AT1 receptor inhibition prevents astrocyte degeneration and restores vascular growth in oxygen-induced retinopathy

Enalaprilat as a new means of preventing the development of retinopathy of prematurity

In a rat model of experimental retinopathy of prematurity (ROP), the safety of enalaprilat and its effect on the level of angiotensin-converting enzyme (ACE) and angiotensin-II (AT-II) in the vitreous body and retina were investigated. The study was performed on 136 newborn Wistar rat pups divided into 2 groups: group A - experimental (animals with ROP, n=64) and group B - control (n=72). Each group was further divided into 2 subgroups: A0 and B0 (n=32 and n=36, respectively) - animals that did not receive injections of enalaprilat, and A1 and B1 (n=32 and n=36, respectively) - animals treated with daily intraperitoneal (i.p.) injections of enalaprilat (0.6 mg/kg of body weight). This treatment started on day 2 and lasted either to day 7 or to day 14 in accordance with the therapeutic scheme. Animals were taken out of the experiment on day 7 and day 14. In samples of the vitreous body and retina, the content of ACE and AT-II was determined by enzyme immunoassay. On day 7 in subgroups A1 and B1 the levels of ACE and AT-II in the vitreous did not differ, while on day 14 were lower than in subgroups A0 and B0, respectively. Changes in the parameters studied in the retina were somewhat different from those found in the vitreous body. On the seventh day, the level of ACE in the retina of animals of subgroup B1 did not differ significantly from subgroup B0, and in subgroup A1 it was increased compared to subgroup A0. On day 14, its significant decrease was noted in subgroups A1 and B1 as compared with subgroups A0 and B0. At the same time, the level of AT-II in the retina of rat pups of subgroup B1 was lower than in subgroup B0, both on day 7 and day 14. On day 7, the concentration of AT-II, as well as the concentration of ACE, increased in subgroup A1 as compared to subgroup A0. On day 14, this parameter in subgroup A1 was significantly lower as compared to subgroup A0, but significantly higher than in subgroup B1. It should be noted that i.p. injections of enalaprilat, increased a death rate of animals of both groups. The use of enalaprilat, starting from the preclinical period of the ROP development, led to a decrease in the activity of the renin-angiotensin system (RAS) in ROP animals at the onset of retinopathy in the experimental model used. This opens up prospects for considering enalaprilat as a means of preventing the development of this pathology; however, the recognized high toxicity of the drug requires further studies and correction of the timing of its administration and dosage in order to achieve a balance of efficacy and safety of use in order to prevent the development of ROP in children.

Classification of diabetic retinopathy: Past, present and future

Diabetic retinopathy (DR) is a leading cause of visual impairment and blindness worldwide. Since DR was first recognized as an important complication of diabetes, there have been many attempts to accurately classify the severity and stages of disease. These historical classification systems evolved as understanding of disease pathophysiology improved, methods of imaging and assessing DR changed, and effective treatments were developed. Current DR classification systems are effective, and have been the basis of major research trials and clinical management guidelines for decades. However, with further new developments such as recognition of diabetic retinal neurodegeneration, new imaging platforms such as optical coherence tomography and ultra wide-field retinal imaging, artificial intelligence and new treatments, our current classification systems have significant limitations that need to be addressed. In this paper, we provide a historical review of different classification systems for DR, and discuss the limitations of our current classification systems in the context of new developments. We also review the implications of new developments in the field, to see how they might feature in a future, updated classification.

Diabetic Retinopathy: From Animal Models to Cellular Signaling

Diabetic retinopathy (DR) is an ocular complication of diabetes mellitus (DM), a metabolic disorder characterized by elevation in blood glucose level. The pathogenesis of DR includes vascular, neuronal, and inflammatory components leading to activation of complex cellular molecular signaling. If untreated, the disease can culminate in vision loss that eventually leads to blindness. Animal models mimicking different aspects of DM complications have been developed to study the development and progression of DR. Despite the significant contribution of the developed DR models to discovering the mechanisms of DR and the recent achievements in the research field, the sequence of cellular events in diabetic retinas is still under investigation. Partially, this is due to the complexity of molecular mechanisms, although the lack of availability of models that adequately mimic all the neurovascular pathobiological features observed in patients has also contributed to the delay in determining a precise molecular trigger. In this review, we provide an update on the status of animal models of DR to help investigators choose an appropriate system to validate their hypothesis. We also discuss the key cellular and physiological events of DR in these models.

Animal models of diabetic microvascular complications: Relevance to clinical features

Diabetes has become more common in recent years worldwide, and this growth is projected to continue in the future. The primary concern with diabetes is developing various complications, which significantly contribute to the disease's mortality and morbidity. Over time, the condition progresses from the pre-diabetic to the diabetic stage and then to the development of complications. Years and enormous resources are required to evaluate pharmacological interventions to prevent or delay the progression of disease or complications in humans. Appropriate screening models are required to gain a better understanding of both pathogenesis and potential therapeutic agents. Different species of animals are used to evaluate the pharmacological potentials and study the pathogenesis of the disease. Animal models are essential for research because they represent most of the structural, functional, and biochemical characteristics of human diseases. An ideal screening model should mimic the pathogenesis of the disease with identifiable characteristics. A thorough understanding of animal models is required for the experimental design to select an appropriate model. Each animal model has certain advantages and limitations. The present manuscript describes the animal models and their diagnostic characteristics to evaluate microvascular diabetic complications.

Diabetic retinal neurodegeneration as a form of diabetic retinopathy

PURPOSE

To review the evidence supporting diabetic retinal neurodegeneration (DRN) as a form of diabetic retinopathy.

METHOD

Review of literature.

RESULTS

DRN is recognized to be a part of retinopathy in patients with diabetes mellitus (DM), in addition to the well-established diabetic retinal vasculopathy (DRV). DRN has been noted in the early stages of DM, before the onset of clinically evident diabetic retinopathy. The occurrence of DRN has been confirmed in animal models of DM, histopathological examination of donor's eyes from diabetic individuals and assessment of neural structure and function in humans. DRN involves alterations in retinal ganglion cells, photoreceptors, amacrine cells and bipolar cells, and is thought to be driven by glutamate, oxidative stress and dysregulation of neuroprotective factors in the retina. Potential therapeutic options for DRN are under evaluation.

CONCLUSIONS

Literature is divided on the temporal relation between DRN and DRV, with evidence of both precedence and simultaneous occurrence. The relationship between DRN and multi-system neuropathy in DM is yet to be evaluated critically.

An Update on Connexin Gap Junction and Hemichannels in Diabetic Retinopathy

Diabetic retinopathy (DR) is one of the main causes of vision loss in the working age population. It is characterized by a progressive deterioration of the retinal microvasculature, caused by long-term metabolic alterations inherent to diabetes, leading to a progressive loss of retinal integrity and function. The mammalian retina presents an orderly layered structure that executes initial but complex visual processing and analysis. Gap junction channels (GJC) forming electrical synapses are present in each retinal layer and contribute to the communication between different cell types. In addition, connexin hemichannels (HCs) have emerged as relevant players that influence diverse physiological and pathological processes in the retina. This article highlights the impact of diabetic conditions on GJC and HCs physiology and their involvement in DR pathogenesis. Microvascular damage and concomitant loss of endothelial cells and pericytes are related to alterations in gap junction intercellular communication (GJIC) and decreased connexin 43 (Cx43) expression. On the other hand, it has been shown that the expression and activity of HCs are upregulated in DR, becoming a key element in the establishment of proinflammatory conditions that emerge during hyperglycemia. Hence, novel connexin HCs blockers or drugs to enhance GJIC are promising tools for the development of pharmacological interventions for diabetic retinopathy, and initial in vitro and in vivo studies have shown favorable results in this regard.

