REVIEW: tau protein pathology in Alzheimer's disease and related disorders

Abundant neurofibrillary lesions made of hyperphosphorylated microtubule-associated protein tau constitute one of the defining neuropathological features of Alzheimer's disease. However, tau containing filamentous inclusions in neurones and/or glial cells also define a number of other neurodegenerative disorders clinically characterized by dementia and/or motor syndromes. All these disorders, therefore, are grouped under the generic term of tauopathies. In the first part of this review we outline the morphological and biochemical features of some major tauopathies, e. g. Alzheimer's disease, argyrophilic grain disease, Pick's disease, progressive supranuclear palsy and corticobasal degeneration. The impact of the recent finding of tau gene mutations in familial frontotemporal dementia and parkinsonism linked to chromosome 17 on other tauopathies is discussed in the second part. The review closes with a look towards a new understanding of neurodegenerative disorders characterized by filamentous nerve cell inclusions. The recent identification of the major protein component of their respective inclusions led to a surprising convergence of seemingly unrelated disorders. The new findings now allow us to classify neurodegenerative disorders with filamentous nerve cell inclusions into four main categories: (i) the tauopathies; (ii) the alpha-synucleinopathies; (iii) the polyglutamine disorders; and (iv) the iquitin disorders'. Within the proposed classification scheme, tauopathies constitute the most frequent type of disorder.     

Relationship between microtubule-binding repeats and morphology of neurofibrillary tangle in Alzheimer's disease

OBJECTIVE: The present study was performed to compare the distributions of three-repeat (3R) and four-repeat (4R) neurofibrillary tangles (NFT) with those of pretangles (p-NFT), intracellular NFT (i-NFT), and extracellular NFT (e-NFT) in the hippocampus of Alzheimer's disease brains. METHODS: NFT labeling was performed using anti-tau antibodies: pSer262 for p-NFT, pSer422 for i-NFT, AT8 for e-NFT, RD3 for 3R, and RD4 for 4R tau, and Gallyas impregnation for the NFT population. RD4- and pSer422-positive NFT were detected predominantly in sectors from CA2 to CA4, while RD3- and pSer262-positive NFT were predominantly present in CA1, the entorhinal cortex, and the subiculum. The tau epitope recognized by pSer262 belongs to 4R tau but it showed a strong correlation with RD3- and AT8-positive NFT. CONCLUSIONS: Sectors CA2-CA4 showed predominantly 4R-NFT containing the pSer422 epitope. pSer262 may detect the process of transformation from p-NFT to i-NFT, and e-NFT consisted predominantly of 3R tau.     

14-3-3 protein beta isoform is associated with 3-repeat tau neurofibrillary tangles in Alzheimer's disease

14-3-3 proteins play roles in phosphorylation of tau proteins in neurofibrillary tangles (NFT) in Alzheimer's disease (AD). Tau is phosphorylated at serine (pSer) and threonine (pThr) in NFT, and NFT morphology varies according to phosphorylated sites and tau isoform. The roles of 14-3-3 proteins in NFT morphology remain unknown. This study was performed to examine the relationships between 14 and 3-3 proteins and tau phosphorylation of NFT. NFT were labeled with Gallyas impregnation, tau and 14-3-3 immunohistochemistry in paraffin-embedded hippocampal sections from seven AD and three control brains. Anti-tau antisera included monoclonal antisera that recognize pSer262 (pSer262), pSer422 (pSer422), pSer202/pThr205 (AT8), Thr231 (AT180), three-repeat (RD3) and four-repeat (RD4) tau isoform. Anti-14-3-3 protein isoform antisera included polyclonal antisera to beta, gamma, zeta, epsilon, tau, mu and sigma isoforms and monoclonal antiserum to beta antiserum (H8-beta). NFT density was obtained by counting labeled NFT in cornu ammonis (CA) 1-CA4, subiculum and entorhinal cortex. H8-beta and zeta isoforms were strongly expressed in NFT. Regional densities of NFT positive for pSer262, AT8, AT180, and Gallyas impregnation were similar to RD3-positive NFT density with high densities in CA1 and entorhinal cortex. NFT positive for pSer422 showed a similar regional distribution to RD4-positive NFT with high NFT density in CA2-CA4. H8-beta-positive NFT showed a similar regional distribution to RD3-positive NFT. In contrast, zeta isoform-positive NFT showed no specific distribution. In conclusion, H8-beta isoform is associated with development of 3-repeats NFT but a role of 14-3-3 zeta isoform in NFT could not be specified.     

