Structure, Function, and Pathology of Bruch's Membrane

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.

Association of torpedo maculopathy and keratoconus in a young patient: A multimodal imaging study

PURPOSE

To report and document a case of torpedo maculopathy found in a patient affected by keratoconus.Case report: An healthy 16-year-old male patient, affected by keratoconus in both eyes, was referred to the cornea service of our hospital for a follow-up visit.During the dilated fundus examination of the left eye, an oval, well-demarcated, hypopigmented lesion was observed in the juxtafoveal temporal region, pointing towards the center of the macula. Multimodal imaging of the lesion was performed, and the diagnosis of Torpedo Maculopathy was established based on the clinical picture.

CONCLUSION

This is the first case of torpedo maculopathy described in a patient affected by keratoconus. This association may be merely fortuitous or the result of developmental abnormalities affecting both corneal and retinal structures.

Engineering a Biomimetic In Vitro Model of Bruch's Membrane Using Hagfish Slime Intermediate Filament Proteins

Bruch's membrane resides in the subretinal tissue and regulates the flow of nutrients and waste between the retinal pigment epithelial (RPE) and vascular layers of the eye. With age, Bruch's membrane becomes thicker, stiffer, and less permeable, which impedes its function as a boundary layer in the subretina. These changes contribute to pathologies such as age-related macular degeneration (AMD). To better understand how aging in Bruch's membrane affects surrounding tissues and to determine the relationship between aging and disease, an in vitro model of Bruch's membrane is needed. An accurate model of Bruch's membrane must be a proteinaceous, semipermeable, and nonporous biomaterial with similar mechanical properties to in vivo conditions. Additionally, this model must support RPE cell growth. While models of subretinal tissue exist, they typically differ from in vivo Bruch's membrane in one or more of these properties. This study evaluates the capability of membranes created from recombinant hagfish intermediate filament (rHIF) proteins to accurately replicate Bruch's membrane in an in vitro model of the subretinal tissue. The physical characteristics of these rHIF membranes were evaluated using mechanical testing, permeability assays, brightfield microscopy, and scanning electron microscopy. The capacity of the membranes to support RPE cell culture was determined using brightfield and fluorescent microscopy, as well as immunocytochemical staining. This study demonstrates that rHIF protein membranes are an appropriate biomaterial to accurately mimic both healthy and aged Bruch's membrane for in vitro modeling of the subretinal tissue.

New Insights into RPE-Photoreceptor Complex Ultrastructure using Focused Ion Beam-Scanning Election Microscopy (FIB-SEM)

Photoreceptors in the retina are specialized neuronal cells that perceive light and play a central role in the visual system. Damage to photoreceptors is a clinical feature often associated with various retinal degenerative disorders. The photoreceptor bed comprises a unique extracellular matrix (ECM) scaffold often described as the interphotoreceptor matrix (IPM) in the subretinal space, vital during retinal development and homeostasis. In this study, we used focused ion beam scanning electron microscopy (FIB-SEM) and transmission electron microscopy (TEM) to analyze the ultrastructural architecture of the retinal pigmented epithelium (RPE)-photoreceptor complex in mice. Additionally, we describe methods for retinal preparation in EM imaging. TEM images display ultrastructural retina layers, including Bruch's membrane and the interdigitation zone (IZ). The 3-dimensional reconstruction of the outer retina revealed individual photoreceptors, the connection between their inner and outer segment via the photoreceptor cilia, and photoreceptor interaction with the RPE ciliary processes. Our findings highlight the importance of FIB-SEM in deciphering the ultrastructural details of RPE-photoreceptor interactions in the IPM complex which are essential for the maintenance of retinal architecture.

Zinc Supplementation Induced Transcriptional Changes in Primary Human Retinal Pigment Epithelium: A Single-Cell RNA Sequencing Study to Understand Age-Related Macular Degeneration

Zinc supplementation has been shown to be beneficial to slow the progression of age-related macular degeneration (AMD). However, the molecular mechanism underpinning this benefit is not well understood. This study used single-cell RNA sequencing to identify transcriptomic changes induced by zinc supplementation. Human primary retinal pigment epithelial (RPE) cells could mature for up to 19 weeks. After 1 or 18 weeks in culture, we supplemented the culture medium with 125 µM added zinc for one week. RPE cells developed high transepithelial electrical resistance, extensive, but variable pigmentation, and deposited sub-RPE material similar to the hallmark lesions of AMD. Unsupervised cluster analysis of the combined transcriptome of the cells isolated after 2, 9, and 19 weeks in culture showed considerable heterogeneity. Clustering based on 234 pre-selected RPE-specific genes divided the cells into two distinct clusters, we defined as more and less differentiated cells. The proportion of more differentiated cells increased with time in culture, but appreciable numbers of cells remained less differentiated even at 19 weeks. Pseudotemporal ordering identified 537 genes that could be implicated in the dynamics of RPE cell differentiation (FDR < 0.05). Zinc treatment resulted in the differential expression of 281 of these genes (FDR < 0.05). These genes were associated with several biological pathways with modulation of ID1/ID3 transcriptional regulation. Overall, zinc had a multitude of effects on the RPE transcriptome, including several genes involved in pigmentation, complement regulation, mineralization, and cholesterol metabolism processes associated with AMD.

2-Hydroxypropyl-β-cyclodextrin mitigates pathological changes in a mouse model of retinal cholesterol dyshomeostasis

CYP46A1 is a CNS-specific enzyme, which eliminates cholesterol from the brain and retina by metabolism to 24-hydroxycholesterol, thus contributing to cholesterol homeostasis in both organs. 2-Hydroxypropyl-β-cyclodextrin (HPCD), a Food and Drug Administration-approved formulation vehicle, is currently being investigated off-label for treatment of various diseases, including retinal diseases. HPCD was shown to lower retinal cholesterol content in mice but had not yet been evaluated for its therapeutic benefits. Herein, we put Cyp46a1^-/- mice on high fat cholesterol-enriched diet from 1 to 14 months of age (control group) and at 12 months of age, started to treat a group of these animals with HPCD until the age of 14 months. We found that as compared with mature and regular chow-fed Cyp46a1^-/- mice, control group had about 6-fold increase in the retinal total cholesterol content, focal cholesterol and lipid deposition in the photoreceptor-Bruch's membrane region, and retinal macrophage activation. In addition, aged animals had cholesterol crystals at the photoreceptor-retinal pigment epithelium interface and changes in the Bruch's membrane ultrastructure. HPCD treatment mitigated all these manifestations of retinal cholesterol dyshomeostasis and altered the abundance of six groups of proteins (genetic information transfer, vesicular transport, and cytoskeletal organization, endocytosis and lysosomal processing, unfolded protein removal, lipid homeostasis, and Wnt signaling). Thus, aged Cyp46a1^-/- mice on high fat cholesterol-enriched diet revealed pathological changes secondary to retinal cholesterol overload and supported further studies of HPCD as a potential therapeutic for age-related macular degeneration and diabetic retinopathy associated with retinal cholesterol dyshomeostasis.

