Cellular Models for the Study of Prions

HTRA1 disaggregates α-synuclein amyloid fibrils and converts them into non-toxic and seeding incompetent species

Parkinson's disease (PD) is closely linked to α-synuclein (α-syn) misfolding and accumulation in Lewy bodies. The PDZ serine protease HTRA1 degrades fibrillar tau, which is associated with Alzheimer's disease, and inactivating mutations to mitochondrial HTRA2 are implicated in PD. Here, we report that HTRA1 inhibits aggregation of α-syn as well as FUS and TDP-43, which are implicated in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia. The protease domain of HTRA1 is necessary and sufficient for inhibiting aggregation, yet this activity is proteolytically-independent. Further, HTRA1 disaggregates preformed α-syn fibrils, rendering them incapable of seeding aggregation of endogenous α-syn, while reducing HTRA1 expression promotes α-syn seeding. HTRA1 remodels α-syn fibrils by targeting the NAC domain, the key domain catalyzing α-syn amyloidogenesis. Finally, HTRA1 detoxifies α-syn fibrils and prevents formation of hyperphosphorylated α-syn accumulations in primary neurons. Our findings suggest that HTRA1 may be a therapeutic target for a range of neurodegenerative disorders.

Amyloidogenic propensity of self-assembling peptides and their adjuvant potential for use as DNA vaccines

De novo designed peptides that self-assemble into cross-β rich fibrillar biomaterials have been pursued as an innovative platform for the development of adjuvant- and inflammation-free vaccines. However, they share structural and morphological properties similar to amyloid species implicated in neurodegenerative diseases, which has been a long-standing concern for their successful translation. Here, we comprehensively characterize the amyloidogenic character of the amphipathic self-assembling cross-β peptide KFE8, compared to pathological amyloid and amyloid-like proteins α-synuclein (α-syn) and TDP-43. Further, we developed plasmid-based DNA vaccines with the KFE8 backbone serving as a scaffold for delivery of a GFP model antigen. We find that expression of tandem repeats of KFE8 is non-toxic and efficiently cleared by autophagy. We also demonstrate that preformed KFE8 fibrils do not cross-seed amyloid formation of α-syn in mammalian cells compared to α-syn preformed fibrils. In mice, vaccination with plasmids encoding the KFE32-GFP fusion protein elicited robust immune responses, inducing production of significantly higher levels of anti-GFP antibodies compared to soluble GFP. Antigen-specific CD8+T cells were also detected in the spleens of vaccinated mice and cytokine profiles from antigen recall assays indicate a balanced Th1/Th2 response. These findings illustrate that cross-β-rich peptide nanofibers have distinct physicochemical properties from those of pathological amyloidogenic proteins, and are an attractive platform for the development of DNA vaccines with self-adjuvanting properties and improved safety profiles. STATEMENT OF SIGNIFICANCE: Biomaterials comprised of self-assembling peptides hold great promise for the development of new vaccines that do not require use of adjuvants. However, these materials have safety concerns, as they self-assemble into cross-β rich fibrils that are structurally similar to amyloid species implicated in disease. Here, we comprehensively study the properties of these biomaterials. We demonstrate that they have distinct properties from pathological proteins. They are non-toxic and do not trigger amyloidogenesis. Vaccination of these materials in mice elicited a robust immune response. Most excitingly, our work suggests that this platform could be used to develop DNA-based vaccines, which have few storage requirements. Further, due to their genetic encoding, longer sequences can be generated and the vaccines will be amenable to modification.

HtrA1 prevents and reverses α-synuclein aggregation, rendering it non-toxic and seeding incompetent

