Microtubule dynamics in axons and dendrites

Abstract

We have investigated the stability, α‐tubulin composition, and polarity orientation of microtubules (MTs) in the axons and dendrites of cultured sympathetic neurons. MT stability was evaluated in terms of sensitivity to nocodazole, a potent anti‐MT drug. Nocodazole sensitivity was assayed by quantifying the loss of MT polymer as a function of time in 2 μg/ml of the drug. MTs in the axon and the dendrite exhibit striking similarities in their drug sensitivity. In both types of neurites, the kinetics of MT loss are biphasic, and are consistent with the existence of two types of MT polymer that depolymerize with half‐times of ≈︁3.5 min and ≈︁130 min. We define the more rapidly depolymerizing polymer as drug‐labile and the more slowly depolymerizing polymer as drug‐stable. The proportion of MT polymer that is drug‐stable is greater in axons (58%) than in dendrites (25%). On the basis of current understanding of the mechanism of action of nocodazole, we suggest that the drug‐labile and drug‐stable polymer observed in both axons and dendrites correspond to two distinct types of polymer that differ in their relative rates of turnover in vivo. In a previous study, we established that in the axon, these drug‐stable and drug‐labile types of MT polymer exist in the form of distinct domains on individual MTs, with the labile domain situated at the plus end of the stable domain (Baas and Black, J Cell Biol 111:495–509, 1990). Because of the great difference in drug sensitivity between the drug‐labile and drug‐stable MT polymer, we were able to dissect them apart by appropriate treatments with nocodazole. This permitted us to evaluate the drug‐labile and drug‐stable polymer in terms of polarity orientation and relative content of α‐tubulin variants generated by posttranslational detyrosination or acetylation. In both the axon and the dendrite, the modified as well as unmodified α‐tubulins are present in both drug‐labile and drug‐stable polymer, but at different levels. Specifically, the modified forms of α‐tubulin are enriched in the drug‐stable MT polymer compared to the drug‐labile MT polymer. In studies on MT polarity orientation, we demonstrate that in axons, MTs are uniformly plus‐end‐distal, whereas in dendrites, MTs are non‐uniform in their polarity orientation, with roughly equal levels of the MTs having each orientation. We also demonstrate that both drug‐stable and drug‐labile polymer are incorporated into MTs of both polarity orientations in the dendrite.