Aggregation of human S100A8 and S100A9 amyloidogenic proteins perturbs proteostasis in a yeast model

Oxidation-sensitive cysteines drive IL-38 amyloid formation

Interleukin (IL)-1 family cytokines are essential for host defense at epithelial barriers. The IL-1 family member IL-33 was recently linked to stress granules (SGs). Formation of SGs and other biomolecular condensates is promoted by proteins containing low-complexity regions (LCRs). Computational analysis predicts LCRs in six of the 11 IL-1 family members. Among these, IL-38 contains a long LCR including two amyloid cores. IL-38 localizes to intracellular granules in keratinocytes under oxidative stress (OS) and forms OS-induced amyloid aggregates in cells and in vitro. Interestingly, soluble and aggregated IL-38 are released from keratinocytes in an exosome-enriched extracellular vesicle fraction. Disulfide-bond mapping, in silico modeling, and mutational analysis suggest that oxidation-sensitive cysteines act as redox switches to alter IL-38 conformation and promote its aggregation. Finally, the presence of IL-38 granules in human epidermis facing environmental OS suggests that oxidation-induced amyloidogenesis, as an intrinsic property of IL-38, supports barrier function.

Inflammatory role of S100A8/A9 in the central nervous system non-neoplastic diseases

S100A8 (MRP8) and S100A9 (MRP14) are critical mediators of the inflammatory response; they are usually present as heterodimers because of the instability of homodimers. Studies have demonstrated that S100A8/A9 expression is significantly upregulated in several central nervous system (CNS) diseases. S100A8/A9 is actively released by neutrophils and monocytes; it plays a key role in regulating the inflammatory response by stimulating leukocyte recruitment and inducing cytokine secretion during inflammation. Additionally, S100A8/A9 can be used as a diagnostic biomarker for several CNS diseases and as a predictor of therapeutic response to inflammation-related diseases. In this work, we reviewed our current understanding of S100A8/A9 overexpression in inflammation and its importance in the development and progression of CNS inflammatory diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), multiple sclerosis (MS), and stroke, and the functional roles and therapeutic applications of S100A8/A9 in these diseases. Finally, we discussed the current barriers and future research directions of S100A8/A9 in CNS diseases.

Formation of Calprotectin Inhibits Amyloid Aggregation of S100A8 and S100A9 Proteins

Calcium-binding S100A8 and S100A9 proteins play a significant role in various disorders due to their pro-inflammatory functions. Substantially, they are also relevant in neurodegenerative disorders via the delivery of signals for the immune response. However, at the same time, they can aggregate and accelerate the progression of diseases. Natively, S100A8 and S100A9 exist as homo- and heterodimers, but upon aggregation, they form amyloid-like oligomers, fibrils, or amorphous aggregates. In this study, we aimed to elucidate the aggregation propensities of S100A8, S100A9, and their heterodimer calprotectin by investigating aggregation kinetics, secondary structures, and morphologies of the aggregates. For the first time, we followed the in vitro aggregation of S100A8, which formed spherical aggregates, unlike the fibrillar structures of S100A9 under the same conditions. The aggregates were sensitive to amyloid-specific ThT and ThS dyes and had a secondary structure composed of β-sheets. Similarly to S100A9, S100A8 protein was stabilized by calcium ions, resulting in aggregation inhibition. Finally, the formation of S100A8 and S100A9 heterodimers stabilized the proteins in the absence of calcium ions and prevented their aggregation.

Natural Compound from Olive Oil Inhibits S100A9 Amyloid Formation and Cytotoxicity: Implications for Preventing Alzheimer's Disease

Polyphenolic compounds in the Mediterranean diet have received increasing attention due to their protective properties in amyloid neurodegenerative and many other diseases. Here, we have demonstrated for the first time that polyphenol oleuropein aglycone (OleA), which is the most abundant compound in olive oil, has multiple potencies for the inhibition of amyloid self-assembly of pro-inflammatory protein S100A9 and the mitigation of the damaging effect of its amyloids on neuroblastoma SH-SY5Y cells. OleA directly interacts with both native and fibrillar S100A9 as shown by intrinsic fluorescence and molecular dynamic simulation. OleA prevents S100A9 amyloid oligomerization as shown using amyloid oligomer-specific antibodies and cross-β-sheet formation detected by circular dichroism. It decreases the length of amyloid fibrils measured by atomic force microscopy (AFM) as well as reduces the effective rate of amyloid growth and the overall amyloid load as derived from the kinetic analysis of amyloid formation. OleA disintegrates already preformed fibrils of S100A9, converting them into nonfibrillar and nontoxic aggregates as revealed by amyloid thioflavin-T dye binding, AFM, and cytotoxicity assays. At the cellular level, OleA targets S100A9 amyloids already at the membranes as shown by immunofluorescence and fluorescence resonance energy transfer, significantly reducing the amyloid accumulation in GM1 ganglioside containing membrane rafts. OleA increases overall cell viability when neuroblastoma cells are subjected to the amyloid load and alleviates amyloid-induced intracellular rise of reactive oxidative species and free Ca²⁺. Since S100A9 is both a pro-inflammatory and amyloidogenic protein, OleA may effectively mitigate the pathological consequences of the S100A9-dependent amyloid-neuroinflammatory cascade as well as provide protection from neurodegeneration, if used within the Mediterranean diet as a potential preventive measure.

