Directional protein secretion by the retinal pigment epithelium: roles in retinal health and the development of age-related macular degeneration

Scaling up polarized RPE cell supernatant production on parylene membrane

Age-related macular degeneration (AMD), a leading cause of vision loss, primarily arises from the degeneration of retinal pigment epithelium (RPE) and photoreceptors. Current therapeutic options for dry AMD are limited. Encouragingly, cultured RPE cells on parylene-based biomimetic Bruch's membrane demonstrate characteristics akin to the native RPE layer. In this study, we cultivated human embryonic stem cell-derived polarized RPE (hESC-PRPE) cells on parylene membranes at both small- and large-scale settings, collecting conditioned supernatant, denoted as PRPE-SF. We conducted a comprehensive analysis of the morphology of the cultured hESC-RPE cells and the secreted growth factors in PRPE-SF. To evaluate the in vivo efficacy of these products, the product was administered via intravitreal injections of PRPE-SF in immunodeficient Royal College of Surgeons (iRCS) rats, a model for retinal degeneration. Our study not only demonstrated the scalability of PRPE-SF production while maintaining RPE cell phenotype but also showed consistent protein concentrations between small- and large-scale batches. We consistently identified 10 key factors in PRPE-SF, including BMP-7, IGFBP-2, IGFBP-3, IGFBP-4, IGFBP-6, MANF, PEDF, PDGF-AA, TGFβ1, and VEGF. Following intravitreal administration of PRPE-SF, we observed a significant increase in the thickness of the outer nuclear layer (ONL) and photoreceptor preservation in iRCS rats. Furthermore, correlation analysis revealed that IGFBP-3, IGFBP-4, MANF, PEDF, and TGFβ1 displayed positive associations with in vivo bioactivity, while GDF-15 exhibited a negative correlation. Overall, this study highlights the feasibility of scaling up PRPE-SF production on parylene membranes without compromising its essential constituents. The outcomes of PRPE-SF administration in an animal model of retinal degeneration present substantial potential for photoreceptor preservation. Moreover, the identification of candidate surrogate potency markers, showing strong positive associations with in vivo bioactivity, lays a solid foundation for the development of a promising therapeutic intervention for retinal degenerative diseases.

Endogenous and Exogenous Regulation of Redox Homeostasis in Retinal Pigment Epithelium Cells: An Updated Antioxidant Perspective

The retinal pigment epithelium (RPE) performs a range of necessary functions within the neural layers of the retina and helps ensure vision. The regulation of pro-oxidative and antioxidant processes is the basis for maintaining RPE homeostasis and preventing retinal degenerative processes. Long-term stable changes in the redox balance under the influence of endogenous or exogenous factors can lead to oxidative stress (OS) and the development of a number of retinal pathologies associated with RPE dysfunction, and can eventually lead to vision loss. Reparative autophagy, ubiquitin-proteasome utilization, the repair of damaged proteins, and the maintenance of their conformational structure are important interrelated mechanisms of the endogenous defense system that protects against oxidative damage. Antioxidant protection of RPE cells is realized as a result of the activity of specific transcription factors, a large group of enzymes, chaperone proteins, etc., which form many signaling pathways in the RPE and the retina. Here, we discuss the role of the key components of the antioxidant defense system (ADS) in the cellular response of the RPE against OS. Understanding the role and interactions of OS mediators and the components of the ADS contributes to the formation of ideas about the subtle mechanisms in the regulation of RPE cellular functions and prospects for experimental approaches to restore RPE functions.

Analysis of Wild Type and Variant B Cystatin C Interactome in Retinal Pigment Epithelium Cells Reveals Variant B Interacting Mitochondrial Proteins

Cystatin C, a secreted cysteine protease inhibitor, is abundantly expressed in retinal pigment epithelium (RPE) cells. A mutation in the protein's leader sequence, corresponding to formation of an alternate variant B protein, has been linked with an increased risk for both age-related macular degeneration (AMD) and Alzheimer's disease (AD). Variant B cystatin C displays intracellular mistrafficking with partial mitochondrial association. We hypothesized that variant B cystatin C interacts with mitochondrial proteins and impacts mitochondrial function. We sought to determine how the interactome of the disease-related variant B cystatin C differs from that of the wild-type (WT) form. For this purpose, we expressed cystatin C Halo-tag fusion constructs in RPE cells to pull down proteins interacting with either the WT or variant B form, followed by identification and quantification by mass spectrometry. We identified a total of 28 interacting proteins, of which 8 were exclusively pulled down by variant B cystatin C. These included 18 kDa translocator protein (TSPO) and cytochrome B5 type B, both of which are localized to the mitochondrial outer membrane. Variant B cystatin C expression also affected RPE mitochondrial function with increased membrane potential and susceptibility to damage-induced ROS production. The findings help us to understand how variant B cystatin C differs functionally from the WT form and provide leads to RPE processes adversely affected by the variant B genotype.

Extracellular Matrix: The Unexplored Aspects of Retinal Pathologies and Regeneration

Nearly a billion people worldwide are affected by vision-impairing conditions, with retinal degenerative diseases being a major cause of blindness. Unfortunately, such diseases are often permanent and progressive, resulting in further degeneration and loss of sight, due to the human retina possessing little, if any, regenerative capacity. Despite numerous efforts and great progress being made to understand the molecular mechanisms of these diseases and possible therapies, the majority of investigations focused on cell-intrinsic factors. However, the microenvironment surrounding retinal cells throughout these processes also plays an important role, though our current understanding of its involvement remains limited. Here we present a brief overview of the current state of the field of extracellular matrix studies within the retina and its potential roles in retinal diseases and potential therapeutic approaches.

Intercellular communication analysis of the human retinal pigment epithelial and choroidal cells predicts pathways associated with aging, cellular senescence and age-related macular degeneration

The retinal pigment epithelium (RPE) and the choroid are ocular tissues with fundamental roles in supporting neuroretinal function. The pathogenesis of age-related macular degeneration (AMD), a leading cause of irreversible blindness for which aging is the highest risk factor is closely linked with progressive impairment of various functions of these tissues. Cellular senescence, marked by cell cycle arrest and secretion of proinflammatory factors, is known to be associated with aging and has been proposed as a potential driver of AMD. Here, we investigated the role played by intercellular communication in the RPE/choroid within the context of aging, senescence and AMD. We inferred cell–cell interactions in the RPE/choroid by applying CellChat and scDiffCom on a publicly available scRNA-seq dataset from three human donors with and without AMD. We identified age-regulated ligand and receptor genes by using limma on a separate publicly available bulk microarray dataset providing RPE/choroid samples at multiple time points. Cellular senescence was investigated by assigning a score to each cell and each sample of these scRNA-seq and microarray datasets, respectively, based on the expression of key signature genes determined by a previous senescence meta-analysis. We identified VEGF-, BMP-and tenascin-mediated pathways supporting some of the strongest cell–cell interactions between RPE cells, fibroblasts and choroidal endothelial cells and as strong intercellular communication pathways related to both aging and senescence. Their signaling strength was enhanced between subpopulations of cells having high senescence scores. Predominant ligands of these pathways were upregulated with age whereas predominant receptors were downregulated. Globally, we also observed that cells from AMD samples presented slightly bigger senescence scores than normal cells and that the senescence score positively correlated with age in bulk samples (R = 0.26, value of p < 0.01). Hence, our analysis provides novel information on RPE/choroid intercellular communication that gives insights into the connection between aging, senescence and AMD.

Cellular and molecular alterations in neurons and glial cells in inherited retinal degeneration

Multiple gene mutations have been associated with inherited retinal dystrophies (IRDs). Despite the spectrum of phenotypes caused by the distinct mutations, IRDs display common physiopathology features. Cell death is accompanied by inflammation and oxidative stress. The vertebrate retina has several attributes that make this tissue vulnerable to oxidative and nitrosative imbalance. The high energy demands and active metabolism in retinal cells, as well as their continuous exposure to high oxygen levels and light-induced stress, reveal the importance of tightly regulated homeostatic processes to maintain retinal function, which are compromised in pathological conditions. In addition, the subsequent microglial activation and gliosis, which triggers the secretion of pro-inflammatory cytokines, chemokines, trophic factors, and other molecules, further worsen the degenerative process. As the disease evolves, retinal cells change their morphology and function. In disease stages where photoreceptors are lost, the remaining neurons of the retina to preserve their function seek out for new synaptic partners, which leads to a cascade of morphological alterations in retinal cells that results in a complete remodeling of the tissue. In this review, we describe important molecular and morphological changes in retinal cells that occur in response to oxidative stress and the inflammatory processes underlying IRDs.

