Treatment with a non-toxic, self-replicating anti-prion delays or prevents prion disease in vivo

Multiple aspects of amyloid dynamics in vivo integrate to establish prion variant dominance in yeast

Prion variants are self-perpetuating conformers of a single protein that assemble into amyloid fibers and confer unique phenotypic states. Multiple prion variants can arise, particularly in response to changing environments, and interact within an organism. These interactions are often competitive, with one variant establishing phenotypic dominance over the others. This dominance has been linked to the competition for non-prion state protein, which must be converted to the prion state via a nucleated polymerization mechanism. However, the intrinsic rates of conversion, determined by the conformation of the variant, cannot explain prion variant dominance, suggesting a more complex interaction. Using the yeast prion system [PSI+ ], we have determined the mechanism of dominance of the [PSI+ ]Strong variant over the [PSI+ ]Weak variant in vivo. When mixed by mating, phenotypic dominance is established in zygotes, but the two variants persist and co-exist in the lineage descended from this cell. [PSI+ ]Strong propagons, the heritable unit, are amplified at the expense of [PSI+ ]Weak propagons, through the efficient conversion of soluble Sup35 protein, as revealed by fluorescence photobleaching experiments employing variant-specific mutants of Sup35. This competition, however, is highly sensitive to the fragmentation of [PSI+ ]Strong amyloid fibers, with even transient inhibition of the fragmentation catalyst Hsp104 promoting amplification of [PSI+ ]Weak propagons. Reducing the number of [PSI+ ]Strong propagons prior to mating, similarly promotes [PSI+ ]Weak amplification and conversion of soluble Sup35, indicating that template number and conversion efficiency combine to determine dominance. Thus, prion variant dominance is not an absolute hierarchy but rather an outcome arising from the dynamic interplay between unique protein conformations and their interactions with distinct cellular proteostatic niches.

Transmissible long-term neuroprotective and pro-cognitive effects of 1-42 beta-amyloid with A2T icelandic mutation in an Alzheimer's disease mouse model

The amyloid cascade hypothesis assumes that the development of Alzheimer's disease (AD) is driven by a self-perpetuating cycle, in which β-amyloid (Aβ) accumulation leads to Tau pathology and neuronal damages. A particular mutation (A673T) of the amyloid precursor protein (APP) was identified among Icelandic population. It provides a protective effect against Alzheimer- and age-related cognitive decline. This APP mutation leads to the reduced production of Aβ with A2T (position in peptide sequence) change (Aβice). In addition, Aβice has the capacity to form protective heterodimers in association with wild-type Aβ. Despite the emerging interest in Aβice during the last decade, the impact of Aβice on events associated with the amyloid cascade has never been reported. First, the effects of Aβice were evaluated in vitro by electrophysiology on hippocampal slices and by studying synapse morphology in cortical neurons. We showed that Aβice protects against endogenous Aβ-mediated synaptotoxicity. Second, as several studies have outlined that a single intracerebral administration of Aβ can worsen Aβ deposition and cognitive functions several months after the inoculation, we evaluated in vivo the long-term effects of a single inoculation of Aβice or Aβ-wild-type (Aβwt) in the hippocampus of transgenic mice (APPswe/PS1dE9) over-expressing Aβ1-42 peptide. Interestingly, we found that the single intra-hippocampal inoculation of Aβice to mice rescued synaptic density and spatial memory losses four months post-inoculation, compared with Aβwt inoculation. Although Aβ load was not modulated by Aβice infusion, the amount of Tau-positive neuritic plaques was significantly reduced. Finally, a lower phagocytosis by microglia of post-synaptic compounds was detected in Aβice-inoculated animals, which can partly explain the increased density of synapses in the Aβice animals. Thus, a single event as Aβice inoculation can improve the fate of AD-associated pathology and phenotype in mice several months after the event. These results open unexpected fields to develop innovative therapeutic strategies against AD.

Binding modes of potential anti-prion phytochemicals to PrPC structures in silico

BACKGROUND

Prion diseases involve the conversion of a normal, cell-surface glycoprotein (PrPC) into a misfolded pathogenic form (PrPSc). One possible strategy to inhibit PrPSc formation is to stabilize the native conformation of PrPC and interfere with the conversion of PrPC to PrPSc. Many compounds have been shown to inhibit the conversion process, however, no promising drugs have been identified to cure prion diseases.

