Zinc causes an apparent increase in rhodopsin phosphorylation

RECENT BIOMEDICAL APPLICATIONS OF THE OXFORD SCANNING PROTON MICROPROBE

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.

Vertebrate membrane proteins: structure, function, and insights from biophysical approaches

Membrane proteins are key targets for pharmacological intervention because they are vital for cellular function. Here, we analyze recent progress made in the understanding of the structure and function of membrane proteins with a focus on rhodopsin and development of atomic force microscopy techniques to study biological membranes. Membrane proteins are compartmentalized to carry out extra- and intracellular processes. Biological membranes are densely populated with membrane proteins that occupy approximately 50% of their volume. In most cases membranes contain lipid rafts, protein patches, or paracrystalline formations that lack the higher-order symmetry that would allow them to be characterized by diffraction methods. Despite many technical difficulties, several crystal structures of membrane proteins that illustrate their internal structural organization have been determined. Moreover, high-resolution atomic force microscopy, near-field scanning optical microscopy, and other lower resolution techniques have been used to investigate these structures. Single-molecule force spectroscopy tracks interactions that stabilize membrane proteins and those that switch their functional state; this spectroscopy can be applied to locate a ligand-binding site. Recent development of this technique also reveals the energy landscape of a membrane protein, defining its folding, reaction pathways, and kinetics. Future development and application of novel approaches during the coming years should provide even greater insights to the understanding of biological membrane organization and function.

Zinc release at the synaptic terminals of rod photoreceptors

The presence of reactive zinc (Zn2+) within photoreceptor terminals, and evidence that exogenous zinc affects the electrophysiological activity of the distal retina, led to the suggestion that its co-release with glutamate could play an essential role in the modulation of information at the first synapse in the visual pathway. Although we had shown previously that zinc release could be visualized in the region of the outer synaptic layer of a retinal slice preparation, it could not be ascertained with certainty that the release sites were at the presynaptic terminal rather than from the mitochondria-rich inner segment or from zinc within the distal processes of photoreceptors and Müller cells. Using membrane permeant and membrane impermeant forms of a fluorescent zinc indicator (Newport green), we show both the intracellular distribution of Zn2+ and its depolarization-dependent discharge from the terminals of isolated zebrafish photoreceptors in culture. Zinc release could be detected in the dark-adapted preparation, and was further enhanced by brief exposures to black widow spider venom or high K+. Synaptically released zinc may significantly influence neural processing in the vertebrate retina by modulating the activity of excitatory and/or inhibitory receptors as well as intracellular signaling proteins.

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.

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.

Recoverin is a zinc-binding protein

Recoverin is an N-myristoylated 23 kDa calcium-binding protein from retina, which modulates the Ca2+-sensitive deactivation of rhodopsin via Ca2+-dependent inhibition of rhodopsin kinase. It was shown by intrinsic and bis-ANS probe fluorescence, circular dichroism, and differential scanning calorimetry that myristoylated recombinant recoverin interacts specifically with zinc ions. Similar to the calcium binding, the binding of zinc to Ca2+-loaded recoverin additionally increases its alpha-helical content, hydrophobic surface area, and environmental mobility/polarity of its tryptophan residues. In contrast to the calcium binding, the binding of zinc decreases thermal stability of the Ca2+-loaded protein. Zn2+-titration of recoverin, traced by bis-ANS fluorescence, reveals binding of a single Zn2+ ion per protein molecule. It was shown that the double-mutant E85Q/E121Q with inactivated Ca2+-binding EF-hands 2 and 3 (Alekseev, A. M.; Shulga-Morskoy, S. V.; Zinchenko, D. V.; Shulga-Morskaya, S. A.; Suchkov, D. V.; Vaganova, S. A.; Senin, I. I.; Zargarov, A. A.; Lipkin, V. M.; Akhtar, M.; Philippov, P. P. FEBS Lett. 1998, 440, 116-118), which can be considered as an analogue of the apo-protein, binds Zn2+ ion as well. Apparent zinc equilibrium binding constants evaluated from spectrofluorimetric Zn2+-titrations of the protein are 1.4 x 10(5) M(-1) (dissociation constant 7.1 microM) for Ca2+-loaded wild-type recoverin and 3.3 x 10(4) M(-1) (dissociation constant 30 microM) for the E85Q/E121Q mutant (analogue of apo-recoverin). Study of the binding of wild-type recoverin to ROS membranes showed a zinc-dependent increase of its affinity for the membranes, without regard to calcium content, suggesting further solvation of a protein myristoyl group upon Zn2+ binding. Possible implications of these findings to the functioning of recoverin are discussed.

Effect of zinc and temperature on the conformation of the gamma subunit of retinal phosphodiesterase: a natively unfolded protein

The cyclic GMP phosphodiesterase gamma-subunit (PDEgamma) was shown to belong to the family of natively unfolded proteins. Increasing temperature transforms the protein into a more ordered (but still relatively disordered) conformation. The C-terminal part of PDEgamma has a high-affinity zinc-binding site (Kd approximately 1 microM), with His75 and His79 being directly involved into the coordination of Zn2+. Zinc-loaded protein remains effectively unfolded. Possible implications of these findings to the functioning of PDEgamma are discussed.

Modulation of cyclic-nucleotide-gated channels and regulation of vertebrate phototransduction

Cyclic-nucleotide-gated (CNG) channels are crucial for sensory transduction in the photoreceptors (rods and cones) of the vertebrate retina. Light triggers a decrease in the cytoplasmic concentration of cyclic GMP in the outer segments of these cells, leading to closure of CNG channels and hyperpolarization of the membrane potential. Hence, CNG channels translate a chemical change in cyclic nucleotide concentration into an electrical signal that can spread through the photoreceptor cell and be transmitted to the rest of the visual system. The sensitivity of phototransduction can be altered by exposing the cells to light, through adaptation processes intrinsic to photoreceptors. Intracellular Ca2+ is a major signal in light adaptation and, in conjunction with Ca2+-binding proteins, one of its targets for modulation is the CNG channel itself. However, other intracellular signals may be involved in the fine-tuning of light sensitivity in response to cues internal to organisms. Several intracellular signals are candidates for mediating changes in cyclic GMP sensitivity including transition metals, such as Ni2+ and Zn2+, and lipid metabolites, such as diacylglycerol. Moreover, CNG channels are associated with protein kinases and phosphatases that catalyze changes in phosphorylation state and allosterically modulate channel activity. Recent studies suggest that the effects of circadian rhythms and retinal transmitters on CNG channels may be mediated by such changes in phosphorylation. The goal of this paper is to review the molecular mechanisms underlying modulation of CNG channels and to relate these forms of modulation to the regulation of light sensitivity.

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.

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.