Identification of amino-terminally cleaved tau fragments that distinguish progressive supranuclear palsy from corticobasal degeneration

Corticobasal degeneration with visual hallucination as an initial symptom: A case report

Although the initial symptoms of corticobasal degeneration (CBD) are varied, psychiatric symptoms are uncommon. Here, we report the autopsy findings of a patient with early CBD who presented with hallucinations. A 68-year-old man developed memory loss and visions of bears and insects. Because of slow vertical eye movement, postural instability, and levodopa-unresponsive parkinsonism, the patient initially was clinically diagnosed with progressive supranuclear palsy. He died of a urinary tract infection 11 months after the onset of the disease. Histopathological examination revealed neuronal loss and gliosis, which were severe in the substantia nigra and moderate in the globus pallidus and subthalamic nucleus. Astrocytic plaques were scattered throughout the amygdala and premotor cortex. The superficial cortical layers lacked ballooned neurons and spongiosis, and tau deposition was greater in glia than in neurons. The amygdala contained a moderate number of argyrophilic grains and pretangles. Western blot analysis showed a 37-kDa band among the low-molecular-weight tau fragments. Because the CBD pathology was mild, we attributed the patient's visual hallucinations to the marked argyrophilic grain pathology. CBD can occur with psychiatric symptoms, including visual hallucinations, and argyrophilic grain pathology may be associated with psychiatric symptoms.

Tau protein profiling in tauopathies: a human brain study

Abnormal accumulation of misfolded and hyperphosphorylated tau protein in brain is the defining feature of several neurodegenerative diseases called tauopathies, including Alzheimer's disease (AD). In AD, this pathological change is reflected by highly specific cerebrospinal fluid (CSF) tau biomarkers, including both phosphorylated and non-phosphorylated variants. Interestingly, despite tau pathology being at the core of all tauopathies, CSF tau biomarkers remain unchanged in certain tauopathies, e.g., progressive supranuclear palsy (PSP), Pick's disease (PiD), and corticobasal neurodegeneration (CBD). To better understand commonalities and differences between tauopathies, we report a multiplex assay combining immunoprecipitation and high-resolution mass spectrometry capable of detecting and quantifying peptides from different tau protein isoforms as well as non-phosphorylated and phosphorylated peptides, including those carrying multiple phosphorylations. We investigated the tau proteoforms in soluble and insoluble fractions of brain tissue from subjects with autopsy-confirmed tauopathies, including sporadic AD (n = 10), PSP (n = 11), PiD (n = 10), and CBD (n = 10), and controls (n = 10). Our results demonstrate that non-phosphorylated tau profiles differ across tauopathies, generally showing high abundance of microtubule-binding region (MTBR)-containing peptides in insoluble protein fractions compared with controls; the AD group showed 12-72 times higher levels of MTBR-containing aggregates. Quantification of tau isoforms showed the 3R being more abundant in PiD and the 4R isoform being more abundant in CBD and PSP in the insoluble fraction. Twenty-three different phosphorylated peptides were quantified. Most phosphorylated peptides were measurable in all investigated tauopathies. All phosphorylated peptides were significantly increased in AD insoluble fraction. However, doubly and triply phosphorylated peptides were significantly increased in AD even in the soluble fraction. Results were replicated using a validation cohort comprising AD (n = 10), CBD (n = 10), and controls (n = 10). Our study demonstrates that abnormal levels of phosphorylation and aggregation do indeed occur in non-AD tauopathies, however, both appear pronouncedly increased in AD, becoming a distinctive characteristic of AD pathology.

Exploring the significance of caspase-cleaved tau in tauopathies and as a complementary pathology to phospho-tau in Alzheimer's disease: implications for biomarker development and therapeutic targeting

Tauopathies are neurodegenerative diseases that typically require postmortem examination for a definitive diagnosis. Detecting neurotoxic tau fragments in cerebrospinal fluid (CSF) and serum provides an opportunity for in vivo diagnosis and disease monitoring. Current assays primarily focus on total tau or phospho-tau, overlooking other post-translational modifications (PTMs). Caspase-cleaved tau is a significant component of AD neuropathological lesions, and experimental studies confirm the high neurotoxicity of these tau species. Recent evidence indicates that certain caspase-cleaved tau species, such as D13 and D402, are abundant in AD brain neurons and only show a modest degree of co-occurrence with phospho-tau, meaning caspase-truncated tau pathology is partially distinct and complementary to phospho-tau pathology. Furthermore, these caspase-cleaved tau species are nearly absent in 4-repeat tauopathies. In this review, we will discuss the significance of caspase-cleaved tau in the development of tauopathies, specifically emphasizing its role in AD. In addition, we will explore the potential of caspase-cleaved tau as a biomarker and the advantages for drug development targeting caspase-6. Developing specific and sensitive assays for caspase-cleaved tau in biofluids holds promise for improving the diagnosis and monitoring of tauopathies, providing valuable insights into disease progression and treatment efficacy.

Distinct tau folds initiate templated seeding and alter the post-translational modification profile

Pathological tau accumulates in the brain in tauopathies such as Alzheimer's disease, Pick's disease, progressive supranuclear palsy and corticobasal degeneration, and forms amyloid-like filaments incorporating various post-translational modifications (PTMs). Cryo-electron microscopic (cryo-EM) studies have demonstrated that tau filaments extracted from tauopathy brains are characteristic of the disease and share a common fold(s) in the same disease group. Furthermore, the tau PTM profile changes during tau pathology formation and disease progression, and disease-specific PTMs are detected in and around the filament core. In addition, templated seeding has been suggested to trigger pathological tau amplification and spreading in vitro and in vivo, although the molecular mechanisms are not fully understood. Recently, we reported that the cryo-EM structures of tau protofilaments in SH-SY5Y cells seeded with patient-derived tau filaments show a core structure(s) resembling that of the original seeds. Here, we investigated PTMs of tau filaments accumulated in the seeded cells by liquid chromatography/tandem mass spectrometry and compared them with the PTMs of patient-derived tau filaments. Examination of insoluble tau extracted from SH-SY5Y cells showed that numerous phosphorylation, deamidation and oxidation sites detected in the fuzzy coat in the original seeds were well reproduced in SH-SY5Y cells. Moreover, templated tau filament formation preceded both truncation of the N-/C-terminals of tau and PTMs in and around the filament core, indicating these PTMs may predominantly be introduced after the degradation of the fuzzy coat.

Clinical course of pathologically confirmed corticobasal degeneration and corticobasal syndrome

The clinical presentation of corticobasal degeneration is diverse, while the background pathology of corticobasal syndrome is also heterogeneous. Therefore, predicting the pathological background of corticobasal syndrome is extremely difficult. Herein, we investigated the clinical findings and course in patients with pathologically, genetically and biochemically verified corticobasal degeneration and corticobasal syndrome with background pathology to determine findings suggestive of background disorder. Thirty-two patients were identified as having corticobasal degeneration. The median intervals from the initial symptoms to the onset of key milestones were as follows: gait disturbance, 0.0 year; behavioural changes, 1.0 year; falls, 2.0 years; cognitive impairment, 2.0 years; speech impairment, 2.5 years; supranuclear gaze palsy, 3.0 years; urinary incontinence, 3.0 years; and dysphagia, 5.0 years. The median survival time was 7.0 years; 50% of corticobasal degeneration was diagnosed as corticobasal degeneration/corticobasal syndrome at the final presentation. Background pathologies of corticobasal syndrome (n = 48) included corticobasal degeneration (33.3%), progressive supranuclear palsy (29.2%) and Alzheimer's disease (12.5%). The common course of corticobasal syndrome was initial gait disturbance and early fall. In addition, corticobasal degeneration-corticobasal syndrome manifested behavioural change (2.5 years) and cognitive impairment (3.0 years), as the patient with progressive supranuclear palsy-corticobasal syndrome developed speech impairment (1.0 years) and supranuclear gaze palsy (6.0 years). The Alzheimer's disease-corticobasal syndrome patients showed cognitive impairment (1.0 years). The frequency of frozen gait at onset was higher in the corticobasal degeneration-corticobasal syndrome group than in the progressive supranuclear palsy-corticobasal syndrome group [P = 0.005, odds ratio (95% confidence interval): 31.67 (1.46-685.34)]. Dysarthria at presentation was higher in progressive supranuclear palsy-corticobasal syndrome than in corticobasal degeneration-corticobasal syndrome [P = 0.047, 6.75 (1.16-39.20)]. Pyramidal sign at presentation and personality change during the entire course were higher in Alzheimer's disease-corticobasal syndrome than in progressive supranuclear palsy-corticobasal syndrome [P = 0.011, 27.44 (1.25-601.61), and P = 0.013, 40.00 (1.98-807.14), respectively]. In corticobasal syndrome, decision tree analysis revealed that 'freezing at onset' or 'no dysarthria at presentation and age at onset under 66 years in the case without freezing at onset' predicted corticobasal degeneration pathology with a sensitivity of 81.3% and specificity of 84.4%. 'Dysarthria at presentation and age at onset over 61 years' suggested progressive supranuclear palsy pathology, and 'pyramidal sign at presentation and personality change during the entire course' implied Alzheimer's disease pathology. In conclusion, frozen gait at onset, dysarthria, personality change and pyramidal signs may be useful clinical signs for predicting background pathologies in corticobasal syndrome.

