Formation pathways for lysine-arginine cross-links derived from hexoses and pentoses by Maillard processes: unraveling the structure of a pentosidine precursor

In vivo glycation-interplay between oxidant and carbonyl stress in bone

Metabolic syndromes (eg, obesity, type 2 diabetes (T2D), atherosclerosis, and neurodegenerative diseases) and aging, they all have a strong component of carbonyl and reductive-oxidative (redox) stress. Reactive carbonyl (RCS) and oxidant (ROS) stress species are commonly generated as products or byproducts of cellular metabolism or are derived from the environment. RCS and ROS can play a dual role in living organisms. Some RCS and ROS function as signaling molecules, which control cellular defenses against biological and environmental assaults. However, due to their high reactivity, RCS and ROS inadvertently interact with different cellular and extracellular components, which can lead to the formation of undesired posttranslational modifications of bone matrix proteins. These are advanced glycation (AGEs) and glycoxidation (AGOEs) end products generated in vivo by non-enzymatic amino-carbonyl reactions. In this review, metabolic processes involved in generation of AGEs and AGOEs within and on protein surfaces including extracellular bone matrix are discussed from the perspective of cellular metabolism and biochemistry of certain metabolic syndromes. The impact of AGEs and AGOEs on some characteristics of mineral is also discussed. Different therapeutic approaches with the potential to prevent the formation of RCS, ROS, and the resulting formation of AGEs and AGOEs driven by these chemicals are also briefly reviewed. These are antioxidants, scavenging agents of reactive species, and newly emerging technologies for the development of synthetic detoxifying systems. Further research in the area of in vivo glycation and glycoxidation should lead to the development of diverse new strategies for halting the progression of metabolic complications before irreversible damage to body tissues materializes.

Slot Blot- and Electrospray Ionization-Mass Spectrometry/Matrix-Assisted Laser Desorption/Ionization-Mass Spectrometry-Based Novel Analysis Methods for the Identification and Quantification of Advanced Glycation End-Products in the Urine

Proteins, saccharides, and low molecular organic compounds in the blood, urine, and saliva could potentially serve as biomarkers for diseases related to diet, lifestyle, and the use of illegal drugs. Lifestyle-related diseases (LSRDs) such as diabetes mellitus (DM), non-alcoholic steatohepatitis, cardiovascular disease, hypertension, kidney disease, and osteoporosis could develop into life-threatening conditions. Therefore, there is an urgent need to develop biomarkers for their early diagnosis. Advanced glycation end-products (AGEs) are associated with LSRDs and may induce/promote LSRDs. The presence of AGEs in body fluids could represent a biomarker of LSRDs. Urine samples could potentially be used for detecting AGEs, as urine collection is convenient and non-invasive. However, the detection and identification of AGE-modified proteins in the urine could be challenging, as their concentrations in the urine might be extremely low. To address this issue, we propose a new analytical approach. This strategy employs a method previously introduced by us, which combines slot blotting, our unique lysis buffer named Takata's lysis buffer, and a polyvinylidene difluoride membrane, in conjunction with electrospray ionization-mass spectrometry (ESI)/matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS). This novel strategy could be used to detect AGE-modified proteins, AGE-modified peptides, and free-type AGEs in urine samples.

1-Amino-1-deoxy-d-fructose ("fructosamine") and its derivatives

Fructosamine has long been considered as a key intermediate of the Maillard reaction, which to a large extent is responsible for specific aroma, taste, and color formation in thermally processed or dehydrated foods. Since the 1980s, however, as a product of the Amadori rearrangement reaction between glucose and biologically significant amines such as proteins, fructosamine has experienced a boom in biomedical research, mainly due to its relevance to pathologies in diabetes and aging. In this chapter, we assess the scope of the knowledge on and applications of fructosamine-related molecules in chemistry, food, and health sciences, as reflected mostly in publications within the past decade. Methods of fructosamine synthesis and analysis, its chemical, and biological properties, and degradation reactions, together with fructosamine-modifying and -recognizing proteins are surveyed.

Pentosinane, a Post-Translational Modification of Human Proteins with Underappreciated Stability

Pentosinane is a structurally complex nonenzymatic post-translational modification of proteins believed to be present in all living things. It falls into the category of advanced glycation end products (AGEs) and is structurally related to the other AGEs pentosidine and glucosepane. Although pentosidine and glucosepane have been widely studied for their role in wide-ranging conditions (e.g., diabetes mellitus, Alzheimer's disease, and human aging), relatively little is known about pentosinane. Interestingly, previous reports have suggested that pentosidine may derive from pentosinane. The (patho)physiological significance of pentosinane in humans is largely unexplored. As a first step to address this knowledge gap, we report herein the first total synthesis of pentosinane. Our synthesis is high yielding (1.7% over seven steps), concise, and enantioselective, and it leverages a strategy for synthesizing 2,5-diaminoimidazoles previously developed by our lab. Access to synthetic pentosinane has allowed us to perform additional studies showing that its oxidation to pentosidine is both pH and oxygen dependent and is substantially slower under physiological conditions than previously believed. Additionally, pentosinane rapidly decomposes under harshly acidic conditions typically employed for pentosidine isolation. Taken together, these results suggest that pentosinane is likely to be more abundant in vivo than previously appreciated. We believe these results represent a critical step toward illuminating the role(s) of pentosinane in human biology.

Molecular conformations and dynamics in the extracellular matrix of mammalian structural tissues: Solid-state NMR spectroscopy approaches

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Solid-state NMR spectroscopy probes molecular conformation and dynamics in intact ECM.

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Collagen conformational dynamics has roles in mechanical properties of fibrils and cell adhesion.

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Solid-state NMR spectroscopy has shed new light on  the chemical structure of bone mineral.

