Histochemical localization of zinc and copper in rat ocular tissues

Is the human sclera a tendon-like tissue? A structural and functional comparison

Collagen rich connective tissues fulfill a variety of important functions throughout the human body, most of which having to resist mechanical challenges. This review aims to compare structural and functional aspects of tendons and sclera, two tissues with distinct location and function, but with striking similarities regarding their cellular content, their extracellular matrix and their low degree of vascularization. The description of these similarities meant to provide potential novel insight for both the fields of orthopedic research and ophthalmology.

A Multi-Omics Approach Identifies Key Regulatory Pathways Induced by Long-Term Zinc Supplementation in Human Primary Retinal Pigment Epithelium

In age-related macular degeneration (AMD), both systemic and local zinc levels decline. Elevation of zinc in clinical studies delayed the progression to end-stage AMD. However, the molecular pathways underpinning this beneficial effect are not yet identified. In this study, we used differentiated primary human fetal retinal pigment epithelium (RPE) cultures and long-term zinc supplementation to carry out a combined transcriptome, proteome and secretome analysis from three genetically different human donors. After combining significant differences, we identified the complex molecular networks using Database for Annotation, Visualization and Integrated Discovery (DAVID) and Ingenuity Pathway Analysis (IPA). The cell cultures from the three donors showed extensive pigmentation, development of microvilli and basal infoldings and responded to zinc supplementation with an increase in transepithelial electrical resistance (TEER) (apical supplementation: 443.2 ± 79.3%, basal supplementation: 424.9 ± 116.8%, compared to control: 317.5 ± 98.2%). Significant changes were observed in the expression of 1044 genes, 151 cellular proteins and 124 secreted proteins. Gene set enrichment analysis revealed changes in specific molecular pathways related to cell adhesion/polarity, extracellular matrix organization, protein processing/transport, and oxidative stress response by zinc and identified a key upstream regulator effect similar to that of TGFB1.

Zinc Nutrition and Inflammation in the Aging Retina

Zinc is an essential nutrient for human health. It plays key roles in maintaining protein structure and stability, serves as catalytic factor for many enzymes, and regulates diverse fundamental cellular processes. Zinc is important in affecting signal transduction and, in particular, in the development and integrity of the immune system, where it affects both innate and adaptive immune responses. The eye, especially the retina-choroid complex, has an unusually high concentration of zinc compared to other tissues. The highest amount of zinc is concentrated in the retinal pigment epithelium (RPE) (RPE-choroid, 292 ± 98.5 µg g^-1 dry tissue), followed by the retina (123 ± 62.2 µg g^-1 dry tissue). The interplay between zinc and inflammation has been explored in other parts of the body but, so far, has not been extensively researched in the eye. Several lines of evidence suggest that ocular zinc concentration decreases with age, especially in the context of age-related disease. Thus, a hypothesis that retinal function could be modulated by zinc nutrition is proposed, and subsequently trialled clinically. In this review, the distribution and the potential role of zinc in the retina-choroid complex is outlined, especially in relation to inflammation and immunity, and the clinical studies to date are summarized.

Copper Intraocular Foreign Body: Diagnosis and Treatment

Copper intraocular foreign bodies (IOFB) are relatively common after penetrating eye injuries. Ocular signs and symptoms vary greatly in relation to the copper content, ranging from chronic uveitis and severe visual loss for IOFB containing more than 85% copper, to local copper deposits with no severe ocular damage for less toxic alloys. Reported here is the case of a ten-year-old boy who suffered from recurrent chronic uveitis five months after a perforating eye injury due to copper wire. Diagnostic ultrasound and computerized tomography revealed an IOFB within the lens and X-ray spectrometry (DXS) indicated the nature of the IOFB as copper, and accurately measured the dissolution of the metal. The patient underwent cataract extraction and standard three-port pars plana vitrectomy with gas-fluid exchange. Twelve months after surgery the retina was flat and visual acuity had risen to 20/60. Undiluted vitreous samples obtained in the course of the intervention, studied by immunohistochemical techniques, showed an inflammatory reaction with a prevalence of PMN-N and CD3 T-lymphocytes. DXS thus appears to be a new and reliable diagnostic tool for the early detection and management of copper and other toxic metal IOFB.