OBJECTIVE

This study aims to identify potential anti-prion compounds from plant phytochemicals by integrating traditional ethnobotanical knowledge with modern in silico drug design approaches.

MATERIALS AND METHODS

In the current study medicinal phytochemicals were docked with swapped and non-swapped crystal structures of PrPCin silico to identify potential anti-prions to determine their binding modes and interactions.

RESULTS

Eleven new phytochemicals were identified based on their binding energies and pharmacokinetic properties. The binding sites and interactions of the known and new anti-prion compounds are similar, and differences in binding modes occur in structures with very subtle differences in side chain conformations. Binding of these compounds poses steric hindrance to neighbouring molecules. Residues shown to be associated with the inhibition of PrPC to PrPSc conversion form interactions with most of the compounds.

CONCLUSION

Identified compounds might act as potent inhibitors of PrPC to PrPSc conversion. These might be attractive candidates for the development of novel anti-prion therapy although further tests in vitro cell cultures and in vivo mouse models are needed to confirm these findings.

Molecular foundations of prion strain diversity

Despite being caused by a single protein, prion diseases are strikingly heterogenous. Individual prion variants, known as strains, possess distinct biochemical properties, form aggregates with characteristic morphologies and preferentially seed certain brain regions, causing markedly different disease phenotypes. Strain diversity is determined by protein structure, post-translational modifications and the presence of extracellular matrix components, with single amino acid substitutions or altered protein glycosylation exerting dramatic effects. Here, we review recent advances in the study of prion strains and discuss how a deeper knowledge of the molecular origins of strain heterogeneity is providing a foundation for the development of anti-prion therapeutics.

α-Synuclein Strains: Does Amyloid Conformation Explain the Heterogeneity of Synucleinopathies?

Synucleinopathies are a heterogeneous group of neurodegenerative diseases with amyloid deposits that contain the α-synuclein (SNCA/α-Syn) protein as a common hallmark. It is astonishing that aggregates of a single protein are able to give rise to a whole range of different disease manifestations. The prion strain hypothesis offers a possible explanation for this conundrum. According to this hypothesis, a single protein sequence is able to misfold into distinct amyloid structures that can cause different pathologies. In fact, a growing body of evidence suggests that conformationally distinct α-Syn assemblies might be the causative agents behind different synucleinopathies. In this review, we provide an overview of research on the strain hypothesis as it applies to synucleinopathies and discuss the potential implications for diagnostic and therapeutic purposes.

Tau immunotherapies: Lessons learned, current status and future considerations

The majority of clinical trials targeting the tau protein in Alzheimer's disease and other tauopathies are tau immunotherapies. Because tau pathology correlates better with the degree of dementia than amyloid-β lesions, targeting tau is likely to be more effective in improving cognition than clearing amyloid-β in Alzheimer's disease. However, the development of tau therapies is in many ways more complex than for amyloid-β therapies as briefly outlined in this review. Most of the trials are on humanized antibodies, which may have very different properties than the original mouse antibodies. The impact of these differences are to a large extent unknown, can be difficult to decipher, and may not always be properly considered. Furthermore, the ideal antibody properties for efficacy are not well established and can depend on several factors. However, considering the varied approaches in clinical trials, there is a general optimism that at least some of these trials may provide functional benefits to patients suffering of various tauopathies. This article is part of the special issue entitled 'The Quest for Disease-Modifying Therapies for Neurodegenerative Disorders'.

Pros and cons in prion diseases abatement: Insights from nanomedicine and transmissibility patterns

Ample research progress with nanotechnology applications in health and medicine implies precision and accuracy in the scenario of neurodegenerative disorders, for which impending research in ultimate and complete cure has been the vision worldwide. The complexity of prion disease has been unravelled by scientists and demarcated for efficient abatement protocols, but which are still under research and clinical trials. Drug delivery strategies combating prion diseases across the blood brain barrier, the efficacy of drugs and biocompatibility remain a serious question to be thoroughly studied for effective diagnosis and treatment. The present review compiles comprehensively the current treatment modalities against prion diseases and future prospects of nanotechnology addressing diagnosis and treatment of prion diseases with a special emphasis on transmissibility. Further, approaches for anti-prion technology, immunotherapy, and hindrances in vaccine development are discussed.