Design, construction and in vivo functional assessment of a hinge truncated sFLT01

Gene therapy for the treatment of ocular neovascularization has reached clinical trial phases. The AAV2-sFLT01 construct was already evaluated in a phase 1 open-label trial administered intravitreally to patients with advanced neovascular age-related macular degeneration. SFLT01 protein functions by binding to VEGF and PlGF molecules and inhibiting their activities simultaneously. It consists of human VEGFR1/Flt-1 (hVEGFR1), a polyglycine linker, and the Fc region of human IgG1. The IgG1 upper hinge region of the sFLT01 molecule makes it vulnerable to radical attacks and prone to causing immune reactions. This study pursued two goals: (i) minimizing the immunogenicity and vulnerability of the molecule by designing a truncated molecule called htsFLT01 (hinge truncated sFLT01) that lacked the IgG1 upper hinge and lacked 2 amino acids from the core hinge region; and (ii) investigating the structural and functional properties of the aforesaid chimeric molecule at different levels (in silico, in vitro, and in vivo). Molecular dynamics simulations and molecular mechanics energies combined with Poisson-Boltzmann and surface area continuum solvation calculations revealed comparable free energy of binding and binding affinity for sFLT01 and htsFLT01 to their cognate ligands. Conditioned media from human retinal pigment epithelial (hRPE) cells that expressed htsFLT01 significantly reduced tube formation in HUVECs. The AAV2-htsFLT01 virus suppressed vascular development in the eyes of newborn mice. The htsFLT01 gene construct is a novel anti-angiogenic tool with promising improvements compared to existing treatments.

Elastin turnover in ocular diseases: A special focus on age-related macular degeneration

The extracellular matrix (ECM) and its turnover play a crucial role in the pathogenesis of several inflammatory diseases, including age-related macular degeneration (AMD). Elastin, a critical protein component of the ECM, not only provides structural and mechanical support to tissues, but also mediates several intracellular and extracellular molecular signaling pathways. Abnormal turnover of elastin has pathological implications. In the eye elastin is a major structural component of Bruch's membrane (BrM), a critical ECM structure separating the retinal pigment epithelium (RPE) from the choriocapillaris. Reduced integrity of macular BrM elastin, increased serum levels of elastin-derived peptides (EDPs), and elevated elastin antibodies have been reported in AMD. Existing reports suggest that elastases, the elastin-degrading enzymes secreted by RPE, infiltrating macrophages or neutrophils could be involved in BrM elastin degradation, thus contributing to AMD pathogenesis. EDPs derived from elastin degradation can increase inflammatory and angiogenic responses in tissues, and the elastin antibodies are shown to play roles in immune cell activity and complement activation. This review summarizes our current understanding on the elastases/elastin fragments-mediated mechanisms of AMD pathogenesis.

Identification of Age-associated Proteins and Functional Alterations in Human Retinal Pigment Epithelium

Retinal pigment epithelium (RPE) has essential functions, such as nourishing and supporting the neural retina, and is of vital importance in the pathogenesis of age-related retinal degeneration. However, the exact molecular changes of RPE during aging remain poorly understood. Here, we isolated human primary RPE (hRPE) cells from 18 eye donors distributed over a wide age range (10-67 years old). A quantitative proteomic analysis was performed to analyze changes in their intracellular and secreted proteins. Age-group related subtypes and age-associated proteins were revealed and potential age-associated mechanisms were validated in ARPE-19 and hRPE cells. The results of proteomic data analysis and verifications suggest that RNF123- and RNF149-related protein ubiquitination plays an important role in protecting hRPE cells from oxidative damage during aging. In older hRPE cells, apoptotic signaling-related pathways were up-regulated, and endoplasmic reticulum organization was down-regulated both in the intracellular and secreted proteomes. Our work paints a detailed molecular picture of hRPE cells during the aging process and provides new insights into the molecular characteristics of RPE during aging and under other related clinical retinal conditions.

Oxidative stress differentially impacts apical and basolateral secretion of angiogenic factors from human iPSC-derived retinal pigment epithelium cells

The retinal pigment epithelium (RPE) is a polarized monolayer that secretes growth factors and cytokines towards the retina apically and the choroid basolaterally. Numerous RPE secreted proteins have been linked to the pathogenesis of age-related macular degeneration (AMD). The purpose of this study was to determine the differential apical and basolateral secretome of RPE cells, and the effects of oxidative stress on directional secretion of proteins linked to AMD and angiogenesis. Tandem mass tag spectrometry was used to profile proteins in human iPSC-RPE apical and basolateral conditioned media. Changes in secretion after oxidative stress induced by H2O2 or tert-butyl hydroperoxide (tBH) were investigated by ELISA and western analysis. Out of 926 differentially secreted proteins, 890 (96%) were more apical. Oxidative stress altered the secretion of multiple factors implicated in AMD and neovascularization and promoted a pro-angiogenic microenvironment by increasing the secretion of pro-angiogenic molecules (VEGF, PTN, and CRYAB) and decreasing the secretion of anti-angiogenic molecules (PEDF and CFH). Apical secretion was impacted more than basolateral for PEDF, CRYAB and CFH, while basolateral secretion was impacted more for VEGF, which may have implications for choroidal neovascularization. This study lays a foundation for investigations of dysfunctional RPE polarized protein secretion in AMD and other chorioretinal degenerative disorders.

Biomechanical regulation of planar cell polarity in endothelial cells

Cell polarity refers to the uneven distribution of certain cytoplasmic components in a cell with a spatial order. The planar cell polarity (PCP), the cell aligns perpendicular to the polar plane, in endothelial cells (ECs) has become a research hot spot. The planar polarity of ECs has a positive significance on the regulation of cardiovascular dysfunction, pathological angiogenesis, and ischemic stroke. The endothelial polarity is stimulated and regulated by biomechanical force. Mechanical stimuli promote endothelial polarization and make ECs produce PCP to maintain the normal physiological and biochemical functions. Here, we overview recent advances in understanding the interplay and mechanism between PCP and ECs function involved in mechanical forces, with a focus on PCP signaling pathways and organelles in regulating the polarity of ECs. And then showed the related diseases caused by ECs polarity dysfunction. This study provides new ideas and therapeutic targets for the treatment of endothelial PCP-related diseases.

Targeted drug delivery to the retinal pigment epithelium: Untapped therapeutic potential for retinal diseases

The retinal pigment epithelium (RPE) plays a crucial part in sight-threatening diseases. In this review, we shed light on the pivotal implication of the RPE in age-related macular degeneration, diabetic retinopathy and retinopathy of prematurity; and explain why a paradigm shift toward targeted RPE therapy is needed to efficiently fight these retinal diseases. We provide guidance for the development of RPE-specific nanotherapeutics by giving a comprehensive overview of the possibilities and challenges of drug delivery to the RPE and highlight successful nanotherapeutic approaches targeting the RPE.

Factors influencing mesenchymal stromal cells in in vitro cellular models to study retinal pigment epithelial cell rescue

Mesenchymal stromal cells (MSC) stop or slow retinal pigment epithelium (RPE) and neuroretina (NR) degeneration by paracrine activity in oxidative stress-induced retinal degenerative diseases. However, it is mandatory to develop adequate in vitro models that allow testing new treatment strategies against oxidative stress before performing in vivo studies. The viable double- and triple-layered setups are composed of separate layers of NR, MSC, and RPE (NR-MSC-RPE, NR-RPE, MSC-RPE) partially mimic in vivo retinal conditions. In this study, the paracrine neuroprotective effect of each setup's microenvironment on hydrogen peroxide (H₂O₂)-stressed was compared with unstressed RPE cells. RPE cell proliferation viability was assessed on day 1, 3, and 6 using Alamar Blue® (10%), MTT (10%) and a cell viability/cytotoxicity assay kit followed by data analysis. The results showed that RPE cells, highly viable (> 90%) in mixed medium of DMEM and neurobasal A (1:1), lost 50% viability on exposure to 400 µM of H₂O₂ (P < 0.05). The unexposed groups differed significantly from exposed groups for RPE cell growth (RPE and [Formula: see text]RPE (P < 0.0001), NR-MSC-RPE, and NR-MSC-[Formula: see text]RPE (P < 0.05), NR-RPE and NR-[Formula: see text]RPE (P < 0.01), and MSC-RPE and MSC-[Formula: see text]RPE (P < 0.01). NR-[Formula: see text]RPE and NR-RPE supported RPE cell proliferation viability better than other setups (P < 0.01) and RPE cells proliferated 0.49-fold more in NR-MSC-[Formula: see text]RPE than NR-MSC-RPE. Thus, NR and MSC presence improved significantly each setup's microenvironment for cell rescue, nevertheless, each setup also showed limitations for its use as an in vitro study tool. Health of microenvironment of such setups depends on many factors including cell-secreted trophic factors.