Solid-state NMR spectroscopy has played an important role in multidisciplinary studies of the extracellular matrix. Here we review how solid-state NMR has been used to probe collagen molecular conformations, dynamics, post-translational modifications and non-enzymatic chemical changes, and in calcified tissues, the molecular structure of bone mineral and its interface with collagen. We conclude that NMR spectroscopy can deliver vital information that in combination with data from structural imaging techniques, can result in significant new insight into how the extracellular matrix plays its multiple roles.

Dietary advanced glycation end-products elicit toxicological effects by disrupting gut microbiome and immune homeostasis

The aging immune system is characterized by a low-grade chronic systemic inflammatory state ("inflammaging") marked by elevated serum levels of inflammatory molecules such as interleukin (IL)-6 and C-reactive protein (CRP). These inflammatory markers were also reported to be strong predictors for the development/severity of Type 2 diabetes, obesity, and COVID-19. The levels of these markers have been positively associated with those of advanced glycation end-products (AGEs) generated via non-enzymatic glycation and oxidation of proteins and lipids during normal aging and metabolism. Based on the above observations, it is clinically important to elucidate how dietary AGEs modulate inflammation and might thus increase the risk for aging-exacerbated diseases. The present narrative review discusses the potential pro-inflammatory properties of dietary AGEs with a focus on the inflammatory mediators CRP, IL-6 and ferritin, and their relations to aging in general and Type 2 diabetes in particular. In addition, underlying mechanisms - including those related to gut microbiota and the receptors for AGEs, and the roles AGEs might play in affecting physiologies of the healthy elderly, obese individuals, and diabetics are discussed in regard to any greater susceptibility to COVID-19.

Novel Amidine Protein Cross-Links Formed by the Reaction of Glyoxal with Lysine

One crucial aspect of the Maillard reaction is the formation of reactive α-dicarbonyl structures like glyoxal, which are prone toward further reactions with proteins, e.g., the N⁶-amino group of lysine. The initially formed labile glyoxal-imine was previously established as a key intermediate in the formation of the advanced glycation end products N⁶-carboxymethyl lysine (CML), glyoxal lysine amide (GOLA), glyoxal lysine dimer (GOLD), and N⁶-glycolyl lysine (GALA). Here, we introduce a novel amidine cross-link structure N¹,N²-bis-(5-amino-5-carboxypentyl)-2-hydroxy-acetamidine (glyoxal lysine amidine, GLA), which is formed exclusively from glyoxal through the same isomerization cascade. After independent synthesis of the authentic reference standard, we were able to quantitate this cross-link in incubations of 40 mM N²-t-Boc-lysine with glyoxal and various sugars (40-100 mM) under mild conditions (pH 7.4, 37 °C) using an HPLC-MS/MS method. Furthermore, incubations of proteins (6 mg/mL) with 50 mM glyoxal confirmed the cross-linking by GLA, which was additionally identified in acidic hydrolyzed proteins of butter biscuits after HPLC enrichment.

Molecular clocks in ancient proteins: Do they reflect the age at death even after millennia?

Age at death estimation in cases of human skeletal finds is an important task in forensic medicine as well as in anthropology. In forensic medicine, methods based on “molecular clocks” in dental tissues and bone play an increasing role. The question, whether these methods are applicable also in cases with post-depositional intervals far beyond the forensically relevant period, was investigated for two “protein clocks”, the accumulation of D-aspartic acid (D-Asp) and the accumulation of pentosidine (Pen) in dentine. Eight teeth of skeletons from different burial sites in Austria and with post-depositional intervals between c. 1216 and c. 8775 years were analysed. The results of age at death estimation based on D-Asp and Pen in dentine were compared to that derived from a classical morphological examination. Age at death estimation based on D-Asp resulted consistently in false high values. This finding can be explained by a post-mortem accumulation of D-Asp that may be enhanced by protein degradation. In contrast, the Pen-based age estimates fitted well with the morphological age diagnoses. The described effect of post-mortem protein degradation is negligible in forensically relevant time horizons, but not for post-depositional intervals of thousands of years. That means that the “D-Asp clock” loses its functionality with increasing post-depositional intervals, whereas Pen seems to be very stable. The “Pen-clock” may have the potential to become an interesting supplement to the existing repertoire of methods even in cases with extremely long post-depositional intervals. Further investigations have to test this hypothesis.

Generation and Characterization of Anti-Glucosepane Antibodies Enabling Direct Detection of Glucosepane in Retinal Tissue

Although there is ample evidence that the advanced glycation end-product (AGE) glucosepane contributes to age-related morbidities and diabetic complications, the impact of glucosepane modifications on proteins has not been extensively explored due to the lack of sufficient analytical tools. Here, we report the development of the first polyclonal anti-glucosepane antibodies using a synthetic immunogen that contains the core bicyclic ring structure of glucosepane. We investigate the recognition properties of these antibodies through ELISAs involving an array of synthetic AGE derivatives and determine them to be both high-affinity and selective in binding glucosepane. We then employ these antibodies to image glucosepane in aging mouse retinae via immunohistochemistry. Our studies demonstrate for the first time accumulation of glucosepane within the retinal pigment epithelium, Bruch's membrane, and choroid: all regions of the eye impacted by age-related macular degeneration. Co-localization studies further suggest that glucosepane colocalizes with lipofuscin, which has previously been associated with lysosomal dysfunction and has been implicated in the development of age-related macular degeneration, among other diseases. We believe that the anti-glucosepane antibodies described in this study will prove highly useful for examining the role of glycation in human health and disease.

