Glycation of lens membrane intrinsic proteins

Structural stability and solubility of glycated camel lens ζ-crystallin

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

Acute transient bilateral diabetic posterior subcapsular cataracts(1)

A 62-year-old man in whom diabetes was recently detected presented with visually significant, bilateral posterior subcapsular cataracts within days of initiating antihyperglycemic therapy. With efficient control and a stable serum glucose level, the cataracts started regressing. Except for a few scattered opacities, the patient was left with essentially clear lenses. Visual acuity of counting fingers at 2 ft in the right eye and 20/63 in the left eye improved to 20/20 in both eyes within 5 weeks.

Glycation decreases calmodulin binding to lens transmembrane protein, MIP

Channels of the major intrinsic protein (MIP) of the lens transport water, thus playing an important role in lens fiber cell homeostasis. Calmodulin (CAM) interacts with MIP and possibly regulates MIP channel permeability. Protein glycation has been implicated in lens opacification. We previously identified sites of glycation of MIP, which are in close proximity to the putative CAM binding site. This study is aimed to show the effect of in vitro and in vivo glycation on CAM binding to MIP. Our results show that MIP and MP20 are the major CAM binding proteins of the lens membrane. In vitro incubation of lens membranes with 1 M glucose decreased CAM binding by 38% (P<0.001). Similarly, there was a progressive decrease in CAM binding to diabetic lens membranes compared to age-matched controls (up to 30% decrease, P<0.01). Mutation of K228 and K238 as well as a triple K mutation (K228N, K238N, K259N) of MIP resulted in a decrease in CAM binding. Thus, post-translational protein modifications of MIP influence CAM binding. Since CAM is the ubiquitous Ca(2+) receptor, decreases in CAM binding to the target protein will affect the Ca(2+)-mediated cellular processes leading to lens opacification in diabetic and aging lenses.

Relative importance of aldose reductase versus nonenzymatic glycosylation on sugar cataract formation in diabetic rats

The relative importance of sorbitol formation versus nonenzymatic glycosylation and advanced glycosylation end products (AGEs) on sugar cataract formation was examined in diabetic rats. Diabetes was experimentally induced in young, 50 g rats with streptozotocin, and aldose reductase inhibitors were administered in the diet for up to 8 weeks at concentrations of 0.06% for tolrestat or ponalrestat and 0.0125% for AL-1576. Cataract formation was monitored by hand-held slit lamp for up to 11 weeks. Lens polyol levels were monitored by GLC, glycosylated protein levels were spectrophotometrically determined, and AGE products were estimated by fluorescence measurements and ELISA. Sugar cataract formation was observed in all untreated diabetic rats while cataract formation was inhibited in all diabetic rats treated with the AR inhibitors. Lens sorbitol levels were reduced in all ARI-treated rats. Glycosylated lens protein levels were elevated in the diabetic rats, and these levels were not significantly lower in the non-cataractous lenses from ARI-treated diabetic rats. Fluorescence measurements of the lens proteins revealed increased lens AGE levels in all diabetic rats, and these were slightly reduced in the aldose reductase inhibitor treated diabetics. With ELISA, immunoreactive AGEs were only detected in cataractous lenses from the untreated diabetic rats. Immunoreactive AGEs were not detected in the clear lenses of the aldose reductase inhibitor treated diabetics or in the non-diabetic controls. These results support the concept that sugar cataract formation is initiated by the aldose reductase catalyzed intracellular accumulation of polyols and that these sugar cataracts can be prevented through inhibition of aldose reductase.

Age and metabolic control influence lens opacity in type I, insulin-dependent diabetic patients

Cataract is a frequent ocular complication in diabetic patients, but few data are available concerning early modifications occurring in the lens of these patients and their relationship with metabolic control and other clinical parameters. We measured lens opacity in 73 type I, insulin-dependent diabetic patients aging 50 years or less and without clinical evidence of cataract, and in 46 healthy volunteers of similar age. We used a quick, simple, and reliable instrument, the Lensmeter 701, which is based on a back-light scattering quantification system and is able to quantify lens transparency along the nuclear axis. Mean lens opacity was significantly (p = 0.0001) higher in diabetic patients than in the control group, and multiple regression analysis showed that it correlated with age (p = 0.0001) and HbA1c levels (p = 0.009). Moreover in the younger group of patients (age < or =20 years) the only observed correlation was that with Hba1c (p = 0.03), whereas in the older ones (age 21-30 and >30 years) lens opacity correlated with age (p = 0.02 and p = 0.01). These data indicate that early opacifications of the lens occur in type I, insulin-dependent diabetic patients and are influenced by the degree of the metabolic control in the younger ones, whereas the well-known role of aging on lens transparency became prevalent in the older patients. Only longitudinal studies, however, can demonstrate whether these alterations represent any early stage of cataractagenesis and the role of good metabolic control in preventing this ocular complication.