Nutritional optic neuropathies

Nutritional deficiency may be the cause of a genuine optic neuropathy, sometimes associated with involvement of the peripheral nervous system. Nutritional optic neuropathies are usually bilateral, painless, chronic, insidious and slowly progressive. Most often, they present as a non-specific retrobulbar optic neuropathy. The differential diagnosis with other causes of optic nerve involvement, in particular of toxic origin, may be particularly difficult. Nutritional deficits are often associated with toxic effects from alcohol and tobacco; therefore, the separation of the nutritional and toxic components is often illusory and artificial. The pathophysiological mechanisms involved in nutritional -- and toxic -- optic neuropathies affect biochemical pathways involved in cell energetic production, correction of oxidative stress and quenching of free radicals. The recognition of these mechanisms could provide future therapeutic alternatives. Currently, the treatment is limited to the intensive use of vitamins with variable results in individual cases, and to the implementation of preventive measures, when feasible.

Stabilizing effect of Zn2+ in native bovine rhodopsin

Single-molecule force spectroscopy (SMFS) is a powerful tool to dissect molecular interactions that govern the stability and function of proteins. We applied SMFS to understand the effect of Zn2+ on the molecular interactions underlying the structure of rhodopsin. Force-distance curves obtained from SMFS assays revealed the strength and location of molecular interactions that stabilize structural segments within this receptor. The inclusion of ZnCl2 in SMFS assay buffer increased the stability of most structural segments. This effect was not mimicked by CaCl2, CdCl2, or CoCl2. Thus, Zn2+ stabilizes the structure of rhodopsin in a specific manner.

Localization of ZnT7 and zinc ions in mouse retina--immunohistochemistry and selenium autometallography

Zinc transporter 7 (ZnT7, Slc30a7), a member of the Slc30 family, is involved in mobilizing zinc ions from the cytoplasm into the Golgi apparatus. In the present study, we examined the distribution and localization of ZnT7 and the labile zinc ions in the mouse retina using immunohistochemistry and in vivo zinc-selenium autometallography (ZnSe(AMG)). Our results showed that ZnT7 is abundantly expressed in the ganglion cells and pigment epithelial cells of the mouse retina. ZnT7 is also expressed in the amacrine cells and the layer of optic fibers of the mouse retina, but to a lesser extent. Weak staining of ZnT7 was detected in the inner plexiform layer, outer plexiform layer, and outer segment of the photoreceptors. However, ZnT7 was not detected in the outer nuclear layer and inner segment of the photoreceptors. A high level of labile zinc pool was detected in the pigment epithelial cells, the inner segment of the photoreceptors, and the marginal region of the inner nuclear layer. Less amount of labile zinc ions were detected in the ganglion cells of the retina. These observations strongly suggest that ZnT7 may play critical roles in retinal zinc homeostasis and that chelatable zinc pools may have multiple functions in the retina.

Hypothèses sur le rôle des facteurs nutritionnels au cours des hypertonies oculaires et du glaucome

Recent notions in connection with oxidative stress and the fat balance of long chain polyunsaturated fatty acids (PUFA) families have brought new insight to a probable role of nutritional factors in glaucoma and intraocular hypertony. The modifications of the extracellular matrix of the trabecula could be influenced by oxidative stress. On the one hand, collagen apoptosis and remodeling (associated with an increase in intraocular pressure) are mainly influenced by hydrosoluble antioxidants such as glutathione. On the other hand, elastin apoptosis and remodeling (correlated with the occurrence of optic atrophy) are particularly influenced by liposoluble antioxidants such as vitamin E. In addition, the dietary ratio of omega3/omega6PUFA intake could influence the balance of intraocular pressure. Omega-3 PUFA could influence cyclooxygenase competition. A diet with increased omega-3 and decreased omega-6 could thus favor an increase in intraocular pressure reducing synthesis of PG-F2, leading to a decrease in uveoscleral outflow. The true importance of these factors has not yet been solidly determined and studies are in progress to clarify the real implication of these nutritional factors.

