Goldis Malek

The Pivotal Role of the Complement System in Aging and Age-related Macular Degeneration: Hypothesis Re-visited

During the past ten years, dramatic advances have been made in unraveling the biological bases of age-related macular degeneration (AMD), the most common cause of irreversible blindness in western populations. In that timeframe, two distinct lines of evidence emerged which implicated chronic local inflammation and activation of the complement cascade in AMD pathogenesis. First, a number of complement system proteins, complement activators, and complement regulatory proteins were identified as molecular constituents of drusen, the hallmark extracellular deposits associated with early AMD. Subsequently, genetic studies revealed highly significant statistical associations between AMD and variants of several complement pathway-associated genes including: Complement factor H (CFH), complement factor H-related 1 and 3 (CFHR1 and CFHR3), complement factor B (CFB), complement component 2 (C2), and complement component 3 (C3). In this article, we revisit our original hypothesis that chronic local inflammatory and immune-mediated events at the level of Bruchs membrane play critical roles in drusen biogenesis and, by extension, in the pathobiology of AMD. Secondly, we report the results of a new screening for additional AMD-associated polymorphisms in a battery of 63 complement-related genes. Third, we identify and characterize the local complement system in the RPE-choroid complex - thus adding a new dimension of biological complexity to the role of the complement system in ocular aging and AMD. Finally, we evaluate the most salient, recent evidence that bears directly on the role of complement in AMD pathogenesis and progression. Collectively, these recent findings strongly re-affirm the importance of the complement system in AMD. They lay the groundwork for further studies that may lead to the identification of a transcriptional disease signature of AMD, and hasten the development of new therapeutic approaches that will restore the complement-modulating activity that appears to be compromised in genetically susceptible individuals.

Reticular pseudodrusen are subretinal drusenoid deposits.

PURPOSE To characterize reticular pseudodrusen, a potential risk factor for late age-related macular degeneration. DESIGN Retrospective, observational case series. PARTICIPANTS Fifty-eight eyes of 33 patients with pseudodrusen (20 female). METHODS Consecutive patients with reticular pseudodrusen, diagnosed by their typical appearance and distribution using ophthalmoscopy, the blue channel of color fundus photographs, and near infrared images. The patients were imaged by spectral domain optical coherence tomography (SD OCT), and correlations were made between the near infrared images and the SD OCT images. The SD OCT findings in patients with pseudodrusen were compared with previously reported histologic findings of subretinal drusenoid deposits. The histologic specimens were reevaluated with the additional knowledge of the clinical information. MAIN OUTCOME MEASURES Spectral domain optical coherence tomography and histologic characteristics of pseudodrusen. RESULTS The mean age of the 33 patients was 81.7 years. The correlating SD OCT scans showed collections of granular hyperreflective material above the retinal pigment epithelium (RPE), in the subretinal space located primarily between the RPE and the boundary between the inner and outer segments of the photoreceptors (IS/OS boundary). In a more advanced stage, this material formed small mounds that broke through the IS/OS boundary. There were no correlates to the deposits seen under the RPE or in the choroid. These findings were similar in character to previously reported histologic characterization of subretinal drusenoid deposits, which had identified the presence of membranous debris, unesterified cholesterol, and complement within the deposits. CONCLUSIONS Pseudodrusen seen by clinical examination may be subretinal drusenoid deposits seen by histologic examination. This unexpected location suggests that potential pathophysiologic mechanisms on both sides of the RPE need to be taken into account in theories related to the development of age-related macular degeneration.

Apolipoprotein B in Cholesterol-Containing Drusen and Basal Deposits of Human Eyes with Age-Related Maculopathy

