Tau seeding activity begins in the transentorhinal/entorhinal regions and anticipates phospho-tau pathology in Alzheimer's disease and PART

Cryo-EM structures of tau filaments from Alzheimer's disease with PET ligand APN-1607

Tau and Aβ assemblies of Alzheimer’s disease (AD) can be visualized in living subjects using positron emission tomography (PET). Tau assemblies comprise paired helical and straight filaments (PHFs and SFs). APN-1607 (PM-PBB3) is a recently described PET ligand for AD and other tau proteinopathies. Since it is not known where in the tau folds PET ligands bind, we used electron cryo-microscopy (cryo-EM) to determine the binding sites of APN-1607 in the Alzheimer fold. We identified two major sites in the β-helix of PHFs and SFs and a third major site in the C-shaped cavity of SFs. In addition, we report that tau filaments from posterior cortical atrophy (PCA) and primary age-related tauopathy (PART) are identical to those from AD. In support, fluorescence labelling showed binding of APN-1607 to intraneuronal inclusions in AD, PART and PCA. Knowledge of the binding modes of APN-1607 to tau filaments may lead to the development of new ligands with increased specificity and binding activity. We show that cryo-EM can be used to identify the binding sites of small molecules in amyloid filaments.

Re-emphasizing early Alzheimer's disease pathology starting in select entorhinal neurons, with a special focus on mitophagy

The entorhinal-hippocampal system contains distinct networks subserving declarative memory. This system is selectively vulnerable to changes of ageing and pathological processes. The entorhinal cortex (EC) is a pivotal component of this memory system since it serves as the interface between the neocortex and the hippocampus. EC is heavily affected by the proteinopathies of Alzheimer's disease (AD). These appear in a stereotypical spatiotemporal manner and include increased levels of intracellular amyloid-beta Aβ (iAβ), parenchymal deposition of Aβ plaques, and neurofibrillary tangles (NFTs) containing abnormally processed Tau. Increased levels of iAβ and the formation of NFTs are seen very early on in a population of neurons belonging to EC layer II (EC LII), and recent evidence leads us to believe that this population is made up of highly energy-demanding reelin-positive (RE+) projection neurons. Mitochondria are fundamental to the energy supply, metabolism, and plasticity of neurons. Evidence from AD postmortem brain tissues supports the notion that mitochondrial dysfunction is one of the initial pathological events in AD, and this is likely to take place in the vulnerable RE + EC LII neurons. Here we review and discuss these notions, anchored to the anatomy of AD, and formulate a hypothesis attempting to explain the vulnerability of RE + EC LII neurons to the formation of NFTs. We attempt to link impaired mitochondrial clearance to iAβ and signaling involving both apolipoprotein 4 and reelin, and argue for their relevance to the formation of NFTs specifically in RE + EC LII neurons during the prodromal stages of AD. We believe future studies on these interactions holds promise to advance our understanding of AD etiology and provide new ideas for drug development.

Simulating the outcome of amyloid treatments in Alzheimer's disease from imaging and clinical data

In this study, we investigate SimulAD, a novel quantitative instrument for the development of intervention strategies for disease-modifying drugs in Alzheimer's disease. SimulAD is based on the modeling of the spatio-temporal dynamics governing the joint evolution of imaging and clinical biomarkers along the history of the disease, and allows the simulation of the effect of intervention time and drug dosage on the biomarkers' progression. When applied to multi-modal imaging and clinical data from the Alzheimer's Disease Neuroimaging Initiative the method enables to generate hypothetical scenarios of amyloid lowering interventions. The results quantify the crucial role of intervention time, and provide a theoretical justification for testing amyloid modifying drugs in the pre-clinical stage. Our experimental simulations are compatible with the outcomes observed in past clinical trials, and suggest that anti-amyloid treatments should be administered at least 7 years earlier than what is currently being done in order to obtain statistically powered improvement of clinical endpoints.

Regulation of tau internalization, degradation, and seeding by LRP1 reveals multiple pathways for tau catabolism

In Alzheimer's disease (AD), pathological forms of tau are transferred from cell to cell and “seed” aggregation of cytoplasmic tau. Phosphorylation of tau plays a key role in neurodegenerative tauopathies. In addition, apolipoprotein E (apoE), a major component of lipoproteins in the brain, is a genetic risk determinant for AD. The identification of the apoE receptor, low-density lipoprotein receptor–related protein 1 (LRP1), as an endocytic receptor for tau raises several questions about the role of LRP1 in tauopathies: is internalized tau, like other LRP1 ligands, delivered to lysosomes for degradation, and does LRP1 internalize pathological tau leading to cytosolic seeding? We found that LRP1 rapidly internalizes ¹²⁵I-labeled tau, which is then efficiently degraded in lysosomal compartments. Surface plasmon resonance experiments confirm high affinity binding of tau and the tau microtubule-binding domain to LRP1. Interestingly, phosphorylated forms of recombinant tau bind weakly to LRP1 and are less efficiently internalized by LRP1. LRP1-mediated uptake of tau is inhibited by apoE, with the apoE4 isoform being the most potent inhibitor, likely because of its higher affinity for LRP1. Employing post-translationally–modified tau derived from brain lysates of human AD brain tissue, we found that LRP1-expressing cells, but not LRP1-deficient cells, promote cytosolic tau seeding in a process enhanced by apoE. These studies identify LRP1 as an endocytic receptor that binds and processes monomeric forms of tau leading to its degradation and promotes seeding by pathological forms of tau. The balance of these processes may be fundamental to the spread of neuropathology across the brain in AD.

Tau seeds are subject to aberrant modifications resulting in distinct signatures

The prion-like spread of tau pathology could underlie a spectrum of clinical syndromes including Alzheimer's disease (AD). Although evidence indicates that tau is transmissible, it is unclear how pathogenic tau seeds are processed in neurons. Here, we analyze fibrillar wild-type and disease-associated P301L tau seeds by using in vitro and neuronal assays. We show that P301L seeds are uniquely modified by post-translational modifications (PTMs) within the microtubule-binding region (MTBR). Although these modifications do not alter tau seed trafficking or localization, acetylated tau variants show accelerated tau aggregation, enhanced tau PTM priming, and prion-like templating. To explain the enhanced tau seed acetylation, we demonstrate that P301L seeds undergo auto-acetylation. Moreover, tau acts generally to inhibit HDAC6 deacetylase activity by preventing HDAC6 phosphorylation, leading to increased substrate acetylation. Our study highlights complex post-translational regulation of transmissible tau seeds and provides insight into the biological properties of tau strains in AD and other tauopathies.

Alzheimer's disease brain contains tau fractions with differential prion-like activities

Neurofibrillary tangles (NFTs) made of abnormally hyperphosphorylated tau are a hallmark of Alzheimer’s disease (AD) and related tauopathies. Regional distribution of NFTs is associated with the progression of the disease and has been proposed to be a result of prion-like propagation of misfolded tau. Tau in AD brain is heterogenous and presents in various forms. In the present study, we prepared different tau fractions by sedimentation combined with sarkosyl solubility from AD brains and analyzed their biochemical and pathological properties. We found that tau in oligomeric fraction (O-tau), sarkosyl-insoluble fractions 1 and 2 (SI₁-tau and SI₂-tau) and monomeric heat-stable fraction (HS-tau) showed differences in truncation, hyperphosphorylation, and resistance to proteinase K. O-tau, SI₁-tau, and SI₂-tau, but not HS-tau, were hyperphosphorylated at multiple sites and contained SDS- and β-mercaptoethanol–resistant high molecular weight aggregates, which lacked the N-terminal portion of tau. O-tau and SI₂-tau displayed more truncation and less hyperphosphorylation than SI₁-tau. Resistance to proteinase K was increased from O-tau to SI₁-tau to SI₂-tau. O-tau and SI₁-tau, but not SI₂-tau or HS-tau, captured tau from cell lysates and seeded tau aggregation in cultured cells. Heat treatment could not kill the prion-like activity of O-tau to capture normal tau. Hippocampal injection of O-tau into 18-month-old FVB mice induced significant tau aggregation in both ipsilateral and contralateral hippocampi, but SI₁-tau only induced tau pathology in the ipsilateral hippocampus, and SI₂-tau and HS-tau failed to induce any detectable tau aggregation. These findings suggest that O-tau and SI₁-tau have prion-like activities and may serve as seeds to recruit tau and template tau to aggregate, resulting in the propagation of tau pathology. Heterogeneity of tau pathology within AD brain results in different fractions with different biological and prion-like properties, which may pose a major challenge in targeting tau for development of effective therapeutic treatments.

