Methylglyoxal inhibits the binding step of collagen phagocytosis

Methylglyoxal alters collagen fibril nanostiffness and surface potential

Collagen fibrils are fundamental to the mechanical strength and function of biological tissues. However, they are susceptible to changes from non-enzymatic glycation, resulting in the formation of advanced glycation end-products (AGEs) that are not reversible. AGEs accumulate with aging and disease and can adversely impact tissue mechanics and cell-ECM interactions. AGE-crosslinks have been related, on the one hand, to dysregulation of collagen fibril stiffness and damage and, on the other hand, to altered collagen net surface charge as well as impaired cell recognition sites. While prior studies using Kelvin probe force microscopy (KPFM) have shown the effect glycation has on collagen fibril surface potential (i.e., net charge), the combined effect on individual and isolated collagen fibril mechanics, hydration, and surface potential has not been documented. Here, we explore how methylglyoxal (MGO) treatment affects the mechanics and surface potential of individual and isolated collagen fibrils by utilizing atomic force microscopy (AFM) nanoindentation and KPFM. Our results reveal that MGO treatment significantly increases nanostiffness, alters surface potential, and modifies hydration characteristics at the collagen fibril level. These findings underscore the critical impact of AGEs on collagen fibril physicochemical properties, offering insights into pathophysiological mechanical and biochemical alterations with implications for cell mechanotransduction during aging and in diabetes. STATEMENT OF SIGNIFICANCE: Collagen fibrils are susceptible to glycation, the irreversible reaction of amino acids with sugars. Glycation affects the mechanical properties and surface chemistry of collagen fibrils with adverse alterations in biological tissue mechanics and cell-ECM interactions. Current research on glycation, at the level of individual collagen fibrils, is sparse and has focused either on collagen fibril mechanics, with contradicting evidence, or surface potential. Here, we utilized a multimodal approach combining Kelvin probe force (KPFM) and atomic force microscopy (AFM) to examine how methylglyoxal glycation induces structural, mechanical, and surface potential changes on the same individual and isolated collagen fibrils. This approach helps inform structure-function relationships at the level of individual collagen fibrils.

Glycation-induced age-related illnesses, antiglycation and drug delivery strategies

OBJECTIVES

Ageing is a major cause of multiple age-related diseases. Several mechanisms have been reported to contribute to these abnormalities including glycation, oxidative stress, the polyol pathway and osmotic stress. Glycation, unlike glycosylation, is an irregular biochemical reaction to the formation of active advanced glycation end-products (AGEs), which are considered to be one of the causes of these chronic diseases. This study provides a recent and comprehensive review on the possible causes, mechanisms, types, analytical techniques, diseases and treatments of the toxic glycation end products.

KEY FINDINGS

Several mechanisms have been found to play a role in generating hyperglycaemia-induced oxidative stress including an increase in the levels of reactive oxygen species (ROS), increase in the levels of AGEs, binding of AGEs and their receptors (RAGE) and the polyol pathway and thus have been investigated as promising novel targets.

SUMMARY

This review focuses on the key mechanisms attributed to cumulative increases of glycation and pathological RAGE expression as a significant cause of multiple age-related diseases, and reporting on different aspects of antiglycation therapy as a novel approach to managing/treating age-related diseases. Additionally, historical, current and possible future antiglycation approaches will be presented focussing on novel drug delivery methods.

DDR1 associates with TRPV4 in cell-matrix adhesions to enable calcium-regulated myosin activity and collagen compaction

Tissue fibrosis manifests as excessive deposition of compacted, highly aligned collagen fibrils, which interfere with organ structure and function. Cells in collagen-rich lesions often exhibit marked overexpression of discoidin domain receptor 1 (DDR1), which is linked to increased collagen compaction through the association of DDR1 with the Ca²⁺ -dependent nonmuscle myosin IIA (NMIIA). We examined the functional relationship between DDR1 and the transient receptor potential vanilloid type 4 (TRPV4) channel, a Ca²⁺ -permeable ion channel that is implicated in collagen compaction. Fibroblasts expressing high levels of DDR1 were used to model cells in lesions with collagen compaction. In these cells, the expression of the β1 integrin was deleted to simplify studies of DDR1 function. Compared with DDR1 wild-type cells, high DDR1 expression was associated with increased Ca²⁺ influx through TRPV4, enrichment of TRPV4 in collagen adhesions, and enhanced contractile activity mediated by NMIIA. At cell adhesion sites to collagen, DDR1 associated with TRPV4, which enhanced DDR1-mediated collagen alignment and compaction. We conclude that DDR1 regulates Ca²⁺ influx through the TRPV4 channel to promote critical, DDR1-mediated processes that are important in lesions with collagen compaction and alignment.

Evaluation of the nephrotoxicity and safety of low-dose aristolochic acid, extending to the use of Xixin (Asurum), by determination of methylglyoxal and d-lactate

ETHNOPHARMACOLOGICAL RELEVANCE

Most Aristolochiaceae plants are prohibited due to aristolochic acid nephropathy (AAN), except Xixin (Asarum spp.). Xixin contains trace amounts of aristolochic acid (AA) and is widely used in Traditional Chinese Medicine. Methylglyoxal and d-lactate are regarded as biomarkers for nephrotoxicity.

AIM OF THE STUDY

The use of Xixin (Asarum spp.) is essential and controversial. This study aimed to evaluate tubulointerstitial injury and interstitial renal fibrosis by determining urinary methylglyoxal and d-lactate after withdrawal of low-dose AA in a chronic mouse model.

MATERIALS AND METHODS

C3H/He mice in the AA group (n = 24/group) were given ad libitum access to distilled water containing 3 μg/mL AA (0.5 mg/kg/day) for 56 days and drinking water from days 57 to 84. The severity of tubulointerstitial injury and fibrosis were evaluated using the tubulointerstitial histological score (TIHS) and Masson's trichrome staining. Urinary and serum methylglyoxal were determined by high-performance liquid chromatography (HPLC); urinary d-lactate were determined by column-switching HPLC.

