Paired helical filaments and straight tubules in astrocytes: an electron microscopic study in dementia of the Alzheimer type

Molecular dynamics simulations reveal the destabilization mechanism of Alzheimer's disease-related tau R3-R4 Protofilament by norepinephrine

Aggregation of Tau protein into neurofibrillary tangles is associated with the pathogenesis of Alzheimer's disease (AD) which has no cure yet. Clearing neurofibrillary tangles is one of major therapeutic strategies. Experimental studies reported that norepinephrine (NE) has the ability to disrupt Tau filament and cause Tau degradation. However, the underlying mechanism remains elusive. Herein, we performed molecular dynamic simulations to investigate the influence of NE on the C-shaped Tau R3-R4 protofilament. Our simulations show that NE compound destabilizes Tau protofilament by mostly disrupting β6/β8 and altering the β2-β3 and β6-β7 angles. NE binds mainly with aromatic residues Y310/P312/H374/F378 through ππ stacking and charged residues E338/E342/D348/D358/E372 via hydrogen-bonding interactions. Our results, together with the findings that exercise can markedly increase NE level, suggest that exercise might be a potent therapy against AD. This study reveals the disruptive mechanism of Tau protofilament by NE molecules, which may provide new clues for AD drug candidate design.

Deciphering the Astrocyte Reaction in Alzheimer's Disease

Reactive astrocytes were identified as a component of senile amyloid plaques in the cortex of Alzheimer's disease (AD) patients several decades ago. However, their role in AD pathophysiology has remained elusive ever since, in part owing to the extrapolation of the literature from primary astrocyte cultures and acute brain injury models to a chronic neurodegenerative scenario. Recent accumulating evidence supports the idea that reactive astrocytes in AD acquire neurotoxic properties, likely due to both a gain of toxic function and a loss of their neurotrophic effects. However, the diversity and complexity of this glial cell is only beginning to be unveiled, anticipating that astrocyte reaction might be heterogeneous as well. Herein we review the evidence from mouse models of AD and human neuropathological studies and attempt to decipher the main conundrums that astrocytes pose to our understanding of AD development and progression. We discuss the morphological features that characterize astrocyte reaction in the AD brain, the consequences of astrocyte reaction for both astrocyte biology and AD pathological hallmarks, and the molecular pathways that have been implicated in this reaction.

Astrocyte and Alzheimer's disease

The past several decades have given rise to more insights into the role of astrocytes in normal brain function and diseases. Astrocytes elicit an effect which may be neuroprotective or deleterious in the process of Alzheimer's disease (AD). Impairments in astrocytes and their other functions, as well as physiological reactions of astrocytes to external injury, can trigger or exacerbate hyperphosphorylated tau and amyloid-beta (Aβ) pathologies, leading to the formation of both amyloid plaques and neurofibrillary tangles (NFTs), as well as neuronal dysfunction. This review addresses the involvement of astrocytes in the Aβ pathology, where the main mechanisms include the generation and clearance of Aβ, and the formation of NFTs. It is also discussed that metabolic dysfunction from astrocytes acts as an initiating factor in the pathogenesis of AD and a contributor to the onset and development of clinical presentation in AD.

Aging-related tau astrogliopathy (ARTAG): harmonized evaluation strategy

Pathological accumulation of abnormally phosphorylated tau protein in astrocytes is a frequent, but poorly characterized feature of the aging brain. Its etiology is uncertain, but its presence is sufficiently ubiquitous to merit further characterization and classification, which may stimulate clinicopathological studies and research into its pathobiology. This paper aims to harmonize evaluation and nomenclature of aging-related tau astrogliopathy (ARTAG), a term that refers to a morphological spectrum of astroglial pathology detected by tau immunohistochemistry, especially with phosphorylation-dependent and 4R isoform-specific antibodies. ARTAG occurs mainly, but not exclusively, in individuals over 60 years of age. Tau-immunoreactive astrocytes in ARTAG include thorn-shaped astrocytes at the glia limitans and in white matter, as well as solitary or clustered astrocytes with perinuclear cytoplasmic tau immunoreactivity that extends into the astroglial processes as fine fibrillar or granular immunopositivity, typically in gray matter. Various forms of ARTAG may coexist in the same brain and might reflect different pathogenic processes. Based on morphology and anatomical distribution, ARTAG can be distinguished from primary tauopathies, but may be concurrent with primary tauopathies or other disorders. We recommend four steps for evaluation of ARTAG: (1) identification of five types based on the location of either morphologies of tau astrogliopathy: subpial, subependymal, perivascular, white matter, gray matter; (2) documentation of the regional involvement: medial temporal lobe, lobar (frontal, parietal, occipital, lateral temporal), subcortical, brainstem; (3) documentation of the severity of tau astrogliopathy; and (4) description of subregional involvement. Some types of ARTAG may underlie neurological symptoms; however, the clinical significance of ARTAG is currently uncertain and awaits further studies. The goal of this proposal is to raise awareness of astroglial tau pathology in the aged brain, facilitating communication among neuropathologists and researchers, and informing interpretation of clinical biomarkers and imaging studies that focus on tau-related indicators.

