Tau immunotherapy modulates both pathological tau and upstream amyloid pathology in an Alzheimer's disease mouse model.

In Alzheimer's disease (AD), the pathological accumulation of tau appears to be a downstream effect of amyloid ß protein (Aß). However, the relationship between these two proteins and memory loss is unclear. In this study, we evaluated the specific removal of pathological tau oligomers in aged Tg2576 mice by passive immunotherapy using tau oligomer-specific monoclonal antibody. Removal of tau oligomers reversed memory deficits and accelerated plaque deposition in the brain. Surprisingly, Aß*56 levels decreased, suggesting a link between tau and Aß oligomers in the promotion of cognitive decline. The results suggest that tau oligomerization is not only a consequence of Aß pathology but also a critical mediator of the toxic effects observed afterward in AD. Overall, these findings support the potential of tau oligomers as a therapeutic target for AD.

Passive immunization with Tau oligomer monoclonal antibody reverses tauopathy phenotypes without affecting hyperphosphorylated neurofibrillary tangles.

Recent findings suggest that tau oligomers, which form before neurofibrillary tangles (NFTs), are the true neurotoxic tau entities in neurodegenerative tauopathies, including Alzheimer's disease (AD). Studies in animal models of tauopathy suggest that tau oligomers play a key role in eliciting behavioral and cognitive impairments. Here, we used a novel tau oligomer-specific monoclonal antibody (TOMA) for passive immunization in mice expressing mutant human tau. A single dose of TOMA administered either intravenously or intracerebroventricularly was sufficient to reverse both locomotor and memory deficits in a mouse model of tauopathy for 60 d, coincident with rapid reduction of tau oligomers but not phosphorylated NFTs or monomeric tau. Our data demonstrate that antibody protection is mediated by extracellular and rapid peripheral clearance. These findings provide the first direct evidence in support of a critical role for tau oligomers in disease progression and validate tau oligomers as a target for the treatment of AD and other neurodegenerative tauopathies.

Amyloid-ß oligomers as a template for secondary amyloidosis in Alzheimer's disease.

Alzheimer's disease is a complex disease characterized by overlapping phenotypes with different neurodegenerative disorders. Oligomers are considered the most toxic species in amyloid pathologies. We examined human AD brain samples using an anti-oligomer antibody generated in our laboratory and detected potential hybrid oligomers composed of amyloid-ß, prion protein, α-synuclein, and TDP-43 phosphorylated at serines 409 and 410. These data and in vitro results suggest that Aß oligomer seeds act as a template for the aggregation of other proteins and generate an overlapping phenotype with other neuronal disorders. Furthermore, these results could explain why anti-amyloid-ß therapy has been unsuccessful.

Differential activation of the ER stress factor XBP1 by oligomeric assemblies.

Several neurodegenerative disorders are characterized by protein misfolding, a phenomenon that results in perturbation of cellular homeostasis. We recently identified the protective activity of the ER stress response factor XBP1 (X-box binding protein 1) against Amyloid-ß1-42 (Aß42) neurotoxicity in cellular and Drosophila models of Alzheimer's disease. Additionally, subtoxic concentrations of Aß42 soluble aggregates (oligomers) induced accumulation of spliced (active) XBP1 transcripts, supporting the involvement of the ER stress response in Aß42 neurotoxicity. Here, we tested the ability of three additional disease-related amyloidogenic proteins to induce ER stress by analyzing XBP1 activation at the RNA level. Treatment of human SY5Y neuroblastoma cells with homogeneous preparations of α-Synuclein (α-Syn), Prion protein (PrP106-126), and British dementia amyloid peptide (ABri1-34) confirmed the high toxicity of oligomers compared to monomers and fibers. Additionally, α-Syn oligomers, but not monomers or fibers, demonstrated potent induction of XBP1 splicing. On the other hand, PrP106-126 and ABri1-34 did not activate XBP1. These results illustrate the biological complexity of these structurally related assemblies and argue that oligomer toxicity depends on the activation of amyloid-specific cellular responses.

The prion-like transmission of tau oligomers via exosomes.