Pro-Inflammatory S100A9 Protein Aggregation Promoted by NCAM1 Peptide Constructs

Amyloid cascade and neuroinflammation are hallmarks of neurodegenerative diseases, and pro-inflammatory S100A9 protein is central to both of them. Here, we have shown that NCAM1 peptide constructs carrying polycationic sequences derived from Aβ peptide (KKLVFF) and PrP protein (KKRPKP) significantly promote the S100A9 amyloid self-assembly in a concentration-dependent manner by making transient interactions with individual S100A9 molecules, perturbing its native structure and acting as catalysts. Since the individual molecule misfolding is a rate-limiting step in S100A9 amyloid aggregation, the effects of the NCAM1 construct on the native S100A9 are so critical for its amyloid self-assembly. S100A9 rapid self-assembly into large aggregated clumps may prevent its amyloid tissue propagation, and by modulating S100A9 aggregation as a part of the amyloid cascade, the whole process may be effectively tuned.

S100A9-Driven Amyloid-Neuroinflammatory Cascade in Traumatic Brain Injury as a Precursor State for Alzheimer's Disease

Pro-inflammatory and amyloidogenic S100A9 protein is an important contributor to Alzheimer's disease (AD) pathology. Traumatic brain injury (TBI) is viewed as a precursor state for AD. Here we have shown that S100A9-driven amyloid-neuroinflammatory cascade was initiated in TBI and may serve as a mechanistic link between TBI and AD. By analyzing the TBI and AD human brain tissues, we demonstrated that in post-TBI tissues S100A9, produced by neurons and microglia, becomes drastically abundant compared to Aβ and contributes to both precursor-plaque formation and intracellular amyloid oligomerization. Conditions implicated in TBI, such as elevated S100A9 concentration, acidification and fever, provide strong positive feedback for S100A9 nucleation-dependent amyloid formation and delay in its proteinase clearance. Consequently, both intracellular and extracellular S100A9 oligomerization correlated with TBI secondary neuronal loss. Common morphology of TBI and AD plaques indicated their similar initiation around multiple aggregation centers. Importantly, in AD and TBI we found S100A9 plaques without Aβ. S100A9 and Aβ plaque pathology was significantly advanced in AD cases with TBI history at earlier age, signifying TBI as a risk factor. These new findings highlight the detrimental consequences of prolonged post-TBI neuroinflammation, which can sustain S100A9-driven amyloid-neurodegenerative cascade as a specific mechanism leading to AD development.

Intranasally Administered S100A9 Amyloids Induced Cellular Stress, Amyloid Seeding, and Behavioral Impairment in Aged Mice

Amyloid formation and neuroinflammation are major features of Alzheimer's disease pathology. Proinflammatory mediator S100A9 was shown to act as a link between the amyloid and neuroinflammatory cascades in Alzheimer's disease, leading together with Aβ to plaque formation, neuronal loss and memory impairment. In order to examine if S100A9 alone in its native and amyloid states can induce neuronal stress and memory impairment, we have administered S100A9 species intranasally to aged mice. Single and sequential immunohistochemistry and passive avoidance behavioral test were conducted to evaluate the consequences. Administered S100A9 species induced widespread cellular stress responses in cerebral structures, including frontal lobe, hippocampus and cerebellum. These were manifested by increased levels of S100A9, Bax, and to a lesser extent activated caspase-3 immunopositive cells. Upon administration of S100A9 fibrils, the amyloid oligomerization was observed in the brain tissues, which can further exacerbate cellular stress. The cellular stress responses correlated with significantly increased training and decreased retention latencies measured in the passive avoidance test for the S100A9 treated animal groups. Remarkably, the effect size in the behavioral tests was moderate already in the group treated with native S100A9, while the effect sizes were large in the groups administered S100A9 amyloid oligomers or fibrils. The findings demonstrate the brain susceptibility to neurotoxic damage of S100A9 species leading to behavioral and memory impairments. Intranasal administration of S100A9 species proved to be an effective method to study amyloid induced brain dysfunctions, and S100A9 itself may be postulated as a target to allay early stage neurodegenerative and neuroinflammatory processes.

