Murine ccl2/cx3cr1 deficiency results in retinal lesions mimicking human age-related macular degeneration

Age-related macular degeneration: suitability of optogenetic therapy for geographic atrophy

Age-related macular degeneration (AMD) is a growing public health concern given the aging population and it is the leading cause of blindness in developed countries, affecting individuals over the age of 55 years. AMD affects the retinal pigment epithelium (RPE) and Bruch's membrane in the macula, leading to secondary photoreceptor degeneration and eventual loss of central vision. Late AMD is divided into two forms: neovascular AMD and geographic atrophy (GA). GA accounts for around 60% of late AMD and has been the most challenging subtype to treat. Recent advances include approval of new intravitreally administered therapeutics, pegcetacoplan (Syfovre) and avacincaptad pegol (Iveric Bio), which target complement factors C3 and C5, respectively, which slow down the rate of enlargement of the area of atrophy. However, there is currently no treatment to reverse the central vision loss associated with GA. Optogenetics may provide a strategy for rescuing visual function in GA by imparting light-sensitivity to the surviving inner retina (i.e., retinal ganglion cells or bipolar cells). It takes advantage of residual inner retinal architecture to transmit visual stimuli along the visual pathway, while a wide range of photosensitive proteins are available for consideration. Herein, we review the anatomical changes in GA, discuss the suitability of optogenetic therapeutic sensors in different target cells in pre-clinical models, and consider the advantages and disadvantages of different routes of administration of therapeutic vectors.

Replenishing IRAK-M expression in retinal pigment epithelium attenuates outer retinal degeneration

Chronic inflammation is a constitutive component of many age-related diseases, including age-related macular degeneration (AMD). Here, we identified interleukin-1 receptor-associated kinase M (IRAK-M) as a key immunoregulator in retinal pigment epithelium (RPE) that declines during the aging process. Rare genetic variants of IRAK3, which encodes IRAK-M, were associated with an increased likelihood of developing AMD. In human samples and mouse models, IRAK-M abundance in the RPE declined with advancing age or exposure to oxidative stress and was further reduced in AMD. Irak3-knockout mice exhibited an increased incidence of outer retinal degeneration at earlier ages, which was further exacerbated by oxidative stressors. The absence of IRAK-M led to a disruption in RPE cell homeostasis, characterized by compromised mitochondrial function, cellular senescence, and aberrant cytokine production. IRAK-M overexpression protected RPE cells against oxidative or immune stressors. Subretinal delivery of adeno-associated virus (AAV)-expressing human IRAK3 rescued light-induced outer retinal degeneration in wild-type mice and attenuated age-related spontaneous retinal degeneration in Irak3-knockout mice. Our data show that replenishment of IRAK-M in the RPE may redress dysregulated pro-inflammatory processes in AMD, suggesting a potential treatment for retinal degeneration.

Antioxidants and Mechanistic Insights for Managing Dry Age-Related Macular Degeneration

Age-related macular degeneration (AMD) severely affects central vision due to progressive macular degeneration and its staggering prevalence is rising globally, especially in the elderly population above 55 years. Increased oxidative stress with aging is considered an important contributor to AMD pathogenesis despite multifaceted risk factors including genetic predisposition and environmental agents. Wet AMD can be managed with routine intra-vitreal injection of angiogenesis inhibitors, but no satisfactory medicine has been approved for the successful management of the dry form. The toxic carbonyls due to photo-oxidative degradation of accumulated bisretinoids within lysosomes initiate a series of events including protein adduct formation, impaired autophagy flux, complement activation, and chronic inflammation, which is implicated in dry AMD. Therapy based on antioxidants has been extensively studied for its promising effect in reducing the impact of oxidative stress. This paper reviews the dry AMD pathogenesis, delineates the effectiveness of dietary and nutrition supplements in clinical studies, and explores pre-clinical studies of antioxidant molecules, extracts, and formulations with their mechanistic insights.

Modeling complex age-related eye disease

Modeling complex eye diseases like age-related macular degeneration (AMD) and glaucoma poses significant challenges, since these conditions depend highly on age-related changes that occur over several decades, with many contributing factors remaining unknown. Although both diseases exhibit a relatively high heritability of >50%, a large proportion of individuals carrying AMD- or glaucoma-associated genetic risk variants will never develop these diseases. Furthermore, several environmental and lifestyle factors contribute to and modulate the pathogenesis and progression of AMD and glaucoma. Several strategies replicate the impact of genetic risk variants, pathobiological pathways and environmental and lifestyle factors in AMD and glaucoma in mice and other species. In this review we will primarily discuss the most commonly available mouse models, which have and will likely continue to improve our understanding of the pathobiology of age-related eye diseases. Uncertainties persist whether small animal models can truly recapitulate disease progression and vision loss in patients, raising doubts regarding their usefulness when testing novel gene or drug therapies. We will elaborate on concerns that relate to shorter lifespan, body size and allometries, lack of macula and a true lamina cribrosa, as well as absence and sequence disparities of certain genes and differences in their chromosomal location in mice. Since biological, rather than chronological, age likely predisposes an organism for both glaucoma and AMD, more rapidly aging organisms like small rodents may open up possibilities that will make research of these diseases more timely and financially feasible. On the other hand, due to the above-mentioned anatomical and physiological features, as well as pharmacokinetic and -dynamic differences small animal models are not ideal to study the natural progression of vision loss or the efficacy and safety of novel therapies. In this context, we will also discuss the advantages and pitfalls of alternative models that include larger species, such as non-human primates and rabbits, patient-derived retinal organoids, and human organ donor eyes.

Endoplasmic reticulum stress: molecular mechanism and therapeutic targets

The endoplasmic reticulum (ER) functions as a quality-control organelle for protein homeostasis, or "proteostasis". The protein quality control systems involve ER-associated degradation, protein chaperons, and autophagy. ER stress is activated when proteostasis is broken with an accumulation of misfolded and unfolded proteins in the ER. ER stress activates an adaptive unfolded protein response to restore proteostasis by initiating protein kinase R-like ER kinase, activating transcription factor 6, and inositol requiring enzyme 1. ER stress is multifaceted, and acts on aspects at the epigenetic level, including transcription and protein processing. Accumulated data indicates its key role in protein homeostasis and other diverse functions involved in various ocular diseases, such as glaucoma, diabetic retinopathy, age-related macular degeneration, retinitis pigmentosa, achromatopsia, cataracts, ocular tumors, ocular surface diseases, and myopia. This review summarizes the molecular mechanisms underlying the aforementioned ocular diseases from an ER stress perspective. Drugs (chemicals, neurotrophic factors, and nanoparticles), gene therapy, and stem cell therapy are used to treat ocular diseases by alleviating ER stress. We delineate the advancement of therapy targeting ER stress to provide new treatment strategies for ocular diseases.

Ocular Vascular Diseases: From Retinal Immune Privilege to Inflammation

The eye is an immune privileged tissue that insulates the visual system from local and systemic immune provocation to preserve homeostatic functions of highly specialized retinal neural cells. If immune privilege is breached, immune stimuli will invade the eye and subsequently trigger acute inflammatory responses. Local resident microglia become active and release numerous immunological factors to protect the integrity of retinal neural cells. Although acute inflammatory responses are necessary to control and eradicate insults to the eye, chronic inflammation can cause retinal tissue damage and cell dysfunction, leading to ocular disease and vision loss. In this review, we summarized features of immune privilege in the retina and the key inflammatory responses, factors, and intracellular pathways activated when retinal immune privilege fails, as well as a highlight of the recent clinical and research advances in ocular immunity and ocular vascular diseases including retinopathy of prematurity, age-related macular degeneration, and diabetic retinopathy.

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.

