Sequestration of toxic oligomers by HspB1 as a cytoprotective mechanism

Reactive astrocytes secrete the chaperone HSPB1 to mediate neuroprotection

Molecular chaperones are protective in neurodegenerative diseases by preventing protein misfolding and aggregation, such as extracellular amyloid plaques and intracellular tau neurofibrillary tangles in Alzheimer's disease (AD). In addition, AD is characterized by an increase in astrocyte reactivity. The chaperone HSPB1 has been proposed as a marker for reactive astrocytes; however, its astrocytic functions in neurodegeneration remain to be elucidated. Here, we identify that HSPB1 is secreted from astrocytes to exert non-cell-autonomous protective functions. We show that in human AD brain, HSPB1 levels increase in astrocytes that cluster around amyloid plaques, as well as in the adjacent extracellular space. Moreover, in conditions that mimic an inflammatory reactive response, astrocytes increase HSPB1 secretion. Concomitantly, astrocytes and neurons can uptake astrocyte-secreted HSPB1, which is accompanied by an attenuation of the inflammatory response in reactive astrocytes and reduced pathological tau inclusions. Our findings highlight a protective mechanism in disease conditions that encompasses the secretion of a chaperone typically regarded as intracellular.

Identification of novel hub genes for Alzheimer's disease associated with the hippocampus using WGCNA and differential gene analysis

Background

Alzheimer's disease (AD) is a common, refractory, progressive neurodegenerative disorder in which cognitive and memory deficits are highly correlated with abnormalities in hippocampal brain regions. There is still a lack of hippocampus-related markers for AD diagnosis and prevention.

Methods

Differently expressed genes were identified in the gene expression profile GSE293789 in the hippocampal brain region. Enrichment analyses GO, KEGG, and GSEA were used to identify biological pathways involved in the DEGs and AD-related group. WGCNA was used to identify the gene modules that are highly associated with AD in the samples. The intersecting genes of the genes in DEGs and modules were extracted and the top ten ranked hub genes were identified. Finally GES48350 was used as a validation cohort to predict the diagnostic efficacy of hub genes.

Results

From GSE293789, 225 DEGs were identified, which were mainly associated with calcium response, glutamatergic synapses, and calcium-dependent phospholipid-binding response. WGCNA analysis yielded dark green and bright yellow modular genes as the most relevant to AD. From these two modules, 176 genes were extracted, which were taken to be intersected with DEGs, yielding 51 intersecting genes. Then 10 hub genes were identified in them: HSPA1B, HSPB1, HSPA1A, DNAJB1, HSPB8, ANXA2, ANXA1, SOX9, YAP1, and AHNAK. Validation of these genes was found to have excellent diagnostic performance.

Conclusion

Ten AD-related hub genes in the hippocampus were identified, contributing to further understanding of AD development in the hippocampus and development of targets for therapeutic prevention.

Misfolded protein oligomers: mechanisms of formation, cytotoxic effects, and pharmacological approaches against protein misfolding diseases

The conversion of native peptides and proteins into amyloid aggregates is a hallmark of over 50 human disorders, including Alzheimer's and Parkinson's diseases. Increasing evidence implicates misfolded protein oligomers produced during the amyloid formation process as the primary cytotoxic agents in many of these devastating conditions. In this review, we analyze the processes by which oligomers are formed, their structures, physicochemical properties, population dynamics, and the mechanisms of their cytotoxicity. We then focus on drug discovery strategies that target the formation of oligomers and their ability to disrupt cell physiology and trigger degenerative processes.

Single-Cell Transcriptional Profiling and Gene Regulatory Network Modeling in Tg2576 Mice Reveal Gender-Dependent Molecular Features Preceding Alzheimer-Like Pathologies

Alzheimer's disease (AD) onset and progression is influenced by a complex interplay of several environmental and genetic factors, one of them gender. Pronounced gender differences have been observed both in the relative risk of developing AD and in clinical disease manifestations. A molecular level understanding of these gender disparities is still missing, but could provide important clues on cellular mechanisms modulating the disease and reveal new targets for gender-oriented disease-modifying precision therapies. We therefore present here a comprehensive single-cell analysis of disease-associated molecular gender differences in transcriptomics data from the neocortex, one of the brain regions most susceptible to AD, in one of the most widely used AD mouse models, the Tg2576 model. Cortical areas are also most commonly used in studies of post-mortem AD brains. To identify disease-linked molecular processes that occur before the onset of detectable neuropathology, we focused our analyses on an age with no detectable plaques and microgliosis. Cell-type specific alterations were investigated at the level of individual genes, pathways, and gene regulatory networks. The number of differentially expressed genes (DEGs) was not large enough to build context-specific gene regulatory networks for each individual cell type, and thus, we focused on the study of cell types with dominant changes and included analyses of changes across the combination of cell types. We observed significant disease-associated gender differences in cellular processes related to synapse organization and reactive oxygen species metabolism, and identified a limited set of transcription factors, including Egr1 and Klf6, as key regulators of many of the disease-associated and gender-dependent gene expression changes in the model. Overall, our analyses revealed significant cell-type specific gene expression changes in individual genes, pathways and sub-networks, including gender-specific and gender-dimorphic changes in both upstream transcription factors and their downstream targets, in the Tg2576 AD model before the onset of overt disease. This opens a window into molecular events that could determine gender-susceptibility to AD, and uncovers tractable target candidates for potential gender-specific precision medicine for AD.

Integrating single-nucleus sequence profiling to reveal the transcriptional dynamics of Alzheimer's disease, Parkinson's disease, and multiple sclerosis

BACKGROUND

Alzheimer's disease (AD), Parkinson's disease (PD), and multiple sclerosis (MS) are three nervous system diseases that partially overlap clinically and genetically. However, bulk RNA-sequencing did not accurately detect the core pathogenic molecules in them. The availability of high-quality single cell RNA-sequencing data of post-mortem brain collections permits the generation of a large-scale gene expression in different cells in human brain, focusing on the molecular features and relationships between diseases and genes. We integrated single-nucleus RNA-sequencing (snRNA-seq) datasets of human brains with AD, PD, and MS to identify transcriptomic commonalities and distinctions among them.

METHODS

The snRNA-seq datasets were downloaded from Gene Expression Omnibus (GEO) database. The Seurat package was used for snRNA-seq data processing. The uniform manifold approximation and projection (UMAP) were utilized for cluster identification. The FindMarker function in Seurat was used to identify the differently expressed genes. Functional enrichment analysis was carried out using the Gene Set Enrichment Analysis (GSEA) and Gene ontology (GO). The protein-protein interaction (PPI) analysis of differentially expressed genes (DEGs) was analyzed using STRING database ( http://string-db.org ). SCENIC analysis was performed using utilizing pySCENIC (v0.10.0) based on the hg19-tss-centered-10 kb-10species databases. The analysis of potential therapeutic drugs was analyzed on Connectivity Map ( https://clue.io ).

RESULTS

The gene regulatory network analysis identified several hub genes regulated in AD, PD, and MS, in which HSPB1 and HSPA1A were key molecules. These upregulated HSP family genes interact with ribosome genes in AD and MS, and with immunomodulatory genes in PD. We further identified several transcriptional regulators (SPI1, CEBPA, TFE3, GRHPR, and TP53) of the hub genes, which has important implications for uncovering the molecular crosstalk among AD, PD, and MS. Arctigenin was identified as a potential therapeutic drug for AD, PD, and MS.

CONCLUSIONS

Together, the integrated snRNA-seq data and findings have significant implications for unraveling the shared and unique molecular crosstalk among AD, PD, and MS. HSPB1 and HSPA1A as promising targets involved in the pathological mechanisms of neurodegenerative diseases. Additionally, the identification of arctigenin as a potential therapeutic drug for AD, PD, and MS further highlights its potential in treating these neurological disorders. These discoveries lay the groundwork for future research and interventions to enhance our understanding and treatment of AD, PD, and MS.

