Detection and Characterization of elementary events underlying force generation in lamellipodia of Dorsal Root Ganglia Neurons

Abstract

Force generation in lamellipodia of growth cones originates from the progressive addition of small polymers to the existing network of actin molecules pushing the cellular membrane forward. Using optical tweezers we have characterized with high temporal resolution and sensitivity the molecular mechanisms by which lamellipodia generate force on encountered obstacles such as silica beads. When beads are positioned in close contact to the lamellipodium, because of adhesion forces, beads can seal on the membrane decreasing the standard deviation σ of Brownian fluctuations to less than 10 nm. Under these conditions, when the lamellipodium leading edge pushes the bead it is possible to detect discrete jumps with a variable shape and amplitude. The amplitude of these jumps varies from 5 to 40 nm. The summation of these jumps leads to a plateau level, during which an almost constant force can be measured for several seconds. During this plateau, asymmetric brief transients are observed, ultimately leading to the collapse of the generated force. These transients have amplitude up to 150 nm and last some hundreds of msec. These jumps and transients constitute different phases of the polymerization and depolymerisation cycles of the actin filament network and constitute also the elementary events underlying force generation in lamellipodia.