Novel lipofuscin bisretinoids prominent in human retina and in a model of recessive Stargardt disease

Natural products for the treatment of age-related macular degeneration

BACKGROUND

Age-related macular degeneration (AMD) is a chronic retinal disease that significantly influences the vision of the elderly.

PURPOSE

There is no effective treatment and prevention method. The pathogenic process behind AMD is complex, including oxidative stress, inflammation, and neovascularization. It has been demonstrated that several natural products can be used to manage AMD, but systematic summaries are lacking.

STUDY DESIGN AND METHODS

PubMed, Web of Science, and ClinicalTrials.gov were searched using the keywords "Biological Products" AND "Macular Degeneration" for studies published within the last decade until May 2023 to summarize the latest findings on the prevention and treatment of age-related macular degeneration through the herbal medicines and functional foods.

RESULTS

The eligible studies were screened, and the relevant information about the therapeutic action and mechanism of natural products used to treat AMD was extracted. Our findings demonstrate that natural substances, including retinol, phenols, and other natural products, prevent the development of new blood vessels and protect the retina from oxidative stress in cells and animal models. However, they have barely been examined in clinical studies.

CONCLUSION

Natural products could be highly prospective candidate drugs used to treat AMD, and further preclinical and clinical research is required to validate it to control the disease.

Iron overload and chelation modulates bisretinoid levels in the retina

Aim

Iron dysregulation in conjunction with other disease processes may exacerbate retinal degeneration. We employed models of iron overload and iron chelation to explore the interactions between iron-catalyzed oxidation and photoreactive bisretinoid lipofuscin.

Methods

The mice were injected intravitreally with ferric ammonium citrate (FAC) or were treated using the iron chelator deferiprone (DFP) from birth to 2 months of age. Short-wavelength fundus autofluorescence (SW-AF) and spectral-domain optical coherence tomography (SD-OCT) scans were acquired. The bisretinoid levels were quantified using ultra performance liquid chromatography (UPLC) and in vivo through quantitative fundus autofluorescence (qAF). In histologic sections, the photoreceptor cell viability was assessed by measuring the thickness of the outer nuclear layer (ONL).

Results

The levels of bisretinoids, all-trans-retinal dimers, and A2PE were significantly increased in the FAC-injected eyes of C57BL/6J mice. Seven days after FAC injection, hyperautofluorescent foci were visible in fundus autofluorescence (488 nm) images, and in SD-OCT scans, aberrant hyperreflectivity was present in the outer retina and ONL thinning was observed. In FAC-injected Abca4-/- mice with pronounced RPE bisretinoid lipofuscin accumulation, the hyperautofluorescent puncta were more abundant than in the wild-type mice, and the extent of ONL thinning was greater. Conversely, the intravitreal injection of FAC in Mertk-/- mice led to a more modest increase in A2PE after 2 days. In contrast to the effect of iron accumulation, chelation with DFP resulted in significantly increased levels of A2E and A2-GPE and qAF due to the reduced iron-catalyzed oxidation of bisretinoids. In Mertk-/- mice, the A2E level was significantly lower and the ONL area was smaller than in DFP-treated mice. DFP chelation did not impair the visual cycle in BALB/cJ mice.

Conclusion

Iron accumulation was associated with progressive impairment in photoreceptor cells that was associated with the increased formation of a bisretinoid species known to form in photoreceptor outer segments as a precursor to A2E. Additionally, disease features such as the development of hyperautofluorescence puncta in fundus AF images, hyperreflectivity in the outer retina of SD-OCT scans, and ONL thinning were more pronounced when iron was delivered to Abca4-/- mice with a greater propensity for bisretinoid formation. Higher bisretinoid levels and enhanced qAF are indicative of lesser bisretinoid loss due to oxidation.

Retina and RPE lipid profile changes linked with ABCA4 associated Stargardt's maculopathy

Stargardt maculopathy, caused predominantly by mutations in the ABCA4 gene, is characterized by an accumulation of non-degradable visual pigment derivative, lipofuscin, in the retinal pigment epithelium (RPE) - resulting in RPE atrophy. RPE is a monolayer tissue located adjacent to retinal photoreceptors and regulates their health and functioning; RPE atrophy triggers photoreceptor cell death and vision loss in Stargardt patients. Previously, ABCA4 mutations in photoreceptors were thought to be the major contributor to lipid homeostasis defects in the eye. Recently, we demonstrated that ABCA4 loss of function in the RPE leads to cell-autonomous lipid homeostasis defects. Our work underscores that an incomplete understanding of lipid metabolism and lipid-mediated signaling in the retina and RPE are potential causes for lacking treatments for this disease. Here we report altered lipidomic in mouse and human Stargardt models. This work provides the basis for therapeutics that aim to restore lipid homeostasis in the retina and the RPE.

A High Fat Diet Fosters Elevated Bisretinoids

High dietary fat intake is associated with metabolic dysregulation, but little is known regarding the effects of a high fat diet (HFD) on photoreceptor cell functioning. We explored the intersection of a high fat diet (HFD) and the visual cycle adducts that form in photoreceptor cells by non-enzymatic reactions. In black C57BL/6J mice and albino C57BL/6J^c2j mice raised on a high fat diet until age 3, 6 or 12 months, chromatographically quantified bisretinoids were increased relative to mice on a standard diet. In vivo measurement of fundus autofluorescence, the source of which is bisretinoid, also revealed a significant increase in the HFD-mice. Additionally, mice provided with a diet high in fat presented with elevated retinol-binding protein 4 (RBP4) the protein responsible for transporting retinol in plasma. Vitamin A was elevated in plasma although not in ocular tissue. Bisretinoids form in photoreceptor cell outer segments by random reactions of retinaldehyde with phosphatidylethanolamine. We found that the latter phospholipid was significantly increased in mice fed a HFD versus mice on a control diet. In leptin-deficient ob/ob mice, a genetic model of obesity, plasma levels of Rbp4 protein were higher but bisretinoids in retina were not elevated. Photoreceptor cell viability measured as outer nuclear layer thickness was reduced in the ob/ob mice relative to wild-type. The accelerated formation of bisretinoid we observed in diet induced obese mice is related to the high fat intake and to increased delivery of vitamin A to the visual cycle.

Galanin receptor 3 - A new pharmacological target in retina degeneration

The neuropeptide galanin receptor 3 (GALR3) is a class A G protein-coupled receptor (GPCR) broadly expressed in the nervous system, including the retina. GALR3 is involved in the modulation of immune and inflammatory responses. Tight control of these processes is critical for maintaining homeostasis in the retina and is required to sustain vision. Here, we investigated the role of GALR3 in retina pathologies triggered by bright light and P23H mutation in the rhodopsin (RHO) gene, associated with the activation of oxidative stress and inflammatory responses. We used a multiphase approach involving pharmacological inhibition of GALR3 with its antagonist SNAP-37889 and genetic depletion of GALR3 to modulate the GALR3 signaling. Our in vitro experiments in the retinal pigment epithelium-derived cells (ARPE19) susceptible to all-trans-retinal toxicity indicated that GALR3 could be involved in the cellular stress response to this phototoxic product. Indeed, blocking the GALR3 signaling in Abca4-/-/Rdh8-/- and wild-type Balb/cJ mice, sensitive to bright light-induced retina damage, protected retina health in these mice exposed to light. The retina morphology and function were substantially improved, and stress response processes were reduced in these mouse models compared to the controls. Furthermore, in P23H Rho knock-in mice, a model of retinitis pigmentosa (RP), both pharmacological inhibition and genetic ablation of GALR3 prolonged the survival of photoreceptors. These results indicate that GALR3 signaling contributes to acute light-induced and chronic RP-linked retinopathies. Together, this work provides the pharmacological knowledge base to evaluate GALR3 as a potential target for developing novel therapies to combat retinal degeneration.

