Extracellular matrix nitration alters growth factor release and activates bioactive complement in human retinal pigment epithelial cells

The extracellular microenvironment in immune dysregulation and inflammation in retinal disorders

Inherited retinal dystrophies (IRDs) as well as genetically complex retinal phenotypes represent a heterogenous group of ocular diseases, both on account of their phenotypic and genotypic characteristics. Therefore, overlaps in clinical features often complicate or even impede their correct clinical diagnosis. Deciphering the molecular basis of retinal diseases has not only aided in their disease classification but also helped in our understanding of how different molecular pathologies may share common pathomechanisms. In particular, these relate to dysregulation of two key processes that contribute to cellular integrity, namely extracellular matrix (ECM) homeostasis and inflammation. Pathological changes in the ECM of Bruch's membrane have been described in both monogenic IRDs, such as Sorsby fundus dystrophy (SFD) and Doyne honeycomb retinal dystrophy (DHRD), as well as in the genetically complex age-related macular degeneration (AMD) or diabetic retinopathy (DR). Additionally, complement system dysfunction and distorted immune regulation may also represent a common connection between some IRDs and complex retinal degenerations. Through highlighting such overlaps in molecular pathology, this review aims to illuminate how inflammatory processes and ECM homeostasis are linked in the healthy retina and how their interplay may be disturbed in aging as well as in disease.

The retinal pigmentation pathway in human albinism: Not so black and white

Albinism is a pigment disorder affecting eye, skin and/or hair. Patients usually have decreased melanin in affected tissues and suffer from severe visual abnormalities, including foveal hypoplasia and chiasmal misrouting. Combining our data with those of the literature, we propose a single functional genetic retinal signalling pathway that includes all 22 currently known human albinism disease genes. We hypothesise that defects affecting the genesis or function of different intra-cellular organelles, including melanosomes, cause syndromic forms of albinism (Hermansky-Pudlak (HPS) and Chediak-Higashi syndrome (CHS)). We put forward that specific melanosome impairments cause different forms of oculocutaneous albinism (OCA1-8). Further, we incorporate GPR143 that has been implicated in ocular albinism (OA1), characterised by a phenotype limited to the eye. Finally, we include the SLC38A8-associated disorder FHONDA that causes an even more restricted "albinism-related" ocular phenotype with foveal hypoplasia and chiasmal misrouting but without pigmentation defects. We propose the following retinal pigmentation pathway, with increasingly specific genetic and cellular defects causing an increasingly specific ocular phenotype: (HPS1-11/CHS: syndromic forms of albinism)-(OCA1-8: OCA)-(GPR143: OA1)-(SLC38A8: FHONDA). Beyond disease genes involvement, we also evaluate a range of (candidate) regulatory and signalling mechanisms affecting the activity of the pathway in retinal development, retinal pigmentation and albinism. We further suggest that the proposed pigmentation pathway is also involved in other retinal disorders, such as age-related macular degeneration. The hypotheses put forward in this report provide a framework for further systematic studies in albinism and melanin pigmentation disorders.

Human iPSC- and Primary-Retinal Pigment Epithelial Cells for Modeling Age-Related Macular Degeneration

Primary cultures of retinal pigment epithelium (RPE) from human adult donors (haRPE) and induced pluripotent stem cell derived-RPE (iPSC-RPE) are valuable model systems for gaining mechanistic insight and for testing potential therapies for age-related macular degeneration (AMD). This study evaluated the treatment response of haRPE and iPSC-RPE to oxidative stress and potential therapeutics addressing mitochondrial defects. haRPE and iSPC-RPE were derived from donors with or without AMD. Mitochondrial function was measured after treatment with menadione, AICAR, or trehalose and the response to treatment was compared between cell models and by disease status. In a subset of samples, haRPE and iPSC-RPE were generated from the same human donor to make a side-by-side comparison of the two cell models' response to treatment. Disease-specific responses to all three treatments was observed in the haRPE. In contrast, iPSC-RPE had a similar response to all treatments irrespective of disease status. Analysis of haRPE and iPSC-RPE generated from the same human donor showed a similar response for donors without AMD, but there were significant differences in treatment response between cell models generated from AMD donors. These results support the use of iPSC-RPE and haRPE when investigating AMD mechanisms and new therapeutics but indicates that attention to experimental conditions is required.

