Characterization of drusen-associated glycoconjugates

Stages, pathogenesis, clinical management and advancements in therapies of age-related macular degeneration

Age-related macular degeneration (AMD) is a retinal degenerative disorder prevalent in the elderly population, which leads to the loss of central vision. The disease progression can be managed, if not prevented, either by blocking neovascularization ("wet" form of AMD) or by preserving retinal pigment epithelium and photoreceptor cells ("dry" form of AMD). Although current therapeutic modalities are moderately successful in delaying the progression and management of the disease, advances over the past years in regenerative medicine using iPSC, embryonic stem cells, advanced materials (including nanomaterials) and organ bio-printing show great prospects in restoring vision and efficient management of either forms of AMD. This review focuses on the molecular mechanism of the disease, model systems (both cellular and animal) used in studying AMD, the list of various regenerative therapies and the current treatments available. The article also highlights on the recent clinical trials using regenerative therapies and management of the disease.

N-Glycosylation Patterns across the Age-Related Macular Degeneration Spectrum

The pathogenesis of age-related macular degeneration (AMD) remains elusive, despite numerous research studies. Therefore, we aimed to investigate the changes of plasma and IgG-specific N-glycosylation across the disease severity spectrum. We examined 2835 subjects from the 10.001 Dalmatians project, originating from the isolated Croatian islands of Vis and Korčula. All subjects were classified into four groups, namely (i) bilateral AMD, (ii) unilateral AMD, (iii) early-onset drusen, and (iv) controls. We analysed plasma and IgG N-glycans measured by HPLC and their association with retinal fundus photographs. There were 106 (3.7%) detected cases of AMD; 66 of them were bilateral. In addition, 45 (0.9%) subjects were recorded as having early-onset retinal drusen. We detected several interesting differences across the analysed groups, suggesting that N-glycans can be used as a biomarker for AMD. Multivariate analysis suggested a significant decrease in the immunomodulatory bi-antennary glycan structures in unilateral AMD (adjusted odds ratio 0.43 (95% confidence interval 0.22–0.79)). We also detected a substantial increase in the pro-inflammatory tetra-antennary plasma glycans in bilateral AMD (7.90 (2.94–20.95)). Notably, some of these associations were not identified in the aggregated analysis, where all three disease stages were collapsed into a single category, suggesting the need for better-refined phenotypes and the use of disease severity stages in the analysis of more complex diseases. Age-related macular degeneration progression is characterised by the complex interplay of various mechanisms, some of which can be detected by measuring plasma and IgG N-glycans. As opposed to a simple case-control study, more advanced and refined study designs are needed to understand the pathogenesis of complex diseases.

The role of hypoxia-inducible factors in neovascular age-related macular degeneration: a gene therapy perspective

Understanding the mechanisms that underlie age-related macular degeneration (AMD) has led to the identification of key molecules. Hypoxia-inducible transcription factors (HIFs) have been associated with choroidal neovascularization and the progression of AMD into the neovascular clinical phenotype (nAMD). HIFs regulate the expression of multiple growth factors and cytokines involved in angiogenesis and inflammation, hallmarks of nAMD. This knowledge has propelled the development of a new group of therapeutic strategies focused on gene therapy. The present review provides an update on current gene therapies in ocular angiogenesis, particularly nAMD, from both basic and clinical perspectives.

Visualization of Mouse Choroidal and Retinal Vasculature Using Fluorescent Tomato Lectin Perfusion

Purpose

To develop a reliable and simplified method to assess choroid and retinal vasculature on whole mount and cross sections in mice using tomato lectin (TL; Lycopersicon esculentum).

Methods

Albino mice (n = 27) received 1 mg/mL of TL (conjugated to Dylight-594) intravascularly through the tail vein, jugular vein, or cardiac left ventricle. Whole mounts of the retina and choroid were evaluated using fluorescence microscopy. Perfusion with GSL-IB4 conjugated to Dylight-594 and fluorescein isothiocyanate was performed to compare against labeling with TL. Co-labeling of choroidal endothelial cells with perfused TL on cross-sections with antibodies directed against the choriocapillaris-restricted endothelial cell marker CA4 was performed. The percentage of perfused choroidal and retinal vessels was assessed semiquantitatively. One mouse was subjected to thermal laser damage before perfusion to cause retinal and choroidal vasculature ablation.

Results

Intravascular injection of TL led to consistent, robust labeling of retinal and choroidal vascular walls. On cross-sections, choriocapillaris was co-labeled with CA4 and TL. On flat mount, TL perfusion resulted in better labeling of choroidal vessels using tail/jugular vein injection compared with cardiac perfusion (P < .01). More consistent labeling of the choroidal and retinal vascular trees was observed with TL than with GSL-IB4. Vascular damage caused by laser ablation was detected readily using this method.

Conclusions

TL injection intravascularly can reliably label normal and ablated choroid and retinal vasculature in mouse in a quick, simple manner.

Translational Relevance

These data will help to facilitate modeling in rodents for diseases such as age-related macular degeneration, diabetes, and other ischemic/angiogenic processes that can also be used for treatment evaluation.

Age-related macular degeneration in the aspect of chronic low-grade inflammation (pathophysiological parainflammation)

The products of oxidative stress trigger chronic low-grade inflammation (pathophysiological parainflammation) process in AMD patients. In early AMD, soft drusen contain many mediators of chronic low-grade inflammation such as C-reactive protein, adducts of the carboxyethylpyrrole protein, immunoglobulins, and acute phase molecules, as well as the complement-related proteins C3a, C5a, C5, C5b-9, CFH, CD35, and CD46. The complement system, mainly alternative pathway, mediates chronic autologous pathophysiological parainflammation in dry and exudative AMD, especially in the Y402H gene polymorphism, which causes hypofunction/lack of the protective complement factor H (CFH) and facilitates chronic inflammation mediated by C-reactive protein (CRP). Microglial activation induces photoreceptor cells injury and leads to the development of dry AMD. Many autoantibodies (antibodies against alpha beta crystallin, alpha-actinin, amyloid, C1q, chondroitin, collagen I, collagen III, collagen IV, elastin, fibronectin, heparan sulfate, histone H2A, histone H2B, hyaluronic acid, laminin, proteoglycan, vimentin, vitronectin, and aldolase C and pyruvate kinase M2) and overexpression of Fcc receptors play role in immune-mediated inflammation in AMD patients and in animal model. Macrophages infiltration of retinal/choroidal interface acts as protective factor in early AMD (M2 phenotype macrophages); however it acts as proinflammatory and proangiogenic factor in advanced AMD (M1 and M2 phenotype macrophages).