Transactivation response DNA-binding protein 43 microvasculopathy in frontotemporal degeneration and familial Lewy body disease

We describe novel transactivation response DNA-binding protein of 43 kd (TDP-43)-positive structures in the brains of patients with frontotemporal lobar degeneration with ubiquitin-positive inclusions and familial Lewy body disease. The TDP-43 immunohistochemistry revealed small round structures closely associated with small blood vessels. By immunoelectron microscopy, these TDP-43-positive structures were unmyelinated cell processes located adjacent to and sometimes enclosed by the capillary basal lamina. Some processes protruded from outside of the vascular basal lamina to a position beneath the basal lamina. The processes contained 10- to 17-nm-diameter straight filaments or filaments coated with granular material similar to those described in neurites in frontotemporal lobar degeneration with ubiquitin-positive inclusions and other disorders. In some of the abnormal structures, electron-dense material formed paracrystalline arrays composed of TDP-43. The inclusions were variably positive by immunostaining for the small heat shock protein alphaB-crystallin and less often glial fibrillary acidic protein. Bundles of astrocytic glial fibrils characteristic of reactive astrocytes were often found in proximity, but glial fibrils were negative for TDP-43. These data suggest that these processes are astrocytic end-feet with abnormal TDP-43 fibrillary inclusions. The significance of this novel TDP-43 microvasculopathy on blood-brain barrier integrity warrants further investigation.

Spinal cord tau pathology in cervical spondylotic myelopathy

We conducted an immunohistochemical and ultrastructural examination of the spinal cords from 11 cases of cervical spondylotic myelopathy (CSM), together with those from 11 age- and sex-matched control subjects. Immunostaining with AT8 antibody revealed various numbers of tau-positive neuropil thread-like structures (NTSs), often demonstrating a conspicuous astrocytic foot-like perivascular or subpial arrangement, and glial cells with short and thick processes, so-called thorn-shaped astrocytes (TSAs), in the affected cervical cords in 8 of the 11 CSM cases (73%). A number of tau-positive neuronal cytoplasmic pretangles/tangles were also found in the gray matter in all the CSM cases (100%). No such astrocytic or neuronal tau lesions were found in the control subjects. The tau deposited in the NTSs and TSAs was predominantly 4-repeat tau, whereas the neuronal cytoplasmic pretangles/tangles contained both 3-repeat and 4-repeat tau. Ultrastructurally, paired helical filaments about 20 nm wide, together with glial filaments, were detected occasionally in the astrocytic processes. In conclusion, the present findings indicate that astrocytic and neuronal tau lesions appear in the affected cervical cord during the disease process of CSM.

Argyrophilic thorny astrocyte clusters in association with Alzheimer's disease pathology in possible primary progressive aphasia

Although most cases of primary progressive aphasia (PPA) have one of the varieties of frontotemporal lobar degeneration (FTLD) as their pathological substrate, a subset shows Alzheimer's disease (AD) pathology. We report that all eight cases in our clinic diagnosed as possible PPA, on account of the presence of episodic memory difficulties in addition to severe language impairment at the onset of disease, showed AD pathology. Neither focal accentuation of AD pathology nor vascular lesions in language-related areas was observed. Seven of these eight patients showed large argyrophilic thorny astrocyte clusters (ATAC) in the fronto-temporo-parietal cortex and subcortical white matter. The intensely tau immunoreactive astrocytes in ATAC were morphologically similar to the perivascular, subpial, and subependymal astrocytes in elderly brains, but ATAC differ from the latter by the cortical and subcortical location, widespread distribution outside the medial temporal lobe, and intense argyrophilia. The location of ATAC was related to neither local variations in the load of AD pathology, nor the myelin density of white matter. ATAC were not seen in a comparison group of six cases of AD without a prominent aphasia syndrome. Because of the similarity of astrocytes in ATAC to those seen independently of AD pathology in several subtypes of FTLD and two reported cases of PPA we hypothesize that they are a marker of a pathological process concurrent with AD, and related to the focality of the clinical presentation.