The Long Pentraxin PTX3 as a New Biomarker and Pharmacological Target in Age-Related Macular Degeneration and Diabetic Retinopathy

Age related macular degeneration (AMD) and diabetic retinopathy (DR) are multifactorial, neurodegenerative and inflammatory diseases of the eye primarily involving cellular and molecular components of the outer and inner blood-retina barriers (BRB), respectively. Largely contributed by genetic factors, particularly polymorphisms in complement genes, AMD is a paradigm of retinal immune dysregulation. DR, a major complication of diabetes mellitus, typically presents with increased vascular permeability and occlusion of the retinal vasculature that leads, in the proliferative form of the disease, to neovascularization, a pathogenic trait shared with advanced AMD. In spite of distinct etiology and clinical manifestations, both pathologies share common drivers, such as chronic inflammation, either of immune (in AMD) or metabolic (in DR) origin, which initiates and propagates degeneration of the neural retina, yet the underlying mechanisms are still unclear. As a soluble pattern recognition molecule with complement regulatory functions and a marker of vascular damage, long pentraxin 3 (PTX3) is emerging as a novel player in ocular homeostasis and a potential pharmacological target in neurodegenerative disorders of the retina. Physiologically present in the human eye and induced in inflammatory conditions, this protein is strategically positioned at the BRB interface, where it acts as a “molecular trap” for complement, and modulates inflammation both in homeostatic and pathological conditions. Here, we discuss current viewpoints on PTX3 and retinal diseases, with a focus on AMD and DR, the roles therein proposed for this pentraxin, and their implications for the development of new therapeutic strategies.

AMD-Like Substrate Causes Epithelial Mesenchymal Transition in iPSC-Derived Retinal Pigment Epithelial Cells Wild Type but Not C3-Knockout

The Bruch's membrane (BrM) is a five-layered extracellular matrix (ECM) that supports the retinal pigment epithelium (RPE). Normal age-related changes in the BrM may lead to RPE cell damage and ultimately to the onset and progression of age-related macular degeneration (AMD), which is the most common cause of visual loss among the elderly. A role for the complement system in AMD pathology has been established, but the disease mechanisms are poorly understood, which hampers the design of efficient therapies to treat millions of patients. In an effort to identify the mechanisms that lead from normal aging to pathology, we have developed a cell-based model using complement deficient human induced pluripotent stem cell (iPSC)-derived RPE cells cultured on an AMD-like ECM that mimics BrM. The data present evidence that changes in the ECM result in loss of differentiation and promote epithelial mesenchymal transition (EMT) of healthy RPE cells. This pathological process is mediated by complement activation and involves the formation of a randomly oriented collagen meshwork that drives the dedifferentiation of the RPE monolayer. Genetic ablation of complement component 3 has a protective effect against EMT but does not prevent the abnormal deposition of collagens. These findings offer new insights into the sequence of events that initiate AMD and may guide the design of efficient therapies to treat this disease with unmet medical needs.