Parkinson disease (PD) is closely linked to the misfolding and accumulation of α-synuclein (α-syn) into Lewy bodies. HtrA1 is a PDZ serine protease that degrades fibrillar tau, which is associated with Alzheimer disease (AD). Further, inactivating mutations to mitochondrial HtrA2 have been implicated in PD. Here, we establish that HtrA1 inhibits the aggregation of α-syn as well as FUS and TDP-43, which are implicated in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). We demonstrate that the protease domain of HtrA1 is necessary and sufficient for inhibition of aggregation, yet this activity is independent of HtrA1 proteolytic activity. Further, we find that HtrA1 also disaggregates preformed α-syn fibrils, which may promote their clearance. Treatment of α-syn fibrils with HtrA1 renders α-syn incapable of seeding the aggregation of endogenous α-syn in mammalian biosensor cells. We find that HtrA1 remodels α-syn by specifically targeting the NAC domain, which is the key domain that catalyzes α-syn oligomerization and fibrillization. Finally, in a primary neuron model of α-syn aggregation, we show that HtrA1 and its proteolytically inactive form both detoxify α-syn and prevent the formation of hyperphosphorylated α-syn accumulations. Our findings suggest that HtrA1 prevents aggregation and promotes disaggregation of multiple disease-associated proteins, and may be a therapeutic target for treating a range of neurodegenerative disorders.

Tau antibody 77G7 targeting microtubule binding domain suppresses proteopathic tau to seed tau aggregation

INTRODUCTION

Neurofibrillary tangle (NFT) of hyperphosphorylated tau is a hallmark of Alzheimer's disease (AD) and related tauopathies. Tau lesion starts in the trans-entorhinal cortex, from where it spreads to limbic regions, followed by neocortical areas. The regional distribution of NFTs associates with the progression of AD. Accumulating evidence suggests that proteopathic tau can seed tau aggregation in a prion-like fashion in vitro and in vivo. Inhibition of tau seeding activity could provide a potential therapeutic opportunity to block the propagation of tau pathology in AD and related tauopathies.

AIMS

In the present study, we investigated the role of 77G7, a monoclonal tau antibody to the microtubule-binding repeats, in repressing the seeding activity of proteopathic tau.

RESULTS

We found that 77G7 had a higher affinity toward aggregated pathological tau fractions than un-aggregated tau derived from AD brain. 77G7 inhibited the internalization of tau aggregates by cells, blocked AD O-tau to capture normal tau, and to seed tau aggregation in vitro and in cultured cells. Tau pathology induced by hippocampal injection of AD O-tau in 3xTg-AD mice was suppressed by mixing 77G7 with AD O-tau. Intravenous administration of 77G7 ameliorated site-specific hyperphosphorylation of tau induced by AD O-tau in the hippocampi of Tg/hTau mice.

CONCLUSION

These findings indicate that 77G7 can effectively suppress the seeding activity of AD O-tau and thus could be developed as a potential immunotherapeutic drug to inhibit the propagation of tau pathology in AD and related tauopathies.

Lysine 63-linked ubiquitination of tau oligomers contributes to the pathogenesis of Alzheimer's disease

Ubiquitin-modified tau aggregates are abundantly found in human brains diagnosed with Alzheimer's disease (AD) and other tauopathies. Soluble tau oligomers (TauO) are the most neurotoxic tau species that propagate pathology and elicit cognitive deficits, but whether ubiquitination contributes to tau formation and spreading is not fully understood. Here, we observed that K63-linked, but not K48-linked, ubiquitinated TauO accumulated at higher levels in AD brains compared with age-matched controls. Using mass spectrometry analyses, we identified 11 ubiquitinated sites on AD brain-derived TauO (AD TauO). We found that K63-linked TauO are associated with enhanced seeding activity and propagation in human tau-expressing primary neuronal and tau biosensor cells. Additionally, exposure of tau-inducible HEK cells to AD TauO with different ubiquitin linkages (wild type, K48, and K63) resulted in enhanced formation and secretion of K63-linked TauO, which was associated with impaired proteasome and lysosome functions. Multipathway analysis also revealed the involvement of K63-linked TauO in cell survival pathways, which are impaired in AD. Collectively, our study highlights the significance of selective TauO ubiquitination, which could influence tau aggregation, accumulation, and subsequent pathological propagation. The insights gained from this study hold great promise for targeted therapeutic intervention in AD and related tauopathies.

