Lamellipodia

Core Concept

The lamellipodium is the broad, flat, sheet-like protrusion at the leading edge of a migrating cell, a thin veil (only ~100–200 nm thick) filled with a dense, branched actin network. Its branched geometry is created by the Arp2/3 complex, which nucleates a new "daughter" filament at a ~70° angle off the side of an existing "mother" filament. As the network grows, actin treadmills — monomers add at the membrane-proximal barbed ends while older ADP-actin is severed and recycled at the rear by cofilin — and this continuous polymerization pushes the plasma membrane forward. Lamellipodium formation is triggered by the Rho-family GTPase Rac, which acts through WAVE to activate Arp2/3.

Key Components

• Actin filaments (F-actin): polarized polymers whose barbed (plus) ends face the leading-edge membrane.
• Arp2/3 complex: a seven-subunit nucleator that binds the side of a mother filament and initiates a new filament at a ~70° branch.
• Nucleation-promoting factors (NPFs): WASP/WAVE/SCAR proteins that deliver actin monomers and activate Arp2/3.
• Rac (Rho-family GTPase): the master switch for lamellipodia; active Rac activates WAVE.
• Profilin: delivers ATP–actin to barbed ends and promotes nucleotide exchange.
• Capping protein: binds and blocks barbed ends, funneling monomers to the few remaining free ends (thus increasing protrusive force and branch density).
• Cofilin/ADF: severs older ADP-actin filaments and promotes pointed-end disassembly at the network rear.
• AIP1, gelsolin: additional severing/capping regulators of the treadmilling cycle.

Mechanism / How It Works

1. Activated Rac (membrane-anchored, GTP-bound) recruits and activates the WAVE complex, which in turn activates Arp2/3 near the plasma membrane.
2. Arp2/3 binds the side of an existing filament and nucleates a new filament growing at ~70°, producing the characteristic Y-branched, dendritic network.
3. Profilin–actin adds to the free barbed ends, which push against the membrane, generating protrusive force.
4. Capping protein terminates many barbed ends after they elongate a short distance; this channels monomer flux into fewer, well-oriented ends and keeps the network dense and branched.
5. As filaments age, actin-bound ATP hydrolyzes to ADP; cofilin severs and disassembles this older ADP-actin at the network's rear, recycling monomers (profilin recharges them with ATP).
6. The net result is treadmilling: assembly at the front, disassembly at the back, with the membrane pushed forward while the cell body catches up.

Energy and Directionality

Lamellipodial protrusion is powered by actin ATP hydrolysis. ATP–actin polymerizes at the barbed (plus) ends; the lagging hydrolysis to ADP marks older filaments for cofilin severing at the rear. This directional treadmilling (barbed-end assembly at the front, pointed-end/severed disassembly at the back) is what converts chemical energy (ATP) into directed mechanical work — pushing the membrane outward.

Experimental Evidence / Technique

• Electron microscopy (platinum replica): shows the dense, ~70° branched (dendritic) network unique to lamellipodia.
• Arp2/3 inhibition (CK-666, RNAi): cells lose lamellipodial branches and slow migration, proving Arp2/3 builds the network.
• Photoactivation / fluorescent speckle microscopy: reveals actin flux — continuous polymerization at the leading edge and retrograde flow/disassembly at the rear.
• Rac dominant-negative/active mutants: active Rac induces lamellipodia; dominant-negative Rac abolishes them, establishing Rac as the trigger.
• WAVE/WASP knockout: loss of NPFs blocks Arp2/3 activation and lamellipodium formation.

How it works

1. Activated Rac (membrane-anchored, GTP-bound) recruits and activates the WAVE complex, which in turn activates Arp2/3 near the plasma membrane.
2. Arp2/3 binds the side of an existing filament and nucleates a new filament growing at ~70°, producing the characteristic Y-branched, dendritic network.
3. Profilin–actin adds to the free barbed ends, which push against the membrane, generating protrusive force.
4. Capping protein terminates many barbed ends after they elongate a short distance; this channels monomer flux into fewer, well-oriented ends and keeps the network dense and branched.
5. As filaments age, actin-bound ATP hydrolyzes to ADP; cofilin severs and disassembles this older ADP-actin at the network's rear, recycling monomers (profilin recharges them with ATP).
6. The net result is treadmilling: assembly at the front, disassembly at the back, with the membrane pushed forward while the cell body catches up.

Common confusions

• "Lamellipodia and filopodia are interchangeable." — Lamellipodia are branched sheets (Arp2/3); filopodia are parallel bundles (formins + fascin).
• "Arp2/3 elongates filaments." — Arp2/3 nucleates branches; elongation is done by profilin–actin addition at barbed ends (and by formins elsewhere).
• "Rac makes filopodia." — Rac → lamellipodia; Cdc42 → filopodia; Rho → stress fibers.
• "Actin pushes by contracting." — Protrusion is from polymerization; contraction (myosin II) happens behind the leading edge in stress fibers.
• "The network is static." — It continuously treadmills; a steady lamellipodium is a dynamic steady state, not a frozen structure.

Quick review

• Lamellipodium = flat, branched Arp2/3 network at the leading edge.
• Rac → WAVE → Arp2/3 → ~70° branch nucleation; profilin feeds barbed ends.
• Capping protein funnels growth; cofilin severs/recycles at the rear → treadmilling.
• Powered by actin ATP hydrolysis; polymerization pushes the membrane.
• Contrast: filopodia (Cdc42/formins/fascin) and stress fibers (Rho/myosin).
• Relevant to migration, wound healing, immunity, and cancer invasion.