Systemic knockout of Tspo in mice does not affect retinal morphology, function and susceptibility to degeneration.

Translocator protein (18 kDa) (TSPO) is a mitochondrial protein expressed by reactive microglia and astrocytes at the site of neuronal injury. Although TSPO function has not been fully determined, synthetic TSPO ligands have beneficial effects on different pathologies of the central nervous system, including the retina. Here, we studied the pattern of Tspo expression in the aging human retina and in two mouse models of retinal degeneration. Using a newly generated Tspo-KO mouse, we investigated the impact of the lack of TSPO on retinal morphology, function and susceptibility to degeneration. We show that TSPO was expressed in both human and mouse retina and retinal pigment epithelium (RPE). Tspo was induced in the mouse retina upon degeneration, but constitutively expressed in the RPE. Similarly, TSPO expression levels in healthy human retina and RPE were not differentially regulated during aging. Tspo-KO mice had normal retinal morphology and function up to 48 weeks of age. Photoreceptor loss caused either by exposure to excessive light levels or by a mutation in the phosphodiesterase 6b gene was not affected by the absence of Tspo. The reactivity states of retinal mononuclear phagocytes following light-damage were comparable in Tspo-KO and control mice. Our data suggest that lack of endogenous TSPO does not directly influence the magnitude of photoreceptor degeneration or microglia activation in these two models of retinal degeneration. We therefore hypothesize that the interaction of TSPO with its ligands may be required to modulate disease progression.

A chronic hypoxic response in photoreceptors alters the vitreous proteome in mice.

Reduced oxygenation of the outer retina in the aging eye may activate a chronic hypoxic response in RPE and photoreceptor cells and is considered as a risk factor for the development of age-related macular degeneration (AMD). In mice, a chronically active hypoxic response in the retinal pigment epithelium (RPE) or photoreceptors leads to age-dependent retinal degeneration. To identify proteins that may serve as accessible markers for a chronic hypoxic insult to photoreceptors, we used proteomics to determine the protein composition of the vitreous humor in genetically engineered mice that lack the von Hippel-Lindau tumor suppressor (Vhl) specifically in rods (rodΔVhl) or cones (all-coneΔVhl). Absence of VHL leads to constitutively active hypoxia-inducible transcription factors (HIFs) and thus to a molecular response to hypoxia even in normal room air. To discriminate between the consequences of a local response in photoreceptors and systemic hypoxic effects, we also evaluated the vitreous proteome of wild type mice after exposure to acute hypoxia. 1'043 of the identified proteins were common to all three hypoxia models. 257, 258 and 356 proteins were significantly regulated after systemic hypoxia, in rodΔVhl and in all-coneΔVhl mice, respectively, at least at one of the analyzed time points. Only few of the regulated proteins were shared by the models indicating that the vitreous proteome is differentially affected by systemic hypoxia and the rod or cone-specific hypoxic response. Similarly, the distinct protein compositions in the individual genetic models at early and late time points suggest regulated, cell-specific and time-dependent processes. Among the proteins commonly regulated in the genetic models, guanylate binding protein 2 (GBP2) showed elevated levels in the vitreous that were accompanied by increased mRNA expression in the retina of both rodΔVhl and all-coneΔVhl mice. We hypothesize that some of the differentially regulated proteins at early time points may potentially be used as markers for the detection of a chronic hypoxic response of photoreceptors.

Light stress affects cones and horizontal cells via rhodopsin-mediated mechanisms.

Retinal degenerations are a major cause of blindness in human patients. The identification of endogenous mechanisms involved in neurodegeneration or neuroprotection helps to understand the response of the retina to stress and provides essential information not only for basic retinal physiology but also for defining molecular targets for neuroprotective strategies. Here we used excessive light exposure as a model system to study mechanisms of photoreceptor degeneration in mice. Using one wild type and four genetically modified mouse strains, we demonstrate that light exposure resulted not only in the degeneration of rods but also in an early but transient repression of several cone-specific genes, in a reversible hyperreflectivity of the outer retina including the outer plexiform layer, and in the loss of horizontal cells. The effects on cones, horizontal cells and the inner retina depended on light absorption by rhodopsin and, at least partially, on leukemia inhibitory factor. This demonstrates the existence of intercellular communication routes that transduce rod stress to other cells, likely to provide support for photoreceptors and increase cell survival in the injured retina.

