Immunoelectron microscopic analyses of Maillard reaction products in bovine anterior lens capsule and Descemet's membrane

Association between Moraxella keratitis and advanced glycation end products

Diabetes mellitus is recognized as a major predisposing factor for Moraxella keratitis. However, how diabetes mellitus contributes to Moraxella keratitis remains unclear. In this study, we examined Moraxella keratitis; based on the findings, we investigated the impact of advanced glycation end products (AGEs) deposition in the cornea of individuals with diabetic mellitus on the adhesion of Moraxella isolates to the cornea. A retrospective analysis of 27 culture-proven cases of Moraxella keratitis at Ehime University Hospital (March 2006 to February 2022) was performed. Moraxella isolates were identified using matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Among the patients, 30.4% had diabetes mellitus and 22.2% had the predominant ocular condition of using steroid eye drops. The species identified were Moraxella nonliquefaciens in 59.3% and Moraxella lacunata in 40.7% of patients. To investigate the underlying mechanisms, we assessed the effects of M. nonliquefaciens adherence to simian virus 40-immortalized human corneal epithelial cells (HCECs) with or without AGEs. The results demonstrated the number of M. nonliquefaciens adhering to HCECs was significantly increased by adding AGEs compared with that in controls (p < 0.01). Furthermore, in the corneas of streptozotocin-induced diabetic C57BL/6 mice treated with or without pyridoxamine, an AGE inhibitor, the number of M. nonliquefaciens adhering to the corneas of diabetic mice was significantly reduced by pyridoxamine treatment (p < 0.05). In conclusion, the development of Moraxella keratitis may be significantly influenced by the deposition of AGEs on the corneal epithelium of patients with diabetes mellitus.

Neurite regeneration in adult rat retinas exposed to advanced glycation end-products and regenerative effects of neurotrophin-4

The purpose of this study was to determine the effect of low concentrations of advanced glycation end-products on neurite regeneration in isolated rat retinas, and to determine the effects of neurotrophin-4 on regeneration in advanced glycation end-products exposed retinas. Retinal explants of 4 adult Sprague-Dawley rats were cultured on collagen gel and were incubated in; (1) serum-free control culture media, (2) glucose-advanced glycation end-products-bovine serum albumin media, (3) glycolaldehyde-advanced glycation end-products-bovine serum albumin media, (4) glyceraldehyde-advanced glycation end-products-bovine serum albumin media, (5) glucose-advanced glycation end-products+neurotrophin-4 media, (6) glycolaldehyde-advanced glycation end-products+neurotrophin-4 media, or (7) glyceraldehyde-advanced glycation end-products+neurotrophin-4 supplemented culture media. After 7 days, the number of regenerating neurites from the explants was counted. Then, explants were fixed, cryosectioned, and stained for TUNEL. The ratio of TUNEL-positive cells to all cells in the ganglion cell layer was determined. Immunohistochemical examinations for the active-form of caspase-9 and apoptosis-inducing factor were performed. In retinas incubated with advanced glycation end-products containing media, the number of regenerating neurites were fewer than in retinas without advanced glycation end-products, and the number of TUNEL-positive cells and caspase-9- and apoptosis-inducing factor-immunopositive cells was significantly higher than in control media. Neurotrophin-4 supplementation increased the numbers of regenerating neuritis, and the number of TUNEL-positives, caspase-9-, and apoptosis-inducing factor-immunopositive cells were significantly fewer than that in advanced glycation end-products without neurotrophin-4 media. Low doses of advanced glycation end-products impede neurite regeneration in the rat retinas. Neurotrophin-4 significantly enhances neurite regeneration in retinas exposed to advanced glycation end-products.

Protection against advanced glycation end products and oxidative stress during the development of diabetic keratopathy by KIOM-79

OBJECTIVES

KIOM-79 is a mixture of 80% ethanol extracts of parched Puerariae radix, gingered Magnoliae cortex, Glycyrrhizae radix and Euphorbiae radix. The preventive effect of KIOM-79 on the development of diabetic keratopathy has been investigated.