The HSPB1-p62/SQSTM1 functional complex regulates the unconventional secretion and transcellular spreading of the HD-associated mutant huntingtin protein

Conformational diseases, such as Alzheimer, Parkinson and Huntington diseases, are part of a common class of neurological disorders characterized by the aggregation and progressive accumulation of proteins bearing aberrant conformations. Huntington disease (HD) has autosomal dominant inheritance and is caused by mutations leading to an abnormal expansion in the polyglutamine (polyQ) tract of the huntingtin (HTT) protein, leading to the formation of HTT inclusion bodies in neurons of affected patients. Interestingly, recent experimental evidence is challenging the conventional view by which the disease pathogenesis is solely a consequence of the intracellular accumulation of mutant protein aggregates. These studies reveal that transcellular transfer of mutated huntingtin protein is able to seed oligomers involving even the wild-type (WT) forms of the protein. To date, there is still no successful strategy to treat HD. Here, we describe a novel functional role for the HSPB1-p62/SQSTM1 complex, which acts as a cargo loading platform, allowing the unconventional secretion of mutant HTT by extracellular vesicles. HSPB1 interacts preferentially with polyQ-expanded HTT compared with the WT protein and affects its aggregation. Furthermore, HSPB1 levels correlate with the rate of mutant HTT secretion, which is controlled by the activity of the PI3K/AKT/mTOR signalling pathway. Finally, we show that these HTT-containing vesicular structures are biologically active and able to be internalized by recipient cells, therefore providing an additional mechanism to explain the prion-like spreading properties of mutant HTT. These findings might also have implications for the turn-over of other disease-associated, aggregation-prone proteins.

Heat Shock Proteins in Tooth Development and Injury Repair

Heat shock proteins (HSPs) are a class of molecular chaperones with expression increased in response to heat or other stresses. HSPs regulate cell homeostasis by modulating the folding and maturation of intracellular proteins. Tooth development is a complex process that involves many cell activities. During tooth preparation or trauma, teeth can be damaged. The damaged teeth start their repair process by remineralizing and regenerating tissue. During tooth development and injury repair, different HSPs have different expression patterns and play a special role in odontoblast differentiation and ameloblast secretion by mediating signaling pathways or participating in protein transport. This review explores the expression patterns and potential mechanisms of HSPs, particularly HSP25, HSP60 and HSP70, in tooth development and injury repair.

A novel regulator in Alzheimer's disease progression: The astrocyte-derived extracellular vesicles

Alzheimer's disease (AD) is known as an age-related irreversible neurodegenerative disease. AD seriously endangers the health of the elderly, but there is still no effective treatment. In the past several decades, the significant role of astrocytes in the process of AD has been universally acknowledged. In addition, extracellular vesicles (EVs) have been recognized as an essential mediator in intercellular communication and participate in various pathophysiological processes by carrying and transporting diverse cargoes. Moreover, specific conditions and stimuli can modulate the amount and properties of astrocyte-derived EVs (ADEVs) to affect AD progression. Thus, recent studies focused on the involvement of ADEVs in the pathogenesis of AD and the potential application of ADEVs in the diagnosis and treatment of AD, which provides a new direction and possibility for revealing the mystery of AD. Interestingly, it can be concluded that ADEVs have both pathogenic and protective effects in the process of AD through a comprehensive generalization. In this review, we aim to summarize the multi-faces of ADEVs effects on AD development, which can provide a novel strategy to investigate the underlying mechanism in AD. We also summarize the current ADEVs clinically relevant studies to raise the potential use of ADEVs in the discovery of novel biomarkers for diagnosis and therapeutic targets for AD.

Folding of heterologous proteins in bacterial cell factories: Cellular mechanisms and engineering strategies

The expression of correctly folded and functional heterologous proteins is important in many biotechnological production processes, whether it is enzymes, biopharmaceuticals or biosynthetic pathways for production of sustainable chemicals. For industrial applications, bacterial platform organisms, such as E. coli, are still broadly used due to the availability of tools and proven suitability at industrial scale. However, expression of heterologous proteins in these organisms can result in protein aggregation and low amounts of functional protein. This review provides an overview of the cellular mechanisms that can influence protein folding and expression, such as co-translational folding and assembly, chaperone binding, as well as protein quality control, across different model organisms. The knowledge of these mechanisms is then linked to different experimental methods that have been applied in order to improve functional heterologous protein folding, such as codon optimization, fusion tagging, chaperone co-production, as well as strain and protein engineering strategies.

Modeling the early stages of Alzheimer's disease by administering intracerebroventricular injections of human native Aβ oligomers to rats

Alzheimer's disease (AD) is a progressive and irreversible neurodegenerative disease characterized by the accumulation of aggregated amyloid beta (Aβ) and hyperphosphorylated tau along with a slow decline in cognitive functions. Unlike advanced AD, the initial steps of AD pathophysiology have been poorly investigated, partially due to limited availability of animal models focused on the early, plaque-free stages of the disease. The aim of this study was to evaluate the early behavioral, anatomical and molecular alterations in wild-type rats following intracerebroventricular injections of human Aβ oligomers (AβOs). Bioactive human AD and nondemented control brain tissue extracts were characterized using ELISA and proteomics approaches. Following a bilateral infusion, rats underwent behavioral testing, including the elevated plus maze, social recognition test, Morris water maze and Y-maze within 6 weeks postinjection. An analysis of brain structure was performed with manganese-enhanced MRI. Collected brain tissues were analyzed using stereology, immunohistochemistry, ELISA and qPCR. No sensorimotor deficits affecting motor performance on different maze tasks were observed, nor was spatial memory disturbed in AD rats. In contrast, a significant impairment of social memory became evident at 21 days postinjection. This deficit was associated with a significantly decreased volume of the lateral entorhinal cortex and a tendency toward a decrease in the total brain volume. Significant increase of cleaved caspase-3-positive cells, microglial activation and proinflammatory responses accompanied by altered expression of synaptic markers were observed in the hippocampus of AD rats with immunohistochemical and qPCR approaches at 6 weeks postinjection. Our data suggest that the social memory impairment observed in AβO-injected rats might be determined by neuroinflammatory responses and synaptopathy. An infusion of native oligomeric Aβ in the rat brain represents a feasible tool to model early plaque-free events associated with AD.

Immunohistochemical Demonstration of the pGlu79 α-Synuclein Fragment in Alzheimer’s Disease and Its Tg2576 Mouse Model

The deposition of β-amyloid peptides and of α-synuclein proteins is a neuropathological hallmark in the brains of Alzheimer’s disease (AD) and Parkinson’s disease (PD) subjects, respectively. However, there is accumulative evidence that both proteins are not exclusive for their clinical entity but instead co-exist and interact with each other. Here, we investigated the presence of a newly identified, pyroglutamate79-modified α-synuclein variant (pGlu79-aSyn)—along with the enzyme matrix metalloproteinase-3 (MMP-3) and glutaminyl cyclase (QC) implicated in its formation—in AD and in the transgenic Tg2576 AD mouse model. In the human brain, pGlu79-aSyn was detected in cortical pyramidal neurons, with more distinct labeling in AD compared to control brain tissue. Using immunohistochemical double and triple labelings and confocal laser scanning microscopy, we demonstrate an association of pGlu79-aSyn, MMP-3 and QC with β-amyloid plaques. In addition, pGlu79-aSyn and QC were present in amyloid plaque-associated reactive astrocytes that were also immunoreactive for the chaperone heat shock protein 27 (HSP27). Our data are consistent for the transgenic mouse model and the human clinical condition. We conclude that pGlu79-aSyn can be generated extracellularly or within reactive astrocytes, accumulates in proximity to β-amyloid plaques and induces an astrocytic protein unfolding mechanism involving HSP27.