Age-related macular degeneration and changes in the extracellular matrix

Age-related macular degeneration (AMD) is the leading cause of permanent, irreversible, central blindness (scotoma in the central visual field that makes reading and writing impossible, stereoscopic vision, recognition of colors and details) in patients over the age of 50 years in European and North America countries, and an important role is attributed to disorders in the regulation of the extracellular matrix (ECM). The main aim of this article is to present the crucial processes that occur on the level of Bruch’s membrane, with special consideration of the metalloproteinase substrates, metalloproteinase, and tissue inhibitor of metalloproteinase (TIMP).

A comprehensive review of the literature was performed through MEDLINE and PubMed searches, covering the years 2005–2012, using the following keywords: AMD, extracellular matrix, metalloproteinases, tissue inhibitors of metalloproteinases, Bruch’s membrane, collagen, elastin.

In the pathogenesis of AMD, a significant role is played by collagen type I and type IV; elastin; fibulin-3, -5, and -6; matrix metalloproteinase (MMP)-2, MMP-9, MMP-14, and MMP-1; and TIMP-3. Other important mechanisms include: ARMS2 and HTR1 proteins, the complement system, the urokinase plasminogen activator system, and pro-renin receptor activation.

Continuous rebuilding of the extracellular matrix occurs in both early and advanced AMD, simultaneously with the dysfunction of retinal pigment epithelium (RPE) cells and endothelial cells. The pathological degradation or accumulation of ECM structural components are caused by impairment or hyperactivity of specific MMPs/TIMPs complexes, and is also endangered by the influence of other mechanisms connected with both genetic and environmental factors.

Microstructure of subretinal drusenoid deposits revealed by adaptive optics imaging

Subretinal drusenoid deposits (SDD), a recently recognized lesion associated with progression of age-related macular degeneration, were imaged with adaptive optics scanning laser ophthalmoscopy (AO-SLO) and optical coherence tomography (AO-OCT). AO-SLO revealed a distinct en face structure of stage 3 SDD, showing a hyporeflective annulus surrounded reflective core packed with hyperreflective dots bearing a superficial similarity to the photoreceptors in the unaffected retina. However, AO-OCT suggested that the speckled appearance over the SDD rendered by AO-SLO was the lesion material itself, rather than photoreceptors. AO-OCT assists proper interpretation and understanding of the SDD structure and the lesions' impact on surrounding photoreceptors produced by AO-SLO and vice versa.

Innate immune network in the retina activated by optic nerve crush

PURPOSE

Innate immunity plays a role in many diseases, including glaucoma and AMD. We have used transcriptome profiling in the mouse to identify a network of genes involved in innate immunity that is present in the normal retina and that is activated by optic nerve crush (ONC).

METHODS

Using a recombinant inbred (RI) mouse strain set (BXD, C57BL/6 crossed with DBA/2J mice), we generate expression datasets (Illumina WG 6.2 arrays) in the normal mouse retina and 2 days after ONC. The normal dataset is constructed from retinas from 80 mouse strains and the ONC dataset is constructed from 62 strains. These large datasets are hosted by GeneNetwork.org, along with a series of powerful bioinformatic tools.

RESULTS

In the retina datasets, one intriguing network involves transcripts associated with the innate immunity. Using C4b to interrogate the normal dataset, we can identify a group of genes that are coregulated across the BXD RI strains. Many of the genes in this network are associated with the innate immune system, including Serping1, Casp1, C3, Icam1, Tgfbr2, Cfi, Clu, C1qg, Aif1, and Cd74. Following ONC, the expression of these genes is upregulated, along with an increase in coordinated expression across the BXD strains. Many of the genes in this network are risk factors for AMD, including C3, EFEMP1, MCDR2, CFB, TLR4, HTA1, and C1QTNF5.

CONCLUSIONS

We found a retina-intrinsic innate immunity network that is activated by injury including ONC. Many of the genes in this network are risk factors for retinal disease.

The role of complement Factor H in age-related macular degeneration: a review

Factor H is a 155kDa sialic acid containing glycoprotein that plays an integral role in the regulation of the complement-mediated immune system that is involved in microbial defense, immune complex processing, and programmed cell death. These events take place primarily in fluid phase and on the cell surface and are particularly important in the context of distinguishing self from non-self. Activation of the complement system occurs within seconds and results in a proteolytic cascade eventually forming the membrane attack complex leading to cell lysis. Factor H protects host cells from injury resulting from unrestrained complement activation. Mutations and SNPs (single nucleotide polymorphisms) in Factor H have been implicated in a variety of human conditions including age-related macular degeneration (AMD), atypical hemolytic uremic syndrome, and membranoproliferative glomuleronephritis type II or dense deposit disease. It should not be surprising that these seemingly unrelated diseases involving mutations in Factor H may share common features. Because the immune process involves, in part, an inflammatory response and common or similar surface antigens, it is also not unexpected to observe features of inflammation, including deposition of bioactive complement fragments such as C3a and C5a, a cellular influx of immune related cells such as lymphocytes, and the potential for multiple organ involvement. We review recent developments in molecular genetics; SNPs, including Y402H; the three-dimensional structure; and mass spectroscopy of Factor H as it relates to the pathogenesis of eye disease. In addition, we discuss the concepts of molecular mimicry, sequestered or hidden antigens, and antigenic cross reactivity, and propose that AMD should not simply be considered to be an eye disease, but rather a systemic vascular disease where the eye has the ability to self regulate a local immune response. Identification of the initial event or inciting antigen has yet to be determined and will significantly advance the understanding of the pathogenesis of AMD.