Tau-proximity ligation assay reveals extensive previously undetected pathology prior to neurofibrillary tangles in preclinical Alzheimer's disease

BACKGROUND

Multimerization is a key process in prion-like disorders such as Alzheimer's disease (AD), since it is a requirement for self-templating tau and beta-amyloid amyloidogenesis. AT8-immunohistochemistry for hyperphosphorylated tau is currently used for the diagnosis and staging of tau pathology. Given that tau-tau interactions can occur in the absence of hyperphosphorylation or other post-translational modifications (PTMs), the direct visualization of tau multimerization could uncover early pathological tau multimers.

METHODS

Here, we used bimolecular fluorescent complementation, rapamycin-dependent FKBP/FRB-tau interaction and transmission electron microscopy to prove the in vitro specificity of tau-proximity ligation assay (tau-PLA). We then analyzed MAPT KO and P301S transgenic mice, and human hippocampus and temporal isocortex of all Braak stages with tau-PLA and compared it with immunohistochemistry for the diagnostic antibody AT8, the early phosphorylation-dependent AT180, and the conformational-dependent antibody MC1. Finally, we performed proteinase-K treatment to infer the content of amyloidogenic beta-sheet fold.

RESULTS

Our novel tau-proximity ligation assay (tau-PLA) directly visualized tau-tau interactions in situ, and exclusively recognized tau multimers but not monomers. It elicited no signal in MAPT KO mouse brains, but extensively labelled P301S transgenic mice and AD brain. Two groups of structures were detected, a previously unreported widespread small-sized diffuse pathology and large, neurofibrillary-like lesions. Tau-PLA-labelled diffuse pathology appeared from the earliest Braak stages, mostly unaccompanied by tangle-like tau-immunohistochemistry, being significantly more sensitive than any small-sized dot-/thread-like pathology labelled by AT180-, AT8- and MC1-immunohistochemistry in most regions quantified at stages 0-II. Tau-PLA-labelled diffuse pathology was extremely sensitive to Proteinase-K, in contrast to large lesions.

CONCLUSIONS

Tau-PLA is the first method to directly visualize tau multimers both in vitro and in situ with high specificity. We find that tau multimerization appears extensively from the earliest presymptomatic Braak stages as a previously unreported type of diffuse pathology. Importantly, in our study multimerization is the earliest detectable molecular event of AD tau pathology. Our findings open a new window to the study of early tau pathology, with potential implications in early diagnosis and the design of therapeutic strategies.

Impact of Tau on Neurovascular Pathology in Alzheimer's Disease

Alzheimer's disease (AD) is a chronic neurodegenerative disorder and the most prevalent cause of dementia. The main cerebral histological hallmarks are represented by parenchymal insoluble deposits of amyloid beta (Aβ plaques) and neurofibrillary tangles (NFT), intracellular filamentous inclusions of tau, a microtubule-associated protein. It is well-established that cerebrovascular dysfunction is an early feature of AD pathology, but the detrimental mechanisms leading to blood vessel impairment and the associated neurovascular deregulation are not fully understood. In 90% of AD cases, Aβ deposition around the brain vasculature, known as cerebral amyloid angiopathy (CAA), alters blood brain barrier (BBB) essential functions. While the effects of vascular Aβ accumulation are better documented, the scientific community has only recently started to consider the impact of tau on neurovascular pathology in AD. Emerging compelling evidence points to transmission of neuronal tau to different brain cells, including astrocytes, as well as to the release of tau into brain interstitial fluids, which may lead to perivascular neurofibrillar tau accumulation and toxicity, affecting vessel architecture, cerebral blood flow (CBF), and vascular permeability. BBB integrity and functionality may therefore be impacted by pathological tau, consequentially accelerating the progression of the disease. Tau aggregates have also been shown to induce mitochondrial damage: it is known that tau impairs mitochondrial localization, distribution and dynamics, alters ATP and reactive oxygen species production, and compromises oxidative phosphorylation systems. In light of this previous knowledge, we postulate that tau can initiate neurovascular pathology in AD through mitochondrial dysregulation. In this review, we will explore the literature investigating tau pathology contribution to the malfunction of the brain vasculature and neurovascular unit, and its association with mitochondrial alterations and caspase activation, in cellular, animal, and human studies of AD and tauopathies.

Hijacking Endocytosis and Autophagy in Extracellular Vesicle Communication: Where the Inside Meets the Outside

Extracellular vesicles, phospholipid bilayer-membrane vesicles of cellular origin, are emerging as nanocarriers of biological information between cells. Extracellular vesicles transport virtually all biologically active macromolecules (e.g., nucleotides, lipids, and proteins), thus eliciting phenotypic changes in recipient cells. However, we only partially understand the cellular mechanisms driving the encounter of a soluble ligand transported in the lumen of extracellular vesicles with its cytosolic receptor: a step required to evoke a biologically relevant response. In this context, we review herein current evidence supporting the role of two well-described cellular transport pathways: the endocytic pathway as the main entry route for extracellular vesicles and the autophagic pathway driving lysosomal degradation of cytosolic proteins. The interplay between these pathways may result in the target engagement between an extracellular vesicle cargo protein and its cytosolic target within the acidic compartments of the cell. This mechanism of cell-to-cell communication may well own possible implications in the pathogenesis of neurodegenerative disorders.

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.

Early Dendritic Dystrophy in Human Brains With Primary Age-Related Tauopathy

Dystrophic neurites (DNs) are found in many neurological conditions such as traumatic brain injury and age-related neurodegenerative diseases. In Alzheimer's disease (AD) specifically, senile plaques containing silver-stained DNs were already described in the original literature defining this disease. These DNs could be both axonal and dendritic in origin, while axonal dystrophy relative to plaque formation has been more extensively studied. Here, we demonstrate an early occurrence of dendritic dystrophy in the hippocampal CA1 and subicular regions in human brains (n = 23) with primary age-related tauopathy (PART), with neurofibrillary tangle (NFT) burden ranging from Braak stages I to III in the absence of cerebral β-amyloid (Aβ) deposition. In Bielschowsky's silver stain, segmented fusiform swellings on the apical dendrites of hippocampal and subicular pyramidal neurons were observed in all the cases, primarily over the stratum radiatum (s.r.). The numbers of silver-stained neuronal somata and dendritic swellings counted over CA1 to subiculum were positively correlated among the cases. Swollen dendritic processes were also detected in sections immunolabeled for phosphorylated tau (pTau) and sortilin. In aged and AD brains with both Aβ and pTau pathologies, silver- and immunolabeled dystrophic-like dendritic profiles occurred around and within individual neuritic plaques. These findings implicate that dendritic dystrophy can occur among hippocampal pyramidal neurons in human brains with PART. Therefore, as with the case of axonal dystrophy reported in literature, dendritic dystrophy can develop prior to Alzheimer-type plaque and tangle formation in the human brain.

A probabilistic atlas of locus coeruleus pathways to transentorhinal cortex for connectome imaging in Alzheimer's disease

According to the latest Braak staging of Alzheimer's disease (AD), tau pathology occurs earliest in the brain in the locus coeruleus (LC) of the brainstem, then propagates to the transentorhinal cortex (TEC), and later to other neocortical regions. Recent animal and in vivo human brain imaging research also support the trans-axonal propagation of tau pathology. In addition, neurochemical studies link norepinephrine to behavioral symptoms in AD. It is thus critical to examine the integrity of the LC-TEC pathway in studying the early development of the disease, but there has been limited work in this direction. By leveraging the high-resolution and multi-shell diffusion MRI data from the Human Connectome Project (HCP), in this work we develop a novel method for the reconstruction of the LC-TEC pathway in a cohort of 40 HCP subjects carefully selected based on rigorous quality control of the residual distortion artifacts in the brainstem. A probabilistic atlas of the LC-TEC pathway of both hemispheres is then developed in the MNI152 space and distributed publicly on the NITRC website. To apply our atlas on clinical imaging data, we develop an automated approach to calculate the medial core of the LC-TEC pathway for localized analysis of connectivity changes. In a cohort of 138 subjects from the Alzheimer's Disease Neuroimaging Initiative (ADNI), we demonstrate the detection of the decreased fiber integrity in the LC-TEC pathways with increasing disease severity.

