Ultrastructural characteristics of tau filaments in tauopathies: Immuno-electron microscopic demonstration of tau filaments in tauopathies

Untangling tau imaging

In vivo imaging of tau deposits is providing a better understanding of the temporal and spatial tau deposition in the brain, allowing a more comprehensive insight into the causes, diagnoses, and potentially treatment of tauopathies such as Alzheimer's disease, progressive supranuclear palsy, corticobasal syndrome, chronic traumatic encephalopathy, and some variants of frontotemporal lobar degeneration. The assessment of tau deposition in the brain over time will allow a deeper understanding of the relationship between tau and other variables such as cognition, genotype, and neurodegeneration, as well as assessing the role tau plays in ageing. Preliminary human studies suggest that tau imaging could also be used as a diagnostic, prognostic, and theranostic biomarker, as well as a surrogate marker for target engagement, patient recruitment, and efficacy monitoring for disease-specific therapeutic trials.

Glial Tau Pathology in Tauopathies: Functional Consequences

Tauopathies are a class of neurodegenerative diseases characterized by the presence of hyperphosphorylated and aggregated tau pathology in neuronal and glial cells. Though the ratio of neuronal and glial tau aggregates varies across diseases, glial tau aggregates can populate the same degenerating brain regions as neuronal tau aggregates. While much is known about the deleterious consequences of tau pathology in neurons, the relative contribution of glial tau pathology to these diseases is less clear. Recent studies using a number of model systems implicate glial tau pathology in contributing to tauopathy pathogenesis. This review aims to highlight the functional consequences of tau overexpression in glial cells and explore the potential contribution of glial tau pathology in the pathogenesis of neurodegenerative tauopathies.

Biochemical classification of tauopathies by immunoblot, protein sequence and mass spectrometric analyses of sarkosyl-insoluble and trypsin-resistant tau

Intracellular filamentous tau pathology is the defining feature of tauopathies, which form a subset of neurodegenerative diseases. We have analyzed pathological tau in Alzheimer’s disease, and in frontotemporal lobar degeneration associated with tauopathy to include cases with Pick bodies, corticobasal degeneration, progressive supranuclear palsy, and ones due to intronic mutations in MAPT. We found that the C-terminal band pattern of the pathological tau species is distinct for each disease. Immunoblot analysis of trypsin-resistant tau indicated that the different band patterns of the 7–18 kDa fragments in these diseases likely reflect different conformations of tau molecular species. Protein sequence and mass spectrometric analyses revealed the carboxyl-terminal region (residues 243–406) of tau comprises the protease-resistant core units of the tau aggregates, and the sequence lengths and precise regions involved are different among the diseases. These unique assembled tau cores may be used to classify and diagnose disease strains. Based on these results, we propose a new clinicopathological classification of tauopathies based on the biochemical properties of tau.

A Novel Tau Mutation in Exon 12, p.Q336H, Causes Hereditary Pick Disease

Pick disease (PiD) is a frontotemporal lobar degeneration with distinctive neuronal inclusions (Pick bodies) that are enriched in 3-repeat (3R) tau. Although mostly sporadic, mutations in the tau gene (MAPT) have been reported. We screened 24 cases of neuropathologically confirmed PiD for MAPT mutations and found a novel mutation (c.1008G>C, p.Q336H) in 1 patient. Pathogenicity was confirmed on microtubule assembly and tau filament formation assays. The patient was compared with sporadic PiD and PiD associated with MAPT mutations from a review of the literature. The patient had behavioral changes at 55 years of age, followed by reduced verbal fluency, parkinsonism, and death at 63 years of age. His mother and maternal uncle had similar symptoms. Recombinant tau with p.Q336H mutation formed filaments faster than wild-type tau, especially with 3R tau. It also promoted more microtubule assembly than wild-type tau. We conclude that mutations in MAPT, including p.Q336H, can be associated with clinical, pathologic, and biochemical features that are similar to those in sporadic PiD. The pathomechanism of p.Q336H, and another previously reported variant at the same codon (p.Q336R), seems to be unique to MAPT mutations in that they not only predispose to abnormal tau filament formation but also facilitate microtubule assembly in a 3R tau-dependent manner.