The complement system in age-related macular degeneration

Age-related macular degeneration (AMD) is a chronic and progressive degenerative disease of the retina, which culminates in blindness and affects mainly the elderly population. AMD pathogenesis and pathophysiology are incredibly complex due to the structural and cellular complexity of the retina, and the variety of risk factors and molecular mechanisms that contribute to disease onset and progression. AMD is driven by a combination of genetic predisposition, natural ageing changes and lifestyle factors, such as smoking or nutritional intake. The mechanism by which these risk factors interact and converge towards AMD are not fully understood and therefore drug discovery is challenging, where no therapeutic attempt has been fully effective thus far. Genetic and molecular studies have identified the complement system as an important player in AMD. Indeed, many of the genetic risk variants cluster in genes of the alternative pathway of the complement system and complement activation products are elevated in AMD patients. Nevertheless, attempts in treating AMD via complement regulators have not yet been successful, suggesting a level of complexity that could not be predicted only from a genetic point of view. In this review, we will explore the role of complement system in AMD development and in the main molecular and cellular features of AMD, including complement activation itself, inflammation, ECM stability, energy metabolism and oxidative stress.

Altered transcriptome and disease-related phenotype emerge only after fibroblasts harvested from patients with age-related macular degeneration are differentiated into retinal pigment epithelium

We have reported previously that retinal pigment epithelium (RPE) differentiated from induced pluripotent stem cells (iPSC) generated from fibroblasts of patients with age-related macular degeneration (AMD) exhibit a retinal degenerative disease phenotype and a distinct transcriptome compared to age-matched controls. Since the genetic composition of the iPSC and RPE are inherited from fibroblasts, we investigated whether differential behavior was present in the parental fibroblasts and iPSC prior to differentiation of the cell lines into RPE. Principal component analyses revealed significant overlap (essentially no differences) in the transcriptome of fibroblasts between AMD and controls. After reprogramming, there was no significant difference in the transcriptome of iPSC generated from AMD versus normal donors. In contrast, the transcriptome of RPE derived from iPSC segregated into two distinct clusters of AMD-derived cells versus controls. Interestingly, mitochondrial dysfunction in AMD-derived RPE was evident after approximately two months in culture. Moreover, these differences in mitochondrial dysfunction were not evident in the parental fibroblasts and iPSC. This study demonstrates an altered transcriptome and impaired mitochondrial function in RPE derived from AMD patients versus controls, and demonstrates these differences are not present in the original fibroblasts or iPSC. These results suggest that pathology in AMD is triggered upon differentiation of parent cells into RPE. More study of this phenomenon could advance the current understandings of the etiology of AMD and the development of novel therapeutic targets.

Impaired Mitochondrial Function in iPSC-Retinal Pigment Epithelium with the Complement Factor H Polymorphism for Age-Related Macular Degeneration

Age-related macular degeneration (AMD), the leading cause of vision loss in the elderly, is characterized by loss of the retinal pigment epithelium (RPE). While the disease mechanism remains unclear, prior studies have linked AMD with RPE mitochondrial defects and genetic polymorphisms in the complement pathway. This study used RPE generated from induced pluripotent stem cells (iPSC-RPE), which were derived from human donors with or without AMD and genotyped for the complement factor H (CFH) AMD high-risk allele (rs1061170, Y402H) to investigate whether donor disease state or genotype had a detrimental effect on mitochondrial function and inflammation. Results show that cells derived from donors with AMD display decreased mitochondrial function under conditions of stress and elevated expression of inflammatory markers compared to iPSC-RPE from individuals without AMD. A more pronounced reduction in mitochondrial function and increased inflammatory markers was observed in CFH high-risk cells, irrespective of disease state. These results provide evidence for a previously unrecognized link between CFH and mitochondrial function that could contribute to RPE loss in AMD patients harboring the CFH high-risk genotype.

Implications of genetic variation in the complement system in age-related macular degeneration

Age-related macular degeneration (AMD) is the main cause of vision loss among the elderly in the Western world. While AMD is a multifactorial disease, the complement system was identified as one of the main pathways contributing to disease risk. The strong link between the complement system and AMD was demonstrated by genetic associations, and by elevated complement activation in local eye tissue and in the systemic circulation of AMD patients. Several complement inhibitors have been and are being explored in clinical trials, but thus far with limited success, leaving the majority of AMD patients without treatment options to date. This indicates that there is still a gap of knowledge regarding the functional implications of the complement system in AMD pathogenesis and how to bring these towards clinical translation. Many different experimental set-ups and disease models have been used to study complement activation in vivo and in vitro, and recently emerging patient-derived induced pluripotent stem cells and genome-editing techniques open new opportunities to study AMD disease mechanisms and test new therapeutic strategies in the future. In this review we provide an extensive overview of methods employed to understand the molecular processes of complement activation in AMD pathogenesis. We discuss the findings, advantages and challenges of each approach and conclude with an outlook on how recent, exciting developments can fill in current knowledge gaps and can aid in the development of effective complement-targeting therapeutic strategies in AMD.

