Characterization of two pathological tau protein, variants in Alzheimer brain cortices

Reduced Tau protein expression is associated with frontotemporal degeneration with progranulin mutation

Reduction of Tau protein expression was described in 2003 by Zhukareva et al. in a variant of frontotemporal lobar degeneration (FTLD) referred to as diagnosis of dementia lacking distinctive histopathology, then re-classified as FTLD with ubiquitin inclusions. However, the analysis of Tau expression in FTLD has not been reconsidered since then. Knowledge of the molecular basis of protein aggregates and genes that are mutated in the FTLD spectrum would enable to determine whether the “Tau-less” is a separate pathological entity or if it belongs to an existing subclass of FTLD. To address this question, we have analyzed Tau expression in the frontal brain areas from control, Alzheimer’s disease and FTLD cases, including FTLD- Tau (MAPT), FTLD-TDP (sporadic, FTLD-TDP-GRN, FTLD-TDP-C9ORF72) and sporadic FTLD-FUS, using western blot and 2D-DIGE (Two-Dimensional fluorescence Difference Gel Electrophoresis) approaches. Surprisingly, we found that most of the FTLD-TDP-GRN brains are characterized by a huge reduction of Tau protein expression without any decrease in Tau mRNA levels. Interestingly, only cases affected by point mutations, rather than cases with total deletion of one GRN allele, seem to be affected by this reduction of Tau protein expression. Moreover, proteomic analysis highlighted correlations between reduced Tau protein level, synaptic impairment and massive reactive astrogliosis in these FTLD-GRN cases. Consistent with a recent study, our data also bring new insights regarding the role of progranulin in neurodegeneration by suggesting its involvement in lysosome and synaptic regulation. Together, our results demonstrate a strong association between progranulin deficiency and reduction of Tau protein expression that could lead to severe neuronal and glial dysfunctions. Our study also indicates that this FTLD-TDP-GRN subgroup could be part as a distinct entity of FTLD classification.

The Neurofibrillary Pathology of Alzheimer's Disease

Abundant neurofibrillary lesions in certain brain regions constitute one of the defining neuropathological characteristics of Alzheimer's disease, where their presence correlates with the degree of dementia. An understanding of the mechanisms that lead to the neurofibrillary pathology is critical for elucidating the pathogenesis of Alzheimer's disease and for developing effective therapeutic strategies. Neurofibrillary lesions consist of neurofibrillary tangles, neuropil threads, and abnormal neurites. Ultrastructurally, each of these lesions consists of abnormal paired helical and straight filaments. These filaments are made of the six brain isoforms of microtubule-associated protein tau in a hyperphosphorylated and an abnormally phosphorylated state. Several candidate protein kinases and protein phosphatases for the hyperphos phorylation of tau have been identified. Moreover, recent results suggest that an interaction between tau protein and sulfated glycosaminoglycans may play an important role in inducing both the hyperphosphor ylation of tau and the formation of paired helical and straight filaments. NEUROSCIENTIST 3:131-141, 1997

Beta-amyloidolysis and glutathione in Alzheimer's disease

In this review, we hypothesized the importance of the interaction between the brain glutathione (GSH) system, the proteolytic tissue plasminogen activator (t-PA)/plasminogen/ plasmin system, regulated by plasminogen activator inhibitor (PAI-1), and neuroserpin in the pathogenesis of Alzheimer’s disease. The histopathological characteristic hallmark that gives personality to the diagnosis of Alzheimer’s disease is the accumulation of neurofibroid tangles located intracellularly in the brain, such as the protein tau and extracellular senile plaques made primarily of amyloidal substance. These formations of complex etiology are intimately related to GSH, brain protective antioxidants, and the proteolytic system, in which t-PA plays a key role. There is scientific evidence that suggests a relationship between aging, a number of neurodegenerative disorders, and the excessive production of reactive oxygen species and accompanying decreased brain proteolysis. The plasminogen system in the brain is an essential proteolytic mechanism that effectively degrades amyloid peptides (“beta-amyloidolysis”) through action of the plasmin, and this physiologic process may be considered to be a means of prevention of neurodegenerative disorders. In parallel to the decrease in GSH levels seen in aging, there is also a decrease in plasmin brain activity and a progressive decrease of t-PA activity, caused by a decrease in the expression of the t-PA together with an increase of the PAI-1 levels, which rise to an increment in the production of amyloid peptides and a lesser clearance of them. Better knowledge of the GSH mechanism and cerebral proteolysis will allow us to hypothesize about therapeutic practices.

Gender effect on the accumulation of hyperphosphorylated tau in the brain of locus-ceruleus-injured APP-transgenic mouse

Locus ceruleus (LC) neurons are preferentially and initially affected in Alzheimer disease (AD); however, the impact of the loss of LC neurons on the pathological sequence of AD, including amyloid beta-protein (Abeta) deposition and neurofibrillary tangle formation, has not been elucidated. In this study, we chemically injured LC neurons of the brains of familial AD-related amyloid precursor protein (APP)-transgenic mice using the LC-noradrenergic neuron-selective neurotoxin, N-(2-chloroethyl)-N-ethyl-bromo-benzylamine (DSP4). The levels of noradrenaline significantly decreased in the cerebral cortices of DSP4-treated mice. The deposition of amyloid fibrils was biochemically observed in the APP-transgenic mouse brains; however, those levels were not significantly altered following DSP4 treatment. In contrast, the levels of accumulated hyperphosphorylated tau markedly increased in the cerebral cortices of DSP4-treated female but not male APP-transgenic mice. Our results suggest that innervation from LC neurons and testosterone secretion are potent and mutually independent suppressors of amyloid-related accumulation of hyperphosphorylated tau in the brain.

Hereditary frontotemporal dementia caused by Tau gene mutations

Tau protein is involved in microtubule assembly and stabilization. Filamentous deposits made of tau constitute a defining characteristic of several neurodegenerative diseases. The relevance of tau dysfunction for neurodegeneration has been clarified through the identification of mutations in the Tau gene in cases with frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). Although the mechanisms by which these mutations lead to nerve cell death are only incompletely understood, it is clear that they cause the formation of tau filaments with distinct morphologies and isoform compositions. The range of tau pathology identified in FTDP-17 recapitulates that in sporadic tauopathies, indicating a major role for tau dysfunction in these diseases.

