Chronic Sustained Hypoxia Leads to Brainstem Tauopathy and Declines the Power of Rhythms in the Ventrolateral Medulla: Shedding Light on a Possible Mechanism

Hypoxia, especially the chronic type, leads to disruptive results in the brain that may contribute to the pathogenesis of some neurodegenerative diseases such as Alzheimer's disease (AD). The ventrolateral medulla (VLM) contains clusters of interneurons, such as the pre-Bötzinger complex (preBötC), that generate the main respiratory rhythm drive. We hypothesized that exposing animals to chronic sustained hypoxia (CSH) might develop tauopathy in the brainstem, consequently changing the rhythmic manifestations of respiratory neurons. In this study, old (20-22 months) and young (2-3 months) male rats were subjected to CSH (10 ± 0.5% O2) for ten consecutive days. Western blotting and immunofluorescence (IF) staining were used to evaluate phosphorylated tau. Mitochondrial membrane potential (MMP or ∆ψm) and reactive oxygen species (ROS) production were measured to assess mitochondrial function. In vivo diaphragm's electromyography (dEMG) and local field potential (LFP) recordings from preBötC were employed to assess the respiratory factors and rhythmic representation of preBötC, respectively. Findings showed that ROS production increased significantly in hypoxic groups, associated with a significant decline in ∆ψm. In addition, tau phosphorylation elevated in the brainstem of hypoxic groups. On the other hand, the power of rhythms declined significantly in the preBötC of hypoxic rats, parallel with changes in the respiratory rate, total respiration time, and expiration time. Moreover, there was a positive and statistically significant correlation between LFP rhythm's power and inspiration time. Our data showed that besides CSH, aging also contributed to mitochondrial dysfunction, tau hyperphosphorylation, LFP rhythms' power decline, and changes in respiratory factors.

Ferritin nanocage-enabled detection of pathological tau in living human retinal cells

Tauopathies, including Alzheimer's disease and Frontotemporal Dementia, are debilitating neurodegenerative disorders marked by cognitive decline. Despite extensive research, achieving effective treatments and significant symptom management remains challenging. Accurate diagnosis is crucial for developing effective therapeutic strategies, with hyperphosphorylated protein units and tau oligomers serving as reliable biomarkers for these conditions. This study introduces a novel approach using nanotechnology to enhance the diagnostic process for tauopathies. We developed humanized ferritin nanocages, a novel nanoscale delivery system, designed to encapsulate and transport a tau-specific fluorophore, BT1, into human retinal cells for detecting neurofibrillary tangles in retinal tissue, a key marker of tauopathies. The delivery of BT1 into living cells was successfully achieved through these nanocages, demonstrating efficient encapsulation and delivery into retinal cells derived from human induced pluripotent stem cells. Our experiments confirmed the colocalization of BT1 with pathological forms of tau in living retinal cells, highlighting the method's potential in identifying tauopathies. Using ferritin nanocages for BT1 delivery represents a significant contribution to nanobiotechnology, particularly in neurodegenerative disease diagnostics. This method offers a promising tool for the early detection of tau tangles in retinal tissue, with significant implications for improving the diagnosis and management of tauopathies. This study exemplifies the integration of nanotechnology with biomedical science, expanding the frontiers of nanomedicine and diagnostic techniques.

Overexpression of pathogenic tau in astrocytes causes a reduction in AQP4 and GLT1, an immunosuppressed phenotype and unique transcriptional responses to repetitive mild TBI without appreciable changes in tauopathy

Epidemiological studies have unveiled a robust link between exposure to repetitive mild traumatic brain injury (r-mTBI) and elevated susceptibility to develop neurodegenerative disorders, notably chronic traumatic encephalopathy (CTE). The pathogenic lesion in CTE cases is characterized by the accumulation of hyperphosphorylated tau in neurons around small cerebral blood vessels which can be accompanied by astrocytes that contain phosphorylated tau, the latter termed tau astrogliopathy. However, the contribution of tau astrogliopathy to the pathobiology and functional consequences of r-mTBI/CTE or whether it is merely a consequence of aging remains unclear. We addressed these pivotal questions by utilizing a mouse model harboring tau-bearing astrocytes, GFAPP301L mice, subjected to our r-mTBI paradigm. Despite the fact that r-mTBI did not exacerbate tau astrogliopathy or general tauopathy, it increased phosphorylated tau in the area underneath the impact site. Additionally, gene ontology analysis of tau-bearing astrocytes following r-mTBI revealed profound alterations in key biological processes including immunological and mitochondrial bioenergetics. Moreover, gene array analysis of microdissected astrocytes accrued from stage IV CTE human brains revealed an immunosuppressed astroglial phenotype similar to tau-bearing astrocytes in the GFAPP301L model. Additionally, hippocampal reduction of proteins involved in water transport (AQP4) and glutamate homeostasis (GLT1) was found in the mouse model of tau astrogliopathy. Collectively, these findings reveal the importance of understanding tau astrogliopathy and its role in astroglial pathobiology under normal circumstances and following r-mTBI. The identified mechanisms using this GFAPP301L model may suggest targets for therapeutic interventions in r-mTBI pathogenesis in the context of CTE.

Reduction of spermine synthase enhances autophagy to suppress Tau accumulation

Precise polyamine metabolism regulation is vital for cells and organisms. Mutations in spermine synthase (SMS) cause Snyder-Robinson intellectual disability syndrome (SRS), characterized by significant spermidine accumulation and autophagy blockage in the nervous system. Emerging evidence connects polyamine metabolism with other autophagy-related diseases, such as Tauopathy, however, the functional intersection between polyamine metabolism and autophagy in the context of these diseases remains unclear. Here, we altered SMS expression level to investigate the regulation of autophagy by modulated polyamine metabolism in Tauopathy in Drosophila and human cellular models. Interestingly, while complete loss of Drosophila spermine synthase (dSms) impairs lysosomal function and blocks autophagic flux recapitulating SRS disease phenotype, partial loss of dSms enhanced autophagic flux, reduced Tau protein accumulation, and led to extended lifespan and improved climbing performance in Tauopathy flies. Measurement of polyamine levels detected a mild elevation of spermidine in flies with partial loss of dSms. Similarly, in human neuronal or glial cells, partial loss of SMS by siRNA-mediated knockdown upregulated autophagic flux and reduced Tau protein accumulation. Importantly, proteomics analysis of postmortem brain tissue from Alzheimer's disease (AD) patients showed a significant albeit modest elevation of SMS level. Taken together, our study uncovers a functional correlation between polyamine metabolism and autophagy in AD: SMS reduction upregulates autophagy, suppresses Tau accumulation, and ameliorates neurodegeneration and cell death. These findings provide a new potential therapeutic target for AD.

Human iPSC 4R tauopathy model uncovers modifiers of tau propagation

Tauopathies are age-associated neurodegenerative diseases whose mechanistic underpinnings remain elusive, partially due to a lack of appropriate human models. Here, we engineered human induced pluripotent stem cell (hiPSC)-derived neuronal lines to express 4R Tau and 4R Tau carrying the P301S MAPT mutation when differentiated into neurons. 4R-P301S neurons display progressive Tau inclusions upon seeding with Tau fibrils and recapitulate features of tauopathy phenotypes including shared transcriptomic signatures, autophagic body accumulation, and reduced neuronal activity. A CRISPRi screen of genes associated with Tau pathobiology identified over 500 genetic modifiers of seeding-induced Tau propagation, including retromer VPS29 and genes in the UFMylation cascade. In progressive supranuclear palsy (PSP) and Alzheimer's Disease (AD) brains, the UFMylation cascade is altered in neurofibrillary-tangle-bearing neurons. Inhibiting the UFMylation cascade in vitro and in vivo suppressed seeding-induced Tau propagation. This model provides a robust platform to identify novel therapeutic strategies for 4R tauopathy.

S100A8-enriched microglia populate the brain of tau-seeded and accelerated aging mice

Long considered to fluctuate between pro- and anti-inflammatory states, it has now become evident that microglia occupy a variegated phenotypic landscape with relevance to aging and neurodegeneration. However, whether specific microglial subsets converge in or contribute to both processes that eventually affect brain function is less clear. To investigate this, we analyzed microglial heterogeneity in a tauopathy mouse model (K18-seeded P301L) and an accelerated aging model (Senescence-Accelerated Mouse-Prone 8, SAMP8) using cellular indexing of transcriptomes and epitopes by sequencing. We found that widespread tau pathology in K18-seeded P301L mice caused a significant change in the number and morphology of microglia, but only a mild overrepresentation of disease-associated microglia. At the cell population-level, we observed a marked upregulation of the calprotectin-encoding genes S100a8 and S100a9. In 9-month-old SAMP8 mice, we identified a unique microglial subpopulation that showed partial similarity with the disease-associated microglia phenotype and was additionally characterized by a high expression of the same calprotectin gene set. Immunostaining for S100A8 revealed that this population was enriched in the hippocampus, correlating with the cognitive impairment observed in this model. However, incomplete colocalization between their residence and markers of neuronal loss suggests regional specificity. Importantly, S100A8-positive microglia were also retrieved in brain biopsies of human AD and tauopathy patients as well as in a biopsy of an aged individual without reported pathology. Thus, the emergence of S100A8-positive microglia portrays a conspicuous commonality between accelerated aging and tauopathy progression, which may have relevance for ensuing brain dysfunction.