Long-term vitamin A supplementation in a preclinical mouse model for RhoD190N-associated retinitis pigmentosa

Retinitis pigmentosa (RP) is caused by one of many possible gene mutations. The National Institutes of Health recommends high daily doses of vitamin A palmitate for RP patients. There is a critical knowledge gap surrounding the therapeutic applicability of vitamin A to patients with the different subtypes of the disease. Here, we present a case report of a patient with RP caused by a p.D190N mutation in Rhodopsin (RHO) associated with abnormally high quantitative autofluorescence values after long-term vitamin A supplementation. We investigated the effects of vitamin A treatment strategy on RP caused by the p.D190N mutation in RHO by exposing Rhodopsin p.D190N (RhoD190N/+) and wild-type (WT) mice to experimental vitamin A-supplemented and standard control diets. The patient's case suggests that the vitamin A treatment strategy should be further studied to determine its effect on RP caused by p.D190N mutation in RHO and other mutations. Our mouse experiments revealed that RhoD190N/+ mice on the vitamin A diet exhibited higher levels of autofluorescence and lipofuscin metabolites compared to WT mice on the same diet and isogenic controls on the standard control diet. Vitamin A supplementation diminished photoreceptor function in RhoD190N/+ mice while preserving cone response in WT mice. Our findings highlight the importance of more investigations into the efficacy of clinical treatments like vitamin A for patients with certain genetic subtypes of disease and of genotyping in the precision care of inherited retinal degenerations.

Vitamin A aldehyde-taurine adducts function in photoreceptor cells

To facilitate the movement of retinoids through the visual cycle and to limit nonspecific chemical reaction, multiple mechanisms are utilized to handle these molecules when not contained within the binding pocket of opsin. Vitamin A aldehyde is sequestered by reversible Schiff base formation with phosphatidylethanolamine (PE) and subsequently undergoes NADPH-dependent reduction. Otherwise inefficient handling of retinaldehyde can lead to the formation of fluorescent di-retinal compounds within the outer segments of photoreceptor cells. These bisretinoid fluorophores initiate photooxidative processes having adverse consequences for retina. Various carrier proteins confer water solubility and maintain the 11-cis-retinoid configuration. Mechanisms for sequestration of retinoid include the formation of a reversible Schiff base between retinaldehyde and taurine (A1-taurine, A1T), the most abundant amino acid in photoreceptor cells. Here we have undertaken to examine the effects of taurine depletion using the transport inhibitors guanidinoethyl sulfonate (GES) and β-alanine. Oral treatment of BALB/cJ mice with β-alanine reduced ocular A1T and the mice exhibited significantly lower scotopic and photopic a-wave amplitudes. As a secondary effect of retinal degeneration, A1T was not detected and taurine was significantly reduced in mice carrying a P23H opsin mutation. The thinning of ONL that is indicative of reduced photoreceptor cell viability in albino Abca4^-/- mice was more pronounced in β-alanine treated mice. Treatment of agouti and albino Abca4^-/- mice with β-alanine and GES was associated with reduced bisretinoid measured chromatographically. Consistent with a reduction in carbonyl scavenging activity by taurine, methylglyoxal-adducts were also increased in the presence of β-alanine. Taken together these findings support the postulate that A1T serves as a reservoir of vitamin A aldehyde, with diminished A1T explaining reduced photoreceptor light-sensitivity, accentuated ONL thinning in Abca4^-/- mice and attenuated bisretinoid formation.

Retinal Pigment Epithelium in Human Donor Eyes Contains Higher Levels of Bisretinoids Including A2E in Periphery than Macula

Purpose

With age, human retinal pigment epithelium (RPE) accumulates bisretinoid fluorophores that may impact cellular function and contribute to age-related macular degeneration (AMD). Bisretinoids are comprised of a central pyridinium, dihydropyridinium, or cyclohexadiene ring. The pyridinium bisretinoid A2E has been extensively studied, and its quantity in the macula has been questioned. Age-changes and distributions of other bisretinoids are not well characterized. We measured levels of three bisretinoids and oxidized A2E in macula and periphery in human donor eyes of different ages.

Methods

Eyes (N = 139 donors, 61 women and 78 men, aged 40–80 years) were dissected into 8 mm diameter macular and temporal periphery punches. Using liquid chromatography – electrospray ionization – mass spectrometry (LC-ESI-MS) and an authentic synthesized standard, we quantified A2E (ng). Using LC-ESI-MS and a 50-eye-extract of A2E, we semiquantified A2E and 3 other compounds (eye extract equivalent units [EEEUs): A2-glycerophosphoethanolamine (A2GPE), dihydropyridine phosphatidyl ethanolamine (A2DHPE), and monofuranA2E (MFA2E).

Results

A2E quantities in ng and EEEUs were highly correlated (r = 0.97, P < 0.001). From 262 eyes, 5 to 9-fold higher levels were observed in the peripheral retina than in the macula for all assayed compounds. A2E, A2DHPE, and MFA2E increased with age, whereas A2GPE remained unaffected. No significant right-left or male-female differences were detected.

Conclusions

Significantly higher levels were observed in the periphery than in the macula for all assayed compounds signifying biologic differences between these regions. Levels of oxidized A2E parallel native A2E and not the distribution of retinal illuminance. Data will assist with the interpretion of clinical trial outcomes of agents targeting bisretinoid-related pathways.

Ionized Radiation-Mediated Retinoid Oxidation in the Retina and Retinal Pigment Epithelium of the Murine Eye

The present study evaluated the effects of proton and gamma-ray ionizing radiation on the mouse eye. The aim of this work was to analyze radiation-mediated retinoid oxidation in the retina and retinal pigment epithelium (RPE). The findings from this analysis can be used to develop a noninvasive method for rapid assessment of the effects of ionizing radiation. Comparative fluorescence and chromatographic analyses of retinoids before and after irradiations were performed. The fluorescent properties of chloroform extracts from irradiated mouse retina and RPE exhibited an increase in fluorescence intensity in the short-wave region of the spectrum (λ < 550 nm). This change is due to increased retinal and RPE retinoid oxidation and degradation products after radiation exposure. Comparative analyses of radiation effects demonstrated that the effect of proton exposure on the retina and RPE was higher than that of gamma-ray exposure. The present study revealed a new approach to assessing the level of radiation exposure in ocular tissues.

Bisretinoids of the Retina: Photo-Oxidation, Iron-Catalyzed Oxidation, and Disease Consequences

The retina and, in particular, retinal pigment epithelial cells are unusual for being encumbered by exposure to visible light, while being oxygen-rich, and also amassing photoreactive molecules. These fluorophores (bisretinoids) are generated as a byproduct of the activity of vitamin A aldehyde-the chromophore necessary for vision. Bisretinoids form in photoreceptor cells due to random reactions of two molecules of vitamin A aldehyde with phosphatidylethanolamine; bisretinoids are subsequently transferred to retinal pigment epithelial (RPE) cells, where they accumulate in the lysosomal compartment with age. Bisretinoids can generate reactive oxygen species by both energy and electron transfer, and they become photo-oxidized and photolyzed in the process. While these fluorescent molecules are accrued by RPE cells of all healthy eyes, they are also implicated in retinal disease.

Lutein and zeaxanthin reduce A2E and iso-A2E levels and improve visual performance in Abca4-/-/Bco2-/- double knockout mice

Accumulation of bisretinoids such as A2E and its isomer iso-A2E is thought to mediate blue light-induced oxidative damage associated with age-related macular degeneration (AMD) and autosomal recessive Stargardt disease (STGD1). We hypothesize that increasing dietary intake of the macular carotenoids lutein and zeaxanthin in individuals at risk of AMD and STGD1 can inhibit the formation of bisretinoids A2E and iso-A2E, which can potentially ameliorate macular degenerative diseases. To study the beneficial effect of macular carotenoids in a retinal degenerative diseases model, we used ATP-binding cassette, sub-family A member 4 (Abca4-/-)/β,β-carotene-9',10'-oxygenase 2 (Bco2-/-) double knockout (KO) mice that accumulate elevated levels of A2E and iso-A2E in the retinal pigment epithelium (RPE) and macular carotenoids in the retina. Abca4-/-/Bco2-/- and Abca4-/- mice were fed a lutein-supplemented chow, zeaxanthin-supplemented chow or placebo chow (~2.6 mg of carotenoid/mouse/day) for three months. Visual function and electroretinography (ERG) were measured after one month and three months of carotenoid supplementation. The lutein and zeaxanthin supplemented Abca4-/-/Bco2-/- mice had significantly lower levels of RPE/choroid A2E and iso-A2E compared to control mice fed with placebo chow and improved visual performance. Carotenoid supplementation in Abca4-/- mice minimally raised retinal carotenoid levels and did not show much difference in bisretinoid levels or visual function compared to the control diet group. There was a statistically significant inverse correlation between carotenoid levels in the retina and A2E and iso-A2E levels in the RPE/choroid. Supplementation with retinal carotenoids, especially zeaxanthin, effectively inhibits bisretinoid formation in a mouse model of STGD1 genetically enhanced to accumulate carotenoids in the retina. These results provide further impetus to pursue oral carotenoids as therapeutic interventions for STGD1 and AMD.

