Dynamic association of tau with neuronal membranes is regulated by phosphorylation

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.

Fyn Kinase in Alzheimer's Disease: Unraveling Molecular Mechanisms and Therapeutic Implications

Alzheimer's disease, characterized by the accumulation of abnormal protein aggregates and neuronal damage in the brain, leads to a gradual decline in cognitive function and memory. As a complex neurodegenerative disorder, it involves disruptions in various biochemical pathways and neurotransmitter systems, contributing to the progressive loss of neurons and synaptic connections. The complexity of Alzheimer's signaling pathways complicates treatment, presenting a formidable challenge in the quest for effective therapeutic interventions. A member of the Src family of kinases (SFKs), Fyn, is a type of non-receptor tyrosine kinase that has been linked to multiple essential CNS processes, such as myelination and synaptic transmission. Fyn is an appealing target for AD treatments because it is uniquely linked to the two major pathologies in AD by its interaction with tau, in addition to being activated by amyloid-beta (Aβ) through PrPC. Fyn mediates neurotoxicity and synaptic impairments caused by Aβ and is involved in regulating the process of Aβ synthesis.Additionally, the tau protein's tyrosine phosphorylation is induced by Fyn. Fyn is also a challenging target because of its widespread body expression and strong homology with other kinases of the Src family, which could cause unintentional off-target effects. This review emphasizes signaling pathways mediated by Fyn that govern neuronal development and plasticity while also summarizing the most noteworthy recent research relevant to Fyn kinase's function in the brain. Additionally, the therapeutic inhibition of Fyn kinase has been discussed, with a focus on the Fyn kinase inhibitors that are in clinical trials, which presents a fascinating opportunity for targeting Fyn kinase in the creation of possible therapeutic approaches for the management of Alzheimer's disease.

Unknown roles of tau pathology in neurological disorders. Challenges and new perspectives

Aging presents progressive changes that increase the susceptibility of the central nervous system (CNS) to suffer neurological disorders (NDs). Several studies have reported that an aged brain suffering from NDs shows the presence of pathological forms of tau protein, a microtubule accessory protein (MAP) critical for neuronal function. In this context, accumulative evidence has shown a pivotal contribution of pathological forms of tau to Alzheimer's disease (AD) and tauopathies. However, current investigations have implicated tau toxicity in other NDs that affect the central nervous system (CNS), including Parkinson's disease (PD), Huntington's disease (HD), Traumatic brain injury (TBI), Multiple sclerosis (MS), and Amyotrophic lateral sclerosis (ALS). These diseases are long-term acquired, affecting essential functions such as motor movement, cognition, hearing, and vision. Previous evidence indicated that toxic forms of tau do not have a critical contribution to the genesis or progression of these diseases. However, recent studies have shown that these tau forms contribute to neuronal dysfunction, inflammation, oxidative damage, and mitochondrial impairment events that contribute to the pathogenesis of these NDs. Recent studies have suggested that these neuropathologies could be associated with a prion-like behavior of tau, which induces a pathological dissemination of these toxic protein forms to different brain areas. Moreover, it has been suggested that this toxic propagation of tau from neurons into neighboring cells impairs the function of glial cells, oligodendrocytes, and endothelial cells by affecting metabolic function and mitochondrial health and inducing oxidative damage by tau pathology. Therefore, in this review, we will discuss current evidence demonstrating the critical role of toxic tau forms on NDs not related to AD and how its propagation and induced-bioenergetics failure may contribute to the pathogenic mechanism present in these NDs.

Interaction of full-length Tau with negatively charged lipid membranes leads to polymorphic aggregates

The Tau protein is implicated in various diseases collectively known as tauopathies, including Alzheimer's disease and frontotemporal dementia. The precise mechanism underlying Tau pathogenicity remains elusive. Recently, the role of lipids has garnered interest due to their implications in Tau aggregation, secretion, uptake, and pathogenic dysregulation. Previous investigations have highlighted critical aspects: (i) Tau's tendency to aggregate into fibers when interacting with negatively charged lipids, (ii) its ability to form structured species upon contact with anionic membranes, and (iii) the potential disruption of the membrane upon Tau binding. In this study, we examine the disease-associated P301L mutation of the 2N4R isoform of Tau and its effects on membranes composed on phosphatidylserine (PS) lipids. Aggregation studies and liposome leakage assays demonstrate Tau's ability to bind to anionic lipid vesicles, leading to membrane disruption. Attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) reveals the accumulation of Tau on the membrane surface without protein insertion, structuration, or lipid removal. Plasmon waveguide resonance (PWR) demonstrates a strong binding of Tau on PS bilayers with an apparent Kd in the micromolar range, indicating the deposition of a thick protein layer. Atomic force microscopy (AFM) real-time imaging allows the observation of partial lipid solubilization and the deposition of polymorphic aggregates in the form of thick patches and fibrillary structures resembling amyloid fibers, which could grow from a combination of extracted anionic phospholipids from the membrane and Tau protein. This study deepens our understanding of full-length Tau's multifaceted interactions with lipids, shedding light on potential mechanisms leading to the formation of pathogenic Tau assemblies.

Serine 31 Phosphorylation-Driven Regulation of AGPase Activity: Potential Implications for Enhanced Starch Yields in Crops

ADP-Glc pyrophosphorylase (AGPase), which catalyzes the transformation of ATP and glucose-1-phosphate (Glc-1-P) into adenosine diphosphate glucose (ADP-Glc), acts as a rate-limiting enzyme in crop starch biosynthesis. Prior research has hinted at the regulation of AGPase by phosphorylation in maize. However, the identification and functional implications of these sites remain to be elucidated. In this study, we identified the phosphorylation site (serine at the 31st position of the linear amino acid sequence) of the AGPase large subunit (Sh2) using iTRAQTM. Subsequently, to ascertain the impact of Sh2 phosphorylation on AGPase, we carried out site-directed mutations creating Sh2-S31A (serine residue replaced with alanine) to mimic dephosphorylation and Sh2-S31D (serine residue replaced with aspartic acid) or Sh2-S31E (serine residue replaced with glutamic acid) to mimic phosphorylation. Preliminary investigations were performed to determine Sh2 subcellular localization, its interaction with Bt2, and the resultant AGPase enzymatic activity. Our findings indicate that phosphorylation exerts no impact on the stability or localization of Sh2. Furthermore, none of these mutations at the S31 site of Sh2 seem to affect its interaction with Bt2 (smaller subunit). Intriguingly, all S31 mutations in Sh2 appear to enhance AGPase activity when co-transfected with Bt2, with Sh2-S31E demonstrating a substantial five-fold increase in AGPase activity compared to Sh2. These novel insights lay a foundational groundwork for targeted improvements in AGPase activity, thus potentially accelerating the production of ADP-Glc (the primary substrate for starch synthesis), promising implications for improved starch biosynthesis, and holding the potential to significantly impact agricultural practices.