The Y402H variant of complement factor H is associated with age-related macular degeneration but not with diabetic retinal disease in the Go-DARTS study

AIMS

The Y402H variant of complement factor H (CFH) is associated with risk of age-related macular degeneration (ARMD). In common with ARMD, diabetic retinal disease also appears to involve complement activation. The aim was to investigate the impact of Y402H on both retinal pathologies in patients with Type 2 diabetes (T2DM) undergoing systematic eye screening.

METHODS

Patients with T2DM (n = 2350) were genotyped for the CFH Y402H variant. The association of genotype with retinal disease was determined in both retrospective and prospective models.

RESULTS

The retrospective study demonstrated that the HH genotype was associated with an age-adjusted odds ratio of 7.4 for ARMD (P = 2.9 x 10(-11)). In a longitudinal study in the disease-free cohort, the age-adjusted hazard ratio was 2.8 (P = 2.4 x 10(-7)). The life-time hazard ratio was 3.4 (P = 2.1 x 10(-16)). We found no association of Y402H with development of referable diabetic retinal disease.

CONCLUSION

The ARMD-associated Y402H variant in CFH does not appear to be associated with diabetic retinal disease, although complement activation is involved in the pathoaetiology of both conditions.

Age-related macular degeneration

Age-related macular degeneration is the leading cause of blindness in elderly populations of European descent. The most consistent risk factors associated with this ocular condition are increasing age and cigarette smoking. Genetic investigations have shown that complement factor H, a regulator of the alternative complement pathway, and LOC387715/HtrA1 are the most consistent genetic risk factors for age-related macular degeneration. Although the pathogenesis of this disease is unknown, oxidative stress might have an important role. Treatment with antioxidant vitamins and zinc can reduce the risk of developing advanced age-related macular degeneration by about a quarter in those at least at moderate risk. Intravitreal injections of ranibizumab, a monoclonal antibody that inhibits all forms of vascular endothelial growth factor, have been shown to stabilise loss of vision and, in some cases, improve vision in individuals with neovascular age-related macular degeneration. These findings, combined with assessments of possible environmental and genetic interactions and new approaches to modulate inflammatory pathways, will hopefully further expand our ability to understand and treat age-related macular degeneration.

Prevalence and morphology of druse types in the macula and periphery of eyes with age-related maculopathy

PURPOSE

Macular drusen are hallmarks of age-related maculopathy (ARM), but these focal extracellular lesions also appear with age in the peripheral retina. The present study was conducted to determine regional differences in morphology that contribute to the higher vulnerability of the macula to advanced disease.

METHODS

Drusen from the macula (n = 133) and periphery (n = 282) were isolated and concentrated from nine ARM-affected eyes. A semiquantitative light microscopic evaluation of 1-mum-thick sections included 12 parameters.

RESULTS

Significant differences were found between the macula and periphery in ease of isolation, distribution of druse type, composition qualities, and substructures. On harvesting, macular drusen were friable, with liquefied or crystallized contents. Peripheral drusen were resilient and never crystallized. On examination, soft drusen appeared in the macula only, had homogeneous content without significant substructures, and had abundant basal laminar deposits (BlamD). Several substructures, previously postulated as signatures of druse biogenesis, were found primarily in hard drusen. Specific to hard drusen, which appeared everywhere, were central subregions and reduced RPE coverage. Macular hard drusen with a rich substructure profile differed from primarily homogeneous peripheral hard drusen. Compound drusen, found in the periphery only, exhibited a composition profile that was not intermediate between hard and soft.

CONCLUSIONS

The data confirm regional differences in druse morphology, composition, and physical properties, most likely based on different formative mechanisms that may contribute to macular susceptibility for ARM progression. Two other reasons that only the macula is at high risk despite having relatively few drusen are the exclusive presence of soft drusen and the abundant BlamD in this region.

Glycoproteins of drusen and drusen-like lesions

Drusen are a marker of age-related macular degeneration (AMD). Lesions similar to drusen, both in histology and their clinical appearance, are also seen in choroidal tumours, chronic inflammatory and degenerative conditions of the eye, and in mesangiocapillary glomerulonephritis type II (MCGN-II). This study aims to compare the saccharide composition of these drusen-like lesions in the various ocular pathological groups and in MCGN-II. Formalin fixed and paraffin wax embedded tissue from 21 eyes was studied. The histological diagnoses included AMD, retinal detachment, phthisis bulbi following failed retinal detachment surgery, malignant melanoma, long-standing uveitis, glaucoma and MCGN II. Glycosylation was examined using a panel of twenty biotinylated lectins and an avidin-peroxidase DAB-cobalt revealing system, with and without neuraminidase pre-treatment. High mannose, bi/tri-nonbisected and bisected complex N-glycan, N-acetyl glucosaminyl, galactosyl and sialyl residues were found to be expressed by drusen, while treatment with neuraminidase exposed subterminal N-acetyl galactosamine and galactosyl residues. Similar binding patterns were found in the various pathological groups studied. As there was no significant difference in the lectin-binding pattern in drusen in different pathologies, a common pathogenesis or at least a final common pathway for the elaboration of carbohydrate components of drusen is suggested.

Progress in defining the molecular biology of age related macular degeneration

Age related macular degeneration (AMD) is an extremely prevalent complex genetic disorder. Its incidence rises exponentially in the elderly to a frequency of 1 in 2 in the general population by age 85. It affects approximately 25 million people and is the commonest cause of irreversible visual loss in the Western world. It is therefore a major public health problem. However, until recently its aetiology was unknown. Our understanding of both the molecular biology of AMD and the relevant clinical treatments has progressed dramatically in the last 2 years. Two genes of large effect have been identified which together contribute to over 70% of the population attributable risk of AMD. Treatments which inhibit expression of vascular endothelial growth factor have been developed which can rescue vision in the "wet" form of the disease. The association of complement factor H with AMD highlights the importance of the alternative complement pathway in the development of AMD whilst the pathophysiology of the serine protease HTRA1 is now under intensive study. This review will give an insight into these developments and will summarise our current knowledge of the molecular biology of AMD.

