1
|
Ballestín R, Torres J, Ponsoda X. TSQ Incubation Enhances Autometallographic Zinc Detection in Cultured Astrocytes. MICROSCOPY AND MICROANALYSIS : THE OFFICIAL JOURNAL OF MICROSCOPY SOCIETY OF AMERICA, MICROBEAM ANALYSIS SOCIETY, MICROSCOPICAL SOCIETY OF CANADA 2024; 30:759-770. [PMID: 39027929 DOI: 10.1093/mam/ozae060] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/27/2023] [Revised: 04/01/2024] [Accepted: 06/18/2024] [Indexed: 07/20/2024]
Abstract
Zinc is a critical ion for a large number of cellular functions. In the central nervous system, zinc ions are involved in synaptic transmission. Therefore, zinc homeostasis is essential, and cells have developed a variety of mechanisms to control cellular zinc concentration, including the zincosome formation. Alterations of free zinc levels have been associated with brain dysfunction and are present in many illnesses and syndromes. Astrocytes are implicated in the maintenance of the neuronal milleu and brain homeostasis. In this work, we have analyzed the combination of direct (TSQ) and indirect (autometallography) zinc detection methods to increase sensitivity for studying zinc uptake by rat astrocytes in vitro. Zincosome formation was visualized with the zinc fluorochrome TSQ by light microscopy. Additionally, we improved both zinc precipitation and cellular fixation methods to preserve zinc ions and make them suitable for autometallography development. Our tests pinpointed paraformaldehyde and sodium sulfide as the more adequate methods for cellular fixation and zinc precipitation, respectively. TSQ incubation and pH of the fixative were shown to be crucial for autometallography. Using this improved method, we visualized the zinc content of zincosomes at the ultrastructural level both as silver autometallographic precipitates and as electrodense sulfide-osmium zinc precipitates.
Collapse
Affiliation(s)
- Raúl Ballestín
- Departamento Biología Celular, Biología Funcional y Antropología Física, Universitat de València, 46100 Burjassot, Valencia, Spain
| | - Josema Torres
- Departamento Biología Celular, Biología Funcional y Antropología Física, Universitat de València, 46100 Burjassot, Valencia, Spain
| | - Xavier Ponsoda
- Departamento Biología Celular, Biología Funcional y Antropología Física, Universitat de València, 46100 Burjassot, Valencia, Spain
| |
Collapse
|
2
|
Goldberg JM, Lippard SJ. Mobile zinc as a modulator of sensory perception. FEBS Lett 2023; 597:151-165. [PMID: 36416529 PMCID: PMC10108044 DOI: 10.1002/1873-3468.14544] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/10/2022] [Revised: 11/15/2022] [Accepted: 11/16/2022] [Indexed: 11/24/2022]
Abstract
Mobile zinc is an abundant transition metal ion in the central nervous system, with pools of divalent zinc accumulating in regions of the brain engaged in sensory perception and memory formation. Here, we present essential tools that we developed to interrogate the role(s) of mobile zinc in these processes. Most important are (a) fluorescent sensors that report the presence of mobile zinc and (b) fast, Zn-selective chelating agents for measuring zinc flux in animal tissue and live animals. The results of our studies, conducted in collaboration with neuroscientist experts, are presented for sensory organs involved in hearing, smell, vision, and learning and memory. A general principle emerging from these studies is that the function of mobile zinc in all cases appears to be downregulation of the amplitude of the response following overstimulation of the respective sensory organs. Possible consequences affecting human behavior are presented for future investigations in collaboration with interested behavioral scientists.
Collapse
Affiliation(s)
| | - Stephen J Lippard
- Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA, USA
| |
Collapse
|
3
|
Gilbert R, Peto T, Lengyel I, Emri E. Zinc Nutrition and Inflammation in the Aging Retina. Mol Nutr Food Res 2019; 63:e1801049. [PMID: 31148351 DOI: 10.1002/mnfr.201801049] [Citation(s) in RCA: 39] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/09/2018] [Revised: 04/18/2019] [Indexed: 12/16/2022]
Abstract
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.
