Di-retinoid-pyridinium-ethanolamine (A2E) Accumulation and the Maintenance of the Visual Cycle Are Independent of Atg7-mediated Autophagy in the Retinal Pigmented Epithelium

Mitophagy in the retina: Viewing mitochondrial homeostasis through a new lens

Mitochondrial function is key to support metabolism and homeostasis in the retina, an organ that has one of the highest metabolic rates body-wide and is constantly exposed to photooxidative damage and external stressors. Mitophagy is the selective autophagic degradation of mitochondria within lysosomes, and can be triggered by distinct stimuli such as mitochondrial damage or hypoxia. Here, we review the importance of mitophagy in retinal physiology and pathology. In the developing retina, mitophagy is essential for metabolic reprogramming and differentiation of retina ganglion cells (RGCs). In basal conditions, mitophagy acts as a quality control mechanism, maintaining a healthy mitochondrial pool to meet cellular demands. We summarize the different autophagy- and mitophagy-deficient mouse models described in the literature, and discuss the potential role of mitophagy dysregulation in retinal diseases such as glaucoma, diabetic retinopathy, retinitis pigmentosa, and age-related macular degeneration. Finally, we provide an overview of methods used to monitor mitophagy in vitro, ex vivo, and in vivo. This review highlights the important role of mitophagy in sustaining visual function, and its potential as a putative therapeutic target for retinal and other diseases.

Ambra1 haploinsufficiency in CD1 mice results in metabolic alterations and exacerbates age-associated retinal degeneration

Macroautophagy/autophagy is a key process in the maintenance of cellular homeostasis. The age-dependent decline in retinal autophagy has been associated with photoreceptor degeneration. Retinal dysfunction can also result from damage to the retinal pigment epithelium (RPE), as the RPE-retina constitutes an important metabolic ecosystem that must be finely tuned to preserve visual function. While studies of mice lacking essential autophagy genes have revealed a predisposition to retinal degeneration, the consequences of a moderate reduction in autophagy, similar to that which occurs during physiological aging, remain unclear. Here, we described a retinal phenotype consistent with accelerated aging in mice carrying a haploinsufficiency for Ambra1, a pro-autophagic gene. These mice showed protein aggregation in the retina and RPE, metabolic underperformance, and premature vision loss. Moreover, Ambra1^+/gt mice were more prone to retinal degeneration after RPE stress. These findings indicate that autophagy provides crucial support to RPE-retinal metabolism and protects the retina against stress and physiological aging.Abbreviations : 4-HNE: 4-hydroxynonenal; AMBRA1: autophagy and beclin 1 regulator 1, AMD: age-related macular degeneration;; GCL: ganglion cell layer; GFAP: glial fibrillary acidic protein; GLUL: glutamine synthetase/glutamate-ammonia ligase; HCL: hierarchical clustering; INL: inner nuclear layer; IPL: inner plexiform layer; LC/GC-MS: liquid chromatography/gas chromatography-mass spectrometry; MA: middle-aged; MTDR: MitoTracker Deep Red; MFI: mean fluorescence intensity; NL: NH₄Cl and leupeptin; Nqo: NAD(P)H quinone dehydrogenase; ONL: outer nuclear layer; OPL: outer plexiform layer; OP: oscillatory potentials; OXPHOS: oxidative phosphorylation; PCR: polymerase chain reaction; PRKC/PKCα: protein kinase C; POS: photoreceptor outer segment; RGC: retinal ganglion cells; RPE: retinal pigment epithelium; SI: sodium iodate; TCA: tricarboxylic acid.

AUTOPHAGY IN THE EYE: FROM PHYSIOLOGY TO PATHOPHYSOLOGY

Autophagy is a catabolic self-degradative pathway that promotes the degradation and recycling of intracellular material through the lysosomal compartment. Although first believed to function in conditions of nutritional stress, autophagy is emerging as a critical cellular pathway, involved in a variety of physiological and pathophysiological processes. Autophagy dysregulation is associated with an increasing number of diseases, including ocular diseases. On one hand, mutations in autophagy-related genes have been linked to cataracts, glaucoma, and corneal dystrophy; on the other hand, alterations in autophagy and lysosomal pathways are a common finding in essentially all diseases of the eye. Moreover, LC3-associated phagocytosis, a form of non-canonical autophagy, is critical in promoting visual cycle function. This review collects the latest understanding of autophagy in the context of the eye. We will review and discuss the respective roles of autophagy in the physiology and/or pathophysiology of each of the ocular tissues, its diurnal/circadian variation, as well as its involvement in diseases of the eye.