The conversion and transmission of misfolded proteins established the basis for the prion concept. Neurodegenerative diseases are considered "prion-like" disorders that lack infectivity. Among them, tauopathies are characterized by the conversion of native tau protein into an abnormally folded aggregate. During the progression of the disease, misfolded tau polymerizes into oligomers and intracellular neurofibrillary tangles (NFTs). While the toxicity of NFTs is an ongoing debate, the contribution of tau oligomers to early onset neurodegenerative pathogenesis is accepted. Tau oligomers are readily transferred from neuron to neuron propagating through the brain inducing neurodegeneration. Recently, transmission of tau oligomers via exosomes is now proposed. There is still too much to uncover about tau misfolding and propagation. Here we summarize novel findings of tau oligomers transmission and propagation via exosomes.

α-Synuclein Oligomers Induce a Unique Toxic Tau Strain.

BACKGROUND: The coexistence of α-synuclein and tau aggregates in several neurodegenerative disorders, including Parkinson's disease and Alzheimer's disease, raises the possibility that a seeding mechanism is involved in disease progression. METHODS: To further investigate the role of α-synuclein in the tau aggregation pathway, we performed a set of experiments using both recombinant and brain-derived tau and α-synuclein oligomers to seed monomeric tau aggregation in vitro and in vivo. Brain-derived tau oligomers were isolated from well-characterized cases of progressive supranuclear palsy (n = 4) and complexes of brain-derived α-synuclein/tau oligomers isolated from patients with Parkinson's disease (n = 4). The isolated structures were purified and characterized by standard biochemical methods, then injected into Htau mice (n = 24) to assess their toxicity and role in tau aggregation. RESULTS: We found that α-synuclein induced a distinct toxic tau oligomeric strain that avoids fibril formation. In vivo, Parkinson's disease brain-derived α-synuclein/tau oligomers administered into Htau mouse brains accelerated endogenous tau oligomer formation concurrent with increasing cell loss. CONCLUSIONS: Our findings provide evidence, for the first time, that α-synuclein enhances the harmful effects of tau, thus contributing to disease progression.

Soluble tau aggregates, not large fibrils, are the toxic species that display seeding and cross-seeding behavior.

Several studies have proposed that fibrillary aggregates of tau and other amyloidogenic proteins are neurotoxic and result in numerous neurodegenerative diseases. However, these studies usually involve sonication or extrusion through needles before experimentation. As a consequence, these methods may fragment large aggregates producing a mixture of aggregated species rather than intact fibrils. Therefore, the results of these experiments may be reflective of other amyloidogenic species, such as oligomers and/or protofibrils/short fibrils. To investigate the effects of sonication on the aggregation of tau and other amyloidogenic proteins, fibrils were prepared and well characterized, then sonicated and evaluated by various biochemical and biophysical methods to identify the aggregated species present. We found that indeed a mixture of aggregated species was present along with short fibrils indicating that sonication leads to impure fibril samples and should be analyzed with caution. Our results corroborate the previous studies showing that sonication of prion and Aß fibrils leads to the formation of toxic, soluble aggregates. We also show that the oligomeric forms are the most toxic species although it is unclear how sonication causes oligomer formation. Recent results suggest that these small toxic oligomers produced by sonication, rather than the stable fibrillar structures, are prion-like in nature displaying seeding and cross-seeding behavior.

Tau Oligomers: Cytotoxicity, Propagation, and Mitochondrial Damage.

Aging has long been considered as the main risk factor for several neurodegenerative disorders including a large group of diseases known as tauopathies. Even though neurofibrillary tangles (NFTs) have been examined as the main histopathological hallmark, they do not seem to play a role as the toxic entities leading to disease. Recent studies suggest that an intermediate form of tau, prior to NFT formation, the tau oligomer, is the true toxic species. However, the mechanisms by which tau oligomers trigger neurodegeneration remain unknown. This review summarizes recent findings regarding the role of tau oligomers in disease, including release from cells, propagation from affected to unaffected brain regions, uptake into cells, and toxicity via mitochondrial dysfunction. A greater understanding of tauopathies may lead to future advancements in regards to prevention and treatment.

Tau Oligomers: The Toxic Player at Synapses in Alzheimer's Disease.