Neuropathology of spinocerebellar ataxia type 8: Common features and unique tauopathy

Spinocerebellar ataxia type 8 (SCA8) is a neurodegenerative condition that presents with several neurological symptoms, such as cerebellar ataxia, parkinsonism, and cognitive impairment. It is caused by a CTA/CTG repeat expansion on chromosome 13q21 (ataxin 8 opposite strand [ATXN8OS]). However, the pathological significance of this expansion remains unclear. Moreover, abnormal CTA/CTG repeat expansions in ATXN8OS have also been reported in other neurodegenerative diseases, including progressive supranuclear palsy. In this study, we analyzed all available autopsy cases in Japan to investigate common pathological features and profiles of tau pathology in each case. Severe neuronal loss in the substantia nigra and prominent loss of Purkinje cells, atrophy of the molecular layer, and proliferation of Bergmann glia in the cerebellum were common features. Regarding tauopathy, one case presented with progressive supranuclear palsy-like 4-repeat tauopathy in addition to mild Alzheimer-type 3- and 4-repeat tauopathy. Another case showed 3- and 4-repeat tauopathy accentuated in the brainstem. The other two cases lacked tauopathy after extensive immunohistochemical studies. The present study confirmed common pathological features of SCA8 as degeneration of the substantia nigra in addition to the cerebellum. Our study also confirmed unique tauopathy in two of four cases, indicating the necessity to further collect autopsy cases.

α-Synuclein Drives Tau's Cytotoxic Aggregates Formation through Hydrophobic Interactions

Tau and α-synuclein are proteins involved in pathologies known as tauopathies and synucleinopathies, respectively. Moreover, evidence shows that there is a crosstalk between them as is seen in the brains of individuals with sporadic neurodegenerative disorders. Based on that, we present data showing that the hydrophobic α-peptide 71 VTGVTAVAQKTV82 induces the aggregation of the full-length tau fragment in the absence of heparin assessed by ThT. Moreover, AFM images reveal the presence of straight filaments and amorphous aggregates of full-length tau in the presence of the α-peptide. Additionally, ITC experiments showed the interaction of the α-peptide with tau full-length (441 amino acids),4R (amino acids from 244 to 369), and both hexapeptides 275 VQIINK280 and 306 VQIVYK311 through hydrophobic interactions. The Raman spectroscopy spectra showed conformational changes in the Amide region in the aggregates formed with full-length tau and α-syn peptide. Furthermore, the incubation of extracellular aggregates with N2a cells showed morphological differences in the cellular body and the nucleus suggesting cell death. Moreover,, the incubation of different types of aggregates in cell culture provokes the release of Lactate dehydrogenase (LDH). Altogether, we found that α-synuclein peptide can drive the aggregation of full-length tau-provoking morphological and structural changes evoking cytotoxic effects.

An autopsy case of progressive supranuclear palsy treated with monoclonal antibody against tau

We report an autopsy case of progressive supranuclear palsy (PSP-Richardson syndrome). The individual had been enrolled in a phase 2 trial and received a monoclonal tau antibody (tilavonemab, ABBV-8E12); he died of intrahepatic cholangiocarcinoma and gastrointestinal bleeding during the clinical trial. Neuropathological examination demonstrated neuronal loss, gliosis, and widespread deposits of phosphorylated tau in the neurofibrillary tangles, tufted astrocytes, coiled bodies, and threads, which mainly occurred in the inferior olive nucleus, dentate nucleus of the cerebellum, substantia nigra, midbrain tegmentum, subthalamic nuclei, globus pallidus, putamen, and precentral gyrus, confirming typical PSP pathology. Phosphorylated tau was also found to accumulate in Betz cells, Purkinje cells, and pencil fibers in the basal ganglia. In conclusion, no additional changes or pathological modifications, which were expected from immunotherapy targeting tau, were visible in the present case.

Case report of a patient with unclassified tauopathy with molecular and neuropathological features of both progressive supranuclear palsy and corticobasal degeneration

Progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) are distinct clinicopathological subtypes of frontotemporal lobar degeneration. They both have atypical parkinsonism, and they usually have distinct clinical features. The most common clinical presentation of PSP is Richardson syndrome, and the most common presentation of CBD is corticobasal syndrome. In this report, we describe a patient with a five-year history of Richardson syndrome and a family history of PSP in her mother and sister. A tau PET scan (¹⁸F-APN-1607) revealed low-to-moderate uptake in the substantia nigra, globus pallidus, thalamus and posterior cortical areas, including temporal, parietal and occipital cortices. Neuropathological evaluation revealed widespread neuronal and glial tau pathology in cortical and subcortical structures, including tufted astrocytes in the motor cortex, striatum and midbrain tegmentum. The subthalamic nucleus had mild-to-moderate neuronal loss with globose neurofibrillary tangles, consistent with PSP. On the other hand, there were also astrocytic plaques, a pathological hallmark of CBD, in the neocortex and striatum. To further characterize the mixed pathology, we applied two machine learning-based diagnostic pipelines. These models suggested diagnoses of PSP and CBD depending on the brain region - PSP in the motor cortex and superior frontal gyrus and CBD in caudate nucleus. Western blots of insoluble tau from motor cortex showed a banding pattern consistent with mixed features of PSP and CBD, whereas tau from the superior frontal gyrus showed a pattern consistent with CBD. Real-time quaking-induced conversion (RT-QuIC) using brain homogenates from the motor cortex and superior frontal gyrus showed ThT maxima consistent with PSP, while reaction kinetics were consistent with CBD. There were no pathogenic variants in MAPT with whole genome sequencing. We conclude that this patient had an unclassified tauopathy and features of both PSP and CBD. The different pathologies in specific brain regions suggests caution in diagnosis of tauopathies with limited sampling.

Induction of Oxidative Stress in SH-SY5Y Cells by Overexpression of hTau40 and Its Mitigation by Redox-Active Nanoparticles

Abnormally phosphorylated tau protein is the principal component of neurofibrillary tangles, accumulating in the brain in many neurodegenerative diseases, including Alzheimer's disease. The aim of this study was to examine whether overexpression of tau protein leads to changes in the redox status of human neuroblastoma SH-SY5Y cells. The level of reactive oxygen species (ROS) was elevated in tau-overexpressing cells (TAU cells) as compared with cells transfected with the empty vector (EP cells). The level of glutathione was increased in TAU cells, apparently due to overproduction as an adaptation to oxidative stress. The TAU cells had elevated mitochondrial mass. They were more sensitive to 6-hydroxydopamine, delphinidin, 4-amino-TEMPO, and nitroxide-containing nanoparticles (NPs) compared to EP controls. These results indicate that overexpression of the tau protein imposes oxidative stress on the cells. The nitroxide 4-amino-TEMPO and nitroxide-containing nanoparticles (NPs) mitigated oxidative stress in TAU cells, decreasing the level of ROS. Nitroxide-containing nanoparticles lowered the level of lipid peroxidation in both TAU and EP cells, suggesting that nitroxides and NPs may mitigate tau-protein-induced oxidative stress.

