Novel tau filament fold in chronic traumatic encephalopathy encloses hydrophobic molecules

The neuropathology of chronic traumatic encephalopathy

Chronic traumatic encephalopathy (CTE) is a progressive tauopathy causally linked to repetitive mild traumatic brain injury. Currently, there are no established clinical diagnostic criteria for CTE, making post-mortem neuropathological examination essential for diagnosis. The pathological hallmark of CTE is the presence of perivascular neuronal p-tau aggregates at cortical sulcal depths. In this commentary, we share our diagnostic experience, highlight emerging diagnostic protocols for CTE, and discuss the challenges in CTE diagnosis and its differentiation from comorbid conditions. Beyond the neuropathological insights, we explore the broader implications of CTE for the community, including the need for prevention strategies and the role of public health policy. By advancing understanding of CTE pathology, neuropathologists are positioned to inform prevention and treatment efforts, ultimately contributing to the mitigation of this preventable disease.

Alzheimer's disease: insights into pathology, molecular mechanisms, and therapy

Alzheimer's disease (AD), the leading cause of dementia, is characterized by the accumulation of amyloid plaques and neurofibrillary tangles in the brain. This condition casts a significant shadow on global health due to its complex and multifactorial nature. In addition to genetic predispositions, the development of AD is influenced by a myriad of risk factors, including aging, systemic inflammation, chronic health conditions, lifestyle, and environmental exposures. Recent advancements in understanding the complex pathophysiology of AD are paving the way for enhanced diagnostic techniques, improved risk assessment, and potentially effective prevention strategies. These discoveries are crucial in the quest to unravel the complexities of AD, offering a beacon of hope for improved management and treatment options for the millions affected by this debilitating disease.

Pseudo-Phosphorylated Tau Forms Paired Helical Filaments in the Presence of High-Curvature Cholesterol-Containing Lipid Membranes

The tau protein misfolds in neurodegenerative diseases such as Alzheimer's disease (AD). These pathological tau aggregates are associated with neuronal membranes, but molecular structural information about how disease-like tau fibrils interact with the lipid membrane is scarce. Here, we use solid-state NMR to investigate the structure of a tau construct bearing four AD-relevant phospho-mimetic mutations (4E tau) with cholesterol-containing high-curvature lipid membranes, which mimic the membrane of synaptic vesicles in neurons. We show that 4E tau adopts the AD paired helical filament (PHF) fold in the presence of the membrane at high protein concentrations. Moreover, it inserts into the membrane-water interface with an orientation that suggests possible bridging of multiple lipid vesicles. At lower protein concentrations, moderate chemical shift changes are observed, indicating a perturbation of the PHF structure by the lipids. Removal of the phospho-mimetic mutations led to a qualitatively different β-sheet conformation. These results indicate that posttranslational modifications impact the tau fibril structure more strongly than lipid membranes, but the membrane modulates the conformational equilibria of the aggregates.

Chronic traumatic encephalopathy: State-of-the-science update and narrative review

OBJECTIVE

The long-recognized association of brain injury with increased risk of dementia has undergone significant refinement and more detailed study in recent decades. Chronic traumatic encephalopathy (CTE) is a specific neurodegenerative tauopathy related to prior exposure to repetitive head impacts (RHI). We aim to contextualize CTE within a historical perspective and among emerging data which highlights the scientific and conceptual evolution of CTE-related research in parallel with the broader field of neurodegenerative disease and dementia.

METHODS

We provide a narrative state-of-the-science update on CTE neuropathology, clinical manifestations, biomarkers, different types and patterns of head impact exposure relevant for CTE, and the complicated influence of neurodegenerative co-pathology on symptoms.

CONCLUSIONS

Now almost 20 years since the initial case report of CTE in a former American football player, the field of CTE continues evolving with increasing clarity but also several ongoing controversies. Our understanding of CTE neuropathology outpaces that of disease-specific clinical correlates or the development of in-vivo biomarkers. Diagnostic criteria for symptoms attributable to CTE are still being validated, but leveraging increasingly available biomarkers for other conditions like Alzheimer's disease may be helpful for informing the CTE differential diagnosis. As diagnostic refinement efforts advance, clinicians should provide care and/or referrals to providers best suited to treat an individual patient's clinical symptoms, many of which have evidence-based behavioral treatment options that are etiologically agnostic. Several ongoing research initiatives and the gradual accrual of gold standard clinico-pathological data will pay dividends for advancing the many existing gaps in the field of CTE.

Cellular Uptake of Tau Aggregates Triggers Disulfide Bond Formation in Four-Repeat Tau Monomers

Oxidative stress is an important driver of aging and has been linked to numerous neurodegenerative disorders, including Alzheimer's disease. A key pathological hallmark of Alzheimer's are filamentous inclusions made of the microtubule associated protein Tau. Based on alternative splicing, Tau protein can feature either three or four microtubule binding repeats. Distinctively, three-repeat Tau contains a single cysteine; four-repeat Tau contains two. Although there is evidence that the cysteines in pathological Tau filaments exist in the reduced form, very little is known about the alternative disulfide-bonded state. It is unclear whether it can exist nontransiently in the reducing environment of the cytosol. Such knowledge, however, is important as different redox states of Tau could modulate aggregation. To address this question, we transfected HEK293 cells expressing the P301S variant of four-repeat Tau with fibril seeds composed of compact, disulfide-bonded Tau monomers. In vitro, these fibrils are observed to recruit only compact Tau, but not Tau in which the cysteines are reduced or replaced by alanines or serines. In line with this characteristic, the fibrils dissociate when treated with a reducing agent. When offered to HEK293 cells, variant Tau protein is recruited to the seeds forming intracellular fibrils with the same seeding properties as the in vitro counterparts. Markedly, the proteins in these fibrils have a compact, disulfide-bonded configuration and dissociate upon reduction. These findings reveal that uptake of exogeneous fibril seeds triggers oxidation of Tau monomers, modulating intracellular aggregation.

Brain-derived tau oligomer polymorphs: distinct aggregations, stability profiles, and biological activities

Aggregation of microtubule-associated tau protein is a distinct hallmark of several neurodegenerative disorders such as Alzheimer's disease (AD), dementia with Lewy bodies (DLB), and progressive supranuclear palsy (PSP). Tau oligomers are suggested to be the primary neurotoxic species that initiate aggregation and propagate prion-like structures. Furthermore, different diseases are shown to have distinct structural characteristics of aggregated tau, denoted as polymorphs. Here, we investigate the structural and functional differences of amplified brain-derived tau oligomers (aBDTOs) from AD, DLB, and PSP. Our results indicate that the aBDTOs possess different structural and morphological features that impact neuronal function, gene regulation, and ultimately disease progression. The distinct tau oligomeric polymorphs may thus contribute to the development of clinical phenotypes and shape the progression of diseases. Our results can provide insight into developing personalized therapy to target a specific neurotoxic tau polymorph.

Inducers and modulators of protein aggregation in Alzheimer's disease - Critical tools for understanding the foundations of aggregate structures

Amyloidogenic protein aggregation is a pathological hallmark of Alzheimer's Disease (AD). As such, this critical feature of the disease has been instrumental in guiding research on the mechanistic basis of disease, diagnostic biomarkers and preventative and therapeutic treatments. Here we review identified molecular triggers and modulators of aggregation for two of the proteins associated with AD: amyloid beta and tau. We aim to provide an overview of how specific molecular factors can impact aggregation kinetics and aggregate structure to promote disease. Looking toward the future, we highlight some research areas of focus that would accelerate efforts to effectively target protein aggregation in AD.

