Effect of light history on retinal antioxidants and light damage susceptibility in the rat

Retinal hyperspectral imaging in the 5xFAD mouse model of Alzheimer's disease

Hyperspectral imaging of the retina has recently been posited as a potentially useful form of spectroscopy of amyloid-beta (Aβ) protein in the eyes of those with Alzheimer's disease (AD). The concept of using the retina as a biomarker for AD is an attractive one, as current screening tools for AD are either expensive or inaccessible. Recent studies have investigated hyperspectral imaging in Aβ models however these studies have been in younger mice. Here we characterised hyperspectral reflectance profile in 6 to 17 months old 5xFAD mice and compare this to Aβ in isolated preparations. Hyperspectral imaging was conducted across two preparations of Aβ using a custom built bench ophthalmoscope. In the in vitro condition, 1 mg of purified human Aβ42 was solubilised and left to aggregate for 72 h. This soluble/insoluble Aβ mixture was then imaged by suspending the solution at a pipette tip and compared against phosphate buffered saline (PBS) control (n = 10 ROIs / group). In the in vivo condition, a 5xFAD transgenic mouse model was used and retinae were imaged at the age of 6 (n = 9), 12 (n = 9) and 17 months (n = 8) with age matched wildtype littermates as control (n = 12, n = 13, n = 15 respectively). In the vitro condition, hyperspectral imaging of the solution showed greater reflectance compared with vehicle (p < 0.01), with the greatest differences occurring in the short visible spectrum (< 500 nm). In the in vivo preparation, 5xFAD showed greater hyperspectral reflectance at all ages (6, 12, 17 months, p < 0.01). These differences were noted most in the short wavelengths at younger ages, with an additional peak appearing at longer wavelengths (~ 550 nm) with advancing age. This study shows that the presence of Aβ (soluble/insoluble mixture) can increase the hyperspectral reflectance profile in vitro as well as in vivo. Differences were evident in the short wavelength spectrum (< 500 nm) in vitro and were preserved when imaged through the ocular media in the in vivo conditions. With advancing age a second hump around ~ 550 nm became more apparent. Hyperspectral imaging of the retina does not require the use of contrast agents and is a potentially useful and non-invasive biomarker for AD.

Threshold protective effect of deuterated polyunsaturated fatty acids on peroxidation of lipid bilayers

Autoxidation of polyunsaturated fatty acids (PUFAs) damages lipid membranes and generates numerous toxic by-products implicated in neurodegeneration, aging, and other pathologies. Abstraction of bis-allylic hydrogen atoms is the rate-limiting step of PUFA autoxidation, which is inhibited by replacing bis-allylic hydrogens with deuterium atoms (D-PUFAs). In cells, the presence of a relatively small fraction of D-PUFAs among natural PUFAs is sufficient to effectively inhibit lipid peroxidation (LPO). Here, we investigate the effect of various D-PUFAs on the stability of liposomes under oxidative stress conditions. The permeability of vesicle membranes to fluorescent dyes was measured as a proxy for bilayer integrity, and the formation of conjugated dienes was monitored as a proxy for LPO. Remarkably, both approaches reveal a similar threshold for the protective effect of D-PUFAs in liposomes. We show that protection rendered by D-PUFAs depends on the structure of the deuterated fatty acid. Our findings suggest that protection of PUFAs against autoxidation depends on the total level of deuterated bi-sallylic (CD₂ ) groups present in the lipid bilayer. However, the phospholipid containing 6,6,9,9,12,12,15,15,18,18-d₁₀ -docosahexaenoic acid exerts a stronger protective effect than should be expected from its deuteration level. These findings further support the application of D-PUFAs as preventive/therapeutic agents in numerous pathologies that involve LPO.

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.

A Drug-Tunable Gene Therapy for Broad-Spectrum Protection against Retinal Degeneration

Retinal degenerations are a large cluster of diseases characterized by the irreversible loss of light-sensitive photoreceptors that impairs the vision of 9.1 million people in the US. An attractive treatment option is to use gene therapy to deliver broad-spectrum neuroprotective factors. However, this approach has had limited clinical translation because of the inability to control transgene expression. To address this problem, we generated an adeno-associated virus vector named RPF2 that was engineered to express domains of leukemia inhibitory factor fused to the destabilization domain of bacterial dihydrofolate reductase. Fusion proteins containing the destabilization domain are degraded in mammalian cells but can be stabilized with the binding of the drug trimethoprim. Our data show that expression levels of RPF2 are tightly regulated by the dose of trimethoprim and can be reversed by trimethoprim withdrawal. We further show that stabilized RPF2 can protect photoreceptors and prevent blindness in treated mice.

The Involvement of the Oxidative Stress in Murine Blue LED Light-Induced Retinal Damage Model

The aim of study was to establish a mouse model of blue light emitting diode (LED) light-induced retinal damage and to evaluate the effects of the antioxidant N-acetylcysteine (NAC). Mice were exposed to 400 or 800 lx blue LED light for 2 h, and were evaluated for retinal damage 5 d later by electroretinogram amplitude and outer nuclear layer (ONL) thickness. Additionally, we investigated the effect of blue LED light exposure on shorts-wave-sensitive opsin (S-opsin), and rhodopsin expression by immunohistochemistry. Blue LED light induced light intensity dependent retinal damage and led to collapse of S-opsin and altered rhodopsin localization from inner and outer segments to ONL. Conversely, NAC administered at 100 or 250 mg/kg intraperitoneally twice a day, before dark adaptation and before light exposure. NAC protected the blue LED light-induced retinal damage in a dose-dependent manner. Further, blue LED light-induced decreasing of S-opsin levels and altered rhodopsin localization, which were suppressed by NAC. We established a mouse model of blue LED light-induced retinal damage and these findings indicated that oxidative stress was partially involved in blue LED light-induced retinal damage.

Light Modulates Ocular Complications in an Albino Rat Model of Type 1 Diabetes Mellitus

PURPOSE

The purpose of the study was to assess potential interactions of light exposure and hyperglycemia upon ocular complications in diabetic rats.

METHODS

Streptozotocin-induced (STZ-induced) diabetic rats (N = 39) and non-diabetic rats (N = 9) were distributed into eight groups according to the irradiance and color of the light phase during the 12/12-hour light/dark regime. Follow-up lasted 90 days and included assessment of cataract development and electroretinogram (ERG) recordings. Stress to the retina was also assessed by glial fibrillary acidic protein immunocytochemistry.

RESULTS

Cataract development was fast in diabetic rats that were exposed to unattenuated white light or to bright colored lights during the light phase. Diabetic rats that were kept under attenuated brown or yellow light during the light phase exhibited slower rate of cataract development. Electroretinogram responses indicated very severe retinal damage in diabetic rats kept under bright colored lights in the blue-yellow range or bright white light during the light phase. Electroretinogram damage was milder in rats kept under bright red light or attenuated yellow or brown light during the light phase. Glial fibrillary acidic protein expression in retinal Müller cells was consistent with ERG assessment of retinal damage.