Autophagy in the retinal pigment epithelium: a new vision and future challenges

The retinal pigment epithelium (RPE) is a highly specialized monolayer of polarized, pigmented epithelial cells that resides between the vessels of the choriocapillaris and the neural retina. The RPE is essential for the maintenance and survival of overlying light-sensitive photoreceptors, as it participates in the formation of the outer blood-retinal barrier, phagocytosis, degradation of photoreceptor outer segment (POS) tips, maintenance of the retinoid cycle, and protection against light and oxidative stress. Autophagy is an evolutionarily conserved 'self-eating' process, designed to maintain cellular homeostasis. The daily autophagy demands in the RPE require precise gene regulation for the digestion and recycling of intracellular and POS components in lysosomes in response to light and stress conditions. In this review, we discuss selective autophagy and focus on the recent advances in our understanding of the mechanism of cell clearance in the RPE for visual function. Understanding how this catabolic process is regulated by both transcriptional and post-transcriptional mechanisms in the RPE will promote the recognition of pathological pathways in genetic disease and shed light on potential therapeutic strategies to treat visual impairments in patients with retinal disorders associated with lysosomal dysfunction.

Myelinosome organelles in pathological retinas: ubiquitous presence and dual role in ocular proteostasis maintenance

The timely and efficient elimination of aberrant proteins and damaged organelles, formed in response to various genetic and environmental stressors, is a vital need for all cells of the body. Recent lines of evidence point out several non-classical strategies employed by ocular tissues to cope with aberrant constituents generated in the retina and in the retinal pigmented epithelium cells exposed to various stressors. Along with conventional strategies relying upon the intracellular degradation of aberrant constituents through ubiquitin-proteasome and/or lysosome-dependent autophagy proteolysis, two non-conventional mechanisms also contribute to proteostasis maintenance in ocular tissues. An exosome-mediated clearing and a myelinosome-driven secretion mechanism do not require intracellular degradation but provide the export of aberrant constituents and "waste proteins" outside of the cells. The current review is centered on the non-degradative myelinosome-driven secretion mechanism, which operates in the retina of transgenic Huntington's disease R6/1 model mice. Myelinosome-driven secretion is supported by rare organelles myelinosomes that are detected not only in degenerative Huntington's disease R6/1 retina but also in various pathological states of the retina and of the retinal pigmented epithelium. The intra-retinal traffic and inter-cellular exchange of myelinosomes was discussed in the context of a dual role of the myelinosome-driven secretion mechanism for proteostasis maintenance in different ocular compartments. Special focus was made on the interplay between degradative and non-degradative strategies in ocular pathophysiology, to delineate potential therapeutic approaches to counteract several vision diseases.

The effect of A2E on lysosome membrane permeability during blue light-induced human RPEs apoptosis

BACKGROUND

To investigate the effect of N-retinyl-N-retinylidene ethanolamine (A2E) on lysosome membrane permeability (LMP) during blue light-induced human retinal pigment epithelium cells (RPEs) apoptosis.

METHODS

By building an A2E and blue light irradiation inducing RPEs damage model, the CCK-8 assay was used to detect RPEs viability loaded with different concentrations of A2E after different culturing time to determine the optimum A2E loading concentration. And the RPEs fluorescence intensity changes were observed by fluorescence microscopy loaded with different concentration of A2E. The RPEs were divided into four groups randomly: control group, A2E-loaded group, blue light irradiation group, and A2E-loaded + blue light irradiation group. Annexin V-FITC/PI and TUNEL/DAPI methods were used to detect RPEs apoptotic rate. Laser scanning confocal microscopy (LSCM) was used to observe RPEs LMP changes stained by acridine orange (AO) method.

RESULTS

The CCK-8 result showed a downward trend in cells viability of RPEs loaded with increasing concentration of A2E and extending culturing time. The optimum A2E loading concentration was determined at 25 μmol/L. With increasing A2E loading concentrations, the intensity of fluorescence in RPEs decreased gradually. The RPEs apoptotic rate in blue light irradiation + A2E-loaded group was significantly higher than those in other three groups detected by Annexin V-FITC/PI method, which was similar to TUNEL/DAPI's result. After AO staining, cytoplasmic and nucleolar RNAs emits green fluorescence; lysosomes emit red fluorescence. Through the interference of A2E and blue light on RPEs, red fluorescent leakage from the lysosomes (means LMP increasing) can be observed. The mean red fluorescence intensity was chosen as the statistics indicator to estimate LMP change in RPEs cultured in vitro. Compared with the control group, the red fluorescence intensity decreased in A2E-loaded group, blue light irradiation group, and blue light irradiation + A2E-loaded group. Meanwhile, the mean red fluorescence intensity in blue light irradiation + A2E-loaded group was the lowest.