Tau strains shape disease

Tauopathies consist of over 25 different neurodegenerative diseases that include argyrophilic grain disease (AGD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and Pick’s disease (PiD). Tauopathies are defined by brain accumulation of microtubule-associated protein tau in fibrillar aggregates, whose prevalence strongly correlates with dementia. Dominant mutations in tau cause neurodegenerative diseases, and most increase its aggregation propensity. Pathogenesis of tauopathies may involve pathological tau conformers that serve as templates to recruit native protein into growing assemblies and also move between brain cells to cause disease progression, similar to prions. Prions adopt pathological conformations, termed “strains,” that stably propagate in living systems, and create unique patterns of neuropathology. Data from multiple laboratories now suggest that tau acts as a prion. It propagates unique strains indefinitely in cultured cells, and when these are inoculated into mouse models, they create defined neuropathological patterns, which establish a direct link between conformation and disease. In humans, distinct fibril structures are associated with different diseases, but causality has not been established as in mice. Cryo-EM structures of tau fibrils isolated from tauopathy brains reveal distinct fibril cores across disease. Interestingly, the conformation of the tau monomer unit within different fibril subtypes from the same patient appears relatively preserved. This is consistent with data that the tau monomer samples an ensemble of conformations that act as distinct pathologic templates in the formation of restricted numbers of strains. The propensity of a tau monomer to adopt distinct conformations appears to be linked to defined local motifs that expose different patterns of amyloidogenic amino acid sequences. The prion hypothesis, which predicts that protein structure dictates resultant disease, has proved particularly useful to understand the diversity of human tauopathies. The challenge now is to develop methods to rapidly classify patients according to the structure of the underlying pathological protein assemblies to achieve more accurate diagnosis and effective therapy.

Risk of Transmissibility From Neurodegenerative Disease-Associated Proteins: Experimental Knowns and Unknowns

Recent studies in animal models demonstrate that certain misfolded proteins associated with neurodegenerative diseases can support templated misfolding of cognate native proteins, to propagate across neural systems, and to therefore have some of the properties of classical prion diseases like Creutzfeldt-Jakob disease. The National Institute of Aging convened a meeting to discuss the implications of these observations for research priorities. A summary of the discussion is presented here, with a focus on limitations of current knowledge, highlighting areas that appear to require further investigation in order to guide scientific practice while minimizing potential exposure or risk in the laboratory setting. The committee concluded that, based on all currently available data, although neurodegenerative disease-associated aggregates of several different non-prion proteins can be propagated from humans to experimental animals, there is currently insufficient evidence to suggest more than a negligible risk, if any, of a direct infectious etiology for the human neurodegenerative disorders defined in part by these proteins. Given the importance of this question, the potential for noninvasive human transmission of proteopathic disorders is deserving of further investigation.

Potential of Microfluidics and Lab-on-Chip Platforms to Improve Understanding of "prion-like" Protein Assembly and Behavior

Human aging is accompanied by a relevant increase in age-associated chronic pathologies, including neurodegenerative and metabolic diseases. The appearance and evolution of numerous neurodegenerative diseases is paralleled by the appearance of intracellular and extracellular accumulation of misfolded proteins in affected brains. In addition, recent evidence suggests that most of these amyloid proteins can behave and propagate among neural cells similarly to infective prions. In order to improve understanding of the seeding and spreading processes of these "prion-like" amyloids, microfluidics and 3D lab-on-chip approaches have been developed as highly valuable tools. These techniques allow us to monitor changes in cellular and molecular processes responsible for amyloid seeding and cell spreading and their parallel effects in neural physiology. Their compatibility with new optical and biochemical techniques and their relative availability have increased interest in them and in their use in numerous laboratories. In addition, recent advances in stem cell research in combination with microfluidic platforms have opened new humanized in vitro models for myriad neurodegenerative diseases affecting different cellular targets of the vascular, muscular, and nervous systems, and glial cells. These new platforms help reduce the use of animal experimentation. They are more reproducible and represent a potential alternative to classical approaches to understanding neurodegeneration. In this review, we summarize recent progress in neurobiological research in "prion-like" protein using microfluidic and 3D lab-on-chip approaches. These approaches are driven by various fields, including chemistry, biochemistry, and cell biology, and they serve to facilitate the development of more precise human brain models for basic mechanistic studies of cell-to-cell interactions and drug discovery.