Repeated Topical Administration of 3 nm Cerium Oxide Nanoparticles Reverts Disease Atrophic Phenotype and Arrests Neovascular Degeneration in AMD Mouse Models

Increased oxidative stress in the retina and retinal pigment epithelium is implicated in age-related macular degeneration (AMD). Antioxidant cerium oxide nanoparticles (CeO2NPs) have been used to treat degenerative retinal pathologies in animal models, although their delivery route is not ideal for chronic patient treatment. In this work, we prepared a formulation for ocular topical delivery that contains small (3 nm), nonaggregated biocompatible CeO2NPs. In vitro results indicate the biocompatible and protective character of the CeO2NPs, reducing oxidative stress in ARPE19 cells and inhibiting neovascularization related to pathological angiogenesis in both HUVEC and in in vitro models of neovascular growth. In the in vivo experiments, we observed the capacity of CeO2NPs to reach the retina after topical delivery and a subsequent reversion of the altered retinal transcriptome of the retinal degenerative mouse model DKOrd8 toward that of healthy control mice, together with signs of decreased inflammation and arrest of degeneration. Furthermore, CeO2NP eye drops' treatment reduced laser-induced choroidal neovascular lesions in mice by lowering VEGF and increasing PEDF levels. These results indicate that CeO2NP eye drops are a beneficial antioxidant and neuroprotective treatment for both dry and wet forms of AMD disease.

More than meets the eye: The role of microglia in healthy and diseased retina

Microglia are the main resident immune cells of the nervous system and as such they are involved in multiple roles ranging from tissue homeostasis to response to insults and circuit refinement. While most knowledge about microglia comes from brain studies, some mechanisms have been confirmed for microglia cells in the retina, the light-sensing compartment of the eye responsible for initial processing of visual information. However, several key pieces of this puzzle are still unaccounted for, as the characterization of retinal microglia has long been hindered by the reduced population size within the retina as well as the previous lack of technologies enabling single-cell analyses. Accumulating evidence indicates that the same cell type may harbor a high degree of transcriptional, morphological and functional differences depending on its location within the central nervous system. Thus, studying the roles and signatures adopted specifically by microglia in the retina has become increasingly important. Here, we review the current understanding of retinal microglia cells in physiology and in disease, with particular emphasis on newly discovered mechanisms and future research directions.

Recent Advances in Proteomics-Based Approaches to Studying Age-Related Macular Degeneration: A Systematic Review

Age-related macular degeneration (AMD) is a common ocular disease characterized by degeneration of the central area of the retina in the elderly population. Progression and response to treatment are influenced by genetic and non-genetic factors. Proteomics is a powerful tool to study, at the molecular level, the mechanisms underlying the progression of the disease, to identify new therapeutic targets and to establish biomarkers to monitor progression and treatment effectiveness. In this work, we systematically review the use of proteomics-based approaches for the study of the molecular mechanisms underlying the development of AMD, as well as the progression of the disease and on-treatment patient monitoring. The Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) reporting guidelines were followed. Proteomic approaches have identified key players in the onset of the disease, such as complement components and proteins involved in lipid metabolism and oxidative stress, but also in the progression to advanced stages, including factors related to extracellular matrix integrity and angiogenesis. Although anti-vascular endothelial growth factor (anti-VEGF)-based therapy has been crucial in the treatment of neovascular AMD, it is necessary to deepen our understanding of the underlying disease mechanisms to move forward to next-generation therapies for later-stage forms of this multifactorial disease.

Curcumin and Wnt/β‑catenin signaling in exudative age‑related macular degeneration (Review)

Curcumin is a natural product widely used due to its pharmacological effects. Nevertheless, only a limited number of studies concerning the effects of curcumin on exudative age‑related macular degeneration (AMD) is currently available. Since ophthalmic diseases, including exudative AMD, have a marked impact on public health, the prevention and therapy of ophthalmic disorders remain of increasing concern. Exudative AMD is characterized by choroidal neovascularization (CNV) invading the subretinal space, ultimately enhancing exudation and hemorrhaging. The exudative AMD subtype corresponds to 10 to 15% of cases of macular degeneration; however, the occurrence of this subtype has been reported as the major cause of vision loss and blindness, with the occurrence of CNV being responsible for 80% of the cases with vision loss. In CNV increased expression of VEGF has been observed, stimulated by the overactivation of Wnt/β‑catenin signaling pathway. The stimulation of the Wnt/β‑catenin signaling pathway is responsible for the activation of several cellular mechanisms, simultaneously enhancing inflammation, oxidative stress and angiogenesis in numerous diseases, including ophthalmic disorders. Some studies have previously demonstrated the possible advantage of the use of curcumin for the inhibition of Wnt/β‑catenin signaling. In the present review article, the different mechanisms of curcumin are described concerning its effects on oxidative stress, inflammation and angiogenesis in exudative AMD, by interacting with Wnt/β‑catenin signaling.

Comparison between sodium iodate and lipid peroxide murine models of age-related macular degeneration for drug evaluation-a narrative review

Objective

In this review, non-transgenic models of age-related macular degeneration (AMD) are discussed, with focuses on murine retinal degeneration induced by sodium iodate and lipid peroxide (HpODE) as preclinical study platforms.

Background

AMD is the most common cause of vision loss in a world with an increasingly aging population. The major phenotypes of early and intermediate AMD are increased drusen and autofluorescence, Müller glia activation, infiltrated subretinal microglia and inward moving retinal pigment epithelium cells. Intermediate AMD may progress to advanced AMD, characterized by geography atrophy and/or choroidal neovascularization. Various transgenic and non-transgenic animal models related to retinal degeneration have been generated to investigate AMD pathogenesis and pathobiology, and have been widely used as potential therapeutic evaluation platforms.

Methods

Two retinal degeneration murine models induced by sodium iodate and HpODE are described. Distinct pathological features and procedures of these two models are compared. In addition, practical protocol and material preparation and assessment methods are elaborated.

Conclusion

Retina degeneration induced by sodium iodate and HpODE in mouse eye resembles many clinical aspects of human AMD and complimentary to the existent other animal models. However, standardization of procedure and assessment protocols is needed for preclinical studies. Further studies of HpODE on different routes, doses and species will be valuable for the future extensive use. Despite many merits of murine studies, differences between murine and human should be always considered.

Retinal Degeneration in a Murine Model of Retinal Ischemia by Unilateral Common Carotid Artery Occlusion

Retinal degeneration is a progressive retinal damage in ocular vascular diseases. There are several reasons for this, such as occlusion of arteries or veins, diabetic retinopathy, or hereditary retinal diseases. To study pathological mechanisms of retinal degeneration, it is required to develop experimentally reproducible and clinically relevant models. In our previous studies, we developed a murine model of retinal hypoperfusion by unilateral common carotid artery occlusion (UCCAO) which mimics the pathophysiology of ocular ischemic syndrome (OIS) in humans, and described broad pathological mechanisms in the retina after UCCAO. However, there still remain missing pieces of the ocular pathologic process by UCCAO. In this study, we examined those unfound mechanisms. UCCAO was performed on adult mice. Ocular dysfunctions, histological deficits, and inflammation were examined after UCCAO, compared with sham-operated mice. Evaluation values were analyzed by electrophysiological, histological, and molecular biological methods. Eyelid drooping was permanently seen after UCCAO. Induction time point of acute reversible cataract under anesthesia was shortened. Retinal/visual dysfunctions were detected 2-4 weeks after UCCAO. Specifically, scotopic b-wave was more affected than a-wave, with the dysfunction of photopic b-wave. Impaired oscillatory potentials and visual evoked potential were constantly observed. Pathological Müller gliosis/inflammation was featured with NeuN-positive cell loss in the ganglion cell layer. Axial length, intraocular pressure, pupillary light reflex, and retinal pigment epithelium/choroidal thickness were not changed by UCCAO. A murine model of retinal ischemia by UCCAO can be useful for studying a series of degenerative process in the ischemic retina.

Plasma C-C Chemokine Concentrations in Intermediate Age-Related Macular Degeneration

Purpose: To determine the relationship between plasma concentrations of the C-C chemokines CCL2, CCL3, CCL4, and CCL5 and intermediate age-related macular degeneration (iAMD) patients compared with control inidividuals to further define the inflammatory pathways associated with age-related macular degeneration.