A look into retinal organoids: methods, analytical techniques, and applications

Inherited retinal diseases (IRDs) cause progressive loss of light-sensitive photoreceptors in the eye and can lead to blindness. Gene-based therapies for IRDs have shown remarkable progress in the past decade, but the vast majority of forms remain untreatable. In the era of personalised medicine, induced pluripotent stem cells (iPSCs) emerge as a valuable system for cell replacement and to model IRD because they retain the specific patient genome and can differentiate into any adult cell type. Three-dimensional (3D) iPSCs-derived retina-like tissue called retinal organoid contains all major retina-specific cell types: amacrine, bipolar, horizontal, retinal ganglion cells, Müller glia, as well as rod and cone photoreceptors. Here, we describe the main applications of retinal organoids and provide a comprehensive overview of the state-of-art analysis methods that apply to this model system. Finally, we will discuss the outlook for improvements that would bring the cellular model a step closer to become an established system in research and treatment development of IRDs.

Protective Effects of a Lutein Ester Prodrug, Lutein Diglutaric Acid, against H2O2-Induced Oxidative Stress in Human Retinal Pigment Epithelial Cells

Oxidative stress-induced cell damage and death of the retinal pigmented epithelium (RPE), a polarized monolayer that maintains retinal health and homeostasis, lead to the development of age-related macular degeneration (AMD). Several studies show that the naturally occurring antioxidant Lutein (Lut) can protect RPE cells from oxidative stress. However, the poor solubility and low oral bioavailability limit the potential of Lut as a therapeutic agent. In this study, lutein diglutaric acid (Lut-DG), a prodrug of Lut, was synthesized and its ability to protect human ARPE-19 cells from oxidative stress was tested compared to Lut. Both Lut and Lut-DG significantly decreased H₂O₂-induced reactive oxygen species (ROS) production and protected RPE cells from oxidative stress-induced death. Moreover, the immunoblotting analysis indicated that both drugs exerted their protective effects by modulating phosphorylated MAPKs (p38, ERK1/2 and SAPK/JNK) and downstream molecules Bax, Bcl-2 and Cytochrome c. In addition, the enzymatic antioxidants glutathione peroxidase (GPx) and catalase (CAT) and non-enzymatic antioxidant glutathione (GSH) were enhanced in cells treated with Lut and Lut-DG. In all cases, Lut-DG was more effective than its parent drug against oxidative stress-induced damage to RPE cells. These findings highlight Lut-DG as a more potent compound than Lut with the protective effects against oxidative stress in RPE cells through the modulation of key MAPKs, apoptotic and antioxidant molecular pathways.

The retinal pigment epithelium: Development, injury responses, and regenerative potential in mammalian and non-mammalian systems

Diseases that result in retinal pigment epithelium (RPE) degeneration, such as age-related macular degeneration (AMD), are among the leading causes of blindness worldwide. Atrophic (dry) AMD is the most prevalent form of AMD and there are currently no effective therapies to prevent RPE cell death or restore RPE cells lost from AMD. An intriguing approach to treat AMD and other RPE degenerative diseases is to develop therapies focused on stimulating endogenous RPE regeneration. For this to become feasible, a deeper understanding of the mechanisms underlying RPE development, injury responses and regenerative potential is needed. In mammals, RPE regeneration is extremely limited; small lesions can be repaired by the expansion of adjacent RPE cells, but large lesions cannot be repaired as remaining RPE cells are unable to functionally replace lost RPE tissue. In some injury paradigms, RPE cells proliferate but do not regenerate a morphologically normal monolayer, while in others, proliferation is pathogenic and results in further disruption to the retina. This is in contrast to non-mammalian vertebrates, which possess tremendous RPE regenerative potential. Here, we discuss what is known about RPE formation during development in mammalian and non-mammalian vertebrates, we detail the processes by which RPE cells respond to injury, and we describe examples of RPE-to-retina and RPE-to-RPE regeneration in non-mammalian vertebrates. Finally, we outline barriers to RPE-dependent regeneration in mammals that could potentially be overcome to stimulate a regenerative response from the RPE.

Cytoprotective Effects of Punicalagin on Hydrogen-Peroxide-Mediated Oxidative Stress and Mitochondrial Dysfunction in Retinal Pigment Epithelium Cells

The retinal pigment epithelium (RPE) is a densely pigmented, monostratified epithelium that provides metabolic and functional support to the outer segments of photoreceptors. Endogenous or exogenous oxidative stimuli determine a switch from physiological to pathological conditions, characterized by an increase of intracellular levels of reactive oxygen species (ROS). Accumulating evidence has elucidated that punicalagin (PUN), the major ellagitannin in pomegranate, is a potent antioxidant in several cell types. The present study aimed to investigate the protective effect of PUN on mitochondrial dysfunction associated with hydrogen peroxide (H₂O₂)-induced oxidative stress. For this purpose, we used a human RPE cell line (ARPE-19) exposed to H₂O₂ for 24 h. The effects of PUN pre-treatment (24 h) were examined on cell viability, mitochondrial ROS levels, mitochondrial membrane potential, and respiratory chain complexes, then finally on caspase-3 enzymatic activity. The results showed that supplementation with PUN: (a) significantly increased cell viability; (b) kept the mitochondrial membrane potential (ΔΨm) at healthy levels and limited ROS production; (c) preserved the activity of respiratory complexes; (d) reduced caspase-3 activity. In conclusion, due to its activity in helping mitochondrial functions, reducing oxidative stress, and subsequent induction of cellular apoptosis, PUN might be considered a useful nutraceutical agent in the treatment of oxidation-associated disorders of RPE.

Basolateral activation with TLR agonists induces polarized cytokine release and reduces barrier function in RPE in vitro

PURPOSE

Systemic inflammation may be of importance in the development of AMD. RPE cells can recognize danger signals with toll-like receptors (TLR) and may react in a pro-inflammatory manner. In this study, we evaluated the basal and apical secretions of TNFα, IL-6, and IL-1β in primary RPE cells and RPE/choroid explant cells under basolateral stimulation of TLR2, 3, and 4; the effects on barrier function; and their influence on neuronal cell viability.

METHODS

RPE/choroid tissue explants were prepared from porcine eyes and cultivated in modified Ussing chambers; primary porcine RPE cells on transwell plates. Cells were basally stimulated with agonists Pam2CSK4 (Pam; TLR2), polyinosinic/polycytidylic acid (Poly I:C; TLR3), and lipopolysaccharide (LPS; TLR4) for 24 h. Supernatants were evaluated with ELISA for cytokines TNFα, IL-6, and IL-1β. Apical supernatants were applied to SHSY-5Y cells, and cell viability was evaluated in MTT assay. Barrier function was tested by measuring transepithelial electrical resistance (TER) and occludin immunostaining.

RESULTS

None of the tested TLR agonists was toxic on RPE cells after 24 h of exposure. Unstimulated RPE cells secreted hardly any cytokines. Pam induced IL-6, IL-1ß, and TNFα on the basal and apical sides at all concentrations tested. Poly I:C induced IL-6 and TNFα primarily at the basal side at lower but on both sides at higher concentrations. LPS induced IL-6, IL-1ß, and TNFα apically and basally at all concentrations tested. In the RPE/choroid, a strong difference between apical and basal secretions could be found. IL-6 was constitutively secreted basally, but not apically, but was induced by all agonists on both sides. IL-1ß and TNFα alpha were strongly induced on the basal side by all agonists. TER was reduced by all agonists, with Pam and LPS being effective in all concentrations tested. Occludin expression was unaltered, but the distribution was influenced by the agonists, with a less distinct localization at the cell borders after treatment. None of the agonists or supernatants of treated RPE and RPE/choroid organ cultures exerted any effect on viability of SHSY-5Y cells.

CONCLUSIONS

Danger signals activating TLRs can induce polarized cytokine expression and contribute to the loss of barrier function in the RPE.

Associations between the Complement System and Choroidal Neovascularization in Wet Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is the leading cause of blindness affecting the elderly in the Western world. The most severe form of AMD, wet AMD (wAMD), is characterized by choroidal neovascularization (CNV) and acute vision loss. The current treatment for these patients comprises monthly intravitreal injections of anti-vascular endothelial growth factor (VEGF) antibodies, but this treatment is expensive, uncomfortable for the patient, and only effective in some individuals. AMD is a complex disease that has strong associations with the complement system. All three initiating complement pathways may be relevant in CNV formation, but most evidence indicates a major role for the alternative pathway (AP) and for the terminal complement complex, as well as certain complement peptides generated upon complement activation. Since the complement system is associated with AMD and CNV, a complement inhibitor may be a therapeutic option for patients with wAMD. The aim of this review is to (i) reflect on the possible complement targets in the context of wAMD pathology, (ii) investigate the results of prior clinical trials with complement inhibitors for wAMD patients, and (iii) outline important considerations when developing a future strategy for the treatment of wAMD.