Characteristics of two distinct clinical phenotypes in pathologically proven progressive supranuclear palsy: Richardson's syndrome and PSP-parkinsonism

The clinical diagnosis of progressive supranuclear palsy (PSP) relies on the identification of characteristic signs and symptoms. A proportion of pathologically diagnosed cases do not develop these classic features, prove difficult to diagnose during life and are considered as atypical PSP. The aim of this study was to examine the apparent clinical dichotomy between typical and atypical PSP, and to compare the biochemical and genetic characteristics of these groups. In 103 consecutive cases of pathologically confirmed PSP, we have identified two clinical phenotypes by factor analysis which we have named Richardson's syndrome (RS) and PSP-parkinsonism (PSP-P). Cases of RS syndrome made up 54% of all cases, and were characterized by the early onset of postural instability and falls, supranuclear vertical gaze palsy and cognitive dysfunction. A second group of 33 (32%) were characterized by asymmetric onset, tremor, a moderate initial therapeutic response to levodopa and were frequently confused with Parkinson's disease (PSP-P). Fourteen cases (14%) could not be separated according to these criteria. In RS, two-thirds of cases were men, whereas the sex distribution in PSP-P was even. Disease duration in RS was significantly shorter (5.9 versus 9.1 years, P < 0.001) and age at death earlier (72.1 versus 75.5 years, P = 0.01) than in PSP-P. The isoform composition of insoluble tangle-tau isolated from the basal pons also differed significantly. In RS, the mean four-repeat:three-repeat tau ratio was 2.84 and in PSP-P it was 1.63 (P < 0.003). The effect of the H1,H1 PSP susceptibility genotype appeared stronger in RS than in PSP-P (odds ratio 13.2 versus 4.5). The difference in genotype frequencies between the clinical subgroups was not significant. There were no differences in apolipoprotein E genotypes. The classic clinical description of PSP, which includes supranuclear gaze palsy, early falls and dementia, does not adequately describe one-third of cases in this series of pathologically confirmed cases. We propose that PSP-P represents a second discrete clinical phenotype that needs to be clinically distinguished from classical PSP (RS). The different tau isoform deposition in the basal pons suggests that this may ultimately prove to be a discrete nosological entity.

Differential involvement and heterogeneous phosphorylation of tau isoforms in progressive supranuclear palsy

We found previously that aggregated insoluble tau protein in progressive supranuclear palsy (PSP) brains exhibits a heterogeneous pattern that is not segregated by the type of clinical presentation. Here we have investigated tau isoform composition from 20 PSP cases and found marked variation between different brains. Cases were classified into three groups, each comprising essentially of (1) 1N4R; (2) 1N4R and 1N3R; or (3) 1N4R, 1N3R and 0N4R tau isoforms. There was also an absence of a simple relationship between isoform composition and the pattern of insoluble tau before dephosphorylation. We conclude that there is distinct molecular heterogeneity in the involvement of tau isoforms in the tau pathology in PSP.

The complex relationship between soluble and insoluble tau in tauopathies revealed by efficient dephosphorylation and specific antibodies

Phosphorylated tau is deposited as insoluble inclusion bodies in the tauopathies. We have used a new efficient method to dephosphorylate tau extracted from control and tauopathy brain. In some tauopathies, including Alzheimer's disease and progressive supranuclear palsy, the pattern of insoluble tau isoforms reflected that of soluble tau. In contrast, in corticobasal degeneration, Pick's disease, and some forms of fronto-temporal dementia, specific tau isoforms were selectively sequestered into insoluble inclusion-forming tau. Therefore the overall expression of individual tau isoforms does not predict which tau isoforms are deposited in all tauopathies and different mechanisms must operate that result in the deposition of specific tau isoforms.

Comparison of A68 levels in Alzheimer diseased and non-Alzheimer's diseased brain by two ALZ50 based methods

A total of 61 human brain specimens were analyzed with both ELISA and Western Blot using the ALZ50 monoclonal antibody. The brain specimens included: Alzheimer's Disease (AD, n=31), AD/Down's (n=2), Normal (n=14), Parkinson's Disease (n=7), Huntington's chorea (n=2), Wernicke-Korsakov's Encephalopathy (n=3), and Motor Neuron Disease (n=2). The non-AD cases (n=28) had no detectable A68 by ELISA, and showed no A68 bands by Western blot. The AD cases (n=33), all were positive for A68 by the ELISA, but only 31 of 33 had visible A68 band by Western blot. Additionally, a method for solubilization of A68 is reported.

Hierarchical phosphorylation of recombinant tau by the paired-helical filament-associated protein kinase is dependent on cyclic AMP-dependent protein kinase

Immunoaffinity-purified paired helical filaments (PHFs) from Alzheimer's disease (AD) brain homogenates contain an associated protein kinase activity that is able to induce the phosphorylation of PHF proteins on addition of exogenous MgCl2 and ATP. PHF kinase activity is shown to be present in immunoaffinity-purified PHFs from both sporadic and familial AD, Down's syndrome, and Pick's disease but not from normal brain homogenates. Although initial studies failed to show that the kinase was able to induce the phosphorylation of tau, additional studies presented in this article show that only cyclic AMP-dependent protein kinase-pretreated recombinant tau is a substrate for the PHF kinase activity. Deletional mutagenesis, phosphopeptide mapping, and site-directed mutagenesis have identified the PHF kinase phosphorylation sites as amino acids Thr361 and Ser412 in htau40. In addition, the cyclic AMP-dependent protein kinase phosphorylation sites that direct the PHF kinase have been mapped to amino acids Ser356 and Ser409 in htau40. Additional data demonstrate that these hierarchical phosphorylations in the extreme C terminus of tau allow for the incorporation of recombinant tau into exogenously added AD-derived PHFs, providing evidence that certain unique phosphorylations of tau may play a role in the pathogenesis of neurofibrillary pathology in AD.