The six brain-specific TAU isoforms and their role in Alzheimer's disease and related neurodegenerative dementia syndromes

INTRODUCTION

Alternative splicing of the human MAPT gene generates six brain-specific TAU isoforms. Imbalances in the TAU isoform ratio can lead to neurodegenerative diseases, underscoring the need for precise control over TAU isoform balance. Tauopathies, characterized by intracellular aggregates of hyperphosphorylated TAU, exhibit extensive neurodegeneration and can be classified by the TAU isoforms present in pathological accumulations.

METHODS

A comprehensive review of TAU and related dementia syndromes literature was conducted using PubMed, Google Scholar, and preprint server.

RESULTS

While TAU is recognized as key driver of neurodegeneration in specific tauopathies, the contribution of the isoforms to neuronal function and disease development remains largely elusive.

DISCUSSION

In this review we describe the role of TAU isoforms in health and disease, and stress the importance of comprehending and studying TAU isoforms in both, physiological and pathological context, in order to develop targeted therapeutic interventions for TAU-associated diseases.

HIGHLIGHTS

MAPT splicing is tightly regulated during neuronal maturation and throughout life. TAU isoform expression is development-, cell-type and brain region specific. The contribution of TAU to neurodegeneration might be isoform-specific. Ineffective TAU-based therapies highlight the need for specific targeting strategies.

Functional Selection of Tau Oligomerization-Inhibiting Aptamers

Pathological hyperphosphorylation and aggregation of microtubule-associated Tau protein contribute to Alzheimer's Disease (AD) and other related tauopathies. Currently, no cure exists for Alzheimer's Disease. Aptamers offer significant potential as next-generation therapeutics in biotechnology and the treatment of neurological disorders. Traditional aptamer selection methods for Tau protein focus on binding affinity rather than interference with pathological Tau. In this study, we developed a new selection strategy to enrich DNA aptamers that bind to surviving monomeric Tau protein under conditions that would typically promote Tau aggregation. Employing this approach, we identified a set of aptamer candidates. Notably, BW1c demonstrates a high binding affinity (Kd=6.6 nM) to Tau protein and effectively inhibits arachidonic acid (AA)-induced Tau protein oligomerization and aggregation. Additionally, it inhibits GSK3β-mediated Tau hyperphosphorylation in cell-free systems and okadaic acid-mediated Tau hyperphosphorylation in cellular milieu. Lastly, retro-orbital injection of BW1c tau aptamer shows the ability to cross the blood brain barrier and gain access to neuronal cell body. Through further refinement and development, these Tau aptamers may pave the way for a first-in-class neurotherapeutic to mitigate tauopathy-associated neurodegenerative disorders.

Mouse serum albumin induces neuronal apoptosis and tauopathies

The elderly frequently present impaired blood-brain barrier which is closely associated with various neurodegenerative diseases. However, how the albumin, the most abundant protein in the plasma, leaking through the disrupted BBB, contributes to the neuropathology remains poorly understood. We here demonstrated that mouse serum albumin-activated microglia induced astrocytes to A1 phenotype to remarkably increase levels of Elovl1, an astrocytic synthase for very long-chain saturated fatty acids, significantly promoting VLSFAs secretion and causing neuronal lippoapoptosis through endoplasmic reticulum stress response pathway. Moreover, MSA-activated microglia triggered remarkable tau phosphorylation at multiple sites through NLRP3 inflammasome pathway. Intracerebroventricular injection of MSA into the brains of C57BL/6J mice to a similar concentration as in patient brains induced neuronal apoptosis, neuroinflammation, increased tau phosphorylation, and decreased the spatial learning and memory abilities, while Elovl1 knockdown significantly prevented the deleterious effect of MSA. Overall, our study here revealed that MSA induced tau phosphorylation and neuron apoptosis based on MSA-activated microglia and astrocytes, respectively, showing the critical roles of MSA in initiating the occurrence of tauopathies and cognitive decline, and providing potential therapeutic targets for MSA-induced neuropathology in multiple neurodegenerative disorders.

Precision proteoform design for 4R tau isoform selective templated aggregation

Prion-like spread of disease-specific tau conformers is a hallmark of all tauopathies. A 19-residue probe peptide containing a P301L mutation and spanning the R2/R3 splice junction of tau folds and stacks into seeding-competent fibrils and induces aggregation of 4R, but not 3R tau. These tau peptide fibrils propagate aggregated intracellular tau over multiple generations, have a high β-sheet content, a colocalized lipid signal, and adopt a well-defined U-shaped fold found in 4R tauopathy brain-derived fibrils. Fully atomistic replica exchange molecular dynamics (MD) simulations were used to compute the free energy landscapes of the conformational ensemble of the peptide monomers. These identified an aggregation-prohibiting β-hairpin structure and an aggregation-competent U-fold unique to 4R tauopathy fibrils. Guided by MD simulations, we identified that the N-terminal-flanking residues to PHF6, which slightly vary between 4R and 3R isoforms, modulate seeding. Strikingly, when a single amino acid switch at position 305 replaced the serine of 4R tau with a lysine from the corresponding position in the first repeat of 3R tau, the seeding induced by the 19-residue peptide was markedly reduced. Conversely, a 4R tau mimic with three repeats, prepared by replacing those amino acids in the first repeat with those amino acids uniquely present in the second repeat, recovered aggregation when exposed to the 19-residue peptide. These peptide fibrils function as partial prions to recruit naive 4R tau-ten times the length of the peptide-and serve as a critical template for 4R tauopathy propagation. These results hint at opportunities for tau isoform-specific therapeutic interventions.

Prominent tauopathy and intracellular β-amyloid accumulation triggered by genetic deletion of cathepsin D: implications for Alzheimer disease pathogenesis

BACKGROUND

Cathepsin D (CatD) is a lysosomal protease that degrades both the amyloid-β protein (Aβ) and the microtubule-associated protein, tau, which accumulate pathognomonically in Alzheimer disease (AD), but few studies have examined the role of CatD in the development of Aβ pathology and tauopathy in vivo.

METHODS

CatD knockout (KO) mice were crossed to human amyloid precursor protein (hAPP) transgenic mice, and amyloid burden was quantified by ELISA and immunohistochemistry (IHC). Tauopathy in CatD-KO mice, as initially suggested by Gallyas silver staining, was further characterized by extensive IHC and biochemical analyses. Controls included human tau transgenic mice (JNPL3) and another mouse model of a disease (Krabbe A) characterized by pronounced lysosomal dysfunction. Additional experiments examined the effects of CatD inhibition on tau catabolism in vitro and in cultured neuroblastoma cells with inducible expression of human tau.

RESULTS

Deletion of CatD in hAPP transgenic mice triggers large increases in cerebral Aβ, manifesting as intense, exclusively intracellular aggregates; extracellular Aβ deposition, by contrast, is neither triggered by CatD deletion, nor affected in older, haploinsufficient mice. Unexpectedly, CatD-KO mice were found to develop prominent tauopathy by just ∼ 3 weeks of age, accumulating sarkosyl-insoluble, hyperphosphorylated tau exceeding the pathology present in aged JNPL3 mice. CatD-KO mice exhibit pronounced perinuclear Gallyas silver staining reminiscent of mature neurofibrillary tangles in human AD, together with widespread phospho-tau immunoreactivity. Striking increases in sarkosyl-insoluble phospho-tau (∼ 1250%) are present in CatD-KO mice but notably absent from Krabbe A mice collected at an identical antemortem interval. In vitro and in cultured cells, we show that tau catabolism is slowed by blockade of CatD proteolytic activity, including via competitive inhibition by Aβ42.

CONCLUSIONS

Our findings support a major role for CatD in the proteostasis of both Aβ and tau in vivo. To our knowledge, the CatD-KO mouse line is the only model to develop detectable Aβ accumulation and profound tauopathy in the absence of overexpression of hAPP or human tau with disease-associated mutations. Given that tauopathy emerges from disruption of CatD, which can itself be potently inhibited by Aβ42, our findings suggest that impaired CatD activity may represent a key mechanism linking amyloid accumulation and tauopathy in AD.

