Bruch's membrane changes in transgenic mice overexpressing the human biglycan and apolipoprotein b-100 genes

Fibroblast growth factor-21 alleviates phenotypic characteristics of dry age-related macular degeneration in mice

Age-related macular degeneration (AMD) is the main cause of blindness in elderly individuals. As a metabolic regulator, fibroblast growth factor 21 (FGF-21) has been proven indicated to have an effect on wet AMD, but whether this cytokine has a therapeutic effect on dry AMD is unclear. The current study aimed to evaluate the preventive effects of FGF-21 against retinal degeneration in mice and provide mechanistic insights. FGF-21-/- mice were raised to 10 months of age. Then, the morphological changes in the retinal pigment epithelium (RPE)/choroid of the mice were observed by transmission electron microscopy (TEM), and iTRAQ was used to detect the variations in the protein profile. Next, FGF-21-/- and wild-type mice of the same age were fed hydroquinone to generate a dry AMD mouse model to examine whether exogenous FGF-21 can interfere with the occurrence and development of dry AMD. In vivo studies revealed that following FGF-21 knockout, there was an increase in the expression of complement in the RPE/choroid concomitant with the occurrence of dry AMD-like pathological changes. Furthermore, exogenous FGF-21 administration effectively reversed this phenomenon. FGF-21 also demonstrated strong anti-inflammatory effects in the RPE/choroid by inhibiting the NF-κB pathway. In conclusion, the present study demonstrates that FGF-21 treatment presents a novel therapeutic approach for the prevention and development of dry AMD by reducing complement.

Exercise training worsens cardiac performance in males but does not change ejection fraction and improves hypertrophy in females in a mouse model of metabolic syndrome

BACKGROUND

Metabolic syndrome (MetS) refers to a cluster of co-existing cardio-metabolic risk factors, including visceral obesity, dyslipidemia, hyperglycemia with insulin resistance, and hypertension. As there is a close link between MetS and cardiovascular diseases, we aimed to investigate the sex-based differences in MetS-associated heart failure (HF) and cardiovascular response to regular exercise training (ET).

METHODS

High-fat diet-fed male and female APOB-100 transgenic (HFD/APOB-100, 3 months) mice were used as MetS models, and age- and sex-matched C57BL/6 wild-type mice on standard diet served as healthy controls (SD/WT). Both the SD/WT and HFD/APOB-100 mice were divided into sedentary and ET groups, the latter running on a treadmill (0.9 km/h) for 45 min 5 times per week for 7 months. At month 9, transthoracic echocardiography was performed to monitor cardiac function and morphology. At the termination of the experiment at month 10, blood was collected for serum low-density lipoprotein (LDL)- and high-density lipoprotein (HDL)-cholesterol measurements and homeostatic assessment model for insulin resistance (HOMA-IR) calculation. Cardiomyocyte hypertrophy and fibrosis were assessed by histology. Left ventricular expressions of selected genes associated with metabolism, inflammation, and stress response were investigated by qPCR.

RESULTS

Both HFD/APOB-100 males and females developed obesity and hypercholesterolemia; however, only males showed insulin resistance. ET did not change these metabolic parameters. HFD/APOB-100 males showed echocardiographic signs of mild HF with dilated ventricles and thinner walls, whereas females presented the beginning of left ventricular hypertrophy. In response to ET, SD/WT males developed increased left ventricular volumes, whereas females responded with physiologic hypertrophy. Exercise-trained HFD/APOB-100 males presented worsening HF with reduced ejection fraction; however, ET did not change the ejection fraction and reversed the echocardiographic signs of left ventricular hypertrophy in HFD/APOB-100 females. The left ventricular expression of the leptin receptor was higher in females than males in the SD/WT groups. Left ventricular expression levels of stress response-related genes were higher in the exercise-trained HFD/APOB-100 males and exercise-trained SD/WT females than exercise-trained SD/WT males.

CONCLUSIONS

HFD/APOB-100 mice showed sex-specific cardiovascular responses to MetS and ET; however, left ventricular gene expressions were similar between the groups except for leptin receptor and several stress response-related genes.