An overview on the toxic morphological changes in the retinal pigment epithelium after systemic compound administration

Many medications that are administered systemically for nonocular conditions may evoke ocular toxicological complications. Therefore, the eye is routinely investigated histopathologically in preclinical in vivo toxicity studies. The retinal pigment epithelium is a likely target for systemically administered compounds, since the underlying choroid is highly vascularized. The specialized pigment epithelium has numerous functions that all maintain the integrity and function of photoreceptors. Consequently, toxic effects on the pigment epithelium will eventually affect the neural retina. The potential of pigment epithelial cells to respond to toxic injury is limited, but a standardized terminology to describe its morphological changes does not exist in the scientific literature. Detailed morphologic analysis, however, might allow early detection of retinotoxicity and may provide evidence on the underlying pathomechanism. We here review toxic effects on the pigment epithelium focusing in particular on the morphology of toxic cell injury. Morphological changes comprise hypertrophy, intracytoplasmic accumulation of cellular components, loss of cell polarity, degeneration, metaplasia, and formation of subretinal membranes. Some of these changes are reversible whereas others are permanent, leading to impaired function of the pigment epithelium and eventually to photoreceptor loss and retinal atrophy.

Dietary glycemic index and carbohydrate in relation to early age-related macular degeneration

BACKGROUND

Several dietary factors have been linked to age-related maculopathy (ARM), the early form of age-related macular degeneration, and there is reason to think that dietary carbohydrate may play a role in the development of ARM.

OBJECTIVE

The purpose of the present study was to examine the relation between dietary carbohydrate quality, as measured by dietary glycemic index (GI) or total carbohydrate intake, and ARM.

DESIGN

From the Nurses' Health Study, 1036 eyes from 526 Boston-area participants without a previous ARM diagnosis were included in the present study. The presence and degree of ARM were classified by the Age-Related Eye Diseases Study system. Long-term dietary information was based on data from an average of 4 food-frequency questionnaires collected over a 10-y period before the assessment of ARM. With eyes as the unit of analysis, we used a generalized estimating approach to logistic regression to estimate the odds ratios for ARM in a manner that accounted for the lack of independence between the 2 eyes from the same subject.

RESULTS

After multivariate adjustment, dietary GI was related to ARM (specifically to retinal pigmentary abnormalities), whereas total carbohydrate intake was not. The odds ratio for ARM being in the highest tertile of dietary GI (> or =77.0) versus the lowest (<74.6) was 2.71 (95% CI: 1.24, 5.93; P for trend = 0.01). Neither dietary GI nor total carbohydrate intake was related to drusen.

CONCLUSION

Our results suggest that dietary GI may be an independent risk factor for ARM.

Age-related macular degeneration--emerging pathogenetic and therapeutic concepts

Today, the average life expectancy in developed nations is over 80 years and climbing. And yet, the quality of life during those additional years is often significantly diminished by the effects of age-related, degenerative diseases, including age-related macular degeneration (AMD), the leading cause of blindness in the elderly worldwide. AMD is characterized by a progressive loss of central vision attributable to degenerative and neovascular changes in the macula, a highly specialized region of the ocular retina responsible for fine visual acuity. Estimates gathered from the most recent World Health Organization (WHO) global eye disease survey conservatively indicate that 14 million persons are blind or severely visually impaired because of AMD. The disease has a tremendous impact on the physical and mental health of the geriatric population and their families and is becoming a major public health burden. Currently, there is neither a cure nor a means to prevent AMD. Palliative treatment options for the less prevalent, late-stage 'wet' form of the disease include anti-neovascular agents, photodynamic therapy and thermal laser. There are no current therapies for the more common 'dry' AMD, except for the use of antioxidants that delay progression in 20%-25% of eyes. New discoveries, however, are beginning to provide a much clearer picture of the relevant cellular events, genetic factors, and biochemical processes associated with early AMD. Recently, compelling evidence has emerged that the innate immune system and, more specifically, uncontrolled regulation of the complement alternative pathway plays a central role in the pathobiology of AMD. The complement Factor H gene--which encodes the major inhibitor of the complement alternative pathway--is the first gene identified in multiple independent studies that confers a significant genetic risk for the development of AMD. The emergence of this new paradigm of AMD pathogenesis should hasten the development of novel diagnostic and therapeutic approaches for this disease that will dramatically improve the quality of our prolonged lifespan.

A common haplotype in the complement regulatory gene factor H (HF1/CFH) predisposes individuals to age-related macular degeneration

Age-related macular degeneration (AMD) is the most frequent cause of irreversible blindness in the elderly in developed countries. Our previous studies implicated activation of complement in the formation of drusen, the hallmark lesion of AMD. Here, we show that factor H (HF1), the major inhibitor of the alternative complement pathway, accumulates within drusen and is synthesized by the retinal pigmented epithelium. Because previous linkage analyses identified chromosome 1q25-32, which harbors the factor H gene (HF1/CFH), as an AMD susceptibility locus, we analyzed HF1 for genetic variation in two independent cohorts comprised of approximately 900 AMD cases and 400 matched controls. We found association of eight common HF1 SNPs with AMD; two common missense variants exhibit highly significant associations (I62V, chi2 = 26.1 and P = 3.2 x 10(-7) and Y402H, chi2 = 54.4 and P = 1.6 x 10(-13)). Haplotype analysis reveals that multiple HF1 variants confer elevated or reduced risk of AMD. One common at-risk haplotype is present at a frequency of 50% in AMD cases and 29% in controls [odds ratio (OR) = 2.46, 95% confidence interval (1.95-3.11)]. Homozygotes for this haplotype account for 24% of cases and 8% of controls [OR = 3.51, 95% confidence interval (2.13-5.78)]. Several protective haplotypes are also identified (OR = 0.44-0.55), further implicating HF1 function in the pathogenetic mechanisms underlying AMD. We propose that genetic variation in a regulator of the alternative complement pathway, when combined with a triggering event, such as infection, underlie a major proportion of AMD in the human population.