Tau reduction with artificial microRNAs modulates neuronal physiology and improves tauopathy phenotypes in mice

Abnormal tau accumulation is the hallmark of several neurodegenerative diseases, named tauopathies. Strategies aimed at reducing tau in the brain are promising therapeutic interventions, yet more precise therapies would require targeting specific nuclei and neuronal subpopulations affected by disease while avoiding global reduction of physiological tau. Here, we developed artificial microRNAs directed against the human MAPT mRNA to dwindle tau protein by engaging the endogenous RNA interference pathway. In human differentiated neurons in culture, microRNA-mediated tau reduction diminished neuronal firing without affecting neuronal morphology or impairing axonal transport. In the htau mouse model of tauopathy, we locally expressed artificial microRNAs in the prefrontal cortex (PFC), an area particularly vulnerable to initiating tau pathology in this model. Tau knockdown prevented the accumulation of insoluble and hyperphosphorylated tau, modulated firing activity of putative pyramidal neurons, and improved glucose uptake in the PFC. Moreover, such tau reduction prevented cognitive decline in aged htau mice. Our results suggest target engagement of designed tau-microRNAs to effectively reduce tau pathology, providing a proof of concept for a potential therapeutic approach based on local tau knockdown to rescue tauopathy-related phenotypes.

A meta-analysis of differentially expressed circulatory micro-RNAs in chronic traumatic encephalopathy and other tauopathies: A significant role of miR-181c-5p

BACKGROUND

Micro-RNA (miRs) targeting kinases and phosphatases regulate the hyper-phosphorylation of tau protein, which is a characteristic feature of Chronic Traumatic Encephalopathy (CTE).

PRIMARY OBJECTIVE

Identification of lead dysregulated miR expressed in CTE, and other similar tauopathies.

METHODS

A search strategy was devised using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines to mine into multiple indexing databases such as Web of Science, Google Scholar, and PubMed spanning from 2005 to June 2022. Seven articles were screened out of 34,221 publications based on inclusion criteria and were categorized into two groups i.e., (1) CTE and its risk factors and (2) Age-related neurodegenerative disorders.

RESULTS

Statistical analysis [RevMan 5.4.1] results showed that the overall risk ratio (RR) of the first group is significant (RR = 0.62, 95% CI = [0.38, 1.00], z = 1.95, p = 0.05) whereas, the second group favours the control population (RR = 1.64, 95% CI = [0.85, 3.16], z = 1.14, p = 0.14).

CONCLUSION

We observed that among all other dysregulated miRs, miR-181c-5p is significantly overexpressed in Alzhimers disease (AD) and CTE. Further, we found that miR-210-3p is also upregulated notably in all groups. In sum, we conclude that these miRs can be considered as potential target and biomarker in the diagnosis and treatment of various tauopathies.

Lymphocyte-Activation Gene 3 Facilitates Pathological Tau Neuron-to-Neuron Transmission

The spread of prion-like protein aggregates is a common driver of pathogenesis in various neurodegenerative diseases, including Alzheimer's disease (AD) and related Tauopathies. Tau pathologies exhibit a clear progressive spreading pattern that correlates with disease severity. Clinical observation combined with complementary experimental studies has shown that Tau preformed fibrils (PFF) are prion-like seeds that propagate pathology by entering cells and templating misfolding and aggregation of endogenous Tau. While several cell surface receptors of Tau are known, they are not specific to the fibrillar form of Tau. Moreover, the underlying cellular mechanisms of Tau PFF spreading remain poorly understood. Here, it is shown that the lymphocyte-activation gene 3 (Lag3) is a cell surface receptor that binds to PFF but not the monomer of Tau. Deletion of Lag3 or inhibition of Lag3 in primary cortical neurons significantly reduces the internalization of Tau PFF and subsequent Tau propagation and neuron-to-neuron transmission. Propagation of Tau pathology and behavioral deficits induced by injection of Tau PFF in the hippocampus and overlying cortex are attenuated in mice lacking Lag3 selectively in neurons. These results identify neuronal Lag3 as a receptor of pathologic Tau in the brain，and for AD and related Tauopathies, a therapeutic target.

[Neuropathology of the Neurodegenerative Diseases]

A definite diagnosis of neurodegenerative diseases is required for neuropathological examination during an autopsy. Each neurodegenerative disease has specific vulnerable regions and affected systems (system degeneration), and is typified by an accumulation of abnormal protein with the formation of characteristic morphological aggregates in the nerve and glial cells, called proteinopathy. The most common neurodegenerative diseases are tauopathy, such as progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and Pick's disease (PiD); α-synucleinopathy, including multiple system atrophy (MSA); and TAR DNA-binding protein of 43 kDa (TDP-43) proteinopathy, including amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). PSP and CBD show characteristic tau-positive astrocytic inclusions known as tufted astrocytes and astrocytic plaques, respectively. PiD shows tau-positive neuronal inclusions termed Pick bodies. MSA is characterized by α-synuclein-positive oligodendroglial inclusions, called glial cytoplasmic inclusions. ALS- and FTLD-TDP show TDP-43-positive neuronal inclusions, such as skein-like and round inclusions. Huntington's disease shows polyglutamine-positive neuronal inclusions, and Creutzfeldt-Jakob disease shows diffuse deposition of granular prions in the neuropil. The atypical proteins in these diseases have abnormal conformational properties. A comprehensive comparison of the clinical findings and neuropathological observations, including neuroanatomy and images acquired during life, is important to improve the sensitivity of clinical diagnosis.

Mathematical topology and geometry-based classification of tauopathies

Neurodegenerative diseases, like Alzheimer's, are associated with the presence of neurofibrillary lesions formed by tau protein filaments in the cerebral cortex. While it is known that different morphologies of tau filaments characterize different neurodegenerative diseases, there are few metrics of global and local structure complexity that enable to quantify their structural diversity rigorously. In this manuscript, we employ for the first time mathematical topology and geometry to classify neurodegenerative diseases by using cryo-electron microscopy structures of tau filaments that are available in the Protein Data Bank. By employing mathematical topology metrics (Gauss linking integral, writhe and second Vassiliev measure) we achieve a consistent, but more refined classification of tauopathies, than what was previously observed through visual inspection. Our results reveal a hierarchy of classification from global to local topology and geometry characteristics. In particular, we find that tauopathies can be classified with respect to the handedness of their global conformations and the handedness of the relative orientations of their repeats. Progressive supranuclear palsy is identified as an outlier, with a more complex structure than the rest, reflected by a small, but observable knotoid structure (a diagrammatic structure representing non-trivial topology). This topological characteristic can be attributed to a pattern in the beginning of the R3 repeat that is present in all tauopathies but at different extent. Moreover, by comparing single filament to paired filament structures within tauopathies we find a consistent change in the side-chain orientations with respect to the alpha carbon atoms at the area of interaction.

Overlaps and divergences between tauopathies and synucleinopathies: a duet of neurodegeneration

Proteinopathy, defined as the abnormal accumulation of proteins that eventually leads to cell death, is one of the most significant pathological features of neurodegenerative diseases. Tauopathies, represented by Alzheimer's disease (AD), and synucleinopathies, represented by Parkinson's disease (PD), show similarities in multiple aspects. AD manifests extrapyramidal symptoms while dementia is also a major sign of advanced PD. We and other researchers have sequentially shown the cross-seeding phenomenon of α-synuclein (α-syn) and tau, reinforcing pathologies between synucleinopathies and tauopathies. The highly overlapping clinical and pathological features imply shared pathogenic mechanisms between the two groups of disease. The diagnostic and therapeutic strategies seemingly appropriate for one distinct neurodegenerative disease may also apply to a broader spectrum. Therefore, a clear understanding of the overlaps and divergences between tauopathy and synucleinopathy is critical for unraveling the nature of the complicated associations among neurodegenerative diseases. In this review, we discuss the shared and diverse characteristics of tauopathies and synucleinopathies from aspects of genetic causes, clinical manifestations, pathological progression and potential common therapeutic approaches targeting the pathology, in the aim to provide a timely update for setting the scheme of disease classification and provide novel insights into the therapeutic development for neurodegenerative diseases.