Nutrigenetic reprogramming of oxidative stress

Retinal disorders such as retinitis pigmentosa, age-related retinal degeneration, oxygen-induced retinopathy, and ischemia-reperfusion injury cause debilitating and irreversible vision loss. While the exact mechanisms underlying these conditions remain unclear, there has been a growing body of evidence demonstrating the pathological contributions of oxidative stress across different cell types within the eye. Nuclear factor erythroid-2-related factor (Nrf2), a transcriptional activator of antioxidative genes, and its regulator Kelch-like ECH-associated protein 1 (Keap1) have emerged as promising therapeutic targets. The purpose of this review is to understand the protective role of the Nrf2-Keap1 pathway in different retinal tissues and shed light on the complex mechanisms underlying these processes. In the photoreceptors, we highlight that Nrf2 preserves their survival and function by maintaining oxidation homeostasis. In the retinal pigment epithelium, Nrf2 similarly plays a critical role in oxidative stabilization but also maintains mitochondrial motility and autophagy-related lipid metabolic processes. In endothelial cells, Nrf2 seems to promote proper vascularization and revascularization through concurrent activation of antioxidative and angiogenic factors as well as inhibition of inflammatory cytokines. Finally, Nrf2 protects retinal ganglion cells against apoptotic cell death. Importantly, we show that Nrf2-mediated protection of the various retinal tissues corresponds to a preservation of functional vision. Altogether, this review underscores the potential of the Nrf2-Keap1 pathway as a powerful tool against retinal degeneration. Key insights into this elegant oxidative defense mechanism may ultimately pave the path toward a universal therapy for various inherited and environmental retinal disorders.

Advancement in Nanostructure-Based Tissue-Engineered Biomaterials for Retinal Degenerative Diseases

The review intends to overview a wide range of nanostructured natural, synthetic and biological membrane implants for tissue engineering to help in retinal degenerative diseases. Herein, we discuss the transplantation strategies and the new development of material in combination with cells such as induced pluripotent stem cells (iPSC), mature retinal cells, adult stem cells, retinal progenitors, fetal retinal cells, or retinal pigment epithelial (RPE) sheets, etc. to be delivered into the subretinal space. Retinitis pigmentosa and age-related macular degeneration (AMD) are the most common retinal diseases resulting in vision impairment or blindness by permanent loss in photoreceptor cells. Currently, there are no therapies that can repair permanent vision loss, and the available treatments can only delay the advancement of retinal degeneration. The delivery of cell-based nanostructure scaffolds has been presented to enrich cell survival and direct cell differentiation in a range of retinal degenerative models. In this review, we sum up the research findings on different types of nanostructure scaffolds/substrate or material-based implants, with or without cells, used to deliver into the subretinal space for retinal diseases. Though, clinical and pre-clinical trials are still needed for these transplants to be used as a clinical treatment method for retinal degeneration.

Chromosome 10q26-driven age-related macular degeneration is associated with reduced levels of HTRA1 in human retinal pigment epithelium

Genome-wide association studies have identified the chromosome 10q26 (Chr10) locus, which contains the age-related maculopathy susceptibility 2 (ARMS2) and high temperature requirement A serine peptidase 1 (HTRA1) genes, as the strongest genetic risk factor for age-related macular degeneration (AMD) [L.G. Fritsche et al., Annu. Rev. Genomics Hum. Genet. 15, 151-171, (2014)]. To date, it has been difficult to assign causality to any specific single nucleotide polymorphism (SNP), haplotype, or gene within this region because of high linkage disequilibrium among the disease-associated variants [J. Jakobsdottir et al. Am. J. Hum. Genet. 77, 389-407 (2005); A. Rivera et al. Hum. Mol. Genet. 14, 3227-3236 (2005)]. Here, we show that HTRA1 messenger RNA (mRNA) is reduced in retinal pigment epithelium (RPE) but not in neural retina or choroid tissues derived from human donors with homozygous risk at the 10q26 locus. This tissue-specific decrease is mediated by the presence of a noncoding, cis-regulatory element overlapping the ARMS2 intron, which contains a potential Lhx2 transcription factor binding site that is disrupted by risk variant rs36212733. HtrA1 protein increases with age in the RPE-Bruch's membrane (BM) interface in Chr10 nonrisk donors but fails to increase in donors with homozygous risk at the 10q26 locus. We propose that HtrA1, an extracellular chaperone and serine protease, functions to maintain the optimal integrity of the RPE-BM interface during the aging process and that reduced expression of HTRA1 mRNA and protein in Chr10 risk donors impairs this protective function, leading to increased risk of AMD pathogenesis. HtrA1 augmentation, not inhibition, in high-risk patients should be considered as a potential therapy for AMD.

FHL-1 interacts with human RPE cells through the α5β1 integrin and confers protection against oxidative stress

Retinal pigment epithelial (RPE) cells that underlie the neurosensory retina are essential for the maintenance of photoreceptor cells and hence vision. Interactions between the RPE and their basement membrane, i.e. the inner layer of Bruch's membrane, are essential for RPE cell health and function, but the signals induced by Bruch's membrane engagement, and their contributions to RPE cell fate determination remain poorly defined. Here, we studied the functional role of the soluble complement regulator and component of Bruch's membrane, Factor H-like protein 1 (FHL-1). Human primary RPE cells adhered to FHL-1 in a manner that was eliminated by either mutagenesis of the integrin-binding RGD motif in FHL-1 or by using competing antibodies directed against the α5 and β1 integrin subunits. These short-term experiments reveal an immediate protein-integrin interaction that were obtained from primary RPE cells and replicated using the hTERT-RPE1 cell line. Separate, longer term experiments utilising RNAseq analysis of hTERT-RPE1 cells bound to FHL-1, showed an increased expression of the heat-shock protein genes HSPA6, CRYAB, HSPA1A and HSPA1B when compared to cells bound to fibronectin (FN) or laminin (LA). Pathway analysis implicated changes in EIF2 signalling, the unfolded protein response, and mineralocorticoid receptor signalling as putative pathways. Subsequent cell survival assays using H2O2 to induce oxidative stress-induced cell death suggest hTERT-RPE1 cells had significantly greater protection when bound to FHL-1 or LA compared to plastic or FN. These data show a non-canonical role of FHL-1 in protecting RPE cells against oxidative stress and identifies a novel interaction that has implications for ocular diseases such as age-related macular degeneration.

Polymeric membranes for biomedical applications

The rapid development of nanotechnology paved the way for further expansion of polymer chemistry and the fabrication of advanced polymeric membranes. Such modifications allowed enhancing or adding some unique properties, including mechanical strength, excellent biocompatibility, easily controlled degradability, and biological activity. This chapter discusses various applications of polymeric membranes in three significant areas of biomedicine, including tissue engineering, drug delivery systems, and diagnostics. It is intended to highlight here possible ways of improvement the properties of polymeric membranes, by modifying with other polymers, functional groups, compounds, drugs, bioactive components, and nanomaterials.

