Thermodynamic and kinetic approaches for drug discovery to target protein misfolding and aggregation.

INTRODUCTION: Protein misfolding diseases, including Alzheimer's and Parkinson's diseases, are characterized by the aberrant aggregation of proteins. These conditions are still largely untreatable, despite having a major impact on our healthcare systems and societies. AREAS COVERED: We describe drug discovery strategies to target protein misfolding and aggregation. We compare thermodynamic approaches, which are based on the stabilization of the native states of proteins, with kinetic approaches, which are based on the slowing down of the aggregation process. This comparison is carried out in terms of the current knowledge of the process of protein misfolding and aggregation, the mechanisms of disease and the therapeutic targets. EXPERT OPINION: There is an unmet need for disease-modifying treatments that target protein misfolding and aggregation for the over 50 human disorders known to be associated with this phenomenon. With the approval of the first drugs that can prevent misfolding or inhibit aggregation, future efforts will be focused on the discovery of effective compounds with these mechanisms of action for a wide range of conditions.

Protein misfolding and related human diseases: A comprehensive review of toxicity, proteins involved, and current therapeutic strategies.

Most of the cell's chemical reactions and structural components are facilitated by proteins. But proteins are highly dynamic molecules, where numerous modifications or changes in the cellular environment can affect their native conformational fold leading to protein aggregation. Various stress conditions, such as oxidative stress, mutations and metal toxicity may cause protein misfolding and aggregation by shifting the conformational equilibrium towards more aggregation-prone states. Most of the protein misfolding diseases (PMDs) involve aggregation of protein. We have discussed such proteins like Aß peptide, α-synuclein, amylin and lysozyme involved in Alzheimer's, Parkinson's, type II diabetes and non-neuropathic systemic amyloidosis respectively. Till date, all advances in PMDs therapeutics help symptomatically but do not prevent the root cause of the disease, i.e., the aggregation of protein involved in the diseases. Current efforts focused on developing therapies for PMDs have employed diverse strategies; repositioning pre-existing drugs as it saves time and money; natural compounds that are touted as potential drug candidates have an advantage of being taken in diet normally and will induce lesser side effects. This review also covers recently developed therapeutic strategies like antisense drugs and disaggregases which has yielded therapeutic agents that have transitioned from preclinical studies into human clinical trials.

DAXX represents a new type of protein-folding enabler.

Protein quality control systems are crucial for cellular function and organismal health. At present, most known protein quality control systems are multicomponent machineries that operate via ATP-regulated interactions with non-native proteins to prevent aggregation and promote folding1, and few systems that can broadly enable protein folding by a different mechanism have been identified. Moreover, proteins that contain the extensively charged poly-Asp/Glu (polyD/E) region are common in eukaryotic proteomes2, but their biochemical activities remain undefined. Here we show that DAXX, a polyD/E protein that has been implicated in diverse cellular processes3-10, possesses several protein-folding activities. DAXX prevents aggregation, solubilizes pre-existing aggregates and unfolds misfolded species of model substrates and neurodegeneration-associated proteins. Notably, DAXX effectively prevents and reverses aggregation of its in vivo-validated client proteins, the tumour suppressor p53 and its principal antagonist MDM2. DAXX can also restore native conformation and function to tumour-associated, aggregation-prone p53 mutants, reducing their oncogenic properties. These DAXX activities are ATP-independent and instead rely on the polyD/E region. Other polyD/E proteins, including ANP32A and SET, can also function as stand-alone, ATP-independent molecular chaperones, disaggregases and unfoldases. Thus, polyD/E proteins probably constitute a multifunctional protein quality control system that operates via a distinctive mechanism.

Pine Bark Polyphenolic Extract Attenuates Amyloid-ß and Tau Misfolding in a Model System of Alzheimer's Disease Neuropathology.

