Force Generation in Lamellipodia is a Discontinuous Process with Nanometer Discrete Forward and Backward Jumps

Abstract

The progressive addition of actin monomers to the existing network of actin filaments underlies force generation in lamellipodia. By using optical tweezers, we have characterized the dynamics by which lamellipodia of Dorsal Root Ganglia neurons exerted force on encountered obstacles such as silica beads. Because of the presence of adhesion forces, beads in close contact with a lamellipodium could seal on its membrane so that the standard deviation of Brownian fluctuations could be reduced by 10 times. In several experiments, the bead remained within 300 nm from the center of the optical trap where voltage sensitivity of the detector and trap stiffness is constant. Under these conditions, if the lamellipodium pushed the bead, we could detect discrete jumps possibly constituting the elementary events underlying force generation. Jumps were detected by using algorithms based on nonlinear diffusion methods and on numerical differentiation. These jumps occurred within 1 ms and had an amplitude varying from 5 to 20 nm. When the lamellipodium retracted, pulling the beads with it, no discrete events were observed. These discrete events were not observed in the presence of Latrunculin A, a blocker of actin polymerization or when neurons were fixed with paraformaldehyde. The amplitude of these jumps increased by 20-40% when cholesterol in cellular membrane was depleted by treatment with cyclodextrin. These jumps show that force generation in lamellipodia is not a smooth process but is a discontinuous process in which bursts of actin polymerization and depolymerization alternate continuously.