Alzheimer's disease (AD) is a progressive disorder in which the most noticeable symptoms are cognitive impairment and memory loss. However, the precise mechanism by which those symptoms develop remains unknown. Of note, neuronal loss occurs at sites where synaptic dysfunction is observed earlier, suggesting that altered synaptic connections precede neuronal loss. The abnormal accumulation of amyloid-ß (Aß) and tau protein is the main histopathological feature of the disease. Several lines of evidence suggest that the small oligomeric forms of Aß and tau may act synergistically to promote synaptic dysfunction in AD. Remarkably, tau pathology correlates better with the progression of the disease than Aß. Recently, a growing number of studies have begun to suggest that missorting of tau protein from the axon to the dendrites is required to mediate the detrimental effects of Aß. In this review we discuss the novel findings regarding the potential mechanisms by which tau oligomers contribute to synaptic dysfunction in AD.

Antibody against Small Aggregated Peptide Specifically Recognizes Toxic Aß-42 Oligomers in Alzheimer's Disease.

Amyloid-beta (Aß) oligomers have emerged as the most toxic species in Alzheimer's disease (AD) and other amyloid pathologies. Also, Aß-42 peptide is more aggregation-prone compared to other Aß isoforms. Thus, we synthesized a small peptide of repeated sequence containing the last three amino acids, Val-40, Ile-41, and Ala-42 of Aß-42 that was subsequently aggregated and used to generate a novel antibody, VIA. In this study, we examined human AD and Tg2576 mouse brain samples using VIA in combination with other amyloid-specific antibodies and confirmed the specificity of VIA to oligomeric Aß-42. Moreover, we found that VIA does not recognize classic amyloid plaques composed of fibrillar Aß or Aß-40 ex vivo. Since VIA recognizes a distinct epitope specific to Aß-42 oligomers, it may have broad use for examining the accumulation of these oligomers in AD and other neurodegenerative diseases. VIA may also be used in immunotherapy studies to prevent neurodegenerative effects associated with Aß-42 oligomers.

Ataxin-1 oligomers induce local spread of pathology and decreasing them by passive immunization slows Spinocerebellar ataxia type 1 phenotypes.

Previously, we reported that ATXN1 oligomers are the primary drivers of toxicity in Spinocerebellar ataxia type 1 (SCA1; Lasagna-Reeves et al., 2015). Here we report that polyQ ATXN1 oligomers can propagate locally in vivo in mice predisposed to SCA1 following intracerebral oligomeric tissue inoculation. Our data also show that targeting these oligomers with passive immunotherapy leads to some improvement in motor coordination in SCA1 mice and to a modest increase in their life span. These findings provide evidence that oligomer propagation is regionally limited in SCA1 and that immunotherapy targeting extracellular oligomers can mildly modify disease phenotypes.

A native interactor scaffolds and stabilizes toxic ATAXIN-1 oligomers in SCA1.

Recent studies indicate that soluble oligomers drive pathogenesis in several neurodegenerative proteinopathies, including Alzheimer and Parkinson disease. Curiously, the same conformational antibody recognizes different disease-related oligomers, despite the variations in clinical presentation and brain regions affected, suggesting that the oligomer structure might be responsible for toxicity. We investigated whether polyglutamine-expanded ATAXIN-1, the protein that underlies spinocerebellar ataxia type 1, forms toxic oligomers and, if so, what underlies their toxicity. We found that mutant ATXN1 does form oligomers and that oligomer levels correlate with disease progression in the Atxn1(154Q/+) mice. Moreover, oligomeric toxicity, stabilization and seeding require interaction with Capicua, which is expressed at greater ratios with respect to ATXN1 in the cerebellum than in less vulnerable brain regions. Thus, specific interactors, not merely oligomeric structure, drive pathogenesis and contribute to regional vulnerability. Identifying interactors that stabilize toxic oligomeric complexes could answer longstanding questions about the pathogenesis of other proteinopathies.

Pathological interface between oligomeric alpha-synuclein and tau in synucleinopathies.

