Light-induced photoreceptor apoptosis in vivo requires neuronal nitric-oxide synthase and guanylate cyclase activity and is caspase-3-independent

Nucleocytoplasmic transport at the crossroads of proteostasis, neurodegeneration and neuroprotection

Nucleocytoplasmic transport comprises the multistep assembly, transport, and disassembly of protein and RNA cargoes entering and exiting nuclear pores. Accruing evidence supports that impairments to nucleocytoplasmic transport are a hallmark of neurodegenerative diseases. These impairments cause dysregulations in nucleocytoplasmic partitioning and proteostasis of nuclear transport receptors and client substrates that promote intracellular deposits - another hallmark of neurodegeneration. Disturbances in liquid-liquid phase separation (LLPS) between dense and dilute phases of biomolecules implicated in nucleocytoplasmic transport promote micrometer-scale coacervates, leading to proteinaceous aggregates. This Review provides historical and emerging principles of LLPS at the interface of nucleocytoplasmic transport, proteostasis, aging and noxious insults, whose dysregulations promote intracellular aggregates. E3 SUMO-protein ligase Ranbp2 constitutes the cytoplasmic filaments of nuclear pores, where it acts as a molecular hub for rate-limiting steps of nucleocytoplasmic transport. A vignette is provided on the roles of Ranbp2 in nucleocytoplasmic transport and at the intersection of proteostasis in the survival of photoreceptor and motor neurons under homeostatic and pathophysiological environments. Current unmet clinical needs are highlighted, including therapeutics aiming to manipulate aggregation-dissolution models of purported neurotoxicity in neurodegeneration.

Rapamycin Inhibits Light-Induced Necrosome Activation Occurring in Wild-Type, but not RPE65-Null, Mouse Retina

Purpose

Both photodamage and aberrant visual cycle contribute to disease progress of many retinal degenerative disorders, whereas the signaling pathways causing photoreceptor death remain unclear. Here we investigated the effects of intense photo-stress on (1) necrosome activation in wild-type and RPE65-null mice, (2) interaction of p62/Sequestosome-1 with the necrosome proteins, and (3) the effects of rapamycin on photodamage-induced necrosome activation and retinal degeneration in wild-type mice.

Methods

Dark-adapted rd12 mice and 129S2/Sv mice with or without rapamycin treatment were exposed to 15,000 lux light for different times. Expression levels and subcellular localization of proteins were determined through immunoblot and immunohistochemical analyses. Cone sheaths were stained with peanut agglutinin. Correlation between photoreceptor degeneration and receptor-interacting protein kinase-1 (RIPK1) expression was assessed with Spearman's correlation analysis. Protein-protein interaction was analyzed by immunoprecipitation.

Results

Intense light caused rod and cone degeneration accompanied by a significant increase in RIPK1-RIPK3 expressions, mixed lineage kinase domain-like protein phosphorylation, damage-associated molecular patterns protein release, and inflammatory responses in wild-type mouse retina. The same intense light did not induce the necrosome activation in rd12 retina, but it did in rd12 mice that received 9-cis-retinal supply. RIPK1 expression levels are positively correlated with the degrees of rod and cone degeneration. Photodamage upregulated expression and interaction of the p62 autophagosome cargo protein with the necrosome proteins, whereas rapamycin treatment attenuated the light-induced necrosome activation and photoreceptor degeneration.

Conclusions

Necrosome activation contributed to photodamage-induced rod and cone degeneration. The visual cycle and autophagy are the important therapeutic targets to alleviate light-induced retinal necroptosis.

Redox Regulation of Signaling Complex between Caveolin-1 and Neuronal Calcium Sensor Recoverin

Caveolin-1 is a cholesterol-binding scaffold protein, which is localized in detergent-resistant membrane (DRM) rafts and interacts with components of signal transduction systems, including visual cascade. Among these components are neuronal calcium sensors (NCSs), some of which are redox-sensitive proteins that respond to calcium signals by modulating the activity of multiple intracellular targets. Here, we report that the formation of the caveolin-1 complex with recoverin, a photoreceptor NCS serving as the membrane-binding regulator of rhodopsin kinase (GRK1), is a redox-dependent process. Biochemical and biophysical in vitro experiments revealed a two-fold decreased affinity of recoverin to caveolin-1 mutant Y14E mimicking its oxidative stress-induced phosphorylation of the scaffold protein. At the same time, wild-type caveolin-1 demonstrated a 5-10-fold increased affinity to disulfide dimer of recoverin (dRec) or its thiol oxidation mimicking the C39D mutant. The formation of dRec in vitro was not affected by caveolin-1 but was significantly potentiated by zinc, the well-known mediator of redox homeostasis. In the MDCK cell model, oxidative stress indeed triggered Y14 phosphorylation of caveolin-1 and disulfide dimerization of recoverin. Notably, oxidative conditions promoted the accumulation of phosphorylated caveolin-1 in the plasma membrane and the recruitment of recoverin to the same sites. Co-localization of these proteins was preserved upon depletion of intracellular calcium, i.e., under conditions reducing membrane affinity of recoverin but favoring its interaction with caveolin-1. Taken together, these data suggest redox regulation of the signaling complex between recoverin and caveolin-1. During oxidative stress, the high-affinity interaction of thiol-oxidized recoverin with caveolin-1/DRMs may disturb the light-induced translocation of the former within photoreceptors and affect rhodopsin desensitization.

Glial cell response to constant low light exposure in rat retina

To study the macroglia and microglia and the immune role in long-time light exposure in rat eyes, we performed glial cell characterization along the time-course of retinal degeneration induced by chronic exposure to low-intensity light. Animals were exposed to light for periods of 2, 4, 6, or 8 days, and the retinal glial response was evaluated by immunohistochemistry, western blot and real-time reverse transcription polymerase chain reaction. Retinal cells presented an increased expression of the macroglia marker GFAP, as well as increased mRNA levels of microglia markers Iba1 and CD68 after 6 days. Also, at this time-point, we found a higher number of Iba1-positive cells in the outer nuclear layer area; moreover, these cells showed the characteristic activated-microglia morphology. The expression levels of immune mediators TNF, IL-6, and chemokines CX3CR1 and CCL2 were also significantly increased after 6 days. All the events of glial activation occurred after 5-6 days of constant light exposure, when the number of photoreceptor cells has already decreased significantly. Herein, we demonstrated that glial and immune activation are secondary to neurodegeneration; in this scenario, our results suggest that photoreceptor death is an early event that occurs independently of glial-derived immune responses.

'Poppers Maculopathy' and the adverse ophthalmic outcomes from the recreational use of alkyl nitrate inhalants: a systematic review

OBJECTIVE

The aim of this systematic review and meta-analysis was to provide an overview of reported cases of poppers maculopathy and a statistical analysis of the clinical presentations, anatomical changes and treatment and follow-up of the rare disease.

METHODS

A systematic search of CINAHL, Embase and PubMed databases was conducted, including a search of the grey literature. Data were pooled to provide a summary of the clinical characteristics and outcomes of poppers maculopathy. Studies were critically appraised using the Joanna Briggs Institute or the Authority, Accuracy, Coverage, Objectivity, Date, Significance checklists.

RESULTS

Of the 493 articles obtained, 32 were included, consisting of 113 participants with a mean age of 40.2. Most patients were male with bilateral maculopathy and presented with either vision loss or a visual disturbance. Cessation of popper use was the most common form of management prescribed. The mean VA at baseline was 0.22 (logMAR units). For those followed-up, the mean VA significantly improved from 0.24 to 0.11 (Z = -5.800, p = <0.001). Bilateral yellow foveal spots were viewed on fundoscopy in 53 patients, and a sub-foveal disturbance of the ellipsoid layer was reported on spectral domain optical coherence tomography in 89 patients. By 6 months, SD-OCT anatomical changes mostly improved or resolved.