Regulated efflux of photoreceptor outer segment-derived cholesterol by human RPE cells.

Genetic studies have linked age-related macular degeneration (AMD) to genes involved in high-density lipoprotein (HDL) metabolism, including ATP-binding cassette transporter A1 (ABCA1). The retinal pigment epithelium (RPE) handles large amounts of lipids, among others cholesterol, partially derived from internalized photoreceptor outer segments (OS) and lipids physiologically accumulate in the aging eye. To analyze the potential function of ABCA1 in the eye, we measured cholesterol efflux, the first step of HDL generation, in RPE cells. We show the expression of selected genes related to HDL metabolism in mouse and human eyecups as well as in ARPE-19 and human primary RPE cells. Immunofluorescence staining revealed localization of ABCA1 on both sides of polarized RPE cells. This was functionally confirmed by directional efflux to apolipoprotein AI (ApoA-I) of 3H-labeled cholesterol given to the cells via serum or via OS. ABCA1 expression and activity was modulated using a liver-X-receptor (LXR) agonist and an ABCA1 neutralizing antibody, demonstrating that the efflux was ABCA1-dependent. We concluded that the ABCA1-mediated lipid efflux pathway, and hence HDL biosynthesis, is functional in RPE cells towards both the basal (choroidal) and apical (subretinal) space. Impaired activity of the pathway might cause age-related perturbations of lipid homeostasis in the outer retina and thus may contribute to disease development and/or progression.

The Expression of Decidual Protein Induced by Progesterone (DEPP) is Controlled by Three Distal Consensus Hypoxia Responsive Element (HRE) in Hypoxic Retinal Epithelial Cells.

Hypoxia affects the development and/or progression of several retinopathies. Decidual protein induced by progesterone (DEPP) has been identified as a hypoxia-responsive gene that may be part of cellular pathways such as autophagy and connected to retinal diseases. To increase our understanding of DEPP regulation in the eye, we defined its expression pattern in mouse and human retina and retinal pigment epithelium (RPE). Interestingly, DEPP expression was increased in an age-dependent way in the central human RPE. We showed that DEPP was regulated by hypoxia in the mouse retina and eyecup and that this regulation was controlled by hypoxia-inducible transcription factors 1 and 2 (HIF1 and HIF2). Furthermore, we identified three hypoxia response elements (HREs) about 3.5 kb proximal to the transcriptional start site that were responsible for hypoxic induction of DEPP in a human RPE cell line. Comparative genomics analysis suggested that one of the three HREs resides in a highly conserved genomic region. Collectively, we defined the molecular elements controlling hypoxic induction of DEPP in an RPE cell line, and provided evidence for an enrichment of DEPP in the aged RPE of human donors. This makes DEPP an interesting gene to study with respect to aging and age-related retinal pathologies.

Hif1a and Hif2a can be safely inactivated in cone photoreceptors.

Impaired tissue oxygenation results in hypoxia and leads to the activation of hypoxia-inducible transcription factors (HIF). A chronic, HIF-triggered molecular response to hypoxia may be an important factor in the etiology of age-related macular degeneration (AMD) and is likely activated before any clinical manifestation of the disease. Thus, HIF1 and HIF2 recently emerged as potential therapeutic targets for AMD. To address and evaluate potential consequences of anti-HIF therapies for retinal physiology and function, we generated mouse lines that have Hif1a, or both Hif1a and Hif2a ablated specifically in cone photoreceptors. The knockdown of Hifs in cones did not cause detectable pathological alterations such as loss of cone photoreceptors, retinal degeneration or abnormalities of the retinal vasculature, had no impact on retinal function and resulted in a similar tolerance to hypoxic exposure. Our data indicate that HIF transcription factors are dispensable for maintaining normal cone function and survival in retinas of adult mice. This study provides the groundwork necessary to establish safety profiles for strategies aiming at antagonizing HIF1A and HIF2A function in cone photoreceptors for the treatment of retinal degenerative diseases that involve a hypoxic component such as AMD.

