Y. Akagi

Localization of Aldose Reductase in the Human Eye

The presence of the enzyme aldose reductase is increasingly being linked to diabetic complications. The distribution of this enzyme in human cornea, lens, retina, and optic nerve has been studied using specific antibodies against purified human placental aldose reductase raised in both rabbit and goat. The antisera from both animals gave equal, specific reactions. In frozen sections of ocular tissues, significant aldose reductase localization was reproducibly demonstrated in the endothelium and epithelium of the cornea and in the basal cell layers of the conjunctiva. In the lens, staining was observed in the epithelium and superficial lens fibers. In retinal sections, the presence of aldose reductase was demonstrated in the Muellers cells, especially near the inner limiting membrane. It was also found in some ganglion and cone cells. In the optic nerve, positive staining was observed in the axon. All other cells of the tissues examined revealed Only weak, nonspecific Staining.

Prevention of Basement Membrane Thickening in Retinal Capillaries by a Novel Inhibitor of Aldose Reductase, Tolrestat

A diabetic-like thickening of retinal capillary basement membranes induced in rats fed for 207 consecutive days a diet containing 50% galactose was prevented by the addition to the diet of tolrestat, a potent, structurally novel inhibitor of aldose reductase. Analysis of electron micrographs (× 25,000) of capillaries from the outer plexiform layer of the retina by computer planimetry showed that the basement membranes were approximately twofold thicker in rats fed galactose than in those fed either a standard diet or a diet containing galactose and tolrestat in doses of 43 or 57 mg/kg/day. The thickening of basement membranes in galactosefed rats was accompanied by other ultrastructural alterations mimicking changes typical of diabetic microangiopathy, such as multilamination and the formation of vacuoles and dense inclusions. Therefore, the galactosemic rat represents a useful model for studying basement membrane-related complications of diabetes and their possible prevention by aldose reductase inhibitors.

The effect of aldose reductase and its inhibition on sugar cataract formation

Cataracts associated with diabetes and galactosemia are characterized by their rapid onset and bilateral appearance. These cataracts display similar morphology and histology and have common biochemical mechanisms initiating the cataractous processes. An understanding of these biochemical mechanisms have been aided both by the ability to reproduce these cataracts in various animal models and by the development of potent inhibitors of aldose reductase. This is a US government work. There are no restrictions on its use.

Aldose reductase localization in dog retinal mural cells

The selective degeneration of retinal mural cells, a hallmark of early human diabetic retinopathy, has also been reported to occur in both diabetic and galactose-fed dogs. By employing antibodies raised against purified dog lens aldose reductase the presence of aldose reductase can be immunohistochemically demonstrated in the cytoplasm of mural cells but not endothelial cells of dog retinal vessels isolated by trypsin digestion. This immunohistochemical staining is similar to that observed with isolated human retinal vessels.

Diabetic cataracts in animal models: Prevention and reversibility with aldose reductase inhibitors

Cataract formation in diabetic and galactosaemic animals i s associated with changes in lens permeability which directly result from osmotic stress caused by the accumulation of polyols initiated by aldose reductase.’ This cataract formation i s accompanied by a number of biochemical changes including a decrease in the normally high intralenticular potassium:sodium ratio and decreases in the levels of lenticular reduced glutathione, adenosine triphosphate (ATP), inositol, and free amino acids (Figure 1). As the cataractous process progresses, growth of the lens i s reduced and protein synthesis declines. This is reflected in the observed decrease in the dry weight of cataractous lenses. Arresting the cataractous process at an early stage when only the cortical region is involved by either removal of the high galactose diet or treatment with aldose reductase inhibitors, results in reversal of the cataract along with a concomitant increase in lens dry weight as new lens fibres are formed. If, however, the process i s allowed to proceed to a stage when a nuclear cataract is formed (about 12 days in young rats fed a 50% galactose diet), reversal of the cataractous process wil l not result in the disappearance of a nuclear pin-head opacity. The synthesis of lens proteins may be monitored through short-term culture of lenses with radiolabelled methionine followed by separation of lens proteins on SDS-polyacrylamide gel electrophoresis and autoradiography.2 Changes in the synthesis of lens proteins can then be assessed through evaluation of densitometric scans of the autoradiographed gels. In rats fed a 50% galactose diet, the levels of crystallin synthesis in the lens decreases as lens opacification increases (crystallins are specific lens proteins that constitute over 90% of total lens proteins). This decrease in crystallin synthesis is reversed when the animal resumes a normal diet, eventually returning to levels similar to that of the control. This renewed synthesis occurs in newly formed lens fibres.