Ultrastructural analysis of submacular choriocapillaris and its transport systems in AMD and aged control eyes

Structural OCT and OCT angiography biomarkers associated with the development and progression of geographic atrophy in AMD

BACKGROUND

Geographic atrophy (GA) is an advanced, irreversible, and progressive form of age-related macular degeneration (AMD). Structural optical coherence tomography (OCT) and OCT angiography (OCTA) have been largely used to characterize this stage of AMD and, more importantly, to define biomarkers associated with the development and progression of GA in AMD.

METHODS

Articles pertaining to OCT and OCTA biomarkers related to the development and progression of GA with relevant key words were used to search in PubMed, Researchgate, and Google Scholar. The articles were selected based on their relevance, reliability, publication year, published journal, and accessibility.

RESULTS

Previous reports have highlighted various OCT and OCTA biomarkers linked to the onset and advancement of GA. These biomarkers encompass characteristics such as the size, volume, and subtype of drusen, the presence of hyperreflective foci, basal laminar deposits, incomplete retinal pigment epithelium and outer retinal atrophy (iRORA), persistent choroidal hypertransmission defects, and the existence of subretinal drusenoid deposits (also referred to as reticular pseudodrusen). Moreover, biomarkers associated with the progression of GA include thinning of the outer retina, photoreceptor degradation, the distance between retinal pigment epithelium and Bruch's membrane, and choriocapillaris loss.

CONCLUSION

The advent of novel treatment strategies for GA underscores the heightened need for prompt diagnosis and precise monitoring of individuals with this condition. The utilization of structural OCT and OCTA becomes essential for identifying distinct biomarkers associated with the initiation and progression of GA.

Age-Related Macular Degeneration, a Mathematically Tractable Disease

A progression sequence for age-related macular degeneration onset may be determinable with consensus neuroanatomical nomenclature augmented by drusen biology and eye-tracked clinical imaging. This narrative review proposes to supplement the Early Treatment of Diabetic Retinopathy Study (sETDRS) grid with a ring to capture high rod densities. Published photoreceptor and retinal pigment epithelium (RPE) densities in flat mounted aged-normal donor eyes were recomputed for sETDRS rings including near-periphery rich in rods and cumulatively for circular fovea-centered regions. Literature was reviewed for tissue-level studies of aging outer retina, population-level epidemiology studies regionally assessing risk, vision studies regionally assessing rod-mediated dark adaptation (RMDA), and impact of atrophy on photopic visual acuity. The 3 mm-diameter xanthophyll-rich macula lutea is rod-dominant and loses rods in aging whereas cone and RPE numbers are relatively stable. Across layers, the largest aging effects are accumulation of lipids prominent in drusen, loss of choriocapillary coverage of Bruch's membrane, and loss of rods. Epidemiology shows maximal risk for drusen-related progression in the central subfield with only one third of this risk level in the inner ring. RMDA studies report greatest slowing at the perimeter of this high-risk area. Vision declines precipitously when the cone-rich central subfield is invaded by geographic atrophy. Lifelong sustenance of foveal cone vision within the macula lutea leads to vulnerability in late adulthood that especially impacts rods at its perimeter. Adherence to an sETDRS grid and outer retinal cell populations within it will help dissect mechanisms, prioritize research, and assist in selecting patients for emerging treatments.

Clinicopathologic Findings in Three Siblings With Geographic Atrophy

Purpose

Age-related macular degeneration (AMD) is a leading cause of blindness among the elderly worldwide. Clinical imaging and histopathologic studies are crucial to understanding disease pathology. This study combined clinical observations of three brothers with geographic atrophy (GA), followed for 20 years, with histopathologic analysis.

Methods

For two of the three brothers, clinical images were taken in 2016, 2 years prior to death. Immunohistochemistry, on both flat-mounts and cross sections, histology, and transmission electron microscopy were used to compare the choroid and retina in GA eyes to those of age-matched controls.

