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First published online 6 February 2003
doi: 10.1242/jcs.00285

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Regulation of membrane expansion at the nerve growth cone

¹ Department of Cellular and Structural Biology, University of Colorado School of Medicine, and University of Colorado Cancer Center, Denver, CO 80262, USA
² Departmento de Química Biológica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, and CIQUIBIC, CONICET, Córdoba, Argentina
³ Instituto Investigación Médica Mercedes y Martín Ferreyra (INIMEC-CONICET), Córdoba, Argentina

^* Author for correspondence (e-mail: squiroga{at}dqbfcq.uncor.edu)

Accepted 20 November 2002

Exocytotic incorporation of plasmalemmal precursor vesicles (PPVs) into the cell surface is necessary for neurite extension and is known to occur mainly at the growth cone. This report examines whether this is a regulated event controlled by growth factors. The Golgi complex and nascent PPVs of hippocampal neurons in culture were pulse-labeled with fluorescent ceramide. We studied the dynamics of labeled PPVs upon arrival at the axonal growth cone. In controls and cultures stimulated with brain-derived neurotrophic factor (BDNF), PPV clusters persisted in growth cones with a half-life (t_1/2) of >14 minutes. Upon challenge with IGF-1, however, fluorescent elements cleared from the growth cones with a t_1/2 of only 6 minutes. Plasmalemmal expansion was measured directly as externalization of membrane glycoconjugates in resealed growth cone particles (GCPs) isolated from fetal forebrain. These assays demonstrated that membrane expansion could be stimulated by IGF-1 in a dose-dependent manner but not by BDNF, even though intact, functional BDNF receptor was present on GCPs. Because both BDNF and IGF-1 are known to enhance neurite growth, but BDNF did not stimulate membrane expansion at the growth cone, we studied the effect of BDNF on the IGF-1 receptor. BDNF was found to cause the translocation of the growth-cone-specific IGF-1 receptor subunit ß_gc to the distal axon, in a KIF2-dependent manner. We conclude that IGF-1 stimulates axonal assembly at the growth cone, and that this occurs via regulated exocytosis of PPVs. This mechanism is affected by BDNF only indirectly, by regulation of the ß_gc level at the growth cone.

Key words: Axonal growth, Growth cone, Membrane expansion, Regulated exocytosis, IGF-1, IGF-1 receptor, BDNF, KIF2

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