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

Bridging Retinal and Cerebral Neurodegeneration: A Focus on Crosslinks between Alzheimer-Perusini's Disease and Retinal Dystrophies

In the early stages of Alzheimer-Perusini's disease (AD), individuals often experience vision-related issues such as color vision impairment, reduced contrast sensitivity, and visual acuity problems. As the disease progresses, there is a connection with glaucoma and age-related macular degeneration (AMD) leading to retinal cell death. The retina's involvement suggests a link with the hippocampus, where most AD forms start. A thinning of the retinal nerve fiber layer (RNFL) due to the loss of retinal ganglion cells (RGCs) is seen as a potential AD diagnostic marker using electroretinography (ERG) and optical coherence tomography (OCT). Amyloid beta fragments (Aβ), found in the eye's vitreous and aqueous humor, are also present in the cerebrospinal fluid (CSF) and accumulate in the retina. Aβ is known to cause tau hyperphosphorylation, leading to its buildup in various retinal layers. However, diseases like AD are now seen as mixed proteinopathies, with deposits of the prion protein (PrP) and α-synuclein found in affected brains and retinas. Glial cells, especially microglial cells, play a crucial role in these diseases, maintaining immunoproteostasis. Studies have shown similarities between retinal and brain microglia in terms of transcription factor expression and morphotypes. All these findings constitute a good start to achieving better comprehension of neurodegeneration in both the eye and the brain. New insights will be able to bring the scientific community closer to specific disease-modifying therapies.

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

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

Zinc Binding to Heliorhodopsin

Heliorhodopsin (HeR), a recently discovered new rhodopsin family, has an inverted membrane topology compared to animal and microbial rhodopsins, and no ion-transport activity. The slow photocycle of HeRs suggests a light-sensor function, although the function remains unknown. HeRs exhibit no specific binding of monovalent cations or anions. Despite this, ATR-FTIR spectroscopy in the present study demonstrates binding of Zn²⁺ to HeR from Thermoplasmatales archaeon (TaHeR). The biding of Zn²⁺ to 0.2 mM K_d is accompanied by helical structural perturbations without altering its color. Even though ion-specific FTIR spectra were observed for many divalent cations, only helical structural perturbations were observed for Zn²⁺-binding. Similar results were obtained for HeR 48C12. These findings suggest a possible modification of HeR function by Zn²⁺.

Rhodopsin: A Potential Biomarker for Neurodegenerative Diseases

Retinal alterations have recently been associated with numerous neurodegenerative diseases. Rhodopsin is a G-protein coupled receptor found in the rod cells of the retina. As a biomarker associated with retinal thinning and degeneration, it bears potential in the early detection and monitoring of several neurodegenerative diseases. In this review article, we summarize the findings of correlations between rhodopsin and several neurodegenerative disorders as well as the potential of a novel technique, cSLO, in the quantification of rhodopsin.

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.

Zinc binding to RNA recognition motif of TDP-43 induces the formation of amyloid-like aggregates

Aggregation of TDP-43 (transactive response DNA binding protein 43 kDa) is a hallmark of certain forms of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). Moreover, intracellular TDP-43-positive inclusions are often found in other neurodegenerative diseases. Recently it was shown that zinc ions can provoke the aggregation of endogenous TDP-43 in cells, allowing to assume a direct interaction of TDP-43 with zinc ions. In this work, we investigated zinc binding to the 102-269 TDP-43 fragment, which comprise the two RNA recognition motifs. Using isothermal titration calorimetry, mass spectrometry, and differential scanning fluorimetry, we showed that zinc binds to this TDP-43 domain with a dissociation constant in the micromolar range and modifies its tertiary structure leading to a decrease of its thermostability. Moreover, the study by dynamic light scattering and negative stain electron microscopy demonstrated that zinc ions induce auto-association process of this TDP-43 fragment into rope-like structures. These structures are thioflavin-T-positive allowing to hypothesize the direct implication of zinc ions in pathological aggregation of TDP-43.

