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Review |
Department of Small Animal Clinical Sciences, Western College of Veterinary Medicine B.H.G.), CANADA
College of Pharmacy and Nutrition (P.G.P., Z.Z.), University of Saskatchewan, Saskatoon, Saskatchewan, CANADA
Department of Family and Nutritional Sciences, University of Prince Edward Island, Charlottetown, Prince Edward Island (K.T.G.-P.), CANADA
Address reprint requests to: Bruce H. Grahn, D.V.M., Department of Small Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon S7N 5C9, CANADA. E-mail: Bruce.Grahn{at}usask.ca
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONLOCALIZATION AND CONCENTRATION...ELECTROPHYSIOLOGIC, GROSS AND...ZINC SUPPLEMENTATION AND...THE FUNCTIONS OF ZINC...CONCLUSIONSREFERENCES |
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Key words: zinc, eye, retina, age-related macular degeneration
Abbreviations: RPE=retinal pigment epithelium • ARMD=age-related macular degeneration • ERG=electroretinogram • OP=oscillatory potential
Key teaching points:
• There is a substantial concentration of zinc in the eye, particularly in the retina, retinal pigment epithelium and choroid of most species of animals
• The ocular manifestations of zinc deficiency include altered vision, electroretinograms, and oscillatory potentials, and, if the deficiency is severe, ultrastructural changes are detected in the retina and retinal pigment epithelium
• Zinc supplementation may alter the progression of some degenerative retinal diseases
• The known functions of zinc in the retina and retinal pigment epithelium include modification of synaptic transmission, regulation of the light rhodopsin reaction, interactions with taurine and vitamin A, modifier of photoreceptor plasma membranes and antioxidant.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONLOCALIZATION AND CONCENTRATION...ELECTROPHYSIOLOGIC, GROSS AND...ZINC SUPPLEMENTATION AND...THE FUNCTIONS OF ZINC...CONCLUSIONSREFERENCES |
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2-macroglobulin, with a small percentage bound to low molecular weight substances such as histidine and cysteine [6]. Until recently, the mechanism of zinc transport by many tissues including enterocytes has been poorly understood. Although differences exist among different cell types, there appears to be both saturable and nonsaturable components to the transport process [7]. There may also be an energy requirement for some aspects of zinc transport [7]. Several proteins directly associated with zinc transport have been described in the last decade, and the family of known zinc transporters has recently beenreviewed [7]. Once these transporters have been completely described, we will have a better basis for understanding zinc absorption and homeostasis. Most zinc in the body is intracellular; the largest pools in adults are in the muscle, bone, skin, hair and liver [2]. Most of the intracellular zinc is stable and does not respond to zinc deprivation. In addition to the plasma zinc pool, the small labileportion in the liver might provide some small reserve during deficiency [2]. Thus, a regular adequate dietary supply of zinc is needed. The metallothioneins, a family of intracellular zinc-binding proteins, have been proposed as playing an important role in controlling whole body zinc metabolism [6,8]. Although severe zinc deficiency is relatively rare in North America, moderate zinc deficiency has been associated with malabsorption syndromes, alcoholism, chronic renal disease and chronic debilitating diseases [1,5]. It is estimated that a milder deficiency presenting with growth retardation or altered taste and olfaction is also relatively widespread in healthy individuals, including the elderly [2,5]. The diagnosis of zinc deficiency is difficult due to the lack of a single specific and sensitive biochemical index of zinc status. The most reliable method for diagnosing marginal zinc deficiency in humans is considered to be a positive response to zinc supplementation. An alternative approach is to use a combination of biochemical and functional tests to evaluate zinc status. Serum or plasma zinc on its own is neither sensitive nor specific. A number of comprehensive reviews have been published on the subject of assessing zinc status in the human population [9–13].
Zinc functions in a multitude of physiologic roles. The clinical manifestations of zinc deficiency depend upon the severity of the deficiency, but the classic features include anorexia, retarded growth, weight loss, impaired immune function, delayed sexual maturation, testicular atrophy, epidermal hyperkeratinization, alopecia, hypogeusia and night blindness [2]. The nonspecificity of these features supports the fundamental role of zinc in cell metabolism [2]. The major biochemical functions of zinc include its catalytic or structural role in at least 300 zinc metalloenzymes, a structural role in a large number of transcription factors and a role in the maintenance of plasma membrane function [6,14,15].