CONCLUSION

Recreational users of poppers who develop maculopathy generally present with vision loss or a visual disturbance secondary to foveal disruption. Patients have a good visual prognosis, with cessation of poppers potentially having a role in improved outcomes. With the high prevalence of use among gay and bisexual men, awareness of related maculopathy and presenting symptoms by clinicians is important to mitigate side effects.

Intravitreal Administration of rhNGF Enhances Regenerative Processes in a Zebrafish Model of Retinal Degeneration

Nerve growth factor (NGF) is the best characterized neurotrophin, and it is known to play an important role in ocular homeostasis. Here, we demonstrated the expression of NGF receptors in adult zebrafish retina and optimized a light-induced retina degeneration (LID) zebrafish model that mimics human cone-rod disorders, demonstrating that intravitreal (IV) administration of rhNGF can boost zebrafish retinal regeneration in this model. Adult zebrafish retinae exposed to 60 h of light irradiation (60 h LID) displayed evident reduction of outer nuclear layer (ONL) thickness and cell number with presence of apoptotic cells. Retinal histologic evaluation at different time points showed that IV therapeutic injection of rhNGF resulted in an increase of ONL thickness and cell number at late time points after damage (14 and 21 days post injury), ultimately accelerating retinal tissue recovery by driving retinal cell proliferation. At a molecular level, rhNGF activated the ERK1/2 pathway and enhanced the regenerative potential of Müller glia gfap- and vim-expressing cells by stimulating at early time points the expression of the photoreceptor regeneration factor Drgal1-L2. Our results demonstrate the highly conserved nature of NGF canonical pathway in zebrafish and thus support the use of zebrafish models for testing new compounds with potential retinal regenerative properties. Moreover, the pro-regenerative effects of IV-injected NGF that we observed pave the way to further studies aimed at evaluating its effects also in mammals, in order to expedite the development of novel rhNGF-based therapeutic approaches for ophthalmological disorders.

Altered transsulfuration pathway enzymes and redox homeostasis in inherited retinal degenerative diseases

Retinal degenerative diseases result from apoptotic photoreceptor cell death. As endogenously produced gaseous molecules such as hydrogen sulfide (H₂S) and nitric oxide (NO) play a key role in apoptosis, we compared the expression levels of genes and proteins involved in the production of these molecules in the retina of normal dogs and three canine models (rcd1, crd2, and xlpra2) of human inherited retinal degeneration (IRD). Using qRT-PCR, Western blot, and immunohistochemistry (IHC), we showed that mRNA and protein levels of cystathionine β-synthase (CBS), an enzyme that produces H₂S in neurons, are increased in retinal degeneration, but those of cystathionine γ-lyase (CSE), an enzyme involved in the production of glutathione (GSH), an antioxidant, are not. Such findings suggest that increased levels of H₂S that are not counterbalanced by increased antioxidant potential may contribute to disease in affected retinas. We also studied the expression of neuronal and inducible nitric oxide synthase (nNOS and iNOS), the enzymes responsible for NO production. Western blot and IHC results revealed increased levels of nNOS and iNOS, resulting in increased NO levels in mutant retinas. Finally, photoreceptors are rich in polyunsaturated fatty acids (PUFAs) that can make these cells vulnerable to oxidative damage through reactive oxygen species (ROS). Our results showed increased levels of acrolein and hydroxynonenal (4HNE), two main toxic products of PUFAs, surrounding the membranes of photoreceptors in affected canines. Increased levels of these toxic products, together with increased NO and ROS, likely render these cells susceptible to an intrinsic apoptotic pathway involving mitochondrial membranes. To assess this possibility, we measured the levels of BCL2, an anti-apoptotic protein in the mitochondrial membrane. Western blot results showed decreased levels of BCL2 protein in affected retinas. Overall, the results of this study identify alterations in the expression of enzymes directly involved in maintaining the normal redox status of the retina during retinal degeneration, thereby supporting future studies to investigate the role of H₂S and NO in retinal degeneration and apoptosis.

Neuroprotective Effect of 4-Phenylbutyric Acid against Photo-Stress in the Retina

Exposure to excessive visible light causes retinal degeneration and may influence the progression of retinal blinding diseases. However, there are currently no applied treatments. Here, we focused on endoplasmic reticulum (ER) stress, which can cause cellular degeneration and apoptosis in response to stress. We analyzed functional, histological, and molecular changes in the light-exposed retina and the effects of administering an ER-stress inhibitor, 4-phenylbutyric acid (4-PBA), in mice. We found that light-induced visual function impairment related to photoreceptor cell loss and outer segment degeneration were substantially suppressed by 4-PBA administration, following attenuated photoreceptor apoptosis. Induction of retinal ER stress soon after light exposure, represented by upregulation of the immunoglobulin heavy chain binding protein (BiP) and C/EBP-Homologous Protein (CHOP), were suppressed by 4-PBA. Concurrently, light-induced oxidative stress markers, Nuclear factor erythroid 2-related factor 2 (Nrf2) and Heme Oxygenase 1 (HO-1), and mitochondrial apoptotic markers, B-cell lymphoma 2 apoptosis regulator (Bcl-2)-associated death promoter (Bad), and Bcl-2-associated X protein (Bax), were suppressed by 4-PBA administration. Increased expression of glial fibrillary acidic protein denoted retinal neuroinflammation, and inflammatory cytokines were induced after light exposure; however, 4-PBA acted as an anti-inflammatory. Suppression of ER stress by 4-PBA may be a new therapeutic approach to suppress the progression of retinal neurodegeneration and protect visual function against photo-stress.

Gradual Increase in Environmental Light Intensity Induces Oxidative Stress and Inflammation and Accelerates Retinal Neurodegeneration

Purpose

Retinitis pigmentosa (RP) is a blinding neurodegenerative disease of the retina that can be affected by many factors. The present study aimed to analyze the effect of different environmental light intensities in rd10 mice retina.

Methods

C57BL/6J and rd10 mice were bred and housed under three different environmental light intensities: scotopic (5 lux), mesopic (50 lux), and photopic (300 lux). Visual function was studied using electroretinography and optomotor testing. The structural and morphological integrity of the retinas was evaluated by optical coherence tomography imaging and immunohistochemistry. Additionally, inflammatory processes and oxidative stress markers were analyzed by flow cytometry and western blotting.

Results

When the environmental light intensity was higher, retinal function decreased in rd10 mice and was accompanied by light-dependent photoreceptor loss, followed by morphological alterations, and synaptic connectivity loss. Moreover, light-dependent retinal degeneration was accompanied by an increased number of inflammatory cells, which became more activated and phagocytic, and by an exacerbated reactive gliosis. Furthermore, light-dependent increment in oxidative stress markers in rd10 mice retina pointed to a possible mechanism for light-induced photoreceptor degeneration.

Conclusions

An increase in rd10 mice housing light intensity accelerates retinal degeneration, activating cell death, oxidative stress pathways, and inflammatory cells. Lighting intensity is a key factor in the progression of retinal degeneration, and standardized lighting conditions are advisable for proper analysis and interpretation of experimental results from RP animal models, and specifically from rd10 mice. Also, it can be hypothesized that light protection could be an option to slow down retinal degeneration in some cases of RP.