Identification of novel tau positron emission tomography tracers for chronic traumatic encephalopathy by comprehensive in silico screening and molecular dynamics simulation

Chronic traumatic encephalopathy (CTE), a neurodegenerative disease associated with repetitive mild traumatic brain injury, is characterized neuropathologically by abnormal hyperphosphorylated tau accumulation. Early detection of tau deposition in the brain is crucial for the prevention and evaluation of CTE. Positron emission tomography (PET) tracers can image specific proteins, while the optimal PET tracer for CTE tau fibrils remains unidentified. In this study, structure-based virtual screening and CNS PET MPO algorithms were utilized to identify candidates for novel tau PET tracers from 23 000 compounds in the ChemDiv CNS BBB library. A total of 8 μs molecular dynamics simulations were then employed to evaluate their binding affinity and atomic-level interaction with CTE tau protofibrils. The results indicate that V017-7820 (CNS-4), S776-0061 (CNS-12), S567-0465 (CNS-18), and T828-0465 (CNS-25) exhibit higher docking scores and binding free energies with CTE tau protofibrils while also satisfying the fundamental physicochemical properties of PET tracers. Further simulation analyses reveal that CNS-4 has the strongest binding affinity to tau protofibrils among the four compounds. Hydrophobic, π-π stacking, and hydrogen bonding interactions are the primary driving forces for the binding of these compounds to CTE tau protofibrils. In particular, CNS-12 and CNS-25 exhibit more intense hydrophobic and π-π stacking interactions, whereas CNS-4 and CNS-25 exhibit stronger hydrogen bonding interactions. This study identifies promising lead compounds for tau PET tracers and highlights their mechanism of binding to CTE tau protofibrils, which provides new insights for further screening and development of novel PET tracers for CTE diagnosis.

Passaging Human Tauopathy Patient Samples in Cells Generates Heterogeneous Fibrils with a Subpopulation Adopting Disease Folds

The discovery by cryo-electron microscopy (cryo-EM) that the neu-ropathological hallmarks of different tauopathies, including Alzheimer's disease, corticobasal degeneration (CBD), and progressive supranuclear palsy (PSP), are caused by unique misfolded conformations of the protein tau is among the most profound developments in neurodegenerative disease research. To capitalize on these discoveries for therapeutic development, one must achieve in vitro replication of tau fibrils that adopt the representative tauopathy disease folds, which represents a grand challenge for the field. A widely used approach has been seeded propagation using pathological tau fibrils derived from post-mortem patient samples in biosensor cells that expresses a fragment of the tau protein known as K18, or Tau4RD, containing the microtubule-binding repeat domain of tau as the substrate. The new insights from cryo-EM raised the question of whether the Tau4RD fragment is capable of adopting characteristic tau folds found in CBD and PSP patient fibrils, and whether cell-passaged and amplified tau fibrils can be used as seeds to achieve cell-free assembly of recombinant 4R tau into fibrils without the addition of cofactors. Using Double Electron Electron Resonance (DEER) spectroscopy, we discovered that cell-passaged pathological seeds generate heterogeneous fibrils that are, however, distinct between the CBD and PSP lysate-seeded fibrils, and vastly different from heparin-induced tau fibril structures. Moreover, the lysate-seeded fibrils contain a characteristic sub-population that resembles the disease fold corresponding to the respective starting patient sample. These findings indicate that templated propagation using CBD and PSP patient-derived fibrils is possible with a tau fragment that does not contain the entire pathological fibril core, but also that additional mechanisms must be tuned to converge on a homogeneous fibril population.

'Knock down the brain': a nonlinear analysis of electroencephalography to study the effects of sub-concussion in boxers

BACKGROUND AND PURPOSE

Boxing is associated with a high risk of head injuries and increases the likelihood of chronic traumatic encephalopathy. This study explores the effects of sub-concussive impacts on boxers by applying both linear and nonlinear analysis methods to electroencephalogram (EEG) data.

METHODS

Twenty-one boxers were selected (mean ± SD, age 28.38 ± 5.5 years; weight 67.55 ± 8.90 kg; years of activity 6.76 ± 5.45; education 14.19 ± 3.08 years) and divided into 'beginner' and 'advanced' groups. The Montreal Cognitive Assessment and the Frontal Assessment Battery were administered; EEG data were collected in both eyes-open (EO) and eyes-closed (EC) conditions during resting states. Analyses of EEG data included normalized power spectral density (nPSD), power law exponent (PLE), detrended fluctuation analysis and multiscale entropy. Statistical analyses were used to compare the groups.

RESULTS

Significant differences in nPSD and PLE were observed between the beginner and advanced boxers, with advanced boxers showing decreased mean nPSD and PLE (nPSD 4-7 Hz, p = 0.013; 8-13 Hz, p = 0.003; PLE frontal lobe F3 EC, p = 0.010). Multiscale entropy analysis indicated increased entropy at lower frequencies and decreased entropy at higher frequencies in advanced boxers (F3 EC, p = 0.024; occipital lobe O1 EO, p = 0.029; occipital lobe O2 EO, p = 0.036). These changes are similar to those seen in Alzheimer's disease.

CONCLUSION

Nonlinear analysis of EEG data shows potential as a neurophysiological biomarker for detecting the asymptomatic phase of chronic traumatic encephalopathy in boxers. This methodology could help monitor athletes' health and reduce the risk of future neurological injuries in sports.

Cryo-EM structure of a novel α-synuclein filament subtype from multiple system atrophy

Multiple system atrophy (MSA) is a progressive neurodegenerative disease characterized by accumulation of α-synuclein cross-β amyloid filaments in the brain. Previous structural studies of these filaments by cryo-electron microscopy (cryo-EM) revealed three discrete folds distinct from α-synuclein filaments associated with other neurodegenerative diseases. Here, we use cryo-EM to identify a novel, low-populated MSA filament subtype (designated Type I2) in addition to a predominant class comprising MSA Type II2 filaments. The 3.3-Å resolution structure of the Type I2 filament reveals a fold consisting of two asymmetric protofilaments, one of which adopts a novel structure that is chimeric between two previously reported protofilaments. These results further define MSA-specific folds of α-synuclein filaments and have implications for designing MSA diagnostics and therapeutics.

Tubulin-Binding Region Modulates Cholesterol-Triggered Aggregation of Tau Proteins

A hallmark of Alzheimer disease (AD) and tauopathies, severe neurodegenerative diseases, is the progressive aggregation of Tau, also known as microtubule-associated Tau protein. Full-length Tau1-441, also known as 2N4R, contains two N-terminal inserts that bind to tubulin. This facilitates the self-assembly of tubulin simultaneously enhancing stability of cell microtubules. Other Tau isoforms have one (1N4R) or zero (0N4R) N-terminal inserts, which makes 2N4R Tau more and 0N4R less effective in promoting microtubule self-assembly. A growing body of evidence indicates that lipids can alter the aggregation rate of Tau isoforms. However, the role of N-terminal inserts in Tau-lipid interactions remains unclear. In this study, we utilized a set of biophysical methods to determine the extent to which N-terminal inserts alter interactions of Tau isoforms with cholesterol, one of the most important lipids in plasma membranes. Our results showed that 2 N insert prevents amyloid-driven aggregation of Tau at the physiological concentration of cholesterol, while the absence of this N-terminal repeat (1N4R and 0N4R Tau) resulted in the self-assembly of Tau into toxic amyloid fibrils. We also found that the presence of cholesterol in the lipid bilayers caused a significant increase in the cytotoxicity of 1N4R and 0N4R Tau to neurons. This effect was not observed for 2N4R Tau fibrils formed in the presence of lipid membranes with low, physiological, and elevated concentrations of cholesterol. Using molecular assays, we found that Tau aggregates primarily exert cytotoxicity by damaging cell endosomes, endoplasmic reticulum, and mitochondria.