The concentration of zinc in ocular tissues is unusually high when compared to other tissues [16]. The descending order of zinc concentration in human ocular tissues is retina and choroid, ciliary body, iris, optic nerve, sclera, cornea, and lens [3]. These unusually high concentrations and conflicting reports of zinc status and vision loss in macular degeneration [17,18] have lead to a wealth of recent research. There are only a few reviews of zinc and the eye, and they were published almost two decades ago [3,19]. The purpose of this paper is to review the ocular manifestations of zinc deficiency, localization and concentration of zinc in the eye and proposed functions of this element in the retina and retinal pigment epithelium (RPE).
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LOCALIZATION AND CONCENTRATION OF ZINC IN THE EYE |
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Zinc has been localized in the photoreceptors of rats and cattle [34,35]. When rats were fed zinc-deficient diets, a weaker histochemical was displayed within the photoreceptors, suggesting a decreased zinc content in the outer segments [20]. Neo-Timm staining of the retinas of larval tiger salamanders has revealed that a significant concentration of zinc is also present in the outer nuclear layer [25].
In vivo and in vitro studies have identified substantial amounts of zinc in the choroid and the RPE [21,22,31,36,37] that alter retinal and RPE morphology [38,39], and both of these tissues are important in the uptake of zinc within the eye [40,41,42]. Zinc is also present in the tapetum lucidum of the cat and dog and, more specifically, in the periphery of the tapetal rods, between the tapetal rod and its membrane [21,22,36].
Zinc concentration in ocular tissues has been determined by flame or flameless atomic absorption spectrophotometry [37,43–52], neutron activation [53,54], X-ray fluorescence [16], electron probe microanalysis [55], inductively coupled plasma-mass spectrometry [56,57] and plasma emission spectrophotometry [58]. The amount of zinc varies greatly with age and the ocular tissue examined; however, the retina and uvea usually contain the highest concentrations (Table 1) [16,37,52,53,56,57,59–62]. A number of investigators have reported an increase in zinc concentration in cataractous lenses [49,50,63], and cataracts have been associated with zinc deficiency in fish [64–67]. The zinc concentration is consistent throughout the normal rabbit lens, despite higher activity of superoxide dismutase, a zinc-dependent metalloenzyme, in the anterior lenticular segment [52,68]. Bentley et al. have characterized zinc exchange in the rabbit lens [68]. Although cornea and aqueous humor zinc concentration declined in the zinc deficient rabbit, these investigators found no change in the zinc concentration of the lens [52]. In contrast, we reported a significant decline in lens zinc concentration in the zinc-deficient rat [57]. This series of reports linking zinc to the lens suggests that the physiologic function of zinc in the lens should be explored as well as the impact of zinc deficiency on cataract development.
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ELECTROPHYSIOLOGIC, GROSS AND ULTRASTRUCTURAL OCULAR MANIFESTATIONS OF ZINC DEFICIENCY |
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TOPABSTRACTINTRODUCTIONLOCALIZATION AND CONCENTRATION...ELECTROPHYSIOLOGIC, GROSS AND...ZINC SUPPLEMENTATION AND...THE FUNCTIONS OF ZINC...CONCLUSIONSREFERENCES |
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Rats
Zinc deficiency in this species dramatically affects ocular development. Severe zinc deficiency, when imposed on rats during gestation, resulted in optic cup invagination failure, colobomata, retinal dysplasia and occasionally anophthalmia in the pups [69,70]. We reported significant retinal dysplasia and depression of the electroretinograms in rats born to dams that were marginally zinc- and taurine-deficient throughout gestation and postnatal life until 7.5 to 8.5 weeks of age [71,72]. These studies indicate that zinc is essential for normal ocular development. When zinc deficiency was imposed at parturition in rats, eyelid opening was delayed; however, morphological changes were not noted in the photoreceptor cells at weaning [73]. Marginal zinc deficiency imposed in our postnatal rat model also failed to produce morphologic ocular changes despite depression of the electroretinogram and oscillatory potentials [74]. Weanling male Sprague-Dawley rats that were maintained for seven weeks on a severely zinc deficient diet did not develop gross ocular manifestations of zinc deficiency. However, they did display retarded growth, alopecia and scaly epidermis and keratitis, and electron microscopy confirmed degeneration of the photoreceptor outer segments and osmiophilic inclusion bodies in the RPE [75–78].