Skin Autofluorescence, a Noninvasive Biomarker for Advanced Glycation End-Products, Is Associated With Prevalent Vertebral and Major Osteoporotic Fractures: The Rotterdam Study

Advanced glycation end-products (AGEs), which bind to type 1 collagen in bone and skin, have been implicated in reduced bone quality. The AGE reader™ measures skin autofluorescence (SAF), which might be regarded as a marker of long-term accumulation of AGEs in tissues. We investigated the association of SAF with bone mineral density (BMD) and fractures in the general population. We studied 2853 individuals from the Rotterdam Study with available SAF measurements (median age, 74.1 years) and with data on prevalent major osteoporotic (MOFs: hip, humerus, wrist, clinical vertebral) and vertebral fractures (VFs: clinical + radiographic Genant's grade 2 and 3). Radiographs were assessed 4 to 5 years before SAF. Multivariate regression models were performed adjusted for age, sex, BMI, creatinine, smoking status, and presence of diabetes and additionally for BMD with interaction terms to test for effect modification. Prevalence of MOFs was 8.5% and of VFs 7%. SAF had a curvilinear association with prevalent MOFs and VFs and therefore, age-adjusted, sex stratified SAF quartiles were used. The odds ratio (OR) (95% confidence interval [CI]) of the second, third and fourth quartiles of SAF for MOFs were as follows: OR 1.60 (95% CI, 1.08-2.35; p = .02); OR 1.30 (95% CI, 0.89-1.97; p = .20), and OR 1.40 (95% CI, 0.95-2.10; p = .09), respectively, with first (lowest) quartile as reference. For VFs the ORs were as follows: OR 1.69 (95% CI, 1.08-2.64; p = .02), OR 1.74(95% CI, 1.11-2.71; p = .01), and OR 1.73 (95% CI, 1.12-2.73; p = .02) for second, third, and fourth quartiles, respectively. When comparing the top three quartiles combined with the first quartile, the OR (95% CI) for MOFs was 1.43 (95% CI, 1.04-2.00; p = .03) and for VFs was 1.72 (95% CI, 1.18-2.53; p = .005). Additional adjustment for BMD did not change the associations. In conclusion, there is evidence of presence of a threshold of skin AGEs below which there is distinctly lower prevalence of fractures. Longitudinal analyses are needed to confirm our cross-sectional findings. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

Enzymatic and non-enzymatic crosslinks found in collagen and elastin and their chemical synthesis

This review summarized the enzymatic and non-enzymatic crosslinks found in collagen and elastin and their organic synthesis.

One-Step Synthesis of 2,5-Diaminoimidazoles and Total Synthesis of Methylglyoxal-Derived Imidazolium Crosslink (MODIC)

Here we describe a general method for the synthesis of 2,5-diaminoimidazoles, which involves a thermal reaction between α-aminoketones and substituted guanylhydrazines without the need for additives. As one of the few known ways to access the 2,5-diaminoimidazole motif, our method greatly expands the number of reported diaminoimidazoles and further supports our previous observations that these compounds spontaneously adopt the non-aromatic 4(H) tautomer. The reaction works successfully on both cyclic and acyclic amino ketone starting materials, as well as a range of substituted guanylhydrazines. Following optimization, the method was applied to the efficient synthesis of the advanced glycation end product (AGE) methylglyoxal-derived imidazolium crosslink (MODIC). We expect that this method will enable rapid access to a variety of biologically important 2,5-diaminoimidazole-containing products.

The fate of dietary advanced glycation end products in the body: from oral intake to excretion

Advanced glycation end products (AGEs), which are closely associated with various chronic diseases, are formed through the Maillard reaction when aldehydes react with amines in heated foods or in living organisms. The fate of dietary AGEs after oral intake plays a crucial role in regulating the association between dietary AGEs and their biological effects. However, the complexity and diversity of dietary AGEs make their fate ambiguous. Glycated modifications can impair the digestion, transport and uptake of dietary AGEs. High and low molecular weight AGEs may exhibit individual differences in their distribution, metabolism and excretion. Approximately 50-60% of free AGEs are excreted after dietary intake, whereas protein-bound AGEs exhibit a limited excretion rate. In this article, we summarize several AGE classification criteria and their abundance in foods, and in the body. A standardized static in vitro digestion method is strongly recommended to obtain comparable results of AGE digestibility. Sophisticated hypotheses regarding the intestinal transportation and absorption of drugs, as well as calculated physicochemical parameters, are expected to alleviate the difficulties determining the digestion, transport and uptake of dietary AGEs. Orally supplied AGEs with low or high molecular weights must be supported by well-defined amounts in investigations of excretion. Furthermore, unequivocal evidence should be obtained regarding the degradation and metabolism products of dietary AGEs.

Glucosepane is associated with changes to structural and physical properties of collagen fibrils

Collagen glycation, and in particular the formation of advanced glycation end-product (AGE) crosslinks, plays a central role in the ageing process and in many of the long-term complications of diabetes. Glucosepane, the most abundant and relevant AGE crosslink, has been suggested to increase the stiffness of tissue and reduce its solubility, although no evidence is available concerning the mechanisms. We have used a combination of computational and experimental techniques to study a collagen-rich tissue with a relatively simple organisation to further our understanding of the impact of glucosepane on the structural and physical properties of collagen fibrils. Our work shows that glucosepane levels increase dramatically in aged tendon tissue and are associated with the reduced density of collagen packing and increased porosity to water molecules. Our studies provide the basis to understand many of the tissue dysfunctions associated with ageing and diabetes across a range of different tissues types.

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Levels of the advanced glycation end-product glucosepane increase in human tendon with increasing chronological age.

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Glucosepane results in a less tightly held helical structure in the collagen molecule and increased porosity to water.

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Water content is higher in Achilles and anterior tibialis tendon tissue from older individuals compared to young people.

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The denaturation temperature of collagen increases in the older age group suggesting a more highly cross-linked structure.

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The enthalpy of collagen denaturation decreases in older donors suggesting molecules are less confined within the fibril.

