AN APLYSIA CELL-ADHESION MOLECULE ASSOCIATED WITH SITE-DIRECTED ACTINFILAMENT ASSEMBLY IN NEURONAL GROWTH CONES

During neuronal growth cone-target interactions, a programmed sequence of cytoskeletal remodeling has been described, involving increased ac tin assembly at the target site and directed microtubule extension int o it. The cell adhesion protein apCAM rapidly accumulates at such inte raction sites, suggesting a possible role in regulating cytoskeletal r emodeling. To test this hypothesis we crosslinked apCAM to varying deg rees with antibodies. Secondary immunocomplexes exhibited a classical patching and capping response; in contrast, high density crosslinking of apCAM by antibody coated beads triggered localized actin assembly a ccompanied by formation of taillike actin structures referred to as in ductopodia, When beads were derivatized with increasing amounts of ant i-apCAM they displayed three sequential dose-dependent kinetic states after binding: (1) lateral diffusion in the plane of the membrane; (2) restricted diffusion due to coupling with underlying F-actin; and (3) translocation in the plane of the membrane driven by de novo actin fi lament assembly local to bead binding sites, i.e. inductopodia formati on. In contrast, lectin coated beads were far less efficient in trigge ring inductopodia formation despite demonstrated membrane protein bind ing. This work provides evidence that crosslinking of a diffusable mem brane protein, apCAM, to threshold levels, can trigger highly localize d actin filament assembly and rapid remodeling of neuronal cytoarchite cture.