Tauopathies: Deciphering Disease Mechanisms to Develop Effective Therapies

Tauopathies are neurodegenerative diseases characterized by the pathological accumulation of microtubule-associated protein tau (MAPT) in the form of neurofibrillary tangles and paired helical filaments in neurons and glia, leading to brain cell death. These diseases include frontotemporal dementia (FTD) and Alzheimer's disease (AD) and can be sporadic or inherited when caused by mutations in the MAPT gene. Despite an incredibly high socio-economic burden worldwide, there are still no effective disease-modifying therapies, and few tau-focused experimental drugs have reached clinical trials. One major hindrance for therapeutic development is the knowledge gap in molecular mechanisms of tau-mediated neuronal toxicity and death. For the promise of precision medicine for brain disorders to be fulfilled, it is necessary to integrate known genetic causes of disease, i.e., MAPT mutations, with an understanding of the dysregulated molecular pathways that constitute potential therapeutic targets. Here, the growing understanding of known and proposed mechanisms of disease etiology will be reviewed, together with promising experimental tau-directed therapeutics, such as recently developed tau degraders. Current challenges faced by the fields of tau research and drug discovery will also be addressed.

Targeting Tau to Treat Clinical Features of Huntington's Disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by severe motor, cognitive and psychiatric impairments. While motor deficits often confirm diagnosis, cognitive dysfunctions usually manifest early in the disease process and are consistently ranked among the leading factors that impact the patients' quality of life. The genetic component of HD, a mutation in the huntingtin (HTT) gene, is traditionally presented as the main contributor to disease pathology. However, accumulating evidence suggests the implication of the microtubule-associated tau protein to the pathogenesis and therefore, proposes an alternative conceptual framework where tau and mutant huntingtin (mHTT) act conjointly to drive neurodegeneration and cognitive dysfunction. This perspective on disease etiology offers new avenues to design therapeutic interventions and could leverage decades of research on Alzheimer's disease (AD) and other tauopathies to rapidly advance drug discovery. In this mini review, we examine the breadth of tau-targeting treatments currently tested in the preclinical and clinical settings for AD and other tauopathies, and discuss the potential application of these strategies to HD.

Cellular Biology of Tau Diversity and Pathogenic Conformers

Tau accumulation is a prominent feature in a variety of neurodegenerative disorders and remarkable effort has been expended working out the biochemistry and cell biology of this cytoplasmic protein. Tau's wayward properties may derive from germline mutations in the case of frontotemporal lobar degeneration (FTLD-MAPT) but may also be prompted by less understood cues—perhaps environmental or from molecular damage as a consequence of chronological aging—in the case of idiopathic tauopathies. Tau properties are undoubtedly affected by its covalent structure and in this respect tau protein is not only subject to changes in length produced by alternative splicing and endoproteolysis, but different types of posttranslational modifications that affect different amino acid residues. Another layer of complexity concerns alternate conformations—“conformers”—of the same covalent structures; in vivo conformers can encompass soluble oligomeric species, ramified fibrillar structures evident by light and electron microscopy and other forms of the protein that have undergone liquid-liquid phase separation to make demixed liquid droplets. Biological concepts based upon conformers have been charted previously for templated replication mechanisms for prion proteins built of the PrP polypeptide; these are now providing useful explanations to feature tau pathobiology, including how this protein accumulates within cells and how it can exhibit predictable patterns of spread across different neuroanatomical regions of an affected brain. In sum, the documented, intrinsic heterogeneity of tau forms and conformers now begins to speak to a fundamental basis for diversity in clinical presentation of tauopathy sub-types. In terms of interventions, emphasis upon subclinical events may be worthwhile, noting that irrevocable cell loss and ramified protein assemblies feature at end-stage tauopathy, whereas earlier events may offer better opportunities for diverting pathogenic processes. Nonetheless, the complexity of tau sub-types, which may be present even within intermediate disease stages, likely mitigates against one-size-fits-all therapeutic strategies and may require a suite of interventions. We consider the extent to which animal models of tauopathy can be reasonably enrolled in the campaign to produce such interventions and to slow the otherwise inexorable march of disease progression.

Patient-centered connectivity-based prediction of tau pathology spread in Alzheimer's disease

In Alzheimer's disease (AD), the Braak staging scheme suggests a stereotypical tau spreading pattern that does, however, not capture interindividual variability in tau deposition. This complicates the prediction of tau spreading, which may become critical for defining individualized tau-PET readouts in clinical trials. Since tau is assumed to spread throughout connected regions, we used functional connectivity to improve tau spreading predictions over Braak staging methods. We included two samples with longitudinal tau-PET from controls and AD patients. Cross-sectionally, we found connectivity of tau epicenters (i.e., regions with earliest tau) to predict estimated tau spreading sequences. Longitudinally, we found tau accumulation rates to correlate with connectivity strength to patient-specific tau epicenters. A connectivity-based, patient-centered tau spreading model improved the assessment of tau accumulation rates compared to Braak stage-specific readouts and reduced sample sizes by ~40% in simulated tau-targeting interventions. Thus, connectivity-based tau spreading models may show utility in clinical trials.

Hypothesis: Tau pathology is an initiating factor in sporadic Alzheimer's disease

The etiology of the common, sporadic form of Alzheimer's disease (sAD) is unknown. We hypothesize that tau pathology within select projection neurons with susceptible microenvironments can initiate sAD. This postulate rests on extensive data demonstrating that in human brains tau pathology appears about a decade before the formation of Aβ plaques (Aβps), especially targeting glutamate projection neurons in the association cortex. Data from aging rhesus monkeys show abnormal tau phosphorylation within vulnerable neurons, associated with calcium dysregulation. Abnormally phosphorylated tau (pTau) on microtubules traps APP‐containing endosomes, which can increase Aβ production. As Aβ oligomers increase abnormal phosphorylation of tau, this would drive vicious cycles leading to sAD pathology over a long lifespan, with genetic and environmental factors that may accelerate pathological events. This hypothesis could be testable in the aged monkey association cortex that naturally expresses characteristics capable of promoting and sustaining abnormal tau phosphorylation and Aβ production.

Disrupted Patterns of Rich-Club and Diverse-Club Organizations in Subjective Cognitive Decline and Amnestic Mild Cognitive Impairment

Background

Subjective cognitive decline (SCD) and amnestic mild cognitive impairment (aMCI) were considered to be a continuum of Alzheimer’s disease (AD) spectrum. The abnormal topological architecture and rich-club organization in the brain functional network can reveal the pathology of the AD spectrum. However, few studies have explored the disrupted patterns of diverse club organizations and the combination of rich- and diverse-club organizations in SCD and aMCI.

Methods

We collected resting-state functional magnetic resonance imaging data of 19 SCDs, 29 aMCIs, and 28 healthy controls (HCs) from the Alzheimer’s Disease Neuroimaging Initiative. Graph theory analysis was used to analyze the network metrics and rich- and diverse-club organizations simultaneously.

Results

Compared with HC, the aMCI group showed altered small-world and network efficiency, whereas the SCD group remained relatively stable. The aMCI group showed reduced rich-club connectivity compared with the HC. In addition, the aMCI group showed significantly increased feeder connectivity and decreased local connectivity of the diverse club compared with the SCD group. The overlapping nodes of the rich club and diverse club showed a significant difference in nodal efficiency and shortest path length (L_p) between groups. Notably, the L_p values of overlapping nodes in the SCD and aMCI groups were significantly associated with episodic memory.

Conclusion

The present study demonstrates that the network properties of SCD and aMCI have varying degrees of damage. The combination of the rich club and the diverse club can provide a novel insight into the pathological mechanism of the AD spectrum. The altered patterns in overlapping nodes might be potential biomarkers in the diagnosis of the AD spectrum.

