Ultrastructural differences in pretangles between Alzheimer disease and corticobasal degeneration revealed by comparative light and electron microscopy

The enigma of depression in corticobasal degeneration, a frequent but poorly understood co-morbidity

Depression is one of the most frequent neuropsychiatric symptoms in corticobasal degeneration (CBD), a rare, sporadic, and late-onset progressive neurodegenerative disorder of unknown etiology. It is clinically characterized by a levodopa-poorly responsible akinetic-rigid syndrome, apraxia, limb dystonia, cognitive, mood, behavioral, and language disorders. This 4-repeat (4R) tauopathy is morphologically featured by asymmetric frontoparietal atrophy, neuronal loss, and gliosis in cortex and subcortex including substantia nigra, ballooned/achromatic neurons with filamentous 4R tau aggregates in cortex and striatum, widespread thread-like structures, pathognomonic "astroglial plaques", "tufted astrocytes", and numerous "coiled bodies" (in astrocytes and oligodendroglia) in cerebral white matter. CBD is non-specific, as pathologically proven cases include several clinical phenotypes. Pubmed and Google Scholar were systematically analyzed until October 2023, with focus on the prevalence, clinical manifestation, neuroimaging data, and treatment options of depression in CBD. Its prevalence is about 30-40% which is more frequent than in most other atypical parkinsonian syndromes. Depression usually does not correlate with motor and other clinical parameters, suggesting different pathophysiological mechanisms. Asymmetric atrophy and hypometabolism of frontoparietal cortical areas are associated with disruption of fronto-subcortical circuits, nigrostriatal dopaminergic, and cholinergic deficiency. Since no specific neuroimaging, neuropathological, or biomarker studies of depression in CBD are available, its pathobiological mechanisms and pathogenesis are poorly understood. Antidepressive therapy may be useful, but is often poorly tolerated. Depression in CBD, like in other parkinsonian syndromes, may be related to multi-regional patterns of cerebral disturbances and complex pathogenic mechanisms that deserve further elucidation as a basis for early diagnosis and adequate treatment to improve the quality of life in this fatal disease.

Pathomechanisms of cognitive and behavioral impairment in corticobasal degeneration

Corticobasal degeneration (CBD) is a rare, sporadic, late-onset progressive neurodegenerative disorder of unknown etiology, clinically characterized by an akinetic-rigid syndrome, behavior and personality disorders, language problems (aphasias), apraxia, executive and cognitive abnormalities and limb dystonia. The syndrome is not specific, as clinical features of pathologically proven CBD include several phenotypes. This 4-repeat (4R) tauopathy is morphologically featured by often asymmetric frontoparietal atrophy, ballooned/achromatic neurons containing filamentous 4R-tau aggregates in cortex and striatum, thread-like processes that are more widespread than in progressive supranuclear palsy (PSP), pathognomonic "astroglial plaques", and numerous inclusions in both astrocytes and oligodendroglia ("coiled bodies") in the white matter. Cognitive deficits in CBD are frequent initial presentations before onset of motor symptoms, depending on the phenotypic variant. They predominantly include executive and visuospatial dysfunction, sleep disorders and language deficits with usually preserved memory domains. Neuroimaging studies showed heterogenous locations of brain atrophy, particularly contralateral to the dominant symptoms, with disruption of striatal connections to prefrontal cortex and basal ganglia circuitry. Asymmetric hypometabolism, mainly involving frontal and parietal regions, is associated with brain cholinergic deficits, and dopaminergic nigrostriatal degeneration. Widespread alteration of cortical and subcortical structures causing heterogenous changes in various brain functional networks support the concept that CBD, similar to PSP, is a brain network disruption disorder. Putative pathogenic factors are hyperphosphorylated tau-pathology, neuroinflammation and oxidative injury, but the basic mechanisms of cognitive impairment in CBD, as in other degenerative movement disorders, are complex and deserve further elucidation as a basis for early diagnosis and adequate treatment of this fatal disorder.

