Pathological proteins Tau 64 and 69 are specifically expressed in the somatodendritic domain of the degenerating cortical neurons during Alzheimer's disease. Demonstration with a panel of antibodies against Tau proteins

Reassessment of Neuronal Tau Distribution in Adult Human Brain and Implications for Tau Pathobiology

Tau is a predominantly neuronal, soluble and natively unfolded protein that can bind and stabilize microtubules in the central nervous system. Tau has been extensively studied over several decades, especially in the context of neurodegenerative diseases where it can aberrantly aggregate to form a spectrum of pathological inclusions. The presence of tau inclusions in the form of neurofibrillary tangles, neuropil threads and dystrophic neurites within senile plaques are essential and defining features of Alzheimer’s disease. The current dogma favors the notion that tau is predominantly an axonal protein, and that in Alzheimer’s disease there is a redistribution of tau towards the neuronal soma that is associated with the formation of pathological inclusions such as neurofibrillary tangles and neuropil threads. Using novel as well as previously established highly specific tau antibodies, we demonstrate that contrary to this overwhelmingly accepted fact, as asserted in numerous articles and reviews, in adult human brain, tau is more abundant in cortical gray matter that is enriched in neuronal soma and dendrites compared to white matter that is predominantly rich in neuronal axons. Additionally, in Alzheimer’s disease tau pathology is significantly more abundant in the brain cortical gray matter of affected brain regions compared to the adjacent white matter regions. These findings have important implications for the biological function of tau as well as the mechanisms involved in the progressive spread of tau associated with the insidious nature of Alzheimer’s disease.

Microtubule-modulating Agents in the Fight Against Neurodegeneration: Will it ever Work?

The microtubule skeleton plays an essential role in nerve cells as the most important structural determinant of morphology and as a highway for axonal transport processes. Many neurodegenerative diseases are characterized by changes in the structure and organization of microtubules and microtubule-regulating proteins such as the microtubule-associated protein tau, which exhibits characteristic changes in a whole class of diseases collectively referred to as tauopathies. Changes in the dynamics of microtubules appear to occur early under neurodegenerative conditions and are also likely to contribute to age-related dysfunction of neurons. Thus, modulating microtubule dynamics and correcting impaired microtubule stability can be a useful neuroprotective strategy to counteract the disruption of the microtubule system in disease and aging. In this article, we review current microtubule- directed approaches for the treatment of neurodegenerative diseases with microtubules as a drug target, tau as a drug target, and post-translational modifications as potential modifiers of the microtubule system. We discuss limitations of the approaches that can be traced back to the rather unspecific mechanism of action, which causes undesirable side effects in non-neuronal cell types or which are due to the disruption of non-microtubule-related interactions. We also develop some thoughts on how the specificity of the approaches can be improved and what further targets could be used for modulating substances.

The Role of NMDA Receptors in Alzheimer's Disease

In Alzheimer’s disease (AD), early synaptic dysfunction is associated with the increased oligomeric amyloid-beta peptide, which causes NMDAR-dependent synaptic depression and spine elimination. Memantine, low-affinity NMDAR channel blocker, has been used in the treatment of moderate to severe AD. However, clear evidence is still deficient in demonstrating the underlying mechanisms and a relationship between NMDARs dysfunction and AD. This review focuses on not only changes in expression of different NMDAR subunits, but also some unconventional modes of NMDAR action.

Near Infrared Light Treatment Reduces Synaptic Levels of Toxic Tau Oligomers in Two Transgenic Mouse Models of Human Tauopathies

Tau oligomers are emerging as a key contributor to the synaptic dysfunction that drives cognitive decline associated with the clinical manifestation and progression of Alzheimer’s disease (AD). Accordingly, there is ample consensus that interventions that target tau oligomers may slow or halt the progression of AD. With this ultimate goal in mind, in the present study, we investigated tau oligomer accumulation and its synaptic and behavioral consequences after an in vivo treatment with near infrared (NIR) light (600–1000 nm) in two transgenic mouse models, overexpressing human tau either alone (hTau mice) or in combination with amyloid beta (3xTgAD mice). We found that a 4-week exposure to NIR light (90 s/day/5 days a week) significantly reduced levels of endogenous total and oligomeric tau in both synaptosomes and total protein extracts from the hippocampus and cortex of hTau mice and improved deteriorating memory function. Similar results were observed in the 3xTgAD mice, which further displayed reduced synaptic Aβ after NIR light treatment. On the other hand, ex vivo binding of tau oligomers in isolated synaptosomes as well as tau oligomer-induced depression of long-term potentiation (LTP) in hippocampal slices from NIR light-treated wt mice were unaffected. Finally, levels of proteins critically involved in two mechanisms associated with clearance of misfolded tau, inducible HSP70 and autophagy, were upregulated in NIR light treated mice. Collectively, these results show that NIR light decreases levels of endogenous toxic tau oligomers and alleviate associated memory deficits, thus furthering the development of NIR light as a possible therapeutic for AD.

