Knockdown of thioredoxin interacting protein in Müller cells attenuates photoreceptor apoptosis in streptozotocin-induced diabetic mouse model

We explored the effect of inhibition of thioredoxin interacting protein (Txnip) on neuroprotection in Müller cells under high glucose. Wild-type (WT) and Txnip knockout (Txnip-/-) mice were used to establish a streptozotocin (STZ)-induced diabetes model and a Müller cells high glucose model. We detected BDNF expression and PI3K/AKT/CREB pathway activation levels in the retina and Müller cells of each group in vivo and in vitro experiments. The Txnip-/- STZ group showed higher expression of BDNF and phosphorylation of PI3K/AKT/CREB in retina, and less retinal photoreceptor apoptosis was observed in Txnip-/- diabetic group than in WT. After using an inhibitor of PI3K signaling pathway, BDNF expression was reduced; In vitro co-cultured with Müller cells in different groups, 661 W cells showed different situations, Txnip-/- Müller cells maximum downregulated Cleaved-caspase 3 expression in 661 W, accompanied by an increase in Bcl-2/Bax ratio. These findings indicate that inhibiting endogenous Txnip in mouse Müller cells can promote their expression and secretion of BDNF, thereby reducing HG induced photoreceptor apoptosis and having important neuroprotective effects on DR. The regulation of BDNF expression by Txnip may be achieved by activating the PI3K/AKT/CREB pathway. This study suggests that regulating Txnip may be a potential target for DR treatment.

Hyaluronan improves photoreceptor differentiation and maturation in human retinal organoids

Human stem cell-derived organoids enable both disease modeling and serve as a source of cells for transplantation. Human retinal organoids are particularly important as a source of human photoreceptors; however, the long differentiation period required and lack of vascularization in the organoid often results in a necrotic core and death of inner retinal cells before photoreceptors are fully mature. Manipulating the in vitro environment of differentiating retinal organoids through the incorporation of extracellular matrix components could influence retinal development. We investigated the addition of hyaluronan (HA), a component of the interphotoreceptor matrix, as an additive to promote long-term organoid survival and enhance retinal maturation. HA treatment had a significant reduction in the proportion of proliferating (Ki67+) cells and increase in the proportion of photoreceptors (CRX+), suggesting that HA accelerated photoreceptor commitment in vitro. HA significantly upregulated genes specific to photoreceptor maturation and outer segment development. Interestingly, prolonged HA-treatment significantly decreased the length of the brush border layer compared to those in control retinal organoids, where the photoreceptor outer segments reside; however, HA-treated organoids also had more mature outer segments with organized discs structures, as revealed by transmission electron microscopy. The brush border layer length was inversely proportional to the molar mass and viscosity of the hyaluronan added. This is the first study to investigate the role of exogenous HA, viscosity, and polymer molar mass on photoreceptor maturation, emphasizing the importance of material properties on organoid culture. STATEMENT OF SIGNIFICANCE: Retinal organoids are a powerful tool to study retinal development in vitro, though like many other organoid systems, can be highly variable. In this work, Shoichet and colleagues investigated the use of hyaluronan (HA), a native component of the interphotoreceptor matrix, to improve photoreceptor maturation in developing human retinal organoids. HA promoted human photoreceptor differentiation leading to mature outer segments with disc formation and more uniform and healthy retinal organoids. These findings highlight the importance of adding components native to the developing retina to generate more physiologically relevant photoreceptors for cell therapy and in vitro models to drive drug discovery and uncover novel disease mechanisms.

Novel CCR3-targeted cyclic peptides as potential therapeutic agents for age-related macular degeneration via inhibiting angiogenesis and reducing retinal photoreceptor damage

The prolonged intravitreal administration of anti-vascular endothelial growth factor (VEGF) drugs is prone to inducing aberrant retinal vascular development and causing damage to retinal neurons. Hence, we have taken an alternative approach by designing and synthesizing a series of cyclic peptides targeting CC motif chemokine receptor 3 (CCR3). Based on the binding mode of the N-terminal region in CCR3 protein to CCL11, we used computer-aided identification of key amino acid sequence, conformational restriction through different cyclization methods, designed and synthesized a series of target cyclic peptides, and screened the preferred compound IB-2 through affinity. IB-2 exhibits excellent anti-angiogenic activity in HRECs. The apoptosis level of 661W cells demonstrated a significant decrease with the escalating concentration of IB-2. This suggests that IB-2 may have a protective effect on photoreceptor cells. In vivo experiments have shown that IB-2 significantly reduces retinal vascular leakage and choroidal neovascularization (CNV) area in a laser-induced mouse model of CNV. These findings indicate the potential of IB-2 as a safe and effective therapeutic agent for AMD, warranting further development.