Insight into the Molecular Imaging of Alzheimer's Disease

Alzheimer's disease is a complex neurodegenerative disease affecting millions of individuals worldwide. Earlier it was diagnosed only via clinical assessments and confirmed by postmortem brain histopathology. The development of validated biomarkers for Alzheimer's disease has given impetus to improve diagnostics and accelerate the development of new therapies. Functional imaging like positron emission tomography (PET), single photon emission computed tomography (SPECT), functional magnetic resonance imaging (fMRI), and proton magnetic resonance spectroscopy provides a means of detecting and characterising the regional changes in brain blood flow, metabolism, and receptor binding sites that are associated with Alzheimer's disease. Multimodal neuroimaging techniques have indicated changes in brain structure and metabolic activity, and an array of neurochemical variations that are associated with neurodegenerative diseases. Radiotracer-based PET and SPECT potentially provide sensitive, accurate methods for the early detection of disease. This paper presents a review of neuroimaging modalities like PET, SPECT, and selected imaging biomarkers/tracers used for the early diagnosis of AD. Neuroimaging with such biomarkers and tracers could achieve a much higher diagnostic accuracy for AD and related disorders in the future.

The domestic cat as a natural animal model of Alzheimer's disease

Introduction

Alzheimer’s disease (AD) is the most dominant neurodegenerative disorder that causes dementia, and no effective treatments are available. To study its pathogenesis and develop therapeutics, animal models representing its pathologies are needed. Although many animal species develop senile plaques (SP) composed of amyloid-β (Aβ) proteins that are identical to those found in humans, none of them exhibit neurofibrillary tangles (NFT) and subsequent neurodegeneration, which are integral parts of the pathology of AD.

Results

The present study shows that Aβ accumulation, NFT formation, and significant neuronal loss all emerge naturally in the hippocampi of aged domestic cats. The NFT that form in the cat brain are identical to those seen in human AD in terms of their spatial distribution, the cells they affect, and the tau isoforms that comprise them. Interestingly, aged cats do not develop mature argyrophilic SP, but instead accumulate intraneuronal Aβ oligomers in their hippocampal pyramidal cells, which might be due to the amino acid sequence of felid Aβ.

Conclusions

These results suggest that Aβ oligomers are more important than SP for NFT formation and the subsequent neurodegeneration. The domestic cat is a unique animal species that naturally replicates various AD pathologies, especially Aβ oligomer accumulation, NFT formation, and neuronal loss.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-015-0258-3) contains supplementary material, which is available to authorized users.

An autopsied case of unclassifiable sporadic four-repeat tauopathy presenting with parkinsonism and speech disturbances

A 48-year-old Japanese woman experienced slow-onset parkinsonism and speech disturbances. Neurological examinations revealed rigidity in the trunk and extremities, bradykinesia and postural instability, although cognitive impairments and psychiatric symptoms were not apparent in the early disease stage. Neuroimaging revealed progressive bilateral frontotemporal lobe atrophy with cerebral blood flow hypoperfusion. No apparent signs of lower motor neuron involvement were observed, such as fasciculation or electromyogram findings. She eventually reached the akinetic mutism state, and gastrostomy and tracheotomy were performed at 4 years after onset. A clinical diagnosis of progressive supranuclear palsy was made prior to her death, which occurred 6 years after onset. Post mortem examinations revealed that the brain weighed 1200 g and showed atrophy of the frontotemporal lobe and brainstem. Severe neuron loss and gliosis were observed in the frontotemporal lobe. The superior and middle frontal gyri were the most severely affected and showed spongiform changes in the superficial layer. The globus pallidus, subthalamic nucleus, cerebellar dentate nucleus, substantia nigra and inferior olivary nucleus also showed neuronal loss with gliosis. Using hyperphosphorylated tau (AT-8) immunostaining, pretangle-like neurons, numerous short threads and glial tau pathology were extensively observed. Using Gallyas-Braak silver staining, thin and short threads were also extensively observed, but considerably fewer than those observed by AT-8 immunostaining. Neither astrocytic plaques nor tuft-shaped astrocytes were observed. Examination by immunoelectron microscopy showed straight fibrils approximately 15 nm in diameter in the neuronal cytoplasmic inclusions in the cerebral cortex and in the fibrillary structures in the cerebral white matter. Western blot analysis of sarkosyl-insoluble tau revealed predominantly four-repeat tau and a banding pattern similar to that seen in progressive supranuclear palsy. No pathogenic mutations were found during the gene analysis of microtubule-associated protein tau. After completing our comprehensive investigation, we diagnosed this patient with unclassifiable four-repeat tauopathy.

Tau imaging in the study of ageing, Alzheimer's disease, and other neurodegenerative conditions

In vivo tau imaging allows a deeper understanding of tau deposition in the brain, providing insights into the causes, diagnosis and treatment of primary and secondary tauopathies such as Alzheimer's disease, progressive supranuclear palsy, corticobasal syndrome, chronic traumatic encephalopathy, and some variants of frontotemporal lobar degeneration. The cross-sectional and longitudinal assessment of the temporal and spatial patterns of tau deposition in the brain will allow a better understanding of the role tau plays in ageing as well as its relationship with cognition, genotype, and neurodegeneration. It is likely that selective tau imaging could be used as a diagnostic and prognostic biomarker of disease progression, as well as a surrogate marker for monitoring of efficacy and patient recruitment for disease-specific therapeutic trials.

