Mechanism of lateral movement of filopodia and radial actin bundles acrossneuronal growth cones

We investigated the motion of filopodia and actin bundles in lamellipodia o f motile cells, using time-lapse sequences of polarized light images. We me asured the velocity of retrograde flow of the actin network and the lateral motion of filopodia and actin bundles of the lamellipodium. Upon noting th at laterally moving filopodia and actin bundles are always tilted with resp ect to the direction of retrograde flow, we propose a simple geometric mode l for the mechanism of lateral motion. The model establishes a relationship between the speed of lateral motion of actin bundles, their tilt angle wit h respect to the direction of retrograde flow, and the speed of retrograde flow in the lamellipodium. Our experimental results verify the quantitative predictions of the model. Furthermore, our observations support the hypoth esis that lateral movement of filopodia is caused by retrograde flow of til ted actin bundles and by their growth through actin polymerization at the t ip of the bundles inside the filopodia. Therefore we conclude that the late ral motion of tilted filopodia and actin bundles does not require a separat e motile mechanism but is the result of retrograde flow and the assembly of actin filaments and bundles near the leading edge of the lamellipodium.