Transmissibility versus Pathogenicity of Self-Propagating Protein Aggregates

The prion-like spreading and accumulation of specific protein aggregates appear to be central to the pathogenesis of many human diseases, including Alzheimer's and Parkinson's. Accumulating evidence indicates that inoculation of tissue extracts from diseased individuals into suitable experimental animals can in many cases induce the aggregation of the disease-associated protein, as well as related pathological lesions. These findings, together with the history of the prion field, have raised the questions about whether such disease-associated protein aggregates are transmissible between humans by casual or iatrogenic routes, and, if so, do they propagate enough in the new host to cause disease? These practical considerations are important because real, and perhaps even only imagined, risks of human-to-human transmission of diseases such as Alzheimer's and Parkinson's may force costly changes in clinical practice that, in turn, are likely to have unintended consequences. The prion field has taught us that a single protein, PrP, can aggregate into forms that can propagate exponentially in vitro, but range from being innocuous to deadly when injected into experimental animals in ways that depend strongly on factors such as conformational subtleties, routes of inoculation, and host responses. In assessing the hazards posed by various disease-associated, self-propagating protein aggregates, it is imperative to consider both their actual transmissibilities and the pathological consequences of their propagation, if any, in recipient hosts.

Prions, prionoids and protein misfolding disorders

Prion diseases are progressive, incurable and fatal neurodegenerative conditions. The term 'prion' was first nominated to express the revolutionary concept that a protein could be infectious. We now know that prions consist of PrP^Sc, the pathological aggregated form of the cellular prion protein PrP^C. Over the years, the term has been semantically broadened to describe aggregates irrespective of their infectivity, and the prion concept is now being applied, perhaps overenthusiastically, to all neurodegenerative diseases that involve protein aggregation. Indeed, recent studies suggest that prion diseases (PrDs) and protein misfolding disorders (PMDs) share some common disease mechanisms, which could have implications for potential treatments. Nevertheless, the transmissibility of bona fide prions is unique, and PrDs should be considered as distinct from other PMDs.

Alzheimer's therapy development: A few points to consider

Development of therapies for Alzheimer's disease has only resulted in a few approved drugs that provide some temporary symptomatic relief in certain patients. None of these compounds in clinical use halts or slows the progression of the disease. To date, several drugs targeting the amyloid-β peptide, and some against the tau protein, have failed in clinical trials. While there are various reasons for these failures, considering the following points may aid in improving the outcome of future trials. First, the tau protein should ideally be targeted intracellularly because most of tau pathology is within cells, neurons in particular. Second, an overriding emphasis in recent years has been on implementing drug-screening models that focus on prevention of seeding/spread of aggregates. Much less attention has been paid to identify compounds that inhibit neurotoxicity of these aggregates, which may not necessarily relate to their seeding/spread propensity. Ideally, all these markers should be readouts in the same assay or model. Third, diversity in conformers/strains of aggregates complicates drug development of small molecule aggregation inhibitors but is likely to be less of an issue for antibody-based treatments. Lastly, other more general targets associated with neurodegeneration should continue to be pursued but are in many ways more difficult to address than clearing amyloid-β and tau, the defining hallmarks of AD.

Protein misfolding, aggregation, and conformational strains in neurodegenerative diseases

A hallmark event in neurodegenerative diseases (NDs) is the misfolding, aggregation, and accumulation of proteins, leading to cellular dysfunction, loss of synaptic connections, and brain damage. Despite the involvement of distinct proteins in different NDs, the process of protein misfolding and aggregation is remarkably similar. A recent breakthrough in the field was the discovery that misfolded protein aggregates can self-propagate through seeding and spread the pathological abnormalities between cells and tissues in a manner akin to the behavior of infectious prions in prion diseases. This discovery has vast implications for understanding the mechanisms involved in the initiation and progression of NDs, as well as for the design of novel strategies for treatment and diagnosis. In this Review, we provide a critical discussion of the role of protein misfolding and aggregation in NDs. Commonalities and differences between distinct protein aggregates will be highlighted, in addition to evidence supporting the hypothesis that misfolded aggregates can be transmissible by the prion principle. We will also describe the molecular basis and implications for prion-like conformational strains, cross-interaction between different misfolded proteins in the brain, and how these concepts can be applied to the development of novel strategies for therapy and diagnosis.