Structural basis for the inhibition of IAPP fibril formation by the co-chaperonin prefoldin

Chaperones, as modulators of protein conformational states, are key cellular actors to prevent the accumulation of fibrillar aggregates. Here, we integrated kinetic investigations with structural studies to elucidate how the ubiquitous co-chaperonin prefoldin inhibits diabetes associated islet amyloid polypeptide (IAPP) fibril formation. We demonstrated that both human and archaeal prefoldin interfere similarly with the IAPP fibril elongation and secondary nucleation pathways. Using archaeal prefoldin model, we combined nuclear magnetic resonance spectroscopy with electron microscopy to establish that the inhibition of fibril formation is mediated by the binding of prefoldin’s coiled-coil helices to the flexible IAPP N-terminal segment accessible on the fibril surface and fibril ends. Atomic force microscopy demonstrates that binding of prefoldin to IAPP leads to the formation of lower amounts of aggregates, composed of shorter fibrils, clustered together. Linking structural models with observed fibrillation inhibition processes opens perspectives for understanding the interference between natural chaperones and formation of disease-associated amyloids.

Integrated kinetic and structural investigations reveal that the ubiquitous co-chaperonin prefoldin interacts with its coiled-coil helices on the islet amyloid polypeptide fibril surface and fibril ends to inhibit fibril elongation and secondary nucleation.

Exposing the Brain Proteomic Signatures of Alzheimer's Disease in Diverse Racial Groups: Leveraging Multiple Data Sets and Machine Learning

Recent studies have highlighted that the proteome can be used to identify potential biomarker candidates for Alzheimer's disease (AD) in diverse cohorts. Furthermore, the racial and ethnic background of participants is an important factor to consider to ensure the effectiveness of potential biomarkers for representative populations. A promising approach to survey potential biomarker candidates for diagnosing AD in diverse cohorts is the application of machine learning to proteomics data sets. Herein, we leveraged six existing bottom-up proteomics data sets, which included non-Hispanic White, African American/Black, and Hispanic participants, to study protein changes in AD and cognitively unimpaired participants. Machine learning models were applied to these data sets and resulted in the identification of amyloid-β precursor protein (APP) and heat shock protein β-1 (HSPB1) as two proteins that have high ability to distinguish AD; however, each protein's performance varied based upon the racial and ethnic background of the participants. HSPB1 particularly was helpful for generating high areas under the curve (AUCs) for African American/Black participants. Overall, HSPB1 improved the performance of the machine learning models when combined with APP and/or participant age and is a potential candidate that should be further explored in AD biomarker discovery efforts.

Small Heat Shock Protein 22 Improves Cognition and Learning in the Tauopathic Brain

The microtubule-associated protein tau pathologically accumulates and aggregates in Alzheimer's disease (AD) and other tauopathies, leading to cognitive dysfunction and neuronal loss. Molecular chaperones, like small heat-shock proteins (sHsps), can help deter the accumulation of misfolded proteins, such as tau. Here, we tested the hypothesis that the overexpression of wild-type Hsp22 (wtHsp22) and its phosphomimetic (S24,57D) Hsp22 mutant (mtHsp22) could slow tau accumulation and preserve memory in a murine model of tauopathy, rTg4510. Our results show that Hsp22 protected against deficits in synaptic plasticity and cognition in the tauopathic brain. However, we did not detect a significant change in tau phosphorylation or levels in these mice. This led us to hypothesize that the functional benefit was realized through the restoration of dysfunctional pathways in hippocampi of tau transgenic mice since no significant benefit was measured in non-transgenic mice expressing wtHsp22 or mtHsp22. To identify these pathways, we performed mass spectrometry of tissue lysates from the injection site. Overall, our data reveal that Hsp22 overexpression in neurons promotes synaptic plasticity by regulating canonical pathways and upstream regulators that have been characterized as potential AD markers and synaptogenesis regulators, like EIF4E and NFKBIA.

Extracellular vesicle-associated small heat shock proteins as therapeutic agents in neurodegenerative diseases and beyond

Increasing evidence points towards using extracellular vesicles (EVs) as a therapeutic strategy in neurodegenerative diseases such as multiple sclerosis, Parkinson's, and Alzheimer's disease. EVs are nanosized carriers that play an essential role in intercellular communication and cellular homeostasis by transporting an active molecular cargo, including a large variety of proteins. Recent publications demonstrate that small heat shock proteins (HSPBs) exhibit a beneficial role in neurodegenerative diseases. Moreover, it is defined that HSPBs target the autophagy and the apoptosis pathway, playing a prominent role in chaperone activity and cell survival. This review elaborates on the therapeutic potential of EVs and HSPBs, in particular HSPB1 and HSPB8, in neurodegenerative diseases. We conclude that EVs and HSPBs positively influence neuroinflammation, central nervous system (CNS) repair, and protein aggregation in CNS disorders. Moreover, we propose the use of HSPB-loaded EVs as advanced nanocarriers for the future development of neurodegenerative disease therapies.

Truncation-Driven Lateral Association of α-Synuclein Hinders Amyloid Clearance by the Hsp70-Based Disaggregase

The aggregation of α-synuclein is the hallmark of a collective of neurodegenerative disorders known as synucleinopathies. The tendency to aggregate of this protein, the toxicity of its aggregation intermediates and the ability of the cellular protein quality control system to clear these intermediates seems to be regulated, among other factors, by post-translational modifications (PTMs). Among these modifications, we consider herein proteolysis at both the N- and C-terminal regions of α-synuclein as a factor that could modulate disassembly of toxic amyloids by the human disaggregase, a combination of the chaperones Hsc70, DnaJB1 and Apg2. We find that, in contrast to aggregates of the protein lacking the N-terminus, which can be solubilized as efficiently as those of the WT protein, the deletion of the C-terminal domain, either in a recombinant context or as a consequence of calpain treatment, impaired Hsc70-mediated amyloid disassembly. Progressive removal of the negative charges at the C-terminal region induces lateral association of fibrils and type B* oligomers, precluding chaperone action. We propose that truncation-driven aggregate clumping impairs the mechanical action of chaperones, which includes fast protofilament unzipping coupled to depolymerization. Inhibition of the chaperone-mediated clearance of C-truncated species could explain their exacerbated toxicity and higher propensity to deposit found in vivo.

The role of amyloid oligomers in neurodegenerative pathologies

Many neurodegenerative diseases are rooted in the activities of amyloid-like proteins which possess conformations that spread to healthy proteins. These include Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD) and amyotrophic lateral sclerosis (ALS). While their clinical manifestations vary, their protein-level mechanisms are remarkably similar. Aberrant monomeric proteins undergo conformational shifts, facilitating aggregation and formation of solid fibrils. However, there is growing evidence that intermediate oligomeric stages are key drivers of neuronal toxicity. Analysis of protein dynamics is complicated by the fact that nucleation and growth of amyloid-like proteins is not a linear pathway. Feedback within this pathway results in exponential acceleration of aggregation, but activities exerted by oligomers and fibrils can alter cellular interactions and the cellular environment as a whole. The resulting cascade of effects likely contributes to the late onset and accelerating progression of amyloid-like protein disorders and the widespread effects they have on the body. In this review we explore the amyloid-like proteins associated with AD, PD, HD and ALS, as well as the common mechanisms of amyloid-like protein nucleation and aggregation. From this, we identify core elements of pathological progression which have been targeted for therapies, and which may become future therapeutic targets.

