Physical Basis of Large Microtubule Aster Growth

TL;DR

This paper develops a biophysical model combining autocatalytic nucleation and polymerization to explain large microtubule aster growth, predicting wave-like expansion and an explosive transition in growth dynamics, confirmed by frog egg experiments.

Contribution

It introduces a new model integrating autocatalytic nucleation with polymerization, explaining aster growth scaling and transitions not captured by previous fixed-microtubule models.

Findings

Asters expand as traveling waves.

Nucleation rate increase causes explosive growth transition.

Experimental validation in frog egg extract.

Abstract

Microtubule asters - radial arrays of microtubules organized by centrosomes - play a fundamental role in the spatial coordination of animal cells. The standard model of aster growth assumes a fixed number of microtubules originating from the centrosomes. However, aster morphology in this model does not scale with cell size, and we recently found evidence for non-centrosomal microtubule nucleation. Here, we combine autocatalytic nucleation and polymerization dynamics to develop a biophysical model of aster growth. Our model predicts that asters expand as traveling waves and recapitulates all major aspects of aster growth. As the nucleation rate increases, the model predicts an explosive transition from stationary to growing asters with a discontinuous jump of the growth velocity to a nonzero value. Experiments in frog egg extract confirm the main theoretical predictions. Our results suggest that asters observed in large frog and amphibian eggs are a meshwork of short, unstable microtubules maintained by autocatalytic nucleation and provide a paradigm for the assembly of robust and evolvable polymer networks.