Relationship between the structures of taxane derivatives and their microtubule polymerization activity.

Paclitaxel (Taxol), one of the most potent anticancer drugs, is a microtubule-stabilizing compound that inhibits microtubule depolymerization within the cell. The structure of paclitaxel is composed of two key elements, a taxane ring and an N-benzoylphenylisoserine side chain at C-13. A number of natural and artificial compounds with taxane skeletons have been isolated, but almost none of their bioactivities have been evaluated. In this study, we focused on compounds having a taxane skeleton structure and examined their effects on tubulin dynamics. Although none of these compounds had an N-benzoylphenylisoserine side chain, three were found to promote tubulin assembly. On the other hand, one compound inhibited tubluin assembly in a way similar to nocodazole. These compounds exhibited novel structure-activity relationships of taxane compounds.

A novel role for hGas7b in microtubular maintenance: possible implication in tau-associated pathology in Alzheimer disease.

Here, we report a novel role for hGas7b (human growth arrest specific protein 7b) in the regulation of microtubules. Using a bioinformatic approach, we studied the actin-binding protein hGas7b with a structural similarity to the WW domain of a peptidyl prolyl cis/trans isomerase, Pin1, that facilitates microtubule assembly. Thus, we have demonstrated that hGas7b binds Tau at the WW motif and that the hGas7b/Tau protein complex interacts with the microtubules, promoting tubulin polymerization. Tau, in turn, contributes to protein stability of hGas7b. Furthermore, we observed decreased levels of hGas7b in the brains from patients with Alzheimer disease. These results suggest an important role for hGas7b in microtubular maintenance and possible implication in Alzheimer disease.

Alzheimer's disease-related protein hGas7b interferes with kinesin motility.

In the previous study, we reported the important properties of hGas7b (i) that binds to phospho-tau and facilitates microtubule polymerization and (ii) the level of hGas7b is very low in the brains of patients with Alzheimer's disease. These results led us to study the function of hGas7b in detail. We focused on the effect of hGas7b on microtubule dynamics in the absence of tau, on the assumption of healthy tau decrease in the brains of Alzheimer's disease. hGas7b binds to microtubule directly without tau, although this binding does not enhance microtubule polymerization. Excess hGas7b interferes with kinesin motility on microtubules. These results suggest that regulation to maintain an appropriate concentration of hGas7b is required for healthy neurotransmission.

Direct optical microscopic observation of the microtubule polymerization intermediate sheet structure in the presence of gas7.

The process of microtubule elongation is thought to consist of two stages-formation of a tubulin sheet structure and its closure into a tube. However, real-time observation of this process has been difficult. Here, by utilizing phospho-tau binding protein Gas7 (growth-arrest-specific protein 7), we visualized the polymer transformation process by dark-field microscopy. Upon elongation, thin and flexible structures, often similar to a curved hook, appeared at the end of microtubules. Electron microscopic observations supported the idea that these flexible structures are tubulin sheets. They maintained their length until they gradually became thick and rigid beginning in the central portion, resulting in straight microtubules. In the absence of Gas7, the sheet-like structure was rarely observed; moreover, when observed, it was fragile and engaged in typical dynamic instability. With Gas7, no catastrophe was observed. These results suggest that Gas7 enhances microtubule polymerization by stabilizing sheet intermediates and is a useful tool for analyzing microtubule transformation.