Methods: The concentrations of CCL2, CCL3, CCL4, and CCL5 were measured using multiplex assays in plasma collected from 210 patients with iAMD and 102 control individuals with no macular degeneration as defined by multi-modal imaging. Non-inflammatory data included in the analysis were: age, sex, family history of AMD, history of smoking, body mass index, presence of reticular pseudo-drusen and cardiovascular disease. Median concentrations as well as a cutoff value for each chemokine were compared between the two groups.

Results: The median concentrations of CCL2 and CCL4 did not differ between control and iAMD groups, however, CCL2 was elevated in iAMD when a cutoff comparison was used (p < 0.05). Median CCL3 and CCL5 concentrations were significantly decreased in the macular degeneration group compared with controls (p < 0.001) as well as when a cutoff value comparison was used. CCL3 and CCL5 were negatively correlated in cases and positively correlated in controls.

Conclusions: Plasma CCL3 and CCL5 concentrations were significantly decreased and CCL2 concentrations were increased in patients with iAMD compared with controls, suggesting a role for C-C chemokines in the systemic inflammatory processes associated with disease development.

An insight on established retinal injury mechanisms and prevalent retinal stem cell activation pathways in vertebrate models

Implementing different tools and injury mechanisms in multiple animal models of retina regeneration, researchers have discovered the existence of retinal stem/progenitor cells. Although they appear to be distributed uniformly across the vertebrate lineage, the reparative potential of the retina is mainly restricted to lower vertebrates. Regenerative repair post-injury requires the creation of a proliferative niche, vital for proper stem cell activation, propagation, and lineage differentiation. This seems to be lacking in mammals. Hence, in this review, we first discuss the many forms of retinal injuries that have been generated using animal models. Next, we discuss how they are utilized to stimulate regeneration and mimic eye disease pathologies. The key to driving stem cell activation in mammals relies on the information we can gather from these models. Lastly, we present a brief update about the genes, growth factors, and signaling pathways that have been brought to light using these models.

Transcriptomics analysis of Ccl2/Cx3cr1/Crb1rd8 deficient mice provides new insights into the pathophysiology of progressive retinal degeneration

Chronic oxidative stress and immune dysregulation are key mechanisms involved in the pathogenesis of most retinal degenerative diseases, including age-related macular degeneration. The Ccl2^-/-/Cx3cr1^-/-/Crb1^rd8/rd8 mouse model develops a progressive degeneration phenotype, with photoreceptor atrophy, drusen-like lesions or pigment alterations at an early age; however, the role of oxidative stress and immune function in the pathogenesis of the model is poorly understood. We performed a comprehensive characterization of the Ccl2^-/-/Cx3cr1^-/-/Crb1^rd8/rd8 mouse to evaluate how these pathways influence pathogenesis. We generated a Ccl2^-/-/Cx3cr1^-/- double-knockout (DKO) mouse on a C57BL/6N background (with the rd8 mutation of the Crb1 gene), assessed its retina status and function during 9 months in both in vivo and post-mortem analysis, and performed a comprehensive transcriptomic analysis. DKOrd8 mice presented focal retinal lesions with increased infiltration of microglia and involvement of Müller cells. Lesions progressed to thinning of the photoreceptor nuclear layer, causing a loss in retinal function. Transcriptomics analysis revealed major differential expression of genes involved in oxidative stress and neuronal function, in particular genes related to the mitochondrial electron transport chain and antioxidant cellular response. Our results suggest that alterations in chemokine signaling combined with the rd8 mutation in Ccl2^-/-/Cx3cr1^-/-/Crb1^rd8/rd8 mice involve early changes in several pathways associated with age-related macular degeneration, highlighting the relevance of these processes in the pathological retinal degeneration in the DKOrd8 model.

Progress in developing rodent models of age-related macular degeneration (AMD)

Age-related macular degeneration (AMD) is the leading cause of irreversible central vision loss, typically affecting individuals from mid-life onwards. Its multifactorial aetiology and the lack of any effective treatments has spurred the development of animal models as research and drug discovery tools. Several rodent models have been developed which recapitulate key features of AMD and provide insights into its underlying pathology. These have contributed to making significant progress in understanding the disease and the identification of novel therapeutic targets. However, a major caveat with existing models is that they do not demonstrate the full disease spectrum. In this review, we outline advances in rodent AMD models from the last decade. These models feature various hallmarks associated with AMD, including oxidative stress, hypoxia, immune dysregulation, genetic mutations and environmental risk factors. The review summarises the methods by which each model was created, its pathological characteristics as well as its relation to the disease in humans.

Retina in a dish: Cell cultures, retinal explants and animal models for common diseases of the retina

For many retinal diseases, including age-related macular degeneration (AMD), glaucoma, and diabetic retinopathy (DR), the exact pathogenesis is still unclear. Moreover, the currently available therapeutic options are often unsatisfactory. Research designed to remedy this situation heavily relies on experimental animals. However, animal models often do not faithfully reproduce human disease and, currently, there is strong pressure from society to reduce animal research. Overall, this creates a need for improved disease models to understand pathologies and develop treatment options that, at the same time, require fewer or no experimental animals. Here, we review recent advances in the field of in vitro and ex vivo models for AMD, glaucoma, and DR. We highlight the difficulties associated with studies on complex diseases, in which both the initial trigger and the ensuing pathomechanisms are unclear, and then delineate which model systems are optimal for disease modelling. To this end, we present a variety of model systems, ranging from primary cell cultures, over organotypic cultures and whole eye cultures, to animal models. Specific advantages and disadvantages of such models are discussed, with a special focus on their relevance to putative in vivo disease mechanisms. In many cases, a replacement of in vivo research will mean that several different in vitro models are used in conjunction, for instance to analyze and validate causative molecular pathways. Finally, we argue that the analytical decomposition into appropriate cell and tissue model systems will allow making significant progress in our understanding of complex retinal diseases and may furthermore advance the treatment testing.

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.

Circadian Rhythms in Exudative Age-Related Macular Degeneration: The Key Role of the Canonical WNT/β-Catenin Pathway

Age-related macular degeneration (AMD) is considered as the main worldwide cause of blindness in elderly adults. Exudative AMD type represents 10 to 15% of macular degeneration cases, but is the main cause of vision loss and blindness. Circadian rhythm changes are associated with aging and could further accelerate it. However, the link between circadian rhythms and exudative AMD is not fully understood. Some evidence suggests that dysregulation of circadian functions could be manifestations of diseases or could be risk factors for the development of disease in elderly adults. Biological rhythms are complex systems interacting with the environment and control several physiological pathways. Recent findings have shown that the dysregulation of circadian rhythms is correlated with exudative AMD. One of the main pathways involved in exudative AMD is the canonical WNT/β-catenin pathway. Circadian clocks have a main role in some tissues by driving the circadian expression of genes involved in physiological and metabolic functions. In exudative AMD, the increase of the canonical WNT/β-catenin pathway is enhanced by the dysregulation of circadian rhythms. Exudative AMD progression is associated with major metabolic reprogramming, initiated by aberrant WNT/β-catenin pathway, of aerobic glycolysis. This review focuses on the interest of circadian rhythm dysregulation in exudative AMD through the aberrant upregulation of the canonical WNT/β-catenin pathway.

Preclinical Evaluation of a Cell-Based Gene Therapy Using the Sleeping Beauty Transposon System in Choroidal Neovascularization

Age-related macular degeneration (AMD) is a progressive retinal disorder characterized by imbalanced pro- and antiangiogenic signals. The aim of this study was to evaluate the effect of ex vivo cell-based gene therapy with stable expression of human pigment epithelium-derived factor (PEDF) release using the non-viral Sleeping Beauty (SB100X) transposon system delivered by miniplasmids free of antibiotic resistance markers (pFAR4). Retinal pigment epithelial (RPE) cells and iris pigment epithelial (IPE) cells were co-transfected with pFAR4-inverted terminal repeats (ITRs) CMV-PEDF-BGH and pFAR4-CMV-SB100X-SV40 plasmids. Laser-induced choroidal neovascularization (CNV) was performed in rats, and transfected primary cells (transfected RPE [tRPE] and transfected IPE [tIPE] cells) were injected into the subretinal space. The leakage and CNV areas, vascular endothelial growth factor (VEGF), PEDF protein expression, metalloproteinases 2 and 9 (MMP-2/9), and microglial/macrophage markers were measured. Injection with tRPE/IPE cells significantly reduced the leakage area at 7 and 14 days and the CNV area at 7 days. There was a significant increase in PEDF and the PEDF/VEGF ratio with tRPE cells and a reduction in the MMP-2 activity. Our data demonstrated that ex vivo non-viral gene therapy reduces CNV and could be an effective and safe therapeutic option for angiogenic retinal diseases.

