Filopodia

Core Concept

Filopodia are thin, finger-like actin protrusions that a migrating or exploring cell extends from its leading edge. Unlike the broad, branched lamellipodium, a filopodium contains a parallel bundle of actin filaments whose barbed (plus) ends point toward the plasma membrane tip. Filopodia act as the cell's "feelers": they probe the environment, sense guidance cues, and form early contacts with other cells or the matrix. Their growth is driven by formins (which processively add profilin–actin to the barbed ends) and Ena/VASP proteins (which protect the ends from capping), while fascin cross-links the filaments into a stiff bundle.

Key Components

• Parallel actin bundle: ~10–30 actin filaments, all with barbed ends at the tip, cross-linked tightly.
• Fascin: the bundling protein that holds filopodial actin filaments in a rigid, parallel array.
• Formins (e.g., mDia2): processive barbed-end polymerases that nucleate and elongate actin using profilin–actin.
• Profilin: binds actin monomers, delivers them to growing barbed ends, and catalyzes nucleotide exchange.
• Ena/VASP proteins: anti-capping factors that accumulate at the filopodial tip and allow continuous elongation.
• Myosin-X: an unconventional myosin that walks to the filopodial tip carrying integrins/adhesion cargo.
• Tip complex: the cluster of formins, Ena/VASP, and other regulators at the membrane-proximal barbed ends.

Mechanism / How It Works

1. At the leading edge, Cdc42 (a Rho-family GTPase) activates formins such as mDia2, which nucleate new actin filaments.
2. Formin remains attached to the growing barbed end, acting as a processive polymerase that adds profilin–actin monomers one at a time.
3. The elongating filaments push against the plasma membrane; Ena/VASP at the tip blocks capping protein, so elongation continues.
4. Neighboring filaments are zippered into a tight parallel bundle by fascin, giving the filopodium its slender, rigid shape (diameter ~100–200 nm, length up to tens of µm).
5. Myosin-X walks toward the barbed ends (the tip), delivering integrins and other cargo to support adhesion and signaling at the leading point.
6. Because all barbed ends point outward and pointed ends are at the base, actin in a filopodium undergoes net treadmilling: assembly at the tip, disassembly at the base (aided by cofilin).

Energy and Directionality

Filopodial growth is powered by ATP hydrolysis on actin monomers. Actin binds ATP; upon polymerization the ATP is hydrolyzed to ADP (with a lag), and ADP-actin at the pointed (minus) end is preferentially depolymerized by cofilin. The barbed (plus) end is the fast-growing, membrane-pushing end, so polymerization there produces the protrusive force — assembly is directional (tip = plus end), and the ATPase cycle sustains the treadmilling flux from tip to base.

Experimental Evidence / Technique

• Electron microscopy (platinum replica): reveals the tight, parallel actin bundle of filopodia versus the branched network of lamellipodia.
• Fluorescent speckle / photoactivation of actin: shows continuous actin assembly at the filopodial tip and retrograde flow toward the base.
• Fascin knockdown: cells lose filopodia but retain lamellipodia, proving fascin-dependent bundling is filopodium-specific.
• Ena/VASP and formin perturbation: blocking these factors stalls filopodial elongation, confirming their tip roles.
• Cdc42 activation assays: constitutively active Cdc42 induces filopodia; dominant-negative Cdc42 suppresses them.

How it works

1. At the leading edge, Cdc42 (a Rho-family GTPase) activates formins such as mDia2, which nucleate new actin filaments.
2. Formin remains attached to the growing barbed end, acting as a processive polymerase that adds profilin–actin monomers one at a time.
3. The elongating filaments push against the plasma membrane; Ena/VASP at the tip blocks capping protein, so elongation continues.
4. Neighboring filaments are zippered into a tight parallel bundle by fascin, giving the filopodium its slender, rigid shape (diameter ~100–200 nm, length up to tens of µm).
5. Myosin-X walks toward the barbed ends (the tip), delivering integrins and other cargo to support adhesion and signaling at the leading point.
6. Because all barbed ends point outward and pointed ends are at the base, actin in a filopodium undergoes net treadmilling: assembly at the tip, disassembly at the base (aided by cofilin).

Common confusions

• "Filopodia and lamellipodia are the same." — Filopodia are parallel-bundled and finger-like; lamellipodia are branched (Arp2/3) and sheet-like.
• "Filopodia grow from the pointed end." — No; the barbed (plus) ends are at the tip and are the growing, membrane-pushing ends.
• "Arp2/3 builds filopodia." — Arp2/3 builds the branched lamellipodial network; filopodia are elongated by formins and bundled by fascin (often initiated from the lamellipodial network).
• "Cdc42 and Rac do the same thing." — Cdc42 → filopodia, Rac → lamellipodia, Rho → stress fibers/focal adhesions.
• "Actin in filopodia is static once built." — It continuously treadmills (adds at tip, disassembles at base) even when length is constant.

Quick review

• Filopodium = slender, fascin-bundled, parallel actin protrusion with barbed ends at the tip.
• Growth: formin + profilin–actin elongation; Ena/VASP anti-capping; myosin-X carries cargo.
• Triggered by Cdc42; distinct from Rac-driven lamellipodia and Rho-driven stress fibers.
• Actin treadmills tip→base; powered by actin ATP hydrolysis.
• Functions: sensing, guidance, adhesion, invasion; fascin = cancer marker/target.