Results

Ulex europaeus agglutinin (UEA) lectin staining of the choroid demonstrated a significant reduction in the percent vascular area and vessel diameter. In one donor, histopathologic analysis demonstrated two separate areas with choroidal neovascularization (CNV). Reevaluation of swept-source optical coherence tomography angiography (SS-OCTA) images revealed CNV in two of the brothers. UEA lectin also revealed a significant reduction in retinal vasculature in the atrophic area. A subretinal glial membrane, composed of processes positive for glial fibrillary acidic protein and/or vimentin, occupied areas identical to those of retinal pigment epithelium (RPE) and choroidal atrophy in all three AMD donors. SS-OCTA also demonstrated presumed calcific drusen in the two donors imaged in 2016. Immunohistochemical analysis and alizarin red S staining verified calcium within drusen, which was ensheathed by glial processes.

Conclusions

This study demonstrates the importance of clinicohistopathologic correlation studies. It emphasizes the need to better understand how the symbiotic relationship between choriocapillaris and RPE, glial response, and calcified drusen impact GA progression.

Bioprinted 3D outer retina barrier uncovers RPE-dependent choroidal phenotype in advanced macular degeneration

Age-related macular degeneration (AMD), a leading cause of blindness, initiates in the outer-blood-retina-barrier (oBRB) formed by the retinal pigment epithelium (RPE), Bruch's membrane, and choriocapillaris. The mechanisms of AMD initiation and progression remain poorly understood owing to the lack of physiologically relevant human oBRB models. To this end, we engineered a native-like three-dimensional (3D) oBRB tissue (3D-oBRB) by bioprinting endothelial cells, pericytes, and fibroblasts on the basal side of a biodegradable scaffold and establishing an RPE monolayer on top. In this 3D-oBRB model, a fully-polarized RPE monolayer provides barrier resistance, induces choriocapillaris fenestration, and supports the formation of Bruch's-membrane-like structure by inducing changes in gene expression in cells of the choroid. Complement activation in the 3D-oBRB triggers dry AMD phenotypes (including subRPE lipid-rich deposits called drusen and choriocapillaris degeneration), and HIF-α stabilization or STAT3 overactivation induce choriocapillaris neovascularization and type-I wet AMD phenotype. The 3D-oBRB provides a physiologically relevant model to studying RPE-choriocapillaris interactions under healthy and diseased conditions.

Caveolar and non-Caveolar Caveolin-1 in ocular homeostasis and disease

Caveolae, specialized plasma membrane invaginations present in most cell types, play important roles in multiple cellular processes including cell signaling, lipid uptake and metabolism, endocytosis and mechanotransduction. They are found in almost all cell types but most abundant in endothelial cells, adipocytes and fibroblasts. Caveolin-1 (Cav1), the signature structural protein of caveolae was the first protein associated with caveolae, and in association with Cavin1/PTRF is required for caveolae formation. Genetic ablation of either Cav1 or Cavin1/PTRF downregulates expression of the other resulting in loss of caveolae. Studies using Cav1-deficient mouse models have implicated caveolae with human diseases such as cardiomyopathies, lipodystrophies, diabetes and muscular dystrophies. While caveolins and caveolae are extensively studied in extra-ocular settings, their contributions to ocular function and disease pathogenesis are just beginning to be appreciated. Several putative caveolin/caveolae functions are relevant to the eye and Cav1 is highly expressed in retinal vascular and choroidal endothelium, Müller glia, the retinal pigment epithelium (RPE), and the Schlemm's canal endothelium and trabecular meshwork cells. Variants at the CAV1/2 gene locus are associated with risk of primary open angle glaucoma and the high risk HTRA1 variant for age-related macular degeneration is thought to exert its effect through regulation of Cav1 expression. Caveolins also play important roles in modulating retinal neuroinflammation and blood retinal barrier permeability. In this article, we describe the current state of caveolin/caveolae research in the context of ocular function and pathophysiology. Finally, we discuss new evidence showing that retinal Cav1 exists and functions outside caveolae.