The advanced glycation endproduct pentosidine induces the expression of PDGF-B in human retinal pigment epithelial cells

Advanced glycation endproducts have been implicated in a number of diabetic and aging changes. Some of these effects occur in part through induction of cytokines such as platelet-derived growth factor (PDGF), which is expressed by the retinal pigment epithelium (RPE). In this study, cultures of RPE were evaluated for PDGF expression after treatment with pentosidine, a well characterized advanced glycation endproduct. Northern analysis provided evidence for the increased expression of a 3.7 kb PDGF-B transcript over unstimulated controls in the established ARPE-19 cell line. Western analysis demonstrated increased PDGF-BB protein in conditioned medium compared to controls of ARPE-19 cells. In addition, two different early passage cultures of RPE showed increased PDGF-BB protein after pentosidine treatment compared to unstimulated controls. The enhanced production of PDGF-BB could play a role in the maintenance of the RPE-Bruch's membrane complex and influence changes associated with diabetes and aging.

Post-translational non-enzymatic modification of proteins. II. Separation of selected protein species after glycation and other carbonyl-mediated modifications

There are two strategies applicable to revealing non-enzymatic post-translational modifications of proteins; while assaying of the hydrolytically stable adducts was the subject of our previous communication [1], here we attempted to review separation technologies for the unfragmented modified proteins. There are a few standard procedures used for this purpose, namely Laemmli gel electrophoresis, different modes of gel permeation chromatography and boronate affinity chromatography. The latter approach makes use of the vicinal hydroxy groups present in glycated proteins. Some (but not all) arising adducts exhibit typical fluorescence which can be exploited for detection. In most cases fluorescence is measured at 370/440 nm for the so-called advanced glycation products or at 335/385 nm for the only so far well characterized glycation marker (pentosidine). Some indication exists that, e.g., synchronous fluorescence detection will probably in the future add to the selectivity and allow the distinction of the different adducts arising during non-enzymatic post-translational modifications (glycation). The proteins reviewed are serum albumin, collagen and lens proteins while glycation of hemoglobin is the subject of another review within the present volume.

Mass spectroscopic identification of in vitro glycated sites of MIP

PURPOSE

Earlier reconstitution studies showed that glycation of the major intrinsic peptide (MIP) of the lens affected the permeability of liposomes. This study is aimed to identify in vitro glycated sites.

METHODS

Urea- and alkali-washed calf lens membranes were incubated with 0 and 1 M glucose for 5 days. Following the incubation, MIP was purified by size exclusion HPLC. The C-terminus peptide of MIP was then cleaved by cyanogen bromide (CNBr) and purified by C4 reversed-phase HPLC. The CNBr peptide was analyzed, either directly or after digestion with trypsin, by electrospray ionization mass spectrometry (ESIMS) and matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS).

RESULTS

The ESIMS and MALDI-MS analyses of the intact C-terminus peptide from 1 M glucose incubated samples showed a mass shift equivalent to one, two and three glucose adducts. The MALDI-MS of the tryptic digest of the same sample showed peptides with mass shift equivalent to one or more glucose adducts. Sequence assignment confirmed that these glycated peptides contained lys238 and lys259. Although the intact C-terminus peptide showed up to three glucose adducts, we could not assign any tryptic peptide that contained glycated lys228. Samples incubated with 0 M glucose did not show any protein modifications.

CONCLUSIONS

The data suggest that in vitro glycation sites of calf lens MIP are lys238 and lys259, and possibly lys228.

The role of alpha- and epsilon-amino groups in the glycation-mediated cross-linking of gammaB-crystallin. Study of three site-directed mutants

In the previous report we demonstrated that gammaB-crystallin is glycated predominantly at the N-terminal alpha-amino group (Casey, E. B., Zhao, H. R., and Abraham, E. C. (1995) J. Biol. Chem. 270, 20781-20786). To investigate the possible role of alpha- and epsilon-amino groups of gammaB-crystallin in glycation-mediated cross-linking, Lys-2 or Lys-163, or both, were mutated to threonine by site-directed mutagenesis in bovine gammaB-crystallin cDNA. Wild type and mutant gammaB-crystallins were expressed in Escherichia coli cells. Cross-linking studies were performed by incubating wild type and mutant gammaB-crystallins with glyceraldehyde, ribose, and galactose followed by SDS-polyacrylamide gel electrophoresis under reducing conditions. When both of the lysines of gammaB-crystallin were mutated to threonines (gammaB-K2T/K163T), the quantity of cross-linked products was greatly reduced, indicating that, despite the fact that the alpha-amino group is a major glycated site, epsilon-amino groups play a predominant role in cross-linking. Therefore, cross-linking ability depends not only upon the level of glycation but also upon which amino group is glycated. Steric hindrance may decrease the cross-linking ability of the alpha-amino group. Our results also show that Lys-2 and Lys-163 play almost equal roles in cross-linking of gammaB-crystallin. By incubating carbonic anhydrase, a protein with a blocked N terminus, and our novel "no lysine" gammaB (gammaB-K2T/K163T) with sugar, we were able to show for the first time that significant cross-linking occurs between lysines and non-lysine sites. The fact that pentosidine and imidazolysine, formed from ribose and methylglyoxal, respectively, were present in the cross-linked gammaB-crystallins revealed the existence of Lys-Arg and Lys-Lys cross-linking.