Neurofibrillary tangles and Alzheimer's disease

The neuropathological diagnosis of Alzheimer's disease relies on the presence of both neurofibrillary tangles and senile plaques. The number of neurofibrillary tangles is tightly linked to the degree of dementia, suggesting that the formation of neurofibrillary tangles more directly correlates with neuronal dysfunction. The regional pattern of areas affected by neurofibrillary tangles formation during the course of the disease is relatively stereotyped. Neurofibrillary tangles are composed of highly phosphorylated forms of the microtubule-associated protein tau. Phosphorylated tau proteins accumulate early in neurons, even before formation of neurofibrillary tangles, suggesting that an imbalance between the activities of protein kinases and phosphatases acting on tau is an early phenomenon. The latter might be related to changes in signalling through transduction cascades, since many of the protein kinases generating phosphorylated tau species participate in signalling pathways. The accumulation of neurofibrillary tangles and phosphorylated tau species is associated with disturbances of the microtubule network and, as a consequence of the latter, of axoplasmic flows. The mechanistic relationship between the formation of neurofibrillary tangles and senile plaques is still little understood and in vivo formation of neurofibrillary tangles in experimental models has not yet been achieved. Future animal models, e.g. transgenic animals expressing combined key human proteins, will hopefully reproduce faithfully all the major cellular lesions of the disease.

Role of phosphorylation in determining the backbone dynamics of the serine/threonine-proline motif and Pin1 substrate recognition

Proline residues provide a backbone switch in a polypeptide chain, which is controlled by the cis/trans isomerization about the peptidyl-prolyl bond. Phosphorylation of serine- and threonine-proline motifs has been shown to be a critical regulatory event for many proteins. The biological significance of these motifs has been further highlighted by the discovery of a novel and essential peptidyl-prolyl cis/trans isomerase Pin1. Pin1 is required for progression through mitosis via catalyzing the isomerization of phosphorylated Ser/Thr-Pro motifs specifically present in mitosis-specific phosphoproteins. However, little is known whether the phosphorylation regulates the conformational switch of the Ser/Thr-Pro bonds. Here, we report the synthesis and conformational characterization of a series of peptides that contain the phosphorylated or nonphosphorylated Ser/Thr-Pro motifs. Phosphorylation affected the rate of the cis to trans isomerization of the Thr/Ser-Pro bonds. As determined by a protease-coupled assay, the isomerization rate of phosphorylated Thr-Pro bond was found to be 8-fold slower than that of the nonphosphorylated analogue. Furthermore, studies of the pH dependence of the isomerization of the phosphopeptides reveal that both cis content and the rate constant of prolyl cis to trans isomerization are lower for the dianionic state of the phosphothreonine-containing peptides. These effects of phosphorylation are specific for phosphorylated Ser/Thr since neither phosphorylated Tyr nor glutamic acid was able to affect the prolyl isomerization. Finally, our experiments provide evidence that effective catalysis of cis/trans isomerization of phosphorylated Ser/Thr-Pro bonds by Pin1 is specific to the dianionic form of the substrate. Thus, our results demonstrate that protein phosphorylation specifically regulates the backbone dynamics of the Ser/Thr-Pro motifs and that Pin1 specifically isomerizes the certain conformation of the phosphorylated Ser/Thr-Pro motifs.

Vulnerable neuronal subsets in Alzheimer's and Pick's disease are distinguished by their tau isoform distribution and phosphorylation

Aggregated tau proteins constitute the basic matrix of neuronal inclusions specific to numerous neurodegenerative disorders. Monodimensional and two-dimensional Western blot analyses performed on cortical brain homogenates allowed discrimination between disease-specific tau protein profiles. These observations raised the issue of the physiopathological significance of such specificities. Alzheimer's disease (AD) pathological tau proteins (PTPs) (tau 74, 69, 64, 55) were compared with those of Pick's disease (PiD) (tau 64, 55) using a panel of antibodies against peptidic sequences of tau isoforms corresponding to exons 2, 3, and 10. AD and PiD could then be critically differentiated by the absence of translated tau isoforms with exon 10 in PiD PTPs, along with the absence of the phosphorylation site on Ser262. Immunohistochemical studies corroborate these findings. Indeed, Pick bodies were strongly immunostained by an anti-"exon 2" antibody but failed to reveal any anti-exon 10 reactive epitope. Tangles in AD contained exon 2, 3, and 10 epitopes. Altogether, our results demonstrated that Pick bodies develop within specific neuronal subsets that express specific patterns of 7 isoforms lacking exon 10 peptidic sequence. We conclude that neurodegenerative disorders imply attrition of selectively vulnerable neuronal subsets, a process revealed, and may be sustained by specific tau isoform patterns.

Familial multiple system tauopathy with presenile dementia: a disease with abundant neuronal and glial tau filaments

Neurofibrillary lesions made of hyperphosphorylated microtubule-associated protein tau constitute not only one of the defining neuropathological features of Alzheimer disease but also are present in a number of other neurodegenerative diseases with dementia. Here we describe a novel autosomal dominant disease named familial "multiple system tauopathy with presenile dementia," which is characterized by abundant fibrillary deposits of tau protein in both neurons and glial cells. There are no detectable deposits of beta-amyloid. The tau deposits are in the form of twisted filaments that differ in diameter and periodicity from the paired helical filaments of Alzheimer disease. They are stained by both phosphorylation-independent and -dependent anti-tau antibodies. Moreover, tau immunoreactivity coexists with heparan sulfate in affected nerve and glial cells. Tau protein extracted from filaments of familial multiple system tauopathy with presenile dementia shows a minor 72-kDa band and two major bands of 64 and 68 kDa that contain mainly hyperphosphorylated four-repeat tau isoforms of 383 and 412 amino acids.