METHODS

Seven-week-old male Zucker diabetic fatty (ZDF) rats were treated with KIOM-79 (50 mg/kg body weight) once a day orally for 13 weeks. The thickness of the cornea was measured and the extent of corneal cell death was detected by a terminal deoxynucleotidyl transferase dUTP nick-end labelling assay. The expression of advanced glycation end products (AGEs), 8-hydroxydeoxyguanosine, nuclear factor-kappaB (NF-κB), Bax and Bcl-2 were evaluated in corneal tissues.

KEY FINDINGS

The administration of KIOM-79 prevented corneal oedema and apoptotic cell death of corneal cells. The accumulation of AGE in corneal tissues was reduced in ZDF rats treated with KIOM-79. Moreover, KIOM-79 attenuated oxidative DNA damage, NF-κB activation and Bax overexpression in the cornea.

CONCLUSIONS

The results suggested that KIOM-79 exhibited corneal protective properties by not only reducing oxidative stress but inhibiting the AGEs/NF-κB downstream signal pathway during the development of diabetic keratopathy.

Involvement of advanced glycation end products, oxidative stress and nuclear factor-kappaB in the development of diabetic keratopathy

BACKGROUND

The purpose of the experiment reported here was to assess the involvement of advanced glycation end products (AGEs), oxidative stress, and nuclear factor kappa-B (NF-κB) activation in the development of diabetic keratopathy.

METHODS

Diabetes was induced by intraperitoneal streptozotocin injection in male Sprague-Dawley rats. The thickness of the cornea was measured. Apoptosis was detected by TUNEL assay and western blot for caspase-3. The expression of AGEs and 8-hydroxydeoxyguanosine (8-OHdG) were studied by immunohistochemistry in corneal tissues of normoglycaemic and diabetic rats. NF-κB activation was evaluated by electrophoretic mobility shift assay and southwestern histochemistry.

RESULTS

Corneal edema was observed in diabetic rats. The thickness of cornea was higher in diabetic than in control rats. AGEs were accumulated in corneal tissues. 8-OHdG and NF-κB were identified in corneal epithelium, stroma and endothelium, and its expressions were greater in diabetic than in those of control rats. Diabetes induces significant alterations in rat corneal tissue structure.

CONCLUSIONS

The higher expression of AGE, 8-OHdG and NF-κB in corneal tissues of diabetic rats suggests that these factors are involved in apoptosis and in subsequent corneal alterations related to diabetic keratopathy.

Effect of Shear Stress on Attachment of Corneal Endothelial Cells in Association With Corneal Endothelial Cell Loss After Laser Iridotomy

PURPOSE

Laser iridotomy often causes bullous keratopathy; however, the mechanism is unclear. We investigated whether changes in aqueous humor hydrodynamics after laser iridotomy have any role in corneal endothelial cell loss.

MATERIALS AND METHODS

Porcine corneal endothelial cells were plated onto glass slides. Following 1 or 3 hours for adhesion, the endothelial cells were exposed to shear stresses (0.1-10 dyne/cm) for 15 minutes, and the number of detached cells was counted. In addition, the pressure and shear stress on corneal endothelial layer were calculated in a virtual model of laser iridotomy.

RESULTS

The number of detached corneal endothelial cells increased with shear stresses in a dose-dependent manner. Significant increase of rate of detached corneal endothelial cells was observed at >0.3 dyne/cm after 1-hour attachment and at 1 dyne/cm after 3-hour attachment. The maximum pressure on corneal endothelial layer was 0.007 mm Hg, which is negligible compared with intraocular pressure. However, the maximum shear stress on the corneal endothelial layer could be> dyne/cm in some conditions of laser iridotomy.

CONCLUSIONS

The resistance of corneal endothelial cell loss to shear stress is time dependent. Shear stress could be a cause of corneal endothelial cell loss in some conditions of laser iridotomy.