Cats, Dogs, Pigs and Fish
An inherited tapetal abnormality has been reported in the dog and cat, in which the tapetal rods contain significantly less zinc than those of control animals. The clinical manifestations included a lack of tapetal development [36,62,79]. Jacobsen et al. fed cats diets that were marginally deficient in zinc (<7 ppm zinc) for four months. They reported reduced weight gain and, although no other morphologic changes were described in these cats, documented inconsistent reductions in the b-wave amplitude of the electroretinogram, which were reversible with zinc repletion [80]. Robertson and Burns reported that dogs fed a zinc-deficient diet containing excess calcium developed conjunctivitis, ocular discharge and keratitis [81]. Samuelson et al. have reported ultrastructural changes in the RPE and choroidal melanocytes of adult pigs when they were maintained on diets deficient in zinc. These included displacement of cone nuclei towards the RPE, swollen outer segments of photoreceptors and altered choroidal melanosomes [31,38,39,82,83]. Cataracts and exophthalmos have been reported in trout and salmon associated with zinc deficiency [64–67].
Humans and Other Primates
Most of the ocular diseases in humans and primates that are associated with zinc deficiency have their pathogenesis linked by decreased serum zinc or by improvement after zinc supplementation and only occasionally by reduced tissue zinc concentrations. Retinas from cynamolgus monkeys with early onset macular degeneration, manifested by drusen and lipofuscin spots, are reported to contain four-fold less zinc and decreased expression of the metallothionein gene as compared to normal monkeys [84]. Olin reported that 47% of a population of aging Rhesus macaques examined had macular drusen; although copper-associated enzymes were the more likely to be associated, serum zinc levels also tended to be lower in monkeys with greater than 10 drusen/fundi [85].
Acrodermatitis enteropathica is a rare early childhood disease, with multiple systemic manifestations caused by an abnormality in zinc metabolism. The ocular abnormalities include blepharitis, photophobia, conjunctivitis, corneal opacities and cataracts [86–89]. Superficial punctate opacities, nebulous subepithelial opacities and linear corneal erosions have also been reported in cases of acrodermatitis enteropathica [86–90]. Gene expression in acrodermatitis enteropathica has been recently investigated with differential display, and it appears that there may be an insertional mutation affecting mRNA of a zinc transport protein, which results in decreased zinc absorption [91]. Recalcitrant corneal ulcers have been reported in conjunction with low serum zinc [92]. In addition, zinc has been linked indirectly with corneal ulcers because it may be required for functional activity of collagenases [93].
Cherry-red maculopathy and visual impairment have been reported in men with Crohn’s disease and zinc deficiency, and the visual function in some patients improves with zinc supplementation [94,95]. Decreased serum zinc has been documented in patients with retinitis pigmentosa [96]. Increased fecal zinc and decreased granulocyte zinc have been described with desferrioxamine use in thalassaemia [97]. Patients receiving long-term total parenteral nutrition have been reported with altered visual function and decreased plasma zinc concentrations [98]. Similarly, abnormal dark adaptation and diminished scotopic retinal responses are associated with zinc deficiency in alcoholism and hepatic cirrhosis. These conditions often respond to zinc supplementation, but also may require supplemental vitamin A [99–105]. While all these studies have advanced the concept of a role for zinc in ocular function, they are complicated by the co-existence of other nutrient deficiencies.
Given the subtle and non-specific ocular manifestations associated with zinc deficiency, the clinical diagnosis is difficult. More importantly, the marginal zinc deficiency associated with chronic diseases may affect the eye by interactions with other key elements, toxins or other diseases, and the clinical signs may not manifest for prolonged periods of time [106–110].