Zinc Is Involved in Depression by Modulating G Protein-Coupled Receptor Heterodimerization

5-Hydroxytryptamine 1A receptor and galanin receptor 1 belong to the G protein-coupled receptors superfamily, and they have been described to heterodimerize triggering an anomalous physiological state that would underlie depression. Zinc supplementation has been widely reported to improve treatment against major depressive disorder. Our work has focused on the study and characterization of these receptors and its relationships with zinc both under purified conditions and in cell culture. To this aim, we have designed a strategy to purify the receptors in a conformationally active state. We have used receptors tagged with the monoclonal Rho-1D4 antibody and employed ligand-assisted purification in order to successfully purify both receptors in a properly folded and active state. The interaction between both purified receptors has been analyzed by surface plasmon resonance in order to determine the kinetics of dimerization. Zinc effect on heteromer has also been tested using the same methodology but exposing the 5-hydroxytryptamine 1A receptor to zinc before the binding experiment. These results, combined with Förster resonance energy transfer (FRET) measurements, in the absence and presence of zinc, suggest that this ion is capable of disrupting this interaction. Moreover, molecular modeling suggests that there is a coincidence between zinc-binding sites and heterodimerization interfaces for the serotonin receptor. Our results establish a rational explanation for the role of zinc in the molecular processes associated with receptor-receptor interactions and its relationship with depression, in agreement with previously reported evidence for the positive effects of zinc in depression treatment, and the involvement of our target dimer in the same disease.

Mercury-induced dark-state instability and photobleaching alterations of the visual g-protein coupled receptor rhodopsin

Mercuric compounds were previously shown to affect the visual phototransduction cascade, and this could result in vision impairment. We have analyzed the effect of mercuric chloride on the structure and stability of the dim light vision photoreceptor rhodopsin. For this purpose, we have used both native rhodopsin immunopurified from bovine retinas and a recombinant mutant rhodopsin carrying several Cys to Ser substitutions in order to investigate the potential binding site of mercury on the receptor. Our results show that mercuric chloride dramatically reduces the stability of dark-state rhodopsin and alters the molecular features of the photoactived conformation obtained upon illumination by eliciting the formation of an altered photointermediate. The thermal bleaching kinetics of native and mutant rhodopsin is markedly accelerated by mercury in a concentration-dependent manner, and its chromophore regeneration ability is severely reduced without significantly affecting its G-protein activation capacity. Furthermore, fluorescence spectroscopic measurements on the retinal release process, ensuing illumination, suggest that mercury impairs complete retinal release from the receptor binding pocket. Our results provide further support for the capacity of mercury as a hazardous metal ion with reported deleterious effect on vision and provide a molecular explanation for such an effect at the rhodopsin photoreceptor level. We suggest that mercury could alter vision by acting in a specific manner on the molecular components of the retinoid cycle, particularly by modifying the ability of the visual photoreceptor protein rhodopsin to be regenerated and to be normally photoactivated by light.

Salt effects on the conformational stability of the visual G-protein-coupled receptor rhodopsin

Membrane protein stability is a key parameter with important physiological and practical implications. Inorganic salts affect protein stability, but the mechanisms of their interactions with membrane proteins are not completely understood. We have undertaken the study of a prototypical G-protein-coupled receptor, the α-helical membrane protein rhodopsin from vertebrate retina, and explored the effects of inorganic salts on the thermal decay properties of both its inactive and photoactivated states. Under high salt concentrations, rhodopsin significantly increased its activation enthalpy change for thermal bleaching, whereas acid denaturation affected the formation of a denatured loose-bundle state for both the active and inactive conformations. This behavior seems to correlate with changes in protonated Schiff-base hydrolysis. However, chromophore regeneration with the 11-cis-retinal chromophore and MetarhodopsinII decay kinetics were slower only in the presence of sodium chloride, suggesting that in this case, the underlying phenomenon may be linked to the activation of rhodopsin and the retinal release processes. Furthermore, the melting temperature, determined by means of circular dichroism and differential scanning calorimetry measurements, was increased in the presence of high salt concentrations. The observed effects on rhodopsin could indicate that salts favor electrostatic interactions in the retinal binding pocket and indirectly favor hydrophobic interactions at the membrane protein receptor core. These effects can be exploited in applications where the stability of membrane proteins in solution is highly desirable.