EB1 Increases the Dynamics of Tau Droplets and Inhibits Tau Aggregation: Implications in Tauopathies

EB1, a microtubule plus end-tracking protein (+TIP), regulates microtubule dynamics. Recent evidence indicates cross-talk between EB proteins and tau, a microtubule-associated neuronal protein that is important for the growth and stability of microtubules. We investigated the interaction between tau and EB1 and the effect of binding of EB1 on tau function and aggregation. EB1 colocalized with tau in SH-SY5Y cells and coimmunoprecipitated with tau. Further, purified EB1 impaired the ability of adult tau to induce tubulin polymerization in vitro. EB1 bound to tau with a dissociation constant of 2.5 ± 0.7 μM. EB1 reduced heparin-induced tau aggregation with a half-maximal inhibitory concentration of 4.3 ± 0.2 μM, and increased the dynamics of tau in phase-separated droplets. The fluorescence recovery rate in tau droplets increased from 0.02 ± 0.01 to 0.07 ± 0.03 s-1, while the half-time of recovery decreased from 44.5 ± 14 to 13.5 ± 6 s in the presence of 8 μM EB1, suggesting a delay in the transition of tau from the soluble to aggregated form in tau liquid-liquid phase separation. EB1 decreased the rate of aggregation and increased the critical concentration of tau aggregation. Dynamic light scattering, atomic force microscopy, dot blot assays, and SDS-PAGE analysis showed that EB1 inhibited the formation of oligomers and higher-order aggregates of tau. The data suggest a novel role for EB1 as a regulator of tau function and aggregation, and the findings indicated the role of the EB family proteins in neuronal function and neurodegeneration.

Crosstalk between tau protein autoproteolysis and amyloid fibril formation

Tau cleavage has been shown to have a significant effect on protein aggregation. Tau truncation results in the formation of aggregation-prone fragments leading to toxic aggregates and also causes the formation of harmful fragments that do not aggregate. Thus, targeting proteolysis of tau would be beneficial for the development of therapeutics for Alzheimer's disease and related tauopathies. In this study, amino-terminal quantification and ThT fluorimetry were respectively used to analyze the kinetics of tau fragmentation and fibril formation. SDS-PAGE analysis of tau protein incubated with a disulfide-reducing agent demonstrated that the cysteines of tau have a crucial role in the fibrillation and autoproteolysis. However, the structures converted to amyloid fibrils were different with conformations that led to autoproteolysis. The quantification of the amino terminal indicated that the double-disulfide parallel structures formed in the presence of heparin did not have protease activity. The survey of possible tau disulfide-mediated dimer configurations suggested that the non-register single disulfide bound conformations were involved in the tau autoproteolysis process. Moreover, the inhibition of autoproteolysis resulted in the increment of aggregation rate; hence it seems that the tau auto-cleavage is the cellular defense mechanism against protein fibrillation.

Disentangling tau: One protein, many therapeutic approaches

The tauopathies encompass over 20 adult neurodegenerative diseases and are characterized by the dysfunction and accumulation of insoluble tau protein. Among them, Alzheimer's disease, frontotemporal dementia, and progressive supranuclear palsy collectively impact millions of patients and their families worldwide. Despite years of drug development using a variety of mechanisms of action, no therapeutic directed against tau has been approved for clinical use. This raises important questions about our current model of tau pathology and invites thoughtful consideration of our approach to nonclinical models and clinical trial design. In this article, we review what is known about the biology and genetics of tau, placing it in the context of current and failed clinical trials. We highlight potential reasons for the lack of success to date and offer suggestions for new pathways in therapeutic development. Overall, our viewpoint to the future is optimistic for this important group of neurodegenerative diseases.

Probable 4-Repeat Tauopathy Criteria Predict Brain Amyloid Negativity, Distinct Clinical Features, and FDG-PET/MRI Neurodegeneneration Patterns in Corticobasal Syndrome

BACKGROUND

Corticobasal syndrome (CBS) is associated with diverse underlying pathologies, including the four-repeat (4R)-tauopathies. The Movement Disorders Society (MDS) criteria for progressive supranuclear palsy (PSP) proposed the novel category "probable 4R-tauopathy" to address the phenotypic overlap between PSP and corticobasal degeneration (CBD).

OBJECTIVES

To investigate the clinical ability of the MDS-PSP criteria for probable 4R-tauopathy in predicting a negative amyloid-PET in CBS. Additionally, this study aims to explore CBS patients classified as 4R-tauopathy concerning their clinical features and neuroimaging degeneration patterns.

METHODS

Thirty-two patients with probable CBS were prospectively evaluated and split into those who fulfilled or did not fulfill the 4R-tauopathy criteria (CBS-4RT+ vs. CBS-4RT-). All patients underwent positron emission tomographies (PET) with [18 F]fluorodeoxyglucose and [11 C]Pittsburgh Compound-B (PIB) on a hybrid PET-MRI scanner to perform multimodal quantitative comparisons with a control group.

RESULTS

Eleven patients were clinically classified as CBS-4RT+, and only one had a positive PIB-PET. The CBS-4RT+ classification had 92% specificity, 52% sensitivity, and 69% accuracy in predicting a negative PIB-PET. The CBS-4RT+ group presented with dysarthria and perseveration more often than the CBS-4RT- group. Moreover, the CBS-4RT+ group showed a prominent frontal hypometabolism extending to the supplementary motor area and striatum, and brain atrophy at the anterior cingulate and bilateral striata.

CONCLUSIONS

The 4R-tauopathy criteria were highly specific in predicting a negative amyloid-PET in CBS. Patients classified as 4R-tauopathy presented distinct clinical aspects, as well as brain metabolism and atrophy patterns previously associated with tauopathies.

In vitro evaluation of [3H]PI-2620 and structural derivatives in non-Alzheimer's tauopathies

Alzheimer's disease (AD) and non-AD tauopathies such as chronic traumatic encephalopathy (CTE), progressive supranuclear palsy (PSP), and corticobasal degeneration (CBD) are characterized by the abnormal aggregation of three-repeat (3R) and/or four-repeat (4R) tau isoforms. Several tau-PET tracers have been applied for human imaging of AD and non-AD tauopathies including [18F]PI-2620. Our objective is to evaluate [3H]PI-2620 and two promising structural derivatives, [3H]PI-2014 and [3H]F-4, using in vitro saturation assays and competitive binding assays against new chemical entities based on this scaffold in human AD tissues for comparison with PSP, CBD and CTE tissues. Thin section autoradiography was employed to assess specific binding and distribution of [3H]PI-2620 and [3H]F-4 in fresh-frozen human post-mortem AD, PSP, CBD and CTE tissues. Immunohistochemistry was performed for phospho-tau (AT8) and 4R-tau (RD4). Homogenate filtration binding assays were performed for saturation analysis and competitive binding studies against [3H]PI-2620. All compounds bound with high affinity in AD tissue. In PSP tissue [3H]PI-2620 demonstrated the highest affinity (5.3 nM) and in CBD tissue [3H]F-4 bound with the highest affinity (9.4 nM). Over 40 fluorinated derivatives based on PI-2620 and F-4 were screened in AD and PSP tissue. Notably, compound 2 was the most potent derivative in PSP tissue (Ki = 7.3 nM). By autoradiography, [3H]PI-2620 and [3H]F-4 demonstrated positive signals similar in intensity in AD, PSP and CTE tissues that were displaced by homologous blockade. Binding of both radiotracers aligned with immunostaining for 4R-tau. This work demonstrates that [3H]PI-2620 and [3H]F-4 show promise for imaging 4R-tau aggregates in non-AD tauopathies. PI-2620 continues to serve as a structural scaffold for PET radiotracers with higher affinity for non-AD tau over AD tau.

Transgenic Dendra2::tau expression allows in vivo monitoring of tau proteostasis in Caenorhabditis elegans

Protein homeostasis is perturbed in aging-related neurodegenerative diseases called tauopathies, which are pathologically characterized by aggregation of the microtubule-associated protein tau (encoded by the human MAPT gene). Transgenic Caenorhabditis elegans serve as a powerful model organism to study tauopathy disease mechanisms, but moderating transgenic expression level has proven problematic. To study neuronal tau proteostasis, we generated a suite of transgenic strains expressing low, medium or high levels of Dendra2::tau fusion proteins by comparing integrated multicopy transgene arrays with single-copy safe-harbor locus strains generated by recombinase-mediated cassette exchange. Multicopy Dendra2::tau strains exhibited expression level-dependent neuronal dysfunction that was modifiable by known genetic suppressors or an enhancer of tauopathy. Single-copy Dendra2::tau strains lacked distinguishable phenotypes on their own but enabled detection of enhancer-driven neuronal dysfunction. We used multicopy Dendra2::tau strains in optical pulse-chase experiments measuring tau turnover in vivo and found that Dendra2::tau turned over faster than the relatively stable Dendra2. Furthermore, Dendra2::tau turnover was dependent on the protein expression level and independent of co-expression with human TDP-43 (officially known as TARDBP), an aggregating protein interacting with pathological tau. We present Dendra2::tau transgenic C. elegans as a novel tool for investigating molecular mechanisms of tau proteostasis.