Invited review: Neuropathology of tauopathies: principles and practice

Tauopathies are clinically, morphologically and biochemically heterogeneous neurodegenerative diseases characterized by the deposition of abnormal tau protein in the brain. The neuropathological phenotypes are distinguished based on the involvement of different anatomical areas, cell types and presence of distinct isoforms of tau in the pathological deposits. The nomenclature of primary tauopathies overlaps with the modern classification of frontotemporal lobar degeneration. Neuropathological phenotypes comprise Pick's disease, progressive supranuclear palsy, corticobasal degeneration, argyrophilic grain disease, primary age-related tauopathy, formerly called also as neurofibrillary tangle-only dementia, and a recently characterized entity called globular glial tauopathy. Mutations in the gene encoding the microtubule-associated protein tau are associated with frontotemporal dementia and parkinsonism linked to chromosome 17. In addition, further neurodegenerative conditions with diverse aetiologies may be associated with tau pathologies. Thus, the spectrum of tau pathologies and tauopathy entities expands beyond the traditionally discussed disease forms. Detailed multidisciplinary studies are still required to understand their significance.

Pathological and Clinical Spectrum of Progressive Supranuclear Palsy: With Special Reference to Astrocytic Tau Pathology

Progressive supranuclear palsy (PSP) is a four-repeat tauopathy with tau-positive, argyrophilic tuft-shaped astrocytes (TAs). We performed a pathological and clinical investigation in 40 consecutive autopsied Japanese patients with pathological diagnoses of PSP or PSP-like disease. Unequivocal TAs were present in 22 cases, all of which were confirmed to be PSP. Such TAs were hardly detected in the other 18 cases, which instead exhibited tau-positive, argyrophilic astrocytes, appearing as comparatively small clusters with central nuclei of irregularly shaped, coarse structures (equivocal TAs). Cluster analysis of the distribution pattern of tau-related pathology for these 18 cases identified two subgroups, pallido-nigro-luysian atrophy (PNLA) Type 1 (n = 9) and Type 2 (n = 9), the former being distinguished from the latter by the presence of tau-related lesions in the motor cortex, pontine nucleus and cerebellar dentate nucleus in addition to the severely affected PNL system. The duration from symptom onset until becoming wheelchair-bound was significantly longer in PNLA Type 1. Immunoblotting of samples from the three disease conditions revealed band patterns of low-molecular-mass tau fragments at ∼35 kDa. These findings shed further light on the wide pathological and clinical spectrum of four-repeat tauopathy, representing PSP in the broad sense rather than classical PSP.

In vivo tau imaging: obstacles and progress

The military conflicts of the last decade have highlighted the growing problem of traumatic brain injury in combatants returning from the battlefield. The considerable evidence pointing at the accumulation of tau aggregates and its recognition as a risk factor in neurodegenerative conditions such as Alzheimer's disease have led to a major effort to develop selective tau ligands that would allow research into the physiopathologic underpinnings of traumatic brain injury and chronic traumatic encephalopathy in military personnel and the civilian population. These tracers will allow new insights into tau pathology in the human brain, facilitating research into causes, diagnosis, and treatment of traumatic encephalopathy and major neurodegenerative dementias, such as Alzheimer's disease and some variants of frontotemporal lobar degeneration, in which tau plays a role. The field of selective tau imaging has to overcome several obstacles, some of them associated with the idiosyncrasies of tau aggregation and others related to radiotracer design. A worldwide effort has focused on the development of imaging agents that will allow selective tau imaging in vivo. Recent progress in the development of these tracers is enabling the noninvasive assessment of the extent of tau pathology in the brain, eventually allowing the quantification of changes in tau pathology over time and its relation to cognitive performance, brain volumetrics, and other biomarkers, as well as assessment of efficacy and patient recruitment for antitau therapeutic trials.

Tau imaging: early progress and future directions

Use of selective in-vivo tau imaging will enable improved understanding of tau aggregation in the brain, facilitating research into causes, diagnosis, and treatment of major tauopathies such as Alzheimer's disease, progressive supranuclear palsy, corticobasal syndrome, chronic traumatic encephalopathy, and some variants of frontotemporal lobar degeneration. Neuropathological studies of Alzheimer's disease show a strong association between tau deposits, decreased cognitive function, and neurodegenerative changes. Selective tau imaging will allow the in-vivo exploration of such associations and measure the global and regional changes in tau deposits over time. Such imaging studies will comprise non-invasive assessment of the spatial and temporal pattern of tau deposition over time, providing insight into the role tau plays in ageing and helping to establish the relation between cognition, genotype, neurodegeneration, and other biomarkers. Once validated, selective tau imaging might be useful as a diagnostic, prognostic, and progression biomarker, and a surrogate marker for the monitoring of efficacy and patient recruitment for anti-tau therapeutic trials.

