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J. Cell Biol.,
Volume 145, Number 6, June 14, 1999 1265-1276
Neurobiology Division, Department of Molecular and Cell Biology, University of California, Berkeley, California 94720
In neuronal growth cones, cycles of filopodial
protrusion and retraction are important in growth cone
translocation and steering. Alteration in intracellular
calcium ion concentration has been shown by several
indirect methods to be critically involved in the regulation of filopodial activity. Here, we investigate whether direct elevation of [Ca2+]i, which is restricted in time
and space and is isolated from earlier steps in intracellular signaling pathways, can initiate filopodial protrusion. We raised [Ca2+]i level transiently in small areas of
nascent axons near growth cones in situ by localized
photolysis of caged Ca2+ compounds. After photolysis,
[Ca2+]i increased from ~60 nM to ~1 µM within the illuminated zone, and then returned to resting level in
~10-15 s. New filopodia arose in this area within 1-5
min, and persisted for ~15 min. Elevation of calcium
concentration within a single filopodium induced
new branch filopodia. In neurons coinjected with
rhodamine-phalloidin, F-actin was observed in dynamic
cortical patches along nascent axons; after photolysis,
new filopodia often emerged from these patches. These
results indicate that local transient [Ca2+]i elevation is
sufficient to induce new filopodia from nascent axons or
from existing filopodia.
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