Differentially Regulated Apolipoproteins and Lipid Profiles as Novel Biomarkers for Polypoidal Choroidal Vasculopathy and Neovascular Age-Related Macular Degeneration

Lipid dyshomeostasis has been implicated in the pathogenesis of various retinal and choroidal vascular diseases. This study aims to investigate whether apolipoprotein (apo) mediated differential regulation of lipid metabolism contributes to the phenotypes of polypoidal choroidal vasculopathy (PCV) and neovascular age-related macular degeneration (nAMD). This study involved 148 subjects including 53 patients with PCV, 44 patients with nAMD, and 51 age-, sex-matched subjects with normal fundus controls. Routine blood biochemistry profile was evaluated. Apolipoproteins was estimated by Luminex technology. After controlling for age, gender, body mass index, duration of hypertension and type 2 diabetes mellitus, apoB/non-high density lipoprotein cholesterol (HDL-C) (p=0.015) was an independent risk factor for nAMD, apoB was an independent risk factor for PCV(p=0.011), compared with control. Low-density lipoprotein cholesterol (LDL-C) was significantly higher in patients with PCV when compared with nAMD (p=0.037). Furthermore, apoB/non-HDL, LDL-C, triglycerides and were significantly correlated with the pathogenesis of subgroups of PCV and nAMD. We concluded that lipid profiles and apos are differential regulated in PCV, nAMD and their subtypes, indicating different pathogenicity contributed to the different phenotypes of PCV and nAMD. Non-pachy PCV shares pathological similarities with nAMD, which is highly correlated with age-related atherosclerosis.

Small Leucine-Rich Proteoglycans (SLRPs) in the Retina

Retinal diseases such as age-related macular degeneration (AMD), retinopathy of prematurity (ROP), and diabetic retinopathy (DR) are the leading causes of visual impairment worldwide. There is a critical need to understand the structural and cellular components that play a vital role in the pathophysiology of retinal diseases. One potential component is the family of structural proteins called small leucine-rich proteoglycans (SLRPs). SLRPs are crucial in many fundamental biological processes involved in the maintenance of retinal homeostasis. They are present within the extracellular matrix (ECM) of connective and vascular tissues and contribute to tissue organization and modulation of cell growth. They play a vital role in cell–matrix interactions in many upstream signaling pathways involved in fibrillogenesis and angiogenesis. In this comprehensive review, we describe the expression patterns and function of SLRPs in the retina, including Biglycan and Decorin from class I; Fibromodulin, Lumican, and a Proline/arginine-rich end leucine-rich repeat protein (PRELP) from class II; Opticin and Osteoglycin/Mimecan from class III; and Chondroadherin (CHAD), Tsukushi and Nyctalopin from class IV.

Metabolic syndrome and the aging retina

PURPOSE OF REVIEW

This review explores metabolic syndrome (MetS) as a risk factor that accelerates aging in retinal neurons and may contribute to the neurodegeneration seen in glaucomatous optic neuropathy (GON) and age-related macular degeneration (AMD).

RECENT FINDINGS

Both animal model experiments and epidemiologic studies suggest that metabolic stress may lead to aberrant regulation of a number of cellular pathways that ultimately lead to premature aging of the cell, including those of a neuronal lineage.

SUMMARY

GON and AMD are each leading causes of irreversible blindness worldwide. Aging is a significant risk factor in the specific retinal neuron loss that is seen with each condition. Though aging at a cellular level is difficult to define, there are many mechanistic modifiers of aging. Metabolic-related stresses induce inflammation, oxidative stress, mitochondrial dysfunction, endoplasmic reticulum stress, alterations to the unfolded protein response, defects in autophagy, alterations to the microbiome, and deposition of advanced glycation end products that can all hasten the aging process. Due to the number of variables related to metabolic health, defining criteria to enable the study of risk factors at a population level is challenging. MetS is a definable constellation of related metabolic risk factors that includes enlarged waist circumference, dyslipidemia, systemic hypertension, and hyperglycemia. MetS has been associated with both GON and AMD and may contribute to disease onset and/or progression in each disease.

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.

