Retinal ultrastructure of murine models of dry age-related macular degeneration (AMD)

TLR9 ligand CpG-ODN applied to the injured mouse cornea elicits retinal inflammation

During bacterial and viral infections, unmethylated CpG-DNA released by proliferating and dying microbes is recognized by toll-like receptor (TLR) 9 in host cells, initiating innate immune responses. Many corneal infections occur secondary to epithelial breaches and represent a major cause of vision impairment and blindness globally. To mimic this clinical situation, we investigated mechanisms of TLR9 ligand-induced corneal inflammation in mice after epithelial debridement. Application of CpG oligodeoxynucleotides (ODNs) resulted in neutrophil and macrophage infiltration to the cornea and loss of transparency. By 6 hours after CpG-ODN administration, TLR9 mRNA was increased in the cornea and retina. In vivo clinical examination at 24 hours revealed inflammatory infiltrates in the vitreous and retina, which were confirmed ex vivo to be neutrophils and macrophages, along with activated resident microglia. CpG-ODN-induced intraocular inflammation was abrogated in TLR9(-/-) and macrophage-depleted mice. Bone marrow reconstitution of irradiated TLR9(-/-) mice with TLR9(+/+) bone marrow led to restored corneal inflammatory responses to CpG-ODN. Fluorescein isothiocyanate-CpG-ODN rapidly penetrated the cornea and ocular media to reach the retina, where it was present within CD68(+) retinal macrophages and microglia. These data show that topically applied CpG-ODN induces intraocular inflammation owing to TLR9 activation of monocyte-lineage cells. These novel findings indicate that microbial CpG-DNA released during bacterial and/or viral keratitis can cause widespread inflammation within the eye, including the retina.

Cellular and 3D optical coherence tomography assessment during the initiation and progression of retinal degeneration in the Ccl2/Cx3cr1-deficient mouse

Retinal pathologies common to human eye diseases, including abnormal retinal pigment epithelial (RPE) cells, drusen-like accumulation, photoreceptor atrophy, and choroidal neovascularization, have been reported in the Ccl2/Cx3cr1-deficient mouse. The Ccl2 gene encodes the pro-inflammatory chemokine CCL2 (MCP-1), which is responsible for chemotactic recruitment of monocyte-derived macrophages to sites of inflammation. The Cx3cr1 gene encodes the fractalkine receptor, CX3CR1, and is required for accumulation of monocytes and microglia recruited via CCL2. Chemokine-mediated inflammation is implicated in retinal degenerative diseases such as diabetic retinopathy, age-related macular degeneration, retinitis pigmentosa, and uveoretinitis, and proper chemokine signaling from the RPE, Müller glia, and astrocytes is necessary to regulate leukocyte trafficking. Therefore, this mouse, possessing aberrant chemokine signaling coupled with retinal degenerative pathologies, presents an ideal opportunity to investigate the effect of altered signaling on retinal homeostasis and photoreceptor degeneration. Since this mouse is a recent development, more data covering the onset, location, and progression rate of pathologies is needed. In the present study we establish these parameters and show two photoreceptor cell death processes. Our observations of decreased glutamine synthetase and increased glial fibrillary acidic protein suggest that Müller cells respond very early within regions where lesions are forming. Finally, we suggest that retinal angiomatous proliferation contributes to pathological angiogenesis in this Ccl2/Cx3cr1-deficient mouse.

Orally active multi-functional antioxidants are neuroprotective in a rat model of light-induced retinal damage

Background

Progression of age-related macular degeneration has been linked to iron dysregulation and oxidative stress that induce apoptosis of neural retinal cells. Since both antioxidants and chelating agents have been reported to reduce the progression of retinal lesions associated with AMD in experimental animals, the present study evaluates the ability of multi-functional antioxidants containing functional groups that can independently chelate redox metals and quench free radicals to protect the retina against light-induced retinal degeneration, a rat model of dry atrophic AMD.

Methods/Results

Proof of concept studies were conducted to evaluate the ability of 4-(5-hydroxypyrimidin-2-yl)-N,N-dimethyl-3,5-dioxopiperazine-1-sulfonamide (compound 4) and 4-(5-hydroxy-4,6-dimethoxypyrimidin-2-yl)-N,N-dimethyl-3,5-dioxopiperazine-1-sulfonamide (compound 8) to reduce retinal damage in 2-week dark adapted Wistar rats exposed to 1000 lx of light for 3 hours. Assessment of the oxidative stress markers 4- hydroxynonenal and nitrotyrosine modified proteins and Thioredoxin by ELISA and Western blots indicated that these compounds reduced the oxidative insult caused by light exposure. The beneficial antioxidant effects of these compounds in providing significant functional and structural protection were confirmed by electroretinography and quantitative histology of the retina.

