Formation of Heterotypic Amyloids: α-Synuclein in Co-Aggregation

Co-Aggregation of TDP-43 with Other Pathogenic Proteins and Their Co-Pathologies in Neurodegenerative Diseases

Pathological aggregation of a specific protein into insoluble aggregates is a common hallmark of various neurodegenerative diseases (NDDs). In the earlier literature, each NDD is characterized by the aggregation of one or two pathogenic proteins, which can serve as disease-specific biomarkers. The aggregation of these specific proteins is thought to be a major cause of or deleterious result in most NDDs. However, accumulating evidence shows that a pathogenic protein can interact and co-aggregate with other pathogenic proteins in different NDDs, thereby contributing to disease onset and progression synergistically. During the past years, more than one type of NDD has been found to co-exist in some individuals, which may increase the complexity and pathogenicity of these diseases. This article reviews and discusses the biochemical characteristics and molecular mechanisms underlying the co-aggregation and co-pathologies associated with TDP-43 pathology. The TDP-43 aggregates, as a hallmark of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD), can often be detected in other NDDs, such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD) and spinocerebellar ataxia type 2 (SCA2). In many cases, TDP-43 is shown to interact and co-aggregate with multiple pathogenic proteins in vitro and in vivo. Furthermore, the co-occurrence and co-aggregation of TDP-43 with other pathogenic proteins have important consequences that may aggravate the diseases. Thus, the current viewpoint that the co-aggregation of TDP-43 with other pathogenic proteins in NDDs and their relevance to disease progression may gain insights into the patho-mechanisms and therapeutic potential of various NDDs.

Proteomic Evidence for Amyloidogenic Cross-Seeding in Fibrinaloid Microclots

In classical amyloidoses, amyloid fibres form through the nucleation and accretion of protein monomers, with protofibrils and fibrils exhibiting a cross-β motif of parallel or antiparallel β-sheets oriented perpendicular to the fibre direction. These protofibrils and fibrils can intertwine to form mature amyloid fibres. Similar phenomena can occur in blood from individuals with circulating inflammatory molecules (and also some originating from viruses and bacteria). Such pathological clotting can result in an anomalous amyloid form termed fibrinaloid microclots. Previous proteomic analyses of these microclots have shown the presence of non-fibrin(ogen) proteins, suggesting a more complex mechanism than simple entrapment. We thus provide evidence against such a simple entrapment model, noting that clot pores are too large and centrifugation would have removed weakly bound proteins. Instead, we explore whether co-aggregation into amyloid fibres may involve axial (multiple proteins within the same fibril), lateral (single-protein fibrils contributing to a fibre), or both types of integration. Our analysis of proteomic data from fibrinaloid microclots in different diseases shows no significant quantitative overlap with the normal plasma proteome and no correlation between plasma protein abundance and their presence in fibrinaloid microclots. Notably, abundant plasma proteins like α-2-macroglobulin, fibronectin, and transthyretin are absent from microclots, while less abundant proteins such as adiponectin, periostin, and von Willebrand factor are well represented. Using bioinformatic tools, including AmyloGram and AnuPP, we found that proteins entrapped in fibrinaloid microclots exhibit high amyloidogenic tendencies, suggesting their integration as cross-β elements into amyloid structures. This integration likely contributes to the microclots' resistance to proteolysis. Our findings underscore the role of cross-seeding in fibrinaloid microclot formation and highlight the need for further investigation into their structural properties and implications in thrombotic and amyloid diseases. These insights provide a foundation for developing novel diagnostic and therapeutic strategies targeting amyloidogenic cross-seeding in blood clotting disorders.

Hotspot site microenvironment in the deubiquitinase OTUB1 drives its stability and aggregation

Lewy bodies (LB) are aberrant protein accumulations observed in the brain cells of individuals affected by Parkinson's disease (PD). A comprehensive analysis of LB proteome identified over a hundred proteins, many co-enriched with α-synuclein, a major constituent of LB. Within this context, OTUB1, a deubiquitinase detected in LB, exhibits amyloidogenic properties, yet the mechanisms underlying its aggregation remain elusive. In this study, we identify two critical sites in OTUB1-namely, positions 133 and 173-that significantly impact its amyloid aggregation. Substituting alanine at position 133 and lysine at position 173 enhances both thermodynamic and kinetic stability, effectively preventing amyloid aggregation. Remarkably, lysine at position 173 demonstrates the highest stability without compromising enzymatic activity. The increased stability and inhibition of amyloid aggregation are attributed mainly to the changes in the specific microenvironment at the hotspot. In our exploration of the in-vivo co-occurrence of α-synuclein and OTUB1 in LB, we observed a synergistic modulation of each other's aggregation. Collectively, our study unveils the molecular determinants influencing OTUB1 aggregation, shedding light on the role of specific residues in modulating aggregation kinetics and structural transition. These findings contribute valuable insights into the complex interplay of amino acid properties and protein aggregation, with potential implications for understanding broader aspects of protein folding and aggregation phenomena.

