Does self-organized criticality drive leading edge protrusion?

Arp2/3 complex nucleates dendritic actin networks and plays a pivotal role in the formation of lamellipodia at the leading edge of motile cells. Mouse fibroblasts lacking functional Arp2/3 complex have the characteristic smooth, veil-like lamellipodial leading edge of wild-type cells replaced by a massive, bifurcating filopodia-like protrusions (FLPs) with fractal geometry. The nanometer-scale actin-network organization of these FLPs can be linked to the fractal geometry of the cell boundary by a self-organized criticality through the bifurcation behavior of cross-linked actin bundles. Despite the pivotal role of the Arp2/3 complex in cell migration, the cells lacking functional Arp2/3 complex migrate at rates similar to wild-type cells. However, these cells display defects in the persistence of a directional movement. We suggest that Arp2/3 complex suppresses the formation of FLPs by locally fine-tuning actin networks and favoring dendritic geometry over bifurcating bundles, giving cells a distinct evolutionary edge by providing the means for a directed movement.