Cerebrospinal Fluid Biomarkers in Alzheimer's Disease-From Brain Starch to Bench and Bedside

Alzheimer’s disease is the most common cause of dementia. Over the last three decades, research has advanced dramatically and provided a detailed understanding of the molecular events underlying the pathogenesis of Alzheimer’s disease. In parallel, assays for the detection of biomarkers that reflect the typical Alzheimer’s disease-associated pathology have been developed and validated in myriads of clinical studies. Such biomarkers complement clinical diagnosis and improve diagnostic accuracy. The use of biomarkers will become even more important with the advent of disease-modifying therapies. Such therapies will likely be most beneficial when administered early in the disease course. Here, we summarise the development of the core Alzheimer’s disease cerebrospinal fluid biomarkers: amyloid-β and tau. We provide an overview of their role in cellular physiology and Alzheimer’s disease pathology, and embed their development as cerebrospinal fluid biomarkers into the historical context of Alzheimer’s disease research. Finally, we summarise recommendations for their use in clinical practice, and outline perspectives for novel cerebrospinal fluid candidate biomarkers.

Anesthesia and tau pathology

Alzheimer's disease (AD) is the most common form of dementia and remains a growing worldwide health problem. As life expectancy continues to increase, the number of AD patients presenting for surgery and anesthesia will steadily rise. The etiology of sporadic AD is thought to be multifactorial, with environmental, biological and genetic factors interacting together to influence AD pathogenesis. Recent reports suggest that general anesthetics may be such a factor and may contribute to the development and exacerbation of this neurodegenerative disorder. Intra-neuronal neurofibrillary tangles (NFT), composed of hyperphosphorylated and aggregated tau protein are one of the main neuropathological hallmarks of AD. Tau pathology is important in AD as it correlates very well with cognitive dysfunction. Lately, several studies have begun to elucidate the mechanisms by which anesthetic exposure might affect the phosphorylation, aggregation and function of this microtubule-associated protein. Here, we specifically review the literature detailing the impact of anesthetic administration on aberrant tau hyperphosphorylation as well as the subsequent development of neurofibrillary pathology and degeneration.

APP/PS1 mice overexpressing SREBP-2 exhibit combined Aβ accumulation and tau pathology underlying Alzheimer's disease

Current evidence indicates that excess brain cholesterol regulates amyloid-β (Aβ) deposition, which in turn can regulate cholesterol homeostasis. Moreover, Aβ neurotoxicity is potentiated, in part, by mitochondrial glutathione (mGSH) depletion. To better understand the relationship between alterations in cholesterol homeostasis and Alzheimer's disease (AD), we generated a triple transgenic mice featuring sterol regulatory element-binding protein-2 (SREBP-2) overexpression in combination with APPswe/PS1ΔE9 mutations (APP/PS1) to examine key biochemical and functional characteristics of AD. Unlike APP/PS1 mice, APP/PS1/SREBP-2 mice exhibited early mitochondrial cholesterol loading and mGSH depletion. Moreover, β-secretase activation and Aβ accumulation, correlating with oxidative damage and neuroinflammation, were accelerated in APP/PS1/SREBP-2 mice compared with APP/PS1 mice. Triple transgenic mice displayed increased synaptotoxicity reflected by loss of synaptophysin and neuronal death, resulting in early object-recognition memory impairment associated with deficits in spatial memory. Interestingly, tau pathology was present in APP/PS1/SREBP-2 mice, manifested by increased tau hyperphosphorylation and cleavage, activation of tau kinases and neurofibrillary tangle (NFT) formation without expression of mutated tau. Importantly, in vivo treatment with the cell permeable GSH ethyl ester, which restored mGSH levels in APP/PS1/SREBP-2 mice, partially prevented the activation of tau kinases, reduced abnormal tau aggregation and Aβ deposition, resulting in attenuated synaptic degeneration. Taken together, these results show that cholesterol-mediated mGSH depletion is a key event in AD progression, accelerating the onset of key neuropathological hallmarks of the disease. Thus, therapeutic approaches to recover mGSH may represent a relevant strategy in the treatment of AD.

Structural and functional changes in tau mutant mice neurons are not linked to the presence of NFTs

In the rTg4510 mouse model, expression of the mutant human tau variant P301L leads to development of neurofibrillary tangles (NFTs), neuronal death, and memory impairment, reminiscent of the pathology observed in human tauopathies. In the present study, we examined the effects of mutant tau expression on the electrophysiology and morphology of individual neurons using whole-cell patch-clamp recordings and biocytin filling of pyramidal cells in cortical slices prepared from rTg4510 (TG) and wild-type (WT) littermate mice. Among the TG cells, 42% contained a clear Thioflavin-S positive inclusion in the soma and were categorized as NFT positive (NFT+), while 58% had no discernable inclusion and were categorized as NFT negative (NFT-). The resting membrane potential (V(r)) was significantly depolarized (+8 mV) in TG cells, and as a consequence, evoked repetitive action potential (AP) firing rates were also significantly increased. Further, single APs were significantly shorter in duration in TG cells and the depolarizing voltage deflection or "sag" evoked by hyperpolarization was significantly greater in amplitude. In addition to these functional electrophysiological changes, TG cells exhibited significant morphological alterations, including loss or significant atrophy of the apical tuft, reduced dendritic complexity and length, and reduced spine density. Importantly, NFT- and NFT+ TG cells were indistinguishable with regard to both morphological and electrophysiological properties. Our observations show that expression of mutated tau results in significant structural and functional changes in neurons, but that these changes occur independent of mature NFT formation.

