Age-related macular degeneration: activation of innate immunity system via pattern recognition receptors

The new paradigm: retinal pigment epithelium cells generated from embryonic or induced pluripotent stem cells

Compared with neural crest-derived melanocytes, retinal pigment epithelium (RPE) cells in the back of the eye are pigment cells of a different kind. They are a part of the brain, form an epithelial monolayer, respond to distinct extracellular signals, and provide functions that far exceed those of a light-absorbing screen. For instance, they control nutrient and metabolite flow to and from the retina, replenish 11-cis-retinal by re-isomerizing all-trans-retinal generated during photoconversion, phagocytose daily a portion of the photoreceptors' outer segments, and secrete cytokines that locally control the innate and adaptive immune systems. Not surprisingly, RPE cell damage is a major cause of human blindness worldwide, with age-related macular degeneration a prevalent example. RPE replacement therapies using RPE cells generated from embryonic or induced pluripotent stem cells provide a novel approach to a rational treatment of such forms of blindness. In fact, RPE-like cells can be obtained relatively easily when stem cells are subjected to a two-step induction protocol, a first step that leads to a neuroectodermal fate and a second to RPE differentiation. Here, we discuss the characteristics of such cells, propose criteria they should fulfill in order to be considered authentic RPE cells, and point out the challenges one faces when using such cells in attempts to restore vision.

Dietary hyperglycemia, glycemic index and metabolic retinal diseases

The glycemic index (GI) indicates how fast blood glucose is raised after consuming a carbohydrate-containing food. Human metabolic studies indicate that GI is related to patho-physiological responses after meals. Compared with a low-GI meal, a high-GI meal is characterized with hyperglycemia during the early postprandial stage (0-2h) and a compensatory hyperlipidemia associated with counter-regulatory hormone responses during late postprandial stage (4-6h). Over the past three decades, several human health disorders have been related to GI. The strongest relationship suggests that consuming low-GI foods prevents diabetic complications. Diabetic retinopathy (DR) is a complication of diabetes. In this aspect, GI appears to be useful as a practical guideline to help diabetic people choose foods. Abundant epidemiological evidence also indicates positive associations between GI and risk for type 2 diabetes, cardiovascular disease, and more recently, age-related macular degeneration (AMD) in people without diabetes. Although data from randomized controlled intervention trials are scanty, these observations are strongly supported by evolving molecular mechanisms which explain the pathogenesis of hyperglycemia. This wide range of evidence implies that dietary hyperglycemia is etiologically related to human aging and diseases, including DR and AMD. In this context, these diseases can be considered as metabolic retinal diseases. Molecular theories that explain hyperglycemic pathogenesis involve a mitochondria-associated pathway and four glycolysis-associated pathways, including advanced glycation end products formation, protein kinase C activation, polyol pathway, and hexosamine pathway. While the four glycolysis-associated pathways appear to be universal for both normoxic and hypoxic conditions, the mitochondria-associated mechanism appears to be most relevant to the hyperglycemic, normoxic pathogenesis. For diseases that affect tissues with highly active metabolism and that frequently face challenge from low oxygen tension, such as retina in which metabolism is determined by both glucose and oxygen homeostases, these theories appear to be insufficient. Several lines of evidence indicate that the retina is particularly vulnerable when hypoxia coincides with hyperglycemia. We propose a novel hyperglycemic, hypoxia-inducible factor (HIF) pathway, to complement the current theories regarding hyperglycemic pathogenesis. HIF is a transcription complex that responds to decrease oxygen in the cellular environment. In addition to playing a significant role in the regulation of glucose metabolism, under hyperglycemia HIF has been shown to increase the expression of HIF-inducible genes, such as vascular endothelial growth factor (VEGF) leading to angiogenesis. To this extent, we suggest that HIF can also be described as a hyperglycemia-inducible factor. In summary, while management of dietary GI appears to be an effective intervention for the prevention of metabolic diseases, specifically AMD and DR, more interventional data is needed to evaluate the efficacy of GI management. There is an urgent need to develop reliable biomarkers of exposure, surrogate endpoints, as well as susceptibility for GI. These insights would also be helpful in deciphering the detailed hyperglycemia-related biochemical mechanisms for the development of new therapeutic agents.

Endoplasmic reticulum stress in age-related macular degeneration: trigger for neovascularization

Age-related macular degeneration (AMD) can be classified into two main categories: the atrophic, dry form and the exudative, wet form. The crucial difference between dry and wet AMD is the development of choroidal neovascularization in wet AMD. One fundamental cause of the neovascularization is the increased expression of VEGF (vascular endothelial growth factor) in retinal pigment epithelial cells. Progression of AMD is linked to augmentation of cellular stress, for example, oxidative stress, proteotoxic stress, inflammation and hypoxia. All these conditions can trigger stress in endoplasmic reticulum (ER), which maintains protein quality control in cells. ER stress induces the unfolded protein response (UPR) via IRE1 (inositol-requiring protein-1), PERK (protein kinase RNA-like ER kinase) and ATF6 (activating transcription factor-6) transducers. UPR signaling is a double-edged sword, that is, it can restore cellular homeostasis as far as possible, but ultimately may lead to chronic, overwhelming stress that can cause apoptotic cell death. Interestingly, ER stress is a well-known inducer of angiogenesis in cancer. Moreover, stress conditions associated with the progress of AMD can induce the expression of VEGF. We discuss the role of ER stress in the regulation of neovascularization and the conversion of dry AMD to its wet, detrimental counterpart.