αB-crystallin and HSP27 in glial cells in tauopathies

Tauopathies are neurodegenerative diseases characterized by hyper-phosphorylated tau deposition in neurons and glial cells. Chaperones, such as small heat shock proteins αB-crystallin and HSP27 highly expressed in normal glial cells, have been postulated as putative molecules preventing abnormal deposition and folding in glial cells in tauopathies. The objective of this work was to assess the expression of αB-crystallin, phosphorylated αB-crystallin at Ser59 and HSP27 in glial cells with and without tau deposits in progressive supranuclear palsy, corticobasal degeneration (CBD), argyrophilic grain disease (AGD), Pick's disease (PiD), Alzheimer's disease, frontotemporal lobar degeneration associated with mutations in the tau gene (FTLD-tau), globular glial tauopathy (GGT) and tauopathy in the elderly. Immunohistochemistry, and double-labeling immunofluorescence and confocal microscopy have been used for this purpose. Increased expression of αB-crystallin and phosphorylated αB-crystallin at Ser59 occurs in a subpopulation of glial cells with and without hyper-phosphorylated tau deposition in all the analyzed tauopathies, but their expression in neurons is restricted to ballooned neurons in CBD, AGD and PiD. HSP27 barely co-localizes with tau and with phosphorylated αB-crystallin at Ser59, thus making the formation of active dimers operating as chaperones unlikely. Results suggest a limited function of αB-crystallin and HSP27 in preventing abnormal tau protein deposition in glial cells and neurons; in addition, the expression of αB-crystallin phosphorylated at Ser59 may act as a protective factor in glial cells.

Characterization of tau oligomeric seeds in progressive supranuclear palsy

Background

Progressive supranuclear palsy (PSP) is a neurodegenerative tauopathy which is primarily defined by the deposition of tau into globose-type neurofibrillary tangles (NFT). Tau in its native form has important functions for microtubule dynamics. Tau undergoes alternative splicing in exons 2, 3, and 10 which results in six different isoforms. Products of splicing on exon 10 are the most prone to mutations. Three repeat (3R) and four repeat (4R) tau, like other disease-associated amyloids, can form oligomers which may then go on to further aggregate and form fibrils. Recent studies from our laboratory and others have provided evidence that tau oligomers, not NFTs, are the most toxic species in neurodegenerative tauopathies and seed the pathological spread of tau.

Results

Analysis of PSP brain sections revealed globose-type NFTs, as well as both phosphorylated and unphosphorylated tau oligomers. Analysis of PSP brains via Western blot and ELISA revealed the presence of increased levels of tau oligomers compared to age-matched control brains. Oligomers were immunoprecipitated from PSP brain and were capable of seeding the oligomerization of both 3R and 4R tau isoforms.

Conclusions

This is the first time tau oligomers have been characterized in PSP. These results indicate that tau oligomers are an important component of PSP pathology, along with NFTs. The ability of PSP brain-derived tau oligomers to seed 3R and 4R tau suggests that these oligomers represent the pathological species responsible for disease propagation and the presence of oligomers in a pure neurodegenerative tauopathy implies a common neuropathological process for tau seen in diseases with other amyloid proteins.

Assessing THK523 selectivity for tau deposits in Alzheimer's disease and non-Alzheimer's disease tauopathies

Introduction

The introduction of tau imaging agents such as ¹⁸F-THK523 offers new hope for the in vivo assessment of tau deposition in tauopathies such as Alzheimer’s disease (AD), where preliminary ¹⁸F-THK523-PET studies have demonstrated significantly higher cortical retention of ¹⁸F-THK523 in AD compared to age-matched healthy individuals. In addition to AD, tau imaging with PET may also be of value in assessing non-AD tauopathies, such as corticobasal degeneration (CBD), progressive supranuclear palsy (PSP) and Pick’s disease (PiD).

Methods

To further investigate the ability of THK523 to recognize tau lesions, we undertook immunohistochemical and fluorescence studies in serial brain sections taken from individuals with AD (n = 3), CBD (n = 2), PSP (n = 1), PiD (n = 2) and Parkinson’s disease (PD; n = 2). In addition to the neuropathological analysis, one PSP patient had undergone a ¹⁸F-THK523 PET scan 5 months before death.