Detecting the Undetectable: Advances in Methods for Identifying Small Tau Aggregates in Neurodegenerative Diseases

Tau, a microtubule-associated protein, plays a critical role in maintaining neuronal structure and function. However, in neurodegenerative diseases such as Alzheimer's disease and other tauopathies, tau misfolds and aggregates into oligomers and fibrils, leading to neuronal damage. Tau oligomers are increasingly recognised as the most neurotoxic species, inducing synaptic dysfunction and contributing to disease progression. Detecting these early-stage aggregates is challenging due to their low concentration and high heterogeneity in biological samples. Traditional methods such as immunostaining and enzyme-linked immunosorbent assay (ELISA) lack the sensitivity and specificity to reliably detect small tau aggregates. Advanced single-molecule approaches, including single-molecule fluorescence resonance energy transfer (smFRET) and single-molecule pull-down (SiMPull), offer improved sensitivity for studying tau aggregation at the molecular level. These emerging tools provide critical insights into tau pathology, enabling earlier detection and characterisation of disease-relevant aggregates, thereby offering potential for the development of targeted therapies and diagnostic approaches for tauopathies.

Traumatic Brain Injury and Alzheimer's Disease Biomarkers: A Systematic Review of Findings from Amyloid and Tau Positron Emission Tomography

Traumatic brain injury (TBI) has been discussed as a risk factor for Alzheimer's disease (AD) due to its association with AD risk and earlier cognitive symptom onset. However, the mechanisms behind this relationship are unclear. Some studies have suggested TBI may increase pathological protein deposition in an AD-like pattern; others have failed to find such associations. This review covers literature that uses positron emission tomography (PET) of β-amyloid (Aβ) and/or tau to examine individuals with a history of TBI who are at increased risk for AD due to age. A comprehensive literature search was conducted on January 9, 2023, and 26 resulting citations met inclusion criteria. Common methodological concerns included small samples, limited clinical detail about participants' TBI, recall bias due to reliance on self-reported TBI, and an inability to establish causation. For both Aβ and tau, results were widespread but inconsistent. The regions that showed the most compelling evidence for increased Aβ deposition were the cingulate gyrus and cuneus/precuneus. Evidence for elevated tau was strongest in the medial temporal lobe, entorhinal cortex, precuneus, and frontal, temporal, parietal, and occipital lobes. However, conflicting findings across most regions in both Aβ- and tau-PET studies indicate the critical need for future work in expanded samples and with greater clinical detail to offer a clearer picture of the relationship between TBI and protein deposition in older individuals at risk for AD.

dGAE(297-391) Tau Fragment Promotes Formation of Chronic Traumatic Encephalopathy-Like Tau Filaments

The microtubule-associated protein tau forms disease-specific filamentous aggregates in several different neurodegenerative diseases. In order to understand how tau undergoes misfolding into a specific filament type and to control this process for drug development purposes, it is crucial to study in vitro tau aggregation methods and investigate the structures of the obtained filaments at the atomic level. Here, we used the tau fragment dGAE, which aggregates spontaneously, to seed the formation of full-length tau filaments. The structures of dGAE and full-length tau filaments were investigated by magic-angle spinning (MAS) solid-state NMR, showing that dGAE allows propagation of a chronic traumatic encephalopathy (CTE)-like fold to the full-length tau. The obtained filaments efficiently seeded tau aggregation in HEK293T cells. This work demonstrates that in vitro preparation of disease-specific types of full-length tau filaments is feasible.

Characterizing heterogeneity in amyloid formation processes

Protein aggregation is a complex process, consisting of a large number of pathways connecting monomers and mature amyloid fibrils. Recent advances in structure determination techniques, such as solid-state NMR and cryoEM, have allowed the determination of atomic resolution structures of fibril polymorphs, but most of the intermediate stages of the process including oligomer formation remain unknown. Proper characterization of the heterogeneity of the process is critical not only for physical and chemical understanding of the aggregation process but also for elucidation of the disease mechanisms and identification of therapeutic targets. This article reviews recent developments in the characterization of heterogeneity in amyloid formation processes.

The importance of prion research

Over the past four decades, prion diseases have received considerable research attention owing to their potential to be transmitted within and across species as well as their consequences for human and animal health. The unprecedented nature of prions has led to the discovery of a paradigm of templated protein misfolding that underlies a diverse range of both disease-related and normal biological processes. Indeed, the "prion-like" misfolding and propagation of protein aggregates is now recognized as a common underlying disease mechanism in human neurodegenerative disorders such as Alzheimer's and Parkinson's disease, and the prion principle has led to the development of novel diagnostic and therapeutic strategies for these illnesses. Despite these advances, research into the fundamental biology of prion diseases has declined, likely due to their rarity and the absence of an acute human health crisis. Given the past translational influence, continued research on the etiology, pathogenesis, and transmission of prion disease should remain a priority. In this review, we highlight several important "unsolved mysteries" in the prion disease research field and how solving them may be crucial for the development of effective therapeutics, preventing future outbreaks of prion disease, and understanding the pathobiology of more common human neurodegenerative disorders.

Lewy-MSA hybrid fold drives distinct neuronal α-synuclein pathology

The ordered assembly of α-synuclein protein into filaments encoded by SNCA characterizes neurodegenerative diseases called synucleinopathies. Lewy body disease (LBD) shows predominantly neuronal α-synuclein pathology and multiple system atrophy (MSA) predominantly oligodendrocytic α-synuclein pathology affecting subcortical brain structures. Based on cryo-electron microscopy, it was reported that structures of α-synuclein filaments from LBD differ from MSA and juvenile onset synucleinopathy (JOS) caused by a 21-nucleotide duplication in the second exon of one allele of SNCA gene1-3. Importantly, a rare subtype of MSA, called atypical MSA4 shows abundant neuronal argyrophilic α-synuclein inclusions in the limbic system. Current concepts indicate that disease entities are characterized by unique protofilament folds. Here we demonstrate that in addition to the MSA fold, α-synuclein can form a new Lewy-MSA hybrid fold in the same brain region, leading to the atypical histopathological form of MSA. Distinct biochemical characteristics of α-synuclein, as demonstrated by protease-sensitivity digestion assay, seed amplification assays (SAAs) and conformational stability assay (CSA), are also linked to cytopathological differences (e.g. neuronal or oligodendroglial). We expand the current structure-based classification of α-synucleinopathies and propose that cell-specific protein pathologies can be associated with distinct filament folds.

Post-Translational Modifications Control Phase Transitions of Tau

The self-assembly of Tau into filaments, which mirror the structures observed in Alzheimer's disease (AD) brains, raises questions about the role of AD-specific post-translational modifications (PTMs) in the formation of paired helical filaments (PHFs). To investigate this, we developed a synthetic approach to produce Tau(291-391) featuring N-acetyllysine, phosphoserine, phosphotyrosine, and N-glycosylation at positions commonly modified in post-mortem AD brains. Using various electron and optical microscopy techniques, we discovered that these modifications generally hinder the in vitro assembly of Tau into PHFs. Interestingly, while acetylation's effect on Tau assembly displayed variability, either promoting or inhibiting phase transitions in cofactor-free aggregation, heparin-induced aggregation, and RNA-mediated liquid-liquid phase separation (LLPS), phosphorylation uniformly mitigated these processes. Our observations suggest that PTMs, particularly those situated outside the rigid core, are pivotal in the nucleation of PHFs. Moreover, with heparin-induced aggregation leading to the formation of heterogeneous aggregates, most AD-specific PTMs appeared to decelerate aggregation. The impact of acetylation on RNA-induced LLPS was notably site-dependent, whereas phosphorylation consistently reduced LLPS across all proteoforms examined. These insights underscore the complex interplay between site-specific PTMs and environmental factors in modulating Tau aggregation kinetics, highlighting the role of PTMs located outside the ordered filament core in driving the self-assembly.