Collapse
Affiliation(s)
- Rosie Gilbert
- Moorfields Eye Hospital NHS Foundation Trust, City Road, London, EC1V 2PD, UK.,UCL Institute of Ophthalmology, Bath Street, London, EC1V 2EL, UK
| | - Tunde Peto
- School of Medicine, Dentistry and Biomedical Science, Queen's University Belfast, 97 Lisburn Road, Belfast, BT9 7BL, Ireland
| | - Imre Lengyel
- UCL Institute of Ophthalmology, Bath Street, London, EC1V 2EL, UK.,School of Medicine, Dentistry and Biomedical Science, Queen's University Belfast, 97 Lisburn Road, Belfast, BT9 7BL, Ireland
| | - Eszter Emri
- School of Medicine, Dentistry and Biomedical Science, Queen's University Belfast, 97 Lisburn Road, Belfast, BT9 7BL, Ireland
| |
Collapse
|
4
|
Pao PJ, Emri E, Abdirahman SB, Soorma T, Zeng HH, Hauck SM, Thompson RB, Lengyel I. The effects of zinc supplementation on primary human retinal pigment epithelium. J Trace Elem Med Biol 2018. [PMID: 29523386 DOI: 10.1016/j.jtemb.2018.02.028] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Population-based and interventional studies have shown that elevated zinc levels can reduce the progression to advanced age-related macular degeneration. The objective of this study was to assess whether elevated extracellular zinc has a direct effect on retinal pigment epithelial cells (RPE), by examining the phenotype and molecular characteristics of increased extracellular zinc on human primary RPE cells. Monolayers of human foetal primary RPE cells were grown on culture inserts and maintained in medium supplemented with increasing total concentrations of zinc (0, 75, 100, 125 and 150 μM) for up to 4 weeks. Changes in cell viability and differentiation as well as expression and secretion of proteins were investigated. RPE cells developed a confluent monolayer with cobblestone morphology and transepithelial resistance (TER) >200 Ω*cm2 within 4 weeks. There was a zinc concentration-dependent increase in TER and pigmentation, with the largest effects being achieved by the addition of 125 μM zinc to the culture medium, corresponding to 3.4 nM available (free) zinc levels. The cells responded to addition of zinc by significantly increasing the expression of Retinoid Isomerohydrolase (RPE65) gene; cell pigmentation; Premelanosome Protein (PMEL17) immunoreactivity; and secretion of proteins including Apolipoprotein E (APOE), Complement Factor H (CFH), and High-Temperature Requirement A Serine Peptidase 1 (HTRA1) without an effect on cell viability. This study shows that elevated extracellular zinc levels have a significant and direct effect on differentiation and function of the RPE cells in culture, which may explain, at least in part, the positive effects seen in clinical settings. The results also highlight that determining and controlling of available, as opposed to total added, zinc will be essential to be able to compare results obtained in different laboratories.
Collapse
Affiliation(s)
- Po-Jung Pao
- Department of Ocular Biology and Therapeutics, UCL Institute of Ophthalmology, 11-43 Bath St, London EC1 V9EL, United Kingdom; Department of Structural and Molecular Biology, University College London, Gower Street, London, WC1E 6BT, United Kingdom.
| | - Eszter Emri
- Department of Ocular Biology and Therapeutics, UCL Institute of Ophthalmology, 11-43 Bath St, London EC1 V9EL, United Kingdom; Centre for Experimental Medicine, School of Medicine, Dentistry and Biomedical Sciences, Queen's University, 97 Lisburn Road, Belfast, BT9 7BL, Northern Ireland, United Kingdom.
| | - Safiya Bishar Abdirahman
- Department of Ocular Biology and Therapeutics, UCL Institute of Ophthalmology, 11-43 Bath St, London EC1 V9EL, United Kingdom.
| | - Talha Soorma
- Department of Ocular Biology and Therapeutics, UCL Institute of Ophthalmology, 11-43 Bath St, London EC1 V9EL, United Kingdom.
| | - Hui-Hui Zeng
- Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, 108 N. Greene Street, Baltimore, MD 21201, United States.
| | - Stefanie M Hauck
- Research Unit Protein Science, Helmholtz Zentrum München, German Research Center for Environmental Health(GmbH), Ingolstädter Landstraße 1, 85764 Oberschleißheim, Germany.
| | - Richard B Thompson
- Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, 108 N. Greene Street, Baltimore, MD 21201, United States.
| | - Imre Lengyel
- Department of Ocular Biology and Therapeutics, UCL Institute of Ophthalmology, 11-43 Bath St, London EC1 V9EL, United Kingdom; Centre for Experimental Medicine, School of Medicine, Dentistry and Biomedical Sciences, Queen's University, 97 Lisburn Road, Belfast, BT9 7BL, Northern Ireland, United Kingdom.
| |
Collapse
|
5
|
McAllister BB, Dyck RH. Zinc transporter 3 (ZnT3) and vesicular zinc in central nervous system function. Neurosci Biobehav Rev 2017. [DOI: 10.1016/j.neubiorev.2017.06.006] [Citation(s) in RCA: 97] [Impact Index Per Article: 13.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
|
6
|
Ugarte M, Osborne NN, Brown LA, Bishop PN. Iron, zinc, and copper in retinal physiology and disease. Surv Ophthalmol 2013; 58:585-609. [DOI: 10.1016/j.survophthal.2012.12.002] [Citation(s) in RCA: 75] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/27/2012] [Revised: 12/09/2012] [Accepted: 12/11/2012] [Indexed: 12/26/2022]
|
7
|
Anastassov I, Shen W, Ripps H, Chappell RL. Zinc modulation of calcium activity at the photoreceptor terminal: a calcium imaging study. Exp Eye Res 2013; 112:37-44. [PMID: 23619034 DOI: 10.1016/j.exer.2013.04.011] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/12/2013] [Revised: 04/12/2013] [Accepted: 04/13/2013] [Indexed: 01/02/2023]
Abstract
There is abundant experimental evidence that zinc ions (Zn(2+)) are present in the synaptic vesicles of vertebrate photoreceptors, and that they are co-released with glutamate. Here we show that increasing the concentration of extracellular zinc (2 μM-2 mM) suppresses the entry of calcium into the synaptic terminals of isolated salamander double cones. The resultant dose-dependent curve was fit by an inverse Hill equation having an IC50 of 38 μM, and Hill coefficient of 1.1. Because there is currently no reliable way to measure the concentration of extracellular zinc, it is not known whether the zinc released under normal circumstances is of physiological significance. In an attempt to circumvent this problem we used zinc chelators to reduce the available pool of endogenous zinc. This enabled us to determine how the absence of zinc affected calcium entry. We found that when intra- or extra-cellular zinc was chelated by 250 μM of membrane-permeable TPEN or 500 μM of membrane-impermeable histidine, there was a significant rise in the depolarization-induced intracellular calcium level within photoreceptor terminals. This increase in internal [Ca(2+)] will undoubtedly lead to a concomitant increase in glutamate release. In addition, we found that blocking the L-type calcium channels that are expressed on the synaptic terminals of photoreceptors with 50 μM nicardipine or 100 μM verapamil abolished the effects of zinc chelation. These findings are a good indication that, when released in vivo, the zinc concentration is sufficient to suppress voltage-gated calcium channels, and reduce the rate of glutamate release from photoreceptor terminals.