Zeaxanthin dipalmitate-enriched wolfberry extract improves vision in a mouse model of photoreceptor degeneration

Zeaxanthin dipalmitate (ZD) is a chemical extracted from wolfberry that protects degenerated photoreceptors in mouse retina. However, the pure ZD is expensive and hard to produce. In this study, we developed a method to enrich ZD from wolfberry on a production line and examined whether it may also protect the degenerated mouse retina. The ZD-enriched wolfberry extract (ZDE) was extracted from wolfberry by organic solvent method, and the concentration of ZD was identified by HPLC. The adult C57BL/6 mice were treated with ZDE or solvent by daily gavage for 2 weeks, at the end of the first week the animals were intraperitoneally injected with N-methyl-N-nitrosourea to induce photoreceptor degeneration. Then optomotor, electroretinogram, and immunostaining were used to test the visual behavior, retinal light responses, and structure. The final ZDE product contained ~30mg/g ZD, which was over 9 times higher than that from the dry fruit of wolfberry. Feeding degenerated mice with ZDE significantly improved the survival of photoreceptors, enhanced the retinal light responses and the visual acuity. Therefore, our ZDE product successfully alleviated retinal morphological and functional degeneration in mouse retina, which may provide a basis for further animal studies for possible applying ZDE as a supplement to treat degenerated photoreceptor in the clinic.

Hyperoside mitigates photoreceptor degeneration in part by targeting cGAS and suppressing DNA-induced microglial activation

Activated microglia play an important role in driving photoreceptor degeneration-associated neuroinflammation in the retina. Controlling pro-inflammatory activation of microglia holds promise for mitigating the progression of photoreceptor degeneration. Our previous study has demonstrated that pre-light damage treatment of hyperoside, a naturally occurring flavonol glycoside with antioxidant and anti-inflammatory activities, prevents photooxidative stress-induced photoreceptor degeneration and neuroinflammatory responses in the retina. However, the direct impact of hyperoside on microglia-mediated neuroinflammation during photoreceptor degeneration remains unknown. Upon verifying the anti-inflammatory effects of hyperoside in LPS-stimulated BV-2 cells, our results here further demonstrated that post-light damage hyperoside treatment mitigated the loss of photoreceptors and attenuated the functional decline of the retina. Meanwhile, post-light damage hyperoside treatment lowered neuroinflammatory responses and dampened microglial activation in the illuminated retinas. With respect to microglial activation, hyperoside mitigated the pro-inflammatory responses in DNA-stimulated BV-2 cells and lowered DNA-stimulated production of 2'3'-cGAMP in BV-2 cells. Moreover, hyperoside was shown to directly interact with cGAS and suppress the enzymatic activity of cGAS in a cell-free system. In conclusion, the current study suggests for the first time that the DNA sensor cGAS is a direct target of hyperoside. Hyperoside is effective at mitigating DNA-stimulated cGAS-mediated pro-inflammatory activation of microglia, which likely contributes to the therapeutic effects of hyperoside at curtailing neuroinflammation and alleviating neuroinflammation-instigated photoreceptor degeneration.

Tamoxifen protects photoreceptors in the sodium iodate model

Because the selective estrogen receptor modulator tamoxifen was shown to be retina-protective in the light damage and rd10 models of retinal degeneration, the purpose of this study was to test whether tamoxifen is retina-protective in a model where retinal pigment epithelium (RPE) toxicity appears to be the primary insult: the sodium iodate (NaIO3) model. C57Bl/6J mice were given oral tamoxifen (in the diet) or the same diet lacking tamoxifen, then given an intraperitoneal injection of NaIO3 at 25 mg/kg. The mice were imaged a week later using optical coherence tomography (OCT). ImageJ with a custom macro was utilized to measure retinal thicknesses in OCT images. Electroretinography (ERG) was used to measure retinal function one week post-injection. After euthanasia, quantitative real-time PCR (qRT-PCR) was performed. Tamoxifen administration partially protected photoreceptors. There was less photoreceptor layer thinning in OCT images of tamoxifen-treated mice. qRT-PCR revealed, in the tamoxifen-treated group, less upregulation of antioxidant and complement factor 3 mRNAs, and less reduction in the rhodopsin and short-wave cone opsin mRNAs. Furthermore, ERG results demonstrated preservation of photoreceptor function for the tamoxifen-treated group. Cone function was better protected than rods. These results indicate that tamoxifen provided structural and functional protection to photoreceptors against NaIO3. RPE cells were not protected. These neuroprotective effects suggest that estrogen-receptor modulation may be retina-protective. The fact that cones are particularly protected is intriguing given their importance for human visual function and their survival until the late stages of retinitis pigmentosa. Further investigation of this protective pathway could lead to new photoreceptor-protective therapeutics.

