Fundus Camera-Delivered Light-Induced Retinal Degeneration in Mice With the RPE65 Leu450Met Variant is Associated With Oxidative Stress and Apoptosis

An overview of retinal light damage models for preclinical studies on age-related macular degeneration: identifying molecular hallmarks and therapeutic targets

Age-related macular degeneration (AMD) is a complex, multifactorial disease leading to progressive and irreversible retinal degeneration, whose pathogenesis has not been fully elucidated yet. Due to the complexity and to the multiple features of the disease, many efforts have been made to develop animal models which faithfully reproduce the overall AMD hallmarks or that are able to mimic the different AMD stages. In this context, light damage (LD) rodent models of AMD represent a suitable and reliable approach to mimic the different AMD forms (dry, wet and geographic atrophy) while maintaining the time-dependent progression of the disease. In this review, we comprehensively reported how the LD paradigms reproduce the main features of human AMD. We discuss the capability of these models to broaden the knowledge in AMD research, with a focus on the mechanisms and the molecular hallmarks underlying the pathogenesis of the disease. We also critically revise the remaining challenges and future directions for the use of LD models.

40 Hz light flickering promotes sleep through cortical adenosine signaling

Flickering light stimulation has emerged as a promising non-invasive neuromodulation strategy to alleviate neuropsychiatric disorders. However, the lack of a neurochemical underpinning has hampered its therapeutic development. Here, we demonstrate that light flickering triggered an immediate and sustained increase (up to 3 h after flickering) in extracellular adenosine levels in the primary visual cortex (V1) and other brain regions, as a function of light frequency and intensity, with maximal effects observed at 40 Hz frequency and 4000 lux. We uncovered cortical (glutamatergic and GABAergic) neurons, rather than astrocytes, as the cellular source, the intracellular adenosine generation from AMPK-associated energy metabolism pathways (but not SAM-transmethylation or salvage purine pathways), and adenosine efflux mediated by equilibrative nucleoside transporter-2 (ENT2) as the molecular pathway responsible for extracellular adenosine generation. Importantly, 40 Hz (but not 20 and 80 Hz) light flickering for 30 min enhanced non-rapid eye movement (non-REM) and REM sleep for 2-3 h in mice. This somnogenic effect was abolished by ablation of V1 (but not superior colliculus) neurons and by genetic deletion of the gene encoding ENT2 (but not ENT1), but recaptured by chemogenetic inhibition of V1 neurons and by focal infusion of adenosine into V1 in a dose-dependent manner. Lastly, 40 Hz light flickering for 30 min also promoted sleep in children with insomnia by decreasing sleep onset latency, increasing total sleep time, and reducing waking after sleep onset. Collectively, our findings establish the ENT2-mediated adenosine signaling in V1 as the neurochemical basis for 40 Hz flickering-induced sleep and unravel a novel and non-invasive treatment for insomnia, a condition that affects 20% of the world population.

E3 ubiquitin ligase Herc3 deficiency leads to accumulation of subretinal microglia and retinal neurodegeneration

Activated microglia have been implicated in the pathogenesis of age-related macular degeneration (AMD), diabetic retinopathy, and other neurodegenerative and neuroinflammatory disorders, but our understanding of the mechanisms behind their activation is in infant stages. With the goal of identifying novel genes associated with microglial activation in the retina, we applied a semiquantitative fundus spot scoring scale to an unbiased, state-of-the-science mouse forward genetics pipeline. A mutation in the gene encoding the E3 ubiquitin ligase Herc3 led to prominent accumulation of fundus spots. CRISPR mutagenesis was used to generate Herc3-/- mice, which developed prominent accumulation of fundus spots and corresponding activated Iba1 + /CD16 + subretinal microglia, retinal thinning on OCT and histology, and functional deficits by Optomotory and electrophysiology. Bulk RNA sequencing identified activation of inflammatory pathways and differentially expressed genes involved in the modulation of microglial activation. Thus, despite the known expression of multiple E3 ubiquitin ligases in the retina, we identified a non-redundant role for Herc3 in retinal homeostasis. Our findings are significant given that a dysregulated ubiquitin-proteasome system (UPS) is important in prevalent retinal diseases, in which activated microglia appear to play a role. This association between Herc3 deficiency, retinal microglial activation and retinal degeneration merits further study.