Collapse
Affiliation(s)
- Ivan Anastassov
- Department of Biological Sciences, Hunter College and the Graduate Center, CUNY, 695 Park Avenue, New York, NY 10065, USA.
| | | | | | | |
Collapse
|
8
|
GRIME GEOFFREYW, PÅLSGÅRD EVA, GARMAN ELSPETHF, UGARTE MARTA, POTTAGE DAVID, WYETH PAUL. RECENT BIOMEDICAL APPLICATIONS OF THE OXFORD SCANNING PROTON MICROPROBE. ACTA ACUST UNITED AC 2011. [DOI: 10.1142/s0129083599000309] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
The Oxford Scanning Proton Microprobe continues to be used in the field of trace element measurement in biological systems, exploiting the unique advantages of sensitive, quantitative trace element analysis using PIXE, high spatial resolution and the long penetrating power of MeV protons. This paper outlines a number of recent applications which highlight these advantages. These include: (a) Analysing the distribution of metals in the pupae of leaf-cutting ants to determine the storage sites and transport mechanism of metals used to harden the edges of the mandibles. (b) A study of the distribution of zinc in the retina of rats to determine the role of zinc in light and dark adaptation of the eye. (c) The analysis of crystals of proteins and other large organic molecules prepared for structure determination using x-ray diffraction. These often contain metal atoms, and the identity and concentration of the metal is an important diagnostic for determining the nature of the protein and the quality of the crystallisation. The crystals are normally small (~100μm) and so microPIXE is being used to characterise them. This technique has wide ranging applications, including qualitative and quantitative identification of metals in reaction centres, in active sites and in metal binding proteins, and of DNA or RNA bound to proteins.
Collapse
Affiliation(s)
- GEOFFREY W. GRIME
- University of Oxford Department of Materials, Parks Road, Oxford, OX1 3PH, UK
| | - EVA PÅLSGÅRD
- University of Oxford Department of Materials, Parks Road, Oxford, OX1 3PH, UK
| | - ELSPETH F. GARMAN
- University of Oxford Department of Biochemistry, South Parks Road, Oxford, UK
| | - MARTA UGARTE
- University of Oxford, Nuffield Laboratory of Ophthalmology, The Eye Hospital, Walton Street, Oxford, UK
| | - DAVID POTTAGE
- Applied Bio-Composites Group, University of Southampton Department of Chemistry, Southampton, UK
| | - PAUL WYETH
- Applied Bio-Composites Group, University of Southampton Department of Chemistry, Southampton, UK
| |
Collapse
|
9
|
Song J, Lee SC, Kim SS, Koh HJ, Kwon OW, Kang JJ, Kim EK, Shin SH, Lee JH. Zn2+ -induced cell death is mediated by the induction of intracellular ROS in ARPE-19 cells. Curr Eye Res 2009; 28:195-201. [PMID: 14977522 DOI: 10.1076/ceyr.28.3.195.26251] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/03/2022]
Abstract
PURPOSE Recent studies have shown that Zn2+ induced cell death in retinal pigment epithelial cells. Here we sought to investigate the mode of Zn2+-induced cell death and the role of reactive oxygen species (ROS) in human retinal pigment epithelial cell line, ARPE-19 cells. METHODS Cell viability was measured by MTT assay. Cell death of ARPE-19 cells was measured by annexin V-fluorescein isothiocyanate (FITC) binding assay, TUNEL assay. The formation of intracellular ROS was measured using 2',7'-dichlorofluorescein diacetate (DCFH-DA). The activation of mitogen-activated protein kinase (MAPK) was examined by Western blot analysis. RESULTS This study demonstrated that Zn2+ treatment induced both necrosis and apoptosis in ARPE-19 cells. Exposure of ARPE-19 cells to Zn2+ led to the activation of ERK1/2, JNK1/2/3, and p38 MAPKs. The activation of these MAPKs was blocked by treatment with the antioxidant, N-acetylcystein (NAC). More importantly, inhibition of ROS production by NAC completely prevented Zn2+-induced cell death in RPE cells. CONCLUSIONS This study suggests that Zn2+ induces both apoptosis and necrosis in ARPE-19 cells and that its cytotoxicity may depend on the induction of intracellular ROS.