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ZINC SUPPLEMENTATION AND DEGENERATIVE EYE DISEASES |
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Recent studies have not clarified the issue of the need for zinc supplementation of patients with degenerative retinal diseases any further. Stur et al. reported a double masked, randomised placebo controlled study that failed to show any effect of zinc supplementation in patients who had an exudative form of ARMD in one eye [112]. Subsequently, Mares-Perlman et al. reported that dietary zinc was weakly protective in the development of early ARMD [113]. In contrast, the Blue Mountains Eye Study failed to document any association between ARMD and intake of zinc from diet or supplements [114]. A recent prospective population-based cohort study demonstrated no relationship between incidence of early ARMD and dietary intake of zinc, whereas a modest inverse association was observed between dietary zinc and development of retinal pigmentary abnormalities [115]. The inconsistencies among these studies could be due to differences in study design, specificity of macular lesions, stage of the disease and methods for assessing dietary zinc intake or status [115].
Further intervention studies are required to establish firm criteria for zinc supplementation aimed at preventing some degenerative retinal diseases [116]. Some authors caution against widespread oral zinc supplementation of humans with ARMD [117]. It is important to stress that the pathogenesis of ARMD is complex and involves multiple genetic, toxic and perhaps nutritional factors. The determination of nutrition influences will require an innovative supplementation study. Such a study is being attempted currently in North America, with the objective to clarify the progression of ARMD with and without supplementation of multi-vitamins, minerals and antioxidants [118]. The prevention of ARMD in most patients in the future will likely involve pharmacologic, genetic and nutritional interventions to change the primary biochemical abnormalities [119].
While numerous studies have documented a relationship between zinc and diabetes mellitus, there are few studies that have investigated the potential link between zinc status and the development of diabetic retinopathy. Chausmer (120) has reviewed the complex interactions between zinc and diabetes. Not only can diabetes disturb zinc homeostasis, mainly through increased urinary zinc loss, but zinc status may also influence insulin action and the development of diabetes mellitus and its chronic complications. Faure et al. (121) suggested that there was a protective effect of zinc supplementation on markers of oxidative stress in patients with diabetic retinopathy. Zinc deficiency and supplementation studies in animal models will be required to unravel the role of zinc in the complex pathogenesis of this and other ocular conditions.
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THE FUNCTIONS OF ZINC IN THE RETINA AND RPE |
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TOPABSTRACTINTRODUCTIONLOCALIZATION AND CONCENTRATION...ELECTROPHYSIOLOGIC, GROSS AND...ZINC SUPPLEMENTATION AND...THE FUNCTIONS OF ZINC...CONCLUSIONSREFERENCES |
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Retina
Zinc is highly concentrated in the retina [37]. Despite a number of electrophysiologic changes noted in mild dietary zinc deficiency in rats [71,72,74], we have not detected a decline in retinal zinc concentration by inductively coupled plasma-mass spectrometry [57]. However, there may be an intracellular shift of zinc which may be significant in terms of function [6]. As previously described, zinc has been mapped to specific retinal regions with histochemistry. Using dithizone, zinc was detected only in rat photoreceptor outer segments, and the staining was reduced in zinc deficiency [20]. The histochemical approaches have limitations in mapping zinc throughout the retina as they can measure only loosely bound zinc [23]. However, this may be the important pool since tightly bound zinc appears to be well-preserved under many zinc deficiency conditions. The results of our studies suggest that effects of zinc are not confined to the photoreceptors, but also extend to inner retinal neurons since zinc deficiency depressed specific oscillatory potentials as well as the b-wave of the electroretinogram (ERG) [71,72,74].
Our studies also revealed that mild zinc deficiency present during prenatal and postnatal life substantially worsened the retinal degeneration observed in the taurine-deficient rat [71,72,74]. When the zinc and taurine deficiencies were combined, retinal dysplasia was documented in the pre-natal model only [71]. Zinc deficiency imposed throughout pre- and postnatal retinal development depresses the ERG, and zinc acts synergistically with taurine in inducing these effects [71,72]. Deficiencies of zinc or taurine also independently depressed the a- and b-waves of the ERG [72]. Vmax represents the maximum amplitude obtained when the wave amplitude is plotted as a function of light intensity [122]. Vmax for the b-wave was decreased, in our studies, suggesting a decrease in the area of functioning retina [72]. Zinc deficiency independently depressed the amplitude of the first oscillatory potential (OP1), but depressed OP2 and OP3 only when the rats were also taurine-deficient [71]. We subsequently evaluated the retinal response to suboptimal zinc supply imposed during the postnatal period until after the retina reached maturity in rats [74]. Zinc deficiency caused a decline in the amplitude of OP1, OP3, OP4 and OP5 and decreased the amplitude of the a- and b- waves of the ERG [74]. The nature of the interaction with taurine differed from that seen in the first study [71].
