The Cell Migration Cycle

Core Concept

Cell migration is a cyclic, coordinated process in which a cell protrudes at its front, adheres to the substrate, contracts to pull its body forward (traction), and finally releases its rear attachments — the four-step cycle of protrusion → adhesion → traction → release. Each step uses a distinct piece of the actin machinery: Arp2/3-branched and formin-driven polymerization pushes the leading edge forward, integrin-based focal adhesions grip the substrate, actomyosin (myosin II) contraction generates the pulling force, and adhesion disassembly frees the rear. Rho-family GTPases (Rac, Cdc42, Rho) act as a spatial "address system" that polarizes these activities into a front and a back.

Key Components

• Leading edge: the front, containing lamellipodia (branched actin) and filopodia (bundled actin).
• Rac / Cdc42: GTPases active at the front; Rac → lamellipodia (via WAVE/Arp2/3), Cdc42 → filopodia (via formins) and polarity.
• RhoA: GTPase active at the rear/sides; drives actomyosin contraction and focal-adhesion maturation.
• Focal adhesions: integrin-based attachments (see focal-adhesion note) that anchor the leading edge.
• Myosin II + actin stress fibers: the contractile apparatus that pulls the cell body forward.
• Adhesion-disassembly machinery: FAK turnover, calpain proteolysis, and microtubule targeting that release rear adhesions.

Mechanism / How It Works

1. Protrusion: external cues activate Rac and Cdc42 at the front, driving Arp2/3-branched actin (lamellipodium) and formin-elongated, fascin-bundled actin (filopodia). Polymerization at barbed ends pushes the membrane forward.
2. Adhesion: new focal adhesions form just behind the leading edge, engaging the actin network to the matrix through integrins, so the protrusion is "committed" to the substrate.
3. Traction (translocation): RhoA → ROCK → myosin II contracts actin stress fibers that span the cell, pulling the cell body and nucleus forward toward the new adhesions while releasing tension at the rear. The adhesions act as a clutch so contraction moves the cell rather than just sliding actin backward.
4. Release (de-adhesion): adhesions at the rear are disassembled — via FAK-mediated turnover, calpain cleavage, and microtubule-directed delivery of disassembly factors — so the cell tail can detach and retract. The cycle repeats, advancing the cell stepwise.

Energy and Directionality

Migration is powered by two coupled ATP-driven processes: actin polymerization (ATP–actin at the front) and actomyosin contraction (myosin II ATPase at the rear/body). Directionality is imposed by spatial Rho-GTPase asymmetry — Rac/Cdc42 at the front vs. RhoA at the rear — which is itself maintained by feedback loops (e.g., PI3K/PIP3 at the front, PTEN at the rear) and external gradients. Net movement results from the cycle running in a polarized, front-to-back sequence; without polarity, the cell "ruffles" in place.

Experimental Evidence / Technique

• Time-lapse phase-contrast / DIC microscopy: directly records the four-step cycle in migrating fibroblasts and keratocytes.
• Fluorescent speckle microscopy (FSM): reveals actin retrograde flow and the clutch behavior at adhesions during traction.
• Traction-force microscopy: maps forces under the cell, showing high stress at the front and release at the rear.
• Rho/Rac/Cdc42 mutants: constitutively active or dominant-negative GTPases selectively abolish protrusion (Rac/Cdc42) or contraction/rear release (Rho), assigning each GTPase to a step.
• Wound-healing / Boyden-chamber assays: quantify migration and show dependence on each cycle component.

How it works

1. Protrusion: external cues activate Rac and Cdc42 at the front, driving Arp2/3-branched actin (lamellipodium) and formin-elongated, fascin-bundled actin (filopodia). Polymerization at barbed ends pushes the membrane forward.
2. Adhesion: new focal adhesions form just behind the leading edge, engaging the actin network to the matrix through integrins, so the protrusion is "committed" to the substrate.
3. Traction (translocation): RhoA → ROCK → myosin II contracts actin stress fibers that span the cell, pulling the cell body and nucleus forward toward the new adhesions while releasing tension at the rear. The adhesions act as a clutch so contraction moves the cell rather than just sliding actin backward.
4. Release (de-adhesion): adhesions at the rear are disassembled — via FAK-mediated turnover, calpain cleavage, and microtubule-directed delivery of disassembly factors — so the cell tail can detach and retract. The cycle repeats, advancing the cell stepwise.

Common confusions

• "Cells move only by pushing forward." — Protrusion alone isn't enough; the cell must adhere, contract, and release the rear to actually translocate.
• "Rho is at the front." — RhoA is at the rear/sides (contraction); Rac and Cdc42 are at the front (protrusion).
• "Adhesions are static anchors that never let go." — Rear adhesions must disassemble for movement; a cell stuck at the rear can't migrate.
• "The order of steps is flexible." — The cycle is sequential and each step is a prerequisite for productive movement.
• "Myosin contraction is only in muscle." — Non-muscle myosin II powers the traction step of crawling in every migrating cell.

Quick review

• Four steps: protrusion, adhesion, traction, release — repeated cyclically.
• Front (Rac/Cdc42): polymerization pushes; rear (RhoA): contraction pulls; adhesions: grip.
• Adhesion disassembly at the rear is essential (FAK, calpain, microtubules).
• ATP drives both actin polymerization and myosin II contraction.
• Steered by chemotactic gradients and front–rear polarity (PIP3/PTEN).
• Underlies development, immunity, wound healing, and metastasis.