Effect of Inhibition of Colony-Stimulating Factor 1 Receptor on Choroidal Neovascularization in Mice

Neovascular age-related macular degeneration is one of the leading causes of blindness. Microglia and macrophages play a critical role in choroidal neovascularization (CNV) and may, therefore, be potential targets to modulate the disease course. This study evaluated the effect of the colony-stimulating factor-1 receptor inhibitor PLX5622 on experimental laser-induced CNV. A 98% reduction of retinal microglia cells was observed in the retina 1 week after initiation of PLX5622 treatment, preventing accumulation of macrophages within the laser site and leading to a reduction of leukocytes within the choroid after CNV induction. Mice treated with PLX5622 had a significantly faster decrease of the CNV lesion size, as revealed by in vivo imaging and immunohistochemistry from day 3 to day 14 compared with untreated mice. Several inflammatory modulators, such as chemokine (C-C motif) ligand 9, granulocyte-macrophage colony-stimulating factor, soluble tumor necrosis factor receptor-I, IL-1α, and matrix metallopeptidase-2, were elevated in the acute phase of the disease when microglia were ablated with PLX5622, whereas other cytokines (eg, interferon-γ, IL-4, and IL-10) were reduced. Our results suggest that colony-stimulating factor-1 receptor inhibition may be a novel therapeutic target in patients with neovascular age-related macular degeneration.

Inhibition of AMD-Like Pathology With a Neurotrophic Compound in Aged Rats and 3xTg-AD Mice

Age-associated macular degeneration (AMD), which leads to loss of vision at its end stage, is one of the most common neurodegenerative diseases among the elderly. However, to date, no effective drug therapy is available for the prevention of AMD. Here, we report the occurrence of AMD pathology and its prevention by chronic treatment with the neurotrophic peptidergic compound P021, in aged rats and 3xTg-AD mice. We found photoreceptor degeneration, lipofuscin granules, vacuoles, and atrophy in retinal pigment epithelium (RPE) as well as Bruch’s membrane (BM) thickening; in aged rats, we even found rosette-like structure formation. Microgliosis and astrogliosis were observed in different retinal layers. In addition, we also found that total tau, phosphorylated tau, Aβ/APP, and VEGF were widely distributed in the sub-retina of aged rats and 3xTg mice. Importantly, chronic treatment with P021 for 3 months in rats and for 18 months in 3xTg mice ameliorated the pathological changes above. These findings indicate the therapeutic potential of P021 for prevention and treatment of AMD and retinal changes associated with aging and Alzheimer’s disease.

Inhibition of Experimental Age-Related Macular Degeneration by ZQMT in Mice

BACKGROUND

Age-related macular degeneration (AMD) is a progressive and irreversible eye disease. The anti-vascular endothelial growth factor (VEGF) therapy has revolutionized the treatment of neovascular AMD. However, the expense for such treatment is quite high.

METHODS

We used a traditional Chinese medicine ZQMT as an alternative therapeutic regimen for AMD. We employed two in vivo animal models mimicking dry and wet AMD respectively to assess the therapeutic efficacy of ZQMT on treating AMD-related retinopathy. AMD-related retinopathy in Crb1rd8 mice was evaluated from week 1 to 8 by fundus photography. Laser-induced choroidal neovascularization (CNV) was evaluated by fluorescein angiography and histopathology.

RESULTS

ZQMT increased CX3CR1 expression in murine CD4+ T cells either cultured in vitro or directly isolated from animals treated with ZQMT. We also performed both in vitro and in vivo studies to confirm that ZQMT has no apparent toxic effects. ZQMT alleviated AMD-related retinopathy in both Crb1rd8 and CNV models. Depletion of CCL2 and CX3CR1 in Crb1rd8 mice abolished the efficacy of ZQMT, suggesting that CCL2 and/or CX3CR1 may underlie the mechanisms of ZQMT in treating AMD-related retinopathy in mice.

CONCLUSION

In summary, our study supports the protective roles of a traditional Chinese medicine ZQMT in AMD.

Polarized retinal pigment epithelium generates electrical signals that diminish with age and regulate retinal pathology

The transepithelial potential difference (TEP) across the retinal pigment epithelial (RPE) is dependent on ionic pumps and tight junction "seals" between epithelial cells. RPE cells release neurotrophic growth factors such as pigment epithelial derived factor (PEDF), which is reduced in age-related macular degeneration (AMD). The mechanisms that control the secretion of PEDF from RPE cells are not well understood. Using the CCL2/CX3CR1 double knockout mouse model (DKO), which demonstrates RPE damage and retinal degeneration, we uncovered an interaction between PEDF and the TEP which is likely to play an important role in retinal ageing and in the pathogenesis of AMD. We found that: (a) the expression of ATP1B1 (the Na+ /K+ -ATPase β1 subunit) was reduced significantly in RPE from aged mice, in patients with CNV (Choroidal Neovascularization) and in DKO mice; (b) the expression of PEDF also was decreased in aged persons and in DKO mice; (c) the TEP across RPE was reduced markedly in RPE cells from DKO mice and (d) an applied electric field (EF) of 50-100 mV/mm, used to mimic the natural TEP, increased the expression and secretion of PEDF in primary RPE cells. In conclusion, the TEP across the RPE depends on the expression of ATP1B1 and this regulates the secretion of PEDF by RPE cells and so may regulate the onset of retinal disease. Increasing the expression of PEDF using an applied EF to replenish a disease or age-reduced TEP may offer a new way of preventing or reversing retinal dysfunction.

An Overview of the Role of Lipofuscin in Age-Related Neurodegeneration

Despite aging being by far the greatest risk factor for highly prevalent neurodegenerative disorders, the molecular underpinnings of age-related brain changes are still not well understood, particularly the transition from normal healthy brain aging to neuropathological aging. Aging is an extremely complex, multifactorial process involving the simultaneous interplay of several processes operating at many levels of the functional organization. The buildup of potentially toxic protein aggregates and their spreading through various brain regions has been identified as a major contributor to these pathologies. One of the most striking morphologic changes in neurons during normal aging is the accumulation of lipofuscin (LF) aggregates, as well as, neuromelanin pigments. LF is an autofluorescent lipopigment formed by lipids, metals and misfolded proteins, which is especially abundant in nerve cells, cardiac muscle cells and skin. Within the Central Nervous System (CNS), LF accumulates as aggregates, delineating a specific senescence pattern in both physiological and pathological states, altering neuronal cytoskeleton and cellular trafficking and metabolism, and being associated with neuronal loss, and glial proliferation and activation. Traditionally, the accumulation of LF in the CNS has been considered a secondary consequence of the aging process, being a mere bystander of the pathological buildup associated with different neurodegenerative disorders. Here, we discuss recent evidence suggesting the possibility that LF aggregates may have an active role in neurodegeneration. We argue that LF is a relevant effector of aging that represents a risk factor or driver for neurodegenerative disorders.