Collagen I Based Enzymatically Degradable Membranes for Organ-on-a-Chip Barrier Models

Organs-on-chips are microphysiological in vitro models of human organs and tissues that rely on culturing cells in a well-controlled microenvironment that has been engineered to include key physical and biochemical parameters. Some systems contain a single perfused microfluidic channel or a patterned hydrogel, whereas more complex devices typically employ two or more microchannels that are separated by a porous membrane, simulating the tissue interface found in many organ subunits. The membranes are typically made of synthetic and biologically inert materials that are then coated with extracellular matrix (ECM) molecules to enhance cell attachment. However, the majority of the material remains foreign and fails to recapitulate the native microenvironment of the barrier tissue. Here, we study microfluidic devices that integrate a vitrified membrane made of collagen-I hydrogel (VC). The biocompatibility of this membrane was confirmed by growing a healthy population of stem cell derived endothelial cells (iPSC-EC) and immortalized retinal pigment epithelium (ARPE-19) on it and assessing morphology by fluorescence microscopy. Moreover, VC membranes were subjected to biochemical degradation using collagenase II. The effects of this biochemical degradation were characterized by the permeability changes to fluorescein. Topographical changes on the VC membrane after enzymatic degradation were also analyzed using scanning electron microscopy. Altogether, we present a dynamically bioresponsive membrane integrated in an organ-on-chip device with which disease-related ECM remodeling can be studied.

Development of 3D Printed Bruch's Membrane-Mimetic Substance for the Maturation of Retinal Pigment Epithelial Cells

Retinal pigment epithelium (RPE) is a monolayer of the pigmented cells that lies on the thin extracellular matrix called Bruch's membrane. This monolayer is the main component of the outer blood-retinal barrier (BRB), which plays a multifunctional role. Due to their crucial roles, the damage of this epithelium causes a wide range of diseases related to retinal degeneration including age-related macular degeneration, retinitis pigmentosa, and Stargardt disease. Unfortunately, there is presently no cure for these diseases. Clinically implantable RPE for humans is under development, and there is no practical examination platform for drug development. Here, we developed porcine Bruch's membrane-derived bioink (BM-ECM). Compared to conventional laminin, the RPE cells on BM-ECM showed enhanced functionality of RPE. Furthermore, we developed the Bruch's membrane-mimetic substrate (BMS) via the integration of BM-ECM and 3D printing technology, which revealed structure and extracellular matrix components similar to those of natural Bruch's membrane. The developed BMS facilitated the appropriate functions of RPE, including barrier and clearance functions, the secretion of anti-angiogenic growth factors, and enzyme formation for phototransduction. Moreover, it could be used as a basement frame for RPE transplantation. We established BMS using 3D printing technology to grow RPE cells with functions that could be used for an in vitro model and RPE transplantation.

Culture surface protein coatings affect the barrier properties and calcium signalling of hESC-RPE

Human pluripotent stem cell-derived retinal pigment epithelium (RPE) transplantation is currently under evaluation as treatment for macular degeneration. For therapeutic applications, cryostorage during cell production is typically needed with potential consequences to cell functionality. We have previously shown that the culture substrate affects human embryonic stem cell-derived RPE (hESC-RPE) properties in fresh cultures. Here, we aimed to further identify the role of RPE basement membrane proteins type IV collagen (Col-IV), laminin (LN), and nidogen-1 in the maturation and functionality of hESC-RPE after cryopreservation. In addition to cell attachment and morphology, transepithelial electrical resistance, expression of key RPE proteins, phagocytosis capacity and Ca²⁺ signalling were analysed. After cryostorage, attachment of hESC-RPE on culture surfaces coated with Col-IV alone was poor. Combining Col-IV and LN with or without nidogen-1 significantly improved cell attachment and barrier properties of the epithelium. Furthermore, functional homogeneity of the hESC-RPE monolayer was enhanced in the presence of nidogen-1. Our results suggest that the choice of coating proteins for the cell culture may have implications to the functional properties of these cells after cryostorage cell banking.

Visible light OCT improves imaging through a highly scattering retinal pigment epithelial wall

Here we provide a counter-example to the conventional wisdom in biomedical optics that longer wavelengths aid deeper imaging in tissue. Specifically, we investigate visible light optical coherence tomography of Bruch's membrane (BM) in the non-pathologic eyes of humans and two mouse strains. Surprisingly, we find that shorter visible wavelengths improve the visualization of BM in pigmented eyes, where it is located behind a highly scattering layer of melanosomes in the retinal pigment epithelium (RPE). Monte Carlo simulations of radiative transport suggest that, while absorption and scattering are higher at shorter wavelengths, detected multiply scattered light from the RPE is preferentially attenuated relative to detected backscattered light from the BM.

Cholesterol Regulation in Age-Related Macular Degeneration: A Framework for Mathematical Modelling of Drusen Biogenesis

In age-related macular degeneration (AMD), there is, in common with many other age-related diseases, the need to distinguish between changes in the ageing eye that lead to disease and those changes that are considered part of a healthy, ageing eye. Various studies investigating the multitude of mechanisms involved in the aetiology of AMD exist within the field of ophthalmology and related medical fields, yet many aspects of it remain poorly understood and only a limited number of therapies are available. A recent study relates drusen's topographically cellular characteristics to the neural retina's metabolic needs and associated cholesterol involvement within the retina. In particular, there is a need to fully understand the maintenance of cholesterol homeostasis in the retina to prevent normal ageing processes from being perturbed towards maculopathy. Here, we present an extensive review of the clinical and physiological features of the ageing retina, as well as mechanisms implicated in pathology, synthesised from a vast body of the published literature. We use this novel synthesis to construct a comprehensive process schematic, encompassing all key species and physiological processes such as nutrients, waste and lipoprotein management. We are therefore able to express these processes in a mathematical language via a comprehensive modelling framework, comprising a set of twenty-three equations spanning three distinct biological compartments. This very general modelling framework may now be adapted to more focused studies on individual mechanisms, processes or components underlying of the many facets of AMD. As an example of such a focused application, we conclude this article with a one-compartment, four-species model of the retinal pigment epithelium, which considers the parametric conditions under which either cholesterol homeostasis or unregulated accumulation of cholesterol may obtain in the ageing eye.