CONCLUSIONS

Attenuating white light and filtering out short wavelengths have a protective effect on the eyes of diabetic rats as evident by slower rate of cataract formation and a smaller degree of retinal damage.

TRANSLATIONAL RELEVANCE

Our findings suggest that special glasses attenuating light exposure and filtering out short wavelengths (400-530 nm) may be beneficial for diabetic patients.

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.

Immunohistochemical Localization of GFAP and Glutamate Regulatory Proteins in Chick Retina and Their Levels of Expressions in Altered Photoperiods

Moderate to intense light is reported to damage the chick retina, which is cone dominated. Light damage alters neurotransmitter pools, such as those of glutamate. Glutamate level in the retina is regulated by glutamate-aspartate transporter (GLAST) and glutamine synthetase (GS). We examined immunolocalization patterns and the expression levels of both markers and of glial fibrillary acidic protein (GFAP, a marker of neuronal stress) in chick retina exposed to 2000 lux under 12-h light:12-h dark (12L:12D; normal photoperiod), 18L:6D (prolonged photoperiod), and 24L:0D (constant light) at post-hatch day 30. Retinal damage (increased death of photoreceptors and inner retinal neurons and Müller cell hypertrophy) and GFAP expression in Müller cells were maximal in 24L:0D condition compared to that seen in 12L:12D and 18L:6D conditions. GS was present in Müller cells and GLAST expressed in Müller cell processes and photoreceptor inner segments. GLAST expression was decreased in 24L:0D condition, and the expression levels between 12L:12D and 18L:6D, though increased marginally, were statistically insignificant. Similar was the case with GS expression that significantly decreased in 24L:0D condition. Our previous study with chicks exposed to 2000 lux reported increased retinal glutamate level in 24L:0D condition. The present results indicate that constant light induces decreased expressions of GLAST and GS, a condition that might aggravate glutamate-mediated neurotoxicity and delay neuroprotection in a cone-dominated retina.

The edaravone and 3-n-butylphthalide ring-opening derivative 10b effectively attenuates cerebral ischemia injury in rats

AIM

Compound 10b is a hybrid molecule of edaravone and a ring-opening derivative of 3-n-butylphthalide (NBP). The aim of this study was to examine the effects of compound 10b on brain damage in rats after focal cerebral ischemia.

METHODS

SD rats were subjected to 2-h-middle cerebral artery occlusion (MCAO). At the onset of reperfusion, the rats were orally treated with NBP (60 mg/kg), edaravone (3 mg/kg), NBP (60 mg/kg)+edaravone (3 mg/kg), or compound 10b (70, 140 mg/kg). The infarct volume, motor behavior deficits, brain water content, histopathological alterations, and activity of GSH, SOD, and MDA were analyzed 24 h after reperfusion. The levels of relevant proteins in the ipsilateral striatum were examined using immunoblotting.

RESULTS

Administration of compound 10b (70 or 140 mg/kg) significantly reduced the infarct volume and neurological deficits in MCAO rats. The neuroprotective effects of compound 10b were more pronounced compared to NBP, edaravone or NBP+edaravone. Furthermore, compound 10b significantly upregulated the protein levels of the cytoprotective molecules Bcl-2, HO-1, Nrf2, Trx, P-NF-κB p65, and IκB-α, while decreasing the expression of Bax, caspase 3, caspase 9, Txnip, NF-κB p65, and P-IκB-α.

CONCLUSION

Oral administration of compound 10b effectively attenuates rat cerebral ischemia injury.

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.

Cytochrome P450 2C epoxygenases mediate photochemical stress-induced death of photoreceptors

Degenerative loss of photoreceptors occurs in inherited and age-related retinal degenerative diseases. A chemical screen facilitates development of new testing routes for neuroprotection and mechanistic investigation. Herein, we conducted a mouse-derived photoreceptor (661W cell)-based high throughput screen of the Food and Drug Administration-approved Prestwick drug library to identify putative cytoprotective compounds against light-induced, synthetic visual chromophore-precipitated cell death. Different classes of hit compounds were identified, some of which target known genes or pathways pathologically associated with retinitis pigmentosa. Sulfaphenazole (SFZ), a selective inhibitor of human cytochrome P450 (CYP) 2C9 isozyme, was identified as a novel and leading cytoprotective compound. Expression of CYP2C proteins was induced by light. Gene-targeted knockdown of CYP2C55, the homologous gene of CYP2C9, demonstrated viability rescue to light-induced cell death, whereas stable expression of functional CYP2C9-GFP fusion protein further exacerbated light-induced cell death. Mechanistically, SFZ inhibited light-induced necrosis and mitochondrial stress-initiated apoptosis. Light elicited calcium influx, which was mitigated by SFZ. Light provoked the release of arachidonic acid from membrane phospholipids and production of non-epoxyeicosatrienoic acid metabolites. Administration of SFZ further stimulated the production of non-epoxyeicosatrienoic acid metabolites, suggesting a metabolic shift of arachidonic acid under inhibition of the CYP2C pathway. Together, our findings indicate that CYP2C genes play a direct causative role in photochemical stress-induced death of photoreceptors and suggest that the CYP monooxygenase system is a risk factor for retinal photodamage, especially in individuals with Stargardt disease and age-related macular degeneration that deposit condensation products of retinoids.

New functions of Müller cells

Müller cells, the major type of glial cells in the retina, are responsible for the homeostatic and metabolic support of retinal neurons. By mediating transcellular ion, water, and bicarbonate transport, Müller cells control the composition of the extracellular space fluid. Müller cells provide trophic and anti-oxidative support of photoreceptors and neurons and regulate the tightness of the blood-retinal barrier. By the uptake of glutamate, Müller cells are more directly involved in the regulation of the synaptic activity in the inner retina. This review gives a survey of recently discoved new functions of Müller cells. Müller cells are living optical fibers that guide light through the inner retinal tissue. Thereby they enhance the signal/noise ratio by minimizing intraretinal light scattering and conserve the spatial distribution of light patterns in the propagating image. Müller cells act as soft, compliant embedding for neurons, protecting them in case of mechanical trauma, and also as soft substrate required for neurite growth and neuronal plasticity. Müller cells release neuroactive signaling molecules which modulate neuronal activity, are implicated in the mediation of neurovascular coupling, and mediate the homeostasis of the extracellular space volume under hypoosmotic conditions which are a characteristic of intense neuronal activity. Under pathological conditions, a subset of Müller cells may differentiate to neural progenitor/stem cells which regenerate lost photoreceptors and neurons. Increasing knowledge of Müller cell function and responses in the normal and diseased retina will have great impact for the development of new therapeutic approaches for retinal diseases.