Lipids accumulate in Bruchs membrane (BrM), a specialized vascular intima of the eye, and in extracellular lesions associated with aging and age-related maculopathy (ARM). We tested the hypothesis that ARM and atherosclerotic cardiovascular disease share molecules and mechanisms pertaining to extracellular lipid accumulation by localizing cholesterol and apolipoprotein B (apo B) in BrM, basal deposits, and drusen. Human donor eyes were preserved <4 hours postmortem and cryosectioned. Sections were stained with traditional lipid stains and filipin for esterified and unesterified cholesterol or probed with antibodies to apo B, apo E, and apo C-III. Normal adult retinal pigment epithelium (RPE) was subjected to RT-PCR and Western blot analysis for apolipoprotein mRNA and protein. Esterified and unesterified cholesterol was present in all drusen and basal deposits of ARM and normal eyes. Both apo B and apo E but not apo C-III were found in BrM, drusen, and basal deposits. Fewer macular drusen were stained by traditional lipid stains and apolipoprotein antibodies than peripheral drusen. RPE contained apo B and apo E mRNA and protein. Finding cholesterol and apo B in sub-RPE deposits links ARM with important molecules and mechanisms in atherosclerosis initiation and progression. The combination of apo B mRNA and protein in RPE raises the possibility that intraocular assembly of apo B-containing lipoproteins is a pathway involved in forming cholesterol-enriched lesions in ARM.

Sub-retinal drusenoid deposits in human retina: Organization and composition

We demonstrate histologically sub-retinal drusenoid debris in three aged human eyes, two of them affected by age-related maculopathy. By postmortem fundus examination, the lesions were drusen-like, i.e., they were pale spots apparently at the level of the retinal pigment epithelium (RPE). Light and electron microscopy revealed aggregations of membranous debris, the principal constituent of soft drusen, in the sub-retinal space. Immunohistochemistry and confocal microscopy confirmed the presence of molecules typically associated with drusen (positive for unesterified cholesterol, apoE, complement factor H, and vitronectin) without evidence for molecules associated with photoreceptors (lectin-binding disaccharide bridges and opsins), Müller cells (glial fibrillary acid protein and cellular retinal binding protein, CRALPB), or RPE (CRALPB). The fact that a drusenoid material, sharing some markers with conventional drusen, can occur on opposite faces of the RPE, suggests deranged polarity of normally highly vectorial processes for basolateral secretion from RPE, and that overproduction of secreted materials and direction of secretion are independently specified processes. In the future, drusenoid sub-retinal debris might be more frequently revealed by emerging high-resolution imaging techniques.

Peripapillary chorioretinal atrophy: Bruch’s membrane changes and photoreceptor loss

PURPOSE To determine relationships among Bruchs membrane ultrastructure, photoreceptor degeneration, and choriocapillaris atrophy with respect to zones of retinal pigment epithelium (RPE) degeneration and atrophy adjacent to the optic nerve head, as a function of age. DESIGN Human tissue study using clinicopathologic correlation. TISSUES: Eyes from patients 36 to 93 years of age lacking clinical evidence of glaucoma, optic nerve abnormalities, severe myopia, age-related macular degeneration, or other macular or peripapillary chorioretinal pathologic condition. METHODS Sections through the retina-choroid complex at the temporal aspect of the optic nerve head were used for light microscopic histopathologic analysis (n = 17), electron microscopy (n = 9), carbonic anhydrase histochemical analysis (n = 7), and lipid histochemical analysis (n = 22). Retinal whole mounts were used for photoreceptor counts (n = 5). MAIN OUTCOME MEASURES We determined the width of RPE degeneration and atrophy, the number of eyes with abnormalities of inner Bruchs membrane, and the number of rod and cone photoreceptors within 1 mm of the disc margin. We determined whether Bruchs membrane changes, photoreceptor degeneration, and choriocapillaris atrophy were associated with RPE degeneration and atrophy. RESULTS All eyes had peripapillary RPE atrophy, degeneration, or both. The zone of RPE atrophy widened significantly after age 75. Thickening of inner Bruchs membrane and abnormalities of the RPE basal lamina were associated with degenerating and atrophic RPE in all eyes. The RPE basal lamina was narrow, reduplicated, or thickened as a basal laminar deposit. All eyes exhibited degeneration and loss of rods but not cones at the peripapillary termination of Bruchs membrane. Diminution of choriocapillaris coverage of Bruchs membrane was associated with RPE degeneration. Complete loss of the choriocapillaris was associated with RPE atrophy. CONCLUSIONS Our results suggest that peripapillary chorioretinal atrophy is an age-related degeneration of the RPE-Bruchs membrane complex that resembles that found in the macula and periphery of normal eyes.