RESULTS

After AA withdrawal, serum methylglyoxal in the AA group increased from day 56 (429.4 ± 48.3 μg/L) to 84 (600.2 ± 99.9 μg/L), and peaked on day 70 (878.3 ± 171.8 μg/L; p < 0.05); TIHS and fibrosis exhibited similar patterns. Urinary methylglyoxal was high on day 56 (3.522 ± 1.061 μg), declined by day 70 (1.583 ± 0.437 μg) and increased by day 84 (2.390 ± 0.130 μg). Moreover, urinary d-lactate was elevated on day 56 (82.10 ± 18.80 μg) and higher from day 70 (201.10 ± 90.82 μg) to 84 (193.28 ± 61.32 μg).

CONCLUSIONS

Methylglyoxal is induced after AA-induced tubulointerstitial injury, so methylglyoxal excretion and metabolism may be a detoxification and repair strategy. A low cumulative AA dose is the key factor that limits tubulointerstitial injury and helps to repair. Thus, AA-containing herbs, especially Xixin, should be used at low doses for short durations (less than one month).

Collagen Biosynthesis, Processing, and Maturation in Lung Ageing

In addition to providing a macromolecular scaffold, the extracellular matrix (ECM) is a critical regulator of cell function by virtue of specific physical, biochemical, and mechanical properties. Collagen is the main ECM component and hence plays an essential role in the pathogenesis and progression of chronic lung disease. It is well-established that many chronic lung diseases, e.g., chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF) primarily manifest in the elderly, suggesting increased susceptibility of the aged lung or accumulated alterations in lung structure over time that favour disease. Here, we review the main steps of collagen biosynthesis, processing, and turnover and summarise what is currently known about alterations upon lung ageing, including changes in collagen composition, modification, and crosslinking. Recent proteomic data on mouse lung ageing indicates that, while the ER-resident machinery of collagen biosynthesis, modification and triple helix formation appears largely unchanged, there are specific changes in levels of type IV and type VI as well as the two fibril-associated collagens with interrupted triple helices (FACIT), namely type XIV and type XVI collagens. In addition, levels of the extracellular collagen crosslinking enzyme lysyl oxidase are decreased, indicating less enzymatically mediated collagen crosslinking upon ageing. The latter contrasts with the ageing-associated increase in collagen crosslinking by advanced glycation endproducts (AGEs), a result of spontaneous reactions of protein amino groups with reactive carbonyls, e.g., from monosaccharides or reactive dicarbonyls like methylglyoxal. Given the slow turnover of extracellular collagen such modifications accumulate even more in ageing tissues. In summary, the collective evidence points mainly toward age-induced alterations in collagen composition and drastic changes in the molecular nature of collagen crosslinks. Future work addressing the consequences of these changes may provide important clues for prevention of lung disease and for lung bioengineering and ultimately pave the way to novel targeted approaches in lung regenerative medicine.

Vimentin tunes cell migration on collagen by controlling β1 integrin activation and clustering

Vimentin is a structural protein that is required for mesenchymal cell migration and directly interacts with actin, β1 integrin and paxillin. We examined how these interactions enable vimentin to regulate cell migration on collagen. In fibroblasts, depletion of vimentin increased talin-dependent activation of β1 integrin by more than 2-fold. Loss of vimentin was associated with reduction of β1 integrin clustering by 50% and inhibition of paxillin recruitment to focal adhesions by more than 60%, which was restored by vimentin expression. This reduction of paxillin was associated with 65% lower Cdc42 activation, a 60% reduction of cell extension formation and a greater than 35% decrease in cell migration on collagen. The activation of PAK1, a downstream effector of Cdc42, was required for vimentin phosphorylation and filament maturation. We propose that vimentin tunes cell migration through collagen by acting as an adaptor protein for focal adhesion proteins, thereby regulating β1 integrin activation, resulting in well-organized, mature integrin clusters.This article has an associated First Person interview with the first author of the paper.

Attenuation of methylglyoxal-induced glycation and cellular dysfunction in wound healing by Centella cordifolia

Current pre-clinical evidences of Centella focus on its pharmacological effects on normal wound healing but there are limited studies on the bioactivity of Centella in cellular dysfunction associated with diabetic wounds. Hence we planned to examine the potential of Centella cordifolia in inhibiting methylglyoxal (MGO)-induced extracellular matrix (ECM) glycation and promoting the related cellular functions. A Cell-ECM adhesion assay examined the ECM glycation induced by MGO. Different cell types that contribute to the healing process (fibroblasts, keratinocytes and endothelial cells) were evaluated for their ability to adhere to the glycated ECM. Methanolic extract of Centella species was prepared and partitioned to yield different solvent fractions which were further analysed by high performance liquid chromatography equipped with photodiode array detector (HPLC-PDA) method. Based on the antioxidant [2,2-diphenyl-1-picrylhydrazyl (DPPH) assay] screening, anti-glycation activity and total phenolic content (TPC) of the different Centella species and fractions, the ethyl acetate fraction of C. cordifolia was selected for further investigating its ability to inhibit MGO-induced ECM glycation and promote cellular distribution and adhesion. Out of the three Centella species (C. asiatica, C. cordifolia and C. erecta), the methanolic extract of C. cordifolia showed maximum inhibition of Advanced glycation end products (AGE) fluorescence (20.20 ± 4.69 %, 25.00 ± 3.58 % and 16.18 ± 1.40 %, respectively). Its ethyl acetate fraction was enriched with phenolic compounds (3.91 ± 0.12 mg CAE/μg fraction) and showed strong antioxidant (59.95 ± 7.18 μM TE/μg fraction) and antiglycation activities. Improvement of cells spreading and adhesion of endothelial cells, fibroblasts and keratinocytes was observed for ethyl acetate treated MGO-glycated extracellular matrix. Significant reduction in attachment capacity of EA.hy926 cells seeded on MGO-glycated fibronectin (41.2%) and attachment reduction of NIH3t3 and HaCaT cells seeded on MGO-glycated collagen (33.7% and 24.1%, respectively) were observed. Our findings demonstrate that ethyl acetate fraction of C. cordifolia was effective in attenuating MGO-induced glycation and cellular dysfunction in the in-vitro wound healing models suggesting that C. cordifolia could be a potential candidate for diabetic wound healing. It could be subjected for further isolation of new phytoconstituents having potential diabetic wound healing properties.