The G-protein-coupled chemoattractant receptor Fpr2 exacerbates neuroglial dysfunction and angiogenesis in diabetic retinopathy

Diabetic retinopathy (DR) as a retinal neovascularization‐related disease is one of the leading causes of irreversible blindness in patients. The goal of this study is to determine the role of a G‐protein‐coupled chemoattractant receptor (GPCR) FPR2 (mouse Fpr2) in the progression of DR, in order to identify novel therapeutic targets. We report that Fpr2 was markedly upregulated in mouse diabetic retinas, especially in retinal vascular endothelial cells, in associated with increased number of activated microglia and Müller glial cells. In contrast, in the retina of diabetic Fpr2^−/− mice, the activation of vascular endothelial cells and glia was attenuated with reduced production of proinflammatory cytokines. Fpr2 deficiency also prevented the formation of acellular capillary during diabetic progression. Furthermore, in oxygen‐induced retinopathy (OIR) mice, the absence of Fpr2 was associated with diminished neovasculature formation and pathological vaso‐obliteration region in the retina. These results highlight the importance of Fpr2 in exacerbating the progression of neuroglial degeneration and angiogenesis in DR and its potential as a therapeutic target.

Endothelial Yes-Associated Protein 1 Promotes Astrocyte Proliferation and Maturation via Cytoplasmic Leukemia Inhibitory Factor Secretion in Oxygen-Induced Retinopathy

Purpose

Purpose The role of endothelial Yes-associated protein 1 (YAP) in the pathogenesis of retinal angiogenesis and the astrocyte network in the mouse oxygen-induced retinopathy (OIR) model is unknown.

Methods

For in vivo studies, OIR was induced in conditional endothelial YAP knockout mice and their wild-type littermates. Retinal vascularization and the astrocyte network were evaluated by whole-mount fluorescence and Western blotting. In vitro experiments were performed in astrocytes cultured with human microvascular endothelial cell-1–conditioned medium to analyze the mechanisms underlying the effect of endothelial YAP on astrocytes.

Results

Endothelial YAP deletion not only impaired retinal blood vessels, but also caused a sparse and disrupted astrocyte network in response to OIR. Levels of the immature astrocyte marker (platelet-derived growth factor A) in the retina were substantially increased owing to YAP deficiency, suggesting a possible failure in astrocyte maturation, whereas retinal expression of leukemia inhibitory factor (LIF) was decreased. In vitro studies suggested that loss or overexpression of YAP resulted in elevated or decreased LIF secretion by human microvascular endothelial cell-1, respectively. Increased LIF levels in the culture medium promoted astrocyte maturation and proliferation and rescued YAP inhibition-induced astrocyte loss. Finally, activating YAP could protect against the pathology of the astrocyte network and even suppress pathologic retinal vascularization in control OIR mice, but not in endothelial YAP-deficient OIR mice.

Conclusions

Endothelial YAP regulation of LIF secretion is required for normalized astrocyte network formation in OIR, thereby providing a novel target for protecting the astrocyte network and thus benefiting retinal blood vessels.

Contribution of microglia and monocytes to the development and progression of age related macular degeneration

PURPOSE

Age related macular degeneration (AMD) is the leading cause of irreversible vision loss in industrialised nations. Based on genetics, as well as proteome analysis of drusen, the role the innate immune system in the development and/or progression of the disease is well established. Mononuclear phagocytes, such as microglia and monocytes, play critical roles in innate immunity. Here, the role of retinal microglia in mediating normal retinal function, and how these cells change with age is discussed, so as to understand their role in the development and progression of AMD.

RECENT FINDINGS

It is now known that microglia dynamically survey the neural environment, responding rapidly to even the most subtle neural injury. The dynamic and phagocytic roles of microglia can change with age contributing to alteration in the response of these cells to damage with age. Accumulation of innate immune cells in the subretinal space is a hallmark feature of the development of AMD, reflecting either an increase in migration of monocytes into the retina, or a failure of immune cell elimination from the retina. Furthermore, changes in phagocytic ability of immune cells could contribute to the accumulation of drusen deposits in the posterior eye.

SUMMARY

An overview of how retinal microglia maintain retinal homeostasis under normal conditions is provided, and then how they contribute to each stage of AMD. In addition, circulating monocytes are altered in those with AMD, contributing to the overall inflammatory state. Understanding the role of cells of the innate immune system in AMD may uncover novel therapeutic targets with which to reduce either the development or progression of disease.

Ablation of Immunoproteasome β5i Subunit Suppresses Hypertensive Retinopathy by Blocking ATRAP Degradation in Mice

Inflammation is associated with retinal diseases. Our recent data demonstrate that immunoproteasome catalytic subunit β2i contributes to angiotensin II (Ang II)-induced retinopathy in mice. Here, we investigated the role of another catalytic subunit β5i in regulating retinopathy and its underlying mechanisms. We induced a murine model of retinopathy by infusing Ang II (3,000 ng/kg/min) for 3 weeks into wild-type (WT) mice, β5i-knockout (KO) mice, or WT mice injected with either adenovirus-expressing β5i (Ad-β5i) or angiotensin II type 1 receptor (AT1R)-associated protein (Ad-ATRAP), which inhibits AT1R. The β5i expression and chymotrypsin-like activity were most significantly elevated in Ang II-infused retinas and serum from patients with hypertensive retinopathy. Moreover, Ang II infusion-induced retinopathy was markedly attenuated in β5i-KO mice but aggravated in Ad-β5i-injected mice. Accordingly, β5i KO markedly restored Ang II-induced downregulation of ATRAP and activation of AT1R downstream mediators, which was further enhanced in Ad-β5i-injected mice. Interestingly, overexpression of ATRAP significantly abrogated Ang II-induced retinopathy in Ad-β5i-injected mice. This study found that β5i promoted Ang II-induced retinopathy by promoting ATRAP degradation and activation of AT1R-mediated signals.

Vascular and Neuronal Protection in the Developing Retina: Potential Therapeutic Targets for Retinopathy of Prematurity

Retinopathy of prematurity (ROP) is a common retinal disease in preterm babies. To prolong the lives of preterm babies, high oxygen is provided to mimic the oxygen level in the intrauterine environment for postnatal organ development. However, hyperoxia-hypoxia induced pathological events occur when babies return to room air, leading to ROP with neuronal degeneration and vascular abnormality that affects retinal functions. With advances in neonatal intensive care, it is no longer uncommon for increased survival of very-low-birth-weight preterm infants, which, therefore, increased the incidence of ROP. ROP is now a major cause of preventable childhood blindness worldwide. Current proven treatment for ROP is limited to invasive retinal ablation, inherently destructive to the retina. The lack of pharmacological treatment for ROP creates a great need for effective and safe therapies in these developing infants. Therefore, it is essential to identify potential therapeutic agents that may have positive ROP outcomes, especially in preserving retinal functions. This review gives an overview of various agents in their efficacy in reducing retinal damages in cell culture tests, animal experiments and clinical studies. New perspectives along the neuroprotective pathways in the developing retina are also reviewed.

The renin-angiotensin system and the retinal neurovascular unit: A role in vascular regulation and disease

The retina is known to have a local renin-angiotensin system (RAS) and dysfunction in the RAS is often associated with diseases of the retinal vasculature that cause irreversible vision loss. Regulation of the retinal vasculature to meet the metabolic needs of the tissues occurs through a mechanism called neurovascular coupling, which is critical for maintaining homeostatic function and support for neurons. Neurovascular coupling is the process by which support cells, including glia, regulate blood vessel calibre and blood flow in response to neural activity. In retinal vascular diseases, this coupling mechanism is often disrupted. However, the role that angiotensin II (Ang II), the main effector peptide of the RAS, has in regulating both the retinal vasculature and neurovascular coupling is not fully understood. As components of the RAS are located on the principal neurons, glia and blood vessels of the retina, it is possible that Ang II has a role in regulating communication and function between these three cell types, and therefore the capacity to regulate neurovascular coupling. This review focuses on components of the RAS located on the retinal neurovascular unit, and the potential of this system to contribute to blood flow modulation in the healthy and compromised retina.