Calcium Dyshomeostasis Drives Pathophysiology and Neuronal Demise in Age-Related Neurodegenerative Diseases

This review postulates that age-related neurodegeneration entails inappropriate activation of intrinsic pathways to enable brain plasticity through deregulated calcium (Ca2+) signalling. Ca2+ in the cytosol comprises a versatile signal controlling neuronal cell physiology to accommodate adaptive structural and functional changes of neuronal networks (neuronal plasticity) and, as such, is essential for brain function. Although disease risk factors selectively affect different neuronal cell types across age-related neurodegenerative diseases (NDDs), these appear to have in common the ability to impair the specificity of the Ca2+ signal. As a result, non-specific Ca2+ signalling facilitates the development of intraneuronal pathophysiology shared by age-related NDDs, including mitochondrial dysfunction, elevated reactive oxygen species (ROS) levels, impaired proteostasis, and decreased axonal transport, leading to even more Ca2+ dyshomeostasis. These core pathophysiological processes and elevated cytosolic Ca2+ levels comprise a self-enforcing feedforward cycle inevitably spiralling toward high levels of cytosolic Ca2+. The resultant elevated cytosolic Ca2+ levels ultimately gear otherwise physiological effector pathways underlying plasticity toward neuronal demise. Ageing impacts mitochondrial function indiscriminately of the neuronal cell type and, therefore, contributes to the feedforward cycle of pathophysiology development seen in all age-related NDDs. From this perspective, therapeutic interventions to safely restore Ca2+ homeostasis would mitigate the excessive activation of neuronal destruction pathways and, therefore, are expected to have promising neuroprotective potential.

Traumatic brain injury and the pathways to cerebral tau accumulation

Tau is a protein that has received national mainstream recognition for its potential negative impact to the brain. This review succinctly provides information on the structure of tau and its normal physiological functions, including in hibernation and changes throughout the estrus cycle. There are many pathways involved in phosphorylating tau including diabetes, stroke, Alzheimer's disease (AD), brain injury, aging, and drug use. The common mechanisms for these processes are put into context with changes observed in mild and repetitive mild traumatic brain injury (TBI). The phosphorylation of tau is a part of the progression to pathology, but the ability for tau to aggregate and propagate is also addressed. Summarizing both the functional and dysfunctional roles of tau can help advance our understanding of this complex protein, improve our care for individuals with a history of TBI, and lead to development of therapeutic interventions to prevent or reverse tau-mediated neurodegeneration.

The Implication of Glial Metabotropic Glutamate Receptors in Alzheimer's Disease

Alzheimer's disease (AD) was first identified more than 100 years ago, yet aspects pertaining to its origin and the mechanisms underlying disease progression are not well known. To this date, there is no therapeutic approach or disease-modifying drug that could halt or at least delay disease progression. Until recently, glial cells were seen as secondary actors in brain homeostasis. Although this view was gradually refuted and the relevance of glial cells for the most diverse brain functions such as synaptic plasticity and neurotransmission was vastly proved, many aspects of its functioning, as well as its role in pathological conditions, remain poorly understood. Metabotropic glutamate receptors (mGluRs) in glial cells were shown to be involved in neuroinflammation and neurotoxicity. Besides its relevance for glial function, glutamatergic receptors are also central in the pathology of AD, and recent studies have shown that glial mGluRs play a role in the establishment and progression of AD. AD-related alterations in Ca2+ signalling, APP processing, and Aβ load, as well as AD-related neurodegeneration, are influenced by glial mGluRs. However, different types of mGluRs play different roles, depending on the cell type and brain region that is being analysed. Therefore, in this review, we focus on the current understanding of glial mGluRs and their implication in AD, providing an insight for future therapeutics and identifying existing research gaps worth investigating.

Amyloids on Membrane Interfaces: Implications for Neurodegeneration

Membrane interfaces are vital for various cellular processes, and their involvement in neurodegenerative disorders such as Alzheimer's and Parkinson's disease has taken precedence in recent years. The amyloidogenic proteins associated with neurodegenerative diseases interact with the neuronal membrane through various means, which has implications for both the onset and progression of the disease. The parameters that regulate the interaction between the membrane and the amyloids remain poorly understood. The review focuses on the various aspects of membrane interactions of amyloids, particularly amyloid-β (Aβ) peptides and Tau involved in Alzheimer's and α-synuclein involved in Parkinson's disease. The genetic, cell biological, biochemical, and biophysical studies that form the basis for our current understanding of the membrane interactions of Aβ peptides, Tau, and α-synuclein are discussed.

Dual role of brain-derived extracellular vesicles in dementia-related neurodegenerative disorders: cargo of disease spreading signals and diagnostic-therapeutic molecules

Neurodegenerative disorders are one of the most common causes of disability and represent 6.3% of the global burden of disease. Among them, Alzheimer's, Parkinson's, and Huntington's diseases cause cognitive decline, representing the most disabling symptom on both personal and social levels. The molecular mechanisms underlying the onset and progression of dementia are still poorly understood, and include secretory factors potentially affecting differentiated neurons, glial cells and neural stem cell niche. In the last decade, much attention has been devoted to exosomes as novel carriers of information exchanged among both neighbouring and distant cells. These vesicles can be generated and internalized by different brain cells including neurons, neural stem cells, astrocytes, and microglia, thereby affecting neural plasticity and cognitive functions in physiological and pathological conditions. Here, we review data on the roles of exosomes as carriers of bioactive molecules potentially involved in the pathogenesis of neurodegenerative disorders and detectable in biological fluids as biomarkers of dementia. We also discuss the experimental evidence of the therapeutic potential of stem cell-derived vesicles in experimental models of neurodegeneration-dependent cognitive decline.

Single-molecule imaging reveals Tau trapping at nanometer-sized dynamic hot spots near the plasma membrane that persists after microtubule perturbation and cholesterol depletion

Accumulation of aggregates of the microtubule‐binding protein Tau is a pathological hallmark of Alzheimer's disease. While Tau is thought to primarily associate with microtubules, it also interacts with and localizes to the plasma membrane. However, little is known about how Tau behaves and organizes at the plasma membrane of live cells. Using quantitative, single‐molecule imaging, we show that Tau exhibits spatial and kinetic heterogeneity near the plasma membrane of live cells, resulting in the formation of nanometer‐sized hot spots. The hot spots lasted tens of seconds, much longer than the short dwell time (∼ 40 ms) of Tau on microtubules. Pharmacological and biochemical disruption of Tau/microtubule interactions did not prevent hot spot formation, suggesting that these are different from the reported Tau condensation on microtubules. Although cholesterol removal has been shown to reduce Tau pathology, its acute depletion did not affect Tau hot spot dynamics. Our study identifies an intrinsic dynamic property of Tau near the plasma membrane that may facilitate the formation of assembly sites for Tau to assume its physiological and pathological functions.

Anesthesia promotes acute expression of genes related to Alzheimer's disease and latent tau aggregation in transgenic mouse models of tauopathy

Background

Exposure to anesthesia in the elderly might increase the risk of dementia. Although the mechanism underlying the association is uncertain, anesthesia has been shown to induce acute tau hyperphosphorylation in preclinical models. We sought to investigate the impact of anesthesia on gene expression and on acute and long-term changes in tau biochemistry in transgenic models of tauopathy in order to better understand how anesthesia influences the pathophysiology of dementia.

Methods

We exposed mice with over-expressed human mutant tau (P301L and hyperdopaminergic COMTKO/P301L) to two hours of isoflurane and compared anesthetized mice to controls at several time points. We evaluated tau hyperphosphorylation with quantitative high-sensitivity enzyme-linked immunosorbent assay and performed differential expression and functional transcriptome analyses following bulk mRNA-sequencing.

Results

Anesthesia induced acute hyperphosphorylation of tau at epitopes related to Alzheimer’s disease (AD) in both P301L-based models. Anesthesia was associated with differential expression of genes in the neurodegenerative pathways (e.g., AD-risk genes ApoE and Trem2) and thermogenesis pathway, which is related to both mammalian hibernation and tau phosphorylation. One and three months after anesthesia, hyperphosphorylated tau aggregates were increased in the anesthetized mice.