Results

Although THK523 labelled tau-containing lesions such as neurofibrillary tangles and neuropil threads in the hippocampus and frontal regions of AD brains, it failed to label tau-containing lesions in non-AD tauopathies. Furthermore, though THK523 faintly labelled dense-cored amyloid-β plaques in the AD frontal cortex, it failed to label α-synuclein-containing Lewy bodies in PD brain sections.

Conclusion

The results of this study suggest that ¹⁸F-THK523 selectively binds to paired helical filament tau in AD brains but does not bind to tau lesions in non-AD tauopathies, or to α-synuclein in PD brains.

Glial and neuronal tau pathology in tauopathies: characterization of disease-specific phenotypes and tau pathology progression

Tauopathies are degenerative diseases characterized by the accumulation of phosphorylated tau in neurons and glial cells. With some exceptions, tau deposits in neurons are mainly manifested as pretangles and tangles unrelated to the tauopathy. It is thought that abnormal tau deposition in neurons occurs following specific steps, but little is known about the progression of tau pathology in glial cells in tauopathies. We compared tau pathology in different astrocyte phenotypes and oligodendroglial inclusions with that in neurons in a large series of tauopathies, including progressive supranuclear palsy, corticobasal degeneration, argyrophilic grain disease, Pick disease, frontotemporal lobar degenerations (FTLD) associated with mutations in the tau gene, globular glial tauopathy (GGT), and tauopathy in the elderly. Our findings indicate that disease-specific astroglial phenotypes depend on i) the primary amino acid sequence of tau (mutated tau, 3Rtau, and 4Rtau); ii) phospho-specific sites of tau phosphorylation, tau conformation, tau truncation, and ubiquitination in that order (which parallel tau modifications related to pretangle and tangle stages in neurons); and iii) modifications of the astroglial cytoskeleton. In contrast to astrocytes, coiled bodies in oligodendrocytes have similar characteristics whatever the tauopathy, except glial globular inclusions in GGT, and coiled bodies and globular oligodendroglial inclusions in FTLD-tau/K317M. These observations indicate that tau pathology in glial cells largely parallels, but is not identical to, that in neurons in many tauopathies.

PET imaging of neuropathology in tauopathies: progressive supranuclear palsy

OBJECTIVE

Currently [18F]FDDNP is the only PET imaging probe with the ability to visualize hyperphosphorylated tau fibrillar aggregates in living subjects. In this work, we evaluate in vivo [18F]FDDNP labeling of brain neuropathology, primarily tau fibrillar aggregates, in patients with progressive supranuclear palsy (PSP), a human tauopathy usually lacking amyloid-β deposits.

METHODS

Fifteen patients with PSP received [18F]FDDNP PET scanning. [18F]FDDNP distribution volume ratios, in reference to cerebellar gray matter, were determined for cortical and subcortical areas and compared with those of patients with Parkinson's disease with short disease duration, and age-matched control subjects without neurodegenerative disorders.

RESULTS

[18F]FDDNP binding was present in subcortical areas (e.g., striatum, thalamus, subthalamic region, midbrain, and cerebellar white matter) regardless of disease severity, with progressive subcortical and cortical involvement as disease severity increased. Brain patterns of [18F]FDDNP binding were entirely consistent with the known pathology distribution for PSP. High midbrain and subthalamic region [18F]FDDNP binding was distinctive for PSP subjects and separated them from controls and patients with Parkinson's disease.

CONCLUSIONS

These results provide evidence that [18F]FDDNP is a sensitive in vivo PET imaging probe to map and quantify the dynamic regional localization of tau fibrillar aggregates in PSP. Furthermore, [18F]FDDNP PET may provide a tool to detect changes in tau pathology distribution either associated with disease progression or as a treatment biomarker for future tau-specific therapies. Patterns of [18F]FDDNP binding may also be useful in diagnosis early in disease presentation when clinical distinction among neurodegenerative disorders is often difficult.

Tangles, Toxicity, and Tau Secretion in AD - New Approaches to a Vexing Problem

When the microtubule (MT)-associated protein tau is not bound to axonal MTs, it becomes hyperphosphorylated and vulnerable to proteolytic cleavage and other changes typically seen in the hallmark tau deposits (neurofibrillary tangles) of tau-associated neurodegenerative diseases (tauopathies). Neurofibrillary tangle formation is preceded by tau oligomerization and accompanied by covalent crosslinking and cytotoxicity, making tangle cytopathogenesis a natural central focus of studies directed at understanding the role of tau in neurodegenerative disease. Recent studies suggest that the formation of tau oligomers may be more closely related to tau neurotoxicity than the presence of the tangles themselves. It has also become increasingly clear that tau pathobiology involves a wide variety of other cellular abnormalities including a disruption of autophagy, vesicle trafficking mechanisms, axoplasmic transport, neuronal polarity, and even the secretion of tau, which is normally a cytosolic protein, to the extracellular space. In this review, we discuss tau misprocessing, toxicity and secretion in the context of normal tau functions in developing and mature neurons. We also compare tau cytopathology to that of other aggregation-prone proteins involved in neurodegeneration (alpha synuclein, prion protein, and APP). Finally, we consider potential mechanisms of intra- and interneuronal tau lesion spreading, an area of particular recent interest.