Suppression of connexin 43 phosphorylation promotes astrocyte survival and vascular regeneration in proliferative retinopathy

Degeneration of retinal astrocytes precedes hypoxia-driven pathologic neovascularization and vascular leakage in ischemic retinopathies. However, the molecular events that underlie astrocyte loss remain unclear. Astrocytes abundantly express connexin 43 (Cx43), a transmembrane protein that forms gap junction (GJ) channels and hemichannels. Cx channels can transfer toxic signals from dying cells to healthy neighbors under pathologic conditions. Here we show that Cx43 plays a critical role in astrocyte apoptosis and the resulting preretinal neovascularization in a mouse model of oxygen-induced retinopathy. Opening of Cx43 hemichannels was not observed following hypoxia. In contrast, GJ coupling between astrocytes increased, which could lead to amplification of injury. Accordingly, conditional deletion of Cx43 maintained a higher density of astrocytes in the hypoxic retina. We also identify a role for Cx43 phosphorylation in mediating these processes. Increased coupling in response to hypoxia is due to phosphorylation of Cx43 by casein kinase 1δ (CK1δ). Suppression of this phosphorylation using an inhibitor of CK1δ or in site-specific phosphorylation-deficient mice similarly protected astrocytes from hypoxic damage. Rescue of astrocytes led to restoration of a functional retinal vasculature and lowered the hypoxic burden, thereby curtailing neovascularization and neuroretinal dysfunction. We also find that absence of astrocytic Cx43 does not affect developmental angiogenesis or neuronal function in normoxic retinas. Our in vivo work directly links phosphorylation of Cx43 to astrocytic coupling and apoptosis and ultimately to vascular regeneration in retinal ischemia. This study reveals that targeting Cx43 phosphorylation in astrocytes is a potential direction for the treatment of proliferative retinopathies.

Retinal vasculature development in health and disease

Development of the retinal vasculature is based on highly coordinated signalling between different cell types of the retina, integrating internal metabolic requirements with external influences such as the supply of oxygen and nutrients. The developing mouse retinal vasculature is a useful model system to study these interactions because it is experimentally accessible for intra ocular injections and genetic manipulations, can be easily imaged and develops in a similar fashion to that of humans. Research using this model has provided insights about general principles of angiogenesis as well as pathologies that affect the developing retinal vasculature. In this review, we discuss recent advances in our understanding of the molecular and cellular mechanisms that govern the interactions between neurons, glial and vascular cells in the developing retina. This includes a review of mechanisms that shape the retinal vasculature, such as sprouting angiogenesis, vascular network remodelling and vessel maturation. We also explore how the disruption of these processes in mice can lead to pathology - such as oxygen induced retinopathy - and how this translates to human retinopathy of prematurity.

Animal Models of Diabetic Retinopathy

PURPOSE OF REVIEW

Diabetic retinopathy (DR) is one of the most common complications associated with chronic hyperglycemia seen in patients with diabetes mellitus. While many facets of DR are still not fully understood, animal studies have contributed significantly to understanding the etiology and progression of human DR. This review provides a comprehensive discussion of the induced and genetic DR models in different species and the advantages and disadvantages of each model.

RECENT FINDINGS

Rodents are the most commonly used models, though dogs develop the most similar morphological retinal lesions as those seen in humans, and pigs and zebrafish have similar vasculature and retinal structures to humans. Nonhuman primates can also develop diabetes mellitus spontaneously or have focal lesions induced to simulate retinal neovascular disease observed in individuals with DR. DR results in vascular changes and dysfunction of the neural, glial, and pancreatic β cells. Currently, no model completely recapitulates the full pathophysiology of neuronal and vascular changes that occur at each stage of diabetic retinopathy; however, each model recapitulates many of the disease phenotypes.

Many Faces of Renin-angiotensin System - Focus on Eye

The renin-angiotensin system (RAS), that is known for its role in the regulation of blood pressure as well as in fluid and electrolyte homeostasis, comprises dozens of angiotensin peptides and peptidases and at least six receptors. Six central components constitute the two main axes of the RAS cascade. Angiotensin (1-7), an angiotensin converting enzyme 2 and Mas receptor axis (ACE2-Ang(1-7)-MasR) counterbalances the harmful effects of the angiotensin II, angiotensin converting enzyme 1 and angiotensin II type 1 receptor axis (ACE1-AngII-AT1R) Whereas systemic RAS is an important factor in blood pressure regulation, tissue-specific regulatory system, responsible for long term regional changes, that has been found in various organs. In other words, RAS is not only endocrine but also complicated autocrine system. The human eye has its own intraocular RAS that is present e.g. in the structures involved in aqueous humor dynamics. Local RAS may thus be a target in the development of new anti-glaucomatous drugs. In this review, we first describe the systemic RAS cascade and then the local ocular RAS especially in the anterior part of the eye.

Dopamine modulates astroglial and microglial activity via glial renin-angiotensin system in cultures

Dopamine is an immunomodulatory molecule that acts on immune effector cells both in the CNS and peripheral tissues. However, the role of changes in dopamine levels in the neuroinflammatory response is controversial. The local/paracrine renin-angiotensin system (RAS) plays a major role in inflammatory processes in peripheral tissues and brain. In the present study, we investigated the possible role of the brain RAS in the effects of dopamine on the glial inflammatory responses. Astrocytes are the major source of the precursor protein angiotensinogen and angiotensin II (AII) in the brain. Neurotoxins such as MPP⁺ (1-methyl-4-phenylpyridinium) can act directly on astrocytes to increase levels of angiotensinogen and AII. Conversely, dopamine, via type-2 (D2) receptors, inhibited production of angiotensinogen, decreased expression of angiotensin type-1 (AT1) receptors and increased expression of AT2 receptors. In microglia, dopamine and dopamine agonists also regulated RAS activity. First, indirectly, via downregulation of the astrocyte-derived AII. Second, via dopamine-induced regulation of microglial angiotensin receptors. Dopamine decreased the microglial AT1/AT2 ratio leading to inhibition of the pro-inflammatory AT1/NADPH-oxidase/superoxide axis. D2 receptors were particularly responsible for microglial RAS inhibition in basal culture conditions. However, both D1 and D2 agonists inhibited the AT1/NADPH-oxidase axis in lipopolysaccharide-treated (LPS; i.e. activated) microglia. The results indicate that the decrease in dopamine levels observed in early stages of Parkinson's disease and aging may promote neuroinflammation and disease progression via glial RAS exacerbation.

Potential of angiotensin II receptor blockers in the treatment of diabetic retinopathy

Impairments in renin-angiotensin-aldosterone system during diabetes mellitus, leading to its dysfunction, have been known since a decade. Hyperglycemia induces several pathological alterations which upregulate this system, accounting for overexpression of its downstream signaling molecules, amongst which angiotensin II (Ang II) and aldosterone hold prime pathological notoriety. Although it is well known that elevated plasma levels of Ang II play a crucial role in the pathophysiology of type-II diabetic mellitus, by inducing and aggravating insulin resistance, it is not quite much known that it equally elevates the possibility/risk of development of diabetic complications. Also, amongst the various diabetic complications, the effects of Ang II upregulation are more widely acknowledged in studies concerning the pathophysiology of diabetic nephropathy; however, its role in exacerbating diabetic retinopathy has not received much attention. Ang II, indeed, plays a detrimental role in the progression of diabetic retinopathy by augmenting the principal events involved in its pathogenesis, namely oxidative stress, angiogenesis and inflammation. The utility of angiotensin receptor blockers has shown positive results in research studies and hence, might potentially provide a novel adjuvant therapy for treating this complication and preventing the associated vision-loss in diabetic patients.

Lutein facilitates physiological revascularization in a mouse model of retinopathy of prematurity

BACKGROUND

Retinopathy of prematurity is one of the leading causes of childhood blindness worldwide, with vessel growth cessation and vessel loss in phase I followed by neovascularization in phase II. Ischaemia contributes to its pathogenesis, and lutein protects against ischaemia-induced retinal damages. We aimed to investigate the effects of lutein on a murine model of oxygen-induced retinopathy.

METHODS

Mouse pups were exposed to 75% oxygen for 5 days and returned to room air for another 5 days. Vascular obliteration, neovascularization and blood vessel leakage were examined. Immunohistochemistry for glial cells and microglia were performed.

