Abstract
Mechanical forces, actin filament turnover, and adhesion to the extracellular environment regulate lamellipodial protrusions. Computational and mathematical models at the continuum level have been used to investigate the molecular clutch mechanism, calculating the stress profile through the lamellipodium and around focal adhesions. However, the forces and deformations of individual actin filaments have not been considered while interactions between actin networks and actin bundles is not easily accounted with such methods. We develop a filament-level model of a lamellipodial actin network undergoing retrograde flow using 3D Brownian dynamics. Retrograde flow is promoted in simulations by pushing forces from the leading edge (due to actin polymerization), pulling forces (due to molecular motors), and opposed by viscous drag in cytoplasm and focal adhesions. Simulated networks have densities similar to measurements in prior electron micrographs. Connectivity between individual actin segments is maintained by permanent and dynamic crosslinkers. Remodeling of the network occurs via the addition of single actin filaments near the leading edge and via filament bond severing. We investigated how several parameters affect the stress distribution, network deformation and retrograde flow speed. The model captures the decrease in retrograde flow upon increase of focal adhesion strength. The stress profile changes from compression to extension across the leading edge, with regions of filament bending around focal adhesions. The model reproduces the observed reduction in retrograde flow speed upon exposure to cytochalasin D, which halts actin polymerization. Changes in crosslinker concentration and dynamics, as well as in the orientation pattern of newly added filaments demonstrate the model's ability to generate bundles of filaments perpendicular (actin arcs) or parallel (microspikes) to the protruding direction.
Highlights
Cells adhered to 2D surfaces can undergo crawling motion by extending a thin, sheet-like network of polymerizing actin near the leading edge of the cell known as the lamellipodium [1,2,3,4]
The balance between protrusion and retrograde flow is hypothesized to be controlled by a clutch mechanism: retrograde flow is impeded by the presence of focal adhesions, connecting actin filaments in the lamellipodium and the lamella to the external substrate via integrins and focal adhesion-associated proteins such as talin and vinculin [6,7,8]
When the clutch is “engaged” the frictional force on the actin network increases leading to a reduction in retrograde flow speed and an increase in the force on the leading edge by the lamellipodial network, helping to increase cell propulsion speed
Summary
Cells adhered to 2D surfaces can undergo crawling motion by extending a thin, sheet-like network of polymerizing actin near the leading edge of the cell known as the lamellipodium [1,2,3,4]. The actin network within the lamellipodium is a highly branched network of filaments which pushes against the plasma membrane at the leading edge as each filament polymerizes, in order to advance the cell. Crawling cells control motility by regulating the fraction of actin polymerization that is converted into membrane protrusion at the leading edge versus retraction of the actin network towards the interior of the cell (known as retrograde flow). When the clutch is “engaged” (i.e. when the lamellipodial network is strongly attached to the focal adhesion) the frictional force on the actin network increases leading to a reduction in retrograde flow speed and an increase in the force on the leading edge by the lamellipodial network, helping to increase cell propulsion speed
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