Regulated Actin Cytoskeleton Assembly at Filopodium Tips Controls Their Extension and Retraction

TLDR

Filopodia extend and retract in response to extracellular cues, a process that directs growth cone advancement. The study investigates how actin cytoskeleton dynamics regulate filopodial extension versus retraction. By tracking fiduciary marks on actin filaments in neuroblastoma growth cones, the authors demonstrate that filopodial behavior is governed by a balance between tip assembly and retrograde flow. Assembly and retrograde flow rates vary over time and among filopodia, yet regulation of assembly rate is the dominant factor controlling filopodial dynamics.

Abstract

The extension and retraction of filopodia in response to extracellular cues is thought to be an important initial step that determines the direction of growth cone advance. We sought to understand how the dynamic behavior of the actin cytoskeleton is regulated to produce extension or retraction. By observing the movement of fiduciary marks on actin filaments in growth cones of a neuroblastoma cell line, we found that filopodium extension and retraction are governed by a balance between the rate of actin cytoskeleton assembly at the tip and retrograde flow. Both assembly and flow rate can vary with time in a single filopodium and between filopodia in a single growth cone. Regulation of assembly rate is the dominant factor in controlling filopodia behavior in our system.