NERVE GROWTH-FACTOR STIMULATES THE ACCUMULATION OF BETA-1 INTEGRIN ATTHE TIPS OF FILOPODIA IN THE GROWTH CONES OF SYMPATHETIC NEURONS

Addition of nerve growth factor (NGF) to sympathetic neurons that have been starved of it causes a rapid induction of growth cone motility a nd the resumption of neurite growth. Using immunofluorescence staining , we show that within 10 min, NGF stimulated the accumulation of dense aggregates of beta 1 integrin [a receptor for extracellular matrix (E CM) proteins] at most of the tips of either newly extended or preexist ing filopodia. This effect occurred in the absence of ECM proteins and in the presence of 1 mg/ml Arg-Gly-Asp-Ser peptide, which blocks ECM binding to integrin, indicating that occupation of the integrin recept or is not necessary for tip localization. In fact, addition of either laminin or fibronectin caused a rapid withdrawal of beta 1 integrin ag gregates from filopodial tips at a rate comparable to that of the rear ward flow of actin filaments in the periphery of the growth cone. Surf ace labeling of the extracellular domain of beta 1 integrin while aggr egated at the tips of filopodia or withdrawing in response to ECM prot eins showed that the receptor is positioned within the membrane. The d rug butanedione monoxime, an inhibitor of myosins, blocked the accumul ation of beta 1 integrin at the tips of filopodia without inhibiting t he formation of fito-podia, suggesting the involvement of a myosin mot or in beta 1 integrin transport. These results provide the first evide nce of NGF-mediated accumulation of ECM receptors to sensory elements of the growth cone and suggest one mechanism whereby soluble and subst rate-bound cues coordinate to produce directed neurite growth.