Selective destruction of stable microtubules and axons by inhibitors of protein serine/threonine phosphatases in cultured human neurons.

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.

Site-specific dephosphorylation of tau protein at Ser202/Thr205 in response to microtubule depolymerization in cultured human neurons involves protein phosphatase 2A.

Tau proteins isolated from paired helical filaments, the major building blocks of Alzheimer's disease neurofibrillary tangle, are abnormally phosphorylated and unable to bind microtubules. To examine the dynamics of tau phosphorylation and to identify specific tau phosphorylation sites involved in the stabilization of microtubules, we treated cultured postmitotic neuron-like cells (NT2N) derived from a human teratocarcinoma cell line (NTera2/D1) with drugs that depolymerize microtubules (i.e. colchicine or nocodazole). This led to the recovery of dephosphorylated tau from the NT2N cells as monitored by a relative increase in the electrophoretic mobility of tau and an increase in the turnover of [32P]PO4-labeled tau. However, not all phosphorylation sites on tau are affected by colchicine or nocodazole. Ser202/Thr205 appears to be completely and specifically dephosphorylated by protein phosphatase 2A since this dephosphorylation was blocked by inhibitors of protein phosphatase 2A but not by inhibitors of protein phosphatase 2B. These findings, together with the recent observation that protein phosphatase 2A is normally bound to microtubules in intact cells, suggest that the polymerization state of microtubules could modulate the phosphorylation state of tau at specific sites in the normal and Alzheimer's disease brain.

Glial cells of the lamprey nervous system contain keratin-like proteins.

Lamprey axons regenerate following spinal cord transection despite the formation of a glial scar. As we were unable to detect a lamprey homologue of glial fibrillary acidic protein (GFAP), a major constituent of astrocytes, we studied the composition of intermediate filament (IF) proteins of lamprey glia. Monoclonal antibodies (mAbs) were raised to lamprey spinal cord cytoskeletal extracts and these mAbs were characterized by using Western blotting and immunocytochemistry. On two-dimensional (2-D) Western blots, five of the mAbs detected three major IF polypeptides in the molecular weight (MW) range of 45-56 kD. Further studies were conducted to determine the relationship between the lamprey glial-specific antigen and other mammalian IF proteins. Antikeratin 8 antibody recognized two of the three polypeptides. Several of the glial-specific mAbs reacted with human keratins 8 and 18 on Western blots. Keratin-like immunoreactivity was found in all parts of the central and peripheral nervous systems in both larval and adult lampreys. The immunocytochemical staining patterns of glial-specific mAbs were indistinguishable on lamprey spinal cord sections. However, on brain sections, two distinct patterns were observed. A subset of mAbs stained only a few glial fibers in the brain, whereas others stained many more brain glia, particularly the ependymal cells. The former group of mAbs recognized only the two lower MW polypeptides on 2-D Western blots, but the latter group of mAbs recognized all three major IF polypeptides. This correlation is supported by the observation that the highest MW IF polypeptide has an increased level of expression in the brain relative to the spinal cord. Thus, in the lamprey, the glial cells of both spinal cord and brain express molecules similar to simple epithelial cytokeratins, but their IFs may contain these keratins in different stoichiometric proportions. The widespread presence in the lamprey of primitive glial cells containing keratin-like intermediate filaments may have significance for the extraordinary ability of lamprey spinal axons to regenerate.

Abnormal tau phosphorylation at Ser396 in Alzheimer's disease recapitulates development and contributes to reduced microtubule binding.

Abnormally phosphorylated tau proteins (A68) are the building blocks of Alzheimer's disease (AD) paired helical filaments. The biological consequences of the conversion of normal adult tau to A68 remain unknown. Here we demonstrate that native A68 does not bind to microtubules (MTs), yet dephosphorylated A68 regains the ability to bind to MTs. Ser396 is phosphorylated in A68, but not in normal adult tau, whereas fetal tau is phosphorylated transiently at this site. Phosphorylation of tau at Ser396 by protein kinases in CHO cells and rat brain produces an electrophoretic mobility similar to that of A68. Using CHO cells transfected with an Ala396 mutant, we show that the phosphorylation of tau at Ser396 reduces its affinity for MTs and its ability to stabilize MTs against nocodazole-induced depolymerization. Our results demonstrate that the abnormal phosphorylation of tau in AD involves Ser396, and we suggest that this may be mediated by the inappropriate activation of fetal kinases or the reduced activity of tau protein phosphatases. Thus, phosphorylation of Ser396 may destabilize MTs in AD, resulting in the degeneration of affected cells.

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