Microtubule assembly in growing dendrites.
Dendritic microtubules (MTs) are nonuniform with respect to polarity orientation, with roughly equal proportions having a plus-end-distal or minus-end-distal orientation. In the present studies, we have microinjected biotin-labeled tubulin (Bt-tub) into cultured sympathetic neurons extending dendrites to explore the contribution of MT assembly to the elaboration and maintenance of the dendritic MT array. Within minutes of injecting Bt-tub, an enormous number of MTs were seen emanating from a point source in the cell body. Over time, this pattern changed such that by 120 min after injection, biotinylated MTs no longer emanated from a discrete site, but were distributed over a broad region that extended from the cell body into the dendrites. The observation that biotinylated MTs emanate from a point source in the soma at relatively short times after injection, but not at longer times, suggests that they undergo a redistribution subsequent to their initial nucleation rather than a simple radial expansion from the somal nucleation site. Bt-tub assembly also occurred in dendrites but, unlike in the cell body, assembly was dispersed throughout the dendrite rather than emanating from a discrete site. Immunoelectron microscopic analyses revealed that assembly in dendrites reflected the addition of Bt-tub onto the ends of both plus-end-distal and minus-end-distal MTs that existed in the cell at the time of injection. The time course of Bt-tub appearance in dendritic MTs suggested an average half-life of approximately 76 min for these MTs. We discuss these observations in the context of a model for generating the MT array of dendrites that combines both MT transport and MT assembly.
Wang
J
J
Department of Anatomy and Cell Biology, Temple University School of Medicine, Philadelphia, Pennsylvania 19140, USA.
Yu
W
W
Baas
P W
PW
Black
M M
MM
D000818:Animals; D001710:Biotin / Q000378:metabolism; D002478:Cells, Cultured; D016922:Cellular Senescence; D003712:Dendrites / Q000378:metabolism / Q000502:physiology*; D008870:Microtubules / Q000378:metabolism / Q000502:physiology*; D009474:Neurons / Q000502:physiology; D051381:Rats; D013564:Sympathetic Nervous System / Q000166:cytology / Q000502:physiology; D014404:Tubulin / Q000378:metabolism
D014404:Tubulin; D001710:Biotin