Bisretinoid phospholipid and vitamin A aldehyde: shining a light

Vitamin A aldehyde covalently bound to opsin protein is embedded in a phospholipid-rich membrane that supports photon absorption and phototransduction in photoreceptor cell outer segments. Following absorption of a photon, the 11-cis-retinal chromophore of visual pigment in photoreceptor cells isomerizes to all-trans-retinal. To maintain photosensitivity 11-cis-retinal must be replaced. At the same time, however, all-trans-retinal has to be handled so as to prevent nonspecific aldehyde activity. Some molecules of retinaldehyde upon release from opsin are efficiently reduced to retinol. Other molecules are released into the lipid phase of the disc membrane where they form a conjugate [N-retinylidene-PE (NRPE)] through a Schiff base linkage with PE. The reversible formation of NRPE serves as a transient sink for retinaldehyde that is intended to return retinaldehyde to the visual cycle. However, if instead of hydrolyzing to PE and retinaldehyde, NRPE reacts with a second molecule of retinaldehyde, a synthetic pathway is initiated that leads to the formation of multiple species of unwanted bisretinoid fluorophores. We report on recently identified members of the bisretinoid family, some of which differ with respect to the acyl chains associated with the glycerol backbone. We discuss processing of the lipid moieties of these fluorophores in lysosomes of retinal pigment epithelial cells, their fluorescence characters, and new findings related to light- and iron-associated oxidation of bisretinoids.

A vicious cycle of bisretinoid formation and oxidation relevant to recessive Stargardt disease

The ability of iron to transfer electrons enables the contribution of this metal to a variety of cellular activities even as the redox properties of iron are also responsible for the generation of hydroxyl radicals (^•OH), the most destructive of the reactive oxygen species. We previously showed that iron can promote the oxidation of bisretinoid by generating highly reactive hydroxyl radical (^•OH). Now we report that preservation of iron regulation in the retina is not sufficient to prevent iron-induced bisretinoid oxidative degradation when blood iron levels are elevated in liver-specific hepcidin knockout mice. We obtained evidence for the perpetuation of Fenton reactions in the presence of the bisretinoid A2E and visible light. On the other hand, iron chelation by deferiprone was not associated with changes in postbleaching recovery of 11-cis-retinal or dark-adapted ERG b-wave amplitudes indicating that the activity of Rpe65, a rate-determining visual cycle protein that carries an iron-binding domain, is not affected. Notably, iron levels were elevated in the neural retina and retinal pigment epithelial (RPE) cells of Abca4^−/− mice. Consistent with higher iron content, ferritin-L immunostaining was elevated in RPE of a patient diagnosed with ABCA4-associated disease and in RPE and photoreceptor cells of Abca4^−/− mice. In neural retina of the mutant mice, reduced Tfrc mRNA was also an indicator of retinal iron overload. Thus iron chelation may defend retina when bisretinoid toxicity is implicated in disease processes.

Retinoid analogs and polyphenols as potential therapeutics for age-related macular degeneration

IMPACT STATEMENT

Age-related macular degeneration (AMD) is a devastating retinal degenerative disease. Epidemiological reports showed an expected increasing prevalence of AMD in the near future. The only one existing FDA-approved pharmacological treatment involves an anti-vascular endothelial growth factor (VEGF) therapy with serious disadvantages. This limitation emphasizes an alarming need to develop new therapeutic approaches to prevent and treat AMD. In this review, we summarize scientific data unraveling the therapeutic potential of the specific retinoid and natural compounds. The experimental results reported by us and other research groups demonstrated that retinoid analogs and compounds with natural product scaffolds could serve as lead compounds for the development of new therapeutic agents with potential to prevent or slow down the pathogenesis of AMD.

Fundus autofluorescence imaging

Fundus autofluorescence (FAF) imaging is an in vivo imaging method that allows for topographic mapping of naturally or pathologically occurring intrinsic fluorophores of the ocular fundus. The dominant sources are fluorophores accumulating as lipofuscin in lysosomal storage bodies in postmitotic retinal pigment epithelium cells as well as other fluorophores that may occur with disease in the outer retina and subretinal space. Photopigments of the photoreceptor outer segments as well as macular pigment and melanin at the fovea and parafovea may act as filters of the excitation light. FAF imaging has been shown to be useful with regard to understanding of pathophysiological mechanisms, diagnostics, phenotype-genotype correlation, identification of prognostic markers for disease progression, and novel outcome parameters to assess efficacy of interventional strategies in chorio-retinal diseases. More recently, the spectrum of FAF imaging has been expanded with increasing use of green in addition to blue FAF, introduction of spectrally-resolved FAF, near-infrared FAF, quantitative FAF imaging and fluorescence life time imaging (FLIO). This article gives an overview of basic principles, FAF findings in various retinal diseases and an update on recent developments.

Fluorescence characteristics of lipofuscin fluorophores from human retinal pigment epithelium

Lipofuscin granules accumulate in the retinal pigment epithelium (RPE) with age, especially in patients with visual diseases, including progressive age-related macular degeneration (AMD). Bisretinoids and their photooxidation and photodegradation products are major sources of lipofuscin granule fluorescence. The present study focused on examining the fluorescence decay characteristics of bisretinoid photooxidation and photodegradation products to evaluate the connection between fluorescence lifetime and spectral characteristics of target fluorophore groups. The primary objective of the study was to apply experimental spectral analysis results of lipofuscin granule fluorescence properties to interpretation of fluorescence lifetime imaging ophthalmoscopy data. Fluorescence analysis of the lipofuscin granule fluorophores in RPE collected from cadaver eyes was performed. The fluorescence lifetimes were measured by picosecond-resolved time correlated single photon counting technique. A global analytical method was applied to analyze data sets. The photooxidation and photodegradation products of bisretinoids exhibited a longer fluorescence lifetime (average value approximately 6 ns) and a shorter wavelength maximum (530-580 nm). Further, these products significantly contributed (more than 30%), to total fluorescence compared to the other fluorophores in lipofuscin granules. Thus, the contribution of oxidized lipofuscin bisretinoids to autofluorescence decay kinetics is an important characteristic for fluorescence lifetime imaging microscopy data analysis. The higher average fluorescence lifetime in AMD eyes was likely due to the higher abundance of oxidized bisretinoids compared with non-oxidized bisretinoids. Because higher level of oxidized bisretinoids is indicative of pathological processes in the retina and RPE, the present findings have the potential to improve fluorescence lifetime imaging approaches for early diagnosis of degenerative processes in the retina and RPE.

Clinical spectrum, genetic complexity and therapeutic approaches for retinal disease caused by ABCA4 mutations

The ABCA4 protein (then called a “rim protein”) was first identified in 1978 in the rims and incisures of rod photoreceptors. The corresponding gene, ABCA4, was cloned in 1997, and variants were identified as the cause of autosomal recessive Stargardt disease (STGD1). Over the next two decades, variation in ABCA4 has been attributed to phenotypes other than the classically defined STGD1 or fundus flavimaculatus, ranging from early onset and fast progressing cone-rod dystrophy and retinitis pigmentosa-like phenotypes to very late onset cases of mostly mild disease sometimes resembling, and confused with, age-related macular degeneration. Similarly, analysis of the ABCA4 locus uncovered a trove of genetic information, including >1200 disease-causing mutations of varying severity, and of all types – missense, nonsense, small deletions/insertions, and splicing affecting variants, of which many are located deep-intronic. Altogether, this has greatly expanded our understanding of complexity not only of the diseases caused by ABCA4 mutations, but of all Mendelian diseases in general. This review provides an in depth assessment of the cumulative knowledge of ABCA4-associated retinopathy – clinical manifestations, genetic complexity, pathophysiology as well as current and proposed therapeutic approaches.