CONCLUSIONS

Both A2E-loaded and blue light irradiation could induce human RPEs apoptosis, and the two factors had a synergistic effect. In addition, both A2E and blue light can lead to LMP increasing, which indicated LMP change might be the upstream part in inducing mitochondrion-dependent apoptotic pathway. These data provided evidence that A2E as the most important auto-fluorescence substance in lipofuscin is an initiator of blue light-mediated damage of RPEs and participate in pathogenesis of retinal degenerative diseases in humans.

New insights into the role of autophagy in retinal and eye diseases

Autophagy is a fundamental homeostatic pathway that mediates the degradation and recycling of intracellular components. It serves as a key quality control mechanism, especially in non-dividing cells such as neurons. Proteins, lipids, and even whole organelles are engulfed in autophagosomes and delivered to the lysosome for elimination. The retina is a light-sensitive tissue located in the back of the eye that detects and processes visual images. Vision is a highly demanding process, making the eye one of the most metabolically active tissues in the body and photoreceptors display glycolytic metabolism, even in the presence of oxygen. The retina and eye are also exposed to other stressors that can impair their function, including genetic mutations and age-associated changes. Autophagy, among other pathways, is therefore a key process for the preservation of retinal homeostasis. Here, we review the roles of both canonical and non-canonical autophagy in normal retinal function. We discuss the most recent studies investigating the participation of autophagy in eye diseases such as age-related macular degeneration, glaucoma, and diabetic retinopathy and its role protecting photoreceptors in several forms of retinal degeneration. Finally, we consider the therapeutic potential of strategies that target autophagy pathways to treat prevalent retinal and eye diseases.

ADAM17 mediates ectodomain shedding of the soluble VLDL receptor fragment in the retinal epithelium

Very low-density lipoprotein receptor (VLDLR) is a multifunctional transmembrane protein. Beyond the function of the full-length VLDLR in lipid transport, the soluble ectodomain of VLDLR (sVLDLR) confers anti-inflammatory and antiangiogenic roles in ocular tissues through inhibition of canonical Wnt signaling. However, it remains unknown how sVLDLR is shed into the extracellular space. In this study, we present the first evidence that a disintegrin and metalloprotease 17 (ADAM17) is responsible for sVLDLR shedding in human retinal pigment epithelium cells using pharmacological and genetic approaches. Among selected proteinase inhibitors, an ADAM17 inhibitor demonstrated the most potent inhibitory effect on sVLDLR shedding. siRNA-mediated knockdown or CRISPR/Cas9-mediated KO of ADAM17 diminished, whereas plasmid-mediated overexpression of ADAM17 promoted sVLDLR shedding. The amount of shed sVLDLR correlated with an inhibitory effect on the Wnt signaling pathway. Consistent with these in vitro findings, intravitreal injection of an ADAM17 inhibitor reduced sVLDLR levels in the extracellular matrix in the mouse retina. In addition, our results demonstrated that ADAM17 cleaved VLDLR only in cells coexpressing these proteins, suggesting that shedding occurs in a cis manner. Moreover, our study demonstrated that aberrant activation of Wnt signaling was associated with decreased sVLDLR levels, along with downregulation of ADAM17 in ocular tissues of an age-related macular degeneration model. Taken together, our observations reveal the mechanism underlying VLDLR cleavage and identify a potential therapeutic target for the treatment of disorders associated with dysregulation of Wnt signaling.

Complement activation, lipid metabolism, and mitochondrial injury: Converging pathways in age-related macular degeneration

The retinal pigment epithelium (RPE) is the primary site of injury in non-neovascular age-related macular degeneration or dry AMD. Polymorphisms in genes that regulate complement activation and cholesterol metabolism are strongly associated with AMD, but the biology underlying disease-associated variants is not well understood. Here, we highlight recent studies that have used molecular, biochemical, and live-cell imaging methods to elucidate mechanisms by which aging-associated insults conspire with AMD genetic risk variants to tip the balance towards disease. We discuss how critical functions including lipid metabolism, autophagy, complement regulation, and mitochondrial dynamics are compromised in the RPE, and how a deeper understanding of these mechanisms has helped identify promising therapeutic targets to preserve RPE homeostasis in AMD.

