CALCIUM CHANNELS IN NEUROBLASTOMA CELL-GROWTH CONES

The concentration of free calcium ions in the cytosol has been shown t o influence many components of growth cone behaviour, including the ex tension of filopodia and veils, the addition of new membrane to the pl asmalemma, the retraction and disappearance of filopodia, and gross co llapse and retraction of the growth cone. A spatially localized modula tion of these processes by very local calcium changes has been propose d to underlie the steering of growth cones by gradients of neurotransm itters, voltage and cell adhesion molecules. Such local control can be studied in mouse neuroblastoma cells, where depolarization causes cal cium to rise in a limited number of spatially restricted hotspots, tri ggering a localized advance. We have studied the simple, club-shaped g rowth cones that are characteristically found on advancing neurites. D epolarization caused calcium to increase most at the distal, leading t ip. Agents that disrupt calcium-induced calcium release do not affect growth cone calcium dynamics, ruling out a local release of calcium at the tip as a cause of the gradient. Using cell-attached patch recordi ng, we find that L-type calcium channels are present at a higher densi ty at the distal tip than in the proximal growth cone. Our results sho w that the calcium gradients seen in depolarized growth cones are a di rect consequence of a gradient of calcium channel density.