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

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

Shapeshifting tau: from intrinsically disordered to paired-helical filaments

Tau is an intrinsically disordered protein that has the ability to self-assemble to form paired helical and straight filaments in Alzheimer's disease, as well as the ability to form additional distinct tau filaments in other tauopathies. In the presence of microtubules, tau forms an elongated form associated with tubulin dimers via a series of imperfect repeats known as the microtubule binding repeats. Tau has recently been identified to have the ability to phase separate in vitro and in cells. The ability of tau to adopt a wide variety of conformations appears fundamental both to its biological function and also its association with neurodegenerative diseases. The recently highlighted involvement of low-complexity domains in liquid-liquid phase separation provides a critical link between the soluble function and the insoluble dysfunctional properties of tau.

Modulation of the Interactions Between α-Synuclein and Lipid Membranes by Post-translational Modifications

Parkinson's disease is characterised by the presence in brain tissue of aberrant inclusions known as Lewy bodies and Lewy neurites, which are deposits composed by α-synuclein and a variety of other cellular components, including in particular lipid membranes. The dysregulation of the balance between lipid homeostasis and α-synuclein homeostasis is therefore likely to be closely involved in the onset and progression of Parkinson's disease and related synucleinopathies. As our understanding of this balance is increasing, we describe recent advances in the characterisation of the role of post-translational modifications in modulating the interactions of α-synuclein with lipid membranes. We then discuss the impact of these advances on the development of novel diagnostic and therapeutic tools for synucleinopathies.

Tau Filament Self-Assembly and Structure: Tau as a Therapeutic Target

Tau plays an important pathological role in a group of neurodegenerative diseases called tauopathies, including Alzheimer's disease, Pick's disease, chronic traumatic encephalopathy and corticobasal degeneration. In each disease, tau self-assembles abnormally to form filaments that deposit in the brain. Tau is a natively unfolded protein that can adopt distinct structures in different pathological disorders. Cryo-electron microscopy has recently provided a series of structures for the core of the filaments purified from brain tissue from patients with different tauopathies and revealed that they share a common core region, while differing in their specific conformation. This structurally resolvable part of the core is contained within a proteolytically stable core region from the repeat domain initially isolated from AD tau filaments. Tau has recently become an important target for therapy. Recent work has suggested that the prevention of tau self-assembly may be effective in slowing the progression of Alzheimer's disease and other tauopathies. Here we review the work that explores the importance of tau filament structures and tau self-assembly mechanisms, as well as examining model systems that permit the exploration of the mode of action of potential inhibitors.

Physiological Tau Interactome in Brain and Its Link to Tauopathies

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

Prevention of tau seeding and propagation by immunotherapy with a central tau epitope antibody

Tauopathies are neurodegenerative diseases characterized by the intraneuronal accumulation of aggregated tau. The staging of this neurodegenerative process is well established for Alzheimer’s disease as well as for other tauopathies. The stereotypical pattern of tau pathology in these diseases is consistent with the hypothesis that the tau protein can spread in a ‘prion-like’ manner. It proposes that extracellular pathological tau species can transmit pathology from cell to cell. Accordingly, by targeting these spreading species with therapeutic antibodies one should be able to slow or halt the progression of tau pathology. To be effective, antibodies should neutralize the pathological species present in Alzheimer’s disease brains and block their cell-to-cell spread. To evaluate both aspects, tau antibody D, which recognizes an epitope in the central region of tau, and was selected for its outstanding ability to block tau seeding in cell based assays, was used in this study. Here, we addressed two fundamental questions: (i) can this anti-tau antibody neutralize the pathological species present in Alzheimer’s disease brains; and (ii) can it block the cell-to-cell spread of tau seeds in vivo? First, antibody D effectively prevented the induction of tau pathology in the brains of transgenic mice that had been injected with human Alzheimer’s disease brain extracts, showing that it could effectively neutralize the pathological species present in these extracts. Second, by using K18 P301L tau fibrils to induce pathology, we further demonstrated that antibody D was also capable of blocking the progression of tau pathology to distal brain regions. In contrast, an amino-terminal tau antibody, which was less effective at blocking tau seeding in vitro showed less efficacy in reducing Alzheimer’s disease patient tau driven pathology in the transgenic mouse model. We did not address whether the same is true for a spectrum of other amino-terminal antibodies that were tested in vitro. These data highlight important differences between tau antibodies and, when taken together with other recently published data, suggest that epitope may be important for function.