Zinc-induced decrease of the thermal stability and regeneration of rhodopsin

Zinc is present at high concentrations in the photoreceptor cells of the retina where it has been proposed to play a role in the visual phototransduction process. In order to obtain more information about this role, the study of the effect of zinc on several properties of the visual photoreceptor rhodopsin has been investigated. A specific effect of Zn(2+) on the thermal stability of rhodopsin, obtained from bovine retinas and solubilized in dodecyl maltoside detergent, in the dark is reported. The thermal stability of rhodopsin in its ground state (dark state) is clearly reduced with increasing Zn(2+) concentrations (0-50 microm Zn(2+)). The thermal bleaching process is accelerated in the presence of Zn(2+) with k rate constants, at 55 degrees C, of 0.028 +/- 0.002 min(-1) (0 microm Zn(2+)) and 0.056 +/- 0.003 min(-1) (50 microm Zn(2+)), corresponding to t(12) values of 24.4 +/- 1.6 min and 11.8 +/- 0.1 min, respectively. Thermodynamic parameters derived from Arrhenius plots show a significant E(a) increase at 50 microm Zn(2+) for the process, with deltaG++ decrease and increase in deltaH++ and deltaS++ possibly reflecting conformational rearrangements and reordering of water molecules. The stability of the metarhodopsin II intermediate is also decreased and changes in the metarhodopsin II decay pathway are also detected. The extent of rhodopsin regeneration in vitro is also reduced by zinc. These effects, specific for zinc, are also seen for rhodopsin in native disc membranes, and may be relevant to the suggested role of Zn(2+) in normal and pathological retinal function.

DNA damage-induced [Zn(2+)](i) transients: correlation with cell cycle arrest and apoptosis in lymphoma cells

Reactive changes in free intracellular zinc cation concentration ([Zn(2+)](i)) were monitored, using the fluorescent probe Zinquin, in human lymphoma cells exposed to the DNA-damaging agent VP-16. Two-photon excitation microscopy showed that Zinquin-Zn(2+) forms complexes in cytoplasmic vesicles. [Zn(2+)](i) increased in both p53(wt) (wild type) and p53(mut) (mutant) cells after exposure to low drug doses. In p53(mut) cells noncompetent for DNA damage-induced apoptosis, elevated [Zn(2+)](i) was maintained at higher drug doses, unlike competent p53(wt) cells that showed a collapse of the transient before apoptosis. In p53(wt) cells, the [Zn(2+)](i) rise paralleled an increase in p53 and bax-to-bcl-2 ratio but preceded an increase in p21(WAF1), active cell cycle arrest in G(2), or nuclear fragmentation. Reducing [Zn(2+)](i), using N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine, caused rapid apoptosis in both p53(wt) and p53(mut) cells, although cotreatment with VP-16 exacerbated apoptosis only in p53(wt) cells. This may reflect changed thresholds for proapoptotic caspase-3 activation in competent cells. We conclude that the DNA damage-induced transient is p53-independent up to a damage threshold, beyond which competent cells reduce [Zn(2+)](i) before apoptosis. Early stress responses in p53(wt) cells take place in an environment of enhanced Zn(2+) availability.

Zinc in the retina

Experimental evidence exists to suggest that zinc can have positive and negative effects on the physiology of cells depending on the "local" concentration, localisation (extracellular vs. intracellular) and/or state (bound vs. free). The retina contains particularly high amounts of zinc suggesting a pivotal role in the tissue. There is also suggestive evidence that zinc deficiency in humans may result in abnormal dark adaptation and/or age-related macular degeneration. The purpose of this article is to provide an overview of various proposed functions for zinc, particularly in the retina. Endogenous chelatable zinc in the retina is localised mainly to the photoreceptors and retinal pigment epithelial cells. Moreover, the zinc localisation in the photoreceptors varies in dark and light, suggesting a role for zinc in a light-regulated process. Some zinc is also located to other areas of the retina but clearly defined zinc-enriched neurones could not be identified as has been shown to occur in certain areas of the brain. Neurones post-synaptic to zinc-enriched neurones in the brain have been suggested to be particularly vulnerable in ischaemia. The role of zinc in retinal ischaemia has been investigated to determine how it is involved in the process. It would appear that when zinc is administered in low concentrations it generally has a positive effect on an insulted retina as in ischaemia. However, higher concentrations of zinc exacerbates the influence of the insult and also acts as a toxin. Use of zinc supplements in diet must, therefore, be taken with caution.

