Glycation induces formation of amyloid cross-beta structure in albumin

Inhibitory Effect of Antidesma bunius Fruit Extract on Carbohydrate Digestive Enzymes Activity and Protein Glycation In Vitro

Antidesma bunius (L.) spreng (Mamao) is widely distributed in Northeastern Thailand. Antidesma bunius has been reported to contain anthocyanins, which possess antioxidant and antihypertensive actions. However, the antidiabetic and antiglycation activity of Antidesma bunius fruit extract has not yet been reported. In this study, we investigated the inhibitory activity of anthocyanin-enriched fraction of Antidesma bunius fruit extract (ABE) against pancreatic α-amylase, intestinal α-glucosidase (maltase and sucrase), protein glycation, as well as antioxidant activity. A liquid chromatography-tandem mass spectrometry (LC-MS/MS) chromatogram revealed that ABE contained phytochemical compounds such as cyanidin-3-glucoside, delphinidin-3-glucoside, ellagic acid, and myricetin-3-galactoside. ABE inhibited intestinal maltase and sucrase activity with the IC₅₀ values of 0.76 ± 0.02 mg/mL and 1.33 ± 0.03 mg/mL, respectively. Furthermore, ABE (0.25 mg/mL) reduced the formation of fluorescent AGEs and the level of N^ε-carboxymethyllysine (N^ε-CML) in fructose and glucose-induced protein glycation during four weeks of incubation. During the glycation process, the protein carbonyl and β-amyloid cross structure were decreased by ABE (0.25 mg/mL). In addition, ABE exhibited antioxidant activity through DPPH radical scavenging activity and Trolox equivalent antioxidant capacity (TEAC) with the IC₅₀ values 15.84 ± 0.06 µg/mL and 166.1 ± 2.40 µg/mL, respectively. Meanwhile, ferric reducing antioxidant power (FRAP) showed an EC₅₀ value of 182.22 ± 0.64 µg/mL. The findings suggest that ABE may be a promising agent for inhibiting carbohydrate digestive enzyme activity, reducing monosaccharide-induced protein glycation, and antioxidant activity.

Glyburide inhibits non-enzymatic glycation of HSA: An approach for the management of AGEs associated diabetic complications

Advanced glycation endproducts (AGEs) are the final product of glycation, highly reactive in nature and contribute directly or indirectly to numerous complications related to diabetes. In this study, the antiglycation activity of glyburide was investigated using HSA as model protein, both against glucose and methylglyoxal mediated glycation. The possible mechanism of action was also deciphered using biophysical and computational tools. Approximately 70% inhibition of both early and advanced glycation end products were recorded in the presence of glyburide. Free lysine modification was reduced by glyburide treatment and improvement in biochemical markers such as free thiol groups and carbonyl content was observed. Interaction studies revealed that glyburide showed moderate to strong binding affinity towards HSA with binding constant in the order of 10⁶ M^-1. The interaction of glyburide with HSA was entropically favourable and spontaneous in nature. Molecular dynamics simulation deciphered that glyburide-HSA complex was quite stable where RMSD, RMSF, R_g, SASA, and secondary structure of HSA remained approximately same over the entire simulation period. The average binding energy of the MD simulation for glyburide-HSA complex was found to be -15.386 kJ mol^-1. The findings demonstrate the antiglycation potential of glyburide and its possible mechanism of action.

Protective actions of bioactive flavonoids chrysin and luteolin on the glyoxal induced formation of advanced glycation end products and aggregation of human serum albumin: In vitro and molecular docking analysis

The post-translational modification of proteins by nonenzymatic glycation (NEG) and the accumulation of AGEs are the two underlying factors associated with the long-term pathogenesis in diabetes. Glyoxal (GO) is a reactive intermediate which has the ability to modify proteins and generate AGEs at a faster rate. Human serum albumin (HSA) being the most abundant serum protein has a higher chance to be modified by NEG. The key objective of the present study is to investigate the potency of chrysin and luteolin as antiglycating and antifibrillating agents in the GO-mediated glycation and fibril formation of HSA. AGEs formation were confirmed from the absorption and fluorescence spectral measurements. Both the flavonoids were able to quench the AGEs fluorescence intensity in vitro indicating the antiglycating nature of the molecules. The formation of fibrils in the GO-modified HSA was confirmed by the Thioflavin T (ThT) fluorescence assay and the flavonoids were found to exihibit the antifibrillation properties in vitro. Docking results suggested that both the flavonoids interact with various amino acid residues of subdomain IIA including glycation prone lysines and arginines via non-covalent forces and further stabilized the structure of HSA, which further explains their mechanisms of action as antiglycating and antifibrillating agents.