[Morphologic and molecular neuropathology of Alzheimer's disease]

Alzheimer disease lesions include the abnormal accumulation of two proteins normally present in neurons: tau protein and Abeta peptide. Tau protein aggregates into fibrils in the cell body of neurons (neurofibrillary tangles), in dendrites (neuropil threads) and in degenerating axons that constitute the corona of the senile plaque. Tau pathology progresses in the brain areas in a stereotyped manner and in parallel with the clinical symptoms. Abeta extracellular deposits may be diffuse or focal. The Abeta focal deposit constitutes the core of the senile plaque. Progression of the Abeta lesions, which initially affect the isocortex, then the hippocampus, basal ganglia, various brainstem nuclei and cerebellum, is not directly correlated with symptoms. Mutations involving the genes implicated in Abeta peptide metabolism are responsible for familial Alzheimer disease. Mutations of the tau gene are not associated with Alzheimer disease but with frontotemporal dementia. The link between altered Abeta peptide metabolism and tau pathology has not been fully elucidated. Animal models mimic several aspects of the disease and have contributed to a better understanding of the mechanisms of the lesions.

Molecular characterization and measurement of Alzheimer's disease pathology: implications for genetic and environmental aetiology

The neuropathological changes seen in Alzheimer's disease represent an interaction between the ageing process in which normal intellectual function is retained, and changes which are specifically associated with severe cognitive deterioration. Molecular analysis of these changes has tended to emphasize the distinction between neurofibrillary pathology, which is intracellular and highly correlated with cognitive deterioration, and the changes associated with the deposition of extracellular amyloid, which appears to be widespread in normal ageing. Extracellular amyloid deposits consist of fibrils composed of a short 42 amino acid peptide (beta/A4) derived by abnormal proteolysis from a much larger precursor molecule (APP). The recent demonstration of a mutation associated with APP in rare cases with familial dementia, neurofibrillary pathology in the hippocampus and atypical cortical Lewy body pathology raises the possibility that abnormal processing of APP could be linked directly with neurofibrillary pathology. Neurofibrillary tangles and neuritic plaques are sites of dense accumulation of pathological paired helical filaments (PHFs) which are composed in part of an antigenically modified form of the microtubule-associated protein tau. The average brain tissue content of PHFs measured biochemically does not increase in the course of normal ageing but increases 10-fold relative to age-matched controls in patients with Alzheimer's disease. There is also a substantial (three-fold) disease-related decline in normal soluble tau protein relative to age-matched controls. This intracellular redistribution of a protein essential for microtubule stability in cortico-cortical association circuits may play an important part in the molecular pathogenesis of dementia in Alzheimer's disease. The role of abnormal proteolysis of APP in this process remains to be elucidated. Immunohistochemical studies on renal dialysis cases have failed to detect evidence of neurofibrillary pathology related to aluminium accumulation in brain tissue. Nevertheless it needs to be seen whether more sensitive biochemical assays of neurofibrillary pathology can demonstrate evidence of an association with aluminium.

Comparative biochemistry of tau in progressive supranuclear palsy, corticobasal degeneration, FTDP-17 and Pick's disease

Neurodegenerative disorders referred to as tauopathies have cellular hyperphosphorylated tau protein aggregates in the absence of amyloid deposits. Comparative biochemistry of tau aggregates shows that they differ in both phosphorylation and content of tau isoforms. The six tau isoforms found in human brain contain either three (3R) or four microtubule-binding domains (4R). In Alzheimer's disease, all six tau isoforms are abnormally phosphorylated and aggregate into paired helical filaments. They are detected by immunoblotting as a major tau triplet (tau55, 64 and 69). In corticobasal degeneration and progressive supranuclear palsy, only 4R-tau isoforms aggregate into twisted and straight filaments respectively. They appear as a major tau doublet (tau64 and 69). Finally, in Pick's disease, only 3R-tau isoforms aggregate into random coiled filaments. They are characterized by another major tau doublet (tau55 and 64). These differences in tau isoforms may be related to either the degeneration of particular cell populations in a given disorder or aberrant cell trafficking of particular tau isoforms. Finally, recent findings provide a direct link between a genetic defect in tau and its abnormal aggregation into filaments in fronto-temporal dementia with Parkinsonism linked to chromosome 17, demonstrating that tau aggregation is sufficient for nerve cell degeneration. Thus, tau mutations and polymorphisms may also be instrumental in many neurodegenerative disorders.

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.

Clinical and molecular aspects of the myotonic dystrophies: a review

Type 1 myotonic dystrophy or DM1 (Steinert's disease), which is the commonest muscular dystrophy in adults, has intrigued physicians for over a century. Unusual features, compared with other dystrophies, include myotonia, anticipation, and involvement of other organs, notably the brain, eyes, smooth muscle, cardiac conduction apparatus, and endocrine system. Morbidity is high, with a substantial mortality relating to cardiorespiratory dysfunction. More recently a second form of multisystem myotonic disorder has been recognized and variously designated as proximal myotonic myopathy (PROMM), proximal myotonic dystrophy (PDM), or DM2. For both DM1 and DM2 the molecular basis is expansion of an unstable repeat sequence in a noncoding part of a gene (DMPK in DM1 and ZNF9 in DM2). There is accumulating evidence that the basic molecular mechanism is disruption of mRNA metabolism, which has far-reaching effects on many other genes, in part through the induction of aberrant splicing, explaining the multisystemic nature of the disease. The unstable nature of the expansion provides a molecular explanation for anticipation. This review emphasizes the clinical similarities and differences between DM1 and DM2. It examines current views about the molecular basis of these disorders, and contrasts them with other repeat expansion disorders that have increasingly been recognized as a cause of neurological disease.

