Prions: generation and spread versus neurotoxicity

A golden age of muscarinic acetylcholine receptor modulation in neurological diseases

Over the past 40 years, the muscarinic acetylcholine receptor family, particularly the M1-receptor and M4-receptor subtypes, have emerged as validated targets for the symptomatic treatment of neurological diseases such as schizophrenia and Alzheimer disease. However, despite considerable effort and investment, no drugs have yet gained clinical approval. This is largely attributable to cholinergic adverse effects that have halted the majority of programmes and resulted in a waning of interest in these G-protein-coupled receptor targets. Recently, this trend has been reversed. Driven by advances in structure-based drug design and an appreciation of the optimal pharmacological properties necessary to deliver clinical efficacy while minimizing adverse effects, a new generation of M1-receptor and M4-receptor orthosteric agonists and positive allosteric modulators are now entering the clinic. These agents offer the prospect of novel therapeutic solutions for 'hard to treat' neurological diseases, heralding a new era of muscarinic drug discovery.

Assessment of the therapeutic potential of Hsp70 activator against prion diseases using in vitro and in vivo models

Introduction

Prion diseases are deadly neurodegenerative disorders in both animals and humans, causing the destruction of neural tissue and inducing behavioral manifestations. Heat shock proteins (Hsps), act as molecular chaperones by supporting the appropriate folding of proteins and eliminating the misfolded proteins as well as playing a vital role in cell signaling transduction, cell cycle, and apoptosis control. SW02 is a potent activator of Hsp 70 kDa (Hsp70).

Methods

In the current study, the protective effects of SW02 against prion protein 106-126 (PrP106-126)-induced neurotoxicity in human neuroblastoma cells (SH-SY5Y) were investigated. In addition, the therapeutic effects of SW02 in ME7 scrapie-infected mice were evaluated.

Results

The results showed that SW02 treatment significantly increased Hsp70 mRNA expression levels and Hsp70 ATPase activity (p < 0.01). SW02 also significantly inhibited cytotoxicity and apoptosis induced by PrP106-126 (p < 0.01) and promoted neurite extension. In vivo, intraperitoneal administration of SW02 did not show a statistically significant difference in survival time (p = 0.16); however, the SW02-treated group exhibited a longer survival time of 223.6 ± 6.0 days compared with the untreated control group survival time of 217.6 ± 5.4 days. In addition, SW02 reduced the PrPSc accumulation in ME7 scrapie-infected mice at 5 months post-injection (p < 0.05). A significant difference was not observed in GFAP expression, an astrocyte marker, between the treated and untreated groups.

Conclusion

In conclusion, the potential therapeutic role of the Hsp70 activator SW02 was determined in the present study and may be a novel and effective drug to mitigate the pathologies of prion diseases and other neurodegenerative diseases. Further studies using a combination of two pharmacological activators of Hsp70 are required to maximize the effectiveness of each intervention.

PolyQ-Expanded Mutant Huntingtin Forms Inclusion Body Following Transient Cold Shock in a Two-Step Aggregation Mechanism

Age-dependent formation of insoluble protein aggregates is a hallmark of many neurodegenerative diseases. We are interested in the cell chemistry that drives the aggregation of polyQ-expanded mutant Huntingtin (mHtt) protein into insoluble inclusion bodies (IBs). Using an inducible cell model of Huntington's disease, we show that a transient cold shock (CS) at 4 °C followed by recovery incubation at temperatures of 25-37 °C strongly and rapidly induces the compaction of diffuse polyQ-expanded HuntingtinExon1-enhanced green fluorescent protein chimera protein (mHtt) into round, micron size, cytosolic IBs. This transient CS-induced mHtt IB formation is independent of microtubule integrity or de novo protein synthesis. The addition of millimolar concentrations of sodium chloride accelerates, whereas urea suppresses this transient CS-induced mHtt IB formation. These results suggest that the low temperature of CS constrains the conformation dynamics of the intrinsically disordered mHtt into labile intermediate structures to facilitate de-solvation and hydrophobic interaction for IB formation at the higher recovery temperature. This work, along with our previous observation of the effects of heat shock protein chaperones and osmolytes in driving mHtt IB formation, underscores the primacy of mHtt structuring and rigidification for H-bond-mediated cross-linking in a two-step mechanism of mHtt IB formation in living cells.

M1 muscarinic receptor activation reduces the molecular pathology and slows the progression of prion-mediated neurodegenerative disease

Many dementias are propagated through the spread of "prion-like" misfolded proteins. This includes prion diseases themselves (such as Creutzfeldt-Jakob disease) and Alzheimer's disease (AD), for which no treatments are available to slow or stop progression. The M1 acetylcholine muscarinic receptor (M1 receptor) is abundant in the brain, and its activity promotes cognitive function in preclinical models and in patients with AD. Here, we investigated whether activation of the M1 receptor might slow the progression of neurodegeneration associated with prion-like misfolded protein in a mouse model of prion disease. Proteomic and transcriptomic analysis of the hippocampus revealed that this model had a molecular profile that was similar to that of human neurodegenerative diseases, including AD. Chronic enhancement of the activity of the M1 receptor with the positive allosteric modulator (PAM) VU0486846 reduced the abundance of prion-induced molecular markers of neuroinflammation and mitochondrial dysregulation in the hippocampus and normalized the abundance of those associated with neurotransmission, including synaptic and postsynaptic signaling components. PAM treatment of prion-infected mice prolonged survival and maintained cognitive function. Thus, allosteric activation of M1 receptors may reduce the severity of neurodegenerative diseases caused by the prion-like propagation of misfolded protein.