Extensive deamidation at asparagine residue 279 accounts for weak immunoreactivity of tau with RD4 antibody in Alzheimer's disease brain

Background

Intracytoplasmic inclusions composed of filamentous tau proteins are defining characteristics of neurodegenerative tauopathies, but it remains unclear why different tau isoforms accumulate in different diseases and how they induce abnormal filamentous structures and pathologies. Two tau isoform-specific antibodies, RD3 and RD4, are widely used for immunohistochemical and biochemical studies of tau species in diseased brains.

Results

Here, we show that extensive irreversible post-translational deamidation takes place at asparagine residue 279 (N279) in the RD4 epitope of tau in Alzheimer’s disease (AD), but not corticobasal degeneration (CBD) or progressive supranuclear palsy (PSP), and this modification abrogates the immunoreactivity to RD4. An antiserum raised against deamidated RD4 peptide specifically recognized 4R tau isoforms, regardless of deamidation, and strongly stained tau in AD brain. We also found that mutant tau with N279D substitution showed reduced ability to bind to microtubules and to promote microtubule assembly.

Conclusion

The biochemical and structural differences of tau in AD from that in 4R tauopathies found in this study may therefore have implications for prion-like propagation of tau.

Neurofibrillary tangles and the deposition of a beta amyloid peptide with a novel N-terminal epitope in the brains of wild Tsushima leopard cats

Beta amyloid (Aβ) deposits are seen in aged individuals in many of the mammalian species that possess the same Aβ amino acid sequence as humans. Conversely, neurofibrillary tangles (NFT), the other hallmark lesion of Alzheimer's disease (AD), are extremely rare in these animals. We detected Aβ deposits in the brains of Tsushima leopard cats (Prionailurus bengalensis euptilurus) that live exclusively on Tsushima Island, Japan. Aβ42 was deposited in a granular pattern in the neuropil of the pyramidal cell layer, but did not form argyrophilic senile plaques. These Aβ deposits were not immunolabeled with antibodies to the N-terminal of human Aβ. Sequence analysis of the amyloid precursor protein revealed an amino acid substitution at the 7th residue of the Aβ peptide. In a comparison with other mammalian animals that do develop argyrophilic senile plaques, we concluded that the alternative Aβ amino acid sequence displayed by leopard cats is likely to be related to its distinctive deposition pattern. Interestingly, most of the animals with these Aβ deposits also developed NFTs. The distributions of hyperphosphorylated tau-positive cells and the two major isoforms of aggregated tau proteins were quite similar to those seen in Alzheimer's disease. In addition, the unphosphorylated form of GSK-3β colocalized with hyperphosphorylated tau within the affected neurons. In conclusion, this animal species develops AD-type NFTs without argyrophilic senile plaques.

Is tau ready for admission to the prion club?

Aggregation-prone proteins associated with neurodegenerative disease, such as α synuclein and β amyloid, now appear to share key prion-like features with mammalian prion protein, such as the ability to recruit normal proteins to aggregates and to translocate between neurons. These features may shed light on the genesis of stereotyped lesion development patterns in conditions such as Alzheimer disease and Lewy Body dementia. We discuss the qualifications of tau protein as a possible "prionoid" mediator of lesion spread based on recent characterizations of the secretion, uptake and transneuronal transfer of human tau isoforms in a variety of tauopathy models, and in human patients. In particular, we consider (1) the possibility that prionoid behavior of misprocessed tau in neurodegenerative disease may involve other aggregation-prone proteins, including PrP itself, and (2) whether "prionlike" tau lesion propagation might include mechanisms other than protein-protein templating.

The challenges of tau imaging

In vivo imaging of tau pathology will provide new insights into tau deposition in the human brain, thus facilitating research into causes, diagnosis and treatment of major dementias, such as Alzheimer’s disease, or some variants of frontotemporal lobar degeneration, in which tau plays a role. Tau imaging poses several challenges, some related to the singularities of tau aggregation, and others related to radiotracer design. Several groups around the world are working on the development of imaging agents that will allow the in vivo assessment of tau deposition in aging and in neurodegeneration. Development of a tau imaging tracer will enable researchers to noninvasively examine the degree and extent of tau pathology in the brain, quantify changes in tau deposition over time, evaluate its relation to cognition and assess the efficacy of anti-tau therapy.