Lewy body densities in the entorhinal and anterior cingulate cortex predict cognitive deficits in Parkinson's disease

Previous studies reported an association between cortical Lewy body (LB) formation and dementia in Parkinson's disease (PD). However, it is unclear whether cognitive decline in this disorder is related to specific patterns of LB distribution within the cerebral cortex. Moreover, the prediction of cognitive status based on concomitant assessment of LB and Alzheimer's disease lesions has led to conflicting results. We performed a clinicopathological study in 22 elderly PD patients in whom parkinsonism preceded cognitive decline by at least 3 years. Cognitive status was assessed prospectively using the clinical dementia rating scale (CDR); quantitative assessment of LB, neurofibrillary tangles (NFT), and senile plaques (SP) was performed in Brodmann areas 9, 21, 24, 40 and the entorhinal cortex. Statistical analysis was performed using both correlation coefficients and logistic regression models. There was a highly significant correlation between CDR scores and regional LB scores in the entorhinal cortex and area 24. LB and SP densities in the entorhinal cortex accounted for 36.2% and 19.3% of the variability in CDR scores. LB densities in area 24 could explain 25.2% of this variability. NFT densities did not predict cognitive status. In multivariate models only LB densities in the entorhinal cortex and anterior cingulate cortex were significantly associated with CDR scores. These results imply that an assessment of LB pathology limited to the entorhinal cortex and area 24 may be sufficient to predict cognition in PD. They also suggest that LB formation in limbic areas may be crucial for the development of PD dementia.

Lysosomal activation is a compensatory response against protein accumulation and associated synaptopathogenesis--an approach for slowing Alzheimer disease?

Previous reports suggest that age-related lysosomal disturbances contribute to Alzheimer-type accumulations of protein species, blockage of axonal/dendritic transport, and synaptic decline. Here, we tested the hypothesis that lysosomal enzymes are upregulated as a compensatory response to pathogenic protein accumulation. In the hippocampal slice model, tau deposits and amyloidogenic fragments induced by the lysosomal inhibitor chloroquine were accompanied by disrupted microtubule integrity and by corresponding declines in postsynaptic glutamate receptors and the presynaptic marker synaptophysin. In the same slices, cathepsins B, D, and L, beta-glucuronidase, and elastase were upregulated by 70% to 135%. To address whether this selective activation of the lysosomal system represents compensatory signaling, N-Cbz-L-phenylalanyl-L-alanyl-diazomethylketone (PADK) was used to enhance the lysosome response, generating 4- to 8-fold increases in lysosomal enzymes. PADK-mediated lysosomal modulation was stable for weeks while synaptic components remained normal. When PADK and chloroquine were co-infused, chloroquine no longer increased cellular tau levels. To assess pre-existing pathology, chloroquine was applied for 6 days after which its removal resulted in continued degeneration. In contrast, enhancing lysosomal activation by replacing chloroquine after 6 days with PADK led to clearance of accumulated protein species and restored microtubule integrity. Transport processes lost during chloroquine exposure were consequently re-established, resulting in marked recovery of synaptic components. These data indicate that compensatory activation of lysosomes follows protein accumulation events, and that lysosomal modulation represents a novel approach for treating Alzheimer disease and other protein deposition diseases.

Tau protein isoforms, phosphorylation and role in neurodegenerative disorders

Tau proteins belong to the family of microtubule-associated proteins. They are mainly expressed in neurons where they play an important role in the assembly of tubulin monomers into microtubules to constitute the neuronal microtubules network. Microtubules are involved in maintaining the cell shape and serve as tracks for axonal transport. Tau proteins also establish some links between microtubules and other cytoskeletal elements or proteins. Tau proteins are translated from a single gene located on chromosome 17. Their expression is developmentally regulated by an alternative splicing mechanism and six different isoforms exist in the human adult brain. Tau proteins are the major constituents of intraneuronal and glial fibrillar lesions described in Alzheimer's disease and numerous neurodegenerative disorders referred to as 'tauopathies'. Molecular analysis has revealed that an abnormal phosphorylation might be one of the important events in the process leading to their aggregation. Moreover, a specific set of pathological tau proteins exhibiting a typical biochemical pattern, and a different regional and laminar distribution could characterize each of these disorders. Finally, a direct correlation has been established between the progressive involvement of the neocortical areas and the increasing severity of dementia, suggesting that pathological tau proteins are reliable marker of the neurodegenerative process. The recent discovery of tau gene mutations in frontotemporal dementia with parkinsonism linked to chromosome 17 has reinforced the predominant role attributed to tau proteins in the pathogenesis of neurodegenerative disorders, and underlined the fact that distinct sets of tau isoforms expressed in different neuronal populations could lead to different pathologies.

Natural and experimental oral infection of nonhuman primates by bovine spongiform encephalopathy agents

Experimental lemurs either were infected orally with the agent of bovine spongiform encephalopathy (BSE) or were maintained as uninfected control animals. Immunohistochemical examination for proteinase-resistant protein (prion protein or PrP) was performed on tissues from two infected but still asymptomatic lemurs, killed 5 months after infection, and from three uninfected control lemurs. Control tissues showed no staining, whereas PrP was detected in the infected animals in tonsil, gastrointestinal tract and associated lymphatic tissues, and spleen. In addition, PrP was detected in ventral and dorsal roots of the cervical spinal cord, and within the spinal cord PrP could be traced in nerve tracts as far as the cerebral cortex. Similar patterns of PrP immunoreactivity were seen in two symptomatic and 18 apparently healthy lemurs in three different French primate centers, all of which had been fed diets supplemented with a beef protein product manufactured by a British company that has since ceased to include beef in its veterinary nutritional products. This study of BSE-infected lemurs early in their incubation period extends previous pathogenesis studies of the distribution of infectivity and PrP in natural and experimental scrapie. The similarity of neuropathology and PrP immunostaining patterns in experimentally infected animals to those observed in both symptomatic and asymptomatic animals in primate centers suggests that BSE contamination of zoo animals may have been more widespread than is generally appreciated.