Sphingolipid biosynthetic inhibitor L-Cycloserine prevents oxidative-stress-mediated death in an in vitro model of photoreceptor-derived 661W cells

Oxidative stress plays a pivotal role in the pathogenesis of several neurodegenerative diseases. Retinal degeneration causes irreversible death of photoreceptor cells, ultimately leading to vision loss. Under oxidative stress, the synthesis of bioactive sphingolipid ceramide increases, triggering apoptosis in photoreceptor cells and leading to their death. This study investigates the effect of L-Cycloserine, a small molecule inhibitor of ceramide biosynthesis, on sphingolipid metabolism and the protection of photoreceptor-derived 661W cells from oxidative stress. The results demonstrate that treatment with L-Cycloserine, an inhibitor of Serine palmitoyl transferase (SPT), markedly decreases bioactive ceramide and associated sphingolipids in 661W cells. A nontoxic dose of L-Cycloserine can provide substantial protection of 661W cells against H2O2-induced oxidative stress by reversing the increase in ceramide level observed under oxidative stress conditions. Analysis of various antioxidant, apoptotic and sphingolipid pathway genes and proteins also confirms the ability of L-Cycloserine to modulate these pathways. Our findings elucidate the generation of sphingolipid mediators of cell death in retinal cells under oxidative stress and the potential of L-Cycloserine as a therapeutic candidate for targeting ceramide-induced degenerative diseases by inhibiting SPT. The promising therapeutic prospect identified in our findings lays the groundwork for further validation in in-vivo and preclinical models of retinal degeneration.

Myosin 1f-mediated activation of microglia contributes to the photoreceptor degeneration in a mouse model of retinal detachment

Photoreceptor death and neurodegeneration is the leading cause of irreversible vision loss. The inflammatory response of microglia plays an important role in the process of neurodegeneration. In this study, we chose retinal detachment as the model of photoreceptor degeneration. We found Myosin 1f was upregulated after retinal detachment, and it was specifically expressed in microglia. Deficiency of myosin 1f protected against photoreceptor apoptosis by inhibiting microglia activation. The elimination of microglia can abolish the protective effect of myosin 1f deficiency. After stimulation by LPS, microglia with myosin 1f deficiency showed downregulation of the MAPK and AKT pathways. Our results demonstrated that myosin 1f plays a crucial role in microglia-induced neuroinflammation after retinal injury and photoreceptor degeneration by regulating two classic inflammatory pathways and thereby decreasing the expression of inflammatory cytokines. Knockout of myosin 1f reduces the intensity of the immune response and prevents cell death of photoreceptor, suggesting that myosin 1f can be inhibited to prevent a decline in visual acuity after retinal detachment.

Alterations of Lipidomes in Rat Photoreceptor Degeneration Induced by N-Methyl-N-nitrosourea

To investigate alterations of lipidomes in the progress of photoreceptor degeneration induced by N-methyl-N-nitrosourea (MNU) in a rat model, retinal lipid molecular species in adult Sprague-Dawley (SD) rats at 1, 3, and 7 days after MNU administration and age-matched controls were analyzed by the shotgun lipidomics technology. Moreover, total fatty acid levels in retinal, liver, and plasma samples of different groups were determined with gas chromatography. Generally, at day 1, the levels of ethanolamine plasmalogen species in retinas were markedly elevated after treatment with MNU, while the contents of other phospholipids and sphingolipids in the retina were not significantly changed than those of the control group. The compositions of almost all of unsaturated fatty acids in the retina increased significantly at day 1 after MNU administration. At day 7, the MNU treatment group has significant increases in lipid species in the retina. However, the majority of lipids containing docosahexaenoic acid (DHA, 22:6n-3) and docosapentaenoic acid (22:5n-6) declined, especially di-DHA phospholipids were dramatically reduced in the retina. In contrast, similar alterations did not occur in plasma or the liver after MNU treatment. These results suggested that at the early stage of photoreceptor degeneration, lipidome remodeling in the retina might involve protection of photoreceptor from apoptosis and continue their transduction of light. However, at the late stage of photoreceptor apoptosis, increases in comprehensive lipid species occurred, likely due to the myelination of the retina. Finally, the deficiency of DHA in photoreceptor degeneration could exacerbate the influence of myelination on retinal function. We further investigated the effects of unsaturated fatty acids on neuronal apoptosis. The preliminary experiments confirmed our observation from lipidomics analysis that unsaturated fatty acids can protect neurons from apoptosis. Collectively, our study suggests that increased levels of DHA should be protective from photoreceptor degeneration.

Minocycline decreases CCR2-positive monocytes in the retina and ameliorates photoreceptor degeneration in a mouse model of retinitis pigmentosa

Retinal inflammation accelerates photoreceptor cell death caused by retinal degeneration. Minocycline, a semisynthetic broad-spectrum tetracycline antibiotic, has been previously reported to rescue photoreceptor cell death in retinal degeneration. We examined the effect of minocycline on retinal photoreceptor degeneration using c-mer proto-oncogene tyrosine kinase (Mertk)^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice, which enabled the observation of CX3CR1-green fluorescent protein (GFP)- and CCR2-red fluorescent protein (RFP)-positive macrophages by fluorescence. Retinas of Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice showed photoreceptor degeneration and accumulation of GFP- and RFP-positive macrophages in the outer retina and subretinal space at 6 weeks of age. Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice were intraperitoneally administered minocycline. The number of CCR2-RFP positive cells significantly decreased after minocycline treatment. Furthermore, minocycline administration resulted in partial reversal of the thinning of the outer nuclear layer and decreased the number of apoptotic cells, as assessed by the TUNEL assay, in Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice. In conclusion, we found that minocycline ameliorated photoreceptor cell death in an inherited photoreceptor degeneration model due to Mertk gene deficiency and has an inhibitory effect on CCR2 positive macrophages, which is likely to be a neuroprotective mechanism of minocycline.