N-methyl D-aspartate (NMDA) receptor antagonists and memantine treatment for Alzheimer's disease, vascular dementia and Parkinson's disease

Memantine, a partial antagonist of N-methyl-D-aspartate receptor (NMDAR), approved for moderate to severe Alzheimer's disease (AD) treatment within the U.S. and Europe under brand name Namenda (Forest), Axura and Akatinol (Merz), and Ebixa and Abixa (Lundbeck), may have potential in alleviating additional neurological conditions, such as vascular dementia (VD) and Parkinson's disease (PD). In various animal models, memantine has been reported to be a neuroprotective agent that positively impacts both neurodegenerative and vascular processes. While excessive levels of glutamate result in neurotoxicity, in part through the over-activation of NMDARs, memantine-as a partial NMDAR antagonist, blocks the NMDA glutamate receptors to normalize the glutamatergic system and ameliorate cognitive and memory deficits. The key to memantine's therapeutic action lies in its uncompetitive binding to the NMDAR through which low affinity and rapid off-rate kinetics of memantine at the level of the NMDAR-channel preserves the physiological function of the receptor, underpinning memantine's tolerability and low adverse event profile. As the biochemical pathways evoked by NMDAR antagonism also play a role in PD and since no other drug is sufficiently effective to substitute for the first-line treatment of L-dopa despite its side effects, memantine may be useful in PD treatment with possibly fewer side effects. In spite of the relative modest nature of its adverse effects, memantine has been shown to provide only a moderate decrease in clinical deterioration in AD and VD, and hence efforts are being undertaken in the design of new and more potent memantine-based drugs to hopefully provide greater efficacy.

Beta amyloid deposition and neurofibrillary tangles spontaneously occur in the brains of captive cheetahs (Acinonyx jubatus)

Alzheimer disease is a dementing disorder characterized pathologically by Aβ deposition, neurofibrillary tangles, and neuronal loss. Although aged animals of many species spontaneously develop Aβ deposits, only 2 species (chimpanzee and wolverine) have been reported to develop Aβ deposits and neurofibrillary tangles in the same individual. Here, the authors demonstrate the spontaneous occurrence of Aβ deposits and neurofibrillary tangles in captive cheetahs (Acinonyx jubatus). Among 22 cheetahs examined in this study, Aβ deposits were observed in 13. Immunostaining (AT8) revealed abnormal intracellular tau immunoreactivity in 10 of the cheetahs with Aβ deposits, and they were mainly distributed in the parahippocampal cortex and CA1 in a fashion similar to that in human patients with Alzheimer disease. Ultrastructurally, bundles of straight filaments filled the neuronal somata and axons, consistent with tangles. Interestingly, 2 of the cheetahs with the most severe abnormal tau immunoreactivity showed clinical cognitive dysfunction. The authors conclude that cheetahs spontaneously develop age-related neurodegenerative disease with pathologic changes similar to Alzheimer disease.

Tau protein assembles into isoform- and disulfide-dependent polymorphic fibrils with distinct structural properties

Tauopathies are neurodegenerative diseases in which insoluble fibrillar aggregates of a microtubule-binding protein, Tau, are abnormally accumulated. Pathological Tau fibrils often exhibit structural polymorphisms that differ among phenotypically distinct tauopathies; however, a molecular mechanism to generate polymorphic Tau fibrils remains obscure. Here, we note the formation of a disulfide bond in isoforms of full-length Tau and show that the thiol-disulfide status as well as the isoform composition determines structural and morphological properties of Tau fibrils in vitro. Mainly two regions in a Tau primary sequence are found to act as structural blocks for building a protease-resistant core of Tau fibrils. Interactions among those two blocks for building a core structure depend upon the thiol-disulfide status in each isoform of Tau, which results in the formation of polymorphic fibrils with distinct structural properties. Furthermore, we have found that more diverse structures of Tau fibrils emerge through a cross-seeded fibrillation between heterologous pairs of Tau isoforms. We thus propose that isoform- and disulfide-dependent combinatorial interactions among multiple regions in a Tau sequence endow Tau fibrils with various structures, i.e. polymorphism.

Tau and neurodegenerative disorders

The mechanisms that render tau a toxic agent are still unclear, although increasing evidence supports the assertion that alterations of tau can directly cause neuronal degeneration. In addition, it is unclear whether neurodegeneration in various tauopathies occurs via a common mechanism or that specific differences exist. The aim of this review is to provide an overview of tauopathies from bench to bedside. The review begins with clinicopathological findings of familial and sporadic tauopathies. It includes a discussion of the similarities and differences between these two conditions. The second part concentrates on biochemical alterations of tau such as phosphorylation, truncation and acetylation. Although pathological phosphorylation of tau has been studied for many years, recently researchers have focused on the physiological role of tau during development. Finally, the review contains a summary of the significance of tauopathy model mice for research on neurofibrillary tangles, axonopathies, and synaptic alteration.