Alzheimer's disease risk modifier genes do not affect tau aggregate uptake, seeding or maintenance in cell models

Alzheimer's disease (AD) afflicts millions of people worldwide and is caused by accumulated amyloid beta and tau pathology. Progression of tau pathology in AD may utilize prion mechanisms of propagation in which pathological tau aggregates released from one cell are taken up by neighboring or connected cells and act as templates for their own replication, a process termed ‘seeding’. We have used HEK293T cells to model various aspects of pathological tau propagation, including uptake of tau aggregates, induced seeding by exogenous aggregates, seeding caused by Lipofectamine‐mediated delivery to the cell interior, and stable maintenance of aggregates in dividing cells. The factors that regulate these processes are not well understood, and we hypothesized that AD risk modifier genes might play a role. We identified 22 genes strongly linked to AD via meta‐analysis of genome‐wide association study (GWAS). We used CRISPR/Cas9 to individually knock out each gene in HEK293T cells and verified disruption using genomic sequencing. We then tested the effect of gene knockout in tau aggregate uptake, naked and Lipofectamine‐mediated seeding, and aggregate maintenance in these cultured cell lines. GWAS gene knockouts had no effect in these models of tau pathology. With obvious caveats due to the model systems used, these results imply that the 22 AD risk modifier genes are unlikely to directly modulate tau uptake, seeding, or aggregate maintenance in a cell‐autonomous fashion.

Defining the Protein Seeds of Neurodegeneration using Real-Time Quaking-Induced Conversion Assays

Neurodegenerative diseases are characterized by the accumulation of disease-related misfolded proteins. It is now widely understood that the characteristic self-amplifying (i.e., seeding) capacity once only attributed to the prions of transmissible spongiform encephalopathy diseases is a feature of other misfolded proteins of neurodegenerative diseases, including tau, Aβ, and αSynuclein (αSyn). Ultrasensitive diagnostic assays, known as real-time quaking-induced conversion (RT-QuIC) assays, exploit these seeding capabilities in order to exponentially amplify protein seeds from various biospecimens. To date, RT-QuIC assays have been developed for the detection of protein seeds related to known prion diseases of mammals, the αSyn aggregates of Parkinson’s disease, dementia with Lewy bodies, and multiple system atrophy, and the tau aggregates of Alzheimer’s disease, chronic traumatic encephalopathy, and other tauopathies including progressive supranuclear palsy. Application of these assays to premortem human biospecimens shows promise for diagnosis of neurodegenerative disease and is an area of active investigation. RT-QuIC assays are also powerful experimental tools that can be used to dissect seeding networks within and between tissues and to evaluate how protein seed distribution and quantity correlate to disease-related outcomes in a host. As well, RT-QuIC application may help characterize molecular pathways influencing protein seed accumulation, transmission, and clearance. In this review we discuss the application of RT-QuIC assays as diagnostic, experimental, and structural tools for detection and discrimination of PrP prions, tau, and αSyn protein seeds.

Neuropathological assessment of the Alzheimer spectrum

Alzheimer disease (AD), the most common form of dementia globally, classically defined a clinicopathological entity, is a heterogenous disorder with various pathobiological subtypes, currently referred to as Alzheimer continuum. Its morphological hallmarks are extracellular parenchymal β-amyloid (amyloid plaques) and intraneuronal (tau aggregates forming neurofibrillary tangles) lesions accompanied by synaptic loss and vascular amyloid deposits, that are essential for the pathological diagnosis of AD. In addition to "classical" AD, several subtypes with characteristic regional patterns of tau pathology have been described that show distinct clinical features, differences in age, sex distribution, biomarker levels, and patterns of key network destructions responsible for cognitive decline. AD is a mixed proteinopathy (amyloid and tau), frequently associated with other age-related co-pathologies, such as cerebrovascular lesions, Lewy and TDP-43 pathologies, hippocampal sclerosis, or argyrophilic grain disease. These and other co-pathologies essentially influence the clinical picture of AD and may accelerate disease progression. The purpose of this review is to provide a critical overview of AD pathology, its defining pathological substrates, and the heterogeneity among the Alzheimer spectrum entities that may provide a broader diagnostic coverage of this devastating disorder as a basis for implementing precision medicine approaches and for ultimate development of successful disease-modifying drugs for AD.

From the Entorhinal Region via the Prosubiculum to the Dentate Fascia: Alzheimer Disease-Related Neurofibrillary Changes in the Temporal Allocortex

The pathological process underlying Alzheimer disease (AD) unfolds predominantly in the cerebral cortex with the gradual appearance and regional progression of abnormal tau. Intraneuronal tau pathology progresses from the temporal transentorhinal and entorhinal regions into neocortical fields/areas of the temporal allocortex. Here, based on 95 cases staged for AD-related neurofibrillary changes, we propose an ordered progression of abnormal tau in the temporal allocortex. Initially, abnormal tau was limited to distal dendritic segments followed by tau in cell bodies of projection neurons of the transentorhinal/entorhinal layer pre-α. Next, abnormal distal dendrites accumulated in the prosubiculum and extended into the CA1 stratum oriens and lacunosum. Subsequently, altered dendrites developed in the CA2/CA3 stratum oriens and stratum lacunosum-moleculare, combined with tau-positive thorny excrescences of CA3/CA4 mossy cells. Finally, granule cells of the dentate fascia became involved. Such a progression might recapitulate a sequence of transsynaptic spreading of abnormal tau from 1 projection neuron to the next: From pre-α cells to distal dendrites in the prosubiculum and CA1; then, from CA1 or prosubicular pyramids to CA2 principal cells and CA3/CA4 mossy cells; finally, from CA4 mossy cells to dentate granule cells. The lesions are additive: Those from the previous steps persist.

Quantitative propagation of assembled human Tau from Alzheimer's disease brain in microfluidic neuronal cultures

Tau aggregation and hyperphosphorylation is a key neuropathological hallmark of Alzheimer's disease (AD), and the temporospatial spread of Tau observed during clinical manifestation suggests that Tau pathology may spread along the axonal network and propagate between synaptically connected neurons. Here, we have developed a cellular model that allows the study of human AD-derived Tau propagation from neuron to neuron using microfluidic devices. We show by using high-content imaging techniques and an in-house developed interactive computer program that human AD-derived Tau seeds rodent Tau that propagates trans-neuronally in a quantifiable manner in a microfluidic culture model. Moreover, we were able to convert this model to a medium-throughput format allowing the user to handle 16 two-chamber devices simultaneously in the footprint of a standard 96-well plate. Furthermore, we show that a small molecule inhibitor of aggregation can block the trans-neuronal transfer of Tau aggregates, suggesting that the system can be used to evaluate mechanisms of Tau transfer and find therapeutic interventions.

Selective Neuronal Vulnerability in Alzheimer's Disease: A Network-Based Analysis

A major obstacle to treating Alzheimer's disease (AD) is our lack of understanding of the molecular mechanisms underlying selective neuronal vulnerability, a key characteristic of the disease. Here, we present a framework integrating high-quality neuron-type-specific molecular profiles across the lifetime of the healthy mouse, which we generated using bacTRAP, with postmortem human functional genomics and quantitative genetics data. We demonstrate human-mouse conservation of cellular taxonomy at the molecular level for neurons vulnerable and resistant in AD, identify specific genes and pathways associated with AD neuropathology, and pinpoint a specific functional gene module underlying selective vulnerability, enriched in processes associated with axonal remodeling, and affected by amyloid accumulation and aging. We have made all cell-type-specific profiles and functional networks available at http://alz.princeton.edu. Overall, our study provides a molecular framework for understanding the complex interplay between Aβ, aging, and neurodegeneration within the most vulnerable neurons in AD.

Crystal structure of a conformational antibody that binds tau oligomers and inhibits pathological seeding by extracts from donors with Alzheimer's disease

Soluble oligomers of aggregated tau accompany the accumulation of insoluble amyloid fibrils, a histological hallmark of Alzheimer disease (AD) and two dozen related neurodegenerative diseases. Both oligomers and fibrils seed the spread of Tau pathology, and by virtue of their low molecular weight and relative solubility, oligomers may be particularly pernicious seeds. Here, we report the formation of in vitro tau oligomers formed by an ionic liquid (IL15). Using IL15-induced recombinant tau oligomers and a dot blot assay, we discovered a mAb (M204) that binds oligomeric tau, but not tau monomers or fibrils. M204 and an engineered single-chain variable fragment (scFv) inhibited seeding by IL15-induced tau oligomers and pathological extracts from donors with AD and chronic traumatic encephalopathy. This finding suggests that M204-scFv targets pathological structures that are formed by tau in neurodegenerative diseases. We found that M204-scFv itself partitions into oligomeric forms that inhibit seeding differently, and crystal structures of the M204-scFv monomer, dimer, and trimer revealed conformational differences that explain differences among these forms in binding and inhibition. The efficiency of M204-scFv antibodies to inhibit the seeding by brain tissue extracts from different donors with tauopathies varied among individuals, indicating the possible existence of distinct amyloid polymorphs. We propose that by binding to oligomers, which are hypothesized to be the earliest seeding-competent species, M204-scFv may have potential as an early-stage diagnostic for AD and tauopathies, and also could guide the development of promising therapeutic antibodies.