A reduced zinc diet or zinc transporter 3 knockout attenuate light induced zinc accumulation and retinal degeneration

Our previous study on retinal light exposure suggests the involvement of zinc (Zn(2+)) toxicity in the death of RPE and photoreceptors (LD) which could be attenuated by pyruvate and nicotinamide, perhaps through restoration of NAD(+) levels. In the present study, we examined Zn(2+) toxicity, and the effects of NAD(+) restoration in primary retinal cultures. We then reduced Zn(2+) levels in rodents by reducing Zn(2+) levels in the diet, or by genetics and measured LD. Sprague Dawley albino rats were fed 2, or 61 mg Zn(2+)/kg of diet for 3 weeks, and exposed to 18 kLux of white light for 4 h. We light exposed (70 kLux of white light for 50 h) Zn(2+) transporter 3 knockout (ZnT3-KO, no synaptic Zn(2+)), or RPE65 knockout mice (RPE65-KO, lack rhodopsin cycling), or C57/BI6/J controls and determined light damage and Zn(2+) staining. Retinal Zn(2+) staining was examined at 1 h and 4 h after light exposure. Retinas were examined after 7 d by optical coherence tomography and histology. After LD, rats fed the reduced Zn(2+) diet showed less photoreceptor Zn(2+) staining and degeneration compared to a normal Zn(2+) diet. Similarly, ZnT3-KO and RPE65-KO mice showed less Zn(2+) staining, NAD(+) loss, and RPE or photoreceptor death than C57/BI6/J control mice. Dietary or ZnT3-dependent Zn(2+) stores, and intracellular Zn(2+) release from rhodopsin recycling are suggested to be involved in light-induced retinal degeneration. These results implicate novel rhodopsin-mediated mechanisms and therapeutic targets for LD. Our companion manuscript demonstrates that pharmacologic, circadian, or genetic manipulations which maintain NAD(+) levels reduce LD.

Müller glial cells in retinal disease

Virtually all pathogenic stimuli activate Müller cells. Reactive Müller cells exert protective and toxic effects on photoreceptors and neurons. They contribute to oxidative stress and glutamate toxicity due to malfunctions of glutamate uptake and glutathione synthesis. Downregulation of potassium conductance disrupts transcellular potassium and water transport, resulting in neuronal hyperexcitability and edema. Protective effects of reactive Müller cells include upregulation of adenosine 5'-triphosphate (ATP)-degrading ectoenzymes, which enhances the extracellular availability of the neuroprotectant adenosine, abrogation of the osmotic release of ATP, which might protect retinal ganglion cells from apoptosis, and the release of antioxidants and neurotrophic factors. The dedifferentiation of reactive Müller cells to progenitor-like cells might have an impact on future therapeutic approaches. A better understanding of the gliotic mechanisms will be helpful in developing efficient therapeutic strategies aiming at increased protective and regenerative properties and decreased toxicity of reactive Müller cells.

Oxygen supply from the bird's eye perspective: globin E is a respiratory protein in the chicken retina

The visual process in the vertebrate eye requires high amounts of metabolic energy and thus oxygen. Oxygen supply of the avian retina is a challenging task because birds have large eyes, thick retinae, and high metabolic rates but neither deep retinal nor superficial capillaries. Respiratory proteins such as myoglobin may enhance oxygen supply to certain tissues, and thus the mammalian retina harbors high amounts of neuroglobin. Globin E (GbE) was recently identified as an eye-specific globin of chicken (Gallus gallus). Orthologous GbE genes were found in zebra finch and turkey genomes but appear to be absent in non-avian vertebrate classes. Analyses of globin phylogeny and gene synteny showed an ancient origin of GbE but did not help to assign it to any specific globin type. We show that the photoreceptor cells of the chicken retina have a high level of GbE protein, which accumulates to ∼10 μM in the total eye. Quantitative real-time RT-PCR revealed an ∼50,000-fold higher level of GbE mRNA in the eye than in the brain. Spectroscopic analysis and ligand binding kinetics of recombinant chicken GbE reveal a penta-coordinated globin with an oxygen affinity of P(50) = 5.8 torrs at 25 °C and 15 torrs at 41 °C. Together these data suggest that GbE helps to sustain oxygen supply to the avian retina.

Differential effects of rapamycin on rods and cones during light-induced stress in albino mice

PURPOSE

Autophagy is a lysosomal machinery-dependent process that catabolizes cellular components/organelles and proteins in an autophagic vacuole (AV)-dependent and -independent manner, respectively. Short-term exposure of the retina to bright light results in shortening of the outer segments, concomitant with AV formation. Autophagy is also induced by continuous long-term light damage, leading to photoreceptor cell death. Here the authors examined two questions: is autophagy induced during light damage proapoptotic or antiapoptotic, and are rods and cones affected differently? To this end, Balb/c mice exposed to light damage were treated with rapamycin to increase autophagy.

METHODS

Balb/c and GFP-LC3 mice were treated with rapamycin/vehicle. Photoreceptor degeneration was induced by 10-day light damage. Autophagy was documented by histologic, biochemical, and molecular tools; rod and cone survival was assessed by histology and electroretinography.

RESULTS

Light damage resulted in rod, but not cone, cell loss. Autophagy and AV formation was elicited in response to light damage, which was amplified by rapamycin. Rapamycin treatment significantly improved rod survival and function, reduced apoptosis, and normalized cytokine production that was increased in light damage. However, AV formation in GFP-LC3 mice revealed that light damage or rapamycin treatment induced AVs in cones, concomitant with reduced cone-mediated electroretinograms.

CONCLUSIONS

Systemic rapamycin treatment provided rod protection; however, AV formation was induced only in cones. Thus, rapamycin may act differentially in stressed photoreceptors; rapamycin might protect rods by normalizing cytokine production, removing damaged proteins by AV-independent autophagy, or both, whereas cones might be protected by AV-dependent autophagy, possibly involving reduced photon capture.

Effects of PACAP in UV-A radiation-induced retinal degeneration models in rats

The retina is constantly exposed to ultraviolet (UV) light with different wavelengths, which may lead to chronic UV-induced retinal injury. In our previous studies, we have shown the protective effects of pituitary adenylate cyclase activating polypeptide (PACAP) in toxic and ischemic retinal injuries. The aim of the present study was to investigate the effects of PACAP in UV-A-induced retinal lesion. We used diffuse UV-A radiation (315-400 nm) to induce acute retinal damage over a short period of exposure. Using standard histological (morphological and morphometrical) analysis, we assessed the actions of intravitreal PACAP (100 pmol/5 µl) treatment on acute UV-A-induced retinal damage. We measured the thickness of nuclear and plexiform layers as well as the number of cells in the outer nuclear and inner nuclear layers and in the ganglion cell layer. Outer limiting membrane-inner limiting membrane distances in the cross-section of the retina were also examined. Our results show that UV-A light-induced retinal damage led to severe degeneration in the photoreceptor layer, and in the outer and inner nuclear layers. Alteration in the plexiform layers was also observed. We found that post-irradiation PACAP treatment significantly attenuated the UV-A-induced retinal damage. Our results provide the basis for future clinical application of PACAP treatment in retinal degeneration and may have clinical implications in several ophthalmic diseases.

Near-infrared light protect the photoreceptor from light-induced damage in rats

BACKGROUND

A project originally developed for NASA plant growth experiments in space demonstrating the Light-Emitting Diode (LED) could promote the wound healing. Further study showed that the LED's could protect cells by stimulating the basic energy processes in the mitochondria of each cell.