PP2B isolated from human brain preferentially dephosphorylates Ser-262 and Ser-396 of the Alzheimer disease abnormally hyperphosphorylated tau

Summary. PP2B is one of the major serine/threonine phosphatases in the brain. We quantitated the dephosphorylation of various sites of Alzheimer disease abnormally hyperphosphorylated tau by PP2B purified from six (three Alzheimer and three control) autopsied human brains. The purified PP2B was essentially homogenous holoenzyme as determined by SDS-PAGE, Western blot analyses and biochemical characterization. Purified PP2B from all six brains efficiently dephosphorylated ³²P-tau with specific activities ranging from 684–1286 pmol ³²Pi/mg/min. Estimated by dot-blot analyses, the purified PP2B (on average from six brains) dephosphorylated Alzheimer tau at pS199, pT217, pS262, pS396 and pS422 by 38%, 32%, 63%, 78%, and 32%, respectively. Dephosphorylation of tau at pT181, pS202, pT205, pT212, pS214, and pS404 by PP2B was undetectable. The preferential dephosphorylation of Ser262 and Ser396 by PP2B suggests a possible involvement of this phosphatase in Alzheimer neurofibrillary degeneration.     

Immunohistochemical detection of tau protein in various non-human animal brains

In order to clarify the tau phosphorylation in aged non-human animal brains, paraffin sections of human and non-human animal brains were examined immunohistochemically using two antibodies against tau. The monoclonal antibody (mAb) tau-2 is known to bind non-phosphorylated and phosphorylated forms of tau protein and to recognize neurofibrillary tangles (NFT) in the human brain, whereas rabbit antiserum against tau recognizes the protein in a wide variety of mammalian species. In this study, both antibodies recognized axons, neurites, and cytoplasm of neuronal cells together with oligodendrocytes in non-human animal brains as reported in human brains, suggesting that the distribution of normal tau is quite similar between human and non-human animal brains. Neurofibrillary tangles detected by the Gallyas method were not found in non-human animal brains. Tau-2 intensely labeled NFT in the human brain and neurons of brain stem nuclei in the canine brain. The results may suggest very early stage of abnormal tau phosphorylation leading to NFT formation in the aged canine brain. It is probable that phosphorylation develops over a long period of time (more than 25–30 years) and as a result abnormal tau becomes apparent in humans because they have a long life-span in comparison with most non-human animals.     

Neuropathological diagnostic criteria for Alzheimer's disease

Neuropathological diagnostic criteria for Alzheimer's disease (AD) are based on tau‐related pathology: NFT or neuritic plaques (NP). The Consortium to Establish a Registry for Alzheimer's disease (CERAD) criterion evaluates the highest density of neocortical NP from 0 (none) to C (abundant). Clinical documentation of dementia and NP stage A in younger cases, B in young old cases and C in older cases fulfils the criterion of AD. The CERAD criterion is most frequently used in clinical outcome studies because of its inclusion of clinical information. Braak and Braak's criterion evaluates the density and distribution of NFT and classifies them into: I/II, entorhinal; III/IV, limbic; and V/VI, neocortical stage. These three stages correspond to normal cognition, cognitive impairment and dementia, respectively. As Braak's criterion is based on morphological evaluation of the brain alone, this criterion is usually adopted in the research setting. The National Institute for Aging and Ronald and Nancy Reagan Institute of the Alzheimer's Association criterion combines these two criteria and categorizes cases into NFT V/VI and NP C, NFT III/IV and NP B, and NFT I/II and NP A, corresponding to high, middle and low probability of AD, respectively. As most AD cases in the aged population are categorized into Braak tangle stage IV and CERAD stage C, the usefulness of this criterion has not yet been determined. The combination of Braak's NFT stage equal to or above IV and Braak's senile plaque Stage C provides, arguably, the highest sensitivity and specificity. In future, the criteria should include in vivo dynamic neuropathological data, including 3D MRI, PET scan and CSF biomarkers, as well as more sensitive and specific immunohistochemical and immunochemical grading of AD.     