Suppressive Effect of Arctium Lappa L. Leaves on Retinal Damage Against A2E-Induced ARPE-19 Cells and Mice

Age-related macular degeneration (AMD) is a major cause of irreversible loss of vision with 80–90% of patients demonstrating dry type AMD. Dry AMD could possibly be prevented by polyphenol-rich medicinal foods by the inhibition of N-retinylidene-N-retinylethanolamine (A2E)-induced oxidative stress and cell damage. Arctium lappa L. (AL) leaves are medicinal and have antioxidant activity. The purpose of this study was to elucidate the protective effects of the extract of AL leaves (ALE) on dry AMD models, including in vitro A2E-induced damage in ARPE-19 cells, a human retinal pigment epithelial cell line, and in vivo light-induced retinal damage in BALB/c mice. According to the total phenolic contents (TPCs), total flavonoid contents (TFCs) and antioxidant activities, ALE was rich in polyphenols and had antioxidant efficacies on 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), ferric reducing antioxidant power (FRAP), and 2′,7′-dichlorofluorescin diacetate (DCFDA) assays. The effects of ALE on A2E accumulation and A2E-induced cell death were also monitored. Despite continued exposure to A2E (10 μM), ALE attenuated A2E accumulation in APRE-19 cells with levels similar to lutein. A2E-induced cell death at high concentration (25 μM) was also suppressed by ALE by inhibiting the apoptotic signaling pathway. Furthermore, ALE could protect the outer nuclear layer (ONL) in the retina from light-induced AMD in BALB/c mice. In conclusion, ALE could be considered a potentially valuable medicinal food for dry AMD.

Effects of deficiency in the RLBP1-encoded visual cycle protein CRALBP on visual dysfunction in humans and mice

Mutations in retinaldehyde-binding protein 1 (RLBP1), encoding the visual cycle protein cellular retinaldehyde-binding protein (CRALBP), cause an autosomal recessive form of retinal degeneration. By binding to 11-cis-retinoid, CRALBP augments the isomerase activity of retinoid isomerohydrolase RPE65 (RPE65) and facilitates 11-cis-retinol oxidation to 11-cis-retinal. CRALBP also maintains the 11-cis configuration and protects against unwanted retinaldehyde activity. Studying a sibling pair that is compound heterozygous for mutations in RLBP1/CRALBP, here we expand the phenotype of affected individuals, elucidate a previously unreported phenotype in RLBP1/CRALBP carriers, and demonstrate consistencies between the affected individuals and Rlbp1/Cralbp^−/− mice. In the RLBP1/CRALBP-affected individuals, nonrecordable rod-specific electroretinogram traces were recovered after prolonged dark adaptation. In ultrawide-field fundus images, we observed radially arranged puncta typical of RLBP1/CRALBP-associated disease. Spectral domain-optical coherence tomography (SD-OCT) revealed hyperreflective aberrations within photoreceptor-associated bands. In short-wavelength fundus autofluorescence (SW-AF) images, speckled hyperautofluorescence and mottling indicated macular involvement. In both the affected individuals and their asymptomatic carrier parents, reduced SW-AF intensities, measured as quantitative fundus autofluorescence (qAF), indicated chronic impairment in 11-cis-retinal availability and provided information on mutation severity. Hypertransmission of the SD-OCT signal into the choroid together with decreased near-infrared autofluorescence (NIR-AF) provided evidence for retinal pigment epithelial cell (RPE) involvement. In Rlbp1/Cralbp^−/− mice, reduced 11-cis-retinal levels, qAF and NIR-AF intensities, and photoreceptor loss were consistent with the clinical presentation of the affected siblings. These findings indicate that RLBP1 mutations are associated with progressive disease involving RPE atrophy and photoreceptor cell degeneration. In asymptomatic carriers, qAF disclosed previously undetected visual cycle deficiency.

A Novel Mouse Model of MYO7A USH1B Reveals Auditory and Visual System Haploinsufficiencies

Usher’s syndrome is the most common combined blindness–deafness disorder with USH1B, caused by mutations in MYO7A, resulting in the most severe phenotype. The existence of numerous, naturally occurring shaker1 mice harboring variable MYO7A mutations on different genetic backgrounds has complicated the characterization of MYO7A knockout (KO) and heterozygote mice. We generated a novel MYO7A KO mouse (Myo7a^–/–) that is easily genotyped, maintained, and confirmed to be null for MYO7A in both the eye and inner ear. Like USH1B patients, Myo7a^–/– mice are profoundly deaf, and display near complete loss of inner and outer cochlear hair cells (HCs). No gross structural changes were observed in vestibular HCs. Myo7a^–/– mice exhibited modest declines in retinal function but, unlike patients, no loss of retinal structure. We attribute the latter to differential expression of MYO7A in mouse vs. primate retina. Interestingly, heterozygous Myo7a^+/– mice had reduced numbers of cochlear HCs and concomitant reductions in auditory function relative to Myo7a^+/+ controls. Notably, this is the first report that loss of a single Myo7a allele significantly alters auditory structure and function and suggests that audiological characterization of USH1B carriers is warranted. Maintenance of vestibular HCs in Myo7a^–/– mice suggests that gene replacement could be used to correct the vestibular dysfunction in USH1B patients. While Myo7a^–/– mice do not exhibit sufficiently robust retinal phenotypes to be used as a therapeutic outcome measure, they can be used to assess expression of vectored MYO7A on a null background and generate valuable pre-clinical data toward the treatment of USH1B.

Lessons learned from quantitative fundus autofluorescence

Quantitative fundus autofluorescence (qAF) is an approach that is built on a confocal scanning laser platform and used to measure the intensity of the inherent autofluorescence of retina elicited by short-wavelength (488 nm) excitation. Being non-invasive, qAF does not interrupt tissue architecture, thus allowing for structural correlations. The spectral features, cellular origin and topographic distribution of the natural autofluorescence of the fundus indicate that it is emitted from retinaldehyde-adducts that form in photoreceptor cells and accumulate, under most conditions, in retinal pigment epithelial cells. The distributions and intensities of fundus autofluorescence deviate from normal in many retinal disorders and it is widely recognized that these changing patterns can aid in the diagnosis and monitoring of retinal disease. The standardized protocol employed by qAF involves the normalization of fundus grey levels to a fluorescent reference installed in the imaging instrument. Together with corrections for magnification and anterior media absorption, this approach facilitates comparisons with serial images and images acquired within groups of patients. Here we provide a comprehensive summary of the principles and practice of qAF and we highlight recent efforts to elucidate retinal disease processes by combining qAF with multi-modal imaging.

The Y227N mutation affects bestrophin-1 protein stability and impairs sperm function in a mouse model of Best vitelliform macular dystrophy

Human bestrophin-1 (BEST1) is an integral membrane protein known to function as a Ca²⁺-activated and volume-regulated chloride channel. The majority of disease-associated mutations in BEST1 constitute missense mutations and were shown in vitro to lead to a reduction in mutant protein half-life causing Best disease (BD), a rare autosomal dominant macular dystrophy. To further delineate BEST1-associated pathology in vivo and to provide an animal model useful to explore experimental treatment efficacies, we have generated a knock-in mouse line (Best1^Y227N). Heterozygous and homozygous mutants revealed no significant ocular abnormalities up to 2 years of age. In contrast, knock-in animals demonstrated a severe phenotype in the male reproductive tract. In heterozygous Best1^Y227N males, Best1 protein was significantly reduced in testis and almost absent in homozygous mutant mice, although mRNA transcription of wild-type and knock-in allele is present and similar in quantity. Degradation of mutant Best1 protein in testis was associated with adverse effects on sperm motility and the capability to fertilize eggs. Based on these results, we conclude that mice carrying the Best1 Y227N mutation reveal a reproducible pathologic phenotype and thus provide a valuable in vivo tool to evaluate efficacy of drug therapies aimed at restoring Best1 protein stability and function.

Investigation of Blood Characteristics in Nonsyndromic Retinitis Pigmentosa: A Retrospective Study

Purpose

To investigate the characteristics of blood in nonsyndromic retinitis pigmentosa (RP) and reveal the pathogenesis of blood cells involved in blood stasis in RP.

Design

This is a retrospective observational study.

Methods

We collected vein blood from 101 cases of patients with nonsyndromic RP and 120 cases of normal individuals according to a single-blind study and used routine clinical examination to detect the indicators of blood. All the subjects were mainly from the central south of China. Data were analyzed statistically between the RP group and normal control.

Results

The indicator of platelet distribution width (PDW) in patients with RP was higher than that in the normal group; the indicators of red blood cell (RBCs), hemoglobin (HGB), hematocrit (HCT), basophils (BASs), platelets (PLTs), and plateletcrit (PCT) in the RP group were lower than those in the normal control. The differences were statistically very significant between the RP group and normal group (p < 0.01). There were no statistical differences in the other indicators between the RP and normal group.

Conclusions

The changes in RBCs and PLTs in patients with RP implied that RP induces RBC aggregation and platelet activation, leading to blood stasis which in turn initiates more apoptosis.