Conclusions

Anesthesia may influence the expression of AD-risk genes and induce biochemical changes in tau that promote aggregation even after single exposure. Further preclinical and human studies are necessary to establish the relevance of our transcriptomic and biochemical findings in these preclinical models to the pathogenesis of dementia following anesthesia.

Trial registration: Not applicable.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10020-022-00506-4.

What's in a Gene? The Outstanding Diversity of MAPT

Tau protein is a microtubule-associated protein encoded by the MAPT gene that carries out a myriad of physiological functions and has been linked to certain pathologies collectively termed tauopathies, including Alzheimer's disease, frontotemporal dementia, Huntington's disease, progressive supranuclear palsy, etc. Alternative splicing is a physiological process by which cells generate several transcripts from one single gene and may in turn give rise to different proteins from the same gene. MAPT transcripts have been proven to be subjected to alternative splicing, generating six main isoforms in the central nervous system. Research throughout the years has demonstrated that the splicing landscape of the MAPT gene is far more complex than that, including at least exon skipping events, the use of 3' and 5' alternative splice sites and, as has been recently discovered, also intron retention. In addition, MAPT alternative splicing has been showed to be regulated spatially and developmentally, further evidencing the complexity of the gene's splicing regulation. It is unclear what would drive the need for the existence of so many isoforms encoded by the same gene, but a wide range of functions have been ascribed to these Tau isoforms, both in physiology and pathology. In this review we offer a comprehensive up-to-date exploration of the mechanisms leading to the outstanding diversity of isoforms expressed from the MAPT gene and the functions in which such isoforms are involved, including their potential role in the onset and development of tauopathies such as Alzheimer's disease.

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

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

Tau and Membranes: Interactions That Promote Folding and Condensation

Tau misfolding and assembly is linked to a number of neurodegenerative diseases collectively described as tauopathies, including Alzheimer’s disease (AD) and Parkinson’s disease. Anionic cellular membranes, such as the cytosolic leaflet of the plasma membrane, are sites that concentrate and neutralize tau, primarily due to electrostatic interactions with tau’s microtubule binding repeat domain (RD). In addition to electrostatic interactions with lipids, tau also has interactions with membrane proteins, which are important for tau’s cellular functions. Tau also interacts with lipid tails to facilitate direct translocation across the membrane and can form stable protein-lipid complexes involved in cell-to-cell transport. Concentrated tau monomers at the membrane surface can form reversible condensates, change secondary structures, and induce oligomers, which may eventually undergo irreversible crosslinking and fibril formation. These β-sheet rich tau structures are capable of disrupting membrane organization and are toxic in cell-based assays. Given the evidence for relevant membrane-based tau assembly, we review the emerging hypothesis that polyanionic membranes may serve as a site for phase-separated tau condensation. Membrane-mediated phase separation may have important implications for regulating tau folding/misfolding, and may be a powerful mechanism to spatially direct tau for native membrane-mediated functions.

Melatonin: Regulation of Biomolecular Condensates in Neurodegenerative Disorders

Biomolecular condensates are membraneless organelles (MLOs) that form dynamic, chemically distinct subcellular compartments organizing macromolecules such as proteins, RNA, and DNA in unicellular prokaryotic bacteria and complex eukaryotic cells. Separated from surrounding environments, MLOs in the nucleoplasm, cytoplasm, and mitochondria assemble by liquid-liquid phase separation (LLPS) into transient, non-static, liquid-like droplets that regulate essential molecular functions. LLPS is primarily controlled by post-translational modifications (PTMs) that fine-tune the balance between attractive and repulsive charge states and/or binding motifs of proteins. Aberrant phase separation due to dysregulated membrane lipid rafts and/or PTMs, as well as the absence of adequate hydrotropic small molecules such as ATP, or the presence of specific RNA proteins can cause pathological protein aggregation in neurodegenerative disorders. Melatonin may exert a dominant influence over phase separation in biomolecular condensates by optimizing membrane and MLO interdependent reactions through stabilizing lipid raft domains, reducing line tension, and maintaining negative membrane curvature and fluidity. As a potent antioxidant, melatonin protects cardiolipin and other membrane lipids from peroxidation cascades, supporting protein trafficking, signaling, ion channel activities, and ATPase functionality during condensate coacervation or dissolution. Melatonin may even control condensate LLPS through PTM and balance mRNA- and RNA-binding protein composition by regulating N6-methyladenosine (m6A) modifications. There is currently a lack of pharmaceuticals targeting neurodegenerative disorders via the regulation of phase separation. The potential of melatonin in the modulation of biomolecular condensate in the attenuation of aberrant condensate aggregation in neurodegenerative disorders is discussed in this review.

Non-Rodent Genetic Animal Models for Studying Tauopathy: Review of Drosophila, Zebrafish, and C. elegans Models

Tauopathy refers to a group of progressive neurodegenerative diseases, including frontotemporal lobar degeneration and Alzheimer's disease, which correlate with the malfunction of microtubule-associated protein Tau (MAPT) due to abnormal hyperphosphorylation, leading to the formation of intracellular aggregates in the brain. Despite extensive efforts to understand tauopathy and develop an efficient therapy, our knowledge is still far from complete. To find a solution for this group of devastating diseases, several animal models that mimic diverse disease phenotypes of tauopathy have been developed. Rodents are the dominating tauopathy models because of their similarity to humans and established disease lines, as well as experimental approaches. However, powerful genetic animal models using Drosophila, zebrafish, and C. elegans have also been developed for modeling tauopathy and have contributed to understanding the pathophysiology of tauopathy. The success of these models stems from the short lifespans, versatile genetic tools, real-time in-vivo imaging, low maintenance costs, and the capability for high-throughput screening. In this review, we summarize the main findings on mechanisms of tauopathy and discuss the current tauopathy models of these non-rodent genetic animals, highlighting their key advantages and limitations in tauopathy research.

Insights Into Parkin-Mediated Mitophagy in Alzheimer's Disease: A Systematic Review

Background: Parkin-mediated mitophagy is the dominant mitophagy pathway of neural cells. Its restoration will result in prevention of cognitive decline, including Alzheimer's disease (AD). The role of this mitophagy pathway in neurodegenerative diseases has drawn attention in recent years. The two main pathological proteins in AD, amyloid β (Aβ) and human Tau (hTau), interfere with mitochondrial dynamics through several pathways. However, taking into consideration the specific interactions between Aβ/hTau and Parkin, special focus is required on this mitophagy pathway and AD. In this review, these interactions are fully discussed, and an overview of the neuroprotective drugs that enhance Parkin-mediated mitophagy is presented. Methods: This systematic review was performed according to PRISMA guidelines, and a comprehensive literature search was done in the electronic databases up to September 2020, using search terms in the titles and abstracts to identify relevant studies. One hundred eighty-six articles were found, and 113 articles were screened by title and abstract. Finally, 25 articles were included in this systematic review according to our inclusion and exclusion criteria. Results: Accumulation of Aβ and hTau affects mitophagy, including Parkin-mediated. Tau seems to prevent Parkin translocation directly. A Parkin level in the cell appears to be of importance in determining the damage caused by Aβ and hTau and in the future therapeutic approaches. Parkin controls the PINK1 level via the presenillins, suggesting that mutations in presenillins affect Parkin mitophagy. Significance: Parkin mitophagy is a process affected by several AD pathological events multidimensionally.