Macroscopic diagnostic clue for parkinsonism

The neuropathological background of parkinsonism includes various neurodegenerative disorders, including Lewy body disease (LBD), multiple system atrophy (MSA), progressive supranuclear palsy (PSP), and corticobasal degeneration (CBD). The pathological diagnostic procedure begins by assessing the macroscopic findings to evaluate the degenerative lesions in brains with the naked eye. Usually, degenerative lesions show variable atrophy and brownish discoloration in accordance with disease-specific profiles. These macroscopic appearances support neuropathologists in identifying the relevant regions for microscopic examination. The neuropathological diagnosis of parkinsonism is based on regional distribution and fundamental proteinopathies in neurons and glia cells. LBD and MSA are synucleinopathies, and PSP and CBD are tauopathies. Among them, glial-predominant proteinopathy (MSA, PSP, and CBD) may play a significant role in volume reduction. Therefore, macroscopic inspection provides the appropriate direction for assessment. The disease duration, the severity of lesions, and mixed pathologies make the validation of macroscopic observations more complicated. In this review, we outline the macroscopic diagnostic clues in LBD, MSA, PSP, and CBD that could help with pathological refinement.

Disruption of tubulin-alpha4a polyglutamylation prevents aggregation of hyper-phosphorylated tau and microglia activation in mice

Dissociation of hyper-phosphorylated Tau from neuronal microtubules and its pathological aggregates, are hallmarks in the etiology of tauopathies. The Tau-microtubule interface is subject to polyglutamylation, a reversible posttranslational modification, increasing negative charge at tubulin C-terminal tails. Here, we asked whether tubulin polyglutamylation may contribute to Tau pathology in vivo. Since polyglutamylases modify various proteins other than tubulin, we generated a knock-in mouse carrying gene mutations to abolish Tuba4a polyglutamylation in a substrate-specific manner. We found that Tuba4a lacking C-terminal polyglutamylation prevents the binding of Tau and GSK3 kinase to neuronal microtubules, thereby strongly reducing phospho-Tau levels. Notably, crossbreeding of the Tuba4a knock-in mouse with the hTau tauopathy model, expressing a human Tau transgene, reversed hyper-phosphorylation and oligomerization of Tau and normalized microglia activation in brain. Our data highlight tubulin polyglutamylation as a potential therapeutic strategy in fighting tauopathies.

Pathologic oligomerization of hyper-phosphorylated Tau is a hallmark of tauopathies. Here the authors show that the loss of tubulin a4 polyglutamylation reverses tau hyperphosphorylation, oligomerization and microglia activation in a tauopathy mouse.

Ultrastructural and biochemical classification of pathogenic tau, α-synuclein and TDP-43

Intracellular accumulation of abnormal proteins with conformational changes is the defining neuropathological feature of neurodegenerative diseases. The pathogenic proteins that accumulate in patients' brains adopt an amyloid-like fibrous structure and exhibit various ultrastructural features. The biochemical analysis of pathogenic proteins in sarkosyl-insoluble fractions extracted from patients' brains also shows disease-specific features. Intriguingly, these ultrastructural and biochemical features are common within the same disease group. These differences among the pathogenic proteins extracted from patients' brains have important implications for definitive diagnosis of the disease, and also suggest the existence of pathogenic protein strains that contribute to the heterogeneity of pathogenesis in neurodegenerative diseases. Recent experimental evidence has shown that prion-like propagation of these pathogenic proteins from host cells to recipient cells underlies the onset and progression of neurodegenerative diseases. The reproduction of the pathological features that characterize each disease in cellular and animal models of prion-like propagation also implies that the structural differences in the pathogenic proteins are inherited in a prion-like manner. In this review, we summarize the ultrastructural and biochemical features of pathogenic proteins extracted from the brains of patients with neurodegenerative diseases that accumulate abnormal forms of tau, α-synuclein, and TDP-43, and we discuss how these disease-specific properties are maintained in the brain, based on recent experimental insights.

Parallel gold enhancement of quantum dots 565/655 for double-labelling correlative light and electron microscopy on human autopsied samples

Cadmium selenide quantum dots (QDs) are fluorescent and electron-dense nanoparticles. When used as reporter of immunolabeling, this dual visibility is essential for direct comparison of its fluorescent signals on light microscopy (LM) and their ultrastructrual counterparts on electron microscopy (EM) as correlative light and electron microscopy (CLEM). To facilitate EM recognition, QDs on EM grid were gold enhanced, which increased their size and electron density. On histological sections as well, gold-enhanced QDs, used as a reporter of immunolabeling, were easily recognized on EM. Because target structures are visible on bright field microscopy, gold enhancement facilitated trimming the target structures into final EM sections. Furthermore, gold enhancement of rod-shaped QD655 on EM grid was accentuated on their tips while spherical QD565 was gold-enhanced as sphere in contrast. This EM distinction was evident on histological sections where QD565 (green fluorescence) and QD655 (red fluorescence) were used as a reporter pair for double immunolabeling. Double-labeled immuno-fluorescent images, initially captured before EM processing, are now compared with their respective immuno EM counterparts. Specific labeling of each epitope was corroborated by mutual comparison between LM and EM. Although fluoronanogold may be a candidate reporter partner with QDs for gold-enhanced, double-labeling CLEM, its limited penetration into fixed tissue hampers universal use for thick histological sections. Gold-enhancement of QD immunolabeling, now expanded to double-labeling CLEM for human brain samples, will pave the way to translate molecular events into ultrastructural morphopathogenesis in situ.