Untangling the Complexity and Impact of Tau Protein Ubiquitination

The microtubule-associated protein tau is an intrinsically disordered protein highly expressed in neuronal axons. In healthy neurons, tau regulates microtubule dynamics and neurite outgrowth. However, pathological conditions can trigger aberrant tau aggregation into insoluble filaments, a hallmark of neurodegenerative disorders known as tauopathies. Tau undergoes diverse posttranslational modifications (PTMs), suggesting complex regulation and potentially varied functions. Among PTMs, the role and mechanisms of ubiquitination in physiology and disease have remained enigmatic. The past three decades have witnessed the emergence of key studies on tau protein ubiquitination. In this concept, we discuss how these investigations have begun to shed light on the ubiquitination patterns of physiological and pathological tau, the responsible enzymatic machinery, and the influence of ubiquitination on tau aggregation. We also provide an overview of the semi-synthetic methods that have enabled in vitro investigations of conformational transitions of tau induced by ubiquitin modification. Finally, we discuss future perspectives in the field necessary to elucidate the molecular mechanisms of tau ubiquitination and clearance.

The prevalence of chronic traumatic encephalopathy in a historical epilepsy post-mortem collection

Previous post-mortem epilepsy series showed phosphorylated tau (pTau) accumulation in relation to traumatic brain injury (TBI) rather than driven by seizure frequency. The Corsellis Epilepsy Collection, established in the mid-20th century, represents brain samples collected from patients living with a range of epilepsies from the 1880s to 1990s. Our aim was to interrogate this historical archive to explore relationships between epilepsy, trauma and tau pathology. AT8 immunohistochemistry for pTau was carried out in 102 cases (55% male, with mean age at death of 62 years) on frontal, temporal, amygdala, hippocampal and lesional cortical regions and evaluated using current NINDS criteria for chronic traumatic encephalopathy (CTE) and Braak staging with beta-amyloid, AT8-GFAP and other pTau markers (CP13, PHF1, AT100, AT180) in selected cases. CTE-neuropathologic change (CTE-NC) was identified in 15.7% and was associated with the presence of astroglial tau, a younger age of onset of epilepsy, evidence of TBI and institutionalisation for epilepsy compared to cases without CTE-NC, but not for seizure type or frequency. Memory impairment was noted in 43% of cases with CTE-NC, and a significantly younger age of death; more frequent reports of sudden and unexpected death (p <0.05-0.001) were noted in cases with CTE-NC. In contrast, a higher Braak stage was associated with late-onset epilepsy and cognitive decline. Of note, 9% of cases showed no pTau, including cases with long epilepsy duration, poor seizure control and a history of prior TBI. In summary, this cohort includes patients with more severe and diverse forms of epilepsy, with CTE-NC observed more frequently than reported in non-epilepsy community-based studies (0%-8%) but lower than published series from contact sports participants (32%-87%). Although the literature does not report increased epilepsy occurring in CTE syndrome, our findings support an increased risk of CTE in epilepsy syndromes, likely primarily related to increased TBI.

Cryo-EM structure of a lysozyme-derived amyloid fibril from hereditary amyloidosis

Systemic ALys amyloidosis is a debilitating protein misfolding disease that arises from the formation of amyloid fibrils from C-type lysozyme. We here present a 2.8 Å cryo-electron microscopy structure of an amyloid fibril, which was isolated from the abdominal fat tissue of a patient who expressed the D87G variant of human lysozyme. We find that the fibril possesses a stable core that is formed by all 130 residues of the fibril precursor protein. There are four disulfide bonds in each fibril protein that connect the same residues as in the globularly folded protein. As the conformation of lysozyme in the fibril is otherwise fundamentally different from native lysozyme, our data provide a structural rationale for the need of protein unfolding in the development of systemic ALys amyloidosis.

NMR studies of amyloid interactions

Amyloid fibrils are insoluble, fibrous nanostructures that accumulate extracellularly in biological tissue during the progression of several human disorders, including Alzheimer's disease (AD) and type 2 diabetes. Fibrils are assembled from protein monomers via the transient formation of soluble, cytotoxic oligomers, and have a common molecular architecture consisting of a spinal core of hydrogen-bonded protein β-strands. For the past 25 years, NMR spectroscopy has been at the forefront of research into the structure and assembly mechanisms of amyloid aggregates. Until the recent boom in fibril structure analysis by cryo-electron microscopy, solid-state NMR was unrivalled in its ability to provide atomic-level models of amyloid fibril architecture. Solution-state NMR has also provided complementary information on the early stages in the amyloid assembly mechanism. Now, both NMR modalities are proving to be valuable in unravelling the complex interactions between amyloid species and a diverse range of physiological metal ions, molecules and surfaces that influence the assembly pathway, kinetics, morphology and clearance in vivo. Here, an overview is presented of the main applications of solid-state and solution-state NMR for studying the interactions between amyloid proteins and biomembranes, glycosaminoglycan polysaccharides, metal ions, polyphenols, synthetic therapeutics and diagnostics. Key NMR methodology is reviewed along with examples of how to overcome the challenges of detecting interactions with aggregating proteins. The review heralds this new role for NMR in providing a comprehensive and pathologically-relevant view of the interactions between protein and non-protein components of amyloid. Coverage of both solid- and solution-state NMR methods and applications herein will be informative and valuable to the broad communities that are interested in amyloid proteins.

Examination of plasma biomarkers of amyloid, tau, neurodegeneration, and neuroinflammation in former elite American football players

INTRODUCTION

Blood-based biomarkers offer a promising approach for the detection of neuropathologies from repetitive head impacts (RHI). We evaluated plasma biomarkers of amyloid, tau, neurodegeneration, and inflammation in former football players.

METHODS

The sample included 180 former football players and 60 asymptomatic, unexposed male participants (aged 45-74). Plasma assays were conducted for beta-amyloid (Aβ) 40, Aβ42, hyper-phosphorylated tau (p-tau) 181+231, total tau (t-tau), neurofilament light (NfL), glial fibrillary acidic protein (GFAP), interleukin-6 (IL-6), Aβ42/p-tau181 and Aβ42/Aβ40 ratios. We evaluated their ability to differentiate the groups and associations with RHI proxies and traumatic encephalopathy syndrome (TES).

RESULTS

P-tau181 and p-tau231(padj = 0.016) were higher and Aβ42/p-tau181 was lower(padj = 0.004) in football players compared to controls. Discrimination accuracy for p-tau was modest (area under the curve [AUC] = 0.742). Effects were not attributable to AD-related pathology. Younger age of first exposure (AFE) correlated with higher NfL (padj = 0.03) and GFAP (padj = 0.033). Plasma GFAP was higher in TES-chronic traumatic encephalopathy (TES-CTE) Possible/Probable (padj = 0.008).

DISCUSSION

Plasma p-tau181 and p-tau231, GFAP, and NfL may offer some usefulness for the characterization of RHI-related neuropathologies.

HIGHLIGHTS

Former football players had higher plasma p-tau181 and p-tau231 and lower Aβ42/ptau-181 compared to asymptomatic, unexposed men. Younger age of first exposure was associated with increased plasma NfL and GFAP in older but not younger participants. Plasma GFAP was higher in participants with TES-CTE possible/probable compared to TES-CTE no/suggestive.

Linking Environmental Genotoxins to Neurodegenerative Diseases Through Transcriptional Mutagenesis

Numerous lines of evidence suggest that DNA damage contributes to the initiation, progression, and severity of neurodegenerative diseases. However, the molecular mechanisms responsible for this relationship remain unclear. This review integrates historical data with contemporary findings to propose that DNA damage exacerbates neurodegenerative diseases by inducing transcription errors. First, we describe the scientific rationale and basic biological concepts that underpin this hypothesis. Then, we provide epidemiological, cellular, and molecular data to support this idea, and we describe new and recently published observations that suggest that the former high incidence of neurodegenerative disease in Guam may have been driven by DNA damage-induced transcription errors. Finally, we explore the long-term implications of these findings on our understanding of the impact of genotoxic stress on human aging and disease.