Biochemical approaches to assess the impact of post-translational modifications on pathogenic tau conformations using recombinant protein

Tau protein is associated with many neurodegenerative disorders known as tauopathies. Aggregates of tau are thought of as a main contributor to neurodegeneration in these diseases. Increasingly, evidence points to earlier, soluble conformations of abnormally modified monomers and multimeric tau as toxic forms of tau. The biological processes driving tau from physiological species to pathogenic conformations remain poorly understood, but certain avenues are currently under investigation including the functional consequences of various pathological tau changes (e.g. mutations, post-translational modifications (PTMs), and protein-protein interactions). PTMs can regulate several aspects of tau biology such as proteasomal and autophagic clearance, solubility, and aggregation. Moreover, PTMs can contribute to the transition of tau from normal to pathogenic conformations. However, our understating of how PTMs specifically regulate the transition of tau into pathogenic conformations is partly impeded by the relative lack of structured frameworks to assess and quantify these conformations. In this review, we describe a set of approaches that includes several in vitro assays to determine the contribution of PTMs to tau's transition into known pathogenic conformations. The approaches begin with different methods to create recombinant tau proteins carrying specific PTMs followed by validation of the PTMs status. Then, we describe a set of biochemical and biophysical assays that assess the contribution of a given PTM to different tau conformations, including aggregation, oligomerization, exposure of the phosphatase-activating domain, and seeding. Together, these approaches can facilitate the advancement of our understanding of the relationships between PTMs and tau conformations.

Quantitative live cell imaging of a tauopathy model enables the identification of a polypharmacological drug candidate that restores physiological microtubule interaction

Tauopathies such as Alzheimer's disease are characterized by aggregation and increased phosphorylation of the microtubule-associated protein tau. Tau's pathological changes are closely linked to neurodegeneration, making tau a prime candidate for intervention. We developed an approach to monitor pathological changes of aggregation-prone human tau in living neurons. We identified 2-phenyloxazole (PHOX) derivatives as putative polypharmacological small molecules that interact with tau and modulate tau kinases. We found that PHOX15 inhibits tau aggregation, restores tau's physiological microtubule interaction, and reduces tau phosphorylation at disease-relevant sites. Molecular dynamics simulations highlight cryptic channel-like pockets crossing tau protofilaments and suggest that PHOX15 binding reduces the protofilament's ability to adopt a PHF-like conformation by modifying a key glycine triad. Our data demonstrate that live-cell imaging of a tauopathy model enables screening of compounds that modulate tau-microtubule interaction and allows identification of a promising polypharmacological drug candidate that simultaneously inhibits tau aggregation and reduces tau phosphorylation.

Atomic Force Microscopy Applied to the Study of Tauopathies

Atomic force microscopy (AFM) is a scanning probe microscopy technique which has a physical principle, the measurement of interatomic forces between a very thin tip and the surface of a sample, allowing the obtaining of quantitative data at the nanoscale, contributing to the surface study and mechanical characterization. Due to its great versatility, AFM has been used to investigate the structural and nanomechanical properties of several inorganic and biological materials, including neurons affected by tauopathies. Tauopathies are neurodegenerative diseases featured by aggregation of phosphorylated tau protein inside neurons, leading to functional loss and progressive neurotoxicity. In the broad universe of neurodegenerative diseases, tauopathies comprise the most prevalent, with Alzheimer's disease as its main representative. This review highlights the use of AFM as a suitable research technique for the study of cellular damages in tauopathies, even in early stages, allowing elucidation of pathogenic mechanisms of these diseases.

Diagnostic and therapeutic targeting of pathological tau proteins in neurodegenerative disorders

Tauopathies, characterized by fibrillar tau accumulation in neurons and glial cells, constitute a major neuropathological category of neurodegenerative diseases. Neurofibrillary tau lesions are strongly associated with cognitive deficits in these diseases, but the causal mechanisms underlying tau-induced neuronal dysfunction remain unresolved. Recent advances in cryo-electron microscopy examination have revealed various core structures of tau filaments from different tauopathy patients, which can be used to classify tauopathies. In vivo visualization of tau pathology is now available using several tau positron emission tomography tracers. Among these radioprobes, PM-PBB3 allows high-contrast imaging of tau deposits in the brains of patients with diverse disorders and tauopathy mouse models. Selective degradation of pathological tau species by the ubiquitin-proteasome system or autophagy machinery is a potential therapeutic strategy. Alternatively, the non-cell-autonomous clearance of pathological tau species through neuron-glia networks could be reinforced as a disease-modifying treatment. In addition, the development of neuroinflammatory biomarkers is required for understanding the contribution of immunocompetent cells in the brain to preventing neurodegeneration. This review provides an overview of the current research and development of diagnostic and therapeutic agents targeting divergent tau pathologies.

KIBRA repairs synaptic plasticity and promotes resilience to tauopathy-related memory loss

Synaptic plasticity is obstructed by pathogenic tau in the brain, representing a key mechanism that underlies memory loss in Alzheimer's disease (AD) and related tauopathies. Here, we found that reduced levels of the memory-associated protein KIdney/BRAin (KIBRA) in the brain and increased KIBRA protein levels in cerebrospinal fluid are associated with cognitive impairment and pathological tau levels in disease. We next defined a mechanism for plasticity repair in vulnerable neurons using the C-terminus of the KIBRA protein (CT-KIBRA). We showed that CT-KIBRA restored plasticity and memory in transgenic mice expressing pathogenic human tau; however, CT-KIBRA did not alter tau levels or prevent tau-induced synapse loss. Instead, we found that CT-KIBRA stabilized the protein kinase Mζ (PKMζ) to maintain synaptic plasticity and memory despite tau-mediated pathogenesis. Thus, our results distinguished KIBRA both as a biomarker of synapse dysfunction and as the foundation for a synapse repair mechanism to reverse cognitive impairment in tauopathy.

Novel aspects of the phosphorylation and structure of pathological tau: implications for tauopathy biomarkers

The deposition of highly phosphorylated and aggregated tau is a characteristic of tauopathies, including Alzheimer's disease. It has long been known that different isoforms of tau are aggregated in different cell types and brain regions in each tauopathy. Recent advances in analytical techniques revealed the details of the biochemical and structural biological differences of tau specific to each tauopathy. In this review, we explain recent advances in the analysis of post-translational modifications of tau, particularly phosphorylation, brought about by the development of mass-spectrometry and Phos-tag technology. We then discuss the structure of tau filaments in each tauopathy revealed by the advent of cryo-EM. Finally, we describe the progress in biofluid and imaging biomarkers for tauopathy. This review summarizes current efforts to elucidate the characteristics of pathological tau and the landscape of the use of tau as a biomarker to diagnose and determine the pathological stage of tauopathy.

Reinstated Activity of Human Tau-induced Enhanced Insulin Signaling Restricts Disease Pathogenesis by Regulating the Functioning of Kinases/Phosphatases and Tau Hyperphosphorylation in Drosophila

Tauopathies such as Alzheimer's disease (AD), Frontotemporal dementia, and parkinsonism linked to chromosome 17 (FTDP-17), etc. are characterized by tau hyperphosphorylation and distinguished accumulation of paired helical filaments (PHFs)/or neurofibrillary tangles (NFTs) in a specific-neuronal subset of the brain. Among different reported risk factors, type 2 diabetes (T2D) has gained attention due to its correlation with tau pathogenesis. However, mechanistic details and the precise contribution of insulin pathway in tau etiology is still debatable. We demonstrate that expression of human tau causes overactivation of insulin pathway in Drosophila disease models. We subsequently noted that tissue-specific downregulation of insulin signaling or even exclusive reduction of its growth-promoting sub-branch effectively reinstates the overactivated insulin signaling pathway in human tau expressing cells, which in turn restricts pathogenic tau hyperphosphorylation and aggregate formation. It was further noted that restored tau phosphorylation was achieved due to a reestablished balance between the levels of different kinase(s) (GSK3β and ERK/P38 MAP kinase) and phosphatase (PP2A). Taken together, our study demonstrates a precise involvement of the insulin pathway and associated molecular events in the pathogenesis of human tauopathies in Drosophila, which will be immensely helpful in developing novel therapeutic options against these devastating human brain disorders. Moreover, our study reveals an interesting link between tau etiology and aberrant insulin signaling, which is a characteristic feature of Type 2 Diabetes.