The Heart of the Alzheimer's: A Mindful View of Heart Disease

Purpose of Review: This review summarizes the current evidence for the involvement of proteotoxicity and protein quality control systems defects in diseases of the central nervous and cardiovascular systems. Specifically, it presents the commonalities between the pathophysiology of protein misfolding diseases in the heart and the brain. Recent Findings: The involvement of protein homeostasis dysfunction has been for long time investigated and accepted as one of the leading pathophysiological causes of neurodegenerative diseases. In cardiovascular diseases instead the mechanistic focus had been on the primary role of Ca2+ dishomeostasis, myofilament dysfunction as well as extracellular fibrosis, whereas no attention was given to misfolding of proteins as a pathogenetic mechanism. Instead, in the recent years, several contributions have shown protein aggregates in failing hearts similar to the ones found in the brain and increasing evidence have highlighted the crucial importance that proteotoxicity exerts via pre-amyloidogenic species in cardiovascular diseases as well as the prominent role of the cellular response to misfolded protein accumulation. As a result, proteotoxicity, unfolding protein response (UPR), and ubiquitin-proteasome system (UPS) have recently been investigated as potential key pathogenic pathways and therapeutic targets for heart disease. Summary: Overall, the current knowledge summarized in this review describes how the misfolding process in the brain parallels in the heart. Understanding the folding and unfolding mechanisms involved early through studies in the heart will provide new knowledge for neurodegenerative proteinopathies and may prepare the stage for targeted and personalized interventions.

Synthesis and in vitro quantification of amyloid fibrils by barbituric and thiobarbituric acid-based chromene derivatives

Neurodegenerative disease is caused by the abnormal build-up of proteins in and around cells called amyloid. The amyloid fibril formation and its mechanism have been investigated with various techniques, including dye-binding assay. Thioflavin T (ThT) has been one of the most widely used dyes for quantifying amyloid deposits, but ThT has a weak fluorescence signal especially at low concentration of amyloid fibrils, low lipophilicity and positive charge that makes it unable to cross the blood-brain barrier (BBB) to detect amyloid fibrils in vivo. Hence, there is a strong motivation for designing and developing the new compounds for in vitro amyloid quantification and in vivo amyloid imaging. The need for new probes to detect amyloid fibrils, especially within the cell, is highlighted by the fact that an accurate understanding of the molecular details of amyloid fibril formation is required to design and develop strategies for controlling the amyloid formation, and this needs more reliable probes for amyloid identification. In this work, we synthesized and applied barbituric and thiobarbituric acid-based chromene derivatives, as new fluorescent dyes to quantitatively detect the amyloid fibrils of bovine serum albumin (BSA) and human insulin in comparison with native soluble proteins or amorphous aggregation. Our results showed that among the 14 synthesized compounds, five compounds 4a, 4h, 4j, 4k, and 4l could selectively and specifically bind to amyloid fibrils while other compounds demonstrated a low-affinity binding. Furthermore, according to the cell viability experiment, compounds 4a, 4j and 4l at low concentration of compounds are not toxic, especially compound 4j which could be used as a suitable candidate for in vivo study. Further studies are needed to determine all the properties of compounds, especially in vivo experiments.

Identification of contributing genes of Huntington's disease by machine learning

Background

Huntington’s disease (HD) is an inherited disorder caused by the polyglutamine (poly-Q) mutations of the HTT gene results in neurodegeneration characterized by chorea, loss of coordination, cognitive decline. However, HD pathogenesis is still elusive. Despite the availability of a wide range of biological data, a comprehensive understanding of HD’s mechanism from machine learning is so far unrealized, majorly due to the lack of needed data density.

Methods

To harness the knowledge of the HD pathogenesis from the expression profiles of postmortem prefrontal cortex samples of 157 HD and 157 controls, we used gene profiling ranking as the criteria to reduce the dimension to the order of magnitude of the sample size, followed by machine learning using the decision tree, rule induction, random forest, and generalized linear model.

Results

These four Machine learning models identified 66 potential HD-contributing genes, with the cross-validated accuracy of 90.79 ± 4.57%, 89.49 ± 5.20%, 90.45 ± 4.24%, and 97.46 ± 3.26%, respectively. The identified genes enriched the gene ontology of transcriptional regulation, inflammatory response, neuron projection, and the cytoskeleton. Moreover, three genes in the cognitive, sensory, and perceptual systems were also identified.

Conclusions

The mutant HTT may interfere with both the expression and transport of these identified genes to promote the HD pathogenesis.

ATP Impedes the Inhibitory Effect of Hsp90 on Aβ40 Fibrillation

Heat shock protein 90 (Hsp90) is a molecular chaperone that assists protein folding in an Adenosine triphosphate (ATP)-dependent way. Hsp90 has been reported to interact with Alzheimeŕs disease associated amyloid-β (Aβ) peptides and to suppress toxic oligomer- and fibril formation. However, the mechanism remains largely unclear. Here we use a combination of atomic force microscopy (AFM) imaging, circular dichroism (CD) spectroscopy and biochemical analysis to quantify this interaction and put forward a microscopic picture including rate constants for the different transitions towards fibrillation. We show that Hsp90 binds to Aβ₄₀ monomers weakly but inhibits Aβ₄₀ from growing into fibrils at substoichiometric concentrations. ATP impedes this interaction, presumably by modulating Hsp90's conformational dynamics and reducing its hydrophobic surface. Altogether, these results might indicate alternative ways to prevent Aβ₄₀ fibrillation by manipulating chaperones that are already abundant in the brain.

Therapeutic Strategies to Reduce the Toxicity of Misfolded Protein Oligomers

The aberrant aggregation of proteins is implicated in the onset and pathogenesis of a wide range of neurodegenerative disorders, including Alzheimer’s and Parkinson’s diseases. Mounting evidence indicates that misfolded protein oligomers produced as intermediates in the aggregation process are potent neurotoxic agents in these diseases. Because of the transient and heterogeneous nature of these elusive aggregates, however, it has proven challenging to develop therapeutics that can effectively target them. Here, we review approaches aimed at reducing oligomer toxicity, including (1) modulating the oligomer populations (e.g., by altering the kinetics of aggregation by inhibiting, enhancing, or redirecting the process), (2) modulating the oligomer properties (e.g., through the size–hydrophobicity–toxicity relationship), (3) modulating the oligomer interactions (e.g., by protecting cell membranes by displacing oligomers), and (4) reducing oligomer toxicity by potentiating the protein homeostasis system. We analyze examples of these complementary approaches, which may lead to the development of compounds capable of preventing or treating neurodegenerative disorders associated with protein aggregation.

Oral Immunization with Soybean Storage Protein Containing Amyloid-β 4-10 Prevents Spatial Learning Decline

 Amyloid-β (Aβ) plays a central role in the pathogenesis of Alzheimer’s disease (AD). Because AD pathologies begin two decades before the onset of dementia, prevention of Aβ amyloidosis has been proposed as a mean to block the pathological cascade. Here, we generate a transgenic plant-based vaccine, a soybean storage protein containing Aβ4–10, named Aβ+, for oral Aβ immunization. One mg of Aβ+ or control protein (Aβ–) was administered to TgCRND8 mice once a week from 9 weeks up to 58 weeks. Aβ+ immunization raised both anti-Aβ antibodies and cellular immune responses. Spatial learning decline was prevented in the Aβ+ immunized group in an extended reference memory version of Morris water maze test from 21 to 57 weeks. In Tris-buffered saline (TBS), sodium dodecyl sulfate (SDS), and formic acid (FA) serial extractions, all sets of Aβ species from Aβ monomer, low to high molecular weight Aβ oligomers, and Aβ smears had different solubility in TgCRND8 brains. Aβ oligomers decreased in TBS fractions, corresponding to an increase in high molecular weight Aβ oligomers in SDS extracts and Aβ smears in FA fraction of the Aβ+ treated group. There was significant inhibition of histological Aβ burden, especially in diffuse plaques, and suppression of microglial inflammation. Processing of amyloid-β protein precursor was not different between Aβ+ and Aβ– groups. No evidence of amyloid-related inflammatory angiopathy was observed. Thus, Aβ+ oral immunization could be a promising, cheap, and long-term safe disease-modifying therapy to prevent the pathological process in AD.