Phosphorylation of specific sets of tau isoforms reflects different neurofibrillary degeneration processes

Tau proteins are the basic components of filaments that accumulate within neurons during neurofibrillary degeneration, a degenerating process with disease-specific phenotypes. This specificity is likely to be sustained by both phosphorylation state and isoform content of tau aggregates that form neuronal inclusions. In the present study, characterization of tau isoforms involved in neurofibrillary degeneration in Alzheimer's disease, Pick's disease, corticobasal degeneration and progressive supranuclear palsy was performed. Both analyses by immunoblotting using specific tau antibodies and cell transfection by tau isoform cDNAs allowed us to demonstrate the aggregation of (1) the six hyperphosphorylated tau isoforms in Alzheimer's disease, (2) tau isoforms without exon 10-encoding sequence in Pick's disease and (3) hyperphosphorylated exon 10-tau isoforms in corticobasal degeneration and progressive supranuclear palsy. Thus, neurofibrillary degeneration phenotypes are likely to be related to the phosphorylation of different combinations of tau isoforms (with and/or without exon 10-encoding sequence) in subpopulations of neurons.

Interhemispheric disconnection syndrome in Alzheimer's disease

It is commonly acknowledged that patients with Alzheimer's disease show memory and cognitive deficits that result from their cerebral histopathological abnormalities. We report new evidence showing that they also manifest deficits in interhemispheric integration of information, probably reflecting a corpus callosum dysfunction. Patients were given a battery of motor, somatosensory, and visual tests that had to be carried out by using either one or both hemispheres. Tasks were chosen such that subjects with Alzheimer's disease performed normally when using intrahemispheric processing. They, however, performed poorly when interhemispheric communication was required. This observation attests to the presence of a disconnection syndrome and suggests that these interhemispheric tasks can serve as diagnostic tools for the early assessment of their dementia.

Apolipoprotein E and Tau phosphorylation in human neuroblastoma cells

Phosphorylation is the major post-translational modification of Tau proteins and it plays an important role in Tau biological functions. Hyperphosphorylation of these proteins occurs during neurodegenerative disorders such as Alzheimer's disease. It was hypothesized that some variants of apolipoprotein E (apo E) may have a protective effect against the normal or pathological phosphorylation of Tau proteins. We have recently shown that apo E synthesis occurs in human SY 5Y and Kelly neuroblastoma cell lines which express different isoforms (E3 for SY 5Y; E3 and E4 for Kelly) [Dupont-Wallois, L., Soulié, C., Sergeant, N., Wavrant-de Wrieze, F., Chartier-Harlin, M.C., Delacourte, A. and Caillet-Boudin, M.L., Neurobiol. Dis., 4 (1997) 356-364]. Therefore, this cellular model makes it possible to study the differential influence, if any, of apo E3 and E4 on Tau phosphorylation. Using a large panel of Tau phosphorylation-dependent antibodies, we were not able to detect a significant difference in Tau immunoreactivity linked to the different apo E genotypes, even when the hyperphosphorylation of Tau proteins was induced by treating cells with Okadaic acid (OA), an inhibitor of phosphatase 1 and 2A proteins. Thus, a difference in apo E isoforms had no dramatic effect upon Tau phosphorylation in native or OA treated cells.

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.

Atypical form of Alzheimer's disease with prominent posterior cortical atrophy: a review of lesion distribution and circuit disconnection in cortical visual pathways

In recent years, the existence of visual variants of Alzheimer's disease characterized by atypical clinical presentation at onset has been increasingly recognized. In many of these cases post-mortem neuropathological assessment revealed that correlations could be established between clinical symptoms and the distribution of neurodegenerative lesions. We have analyzed a series of Alzheimer's disease patients presenting with prominent visual symptomatology as a cardinal sign of the disease. In these cases, a shift in the distribution of pathological lesions was observed such that the primary visual areas and certain visual association areas within the occipito-parieto-temporal junction and posterior cingulate cortex had very high densities of lesions, whereas the prefrontal cortex had fewer lesions than usually observed in Alzheimer's disease. Previous quantitative analyses have demonstrated that in Alzheimer's disease, primary sensory and motor cortical areas are less damaged than the multimodal association areas of the frontal and temporal lobes, as indicated by the laminar and regional distribution patterns of neurofibrillary tangles and senile plaques. The distribution of pathological lesions in the cerebral cortex of Alzheimer's disease cases with visual symptomatology revealed that specific visual association pathways were disrupted, whereas these particular connections are likely to be affected to a less severe degree in the more common form of Alzheimer's disease. These data suggest that in some cases with visual variants of Alzheimer's disease, the neurological symptomatology may be related to the loss of certain components of the cortical visual pathways, as reflected by the particular distribution of the neuropathological markers of the disease.

