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Volume 17, Number 15, Issue of August 1, 1997 pp. 5726-5737
Copyright ©1997 Society for Neuroscience

Selective Destruction of Stable Microtubules and Axons by Inhibitors of Protein Serine/Threonine Phosphatases in Cultured Human Neurons (NT2N Cells)

Received April 11, 1997; revised May 8, 1997; accepted May 13, 1997.

Sandra E. Merrick, John Q. Trojanowski, and Virginia M.-Y. Lee

Department of Pathology and Laboratory Medicine, The Center for Neurodegenerative Disease Research, The University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-4283

Paired helical filaments (PHFs) in the neurofibrillary tangles (NFTs) in Alzheimer's disease (AD) brains are composed of highly phosphorylated isoforms of tau (PHFtau) that fail to bind microtubules (MTs), and the levels of MT-binding competent tau are decreased in AD brains with abundant PHFtau. Because this loss of MT binding could compromise the viability of tangle-bearing AD neurons by destabilizing MTs, we asked whether these events could be initiated by inhibiting protein phosphatase 1 (PP1) and PP2A in cultured human neurons (NT2N cells) using okadaic acid (OK) and calyculin-A (CL-A). The treatment of NT2N cells with OK and CL-A increased tau phosphorylation, decreased the binding of tau to MTs, and selectively depolymerized the more stable detyrosinated MTs but not the more labile tyrosinated MTs. Significantly, this led to the rapid degeneration of axons, which are enriched in the more stable detyrosinated MTs, and PP2A was implicated in the initiation of this cascade of events because PP2A but not PP1 was closely associated with MTs in the NT2N cells. These studies imply that inactivation of PP2A in vulnerable neurons of the AD brain may play a mechanistic role in the conversion of normal tau into PHFtau, in the depolymerization of stable MTs, and in the degeneration of axons emanating from tangle-bearing neurons.

Key words: Alzheimer's disease; paired helical filaments; protein phosphatase 1; protein phosphatase 2A; tau; cytoskeleton

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