Expression of glia maturation factor in neuropathological lesions of Alzheimer's disease

R. Thangavel, D. Stolmeier, X. Yang, P. Anantharam and A. Zaheer (2012) Neuropathology and Applied Neurobiology 38, 572–581 Expression of glia maturation factor in neuropathological lesions of Alzheimer's disease Aims: The pathology of Alzheimer's disease (AD) is characterized by the presence of amyloid plaques (APs), neurofibrillary tangles (NFTs), degenerating neurones, and an abundance of reactive astrocytes and microglia. We aim to examine the association between glia maturation factor (GMF) expression, activated astrocytes/microglia, APs and NFTs in AD‐affected brain regions. Methods: Brain sections were stained with Thioflavin‐S to study AD pathology and sequentially immunolabeled with antibodies against GMF, glial fibrillary acidic protein (marker for reactive astrocytes), and Ionized calcium binding adaptor molecule 1 (Iba‐1, marker for activated microglia) followed by visualization with avidin‐biotin peroxidase complex. Results: Our double immunofluorescence labelling with cell‐specific markers demonstrated the glial localization of GMF. The immunohistochemical data showed that APs and NFTs are associated with increased expression of GMF in reactive glia of AD brains compared with non‐AD controls. Conclusions: This is the first report that shows GMF, a mediator of central nervous system inflammation, is expressed in the brain regions affected in AD and that GMF is mainly localized in reactive astrocytes surrounding APs/NFTs. The distribution of GMF‐immunoreactive cells in and around Thioflavin‐S stained APs and NFTs suggests involvement of GMF in inflammatory responses through reactive glia and a role of GMF in AD pathology.     

Relationship of neurofibrillary pathology to cerebral amyloid angiopathy in Alzheimer's disease

Over 90% of patients with Alzheimer's disease (AD) develop cerebral amyloid angiopathy (CAA). Severe dyshoric CAA, in which amyloid extends into the surrounding brain parenchyma, may be associated with adjacent clustering of tau-immunopositive neurites but the relationship of CAA to neurofibrillary pathology has not been systematically investigated. In the present study this relationship was examined in sections of frontal, temporal and parietal cortex from 25 AD patients with moderate to severe CAA and 26 with mild or absent CAA. We measured immunolabelling of abnormally phosphorylated tau adjacent to A beta-laden and non-A beta-laden arteries and arterioles, and in cortex away from arteries and arterioles. We also analysed the possible influence of APOE genotype on these measurements. There were no significant differences between the lobes in measurements of tau labelling, either around blood vessels or elsewhere in the cortex. However, tau labelling around A beta-laden arteries and arterioles significantly exceeded that around non-A beta-laden blood vessels (P<0.001) and this, in turn was greater than the labelling of cortex away from blood vessels (P<0.001). There was no association between APOE epsilon 4 and the immunolabelling density for tau, whether around amyloid- or non-amyloid-laden arteries and arterioles, or in the cerebral cortex away from these. We propose that both CAA and peri-vascular accumulation of hyperphosphorylated tau may be a consequence of elevated levels of soluble A beta around cortical arteries and arterioles.     

Monoclonal antibody PHF-9 recognizes phosphorylated serine 404 of tau protein and labels paired helical filaments

Paired helical filaments (PHFs) purified from Alzheimer's brain consist of hyperphosphorylated microtubule-associated protein tau. In PHF, phosphorylation occurs at ser/thr tau residues. Several of these ser/thr phosphorylation sites lie immediately C-terminal to the tau tubulin binding domain. The C-terminal ser³⁹⁶ to thr⁴¹³ tau region contains two or more phosphorylated residues and eight possible ser/thr phosphorylation sites. Immunologic studies and mass spectroscopy have identified ser³⁹⁶ as one of the phosphorylation sites but identification of more C-terminal phosphorylated residues has been hampered by the lack of monoclonal antibodies (Mabs) that recognize defined epitopes in this region. We have raised Mabs against PHF purified from Alzheimer's brain. One of these Mabs, PHF-9, showed phosphorylation-dependent binding to purified PHF and recognized a phosphorylated epitope in the C-terminal portion of cyanogen bromide-digested PHF. Epitope mapping studies employing synthetic tau phosphopeptides indicated that PHF-9 labeled a 13-mer tau peptide phosphorylated at ser⁴⁰⁴ but not the corresponding non-phosphorylated peptide. PHF-9 demonstrated no immunoreactivity with a synthetic peptide phosphorylated at ser³⁹⁶ indicating that the PHF-9 epitope is C-terminal to ser³⁹⁶. In conclusion, the present study describes a Mab, PHF-9, which recognizes phosphorylated ser⁴⁰⁴ of tau independently of phosphorylated ser³⁹⁶ and indicates that tau ser⁴⁰⁴ is phosphorylated in PHF. © 1996 Wiley-Liss, Inc.     