Diverse, evolving conformer populations drive distinct phenotypes in frontotemporal lobar degeneration caused by the same MAPT-P301L mutation

Tau protein accumulation is a common denominator of major dementias, but this process is inhomogeneous, even when triggered by the same germline mutation. We considered stochastic misfolding of human tau conformers followed by templated conversion of native monomers as an underlying mechanism and derived sensitive conformational assays to test this concept. Assessments of brains from aged TgTauP301L transgenic mice revealed a prodromal state and three distinct signatures for misfolded tau. Frontotemporal lobar degeneration (FTLD)-MAPT-P301L patients with different clinical phenotypes also displayed three signatures, two resembling those found in TgTauP301L mice. As physicochemical and cell bioassays confirmed diverse tau strains in the mouse and human brain series, we conclude that evolution of diverse tau conformers is intrinsic to the pathogenesis of this uni-allelic form of tauopathy. In turn, effective therapeutic interventions in FTLD will need to address evolving repertoires of misfolded tau species rather than singular, static molecular targets.

The synaptic pathology of cognitive life

Prospective, community-based studies allow evaluation of associations between cognitive functioning and synaptic measures, controlled for age-related pathologies. Findings from >400 community-based participants are reviewed. Levels of two presynaptic proteins, complexin-I (inhibitory terminals), and complexin-II (excitatory terminals) contributed to cognitive variation from normal to dementia. Adding the amount of protein-protein interaction between two others, synaptosome-associated protein-25 and syntaxin, explained 6% of overall variance. The presynaptic protein Munc18-1 long variant was localized to inhibitory terminals, and like complexin-I, was positively associated with cognition. Associations depended on Braak stage, with the level of complexin-I contributing nearly 15% to cognitive variation in stages 0-II, while complexin-II contributed 7% in stages V-VI. Non-denaturing gels identified multiple soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein-protein (SNARE) complexes in frontal and in temporal lobes, making specific contributions to cognitive functions. Multiple mechanisms of presynaptic plasticity contribute to cognitive function during aging.


Propagation of Tau Pathology: Integrating Insights From Postmortem and In Vivo Studies

Cellular accumulation of aggregated forms of the protein tau is a defining feature of so-called tauopathies such as Alzheimer's disease, progressive supranuclear palsy, and chronic traumatic encephalopathy. A growing body of literature suggests that conformational characteristics of tau filaments, along with regional vulnerability to tau pathology, account for the distinct histopathological morphologies, biochemical composition, and affected cell types seen across these disorders. In this review, we describe and discuss recent evidence from human postmortem and clinical biomarker studies addressing the differential vulnerability of brain areas to tau pathology, its cell-to-cell transmission, and characteristics of the different strains that tau aggregates can adopt. Cellular biosensor assays are increasingly used in human tissue to detect the earliest forms of tau pathology, before overt histopathological lesions (i.e., neurofibrillary tangles) are apparent. Animal models with localized tau expression are used to uncover the mechanisms that influence spreading of tau aggregates. Further, studies of human postmortem-derived tau filaments from different tauopathies injected in rodents have led to striking findings that recapitulate neuropathology-based staging of tau. Furthermore, the recent advent of tau positron emission tomography and novel fluid-based biomarkers render it possible to study the temporal progression of tau pathology in vivo. Ultimately, evidence from these approaches must be integrated to better understand the onset and progression of tau pathology across tauopathies. This will lead to improved methods for the detection and monitoring of disease progression and, hopefully, to the development and refinement of tau-based therapeutics.

Protein transmission in neurodegenerative disease

Most neurodegenerative diseases are characterized by the intracellular or extracellular aggregation of misfolded proteins such as amyloid-β and tau in Alzheimer disease, α-synuclein in Parkinson disease, and TAR DNA-binding protein 43 in amyotrophic lateral sclerosis. Accumulating evidence from both human studies and disease models indicates that intercellular transmission and the subsequent templated amplification of these misfolded proteins are involved in the onset and progression of various neurodegenerative diseases. The misfolded proteins that are transferred between cells are referred to as 'pathological seeds'. Recent studies have made exciting progress in identifying the characteristics of different pathological seeds, particularly those isolated from diseased brains. Advances have also been made in our understanding of the molecular mechanisms that regulate the transmission process, and the influence of the host cell on the conformation and properties of pathological seeds. The aim of this Review is to summarize our current knowledge of the cell-to-cell transmission of pathological proteins and to identify key questions for future investigation.

Intermediate flortaucipir uptake is associated with Aβ-PET and CSF tau in asymptomatic adults

OBJECTIVE

To investigate relationships between flortaucipir (FTP) uptake, age, and established Alzheimer disease (AD) markers in asymptomatic adults at increased risk of AD.

METHODS

One-hundred nineteen individuals with a family history of AD (Presymptomatic Evaluation of Experimental or Novel Treatments of Alzheimer's Disease [PREVENT-AD] cohort, mean age 67 ± 5 years) underwent tau-PET ([¹⁸F]FTP), β-amyloid (Aβ)-PET ([¹⁸F]NAV4694 [NAV]), and cognitive assessment. Seventy-four participants also had CSF phosphorylated tau and total tau data available. We investigated the association between age and FTP in this relatively young cohort of older adults. We also investigated regional FTP standardized uptake value ratio (SUVR) differences between Aβ-positive and Aβ-negative individuals and regional correlations between FTP and NAV retention. In cortical regions showing consistent associations across analyses, we assessed whether FTP was in addition related to CSF tau and cognitive performance. Lastly, we identified the lowest FTP value at which associations with Aβ-PET, CSF, and cognition were detectable.

RESULTS

Increased age was associated only with amygdala and transverse temporal lobe FTP retention. Aβ-positive individuals had higher FTP SUVR values in several brain regions, further showing correlation with NAV load through the cortex. Increased FTP SUVRs in medial temporal regions were associated with increased CSF tau values and worse cognition. The SUVRs at which associations between entorhinal FTP SUVR and other AD markers were first detected differed by modality, with a detection point of 1.12 for CSF values, 1.2 for Aβ-PET, and 1.4 for cognition.

CONCLUSIONS

Relatively low FTP-PET SUVRs are associated with pathologic markers of AD in the preclinical phase of the disease. Adjustment in the tau threshold should be considered, depending on the purpose of the tau classification.

Functional brain architecture is associated with the rate of tau accumulation in Alzheimer's disease

In Alzheimer's diseases (AD), tau pathology is strongly associated with cognitive decline. Preclinical evidence suggests that tau spreads across connected neurons in an activity-dependent manner. Supporting this, cross-sectional AD studies show that tau deposition patterns resemble functional brain networks. However, whether higher functional connectivity is associated with higher rates of tau accumulation is unclear. Here, we combine resting-state fMRI with longitudinal tau-PET in two independent samples including 53 (ADNI) and 41 (BioFINDER) amyloid-biomarker defined AD subjects and 28 (ADNI) vs. 16 (BioFINDER) amyloid-negative healthy controls. In both samples, AD subjects show faster tau accumulation than controls. Second, in AD, higher fMRI-assessed connectivity between 400 regions of interest (ROIs) is associated with correlated tau-PET accumulation in corresponding ROIs. Third, we show that a model including baseline connectivity and tau-PET is associated with future tau-PET accumulation. Together, connectivity is associated with tau spread in AD, supporting the view of transneuronal tau propagation.

Brainstem Volumetric Integrity in Preclinical and Prodromal Alzheimer's Disease

BACKGROUND

Neuropathological studies have suggested the tau pathology observed in Alzheimer's disease (AD) originates in brainstem nuclei, but no studies to date have quantified brainstem volumes in clinical populations with biomarker-confirmed mild cognitive impairment (MCI) or dementia due to AD or determined the value of brainstem volumetrics in predicting dementia.