RESULTS

Compared with vehicle controls, mouse pups receiving lutein treatment displayed smaller central vaso-obliterated area and reduced blood vessel leakage. No significant difference in neovascular area was found between lutein and vehicle controls. Lutein promoted endothelial tip cell formation and maintained the astrocytic template in the avascular area in oxygen-induced retinopathy. No significant changes in Müller cell gliosis and microglial activation in the central avascular area were found in lutein-treated pups.

CONCLUSIONS

Our observations indicated that lutein significantly promoted normal retinal vascular regrowth in the central avascular area, possibly through promoting endothelial tip cell formation and preserving astrocytic template. Our results indicated that lutein might be considered as a supplement for the treatment of proliferative retinopathy of prematurity because of its role in facilitating the revascularization of normal vasculature.

Growth Factors in the Pathogenesis of Retinal Neurodegeneration in Diabetes Mellitus

Neurodegeneration is an initial process in the development of diabetic retinopathy (DR).

High quantities of glutamate, oxidative stress, induction of the renin-angiotensin system (RAS) and elevated levels of RAGE are crucial elements in the retinal neurodegeneration caused by diabetes mellitus. At least, there is emerging proof to indicate that the equilibrium between the neurotoxic and neuroprotective components will affect the state of the retinal neurons.

Somatostatin (SST), pigment epithelium-derived factor (PEDF), and erythropoietin (Epo) are endogenous neuroprotective peptides that are decreased in the eye of diabetic persons and play an essential role in retinal homeostasis. On the other hand, insulin-like growth factor 1 (IGF-1), and vascular endothelial growth factor (VEGF) are pivotal proteins which participate in the development of new capillaries and finally cause damage to the retinal neurons. During recent years, our knowledge about the function of growth factors in the pathogenesis of retinal neurodegeneration has increased. However, intensive investigations are needed to clarify the basic processes that contribute to retinal neurodegeneration and its association with damage to the capillary blood vessels. The objective of this review article is to show new insights on the role of neurotransmitters and growth factors in the pathogenesis of diabetic retinopathy. The information contained in this manuscript may provide the basis for novel strategies based on the factors of neurodegeneration to diagnose, prevent and treat DR in its earliest phases.

Adenosine receptors and caffeine in retinopathy of prematurity

Retinopathy of prematurity (ROP) is a major cause of childhood blindness in the world and is caused by oxygen-induced damage to the developing retinal vasculature, resulting in hyperoxia-induced vaso-obliteration and subsequent delayed retinal vascularization and hypoxia-induced pathological neovascularization driven by vascular endothelial growth factor (VEGF) signaling pathway in retina. Current anti-VEGF therapy has shown some effective in a clinical trial, but is associated with the unintended effects on delayed eye growth and retinal vasculature development of preterm infants. Notably, cellular responses to hypoxia are characterized by robust increases in extracellular adenosine production and the markedly induced adenosine receptors, which provide a novel target for preferential control of pathological angiogenesis without affecting normal vascular development. Here, we review the experimental evidence in support of adenosine receptor-based therapeutic strategy for ROP, including the aberrant adenosine signaling in oxygen-induced retinopathy and the role of three adenosine receptor subtypes (A₁R, A_2AR, A_2BR) in development and treatment of ROP using oxygen-induced retinopathy models. The clinical and initial animal evidence that implicate the therapeutic effect of caffeine (a non-selective adenosine receptor antagonist) in treatment of ROP are highlighted. Lastly, we discussed the translational potential as well therapeutic advantage of adenosine receptor- and caffeine-based therapy for ROR and possibly other proliferative retinopathy.

Effects of the neuroprotective drugs somatostatin and brimonidine on retinal cell models of diabetic retinopathy

AIMS

Diabetic retinopathy is considered a microvascular disease, but recent evidence has underlined early involvement of the neuroretina with interactions between microvascular and neural alterations. Topical administration of somatostatin (SST), a neuroprotective molecule with antiangiogenic properties, prevents diabetes-induced retinal neurodegeneration in animals. The α₂-adrenergic receptor agonist brimonidine (BRM) decreases vitreoretinal vascular endothelial growth factor and inhibits blood-retinal barrier breakdown in diabetic rats. However, SST and BRM effects on microvascular cells have not yet been studied. We investigated the behaviour of these drugs on the crosstalk between microvasculature and neuroretina.

METHODS

Expression of SST receptors 1-5 in human retinal pericytes (HRP) was checked. We subsequently evaluated the effects of diabetic-like conditions (high glucose and/or hypoxia) with/without SST/BRM on HRP survival. Endothelial cells (EC) and photoreceptors were maintained in the above conditions and their conditioned media (CM) used to culture HRP. Vice versa, HRP-CM was used on EC and photoreceptors. Survival parameters were assessed.

RESULTS

HRP express the SST receptor 1 (SSTR1). Glucose fluctuations mimicking those occurring in diabetic subjects are more damaging for pericytes and photoreceptors than stable high glucose and hypoxic conditions. SST/BRM added to HRP in diabetic-like conditions decrease EC apoptosis. However, neither SST nor BRM changed the response of pericytes and neuroretina-vascular crosstalk under diabetic-like conditions.

CONCLUSIONS

Retinal pericytes express SSTR1, indicating that they can be a target for SST. Exposure to SST/BRM had no adverse effects, direct or mediated by the neuroretina, suggesting that these molecules could be safely evaluated for the treatment of ocular diseases.

Renin-angiotensin system as a potential therapeutic target in stroke and retinopathy: experimental and clinical evidence

As our knowledge expands, it is now clear that the renin-angiotensin (Ang) system (RAS) mediates functions other than regulating blood pressure (BP). The RAS plays a central role in the pathophysiology of different neurovascular unit disorders including stroke and retinopathy. Moreover, the beneficial actions of RAS modulation in brain and retina have been documented in experimental research, but not yet exploited clinically. The RAS is a complex system with distinct yet interconnected components. Understanding the different RAS components and their functions under brain and retinal pathological conditions is crucial to reap their benefits. The aim of the present review is to provide an experimental and clinical update on the role of RAS in the pathophysiology and treatment of stroke and retinopathy. Combining the evidence from both these disorders allows a unique opportunity to move both fields forward.

Adenosine A1 Receptors Selectively Modulate Oxygen-Induced Retinopathy at the Hyperoxic and Hypoxic Phases by Distinct Cellular Mechanisms

PURPOSE

We critically evaluated the role of the adenosine A1 receptor (A1R) in normal development of retinal vasculature and pathogenesis of retinopathy of prematurity (ROP) by using the A1R knockout (KO) mice and oxygen-induced retinopathy (OIR) model.

METHODS

Mice deficient in A1Rs and their wild-type (WT) littermates were examined during normal postnatal development or after being subjected to 75% oxygen from postnatal day (P) 7 to P12 and to room air from P12 to P17 (OIR model of ROP). Retinal vascularization was examined by whole-mount fluorescence and cross-sectional hematoxylin-eosin staining. Cellular proliferation, astrocyte and microglial activation, and tip cell function were determined by isolectin staining and immunohistochemistry. Apoptosis was determined by TUNEL assay.

RESULTS

Genetic deletion of the A1R did not affect normal retinal vascularization during postnatal development with indistinguishable three-layer vascularization patterns in retina between WT and A1R KO mice. In the OIR model, genetic deletion of the A1R resulted in stage-specific effects: reduced hyperoxia-induced retinal vaso-obliteration at P12, but reduced avascular area and attenuated hypoxia-induced intraretinal revascularization without affecting intravitreal neovascularization at P17 and reduced avascular areas in retina at P21. These distinct effects of A1Rs on OIR were associated with A1R control of apoptosis mainly in inner and outer nuclear layers at the vaso-obliterative phase (P12) and the growth of endothelium tip cells at the vasoproliferative phase (P17), without modification of cellular proliferation, astrocytic activation, and tissue inflammation.

CONCLUSIONS

Adenosine A1 receptor activity is not required for normal postnatal development of retinal vasculature but selectively controls hyperoxia-induced vaso-obliteration and hypoxia-driven revascularization by distinct cellular mechanisms.