Diet Mimicking "Fast Food" Causes Structural Changes to the Retina Relevant to Age-Related Macular Degeneration

Introduction: Metabolic syndrome is a disorder characterized by a constellation of findings including truncal obesity, elevated blood pressure, abnormal cholesterol levels, and high blood glucose. Recent evidence suggests that metabolic syndrome may be associated with increased risk of age-related macular degeneration (AMD) and other eye diseases. Recently, C57BL/6J wild-type mice fed with a "fast food" diet consisting of high fat, cholesterol, and fructose-supplemented water showed unique systemic pathology consistent with metabolic syndrome and nonalcoholic steatohepatitis. Additionally, these mice showed higher levels of fibrosis, inflammation, endoplasmic reticulum stress, and mitochondrial dysfunction compared to mice fed with only a high-fat diet alone. Since similar pathways are activated in AMD, we sought to determine whether mice fed a "fast food" diet exhibited retinal changes.Methods: 3-month-old wild-type mice were randomized to a standard chow (n = 11) or a "fast food" (n = 18) diet and fed for 9 months. At 1 year of age, tissues were collected and retinas were analyzed using transmission electron microscopy. Quantitative measures of Bruch's membrane thickness and retinal pigment epithelium (RPE) cell counts were performed.Results: "Fast food" fed mice showed ocular pathology relevant to various stages of AMD including basal laminar deposits, focal thickening of Bruch's membrane, and a significant loss of RPE cells.Discussion/conclusion: A wild-type mouse model of metabolic syndrome fed a "fast food" diet developed changes to the retina similar to some of the pathologic features seen in AMD. Further investigations into this and similar animal models as well as further epidemiological studies are needed to more clearly define the association between metabolic syndrome and AMD.

A new perspective on lipid research in age-related macular degeneration

There is an urgency to find new treatment strategies that could prevent or delay the onset or progression of AMD. Different classes of lipids and lipoproteins metabolism genes have been associated with AMD in a multiple ways, but despite the ever-increasing knowledge base, we still do not understand fully how circulating lipids or local lipid metabolism contribute to AMD. It is essential to clarify whether dietary lipids, systemic or local lipoprotein metabolismtrafficking of lipids in the retina should be targeted in the disease. In this article, we critically evaluate what has been reported in the literature and identify new directions needed to bring about a significant advance in our understanding of the role for lipids in AMD. This may help to develop potential new treatment strategies through targeting the lipid homeostasis.

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.

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.

Cellular and Molecular Pathology of Age-Related Macular Degeneration: Potential Role for Proteoglycans

Age-related macular degeneration (AMD) is a retinal disease evident after the age of 50 that damages the macula in the centre of retina. It leads to a loss of central vision with retained peripheral vision but eventual blindness occurs in many cases. The initiation site of AMD development is Bruch's membrane (BM) where multiple changes occur including the deposition of plasma derived lipids, accumulation of extracellular debris, changes in cell morphology, and viability and the formation of drusen. AMD manifests as early and late stage; the latter involves cell proliferation and neovascularization in wet AMD. Current therapies target the later hyperproliferative and invasive wet stage whilst none target early developmental stages of AMD. In the lipid deposition disease atherosclerosis modified proteoglycans bind and retain apolipoproteins in the artery wall. Chemically modified trapped lipids are immunogenic and can initiate a chronic inflammatory process manifesting as atherosclerotic plaques and subsequent artery blockages, heart attacks, or strokes. As plasma derived lipoprotein deposits are found in BM in early AMD, it is possible that they arise by a similar process within the macula. In this review we consider aspects of the pathological processes underlying AMD with a focus on the potential role of modifications to secreted proteoglycans being a cause and therefore a target for the treatment of early AMD.

Gene-Diet Interactions in Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is a prevalent blinding disease, accounting for roughly 50 % of blindness in developed nations. Very significant advances have been made in terms of discovering genetic susceptibilities to AMD as well as dietary risk factors. To date, nutritional supplementation is the only available treatment option for the dry form of the disease known to slow progression of AMD. Despite an excellent understanding of genes and nutrition in AMD, there is remarkably little known about gene-diet interactions that may identify efficacious approaches to treat individuals. This review will summarize our current understanding of gene-diet interactions in AMD with a focus on animal models and human epidemiological studies.

