SELECTIVE DESTRUCTION OF STABLE MICROTUBULES AND AXONS BY INHIBITORS OF PROTEIN SERINE THREONINE PHOSPHATASES IN CULTURED HUMAN NEURONS (NT2N CELLS)/

Paired helical filaments (PHFs) in the neurofibrillary tangles (NFTs) in Alzheimer's disease (AD) brains are composed oi highly phosphorylat ed isoforms of tau (PHFtau) that fail to bind microtubules (MTs), and the levels of MT-binding competent tau are decreased in Aa brains with abundant PHFtau. Because this loss of MT binding could compromise the viability oi tangle-bearing AD neurons by destabilizing MTs, we asked whether these events could be initiated by inhibiting protein phospha tase 1 (PP1) and PP2A in cultured human neurons (NT2N cells) using oka daic acid (OK) and calyculin-A (CL-A). The treatment of NT2N cells wit h OK and GL-A increased iau phosphorylation, decreased the binding of tau to MTs, and selectively depolymerized the more stable detyrosinate d MTs but not the more labile tyrosinated MTs. Significantly, this led to the rapid degeneration of axons, which are enriched in the more st able detyrosinated MTs, and PP2A was implicated in the initiation of t his cascade of events because PP2A but not PP1 was closely associated with MTs in the NT2N cells. These studies imply that inactivation of P P2A in vulnerable neurons of the AD brain may play a mechanistic role in the conversion oi normal tau into PHFtau, in the depolymerization o f stable MTs, and in the degeneration of axons emanating from tangle-b earing neurons.