Identification of angiotensin converting enzyme 2 in the rodent retina

Glaucoma and the ocular renin-angiotensin-aldosterone system: Update on molecular signalling and treatment perspectives

Glaucoma, a leading cause of blindness worldwide, encompasses a group of pathological conditions affecting the optic nerve and is characterized by progressive retinal ganglion cell loss, cupping of the optic nerve head, and distinct visual field defects. While elevated intraocular pressure (IOP) is the main risk factor for glaucoma, many patients do not have elevated IOP. Consequently, other risk factors, such as ocular blood flow abnormalities and immunological factors, have been implicated in its pathophysiology. Traditional therapeutic strategies primarily aim to reduce IOP, but there is growing interest in developing novel treatment approaches to improve disease management and reduce the high rates of severe visual impairment. In this context, targeting the ocular renin-angiotensin-aldosterone system (RAAS) has been found as a potential curative strategy. The RAAS contributes to glaucoma development through key effectors such as prorenin, angiotensin II, and aldosterone. Recent evidence has highlighted the potential of using RAAS modulators to combat glaucoma, yielding encouraging results. Our study aims to explore the molecular pathways linking the ocular RAAS and glaucoma, summarizing recent advances that elucidate the role of the RAAS in triggering oxidative stress, inflammation, and remodelling in the pathogenesis of glaucoma. Additionally, we will present emerging therapeutic approaches that utilize RAAS modulators and antioxidants to slow the progression of glaucoma.

Bibliometric and visual analysis of ACE2/Ang 1-7/MasR axis in diabetes and its microvascular complications from 2000 to 2023

Background

The pathogenesis of diabetes and its microvascular complications are intimately associated with renin angiotensin system dysregulation. Evidence suggests the angiotensin converting enzyme 2 (ACE2)/angiotensin 1-7 (Ang 1-7)/Mas receptor (MasR) axis regulates metabolic imbalances, inflammatory responses, reduces oxidative stress, and sustains microvascular integrity, thereby strengthening defences against diabetic conditions. This study aims to conduct a comprehensive analysis of the ACE2/Ang 1-7/MasR axis in diabetes and its microvascular complications over the past two decades, focusing on key contributors, research hotspots, and thematic trends.

Methods

This cross-sectional bibliometric analysis of 349 English-language publications was performed using HistCite, VOSviewer, CiteSpace, and Bibliometrix R for visualization and metric analysis. Primary analytical metrics included publication count and keyword trend dynamics.

Results

The United States, contributing 105 articles, emerged as the most productive country, with the University of Florida leading institutions with 18 publications. Benter IF was the most prolific author with 14 publications, and Clinical Science was the leading journal with 13 articles. A total of 151 of the 527 author's keywords with two or more occurrences clustered into four major clusters: diabetic microvascular pathogenesis, metabolic systems, type 2 diabetes, and coronavirus infections. Keywords such as "SARS", "ACE2", "coronavirus", "receptor" and "infection" displayed the strongest citation bursts. The thematic evolution in this field expanded from focusing on the renin angiotensin system (2002-2009) to incorporating ACE2 and diabetes metabolism (2010-2016). The latter period (2017-2023) witnessed a significant surge in diabetes research, reflecting the impact of COVID-19 and associated conditions such as diabetic retinopathy and cardiomyopathy.

Conclusions

This scientometric study offers a detailed analysis of the ACE2/Ang 1-7/MasR axis in diabetes and its microvascular complications, providing valuable insights for future research directions.

Different responses of the MIO‑M1 Mueller cell line to angiotensin II under hyperglycemic or hypoxic conditions

Members of the renin-angiotensin aldosterone system (RAAS) are expressed by various retinal tissues including Mueller glial cells. As the RAAS is hypothesized to play an important role in the pathogenesis of diseases that threaten vision, such as diabetic macular edema or retinal vein occlusion, the possible changes induced by exposure of the human cell line MIO-M1, an established model of Mueller cells, to angiotensin II or aldosterone for 6 h under hypoxic and/or hyperglycemic conditions were investigated. The mRNA expression levels of the members of the RAAS were assessed by reverse transcription-quantitative PCR, and the secretion of cytokines was assessed by ELISA. Under hyperglycemic conditions, the mRNA expression levels of the angiotensin-converting enzyme 2 (ACE2), angiotensin II receptors, AT1 and AT2, and the receptor of angiotensin (1-7) MAS1 were significantly higher after exposure to angiotensin II, and the expression of ACE2, AT2, and IL-6 (a marker of inflammation) was significantly increased after treatment with aldosterone; the expression of the other targets investigated remained unchanged. Significantly more IL-6 was secreted by MIO-M1 cells exposed to hyperglycemia and angiotensin. When cells were cultured in a hypoxic environment, additional treatment with aldosterone significantly increased the mRNA expression levels of ACE, but significantly more ACE2 mRNA was expressed in the presence of angiotensin II. Under hypoxic plus hyperglycemic conditions, significantly less ACE but more AT2 was expressed after treatment with angiotensin II, which also led to strongly elevated expression of IL-6. The mRNA expression levels of the angiogenic growth factor VEGF-A and secretion of the encoded protein were notably increased under hypoxic and hypoxic plus hyperglycemic conditions, irrespective of additional treatment with angiotensin II or aldosterone. These findings suggest that angiotensin II induces a pro-inflammatory response in MIO-M1 cells under hyperglycemic conditions despite activation of the counteracting ACE2/MAS1 signaling cascade. However, hypoxia results in an increased expression of angiogenic VEGF-A by these cells, which is not altered by angiotensin II or aldosterone.