Regulation of tau by peptidyl-prolyl isomerases

Tau is an intrinsically disordered protein found abundantly in axons, where it binds to microtubules. Since tau is a central player in the dynamic microtubule network, it is highly regulated by post-translational modifications. Abnormal hyperphosphorylation and aggregation of tau characterize a group of diseases called tauopathies. A specific protein family of cis/trans peptidyl-prolyl isomerases (PPIases) can interact with tau to regulate its aggregation and neuronal resilience. Structural interactions between tau and specific PPIases have been determined, establishing possible mechanisms for tau regulation and modification. While there have been numerous in vivo studies evaluating the impact of PPIase expression on tau biology/pathology, the direct roles of PPIases have yet to be fully characterized. Different PPIases correlate to either increased or decreased levels of tau-associated degeneration. Therefore, the ability of PPIases to structurally modify and regulate tau should be further investigated due to its potential therapeutic implications for Alzheimer's disease and other tauopathies.

Galectin-3 aggravates microglial activation and tau transmission in tauopathy

Alzheimer's disease is characterized by the accumulation of amyloid-β plaques, aggregation of hyperphosphorylated tau (pTau), and microglia activation. Galectin-3 (Gal3) is a β-galactoside-binding protein that has been implicated in amyloid pathology. Its role in tauopathy remains enigmatic. Here, we showed that Gal3 was upregulated in the microglia of humans and mice with tauopathy. pTau triggered the release of Gal3 from human induced pluripotent stem cell-derived microglia in both its free and extracellular vesicular-associated (EV-associated) forms. Both forms of Gal3 increased the accumulation of pathogenic tau in recipient cells. Binding of Gal3 to pTau greatly enhanced tau fibrillation. Besides Gal3, pTau was sorted into EVs for transmission. Moreover, pTau markedly enhanced the number of EVs released by iMGL in a Gal3-dependent manner, suggesting a role of Gal3 in biogenesis of EVs. Single-cell RNA-Seq analysis of the hippocampus of a mouse model of tauopathy (THY-Tau22) revealed a group of pathogenic tau-evoked, Gal3-associated microglia with altered cellular machineries implicated in neurodegeneration, including enhanced immune and inflammatory responses. Genetic removal of Gal3 in THY-Tau22 mice suppressed microglia activation, reduced the level of pTau and synaptic loss in neurons, and rescued memory impairment. Collectively, Gal3 is a potential therapeutic target for tauopathy.

The O-GlcNAc Modification of Recombinant Tau Protein and Characterization of the O-GlcNAc Pattern for Functional Study

The neuronal microtubule-associated tau protein is characterized in vivo by a large number of post-translational modifications along the entire primary sequence that modulates its function. The primary modification of tau is phosphorylation of serine/threonine or tyrosine residues that is involved in the regulation of microtubule binding and polymerization. In neurodegenerative disorders referred to as tauopathies including Alzheimer's disease, tau is abnormally hyperphosphorylated and forms fibrillar inclusions in neurons progressing throughout different brain area during the course of the disease. The O-β-linked N-acetylglucosamine (O-GlcNAc) is another reversible post-translational modification of serine/threonine residues that is installed and removed by the unique O-GlcNAc transferase (OGT) and O-GlcNAc hydrolase (OGA), respectively. This modification was described as a potential modulator of tau phosphorylation and functions in the physiopathology. Moreover, reducing protein O-GlcNAc levels in the brain upon treatment of tauopathy mouse models with an OGA inhibitor reveals a beneficial effect on tau pathology and neurodegeneration. However, whether the role of tau O-GlcNAcylation is responsible of the protective effect against tau toxicity remains to be determined. The production of O-GlcNAc modified recombinant tau protein is a valuable tool for the investigations of the impact of O-GlcNAcylation on tau functions, modulation of interactions with partners and crosstalk with other post-translational modifications, including but not restricted to phosphorylation. We describe here the in vitro O-GlcNAcylation of tau with recombinant OGT for which we provide an expression and purification protocol. The use of the O-GlcNAc tau protein in functional studies requires the analytical characterization of the O-GlcNAc pattern. Here, we describe a method for the O-GlcNAc modification of tau protein with recombinant OGT and the analytical characterization of the resulting O-GlcNAc pattern by a combination of methods for the overall characterization of tau O-GlcNAcylation by chemoenzymatic labeling and mass spectrometry, as well as the quantitative, site-specific pattern by NMR spectroscopy.

Tau and Alzheimer's disease: Past, present and future

My journey with tau started when in 1974 for the first time I isolated neurofibrillary tangles of paired helical filaments (PHFs) from autopsied Alzheimer's disease (AD) brains and discovered that they were made up of a ~50-70 KDa protein on SDS-polyacrylamide gels. Subsequently my team discovered that this PHF protein and the microtubule-associated factor called tau were one and the same protein. However, we found that tau in neurofibrillary tangles/PHFs in AD brain was abnormally hyperphosphorylated, and unlike normal tau, which promoted the assembly of tubulin into microtubules, the AD-hyperphosphorylated tau inhibited microtubule assembly. These discoveries of tau pathology in AD opened a new and a major area of research on tau and on the molecular pathology of this major cause of dementia in middle- and old-age individuals. Tau pathology, which without fail is made up of the aggregated hyperphosphorylated state of the protein, is also the hallmark lesion of a family of around 20 related neurodegenerative diseases, called tauopathies. Currently, tau pathology is a major drug target for the treatment of AD and related tauopathies. Both active and passive tau immunization human clinical trials at various stages are underway. Initial results range from negative to partially promising. Future studies will reveal whether tau therapy alone or in combination with drugs targeting Aβ and/or neurodegeneration will be required to achieve the most effective treatment for AD and related disorders.

Traumatic brain injury derived pathological tau polymorphs induce the distinct propagation pattern and neuroinflammatory response in wild type mice

The misfolding and aggregation of the tau protein into neurofibrillary tangles constitutes a central feature of tauopathies. Traumatic brain injury (TBI) has emerged as a potential risk factor, triggering the onset and progression of tauopathies. Our previous research revealed distinct polymorphisms in soluble tau oligomers originating from single versus repetitive mild TBIs. However, the mechanisms orchestrating the dissemination of TBI brain-derived tau polymorphs (TBI-BDTPs) remain elusive. In this study, we explored whether TBI-BDTPs could initiate pathological tau formation, leading to distinct pathogenic trajectories. Wild-type mice were exposed to TBI-BDTPs from sham, single-blast (SB), or repeated-blast (RB) conditions, and their memory function was assessed through behavioral assays at 2- and 8-month post-injection. Our findings revealed that RB-BDTPs induced cognitive and motor deficits, concurrently fostering the emergence of toxic tau aggregates within the injected hippocampus. Strikingly, this tau pathology propagated to cortical layers, intensifying over time. Importantly, RB-BDTP-exposed animals displayed heightened glial cell activation, NLRP3 inflammasome formation, and increased TBI biomarkers, particularly triggering the aggregation of S100B, which is indicative of a neuroinflammatory response. Collectively, our results shed light on the intricate mechanisms underlying TBI-BDTP-induced tau pathology and its association with neuroinflammatory processes. This investigation enhances our understanding of tauopathies and their interplay with neurodegenerative and inflammatory pathways following traumatic brain injury.

Assays for the Screening and Characterization of Tau Aggregation Inhibitors

Aggregation of tau protein is a pathological hallmark of Alzheimer's disease and other neurodegenerative tauopathies. Inhibition of tau aggregation may provide a method for treatment of these disorders. Methods to identify tau aggregation inhibitors (TAIs) in vitro are useful and here we describe assays for TAIs using purified recombinant tau protein fragments in a cell-free immunoassay format and in a stably transfected cell model to create a more physiological environment.

Why kiss-and-hop explains that tau does not stabilize microtubules and does not interfere with axonal transport (at physiological conditions)

Tau is a microtubule-associated protein that is enriched in the axonal process of neurons. Post-translational modifications of tau have been implicated in the development of tauopathies characterized by defects in axonal transport, neuronal atrophy, and microtubule disassembly. Although tau is almost quantitatively bound to microtubules under physiological conditions, it does not significantly affect axonal transport. Furthermore, acute or chronic tau deficiency does not result in significant destabilization of neuronal microtubules, challenging the classical view that disease-related tau modifications directly cause axonal microtubule collapse. Here, we discuss how the rapid interaction kinetics of the tau-microtubule interaction, which we previously termed the kiss-and-hop interaction, explains why tau does not affect microtubule-dependent axonal transport but still allows tau to modulate microtubule polymerization. In contrast, tau modifications that slow down the kinetics of the tau-microtubule interaction and increase the residence time of tau at a microtubule interaction site can disrupt axonal transport and cause dendritic atrophy. We discuss the consequences of such a gain-of-toxicity mechanism in terms of the development of disease-modulating drugs that target the tau protein.