Collapse
Affiliation(s)
- Jeongmin Song
- Department of Ophthalmology, Institute of Vision Research, Yonsei University College of Medicine, Seoul, Korea
| | | | | | | | | | | | | | | | | |
Collapse
|
10
|
Human retinal cadmium accumulation as a factor in the etiology of age-related macular degeneration. Exp Eye Res 2009; 89:79-87. [PMID: 19254715 DOI: 10.1016/j.exer.2009.02.014] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/29/2008] [Revised: 02/05/2009] [Accepted: 02/19/2009] [Indexed: 11/22/2022]
Abstract
Cadmium is a naturally occurring, highly toxic, metallic element. It pollutes the environment as a result of industrial activity and accumulates in human tissues with a long biological half-life. Cadmium content has been demonstrated to increase in human retinal tissues as a function of age and tobacco smokers have approximately twice as much cadmium in retinal tissues than non-smokers. Smoking is also a key environmental risk factor for the retinal disease age-related macular degeneration (AMD). Recent studies have shown that urinary cadmium levels (a measure of Cd body burden) are higher in smokers who have AMD. We now report the Cd measurements in human retinal tissues from eyes afflicted with AMD compared to non-diseased eyes (controls) from age-matched donors. Human donor eyes frozen under argon gas were assessed for AMD severity using color stereoscopic fundus photographs and the Minnesota Grading System. Cadmium, zinc and, copper levels were measured in retinal tissues (neural retina, retinal pigment epithelium and choroid) using inductively coupled plasma mass spectrometry and graphite furnace spectrophotometry and values were normalized to tissue protein levels. Higher Cd levels were found in the neural retina and RPE for eyes afflicted with AMD compared to controls in males, differences were not statistically significant in females. The results indicate that higher retinal cadmium burdens are associated with the presence of AMD at least in males and suggest possible gender differences in the metabolism of metals in the human retina.
Collapse
|
11
|
Localization of Taurine Transporter, Taurine, and Zinc in Goldfish Retina. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2009; 643:233-42. [DOI: 10.1007/978-0-387-75681-3_24] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/13/2023]
|
12
|
Biesemeier A, Kokkinou D, Julien S, Heiduschka P, Berneburg M, Bartz-Schmidt KU, Schraermeyer U. UV-A induced oxidative stress is more prominent in naturally pigmented aged human RPE cells compared to non-pigmented human RPE cells independent of zinc treatment. JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY B-BIOLOGY 2007; 90:113-20. [PMID: 18203614 DOI: 10.1016/j.jphotobiol.2007.11.005] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/18/2007] [Revised: 11/23/2007] [Accepted: 11/23/2007] [Indexed: 11/29/2022]
Abstract
To investigate the effects of zinc supplementation on human amelanotic (ARPE-19) and native pigmented retinal pigment epithelial cells (hRPE) under normal light conditions and after ultraviolet A light exposure. hRPE cells, containing both melanin and lipofuscin granules, were prepared from human donor eyes of 60-70 year old patients. Cells of the amelanotic ARPE-19 cell line and pigmented hRPE cells were treated with zinc chloride and subjected to oxidative stress by UV-A irradiation. Intracellular H(2)O(2) formation was measured using a fluorescence oxidation assay. Additionally, apoptosis and viability assays were performed. Control cells were treated identically except for irradiation and zinc supplementation. Under normal light conditions, zinc treated hRPE cells produced less H(2)O(2) than unsupplemented hRPE cells. Viability and apoptosis events did not change. After UV-A irradiation, ARPE and hRPE cells were greatly impaired in all tests performed compared to the non-irradiated controls. No differences were found after zinc supplementation. hRPE cells showed a higher apoptosis and mortality rate than non-pigmented cells when stressed by UV-A light. ARPE cells never showed any zinc related effects. In contrast, without irradiation, zinc supplementation reduced H(2)O(2) production in pigmented hRPE cells slightly. We did not find any zinc effect in irradiated hRPE cells. After UV light exposure, pigmented cells showed a higher apoptosis and mortality than cells lacking any pigmentation. We conclude that cells with pigmentation consisting of melanin and lipofuscin granules have more prooxidative than antioxidative capacity when stressed by UV light exposure compared to cells lacking any pigmentation.
Collapse
Affiliation(s)
- Antje Biesemeier
- Section of Experimental Vitreoretinal Surgery, University Eye Hospital Tuebingen, Schleichstr. 12/1, 72076 Tuebingen, Germany.
| | | | | | | | | | | | | |
Collapse
|
13
|
Park PSH, Sapra KT, Koliński M, Filipek S, Palczewski K, Muller DJ. Stabilizing effect of Zn2+ in native bovine rhodopsin. J Biol Chem 2007; 282:11377-85. [PMID: 17303564 PMCID: PMC2043472 DOI: 10.1074/jbc.m610341200] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
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.