Evaluation of acute effects of pulmonary involvement and hypoxia on retina and choroid in coronavirus disease 2019: An optic coherence tomography study

PURPOSE

We investigated the acute subclinical choroidal and retinal changes caused by Coronavirus Disease 2019 (COVID-19) in patients with and without pulmonary involvement, using spectral domain optic coherence tomography.

METHODS

This prospective case-control study included COVID-19 patients: 50 with pulmonary involvement and 118 with non-pulmonary involvement. All patients were examined 1 month after recovering from COVID-19. The changes were followed using optic coherence tomography parameters such as choroidal and macular thickness and retinal nerve fibre layer and ganglion cell complex measurements.

RESULTS

All choroidal thicknesses in the pulmonary involvement group were lower than in the non-pulmonary involvement group and the subfoveal choroidal thickness differed significantly (p=0.036). Although there were no significant differences between the central and average macular thicknesses in the two groups, they were slightly thicker in the pulmonary involvement group (p=0.152 and p=0.180, respectively). A significant decrease was detected in the pulmonary involvement group in all ganglion cell complex segments, except for the outer nasal inferior segment (p<0.05). In addition, a thinning tendency was observed in all retinal nerve fibre layer quadrants in the pulmonary involvement group compared to the non-pulmonary involvement group.

CONCLUSION

In COVID-19 patients with pulmonary involvement, subclinical choroidal and retinal changes may occur due to hypoxia and ischemia in the acute period. These patients may be predisposed to ischemic retinal and optic nerve diseases in the future. Therefore, COVID-19 patients with pulmonary involvement should be followed for ophthalmological diseases.

SARS-CoV-2 infects and replicates in photoreceptor and retinal ganglion cells of human retinal organoids

Several studies have pointed to retinal involvement in COVID-19, yet many questions remain regarding the ability of SARS-CoV-2 to infect and replicate in retinal cells and its effects on the retina. Here, we have used human pluripotent stem cell-derived retinal organoids to study retinal infection by SARS-CoV-2. Indeed, SARS-CoV-2 can infect and replicate in retinal organoids, as it is shown to infect different retinal lineages, such as retinal ganglion cells and photoreceptors. SARS-CoV-2 infection of retinal organoids also induces the expression of several inflammatory genes, such as interleukin 33, a gene associated with acute COVID-19 and retinal degeneration. Finally, we show that the use of antibodies to block ACE2 significantly reduces SARS-CoV-2 infection of retinal organoids, indicating that SARS-CoV-2 infects retinal cells in an ACE2-dependent manner. These results suggest a retinal involvement in COVID-19 and emphasize the need to monitor retinal pathologies as potential sequelae of “long COVID.”

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SARS-CoV-2 can infect and replicate in retinal organoids

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Retinal ganglion cells are particularly open to SARS-CoV-2 infection

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SARS-CoV-2 infection of retinal organoids induces expression of inflammatory genes

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SARS-CoV-2 infection of retinal organoids is dependent on functional ACE2 receptors

Retinal and choroidal manifestations of COVID-19

PURPOSE OF REVIEW

The novel coronavirus disease in 2019 (COVID-19) has a wide range of systemic manifestations. Ocular manifestations are now well recognized, with conjunctivitis being the most common. Posterior segment involvement is uncommon and has mainly been described in case reports. This review provides an overview of the posterior segment involvement of COVID-19.

RECENT FINDINGS

Severe acute respiratory syndrome coronavirus 2 affects the retina and the choroid through either direct entry or triggering an indirect inflammatory response. Majority of the retinal findings is a result of microvascular derangement leading to cotton wool spots, intraretinal hemorrhages, paracentral acute middle maculopathy, acute macular neuroretinopathy, or retinal vein occlusions. Rarely, inflammation involving the retina or the choroid, or reactivation of previously quiescent uveitis, can be seen.

SUMMARY

It is important to recognize the possible correlation between ophthalmic conditions and COVID-19 as it can aid in diagnosis, management, and mitigation of the disease.

Serial swept source optical coherence tomography of a cotton wool spot following SARS CoV2 infection

A 33-year-old male presented with history of blurring of vision in the left eye for two months. He was seen by the local ophthalmologist and diagnosed as nongranulomatous anterior uveitis in the left eye with normal fundus. He had history of fever 20 days back and was diagnosed positive for COVID-19 by RT-PCR. He presented with acute anterior uveitis in the left eye. Fundus examination revealed a cotton wool spot close to the fovea, which was confirmed by swept-source optical coherence tomography (SS-OCT) of the macula. The patient after two months' follow-up showed resolution of the cotton wool spot, which was confirmed by SS-OCT. Our case indicates that cotton wool spot can be an ocular manifestation of COVID-19 infection, and swept-source optical coherence tomography can precisely document the resolution of the lesion.

Viral S protein histochemistry reveals few potential SARS-CoV-2 entry sites in human ocular tissues

Despite the reported low expression of the primary SARS-CoV-2 receptor ACE2 in distinct ocular tissues, some clinical evidence suggests that SARS-CoV-2 can infect the eye. In this study, we explored potential entry sites for SARS-CoV-2 by viral S protein histochemistry on various ocular tissues and compared the staining patterns with RNA and protein expression of TMPRSS2 and ACE2. Potential viral entry sites were investigated by histochemistry using tagged recombinant viral S protein on 52 ocular tissue samples including specimens of the cornea, conjunctiva, lid margin, lacrimal gland tissue, retina, choroid, and RPE. In addition, ACE2 and TMPRSS2 immunohistochemistry were performed on the same ocular tissue, each with distinct antibodies binding to different epitopes. Lung tissue samples were used as positive controls. Finally, bulk RNA sequencing (RNA-Seq) was used to determine the expression of ACE2 and its auxiliary factors in the tissues mentioned above. S protein histochemistry revealed a positive staining in lung tissue but absent staining in the cornea, the conjunctiva, eye lid samples, the lacrimal glands, the retina and the optic nerve which was supported by hardly any immunoreactivity for ACE2 and TMPRSS2 and scarce ACE2 and TMPRSS2 RNA expression. Negligible staining with antibodies targeting ACE2 or TMPRSS2 was seen in the main and accessory lacrimal glands. In contrast, ocular staining (S protein, ACE2, TMPRSS2) was distinctly present in pigmented cells of the RPE and choroid, as well as in the ciliary body and the iris stroma. S protein histochemistry revealed hardly any SARS-CoV-2 entry sites in all ocular tissues examined. Similarly, no significant ACE2 or TMPRSS2 expression was found in extra- and intraocular tissue. While this study suggest a rather low risk of ocular infection with SARS-CoV-2, it should be noted, that potential viral entry sites may increase in response to inflammation or in certain disease states.