Tau protein and tau aggregation inhibitors

Alzheimer disease is characterized by pathological aggregation of two proteins, tau and Abeta-amyloid, both of which are considered to be toxic to neurons. In this review we summarize recent advances on small molecule inhibitors of protein aggregation with emphasis on tau, with activities mediated by the direct interference of self-assembly. The inhibitors can be clustered in several compound classes according to their chemical structure, with subsequent description of the structure-activity relationships, showing that hydrophobic interactions are prevailing. The description is extended to the pharmacological profile of the compounds in order to evaluate their drug-likeness, with special attention to toxicity and bioavailability. The collected data indicate that following the improvements of the in vitro inhibitory potencies, the consideration of the in vivo pharmacokinetics is an absolute prerequisite for the development of compounds suitable for a transfer from bench to bedside.

Development of tau aggregation inhibitors for Alzheimer's disease

A variety of human diseases are suspected to be directly linked to protein misfolding. Highly organized protein aggregates, called amyloid fibrils, and aggregation intermediates are observed; these are considered to be mediators of cellular toxicity and thus attract a great deal of attention from investigators. Neurodegenerative pathologies such as Alzheimer's disease account for a major part of these protein misfolding diseases. The last decade has witnessed a renaissance of interest in inhibitors of tau aggregation as potential disease-modifying drugs for Alzheimer's disease and other "tauopathies". The recent report of a phase II clinical trial with the tau aggregation inhibitor MTC could hold promise for the validation of the concept. This Review summarizes the available data concerning small-molecule inhibitors of tau aggregation from a medicinal chemistry point of view.

MAPT S305I mutation: implications for argyrophilic grain disease

Frontotemporal lobar degeneration (FTLD) with mutations in the tau gene (MAPT) causes familial frontotemporal dementia with tau pathology. Many of these mutations result in morphological phenotypes resembling sporadic tauopathies, although, to date, no such cases mimicking argyrophilic grain disease (AgD) have been documented. We now present a case with a novel S305I MAPT mutation and a morphological phenotype showing resemblance to AgD. At the age of 39, the patient developed behavioural and personality changes and lack of verbal fluency with later poor performance on naming tasks and rigidity in the extremities. After a short disease course of 1.5 years, the patient died. A unique neuropathological phenotype with neuronal diffuse cytoplasmic tau immunoreactivity, oligodendroglial-coiled bodies, argyrophilic grains, and non-argyrophilic, but tau-immunopositive and ubiquitin-immunonegative pre-grains were observed, whereas classical neurofibrillary tangles, Pick bodies, and neuritic plaques were absent. The tau-positive abnormal structures were composed only of 4R-tau isoforms and, ultrastructurally, straight filaments. Neuronal loss was greatest in the medial temporal cortex, hippocampus, and amygdala. These pathological features resemble AgD. The novel S305I substitution has a strong effect on MAPT exon 10 splicing, thereby causing a striking increase in 4R-tau isoforms. Our observation not only widens the phenotypic spectrum of FTLD with MAPT mutation but also underpins the notion that the predominance of similar neuropathological findings in sporadic AgD cases may be viewed as features of a distinct disease entity.

Tau-based treatment strategies in neurodegenerative diseases

Neurofibrillary tangles are a characteristic hallmark of Alzheimer's and other neurodegenerative diseases, such as Pick's disease (PiD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). These diseases are summarized as tauopathies, because neurofibrillary tangles are composed of intracellular aggregates of the microtubule-associated protein tau. The molecular mechanisms of tau-mediated neurotoxicity are not well understood; however, pathologic hyperphosphorylation and aggregation of tau play a central role in neurodegeneration and neuronal dysfunction. The present review, therefore, focuses on therapeutic approaches that aim to inhibit tau phosphorylation and aggregation or to dissolve preexisting tau aggregates. Further experimental therapy strategies include the enhancement of tau clearance by activation of proteolytic, proteasomal, or autophagosomal degradation pathways or anti-tau directed immunotherapy. Hyperphosphorylated tau does not bind microtubules, leading to microtubule instability and transport impairment. Pharmacological stabilization of microtubule networks might counteract this effect. In several tauopathies there is a shift toward four-repeat tau isoforms, and interference with the splicing machinery to decrease four-repeat splicing might be another therapeutic option.