Age and light damage influence Fzd5 regulation of ocular growth-related genes

Genetic and environmental factors can independently or coordinatively drive ocular axis growth. Mutations in FRIZZLED5 (FZD5) have been associated with microphthalmia, coloboma, and, more recently, high myopia. The molecular mechanism of how Fzd5 participates in ocular growth remains unknown. In this study, we compiled a list of human genes associated with ocular growth abnormalities based on public databases and a literature search. We identified a set of ocular growth-related genes from the list that was altered in the Fzd5 mutant mice by RNAseq analysis at different time points. The Fzd5 regulation of this set of genes appeared to be impacted by age and light damage. Further bioinformatical analysis indicated that these genes are extracellular matrix (ECM)-related; and meanwhile an altered Wnt signaling was detected. Altogether, the data suggest that Fzd5 may regulate ocular growth through regulating ECM remodeling, hinting at a genetic-environmental interaction in gene regulation of ocular axis control.

Single Cell RNA Sequencing Analysis of Mouse Retina Identifies a Subpopulation of Muller Glia Involved in Retinal Recovery From Injury in the FCD-LIRD Model

Purpose

Although retinal light injury models have been useful in understanding aspects of retinal degeneration and retinal oxidative stress, information on retinal recovery from oxidative/photoinflammatory retinal injury is scarce. The fundus camera-delivered light-induced retinal degeneration model is a simple and reproducible retinal light injury model developed to recapitulate not only the retinal degeneration aspect, but also the retinal recovery from injury. In this study, we used the fundus camera-delivered light-induced retinal degeneration model to perform cell type-specific analyses of the acute and subacute retinal responses to light injury.

Methods

C57BL/6J eyes were collected before or after light injury (4 hours, 48 hours, and day 5). Retina samples were processed into single-cell suspensions. Droplet-based encapsulation of single cells was performed to generate libraries for sequencing.

Results

Gene expression analysis generated 23 clusters encompassing all known major retinal cell populations. Using unbiased analyses, we identified genes and pathways that were significantly altered in each cell type after light injury, including some cellular processes suggestive of activation of pathways for retinal recovery (e.g., synaptogenesis signaling, ephrin receptor signaling, and Reelin signaling in neurons). More importantly, our data show that a subpopulation of Muller glia cells may play an important role in the cellular recovery process.

Conclusions

This work identifies acute and subacute cell type-specific responses to retinal photo-oxidative injury. A subpopulation of Muller glia seems to initiate the cellular recovery process. A better understanding of these responses may be helpful in identifying therapeutic approaches to minimize retinal damage and maximize recovery after exposure to injury.

Antibody blockade of Jagged1 attenuates choroidal neovascularization

Antibody-based blocking of vascular endothelial growth factor (VEGF) reduces choroidal neovascularization (CNV) and retinal edema, rescuing vision in patients with neovascular age-related macular degeneration (nAMD). However, poor response and resistance to anti-VEGF treatment occurs. We report that targeting the Notch ligand Jagged1 by a monoclonal antibody reduces neovascular lesion size, number of activated phagocytes and inflammatory markers and vascular leakage in an experimental CNV mouse model. Additionally, we demonstrate that Jagged1 is expressed in mouse and human eyes, and that Jagged1 expression is independent of VEGF signaling in human endothelial cells. When anti-Jagged1 was combined with anti-VEGF in mice, the decrease in lesion size exceeded that of either antibody alone. The therapeutic effect was solely dependent on blocking, as engineering antibodies to abolish effector functions did not impair the therapeutic effect. Targeting of Jagged1 alone or in combination with anti-VEGF may thus be an attractive strategy to attenuate CNV-bearing diseases.

Forward genetic screening using fundus spot scale identifies an essential role for Lipe in murine retinal homeostasis

Microglia play a role in the pathogenesis of many retinal diseases. Fundus spots in mice often correlate with the accumulation of activated subretinal microglia. Here we use a semiquantitative fundus spot scoring scale in combination with an unbiased, state-of-the-science forward genetics pipeline to identify causative associations between chemically induced mutations and fundus spot phenotypes. Among several associations, we focus on a missense mutation in Lipe linked to an increase in yellow fundus spots in C57BL/6J mice. Lipe^-/- mice generated using CRISPR-Cas9 technology are found to develop accumulation of subretinal microglia, a retinal degeneration with decreased visual function, and an abnormal retinal lipid profile. We establish an indispensable role of Lipe in retinal/RPE lipid homeostasis and retinal health. Further studies using this new model will be aimed at determining how lipid dysregulation results in the activation of subretinal microglia and whether these microglia also play a role in the subsequent retinal degeneration.