Plant-derived polyphenolic compounds possess diverse biological activities, including strong anti-oxidant, anti-inflammatory, anti-microbial, and anti-tumorigenic activities. There is a growing interest in the development of polyphenolic compounds for preventing and treating chronic and degenerative diseases, such as cardiovascular disorders, cancer, and neurological diseases including Alzheimer's disease (AD). Two neuropathological changes of AD are the appearance of neurofibrillary tangles containing tau and extracellular amyloid deposits containing amyloid-ß protein (Aß). Our laboratory and others have found that polyphenolic preparations rich in proanthocyanidins, such as grape seed extract, are capable of attenuating cognitive deterioration and reducing brain neuropathology in animal models of AD. Oligopin is a pine bark extract composed of low molecular weight proanthocyanidins oligomers (LMW-PAOs), including flavan-3-ol units such as catechin (C) and epicatechin (EC). Based on the ability of its various components to confer resilience to the onset of AD, we tested whether oligopin can specifically prevent or attenuate the progression of AD dementia preclinically. We also explored the underlying mechanism(s) through which oligopin may exert its biological activities. Oligopin inhibited oligomer formation of not only Aß1-40 and Aß1-42, but also tau in vitro. Our pharmacokinetics analysis of metabolite accumulation in vivo resulted in the identification of Me-EC-O-ß-Glucuronide, Me-(±)-C-O-ß-glucuronide, EC-O-ß-glucuronide, and (±)-C-O-ß-glucuronide in the plasma of mice. These metabolites are primarily methylated and glucuronidated C and EC conjugates. The studies conducted provide the necessary impetus to design future clinical trials with bioactive oligopin to prevent both prodromal and residual forms of AD.

Structure of Calcarisporiella thermophila Hsp104 Disaggregase that Antagonizes Diverse Proteotoxic Misfolding Events.

Hsp104 is an AAA+ protein disaggregase with powerful amyloid-remodeling activity. All nonmetazoan eukaryotes express Hsp104 while eubacteria express an Hsp104 ortholog, ClpB. However, most studies have focused on Hsp104 from Saccharomyces cerevisiae and ClpB orthologs from two eubacterial species. Thus, the natural spectrum of Hsp104/ClpB molecular architectures and protein-remodeling activities remains largely unexplored. Here, we report two structures of Hsp104 from the thermophilic fungus Calcarisporiella thermophila (CtHsp104), a 2.70Å crystal structure and 4.0Å cryo-electron microscopy structure. Both structures reveal left-handed, helical assemblies with all domains clearly resolved. We thus provide the highest resolution and most complete view of Hsp104 hexamers to date. We also establish that CtHsp104 antagonizes several toxic protein-misfolding events in vivo where S. cerevisiae Hsp104 is ineffective, including rescue of TDP-43, polyglutamine, and α-synuclein toxicity. We suggest that natural Hsp104 variation is an invaluable, untapped resource for illuminating therapeutic disaggregases for fatal neurodegenerative diseases.

Inhibition of protein misfolding and aggregation by natural phenolic compounds.

Protein misfolding and aggregation into fibrillar deposits is a common feature of a large group of degenerative diseases affecting the central nervous system or peripheral organs, termed protein misfolding disorders (PMDs). Despite their established toxic nature, clinical trials aiming to reduce misfolded aggregates have been unsuccessful in treating or curing PMDs. An interesting possibility for disease intervention is the regular intake of natural food or herbal extracts, which contain active molecules that inhibit aggregation or induce the disassembly of misfolded aggregates. Among natural compounds, phenolic molecules are of particular interest, since most have dual activity as amyloid aggregation inhibitors and antioxidants. In this article, we review many phenolic natural compounds which have been reported in diverse model systems to have the potential to delay or prevent the development of various PMDs, including Alzheimer's and Parkinson's diseases, prion diseases, amyotrophic lateral sclerosis, systemic amyloidosis, and type 2 diabetes. The lower toxicity of natural compounds compared to synthetic chemical molecules suggest that they could serve as a good starting point to discover protein misfolding inhibitors that might be useful for the treatment of various incurable diseases.

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

Transmissible spongiform encephalopathies (TSEs) are fatal neurological disorders caused by prions, which are composed of a misfolded protein (PrPSc) that self-propagates in the brain of infected individuals by converting the normal prion protein (PrPC) into the pathological isoform. Here, we report a novel experimental strategy for preventing prion disease based on producing a self-replicating, but innocuous PrPSc-like form, termed anti-prion, which can compete with the replication of pathogenic prions. Our results show that a prophylactic inoculation of prion-infected animals with an anti-prion delays the onset of the disease and in some animals completely prevents the development of clinical symptoms and brain damage. The data indicate that a single injection of the anti-prion eliminated ~99% of the infectivity associated to pathogenic prions. Furthermore, this treatment caused significant changes in the profile of regional PrPSc deposition in the brains of animals that were treated, but still succumbed to the disease. Our findings provide new insights for a mechanistic understanding of prion replication and support the concept that prion replication can be separated from toxicity, providing a novel target for therapeutic intervention.