Modulatory Effect of Glycated Collagen on Oral Streptococcal Nanoadhesion

Biofilm-mediated oral diseases such as dental caries and periodontal disease remain highly prevalent in populations worldwide. Biofilm formation initiates with the attachment of primary colonizers onto surfaces, and in the context of caries, the adhesion of oral streptococci to dentinal collagen is crucial for biofilm progression. It is known that dentinal collagen suffers from glucose-associated crosslinking as a function of aging or disease; however, the effect of collagen crosslinking on the early adhesion and subsequent biofilm formation of relevant oral streptococci remains unknown. Therefore, the aim of this work was to determine the impact of collagen glycation on the initial adhesion of primary colonizers such as Streptococcus mutans UA159 and Streptococcus sanguinis SK 36, as well as its effect on the early stages of streptococcal biofilm formation in vitro. Type I collagen matrices were crosslinked with either glucose or methylglyoxal. Atomic force microscopy nanocharacterization revealed morphologic and mechanical changes within the collagen matrix as a function of crosslinking, such as a significantly increased elastic modulus in crosslinked fibrils. Increased nanoadhesion forces were observed for S. mutans on crosslinked collagen surfaces as compared with the control, and retraction curves obtained for both streptococcal strains demonstrated nanoscale unbinding behavior consistent with bacterial adhesin-substrate coupling. Overall, glucose-crosslinked substrates specifically promoted the initial adhesion, biofilm formation, and insoluble extracellular polysaccharide production of S. mutans, while methylglyoxal treatment reduced biofilm formation for both strains. Changes in the adhesion behavior and biofilm formation of oral streptococci as a function of collagen glycation could help explain the biofilm dysbiosis seen in older people and patients with diabetes. Further studies are necessary to determine the influence of collagen crosslinking on the balance between acidogenic and nonacidogenic streptococci to aid in the development of novel preventive and therapeutic treatment against dental caries in these patients.

The Diabetic Cardiac Fibroblast: Mechanisms Underlying Phenotype and Function

Diabetic cardiomyopathy involves remodeling of the heart in response to diabetes that includes microvascular damage, cardiomyocyte hypertrophy, and cardiac fibrosis. Cardiac fibrosis is a major contributor to diastolic dysfunction that can ultimately result in heart failure with preserved ejection fraction. Cardiac fibroblasts are the final effector cell in the process of cardiac fibrosis. This review article aims to describe the cardiac fibroblast phenotype in response to high-glucose conditions that mimic the diabetic state, as well as to explain the pathways underlying this phenotype. As such, this review focuses on studies conducted on isolated cardiac fibroblasts. We also describe molecules that appear to oppose the pro-fibrotic actions of high glucose on cardiac fibroblasts. This represents a major gap in knowledge in the field that needs to be addressed.

Effect of prednisolone on glyoxalase 1 in an inbred mouse model of aristolochic acid nephropathy using a proteomics method with fluorogenic derivatization-liquid chromatography-tandem mass spectrometry

Prednisolone is involved in glucose homeostasis and has been used for treatment for aristolochic acid (AA) nephropathy (AAN), but its effect on glycolysis in kidney has not yet been clarified. This study aims to investigate the effect in terms of altered proteins after prednisolone treatment in a mice model of AAN using a proteomics technique. The six-week C3H/He female mice were administrated AA (0.5 mg/kg/day) for 56 days. AA+P group mice were then given prednisolone (2 mg/kg/day) via oral gavage for the next 14 days, and AA group mice were fed water instead. The tubulointerstitial damage was improved after prednisolone treatment comparing to that of AA group. Kidney homogenates were harvested to perform the proteomics analysis with fluorogenic derivatization-liquid chromatography-tandem mass spectrometry method (FD-LC-MS/MS). On the other hand, urinary methylglyoxal and D-lactate levels were determined by high performance liquid chromatography with fluorescence detection. There were 47 altered peaks and 39 corresponding proteins on day 14 among the groups, and the glycolysis-related proteins, especially glyoxalase 1 (GLO1), fructose-bisphosphate aldolase B (aldolase B), and triosephosphate isomerase (TPI), decreased in the AA+P group. Meanwhile, prednisolone decreased the urinary amount of methylglyoxal (AA+P: 2.004 ± 0.301 μg vs. AA: 2.741 ± 0.630 μg, p < 0.05), which was accompanied with decrease in urinary amount of D-lactate (AA+P: 54.07 ± 5.45 μmol vs. AA: 86.09 ± 8.44 μmol, p < 0.05). Prednisolone thus alleviated inflammation and interstitial renal fibrosis. The renal protective mechanism might be associated with down-regulation of GLO1 via reducing the contents of methylglyoxal derived from glycolysis. With the aid of proteomics analysis and the determination of methylglyoxal and its metabolite-D-lactate, we have demonstrated for the first time the biochemical efficacy of prednisolone, and urinary methylglyoxal and its metabolite-D-lactate might be potential biomarkers for AAN.