OBJECTIVE

The present study examined whether MRI-based brainstem volumes differ among cognitively normal older adults and those with MCI or dementia due to AD and whether preclinical brainstem volumes predict future progression to dementia.

METHODS

Alzheimer's Disease Neuroimaging Initiative participants (N = 1,629) underwent baseline MRI scanning with variable clinical follow-up (6-120 months). Region of interest and voxel-based morphometric methods assessed brainstem volume differences among cognitively normal (n = 814), MCI (n = 542), and AD (n = 273) participants, as well as subsets of cerebrospinal fluid biomarker-confirmed MCI (n = 203) and AD (n = 160) participants.

RESULTS

MCI and AD cases showed smaller midbrain volumes relative to cognitively normal participants when normalizing to whole brainstem volume, and showed smaller midbrain, locus coeruleus, pons, and whole brainstem volumes when normalizing to total intracranial volume. Cognitively normal individuals who later progressed to AD dementia diagnosis exhibited smaller baseline midbrain volumes than individuals who did not develop dementia, and voxel-wise analyses revealed specific volumetric reduction of the locus coeruleus.

CONCLUSION

Findings are consistent with neuropathological observations of early AD-related pathology in brainstem nuclei and further suggest the clinical relevance of brainstem substructural volumes in preclinical and prodromal AD.

Is tau in the absence of amyloid on the Alzheimer's continuum?: A study of discordant PET positivity

The amyloid cascade model of Alzheimer’s disease posits the primacy of amyloid beta deposition preceding tau-mediated neurofibrillary tangle formation. The amyloid-tau-neurodegeneration biomarker-only diagnostic framework similarly requires the presence of amyloid beta for a diagnosis on the Alzheimer’s continuum. However, medial temporal lobe tau pathology in the absence of amyloid beta is frequently observed at autopsy in cognitively normal individuals, a phenomenon that may reflect a consequence of aging and has been labelled ‘primary age-related tauopathy’. Alternatively, others argue that this tauopathy reflects an early stage of the developmental continuum leading to Alzheimer’s disease. We used positron emission tomography imaging to investigate amyloid beta and tau positivity and associations with cognition to better inform the conceptualization of biomarker changes in Alzheimer’s pathogenesis. Five hundred twenty-three individuals from the Alzheimer’s Disease Neuroimaging Initiative who had undergone flortaucipir positron emission tomography imaging were selected to derive positron emission tomography positivity thresholds using conditional inference decision tree regression. A subsample of 301 individuals without dementia (i.e. those with normal cognition or mild cognitive impairment) had also undergone florbetapir positron emission tomography imaging within 12 months and were categorized into one of the four groups based on cortical amyloid and Braak stage I/II tau positivity: A−/T−, A+/T−, A−/T+, or A+/T+. Tau positivity in the absence of amyloid beta positivity (i.e. A−/T+) comprised the largest group, representing 45% of the sample. In contrast, only 6% of the sample was identified as A+/T−, and the remainder of the sample fell into A−/T− (22%) or A+/T+ (27%) categories. A−/T− and A+/T− groups had the best cognitive performances across memory, language and executive function; the A−/T+ group showed small-to-moderate relative decreases in cognition; and the A+/T+ group had the worst cognitive performances. Furthermore, there were negative associations between Braak stage I/II tau values and all cognitive domains only in the A−/T+ and A+/T+ groups, with strongest associations for the A+/T+ group. Among our sample of older adults across the Alzheimer’s pathological spectrum, 7-fold fewer individuals have positron emission tomography evidence of amyloid beta pathology in the absence of tau pathology than the converse, challenging prevailing models of amyloid beta’s primacy in Alzheimer’s pathogenesis. Given that cognitive performance in the A−/T+ group was poorer than in individuals without either pathology, our results suggest that medial temporal lobe tau without cortical amyloid beta may reflect an early stage on the Alzheimer’s pathological continuum.

To stage, or not to stage

Staging of neurodegenerative diseases is based chiefly on the topographical or anatomical extent of aggregated proteinaceous inclusions, and the density or severity of the lesions in a given region is usually assessed semiquantitatively. Associated phenomena, such as cell loss and synapse loss, are evaluated but not staged. This article reviews the development of neuropathological staging of the sporadic Alzheimer's and sporadic Parkinson's diseases. It considers challenges for staging systems, and it poses the question whether neuropathological staging as practiced up to now is still relevant.

Hypoxia induces HT-22 neuronal cell death via Orai1/CDK5 pathway-mediated Tau hyperphosphorylation

Hypoxia and apoptosis are involved in the pathogenesis of Alzheimer's disease (AD). Hypoxia induces the formation of amyloid precursor protein in neurons, leading to the abnormal deposition of β-amyloid protein and hyperphosphorylation of Tau. Such changes increase the risk of AD. In the present study, a cellular model of hypoxia-induced AD was established by exposing HT-22 mouse hippocampal neurons to the chemical hypoxia-mimicking agent cobalt chloride (CoCl₂). It was found that hypoxia increased neuronal apoptosis. Hypoxia caused an abnormal increase in the expression of the intracellular calcium channel protein Orai1 and cyclin-dependent kinase 5 (CDK5), resulting in hyperphosphorylation of Tau. Treatment with small-interfering RNA against Orai1 (siOrai1) or an Orai1-overexpression plasmid effectively intervened the CDK5-mediated hyperphosphorylation of Tau. In summary, following hypoxic injury of neuron, the Orai1-induced expression of CDK5 leads to Tau hyperphosphorylation. Tau hyperphosphorylation is an important pathophysiological manifestation in AD patients. These results indicated that hypoxia induces HT-22 cell death by Orai1/CDK5 pathway mediated Tau hyperphosporylation. This study simulated the pathological process associated with AD and proposed that hypoxia of intravascular cells with normal blood oxygen saturation might be one of a pathogenic mechanisms of AD. Therefore, this work may provide a new theoretical basis for AD prevention and treatment.

Advances and considerations in AD tau-targeted immunotherapy

The multifactorial and complex nature of Alzheimer’s disease (AD) has made it difficult to identify therapeutic targets that are causally involved in the disease process. However, accumulating evidence from experimental and clinical studies that investigate the early disease process point towards the required role of tau in AD etiology. Importantly, a large number of studies investigate and characterize the plethora of pathological forms of tau protein involved in disease onset and propagation. Immunotherapy is one of the most clinical approaches anticipated to make a difference in the field of AD therapeutics. Tau –targeted immunotherapy is the new direction after the failure of amyloid beta (Aß)-targeted immunotherapy and the growing number of studies that highlight the Aß-independent disease process. It is now well established that immunotherapy alone will most likely be insufficient as a monotherapy. Therefore, this review discusses updates on tau-targeted immunotherapy studies, AD-relevant tau species, updates on promising biomarkers and a prospect on combination therapies to surround the disease propagation in an efficient and timely manner.

A Longitudinal Study of Epileptic Seizures in Alzheimer's Disease

The prevalence of epileptic seizures is increased in patients in the clinical stages of Alzheimer's disease (AD) when compared to age-matched cognitively normal populations. In previously reported work from the Presentation of Epileptic Seizures in Dementia (PrESIDe) study, we identified a clinical suspicion of epilepsy in between 12.75 and 28.43% of patients with AD recruited from a memory clinic. EEGs were not performed in this study. Patients with epilepsy performed similarly to patients without epilepsy on cognitive testing at the time of recruitment but were more impaired on two measures of everyday functioning [Cambridge Behavioral Inventory—Revised and Clinical Dementia Rating (CBI-R and CDR)]. On repeated testing in this 12-month follow-up study, patients in whom a suspicion of epilepsy was identified performed significantly worse on cognitive function testing (p = 0.028) in addition to maintaining a difference on the informant questionnaires (CBI-R p < 0.001, CDR p = 0.020). These findings suggest that seizures in this population could be a marker of a more rapid decline and worse prognosis.