13C- and 15N-labeling of amyloid-β and inhibitory peptides to study their interaction via nanoscale infrared spectroscopy

Interactions between molecules are fundamental in biology. They occur also between amyloidogenic peptides or proteins that are associated with different amyloid diseases, which makes it important to study the mutual influence of two polypeptides on each other's properties in mixed samples. However, addressing this research question with imaging techniques faces the challenge to distinguish different polypeptides without adding artificial probes for detection. Here, we show that nanoscale infrared spectroscopy in combination with 13C, 15N-labeling solves this problem. We studied aggregated amyloid-β peptide (Aβ) and its interaction with an inhibitory peptide (NCAM1-PrP) using scattering-type scanning near-field optical microscopy. Although having similar secondary structure, labeled and unlabeled peptides could be distinguished by comparing optical phase images taken at wavenumbers characteristic for either the labeled or the unlabeled peptide. NCAM1-PrP seems to be able to associate with or to dissolve existing Aβ fibrils because pure Aβ fibrils were not detected after mixing.

Coaggregation of polyglutamine (polyQ) proteins is mediated by polyQ-tract interactions and impairs cellular proteostasis

Nine polyglutamine (polyQ) proteins have already been identified that are considered to be associated with the pathologies of neurodegenerative disorders called polyQ diseases, but whether these polyQ proteins mutually interact and synergize in proteinopathies remains to be elucidated. In this study, 4 polyQ-containing proteins, androgen receptor (AR), ataxin-7 (Atx7), huntingtin (Htt) and ataxin-3 (Atx3), are used as model molecules to investigate their heterologous coaggregation and consequent impact on cellular proteostasis. Our data indicate that the N-terminal fragment of polyQ-expanded (PQE) Atx7 or Htt can coaggregate with and sequester AR and Atx3 into insoluble aggregates or inclusions through their respective polyQ tracts. In vitro coprecipitation and NMR titration experiments suggest that this specific coaggregation depends on polyQ lengths and is probably mediated by polyQ-tract interactions. Luciferase reporter assay shows that these coaggregation and sequestration effects can deplete the cellular availability of AR and consequently impair its transactivation function. This study provides valid evidence supporting the viewpoint that coaggregation of polyQ proteins is mediated by polyQ-tract interactions and benefits our understanding of the molecular mechanism underlying the accumulation of different polyQ proteins in inclusions and their copathological causes of polyQ diseases.

Linking Alzheimer’s Disease and Type 2 Diabetes: Characterization and Inhibition of Cytotoxic Aβ and IAPP Hetero-Aggregates

The cytotoxic self-aggregation of β-amyloid (Aβ) peptide and islet amyloid polypeptide (IAPP) is implicated in the pathogenesis of Alzheimer’s disease (AD) and Type 2 diabetes (T2D), respectively. Increasing evidence, particularly the co-deposition of Aβ and IAPP in both brain and pancreatic tissues, suggests that Aβ and IAPP cross-interaction may be responsible for a pathological link between AD and T2D. Here, we examined the nature of IAPP-Aβ40 co-aggregation and its inhibition by small molecules. In specific, we characterized the kinetic profiles, morphologies, secondary structures and toxicities of IAPP-Aβ40 hetero-assemblies and compared them to those formed by their homo-assemblies. We demonstrated that monomeric IAPP and Aβ40 form stable hetero-dimers and hetero-assemblies that further aggregate into β-sheet-rich hetero-aggregates that are toxic (cell viability <50%) to both PC-12 cells, a neuronal cell model, and RIN-m5F cells, a pancreatic cell model for β-cells. We then selected polyphenolic candidates to inhibit IAPP or Aβ40 self-aggregation and examined the inhibitory effect of the most potent candidate on IAPP-Aβ40 co-aggregation. We demonstrated that epigallocatechin gallate (EGCG) form inter-molecular hydrogen bonds with each of IAPP and Aβ40. We also showed that EGCG reduced hetero-aggregate formation and resulted in lower β-sheets content and higher unordered structures in IAPP-Aβ40-EGCG samples. Importantly, we showed that EGCG is highly effective in reducing the toxicity of IAPP-Aβ40 hetero-aggregates on both cell models, specifically at concentrations that are equivalent to or are 2.5-fold higher than the mixed peptide concentrations. To the best of our knowledge, this is the first study to report the inhibition of IAPP-Aβ40 co-aggregation by small molecules. We conclude that EGCG is a promising candidate to prevent co-aggregation and cytotoxicity of IAPP-Aβ40, which in turn, contribute to the pathological link between AD and T2D.