A review on protein misfolding, aggregation and strategies to prevent related ailments.

This review aims to highlight the fundamental mechanism of protein misfolding leading to protein aggregation and associated diseases. It also aims to anticipate novel therapeutic strategies with which to prevent or treat these highly debilitating conditions linked to these pathologies. The failure of a protein to correctly fold de novo or to remain correctly folded can have profound consequences on a living system especially when the cellular quality control processes fail to eliminate the rogue proteins. The core cause of over 20 different human diseases which have now been designated as 'conformational diseases' including neurodegenerative diseases such as Alzheimer's disease (AD), Huntington's disease (HD) and Parkinson's disease (PD) etc. A comprehensive study on protein misfolding, aggregation, and the outcomes of the effects of cytotoxic aggregates will lead to understand the aggregation-mediated cell toxicity and serves as a foundation for future research in development of promising therapies and drugs. This review has also shed light on the mechanism of protein misfolding which leads to its aggregation and hence the neurodegeneration. From these considerations, one could also envisage the possibility that protein aggregation may be exploited by nature to perform specific physiological functions in differing biological contexts.

Protein Misfolding, Amyloid Formation, and Human Disease: A Summary of Progress Over the Last Decade.

Peptides and proteins have been found to possess an inherent tendency to convert from their native functional states into intractable amyloid aggregates. This phenomenon is associated with a range of increasingly common human disorders, including Alzheimer and Parkinson diseases, type II diabetes, and a number of systemic amyloidoses. In this review, we describe this field of science with particular reference to the advances that have been made over the last decade in our understanding of its fundamental nature and consequences. We list the proteins that are known to be deposited as amyloid or other types of aggregates in human tissues and the disorders with which they are associated, as well as the proteins that exploit the amyloid motif to play specific functional roles in humans. In addition, we summarize the genetic factors that have provided insight into the mechanisms of disease onset. We describe recent advances in our knowledge of the structures of amyloid fibrils and their oligomeric precursors and of the mechanisms by which they are formed and proliferate to generate cellular dysfunction. We show evidence that a complex proteostasis network actively combats protein aggregation and that such an efficient system can fail in some circumstances and give rise to disease. Finally, we anticipate the development of novel therapeutic strategies with which to prevent or treat these highly debilitating and currently incurable conditions.

Dimethyl fumarate ameliorates myoclonus stemming from protein misfolding in oligodendrocytes.

Multiple sclerosis (MS) is considered a primary autoimmune disease; however, this view is increasingly being challenged in basic and clinical science arenas because of the growing body of clinical trials' data showing that exclusion of immune cells from the CNS only modestly slows disease progression to disability. Accordingly, there is significant need for expanding the scope of potential disease mechanisms to understand the etiology of MS. Concomitantly, the use of a broader range of pre-clinical animal models for characterizing existing efficacious clinical treatments may elucidate additional or unexpected mechanisms of action for these drugs that augment insight into MS etiology. Herein, we explore the in vivo mechanism of action of dimethyl fumarate, which has been shown to suppress oxidative stress and immune cell responses in psoriasis and MS. Rather than studying this compound in the context of an experimental autoimmune-induced attack on the CNS, we have used a genetic model of hypomyelination, male rumpshaker (rsh) mice, which exhibit oligodendrocyte metabolic stress and startle-induced subcortical myoclonus during development and into adulthood. We find that myoclonus is reduced 30-50% in treated mutants but we do not detect substantial changes in metabolic or oxidative stress response pathways, cytokine modulation, or myelin thickness (assessed by anova). All procedures involving vertebrate animals in this study were reviewed and approved by the IACUC committee at Wayne State University.

Endoplasmic reticulum proteostasis impairment in aging.