Tau secretion and propagation: Perspectives for potential preventive interventions in Alzheimer's disease and other tauopathies

Alzheimer's disease (AD) is characterised by the accumulation of intracytoplasmic aggregates of tau protein, which are suggested to spread in a prion-like manner between interconnected brain regions. This spreading is mediated by the secretion and uptake of tau from the extracellular space or direct cell-to-cell transmission through cellular protrusions. The prion-like tau then converts the endogenous, normal tau into pathological forms, resulting in neurodegeneration. The endoplasmic reticulum/Golgi-independent tau secretion through unconventional secretory pathways involves delivering misfolded and aggregated tau to the plasma membrane and its release into the extracellular space by non-vesicular and vesicular mechanisms. Although cytoplasmic tau was thought to be released only from degenerating cells, studies now show that cells constitutively secrete tau at low levels under physiological conditions. The mechanisms of secretion of tau under physiological and pathological conditions remain unclear. Therefore, a better understanding of these pathways is essential for developing therapeutic approaches that can target prion-like tau forms to prevent neurodegeneration progression in AD. This review focuses on unconventional secretion pathways involved in the spread of tau pathology in AD and presents these pathways as prospective areas for future AD drug discovery and development.

Role of the Lipid Membrane and Membrane Proteins in Tau Pathology

Abnormal accumulation of misfolded tau aggregates is a pathological hallmark of various tauopathies including Alzheimer’s disease (AD). Although tau is a cytosolic microtubule-associated protein enriched in neurons, it is also found in extracellular milieu, such as interstitial fluid, cerebrospinal fluid, and blood. Accumulating evidence showed that pathological tau spreads along anatomically connected areas in the brain through intercellular transmission and templated misfolding, thereby inducing neurodegeneration and cognitive dysfunction. In line with this, the spatiotemporal spreading of tau pathology is closely correlated with cognitive decline in AD patients. Although the secretion and uptake of tau involve multiple different pathways depending on tau species and cell types, a growing body of evidence suggested that tau is largely secreted in a vesicle-free forms. In this regard, the interaction of vesicle-free tau with membrane is gaining growing attention due to its importance for both of tau secretion and uptake as well as aggregation. Here, we review the recent literature on the mechanisms of the tau-membrane interaction and highlights the roles of lipids and proteins at the membrane in the tau-membrane interaction as well as tau aggregation.

Astrocytic Propagation of Tau in the Context of Alzheimer's Disease

More than 6 million Americans are currently living with Alzheimer's disease (AD), and the incidence is growing rapidly with our aging population. Numerous therapeutics have failed to make it to the clinic, potentially due to a focus on presumptive pathogenic proteins instead of cell-type-specific signaling mechanisms. The tau propagation hypothesis that inter-neuronal tau transfer drives AD pathology has recently garnered attention, as accumulation of pathological tau in the brain has high clinical significance in correlating with progression of cognitive AD symptoms. However, studies on tau pathology in AD are classically neuron-centric and have greatly overlooked cell-type specific effects of tau internalization, degradation, and propagation. While the contribution of microglia to tau processing and propagation is beginning to be recognized and understood, astrocytes, glial cells in the brain important for maintaining neuronal metabolic, synaptic, trophic, and immune function which can produce, internalize, degrade, and propagate tau are understudied in their ability to affect AD progression through tau pathology. Here, we showcase evidence for whether tau uptake by astrocytes may be beneficial or detrimental to neuronal health and how astrocytes and their immunometabolic functions may be key targets for future successful AD therapies.

Tau in the gut, does it really matter?

The enteric nervous system plays a critical role in the regulation of gastrointestinal tract functions and is often referred to as the 'second brain' because it shares many features with the central nervous system. These similarities include among others a large panel of neurotransmitters, a large population of glial cells and a susceptibility to neurodegeneration. This close homology between the central and enteric nervous systems suggests that a disease process affecting the central nervous system could also involve its enteric counterpart. This was already documented in Parkinson's disease, the most common synucleinopathy, in which alpha-synuclein deposits are reported in the enteric nervous system in the vast majority of patients. Tau is another key protein involved in neurodegenerative disorders of the brain. Whether changes in tau also occur in the enteric nervous system during gut or brain disorders has just begun to be explored. The scope of the present article is therefore to review existing studies on the expression and phosphorylation pattern of tau in the enteric nervous system under physiological and pathological conditions and to discuss the possible occurrence of 'enteric tauopathies'.

Filamentous recombinant human Tau activates primary astrocytes via an integrin receptor complex

Microtubule-associated protein Tau can form protein aggregates transmissible within the brain, correlating with the progression of tauopathies in humans. The transmission of aggregates requires neuron-released Tau to interact with surface receptors on target cells. However, the underlying molecular mechanisms in astrocytes and downstream effects are unclear. Here, using a spatially resolved proteomic mapping strategy, we show that integrin αV/β1 receptor binds recombinant human Tau, mediating the entry of Tau fibrils in astrocytes. The binding of distinct Tau species to the astrocytic αV/β1 receptor differentially activate integrin signaling. Furthermore, Tau-mediated activation of integrin signaling results in NFκB activation, causing upregulation of pro-inflammatory cytokines and chemokines, induction of a sub-group of neurotoxic astrocytic markers, and release of neurotoxic factors. Our findings suggest that filamentous recombinant human Tau-mediated activation of integrin signaling induces astrocyte conversion towards a neurotoxic state, providing a mechanistic insight into tauopathies.

Exosomal tau with seeding activity is released from Alzheimer's disease synapses, and seeding potential is associated with amyloid beta

Synaptic transfer of tau has long been hypothesized from the human pathology pattern and has been demonstrated in vitro and in vivo, but the precise mechanisms remain unclear. Extracellular vesicles such as exosomes have been suggested as a mechanism, but not all tau is exosomal. The present experiments use a novel flow cytometry assay to quantify depolarization of synaptosomes by KCl after loading with FM2-10, which induces a fluorescence reduction associated with synaptic vesicle release; the degree of reduction in cryopreserved human samples equaled that seen in fresh mouse synaptosomes. Depolarization induced the release of vesicles in the size range of exosomes, along with tetraspanin markers of extracellular vesicles. A number of tau peptides were released, including tau oligomers; released tau was primarily unphosphorylated and C-terminal truncated, with Aβ release just above background. When exosomes were immunopurified from release supernatants, a prominent tau band showed a dark smeared appearance of SDS-stable oligomers along with the exosomal marker syntenin-1, and these exosomes induced aggregation in the HEK tau biosensor assay. However, the flow-through did not seed aggregation. Size exclusion chromatography of purified released exosomes shows faint signals from tau in the same fractions that show a CD63 band, an exosomal size signal, and seeding activity. Crude synaptosomes from control, tauopathy, and AD cases demonstrated lower seeding in tauopathy compared to AD that is correlated with the measured Aβ42 level. These results show that AD synapses release exosomal tau that is C-terminal-truncated, oligomeric, and with seeding activity that is enhanced by Aβ. Taken together with previous findings, these results are consistent with a direct prion-like heterotypic seeding of tau by Aβ within synaptic terminals, with subsequent loading of aggregated tau onto exosomes that are released and competent for tau seeding activity.