Minimal change prion retinopathy: Morphometric comparison of retinal and brain prion deposits in Creutzfeldt-Jakob disease

Sporadic Creutzfeldt-Jakob disease (sCJD) is the most commonly diagnosed human prion disease caused by the abnormal misfolding of the 'cellular' prion protein (PrPC) into the transmissible 'scrapie-type' prion form (PrPSc). Neuropathologic evaluation of brains with sCJD reveals abnormal PrPSc deposits primarily in grey matter structures, often associated with micro-vacuolar spongiform changes in neuropil, neuronal loss, and gliosis. Abnormal PrPSc deposits have also been reported in the retina of patients with sCJD, but few studies have characterized the morphology of these retinal PrPSc deposits or evaluated for any retinal neurodegenerative changes. We performed histopathologic and morphometric analyses of retinal and brain prion deposits in 14 patients with sCJD. Interestingly, we discovered that the morphology of retinal PrPSc deposits generally differs from that of brain PrPSc deposits in terms of size and shape. We found that retinal PrPSc deposits consistently localize to the outer plexiform layer of the retina. Additionally, we observed that the retinal PrPSc deposits are not associated with the spongiform change, neuronal loss, and gliosis often seen in the brain. The stereotypic morphology and location of PrPSc deposits in sCJD retinas may help guide the use of ocular imaging devices in the detection of these deposits for a clinical diagnosis.

The Possible Mechanism of Amyloid Transformation Based on the Geometrical Parameters of Early-Stage Intermediate in Silico Model for Protein Folding

The specificity of the available experimentally determined structures of amyloid forms is expressed primarily by the two- and not three-dimensional forms of a single polypeptide chain. Such a flat structure is possible due to the β structure, which occurs predominantly. The stabilization of the fibril in this structure is achieved due to the presence of the numerous hydrogen bonds between the adjacent chains. Together with the different forms of twists created by the single R- or L-handed α-helices, they form the hydrogen bond network. The specificity of the arrangement of these hydrogen bonds lies in their joint orientation in a system perpendicular to the plane formed by the chain and parallel to the fibril axis. The present work proposes the possible mechanism for obtaining such a structure based on the geometric characterization of the polypeptide chain constituting the basis of our early intermediate model for protein folding introduced formerly. This model, being the conformational subspace of Ramachandran plot (the ellipse path), was developed on the basis of the backbone conformation, with the side-chain interactions excluded. Our proposal is also based on the results from molecular dynamics available in the literature leading to the unfolding of α-helical sections, resulting in the β-structural forms. Both techniques used provide a similar suggestion in a search for a mechanism of conformational changes leading to a formation of the amyloid form. The potential mechanism of amyloid transformation is presented here using the fragment of the transthyretin as well as amyloid Aβ.

On the Dependence of Prion and Amyloid Structure on the Folding Environment

Currently available analyses of amyloid proteins reveal the necessity of the existence of radical structural changes in amyloid transformation processes. The analysis carried out in this paper based on the model called fuzzy oil drop (FOD) and its modified form (FOD-M) allows quantifying the role of the environment, particularly including the aquatic environment. The starting point and basis for the present presentation is the statement about the presence of two fundamentally different methods of organizing polypeptides into ordered conformations—globular proteins and amyloids. The present study shows the source of the differences between these two paths resulting from the specificity of the external force field coming from the environment, including the aquatic and hydrophobic one. The water environment expressed in the fuzzy oil drop model using the 3D Gauss function directs the folding process towards the construction of a micelle-like system with a hydrophobic core in the central part and the exposure of polarity on the surface. The hydrophobicity distribution of membrane proteins has the opposite characteristic: Exposure of hydrophobicity at the surface of the membrane protein with an often polar center (as in the case of ion channels) is expected. The structure of most proteins is influenced by a more or less modified force field generated by water through the appropriate presence of a non-polar (membrane-like) environment. The determination of the proportion of a factor different from polar water enables the assessment of the protein status by indicating factors favoring the structure it represents.

Understanding the Role of Protein Glycation in the Amyloid Aggregation Process

Protein function and flexibility is directly related to the native distribution of its structural elements and any alteration in protein architecture leads to several abnormalities and accumulation of misfolded proteins. 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 amyloidosis characterized by the accumulation of amyloid aggregates both in the extracellular space of tissues and as intracellular deposits. Post-translational modifications are known to have an active role in the in vivo amyloid aggregation as able to affect protein structure and dynamics. Among them, a key role seems to be played by non-enzymatic glycation, the most unwanted irreversible modification of the protein structure, which strongly affects long-living proteins throughout the body. This study provided an overview of the molecular effects induced by glycation on the amyloid aggregation process of several protein models associated with misfolding diseases. In particular, we analyzed the role of glycation on protein folding, kinetics of amyloid formation, and amyloid cytotoxicity in order to shed light on the role of this post-translational modification in the in vivo amyloid aggregation process.

Analysis of the Neuroproteome Associated With Cell Therapy After Intranigral Grafting in a Mouse Model of Parkinson Disease

Advances in large-scale proteomics analysis have been very useful in understanding pathogenesis of diseases and elaborating therapeutic strategies. Proteomics has been employed to study Parkinson disease (PD); however, sparse studies reported proteome investigation after cell therapy approaches. In this study, we used liquid chromatography–tandem mass spectrometry and systems biology to identify differentially expressed proteins in a translational mouse model of PD after cell therapy. Proteins were extracted from five nigrostriatal-related brain regions of mice previously lesioned with 6-hydroxydopamine in the substantia nigra. Protein expression was compared in non-grafted brain to 1 and 7 days after intranigral grafting of E12.5 embryonic ventral mesencephalon (VM). We found a total of 277 deregulated proteins after transplantation, which are enriched for lipid metabolism, oxidative phosphorylation and PD, thus confirming that our animal model is similar to human PD and that the presence of grafted cells modulates the expression of these proteins. Notably, seven proteins (Acta1, Atp6v1e1, Eci3, Lypla2, Pip4k2a, Sccpdh, and Sh3gl2) were commonly down-regulated after engraftment in all studied brain regions. These proteins are known to be involved in the formation of lipids and recycling of dopamine (DA) vesicle at the synapse. Moreover, intranigral transplantation of VM cells decreased the expression of proteins related to oxidative stress, especially in the nigrostriatal pathway containing the DA grafted neurons. In the same regions, an up-regulation of several proteins including α-synuclein and tyrosine hydroxylase was observed, whereas expression of tetraspanin 7 was shut down. Overall, these results suggest that intranigral transplantation of VM tissue in an animal model of PD may induce a decrease of oxidative stress in the nigrostriatal pathway and a restoration of the machinery of neurotransmitters, particularly DA release to promote DA transmission through a decrease of D2 DA receptors endocytosis. Identification of new mechanistic elements involved in the nigrostriatal reconstruction process, using translational animal models and systems biology, is a promising approach to enhance the repair of this pathway in PD patients undergoing cell therapy.