Assessment of multifocal electroretinogram abnormalities and their relation to morphologic characteristics in patients with large drusen

OBJECTIVES

To determine the extent of functional changes in the first-order kernel multifocal electroretinogram (mfERG) responses in patients with large drusen by means of a localized analysis and to determine correlations between mfERG responses and morphologic changes.

METHODS

Thirty-one eyes from 20 patients ages 58 to 84 years with large drusen (> or =5 drusen > or =63 microm diameter) were studied. The mfERGs were recorded with a stimulus of 103 hexagons and a flash intensity of 2.67 candela (cd).s-1.m-2. Each of the 103 single first-order kernel mfERG responses was analyzed and compared with those of age-matched healthy control subjects. Imaging studies, including color stereo fundus photography, red-free fundus photography, and fluorescein angiography, were performed in all patients, and morphologic changes (drusen in red-free fundus photography, staining or window defect in fluorescein angiography) were determined with a digital measurement tool. The mfERG responses were correlated to areas with and without morphologic changes.

RESULTS

Reduced responses were found in 10.0% (scalar products) and 4.0% (response densities) and delayed implicit times in 13.8% (N1), 18.9% (P1), and 23.8% (N2) of all mfERGs. Abnormal mfERG responses extended up to 25 degrees in radius. Significant morphologic-functional relations were detected in only a few patients. Abnormal mfERG variables were present in areas without morphologic changes.

CONCLUSIONS

Patients with large drusen exhibit functional changes in the cone-driven pathways evaluated by the mfERG, indexed particularly by implicit times. Morphologically visible changes do not predict retinal function. Large drusen are associated with a more general retinal dysfunction.

Age-dependent variations in the drusen fluorescence on indocyanine green angiography

PURPOSE

To examine the fluorescence patterns of macular drusen using indocyanine green (ICG) angiography in human subjects of different ages.

METHODS

A total of 180 eyes in 100 consecutive patients aged 34-90 years with macula drusen were evaluated with ICG angiography using the Topcon 1024 ImageNet system. Eyes were divided into two groups depending on age. The older group (subjects over the age of 55 years) comprised 170 eyes. The younger group (subjects 55 years or younger) comprised 10 eyes. The fluorescence patterns of the drusen were graded as hyper-fluorescent (brighter than the background fluorescence), hypofluorescent (darker than the background fluorescence) or isofluorescent (unable to be distinguished from the background fluorescence). The ICG angiogram appearances were correlated with the clinical types of drusen in differing age groups.

RESULTS

An age-dependent difference in the fluorescence pattern of drusen was observed. Drusen in the older group were hypofluorescent or isofluorescent (169/170 eyes). In contrast, drusen in the younger group (55 years or younger) were hyperfluorescent (10/10 eyes).

CONCLUSIONS

This study defines the distinct age-dependent differences in the ICG fluorescence pattern of drusen. This heterogeneity might be due to differential binding of extravasated ICG to the constituents and histological site of the drusen deposits. Furthermore, ICG angiography is able to differentiate drusen types not possible by clinical and fluorescein angiographic evaluation.

Identification of lesion components that influence visual function in age related macular degeneration

AIMS

To explore the relation between lesion composition as assessed by fundus photography and fluorescein angiography with clinical measures of vision in eyes of patients with age related macular degeneration (AMD).

METHODS

A standardised visual function assessment along with colour stereo pair fundus photography was carried out in both eyes of 58 subjects with a confirmed clinical diagnosis of AMD. The size, location, and composition of the macular lesion (blood, exudate, subretinal fluid, pigment, membrane, atrophy, and fibrosis) were measured on the colour photographs using computer assisted image analysis. Of the 58 subjects, 44 also had concurrent fluorescein angiography. Classic and occult choroidal neovascularisation (CNV), blood, blocked fluorescence, fibrosis, geographic atrophy, and the total area of abnormal fluorescence were measured. Multiple linear regression was used to examine the relation between clinical measures of vision and the location and extent of lesion components identified by both colour and fluorescein image capture.

RESULTS

The composition of the macular lesion strongly influenced visual function, with atrophy (p=0.001) and fibrosis (p=0.002) accounting for most of the variation. When the location of the lesion with respect to the fovea was examined, fibrosis within the fovea significantly influenced all clinical measures of vision (p=0.008). The regression model selected the total area of abnormal fluorescence and a composite parameter (a semiquantitative measure of the following characteristics: atrophy, exudates, blood, and fibrosis ) from colour photography (r(2) =0.52) as the variables that explained most of the variation in clinical measures of vision.

CONCLUSIONS

The composition and extent of the macular lesion strongly influences visual function in eyes with AMD. Both colour photography and angiography yielded information, which together explained considerably more of the variation in the clinical measures of vision than either on its own.

A role for local inflammation in the formation of drusen in the aging eye

PURPOSE

The accumulation of numerous or confluent drusen, especially in the macula, is a significant risk factor for the development of age-related macular degeneration (AMD). Identifying the origin and molecular composition of these deposits, therefore, has been an important, yet elusive, objective for many decades. Recently, a more complete profile of the molecular composition of drusen has emerged.

DESIGN

In this focused review, we discuss these new findings and their implications for the pathogenic events that give rise to drusen and AMD.

METHODS

Tissue specimens from one or both eyes of more than 400 human donors were examined by light, confocal or electron microscopy, in conjunction with antibodies to specific drusen-associated proteins, to help characterize the transitional events in drusen biogenesis. Quantification of messenger RNA from the retinal pigment epithelium (RPE)/choroid of donor eyes was used to determine if local ocular sources for drusen-associated molecules exist.

RESULTS

The results indicate that cellular remnants and debris derived from degenerate RPE cells become sequestered between the RPE basal lamina and Bruch's membrane. We propose that this cellular debris constitutes a chronic inflammatory stimulus, and a potential "nucleation" site for drusen formation. The entrapped cellular debris then becomes the target of encapsulation by a variety of inflammatory mediators, some of which are contributed by the RPE and, perhaps, other local cell types; and some of which are extravasated from the choroidal circulation.