p38 mitogen activated protein kinase as a therapeutic target for Alzheimer's disease

The characteristic pathological hallmarks of Alzheimer’s disease (AD) include neuritic plaques, neurofibrillary tangles, and inflammatory changes. Current therapies, such as molecules that target enhancing cholinergic activity, can improve cognitive function in the short term but, unfortunately, have no impact on progression of the disease. Although many molecular targets have been suggested to play a causative role in AD progression, clinical data demonstrating a link between the blockade of such targets and amelioration or halting of disease progression are lacking. Even so, there are many interesting candidate targets, and current research efforts in these areas promises to deliver a wealth of new possibilities for treating AD in the future. This brief review will focus on p38 mitogen-activated protein kinase as a possible target for therapeutic intervention in AD.     

Alzheimer disease hyperphosphorylated tau aggregates hydrophobically

The chemical interaction that condenses the hyperphosphorylated protein tau in Alzheimer's disease (AD P-tau) into neurofibrillary tangles and cripples synaptic transmission remains unknown. Only β-sheet, positive ion salt bridges between phosphates, and hydrophobic association can create tangles of just AD P-tau. We have correlated transmission electron microscope (TEM) images of tau aggregation with different percentages of β-sheet in aqueous suspensions of tau while using buffers that block dispositive or tripositive ionic bridges between intermolecular phosphates. Circular dichroism (CD) studies were performed at different temperatures from 5–85°C using AD P-tau, AD P-tau dephosphorylated with hydrofluoric acid (HF AD P-tau) or alkaline phosphatase (AP AD P-tau), and recombinant human tau with 3-repeats and two amino terminal inserts (R-39) and using bovine tau (B tau) isolated without heat or acid treatment. Secondary structure was estimated from CD spectra at 5°C using the Lincomb algorithm. Each preparation except one demonstrated an inverse temperature transition, T_i, in the CD at 197 nm. No correlation was found between β-sheet content and aggregation, leaving only hydrophobic interaction as the remaining possibility. Thirteen of 21 possible phophorylation sites in AD P-tau lie adjacent to positive residues in tau's primary structure. Occupation of five to nine phosphate sites on AD P-tau appears sufficient to reduce or neutralize tau's basic character. AD P-tau's hydrophobic character is indicated by its low inverse temperature transition, T_i. The T_i for AD P-tau was 24.5°C or 28°C, whereas for B tau with three phosphates it was 32°C, for unphosphorylated tau R-39 it was 38°C, and for dephosphorylated HF AD P-tau it was 37.5°C. The hydrophobic protein elastin and its analogs coalesce and precipitate at their T_i of 24–29°C, well below body temperature. We hypothesize that AD P-tau causes tangle accumulation by this mechanism. Synapse 27:208–229, 1997. © 1997 Wiley-Liss, Inc.     

Developmental exposure to lead (Pb) alters the expression of the human tau gene and its products in a transgenic animal model

Tauopathies are a class of neurodegenerative diseases associated with the pathological aggregation of the tau protein in the human brain. The best known of these illnesses is Alzheimer’s disease (AD); a disease where the microtubule associated protein tau (MAPT) becomes hyperphosphorylated (lowering its binding affinity to microtubules) and aggregates within neurons in the form of neurofibrillary tangles (NFTs). In this paper we examine whether environmental factors play a significant role in tau pathogenesis. Our studies were conducted in a double mutant mouse model that expressed the human tau gene and lacked the gene for murine tau. The human tau mouse model was tested for the transgene’s ability to respond to an environmental toxicant. Pups were developmentally exposed to lead (Pb) from postnatal day (PND) 1-20 with 0.2% Pb acetate. Mice were then sacrificed at PND 20, 30, 40 and 60. Protein and mRNA levels for tau and CDK5 as well as tau phosphorylation at Ser396 were determined. In addition, the potential role of miRNA in tau expression was investigated by measuring levels of miR-34c, a miRNA that targets the mRNA for human tau, at PND20 and 50. The expression of the human tau transgene was altered by developmental exposure to Pb. This exposure also altered the expression of miR-34c. Our findings are the first of their kind to test the responsiveness of the human tau gene to an environmental toxicant and to examine an epigenetic mechanism that may be involved in the regulation of this gene’s expression.     