Retinal microglia - A key player in healthy and diseased retina

Microglia, the resident immune cells of the brain and retina, are constantly engaged in the surveillance of their surrounding neural tissue. During embryonic development they infiltrate the retinal tissues and participate in the phagocytosis of redundant neurons. The contribution of microglia in maintaining the purposeful and functional histo-architecture of the adult retina is indispensable. Within the retinal microenvironment, robust microglial activation is elicited by subtle changes caused by extrinsic and intrinsic factors. When there is a disturbance in the cell-cell communication between microglia and other retinal cells, for example in retinal injury, the activated microglia can manifest actions that can be detrimental. This is evidenced by activated microglia secreting inflammatory mediators that can further aggravate the retinal injury. Microglial activation as a harbinger of a variety of retinal diseases is well documented by many studies. In addition, a change in the microglial phenotype which may be associated with aging, may predispose the retina to age-related diseases. In light of the above, the focus of this review is to highlight the role played by microglia in the healthy and diseased retina, based on findings of our own work and from that of others.

Models of retinal diseases and their applicability in drug discovery

INTRODUCTION

The impact of vision debilitating diseases is a global public health concern, which will continue until effective preventative and management protocols are developed. Two retinal diseases responsible for the majority of vision loss in the working age adults and elderly populations are diabetic retinopathy (DR) and age-related macular degeneration (AMD), respectively. Model systems, which recapitulate aspects of human pathology, are valid experimental modalities that have contributed to the identification of signaling pathways involved in disease development and consequently potential therapies. Areas covered: The pathology of DR and AMD, which serve as the basis for designing appropriate models of disease, is discussed. The authors also review in vitro and in vivo models of DR and AMD and evaluate the utility of these models in exploratory and pre-clinical studies. Expert opinion: The complex nature of non-Mendelian diseases such as DR and AMD has made identification of effective therapeutic treatments challenging. However, the authors believe that while in vivo models are often criticized for not being a 'perfect' recapitulation of disease, they have been valuable experimentally when used with consideration of the strengths and limitations of the experimental model selected and have a place in the drug discovery process.

Molecular Chaperone ERp29: A Potential Target for Cellular Protection in Retinal and Neurodegenerative Diseases

The molecular chaperone endoplasmic reticulum protein 29 (ERp29) plays a critical role in protein folding, trafficking, and secretion. Though ubiquitously expressed, ERp29 is upregulated in response to ER stress and is found at higher levels in certain cell types such as secretory epithelial cells and neurons. As an ER resident protein, ERp29 shares many structural and functional similarities with protein disulfide isomerases, but is not regarded as part of this family due to several key differences. The broad expression and myriad roles of ERp29 coupled with its upregulation via the unfolded protein response (UPR) upon ER stress have implicated ERp29 in a range of cellular processes and diseases. We summarize the diverse activities of ERp29 in protein trafficking, cell survival and apoptosis, and ER homeostasis and highlight a potential role of ERp29 in neuroprotection in retinal and neurodegenerative diseases.

Assessment of Safety and Functional Efficacy of Stem Cell-Based Therapeutic Approaches Using Retinal Degenerative Animal Models

Dysfunction and death of retinal pigment epithelium (RPE) and or photoreceptors can lead to irreversible vision loss. The eye represents an ideal microenvironment for stem cell-based therapy. It is considered an “immune privileged” site, and the number of cells needed for therapy is relatively low for the area of focused vision (macula). Further, surgical placement of stem cell-derived grafts (RPE, retinal progenitors, and photoreceptor precursors) into the vitreous cavity or subretinal space has been well established. For preclinical tests, assessments of stem cell-derived graft survival and functionality are conducted in animal models by various noninvasive approaches and imaging modalities. In vivo experiments conducted in animal models based on replacing photoreceptors and/or RPE cells have shown survival and functionality of the transplanted cells, rescue of the host retina, and improvement of visual function. Based on the positive results obtained from these animal experiments, human clinical trials are being initiated. Despite such progress in stem cell research, ethical, regulatory, safety, and technical difficulties still remain a challenge for the transformation of this technique into a standard clinical approach. In this review, the current status of preclinical safety and efficacy studies for retinal cell replacement therapies conducted in animal models will be discussed.

Aerobic Glycolysis Hypothesis Through WNT/Beta-Catenin Pathway in Exudative Age-Related Macular Degeneration

Exudative age-related macular degeneration (AMD) is characterized by molecular mechanisms responsible for the initiation of choroidal neovascularization (CNV). Inflammatory processes are associated with upregulation of the canonical WNT/beta-catenin pathway in exudative AMD. We focus this review on the link between WNT/beta-catenin pathway activation and neovascular progression in exudative AMD through activation of aerobic glycolysis for production of angiogenic factors. Increased WNT/beta-catenin pathway involves hexokinase 2 (HK2) and pyruvate kinase M2 (PKM2). WNT/beta-catenin pathway stimulates PI3K/Akt pathway and then HIF-1alpha which activates glycolytic enzymes: glucose transporter (Glut), pyruvate dehydrogenase kinase 1 (PDK1), lactate dehydrogenase A (LDH-A), and monocarboxylate lactate transporter (MCT-1). This phenomenon is called aerobic glycolysis or the Warburg effect. Consequently, phosphorylation of PDK-1 inhibits the pyruvate dehydrogenase complex (PDH). Thus, a large part of pyruvate cannot be converted into acetyl-CoA in mitochondria and only a part of acetyl-CoA can enter the tricarboxylic acid cycle. Cytosolic pyruvate is converted into lactate through the action of LDH-A. In exudative AMD, high level of cytosolic lactate is correlated with increase of VEGF expression, the angiogenic factor of CNV. Photoreceptors in retina cells can metabolize glucose through aerobic glycolysis to protect them against oxidative damage, as cancer cells do.

Cell-based therapeutic strategies for replacement and preservation in retinal degenerative diseases

Cell-based therapeutics offer diverse options for treating retinal degenerative diseases, such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP). AMD is characterized by both genetic and environmental risks factors, whereas RP is mainly a monogenic disorder. Though treatments exist for some patients with neovascular AMD, a majority of retinal degenerative patients have no effective therapeutics, thus indicating a need for universal therapies to target diverse patient populations. Two main cell-based mechanistic approaches are being tested in clinical trials. Replacement therapies utilize cell-derived retinal pigment epithelial (RPE) cells to supplant lost or defective host RPE cells. These cells are similar in morphology and function to native RPE cells and can potentially supplant the responsibilities of RPE in vivo. Preservation therapies utilize supportive cells to aid in visual function and photoreceptor preservation partially by neurotrophic mechanisms. The goal of preservation strategies is to halt or slow the progression of disease and maintain remaining visual function. A number of clinical trials are testing the safety of replacement and preservation cell therapies in patients; however, measures of efficacy will need to be further evaluated. In addition, a number of prevailing concerns with regards to the immune-related response, longevity, and functionality of the grafted cells will need to be addressed in future trials. This review will summarize the current status of cell-based preclinical and clinical studies with a focus on replacement and preservation strategies and the obstacles that remain regarding these types of treatments.

A Review: Proteomics in Retinal Artery Occlusion, Retinal Vein Occlusion, Diabetic Retinopathy and Acquired Macular Disorders

Retinal artery occlusion (RAO), retinal vein occlusion (RVO), diabetic retinopathy (DR) and age-related macular degeneration (AMD) are frequent ocular diseases with potentially sight-threatening outcomes. In the present review we discuss major findings of proteomic studies of RAO, RVO, DR and AMD, including an overview of ocular proteome changes associated with anti-vascular endothelial growth factor (VEGF) treatments. Despite the severe outcomes of RAO, the proteome of the disease remains largely unstudied. There is also limited knowledge about the proteome of RVO, but proteomic studies suggest that RVO is associated with remodeling of the extracellular matrix and adhesion processes. Proteomic studies of DR have resulted in the identification of potential therapeutic targets such as carbonic anhydrase-I. Proliferative diabetic retinopathy is the most intensively studied stage of DR. Proteomic studies have established VEGF, pigment epithelium-derived factor (PEDF) and complement components as key factors associated with AMD. The aim of this review is to highlight the major milestones in proteomics in RAO, RVO, DR and AMD. Through large-scale protein analyses, proteomics is bringing new important insights into these complex pathological conditions.