Current Progress and Future Directions for Tau-Based Fluid Biomarker Diagnostics in Alzheimer's Disease

Despite continued efforts, there remain no disease-modifying drugs approved by the United States Food and Drug Administration (FDA) or European Medicines Agency (EMA) to combat the global epidemic of Alzheimer's disease. Currently approved medicines are unable to delay disease progression and are limited to symptomatic treatment. It is well established that the pathophysiology of this disease remains clinically silent for decades prior to symptomatic clinical decline. Identifying those at risk of disease progression could allow for effective treatment whilst the therapeutic window remains open for preservation of quality of life. This review aims to evaluate critically the current advances in the interpretation of tau-based biomarkers and their use to provide insights into the onset and progression of Alzheimer's disease, whilst highlighting important future directions for the field. This review emphasises the need for a more comprehensive analysis and interrogation of tau within biological fluids, to aid in obtaining a disease specific molecular signature for each stage of Alzheimer's disease. Success in achieving this could provide essential utility for presymptomatic patient selection for clinical trials, monitoring disease progression, and evaluating disease modifying therapies.

Much More Than a Cytoskeletal Protein: Physiological and Pathological Functions of the Non-microtubule Binding Region of Tau

Tau protein (MAPT) is classified as a microtubule-associated protein (MAP) and is believed to regulate the axonal microtubule arrangement. It belongs to the tau/MAP2/MAP4 family of MAPs that have a similar microtubule binding region at their carboxy-terminal half. In tauopathies, such as Alzheimer's disease, tau is distributed more in the somatodendritic compartment, where it aggregates into filamentous structures, the formation of which correlates with cognitive impairments in patients. While microtubules are the dominant interaction partners of tau under physiological conditions, tau has many additional interaction partners that can contribute to its physiological and pathological role. In particular, the amino-terminal non-microtubule binding domain (N-terminal projection region, NTR) of tau interacts with many partners that are involved in membrane organization. The NTR contains intrinsically disordered regions (IDRs) that show a strong evolutionary increase in the disorder and may have been the basis for the development of new, tau-specific interactions. In this review we discuss the functional organization of the tau protein and the special features of the tau non-microtubule binding region also in the connection with the results of Tau KO models. We consider possible physiological and pathological functions of tau's non-microtubule interactions, which could indicate that interactions mediated by tau's NTR and regulated by far-reaching functional interactions of the PRR and the extreme C-terminus of tau contribute to the pathological processes.

Microglia in Alzheimer's Disease in the Context of Tau Pathology

Microglia are the cells that comprise the innate immune system in the brain. First described more than a century ago, these cells were initially assigned a secondary role in the central nervous system (CNS) with respect to the protagonists, neurons. However, the latest advances have revealed the complexity and importance of microglia in neurodegenerative conditions such as Alzheimer’s disease (AD), the most common form of dementia associated with aging. This pathology is characterized by the accumulation of amyloid-β peptide (Aβ), which forms senile plaques in the neocortex, as well as by the aggregation of hyperphosphorylated tau protein, a process that leads to the development of neurofibrillary tangles (NFTs). Over the past few years, efforts have been focused on studying the interaction between Aβ and microglia, together with the ability of the latter to decrease the levels of this peptide. Given that most clinical trials following this strategy have failed, current endeavors focus on deciphering the molecular mechanisms that trigger the tau-induced inflammatory response of microglia. In this review, we summarize the most recent studies on the physiological and pathological functions of tau protein and microglia. In addition, we analyze the impact of microglial AD-risk genes (APOE, TREM2, and CD33) in tau pathology, and we discuss the role of extracellular soluble tau in neuroinflammation.

Tau Protein and Zebrafish Models for Tau-Induced Neurodegeneration

Tauopathies are a specific type of slow and progressive neurodegeneration, which involves intracellular deposition of fibrillar material composed of abnormal hyperphosphorylation of the microtubule associated protein (MAP) tau. Despite many years of intensive research, our understanding of the molecular events that lead to neurodegeneration is far from complete. No effective therapeutic treatments have been defined, and questions surround the validity and utility of existing animal models. It is an urgent need to develop a novel animal model to study the underlying neurodegenerative mechanisms of tauopathies. Zebrafish models of tauopathies could complement existing models by providing an in vivo platform for genetic and chemical screens in order to identify new therapeutic targets and compounds, meanwhile zebrafish models have permitted discovery of unique characteristics of these genes that could have been difficultly observed in other models. Novel transgenic zebrafish models expressing wild-type or mutant forms of human 4R-tau in neurons have recently been reported. These studies show disease-relevant changes including tau hyperphosphorylation, aggregation and somato-dendritic relocalization. This review highlights the availability of transgenic tau zebrafish models that allow more detailed biochemical studies of tau in the zebrafish CNS to characterize solubility, fibril morphology and further clarify phosphorylation proceedings. Furthermore, a deeper knowledge of the zebrafish brain and a better characterization of tau caused by alterations in neurodegenerative disorders are needed.

Mechanisms of Pathogenic Tau and Aβ Protein Spreading in Alzheimer's Disease

Alzheimer’s disease (AD) is pathologically defined by extracellular accumulation of amyloid-β (Aβ) peptides generated by the cleavage of amyloid precursor protein (APP), strings of hyperphosphorylated Tau proteins accumulating inside neurons known as neurofibrillary tangles (NFTs) and neuronal loss. The association between the two hallmarks and cognitive decline has been known since the beginning of the 20th century when the first description of the disease was carried out by Alois Alzheimer. Today, more than 40 million people worldwide are affected by AD that represents the most common cause of dementia and there is still no effective treatment available to cure the disease. In general, the aggregation of Aβ is considered an essential trigger in AD pathogenesis that gives rise to NFTs, neuronal dysfunction and dementia. During the process leading to AD, tau and Aβ first misfold and form aggregates in one brain region, from where they spread to interconnected areas of the brain thereby inducing its gradual morphological and functional deterioration. In this mini-review article, we present an overview of the current literature on the spreading mechanisms of Aβ and tau pathology in AD since a more profound understanding is necessary to design therapeutic approaches aimed at preventing or halting disease progression.

Role of tau protein in Alzheimer's disease: The prime pathological player

Alzheimer's disease (AD) is a prevalently found tauopathy characterized by memory loss and cognitive insufficiency. AD is an age-related neurodegenerative disease with two major hallmarks which includes extracellular amyloid plaques made of amyloid-β (Aβ) and intracellular neurofibrillary tangles of hyperphosphorylated tau. With population aging worldwide, there is an indispensable need for treatment strategies that can potentially manage this developing dementia. Despite broad researches on targeting Aβ in the past two decades, research findings on Aβ targeted therapeutics failed to prove efficacy in the treatment of AD. Tau protein with its extensive pathological role in several neurodegenerative diseases can be considered as a promising target candidate for developing therapeutic interventions. The abnormal hyperphosphorylation of tau plays detrimental pathological functions which ultimately lead to neurodegeneration. This review will divulge the importance of tau in AD pathogenesis, the interplay of Aβ and tau, the pathological functions of tau, and potential therapeutic strategies for an effective management of neuronal disorders.