Glutathionylation of lens proteins through the formation of thioether bond

Formation of lanthionine, a dehydroalanine crosslink, is associated with aging of the human lens and cataractogenesis. In this study we investigated whether modification of lens proteins by glutathione could proceed through an alternative pathway: that is, by the formation of a nonreducible thioether bond between protein and glutathione. Direct ELISA of the reduced water-soluble and water-insoluble lens proteins from human cataractous, aged and bovine lenses showed a concentration-dependent immunoreactivity toward human nonreducible glutathionyl-lens proteins only. The reduced water-insoluble cataractous lens proteins showed the highest immunoreactivity, while bovine lens protein exhibited no reaction. These data were confirmed by dot-blot analysis. The level of this modification ranged from 0.7 to 1.6 nmol/mg protein in water-insoluble proteins from aged and cataractous lenses. N-terminal amino acid determination in the reduced and alkylated lens proteins, performed by derivatization of these preparations with dansyl chloride followed by an exhaustive dialysis, acid hydrolysis and fluorescence detection of dansylated amino acids by RP-HPLC, showed that N-terminal glutamic acid was present in concentration of approximately 0.2 nmol/mg of lens protein. This evidence points out that at least some of the N-terminal amino groups of nonreducible glutathione in the reduced human lens proteins are not involved in a covalent bond formation. Since disulfides were not detected in the reduced and alkylated human lens proteins, GSH is most likely attached to lens proteins through thioether bonds. These results provide, for the first time, evidence that glutathiolation of human lens proteins can occur through the formation of nonreducible thioether bonds.

Advanced glycation end products in Descemet's membrane and their effect on corneal endothelial cell

PURPOSE

[corrected] The purpose of this study was to evaluate the effect of advanced glycation end products (AGEs) in Descemet's membrane on the attachment and spreading of the corneal endothelial cells.

METHODS

An anti-AGEs monoclonal antibody (6D12), which recognizes a N(epsilon)-carboxymethyl lysine (CML)-protein adduct as an epitope, was used for immunohistochemistry and enzyme-linked immunosorbent assay (ELISA). Fresh bovine Descemet's membrane was incubated for 4 weeks in the buffered solution with 500 mM of glucose-6-phosphate (G-6-P). In the incubated Descemet's membrane, the immunohistochemical localization of CML was examined. Type I collagen-, type IV collagen-, fibronectin-, or laminin-coated 96-well plates were glycated by G-6-P. The amount of CML was determined by ELISA using 6D12. Cultured bovine corneal endothelial cells were seeded onto glycated or non-glycated extracellular matrix (ECM) in 96-well plates and allowed to attach for 3 hours. The number and the surface area of the attached cells were examined.

RESULTS

Immunoreactivity to CML was detected in Descemet's membrane incubated in the buffered solution containing G-6-P. Glycation of fibronectin and laminin decreased the number and the surface area of the attached corneal endothelial cells. Aminoguanidine in the incubation mixture inhibited CML formation of ECM components and increased the number and the surface area of the attached corneal endothelial cells in a dose-dependent manner.

CONCLUSIONS

AGE formation on fibronectin and laminin attenuated the attachment and spreading of the corneal endothelial cells. AGEs' formation in Descemet's membrane may be responsible for the corneal endothelial cell loss with aging and corneal endothelial abnormalities in diabetic patients

Microscopic and histochemical manifestations of hyaline cartilage dynamics

Structure and function of hyaline cartilages has been the focus of many correlative studies for over a hundred years. Much of what is known regarding dynamics and function of cartilage constituents has been derived or inferred from biochemical and electron microscopic investigations. Here we show that in conjunction with ultrastructural, and high-magnification transmission light and polarization microscopy, the well-developed histochemical methods are indispensable for the analysis of cartilage dynamics. Microscopically demonstrable aspects of cartilage dynamics include, but are not limited to, formation of the intracellular liquid crystals, phase transitions of the extracellular matrix and tubular connections between chondrocytes. The role of the interchondrocytic liquid crystals is considered in terms of the tensegrity hypothesis and non-apoptotic cell death. Phase transitions of the extracellular matrix are discussed in terms of self-alignment of chondrons, matrix guidance pathways and cartilage growth in the absence of mitosis. The possible role of nonenzymatic glycation reactions in cartilage dynamics is also reviewed.