Oral Ursodeoxycholic Acid Crosses the Blood Retinal Barrier in Patients with Retinal Detachment and Protects Against Retinal Degeneration in an Ex Vivo Model

Rhegmatogenous retinal detachment (RD) is a threatening visual condition and a human disease model for retinal degenerations. Despite successful reattachment surgery, vision does not fully recover, due to subretinal fluid accumulation and subsequent photoreceptor cell death, through mechanisms that recapitulate those of retinal degenerative diseases. Hydrophilic bile acids are neuroprotective in animal models, but whether they can be used orally for retinal diseases is unknown. Ursodeoxycholic acid (UDCA) being approved for clinical use (e.g., in cholestasis), we have evaluated the ocular bioavailability of oral UDCA, administered to patients before RD surgery. The level of UDCA in ocular media correlated with the extent of blood retinal barrier disruption, evaluated by the extent of detachment and the albumin concentration in subretinal fluid. UDCA, at levels measured in ocular media, protected photoreceptors from apoptosis and necrosis in rat retinal explants, an ex vivo model of RD. The subretinal fluid from UDCA-treated patients, collected during surgery, significantly protected rat retinal explants from cell death, when compared to subretinal fluid from control patients. Pan-transcriptomic analysis of the retina showed that UDCA upregulated anti-apoptotic, anti-oxidant, and anti-inflammatory genes. Oral UDCA is a potential neuroprotective adjuvant therapy in RD and other retinal degenerative diseases and should be further evaluated in a clinical trial.

IGFBPs mediate IGF-1's functions in retinal lamination and photoreceptor development during pluripotent stem cell differentiation to retinal organoids

Development of the retina is regulated by growth factors, such as insulin-like growth factors 1 and 2 (IGF-1/2), which coordinate proliferation, differentiation, and maturation of the neuroepithelial precursors cells. In the circulation, IGF-1/2 are transported by the insulin growth factor binding proteins (IGFBPs) family members. IGFBPs can impact positively and negatively on IGF-1, by making it available or sequestering IGF-1 to or from its receptor. In this study, we investigated the expression of IGFBPs and their role in the generation of human retinal organoids from human pluripotent stem cells, showing a dynamic expression pattern suggestive of different IGFBPs being used in a stage-specific manner to mediate IGF-1 functions. Our data show that IGF-1 addition to culture media facilitated the generation of retinal organoids displaying the typical laminated structure and photoreceptor maturation. The organoids cultured in the absence of IGF-1, lacked the typical laminated structure at the early stages of differentiation and contained significantly less photoreceptors and more retinal ganglion cells at the later stages of differentiation, confirming the positive effects of IGF-1 on retinal lamination and photoreceptor development. The organoids cultured with the IGFBP inhibitor (NBI-31772) and IGF-1 showed lack of retinal lamination at the early stages of differentiation, an increased propensity to generate horizontal cells at mid-stages of differentiation and reduced photoreceptor development at the later stages of differentiation. Together these data suggest that IGFBPs enable IGF-1's role in retinal lamination and photoreceptor development in a stage-specific manner.

Systemic Treatment With Nicotinamide Riboside Is Protective in a Mouse Model of Light-Induced Retinal Degeneration

Purpose

Maintaining levels of nicotinamide adenine dinucleotide (NAD+), a coenzyme critical for cellular energetics and biosynthetic pathways, may be therapeutic in retinal disease because retinal NAD+ levels decline during retinal damage and degeneration. The purpose of this study was to investigate whether systemic treatment with nicotinamide riboside (NR), a NAD+ precursor that is orally deliverable and well-tolerated by humans, is protective in a mouse model of light-induced retinal degeneration.

Methods

Mice were injected intraperitoneally with vehicle or NR the day before and the morning of exposure to degeneration-inducing levels of light. Retinal function was assessed by electroretinography and in vivo retinal morphology and inflammation was assessed by optical coherence tomography. Post mortem retina sections were assessed for morphology, TUNEL, and inflammatory markers Iba1 and GFAP. Retinal NAD+ levels were enzymatically assayed.

Results

Exposure to degeneration-inducing levels of light suppressed retinal NAD+ levels. Mice undergoing light-induced retinal degeneration exhibited significantly suppressed retinal function, severely disrupted photoreceptor cell layers, and increased apoptosis and inflammation in the outer retina. Treatment with NR increased levels of NAD+ in retina and prevented these deleterious outcomes.