Retinal Microcirculation as a Correlate of a Systemic Capillary Impairment After Severe Acute Respiratory Syndrome Coronavirus 2 Infection

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes coronavirus disease 2019 (COVID-19), affects the pulmonary systems via angiotensin-converting enzyme-2 (ACE-2) receptor, being an entry to systemic infection. As COVID-19 disease features ACE-2 deficiency, a link to microcirculation is proposed. Optical coherence tomography angiography (OCT-A) enables non-invasive analysis of retinal microvasculature. Thus, an impaired systemic microcirculation might be mapped on retinal capillary system. As recent OCT-A studies, analyzing microcirculation in two subdivided layers, yielded contrary results, an increased subdivision of retinal microvasculature might offer an even more fine analysis. The aim of the study was to investigate retinal microcirculation by OCT-A after COVID-19 infection in three subdivided layers (I). In addition, short-term retinal affections were monitored during COVID-19 disease (II). Considering (I), a prospective study (33 patients_post-COVID and 28 controls) was done. Macula and peripapillary vessel density (VD) were scanned with the Spectralis II. Macula VD was measured in three layers: superficial vascular plexus (SVP), intermediate capillary plexus (ICP), and deep capillary plexus (DCP). Analysis was done by the EA-Tool, including an Anatomical Positioning System and an analysis of peripapillary VD by implementing Bruch's membrane opening (BMO) landmarks. Overall, circular (c₁, c₂, and c₃) and sectorial VD (s₁-s₁₂) was analyzed. Considering (II), in a retrospective study, 29 patients with severe complications of COVID-19 infection, hospitalized at the intensive care unit, were monitored for retinal findings at bedside during hospitalization. (I) Overall (p = 0.0133) and circular (c₁, p = 0.00257; c₂, p = 0.0067; and c₃, p = 0.0345). VD of the ICP was significantly reduced between patients_post-COVID and controls, respectively. Overall (p = 0.0179) and circular (c₁, p = 0.0189) peripapillary VD was significantly reduced between both groups. Subgroup analysis of hospitalized vs. non-hospitalized patients_post-COVID yielded a significantly reduced VD of adjacent layers (DCP and SVP) with increased severity of COVID-19 disease. Clinical severity parameters showed a negative correlation with VD (ICP) and peripapillary VD. (II) Funduscopy yielded retinal hemorrhages and cotton wool spots in 17% of patients during SARS-CoV-2 infection. As VD of the ICP and peripapillary regions was significantly reduced after COVID-19 disease and showed a link to clinical severity markers, we assume that the severity of capillary impairment after COVID-19 infection is mapped on retinal microcirculation, visualized by non-invasive OCT-A.

Ocular Manifestations of Patients with Coronavirus Disease 2019: A Comprehensive Review

Apart from conjunctival involvement which is the most well-known ocular manifestation of coronavirus infectious disease 2019 (COVID-19), there are multiple reports of the involvement of other ocular structures by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). We comprehensively reviewed PubMed, Scopus, Embase, and Google Scholar for available evidence regarding COVID-19 various ocular manifestations, with special focus on less known and unusual ocular findings. We then categorized the findings based on the parts of the eye which was involved. In anterior sections of the eye, the involvement of the eyelid (tarsadenitis), conjunctiva and cornea (follicular conjunctivitis, pseudomembranous conjunctivitis, and keratoconjunctivitis), episclera (nodular episcleritis), uvea (anterior uveitis) were reported. Also, third, fourth, and sixth nerve palsy, retinal vasculitis, retinal optical coherence tomography (OCT) changes (hyper-reflective lesions and increased retinal nerve fiber layer thickness [RNFLT]), optic neuritis, papillophlebitis, Miller Fisher syndrome, posterior reversible leukoencephalopathy (PRES), ophthalmic artery and central retinal artery occlusion, and polyneuritis cranialis were reported in different studies. Postmortem evaluation of COVID-19 patients detected no viral RNA in different anterior and posterior segments of the eyes. However, another study revealed a 21.4% positivity of the retinal biopsies of dead patients. The results of this study can help ophthalmologists to be vigilant when they see these findings in a suspected case of COVID-19. In addition, wearing face masks and protective goggles or eye shields are recommended, especially in high risk contacts.

The eye: "An organ that must not be forgotten in coronavirus disease 2019 (COVID-2019) pandemic"

The coronavirus family is a group of zoonotic viruses with some recognized reservoirs particularly some bats. A novel coronavirus emerged in the province of Wuhan (China) in December of 2019.The number of infected patient with serious respiratory infection quickly spread around the world to become a global pandemic. The clinical presentation and viral pathogenesis of the coronavirus disease named COVID-19 indicated that the virus is transmitted from person to person through infected droplets entering the respiratory mucosa. Close contact with infected individuals particularly in crowded environments has characterized the rapid spread of the infection. Clinical manifestations of the viral infection have mentioned the presence of some ocular findings such as conjunctival congestion, conjunctivitis and even corneal injury associated with the classical COVID-19 infection. Some animal models of different coronaviruses eye infections have described the viral pathogenesis through tear and conjunctival sampling. On the other hand, we are recommended protective measure to prevent contagion and limit the spread of the virus in health care professionals and contact lenses wearers.