OBJECTIVE

The purpose of this study was to assess the effects of 670 nm LED to protect the photoreceptor from the light-induced damage in a rodent model.

METHODS

SD rats were randomly assigned to one of eight groups: untreated control group, the LED-treated control group, three light-induced damage groups, and three LED-protected groups. The rats were exposed to constant light for 3 h of different illuminations of 900, 1,800 and 2,700 lux, respectively. The LED treatment (50 mW) were done for 30 min, 3 h before the light damage and 0, 24 and 48 h after the light damage. Using the electroretinogram as a sensitive indicator of retinal function, and the histopathologic change was showed as a proof of the protective effect of LED treatment.

RESULTS

The 900 lux illumination for 3 h did not cause damage to the retina of rats, however, the 1,800 lux illumination for 3 h caused significant damage to ONL of an approximate half retina, which caused the swing of ERG b wave to be 431 muV. With the LED protection: the damage of ONL was near 1/6 of retina, which was significantly reduced than the ones without LED protection (P < 0.01); and the swing of ERG b wave was recorded to be 1,011 muV, which was increased significantly than the ones without LED protection (P < 0.01). The illumination of 2,700 lux for 3 h caused severe damage to the rats' retinas and the LED could not protect them significantly in both of morphology and function (P > 0.05, P > 0.05).

CONCLUSIONS

670 nm LED treatment has an evident protective effect on retinal cells against light-induced damage, which may be an innovative and non-invasive therapeutic approach to prevent or to delay age-related macular degeneration.

Retinal light damage: mechanisms and protection

By its action on rhodopsin, light triggers the well-known visual transduction cascade, but can also induce cell damage and death through phototoxic mechanisms - a comprehensive understanding of which is still elusive despite more than 40 years of research. Herein, we integrate recent experimental findings to address several hypotheses of retinal light damage, premised in part on the close anatomical and metabolic relationships between the photoreceptors and the retinal pigment epithelium. We begin by reviewing the salient features of light damage, recently joined by evidence for retinal remodeling which has implications for the prognosis of recovery of function in retinal degenerations. We then consider select factors that influence the progression of the damage process and the extent of visual cell loss. Traditional, genetically modified, and emerging animal models are discussed, with particular emphasis on cone visual cells. Exogenous and endogenous retinal protective factors are explored, with implications for light damage mechanisms and some suggested avenues for future research. Synergies are known to exist between our long term light environment and photoreceptor cell death in retinal disease. Understanding the molecular mechanisms of light damage in a variety of animal models can provide valuable insights into the effects of light in clinical disorders and may form the basis of future therapies to prevent or delay visual cell loss.

Preconditioning-induced protection of photoreceptors requires activation of the signal-transducing receptor gp130 in photoreceptors

Retinal degenerations are a class of neurodegenerative disorders that ultimately lead to blindness due to the death of retinal photoreceptors. In most cases, death is the result of long-term exposure to environmental, inflammatory, and genetic insults. In age-related macular degeneration, significant vision loss may take up to 70-80 years to develop. The protracted time to develop blindness suggests that retinal neurons have an endogenous mechanism for protection from chronic injury. Previous studies have shown that endogenous protective mechanisms can be induced by preconditioning animals with sublethal bright cyclic light. Such preconditioning can protect photoreceptors from a subsequent damaging insult and is thought to be accomplished through induced expression of protective factors. Some of the factors shown to be associated with protection bind and activate the signal transducing receptor gp130. To determine whether stress-induced endogenous protection of photoreceptors requires gp130, we generated conditional gp130 knockout (KO) mice with the Cre/lox system and used light-preconditioning to induce neuroprotection in these mice. Functional and morphological analyses demonstrated that the retina-specific gp130 KO impaired preconditioning-induced endogenous protection. Photoreceptor-specific gp130 KO mice had reduced protection, although the Müller cell KO mice did not, thus gp130-induced protection was restricted to photoreceptors. Using an animal model of retinitis pigmentosa, we found that the photoreceptor-specific gp130 KO increased sensitivity to genetically induced photoreceptor cell death, demonstrating that gp130 activation in photoreceptors had a general protective role independent of whether stress was caused by light or genetic mutations.

Optimising the structure and function of the adult P23H-3 retina by light management in the juvenile and adult

This study tests the potential of light restriction to optimise retinal structure and function in adulthood, using the P23H-3 rhodopsin-mutant transgenic rat as a model. P23H-3 rats were reared in scotopic (5 lux) or mesopic (40-60 lux) cyclic (12 h/12 h light/dark) light. A further 2 groups were reared in one of these light conditions to P(postnatal day)30, and then were transferred to the other condition. Retinae were examined at P30-365. Rod and cone function were assessed by the dark-adapted flash electroretinogram. The rate of photoreceptor death was assessed with the TUNEL technique, and photoreceptor survival by the thickness of the outer nuclear layer (ONL). Photoreceptor structural changes were assessed by immunohistochemistry. Mesopic rearing severely reduced the number, function and outer segment (OS) length of photoreceptors. Light restriction in the adult (achieved by moving mesopic-reared animals to scotopic conditions at P30) slowed photoreceptor death, induced recovery of the ERG and of OS length in survivors, resulting in an adult retina that matched the scotopic-reared in function, photoreceptor survival (stability) and structure. Conversely, light exposure in the adult (achieved by moving scotopic-reared animals to mesopic conditions at P30) accelerated photoreceptor death, shortened OSs and reduced the ERG, resulting in a retina that was as damaged and dysfunctional as a mesopic-reared retina, and showed greater photoreceptor instability. Present observations suggest, that the stability and function of adult photoreceptors are determined by both early and adult ambient light experience. Light restriction in the adult was effective in inducing the self-repair of photoreceptors, and the recovery of their function and stability. Light restriction in the juvenile (before P30) improved early photoreceptor survival but made adult photoreceptors vulnerable to brighter light experienced in adulthood. For comparable human dystrophies, these results suggest that light restriction begun after retinal maturation may be effective in optimising the structure, function and stability of the adult retina.

Cellular signaling and factors involved in Müller cell gliosis: neuroprotective and detrimental effects

Müller cells are active players in normal retinal function and in virtually all forms of retinal injury and disease. Reactive Müller cells protect the tissue from further damage and preserve tissue function by the release of antioxidants and neurotrophic factors, and may contribute to retinal regeneration by the generation of neural progenitor/stem cells. However, Müller cell gliosis can also contribute to neurodegeneration and impedes regenerative processes in the retinal tissue by the formation of glial scars. This article provides an overview of the neuroprotective and detrimental effects of Müller cell gliosis, with accounts on the cellular signal transduction mechanisms and factors which are implicated in Müller cell-mediated neuroprotection, immunomodulation, regulation of Müller cell proliferation, upregulation of intermediate filaments, glial scar formation, and the generation of neural progenitor/stem cells. A proper understanding of the signaling mechanisms implicated in gliotic alterations of Müller cells is essential for the development of efficient therapeutic strategies that increase the supportive/protective and decrease the destructive roles of gliosis.

