Metallothionein in human retinal pigment epithelial cells: expression, induction and zinc uptake

Zinc Supplementation Induced Transcriptional Changes in Primary Human Retinal Pigment Epithelium: A Single-Cell RNA Sequencing Study to Understand Age-Related Macular Degeneration

Zinc supplementation has been shown to be beneficial to slow the progression of age-related macular degeneration (AMD). However, the molecular mechanism underpinning this benefit is not well understood. This study used single-cell RNA sequencing to identify transcriptomic changes induced by zinc supplementation. Human primary retinal pigment epithelial (RPE) cells could mature for up to 19 weeks. After 1 or 18 weeks in culture, we supplemented the culture medium with 125 µM added zinc for one week. RPE cells developed high transepithelial electrical resistance, extensive, but variable pigmentation, and deposited sub-RPE material similar to the hallmark lesions of AMD. Unsupervised cluster analysis of the combined transcriptome of the cells isolated after 2, 9, and 19 weeks in culture showed considerable heterogeneity. Clustering based on 234 pre-selected RPE-specific genes divided the cells into two distinct clusters, we defined as more and less differentiated cells. The proportion of more differentiated cells increased with time in culture, but appreciable numbers of cells remained less differentiated even at 19 weeks. Pseudotemporal ordering identified 537 genes that could be implicated in the dynamics of RPE cell differentiation (FDR < 0.05). Zinc treatment resulted in the differential expression of 281 of these genes (FDR < 0.05). These genes were associated with several biological pathways with modulation of ID1/ID3 transcriptional regulation. Overall, zinc had a multitude of effects on the RPE transcriptome, including several genes involved in pigmentation, complement regulation, mineralization, and cholesterol metabolism processes associated with AMD.

Biomolecular modifications in the sacfry of Mogurnda adspersa in response to copper stress

Copper (Cu) is one of the most harmful contaminants in fresh-water systems. Fish larvae such as sacfry are particularly vulnerable to metals such as copper (Cu) due to a less-developed excretory organ system and permeable skin that can absorb metals directly from the water. However, the sublethal effects of metals on this life stage are not well understood. This study assessed the sublethal toxicity of Cu on purple-spotted gudgeon sacfry (PSG, Mogurnda adspersa). For this purpose, 96 h Cu toxicity bioassays were performed and toxic effects of Cu on PSG were measured at different levels of biological organization, from the individual (loss of equilibrium, wet weight), to tissue (chemical changes in retinal tissue composition) and molecular responses (whole body amino acid (AA) profiles). The EC₁₀ and EC₅₀ (ECx: effect concentration that affected X% of test organisms) were found to be 12 (9 - 15) µg Cu L^-1 and 22 (19 - 24) µg Cu L^-1, respectively. Copper stress caused a decrease in total amino acid content and changed the AA profile of PSG compared to the controls. Proton-induced X-ray emission (PIXE) mapping techniques showed accumulation of Cu in the retinal tissues disturbing the distribution of other elements such as zinc, sulfur, phosphorus and potassium. Fourier-transform infrared (FTIR) microspectroscopy of control and Cu treated eye tissues revealed a change in protein secondary structure in retinal tissues in response to Cu accumulation, as well as decreased levels of the molecular retinal, consistent with the degradation of rhodopsin, a key protein in the visual sensory system. This is the first study to demonstrate the multi-level responses of PSG arising from exposure to environmentally realistic Cu concentrations and suggests that AA profiling can serve as a useful tool to assess the impacts of metals on fresh-water organisms.