Mapping the differential distribution of proteoglycan core proteins in the adult human retina, choroid, and sclera

PURPOSE

To examine the presence and distribution of proteoglycan (PG) core proteins in the adult human retina, choroid, and sclera.

METHODS

Postmortem human eye tissue was dissected into Bruch's membrane/choroid complex, isolated Bruch's membrane, or neurosensory retina. PGs were extracted and partially purified by anion exchange chromatography. Trypsinized peptides were analyzed by tandem mass spectrometry and PG core proteins identified by database search. The distribution of PGs was examined by immunofluorescence microscopy on human macular tissue sections.

RESULTS

The basement membrane PGs perlecan, agrin, and collagen-XVIII were identified in the human retina, and were present in the internal limiting membrane, blood vessel walls, and Bruch's membrane. The hyalectans versican and aggrecan were also detected. Versican was identified in Bruch's membrane, while aggrecan was distributed throughout the retina, choroid, and sclera. The cartilage link protein HAPLN1 was abundant in the interphotoreceptor matrix and sclera, while HAPLN4 (brain link protein 2) was found throughout the retina and choroid. The small leucine-rich repeat PG (SLRP) family members biglycan, decorin, fibromodulin, lumican, mimecan, opticin, and prolargin were present, with different patterns of distribution in the retina, choroid, and sclera.

CONCLUSIONS

A combination of proteomics and immunohistochemistry approaches has provided for the first time a comprehensive analysis of the presence and distribution of PG core proteins throughout the human retina, choroid, and sclera. This complements our knowledge of glycosaminoglycan chain distribution in the human eye, and has important implications for understanding the structure and functional regulation of the eye in health and disease.

Animal models of age related macular degeneration

Age related macular degeneration (AMD) is the leading cause of vision loss of those over the age of 65 in the industrialized world. The prevalence and need to develop effective treatments for AMD has lead to the development of multiple animal models. AMD is a complex and heterogeneous disease that involves the interaction of both genetic and environmental factors with the unique anatomy of the human macula. Models in mice, rats, rabbits, pigs and non-human primates have recreated many of the histological features of AMD and provided much insight into the underlying pathological mechanisms of this disease. In spite of the large number of models developed, no one model yet recapitulates all of the features of human AMD. However, these models have helped reveal the roles of chronic oxidative damage, inflammation and immune dysregulation, and lipid metabolism in the development of AMD. Models for induced choroidal neovascularization have served as the backbone for testing new therapies. This article will review the diversity of animal models that exist for AMD as well as their strengths and limitations.

DNA methylation is associated with altered gene expression in AMD

PURPOSE

Age-related macular degeneration (AMD) is the leading cause of blindness in the elderly. Evidence suggests oxidative stress plays a role in the disease. To assess the potential contribution of epigenetic regulation of antioxidant genes relevant to AMD pathogenesis, we evaluated DNA methylation, a tissue-specific genetic modulation that affects gene expression.

METHODS

Using the Infinium HumanMethylation27 Illumina platform, we performed DNA bisulfite sequencing to compare the methylation status in postmortem retina pigment epithelium (RPE)/choroid between patients with AMD and age-matched controls. Gene expression was assessed with the Affymetrix Exon Array. TaqMan gene expression assays were used for relative quantification (RT-PCR) confirmation of the expression array results: Glutathione S-transferase isoform mu1 (GSTM1) and mu5 (GSTM5) promoter methylation was confirmed by CpG island bisulfite pyrosequencing. To assess protein levels and localization, we used Western analysis, immunohistochemistry, and immunofluorescence with murine and human samples.

RESULTS

The mRNA levels of GSTM1 and GSTM5 were significantly reduced in AMD versus age-matched controls in RPE/choroid and neurosensory retina (NSR), which corresponded to hypermethylation of the GSTM1 promoter. mRNA and protein levels were decreased (RPE to a greater extent than NSR) in AMD postmortem samples, irrespective of age. Immunohistochemistry and immunofluorescence confirm the presence of the enzymes in the NSR and RPE.