Glutathione-Related Enzymes and the Eye

Glutathione and the related enzymes belong to the defence system protecting the eye against chemical and oxidative stress. This review focuses on GSH and two key enzymes, glutathione reductase and glucose-6-phosphate dehydrogenase in lens, cornea, and retina. Lens contains a high concentration of reduced glutathione, which maintains the thiol groups in the reduced form. These contribute to lens complete transparency as well as to the transparent and refractive properties of the mammalian cornea, which are essential for proper image formation on the retina. In cornea, gluthatione also plays an important role in maintaining normal hydration level, and in protecting cellular membrane integrity. In retina, glutathione is distributed in the different types of retinal cells. Intracellular enzyme, glutathione reductase, involved in reducing the oxidized glutathione has been found at highest activity in human and primate lenses, as compared to other species. Besides the enzymes directly involved in maintaining the normal redox status of the cell, glucose-6-phosphate dehydrogenase which catalyzes the first reaction of the pentose phosphate pathway, plays a key role in protection of the eye against reactive oxygen species. Cornea has a high activity of the pentose phosphate pathway and glucose-6-phosphate dehydrogenase activity. Glycation, the non-enzymic reaction between a free amino group in proteins and a reducing sugar, slowly inactivates gluthathione-related and other enzymes. In addition, glutathione can be also glycated. The presence of glutathione, and of the related enzymes has been also reported in other parts of the eye, such as ciliary body and trabecular meshwork, suggesting that the same enzyme systems are present in all tissues of the eye to generate NADPH and to maintain gluthatione in the reduced form. Changes of glutathione and related enzymes activity in lens, cornea, retina and other eye tissues, occur with ageing, cataract, diabetes, irradiation and administration of some drugs.

The impact of light, noise, cage cleaning and in-house transport on welfare and stress of laboratory rats

Human interaction and physical environmental factors are part of the stimuli presented to laboratory animals everyday, influencing their behaviour and physiology and contributing to their welfare. Certain environmental conditions and routine procedures in the animal facility might induce stress responses and when the animal is unable to maintain its homeostasis in the presence of a particular stressor, the animal's wellbeing is threatened. This review article summarizes several published studies on the impact of environmental factors such as light, noise, cage cleaning and in-house transport on welfare and stress of laboratory rats. The behaviour and physiological responses of laboratory rats to different environmental housing conditions and routine procedures are reviewed. Recommendations on the welfare of laboratory rats and refinements in experimental design are discussed and how these can influence and improve the quality of scientific data.

Preconditioning-induced protection from oxidative injury is mediated by leukemia inhibitory factor receptor (LIFR) and its ligands in the retina

Preconditioning with moderate oxidative stress (e.g., moderate bright light or mild hypoxia) can induce changes in retinal tissue that protect photoreceptors from a subsequent dose of lethal oxidative stress. The mechanism underlying this induced protection is likely a general mechanism of endogenous protection which has been demonstrated in heart and brain using ischemia and reperfusion. While multiple factors like bFGF, CNTF, LIF and BDNF have been hypothesized to play a role in preconditioning-induced endogenous neuroprotection, it has not yet been demonstrated which factors or receptors are playing an essential role. Using quantitative PCR techniques we provide evidence that in the retina, LIFR activating cytokines leukemia inhibitory factor (LIF), cardiotrophin-1 (CT-1) and cardiotrophin like cytokine (CLC) are strongly upregulated in response to preconditioning with bright cyclic light leading to robust activation of signal transducer and activator of transcription-3 (STAT3) in a time-dependent manner. Further, we found that blocking LIFR activation during preconditioning using a LIFR antagonist (LIF05) attenuated the induced STAT3 activation and also resulted in reduced preconditioning-induced protection of the retinal photoreceptors. These data demonstrate that LIFR and its ligands play an essential role in endogenous neuroprotective mechanisms triggered by preconditioning-induced stress.

Sustained delivery of NT-3 from lens fiber cells in transgenic mice reveals specificity of neuroprotection in retinal degenerations

Several neurotrophic factors (NTFs) are effective in protecting retinal photoreceptor cells from the damaging effects of constant light and slowing the rate of inherited photoreceptor degenerations. It is currently unclear whether, if continuously available, all NTFs can be protective for many or most retinal degenerations (RDs). We used transgenic mice that continuously overexpress the neurotrophin NT-3 from lens fibers under the control of the alphaA-crystallin promoter to test for neuroprotection in light-damage experiments and in four naturally occurring or transgenically induced RDs in mice. Lens-specific expression of NT-3 mRNA was demonstrated both by in situ hybridization in embryos and by reverse-transcriptase polymerase chain reaction (RT-PCR) in adult mice. Furthermore, NT-3 protein was found in abundance in the lens, ocular fluids, and retina by enzyme-linked immunosorbent assay (ELISA) and immunocytochemistry. Overexpression of NT-3 had no adverse effects on the structure or function of the retina for up to at least 14 months of age. Mice expressing the NT-3 transgene were protected from the damaging effects of constant light to a much greater degree than those receiving bolus injections of NT-3. When the NT-3 transgene was transferred into rd/rd, Rds/+, Q344ter mutant rhodopsin or Mertk knockout mice, overexpression of NT-3 had no protective effect on the RDs in these mice. Thus, specificity of the neuroprotective effect of NT-3 is clearly demonstrated, and different molecular mechanisms are inferred to mediate the protective effect in light-induced and inherited RDs.

Orally administered epigallocatechin gallate attenuates light-induced photoreceptor damage

EGCG, a major component of green tea, has a number of properties which includes it being a powerful antioxidant. The purpose of this investigation was to deduce whether inclusion of EGCG in the drinking water of albino rats attenuates the effect of a light insult (2200lx, for 24h) to the retina. TUNEL-positive cells were detected in the outer nuclear layer of the retina, indicating the efficacy of the light insult in inducing photoreceptor degeneration. Moreover, Ret-P1 and the mRNA for rhodopsin located at photoreceptors were also significantly reduced as well as the amplitude of both the a- and b-waves of the electroretinogram was also reduced showing that photoreceptors in particular are affected by light. An increase in protein/mRNA of GFAP located primarily to Müller cells caused by light shows that other retinal components are also influenced by the light insult. However, antigens associated with bipolar (alpha-PKC), ganglion (Thy-1) and amacrine (GABA) cells, in contrast, appeared unaffected. The light insult also caused a change in the content of various proteins (caspase-3, caspase-8, PARP, Bad, and Bcl-2) involved in apoptosis. A number of the changes to the retina caused by a light insult were significantly attenuated when EGCG was in the drinking water. The reduction of the a- and b-waves and photoreceptor specific mRNAs/protein caused by light were significantly less. In addition, EGCG attenuated the changes caused by light to certain apoptotic proteins (especially at after 2 days) but did not appear to significantly influence the light-induced up-regulation of GFAP protein/mRNA. It is concluded that orally administered EGCG blunts the detrimental effect of light to the retina of albino rats where the photoreceptors are primarily affected.