Calcium channel blockers and Alzheimer’s disease

Alzheimer’s disease is characterized by two pathological hallmarks: amyloid plaques and neurofi-brillary tangles. In addition, calcium homeostasis is disrupted in the course of human aging. Recent research shows that dense plaques can cause functional alteration of calcium signals in mice with Alzheimer’s disease. Calcium channel blockers are effective therapeutics for treating Alzheimer’s disease. This review provides an overview of the current research of calcium channel blockers in-volved in Alzheimer’s disease therapy.     

Colocalization of phosphorylated forms of WAVE1, CRMP2, and tau in Alzheimer's disease model mice: Involvement of Cdk5 phosphorylation and the effect of ATRA treatment

Alzheimer's disease (AD) is the most common type of dementia among the elderly. Neurofibrillary tangles (NFTs), a major pathological hallmark of AD, are composed of tau protein that is hyperphosphorylated by cyclin‐dependent kinase 5 (Cdk5) and glycogen synthase kinase 3β (GSK3β). NFTs also contain Wiskott‐Aldrich syndrome protein family verprolin‐homologous protein 1 (WAVE1) and collapsin response‐mediator protein 2 (CRMP2). Although Cdk5 is known to phosphorylate tau, WAVE1, and CRMP2, the significance of this with respect to NFT formation remains to be elucidated. This study examines the involvement of phosphorylated (p‐) CRMP2 and WAVE1 in p‐tau aggregates using a triple‐transgenic (3×Tg; APPswe/PS1M146V/tauP301L) AD mouse model. First, we verified the colocalization of p‐WAVE1 and p‐CRMP2 with aggregated hyperphosphorylated tau in the hippocampus at 23 months of age. Biochemical analysis revealed the inclusion of p‐WAVE1, p‐CRMP2, and tau in the sarkosyl‐insoluble fractions of hippocampal homogenates. To test the significance of phosphorylation of these proteins further, we administered all‐trans‐retinoic acid (ATRA) to the 3×Tg mice, which downregulates Cdk5 and GSK3β activity. In ATRA‐treated mice, fewer and smaller tau aggregates were observed compared with non‐ATRA‐treated mice. These results suggest the possibility of novel therapeutic target molecules for preventing tau pathology. © 2015 Wiley Periodicals, Inc.     

Differential expression of phosphorylated translation initiation factor 2 alpha in Alzheimer's disease and Creutzfeldt-Jakob's disease

Studies in vitro have shown that phosphorylated translation initiation factor 2 alpha (TIF 2 alpha) may have several functions, including regulation of protein synthesis, control of cell death and procurement of resistance to oxidative stress in nerve cells. These properties may have implications in certain human neurodegenerative diseases, such as Alzheimer's disease (AD) and Creutzfeldt-Jakob's disease (CJD), in which oxidative stress appears to be involved in the process of neurodegeneration and neurone death. Single and double-labelling immunohistochemistry to phosphorylated TIF 2 alpha, phosphorylated SAPK/JNK, phosphorylated p38, tau, Cu/Zn superoxide dismutase 1 (SOD 1) and cleaved caspase-3 (17 kDa), and in situ end-labelling of nuclear DNA fragmentation, was carried out in postmortem samples of 10 patients with AD (stages III and VI of Braak and Braak), seven patients with CJD (five cases with methionine/methionine and two cases with methionine/valine at the codon 129 of the PrP gene) and eight age-matched controls. No phosphorylated TIF 2 alpha immunoreactivity was found in control brains, but strong phosphorylated TIF 2 alpha expression was observed in subpopulations of neurones bearing neurofibrillary tangles (NFTs) or pretangles in the hippocampus, entorhinal cortex and isocortex in AD. Phosphorylated TIF 2 alpha is restricted to neurones with abnormal tau deposition, but only approximately 80% of neurones with NFTs in the hippocampus and 60% in the isocortex colocalize phosphorylated TIF 2 alpha, thus indicating that not all neurones with NFTs over-express phosphorylated TIF 2 alpha. Moreover, phosphorylated TIF 2 alpha immunoreactivity was found in a percentage of neurones expressing phosphorylated SAPK/JNK and p38, which, in turn, are involved in tau phosphorylation in AD. However, dystrophic neurites of senile plaques that contain abnormal tau and express SOD 1 are negative to antiphosphorylated TIF 2 alpha antibodies. Smooth muscle cells in blood vessels affected by amyloid angiopathy, which are putative targets of beta A 4 amyloid-derived oxidative stress, are not associated with phosphorylated TIF 2 alpha immunoreactivity. Double-staining with the method of in situ end-labelling of nuclear DNA fragmentation demonstrated no relationship between phosphorylated TIF 2 alpha expression and increased nuclear DNA vulnerability in individual cells. Moreover, no single caspase-3-immunoreactive cell in AD expressed phosphorylated TIF 2 alpha. Oxidative stress response, manifested as positive SOD 1 expression in Bergmann glia and in a few reactive astrocytes, has been demonstrated in CJD. No phosphorylated SAPK/JNK or phosphorylated p38 kinase immunoreactivity was observed in these cases. Moreover, neurones and glial cells do not over-express phosphorylated TIF 2 alpha in CJD. The present results demonstrate selective expression of phosphorylated TIF 2 alpha in subpopulations of nerve cells with abnormal tau deposition, and suggest that factors linked with tau deposition regulate protein synthesis throughout TIF 2 alpha phosphorylation in certain neurones sensitive to oxidative stress in AD.     