Antioxidant Defenses in the Human Eye: A Focus on Metallothioneins

The human eye, the highly specialized organ of vision, is greatly influenced by oxidants of endogenous and exogenous origin. Oxidative stress affects all structures of the human eye with special emphasis on the ocular surface, the lens, the retina and its retinal pigment epithelium, which are considered natural barriers of antioxidant protection, contributing to the onset and/or progression of eye diseases. These ocular structures contain a complex antioxidant defense system slightly different along the eye depending on cell tissue. In addition to widely studied enzymatic antioxidants, including superoxide dismutase, glutathione peroxidase, catalase, peroxiredoxins and selenoproteins, inter alia, metallothioneins (MTs) are considered antioxidant proteins of growing interest with further cell-mediated functions. This family of cysteine rich and low molecular mass proteins captures and neutralizes free radicals in a redox-dependent mechanism involving zinc binding and release. The state of the art of MTs, including the isoforms classification, the main functions described to date, the Zn-MT redox cycle as antioxidant defense system, and the antioxidant activity of Zn-MTs in the ocular surface, lens, retina and its retinal pigment epithelium, dependent on the number of occupied zinc-binding sites, will be comprehensively reviewed.

Light-induced retinal degeneration is prevented by zinc, a component in the age-related eye disease study formulation

Mineral supplements are often included in multivitamin preparations because of their beneficial effects on metabolism. In this study, we used an animal model of light-induced retinal degeneration to test for photoreceptor cell protection by the essential trace element zinc. Rats were treated with various doses of zinc oxide and then exposed to intense visible light for as long as 8 h. Zinc treatment effectively prevented retinal light damage as determined by rhodopsin and retinal DNA recovery, histology and electrophoretic analysis of DNA damage and oxidized retinal proteins. Zinc oxide was particularly effective when given before light exposure and at doses two- to four-fold higher than recommended by the age-related eye disease study group. Treated rats exhibited higher serum and retinal pigment epithelial zinc levels and an altered retinal gene expression profile. Using an Ingenuity database, 512 genes with known functional annotations were found to be responsive to zinc supplementation, with 45% of these falling into a network related to cellular growth, proliferation, cell cycle and death. Although these data suggest an integrated and extensive regulatory response, zinc induced changes in gene expression also appear to enhance antioxidative capacity in retina and reduce oxidative damage arising from intense light exposure.

Cortisol stimulates the zinc signaling pathway and expression of metallothioneins and ZnT1 in rainbow trout gill epithelial cells

Intracellular zinc signaling is important in the control of a number of cellular processes. Hormonal factors that regulate cellular zinc influx and initiate zinc signals are poorly understood. The present study investigates the possibility for cross talk between the glucocorticoid and zinc signaling pathways in cultured rainbow trout gill epithelial cells. The rainbow trout metallothionein A (MTA) gene possesses a putative glucocorticoid response element and multiple metal response elements 1042 base pairs upstream of the start codon, whereas metallothionein B (MTB) and zinc transporter-1 (ZnT1) have multiple metal response elements but no glucocorticoid response elements in this region. Cortisol increased MTA, MTB, and ZnT1 gene expression, and this stimulation was enhanced if cells were treated with cortisol together with zinc. Cells treated with zinc showed increased zinc accumulation, transepithelial zinc influx (apical to basolateral), and intracellular labile zinc concentrations. These responses were also significantly enhanced in cells pretreated with cortisol and zinc. The cortisol-mediated effects were blocked by the glucocorticoid receptor (GR) antagonist RU-486, indicating mediation via a GR. In reporter gene assays, zinc stimulated MTA promoter activity, whereas cortisol did not. Furthermore, cortisol significantly reduced zinc-stimulated MTA promoter activity in cells expressing exogenous rainbow trout GR. These results demonstrate that cortisol enhances cellular zinc uptake, which in turn stimulates expression of MTA, MTB, and ZnT1 genes.

Zinc and the Eye

Zinc, a trace element that influences cell metabolism through a variety of mechanisms, appears to play an integral role in maintaining normal ocular function. This element is present in high concentrations in ocular tissue, particularly in retina and choroid. Zinc deficiency has been shown in a number of species to result in a variety of gross, ultrastructural and electrophysiologic ocular manifestations. The physiological functions for zinc have been studied predominantly in retina and retinal pigment epithelium where zinc is believed to interact with taurine and vitamin A. modify photoreceptor plasma membranes, regulate the light-rhodopsin reaction, modulate synaptic transmission and serve as an antioxidant. Suboptimal zinc status in North America may influence the development and progression of several chronic eye diseases. Zinc supplementation trials and epidemiological studies have produced conflicting results concerning the role of zinc in age-related macular degeneration. Additional well-controlled supplementation trials are indicated to clarify the role of zinc in this disease. Future investigations must also expand our understanding of the mechanisms by which zinc regulates ocular morphology and function.