Protection against light-induced retinal degeneration via dual anti-inflammatory and anti-angiogenic functions of thrombospondin-1

BACKGROUND AND PURPOSE

Retinal photodamage is a high-risk factor for age-related macular degeneration (AMD), the leading cause of irreversible blindness worldwide. However, both the pathogenesis and effective therapies for retinal photodamage are still unclear and debated.

EXPERIMENTAL APPROACH

The anti-inflammatory effects of thrombospondin-1 on blue light-induced inflammation in ARPE-19 cells and in retinal inflammation were evaluated. Furthermore, the anti-angiogenic effects of thrombospondin-1 on human microvascular endothelial cells (hMEC-1 cells) and a laser-induced choroidal neovascularisation (CNV) mouse model were evaluated. in vitro experiments, including western blotting, immunocytochemistry, migration assays and tube formation assays, as well as in vivo experiments, including immunofluorescence, visual electrophysiology, spectral-domain optical coherence tomography, and fluorescein angiography, were employed to evaluate the anti-inflammatory and anti-angiogenic effects of thrombospondin-1.

KEY RESULTS

Specific effects of blue light-induced retinal inflammation and pathological angiogenesis were reflected by up-regulation of pro-inflammatory factors and activation of angiogenic responses, predominantly regulated by the NF-κB and VEGFR2 pathways respectively. During the blue light-induced pathological progress, THBS-1 derived from retinal pigment epithelium down-regulated proteomics and biological assays. Thrombospondin-1 treatment also suppressed inflammatory infiltration and neovascular leakage. The protective effect of Thrombospondin-1 was additionally demonstrated by a substantial rescue of visual function. Mechanistically, thrombospondin-1 reversed blue light-induced retinal inflammation and angiogenesis by blocking the activated NF-κB and VEGFR2 pathways, respectively.

CONCLUSION AND IMPLICATIONS

Thrombospondin-1, with dual anti-inflammatory and anti-neovascularisation properties, is a promising agent for protection against blue light-induced retinal damage and retinal degenerative disorders which are pathologically associated with inflammatory and angiogenic progress.

Ethanol-Induced Oxidative Stress Modifies Inflammation and Angiogenesis Biomarkers in Retinal Pigment Epithelial Cells (ARPE-19): Role of CYP2E1 and its Inhibition by Antioxidants

The retinal pigment epithelium (RPE) plays a key role in retinal health, being essential for the protection against reactive oxygen species (ROS). Nevertheless, excessive oxidative stress can induce RPE dysfunction, promoting visual loss. Our aim is to clarify the possible implication of CYP2E1 in ethanol (EtOH)-induced oxidative stress in RPE alterations. Despite the increase in the levels of ROS, measured by fluorescence probes, the RPE cells exposed to the lowest EtOH concentrations were able to maintain cell survival, measured by the Cell Proliferation Kit II (XTT). However, EtOH-induced oxidative stress modified inflammation and angiogenesis biomarkers, analyzed by proteome array, ELISA, qPCR and Western blot. The highest EtOH concentration used stimulated a large increase in ROS levels, upregulating the cytochrome P450-2E1 (CYP2E1) and promoting cell death. The use of antioxidants such as N-acetylcysteine (NAC) and diallyl sulfide (DAS), which is also a CYP2E1 inhibitor, reverted cell death and oxidative stress, modulating also the upstream angiogenesis and inflammation regulators. Because oxidative stress plays a central role in most frequent ocular diseases, the results herein support the proposal that CYP2E1 upregulation could aggravate retinal degeneration, especially in those patients with high baseline oxidative stress levels due to their ocular pathology and should be considered as a risk factor.

An in vitro model of chronic wounding and its implication for age-related macular degeneration

Degeneration of the retinal pigment epithelium (RPE) plays a central role in age-related macular degeneration (AMD). Throughout life, RPE cells are challenged by a variety of cytotoxic stressors, some of which are cumulative with age and may ultimately contribute to drusen and lipofuscin accumulation. Stressors such as these continually damage RPE cells resulting in a state of chronic wounding. Current cell-based platforms that model a state of chronic RPE cell wounding are limited, and the RPE cellular response is not entirely understood. Here, we used the electric cell-substrate impedance sensing (ECIS) system to induce a state of acute or chronic wounding on differentiated human fetal RPE cells to analyze changes in the wound repair response. RPE cells surrounding the lesioned area employ both cell migration and proliferation to repair wounds but fail to reestablish their original cell morphology or density after repetitive wounding. Chronically wounded RPE cells develop phenotypic AMD characteristics such as loss of cuboidal morphology, enlarged size, and multinucleation. Transcriptomic analysis suggests a systemic misregulation of RPE cell functions in bystander cells, which are not directly adjacent to the wound. Genes associated with the major RPE cell functions (LRAT, MITF, RDH11) significantly downregulate after wounding, in addition to differential expression of genes associated with the cell cycle (CDK1, CDC6, CDC20), inflammation (IL-18, CCL2), and apoptosis (FAS). Interestingly, repetitive wounding resulted in prolonged misregulation of genes, including FAS, LRAT, and PEDF. The use of ECIS to induce wounding resulted in an over-representation of AMD-associated genes among those dysregulated genes, particularly genes associated with advanced AMD. This simple system provides a new model for further investigation of RPE cell wound response in AMD pathogenesis.

Increased Rate of Retinal Pigment Epithelial Cell Migration and Pro-Angiogenic Potential Ensuing From Reduced Cystatin C Expression

Purpose

Variant B precursor cysteine protease inhibitor cystatin C, a known recessive risk factor for developing exudative age-related macular degeneration (AMD), presents altered intracellular trafficking and reduced secretion from retinal pigment epithelial (RPE) cells. Because cystatin C inhibits multiple extracellular matrix (ECM)-degrading cathepsins, this study evaluated the role of this mutation in inducing ECM-related functional changes in RPE cellular behavior.

Methods

Induced pluripotent stem cells gene-edited bi-allelically by CRISPR/Cas9 to express the AMD-linked cystatin C variant were differentiated to RPE cells and assayed for their ability to degrade fluorescently labeled ECM proteins. Cellular migration and adhesion on multiple ECM proteins, differences in transepithelial resistance and polarized protein secretion were tested. Vessel formation induced by gene edited cells-conditioned media was quantified using primary human dermal microvascular epithelial cells.

Results

Variant B cystatin C-expressing induced pluripotent stem cells-derived RPE cells displayed a significantly higher rate of laminin and fibronectin degradation 3 days after seeding on fluorescently labeled ECM (P < 0.05). Migration on matrigel, collagen IV and fibronectin was significantly faster for edited cells compared with wild-type (WT) cells. Both edited and WT cells displayed polarized secretion of cystatin C, but transepithelial resistance was lower in gene-edited cells after 6 weeks culture, with significantly lower expression of tight junction protein claudin-3. Media conditioned by gene-edited cells stimulated formation of significantly longer microvascular tubes (P < 0.05) compared with WT-conditioned media.

Conclusions

Reduced levels of cystatin C lead to changes in the RPE ability to degrade, adhere, and migrate supporting increased invasiveness and angiogenesis relevant for AMD pathology.

Secretory proteostasis of the retinal pigmented epithelium: Impairment links to age-related macular degeneration

Secretory proteostasis integrates protein synthesis, processing, folding and trafficking pathways that are essential for efficient cellular secretion. For the retinal pigment epithelium (RPE), secretory proteostasis is of vital importance for the maintenance of the structural and functional integrity of apical (photoreceptors) and basal (Bruch's membrane/choroidal blood supply) sides of the environment it resides in. This integrity is achieved through functions governed by RPE secreted proteins, which include extracellular matrix modelling/remodelling, angiogenesis and immune response modulation. Impaired RPE secretory proteostasis affects not only the extracellular environment, but leads to intracellular protein aggregation and ER-stress with subsequent cell death. Ample recent evidence implicates dysregulated proteostasis as a key factor in the development of age-related macular degeneration (AMD), the leading cause of blindness in the developed world, and research aiming to characterise the roles of various proteins implicated in AMD-associated dysregulated proteostasis unveiled unexpected facets of the mechanisms involved in degenerative pathogenesis. This review analyses cellular processes unveiled by the study of the top 200 transcripts most abundantly expressed by the RPE/choroid in the light of the specialised secretory nature of the RPE. Functional roles of these proteins and the mechanisms of their impaired secretion, due to age and genetic-related causes, are analysed in relation to AMD development. Understanding the importance of RPE secretory proteostasis in relation to maintaining retinal health and how it becomes impaired in disease is of paramount importance for the development and assessment of future therapeutic advancements involving gene and cell therapies.