Age-related macular degeneration phenotypes are associated with increased tumor necrosis-alpha and subretinal immune cells in aged Cxcr5 knockout mice

The role of chemokine receptor in age-related macular degeneration (AMD) remains elusive. The objective of this study is to investigate the role of chemokine receptor Cxcr5 in the pathogenesis of AMD. Cxcr5 gene expression levels (mRNA and protein) are higher in the retina and retinal pigment epithelium (RPE) of aged C57BL/6 wild type mice than younger ones. Vascular and glial cells express Cxcr5 and its ligand Cxcl13 in mouse retina. Aged Cxcr5 knockout (-/-) mice develop both early and late AMD-like pathological features. White and yellow spots, which look like drusen in humans, were identified with fundscopic examination. Drusen-like sub-RPE deposits with dome-shaped morphology were characterized on the sections. RPE vacuolization, swelling, and sub-RPE basal deposits were illustrated with light and transmission electron microscope (TEM). TEM further illustrated degenerated and disorganized RPE basal infoldings, phagosomes and melanosomes inside RPE, as well as abnormal photoreceptor outer segments. Lipofuscin granules and lipid droplets in the subretinal space, RPE, and choroid were revealed with fluorescence microscope and oil-red-O staining. Increased IgG in RPE/choroid were determined with Western blots (WB). WB and immunofluorescence staining determined RPE zona occuldens (ZO)-1 protein reduction and abnormal subcellular localization. TUNEL staining, outer nuclear layer (ONL) measurement and electroretinogram (ERG) recording indicated that photoreceptors underwent apoptosis, degeneration, and functional impairment. Additionally, spontaneous neovascularization (NV)-like lesions develop in the subretinal space of aged Cxcr5-/- mice. The underlying mechanisms are associated with increased subretinal F4/80+ immune cells, some of which contain RPE marker RPE65, and up-regulation of the multifunctional cytokine tumor necrosis factor-alpha (TNF-α) in RPE/choroid and retina. These findings suggest that Cxcr5 itself may be involved in the protection of RPE and retinal cells during aging and its loss may lead to AMD-like pathological changes in aged mice.

Evaluation of CC-cytokine ligand 2 and complementary factor H Y402H polymorphisms and their interactional association with age-related macular degeneration

PURPOSE

To evaluate the association of CC-cytokine ligand 2 CCL2-2518 (rs1024611) single nucleotide polymorphism, complement factor H (CFH Y402H) and their possible interaction in developing advanced age-related macular degeneration (AMD).

METHODS

In this case-control study, DNA samples from 266 patients with advanced AMD and 229 healthy controls were genotyped for CCL2 polymorphism and also 254 patients and 164 healthy controls were genotyped for CFH polymorphism. The possible associations of these polymorphisms with susceptibility to AMD independently and in different joint combinations were evaluated.

RESULTS

The genotype frequency for CFH was found to be significantly different between AMD and normal controls (31.5% versus 20.7%, OR = 3.56, p < 0.001 for CC and 52.4% versus 41.5%, OR = 2.96, p < 0.001 for CT genotype). However, no significant association between CCL2 polymorphism and AMD was observed in this cohort (OR = 1.15 and OR = 0.8, p = 0.172). Interestingly, studying the joint effects of two genotypes (TT genotype of CFH Y402H and AG genotype of CCL2-2518) showed more significant protective effect against AMD (p = 0.0001), while the risk effect of CC and CT genotypes of CFH was only visible in the presence of AA genotype of CCL2-2518 (p = 0.044 and p = 0.05).

CONCLUSION

Complement factor H Y402H polymorphism is strongly associated with advanced type AMD. Although this study revealed no association of CCL2-2518 with AMD, the risk effect of CFH genotypes was only visible in the presence of AA genotype of CCL2-2518. AG genotype of CCL2-2518 in combination with TT genotype of CFH Y402H showed significant protective effect against AMD.

Humanin Protects RPE Cells from Endoplasmic Reticulum Stress-Induced Apoptosis by Upregulation of Mitochondrial Glutathione

Humanin (HN) is a small mitochondrial-encoded peptide with neuroprotective properties. We have recently shown protection of retinal pigmented epithelium (RPE) cells by HN in oxidative stress; however, the effect of HN on endoplasmic reticulum (ER) stress has not been evaluated in any cell type. Our aim here was to study the effect of HN on ER stress-induced apoptosis in RPE cells with a specific focus on ER-mitochondrial cross-talk. Dose dependent effects of ER stressors (tunicamycin (TM), brefeldin A, and thapsigargin) were studied after 12 hr of treatment in confluent primary human RPE cells with or without 12 hr of HN pretreatment (1-20 μg/mL). All three ER stressors induced RPE cell apoptosis in a dose dependent manner. HN pretreatment significantly decreased the number of apoptotic cells with all three ER stressors in a dose dependent manner. HN pretreatment similarly protected U-251 glioma cells from TM-induced apoptosis in a dose dependent manner. HN pretreatment significantly attenuated activation of caspase 3 and ER stress-specific caspase 4 induced by TM. TM treatment increased mitochondrial superoxide production, and HN co-treatment resulted in a decrease in mitochondrial superoxide compared to TM treatment alone. We further showed that depleted mitochondrial glutathione (GSH) levels induced by TM were restored with HN co-treatment. No significant changes were found for the expression of several antioxidant enzymes between TM and TM plus HN groups except for the expression of glutamylcysteine ligase catalytic subunit (GCLC), the rate limiting enzyme required for GSH biosynthesis, which is upregulated with TM and TM+HN treatment. These results demonstrate that ER stress promotes mitochondrial alterations in RPE that lead to apoptosis. We further show that HN has a protective effect against ER stress-induced apoptosis by restoring mitochondrial GSH. Thus, HN should be further evaluated for its therapeutic potential in disorders linked to ER stress.

The Complement Regulatory Protein CD46 Deficient Mouse Spontaneously Develops Dry-Type Age-Related Macular Degeneration-Like Phenotype

In the mouse, membrane cofactor protein (CD46), a key regulator of the alternative pathway of the complement system, is only expressed in the eye and on the inner acrosomal membrane of spermatozoa. We noted that although Cd46(-/-) mice have normal systemic alternative pathway activating ability, lack of CD46 leads to dysregulated complement activation in the eye, as evidenced by increased deposition of C5b-9 in the retinal pigment epithelium (RPE) and choroid. A knockout of CD46 induced the following cardinal features of human dry age-related macular degeneration (AMD) in 12-month-old male and female mice: accumulation of autofluorescent material in and hypertrophy of the RPE, dense deposits in and thickening of Bruch's membrane, loss of photoreceptors, cells in subretinal space, and a reduction of choroidal vessels. Collectively, our results demonstrate spontaneous age-related degenerative changes in the retina, RPE, and choroid of Cd46(-/-) mice that are consistent with human dry AMD. These findings provide the exciting possibility of using Cd46(-/-) mice as a convenient and reliable animal model for dry AMD. Having such a relatively straight-forward model for dry AMD should provide valuable insights into pathogenesis and a test model system for novel drug targets. More important, tissue-specific expression of CD46 gives the Cd46(-/-) mouse model of dry AMD a unique advantage over other mouse models using knockout strains.

Risk factors and biomarkers of age-related macular degeneration

A biomarker can be a substance or structure measured in body parts, fluids or products that can affect or predict disease incidence. As age-related macular degeneration (AMD) is the leading cause of blindness in the developed world, much research and effort has been invested in the identification of different biomarkers to predict disease incidence, identify at risk individuals, elucidate causative pathophysiological etiologies, guide screening, monitoring and treatment parameters, and predict disease outcomes. To date, a host of genetic, environmental, proteomic, and cellular targets have been identified as both risk factors and potential biomarkers for AMD. Despite this, their use has been confined to research settings and has not yet crossed into the clinical arena. A greater understanding of these factors and their use as potential biomarkers for AMD can guide future research and clinical practice. This article will discuss known risk factors and novel, potential biomarkers of AMD in addition to their application in both academic and clinical settings.