CONCLUSIONS

The results support a role for local inflammation in drusen biogenesis, and suggest that it is analogous to the process that occurs in other age-related diseases, such as Alzheimer's disease and atherosclerosis, where accumulation of extracellular plaques and deposits elicits a local chronic inflammatory response that exacerbates the effects of primary pathogenic stimuli.

Drusen are Cold Spots for Proteolysis: Expression of Matrix Metalloproteinases and Their Tissue Inhibitor Proteins in Age-related Macular Degeneration

Drusen are abnormal extracellular matrix deposits characteristic of age-related macular degeneration (AMD), a leading cause of blindness in the aging human population. The mechanisms underlying drusen formation are not well characterized. The purpose of this study was to examine the expression of matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) in drusen, and in the surrounding cells and tissue. To assess the extent of MMP and TIMP expression by retinal pigment epithelial (RPE) cells, cDNA arrays were screened with probes generated from cultured human RPE cells. The distribution of MMP-1, -2 and -3 and TIMP-1, -2, -3 and -4 was determined using immunohistochemistry in human RPE choroid from donor eyes with and without a clinical history of AMD. Gelatinase activity was assessed in unfixed frozen sections using in situ zymography. In cultured RPE cells, expression of 10 MMP and all four known TIMP mRNAs was detected. MMP immunoreactivity was widespread in the RPE choroid, but was absent from the interior of drusen. TIMP-3, but not other TIMPs, was detected in the drusen interior. Likewise, metal ion dependent gelatinase activity could be detected in RPE choroid, but not in drusen. These results show that, while metalloproteinase activity is widespread throughout the RPE choroid, drusen are cold spots for proteolysis. The data lead to the speculation that high TIMP-3 concentrations within drusen could inhibit MMPs and as a result slow the proteolytic degradation of these deposits.

Complement activation and inflammatory processes in Drusen formation and age related macular degeneration

Recent studies implicate inflammation and complement mediated attack as early events in drusen biogenesis. The investigations described here sought to determine whether primary sites of complement activation could be identified within drusen substructure, and whether known inhibitors of the terminal pathway of complement are present in drusen and/or retinal pigmented epithelial (RPE) cells that lie in close proximity to drusen. Immunohistochemical examination shows two fluid phase regulators of the terminal pathway, vitronectin (Vn, S-protein) and clusterin (apolipoprotein J), to be present in drusen; Vn also accumulates in the cytoplasm of RPE cells that are closely associated with drusen. The membrane associated complement inhibitor, complement receptor 1, is also localized in drusen, but it is not detected in RPE cells immunohistochemically. In contrast, a second membrane associated complement inhibitor, membrane cofactor protein, is present in drusen associated RPE cells, as well as in small, spherical substructural elements within drusen. These previously unidentified elements also show strong immunoreactivity for proteolytic fragments of complement component C3 that are characteristically deposited at sites of complement activation. It is proposed that these structures represent residual debris from degenerating RPE cells that are the targets of complement attack. It is likely that RPE cell debris entrapped between the RPE monolayer and Bruch's membrane serves as a chronic inflammatory stimulus and a potential nucleation site for drusen formation. Thus, the process of drusen biogenesis may be envisaged as a secondary manifestation of primary RPE pathology that is exacerbated by consequences of local inflammatory processes.

Local cellular sources of apolipoprotein E in the human retina and retinal pigmented epithelium: implications for the process of drusen formation

PURPOSE

The inheritance of specific apolipoprotein E allelles has been linked to atherosclerosis, Alzheimer disease, and, most recently, to the incidence of age-related macular degeneration. Apolipoprotein E is a common component of the extracellular plaques and deposits characteristic of these disorders, including drusen, which are a hallmark of age-related macular degeneration. Accordingly, we assessed the potential biosynthetic contribution of local ocular cell types to the apolipoprotein E found in drusen.

METHODS

We measured apolipoprotein E mRNA levels in human donor tissues using a quantitative assay of apolipoprotein E transcription, and we localized apolipoprotein E protein to specific cell types and compartments in the neural retina, retinal pigmented epithelium, and choroid using laser scanning confocal immunofluorescence microscopy.

RESULTS

Apolipoprotein E immunoreactivity is associated with photoreceptor outer segments, the retinal ganglion cell layer, the retinal pigmented epithelium basal cytoplasm and basal lamina, and with both collagenous layers of Bruch membrane. Apolipoprotein E appears to be a ubiquitous component of drusen, irrespective of clinical phenotype. It also accumulates in the cytoplasm of a subpopulation of retinal pigmented epithelial cells, many of which overlie or flank drusen. Mean levels of apolipoprotein E mRNA in the adult human retina are 45% and 150% of the levels measured in liver and adult brain, the two most abundant biosynthetic sources of apolipoprotein E. Apolipoprotein E mRNA levels are highest in the inner retina, and lowest in the outer retina where photoreceptors predominate. Significant levels of apolipoprotein E mRNA are also present in the retinal pigmented epithelium/choroid complex and in cultured human retinal pigmented epithelial cells.

CONCLUSIONS

Apolipoprotein E protein is strategically located at the same anatomic locus where drusen are situated, and the retinal pigmented epithelium is the most likely local biosynthetic source of apolipoprotein E at that location. Age-related alteration of lipoprotein biosynthesis and/or processing at the level of the retinal pigmented epithelium and/or Bruch membrane may be a significant contributing factor in drusen formation and age-related macular degeneration pathogenesis.

Structure and composition of drusen associated with glomerulonephritis: implications for the role of complement activation in drusen biogenesis

PURPOSE

The ocular fundi of many patients with membranoproliferative glomerulonephritis type II (MPGN-II) are characterised by the presence of deposits within Bruch's membrane that resemble drusen, hallmark lesions associated with age-related macular degeneration (AMD). Glomerulonephritis (GN)-associated drusen appear at a younger age, however, than do drusen in individuals with AMD. In light of recent evidence that immune-mediated events participate in drusen biogenesis and AMD, we examined the structure and composition of drusen in eyes obtained from human donors with two distinct glomerulopathies, both of which involve complement deposition within glomeruli. These features were compared with those of drusen from patients with clinically documented AMD.