Dysfunctional Autophagy, Proteostasis, and Mitochondria as a Prelude to Age-Related Macular Degeneration

Retinal pigment epithelial (RPE) cell dysfunction is a key driving force of AMD. RPE cells form a metabolic interface between photoreceptors and choriocapillaris, performing essential functions for retinal homeostasis. Through their multiple functions, RPE cells are constantly exposed to oxidative stress, which leads to the accumulation of damaged proteins, lipids, nucleic acids, and cellular organelles, including mitochondria. As miniature chemical engines of the cell, self-replicating mitochondria are heavily implicated in the aging process through a variety of mechanisms. In the eye, mitochondrial dysfunction is strongly associated with several diseases, including age-related macular degeneration (AMD), which is a leading cause of irreversible vision loss in millions of people globally. Aged mitochondria exhibit decreased rates of oxidative phosphorylation, increased reactive oxygen species (ROS) generation, and increased numbers of mitochondrial DNA mutations. Mitochondrial bioenergetics and autophagy decline during aging because of insufficient free radical scavenger systems, the impairment of DNA repair mechanisms, and reductions in mitochondrial turnover. Recent research has uncovered a much more complex role of mitochondrial function and cytosolic protein translation and proteostasis in AMD pathogenesis. The coupling of autophagy and mitochondrial apoptosis modulates the proteostasis and aging processes. This review aims to summarise and provide a perspective on (i) the current evidence of autophagy, proteostasis, and mitochondrial dysfunction in dry AMD; (ii) current in vitro and in vivo disease models relevant to assessing mitochondrial dysfunction in AMD, and their utility in drug screening; and (iii) ongoing clinical trials targeting mitochondrial dysfunction for AMD therapeutics.

Noninvasive Ophthalmic Imaging Measures Retinal Degeneration and Vision Deficits in Ndufs4-/- Mouse Model of Mitochondrial Complex I Deficiency

Purpose

To characterize postnatal ocular pathology in a Ndufs4^−/− mouse model of complex I deficiency using noninvasive retinal imaging and visual testing.

Methods

Ndufs4^−/− mice and wild-type (WT) littermates were analyzed at 3, 5, and 7 weeks postnatal. Retinal morphology was visualized by optical coherence tomography (OCT). OCT images were analyzed for changes in retinal thickness and reflectivity profiles. Visual function was assessed by electroretinogram (ERG) and optomotor reflex (OMR).

Results

Ndufs4^−/− animals have normal OCT morphology at weaning and develop inner plexiform layer atrophy over weeks 5 to 7. Outer retinal layers show hyporeflectivity of the external limiting membrane (ELM) and photoreceptor ellipsoid zone (EZ). Retinal function is impaired at 3 weeks, with profound deficits in b-wave, a-wave, and oscillatory potential amplitudes. The b-wave and oscillatory potential implicit times are delayed, but the a-wave implicit time is unaffected. Ndufs4^−/− animals have normal OMR at 3 weeks and present with increasing acuity and contrast OMR deficits at 5 and 7 weeks. Physiological thinning of inner retinal layers, attenuation of ELM reflectivity, and attenuation of ERG b- and a-wave amplitudes occur in WT C57BL/6 littermates between weeks 3 and 7.

Conclusions

Noninvasive ocular imaging captures early-onset retinal degeneration in Ndufs4^−/− mice and is a tractable approach for investigating retinal pathology subsequent to complex I deficiency.

Translational Relevance

Ophthalmic imaging captures clinically relevant measures of retinal disease in a fast-progressing mouse model of complex I deficiency consistent with human Leigh syndrome.

Utility of the DHFR-based destabilizing domain across mouse models of retinal degeneration and aging

The Escherichia coli dihydrofolate reductase (DHFR) destabilizing domain (DD) serves as a promising approach to conditionally regulate protein abundance in a variety of tissues. To test whether this approach could be effectively applied to a wide variety of aged and disease-related ocular mouse models, we evaluated the DHFR DD system in the eyes of aged mice (up to 24 months), a light-induced retinal degeneration (LIRD) model, and two genetic models of retinal degeneration (rd2 and Abca4^−/− mice). The DHFR DD was effectively degraded in all model systems, including rd2 mice, which showed significant defects in chymotrypsin proteasomal activity. Moreover, trimethoprim (TMP) administration stabilized the DHFR DD in all mouse models. Thus, the DHFR DD-based approach allows for control of protein abundance in a variety of mouse models, laying the foundation to use this strategy for the conditional control of gene therapies to potentially treat multiple eye diseases.