The future of bladder cancer therapy: Optimizing the inhibition of the fibroblast growth factor receptor

Therapeutic options for metastatic bladder cancer (BC) have seen minimal evolution over the past 30 years, with platinum-based chemotherapy remaining the mainstay of standard of care for metastatic BC. Recently, five immune checkpoint inhibitors (ICIs) have been approved by the FDA as second-line therapy, and two ICIs are approved as first-line treatment in selected patients. Molecular alterations of muscle-invasive bladder cancer (MIBC) have been reported by The Cancer Genome Atlas. About 15% of patients with MIBC have molecular alterations in the fibroblast growth factor (FGF) axis. Several ongoing trials are testing novel FGF receptor (FGFR) inhibitors in patients with FGFR genomic aberrations. Recently, erdafitinib, a pan-FGFR inhibitor, was approved by the FDA in patients with metastatic BC who have progressed on platinum-based chemotherapy. We reviewed the literature over the last decade and provide a summary of current knowledge of FGF signaling, and the prognosis associated with FGFR mutations in BC. We cover the role of FGFR inhibition with non-selective and selective tyrosine kinase inhibitors as well as novel agents in metastatic BC. Efficacy and safety data including insights from mechanism-based toxicity are reported for selected populations of metastatic BC with FGFR aberrations. Current strategies to managing resistance to anti-FGFR agents is addressed, and the importance of developing reliable biomarkers as the therapeutic landscape moves towards an individualized therapeutic approach.

Roles of Drug Transporters in Blood-Retinal Barrier

Blood-retinal barrier (BRB) includes inner BRB (iBRB) and outer BRB (oBRB), which are formed by retinal capillary endothelial (RCEC) cells and by retinal pigment epithelial (RPE) cells in collaboration with Bruch's membrane and the choriocapillaris, respectively. Functions of the BRB are to regulate fluids and molecular movement between the ocular vascular beds and retinal tissues and to prevent leakage of macromolecules and other potentially harmful agents into the retina, keeping the microenvironment of the retina and retinal neurons. These functions are mainly attributed to absent fenestrations of RCECs, tight junctions, expression of a great diversity of transporters, and coverage of pericytes and glial cells. BRB existence also becomes a reason that systemic administration for some drugs is not suitable for the treatment of retinal diseases. Some diseases (such as diabetes and ischemia-reperfusion) impair BRB function via altering tight junctions, RCEC death, and transporter expression. This chapter will illustrate function of BRB, expressions and functions of these transporters, and their clinical significances.

Protective Effects of Curcumin Ester Prodrug, Curcumin Diethyl Disuccinate against H2O2-Induced Oxidative Stress in Human Retinal Pigment Epithelial Cells: Potential Therapeutic Avenues for Age-Related Macular Degeneration

Oxidative stress-induced damage to the retinal pigmented epithelium (RPE), a specialised post-mitotic monolayer that maintains retinal homeostasis, contributes to the development of age-related macular degeneration (AMD). Curcumin (Cur), a naturally occurring antioxidant, was previously shown to have the ability to protect RPE cells from oxidative stress. However, poor solubility and bioavailability makes Cur a poor therapeutic agent. As prodrug approaches can mitigate these limitations, we compared the protective properties of the Cur prodrug curcumin diethyl disuccinate (CurDD) against Cur in relation to oxidative stress induced in human ARPE-19 cells. Both CurDD and Cur significantly decreased H2O2-induced reactive oxygen species (ROS) production and protected RPE cells from oxidative stress-induced death. Both drugs exerted their protective effects through the modulation of p44/42 (ERK) and the involvement of downstream molecules Bax and Bcl-2. Additionally, the expression of antioxidant enzymes HO-1 and NQO1 was also enhanced in cells treated with CurDD and Cur. In all cases, CurDD was more effective than its parent drug against oxidative stress-induced damage to ARPE-19 cells. These findings highlight CurDD as a more potent drug compared to Cur against oxidative stress and indicate that its protective effects are exerted through modulation of key apoptotic and antioxidant molecular pathways.

Comparison of histologic findings in age-related macular degeneration with RPE flatmount images

Purpose

To visualize and analyze ex vivo flatmounted human RPE morphology from patients with age-related macular degeneration (AMD), and to compare the morphology with histologic findings. To establish whether the sub-RPE structures identified en face in RPE flatmount preparations are drusen with histopathological registration in serial sections. To detect characteristic patterns found en face in RPE with the same structures in histological cross sections from eyes from cadavers of patients with AMD.

Methods

Twenty-eight postmortem eyes from 14 patients (16 eyes with AMD and 12 age-matched control eyes) were oriented and microdissected yielding a RPE-choroid preparation. The tissues were flatmounted, stained with Alexa Fluor 635 Phalloidin (AF635-phalloidin) for f-actin and propidium iodide for DNA, and imaged using confocal microscopy. Portions of tissue from macular regions were processed for electron microscopic examination. After confocal imaging, the samples were remounted for histologic processing, embedded in paraffin, and serially sectioned perpendicular to the plane of the RPE-choroid sheet. Scaled two-dimensional (2D) maps of drusen locations found with the histological cross sections were constructed and correlated with the en face confocal microscopic images.

Results

Twenty-eight postmortem eyes with a mean time of death to tissue preservation of 23.7 h (range 8.0–51 h) from 14 donors (seven women and seven men) with an average age of 78 years (range 60–93 years) were evaluated. Eight donors had AMD, and six served as controls. Scattered small, hard drusen were present in the periphery of the eyes with AMD and the healthy eyes. The macular region of the eyes with AMD contained small (<63 µm), medium (63.0–124 µm), and large ( ≥ 125 µm) drusen. The RPE was arranged in rosette-like structures overlying small drusen, attenuated overlying medium-sized drusen, and consisted of large multinucleated cells overlying large drusen. The RPE in the area of geographic atrophy was attenuated and depigmented.

Conclusions

Confocal images of flatmounts from eyes with AMD showed RPE patterns overlying various types of drusen and geographic atrophy that correlated with histologic characteristics. We propose RPE repair mechanisms that may result in the patterns that we observed.

Zika virus induces strong inflammatory responses and impairs homeostasis and function of the human retinal pigment epithelium

BACKGROUND

Zika virus (ZIKV) has recently re-emerged as a pathogenic agent with epidemic capacities as was well illustrated in South America. Because of the extent of this health crisis, a number of more serious symptoms have become associated with ZIKV infection than what was initially described. In particular, neuronal and ocular disorders have been characterized, both in infants and in adults. Notably, the macula and the retina can be strongly affected by ZIKV, possibly by a direct effect of the virus. This is supported by the detection of replicative and infectious virus in lachrimal fluid in human patients and mouse models.

METHODS

Here, we used an innovative, state-of-the-art iPSC-derived human retinal pigment epithelium (RPE) model to study ZIKV retinal impairment.

FINDINGS

We showed that the human RPE is highly susceptible to ZIKV infection and that a ZIKV African strain was more virulent and led to a more potent epithelium disruption and stronger anti-viral response than an Asian strain, suggesting lineage differences. Moreover, ZIKV infection led to impaired membrane dynamics involved in endocytosis, organelle biogenesis and potentially secretion, key mechanisms of RPE homeostasis and function.

INTERPRETATION

Taken together, our results suggest that ZIKV has a highly efficient ocular tropism, which creates a strong inflammatory environment that could have acute or chronic adverse effects. FUND: This work was funded by Retina France, REACTing and La Région Languedoc-Roussillon.

Quantitative proteomic analysis of aqueous humor from patients with drusen and reticular pseudodrusen in age-related macular degeneration

Background

To identify novel biomarkers related to the pathogenesis of dry age-related macular degeneration (AMD), we adopted a human retinal pigment epithelial (RPE) cell culture model that mimics some features of dry AMD including the accumulation of intra- and sub-RPE deposits. Then, we investigated the aqueous humor (AH) proteome using a data-independent acquisition method (sequential window acquisition of all theoretical fragment ion mass spectrometry) for dry AMD patients and controls.

Methods

After uniformly pigmented polarized monolayers of human fetal primary RPE (hfRPE) cells were established, the cells were exposed to 4-hydroxy-2-nonenal (4-HNE), followed by Western blotting, immunofluorescence analysis and ELISA of cells or conditioned media for several proteins of interest. Data-dependent acquisition for identification of the AH proteome and SWATH-based mass spectrometry were performed for 11 dry AMD patients according to their phenotypes (including soft drusen and reticular pseudodrusen [RPD]) and 2 controls (3 groups).