Differentiating SH-SY5Y Cells into Polarized Human Neurons for Studying Endogenous and Exogenous Tau Trafficking: Four Protocols to Obtain Neurons with Noradrenergic, Dopaminergic, and Cholinergic Properties

Pathological alterations of the neuronal Tau protein are characteristic for many neurodegenerative diseases, called tauopathies. To investigate the underlying mechanisms of tauopathies, human neuronal cell models are required to study Tau physiology and pathology in vitro. Primary rodent neurons are an often used model for studying Tau, but rodent Tau differs in sequence, splicing, and aggregation propensity, and rodent neuronal physiology cannot be compared to humans. Human-induced pluripotent stem cell (hiPSC)-derived neurons are expensive and time-consuming. Therefore, the human neuroblastoma SH-SY5Y cell line is a commonly used cell model in neuroscience as it combines convenient handling and low costs with the advantages of human-derived cells. Since naïve SH-SY5Y cells show little similarity to human neurons and almost no Tau expression, differentiation is necessary to obtain human-like neurons for studying Tau protein-related aspects of health and disease. As they express in principle all six Tau isoforms seen in the human brain, differentiated SH-SY5Y-derived neurons are suitable for investigating the human microtubule-associated protein Tau and, for example, its sorting and trafficking. Here, we describe and discuss a general cultivation procedure as well as four differentiation methods to obtain SH-SY5Y-derived neurons resembling noradrenergic, dopaminergic, and cholinergic properties, based on the treatment with retinoic acid (RA), brain-derived neurotrophic factor (BDNF), and 12-O-tetrade canoylphorbol-13-acetate (TPA). TPA and RA-/TPA-based protocols achieve differentiation efficiencies of 40-50% after 9 days of treatment. The highest differentiation efficiency (~75%) is accomplished by a combination of RA and BDNF; treatment only with RA is the most time-efficient method as ~50% differentiated cells can be obtained already after 7 days.

Genetic and sporadic forms of tauopathies-TAU as a disease driver for the majority of patients but the minority of tauopathies

Ageing-associated tauopathies like frontotemporal dementia (FTD), variants thereof (like progressive supranuclear palsy (PSP), pick diseases (PiD), corticobasal degeneration (CBD)), and of course the most prevalent form of dementia, Alzheimer Disease (AD), are widely recognized forms of tauopathies. The list of tauopathies is expanding. We now include: (i) tauopathies where the disease cause or trigger is clearly either physical, such as in Traumatic Brain Injury (TBI) or Chronic Traumatic Encephalopathy (CTE), and (ii) genetic diseases that result in tauopathy but have pathogenic genetic variants in genes not related to TAU. Examples of the latter are myotonic dystrophy Type 1 and Type 2 (DM1, DM2, due to pathogenic genetic variants in the genes DMPK and CNBP, respectively), Niemann-Pick Disease Type C (NPD, due to mutations in NPC1 or NPC2), Kufs Disease (CLN6), Christianson Syndrome (SLC9A6), familial forms of Parkinson Disease (PD), and many others. In terms of affected brain regions and cell types, intracellular distribution of TAU pathology/aggregates, age of disease onset, velocity of disease progression and spreading of TAU pathology, there is, however, little in common in most of these disease entities. Here, I reason that TAU/MAPT is causative for the minority of tauopathies (e.g., MAPT-related FTD/PSP and Vacuolar Tauopathy (VCP)) and a critical mediator for others, like shown by overwhelming evidence for AD. However, TAU may also be a mere bystander or even protective in other settings. Improved understanding of rare tauopathies is necessary to develop specific treatments, but also to improve our understanding of the pathomechanistic role of TAU and to identify diseases that may profit from TAU-based therapies.

Tau seeding without tauopathy

Neurodegenerative tauopathies such as Alzheimer's disease (AD) are caused by brain accumulation of tau assemblies. Evidence suggests tau functions as a prion, and cells and animals can efficiently propagate unique, transmissible tau pathologies. This suggests a dedicated cellular replication machinery, potentially reflecting a normal physiologic function for tau seeds. Consequently, we hypothesized that healthy control brains would contain seeding activity. We have recently developed a novel monoclonal antibody (MD3.1) specific for tau seeds. We used this antibody to immunopurify tau from the parietal and cerebellar cortices of 19 healthy subjects without any neuropathology, ranging 19 to 65 years. We detected seeding in lysates from the parietal cortex, but not in the cerebellum. We also detected no seeding in brain homogenates from wildtype or human tau knockin mice, suggesting that cellular/genetic context dictates development of seed-competent tau. Seeding did not correlate with subject age or brain tau levels. We confirmed our essential findings using an orthogonal assay, real-time quaking-induced conversion, which amplifies tau seeds in vitro. Dot blot analyses revealed no AT8 immunoreactivity above background levels in parietal and cerebellar extracts and ∼1/100 of that present in AD. Based on binding to a panel of antibodies, the conformational characteristics of control seeds differed from AD, suggesting a unique underlying assembly, or structural ensemble. Tau's ability to adopt self-replicating conformations under nonpathogenic conditions may reflect a normal function that goes awry in disease states.

Isolation and Detection of Pathological Tau Species in a Tauopathy Mouse Model

Tau protein in Alzheimer's disease (AD) and tauopathies becomes insoluble due to hyperphosphorylation, conformational alterations, and aggregation. To analyze insoluble tau and pathological tau species, this study employs a methodology that utilizes wild-type and transgenic tau mice (P310S Tau) tissue extraction using 1% Sarkosyl or N-Lauroylsarcosine sodium salt and the radio immunoprecipitation assay (RIPA) buffer. However, the commonly used methods to study the insoluble tau fraction using detergents like Sarkosyl and RIPA require a large amount of homogenate, which can pose challenges when dealing with small tissue samples. Additionally, the study employs immunohistochemistry to visualize and quantify the pathological tau species in the brain tissue of transgenic mice, aiming to identify and analyze pathological tau species such as hyperphosphorylated tau to further our understanding of tauopathies such as Alzheimer's disease.

Quantification of Methylation and Phosphorylation Stoichiometry

Tauopathies including Alzheimer's disease (AD) are neurodegenerative disorders accompanied by the conversion of functional forms of the microtubule associated protein Tau into non-functional aggregates. A variety of post-translational modifications (PTMs) on Tau precede or accompany the conversion, placing them in position to modulate Tau function as well as its propensity to aggregate. Although Tau PTMs can be characterized by their sites of modification, their total stoichiometry when summed over all sites also is an important metric of their potential impact on function. Here we provide a protocol for rapidly producing recombinant Tau with enzyme-specific PTMs at high stoichiometry in vitro and demonstrate its utility in the context of hyperphosphorylation. Additionally, protocols for estimating phosphorylation and methylation stoichiometry on Tau proteins isolated from any source are presented. Together these methods support experimentation on Tau PTM function over a wide range of experimental conditions.

Studying Microtubule Dynamics in Human Neurons: Two-Dimensional Microtubule Tracing and Kymographs in iPSC- and SH-SY5Y-Derived Neurons for Tau Research

The study of microtubule (MT) dynamics is essential for the understanding of cellular transport, cell polarity, axon formation, and other neurodevelopmental mechanisms. All these processes rely on the constant transition between assembly and disassembly of tubulin polymers to/from MTs, known as dynamic instability. This process is well-regulated, among others, by phosphorylation of microtubule-associated proteins (MAP), including the Tau protein. Protein kinases, in particular the microtubule affinity regulating kinase (MARK), regulate the MT-Tau interaction, inducing Tau dissociation by phosphorylation. Phosphorylated Tau dissociates from microtubules forming insoluble aggregates known as neurofibrillary tangles. These accumulations of hyperphosphorylated Tau in the neurons disrupt the physiological MT-based transport machinery within the cell and can potentially lead to the development of neurodegenerative disorders, such as Alzheimer's disease (AD) and related tauopathies. Further investigations on the MT cytoskeleton dynamics are essential as they may elucidate pathomechanisms of neurodegenerative diseases - particularly tauopathies - as well as fundamental neurodevelopmental processes.The study of the dynamic assembly and disassembly of the MT network requires live-cell imaging rather than conventional immunocytochemistry based on fixed samples. To investigate MT dynamics, we perform live-cell imaging of neurons transfected with a fluorescently tagged version of the microtubule plus-end tracking protein (+TIP) EB3. This protein associates with the growing ends of MTs and thus visualizes MT growth in real time. Our imaging analysis protocol allows the determination of quantity, orientation, and velocity of MT growth in the soma and neurites of transfected neurons, using ImageJ-based tracking software and kymographs. Furthermore, functional effects of Tau and MARK kinases on the MT cytoskeleton can be assessed by overexpression or downregulation experiments of the respective protein prior to the live imaging assay. We use two different human neuronal cell models, naive and differentiated SH-SY5Y neuroblastoma cells, and neurons derived from induced pluripotent stem cells (iPSCs), both of which have shown success as models to study Tau-related pathologies.This protocol describes an optimized method for analysis of microtubule dynamics using fluorescent tagged EB3 protein as microtubule plus end marker. In this chapter, we outline the process of neuronal transfection, live-cell imaging, and necessary time-lapse image analysis based on ImageJ in two human-derived neuronal systems, which are suitable for the analysis of Tau trafficking and sorting studies.