Tau as a Biomarker of Neurodegeneration

Less than 50 years since tau was first isolated from a porcine brain, its detection in femtolitre concentrations in biological fluids is revolutionizing the diagnosis of neurodegenerative diseases. This review highlights the molecular and technological advances that have catapulted tau from obscurity to the forefront of biomarker diagnostics. Comprehensive updates are provided describing the burgeoning clinical applications of tau as a biomarker of neurodegeneration. For the clinician, tau not only enhances diagnostic accuracy, but holds promise as a predictor of clinical progression, phenotype, and response to drug therapy. For patients living with neurodegenerative disorders, characterization of tau dysregulation could provide much-needed clarity to a notoriously murky diagnostic landscape.

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.

Structures of tau and α-synuclein filaments from brains of patients with neurodegenerative diseases

Intracellular accumulations and aggregates of abnormal protein, consisting of amyloid-like fibrils, are common neuropathological features of many neurodegenerative diseases. The distributions and spreading of these pathological proteins are closely correlated with clinical symptoms and progression. Recent evidence supports the idea that template-mediated amplification of amyloid-like fibrils and intracellular propagation of fibril seeds are the main mechanisms by which pathological features spread along the neural circuits in the brain. Here, we review recent developments in the structural analysis of amyloid-like fibrils from brains of patients with various types of tauopathy and alpha-synucleinopathy, focusing on cryo-electron microscopy and mass analysis, and we discuss their relevance to the mechanisms of template-mediated amplification and intracellular propagation.

SUMO1 Modification of Tau in Progressive Supranuclear Palsy

Small ubiquitin-like modifiers (SUMO) have been implicated in several neurodegenerative diseases. SUMO1 conjugation has been shown to promote aggregation and regulate phosphorylation of the tau protein linked to Alzheimer’s disease and related tauopathies. The current study has demonstrated that SUMO1 co-localizes with intraneuronal tau inclusions in progressive supranuclear palsy (PSP). Immunoprecipitation of isolated and solubilized tau fibrils from PSP tissues revealed SUMO1 conjugation to a cleaved and N-terminally truncated tau. The effects of SUMOylation were examined using tau-SUMO fusion proteins which showed a higher propensity for tau oligomerization of PSP-truncated tau and accumulation on microtubules as compared to the full-length protein. This was found to be specific for SUMO1 as the corresponding SUMO2 fusion protein did not display a significantly altered cytoplasmic distribution or aggregation of tau. Blocking proteasome-mediated degradation promoted the aggregation of the tau fusion proteins with the greatest effect observed for truncated tau-SUMO1. The SUMO1 modification of the truncated tau in PSP may represent a detrimental event that promotes aggregation and impedes the ability of cells to remove the resulting protein deposits. This combination of tau truncation and SUMO1 modification may be a contributing factor in PSP pathogenesis.

Independent distribution between tauopathy secondary to subacute sclerotic panencephalitis and measles virus: An immunohistochemical analysis in autopsy cases including cases treated with aggressive antiviral therapies

Subacute sclerotic panencephalitis (SSPE) is a refractory neurological disorder after exposure to measles virus. Recently, SSPE cases have been treated with antiviral therapies, but data on the efficacy are inconclusive. Abnormal tau accumulation has been reported in the brain tissue of SSPE cases, but there are few reports in which this is amply discussed. Five autopsied cases diagnosed as definite SSPE were included in this study. The subject age or disease duration ranged from 7.6 to 40.9 years old or from 0.5 to 20.8 years, respectively. Cases 3 and 4 had been treated with antiviral therapies. All evaluated cases showed marked brain atrophy with cerebral ventricle dilatation; additionally, marked demyelination with fibrillary gliosis were observed in the cerebral white matter. The brainstem, cerebellum, and spinal cord were relatively preserved. Immunoreactivity (IR) against measles virus was seen in the brainstem tegmentum, neocortex, and/or limbic cortex of the untreated cases but was rarely seen in the two treated cases. Activated microglia were broadly observed from the cerebrum to the spinal cord and had no meaningful difference among cases. Neurofibrillary tangles characterized by a combination of 3‐ and 4‐repeat tau were observed mainly in the oculomotor nuclei, locus coeruleus, and limbic cortex. IR against phosphorylated tau was seen mainly in the cingulate gyrus, oculomotor nuclei, and pontine tegmentum, and tended to be observed frequently in cases with long disease durations but also tended to decrease along with neuronal loss, as in Case 5, which had the longest disease duration. Since the distribution of phosphorylated tau was independent from that of measles virus, the tauopathy following SSPE was inferred to be the result of diffuse brain inflammation triggered by measles rather than a direct result of measles virus. Moreover, antiviral therapies seemed to suppress measles virus but not the progression of tauopathy.

Pathologically verified corticobasal degeneration mimicking Richardson's syndrome coexisting clinically and radiologically shunt‐responsive normal pressure hydrocephalus

Background

Normal pressure hydrocephalus (NPH) manifests as gait instability, cognitive impairment, and urinary incontinence. This clinical triad of NPH sometimes occurs with ventriculomegaly in patients with neurodegenerative disease. Patients with pathologically verified neurodegenerative diseases, such as progressive supranuclear palsy (PSP), have received antemortem diagnoses of NPH.

Objectives

This study presents clinical and pathological features of a patient with pathologically verified corticobasal degeneration (CBD) coexisting with clinically shunt‐responsive NPH.

Methods

We performed clinical, radiological, and pathological evaluations in a patient with CBD whose antemortem diagnosis was PSP Richardson's syndrome (PSP‐RS) coexisting with shunt‐responsive NPH.

Results

A 59‐year‐old woman developed bradykinesia and gait instability and then frequent falls, urinary incontinence, and supranuclear vertical gaze palsy followed. At 63 years of age, her gait disturbance and urinary incontinence had deteriorated rapidly, and cognitive impairment was disclosed. There were typical findings of NPH with ventriculomegaly and disproportionately enlarged subarachnoid space hydrocephalus as well as a 2‐layer appearance with decreased and increased cerebral blood perfusion. Shunt placement ameliorated gait instability for more than 1 year and improved radiological indicators of NPH. However, atrophy of the midbrain progressed with time after transient increases in size. Although the antemortem diagnosis was probable PSP‐RS, pathological evaluation verified CBD. There were severe discontinuities of the ependymal lining of the lateral ventricles and subependymal rarefaction and gliosis with tau‐positive deposition.

Conclusions

Shunt surgery could ameliorate NPH symptoms in patients with 4‐repeat tauopathies. Careful assessments of clinical findings are necessary to predict the benefits of shunts as a therapeutic option for patients with neurodegenerative diseases coexisting with NPH.

Tau deposition patterns are associated with functional connectivity in primary tauopathies

Tau pathology is the main driver of neuronal dysfunction in 4-repeat tauopathies, including cortico-basal degeneration and progressive supranuclear palsy. Tau is assumed to spread prion-like across connected neurons, but the mechanisms of tau propagation are largely elusive in 4-repeat tauopathies, characterized not only by neuronal but also by astroglial and oligodendroglial tau accumulation. Here, we assess whether connectivity is associated with 4R-tau deposition patterns by combining resting-state fMRI connectomics with both 2^nd generation ¹⁸F-PI-2620 tau-PET in 46 patients with clinically diagnosed 4-repeat tauopathies and post-mortem cell-type-specific regional tau assessments from two independent progressive supranuclear palsy patient samples (n = 97 and n = 96). We find that inter-regional connectivity is associated with higher inter-regional correlation of both tau-PET and post-mortem tau levels in 4-repeat tauopathies. In regional cell-type specific post-mortem tau assessments, this association is stronger for neuronal than for astroglial or oligodendroglial tau, suggesting that connectivity is primarily associated with neuronal tau accumulation. Using tau-PET we find further that patient-level tau patterns are associated with the connectivity of subcortical tau epicenters. Together, the current study provides combined in vivo tau-PET and histopathological evidence that brain connectivity is associated with tau deposition patterns in 4-repeat tauopathies.