BACKGROUND: Aberrant accumulation of α-synuclein constitutes inclusion bodies that are considered a characteristic feature of a group of neurological disorders described as synucleinopathies. Often, multiple disease-causing proteins overlap within a given disease pathology. An emerging body of research focuses on the oligomeric populations of various pathogenic proteins, considering them as the culprits causing neuronal damage and degeneration. To this end, the use of conformation-specific antibodies has proven to be an effective tool. Previous work from our laboratory and others has shown that oligomeric entities of α-synuclein and tau accumulate in their respective diseases, but their interrelationship at this higher order has yet to be shown in synucleinopathies. METHODS: Here, we used two novel conformation-specific antibodies, F8H7 and Syn33, which recognize α-synuclein oligomers and were developed in our laboratory. We investigated brain tissue from five of each Parkinson's disease and dementia with Lewy bodies patients by performing biophysical and biochemical assays using these antibodies, in addition to the previously characterized anti-tau oligomer antibody T22. RESULTS: We demonstrate that in addition to the deposition of oligomeric α-synuclein, tau oligomers accumulate in these diseased brains compared with control brains. Moreover, we observed that oligomers of tau and α-synuclein exist in the same aggregates, forming hybrid oligomers in these patients' brains. CONCLUSIONS: In addition to the deposition of tau oligomers, our results also provide compelling evidence of co-occurrence of α-synuclein and tau into their most toxic forms, i.e., oligomers suggesting that these species interact and influence each other's aggregation via an interface in synucleinopathies.

Immunotherapy for the treatment of Alzheimer's disease: amyloid-ß or tau, which is the right target?

Alzheimer's disease (AD) is characterized by the presence of amyloid plaques composed mainly of amyloid-ß (Aß) protein. Overproduction or slow clearance of Aß initiates a cascade of pathologic events that may lead to formation of neurofibrillary tangles, neuronal cell death, and dementia. Although immunotherapy in animal models has been demonstrated to be successful at removing plaques or prefibrillar forms of Aß, clinical trials have yielded disappointing results. The lack of substantial cognitive improvement obtained by targeting Aß raises the question of whether or not this is the correct target. Another important pathologic process in the AD brain is tau aggregation, which seems to become independent once initiated. Recent studies targeting tau in AD mouse models have displayed evidence of cognitive improvement, providing a novel therapeutic approach for the treatment of AD. In this review, we describe new advances in immunotherapy targeting Aß peptide and tau protein, as well as future directions.

Therapeutic approaches against common structural features of toxic oligomers shared by multiple amyloidogenic proteins.

Impaired proteostasis is one of the main features of all amyloid diseases, which are associated with the formation of insoluble aggregates from amyloidogenic proteins. The aggregation process can be caused by overproduction or poor clearance of these proteins. However, numerous reports suggest that amyloid oligomers are the most toxic species, rather than insoluble fibrillar material, in Alzheimer's, Parkinson's, and Prion diseases, among others. Although the exact protein that aggregates varies between amyloid disorders, they all share common structural features that can be used as therapeutic targets. In this review, we focus on therapeutic approaches against shared features of toxic oligomeric structures and future directions.

Specific targeting of tau oligomers in Htau mice prevents cognitive impairment and tau toxicity following injection with brain-derived tau oligomeric seeds.

Neurodegenerative disease is one of the greatest health crises in the world and as life expectancy rises, the number of people affected will continue to increase. The most common neurodegenerative disease, Alzheimer's disease, is a tauopathy, characterized by the presence of aggregated tau, namely in the form of neurofibrillary tangles. Historically, neurofibrillary tangles have been considered the main tau species of interest in Alzheimer's disease; however, we and others have shown that tau oligomers may be the most toxic form and the species responsible for the spread of pathology. We developed a novel anti-tau oligomer-specific mouse monoclonal antibody (TOMA) and investigated the potential of anti-tau oligomer passive immunization in preventing the toxicity of tau pathology in Htau mice. We injected pure brain-derived tau oligomers intracerebrally in 3-month-old wild-type and Htau mice and investigated the protective effects of a single 60 µg TOMA injection when compared to the same dose of non-specific IgG and found that TOMA conferred protection against the accumulation of tau oligomers and cognitive deficits for up to 1 month after treatment. Additionally, we injected pure brain-derived tau oligomers intracerebrally in 3-month-old wild-type and Htau mice and treated animals with biweekly injections of 60 µg TOMA or non-specific IgG. We found that long-term administration of TOMA was effective as a preventative therapy, inhibiting oligomeric tau and preserving memory function. These results support the critical role of oligomeric tau in disease progression and validate tau oligomers as a potential drug target.

Characterization of tau oligomeric seeds in progressive supranuclear palsy.