3-Deoxyglucosone: metabolism, analysis, biological activity, and clinical implication

3-Deoxyglucosone (3-DG) is synthesized via the Maillard reaction and the polyol pathway, and is detoxified to 3-deoxyfructose and 2-keto-3-deoxygluconic acid. 3-DG rapidly reacts with protein amino groups to form advanced glycation end products (AGEs) such as imidazolone, pyrraline, N'-(carboxymethyl)lysine and pentosidine, among which imidazolone is the AGE most specific for 3-DG. As demonstrated by using gas chromatography-mass spectrometry or high-performance liquid chromatography, plasma 3-DG levels are markedly increased in diabetes and uremia. Although the plasma 3-DG levels had been controversial, it was clearly demonstrated that its plasma level depends on the deproteinization method by which either free or total 3-DG, presumably bound to proteins, is measured. In diabetes, hyperglycemia enhances the synthesis of 3-DG via the Maillard reaction and the polyol pathway, and thereby leads to its high plasma and erythrocyte levels. In uremia, however, the decreased catabolism of 3-DG, which may be due to the loss of 3-DG reductase activity in the end-stage kidneys, may lead to high plasma 3-DG level. The elevated 3-DG levels in plasma and erythrocytes may promote the formation of AGEs such as imidazolone, as demonstrated by immunohistochemistry and immunochemistry using an anti-imidazolone antibody. Although AGE-modified proteins prepared in vitro exhibit a variety of biological activities, known AGE structures have not yet been demonstrated to show any biological activities. Because 3-DG is potent in the formation of AGEs and has some biological activities, such as cellular toxicity, it may be more important in the development of diabetic and uremic complications than the known AGE structures. By demonstrating that treatment with an aldose reductase inhibitor reduces the erythrocyte levels of 3-DG and AGEs, such as imidazolone, light is shed on the mystery of how aldose reductase inhibitors may prove beneficial in diabetic complications. These evidences suggest that 3-DG plays a principal role in the development of diabetic and uremic complications.

Mechanisms of maturation and ageing of collagen

The deleterious age-related changes in collagen that manifest in the stiffening of the joints, the vascular system and the renal and retinal capillaries are primarily due to the intermolecular cross-linking of the collagen molecules within the tissues. The formation of cross-links was elegantly demonstrated by Verzar over 40 years ago but the nature and mechanisms are only now being unravelled. Cross-linking involves two different mechanisms, one a precise enzymically controlled cross-linking during development and maturation and the other an adventitious non-enzymic mechanism following maturation of the tissue. It is this additional non-enzymic cross-linking, known as glycation, involving reaction with glucose and subsequent oxidation products of the complex, that is the major cause of dysfunction of collagenous tissues in old age. The process is accelerated in diabetic subjects due to the higher levels of glucose. The effect of glycation on cell-matrix interactions is now being studied and may be shown to be an equally important aspect of ageing of collagen. An understanding of these mechanisms is now leading to the development of inhibitors of glycation and compounds capable of cleaving the cross-links, thus alleviating the devastating effects of ageing.

Post-translational non-enzymatic modification of proteins. I. Chromatography of marker adducts with special emphasis to glycation reactions

Analytical methods for marker compounds formed during post-translational modifications of proteins are reviewed. Only adducts arising either in vivo or under in vitro conditions simulating the in vivo situations are discussed. All of these compounds stem from either the reaction of free amino groups (i.e., lysine, arginine or N-terminal amino acid). In most cases the reactive counterpart is an aldehydic moiety containing endogenous compound; however, other functional groups containing metabolites are considered as well. The main demand put upon such marker compounds is that they are stable in acid or enzymatic hydrolysis or, alternatively, can be stabilized by simple sample pretreatment (e.g., by reduction). Practically all categories of separation procedures can be applied provided that the chemical characteristics of a particular marker are adequately respected; frequently combination of two different separation procedures based on different principles must be used. Because of the low level of such marker compounds under in vivo conditions, an appropriate sample enrichment step must be involved. Emphasis is put upon the analysis of Amadori products, pentosidine (and pentosidine related compounds), pyrraline, furosine, N-carboxymethylamino acids, amino acid hydantoins and stabilized dicarbonyl intermediates.

Urinary pyrraline as a biochemical marker of non-oxidative Maillard reactions in vivo

The presence of pyrraline, a non-oxidative glucose-derived Maillard reaction product in plasma proteins has been established previously. In this study we have investigated the presence of pyrraline in human urine to determine whether pyrraline-containing proteins are metabolized or selectively retained. Pyrraline was detected by means of HPLC, and its presence was confirmed by UV and electrospray-mass spectrometry. The quantification of pyrraline in urine from healthy individuals showed 1.21 +/- 0.4 micrograms/mg creatinine. In urine from diabetic patients, pyrraline levels varied considerably, although the mean level was higher than in healthy subjects (1.37 +/- 0.6 micrograms/mg creatinine). These data further support the presence of a catabolic pathway for advanced non-oxidative Maillard reaction products in vivo and suggest their role in the pathogenesis of diabetes.