Motor neuron TDP-43 proteinopathy in progressive supranuclear palsy and corticobasal degeneration

Transactive response DNA-binding protein 43 kDa (TDP-43) is mislocalized from the nucleus and aggregates within the cytoplasm of affected neurons in amyotrophic lateral sclerosis (ALS) cases. TDP-43 pathology has also been found in brain tissues under non-ALS conditions, suggesting mechanistic links between TDP-43-related ALS (ALS-TDP) and various neurological disorders. This study aimed to assess TDP-43 pathology in the spinal cord motor neurons of tauopathies. We examined 106 spinal cords from consecutively autopsied cases with progressive supranuclear palsy (PSP, n = 26), corticobasal degeneration (CBD, n = 12), globular glial tauopathy (GGT, n = 5), Alzheimer's disease (AD, n = 21), or Pick disease (PiD, n = 6) and neurologically healthy controls (n = 36). Ten of the PSP cases (38%) and seven of the CBD cases (58%) showed mislocalization and cytoplasmic aggregation of TDP-43 in spinal cord motor neurons, which was prominent in the cervical cord. TDP-43-aggregates were found to be skein-like, round-shaped, granular, or dot-like and contained insoluble C-terminal fragments showing blotting pattern of ALS or frontotemporal lobar degeneration (FTLD). The lower motor neurons also showed cystatin-C aggregates, although Bunina bodies were absent in hematoxylin-eosin staining. The spinal cord TDP-43 pathology was often associated with TDP-43 pathology of the primary motor cortex. Positive correlations were shown between the severities of TDP-43 and 4-repeat (4R)-tau aggregates in the cervical cord. TDP-43 and 4R-tau aggregates burdens positively correlated with microglial burden in anterior horn. TDP-43 pathology of spinal cord motor neuron did not develop in an age-dependent manner and was not found in the AD, PiD, GGT, and control groups. Next, we assessed splicing factor proline/glutamine rich (SFPQ) expression in spinal cord motor neurons; SFPQ is a recently-identified regulator of ALS/FTLD pathogenesis, and it is also reported that interaction between SFPQ and fused-in-sarcoma (FUS) regulates splicing of microtubule-associated protein tau exon 10. Immunofluorescent and proximity-ligation assays revealed altered SFPQ/FUS-interactions in the neuronal nuclei of PSP, CBD, and ALS-TDP cases but not in AD, PiD, and GGT cases. Moreover, SFPQ expression was depleted in neurons containing TDP-43 or 4R-tau aggregates of PSP and CBD cases. Our results indicate that PSP and CBD may have properties of systematic motor neuron TDP-43 proteinopathy, suggesting mechanistic links with ALS-TDP. SFPQ dysfunction, arising from altered interaction with FUS, may be a candidate of the common pathway.

OUP accepted manuscript

Precise immunolocalization of molecules in relation to ultrastructural features is challenging, especially when the target is small and not frequent enough to be included in tiny ultrathin sections randomly selected for electron microscopy (EM). Glucose transporter 1 (GLUT1) is in charge of transporting glucose across brain capillary endothelial cells (BCECs). Paraformaldehyde-fixed floating sections (50 μm thick) of mouse brain were immunolabeled with anti-GLUT1 antibody and visualized with fluoronanogold. Fluorescent images encompassing the entire hemisphere were tiled to enable selection of GLUT1-positive BCECs suitable for subsequent EM and landmark placement with laser microdissection to guide trimming. Sections were then fixed with glutaraldehyde, gold enhanced to intensify the labeling and fixed with osmium tetroxide to facilitate ultrastructural recognition. Even though a region that contained target BCECs was successfully trimmed in the resin block, it was only after observation of serial ultrathin sections that GLUT1 signals in coated vesicles on the same cross section corresponding to the cross section preidentified by confocal laser microscope. This is the first ultrastructural demonstration of GLUT1 molecules in coated vesicles, which may well explain its functional relevance to transport glucose across BCECs. Successful ultrastructural localization of molecules in relation to well-preserved target structure in native tissue samples, as achieved in this study, will pave the way to understand the functional relevance of molecules and their relation to ultrastructural details.