Cryo-EM structures of τ filaments from human brain

Electron cryo-microscopy (cryo-EM) has made it possible to determine near-atomic structures of τ filaments from human brain. Previous work had shown that the cores of paired helical and straight filaments of Alzheimer's disease are made of two identical, but differently arranged C-shaped protofilaments. In recent years, cryo-EM has shown that the Alzheimer τ fold is 79 amino acids long. Five of the eight β-strands give rise to two antiparallel β-sheets, with the other three forming a β-helix. High-affinity binding sites of positron emission tomography ligand APN-1607 (PM-PBB3) are in the β-helix region. The Alzheimer fold contrasts with the 94 amino acid-long Pick fold, which is J-shaped and comprises nine β-strands that give rise to four antiparallel β-sheets, in the absence of a β-helix. Chronic traumatic encephalopathy τ fold is similar to the Alzheimer fold, but differs in the β-helix region, which is larger and contains a non-proteinaceous density that is probably hydrophobic. These folds are mostly two-layered. By contrast, the 107 amino acid τ fold of the 4R tauopathy corticobasal degeneration is four-layered and comprises 11 β-strands. It contains an internal, probably hydrophilic, density that is surrounded by τ. The τ folds described here share the presence of microtubule-binding repeats 3 and 4, as well as 10-13 amino acids after repeat 4.

Revisiting the grammar of Tau aggregation and pathology formation: how new insights from brain pathology are shaping how we study and target Tauopathies

Converging evidence continues to point towards Tau aggregation and pathology formation as central events in the pathogenesis of Alzheimer's disease and other Tauopathies. Despite significant advances in understanding the morphological and structural properties of Tau fibrils, many fundamental questions remain about what causes Tau to aggregate in the first place. The exact roles of cofactors, Tau post-translational modifications, and Tau interactome in regulating Tau aggregation, pathology formation, and toxicity remain unknown. Recent studies have put the spotlight on the wide gap between the complexity of Tau structures, aggregation, and pathology formation in the brain and the simplicity of experimental approaches used for modeling these processes in research laboratories. Embracing and deconstructing this complexity is an essential first step to understanding the role of Tau in health and disease. To help deconstruct this complexity and understand its implication for the development of effective Tau targeting diagnostics and therapies, we firstly review how our understanding of Tau aggregation and pathology formation has evolved over the past few decades. Secondly, we present an analysis of new findings and insights from recent studies illustrating the biochemical, structural, and functional heterogeneity of Tau aggregates. Thirdly, we discuss the importance of adopting new experimental approaches that embrace the complexity of Tau aggregation and pathology as an important first step towards developing mechanism- and structure-based therapies that account for the pathological and clinical heterogeneity of Alzheimer's disease and Tauopathies. We believe that this is essential to develop effective diagnostics and therapies to treat these devastating diseases.

To target Tau pathologies, we must embrace and reconstruct their complexities

The accumulation of hyperphosphorylated fibrillar Tau aggregates in the brain is one of the defining hallmarks of Tauopathy diseases, including Alzheimer's disease. However, the primary events or molecules responsible for initiation of the pathological Tau aggregation and spreading remain unknown. The discovery of heparin as an effective inducer of Tau aggregation in vitro was instrumental to enabling different lines of research into the role of Tau aggregation in the pathogenesis of Tauopathies. However, recent proteomics and cryogenic electron microscopy (cryo-EM) studies have revealed that heparin-induced Tau fibrils generated in vitro do not reproduce the biochemical and ultrastructural properties of disease-associated brain-derived Tau fibrils. These observations demand that we reassess our current approaches for investigating the mechanisms underpinning Tau aggregation and pathology formation. Our review article presents an up-to-date survey and analyses of 1) the evolution of our understanding of the interactions between Tau and heparin, 2) the various structural and mechanistic models of the heparin-induced Tau aggregation, 3) the similarities and differences between brain-derived and heparin-induced Tau fibrils; and 4) emerging concepts on the biochemical and structural determinants underpinning Tau pathological heterogeneity in Tauopathies. Our analyses identify specific knowledge gaps and call for 1) embracing the complexities of Tau pathologies; 2) reassessment of current approaches to investigate, model and reproduce pathological Tau aggregation as it occurs in the brain; 3) more research towards a better understanding of the naturally-occurring cofactor molecules that are associated with Tau brain pathology initiation and propagation; and 4) developing improved approaches for in vitro production of the Tau aggregates and fibrils that recapitulate and/or amplify the biochemical and structural complexity and diversity of pathological Tau in Tauopathies. This will result in better and more relevant tools, assays, and mechanistic models, which could significantly improve translational research and the development of drugs and antibodies that have higher chances for success in the clinic.

Evaluation of [18F]PI-2620, a second-generation selective tau tracer, for assessing four-repeat tauopathies

Tau aggregates represent a key pathologic feature of Alzheimer's disease and other neurodegenerative diseases. Recently, PET probes have been developed for in vivo detection of tau accumulation; however, they are limited because of off-target binding and a reduced ability to detect tau in non-Alzheimer's disease tauopathies. The novel tau PET tracer, [¹⁸F]PI-2620, has a high binding affinity and specificity for aggregated tau; therefore, it was hypothesized to have desirable properties for the visualization of tau accumulation in Alzheimer's disease and non-Alzheimer's disease tauopathies. To assess the ability of [¹⁸F]PI-2620 to detect regional tau burden in non-Alzheimer's disease tauopathies compared with Alzheimer's disease, patients with progressive supranuclear palsy (n = 3), corticobasal syndrome (n = 2), corticobasal degeneration (n = 1) or Alzheimer's disease (n = 8), and healthy controls (n = 7) were recruited. All participants underwent MRI, amyloid β assessment and [¹⁸F]PI-2620 PET (Image acquisition at 60-90 min post-injection). Cortical and subcortical tau accumulations were assessed by calculating standardized uptake value ratios using [¹⁸F]PI-2620 PET. For pathologic validation, tau pathology was assessed using tau immunohistochemistry and compared with [¹⁸F]PI-2620 retention in an autopsied case of corticobasal degeneration. In Alzheimer's disease, focal retention of [¹⁸F]PI-2620 was evident in the temporal and parietal lobes, precuneus, and cingulate cortex. Standardized uptake value ratio analyses revealed that patients with non-Alzheimer's disease tauopathies had elevated [¹⁸F]PI-2620 uptake only in the globus pallidus, as compared to patients with Alzheimer's disease, but not healthy controls. A head-to-head comparison of [¹⁸F]PI-2620 and [¹⁸F]PM-PBB3, another tau PET probe for possibly visualizing the four-repeat tau pathogenesis in non-Alzheimer's disease, revealed different retention patterns in one subject with progressive supranuclear palsy. Imaging-pathology correlation analysis of the autopsied patient with corticobasal degeneration revealed no significant correlation between [¹⁸F]PI-2620 retention in vivo. High [¹⁸F]PI-2620 uptake at 60-90 min post-injection in the globus pallidus may be a sign of neurodegeneration in four-repeat tauopathy, but not necessarily practical for diagnosis of non-Alzheimer's disease tauopathies. Collectively, this tracer is a promising tool to detect Alzheimer's disease-tau aggregation. However, late acquisition PET images of [¹⁸F]PI-2620 may have limited utility for reliable detection of four-repeat tauopathy because of lack of correlation between post-mortem tau pathology and different retention pattern than the non-Alzheimer's disease-detectable tau radiotracer, [¹⁸F]PM-PBB3. A recent study reported that [¹⁸F]PI-2620 tracer kinetics curves in four-repeat tauopathies peak earlier (within 30 min) than Alzheimer's disease; therefore, further studies are needed to determine appropriate PET acquisition times that depend on the respective interest regions and diseases.