Molecular genetics of AMD and current animal models

During the past few years systematic investigation into the epidemiology, genetics, and pathophysiology of age-related macular degeneration (AMD) has provided important new insight into this leading cause of vision loss in older persons. These studies provide a view of AMD as a complex trait influenced by well-established genetic and environmental risks that leads to the deposition of inflammatory deposits in the outer retina. This maculopathy leads to visual dysfunction through a variety of mechanisms and complications that can be observed in both humans and animal models. In this review, the risks associated with AMD in humans and the animal models used to study AMD and its complications will be summarized. No effort has been made to perform a comprehensive citation of all areas of AMD genetics and animal models, but rather a selection of observations and supporting references illustrative of the current state of the field is presented.

Dominant Late-onset Retinal Degeneration with Regional Variation of Sub-Retinal Pigment Epithelium Deposits, Retinal Function, and Photoreceptor Degeneration

PURPOSE To clarify the pathogenesis of late-onset retinal degeneration (L-ORD), an autosomal dominant disorder characterized by thick deposits of lipid-rich material between the retinal pigment epithelium (RPE) and Bruchs membrane. STUDY DESIGN Comparative clinicopathologic case report and case series. TISSUES: Eyes of an 82-year-old L-ORD eye donor and an age-matched control. SUBJECTS Five descendants of the eye donor and his affected sister. METHODS The eyes were processed for histopathologic examination, including electron microscopy and immunohistochemistry. Family members were examined clinically and with retinal function tests. RESULTS The L-ORD eye had sub-RPE deposits that were positive for lipid, including esterified and unesterified cholesterol. The deposits were thinnest in the macula, which retained the highest percentage of photoreceptors. In the periphery, RPE thinning and photoreceptor loss correlated with thickness of the sub-RPE deposits. The eye donor was asymptomatic until his late 50s, when he developed problems with adapting to darkness. At age 68, the eye donor had normal acuity but a midperipheral scotoma and subnormal electroretinograms (ERGs); visual loss was progressive. The five descendants (at the time of examination ages 44-58) of the eye donor and his affected sister, who were at 50/50 risk of inheriting L-ORD, had normal ERGs, but four showed defects in dark adaptation. The dark adaptation abnormalities had a distribution similar to the thickness of the sub-RPE deposits in the eye donor, with slow kinetics in the midperiphery and normal kinetics centrally. CONCLUSIONS The L-ORD donor eye differed from a previous case in the regional distribution of sub-RPE deposits and photoreceptors. In the next generation of this L-ORD family, the first expression of disease, abnormal dark adaptation, mirrored the regional distribution of the deposits in the donor eye. The fine structure and staining characteristics of the sub-RPE deposits in L-ORD resemble those in age-related macular degeneration and Sorsby fundus dystrophy.

Aryl hydrocarbon receptor deficiency causes dysregulated cellular matrix metabolism and age-related macular degeneration-like pathology

Significance Age-related Macular Degeneration (AMD) is the leading cause of vision loss. In its early stage, extracellular deposits accumulate below the retinal pigment epithelial layer (RPE), nurse cells to the retina. Identification of therapeutic treatments targeting deposit removal, which when left untreated exacerbate RPE and retinal damage, necessitates the discovery of pathways regulating deposit formation. We show that the activity of a nuclear receptor, essential to xenobiotic/toxin metabolism and cellular debris clearance, is critical to maintaining RPE cell health and that its deficiency in mice causes AMD pathology. This model provides a better understanding of AMD pathogenic mechanisms and a platform for testing novel therapeutics. The aryl hydrocarbon receptor (AhR) is a nuclear receptor that regulates xenobiotic metabolism and detoxification. Herein, we report a previously undescribed role for the AhR signaling pathway as an essential defense mechanism in the pathogenesis of early dry age-related macular degeneration (AMD), the leading cause of vision loss in the elderly. We found that AhR activity and protein levels in human retinal pigment epithelial (RPE) cells, cells vulnerable in AMD, decrease with age. This finding is significant given that age is the most established risk factor for development of AMD. Moreover, AhR−/− mice exhibit decreased visual function and develop dry AMD-like pathology, including disrupted RPE cell tight junctions, accumulation of RPE cell lipofuscin, basal laminar and linear-like deposit material, Bruch’s membrane thickening, and progressive RPE and choroidal atrophy. High-serum low-density lipoprotein levels were also observed in AhR−/− mice. In its oxidized form, this lipoprotein can stimulate increased secretion of extracellular matrix molecules commonly found in deposits from RPE cells, in an AhR-dependent manner. This study demonstrates the importance of cellular clearance via the AhR signaling pathway in dry AMD pathogenesis, implicating AhR as a potential target, and the mouse model as a useful platform for validating future therapies.