CONCLUSIONS

Comparison of DNA methylation, together with mRNA levels, revealed significant differences between AMD versus normal retinas. The evidence presented suggests that GSTM1 and GSTM5 undergo epigenetic repression in AMD RPE/choroid, which may increase susceptibility to oxidative stress in AMD retinas.

Identifying subtypes of patients with neovascular age-related macular degeneration by genotypic and cardiovascular risk characteristics

BACKGROUND

One of the challenges in the interpretation of studies showing associations between environmental and genotypic data with disease outcomes such as neovascular age-related macular degeneration (AMD) is understanding the phenotypic heterogeneity within a patient population with regard to any risk factor associated with the condition. This is critical when considering the potential therapeutic response of patients to any drug developed to treat the condition. In the present study, we identify patient subtypes or clusters which could represent several different targets for treatment development, based on genetic pathways in AMD and cardiovascular pathology.

METHODS

We identified a sample of patients with neovascular AMD, that in previous studies had been shown to be at elevated risk for the disease through environmental factors such as cigarette smoking and genetic variants including the complement factor H gene (CFH) on chromosome 1q25 and variants in the ARMS2/HtrA serine peptidase 1 (HTRA1) gene(s) on chromosome 10q26. We conducted a multivariate segmentation analysis of 253 of these patients utilizing available epidemiologic and genetic data.

RESULTS

In a multivariate model, cigarette smoking failed to differentiate subtypes of patients. However, four meaningfully distinct clusters of patients were identified that were most strongly differentiated by their cardiovascular health status (histories of hypercholesterolemia and hypertension), and the alleles of ARMS2/HTRA1 rs1049331.

CONCLUSIONS

These results have significant personalized medicine implications for drug developers attempting to determine the effective size of the treatable neovascular AMD population. Patient subtypes or clusters may represent different targets for therapeutic development based on genetic pathways in AMD and cardiovascular pathology, and treatments developed that may elevate CV risk, may be ill advised for certain of the clusters identified.

Advanced glycation endproduct changes to Bruch's membrane promotes lipoprotein retention by lipoprotein lipase

Lipoprotein particles accumulate in Bruch's membrane before the development of basal deposits and drusen, two histopathologic lesions that define age-related macular degeneration (AMD). We therefore, sought to determine which molecules could participate in lipoprotein retention. Wild-type or lipoprotein lipase-deficient mice were injected with low-dose D-galactose or PBS subcutaneously for 8 weeks to induce advanced glycation endproduct (AGE) formation. Some mice were also injected with the AGE breaker phenacylphiazolium bromide and D-galactose. Rhodamine-labeled low-density lipoproteins were injected into mice, and the fluorescence was measured up to 72 hours later. AGEs, proteoglycans, and other lipid-retaining molecules were evaluated by IHC. Lipoprotein lipase distribution was assessed in AMD samples by IHC. D-galactose-treated mice retained lipoproteins in the retinal pigment epithelial and Bruch's membrane to a greater extent than either PBS- or phenacylphiazolium bromide/D-galactose-treated mice at 24 and 72 hours after injection (P ≤ 0.04). Immunolabeling for carboxymethyllysine, biglycan, and lipoprotein lipase was found in D-galactose-treated mice only. Mice deficient for lipoprotein lipase treated with D-galactose did not retain lipoproteins to any measureable extent. Human AMD samples had lipoprotein lipase labeling within drusen, basal deposits, and the choroid. Mice treated with D-galactose to induce AGE formation in Bruch's membrane retain intravenously injected lipoproteins. Our results suggest that lipoprotein retention in Bruch's membrane is mediated by lipoprotein lipase.

Abundant lipid and protein components of drusen

Background

Drusen are extracellular lesions characteristic of aging and age-related maculopathy, a major retinal disease of the elderly. We determined the relative proportions of lipids and proteins in drusen capped with retinal pigment epithelium (RPE) and in RPE isolated from non-macular regions of 36 human retinas with grossly normal maculas obtained <6 hr after death.