Glycation of collagen: the basis of its central role in the late complications of ageing and diabetes

The most serious late complication of ageing and diabetes mellitus follow similar patterns in the dysfunction of retinal capillaries, renal tissue, and the cardiovascular system. The changes are accelerated in diabetic patients owing to hyerglycaemia and are the major cause of premature morbidity and mortality. These tissues and their optimal functioning are dependent on the integrity of their supporting framework of collagen. It is the modification of the properties by glycation that results in many of the damaging late complications. Initially glycation affects the interactions of collagen with cells and other matrix components, but the most damaging effects are caused by the formation of glucose-mediated intermolecular cross-links. These cross-links decrease the critical flexibility and permeability of the tissues and reduce turnover. In contrast to the renal and retinal tissue, the cardiovascular system also contains a significant proportion of other fibrous connective tissue protein elastin, and its properties are similarly modified by glycation. The nature of these glycation cross-links is now being unravelled and this knowledge is crucial in any attempt to inhibit these deleterious glycation reactions.

Mechanism of protein modification by glyoxal and glycolaldehyde, reactive intermediates of the Maillard reaction

The role of glyoxal and glycolaldehyde in protein cross-linking and N epsilon-(carboxymethyl)lysine (CML) formation during Maillard reaction under physiological conditions was investigated. Incubation of bovine serum albumin with these reagents lead to rapid formation of C-2-imine cross-links and CML. Initial CML formation rate from glyoxal was not dependent on oxidation, suggesting an intramolecular Cannizzaro reaction. CML formation from glucose/lysine or Amadori product of both was strongly dependent on oxidation. Blocking of Amadori product by boric acid totally suppressed CML formation from Amadori product, but only by 37% in the glucose/lysine system. Trapping of glyoxal with aminoguanidine hardly suppressed CML formation from Amadori product, whereas it blocked 50% of CML production in the glucose/lysine system. While these results would support a significant role for glucose autoxidation in CML formation, the addition of lysine to a glucose/aminoguanidine incubation system catalyzed glyoxal-triazine formation 7-fold, thereby strongly suggesting that glucose autoxidation is not a factor for glyoxal-mediated CML formation. Based on these results, it can be estimated that approximately 50% of the CML forming in a glucose/lysine system originates from oxidation of Amadori product, and 40-50% originates from a pre-Amadori stage largely independent from glucose autoxidation. This step may be related to the so-called Namiki pathway of the Maillard reaction.

Chromatographic evidence for pyrraline formation during protein glycation in vitro and in vivo

Pyrraline (epsilon-2-(formyl-5-hydroxymethyl-pyrrol-1-yl)-L-norleucine) is an advanced Maillard reaction product formed from 3-deoxyglucosone in the non-enzymatic reaction between glucose and the epsilon-amino group of lysine residues on proteins. Although its presence in vivo as well as in in vitro incubations of proteins with sugars has been documented by immunochemical methods using polyclonal and monoclonal antibodies, its formation in proteins has recently been questioned by similar methodology. To clarify this issue, we investigated pyrraline formation in proteins following alkaline hydrolysis and quantitation by high-performance liquid chromatography on a C18 reverse-phase column. Time- and sugar concentration-dependent increase in pyrraline formation was noted in serum albumin incubated with either 100 mM glucose or 50 mM 3-deoxyglucosone. Formation of pyrraline from 3-deoxyglucosone was rapid at slightly acidic pH, confirming its synthetic pathway through this Maillard reaction intermediate. Low levels of pyrraline (< 10 pmol/mg protein) were also detected in a pool of human skin collagen by this method, but no age effect was apparent. Using a slightly different approach, pyrraline-like material was detected in human plasma proteins following enzyme digestion and analysis by high performance liquid chromatography. Plasma from diabetic patients showed a significant increase in pyrraline-like material compared to controls. The levels in diabetic and normal individuals were 21.6 +/- 9.56 and 12.8 +/- 5.6 pmol per mg protein, respectively (P = 0.005), reflecting thereby the elevated levels of the immediate precursor of pyrraline, 3-deoxyglucosone, in diabetic plasma.