Optic nerve changes in zinc-deficient rats

In this study the optic nerve changes in zinc (Zn)-deficient rats are examined. Zinc is one of the essential trace elements and is known to be related to optic nerve diseases such as ethambutol neuropathy. However, the effect of Zn on the optic nerve has not been studied experimentally in animals. We used 3 week old weanling male Wistar Kyoto rats weighing 40-50 g. Rats were fed a Zn-deficient diet containing 0.007 mg of Zn per 100 g, all other nutrients and distilled and deionized water. The same water supplemented with 3 mg Zn per 100 g was given to the control group. After 4 or 7 weeks on a Zn-deficient diet, the optic nerve was examined with an electron microscope. A recovery group was fed a Zn-containing diet for 5 weeks after 7 weeks on the Zn-deficient diet. The serum Zn levels of the deficient group were significantly decreased at both 4 and 7 weeks. Most of the Zn-deficient rats showed hair loss around the eyes and on the extremities. Ultrastructural findings were as follows. The number of myelinated axons of Zn-deficient rats at 4 and 7 weeks were significantly decreased and the myelin sheaths were significantly thinner in the Zn-deficient groups and in the recovery group. Unmyelinated axons were more numerous than in the control rats. Destruction of myelin and proliferation of glial cells were found in the optic nerves of Zn-deficient rats. This study suggests that the optic nerve needs Zn for the maintenance of its cell structure and even if Zn is supplied to the Zn-deficient rats, destruction of the myelin structure may continue. Zn-deficiency induce a decrease of myelinated nerve fibers, and it is thought that optic neuropathy in patients treated with some drugs such as ethambutol may be a secondary change due to Zn-deficiency following drug administration.

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.

Depletion of intracellular zinc induces macromolecule synthesis- and caspase-dependent apoptosis of cultured retinal cells

Although zinc deficiency may contribute to age-related macular degeneration (ARMD), the pathogenic mechanism is as yet uncertain. In light of evidence that cellular zinc depletion induces apoptosis in cortical neurons and thymocytes, in the present study, we examined the possibility that the same phenomenon occurs also in retinal cells. Exposure of primary retinal cell cultures to 1-3 microM of a cell membrane-permeant zinc chelator TPEN for 24 h induced concentration-dependent death of neurons, photoreceptor cells, and astrocytes. Addition of zinc or copper reversed TPEN toxicity to all cell components, indicating the particular involvement of zinc chelation in cell death. Consistent with apoptosis, oligonucleosomal DNA fragmentation and chromatin condensation accompanied, and the protein synthesis inhibitor cycloheximide blocked the TPEN-induced retinal cell death. During TPEN-induced retinal cell apoptosis, cleavage/activation of procaspase-1, but little of procaspase-3, was observed. Consistent with this finding, a broad-spectrum caspase inhibitor (zVAD-fmk) was significantly more protective than a caspase-3-selective inhibitor (DEVD-fmk). The present study has demonstrated that depletion of intracellular zinc is sufficient to induce macromolecule synthesis- and caspase-dependent apoptosis of cultured retinal cells. In light of the possibility that zinc depletion may contribute to the pathogenesis of ARMD, the current culture model may be a useful tool for the investigation of the mechanism of zinc depletion-induced retinal cell death.

Reduced voltage-dependent Ca2+ signaling in CA1 neurons after brief ischemia in gerbils

An initial overload of intracellular Ca2+ plays a critical role in the delayed death of hippocampal CA1 neurons that die a few days after transient ischemia. Without direct evidence, the prevailing hypothesis has been that Ca2+ overload may recur until cell death. Here, we report the first measurements of intracellular Ca2+ in living CA1 neurons within brain slices prepared 1, 2, and 3 days after transient (5 min) ischemia. With no sign of ongoing Ca2+ overload, voltage-dependent Ca2+ transients were actually reduced after 2-3 days of reperfusion. Resting Ca2+ levels and recovery rate after loading were similar to neurons receiving no ischemic insult. The tetrodotoxin-insensitive Ca spike, normally generated by these neurons, was absent at 2 days postischemia, as was a large fraction of Ca2+-dependent spike train adaptation. These surprising findings may lead to a new perspective on delayed neuronal death and intervention.

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.