Tissue engineering in age-related macular degeneration: a mini-review

Age-related macular degeneration (AMD) is a progressive, degenerative disease of the macula, leading to severe visual loss in the elderly population. There are two types of AMD: non-exudative ('dry') AMD and exudative ('wet') AMD. Non-exudative AMD is characterized by drusen formation and macular atrophy, while the blood vessels are not leaky. Exudative AMD is a more advanced form of the disease, featured with abnormal blood vessel growth and vascular leakage. Even though anti-angiogenic therapies have been effective in treating wet AMD by normalizing blood vessels, there is no treatment available to prevent or treat dry AMD. Currently, the mechanisms of drusen formation and macular atrophy in the dry AMD are poorly understood, in part because the currently available in vivo models of AMD could not decouple and isolate the complex biological and biophysical factors in the macular region for a detailed mechanism study, including the complement system, angiogenesis factors, extracellular matrix, etc. In the present review article, we describe the biological background of AMD and the key cells and structures in AMD, including retinal epithelium, photoreceptor, Bruch's membrane, and choriocapillaris. We also discuss pre-clinical animal models of AMD and in vivo tissue-engineered approaches, including cell suspension injection and organoid-derived cell sheet transplantation. We also discuss in vitro tissue-engineered models for AMD research. Specifically, we evaluate and compare currently available two- and three-dimensional AMD tissue-engineered models that mimic key anatomical players in AMD progression, including pathophysiological characteristics in Bruch's membrane, photoreceptor, and choriocapillaris. Finally, we discuss the limitation of current AMD models and future directions.

An Update on the Hemodynamic Model of Age-Related Macular Degeneration

PURPOSE

To provide an update on the hemodynamic model of age-related macular degeneration (AMD).

DESIGN

Evidence-based perspective.

METHODS

Review of the literature and experience of the authors.

RESULTS

Choroidal hemodynamics are not the primary cause of AMD as proposed by Ephraim Friedman in 1997. However, evidence is accumulating to suggest that choroidal perfusion is an important environmental influence that contributes to our understanding of disease progression in this complex genetic disorder. Although early and intermediate AMD seem to be influenced to a large extent by the underlying genetics, the asymmetry of disease progression to the later stages of AMD cannot be explained by genetics alone. The progression of disease and the asymmetry of this progression seem to correlate with abnormalities in choroidal perfusion that can be documented by optical coherence tomography. These perfusion abnormalities in the setting of a thickened Bruch's membrane are thought to exacerbate the impaired nutritional exchange between the retinal pigment epithelium and the choriocapillaris. We propose that the genetic susceptibility to develop AMD combined with age-related changes in macular choroidal hemodynamics, such as increasing choriocapillaris perfusion deficits and decreasing choroidal vascular densities, play an important role in disease progression and may help to explain the asymmetry between eyes, particularly in the later stages of AMD.

CONCLUSIONS

This updated hemodynamic model of AMD focuses on disease progression and highlights the importance of age-related changes in the choroidal circulation as a major environmental influence on disease severity in eyes that are genetically susceptible to develop AMD.

Variability in Retinal Neuron Populations and Associated Variations in Mass Transport Systems of the Retina in Health and Aging

Aging is associated with a broad range of visual impairments that can have dramatic consequences on the quality of life of those impacted. These changes are driven by a complex series of alterations affecting interactions between multiple cellular and extracellular elements. The resilience of many of these interactions may be key to minimal loss of visual function in aging; yet many of them remain poorly understood. In this review, we focus on the relation between retinal neurons and their respective mass transport systems. These metabolite delivery systems include the retinal vasculature, which lies within the inner portion of the retina, and the choroidal vasculature located externally to the retinal tissue. A framework for investigation is proposed and applied to identify the structures and processes determining retinal mass transport at the cellular and tissue levels. Spatial variability in the structure of the retina and changes observed in aging are then harnessed to explore the relation between variations in neuron populations and those seen among retinal metabolite delivery systems. Existing data demonstrate that the relation between inner retinal neurons and their mass transport systems is different in nature from that observed between the outer retina and choroid. The most prominent structural changes observed across the eye and in aging are seen in Bruch’s membrane, which forms a selective barrier to mass transfers at the interface between the choroidal vasculature and the outer retina.