Cell-Matrix Interactions and Matricrine Signaling in the Pathogenesis of Vascular Calcification

Vascular calcification is a complex pathological process occurring in patients with atherosclerosis, type 2 diabetes, and chronic kidney disease. The extracellular matrix, via matricrine-receptor signaling plays important roles in the pathogenesis of calcification. Calcification is mediated by osteochondrocytic-like cells that arise from transdifferentiating vascular smooth muscle cells. Recent advances in our understanding of the plasticity of vascular smooth muscle cell and other cells of mesenchymal origin have furthered our understanding of how these cells transdifferentiate into osteochondrocytic-like cells in response to environmental cues. In the present review, we examine the role of the extracellular matrix in the regulation of cell behavior and differentiation in the context of vascular calcification. In pathological calcification, the extracellular matrix not only provides a scaffold for mineral deposition, but also acts as an active signaling entity. In recent years, extracellular matrix components have been shown to influence cellular signaling through matrix receptors such as the discoidin domain receptor family, integrins, and elastin receptors, all of which can modulate osteochondrocytic differentiation and calcification. Changes in extracellular matrix stiffness and composition are detected by these receptors which in turn modulate downstream signaling pathways and cytoskeletal dynamics, which are critical to osteogenic differentiation. This review will focus on recent literature that highlights the role of cell-matrix interactions and how they influence cellular behavior, and osteochondrocytic transdifferentiation in the pathogenesis of cardiovascular calcification.

Tannerella forsythia-produced methylglyoxal causes accumulation of advanced glycation endproducts to trigger cytokine secretion in human monocytes

The periodontal pathogen Tannerella forsythia has the unique ability to produce methylglyoxal (MGO), an electrophilic compound which can covalently modify amino acid side chains and generate inflammatory adducts known as advanced glycation endproducts (AGEs). In periodontitis, concentrations of MGO in gingival-crevicular fluid are increased and are correlated with the T. forsythia load. However, the source of MGO and the extent to which MGO may contribute to periodontal inflammation has not been fully explored. In this study we identified a functional homolog of the enzyme methylglyoxal synthase (MgsA) involved in the production of MGO in T. forsythia. While wild-type T.forsythia produced a significant amount of MGO in the medium, a mutant lacking this homolog produced little to no MGO. Furthermore, compared with the spent medium of the T. forsythia parental strain, the spent medium of the T. forsythia mgsA-deletion strain induced significantly lower nuclear factor-kappa B activity as well as proinflammogenic and pro-osteoclastogenic cytokines from THP-1 monocytes. The ability of T. forsythia to induce protein glycation endproducts via MGO was confirmed by an electrophoresis-based collagen chain mobility shift assay. Together these data demonstrated that T. forsythia produces MGO, which may contribute to inflammation via the generation of AGEs and thus act as a potential virulence factor of the bacterium.

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.

Intracellular Accumulation of Methylglyoxal by Glyoxalase 1 Knock Down Alters Collagen Homoeostasis in L6 Myoblasts

Hyperglycemia results in accumulation of the reactive dicarbonyl methylglyoxal (MG). Methylglyoxal is detoxified by the glyoxalase system (glyoxalase 1 and 2). The influence of glyoxalase 1 knockdown on expression of collagens 1, 3, 4, and 5 in L6 myoblasts under hyperglycemic conditions was investigated. Increased biosynthesis of collagens 1, 3, 4, and 5 was detected at mRNA-level following knockdown of glyoxalase 1 (GLO1). At the protein level a significant elevation of the concentration of collagen 1 and 4 was shown, whereas no increase of collagen 5 and a non-significant increase in collagen 3 were detectable. These results could partially explain MG-induced changes in the extracellular matrix (ECM) which account for increased fibrosis and impaired function in myocytes. The mechanisms by which reactive glucose metabolites influence ECM composition deserve further investigation.

Glycated Collagen Induces α11 Integrin Expression Through TGF-β2 and Smad3

The adhesion of cardiac fibroblasts to the glycated collagen interstitium in diabetics is associated with de novo expression of the α11 integrin, myofibroblast formation and cardiac fibrosis. We examined how methylglyoxal-glycated collagen regulates α11 integrin expression. In cardiac fibroblasts plated on glycated collagen but not glycated fibronectin, there was markedly increased α11 integrin and α-smooth muscle actin expression. Compared with native collagen, binding of purified α11β1 integrin to glycated collagen was reduced by >fourfold, which was consistent with reduced fibroblast attachment to glycated collagen. Glycated collagen strongly enhanced the expression of TGF-β2 but not TGF-β1 or TGF-β3. The increased expression of TGF-β2 was inhibited by triple helical collagen peptides that mimic the α11β1 integrin binding site on type I collagen. In cardiac fibroblasts transfected with α11 integrin luciferase promoter constructs, glycated collagen activated the α11 integrin promoter. Analysis of α11 integrin promoter truncation mutants showed a novel Smad2/3 binding site located between -809 and -1300 nt that was required for promoter activation. We conclude that glycated collagen in the cardiac interstitium triggers an autocrine TGF-β2 signaling pathway that stimulates α11 integrin expression through Smad2/3 binding elements in the α11 integrin promoter, which is important for myofibroblast formation and fibrosis.

Skin collagen advanced glycation endproducts (AGEs) and the long-term progression of sub-clinical cardiovascular disease in type 1 diabetes

Background

We recently reported strong associations between eight skin collagen AGEs and two solubility markers from skin biopsies obtained at DCCT study closeout and the long-term progression of microvascular disease in EDIC, despite adjustment for mean glycemia. Herein we investigated the hypothesis that some of these AGEs (fluorescence to be reported elsewhere) correlate with long-term subclinical cardiovascular disease (CVD) measurements, i.e. coronary artery calcium score (CAC) at EDIC year 7–9 (n = 187), change of carotid intima-media thickness (IMT) from EDIC year 1 to year 6 and 12 (n = 127), and cardiac MRI outcomes at EDIC year 15–16 (n = 142).

