The Journal of Neuroscience, September 15, 1999, 19(18):7971-7982
Calcium Influx Alters Actin Bundle Dynamics and Retrograde Flow
in Helisoma Growth Cones
Elizabeth A.
Welnhofer,
Lin
Zhao, and
Christopher S.
Cohan
Department of Anatomy and Cell Biology, University at Buffalo,
State University of New York, Buffalo, New York 14214
The ability of calcium (Ca2+) to effect changes
in growth cone motility requires remodeling of the actin cytoskeleton.
To understand the mechanisms involved, we evaluated the effect of
elevated intracellular calcium
([Ca2+]i) on actin bundle
dynamics, organization, and retrograde flow in the large growth cones
of identified Helisoma neurons. Depolarization with 15 mM KCl (high K+) for 30 min caused a
rapid and sustained increase in
[Ca2+]i and resulted in longer
filopodia, shorter actin ribs, and a decrease in lamellipodia width.
Time-lapse microscopy revealed that increasing
[Ca2+]i affected actin bundle dynamics
differently at the proximal and distal ends. Filopodial lengthening
resulted from assembly-driven elongation of actin bundles whereas actin
rib shortening resulted from a distal shift in the location of
breakage. Buckling of ribs occurred before breakage, suggesting
nonuniform forces were applied to ribs before shortening. Calcium
(Ca2+) influx also resulted in a decrease in density
of F-actin in bundles, as determined by contrast changes in ribs imaged
by differential interference contrast microscopy and fluorescent
intensity changes in rhodamine-labeled ribs. The velocity of retrograde
flow decreased by 50% after elevation of
[Ca2+]i. However, no significant
change in retrograde flow occurred when the majority of changes in
actin bundles were blocked by phalloidin. This suggests that inhibition
of retrograde flow resulted from Ca2+-induced
changes in the actin cytoskeleton. These results implicate Ca2+ as a regulator of actin dynamics and, as such,
provide a mechanism by which Ca2+ can influence
growth cone motility and behavior.
Key words:
actin dynamics; filament severing; retrograde flow; growth cone motility; calcium; cytoskeleton; Helisoma
Copyright © 1999 Society for Neuroscience 0270-6474/99/19187971-12$05.00/0
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