Carbamylation of human serum albumin generates high-molecular weight aggregates: fine characterization by multi-spectroscopic methods and electron microscopy

Carbamylation is the non-enzymatic reaction between isocyanic acid and macromolecules (mainly proteins) which results in carbamylation-derived products (CDPs) generation, wherein the macromolecules show altered structure and function. In this study, we examined the modifications caused in human serum albumin (HSA) upon interaction with potassium cyanate (KCNO). HSA was incubated with varying concentrations of KCNO for 6 h at 37 °C. The resultant product was characterized by biochemical and biophysical techniques. Among other changes, the carbamylated-HSA showed homocitrulline generation (LC-MS), increase in mass (DLS), and amyloidogenic aggregate formation (Congo red, SEM, TEM). The Gibb's free energy was calculated to be -2.91 to -3.95 kcal mol^-1, suggesting that the binding was spontaneous and energetically favourable. The results indicate that in chronic kidney disease patients, elevated levels of isocyanic acid (formed from urea) may modify the albumin structure and lead to its conversion into amyloidogenic aggregates, thus accelerating kidney damage.

Protein aggregation as a consequence of non-enzymatic glycation: Therapeutic intervention using aspartic acid and arginine

Non-enzymatic glycation tempted AGEs of proteins are currently at the heart of a number of pathological conditions. Production of chemically stable AGEs can permanently alter the protein structure and function, concomitantly leading to dilapidated situations. Keeping in perspective, present study aims to report the glycation induced structural and functional modification of a cystatin type isolated from rai mustard seeds, using RSC-glucose and RSC-ribose as model system. Among the sugars studied, ribose was found to be most potent glycating agent as evident from different biophysical assays. During the course of incubation, RSC was observed to pass through a series of structural intermediates as revealed by circular dichroism, altered intrinsic fluorescence and high ANS binding. RSC incubation with ribose post day 36 revealed the possible buildup of β structures as observed in CD spectral analysis, hinting towards the generation of aggregated structures in RSC. High thioflavin T fluorescence and increased Congo red absorbance together with enhanced turbidity of the modified form confirmed the aggregation of RSC. The study further revealed anti-glycation and anti-aggregation potential of amino acids; aspartic acid and arginine as they prevented and/or slowed down the process of AGEs and β structure buildup in a concentration dependent manner with arginine proving to be the most effective one.

Ribosylation induced structural changes in Bovine Serum Albumin: understanding high dietary sugar induced protein aggregation and amyloid formation

Non-enzymatic glycation of proteins is believed to be the root cause of high dietary sugar associated pathophysiological maladies. We investigated the structural changes in protein during progression of glycation using ribosylated Bovine Serum Albumin (BSA). Non enzymatic attachment of about 45 ribose molecules to BSA resulted in gradual reduction of hydrophobicity and aggregation as indicated by red-shifted tryptophan fluorescence, reduced ANS binding and lower anisotropy of FITC-conjugated protein. Parallely, there was a significant decrease of alpha helicity as revealed by Circular Dichroism (CD) and Fourier transformed-Infra Red (FT-IR) spectra. The glycated proteins assumed compact globular structures with enhanced Thioflavin-T binding resembling amyloids. The gross structural transition affected by ribosylation led to enhanced thermostability as indicated by melting temperature and Transmission Electron Microscopy. At a later stage of glycation, the glycated proteins developed non-specific aggregates with increase in size and loss of amyloidogenic behaviour. A parallel non-glycated control incubated under similar conditions indicated that amyloid formation and associated changes were specific for ribosylation and not driven by thermal denaturation due to incubation at 37 °C. Functionality of the glycated protein was significantly altered as probed by Isothermal Titration Calorimetry using polyphenols as substrates. The studies demonstrated that glycation driven globular amyloids form and persist as transient intermediates during formation of misfolded glycated adducts. To the best of our knowledge, the present study is the first systematic attempt to understand glycation associated changes in a protein and provides important insights towards designing therapeutics for arresting dietary sugar induced amyloid formation.

Transcriptomic Analysis Reveals the Protection of Astragaloside IV against Diabetic Nephropathy by Modulating Inflammation

Background

Diabetic nephropathy (DN) is one of the leading causes of end-stage kidney disease. Recently, there is no specific drug available to block the kidney damage. Astragaloside IV (AS-IV) is a major active component of Astragalus membranaceus (Fisch) Bge and has been demonstrated to benefit the kidney functions. This study explores the potential pharmacological action of AS-IV in DN of rats.

Methods

Male Sprague-Dawley rats were fed with high-fat diet and injected with streptozotocin to induce diabetes. The diabetic rats were randomized and treated with vehicle or AS-IV (80 mg/kg) daily by gavage for 12 weeks as the DN or AS-IV group, respectively. The normal control rats were fed with normal chow and injected with vehicles (n = 8 per group). These rats were monitored for diabetes- and kidney function-related measures. The expression profiles of gene mRNA transcripts in the kidney tissues were analyzed by RNA-seq and quantitative RT-PCR. The levels of advanced glycation end products (AGEs), IL-1β, and IL-18 in the serum samples and kidney tissues were quantified by ELISA. The levels of collagen IV (COL-4) and fibronectin (FN) expression in kidney tissues were examined by immunohistochemistry and Western blot.

