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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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