Autophagy in Age-Related Macular Degeneration: A Regulatory Mechanism of Oxidative Stress

Age-related macular degeneration (AMD) is a leading cause of severe visual loss and irreversible blindness in the elderly population worldwide. Retinal pigment epithelial (RPE) cells are the major site of pathological alterations in AMD. They are responsible for the phagocytosis of shed photoreceptor outer segments (POSs) and clearance of cellular waste under physiological conditions. Age-related, cumulative oxidative stimuli contribute to the pathogenesis of AMD. Excessive oxidative stress induces RPE cell degeneration and incomplete digestion of POSs, leading to the continuous accumulation of cellular waste (such as lipofuscin). Autophagy is a major system of degradation of damaged or unnecessary proteins. However, degenerative RPE cells in AMD patients cannot perform autophagy sufficiently to resist oxidative damage. Increasing evidence supports the idea that enhancing the autophagic process can properly alleviate oxidative injury in AMD and protect RPE and photoreceptor cells from degeneration and death, although overactivated autophagy may lead to cell death at early stages of retinal degenerative diseases. The crosstalk among the NFE2L2, PGC-1, p62, AMPK, and PI3K/Akt/mTOR pathways may play a crucial role in improving disturbed autophagy and mitigating the progression of AMD. In this review, we discuss how autophagy prevents oxidative damage in AMD, summarize potential neuroprotective strategies for therapeutic interventions, and provide an overview of these neuroprotective mechanisms.

The cell biology of the retinal pigment epithelium

The retinal pigment epithelium (RPE), a monolayer of post-mitotic polarized epithelial cells, strategically situated between the photoreceptors and the choroid, is the primary caretaker of photoreceptor health and function. Dysfunction of the RPE underlies many inherited and acquired diseases that cause permanent blindness. Decades of research have yielded valuable insight into the cell biology of the RPE. In recent years, new technologies such as live-cell imaging have resulted in major advancement in our understanding of areas such as the daily phagocytosis and clearance of photoreceptor outer segment tips, autophagy, endolysosome function, and the metabolic interplay between the RPE and photoreceptors. In this review, we aim to integrate these studies with an emphasis on appropriate models and techniques to investigate RPE cell biology and metabolism, and discuss how RPE cell biology informs our understanding of retinal disease.

Clinical Course and Electron Microscopic Findings in Lymphocytes of Patients with DRAM2-Associated Retinopathy

DRAM2-associated retinopathy is a rare inherited retinal dystrophy, and its outcome has not been determined. A single retinal involvement by a mutation of the DRAM2 gene is unexplained. We found three unrelated patients with a disease-causing DRAM2 variant in a biallelic state from 1555 Japanese individuals of 1314 families with inherited retinal dystrophy. We reviewed their medical records and examined their peripheral lymphocytes by transmission electron microscopy (TEM). Patient 1 was a 38-year-old woman who complained of night blindness and reduced vision. She developed macular degeneration at age 43 years. Patients 2 and 3 were a man and a woman both of whom noticed night blindness in their 30s. Both had a degeneration in the macula and midperiphery in their 40s, which progressed to a diffuse retinal degeneration in their 60s when their vision was reduced to hand motions. Three novel DRAM2 variants were identified. TEM of the lymphocytes of Patients 1 and 2 showed abnormal structures in 40.6% and 0.3% of the peripheral lymphocytes, respectively. We concluded that the DRAM2-associated retinopathy of our patients was a progressive rod-cone dystrophy, and the visual outcome was poor. The systemic effect of DRAM2 mutations may be compensable and have variations.

Autophagy and Age-Related Eye Diseases

Background

Autophagy is a catabolic process that depends on the lysosome. It is usually used to maintain cellular homeostasis, survival and development by degrading abnormal substances and dysfunctional organelles, especially when the cell is exposed to starvation or other stresses. Increasing studies have reported that autophagy is associated with various eye diseases, of which aging is one of the important factors.

Objective

To summarize the functional and regulatory role of autophagy in ocular diseases with aging, and discuss the possibility of autophagy-targeted therapy in age-related diseases.

Methods

PubMed searches were performed to identify relevant articles published mostly in the last 5 years. The key words were used to retrieve including “autophagy”, “aging”, “oxidative stress AND autophagy”, “dry eye AND autophagy”, “corneal disease AND autophagy”, “glaucoma AND autophagy”, “cataract AND autophagy”, “AMD AND autophagy”, “cardiovascular diseases AND autophagy”, “diabetes AND autophagy”. After being classified and assessed, the most relevant full texts in English were chosen.

Results

Apart from review articles, more than two research articles for each age-related eye diseases related to autophagy were retrieved. We only included the most relevant and recent studies for summary and discussion.

Conclusion

Autophagy has both protective and detrimental effects on the progress of age-related eye diseases. Different types of studies based on certain situations in vitro showed distinct results, which do not necessarily coincide with the actual situation in human bodies completely. It means the exact role and regulatory function of autophagy in ocular diseases remains largely unknown. Although autophagy as a potential therapeutic target has been proposed, many problems still need to be solved before it applies to clinical practice.