Conclusions

This study is the first to report the protective effects of NR treatment in a mouse model of retinal degeneration. The positive outcomes, coupled with human tolerance to NR dosing, suggest that maintaining retinal NAD+ via systemic NR treatment should be further explored for clinical relevance.

Protective Effect of Astaxanthin on Blue Light Light-Emitting Diode-Induced Retinal Cell Damage via Free Radical Scavenging and Activation of PI3K/Akt/Nrf2 Pathway in 661W Cell Model

Light-emitting diodes (LEDs) are widely used and energy-efficient light sources in modern life that emit higher levels of short-wavelength blue light. Excessive blue light exposure may damage the photoreceptor cells in our eyes. Astaxanthin, a xanthophyll that is abundantly available in seafood, is a potent free radical scavenger and anti-inflammatory agent. We used a 661W photoreceptor cell line to investigate the protective effect of astaxanthin on blue light LED-induced retinal injury. The cells were treated with various concentrations of astaxanthin and then exposed to blue light LED. Our results showed that pretreatment with astaxanthin inhibited blue light LED-induced cell apoptosis and prevented cell death. Moreover, the protective effect was concentration dependent. Astaxanthin suppressed the production of reactive oxygen species and oxidative stress biomarkers and diminished mitochondrial damage induced by blue light exposure. Western blot analysis confirmed that astaxanthin activated the PI3K/Akt pathway, induced the nuclear translocation of Nrf2, and increased the expression of phase II antioxidant enzymes. The expression of antioxidant enzymes and the suppression of apoptosis-related proteins eventually protected the 661W cells against blue light LED-induced cell damage. Thus, our results demonstrated that astaxanthin exerted a dose-dependent protective effect on photoreceptor cells against damage mediated by blue light LED exposure.

Suprachoroidal Delivery of Viral and Nonviral Gene Therapy for Retinal Diseases

Retinal gene therapy is a rapidly growing field with numerous clinical trials underway, and route of delivery is a critical contributor to its success. Subretinal administration, which involves pars plana vitrectomy in the operating room, offers targeted delivery to retinal-pigment epithelium cells and photoreceptors. Due to the immune-privileged nature of the subretinal space, the risk of an immune reaction against viral capsid antigens is minimized, an advantage of subretinal administration in patients with preexisting neutralizing antibodies. Intravitreal administration, with fewer procedure-related complications, is challenged by potential immune response and incomplete vector penetration through the internal limiting membrane. However, novel vectors, optimized by "directed evolution" may address these issues. Nonsurgical in-office suprachoroidal gene delivery offers the potential for greater surface-area coverage of the posterior segment compared to focal subretinal injection, and is not hindered by the internal limiting membrane. However, the vector must pass through multiple layers to reach the targeted retinal layers, and there is a risk of immune response. This review highlights recent developments, challenges, and future opportunities associated with viral and nonviral suprachoroidal gene delivery for the treatment of chorioretinal diseases. While ocular tolerability and short-term effectiveness of suprachoroidal gene delivery have been demonstrated in preclinical models, durability of gene expression, long-term safety, potential systemic exposure, and effective delivery to the macula require further exploration. Although the safety and efficacy of suprachoroidal gene delivery are yet to be proven in clinical trials, further optimization could facilitate nonsurgical in-office suprachoroidal gene therapy.

Evaluation of antioxidant treatments for the modulation of macrophage function in the context of retinal degeneration

Purpose

Oxidative stress and macrophages have been implicated in the pathogenesis of atrophic and neovascular age-related macular degeneration (aAMD and nvAMD). It is unclear whether oxidative injury mediates macrophage involvement in AMD. We aimed to investigate the effect of antioxidant treatments on human monocyte-derived macrophages (hMDMs) from patients with AMD in models for the disease.

Methods

Four antioxidant treatments were evaluated (G1: lutein + zeaxanthin, G2: lutein + zeaxanthin and zinc, G3: lutein + zeaxanthin, zinc, Lyc-O-Mato, and carnosic acid, G4: lutein + zeaxanthin, carnosic acid, and beta-carotene, G5: olive oil as vehicle control). The compounds were added to the culture medium of M1 (interferon-gamma [IFN-Ɣ] and lipopolysaccharide [LPS]) and M2a (interleukin-13 [IL-13] and IL-4) hMDMs from patients with AMD (n=7 and n=8, respectively). Mouse choroidal tissue was cultured with supernatants from treated M1/M2a hMDMs, to evaluate the effect of treatments on the angiogenic properties of macrophages with choroidal sprouting assay (CSA). Mouse retinal explants were cultured with treated hMDMs for 18 h, and evaluated for photoreceptor apoptosis using terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) labeling. Adult BALB/c mice (n=8) were exposed to 8,000 lux bright light for 3 h, and treated orally with antioxidant supplements for 7 days that preceded light injury and following it. Oxidative stress was assessed using an anti-4 hydroxynonenal (4-HNE) antibody. Retinal function and the thickness of the outer nuclear layer were evaluated with electroretinography (ERG) and histological analysis, respectively.