Phosphorylation of Extracellular Bone Matrix Proteins and Its Contribution to Bone Fragility

Phosphorylation of bone matrix proteins is of fundamental importance to all vertebrates including humans. However, it is currently unknown whether increase or decline of total protein phosphorylation levels, particularly in hypophosphatemia-related osteoporosis, osteomalacia, and rickets, contribute to bone fracture. To address this gap, we combined biochemical measurements with mechanical evaluation of bone to discern fracture characteristics associated with age-related development of skeletal fragility in relation to total phosphorylation levels of bone matrix proteins and one of the key representatives of bone matrix phosphoproteins, osteopontin (OPN). Here for the first time, we report that as people age the total phosphorylation level declines by approximately 20% for bone matrix proteins and approximately 30% for OPN in the ninth decade of human life. Moreover, our results suggest that the decline of total protein phosphorylation of extracellular matrix (ECM) contributes to bone fragility, but less pronouncedly than glycation. We theorize that the separation of two sources of OPN negative charges, acidic backbone amino acids and phosphorylation, would be nature's means of assuring that OPN functions in both energy dissipation and biomineralization. We propose that total phosphorylation decline could be an important contributor to the development of osteoporosis, increased fracture risk and skeletal fragility. Targeting the enzymes kinase FamC20 and bone alkaline phosphatase involved in the regulation of matrix proteins' phosphorylation could be a means for the development of suitable therapeutic treatments. © 2018 American Society for Bone and Mineral Research.

Proteomic and immunochemical approaches to understanding the glycation behaviour of the casein and β-lactoglobulin fractions of flavoured drinks under UHT processing conditions

Dairy technology used to produce sweetened milk products might introduce additional advanced glycation end products (AGEs) into the diet. These molecular messengers are linked to detrimental health effects. Using a model accurate to the thermal treatment, reducing sugars, main protein content, and prolonged storage of ultra-high-temperature-sterilized (UHT) milk, we studied the behaviour of milk proteins during glycation. Two-dimensional electrophoresis (2-DE) profiles and western blots of glycated total casein revealed the major contributions of αs2-casein and β-casein and the relatively minor contributions of κ-casein towards the formation of Nε-carboxymethyllysine (CML)-positive aggregates. Glycated κ-casein had the lowest furosine (FUR), 5-hydroxymethylfurfural (HMF) and AGEs content. Conversely, the α-casein fraction demonstrated a high susceptibility to glycation, having the highest FUR, HMF and AGE levels. The gel-filtration elution profiles and the corresponding fraction fluorescence revealed that glycated casein aggregates were highly fluorescent, while the β-lactoglobulin glycation profile was similar to that of bovine serum albumin, and fluorescence was detected mainly in tetramers. Although CML is not a cross-linking AGE, it was only detected in large molecular aggregates and not in glycated monomers. Our results also indicate that in casein, glycation-induced changes in the UHT conditions were less deleterious than the subsequent 90 day storage period.

Analysis of Advanced Glycation Endproducts in Rat Tail Collagen and Correlation to Tendon Stiffening

Methylglyoxal is a major 1,2-dicarbonyl compound in vivo and leads to nonenzymatic protein modifications, known as advanced glycation endproducts. Especially long-lived proteins like collagen are prone to changes of the mechanical or biological function, respectively, by accumulation of Maillard-derived modifications. Specifically, the resulting nonenzymatic cross-link structures in parallel to the natural maturation process of collagen fibrils lead to complications with age or during disease. A novel lysine-lysine amide cross-link derived from methylglyoxal, 2,15-diamino-8-methyl-9-oxo-7,10-diaza-1,16-hexadecanedioic acid, named MOLA, was synthesized and identified in vitro and in vivo. Tail tendons of young, adult, and old rats (3, 12, and 22 months) were enzymatically digested prior to analysis of acid-labile glycation products via liquid chromatography-tandem mass spectrometry (LC-MS/MS). As a result, nine monovalent amino acid modifications, mostly originating from methylglyoxal (36 μmol/mol leucine-equivalents in total), and four glycation cross-links (0.72 μmol/mol glucosepane, 0.24 μmol/mol DODIC (3-deoxyglucosone-derived imidazoline cross-link), 0.04 μmol/mol MODIC (methylglyoxal-derived imidazoline cross-link), 0.34 μmol/mol MOLA) were quantitated in senescent tendon collagen. The results correlated with increased tail tendon breaking time from 10 to 190 min and indicate that methylglyoxal is a major player in the aging process of connective tissue.

Extracellular Matrix and Ageing

The extracellular matrix (ECM) provides the environment for many cells types within the body and, in addition to the well recognised role as a structural support, influences many important cell process within the body. As a result, age-related changes to the proteins of the ECM have far reaching consequences with the potential to disrupt many different aspects of homeostasis and healthy function. The proteins collagen and elastin are the most abundant in the ECM and their ability to function as a structural support and provide mechanical stability results from the formation of supra-molecular structures. Collagen and elastin have a long half-life, as required by their structural role, which leaves them vulnerable to a range of post-translational modifications. In this chapter the role of the ECM is discussed and the component proteins introduced. Major age-related modifications including glycation, carbamylation and fragmentation and the impact these have on ECM function are reviewed.

In situ characterization of advanced glycation end products (AGEs) in collagen and model extracellular matrix by solid state NMR

Non-enzymatic glycation of extracellular matrix with (U-13C5)-d-ribose-5-phosphate (R5P), enables in situ 2D ssNMR identification of many deleterious protein modifications and crosslinks, including previously unreported oxalamido and hemiaminal (CH3-CH(OH)NHR) substructures. Changes in charged residue proportions and distribution may be as important as crosslinking in provoking and understanding harmful tissue changes.