Autophagy Dysfunction, Cellular Senescence, and Abnormal Immune-Inflammatory Responses in AMD: From Mechanisms to Therapeutic Potential

Age-related macular degeneration (AMD) is a blinding disease caused by multiple factors and is the primary cause of vision loss in the elderly. The morbidity of AMD increases every year. Currently, there is no effective treatment option for AMD. Intravitreal injection of antivascular endothelial growth factor (anti-VEGF) is currently the most widely used therapy, but it only aims at neovascularization, which is an intermediate pathological phenomenon of wet AMD, not at the etiological treatment. Anti-VEGF therapy can only temporarily delay the degeneration process of wet AMD, and AMD is easy to relapse after drug withdrawal. Therefore, it is urgent to deepen our understanding of the pathophysiological processes underlying AMD and to identify integrated or new strategies for AMD prevention and treatment. Recent studies have found that autophagy dysfunction in retinal pigment epithelial (RPE) cells, cellular senescence, and abnormal immune-inflammatory responses play key roles in the pathogenesis of AMD. For many age-related diseases, the main focus is currently the clearing of senescent cells (SNCs) as an antiaging treatment, thereby delaying diseases. However, in AMD, there is no relevant antiaging application. This review will discuss the pathogenesis of AMD and how interactions among RPE autophagy dysfunction, cellular senescence, and abnormal immune-inflammatory responses are involved in AMD, and it will summarize the three antiaging strategies that have been developed, with the aim of providing important information for the integrated prevention and treatment of AMD and laying the ground work for the application of antiaging strategies in AMD treatment.

RPE Visual Cycle and Biochemical Phenotypes of Mutant Mouse Models

The retinal pigmented epithelium (RPE) is a single layer of polarized epithelial cells which plays many important roles for visual function. One of such roles is production of visual chromophore, 11-cis-retinal through the visual cycle. The visual cycle consists of biochemical processes for regenerating chromophore by a collective action of the RPE and photoreceptor. Photoreceptors harbor the G protein-coupled receptors, opsin which enables to receive light when it bounds to 11-cis-retinal. With absorption of a photon of light, 11-cis-retinal photoisomerizes to all-trans-retinal. All-trans-retinal reduces to all-trans-retinol in the photoreceptor and further recycles back to 11-cis-retinal in the RPE. Acyltransferases and isomerohydrolase(s) along with retinol dehydrogenases sequentially convert all-trans-retinol to 11-cis-retinal in the RPE. Dysfunctions of any retinoid cycle enzymes in the RPE can cause retinal diseases. Phenotyping RPE functions by the use of mutant mouse models will provide great detailed biochemical insights of the visual cycle and further manipulative strategies to protect against retinal degeneration. Here, we describe biochemical analyses of the visual cycle in mouse models using RPE cells.

Autophagy resists EMT process to maintain retinal pigment epithelium homeostasis

Proliferative vitreoretinopathy (PVR) is the most serious fibrous complication that causes vision loss after intraocular surgery, and there is currently no effective treatment in clinical. Autophagy is an important cell biological mechanism in maintaining the homeostasis of tissues and cells, resisting the process of EMT. However, it is still unclear whether autophagy could resist intraocular fibrosis and prevent PVR progression. In this study, we investigated the expression of mesenchymal biomarkers in autophagy deficiency cells and found these proteins were increased. The mesenchymal protein transcription factor Twist can bind to autophagy related protein p62 and promote the degradation of Twist, which reduced the expression of mesenchymal markers. By constructing an EMT model of retinal pigment epithelial (RPE) cells in vitro, we found that autophagy was activated in the EMT process of RPE cells. Moreover, in autophagy deficient RPE cell line via knockdown autophagy related protein 7 (Atg7), the expression of epithelial marker claudin-1 was suppressed and the mesenchymal markers were increased, accompanied by an increase in cell migration and contractility. Importantly, RPE epithelial properties can be maintained by promoting autophagy and effectively reversing TFG-β2-induced RPE fibrosis. These observations reveal that autophagy may be an effective way to treat PVR.

Lipocalin 2 Plays an Important Role in Regulating Inflammation in Retinal Degeneration