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

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

Therapeutic antibody targeting microtubule-binding domain prevents neuronal internalization of extracellular tau via masking neuron surface proteoglycans

Pathologically altered tau protein is a common denominator of neurodegenerative disorders including Alzheimer's disease (AD) and other tauopathies. Therefore, promising immunotherapeutic approaches target and eliminate extracellular pathogenic tau species, which are thought to be responsible for seeding and propagation of tau pathology. Tau isoforms in misfolded states can propagate disease pathology in a template-dependent manner, proposed to be mediated by the release and internalization of extracellular tau. Monoclonal antibody DC8E8, binding four highly homologous and independent epitopes in microtubule-binding domain (MTBD) of diseased tau, inhibits tau-tau interaction, discriminates between healthy and pathologically truncated tau and reduces tau pathology in animal model in vivo. Here, we show that DC8E8 antibody acts via extracellular mechanism and does not influence viability and physiological functions of neurons. Importantly, in vitro functional assays showed that DC8E8 recognises pathogenic tau proteins of different size and origin, and potently blocks their entry into neurons. Next, we examined the mechanisms by which mouse antibody DC8E8 and its humanized version AX004 effectively block the neuronal internalization of extracellular AD tau species. We determined a novel mode of action of a therapeutic candidate antibody, which potently inhibits neuronal internalization of AD tau species by masking of epitopes present in MTBD important for interaction with neuron surface Heparan Sulfate Proteoglycans (HSPGs). We show that interference of tau-heparane sulfate interaction with DC8E8 antibody via steric hindrance represents an efficient and important therapeutic approach halting tau propagation.

Structure and Functions of Microtubule Associated Proteins Tau and MAP2c: Similarities and Differences

The stability and dynamics of cytoskeleton in brain nerve cells are regulated by microtubule associated proteins (MAPs), tau and MAP2. Both proteins are intrinsically disordered and involved in multiple molecular interactions important for normal physiology and pathology of chronic neurodegenerative diseases. Nuclear magnetic resonance and cryo-electron microscopy recently revealed propensities of MAPs to form transient local structures and long-range contacts in the free state, and conformations adopted in complexes with microtubules and filamentous actin, as well as in pathological aggregates. In this paper, we compare the longest, 441-residue brain isoform of tau (tau40), and a 467-residue isoform of MAP2, known as MAP2c. For both molecules, we present transient structural motifs revealed by conformational analysis of experimental data obtained for free soluble forms of the proteins. We show that many of the short sequence motifs that exhibit transient structural features are linked to functional properties, manifested by specific interactions. The transient structural motifs can be therefore classified as molecular recognition elements of tau40 and MAP2c. Their interactions are further regulated by post-translational modifications, in particular phosphorylation. The structure-function analysis also explains differences between biological activities of tau40 and MAP2c.

The elusive tau molecular structures: can we translate the recent breakthroughs into new targets for intervention?

Insights into tau molecular structures have advanced significantly in recent years. This field has been the subject of recent breakthroughs, including the first cryo-electron microscopy structures of tau filaments from Alzheimer’s and Pick’s disease inclusions, as well as the structure of the repeat regions of tau bound to microtubules. Tau structure covers various species as the tau protein itself takes many forms. We will here address a range of studies that help to define the many facets of tau protein structures and how they translate into pathogenic forms. New results shed light on previous data that need now to be revisited in order to up-date our knowledge of tau molecular structure. Finally, we explore how these data can contribute the important medical aspects of this research - diagnosis and therapeutics.

A walk through tau therapeutic strategies

Tau neuronal and glial pathologies drive the clinical presentation of Alzheimer's disease and related human tauopathies. There is a growing body of evidence indicating that pathological tau species can travel from cell to cell and spread the pathology through the brain. Throughout the last decade, physiological and pathological tau have become attractive targets for AD therapies. Several therapeutic approaches have been proposed, including the inhibition of protein kinases or protein-3-O-(N-acetyl-beta-D-glucosaminyl)-L-serine/threonine Nacetylglucosaminyl hydrolase, the inhibition of tau aggregation, active and passive immunotherapies, and tau silencing by antisense oligonucleotides. New tau therapeutics, across the board, have demonstrated the ability to prevent or reduce tau lesions and improve either cognitive or motor impairment in a variety of animal models developing neurofibrillary pathology. The most advanced strategy for the treatment of human tauopathies remains immunotherapy, which has already reached the clinical stage of drug development. Tau vaccines or humanised antibodies target a variety of tau species either in the intracellular or extracellular spaces. Some of them recognise the amino-terminus or carboxy-terminus, while others display binding abilities to the proline-rich area or microtubule binding domains. The main therapeutic foci in existing clinical trials are on Alzheimer's disease, progressive supranuclear palsy and non-fluent primary progressive aphasia. Tau therapy offers a new hope for the treatment of many fatal brain disorders. First efficacy data from clinical trials will be available by the end of this decade.