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Destabilizing domains (DDs) confer conditional control of ocular protein abundance

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The DHFR DD is effectively turned over and stabilized in aged mouse’s retina

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DHFR DDs perform well in environmental and genetic retinal degenerative models

Quantitative Assessment of Experimental Ocular Inflammatory Disease

Ocular inflammation imposes a high medical burden on patients and substantial costs on the health-care systems that mange these often chronic and debilitating diseases. Many clinical phenotypes are recognized and classifying the severity of inflammation in an eye with uveitis is an ongoing challenge. With the widespread application of optical coherence tomography in the clinic has come the impetus for more robust methods to compare disease between different patients and different treatment centers. Models can recapitulate many of the features seen in the clinic, but until recently the quality of imaging available has lagged that applied in humans. In the model experimental autoimmune uveitis (EAU), we highlight three linked clinical states that produce retinal vulnerability to inflammation, all different from healthy tissue, but distinct from each other. Deploying longitudinal, multimodal imaging approaches can be coupled to analysis in the tissue of changes in architecture, cell content and function. This can enrich our understanding of pathology, increase the sensitivity with which the impacts of therapeutic interventions are assessed and address questions of tissue regeneration and repair. Modern image processing, including the application of artificial intelligence, in the context of such models of disease can lay a foundation for new approaches to monitoring tissue health.

Forward genetic analysis using OCT screening identifies Sfxn3 mutations leading to progressive outer retinal degeneration in mice

Retinal disease and loss of vision can result from any disruption of the complex pathways controlling retinal development and homeostasis. Forward genetics provides an excellent tool to find, in an unbiased manner, genes that are essential to these processes. Using N-ethyl-N-nitrosourea mutagenesis in mice in combination with a screening protocol using optical coherence tomography (OCT) and automated meiotic mapping, we identified 11 mutations presumably causative of retinal phenotypes in genes previously known to be essential for retinal integrity. In addition, we found multiple statistically significant gene-phenotype associations that have not been reported previously and decided to target one of these genes, Sfxn3 (encoding sideroflexin-3), using CRISPR/Cas9 technology. We demonstrate, using OCT, light microscopy, and electroretinography, that two Sfxn3 ^-/- mouse lines developed progressive and severe outer retinal degeneration. Electron microscopy showed thinning of the retinal pigment epithelium and disruption of the external limiting membrane. Using single-cell RNA sequencing of retinal cells isolated from C57BL/6J mice, we demonstrate that Sfxn3 is expressed in several bipolar cell subtypes, retinal ganglion cells, and some amacrine cell subtypes but not significantly in Müller cells or photoreceptors. In situ hybridization confirmed these findings. Furthermore, pathway analysis suggests that Sfxn3 may be associated with synaptic homeostasis. Importantly, electron microscopy analysis showed disruption of synapses and synaptic ribbons in the outer plexiform layer of Sfxn3 ^-/- mice. Our work describes a previously unknown requirement for Sfxn3 in retinal function.

Central Role of Oxidative Stress in Age-Related Macular Degeneration: Evidence from a Review of the Molecular Mechanisms and Animal Models

Age-related macular degeneration (AMD) is a common cause of visual impairment in the elderly. There are very limited therapeutic options for AMD with the predominant therapies targeting vascular endothelial growth factor (VEGF) in the retina of patients afflicted with wet AMD. Hence, it is important to remind readers, especially those interested in AMD, about current studies that may help to develop novel therapies for other stages of AMD. This study, therefore, provides a comprehensive review of studies on human specimens as well as rodent models of the disease, to identify and analyze the molecular mechanisms behind AMD development and progression. The evaluation of this information highlights the central role that oxidative damage in the retina plays in contributing to major pathways, including inflammation and angiogenesis, found in the AMD phenotype. Following on the debate of oxidative stress as the earliest injury in the AMD pathogenesis, we demonstrated how the targeting of oxidative stress-associated pathways, such as autophagy and nuclear factor erythroid 2-related factor 2 (Nrf2) signaling, might be the futuristic direction to explore in the search of an effective treatment for AMD, as the dysregulation of these mechanisms is crucial to oxidative injury in the retina. In addition, animal models of AMD have been discussed in great detail, with their strengths and pitfalls included, to assist inform in the selection of suitable models for investigating any of the molecular mechanisms.

Mice With a Combined Deficiency of Superoxide Dismutase 1 (Sod1), DJ-1 (Park7), and Parkin (Prkn) Develop Spontaneous Retinal Degeneration With Aging

Purpose

Chronic oxidative stress is an important mechanism of disease in aging disorders. We do not have a good model to recapitulate AMD and other retinal disorders in which chronic oxidative stress plays an important role. We hypothesized that mice with a combined deficiency in superoxide dismutase 1 (Sod1), DJ-1 (Park-7), and Parkin (Prkn) (triple knock out, TKO) would have an increased level of chronic oxidative stress in the retina, with anatomic and functional consequences just with aging.