It has become increasingly important to understand how retinal inflammation is regulated because inflammation plays a role in retinal degenerative diseases. Lipocalin 2 (LCN2), an acute stress response protein with multiple innate immune functions, is increased in ATP-binding cassette subfamily A member 4 (Abca4) ^-/- retinol dehydrogenase 8 (Rdh8) ^-/- double-knockout mice, an animal model for Stargardt disease and age-related macular degeneration (AMD). To examine roles of LCN2 in retinal inflammation and degeneration, Lcn2^-/-Abca4^-/-Rdh8^-/- triple-knockout mice were generated. Exacerbated inflammation following light exposure was observed in Lcn2^-/-Abca4^-/-Rdh8^-/- mice as compared with Abca4^-/-Rdh8^-/- mice, with upregulation of proinflammatory genes and microglial activation. RNA array analyses revealed an increase in immune response molecules such as Ccl8, Ccl2, and Cxcl10 To further probe a possible regulatory role for LCN2 in retinal inflammation, we examined the in vitro effects of LCN2 on NF-κB signaling in human retinal pigmented epithelial (RPE) cells differentiated from induced pluripotent stem cells derived from healthy donors. We found that LCN2 induced expression of antioxidant enzymes heme oxygenase 1 and superoxide dismutase 2 in these RPE cells and could inhibit the cytotoxic effects of H₂O₂ and LPS. ELISA revealed increased LCN2 levels in plasma of patients with Stargardt disease, retinitis pigmentosa, and age-related macular degeneration as compared with healthy controls. Finally, overexpression of LCN2 in RPE cells displayed protection from cell death. Overall these results suggest that LCN2 is involved in prosurvival responses during cell stress and plays an important role in regulating inflammation during retinal degeneration.

Involvement of the autophagic pathway in the progression of AMD-like retinopathy in senescence-accelerated OXYS rats

Age-related macular degeneration (AMD) is a complex neurodegenerative disease resulting in a loss of central vision in the elderly. It is currently assumed that impairment of autophagy may be one of the key mechanisms leading to AMD. Here we estimated the influence of age-related autophagy alterations in the retina on the development of AMD-like retinopathy in senescence-accelerated OXYS rats. Significant changes in the expression of the autophagy proteins were absent at the age preceding the development of retinopathy (age 20 days). We found increased levels of LC3A/B, Atg7, and Atg12-Atg5 conjugated proteins in the OXYS retina during manifestation of this retinopathy at the age of 3 months. By contrast, in the retina of 18-month-old OXYS rats with a progressive stage of retinopathy, we revealed significantly decreased protein levels of Atg7 and Atg12-Atg5 as compared to age-matched Wistar rats. Simultaneously with perturbation of the autophagic response, the necrosome subunits Ripk1 and Ripk3 were detected in the OXYS retina. The downregulation of autophagy markers coincided with amyloid β accumulation (Moab-2) in the retinal pigment epithelium and choroid. Using high-throughput RNA sequencing, we found a missense single-nucleotide polymorphism (SNP) in the Pik3c2b gene associated with autophagy regulation. This SNP was predicted to significantly affect protein structure. Our data prove participation of the autophagic pathway in the progression of AMD-like retinopathy.

A2E-associated cell death and inflammation in retinal pigmented epithelial cells from human induced pluripotent stem cells

Accumulation of lipofuscin in the retinal pigmented epithelium (RPE) is observed in retinal degenerative diseases including Stargardt disease and age-related macular degeneration. Bis-retinoid N-retinyl-N-retinylidene ethanolamine (A2E) is a major component of lipofuscin. A2E has been implicated in RPE atrophy and retinal inflammation; however, mice with A2E accumulation display only a mild retinal phenotype. In the current study, human iPSC-RPE (hiPSC-RPE) cells were generated from healthy individuals to examine effects of A2E in human RPE cells. hiPSC-RPE cells displayed RPE-specific features, which include expression of RPE-specific genes, tight junction formation and ability to carry out phagocytosis. hiPSC-RPE cells demonstrated cell death and increased VEGF-A production in a time-dependent manner when they were cocultured with 10 μM of A2E. PCR array analyses revealed upregulation of 26 and 12 pro-inflammatory cytokines upon A2E and H₂O₂ exposure respectively, indicating that A2E and H₂O₂ can cause inflammation in human retinas. Notably, identified gene profiles were different between A2E- and H₂O₂-treated hiPSC-RPE cells. A2E caused inflammatory changes observed in retinal degenerative diseases more closely as compared to H₂O₂. Collectively, these data obtained with hiPSC-RPE cells provide evidence that A2E plays an important role in pathogenesis of retinal degenerative diseases in humans.

Adaptations in rod outer segment disc membranes in response to environmental lighting conditions

The light-sensing rod photoreceptor cell exhibits several adaptations in response to the lighting environment. While adaptations to short-term changes in lighting conditions have been examined in depth, adaptations to long-term changes in lighting conditions are less understood. Atomic force microscopy was used to characterize the structure of rod outer segment disc membranes, the site of photon absorption by the pigment rhodopsin, to better understand how photoreceptor cells respond to long-term lighting changes. Structural properties of the disc membrane changed in response to housing mice in constant dark or light conditions and these adaptive changes required output from the phototransduction cascade initiated by rhodopsin. Among these were changes in the packing density of rhodopsin in the membrane, which was independent of rhodopsin synthesis and specifically affected scotopic visual function as assessed by electroretinography. Studies here support the concept of photostasis, which maintains optimal photoreceptor cell function with implications in retinal degenerations.

