Fig. 5.

Hypothetical model for the coordinated action of NGF and ECM proteins in a filopodium. A, In the quiescent state, β1 integrin (β) is distributed evenly, and the actin cytoskeleton (open triangles) is static.B, NGF induces forward transport of β1 integrin to the tip of the filopodium. It also promotes the polymerization of actin, which can lead to protrusion. It is hypothesized that retrograde flow of actin is also stimulated by NGF, but because β1 integrin couples to this retrograde flow only in the presence of ECM ligand, β1 integrin accumulates at the tip. C, In the presence of an ECM substrate (open circles) β1 integrin continues to be transported forward. Aggregates located at the tip of the filopodium bind to rearward flowing actin filaments probably via a complex of other proteins (gray shapes) creating tension and/or promoting growth.