Epithelial phenotype restoring drugs suppress macular degeneration phenotypes in an iPSC model

Age-related Macular Degeneration (AMD), a blinding eye disease, is characterized by pathological protein- and lipid-rich drusen deposits underneath the retinal pigment epithelium (RPE) and atrophy of the RPE monolayer in advanced disease stages - leading to photoreceptor cell death and vision loss. Currently, there are no drugs that stop drusen formation or RPE atrophy in AMD. Here we provide an iPSC-RPE AMD model that recapitulates drusen and RPE atrophy. Drusen deposition is dependent on AMD-risk-allele CFH(H/H) and anaphylatoxin triggered alternate complement signaling via the activation of NF-κB and downregulation of autophagy pathways. Through high-throughput screening we identify two drugs, L-745,870, a dopamine receptor antagonist, and aminocaproic acid, a protease inhibitor that reduce drusen deposits and restore RPE epithelial phenotype in anaphylatoxin challenged iPSC-RPE with or without the CFH(H/H) genotype. This comprehensive iPSC-RPE model replicates key AMD phenotypes, provides molecular insight into the role of CFH(H/H) risk-allele in AMD, and discovers two candidate drugs to treat AMD.

Localized Structural and Functional Deficits in a Nonhuman Primate Model of Outer Retinal Atrophy

Purpose

Cell-based therapy development for geographic atrophy (GA) in age-related macular degeneration (AMD) is hampered by the paucity of models of localized photoreceptor and retinal pigment epithelium (RPE) degeneration. We aimed to characterize the structural and functional deficits in a laser-induced nonhuman primate model, including an analysis of the choroid.

Methods

Macular laser photocoagulation was applied in four macaques. Fundus photography, optical coherence tomography (OCT), dye angiography, and OCT-angiography were conducted over 4.5 months, with histological correlation. Longitudinal changes in spatially resolved macular dysfunction were measured using multifocal electroretinography (MFERG).

Results

Lesion features, depending on laser settings, included photoreceptor layer degeneration, inner retinal sparing, skip lesions, RPE elevation, and neovascularization. The intralesional choroid was degenerated. The normalized mean MFERG amplitude within lesions was consistently lower than control regions (0.94 ± 0.35 vs. 1.10 ± 0.27, P = 0.032 at month 1, 0.67 ± 0.22 vs. 0.83 ± 0.15, P = 0.0002 at month 2, and 0.97 ± 0.31 vs. 1.20 ± 0.21, P < 0.0001 at month 3.5). The intertest variation of mean MFERG amplitudes in rings 1 to 5 ranged from 13.0% to 26.0% in normal eyes.

Conclusions

Laser application in this model caused localized outer retinal, RPE, and choriocapillaris loss. Localized dysfunction was apparent by MFERG in the first month after lesion induction. Correlative structure-function testing may be useful for research on the functional effects of stem cell-based therapy for GA. MFERG amplitude data should be interpreted in the context of relatively high intertest variability of the rings that correspond to the central macula. Sustained choroidal insufficiency may limit long-term subretinal graft viability in this model.

Choriocapillaris: Fundamentals and advancements

The choriocapillaris is the innermost structure of the choroid that directly nourishes the retinal pigment epithelium and photoreceptors. This article provides an overview of its hemovasculogenesis development to achieve its final architecture as a lobular vasculature, and also summarizes the current histological and molecular knowledge about choriocapillaris and its dysfunction. After describing the existing state-of-the-art tools to image the choriocapillaris, we report the findings in the choriocapillaris encountered in the most frequent retinochoroidal diseases including vascular diseases, inflammatory diseases, myopia, pachychoroid disease spectrum disorders, and glaucoma. The final section focuses on the development of imaging technology to optimize visualization of the choriocapillaris as well as current treatments of retinochoroidal disorders that specifically target the choriocapillaris. We conclude the article with pertinent unanswered questions and future directions in research for the choriocapillaris.

