Quantitative Image Analysis and Modeling of Actin-Dependent Chromosome Transport in Starfish Oocytes

Abstract

A dynamic filamentous actin (F-actin) meshwork in the nuclear region of starfish oocytes is essential for chromosome transport to the spindle pole during meiosis. To understand the molecular mechanism by which this meshwork transports chromosomes, we have developed computational image processing methods to analyze high spatiotemporal resolution timelapse movies of the dynamics of chromosomes, inert beads, and F-actin filaments in living cells during the congression process. A novel method for reliable 3D tracking of dynamic and amorphously-shaped chromosomes extracts sub-pixel resolution chromosome (and bead) trajectories with high temporal sampling, permitting analysis of their motion at a range of timescales. On long timescales chromosomes and beads exhibit directed motion with velocities that increase linearly with their starting distance from the spindle pole, consistent with a model of congression in which chromosomes are embedded in a homogeneously contracting meshwork of contractile units anchored at the spindle pole. We use Bayesian hypothesis testing to evaluate competing models of diffusive motion superimposed on advective transport and find that on short timescales chromosomes and beads exhibit confined/anomalous diffusion, suggesting that they are transported by passive diffusion inside confined spaces within the actively flowing actin meshwork. Finally, we use spatiotemporal image-correlation spectroscopy (STICS) to probe the spatiotemporal dynamics of the actin meshwork flow field surrounding the choromosomes, revealing changes in the actin meshwork velocity profile over time during the transport process. These quantitative analyses support a novel chromosome transport mechanism in which transport is accomplished by confinement of diffusing chromosomes within a dynamic actin meshwork that is organized into contractile units.