Normal and pathological Tau proteins as factors for microtubule assembly

Tau proteins are microtubule-associated proteins. They regulate the dynamics of the microtubule network, especially involved in the axonal transport and neuronal plasticity. Tau proteins belong to a family of developmentally regulated isoforms generated by alternative splicing and phosphorylation. This generates several Tau variants that interact with tubulin and other proteins. Therefore, Tau proteins are influenced by many physiological regulations. Tau proteins are also powerful markers of the neuronal physiological state. Their degree of phosphorylation is a good marker of cell integrity. It is heavily disturbed in numerous neurodegenerative disorders, leading to a collapse of the microtubule network and the presence of intraneuronal lesions resulting from Tau aggregation. However, different biochemical and immunological patterns of pathological Tau proteins found among neurodegenerative disorders are useful markers for the understanding of the role of Tau protein isoforms and the diagnosis of these pathological conditions.

AD2, a phosphorylation-dependent monoclonal antibody directed against tau proteins found in Alzheimer's disease

Alzheimer's disease is characterized by an intraneuronal aggregation of hyperphosphorylated tau proteins into paired helical filaments. The hyperphosphorylation of tau proteins induces a decrease in their electrophoretic mobility, resulting in a pathological tau triplet referred to as tau 55, 64 and 69 or tau-PHF. We have developed monoclonal antibodies directed against this pathological tau triplet. In the present article, we report the properties of antibody AD2, which detects the hyperphosphorylated tau proteins forming paired helical filaments during Alzheimer's disease. Using immunoblotting, AD2 exclusively labeled the tau triplet, while normal tau proteins from control cases were not immunodetected. Furthermore, AD2 is highly specific in that it was able to detect the triplet not only in tau preparations but also in total brain homogenates from Alzheimer's disease patients. The binding of this monoclonal antibody to tau proteins is phosphorylation dependent. Characterization of this antibody allowed us to identify its epitope as containing phosphorylated Ser-396 with the participation of phosphorylated Ser-404. AD2 was also shown to label normal tau proteins from rapidly processed brain tissues, but its epitope is rapidly dephosphorylated during postmortem intervals. However, in autopsic brains, AD2 still represents a valuable tool to investigate neurofibrillary degeneration at the biochemical and immunocytochemical levels.

Distribution of isoforms of the microtubule-associated protein tau in grey and white matter areas of human brain: a two-dimensional gelelectrophoretic analysis

The microtubule-associated protein tau in human brain consists of six molecular isoforms derived from a single gene by alternative mRNA-splicing and further modified by posttranslational processing. In the present study, the distribution of tau isoforms in grey and white matter of human temporal cortex was investigated by two-dimensional gelelectrophoresis. More than 80 isoforms were detected. The pattern of isoforms obtained after treatment with alkaline phosphatase was still more complex than those of recombinant tau, indicating that posttranslational modifications other than phosphorylation contribute to the molecular heterogeneity of tau. The tau isoform D according to Goedert containing four tubulin-binding regions shown to promote tubulin polymerisation most efficiently was present in higher amounts in white as compared to grey matter. The pattern of isoform distribution was not significantly altered in Alzheimer's disease. It is concluded that molecular isoforms that differ in their tubulin-binding characteristics are differentially distributed in subcellular neuronal compartments and/or neuronal types.

Immunogold labelling of paired helical filaments and amyloid fibrils by specific monoclonal and polyclonal antibodies

Senile plaque and paired helical filament (PHF) formation are characteristic of Alzheimer's disease, but the mechanisms leading to these lesions still remain unclear. To understand them better, we have performed different immunolabellings of amyloid protein and PHF. We describe a very specific immunodetection of PHF with AD2, a monoclonal antibody directed against a hyperphosphorylated epitope of PHF-tau, and use double immunolabelling to show that PHF and plaque amyloid are discretely labelled by different antibodies. We also discuss different mechanisms of PHF maturation.

Cortical mapping of Alzheimer pathology in brains of aged non-demented subjects

1. The presence of Alzheimer-type neurofibrillary pathology and amyloid deposits within the brains of 27 aged non-demented subjects was investigated by immunoblotting and immunohistochemistry using antibodies directed against pathological Tau proteins 55, 64 and 69 and beta A4 respectively. 2. The abnormal Tau triplet, a biochemical marker of neurofibrillary degeneration was quantified by western blot and densitometric analysis in several cortical areas including the entorhinal cortex (EC), hippocampus and Brodmann areas (BA) 38, 20, 22, 35, 9, 44 and 39. 3. The abnormal Tau triplet was detected in the EC and the hippocampus of most of the controls aged over 70 years. In few control cases abnormal Tau proteins were also detected in the isocortex, in BA38 alone or also in BA20. Some cases and especially those with Tau pathology in the temporal lobe contained numerous senile plaques (SP) in the neocortex. 4. The authors conclude that control cases with Tau pathology in the temporal lobe and numerous SP in the neocortex were likely to be subclinical stages of AD whereas others with Tau pathology exclusively detected in the EC and hippocampus and without or few SP in the neocortex were related to normal aging.

The role of tau phosphorylation in transfected COS-1 cells

Tau cDNAs from each of the six human isoforms were transfected into COS-1 cells and, in every case, more than one peptide was observed. The diversity of expressed isoforms was due to different levels of tau phosphorylation. Tau phosphorylation results in a decrease of the protein electrophoretic mobility. The major contribution to this mobility shift is due to the phosphorylation at the at the C-terminus of the molecule, as inferred from the expression of tau fragments. Phosphorylation takes place in some of the sites modified in neural cells and in the basis of AD patients. Copolymerization studies indicate that the level of phosphorylation, as well as the localization of the modified residues, may affect the binding of the protein to microtubules. These results indicate that phosphorylation regulates tau function inside the cell.