Intracellular tau fragment droplets serve as seeds for tau fibrils

Intracellular tau aggregation requires a local protein concentration increase, referred to as "droplets". However, the cellular mechanism for droplet formation is poorly understood. Here, we expressed OptoTau, a P301L mutant tau fused with CRY2olig, a light-sensitive protein that can form homo-oligomers. Under blue light exposure, OptoTau increased tau phosphorylation and was sequestered in aggresomes. Suppressing aggresome formation by nocodazole formed tau granular clusters in the cytoplasm. The granular clusters disappeared by discontinuing blue light exposure or 1,6-hexanediol treatment suggesting that intracellular tau droplet formation requires microtubule collapse. Expressing OptoTau-ΔN, a species of N-terminal cleaved tau observed in the Alzheimer's disease brain, formed 1,6-hexanediol and detergent-resistant tau clusters in the cytoplasm with blue light stimulation. These intracellular stable tau clusters acted as a seed for tau fibrils in vitro. These results suggest that tau droplet formation and N-terminal cleavage are necessary for neurofibrillary tangles formation in neurodegenerative diseases.

The future of integrated structural biology

Instruct-ERIC, "the European Research Infrastructure Consortium for Structural biology research," is a pan-European distributed research infrastructure making high-end technologies and methods in structural biology available to users. Here, we describe the current state-of-the-art of integrated structural biology and discuss potential future scientific developments as an impulse for the scientific community, many of which are located in Europe and are associated with Instruct. We reflect on where to focus scientific and technological initiatives within the distributed Instruct research infrastructure. This review does not intend to make recommendations on funding requirements or initiatives directly, neither at the national nor the European level. However, it addresses future challenges and opportunities for the field, and foresees the need for a stronger coordination within the European and international research field of integrated structural biology to be able to respond timely to thematic topics that are often prioritized by calls for funding addressing societal needs.

Amyloid accelerator polyphosphate fits as the mystery density in α-synuclein fibrils

Aberrant aggregation of α-Synuclein is the pathological hallmark of a set of neurodegenerative diseases termed synucleinopathies. Recent advances in cryo-electron microscopy have led to the structural determination of the first synucleinopathy-derived α-Synuclein fibrils, which contain a non-proteinaceous, "mystery density" at the core of the protofilaments, hypothesized to be highly negatively charged. Guided by previous studies that demonstrated that polyphosphate (polyP), a universally conserved polyanion, significantly accelerates α-Synuclein fibril formation, we conducted blind docking and molecular dynamics simulation experiments to model the polyP binding site in α-Synuclein fibrils. Here, we demonstrate that our models uniformly place polyP into the lysine-rich pocket, which coordinates the mystery density in patient-derived fibrils. Subsequent in vitro studies and experiments in cells revealed that substitution of the 2 critical lysine residues K43 and K45 with alanine residues leads to a loss of all previously reported effects of polyP binding on α-Synuclein, including stimulation of fibril formation, change in filament conformation and stability as well as alleviation of cytotoxicity. In summary, our study demonstrates that polyP fits the unknown electron density present in in vivo α-Synuclein fibrils and suggests that polyP exerts its functions by neutralizing charge repulsion between neighboring lysine residues.

In situ seeding assay: A novel technique for direct tissue localization of bioactive tau

Proteins exhibiting prion-like properties are implicated in tauopathies. The prion-like traits of tau influence disease progression and correlate with severity. Techniques to measure tau bioactivity such as RT-QuIC and biosensor cells lack spatial specificity. Therefore, we developed a histological probe aimed at detecting and localizing bioactive tau in situ. We first induced the recruitment of a tagged probe by bioactive Tau in human brain tissue slices using biosensor cell lysates containing a fluorescent probe. We then enhanced sensitivity and flexibility by designing a recombinant probe with a myc tag. The probe design aimed to replicate the recruitment process seen in prion-like mechanisms based on the cryo-EM structure of tau aggregates in Alzheimer disease (AD). Using this novel probe, we observed selective staining of misfolded tau in pre- and post-synaptic structures within neurofibrillary tangles and neurites, whether or not associated with neuritic plaques. The probe specifically targeted AD-associated bioactive tau and did not recognize bioactive tau from other neurodegenerative diseases. Electron microscopy and immunolabeling further confirmed the identification of fibrillar and non-fibrillar tau. Finally, we established a correlation between quantifying bioactive tau using this technique and gold standard biosensor cells. This technique presents a robust approach for detecting bioactive tau in AD tissues and has potential applications for deciphering mechanisms of tau propagation and degradation pathways.

Connecting cellular mechanisms and extracellular vesicle cargo in traumatic brain injury

Traumatic brain injury is followed by a cascade of dynamic and complex events occurring at the cellular level. These events include: diffuse axonal injury, neuronal cell death, blood-brain barrier break down, glial activation and neuroinflammation, edema, ischemia, vascular injury, energy failure, and peripheral immune cell infiltration. The timing of these events post injury has been linked to injury severity and functional outcome. Extracellular vesicles are membrane bound secretory vesicles that contain markers and cargo pertaining to their cell of origin and can cross the blood-brain barrier. These qualities make extracellular vesicles intriguing candidates for a liquid biopsy into the pathophysiologic changes occurring at the cellular level post traumatic brain injury. Herein, we review the most commonly reported cargo changes in extracellular vesicles from clinical traumatic brain injury samples. We then use knowledge from animal and in vitro models to help infer what these changes may indicate regrading cellular responses post traumatic brain injury. Future research should prioritize labeling extracellular vesicles with markers for distinct cell types across a range of timepoints post traumatic brain injury.

Membrane-assisted Aβ40 aggregation pathways

Alzheimer's disease (AD) is caused by the assembly of amyloid-beta (Aβ) peptides into oligomers and fibrils. Endogenous Aβ aggregation may be assisted by cell membranes, which can accelerate the nucleation step enormously, but knowledge of membrane-assisted aggregation is still very limited. Here we used extensive MD simulations to structurally and energetically characterize key intermediates along the membrane-assisted aggregation pathways of Aβ40. Reinforcing experimental observations, the simulations reveal unique roles of GM1 ganglioside and cholesterol in stabilizing membrane-embedded β-sheets and of Y10 and K28 in the ordered release of a small oligomeric seed into solution. The same seed leads to either an open-shaped or R-shaped fibril, with significant stabilization provided by inter- or intra-subunit interfaces between a straight β-sheet (residues Q15-D23) and a bent β-sheet (residues A30-V36). This work presents the first comprehensive picture of membrane-assisted aggregation of Aβ40, with broad implications for developing AD therapies and rationalizing disease-specific polymorphisms of amyloidogenic proteins.