Perturbed neuronal proteostasis is a salient feature shared by both aging and protein misfolding disorders. The proteostasis network controls the health of the proteome by integrating pathways involved in protein synthesis, folding, trafficking, secretion, and their degradation. A reduction in the buffering capacity of the proteostasis network during aging may increase the risk to undergo neurodegeneration by enhancing the accumulation of misfolded proteins. As almost one-third of the proteome is synthetized at the endoplasmic reticulum (ER), maintenance of its proper function is fundamental to sustain neuronal function. In fact, ER stress is a common feature of most neurodegenerative diseases. The unfolded protein response (UPR) operates as central player to maintain ER homeostasis or the induction of cell death of chronically damaged cells. Here, we discuss recent evidence placing ER stress as a driver of brain aging, and the emerging impact of neuronal UPR in controlling global proteostasis at the whole organismal level. Finally, we discuss possible therapeutic interventions to improve proteostasis and prevent pathological brain aging.

Cytosolic Fc receptor TRIM21 inhibits seeded tau aggregation.

Alzheimer's disease (AD) and other neurodegenerative disorders are associated with the cytoplasmic aggregation of microtubule-associated protein tau. Recent evidence supports transcellular transfer of tau misfolding (seeding) as the mechanism of spread within an affected brain, a process reminiscent of viral infection. However, whereas microbial pathogens can be recognized as nonself by immune receptors, misfolded protein assemblies evade detection, as they are host-derived. Here, we show that when misfolded tau assemblies enter the cell, they can be detected and neutralized via a danger response mediated by tau-associated antibodies and the cytosolic Fc receptor tripartite motif protein 21 (TRIM21). We developed fluorescent, morphology-based seeding assays that allow the formation of pathological tau aggregates to be measured in situ within 24 h in the presence of picomolar concentrations of tau seeds. We found that anti-tau antibodies accompany tau seeds into the cell, where they recruit TRIM21 shortly after entry. After binding, TRIM21 neutralizes tau seeds through the activity of the proteasome and the AAA ATPase p97/VCP in a similar manner to infectious viruses. These results establish that intracellular antiviral immunity can be redirected against host-origin endopathogens involved in neurodegeneration.

Endoplasmic Reticulum-Targeted Subunit Toxins Provide a New Approach to Rescue Misfolded Mutant Proteins and Revert Cell Models of Genetic Diseases.

Many germ line diseases stem from a relatively minor disturbance in mutant protein endoplasmic reticulum (ER) 3D assembly. Chaperones are recruited which, on failure to correct folding, sort the mutant for retrotranslocation and cytosolic proteasomal degradation (ER-associated degradation-ERAD), to initiate/exacerbate deficiency-disease symptoms. Several bacterial (and plant) subunit toxins, retrograde transport to the ER after initial cell surface receptor binding/internalization. The A subunit has evolved to mimic a misfolded protein and hijack the ERAD membrane translocon (dislocon), to effect cytosolic access and cytopathology. We show such toxins compete for ERAD to rescue endogenous misfolded proteins. Cholera toxin or verotoxin (Shiga toxin) containing genetically inactivated (± an N-terminal polyleucine tail) A subunit can, within 2-4 hrs, temporarily increase F508delCFTR protein, the major cystic fibrosis (CF) mutant (5-10x), F508delCFTR Golgi maturation (<10x), cell surface expression (20x) and chloride transport (2x) in F508del CFTR transfected cells and patient-derived F508delCFTR bronchiolar epithelia, without apparent cytopathology. These toxoids also increase glucocerobrosidase (GCC) in N370SGCC Gaucher Disease fibroblasts (3x), another ERAD-exacerbated misfiling disease. We identify a new, potentially benign approach to the treatment of certain genetic protein misfolding diseases.

Dietary Protein, Metabolism, and Aging.

Dietary restriction (DR), a moderate reduction in food intake, improves health during aging and extends life span across multiple species. Specific nutrients, rather than overall calories, mediate the effects of DR, with protein and specific amino acids (AAs) playing a key role. Modulations of single dietary AAs affect traits including growth, reproduction, physiology, health, and longevity in animals. Epidemiological data in humans also link the quality and quantity of dietary proteins to long-term health. Intricate nutrient-sensing pathways fine tune the metabolic responses to dietary AAs in a highly conserved manner. In turn, these metabolic responses can affect the onset of insulin resistance, obesity, neurodegenerative disease, and other age-related diseases. In this review we discuss how AA requirements are shaped and how ingested AAs regulate a spectrum of homeostatic processes. Finally, we highlight the resulting opportunity to develop nutritional strategies to improve human health during aging.

