Modeling a patch of Cortex in the C. elegans embryo

Gile, Munro, Alberts

This ongoing project extends previous work by Alberts and Odell on actin assembly and Listeria propulsion to explore the functional dynamics of cross-linked actomyosin networks. We have three major goals. The first is to explore in general terms how meso-scale contractile and viscoelastic properties of actomyosin networks emerge from the micro-scale dynamical interactions of actin-filaments, cross-linkers, myosin motors, and factors that regulate their assembly and activities. The second is to couple detailed simulations to our ongoing studies of cortical actomyosin dynamics in early C. elegans embryos. A third, and longer-term, goal is to infer from detailed simulations of a small patch of cortex the appropriate constitutive laws for a continuum-style model of the whole cell.

The agents in our simulations are actin filaments (modeled as semi-flexible polymers built from discrete subunits), various cross-linkers, and myosin mini-filaments. Individual agents move/writhe in response to thermal forces, collide and exchange forces with one another. At every timestep, each cytoskeletal part in the simulation decides what to do next by numerically solving its zero-inertia Newtonian force/torque balance equations of motion. A very flexible framework allows us to endow cross-linkers and motors with variable structures, force-dependent binding kinetics, duty cycles, and preferred angles for filament binding, and to assert different empirically-based rules for local filament assembly/disassembly.

1)  (Ryan Gile) A single (thermally forced) actin filament, tuned to match experimentally determined values
for persistence length, a theoretically derived elastic modulus, and relaxation times during deformation
(click here to check out Jon Albert's demo of how this works called SImFil).
Click on the image to see a quick time movie.

2)  (Ryan Gile) A network of cross-linked actin filaments. A single filament is highlighted in white.
The large balls are tracer beads that can be used to calibrate our simulations against microrheology measurements made in vitro.
Click on the image to see a quick time movie.

3)  (Ryan Gile) A network of actin filaments and cross-linkers (modeled on alpha-actinin) self-organize filament bundles.
Click on the image to see a quick time movie.

4)  A network of actin filaments, cross-linkers and myosin mini-filaments self-organize contractile foci
that resemble (superficially at least) foci that form during polarization in C. elegans.
This simpler 2D simulation was made by Devin Strickland, Jessica Polka, Ryan Gile and on Alberts
during a 2-week session of the Woods Hole physiology course in the summer of 2007.
Click on the image to see a quick time movie.

Page written by Ed Munro
Last Updated Nov 30, 2009