Stochastic actin dynamics in lamellipodia reveal parameter space for cell type classification

Abstract

The lamellipodium, a thin veil-like structure at the leading edge of motile cells, is fundamental for cell migration and growth. Orchestrated activities of membrane components and an underlying biopolymer film result in a controlled movement of the whole system. Dynamics in two-dimensional cell motility are primarily driven by the actively moving protein film in the lamellipodium. Polymerization of actin filaments at the leading edge, back-transport of the actin network due to myosin motor activity, depolymerization in the back, and diffusive transport of actin monomers to the front control these dynamics. The same molecular prerequisites for lamellipodial motion are found in most eukaryotic cells and can function independently of the cell body. Here we show that lamellipodial dynamics differ strongly in different cell types according to their function. Path finding neuronal growth cones display strong stochastic fluctuations, wound healing fibroblasts that locally migrate in tissues exhibit reduced fluctuations while fish keratocytes move highly persistently. Nevertheless, experimental analysis and computer simulations show that changes in the parameters for actin polymerization and retrograde actin transport alone are sufficient for the cell to utilize the same, highly adaptive machinery to display this rich variety of behaviors.

Forces from the rear: deformed microtubules in neuronal growth cones influence retrograde flow and advancement

“…A comparison of subsequent frames of a time series using feature-tracking and cross-correlation algorithms allows one to determine RF velocity within the P-domain of a GC. In three of the GCs under investigation, we were able to detect RF velocities ranging from 1 to 5 µm min −1 , which is in good agreement with values previously measured in GCs of the same cell type [28,29]. We found that RF varies temporally and spatially and often increases for tens of seconds in confined areas.…”

“…If we assume constant MT stiffness, the two different buckling conditions must originate from variations in the surrounding actin gel. The lamellipodium is generally not homogeneous since two different states (on/off) of actin polymerization regulate edge dynamics [28,31] and result in different types of networks. These differ strongly in actin network structure and density [32] which very likely leads to two typical values for network elasticity.…”

Causes of retrograde flow in fish keratocytes

“…Edge protrusion velocity and retrograde flow data were obtained by phase contrast feature tracking [Betz et al, ; Knorr et al, ] of the internal actin transport/flow [Figs. C and D).…”

“…Traction forces measurements were performed as described in Fuhs et al (2013), using polyacrylamide hydrogels with a Young's modulus of 6 kPa. Edge protrusion velocity and retrograde flow data were obtained by phase contrast feature tracking [Betz et al, 2006;Knorr et al, 2011] of the internal actin transport/ flow [Figs. 3C and 3D).…”

Different modes of growth cone collapse in NG 108-15 cells

“…Consequently, alternating phases of active growth cone collapse or neurite retraction and outgrowth are integral features of neurite pathfinding (Kalil et al 2000). On the one hand, many details are known about different modes of growth cone advancement and steering, especially concerning the interplay of actin polymerization and retrograde flow (Betz et al 2009; Knorr et al 2011) as well as actin-microtubule interactions (Zhou et al 2002; Zhou and Cohan 2004). Collapse and retraction processes, on the other hand, are mostly evaluated in cases of partial or full retraction of the respective neurite.…”