Our results support an earlier study which revealed that zinc and taurine interact in the retina in vitro [123]. Jacobsen et al. also reported an elevated dark-adapted ERG threshold in zinc-deficient cats that was reversible with zinc supplementation. The rod-mediated electroretinograms from the cats with zinc deficiency differed from those of cats with taurine deficiency [80]. These studies demonstrate that mild zinc deficiency, consistent with that which may occur in selected human populations, produces detrimental functional consequences in the retina.
A number of hypotheses have been offered to explain the mechanism of action of zinc in the retina, including its role in zinc metalloenzymes. While there are a vast number of zinc metalloenzymes, only a few have been shown in other tissues to have reduced activity in response to zinc deficiency, and this is likely because the zinc is tightly bound to the metalloenzyme [6]. Convincing correlations between changes in the activity of specific zinc metalloenzymes and the pathology of zinc deficiency are also lacking. Therefore we suggest that alternate mechanisms appear more viable in explaining the functional activity of zinc in retina.
It has been proposed that zinc exerts an effect on retinal function through its involvement in vitamin A metabolism. Smith has reviewed the complex and controversial interaction between zinc and vitamin A [124]. Impaired dark adaptation reported in human zinc deficiency [99,125] has been thought to occur due to decreased rhodopsin formation [126]. Conflicting results have been obtained in severely zinc-deficient rats as to whether this is a consequence of a decreased rate of oxidation of retinol to retinal by a zinc-dependent alcohol dehydrogenase [124,126–128]. It has become apparent that much of the effect of zinc on this enzyme can be attributed to the depressed food intake and growth retardation associated with severe zinc deficiency [124]. These studies highlight the importance of including pair-fed controls in animal studies of zinc deficiency. Zinc deficiency also depresses plasma and liver retinol binding protein concentrations, which could result in a failure to mobilize vitamin A from liver stores [124,129]. The impact of the latter on vitamin A metabolism in the retina has not been studied. It has been proposed that zinc deficiency must be of a severe nature to impair vitamin A metabolism at the cellular level [124]. An independent role for zinc in retinal function was suggested by a study in which supplementation of both zinc and vitamin A was necessary for the greatest improvement in the electroretinogram of rats deficient in both nutrients [130]. Christian and West have also concluded in a recent review that there remains to be established evidence of an interaction between zinc and vitamin A of proven health significance in humans [131]. Since our prenatal model [71,72] has shown depressed retinal function in mild zinc deficiency in the absence of depressed growth, we suggest that other functions of zinc in the retina exist and should be explored further.
Zinc is important in plasma membrane function through its effects membrane protein conformation and protein-protein interactions [6]. This may be a critical function for zinc in the outer segments of photoreceptors. The discs, comprising stacks of double membranes, are derived through constant growth of the plasma membrane [132]. Loss of zinc from disc proteins that control disc shedding, or maintain structure, may compromise membrane integrity and depress the ERG. Zinc is a normal component of the disc membrane that can be measured by particle-induced X-ray emission [133]. It has been reported that zinc is associated with rod outer segment proteins and that increased amounts were present when the samples were photobleached [34]. Takahashi et al. have also reported a nonuniform subcellular distribution of zinc, with the largest amounts found in the rod outer segments and also in the photoreceptor cell synaptosomes [134]. A metallothionein-like protein may function in transport and compartmentalization of zinc within the retina [134].