Impaired ABCA1/ABCG1-mediated lipid efflux in the mouse retinal pigment epithelium (RPE) leads to retinal degeneration.

Age-related macular degeneration (AMD) is a progressive disease of the retinal pigment epithelium (RPE) and the retina leading to loss of central vision. Polymorphisms in genes involved in lipid metabolism, including the ATP-binding cassette transporter A1 (ABCA1), have been associated with AMD risk. However, the significance of retinal lipid handling for AMD pathogenesis remains elusive. Here, we study the contribution of lipid efflux in the RPE by generating a mouse model lacking ABCA1 and its partner ABCG1 specifically in this layer. Mutant mice show lipid accumulation in the RPE, reduced RPE and retinal function, retinal inflammation and RPE/photoreceptor degeneration. Data from human cell lines indicate that the ABCA1 AMD risk-conferring allele decreases ABCA1 expression, identifying the potential molecular cause that underlies the genetic risk for AMD. Our results highlight the essential homeostatic role for lipid efflux in the RPE and suggest a pathogenic contribution of reduced ABCA1 function to AMD.

Hif1a inactivation rescues photoreceptor degeneration induced by a chronic hypoxia-like stress.

Reduced choroidal blood flow and tissue changes in the ageing human eye impair oxygen delivery to photoreceptors and the retinal pigment epithelium. As a consequence, mild but chronic hypoxia may develop and disturb cell metabolism, function and ultimately survival, potentially contributing to retinal pathologies such as age-related macular degeneration (AMD). Here, we show that several hypoxia-inducible genes were expressed at higher levels in the aged human retina suggesting increased activity of hypoxia-inducible transcription factors (HIFs) during the physiological ageing process. To model chronically elevated HIF activity and investigate ensuing consequences for photoreceptors, we generated mice lacking von Hippel Lindau (VHL) protein in rods. This activated HIF transcription factors and led to a slowly progressing retinal degeneration in the ageing mouse retina. Importantly, this process depended mainly on HIF1 with only a minor contribution of HIF2. A gene therapy approach using AAV-mediated RNA interference through an anti-Hif1a shRNA significantly mitigated the degeneration suggesting a potential intervention strategy that may be applicable to human patients.

Digoxin-induced retinal degeneration depends on rhodopsin.

Na,K-ATPases are energy consuming ion pumps that are required for maintaining ion homeostasis in most cells. In the retina, Na,K-ATPases are especially important to sustain the dark current in photoreceptor cells needed for rapid hyperpolarization of rods and cones in light. Cardiac glycosides like digoxin inhibit the activity of Na,K-ATPases by targeting their catalytic alpha subunits. This leads to a disturbed ion balance, which can affect cellular function and survival. Here we show that the treatment of wild-type mice with digoxin leads to severe retinal degeneration and loss of vision. Digoxin induced cell death specifically in photoreceptor cells with no or only minor effects in other retinal cell types. Photoreceptor-specific cytotoxicity depended on the presence of bleachable rhodopsin. Photoreceptors of Rpe65 knockouts, which have no measurable rhodopsin and photoreceptors of Rpe65R91W mice that have <10% of the rhodopsin found in retinas of wild-type mice were not sensitive to digoxin treatment. Similarly, cones in the all-cone retina of Nrl knockout mice were also not affected. Digoxin induced expression of several genes involved in stress signaling and inflammation. It also activated proteins such as ERK1/2, AKT, STAT1, STAT3 and CASP1 during a period of up to 10 days after treatment. Activation of signaling genes and proteins, as well as the dependency on bleachable rhodopsin resembles mechanisms of light-induced photoreceptor degeneration. Digoxin-mediated photoreceptor cell death may thus be used as an inducible model system to study molecular mechanisms of retinal degeneration.