Distribution and expression of the alpha-tubulin mRNA in the hippocampus and the temporal cortex in Alzheimer's disease

The distribution of the messenger RNA for alpha-tubulin has been investigated by in situ hybridization in the human hippocampus and temporal cortex in normal subjects and in Alzheimer's disease. The alpha-tubulin mRNA was strongly expressed in neurons in the gyrus dentatus, in the Ammon's horn and in cortical layers of the temporal cortex. The same distribution was observed in Alzheimer's disease. An important reduction of the hybridization signal was apparent, however, in areas rich in neurofibrillary lesions, e.g. as in layer II of entorhinal cortex. Neurons containing neurofibrillary tangles exhibited a weaker hybridization signal than adjacent neurons devoid of neurofibrillary tangles. The immunoreactivity for alpha-tubulin was drastically reduced in tangles-bearing neurons. These results indicate that tubulin transcription is reduced in tangles-bearing neurons, a reduction which might play a role in the reported decrease in the number of microtubules in neurons containing neurofibrillary tangles.

Molecular dissection of the paired helical filament

Abundant neurofibrillary tangles, neuropil threads and plaque neurites constitute the neurofibrillary pathology of Alzheimer's disease. They form in the nerve cells that undergo degeneration in the disease where their regional distribution correlates with the degree of dementia. Each lesion contains the paired helical filament (PHF) as its major fibrous component. Recent work has shown that PHFs are composed of the microtubule-associated protein tau in a hyperphosphorylated state. PHF-tau is hyperphosphorylated on six adult brain tau isoforms. As a consequence, tau is unable to bind to microtubules and is believed to self-assemble into the PHF. Current evidence suggests that protein kinases or protein phosphatases with a specificity for serine/threonine-proline residues play an important role in the hyperphosphorylation of tau. Candidate protein kinases include mitogen-activated protein kinase, glycogen synthase kinase-3 and cyclin-dependent kinase 5, whereas the trimeric form of protein phosphatase 2A is a candidate phosphatase.

Abnormally phosphorylated tau protein in Alzheimer's disease: heterogeneity of individual regional distribution and relationship to clinical severity

The distribution of abnormally phosphorylated tau proteins was investigated in a number of cortical areas and in the basal nucleus of Meynert of 12 patients with Alzheimer's disease. Quantification was performed with a sensitive sandwich enzyme-linked immunosorbent assay employing the monoclonal antibody B5-2, which immunoreacts with neurofibrillary tangles, dystrophic neurites surrounding amyloid cores of plaques and neuropil threads. On western blots, the monoclonal antibody B5-2 specifically labels the abnormally phosphorylated paired helical filament-tau species in a phosphorylation-dependent manner but is not cross-reactive to normal tau proteins. The preferential involvement of cytoarchitecturally defined cortical areas showed marked individual differences in Alzheimer's disease, and no unique distribution pattern of abnormally phosphorylated tau proteins could be established. While regional heterogeneities along the rostrocaudal extension of the brain declined with the progression of the disease, lateral differences which were largely non-directional appeared to be more stable over time. The mean content of abnormally phosphorylated tau proteins of individual cases was significantly related to the severity of the disease. On a regional scale, this correlation was highest for the basal nucleus. The formation of abnormally phosphorylated tau was associated with a loss of normal soluble tau proteins. Those cortical areas which in normal brain showed the highest amount of normal soluble tau proteins appeared to be particularly prone to deposition of abnormally phosphorylated tau proteins. The present results indicate that the formation of abnormally phosphorylated tau proteins can be initiated in the neuropil at more than one brain area. The spreading of the pathology appears to involve both intracortical and subcortical pathways but largely spares interhemispheric pathways. It is hypothesized that regional differences in the compartmentation and metabolism of tau proteins might reflect the molecular basis for the regionally different vulnerability in Alzheimer's disease.

Alterations in neurofilament protein immunoreactivity in human hippocampal neurons related to normal aging and Alzheimer's disease

The distribution of immunoreactivity for the neurofilament triplet class of intermediate filament proteins was examined in the hippocampus of young, adult and elderly control cases and compared to that of Alzheimer's disease cases. In a similar fashion to non-human mammalian species, pyramidal neurons in the CA1 region showed a very low degree of neurofilament triplet immunoreactivity in the three younger control cases examined. However, in the other control cases of 49 years of age and older, many CA1 pyramidal neurons showed elevated neurofilament immunoreactivity. In the Alzheimer's disease cases, most of the surviving CA1 neurons showed intense labeling for the neurofilament triplet proteins, with many of these neurons giving off abnormal "sprouting" processes. Double labeling demonstrated that many of these neurons contained tangle-like or granular material that was immunoreactive for abnormal forms of tau and stained with thioflavine S, indicating that these neurons are in a transitional degenerative stage. An antibody to phosphorylated neurofilament proteins labeled a subset of neurofibrillary tangles in the Alzheimer's disease cases. However, following formic acid pre-treatment, the number of neurofibrillary tangles showing phosphorylated neurofilament protein immunoreactivity increased, with double labeling confirming that all of the tau-immunoreactive neurofibrillary tangles were also immunoreactive for phosphorylated neurofilament proteins. Immunoblotting demonstrated that there was a proportionately greater amount of the neurofilament triplet subunit proteins in hippocampal tissue from Alzheimer's disease cases as compared to controls. These results indicate that there are changes in the cytoskeleton of CA1 neurons associated with age which are likely to involve an increase in the level of neurofilament proteins and may be a predisposing factor contributing towards their high degree of vulnerability in degenerative conditions such as Alzheimer's disease. The cellular factors affecting hippocampal neurons during aging may be potentiated in Alzheimer's disease to result in even higher levels of intracellular neurofilament proteins and the progressive alterations of neurofilaments and other cytoskeletal proteins that finally results in neurofibrillary tangle formation and cellular degeneration.