Zinc protects against oxidative damage in cultured human retinal pigment epithelial cells

This study was undertaken to determine whether bioavailable zinc can influence the effects of oxidative stress on cultured human retinal pigment epithelial (RPE) cells. RPE cells were maintained for 7 d in culture medium containing 14 microM total zinc, or in medium containing 0.55 microM total zinc. After 1 week, MTT assays were performed to determine the relative cytotoxicity of H2O2 or paraquat on RPE cells. Conjugated dienes and thiobarbituric acid reactive substances (TBARS) were measured in RPE cells treated with 0, 0.5 mM H2O2, 10 microM FeSO4 + 0.5 mM H2O2 or 10 microM FeSO4 + xanthine/xanthine oxidase for 24 h or paraquat for 7 d. Oxidized proteins were determined by the formation of carbonyl residues. The antioxidants metallothionein, catalase, superoxide dismutase, and glutathione peroxidase were also measured. The MTT assays showed that zinc protected cultured RPE from the toxicity of H2O2 and paraquat. RPE cells in 0.55 microM zinc medium contained higher levels of TBARS, conjugated dienes and protein carbonyls due to the oxidative stresses, compared to cells in 14 microM zinc. Catalase and MT content were reduced in cells cultured in 0.55 microM zinc medium and were reduced additionally when treated with above stresses. Superoxide dismutase activity increased in 0.55 microM zinc medium in response to these stresses. Our results show RPE cells cultured in zinc-reduced medium are more susceptible to oxidative insult.

Expression of the protective proteins hemopexin and haptoglobin by cells of the neural retina

The blood-retinal barrier, consisting of retinal pigment epithelial cells and retinal endothelial cells, prevents hemopexin and haptoglobin, anti-oxidant protective plasma proteins normally synthesized by the liver, from entering the neural retina. If present, these proteins must, therefore, be made locally. The cell types within the retina in which hemopexin and haptoglobin mRNAs are made have been investigated. RNA was extracted from both the neural retina and pigment epithelium obtained by dissection of human donor eyes as well as from cultured pigment epithelial and photoreceptor cells. The mRNAs for both haptoglobin and hemopexin were detected, using reverse-transcriptase polymerase chain reaction, in the neural retina and cultured photoreceptors but not in pigment epithelial cells. The cellular location of these mRNAs was determined using in situ hybridization of sections of human retina which revealed that haptoglobin mRNA was located principally in the photoreceptor cells, cells of the inner nuclear layer and some cells of the ganglion cell layer. Hemopexin mRNA, previously shown to be made in the human neural retina (Hunt et al., 1996. Journal of Cellular Physiology 168: 71-80), is expressed by most of the cells of neural retina including the photoreceptors and, notably, the ganglion cells.

The localization of endogenous zinc and the in vitro effect of exogenous zinc on the GABA immunoreactivity and formation of reactive oxygen species in the retina

1. Endogenous zinc is localized mainly in the retinal photoreceptors and retinal pigment epithelial cells in the mammalian retina. No other types of retinal neurons contain large amounts of zinc. 2. Low concentrations of exogenous zinc, like the N-methyl-D-aspartate (NMDA) antagonist MK-801, counteract the NMDA-induced changes in the gamma-aminobutyric acid (GABA) immunoreactivity in the rabbit retina. However, greater concentrations of zinc exacerbate the effects of NMDA and ischemic-like insults (lack of glucose and oxygen) on GABA immunoreactivity. The data suggest that low concentrations of zinc are neuroprotective, but higher concentrations of zinc have a negative effect. 3. When low concentrations of zinc are present during ischemic-like insults to the retina, the GABA immunoreactivity is localized to the Müller cells, suggesting that the metabolism of GABA in the Müller glial cells is prevented. 4. Ascorbate/iron-induced generation of reactive oxygen species (ROS) in the retina is prevented by deferoxamine but not by zinc. High concentrations of zinc potentiate the ascorbate/iron induced formation of ROS.