Methylene blue prevents retinal damage in an experimental model of ischemic proliferative retinopathy

Perinatal asphyxia induces retinal lesions, generating ischemic proliferative retinopathy, which may result in blindness. Previously, we showed that the nitrergic system was involved in the physiopathology of perinatal asphyxia. Here we analyze the application of methylene blue, a well-known soluble guanylate cyclase inhibitor, as a therapeutic strategy to prevent retinopathy. Male rats (n = 28 per group) were treated in different ways: 1) control group comprised born-to-term animals; 2) methylene blue group comprised animals born from pregnant rats treated with methylene blue (2 mg/kg) 30 and 5 min before delivery; 3) perinatal asphyxia (PA) group comprised rats exposed to perinatal asphyxia (20 min at 37°C); and 4) methylene blue-PA group comprised animals born from pregnant rats treated with methylene blue (2 mg/kg) 30 and 5 min before delivery, and then the pups were subjected to PA as above. For molecular studies, mRNA was obtained at different times after asphyxia, and tissue was collected at 30 days for morphological and biochemical analysis. Perinatal asphyxia produced significant gliosis, angiogenesis, and thickening of the inner retina. Methylene blue treatment reduced these parameters. Perinatal asphyxia resulted in a significant elevation of the nitrergic system as shown by NO synthase (NOS) activity assays, Western blotting, and (immuno)histochemistry for the neuronal isoform of NOS and NADPH-diaphorase activity. All these parameters were also normalized by the treatment. In addition, methylene blue induced the upregulation of the anti-angiogenic peptide, pigment epithelium-derived factor. Application of methylene blue reduced morphological and biochemical parameters of retinopathy. This finding suggests the use of methylene blue as a new treatment to prevent or decrease retinal damage in the context of ischemic proliferative retinopathy.

Losartan Treatment Protects Retinal Ganglion Cells and Alters Scleral Remodeling in Experimental Glaucoma

Purpose

To determine if oral losartan treatment decreases the retinal ganglion cell (RGC) death caused by experimental intraocular pressure (IOP) elevation in mice.

Methods

We produced IOP increase in CD1 mice and performed unilateral optic nerve crush. Mice received oral losartan, spironolactone, enalapril, or no drug to test effects of inhibiting angiotensin receptors. IOP was monitored by Tonolab, and blood pressure was monitored by tail cuff device. RGC loss was measured in masked axon counts and RGC bodies by β-tubulin labeling. Scleral changes that could modulate RGC injury were measured including axial length, scleral thickness, and retinal layer thicknesses, pressure-strain behavior in inflation testing, and study of angiotensin receptors and pathways by reverse transcription polymerase chain reaction, Western blot, and immunohistochemistry.

Results

Losartan treatment prevented significant RGC loss (median loss = 2.5%, p = 0.13), while median loss with water, spironolactone, and enalapril treatments were 26%, 28% and 43%; p < 0.0001). The lower RGC loss with losartan was significantly less than the loss with spironolactone or enalapril (regression model p = 0.001; drug treatment group term p = 0.01). Both losartan and enalapril significantly lowered blood pressure (p< 0.001), but losartan was protective, while enalapril led to worse than water-treated RGC loss. RGC loss after crush injury was unaffected by losartan treatment (difference from control p = 0.9). Survival of RGC in cell culture was not prolonged by sartan treatment. Axonal transport blockade after 3 day IOP elevations was less in losartan-treated than in control glaucoma eyes (p = 0.007). Losartan inhibited effects of glaucoma, including reduction in extracellular signal-related kinase activity and modification of glaucoma-related changes in scleral thickness and creep under controlled IOP.

Conclusions

The neuroprotective effect of losartan in mouse glaucoma is associated with adaptive changes in the sclera expressed at the optic nerve head.

Ocular renin-angiotensin system with special reference in the anterior part of the eye

The renin-angiotensin system (RAS) regulates blood pressure (BP) homeostasis, systemic fluid volume and electrolyte balance. The RAS cascade includes over twenty peptidases, close to twenty angiotensin peptides and at least six receptors. Out of these, angiotensin II, angiotensin converting enzyme 1 and angiotensin II type 1 receptor (AngII-ACE1-AT1R) together with angiotensin (1-7), angiotensin converting enzyme 2 and Mas receptor (Ang(1-7)-ACE2-MasR) are regarded as the main components of RAS. In addition to circulating RAS, local RA-system exists in various organs. Local RA-systems are regarded as tissue-specific regulatory systems accounting for local effects and long term changes in different organs. Many of the central components such as the two main axes of RAS: AngII-ACE1-AT1R and Ang(1-7)-ACE2-MasR, have been identified in the human eye. Furthermore, it has been shown that systemic antihypertensive RAS- inhibiting medications lower intraocular pressure (IOP). These findings suggest the crucial role of RAS not only in the regulation of BP but also in the regulation of IOP, and RAS potentially plays a role in the development of glaucoma and antiglaucomatous drugs.

Candesartan stimulates reparative angiogenesis in ischemic retinopathy model: role of hemeoxygenase-1 (HO-1)

Ischemic diseases such as stroke and proliferative retinopathy are characterized by hypoxia-driven release of angiogenic factors such as vascular endothelial growth factor (VEGF). However, revascularization of the ischemic areas is inadequate, resulting in impaired neuro-vascular function. We aim to examine the vascular protective effects of candesartan, an angiotensin receptor blocker, in an ischemic retinopathy mouse model. Vascular density, number of tip cells, and perfusions of capillaries were assessed. Activation of Muller glial cells and levels of peroxynitrite, VEGF, VEGFR2, inducible nitric oxide synthase, hemeoxygenase-1 (HO-1) were assessed. Proangiogenic effects of candesartan were examined in human endothelial cells (EC) that were cultured in normoxia or hypoxia and transduced with siRNA against HO-1. Candesartan (1 mg/kg) and (10 mg/kg) decreased hypoxia-induced neovascularization by 67 and 70%, respectively. Candesartan (10 mg/kg) significantly stimulated the number of tip cells and physiological revascularization of the central retina (45%) compared with untreated pups. The effects of candesartan coincided with reduction of hypoxia-induced Muller glial activation, iNOS expression and restoration of HO-1 expression with no significant change in VEGF levels. In vitro, silencing HO-1 expression blunted the ability of candesartan to induce VEGF expression under normoxia and VEGFR2 activation and angiogenic response under both normoxia and hypoxia. These findings suggest that candesartan improved reparative angiogenesis and hence prevented pathological angiogenesis by modulating HO-1 and iNOS levels in ischemic retinopathy. HO-1 is required for VEGFR2 activation and proangiogenic action of candesartan in EC. Candesartan, an FDA-approved drug, could be repurposed as a potential therapeutic agent for the treatment of ischemic diseases.

Choosing preclinical study models of diabetic retinopathy: key problems for consideration

Diabetic retinopathy (DR) is the most common complication of diabetes mellitus in the eye. Although the clinical treatment for DR has already developed to a relative high level, there are still many urgent problems that need to be investigated in clinical and basic science. Currently, many in vivo animal models and in vitro culture systems have been applied to solve these problems. Many approaches have also been used to establish different DR models. However, till now, there has not been a single study model that can clearly and exactly mimic the developmental process of the human DR. Choosing the suitable model is important, not only for achieving our research goals smoothly, but also, to better match with different experimental proposals in the study. In this review, key problems for consideration in choosing study models of DR are discussed. These problems relate to clinical relevance, different approaches for establishing models, and choice of different species of animals as well as of the specific in vitro culture systems. Attending to these considerations will deepen the understanding on current study models and optimize the experimental design for the final goal of preventing DR.