Effect on the mechanical properties of type I collagen of intra-molecular lysine-arginine derived advanced glycation end-product cross-linking

Non-enzymatic advanced glycation end product (AGE) cross-linking of collagen molecules has been hypothesised to result in significant changes to the mechanical properties of the connective tissues within the body, potentially resulting in a number of age related diseases. We have investigated the effect of two of these cross-links, glucosepane and DOGDIC, on the tensile and lateral moduli of the collagen molecule through the use of a steered molecular dynamics approach, using previously identified preferential formation sites for intra-molecular cross-links. Our results show that the presence of intra-molecular AGE cross-links increases the tensile and lateral Young's moduli in the low strain domain by between 3.0-8.5% and 2.9-60.3% respectively, with little effect exhibited at higher strains.

Dicarbonyls and Advanced Glycation End-Products in the Development of Diabetic Complications and Targets for Intervention

Advanced glycation end-products (AGEs) are non-enzymatic protein and amino acid adducts as well as DNA adducts which form from dicarbonyls and glucose. AGE formation is enhanced in diabetes and is associated with the development of diabetic complications. In the current review, we discuss mechanisms that lead to enhanced AGE levels in the context of diabetes and diabetic complications. The methylglyoxal-detoxifying glyoxalase system as well as alternative pathways of AGE detoxification are summarized. Therapeutic approaches to interfere with different pathways of AGE formation are presented.

Non-enzymatic glycation mediated structure–function changes in proteins: case of serum albumin

Albumin, a major plasma protein with extraordinary ligand binding properties, transports various ligands ranging from drugs, hormones, fatty acids, and toxins to different tissues and organs in the body.

Effects of 18-month treatment with bazedoxifene on enzymatic immature and mature cross-links and non-enzymatic advanced glycation end products, mineralization, and trabecular microarchitecture of vertebra in ovariectomized monkeys

Bazedoxifene (BZA) is used for the treatment of post-menopausal osteoporosis. To elucidate changes in collagen, mineralization, and structural properties and their relationship to bone strength after treatment with BZA in ovariectomized (OVX) monkeys, the levels of collagen and enzymatic immature, mature, and non-enzymatic cross-links were simultaneously examined, as well as trabecular architecture and mineralization of vertebrae. Adult female cynomolgus monkeys were divided into 4 groups (n=18 each) as follows: Sham group, OVX group, and OVX monkeys given either 0.2 or 0.5mg/kg BZA for 18 months. Collagen concentration, enzymatic and non-enzymatic pentosidine cross-links, whole fluorescent advanced glycation end products (AGEs), trabecular architecture, mineralization, and cancellous bone strength of vertebrae were analyzed. The levels of enzymatic immature and mature cross-links, bone volume (BV/TV), and trabecular thickness (Tb.Th) in BZA-treated groups were significantly higher than those in the OVX control group. In contrast, the trabecular bone pattern factor (TBPf), the structure model index (SMI), the enzymatic cross-link ratio, and the levels of pentosidine and whole AGEs in BZA-treated groups were significantly lower than those in the OVX control group. Stepwise logistic regression analysis revealed that BV/TV, Tb.Th, TbPf, and pentosidine or whole AGEs independently affected ultimate load (model R(2)=0.748, p<0.001) and breaking energy (model R(2)=0.702). Stiffness was affected by Tb.Th, enzymatic immature cross-link levels and their ratio (model R(2)=0.400). Treatment with BZA prevented OVX-induced deterioration in the total levels of immature enzymatic cross-links and AGEs accumulation and structural properties such as BV/TV, Tb.Th, and TbPf, which contribute significantly to vertebral cancellous bone strength.

Effects of Collagen Crosslinking on Bone Material Properties in Health and Disease

Data have accumulated to show that various types of collagen crosslinking are implicated in the health of individuals, as well as in a number of disease states, such as osteoporosis, diabetes mellitus, chronic kidney disease, inflammatory bowel disease, or in conditions of mild hyperhomocysteinemia, or when glucocorticoid use is indicated. Collagen crosslinking is a posttranslational modification of collagen molecules and plays important roles in tissue differentiation and in the mechanical properties of collagenous tissue. The crosslinking of collagen in the body can form via two mechanisms: one is enzymatic crosslinking and the other is nonenzymatic crosslinking. Lysyl hydroxylases and lysyl oxidases regulate tissue-specific crosslinking patterns and quantities. Enzymatic crosslinks initially form via immature divalent crosslinking, and a portion of them convert into mature trivalent forms such as pyridinoline and pyrrole crosslinks. Nonenzymatic crosslinks form as a result of reactions which create advanced glycation end products (AGEs), such as pentosidine and glucosepane. These types of crosslinks differ in terms of their mechanisms of formation and function. Impaired enzymatic crosslinking and/or an increase of AGEs have been proposed as a major cause of bone fragility associated with aging and numerous disease states. This review focuses on the effects of collagen crosslinking on bone material properties in health and disease.

AGEs-Induced IL-6 Synthesis Precedes RAGE Up-Regulation in HEK 293 Cells: An Alternative Inflammatory Mechanism?

Advanced glycation end products (AGEs) can activate the inflammatory pathways involved in diabetic nephropathy. Understanding these molecular pathways could contribute to therapeutic strategies for diabetes complications. We evaluated the modulation of inflammatory and oxidative markers, as well as the protective mechanisms employed by human embryonic kidney cells (HEK 293) upon exposure to 200 μg/mL bovine serum albumine (BSA) or AGEs–BSA for 12, 24 and 48 h. The mRNA and protein expression levels of AGEs receptor (RAGE) and heat shock proteins (HSPs) 27, 60 and 70, the activity of antioxidant enzymes and the expression levels of eight cytokines were analysed. Cell damage via oxidative mechanisms was evaluated by glutathione and malondialdehyde levels. The data revealed two different time scale responses. First, the up-regulation of interleukin-6 (IL-6), HSP 27 and high catalase activity were detected as early as 12 h after exposure to AGEs–BSA, while the second response, after 24 h, consisted of NF-κB p65, RAGE, HSP 70 and inflammatory cytokine up-regulation, glutathione depletion, malondialdehyde increase and the activation of antioxidant enzymes. IL-6 might be important in the early ignition of inflammatory responses, while the cellular redox imbalance, RAGE activation and NF-κB p65 increased expression further enhance inflammatory signals in HEK 293 cells.

