Mechanism of Filopodia Formation

Cdc42, a small Rho family protein, initiates filopodia formation by switching to its GTP-bound active state when the cell receives an appropriate signal. Active Cdc42 recruits IRSp53 dimers to the cell membrane, forming activated Cdc42-IRSp53 complexes that trigger downstream actin reorganization and membrane deformation necessary for filopodial extension and cell polarization.

IRSp53, a multi-domain protein, impacts both the actin cytoskeleton and membrane during filopodia formation. It mobilizes actin nucleators like Ena/VASP proteins to filament ends and clusters phosphatidylinositol 4,5-bisphosphate molecules at the membrane's inner leaflet, inducing curvature that deforms the membrane into a filopodial structure.

Fascin proteins cross-link actin filaments to form tight bundles as the filopodium extends. This bundling gives rigidity to the filopodial structure, enabling it to sustain extracellular force and membrane tension. The filopodia core comprises 12-20 actin filaments spaced 12 nanometers apart, held together by fascin cross-linking.

Ena/VASP accumulation displaces capping proteins from filament ends, promoting actin elongation during filopodia formation. Conversely, capping proteins promote filopodial retraction by shielding polymerizing filament ends from further elongation, controlling the balance between filopodial growth and disassembly during the retraction phase.

Filopodia guide migrating cells during tissue morphogenesis and cancer metastasis by recognizing and making initial contacts with the extracellular matrix. They also act as stationary cell anchors, establish communication between cells through filopodial bridges, and help cells reach and internalize distant targets such as pathogens.

IRSp53 clusters phosphatidylinositol 4,5-bisphosphate molecules at the inner membrane leaflet, inducing membrane curvature and asymmetry. Combined with actin filament elongation, this increased membrane asymmetry deforms the membrane, structuring the filopodium and enabling its characteristic thin, elongated morphology and cellular protrusion.

Filopodia retraction involves periodic helical and rotational motion of the actin shaft, regulated by several factors including capping proteins and RhoA kinase activity. RhoA kinase regulates actin polymerization within filopodia, controlling the dynamics of filament assembly and disassembly during the retraction phase.