An Integrated Hypothesis That Considers Drusen as Biomarkers of Immune-Mediated Processes at the RPE-Bruch's Membrane Interface in Aging and Age-Related Macular Degeneration

Objective To review and synthesize information concerning the pathogenesis ofage-related macular degeneration (AMD). Methods Review of the English-language literature. Results Five concepts relevant to the cell biology of AMD are as follows: (1)AMD involves aging changes plus additional pathological changes (ie, AMD isnot just an aging change); (2) in aging and AMD, oxidative stress causes retinalpigment epithelial (RPE) and, possibly, choriocapillaris injury; (3) in AMD(and perhaps in aging), RPE and, possibly, choriocapillaris injury resultsin a chronic inflammatory response within the Bruch membrane and the choroid;(4) in AMD, RPE and, possibly, choriocapillaris injury and inflammation leadto formation of an abnormal extracellular matrix (ECM), which causes altereddiffusion of nutrients to the retina and RPE, possibly precipitating furtherRPE and retinal damage; and (5) the abnormal ECM results in altered RPE-choriocapillarisbehavior leading ultimately to atrophy of the retina, RPE, and choriocapillarisand/or choroidal new vessel growth. In this sequence of events, both the environmentand multiple genes can alter a patient's susceptibility to AMD. Implicit inthis characterization of AMD pathogenesis is the concept that there is linearprogression from one stage of the disease to the next. This assumption maybe incorrect, and different biochemical pathways leading to geographic atrophyand/or choroidal new vessels may operate simultaneously. Conclusions Better knowledge of AMD cell biology will lead to better treatmentsfor AMD at all stages of the disease. Many unanswered questions regardingAMD pathogenesis remain. Multiple animal models and in vitro models of specificaspects of AMD are needed to make rapid progress in developing effective therapiesfor different stages of the disease.

https://jamanetwork.com/journals/jamaophthalmology/articlepdf/416250/EMO20000.pdf

Current Concepts in the Pathogenesis of Age-Related Macular Degeneration

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. (Am J Ophthalmol 2002;134:411‐ 431. © 2002 by Elsevier

PERSPECTIVE A Role for Local Inflammation in the Formation of Drusen in the Aging Eye

Age-related macular degeneration: etiology, pathogenesis, and therapeutic strategies.

Purpose The authors showed previously that parafoveal rods, but not cones, decrease during the course of adulthood in donor eyes that were screened to exclude the grossly visible macular drusen and pigmentary disturbances typical of age-related macular degeneration (AMD). Because AMD begins in the parafovea, this selective loss of rods actually may be subclinical AMD not yet visible in the fundus. If so, AMD must have a predilection for rods over cones. The authors tested this hypothesis by determining the relative numbers of cones and rods in donor eyes with mid-to late-stage AMD and in age-matched controls. Methods Thirteen eyes (from seven donors) with grossly visible macular drusen and pigmentary disturbances were either wholemounted for photoreceptor counts or sectioned through the fovea for histopathology and carbonic anhydrase histochemistry to label red-green cones. Eyes were assigned to AMD or control groups on the basis of histopathology and clinical history. Results Five nonexudative AMD (NE-AMD) eyes from three donors showed sparing of foveal cones and loss of rods and cones in the parafovea. In two donors, rod loss exceeded cone loss at most parafoveal locations, and in one donor, rod density was normal and cone density was reduced. In eight exudative AMD (EX-AMD) eyes from five donors, photoreceptors surviving along the margins of and overlying disciform scars were largely cones. Conclusions Photoreceptors are lost in NE-AMD as well as in the more severe exudative form, consistent with functional and clinical studies. The authors propose that rods die in older eyes without evidence of overt retinal pigment epithelial disease. In persons susceptible to AMD, the retinal pigment epithelium becomes dysfunctional. Secondarily, rod loss continues and cones begin to degenerate. Eventually, only degenerate cones remain; ultimately, all photoreceptors may disappear.

Photoreceptor loss in age-related macular degeneration.

