Antioxidant Defenses in the Human Eye: A Focus on Metallothioneins

Abstract

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.

Keywords: antioxidants; eye; metallothioneins; natural barriers; ocular diseases; oxidative stress.

Figure 1

Schematic view of the human eye anatomy and representation of the absorption of UV radiation by the main ocular barriers.

Figure 2

Hierarchical cluster and heat map (whole-genome expression microarrays) of the main antioxidant enzymes in human ocular tissues. The row at the top shows the clustering information in the form of a dendogram and the similarity relationships among the genes and tissues: cornea (n = 11), trabecular meshwork (TM; n = 9), CB (n = 12), sclera (n = 7), iris (11), RPE (n = 8), retina (n = 12) and lens (n = 10). The column at the left of the heat map shows four clusters (A to D), each with antioxidant enzymes expressed at different abundance. Mean values of 6.6–14.9 from biological replicas per tissue are indicated according to the log2 scale, in arbitrary units, depicted at the bottom. Hierarchical cluster and heat maps were created using Illumina BeadChip array platform (HumanHT-12 v4.0 Expression BeadChip Kit, Illumina, San Diego, CA, USA) and ArrayStar software, version 4 (DNASTAR, Inc, Madison, WI, USA) according to Alvarez et al., 2012 [15]. n: number of samples per tissue.

Figure 3

(a) Mammalian MTs contain 60–68 amino acids, 20 of which are cysteines that bind seven divalent metal ions in two metal/thiolate clusters, the α- and β-metal-binding domains. Eleven cysteines bind four zinc ions in the C-terminal α-cluster, whereas nine cysteines bind three zinc ions in the N-terminal β-cluster. (b) Zinc–metallothionein redox cycle: zinc bound to MT (Zn–MT) is released under physiological oxidative conditions, forming MT-disulfide (thionin), process boosted by free radicals derived from nitric oxide, ROS and oxidized glutathione. MT-disulfide may be degraded or reduced to MT-thiol (thionein) in the presence of a selenium-derived catalyst. MT-thiol binds zinc to form the thermodynamically stable Zn–MT system. Adapted from Gonzalez-Iglesias et al., 2014 [138].

Figure 4

Gene expression profiling of MT1 (MT1A, MT1B, MT1E, MT1F, MT1G, MT1H, MT1M AND MT1X), MT2 (MT2A) and MT3 isoforms in human eye tissues, according to Alvarez et al., 2012 [15] and adapted, expressed in arbitrary units (AU) after normalization with internal controls. Scale bars of Y-axis range from 0 to 35,000 AU for each eye tissue, with the exception of lens, which Y-axis scale bar ranges from 0 to 90,000 AU.