Visualization of neurofibrillary tangle maturity in Alzheimer's disease: A clinicopathologic perspective for biomarker research

Neurofibrillary tangles, one of the neuropathologic hallmarks of Alzheimer's disease, have a dynamic lifespan of maturity that associates with progressive neuronal dysfunction and cognitive deficits. As neurofibrillary tangles mature, the biology of the neuron undergoes extensive changes that may impact biomarker recognition and therapeutic targeting. Neurofibrillary tangle maturity encompasses three levels: pretangles, mature tangles, and ghost tangles. In this review, we detail distinct and overlapping characteristics observed in the human brain regarding morphologic changes the neuron undergoes, conversion from intracellular to extracellular space, tau immunostaining patterns, and tau isoform expression changes across the lifespan of the neurofibrillary tangle. Post-translational modifications of tau such as phosphorylation, ubiquitination, conformational events, and truncations are discussed to contextualize tau immunostaining patterns. We summarize accumulated and emerging knowledge of neurofibrillary tangle maturity, discuss the current tools used to interpret the dynamic nature in the postmortem brain, and consider implications for cognitive dysfunction and tau biomarkers.

18F-AV-1451 positron emission tomography in neuropathological substrates of corticobasal syndrome

Multiple neuropathological processes can manifest in life as a corticobasal syndrome. We sought to relate retention of the tau-PET tracer 18F-AV-1451 and structural magnetic resonance measures of regional atrophy to clinical features in clinically diagnosed and neuropathologically confirmed cases of corticobasal syndrome and to determine whether these vary with the underlying neuropathological changes. In this observational, cross-sectional study, 11 subjects (eight female and three male, median age 72 years) with corticobasal syndrome underwent structural MRI, tau-PET with 18F-AV-1451, amyloid-PET with 11C-Pittsburgh compound B, detailed clinical examinations and neuropsychological testing. Of the 11, three had evidence of high amyloid burden consistent with Alzheimer's disease while eight did not. Neuropathological evaluations were acquired in six cases. Mixed effects general linear models were used to compare 18F-AV-1451 retention and atrophy in amyloid-negative corticobasal syndrome cases to 32 age-matched healthy control subjects and to relate cortical and subcortical 18F-AV-1451 retention and atrophy to clinical features. Subjects without amyloid, including three with pathologically confirmed corticobasal degeneration, showed greater regional 18F-AV-1451 retention and associated regional atrophy in areas commonly associated with corticobasal degeneration pathology than healthy control subjects [retention was higher compared to healthy controls (P = 0.0011), driven especially by the precentral gyrus (P = 0.011) and pallidum (P < 0.0001), and greater atrophy was seen in subjects compared to control subjects (P = 0.0004)]. Both 18F-AV-1451 retention and atrophy were greater in the clinically more affected hemisphere [on average, retention was 0.173 standardized uptake value ratio units higher on the more affected side (95% confidence interval, CI 0.11-0.24, P < 0.0001), and volume was 0.719 lower on the more affected side (95% CI 0.35-1.08, P = 0.0001)]. 18F-AV-1451 retention was greater in subcortical than in cortical regions, P < 0.0001. In contrast to these findings, subjects with amyloid-positive corticobasal syndrome, including two neuropathologically confirmed cases of Alzheimer's disease, demonstrated greater and more widespread 18F-AV-1451 retention and regional atrophy than observed in the amyloid-negative cases. There was thalamic 18F-AV-1451 retention but minimal cortical and basal ganglia uptake in a single corticobasal syndrome subject without neuropathological evidence of tau pathology, likely representing non-specific signal. Asymmetric cortical and basal ganglia 18F-AV-1451 retention consonant with the clinical manifestations characterize corticobasal syndrome due to corticobasal degeneration, whereas the cortical retention in cases associated with Alzheimer's disease is greater and more diffuse.