Finke-Watzky Two-Step Nucleation-Autocatalysis Model of S100A9 Amyloid Formation: Protein Misfolding as "Nucleation" Event

Quantitative kinetic analysis is critical for understanding amyloid mechanisms. Here we demonstrate the application of generic Finke-Watzky (F-W) two-step nucleation-autocatalytic growth model to the concentration-dependent amyloid kinetics of proinflammatory α-helical S100A9 protein at pH 7.4 and at 37 and 42 °C. The model is based on two pseudoelementary reaction steps applied without further analytical constraints, and its treatment of S100A9 amyloid self-assembly demonstrates that initial misfolding and β-sheet formation, defined as "nucleation" step, spontaneously takes place within individual S100A9 molecules at higher rate than the subsequent fibrillar growth. The latter, described as an autocatalytic process, will proceed if misfolded amyloid-prone S100A9 is populated on a macroscopic time scale. Short lengths of S100A9 fibrils are consistent with the F-W model. The analysis of fibrillar length distribution by the Beker-Döring model demonstrates independently that such distribution is solely determined by slow fibril growth and there is no fragmentation or secondary pathways decreasing fibrillar length.

Platelet-Derived S100A8/A9 and Cardiovascular Disease in Systemic Lupus Erythematosus

OBJECTIVE

Levels of S100A8/A9, a proinflammatory and prothrombotic protein complex, are increased in several diseases, and high levels predispose to cardiovascular disease (CVD). Recently, platelet S100A8/A9 synthesis was described in mice and humans in relation to CVD. The aim of this study was to investigate the role of platelet S100A8/A9 in systemic lupus erythematosus (SLE), a disease with markedly increased cardiovascular morbidity, as well as the exact platelet distribution of the S100A8/A9 proteins.

METHODS

The occurrence and distribution of platelet S100A8/A9 protein were detected by enzyme-linked immunosorbent assay, electron microscopy, Western blotting, and flow cytometry in healthy controls (n = 79) and in 2 individual cohorts of SLE patients (n = 148 and n = 318, respectively) and related to cardiovascular morbidity.

RESULTS

We observed that human platelets expressed S100A8/A9 proteins, and that these were localized in close proximity to intracellular membranes and granules as well as on the cell surface upon activation with physiologic and pathophysiologic stimuli. Interestingly, S100A8/A9 was enriched at sites of membrane interactions, indicating a role of S100A8/A9 in cell-cell communication. S100A8/A9 levels were highly regulated by interferon-α, both in vivo and in vitro. Patients with SLE had increased platelet S100A8/A9 content compared with healthy individuals. Increased levels of platelet S100A8/A9 were associated with CVD, particularly myocardial infarction (odds ratio 4.8, 95% confidence interval 1.5-14.9, P = 0.032 [adjusted for age, sex, and smoking]).

CONCLUSION

Platelets contain S100A8/A9 in membrane-enclosed vesicles, enabling rapid cell surface deposition upon activation. Furthermore, platelet S100A8/A9 protein levels were increased in SLE patients, particularly in those with CVD, and may be a future therapeutic target.

Uncoupling the Trade-Off between Somatic Proteostasis and Reproduction in Caenorhabditis elegans Models of Polyglutamine Diseases

Caenorhabditis elegans somatic protein homeostasis (proteostasis) is actively remodeled at the onset of reproduction. This proteostatic collapse is regulated cell-nonautonomously by signals from the reproductive system that transmit the commitment to reproduction to somatic cells. Here, we asked whether the link between the reproductive system and somatic proteostasis could be uncoupled by activating downstream effectors in the gonadal longevity cascade. Specifically, we examined whether over-expression of lipl-4 (lipl-4(oe)), a target gene of the gonadal longevity pathway, or increase in arachidonic acid (AA) levels, associated with lipl-4(oe), modulated proteostasis and reproduction. We found that lipl-4(oe) rescued somatic proteostasis and postponed the onset of aggregation and toxicity in C. elegans models of polyglutamine (polyQ) diseases. However, lipl-4(oe) also disrupted fatty acid transport into developing oocytes and reduced reproductive success. In contrast, diet supplementation of AA recapitulated lipl-4(oe)-mediated proteostasis enhancement in wild type animals but did not affect the reproductive system. Thus, the gonadal longevity pathway mediates a trade-off between somatic maintenance and reproduction, in part by regulating the expression of genes, such as lipl-4, with inverse effects on somatic maintenance and reproduction. We propose that AA could uncouple such germline to soma crosstalk, with beneficial implications protein misfolding diseases.