Neovascular Macular Degeneration: A Review of Etiology, Risk Factors, and Recent Advances in Research and Therapy

Neovascular age-related macular degeneration (exudative or wet AMD) is a prevalent, progressive retinal degenerative macular disease that is characterized by neovascularization of the choroid, mainly affecting the elderly population causing gradual vision impairment. Risk factors such as age, race, genetics, iris color, smoking, drinking, BMI, and diet all play a part in nvAMD's progression, with anti-vascular endothelial growth factor (anti-VEGF) therapy being the mainstay of treatment. Current therapeutic advancements slow the progression of the disease but do not cure or reverse its course. Newer therapies such as gene therapies, Rho-kinase inhibitors, and levodopa offer potential new targets for treatment.

Retinal capillary basement membrane thickening: Role in the pathogenesis of diabetic retinopathy

Vascular basement membrane (BM) thickening has been hailed over half a century as the most prominent histological lesion in diabetic microangiopathy, and represents an early ultrastructural change in diabetic retinopathy (DR). Although vascular complications of DR have been clinically well established, specific cellular and molecular mechanisms underlying dysfunction of small vessels are not well understood. In DR, small vessels develop insidiously as BM thickening occurs. Studies examining high resolution imaging data have established BM thickening as one of the foremost structural abnormalities of retinal capillaries. This fundamental structural change develops, at least in part, from excess accumulation of BM components. Although BM thickening is closely associated with the development of DR, its contributory role in the pathogenesis of DR is coming to light recently. DR develops over several years before clinical manifestations appear, and it is during this clinically silent period that hyperglycemia induces excess synthesis of BM components, contributes to vascular BM thickening, and promotes structural and functional lesions including cell death and vascular leakage in the diabetic retina. Studies using animal models show promising results in preventing BM thickening with subsequent beneficial effects. Several gene regulatory approaches are being developed to prevent excess synthesis of vascular BM components in an effort to reduce BM thickening. This review highlights current understanding of capillary BM thickening development, role of BM thickening in retinal vascular lesions, and strategies for preventing vascular BM thickening as a potential therapeutic strategy in alleviating characteristic lesions associated with DR.

Properdin Modulates Complement Component Production in Stressed Human Primary Retinal Pigment Epithelium Cells

The retinal pigment epithelium (RPE) maintains visual function and preserves structural integrity of the retina. Chronic dysfunction of the RPE is associated with retinal degeneration, including age-related macular degeneration (AMD). The AMD pathogenesis includes both increased oxidative stress and complement dysregulation. Physiological sources of oxidative stress in the retina are well known, while complement sources and regulation are still under debate. Using human primary RPE (hpRPE) cells, we have established a model to investigate complement component expression on transcript and protein level in AMD-risk and non-risk hpRPE cells. We evaluated the effect of properdin, a complement stabilizer, on the hpRPE cell-dependent complement profile exposed to oxidative stress. hpRPE cells expressed complement components, receptors and regulators. Complement proteins were also stored and secreted by hpRPE cells. We associated AMD-risk single nucleotide polymorphisms with an increased secretion of complement factors D (CFD) and I (CFI). Furthermore, we detected hpRPE cell-associated complement activation products (C3a, C5a) independent of any extracellularly added complement system. Exogenous properdin increased the mRNA expression of CFI and CFD, but decreased levels of complement components (C1Q, C3), receptors (C3AR, C5AR1, CD11B) and inflammation-associated transcripts (NLRP3, IL1B) in hpRPE cells exposed to oxidative stress. This properdin effect was time-dependently counter regulated. In conclusion, our data unveiled a local, genotype-associated complement component production in hpRPE cells, regulated by exogenous properdin. The local complement production and activation via blood-independent mechanisms can be a new therapeutic target for AMD.