Collapse
Affiliation(s)
- Paul S-H Park
- Department of Pharmacology, Case Western Reserve University, Cleveland, Ohio 44106, USA.
| | | | | | | | | | | |
Collapse
|
14
|
Wang X, Wang ZY, Gao HL, Danscher G, Huang L. Localization of ZnT7 and zinc ions in mouse retina--immunohistochemistry and selenium autometallography. Brain Res Bull 2006; 71:91-6. [PMID: 17113933 DOI: 10.1016/j.brainresbull.2006.08.002] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2006] [Revised: 07/11/2006] [Accepted: 08/07/2006] [Indexed: 10/24/2022]
Abstract
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.
Collapse
Affiliation(s)
- Xin Wang
- Department of Histology and Embryology, China Medical University, Shenyang 110001, PR China
| | | | | | | | | |
Collapse
|
15
|
Osborne NN, Casson RJ, Wood JPM, Chidlow G, Graham M, Melena J. Retinal ischemia: mechanisms of damage and potential therapeutic strategies. Prog Retin Eye Res 2004; 23:91-147. [PMID: 14766318 DOI: 10.1016/j.preteyeres.2003.12.001] [Citation(s) in RCA: 737] [Impact Index Per Article: 36.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
Retinal ischemia is a common cause of visual impairment and blindness. At the cellular level, ischemic retinal injury consists of a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and increased glutamatergic stimulation. There is a cell-specific sensitivity to ischemic injury which may reflect variability in the balance of excitatory and inhibitory neurotransmitter receptors on a given cell. A number of animal models and analytical techniques have been used to study retinal ischemia, and an increasing number of treatments have been shown to interrupt the "ischemic cascade" and attenuate the detrimental effects of retinal ischemia. Thus far, however, success in the laboratory has not been translated to the clinic. Difficulties with the route of administration, dosage, and adverse effects may render certain experimental treatments clinically unusable. Furthermore, neuroprotection-based treatment strategies for stroke have so far been disappointing. However, compared to the brain, the retina exhibits a remarkable natural resistance to ischemic injury, which may reflect its peculiar metabolism and unique environment. Given the increasing understanding of the events involved in ischemic neuronal injury it is hoped that clinically effective treatments for retinal ischemia will soon be available.
Collapse
Affiliation(s)
- Neville N Osborne
- Nuffield Laboratory of Ophthalmology, University of Oxford, Walton Street, Oxford OX2 6AW, UK.
| | | | | | | | | | | |
Collapse
|
16
|
Wood JPM, Osborne NN. Zinc and energy requirements in induction of oxidative stress to retinal pigmented epithelial cells. Neurochem Res 2003; 28:1525-33. [PMID: 14570397 DOI: 10.1023/a:1025622425501] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
In age-related macular degeneration (AMD), retinal pigmented epithelium (RPE) cells are believed to be detrimentally affected. It is thought that zinc may play a part in this process. In the past, therefore, zinc supplementation has been suggested as a treatment for AMD. Experimental data shown here confound this view by indicating that whereas low amounts of zinc do protect RPE cells in culture from stress-induced effects, greater amounts of zinc have the opposite influence. These effects are partly dependent upon the "health status" of the cells. Experimental data presented herein also show that zinc-induced death of RPE cells can, however, be attenuated by compounds such as antioxidants (alpha-tocopherol, trolox, and metipranolol), or cellular energy substrates (pyruvate and oxaloacetate). It is therefore concluded that a combination of zinc and antioxidants or energy substrates rather that zinc alone should provide a safer and more effective way to treat a disease such as AMD.
Collapse
Affiliation(s)
- John P M Wood
- Nuffield Laboratory of Ophthalmology, University of Oxford, Walton Street, Oxford, United Kingdom GB-OX2 6AW
| | | |
Collapse
|
17
|
Nakamichi N, Chidlow G, Osborne NN. Effects of intraocular injection of a low concentration of zinc on the rat retina. Neuropharmacology 2003; 45:637-48. [PMID: 12941377 DOI: 10.1016/s0028-3908(03)00206-5] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The main aim of this study was to investigate whether intraocular injection of low concentrations of zinc (no greater than 10 microM) aid the survival of ganglion cells in the rat retina after excitotoxic (NMDA) and ischemia/reperfusion injuries. We also determined whether low amounts of zinc cause any detectable retinal toxicity. Intraocular injection of NMDA caused substantial reductions in the mRNA levels of the ganglion cell-specific markers Thy-1 and neurofilament light (NF-L). Co-injection of 0.1 or 1 nmol zinc neither diminished nor exacerbated the effect of NMDA on the levels of these mRNAs. Likewise, ischemia/reperfusion caused significant decreases in the levels of Thy-1 and NF-L mRNAs and in the b-wave amplitude of the electroretinogram. These effects were not counteracted by injection of zinc. Intraocular injection of NMDA caused marked toxicological effects in retinal glial cells, including upregulations of cyclooxygenase-2 (COX-2), inducible nitric oxide synthase (iNOS), glial fibrial acidic protein (GFAP), basic fibroblast growth factor (FGF-2) and ciliary neurotrophic factor (CNTF). Interestingly, injection of 1 nmol zinc caused no changes in the levels of COX-2 and iNOS, yet produced similar, although quantitatively less pronounced, changes in FGF-2, GFAP and CNTF. The upregulations of FGF-2 and CNTF suggest that increasing zinc intake may benefit injured retinal neurons. However, this was not found to be the case in the present studies, perhaps due to the acute nature of the injury paradigms utilised.