Secreted Chaperones in Neurodegeneration

Protein homeostasis, or proteostasis, is a combination of cellular processes that govern protein quality control, namely, protein translation, folding, processing, and degradation. Disruptions in these processes can lead to protein misfolding and aggregation. Proteostatic disruption can lead to cellular changes such as endoplasmic reticulum or oxidative stress; organelle dysfunction; and, if continued, to cell death. A majority of neurodegenerative diseases involve the pathologic aggregation of proteins that subverts normal neuronal function. While prior reviews of neuronal proteostasis in neurodegenerative processes have focused on cytoplasmic chaperones, there is increasing evidence that chaperones secreted both by neurons and other brain cells in the extracellular – including transsynaptic – space play important roles in neuronal proteostasis. In this review, we will introduce various secreted chaperones involved in neurodegeneration. We begin with clusterin and discuss its identification in various protein aggregates, and the use of increased cerebrospinal fluid (CSF) clusterin as a potential biomarker and as a potential therapeutic. Our next secreted chaperone is progranulin; polymorphisms in this gene represent a known genetic risk factor for frontotemporal lobar degeneration, and progranulin overexpression has been found to be effective in reducing Alzheimer’s- and Parkinson’s-like neurodegenerative phenotypes in mouse models. We move on to BRICHOS domain-containing proteins, a family of proteins containing highly potent anti-amyloidogenic activity; we summarize studies describing the biochemical mechanisms by which recombinant BRICHOS protein might serve as a therapeutic agent. The next section of the review is devoted to the secreted chaperones 7B2 and proSAAS, small neuronal proteins which are packaged together with neuropeptides and released during synaptic activity. Since proteins can be secreted by both classical secretory and non-classical mechanisms, we also review the small heat shock proteins (sHsps) that can be secreted from the cytoplasm to the extracellular environment and provide evidence for their involvement in extracellular proteostasis and neuroprotection. Our goal in this review focusing on extracellular chaperones in neurodegenerative disease is to summarize the most recent literature relating to neurodegeneration for each secreted chaperone; to identify any common mechanisms; and to point out areas of similarity as well as differences between the secreted chaperones identified to date.

Trodusquemine displaces protein misfolded oligomers from cell membranes and abrogates their cytotoxicity through a generic mechanism

The onset and progression of numerous protein misfolding diseases are associated with the presence of oligomers formed during the aberrant aggregation of several different proteins, including amyloid-β (Aβ) in Alzheimer’s disease and α-synuclein (αS) in Parkinson’s disease. These small, soluble aggregates are currently major targets for drug discovery. In this study, we show that trodusquemine, a naturally-occurring aminosterol, markedly reduces the cytotoxicity of αS, Aβ and HypF-N oligomers to human neuroblastoma cells by displacing the oligomers from cell membranes in the absence of any substantial morphological and structural changes to the oligomers. These results indicate that the reduced toxicity results from a mechanism that is common to oligomers from different proteins, shed light on the origin of the toxicity of the most deleterious species associated with protein aggregation and suggest that aminosterols have the therapeutically-relevant potential to protect cells from the oligomer-induced cytotoxicity associated with numerous protein misfolding diseases.

Limbocker et al. show that trodusquemine, an aminosterol, reduces the cytotoxicity of protein misfolded oligomers by displacing them from cell membranes in the absence of any overt structural/ morphological changes in them. This mechanism appears to be general, as they test it for oligomers of αS, Aβ and the model protein HypF-N to human neuroblastoma cells.

Small heat shock proteins in neurodegenerative diseases

Small heat shock proteins are ubiquitously expressed chaperones, yet mutations in some of them cause tissue-specific diseases. Here, we will discuss how small heat shock proteins give rise to neurodegenerative disorders themselves while we will also highlight how these proteins can fulfil protective functions in neurodegenerative disorders caused by protein aggregation. The first half of this paper will be focused on how mutations in HSPB1, HSPB3, and HSPB8 are linked to inherited peripheral neuropathies like Charcot-Marie-Tooth (CMT) disease and distal hereditary motor neuropathy (dHMN). The second part of the paper will discuss how small heat shock proteins are linked to neurodegenerative disorders like Alzheimer's, Parkinson's, and Huntington's disease.

Rationally Designed Antibodies as Research Tools to Study the Structure-Toxicity Relationship of Amyloid-β Oligomers

Alzheimer's disease is associated with the aggregation of the amyloid-β peptide (Aβ), resulting in the deposition of amyloid plaques in brain tissue. Recent scrutiny of the mechanisms by which Aβ aggregates induce neuronal dysfunction has highlighted the importance of the Aβ oligomers of this protein fragment. Because of the transient and heterogeneous nature of these oligomers, however, it has been challenging to investigate the detailed mechanisms by which these species exert cytotoxicity. To address this problem, we demonstrate here the use of rationally designed single-domain antibodies (DesAbs) to characterize the structure-toxicity relationship of Aβ oligomers. For this purpose, we use Zn2+-stabilized oligomers of the 40-residue form of Aβ (Aβ40) as models of brain Aβ oligomers and two single-domain antibodies (DesAb18-24 and DesAb34-40), designed to bind to epitopes at residues 18-24 and 34-40 of Aβ40, respectively. We found that the DesAbs induce a change in structure of the Zn2+-stabilized Aβ40 oligomers, generating a simultaneous increase in their size and solvent-exposed hydrophobicity. We then observed that these increments in both the size and hydrophobicity of the oligomers neutralize each other in terms of their effects on cytotoxicity, as predicted by a recently proposed general structure-toxicity relationship, and observed experimentally. These results illustrate the use of the DesAbs as research tools to investigate the biophysical and cytotoxicity properties of Aβ oligomers.

The noncanonical small heat shock protein HSP-17 from Caenorhabditis elegans is a selective protein aggregase

Small heat shock proteins (sHsps) are conserved, ubiquitous members of the proteostasis network. Canonically, they act as "holdases" and buffer unfolded or misfolded proteins against aggregation in an ATP-independent manner. Whereas bacteria and yeast each have only two sHsps in their genomes, this number is higher in metazoan genomes, suggesting a spatiotemporal and functional specialization in higher eukaryotes. Here, using recombinantly expressed and purified proteins, static light-scattering analysis, and disaggregation assays, we report that the noncanonical sHsp HSP-17 of Caenorhabditis elegans facilitates aggregation of model substrates, such as malate dehydrogenase (MDH), and inhibits disaggregation of luciferase in vitro Experiments with fluorescently tagged HSP-17 under the control of its endogenous promoter revealed that HSP-17 is expressed in the digestive and excretory organs, where its overexpression promotes the aggregation of polyQ proteins and of the endogenous kinase KIN-19. Systemic depletion of hsp-17 shortens C. elegans lifespan and severely reduces fecundity and survival upon prolonged heat stress. HSP-17 is an abundant protein exhibiting opposing chaperone activities on different substrates, indicating that it is a selective protein aggregase with physiological roles in development, digestion, and osmoregulation.

Nanoscopic insights into the surface conformation of neurotoxic amyloid β oligomers

Raman spectroscopy assisted by localized plasmon resonances generating effective hot spots at the gaps between intertwined silver nanowires is herein adopted to unravel characteristic molecular motifs on the surface of Aβ₄₂ misfolded oligomers that are critical in driving intermolecular interactions in neurodegeneration.

Unraveling characteristic structural determinants at the basis of Aβ₄₂ oligomers' neurotoxicity by a sub-molecular SERS investigation of their surface.