Inflammation and its role in age-related macular degeneration

Inflammation is a cellular response to factors that challenge the homeostasis of cells and tissues. Cell-associated and soluble pattern-recognition receptors, e.g. Toll-like receptors, inflammasome receptors, and complement components initiate complex cellular cascades by recognizing or sensing different pathogen and damage-associated molecular patterns, respectively. Cytokines and chemokines represent alarm messages for leukocytes and once activated, these cells travel long distances to targeted inflamed tissues. Although it is a crucial survival mechanism, prolonged inflammation is detrimental and participates in numerous chronic age-related diseases. This article will review the onset of inflammation and link its functions to the pathogenesis of age-related macular degeneration (AMD), which is the leading cause of severe vision loss in aged individuals in the developed countries. In this progressive disease, degeneration of the retinal pigment epithelium (RPE) results in the death of photoreceptors, leading to a loss of central vision. The RPE is prone to oxidative stress, a factor that together with deteriorating functionality, e.g. decreased intracellular recycling and degradation due to attenuated heterophagy/autophagy, induces inflammation. In the early phases, accumulation of intracellular lipofuscin in the RPE and extracellular drusen between RPE cells and Bruch's membrane can be clinically detected. Subsequently, in dry (atrophic) AMD there is geographic atrophy with discrete areas of RPE loss whereas in the wet (exudative) form there is neovascularization penetrating from the choroid to retinal layers. Elevations in levels of local and systemic biomarkers indicate that chronic inflammation is involved in the pathogenesis of both disease forms.

The retinal pigment epithelium as a gateway for monocyte trafficking into the eye

The choroid plexus epithelium within the brain ventricles orchestrates blood-derived monocyte entry to the central nervous system under injurious conditions, including when the primary injury site is remote from the brain. Here, we hypothesized that the retinal pigment epithelium (RPE) serves a parallel role, as a gateway for monocyte trafficking to the retina following direct or remote injury. We found elevated expression of genes encoding leukocyte trafficking determinants in mouse RPE as a consequence of retinal glutamate intoxication or optic nerve crush (ONC). Blocking VCAM-1 after ONC interfered with monocyte infiltration into the retina and resulted in a local pro-inflammatory cytokine bias. Live imaging of the injured eye showed monocyte accumulation first in the RPE, and subsequently in the retina, and peripheral leukocytes formed close contact with the RPE Our findings further implied that the ocular milieu can confer monocytes a phenotype advantageous for neuroprotection. These results suggest that the eye utilizes a mechanism of crosstalk with the immune system similar to that of the brain, whereby epithelial barriers serve as gateways for leukocyte entry.

Repeatability and Reproducibility of Spectral-Domain Optical Coherence Tomography Measurements of Retinal Thickness in Rats

PURPOSE

To evaluate the in vivo repeatability and reproducibility of spectral-domain optical coherence tomography (SD-OCT) measurements of total retinal thickness (TRT) and retinal nerve fiber layer (RNFL) thickness in rats.

METHODS

Retinal thickness was measured using an RNFL circular scan with AutoRescan and TruTracking modes in 20 eyes of Long-Evans Tokushima Otsuka rats. Three RNFL circular scan images were acquired with a brief rest between measurements to evaluate intra-session repeatability. Three additional RNFL circular scans were acquired 1 day later to evaluate intersession repeatability. Two experienced examiners independently measured TRT using automatic alignment and RNFL thickness using manual alignment according to the same procedure to evaluate inter-examiner reproducibility 1 week later. The correlation of measurements by OCT and histology was determined.

RESULTS

For TRT, the intra-session and inter-session intraclass correlation coefficient (ICC) with SD-OCT was between 0.85 and 0.93. The coefficient of variance (CV) ranged from 3.9% to 4.8%. For RNFL thickness, intra-session, and inter-session ICC with SD-OCT were between 0.61 and 0.91. The inter-examiner ICC was 0.74 for TRT and 0.70 for RNFL. The CV was 4.4% for TRT and 6.0% for RNFL. OCT and histologically determined TRT and RNFL thicknesses were significantly positively related based on Pearson's correlation coefficient (TRT, r = 0.781, p < 0.001; RNFL, r = 0.517, p = 0.019).

CONCLUSIONS

TRT and RNFL thickness measurements using SD-OCT in rats had high intra-session and inter-session repeatability and inter-examiner reproducibility in vivo. This method will facilitate longitudinal studies to follow disease processes over time and to evaluate therapeutic effects after experimental intervention.

NLRP3 Upregulation in Retinal Pigment Epithelium in Age-Related Macular Degeneration

Inflammation and oxidative stress are involved in age-related macular degeneration (AMD) and possibly associated with an activation of neuronal apoptosis inhibitor protein/class II transcription activator of the Major Histocompatibility Complex (MHC)/heterokaryon incompatibility/telomerase-associated protein 1, leucine-rich repeat or nucleotide-binding domain, leucine-rich repeat-containing family, and pyrin domain-containing 3 (NLRP3) inflammasome. In the present study, we used a translational approach to address this hypothesis. In patients with AMD, we observed increased mRNA levels of NLRP3, pro-interleukin-1 beta (IL-1β) and pro-IL-18 in AMD lesions of the retinal pigment epithelium (RPE) and photoreceptor. In vitro, a similar increase was evoked by oxidative stress or lipopolysaccharide (LPS) stimulation in the adult retinal pigment epithelium (ARPE-19) cell line, and the increase was reduced in siRNA transfected cells to knockdown NLRP3. Ultrastructural studies of ARPE-19 cells showed a swelling of the cytoplasm, mitochondrial damage, and occurrence of autophagosome-like structures. NLRP3 positive dots were detected within autophagosome-like structures or in the extracellular space. Next, we used a mouse model of AMD, Ccl2/Cx3cr1 double knockout on rd8 background (DKO rd8) to ascertain the in vivo relevance. Ultrastructural studies of the RPE of these mice showed damaged mitochondria, autophagosome-like structures, and cytoplasmic vacuoles, which are reminiscent of the pathology seen in stressed ARPE-19 cells. The data suggest that the NLRP3 inflammasome may contribute in AMD pathogenesis.

Erp29 Attenuates Cigarette Smoke Extract-Induced Endoplasmic Reticulum Stress and Mitigates Tight Junction Damage in Retinal Pigment Epithelial Cells

PURPOSE

Endoplasmic reticulum protein 29 (ERp29) is a novel chaperone that was recently found decreased in human retinas with AMD. Herein, we examined the effect of ERp29 on cigarette smoke-induced RPE apoptosis and tight junction disruption.

METHODS

Cultured human RPE (HRPE) cells (ARPE-19) or mouse RPE eyecup explants were exposed to cigarette smoke extract (CSE) for short (up to 24 hours) or long (up to 3 weeks) periods. Expression of ERp29 was up- and downregulated by adenovirus and siRNA, respectively. Endoplasmic reticulum stress markers, apoptosis, and cell death, the expression and distribution of tight junction protein ZO-1, transepithelial electrical resistance (TEER), and F-actin expression were examined.

RESULTS

Endoplasmic reticulum protein 29 was significantly increased by short-term exposure to CSE in ARPE-19 cells or eyecup explants but was reduced after 3-week exposure. Overexpression of ERp29 increased the levels of GRP78, p58(IPK), and Nrf-2, while reducing p-eIF2α and C/EBP homologous protein (CHOP), and protected RPE cells from CSE-induced apoptosis. In contrast, knockdown of ERp29 decreased the levels of p58(IPK) and Nrf2, but increased p-eIF2α and CHOP and exacerbated CSE-triggered cell death. In addition, overexpression of ERp29 attenuated CSE-induced reduction in ZO-1 and enhanced the RPE barrier function, as measured by TEER. Knockdown of ERp29 decreased the level of ZO-1 protein. These effects were associated with changes in the expression of cytoskeleton F-actin.

CONCLUSIONS

Endoplasmic reticulum protein 29 attenuates CSE-induced ER stress and enhances cell viability and barrier integrity of RPE cells, and therefore may act as a protective mechanism for RPE survival and activity.