Methods

Skin collagen AGE measurements obtained from stored specimens were related to clinical data from the DCCT/EDIC using Spearman correlations and multivariable logistic regression analyses.

Results

Spearman correlations showed furosine (early glycation) was associated with future mean CAC (p < 0.05) and CAC >0 (p = 0.39), but not with CAC score <100 vs. >100. Glucosepane and pentosidine crosslinks, methylglyoxal hydroimidazolones (MG-H1) and pepsin solubility (inversely) correlated with IMT change from year 1 to 6(all P < 0.05). Left ventricular (LV) mass (cMRI) correlated with MG-H1, and inversely with pepsin solubility (both p < 0.05), while the ratio LV mass/end diastolic volume correlated with furosine and MG-H1 (both p < 0.05), and highly with CML (p < 0.01). In multivariate analysis only furosine (p = 0.01) was associated with CAC. In contrast IMT was inversely associated with lower collagen pepsin solubility and positively with glucosepane,

Conclusions

In type 1 diabetes, multiple AGEs are associated with IMT progression in spite of adjustment for A1c implying a likely participatory role of glycation and AGE mediated crosslinking on matrix accumulation in coronary arteries. This may also apply to functional cardiac MRI outcomes, especially left ventricular mass. In contrast, early glycation measured by furosine, but not AGEs, was associated with CAC score, implying hyperglycemia as a risk factor in calcium deposition perhaps via processes independent of glycation.

Trial registration: Registered at Clinical trial reg. nos. NCT00360815 and NCT00360893, http://www.clinicaltrials.gov

Advanced glycation end products and oxidative stress in type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) is a very complex and multifactorial metabolic disease characterized by insulin resistance and β cell failure leading to elevated blood glucose levels. Hyperglycemia is suggested to be the main cause of diabetic complications, which not only decrease life quality and expectancy, but are also becoming a problem regarding the financial burden for health care systems. Therefore, and to counteract the continually increasing prevalence of diabetes, understanding the pathogenesis, the main risk factors, and the underlying molecular mechanisms may establish a basis for prevention and therapy. In this regard, research was performed revealing further evidence that oxidative stress has an important role in hyperglycemia-induced tissue injury as well as in early events relevant for the development of T2DM. The formation of advanced glycation end products (AGEs), a group of modified proteins and/or lipids with damaging potential, is one contributing factor. On the one hand it has been reported that AGEs increase reactive oxygen species formation and impair antioxidant systems, on the other hand the formation of some AGEs is induced per se under oxidative conditions. Thus, AGEs contribute at least partly to chronic stress conditions in diabetes. As AGEs are not only formed endogenously, but also derive from exogenous sources, i.e., food, they have been assumed as risk factors for T2DM. However, the role of AGEs in the pathogenesis of T2DM and diabetic complications—if they are causal or simply an effect—is only partly understood. This review will highlight the involvement of AGEs in the development and progression of T2DM and their role in diabetic complications.

Role of discoidin domain receptor 1 in dysregulation of collagen remodeling by cyclosporin A

The anti-transplant rejection drug cyclosporin A (CsA) causes loss of collagen homeostasis in rapidly remodeling connective tissues, such as human gingiva. As a result of CsA treatment, collagen degradation by fibroblasts is inhibited, which leads to a net increase of tissue collagen and gingival overgrowth. Since fibrillar collagen is the primary ligand for the discoidin domain receptor 1 (DDR1), we hypothesized that CsA perturbs DDR1-associated functions that affect collagen homeostasis. For these experiments, human fibroblasts obtained from gingival explants or mouse 3T3 fibroblasts (wild type, over-expressing DDR1 or DDR1 knockdown) or mouse GD25 cells (expressing DDR1 but null for β1 integrin), were treated with vehicle (dimethyl sulfoxide) or with CsA. The effect of CsA on cell binding to collagen was examined by flow cytometry; cell-mediated collagen remodeling was analyzed with contraction, compaction and migration assays. We found that CsA inhibited cell binding to collagen, internalization of collagen, contraction of collagen gels and cell migration over collagen in a DDR1-dependent manner. CsA also enhanced collagen compaction around cell extensions. Treatment with CsA strongly reduced surface levels of β1 integrins in wild type and DDR1 over-expressing 3T3 cells but did not affect β1 integrin activation or focal adhesion formation. We conclude that CsA inhibition of collagen remodeling is mediated through its effects on both DDR1 and cell surface levels of the β1 integrin.

Glycation of extracellular matrix proteins impairs migration of immune cells

The immune response during aging and diabetes is disturbed and may be due to the altered migration of immune cells in an aged tissue. Our study should prove the hypothesis that age and diabetes-related advanced glycation end products (AGEs) have an impact on the migration and adhesion of human T-cells. To achieve our purpose, we used in vitro AGE-modified proteins (soluble albumin and fibronectin [FN]), as well as human collagen obtained from bypass graft. A Boyden chamber was used to study cell migration. Migrated Jurkat T-cells were analyzed by flow cytometry and cell adhesion by crystal violet staining. Actin polymerization was determined by phalloidin-Alexa-fluor 488-labeled antibody and fluorescence microscopy. We found that significantly fewer cells (50%, p = 0.003) migrated through methylglyoxal modified FN. The attachment to FN in the presence of AGE-bovine serum albumin (BSA) was also reduced (p < 0.05). In ex vivo experiments, isolated collagen from human vein graft material negatively affected the migration of the cells depending on the grade of AGE modification of the collagen. Collagen with a low AGE level reduced the cell migration by 30%, and collagen with a high AGE level by 60%. Interaction of the cells with an AGE-modified matrix, but not with soluble AGEs like BSA-AGE per se, was responsible for a disturbed migration. The reduced migration was accompanied by an impaired actin polymerization. We conclude that AGEs-modified matrix protein inhibits cell migration and adhesion of Jurkat T-cells.

