Nerve Growth Factor Stimulates the Accumulation of β1 Integrin at the Tips of Filopodia in the Growth Cones of Sympathetic Neurons

Abstract

Addition of nerve growth factor (NGF) to sympathetic neurons that have been starved of it causes a rapid induction of growth cone motility and the resumption of neurite growth. Using immunofluorescence staining, we show that within 10 min, NGF stimulated the accumulation of dense aggregates of beta1 integrin [a receptor for extracellular matrix (ECM) proteins] at most of the tips of either newly extended or preexisting 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 receptor is not necessary for tip localization. In fact, addition of either laminin or fibronectin caused a rapid withdrawal of beta1 integrin aggregates from filopodial tips at a rate comparable to that of the rearward flow of actin filaments in the periphery of the growth cone. Surface labeling of the extracellular domain of beta1 integrin while aggregated at the tips of filopodia or withdrawing in response to ECM proteins showed that the receptor is positioned within the membrane. The drug butanedione monoxime, an inhibitor of myosins, blocked the accumulation of beta1 integrin at the tips of filopodia without inhibiting the formation of filo-podia, suggesting the involvement of a myosin motor in beta1 integrin transport. These results provide the first evidence of NGF-mediated accumulation of ECM receptors to sensory elements of the growth cone and suggest one mechanism whereby soluble and substrate-bound cues coordinate to produce directed neurite growth.