Transmissibility versus Pathogenicity of Self-Propagating Protein Aggregates

The prion-like spreading and accumulation of specific protein aggregates appear to be central to the pathogenesis of many human diseases, including Alzheimer's and Parkinson's. Accumulating evidence indicates that inoculation of tissue extracts from diseased individuals into suitable experimental animals can in many cases induce the aggregation of the disease-associated protein, as well as related pathological lesions. These findings, together with the history of the prion field, have raised the questions about whether such disease-associated protein aggregates are transmissible between humans by casual or iatrogenic routes, and, if so, do they propagate enough in the new host to cause disease? These practical considerations are important because real, and perhaps even only imagined, risks of human-to-human transmission of diseases such as Alzheimer's and Parkinson's may force costly changes in clinical practice that, in turn, are likely to have unintended consequences. The prion field has taught us that a single protein, PrP, can aggregate into forms that can propagate exponentially in vitro, but range from being innocuous to deadly when injected into experimental animals in ways that depend strongly on factors such as conformational subtleties, routes of inoculation, and host responses. In assessing the hazards posed by various disease-associated, self-propagating protein aggregates, it is imperative to consider both their actual transmissibilities and the pathological consequences of their propagation, if any, in recipient hosts.

Different Amyloid-β Self-Assemblies Have Distinct Effects on Intracellular Tau Aggregation

Alzheimer's disease (AD) pathology is characterized by the aggregation of beta-amyloid (Aβ) and tau in the form of amyloid plaques and neurofibrillary tangles in the brain. It has been found that a synergistic relationship between these two proteins may contribute to their roles in disease progression. However, how Aβ and tau interact has not been fully characterized. Here, we analyze how tau seeding or aggregation is influenced by different Aβ self-assemblies (fibrils and oligomers). Our cellular assays utilizing tau biosensor cells show that transduction of Aβ oligomers into the cells greatly enhances seeded tau aggregation in a concentration-dependent manner. In contrast, transduced Aβ fibrils slightly reduce tau seeding while untransduced Aβ fibrils promote it. We also observe that the transduction of α-synuclein fibrils, another amyloid protein, has no effect on tau seeding. The enhancement of tau seeding by Aβ oligomers was confirmed using tau fibril seeds derived from both recombinant tau and PS19 mouse brain extracts containing human tau. Our findings highlight the importance of considering the specific form and cellular location of Aβ self-assembly when studying the relationship between Aβ and tau in future AD therapeutic development.

Structure-based inhibitors halt prion-like seeding by Alzheimer's disease-and tauopathy-derived brain tissue samples

In Alzheimer's disease (AD) and tauopathies, tau aggregation accompanies progressive neurodegeneration. Aggregated tau appears to spread between adjacent neurons and adjacent brain regions by prion-like seeding. Hence, inhibitors of this seeding offer a possible route to managing tauopathies. Here, we report the 1.0 Å resolution micro-electron diffraction structure of an aggregation-prone segment of tau with the sequence SVQIVY, present in the cores of patient-derived fibrils from AD and tauopathies. This structure illuminates how distinct interfaces of the parent segment, containing the sequence VQIVYK, foster the formation of distinct structures. Peptide-based fibril-capping inhibitors designed to target the two VQIVYK interfaces blocked proteopathic seeding by patient-derived fibrils. These VQIVYK inhibitors add to a panel of tau-capping inhibitors that targets specific polymorphs of recombinant and patient-derived tau fibrils. Inhibition of seeding initiated by brain tissue extracts differed among donors with different tauopathies, suggesting that particular fibril polymorphs of tau are associated with certain tauopathies. Donors with progressive supranuclear palsy exhibited more variation in inhibitor sensitivity, suggesting that fibrils from these donors were more polymorphic and potentially vary within individual donor brains. Our results suggest that a subset of inhibitors from our panel could be specific for particular disease-associated polymorphs, whereas inhibitors that blocked seeding by extracts from all of the tauopathies tested could be used to broadly inhibit seeding by multiple disease-specific tau polymorphs. Moreover, we show that tau-capping inhibitors can be transiently expressed in HEK293 tau biosensor cells, indicating that nucleic acid-based vectors can be used for inhibitor delivery.

Cellular and regional vulnerability in frontotemporal tauopathies

The frontotemporal tauopathies all deposit abnormal tau protein aggregates, but often of only certain isoforms and in distinguishing pathologies of five main types (neuronal Pick bodies, neurofibrillary tangles, astrocytic plaques, tufted astrocytes, globular glial inclusions and argyrophilic grains). In those with isoform specific tau aggregates glial pathologies are substantial, even though there is limited evidence that these cells normally produce tau protein. This review will assess the differentiating features and clinicopathological correlations of the frontotemporal tauopathies, the genetic predisposition for these different pathologies, their neuroanatomical selectivity, current observations on how they spread through the brain, and any potential contributing cellular and molecular changes. The findings show that diverse clinical phenotypes relate most to the brain region degenerating rather than the type of pathology involved, that different regions on the MAPT gene and novel risk genes are associated with specific tau pathologies, that the 4-repeat glial tauopathies do not follow individual patterns of spreading as identified for neuronal pathologies, and that genetic and pathological data indicate that neuroinflammatory mechanisms are involved. Each pathological frontotemporal tauopathy subtype with their distinct pathological features differ substantially in the cell type affected, morphology, biochemical and anatomical distribution of inclusions, a fundamental concept central to future success in understanding the disease mechanisms required for developing therapeutic interventions. Tau directed therapies targeting genetic mechanisms, tau aggregation and pathological spread are being trialled, although biomarkers that differentiate these diseases are required. Suggested areas of future research to address the regional and cellular vulnerabilities in frontotemporal tauopathies are discussed.

Tau deposition is associated with functional isolation of the hippocampus in aging

The tau protein aggregates in aging and Alzheimer disease and may lead to memory loss through disruption of medial temporal lobe (MTL)-dependent memory systems. Here, we investigated tau-mediated mechanisms of hippocampal dysfunction that underlie the expression of episodic memory decline using fMRI measures of hippocampal local coherence (regional homogeneity; ReHo), distant functional connectivity and tau-PET. We show that age and tau pathology are related to higher hippocampal ReHo. Functional disconnection between the hippocampus and other components of the MTL memory system, particularly an anterior-temporal network specialized for object memory, is also associated with higher hippocampal ReHo and greater tau burden in anterior-temporal regions. These associations are not observed in the posteromedial network, specialized for context/spatial information. Higher hippocampal ReHo predicts worse memory performance. These findings suggest that tau pathology plays a role in disconnecting the hippocampus from specific MTL memory systems leading to increased local coherence and memory decline.

Deposition of tau protein aggregates occurs during aging and Alzheimer disease. Here, the authors show that tau burden in the anterior-temporal memory network is associated with disrupted fMRI connectivity and functional isolation of the hippocampus from other memory network components.

Restoration of Olfactory Memory in Drosophila Overexpressing Human Alzheimer's Disease Associated Tau by Manipulation of L-Type Ca2+ Channels

The cellular underpinnings of memory deficits in Alzheimer’s disease (AD) are poorly understood. We utilized the tractable neural circuits sub-serving memory in Drosophila to investigate the role of impaired Ca²⁺ handling in memory deficits caused by expression of human 0N4R isoform of tau which is associated with AD. Expression of tau in mushroom body neuropils, or a subset of mushroom body output neurons, led to impaired memory. By using the Ca²⁺ reporter GCaMP6f, we observed changes in Ca²⁺ signaling when tau was expressed in these neurons, an effect that could be blocked by the L-type Ca²⁺ channel antagonist nimodipine or reversed by RNAi knock-down of the L-type channel gene. The L-type Ca²⁺ channel itself is required for memory formation, however, RNAi knock-down of the L-type Ca²⁺ channel in neurons overexpressing human tau resulted in flies whose memory is restored to levels equivalent to wild-type. Expression data suggest that Drosophila L-type Ca²⁺ channel mRNA levels are increased upon tau expression in neurons, thus contributing to the effects observed on memory and intracellular Ca²⁺ homeostasis. Together, our Ca²⁺ imaging and memory experiments suggest that expression of the 0N4R isoform of human tau increases the number of L-type Ca²⁺ channels in the membrane resulting in changes in neuronal excitability that can be ameliorated by RNAi knockdown or pharmacological blockade of L-type Ca²⁺ channels. This highlights a role for L-type Ca²⁺ channels in tauopathy and their potential as a therapeutic target for AD.