The efficiency of compounds with α-amino-β-mercapto-ethane group in protection of human serum albumin carbonylation and cross-linking with methylglyoxal

Substances containing an α-amino-β-mercapto-ethane pharmacophore may be used as effective methylglyoxal scavengers and inhibitors of protein carbonylation and cross-linking.

Interactions between the discoidin domain receptor 1 and β1 integrin regulate attachment to collagen

Collagen degradation by phagocytosis is essential for physiological collagen turnover and connective tissue homeostasis. The rate limiting step of phagocytosis is the binding of specific adhesion receptors, which include the integrins and discoidin domain receptors (DDR), to fibrillar collagen. While previous data suggest that these two receptors interact, the functional nature of these interactions is not defined. In mouse and human fibroblasts we examined the effects of DDR1 knockdown and over-expression on β1 integrin subunit function. DDR1 expression levels were positively associated with enhanced contraction of floating and attached collagen gels, increased collagen binding and increased collagen remodeling. In DDR1 over-expressing cells compared with control cells, there were increased numbers, area and length of focal adhesions immunostained for talin, paxillin, vinculin and activated β1 integrin. After treatment with the integrin-cleaving protease jararhagin, in comparison to controls, DDR1 over-expressing cells exhibited increased β1 integrin cleavage at the cell membrane, indicating that DDR1 over-expression affected the access and susceptibility of cell-surface β1 integrin to the protease. DDR1 over-expression was associated with increased glycosylation of the β1 integrin subunit, which when blocked by deoxymannojirimycin, reduced collagen binding. Collectively these data indicate that DDR1 regulates β1 integrin interactions with fibrillar collagen, which positively impacts the binding step of collagen phagocytosis and collagen remodeling.

IL-1β enhances cell adhesion to degraded fibronectin

IL-1β is a prominent proinflammatory cytokine that mediates degradation of extracellular matrix proteins through increased expression of matrix metalloproteinases, which involves a signaling pathway in adherent cells that is restricted by focal adhesions. Currently, the mechanism by which IL-1β affects cell adhesion to matrix proteins is not defined, and it is not known whether degraded matrix proteins affect IL-1β signaling. We examined adhesion-related IL-1β signaling in fibroblasts attaching to native or MMP3-degraded fibronectin. IL-1β increased cell attachment, resistance to shear force and the numbers of focal adhesions containing activated β(1) integrins. IL-1β-enhanced attachment required FAK, kindlins 1/2, and talin. MMP3-degraded fibronectin-inhibited IL-1β-enhanced cell adhesion and promoted spontaneous ERK activation that was independent of IL-1β treatment. We conclude that IL-1β enhances the adhesion of anchorage-dependent cells to MMP3-degraded fibronectin, which, in turn, is associated with deregulated cellular responses to IL-1β. These data point to a novel role of IL-1β as a proadhesive signaling molecule in inflammation that employs kindlins and talin to regulate adhesion.

α11 integrin stimulates myofibroblast differentiation in diabetic cardiomyopathy

AIMS

Diabetic cardiomyopathy is characterized by the production of a disorganized fibrotic matrix in the absence of coronary atherosclerosis and hypertension. We examined whether adhesion of cardiac fibroblasts to glycated collagens mediates the differentiation of pro-fibrotic myofibroblasts, which may contribute to cardiac fibrosis.

METHODS AND RESULTS

By microarray, we found that methylglyoxal-treated collagen selectively enhanced α11 integrin expression in human cardiac fibroblasts, while levels of other collagen-binding integrins (α1, α2, and α10) were unchanged. Similar increases in α11 integrin mRNA and protein expression were observed in cardiac fibroblasts from streptozotocin (STZ)-treated Sprague-Dawley rats. In human cardiac fibroblasts plated on methyglyoxal-treated collagen and in cardiac fibroblasts from diabetic rats, transforming growth factor (TGF)-β2 but not TGF-β1 or TGF-β3 was increased compared with controls. Knock-down of α11 integrin and TGF-β receptors with small-interfering RNA blocked the increased expression of TGF-β2, α-smooth muscle actin (α-SMA), and α11 integrin that were induced in cells plated on methylglyoxal-treated collagen. Further, inhibition of Smad3 signalling blocked methylglyoxal-collagen up-regulation of α11 integrin and α-SMA expression. Rats with STZ-induced diabetes exhibited increased phosphorylation of Smad3 in cardiac tissues compared with control rats.

CONCLUSION

Interactions between α11 integrins and the Smad-dependent TGF-β2 signalling may contribute to the formation of pro-fibrotic myofibroblasts and the development of a fibrotic interstitium in diabetic cardiomyopathy.

Flightless I is a focal adhesion-associated actin-capping protein that regulates cell migration

The role of adhesion-associated actin-binding proteins in cell migration is not well defined. In mouse fibroblasts we screened for focal adhesion-associated proteins that were isolated with collagen-coated beads and detected by tandem mass spectrometry. We identified flightless I (FliI) as an actin-binding protein in focal adhesion fractions, which was verified by immunoblotting. By confocal microscopy most FliI was distributed throughout the cytosol and in focal adhesions. By sedimentation assays and in vitro binding assays, we found that FliI associates with actin filaments and actin monomers. Assays using purified proteins showed that FliI inhibits actin polymerization and caps but does not sever actin filaments. Cells with FliI knockdown or cells overexpressing FliI migrated more or less rapidly, respectively, than wild-type controls. Compared with controls, cells with FliI knockdown were less adherent than wild-type cells, exhibited reduced numbers of focal adhesions containing activated β1 integrins and vinculin, and exhibited increased incorporation of actin monomers into nascent filaments at focal adhesions. These data indicate that FliI regulates cell migration through its localization to focal adhesions and its ability to cap actin filaments, which collectively affect focal adhesion maturation.