A Brief Discussion on Lipid Activated Nuclear Receptors and their Potential Role in Regulating Microglia in Age-Related Macular Degeneration (AMD)

Age-related macular degeneration (AMD) is the leading cause of legal blindness and visual impairment in individuals over 60 years of age in the Western World. A common morphological denominator in all forms of AMD is the accumulation of microglia within the sub-retinal space, which is believed to be a contributing factor to AMD progression. However, the signaling pathway and molecular players regulating microglial recruitment have not been completely identified. Multiple in-vitro and in-vivo studies, to date, have highlighted the contributions of nuclear receptor ligands in the treatment of inflammation related disorders such as atherosclerosis and Alzheimer's disease. Given that inflammation and the immune response play a vital role in the initiation and progression of AMD, in this brief review we will highlight some of these studies with a particular focus on the lipid activated "adopted orphan" nuclear receptors, the liver x receptors (LXRs) and the peroxisome proliferator-activated receptors (PPARs). The results of these studies strongly support the rationale that treatment with LXR and PPAR ligands may ameliorate microglial activation in the sub-retinal space and ultimately slow down or reverse the progression of AMD.

Mthfr as a modifier of the retinal phenotype of Crb1(rd8/rd8) mice

Mutations in crumb homologue 1 (CRB1) in humans are associated with Leber's congenital amaurosis (LCA) and retinitis pigmentosa (RP). There is no clear genotype-phenotype correlation for human CRB1 mutations in RP and LCA. The high variability in clinical features observed in CRB1 mutations suggests that environmental factors or genetic modifiers influence severity of CRB1 related retinopathies. Retinal degeneration 8 (rd8) is a spontaneous mutation in the Crb1 gene (Crb1(rdr/rd8)). Crb1(rdr/rd8) mice present with focal disruption in the outer retina manifesting as white spots on fundus examination. Mild retinal dysfunction with decreased b-wave amplitude has been reported in Crb1(rdr/rd8) mice at 18 months. Methylene tetrahydrofolate reductase (MTHFR) is a crucial enzyme of homocysteine metabolism. MTHFR mutations are prevalent in humans and are linked to a broad spectrum of disorders including cardiovascular and neurodegenerative diseases. We recently reported the retinal phenotype in Mthfr-deficient (Mthfr(+/-)) heterozygous mice. At 24 weeks the mice showed decreased RGC function, thinner nerve fiber layer, focal areas of vascular leakage and 20% fewer cells in the ganglion cell layer (GCL). Considering the variability in CRB1-related retinopathies and the high occurrence of human MTHFR mutations we evaluated whether Mthfr deficiency influences rd8 retinal phenotype. Mthfr heterozygous mice with rd8 mutations (Mthfr(+/-)(rd8/rd8)) and Crb(rd8/rd8) mice (Mthfr(+/+rd8/rd8)) mice were subjected to comprehensive retinal evaluation using ERG, fundoscopy, fluorescein angiography (FA), morphometric and retinal flat mount immunostaining analyses of isolectin-B4 at 8-54 wks. Assessment of retinal function revealed a significant decrease in the a-, b- and c-wave amplitudes in Mthfr(+/-)(rd8/rd8) mice at 52 wks. Fundoscopic evaluation demonstrated the presence of signature rd8 spots in Mthfr(+/+rd8/rd8) mice and an increase in the extent of these rd8 spots in Mthfr(+/-)(rd8/rd8) mice at 24 weeks and beyond. FA revealed marked vascular leakage, ischemia and vascular tortuosity in Mthfr(+/-)(rd8/rd8) mice at 24 and 52 weeks. Retinal dysplasia was observed in ∼14-33% Mthfr(+/-)(rd8/rd8) mice by morphometric analysis. This was accompanied by a ∼20% reduction in cells of the GCL of Mthfr(+/-)(rd8/rd8) mice at 24 and 52 weeks. Retinal flat mount immunostaining with isolectin-B4 showed neovascularization and loss of blood vessel integrity in Mthfr(+/-)(rd8/rd8) mice in contrast to mild vasculopathy in Mthfr(+/+rd8/rd8) mice. Taken together, our data support an earlier onset and worsened retinal phenotype when Mthfr and rd8 mutations coexist. Our study sets the stage for future studies to investigate the role of MTHFR deficiency in human CRB1 retinopathies.

RPE necroptosis in response to oxidative stress and in AMD

Age-related macular degeneration (AMD) is the leading cause of irreversible blindness in the elderly. The underlying mechanism of non-neovascular AMD (dry AMD), also named geographic atrophy (GA) remains unclear and the mechanism of retinal pigment epithelial (RPE) cell death in AMD is controversial. We review the history and recent progress in understanding the mechanism of RPE cell death induced by oxidative stress, in AMD mouse models, and in AMD patients. Due to the limitation of toolsets to distinguish between apoptosis and necroptosis (or necrosis), most previous research concludes that apoptosis is a major mechanism for RPE cell death in response to oxidative stress and in AMD. Recent studies suggest necroptosis as a major mechanism of RPE cell death in response to oxidative stress. Moreover, ultrastructural and histopathological studies support necrosis as major mechanism of RPE cells death in AMD. In this review, we discuss the mechanism of RPE cell death in response to oxidative stress, in AMD mouse models, and in human AMD patients. Based on the literature, we hypothesize that necroptosis is a major mechanism for RPE cell death in response to oxidative stress and in AMD.

Wnt signaling in age-related macular degeneration: human macular tissue and mouse model

Background

The wingless-type MMTV integration site (Wnt) signaling is a group of signal transduction pathways. In canonical Wnt pathway, Wnt ligands bind to low-density lipoprotein receptor-related protein 5 or 6 (LRP5 or LRP6), resulting in phosphorylation and activation of the receptor. We hypothesize that canonical Wnt pathway plays a role in the retinal lesion of age-related macular degeneration (AMD), a leading cause of irreversible central visual loss in elderly.

Methods

We examined LRP6 phosphorylation and Wnt signaling cascade in human retinal sections and plasma kallistatin, an endogenous inhibitor of the Wnt pathway in AMD patients and non-AMD subjects. We also used the Ccl2^−/−/Cx3cr1^−/−/rd8 and Ccl2^−/−/Cx3cr1^gfp/gfp mouse models with AMD-like retinal degeneration to further explore the involvement of Wnt signaling activation in the retinal lesions in those models and to preclinically evaluate the role of Wnt signaling suppression as a potential therapeutic option for AMD.

Results

We found higher levels of LRP6 (a key Wnt signaling receptor) protein phosphorylation and transcripts of the Wnt pathway-targeted genes, as well as higher beta-catenin protein in AMD macula compared to controls. Kallistatin was decreased in the plasma of AMD patients. Retinal non-phosphorylated-β-catenin and phosphorylated-LRP6 were higher in Ccl2^−/−/Cx3cr1^−/−/rd8 mice than that in wild type. Intravitreal administration of an anti-LRP6 antibody slowed the progression of retinal lesions in Ccl2^−/−/Cx3cr1^−/−/rd8 and Ccl2^−/−/Cx3cr1^gfp/gfp mice. Electroretinography of treated eyes exhibited larger amplitudes compared to controls in both mouse models. A2E, a retinoid byproduct associated with AMD was lower in the treated eyes of Ccl2^−/−/Cx3cr1^−/−/rd8 mice. Anti-LRP6 also suppressed the expression of Tnf-α and Icam-1 in Ccl2^−/−/Cx3cr1^−/−/rd8 retinas.

Conclusions

Wnt signaling may be disturbed in AMD patients, which could contribute to the retinal inflammation and increased A2E levels found in AMD. Aberrant activation of canonical Wnt signaling might also contribute to the focal retinal degenerative lesions of mouse models with Ccl2 and Cx3cr1 deficiency, and intravitreal administration of anti-LRP6 antibody could be beneficial by deactivating the canonical Wnt pathway.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-015-0683-x) contains supplementary material, which is available to authorized users.