Genetic, clinical, molecular, and pathogenic aspects of the South Asian-specific polymorphic MYBPC3Δ25bp variant

Hypertrophic cardiomyopathy (HCM) is a cardiac genetic disease characterized by ventricular enlargement, diastolic dysfunction, and increased risk for sudden cardiac death. Sarcomeric genetic defects are the predominant known cause of HCM. In particular, mutations in the myosin-binding protein C gene (MYBPC3) are associated with ~ 40% of all HCM cases in which a genetic basis has been established. A decade ago, our group reported a 25-base pair deletion in intron 32 of MYBPC3 (MYBPC3Δ25bp) that is uniquely prevalent in South Asians and is associated with autosomal dominant cardiomyopathy. Although our studies suggest that this deletion results in left ventricular dysfunction, cardiomyopathies, and heart failure, the precise mechanism by which this variant predisposes to heart disease remains unclear. Increasingly appreciated, however, is the contribution of secondary risk factors, additional mutations, and lifestyle choices in augmenting or modifying the HCM phenotype in MYBPC3Δ25bp carriers. Therefore, the goal of this review article is to summarize the current research dedicated to understanding the molecular pathophysiology of HCM in South Asians with the MYBPC3Δ25bp variant. An emphasis is to review the latest techniques currently applied to explore the MYBPC3Δ25bp pathogenesis and to provide a foundation for developing new diagnostic strategies and advances in therapeutics.

The PERK-Dependent Molecular Mechanisms as a Novel Therapeutic Target for Neurodegenerative Diseases

Higher prevalence of neurodegenerative diseases is strictly connected with progressive aging of the world population. Interestingly, a broad range of age-related, neurodegenerative diseases is characterized by a common pathological mechanism-accumulation of misfolded and unfolded proteins within the cells. Under certain circumstances, such protein aggregates may evoke endoplasmic reticulum (ER) stress conditions and subsequent activation of the unfolded protein response (UPR) signaling pathways via the protein kinase RNA-like endoplasmic reticulum kinase (PERK)-dependent manner. Under mild to moderate ER stress, UPR has a pro-adaptive role. However, severe or long-termed ER stress conditions directly evoke shift of the UPR toward its pro-apoptotic branch, which is considered to be a possible cause of neurodegeneration. To this day, there is no effective cure for Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), or prion disease. Currently available treatment approaches for these diseases are only symptomatic and cannot affect the disease progression. Treatment strategies, currently under detailed research, include inhibition of the PERK-dependent UPR signaling branches. The newest data have reported that the use of small-molecule inhibitors of the PERK-mediated signaling branches may contribute to the development of a novel, ground-breaking therapeutic approach for neurodegeneration. In this review, we critically describe all the aspects associated with such targeted therapy against neurodegenerative proteopathies.

Amyloid aggregates of the deubiquitinase OTUB1 are neurotoxic, suggesting that they contribute to the development of Parkinson's disease

Parkinson's disease (PD) is a multifactorial malady and the second most common neurodegenerative disorder, characterized by loss of dopaminergic neurons in the midbrain. A hallmark of PD pathology is the formation of intracellular protein inclusions, termed Lewy bodies (LBs). Recent MS studies have shown that OTU deubiquitinase ubiquitin aldehyde-binding 1 (OTUB1), a deubiquitinating enzyme of the OTU family, is enriched together with α-synuclein in LBs from individuals with PD and is also present in amyloid plaques associated with Alzheimer's disease. In the present study, using mammalian cell cultures and a PD mouse model, along with CD spectroscopy, atomic force microscopy, immunofluorescence-based imaging, and various biochemical assays, we demonstrate that after heat-induced protein aggregation, OTUB1 reacts strongly with both anti-A11 and anti-osteocalcin antibodies, detecting oligomeric, prefibrillar structures or fibrillar species of amyloidogenic proteins, respectively. Further, recombinant OTUB1 exhibited high thioflavin-T and Congo red binding and increased β-sheet formation upon heat induction. The oligomeric OTUB1 aggregates were highly cytotoxic, characteristic of many amyloid proteins. OTUB1 formed inclusions in neuronal cells and co-localized with thioflavin S and with α-synuclein during rotenone-induced stress. It also co-localized with the disease-associated variant pS129-α-synuclein in rotenone-exposed mouse brains. Interestingly, OTUB1 aggregates were also associated with severe cytoskeleton damage, rapid internalization inside the neuronal cells, and mitochondrial damage, all of which contribute to neurotoxicity. In conclusion, the results of our study indicate that OTUB1 may contribute to LB pathology through its amyloidogenic properties.

Prion-Like Mechanisms in Parkinson's Disease

Formation and aggregation of misfolded proteins in the central nervous system (CNS) is a key hallmark of several age-related neurodegenerative diseases, including Parkinson’s disease (PD), Alzheimer’s disease (AD), and amyotrophic lateral sclerosis (ALS). These diseases share key biophysical and biochemical characteristics with prion diseases. It is believed that PD is characterized by abnormal protein aggregation, mainly that of α-synuclein (α-syn). Of particular importance, there is growing evidence indicating that abnormal α-syn can spread to neighboring brain regions and cause aggregation of endogenous α-syn in these regions as seeds, in a “prion-like” manner. Abundant studies in vitro and in vivo have shown that α-syn goes through a templated conformational change, propagates from the original region to neighboring regions, and eventually cause neuron degeneration in the substantia nigra and striatum. The objective of this review is to summarize the mechanisms involved in the aggregation of abnormal intracellular α-syn and its subsequent cell-to-cell transmission. According to these findings, we look forward to effective therapeutic perspectives that can block the progression of neurodegenerative diseases.