Rescue of synaptosomal glutamate release defects in tau transgenic mice by the tau aggregation inhibitor hydromethylthionine

Glutamatergic neurotransmission, important for learning and memory, is disrupted in different ways in patients with Alzheimer's disease (AD) and frontotemporal dementia (FTD) tauopathies. We have previously reported that two tau transgenic mouse models, L1 and L66, produce different phenotypes resembling AD and FTD, respectively. The AD-like L1 model expresses the truncated core aggregation domain of the AD paired helical filament (PHF) form of tau (tau296-390) whereas the FTD-like L66 model expresses full-length tau carrying two mutations at P301S/G335D. We have used synaptosomes isolated from these mice to investigate K+-evoked glutamate release and, if abnormal, to determine responsiveness to hydromethylthionine, a tau aggregation inhibitor previously shown to reduce tau pathology in these models. We report that the transgenes in these two mouse lines cause opposite abnormalities in glutamate release. Over-expression of the core tau unit in L1 produces a significant reduction in glutamate release and a loss of Ca2+-dependency compared with wild-type control mice. Full-length mutant tau produces an increase in glutamate release that retains normal Ca2+-dependency. Chronic pre-treatment with hydromethylthionine normalises both reduced (L1) and excessive glutamate (L66) and restores normal Ca2+-dependency in L1 mice. This implies that both patterns of impairment are the result of tau aggregation, but that the direction and Ca2+-dependency of the abnormality is determined by expression of the disease-specific transgene. Our results lead to the conclusion that the tauopathies need not be considered a single entity in terms of the downstream effects of pathological aggregation of tau protein. In this case, directionally opposite abnormalities in glutamate release resulting from different types of tau aggregation in the two mouse models can be corrected by hydromethylthionine. This may help to explain the activity of hydromethylthionine on cognitive decline and brain atrophy in both AD and behavioural-variant FTD.

18F-Flortaucipir (AV1451) imaging identifies grey matter atrophy in retired athletes

BACKGROUND

The long-term consequences of concussions may include pathological neurodegeneration as seen in Alzheimer's disease (AD) and chronic traumatic encephalopathy (CTE). Tau-PET showed promise as a method to detect tau pathology of CTE, but more studies are needed OBJECTIVE: This study aimed (1) to assess the association of imaging evidence of tau pathology with brain volumes in retired athletes and (2) to examine the relationship between tau-PET and neuropsychological functioning.

METHODS

Former contact sport athletes were recruited through the Canadian Football League Alumni Association or the Canadian Concussion Centre clinic. Athletes completed MRI, [18F]flortaucipir tau-PET, and a neuropsychological battery. Memory composite was created by averaging the Rey Auditory Verbal Learning Test and Rey Visual Design Learning Test z-scores. Grey matter (GM) volumes were age/intracranial volume corrected using normal control MRIs. Tau-PET % positivity in GM was calculated as the number of positive voxels (≥ 1.3 standardized uptake value ratio (SUVR)/total voxels).

RESULTS

47 retired contact sport athletes negative for AD (age:51 ± 14; concussions/athlete:15 ± 2) and 54 normal controls (age:50 ± 13) were included. Tau-PET positive voxels had significantly lower GM volumes, compared to tau-PET negative voxels (- 0.37 ± 0.41 vs. - 0.31 ± 0.37, paired p = .006). There was a significant relationship between GM tau-PET % positivity and memory composite score (r =  - .366, p = .02), controlled for age, PET scanner, and PET scan duration. There was no relationship between tau-PET measures and concussion number, or years of sport played.

CONCLUSION

A higher tau-PET signal was associated with reduced GM volumes and lower memory scores. Tau-PET may be useful for identifying those at risk for neurodegeneration.

Alzheimer's disease seeded tau forms paired helical filaments yet lacks seeding potential

Alzheimer's disease (AD) and many other neurodegenerative diseases are characterized by pathological aggregation of the protein tau. These tau aggregates spread in a stereotypical spatiotemporal pattern in the brain of each disease, suggesting that the misfolded tau can recruit soluble monomers to adopt the same pathological structure. To investigate whether recruited tau indeed adopts the same structure and properties as the original seed, here we template recombinant full-length 0N3R tau, 0N4R tau, and an equimolar mixture of the two using sarkosyl-insoluble tau extracted from AD brain and determine the structures of the resulting fibrils using cryoelectron microscopy. We show that these cell-free amplified tau fibrils adopt the same molecular structure as the AD paired-helical filament (PHF) tau but are unable to template additional monomers. Therefore, the PHF structure alone is insufficient for defining the pathological properties of AD tau, and other biochemical components such as tau posttranslational modifications, other proteins, polyanionic cofactors, and salt are required for the prion-like serial propagation of tauopathies.

Under Heparin-Free Conditions Unsaturated Phospholipids Inhibit the Aggregation of 1N4R and 2N4R Tau

A progressive aggregation of Tau proteins in the brain is linked to both Alzheimer's disease (AD) and various Tauopathies. This pathological process can be enhanced by several substances, including heparin. However, very little if anything is known about molecules that can inhibit the aggregation of Tau isoforms. In this study, we examined the effect of phosphatidylserines (PSs) with various lengths and saturations of fatty acids (FAs) on the aggregation properties of Tau isoforms with one (1N4R) and two (2N4R) N-terminal inserts that enhance binding of Tau to tubulin. We found that PS with unsaturated and short-length FAs inhibited Tau aggregation and drastically lowered the toxicity of Tau oligomers that were formed in the presence of such phospholipids. Such an effect was not observed for PS with fully saturated long-chain FAs. These results suggest that a short-chain irreversible disbalance between saturated and unsaturated lipids in the brain could be the trigger of Tau aggregation.

Single-domain antibodies and aptamers drive new opportunities for neurodegenerative disease research

Neurodegenerative diseases (NDs) in mammals, such as Alzheimer's disease (AD), Parkinson's disease (PD), and transmissible spongiform encephalopathies (TSEs), are characterized by the accumulation of misfolded proteins in the central nervous system (CNS). Despite the presence of these pathogenic proteins, the immune response in affected individuals remains notably muted. Traditional immunological strategies, particularly those reliant on monoclonal antibodies (mAbs), face challenges related to tissue penetration, blood-brain barrier (BBB) crossing, and maintaining protein stability. This has led to a burgeoning interest in alternative immunotherapeutic avenues. Notably, single-domain antibodies (or nanobodies) and aptamers have emerged as promising candidates, as their reduced size facilitates high affinity antigen binding and they exhibit superior biophysical stability compared to mAbs. Aptamers, synthetic molecules generated from DNA or RNA ligands, present both rapid production times and cost-effective solutions. Both nanobodies and aptamers exhibit inherent qualities suitable for ND research and therapeutic development. Cross-seeding events must be considered in both traditional and small-molecule-based immunodiagnostic and therapeutic approaches, as well as subsequent neurotoxic impacts and complications beyond protein aggregates. This review delineates the challenges traditional immunological methods pose in ND research and underscores the potential of nanobodies and aptamers in advancing next-generation ND diagnostics and therapeutics.

Severe neurodegeneration in brains of transgenic rats producing human tau prions

Both wild-type and mutant tau proteins can misfold into prions and self-propagate in the central nervous system of animals and people. To extend the work of others, we investigated the molecular basis of tau prion-mediated neurodegeneration in transgenic (Tg) rats expressing mutant human tau (P301S); this line of Tg rats is denoted Tg12099. We used the rat Prnp promoter to drive the overexpression of mutant tau (P301S) in the human 0N4R isoform. In Tg12099(+/+) rats homozygous for the transgene, ubiquitous expression of mutant human tau resulted in the progressive accumulation of phosphorylated tau inclusions, including silver-positive tangles in the frontal cortices and limbic system. Signs of central nervous system dysfunction were found in terminal Tg12099(+/+) rats exhibiting severe neurodegeneration and profound atrophy of the amygdala and piriform cortex. The greatest increases in tau prion activity were found in the corticolimbic structures. In contrast to the homozygous Tg12099(+/+) rats, we found lower levels of mutant tau in the hemizygous rats, resulting in few neuropathologic changes up to 2 years of age. Notably, these hemizygous rats could be infected by intracerebral inoculation with recombinant tau fibrils or precipitated tau prions from the brain homogenates of sick, aged homozygous Tg12099(+/+) rats. Our studies argue that the regional propagation of tau prions and neurodegeneration in the Tg12099 rats resembles that found in human primary tauopathies. These findings seem likely to advance our understanding of human tauopathies and may lead to effective therapeutics for Alzheimer's disease and other tau prion disorders.