Early AMD-like defects in the RPE and retinal degeneration in aged mice with RPE-specific deletion of Atg5 or Atg7

PURPOSE

To examine the effects of autophagy deficiency induced by RPE-specific deletion of Atg5 or Atg7 in mice as a function of age.

METHODS

Conditional knockout mice with a floxed allele of Atg5 or Atg7 were crossed with inducible VMD2-rtTA/Cre transgenic mice. VMD2-directed RPE-specific Cre recombinase expression was induced with doxycycline feeding in the resulting mice. Cre-mediated deletion of floxed Atg5 or Atg7 resulted in RPE-specific inactivation of the Atg5 or Atg7 gene. Plastic and thin retinal sections were analyzed with light and electron microscopy for histological changes. Photoreceptor outer segment (POS) thickness in plastic sections was measured using the Adobe Photoshop CS4 extended ruler tool. Autophagic adaptor p62/SQSTM1 and markers for oxidatively damaged lipids, proteins, and DNA were examined with immunofluorescence staining of cryosections. Fluorescence signals were quantified using Image J software.

RESULTS

Accumulation of p62/SQSTM1 reflecting autophagy deficiency was observed in the RPE of the Atg5^ΔRPE and Atg7^ΔRPE mice. 3-nitrotyrosine, advanced glycation end products (AGEs), and 8-hydroxy-2'-deoxyguanosine (8-OHdG), markers for oxidatively damaged proteins and DNA, were also found to accumulate in the RPE of these mice. We observed retinal degeneration in 35% of the Atg5^ΔRPE mice and 45% of the Atg7^ΔRPE mice at 8 to 24 months old. Degeneration severity and the number of mice with degeneration increased with age. The mean POS thickness of these mice was 25 µm at 8-12 months, 15 µm at 13-18 months, and 3 µm at 19-24 months, compared to 35 µm, 30 µm, and 24 µm in the wild-type mice, respectively. Early age-related macular degeneration (AMD)-like RPE defects were found in all the Atg5^ΔRPE and Atg7^ΔRPE mice 13 months old or older, including vacuoles, uneven RPE thickness, diminished basal infoldings, RPE hypertrophy/hypotrophy, pigmentary irregularities, and necrosis. The severity of the RPE defects increased with age and in the mice with retinal degeneration. RPE atrophy and choroidal neovascularization (CNV) were occasionally observed in the Atg5^ΔRPE and Atg7^ΔRPE mice with advanced age.

CONCLUSIONS

Autophagy deficiency induced by RPE-specific deletion of Atg5 or Atg7 predisposes but does not necessarily drive the development of AMD-like phenotypes or retinal degeneration.

Absence of DJ-1 causes age-related retinal abnormalities in association with increased oxidative stress

Oxidative stress alters physiological function in most biological tissues and can lead to cell death. In the retina, oxidative stress initiates a cascade of events leading to focal loss of RPE and photoreceptors, which is thought to be a major contributing factor to geographic atrophy. Despite these implications, the molecular regulation of RPE oxidative stress under normal and pathological conditions remains largely unknown. A better understanding of the mechanisms involved in regulating RPE and photoreceptors oxidative stress response is greatly needed. To this end we evaluated photoreceptor and RPE changes in mice deficient in DJ-1, a protein that is thought to be important in protecting cells from oxidative stress. Young (3 months) and aged (18 months) DJ-1 knockout (DJ-1 KO) and age-matched wild-type mice were examined. In both group of aged mice, scanning laser ophthalmoscopy (SLO) showed the presence of a few autofluorescent foci. The 18 month-old DJ-1 KO retinas were also characterized by a noticeable increase in RPE fluorescence to wild-type. Optical coherence tomography (OCT) imaging demonstrated that all retinal layers were present in the eyes of both DJ-1 KO groups. ERG comparisons showed that older DJ-1 KO mice had reduced sensitivity under dark- and light-adapted conditions compared to age-matched control. Histologically, the RPE contained prominent vacuoles in young DJ-1 KO group with the appearance of enlarged irregularly shaped RPE cells in the older group. These were also evident in OCT and in whole mount RPE/choroid preparations labeled with phalloidin. Photoreceptors in the older DJ-1 KO mice displayed decreased immunoreactivity to rhodopsin and localized reduction in cone markers compared to the wild-type control group. Lower levels of activated Nrf2 were evident in retina/RPE lysates in both young and old DJ-1 KO mouse groups compared to wild-type control levels. Conversely, higher levels of protein carbonyl derivatives and iNOS immunoreactivity were detected in retina/RPE lysates from both young and old DJ-1 KO mice. These results demonstrate that DJ-1 KO mice display progressive signs of retinal/RPE degeneration in association with higher levels of oxidative stress markers. Collectively this analysis indicates that DJ-1 plays an important role in protecting photoreceptors and RPE from oxidative damage during aging.