Glycation of human cortical and cancellous bone captures differences in the formation of Maillard reaction products between glucose and ribose

To better understand some aspects of bone matrix glycation, we used an in vitro glycation approach. Within two weeks, our glycation procedures led to the formation of advanced glycation end products (AGEs) at the levels that corresponded to approx. 25-30 years of the natural in vivo glycation. Cortical and cancellous bones from human tibias were glycated in vitro using either glucose (glucosylation) or ribose (ribosylation). Both glucosylation and ribosylation led to the formation of higher levels of AGEs and pentosidine (PEN) in cancellous than cortical bone dissected from all tested donors (young, middle-age and elderly men and women). More efficient glycation of bone matrix proteins in cancellous bone most likely depended on the higher porosity of this tissue, which facilitated better accessibility of the sugars to the matrix proteins. Notably, glycation of cortical bone from older donors led to much higher AGEs levels as compared to young donors. Such efficient in vitro glycation of older cortical bone could result from aging-related increase in porosity caused by the loss of mineral content. In addition, more pronounced glycation in vivo would be driven by elevated oxidation processes. Interestingly, the levels of PEN formation differed pronouncedly between glucosylation and ribosylation. Ribosylation generated very high levels of PEN (approx. 6- vs. 2.5-fold higher PEN level than in glucosylated samples). Kinetic studies of AGEs and PEN formation in human cortical and cancellous bone matrix confirmed higher accumulation of fluorescent crosslinks for ribosylation. Our results suggest that in vitro glycation of bone using glucose leads to the formation of lower levels of AGEs including PEN, whereas ribosylation appears to support a pathway toward PEN formation. Our studies may help to understand differences in the progression of bone pathologies related to protein glycation by different sugars, and raise awareness for excessive sugar supplementation in food and drinks.

Proteomic characterization of intermediate and advanced glycation end-products in commercial milk samples

The Maillard reaction consists of a number of chemical processes affecting the structure of the proteins present in foods. We previously accomplished the proteomic characterization of the lactosylation targets in commercial milk samples. Although characterizing the early modification derivatives, this analysis did not describe the corresponding advanced glycation end-products (AGEs), which may be formed from the further oxidation of former ones or by reaction of oxidized sugars with proteins, when high temperatures are exploited. To fill this gap, we have used combined proteomic procedures for the systematic characterization of the lactosylated and AGE-containing proteins from the soluble and milk fat globule membrane fraction of various milk products. Besides to confirm all lactulosyl-lysines described previously, 40 novel lactosylation sites were identified. More importantly, 308 additional intermediate and advanced glyco-oxidation derivatives (including cross-linking adducts) were characterized in 31 proteins, providing the widest qualitative inventory of modified species ascertained in commercial milk samples so far. Amadori adducts with glucose/galactose, their dehydration products, carboxymethyllysine and glyoxal-, 3-deoxyglucosone/3-deoxygalactosone- and 3-deoxylactosone-derived dihydroxyimidazolines and/or hemiaminals were the most frequent derivatives observed. Depending on thermal treatment, a variable number of modification sites was identified within each protein; their number increased with harder food processing conditions. Among the modified proteins, species involved in assisting the delivery of nutrients, defense response against pathogens and cellular proliferation/differentiation were highly affected by AGE formation. This may lead to a progressive decrease of the milk nutritional value, as it reduces the protein functional properties, abates the bioavailability of the essential amino acids and eventually affects food digestibility. These aspects are of particular importance in products intended for infant diet, such as milk powders and infant formulas.

BIOLOGICAL SIGNIFICANCE

We used combined shotgun proteomic procedures for the systematic characterization of intermediate and advanced glycoxidation protein products in various raw and commercial milk samples. Several hundreds of modified species were characterized as deriving from 31 milk proteins, providing the widest qualitative inventory of assigned components in this fluid. Amadori adducts with glucose/galactose, their dehydration products, carboxymethyl-lysine, and glyoxal-, 3-deoxyglucosone/3-deoxygalactosone- and 3-deoxylactosone-derived dihydroxyimidazolines and/or hemiaminals were the most frequent derivatives observed. Proteins involved in nutrient delivery, defense response against pathogens and cellular proliferation/differentiation were highly subjected to intermediate and advanced glyco-oxidation modification. This may lead to a progressive decrease of the milk nutritional value, as it reduces the protein functional properties, diminishes the bioavailability of the essential amino acids, eventually affects food digestibility and determines a potential increase of specific allergens. These information are important points of interest to connect the extent of the Maillard reaction present in different commercial samples with the potential nutritional aspects mentioned above. These themes have to be fully evaluated in a next future for a complete estimation of the nutritional and toxicological properties of the dairy products deriving from severe heat processing.