Aggregation-induced emission-based ionic liquids for bacterial killing, imaging, cell labeling, and bacterial detection in blood cells

A series of aggregation-induced emission (AIE)-based imidazolium-type ionic liquids (ILs) were designed and synthesized for bacterial killing and imaging, cell labeling, and bacterial detection in blood cells. The AIE-based ILs showed antibacterial activities against both Escherichia coli and Staphylococcus aureus. The carbon chain length of substitution at the N3 position of the imidazolium cations highly affects the antibacterial properties of ILs. Owing to their AIE characteristics, the ILs could selectively capture fluorescence image of dead bacteria while killing the bacteria. The fluorescence intensity varied with the concentration of bacteria, indicating that AIE-based ILs has potential as an antibacterial material and an efficient probe for bacterial viability assay. In addition, the synthesized AIE-based ILs exhibit relatively low cytotoxicity and hemolysis rate and therefore potential for cell labeling, as well as bacterial detection in blood cells. STATEMENT OF SIGNIFICANCE: Bacteria are ubiquitous, especially the pathogenic bacteria, which pose a serious threat to human health. There is an urgent need for materials with efficient antibacterial properties and biocompatibility and without causing drug resistance. In this work, we synthesized a series of aggregation-induced emission (AIE)-doped imidazolium type ionic liquids (ILs) with multifunction potential of bacterial killing and imaging, cell labeling, and detection of bacteria from blood cells. The synthesized AIE-based ILs can image dead bacteria at the same time of killing these bacteria, which can avoid the fluorescent dyeing process. Simultaneously, the fluorescent imaging of dead bacteria can be distinguished by the naked eye, and the fluorescence intensity from the AIE-based ILs varied with the concentration of bacteria. In addition, the AIE-based ILs exhibit relatively low cytotoxicity and hemolysis rate and therefore potential for cell labeling as well as detection of bacteria from red blood cell suspension.

Small Heat Shock Proteins, Big Impact on Protein Aggregation in Neurodegenerative Disease

Misfolding, aggregation, and aberrant accumulation of proteins are central components in the progression of neurodegenerative disease. Cellular molecular chaperone systems modulate proteostasis, and, therefore, are primed to influence aberrant protein-induced neurotoxicity and disease progression. Molecular chaperones have a wide range of functions from facilitating proper nascent folding and refolding to degradation or sequestration of misfolded substrates. In disease states, molecular chaperones can display protective or aberrant effects, including the promotion and stabilization of toxic protein aggregates. This seems to be dependent on the aggregating protein and discrete chaperone interaction. Small heat shock proteins (sHsps) are a class of molecular chaperones that typically associate early with misfolded proteins. These interactions hold proteins in a reversible state that helps facilitate refolding or degradation by other chaperones and co-factors. These sHsp interactions require dynamic oligomerization state changes in response to diverse cellular triggers and, unlike later steps in the chaperone cascade of events, are ATP-independent. Here, we review evidence for modulation of neurodegenerative disease-relevant protein aggregation by sHsps. This includes data supporting direct physical interactions and potential roles of sHsps in the stewardship of pathological protein aggregates in brain. A greater understanding of the mechanisms of sHsp chaperone activity may help in the development of novel therapeutic strategies to modulate the aggregation of pathological, amyloidogenic proteins. sHsps-targeting strategies including modulators of expression or post-translational modification of endogenous sHsps, small molecules targeted to sHsp domains, and delivery of engineered molecular chaperones, are also discussed.

The Complex Phosphorylation Patterns that Regulate the Activity of Hsp70 and Its Cochaperones

Proteins must fold into their native structure and maintain it during their lifespan to display the desired activity. To ensure proper folding and stability, and avoid generation of misfolded conformations that can be potentially cytotoxic, cells synthesize a wide variety of molecular chaperones that assist folding of other proteins and avoid their aggregation, which unfortunately is unavoidable under acute stress conditions. A protein machinery in metazoa, composed of representatives of the Hsp70, Hsp40, and Hsp110 chaperone families, can reactivate protein aggregates. We revised herein the phosphorylation sites found so far in members of these chaperone families and the functional consequences associated with some of them. We also discuss how phosphorylation might regulate the chaperone activity and the interaction of human Hsp70 with its accessory and client proteins. Finally, we present the information that would be necessary to decrypt the effect that post-translational modifications, and especially phosphorylation, could have on the biological activity of the Hsp70 system, known as the “chaperone code”.

The Toxicity of Misfolded Protein Oligomers Is Independent of Their Secondary Structure

The self-assembly of proteins into structured fibrillar aggregates is associated with a range of neurodegenerative diseases, including Alzheimer's and Parkinson's diseases, in which an important cytotoxic role is thought to be played by small soluble oligomers accumulating during the aggregation process or released by mature fibrils. As the structural characteristics of such species and their links with toxicity are still not fully defined, we have compared six examples of preformed misfolded protein oligomers with different β-sheet content, as determined using Fourier transform infrared spectroscopy, and with different toxicity, as determined by three cellular readouts of cell viability. The results show the absence of any measurable correlation between the nature of their secondary structure and their cellular toxicity, both when comparing the six types of oligomers as a group and when comparing species in subgroups characterized by either the same size or the same exposure of hydrophobic moieties.

Modulation of Amyloid States by Molecular Chaperones

Aberrant protein aggregation is a defining feature of most neurodegenerative diseases. During pathological aggregation, key proteins transition from their native state to alternative conformations, which are prone to oligomerize into highly ordered fibrillar states. As part of the cellular quality control machinery, molecular chaperones can intervene at many stages of the aggregation process to inhibit or reverse aberrant protein aggregation or counteract the toxicity associated with amyloid species. Although the action of chaperones is considered cytoprotective, essential housekeeping functions can be hijacked for the propagation and spreading of protein aggregates, suggesting the cellular protein quality control system constitutes a double-edged sword in neurodegeneration. Here, we discuss the various mechanisms used by chaperones to influence protein aggregation into amyloid fibrils to understand how the interplay of these activities produces specific cellular outcomes and to define mechanisms that may be targeted by pharmacological agents for the treatment of neurodegenerative conditions.

Human islet amyloid polypeptide (hIAPP) aggregation in type 2 diabetes: Correlation between intrinsic physicochemical properties of hIAPP aggregates and their cytotoxicity

A large number of pathological diseases are known now to be associated with the misfolding and the aberrant oligomerization and deposition of peptides and proteins into various aggregates. One of these peptides is islet amyloid polypeptide (IAPP), which is responsible for amyloid formation in type 2 diabetes. The mechanism of IAPP amyloid formation in vivo and in vitro is not well understood and the factors behind the peptide aggregates toxicity are not fully defined. Therefore, the precise nature of toxic agents still remains to be elucidated. In this context, first we used a complementary biophysical approach to undertake a systematic study of the hIAPP aggregation process with focus on the lag phase, followed by the study of their degrees of toxicity when added to the extracellular medium of pancreatic cells. The structural properties of hIAPP aggregates are characterized by evaluating their size with DLS, their surface hydrophobicity with ANS, and the interactions between monomers through the intrinsic fluorescence of aromatic residues or by the quenching of these residues mainly the tyrosine in position 37. Our results indicate that despite the method used to study hIAPP aggregation, the obtained curve is easily well fitted in a sigmoidal curve but with some differences. In fact, the analysis of the kinetic parameters gives different information about the hIAPP aggregation process such as lag time and growth rate. Moreover, a high surface hydrophobicity and small size of the aggregates, mainly for the species formed during the lag time, shows strong correlation with the cytotoxicity. These findings provide new insights into the structural changes during hIAPP aggregation and are consistent with a model in which the exposure of hydrophobic surfaces and the small size of aggregates formed during the early stage of the process are crucial for their cytotoxicity.

Memory Loss and the Onset of Alzheimer's Disease Could Be Under the Control of Extracellular Heat Shock Proteins

Alzheimer's disease (AD) is a major contemporary and escalating malady in which amyloid-β (Aβ) peptides are the most likely causative agent. Aβ peptides spontaneously tend to aggregate in extracellular fluids following a progression from a monomeric state, through intermediate forms, ending in amyloid fibers and plaques. It is generally accepted now that the neurotoxic agents leading to cellular death, memory loss, and other AD characteristics are the Aβ intermediate aggregated states. However, Aβ peptides are continuously produced, released into the extracellular space, and rapidly cleared from healthy brains. Coincidentally, members of the heat shock proteins (hsp) family are present in the extracellular medium of healthy cells and body fluids, opening the possibility that hsps and Aβ could meet and interact in the extracellular milieu of the brain. In this perspective and reflection article, we place our investigation showing that the presence of Hsp70s mitigate the formation of low molecular weight Aβ peptide oligomers resulting in a reduction of cellular toxicity, in context of the current understanding of the disease. We propose that it may be an inverse relationship between the presence of Hsp70, the stage of Aβ oligomers, neurotoxicity, and the incidence of AD, particularly since the expression and circulating levels of hsp decrease with aging. Combining these observations, we propose that changes in the dynamics of Hsp70s and Aβ concentrations in the circulating brain fluids during aging defines the control of the formation of Aβ toxic aggregates, thus determining the conditions for neuron degeneration and the incidence of AD.