Prions Strongly Reduce NMDA Receptor S-Nitrosylation Levels at Pre-symptomatic and Terminal Stages of Prion Diseases

Prion diseases are fatal neurodegenerative disorders characterized by the cellular prion protein (PrP^C) conversion into a misfolded and infectious isoform termed prion or PrP^Sc. The neuropathological mechanism underlying prion toxicity is still unclear, and the debate on prion protein gain- or loss-of-function is still open. PrP^C participates to a plethora of physiological mechanisms. For instance, PrP^C and copper cooperatively modulate N-methyl-D-aspartate receptor (NMDAR) activity by mediating S-nitrosylation, an inhibitory post-translational modification, hence protecting neurons from excitotoxicity. Here, NMDAR S-nitrosylation levels were biochemically investigated at pre- and post-symptomatic stages of mice intracerebrally inoculated with RML, 139A, and ME7 prion strains. Neuropathological aspects of prion disease were studied by histological analysis and proteinase K digestion. We report that hippocampal NMDAR S-nitrosylation is greatly reduced in all three prion strain infections in both pre-symptomatic and terminal stages of mouse disease. Indeed, we show that NMDAR S-nitrosylation dysregulation affecting prion-inoculated animals precedes the appearance of clinical signs of disease and visible neuropathological changes, such as PrP^Sc accumulation and deposition. The pre-symptomatic reduction of NMDAR S-nitrosylation in prion-infected mice may be a possible cause of neuronal death in prion pathology, and it might contribute to the pathology progression opening new therapeutic strategies against prion disorders.

Endoplasmic Reticulum Stress: Implications for Neuropsychiatric Disorders

The Endoplasmic reticulum (ER), an indispensable sub-cellular component of the eukaryotic cell carries out essential functions, is critical to the survival of the organism. The chaperone proteins and the folding enzymes which are multi-domain ER effectors carry out 3-dimensional conformation of nascent polypeptides and check misfolded protein aggregation, easing the exit of functional proteins from the ER. Diverse conditions, for instance redox imbalance, alterations in ionic calcium levels, and inflammatory signaling can perturb the functioning of the ER, leading to a build-up of unfolded or misfolded proteins in the lumen. This results in ER stress, and aiming to reinstate protein homeostasis, a well conserved reaction called the unfolded protein response (UPR) is elicited. Equally, in protracted cellular stress or inadequate compensatory reaction, UPR pathway leads to cell loss. Dysfunctional ER mechanisms are responsible for neuronal degeneration in numerous human diseases, for instance Alzheimer's, Parkinson's and Huntington's diseases. In addition, mounting proof indicates that ER stress is incriminated in psychiatric diseases like major depressive disorder, bipolar disorder, and schizophrenia. Accumulating evidence suggests that pharmacological agents regulating the working of ER may have a role in diminishing advancing neuronal dysfunction in neuropsychiatric disorders. Here, new findings are examined which link the foremost mechanisms connecting ER stress and cell homeostasis. Furthermore, a supposed new pathogenic model of major neuropsychiatry disorders is provided, with ER stress proposed as the pivotal step in disease development.

Heat shock promotes inclusion body formation of mutant huntingtin (mHtt) and alleviates mHtt-induced transcription factor dysfunction

PolyQ-expanded huntingtin (mHtt) variants form aggregates, termed inclusion bodies (IBs), in individuals with and models of Huntington's disease (HD). The role of IB versus diffusible mHtt in neurotoxicity remains unclear. Using a ponasterone (PA)-inducible cell model of HD, here we evaluated the effects of heat shock on the appearance and functional outcome of Htt103QExon1-EGFP expression. Quantitative image analysis indicated that 80-90% of this mHtt protein initially appears as "diffuse" signals in the cytosol, with IBs forming at high mHtt expression. A 2-h heat shock during the PA induction reduced the diffuse signal, but greatly increased mHtt IB formation in both cytosol and nucleus. Dose- and time-dependent mHtt expression suggested that nucleated polymerization drives IB formation. RNA-mediated knockdown of heat shock protein 70 (HSP70) and heat shock cognate 70 protein (HSC70) provided evidence for their involvement in promoting diffuse mHtt to form IBs. Reporter gene assays assessing the impacts of diffuse versus IB mHtt showed concordance of diffuse mHtt expression with the repression of heat shock factor 1, cAMP-responsive element-binding protein (CREB), and NF-κB activity. CREB repression was reversed by heat shock coinciding with mHtt IB formation. In an embryonic striatal neuron-derived HD model, the chemical chaperone sorbitol similarly promoted the structuring of diffuse mHtt into IBs and supported cell survival under stress. Our results provide evidence that mHtt IB formation is a chaperone-supported cellular coping mechanism that depletes diffusible mHtt conformers, alleviates transcription factor dysfunction, and promotes neuron survival.