Collapse
Affiliation(s)
- N Nakamichi
- Laboratory of Molecular Pharmacology, Kanazawa University Graduate School of Natural Science and Technology, 13-1 Takara-machi, Kanazawa, Ishikawa 920-0934, Japan
| | | | | |
Collapse
|
18
|
Zhang DQ, Ribelayga C, Mangel SC, McMahon DG. Suppression by zinc of AMPA receptor-mediated synaptic transmission in the retina. J Neurophysiol 2002; 88:1245-51. [PMID: 12205145 DOI: 10.1152/jn.2002.88.3.1245] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023] Open
Abstract
Zinc is strikingly co-localized with glutamate-containing vesicles in the synaptic terminals of retinal photoreceptors, and it is thought to be co-released with glutamate onto postsynaptic neurons such as horizontal cells and bipolar cells. Here we examined exogenous zinc modulation of glutamate receptors on cultured retinal horizontal cells using patch-clamp recording and endogenous zinc effect on intact horizontal cells using intracellular recording techniques. Application of 3, 30, and 300 microM zinc reduced the whole cell peak current of response to 200 microM glutamate by 2, 30, and 56%, respectively. Zinc suppression of glutamate response persisted in the presence of 10 microM cyclothiazide (CTZ). Glutamate responses of outside-out patches were completely abolished by 30 microM 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466), and the receptor desensitization was blocked by 30 microM CTZ, indicating that receptor target for the zinc action on horizontal cells is alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproponic acid (AMPA) receptors. Zinc decreased the amplitude of outside-out patch peak current without an effect on either its 10-90% rise time or the rate of receptor desensitization. Dose-response curves for glutamate show that zinc reduced the maximal current evoked by glutamate and increased EC(50) from 50 +/- 3 to 70 +/- 6 microM without changing the Hill coefficient. Chelation of endogenous zinc with 1 mM Ca-EDTA depolarized horizontal cells in the intact retina by 3 mV, consistent with relief of the partial glutamate receptor inhibition by zinc. Overall, the results describe a unimodal form of zinc modulation of AMPA-type glutamate receptor responses not previously described in native neuronal preparations and a novel role for endogenous zinc in modulating neurotransmission.
Collapse
Affiliation(s)
- Dao-Qi Zhang
- Department of Physiology, University of Kentucky, Lexington, Kentucky 40536-0084, USA
| | | | | | | |
Collapse
|
19
|
López-García C, Varea E, Palop JJ, Nacher J, Ramirez C, Ponsoda X, Molowny A. Cytochemical techniques for zinc and heavy metals localization in nerve cells. Microsc Res Tech 2002; 56:318-31. [PMID: 11877810 DOI: 10.1002/jemt.10037] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
Zinc is one of the most abundant oligoelements in the living cell. It appears tightly bound to metallothioneins, loosely bound to some metalloproteins and nucleic acids, or even as free ion. Small amounts of zinc ions (in the nanomolar range) regulate a plentitude of enzymatic proteins, receptors, and transcription factors; thus, cells need accurate homeostasis of zinc ions. Some neurons have developed mechanisms to accumulate zinc in specific membrane compartments ("vesicular zinc") which can be revealed using histochemical techniques. This article is a short report on the different direct-indirect experimental approaches for zinc and heavy metal detection in neurons. Substances giving a bright color or emitting fluorescence when in contact with divalent metal ions are currently used to detect them inside cells; their use leads to the so called "direct" methods. The fixation and precipitation of metal ions as insoluble salt precipitates, their maintenance along the histological process, and their demonstration after autometallographic development are essential steps for other methods, the so-called "indirect methods" (Timm and Danscher Neo-Timm methods).
Collapse
Affiliation(s)
- Carlos López-García
- Neurobiology, Department of Cell Biology, University of Valencia, 46100 Burjassot, Valencia, Spain.
| | | | | | | | | | | | | |
Collapse
|
20
|
Provinciali M, Donnini A, Argentati K, Di Stasio G, Bartozzi B, Bernardini G. Reactive oxygen species modulate Zn(2+)-induced apoptosis in cancer cells. Free Radic Biol Med 2002; 32:431-45. [PMID: 11864783 DOI: 10.1016/s0891-5849(01)00830-9] [Citation(s) in RCA: 42] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
Some recent evidence has suggested a protective role of zinc against cancer. The mechanism by which zinc exerts this action has not been defined and, in particular, it has not been clarified whether zinc may directly act on cancer cells and the molecular mechanisms involved in this effect. In this study, we examined the in vitro effect of zinc on the apoptosis of mouse TS/A mammary adenocarcinoma cells, studying the zinc-dependent modulation of the intracellular levels of reactive oxygen species (ROS) and of p53 and Fas/Fas ligand pathways. We showed that zinc concentrations ranging from 33.7 to 75 muM Zn(2+) induced apoptosis in mammary cancer cells. The apoptosis was associated with an increased production of intracellular ROS, and of p53 and Fas/Fas ligand mRNA and protein. Zn(2+) induced a faint metallothionein response in TS/A cells in comparison with mouse lymphocytes. The treatment of tumor cells with the antioxidant N-acetylcysteine was able to prevent Zn(2+)-induced apoptosis, as well as the increase of p53 and Fas ligand protein induced by zinc. The data demonstrate that zinc exerts a direct action on mammary cancer cells inducing ROS-mediated apoptosis and that the effect may be mediated by the ROS-dependent induction of p53 and Fas/Fas ligand.