Purpose Because previous studies suggested degeneration and loss of photoreceptors in aged human retina, the spatial density of cones and rods subserving the central 43 degrees of vision as a function of age was determined. Methods Cones and rods were counted in 27 whole mounted retinas from donors aged 27 to 90 years with macroscopically normal fundi. Photoreceptor topography was analyzed with new graphic and statistical techniques. Results Changes in cone density throughout this age span showed no consistent relationship to age or retinal location, and the total number of foveal cones was remarkably stable. In contrast, rod density decreased by 30%, beginning inferior to the fovea in midlife and culminating in an annulus of deepest loss at 0.5 to 3 mm eccentricity by the ninth decade. Space vacated by dying rods was filled in by larger rod inner segments, resulting in a similar rod coverage at all ages. At the temporal equator, cone density declined by 23%, but rods were stable throughout adulthood. Conclusions The stability of both rod coverage and rhodopsin content despite decreasing cell number suggests plasticity of the adult rod system and that age-related declines in scotopic sensitivity may be due to postreceptoral factors. There is no evidence for the massive loss of foveal cones required to explain even modest decrements in acuity, consistent with evidence that visual deficits at high photopic levels may be largely due to optical factors. Why the rods of central retina, which share a common support system and light exposure with the neighboring cones, are preferentially vulnerable to aging remains to be determined.

Aging of the human photoreceptor mosaic: evidence for selective vulnerability of rods in central retina.

Purpose To determine the cholesterol composition of normal human Bruch's membrane and choroid as a function of age and retinal location. Methods Human eyes with grossly normal maculas were preserved 70 years). Results EC and UC in Bruch's membrane increased with age in the macula. EC was sevenfold higher in macula than in periphery. Sixty percent of total cholesterol was esterified, and Bruch's membrane EC was 16- to 40-fold enriched relative to plasma. Solid, 100-nm-diameter particles occupied >30% of the inner collagenous layer in eyes >60 years. Cholesterol accumulated in choroidal arteries and in small age-related drusen. Conclusions Human Bruch's membrane ages like arterial intima and other connective tissues for which plasma lipoproteins are the known source of extracellular cholesterol. Age-related maculopathy and atherosclerotic cardiovascular disease may share common pathogenic mechanisms.

Accumulation of cholesterol with age in human Bruch's membrane.

Objective To determine the distributions of basal laminar and basal linear deposits in Bruch membrane (BM) with respect to age and early age-related maculopathy (ARM). Methods The foveas of 41 human eyes ( Results Both deposits occurred only in eyes older than 60 years. The highest specificities and sensitivities for early ARM were attained for eyes that had basal linear deposits or large (>125 µm) drusen, followed by eyes with any quantity of basal laminar deposits that also contained membranous debris. Eyes with ARM were 24 times more likely than age-matched control eyes to have basal linear deposits or large drusen ( P= .002). Conclusions Basal linear deposits and large drusen with membranous contents constitute different morphologic forms of the same ARM-associated lesion and may be significant for progression to late ARM.

https://jamanetwork.com/journals/jamaophthalmology/articlepdf/411601/ecs8295.pdf

Basal linear deposit and large drusen are specific for early age-related maculopathy.

Purpose To develop a systematic method for identifying and grading age-related macular degeneration (ARMD) in human donor eyes, postmortem fundus appearance was compared with histopathologic assessment in eyes with a spectrum of age-related macular change. Methods Eyes without grossly visible, late ARMD were obtained from 8 cancer patients and 26 donors older than 50 years. Postmortem fundus appearance was graded for drusen and pigmentary change, using stereo color photographs. Eyes were processed and sectioned at 1 microm for histopathologic evaluation of macular retinal pigment epithelium-Bruch's membrane complex. The histologic diagnosis was compared with gross fundus appearance, clinical ophthalmic histories (n = 25), and clinical fundus photographs that were graded using the Wisconsin Age-related Maculopathy Grading System (n = 5). Results Ten eyes met histopathologic criteria for early ARMD. A similar proportion of eyes (27%-32%) was identified as affected by ARMD by other published histopathologic criteria. By choosing eyes with at least one druse larger than 125 microm in diameter or an area of pigment-clumping 500 microm in diameter that was visible in the postmortem fundus, ARMD cases were identified with 90% sensitivity and 95% specificity. Conclusions The Alabama ARMD Grading System permits rational and standardized use of donor eyes in studies that are directed toward understanding the pathogenesis of ARMD.

C L Millican

Papers

Photoreceptor loss in age-related macular degeneration.

Aging of the human photoreceptor mosaic: evidence for selective vulnerability of rods in central retina.

Accumulation of cholesterol with age in human Bruch's membrane.

Basal linear deposit and large drusen are specific for early age-related maculopathy.

The Alabama Age-Related Macular Degeneration Grading System for donor eyes.