Inflammasome signaling at the heart of central nervous system pathology

Neuroinflammation is a complex innate response of neural tissue against harmful effects of diverse stimuli viz., pathogens, damaged cells and irritants within the Central Nervous System (CNS). Studies show that multiple inflammatory mediators including cytokines, chemokines and prostaglandins are elevated in the Cerebrospinal Fluid (CSF) and in post-mortem brain tissues of patients with history of neuroinflammatory conditions as well as neurodegenerative disorders like Alzheimer's disease, Parkinson's disease and Multiple Sclerosis. The innate immunity mediators in the brain, namely microglia and astrocytes, express certain Pattern Recognition Receptors (PRRs), which are always on 'high-alert' for pathogens or other inflammatory triggers and participate in the assembly and activation of the inflammasome. The inflammasome orchestrates the activation of the precursors of proinflammatory caspases, which in turn, cleave the precursor forms of interleukin-1beta, IL-18 and IL-33 into their active forms; the secretion of which leads to a potent inflammatory response, and/or influences the release of toxins from glial and endothelial cells. Altered expression of inflammasome mediators can either promote or inhibit neurodegenerative processes. Therefore, modulating the inflammasome machinery seems a better combat strategy than summarily suppressing all inflammation in most neuroinflammatory conditions. In the current review we have surveyed the identified triggers and pathways of inflammasome activation and the following events which ultimately accomplish the innate inflammatory response in the CNS, with a goal to provide an analytical insight into disease pathogenesis that might provide cues for devising novel therapeutic strategies.

Complement-mediated inhibition of neovascularization reveals a point of convergence between innate immunity and angiogenesis

Beyond its role in immunity, complement mediates a wide range of functions in the context of morphogenetic or tissue remodeling processes. Angiogenesis is crucial during tissue remodeling in multiple pathologies; however, the knowledge about the regulation of neovascularization by the complement components is scarce. Here we studied the involvement of complement in pathological angiogenesis. Strikingly, we found that mice deficient in the central complement component C3 displayed increased neovascularization in the model of retinopathy of prematurity (ROP) and in the in vivo Matrigel plug assay. In addition, antibody-mediated blockade of C5, treatment with C5aR antagonist, or C5aR deficiency in mice resulted in enhanced pathological retina angiogenesis. While complement did not directly affect angiogenesis-related endothelial cell functions, we found that macrophages mediated the antiangiogenic activity of complement. In particular, C5a-stimulated macrophages were polarized toward an angiogenesis-inhibitory phenotype, including the up-regulated secretion of the antiangiogenic soluble vascular endothelial growth factor receptor-1. Consistently, macrophage depletion in vivo reversed the increased neovascularization associated with C3- or C5aR deficiency. Taken together, complement and in particular the C5a-C5aR axes are potent inhibitors of angiogenesis.

Induction of interleukin-8 gene expression and protein secretion by C-reactive protein in ARPE-19 cells

C-reactive protein (CRP) is an acute phase reactant and its level rises rapidly during inflammation. Recent studies have suggested the potential involvement of CRP in the pathogenesis of age-related macular degeneration (AMD). To delineate the functional roles of CRP in inflammatory response by the ocular posterior segments, the effects of CRP on ARPE-19, an immortalized human retinal pigment epithelia (hRPE) cell line, were investigated in the present study. Treatment of ARPE-19 cells with CRP resulted in enhanced NF-kB nuclear translocation and dose-dependent transient induction of IL-8 mRNA synthesis and protein secretion. Stimulated expression of VEGF, but not MCP-1 by CRP was also observed. The induced IL-8 expression was transient and peaked at 12h post stimulation. In the presence of inhibitors for NF-kB, p38, MEK and JNK, the CRP-induced IL-8 production was abolished by 99.5+/-2.3, 97.8+/-2.1, 55.3+/-2.5 and 37.3+/-1.3%, respectively. Neutralization of Fc gamma receptors by anti-CD32 and CD64 antibodies produced 39.9+/-1.6 and 59.5+/-2.6% reduction, respectively, of CRP-stimulated IL-8 secretion, whereas that by anti-CD16 antibody had no effect. This study suggests that the pro-inflammatory effects of CRP in ARPE-19 cells may contribute to the inflammatory retinal diseases by induction of pro-inflammatory cytokines such as IL-8. This induction is mediated by NF-kB and multiple MAPK pathways through Fc gamma receptors.

Regulatory role of HIF-1alpha in the pathogenesis of age-related macular degeneration (AMD)

Age-related macular degeneration (AMD) is a leading cause of irreversible blindness in the elderly throughout the world. AMD is attributed to a complex interaction of genetic and environmental factors. It is characterized by degeneration involving the retinal photoreceptors, retinal pigment epithelium (RPE), and Bruch's membrane, as well as alterations in choroidal capillaries. Aging and age-associated degenerative diseases, such as AMD, are intimately associated with decreased levels of tissue oxygenation and hypoxia that may induce accumulation of detrimental RPE-associated deposits, inflammation and neovascularization processes in retina. Hypoxia-inducible factor (HIF) is the master regulator for hypoxia-induced cellular adaptation that is involved in NF-kappaB signaling and the autophagic protein clearance system. In this review, we discuss role of HIF in AMD pathology and as a possible therapeutic target.