Rostral locus coeruleus integrity is associated with better memory performance in older adults

For decades, research into memory decline in human cognitive ageing has focused on neocortical regions, the hippocampus and dopaminergic neuromodulation. Recent findings indicate that the locus coeruleus (LC) and noradrenergic neuromodulation may also play an important role in shaping memory development in later life. However, technical challenges in quantification of LC integrity have hindered the study of LC-cognition associations in humans. Using high-resolution, neuromelanin-sensitive magnetic resonance imaging, we found that individual differences in learning and memory were positively associated with LC integrity across a variety of memory tasks in both younger (n = 66) and older adults (n = 228). Moreover, we observed functionally relevant age differences confined to rostral LC. Older adults with a more 'youth-like' rostral LC also showed higher memory performance. These findings link non-invasive, in vivo indices of LC integrity to memory in ageing and highlight the role of the LC norepinephrine system in the decline of cognition.

Cortical tau deposition follows patterns of entorhinal functional connectivity in aging

Tau pathology first appears in the transentorhinal and anterolateral entorhinal cortex (alEC) in the aging brain. The transition to Alzheimer's disease (AD) is hypothesized to involve amyloid-β (Aβ) facilitated tau spread through neural connections. We contrasted functional connectivity (FC) of alEC and posteromedial EC (pmEC), subregions of EC that differ in functional specialization and cortical connectivity, with the hypothesis that alEC-connected cortex would show greater tau deposition than pmEC-connected cortex. We used resting state fMRI to measure FC, and PET to measure tau and Aβ in cognitively normal older adults. Tau preferentially deposited in alEC-connected cortex compared to pmEC-connected or non-connected cortex, and stronger connectivity was associated with increased tau deposition. FC-tau relationships were present regardless of Aβ, although strengthened with Aβ. These results provide an explanation for the anatomic specificity of neocortical tau deposition in the aging brain and reveal relationships between normal aging and the evolution of AD.

Electrophysiological evidence of altered facial expressions recognition in Alzheimer's disease: A comprehensive ERP study

OBJECTIVES

The present study aims to evaluate the amplitude and latency of event-related potentials (ERPs) P100, N170, VPP and N230 in individuals with Alzheimer's disease (AD) compared to healthy elderly controls, using a passive viewing task of emotional facial expressions.

METHODS

Twenty-four individuals with mild to moderate AD and 23 demographically matched healthy elderly controls were included in the study. ERP P100, N170, VPP and N230 amplitude and latency values were compared between groups.

RESULTS

The categorization of emotional facial expressions was intact; yet, increased P100 amplitude and latency, decreased N170 amplitude, and increased VPP amplitude were observed in AD compared to controls. Increased N230 amplitude and latency were observed in response to angry expressions, while neutral expressions elicited decreased amplitude and latency.

CONCLUSIONS

Increased P100 amplitude and latency may reflect reduced amygdala volume and disruptions in the visual system, while decreased N170 and increased VPP amplitudes may reflect impaired perceptual processing, mitigated by a greater involvement of prefrontal areas for task performance in AD.

SIGNIFICANCE

This study is the first to report a complex pattern of ERPs to emotional facial expressions in individuals with AD.

Alzheimer's pathology targets distinct memory networks in the ageing brain

Alzheimer's disease researchers have been intrigued by the selective regional vulnerability of the brain to amyloid-β plaques and tau neurofibrillary tangles. Post-mortem studies indicate that in ageing and Alzheimer's disease tau tangles deposit early in the transentorhinal cortex, a region located in the anterior-temporal lobe that is critical for object memory. In contrast, amyloid-β pathology seems to target a posterior-medial network that subserves spatial memory. In the current study, we tested whether anterior-temporal and posterior-medial brain regions are selectively vulnerable to tau and amyloid-β deposition in the progression from ageing to Alzheimer's disease and whether this is reflected in domain-specific behavioural deficits and neural dysfunction. 11C-PiB PET and 18F-flortaucipir uptake was quantified in a sample of 131 cognitively normal adults (age: 20-93 years; 47 amyloid-β-positive) and 20 amyloid-β-positive patients with mild cognitive impairment or Alzheimer's disease dementia (65-95 years). Tau burden was relatively higher in anterior-temporal regions in normal ageing and this difference was further pronounced in the presence of amyloid-β and cognitive impairment, indicating exacerbation of ageing-related processes in Alzheimer's disease. In contrast, amyloid-β deposition dominated in posterior-medial regions. A subsample of 50 cognitively normal older (26 amyloid-β-positive) and 25 young adults performed an object and scene memory task while functional MRI data were acquired. Group comparisons showed that tau-positive (n = 18) compared to tau-negative (n = 32) older adults showed lower mnemonic discrimination of object relative to scene images [t(48) = -3.2, P = 0.002]. In a multiple regression model including regional measures of both pathologies, higher anterior-temporal flortaucipir (tau) was related to relatively worse object performance (P = 0.010, r = -0.376), whereas higher posterior-medial PiB (amyloid-β) was related to worse scene performance (P = 0.037, r = 0.309). The functional MRI data revealed that tau burden (but not amyloid-β) was associated with increased task activation in both systems and a loss of functional specificity, or dedifferentiation, in posterior-medial regions. The loss of functional specificity was related to worse memory. Our study shows a regional dissociation of Alzheimer's disease pathologies to distinct memory networks. While our data are cross-sectional, they indicate that with ageing, tau deposits mainly in the anterior-temporal system, which results in deficits in mnemonic object discrimination. As Alzheimer's disease develops, amyloid-β deposits preferentially in posterior-medial regions additionally compromising scene discrimination and anterior-temporal tau deposition worsens further. Finally, our findings propose that the progression of tau pathology is linked to aberrant activation and dedifferentiation of specialized memory networks that is detrimental to memory function.

Brain somatic mutations observed in Alzheimer's disease associated with aging and dysregulation of tau phosphorylation

The role of brain somatic mutations in Alzheimer’s disease (AD) is not well understood. Here, we perform deep whole-exome sequencing (average read depth 584×) in 111 postmortem hippocampal formation and matched blood samples from 52 patients with AD and 11 individuals not affected by AD. The number of somatic single nucleotide variations (SNVs) in AD brain specimens increases significantly with aging, and the rate of mutation accumulation in the brain is 4.8-fold slower than that in AD blood. The putatively pathogenic brain somatic mutations identified in 26.9% (14 of 52) of AD individuals are enriched in PI3K-AKT, MAPK, and AMPK pathway genes known to contribute to hyperphosphorylation of tau. We show that a pathogenic brain somatic mutation in PIN1 leads to a loss-of-function mutation. In vitro mimicking of haploinsufficiency of PIN1 aberrantly increases tau phosphorylation and aggregation. This study provides new insights into the genetic architecture underlying the pathogenesis of AD.

The role of brain somatic mutations in neurodegenerative diseases such as Alzheimer’s disease (AD) is not well understood. Here the authors carry out high-depth exome sequencing ~500× on brain tissue from patients with AD and controls, and identify mutations in a number of genes that are known to contribute to phosphorylation and aggregation of tau, including PIN1.

Intersection of pathological tau and microglia at the synapse

Tauopathies are a heterogenous class of diseases characterized by cellular accumulation of aggregated tau and include diseases such as Alzheimer’s disease (AD), progressive supranuclear palsy and chronic traumatic encephalopathy. Tau pathology is strongly linked to neurodegeneration and clinical symptoms in tauopathy patients. Furthermore, synapse loss is an early pathological event in tauopathies and is the strongest correlate of cognitive decline. Tau pathology is additionally associated with chronic neuroinflammatory processes, such as reactive microglia, astrocytes, and increased levels of pro-inflammatory molecules (e.g. complement proteins, cytokines). Recent studies show that as the principal immune cells of the brain, microglia play a particularly important role in the initiation and progression of tau pathology and associated neurodegeneration. Furthermore, AD risk genes such as Triggering receptor expressed on myeloid cells 2 (TREM2) and Apolipoprotein E (APOE) are enriched in the innate immune system and modulate the neuroinflammatory response of microglia to tau pathology. Microglia can play an active role in synaptic dysfunction by abnormally phagocytosing synaptic compartments of neurons with tau pathology. Furthermore, microglia are involved in synaptic spreading of tau – a process which is thought to underlie the progressive nature of tau pathology propagation through the brain. Spreading of pathological tau is also the predominant target for tau-based immunotherapy. Active tau vaccines, therapeutic tau antibodies and other approaches targeting the immune system are actively explored as treatment options for AD and other tauopathies. This review describes the role of microglia in the pathobiology of tauopathies and the mechanism of action of potential therapeutics targeting the immune system in tauopathies.