Retinal Microvascular Impairment in COVID-19 Bilateral Pneumonia Assessed by Optical Coherence Tomography Angiography

The purpose of this study was to evaluate the presence of retinal and microvascular alterations in COVID-19 patients with bilateral pneumonia due to SARS-COV-2 that required hospital admission and compare this with a cohort of age- and sex-matched controls. COVID-19 bilateral pneumonia patients underwent retinal imaging 14 days after hospital discharge with structural optical coherence tomography (OCT) and optical coherence tomography angiography (OCTA) measurements. Vessel density (VD) and foveal avascular zone (FAZ) area were evaluated in the superficial, deep capillary plexus (SCP, DCP), and choriocapillaris (CC). After exclusion criteria, only one eye per patient was selected, and 50 eyes (25 patients and 25 controls) were included in the analysis. COVID-19 patients presented significantly thinner ganglion cell layer (GCL) (p = 0.003) and thicker retinal nerve fiber layer (RNFL) compared to controls (p = 0.048), and this RNFL thickening was greater in COVID-19 cases with cotton wool spots (CWS), when compared with patients without CWS (p = 0.032). In both SCP and DCP, COVID-19 patients presented lower VD in the foveal region (p < 0.001) and a greater FAZ area than controls (p = 0.007). These findings suggest that thrombotic and inflammatory phenomena could be happening in the retina of COVID-19 patients. Further research is warranted to analyze the longitudinal evolution of these changes over time as well as their correlation with disease severity.

Can SARS-CoV-2 infect the eye? An overview of the receptor status in ocular tissue

Is the new coronavirus SARS-CoV‑2 able to infect ocular tissue and thus poses a risk of infection through the tissue in addition to the risk of contact? This is the question that has occupied ophthalmologists since the beginning of the outbreak. In order to infect a certain type of tissue specific receptors for each virus and sometimes also coreceptors or other proteins must be present. The aim of this review was to summarize and reflect the current state of research with the help of the currently available literature as of 28 May 2020. At the time of the research, angiotensin-converting enzyme 2 (ACE2) was clearly identified as the receptor and transmembrane serine protease 2 (TMPRSS2) as the necessary protease to enable the infection of human cells with SARS-CoV‑2. In the eye both ACE2 and TMPRSS2 are expressed, although sometimes very weakly and with varying degrees in different tissues. It is noteworthy that very different results were obtained with different methods. Several reasons can account for this effect: Firstly, the method of detection or preservation of the tissue, secondly, the possibly different expression of the tested tissue samples and thirdly, a possibly rapid loss of receptor expression post-mortem. Therefore, an infection of the eye seems possible, which has already been reported in various publications. The amount of virus or receptor expression necessary to cause an infection is not known. According to current state of knowledge the eye is not considered to be a high-risk tissue due to the low ACE2 and TMPRSS2 expression. Nevertheless, appropriate protective measures are necessary for both medical personnel and patients. In cases of corneal transplantation an infection of the donor tissue with SARS-CoV‑2 must be excluded.

Human coronaviruses: ophthalmic manifestations

The 2019 novel coronavirus which causes severe acute respiratory syndrome (SARS) known as SARS-CoV-2 still remains as a global pandemic since its discovery and continues to spread across the world, given how highly contagious the virus is. We reviewed various articles that explore eye involvement in COVID-19 and other human coronaviruses, its human manifestations in comparison to animal studies and potential mechanism of viral entry into the eye surface. Evidence of animal studies depicted various complications of coronaviruses infection into the eyes, in both anterior and posterior segments of the eye. Conjunctival inflammation remains uncommon in association with COVID-19, with other ophthalmic findings. The risk of transmission via the ocular surface remains likely low, though it is inarguably present based on preliminary finding of viral load in ocular samples and expression of ACE2 on the ocular surface. Testing the tears sample for diagnosing SARS-CoV-2 was unreliable due to limitations of the testing kits and conflicting evidence of the viral titre in the ocular samples. Further larger, more precise and specific studies are required to allow us to better understand the pattern of virulence underlying the associations of SARS-CoV-2 in the eye despite its rare occurrence. This review article aims to enhance better awareness among clinicians regarding ocular manifestations associated with COVID-19 and necessary precautions should be implemented to minimise the risk of person-to-person especially in the nosocomial setting.

Potential neuroinvasive pathways of SARS-CoV-2: Deciphering the spectrum of neurological deficit seen in coronavirus disease-2019 (COVID-19)

Coronavirus disease-2019 (COVID-19) was declared a global pandemic on 11 March 2020. Scientists and clinicians must acknowledge that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has the potential to attack the human body in multiple ways simultaneously and exploit any weaknesses of its host. A multipronged attack could potentially explain the severity and extensive variety of signs and symptoms observed in patients with COVID-19. Understanding the diverse tactics of this virus to infect the human body is both critical and incredibly complex. Although patients diagnosed with COVID-19 have primarily presented with pulmonary involvement, viral invasion, and injury to diverse end organs is also prevalent and well documented in these patients, but has been largely unheeded. Human organs known for angiotensin-converting enzyme 2 (ACE2) expression including the gastrointestinal tract, kidneys, heart, adrenals, brain, and testicles are examples of extra pulmonary tissues with confirmed invasion by SARS-CoV-2. Initial multiple organ involvement may present with vague signs and symptoms to alert health care professionals early in the course of COVID-19. Another example of an ongoing, yet neglected element of the syndromic features of COVID-19, are the reported findings of loss of smell, altered taste, ataxia, headache, dizziness, and loss of consciousness, which suggest a potential for neural involvement. In this review, we further deliberate on the neuroinvasive potential of SARS-CoV-2, the neurologic symptomology observed in COVID-19, the host-virus interaction, possible routes of SARS-CoV-2 to invade the central nervous system, other neurologic considerations for patients with COVID-19, and a collective call to action.