Blue light versus green light fundus autofluorescence in normal subjects and in patients with retinochoroidopathy secondary to retinal and uveitic diseases

PURPOSE

The aim of this study is to evaluate the differences in the fundus autofluorescence (FAF) signal between the blue light autofluorescence (BAF) from Spectralis® (Heidelberg, CA) and green light autofluorescence (GAF) 200TxTM (OPTOS, UK, in normal subjects and in patients with retinochoroidopathies (RC).

METHODS

In this prospective study, FAF was performed using BL (λ = 488 nm) and GL (λ = 532 nm) on normal subjects and patients with RC. The corresponding pairs of BAF and GAF images from both groups were analyzed using Photoshop. The strength of the FAF signal was measured on a gray scale, where optic disc was a standard to indicate absence of AF. In addition, gray values obtained from three identical points (foveal center, and points of hypo and hyper autofluorescence) in the corresponding BAF and GAF images of normal and RC subjects were divided by the optic disc value to calculate autofluorescence signal ratio (R). The R values at fovea (R1), hypoautofluorescent point (R2), and hyperautofluorescent point (R3) were compared between BAF and GAF modalities, in normal and in RC subjects separately.

RESULTS

One hundred six pairs (106 eyes) of FAF images analyzed (37 pairs: normal and 69 pairs: RC subjects). In normal subjects, the mean R1, R2, and R3 values for BAF were (1.5 ± 0.88, 1.23 ± 0.58, and 4.73 ± 2.85, respectively) and for GAF were (0.78 ± 0.20, 0.78 ± 0.20, and 1.62 ± 0.39, respectively). Similarly, in subjects with RC, the mean R1, R2, and R3 values for BAF were (1.68 ± 1.02, 1.66 ± 1.15, and 7.75 ± 6.82, respectively) and for GAF were (0.95 ± 0.59, 0.79 ± 0.45, and 2.50 ± 1.65, respectively). The mean difference in the R1, R2, and R3 ratios between BAF and GAF in normal and in RC subjects was statistically significant (p < 0.001). The strength of the correlation (r) between ratios for BAF and GAF was weak or not statistically significant in both normal and RC subjects (p > 0.05).

CONCLUSION

The distribution and intensity of the AF signal differ in BAF and GAF and cannot be used interchangeably. In BAF, optic disc signal is always weaker than in other areas, which was not true for GAF where optic disc signal was stronger than fovea and hypoautofluorescent point in both groups.

Bisretinoids mediate light sensitivity resulting in photoreceptor cell degeneration in mice lacking the receptor tyrosine kinase Mer

The receptor tyrosine kinase Mer is expressed by retinal pigment epithelial (RPE) cells and participates in photoreceptor outer-segment phagocytosis, a process enabling membrane renewal. Mutations in the gene encoding MERTK cause blinding retinitis pigmentosa in humans. Targeted Mertk disruption in mice causes defective RPE-mediated phagocytosis of the outer segments, leading to deposition of autofluorescent debris at the RPE-photoreceptor cell interface, followed by photoreceptor cell degeneration. Here, we show that retinaldehyde adducts (bisretinoid fluorophores) that form in photoreceptor outer segments occupy the unphagocytosed outer-segment debris that accumulates in Mertk ^-/- mice. Bisretinoids measured by HPLC were elevated in Mertk ^-/- mice compared with WT animals. Bisretinoids were also more abundant in albino Mertk ^-/- mice expressing leucine at position 450 of the isomerase RPE65 (Rpe65-Leu450) rather than the variant methionine (Rpe65-450Met) that yields lower bisretinoid levels. In Royal College of Surgeons rats having dysfunctional Mertk, bisretinoids were higher than in WT rats. Intensities of in vivo fundus autofluorescence were higher in Mertk ^-/- mice than in WT mice and peaked earlier in albino Mertk ^-/-/Rpe65-Leu450 mice than in albino Mertk ^-/-/Rpe65-450Met mice. Of note, the rate of photoreceptor cell degeneration was more rapid in albino Mertk ^-/- mice exposed to higher levels of intraocular light (albino versus pigmented mice) and in mice carrying Rpe65-Leu450 than in Rpe65-450Met mice, revealing a link between bisretinoid accumulation and light-mediated acceleration of photoreceptor cell degeneration. In conclusion, the light sensitivity of photoreceptor cell degeneration arising from Mertk deficiency is consistent with the known phototoxicity of bisretinoids.

Conversion of all-trans-retinal into all-trans-retinal dimer reflects an alternative metabolic/antidotal pathway of all-trans-retinal in the retina

Free all-trans-retinal (atRAL) and retinal pigment epithelium (RPE) lipofuscin are both considered to play etiological roles in Stargardt disease and age-related macular degeneration. A2E and all-trans-retinal dimer (atRAL-dimer) are two well characterized bisretinoid constituents of RPE lipofuscin. In this study, we found that, after treatment of primary porcine RPE (pRPE) cells with atRAL, atRAL-dimer readily formed and accumulated in a concentration- and time-dependent manner, but A2E was barely detected. Cell-based assays revealed that atRAL, the precursor of atRAL-dimer, significantly altered the morphology of primary pRPE cells and decreased cell viability at a concentration of 80 μm regardless of light exposure. By contrast, atRAL-dimer was not cytotoxic and phototoxic to primary pRPE cells. Compared with atRAL and A2E, atRAL-dimer was more vulnerable to light, followed by the generation of its photocleaved products. Moreover, we observed the presence of atRAL-dimer in reaction mixtures of atRAL with porcine rod outer segments (ROS), RPE/choroid, or neural retina. Taken together, we here proposed an alternative metabolic/antidotal pathway of atRAL in the retina: atRAL that evades participation of the visual (retinoid) cycle undergoes a condensation reaction to yield atRAL-dimer in both ROS and RPE. Translocation of atRAL, all-trans N-retinylidene-phosphatidylethanolamine (NR-PE), atRAL-dimer, and photocleavage products of atRAL-dimer from ROS into RPE is accomplished by phagocytosing shed ROS on a daily basis. Without causing damage to RPE cells, light breaks up total atRAL-dimer within RPE cells to release low-molecular-weight photocleavage fragments. The latter, together with ROS-atRAL-dimer photocleavage products, may easily move across membranes and thereby be metabolically eliminated.

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.

Novel bisretinoids of human retina are lyso alkyl ether glycerophosphoethanolamine-bearing A2PE species

Bisretinoids are a family of fluorophores that form in photoreceptor cells' outer segments by nonenzymatic reaction of two vitamin A aldehydes (A2) with phosphatidylethanolamine (PE). Bisretinoid fluorophores are the major constituents of the lipofuscin of retinal pigment epithelium (RPE) that accumulate with age and contribute to some retinal diseases. Here, we report the identification of a previously unknown fluorescent bisretinoid. By ultra-performance LC (UPLC) coupled to photodiode array detection, fluorescence (FLR), and ESI-MS, we determined that this novel bisretinoid is 1-octadecyl-2-lyso-sn-glycero A2PE (alkyl ether lysoA2PE). This structural assignment was based on molecular mass (m/z 998), UV-visible absorbance maxima (340 and 440 nm), and retention time (73 min) and was corroborated by biomimetic synthesis using all-trans-retinal and glycerophosphoethanolamine analogs as starting materials. UPLC profiles of ocular extracts acquired from human donor eyes revealed that alkyl ether lysoA2PE was detectable in RPE, but not neural retina. LysoA2PE FLR spectra exhibited a significant hyperchromic shift in hydrophobic environments. The propensity for lysoA2PE to undergo photooxidation/degradation was less pronounced than A2E. In mechanistic studies, A2PE was hydrolyzed by phospholipase A₂ and plasmalogen lysoA2PE was cleaved under acidic conditions. The characterization of these additional members of the bisretinoid family advances our understanding of the mechanisms underlying bisretinoid biogenesis.

Spectral analysis of fundus autofluorescence pattern as a tool to detect early stages of degeneration in the retina and retinal pigment epithelium

PURPOSE

The aim of this work is the determination of quantitative diagnostic criteria based on the spectral characteristics of fundus autofluorescence to detect early stages of degeneration in the retina and retinal pigment epithelium (RPE).

METHODS

RPE cell suspension samples were obtained from the cadaver eyes with and without signs of age-related macular degeneration (AMD). Fluorescence analysis at an excitation wavelength of 488 nm was performed. The fluorescence lifetimes of lipofuscin-granule fluorophores were measured by counting time-correlated photon method.

