30.4: Mechanism of Lamellipodia Formation
Cells migrating in response to external stimuli form lamellipodia, which are thin membrane protrusions supported by a mesh of linked, branched, or unbranched actin filaments. These actin filaments interact with myosin motor proteins, creating the dynamic actomyosin complex within the cytoskeleton. Contractility, or the ability to generate contractile stress, is inherent to the actomyosin complex. It helps cells detect the stiffness of the surrounding ECM and exert contractile force for migration.
At the leading edge, the binding of an activated Rac GTPase to the WAVE complex initiates lamellipodia formation. Subsequently, the WAVE complex changes conformation to prime actin nucleators like the Arp2/3 complex for polymerization and branching of filaments. Additionally, members of the formin family localize at the sites of lamellipodia formation, elongating the unbranched filaments. The force generated by continuous actin polymerization pushes the membrane outwards and forms the membrane protrusion. The actin mesh at the leading edge contributes to the formation of focal adhesion points on the ECM, preventing the retraction of lamellipodia.
After lamellipodia formation, the dissociation of filaments begins at the rear end of the dynamic actin meshwork. The actin monomers steadily detach from the minus ends and attach to the plus ends of filaments in the actin network, resulting in treadmilling. This cyclic assembly and disassembly causes the forward movement of the entire actin filament network even though the position of individual filaments remains stationary to the substratum.
Behind the dynamic lamellipodial meshwork lies a stable network of bundled linear actin filaments called the lamella. This actin network supports the lamellipodium by developing mature and strong adhesion sites, which resist cell compression during migration. Additionally, stable actin filaments of the lamella associate with myosin proteins and use their inherent contractibility to propel the cell forward.