Choroidal changes in intermediate age-related macular degeneration patients with drusen or pseudodrusen

Reticular pseudodrusen are associated with a thinner choroid. The aim of our study was to determine the differences in central choroidal thickness and choriocapillaris vascular flow area between eyes with and without reticular pseudodrusen using swept-source optical coherence tomography and swept-source optical coherence tomography angiography. We conducted a retrospective case control study which included 27 eyes from 27 consecutive patients with intermediate age-related macular degeneration and 17 eyes from 17 healthy participants. Complete ophthalmic examinations were carried out including axial length measurements; fundus color retinography; fundus autofluorescence; swept-source optical coherence tomography and swept-source optical coherence tomography angiography; central choroidal thickness and choriocapillaris vascular flow area. Patients were classified as no reticular pseudodrusen, mild reticular pseudodrusen, and severe reticular pseudodrusen. Mean central choroidal thickness in patients exhibiting severe reticular pseudodrusen (110 ± 56 μm) was significantly smaller than in patients with no reticular pseudodrusen (201 ± 76 μm, p < 0.01). Mean choriocapillaris vascular flow area in severe reticular pseudodrusen patients (45.2% ± 3.0%) was also significantly less than in patients with no (47.9% ± 1.6%, p < 0.001) and mild reticular pseudodrusen (47.7% ± 1.0%, p < 0.05). Stepwise multiple regression models confirmed the association of reticular pseudodrusen with central choroidal thickness (p < 0.001) and choriocapillaris vascular flow area (p < 0.01) even after accounting for age, axial length, and refractive error. Soft drusen were not associated with changes in either central choroidal thickness (p = 0.13) nor choriocapillaris vascular flow area (p = 0.29). A significant, positive relationship was found between central choroidal thickness and choriocapillaris vascular flow area (r = 0.44, p = 0.01). Therefore, both central choroidal thickness and choriocapillaris vascular flow area are decreased in eyes with reticular pseudodrusen, as compared to healthy eyes and intermediate age-related macular degeneration eyes not exhibiting reticular pseudodrusen. In addition, central choroidal thickness and choriocapillaris vascular flow area are related, and the reduction of either is directly associated to the severity of reticular pseudodrusen. Further studies are needed to assess the clinical significance of these findings.

Factors affecting optic nerve head biomechanics in a rat model of glaucoma

Glaucoma is the leading cause of irreversible blindness and is characterized by the death of retinal ganglion cells, which carry vision information from the retina to the brain. Although it is well accepted that biomechanics is an important part of the glaucomatous disease process, the mechanisms by which biomechanical insult, usually due to elevated intraocular pressure (IOP), leads to retinal ganglion cell death are not understood. Rat models of glaucoma afford an opportunity for learning more about these mechanisms, but the biomechanics of the rat optic nerve head (ONH), a primary region of damage in glaucoma, are only just beginning to be characterized. In a previous study, we built finite-element models with individual-specific rat ONH geometries. Here, we developed a parametrized model of the rat ONH and used it to perform a sensitivity study to determine the influence that six geometric parameters and 13 tissue material properties have on rat optic nerve biomechanical strains due to IOP elevation. Strain magnitudes and patterns in the parametrized model generally matched those from individual-specific models, suggesting that the parametrized model sufficiently approximated rat ONH anatomy. Similar to previous studies in human eyes, we found that scleral properties were highly influential: the six parameters with highest influence on optic nerve strains were optic nerve stiffness, IOP, scleral thickness, the degree of alignment of scleral collagen fibres, scleral ground substance stiffness and the scleral collagen fibre uncrimping coefficient. We conclude that a parametrized modelling strategy is an efficient approach that allows insight into rat ONH biomechanics. Further, scleral properties are important influences on rat ONH biomechanics, and additional efforts should be made to better characterize rat scleral collagen fibre organization.

Unstimulated, Serum-free Cultures of Retinal Pigment Epithelium Excrete Large Mounds of Drusen-like Deposits

Purpose: A hallmark of age-related macular degeneration is the accumulation of deposits of lipids and proteins, called drusen, in Bruch's membrane. Several culture models of retinal pigment epithelia (RPE) develop drusen-like deposits. We examined whether prolonged culture of RPE with a retina-like tissue affected the number or size of these deposits. Methods: RPE and retinal progenitor cells (RPC) were differentiated from induced pluripotent stem cells derived from fetal tissue and maintained in serum-free medium containing the B27 supplement. RPE was cultured on Transwell filter inserts, and RPC were cultured on a planar matrix composed of gelatin, hyaluronic acid, and chondroitin sulfate. After seeding the filter, RPC were layered on top of the RPE. RPE ± RPC were cultured for six months. The function of RPE tight junctions was assessed by the transepithelial electrical resistance. Cultures were stained for actin, neutral lipids, APOE, TIMP3, vitronectin, and calcium deposits. Morphometric analysis was used to determine the number and volume of the "druse". Results: After six months, the TER was greater for the co-cultures (304 ± 11 Ω× cm² vs 243 ± 7 Ω× cm², p < .01). RPE formed mounds of druse-like deposits that contained, vitronectin, APOE, TIMP3 and calcium deposits, but lipids were undetected. The mounds overlay areas of the filter where no lipid was detected in the pores, and the RPE overlying the mounds was often thin. The number of "druse"/100,000 μm² was 5.0 ± 0.4 (co-cultures) vs 2.3 ± 0.1 (monocultures) (p < .05). The total volume of "drusen"/100,000 μm³ was 15,133 ± 1544 (co-cultures) vs 5,993 ± 872 (monocultures) (p < .05). There was no statistical difference between the size-distribution of druse-like particles formed by each culture. Conclusions: Covering the apical membrane of RPE with a thick tissue increased the number of druse-like deposits. The apparent size limitation of the deposits may reflect the apparent interruption of the of lipid cycle found at the basal membrane of the RPE.

Ultrastructural changes in the choriocapillaris of N-methyl-N-nitrosourea-induced retinal degeneration in C57BL/6 mice

N-methyl-N-nitrosourea (MNU) is known to cause apoptosis of photoreceptor cells and changes in retinal pigment epithelium (RPE). However, the changes in choriocapillaris, which nourishes photoreceptor cells by diffusing tissue fluid through RPE, have not been reported in detail. Therefore, we studied the ultrastructural transformation in and around the choriocapillaris to characterize the interdependence between choriocapillaris and surrounding tissue components in a mouse model. Seven-week-old male C57BL/6 mice were given a single intraperitoneal injection of MNU (60 mg/kg of body weight). Perfusion-fixed eyeballs were examined chronologically using immunohistochemistry and electron microscopy until the photoreceptor cells were lost. Sequential ultrastructural changes were observed in photoreceptor cells, RPE, Bruch's membrane, choriocapillaris, and choroidal melanocytes after an MNU injection. The lumens of the choriocapillaris narrowed following dilation, and the vascular endothelium showed structural alterations. When the photoreceptor cells were completely lost, the choriocapillaris appeared to be in a recovery process. Our results suggest that transport abnormality through Bruch's membrane and structural changes in the choroid might have influenced the morphology of choriocapillaris. The thin wall of the choriocapillaris appears to be the cause of the vulnerability with its altered morphology.