Methodology/Principal Findings

Druse pellets were examined by light and electron microscopy. Component proteins were extracted using novel methods for preserved tissues, separated, subjected to tryptic digestion and LC-MS(MS)² analysis using an ion trap mass spectrometer, and identified with reference to databases. Lipid classes were separated using thin layer chromatography and quantified by densitometry. Major druse components were esterified cholesterol (EC), phosphatidylcholine (PC), and protein (37.5±13.7, 36.9±12.9, and 43.0±11.5 ng/druse, respectively). Lipid-containing particles (median diameter, 77 nm) occupied 37–44% of druse volume. Major proteins include vitronectin, complement component 9, apoE, and clusterin, previously seen in drusen, and ATP synthase subunit β, scavenger receptor B2, and retinol dehydrogenase 5, previously seen in RPE. Drusen and RPE had similar protein profiles, with higher intensities and greater variability in drusen. C8, part of the complement membrane attack complex, was localized in drusen by immunofluorescence.

Conclusions/Significance

At least 40% of druse content is comprised by lipids dominated by EC and PC, 2 components that are potentially accounted for by just one pathway, the secretion of lipoproteins by RPE. Manipulating genes encoding apolipoprotein pathways would be a fruitful approach to producing drusen with high EC content in laboratory animals. Therapies that directly mitigate drusen should prepare for the substantial volume of neutral lipids. The catalog of major druse proteins is nearing completion.

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

Age-related macular degeneration (AMD) is the most prevalent form of irreversible blindness worldwide in the elderly population. The pathology of dry AMD consists of macular degeneration of photoreceptors and the RPE, lipofuscin (A2E) accumulation, and drusen formation. Mice have been widely used for generating models that simulate human AMD features for investigating the pathogenesis, treatment and prevention of the disease. Although the mouse has no macula, focal atrophy of photoreceptors and RPE, lipofuscin accumulation, and increased A2E can develop in aged mouse eyes. However, drusen are rarely seen in mice because of their simpler Bruch's membrane and different process of lipofuscin extrusion compared with humans. Thus, analyzing basal deposits at the ultrastructural level and understanding the ultrastructural pathologic differences between various mouse AMD models are critical to comprehending the significance of research findings and response to possible therapeutic options for dry AMD. Based on the multifactorial pathogenesis of AMD, murine dry AMD models can be classified into three groups. First, genetically engineered mice that target genes related to juvenile macular dystrophies are the most common models, and they include abcr(-/-) (Stargardt disease), transgenic ELOVL4 (Stargardt-3 dominant inheritary disease), Efemp1(R345W/R345W) (Doyne honeycomb retinal dystrophy), and Timp3(S156C/S156C) (Sorsby fundus dystrophy) mice. Other murine models target genes relevant to AMD, including inflammatory genes such as Cfh(-/-), Ccl2(-/-), Ccr2(-/-), Cx3cr1(-/-), and Ccl2(-/-)/cx3cr1(-/-), oxidative stress associated genes such as Sod1(-/-) and Sod2 knockdown, metabolic pathway genes such as neprilysin(-/-) (amyloid beta), transgenic mcd/mcd (cathepsin D), Cp(-/-)/Heph(-/Y) (ferroxidase ceruloplasmin/hepaestin, iron metabolism), and transgenic ApoE4 on high fat and high cholesterol diet (lipid metabolism). Second, mice have also been immunologically manipulated by immunization with carboxyethylpyrrole (CEP), an oxidative fragment of DHA found in drusen, and found to present with dry AMD features. Third, natural mouse strains such as arrd2/arrd2 (Mdm gene mutation) and the senescence accelerated mice (SAM) spontaneously develop features of dry AMD like photoreceptor atrophy and thickening of Bruch's membrane. All the aforementioned models develop retinal lesions with various features that simulate dry AMD lesions: focal photoreceptor degeneration, abnormal RPE with increased lipofuscin, basal infolding, decreased melanosomes and degeneration. However, Bruch's membrane changes are less common. Most mice develop retinal lesions at an older age (6-24 months, depending on the models), while the Ccl2(-/-)/cx3cr1(-/-) mice develop lesions by 4-6 weeks. Although murine models present various degrees of retinal and/or RPE degeneration, classical drusen is extremely rare. Using electron microscopy, small drusenoid deposits are found between RPE and Bruch's membrane in a few models including Efemp1(R345W/R345W), Ccl2(-/-)/cx3cr1(-/-), neprilysin(-/-), transgenic mcd/mcd, and ApoE4 transgenic mice on a high fat diet. High A2E levels are measured in the retinas of abcr(-/-), transgenic ELOVL4, and Ccl2(-/-)/cx3cr1(-/-) mice. In summary, murine models provide useful tools for studying AMD pathogenesis and evaluating novel therapies for this disease. This review compares the major dry AMD murine models and discusses retinal pathology at the ultrastructural level.