Retrograde Transport by Clathrin-Coated Vesicles is Involved in Intracellular Transport of PrPSc in Persistently Prion-Infected Cells

Intracellular dynamics of an abnormal isoform of prion protein (PrP^Sc) are tightly associated with prion propagation. However, the machineries involved in the intracellular trafficking of PrP^Sc are not fully understood. Our previous study suggested that PrP^Sc in persistently prion-infected cells dynamically circulates between endocytic-recycling compartments (ERCs) and peripheral regions of the cells. To investigate these machineries, we focused on retrograde transport from endosomes to the trans-Golgi network, which is one of the pathways involved in recycling of molecules. PrP^Sc was co-localized with components of clathrin-coated vesicles (CCVs) as well as those of the retromer complex, which are known as machineries for retrograde transport. Fractionation of intracellular compartments by density gradient centrifugation showed the presence of PrP^Sc and the components of CCVs in the same fractions. Furthermore, PrP^Sc was detected in CCVs isolated from intracellular compartments of prion-infected cells. Knockdown of clathrin interactor 1, which is one of the clathrin adaptor proteins involved in retrograde transport, did not change the amount of PrP^Sc, but it altered the distribution of PrP^Sc from ERCs to peripheral regions, including late endosomes/lysosomes. These data demonstrated that some PrP^Sc is transported from endosomes to ERCs by CCVs, which might be involved in the recycling of PrP^Sc.

Neuronal Cell Death

Neuronal cell death occurs extensively during development and pathology, where it is especially important because of the limited capacity of adult neurons to proliferate or be replaced. The concept of cell death used to be simple as there were just two or three types, so we just had to work out which type was involved in our particular pathology and then block it. However, we now know that there are at least a dozen ways for neurons to die, that blocking a particular mechanism of cell death may not prevent the cell from dying, and that non-neuronal cells also contribute to neuronal death. We review here the mechanisms of neuronal death by intrinsic and extrinsic apoptosis, oncosis, necroptosis, parthanatos, ferroptosis, sarmoptosis, autophagic cell death, autosis, autolysis, paraptosis, pyroptosis, phagoptosis, and mitochondrial permeability transition. We next explore the mechanisms of neuronal death during development, and those induced by axotomy, aberrant cell-cycle reentry, glutamate (excitoxicity and oxytosis), loss of connected neurons, aggregated proteins and the unfolded protein response, oxidants, inflammation, and microglia. We then reassess which forms of cell death occur in stroke and Alzheimer's disease, two of the most important pathologies involving neuronal cell death. We also discuss why it has been so difficult to pinpoint the type of neuronal death involved, if and why the mechanism of neuronal death matters, the molecular overlap and interplay between death subroutines, and the therapeutic implications of these multiple overlapping forms of neuronal death.

Protein Disaggregation in Multicellular Organisms

Protein aggregates are formed in cells with profoundly perturbed proteostasis, where the generation of misfolded proteins exceeds the cellular refolding and degradative capacity. They are a hallmark of protein conformational disorders and aged and/or environmentally stressed cells. Protein aggregation is a reversible process in vivo, which counteracts proteotoxicities derived from aggregate persistence, but the chaperone machineries involved in protein disaggregation in Metazoa were uncovered only recently. Here we highlight recent advances in the mechanistic understanding of the major protein disaggregation machinery mediated by the Hsp70 chaperone system and discuss emerging alternative disaggregation activities in multicellular organisms.

Flow Cytometric Detection of PrPSc in Neurons and Glial Cells from Prion-Infected Mouse Brains

In prion diseases, an abnormal isoform of prion protein (PrP^Sc) accumulates in neurons, astrocytes, and microglia in the brains of animals affected by prions. Detailed analyses of PrP^Sc-positive neurons and glial cells are required to clarify their pathophysiological roles in the disease. Here, we report a novel method for the detection of PrP^Sc in neurons and glial cells from the brains of prion-infected mice by flow cytometry using PrP^Sc-specific staining with monoclonal antibody (MAb) 132. The combination of PrP^Sc staining and immunolabeling of neural cell markers clearly distinguished neurons, astrocytes, and microglia that were positive for PrP^Sc from those that were PrP^Sc negative. The flow cytometric analysis of PrP^Sc revealed the appearance of PrP^Sc-positive neurons, astrocytes, and microglia at 60 days after intracerebral prion inoculation, suggesting the presence of PrP^Sc in the glial cells, as well as in neurons, from an early stage of infection. Moreover, the kinetic analysis of PrP^Sc revealed a continuous increase in the proportion of PrP^Sc-positive cells for all cell types with disease progression. Finally, we applied this method to isolate neurons, astrocytes, and microglia positive for PrP^Sc from a prion-infected mouse brain by florescence-activated cell sorting. The method described here enables comprehensive analyses specific to PrP^Sc-positive neurons, astrocytes, and microglia that will contribute to the understanding of the pathophysiological roles of neurons and glial cells in PrP^Sc-associated pathogenesis.IMPORTANCE Although formation of PrP^Sc in neurons is associated closely with neurodegeneration in prion diseases, the mechanism of neurodegeneration is not understood completely. On the other hand, recent studies proposed the important roles of glial cells in PrP^Sc-associated pathogenesis, such as the intracerebral spread of PrP^Sc and clearance of PrP^Sc from the brain. Despite the great need for detailed analyses of PrP^Sc-positive neurons and glial cells, methods available for cell type-specific analysis of PrP^Sc have been limited thus far to microscopic observations. Here, we have established a novel high-throughput method for flow cytometric detection of PrP^Sc in cells with more accurate quantitative performance. By applying this method, we succeeded in isolating PrP^Sc-positive cells from the prion-infected mouse brains via fluorescence-activated cell sorting. This allows us to perform further detailed analysis specific to PrP^Sc-positive neurons and glial cells for the clarification of pathological changes in neurons and pathophysiological roles of glial cells.

What Is Our Current Understanding of PrPSc-Associated Neurotoxicity and Its Molecular Underpinnings?