RESULTS

Comparative analysis of fluorescence spectra of RPE cell suspensions from the cadaver eyes with and without signs of AMD showed a significant difference in fluorescence intensity at 530-580 nm in response to fluorescence excitation at 488 nm. It was notably higher in eyes with visual pathology than in normal eyes regardless of the age of the eye donor. Measurements of fluorescence lifetimes of lipofuscin fluorophores showed that the contribution of photooxidation and photodegradation products of bisretinoids to the total fluorescence at 530-580 nm of RPE cell suspensions was greater in eyes with visual pathology than in normal eyes.

CONCLUSION

Because photooxidation and photodegradation products of bisretinoids are markers of photodestructive processes, which can cause RPE cell death and initiate degenerative processes in the retina, quantitative determination of increases in these bisretinoid products in lipofuscin granules may be used to establish quantitative diagnostic criteria for degenerative processes in the retina and RPE.

Photoreceptor cells as a source of fundus autofluorescence in recessive Stargardt disease

Bisretinoid fluorophores form in photoreceptor outer segments from nonenzymatic reactions of vitamin A aldehyde. The short-wavelength autofluorescence (SW-AF) of fundus flecks in recessive Stargardt disease (STGD1) suggests a connection to these fluorophores. Through multimodal imaging, we sought to elucidate this link. Flecks observed in SW-AF images often colocalized with foci exhibiting reduced or absent near-infrared autofluorescence signal, the source of which is melanin in retinal pigment epithelial (RPE) cells. With serial imaging, changes in near-infrared autofluorescence (NIR-AF) preceded the onset of fleck hyperautofluorescence in SW-AF images and fleck profiles in NIR-AF images tended to be larger. Flecks in SW-AF and NIR-AF images also corresponded to hyperreflective lesions traversing photoreceptor-attributable bands in horizontal SD-OCT scans. The hyperreflective lesions interrupted adjacent OCT reflectivity bands and were associated with thinning of the outer nuclear layer. These SD-OCT findings are attributable to photoreceptor cell degeneration. Progressive increases and decreases in the SW-AF intensity of flecks were evident in color-coded quantitative fundus autofluorescence maps. In some cases, flecks appeared to spread radially from the fovea to approximately 8° of eccentricity, beyond which a circumferential spread characterized the distribution. Since the NIR-AF signal is derived from melanin and loss of this autofluorescence is indicative of RPE atrophy, the SW-AF of flecks cannot be accounted for by bisretinoid lipofuscin in RPE. Instead, we suggest that the bisretinoid serving as the source of the SW-AF signal, resides in photoreceptors, the cell that is also the site of bisretinoid synthesis.

A Brief Review on the Pathological Role of Decreased Blood Flow Affected in Retinitis Pigmentosa

Retinitis pigmentosa (RP) represents a clinically and genetically heterogeneous disease characterized by progressive photoreceptor loss. In recent years, research has been rarely made in blood flow affected in RP. The specific mechanism of blood flow affected in RP is not completely clear. A number of studies indicated that the decreased blood flow was related to RP. According to clinical observation and treatment experience, Chinese medicine considered that blood stasis runs throughout the RP disease progression, and the blood stasis corresponding to Chinese herbal medicine has a positive effect on the clinical treatment of RP. Therefore, we proposed that the decreased blood flow may participate in the lesion. In this article, we will review the findings on the decreased blood flow affected in RP from the perspective of modern medicine and Chinese medicine.

Microwave assisted synthesis for A2E and development of LC-ESI-MS method for quantification of ocular bisretinoids in human retina

PURPOSE

To develop a microwave assisted method for the rapid synthesis of A2E and also to develop a method to quantify N-retinylidene-N-retinylethanolamine(A2E), all-trans retinal dimer (ATRD), A2-glycerophospho ethanolamine (A2GPE), dihydropyridine phosphatidyl ethanolamine (A2DHPE) and monofuran A2E (MFA2E) in age matched retina.

METHODS

The development of microwave assisted synthesis of A2E, its purification and characterization for its utility in quantification in human retina. The semi-quantitative method development using LC-ESI-MS, LC-ESI-MS/MS and LC-APCI-MS/MS from pooled macula and peripheral retina for the bisretinoid analysis has been done.

RESULTS

Maximum A2E conversion using microwave assisted process took place at 80°C for 45min with a yield of 55.01%. Highly sensitive and specific mass spectrometric method was developed using reverse phase C-18 separation with positive electrospray ionization and positive atmospheric phase chemical ionization of tandom mass spectrometry. A gradient mobile phase separation was achieved using water and methanol with 0.1% TFA. Multiple reaction monitoring acquisition for ESI and APCI was performed at ATRD m/z 551.2/522.2, A2GPE m/z 746.4/729.5, A2DHPEm/z 594.4/576.5, MFA2E m/z 608.2/591.2, A2E m/z 592.4/418.2. Method was validated using LC-ESI-SIM mode to determine selectivity, linearity, sensitivity, precision and accuracy.

CONCLUSION

An attempt towards optimization of the synthetic procedure of A2E was made so as to reduce the lengthy reaction time without compromising the yield. Developed method was capable enough for the detection of low level of bisretinids in retina.

Towards Treatment of Stargardt Disease: Workshop Organized and Sponsored by the Foundation Fighting Blindness

Accumulation of fluorescent metabolic byproducts of the visual (retinoid) cycle is associated with photoreceptor and retinal pigment epithelial cell death in both Stargardt disease and atrophic (nonneovascular) age-related macular degeneration (AMD). As a consequence of this observation, small molecular inhibitors of enzymes in the visual cycle were recently tested in clinical trials as a strategy to protect the retina and retinal pigment epithelium in patients with atrophic AMD.

To address the clinical translational needs for therapies aimed at both diseases, a workshop organized by the Foundation Fighting Blindness was hosted by the Department of Pharmacology at Case Western Reserve University on February 17, 2017, at the Tinkham Veale University Center, Cleveland, OH, USA. Invited speakers highlighted recent advances in the understanding of the pathophysiology of Stargardt disease, in terms of its clinical characterization and the development of endpoints for clinical trials, and discussed the comparability of therapeutic strategies between atrophic age-related macular degeneration (AMD) and Stargardt disease. Investigators speculated that reducing the concentrations of visual cycle precursor substances and/or their byproducts may provide valid therapeutic options for the treatment of Stargardt disease. Here we review the workshop's presentations in the context of published literature to help shape the aims of ongoing research endeavors and aid the development of therapies for Stargardt disease.

Complement modulation in the retinal pigment epithelium rescues photoreceptor degeneration in a mouse model of Stargardt disease

Recessive Stargardt macular degeneration (STGD1) is caused by mutations in the gene for the ABCA4 transporter in photoreceptor outer segments. STGD1 patients and Abca4^-/- (STGD1) mice exhibit buildup of bisretinoid-containing lipofuscin pigments in the retinal pigment epithelium (RPE), increased oxidative stress, augmented complement activation and slow degeneration of photoreceptors. A reduction in complement negative regulatory proteins (CRPs), possibly owing to bisretinoid accumulation, may be responsible for the increased complement activation seen on the RPE of STGD1 mice. CRPs prevent attack on host cells by the complement system, and complement receptor 1-like protein y (CRRY) is an important CRP in mice. Here we attempted to rescue the phenotype in STGD1 mice by increasing expression of CRRY in the RPE using a gene therapy approach. We injected recombinant adeno-associated virus containing the CRRY coding sequence (AAV-CRRY) into the subretinal space of 4-wk-old Abca4^-/- mice. This resulted in sustained, several-fold increased expression of CRRY in the RPE, which significantly reduced the complement factors C3/C3b in the RPE. Unexpectedly, AAV-CRRY-treated STGD1 mice also showed reduced accumulation of bisretinoids compared with sham-injected STGD1 control mice. Furthermore, we observed slower photoreceptor degeneration and increased visual chromophore in 1-y-old AAV-CRRY-treated STGD1 mice. Rescue of the STGD1 phenotype by AAV-CRRY gene therapy suggests that complement attack on the RPE is an important etiologic factor in STGD1. Modulation of the complement system by locally increasing CRP expression using targeted gene therapy represents a potential treatment strategy for STGD1 and other retinopathies associated with complement dysregulation.