ACE2 in the Era of SARS-CoV-2: Controversies and Novel Perspectives

Angiotensin-converting enzyme 2 (ACE2) is related to ACE but turned out to counteract several pathophysiological actions of ACE. ACE2 exerts antihypertensive and cardioprotective effects and reduces lung inflammation. ACE2 is subjected to extensive transcriptional and post-transcriptional modulation by epigenetic mechanisms and microRNAs. Also, ACE2 expression is regulated post-translationally by glycosylation, phosphorylation, and shedding from the plasma membrane. ACE2 protein is ubiquitous across mammalian tissues, prominently in the cardiovascular system, kidney, and intestine. ACE2 expression in the respiratory tract is of particular interest, in light of the discovery that ACE2 serves as the initial cellular target of severe acute respiratory syndrome (SARS)-coronaviruses, including the recent SARS-CoV2, responsible of the COronaVIrus Disease 2019 (COVID-19). Since the onset of the COVID-19 pandemic, an intense effort has been made to elucidate the biochemical determinants of SARS-CoV2-ACE2 interaction. It has been determined that SARS-CoV2 engages with ACE2 through its spike (S) protein, which consists of two subunits: S1, that mediates binding to the host receptor; S2, that induces fusion of the viral envelope with the host cell membrane and delivery of the viral genome. Owing to the role of ACE2 in SARS-CoV2 pathogenicity, it has been speculated that medical conditions, i.e., hypertension, and/or drugs, i.e., ACE inhibitors and angiotensin receptor blockers, known to influence ACE2 density could alter the fate of SARS-CoV-2 infection. The debate is still open and will only be solved when results of properly designed experimental and clinical investigations will be made public. An interesting observation is, however that, upon infection, ACE2 activity is reduced either by downregulation or by shedding. These events might precipitate the so-called "cytokine storm" that characterizes the most severe COVID-19 forms. As evidence accumulates, ACE2 appears a druggable target in the attempt to limit virus entry and replication. Strategies aimed at blocking ACE2 with antibodies, small molecules or peptides, or at neutralizing the virus by competitive binding with exogenously administered ACE2, are currently under investigations. In this review, we will present an overview of the state-of-the-art knowledge on ACE2 biochemistry and pathophysiology, outlining open issues in the context of COVID-19 disease and potential experimental and clinical developments.

SARS-CoV-2 Targeting the Retina: Host-virus Interaction and Possible Mechanisms of Viral Tropism

Background: Coronavirus disease 2019 (COVID-19) is caused by Severe Acute Respiratory Syndrome Coronavirus 2 10 (SARS-CoV-2). Recent studies demonstrated not only retinal impairments but also detected SARS-CoV-2 in the retina of patients with COVID-19. Purpose: This letter discusses the retinal tropism of SARS-CoV-2, describing possible routes for this coronavirus to reach the retina and cellular mechanisms involved in the retinal cell infection. Conclusions: Determining how SARS-CoV-2 can affect the retinal tissue is essential for the development of new therapeutic strategies and preventive measures, as well as for understanding the possible relationship between COVID-19 damage to the retina and to the brain.

The ACE2-deficient mouse: A model for a cytokine storm-driven inflammation

Angiotensin converting enzyme 2 (ACE2) plays an important role in inflammation, which is attributable at least, in part, to the conversion of the pro-inflammatory angiotensin (Ang) II peptide into angiotensin 1-7 (Ang 1-7), a peptide which opposes the actions of AngII. ACE2 and AngII are present in many tissues but information on the cornea is lacking. We observed that mice deficient in the Ace2 gene (Ace2-/- ), developed a cloudy cornea phenotype as they aged. Haze occupied the central cornea, accompanied by corneal edema and neovascularization. In severe cases with marked chronic inflammation, a cell-fate switch from a transparent corneal epithelium to a keratinized, stratified squamous, psoriasiform-like epidermis was observed. The stroma contained a large number of CD11c, CD68, and CD3 positive cells. Corneal epithelial debridement experiments in young ACE2-deficient mice showed normal appearing corneas, devoid of haze. We hypothesized, however, that these mice are "primed" for a corneal inflammatory response, which once initiated, would persist. In vitro studies reveal that interleukins (IL-1a, IL-1b), chemokines (CCL2, CXCL8), and TNF-α, are all significantly elevated, resulting in a cytokine storm-like phenotype. This phenotype could be partially rescued by treatment with the AngII type 1 receptor (AT1R) antagonist, losartan, suggesting that the observed effect was mediated by AngII acting on its main receptor. Since the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) utilizes human ACE2 as the receptor for entry with subsequent downregulation of ACE2, corneal inflammation in Ace2-/- mice may have a similar mechanism with that in COVID-19 patients. Thus the Ace2-/- cornea, because of easy accessibility, may provide an attractive model to explore the molecular mechanisms, immunological changes, and treatment modalities in patients with COVID-19.