Retinitis Pigmentosa and Retinal Degenerations: Deciphering Pathways and Targets for Drug Discovery and Development

Inherited retinal diseases (IRDs) are a group of retinopathies generally caused by genetic mutations. Retinitis pigmentosa (RP) represents one of the most studied IRDs. RP leads to intense vision loss or blindness resulting from the degeneration of photoreceptor cells. To date, RP is mainly treated with palliative supplementation of vitamin A and retinoids, gene therapies, or surgical interventions. Therefore, a pharmacologically based therapy is an urgent need requiring a medicinal chemistry approach, to validate molecular targets able to deal with retinal degeneration. This Review aims at outlining the recent research efforts in identifying new drug targets for RP, especially focusing on the neuroprotective role of the Wnt/β-catenin/GSK3β pathway and apoptosis modulators (in particular PARP-1) but also on growth factors such as VEGF and BDNF. Furthermore, the role of spatiotemporally expressed G protein-coupled receptors (GPR124) in the retina and the emerging function of histone deacetylase inhibitors in promoting retinal neuroprotection will be discussed.

KIT ligand protects against both light-induced and genetic photoreceptor degeneration

Photoreceptor degeneration is a major cause of blindness and a considerable health burden during aging but effective therapeutic or preventive strategies have not so far become readily available. Here, we show in mouse models that signaling through the tyrosine kinase receptor KIT protects photoreceptor cells against both light-induced and inherited retinal degeneration. Upon light damage, photoreceptor cells upregulate Kit ligand (KITL) and activate KIT signaling, which in turn induces nuclear accumulation of the transcription factor NRF2 and stimulates the expression of the antioxidant gene Hmox1. Conversely, a viable Kit mutation promotes light-induced photoreceptor damage, which is reversed by experimental expression of Hmox1. Furthermore, overexpression of KITL from a viral AAV8 vector prevents photoreceptor cell death and partially restores retinal function after light damage or in genetic models of human retinitis pigmentosa. Hence, application of KITL may provide a novel therapeutic avenue for prevention or treatment of retinal degenerative diseases.

IL-1β induces rod degeneration through the disruption of retinal glutamate homeostasis

BACKGROUND

Age-related macular degeneration is characterized by the accumulation of subretinal macrophages and the degeneration of cones, but mainly of rods. We have previously shown that Mononuclear Phagocytes-derived IL-1β induces rod photoreceptor cell death during experimental subretinal inflammation and in retinal explants exposed to IL-1β but the mechanism is unknown.

METHODS

Retinal explants were culture in the presence of human monocytes or IL-1β and photoreceptor cell survival was analyzed by TUNEL labeling. Glutamate concentration and transcription levels of gene involved in the homeostasis of glutamate were analyzed in cell fractions of explant cultured or not in the presence of IL-1β. Glutamate receptor antagonists were evaluated for their ability to reduce photoreceptor cell death in the presence of IL1-β or monocytes.

RESULTS

We here show that IL-1β does not induce death in isolated photoreceptors, suggesting an indirect effect. We demonstrate that IL-1β leads to glutamate-induced rod photoreceptor cell death as it increases the extracellular glutamate concentrations in the retina through the inhibition of its conversion to glutamine in Müller cells, increased release from Müller cells, and diminished reuptake. The inhibition of non-NMDA receptors completely and efficiently prevented rod apoptosis in retinal explants cultured in the presence of IL-1β or, more importantly, in vivo, in a model of subretinal inflammation.

CONCLUSIONS

Our study emphasizes the importance of inflammation in the deregulation of glutamate homeostasis and provides a comprehensive mechanism of action for IL-1β-induced rod degeneration.

Photoreceptor Survival Is Regulated by GSTO1-1 in the Degenerating Retina

Purpose

Glutathione-S-transferase omega 1-1 (GSTO1-1) is a cytosolic glutathione transferase enzyme, involved in glutathionylation, toll-like receptor signaling, and calcium channel regulation. GSTO1-1 dysregulation has been implicated in oxidative stress and inflammation, and contributes to the pathogenesis of several diseases and neurological disorders; however, its role in retinal degenerations is unknown. The aim of this study was to investigate the role of GSTO1-1 in modulating oxidative stress and consequent inflammation in the normal and degenerating retina.

Methods

The role of GSTO1-1 in retinal degenerations was explored by using Gsto1-/- mice in a model of retinal degeneration. The expression and localization of GSTO1-1 were investigated with immunohistochemistry and Western blot. Changes in the expression of inflammatory (Ccl2, Il-1β, and C3) and oxidative stress (Nox1, Sod2, Gpx3, Hmox1, Nrf2, and Nqo1) genes were investigated via quantitative real-time polymerase chain reaction. Retinal function in Gsto1-/- mice was investigated by using electroretinography.

Results

GSTO1-1 was localized to the inner segment of cone photoreceptors in the retina. Gsto1-/- photo-oxidative damage (PD) mice had decreased photoreceptor cell death as well as decreased expression of inflammatory (Ccl2, Il-1β, and C3) markers and oxidative stress marker Nqo1. Further, retinal function in the Gsto1-/- PD mice was increased as compared to wild-type PD mice.

Conclusions

These results indicate that GSTO1-1 is required for inflammatory-mediated photoreceptor death in retinal degenerations. Targeting GSTO1-1 may be a useful strategy to reduce oxidative stress and inflammation and ameliorate photoreceptor loss, slowing the progression of retinal degenerations.

Broad spectrum metabolomics for detection of abnormal metabolic pathways in a mouse model for retinitis pigmentosa

Retinitis pigmentosa (RP) is a degenerative disease of the retina that affects approximately 1 million people worldwide. There are multiple genetic causes of this disease, for which, at present, there are no effective therapeutic strategies. In the present report, we utilized broad spectrum metabolomics to identify perturbations in the metabolism of the rd10 mouse, a genetic model for RP that contains a mutation in Pde6β. These data provide novel insights into mechanisms that are potentially critical for retinal degeneration. C57BL/6J and rd10 mice were raised in cyclic light followed by either light or dark adaptation at postnatal day (P) 18, an early stage in the degeneration process. Mice raised entirely in the dark until P18 were also evaluated. After euthanasia, retinas were removed and extracted for analysis by ultra-performance liquid chromatography-time of flight-mass spectrometry (UPLC-QTOF-MS). Compared to wild type mice, rd10 mice raised in cyclic light or in complete darkness demonstrate significant alterations in retinal pyrimidine and purine nucleotide metabolism, potentially disrupting deoxynucleotide pools necessary for mitochondrial DNA replication. Other metabolites that demonstrate significant increases are the Coenzyme A intermediate, 4'-phosphopantothenate, and acylcarnitines. The changes in these metabolites, identified for the first time in a model of RP, are highly likely to disrupt normal energy metabolism. High levels of nitrosoproline were also detected in rd10 retinas relative to those from wild type mice. These results suggest that nitrosative stress may be involved in retinal degeneration in this mouse model.

Cell Death Mechanisms in a Mouse Model of Retinal Degeneration in Spinocerebellar Ataxia 7

Spino-cerebellar ataxia type 7 (SCA7) is a polyglutamine (polyQ) disorder characterized by neurodegeneration of the brain, cerebellum, and retina caused by a polyglutamine expansion in ataxin7. The presence of an expanded polyQ tract in a mutant protein is known to induce protein aggregation, cellular stress, toxicity, and finally cell death. However, the consequences of the presence of mutant ataxin7 in the retina and the mechanisms underlying photoreceptor degeneration remain poorly understood. In this study, we show that in a retinal SCA7 mouse model, polyQ ataxin7 induces stress within the retina and activates Muller cells. Moreover, unfolded protein response and autophagy are activated in SCA7 photoreceptors. We have also shown that the photoreceptor death does not involve a caspase-dependent apoptosis but instead involves apoptosis inducing factor (AIF) and Leukocyte Elastase Inhibitor (LEI/L-DNase II). When these two cell death effectors are downregulated by their siRNA, a significant reduction in photoreceptor death is observed. These results highlight the consequences of polyQ protein expression in the retina and the role of caspase-independent pathways involved in photoreceptor cell death.