Life and death of neurons in the aging brain

Neurodegenerative disorders are characterized by extensive neuron death that leads to functional decline, but the neurobiological correlates of functional decline in normal aging are less well defined. For decades, it has been a commonly held notion that widespread neuron death in the neocortex and hippocampus is an inevitable concomitant of brain aging, but recent quantitative studies suggest that neuron death is restricted in normal aging and unlikely to account for age-related impairment of neocortical and hippocampal functions. In this article, the qualitative and quantitative differences between aging and Alzheimer's disease with respect to neuron loss are discussed, and age-related changes in functional and biochemical attributes of hippocampal circuits that might mediate functional decline in the absence of neuron death are explored. When these data are viewed comprehensively, it appears that the primary neurobiological substrates for functional impairment in aging differ in important ways from those in neurodegenerative disorders such as Alzheimer's disease.

Cerebral cortex pathology in aging and Alzheimer's disease: a quantitative survey of large hospital-based geriatric and psychiatric cohorts

In order to explore the relationships between the involvement of specific neuronal populations and cognitive deterioration, and to compare the hierarchical patterns of cortical involvement in normal brain aging and Alzheimer's disease, over 1200 brains from elderly subjects without cognitive deficits, as well as from patients with age-associated memory impairment and Alzheimer's disease, were examined. Our results suggest that the neuropathological changes associated with normal brain aging and Alzheimer's disease affect select cortical circuits at different points in time. Extensive hippocampal alterations are correlated with age-associated memory impairment, whereas substantial neurofibrillary tangle formation in neocortical association areas of the temporal lobe is a prerequisite for the development of Alzheimer's disease. Despite several lines of evidence involving amyloid deposit in the pathogenesis of Alzheimer's disease and Down's syndrome, our observations indicate that there is no correlation between senile plaque densities and degree of dementia in both disorders. In contrast to younger elderly cases, in the ninth and tenth decades of life, there is a differential cortical involvement in that parietal and cingulate areas are early affected in the course of Alzheimer's disease, and neocortical senile plaques densities are strongly correlated with the severity of dementia. Moreover, Alzheimer's disease symptomatology is characterized in these very old patients by high neurofibrillary tangle densities in the anterior CA1 field, but not in the entorhinal cortex and inferior temporal cortex. These observations are discussed in the light of the hypothesis of global corticocortical disconnection and with respect to the notion of selective neuronal vulnerability in Alzheimer's disease.

Brain microvascular changes in Alzheimer's disease and other dementias

Vasculopathy in Alzheimer's disease (AD) may represent an important pathogenetic factor of this disorder. In the present study, microvasculature was studied by immunohistochemistry using a monoclonal antibody against a vascular heparan sulfate proteoglycan. Vascular changes were consistently observed in AD and included decrease in vascular density, presence of atrophic and coiling vessels, and glomerular loop formations. The laminar and regional distribution of these vascular alterations was correlated with the presence of neurofibrillary tangles. However, vascular changes may also follow neuronal loss. Vascular density may be related to a decrease in brain metabolism. Furthermore, one of the main features of AD is the presence of amyloid deposits within brain parenchyma and blood vessel walls. It is not yet clear whether amyloid components are derived from the blood or the central nervous system. Because AD is clearly heterogeneous, based on clinical and genetic data, evidence for either a brain or peripheral origin is discussed. Microvasculature was also analyzed in other neurodegenerative disorders devoid of amyloid deposits including amyotrophic lateral sclerosis/parkinsonism-dementia complex of Guam and Pick's disease. In conclusion, if vasculopathy in neurodegenerative disorders is not directly involved in pathogenesis, it may act synergistically with other pathogenetic mechanisms including genetic and environmental factors. This aspect of pathology is particularly interesting in view of its accessibility to therapeutic interventions.

Pathological tau proteins in postencephalitic parkinsonism: comparison with Alzheimer's disease and other neurodegenerative disorders

Immunohistochemical and biochemical analyses of hyperphosphorylated tau proteins, the major component of neurofibrillary tangles, were performed in different brain regions from patients presenting with postencephalitic parkinsonism. Neurofibrillary tangles were found in hippocampus, neocortical areas (mostly in supragranular layers), and several subcortical structures. By immunoblotting, a tau protein triplet similar to the one seen in Alzheimer's disease was observed. This biochemical approach allows for the definition of postencephalitic parkinsonism from certain neurodegenerative disorders such as progressive supranuclear palsy and corticobasal degeneration.

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.

The neurochemistry of Alzheimer's disease

The last 15-20 years have seen a wealth of studies to characterize the neurochemical abnormalities of Alzheimer's disease, in particular those involving the beta-amyloid and tau proteins, as well as more recently, apolipoprotein E4. This article provides a summary of the evidence for the involvement of these proteins in Alzheimer's disease pathogenesis based on postmortem brain and CSF studies.

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.

Dephosphorylation studies of SKNSH-SY 5Y cell Tau proteins by endogenous phosphatase activity

Recent data have shown that the microtubule-associated Tau proteins are phosphorylated but to a lesser extent than PHF-Tau proteins which are the major components of Alzheimer's disease paired helical filaments. These normal Tau proteins are highly sensitive to the endogenous phosphatase activity during post-mortem delay. In order to understand the basic equilibrium between phosphatase and kinase activities, phosphorylation and dephosphorylation mechanisms of Tau proteins were studied in neuroblastoma cells. The present results demonstrate that an endogenous phosphatase activity is present and directed on Tau proteins in the SKNSH-SY 5Y cell extracts. Interestingly, the okadaic acid-induced hyperphosphorylated Tau proteins are more resistant to the phosphatase activity than the control Tau proteins. Our data emphasize the value of this in vitro cellular model for the study of biological conditions that control Tau protein phosphorylation levels.