Human pluripotent stem cells: A toolbox to understand and treat retinal degeneration

Age-related Macular Degeneration (AMD) and Retinitis Pigmentosa (RP) are retinal degenerative disorders that dramatically damage the retina. As there is no therapeutic option for the majority of patients, vision is progressively and irremediably lost. Owing to their unlimited renewal and potency to give rise to any cell type of the human adult body, human pluripotent stem cells (hPSCs) have been extensively studied in recent years to develop more physiologically relevant in vitro cellular models. Such models open new perspectives to investigate the pathological molecular mechanisms of AMD and RP but also in drug screening. Moreover, proof-of-concept of hPSC-derived retinal cell therapy in animal models have led to first clinical trials. This review outlines the recent advances in the use of hPSCs in pathological modeling of retinal degeneration and their use in regenerative medicine. We also address the associated limitations and challenges that need to be overcome when using hPSCs.

Pigment Epithelium-Derived Factor as a Possible Treatment Agent for Choroidal Neovascularization

Choroidal neovascularization (CNV) is a sight-threatening disease and is characterized by the formation of pathological neovascularization in the choroid which extends into the subretinal space. Exudative age-related macular degeneration (AMD) is the formation of CNV in the macular area which leads to irreversible blindness. Continuous leakage and hemorrhage of the CNV lesion may eventually result in scarring or later fibrosis, which could result in photoreceptor cell atrophy. The current strategy for treating CNV is the use of antivascular endothelial growth factor (VEGF) agents. Many studies have demonstrated the efficacy of intravitreal anti-VEGF therapy. Other studies have also reported the side effects of single anti-VEGF treatment. And long-term inhibition of a single system may result in collateral damage to other visual elements. Pigment epithelium-derived factor (PEDF) is a 50 kDa protein that was first isolated from the conditioned medium of human RPE cells. PEDF has both antiangiogenesis and neuroprotective functions for photoreceptor cells. It may be a potential ocular antiangiogenic agent. This review outlines the distribution of PEDF in the eye, the mechanism of antiangiogenesis, the protective effect on the retina, and the relationship between PEDF and VEGF.

Extracellular matrix and oxidative stress regulate human retinal pigment epithelium growth

Age-related macular degeneration (AMD), the most common cause of vision loss with ageing, is characterised by degeneration of the photoreceptors and retinal pigment epithelium (RPE) and changes in the extracellular matrix (ECM) underlying the RPE. The pathogenesis of AMD is still not fully understood. In this study we investigated the in vitro growth and function of primary human RPE cells in response to different ECM substrates, including nitrite-modified ECM. We initially confirmed the presence of disorganised retinal glial and photoreceptor cells, marked retinal cytoplasmic and Bruch's membrane expression of nitro-tyrosine (an oxidative stress marker) and increased numbers of Iba1⁺ macrophages/microglia in human donor eye sections (aged and AMD) using multi-marker immunohistochemistry (n = 3). Concurrently, we utilised two-photon microscopy to reveal topographical changes in flatmounts of RPE-associated ECM and in the underlying choroid of aged and AMD donor eyes (n = 3). To recapitulate these observations in vitro, we then used primary human RPE cells to investigate how different ECM proteins, including nitrite cross-linked RPE-secreted ECM, modified RPE cell growth and function. Collagen I or IV increased RPE attachment and spreading two-to three-fold, associated with significantly increased cell migration and proliferation, consistent with a preferential interaction with these matrix substrates. Primary human RPE cells grown on collagen I and IV also showed increased secretion of pro-inflammatory cytokines, MCP-1 and IL-8. Nitrite-modification of RPE-secreted ECM (simulating ageing of Bruch's membrane) significantly reduced in vitro RPE attachment to the ECM and this was mitigated with collagen IV coating of the modified ECM. Taken together, our observations confirm the importance of RPE-ECM interactions for normal RPE growth and function, and for inducing RPE secretion of pro-inflammatory cytokines. Furthermore, the findings are consistent with ageing and/or oxidative stress-induced disruption of RPE-ECM interactions contributing to the pathogenesis of AMD.

