Amino-Carbonyl Reactions in the Aging Human Lens

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This research program addresses the hypothesis that extensive damage to lens crystallins by amino-carbonyl reactions between reducing sugars, ascorbic acid and proteins (so-called Maillard reaction) contributes to the aging of the human lens. Our key hypothesis is that the accumulation of these glycation products predisposes the lens toward cataractogenesis, and that much of the reaction is catalyzed by ascorbic acid oxidation products. Toward this hypothesis progress several breakthroughs have been achieved in the previous funding period. First, the structure of the fluorophore LM-1 was identified as Vesperlysine A, an ascorbate catalyzed protein crosslink that is specifically increased in diabetic human lenses. Second, we discovered carboxymethyl-lysine, a major ascorbylation product, bindsredox active Cu 2+ in the human lens. Third, we have pioneered a novel "metabolomics" approach based on fluorine-labeled ascorbic acid (F-ASA) and high resolution19F-NMR spectroscopy and found that oxidatively stressed lens epithelial cells and cataracts accumulate F-ASA degradation products. Fourth, we discovered, cloned and elucidated the catalytic mechanism of deglycating enzymes for future use in transgenic animal models of excessive glycation. Finally and most recently, we discovered evidence of extensive glycation-catalyzed conversion of arginine into ornithine in the aging human lens. In the coming years we propose an entirely new and, we believe, highly promising approach to the problem of lenticular aging by ascorbylation and glycation through the creation of three transgenic/conditionalknockout models of accelerated damage to the lens. Based on the discovery that rodent lenses are deficient in ascorbic acid uptake, we propose to first create a mouse model of the human lens by overexpressing the human Vitamin C Transporter 2 (hSVCT2) in the lens. The impact of this transgene on lenticular homoestasis, protein pigmentation, fragmentation and crosslinkingwill be studied. This humanized hSVCT+/+ mouse will be then be crossbred with a mouse deficient in glutathionesynthesis obtained by conditionally knocking out y-glutamyl cysteine ligase (yGclc)from the lens. The impact of the knockouton lens crystallin changes with be similarly studied. The homozygous hSVCT^/yGcIc"'" hybrid mouse lens is expected to accumulate substantial levels of reactive ascorbylation products due to impairment of ascorbic acid recycling. The approach taken will allow us to study separately the impact of multipleforms of protein damage, i.e. ascorbylation, glycation and oxidation on lens chaperone and other properties. With the help of these genetic manipulations, it is hoped that the 70-yr long aging process of the human lens can be condensed into 24 mos. This mouse model is expected to become a potent tool for the development of novel genetic and pharmacological interventions against lenticular aging.