Ten Years of Tau-Targeted Immunotherapy: The Path Walked and the Roads Ahead

Neurofibrillary pathology comprised of pathological tau protein is closely tied to a range of neurodegenerative disorders, the most common of which is Alzheimer's disease. While they are individually rarer, a range of other disorders, the tauopathies (including Pick's disease, progressive supranuclear palsy, corticobasal degeneration, primary progressive aphasia, and ∼50% of behavioral variant frontotemporal dementia cases) display pronounced underlying tau pathology. In all cases, the distribution and amount of tau pathology closely correlates with the severity and phenotype of cognitive impairment, and with the pattern and degree of brain atrophy. Successfully counteracting tau pathology is likely to halt or slow the progression of these debilitating disorders. This makes tau a target of prime importance, yet an elusive one. The diversity of the tau proteome and post-translational modifications, as well as pathophysiology of tau are reviewed. Beginning 2013, a range of tau-targeted immunotherapies have entered clinical development; these therapies, and their common themes and differences are reviewed. The manuscript provides an extensive discussion on epitope selection for immunotherapies against tau pathology, on immunological mechanisms involved in their action, and challenges such as immune senescence, vaccine design, or evolution of epitopes. Furthermore, we provide methodological recommendations for the characterization of active vaccines and antibodies, animal models, and the target itself - the diseased tau proteome.

Neuronal Expression of Truncated Tau Efficiently Promotes Neurodegeneration in Animal Models: Pitfalls of Toxic Oligomer Analysis

Animal models of neurodegeneration induced by neuronal expression of truncated tau protein emerge as an important tool for understanding the pathogenesis of human tauopathies and for therapy development. Here we highlight common features of truncated tau models and make a critical assessment of possible pitfalls in their analysis. Particularly, the amount of soluble tau oligomers, which are suspected to be neurotoxic agents participating on the spreading of pathology inside the brain, may be overestimated due to a post-lysis oxidative tau oligomerization. Using a mouse brain lysate spiked with recombinant truncated and full length tau forms, we show that tau oligomers might inadvertently be produced during the isolation procedure. This finding is further corroborated by the analysis of brain lysates originated from a mouse model expressing truncated tau variant. Our results underline the necessity of thiol-protecting conditions during the analysis of tau oligomers involved in the etiopathogenesis of various tauopathies including Alzheimer's disease.

First-in-Rat Study of Human Alzheimer's Disease Tau Propagation

One of the key features of misfolded tau in human neurodegenerative disorders is its propagation from one brain area into many others. In the last decade, in vivo tau spreading has been replicated in several mouse transgenic models expressing mutated human tau as well as in normal non-transgenic mice. In this study, we demonstrate for the first time that insoluble tau isolated from human AD brain induces full-blown neurofibrillary pathology in a sporadic rat model of tauopathy expressing non-mutated truncated tau protein. By using specific monoclonal antibodies, we were able to monitor the spreading of tau isolated from human brain directly in the rat hippocampus. We found that exogenous human AD tau was able to spread from the area of injection and induce tau pathology. Interestingly, solubilisation of insoluble AD tau completely abolished the capability of tau protein to induce and spread of neurofibrillary pathology in the rat brain. Our results show that exogenous tau is able to induce and drive neurofibrillary pathology in rat model for human tauopathy in a similar way as it was described in various mouse transgenic models. Rat tau spreading model has many advantages over mouse and other organisms including size and complexity, and thus is highly suitable for identification of pathogenic mechanism of tau spreading.

Tau Proteins Cross the Blood-Brain Barrier

Tauopathies are a hallmark of many neurodegenerative diseases, including Alzheimer's disease and traumatic brain injuries. It has been demonstrated that amyloid-beta peptides, alpha-synuclein, and prion proteins cross the blood-brain barrier (BBB), contributing to their abilities to induce disease. Very little is known about whether tau proteins can cross the BBB. Here we systematically characterized several key forms of tau proteins to cross the BBB, including Tau-441 (2N4R), Tau-410 (2N3R), truncated tau 151-391 (0N4R), and truncated tau 121-227. All of these tau proteins crossed the BBB readily and bidirectonally; however, only Tau-410 had a saturable component to its influx. The tau proteins also entered the blood after their injection into the brain, with Tau 121-227 having the slowest exit from brain. The tau proteins varied in regards to their enzymatic stability in brain and blood and in their peripheral pharmacokinetics. These results show that blood-borne tau proteins could contribute to brain tauopathies. The result also suggest that the CNS can contribute to blood levels of tau, raising the possibility that, as suggested for other misfolded proteins, blood levels of tau proteins could be used as a biomarker of CNS disease.

What is the evidence that tau pathology spreads through prion-like propagation?