Collapse
Affiliation(s)
- Mauro Provinciali
- Laboratory of Tumor Immunology, Immunology Centre, Gerontology Research Department, I.N.R.C.A., Ancona, Italy.
| | | | | | | | | | | |
Collapse
|
21
|
Abstract
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.
Collapse
Affiliation(s)
- M Ugarte
- Nuffield Laboratory of Ophthalmology, University of Oxford, Walton Street, Oxford OX2 6AW, UK
| | | |
Collapse
|
22
|
Luo DG, Yang XL. Zn2+ differentially modulates signals from red- and short wavelength-sensitive cones to horizontal cells in carp retina. Brain Res 2001; 900:95-102. [PMID: 11325351 DOI: 10.1016/s0006-8993(01)02268-5] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The effects of Zn2+ were studied while recording intracellularly from L-type horizontal cells (LHCs) in the isolated, superfused carp retina. In darkness, 25 microM Zn2+ hyperpolarized LHCs and potentiated responses of these cells to 500 nm flashes, but decreased those to 680 nm flashes. Zn2+ did not change photopic electroretinographic P III responses. The differential modulation by Zn2+ persisted when the Zn2+-induced membrane hyperpolarization was compensated by lowering Ca2+ concentration in the perfusate, but it was abolished in the presence of background illumination. Furthermore, the differential modulation no longer existed in the presence of bicuculline, suggesting the involvement of gamma-aminobutyric acid(A) (GABA(A)) receptors. We speculate that the differential modulation may be a consequence of multiple changes caused by Zn2+. Decreased glutamate release from the cone terminal by Zn2+ results in a reduction of cone signals. Zn2+ antagonizes GABA receptors on LHCs, leading to cone signal reduction. On the other hand, Zn2+ may reduce the strength of the negative feedback from LHCs to cones by downregulating the activity of GABA receptors on the cone terminal, which causes a potentiation of LHC light responses. Cone- or wavelength-relevance of the Zn2+-induced feedback strength change may account for the differential modulation.
Collapse
Affiliation(s)
- D G Luo
- Institute of Neurobiology, Fudan University and Shanghai Institute of Physiology, Chinese Academy of Sciences, 220 Han-Dan Road, 200433, Shanghai, People's Republic of China
| | | |
Collapse
|
23
|
Abstract
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.
Collapse
Affiliation(s)
- B H Grahn
- Department of Small Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Canada.
| | | | | | | |
Collapse
|
24
|
Akagi T, Kaneda M, Ishii K, Hashikawa T. Differential subcellular localization of zinc in the rat retina. J Histochem Cytochem 2001; 49:87-96. [PMID: 11118481 DOI: 10.1177/002215540104900109] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
Abstract
In the retina, zinc is believed to be a modulator of synaptic transmission and a constituent of metalloenzymes. To determine whether the intracellular localization of zinc correlates with function, we examined the localization of endogenous zinc in the rat retina using the silver amplification method. By light microscopy, reaction products were detected in the pigment epithelial cells (PE), the inner segment of photoreceptors (IS), the outer nuclear layer (ONL) and the inner nuclear layer (INL), the outer plexiform layer (OPL) and the inner plexiform layer (IPL), and the ganglion cell layer (GC). The heaviest accumulation of precipitate was observed in PE and IS, whereas only a little precipitate was found in GC. When the intracellular zinc was chelated with diethyldithiocarbamate, a small amount of precipitate was observed only in ONL. By electron microscopy, zinc was associated with three compartments. In OPL and IPL, zinc was associated with neural processes, while in PE, IS, INL, and GC it was associated with the Golgi apparatus. In ONL, zinc was associated with the nucleus. Zinc in the neural processes is believed to act as a modulator of synaptic transmission, and zinc associated with the Golgi apparatus is assumed to catalyze metalloenzyme reactions.
Collapse
Affiliation(s)
- T Akagi
- Laboratory for Neural Architecture, Brain Science Institute, RIKEN, Wako, Saitama, Japan
| | | | | | | |
Collapse
|
25
|
Hyun HJ, Sohn J, Ahn YH, Shin HC, Koh JY, Yoon YH. Depletion of intracellular zinc induces macromolecule synthesis- and caspase-dependent apoptosis of cultured retinal cells. Brain Res 2000; 869:39-48. [PMID: 10865057 DOI: 10.1016/s0006-8993(00)02340-4] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
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.