Tau protein in cerebrospinal fluid: a biochemical marker for axonal degeneration in Alzheimer disease?

Cerebrospinal fluid (CSF) biochemical markers for Alzheimer disease (AD) would be of great value to improve the clinical diagnostic accuracy of the disorder. As abnormally phosphorylated forms of the microtubule-associated protein tau have been consistently found in the brains of AD patients, and since tau can be detected in CSF, two assays based on several well-defined monoclonal tau antibodies were used to study these proteins in CSF. One assay detects most normal and abnormal forms of tau (CSF-tau), while the other is highly specific for phosphorylated tau (CSF-PHFtau). A marked increase in CSF-PHFtau was found in AD (2230 +/- 930 pg/mL), as compared with controls (640 +/- 230 pg/mL; p < 0.0001), vascular dementia, VAD (1610 +/- 840 pg/mL; p < 0.05), frontal lobe dementia, FLD (1530 +/- 1000 pg/mL; p < 0.05), Parkinson disease, PD (720 +/- 590 pg/mL; p < 0.0001), and patients with major depression (230 +/- 130 pg/mL; p < 0.0001). Parallel results were obtained for CSF-tau. No less than 35/40 (88%) of AD patients had a CSF-PHFtau value higher than the cutoff level of 1140 pg/mL in controls. The present study demonstrates that elevated tau/PHFtau levels are consistently found in CSF of AD patients. However, a considerable overlap is still present with other forms of dementia, both VAD and FLD. CSF-tau and CSF-PHFtau may therefore be useful as a positive biochemical marker, to discriminate AD from normal aging, PD, and depressive pseudodementia. Further studies are needed to clarify the sensitivity and specificity of these assays, including follow-up studies with neuropathological examinations.

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.

Increased expression and subcellular translocation of the mitogen activated protein kinase kinase and mitogen-activated protein kinase in Alzheimer's disease

The sequential activation of the mitogen-activated protein kinase kinase and its substrate, the mitogen-activated protein kinase is involved in a cascade of protein kinases which link a number of cell surface signals to intracellular changes in enzyme activity and gene expression. In vitro, mitogen-activated protein kinase is able to phosphorylate the microtubule-associated protein tau at Ser-Pro and Thr-Pro sites, thereby generating abnormally hyperphosphorylated tau species that are similar to paired helical filament-tau found in Alzheimer's disease. In the present study, we analysed the levels of immunoreactive mitogen-activated protein kinase kinase and mitogen-activated protein kinase in the temporal cortex (area 22) of patients with Alzheimer's disease by means of enzyme-linked immuno-sorbent assays and compared these changes with the content of abnormally phosphorylated paired helical filament-tau. The levels of immunochemically detected mitogen-activated protein kinase kinase and mitogen-activated protein kinase were both increased in Alzheimer's disease by between 35 and 40% compared with age-matched controls. Elevation of mitogen-activated protein kinase kinase was most pronounced during early stages of Alzheimer's disease and was inversely related to the tissue content of abnormally phosphorylated paired helical filament-tau. Pronounced immunoreactivity of mitogen-activated protein kinase kinase and mitogen-activated protein kinase was present in both tangle bearing neurons and unaffected neurons of the temporal cortex. Immunoreactive neurons were most often localized in the direct vicinity of neuritic plaques. In Alzheimer's disease, the subcellular distribution of mitogen-activated protein kinase kinase and mitogen-activated protein kinase showed a striking translocation from the cytoplasmic to the nuclear compartment. It is suggested that the activation of the mitogen-activated protein kinase cascade which appears to be an early feature of Alzheimer's disease might be critically involved in self-stimulating processes of neurodegeneration and aberrant repair under these conditions.

Quantitation of Alzheimer's amyloid peptide and identification of related amyloid proteins by dot-blot immunoassay

In Alzheimer's disease, the main component of amyloid deposits is a 39-43 amino acid peptide referred to as amyloid peptide or A beta. A crucial issue in the study of this disorder is to define the sequence of events that lead to amyloid deposition. In the present study, a new approach was developed that allows to specifically solubilize A beta peptide trapped within amyloid deposits and to quantify its amount by dot-blot immunoassay. The present method also permits to isolate components tightly bound to A beta and that are likely to catalyze its aggregation. Biochemical A beta quantitation was performed in 4 Brodmann areas from 17 elderly individuals exhibiting different degrees of amyloidosis. In parallel, classical neuropathology was done by histochemical and immunohistochemical methods. A beta amounts (pmol) were correlated to the number of amyloid deposits determined by neuropathology showing high statistical significance. Moreover, amyloid-binding proteins including apolipoprotein E and heparan sulfate proteoglycans were also found associated to A beta in the amyloid preparation. The present biochemical procedure is a new and reliable method to quantify amyloid deposition in brain. Furthermore, it allows to detect amyloid-associated components such as apolipoprotein E, that may be involved in the pathological process of amyloidogenesis.

