Journal of Neuroscience, Vol 11, 1300-1308, Copyright © 1991 by Society for Neuroscience
Calcium ion distribution in nascent pioneer axons and coupled preaxonogenesis neurons in situ
D Bentley, PB Guthrie and SB Kater
Department of Molecular and Cell Biology, University of California, Berkeley 94720.
The "calcium hypothesis" of regulation of growth cone motility and neurite
elongation has derived from analysis of a variety of neurons growing in
vitro. It proposes that calcium ion concentration within growth cones is an
important regulator of motility and growth. We now extend this analysis by
investigating calcium concentrations within growth cones and nascent
neurites of identified embryonic neurons growing on their normal substrate
in situ. The pair of Ti1 pioneer neurons are the first to extend axons in
limb buds of grasshopper embryos. Their growth cones migrate along a
stereotyped pathway, where they encounter a series of guidance cues,
including preaxonogenesis afferent neurons (guidepost cells). By injecting
a pioneer neuron with fura-2 dye, we measured calcium concentration in the
injected neuron, and in cells to which it became dye-coupled, at successive
stages of pioneer outgrowth through the limb. Pioneer neurons undergoing
axonogenesis had calcium concentrations in the above-100-nM range
characteristic of a variety of neuron types studied in vitro. In pioneer
neurons not yet in contact with guidepost cells, a shallow calcium
concentration gradient, highest at the growth cone, was often observed.
Guidepost cells that had not begun axonogenesis had lower calcium
concentrations, in the 65 nM range. Therefore, an increase in cytosolic
calcium concentration may be associated with the onset of axonogenesis.
Nascent pioneer neurons were fura-2 dye-coupled to each other and
established coupling with contacted guidepost cells. Calcium concentration
measurements along pioneer neurites suggest that calcium ions also are
transferred from pioneer neurons to these coupled guidepost cells.(ABSTRACT
TRUNCATED AT 250 WORDS)
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