Emergence of Large-Scale Cell Morphology and Movement from Local Actin Filament Growth Dynamics
We developed a method to generate nascent lamellipodia during recovery of protrusion after temporarily stalling a section of the lamellipodium. A glass micropipette, acting as a barrier, was lowered into the path of movement of a migrating keratocyte overexpressing EGFP-VASP and forced down until a section flexed parallel to the surface. The pipette was left in place until the cell was in firm contact with the pipette and subsequently removed by translating the pipette in the direction of cell migration. A coherent keratocyte shows EGFP-VASP as a uniform thin line at the leading (time: 0:00, min:s). Fluorescent images are shown in both regular and inverted contrast for clarity. Insets show inversed and zoomed in images of the corresponding boxed areas of the leading edge. When this keratocyte reaches the edge of the barrier, the lamellipodium temporarily stops protruding forward and acquires a very flat shape corresponding to the shape of the barrier (time: 0:48, 0:57). The cell continues migrating in the original direction of motion while EGFP-VASP becomes displaced from the edge of the region in contact with the barrier (see inset). When the micropipette barrier is removed (between 0:57 and 1:00), the leading edge of the lamellipodium immediately resumes protrusion and appears rough with several protruding microregions enriched in EGFP-VASP (time: 1:00). The levels of EGFP-VASP quickly recover along the impacted region and become uniform (time: 1:06). Only 18 s after removal of the barrier, the keratocyte's original shape and EGFP-VASP localization at the leading edge are restored (time: 1:18). Scale bar = 20 μm.