Results

In comparison with the DN group, AS-IV treatment significantly reduced blood glucose levels, food and water consumption, 24 h urine, renal index values, 24 h urine total proteins, blood urea nitrogen (BUN) levels, and creatinine clearance rates (CCR), accompanied by minimizing the DN-induced early kidney damages, fibrosis, and microstructural changes. Furthermore, AS-IV treatment significantly modulated the DN-altered gene transcription profiles in the kidney of rats, particularly for inflammation-related genes, including the nucleotide-binding oligomerization domain-like receptor signaling, which was validated by quantitative RT-PCR. AS-IV treatment significantly decreased the levels of serum and kidney AGEs, IL-1β, and IL-18 expression and fibrosis indexes in the kidney of rats.

Conclusion

AS-IV treatment ameliorated the severity of DN by inhibiting inflammation-related gene expression in the kidney of rats.

Glycated albumin precipitation using aptamer conjugated magnetic nanoparticles

To develop a strategy for the elimination of prefibrillar amyloid aggregates, a three-step non-modified DNA aptamer conjugation on silica-coated magnetic nanoparticles was carried out to achieve aptamer conjugated on MNP (Ap-SiMNP). Prefibrillar amyloid aggregates are generated under a diabetic condition which are prominently participated in developing diabetic complications. The binding properties of candidate DNA aptamer against serum albumin prefibrillar amyloid aggregates (AA20) were verified using electrophoretic mobility shift assay (EMSA) and surface plasmon resonance spectroscopy (SPR) analysis. The chloro-functionalized silica-coated MNPs were synthesized then a nano-targeting structure as aptamer conjugated on MNP (Ap-SiMNP) was constructed. Finally, Ap-SiMNP was verified for specific binding efficiency and AA20 removal using an external magnetic field. The candidate aptamer showed a high binding capacity at EMSA and SPR analysis (KD = 3.4 × 10─9 M) and successfully used to construct Ap-SiMNP. Here, we show a proof of concept for an efficient bio-scavenger as Ap-SiMNP to provide a promising opportunity to consider as a possible strategy to overcome some diabetic complications through specific binding/removal of toxic AA20 species.

Poncirin, an orally active flavonoid exerts antidiabetic complications and improves glucose uptake activating PI3K/Akt signaling pathway in insulin resistant C2C12 cells with anti-glycation capacities

Poncirin, a natural flavanone glycoside present abundantly in many citrus fruits, contains an extensive range of biological activities. However, the antidiabetic mechanism of poncirin is unexplored yet. In this study, we examined the anti-diabetic prospective of poncirin by evaluating its ability to inhibit protein tyrosine phosphatase 1B (PTP1B), α-glucosidase, human recombinant AR (HRAR), rat lens aldose reductase (RLAR), and advanced glycation end-product (AGE) formation (IC₅₀ = 7.76 ± 0.21, 21.31 ± 1.26, 3.56 ± 0.33, 11.91 ± 0.21, and 3.23 ± 0.09 µM, respectively). Kinetics data and docking studies showed the lowest binding energy and highestaffinityforthemixed and competitivetypeof inhibitorsof poncirin. Moreover, the molecular mechanisms underlying the antidiabetic outcomes of poncirin in insulin resistant C2C12 skeletal muscle cells were explored, which significantly increased glucose uptake and decreased the expression of PTP1B in C2C12 cells. Consequently, poncirin increased GLUT-4 expression level by activating the IRS-1/PI3K/Akt/GSK-3 signaling pathway. Moreover, poncirin (0.5-50 µM) remarkably inhibited the formation of fluorescent AGE, nonfluorescent CML, fructosamine, and β-cross amyloid structures in glucose-fructose-induced BSA glycation during 4 weeks of study. Poncirin also notably prevented protein oxidation demonstrated with decreasing the protein carbonyl and the consumption of protein thiol in the dose-dependent manner. The results clearly expressed the promising activity of poncirin for the therapy of diabetes and its related complications.

Antirhea borbonica Aqueous Extract Protects Albumin and Erythrocytes from Glycoxidative Damages

Diabetes constitutes a major health problem associated with severe complications. In hyperglycemic conditions, chronically increased oxidation and glycation of circulating components lead to advanced glycation end-products (AGEs) formation, a key contributor in diabetes complication progression. In line with literature documenting the beneficial properties of herbal teas, this study evaluates the antioxidant/glycant properties of Antirhea borbonica (Ab). Ab aqueous extract effects were tested on human albumin or erythrocytes submitted to methyl glyoxal-mediated glycoxidative damages. By using mass spectrometry, Ab aqueous extracts revealed to be rich in polyphenols. All tested biomarkers of oxidation and glycation, such as AGE, ketoamine, oxidized thiol groups, were decreased in albumin when glycated in the presence of Ab aqueous extract. Ab extract preserve erythrocyte from methylglyoxal (MGO)-induced damages in terms of restored membrane deformability, reduced oxidative stress and eryptosis phenomenon. Antioxidant capacities of Ab extract on erythrocytes were retrieved in vivo in zebrafish previously infused with MGO. These results bring new evidences on the deleterious impacts of glycation on albumin and erythrocyte in diabetes. Furthermore, it reveals antioxidant and antiglycant properties of Ab that could be used for the dietary modulation of oxidative stress and glycation in hyperglycemic situations.