Results

The G3 treatment reduced M2a hMDMs-associated sprouting in the CSA compared to the untreated group (n=7, -1.52-fold, p=0.05). Conversely, the G2 treatment was associated with an increased neurotoxic effect of M2a hMDMs in the retinal explant assay compared to the control group (n=7, 1.37-fold, p=0.047), as well as compared to the G3 treatment group (1.46-fold, p=0.01). The G4 treatment was also associated with increased cytotoxicity compared to the control group (1.48-fold, p=0.004), and compared to the G3 treatment group (1.58-fold, p=0.001). In the in vivo light damage model, mice (n=8) supplemented with G2, G3, and G4 had decreased levels of oxidative injury assessed using 4-HNE labeling (-2.32-fold, -2.17-fold, and -2.18-fold, respectively, p<0.05 for all comparisons). None of the treatments were associated with reduced photoreceptor cell loss, as shown with histology and ERG.

Conclusions

Antioxidant treatment modulates M2a hMDMs at the functional level. In particular, we found that the G3 combination has a beneficial effect on M2a macrophages in reducing their angiogenic and neurotoxic capacity ex vivo. In addition, antioxidant treatments considerably reduced the oxidative stress level in light-damaged retinas. Further research is required to assess whether such therapies may curb macrophage-driven photoreceptor loss and neovascularization in AMD.

Drug delivery to retinal photoreceptors

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Routes of administration to retinal photoreceptors.

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The blood–retinal barrier as a challenge for photoreceptor drug delivery.

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Review of nanoparticle drug delivery systems used for intraocular applications.

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Perspectives for topical drug delivery to the retina.

The photoreceptors of the retina are afflicted by diseases that still often lack satisfactory treatment options. Although suitable drugs might be available in some cases, the delivery of these compounds into the eye and across the blood–retinal barrier remains a significant challenge for therapy development. Here, we review the routes of drug administration to the retina and highlight different options for drug delivery to the photoreceptor cells.

Apoptosis inducing factor deficiency causes retinal photoreceptor degeneration. The protective role of the redox compound methylene blue

Dysfunction in mitochondrial oxidative phosphorylation (OXPHOS) underlies a wide spectrum of human ailments known as mitochondrial diseases. Deficiencies in complex I of the electron transport chain (ETC) contribute to 30–40% of all cases of mitochondrial diseases, and leads to eye disease including optic nerve atrophy and retinal degeneration. The mechanisms responsible for organ damage in mitochondrial defects may include energy deficit, oxidative stress, and an increase in the NADH/NAD⁺ redox ratio due to decreased NAD⁺ regeneration. Currently, there is no effective treatment to alleviate human disease induced by complex I defect.

Photoreceptor cells have the highest energy demand and dependence on OXPHOS for survival, and the lowest reserve capacity indicating that they are sensitive to OXPHOS defects. We investigated the effect of mitochondrial OXPHOS deficiency on retinal photoreceptors in a model of mitochondrial complex I defect (apoptosis inducing factor, AIF-deficient mice, Harlequin mice), and tested the protective effect of a mitochondrial redox compound (methylene blue, MB) on mitochondrial and photoreceptor integrity. MB prevented the reduction in the retinal thickness and protein markers for photoreceptor outer segments, Muller and ganglion cells, and altered mitochondrial integrity and function induced by AIF deficiency. In rotenone-induced complex I deficient 661 W cells (an immortalized mouse photoreceptor cell line) MB decreased the NADH/NAD⁺ ratio and oxidative stress without correcting the energy deficit, and improved cell survival. MB deactivated the mitochondrial stress response pathways, the unfolding protein response and mitophagy. In conclusion, preserving mitochondrial structure and function alleviates retinal photoreceptor degeneration in mitochondrial complex I defect.

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Mitochondrial complex I causes damage of the retinal photoreceptor cells and their outer segments.

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Methylene blue decreases the NADH/ NAD⁺ ratio and oxidative stress induced by complex I defect.

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Methylene blue deactivates the mitochondrial stress response pathways.

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Methylene blue maintains mitochondrial integrity and function.

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Methylene blue improves photoreceptor cell survival and outer segment integrity.

Fibroblast Growth Factor 21 Protects Photoreceptor Function in Type 1 Diabetic Mice

Retinal neuronal abnormalities occur before vascular changes in diabetic retinopathy. Accumulating experimental evidence suggests that neurons control vascular pathology in diabetic and other neovascular retinal diseases. Therefore, normalizing neuronal activity in diabetes may prevent vascular pathology. We investigated whether fibroblast growth factor 21 (FGF21) prevented retinal neuronal dysfunction in insulin-deficient diabetic mice. We found that in diabetic neural retina, photoreceptor rather than inner retinal function was most affected and administration of the long-acting FGF21 analog PF-05231023 restored the retinal neuronal functional deficits detected by electroretinography. PF-05231023 administration protected against diabetes-induced disorganization of photoreceptor segments seen in retinal cross section with immunohistochemistry and attenuated the reduction in the thickness of photoreceptor segments measured by optical coherence tomography. PF-05231023, independent of its downstream metabolic modulator adiponectin, reduced inflammatory marker interleukin-1β (IL-1β) mRNA levels. PF-05231023 activated the AKT-nuclear factor erythroid 2-related factor 2 pathway and reduced IL-1β expression in stressed photoreceptors. PF-05231023 administration did not change retinal expression of vascular endothelial growth factor A, suggesting a novel therapeutic approach for the prevention of early diabetic retinopathy by protecting photoreceptor function in diabetes.