Angiotensin II-related hypertension and eye diseases

Systemic vascular disease, especially hypertension, has been suspected as a risk factor for some eye diseases including, diabetic retinopathy and age-related macular degeneration. Hypertension can contribute to chronic diseases by hemodynamic injury and/or cellular actions induced by hypertension-related hormones or growth factors. Among the most important is Angiotensin II (Ang II), which controls blood pressure and induces different cellular functions that may be dependent or independent of its effect on blood pressure. Importantly, as is true for heart, kidney and other organs, the renin-angiotensin system (RAS) is present in the eye. So, even in the absence of hypertension, local production of Ang II could be involved in eye diseases. The goal of this manuscript is to review the most relevant scientific evidence supporting the role of the RAS activation, in the development of age-related macular degeneration and diabetic retinopathy, and highlight the importance of Ang II in the etiology of these diseases.

Quantification of vascular tortuosity as an early outcome measure in oxygen induced retinopathy (OIR)

Oxygen-induced retinopathy (OIR) in mice is a popular model system to study pathological angiogenesis in the retinal vasculature. The system is based on vessel depletion by exposure to hyperoxia, which results in acute retinal hypoxia upon return to room air. This hypoxia then triggers neovascularization in the remaining vessels after 5 days. Here we aimed to establish an additional and earlier experimental readout of the vascular response to hypoxia by quantifying the tortuosity of retinal arteries after 2 days. Mouse pups from three different mouse strains were exposed to hyperoxia from postnatal day (P) 7 to P12 and retinas were analysed at P12, P14 and P17. Hypoxia was assessed by staining with the hypoxia marker EF5 and by measuring Vegf mRNA by qPCR. The retinal vasculature was stained in whole mount retinas and tortuosity of radial arterioles was quantified. C57BL/6J mice were used because the vascular response at P17 is well characterised in this strain. We also used C3H/HeJ mice, which contain the retinal degeneration 1 (Rd1) mutation (Pde6b(Rd1)) and have abnormally thin retinas. These thinner, C3H/HeJ retinas do not become ischemic during the OIR model and do not develop neovascularization. They can therefore be used as a control. In addition, we included C3H/HeJ mice that lack the Rd1 mutation (C3H/He(Rd1-)), with normal thickness retinas, to control for strain differences between C57BL/6J and C3H/HeJ. Quantification of vessel tortuosity at P14 showed tortuous arteries in normal thickness retinas (C57BL/6J and C3H/He(Rd1-)) and straight arteries in the thin C3H/HeJ retinas. This correlated with hypoxia, which was severe in normal thickness retinas and mild in the thin C3H/HeJ retinas. Furthermore, at P17 the normal thickness retinas showed strong neovascularisation whereas in the thin C3H/HeJ retinas the retinal vasculature regenerated normally. In conclusion we have demonstrated that arterial tortuosity can act as an early readout for hypoxia in the OIR model before neovascularisation develops.

Neurodegeneration in the diabetic eye: new insights and therapeutic perspectives

Diabetic retinopathy (DR), one of the leading causes of preventable blindness, has been considered a microcirculatory disease of the retina. However, there is emerging evidence to suggest that retinal neurodegeneration is an early event in the pathogenesis of DR, which participates in the development of microvascular abnormalities. Therefore, the study of the underlying mechanisms leading to neurodegeneration and the identification of the mediators in the crosstalk between neurodegeneration and microangiopathy will be essential for the development of new therapeutic strategies. In this review, an updated discussion of the mechanisms involved in neurodegeneration, as well as the link between neurodegeneration and microangiopathy, is presented. Finally, the therapeutic implications and new perspectives based on identifying those patients with retinal neurodegeneration are given.

The ocular renin-angiotensin system: a therapeutic target for the treatment of ocular disease

The renin-angiotensin system (RAS) is most well-known for its role in regulation and dysregulation of blood pressure as well as fluid and electrolyte homeostasis. Due to its ability to cause cardiovascular disease, the RAS is the target of a multitude of drugs that antagonize its pathophysiological effects. While the "classical" RAS is a systemic hormonal system, there is an increasing awareness of the existence and functional significance of local RASs in a number of organs, e.g., liver, kidney, heart, lungs, reproductive organs, adipose tissue and adrenal. The eye is one of these organs where a compelling body of evidence has demonstrated the presence of a local RAS. Individual components of the RAS have been shown to be present in many structures of the eye and their potential functional significance in ocular disease states is described. Because the eye is one of the most important and complex organs in the body, this review also discusses the implications of dysregulation of the systemic RAS on the pathogenesis of ocular diseases and how pharmacological manipulation of the RAS might lead to novel or adjunctive therapies for ocular disease states.

Topical administration of somatostatin prevents retinal neurodegeneration in experimental diabetes

Retinal neurodegeneration is an early event in the pathogenesis of diabetic retinopathy (DR). Somatostatin (SST) is an endogenous neuroprotective peptide that is downregulated in the diabetic eye. The aim of the study was to test the usefulness of topical administration of SST in preventing retinal neurodegeneration. For this purpose, rats with streptozotocin-induced diabetes mellitus (STZ-DM) were treated with either SST eye drops or vehicle for 15 days. Nondiabetic rats treated with vehicle served as a control group. Functional abnormalities were assessed by electroretinography (ERG), and neurodegeneration was assessed by measuring glial activation and the apoptotic rate. In addition, proapoptotic (FasL, Bid, and activation of caspase-8 and caspase-3) and survival signaling pathways (BclxL) were examined. Intraretinal concentrations of glutamate and its main transporter glutamate/aspartate transporter (GLAST) were also determined. Treatment with SST eye drops prevented ERG abnormalities, glial activation, apoptosis, and the misbalance between proapoptotic and survival signaling detected in STZ-DM rats. In addition, SST eye drops inhibited glutamate accumulation in the retina and GLAST downregulation induced by diabetes mellitus. We conclude that topical administration of SST has a potent effect in preventing retinal neurodegeneration induced by diabetes mellitus. In addition, our findings open up a new preventive pharmacological strategy targeted to early stages of DR.

Hyperoxia causes reduced density of retinal astrocytes in the central avascular zone in the mouse model of oxygen-induced retinopathy

The mouse model of oxygen-induced retinopathy (OIR) is commonly used to investigate various aspects of the pathogenesis of the retinopathy of prematurity (ROP) as well as angiogenesis in general. Retinal astrocytes were suggested to be involved in retinal angiogenesis. This study aimed to describe their localization and cell density during the course of physiological vascularization and pathological revascularization. Mice expressing H2B-GFP (green fluorescent protein fused to histone 2B) from the endogenous Pdgfra promoter were kept in 75% oxygen from P7 (post natal day 7) to P12 (mouse model of OIR). Retinal flatmounts or cryosections were immunostained for glial fibrillary acidic protein (Gfap), glutamine synthetase (Glul), collagen IV (Col IV), desmin (Des), caspase 3 (Casp3), paired box 2 (Pax2), or Ki67. Astrocytic nuclei were counted with the ImageJ macro AuTOCellQuant. The hypoxic state of the retina was investigated by Hypoxyprobe. The GFP signal of the Pdgfra reporter mice co-localized with Pax2, a nuclear marker for retinal astrocytes. This bright label was much easier to quantify than Gfap or Pax2 staining. Quantification of the cell density of astrocytes during physiological development specified the spreading of astrocytes in a concentrical wave from the optic nerve head towards the periphery. Astrocyte density was reduced during the remodelling of the primary vascular plexus into a hierarchical vascular tree (maximal astrocyte density at P1: 2800 astrocytes/mm2, final astrocyte density: 800 astrocytes/mm2). In the OIR model, cell density of astrocytes was elevated in the peripheral vascularized zone. In contrast, astrocyte density dropped to a half (400 astrocytes/mm2) of the normal value in the central avascular zone during the hyperoxic phase between P8 and P10 by apoptosis and rose only after P17 as the retinal network normalized. An additional drop of astrocyte density was observed within the angles between the large vessels of the central avascular zone during hypoxia between P12 and P17. Astrocyte density was not altered at vascular tufts. The hyperoxia effect on astrocytes including the reduced astrocyte density is not the reason for vascular tuft formation. Hypoxia-affected astrocytes in combination with a reduced astrocytic network in the central avascular zone during the hypoxic phase are important determinants in the formation of pathological features during retinal revascularization.