Hsp90 co-chaperones, FKBP52 and Aha1, promote tau pathogenesis in aged wild-type mice

The microtubule associated protein tau is an intrinsically disordered phosphoprotein that accumulates under pathological conditions leading to formation of neurofibrillary tangles, a hallmark of Alzheimer's disease (AD). The mechanisms that initiate the accumulation of phospho-tau aggregates and filamentous deposits are largely unknown. In the past, our work and others' have shown that molecular chaperones play a crucial role in maintaining protein homeostasis and that imbalance in their levels or activity can drive tau pathogenesis. We have found two co-chaperones of the 90 kDa heat shock protein (Hsp90), FK506-binding protein 52 (FKBP52) and the activator of Hsp90 ATPase homolog 1 (Aha1), promote tau aggregation in vitro and in the brains of tau transgenic mice. Based on this, we hypothesized that increased levels of these chaperones could promote tau misfolding and accumulation in the brains of aged wild-type mice. We tested this hypothesis by overexpressing Aha1, FKBP52, or mCherry (control) proteins in the hippocampus of 9-month-old wild-type mice. After 7 months of expression, mice were evaluated for cognitive and pathological changes. Our results show that FKBP52 overexpression impaired spatial reversal learning, while Aha1 overexpression impaired associative learning in aged wild-type mice. FKBP52 and Aha1 overexpression promoted phosphorylation of distinct AD-relevant tau species. Furthermore, FKBP52 activated gliosis and promoted neuronal loss leading to a reduction in hippocampal volume. Glial activation and phospho-tau accumulation were also detected in areas adjacent to the hippocampus, including the entorhinal cortex, suggesting that after initiation these pathologies can propagate through other brain regions. Overall, our findings suggest a role for chaperone imbalance in the initiation of tau accumulation in the aging brain.

The 'a, b, c's of pretangle tau and their relation to aging and the risk of Alzheimer's Disease

Braak has described the beginnings of Alzheimer's Disease as occurring in the locus coeruleus. Here we review these pretangle stages and relate their expression to recently described normal features of tau biology. We suggest pretangle tau depends on characteristics of locus coeruleus operation that promote tau condensates. We examine the timeline of pretangle and tangle appearance in locus coeruleus. We find catastrophic loss of locus coeruleus neurons is a late event. The strong relationship between locus coeruleus neuron number and human cognition underscores the utility of a focus on locus coeruleus. Promoting locus coeruleus health will benefit normal aging as well as aid in the prevention of dementia. Two animal models offering experimental approaches to understanding the functional change initiated by pretangles in locus coeruleus neurons are discussed.

Catecholamines in Alzheimer's Disease: A Systematic Review and Meta-Analysis

Background and Purpose: Previous studies found inconsistent results regarding the relationship between Alzheimer's disease (AD) and catecholamines, such as dopamine (DA), norepinephrine (NE), and epinephrine (EPI). Therefore, the purpose of this study was to perform a systematic review and meta-analysis to evaluate the results of previous studies on this relationship.

Method: Literature retrieval of eligible studies was performed in four databases (Web of Science, PubMed, Embase, and PsycARTICLES). Standardized mean differences (SMDs) were calculated to assess differences in catecholamine concentrations between the AD groups and controls.

Results: Thirteen studies met the eligibility criteria. Compared with the controls, significant lower concentrations of NE (SMD = −1.10, 95% CI: −2.01 to −0.18, p = 0.019) and DA (SMD = −1.12, 95% CI: −1.88 to −0.37, p = 0.003) were observed in patients with AD. No difference was found in the concentrations of EPI between the two groups (SMD = −0.74, 95% CI: −1.85 to 0.37, p = 0.189).

Conclusion: Overall, these findings are in line with the hypothesis that reduced NE and DA may be an important indicator for AD (Registration number CRD42018112816).

Potential Use of Exfoliated and Cultured Olfactory Neuronal Precursors for In Vivo Alzheimer's Disease Diagnosis: A Pilot Study

Histopathological hallmarks of dementia have been described postmortem in the brain of patients with Alzheimer's disease (AD). Tau, a microtubule associated protein, is abnormally arranged in neurofibrillary tangles. In living AD patients, total tau (t-tau) and hyperphosphorylated tau (p-tau) levels are increased in the cerebrospinal fluid obtained by lumbar puncture. Herein, we studied the t-tau and p-tau levels as well as the subcellular distribution of t-tau in olfactory neuronal precursors obtained by exfoliation of the nasal cavity of AD patients and control participants. Data showed that t-tau and p-tau levels were increased in cell homogenates from AD patients. Also, t-tau immunoreactivity was arranged in a punctate pattern in olfactory neuronal precursors derived from an AD participant with 5 years of evolution and in the oldest participants, either control subjects or those with Alzheimer's disease. Results support that exfoliated neuronal precursors have tau alterations demonstrated in postmortem brain and in the cerebrospinal fluid. This evidence and because the obtainment of olfactory neuronal precursors is a noninvasive procedure, detection of tau alterations shown here might be useful for an early diagnosis of AD-type dementia.