Heme-mediated reactive oxygen species toxicity to retinal pigment epithelial cells is reduced by hemopexin

Catalysis of the formation of reactive oxygen species (RO2S) by low molecular weight complexes of iron has been implicated in several pathological conditions in the retina since photoreceptors and retinal pigment epithelial cells are likely to be especially sensitive to RO2S. Since protective proteins cannot cross the blood-retinal barrier, it is likely that the retina performs its own protective functions by synthesizing proteins that bind iron and nonprotein iron complexes, the major catalysts of RO2S generation. Investigations were carried out to determine whether pigment epithelial cells are themselves sensitive to iron-generated RO2S and whether apo-transferrin and apo-hemopexin, known to be made locally in the retina, can perform a protective function. In 51Cr release assays, the toxicity of exogenous RO2S including hydrogen peroxide or superoxide (generated by xanthine oxidase/hypoxanthine) to human retinal pigment epithelial cells was inhibited by the iron chelators, desferrioxamine and apo-transferrin. Free but not protein-bound ferric iron and heme exacerbated the toxic effect. The toxic effect of heme was abolished by the heme-scavenging, extracellular antioxidant, apo-hemopexin, and also by exogenous bovine serum albumin. In addition, heme toxicity was inhibited by a 3 h preincubation of cells with either heme, apo-hemopexin, or heme-hemopexin 24 h prior to the toxicity assay. It is concluded, first, that toxic effects of iron and heme can be prevented by apo-transferrin or apo-hemopexin and, second, that exposure of RPE cells to free heme or hemopexin sets in motion a series of biochemical events resulting in protection against oxidative stress. It is probable that these include heme oxygenase induction.

Influence of zinc on selected cellular functions of cultured human retinal pigment epithelium

Zinc is a necessary micronutrient, usually abundant in human RPE. Our study was undertaken to determine the effects of short-term, zinc deficiency on human retinal pigment epithelium (RPE) using a culture model of fetal human RPE cells. Human fetal RPE cells were isolated and cultured in Coon's modified Ham's F-12 medium. For zinc depletion studies, cells were cultured for 1 week in Chelex-treated Dulbecco's modified Eagle's medium containing low (0.25 microM) or physiologic (11 microM) total zinc concentrations as determined by flame atomic absorption spectroscopy. Protein synthesis was determined by incorporation of 35S-cysteine/methionine and labeled proteins analysed by polyacrylamide gel electrophoresis. Several cell parameters and enzymes were significantly reduced below control when cultured in low zinc: zinc content (40%), proliferation (63%), protein/well (50%), catalase activity (68%), alkaline phosphatase activity (61%), alpha-mannosidase activity (68%), and metallothionein (82%). No statistically significant decline was seen in acid phosphatase activity, superoxide dismutase activity, glutathione peroxidase activity and dexamethasone induction of metallothionein. Zinc repletion (100 microM, 1 h) increased catalase and alpha-mannosidase activities from 32% and 33% of control to 75% and 73%, respectively. Cycloheximide did not inhibit this short-term zinc-induced repletion of catalase or alpha-mannosidase. Protein synthesis in low zinc medium was depressed, but not significantly, as shown by incorporation of radiolabeled 35S-cysteine/methionine into newly synthesized proteins. The effects of zinc deficiency in cultured human RPE are selective. Adequate intracellular zinc was required for maximal activity of some enzymes. The dependence of catalase activity on zinc was not predicted and may help explain the observed decline in catalase activity seen with age in RPE. Our model of zinc deficiency should prove useful in elucidating the complex effects of zinc deficiency and repletion in human RPE.