Potential applications of stress solutes from extremophiles in protein folding diseases and healthcare.

Protein misfolding, aggregation and deposition in the brain, in the form of amyloid, are implicated in the etiology of several neurodegenerative disorders, such as Alzheimer's, Parkinson's and prion diseases. Drugs available on the market reduce the symptoms, but they are not a cure. Therefore, it is urgent to identify promising targets and develop effective drugs. Preservation of protein native conformation and/or inhibition of protein aggregation seem pertinent targets for drug development. Several studies have shown that organic solutes, produced by extremophilic microorganisms in response to osmotic and/or heat stress, prevent denaturation and aggregation of model proteins. Among these stress solutes, mannosylglycerate, mannosylglyceramide, di-myo-inositol phosphate, diglycerol phosphate and ectoine are effective in preventing amyloid formation by Alzheimer's Aß peptide and/or α-synuclein in vitro. Moreover, mannosylglycerate is a potent inhibitor of Aß and α-synuclein aggregation in living cells, and mannosylglyceramide and ectoine inhibit aggregation and reduce prion peptide-induced toxicity in human cells. This review focuses on the efficacy of stress solutes from hyper/thermophiles and ectoines to prevent amyloid formation in vitro and in vivo and their potential application in drug development against protein misfolding diseases. Current and envisaged applications of these extremolytes in neurodegenerative diseases and healthcare will also be addressed.

Preventing proteostasis diseases by selective inhibition of a phosphatase regulatory subunit.

Protein phosphorylation regulates virtually all biological processes. Although protein kinases are popular drug targets, targeting protein phosphatases remains a challenge. Here, we describe Sephin1 (selective inhibitor of a holophosphatase), a small molecule that safely and selectively inhibited a regulatory subunit of protein phosphatase 1 in vivo. Sephin1 selectively bound and inhibited the stress-induced PPP1R15A, but not the related and constitutive PPP1R15B, to prolong the benefit of an adaptive phospho-signaling pathway, protecting cells from otherwise lethal protein misfolding stress. In vivo, Sephin1 safely prevented the motor, morphological, and molecular defects of two otherwise unrelated protein-misfolding diseases in mice, Charcot-Marie-Tooth 1B, and amyotrophic lateral sclerosis. Thus, regulatory subunits of phosphatases are drug targets, a property exploited here to safely prevent two protein misfolding diseases.

Reversing deleterious protein aggregation with re-engineered protein disaggregases.

Aberrant protein folding is severely problematic and manifests in numerous disorders, including amyotrophic lateral sclerosis (ALS), Parkinson disease (PD), Huntington disease (HD), and Alzheimer disease (AD). Patients with each of these disorders are characterized by the accumulation of mislocalized protein deposits. Treatments for these disorders remain palliative, and no available therapeutics eliminate the underlying toxic conformers. An intriguing approach to reverse deleterious protein misfolding is to upregulate chaperones to restore proteostasis. We recently reported our work to re-engineer a prion disaggregase from yeast, Hsp104, to reverse protein misfolding implicated in human disease. These potentiated Hsp104 variants suppress TDP-43, FUS, and α-synuclein toxicity in yeast, eliminate aggregates, reverse cellular mislocalization, and suppress dopaminergic neurodegeneration in an animal model of PD. Here, we discuss this work and its context, as well as approaches for further developing potentiated Hsp104 variants for application in reversing protein-misfolding disorders.

Safety, specificity and immunogenicity of a PrP(Sc)-specific prion vaccine based on the YYR disease specific epitope.

Prions are a novel form of infectivity based on the misfolding of a self-protein (PrP(C)) into a pathological, infectious isomer (PrP(Sc)). The current uncontrolled spread of chronic wasting disease in cervids, coupled with the demonstrated zoonotic nature of select livestock prion diseases, highlights the urgent need for disease management tools. While there is proof-of-principle evidence for a prion vaccine, these efforts are complicated by the challenges and risks associated with induction of immune responses to a self-protein. Our priority is to develop a PrP(Sc)-specific prion vaccine based on epitopes that are uniquely exposed upon misfolding. These disease specific epitopes (DSEs) have the potential to enable specific targeting of the pathological species through immunotherapy. Here we review outcomes of the translation of a prion DSE into a PrP(Sc)-specific vaccine based on the criteria of immunogenicity, safety and specificity.