Emerging experimental evidence suggests that the spread of tau pathology in the brain in Tauopathies reflects the propagation of abnormal tau species along neuroanatomically connected brain areas. This propagation could occur through a "prion-like" mechanism involving transfer of abnormal tau seeds from a "donor cell" to a "recipient cell" and recruitment of normal tau in the latter to generate new tau seeds. This review critically appraises the evidence that the spread of tau pathology occurs via such a "prion-like" mechanism and proposes a number of recommendations for directing future research. Recommendations for definitions of frequently used terms in the tau field are presented in an attempt to clarify and standardize interpretation of research findings. Molecular and cellular factors affecting tau aggregation are briefly reviewed, as are potential contributions of physiological and pathological post-translational modifications of tau. Additionally, the experimental evidence for tau seeding and "prion-like" propagation of tau aggregation that has emerged from cellular assays and in vivo models is discussed. Propagation of tau pathology using "prion-like" mechanisms is expected to incorporate several steps including cellular uptake, templated seeding, secretion and intercellular transfer through synaptic and non-synaptic pathways. The experimental findings supporting each of these steps are reviewed. The clinical validity of these experimental findings is then debated by considering the supportive or contradictory findings from patient samples. Further, the role of physiological tau release in this scenario is examined because emerging data shows that tau is secreted but the physiological function (if any) of this secretion in the context of propagation of pathological tau seeds is unclear. Bona fide prions exhibit specific properties, including transmission from cell to cell, tissue to tissue and organism to organism. The propagation of tau pathology has so far not been shown to exhibit all of these steps and how this influences the debate of whether or not abnormal tau species can propagate in a "prion-like" manner is discussed. The exact nature of tau seeds responsible for propagation of tau pathology in human tauopathies remains controversial; it might be tightly linked to the existence of tau strains stably propagating peculiar patterns of neuropathological lesions, corresponding to the different patterns seen in human tauopathies. That this is a property shared by all seed-competent tau conformers is not yet firmly established. Further investigation is also required to clarify the relationship between propagation of tau aggregates and tau-induced toxicity. Genetic variants identified as risks factors for tauopathies might play a role in propagation of tau pathology, but many more studies are needed to document this. The contribution of selective vulnerability of neuronal populations, as an alternative to prion-like mechanisms to explain spreading of tau pathology needs to be clarified. Learning from the prion field will be helpful to enhance our understanding of propagation of tau pathology. Finally, development of better models is expected to answer some of these key questions and allow for the testing of propagation-centred therapies.

Tau can switch microtubule network organizations: from random networks to dynamic and stable bundles

Tau is a neuronal microtubule bundler that is known to stabilize microtubules by promoting their growth and inhibiting their shrinkage. This study reveals novel mechanisms by which tau is able to switch microtubule network organizations via the differential regulation of microtubule bundling and dynamics.

In neurons, microtubule networks alternate between single filaments and bundled arrays under the influence of effectors controlling their dynamics and organization. Tau is a microtubule bundler that stabilizes microtubules by stimulating growth and inhibiting shrinkage. The mechanisms by which tau organizes microtubule networks remain poorly understood. Here, we studied the self-organization of microtubules growing in the presence of tau isoforms and mutants. The results show that tau’s ability to induce stable microtubule bundles requires two hexapeptides located in its microtubule-binding domain and is modulated by its projection domain. Site-specific pseudophosphorylation of tau promotes distinct microtubule organizations: stable single microtubules, stable bundles, or dynamic bundles. Disease-related tau mutations increase the formation of highly dynamic bundles. Finally, cryo–electron microscopy experiments indicate that tau and its variants similarly change the microtubule lattice structure by increasing both the protofilament number and lattice defects. Overall, our results uncover novel phosphodependent mechanisms governing tau’s ability to trigger microtubule organization and reveal that disease-related modifications of tau promote specific microtubule organizations that may have a deleterious impact during neurodegeneration.

Safety and immunogenicity of the tau vaccine AADvac1 in patients with Alzheimer's disease: a randomised, double-blind, placebo-controlled, phase 1 trial

BACKGROUND

Neurofibrillary pathology composed of tau protein is a main correlate of cognitive impairment in patients with Alzheimer's disease. Immunotherapy targeting pathological tau proteins is therefore a promising strategy for disease-modifying treatment of Alzheimer's disease. We have developed an active vaccine, AADvac1, against pathological tau proteins and assessed it in a phase 1 trial.

METHODS

We did a first-in-man, phase 1, 12 week, randomised, double-blind, placebo-controlled study of AADvac1 with a 12 week open-label extension in patients aged 50-85 years with mild-to-moderate Alzheimer's disease at four centres in Austria. We randomly assigned patients with a computer-generated sequence in a 4:1 ratio overall to receive AADvac1 or placebo. They received three subcutaneous doses of AADvac1 or placebo from masked vaccine kits at monthly intervals, and then entered the open-label phase, in which all patients were allocated to AADvac1 treatment and received another three doses at monthly intervals. Patients, carers, and all involved with the trial were masked to treatment allocation. The primary endpoint was all-cause treatment-emergent adverse events, with separate analyses for injection site reactions and other adverse events. We include all patients who received at least one dose of AADvac1 in the safety assessment. Patients who had a positive IgG titre against the tau peptide component of AADvac1 at least once during the study were classified as responders. The first-in-man study is registered with EU Clinical Trials Register, number EudraCT 2012-003916-29, and ClinicalTrials.gov, number NCT01850238; the follow-up study, which is ongoing, is registered with EU Clinical Trials Register, number EudraCT 2013-004499-36, and ClinicalTrials.gov, number NCT02031198.