Structural stability and solubility of glycated camel lens ζ-crystallin

The camel has several biochemical, physiological, and anatomical features to withstand the harsh desert climate. Camel eye lens contains a novel protein (ζ-crystallin) in bulk quantity. Previous reports suggest that non-enzymatic glycation of eye lens proteins plays an important role in the etiology of cataract. In this study, we have characterized the role of glucose, fructose, and methylglyoxal (MGO) in the glycation of camel lens ζ-crystallin. From the results obtained, it was found that MGO reacted rapidly, fructose reacted moderately, and glucose was the least reactive even after prolonged incubation (>100 days). Glycation with MGO and fructose led to changes in the structure of ζ-crystallin, while glucose had no remarkable effect. The surface hydrophobicity did not change and no aggregates or amyloid fibrils were observed in the glycated ζ-crystallin. Moreover, the secondary structure of glycated ζ-crystallin remained similar after glycation. Our results suggested that due to natural adaptation, the camel lens protein ζ-crystallin retained its structure and solubility even after glycation to perform the single known function of the lens proteins: to focus unscattered light on the retina.

Advanced glycation end products (AGEs), protein aggregation and their cross talk: new insight in tumorigenesis

Protein glycation and protein aggregation are two distinct phenomena being observed in cancer cells as factors promoting cancer cell viability. Protein aggregation is an abnormal interaction between proteins caused as a result of structural changes in them after any mutation or environmental assault. Protein aggregation is usually associated with neurodegenerative diseases like Alzheimer's and Parkinson's, but of late, research findings have shown its association with the development of different cancers like lung, breast and ovarian cancer. On the contrary, protein glycation is a cascade of irreversible nonenzymatic reaction of reducing sugar with the amino group of the protein resulting in the modification of protein structure and formation of advanced glycation end products (AGEs). These AGEs are reported to obstruct the normal function of proteins. Lately, it has been reported that protein aggregation occurs as a result of AGEs. This aggregation of protein promotes the transformation of healthy cells to neoplasia leading to tumorigenesis. In this review, we underline the current knowledge of protein aggregation and glycation along with the cross talk between the two, which may eventually lead to the development of cancer.

The Microbiota-Gut-Brain Axis-Heart Shunt Part II: Prosaic Foods and the Brain-Heart Connection in Alzheimer Disease

There is a strong cerebrovascular component to brain aging, Alzheimer disease, and vascular dementia. Foods, common drugs, and the polyphenolic compounds contained in wine modulate health both directly and through the gut microbiota. This observation and novel findings centered on nutrition, biochemistry, and metabolism, as well as the newer insights we gain into the microbiota-gut-brain axis, now lead us to propose a shunt to this classic triad, which involves the heart and cerebrovascular systems. The French paradox and prosaic foods, as they relate to the microbiota-gut-brain axis and neurodegenerative diseases, are discussed in this manuscript, which is the second part of a two-part series of concept papers addressing the notion that the microbiota and host liver metabolism all play roles in brain and heart health.

The inhibitory effect of the catechin structure on advanced glycation end product formation in alcoholic media

Catechin has a good inhibitory effect on advanced glycation end product (AGE) formation in alcoholic media, which is generated by Maillard reaction is closely related to diabetes.

Comprehensive Analysis of Protein Glycation Reveals Its Potential Impacts on Protein Degradation and Gene Expression in Human Cells

Glycation as a type of non-enzymatic protein modification is related to aging and chronic diseases, especially diabetes. Global analysis of protein glycation will aid in a better understanding of its formation mechanism and biological significance. In this work, we comprehensively investigated protein glycation in human cells (HEK293T, Jurkat, and MCF7 cells). The current results indicated that this non-enzymatic modification was not random, and protein at the extracellular regions and the nucleus were more frequently glycated. Systematic and site-specific analysis of glycated proteins allowed us to study the effect of the primary sequences and secondary structures of proteins on glycation. Furthermore, nearly every enzyme in the glycolytic pathway was found to be glycated and a possible mechanism was proposed. Many glycation sites were also previously reported as acetylation and ubiquitination sites, which strongly suggested that this non-enzymatic modification may disturb protein degradation and gene expression. The current results will facilitate further studies of protein glycation in biomedical and clinical research.