Utilizing Recombinant Spider Silk Proteins To Develop a Synthetic Bruch's Membrane for Modeling the Retinal Pigment Epithelium

Spider silks are intriguing biomaterials that have a high potential as innovative biomedical processes and devices. The intent of this study was to evaluate the capacity of recombinant spider silk proteins (rSSps) as a synthetic Bruch's membrane. Nonporous silk membranes were prepared with comparable thicknesses (<10 μm) to that of native Bruch's membrane. Biomechanical characterization was performed prior to seeding cells. The ability of RPE cells (ARPE-19) to attach and grow on the membranes was then evaluated with bright-field and electron microscopy, intracellular DNA quantification, and immunocytochemical staining (ZO-1 and F-actin). Controls were cultured on permeable Transwell support membranes and characterized with the same methods. A size-dependent permeability assay, using FITC-dextran, was used to determine cell-membrane barrier function. Compared to Transwell controls, RPE cells cultured on rSSps membranes developed more native-like "cobblestone" morphologies, exhibited higher intracellular DNA content, and expressed key organizational proteins more consistently. Comparisons of the membranes to native structures revealed that the silk membranes exhibited equivalent thicknesses, biomechanical properties, and barrier functions. These findings support the use of recombinant spider silk proteins to model Bruch's membrane and develop more biomimetic retinal models.

Characterization of Human iPSC-RPE on a Prosthetic Bruch's Membrane Manufactured From Silk Fibroin

Purpose

RPE cell transplantation as a potential treatment for AMD has been extensively investigated; however, in AMD, ultrastructural damage affects both the RPE and its underlying matrix support, the Bruch's membrane (BrM). An RPE monolayer supported by a surrogate scaffold could thus provide a more effective approach to cell-based therapy for AMD. Toward this goal, we aimed to establish a functional human induced pluripotent stem cell-derived (hiPSC)-RPE monolayer on a Bombyx mori silk fibroin (BMSF) scaffold.

Methods

RPE differentiated from five distinct hiPSC lines were cultured on BMSF membrane coated with extracellular matrix (ECM, COL1), and either regular tissue culture plastic or Transwell coated with ECM (LAM-TCP). Morphologic, gene and protein expression, and functional characteristics of the hiPSC-RPE cultured on different membranes were compared in longitudinal experiments spanning 1 day to ≥3 months.

Results

The hiPSC-RPE monolayers on ECM-coated BMSF and TCP could be maintained in culture for ≥3 months and displayed RPE-characteristic morphology, pigmentation, polarity, and expression of RPE signature genes and proteins. Furthermore, hiPSC-RPE on both ECM-coated BMSF and TCP displayed robust expression and secretion of several basement membrane proteins. Importantly, hiPSC-RPE cells on COL1-BMSF and LAM-TCP showed similar efficacy in the phagocytosis and degradation of photoreceptor outer segments.

Conclusions

A biomaterial scaffold manufactured from silk fibroin supports the maturation and long-term survival of a functional hiPSC-RPE monolayer. This has significant implications for both in vitro disease modeling and in vivo cell replacement therapy.

Complement factor H regulates retinal development and its absence may establish a footprint for age related macular degeneration

Age related macular degeneration (AMD) is the most common blinding disease in those over 60 years. In 50% of cases it is associated with polymorphisms of complement factor H (FH), implicating immune vulnerability. But such individuals may exhibit abnormal outer retinal blood flow decades before disease initiation, suggesting an early disease footprint. FH is expressed in the retinal pigmented epithelium (RPE). During development the RPE is adjacent to the site of retinal mitosis and complex regulatory interactions occur between the relatively mature RPE and retinal neuronal precursors that control the cell cycle. Here we ask if the absence of FH from the RPE influences retinal development using a mouse CFH knockout (Cfh^−/−) with an aged retinal degenerative phenotype. We reveal that from birth, these mice have significantly disrupted and delayed retinal development. However, once development is complete, their retinae appear relatively normal, although many photoreceptor and RPE mitochondria are abnormally large, suggesting dysfunction consistent with premature ATP decline in Cfh^−/−. Total retinal mtDNA is also reduced and these deficits are associated shortly after with reduced retinal function. Cfh^−/+ mice also show significant abnormal patterns of cell production but not as great as in Cfh^−/−. These results reveal that not only is FH an important player in sculpting retinal development but also that the developmental abnormality in Cfh^−/− likely establishes critical vulnerability for later aged retinal degeneration.

Isolation of Retinal Exosome Biomarkers from Blood by Targeted Immunocapture

The retinal pigmented epithelium (RPE) forms the outer blood-retinal barrier, provides nutrients, recycles visual pigment, and removes spent discs from the photoreceptors, among many other functions. Because of these critical roles in visual homeostasis, the RPE is a principal location of disease-associated changes in age-related macular degeneration (AMD), emphasizing its importance for study in both visual health and disease. Unfortunately, there are no early indicators of AMD or disease progression, a void that could be filled by the development of early AMD biomarkers. Exosomes are lipid bilayer membrane vesicles of nanoscale sizes that are released in a controlled fashion by cells and carry out a number of extra- and intercellular activities. In the RPE they are released from both the apical and basal sides, and each source has a unique signature/content. Exosomes released from the basolateral side of RPE cells enter the systemic circulation via the choroid and thus represent a potential source of retinal disease biomarkers in blood. Here we discuss the potential of targeted immunocapture of eye-derived exosomes and other small extracellular vesicles from blood for eye disease biomarker discovery.

Soft Drusen in Age-Related Macular Degeneration: Biology and Targeting Via the Oil Spill Strategies

AMD is a major cause of legal blindness in older adults approachable through multidisciplinary research involving human tissues and patients. AMD is a vascular-metabolic-inflammatory disease, in which two sets of extracellular deposits, soft drusen/basal linear deposit (BLinD) and subretinal drusenoid deposit (SDD), confer risk for end-stages of atrophy and neovascularization. Understanding how deposits form can lead to insights for new preventions and therapy. The topographic correspondence of BLinD and SDD with cones and rods, respectively, suggest newly realized exchange pathways among outer retinal cells and across Bruch's membrane and the subretinal space, in service of highly evolved, eye-specific physiology. This review focuses on soft drusen/BLinD, summarizing evidence that a major ultrastructural component is large apolipoprotein B,E-containing, cholesterol-rich lipoproteins secreted by the retinal pigment epithelium (RPE) that offload unneeded lipids of dietary and outer segment origin to create an atherosclerosis-like progression in the subRPE-basal lamina space. Clinical observations and an RPE cell culture system combine to suggest that soft drusen/BLinD form when secretions of functional RPE back up in the subRPE-basal lamina space by impaired egress across aged Bruch's membrane-choriocapillary endothelium. The soft drusen lifecycle includes growth, anterior migration of RPE atop drusen, then collapse, and atrophy. Proof-of-concept studies in humans and animal models suggest that targeting the “Oil Spill in Bruch's membrane” offers promise of treating a process in early AMD that underlies progression to both end-stages. A companion article addresses the antecedents of soft drusen within the biology of the macula.