Light-Induced Thiol Oxidation of Recoverin Affects Rhodopsin Desensitization

The excessive light illumination of mammalian retina is known to induce oxidative stress and photoreceptor cell death linked to progression of age-related macular degeneration. The photochemical damage of photoreceptors is suggested to occur via two apoptotic pathways that involve either excessive rhodopsin activation or constitutive phototransduction, depending on the light intensity. Both pathways are dramatically activated in the absence of rhodopsin desensitization by GRK1. Previously, we have shown that moderate illumination (halogen lamp, 1,500 lx, 1–5 h) of mammalian eyes provokes disulfide dimerization of recoverin, a calcium-dependent regulator of GRK1. Here, we demonstrate under in vivo conditions that both moderate long-term (metal halide lamp, 2,500 lx, 14 h, rat model) and intense short-term (halogen lamp, 30,000 lx for 3 h, rabbit model) illumination of the mammalian retina are accompanied by accumulation of disulfide dimer of recoverin. Furthermore, in the second case we reveal alternatively oxidized derivatives of the protein, apparently including its monomer with sulfinic group. Histological data indicate that thiol oxidation of recoverin precedes apoptosis of photoreceptors. Both disulfide dimer and oxidized monomer (or oxidation mimicking C39D mutant) of recoverin exhibit lowered α-helical content and thermal stability of their apo-forms, as well as increased Ca²⁺ affinity. Meanwhile, the oxidized monomer and C39D mutant of recoverin demonstrate impaired ability to bind photoreceptor membranes and regulate GRK1, whereas disulfide dimer exhibits notably improved membrane binding and GRK1 inhibition in absence of Ca²⁺. The latter effect is expected to slow down rhodopsin desensitization in the light, thereby favoring support of the light-induced oxidative stress, ultimately leading to photoreceptor apoptosis. Overall, the intensity and duration of illumination of the retina affect thiol oxidation of recoverin likely contributing to propagation of the oxidative stress and photoreceptor damage.

Suppression of Light-Induced Oxidative Stress in the Retina by Mitochondria-Targeted Antioxidant

Light-induced oxidation of lipids and proteins provokes retinal injuries and results in progression of degenerative retinal diseases, such as, for instance, iatrogenic photic maculopathies. Having accumulated over years retinal injuries contribute to development of age-related macular degeneration (AMD). Antioxidant treatment is regarded as a promising approach to protecting the retina from light damage and AMD. Here, we examine oxidative processes induced in rabbit retina by excessive light illumination with or without premedication using mitochondria-targeted antioxidant SkQ1 (10-(6'-plastoquinonyl)decyltriphenyl-phosphonium). The retinal extracts obtained from animals euthanized within 1⁻7 days post exposure were analyzed for H₂O₂, malondialdehyde (MDA), total antioxidant activity (AOA), and activities of glutathione peroxidase (GPx) and superoxide dismutase (SOD) using colorimetric and luminescence assays. Oxidation of visual arrestin was monitored by immunoblotting. The light exposure induced lipid peroxidation and H₂O₂ accumulation in the retinal cells. Unexpectedly, it prominently upregulated AOA in retinal extracts although SOD and GPx activities were compromised. These alterations were accompanied by accumulation of disulfide dimers of arrestin revealing oxidative stress in the photoreceptors. Premedication of the eyes with SkQ1 accelerated normalization of H₂O₂ levels and redox-status of lipids and proteins, contemporarily enhancing AOA and, likely, sustaining normal activity of GPx. Thus, SkQ1 protects the retina from light-induced oxidative stress and could be employed to suppress oxidative damage of proteins and lipids contributing to AMD.

Macular Hole After Poppers (Alkyl Nitrate) Inhalation in a Child

The authors present the first case of macular hole (MH) after a single inhalation of poppers. A 13-year-old girl presented with vision loss in the left eye (OS). Pediatric and neurology exams were normal. Funduscopy revealed bilateral papilledema and yellow foveal spot OS. Optic neuritis was diagnosed and treated. Due to foveal alteration, optical coherence tomography was performed, and MH was diagnosed. Twenty-three-gauge pars plana vitrectomy, peeling of the internal limiting membrane, and SF₆ exchange were performed. Postoperatively, vision restoration and MH closure were observed. Although MH mechanism in the poppers context is unknown, the classic surgery is effective. [Ophthalmic Surg Lasers Imaging Retina. 2018;49:897-900.].

Protective Effect of a Locked Retinal Chromophore Analog against Light-Induced Retinal Degeneration

Continuous regeneration of the 11-cis-retinal visual chromophore from all-trans-retinal is critical for vision. Insufficiency of 11-cis-retinal arising from the dysfunction of key proteins involved in its regeneration can impair retinal health, ultimately leading to loss of human sight. Delayed recovery of visual sensitivity and night blindness caused by inadequate regeneration of the visual pigment rhodopsin are typical early signs of this condition. Excessive concentrations of unliganded, constitutively active opsin and increased levels of all-trans-retinal and its byproducts in photoreceptors also accelerate retinal degeneration after light exposure. Exogenous 9-cis-retinal iso-chromophore can reduce the toxicity of ligand-free opsin but fails to prevent the buildup of retinoid photoproducts when their clearance is defective in human retinopathies, such as Stargardt disease or age-related macular degeneration. Here we evaluated the effect of a locked chromophore analog, 11-cis-6-membered ring-retinal against bright light-induced retinal degeneration in Abca4^-/-Rdh8^-/- mice. Using in vivo imaging techniques, optical coherence tomography, scanning laser ophthalmoscopy, and two-photon microscopy, along with in vitro histologic analysis of retinal morphology, we found that treatment with 11-cis-6-membered ring-retinal before light stimulation prevented rod and cone photoreceptor degradation and preserved functional acuity in these mice. Moreover, additive accumulation of 11-cis-6-membered ring-retinal measured in the eyes of these mice by quantitative liquid chromatography-mass spectrometry indicated stable binding of this retinoid to opsin. Together, these results suggest that eliminating excess of unliganded opsin can prevent light-induced retinal degeneration in Abca4^-/-Rdh8^-/- mice.

Neuroprotective effect of tetramethylpyrazine against all-trans-retinal toxicity in the differentiated Y-79 cells via upregulation of IRBP expression

It is estimated that abnormal accumulation of all-trans-retinal (atRAL) is a leading cause of photoreceptor degeneration in retinal degenerative diseases. Deficiency of interphotoreceptor retinoid-binding protein (IRBP), a retinoid transporter in the visual cycle, is responsible for the impaired clearance of atRAL and results in atRAL toxicity in retina. Therefore, IRBP has been proposed to be a potent target in preventing atRAL-induced photoreceptor degeneration. In this study, the neuroprotective effect of tetramethylpyrazine (TMP) against atRAL toxicity in the differentiated Y-79 cells, a in vitro model of photoreceptor, was first investigated. Our findings showed that atRAL could induce cytotoxicity, oxidative/nitrosative stresses, apoptosis and leukostasis in the differentiated Y-79 cells; however, the pre-treatment of TMP significantly attenuated such effects in a dose-dependent manner. Furthermore, our results indicated that TMP exerted its neuroprotective effect mainly through upregulating IRBP expression. The present study significantly contributes to better understanding the important role of IRBP in retinal degenerative diseases and forms the basis of the therapeutic development of TMP in such diseases in the future.

Molecular Pathway to Protection From Age-Dependent Photoreceptor Degeneration in Mef2 Deficiency

Purpose

Photoreceptor degeneration in the retina is a major cause of blindness in humans. Elucidating mechanisms of degenerative and neuroprotective pathways in photoreceptors should afford identification and development of therapeutic strategies.