Mechanisms of secretion and spreading of pathological tau protein

Accumulation of misfolded and aggregated forms of tau protein in the brain is a neuropathological hallmark of tauopathies, such as Alzheimer's disease and frontotemporal lobar degeneration. Tau aggregates have the ability to transfer from one cell to another and to induce templated misfolding and aggregation of healthy tau molecules in previously healthy cells, thereby propagating tau pathology across different brain areas in a prion-like manner. The molecular mechanisms involved in cell-to-cell transfer of tau aggregates are diverse, not mutually exclusive and only partially understood. Intracellular accumulation of misfolded tau induces several mechanisms that aim to reduce the cellular burden of aggregated proteins and also promote secretion of tau aggregates. However, tau may also be released from cells physiologically unrelated to protein aggregation. Tau secretion involves multiple vesicular and non-vesicle-mediated pathways, including secretion directly through the plasma membrane. Consequently, extracellular tau can be found in various forms, both as a free protein and in vesicles, such as exosomes and ectosomes. Once in the extracellular space, tau aggregates can be internalized by neighboring cells, both neurons and glial cells, via endocytic, pinocytic and phagocytic mechanisms. Importantly, accumulating evidence suggests that prion-like propagation of misfolding protein pathology could provide a general mechanism for disease progression in tauopathies and other related neurodegenerative diseases. Here, we review the recent literature on cellular mechanisms involved in cell-to-cell transfer of tau, with a particular focus in tau secretion.

Synaptic Localisation of Tau

The microtubule-associated protein tau has been identified in several intraneuronal compartments, including in association with synapses. In Alzheimer's disease, frontotemporal dementia and related tauopathies, highly phosphorylated tau accumulates as intraneuronal protein aggregates that are likely responsible for the demise of neurons and the subsequent progressive cognitive decline. However, the molecular mechanisms underlying such tau-mediated damage in the tauopathies is not fully understood. Tauopathy induces loss of synapses, which is one of the earliest structural correlates of cognitive dysfunction and disease progression. Notably, altered post-translational modifications of tau, including increased phosphorylation and acetylation, augment the mislocalisation of tau to synapses, impair synaptic vesicle release and might influence the activity-dependent release of tau from neurons. Thus, disease-associated accumulation of modified tau at the synapse adversely affects critical neuronal processes that are linked to neuronal activity and synaptic function. These findings emphasise the importance of gaining a comprehensive understanding of the diverse roles of tau at distinct intraneuronal locations. An improved knowledge of the impact of synaptic tau under physiological and pathological conditions and how tau localisation impacts on neuronal function will provide valuable insights that may lead to the development of new therapies for the tauopathies.

Bridging Integrator-1 protein loss in Alzheimer's disease promotes synaptic tau accumulation and disrupts tau release

Polymorphisms associated with BIN1 (bridging integrator 1) confer the second greatest risk for developing late-onset Alzheimer’s disease. The biological consequences of this genetic variation are not fully understood; however, BIN1 is a binding partner for tau. Tau is normally a highly soluble cytoplasmic protein, but in Alzheimer’s disease, tau is abnormally phosphorylated and accumulates at synapses to exert synaptotoxicity. The purpose of this study was to determine whether alterations in BIN1 and tau in Alzheimer’s disease promote the damaging redistribution of tau to synapses, as a mechanism by which BIN1 polymorphisms may increase the risk of developing Alzheimer’s disease. We show that BIN1 is lost from the cytoplasmic fraction of Alzheimer’s disease cortex, and this is accompanied by the progressive mislocalization of phosphorylated tau to synapses. We confirmed proline 216 in tau as critical for tau interaction with the BIN1-SH3 domain and showed that the phosphorylation of tau disrupts this binding, suggesting that tau phosphorylation in Alzheimer’s disease disrupts tau–BIN1 associations. Moreover, we show that BIN1 knockdown in rat primary neurons to mimic BIN1 loss in Alzheimer’s disease brain causes the damaging accumulation of phosphorylated tau at synapses and alterations in dendritic spine morphology. We also observed reduced release of tau from neurons upon BIN1 silencing, suggesting that BIN1 loss disrupts the function of extracellular tau. Together, these data indicate that polymorphisms associated with BIN1 that reduce BIN1 protein levels in the brain likely act synergistically with increased tau phosphorylation to increase the risk of Alzheimer’s disease by disrupting cytoplasmic tau–BIN1 interactions, promoting the damaging mis-sorting of phosphorylated tau to synapses to alter synapse structure and reducing the release of physiological forms of tau to disrupt tau function.

Secretion of Tau via an Unconventional Non-vesicular Mechanism

Tauopathies are characterized by cerebral accumulation of Tau protein aggregates that appear to spread throughout the brain via a cell-to-cell transmission process that includes secretion and uptake of pathological Tau, followed by templated misfolding of normal Tau in recipient cells. Here, we show that phosphorylated, oligomeric Tau clusters at the plasma membrane in N2A cells and is secreted in vesicle-free form in an unconventional process sensitive to changes in membrane properties, particularly cholesterol and sphingomyelin content. Cell surface heparan sulfate proteoglycans support Tau secretion, possibly by facilitating its release after membrane penetration. Notably, secretion of endogenous Tau from primary cortical neurons is mediated, at least partially, by a similar mechanism. We suggest that Tau is released from cells by an unconventional secretory mechanism that involves its phosphorylation and oligomerization and that membrane interaction may help Tau to acquire properties that allow its escape from cells directly through the plasma membrane.

Tau Phosphorylation and Aggregation in the Developing Human Brain

Tau hyperphosphorylation, mostly at serine (Ser) or threonine (Thr) residues, plays a key role in the pathogenesis of Alzheimer disease (AD) and other tauopathies. Rodent studies show similar hyperphosphorylation in the developing brain, which may be involved in regulating axonal growth and plasticity, but detailed human studies are lacking. Here, we examine tau phosphorylation by immunohistochemistry and immunoblotting in human fetal and adult autopsy brain tissue. Of the 20 cases with sufficient tissue preservation, 18 (90%) showed positive staining for S214 (pSer214), with the majority also positive for CP13 (pSer202), and PHF-1 (pSer396/pSer404). AT8 (pSer202/pThr205) and RZ3 (pThr231) were largely negative while PG5 (pSer409) was negative in all cases. Immunoblotting showed tau monomers with a similar staining pattern. We also observed phospho-tau aggregates in the fetal molecular layer, staining positively for S214, CP13, and PHF1 and negative for thioflavin S. These corresponded to high-molecular weight (∼150 kD) bands seen on Western blots probed with S214, PHF1, and PG5. We therefore conclude that fetal phosphorylation overlaps with AD in some residues, while others (e.g. T231, S409) appear to be unique to AD, and that tau is capable of forming nontoxic aggregates in the developing brain. These findings suggest that the fetal brain is resilient to formation of toxic aggregates, the mechanism for which may yield insights into the pathogenesis of tau aggregation and toxicity in the aging brain.

Current and emerging avenues for Alzheimer's disease drug targets

Alzheimer's disease (AD), the most frequent cause of dementia, is escalating as a global epidemic, and so far, there is neither cure nor treatment to alter its progression. The most important feature of the disease is neuronal death and loss of cognitive functions, caused probably from several pathological processes in the brain. The main neuropathological features of AD are widely described as amyloid beta (Aβ) plaques and neurofibrillary tangles of the aggregated protein tau, which contribute to the disease. Nevertheless, AD brains suffer from a variety of alterations in function, such as energy metabolism, inflammation and synaptic activity. The latest decades have seen an explosion of genes and molecules that can be employed as targets aiming to improve brain physiology, which can result in preventive strategies for AD. Moreover, therapeutics using these targets can help AD brains to sustain function during the development of AD pathology. Here, we review broadly recent information for potential targets that can modify AD through diverse pharmacological and nonpharmacological approaches including gene therapy. We propose that AD could be tackled not only using combination therapies including Aβ and tau, but also considering insulin and cholesterol metabolism, vascular function, synaptic plasticity, epigenetics, neurovascular junction and blood-brain barrier targets that have been studied recently. We also make a case for the role of gut microbiota in AD. Our hope is to promote the continuing research of diverse targets affecting AD and promote diverse targeting as a near-future strategy.