New immunoassay for the mapping of neurofibrillary degeneration in Alzheimer's disease using two monoclonal antibodies against human paired helical filament tau proteins

Monoclonal antibodies against human paired helical filament tau (PHF-tau) proteins were produced. Two of these antibodies, AD1 and AD2, were shown by immunoblot to be directed against distinct hyperphosphorylated epitopes of the PHF-tau proteins. Using AD1 and AD2, an antigen-capture ELISA specific for PHF-tau proteins was developed and used to map the neurofibrillary degeneration of several Broadmann areas from an Alzheimer's disease patient. The results confirm that the neurofibrillary degeneration predominates in parietal and temporal lobes.

beta-amyloid fibrils induce tau phosphorylation and loss of microtubule binding

A central issue in the pathogenesis of Alzheimer's disease (AD) is the relationship between amyloid deposition and neurofibrillary tangle formation. To determine whether amyloid fibril formation affects the phosphorylation state of tau, primary cultures of fetal rat hippocampal and human cortical neurons were treated with beta-amyloid (beta A) in a soluble, amorphous-aggregated, or fibrillar form. Fibrillar beta A, but not soluble or amorphous-aggregated beta A, markedly induces the phosphorylation of tau at Ser-202 and Ser-396/Ser-404, resulting in a shift in the tau M(r) in human cortical neurons. Hyperphosphorylated tau accumulates in the somatodendritic compartment of fibrillar beta A-treated neurons in a soluble form that is not associated with microtubules and is incapable of binding to microtubules in vitro. Dephosphorylation of beta A-induced tau restores its capacity to bind to microtubules. Thus, amyloid fibril formation alters the phosphorylation state of tau, resulting in the loss of microtubule binding capacity and somatodendritic accumulation, properties also exhibited by tau in the AD brain. Amyloid fibril formation may therefore be a cause of abnormal tau phosphorylation in AD.

The morphologic and neurochemical basis of dementia: aging, hierarchical patterns of lesion distribution and vulnerable neuronal phenotype

Alzheimer's disease is the most common form of dementia in elderly individuals. Approximately 11% of the population older than 65, and up to 50% of individuals over 85 qualify as having "probable Alzheimer's disease" on the basis of clinical evaluation. Since the early description of the clinical symptoms and neuropathologic features of Alzheimer's disease, there has been an extraordinary growth in the knowledge of the morphologic and molecular characteristics of Alzheimer's disease. Although the pathogenetic events that lead to dementia are not yet fully understood, several hypotheses regarding the formation of the hallmark pathologic structures of Alzheimer's disease have been proposed. In this context, the use of specific histochemical techniques in the primate brain has greatly expanded our understanding of neuron typology, connectivity and circuit distribution in relation to neurochemical identity. In this respect, very specific subsets of cortical neurons and cortical afferents can be identified by their particular content of certain neurotransmitters and structural proteins. In this article, we discuss the possible relationships between the distribution of pathologic changes in aging, Alzheimer's disease, and possibly related dementing conditions, in the context of the specific elements of the cortical circuitry that are affected by these alterations. Also, evidence for links between the neurochemical phenotype of a given neuron and its relative vulnerability or resistance to the degenerative process are presented in order to correlate the distribution of cellular pathologic changes, neurochemical characteristics related to vulnerability, and affected cortical circuits.

Dementia lacking distinctive histopathology: clinicopathological evaluation of 32 cases

We report the neuropathological findings in 32 patients, aged 46-86 years, with dementia lacking distinctive histopathology. All of the patients were classified clinically as having Pick's or atypical Pick's disease, but the routine neuropathological evaluation showed no specific histopathological changes such as Pick bodies, senile plaques, neurofibrillary tangles or Lewy bodies. In 50% of the cases the first symptoms appeared before 65 years of age. However, there were 9 patients with onset in the eighth decade. Positive family history was found only in 6 presenile cases. The retrospective evaluation of the clinical records revealed the consistent presence of "frontal" symptomatology, including loss of personal awareness, inappropriate euphoria and stereotyped behavior. Speech disorders were observed in 80% of the cases, whereas temporospatial disorientation and memory impairment were less frequent. Praxis and gnosis were strikingly preserved in most of the cases. The macroscopic neuropathological examination revealed frontal or temporopolar atrophy in 97% of the cases, while the hippocampus and subcortical structures were relatively spared in the majority of the cases. Histologically, four groups were recognized. Group A showed moderate to severe neuron loss and gliosis in the frontal and/or temporopolar cortex without subcortical involvement. In group B, the neocortical cell loss was widespread, and the striatum and substantia nigra displayed differential degrees of gliosis but no neuron loss. Group C patients showed a lesion distribution comparable to that observed in group B but with severe neuron loss in at least one subcortical region. Four cases formed group D, which was characterized by the preservation of the pyramidal neurons in the neocortex and variable subcortical changes. Despite these differences in the topography of pathological changes, all of the cases shared a similar clinical profile. These findings further demonstrate the epidemiological and neuropathological heterogeneity of dementia lacking distinctive histopathology. Furthermore, they suggest that the same clinical manifestations may correspond to several distinct pathological processes in this condition.

Shift from fetal-type to Alzheimer-type phosphorylated Tau proteins in SKNSH-SY 5Y cells treated with okadaic acid

Tau proteins are abnormally phosphorylated in Alzheimer's disease. Pathological Tau proteins named PHF-Tau 55, PHF-Tau 64, and PHF-Tau 69, are the main constituents of the paired helical filaments (PHF). When treating SKNSH-SY 5Y cells with okadaic acid (OA), Tau 55 protein was clearly induced whereas Tau 64 protein was only faintly induced. Here, we show that the absence of Tau 69 could be explained by the fact that adult isoforms containing N-terminal inserts are not detected. Phosphorylation is similar for untreated cellular Tau proteins and fetal Tau proteins, while OA cell treatment transformed fetal-type into Alzheimer-type phosphorylated proteins.