Methods

We used mouse genetic models and improved methods for retinal explant cultures. Retinas were enucleated from Mef2d^+/+ and Mef2d^−/− mice, stained for MEF2 proteins and outer nuclear layer thickness, and assayed for apoptotic cells. Chromatin immunoprecipitation (ChIP) assays revealed MEF2 binding, and RT-qPCR showed levels of transcription factors. We used AAV2 and electroporation to express genes in retinal explants and electroretinograms to assess photoreceptor functionality.

Results

We identify a prosurvival MEF2D-PGC1α pathway that plays a neuroprotective role in photoreceptors. We demonstrate that Mef2d^−/− mouse retinas manifest decreased expression of PGC1α and increased photoreceptor cell loss, resulting in the absence of light responses. Molecular repletion of PGC1α protects Mef2d^−/− photoreceptors and preserves light responsivity.

Conclusions

These results suggest that the MEF2-PGC1α cascade may represent a new therapeutic target for drugs designed to protect photoreceptors from developmental- and age-dependent loss.

NADPH Oxidase Contributes to Photoreceptor Degeneration in Constitutively Active RAC1 Mice

Purpose

The active form of small GTPase RAC1 is required for activation of NADPH oxidase (NOX), which in turn generates reactive oxygen species (ROS) in nonphagocytic cells. We explored whether NOX-induced oxidative stress contributes to rod degeneration in retinas expressing constitutively active (CA) RAC1.

Methods

Transgenic (Tg)–CA-RAC1 mice were given apocynin (10 mg/kg, intraperitoneal), a NOX inhibitor, or vehicle daily for up to 13 weeks. Superoxide production and oxidative damage were assessed by dihydroethidium staining and by protein carbonyls and malondialdehyde levels, respectively. Outer nuclear layer (ONL) cells were counted and electroretinogram (ERG) amplitudes measured in Tg-CA-RAC1 mice. Outer nuclear layer cells were counted in wild-type (WT) mice after transfer of CA-Rac1 gene by subretinal injection of AAV8-pOpsin-CA Rac1-GFP.

Results

Transgenic-CA-RAC1 retinas had significantly fewer photoreceptor cells and more apoptotic ONL cells than WT controls from postnatal week (Pw) 3 to Pw13. Superoxide accumulation and protein and lipid oxidation were increased in Tg-CA-RAC1 retinas and were reduced in mice treated with apocynin. Apocynin reduced the loss of photoreceptors and increased the rod ERG a- and b-wave amplitudes when compared with vehicle-injected transgenic controls. Photoreceptor loss was also observed in regions of adult WT retina transduced with AAV8-pOpsin-CA Rac1-GFP but not in neighboring regions that were not transduced or in AAV8-pOpsin-GFP–transduced retinas.

Conclusions

Constitutively active RAC1 promotes photoreceptor cell death by oxidative damage that occurs, at least partially, through NOX-induced ROS. Reactive oxygen species are likely involved in multiple forms of retinal degenerations, and our results support investigating RAC1 inhibition as a therapeutic approach that targets this disease pathway.

AAV-Nrf2 Promotes Protection and Recovery in Animal Models of Oxidative Stress

NRF2 is a transcription factor that drives antioxidant gene expression in multiple organ systems. We hypothesized that Nrf2 overexpression could be therapeutically applied toward diseases in which redox homeostasis is disrupted. In this study, adeno-associated virus (AAV)-Nrf2 was tested in a mouse model of acute acetaminophen-induced liver toxicity and successfully conferred protection from hepatotoxicity, validating the vector design and early onset of NRF2-mediated protection. Furthermore, therapeutic potential of AAV-Nrf2 in chronic disease also was tested in a light-induced mouse model of age-related macular degeneration. Adult BALB/c mice were intravitreally injected with AAV-Nrf2 and subject to light damage following injection. Retinal thickness and function were monitored following light damage using optical coherence tomography and electroretinography, respectively. By 3 months post-damage, injected eyes had greater retinal thickness compared to uninjected controls. At 1 month post-damage, AAV-Nrf2 injection facilitated full functional recovery from light damage. Our results suggest a therapeutic potential for Nrf2 overexpression in acute and long-term capacities in multiple organ systems, opening up doors for combination gene therapy where replacement gene therapy requires additional therapeutic support to prevent further degeneration.

Retinal metabolic events in preconditioning light stress as revealed by wide-spectrum targeted metabolomics

INTRODUCTION

Light is the primary stimulus for vision, but may also cause damage to the retina. Pre-exposing the retina to sub-lethal amount of light (or preconditioning) improves chances for retinal cells to survive acute damaging light stress.

OBJECTIVES

This study aims at exploring the changes in retinal metabolome after mild light stress and identifying mechanisms that may be involved in preconditioning.

METHODS

Retinas from 12 rats exposed to mild light stress (1000 lux × for 12 h) and 12 controls were collected one and seven days after light stress (LS). One retina was used for targeted metabolomics analysis using the Biocrates p180 kit while the fellow retina was used for histological and immunohistochemistry analysis.

RESULTS

Immunohistochemistry confirmed that in this experiment, a mild LS with retinal immune response and minimal photoreceptor loss occurred. Compared to controls, LS induced an increased concentration in phosphatidylcholines. The concentration in some amino acids and biogenic amines, particularly those related to the nitric oxide pathway (like asymmetric dimethylarginine (ADMA), arginine and citrulline) also increased 1 day after LS. 7 days after LS, the concentration in two sphingomyelins and phenylethylamine was found to be higher. We further found that in controls, retina metabolome was different between males and females: male retinas had an increased concentration in tyrosine, acetyl-ornithine, phosphatidylcholines and (acyl)-carnitines.

CONCLUSIONS

Besides retinal sexual metabolic dimorphism, this study shows that preconditioning is mostly associated with re-organisation of lipid metabolism and changes in amino acid composition, likely reflecting the involvement of arginine-dependent NO signalling.

Mutated olfactomedin 1 in the interphotoreceptor matrix of the mouse retina causes functional deficits and vulnerability to light damage

Olfactomedin 1 (OLFM1) is a secreted glycoprotein and member of the olfactomedin protein family, which is preferentially expressed in various areas throughout the central nervous system. To learn about the functional properties of OLFM1 in the eye, we investigated its localization in the mouse and pig eye. In addition, we analyzed the ocular phenotype of Olfm1 mutant mice in which 52 amino acids were deleted in the central part (M2 region) of OLFM1. OLFM1 was detected in cornea, sclera, retina, and optic nerve of both wild-type and Olfm1 mutant littermates. By immunohistochemistry and double labeling with the lectin peanut agglutinin, OLFM1 was found in the interphotoreceptor matrix (IPM) of mouse and pig retina where it was directly localized to the inner segments of photoreceptors. Western blotting confirmed the presence of the OLFM1 isoforms pancortin 1 (BMY) and pancortin 2 (BMZ) in the IPM. The retinal phenotype of Olfm1 mutant mice did not obviously differ from that of wild-type littermates. In addition, outer nuclear layer (ONL) and total retinal thickness were not different, and the same was true for the area of the optic nerve in cross sections. Functional changes were observed though by electroretinography, which showed significantly lower a- and b-wave amplitudes in Olfm1 mutant mice when compared to age-matched wild-type mice. When light damage experiments were performed as an experimental paradigm of photoreceptor apoptosis, significantly more TUNEL-positive cells were observed in Olfm1 mutant mice 30 h after light exposure. One week after light exposure, the ONL was significantly thinner in Olfm1 mutant mice than in wild-type littermates indicating increased photoreceptor loss. No differences were observed when rhodopsin turnover or ERK1/2 signaling was investigated. We conclude that OLFM1 is a newly identified IPM molecule that serves an important role for photoreceptor homeostasis, which is significantly compromised in the eyes of Olfm1 mutant mice.