Comprehensive identification of glycated peptides and their glycation motifs in plasma and erythrocytes of control and diabetic subjects

Nonenzymatic glycation of proteins sets the stage for formation of advanced glycation end-products and development of chronic complications of diabetes. In this report, we extended our previous methods on proteomics analysis of glycated proteins to comprehensively identify glycated proteins in control and diabetic human plasma and erythrocytes. Using immunodepletion, enrichment, and fractionation strategies, we identified 7749 unique glycated peptides, corresponding to 3742 unique glycated proteins. Semiquantitative comparisons showed that glycation levels of a number of proteins were significantly increased in diabetes and that erythrocyte proteins were more extensively glycated than plasma proteins. A glycation motif analysis revealed that some amino acids were favored more than others in the protein primary structures in the vicinity of the glycation sites in both sample types. The glycated peptides and corresponding proteins reported here provide a foundation for potential identification of novel markers for diabetes, hyperglycemia, and diabetic complications in future studies.

Methylglyoxal-a potential risk factor of manuka honey in healing of diabetic ulcers

Honey has been considered as a remedy in wound healing since ancient times. However, as yet, there are inadequate supportive robust randomized trials and experimental data to fully accept honey as an effective medical product in wound care. Manuka honey has been claimed to have therapeutic advantages over other honeys. Recently, it has been documented that the pronounced antibacterial activity of manuka honey is due, at least in part, to reactive methylglyoxal (MG). The concentration of MG in manuka honeys is up to 100-fold higher than in conventional honeys. MG is a potent protein-glycating agent and an important precursor of advanced glycation end products (AGEs). MG and AGEs play a role in the pathogenesis of impaired diabetic wound healing and can modify the structure and function of target molecules. This commentary describes the concern that MG in manuka honey may delay wound healing in diabetic patients. Further detailed research is needed to fully elucidate the participation of honey/derived MG in healing diabetic ulcers. We advocate randomized controlled trials to determine efficacy and safety of manuka honey in this population.

Skin wound healing in diabetic β6 integrin-deficient mice

Integrin αvβ6 is a heterodimeric cell surface receptor, which is absent from the normal epithelium, but is expressed in wound-edge keratinocytes during re-epithelialization. However, the function of the αvβ6 integrin in wound repair remains unclear. Impaired wound healing in patients with diabetes constitutes a major clinical problem worldwide and has been associated with the accumulation of advanced glycated endproducts (AGEs) in the tissues. AGEs may account for aberrant interactions between integrin receptors and their extracellular matrix ligands such as fibronectin (FN). In this study, we compared healing of experimental excisional skin wounds in wild-type (WT) and β6-knockout (β6(-/-) ) mice with streptozotocin-induced diabetes. Results showed that diabetic β6(-/-) mice had a significant delay in early wound closure rate compared with diabetic WT mice, suggesting that αvβ6 integrin may serve as a protective role in re-epithelialization of diabetic wounds. To mimic the glycosylated wound matrix, we generated a methylglyoxal (MG)-glycated variant of FN. Keratinocytes utilized αvβ6 and β1 integrins for spreading on both non-glycated and FN-MG, but their spreading was reduced on FN-MG. These findings indicated that glycation of FN and possibly other integrin ligands could hamper keratinocyte interactions with the provisional matrix proteins during re-epithelialization of diabetic wounds.

Two dicarbonyl compounds, 3-deoxyglucosone and methylglyoxal, differentially modulate dermal fibroblasts

Advanced glycation endproducts accumulate on long-lived proteins such as collagens as a function of diet and age and mediate the cross-linking of those proteins causing changes in collagen pathophysiology resulting in the disruption of normal collagen matrix remodeling. Two commonly studied advanced glycation endproduct precursors 3-deoxyglucosone and methylglyoxal were investigated for their role in the modification of collagen and on extracellular matrix expression. Fibroblasts cultured on methylglyoxal cross-linked matrices increased the expression of collagen, active TGF-beta1, beta1-integrin, and decreased Smad7; whereas 3-deoxyglucosone decreased collagen, active TGF-beta1, beta1-integrin but increased Smad7. Purified collagen modified by 3-deoxyglucosone or methylglyoxal had different molecular weights; methylglyoxal increased the apparent molecular weight by approximately 20 kDa, whereas 3-deoxyglucosone did not. The differences in collagen expression by 3-deoxyglucosone and methylglyoxal raise the provocative idea that a genetic or environmental background leading to the predominance of one of these advanced glycation endproduct precursors may precipitate a fibrotic or chronic wound in susceptible individuals, particularly in the diabetic.

Filamin A is required for vimentin-mediated cell adhesion and spreading

Cell adhesion and spreading are regulated by complex interactions involving the cytoskeleton and extracellular matrix proteins. We examined the interaction of the intermediate filament protein vimentin with the actin cross-linking protein filamin A in regulation of spreading in HEK-293 and 3T3 cells. Filamin A and vimentin-expressing cells were well spread on collagen and exhibited numerous cell extensions enriched with filamin A and vimentin. By contrast, cells treated with small interfering RNA (siRNA) to knock down filamin A or vimentin were poorly spread; both of these cell populations exhibited >50% reductions of cell adhesion, cell surface beta1 integrin expression, and beta1 integrin activation. Knockdown of filamin A reduced vimentin phosphorylation and blocked recruitment of vimentin to cell extensions, whereas knockdown of filamin and/or vimentin inhibited the formation of cell extensions. Reduced vimentin phosphorylation, cell spreading, and beta1 integrin surface expression, and activation were phenocopied in cells treated with the protein kinase C inhibitor bisindolylmaleimide; cell spreading was also reduced by siRNA knockdown of protein kinase C-epsilon. By immunoprecipitation of cell lysates and by pull-down assays using purified proteins, we found an association between filamin A and vimentin. Filamin A also associated with protein kinase C-epsilon, which was enriched in cell extensions. These data indicate that filamin A associates with vimentin and to protein kinase C-epsilon, thereby enabling vimentin phosphorylation, which is important for beta1 integrin activation and cell spreading on collagen.