Advanced Glycation End Products of Bovine Serum Albumin Suppressed Th1/Th2 Cytokine but Enhanced Monocyte IL-6 Gene Expression via MAPK-ERK and MyD88 Transduced NF-κB p50 Signaling Pathways

Advanced glycation end products (AGE), the most known aging biomarker, may cause “inflamm-aging” (i.e., chronic low-grade inflammation that develops with aging) in both aged and diabetes groups. However, the molecular bases of inflamm-aging remain obscure. We prepared AGE by incubating BSA (0.0746 mmol/L) + glucose (0.5 mol/L) at 37 °C in 5% CO₂–95% air for 1–180 days. The lysine glycation in BSA–AGE reached 77% on day 30 and 100% after day 130, whereas the glycation of arginine and cysteine was minimal. The N^ε-(carboxymethyl)-lysine content in BSA–AGE was also increased with increasing number of incubation days. The lectin-binding assay revealed that the glycation of BSA not only altered the conformational structure, but lost binding capacity with various lectins. An immunological functional assay showed that BSA–AGE > 8 μg/mL significantly suppressed normal human Th1 (IL-2 and IFN-γ) and Th2 (IL-10) mRNA expression, whereas AGE > 0.5 μg/mL enhanced monocyte IL-6 production irrelevant to cell apoptosis. The AGE-enhanced monocyte IL-6 production was via MAPK–ERK and MyD88-transduced NF-κBp50 signaling pathways. To elucidate the structure–function relationship of BSA–AGE-enhanced IL-6 production, we pre-preincubated BSA–AGE with different carbohydrate-degrading, protein-degrading, and glycoprotein-degrading enzymes. We found that trypsin and carboxypeptidase Y suppressed whereas β-galactosidase enhanced monocyte IL-6 production. In conclusion, BSA–AGE exerted both immunosuppressive and pro-inflammatory effects that are the molecular basis of inflamm-aging in aged and diabetes groups.

Differential Effects of Dry vs. Wet Heating of β-Lactoglobulin on Formation of sRAGE Binding Ligands and sIgE Epitope Recognition

The effect of glycation and aggregation of thermally processed β-lactoglobulin (BLG) on binding to sRAGE and specific immunoglobulin E (sIgE) from cow milk allergic (CMA) patients were investigated. BLG was heated under dry conditions (water activity < 0.7) and wet conditions (in phosphate buffer at pH 7.4) at low temperature (<73 °C) and high temperatures (>90 °C) in the presence or absence of the milk sugar lactose. Nε-(carboxymethyl)-l-lysine (CML) western blot and glycation staining were used to directly identify glycation structures on the protein fractions on SDS-PAGE. Western blot was used to specify sRAGE and sIgE binding fractions. sRAGE binding was highest under wet-heated BLG independent of the presence of the milk sugar lactose. Under wet heating, high-molecular-weight aggregates were most potent and did not require the presence of CML to generate sRAGE binding ligands. In the dry system, sRAGE binding was observed only in the presence of lactose. sIgE binding affinity showed large individual differences and revealed four binding profiles. Dependent on the individual, sIgE binding decreased or increased by wet heating independent of the presence of lactose. Dry heating required the presence of lactose to show increased binding to aggregates in most individuals. This study highlights an important role of heating condition-dependent protein aggregation and glycation in changing the immunogenicity and antigenicity of cow’s milk BLG.

Amyloid-Targeting PET Tracer [18F]Flutemetamol Accumulates in Atherosclerotic Plaques

Atherosclerosis is characterized by the accumulation of oxidized lipids in the artery wall, which triggers an inflammatory response. Oxidized low-density lipoprotein (ox-LDL) presents amyloid-like structural properties, and different amyloid species have recently been recognized in atherosclerotic plaques. Therefore, we studied the uptake of the amyloid imaging agent [¹⁸F]Flutemetamol in atherosclerotic plaques. The binding of [¹⁸F]Flutemetamol to human carotid artery plaque was studied in vitro. In vivo uptake of the tracer was studied in hypercholesterolemic IGF-II/LDLR^−/−ApoB^100/100 mice and C57BL/6N controls. Tracer biodistribution was studied in vivo with PET/CT, and ex vivo by gamma counter and digital ex vivo autoradiography. The presence of amyloid, ox-LDL, and macrophages in the plaques was examined by immunohistochemistry. [¹⁸F]Flutemetamol showed specific accumulation in human carotid plaque, especially in areas positive for amyloid beta. The aortas of IGF-II/LDLR^−/−ApoB^100/100 mice showed large thioflavin-S-positive atherosclerotic plaques containing ox-LDL and macrophages. Autoradiography revealed 1.7-fold higher uptake in the plaques than in a lesion-free vessel wall, but no difference in aortic tissue uptake between mouse strains were observed in the in vivo PET/CT. In conclusion, [¹⁸F]Flutemetamol binds to amyloid-positive areas in human atherosclerotic plaques. Further studies are warranted to clarify the uptake mechanisms, and the potential of the tracer for in vivo imaging of atherosclerosis in patients.

Covalent Modification by Glyoxals Converts Cytochrome c Into its Apoptotically Competent State

The effects of glycation by glyoxal (Gly) and methylglyoxal (MGly) on the early and late conformational alterations in Cytochrome c (Cyt c) were studied. Spectroscopic measurements revealed that Cyt c undergo certain conformational alterations and exposure of heme upon overnight incubation with Gly and MGly. These were followed by the reduction of heme centre and activation of its peroxidase-like, which is crucial for initiation of the intrinsic apoptotic pathway. An extended incubation resulted in appearance of AGE-like fluorescence, with significant alterations in secondary structural compositions. However, no amyloidogenic conversions were observed as suggested by TEM analyses. The study provides an insight to the role of glycating agents, elevated under oxidative stress in inducing Cyt c release and apoptosis.