Conserved rhodopsin intradiscal structural motifs mediate stabilization: effects of zinc

Retinitis pigmentosa (RP), a neurodegenerative disorder, can arise from single point mutations in rhodopsin, leading to a cascade of protein instability, misfolding, aggregation, rod cell death, retinal degeneration, and ultimately blindness. Divalent cations, such as zinc and copper, have allosteric effects on misfolded aggregates of comparable neurodegenerative disorders including Alzheimer disease, prion diseases, and ALS. We report that two structurally conserved low-affinity zinc coordination motifs, located among a cluster of RP mutations in the intradiscal loop region, mediate dose-dependent rhodopsin destabilization. Disruption of native interactions involving histidines 100 and 195, through site-directed mutagenesis or exogenous zinc coordination, results in significant loss of receptor stability. Furthermore, chelation with EDTA stabilizes the structure of both wild-type rhodopsin and the most prevalent rhodopsin RP mutation, P(23)H. These interactions suggest that homeostatic regulation of trace metal concentrations in the rod outer segment of the retina may be important both physiologically and for an important cluster of RP mutations. Furthermore, with a growing awareness of allosteric zinc binding domains on a diverse range of GPCRs, such principles may apply to many other receptors and their associated diseases.

CXCR4, inhibitors and mechanisms of action

In this review, the author discusses recent advances in anti-HIV inhibitors, targeting CXCR4, including natural and modified chemokines, peptides and organic compounds, their mechanisms of action, and the molecular process of virus invasion of immune cells. Peptides with strong anti-HIV activity exhibit several common features, such as electrostatic charges, cyclization, beta-turns and dimerization induced by a sulphide bond. Organic compounds, such as cyclams, display a unique metal-mediated mechanism in the binding process to its target CXCR4. Understanding of their mechanisms of action may be useful for the design of more effective drugs. Consecutive interactions of viral glycoprotein gp120 with CD4 and the co-receptor, CXCR4 or another co-receptor CCR5 on the cell surface leads to virus invasion into host cells. The molecular details of the binding between HIV glycoproteins and the co-receptors also provide a basis for anti-HIV therapy.

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.

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.

G protein-coupled receptor rhodopsin

The rhodopsin crystal structure provides a structural basis for understanding the function of this and other G protein-coupled receptors (GPCRs). The major structural motifs observed for rhodopsin are expected to carry over to other GPCRs, and the mechanism of transformation of the receptor from inactive to active forms is thus likely conserved. Moreover, the high expression level of rhodopsin in the retina, its specific localization in the internal disks of the photoreceptor structures [termed rod outer segments (ROS)], and the lack of other highly abundant membrane proteins allow rhodopsin to be examined in the native disk membranes by a number of methods. The results of these investigations provide evidence of the propensity of rhodopsin and, most likely, other GPCRs to dimerize, a property that may be pertinent to their function.

A methyl group at C7 of 11-cis-retinal allows chromophore formation but affects rhodopsin activation

The newly synthesized 11-cis-7-methylretinal can form an artificial visual pigment with kinetic and spectroscopic properties similar to the native pigment in the dark-state. However, its photobleaching behavior is altered, showing a Meta I-like photoproduct. This behavior reflects a steric constraint imposed by the 7-methyl group that affects the conformational change in the binding pocket as a result of retinal photoisomerization. Transducin activation is reduced, when compared to the native pigment with 11-cis-retinal. Molecular dynamics simulations suggest coupling of the C7 methyl group and the beta-ionone ring with Met207 in transmembrane helix 5 in agreement with recent experimental results.

