Cell Polarization by Rho Proteins

Rho proteins are small GTPases that act as molecular switches, activated by chemical signals to establish distinct cell front and rear regions. Rac and Cdc42 activate at the leading edge to promote actin polymerization and branching, forming membrane protrusions. Rho activates at the trailing edge to increase myosin-driven contraction, retracting the cell rear. This spatial separation of Rho protein activity creates and maintains cell polarity throughout migration.

Rac and Cdc42 are locally activated on the cell side facing the chemical stimulus. Rac triggers actin branching via the Arp 2/3 complex to form lamellipodia, while Cdc42 promotes actin nucleation and bundling to form filopodia. Together, these Rho proteins reorganize the actin cytoskeleton to create membrane protrusions that push the cell forward during migration.

Rho is activated at the opposite end of the cell from the stimulus. It triggers formins and Rho-dependent kinase (ROCK), which promote parallel actin bundling and myosin-driven contraction. This increased contractility retracts the cell rear, pulling the cell in the direction of migration while establishing a distinct trailing edge.

Rac and Rho inhibit each other through negative feedback regulation, which stabilizes cell polarity. Rac inhibits Rho activation at the leading edge, restricting Rho activity to the rear and maintaining Rac activity at the cell front. This mutual inhibition prevents conflicting signals and ensures the cell maintains distinct, stable front and rear regions during migration.

Chemical signals bind to G-protein coupled receptors on the cell surface, activating secondary messengers like PIP3 and G-proteins 12/13. Because PIP3 is membrane-bound, it activates Rac and Cdc42 only on the side facing the stimulus, establishing the leading edge. G-proteins 12/13 diffuse through the cytoplasm to activate Rho at the opposite end, creating spatial specificity in Rho protein activation.

Cell polarity is the asymmetric distribution of cellular and membrane components that creates distinct cell regions. During migration, polarity establishes a leading edge with membrane protrusions that push the cell forward and a trailing edge that retracts. This organization enables directional cell movement in response to chemical signals and is fundamental to embryogenesis, axon migration, and other cellular processes.

Cdc42 promotes actin nucleation and filament bundling via the Wasp-Arp 2/3 pathway, forming filopodia—thin, spike-like protrusions. Rac triggers actin branching through the WAVE pathway, creating lamellipodia—broad, sheet-like membrane extensions. Both contribute to the leading edge but through distinct actin polymerization mechanisms that generate different types of membrane protrusions.