Cellular tau pathology and immunohistochemical study of tau isoforms in sporadic tauopathies

Pathological inclusions in neurons and glial cells containing fibrillary aggregates of abnormally hyperphosphorylated tau protein are characteristic features in sporadic tauopathies. In the first part of this paper we outline the morphological features of some major sporadic tauopathies. In the second part, to better define the tau isoform composition, we report on the immunohistochemistry of tau isoforms in autopsied brains, including two cases with AD, two with diffuse neurofibrillary tangles with calcification, four with Pick's disease with Pick bodies (PiD), seven with progressive supranuclear palsy (PSP), six with corticobasal degeneration (CBD) and seven cases with argyrophilic grain disease. We used two monoclonal antibodies, RD3 and RD4, and a polyclonal antibody for exon 10 that effectively distinguish between three-repeat (3R) tau and four-repeat (4R) tau. Neuronal neurofibrillary tangles (NFT) in AD and diffuse neurofibrillary tangles with calcification contained both 3R-tau and 4R-tau. The Pick bodies were immunopositive for 3R-tau in two cases; however, in two other cases they were mainly immunopositive for 4R-tau. Thus, Pick bodies demonstrated heterogeneity. 3R-tau PiD contained 3R-tau glial inclusions, and 4R-tau PiD contained mainly 4R-tau glial inclusions. Glial inclusions were more abundant in 4R-tau PiD cases. In progressive supranuclear palsy and CBD, both neuronal and glial tau accumulation forming NFF, pretangles, tuft-shaped astrocytes, astrocytic plaques, coiled bodies and threads demonstrated 4R-tau in the cerebral cortices, although in the basal ganglia and brainstem neuronal and glial inclusions were occasionally immunopositive for 3R-tau in addition to 4R-tau. Argyrophilic grains (AG) were immunopositive for 4R-tau, although pretangles were weakly stained for 4R-tau. Thus the immunoreactivity for 4R-tau was different between AG and pretangles. Therefore, the isoform composition on immunohistochemical study showed heterogeneity in PiD, and was not uniform in the basal ganglia and brain stem in PSP and CBD. It is suggested that the isoform composition of sporadic tauopathies may have a spectrum in individual cases, and cellular isoform composition may differ in various brain regions.

Dementia with grains (argyrophilic grain disease)

Dementia with grains, also referred to as argyrophilic grain disease, is a morphological condition in elderly individuals histologically characterised by the widespread occurrence of minute, spindle or comma-shaped argyrophilic, tau-immunoreactive structures distinct from neuropil threads that are predominantly located in the hippocampus and related limbic areas including the amygdala. They are suggested to arise mainly in dendrites of neurons showing accumulation of hyperphosphorylated tau proteins (pretangle stage) but not necessarily forming paired helical filaments. Argyrophilic grains are associated with argyrophilic, tau-positive oligodendroglial inclusions ("coiled bodies") in the white matter, while astroglia are not affected. Argyrophilic grain disease is considered to be a progressive disorder that may or may not be associated with dementia, the grains occasionally being the only morphologic substrates of cognitive decline. They often occur in combination with neuritic Alzheimer-type lesions (many corresponding to "limbic" Braak stages III and IV) or other neurodegenerative disorders, such as progressive supranuclear palsy, corticobasal degeneration, or Pick's disease. The prevalence and pathogenesis of this condition, its clinicopathologic correlations and nosological position among tau-pathology related disorders await further elucidation.

Senile dementia with tangles (tangle predominant form of senile dementia)

Senile dementia with tangles is a sporadic subset of very late onset dementia with preponderance in females over age 80 years. Neuropathology shows diffuse cerebral atrophy with neurofibrillary tangles, often ghost tangles, and neuropil threads almost limited to limbic areas (transentorhinal, entorhinal area, hippocampuS--not exclusively sector CA 1--and amygdala) with only rare and mild involvement of the neocortex, basal ganglia and brainstem (except nucleus basalis and locus ceruleus), absence of neuritic plaques and absence or scarcety of amyloid deposits. This pattern of fibrillary pathology corresponds to Braak stages III and IV or the "limbic" type of Alzheimer disease that is considered the main form in the oldest-old but escapes the current criteria for the morphologic diagnosis of Alzheimer disease. It is distinct from other tau- or tangle-pathology related conditions, e.g. progressive supranuclear palsy, autosomal dominant dementia with tangles, and diffuse tangles with calcification. Very low prevalence of ApoE e4 allele (0.03-0.11%) and higher frequency of ApoE e3 and/or e2 suggest a lack of promoting effect of e4 and a possible protecting effect of e2/3 on amyloidogenesis. Senile dementia with tangles is suggested to be a variant of Alzheimer disease occurring in the oldest-old, but its nosological position within aging disorders of the brain is still controversy.