FINDINGS

This study was done between June 9, 2013, and March 26, 2015. 30 patients were randomly assigned in the double-blind phase: 24 patients to the AADvac1 group and six to the placebo group. A total of 30 patients received AADvac1. Two patients withdrew because of serious adverse events. The most common adverse events were injection site reactions after administration (reported in 16 [53%] vaccinated patients [92 individual events]). No cases of meningoencephalitis or vasogenic oedema occurred after administration. One patient with pre-existing microhaemorrhages had newly occurring microhaemorrhages. Of 30 patients given AADvac1, 29 developed an IgG immune response. A geometric mean IgG antibody titre of 1:31415 was achieved. Baseline values of CD3+ CD4+ lymphocytes correlated with achieved antibody titres.

INTERPRETATION

AADvac1 had a favourable safety profile and excellent immunogenicity in this first-in-man study. Further trials are needed to corroborate the safety assessment and to establish proof of clinical efficacy of AADvac1.

FUNDING

AXON Neuroscience SE.

Physiological functions and clinical implications of the N-end rule pathway

The N-end rule pathway is a unique branch of the ubiquitin-proteasome system in which the determination of a protein's half-life is dependent on its N-terminal residue. The N-terminal residue serves as the degradation signal of a protein and thus called N-degron. N-degron can be recognized and modifed by several steps of post-translational modifications, such as oxidation, deamination, arginylation or acetylation, it then polyubiquitinated by the N-recognin for degradation. The molecular basis of the N-end rule pathway has been elucidated and its physiological functions have been revealed in the past 30 years. This pathway is involved in several biological aspects, including transcription, differentiation, chromosomal segregation, genome stability, apoptosis, mitochondrial quality control, cardiovascular development, neurogenesis, carcinogenesis, and spermatogenesis. Disturbance of this pathway often causes the failure of these processes, resulting in some human diseases. This review summarized the physiological functions of the N-end rule pathway, introduced the related biological processes and diseases, with an emphasis on the inner link between this pathway and certain symptoms.

Subacute Changes in Cleavage Processing of Amyloid Precursor Protein and Tau following Penetrating Traumatic Brain Injury

Traumatic brain injury (TBI) is an established risk factor for the development of Alzheimer's disease (AD). Here the effects of severe penetrating TBI on APP and tau cleavage processing were investigated in a rodent model of penetrating ballistic-like brain injury (PBBI). PBBI was induced by stereotactically inserting a perforated steel probe through the right frontal cortex of the anesthetized rat and rapidly inflating/deflating the probe's elastic tubing into an elliptical shaped balloon to 10% of total rat brain volume causing temporary cavitation injury. Separate animals underwent probe injury (PrI) alone without balloon inflation. Shams underwent craniectomy. Brain tissue was collected acutely (4h, 24h, 3d) and subacutely (7d) post-injury and analyzed by immunoblot for full length APP (APP-FL) and APP beta c-terminal fragments (βCTFs), full length tau (tau-FL) and tau truncation fragments and at 7d for cytotoxic Beta amyloid (Aβ) peptides Aβ40 and Aβ42 analysis. APP-FL was significantly decreased at 3d and 7d following PBBI whereas APP βCTFs were significantly elevated by 4h post-injury and remained elevated through 7d post-injury. Effects on βCTFs were mirrored with PrI, albeit to a lesser extent. Aβ40 and Aβ42 were significantly elevated at 7d following PBBI and PrI. Tau-FL decreased substantially 3d and 7d post-PBBI and PrI. Importantly, a 22 kDa tau fragment (tau22), similar to that found in AD, was significantly elevated by 4h and remained elevated through 7d post-injury. Thus both APP and tau cleavage was dramatically altered in the acute and subacute periods post-injury. As cleavage of these proteins has also been implicated in AD, TBI pathology shown here may set the stage for the later development of AD or other tauopathies.

Nuclear Tau and Its Potential Role in Alzheimer's Disease

Tau protein, found in both neuronal and non-neuronal cells, forms aggregates in neurons that constitutes one of the hallmarks of Alzheimer’s disease (AD). For nearly four decades, research efforts have focused more on tau’s role in physiology and pathology in the context of the microtubules, even though, for over three decades, tau has been localised in the nucleus and the nucleolus. Its nuclear and nucleolar localisation had stimulated many questions regarding its role in these compartments. Data from cell culture, mouse brain, and the human brain suggests that nuclear tau could be essential for genome defense against cellular distress. However, its nature of translocation to the nucleus, its nuclear conformation and interaction with the DNA and other nuclear proteins highly suggest it could play multiple roles in the nucleus. To find efficient tau-based therapies, there is a need to understand more about the functional relevance of the varied cellular distribution of tau, identify whether specific tau transcripts or isoforms could predict tau’s localisation and function and how they are altered in diseases like AD. Here, we explore the cellular distribution of tau, its nuclear localisation and function and its possible involvement in neurodegeneration.

