time to catastrophe, the Howard Lab studied time to catastrophe depending on different concentrations of labeled tubulin. All of the data was made available to us via Justin Bois.

Acknowledgments

Thank you to Griffin Chure, Michelle Cua, Sanjana Kulkarni, Liana Merk, Sophie Miller, Ankita Roychoudhury and especially Justin Bois for making this awesome class possible! Also, thank you Rosita Fu for helping us start the website. All data was obtained from the Howard Lab in this paper .