Journal of Neuroscience, Vol 13, 856-866, Copyright © 1993 by Society for Neuroscience
Regional differences in microtubule dynamics in the axon
FJ Ahmad, TP Pienkowski and PW Baas
Department of Anatomy, University of Wisconsin Medical School, Madison 53706.
We have used an indirect method to compare the dynamic properties of
microtubules (MTs) in the main shaft and distal regions of the axon.
Individual MTs are staggered along the length of the axon and consist of a
labile domain situated at the plus end of a stable domain (Baas and Black,
1990). As a result of this organization and the plus-end- distal
orientation of axonal MTs, the most distal region of the axon consists
entirely of labile domains, while the main shaft consists of a mixture of
labile and stable domains. In this study, we wished to determine whether
the labile domains extending into the distal axon differ in their dynamic
properties from the labile domains terminating in the main shaft. To
address this issue, we used immunoelectron microscopy to compare the
tyrosination state of the labile domains terminating in these 2 axon
regions. Because detyrosination is a polymerspecific modification of
alpha-tubulin that accumulates with time, the levels of tyrosinated
alpha-tubulin will be a reflection of the age, and hence dynamic
properties, of the polymer. To maximize our chances of visualizing
potential differences, we varied the concentration of the primary antibody
in these experiments. Our studies indicate that the stable domains are
generally deficient in tyrosinated alpha-tubulin, while the labile domains
contain clearly detectable levels. Within the labile domain, the subsection
closer to the plus end of the MT contains relatively higher levels of
tyrosinated alpha- tubulin than does the subsection farther from the plus
end, suggesting that the levels of tyrosinated alpha-tubulin in the labile
domain may gradually increase as one moves away from the stable domain
toward the plus end of the MT. Although these observations apply to the
labile domains in both regions of the axon, the labile domains extending
into the distal region contain comparatively higher levels of tyrosinated
alpha-tubulin than do the labile domains terminating in the main shaft.
These results are consistent with the view that highly dynamic MT polymer
is present throughout the axon, but that the polymer nearest the advancing
growth cone is particularly dynamic.
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