A Methodology for Individual-Specific Modeling of Rat Optic Nerve Head Biomechanics in Glaucoma

Glaucoma is the leading cause of irreversible blindness and involves the death of retinal ganglion cells (RGCs). Although biomechanics likely contributes to axonal injury within the optic nerve head (ONH), leading to RGC death, the pathways by which this occurs are not well understood. While rat models of glaucoma are well-suited for mechanistic studies, the anatomy of the rat ONH is different from the human, and the resulting differences in biomechanics have not been characterized. The aim of this study is to describe a methodology for building individual-specific finite element (FE) models of rat ONHs. This method was used to build three rat ONH FE models and compute the biomechanical environment within these ONHs. Initial results show that rat ONH strains are larger and more asymmetric than those seen in human ONH modeling studies. This method provides a framework for building additional models of normotensive and glaucomatous rat ONHs. Comparing model strain patterns with patterns of cellular response seen in studies using rat glaucoma models will help us to learn more about the link between biomechanics and glaucomatous cell death, which in turn may drive the development of novel therapies for glaucoma.

C3a triggers formation of sub-retinal pigment epithelium deposits via the ubiquitin proteasome pathway

The mechanisms that connect complement system activation and basal deposit formation in early stages of age-related macular degeneration (AMD) are insufficiently understood, which complicates the design of efficient therapies to prevent disease progression. Using human fetal (hf) retinal pigment epithelial (RPE) cells, we have established an in vitro model to investigate the effect of complement C3a on RPE cells and its role in the formation of sub-RPE deposits. The results of these studies revealed that C3a produced after C3 activation is sufficient to induce the formation of sub-RPE deposits via complement-driven proteasome inhibition. C3a binds the C3a receptor (C3aR), stimulates deposition of collagens IV and VI underneath the RPE, and impairs the extracellular matrix (ECM) turnover by increased MMP-2 activity, all mediated by downregulation of the ubiquitin proteasome pathway (UPP). The formation of basal deposits can be prevented by the addition of a C3aR antagonist, which restores the UPP activity and ECM turnover. These findings indicate that the cell-based model can be used to test potential therapeutic agents in vitro. The data suggest that modulation of C3aR-mediated events could be a therapeutic approach for treatment of early AMD.

Polarized Exosome Release from the Retinal Pigmented Epithelium

The retinal pigmented epithelium (RPE) forms the outer blood-retinal barrier and provides nutrients and recycling of visual pigment to the photoreceptors, among many other functions. The RPE is also a key site of pathophysiological changes in age-related macular degeneration, making it an important focus of study in both visual health and disease. Exosomes are nanometer-sized vesicles that are released by cells in a controlled fashion and mediate a range of extra- and intercellular activities. Some key exosome actions include cell-cell communication, immune modulation, extracellular matrix turnover, stem cell division/differentiation, neovascularization, and cellular waste removal. While much is known about their role in cancer and cardiovascular disease, exosome function in the many specialized tissues of the eye is just beginning to undergo rigorous study. Here we review current knowledge of the functions and roles of exosomes and other small extracellular vesicles released from the RPE. In particular, we discuss the potential role and importance of polarized exosome release from the RPE.

Effect of aging and lifestyle on photoreceptors and retinal pigment epithelium: cross-sectional study in a healthy Danish population

Photoreceptors and their supporting retinal pigment epithelium constitute the key functional parts of the retina. Here, a study was undertaken to show how aging and lifestyle factors affect the photoreceptor layer and the retinal pigment epithelium and Bruch's membrane complex (RPE-BM) in vivo in a healthy Danish population using spectral-domain optical coherence tomography. This was a cross-sectional study of healthy humans aged ≥50 years. All participants were interviewed for medical history and lifestyle factors. Maculae of all participants were scanned using spectral-domain optical coherence tomography. The thickness of the photoreceptor layer and the RPE-BM was measured on one eye from each participant. In 150 eyes of 150 participants, it was found that aging was associated with a decrease in the thickness of the photoreceptor layer (-0.143 μm/year, P = 0.031) and an increase in the thickness of the RPE-BM layer (0.100 μm/year, P = 0.029) at the foveal minimum. Regarding lifestyle factors, alcohol intake or BMI were not associated with any significant trend, but physical inactivity and smoking had effects on the photoreceptor layer (decreased thickness) and the RPE-BM layer (increased thickness) indicating an accelerated aging process of the macula. Taken together, aging affects photoreceptors and the RPE-BM, and these aging trends are accelerated in smokers and the physically inactive.

ApoA-I Mimetic Peptide 4F Reduces Age-Related Lipid Deposition in Murine Bruch's Membrane and Causes Its Structural Remodeling

PURPOSE

Accumulation of lipoprotein-derived lipids including esterified and unesterified cholesterol in Bruch's membrane of human eyes is a major age-related change involved in initiating and sustaining soft drusen in age-related macular degeneration (AMD). The apolipoprotein (apo) A-I mimetic peptide 4F is a small anti-inflammatory and anti-atherogenic agent, and potent modifier of plasma membranes. We evaluated the effect of intravitreally-injected 4F on murine Bruch's membrane.

METHODS

We tested single intravitreal injections of 4F doses (0.6 µg, 1.2 µg, 2.4 µg, and placebo scrambled peptide) in ApoE^null mice ≥10 months of age. After 30 days, mice were euthanized. Eyes were processed for either direct immunofluorescence detection of esterified cholesterol (EC) in Bruch's membrane whole mounts via a perfringolysin O-based marker linked to green fluorescent protein or by transmission electron microscopic visualization of Bruch's membrane integrity. Fluorescein isothiocyanate-conjugated 4F was traced after injection.

RESULTS

All injected eyes showed a dose-dependent reduction of Bruch's membrane EC with a concomitant ultrastructural improvement compared to placebo treated eyes. At a 2.4 µg dose of 4F, EC was reduced on average by ~60% and Bruch's membrane returned to a regular pentalaminar structure and thickness. Tracer studies confirmed that injected 4F reached intraocular targets.

CONCLUSION

We demonstrated a highly effective pharmacological reduction of EC and restoration of Bruch's membrane ultrastructure. The apoA-I mimetic peptide 4F is a novel way to treat a critical AMD disease process and thus represents a new candidate for treating the underlying cause of AMD.

Directional Exosome Proteomes Reflect Polarity-Specific Functions in Retinal Pigmented Epithelium Monolayers

The retinal pigmented epithelium (RPE) forms the outer blood-retinal barrier in the eye and its polarity is responsible for directional secretion and uptake of proteins, lipoprotein particles and extracellular vesicles (EVs). Such a secretional division dictates directed interactions between the systemic circulation (basolateral) and the retina (apical). Our goal is to define the polarized proteomes and physical characteristics of EVs released from the RPE. Primary cultures of porcine RPE cells were differentiated into polarized RPE monolayers on permeable supports. EVs were isolated from media bathing either apical or basolateral RPE surfaces, and two subpopulations of small EVs including exosomes, and dense EVs, were purified and processed for proteomic profiling. In parallel, EV size distribution and concentration were determined. Using protein correlation profiling mass spectrometry, a total of 631 proteins were identified in exosome preparations, 299 of which were uniquely released apically, and 94 uniquely released basolaterally. Selected proteins were validated by Western blot. The proteomes of these exosome and dense EVs preparations suggest that epithelial polarity impacts directional release. These data serve as a foundation for comparative studies aimed at elucidating the role of exosomes in the molecular pathophysiology of retinal diseases and help identify potential therapeutic targets and biomarkers.