Stem cell-derived retinal pigment epithelium from patients with age-related macular degeneration exhibit reduced metabolism and matrix interactions

Modeling age‐related macular degeneration (AMD) is challenging, because it is a multifactorial disease. To focus on interactions between the retinal pigment epithelium (RPE) and Bruch's membrane, we generated RPE from AMD patients and used an altered extracellular matrix (ECM) that models aged Bruch's membrane. Induced pluripotent stem cells (iPSCs) were generated from fibroblasts isolated from AMD patients or age‐matched (normal) controls. RPE derived from iPSCs were analyzed by morphology, marker expression, transepithelial electrical resistance (TER), and phagocytosis of rod photoreceptor outer segments. Cell attachment and viability was tested on nitrite‐modified ECM, a typical modification of aged Bruch's membrane. DNA microarrays with hierarchical clustering and analysis of mitochondrial function were used to elucidate possible mechanisms for the observed phenotypes. Differentiated RPE displayed cell‐specific morphology and markers. The TER and phagocytic capacity were similar among iPSC‐derived RPE cultures. However, distinct clusters were found for the transcriptomes of AMD and control iPSC‐derived RPE. AMD‐derived iPSC‐RPE downregulated genes responsible for metabolic‐related pathways and cell attachment. AMD‐derived iPSC‐RPE exhibited reduced mitochondrial respiration and ability to attach and survive on nitrite‐modified ECM. Cells that did attach induced the expression of complement genes. Despite reprogramming, iPSC derived from AMD patients yielded RPE with a transcriptome that is distinct from that of age‐matched controls. When challenged with an AMD‐like modification of Bruch's membrane, AMD‐derived iPSC‐RPE activated the complement immune system.

Oxidative Stress Increases Endogenous Complement-Dependent Inflammatory and Angiogenic Responses in Retinal Pigment Epithelial Cells Independently of Exogenous Complement Sources

Oxidative stress-induced damage of the retinal pigment epithelium (RPE) and chronic inflammation have been suggested as major contributors to a range of retinal diseases. Here, we examined the effects of oxidative stress on endogenous complement components and proinflammatory and angiogenic responses in RPE cells. ARPE-19 cells exposed for 1–48 h to H₂O₂ had reduced cell–cell contact and increased markers for epithelial–mesenchymal transition but showed insignificant cell death. Stressed ARPE-19 cells increased the expression of complement receptors CR3 (subunit CD11b) and C5aR1. CD11b was colocalized with cell-derived complement protein C3, which was present in its activated form in ARPE-19 cells. C3, as well as its regulators complement factor H (CFH) and properdin, accumulated in the ARPE-19 cells after oxidative stress independently of external complement sources. This cell-associated complement accumulation was accompanied by increased nlrp3 and foxp3 expression and the subsequently enhanced secretion of proinflammatory and proangiogenic factors. The complement-associated ARPE-19 reaction to oxidative stress, which was independent of exogenous complement sources, was further augmented by the poly(ADP-ribose) polymerase (PARP) inhibitor olaparib. Our results indicate that ARPE-19 cell-derived complement proteins and receptors are involved in ARPE-19 cell homeostasis following oxidative stress and should be considered as targets for treatment development for retinal degeneration.