Results

Increased intra- and sub-RPE deposits were observed in 4-HNE-treated hfRPE cells compared with control cultures based on APOA1, cathepsin D, and clusterin immunoreactivity. Additionally, the differential abundance of proteins in apical and basal chambers with or without 4-HNE treatment confirmed the polarized secretion of proteins from hfRPE cells. A total of 119 proteins were quantified in dry AMD patients and controls by SWATH-MS. Sixty-five proteins exhibited significantly altered abundance among the three groups. A two-dimensional principal component analysis plot was generated to identify typical proteins related to the pathogenesis of dry AMD. Among the identified proteins, eight proteins, including APOA1, CFHR2, and CLUS, were previously considered major components or regulators of drusen. Three proteins (SERPINA4, LUM, and KERA proteins) have not been previously described as components of drusen or as being related to dry AMD. Interestingly, the LUM and KERA proteins, which are related to extracellular matrix organization, were upregulated in both RPD and soft drusen.

Conclusions

Differential protein expression in the AH between patients with drusen and RPD was quantified using SWATH-MS in the present study. Detailed proteomic analyses of dry AMD patients might provide insights into the in vivo biology of drusen and RPD.

Electronic supplementary material

The online version of this article (10.1186/s12886-018-0941-9) contains supplementary material, which is available to authorized users.

The effects of zinc supplementation on primary human retinal pigment epithelium

Population-based and interventional studies have shown that elevated zinc levels can reduce the progression to advanced age-related macular degeneration. The objective of this study was to assess whether elevated extracellular zinc has a direct effect on retinal pigment epithelial cells (RPE), by examining the phenotype and molecular characteristics of increased extracellular zinc on human primary RPE cells. Monolayers of human foetal primary RPE cells were grown on culture inserts and maintained in medium supplemented with increasing total concentrations of zinc (0, 75, 100, 125 and 150 μM) for up to 4 weeks. Changes in cell viability and differentiation as well as expression and secretion of proteins were investigated. RPE cells developed a confluent monolayer with cobblestone morphology and transepithelial resistance (TER) >200 Ω*cm² within 4 weeks. There was a zinc concentration-dependent increase in TER and pigmentation, with the largest effects being achieved by the addition of 125 μM zinc to the culture medium, corresponding to 3.4 nM available (free) zinc levels. The cells responded to addition of zinc by significantly increasing the expression of Retinoid Isomerohydrolase (RPE65) gene; cell pigmentation; Premelanosome Protein (PMEL17) immunoreactivity; and secretion of proteins including Apolipoprotein E (APOE), Complement Factor H (CFH), and High-Temperature Requirement A Serine Peptidase 1 (HTRA1) without an effect on cell viability. This study shows that elevated extracellular zinc levels have a significant and direct effect on differentiation and function of the RPE cells in culture, which may explain, at least in part, the positive effects seen in clinical settings. The results also highlight that determining and controlling of available, as opposed to total added, zinc will be essential to be able to compare results obtained in different laboratories.

N-Terminomics identifies HtrA1 cleavage of thrombospondin-1 with generation of a proangiogenic fragment in the polarized retinal pigment epithelial cell model of age-related macular degeneration

Age-related macular degeneration (AMD) is the leading cause of irreversible blindness in the elderly population. Variants in the HTRA1-ARMS2 locus have been linked to increased AMD risk. In the present study we investigated the impact of elevated HtrA1 levels on the retina pigment epithelial (RPE) secretome using a polarized culture system. Upregulation of HtrA1 alters the abundance of key proteins involved in angiogenesis and extracellular matrix remodeling. Thrombospondin-1, an angiogenesis modulator, was identified as a substrate for HtrA1 using terminal amine isotope labeling of substrates in conjunction with HtrA1 specificity profiling. HtrA1 cleavage of thrombospondin-1 was further corroborated by in vitro cleavage assays and targeted proteomics together with small molecule inhibition of HtrA1. While thrombospondin-1 is anti-angiogenic, the proteolytically released N-terminal fragment promotes the formation of tube-like structure by endothelial cells. Taken together, our findings suggest a mechanism by which increased levels of HtrA1 may contribute to AMD pathogenesis. The proteomic data has been deposited to the ProteomeXchange Consortium via the PRIDE partner repository with the data set identifier. For quantitative secretome analysis, project accession: PXD007691, username: reviewer45093@ebi.ac.uk, password: 1FUpS6Yq. For TAILS analysis, project accession: PXD007139, username: reviewer76731@ebi.ac.uk, password: sNbMp7xK.

Directional ABCA1-mediated cholesterol efflux and apoB-lipoprotein secretion in the retinal pigment epithelium

Cholesterol-containing soft drusen and subretinal drusenoid deposits (SDDs) occur at the basolateral and apical side of the retinal pigment epithelium (RPE), respectively, in the chorioretina and are independent risk factors for late age-related macular degeneration (AMD). Cholesterol in these deposits could originate from the RPE as nascent HDL or apoB-lipoprotein. We characterized cholesterol efflux and apoB-lipoprotein secretion in RPE cells. Human RPE cells, ARPE-19, formed nascent HDL that was similar in physicochemical properties to nascent HDL formed by other cell types. In highly polarized primary human fetal RPE (phfRPE) monolayers grown in low-lipid conditions, cholesterol efflux to HDL was moderately directional to the apical side and much stronger than ABCA1-mediated efflux to apoA-I at both sides; ABCA1-mediated efflux was weak and equivalent between the two sides. Feeding phfRPE monolayers with oxidized or acetylated LDL increased intracellular levels of free and esterified cholesterol and substantially raised ABCA1-mediated cholesterol efflux at the apical side. phfRPE monolayers secreted apoB-lipoprotein preferentially to the apical side in low-lipid and oxidized LDL-feeding conditions. These findings together with evidence from human genetics and AMD pathology suggest that RPE-generated HDL may contribute lipid to SDDs.

A convenient protocol for establishing a human cell culture model of the outer retina

The retinal pigment epithelium (RPE) plays a key role in the pathogenesis of several blinding retinopathies. Alterations to RPE structure and function are reported in Age-related Macular Degeneration, Stargardt and Best disease as well as pattern dystrophies. However, the precise role of RPE cells in disease aetiology remains incompletely understood. Many studies into RPE pathobiology have utilised animal models, which only recapitulate limited disease features. Some studies are also difficult to carry out in animals as the ocular space remains largely inaccessible to powerful microscopes. In contrast, in-vitro models provide an attractive alternative to investigating pathogenic RPE changes associated with age and disease. In this article we describe the step-by-step approach required to establish an experimentally versatile in-vitro culture model of the outer retina incorporating the RPE monolayer and supportive Bruch's membrane (BrM). We show that confluent monolayers of the spontaneously arisen human ARPE-19 cell-line cultured under optimal conditions reproduce key features of native RPE. These models can be used to study dynamic, intracellular and extracellular pathogenic changes using the latest developments in microscopy and imaging technology. We also discuss how RPE cells from human foetal and stem-cell derived sources can be incorporated alongside sophisticated BrM substitutes to replicate the aged/diseased outer retina in a dish. The work presented here will enable users to rapidly establish a realistic in-vitro model of the outer retina that is amenable to a high degree of experimental manipulation which will also serve as an attractive alternative to using animals. This in-vitro model therefore has the benefit of achieving the 3Rs objective of reducing and replacing the use of animals in research. As well as recapitulating salient structural and physiological features of native RPE, other advantages of this model include its simplicity, rapid set-up time and unlimited scope for detailed single-cell resolution and matrix studies.

The role of the retinal pigment epithelium and Müller cells secretome in neovascular retinal pathologies

Secreted trophic factors are key to maintain the structural and functional integrity of the retina, as they regulate cellular pathways responsible for survival, function, and response to injury. Nevertheless, these same factors can also be involved in retinal pathologies, as a consequence of the impairment of the secretory function of cells. The cells considered as major contributors to the retinal secretome are the retinal pigmented epithelium (RPE) and Müller cells. Their role in the pathophysiology of the most common neovascular pathologies in the retina - Age-related Macular Degeneration (AMD), Diabetic Retinopathy (DR), and Retinopathy of Prematurity (ROP) - is highlighted in this short review, together with current trophic factor-based therapies, which are mainly focused on controlling inflammation, cell survival, and angiogenesis.