The complexity of tau in Alzheimer's disease

Alzheimer's disease (AD) is characterized by two major pathological lesions in the brain, amyloid plaques and neurofibrillary tangles (NFTs) composed mainly of amyloid-β (Aβ) peptides and hyperphosphorylated tau, respectively. Although accumulation of toxic Aβ species in the brain has been proposed as one of the important early events in AD, continued lack of success of clinical trials based on Aβ-targeting drugs has triggered the field to seek out alternative disease mechanisms and related therapeutic strategies. One of the new approaches is to uncover novel roles of pathological tau during disease progression. This review will primarily focus on recent advances in understanding the contributions of tau to AD.

Dale Schenk One Year Anniversary: Fighting to Preserve the Memories

It has been a year since we lost Dale Schenk on September 30, 2016. Dale's visionary work resulted in the remarkable discovery in 1999 that an experimental amyloid-β (Aβ) vaccine reduced the neurodegeneration in a transgenic model of Alzheimer's disease (AD). Following Dale's seminal work, several active and passive immunotherapies have since been developed and tested in the clinic for AD, Parkinson's disease (PD), and other neurodegenerative disorders. Here we provide a brief overview of the current state of development of immunotherapy for AD, PD, and other neurodegenerative disorders in the context of this anniversary. The next steps in the development of immunotherapies will require combinatorial approaches mixing antibodies against various targets (e.g., Aβ, α-syn, Tau, and TDP43) with small molecules that block toxicity, aggregation, inflammation, and promote cell survival.

It's all about tau

Tau is a protein that is highly enriched in neurons and was originally defined by its ability to bind and stabilize microtubules. However, it is now becoming evident that the functions of tau extend beyond its ability to modulate microtubule dynamics. Tau plays a role in mediating axonal transport, synaptic structure and function, and neuronal signaling pathways. Although tau plays important physiological roles in neurons, its involvement in neurodegenerative diseases, and most prominently in the pathogenesis of Alzheimer disease (AD), has directed the majority of tau studies. However, a thorough knowledge of the physiological functions of tau and its post-translational modifications under normal conditions are necessary to provide the foundation for understanding its role in pathological settings. In this review, we will focus on human tau, summarizing tau structure and organization, as well as its posttranslational modifications associated with physiological processes. We will highlight possible mechanisms involved in mediating the turnover of tau and finally discuss newly elucidated tau functions in a physiological context.

The Involvement of Aβ42 and Tau in Nucleolar and Protein Synthesis Machinery Dysfunction

Alzheimer’s disease (AD) is the most common form of dementia and is distinguished from other dementias by observation of extracellular Amyloid-β (Aβ) plaques and intracellular neurofibrillary tangles, comprised of fibrils of Aβ and tau protein, respectively. At early stages, AD is characterized by minimal neurodegeneration, oxidative stress, nucleolar stress, and altered protein synthesis machinery. It is generally believed that Aβ oligomers are the neurotoxic species and their levels in the AD brain correlate with the severity of dementia suggesting that they play a critical role in the pathogenesis of the disease. Here, we show that the incubation of differentiated human neuroblastoma cells (SHSY5Y) with freshly prepared Aβ42 oligomers initially resulted in oxidative stress and subtle nucleolar stress in the absence of DNA damage or cell death. The presence of exogenous Aβ oligomers resulted in altered nuclear tau levels as well as phosphorylation state, leading to altered distribution of nucleolar tau associated with nucleolar stress. These markers of cellular dysfunction worsen over time alongside a reduction in ribosomal RNA synthesis and processing, a decrease in global level of newly synthesized RNA and reduced protein synthesis. The interplay between Aβ and tau in AD remains intriguing and Aβ toxicity has been linked to tau phosphorylation and changes in localization. These findings provide evidence for the involvement of Aβ42 effects on nucleolar tau and protein synthesis machinery dysfunction in cultured cells. Protein synthesis dysfunction is observed in mild cognitive impairment and early AD in the absence of significant neuronal death.

Characterisation of tau in the human and rodent enteric nervous system under physiological conditions and in tauopathy

Tau is normally a highly soluble phosphoprotein found predominantly in neurons. Six different isoforms of tau are expressed in the adult human CNS. Under pathological conditions, phosphorylated tau aggregates are a defining feature of neurodegenerative disorders called tauopathies. Recent findings have suggested a potential role of the gut-brain axis in CNS homeostasis, and therefore we set out to examine the isoform profile and phosphorylation state of tau in the enteric nervous system (ENS) under physiological conditions and in tauopathies. Surgical specimens of human colon from controls, Parkinson's disease (PD) and progressive supranuclear palsy (PSP) patients were analyzed by Western Blot and immunohistochemistry using a panel of anti-tau antibodies. We found that adult human ENS primarily expresses two tau isoforms, localized in the cell bodies and neuronal processes. We did not observe any difference in the enteric tau isoform profile and phosphorylation state between PSP, PD and control subjects. The htau mouse model of tauopathy also expressed two main isoforms of human tau in the ENS, and there were no apparent differences in ENS tau localization or phosphorylation between wild-type and htau mice. Tau in both human and mouse ENS was found to be phosphorylated but poorly susceptible to dephosphorylation with lambda phosphatase. To investigate ENS tau phosphorylation further, primary cultures from rat enteric neurons, which express four isoforms of tau, were pharmacologically manipulated to show that ENS tau phosphorylation state can be regulated, at least in vitro. Our study is the first to characterize tau in the rodent and human ENS. As a whole, our findings provide a basis to unravel the functions of tau in the ENS and to further investigate the possibility of pathological changes in enteric neuropathies and tauopathies.

Preparation of organotypic brain slice cultures for the study of Alzheimer's disease

Alzheimer's disease, the most common cause of dementia, is a progressive neurodegenerative disorder characterised by amyloid-beta deposits in extracellular plaques, intracellular neurofibrillary tangles of aggregated tau, synaptic dysfunction and neuronal death.

Transgenic rodent models to study Alzheimer’s mimic features of human disease such as age-dependent accumulation of abnormal beta-amyloid and tau, synaptic dysfunction, cognitive deficits and neurodegeneration. These models have proven vital for improving our understanding of the molecular mechanisms underlying AD and for identifying promising therapeutic approaches. However, modelling neurodegenerative disease in animals commonly involves aging animals until they develop harmful phenotypes, often coupled with invasive procedures.

We have developed a novel organotypic brain slice culture model to study Alzheimer’s disease using 3xTg-AD mice which brings the potential of substantially reducing the number of rodents used in dementia research from an estimated 20,000 per year. Using a McIllwain tissue chopper, we obtain 36 x 350 micron slices from each P8-P9 mouse pup for culture between 2 weeks and 6 months on semi-permeable 0.4 micron pore membranes, considerably reducing the numbers of animals required to investigate multiple stages of disease. This tractable model also allows the opportunity to modulate multiple pathways in tissues from a single animal. We believe that this model will most benefit dementia researchers in the academic and drug discovery sectors.