Melatonin delays photoreceptor degeneration in a mouse model of autosomal recessive retinitis pigmentosa

Retinitis pigmentosa (RP) comprises a group of incurable inherited retinal degenerations. Targeting common processes, instead of mutation-specific treatment, has proven to be an innovative strategy to combat debilitating retinal degeneration. Growing evidence indicates that melatonin possesses a potent activity against neurodegenerative disorders by mitigating cell damage associated with apoptosis and inflammation. Given the pleiotropic role of melatonin in central nervous system, the aim of the present study was to investigate whether melatonin would afford protection against retinal degeneration in autosomal recessive RP (arRP). Rd10, a well-characterized murine model of human arRP, received daily intraperitoneal injection of melatonin (15 mg/kg) between postnatal day (P) 13 and P30. Retinas treated with melatonin or vehicle were harvested for analysis at P30 and P45, respectively. The findings showed that melatonin could dampen the photoreceptors death and delay consequent retinal degeneration. We also observed that melatonin weakened the expression of glial fibrillary acidic protein (GFAP) in Müller cells. Additionally, melatonin could alleviate retinal inflammatory response visualized by IBA1 staining, which was further corroborated by downregulation of inflammation-related genes, such as tumor necrosis factor alpha (Tnf-α), chemokine (C-C motif) ligand 2 (Ccl2), and chemokine (C-X-C motif) ligand 10 (Cxcl10). These data revealed that melatonin could ameliorate retinal degeneration through potentially attenuating apoptosis, reactive gliosis, and microglial activation in rd10 mice. Moreover, these results suggest melatonin as a promising agent improving photoreceptors survival in human RP.

A colour preference technique to evaluate acrylamide-induced toxicity in zebrafish

The zebrafish has become a commonly used vertebrate model for toxicity assessment, of particular relevance to the study of toxic effects on the visual system because of the structural similarities shared by zebrafish and human retinae. In this article we present a colour preference-based technique that, by assessing the functionality of photoreceptors, can be used to evaluate the effects of toxicity on behaviour. A digital camera was used to record the locomotor behaviour of individual zebrafish swimming in a water tank consisting of two compartments separated by an opaque perforated wall through which the fish could pass. The colour of the lighting in each compartment could be altered independently (producing distinct but connected environments of white, red or blue) to allow association of the zebrafish's swimming behaviour with its colour preference. The functionality of the photoreceptors was evaluated based on the ability of the zebrafish to sense the different colours and to swim between the compartments. The zebrafish tracking was carried out using our algorithm developed with MATLAB. We found that zebrafish preferred blue illumination to white, and white illumination to red. Acute treatment with acrylamide (2mM for 36h) resulted in a marked reduction in locomotion and a concomitant loss of colour-preferential swimming behaviour. Histopathological examination of acrylamide-treated zebrafish eyes showed that acrylamide exposure had caused retinal damage. The colour preference tracking technique has applications in the assessment of neurodegenerative disorders, as a method for preclinical appraisal of drug efficacy and for behavioural evaluation of toxicity.

Protective effects on the retina after ranibizumab treatment in an ischemia model