Reactive oxygen species, Nox and angiotensin II in angiogenesis: implications for retinopathy

Pathological angiogenesis is a key feature of many diseases including retinopathies such as ROP (retinopathy of prematurity) and DR (diabetic retinopathy). There is considerable evidence that increased production of ROS (reactive oxygen species) in the retina participates in retinal angiogenesis, although the mechanisms by which this occurs are not fully understood. ROS is produced by a number of pathways, including the mitochondrial electron transport chain, cytochrome P450, xanthine oxidase and uncoupled nitric oxide synthase. The family of NADPH oxidase (Nox) enzymes are likely to be important given that their primary function is to produce ROS. Seven isoforms of Nox have been identified named Nox1-5, Duox (dual oxidase) 1 and Duox2. Nox1, Nox2 and Nox4 have been most extensively studied and are implicated in the development of conditions such as hypertension, cardiovascular disease and diabetic nephropathy. In recent years, evidence has accumulated to suggest that Nox1, Nox2 and Nox4 participate in pathological angiogenesis; however, there is no clear consensus about which Nox isoform is primarily responsible. In terms of retinopathy, there is growing evidence that Nox contribute to vascular injury. The RAAS (renin-angiotensin-aldosterone system), and particularly AngII (angiotensin II), is a key stimulator of Nox. It is known that a local RAAS exists in the retina and that blockade of AngII and aldosterone attenuate pathological angiogenesis in the retina. Whether the RAAS influences the production of ROS derived from Nox in retinopathy is yet to be fully determined. These topics will be reviewed with a particular emphasis on ROP and DR.

Animal models of diabetic retinopathy: summary and comparison

Diabetic retinopathy (DR) is a microvascular complication associated with chronic exposure to hyperglycemia and is a major cause of blindness worldwide. Although clinical assessment and retinal autopsy of diabetic patients provide information on the features and progression of DR, its underlying pathophysiological mechanism cannot be deduced. In order to have a better understanding of the development of DR at the molecular and cellular levels, a variety of animal models have been developed. They include pharmacological induction of hyperglycemia and spontaneous diabetic rodents as well as models of angiogenesis without diabetes (to compensate for the absence of proliferative DR symptoms). In this review, we summarize the existing protocols to induce diabetes using STZ. We also describe and compare the pathological presentations, in both morphological and functional aspects, of the currently available DR animal models. The advantages and disadvantages of using different animals, ranging from zebrafish, rodents to other higher-order mammals, are also discussed. Until now, there is no single model that displays all the clinical features of DR as seen in human. Yet, with the understanding of the pathological findings in these animal models, researchers can select the most suitable models for mechanistic studies or drug screening.

Angiotensin-2-mediated Ca2+ signaling in the retinal pigment epithelium: role of angiotensin-receptor-associated-protein and TRPV2 channel

Angiotensin II (AngII) receptor (ATR) is involved in pathologic local events such as neovascularisation and inflammation including in the brain and retina. The retinal pigment epithelium (RPE) expresses ATR in its AT1R form, angiotensin-receptor-associated protein (Atrap), and transient-receptor-potential channel-V2 (TRPV2). AT1R and Atrap co-localize to the basolateral membrane of the RPE, as shown by immunostaining. Stimulation of porcine RPE (pRPE) cells by AngII results in biphasic increases in intracellular free Ca(2+)inhibited by losartan. Xestospongin C (xest C) and U-73122, blockers of IP3R and PLC respectively, reduced AngII-evoked Ca(2+)response. RPE cells from Atrap(-/-) mice showed smaller AngII-evoked Ca(2+)peak (by 22%) and loss of sustained Ca(2+)elevation compared to wild-type. The TRPV channel activator cannabidiol (CBD) at 15 µM stimulates intracellular Ca(2+)-rise suggesting that porcine RPE cells express TRPV2 channels. Further evidence supporting the functional expression of TRPV2 channels comes from experiments in which 100 µM SKF96365 (a TRPV channel inhibitor) reduced the cannabidiol-induced Ca(2+)-rise. Application of SKF96365 or reduction of TRPV2 expression by siRNA reduced the sustained phase of AngII-mediated Ca(2+)transients by 53%. Thus systemic AngII, an effector of the local renin-angiotensin system stimulates biphasic Ca(2+)transients in the RPE by releasing Ca(2+)from cytosolic IP3-dependent stores and activating ATR/Atrap and TRPV2 channels to generate a sustained Ca(2+)elevation.

Alternative pathways in the development of diabetic retinopathy: the renin-angiotensin and kallikrein-kinin systems

Diabetic retinopathy is a common complication of both type 1 and type 2 diabetes and is the leading cause of blindness in people of working age. Current treatment strategies are mostly limited to laser photocoagulation, which restricts proliferative retinopathic changes but also causes irreversible damage to the retina. This review examines two important pathways involved in regulating vascular function and their role in the development of diabetic retinopathy. One, the renin-angiotensin system, is well known and has established angiogenic effects on the retina that increase in diabetic retinopathy. The other, the kallikrein-kinin system, has recently been found to be important in the development of diabetic retinal complications. This review describes the components of the two signalling networks, examines the current animal model studies investigating the role of these pathways in diabetic retinopathy and reviews the clinical studies that have been undertaken examining systemic inhibition of different points in these pathways. These systems are promising targets for therapies aimed at inhibiting the development of diabetic retinopathy and in the future, combination therapies that take advantage of both pathways might result in new treatment options for this debilitating complication of diabetes.

Neuroprotection in diabetic retinopathy

Diabetic retinopathy (DR) has been considered to be a microcirculatory disease of the retina. However, there is emerging evidence to suggest that retinal neurodegeneration is an early event in the pathogenesis of DR, which may antedate, and also participates in, the microcirculatory abnormalities that occur in DR. Therefore, the study of the underlying mechanisms that lead to neurodegeneration will be essential for identifying new therapeutic targets in the early stages of DR. Elevated levels of glutamate, oxidative stress, the overexpression of the renin-angiotensin system and the upregulation of RAGE play an essential role in the retinal neurodegeneration induced by diabetes. Finally, the balance between the neurotoxic and neuroprotective factors is crucial in determining the survival of retinal neurons. In this review we will focus on neurotrophic factors already synthesized by the retina in physiological conditions as a new therapy strategy for neuroprotection.

The retinal renin-angiotensin system: roles of angiotensin II and aldosterone

In the present review we examine the experimental and clinical evidence for the presence of a local renin-angiotensin system within the retina. Interest in a pathogenic role for the renin-angiotensin system in retinal disease originally stemmed from observations that components of the pathway were elevated in retina during the development of certain retinal pathologies. Since then, our knowledge about the contribution of the RAS to retinal disease has greatly expanded. We discuss the known functions of the renin-angiotensin system in retinopathy of prematurity and diabetic retinopathy. This includes the promotion of retinal neovascularization, inflammation, oxidative stress and neuronal and glial dysfunction. The contribution of specific components of the renin-angiotensin system is evaluated with a particular focus on angiotensin II and aldosterone and their cognate receptors. The therapeutic utility of inhibiting key components of the renin-angiotensin system is complex, but may hold promise for the prevention and improvement of vision threatening diseases.

TMP prevents retinal neovascularization and imparts neuroprotection in an oxygen-induced retinopathy model

PURPOSE

To evaluate the effects of tetramethylpyrazine (TMP) on retinal neovascularization (NV) and neuroprotection in an oxygen-induced retinopathy (OIR) model.

METHODS

Neonatal C57BL/6J mice were subjected to 75% oxygen from postnatal day 7 (P7) to P12 and then returned to room air. TMP (200 mg/kg) or normal saline was given daily from P12 to P17. Immunostaining, HE staining, TUNEL assay, and RT-PCR were used to assess the effects of TMP on retinal neurovascular repair.