We validated the slice culture model against aged mice, showing that the molecular phenotype closely mimics that displayed in vivo, albeit in an accelerated timescale. We showed beneficial outcomes following treatment of slices with agents previously shown to have therapeutic effects in vivo, and we also identified new mechanisms of action of other compounds. Thus, organotypic brain slice cultures from transgenic mouse models expressing Alzheimer’s disease-related genes may provide a valid and sensitive replacement for in vivo studies that do not involve behavioural analysis.

The Deacetylase HDAC6 Mediates Endogenous Neuritic Tau Pathology

The initiating events that promote tau mislocalization and pathology in Alzheimer's disease (AD) are not well defined, partly because of the lack of endogenous models that recapitulate tau dysfunction. We exposed wild-type neurons to a neuroinflammatory trigger and examined the effect on endogenous tau. We found that tau re-localized and accumulated within pathological neuritic foci, or beads, comprised of mostly hypo-phosphorylated, acetylated, and oligomeric tau. These structures were detected in aged wild-type mice and were enhanced in response to neuroinflammation in vivo, highlighting a previously undescribed endogenous age-related tau pathology. Strikingly, deletion or inhibition of the cytoplasmic shuttling factor HDAC6 suppressed neuritic tau bead formation in neurons and mice. Using mass spectrometry-based profiling, we identified a single neuroinflammatory factor, the metalloproteinase MMP-9, as a mediator of neuritic tau beading. Thus, our study uncovers a link between neuroinflammation and neuritic tau beading as a potential early-stage pathogenic mechanism in AD.

Ischemic tau protein gene induction as an additional key factor driving development of Alzheimer's phenotype changes in CA1 area of hippocampus in an ischemic model of Alzheimer's disease

BACKGROUND

Tauopathies are a class of neurodegenerative illnesses associated with the aberrant accumulation of the tau protein in the brain. The best known out of these diseases is Alzheimer's disease, a disorder where the microtubule associated tau protein becomes hyperphosphorylated (which lowers its binding affinity to microtubules) and accumulates inside neurons in the form of tangles. In this study, we attempt to find out whether brain ischemia may play an important role in tau protein gene alterations.

METHODS

We have investigated the relationship between hippocampal ischemia and Alzheimer's disease by means of a transient 10-min global brain ischemia in rats and determining the effect on Alzheimer's disease tau protein gene expression during 2, 7 and 30 days post injury.

RESULTS

We found the significant overexpression of tau protein gene on the 2nd day, but on day's 7 and 30 post-ischemia there a significant opposite tendency was observed.

CONCLUSION

The obtained results offer a novel insight into tau protein gene in regulating delayed neuronal death in the ischemic hippocampus. Finally, these findings further elucidate the long-term impact of brain ischemia on Alzheimer's disease development.

Vesicular Axonal Transport is Modified In Vivo by Tau Deletion or Overexpression in Drosophila

Structural microtubule associated protein Tau is found in high amount in axons and is involved in several neurodegenerative diseases. Although many studies have highlighted the toxicity of an excess of Tau in neurons, the in vivo understanding of the endogenous role of Tau in axon morphology and physiology is poor. Indeed, knock-out mice display no strong cytoskeleton or axonal transport phenotype, probably because of some important functional redundancy with other microtubule-associated proteins (MAPs). Here, we took advantage of the model organism Drosophila, which genome contains only one homologue of the Tau/MAP2/MAP4 family to decipher (endogenous) Tau functions. We found that Tau depletion leads to a decrease in microtubule number and microtubule density within axons, while Tau excess leads to the opposite phenotypes. Analysis of vesicular transport in tau mutants showed altered mobility of vesicles, but no change in the total amount of putatively mobile vesicles, whereas both aspects were affected when Tau was overexpressed. In conclusion, we show that loss of Tau in tau mutants not only leads to a decrease in axonal microtubule density, but also impairs axonal vesicular transport, albeit to a lesser extent compared to the effects of an excess of Tau.

Phospho-Tau Bar Code: Analysis of Phosphoisotypes of Tau and Its Application to Tauopathy

Tau is a microtubule-associated protein which regulates the assembly and stability of microtubules in the axons of neurons. Tau is also a major component of neurofibrillary tangles (NFTs), a pathological hallmark in Alzheimer's disease (AD). A characteristic of AD tau is hyperphosphorylation with more than 40 phosphorylation sites. Aggregates of hyperphosphorylated tau are also found in other neurodegenerative diseases which are collectively called tauopathies. Although a large number of studies have been performed on the phosphorylation of AD tau, it is not known if there is disease-specific phosphorylation among tauopathies. This is due to the lack of a proper method for analyzing tau phosphorylation in vivo. Most previous phosphorylation studies were conducted using a range of phosphorylation site-specific antibodies. These studies describe relative changes of different phosphorylation sites, however, it is hard to estimate total, absolute and collective changes in phosphorylation. To overcome these problems, we have recently applied the Phos-Tag technique to the analysis of tau phosphorylation in vitro and in vivo. This method separates tau into many bands during SDS-PAGE depending on its phosphorylation states, creating a bar code appearance. We propose calling this banding pattern of tau the "phospho-tau bar code." In this review article, we describe what is newly discovered regarding tau phosphorylation through the use of the Phos-Tag. We would like to propose its use for the postmortem diagnosis of tauopathy which is presently done by immunostaining diseased brains with anti-phospho-antibodies. While Phos-tag SDS-PAGE, like other biochemical assays, will lose morphological information, it could provide other types of valuable information such as disease-specific phosphorylation.

Vitamin B12 Inhibits Tau Fibrillization via Binding to Cysteine Residues of Tau

Two mechanisms underlie the inhibitory/acceleratory action of chemical compounds on tau aggregation including the regulation of cellular kinases and phosphatases activity and direct binding to tau protein. Vitamin B12 is one of the tau polymerization inhibitors, and its deficiency is linked to inactivation of protein phosphatase 2A and subsequently hyperphosphorylation and aggregation of tau protein. Regarding the structure and function of vitamin B12 and tau protein, we assumed that vitamin B12 is also able to directly bind to tau protein. Hence, we investigated the interaction of vitamin B12 with tau protein in vitro using fluorometry and circular dichrosim. Interaction studies was followed by investigation into the effect of vitamin B12 on tau aggregation using ThT fluorescence, circular dichroism, transmission electron microscopy, and SDS-PAGE. The results indicated that vitamin B12 interacts with tau protein and prevents fibrillization of tau protein. Blocking the cysteine residues of tau confirmed the cysteine-mediated binding of vitamin B12 to tau and showed that binding to cysteine is essential for inhibitory effect of vitamin B12 on tau aggregation. SDS-PAGE analysis indicated that vitamin B12 inhibits tau aggregation and that tau oligomers formed in the presence of vitamin B12 are mostly SDS-soluble. We propose that direct binding of vitamin B12 is another mechanism underlying the inhibitory role of vitamin B12 on tau aggregation and neurodegeneration.

Development of dual casein kinase 1δ/1ε (CK1δ/ε) inhibitors for treatment of breast cancer

Casein kinase 1δ/ε have been identified as promising therapeutic target for oncology application, including breast and brain cancer. Here, we described our continued efforts in optimization of a lead series of purine scaffold inhibitors that led to identification of two new CK1δ/ε inhibitors 17 and 28 displaying low nanomolar values in antiproliferative assays against the human MDA-MB-231 triple negative breast cancer cell line and have physical, in vitro and in vivo pharmacokinetic properties suitable for use in proof of principle animal xenograft studies against human cancers.