The role of hydrodynamics in branching microtubule nucleation and the role of branching microtubule nucleation in acentrosomal spindle assembly

Abstract

Branching microtubule nucleation is key for properly assembling the spindle dur- ing eukaryotic cell division, yet much remains to be learned about how and where branched microtubules are nucleated in the cell to find and move chromosomes. To improve our understanding of how and where branching microtubule nucleation oc- curs and how the spindle moves chromosomes during cell division, I engineered re- constitutions to precisely isolate specific parts of cell division, reconstitutions that were both in vitro and ex vivo. Then, I tested various hypotheses concerning the physical and molecular mechanisms at play. In Chapter 2, we discovered that TPX2 undergoes the hydrodynamic Rayleigh-Plateau instability to form droplets on mi- crotubules, droplets from which branched microtubules nucleate and which make branching nucleation more efficient. In Chapter 3, we discovered that chromosomes alone can generate spindles, and that branching microtubule nucleation is the chief source of microtubules generated at chromosomes. Ongoing work seeks to fit the ex- perimentally measured distribution of microtubules over time to theory, specifically a model that predicts the architecture and dynamics of branched microtubule networks that assemble around chromosomes. In Chapter 4, we discovered how to reconstitute metaphase chromosome movement ex vivo in centrosomal microtubule asters and observed chromosomes moving poleward toward centrosomes, at a range of speeds similar to what has been observed in vivo. Ongoing work seeks to estimate the force applied on chromosomes by measuring the hydrodynamic radius of the chromosomes and then calculating the viscous drag opposing their motion.

I also did work related to bacterial cell biology. In Chapter 5, using biochem- istry, fluorescence microscopy, and mean-squared-displacement analysis, we discov- ered that the bacterial cytoskeleton spatially confines phase separated microdomains, also known as lipid rafts, within the bacterial membrane.