Travels with tau prions

Tau was originally identified as a microtubule associated protein, and subsequently recognized to constitute the fibrillar assemblies found in Alzheimer disease and related neurodegenerative tauopathies. Point mutations in the microtubule associated protein tau (MAPT) gene cause dominantly inherited tauopathies, and most predispose it to aggregate. This indicates tau aggregation underlies pathogenesis of tauopathies. Our work has suggested that tau functions as a prion, forming unique intracellular pathological assemblies that subsequently move to other cells, inducing further aggregation that underlies disease progression. Remarkably, in simple cells tau forms stably propagating aggregates of distinct conformation, termed strains. Each strain induces a unique and, in some cases, transmissible, neuropathological phenotype upon inoculation into a mouse model. After binding heparan sulfate proteoglycans on the plasma membrane, tau assemblies enter cells via macropinocytosis. From within a vesicle, if not trafficked to the endolysosomal system, tau subsequently enters the cytoplasm, where it becomes a template for its own replication, apparently after processing by valosin containing protein. The smallest seed unit is a stable monomer, which suggests that initial folding events in tau presage subsequent pathological aggregation. The study of tau prions has raised important questions about basic cell biological processes that underlie their replication and propagation, with implications for therapy of tauopathies.

Big tau: What, how, where and why

Over the last 50 years the different isoforms of tau proteins (45-60 kDa) have been a focus of research because of their roles in modulating the dynamic properties of microtubules shaping the structure and function of neurons but also becoming a center of attention in the pathology of neurodegeneration associated with tauopathies. Much less attention has been given to Big tau, a unique isoform containing exon 4a encoding about 250 amino acids to form a much longer projection domain of a protein of 110 kDa. Big tau is expressed in peripheral neurons and selective regions of the central nervous system in a defined transition during postnatal developmental stages. Although Big tau was discovered 30 years ago, there has been a persistent gap of knowledge regarding its physiological properties and pathological implications. This Perspective summarizes the progress so far in defining the structure and expression of Big tau within and outside the nervous system, proposes a role for Big tau in improving axonal transport in projecting axons, considers its potential in averting tau aggregation in tauopathies and highlights the need for further progress.

TFEB-vacuolar ATPase signaling regulates lysosomal function and microglial activation in tauopathy

Transcription factor EB (TFEB) mediates gene expression through binding to the coordinated lysosome expression and regulation (CLEAR) sequence. TFEB targets include subunits of the vacuolar ATPase (v-ATPase), which are essential for lysosome acidification. Single-nucleus RNA sequencing of wild-type and PS19 (Tau) transgenic mice expressing the P301S mutant tau identified three unique microglia subclusters in Tau mice that were associated with heightened lysosome and immune pathway genes. To explore the lysosome-immune relationship, we specifically disrupted the TFEB-v-ATPase signaling by creating a knock-in mouse line in which the CLEAR sequence of one of the v-ATPase subunits, Atp6v1h, was mutated. CLEAR mutant exhibited a muted response to TFEB, resulting in impaired lysosomal acidification and activity. Crossing the CLEAR mutant with Tau mice led to higher tau pathology but diminished microglia response. These microglia were enriched in a subcluster low in mTOR and HIF-1 pathways and were locked in a homeostatic state. Our studies demonstrate a physiological function of TFEB-v-ATPase signaling in maintaining lysosomal homeostasis and a critical role of the lysosome in mounting a microglia and immune response in tauopathy and Alzheimer's disease.

Suspecting Non-Alzheimer's Pathologies and Mixed Pathologies: A Comparative Study Between Brain Metabolism and Tau Images

BACKGROUND

Alzheimer's disease (AD) pathology can be disclosed in vivo using amyloid and tau imaging, unlike non-AD neuropathologies for which no specific markers exist.

OBJECTIVE

We aimed to compare brain hypometabolism and tauopathy to unveil non-AD pathologies.

METHODS

Sixty-one patients presenting cognitive complaints (age 48-90), including 32 with positive AD biomarkers (52%), performed [18F]-Fluorodeoxyglucose (FDG)-PET (brain metabolism) and [18F]-MK-6240-PET (tau). We normalized these images using data from clinically normal individuals (n = 30), resulting in comparable FDG and tau z-scores. We computed between-patients correlations to evaluate regional associations. For each patient, a predominant biomarker (i.e., Hypometabolism > Tauopathy or Hypometabolism≤Tauopathy) was determined in the temporal and frontoparietal lobes. We computed within-patient correlations between tau and metabolism and investigated their associations with demographics, cognition, cardiovascular risk factors (CVRF), CSF biomarkers, and white matter hypointensities (WMH).

RESULTS

We observed negative associations between tau and FDG in 37 of the 68 cortical regions-of-interest (average Pearson's r = -0.25), mainly in the temporal lobe. Thirteen patients (21%) had Hypometabolism > Tauopathy whereas twenty-five patients (41%) had Hypometabolism≤Tauopathy. Tau-predominant patients were more frequently females and had greater amyloid burden. Twenty-three patients (38%) had Hypometabolism≤Tauopathy in the temporal lobe, but Hypometabolism > Tauopathy in the frontoparietal lobe. This group was older and had higher CVRF than Tau-predominant patients. Patients with more negative associations between tau and metabolism were younger, had worse cognition, and greater amyloid and WMH burdens.

CONCLUSIONS

Tau-FDG comparison can help suspect non-AD pathologies in patients presenting cognitive complaints. Stronger Tau-FDG correlations are associated with younger age, worse cognition, and greater amyloid and WMH burdens.

Western Blot of Tau Protein from Mouse Brains Extracts: How to Avoid Signal Artifacts

Tau is a microtubule-associated protein enriched in the axonal compartment. Its most well-known function is to bind and stabilize microtubules. In Alzheimer's disease and other neurodegenerative diseases known as tauopathies, tau undergoes several abnormal post-translational modifications including hyperphosphorylation, conformational changes, oligomerization, and aggregation. Numerous mouse models of tauopathies have been developed, and Western blotting remains an invaluable tool in studying tau protein physiological and pathological changes in these models. However, many of the antibodies that have been developed to analyze tau post-translational modifications are mouse monoclonal, which are at risk of producing artifactual signals in Western blotting procedures. This risk does not arise due to their lack of specificity, but rather because the secondary antibodies used to detect them will also react with the heavy chain of endogenous mouse immunoglobulins (Igs), leading to a non-specific signal at the same molecular weight as tau protein (around 50 kDa). Here, we present the use of anti-light-chain secondary antibodies as a simple and efficient technique to prevent non-specific Ig signals around 50 kDa. We demonstrate the efficacy of this method by either eliminating or identifying artifactual signals when using monoclonal antibodies directed at non-phosphorylated epitopes (T49, Tau3R, Tau4R), phosphorylated epitopes (MC6, AT180, CP13), or an abnormal tau conformation (MC1), in wild-type (WT) mice with tau hyperphosphorylation (hypothermic), transgenic mice overexpressing human tau (hTau mice), and tau knockout (TKO) mice.

Methods for Biochemical Isolation of Insoluble Tau in Rodent Models of Tauopathies

The intracellular accumulation of microtubule-associated protein tau is a characteristic feature of tauopathies, a group of neurodegenerative diseases including Alzheimer's disease. Formation of insoluble tau aggregates is initiated by the abnormal hyperphosphorylation and oligomerization of tau. Over the past decades, multiple transgenic rodent models mimicking tauopathies have been develop, showcasing this neuropathological hallmark. The biochemical analysis of insoluble tau in these models has served as a valuable tool to understand the progression of tau-related pathology. In this chapter, we provide a comprehensive review of the two primary methods for isolating insoluble tau, namely, sarkosyl and formic acid extraction (and their variants), which are employed for biochemical analysis in transgenic mouse models of tauopathy. We also analyze the strengths and limitations of these methods.