Phosphoryl group wires stabilize pathological tau fibrils as revealed by multiple quantum spin counting NMR

Hyperphosphorylation of the protein tau is one of the biomarkers of neurodegenerative diseases in the category of tauopathies. However, the molecular level, mechanistic, role of this common post-translational modification (PTM) in enhancing or reducing the aggregation propensity of tau is unclear, especially considering that combinatorial phosphorylation of multiple sites can have complex, non-additive, effects on tau protein aggregation. Since tau proteins stack in register and parallel to elongate into pathological fibrils, phosphoryl groups from adjacent tau strands with 4.8 Å separation must find an energetically favorable spatial arrangement. At first glance, this appears to be an unfavorable configuration due to the proximity of negative charges between phosphate groups from adjacent neighboring tau fibrils. However, this study tests a counterhypothesis that phosphoryl groups within the fibril core-forming segments favorably assemble into highly ordered, hydrogen-bonded, one-dimensionally extended wires under biologically relevant conditions. We selected two phosphorylation sites associated with neurodegeneration, serine 305 (S305p) and tyrosine 310 (Y310p), on a model tau peptide jR2R3-P301L (tau295-313) spanning the R2/R3 splice junction of tau, that readily aggregate into a fibril with characteristics of a seed-competent mini prion. Using multiple quantum spin counting (MQ-SC) by 31P solid-state NMR of phosphorylated jR2R3-P301L tau peptide fibrils, enhanced by dynamic nuclear polarization, we find that at least six phosphorous spins must neatly arrange in 1D within fibrils or in 2D within a protofibril to yield the experimentally observed MQ-coherence orders of four. We found that S305p stabilizes the tau fibrils and leads to more seeding-competent fibrils compared to jR2R3 P301L or Y310p. This study introduces a new concept that phosphorylation of residues within a core forming tau segment can mechanically facilitate fibril registry and stability due a hitherto unrecognized role of phosphoryl groups to form highly ordered, extended, 1D wires that stabilize pathological tau fibrils.

Tau in neurodegenerative diseases: molecular mechanisms, biomarkers, and therapeutic strategies

The deposition of abnormal tau protein is characteristic of Alzheimer's disease (AD) and a class of neurodegenerative diseases called tauopathies. Physiologically, tau maintains an intrinsically disordered structure and plays diverse roles in neurons. Pathologically, tau undergoes abnormal post-translational modifications and forms oligomers or fibrous aggregates in tauopathies. In this review, we briefly introduce several tauopathies and discuss the mechanisms mediating tau aggregation and propagation. We also describe the toxicity of tau pathology. Finally, we explore the early diagnostic biomarkers and treatments targeting tau. Although some encouraging results have been achieved in animal experiments and preclinical studies, there is still no cure for tauopathies. More in-depth basic and clinical research on the pathogenesis of tauopathies is necessary.

"Prion-like" seeding and propagation of oligomeric protein assemblies in neurodegenerative disorders

Intra- or extracellular aggregates of proteins are central pathogenic features in most neurodegenerative disorders. The accumulation of such proteins in diseased brains is believed to be the end-stage of a stepwise aggregation of misfolded monomers to insoluble cross-β fibrils via a series of differently sized soluble oligomers/protofibrils. Several studies have shown how α-synuclein, amyloid-β, tau and other amyloidogenic proteins can act as nucleating particles and thereby share properties with misfolded forms, or strains, of the prion protein. Although the roles of different protein assemblies in the respective aggregation cascades remain unclear, oligomers/protofibrils are considered key pathogenic species. Numerous observations have demonstrated their neurotoxic effects and a growing number of studies have indicated that they also possess seeding properties, enabling their propagation within cellular networks in the nervous system. The seeding behavior of oligomers differs between the proteins and is also affected by various factors, such as size, shape and epitope presentation. Here, we are providing an overview of the current state of knowledge with respect to the "prion-like" behavior of soluble oligomers for several of the amyloidogenic proteins involved in neurodegenerative diseases. In addition to providing new insight into pathogenic mechanisms, research in this field is leading to novel diagnostic and therapeutic opportunities for neurodegenerative diseases.

Cryo-EM structures of pathogenic fibrils and their impact on neurodegenerative disease research

Neurodegenerative diseases are commonly associated with the formation of aberrant protein aggregates within the brain, and ultrastructural analyses have revealed that the proteins within these inclusions often assemble into amyloid filaments. Cryoelectron microscopy (cryo-EM) has emerged as an effective method for determining the near-atomic structure of these disease-associated filamentous proteins, and the resulting structures have revolutionized the way we think about aberrant protein aggregation and propagation during disease progression. These structures have also revealed that individual fibril conformations may dictate different disease conditions, and this newfound knowledge has improved disease modeling in the lab and advanced the ongoing pursuit of clinical tools capable of distinguishing and targeting different pathogenic entities within living patients. In this review, we summarize some of the recently developed cryo-EM structures of ex vivo α-synuclein, tau, β-amyloid (Aβ), TAR DNA-binding protein 43 (TDP-43), and transmembrane protein 106B (TMEM106B) fibrils and discuss how these structures are being leveraged toward mechanistic research and therapeutic development.

Role of Amyloidogenic and Non-Amyloidogenic Protein Spaces in Neurodegenerative Diseases and their Mitigation Using Theranostic Agents

Neurodegenerative diseases (NDDs) refer to a complex heterogeneous group of diseases which are associated with the accumulation of amyloid fibrils or plaques in the brain leading to progressive loss of neuronal functions. Alzheimer's disease is one of the major NDD responsible for 60-80 % of all dementia cases. Currently, there are no curative or disease-reversing/modifying molecules for many of the NDDs except a few such as donepezil, rivastigmine, galantamine, carbidopa and levodopa which treat the disease-associated symptoms. Similarly, there are very few FDA-approved tracers such as flortaucipir (Tauvid) for tau fibril imaging and florbetaben (Neuraceq), flutemetamol (Vizamyl), and florbetapir (Amyvid) for amyloid imaging available for diagnosis. Recent advances in the cryogenic electron microscopy reported distinctly different microstructures for tau fibrils associated with different tauopathies highlighting the possibility to develop tauopathy-specific imaging agents and therapeutics. In addition, it is important to identify the proteins that are associated with disease development and progression to know about their 3D structure to develop various diagnostics, therapeutics and theranostic agents. The current article discusses in detail the disease-associated amyloid and non-amyloid proteins along with their structural insights. We comprehensively discussed various novel proteins associated with NDDs and their implications in disease pathology. In addition, we document various emerging chemical compounds developed for diagnosis and therapy of different NDDs with special emphasis on theranostic agents for better management of NDDs.

Tubulin-binding region alters tau-lipid interactions and changes toxicity of tau fibrils formed in the presence of phosphatidylserine lipids

Alzheimer's disease is the fastest-growing neurodegenerative disease that affects over six million Americans. The abnormal aggregation of amyloid β peptide and Tau protein is the expected molecular cause of the loss of neurons in brains of AD patients. A growing body of evidence indicates that lipids can alter the aggregation rate of amyloid β peptide and modify the toxicity of amyloid β aggregates. However, the role of lipids in Tau aggregation remains unclear. In this study, we utilized a set of biophysical methods to determine the extent to which phospatidylserine (PS) altered the aggregation properties of Tau isoforms with one (1N4R) and two (2N4R) N terminal inserts that enhance the binding of Tau to tubulin. We found that the length and saturation of fatty acids (FAs) in PS altered the aggregation rate of 2N4R isoform, while no changes in the aggregation rate of 1N4R were observed. These results indicate that N terminal inserts play an important role in protein-lipid interactions. We also found that PS could change the toxicity of 1N4R and 2N4R Tau fibrils, as well as alter molecular mechanisms by which these aggregates exert cytotoxicity to neurons. Finally, we found that although Tau fibrils formed in the presence and absence of PS endocytosed by cells, only fibril species that were formed in the presence of PS exert strong impairment of the cell mitochondria.