Exploring the choroidal vascular labyrinth and its molecular and structural roles in health and disease

The choroid is a key player in maintaining ocular homeostasis and plays a role in a variety of chorioretinal diseases, many of which are poorly understood. Recent advances in the field of single-cell RNA sequencing have yielded valuable insights into the properties of choroidal endothelial cells (CECs). Here, we review the role of the choroid in various physiological and pathophysiological mechanisms, focusing on the role of CECs. We also discuss new insights regarding the phenotypic properties of CECs, CEC subpopulations, and the value of measuring transcriptomics in primary CEC cultures derived from post-mortem eyes. In addition, we discuss key phenotypic, structural, and functional differences that distinguish CECs from other endothelial cells such as retinal vascular endothelial cells. Understanding the specific clinical and molecular properties of the choroid will shed new light on the pathogenesis of the broad clinical range of chorioretinal diseases such as age-related macular degeneration, central serous chorioretinopathy and other diseases within the pachychoroid spectrum, uveitis, and diabetic choroidopathy. Although our knowledge is still relatively limited with respect to the clinical features and molecular pathways that underlie these chorioretinal diseases, we summarise new approaches and discuss future directions for gaining new insights into these sight-threatening diseases and highlight new therapeutic strategies such as pluripotent stem cell‒based technologies and gene therapy.

The Impact of Oxidative Stress on Blood-Retinal Barrier Physiology in Age-Related Macular Degeneration

The blood retinal barrier (BRB) is a fundamental eye component, whose function is to select the flow of molecules from the blood to the retina and vice-versa, and its integrity allows the maintenance of a finely regulated microenvironment. The outer BRB, composed by the choriocapillaris, the Bruch's membrane, and the retinal pigment epithelium, undergoes structural and functional changes in age-related macular degeneration (AMD), the leading cause of blindness worldwide. BRB alterations lead to retinal dysfunction and neurodegeneration. Several risk factors have been associated with AMD onset in the past decades and oxidative stress is widely recognized as a key factor, even if the exact AMD pathophysiology has not been exactly elucidated yet. The present review describes the BRB physiology, the BRB changes occurring in AMD, the role of oxidative stress in AMD with a focus on the outer BRB structures. Moreover, we propose the use of cerium oxide nanoparticles as a new powerful anti-oxidant agent to combat AMD, based on the relevant existing data which demonstrated their beneficial effects in protecting the outer BRB in animal models of AMD.

Bruch's Membrane and the Choroid in Age-Related Macular Degeneration

A healthy choroidal vasculature is necessary to support the retinal pigment epithelium (RPE) and photoreceptors, because there is a mutualistic symbiotic relationship between the components of the photoreceptor/retinal pigment epithelium (RPE)/Bruch's membrane (BrMb)/choriocapillaris (CC) complex. This relationship is compromised in age-related macular degeneration (AMD) by the dysfunction or death of the choroidal vasculature. This chapter will provide a basic description of the human Bruch's membrane and choroidal anatomy and physiology and how they change in AMD.The choriocapillaris is the lobular, fenestrated capillary system of choroid. It lies immediately posterior to the pentalaminar Bruch's membrane (BrMb). The blood supply for this system is the intermediate blood vessels of Sattler's layer and the large blood vessels in Haller's layer.In geographic atrophy (GA), an advanced form of dry AMD, large confluent drusen form on BrMb, and hyperpigmentation (presumably dysfunction in RPE) appears to be the initial insult. The resorption of these drusen and loss of RPE (hypopigmentation) can be predictive for progression of GA. The death and dysfunction of CC and photoreceptors appear to be secondary events to loss in RPE. The loss of choroidal vasculature may be the initial insult in neovascular AMD (nAMD). We have observed a loss of CC with an intact RPE monolayer in nAMD, by making RPE hypoxic. These hypoxic cells then produce angiogenic substances like vascular endothelial growth factor (VEGF), which stimulate growth of new vessels from CC, resulting in choroidal neovascularization (CNV). Reduction in blood supply to the CC, often stenosis of intermediate and large blood vessels, is associated with CC loss.The polymorphisms in the complement system components are associated with AMD. In addition, the environment of the CC, basement membrane and intercapillary septa, is a proinflammatory milieu with accumulation of proinflammatory molecules like CRP and complement components during AMD. In this toxic milieu, CC die or become dysfunctional even early in AMD. The loss of CC might be a stimulus for drusen formation since the disposal system for retinal debris and exocytosed material from RPE would be limited. Ultimately, the photoreceptors die of lack of nutrients, leakage of serum components from the neovascularization, and scar formation.Therefore, the mutualistic symbiotic relationship of the photoreceptor/RPE/BrMb/CC complex is lost in both forms of AMD. Loss of this functionally integrated relationship results in death and dysfunction of all of the components in the complex.