Concurrent alterations of O-GlcNAcylation and phosphorylation of tau in mouse brains during fasting

Impaired brain glucose uptake/metabolism precedes the symptoms of Alzheimer disease (AD) and is likely to play a role in the development of the disease, but the mechanism by which it contributes to AD is not understood. Because glucose uptake/metabolism regulates protein O-GlcNAcylation, and the latter modulates phosphorylation of tau inversely, we investigated, in fasting Kunming mice, whether impaired brain glucose uptake/metabolism causes abnormal hyperphosphorylation of tau and, consequently, facilitates the neurofibrillary degeneration of AD via down-regulation of tau O-GlcNAcylation. We found that fasting caused decreased tau O-GlcNAcylation and concurrent hyperphosphorylation of tau at most of the phosphorylation sites studied. The hippocampus was found more vulnerable to the tau alterations than the cerebral cortex, which is consistent with the fact that it is the hippocampus that is first affected in AD. Furthermore, hyperphosphorylation of tau induced by fasting was reversible in the brain after re-feeding. These findings provide a novel mechanism explaining how impaired brain glucose uptake/metabolism contributes to AD and suggest that it may be feasible to treat AD by reversing the abnormal hyperphosphorylation of tau at early stages of the disease.     

Differential expression of exons 10 and 11 in normal tau and tau associated with paired helical filaments

Antibodies were raised to two synthetic peptides with amino acid sequences encoded by a variable region of exons 10 and 11 of the tau gene. The affinity-purified antibodies, designated E-10 and E-11, were used to determine whether PHF-tau and normal tau differ in variants containing three or four repeats in the microtubule-binding domain, respectively. Normal adult human brain was shown by gel electrophoresis to contain six isoforms of tau. All of the isoforms reacted with E-11, whereas only four of them with slower electrophoretic mobility were recognized by E-10. Fetal brain tau was readily recognized by E-11 but reacted poorly with E-10. In PHF preparations, E-11 bound to all three polypeptides of PHF-tau of 68 kD, 64 kD, and 60 kD and reacted intensely with a material smearing from the top of the gel to about the 50-kD region. In contrast, E-10 only weakly recognized the two higher molecular weight PHF-tau polypeptides of 68 kD and 64 kD, as well as smeared material, and the binding was not affected by phosphatase treatment. Using recombinant tau with four repeats as a reference, the immunoreactivity of E-10 with PHF-tau was estimated to be approximately 5% of that of E-11. By comparison, the immunoreactivity of E-10 with four isoforms of normal tau was comparable to that of E-11. These results indicate that the ratio of three vs. four repeat variants in PHF-tau is higher than in normal tau and suggest that Alzheimer disease may be associated with the disproportional expression of fetal (or juvenile) forms of tau. Alternatively, the weak reactivity of PHF-tau with E-10 antibody could be due to post-translational modifications other than phosphorylation. © 1995 Wiley-Liss, Inc.