A2A adenosine receptor deletion is protective in a mouse model of Tauopathy

Consumption of caffeine, a non-selective adenosine A2A receptor (A2AR) antagonist, reduces the risk of developing Alzheimer's disease (AD) in humans and mitigates both amyloid and Tau burden in transgenic mouse models. However, the impact of selective A2AR blockade on the progressive development of AD-related lesions and associated memory impairments has not been investigated. In the present study, we removed the gene encoding A2AR from THY-Tau22 mice and analysed the subsequent effects on both pathological (Tau phosphorylation and aggregation, neuro-inflammation) and functional impairments (spatial learning and memory, hippocampal plasticity, neurotransmitter profile). We found that deleting A2ARs protect from Tau pathology-induced deficits in terms of spatial memory and hippocampal long-term depression. These effects were concomitant with a normalization of the hippocampal glutamate/gamma-amino butyric acid ratio, together with a global reduction in neuro-inflammatory markers and a decrease in Tau hyperphosphorylation. Additionally, oral therapy using a specific A2AR antagonist (MSX-3) significantly improved memory and reduced Tau hyperphosphorylation in THY-Tau22 mice. By showing that A2AR genetic or pharmacological blockade improves the pathological phenotype in a Tau transgenic mouse model, the present data highlight A2A receptors as important molecular targets to consider against AD and Tauopathies.

Mislocalization of neuronal tau in the absence of tangle pathology in phosphomutant tau knockin mice

Hyperphosphorylation and fibrillar aggregation of the microtubule-associated protein tau are key features of Alzheimer's disease and other tauopathies. To investigate the involvement of tau phosphorylation in the pathological process, we generated a pair of complementary phosphomutant tau knockin mouse lines. One exclusively expresses phosphomimetic tau with 18 glutamate substitutions at serine and/or threonine residues in the proline-rich and first microtubule-binding domains to model hyperphosphorylation, whereas its phosphodefective counterpart has matched alanine substitutions. Consistent with expected effects of genuine phosphorylation, association of the phosphomimetic tau with microtubules and neuronal membranes is severely disrupted in vivo, whereas the phosphodefective mutations have more limited or no effect. Surprisingly, however, age-related mislocalization of tau is evident in both lines, although redistribution appears more widespread and more pronounced in the phosphomimetic tau knockin. Despite these changes, we found no biochemical or immunohistological evidence of pathological tau aggregation in mice of either line up to at least 2 years of age. These findings raise important questions about the role of tau phosphorylation in driving pathology in human tauopathies.

Invited review: Drug development for tauopathies

Many different approaches to treating tauopathies are currently being explored, with a few compounds already in clinical development (including small molecules such as anti-aggregation compound LMTX and active vaccines AADvac1 and ACI-35). This review aims to summarize the status of the clinical candidates and to highlight the emerging areas of research that hold promise for drug development. Tau is post-translationally modified in several different ways (phosphorylated, acetylated, glycosylated and truncated). The extent of these modifications can be manipulated to influence tau aggregation state and pathogenesis and the enzymes involved provide tractable targets for drug intervention. In addition, modulation of tau expression levels is an attractive therapeutic approach. Finally, the recently described prion-like spreading of tau between cells opens up novel avenues from the tau drug development perspective. The review compares the merits of small-molecule and antibody-based therapies and emphasizes the need for amenable clinical biomarkers for drug development, particularly PET imaging.

Role of the Tau N-terminal region in microtubule stabilization revealed by new endogenous truncated forms

Tau is a central player in Alzheimer's disease (AD) and related Tauopathies, where it is found as aggregates in degenerating neurons. Abnormal post-translational modifications, such as truncation, are likely involved in the pathological process. A major step forward in understanding the role of Tau truncation would be to identify the precise cleavage sites of the several truncated Tau fragments that are observed until now in AD brains, especially those truncated at the N-terminus, which are less characterized than those truncated at the C-terminus. Here, we optimized a proteomics approach and succeeded in identifying a number of new N-terminally truncated Tau species from the human brain. We initiated cell-based functional studies by analyzing the biochemical characteristics of two N-terminally truncated Tau species starting at residues Met11 and Gln124 respectively. Our results show, interestingly, that the Gln124-Tau fragment displays a stronger ability to bind and stabilize microtubules, suggesting that the Tau N-terminal domain could play a direct role in the regulation of microtubule stabilization. Future studies based on our new N-terminally truncated-Tau species should improve our knowledge of the role of truncation in Tau biology as well as in the AD pathological process.