The prion diseases are a collection of fatal, transmissible neurodegenerative diseases that cause rapid onset dementia and ultimately death. Uniquely, the infectious agent is a misfolded form of the endogenous cellular prion protein, termed PrP^Sc. Despite the identity of the molecular agent remaining the same, PrP^Sc can cause a range of diseases with hereditary, spontaneous or iatrogenic aetiologies. However, the link between PrP^Sc and toxicity is complex, with subclinical cases of prion disease discovered, and prion neurodegeneration without obvious PrP^Sc deposition. The toxic mechanisms by which PrP^Sc causes the extensive neuropathology are still poorly understood, although recent advances are beginning to unravel the molecular underpinnings, including oxidative stress, disruption of proteostasis and induction of the unfolded protein response. This review will discuss the diseases caused by PrP^Sc toxicity, the nature of the toxicity of PrP^Sc, and our current understanding of the downstream toxic signaling events triggered by the presence of PrP^Sc.

Prion Peptides Are Extremely Sensitive to Copper Induced Oxidative Stress

Copper(II) binding to prion peptides does not prevent Cu redox cycling and formation of reactive oxygen species (ROS) in the presence of reducing agents. The toxic effects of these species are exacerbated in the presence of catecholamines, indicating that dysfunction of catecholamine vesicular sequestration or recovery after synaptic release is a dangerous amplifier of Cu induced oxidative stress. Cu bound to prion peptides including the high affinity site involving histidines adjacent to the octarepeats exhibits marked catalytic activity toward dopamine and 4-methylcatechol. The resulting quinone oxidation products undergo parallel oligomerization and endogenous peptide modification yielding catechol adducts at the histidine binding ligands. These modifications add to the more common oxidation of Met and His residues produced by ROS. Derivatization of Cu-prion peptides is much faster than that undergone by Cu-β-amyloid and Cu-α-synuclein complexes in the same conditions.

The relevance of contact-independent cell-to-cell transfer of TDP-43 and SOD1 in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease involving the formation of cytoplasmic aggregates by proteins including TDP-43 and SOD1, in affected cells in the central nervous system (CNS). Pathology spreads from an initial site of onset to contiguous anatomical regions. There is evidence that for disease-associated proteins, including TDP-43 and SOD1, non-native protein conformers can promote misfolding of the natively folded counterparts, and cell-to-cell transfer of pathological aggregates may underlie the spread of the disease throughout the CNS. A variety of studies have demonstrated that SOD1 is released by neuron-like cells into the surrounding culture medium, either in their free state or encapsulated in extracellular vesicles such as exosomes. Extracellular SOD1 can then be internalised by naïve cells incubated in this conditioned medium, leading to the misfolding and aggregation of endogenous intracellular SOD1; an effect that propagates over serial passages. A similar phenomenon has also been observed with other proteins associated with protein misfolding and progressive neurological disorders, including tau, α-synuclein and both mammalian and yeast prions. Conditioned media experiments using TDP-43 have been less conclusive, with evidence for this protein undergoing intercellular transfer being less straightforward. In this review, we describe the properties of TDP-43 and SOD1 and look at the evidence for their respective abilities to participate in cell-to-cell transfer via conditioned medium, and discuss how variations in the nature of cell-to-cell transfer suggests that a number of different mechanisms are involved in the spreading of pathology in ALS.

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Protein aggregates transfer between cells in motor neuron disease.

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Cell contact-independent mechanisms may be a route of transfer.

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SOD1 undergoes cell-to-cell transfer via conditioned medium in cell culture.

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It is still unclear whether TDP-43 consistently undergoes cell-to-cell transfer

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Differences between the two proteins may explain this observation.

Common therapeutic strategies for prion and Alzheimer's diseases

A number of unexpected pathophysiological connections linking different neurodegenerative diseases have emerged over the past decade. An example is provided by prion and Alzheimer's diseases. Despite being distinct pathologies, these disorders share several neurotoxic mechanisms, including accumulation of misfolded protein isoforms, stress of the protein synthesis machinery, and activation of a neurotoxic signaling mediated by the cellular prion protein. Here, in addition to reviewing these mechanisms, we will discuss the potential therapeutic interventions for prion and Alzheimer's diseases that are arising from the comprehension of their common neurodegenerative pathways.

Cellular Regulation of Amyloid Formation in Aging and Disease

As the population is aging, the incidence of age-related neurodegenerative diseases, such as Alzheimer and Parkinson disease, is growing. The pathology of neurodegenerative diseases is characterized by the presence of protein aggregates of disease specific proteins in the brain of patients. Under certain conditions these disease proteins can undergo structural rearrangements resulting in misfolded proteins that can lead to the formation of aggregates with a fibrillar amyloid-like structure. Cells have different mechanisms to deal with this protein aggregation, where the molecular chaperone machinery constitutes the first line of defense against misfolded proteins. Proteins that cannot be refolded are subjected to degradation and compartmentalization processes. Amyloid formation has traditionally been described as responsible for the proteotoxicity associated with different neurodegenerative disorders. Several mechanisms have been suggested to explain such toxicity, including the sequestration of key proteins and the overload of the protein quality control system. Here, we review different aspects of the involvement of amyloid-forming proteins in disease, mechanisms of toxicity, structural features, and biological functions of amyloids, as well as the cellular mechanisms that modulate and regulate protein aggregation, including the presence of enhancers and suppressors of aggregation, and how aging impacts the functioning of these mechanisms, with special attention to the molecular chaperones.

A New Folding Kinetic Mechanism for Human Transthyretin and the Influence of the Amyloidogenic V30M Mutation

Protein aggregation into insoluble amyloid fibrils is the hallmark of several neurodegenerative diseases, chief among them Alzheimer's and Parkinson's. Although caused by different proteins, these pathologies share some basic molecular mechanisms with familial amyloidotic polyneuropathy (FAP), a rare hereditary neuropathy caused by amyloid formation and deposition by transthyretin (TTR) in the peripheral and autonomic nervous systems. Among the amyloidogenic TTR mutations known, V30M-TTR is the most common in FAP. TTR amyloidogenesis (ATTR) is triggered by tetramer dissociation, followed by partial unfolding and aggregation of the low conformational stability monomers formed. Thus, tetramer dissociation kinetics, monomer conformational stability and competition between refolding and aggregation pathways do play a critical role in ATTR. Here, we propose a new model to analyze the refolding kinetics of WT-TTR and V30M-TTR, showing that at pH and protein concentrations close to physiological, a two-step mechanism with a unimolecular first step followed by a second-order second step adjusts well to the experimental data. Interestingly, although sharing the same kinetic mechanism, V30M-TTR refolds at a much slower rate than WT-TTR, a feature that may favor the formation of transient species leading to kinetic partition into amyloidogenic pathways and, thus, significantly increasing the probability of amyloid formation in vivo.