METHODS

Eyes obtained from two human human donors diagnosed with membranous and post-streptococcal GN, respectively, were analysed histochemically, immunohistochemically and ultrastructurally.

RESULTS

Subretinal pigment epithelial (RPE) deposits in both types of GN are numerous and indistinguishable, both structurally and compositionally, from drusen in donors with AMD. GN-associated drusen exhibit sudanophilia, bind filipin, and react with antibodies directed against vitronectin, complement C5 and C5b-9 complexes, TIMP-3 and amyloid P component. Drusen from the membranous GN donor, but not the post-streptococcal GN donor, reacted with peanut agglutinin and antibodies directed against MHC class II antigens and IgG. The ultrastructural characteristics of these deposits were also identical with those of AMD-associated drusen.

CONCLUSIONS

The composition and structure of ocular drusen associated with membranous and post-streptococcal/segmental GN are generally similar to those of drusen in individuals with AMD. In view of the recent data supporting the involvement of complement activation in drusen biogenesis and the pathobiology of AMD, further studies of the biological relationships between AMD and diseases associated with complement activation are warranted.

The APO(*)E3-Leiden mouse as an animal model for basal laminar deposit

AIM

To investigate the APO(*)E3-Leiden mouse as an animal model for age related maculopathy (ARM) related extracellular deposits.

METHODS

Eyes were obtained from APO(*)E3-Leiden transgenic mice on a high fat/cholesterol (HFC) diet (n=12) or on a normal mouse chow (n=6), for 9 months. As controls, eyes were collected from APO-E knockout mice on the same diets. From each mouse one eye was processed for microscopic evaluation and immunohistochemistry with a polyclonal antibody directed against human apo-E. Electron microscopy was also performed.

RESULTS

All 12 eyes of the APO(*)E3-Leiden mice on an HFC diet contained basal laminar deposit (BLD; class 1 to class 3), whereas two of six APO(*)E3-Leiden mice on normal chow showed BLD class 1. The ultrastructural aspects of this BLD were comparable with those seen in early BLD in humans, and BLD showed immunoreaction with anti-human-apo-E antibodies. No BLD was found in any of the control mice. Drusen were not detected in any of the mice.

CONCLUSION

These results indicate that APO(*)E3-Leiden mice can be used as animal model for the pathogenesis of BLD, and that a HFC diet enhances the accumulation of this deposit. Furthermore, this study supports the previously suggested involvement of dysfunctional apo-E in the accumulation of extracellular deposits in ARM.

A potential role for immune complex pathogenesis in drusen formation

Drusen are abnormal extracellular deposits that accumulate between the retinal pigmented epithelium and Bruch's membrane and are commonly associated with age-related macular degeneration. Our recent work has identified a number of plasma proteins as molecular components of drusen. Of interest is the fact that many of these drusen-associated molecules are acute phase reactant proteins and some have established roles in mediating immune responsiveness. As immune and inflammatory responses appear to play a role in the formation of other pathologic age-related deposits, we examined the distribution of immunoglobulin molecules and terminal complement complexes at sites of drusen deposition. Here, we report that concentrations of immunoglobulin G and terminal C5b-9 complement complexes are present in drusen. In addition, we observe that retinal pigmented epithelial cells overlying or directly adjacent to drusen, as well as some within apparently normal epithelia, exhibit cytoplasmic immunoreactivity for immunoglobulin and the C5 component of complement. Taken together, these results suggest that drusen biogenesis may be a byproduct of immune responsiveness, and they implicate immune complex-mediated pathogenesis involving retinal pigmented epithelial cells as an initiating event in drusen formation.

Location, substructure, and composition of basal laminar drusen compared with drusen associated with aging and age-related macular degeneration

PURPOSE

To determine whether basal laminar drusen differ in their location, ultrastructure, or composition from drusen associated with aging and age-related macular degeneration.

METHODS

A paraffin-embedded block from an eye of a patient with basal laminar drusen was obtained. Sections were examined immunohistochemically using a battery of antibodies and lectins directed against drusen-associated proteins and glycoconjugates, respectively. Thin sections were examined by electron microscopy and compared with eyes with age-related macular degeneration.

RESULTS

Drusen in the eye with basal laminar drusen are located between the basal lamina of the retinal pigment epithelium and the inner collagenous layer of Bruch membrane, just as they are in age-related macular degeneration. Two distinct ultrastructural phenotypes are observed in the eye with basal laminar drusen; their substructure is indistinguishable from drusen phenotypes in age-related macular degeneration. Both basal laminar drusen and drusen associated with age-related macular degeneration are bound by the lectins Ricinis communis agglutinin and Arachis hypogea agglutinin (after neuraminidase digestion) and by antivitronectin, anti-HLA-DR, anti-serum amyloid P, and anti-C5 antibodies, but not by antibodies directed against basement membrane-associated heparan sulfate proteoglycan, laminin, fibrinogen, or collagen type IV.

CONCLUSIONS

These data support the notion that cuticular or basal laminar drusen are similar to, and perhaps indistinguishable from, drusen associated with age-related macular degeneration and are not nodular or diffuse thickenings of Bruch membrane, as previously suggested. Thus, we suggest basal laminar drusen is a misnomer. This clinical phenotype should be identified as "early adult onset, grouped drusen" or by the eponym "Gass syndrome." Features of basal laminar drusen, such as uniform drusen size, clustered distribution, and angiographic features, do not appear to be related to differences in drusen location, composition, or substructure.