Interactions of the choroid, Bruch's membrane, retinal pigment epithelium, and neurosensory retina collaborate to form the outer blood-retinal-barrier

The three interacting components of the outer blood-retinal barrier are the retinal pigment epithelium (RPE), choriocapillaris, and Bruch's membrane, the extracellular matrix that lies between them. Although previously reviewed independently, this review integrates these components into a more wholistic view of the barrier and discusses reconstitution models to explore the interactions among them. After updating our understanding of each component's contribution to barrier function, we discuss recent efforts to examine how the components interact. Recent studies demonstrate that claudin-19 regulates multiple aspects of RPE's barrier function and identifies a barrier function whereby mutations of claudin-19 affect retinal development. Co-culture approaches to reconstitute components of the outer blood-retinal barrier are beginning to reveal two-way interactions between the RPE and choriocapillaris. These interactions affect barrier function and the composition of the intervening Bruch's membrane. Normal or disease models of Bruch's membrane, reconstituted with healthy or diseased RPE, demonstrate adverse effects of diseased matrix on RPE metabolism. A stumbling block for reconstitution studies is the substrates typically used to culture cells are inadequate substitutes for Bruch's membrane. Together with human stem cells, the alternative substrates that have been designed offer an opportunity to engineer second-generation culture models of the outer blood-retinal barrier.

Retinal pigment epithelium polarity in health and blinding diseases

The polarized phenotype of the retinal pigment epithelium is crucial for the outer retina-blood barrier and support of photoreceptors and underlying choroid, and its disruption plays a central role in degenerative retinopathies. Although the mechanisms of polarization remain mostly unknown, they are fundamental for homeostasis of the outer retina. Recent research is revealing a growing picture of interconnected tissues in the outer retina, with the retinal pigment epithelium at the center. This review discusses how elements of epithelial polarity relate to emerging apical interactions with the neural retina, basolateral cross-talk with the underlying Bruch's membrane and choriocapillaris, and tight junction biology. An integrated view of outer retina physiology is likely to provide insights into the pathogenesis of blinding diseases.

Nitrite ion modifies tyrosine and lysine residues of extracellular matrix proteins

Age-related macular degeneration (AMD) is a disease characterized by degenerative changes in the retinal pigment epithelium and Bruch's membrane. Inflammation is considered a major risk factor for the development and progression of AMD. Nitrite is a potent byproduct of inflammation and has been detected at elevated concentrations in AMD donor tissue. We hypothesize that nitrite chemically modifies the extracellular matrix (ECM) of Bruch's membrane as an initial step to degenerative changes observed in AMD. Non-enzymatically nitrated synthetic ECM peptides, fibronectin and laminin, were used as model systems for inflammation. Using LC/MS, we identified that nitration preferentially occurred on tyrosine and deamination of lysine under the studied conditions. At tyrosine residues, 3-nitrotyrosine was produced and shifted the total mass by the addition of 45 amu. Deamination of lysine occurred and resulted in the formation of either an alkene or alcohol group. The alkene group was observed with a loss of 17 amu. An addition of 1 amu was observed with alcohol formation. We hypothesize that these initial chemical modifications to the structure of ECM proteins may be the responsible for altering the structure and consequent function of Bruch's membrane.

Differentiation, Evaluation, and Application of Human Induced Pluripotent Stem Cell-Derived Endothelial Cells

The emergence of induced pluripotent stem cell (iPSC) technology paves the way to generate large numbers of patient-specific endothelial cells (ECs) that can be potentially delivered for regenerative medicine in patients with cardiovascular disease. In the last decade, numerous protocols that differentiate EC from iPSC have been developed by many groups. In this review, we will discuss several common strategies that have been optimized for human iPSC-EC differentiation and subsequent studies that have evaluated the potential of human iPSC-EC as a cell therapy or as a tool in disease modeling. In addition, we will emphasize the importance of using in vivo vessel-forming ability and in vitro clonogenic colony-forming potential as a gold standard with which to evaluate the quality of human iPSC-EC derived from various protocols.