FRET-based Tau seeding assay does not represent prion-like templated assembly of Tau filaments

Tau aggregation into amyloid fibers based on the cross-beta structure is a hallmark of several Tauopathies, including Alzheimer Disease (AD). Trans-cellular propagation of Tau with pathological conformation has been suggested as a key disease mechanism. This is thought to cause the spreading of Tau pathology in AD by templated conversion of naive Tau in recipient cells into a pathological state, followed by assembly of pathological Tau fibers, similar to the mechanism of nucleated polymerization proposed for prion pathogenesis. In cell cultures, the process is often monitored by a FRET assay where the recipient cell expresses the Tau repeat domain (Tau^RD) with a pro-aggregant mutation, fused to GFP-based FRET pairs. Since the size of the reporter GFP (barrel of ~ 3 nm × 4 nm) is ~ 7 times larger than the β-strand distance (0.47 nm), this points to a potential steric clash. Hence, we investigated the influence of the GFP tag on Tau^FL or Tau^RD aggregation. Using biophysical methods (light scattering, atomic force microscopy (AFM), and scanning-transmission electron microscopy (STEM)), we found that the assembly of Tau^RD-GFP was severely inhibited and incompatible with that of Alzheimer filaments. These observations argue against the hypothesis that the propagation of Tau pathology in AD is caused by the prion-like templated aggregation of Tau protein, transmitted via cell-to-cell spreading of Tau. Thus, even though the observed local increase of FRET in recipient cells may be a valid hallmark of a pathological reaction, our data argue that it is caused by a process distinct from assembly of Tau^RD filaments.

Cellular models of alpha-synuclein toxicity and aggregation

Misfolding and aggregation of alpha-synuclein (α-synuclein) with concomitant cytotoxicity is a hallmark of Lewy body related disorders such as Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. Although it plays a pivotal role in pathogenesis and disease progression, the function of α-synuclein and the molecular mechanisms underlying α-synuclein-induced neurotoxicity in these diseases are still elusive. Many in vitro and in vivo experimental models mimicking α-synuclein pathology such as oligomerization, toxicity and more recently neuronal propagation have been generated over the years. In particular, cellular models have been crucial for our comprehension of the pathogenic process of the disease and are beneficial for screening of molecules capable of modulating α-synuclein toxicity. Here, we review α-synuclein based cell culture models that reproduce some features of the neuronal populations affected in patients, from basic unicellular organisms to mammalian cell lines and primary neurons, to the cutting edge models of patient-specific cell lines. These reprogrammed cells known as induced pluripotent stem cells (iPSCs) have garnered attention because they closely reproduce the characteristics of neurons found in patients and provide a valuable tool for mechanistic studies. We also discuss how different cell models may constitute powerful tools for high-throughput screening of molecules capable of modulating α-synuclein toxicity and prevention of its propagation. This article is part of the Special Issue "Synuclein".

α-Synuclein: Multiple System Atrophy Prions

Multiple system atrophy (MSA) is a rapidly progressive neurodegenerative disease arising from the misfolding and accumulation of the protein α-synuclein in oligodendrocytes, where it forms glial cytoplasmic inclusions (GCIs). Several years of studying synthetic α-synuclein fibrils has provided critical insight into the ability of α-synuclein to template endogenous protein misfolding, giving rise to fibrillar structures capable of propagating from cell to cell. However, more recent studies with MSA-derived α-synuclein aggregates have shown that they have a similar ability to undergo template-directed propagation, like PrP prions. Almost 20 years after α-synuclein was discovered as the primary component of GCIs, α-synuclein aggregates isolated from MSA patient samples were shown to infect cultured mammalian cells and also to transmit neurological disease to transgenic mice. These findings argue that α-synuclein becomes a prion in MSA patients. In this review, we discuss the in vitro and in vivo data supporting the recent classification of MSA as a prion disease.

Chronic Traumatic Encephalopathy: Is Latency in Symptom Onset Explained by Tau Propagation?