Evolving concepts in progressive supranuclear palsy and other 4-repeat tauopathies

Tauopathies are classified according to whether tau deposits predominantly contain tau isoforms with three or four repeats of the microtubule-binding domain. Those in which four-repeat (4R) tau predominates are known as 4R-tauopathies, and include progressive supranuclear palsy, corticobasal degeneration, argyrophilic grain disease, globular glial tauopathies and conditions associated with specific MAPT mutations. In these diseases, 4R-tau deposits are found in various cell types and anatomical regions of the brain and the conditions share pathological, pathophysiological and clinical characteristics. Despite being considered 'prototype' tauopathies and, therefore, ideal for studying neuroprotective agents, 4R-tauopathies are still severe and untreatable diseases for which no validated biomarkers exist. However, advances in research have addressed the issues of phenotypic overlap, early clinical diagnosis, pathophysiology and identification of biomarkers, setting a road map towards development of treatments. New clinical criteria have been developed and large cohorts with early disease are being followed up in prospective studies. New clinical trial readouts are emerging and biomarker research is focused on molecular pathways that have been identified. Lessons learned from failed trials of neuroprotective drugs are being used to design new trials. In this Review, we present an overview of the latest research in 4R-tauopathies, with a focus on progressive supranuclear palsy, and discuss how current evidence dictates ongoing and future research goals.

Structure-based classification of tauopathies

The ordered assembly of tau protein into filaments characterizes several neurodegenerative diseases, which are called tauopathies. It was previously reported that, by cryo-electron microscopy, the structures of tau filaments from Alzheimer's disease^1,2, Pick's disease³, chronic traumatic encephalopathy⁴ and corticobasal degeneration⁵ are distinct. Here we show that the structures of tau filaments from progressive supranuclear palsy (PSP) define a new three-layered fold. Moreover, the structures of tau filaments from globular glial tauopathy are similar to those from PSP. The tau filament fold of argyrophilic grain disease (AGD) differs, instead resembling the four-layered fold of corticobasal degeneration. The AGD fold is also observed in ageing-related tau astrogliopathy. Tau protofilament structures from inherited cases of mutations at positions +3 or +16 in intron 10 of MAPT (the microtubule-associated protein tau gene) are also identical to those from AGD, suggesting that relative overproduction of four-repeat tau can give rise to the AGD fold. Finally, the structures of tau filaments from cases of familial British dementia and familial Danish dementia are the same as those from cases of Alzheimer's disease and primary age-related tauopathy. These findings suggest a hierarchical classification of tauopathies on the basis of their filament folds, which complements clinical diagnosis and neuropathology and also allows the identification of new entities-as we show for a case diagnosed as PSP, but with filament structures that are intermediate between those of globular glial tauopathy and PSP.

Development of a novel tau propagation mouse model endogenously expressing 3 and 4 repeat tau isoforms

The phenomenon of "prion-like propagation" in which aggregates of abnormal amyloid-fibrilized protein propagate between neurons and spread pathology, is attracting attention as a new mechanism in neurodegenerative diseases. There is a strong correlation between the accumulation or spread of abnormal tau aggregates and the clinical symptoms of tauopathies. Microtubule-associated protein of tau contains a microtubule-binding domain which consists of 3-repeats or 4-repeats due to alternative mRNA splicing of transcripts for the Microtubule-associated protein of tau gene. Although a number of models for tau propagation have been reported, most utilize 4-repeat human tau transgenic mice or adult wild-type mice expressing only endogenous 4-repeat tau and these models have not been able to reproduce the pathology of Alzheimer's disease in which 3-repeat and 4-repeat tau accumulate simultaneously, or that of Pick's disease in which only 3-repeat tau is aggregated. These deficiencies may reflect differences between human and rodent tau isoforms in the brain. To overcome this problem, we used genome editing techniques to generate mice that express an equal ratio of endogenous 3-repeat and 4-repeat tau, even after they become adults. We injected these mice with sarkosyl-insoluble fractions derived from the brains of human tauopathy patients such as those afflicted with Alzheimer's disease (3- and 4-repeat tauopathy), corticobasal degeneration (4-repeat tauopathy) or Pick's disease (3-repeat tauopathy). At 8-9 months following intracerebral injection of mice, histopathological and biochemical analyses revealed that the abnormal accumulation of tau was seed-dependent, with 3- and 4-repeat tau in Alzheimer's disease-injected brains, 4-repeat tau only in corticobasal degeneration-injected brains, and 3-repeat tau only in Pick disease-injected brains, all of which contained isoforms related to those found in the injected seeds. The injected abnormal tau was seeded, and accumulated at the site of injection and at neural connections, predominantly within the same site. The abnormal tau newly accumulated was found to be endogenous in these mice and to have crossed the species barrier. Of particular importance, Pick's body-like inclusions were observed in Pick's disease-injected mice, and accumulations characteristic of Pick's disease were reproduced, suggesting that we have developed the first model that recapitulates the pathology of Pick's disease. These models are not only useful for elucidating the mechanism of propagation of tau pathology involving both 3- and 4-repeat-isoforms, but can also reproduce the pathology of tauopathies, which should lead to the discovery of new therapeutic agents.

Tau Protein Interaction Partners and Their Roles in Alzheimer's Disease and Other Tauopathies

Tau protein plays a critical role in the assembly, stabilization, and modulation of microtubules, which are important for the normal function of neurons and the brain. In diseased conditions, several pathological modifications of tau protein manifest. These changes lead to tau protein aggregation and the formation of paired helical filaments (PHF) and neurofibrillary tangles (NFT), which are common hallmarks of Alzheimer's disease and other tauopathies. The accumulation of PHFs and NFTs results in impairment of physiological functions, apoptosis, and neuronal loss, which is reflected as cognitive impairment, and in the late stages of the disease, leads to death. The causes of this pathological transformation of tau protein haven't been fully understood yet. In both physiological and pathological conditions, tau interacts with several proteins which maintain their proper function or can participate in their pathological modifications. Interaction partners of tau protein and associated molecular pathways can either initiate and drive the tau pathology or can act neuroprotective, by reducing pathological tau proteins or inflammation. In this review, we focus on the tau as a multifunctional protein and its known interacting partners active in regulations of different processes and the roles of these proteins in Alzheimer's disease and tauopathies.

A molecular pathology, neurobiology, biochemical, genetic and neuroimaging study of progressive apraxia of speech

Progressive apraxia of speech is a neurodegenerative syndrome affecting spoken communication. Molecular pathology, biochemistry, genetics, and longitudinal imaging were investigated in 32 autopsy-confirmed patients with progressive apraxia of speech who were followed over 10 years. Corticobasal degeneration and progressive supranuclear palsy (4R-tauopathies) were the most common underlying pathologies. Perceptually distinct speech characteristics, combined with age-at-onset, predicted specific 4R-tauopathy; phonetic subtype and younger age predicted corticobasal degeneration, and prosodic subtype and older age predicted progressive supranuclear palsy. Phonetic and prosodic subtypes showed differing relationships within the cortico-striato-pallido-nigro-luysial network. Biochemical analysis revealed no distinct differences in aggregated 4R-tau while tau H1 haplotype frequency (69%) was lower compared to 1000+ autopsy-confirmed 4R-tauopathies. Corticobasal degeneration patients had faster rates of decline, greater cortical degeneration, and shorter illness duration than progressive supranuclear palsy. These findings help define the pathobiology of progressive apraxia of speech and may have consequences for development of 4R-tau targeting treatment.

Progressive apraxia of speech (PAOS) is a neurodegenerative syndrome of multiple etiologies which affects spoken communication. Here, the authors characterized the molecular pathology, biochemistry, genetics and longitudinal neuroimaging of 32 autopsy-confirmed patients with PAOS who were followed over 10 years.

Genetic pleiotropy and the shared pathological features of corticobasal degeneration and progressive supranuclear palsy: a case report and a review of the literature

Though distinct pathological entities, corticobasal degeneration (CBD) and progressive supranuclear palsy (PSP) share multiple biochemical and genetic features suggesting overlapping pathophysiology. We report the case of a patient with an 18-year clinical course consistent with behavioral variant frontotemporal dementia. The neuropathological assessment revealed unclassifiable frontotemporal lobar degeneration with tau-immunoreactive inclusions sharing features of both CBD and PSP. Whole-genome sequencing revealed a unique combination of pleiotropic genetic risk variants associated with both PSP and CBD. These findings support the observation that CBD and PSP share genetic co-expression networks that influence neurodegenerative pathogenesis common to 4R tauopathies.