First-in-man tau vaccine targeting structural determinants essential for pathological tau-tau interaction reduces tau oligomerisation and neurofibrillary degeneration in an Alzheimer's disease model

INTRODUCTION

We have identified structural determinants on tau protein that are essential for pathological tau-tau interaction in Alzheimer's disease (AD). These regulatory domains, revealed by monoclonal antibody DC8E8, represent a novel target for tau-directed therapy. In order to validate this target, we have developed an active vaccine, AADvac1.

METHODS

A tau peptide encompassing the epitope revealed by DC8E8 was selected for the development of an active vaccine targeting structural determinants on mis-disordered tau protein that are essential for pathological tau-tau interaction. The efficacy of the vaccine was tested in a transgenic rat model of human tauopathies. Toxicology and safety pharmacology studies were conducted under good laboratory practice conditions in multiple rodent and nonrodent species.

RESULTS

We have administered the tau peptide vaccine to a rat model of AD to investigate whether the vaccine can improve its clinical, histopathological and biochemical AD phenotype. Our results show that vaccination induced a robust protective humoral immune response, with antibodies discriminating between pathological and physiological tau. Active immunotherapy reduced the levels of tau oligomers and the extent of neurofibrillary pathology in the brains of transgenic rats. Strikingly, immunotherapy has reduced AD-type hyperphosphorylation of tau by approximately 95%. Also, the tau peptide vaccine improved the clinical phenotype of transgenic animals. Toxicology and safety pharmacology studies showed an excellent safety and tolerability profile of the AADvac1 vaccine.

CONCLUSIONS

Active immunisation targeting crucial domains of Alzheimer tau eliminated tau aggregation and neurofibrillary pathology. Most importantly, the AD type of tau hyperphosphorylation was abolished by vaccination across a wide range of AD phospho-epitopes. Our results demonstrate that active immunisation led to elimination of all major hallmarks of neurofibrillary pathology, which was reflected by a profound improvement in the clinical presentation of transgenic rats. This makes the investigated tau peptide vaccine a highly promising candidate therapeutic for the disease-modifying treatment of AD. The tested vaccine displayed a highly favourable safety profile in preclinical toxicity studies, which opens up the possibility of using it for AD prophylaxis in the future. The vaccine has already entered phase I clinical trial under the name AADvac1.

TRIAL REGISTRATION

Current Controlled Trials NCT01850238. Registered 7 May 2013.

Identification of structural determinants on tau protein essential for its pathological function: novel therapeutic target for tau immunotherapy in Alzheimer's disease

Introduction

Pathologically modified tau protein is the main feature of Alzheimer’s disease (AD) and related tauopathies. Therefore, immunotherapies that target mis-disordered tau represent a promising avenue for the disease-modifying treatment of AD. In this report, we present our discovery of (1) a novel target for tau immunotherapy; (2) monoclonal antibody DC8E8, which neutralizes this target; and (3) the results of efficacy studies of DC8E8 in a murine model of tauopathy.

Methods

In vitro tau oligomerisation assays were used for the selection of antibodies. The therapeutic efficacy of DC8E8 was evaluated in transgenic mice. The structure of the DC8E8 epitope was determined by X-ray crystallography.

Results

Screening of a panel of monoclonal antibodies for their inhibitory activity in an in vitro pathological tau–tau interaction assay yielded DC8E8, which reduced the amount of oligomeric tau by 84%. DC8E8 recognised all developmental stages of tau pathology in AD human brains, including pretangles and intra- and extracellular tangles. Treatment with DC8E8 in a mouse AD model expressing mis-disordered human tau significantly reduced the amount of insoluble oligomerised tau and the number of early and mature neurofibrillary tangles in the transgenic mouse brains. By using a panel of tau-derived peptides in a competitive enzyme-linked immunosorbent assay, we identified the tau domain essential for pathological tau–tau interaction, which is targeted by DC8E8. The antibody was capable of binding to four highly homologous and yet independent binding regions on tau, each of which is a separate epitope. The X-ray structure of the DC8E8 Fab apo form, solved at 3.0 Å, suggested that the four DC8E8 epitopes form protruding structures on the tau molecule. Finally, by kinetic measurements with surface plasmon resonance, we determined that antibody DC8E8 is highly discriminatory between pathological and physiological tau.

Conclusions

We have discovered defined determinants on mis-disordered truncated tau protein which are responsible for tau oligomerisation leading to neurofibrillary degeneration. Antibody DC8E8 reactive with these determinants is able to inhibit tau–tau interaction in vitro and in vivo. DC8E8 is able to discriminate between the healthy and diseased tau proteome, making its epitopes suitable targets, and DC8E8 a suitable candidate molecule, for AD immunotherapy.

Microtubule-associated protein tau as a therapeutic target in Alzheimer's disease

INTRODUCTION

Alzheimer's disease (AD) is a major public health problem in modern society and as yet, other than a few symptomatic drugs, there is no disease-modifying treatment for this disease available.