Aging, age-related macular degeneration, and the response-to-retention of apolipoprotein B-containing lipoproteins

The largest risk factor for age-related macular degeneration (ARMD) is advanced age. A prominent age-related change in the human retina is the accumulation of histochemically detectable neutral lipid in normal Bruch's membrane (BrM) throughout adulthood. This change has the potential to have a major impact on physiology of the retinal pigment epithelium (RPE). It occurs in the same compartment as drusen and basal linear deposit, the pathognomonic extracellular, lipid-containing lesions of ARMD. Here we present evidence from light microscopic histochemistry, ultrastructure, lipid profiling of tissues and isolated lipoproteins, and gene expression analysis that this deposition can be accounted for by esterified cholesterol-rich, apolipoprotein B-containing lipoprotein particles constitutively produced by the RPE. This work collectively allows ARMD lesion formation and its aftermath to be conceptualized as a response to the retention of a sub-endothelial apolipoprotein B lipoprotein, similar to a widely accepted model of atherosclerotic coronary artery disease (CAD) (Tabas et al., 2007). This approach provides a wide knowledge base and sophisticated clinical armamentarium that can be readily exploited for the development of new model systems and the future benefit of ARMD patients.

Apolipoprotein B-containing lipoproteins in retinal aging and age-related macular degeneration

The largest risk factor for age-related macular degeneration (ARMD) is advanced age. With aging, there is a striking accumulation of neutral lipids in Bruch's membrane (BrM) of normal eye that continues through adulthood. This accumulation has the potential to significantly impact the physiology of the retinal pigment epithelium (RPE). It also ultimately leads to the creation of a lipid wall at the same locations where drusen and basal linear deposit, the pathognomonic extracellular, lipid-containing lesions of ARMD, subsequently form. Here, we summarize evidence obtained from light microscopy, ultrastructural studies, lipid histochemistry, assay of isolated lipoproteins, and gene expression analysis. These studies suggest that lipid deposition in BrM is at least partially due to accumulation of esterified cholesterol-rich, apolipoprotein B-containing lipoprotein particles produced by the RPE. Furthermore, we suggest that the formation of ARMD lesions and their aftermath may be a pathological response to the retention of a sub-endothelial apolipoprotein B lipoprotein, similar to a widely accepted model of atherosclerotic coronary artery disease (Tabas, I., K. J. Williams, and J. Borén. 2007. Subendothelial lipoprotein retention as the initiating process in atherosclerosis: update and therapeutic implications. Circulation. 116:1832-1844). This view provides a conceptual basis for the development of novel treatments that may benefit ARMD patients in the future.

Convergence of linkage, gene expression and association data demonstrates the influence of the RAR-related orphan receptor alpha (RORA) gene on neovascular AMD: a systems biology based approach

To identify novel genes and pathways associated with AMD, we performed microarray gene expression and linkage analysis which implicated the candidate gene, retinoic acid receptor-related orphan receptor alpha (RORA, 15q). Subsequent genotyping of 159 RORA single nucleotide polymorphisms (SNPs) in a family-based cohort, followed by replication in an unrelated case-control cohort, demonstrated that SNPs and haplotypes located in intron 1 were significantly associated with neovascular AMD risk in both cohorts. This is the first report demonstrating a possible role for RORA, a receptor for cholesterol, in the pathophysiology of AMD. Moreover, we found a significant interaction between RORA and the ARMS2/HTRA1 locus suggesting a novel pathway underlying AMD pathophysiology.