Mechanism of non-enzymatic antiglycation action by coumarin: a biophysical study

Coumarin inhibited non-enzymatic glycation by masking the free amino groups and scavenging carbonyl groups of protein.

Structural effects of methylglyoxal glycation, a study on the model protein MNEI

The reaction of free amino groups in proteins with reactive carbonyl species, known as glycation, leads to the formation of mixtures of products, collectively referred to as advanced glycation endproducts (AGEs). These compounds have been implicated in several important diseases, but their role in pathogenesis and clinical symptoms' development is still debated. Particularly, AGEs are often associated to the formation of amyloid deposits in conformational diseases, such as Alzheimer's and Parkinson's disease, and it has been suggested that they might influence the mechanisms and kinetics of protein aggregation. We here present the characterization of the products of glycation of the model protein MNEI with methylglyoxal and their effect on the protein structure. We demonstrate that, despite being an uncontrolled process, glycation occurs only at specific residues of the protein. Moreover, while not affecting the protein fold, it alters its shape and hydrodynamic properties and increases its tendency to fibrillar aggregation. Our study opens the way to in deep structural investigations to shed light on the complex link between protein post-translational modifications, structure, and stability.

The receptor for advanced glycation endproducts is a mediator of toxicity by IAPP and other proteotoxic aggregates: Establishing and exploiting common ground for novel amyloidosis therapies

Proteotoxicity plays a key role in many devastating human disorders, including Alzheimer's, Huntington's and Parkinson's diseases; type 2 diabetes; systemic amyloidosis; and cardiac dysfunction, to name a few. The cellular mechanisms of proteotoxicity in these disorders have been the focus of considerable research, but their role in prevalent and morbid disorders, such as diabetes, is less appreciated. There is a large body of literature on the impact of glucotoxicity and lipotoxicity on insulin-producing pancreatic β-cells, and there is increasing recognition that proteotoxicty plays a key role. Pancreatic islet amyloidosis by the hormone IAPP, the production of advanced glycation endproducts (AGE), and insulin misprocessing into cytotoxic aggregates are all sources of β-cell proteotoxicity in diabetes. AGE, produced by the reaction of reducing sugars with proteins and lipids are ligands for the receptor for AGE (RAGE), as are the toxic pre-fibrillar aggregates of IAPP produced during amyloid formation. The mechanisms of amyloid formation by IAPP in vivo or in vitro are not well understood, and the cellular mechanisms of IAPP-induced β-cell death are not fully defined. Here, we review recent findings that illuminate the factors and mechanisms involved in β-cell proteotoxicity in diabetes. Together, these new insights have far-reaching implications for the establishment of unifying mechanisms by which pathological amyloidoses imbue their injurious effects in vivo.

Variation in Polyphenolic Profiles, Antioxidant and Antimicrobial Activity of Different Achillea Species as Natural Sources of Antiglycative Compounds

A comparative study was carried out on the methanolic extracts from six Achillea species and the examined polyphenols from these plants on the formation of advanced glycation end-products (AGE) in vitro. A. pachycephala which was richer in flavonoids (15 mg quercetin/g W) and phenolics (111.10 mg tannic acid/g DW) with substantial antioxidant activity (IC₅₀  = 365.5 μg/ml) presented strong anti-AGE properties. Chlorogenic acid, luteolin, quercetin and caffeic acid were identified as the major polyphenols in the extracts by HPLC. In general, polyphenolic content follows the order of A. pachycephalla > A. nobilis > A. filipendulina > A. santolina > A. aucheri > A. millefolium. Most extracts exhibited marked anti-AGE ability in the bovine serum albumin (BSA)/methylglyoxal (MG) system, though A. pachycephala showed the highest potential. The formation of AGEs was assessed by monitoring the production of fluorescent products and circular dichroism (CD) spectroscopy. Diminution in free radical production (assessed by 2,2-diphenyl-1-picrylhydrazyl (DPPH) assays) is discussed as potential mechanism for delay or reduced AGE. The results demonstrate the antiglycative, antioxidant and antimicrobial potential of Achillea species which can be attributed to polyphenols content and the effectiveness on generation of AGEs, thus Achillea species can be considered as natural sources for slowing down glycation related diseases.

Digestibility of Bovine Serum Albumin and Peptidomics of the Digests: Effect of Glycation Derived from α-Dicarbonyl Compounds

α-Dicarbonyl compounds, which are widely generated during sugar fragmentation and oil oxidation, are important precursors of advanced glycation end products (AGEs). In this study, the effect of glycation derived from glyoxal (GO), methylglyoxal (MGO) and diacetyl (DA) on the in vitro digestibility of bovine serum albumin (BSA) was investigated. Glycation from α-dicarbonyl compounds reduced digestibility of BSA in both gastric and intestinal stage of digestion according to measurement of degree of hydrolysis. Changes in peptide composition of digests induced by glycation were displayed, showing absence of peptides, occurrence of new peptides and formation of peptide-AGEs, based on the results obtained using liquid chromatography electron-spray-ionization tandem mass spectrometry (LC-ESI-MS/MS). Crosslinked glycation structures derived from DA largely reduced the sensitivity of glycated BSA towards digestive proteases based on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) results. Network structures were found to remain in the digests of glycated samples by transmission electron microscope (TEM), thus the impact of AGEs in unabsorbed digests on the gut flora should be an interest for further studies.