Neuropathology and pathogenesis of extrapyramidal movement disorders: a critical update-I. Hypokinetic-rigid movement disorders

Extrapyramidal movement disorders include hypokinetic rigid and hyperkinetic or mixed forms, most of them originating from dysfunction of the basal ganglia (BG) and their information circuits. The functional anatomy of the BG, the cortico-BG-thalamocortical, and BG-cerebellar circuit connections are briefly reviewed. Pathophysiologic classification of extrapyramidal movement disorder mechanisms distinguish (1) parkinsonian syndromes, (2) chorea and related syndromes, (3) dystonias, (4) myoclonic syndromes, (5) ballism, (6) tics, and (7) tremor syndromes. Recent genetic and molecular-biologic classifications distinguish (1) synucleinopathies (Parkinson's disease, dementia with Lewy bodies, Parkinson's disease-dementia, and multiple system atrophy); (2) tauopathies (progressive supranuclear palsy, corticobasal degeneration, FTLD-17; Guamian Parkinson-dementia; Pick's disease, and others); (3) polyglutamine disorders (Huntington's disease and related disorders); (4) pantothenate kinase-associated neurodegeneration; (5) Wilson's disease; and (6) other hereditary neurodegenerations without hitherto detected genetic or specific markers. The diversity of phenotypes is related to the deposition of pathologic proteins in distinct cell populations, causing neurodegeneration due to genetic and environmental factors, but there is frequent overlap between various disorders. Their etiopathogenesis is still poorly understood, but is suggested to result from an interaction between genetic and environmental factors. Multiple etiologies and noxious factors (protein mishandling, mitochondrial dysfunction, oxidative stress, excitotoxicity, energy failure, and chronic neuroinflammation) are more likely than a single factor. Current clinical consensus criteria have increased the diagnostic accuracy of most neurodegenerative movement disorders, but for their definite diagnosis, histopathological confirmation is required. We present a timely overview of the neuropathology and pathogenesis of the major extrapyramidal movement disorders in two parts, the first one dedicated to hypokinetic-rigid forms and the second to hyperkinetic disorders.

How to demix Alzheimer-type and PSP-type tau lesions out of their mixture -hybrid approach to dissect comorbidity

Neurofibrillary tangles (NFTs), are shared between progressive supranuclear palsy (PSP) and Alzheimer disease (AD). Histological distinction of PSP and AD is possible based on the distribution of NFTs. However, neuropathologists may encounter diagnostic difficulty with comorbidity of PSP and AD. In this study, we tried to circumvent this difficulty by analyzing five autopsied brains harboring both PSP and AD pathology. Tau-positive lesions were sorted based on their cell type (neuron versus glia), and tau isoforms: three-repeat (3R) versus four-repeat (4R) tau. 16 regions were selected to map these lesions throughout the brain. 4R-tau lesions were present in all areas examined. Among them, 3R-tau lesions were absent in some areas. These 4R selective (4R+/3R-) areas dictate prototypic distribution of PSP, not usually found in AD, such as pontine nucleus, red nucleus, inferior olivary nucleus, dentate nucleus, globus pallidus and putamen, each contained both glial and neuronal lesions. In contrast, additional 3R-tau lesions were found in hippocampal formation to neocortex, where 3R immunoreactivity (IR) was predominant over the 4R counterpart mainly in neurons as found in AD but not in PSP. Although tau lesions in central grey matter, substantia nigra and locus coeruleus are found in both AD and PSP, 4R-selectivity with glial component suggests PSP origin. Even if the presence of 3 R IR in these areas suggests AD pathology, it does not exclude the involvement of PSP-type lesion because distinction of 4R IR into PSP or AD is not yet possible. Further demixing may be possible if biochemical difference of 4R tau between PSP and AD is identified.