A Bystander Mechanism Explains the Specific Phenotype of a Broadly Expressed Misfolded Protein

Misfolded proteins in transgenic models of conformational diseases interfere with proteostasis machinery and compromise the function of many structurally and functionally unrelated metastable proteins. This collateral damage to cellular proteins has been termed 'bystander' mechanism. How a single misfolded protein overwhelms the proteostasis, and how broadly-expressed mutant proteins cause cell type-selective phenotypes in disease are open questions. We tested the gain-of-function mechanism of a R37C folding mutation in an endogenous IGF-like C.elegans protein DAF-28. DAF-28(R37C) is broadly expressed, but only causes dysfunction in one specific neuron, ASI, leading to a distinct developmental phenotype. We find that this phenotype is caused by selective disruption of normal biogenesis of an unrelated endogenous protein, DAF-7/TGF-β. The combined deficiency of DAF-28 and DAF-7 biogenesis, but not of DAF-28 alone, explains the gain-of-function phenotype—deficient pro-growth signaling by the ASI neuron. Using functional, fluorescently-tagged protein, we find that, in animals with mutant DAF-28/IGF, the wild-type DAF-7/TGF-β is mislocalized to and accumulates in the proximal axon of the ASI neuron. Activation of two different branches of the unfolded protein response can modulate both the developmental phenotype and DAF-7 mislocalization in DAF-28(R37C) animals, but appear to act through divergent mechanisms. Our finding that bystander targeting of TGF-β explains the phenotype caused by a folding mutation in an IGF-like protein suggests that, in conformational diseases, bystander misfolding may specify the distinct phenotypes caused by different folding mutations.

Correct protein folding and localization ensures cellular health. Dedicated proteostasis machinery assists in protein folding and protects against misfolding. Yet, folding mutations cause many conformational diseases, including neurodegenerative diseases and certain types of diabetes and cancer. Misfolded disease-related proteins interfere with proteostasis machinery, causing global misfolding in the cell. How this global mechanism leads to the specific phenotypes in different conformational diseases is unknown. Moreover, mutant misfolded proteins that only damage specific cell-types in disease often lose this cell-selectivity when overexpressed in genetic models. Here we use an endogenous folding mutation in a C. elegans secreted IGF-like protein, DAF-28, that causes dysfunction in one neuron and a specific developmental phenotype, despite expression in many cells. We find that misfolding of mutant DAF-28 causes mislocalization and defective function of another, wild-type growth factor that is expressed in the affected neuron, the TGF-β protein DAF-7. Decrease in DAF-7 function explains the observed developmental phenotype. This targeting of the bystander protein DAF-7 by the misfolded mutant DAF-28 is specific and is not caused by the global stress. Our data suggest that rather than global effects, it is the selective targeting of specific susceptible bystander proteins that defines the specific phenotypes in conformational diseases.

The role of pro-inflammatory S100A9 in Alzheimer's disease amyloid-neuroinflammatory cascade

Pro-inflammatory S100A9 protein is increasingly recognized as an important contributor to inflammation-related neurodegeneration. Here, we provide insights into S100A9 specific mechanisms of action in Alzheimer's disease (AD). Due to its inherent amyloidogenicity S100A9 contributes to amyloid plaque formation together with Aβ. In traumatic brain injury (TBI) S100A9 itself rapidly forms amyloid plaques, which were reactive with oligomer-specific antibodies, but not with Aβ and amyloid fibrillar antibodies. They may serve as precursor-plaques for AD, implicating TBI as an AD risk factor. S100A9 was observed in some hippocampal and cortical neurons in TBI, AD and non-demented aging. In vitro S100A9 forms neurotoxic linear and annular amyloids resembling Aβ protofilaments. S100A9 amyloid cytotoxicity and native S100A9 pro-inflammatory signaling can be mitigated by its co-aggregation with Aβ, which results in a variety of micron-scale amyloid complexes. NMR and molecular docking demonstrated transient interactions between native S100A9 and Aβ. Thus, abundantly present in AD brain pro-inflammatory S100A9, possessing also intrinsic amyloidogenic properties and ability to modulate Aβ aggregation, can serve as a link between the AD amyloid and neuroinflammatory cascades and as a prospective therapeutic target.