Emergent functions of proteins in non-stoichiometric supramolecular assemblies

Proteins are the basic functional units of the cell, carrying out myriads of functions essential for life. There are countless reports in molecular cell biology addressing the functioning of proteins under physiological and pathological conditions, aiming to understand life at the atomistic-molecular level and thereby being able to develop remedies against diseases. The central theme in most of these studies is that the functional unit under study is the protein itself. Recent rapid progress has radically challenged and extended this protein-function paradigm, by demonstrating that novel function(s) may emerge when proteins form dynamic and non-stoichiometric supramolecular assemblies. There is an increasing number of cases for such collective functions, such as targeting, localization, protection/shielding and filtering effects, as exemplified by signaling complexes and prions, biominerals and mucus, amphibian adhesions and bacterial biofilms, and a broad range of membraneless organelles (bio-condensates) formed by liquid-liquid phase separation in the cell. In this short review, we show that such non-stoichiometric organization may derive from the heterogeneity of the system, a mismatch in valency and/or geometry of the partners, and/or intrinsic structural disorder and multivalency of the component proteins. Either way, the resulting functional features cannot be simply described by, or predicted from, the properties of the isolated single protein(s), as they belong to the collection of proteins.

Proteomic analysis of protein homeostasis and aggregation

Protein homeostasis (proteostasis) refers to the ability of cells to preserve the correct balance between protein synthesis, folding and degradation. Proteostasis is essential for optimal cell growth and survival under stressful conditions. Various extracellular and intracellular stresses including heat shock, oxidative stress, proteasome malfunction, mutations and aging-related modifications can result in disturbed proteostasis manifested by enhanced misfolding and aggregation of proteins. To limit protein misfolding and aggregation cells have evolved various strategies including molecular chaperones, proteasome system and autophagy. Molecular chaperones assist folding of proteins, protect them from denaturation and facilitate renaturation of the misfolded polypeptides, whereas proteasomes and autophagosomes remove the irreversibly damaged proteins. The impairment of proteostasis results in protein aggregation that is a major pathological hallmark of numerous age-related disorders, such as cataract, Alzheimer's, Parkinson's, Huntington's, and prion diseases. To discover protein markers and speed up diagnosis of neurodegenerative diseases accompanied by protein aggregation, proteomic tools have increasingly been used in recent years. Systematic and exhaustive analysis of the changes that occur in the proteomes of affected tissues and biofluids in humans or in model organisms is one of the most promising approaches to reveal mechanisms underlying protein aggregation diseases, improve their diagnosis and develop therapeutic strategies. Significance: In this review we outline the elements responsible for maintaining cellular proteostasis and present the overview of proteomic studies focused on protein-aggregation diseases. These studies provide insights into the mechanisms responsible for age-related disorders and reveal new potential biomarkers for Alzheimer's, Parkinson's, Huntigton's and prion diseases.

Crucial role of protein oligomerization in the pathogenesis of Alzheimer's and Parkinson's diseases

Misfolding and aggregation of the proteins amyloid-β, tau and alpha-synuclein is the predominant pathology underlying the neurodegenerative disorders, Alzheimer's and Parkinson's disease. While end stage insoluble products of aggregation have been well characterised in human and animal models of disease, accumulating evidence from biophysical, cellular and in vivo studies has shown that soluble intermediates of aggregation, or oligomers, may be the key species that mediate toxicity and underlie seeding and spreading in disease. Here, we review the process of protein misfolding, and the intrinsic and extrinsic processes that cause the native states of the key aggregating proteins to undergo conformational change to form oligomers and ultimately fibrils. We discuss the structural features of the key toxic intermediate, and describe the putative mechanisms by which oligomers may cause cell toxicity. Finally, we explore the potential therapeutic approaches raised by the oligomer hypothesis in neurodegenerative disease.

Structural differences between toxic and nontoxic HypF-N oligomers

We have studied two misfolded oligomeric forms of the protein HypF-N, which show similar morphologies but very different toxicities. We measured over 80 intermolecular distance-dependent parameters for each oligomer type using FRET, in conjunction with solution- and solid-state NMR and other biophysical techniques. The results indicate that the formation of a highly organised hydrogen bonded core in the toxic oligomers results in the exposure of a larger number of hydrophobic residues than in the nontoxic species, causing the former to form aberrant interactions with cellular components.

Modulation of Alzheimer's amyloid β peptide oligomerization and toxicity by extracellular Hsp70

Alzheimer's disease (AD) is a progressive neurodegenerative disorder leading to dementia caused by advanced neuronal dysfunction and death. The most significant symptoms of AD are observed at late stages of the disease when interventions are most likely too late to ameliorate the condition. Currently, the predominant theory for AD is the "amyloid hypothesis," which states that abnormally increased levels of amyloid β (Aβ) peptides result in the production of a variety of aggregates that are neurotoxic. The specific mechanisms for Aβ peptide-induced cytotoxicity have not yet been completely elucidated. However, since the majority of Aβ is released into the extracellular milieu, it is reasonable to assume that toxicity begins outside the cells and makes its way inside where it disrupts the basic cellular process resulting in cell death. There is increasing evidence that hsp, particularly Hsp70, are exported into the extracellular milieu by an active export mechanism independent of cell death. Therefore, both Aβ peptides and Hsp70 may coexist in a common environment during pathological conditions. We observed that Hsp70 affected the Aβ assembling process in vitro preventing oligomer formation. Moreover, the presence of Hsp70 reduced the Aβ peptide-induced toxicity of cultured neurons (N2A cells). These results suggest a potential mechanism for the reduction of the detrimental effects of Aβ peptides in AD.

Hepatocyte-like cells reveal novel role of SerpinA1 in transthyretin amyloidosis

Transthyretin (TTR)-related familial amyloid polyneuropathy (ATTR) results from aggregation and extracellular disposition of misfolded TTR variants. Growing evidence suggests the importance of hepatic chaperones for modulation of pathogenesis. We took advantage of iPSC-derived hepatocyte-like cells (HLCs) derived from ATTR patients (ATTR-HLCs) to compare chaperone gene expression to healthy individuals (H-HLCs). From the set of genes analyzed, chaperones that are predominantly located extracellularly were differently expressed. Expression of the chaperones showed a high correlation with TTR in both ATTR-HLCs and H-HLCs. In contrast, after TTR knockdown, the correlation was mainly affected in ATTR-HLCs suggesting that variant TTR expression triggers abberant chaperone expression. Serpin peptidase inhibitor clade A member 1 (SERPINA1/alpha-1 antitrypsin) was the only extracellular chaperone that was markedly upregulated after TTR knockdown in ATTR-HLCs. Co-immunoprecipitation revealed that SerpinA1 physically interacts with TTR. In vitro assays indicated that SerpinA1 can interfere with TTR aggregation. Taken together, our results suggest that extracellular chaperones play a crucial role in ATTR pathogenesis, in particular SerpinA1, which may affect amyloid formation.

