Exocytosis of integrin containing vesicles contributes to the formation of adhesive contacts at growth cones of cultured hippocampal neurons

Adhesive contact formation at growth cones is essential for neurite extension. It has been suggested that incorporation of cell adhesion molecules to the cell membrane is mediated by exocytosis of vesicles that contained molecules for cell adhesion, such as integrin. Rat hippocampal neurons were cultured for 1 to 3 days until they extended neurites with growth cones. Exocytosis of vesicles was monitored with FM4-64, a fluorescent marker for exocytosis, and adhesive contact formation by reflection interference contrast microscopy (RICM). High K⁺ solution (60mM or 120mM) was applied to the growth cone through a glass pipette to induce exocytosis at the growth cone. The number of FM4-64 fluorescence spots increased remarkably during the high K⁺ stimulation. These FM4-64 fluorescence spots disappeared within several seconds, denoting the exocytosis of the FM4-64 containing vesicle. These bright spots were localized at cell adhesion sites that were imaged by RICM. The area of the cell adhesion increased gradually during the stimulation, suggesting close correlation between the vesicle exocytosis and adhesive contact formation at the growth cone. Furthermore immunostaining of integrin and synapsin showed that these molecules are colocalized in the vesicle at the growth cone. These results suggest that vesicles containing synapsin and integrin are transported to growth cones and integrin is incorporated into the basal cell membrane by exocytosis. [Jpn J Physiol 54 Suppl:S144 (2004)]