Human ocular drusen possess novel core domains with a distinct carbohydrate composition

Ocular drusen are extracellular deposits that form between the retinal pigmented epithelium (RPE) and Bruch's membrane. Although the presence of large and/or numerous drusen in the macula is a significant risk factor for development of age-related macular degeneration (AMD), a major cause of irreversible blindness, little is known about their origin or composition. We have expanded on our previous investigations related to drusen-associated glycoconjugates by examining lectin binding patterns after removal of terminal sialic acid residues. Strikingly, intense and distinct labeling of drusen subdomains is revealed by Arachea hypogea agglutinin (PNA) after neuraminidase treatment. PNA binding is confined to discrete domains within both hard and soft drusen. These "cores" are positioned centrally within drusen and are typically juxtaposed to Bruch's membrane. Only one core per druse is observed. PNA labeling of drusen cores does not co-localize with associated lipids and is abrogated by digestion with O-glycosidase but not N-glycosidase. The association of cores with small drusen suggests that they may participate in drusen biogenesis. (J Histochem Cytochem 47:1533-1539, 1999)

Drusen in age-related macular degeneration: pathogenesis, natural course, and laser photocoagulation-induced regression

Drusen are subretinal pigment epithelial deposits that are characteristic of but not uniquely associated with age-related macular degeneration (AMD). Age-related macular degeneration is associated with two types of drusen that have different clinical appearances and different prognoses. Hard drusen appear as small, punctate, yellow nodules and can precede the development of atrophic AMD. Areolar atrophy of the retinal pigment epithelium (RPE), choriocapillaris, and outer retina develop as the drusen disappear, but drusen can regress without evidence of atrophy. Soft drusen appear as large (usually larger than 63 microm in diameter), pale yellow or grayish-white, dome-shaped elevations that can resemble localized serous RPE detachments. They tend to precede the development of clinically evident RPE detachments and choroidal neovascularization. Drusen characteristics correlated with progression to exudative maculopathy include drusen number (five or more), drusen size (larger than 63 microm in diameter), and confluence of drusen. Focal hyperpigmentation in the macula and systemic hypertension also are associated with an increased risk of developing choroidal new vessels (CNVs). Large drusen are usually a sign of diffuse thickening of Bruch's membrane with basal linear deposit, a vesicular material that probably arises from the RPE, constitutes a diffusion barrier to water-soluble constituents in the plasma, results in lipidization of Bruch's membrane, and creates a potential cleavage plane between the RPE basement membrane and the inner collagenous layer of Bruch's membrane through which CNVs can grow. Disappearance of drusen spontaneously and in areas adjacent to laser photocoagulation scars was first noted by Gass (Gass JD: Arch Ophthalmol 90:206-217, 1973; Trans Am Acad Ophthalmol Otolaryngol 75:580-608, 1971). Subsequent reports have confirmed these observations. Photocoagulation-induced drusen regression might prevent patients with drusen from developing exudative maculopathy. The mechanism for spontaneous drusen regression probably involves RPE atrophy. The mechanism for photocoagulation-induced drusen regression is unknown. If photocoagulation-induced drusen regression is anatomically similar to atrophy-associated drusen regression, then the former will be associated with dissolution of basal linear deposit and a residuum of basal laminar deposit. Sarks and coworkers (Sarks JP, Sarks SH, Killingsworth MC: Eye 11:515-522, 1997) proposed that this in turn will eliminate the potential cleavage plane between the RPE basement membrane and inner collagenous layer of Bruch's membrane through which CNVs grow, thus retarding the growth of CNVs.

Vitronectin is a constituent of ocular drusen and the vitronectin gene is expressed in human retinal pigmented epithelial cells

Age-related macular degeneration (AMD) leads to dysfunction and degeneration of retinal photoreceptor cells. This disease is characterized, in part, by the development of extracellular deposits called drusen. The presence of drusen is correlated with the development of AMD, although little is known about drusen composition or biogenesis. Drusen form within Bruch's membrane, a stratified extracellular matrix situated between the retinal pigmented epithelium and choriocapillaris. Because of this association, we sought to determine whether drusen contain known extracellular matrix constituents. Antibodies directed against a battery of extracellular matrix molecules were screened on drusen-containing sections from human donor eyes, including donors with clinically documented AMD. Antibodies directed against vitronectin, a plasma protein and extracellular matrix component, exhibit intense and consistent reactivity with drusen; antibodies to the conformationally distinct, heparin binding form of human vitronectin are similarly immunoreactive. No differences in vitronectin immunoreactivity between hard and soft drusen, or between macular and extramacular regions, have been observed. RT-PCR analyses revealed that vitronectin mRNA is expressed in the retinal pigmented epithelium (RPE)-choroidal complex and cultured RPE cells. These data document that vitronectin is a major constituent of human ocular drusen and that vitronectin mRNA is synthesized locally. Based on these data, we propose that vitronectin may participate in the pathogenesis of AMD.

Early drusen formation in the normal and aging eye and their relation to age related maculopathy: a clinicopathological study

AIM

To describe the early formation of drusen and their relation to normal aging changes at the macula and to the development of age related maculopathy (ARM).

METHOD

Histopathological features of 353 eyes without histological evidence of ARM are described and correlated with the clinical appearance. In addition, 45 of these eyes were examined by transmission electron microscopy.

RESULTS

Drusen were detected histopathologically in 177 (50%) eyes but were seen clinically in only 34% of these. Drusen were mainly small hard drusen with an occasional soft distinct drusen: no soft indistinct drusen were seen. Only those drusen deposits larger than 25-30 microns in diameter were detectable clinically. Preclinical drusen in eyes with only an occasional drusen were seen on electron microscopy as entrapment sites of coated membrane bound bodies which formed adjacent to the inner collagenous zone of Bruch's membrane. In contrast, preclinical drusen deposits in eyes with many drusen were seen as accumulations of amorphous material which appeared hyalinised by light microscopy. A distinct feature were rows of dense hyalinised microdrusen (1-2 microns in diameter), over which larger globular hyalinised drusen formed.

CONCLUSION

Histological and ultrastructural examination can recognise and distinguish the earliest drusen formed as a result of normal aging from those associated with ARM. In eyes without diffuse deposits, histologically all drusen were of the hard hyalinised variety or their derivatives; no soft drusen composed of membranous debris were found. These findings support and explain those of other authors who do not consider the presence of a few small hard drusen to be a risk factor for the development of ARM.