Differences in polypeptide composition and enzyme activity between cold-stable and cold-labile microtubules and study of microtubule alkaline phosphatase activity

Zinc potentiates agonist-induced currents at certain splice variants of the NMDA receptor

We have determined the gene structure for the NMDA receptor subunit gene NMDAR1. We found eight splice variants that arise from different combinations of a single 5' terminal exon insertion and three different 3' terminal exon deletions, relative to NMDAR1. We analyzed the modulation by Zn2+ of currents through homomeric receptors assembled from these splice variants and found that, in addition to its well-known inhibitory effect at high concentrations, Zn2+ potentiates agonist-induced currents at submicromolar concentrations (EC50 = 0.50 microM). This potentiation is observed only with a subset of NMDAR1 splice variants that show additional differences in pharmacological properties. Zn2+ potentiation is rapidly reversible, noncompetitive with either glutamate or glycine, and voltage independent. Zn2+ potentiation is mimicked by Cd2+, Cu2+, and Ni2+, but not by Mn2+, Co2+, Fe3+, Sn2+, or Hg2+. Our results suggest a possible role for Zn2+ as a positive modulator of NMDA receptors in certain regions of the brain.

Changes in the organization of the neuritic cytoskeleton during nerve growth factor-activated differentiation of PC12 cells: a serial electron microscopic study of the development and control of neurite shape

After exposure to nerve growth factor, PC12 cells differentiate within a period of only a few days into cholinergic sympathetic neurons. Using computer-assisted three-dimensional serial electron microscopic reconstruction, we describe the progressive cytoskeletal and structural changes of PC12 neurites at different stages in their differentiation. Developmental changes in these neurites can be characterized by two major transitions. First, microtubules (MTs), which define the longitudinal axis of the neurite, increase in number leading to a more cylindrical and uniform neurite shape. Second, there are major changes in the relative numbers of other organelle types, which reflect the functional specialization of the neurite. These changes do not in themselves seriously affect shape change of the neurite during development, however the presence of these organelles and their associated obligatory volumes (volumes surrounding organelle) account for well over 50% of the neurite's volume at all stages of development. The MT-MT distances and obligatory volumes associated with the organelles remain constant throughout development. Thus, we can conclude that many of the observed changes seen in developing PC12 neurites are due simply to the production of a greater number of MTs in the cell, and that many of the other important parameters that can be measured and contribute to neurite shape remain constant during development.

Microtubule reassembly from nucleating fragments during the regrowth of amputated neurites

We have proposed that stable microtubule (MT) fragments that resist depolymerization may serve as nucleating elements for the local control of MT dynamics in the axon (Heidemann, S. R., M. A. Hamborg, S. J. Thomas, B. Song, S. Lindley, and D. Chu, 1984, J. Cell Biol., 99:1289-1295). Here we report evidence that supports this proposal in studies on the role of MTs in the regrowth of neurites from the distal segments of amputated chick sensory neurites. Amputated neurites collapse to "beads" of axoplasm that rapidly regrow (Shaw, G., and D. Bray, 1977, Exp. Cell Res., 104:55-62). We examined both unarrested regrowth and regrowth after MT disassembly by either cold (-5 degrees C for 2 h) or nocodazole (0.1 microgram/ml for 15-20 min). In all these cases regrowth occurred at 3.5-4.5 micron/min with no delay times other than the times to reach 37 degrees C or rinse out the nocodazole. Electron micrographs of untreated beads show many MTs of varying lengths, while those of cold- and nocodazole-treated beads show markedly shorter MTs. The robust regrowth of neurites from beads containing only very short MTs argues against unfurling of intact MTs from the bead into the growing neurite. Electron micrographs of cold-treated beads lysed under conditions that cause substantial MT depolymerization in untreated intact neurites show persistent MT fragments similar to those in unlysed cold-treated beads. We interpret this as evidence that the MT fragments in cold-treated beads are somehow distinct from the majority of the MT mass that had depolymerized. Collapsed neurites treated with a higher dose of nocodazole (1.0 microgram/ml for 15-20 min) were completely devoid of MTs and regrew only after a 15-20 min delay in two cases but never regrew in 11 other cases. We found that MTs did not return in beads treated with 1.0 microgram/ml nocodazole even 30 min after removal of the drug. It was unlikely that the inability of these beads to reassemble MTs was due to incomplete removal of nocodazole in that a much higher dose (20 micrograms/ml nocodazole) could be quickly rinsed from intact neurites. Beads treated with 1.0 microgram/ml nocodazole could, however, be stimulated to reassemble MTs and regrow neurites by treatment with taxol. We conclude that the immediate, robust regrowth of neurites from collapsed beads of axoplasm requires MT nucleation sites to support MT reassembly.(ABSTRACT TRUNCATED AT 400 WORDS)

Microtubule assembly in rat brain extracts. Further characterization of the effects of zinc on assembly and cold stability

Microtubule assembly has been studied turbidometrically in supernatant fluids prepared from rat brain by high-speed centrifugation. It was confirmed that assembly occurred in the absence of added GTP. Zinc ions (500 microM, but in the presence of 1 mM EGTA) stimulated assembly under these conditions. Zinc-stimulated assembly produced microtubules with normal characteristics, as judged by electron microscopy, SDS-polyacrylamide gel electrophoresis and inhibition of assembly by fructose-6-phosphate or colchicine. However, microtubules formed in the presence of such zinc concentrations were more stable to cold than controls, although the rate constant for the disassembly reaction was unchanged. Neither the stimulation of assembly by zinc nor the effect on cold stability was affected by trifluoperazine suggesting that a calmodulin-related mechanism is not involved. Microtubule "seeds" had little effect in the presence of zinc, suggesting that it may be acting on the nucleation phase of the assembly reaction. This was supported by the findings that zinc reduced the critical concentration of brain supernatant necessary for assembly and that zinc did not affect the rate constant for assembly. The results suggest zinc can in some way stabilize microtubules; possible mechanisms are discussed.