Retinal Neuroprotective Effects of Flibanserin, an FDA-Approved Dual Serotonin Receptor Agonist-Antagonist

PURPOSE

To assess the neuroprotective effects of flibanserin (formerly BIMT-17), a dual 5-HT1A agonist and 5-HT2A antagonist, in a light-induced retinopathy model.

METHODS

Albino BALB/c mice were injected intraperitoneally with either vehicle or increasing doses of flibanserin ranging from 0.75 to 15 mg/kg flibanserin. To assess 5-HT1A-mediated effects, BALB/c mice were injected with 10 mg/kg WAY 100635, a 5-HT1A antagonist, prior to 6 mg/kg flibanserin and 5-HT1A knockout mice were injected with 6 mg/kg flibanserin. Injections were administered once immediately prior to light exposure or over the course of five days. Light exposure lasted for one hour at an intensity of 10,000 lux. Retinal structure was assessed using spectral domain optical coherence tomography and retinal function was assessed using electroretinography. To investigate the mechanisms of flibanserin-mediated neuroprotection, gene expression, measured by RT-qPCR, was assessed following five days of daily 15 mg/kg flibanserin injections.

RESULTS

A five-day treatment regimen of 3 to 15 mg/kg of flibanserin significantly preserved outer retinal structure and function in a dose-dependent manner. Additionally, a single-day treatment regimen of 6 to 15 mg/kg of flibanserin still provided significant protection. The action of flibanserin was hindered by the 5-HT1A antagonist, WAY 100635, and was not effective in 5-HT1A knockout mice. Creb, c-Jun, c-Fos, Bcl-2, Cast1, Nqo1, Sod1, and Cat were significantly increased in flibanserin-injected mice versus vehicle-injected mice.

CONCLUSIONS

Intraperitoneal delivery of flibanserin in a light-induced retinopathy mouse model provides retinal neuroprotection. Mechanistic data suggests that this effect is mediated through 5-HT1A receptors and that flibanserin augments the expression of genes capable of reducing mitochondrial dysfunction and oxidative stress. Since flibanserin is already FDA-approved for other indications, the potential to repurpose this drug for treating retinal degenerations merits further investigation.

Ion Channels in the Eye: Involvement in Ocular Pathologies

The eye is the sensory organ of vision. There, the retina transforms photons into electrical signals that are sent to higher brain areas to produce visual sensations. In the light path to the retina, different types of cells and tissues are involved in maintaining the transparency of avascular structures like the cornea or lens, while others, like the retinal pigment epithelium, have a critical role in the maintenance of photoreceptor function by regenerating the visual pigment. Here, we have reviewed the roles of different ion channels expressed in ocular tissues (cornea, conjunctiva and neurons innervating the ocular surface, lens, retina, retinal pigment epithelium, and the inflow and outflow systems of the aqueous humor) that are involved in ocular disease pathophysiologies and those whose deletion or pharmacological modulation leads to specific diseases of the eye. These include pathologies such as retinitis pigmentosa, macular degeneration, achromatopsia, glaucoma, cataracts, dry eye, or keratoconjunctivitis among others. Several disease-associated ion channels are potential targets for pharmacological intervention or other therapeutic approaches, thus highlighting the importance of these channels in ocular physiology and pathophysiology.

Uncoupling phototoxicity-elicited neural dysmorphology and death by insidious function and selective impairment of Ran-binding protein 2 (Ranbp2)

Morphological disintegration of neurons is coupled invariably to neural death. In particular, disruption of outer segments of photoreceptor neurons triggers photoreceptor death regardless of the pathological stressors. We show that Ranbp2(-/-)::Tg-Ranbp2(CLDm-HA) mice with mutations in SUMO-binding motif (SBM) of cyclophilin-like domain (CLD) of Ran-binding protein 2 (Ranbp2) expressed in a null Ranbp2 background lack untoward effects in photoreceptors in the absence of light-stress. However, compared to wild type photoreceptors, light-stress elicits profound disintegration of outer segments of Ranbp2(-/-)::Tg-Ranbp2(CLDm-HA) with paradoxical age-dependent resistance of photoreceptors to death and genotype-independent activation of caspases. Ranbp2(-/-)::Tg-Ranbp2(CLDm-HA) exhibit photoreceptor death-independent changes in ubiquitin-proteasome system (UPS), but death-dependent increase of ubiquitin carrier protein 9(ubc9) levels. Hence, insidious functional impairment of SBM of Ranbp2's CLD promotes neuroprotection and uncoupling of photoreceptor degeneration and death against phototoxicity.

Light, lipids and photoreceptor survival: live or let die?

Due to its constant exposure to light and its high oxygen consumption the retina is highly sensitive to oxidative damage, which is a common factor in inducing the death of photoreceptors after light damage or in inherited retinal degenerations. The high content of docosahexaenoic acid (DHA), the major polyunsaturated fatty acid in the retina, has been suggested to contribute to this sensitivity. DHA is crucial for developing and preserving normal visual function. However, further roles of DHA in the retina are still controversial. Current data support that it can tilt the scale either towards degeneration or survival of retinal cells. DHA peroxidation products can be deleterious to the retina and might lead to retinal degeneration. However, DHA has also been shown to act as, or to be the source of, a survival molecule that protects photoreceptors and retinal pigment epithelium cells from oxidative damage. We have established that DHA protects photoreceptors from oxidative stress-induced apoptosis and promotes their differentiation in vitro. DHA activates the retinoid X receptor (RXR) and the ERK/MAPK pathway, thus regulating the expression of anti and pro-apoptotic proteins. It also orchestrates a diversity of signaling pathways, modulating enzymatic pathways that control the sphingolipid metabolism and activate antioxidant defense mechanisms to promote photoreceptor survival and development. A deeper comprehension of DHA signaling pathways and context-dependent behavior is required to understand its dual functions in retinal physiology.

Retinal damage induced by commercial light emitting diodes (LEDs)

Spectra of "white LEDs" are characterized by an intense emission in the blue region of the visible spectrum, absent in daylight spectra. This blue component and the high intensity of emission are the main sources of concern about the health risks of LEDs with respect to their toxicity to the eye and the retina. The aim of our study was to elucidate the role of blue light from LEDs in retinal damage. Commercially available white LEDs and four different blue LEDs (507, 473, 467, and 449nm) were used for exposure experiments on Wistar rats. Immunohistochemical stain, transmission electron microscopy, and Western blot were used to exam the retinas. We evaluated LED-induced retinal cell damage by studying oxidative stress, stress response pathways, and the identification of cell death pathways. LED light caused a state of suffering of the retina with oxidative damage and retinal injury. We observed a loss of photoreceptors and the activation of caspase-independent apoptosis, necroptosis, and necrosis. A wavelength dependence of the effects was observed. Phototoxicity of LEDs on the retina is characterized by a strong damage of photoreceptors and by the induction of necrosis.

The activation of the atypical PKC zeta in light-induced retinal degeneration and its involvement in L-DNase II control

Light-induced retinal degeneration is characterized by photoreceptor cell death. Many studies showed that photoreceptor demise is caspase-independent. In our laboratory we showed that leucocyte elastase inhibitor/LEI-derived DNase II (LEI/L-DNase II), a caspase-independent apoptotic pathway, is responsible for photoreceptor death. In this work, we investigated the activation of a pro-survival kinase, the protein kinase C (PKC) zeta. We show that light exposure induced PKC zeta activation. PKC zeta interacts with LEI/L-DNase II and controls its DNase activity by impairing its nuclear translocation. These results highlight the role of PKC zeta in retinal physiology and show that this kinase can control caspase-independent pathways.