Brainstem tau pathology in Alzheimer's disease is characterized by increase of three repeat tau and independent of amyloid β

INTRODUCTION

Alzheimer-type neuropil threads (NTs) and neurofibrillary tangles (NFTs) are comprised of either 4 repeat (4R)-tau, 3 repeat (3R)-tau, or a mixture of both. In the hippocampus, the number of NFTs, and the proportion of 3R tau progressively increases. If this preferential accumulation of 3R tau also occurs in the brainstem, it may be fundamentally related to progression of Alzheimer pathology.

METHODS

Midbrain and pontine sections of brainstems from 23 cases (Braak-NFT stages I/II: 8, III/IV: 8, and V/VI: 7) were double immunofluorolabeled for 4R and 3R tau. High-resolution (0.645 μm/pixel), in-focus snapshots were tiled to cover entire brain sections using a virtual slide system. Each lesion was classified by size (NT < 200 μm² < NFT) and staining profile (3R/4R). In addition, the localization and quantity of amyloid β (Aβ) deposits were examined in adjacent sections for comparison with tau.

RESULTS

The data sets obtained from approximately 286 gigabytes of image files consisted of 847,763 NTs and 7859 NFTs. The proportion of 3R tau-positive NTs and NFTs in the midbrain, and 3R tau-positive NTs in the pons gradually increased with advancing NFT stages, while the proportion of 3R tau-positive NFTs in the pons was already elevated at early stages. Aβ deposits were absent at NFT stages I/II, and when present at later stages, their regional distribution was different from that of tau. These observations suggest that a progressive increase in the proportion of 3R tau occurs independently of Aβ deposits.

CONCLUSIONS

This is the first quantitative analysis of NFTs and NTs in the human brainstem. We demonstrate that the proportion of 3R tau in the brainstem neurofibrillary changes increases with disease progression. Because this phenomenon is shared between the brainstem and the hippocampus, this increase may be fundamental to the pathogenesis of Alzheimer disease.

Tau pathology in aged cynomolgus monkeys is progressive supranuclear palsy/corticobasal degeneration- but not Alzheimer disease-like -Ultrastructural mapping of tau by EDX

Concomitant deposition of amyloid -beta protein (Aβ) and neuronal tau as neurofibrillary tangles in the human brain is a hallmark of Alzheimer disease (AD). Because these deposits increase during normal aging, it has been proposed that aging brains may also undergo AD-like changes. To investigate the neuropathological changes that occur in the aging primate brain, we examined 21 brains of cynomolgus monkeys (7–36 years old) for Aβ- and tau-positive lesions. We found, 1) extensive deposition of Aβ in brains of cynomolgus monkeys over 25 years of age, 2) selective deposition of 4-repeat tau as pretangles in neurons, and as coiled body-like structures in oligodendroglia-like cells and astrocytes, 3) preferential distribution of tau in the basal ganglia and neocortex rather than the hippocampus, and 4) age-associated increases in 30–34 kDa AT8- and RD4-positive tau fragments in sarkosyl-insoluble fractions. We further labeled tau-positive structures using diaminobezidine enhanced with nickel, and visualized nickel-labeled structures by energy-dispersive X-ray (EDX) analysis of ultrathin sections. This allowed us to distinguish between nickel-labeled tau and background electron-dense structures, and we found that tau localized to 20–25 nm straight filaments in oligodendroglia-like cells and neurons. Our results indicate that the cytopathology and distribution of tau deposits in aged cynomolgus brains resemble those of progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) rather than AD. Thus, even in the presence of Aβ, age-associated deposition of tau in non-human primates likely does not occur through AD-associated mechanisms.

Characteristics of Tau and Its Ligands in PET Imaging

Tau deposition is one of the neuropathological hallmarks in Alzheimer’s disease as well as in other neurodegenerative disorders called tauopathies. Recent efforts to develop selective tau radiopharmaceuticals have allowed the visualization of tau deposits in vivo. In vivo tau imaging allows the assessment of the regional distribution of tau deposits in a single human subject over time for determining the pathophysiology of tau accumulation in aging and neurodegenerative conditions as well as for application in drug discovery of anti-dementia drugs as surrogate markers. However, tau deposits show complicated characteristics because of different isoform composition, histopathology, and ultrastructure in various neurodegenerative conditions. In addition, since tau radiopharmaceuticals possess different chemotype classes, they may show different binding characteristics with heterogeneous tau deposits. In this review, we describe the characteristics of tau deposits and their ligands that have β-sheet binding properties, and the status of tau imaging in clinical studies.