HYPERSPECTRAL AUTOFLUORESCENCE IMAGING OF DRUSEN AND RETINAL PIGMENT EPITHELIUM IN DONOR EYES WITH AGE-RELATED MACULAR DEGENERATION

PURPOSE

To elucidate the molecular pathogenesis of age-related macular degeneration (AMD) and interpretation of fundus autofluorescence imaging, the authors identified spectral autofluorescence characteristics of drusen and retinal pigment epithelium (RPE) in donor eyes with AMD.

METHODS

Macular RPE/Bruch membrane flat mounts were prepared from 5 donor eyes with AMD. In 12 locations (1-3 per eye), hyperspectral autofluorescence images in 10-nm-wavelength steps were acquired at 2 excitation wavelengths (λex 436, 480 nm). A nonnegative tensor factorization algorithm was used to recover 5 abundant emission spectra and their corresponding spatial localizations.

RESULTS

At λex 436 nm, the authors consistently localized a novel spectrum (SDr) with a peak emission near 510 nm in drusen and sub-RPE deposits. Abundant emission spectra seen previously (S0 in Bruch membrane and S1, S2, and S3 in RPE lipofuscin/melanolipofuscin, respectively) also appeared in AMD eyes, with the same shapes and peak wavelengths as in normal tissue. Lipofuscin/melanolipofuscin spectra localizations in AMD eyes varied widely in their overlap with drusen, ranging from none to complete.

CONCLUSION

An emission spectrum peaking at ∼510 nm (λex 436 nm) appears to be sensitive and specific for drusen and sub-RPE deposits. One or more abundant spectra from RPE organelles exhibit characteristic relationships with drusen.

All-trans retinal levels and formation of lipofuscin precursors after bleaching in rod photoreceptors from wild type and Abca4-/- mice

The accumulation of lipofuscin in the cells of the retinal pigment epithelium (RPE) is thought to play an important role in the development and progression of degenerative diseases of the retina. The bulk of RPE lipofuscin originates in reactions of the rhodopsin chromophore, retinal, with components of the photoreceptor outer segment. The 11-cis retinal isomer is generated in the RPE and supplied to rod photoreceptor outer segments where it is incorporated as the chromophore of rhodopsin. It is photoisomerized during light detection to all-trans and subsequently released by photoactivated rhodopsin as all-trans retinal, which is removed through reduction to all-trans retinol in a reaction requiring metabolic input in the form of NADPH. Both 11-cis and all-trans retinal can form lipofuscin precursor fluorophores in rod photoreceptor outer segments. Increased accumulation of lipofuscin has been suggested to result from excess formation of lipofuscin precursors due to buildup of all-trans retinal released by light exposure. In connection with this suggestion, the Abca4 transporter protein, an outer segment protein defects in which result in recessive Stargardt disease, has been proposed to promote the removal of all-trans retinal by facilitating its availability for reduction. To examine this possibility, we have measured the outer segment levels of all-trans retinal, all-trans retinol, and of lipofuscin precursors after bleaching by imaging the fluorescence of single rod photoreceptors isolated from wild type and Abca4^-/- mice. We found that all-trans retinol and all-trans retinal levels increased after bleaching in both wild type and Abca4^-/- rods. At all times after bleaching, there was no significant difference in all-trans retinal levels between the two strains. All-trans retinol levels were not significantly different between the two strains at early times, but were lower in Abca4^-/- rods at times longer than 20 min after bleaching. Bleaching in the presence of lower metabolic substrate concentrations resulted in higher all-trans retinal levels and increased formation of lipofuscin precursors in both wild type and Abca4^-/- rods. The results show that conditions that result in buildup of all-trans retinal levels result in increased generation of lipofuscin precursors in both wild type and Abca4^-/- rods. The results are consistent with the proposal that Abca4 facilitates the reduction of all-trans retinal to retinol; absence of Abca4 however does not appear to be associated with higher all-trans retinal levels compared to wild type.

Protective effects of resveratrol and its analogs on age-related macular degeneration in vitro

Damage of retinal pigment epithelial (RPE) cells by A2E may be critical for age-related macular degeneration (AMD) management. Accumulation and photooxidation of A2E are known to be one of the critical causes in AMD. Here, we evaluated the protective effect of resveratrol (RES), piceatannol (PIC) and RES glycones on blue-light-induced RPE cell death caused by A2E photooxidation. A2E treatment followed by blue light exposure caused significant damages on human RPE cells (ARPE-19). But the damages were attenuated by post- and pre-treatment of RES and PIC in our in vitro models. The results of cell free system and FAB-MS analysis clearly showed that the reduction of A2E by blue light exposure was significantly rescued, and that oxidized forms of A2E were significantly reduced by RES or PIC treatment. Besides, RES or PIC inhibited the intracellular accumulation of A2E. Not only RES and PIC but RES glycones showed protection of ARPE-19 cells against A2E and blue-light-induced photo-damage. These findings demonstrate that RES and its analogs may have protective effects against A2E and blue-light-induced ARPE-19 cell death through regulation of A2E accumulation as well as photooxidation of A2E. Thus RES and its analogs may be beneficial for AMD treatment.

Photodegradation of retinal bisretinoids in mouse models and implications for macular degeneration

Adducts of retinaldehyde (bisretinoids) form nonenzymatically in photoreceptor cells and accumulate in retinal pigment epithelial (RPE) cells as lipofuscin; these fluorophores are implicated in the pathogenesis of inherited and age-related macular degeneration (AMD). Here we demonstrate that bisretinoid photodegradation is ongoing in the eye. High-performance liquid chromatography (HPLC) analysis of eyes of dark-reared and cyclic light-reared wild-type mice, together with comparisons of pigmented versus albino mice, revealed a relationship between intraocular light and reduced levels of the bisretinoids A2E and A2-glycero-phosphoethanolamine (A2-GPE). Analysis of the bisretinoids A2E, A2-GPE, A2-dihydropyridine-phosphatidylethanolamine (A2-DHP-PE), and all-trans-retinal dimer-phosphatidylethanolamine (all-trans-retinal dimer-PE) also decreases in albino Abca4(-/-) mice reared in cyclic light compared with darkness. In albino Abca4(-/-) mice receiving a diet supplemented with the antioxidant vitamin E, higher levels of RPE bisretinoid were evidenced by HPLC analysis and quantitation of fundus autofluorescence; this effect is consistent with photooxidative processes known to precede bisretinoid degradation. Amelioration of outer nuclear layer thinning indicated that vitamin E treatment protected photoreceptor cells. Conversely, in-cage exposure to short-wavelength light resulted in reduced fundus autofluorescence, decreased HPLC-quantified A2E, outer nuclear layer thinning, and increased methylglyoxal (MG)-adducted protein. MG was also released upon bisretinoid photodegradation in cells. We suggest that the lower levels of these diretinal adducts in cyclic light-reared and albino mice reflect photodegradative loss of bisretinoid. These mechanisms may underlie associations among AMD risk, oxidative mechanisms, and lifetime light exposure.

Determination of N-retinylidene-N-retinylethanolamine (A2E) levels in central and peripheral areas of human retinal pigment epithelium

The bis-retinoid N-retinylidene-N-retinylethanolamine (A2E) is one of the major components of lipofuscin, a fluorescent material that accumulates with age in the lysosomes of the retinal pigment epithelium (RPE) of the human eye. Lipofuscin, as well as A2E, exhibit a range of cytotoxic properties, which are thought to contribute to the pathogenesis of degenerative diseases of the retina such as Age-related Macular Degeneration. Consistent with such a pathogenic role, high levels of lipofuscin fluorescence are found in the central area of the human RPE, and decline toward the periphery. Recent reports have however suggested a surprising incongruence between the distributions of lipofuscin and A2E in the human RPE, with A2E levels being lowest in the central area and increasing toward the periphery. To appraise such a possibility, we have quantified the levels of A2E in the central and peripheral RPE areas of 10 eyes from 6 human donors (ages 75-91 years) with HPLC and UV/VIS spectroscopy. The levels of A2E in the central area were on average 3-6 times lower than in peripheral areas of the same eye. Furthermore, continuous accumulation of selected ions (CASI) imaging mass spectrometry showed the presence of A2E in the central RPE, and at lower intensities than in the periphery. We have therefore corroborated that in human RPE the levels of A2E are lower in the central area compared to the periphery. We conclude that the levels of A2E cannot by themselves provide an explanation for the higher lipofuscin fluorescence found in the central area of the human RPE.