Chronic traumatic encephalopathy (CTE) is a neurodegenerative tauopathy associated with repetitive mild brain trauma. CTE, previously termed "dementia pugilistica," has been identified in American football, ice hockey, baseball, rugby and soccer players, boxers, wrestlers, and military personnel exposed to blast and other traumatic brain injuries. There is often a long latency period between an individual's exposure to repetitive brain trauma and the clinical symptoms of CTE. The pathology of CTE is characterized by a progression from isolated focal perivascular hyperphosphorylated tau lesions in the cerebral cortex to a widespread tauopathy that involves diffuse cortical and medial temporal lobe regions. We hypothesize that the spread of tau from focal perivascular lesions to a widespread tauopathy occurs as a result of intraneuronal and intrasynaptic prion-like protein templating, as well as tau secretion and propagation along glymphatic and cerebrospinal fluid pathways.

Seeking a New Paradigm for Alzheimer's Disease: Considering the Roles of Inflammation, Blood-Brain Barrier Dysfunction, and Prion Disease

There is no effective etiologic treatment for Alzheimer's disease, nor is there a prophylactic medication which delays or prevents its onset. The lack of an accurate paradigm is undoubtedly related to the lack of effective means of prophylaxis and treatment. The current paradigm of beta amyloid in Alzheimer's brains causing cognitive dysfunction must be modified. Despite failed clinical trials, research continues into amyloid-oriented treatments. The persistence of the amyloid hypothesis/paradigm is an example of anchoring and representativeness heuristics described by Kahneman and Tversky in their classic 1974 Science paper. Economic factors also contribute to the persistence of this paradigm. Paradigms impact the scientific process by the following: (1) what is studied; (2) the types of questions that are asked; (3) the structure and nature of the questions; (4) the interpretations of research findings. We review the contribution of inflammation, malfunction of the neurovascular unit, and prion disease to Alzheimer's disease manifestations. Any or all of these are candidates for inclusion into a more accurate, inclusive, and useful new paradigm. By incorporating emerging facts and understanding into a new paradigm, we will enhance our ability to move toward effective prophylaxis and therapy for this tragic disease.

The Prion-Like Behavior of Assembled Tau in Transgenic Mice

Tauopathies constitute neurodegenerative diseases that are characterized by the intracellular deposition of filaments made of hyperphosphorylated tau protein. The pattern of tau deposition in Alzheimer's disease follows a stereotypical progression, with the first lesions appearing in the locus coeruleus and entorhinal cortex, from where they appear to spread to the hippocampus and neocortex. Propagation of pathological tau is also characteristic of argyrophilic grain disease, where the lesions seem to spread through distinct regions of the limbic system. In chronic traumatic encephalopathy, tauopathy appears to spread from the neocortex to the brainstem. These findings implicate neuron-to-neuron propagation of tau aggregates. Isoform compositions and morphologies of tau filaments can differ between tauopathies, which is consistent with the existence of distinct tau strains. Here, we review recent findings that support prion-like mechanisms in the pathogenesis of tauopathies through the experimental use of transgenic mice.

Bioassays and Inactivation of Prions

The experimental study of prions requires a model for their propagation. However, because prions lack nucleic acids, the simple techniques used to replicate bacteria and viruses are not applicable. For much of the history of prion research, time-consuming bioassays in animals were the only option for measuring infectivity. Although cell models and other in vitro tools for the propagation of prions have been developed, they all suffer limitations, and animal bioassays remain the gold standard for measuring infectivity. A wealth of recent data argues that both β-amyloid (Aβ) and tau proteins form prions that cause Alzheimer's disease, and α-synuclein forms prions that cause multiple system atrophy and Parkinson's disease. Cell and animal models that recapitulate some of the key features of cell-to-cell spreading and distinct strains of prions can now be measured.

Interactions between Microtubule-Associated Protein Tau (MAPT) and Small Molecules

Tau aggregation is linked to multiple neurodegenerative disorders that are collectively termed tauopathies. Small molecules are powerful probes of the aggregation process, helping to reveal the key steps and serving as diagnostics and reporters. Moreover, some of these small molecules may have potential as therapeutics. This review details how small molecules and chemical biology have helped to elucidate the mechanisms of tau aggregation and how they are being used to detect and prevent tau aggregation. In addition, we comment on how new insights into tau prions are changing the approach to small molecule discovery.