THE IG SUPERFAMILY CELL-ADHESION MOLECULE, APCAM, MEDIATES GROWTH CONE STEERING BY SUBSTRATE-CYTOSKELETAL COUPLING

Dynamic cytoskeletal rearrangements are involved in neuronal growth co ne motility and guidance. To investigate how cell surface receptors tr anslate guidance cue recognition into these cytoskeletal changes, we d eveloped a novel in vitro assay where beads, coated with antibodies to the immunoglobulin superfamily cell adhesion molecule apCAM or with p urified native apCAM, replaced cellular substrates. These beads associ ated with retrograde F-actin flow, but in contrast to previous studies , were then physically restrained with a microneedle to simulate inter actions with noncompliant cellular substrates. After a latency period of similar to 10 min, we observed an abrupt increase in bead-restraini ng tension accompanied by direct extension of the microtubule-rich cen tral domain toward sites of apCAM bead binding. Most importantly, we f ound that retrograde F-actin flow was attenuated only after restrainin g tension had increased and only in the bead interaction axis where pr eferential microtubule extension occurred. These cytoskeletal and stru ctural changes are very similar to those reported for growth cone inte ractions with physiological targets. Immunolocalization using an antib ody against the cytoplasmic domain of apCAM revealed accumulation of t he transmembrane isoform of apCAM around bead-binding sites. Our resul ts provide direct evidence for a mechanical continuum from apCAM bead substrates through the peripheral domain to the central cytoplasmic do main. By modulating functional linkage to the underlying actin cytoske leton, cell surface receptors such as apCAM appear to enable the appli cation of tensioning forces to extracellular substrates, providing a m echanism for transducing retrograde flow into guided growth cone movem ent.