Age-dependent accumulation of lipofuscin in perivascular and subretinal microglia in experimental mice

Fundus autofluorescence (AF) imaging by confocal scanning laser ophthalmoscopy has been widely used by ophthalmologists in the diagnosis/monitoring of various retinal disorders. It is believed that fundus AF is derived from lipofuscin in retinal pigment epithelial (RPE) cells; however, direct clinicopathological correlation has not been possible in humans. We examined fundus AF by confocal scanning laser ophthalmoscopy and confocal microscopy in normal C57BL/6 mice of different ages. Increasingly strong AF signals were observed with age in the neuroretina and subretinal/RPE layer by confocal scanning laser ophthalmoscopy. Unlike fundus AF detected in normal human subjects, mouse fundus AF appeared as discrete foci distributed throughout the retina. Most of the AF signals in the neuroretina were distributed around retinal vessels. Confocal microscopy of retinal and choroid/RPE flat mounts demonstrated that most of the AF signals were derived from Iba-1+ perivascular and subretinal microglia. An age-dependent accumulation of Iba-1+ microglia at the subretinal space was observed. Lipofuscin granules were detected in large numbers in subretinal microglia by electron microscopy. The number of AF+ microglia and the amount of AF granules/cell increased with age. AF granules/lipofuscin were also observed in RPE cells in mice older than 12 months, but the number of AF+ RPE cells was very low (1.48 mm(-2) and 5.02 mm(-2) for 12 and 24 months, respectively) compared to the number of AF+ microglial cells (20.63 mm(-2) and 76.36 mm(-2) for 6 and 24 months, respectively). The fluorescence emission fingerprints of AF granules in subretinal microglia were the same as those in RPE cells. Our observation suggests that perivascular and subretinal microglia are the main cells producing lipofuscin in normal aged mouse retina and are responsible for in vivo fundus AF. Microglia may play an important role in retinal aging and age-related retinal diseases.

Basic fibroblast growth factor-induced protection from light damage in the mouse retina in vivo

Basic fibroblast growth factor (bFGF) has proven neuroprotective efficacy in the rodent retina against a diverse array of injurious stimuli. However, there is no consensus to date as to the molecular mechanisms underlying this neuroprotection. The study presented herein demonstrates increased expression of endogenous bFGF in the albino mouse retina in response to acute exposure to sublethal levels of light stress. The increased expression correlates with significant photoreceptor protection from light damage. The neuroprotection is likely to be mediated by bFGF as we demonstrate that a shorter exposure to bright light stress that does not up-regulate bFGF fails to protect photoreceptors from light damage. Furthermore, intravitreal bFGF injection into the retina of mice 3 h prior to light damage affords almost complete photoreceptor protection from light-induced degeneration. In addition, injected bFGF induces the activation of protein kinase B and extracellular signal-regulated kinase 1/2 signalling which correlate directly with the pathways we find to be activated in response to light stress and up-regulated bFGF. Moreover, we demonstrate that both bright light pre-conditioning and intravitreal bFGF injection result in dramatic increases in levels of inactive glycogen synthase kinase 3beta and cyclic AMP response element binding protein phosphorylation indicating a potential mechanism by which bFGF promotes survival of photoreceptors in vivo.

Visible light induces matrix metalloproteinase-9 expression in rat eye

Up-regulation of matrix metalloproteinase-9 (MMP-9, gelatinase B) in the nervous system has been demonstrated when excitotoxicity-induced tissue remodeling and neuronal death occurs. Induction of MMP-9 by a natural stimulus has not been observed yet. Using RT-PCR and gelatin-zymography we demonstrated MMP-9 induction at transcriptional and protein levels in different structures of the rat eye following over-stimulation with white light. MMP-9 elevation occurred in the retina without reduction in photoreceptor number or major anatomical reorganization. A transient decrease in electroretinogram b-wave indicated the functional recovery. Retrobulbar injection of a broad-spectrum MMP-inhibitor GM6001, slowed the recovery rate of b-wave amplitude. Even room-light applied to dark-adapted awake animals induced MMP-9 increase in the retina, which suggests a role for MMP-9 in physiological functional plasticity of the nervous system, such as light adaptation. This is the first demonstration of MMP-9 induction by a sensory stimulus.

Paradoxical role of BDNF: BDNF+/- retinas are protected against light damage-mediated stress

PURPOSE

Photoreceptors can be prevented from undergoing apoptosis in response to constant light by the application of exogenous neuroprotective agents, including brain-derived neurotrophic factor (BDNF). BDNF, however, cannot exert its effect directly on photoreceptors because they do not express receptors for BDNF. It has been proposed that BDNF released from Müller cells provides a feed-forward loop, increasing ciliary neurotrophic factor (CNTF) and basic fibroblast growth factor (bFGF) production in Müller cells, which may enhance photoreceptor survival. The authors hypothesized that retinas with reduced BDNF levels in which the BDNF-mediated release of neuroprotective signals is dampened are more susceptible to light-induced photoreceptor degeneration.

METHODS

Young adult BDNF+/+ and BDNF+/- littermates (B6.129-BDNF(tm1-LT)) were analyzed. Retinal neurotrophin and growth factor mRNA levels were determined by quantitative RT-PCR, photoreceptor function was assessed through electroretinography, and survival was documented in morphologic sections and in TUNEL assays. Oxidative stress was assayed by measuring glutathione peroxidase activity.

RESULTS

At baseline, BDNF+/- animals had significantly increased levels of glial-derived neurotrophic factor (GDNF) mRNA compared with their wild-type littermates. After light damage GDNF, CNTF, and BDNF mRNA levels dropped 14- to 16-fold in the BDNF+/+ mice but remained almost unchanged compared with baseline levels in the BDNF+/- mice. Preservation of neurotrophin levels in BDNF+/- mice correlated with photoreceptor cell survival, preservation of function, and reduced oxidative stress.

CONCLUSIONS

Contrary to the hypothesis, reducing BDNF levels resulted in photoreceptor protection against light damage. Survival was paralleled by a reduction in oxidative stress and the preservation of neurotrophin levels, suggesting that chronic reduction of BDNF in the retina provides a level of preconditioning against stress.

Oxidative stress-induced expression and modulation of Phosphatase of Regenerating Liver-1 (PRL-1) in mammalian retina

The phosphatase of regenerating liver-1, PRL-1, gene was detected in a screen for foveal cone photoreceptor-associated genes. It encodes a small protein tyrosine phosphatase that was previously immunolocalized to the photoreceptors in primate retina. Here we report that in cones and cone-derived cultured cells both PRL-1 activity and PRL-1 gene expression are modulated under oxidative stress. Oxidation reversibly inhibited the phosphatase activity of PRL-1 due to the formation of an intramolecular disulfide bridge between Cys104 within the active site and another conserved Cys, Cys49. This modulation was observed in vitro, in cell culture and in isolated retinas exposed to hydrogen peroxide. The same treatment caused a rapid increase in PRL-1 expression levels in cultured cells which could be blocked by the protein translation inhibitor, cycloheximide. Increased PRL-1 expression was also observed in living rats subjected to constant light exposure inducing photooxidative stress. We further demonstrated that both oxidation and overexpression of PRL-1 upon oxidative stress are greatly enhanced by inhibition of the glutathione system responsible for cellular redox regulation. These findings suggest that PRL-1 is a molecular component of the photoreceptor's response to oxidative stress acting upstream of the glutathione system.