Tau-positive glial inclusions in progressive supranuclear palsy, corticobasal degeneration and Pick's disease

The presence of tau-positive glial inclusions has been recently found a consistent feature in the brains of patients with progressive supranuclear palsy (PSP), corticobasal degeneration (CBD) and Pick's disease (PiD). These inclusions are classified based on cellular origin as tau-positive astrocytes, presumably either fibrillary or protoplasmic, coiled bodies and glial threads. Immunohistochemically, their major structural component is abnormal tau proteins, similar to those found in Alzheimer's disease. Nevertheless, their morphology, including ultrastructural profile, has been suggested to be distinctive for each disease. The profile and extent of particular glial inclusions correlate well with disease phenotype. Highly characteristic correlations include tufts of abnormal fibers in PSP, astrocytic plaques and dense glial threads in CBD and ramified astrocytes and small Pick body-like inclusions in PiD. The significance of the inclusions in disease pathogenesis and their biochemical characteristics remain to be clarified. Nevertheless, these distinctive glial lesions most likely reflect fundamental alterations in isoform composition of tau as well as its specific cellular and regional expression in sporadic tauopathies.

Tau-Positive Fine Granules in the Cerebral White Matter: A Novel Finding Among the Tauopathies Exclusive to Parkinsonism-Dementia Complex of Guam

We examined the autopsied brains of cases of 6 types of tauopathy: parkinsonism-dementia complex of Guam (PDC), corticobasal degeneration (CBD), progressive supranuclear palsy (PSP), Pick disease, Alzheimer disease (AD), and myotonic dystrophy together with Guamanian controls. Light microscopy sections of these brains were examined using anti-tau antibodies. Tau-positive fine granules (TFGs) were globe-shaped, and 3 to 6 mum in diameter, were observed predominantly in the frontal white matter in 30 of the 35 patients with PDC. However, no TFGs were found in association with PSP, myotonic dystrophy, Pick disease, AD, or CBD. Western blot analysis of frozen brain tissue taken from the PDC cases revealed that the frontal cortex was hyperphosphorylated and contained 6 tau isoforms (3R+4R tau). However, in the present study, it was revealed that the novel TFGs in the white matter of patients with PDC was composed of 4R tau. Western blot analysis of sarkosyl-insoluble tau from the white matter of the PDC cases showed 2 major bands of 60 and 64 kDa and one minor band of 67 kDa. After dephosphorylation, these bands resolved into one major band of 4-repeat (4R) tau isoform and 3 minor bands of 3-repeat (3R) and 4R tau isoforms. Moreover, the TFGs observed in cases in which the number of neurofibrillary tangles (NFTs) was higher than the threshold level were not correlated with the presence of cortical NFTs. In conclusion, these novel TFGs were found almost exclusively in PDC brains and could therefore be considered as a characteristic neuropathologic marker of this particular tauopathy. The TFGs were hyperphosphorylated tau-positive structures that may be formed by a different mechanism from that used to produce cortical NFTs.

Pathological glial tau accumulations in neurodegenerative disease: review and case report

Abnormal deposits of tau protein accumulate in glia in many neurodegenerative diseases. This suggests that in some instances the disease process may target glial tau, with neuronal degeneration a secondary consequence of this process. In this report, we summarize the pattern of glial tau pathology in various neurodegenerative disorders and add original findings from a case of sporadic frontotemporal dementia that exhibits astrocytic tau pathology. The neurodegenerative diseases span the spectrum of relative neuronal and glial tau involvement, from disorders affecting only neuronal tau to those in which abnormal tau deposits are found only in glia. From this, we conclude that glial tau can be a primary target of the disease process, and that this can lead to neuronal degeneration.

Transgenic expression of the shortest human tau affects its compartmentalization and its phosphorylation as in the pretangle stage of Alzheimer's disease

We have generated transgenic mice expressing the shortest human tau protein, the microtubule-associated protein that composes paired helical filaments in Alzheimer's disease. Transgenic tau transcripts and proteins were strongly expressed in neurons in the developing and adult brain. In contrast to the endogenous tau that progressively disappeared from neuronal cell bodies during development, the human transgenic tau remained abundant in cell bodies and dendrites of a subset of neurons in the adult. This somatodendritic transgenic tau was immunoreactive with antibodies to tau phosphorylated on Thr181 and Thr231 and with the conformation-dependent Alz50 antibody. A few astrocytes expressing the transgenic tau were strongly immunoreactive with antibodies to additional tau phosphorylation sites, ie, at Ser262/ 356 and Ser396/404. All of these phosphorylation sites have been identified in paired helical filaments-tau proteins. In electron microscopy, the transgenic tau was detected into microtubules in axons and in dendrites but not in cell bodies. Neurofibrillary tangles were not detected in transgenic animals examined up to the age of 19 months. These results indicate that transgenic manipulation of tau expression and intracellular targeting is sufficient per se to affect tau compartmentalization, phosphorylation, and conformation partly as it is observed at the pretangle stage in Alzheimer's disease.

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.