[Can SARS-CoV-2 infect the eye?-An overview of the receptor status in ocular tissue]

Is the new coronavirus SARS-CoV‑2 able to infect ocular tissue and thus poses a risk of infection through the tissue in addition to the risk of contact? This is the question that has occupied ophthalmologists since the beginning of the outbreak. In order to infect a certain type of tissue specific receptors for each virus and sometimes also coreceptors or other proteins must be present. The aim of this review was to summarize and reflect the current state of research with the help of the currently available literature as of 28 May 2020. At the time of the research, angiotensin-converting enzyme 2 (ACE2) was clearly identified as the receptor and transmembrane serine protease 2 (TMPRSS2) as the necessary protease to enable the infection of human cells with SARS-CoV‑2. In the eye both ACE2 and TMPRSS2 are expressed, although sometimes very weakly and with varying degrees in different tissues. It is noteworthy that very different results were obtained with different methods. Several reasons can account for this effect: Firstly, the method of detection or preservation of the tissue, secondly, the possibly different expression of the tested tissue samples and thirdly, a possibly rapid loss of receptor expression post-mortem. Therefore, an infection of the eye seems possible, which has already been reported in various publications. The amount of virus or receptor expression necessary to cause an infection is not known. According to current state of knowledge the eye is not considered to be a high-risk tissue due to the low ACE2 and TMPRSS2 expression. Nevertheless, appropriate protective measures are necessary for both medical personnel and patients. In cases of corneal transplantation an infection of the donor tissue with SARS-CoV‑2 must be excluded.

Expression of SARS-CoV-2 receptor ACE2 and TMPRSS2 in human primary conjunctival and pterygium cell lines and in mouse cornea

PURPOSE

To determine the expressions of SARS-CoV-2 receptor angiotensin-converting enzyme 2 (ACE2) and type II transmembrane serine protease (TMPRSS2) genes in human and mouse ocular cells and comparison to other tissue cells.

METHODS

Human conjunctiva and primary pterygium tissues were collected from pterygium patients who underwent surgery. The expression of ACE2 and TMPRSS2 genes was determined in human primary conjunctival and pterygium cells, human ocular and other tissue cell lines, mesenchymal stem cells as well as mouse ocular and other tissues by reverse transcription-polymerase chain reaction (RT-PCR) and SYBR green PCR.

RESULTS

RT-PCR analysis showed consistent expression by 2 ACE2 gene primers in 2 out of 3 human conjunctival cells and pterygium cell lines. Expression by 2 TMPRSS2 gene primers could only be found in 1 out of 3 pterygium cell lines, but not in any conjunctival cells. Compared with the lung A549 cells, similar expression was noted in conjunctival and pterygium cells. In addition, mouse cornea had comparable expression of Tmprss2 gene and lower but prominent Ace2 gene expression compared with the lung tissue.

CONCLUSION

Considering the necessity of both ACE2 and TMPRSS2 for SARS-CoV-2 infection, our results suggest that conjunctiva would be less likely to be infected by SARS-CoV-2, whereas pterygium possesses some possibility of SARS-CoV-2 infection. With high and consistent expression of Ace2 and Tmprss2 in cornea, cornea rather than conjunctiva has higher potential to be infected by SARS-CoV-2. Precaution is necessary to prevent possible SARS-CoV-2 infection through ocular surface in clinical practice.

Detection of SARS-CoV-2 in Human Retinal Biopsies of Deceased COVID-19 Patients

PURPOSE

To report the presence of viral ribonucleic acid (RNA) of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in human retina in deceased patients with confirmed novel coronavirus disease 2019 (COVID-19).

PATIENTS AND METHODS

Fourteen eyes of 14 deceased patients with confirmed COVID-19 disease were enucleated during autopsy. A sample of human retina was secured and fixed in RNAlater™. Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) was performed to detect three different viral RNA sequences (RdRp-gene, E-gene and Orf1 gene) of SARS-CoV-2.

RESULTS

In three out of 14 eyes SARS-CoV-2 viral RNA was detected in the retina of deceased COVID-19 patients. As analysis for three different sequences (RdRp-gene, E-gene and Orf1 gene) revealed positive results in RT-PCR, the existence of SARS-CoV-2 viral RNA in human retina is proven according to the standards of the World-Health-Organization.

CONCLUSION

Viral RNA of SARS-CoV-2 is detectable in the retina of COVID-19 patients.

Control of the retinal local RAS by the RPE: An interface to systemic RAS activity

As many other organs, the retina has a local renin-angiotensin-system (RAS). All main elements of the RAS are active in the retina: renin, angiotensinogen, angiotensin-converting enzymes. The functional role of the intraretinal RAS is not fully understood. So far, histological and functional analysis point to a regulation of ganglion cell activity and maybe also of bipolar cell activity, but it is not clear how RAS contributes to retinal signal processing. In contrast to local RAS in other organs, the retinal RAS is clearly separated from the systemic RAS. The angiotensin-2 (AngII)/AngI ratio in the retina is different to that in the plasma. However, it appears that the retinal pigment epithelium (RPE), that forms the outer blood/retina barrier, is a major regulator of the retinal RAS by producing renin. Interestingly, comparable to the kidney, the renin production in the RPE is under control of the angiotensin-2 receptor type-1 (AT1). AT1 localizes to the basolateral membrane of the RPE and faces the blood side of the blood/retina barrier. Increases in systemic AngII reduce renin production in the RPE and therefore decrease the intraretinal RAS activity. The relevance of the local RAS for retinal function remains unclear. Nevertheless, it is of fundamental significance to understand the pathology of systemically induced retinal diseases such as hypertension or diabetes.