Innate Immunity in Age-Related Macular Degeneration

Multiple lines of investigation have demonstrated that inflammation plays significant roles in etiology of age-related macular degeneration (AMD). Although interventional trials in AMD therapy targeting inflammatory pathways have been conducted, they have not yet been successful and a detailed understanding as to why some have failed is still elusive. One limitation is the relative dearth of information on how immune cells interact with retinal cells to generate AMD phenotypes at each disease stage. Here, we summarize current research evidence and hypotheses regarding potential pathogenic roles of innate immune cells in the eye, which include resident retinal microglia, macrophages derived from infiltrating systemic monocytes, and macrophages resident in the choroid. We relate recent findings regarding the physiology, function, and cellular interactions involving innate immune cells in the retina and choroid to AMD-related processes, including: (1) drusen formation and regression, (2) the onset and spread of degeneration in late atrophic AMD, and (3) the initiation, growth, and exudation of neovascular vessels in late "wet" AMD. Understanding how innate immune cells contribute to specific AMD phenotypes can assist in generating a comprehensive view on the inflammatory etiology of AMD and aid in identifying anti-inflammatory therapeutic strategies and selecting appropriate clinical outcomes for the planned interventions.

Discovery of 4-((2S,4S)-4-Ethoxy-1-((5-methoxy-7-methyl-1H-indol-4-yl)methyl)piperidin-2-yl)benzoic Acid (LNP023), a Factor B Inhibitor Specifically Designed To Be Applicable to Treating a Diverse Array of Complement Mediated Diseases

The alternative pathway (AP) of the complement system is a key contributor to the pathogenesis of several human diseases including age-related macular degeneration, paroxysmal nocturnal hemoglobinuria (PNH), atypical hemolytic uremic syndrome (aHUS), and various glomerular diseases. The serine protease factor B (FB) is a key node in the AP and is integral to the formation of C3 and C5 convertase. Despite the prominent role of FB in the AP, selective orally bioavailable inhibitors, beyond our own efforts, have not been reported previously. Herein we describe in more detail our efforts to identify FB inhibitors by high-throughput screening (HTS) and leveraging insights from several X-ray cocrystal structures during optimization efforts. This work culminated in the discovery of LNP023 (41), which is currently being evaluated clinically in several diverse AP mediated indications.

Nonexudative Macular Neovascularization Supporting Outer Retina in Age-Related Macular Degeneration: A Clinicopathologic Correlation

PURPOSE

Type 1 macular neovascularization (MNV) secondary to age-related macular degeneration (AMD) may sustain hypoxic and micronutrient-insufficient outer retinal cells compensatorily. We explored this hypothesis via histologic analysis of an eye with a shallow irregular retinal pigment epithelial elevation (SIRE) on OCT and good vision.

DESIGN

Case study and clinicopathologic correlation.

PARTICIPANT

A white woman with untreated nonexudative neovascular AMD and 20/30 visual acuity (left eye) and neovascular AMD (right eye), with 9 years' multimodal imaging before dying at 90 years of age.

METHODS

The left eye was preserved 6.25 hours after death and prepared for submicrometer epoxy resin sections and transmission electron microscopy aligned to clinical OCT B-scans. Inside and outside the MNV area, layer thicknesses, phenotypes, and vascular density of native choriocapillaris and neovessels were measured. Lengths of choriocapillaries and intervening gaps in the index eye and in early AMD eyes and healthy eyes with similar age (n = 19 each) from the Project MACULA (Maculopathy Unveiled by Laminar Analysis) online histopathologic resource (http://projectmacula.cis.uab.edu/) were measured with custom software (Caps and Gaps).

MAIN OUTCOME MEASURES

Descriptive features, vascular density, histologic and OCT layer thicknesses, and distribution of choriocapillaries and intervening gaps.

