Spindle Micro-Fluctuations of Length Reveal its Dynamics Over Cell Division

Abstract

The mitotic spindle ensures correct segregation of sister chromatids and correct partitioning in daughter cells. It comprises dynamical microtubules (alternating polymerizing and depolymerizing), a variety of molecular motors and their regulators. Although spindle structure is well known, the link to its functions remains elusive, calling for including the dynamics of its components and their interactions. This question was mostly investigated by in silico or in vitro approaches. But a detailed characterizing of spindle mechanics, in physiological conditions, is missing. We propose an image processing based, non invasive, method combined to an heuristic model to measure mechanical parameters along time.We tracked fluorescently labeled spindle pole at high temporal and spatial resolution and measured the micro-fluctuations of spindle length, in vivo. We computed its power density spectrum using short time Fourier transform (sliding window) — a blueprint of spindle mechanics. Such a spectrum is then fitted with a Kelvin Voigt model with inertia (a spring, a damper, an inertial element in parallel) convolved with the window. We validated this method by recovering the mechanical parameters over time from simulated data. Then we measured them, in vivo, in two model organisms, nematode C. elegans one-cell embryo and fission yeast S. pombe.In control C. elegans embryos, metaphase appeared dominated by damping element, consistent with the slow spindle elongation observed. At anaphase onset, all three parameters collapsed, before increasing about 50s later to reach a regime where damping and inertia dominated, suggesting a dramatic re-arrangement of the spindle. Using a gene candidate approach, we could relate these mechanical behaviors to known structures at work, overlapping and kinetochore microtubules during metaphase and centralspindle at anaphase.With such a tool, we will propose a model of the spindle accounting for the dynamics of its component.