Prion-like disorders and Transmissible Spongiform Encephalopathies: An overview of the mechanistic features that are shared by the various disease-related misfolded proteins

Prion diseases or Transmissible Spongiform Encephalopathies (TSEs) are a group of fatal neurodegenerative disorders affecting several mammalian species. Its causative agent, disease-associated prion protein (PrP^d), is a self-propagating β-sheet rich aberrant conformation of the cellular prion protein (PrP^C) with neurotoxic and aggregation-prone properties, capable of inducing misfolding of PrP^C molecules. PrP^d is the major constituent of prions and, most importantly, is the first known example of a protein with infectious attributes. It has been suggested that similar molecular mechanisms could be shared by other proteins implicated in diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis or systemic amyloidoses. Accordingly, several terms have been proposed to collectively group all these disorders. Through the stringent evaluation of those aspects that characterise TSE-causing prions, in particular propagation and spread, strain variability or transmissibility, we will discuss whether terms such as "prion", "prion-like", "prionoid" or "propagon" can be used when referring to the aetiological agents of the above other disorders. Moreover, it will also be discussed whether the term "infectious", which defines a prion essential trait, is currently misused when referring to the other misfolded proteins.

A critical appraisal of the pathogenic protein spread hypothesis of neurodegeneration

There has been an explosion in the number of papers discussing the hypothesis of 'pathogenic spread' in neurodegenerative disease - the idea that abnormal forms of disease-associated proteins, such as tau or α-synuclein, physically move from neuron to neuron to induce disease progression. However, whether inter-neuronal spread of protein aggregates actually occurs in humans and, if so, whether it causes symptom onset remain uncertain. Even if pathogenic spread is proven in humans, it is unclear how much this would alter the specific therapeutic approaches that are in development. A critical appraisal of this increasingly popular hypothesis thus seems both important and timely.

Key Points Concerning Amyloid Infectivity and Prion-Like Neuronal Invasion

Amyloid aggregation has been related to an increasing number of human illnesses, from Alzheimer’s and Parkinson’s diseases (AD/PD) to Creutzfeldt-Jakob disease. Commonly, only prions have been considered as infectious agents with a high capacity of propagation. However, recent publications have shown that many amyloid proteins, including amyloid β-peptide, α-synuclein (α-syn) and tau protein, also propagate in a “prion-like” manner. Meanwhile, no link between propagation of pathological proteins and neurotoxicity has been demonstrated. The extremely low infectivity under natural conditions of most non-prion amyloids is far below the capacity to spread exhibited by prions. Nonetheless, it is important to elucidate the key factors that cause non-prion amyloids to become infectious agents. In recent years, important advances in our understanding of the amyloid processes of amyloid-like proteins and unrelated prions (i.e., yeast and fungal prions) have yielded essential information that can shed light on the prion phenomenon in mammals and humans. As shown in this review, recent evidence suggests that there are key factors that could dramatically modulate the prion capacity of proteins in the amyloid conformation. The concentration of nuclei, the presence of oligomers, and the toxicity, resistance and localization of these aggregates could all be key factors affecting their spread. In short, those factors that favor the high concentration of extracellular nuclei or oligomers, characterized by small size, with a low toxicity could dramatically increase prion propensity; whereas low concentrations of highly toxic intracellular amyloids, with a large size, would effectively prevent infectivity.

Mechanisms of prion-induced neurodegeneration

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are fatal neurodegenerative disorders characterised by long incubation period, short clinical duration, and transmissibility to susceptible species. Neuronal loss, spongiform changes, gliosis and the accumulation in the brain of the misfolded version of a membrane-bound cellular prion protein (PrP(C)), termed PrP(TSE), are diagnostic markers of these diseases. Compelling evidence links protein misfolding and its accumulation with neurodegenerative changes. Accordingly, several mechanisms of prion-mediated neurotoxicity have been proposed. In this paper, we provide an overview of the recent knowledge on the mechanisms of neuropathogenesis, the neurotoxic PrP species and the possible therapeutic approaches to treat these devastating disorders.

PKR Inhibition Rescues Memory Deficit and ATF4 Overexpression in ApoE ε4 Human Replacement Mice

Sporadic Alzheimer's disease (AD) is an incurable neurodegenerative disease with clear pathological hallmarks, brain dysfunction, and unknown etiology. Here, we tested the hypothesis that there is a link between genetic risk factors for AD, cellular metabolic stress, and transcription/translation regulation. In addition, we aimed at reversing the memory impairment observed in a mouse model of sporadic AD. We have previously demonstrated that the most prevalent genetic risk factor for AD, the ApoE4 allele, is correlated with increased phosphorylation of the translation factor eIF2α. In the present study, we tested the possible involvement of additional members of the eIF2α pathway and identified increased mRNA expression of negative transcription factor ATF4 (aka CREB2) both in human and a mouse model expressing the human ApoE4 allele. Furthermore, injection of a PKR inhibitor rescued memory impairment and attenuated ATF4 mRNA increased expression in the ApoE4 mice. The results propose a new mechanism by which ApoE4 affects brain function and further suggest that inhibition of PKR is a way to restore ATF4 overexpression and memory impairment in early stages of sporadic AD. Significance statement: ATF4 mRNA relative quantities are elevated in ApoE4 allele carriers compared with noncarrier controls. This is true also for the ApoE ε4 human replacement mice. ApoE4 mice injected with PKR inhibitor (PKRi) demonstrate a significant reduction in ATF4 expression levels 3 h after one injection of PKRi. Treatment of ApoE4 human replacement mice with the PKRi before learning rescues the memory impairment of the ApoE4 AD model mice. We think that these results propose a new mechanism by which ApoE4 affects brain function and suggest that inhibition of PKR is a way to restore memory impairment in early stages of sporadic AD.