AREAS COVERED

Neurofibrillary pathology, which is made up from abnormally hyperphosphorylated microtubule-associated protein tau, is both a hallmark and key lesion of AD and related tauopathies. The density of neurofibrillary pathology in the cerebral cortex correlates with the degree of dementia. Both experimental and transgenic animal studies have consistently shown that abnormal hyperphosphorylation of tau causes cognitive impairment. Abnormal hyperphosphorylation of tau converts it from a microtubule assembly-promoting to a microtubule-disrupting protein and promotes its self-assembly into paired helical filaments. To date, the bulk of studies have shown that abnormal hyperphosphorylation is the key gain of toxic function step though some cell culture and transgenic mouse studies have also reported that aggregated tau can lead to neurodegeneration. In this article, we have reviewed data from our lab and that from PubMed search on the molecular mechanism of tau pathology and the potential of tau as a therapeutic target for AD and related disorders.

EXPERT OPINION

In our opinion, inhibition of abnormal hyperphosphorylation of tau is the most rational therapeutic target. Therapeutic approaches include restoration of the activity of protein phosphatase-2A, which is the major regulator of tau phosphorylation and the activity of which is compromised in AD brain, inhibition of one or more tau protein kinases which include GSK-3β, cyclin-dependent protein kinase-5, dual-specificity tyrosine phosphorylated-regulated kinase 1A, Ca(2+)/calmodulin-activated protein kinase II and casein kinase I, enhancement of O-GlcNAcylation of tau, and tau immunization.

Hyperglycemia-induced tau cleavage in vitro and in vivo: a possible link between diabetes and Alzheimer's disease

Multiple lines of evidence link the incidence of diabetes to the development of Alzheimer's disease (AD). Patients with diabetes have a 50 to 75% increased risk of developing AD. In parallel, AD patients have a higher than normal tendency to develop type 2 diabetes or impaired fasting glucose. Tau is the major component of neurofibrillary tangles, one of the hallmarks of AD pathology. The current study examined the effect of hyperglycemia on tau modification. Glucose treatment of rat embryonic cortical neurons results in concentration-dependent apoptosis and caspase-3 activation. These changes are well correlated with glucose time- and concentration-dependent tau cleavage. Aβ treatment induces tau cleavage and when added together with glucose, there is an additive effect on caspase activation, apoptosis, and tau cleavage. Tau cleavage is partially blocked by the caspase inhibitor, ZVAD. Cleaved tau displays a punctate staining along the neurites and colocalizes with cleaved caspase-3 in the cytoplasm. Both type 1 and type 2 diabetic mice display increased tau phosphorylation in the brain. In agreement with the effects of glucose on tau modifications in vitro, there is increased tau cleavage in the brains of ob/ob mice; however, tau cleavage is not observed in type 1 diabetic mouse brains. Our study demonstrates that hyperglycemia is one of major factors that induce tau modification in both in vitro and in vivo models of diabetes. We speculate that tau cleavage in diabetic conditions (especially in type 2 diabetes) may be a key link for the increased incidence of AD in diabetic patients.

What Renders TAU Toxic

TAU is a microtubule-associated protein that under pathological conditions such as Alzheimer's disease (AD) forms insoluble, filamentous aggregates. When 20 years after TAU's discovery the first TAU transgenic mouse models were established, one declared goal that was achieved was the modeling of authentic TAU aggregate formation in the form of neurofibrillary tangles. However, as we review here, it has become increasingly clear that TAU causes damage much before these filamentous aggregates develop. In fact, because TAU is a scaffolding protein, increased levels and an altered subcellular localization (due to an increased insolubility and impaired clearance) result in the interaction of TAU with cellular proteins with which it would otherwise either not interact or do so to a lesser degree, thereby impairing their physiological functions. We specifically discuss the non-axonal localization of TAU, the role phosphorylation has in TAU toxicity and how TAU impairs mitochondrial functions. A major emphasis is on what we have learned from the four available TAU knock-out models in mice, and the knock-out of the TAU/MAP2 homolog PTL-1 in worms. It has been proposed that in human pathological conditions such as AD, a rare toxic TAU species exists which needs to be specifically removed to abrogate TAU's toxicity and restore neuronal functions. However, what is toxic in one context may not be in another, and simply reducing, but not fully abolishing TAU levels may be sufficient to abrogate TAU toxicity.

Twice is better: highlights of the second meeting focused on tau biology and pathology

It is an exciting time for tau researchers as it is now generally accepted that abnormal tau species are required to mediate the toxic effects of amyloid β-peptide oligomers in Alzheimer's disease. Tau may play multiple roles in neurophysiology and there may be further pathologically relevant tau alterations, besides hyperphosphorylation and aggregation. The recent Biology and Pathology of Tau and its Role in Tauopathies II meeting explored these various aspects of tau, and presentations at the meeting, described in the following articles in this issue of Biochemical Society Transactions, are outlined in the present paper.