RESULTS

The SIRE correlated to a type 1 MNV that expanded slowly without evidence of exudation and with numerous choroidal vessels traversing Bruch's membrane defects, some visible on OCT. Tissue layers in and adjacent to the MNV area showed continuous RPE and characteristic AMD deposits. Capillary-like neovessels with fenestrations and caveolae resembling native choriocapillaris lined the retinal pigment epithelium (RPE) with a vascular density comparable with surrounding non-MNV areas. Relative to early AMD and healthy aged eyes, the index eye showed similar capillary lengths but larger gaps between vessels, indicating dropout. Outer nuclear layer thickness was preserved and showed less photoreceptor degeneration over areas of relative choriocapillaris health, including the type 1 MNV.

CONCLUSIONS

Eyes with nonexudative type 1 MNV in AMD may progress to exudation, yet this stable MNV complex supported outer retinal structure for 9 years. Distinguishing features were numerous connecting vessels, high density of neovessels, continuous RPE, and slow growth. Maintaining beneficial type 1 MNV may be a therapeutic strategy.

Choriocapillaris dropout in early age-related macular degeneration

Loss of choriocapillaris (CC) in advanced age-related macular degeneration (AMD) is well documented but changes in early AMD have not been quantified. Postmortem eyes from donors with clinically documented early AMD were examined in choroidal whole mounts to determine the area, pattern, and severity of CC loss. Choroids from postmortem human eyes without AMD (n = 7; mean age = 86.1) and from eyes with a Grade 2 clinical classification of early AMD (n = 7; mean age = 87) were immunolabeled with Ulex europaeus agglutinin (UEA) lectin-FITC to stain blood vessels. Whole mounts were imaged using confocal microscopy and image analysis was performed to determine the area of vascular changes and density of vasculature (percent vascular area, %VA). All areas evaluated had a complete RPE monolayer upon gross examination. In age-matched control eyes, the CC had broad lumens and a homogenous pattern of freely interconnecting capillaries. The mean %VA ± standard deviation in submacula of control subjects was 78.1 ± 3.25%. In eyes with early AMD, there was a significant decrease in mean %VA to 60.1 ± 10.4% (p < 0.0001). The paramacular %VA was not significantly different in eyes with or without AMD. The area of submacular choroid affected by CC dropout was 0.04 ± 0.09 mm² in control eyes. In eyes with early AMD, the mean area affected by CC dropout was significantly increased (10.4 ± 6.1 mm²; p < 0.001). In some cases, incipient neovascular buds were observed at the border of regions with CC dropout in early AMD choroids. In conclusion, UEA lectin-labeled choroidal whole mounts from donors with clinically documented early AMD has provided a unique opportunity to examine regional changes in vascular pathology associated with choriocapillaris. The study demonstrated attenuation of submacular CC in early AMD subjects but no vascular pathology was observed outside the submacular region. While the affected area in some eyes was quite extensive histologically, these changes may not be detectable clinically using standard in vivo imaging.

Microvascular contributions to age-related macular degeneration (AMD): from mechanisms of choriocapillaris aging to novel interventions

Aging of the microcirculatory network plays a central role in the pathogenesis of a wide range of age-related diseases, from heart failure to Alzheimer's disease. In the eye, changes in the choroid and choroidal microcirculation (choriocapillaris) also occur with age, and these changes can play a critical role in the pathogenesis of age-related macular degeneration (AMD). In order to develop novel treatments for amelioration of choriocapillaris aging and prevention of AMD, it is essential to understand the cellular and functional changes that occur in the choroid and choriocapillaris during aging. In this review, recent advances in in vivo analysis of choroidal structure and function in AMD patients and patients at risk for AMD are discussed. The pathophysiological roles of fundamental cellular and molecular mechanisms of aging including oxidative stress, mitochondrial dysfunction, and impaired resistance to molecular stressors in the choriocapillaris are also considered in terms of their contribution to the pathogenesis of AMD. The pathogenic roles of cardiovascular risk factors that exacerbate microvascular aging processes, such as smoking, hypertension, and obesity as they relate to AMD and choroid and choriocapillaris changes in patients with these cardiovascular risk factors, are also discussed. Finally, future directions and opportunities to develop novel interventions to prevent/delay AMD by targeting fundamental cellular and molecular aging processes are presented.