Spatial and Spectral Characterization of Human Retinal Pigment Epithelium Fluorophore Families by Ex Vivo Hyperspectral Autofluorescence Imaging

Purpose

Discovery of candidate spectra for abundant fluorophore families in human retinal pigment epithelium (RPE) by ex vivo hyperspectral imaging.

Methods

Hyperspectral autofluorescence emission images were captured between 420 and 720 nm (10-nm intervals), at two excitation bands (436–460, 480–510 nm), from three locations (fovea, perifovea, near-periphery) in 20 normal RPE/Bruch's membrane (BrM) flatmounts. Mathematical factorization extracted a BrM spectrum (S0) and abundant lipofuscin/melanolipofuscin (LF/ML) spectra of RPE origin (S1, S2, S3) from each tissue.

Results

Smooth spectra S1 to S3, with perinuclear localization consistent with LF/ML at all three retinal locations and both excitations in 14 eyes (84 datasets), were included in the analysis. The mean peak emissions of S0, S1, and S2 at λ_ex 436 nm were, respectively, 495 ± 14, 535 ± 17, and 576 ± 20 nm. S3 was generally trimodal, with peaks at either 580, 620, or 650 nm (peak mode, 650 nm). At λ_ex 480 nm, S0, S1, and S2 were red-shifted to 526 ± 9, 553 ± 10, and 588 ± 23 nm, and S3 was again trimodal (peak mode, 620 nm). S1 often split into two spectra, S1A and S1B. S3 strongly colocalized with melanin. There were no significant differences across age, sex, or retinal location.

Conclusions

There appear to be at least three families of abundant RPE fluorophores that are ubiquitous across age, retinal location, and sex in this sample of healthy eyes. Further molecular characterization by imaging mass spectrometry and localization via super-resolution microscopy should elucidate normal and abnormal RPE physiology involving fluorophores.

Translational Relevance

Our results help establish hyperspectral autofluorescence imaging of the human retinal pigment epithelium as a useful tool for investigating retinal health and disease.

Correlations between Photodegradation of Bisretinoid Constituents of Retina and Dicarbonyl Adduct Deposition

Non-enzymatic collagen cross-linking and carbonyl adduct deposition are features of Bruch's membrane aging in the eye, and disturbances in extracellular matrix turnover are considered to contribute to Bruch's membrane thickening. Because bisretinoid constituents of the lipofuscin of retinal pigment epithelial (RPE) cells are known to photodegrade to mixtures of aldehyde-bearing fragments and small dicarbonyls (glyoxal (GO) and methylglyoxal (MG)), we investigated RPE lipofuscin as a source of the reactive species that covalently modify protein side chains. Abca4(-/-) and Rdh8(-/-)/Abca4(-/-) mice that are models of accelerated bisretinoid formation were studied and pre-exposure of mice to 430 nm light enriched for dicarbonyl release by bisretinoid photodegradation. MG protein adducts were elevated in posterior eyecups of mutant mice, whereas carbonylation of an RPE-specific protein was observed in Abca4(-/-) but not in wild-type mice under the same conditions. Immunolabeling of cryostat-sectioned eyes harvested from Abca4(-/-) mice revealed that carbonyl adduct deposition in Bruch's membrane was accentuated. Cell-based assays corroborated these findings in mice. Moreover, the receptor for advanced glycation end products that recognizes MG and GO adducts and glyoxylase 1 that metabolizes MG and GO were up-regulated in Abca4(-/-) mice. Additionally, in acellular assays, peptides were cross-linked in the presence of A2E (adduct of two vitamin A aldehyde and ethanolamine) photodegradation products, and in a zymography assay, reaction of collagen IV with products of A2E photodegradation resulted in reduced cleavage by the matrix metalloproteinases MMP2 and MMP9. In conclusion, these mechanistic studies demonstrate a link between the photodegradation of RPE bisretinoid fluorophores and aging changes in underlying Bruch's membrane that can confer risk of age-related macular degeneration.

Fundus Autofluorescence and RPE Lipofuscin in Age-Related Macular Degeneration

Genes that increase susceptibility to age-related macular degeneration (AMD) have been identified; however, since many individuals carrying these risk alleles do not develop disease, other contributors are involved. One additional factor, long implicated in the pathogenesis of AMD, is the lipofuscin of retinal pigment epithelium (RPE). The fluorophores that constitute RPE lipofuscin also serve as a source of autofluorescence (AF) that can be imaged by confocal laser ophthalmoscopy. The AF originating from lipofuscin is excited by the delivery of short wavelength (SW) light. A second autofluorescence is emitted from the melanin of RPE (and choroid) upon near-infrared (NIR-AF) excitation. SW-AF imaging is currently used in the clinical management of retinal disorders and the advantages of NIR-AF are increasingly recognized. Here we visit the damaging properties of RPE lipofuscin that could be significant when expressed on a background of genetic susceptibility. To advance interpretations of disease-related patterns of fundus AF in AMD, we also consider the photochemical and spectrophotometric features of the lipofuscin compounds responsible for generating the fluorescence emission.

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.

Changes in spectral properties and composition of lipofuscin fluorophores from human-retinal-pigment epithelium with age and pathology

Fundus autofluorescence mostly originates from bisretinoid fluorophores in lipofuscin granules, which accumulate in retinal-pigment-epithelium cells with age. The dynamics of accumulation, photo-oxidation, and photodegradation of bisretinoids during aging or in the presence of pathology have been insufficiently investigated. Changes in spectral properties and composition of human lipofuscin-granule fluorophores with age and pathology have now been investigated by a high-performance liquid chromatography method using spectrophotometric and fluorescent detectors connected in series. It was found that: (i) N-retinylidene-N-retinylethanolamine (A2E) fluorescence intensity is not predominant in the chloroform extract of human-cadaver-eye retinal pigment epithelium studied; bisretinoid photo-oxidation and photodegradation products have much higher fluorescent properties; (ii) the relative emission maximum in the fluorescence spectrum of suspended retinal-pigment-epithelium cells obtained from an individual human-cadaver eye without pathology is irrespective of donor age and falls within the range 575 ± 15 nm; in two cadaver eyes with signs of age-related macular degeneration, emission maxima were shifted by 23-36 nm towards the shortwave region; and (iii) the ratio of bisretinoid photo-oxidation and photodegradation products to unoxidized bisretinoids in the chloroform extract of cadaver-eye retinal pigment epithelium increases with donor age, from 0.69 ± 0.03 to 1.32 ± 0.04. The differences in fluorescence properties between chloroform extracts obtained from cadaver eyes with and without signs of age-related macular degeneration could be used to increase the potential of fundus autofluorescence imaging as a noninvasive diagnostic method.

Simultaneous decomposition of multiple hyperspectral data sets: signal recovery of unknown fluorophores in the retinal pigment epithelium

Upon excitation with different wavelengths of light, biological tissues emit distinct but related autofluorescence signals. We used non-negative matrix factorization (NMF) to simultaneously decompose co-registered hyperspectral emission data from human retinal pigment epithelium/Bruch's membrane specimens illuminated with 436 and 480 nm light. NMF analysis was initialized with Gaussian mixture model fits and constrained to provide identical abundance images for the two excitation wavelengths. Spectra recovered this way were smoother than those obtained separately; fluorophore abundances more clearly localized within tissue compartments. These studies provide evidence that leveraging multiple co-registered hyperspectral emission data sets is preferential for identifying biologically relevant fluorophore information.

Retinal metabolism in humans induces the formation of an unprecedented lipofuscin fluorophore ‘pdA2E’

We identified a novel by-product of retinal metabolism called pdA2E in the lipofuscin of the retinal pigment epithelium, demonstrated the photoisomerization and oxidation of pdA2E, and proposed a non-enzymatic mechanism for its formation in the retina.

Structures and biogenetic analysis of lipofuscin bis-retinoids

Age-related macular degeneration (AMD) is still an incurable blinding eye disease because of complex pathogenic mechanisms and unusual diseased regions. With the use of chemical biology tools, great progress has been achieved in improving the understanding of AMD pathogenesis. The severity of AMD is, at least in part, linked to the non-degradable lipofuscin bis-retinoids in retinal pigment epithelial (RPE). This material is thought to result from the lifelong accumulation of lysosomal residual bodies containing the end products derived from the daily phagocytosis of rod outer segments by RPE cells. Here, we present previously recognized bis-retinoids with focus on structures and biosynthetic pathways. In addition to a brief discussion on the mutual conversion relationships of bis-retinoids, future perspectives and the medical relevance of such studies on these lipofuscin constituents are also highlighted.