G-protein-coupled receptor rhodopsin regulates the phosphorylation of retinal insulin receptor

We have shown previously that phosphoinositide 3-kinase in the retina is activated in vivo through light-induced tyrosine phosphorylation of the insulin receptor (IR). The light effect is localized to photoreceptor neurons and is independent of insulin secretion (Rajala, R. V., McClellan, M. E., Ash, J. D., and Anderson, R. E. (2002) J. Biol. Chem. 277, 43319-43326). These results suggest that there exists a cross-talk between phototransduction and other signal transduction pathways. In this study, we examined the stage of phototransduction that is coupled to the activation of the IR. We studied IR phosphorylation in mice lacking the rod-specific alpha-subunit of transducin to determine if phototransduction events are required for IR activation. To confirm that light-induced tyrosine phosphorylation of the IR is signaled through bleachable rhodopsin, we examined IR activation in retinas from RPE65(-/-) mice that are deficient in opsin chromophore. We observed that IR phosphorylation requires the photobleaching of rhodopsin but not transducin signaling. To determine whether the light-dependent activation of IR is mediated through the rod or cone transduction pathway, we studied the IR activation in mice lacking opsin, a mouse model of pure cone function. No light-dependent activation of the IR was found in the retinas of these mice. We provide evidence for the existence of a light-mediated IR pathway in the retina that is different from the known insulin-mediated pathway in nonneuronal tissues. These results suggest that IR phosphorylation in rod photoreceptors is signaled through the G-protein-coupled receptor rhodopsin. This is the first study demonstrating that rhodopsin can initiate signaling pathway(s) in addition to its classical phototransduction.

Effects of ascorbic acid on UV light-mediated photoreceptor damage in isolated rat retina

Concerns have been raised about whether operating microscopes and endoillumination used during ophthalmic surgeries contribute to retinal damage. Despite the recognition that ascorbic acid (vitamin C) helps to protect the eye from light and the abundance of vitamin C in the retina, artificial aqueous humors used during surgery only contain the antioxidant glutathione. To test whether inclusion of antioxidants other than glutathione in surgical solutions might help to preserve retinal integrity, we studied the effects of vitamin C on acute toxicity in isolated rat retinas. Male Sprague-Dawley rats (PND 30+/-2) were sacrificed for retinal isolation. In the presence or absence of vitamin C (1 or 3 mM), retinas were exposed to 302 nm ultraviolet B (UVB) light for 1 h and were incubated for a total of 5 h at 30 degrees C. Retinal damage was assessed by morphological examination and biochemical assay measuring the amount of lactate dehydrogenase (LDH) released from injured cells. In control retinas, LDH release was significantly increased after UVB exposure. The presence of 1 mM vitamin C in the incubation media significantly reduced LDH release during the post-incubation period following UV exposure. No difference was found between 1 and 3 mM vitamin C. Microscopic examination revealed that disorganization in the outer nuclear layer after UVB exposure was markedly attenuated by administration of 1 mM vitamin C. Vitamin C (1 mM), a concentration found in the anterior chamber in humans, but not glutathione, prevented phototoxic injury following UV exposure. Although vitamin C itself cannot be used in intraocular irrigating solutions because of adverse interactions with iron released during bleeding, inclusion of antioxidants equivalent to vitamin C should be considered to help protect the retina from intraoperative light toxicity.

Photochemical damage of the retina

Visual perception occurs when radiation with a wavelength between 400 and 760 nm reaches the retina. The retina has evolved to capture photons efficiently and initiate visual transduction. The retina, however, is vulnerable to damage by light, a vulnerability that has long been recognized. Photochemical damage has been widely studied, because it can cause retinal damage within the intensity range of natural light. Photochemical lesions are primarily located in the outer layers at the central region of the retina. Two classes of photochemical damage have been recognized: Class I damage, which is characterized by the rhodopsin action spectrum, is believed to be mediated by visual pigments, with the primary lesions located in the photoreceptors; whereas Class II damage is generally confined to the retinal pigment epithelium. The action spectrum peaks in the short wavelength region, providing the basis for the concept of blue light hazard. Several factors can modify the susceptibility of the retina to photochemical damage. Photochemical mechanisms, in particular mechanisms that arise from illumination with blue light, are responsible for solar retinitis and for iatrogenic retinal insult from ophthalmological instruments. Further, blue light may play a role in the pathogenesis of age-related macular degeneration. Laboratory studies have suggested that photochemical damage includes oxidative events. Retinal cells die by apoptosis in response to photic injury, and the process of cell death is operated by diverse damaging mechanisms. Modern molecular biology techniques help to study in-depth the basic mechanism of photochemical damage of the retina and to develop strategies of neuroprotection.

Structural and functional consequences of bright light exposure on the retina of neonatal rats

In a previous study we showed that juvenile rats exposed, for various durations of time, to a bright luminous environment between P14 (eye opening) and P34 developed a light-induced retinopathy (LIR), the severity of which depending on the duration of exposure as well as the age of the rat at the onset of exposure. Our study also revealed that the severity of the LIR increased as the time elapsed between the cessation of exposure and the structural/functional evaluation increased, suggesting that the LIR degenerative process proceeded in two distinct steps namely, an initial (rapid) acute phase that was followed by a (slower) chronic phase. In view of the above, the purpose of the present study was to reinvestigate previous claims suggesting that exposure to bright light prior to eyelid opening had no measurable consequences on the retinal structure and function; the claim being that despite a non-detectable acute phase, bright light exposure prior to eyelid opening could nonetheless yield a significant retinopathy during the chronic phase of development of LIR. In order to test our hypothesis, neonatal rats were raised in a bright luminous environment from birth to P14. At P30, analysis of the results obtained from rats exposed between P0-P14 did not reveal, as previously acknowledged by others, significant LIR damages. However, results obtained at P60 disclosed significant functional anomalies with relative sparing of the retinal ultrastructure. Our results confirm that, in spite of closed eyelids, postnatal exposure to bright environment did trigger a slow degenerative process.

Light-induced damage to the retina: role of rhodopsin chromophore revisited

The presence of the regenerable visual pigment rhodopsin has been shown to be primarily responsible for the acute photodamage to the retina. The photoexcitation of rhodopsin leads to isomerization of its chromophore 11-cis-retinal to all-trans-retinal (ATR). ATR is a potent photosensitizer and its role in mediating photodamage has been suspected for over two decades. However, there was lack of experimental evidence that free ATR exists in the retina in sufficient concentrations to impose a risk of photosensitized damage. Identification in the retina of a retinal dimer and a pyridinium bisretinoid, so called A2E, and determination of its biosynthetic pathway indicate that substantial amounts of ATR do accumulate in the retina. Both light damage and A2E accumulation are facilitated under conditions where efficient retinoid cycle operates. Efficient retinoid cycle leads to rapid regeneration of rhodopsin, which may result in ATR release from the opsin "exit site" before its enzymatic reduction to all-trans-retinol. Here we discuss photodamage to the retina where ATR could play a role as the main toxic and/or phototoxic agent. Moreover, we discuss secondary products of (photo)toxic properties accumulating within retinal lipofuscin as a result of ATR accumulation.