Ligand-based design of [18F]OXD-2314 for PET imaging in non-Alzheimer's disease tauopathies

Positron emission tomography (PET) imaging of tau aggregation in Alzheimer's disease (AD) is helping to map and quantify the in vivo progression of AD pathology. To date, no high-affinity tau-PET radiopharmaceutical has been optimized for imaging non-AD tauopathies. Here we show the properties of analogues of a first-in-class 4R-tau lead, [18F]OXD-2115, using ligand-based design. Over 150 analogues of OXD-2115 were synthesized and screened in post-mortem brain tissue for tau affinity against [3H]OXD-2115, and in silico models were used to predict brain uptake. [18F]OXD-2314 was identified as a selective, high-affinity non-AD tau PET radiotracer with favorable brain uptake, dosimetry, and radiometabolite profiles in rats and non-human primate and is being translated for first-in-human PET studies.

Molecular Dynamics Simulation Study of the Self-Assembly of Tau-Derived PHF6 and Its Inhibition by Oleuropein Aglycone from Extra Virgin Olive Oil

Alzheimer's disease (AD) and other taupathies are neurodegenerative disorders associated with the amyloid deposition of the Tau protein in the brain. This amyloid formation may be inhibited by small molecules, which is recognized as one of the best therapeutic strategies to stop the progression of the disease. This work focuses on the small nucleating segment, hexapeptide-paired helical filament 6 (PHF6), responsible for Tau aggregation. Using computational modeling and classical molecular dynamics simulations, we show that PHF6 monomers collapse in water to form β-sheet rich structures, and the main olive oil polyphenol oleuropein aglycone (OleA) prevents peptide aggregation significantly. We gradually increase the ratio of the PHF6-OleA from 1:1 to 1:3 and find that for the 1:1 ratio, the peptide monomers are prone to form aggregated structures, while for the 1:2 ratio, the formation of the extended β-sheet structure is significantly less. For a 1:3 ratio of protein/OleA, the peptide residues are sufficiently crowded by OleA molecules through hydrogen bonding, hydrophobic interactions, and π-π stacking; hence, the peptide chains prefer to exist in a monomeric random coil conformation.

Cryo-EM structures reveal tau filaments from Down syndrome adopt Alzheimer's disease fold

Down syndrome (DS) is a common genetic condition caused by trisomy of chromosome 21. Among their complex clinical features, including musculoskeletal, neurological, and cardiovascular disabilities, individuals with DS have an increased risk of developing progressive dementia and early-onset Alzheimer's disease (AD). This dementia is attributed to the increased gene dosage of the amyloid-β (Aβ) precursor protein gene, the formation of self-propagating Aβ and tau prion conformers, and the deposition of neurotoxic Aβ plaques and tau neurofibrillary tangles. Tau amyloid fibrils have previously been established to adopt many distinct conformations across different neurodegenerative conditions. Here, we report the characterization of brain samples from four DS cases spanning 36-63 years of age by spectral confocal imaging with conformation-specific dyes and cryo-electron microscopy (cryo-EM) to determine structures of isolated tau fibrils. High-resolution structures revealed paired helical filament (PHF) and straight filament (SF) conformations of tau that were identical to those determined from AD cases. The PHFs and SFs are made of two C-shaped protofilaments, each containing a cross-β/β-helix motif. Similar to filaments from AD cases, most filaments from the DS cases adopted the PHF form, while a minority (approximately 20%) formed SFs. Samples from the youngest individual with no documented dementia had sparse tau deposits. To isolate tau for cryo-EM from this challenging sample we used a novel affinity-grid method involving a graphene oxide surface derivatized with anti-tau antibodies. This method improved isolation and revealed that primarily tau PHFs and a minor population of chronic traumatic encephalopathy type II-like filaments were present in this youngest case. These findings expand the similarities between AD and DS to the molecular level, providing insight into their related pathologies and the potential for targeting common tau filament folds by small-molecule therapeutics and diagnostics.

The structural line between prion and "prion-like": Insights from prion protein and tau

The concept of 'prion-like' behavior has emerged in the study of diseases involving protein misfolding where fibrillar structures, called amyloids, self-propagate and induce disease in a fashion similar to prions. From a biological standpoint, in order to be considered 'prion-like,' a protein must traverse cells and tissues and further propagate via a templated conformational change. Since 2017, cryo-electron microscopy structures from patient-derived 'prion-like' amyloids, in particular tau, have been presented and revealed structural similarities shared across amyloids. Since 2021, cryo-EM structures from prions of known infectivity have been added to the ex vivo amyloid structure family. In this review, we discuss current proposals for the 'prion-like' mechanisms of spread for tau and prion protein as well as discuss different influencers on structures of aggregates from tauopathies and prion diseases. Lastly, we discuss some of the current hypotheses for what may distinguish structures that are 'prion-like' from transmissible prion structures.

Cryo-EM Structures Reveal Tau Filaments from Down Syndrome Adopt Alzheimer's Disease Fold

Down syndrome (DS) is a common genetic condition caused by trisomy of chromosome 21. Among the complex clinical features including musculoskeletal, neurological and cardiovascular disabilities, individuals with DS have an increased risk of developing progressive dementia and early onset Alzheimer's Disease (AD). This is attributed to the increased gene dosage of amyloid-β (Aβ) precursor protein gene, the formation of self-propagating Aβ and tau prion conformers, and the deposition of neurotoxic Aβ plaques and tau neurofibrillary tangles. Tau amyloid fibrils have previously been established to adopt many distinct conformations across different neurodegenerative conditions. Here we report the characterization of brain samples from four DS cases spanning 36 to 63 years of age by spectral confocal imaging with conformation-specific dyes and cryo-electron microscopy (cryo-EM) to determine structures of isolated tau fibrils. High-resolution structures reveal paired helical filament (PHF) and straight filament (SF) conformations of tau that are identical to those determined from AD. The PHFs and SFs are made of two C-shaped protofilaments with a cross-β/β-helix motif. Similar to filaments from AD cases, most filaments from the DS cases adopted the PHF form, while a minority (~20%) formed SFs. Samples from the youngest individual with no documented dementia had sparse tau deposits. To isolate tau for cryo-EM from this challenging sample we used a novel affinity-grid method involving a graphene-oxide surface derivatized with anti-tau antibodies. This improved isolation and revealed primarily tau PHFs and a minor population of chronic traumatic encephalopathy type II-like filaments were present in this youngest case. These findings expand the similarities between AD and DS to the molecular level, providing insight into their related pathologies and the potential for targeting common tau filament folds by small-molecule therapeutics and diagnostics.

Tau filaments with the chronic traumatic encephalopathy fold in a case of vacuolar tauopathy with VCP mutation D395G

Dominantly inherited mutation D395G in the gene encoding valosin-containing protein causes vacuolar tauopathy, a type of behavioural-variant frontotemporal dementia, with marked vacuolation and abundant filamentous tau inclusions made of all six brain isoforms. Here we report that tau inclusions were concentrated in layers II/III of the frontotemporal cortex in a case of vacuolar tauopathy. By electron cryomicroscopy, tau filaments had the chronic traumatic encephalopathy (CTE) fold. Tau inclusions of vacuolar tauopathy share this cortical location and the tau fold with CTE, subacute sclerosing panencephalitis and amyotrophic lateral sclerosis/parkinsonism-dementia complex, which are believed to be environmentally induced. Vacuolar tauopathy is the first inherited disease with the CTE tau fold.