Research Resource: Nuclear Receptor Atlas of Human Retinal Pigment Epithelial Cells: Potential Relevance to Age-Related Macular Degeneration

Retinal pigment epithelial (RPE) cells play a vital role in retinal physiology by forming the outer blood-retina barrier and supporting photoreceptor function. Retinopathies including age-related macular degeneration (AMD) involve physiological and pathological changes in the epithelium, severely impairing the retina and effecting vision. Nuclear receptors (NRs), including peroxisome proliferator-activated receptor and liver X receptor, have been identified as key regulators of physiological pathways such as lipid metabolic dysregulation and inflammation, pathways that may also be involved in development of AMD. However, the expression levels of NRs in RPE cells have yet to be systematically surveyed. Furthermore, cell culture lines are widely used to study the biology of RPE cells, without knowledge of the differences or similarities in NR expression and activity between these in vitro models and in vivo RPE. Using quantitative real-time PCR, we assessed the expression patterns of all 48 members of the NR family plus aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator in human RPE cells. We profiled freshly isolated cells from donor eyes (in vivo), a spontaneously arising human cell line (in vitro), and primary cell culture lines (in vitro) to determine the extent to which NR expression in the cultured cell lines reflects that of in vivo. To evaluate the validity of using cell culture models for investigating NR receptor biology, we determined transcriptional activity and target gene expression of several moderately and highly expressed NRs in vitro. Finally, we identified a subset of NRs that may play an important role in pathobiology of AMD.

Aryl hydrocarbon receptor knock-out exacerbates choroidal neovascularization via multiple pathogenic pathways

The aryl hydrocarbon receptor (AhR) is a heterodimeric transcriptional regulator with pleiotropic functions in xenobiotic metabolism and detoxification, vascular development and cancer. Herein, we report a previously undescribed role for the AhR signalling pathway in the pathogenesis of the wet, neovascular subtype of age‐related macular degeneration (AMD), the leading cause of vision loss in the elderly in the Western world. Comparative analysis of gene expression profiles of aged AhR−/− and wild‐type (wt) mice, using high‐throughput RNA sequencing, revealed differential modulation of genes belonging to several AMD‐related pathogenic pathways, including inflammation, angiogenesis and extracellular matrix regulation. To investigate AhR regulation of these pathways in wet AMD, we experimentally induced choroidal neovascular lesions in AhR−/− mice and found that they measured significantly larger in area and volume compared to age‐matched wt mice. Furthermore, these lesions displayed a higher number of ionized calcium‐binding adaptor molecule 1‐positive (Iba1+) microglial cells and a greater amount of collagen type IV deposition, events also seen in human wet AMD pathology specimens. Consistent with our in vivo observations, AhR knock‐down was sufficient to increase choroidal endothelial cell migration and tube formation in vitro. Moreover, AhR knock‐down caused an increase in collagen type IV production and secretion in both retinal pigment epithelial (RPE) and choroidal endothelial cell cultures, increased expression of angiogenic and inflammatory molecules, including vascular endothelial growth factor A (VEGFA) and chemokine (C–C motif) ligand 2 (CCL2) in RPE cells, and increased expression of secreted phosphoprotein 1 (SPP1) and transforming growth factor‐β1 (TGFβ1) in choroidal endothelial cells. Collectively, our findings identify AhR as a regulator of multiple pathogenic pathways in experimentally induced choroidal neovascularization, findings that are consistent with a possible role of AhR in wet AMD. The data discussed in this paper have been deposited in NCBIs Gene Expression Omnibus; GEO Submission No. GSE56983, NCBI Tracking System No. 17021116.