Prion protein localizes at the ciliary base during neural and cardiovascular development, and its depletion affects α-tubulin post-translational modifications

Although conversion of the cellular form of the prion protein (PrP^C) into a misfolded isoform is the underlying cause of prion diseases, understanding PrP^C physiological functions has remained challenging. PrP^C depletion or overexpression alters the proliferation and differentiation properties of various types of stem and progenitor cells in vitro by unknown mechanisms. Such involvement remains uncertain in vivo in the absence of any drastic phenotype of mice lacking PrP^C. Here, we report PrP^C enrichment at the base of the primary cilium in stem and progenitor cells from the central nervous system and cardiovascular system of developing mouse embryos. PrP^C depletion in a neuroepithelial cell line dramatically altered key cilium-dependent processes, such as Sonic hedgehog signalling and α-tubulin post-translational modifications. These processes were also affected over a limited time window in PrP^C–ablated embryos. Thus, our study reveals PrP^C as a potential actor in the developmental regulation of microtubule dynamics and ciliary functions.

From eyeless to neurological diseases

Age-related cataracts are frequently associated with degenerative changes in the ocular lens including the aggregation of proteins - mainly crystallins, but also other proteins including amyloids (Aβ) leading to the hypothesis that cataracts could be used as "biomarkers" for Alzheimer disease. Even if this hypothesis was rejected by David Beebe's last paper (Bei et al., Exp. Eye Res., 2015), it is a fascinating aspect to look for commonalities between eye diseases and neurological disorders. In this review, I discuss such commonalities between eye and brain mainly from a developmental point of view. The finding of the functional homology of the Drosophila eyeless gene with the mammalian Pax6 gene marks a first highlight in the developmental genetics of the eye - this result destroyed the "dogma" of the different evolutionary routes of eye development in flies and mammals. The second highlight was the finding that Pax6 is also involved in the development of the forebrain supporting the pleiotropic role of many genes. These findings opened a new avenue for research showing that a broad variety of transcription factors, but also structural proteins are involved both, in eye and brain development as well as into the maintenance of the functional integrity of the corresponding tissue(s). In this review recent findings are summarized demonstrating that genes whose mutations have been identified first to be causative for congenital or juvenile eye disorders are also involved in regenerative processes and neurogenesis (Pax6), but also in neurodegenerative diseases like Parkinson (e.g. Pitx3) or in neurological disorders like Schizophrenia (e.g. Crybb1, Crybb2).

Animal models for prion-like diseases

Prion diseases or Transmissible Spongiform Encephalopathies (TSEs) are a group of fatal neurodegenerative disorders affecting several mammalian species being Creutzfeldt-Jacob Disease (CJD) the most representative in human beings, scrapie in ovine, Bovine Spongiform Encephalopathy (BSE) in bovine and Chronic Wasting Disease (CWD) in cervids. As stated by the "protein-only hypothesis", the causal agent of TSEs is a self-propagating aberrant form of the prion protein (PrP) that through a misfolding event acquires a β-sheet rich conformation known as PrP(Sc) (from scrapie). This isoform is neurotoxic, aggregation prone and induces misfolding of native cellular PrP. Compelling evidence indicates that disease-specific protein misfolding in amyloid deposits could be shared by other disorders showing aberrant protein aggregates such as Alzheimer's Disease (AD), Parkinson's Disease (PD), Amyotrophic lateral sclerosis (ALS) and systemic Amyloid A amyloidosis (AA amyloidosis). Evidences of shared mechanisms of the proteins related to each disease with prions will be reviewed through the available in vivo models. Taking prion research as reference, typical prion-like features such as seeding and propagation ability, neurotoxic species causing disease, infectivity, transmission barrier and strain evidences will be analyzed for other protein-related diseases. Thus, prion-like features of amyloid β peptide and tau present in AD, α-synuclein in PD, SOD-1, TDP-43 and others in ALS and serum α-amyloid (SAA) in systemic AA amyloidosis will be reviewed through models available for each disease.

Dicer and Hsp104 function in a negative feedback loop to confer robustness to environmental stress

Epigenetic mechanisms can be influenced by environmental cues and thus evoke phenotypic variation. This plasticity can be advantageous for adaptation but also detrimental if not tightly controlled. Although having attracted considerable interest, it remains largely unknown if and how environmental cues such as temperature trigger epigenetic alterations. Using fission yeast, we demonstrate that environmentally induced discontinuous phenotypic variation is buffered by a negative feedback loop that involves the RNase Dicer and the protein disaggregase Hsp104. In the absence of Hsp104, Dicer accumulates in cytoplasmic inclusions and heterochromatin becomes unstable at elevated temperatures, an epigenetic state inherited for many cell divisions after the heat stress. Loss of Dicer leads to toxic aggregation of an exogenous prionogenic protein. Our results highlight the importance of feedback regulation in building epigenetic memory and uncover Hsp104 and Dicer as homeostatic controllers that buffer environmentally induced stochastic epigenetic variation and toxic aggregation of prionogenic proteins.

Prions and prion-like proteins

Prions are self-replicating protein aggregates and are the primary causative factor in a number of neurological diseases in mammals. The prion protein (PrP) undergoes a conformational transformation leading to aggregation into an infectious cellular pathogen. Prion-like protein spreading and transmission of aggregates between cells have also been demonstrated for other proteins associated with Alzheimer disease and Parkinson disease. This protein-only phenomenon may therefore have broader implications in neurodegenerative disorders. The minireviews in this thematic series highlight the recent advances in prion biology and the roles these unique proteins play in disease.