Antifibrotic Roles of RAAS Blockers: Update

The rennin-angiotensin-aldosterone system (RAAS) has been well documented in regulating blood pressure, fluid volume, and sodium balance. Overactivity of RAAS promotes both systemic and regional glomerular capillary hypertension, which could induce hemodynamic injury to the glomerulus, leading to kidney damage and renal fibrosis via profibrotic and proinflammatory pathway. Therefore, the use of RAAS inhibitors (i.e., ACEIs, ARBs, and MRAs) as the optional therapy has been demonstrated to prevent proteinuria, and kidney fibrosis and slow the decline of renal function effectively in the process of kidney disease during the last few decades. Recently, several new components of the RAAS have been discovered, including ACE2 and the corresponding ACE2/Ang (1-7)/Mas axis, which are also present in the kidney. Besides the classic RAAS inhibitors target the angiotensin-AT1-aldosterone axis, with the expanding knowledge about RAAS, a number of potential therapeutic targets in this system is emerging. Newer agents that are more specific are being developed. The present chapter outlines the insights of the RAAS agents (classic RAAS antagonists/the new RAAS drugs), and discusses its clinical application in the combat of renal fibrosis.

Retinal angiotensin II and angiotensin-(1-7) response to hyperglycemia and an intervention with captopril

Hypothesis:

Hyperglycemia decreases angiotensin-(1-7), the endogenous counter-regulator of angiotensin II in the retina.

Materials and methods:

The distribution and levels of retinal angiotensin II (Ang II) and angiotensin-(1-7) (Ang-(1-7)) were evaluated by confocal imaging and quantitative immunohistochemistry during the development of streptozotocin-induced diabetes in rats.

Results:

In the nondiabetic eye, Ang II was localized to the endfeet of Müller cells, extending into the cellular processes of the inner plexiform layer and inner nuclear layer; Ang-(1-7) showed a wider distribution, extending from the foot plates of the Müller cells to the photoreceptor layer. Eyes from diabetic animals showed a higher intensity and extent of Ang II staining compared with nondiabetic eyes, but lower intensity with a reduced distribution of Ang-(1-7) immunoreactivity. Treatment of the diabetic animals with the angiotensin-converting enzyme inhibitor (ACEI) captopril showed a reduced intensity of Ang II staining, whereas increased intensity and distribution were evident with Ang-(1-7) staining.

Conclusions:

These studies reveal that pharmacological inhibition with ACEIs may provide a specific intervention for the management of the diabetes-induced decline in retinal function, reversing the profile of the endogenous angiotensin peptides closer to the normal condition.

Role of Tissue Renin-angiotensin System and the Chymase/angiotensin-( 1-12) Axis in the Pathogenesis of Diabetic Retinopathy

Diabetic retinopathy (DR) is a major diabetes complication and the leading cause for vision loss and blindness in the adult human population. Diabetes, being an endocrinological disorder dysregulates a number of hormonal systems including the renin angiotensin system (RAS), which thereby may damage both vascular and neuronal cells in the retina. Angiotensin II (Ang II), an active component of the RAS is increased in diabetic retina, and may play a significant role in neurovascular damage leading to the progression of DR. In this review article, we highlight the role of Ang II in the pathogenesis of retinal damage in diabetes and discuss a newly identified mechanism involving tissue chymase and angiotensin-(1-12) [Ang-(1-12)] pathways. We also discuss the therapeutic effects of potential RAS inhibitors targeting blockade of cellular Ang II formation to prevent/ protect the retinal damage. Thus, a better understanding of Ang II formation pathways in the diabetic retina will elucidate early molecular mechanism of vision loss. These concepts may provide a novel strategy for preventing and/or treating diabetic retinopathy, a leading cause of blindness worldwide.

Therapeutic targets of renin-angiotensin system in ocular disorders

Purpose

To review current literature on the renin-angiotensin system (RAS)-mediated pathogenic mechanisms and therapeutic targets in ocular diseases.

Methods

A comprehensive literature survey was performed on PubMed, Scopus, and Google Scholar databases published from 1977 to 2016. The search terms were a RAS, angiotensin, angiotensin receptor, prorenin, pro (renin) receptor, angiotensin converting enzyme inhibitor, angiotensin receptor blocker associated with ocular disorders like cataract, glaucoma, diabetic retinopathy (DR), macular degeneration, and uveitis. Articles were reviewed on the basis of the association between ocular disorders and RAS and relevant articles were discussed.

Results

The literature revealed that the individual RAS components including renin, angiotensins, angiotensin converting enzymes, and RAS receptors have been expressed in the specific ocular tissues like retina, choroid, and ciliary body. The activation of both circulatory and local RAS potentiate the various inflammatory and angiogenic signaling molecules, including vascular endothelial growth factor (VEGF), extracellular signal-regulated kinase, and advanced glycation end products (AGE) in the ocular tissues and leads to several blinding disorders like DR, glaucoma, and macular degeneration. The classical and newer RAS inhibitors have illustrated protective effects on blinding disorders, including DR, glaucoma, macular degeneration, uveitis, and cataract.

Conclusions

The RAS components are present in the extrarenal tissues including ocular tissue and have an imperative role in the ocular pathophysiology. The clinical studies are needed to show the role of therapeutic modalities targeting RAS in the treatment of different ocular disorders.

Role of the ACE2/Angiotensin 1-7 Axis of the Renin-Angiotensin System in Heart Failure

Heart failure (HF) remains the most common cause of death and disability, and a major economic burden, in industrialized nations. Physiological, pharmacological, and clinical studies have demonstrated that activation of the renin-angiotensin system is a key mediator of HF progression. Angiotensin-converting enzyme 2 (ACE2), a homolog of ACE, is a monocarboxypeptidase that converts angiotensin II into angiotensin 1-7 (Ang 1-7) which, by virtue of its actions on the Mas receptor, opposes the molecular and cellular effects of angiotensin II. ACE2 is widely expressed in cardiomyocytes, cardiofibroblasts, and coronary endothelial cells. Recent preclinical translational studies confirmed a critical counter-regulatory role of ACE2/Ang 1-7 axis on the activated renin-angiotensin system that results in HF with preserved ejection fraction. Although loss of ACE2 enhances susceptibility to HF, increasing ACE2 level prevents and reverses the HF phenotype. ACE2 and Ang 1-7 have emerged as a key protective pathway against HF with reduced and preserved ejection fraction. Recombinant human ACE2 has been tested in phase I and II clinical trials without adverse effects while lowering and increasing plasma angiotensin II and Ang 1-7 levels, respectively. This review discusses the transcriptional and post-transcriptional regulation of ACE2 and the role of the ACE2/Ang 1-7 axis in cardiac physiology and in the pathophysiology of HF. The pharmacological and therapeutic potential of enhancing ACE2/Ang 1-7 action as a novel therapy for HF is highlighted.