Polarized Human Retinal Pigment Epithelium Exhibits Distinct Surface Proteome on Apical and Basal Plasma Membranes

Surface proteins localized on the apical and basal plasma membranes are required for a cell to sense its environment and relay changes in ionic, cytokine, chemokine, and hormone levels to the inside of the cell. In a polarized cell, surface proteins are differentially localized on the apical or the basolateral sides of the cell. The retinal pigment epithelium (RPE) is an example of a polarized cell that performs a variety of functions that are dependent on its polarized state including trafficking of ions, fluid, and metabolites across the RPE monolayer. These functions are absolutely crucial for maintaining the health and integrity of adjacent photoreceptors, the photosensitive cells of the retina. Here we present a series of approaches to identify and validate the polarization state of cultured primary human RPE cells using immunostaining for RPE apical/basolateral markers, polarized cytokine secretion, electrophysiology, fluid transport, phagocytosis, and identification of plasma membrane proteins through cell surface capturing technology. These approaches are currently being used to validate the polarized state and the epithelial phenotype of human induced pluripotent stem (iPS) cell derived RPE cells. This work provides the basis for developing an autologous cell therapy for age-related macular degeneration using patient specific iPS cell derived RPE.

Design, development and characterization of synthetic Bruch's membranes

Age-related macular degeneration (AMD) is a leading cause of blindness, and dry AMD has no effective treatment. Retinal constructs comprising retinal pigment epithelium (RPE) cells supported by electrospun scaffolds have been investigated to treat dry AMD. However, electrospun scaffolds studied to-date do not mimic the structural microenvironment of human Bruch's membrane (BM), essential for native-like RPE monolayers. The aim of this study was to develop a structurally biomimetic scaffold designed to support a functional RPE monolayer, comprising porous, electrospun nanofibrous membranes (ENMs), coated with laminin, mimicking the inner collagenous layer (ICL) and basal RPE lamina respectively, the cell supporting layers of the BM. In vitro evaluation showed 70nm PLLA ENMs adsorbed high amounts of laminin and supported functional RPE monolayers, exhibiting 3D polygonal-cobblestone morphology, apical microvilli, basal infoldings, high transepithelial resistance (TER), phagocytic activity and expression of signature RPE markers. 70nm PLLA ENMs were successfully implanted into the subretinal space of RCS-rdy+p+/LAV rats, also commonly know as rdy rats. At week 4, in the absence of immunosuppressants, implanted PLLA ENMs were surrounded by a significantly low number of activated microglial cells, compared to week 1, indicating no adverse long-term immune response. In conclusion, we successfully designed and tested ENMs emulating the RPE cell supporting layers of the BM, and found 70nm PLLA ENMs to be best suited as scaffolds for fabricating retinal constructs.

STATEMENT OF SIGNIFICANCE

Age related macular degeneration (AMD) is a leading cause of vision loss in the developed world, with an increasing number of people suffering from blindness or severe visual impairment. Transplantation of retinal pigment epithelium (RPE) cells supported on a synthetic, biomimetic-like Bruch's membrane (BM) is considered a promising treatment. However, the synthetic scaffolds used do not mimic the microenvironment of the RPE cell supporting layers, required for the development of a functional RPE monolayer. This study indicated that porous, laminin coated, 70nm PLLA ENMs supported functional RPE monolayers, exhibiting 3D polygonal-cobblestone morphology, apical microvilli, basal infoldings, high transepithelial resistance (TER), phagocytic activity and expression of signature RPE markers. These findings indicate the potential clinical use of porous, laminin coated, 70nm PLLA ENMs in fabricating retinal constructs aimed at treating dry AMD.

Identifying characteristic features of the retinal and choroidal vasculature in choroideremia using optical coherence tomography angiography

PurposeUsing optical coherence tomography angiography (OCTA) to investigate the area with flow in the superficial retinal vessel network (SVRN) and choriocapillaris (CC) layer among male subjects with choroideremia (CHM), female carriers, and normal controls to identify vascular changes.Patients and methodsImages of SRVN and CC layer were acquired in 9 affected males, 5 female carriers, and 14 age- and gender-matched controls using the Angiovue software of the RTVue XR Avanti.ResultsThe mean age was 33 years for affected male CHM patients (median 30 years), 46 years for female carriers (median 53 years), and 39 years for controls (median 38.5). Mean SRVN area±SD in subjects with CHM was 12.93±2.06 mm², in carrier subjects 15.36±0.60 mm², and in controls 15.30±1.35 mm² (P<0.01). The mean CC area±SD with flow was 6.97±5.26 mm² in CHM subjects, 21.65±0.17 mm² in carriers and 21.36±0.76 mm² in controls (P<0.01). SRVN and CC area with flow showed a negative correlation in CHM subjects with the age (r=-0.86; P<0.003 and r=-0.77; P<0.01, respectively). CC area with flow had a positive correlation with SRVN (r=0.83, P<0.001). Overall, visual acuity had a negative correlation with SRVN and CC area with flow (r=-0.67, P<0.001 and r=-0.57, P<0.002, respectively).

CONCLUSIONS

This is the first study to highlight changes in the SRVN in CHM subjects. OCTA detected a reduced area with flow in both retinal and choroidal circulations, and may be a useful tool for monitoring natural history and disease progression in forthcoming clinical trials.

The pigmented epithelium, a bright partner against photoreceptor degeneration

Sight depends on the intimate association between photoreceptors and pigment epithelial cells. The evolutionary origin of this cellular tandem can be traced back to the emergence of bilateral animals, at least 450 million years ago, as they define the minimal unit of the ancestral prototypic eye. Phototransduction is a demanding process from the energetic and homeostatic points of view, and not surprisingly photoreceptive cells are particularly susceptible to damage and degeneration. Here, we will examine the different ancillary roles that the pigmented cells play in the physiology and homeostasis of photoreceptors, linking each one of these processes to the most common hereditary retinal diseases. We will discuss the challenges and opportunities of recent therapeutic advances based on cell and gene replacement. The transition from animal models to clinical trials will be addressed for each one of the different therapeutic strategies with a special focus on those depending on retinal-pigmented epithelial cells. Finally, we will discuss the potential impact of combining CRISPR technologies with gene and cell therapy approaches, which - in the frame of the personalized medicine revolution - may constitute a leap forward in the treatment of retinal dystrophies.

Atorvastatin Promotes Phagocytosis and Attenuates Pro-Inflammatory Response in Human Retinal Pigment Epithelial Cells

Phagocytosis of daily shed photoreceptor outer segments is an important function of the retinal pigment epithelium (RPE) and it is essential for retinal homeostasis. RPE dysfunction, especially impairment of its phagocytic ability, plays an essential role in the pathogenesis of age-related macular degeneration (AMD). Statins, or HMG CoA (3-hydroxy-3-methylglutaryl-coenzyme A) reductase inhibitors, are drugs with multiple properties that have been extensively used to treat hyperlipidemia. However, their effect on RPE cells has not been fully elucidated. Here we report that high dose atorvastatin increased the phagocytic function of ARPE-19 cells, as well as rescue the cells from the phagocytic dysfunction induced by cholesterol crystals and oxidized low-density lipoproteins (ox-LDL), potentially by increasing the cellular membrane fluidity. Similar effects were observed when evaluating two other hydrophobic statins, lovastatin and simvastatin. Furthermore, atorvastatin was able to block the induction of interleukins IL-6 and IL-8 triggered by pathologic stimuli relevant to AMD, such as cholesterol crystals and ox-LDL. Our study shows that statins, a well-tolerated class of drugs with rare serious adverse effects, help preserve the phagocytic function of the RPE while also exhibiting anti-inflammatory properties. Both characteristics make statins a potential effective medication for the prevention and treatment of AMD.

Methods for culturing retinal pigment epithelial cells: a review of current protocols and future recommendations

The retinal pigment epithelium is an important part of the vertebrate eye, particularly in studying the causes and possible treatment of age-related macular degeneration. The retinal pigment epithelium is difficult to access in vivo due to its location at the back of the eye, making experimentation with age-related macular degeneration treatments problematic. An alternative to in vivo experimentation is cultivating the retinal pigment epithelium in vitro, a practice that has been going on since the 1970s, providing a wide range of retinal pigment epithelial culture protocols, each producing cells and tissue of varying degrees of similarity to natural retinal pigment epithelium. The purpose of this review is to provide researchers with a ready list of retinal pigment epithelial protocols, their effects on cultured tissue, and their specific possible applications. Protocols using human and animal retinal pigment epithelium cells, derived from tissue or cell lines, are discussed, and recommendations for future researchers included.

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).