Mechanisms of Neurodegeneration in Various Forms of Parkinsonism-Similarities and Differences

Parkinson’s disease (PD), dementia with Lewy body (DLB), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD) and multiple system atrophy (MSA) belong to a group of neurodegenerative diseases called parkinsonian syndromes. They share several clinical, neuropathological and genetic features. Neurodegenerative diseases are characterized by the progressive dysfunction of specific populations of neurons, determining clinical presentation. Neuronal loss is associated with extra- and intracellular accumulation of misfolded proteins. The parkinsonian diseases affect distinct areas of the brain. PD and MSA belong to a group of synucleinopathies that are characterized by the presence of fibrillary aggregates of α-synuclein protein in the cytoplasm of selected populations of neurons and glial cells. PSP is a tauopathy associated with the pathological aggregation of the microtubule associated tau protein. Although PD is common in the world’s aging population and has been extensively studied, the exact mechanisms of the neurodegeneration are still not fully understood. Growing evidence indicates that parkinsonian disorders to some extent share a genetic background, with two key components identified so far: the microtubule associated tau protein gene (MAPT) and the α-synuclein gene (SNCA). The main pathways of parkinsonian neurodegeneration described in the literature are the protein and mitochondrial pathways. The factors that lead to neurodegeneration are primarily environmental toxins, inflammatory factors, oxidative stress and traumatic brain injury.

Human tauopathy-derived tau strains determine the substrates recruited for templated amplification

Tauopathies are a subset of neurodegenerative diseases characterized by abnormal tau inclusions. Specifically, three-repeat tau and four-repeat tau in Alzheimer’s disease, three-repeat tau in Pick’s disease (PiD) and four-repeat tau in progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) form amyloid-like fibrous structures that accumulate in neurons and/or glial cells. Amplification and cell-to-cell transmission of abnormal tau based on the prion hypothesis are believed to explain the onset and progression of tauopathies. Recent studies support not only the self-propagation of abnormal tau, but also the presence of conformationally distinct tau aggregates, namely tau strains. Cryogenic electron microscopy analyses of patient-derived tau filaments have revealed disease-specific ordered tau structures. However, it remains unclear whether the ultrastructural and biochemical properties of tau strains are inherited during the amplification of abnormal tau in the brain. In this study, we investigated template-dependent amplification of tau aggregates using a cellular model of seeded aggregation. Tau strains extracted from human tauopathies caused strain-dependent accumulation of insoluble filamentous tau in SH-SY5Y cells. The seeding activity towards full-length four-repeat tau substrate was highest in CBD-tau seeds, followed by PSP-tau and Alzheimer’s disease (AD)-tau seeds, while AD-tau seeds showed higher seeding activity than PiD-tau seeds towards three-repeat tau substrate. Abnormal tau amplified in cells inherited the ultrastructural and biochemical properties of the original seeds. These results strongly suggest that the structural differences of patient-derived tau strains underlie the diversity of tauopathies, and that seeded aggregation and filament formation mimicking the pathogenesis of sporadic tauopathy can be reproduced in cultured cells. Our results indicate that the disease-specific conformation of tau aggregates determines the tau isoform substrate that is recruited for templated amplification, and also influences the prion-like seeding activity.

Strain-specific clearance of seed-dependent tau aggregation by lithium-induced autophagy

Different conformational strains of tau have been implicated in the clinicopathological heterogeneity of tauopathies. In this study, we hypothesized that distinct strains are degraded in a different manner. Lithium, a drug for bipolar disorder, had previously been reported to reduce aggregation-prone protein content by promoting autophagy. Here, we assessed the effects of lithium on tau aggregates using different tauopathy brain seeds. SH-SY5Y cells were transfected with C-terminal tau fragment Tau-CTF24 (residues 243-441), and Alzheimer's disease (AD), progressive supranuclear palsy (PSP), and corticobasal degeneration (CBD) brain seeds were introduced. After 48-h lithium treatment, sarkosyl-insoluble fractions were prepared. Lithium treatment was found to reduce the amount of insoluble tau and p62, and increase LC3-II levels along with the number of autophagic vacuoles in AD-seeded cells. The effects were lower in case of CBD seeds, and comparable between PSP and AD seeds. An inhibitor of myo-inositol monophosphatase (IMPase) also demonstrated similar effects. Overall, the study suggested that aggregated tau protein is degraded by lithium-induced autophagy, influencing IMPase in a strain-specific manner.

Tau Protein and Frontotemporal Dementias

Filamentous inclusions of tau protein are found in cases of inherited and sporadic frontotemporal dementias (FTDs). Mutations in MAPT, the tau gene, cause approximately 5% of cases of FTD. They proved that dysfunction of tau protein is sufficient to cause neurodegeneration and dementia. Clinically and pathologically, cases with MAPT mutations can resemble sporadic diseases, such as Pick's disease, globular glial tauopathy, progressive supranuclear palsy and corticobasal degeneration. The structures of tau filaments from Pick's disease and corticobasal degeneration, determined by electron cryo-microscopy, revealed the presence of specific tau folds in each disease, with no inter-individual variation. The same was true of chronic traumatic encephalopathy.

Comparison of Common and Disease-Specific Post-translational Modifications of Pathological Tau Associated With a Wide Range of Tauopathies

Tauopathies are the most common type of neurodegenerative proteinopathy, being characterized by cytoplasmic aggregates of hyperphosphorylated tau protein. The formation and morphologies of these tau inclusions, the distribution of the lesions and related metabolic changes in cytoplasm differ among different tauopathies. The aim of this study was to examine whether there are differences in the post-translational modifications (PTMs) in the pathological tau proteins. We analyzed sarkosyl-insoluble pathological tau proteins prepared from brains of patients with Alzheimer's disease, Pick's disease, progressive supranuclear palsy, corticobasal degeneration, globular glial tauopathy, and frontotemporal dementia and parkinsonisms linked to chromosome 17 with tau inclusions using liquid chromatography mass spectrometry. In pathological tau proteins associated with a wide range of tauopathies, 170 PTMs in total were identified including new PTMs. Among them, common PTMs were localized in the N- and C-terminal flanking regions of the microtubule binding repeats and PTMs, which were considered to be disease-specific, were found in microtubule binding repeats forming filament core. These suggested that the differences in PTMs reflected the differences in tau filament core structures in each disease.

Tau proteinopathies and the prion concept

The ordered assembly of a small number of proteins into amyloid filaments is central to age-related neurodegenerative diseases. Tau is the most commonly affected of these proteins. In sporadic diseases, assemblies of tau form in a stochastic manner in certain brain regions, from where they appear to spread in a deterministic way, giving rise to disease symptoms. Over the past decade, multiple lines of evidence have shown that assembled tau behaves like a prion. More recently, electron cryo-microscopy of tau filaments has shown that distinct conformers are present in different diseases, with no inter-individual variation for a given disease.

Unclassified four-repeat tauopathy associated with familial parkinsonism and progressive respiratory failure

We describe an autopsied patient with familial parkinsonism and unclassified four repeat-tau (4R-tau) aggregation. She presented with bradykinesia, truncal dystonia, and mild amnesia at the age of 61 and then exhibited body weight loss (15 kg over 8 months), sleep disturbances, and progressive respiratory failure with CO₂ narcosis. She died of respiratory failure at the age of 62, 14 months after disease onset. Her brother also showed parkinsonism at the age of 58 and suddenly died 6 months later. Postmortem examination revealed 4R-tau aggregation, which was characterized by neuronal globose-type tangles or pretangles, bush-like or miscellaneous astrocytic inclusions, and coiled bodies. The temporal tip, the striatum, the substantia nigra, the tegmentum of the midbrain, the medullary reticular formation, and the spinal cord were severely involved with tau aggregation. Argyrophilic grains and ballooned neurons were also found in the medial temporal structures, however, extensions of the 4R-aggregations in the case were clearly broader than those of the argyrophilic grains. Western blot analysis of sarkosyl-insoluble fractions from brain lysates revealed prominent bands of tau at both 33 kDa and 37 kDa. Genetic examinations did not reveal any known pathogenic mutations in MAPT, DCTN-1, PSEN-1, or familial or young-onset parkinsonism-related genes. The clinical manifestations, pathologic findings, and biochemical properties of aggregated tau in our patient cannot be explained by argyrophilic grain disease or other known 4R-tauopathies alone. Our results further extend the clinical and neuropathologic spectra of 4R-tauopathy.