Insulin-like growth factor 1, glycation and bone fragility: implications for fracture resistance of bone

Despite our extensive knowledge of insulin-like growth factor 1 (IGF1) action on the growing skeleton, its role in skeletal homeostasis during aging and age-related development of certain diseases is still unclear. Advanced glycation end products (AGEs) derived from glucose are implicated in osteoporosis and a number of diabetic complications. We hypothesized that because in humans and rodents IGF1 stimulates uptake of glucose (a glycation substrate) from the bloodstream in a dose-dependent manner, the decline of IGF1 could be associated with the accumulation of glycation products and the decreasing resistance of bone to fracture. To test the aforementioned hypotheses, we used human tibial posterior cortex bone samples to perform biochemical (measurement of IGF1, fluorescent AGEs and pentosidine (PEN) contents) and mechanical tests (crack initiation and propagation using compact tension specimens). Our results for the first time show a significant, age-independent association between the levels of IGF1 and AGEs. Furthermore, AGEs (fAGEs, PEN) predict propensity of bone to fracture (initiation and propagation) independently of age in human cortical bone. Based on these results we propose a model of IGF1-based regulation of bone fracture. Because IGF1 level increases postnatally up to the juvenile developmental phase and decreases thereafter with aging, we propose that IGF1 may play a protective role in young skeleton and its age-related decline leads to bone fragility and an increased fracture risk. Our results may also have important implications for current understanding of osteoporosis- and diabetes-related bone fragility as well as in the development of new diagnostic tools to screen for fragile bones.

Collagen modifications in postmenopausal osteoporosis: advanced glycation endproducts may affect bone volume, structure and quality

The classic model of postmenopausal osteoporosis (PM-OP) starts with the depletion of estrogen, which in turn stimulates imbalanced bone remodeling, resulting in loss of bone mass/volume. Clinically, this leads to fractures because of structural weakness. Recent work has begun to provide a more complete picture of the mechanisms of PM-OP involving oxidative stress and collagen modifications known as advanced glycation endproducts (AGEs). On one hand, AGEs may drive imbalanced bone remodeling through signaling mediated by the receptor for AGEs (RAGE), stimulating resorption and inhibiting formation. On the other hand, AGEs are associated with degraded bone material quality. Oxidative stress promotes the formation of AGEs, inhibits normal enzymatically derived crosslinking and can degrade collagen structure, thereby reducing fracture resistance. Notably, there are multiple positive feedback loops that can exacerbate the mechanisms of PM-OP associated with oxidative stress and AGEs. Anti-oxidant therapies may have the potential to inhibit the oxidative stress based mechanisms of this disease.

Fucoidan as an inhibitor of thermally induced collagen glycation examined by acetate electrophoresis

Non-enzymatic glycation (Maillard reaction) in vitro could be a simple method to obtain glycoconjugates for studying their biological properties. Hence, fucoidan was retained by acetate electrophoresis indicating a strong interaction with the protein. A loss of colour in fucoidan bands was found for samples incubated with collagen as compared with samples of free fucoidan. Also under in vitro conditions at 100°C - simulating a sudden burn incident - fucoidan binds with collagen as a result of the Maillard reaction. In contrast, the colour of the fucoidan bands intensified for samples incubated with collagen, with the addition of glucose. Electrophoretic analyses were carried out after heating the samples to a temperature simulating a burn incident. The bands were found to intensify for samples incubated with collagen during a 30-day-long incubation. Thus, spontaneous in vitro glycation - i.e. without the addition of glucose - was confirmed. This process is highly intensified both by the temperature and time of incubation. For a sample incubated in vitro in a fucoidan solution containing glucose, glycation was confirmed in a preliminary FTIR and acetate electrophoresis examinations, occurring in collagen obtained from chicken skins. In particular, a new band emerging around 1746 cm(-1) was observed for above samples, as was its increasing intensity, as compared with samples without the addition of glucose. In the collagen glycation assay, while glucose reacts with collagen and forms cross-linked aggregates, fucoidan decreases the process of aggregation and recovery of native collagen.

A new glycation product 'norpronyl-lysine,' and direct characterization of cross linking and other glycation adducts: NMR of model compounds and collagen

NMR is ideal for characterizing non-enzymatic protein glycation, including AGEs (advanced glycation endproducts) underlying tissue pathologies in diabetes and ageing. Ribose, R5P (ribose-5-phosphate) and ADPR (ADP-ribose), could be significant and underinvestigated biological glycating agents especially in chronic inflammation. Using [U-¹³C]ribose we have identified a novel glycoxidation adduct, 5-deoxy-5-desmethylpronyl-lysine, 'norpronyl-lysine', as well as numerous free ketones, acids and amino group reaction products. Glycation by R5P and ADPR proceeds rapidly with R5P generating a brown precipitate with PLL (poly-L-lysine) within hours. ssNMR (solid-state NMR) ¹³C-¹³C COSY identifies several crosslinking adducts such as the newly identified norpronyl-lysine, in situ, from the glycating reaction of ¹³C₅-ribose with collagen. The same adducts are also identifiable after reaction of collagen with R5P. We also demonstrate for the first time bio-amine (spermidine, N-acetyl lysine, PLL) catalysed ribose 2-epimerization to arabinose at physiological pH. This work raises the prospect of advancing understanding of the mechanisms and consequences of glycation in actual tissues, in vitro or even ex vivo, using NMR isotope-labelled glycating agents, without analyses requiring chemical or enzymatic degradations, or prior assumptions about glycation products.

Advanced glycoxidation and lipoxidation end products (AGEs and ALEs): an overview of their mechanisms of formation

Advanced lipoxidation end products (ALEs) and advanced glycation end products (AGEs) have a pathogenetic role in the development and progression of different oxidative-based diseases including diabetes, atherosclerosis, and neurological disorders. AGEs and ALEs represent a quite complex class of compounds that are formed by different mechanisms, by heterogeneous precursors and that can be formed either exogenously or endogenously. There is a wide interest in AGEs and ALEs involving different aspects of research which are essentially focused on set-up and application of analytical strategies (1) to identify, characterize, and quantify AGEs and ALEs in different pathophysiological conditions; (2) to elucidate the molecular basis of their biological effects; and (3) to discover compounds able to inhibit AGEs/ALEs damaging effects not only as biological tools aimed at validating AGEs/ALEs as drug target, but also as promising drugs. All the above-mentioned research stages require a clear picture of the chemical formation of AGEs/ALEs but this is not simple, due to the complex and heterogeneous pathways, involving different precursors and mechanisms. In view of this intricate scenario, the aim of the present review is to group the main AGEs and ALEs and to describe, for each of them, the precursors and mechanisms of formation.