Role of glycine 1 and lysine 2 in the glycation of bovine gamma B-crystallin. Site-directed mutagenesis of lysine to threonine

To determine the role of Gly-1 and Lys-2 of bovine gamma B-crystallin in glycation and cross-linking, Lys-2 was changed to Thr by site-directed mutagenesis. A polymerase chain reaction was used to perform site-directed mutagenesis on the third codon (AAG-->ACG) of bovine gamma B-crystallin cDNA. The wild type and mutant cDNAs were cloned into pMON5743 and expressed in JM101 Escherichia coli cells, and the identity of gamma B-crystallin was confirmed by Western blotting after purification by cation exchange high performance liquid chromatography. Glycation of gamma B-crystallin by [14C]glucose was reduced significantly due to the mutation of Lys-2, supporting the view that Lys-2 is a major glycation site. Peptide mapping showed the presence of two major labeled peptides containing N-terminal sequences, and in the mutant these peptides had longer retention times and reduced radioactivity. Amino acid analysis, after glycation with [14C]glucose, revealed N-terminal glycine as the most predominant glycation site. Lys-2 was glycated slower than Gly-1 but faster than Lys-163. Glycation with DL-glyceraldehyde showed an important role for both Gly-1 and Lys-2 in the glycation-mediated gamma B-crystallin cross-linking.

Effect of germanium-132 on galactose cataracts and glycation in rats

Germanium compounds have been shown to be effective in preventing the formation of advanced glycation end-products and for reversible solubilization of glycated proteins. As protein glycation has been proposed to play a role in lens opacification, we initiated studies to evaluate the effects of 2-carboxyethyl germanium sesquioxide (germanium compound 132 or Ge-132) on galactose-induced cataractogenesis. For this study young Sprague-Dawley rats were fed a 50% galactose diet. One group of rats received topical saline and another group was administered Ge-132 in saline four times a day. The lenses were periodically examined with an ophthalmoscope and at desired intervals processed for light and scanning electron microscopy. Our observations, beginning at 3 days and continuing to 21 days of galactose feeding, exhibited the characteristic galactose-induced morphological alterations, which include the formation of vacuoles, cysts, membrane disruption and swelling of fibers and epithelial cells as well as disorganization of the bow in lenses of rats in both groups. However, in the majority of rats administered Ge-132 these alterations were delayed as compared to the lenses of rats administered saline. Our findings show that, although the initiation, progression and pattern of lens opacification in rats receiving saline and Ge-132 were similar, in the majority of lenses the progression and establishment of mature cataracts in the Ge-132 group of rats were delayed. Analysis of the water-soluble and water-insoluble lens-protein fractions for glycated proteins showed increased levels of the Amadori products and advanced glycation related fluorescent products in galactosemic rats treated with saline eye drops. In rats receiving the topical Ge-132 treatment the levels of these glycation products were substantially reduced to levels lower than control values. Prevention of glycation seems to be a mechanism by which cataract progression is delayed.

Heterologous expression in Escherichia coli of native and mutant forms of the major intrinsic protein of rat eye lens (MIP26)

The complete cDNA of rat eye lens major intrinsic protein (MIP26) was sequenced using the dideoxy chain termination method. The sequence displayed 89% nucleotide identity and 95% identity at the amino acid level with bovine MIP26 [Gorin, Yancey, Cline, Revel and Horwitz (1984) Cell, 39, 49-54]. Both native and mutant cDNAs coding for rat MIP26 were amplified by PCR and subcloned into the pPOW expression vector for expression of Escherichia coli. A membrane signal peptide (PelB) was used for secretion of MIP26 into the cytoplasmic membrane. A hydrophilic octapeptide tail (FLAG) was fused to either the N- or C-terminus of MIP26 to aid monoclonal antibody-mediated identification and purification. Heterologously expressed MIP26 was identified by using a monoclonal antibody corresponding to the FLAG peptide located at the termini of MIP26. Immunofluorescently labelled monoclonal antibody was used to determine the localization of MIP26 in the cytoplasmic membrane. The majority of the protein was integrated into cell plasma membrane. MIP26 was extracted with n-octyl beta-D-glucopyranoside and then purified on an affinity gel column. Rat MIP26 cDNA contains an -Asn-Gly- sequence at the C-terminus, which has been shown in other proteins to be particularly susceptible to spontaneous deamidation [Takemoto and Emmons (1991) Curr. Eye Res. 10, 863-869]. We therefore modified the MIP26 molecule using a site-directed mutagenesis method to generate a mutant MIP26 at the appropriate asparagine residue (Asn244-->Asp) near the C-terminus. The mutation was confirmed by DNA sequencing. The mutant MIP26 protein was also expressed in E. coli and incorporated predominantly into the cytoplasmic membrane.