Globular glial tauopathy Type I presenting with behavioral variant frontotemporal dementia

Globular glial tauopathy (GGT) is a recently proposed tauopathy characterized by the globular accumulation of four-repeat (4R) tau in the oligodendroglia (globular oligodendroglial inclusion (GOI)) and astrocytes (globular astrocytic inclusion (GAI)), in addition to deposition in neurons. Although it is proposed that GGT should be classified into three different neuropathological subtypes, previous reports have indicated that subclassification might be difficult in some cases. We report an autopy case of a 79-year-old man with behavioral variant frontotemporal dementia (bvFTD). He developed behavioral changes at 67 years of age and had auditory hallucinations and persecutory delusions at admission to a psychiatric hospital at 69 years of age. Neuropathologically, marked atrophy of the frontotemporal lobes and severe degeneration of the white matter and frontopontine tract were observed. The present case corresponded to GGT Type I, as numerous GOIs were observed, predominantly in the frontotemporal region. However, concurrent degeneration of the motor cortex and corticospinal tract suggest characteristics of Type II. Although the relationship between psychotic symptoms and GGT remains unclear, the present case demonstrates heterogeneity of GGT subtypes.

Cerebrospinal fluid tau fragment correlates with tau PET: a candidate biomarker for tangle pathology

To date, there is no validated fluid biomarker for tau pathology in Alzheimer's disease, with contradictory results from studies evaluating the correlation between phosphorylated tau in CSF with tau PET imaging. Tau protein is subjected to proteolytic processing into fragments before being secreted to the CSF. A recent study suggested that tau cleavage after amino acid 368 by asparagine endopeptidase (AEP) is upregulated in Alzheimer's disease. We used immunoprecipitation followed by mass spectrometric analyses to evaluate the presence of tau368 species in CSF. A novel Simoa® assay for quantification of tau368 in CSF was developed, while total tau (t-tau) was measured by ELISA and the presence of tau368 in tangles was evaluated using immunohistochemistry. The diagnostic utility of tau368 was first evaluated in a pilot study (Alzheimer's disease = 20, control = 20), then in a second cohort where the IWG-2 biomarker criteria were applied (Alzheimer's disease = 37, control = 45), and finally in a third cohort where the correlation with 18F-GTP1 tau PET was evaluated (Alzheimer's disease = 38, control = 11). The tau368/t-tau ratio was significantly decreased in Alzheimer's disease (P < 0.001) in all cohorts. Immunohistochemical staining demonstrated that tau fragments ending at 368 are present in tangles. There was a strong negative correlation between the CSF tau368/t-tau ratio and 18F-GTP1 retention. Our data suggest that tau368 is a tangle-enriched fragment and that the CSF ratio tau368/t-tau reflects tangle pathology. This novel tau biomarker could be used to improve diagnosis of Alzheimer's disease and to facilitate the development of drug candidates targeting tau pathology. Furthermore, future longitudinal studies will increase our understanding of tau pathophysiology in Alzheimer's disease and other tauopathies.

Physiological Tau Interactome in Brain and Its Link to Tauopathies

Alzheimer's disease (AD) and most of the other tauopathies are incurable neurodegenerative diseases with unpleasant symptoms and consequences. The common hallmark of all of these diseases is tau pathology, but its connection with disease progress has not been completely understood so far. Therefore, uncovering novel tau-interacting partners and pathology affected molecular pathways can reveal the causes of diseases as well as potential targets for the development of AD treatment. Despite the large number of known tau-interacting partners, a limited number of studies focused on in vivo tau interactions in disease or healthy conditions are available. Here, we applied an in vivo cross-linking approach, capable of capturing weak and transient protein-protein interactions, to a unique transgenic rat model of progressive tau pathology similar to human AD. We have identified 175 potential novel and known tau-interacting proteins by MALDI-TOF mass spectrometry. Several of the most promising candidates for possible drug development were selected for validation by coimmunoprecipitation and colocalization experiments in animal and cellular models. Three proteins, Baiap2, Gpr37l1, and Nptx1, were confirmed as novel tau-interacting partners, and on the basis of their known functions and implications in neurodegenerative or psychiatric disorders, we proposed their potential role in tau pathology.

Corticobasal degeneration: advances in clinicopathology and biomarkers

PURPOSE OF REVIEW

Corticobasal degeneration (CBD) is a rapidly progressive neurodegenerative tauopathy diagnosed postmortem by pathological examination. The clinical presentation of corticobasal syndrome (CBS) is an apraxic, dystonic, and rigid limb with asymmetrical cortical signs and myoclonus. However, less than half of the patients with CBS receive a CBD diagnosis. As tau-lowering therapies have entered clinical trials, improved antemortem diagnosis of CBD is needed. Here, clinicopathological, neuroimaging, and biofluid data in CBS and/or CBD patients are briefly summarized and some knowledge gaps identified.

RECENT FINDINGS

Developments of MRI-based and nuclear medicine imaging modalities have increased pathophysiological insights of CBS and may improve diagnostic accuracy. In particular, several tau-PET ligands have been evaluated in CBS patients. Cerebrospinal fluid and plasma levels of neurofilament light chain can distinguish CBS from Parkinson's disease but not from other atypical forms of Parkinsonism.

SUMMARY

Structural and functional imaging approaches provide some aid in the diagnosis of CBD but have low-content validity. None of the currently available tau-PET ligands is suitable for detecting straight filament 4repeat tau disease in clinical routine. Biofluid markers reflecting the distinct tau and/or astrocyte disease of CBD are needed. Examining biosamples along with clinical parameters from longitudinally followed patients with autopsy-confirmed CBD diagnosis shall hopefully delineate improved biomarkers.

Familial globular glial tauopathy linked to MAPT mutations: molecular neuropathology and seeding capacity of a prototypical mixed neuronal and glial tauopathy

Globular glial tauopathy (GGT) is a progressive neurodegenerative disease involving the grey matter and white matter (WM) and characterized by neuronal deposition of hyper-phosphorylated, abnormally conformed, truncated, oligomeric 4Rtau in neurons and in glial cells forming typical globular astrocyte and oligodendrocyte inclusions (GAIs and GOIs, respectively) and coiled bodies. Present studies centre on four genetic GGT cases from two unrelated families bearing the P301T mutation in MAPT and one case of sporadic GGT (sGGT) and one case of GGT linked to MAPT K317M mutation, for comparative purposes. Clinical and neuropathological manifestations and biochemical profiles of phospho-tau are subjected to individual variations in patients carrying the same mutation, even in carriers of the same family, independently of the age of onset, gender, and duration of the disease. Immunohistochemistry, western blotting, transcriptomic, proteomics and phosphoproteomics, and intra-cerebral inoculation of brain homogenates to wild-type (WT) mice were the methods employed. In GGT cases linked to MAPT P301T mutation, astrocyte markers GFAP, ALDH1L1, YKL40 mRNA and protein, GJA1 mRNA, and AQ4 protein are significantly increased; glutamate transporter GLT1 (EAAT2) and glucose transporter (SLC2A1) decreased; mitochondrial pyruvate carrier 1 (MPC1) increased, and mitochondrial uncoupling protein 5 (UCP5) almost absent in GAIs in frontal cortex (FC). Expression of oligodendrocyte markers OLIG1 and OLIG2mRNA, and myelin-related genes MBP, PLP1, CNP, MAG, MAL, MOG, and MOBP are significantly decreased in WM; CNPase, PLP1, and MBP antibodies reveal reduction and disruption of myelinated fibres; and SMI31 antibodies mark axonal damage in the WM. Altered expression of AQ4, GLUC-t, and GLT-1 is also observed in sGGT and in GGT linked to MAPT K317M mutation. These alterations point to primary astrogliopathy and oligodendrogliopathy in GGT. In addition, GGT linked to MAPT P301T mutation proteotypes unveil a proteostatic imbalance due to widespread (phospho)proteomic dearrangement in the FC and WM, triggering a disruption of neuron projection morphogenesis and synaptic transmission. Identification of hyper-phosphorylation of variegated proteins calls into question the concept of phospho-tau-only alteration in the pathogenesis of GGT. Finally, unilateral inoculation of sarkosyl-insoluble fractions of GGT homogenates from GGT linked to MAPT P301T, sGGT, and GGT linked to MAPT K317M mutation in the hippocampus, corpus callosum, or caudate/putamen in wild-type mice produces seeding, and time- and region-dependent spreading of phosphorylated, non-oligomeric, and non-truncated 4Rtau and 3Rtau, without GAIs and GOIs but only of coiled bodies. These experiments prove that host tau strains are important in the modulation of cellular vulnerability and phenotypes of phospho-tau aggregates.