Dephosphorylation of Alzheimer's disease abnormally phosphorylated tau by protein phosphatase-2A

Microtubule-associated protein tau is abnormally hyperphosphorylated in the brain of patients with Alzheimer's disease, and is the major protein subunit of paired helical filaments. There is also a significant pool of non-paired helical filament abnormally phosphorylated tau in Alzheimer's disease brain. In the present study, the site-specific dephosphorylation of this Alzheimer's disease abnormally phosphorylated tau by protein phosphatase-2A was studied and compared with that by protein phosphatase-2B. The dephosphorylation was detected by its interaction with several phosphorylation-dependent antibodies to various abnormal phosphorylation sites. Protein phosphatase-2A was able to dephosphorylate the abnormally phosphorylated tau at Ser-46, Ser-199, Ser-202, Ser-396 and Ser-404, but not at Ser-235 (the amino acids are numbered according to the largest isoform of human tau, tau441). Two major types of protein phosphatase-2A, protein phosphatase-2A1 and -2A2, dephosphorylated the abnormally phosphorylated tau at approximately the same rate. After the abnormally phosphorylated tau was dephosphorylated by protein phosphatase-2A, its relative mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis increased. The dephosphorylation of the abnormal tau by protein phosphatase-2A1 and -2A2 was markedly stimulated by Mn2+. These results suggest that tau dephosphorylation is catalysed by protein phosphatase-2A in addition to protein phosphatase-2B. A deficiency of either protein phosphatase-2A or -2B, or both, may be involved in abnormal phosphorylation of tau in Alzheimer's disease.

A subset of calretinin-positive neurons are abnormal in Alzheimer's disease

The distribution of the calcium-binding protein calretinin was investigated by immunohistochemistry in the hippocampus, the subicular areas, and the entorhinal cortex in patients with Alzheimer's disease and in control subjects. By double immunolabelling, the calretinin immunoreactivity was compared to the immunoreactivity for beta/A4 amyloid or for tau proteins. Calretinin-positive neurons were mainly observed in the molecular layer of the gyrus dentatus, the stratum radiatum of the Ammon's horn, and in layers II and III of the entorhinal cortex. The general pattern of calretinin immunoreactivity was conserved in Alzheimer's disease. Calretinin-positive neurons appeared normal in the hippocampus but had a reduced dendritic tree in the entorhinal cortex. Dystrophic calretinin immunoreactive fibres were often observed in the outer molecular layer of the gyrus dentatus and in the CA4 sector in Alzheimer's disease. Most neurons containing neurofibrillary tangles were not calretinin immunoreactive and most senile plaques were not associated with calretinin positive fibres. These results show that entorhinal calretinin-positive neurons are affected in Alzheimer's disease in spite of an absence of systematic association with neurofibrillary tangles and senile plaques.

Biochemical mapping of neurofibrillary degeneration in a case of progressive supranuclear palsy: evidence for general cortical involvement

A biochemical study was performed to quantify and map the neurodegenerating process in cortical and subcortical brain areas from a case of progressive supranuclear palsy (PSP). Our approach was based on a Western blot analysis of pathological Tau proteins, which are the basic components of neurofibrillary lesions. We found that: (i) the abnormal Tau proteins can be detected in all cortical areas, sometimes in larger amounts than in some subcortical areas; (ii) these abnormal Tau proteins consist of a doublet called Tau 64 and 69, except for in the entorhinal cortex where we detected, as for Alzheimer brains, the triplet of Tau proteins called Tau 55, 64 and 69; (iii) the amounts of abnormal Tau proteins were higher in some neocortical regions, especially in the frontal lobe, than in the hippocampal formation. Our results show that the neocortical pathology in PSP, as revealed by the presence of pathological proteins, is more extended than thought so far. Our biochemical approach appears to be more sensitive than the immunohistochemical one and can clearly differentiates between two types of neurofibrillary pathology, the Alzheimer type with a triplet of abnormal Tau proteins (Tau 55, 64 and 69) and the PSP type with a characteristic doublet (Tau 64 and 69).

Dephosphorylation of tau protein from Alzheimer's disease patients

Tau protein-prepared post-mortem from brains of Alzheimer's disease patients was treated with protein phosphatase 1 catalytic subunit, 2A catalytic subunit, and 2B (calcineurin). Dephosphorylation was monitored by immunoblotting with two monoclonal antibodies, TAU-1 and SMI31, which recognize in tau the dephospho- and phospho-states, respectively, of proline-directed protein kinase phosphorylation sites. Out of the three enzymes tested, protein phosphatase 2A was only effective in dephosphorylating tau at these Alzheimer-type epitopes.

Role of the beta-amyloid precursor protein in Alzheimer's disease

Deposition of beta-amyloid peptide in the brain is an early event in Alzheimer's disease, the most common cause of dementia. Studies of the beta-amyloid precursor protein (APP), which gives rise to beta-amyloid, are rapidly leading to a better understanding of the biochemical basis of the disease--a prerequisite for rational drug development.

Pathological Tau proteins of Alzheimer's disease as a biochemical marker of neurofibrillary degeneration

Paired Helical Filaments (PHF) accumulate in the degenerating neurons from the associative cortical brain areas during Alzheimer's disease. They are composed of a triplet of hyperphosphorylated microtubule-associated protein Tau, called Tau 55, 64, 69 or PHF-Tau. The distribution of PHF-Tau in the different brain areas corroborates neuropathological observations and specifies that: the entorhinal cortex and hippocampus are vulnerable regions specifically affected by Alzheimer-type neurofibrillary degeneration during aging, the temporal cortex is already affected at the very first stage of clinical manifestations, almost the whole brain is concerned by neurofibrillary degeneration at the end-stages of the disease. Tau-PHF are also observed in the cortical areas from Parkinson patients with dementia, and more especially in the prefrontal cortex. Tau pathology for Progressive Supranuclear Palsy is significantly different, with a doublet of pathological Tau, namely Tau 64 and 69, in almost all cortical and subcortical areas. Therefore, the presence of pathological Tau proteins in several associative cortical areas is always associated with severe intellectual impairment. Finally, PHF-Tau are powerful biochemical markers of the degenerating process which could be used for setting up an early biological diagnosis test of Alzheimer's disease based upon the immunodetection of PHF antigens in the CSF, as well as for developing experimental models of neurodegeneration.