GABA-gated Cl- channels in the rat retina

gamma-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the mammalian retina, and its physiological action is well established. GABA receptors have been localized immunocytochemically in the retina of different mammalian species, and all major retinal cell types have been found to express GABAA receptor subunits. Recently, a new type of GABA receptor with pharmacological and electrophysiological properties different from the known GABAA and GABAB receptors, has been described. These GABAC receptors are found predominantly in the vertebrate retina. This review concentrates on the electrophysiological characterization of GABA receptors expressed by amacrine and bipolar cells of the rat retina. We recorded GABA-induced currents from cultured neonatal amacrine and bipolar cells as well as from isolated bipolar cells of adult animals. While amacrine cells contain a homogeneous population of GABAA receptors, bipolar cells exhibit both GABAA and GABAC responses. Although both receptors gate chloride-selective ion channels, their biophysical and pharmacological properties differ markedly. These functional differences and the cellular distribution of GABAA and GABAC receptors suggest that they have different inhibitory functions in the rat retina.

Oscillatory potentials and light microscopic changes demonstrate an interaction between zinc and taurine in the developing rat retina

Our objective was to investigate whether zinc interacts with taurine to influence the development of retinal structure and function. Virgin female Sprague-Dawley rats were bred overnight and assigned to one of four treatments in a 2 x 2 factorial design with two levels of zinc (50 micrograms/g through gestation and 50 micrograms/g after parturition; 15 micrograms/g through gestation and 7.5 micrograms/g after parturition) and two levels of taurine (2 or 0 mumol/g). The control diet contained 50 micrograms/g zinc and 2 mumol/g taurine. Guanidinoethyl sulfonate (10 g/L), a taurine transport inhibitor, was added to the drinking water of the rats receiving 0 mumol/g taurine. At postnatal d 23, male pups (n = 10) were weaned onto their respective diets. Pup eyes were examined by biomicroscope and indirect ophthalmoscope at 4 and 7 wk; retinal folds and choroidal atrophy were detected in the pups deficient in zinc and taurine. Analysis of plasma zinc and tibial zinc concentrations revealed a significant interaction in these tissues (P < 0.05). Dark-adapted oscillatory potentials (OP) were recorded at 7.5-8.5 wk. Two-way ANOVA showed a significant interaction between zinc and taurine for OP2 and OP3 amplitudes; marginal zinc deficiency decreased the amplitude of the OP only when rats were also deficient in taurine. A significant depressing effect of marginal zinc deficiency was noted for OP1 amplitude. Taurine deficiency significantly depressed the amplitude of OP1 and OP4. Histological examination of the retinas from rats deficient in both zinc and taurine revealed photoreceptor degeneration and confirmed retinal dysplasia. These data provide evidence for an interaction between zinc and taurine in retinal morphology and function.

Zinc causes an apparent increase in rhodopsin phosphorylation

PURPOSE

Rhodopsin is a zinc-binding protein. We investigated the effect of low concentrations of zinc on the initial phosphorylation of rhodopsin.

METHODS

Dark-adapted bovine rod outer segments (ROS) were incubated with (gamma 32P) ATP and 5 mM magnesium in the presence and absence of micromolar amounts of zinc. The ROS were exposed to light to initiate phosphorylation under conditions which allow only limited initial phosphorylation.

RESULTS

We found that zinc enhanced the rhodopsin phosphorylation apparent on autoradiographies by several fold. Phosphorylation reactions conducted in the presence of potent phospho-opsin phosphatase inhibitors show a comparable zinc-enhanced phosphorylation of rhodopsin. Under our reaction conditions, ROS membranes also appear more red upon initial exposure to light when zinc is present.

CONCLUSIONS

Zinc can increase initial rhodopsin phosphorylation, apparently acting at the substrate rhodopsin and not at relevant phosphatases or rhodopsin kinase. How zinc binding to rhodopsin might increase its ability to serve as a substrate for phosphorylation is under investigation.

Cytochrome oxidase in rat ocular tissues with special reference to copper

The localization of cytochrome oxidase, one of the copper metalloenzymes, was determined histochemically and compared with that of copper. Copper and cytochrome oxidase coexisted in the corneal epithelium and endothelium, iris, ciliary body, lens epithelium, and retinal photoreceptor inner segment. In spite of the presence of copper, no cytochrome oxidase was demonstrated histochemically in the retinal pigment epithelium, choroid, sclera, or optic nerve. The coexistence of copper and cytochrome oxidase suggests that copper plays a role in this copper metalloenzyme, while the non-coincidence of localizations of copper and cytochrome oxidase may be attributed to histochemical problems or to some unknown function of copper.