Why photoreceptors die (and why they don't)

Light can kill the photoreceptors of the eye, not only very bright direct sunlight, but more moderate illumination if the light is present continuously. Recent experiments show that rod apoptosis can be triggered by strong and constant activation of transduction, and that death can be prevented if transduction is inhibited even though the eye is illuminated. Vitamin A deficiency and genetically inherited diseases, such as some forms of retinitis pigmentosa and Leber congenital amaurosis, appear to kill like this: transduction is activated at a high rate and continuously, and this causes the rods to die. Why does transduction kill? Our best guess is that continuous activation produces a prolonged lowering of the Ca(2+) concentration, which is also thought to kill neurons in tissue culture and during the development of the nervous system. To prevent death in constant light, rods have evolved protective mechanisms including modulation of channels and ion transport to keep the Ca(2+) from going too low. Prolonged light exposure also causes migration of transduction proteins from one part of the cell to another and a reversible shortening of the rod outer segments, the part of the cell that contains the pigment rhodopsin. All of these mechanisms are at work in the normal eye to reduce transduction and prevent the Ca(2+) concentration from dropping too low for too long a time. That most of us retain our vision our entire lives is a testament to their effectiveness.

Bright cyclic light rearing-mediated retinal protection against damaging light exposure in adrenalectomized mice

Previous studies have shown that albino rats and mice raised in bright cyclic light are protected from light-induced retinal damage. We tested if the stress response mediated by the adrenal grand is involved in the initiation of this neuroprotective phenomenon. Balb/c mice that were adrenalectomized (Adrex) or sham operated at 28 days of age were kept under dim (5 lux) or bright (400 lux) cyclic light (12h on/off) for 2 weeks. Thereafter, their electroretinogram (ERG), outer nuclear layer (ONL) thickness and area, and plasma corticosterone levels were measured in animals with (dim+light and bright+light groups) and without (dim and bright groups) damaging light exposure (3000 lux for 24 h). In the dim+light group, a- and b-wave amplitudes and the ONL thicknesses and areas were significantly higher in the Adrex animals than the sham animals, indicating that adrenalectomy itself yielded retinal protection. In the Adrex animals, the ONL areas were significantly larger in the bright+light group than the dim+light group, indicating that bright cyclic light rearing yielded further retinal tolerance, even in the absence of the adrenal gland. In sham animals, the plasma corticosterone concentration did not change between the dim and the light groups. Accordingly, glucocorticoid secreted from the adrenal gland is not likely to be required for the mechanisms of the light-adaptation neuroprotection phenomenon in mice.

Divergent Distribution in Vascular and Avascular Mammalian Retinae Links Neuroglobin to Cellular Respiration

The visual function of the vertebrate retina relies on sufficient supply with oxygen. Neuroglobin is a respiratory protein thought to play an essential role in oxygen homeostasis of neuronal cells. For further understanding of its function, we compared the distribution of neuroglobin and mitochondria in both vascular and avascular mammalian retinae. In the vascular retinae of mouse and rat, oxygen is supplied by the outer choroidal, deep retinal, and inner capillaries. We show that in this type of retina, mitochondria are concentrated in the inner segments of photoreceptor cells, the outer and the inner plexiform layers, and the ganglion cell layer. These are the same regions in which oxygen consumption takes place and in which neuroglobin is present at high levels. In the avascular retina of guinea pig the deep retinal and inner capillaries are absent. Therefore, only the inner segments of the photoreceptors adjacent to choroidal capillaries display an oxidative metabolism. We demonstrate that in the retina of guinea pigs both neuroglobin and mitochondria are restricted to this layer. Our results clearly demonstrate an association of neuroglobin and mitochondria, thus supporting the hypothesis that neuroglobin is a respiratory protein that supplies oxygen to the respiratory chain.

Identification of mouse retinal genes differentially regulated by dim and bright cyclic light rearing

Bright cyclic light rearing protects BALB/c mice from light-induced photoreceptor apoptosis compared to dim cyclic light rearing. We used a microarray approach to search for putative neuroprotection genes that were up- or down-regulated under these environmental conditions. Retinal protection by bright cyclic rearing was determined by quantitative histology and DNA fragmentation analysis. Total RNA was isolated from 5-week-old mice raised in bright (400 lux) or dim (5 lux) cyclic light and prepared for analysis on microarrays produced using a 70-mer oligonucleotide library that represented 16,463 mouse genes. Genes of interest were identified using statistically robust bioinformatics analysis methods that were developed in-house. Changes in some genes were confirmed with quantitative real time PCR. We found that 952 genes were up- or down-regulated by bright cyclic light rearing compared to dim cyclic light rearing. One hundred and eighty-four of them, having >/=2-fold differences, were grouped into 13 categories, and selected for further consideration. Eleven up-regulated and two down-regulated genes were confirmed by semi-quantitative PCR. Five neuroprotection-associated genes were up-regulated by bright cyclic light rearing as confirmed by real-time PCR. The human orthologue chromosomal location of 22 differentially expressed genes map to known retinal degeneration loci. Using PathwayAssist software, we modeled the pathway networks of up- and down-regulated genes that are functionally related to the retina. We identified retinal genes that are differentially regulated by environmental light history. Those that directly affect cell processes such as survival, apoptosis, and transcription are likely play a pivotal role in the regulation of retinal neuroprotection against light-induced photoreceptor apoptosis.

Divergent distribution of cytoglobin and neuroglobin in the murine eye

Neuroglobin (Ngb) and cytoglobin (Cygb) are two vertebrate globins with yet poorly defined functions. Previous studies had demonstrated a high expression level of neuroglobin in the mammalian retina, being in line with a respiratory function. Here we show that in the mouse eye, cytoglobin is localised in fibroblasts of the ciliary processes and the choroidea. In the neuronal retina, cytoglobin is expressed in a subset of neurons of the ganglion cell and inner nuclear layers. Cytoglobin is also present in the inner plexiform layer, but absent from the pigment cells. Neuroglobin is localised in photoreceptor inner segments, the plexiform layers and the ganglion cell layer. The divergent distribution of neuroglobin and cytoglobin in the mammalian retina suggests distinct functions of these proteins in the vertebrate nervous system. While neuroglobin seems to be associated with oxygen consumption, a respiratory function of cytoglobin is unlikely.

Environmental light and heredity are associated with adaptive changes in retinal DHA levels that affect retinal function

Retinas of rats and mice react to environmental and genetic stimuli by altering the level of DHA in their rod outer segment membranes. We propose that this adaptation is a neuroprotective response to control the number of photons captured by rhodopsin and the efficiency of visual transduction, under conditions where excessive activation of the transduction cascade could lead to cell death.