Collapse
Affiliation(s)
- H J Hyun
- Department of Neurology, University of Ulsan College of Medicine, 388-1 Poongnap-Dong Songpa-gu, 138-040, Seoul, South Korea
| | | | | | | | | | | |
Collapse
|
26
|
Ugarte M, Osborne NN. The localization of free zinc varies in rat photoreceptors during light and dark adaptation. Exp Eye Res 1999; 69:459-61. [PMID: 10504280 DOI: 10.1006/exer.1999.0727] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
|
27
|
Abstract
Extracellular Zn2+ modulates current passage through voltage- and neurotransmitter-gated ion channels, at concentrations less than, or near, those produced by release at certain synapses. Electrophysiological effects of cytoplasmic Zn2+ are less well understood, and effects have been observed at concentrations that are orders of magnitude greater than those found in resting and stimulated neurons. To examine whether and how neurons are affected by lower levels of cytoplasmic Zn2+, we tested the effect of Zn2+-selective chelators, Zn2+-preferring ionophores, and exogenous Zn2+ on neuronal somata during whole-cell patch-clamp recordings. We report here that cytoplasmic zinc facilitates the downward regulation of a background Cl- conductance by an endogenous protein kinase C (PKC) in fish retinal ganglion cell somata and that this regulation is maintained if nanomolar levels of free Zn2+ are available. This regulation has not been described previously in any tissue, as other Cl- currents have been described as reduced by PKC alone, reduced by Zn2+ alone, or reduced by both independently. Moreover, control of cation currents by a zinc-dependent PKC has not been reported previously. The regulation we have observed thus provides the first electrophysiological measurements consistent with biochemical measurements of zinc-dependent PKC activity in other systems. These results suggest that contributions of background Cl- conductances to electrical properties of neurons are susceptible to modulation.
Collapse
|
28
|
Osborne NN, Ugarte M, Chao M, Chidlow G, Bae JH, Wood JP, Nash MS. Neuroprotection in relation to retinal ischemia and relevance to glaucoma. Surv Ophthalmol 1999; 43 Suppl 1:S102-28. [PMID: 10416754 DOI: 10.1016/s0039-6257(99)00044-2] [Citation(s) in RCA: 259] [Impact Index Per Article: 10.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
Abstract
Management of glaucoma is directed at the control of intraocular pressure (IOP), yet it is recognized now that increased IOP isjust an important risk factor in glaucoma. Therapy that prevents the death of ganglion cells is the main goal of treatment, but an understanding of the causes of ganglion cell death and precisely how it occurs remains speculative. Present information supports the working hypothesis that ganglion cell death may result from a particular form of ischemia. Support for this view comes from the fact that not all types of retinal ischemia lead to the pathologic findings seen in glaucomatous retinas or to cupping in the optic disk area. Moreover, in animal experiments in which ischemia is caused by elevated IOP, a retinal abnormality similar to that seen in true glaucoma is produced, whereas after occlusion of the carotid arteries a different pattern of damage is found. In ischemia, glutamate is released, and this initiates the death of neurons that contain ionotropic glutamate (NMDA) receptors. Elevated glutamate levels exist in the vitreous humor of patients with glaucoma, and NMDA receptors exist on ganglion cells and a subset of amacrine cells. Experimental studies have shown that a variety of agents can be used to prevent the death of retinal neurons (particularly ganglion cells) induced by ischemia. These agents are generally those that block NMDA receptors to prevent the action of the released glutamate or substances that interfere with the subsequent cycle of events that lead to cell death. The major causes of cell death after activation of NMDA receptors are the influx of calcium into cells and the generation of free radicals. Substances that prevent this cascade of events are, therefore, often found to act as neuroprotective agents. For a substance to have a role as a neuroprotective agent in glaucoma, it would ideally be delivered topically to the eye and used repeatedly. It is, therefore, of interest that betaxolol, a beta-blocker presently used to reduce IOP in humans, also has calcium channel-blocking functions. Moreover, experimental studies show that betaxolol is an efficient neuro protective agent against retinal ischemia in animals, when injected directly into the eye or intraperitoneally.
Collapse
Affiliation(s)
- N N Osborne
- Nuffield Laboratory of Ophthalmology, University of Oxford, UK
| | | | | | | | | | | | | |
Collapse
|
29
|
Abstract
Effects of zinc, an endogenous neuromodulator in the central nervous system, on glycine receptors (GlyRs) in retinal ganglion cells were investigated by using the whole-cell voltage-clamp technique. Zn2+ at low concentration (<2 microM) potentiated the glycine-induced chloride current and at higher concentration (>10 microM) suppressed it. This biphasic regulatory action of zinc acted selectively on the fast component of the glycine-induced current mediated by the strychnine-sensitive GlyRs, but not on the slow component mediated by the 5,7-dichlorokynurenic acid-sensitive GlyRs. Dose-response studies showed that 1 microM Zn2+ increased the maximum glycine response (I approximately) and shifted the EC50 to the left, suggesting that Zn2+ at low concentrations acts as an allosteric activator of the strychnine-sensitive GlyRs. Zn2+ at a concentration of 100 microM did not alter I approximately and shifted the EC50 to the right, indicating that Zn2+ at high concentrations acts as a competitive inhibitor of the GlyRs. Physiological functions of zinc modulation of GlyRs in retinal ganglion cells are discussed.
Collapse
Affiliation(s)
- Y Han
- Cullen Eye Institute, Baylor College of Medicine, Houston, TX 77030, USA.
| | | |
Collapse
|