Polarized, Cobblestone, Human Retinal Pigment Epithelial Cell Maturation on a Synthetic PEG Matrix

Cell attachment is essential for the growth and polarization of retinal pigment epithelial (RPE) cells. Currently, surface coatings derived from biological proteins are used as the gold standard for cell culture. However, downstream processing and purification of these biological products can be cumbersome and expensive. In this study, we constructed a library of chemically modified nanofibers to mimic the Bruch's membrane of the retinal pigment epithelium. Using atmospheric-pressure plasma-induced graft polymerization with a high-throughput screening platform to modify the nanofibers, we identified three polyethylene glycol (PEG)-grafted nanofiber surfaces (PEG methyl ether methacrylate, n = 4, 8, and 45) from a library of 62 different surfaces as favorable for RPE cell attachment, proliferation, and maturation in vitro with cobblestone morphology. Compared with the biologically derived culture matrices such as vitronectin-based peptide Synthemax, our newly discovered synthetic PEG surfaces exhibit similar growth and polarization of retinal pigment epithelial (RPE) cells. However, they are chemically defined, are easy to synthesize on a large scale, are cost-effective, are stable with long-term storage capability, and provide a more physiologically accurate environment for RPE cell culture. To our knowledge, no one has reported that PEG derivatives directly support attachment and growth of RPE cells with cobblestone morphology. This study offers a unique PEG-modified 3D cell culture system that supports RPE proliferation, differentiation, and maturation with cobblestone morphology, providing a new avenue for RPE cell culture, disease modeling, and cell replacement therapy.

Investigation of the Etiology of Central Serous Chorioretinopathy Using En-Face Optical Coherence Tomography and Indocyanine Green Angiography

PURPOSE

To identify locations of hypofluorescent lesions on late-phase indocyanine green angiography (ICGA) in patients with central serous chorioretinopathy (CSC) using en-face optical coherence tomography (OCT).

PROCEDURES

We retrospectively studied 25 consecutive untreated CSC patients, using swept-source OCT and ICGA. En-face swept-source OCT images were automatically segmented and flattened with Bruch's membrane (BrM). We compared the sizes of hyperreflective areas in the 25 CSC and 25 contralateral eyes on en-face images and hypofluorescent areas on ICGA after 30 min.

RESULTS

All 25 CSC eyes and 13 contralateral eyes showed abnormal hypofluorescent areas on late-phase ICGA and hyperreflective areas on en-face OCT from BrM to the choriocapillaris, and these findings correlated with the abnormal areas (r = 0.9988; p < 0.001).

CONCLUSIONS

In CSC patients, we detected abnormal hypofluorescence on ICGA in the late phase, which corresponded to abnormal hyperreflective areas from BrM to the choriocapillaris level in en-face images.

Fibrosis of the Choroid Plexus Filtration Membrane

We report a previously undescribed inflammatory lesion consisting of deposition of activated complement (C3d and C9neo) in association with major histocompatibility complex type II (MHC2)-positive activated microglia in choroid plexus villi exhibiting classical fibrous thickening of the pericapillary filtration membrane. The proportion of villi affected ranged from 5% to 90% in 56 adult subjects with diseases of the CNS and 11 subjects with no preexisting disease of the CNS. In 3 of the 4 children studied, 2% or less of examined villi showed stromal thickening, complement deposition, and the presence of MHC2-positive microglia; in adults, the proportion of villi affected increased with age. Other features of the lesion included loss of capillaries and failure by macrophages to clear extracellular particulate electron-dense material by clathrin-mediated phagocytosis. This choroid plexus lesion may relate pathogenetically to age-related macular degeneration and to Alzheimer disease, 2 other conditions with no known risk factors other than increasing age. All 3 conditions are characterized by the presence of damaged capillaries, inflammatory extracellular aggregates of mixed molecular composition and defective clearance of the deposits by macrophages.

Isolation, culture and characterization of primary mouse RPE cells

Mouse models are powerful tools for the study of ocular diseases. Alterations in the morphology and function of the retinal pigment epithelium (RPE) are common features shared by many ocular disorders. We report a detailed protocol to collect, seed, culture and characterize RPE cells from mice. We describe a reproducible method that we previously developed to collect and culture murine RPE cells on Transwells as functional polarized monolayers. The collection of RPE cells takes ∼3 h, and the cultures mimic in vivo RPE cell features within 1 week. This protocol also describes methods to characterize the cells on Transwells within 1-2 weeks by transmission and scanning electron microscopy (TEM and SEM, respectively), immunostaining of vibratome sections and flat mounts, and measurement of transepithelial electrical resistance. The RPE cell cultures are suitable to study the biology of the RPE from wild-type and genetically modified strains of mice between the ages of 10 d and 12 months. The RPE cells can also be manipulated to investigate molecular mechanisms underlying the RPE pathology in the numerous mouse models of ocular disorders. Furthermore, modeling the RPE pathology in vitro represents a new approach to testing drugs that will help accelerate the development of therapies for vision-threatening disorders such as macular degeneration (MD).

Nutrition and Vascular Supply of Retinal Ganglion Cells during Human Development

PURPOSE

To review the roles of the different vascular beds nourishing the inner retina [retinal ganglion cells (RGCs)] during normal development of the human eye, using our own tissue specimens to support our conclusions.

METHODS

An extensive search of the appropriate literature included PubMed, Google scholar, and numerous available textbooks. In addition, choroidal and retinal NADPH-diaphorase stained whole mount preparations were investigated.

RESULTS

The first critical interaction between vascular bed and RGC formation occurs in the sixth to eighth month of gestation leading to a massive reduction of RGCs mainly in the peripheral retina. The first 3 years of age are characterized by an intense growth of the eyeball to near adult size. In the adult eye, the influence of the choroid on inner retinal nutrition was determined by examining the peripheral retinal watershed zones in more detail.

CONCLUSION

This delicately balanced situation of RGC nutrition is described in the different regions of the eye, and a new graphic presentation is introduced to combine morphological measurements and clinical visual field data.

Cellular models and therapies for age-related macular degeneration

Age-related macular degeneration (AMD) is a complex neurodegenerative visual disorder that causes profound physical and psychosocial effects. Visual impairment in AMD is caused by the loss of retinal pigmented epithelium (RPE) cells and the light-sensitive photoreceptor cells that they support. There is currently no effective treatment for the most common form of this disease (dry AMD). A new approach to treating AMD involves the transplantation of RPE cells derived from either human embryonic or induced pluripotent stem cells. Multiple clinical trials are being initiated using a variety of cell therapies. Although many animal models are available for AMD research, most do not recapitulate all aspects of the disease, hampering progress. However, the use of cultured RPE cells in AMD research is well established and, indeed, some of the more recently described RPE-based models show promise for investigating the molecular mechanisms of AMD and for screening drug candidates. Here, we discuss innovative cell-culture models of AMD and emerging stem-cell-based therapies for the treatment of this vision-robbing disease.