Methods

Eyes of TKO and B6J control mice were (1) monitored with optical coherence tomography (OCT) and electroretinography (ERG) over time, and (2) collected for oxidative marker protein analysis by ELISA or immunohistochemistry and for transmission electron microscopy studies.

Results

TKO mice developed qualitative disruptions in outer retinal layers in OCT by 3 months, increased accumulation of fundus spots and subretinal microglia by 6 months of age, significant retinal thinning by 9 months, and decreased ERG signal by 12 months. Furthermore, we found increased accumulation of the oxidative marker malondialdehyde (MDA) in the retina and increased basal laminal deposits (BLD) and mitochondria number and size in the retinal pigment epithelium of aging TKO mice.

Conclusions

TKO mice can serve as a platform to study retinal diseases that involve chronic oxidative stress, including macular degeneration, retinal detachment, and ischemic retinopathies. In order to model each of these diseases, additional disease-specific catalysts or triggers could be superimposed onto the TKO mice. Such studies could provide better insight into disease mechanisms and perhaps lead to new therapeutic approaches.

A Mouse Model of Retinal Recovery From Photo-Oxidative/Photo-Inflammatory Injury: Nrf2, SOD1, DJ-1, and Parkin Are Not Essential to Recovery

Purpose

To determine if there is structural and functional recovery of the retina from light induced retinal degeneration, and to evaluate the role of the oxidative stress response elements Nrf2, SOD1, DJ-1, and Parkin in such a recovery process.

Methods

Eyes from C57BL/6J (B6J) mice and from oxidative stress response-deficient strains of mice were treated with intense light using the fundus camera-delivered light-induced retinal degeneration (FCD-LIRD) model. Fundus photographs, optical coherence tomography (OCT) images, and electroretinography (ERG) responses were obtained before the injury, during the "maximal injury phase" (days 4-7) and during the "recovery phase" (days 14-16) post light exposure and were evaluated for retinal damage and assessed for evidence of recovery from the injury.

Results

We demonstrate that mice treated with a sub-lethal FCD-LIRD protocol show an initial acute retina injury phase peaking between days 4 to 7 followed by a recovery phase in which the outer retinal thickness/volume and retinal function partially recover. These observations are reproduced in B6J mice and in mice lacking oxidative stress response enzymes (SOD1, DJ-1, and Parkin) or the oxidative stress response master regulator Nrf2.

Conclusions

Our data indicate that retinal recovery from injury can proceed via pathways that are independent from the common oxidative stress response elements Nrf2, SOD1, DJ-1, and Parkin. Furthermore, the model of retinal recovery from injury that we describe here mimics changes seen in a variety of clinical entities and may provide an excellent platform for dissecting general pathways of retinal recovery from sub-lethal injury.

Increased susceptibility to fundus camera-delivered light-induced retinal degeneration in mice deficient in oxidative stress response proteins

Oxidative stress is an important contributor to the pathogenesis of many retinal diseases including age-related macular degeneration and retinal dystrophies. Light-induced retinal degeneration (LIRD) can serve as a model in which to study the response of the retina to stress. Of note, many genetic mutant mice are in a C57BL/6 J background and are thus resistant to the usual LIRD models. We recently developed a new model of fundus camera-delivered light-induced retinal degeneration (FCD-LIRD) which is effective in strains of mice expressing the light-resistant variant of RPE65 (450Met), including C57BL/6 J. In this work we investigated whether FCD-LIRD would be useful as a model in which to test the effect of genetic mutations on the response of the retina to stress. Furthermore, we tested whether oxidative stress plays an important role in the setting of this new FCD-LIRD model. FCD-LIRD was applied to C57BL/6 J mice and to mice simultaneously deficient in three proteins that are important in the response of the retina to oxidative stress (SOD1, DJ-1 and Parkin). Using fundus photography, we found that retinal damage was dramatically increased in the SOD1/DJ-1/Parkin deficient mice compared to C57BL/6 J. Outer retinal OCT volume and RPE cell morphology analysis in ZO-1-stained flat mounts added support to these findings. Gene expression analysis confirmed a strong oxidative stress response after FCD-LIRD, which was differentially altered in the SOD1/DJ1/Parkin deficient mice. We conclude that FCD-LIRD is useful to study the effect of genetic mutations on the response of the retina to light stress in light-resistant strains of mice. Furthermore, oxidative stress seems to be an important component of FCD-LIRD. Finally, we have established protocols to quantify the effect of FCD-LIRD on the retina and RPE which will be useful for future studies. Further dissection of the mechanisms by which the retina responds to light-induced oxidative stress may result in new strategies to modulate this response, which could lead to a reduction in retinal and RPE damage.