Figure 5: 2D in silico actomyosin cortex predicts that external constraints alter cytoskeleton organization and force direction.

(a) F-actin is modelled by polar filaments (red) and motors by springs (green). As motor exerts force on filaments, filaments translocate and/or rotate. (b) Within a perimeter of 0.5 filament length, the plus ends of the filaments are caught by springs with defined stiffness to model boundary resistance. (c, left) To simulate isotropic constraints, boundary springs (stiffness=100 kPa) were equally distributed around the boundary. To simulate anisotropic constraints, softer springs (20 kPa) were placed at the top and bottom of the boundary. The rose diagrams illustrate the average force generated on boundary springs for each condition (n≥;10 simulations), and circle divisions represent 10 nN. (Right) The ratio of force production along stiff (left/right) over soft (top/bottom) axes was calculated and plotted depending on increasing values of stiffness anisotropy between left/right and top/bottom boundary springs; n≥;10 simulations per conditions. Error bars represent s.d. (d) Kymographs representing the contraction of the network into an aster (top) or the stabilization of a ring-like structure (bottom) depending on the resistance of the boundary. (e) Images of representative simulations at two time points showing motor (green) and actin filament (red) organization. The associated rose diagram illustrates forces generated on the boundary by these networks, and circle divisions represent 10 nN. Top, isotropic boundary. Bottom, anisotropic boundary. (f) Motor alignment for simulations with isotropic or anisotropic boundaries (t=250 time-steps). The motors are colour coded based on angle relative to the horizontal axis, with red motors being most aligned with that axis. Anisotropic boundary constraints resulted in motors aligned with the stiff (horizontal) axis along the top and bottom of the oval-like myosin structure (arrows). Quantification of motor alignment is given in the table. Numbers highlighted in grey correspond to the isotropic case and numbers highlighted in purple to the anisotropic case. There is a 20% enrichment bias in motors aligned along the stiff horizontal axis when the resistance of the boundary is anisotropic. (g) Model illustrating the factors that influence motor movement and alignment, and force production in response to asymmetric boundary stiffness.