Effect of a sigma-1 receptor agonist, cutamesine dihydrochloride (SA4503), on photoreceptor cell death against light-induced damage

Cutamesine dihydrochloride is an agonist of sigma-1 receptor, which is a ligand-operated receptor chaperone at the mitochondrion-associated endoplasmic reticulum (ER) membrane. ER stress plays a pivotal role in light irradiation-induced retinal damage. In the present study, we examined whether cutamesine is effective against experimental degenerative retinal damages in vitro and in vivo. The effects of cutamesine against white light-induced retinal photoreceptor damage were evaluated in vitro by measuring cell death. The expression of sigma-1 receptor after the light exposure was examined by immunoblot analysis. The disruption of the mitochondrial membrane potential and caspase-3/7 activation after excessive light exposure were also examined. In addition, retinal damage in mice induced by irradiation to white light was evaluated using histological staining and electroretinography. Cutamesine reduced the cell death rate induced by light exposure, and the protective effect was prevented by N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino)ethylamine (BD-1047) dihydrobromide, a sigma-1 receptor antagonist. Sigma-1 receptor expression was decreased by light exposure, and cutamesine suppressed the decreased expression of sigma-1 receptor protein. Cutamesine also reduced the mitochondrial damage and reduced the elevated level of caspase 3/7 activity; this effect was attenuated by BD-1047. In in vivo studies, cutamesine suppressed the light-induced retinal dysfunction and thinning of the outer nuclear layer in the mouse retina. These findings indicate that cutamesine protects against retinal cell death in vitro and in vivo by the agonistic effect of sigma-1 receptor. Therefore, sigma-1 receptor may have a potential as a therapeutic target in retinal diseases mediated by photoreceptor degeneration.

Photo-damage, photo-protection and age-related macular degeneration

The course of Age-related Macular Degeneration (AMD) is described as the effect of light (400–580 nm) on various molecular targets in photoreceptors and the retinal pigment epithelium (RPE). Photo-damage is followed by inflammation, increasing oxidative stress and, probably, unveiling new photosensitive molecules.

Induction of the unfolded protein response by constitutive G-protein signaling in rod photoreceptor cells

Phototransduction is a G-protein signal transduction cascade that converts photon absorption to a change in current at the plasma membrane. Certain genetic mutations affecting the proteins in the phototransduction cascade cause blinding disorders in humans. Some of these mutations serve as a genetic source of "equivalent light" that activates the cascade, whereas other mutations lead to amplification of the light response. How constitutive phototransduction causes photoreceptor cell death is poorly understood. We showed that persistent G-protein signaling, which occurs in rod arrestin and rhodopsin kinase knock-out mice, caused a rapid and specific induction of the PERK pathway of the unfolded protein response. These changes were not observed in the cGMP-gated channel knock-out rods, an equivalent light condition that mimics light-stimulated channel closure. Thus transducin signaling, but not channel closure, triggers rapid cell death in light damage caused by constitutive phototransduction. Additionally, we show that in the albino light damage model cell death was not associated with increase in global protein ubiquitination or unfolded protein response induction. Taken together, these observations provide novel mechanistic insights into the cell death pathway caused by constitutive phototransduction and identify the unfolded protein response as a potential target for therapeutic intervention.

Damage of photoreceptor-derived cells in culture induced by light emitting diode-derived blue light

Our eyes are increasingly exposed to light from the emitting diode (LED) light of video display terminals (VDT) which contain much blue light. VDTs are equipped with televisions, personal computers, and smart phones. The present study aims to clarify the mechanism underlying blue LED light-induced photoreceptor cell damage. Murine cone photoreceptor-derived cells (661 W) were exposed to blue, white, or green LED light (0.38 mW/cm²). In the present study, blue LED light increased reactive oxygen species (ROS) production, altered the protein expression level, induced the aggregation of short-wavelength opsins (S-opsin), resulting in severe cell damage. While, blue LED light damaged the primary retinal cells and the damage was photoreceptor specific. N-Acetylcysteine (NAC), an antioxidant, protected against the cellular damage induced by blue LED light. Overall, the LED light induced cell damage was wavelength-, but not energy-dependent and may cause more severe retinal photoreceptor cell damage than the other LED light.

Angiotensin II type 1 receptor blockade suppresses light-induced neural damage in the mouse retina

Exposure to light contributes to the development and progression of retinal degenerative diseases. However, the mechanisms underlying light-induced tissue damage are not fully understood. Here, we examined the role of angiotensin II type 1 receptor (AT1R) signaling, which is part of the renin-angiotensin system, in light-induced retinal damage. Light-exposed Balb/c mice that were treated with the AT1R blockers (angiotensin II receptor blockers; ARBs) valsartan, losartan, and candesartan before and after the light exposure exhibited attenuated visual function impairment, compared to vehicle-treated mice. This effect was dose-dependent and observed across the ARB class of inhibitors. Further evaluation of valsartan showed that it suppressed a number of light-induced retinal effects, including thinning of the photoreceptor cell layer caused by apoptosis, shortening of the photoreceptor cell outer segment, and increased levels of reactive oxygen species (ROS). The role of ROS in retinal pathogenesis was investigated further using the antioxidant N-acetyl-l-cysteine (NAC). Treatment of light-exposed mice with NAC before the light exposure suppressed the visual function impairment and photoreceptor cell histological changes due to apoptosis. Moreover, treatment with valsartan or NAC suppressed the induction of c-fos (a component of the AP-1 transcription factor) and the upregulation of fasl (a proapoptotic molecule whose transcript is regulated downstream of AP-1). Our results suggest that AT1R signaling mediates light-induced apoptosis, by increasing the levels of ROS and proapoptotic molecules in the retina. Thus, AT1R blockade may represent a new therapeutic approach for preventing light-induced retinal neural tissue damage.

Protective effect of light emitting diode phototherapy on fluorescent light induced retinal damage in Wistar strain albino rats

BACKGROUND

Artificial light at night alters retinal physiology. Several studies have shown that light emitting diode phototherapy protects the retina from the damaging effects of acute light exposure.

OBJECTIVE

The aim of this study has been to elucidate the protective effects of 670 nm LED light on retinal damage induced by chronic fluorescent light in Wistar rats.

METHODS

Male Wistar albino rats were divided into four groups: group 1 were control (CL), group 2, 3 and 4 were exposed to fluorescent light (FL), LED preexposure+fluorescent light exposure (LL) and only LED light exposure (OL) respectively. All animals were maintained in their specific exposure regime for 30 days. Fluorescent light of 1800 lx was exposed between 8 pm to 8 am. Rats were exposed to therapeutic LED light of 670 nm of 9 J/cm2 at 25 mW/cm2 for 6 min duration. Histopathological changes in the retina were studied.

RESULTS

Animals of the FL group showed a significant reduction in the outer nuclear layer thickness and cell count in addition to the total thickness of the retina. LL group which were exposed to 670 nm LED prior to exposure to fluorescent light showed a significant decrease in the degree of damage.

CONCLUSIONS

670 nm LED light preexposure is protective to retinal cells against fluorescent light-induced damage.

Ras homolog enriched in the brain is linked to retinal ganglion cell apoptosis after light injury in rats

Ras homolog enriched in the brain (Rheb) is a small GTPase of the Ras family. It has been confirmed that Rheb activation not only regulates cell growth and migration but also induces neuron apoptosis after toxic stimuli. However, the function of Rheb in the retina is still not fully understood. To find out whether Rheb was involved in retinal neuron death, the expression profile of Rheb in light-damaged retinal ganglion cells (RGCs) of adult rats was investigated. Western blotting showed the expression of Rheb was significantly upregulated in the injured retina. Rheb was mainly detected in apoptotic RGCs by using double immunofluorescent staining. Active caspase-3 was upregulated and co-labeled with Rheb. Meanwhile, terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) showed that Rheb-positive RGCs underwent apoptosis after light exposure, which suggested that Rheb might be relevant to RGC apoptosis following phototoxicity. Furthermore, Western blotting and immunofluorescence showed that the expression profiles of CyclinD1 and cyclin-dependent kinase 4 (CDK4) were parallel with that of Rheb in a time-space dependent manner. Based on this study, it is speculated that Rheb might play an important role in physiological and pathological process in light-induced retina damage, which might provide a potential therapeutic avenue of retinal degeneration.

