Filopodial Initiation and a Novel Filament-organizing Center, the Focal Ring

This study examines filopodial initiation and implicates a putative actin filament organizer, the focal ring. Filopodia were optically recorded as they emerged from veils, the active lamellar extensions of growth cones. Motile histories revealed three events that consistently preceded filopodial emergence: an influx of cytoplasm into adjacent filopodia, a focal increase in phase density at veil margins, and protrusion of nubs that transform into filopodia. The cytoplasmic influx probably supplies materials needed for initiation. In correlated time lapse-immunocytochemistry, these focal phase densities corresponded to adhesions. These adhesions persisted at filopodial bases, regardless of subsequent movements. In correlated time lapse-electron microscopy, these adhesion sites contained a focal ring (an oblate, donut-shaped structure ∼120 nm in diameter) with radiating actin filaments. Filament geometry may explain filopodial emergence at 30 degree angles relative to adjacent filopodia. A model is proposed in which focal rings play a vital role in initiating and stabilizing filopodia: 1) they anchor actin filaments at adhesions, thereby facilitating tension development and filopodial emergence; 2) “axial” filaments connect focal rings to nub tips, thereby organizing filament bundling and ensuring the bundle intersects an adhesion; and 3) “lateral” filaments interconnect focal rings and filament bundles, thereby helping stabilize lamellar margins and filopodia.