Novel tau filament fold in corticobasal degeneration

Corticobasal degeneration (CBD) is a neurodegenerative tauopathy that is characterised by motor and cognitive disturbances (1–3). A higher frequency of the H1 haplotype of MAPT, the tau gene, is present in cases of CBD than in controls (4,5) and genome-wide association studies have identified additional risk factors (6). By histology, astrocytic plaques are diagnostic of CBD (7,8), as are detergent-insoluble tau fragments of 37 kDa by SDS-PAGE (9). Like progressive supranuclear palsy (PSP), globular glial tauopathy (GGT) and argyrophilic grain disease (AGD) (10), CBD is characterised by abundant filamentous tau inclusions that are made of isoforms with four microtubule-binding repeats (4R) (11–15). This distinguishes 4R tauopathies from Pick’s disease, filaments of which are made of three-repeat (3R) tau isoforms, and from Alzheimer’s disease and chronic traumatic encephalopathy (CTE), where both 3R and 4R tau isoforms are found in the filaments (16). Here we report the structures of tau filaments extracted from the brains of three individuals with CBD using electron cryo-microscopy (cryo-EM). They were identical between cases, but distinct from those of Alzheimer’s disease, Pick’s disease and CTE (17–19). The core of CBD filaments comprises residues K274-E380 of tau, spanning the last residue of R1, the whole of R2, R3 and R4, as well as 12 amino acids after R4. It adopts a novel four-layered fold, which encloses a large non-proteinaceous density. The latter is surrounded by the side chains of lysine residues 290 and 294 from R2 and 370 from the sequence after R4. CBD is the first 4R tauopathy with filaments of known structure.

Pre-clinical characterisation of E2814, a high-affinity antibody targeting the microtubule-binding repeat domain of tau for passive immunotherapy in Alzheimer's disease

Tau deposition in the brain is a pathological hallmark of many neurodegenerative disorders, including Alzheimer's disease (AD). During the course of these tauopathies, tau spreads throughout the brain via synaptically-connected pathways. Such propagation of pathology is thought to be mediated by tau species ("seeds") containing the microtubule binding region (MTBR) composed of either three repeat (3R) or four repeat (4R) isoforms. The tau MTBR also forms the core of the neuropathological filaments identified in AD brain and other tauopathies. Multiple approaches are being taken to limit tau pathology, including immunotherapy with anti-tau antibodies. Given its key structural role within fibrils, specifically targetting the MTBR with a therapeutic antibody to inhibit tau seeding and aggregation may be a promising strategy to provide disease-modifying treatment for AD and other tauopathies. Therefore, a monoclonal antibody generating campaign was initiated with focus on the MTBR. Herein we describe the pre-clinical generation and characterisation of E2814, a humanised, high affinity, IgG1 antibody recognising the tau MTBR. E2814 and its murine precursor, 7G6, as revealed by epitope mapping, are antibodies bi-epitopic for 4R and mono-epitopic for 3R tau isoforms because they bind to sequence motif HVPGG. Functionally, both antibodies inhibited tau aggregation in vitro. They also immunodepleted a variety of MTBR-containing tau protein species. In an in vivo model of tau seeding and transmission, attenuation of deposition of sarkosyl-insoluble tau in brain could also be observed in response to antibody treatment. In AD brain, E2814 bound different types of tau filaments as shown by immunogold labelling and recognised pathological tau structures by immunohistochemical staining. Tau fragments containing HVPGG epitopes were also found to be elevated in AD brain compared to PSP or control. Taken together, the data reported here have led to E2814 being proposed for clinical development.

Corticobasal degeneration with deep white matter lesion diagnosed by brain biopsy

Corticobasal degeneration (CBD) is a rare progressive neurodegenerative disorder characterized by asymmetric presentation of cerebral cortex signs, cortical sensory disturbance and extrapyramidal signs. Herein, we report a case of a 66-year-old Japanese woman who presented with apraxia of the right hand. She subsequently developed postural instability and cognitive impairments that rapidly worsened. One and a half years later, the patient was wheelchair-bound and severely demented. Brain magnetic resonance imaging revealed left dominant atrophy of the frontoparietal lobe. There was a hyperintense lesion in the deep white matter expanding toward the subcortical area on fluid-attenuated inversion recovery (FLAIR) images. In order to rule out the possibility of an intracranial tumor such as an astrocytoma or malignant lymphoma, we performed a brain biopsy of the left frontal middle gyrus. The patient became bedridden and showed akinetic mutism 1 year after biopsy. Pathological examination revealed a large amount of 4-repeat tau-immunoreactive neuropil threads scattered predominantly in the corticomedullary junction and tau-immunoreactive structures, consistent with CBD. Immunostaining for p53 showed no positive cells, and there were very few Ki-67-positive cells. On immunoblots of sarkosyl-insoluble brain extracts, a major doublet of 64 and 68 kDa full-length tau with two closely related fragments of approximately 37 kDa were detected. Based on these results, the patient was pathologically diagnosed as having CBD, excluding the possibility of tumor. Taken together with previous similar case reports, our findings indicate that a deep white matter hyperintense lesion on FLAIR images may be a useful clue to CBD, predicting rapid clinical progression with severe dementia based on severe white matter degeneration with a large amount of tau accumulation on pathological examination.

Corticobasal degeneration

Corticobasal degeneration (CBD) is a rare neurodegenerative disease characterized by the predominance of pathological 4 repeat tau deposition in various cell types and anatomical regions. Corticobasal syndrome (CBS) is one of the clinical phenotypes associated with CBD pathology, manifesting as a progressive asymmetric akinetic-rigid, poorly levodopa-responsive parkinsonism, with cerebral cortical dysfunction. CBD can manifest as several clinical phenotypes, and similarly, CBS can also have a pathologic diagnosis other than CBD. This chapter discusses the clinical manifestations of pathologically confirmed CBD cases, the current diagnostic criteria, as well as the pathologic and neuroimaging findings of CBD/CBS. At present, therapeutic options for CBD remain symptomatic. Further research is needed to improve the clinical diagnosis of CBD, as well as studies on disease-modifying therapies for this relentlessly progressive neurodegenerative disorder.

A Comprehensive Resource for Induced Pluripotent Stem Cells from Patients with Primary Tauopathies

Primary tauopathies are characterized neuropathologically by inclusions containing abnormal forms of the microtubule-associated protein tau (MAPT) and clinically by diverse neuropsychiatric, cognitive, and motor impairments. Autosomal dominant mutations in the MAPT gene cause heterogeneous forms of frontotemporal lobar degeneration with tauopathy (FTLD-Tau). Common and rare variants in the MAPT gene increase the risk for sporadic FTLD-Tau, including progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). We generated a collection of fibroblasts from 140 MAPT mutation/risk variant carriers, PSP, CBD, and cognitively normal controls; 31 induced pluripotent stem cell (iPSC) lines from MAPT mutation carriers, non-carrier family members, and autopsy-confirmed PSP patients; 33 genome engineered iPSCs that were corrected or mutagenized; and forebrain neural progenitor cells (NPCs). Here, we present a resource of fibroblasts, iPSCs, and NPCs with comprehensive clinical histories that can be accessed by the scientific community for disease modeling and development of novel therapeutics for tauopathies.

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A collection of fibroblasts from 140 MAPT mutation carriers, PSP, CBD, and controls

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31 iPSC lines reprogrammed from MAPT mutation carriers, PSP patients, and controls

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33 iPSC lines engineered with CRISPR/Cas9 or TALENs

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Comprehensive resource for tauopathy modeling and discovery of novel therapeutics