BACKGROUND: Progressive supranuclear palsy (PSP) is a neurodegenerative tauopathy which is primarily defined by the deposition of tau into globose-type neurofibrillary tangles (NFT). Tau in its native form has important functions for microtubule dynamics. Tau undergoes alternative splicing in exons 2, 3, and 10 which results in six different isoforms. Products of splicing on exon 10 are the most prone to mutations. Three repeat (3R) and four repeat (4R) tau, like other disease-associated amyloids, can form oligomers which may then go on to further aggregate and form fibrils. Recent studies from our laboratory and others have provided evidence that tau oligomers, not NFTs, are the most toxic species in neurodegenerative tauopathies and seed the pathological spread of tau. RESULTS: Analysis of PSP brain sections revealed globose-type NFTs, as well as both phosphorylated and unphosphorylated tau oligomers. Analysis of PSP brains via Western blot and ELISA revealed the presence of increased levels of tau oligomers compared to age-matched control brains. Oligomers were immunoprecipitated from PSP brain and were capable of seeding the oligomerization of both 3R and 4R tau isoforms. CONCLUSIONS: This is the first time tau oligomers have been characterized in PSP. These results indicate that tau oligomers are an important component of PSP pathology, along with NFTs. The ability of PSP brain-derived tau oligomers to seed 3R and 4R tau suggests that these oligomers represent the pathological species responsible for disease propagation and the presence of oligomers in a pure neurodegenerative tauopathy implies a common neuropathological process for tau seen in diseases with other amyloid proteins.

Alzheimer's disease imaging with a novel Tau targeted near infrared ratiometric probe.

Neurofibrillary tangles (NFTs) have long been recognized as one of the pathological hallmarks in Alzheimer's disease (AD). Recent studies, however, showed that soluble aggregated Tau species, especially hyperphosphorylated Tau oligomers, which are formed at early stage of AD prior to the formation of NFT, disrupted neural system integration. Unfortunately, little is known about Tau aggregates, and few Tau targeted imaging probe has been reported. Successful development of new imaging methods that can visualize early stages of Tau aggregation specifically will obviously be important for AD imaging, as well as understanding Tau-associated neuropathology of AD. Here, we report the first NIR ratiometric probe, CyDPA2, that targets Tau aggregates. The specificity of CyPDA2 to aggregated Tau was evaluated with in vitro hyperphosphorylated Tau proteins (pTau), as well as ex vivo Tau samples from AD human brain samples and the tauopathy transgenic mouse model, P301L. The characteristic enhancements of absorption ratio and fluorescence intensity in CyDPA2 were observed in a pTau concentration-dependent manner. In addition, fluorescence microscopy and gel staining studies demonstrated CyDPA2-labeled Tau aggregates. These data indicate that CyDPA2 is a promising imaging probe for studying Tau pathology and diagnosing AD at an early stage.

Design of metastable ß-sheet oligomers from natively unstructured peptide.

Amyloid oligomers represent the primary pathological species for neurodegenerative diseases such as Alzheimer's and Parkinson's diseases. Toxic oligomers are formed by many different proteins and peptides, but their polydispersity makes them highly dynamic and heterogeneous. One way to stabilize these structures is to prepare constrained peptides that can be used to study amyloid intermediates, to identify oligomer-specific drugs, and to generate conformational antibodies. These conformational antibodies have demonstrated that oligomers share a common epitope. In this research, we used a 40-amino acid unstructured segment of prion protein (Prp) 109-148 with substitutions of methionine for glycine (Prp-G) residues to prepare a stable and homogeneous population of ß-sheet oligomer mimics. These structures were characterized by multiple biophysical and biochemical techniques that show characteristic features of oligomers. Finally, this preparation was not detected by three different sequence dependent prion antibodies.

Alzheimer brain-derived tau oligomers propagate pathology from endogenous tau.

Intracerebral injection of brain extracts containing amyloid or tau aggregates in transgenic animals can induce cerebral amyloidosis and tau pathology. We extracted pure populations of tau oligomers directly from the cerebral cortex of Alzheimer disease (AD) brain. These oligomers are potent inhibitors of long term potentiation (LTP) in hippocampal brain slices and disrupt memory in wild type mice. We observed for the first time that these authentic brain-derived tau oligomers propagate abnormal tau conformation of endogenous murine tau after prolonged incubation. The conformation and hydrophobicity of tau oligomers play a critical role in the initiation and spread of tau pathology in the naïve host in a manner reminiscent of sporadic AD.