Alzheimer's Disease, Oligomers, and Inflammation

The production of soluble amyloid-β oligomers (AβOs) and the activation of inflammation are two important early steps in the pathogenesis of Alzheimer's disease (AD). The central role of oligomers as responsible for the neuronal dysfunction associated with the clinical features has been extended to the other protein misfolding disorders definable, on this basis, as oligomeropathies. In AD, recent evidence indicates that the mechanism of inflammation as a consequence of neurodegeneration must be assessed in favor of a more direct role of glial activation in the alteration of synaptic function. Our own experimental models demonstrate the efficacy of anti-inflammatory treatments in preventing the cognitive deficits induced acutely by AβOs applied directly in the brain. Moreover, some promising clinical tools are based on immunological activation reducing the presence of cerebral Aβ deposits. However, the strategies based on the control of inflammatory factors as well as the amyloid aggregation show poor or non-therapeutic efficacy. Numerous studies have examined inflammatory factors in biological fluids as possible markers of the neuroinflammation in AD. In some cases, altered levels of cytokines or other inflammatory markers in cerebrospinal fluid correlate with the severity of the disease. Here we propose, according to the precision medicine principles, innovative therapeutic approaches to AD based on the patient's inflammatory profile/state. The earlier intervention and a multifactor approach are two other elements considered essential to improve the chances of effective therapy in AD.

Evaluating the Effect of Phosphorylation on the Structure and Dynamics of Hsp27 Dimers by Means of Ion Mobility Mass Spectrometry

The quaternary structure and dynamics of the human small heat-shock protein Hsp27 are linked to its molecular chaperone function and influenced by post-translational modifications, including phosphorylation. Phosphorylation of Hsp27 promotes oligomer dissociation and can enhance chaperone activity. This study explored the impact of phosphorylation on the quaternary structure and dynamics of Hsp27. Using mutations that mimic phosphorylation, and ion mobility mass spectrometry, we show that successive substitutions result in an increase in the conformational heterogeneity of Hsp27 dimers. In contrast, we did not detect any changes in the structure of an Hsp27 12-mer, representative of larger Hsp27 oligomers. Our data suggest that oligomer dissociation and increased flexibility of the dimer contribute to the enhanced chaperone activity of phosphorylated Hsp27. Thus, post-translational modifications such as phosphorylation play a crucial role in modulating both the tertiary and quaternary structure of Hsp27, which is pivotal to its function as a key component of the proteostasis network in cells. Our data demonstrate the utility of ion mobility mass spectrometry for probing the structure and dynamics of heterogeneous proteins.

Chaperonin GroEL accelerates protofibril formation and decorates fibrils of the Het-s prion protein

We have studied the interaction of the prototypical chaperonin GroEL with the prion domain of the Het-s protein using solution and solid-state NMR, electron and atomic force microscopies, and EPR. While GroEL accelerates Het-s protofibril formation by several orders of magnitude, the rate of appearance of fibrils is reduced. GroEL remains bound to Het-s throughout the aggregation process and densely decorates the fibrils at a regular spacing of ∼200 Å. GroEL binds to the Het-s fibrils via its apical domain located at the top of the large open ring. Thus, apo GroEL and bullet-shaped GroEL/GroES complexes in which only a single ring is capped by GroES interact with the Het-s fibrils; no evidence is seen for any interaction with football-shaped GroEL/GroES complexes in which both rings are capped by GroES. EPR spectroscopy shows that rotational motion of a nitroxide spin label, placed at the N-terminal end of the first β-strand of Het-s fibrils, is significantly reduced in both Het-s/GroEL aggregates and Het-s fibrils, but virtually completely eliminated in Het-s/GroEL fibrils, suggesting that in the latter, GroEL may come into close proximity to the nitroxide label. Solid-state NMR measurements indicate that GroEL binds to the mobile regions of the Het-s fibril comprising the N-terminal tail and a loop connecting β-strands 4 and 5, consistent with interactions involving GroEL binding consensus sequences located therein.

The small heat shock proteins αB-crystallin (HSPB5) and Hsp27 (HSPB1) inhibit the intracellular aggregation of α-synuclein

Protein homeostasis, or proteostasis, is the process of maintaining the conformational and functional integrity of the proteome. Proteostasis is preserved in the face of stress by a complex network of cellular machinery, including the small heat shock molecular chaperone proteins (sHsps), which act to inhibit the aggregation and deposition of misfolded protein intermediates. Despite this, the pathogenesis of several neurodegenerative diseases has been inextricably linked with the amyloid fibrillar aggregation and deposition of α-synuclein (α-syn). The sHsps are potent inhibitors of α-syn aggregation in vitro. However, the limited availability of a robust, cell-based model of α-syn aggregation has, thus far, restricted evaluation of sHsp efficacy in the cellular context. As such, this work sought to establish a robust model of intracellular α-syn aggregation using Neuro-2a cells. Aggregation of α-syn was found to be sensitive to inhibition of autophagy and the proteasome, resulting in a significant increase in the proportion of cells containing α-syn inclusions. This model was then used to evaluate the capacity of the sHsps, αB-c and Hsp27, to prevent α-syn aggregation in cells. To do so, we used bicistronic expression plasmids to express the sHsps. Unlike traditional fluorescent fusion constructs, these bicistronic expression plasmids enable only individual transfected cells expressing the sHsps (via expression of the fluorescent reporter) to be analysed, but without the need to tag the sHsp, which can affect its oligomeric structure and chaperone activity. Overexpression of both αB-c and Hsp27 significantly reduced the intracellular aggregation of α-syn. Thus, these findings suggest that overexpressing or boosting the activity of sHsps may be a way of preventing amyloid fibrillar aggregation of α-syn in the context of neurodegenerative disease.

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.

Chaperones as Suppressors of Protein Misfolded Oligomer Toxicity

Chaperones have long been recognized to play well defined functions such as to: (i) assist protein folding and promote formation and maintenance of multisubunit complexes; (ii) mediate protein degradation; (iii) inhibit protein aggregation; and (iv) promote disassembly of undesired aberrant protein aggregates. In addition to these well-established functions, it is increasingly clear that chaperones can also interact with aberrant protein aggregates, such as pre-fibrillar oligomers and fibrils, and inhibit their toxicity commonly associated with neurodegenerative diseases without promoting their disassembly. In particular, the evidence collected so far in different labs, exploiting different experimental approaches and using different chaperones and client aggregated proteins, indicates the existence of two distinct mechanisms of action mediated by the chaperones to neutralize the toxicity of aberrant proteins oligomers: (i) direct binding of the chaperones to the hydrophobic patches exposed on the oligomer/fibril surface, with resulting shielding or masking of the moieties responsible for the aberrant interactions with cellular targets; (ii) chaperone-mediated conversion of aberrant protein aggregates into large and more innocuous species, resulting in a decrease of their surface-to-volume ratio and diffusibility and in deposits more easily manageable by clearance mechanisms, such as autophagy. In this review article we will describe the in vitro and in vivo evidence supporting both mechanisms and how this results in a suppression of the detrimental effects caused by protein misfolded aggregates.

Effect of molecular chaperones on aberrant protein oligomers in vitro: super-versus sub-stoichiometric chaperone concentrations

Living systems protect themselves from aberrant proteins by a network of chaperones. We have tested in vitro the effects of different concentrations, ranging from 0 to 16 μm, of two molecular chaperones, namely αB-crystallin and clusterin, and an engineered monomeric variant of transthyretin (M-TTR), on the morphology and cytotoxicity of preformed toxic oligomers of HypF-N, which represent a useful model of misfolded protein aggregates. Using atomic force microscopy imaging and static light scattering analysis, all were found to bind HypF-N oligomers and increase the size of the aggregates, to an extent that correlates with chaperone concentration. SDS-PAGE profiles have shown that the large aggregates were predominantly composed of the HypF-N protein. ANS fluorescence measurements show that the chaperone-induced clustering of HypF-N oligomers does not change the overall solvent exposure of hydrophobic residues on the surface of the oligomers. αB-crystallin, clusterin and M-TTR can diminish the cytotoxic effects of the HypF-N oligomers at all chaperone concentration, as demonstrated by MTT reduction and Ca2+ influx measurements. The observation that the protective effect is primarily at all concentrations of chaperones, both when the increase in HypF-N aggregate size is minimal and large, emphasizes the efficiency and versatility of these protein molecules.