Glycation induces conformational changes in the amyloid-β peptide and enhances its aggregation propensity: molecular insights

The cytotoxicity of the amyloid beta (Aβ) peptide, implicated in the pathogenesis of Alzheimer's disease (AD), can be enhanced by its post-translational glycation, a series of non-enzymatic reactions with reducing sugars and reactive dicarbonyls. However, little is known about the underlying mechanisms that potentially enhance the cytotoxicity of the advanced glycation modified Aβ. In this work, fully atomistic molecular dynamics (MD) simulations are exploited to obtain direct molecular insights into the process of early Aβ self-assembly in the presence and absence of glycated lysine residues. Analyses of data exceeding cumulative timescales of 1 microsecond for each system reveal that glycation results in a stronger enthalpy of association between Aβ monomers and lower conformational entropy, in addition to a sharp overall increase in the beta-sheet content. Further analyses reveal that the enhanced interactions originate, in large part, due to markedly stronger, as well as new, inter-monomer salt bridging propensities in the glycated variety. Interestingly, these conformational and energetic effects are broadly reflected in preformed protofibrillar forms of Aβ small oligomers modified with glycation. Our combined results imply that glycation consolidates Aβ self-assembly regardless of its point of occurrence in the pathway. They provide a basis for further mechanistic studies and therapeutic endeavors that could potentially result in novel ways of combating AGE related AD progression.

Methylglyoxal induced glycation and aggregation of human serum albumin: Biochemical and biophysical approach

Serum protein glycation and formation of advanced glycation end products (AGEs) correlates with many diseases viz. diabetes signifying the importance of studying the glycation pattern of serum proteins. In our present study, methylglyoxal was investigated for its effect on the structure of human serum albumin (HSA); exploring the formation of AGEs and aggregates of HSA. The analytical tools employed includes intrinsic and extrinsic fluorescence, UV spectroscopy, far UV circular dichroism, Thioflavin T fluorescence, congo red binding, polyacrylamide gel electrophoresis (PAGE). UV and fluorescence spectroscopy revealed the structural transition of native HSA evident by new peaks and increased absorbance in UV spectra and quenched fluorescence in the presence of MG. Far UV CD spectroscopy revealed MG induced secondary structural alteration evident by reduced α-helical content. AGEs formation was confirmed by AGEs specific fluorescence. Increased ThT fluorescence and CR absorbance of 10mM MG incubated HSA suggests that glycated HSA results in the formation of aggregates of HSA. SEM and TEM were reported to have an insight of these aggregates. Molecular docking was also utilized to see site specific interaction of MG-HSA. This study is clinically significant as HSA is a clinically relevant protein which plays a crucial role in many diseases.

The digestibility of hydrothermally-treated bovine serum albumin glycated by glyoxal

The digestion of dietary advanced glycation end products (AGEs) largely determines their absorption in humans. To help elucidate the health effects of dietary AGEs, changes in the digestive behavior of bovine serum albumin (BSA, dietary protein) caused by glycation derived from glyoxal (GO, an important precursor of AGEs) in a simulated food heating system have been investigated. The hydrothermal aggregation of BSA was suppressed by GO derived glycation, generating glycated aggregates of loose and branched structures, according to dynamic light scattering (DLS), circular dichroism (CD) spectroscopy, free sulfhydryl group, transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) results. Analysis of protein digests showed that glycation reduced the gastric and gastrointestinal digestibility of BSA and the bioavailability of all seven detected amino acids. A comparative analysis of the distribution of CML and lysine in glycated BSA digests with different molecular weights showed that carboxymethylation directly blocked the action of proteases on Lys residues.

The overall gastrointestinal digestibility of hydrothermally-treated bovine serum albumin drops dramatically following GO-derived glycation.

Binding of thioflavin T by albumins: An underestimated role of protein oligomeric heterogeneity

Amyloid fibrils formation is the well-known hallmark of various neurodegenerative diseases. Thioflavin T (ThT)-based fluorescence assays are widely used to detect and characterize fibrils, however, if performed in bioliquids, the analysis can be biased due to the presence of other, especially abundant, proteins. Particularly, it is known that albumin may bind ThT, although the binding mechanism remains debatable. Here the role of low-order albumin oligomers in ThT binding is investigated using time-resolved fluorometry and size-exclusion chromatography. Under conditions used, the fraction of dimers in human (HSA) and bovine (BSA) serum albumin solutions is as low as ∼7%, however, it is responsible for ∼50% of ThT binding. For both albumins, the binding affinity was estimated to be ∼200 and ∼40μM for monomeric and dimeric species, respectively. Molecular docking suggested that ThT preferentially binds in the hydrophobic pocket of subdomain IB of albumin monomer in a similar position but with a variable torsion angle, resulting in a lower fluorescence enhancement (∼40-fold) compared to amyloid fibrils (∼1000-fold). Dimerization of albumin presumably creates an extra binding site at the subunit interface. These results demonstrate the underestimated role of low-order albumin oligomers that can be highly relevant when analyzing drugs binding using fluorescence spectroscopy.