RESULTS

TMP effectively prevented pathologic NV and accelerated physiologic revascularization by enhancing the formation of endothelial tip cells at the edges of the repairing capillary networks and preserving the astrocytic template in the avascular retina. TMP also prevented morphologic changes and significantly decreased TUNEL-positive cells in the avascular retina by rescuing neurons such as amacrine, rod bipolar, horizontal, and Müller cells. In TMP-treated mice retinas, there was a less obvious loss of amacrine cell bodies and their distinct bands; the number of both rod bipolar and horizontal cell bodies, as well as the density of their dendrites in the outer plexiform layer, was greater than that in OIR control mice. TMP not only decreased the loss of alignment of Müller cell bodies and distortion of processes but reduced the reactive expression of GFAP in Müller cells. Furthermore, HIF-1α and VEGF mRNA expression were downregulated in TMP-treated mice retinas.

CONCLUSIONS

TMP improved neurovascular recovery by preventing NV and protecting retinal astroglia cells and neurons from ischemia-induced cell death partially due to its downregulation of HIF-1α and VEGF mRNA expression.

The role of glia in retinal vascular disease

Retinal vascular diseases collectively represent a leading cause of blindness. Unsurprisingly, pathological characterisation and treatment of retinal 'vascular' diseases have primarily focused on the aetiology and consequences of vascular dysfunction. Far less research has addressed the contribution of neuronal and glial dysfunction to the disease process of retinal vascular disorders. Ample evidence now suggests that retinal vasculopathy only uncommonly occurs in isolation, usually existing in concert with neuropathy and gliopathy. Retinal glia (Müller cells, astrocytes and microglia) have been reported to exhibit morphological and functional changes in both early and advanced phases of almost every retinal vascular disease. It is anticipated that identifying the causes of glial activation and dysfunction, and their contribution to loss of vision in retinal vascular disease, will lead to a better understanding of retinal vascular diseases, which might ultimately be translated into novel clinical therapies.

Candesartan, an angiotensin II type 1 receptor antagonist, inhibits pathological retinal neovascularization by downregulating VEGF receptor-2 expression

Several studies have examined the anti-angiogenic effects of angiotensin II type 1 (AT(1)) receptor antagonists; however, the mechanisms underlying these effects are currently unclear. In the present study, we examined the efficacy and the mechanism of candesartan, an AT(1) receptor antagonist, in suppressing pathological retinal neovascularization. We used an in vivo murine oxygen-induced retinopathy (OIR) model and also studied the in vitro proliferation and migration of human retinal microvascular endothelial cells (HRMECs) induced by vascular endothelial growth factor (VEGF)-A. The regulation of angiogenesis-associated genes such as hypoxia-inducible factor (HIF-1α), VEGF-A, VEGF receptor-1, and VEGF receptor-2 was evaluated with real-time RT-PCR in the OIR model. In the OIR model, candesartan suppressed the pathological neovascularization in a dose-dependent manner, but did not prevent the physiological angiogenesis. However, candesartan did not inhibit VEGF-A-induced proliferation or migration in HRMECs in the in vitro study. When administered interperitoneally in the OIR model, candesartan reduced the upregulation of VEGF receptor-2 in the retina, but had no effects in the other angiogenesis-related genes, such as HIF-1α, VEGF-A, and VEGF receptor-1. These findings indicate that candesartan inhibited the retinal pathological neovascularization, at least in part, by suppressing the expression of VEGF receptor-2, independent of VEGF signaling cascade. Therefore, candesartan may be a useful therapeutic target for the inhibition of retinal neovascularization that has a low risk of serious side effects.

Renin-Angiotensin system hyperactivation can induce inflammation and retinal neural dysfunction

The renin-angiotensin system (RAS) is a hormone system that has been classically known as a blood pressure regulator but is becoming well recognized as a proinflammatory mediator. In many diverse tissues, RAS pathway elements are also produced intrinsically, making it possible for tissues to respond more dynamically to systemic or local cues. While RAS is important for controlling normal inflammatory responses, hyperactivation of the pathway can cause neural dysfunction by inducing accelerated degradation of some neuronal proteins such as synaptophysin and by activating pathological glial responses. Chronic inflammation and oxidative stress are risk factors for high incidence vision-threatening diseases such as diabetic retinopathy (DR), age-related macular degeneration (AMD), and glaucoma. In fact, increasing evidence suggests that RAS inhibition may actually prevent progression of various ocular diseases including uveitis, DR, AMD, and glaucoma. Therefore, RAS inhibition may be a promising therapeutic approach to fine-tune inflammatory responses and to prevent or treat certain ocular and neurodegenerative diseases.

Angiotensin II modulates VEGF-driven angiogenesis by opposing effects of type 1 and type 2 receptor stimulation in the microvascular endothelium

Vascular endothelial growth factor (VEGF) is a main stimulator of pathological vessel formation. Nevertheless, increasing evidence suggests that Angiotensin II (Ang II) can play an augmentory role in this process. We thus analyzed the contribution of the two Ang II receptor types, AT(1)R and AT(2)R, in a mouse model of VEGF-driven angiogenesis, i.e. oxygen-induced proliferative retinopathy. Application of the AT(1)R antagonist telmisartan but not the AT(2)R antagonist PD123,319 largely attenuated the pathological response. A direct effect of Ang II on endothelial cells (EC) was analyzed by assessing angiogenic responses in primary bovine retinal and immortalized rat microvascular EC. Selective stimulation of the AT(1)R by Ang II in the presence of PD123,319 revealed a pro-angiogenic activity which further increased VEGF-driven EC sprouting and migration. In contrast, selective stimulation of the AT(2)R by either CGP42112A or Ang II in the presence of telmisartan inhibited the VEGF-driven angiogenic response. Using specific inhibitors (pertussis toxin, RGS proteins, kinase inhibitors) we identified G(12/13) and G(i) dependent signaling pathways as the mediators of the AT(1)R-induced angiogenesis and the AT(2)R-induced inhibition, respectively. As AT(1)R and AT(2)R stimulation displays opposing effects on the activity of the monomeric GTPase RhoA and pro-angiogenic responses to Ang II and VEGF requires activation of Rho-dependent kinase (ROCK), we conclude that the opposing effects of the Ang II receptors on VEGF-driven angiogenesis converge on the regulation of activity of RhoA-ROCK-dependent EC migration.

Neovascularization is attenuated with aldosterone synthase inhibition in rats with retinopathy

Neovascularization is a hallmark feature of retinopathy of prematurity and diabetic retinopathy. Type 1 angiotensin receptor blockade reduces neovascularization in experimental retinopathy of prematurity, known as oxygen-induced retinopathy (OIR). We investigated in OIR whether inhibiting aldosterone with the aldosterone synthase inhibitor FAD286 reduced neovascularization as effectively as angiotensin receptor blockade (valsartan). OIR was induced in neonatal Sprague-Dawley rats, and they were treated with FAD286 (30 mg/kg per day), valsartan (10 mg/kg per day), or FAD286+valsartan. The cellular sources of aldosterone synthase, the mineralocorticoid receptor, and 11β-hydroxysteroid dehydrogenase 2 were evaluated in retinal cells involved in neovascularization (primary endothelial cells, pericytes, microglia, ganglion cells, and glia). In OIR, FAD286 reduced neovascularization and neovascular tufts by 89% and 67%, respectively, and normalized the increase in vascular endothelial growth factor mRNA (1.74-fold) and protein (4.74-fold) and was as effective as valsartan and FAD286+valsartan. In retina, aldosterone synthase mRNA was reduced with FAD286 but not valsartan. Aldosterone synthase was detected in microglia, ganglion cells, and glia, whereas mineralocorticoid receptor and 11β-hydroxysteroid dehydrogenase 2 were present in all of the cell types studied. Given the location of aldosterone synthase in microglia and their contribution to retinal inflammation and neovascularization in OIR, the effects of FAD286 on microglial density were studied. The increase in microglial density (ionized calcium binding adaptor protein 1 immunolabeling) in OIR was reduced with all of the treatments. In OIR, FAD286 reduced the increase in mRNA for tumor necrosis factor-α, intercellular adhesion molecule 1, vascular cell adhesion molecule 1, and monocyte chemoattractant molecule 1. These findings indicate that aldosterone inhibition may be a potential treatment for retinal neovascularization.