Conclusions/Significance

The present study suggests that multi-functional compounds may be effective candidates for preventive therapy of AMD.

Age related macular degeneration and drusen: neuroinflammation in the retina

Inflammation protects from dangerous stimuli, restoring normal tissue homeostasis. Inflammatory response in the nervous system ("neuroinflammation") has distinct features, which are shared in several diseases. The retina is an immune-privileged site, and the tight balance of immune reaction can be disrupted and lead to age-related macular disease (AMD) and to its peculiar sign, the druse. Excessive activation of inflammatory and immunological cascade with subsequent induction of damage, persistent activation of resident immune cells, accumulation of byproducts that exceeds the normal capacity of clearance giving origin to a chronic local inflammation, alterations in the activation of the complement system, infiltration of macrophages, T-lymphocytes and mast-cells from the bloodstream, participate in the mechanisms which originate the drusen. In addition, aging of the retina and AMD involve also para-inflammation, by which immune cells react to persistent stressful stimuli generating low-grade inflammation, aimed at restoring function and maintaining tissue homeostasis by varying the set point in relation to the new altered conditions. This mechanism is also seen in the normal aging retina, but, in the presence of noxious stimuli as in AMD, it can become chronic and have an adverse outcome. Finally, autophagy may provide new insights to understand AMD pathology, due to its contribution in the removal of defective proteins. Therefore, the AMD retina can represent a valuable model to study neuroinflammation, its mechanisms and therapy in a restricted and controllable environment. Targeting these pathways could represent a new way to treat and prevent both exudative and dry forms of AMD.

Accumulation of murine subretinal macrophages: effects of age, pigmentation and CX3CR1

Macrophages or activated microglia in the subretinal space are considered a hallmark of some retinal pathologies. We investigated the effects of age, pigmentation and CX(3)CR1 deficiency on the accumulation of macrophages/activated microglia in the outer retina of young and old Cx(3)cr1(gfp/gfp) (CX(3)CR1-deficient) or Cx(3)cr1(gfp/+) mice on either a pigmented (C57BL/6) or albino (BALB/c) background. Quantitative analysis of immunostained retinal-choroidal whole mounts revealed an increase in subretinal macrophage (SRMΦ) numbers in young Cx(3)cr1(gfp/gfp) mice compared with Cx(3)cr1(gfp/+) mice, however the increase was more marked in albino Cx(3)cr1(gfp/gfp) mice. In aged mice, large numbers of SRMΦ/activated microglia replete with autofluorescent debris were noted in both old pigmented Cx(3)cr1(gfp/gfp) and Cx(3)cr1(gfp/+) mice proving this accumulation was not CX(3)CR1-dependent. While CX(3)CR1 deficiency leads to an early onset of SRMΦ accumulation, our data reveal that this change occurs in both aged Cx(3)cr1(gfp/+) and Cx(3)cr1(gfp/gfp) pigmented mice in the absence of marked retinal degeneration and is likely a normal response to aging.

Animal models of retinal disease

Diseases of the retina are the leading causes of blindness in the industrialized world. The recognition that animals develop retinal diseases with similar traits to humans has led to not only a dramatic improvement in our understanding of the pathogenesis of retinal disease but also provided a means for testing possible treatment regimes and successful gene therapy trials. With the advent of genetic and molecular biological tools, the association between specific gene mutations and retinal signs has been made. Animals carrying natural mutations usually in one gene now provide well-established models for a host of inherited retinal diseases, including retinitis pigmentosa, Leber congenital amaurosis, inherited macular degeneration, and optic nerve diseases. In addition, the development of transgenic technologies has provided a means by which to study the effects of these and novel induced mutations on retinal structure and function. Despite these advances, there is a paucity of suitable animal models for complex diseases, including age-related macular degeneration (AMD) and diabetic retinopathy, largely because these diseases are not caused by single gene defects, but involve complex genetics and/or exacerbation through environmental factors, epigenetic, or other modes of genetic influence. In this review, we outline in detail the available animal models for inherited retinal diseases and how this information has furthered our understanding of retinal diseases. We also examine how transgenic technologies have helped to develop our understanding of the role of isolated genes or pathways in complex diseases like AMD, diabetes, and glaucoma.