Fructose-human serum albumin interaction undergoes numerous biophysical and biochemical changes before forming AGEs and aggregates

Fructose is a reducing and highly lipogenic sugar that has unique metabolic effects in the liver. Non-enzymatic fructosylation of proteins generates advanced glycation end products (AGEs). Human serum albumin (HSA) may undergo fructosylation vis-à-vis AGEs formation. High fructose consumption may lead to structurally altered and functionally compromised fructosylated-HSA-AGEs, which can cause damage to hepatocytes resulting in hepatic macro- and microvesicular steatosis. In this study, HSA was incubated with varying concentrations of fructose for 10days and the induced changes were studied. Fructosylated-HSA exhibited hyperchromicity, increased AGE-specific fluorescence, quenching of tryptophan fluorescence and increased melting temperature. Nε-[carboxymethyl]-lysine (CML), was detected by liquid chromatography mass spectrometry (LC-MS). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) results showed decreased mobility in fructosylated-HSA. Perturbations in secondary and tertiary structure were revealed by fourier transform-infrared spectroscopy (FT-IR), supported by far- and near-UV circular dichroism (CD). Dynamic light scattering (DLS) and Matrix-Assisted Laser Desorption Ionization-Time of Flight (MALDI-TOF) mass spectrometry studies suggested increase in molecular mass of fructosylated-HSA. Amyloidogenic aggregates were confirmed from Congo red, Thioflavin T assay and Scanning electron microscope (SEM). These investigations confirmed the structural alterations in fructosylated-HSA and warrants further study to probe the role of fructosylated-HSA-AGEs in hepatopathy vis-à-vis fatty liver diseases.

High-resolution physicochemical characterization of different intravenous immunoglobulin products

Intravenous immunoglobulin (IVIg) is a complex mixture drug comprising diverse immunoglobulins and non-IgG proteins purified from the plasma of thousands of healthy donors. Approved IVIg products on the market differ regarding source of plasma, isolation process, and formulation. These products are used widely, and often interchangeably, for the treatment of immunodeficiency and autoimmune and inflammatory diseases, but their mechanisms of action in different indications are not well understood. A primary limitation to understanding the therapeutic relevance of specific components within IVIg has been the limited resolution of analytics historically implemented to characterize its complex mixture. In this study, high-resolution analytics were applied to better understand the composition of IVIg and product variations. We characterized three approved IVIg products: Gammagard®, Privigen®, and Octagam®. Differences in the distribution of molecular weight species, IgG sequence variants, isoforms, glycoforms, and the repertoire of previously reported antibody specificities were identified. We also compared the effect of aging on these products to identify changes in size distribution and posttranslational modifications. This type of characterization may provide insights into the specific factors and components of IVIg that may influence its activity and ultimately lead to optimization of IVIg products for use in autoimmune diseases.

Glycation of Lysozyme by Glycolaldehyde Provides New Mechanistic Insights in Diabetes-Related Protein Aggregation

Glycation occurs in vivo as a result of the nonenzymatic reaction of carbohydrates (and/or their autoxidation products) with proteins, DNA, or lipids. Protein glycation causes loss-of-function and, consequently, the development of diabetic-related diseases. Glycation also boosts protein aggregation, which can be directly related with the higher prevalence of aggregating diseases in diabetic people. However, the molecular mechanism connecting glycation with aggregation still remains unclear. Previously we described mechanistically how glycation of hen egg-white lysozyme (HEWL) with ribose induced its aggregation. Here we address the question of whether the ribose-induced aggregation is a general process or it depends on the chemical nature of the glycating agent. Glycation of HEWL with glycolaldehyde occurs through two different scenarios depending on the HEWL concentration regime (both within the micromolar range). At low HEWL concentration, non-cross-linking fluorescent advanced glycation end-products (AGEs) are formed on Lys side chains, which do not change the protein structure but inhibit its enzymatic activity. These AGEs have little impact on HEWL surface hydrophobicity and, therefore, a negligible effect on its aggregation propensity. Upon increasing HEWL concentration, the glycation mechanism shifts toward the formation of intermolecular cross-links, which triggers a polymerization cascade involving the formation of insoluble spherical-like aggregates. These results notably differ with the aggregation-modulation mechanism of ribosylated HEWL directed by hydrophobic interactions. Additionally, their comparison constitutes the first experimental evidence showing that the mechanism underlying the aggregation of a glycated protein depends on the chemical nature of the glycating agent.