Retinol Dehydrogenases Regulate Vitamin A Metabolism for Visual Function

The visual system produces visual chromophore, 11-cis-retinal from dietary vitamin A, all-trans-retinol making this vitamin essential for retinal health and function. These metabolic events are mediated by a sequential biochemical process called the visual cycle. Retinol dehydrogenases (RDHs) are responsible for two reactions in the visual cycle performed in retinal pigmented epithelial (RPE) cells, photoreceptor cells and Müller cells in the retina. RDHs in the RPE function as 11-cis-RDHs, which oxidize 11-cis-retinol to 11-cis-retinal in vivo. RDHs in rod photoreceptor cells in the retina work as all-trans-RDHs, which reduce all-trans-retinal to all-trans-retinol. Dysfunction of RDHs can cause inherited retinal diseases in humans. To facilitate further understanding of human diseases, mouse models of RDHs-related diseases have been carefully examined and have revealed the physiological contribution of specific RDHs to visual cycle function and overall retinal health. Herein we describe the function of RDHs in the RPE and the retina, particularly in rod photoreceptor cells, their regulatory properties for retinoid homeostasis and future therapeutic strategy for treatment of retinal diseases.

Autophagy in the eye: Development, degeneration, and aging

Autophagy is a catabolic pathway that promotes the degradation and recycling of cellular components. Proteins, lipids, and even whole organelles are engulfed in autophagosomes and delivered to the lysosome for elimination. In response to stress, autophagy mediates the degradation of cell components, which are recycled to generate the nutrients and building blocks required to sustain cellular homeostasis. Moreover, it plays an important role in cellular quality control, particularly in neurons, in which the total burden of altered proteins and damaged organelles cannot be reduced by redistribution to daughter cells through cell division. Research has only begun to examine the role of autophagy in the visual system. The retina, a light-sensitive tissue, detects and transmits electrical impulses through the optic nerve to the visual cortex in the brain. Both the retina and the eye are exposed to a variety of environmental insults and stressors, including genetic mutations and age-associated alterations that impair their function. Here, we review the main studies that have sought to explain autophagy's importance in visual function. We describe the role of autophagy in retinal development and cell differentiation, and discuss the implications of autophagy dysregulation both in physiological aging and in important diseases such as age-associated macular degeneration and glaucoma. We also address the putative role of autophagy in promoting photoreceptor survival and discuss how selective autophagy could provide alternative means of protecting retinal cells. The findings reviewed here underscore the important role of autophagy in maintaining proper retinal function and highlight novel therapeutic approaches for blindness and other diseases of the eye.

Tyrosinase-Cre-Mediated Deletion of the Autophagy Gene Atg7 Leads to Accumulation of the RPE65 Variant M450 in the Retinal Pigment Epithelium of C57BL/6 Mice

Targeted gene knockout mouse models have helped to identify roles of autophagy in many tissues. Here, we investigated the retinal pigment epithelium (RPE) of Atg7f/f Tyr-Cre mice (on a C57BL/6 background), in which Cre recombinase is expressed under the control of the tyrosinase promoter to delete the autophagy gene Atg7. In line with pigment cell-directed blockade of autophagy, the RPE and the melanocytes of the choroid showed strong accumulation of the autophagy adaptor and substrate, sequestosome 1 (Sqstm1)/p62, relative to the levels in control mice. Immunofluorescence and Western blot analysis demonstrated that the RPE, but not the choroid melanocytes, of Atg7f/f Tyr-Cre mice also had strongly increased levels of retinoid isomerohydrolase RPE65, a pivotal enzyme for the maintenance of visual perception. In contrast to Sqstm1, genes involved in retinal regeneration, i.e. Lrat, Rdh5, Rgr, and Rpe65, were expressed at higher mRNA levels. Sequencing of the Rpe65 gene showed that Atg7f/f and Atg7f/f Tyr-Cre mice carry a point mutation (L450M) that is characteristic for the C57BL/6 mouse strain and reportedly causes enhanced degradation of the RPE65 protein by an as-yet unknown mechanism. These results suggest that the increased abundance of RPE65 M450 in the RPE of Atg7f/f Tyr-Cre mice is, at least partly, mediated by upregulation of Rpe65 transcription; however, our data are also compatible with the hypothesis that the RPE65 M450 protein is degraded by Atg7-dependent autophagy in Atg7f/f mice. Further studies in mice of different genetic backgrounds are necessary to determine the relative contributions of these mechanisms.