Phosphorylation of tau protein in tau-transfected 3T3 cells

The tau protein of Alzheimer paired helical filaments (PHFs) is aberrantly phosphorylated, as evidenced by its reactivity with several phosphate-dependent antibodies. We sought to identify whether this unusual phosphorylation state exists in tau expressed by transfected NIH 3T3 fibroblasts. Immunoblot analysis of cell clones transfected with constructs for either the 3-repeat or 4-repeat isoforms of tau revealed two tau bands, with the lower band migrating with unmodified tau in each case. Antibodies T3P and tau-1 were used to probe these bands, as they also react with PHF-tau in a phosphate-dependent manner. The epitopes for both antibodies were phosphorylated in both tau isoforms. Only the upper band was phosphorylated at the T3P site whereas phosphorylation at the tau-1 site was not always associated with a shift of tau mobility on gels. Tau in both bands was soluble, in contrast to PHF-tau, and was competent to bind to exogenously added bovine microtubules. Colchicine treatment of the cells resulted in an inhibition of phosphorylation at both sites, through an unknown mechanism. In conclusion human tau expressed in 3T3 cells was phosphorylated at the T3P and tau-1 sites as is PHF-tau, although no PHFs formed and the phosphorylated tau was competent to bind to microtubules.

Tau protein and the neurofibrillary pathology of Alzheimer's disease

Abundant neurofibrillary tangles, neuropil threads and senile plaque neurites constitute the neurofibrillary pathology of Alzheimer's disease. They form in the nerve cells that undergo degeneration in the disease, in which their regional distribution correlates with the degree of dementia. Each lesion contains the paired helical filament (PHF) as its major fibrous component. Recent work has shown that PHFs are composed of the microtubule-associated protein tau in an abnormally phosphorylated state. PHF-tau is hyperphosphorylated on all six adult brain isoforms. As a consequence, tau is unable to bind to microtubules and is believed to self-assemble into the PHF. Current evidence suggests that protein kinases or protein phosphatases with a specificity for serine/threonine-proline residues are involved in the abnormal phosphorylation of tau.

Identification and distribution of axonal dystrophic neurites in Alzheimer's disease

Dystrophic neurites (DNs) are one of the neuropathological characteristics of Alzheimer's disease (AD). Previously, it has been suggested that tau-immunoreactive DNs are of dendritic origin and that axonal and dendritic dystrophic neurites are morphologically indistinguishable. In the present study, two monoclonal antibodies, tau-1 and PHF-1, were used to examine sections of the hippocampal formation from AD and normal aged brains. Both antibodies stained dendritic DNs as well as axonal DNs. Axonal DNs were clearly seen in axonal fiber tracts, white matter and hippocampal terminal regions. Axonal DNs arising from neurofibrillary tangles were easily detected in CA3 and CA1. The morphological appearance of axonal DNs varied with the neuron type from which it originated. The most distinctive feature of tau-1 or PHF-1 immunostained axonal DNs was their uneven contour, alternating swollen and shrunken segments and short rod or cone shaped fragments. In contrast, dendritic dystrophic neurites are thicker and more tortuous. It appears that while DNs are both dendritic and axonal in origin, axonal DNs are more prevalent and widespread in the AD brain than previously realized and may represent one of the main pathological lesions in AD.

Phosphorylation of tau by proline-directed protein kinase (p34cdc2/p58cyclin A) decreases tau-induced microtubule assembly and antibody SMI33 reactivity

Tau protein was evaluated as a substrate for a proline-directed protein kinase (p34cdc2/p58cyclin A) which recognizes the phosphorylation site motif X-Ser/Thr-Pro-X. The shortest human tau isoform, expressed as a recombinant protein, was phosphorylated to a stoichiometry of 2 mol phosphate/mol tau. Phosphoamino acid analysis revealed phosphorylation of both serine and threonine residues. Phosphorylation of recombinant tau resulted in a decreased ability to induce microtubule assembly but had no effect on the final extent of microtubule formation or on the rate of cold-induced microtubule disassembly. Phosphorylation of tau by the proline-directed protein kinase completely blocked immunoreactivity with antibody SMI33. Phosphorylation did not create the epitopes for the phosphate-dependent antibodies SMI31 or SMI34. Antibody SMI33 recognizes neurofibrillary tangles after treatment with alkaline phosphatase, suggesting that the proline-directed protein kinase may phosphorylate tau at sites that are phosphorylated in Alzheimer's disease.

Dementia in Parkinson's disease: biochemical evidence for cortical involvement using the immunodetection of abnormal Tau proteins

In order to elucidate the neurochemical basis of the dementia of Parkinson's disease, we compared samples of cerebral cortex from 24 nondemented parkinsonian patients and parkinsonian patients with various degrees of dementia, with those from patients with Alzheimer's disease and control subjects, using a quantitative Western blot analysis. An anti-paired helical filaments antibody was used for the immunodetection of the abnormally phosphorylated Tau proteins 55, 64, and 69, which are known to be specific and reliable biochemical markers of Alzheimer-type neurofibrillary degeneration. The frequency and intensity of immunodetection of the abnormal Tau triplet were higher in the demented parkinsonian subgroups than in the nondemented parkinsonian subgroup in the prefrontal area, temporal cortex, and entorhinal cortex but not in either the occipital or the cingular cortex. A quantification of abnormal Tau triplet by densitometry showed that unlike the results obtained in Alzheimer patients, the intensity of lesions in the cerebral cortex of the most demented parkinsonian patients was more severe in the prefrontal area versus the temporal area. This study (1) gives biochemical evidence for Alzheimer-type changes in the cortex of demented parkinsonian patients and (2) suggests that lesions of the prefrontal cortex may significantly contribute to the occurrence of cognitive changes at least in some patients with Parkinson's disease.

General cortical involvement in a late-onset case of Alzheimer disease. A biochemical approach by quantitation of abnormal tau proteins

We have performed a biochemical mapping of the neurofibrillary degeneration in all cortical areas of Alzheimer patients, using the immunological quantification of pathological tau 55, 64, and 69. These abnormally phosphorylated proteins, which are the basic components of PHF, are reliable markers of the degenerating process in Alzheimer disease. Here, we report our biochemical findings on a brain from a 90-yr-old woman with an 8-yr history of Alzheimer disease who exhibited dramatic and general cortical involvement. The detection of these markers was very high in all Brodmann areas, even in primary motor, somatosensory, or visual cortex. This case report contrasts with other studies, which suggested that a more virulent disease process is generally associated with an early onset and argues for the heterogeneity of the disease. Moreover, we show here that the immunodetection of abnormal tau proteins using the western blot method is a precise, reliable, and reproducible way to quantify the degenerating process in AD.