Epigenetic Mechanisms of the Aging Human Retina

Degenerative retinal diseases, such as glaucoma, age-related macular degeneration, and diabetic retinopathy, have complex etiologies with environmental, genetic, and epigenetic contributions to disease pathology. Much effort has gone into elucidating both the genetic and the environmental risk factors for these retinal diseases. However, little is known about how these genetic and environmental risk factors bring about molecular changes that lead to pathology. Epigenetic mechanisms have received extensive attention of late for their promise of bridging the gap between environmental exposures and disease development via their influence on gene expression. Recent studies have identified epigenetic changes that associate with the incidence and/or progression of each of these retinal diseases. Therefore, these epigenetic modifications may be involved in the underlying pathological mechanisms leading to blindness. Further genome-wide epigenetic studies that incorporate well-characterized tissue samples, consider challenges similar to those relevant to gene expression studies, and combine the genome-wide epigenetic data with genome-wide genetic and expression data to identify additional potentially causative agents of disease are needed. Such studies will allow researchers to create much-needed therapeutics to prevent and/or intervene in disease progression. Improved therapeutics will greatly enhance the quality of life and reduce the burden of disease management for millions of patients living with these potentially blinding conditions.

Pathological Effects of Mutant C1QTNF5 (S163R) Expression in Murine Retinal Pigment Epithelium

PURPOSE

The mutation S163R in complement C1q tumor necrosis factor-related protein-5 (C1QTNF5) causes an autosomal dominant disorder known as late-onset retinal degeneration (L-ORD). In this study, our goal is to evaluate the consequences of mutant S163R C1QTNF5 expression in mouse RPE following its delivery using an adeno-associated viral (AAV) vector.

METHODS

We generated AAV vectors containing either human wild-type C1QTNF5 or mutant S163R C1QTNF5 driven by an RPE-specific BEST1 promoter, and delivered them subretinally into one eye of adult C57BL/6 mice. Transgene expression was detected by immunohistochemistry. Retinal function was assessed by full-field ERG. Pathological changes were further examined by digital fundus imaging and spectral-domain optical coherence tomography (SD-OCT).

RESULTS

We show that the AAV-expressed mutant S163R leads to pathological effects similar to some of those found in patients with advanced L-ORD, including RPE thinning, RPE cell loss, and retinal degeneration. In addition, we provide in vivo evidence that mutant S163R C1QTNF5 can form large, transparent, spherical intracellular aggregates throughout the RPE, which are detectable by light microscopy. In contrast to AAV-expressed wild-type C1QTNF5, which is secreted apically from the RPE toward the photoreceptor cells and the outer limiting membrane, the S163R mutant is primarily routed toward the basal side of RPE, where it forms thick, extracellular deposits over time.

CONCLUSIONS

Adeno-associated viral-targeted expression of mutant S163R in the RPE represents a useful approach for quickly generating animal models that mimic pathological features of L-ORD and offers the potential to understand disease mechanisms and develop therapeutic strategies.

A Bruch's membrane substitute fabricated from silk fibroin supports the function of retinal pigment epithelial cells in vitro

Silk fibroin provides a promising biomaterial for ocular tissue reconstruction, including the damaged outer blood-retinal barrier of patients afflicted with age-related macular degeneration (AMD). The aim of the present study was to evaluate the function of retinal pigment epithelial (RPE) cells in vitro, when grown on fibroin membranes manufactured to a thickness similar to that of Bruch's membrane (3 µm). Confluent cultures of RPE cells (ARPE-19) were established on fibroin membranes and maintained under conditions designed to promote maturation over 4 months. Control cultures were grown on polyester cell culture well inserts (Transwell^® ). Cultures established on either material developed a cobblestone morphology, with partial pigmentation, within 12 weeks. Immunocytochemistry at 16 weeks revealed a similar distribution pattern between cultures for F-actin, ZO-1, ezrin, cytokeratin pair 8/18, RPE-65 and Na⁺ /K⁺ -ATPase. Electron microscopy revealed that cultures grown on fibroin displayed a rounder apical surface with a more dense distribution of microvilli. Both cultures avidly ingested fluorescent microspheres coated with vitronectin and bovine serum albumin (BSA), but not controls coated with BSA alone. VEGF and PEDF were detected in the conditioned media collected from above and below the two membrane types. Levels of PEDF were significantly higher than for VEGF on both membranes and a trend was observed towards larger amounts of PEDF in apical compartments. These findings demonstrated that RPE cell functions on fibroin membranes are equivalent to those observed for standard test materials (polyester membranes). As such, these studies support advancement to studies of RPE cell implantation on fibroin membranes in a preclinical model. Copyright © 2015 John Wiley & Sons, Ltd.

Focus on Kir7.1: physiology and channelopathy

Genetic studies have linked alterations in Kir7.1 channel to diverse pathologies. We summarize functional relevance of Kir7.1 channel in retinal pigment epithelium (RPE), regulation of channel function by various cytoplasmic metabolites, and mutations that cause channelopathies. At the apical membrane of RPE, K(+) channels contribute to subretinal K(+) homeostasis and support Na(+)/K(+) pump and Na(+)-K(+)-2Cl(-) cotransporter function by providing a pathway for K(+) secretion. Electrophysiological studies have established that barium- and cesium-sensitive inwardly rectifying K(+) (Kir) channels make up a major component of the RPE apical membrane K(+) conductance. Native human RPE expresses transcripts for Kir1.1, Kir2.1, Kir2.2, Kir3.1, Kir3.4, Kir4.2, and Kir6.1, albeit at levels at least 50-fold lower than Kir7.1. Kir7.1 is structurally similar to other Kir channels, consisting of 2 trans-membrane domains, a pore-forming loop that contains the selectivity filter, and 2 cytoplasmic polar tails. Within the cytoplasmic structure, clusters of amino acid sequences form regulatory domains that interact with cellular metabolites and control the opening and closing of the channel. Recent evidence indicated that intrinsic sequence motifs present in Kir7.1 control surface expression. Mutant Kir7.1 channels are associated with inherited eye pathologies such as Snowflake Vitreoretinal Degeneration (SVD) and Lebers Congenital Amaurosis (LCA16). Based on the current evidence, mutations implicated in channelopathies have the potential to be used for genetic testing to diagnose blindness due to Kir7.1.

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.

Basal and apical regulation of VEGF-A and placenta growth factor in the RPE/choroid and primary RPE

PURPOSE

Members of the vascular endothelial growth factor (VEGF) family are strongly involved in pathological processes in the retina, such as age-related macular degeneration and diabetic retinopathy. Cells of the retinal pigment epithelium (RPE) constitutively secrete VEGF-A, and the secretion of placental growth factor (PlGF) has also been described. RPE cells are strongly polarized cells with different secretome at the apical and basal side. In this study, we evaluated the basal and apical regulation of VEGF-A and PlGF secretion in RPE/choroid explants and primary RPE cells.

METHODS

RPE/choroid tissue explants were prepared from porcine eyes and cultivated in modified Ussing chambers, separating apical (RPE) and basal (choroid) supernatant. Primary RPE cells were also prepared from porcine eyes and cultivated on Transwell plates. Explants and cells were treated with inhibitors for VEGFR-2 (SU1498), p38 (SB203580), and the transcription factors nuclear factor-kappa B (NF-κB) and SP-1 (mithramycin), respectively. VEGF-A and PlGF content was evaluated with enzyme-linked immunosorbent assay (ELISA). In addition, western blots were performed.

RESULTS

In the RPE/choroid, VEGF-A can initially be found on the apical and basal sides with significantly more pronounced secretion on the basal side. VEGF-A secretion is differentially regulated on the apical and basal sides, with the inhibition of SP-1 and NF-κB showing strong effects apically and basally after 24 h and 48 h, the inhibition of p38 displaying its effect mainly on the basal side with some effect apically after 48 h, and the inhibition of VEGFR-2 reducing the secretion of VEGF only on the apical side at 24 h and 48 h. In the RPE cell culture, similar effects were found, with inhibition of NF-κB or SP-1 displaying a strong decrease in VEGF-A on both sides, and p38 inhibition displaying only an inhibitory effect on the basal side. In contrast, an apical effect of VEGFR-2 inhibition was not found. However, the western blot experiments exhibited a significant decrease in the VEGF-A protein under SU1498 treatment. In the RPE/choroid organ cultures, PlGF was initially found mainly on the basal site with only minute amounts of PlGF found apically. NF-κB and SP-1 were strongly involved in PlGF regulation apically and basally, while VEGFR2 and to a lesser degree p38 displayed some regulation at the basal site. In the primary RPE cell culture, PlGF was not found on the apical or basal side.

CONCLUSIONS

VEGF-A and PlGF were constitutively secreted and regulated by the RPE/choroid complex, with PlGF secreted mainly by the choroid. Although the transcription factors NF-κB and SP-1 were involved in apical and basal regulation of both growth factors, VEGFR-2 displayed a strong polarity, with regulation of apical VEGF-A and basal PlGF secretion.