Ion-current-based proteomic profiling of the retina in a rat model of Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS) is one of the most common recessive human disorders and is characterized by multiple congenital malformations as well as neurosensory and cognitive abnormalities. A rat model of SLOS has been developed that exhibits progressive retinal degeneration and visual dysfunction; however, the molecular events underlying the degeneration and dysfunction remain poorly understood. Here, we employed a well-controlled, ion-current-based approach to compare retinas from the SLOS rat model to retinas from age- and sex-matched control rats (n = 5/group). Retinas were subjected to detergent extraction and subsequent precipitation and on-pellet-digestion procedures and then were analyzed on a long, heated column (75 cm, with small particles) with a 7-h gradient. The high analytical reproducibility of the overall proteomics procedure enabled reliable expression profiling. In total, 1,259 unique protein groups, ~40% of which were membrane proteins, were quantified under highly stringent criteria, including a peptide false discovery rate of 0.4%, with high quality ion-current data (e.g. signal-to-noise ratio ≥ 10) obtained independently from at least two unique peptides for each protein. The ion-current-based strategy showed greater quantitative accuracy and reproducibility over a parallel spectral counting analysis. Statistically significant alterations of 101 proteins were observed; these proteins are implicated in a variety of biological processes, including lipid metabolism, oxidative stress, cell death, proteolysis, visual transduction, and vesicular/membrane transport, consistent with the features of the associated retinal degeneration in the SLOS model. Selected targets were further validated by Western blot analysis and correlative immunohistochemistry. Importantly, although photoreceptor cell death was validated by TUNEL analysis, Western blot and immunohistochemical analyses suggested a caspase-3-independent pathway. In total, these results provide compelling new evidence implicating molecular changes beyond the initial defect in cholesterol biosynthesis in this retinal degeneration model, and they might have broader implications with respect to the pathobiological mechanism underlying SLOS.

Recent advances in ophthalmic molecular imaging

The aim of molecular imaging techniques is the visualization of molecular processes and functional changes in living animals and human patients before morphological changes occur at the cellular and tissue level. Ophthalmic molecular imaging is still in its infancy and has mainly been used in small animals for pre-clinical research. The goal of most of these pre-clinical studies is their translation into ophthalmic molecular imaging techniques in clinical care. We discuss various molecular imaging techniques and their applications in ophthalmology.

The potential neuroprotective effect of human adipose stem cells conditioned medium against light-induced retinal damage

Human adipose-derived stem cells (hASCs) are present in adult adipose tissue and have been reported to secrete various factors that have neuroprotective effects. In the present study, we examined whether hASC-conditioned medium (hASC-CM) was effective against experimental degenerative retinal disease. Mature adipocytes (MAs) and hASCs were isolated from human subcutaneous adipose tissue. The isolated hASCs were identified based on their capacity for bone and neural differentiation. The effects of hASC-CM against tunicamycin-, H2O2-, and light-induced retinal photoreceptor damage were evaluated in vitro by measuring cell death. Moreover, we identified various factors present in hASC-CM using antibody arrays. Retinal damage induced in mice by exposure to white light was studied in vivo, and photoreceptor damage was evaluated according to the thickness of the outer nuclear layer and electroretinography results. In addition, the effect of hASC-CM on Akt phosphorylation at Ser473 was confirmed by western blotting. Finally, the effects of the secreted proteins identified in the hASC-CM on light-induced damage were evaluated in vivo. Isolated hASCs differentiated to osteocytes and neurons. hASC-CM protected against tunicamycin-, H2O2-, and light-induced cell death. In addition, hASC-CM inhibited photoreceptor degeneration and retinal dysfunction after exposure to light. Several proteins secreted by hASCs, such as the tissue inhibitor of metalloproteinase-1 (TIMP-1) and the secreted protein acidic and rich in cysteine (SPARC), protected against light-induced damage in vitro and in vivo. The results of the present study showed that hASC-CM has neuroprotective effects against light-induced retinal damage and suggest that hASCs have a therapeutic potential in retinal degenerative diseases via their secreted proteins, without requiring transplantation.

Spaceflight environment induces mitochondrial oxidative damage in ocular tissue

A recent report shows that more than 30% of the astronauts returning from Space Shuttle missions or the International Space Station (ISS) were diagnosed with eye problems that can cause reduced visual acuity. We investigate here whether spaceflight environment-associated retinal damage might be related to oxidative stress-induced mitochondrial apoptosis. Female C57BL/6 mice were flown in the space shuttle Atlantis (STS-135), and within 3-5 h of landing, the spaceflight and ground-control mice, similarly housed in animal enclosure modules (AEMs) were euthanized and their eyes were removed for analysis. Changes in expression of genes involved in oxidative stress, mitochondrial and endothelial cell biology were examined. Apoptosis in the retina was analyzed by caspase-3 immunocytochemical analysis and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) assay. Levels of 4-hydroxynonenal (4-HNE) protein, an oxidative specific marker for lipid peroxidation were also measured. Evaluation of spaceflight mice and AEM ground-control mice showed that expression of several genes playing central roles in regulating the mitochondria-associated apoptotic pathway were significantly altered in mouse ocular tissue after spaceflight compared to AEM ground-control mice. In addition, the mRNA levels of several genes, which are responsible for regulating the production of reactive oxygen species were also significantly up-regulated in spaceflight samples compared to AEM ground-control mice. Further more, the level of HNE protein was significantly elevated in the retina after spaceflight compared to controls. Our results also revealed that spaceflight conditions induced significant apoptosis in the retina especially inner nuclear layer (INL) and ganglion cell layer (GCL) compared to AEM ground controls. The data provided the first evidence that spaceflight conditions induce oxidative damage that results in mitochondrial apoptosis in the retina. This data suggest that astronauts may be at increased risk for late retinal degeneration.

In vitro toxicity of colistin on primary chick cortex neurons and its potential mechanism

Colistin is increasingly used as the last-line therapy for infections caused by Gram-negative 'superbugs'. Although colistin neurotoxicity was reported in the literature, there has no data on its mechanism. In the present study, we examined the effect of colistin on primary chick neuron cells, which were treated with 0.83, 4.15 and 8.3μg/mL colistin for 6, 12 and 24h. Cell viability was evaluated with 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) assays after exposure to colistin. Formation of reactive oxygen species (ROS), nuclear morphology, caspase-3 activity and internucleosomal DNA fragmentation were examined. The results showed that, compared with the control, no significant change was observed in cell viability, ROS formation and caspase-3 activity in cells treated for 6, 12 and 24h with 0.83μg/mL colistin. However, in the 4.15 and 8.3μg/mL colistin-treated groups, the viability of chick primary neurons significantly decreased at 12 and 24h, respectively; caspase-3 activities were significantly increased to 5.1 and 7.4 fold at 6h, more earlier than the changes of ROS, which was significant increased to 124.5% and 143.5% (P<0.01) of control at 12h, respectively. The apoptosis of neuron cells was revealed by both nuclear morphological observations and internucleosomal DNA fragmentation in the 4.15 and 8.3μg/mL colistin-treated groups at 6, 12 and 24h. Our data demonstrated that colistin can induce apoptosis in primary chick cortex neurons through caspase-3 activation, which may be contributed with ROS-dependent and independent mechanism.