Neurofilament monoclonal antibodies RT97 and 8D8 recognize different modified epitopes in paired helical filament-tau in Alzheimer's disease

Neurofibrillary tangles in Alzheimer's disease have been previously found to be labeled by some neurofilament antibodies that also recognize tau proteins. We have studied the reactivity of two such monoclonal antibodies, RT97 and 8D8, and of an anti-ubiquitin serum with the abnormal paired helical filaments (PHF)-tau (A68) polypeptides known to be the main component of the PHFs constituting the neurofibrillary tangles. 8D8 recognized the three major PHF-tau polypeptides, but RT97 reacted only with the two larger PHF-tau species. PHF-tau polypeptides were labeled by 8D8 and RT97 much more strongly than normal human tau and this labeling was decreased after alkaline phosphatase treatment. Anti-ubiquitin and anti-phosphotyrosine antibodies did not label PHF-tau polypeptides. The immunoreactivity of proteolytic fragments of PHF-tau polypeptides was studied with RT97, 8D8, and a panel of tau antibodies. The epitope for 8D8 on PHF-tau was localized between amino acids 222 and 427 in the carboxyl half of tau. The RT97 epitope on PHF-tau was localized in the amino domain of tau, probably in the 29-amino-acid insertion (insert 1) found towards the amino terminus of some tau isoforms. These results show that the basis for the labeling of neurofibrillary tangles by antibodies 8D8 and RT97 to neurofilament is their ability to react with PHF-tau polypeptides by recognizing sites specifically modified on PHF-tau, including a site specific to some tau isoforms.

Mapping of neurofibrillary degeneration in Alzheimer's disease: evaluation of heterogeneity using the quantification of abnormal tau proteins

A biochemical mapping of neurofibrillary degeneration was performed in Brodmann areas of the brains of five patients with senile dementia of the Alzheimer type (AD). To quantify the degenerating process, we used an immunoblot method with antibodies directed against the abnormally phosphorylated tau proteins named Tau 55, 64 and 69, known to be early and reliable markers of the degenerating process in AD. The detection intensities were assessed using a numerical rating scale for each area and scores were grouped by lobe. In all cases, the detection of Tau 55, 64 and 69 was positive in all areas except in primary visual cortex (area 17) for two patients. The detections were especially strong in temporal neocortical and limbic areas and were higher in associative cortex than in primary sensory cortex. Scores from the occipital and frontal lobes differed strongly between patients as compared to the uniform degree of detection in the limbic, temporal and parietal lobes. Despite a relatively identical duration of the disease and an apparently global involvement of the cerebral cortex, the distribution of neurofibrillary degeneration in AD varies significantly across cortical areas and displays striking heterogeneity patterns along the rostrocaudal axis. The immunodetection of abnormal tau proteins using the Western blot method may provide complete and rapid quantitative data of the degenerating process in AD.

The pathology of the neuronal cytoskeleton in Alzheimer's disease

The two characteristic neuropathological lesions of Alzheimer's disease are the neurofibrillary tangles and the senile plaques. Neurofibrillary tangles are made of abnormal filaments (PHF) accumulating in neurons and mainly composed of a modified form of the microtubule-associated protein tau (PHF-tau). Senile plaques are composed of a cluster of dystrophic neurites surrounding an extracellular deposit of amyloid fibers made of a 42 amino-acid peptide (beta-amyloid peptide). The abnormal filaments contain the complete sequences of the different tau isoforms. The PHF-tau proteins can be distinguished from the normal tau proteins by the presence of several phosphorylated sites. One of these sites is phosphorylated by a calcium-calmodulin-dependent kinase. The relationship between PHF-tau and the cytoskeletal pathology in Alzheimer's disease is further discussed.

Immunochemical characterization of a monoclonal antibody specific for Alzheimer's disease associated protein

In this study, the monoclonal antibody PHF-1 which recognizes epitopes unique to Alzheimer's disease associated proteins (ADAP) has been characterized. Crossed affinity immunoelectrophoresis was used to estimate the binding constant for the interaction of PHF-1 with ADAP and to estimate the fraction of PHF-1 reactive protein. The binding constant of PHF-1 was determined to be 1.3 x 10(-8) M. Furthermore, the effect of dephosphorylation on the electrophoretic pattern of the PHF-1 reactive protein and the ensuing changes in its immunoreactivity were demonstrated.

Progressive transformation of the cytoskeleton associated with normal aging and Alzheimer's disease

Transitional and end-stage forms of neurofibrillary tangles associated with normal aging and Alzheimer's disease were identified using thioflavine staining combined with tau and neurofilament protein immunofluorescence. Normal aging was marked by transitional pathology in layer II of the entorhinal cortex but no neurofibrillary tangles in prefrontal cortex, whereas, in Alzheimer's disease cases, layer II entorhinal neurons had progressed to end-stage neurofibrillary tangles and the prefrontal cortex contained a high representation of transitional forms of the neurofibrillary tangle.

Tau pathology in a case of familial Alzheimer's disease with a valine to glycine mutation at position 717 in the amyloid precursor protein

The brain tissue from a case of familial Alzheimer's disease (FAD) caused by a missense (valine to glycine) mutation at codon 717 of the amyloid precursor protein (APP) gene has been examined for the presence of abnormally phosphorylated paired helical filament tau (PHF-tau). There was abundant PHF-tau present, which on Western blots, was indistinguishable from the PHF-tau typical of cases of sporadic Alzheimer's disease and that of another FAD mutation (valine to isoleucine), previously (Neurosci. Lett., 137 (1992) 221-224). This result implies that the cytoskeletal pathology in Alzheimer's disease is biochemically linked to abnormal APP processing and amyloid deposition.