Recombinant PrP and Its Contribution to Research on Transmissible Spongiform Encephalopathies

The misfolding of the cellular prion protein (PrPC) into the disease-associated isoform (PrPSc) and its accumulation as amyloid fibrils in the central nervous system is one of the central events in transmissible spongiform encephalopathies (TSEs). Due to the proteinaceous nature of the causal agent the molecular mechanisms of misfolding, interspecies transmission, neurotoxicity and strain phenomenon remain mostly ill-defined or unknown. Significant advances were made using in vivo and in cellula models, but the limitations of these, primarily due to their inherent complexity and the small amounts of PrPSc that can be obtained, gave rise to the necessity of new model systems. The production of recombinant PrP using E. coli and subsequent induction of misfolding to the aberrant isoform using different techniques paved the way for the development of cell-free systems that complement the previous models. The generation of the first infectious recombinant prion proteins with identical properties of brain-derived PrPSc increased the value of cell-free systems for research on TSEs. The versatility and ease of implementation of these models have made them invaluable for the study of the molecular mechanisms of prion formation and propagation, and have enabled improvements in diagnosis, high-throughput screening of putative anti-prion compounds and the design of novel therapeutic strategies. Here, we provide an overview of the resultant advances in the prion field due to the development of recombinant PrP and its use in cell-free systems.

How do PrPSc Prions Spread between Host Species, and within Hosts?

Prion diseases are sub-acute neurodegenerative diseases that affect humans and some domestic and free-ranging animals. Infectious prion agents are considered to comprise solely of abnormally folded isoforms of the cellular prion protein known as PrP^Sc. Pathology during prion disease is restricted to the central nervous system where it causes extensive neurodegeneration and ultimately leads to the death of the host. The first half of this review provides a thorough account of our understanding of the various ways in which PrP^Sc prions may spread between individuals within a population, both horizontally and vertically. Many natural prion diseases are acquired peripherally, such as by oral exposure, lesions to skin or mucous membranes, and possibly also via the nasal cavity. Following peripheral exposure, some prions accumulate to high levels within the secondary lymphoid organs as they make their journey from the site of infection to the brain, a process termed neuroinvasion. The replication of PrP^Sc prions within secondary lymphoid organs is important for their efficient spread to the brain. The second half of this review describes the key tissues, cells and molecules which are involved in the propagation of PrP^Sc prions from peripheral sites of exposure (such as the lumen of the intestine) to the brain. This section also considers how additional factors such as inflammation and aging might influence prion disease susceptibility.

Reduced Abundance and Subverted Functions of Proteins in Prion-Like Diseases: Gained Functions Fascinate but Lost Functions Affect Aetiology

Prions have served as pathfinders that reveal many aspects of proteostasis in neurons. The recent realization that several prominent neurodegenerative diseases spread via a prion-like mechanism illuminates new possibilities for diagnostics and therapeutics. Thus, key proteins in Alzheimer Disease and Amyotrophic lateral sclerosis (ALS), including amyloid-β precursor protein, Tau and superoxide dismutase 1 (SOD1), spread to adjacent cells in their misfolded aggregated forms and exhibit template-directed misfolding to induce further misfolding, disruptions to proteostasis and toxicity. Here we invert this comparison to ask what these prion-like diseases can teach us about the broad prion disease class, especially regarding the loss of these key proteins' function(s) as they misfold and aggregate. We also consider whether functional amyloids might reveal a role for subverted protein function in neurodegenerative disease. Our synthesis identifies SOD1 as an exemplar of protein functions being lost during prion-like protein misfolding, because SOD1 is inherently unstable and loses function in its misfolded disease-associated form. This has under-appreciated parallels amongst the canonical prion diseases, wherein the normally folded prion protein, PrPC, is reduced in abundance in fatal familial insomnia patients and during the preclinical phase in animal models, apparently via proteostatic mechanisms. Thus while template-directed misfolding and infectious properties represent gain-of-function that fascinates proteostasis researchers and defines (is required for) the prion(-like) diseases, loss and subversion of the functions attributed to hallmark proteins in neurodegenerative disease needs to be integrated into design towards effective therapeutics. We propose experiments to uniquely test these ideas.