Wheat bran biorefinery--a detailed investigation on hydrothermal and enzymatic treatment

Due to the enormous quantities arising in the milling industry and its specific properties, wheat bran can be considered as a feedstock for future biorefineries. In the present work, a detailed investigation was carried out on the hydrothermal (140-200°C) and enzymatic treatment of wheat bran. After hydrothermal pretreatment and a subsequent enzymatic hydrolysis a glucose yield of 65% and 90% was achieved, respectively. Interestingly, the hemicelluloses could be disintegrated to monomers only to approx. 50%. About 70% of the proteins were dissolved, however, practically no free amino acids were obtained under given conditions. Severe treatment conditions induce elevated losses of some amino acids. Minerals could be extracted almost completely. To disintegrate fat into glycerol and fatty acids severe process conditions were necessary. The formation of undesired by-products such as furfural or hydroxymethylfurfural starts at approx. 180°C.

In vitro non-enzymatic ribation reduces post-yield strain accommodation in cortical bone

Non-enzymatic glycation (NEG) and advanced glycation endproducts (AGEs) may contribute to bone fragility in various diseases, ageing, and other conditions by modifying bone collagen and causing degraded mechanical properties. In this study, we sought to further understand how collagen modification in an in vitro non-enzymatic ribation model leads to loss of cortical bone toughness. Previous in vitro studies using non-enzymatic ribation reported loss of ductility in the cortical bone. Increased crosslinking is most commonly blamed for these changes; however, some studies report positive correlations between measures of total collagen crosslinking and work-to-fracture/toughness measurements whilst correlations between general NEG and measures of ductility are often negative. Fifteen bone beam triplets were cut from bovine metatarsi. Each provided one native non-incubated control, one incubated control and one ribated specimen. Incubation involved simulated body fluid±ribose for fourteen days at 37°C. Pentosidine and pyridinoline crosslinks were measured using HPLC. Three-point bending tests quantified mechanical properties. Fracture surfaces were examined using scanning electron microscopy. The effects of ribation on bone collagen molecular stability and intermolecular connectivity were investigated using differential scanning calorimetry and hydrothermal isometric tension testing. Ribation caused increased non-enzymatic collagen modification and pentosidine content (16mmol/mol collagen) and inferior post-yield mechanical behaviour, especially post-yield strain and flexural toughness. Fracture surfaces were smoother with less collagen fibril deformation or tearing than observed in controls. In the ribated group only, pentosidine content and thermomechanical measures of crosslinking were positively correlated with measures of strain accommodation and energy absorption before failure. Non-enzymatic ribation and the resulting modifications reduce cortical bone pseudo-plasticity through a reduced capacity for post-yield strain accommodation. However, the positive correlations we have found suggest that increased crosslinking may not provide a complete explanation for this embrittlement.

Effects of pretreatment of wheat bran on the quality of protein-rich residue for animal feeding and on monosaccharide release for ethanol production

The effects of hydrothermal conditions for pretreating wheat bran on the quality of residual protein for animal feeding, and on monosaccharide release for ethanol production were studied according to a 4×2×2 design with the factors, temperature (120, 140, 160, and 180°C), acidity (pH 2.3 and 3.9), and retention time (5 and 10 min). Temperature affected the quality of residual protein for animal feeding. Pretreatment at 120 and 140°C did not affect O-methylisourea-reactive lysine in protein-rich wheat bran residue, although total lysine decreased with increasing temperature at pH 2.3. At temperatures higher than 140°C, reactive lysine decreased and melanoidins, furfural and 5-HMF increased. Lower acidity during pretreatment at 120 and 140°C increased the digestibility of the residual wheat protein in vitro by 36%. Pretreatment conditions did not substantially affect the release of monomeric xylose and arabinose by hemicellulases, which suggests that arabinoxylans in wheat bran are well accessible for enzymes.

Enhanced levels of non-enzymatic glycation and pentosidine crosslinking in spontaneous osteoarthritis progression

OBJECTIVE

To test the hypothesis that heightened advanced glycation endproducts (AGEs) content in cartilage accelerates the progression of spontaneous osteoarthritis (OA) in the Hartley guinea pig (HGP) model.

METHODS

Twenty-eight male, 3-month-old HGPs were used. Eight were left untreated as a baseline control group and sacrificed at 3 months of age (n = 4) and 9 months of age (n = 4; age-matched controls). The other 20 HGPs received intra-articular knee injections in the right knee whereas the left knees acted as contra-lateral non-injected controls. Injections consisted of 100 μl phosphate buffered saline (PBS; n = 10) or PBS+2.0 M D-(-)-Ribose (n = 10). Injections were given once weekly for 24 weeks. At the end of the treatment period, the tibiae were fixed with formalin, scanned with microCT for sub-chondral bone mineral density, and then histological slides were prepared, stained with Safranin-O with Fast Green counter stain and scored using the OARSI-HISTOgp scheme. Cartilage pentosidine (established biomarker for AGEs) content, collagen content (% dry mass), glucosaminoglycan GAG-to-collagen ratio (μg/μg), GAG-to-DNA ratio and DNA-to-collagen ratio were measured.

RESULTS

Pentosidine content increased greatly due to PBS + Ribose injection (P < 0.0001) and reached levels found in cartilage from 80-year-old humans. Surprisingly, mean OARSI-HISTOgp scores for both the injected and contra-lateral controls in the PBS + Ribose group were not detectably different, nor were they different from the mean score for the age-matched control group.

CONCLUSION

AGEs accumulation due to intra-articular ribose-containing injections in the HGP model of spontaneous knee OA did not enhance disease progression.