Extract from the Marine Seaweed Padina pavonica Protects Mitochondrial Biomembranes from Damage by Amyloidogenic Peptides

The identification of compounds which protect the double-membrane of mitochondrial organelles from disruption by toxic confomers of amyloid proteins may offer a therapeutic strategy to combat human neurodegenerative diseases. Here, we exploited an extract from the marine brown seaweed Padina pavonica (PPE) as a vital source of natural bioactive compounds to protect mitochondrial membranes against insult by oligomeric aggregates of the amyloidogenic proteins amyloid-β (Aβ), α-synuclein (α-syn) and tau, which are currently considered to be major targets for drug discovery in Alzheimer's disease (AD) and Parkinson's disease (PD). We show that PPE manifested a significant inhibitory effect against swelling of isolated mitochondria exposed to the amyloid oligomers, and attenuated the release of cytochrome c from the mitochondria. Using cardiolipin-enriched synthetic lipid membranes, we also show that dye leakage from fluorophore-loaded vesicles and formation of channel-like pores in planar bilayer membranes are largely prevented by incubating the oligomeric aggregates with PPE. Lastly, we demonstrate that PPE curtails the ability of Aβ42 and α-syn monomers to self-assemble into larger β-aggregate structures, as well as potently disrupts their respective amyloid fibrils. In conclusion, the mito-protective and anti-aggregator biological activities of Padina pavonica extract may be of therapeutic value in neurodegenerative proteinopathies, such as AD and PD.

Lysozyme Fibrils Alter the Mechanism of Insulin Amyloid Aggregation

Protein aggregation into amyloid fibrils is linked to multiple disorders. The understanding of how natively non-harmful proteins convert to these highly cytotoxic amyloid aggregates is still not sufficient, with new ideas and hypotheses being presented each year. Recently it has been shown that more than one type of protein aggregates may co-exist in the affected tissue of patients suffering from amyloid-related disorders, sparking the idea that amyloid aggregates formed by one protein may induce another protein’s fibrillization. In this work, we examine the effect that lysozyme fibrils have on insulin amyloid aggregation. We show that not only do lysozyme fibrils affect insulin nucleation, but they also alter the mechanism of its aggregation.

Neuropathological Mechanisms Associated with Pesticides in Alzheimer's Disease

Environmental toxicants have been implicated in neurodegenerative diseases, and pesticide exposure is a suspected environmental risk factor for Alzheimer’s disease (AD). Several epidemiological analyses have affirmed a link between pesticides and incidence of sporadic AD. Meanwhile, in vitro and animal models of AD have shed light on potential neuropathological mechanisms. In this paper, a perspective on neuropathological mechanisms underlying pesticides’ induction of AD is provided. Proposed mechanisms range from generic oxidative stress induction in neurons to more AD-specific processes involving amyloid-beta (Aβ) and hyperphosphorylated tau (p-tau). Mechanisms that are more speculative or indirect in nature, including somatic mutation, epigenetic modulation, impairment of adult neurogenesis, and microbiota dysbiosis, are also discussed. Chronic toxicity mechanisms of environmental pesticide exposure crosstalks in complex ways and could potentially be mutually enhancing, thus making the deciphering of simplistic causal relationships difficult.

Cell Responses to Extracellular α-Synuclein

Synucleins are small naturally unfolded proteins involved in neurodegenerative diseases and cancer. The family contains three members: α-, β-, and -synuclein. α-Synuclein is the most thoroughly investigated because of its close association with Parkinson's disease (PD), dementia with Lewy bodies and multiple system atrophy. Until recently, the synuclein's research was mainly focused on their intracellular forms. However, new studies highlighted the important role of extracellular synucleins. Extracellular forms of synucleins propagate between various types of cells, bind to cell surface receptors and transmit signals, regulating numerous intracellular processes. Here we give an update of the latest results about the mechanisms of action of extracellular synucleins, their binding to cell surface receptors, effect on biochemical pathways and the role in neurodegeneration and neuroinflammation.