Changes in interhelical hydrogen bonding upon rhodopsin activation

Hydrogen bonding interactions between transmembrane helices stabilize the visual pigment rhodopsin in an inactive conformation in the dark. The crystal structure of rhodopsin has previously revealed that Glu122 and Trp126 on transmembrane helix H3 form a complex hydrogen bonding network with Tyr206 and His211 on H5, while the indole nitrogen of Trp265 on H6 forms a water-mediated hydrogen bond with Asn302 on H7. Here, we use solid-state magic angle spinning NMR spectroscopy to probe the changes in hydrogen bonding upon rhodopsin activation. The NMR chemical shifts of 15N-labeled tryptophan are consistent with the indole nitrogens of Trp126 and Trp265 becoming more weakly hydrogen bonded between rhodopsin and metarhodopsin II. The NMR chemical shifts of 15N-labeled histidine show that His211 is neutral; the unprotonated imidazole nitrogen is not coordinated to zinc in rhodopsin and becomes more strongly hydrogen bonded in metarhodopsin II. Moreover, measurements of rhodopsin containing 13C-labeled histidine show that a strong hydrogen bond between the side-chain of Glu122 and the backbone carbonyl of His211 is disrupted in metarhodopsin II. The implications of these observations for the activation mechanism of rhodopsin are discussed.

Critical role of transmembrane segment zinc binding in the structure and function of rhodopsin

Zinc deficiency and retinitis pigmentosa are both important factors resulting in retinal dysfunction and night blindness. In this study, we address the critical biochemical and structural relevance of zinc ions in rhodopsin and examine whether zinc deficiency can lead to rhodopsin dysfunction. We report the identification of a high-affinity zinc coordination site within the transmembrane domain of rhodopsin, coordinated by the side chains of two highly conserved residues, Glu(122) in transmembrane helix III and His(211) in transmembrane helix V. We also demonstrate that this zinc coordination is critical for rhodopsin folding, 11-cis-retinal binding, and the stability of the chromophore-receptor interaction, defects of which are observed in retinitis pigmentosa. Furthermore, a cluster of retinitis pigmentosa mutations is localized within and around this zinc binding site. Based on these studies, we believe that improvement in zinc binding to rhodopsin at this site within the transmembrane domain may be a pharmacological approach for the treatment of select retinitis pigmentosa mutations. Transmembrane coordination of zinc may also be an important common principle across G-protein-coupled receptors.

Assessing structural elements that influence Schiff base stability: mutants E113Q and D190N destabilize rhodopsin through different mechanisms

The stability of the retinal chromophore attachment varies between different visual pigments and may factor in some retinal disease states. Opsin appears to stabilize this Schiff base linkage by: (i) affecting the hydrolysis chemistry, (ii) shielding the retinal linkage from solvent, or (iii) acting as a kinetic trap to slow retinal release. Here we describe methods to determine Schiff base stability in rhodopsin, present examples of dark state and MII rhodopsin stability differences, and show that studies of mutants E113Q and D190N demonstrate different parts of rhodopsin influence Schiff base stability in different ways.

Structural and functional role of helices I and II in rhodopsin. A novel interplay evidenced by mutations at Gly-51 and Gly-89 in the transmembrane domain

The naturally occurring mutations G51A and G51V in transmembrane helix I and G89D in the transmembrane helix II of rhodopsin are associated with the retinal degenerative disease autosomal dominant retinitis pigmentosa. To probe the orientation and packing of helices I and II a number of replacements at positions 51 and 89 were prepared by using site-directed mutagenesis, and the corresponding proteins expressed in COS-1 cells were characterized. Mutations at position 51 (G51V and G51L) bound retinal like wild-type rhodopsin but had thermally destabilized structures in the dark, altered photobleaching behavior, destabilized metarhodopsin II active conformations, and were severely defective in signal transduction. The effects observed can be correlated with the size of the mutated side chains that would interfere with specific interhelical interaction with Val-300 in helix VII. Mutations at position 89 had sensitivity to charge, as in G89K and G89D mutants, which showed reduced transducin activation. G89K showed a second absorbing species in the UV region at 350 nm, suggesting a charge effect of the introduced lysine. Increased formation of non-active forms of rhodopsin, like metarhodopsin III, may have some influence in the molecular defect underlying retinitis pigmentosa in the mutants studied. At the structural level, the effect of the mutations analyzed can be rationalized assuming a very specific set of tertiary interactions in the interhelical packing of the transmembrane segments of rhodopsin.