Carboxyethylpyrrole plasma biomarkers in age-related macular degeneration

Age-related macular degeneration causes irreversible central blindness in people over the age of 50 and is increasing in prevalence among elderly populations. There are currently limited treatment options available for the exudative form of the disease and no formal treatments for the geographic atrophy form aside from lifestyle change and incorporation of antioxidant supplements in the diet. As such, it is important to be able to assess high-risk AMD patients as early as possible in order to prescribe preventative measures. Carboxyethylpyrrole (CEP) is a promising plasma biomarker suited to this purpose. Both CEP immunoreactivity levels as well as anti-CEP autoantibody titers are significantly elevated in AMD patients and thus provide the potential to assess AMD susceptibility with approximately 80% accuracy when evaluated alongside genomic AMD markers. Moreover, strong evidence implicates CEP as functionally related to AMD pathogenesis, a role which must be explored further. This avenue of research will foster improved understanding of the disease itself and perhaps reveal better therapeutic targets and options. Further research into the role of CEP in AMD pathogenesis and the application of CEP as an AMD biomarker is merited.

mTOR-mediated dedifferentiation of the retinal pigment epithelium initiates photoreceptor degeneration in mice

Retinal pigment epithelial (RPE) cell dysfunction plays a central role in various retinal degenerative diseases, but knowledge is limited regarding the pathways responsible for adult RPE stress responses in vivo. RPE mitochondrial dysfunction has been implicated in the pathogenesis of several forms of retinal degeneration. Here we have shown that postnatal ablation of RPE mitochondrial oxidative phosphorylation in mice triggers gradual epithelium dedifferentiation, typified by reduction of RPE-characteristic proteins and cellular hypertrophy. The electrical response of the retina to light decreased and photoreceptors eventually degenerated. Abnormal RPE cell behavior was associated with increased glycolysis and activation of, and dependence upon, the hepatocyte growth factor/met proto-oncogene pathway. RPE dedifferentiation and hypertrophy arose through stimulation of the AKT/mammalian target of rapamycin (AKT/mTOR) pathway. Administration of an oxidant to wild-type mice also caused RPE dedifferentiation and mTOR activation. Importantly, treatment with the mTOR inhibitor rapamycin blunted key aspects of dedifferentiation and preserved photoreceptor function for both insults. These results reveal an in vivo response of the mature RPE to diverse stressors that prolongs RPE cell survival at the expense of epithelial attributes and photoreceptor function. Our findings provide a rationale for mTOR pathway inhibition as a therapeutic strategy for retinal degenerative diseases involving RPE stress.

CCL2/CCR2 and CX3CL1/CX3CR1 chemokine axes and their possible involvement in age-related macular degeneration

The causes of age-related macular degeneration (AMD) are not well understood. Due to demographic shifts in the industrialized world a growing number of people will develop AMD in the coming decades. To develop treatments it is essential to characterize the disease's pathogenic process. Over the past few years, numerous studies have focused on the role of chemotactic cytokines, also known as chemokines. Certain chemokines, such as CCL2 and CX3CL1, appear to be crucial in subretinal microglia and macrophage accumulation observed in AMD, and participate in the development of retinal degeneration as well as in choroidal neovascularization. This paper reviews the possible implications of CCL2 and CX3CL1 signaling in AMD. Expression patterns, single nucleotide polymorphisms (SNPs) association studies, chemokine and chemokine receptor knockout models are discussed. Future AMD treatments could target chemokines and/or their receptors.

Novel rodent models for macular research

Background

Many disabling human retinal disorders involve the central retina, particularly the macula. However, the commonly used rodent models in research, mouse and rat, do not possess a macula. The purpose of this study was to identify small laboratory rodents with a significant central region as potential new models for macular research.

Methodology/Principal Findings

Gerbillus perpallidus, Meriones unguiculatus and Phodopus campbelli, laboratory rodents less commonly used in retinal research, were subjected to confocal scanning laser ophthalmoscopy (cSLO), fluorescein and indocyanine green angiography, and spectral-domain optical coherence tomography (SD-OCT) using standard equipment (Heidelberg Engineering HRA1 and Spectralis™) adapted to small rodent eyes. The existence of a visual streak-like pattern was assessed on the basis of vascular topography, retinal thickness, and the topography of retinal ganglion cells and cone photoreceptors. All three species examined showed evidence of a significant horizontal streak-like specialization. cSLO angiography and retinal wholemounts revealed that superficial retinal blood vessels typically ramify and narrow into a sparse capillary net at the border of the respective area located dorsal to the optic nerve. Similar to the macular region, there was an absence of larger blood vessels in the streak region. Furthermore, the thickness of the photoreceptor layer and the population density of neurons in the ganglion cell layer were markedly increased in the visual streak region.

Conclusions/Significance

The retinal specializations of Gerbillus perpallidus, Meriones unguiculatus and Phodopus campbelli resemble features of the primate macula. Hence, the rodents reported here may serve to study aspects of macular development and diseases like age-related macular degeneration and diabetic macular edema, and the preclinical assessment of therapeutic strategies.