Germline stem cell arrest inhibits the collapse of somatic proteostasis early in Caenorhabditis elegans adulthood

All cells rely on highly conserved protein folding and clearance pathways to detect and resolve protein damage and to maintain protein homeostasis (proteostasis). Because age is associated with an imbalance in proteostasis, there is a need to understand how protein folding is regulated in a multicellular organism that undergoes aging. We have observed that the ability of Caenorhabditis elegans to maintain proteostasis declines sharply following the onset of oocyte biomass production, suggesting that a restricted protein folding capacity may be linked to the onset of reproduction. To test this hypothesis, we monitored the effects of different sterile mutations on the maintenance of proteostasis in the soma of C. elegans. We found that germline stem cell (GSC) arrest rescued protein quality control, resulting in maintenance of robust proteostasis in different somatic tissues of adult animals. We further demonstrated that GSC-dependent modulation of proteostasis requires several different signaling pathways, including hsf-1 and daf-16/kri-1/tcer-1, daf-12, daf-9, daf-36, nhr-80, and pha-4 that differentially modulate somatic quality control functions, such that each signaling pathway affects different aspects of proteostasis and cannot functionally complement the other pathways. We propose that the effect of GSCs on the collapse of proteostasis at the transition to adulthood is due to a switch mechanism that links GSC status with maintenance of somatic proteostasis via regulation of the expression and function of different quality control machineries and cellular stress responses that progressively lead to a decline in the maintenance of proteostasis in adulthood, thereby linking reproduction to the maintenance of the soma.

Natural genetic variation determines susceptibility to aggregation or toxicity in a C. elegans model for polyglutamine disease

Background

Monogenic gain-of-function protein aggregation diseases, including Huntington’s disease, exhibit substantial variability in age of onset, penetrance, and clinical symptoms, even between individuals with similar or identical mutations. This difference in phenotypic expression of proteotoxic mutations is proposed to be due, at least in part, to the variability in genetic background. To address this, we examined the role of natural variation in defining the susceptibility of genetically diverse individuals to protein aggregation and toxicity, using the Caenorhabditis elegans polyglutamine model.

Results

Introgression of polyQ40 into three wild genetic backgrounds uncovered wide variation in onset of aggregation and corresponding toxicity, as well as alteration in the cell-specific susceptibility to aggregation. To further dissect these relationships, we established a panel of 21 recombinant inbred lines that showed a broad range of aggregation phenotypes, independent of differences in expression levels. We found that aggregation is a transgressive trait, and does not always correlate with measures of toxicity, such as early onset of muscle dysfunction, egg-laying deficits, or reduced lifespan. Moreover, distinct measures of proteotoxicity were independently modified by the genetic background.

Conclusions

Resistance to protein aggregation and the ability to restrict its associated cellular dysfunction are independently controlled by the natural variation in genetic background, revealing important new considerations in the search for targets for therapeutic intervention in conformational diseases. Thus, our C. elegans model can serve as a powerful tool to dissect the contribution of natural variation to individual susceptibility to proteotoxicity.

Please see related commentary by Kaeberlein, http://www.biomedcentral.com/1741-7007/11/102.

Protein folding and aggregation into amyloid: the interference by natural phenolic compounds

Amyloid aggregation is a hallmark of several degenerative diseases affecting the brain or peripheral tissues, whose intermediates (oligomers, protofibrils) and final mature fibrils display different toxicity. Consequently, compounds counteracting amyloid aggregation have been investigated for their ability (i) to stabilize toxic amyloid precursors; (ii) to prevent the growth of toxic oligomers or speed that of fibrils; (iii) to inhibit fibril growth and deposition; (iv) to disassemble preformed fibrils; and (v) to favor amyloid clearance. Natural phenols, a wide panel of plant molecules, are one of the most actively investigated categories of potential amyloid inhibitors. They are considered responsible for the beneficial effects of several traditional diets being present in green tea, extra virgin olive oil, red wine, spices, berries and aromatic herbs. Accordingly, it has been proposed that some natural phenols could be exploited to prevent and to treat amyloid diseases, and recent studies have provided significant information on their ability to inhibit peptide/protein aggregation in various ways and to stimulate cell defenses, leading to identify shared or specific mechanisms. In the first part of this review, we will overview the significance and mechanisms of amyloid aggregation and aggregate toxicity; then, we will summarize the recent achievements on protection against amyloid diseases by many natural phenols.