A perspective on the Maillard reaction and the analysis of protein glycation by mass spectrometry: probing the pathogenesis of chronic disease

The Maillard reaction, starting from the glycation of protein and progressing to the formation of advanced glycation end-products (AGEs), is implicated in the development of complications of diabetes mellitus, as well as in the pathogenesis of cardiovascular, renal, and neurodegenerative diseases. In this perspective review, we provide an overview on the relevance of the Maillard reaction in the pathogenesis of chronic disease and discuss traditional approaches and recent developments in the analysis of glycated proteins by mass spectrometry. We propose that proteomics approaches, particularly bottom-up proteomics, will play a significant role in analyses of clinical samples leading to the identification of new markers of disease development and progression.

Advanced glycation of type I collagen and fibronectin modifies periodontal cell behavior

BACKGROUND

Advanced glycation end products (AGEs) have been linked to pathogenic mechanisms of diabetes mellitus. However, little is known about the contribution of protein glycation to periodontal disease in patients with diabetes. Therefore, this study investigated whether glycation of type I collagen (COLI) and fibronectin (FN) modified the behavior of human gingival fibroblasts (hGFs) and periodontal ligament fibroblasts (hPDLs).

METHODS

Procedures for rapid in vitro glycation of COLI and FN used methylglyoxal (MG). Formation of AGEs was analyzed by changes in protein migration using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting with antibodies specific for MG-glycated proteins. Experiments then characterized the effects of glycated FN and COLI on the behavior of hGFs and hPDLs.

RESULTS

MG glycated COLI and FN in <6 hours. Confirming the specificity of the reactions, antibodies specific for MG-induced AGEs reacted with glycated FN and COLI but not with control proteins. In cell culture experiments, glycated FN was significantly less efficient in supporting the attachment of hGFs and hPDLs (P <0.05). Moreover, the morphologic parameters, including length, area, perimeter, and shape factor, were altered (P <0.001) for cells on both glycated proteins. Finally, cell migration was reduced on glycated FN and COLI (P <0.001).

CONCLUSIONS

MG treatment efficiently glycated COLI and FN, providing a new tool to study the effects of diabetes on periodontal disease. The substantial effects of glycated COLI and FN on hGF and hPDL behavior indicated that protein glycation contributed to the pathogenesis and altered periodontal wound healing observed in patients with diabetes.

Filamin A regulates cell spreading and survival via beta1 integrins

Cell spreading and exploration of topographically complex substrates require tightly-regulated interactions between extracellular matrix receptors and the cytoskeleton, but the molecular determinants of these interactions are not defined. We examined whether the actin-binding proteins cortactin, vinculin and filamin A are involved in the formation of the earliest extensions of cells spreading over collagen or poly-L-lysine-coated smooth and beaded substrates. Spreading of human gingival fibroblasts was substantially reduced on beaded or poly-L-lysine-coated substrates. Filamin A, vinculin and cortactin were found in cell extensions on smooth collagen. HEK-293 cells also spread rapidly on smooth collagen and formed numerous cell extensions enriched with filamin A. Knockdown of filamin A in HEK-293 cells by short hairpin RNA reduced spreading and the number of cell extensions. Blocking beta1 integrin function significantly reduced cell spreading and localization of filamin A to cell extensions. Conversely, filamin A-knockdown reduced beta1 integrin-collagen binding as measured by 12G10 antibody, suggesting co-dependence between filamin A and beta1 integrin functions. TUNEL staining showed higher percentages of apoptosis after filamin A-knockdown in spreading cells. Chelation of [Ca2+]i with BAPTA/AM reduced spreading of wild-type and filamin A-knockdown cells, however wild-type cells showed recruitment of filamin A to the subcortex, indicating independent roles of filamin A and [Ca2+]i in cell spreading. We conclude that filamin A integrates with beta1 integrins to mediate cell spreading and prevent apoptosis.

Energy metabolism, altered proteins, sirtuins and ageing: converging mechanisms?

The predominant molecular symptom of ageing is the accumulation of altered gene products. Nutritional studies show that ageing in animals can be significantly influenced by dietary restriction. Genetics has revealed that ageing may be controlled by changes in intracellular NAD/NADH ratio regulating sirtuin activity. Physiological and other approaches indicate that mitochondria may also regulate ageing. A mechanism is proposed which links diet, exercise and mitochondria-dependent changes in NAD/NADH ratio to intracellular generation of altered proteins. It is suggested that ad libitum feeding conditions decrease NAD availability which also decreases metabolism of the triose phosphate glycolytic intermediates, glyceraldehyde-3-phosphate and dihydroxyacetone-phosphate, which can spontaneously decompose into methylglyoxal (MG). MG is a highly toxic glycating agent and a major source of protein advanced-glycosylation end-products (AGEs). MG and AGEs can induce mitochondrial dysfunction and formation of reactive oxygen species (ROS), as well as affect gene expression and intracellular signalling. In dietary restriction–induced fasting, NADH would be oxidised and NAD regenerated via mitochondrial action. This would not only activate sirtuins and extend lifespan but also suppress MG formation. This proposal can also explain the apparent paradox whereby increased aerobic activity suppresses formation of glycoxidized proteins and extends lifespan. Variation in mitochondrial DNA composition and consequent mutation rate, arising from dietary-controlled differences in DNA precursor ratios, could also contribute to tissue differences in age-related mitochondrial dysfunction.