Another particularly attractive hypothesis is that of Shuster et al., who suggest that zinc might help to stabilize the light-activated protein, rhodopsin, or modulate reactions that are initiated by light [135]. Zinc was first found to alter binding of azido-(32P)ATP to rhodopsin [136]. Subsequently zinc was shown to bind detergent-solubilized rhodopsin and enhance rhodopsin phosphorylation at micromolar concentrations [35]. The latter is important for initiating quenching of the light response [135]. The work of Shuster et al. suggests that this effect of zinc appears to occur through an effect on the substrate rather than by enhancing rhodopsin kinase or depressing phosphatase activity [135]. In contrast, studies by Ou et al. suggest that metallothionein in bovine retina can donate zinc to activate protein kinase C which can phosphorylate rhodopsin [137]. Zinc deficiency in humans has also been shown to alter dark adaptation [99,125]. These studies suggest that zinc is associated with the primary photochemical event. This is further supported by a recent report that the localization of histochemically visualized zinc shifts when rat retinas are dark- or light-adapted [138]. These investigators found chelatable zinc in the dark-adapted retina to be primarily associated with the RPE cells and photoreceptor perikarya and some of the perikarya of the inner nuclear layer, both plexiform layers and the ganglion cell layer. When the retina was light adapted, the chelatable zinc was detectable in the inner segments of the photoreceptors only. It was suggested that during light adaptation, zinc may be translocated from the perikarya into the inner and outer segments, but that the zinc in the outer segments was not visualized histochemically because of binding to molecules like rhodopsin [138]. It has also been suggested that an alteration in retinal zinc metabolism may be responsible for the rhodopsin loss seen in fetal alchohol syndrome [139].
Neo-Timm staining also shows zinc to be concentrated in the synaptic terminal regions of photoreceptors, where it has been suggested to function as a modulator of synaptic transmission [25,140]. Zinc and cadmium preferentially occupy open channels in salamander rods and the divalent cations Ni2, Cd2, Zn2 and Mn2 potentiate the activity of cGMP by more than threefold in the outer segments [141]. Using larval tiger salamanders, Wu et al. localized zinc ions within glutamatergic synaptic vesicles in the base of inner segments of the photoreceptors and postulated that zinc was a diffusable switch that regulated neurotransmitter signalling in the outer retina [25]. In addition, they demonstrated that Zn2+ blocks the depolarizing effects of GABA in horizontal cells and suggested that both rod and cone photoreceptors use vesicular Zn2+ to modify the response of the first retinal synapse [25]. Subsequently, Qian et al. documented GABAa in glial cells of the Skate and bipolar cells of Bass and reported that zinc enhanced the response to GABA significantly in these cells [140,142–144]. Zinc has also been shown to inhibit the GABAc response of bipolar cells of bass and the horizontal cells of catfish [140, 142–147]. Shen and Yang reported that low concentrations of zinc stimulated AMPA receptors of carp retinal horizontal cells while higher concentrations inhibited the same response [148].
Zinc may also serve a critical function as an antioxidant in the retina. Although zinc appears to act as an antioxidant in in vitro systems, the specific antioxidant functions for zinc in vivo have not been identified in other tissues [149] and have generally not been studied in the retina. One in vitro study utilizing rod outer segments of frogs showed a significant protective effect by zinc and taurine against peroxidative damage induced by ferrous sulfate [123]. What is striking in other tissues is that zinc-deficient animals are less able to tolerate an increase in oxidative stress, as has been shown in lung exposed to hyperoxia [149,150]. The photoreceptors are particularly susceptible to oxidative damage because of their high polyunsaturated fatty acid content, high metabolic rate and constant bombardment by light [151]. The retina also has relatively low peroxide scavenging capabilities as shown by low glutathione concentration and glutathione peroxidase activity [152,153]. These properties in addition to its high zinc concentration suggest that the retina may be an ideal tissue in which to study the antioxidant functions of zinc.
Retinal Pigment Epithelium
Zinc is also highly concentrated in this cell layer [29], and the concentration decreases during periods of zinc deficiency [31]. Previous data in the pig and rat are conflicting as to whether the morphologic damage associated with zinc deficiency develops first in the neural retina or occurs secondary to alterations in the retinal pigment epithelium [39,77]. The RPE appears to be a significant cell that maintains retinal zinc pools during dietary deficiency. Studies in our laboratories reveal that retinal zinc concentrations are conserved in marginal zinc deficiency in the rat despite significant decreases in serum, liver and bone zinc concentrations [57]. Leure-duPree gave zinc chelators intra-peritoneally and produced RPE electron opaque inclusions. They concluded that sequestration of zinc in Spraque-Dawley rats resulted in pronounced effects on the RPE [75]. They provided further evidence that these effects were specific to zinc deficiency in their subsequent report of marked ultrastructural abnormalities, including deep basal infolding of the RPE, vesiculation and degeneration of the rod outer segments in rats maintained on severely zinc deficient diets [76,77].