Retinal ischemia is common in eye disorders, like diabetic retinopathy or retinal vascular occlusion. The goal of this study was to evaluate the potential protective effects of an intravitreally injected vascular endothelial growth factor (VEGF) inhibitor (ranibizumab) on retinal cells in an ischemia animal model via immunohistochemistry (IF) and quantitative real-time PCR (PCR). A positive binding of ranibizumab to rat VEGF-A was confirmed via dot blot. One eye underwent ischemia and a subgroup received ranibizumab. A significant VEGF increase was detected in aqueous humor of ischemic eyes (p = 0.032), whereas VEGF levels were low in ranibizumab eyes (p = 0.99). Ischemic retinas showed a significantly lower retinal ganglion cell number (RGC; IF Brn-3a: p<0.001, IF RBPMS: p<0.001; PCR: p = 0.002). The ranibizumab group displayed fewer RGCs (IF Brn-3a: 0.3, IF RBPMS: p<0.001; PCR: p = 0.007), but more than the ischemia group (IF Brn-3a: p = 0.04, IF RBPMS: p = 0.03). Photoreceptor area was decreased after ischemia (IF: p = 0.049; PCR: p = 0.511), while the ranibizumab group (IF: p = 0.947; PCR: p = 0.122) was comparable to controls. In the ischemia (p<0.001) and ranibizumab group (p<0.001) a decrease of ChAT⁺ amacrine cells was found, which was less prominent in the ranibizumab group. VEGF-receptor 2 (VEGF-R2; IF: p<0.001; PCR: p = 0.021) and macroglia (GFAP; IF: p<0.001; PCR: p<0.001) activation was present in ischemic retinas. The activation was weaker in ranibizumab retinas (VEGF-R2: IF: p = 0.1; PCR: p = 0.03; GFAP: IF: p = 0.1; PCR: p = 0.015). An increase in the number of total (IF: p = 0.003; PCR: p = 0.023) and activated microglia (IF: p<0.001; PCR: p = 0.009) was detected after ischemia. These levels were higher in the ranibizumab group (Iba1: IF: p<0.001; PCR: p = 0.018; CD68: IF: p<0.001; PCR: p = 0.004). Our findings demonstrate that photoreceptors and RGCs are protected by ranibizumab treatment. Only amacrine cells cannot be rescued. They seem to be particularly sensitive to ischemic damage and need maybe an earlier intervention.

A Novel Neuroprotective Small Molecule for Glial Cell Derived Neurotrophic Factor Induction and Photoreceptor Rescue

PURPOSE

Degenerative diseases of the retina, such as retinitis pigmentosa and age-related macular degeneration, are characterized by the irreversible loss of photoreceptors. Several growth factors, including glial cell derived neurotrophic factor (GDNF), have been shown to rescue retinal neurons. An alternative strategy to direct GDNF administration is its induction in host retina by small molecules. Here we studied the ability of a novel small molecule GSK812 to induce GDNF in vitro/in vivo and rescue photoreceptors.

METHODS

GDNF induction in vitro was assessed in human ARPE-19, human retinal progenitor cells (RPCs) and mouse pluripotent cell-derived eyecups. For time course pharmacokinetic and GDNF induction studies in C57Bl/6 mice, GSK812 sustained release formulation was injected intravitreally. The same delivery approach was used in the rhodopsin knockout mice and Royal College of Surgeon (RCS) rats to assess long-term GDNF induction and photoreceptor rescue.

RESULTS

The suspension provided sustained GSK812 delivery with 28 μg of drug remaining in the eye 2 weeks after a single injection. GSK812 suspension injection in C57Bl/6 mice resulted in significant upregulation of GDNF mRNA (>1.8-fold) and protein levels (>2.8-fold). Importantly, GSK812 treatment resulted in outer nuclear layer preservation in rho-/- mice with a 2-fold difference in photoreceptor number. In the RCS rat, the GSK812 injection provided long-term rescue of photoreceptors and outer segments, accompanied by function preservation as well.

CONCLUSIONS

GSK812 is a potent neuroprotectant that can induce GDNF in normal and diseased retina. This induction results in photoreceptor rescue in 2 models of retinal degeneration.

Controlled delivery of tauroursodeoxycholic acid from biodegradable microspheres slows retinal degeneration and vision loss in P23H rats

Successful drug therapies for treating ocular diseases require effective concentrations of neuroprotective compounds maintained over time at the site of action. The purpose of this work was to assess the efficacy of intravitreal controlled delivery of tauroursodeoxycholic acid (TUDCA) encapsulated in poly(D,L-lactic-co-glycolic acid) (PLGA) microspheres for the treatment of the retina in a rat model of retinitis pigmentosa. PLGA microspheres (MSs) containing TUDCA were produced by the O/W emulsion-solvent evaporation technique. Particle size and morphology were assessed by light scattering and scanning electronic microscopy, respectively. Homozygous P23H line 3 rats received a treatment of intravitreal injections of TUDCA-PLGA MSs. Retinal function was assessed by electroretinography at P30, P60, P90 and P120. The density, structure and synaptic contacts of retinal neurons were analyzed using immunofluorescence and confocal microscopy at P90 and P120. TUDCA-loaded PLGA MSs were spherical, with a smooth surface. The production yield was 78%, the MSs mean particle size was 23 μm and the drug loading resulted 12.5 ± 0.8 μg TUDCA/mg MSs. MSs were able to deliver the loaded active compound in a gradual and progressive manner over the 28-day in vitro release study. Scotopic electroretinografic responses showed increased ERG a- and b-wave amplitudes in TUDCA-PLGA-MSs-treated eyes as compared to those injected with unloaded PLGA particles. TUDCA-PLGA-MSs-treated eyes showed more photoreceptor rows than controls. The synaptic contacts of photoreceptors with bipolar and horizontal cells were also preserved in P23H rats treated with TUDCA-PLGA MSs. This work indicates that the slow and continuous delivery of TUDCA from PLGA-MSs has potential neuroprotective effects that could constitute a suitable therapy to prevent neurodegeneration and visual loss in retinitis pigmentosa.