Summary: Here, we discuss the emerging cell-culture models and potential stem-cell-based therapies for AMD, a blinding disorder that affects millions of people worldwide.

Imaging mass spectrometry of the visual system: Advancing the molecular understanding of retina degenerations

Visual sensation is fundamental for quality of life, and loss of vision to retinal degeneration is a debilitating condition. The eye is the only part of the central nervous system that can be noninvasively observed with optical imaging. In the clinics, various spectroscopic methods provide high spatial resolution images of the fundus and the developing degenerative lesions. However, the currently utilized tools are not specific enough to establish the molecular underpinnings of retinal diseases. In contrast, mass spectrometric imaging (MSI) is a powerful tool to identify molecularly specific disease indicators and classification markers. This technique is particularly well suited to the eye, where molecular information can be correlated with clinical data collected via noninvasive diagnostic imaging modalities. Recent studies during the last few recent years have uncovered a plethora of new spatially defined molecular information on several vision-threatening diseases, including age-related macular degeneration, Stargardt disease, glaucoma, cataract, as well as lipid disorders. Even though MS inside the eye cannot be performed noninvasively, by linking diagnostic and molecular information, these studies are the first step toward the development of smart ophthalmic diagnostic and surgical tools. Here, we provide an overview of current approaches applying MSI technology to ocular pathology.

Extracellular Matrix Alterations and Deposit Formation in AMD

Age related macular degeneration (AMD) is the primary cause of vision loss in the western world (Friedman et al., Arch Ophthalmol 122:564-572, 2004). The first clinical indication of AMD is the presence of drusen. However, with age and prior to the formation of drusen, extracellular basal deposits accumulate between the retinal pigment epithelium (RPE) and Bruch's membrane (BrM). Many studies on the molecular composition of the basal deposits and drusen have demonstrated the presence of extracellular matrix (ECM) proteins, complement components and cellular debris. The evidence reviewed here suggests that alteration in RPE cell function might be the primary cause for the accumulation of ECM and cellular debri found in basal deposits. Further studies are obviously needed in order to unravel the specific pathways that lead to abnormal formation of ECM and complement activation.

A local complement response by RPE causes early-stage macular degeneration

Inherited and age-related macular degenerations (AMDs) are important causes of vision loss. An early hallmark of these disorders is the formation of sub-retinal pigment epithelium (RPE) basal deposits. A role for the complement system in MDs was suggested by genetic association studies, but direct functional connections between alterations in the complement system and the pathogenesis of MD remain to be defined. We used primary RPE cells from a mouse model of inherited MD due to a p.R345W mutation in EGF-containing fibulin-like extracellular matrix protein 1 (EFEMP1) to investigate the role of the RPE in early MD pathogenesis. Efemp1(R345W) RPE cells recapitulate the basal deposit formation observed in vivo by producing sub-RPE deposits in vitro. The deposits share features with basal deposits, and their formation was mediated by EFEMP1(R345W) or complement component 3a (C3a), but not by complement component 5a (C5a). Increased activation of complement appears to occur in response to an abnormal extracellular matrix (ECM), generated by the mutant EFEMP1(R345W) protein and reduced ECM turnover due to inhibition of matrix metalloproteinase 2 by EFEMP1(R345W) and C3a. Increased production of C3a also stimulated the release of cytokines such as interleukin (IL)-6 and IL-1B, which appear to have a role in deposit formation, albeit downstream of C3a. These studies provide the first direct indication that complement components produced locally by the RPE are involved in the formation of basal deposits. Furthermore, these results suggest that C3a generated by RPE is a potential therapeutic target for the treatment of EFEMP1-associated MD as well as AMD.

Regulation of age-related macular degeneration-like pathology by complement factor H

Age-related macular degeneration (AMD) affects approximately one-third of Americans over 70 and is characterized by lipoprotein-rich sub-retinal pigmented epithelium (sub-RPE) deposits. Substantial evidence has emerged that implicates complement factor H (CFH) in the pathogenesis of AMD. Here, we conduct an in vivo analysis to elucidate the role of CFH in AMD pathology. We show that (i) CFH and lipoproteins compete for binding in the sub-RPE extracellular matrix such that decreasing CFH leads to lipoprotein accumulation and sub-RPE deposit formation; and (ii) detrimental complement activation within sub-RPE deposits leads to RPE damage and vision loss. This new understanding of the complicated interactions of CFH in development of AMD-like pathology paves the way for identifying more targeted therapeutic strategies for AMD.

Complement factor H (CFH) is a major susceptibility gene for age-related macular degeneration (AMD); however, its impact on AMD pathobiology is unresolved. Here, the role of CFH in the development of AMD pathology in vivo was interrogated by analyzing aged Cfh^+/− and Cfh^−/− mice fed a high-fat, cholesterol-enriched diet. Strikingly, decreased levels of CFH led to increased sub-retinal pigmented epithelium (sub-RPE) deposit formation, specifically basal laminar deposits, following high-fat diet. Mechanistically, our data show that deposits are due to CFH competition for lipoprotein binding sites in Bruch’s membrane. Interestingly and despite sub-RPE deposit formation occurring in both Cfh^+/− and Cfh^−/− mice, RPE damage accompanied by loss of vision occurred only in old Cfh^+/− mice. We demonstrate that such pathology is a function of excess complement activation in Cfh^+/− mice versus complement deficiency in Cfh^−/− animals. Due to the CFH-dependent increase in sub-RPE deposit height, we interrogated the potential of CFH as a previously unidentified regulator of Bruch’s membrane lipoprotein binding and show, using human Bruch’s membrane explants, that CFH removes endogenous human lipoproteins in aged donors. Thus, advanced age, high-fat diet, and decreased CFH induce sub-RPE deposit formation leading to complement activation, which contributes to RPE damage and visual function impairment. This new understanding of the complicated interactions of CFH in AMD-like pathology provides an improved foundation for the development of targeted therapies for AMD.

Age-related macular degeneration: genetics and biology coming together

Genetic and genomic studies have enhanced our understanding of complex neurodegenerative diseases that exert a devastating impact on individuals and society. One such disease, age-related macular degeneration (AMD), is a major cause of progressive and debilitating visual impairment. Since the pioneering discovery in 2005 of complement factor H (CFH) as a major AMD susceptibility gene, extensive investigations have confirmed 19 additional genetic risk loci, and more are anticipated. In addition to common variants identified by now-conventional genome-wide association studies, targeted genomic sequencing and exome-chip analyses are uncovering rare variant alleles of high impact. Here, we provide a critical review of the ongoing genetic studies and of common and rare risk variants at a total of 20 susceptibility loci, which together explain 40-60% of the disease heritability but provide limited power for diagnostic testing of disease risk. Identification of these susceptibility loci has begun to untangle the complex biological pathways underlying AMD pathophysiology, pointing to new testable paradigms for treatment.