Newsome and Rothman have documented zinc uptake by human RPE in vitro by what appears to be facilitated transport [41]. Newsome et al. demonstrated uptake and retention by RPE and other ocular tissues in rhesus monkeys when 65Zn was administered intravenously or orally [42]. Zinc sulfate concentrations ranging from 0.5 to 5 mM have been shown to activate alpha-mannosidase by twofold when added to human RPE cell cultures [154]. Alpha-mannosidase is concentrated in the RPE and is likely a key enzyme that digests thousands of effete disc membranes each day. Oliver et al. have confirmed the expression of metallothionein in cultured human RPE cells and have shown that its induction is associated with increased capacity for 65Zn uptake in these cells [40]. The presence of metallothionein in the RPE of rats has also been demonstrated by immunohistochemistry [26]. Tate et al. determined that the macular region contains significantly less zinc and metallothionein than the peripheral RPE and that macular RPE metallothionein decreases significantly in patients older than 70 years of age [27]. Culturing human RPE cells in low zinc media resulted in a reduction in metallothionein and zinc concentration, decreased cell proliferation, reduced protein production and decreased activity of catalase, alkaline phosphatase and alpha-mannosidase [155]. The decrease in catalase activity was unexpected since it is not a zinc-dependent enzyme. A follow-up study of zinc supplementation in cultured fetal RPE cells suggested that zinc may play a role in transcriptional regulation of catalase [156]. Subsequent research by this group revealed that human RPE cells cultured in low zinc medium were more susceptible to oxidative insult induced by H2O2 and paraquat [157]. This was shown by an increase in conjugated dienes, thiobarbituric acid reactive substances and protein carbonyls which was coincident with a decrease in catalase activity and reduced metallothionein concentration. In dietary zinc deficiency in pigmented rats, these researchers reported the depression in metallothionein concentration seen in their RPE culture studies, but no change in catalase activity. An increase in retinal oxidative stress was also evident as measured by thiobarbituric acid reactive substances [158]. While they also suggested that dietary zinc deficiency decreases neuroretinal zinc, their data were presented as total retinal zinc; this makes it difficult to interpret whether the decrease is due to zinc deficiency or the smaller size of the retina since zinc deficiency causes growth retardation.
While the mechanism by which zinc serves as an antioxidant in any tissue is uncertain [149], one indirect role of zinc in RPE cells may be its ability to induce the synthesis of metallothionein, which may scavenge hydroxyl radicals [159].
In summary, a number of potential mechanisms have been put forward by which zinc may play a central role in the function of the retina and RPE. These are mainly based on in vitro studies in which zinc has been added in concentrations ranging from physiological to pharmacological. These mechanisms should be further explored in zinc-deficient animal models to determine which of the biochemical functions can account for the altered retinal physiology associated with zinc deficiency.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONLOCALIZATION AND CONCENTRATION...ELECTROPHYSIOLOGIC, GROSS AND...ZINC SUPPLEMENTATION AND...THE FUNCTIONS OF ZINC...CONCLUSIONSREFERENCES |
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A complete picture of the many mechanisms by which zinc influences cell metabolism is still being explored. Future investigations must address these mechanisms in the eye as a means of understanding the role of zinc in ocular function and degenerative eye diseases.
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ACKNOWLEDGMENTS |
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Received May 22, 2000. Accepted December 5, 2000.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONLOCALIZATION AND CONCENTRATION...ELECTROPHYSIOLOGIC, GROSS AND...ZINC SUPPLEMENTATION AND...THE FUNCTIONS OF ZINC...CONCLUSIONSREFERENCES |
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32P) binding to rod outer segment proteins.
Exp Eye Res
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475–484,
1988.[Medline]