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.

Pharmacology of the retinal pigment epithelium, the interface between retina and body system

The retinal pigment epithelium (RPE) is a close, interactive partner to the photoreceptors as well as an interface with the endothelium of the choroid and thus with the body's circulatory system. To fulfill these roles, the RPE communicates with neighboring tissue by secretion of a large variety of factors and is able to react to secreted factors via a plethora of transmembrane receptors. Clinically relevant local pharmacological effects are caused by anti-VEGF-A treatment in choroidal neovascularization or by carboanhydrase inhibitors reducing fluid accumulation in the macula. Being exposed to the bloodstream, the RPE reacts to systemic disease, such as diabetes or hypertension, but also to systemic pharmacological intervention, for example to hypotensive drugs acting on the renin-angiotensin-system. Sustained pharmacological treatments, in particular, cause side effects at the RPE with consequences for both RPE function and photoreceptor survival. Among these are systemic inhibition of angiotensin-converting enzyme, insulin treatment in diabetes and anti-VEGF-A therapy. Given the special anatomical and functional relationships of the RPE, pharmacological intervention targeting either the eye or the body systemically should take potential alteration of RPE and subsequently photoreceptor function into account.

Activation of endogenous angiotensin converting enzyme 2 prevents early injuries induced by hyperglycemia in rat retina

Diabetic retinopathy (DR) is a serious complication of diabetes mellitus that may result in blindness. We evaluated the effects of activation of endogenous angiotensin converting enzyme (ACE) 2 on the early stages of DR. Rats were administered an intravenous injection of streptozotocin to induce hyperglycemia. The ACE2 activator 1-[[2-(dimethylamino) ethyl] amino]-4-(hydroxymethyl)-7-[[(4-methylphenyl) sulfonyl] oxy]-9H-xanthone 9 (XNT) was administered by daily gavage. The death of retinal ganglion cells (RGC) was evaluated in histological sections, and retinal ACE2, caspase-3, and vascular endothelial growth factor (VEGF) expressions were analyzed by immunohistochemistry. XNT treatment increased ACE2 expression in retinas of hyperglycemic (HG) rats (control: 13.81±2.71 area%; HG: 14.29±4.30 area%; HG+XNT: 26.87±1.86 area%; P<0.05). Importantly, ACE2 activation significantly increased the RCG number in comparison with HG animals (control: 553.5±14.29; HG: 530.8±10.3 cells; HG+XNT: 575.3±16.5 cells; P<0.05). This effect was accompanied by a reduction in the expression of caspase-3 in RGC of the HG+XNT group when compared with untreated HG rats (control: 18.74±1.59; HG: 38.39±3.39 area%; HG+XNT: 27.83±2.80 area%; P<0.05). Treatment with XNT did not alter the VEGF expression in HG animals (P>0.05). Altogether, these findings indicate that activation of ACE2 reduced the death of retinal ganglion cells by apoptosis in HG rats.

Ocular Inserts for Sustained Release of the Angiotensin-Converting Enzyme 2 Activator, Diminazene Aceturate, to Treat Glaucoma in Rats

The aim of this study was to develop and evaluate the effects of chitosan inserts for sustained release of the angiotensin-converting enzyme 2 (ACE2) activator, diminazene aceturate (DIZE), in experimental glaucoma. Monolayer DIZE loaded inserts (D+I) were prepared and characterized through swelling, attenuated total reflectance Fourier transformed infrared spectroscopy (ATR-FTIR), differential scanning calorimetry (DSC) and in vitro drug release. Functionally, the effects of D+I were tested in glaucomatous rats. Glaucoma was induced by weekly injections of hyaluronic acid (HA) into the anterior chamber and intraocular pressure (IOP) measurements were performed. Retinal ganglion cells (RGC) and optic nerve head cupping were evaluated in histological sections. Biodistribution of the drug was accessed by scintigraphic images and ex vivo radiation counting. We found that DIZE increased the swelling index of the inserts. Also, it was molecularly dispersed and interspersed in the polymeric matrix as a freebase. DIZE did not lose its chemical integrity and activity when loaded in the inserts. The functional evaluation demonstrated that D+I decreased the IOP and maintained the IOP lowered for up to one month (last week: 11.0±0.7 mmHg). This effect of D+I prevented the loss of RGC and degeneration of the optic nerve. No toxic effects in the eyes related to application of the inserts were observed. Moreover, biodistribution studies showed that D+I prolonged the retention of DIZE in the corneal site. We concluded that D+I provided sustained DIZE delivery in vivo, thereby evidencing the potential application of polymeric-based DIZE inserts for glaucoma management.

Novel potential treatment modalities for ocular hypertension: focus on angiotensin and bradykinin system axes

Despite the availability of modern surgical procedures, new drug delivery techniques, health authority-approved single topical ocular drugs, and combination products thereof, there continues to be an unmet medical need for novel treatment modalities for preserving vision. This is especially true for the treatment of glaucoma and the high risk factor often associated with this ocular disease, elevated intraocular pressure (IOP). Undesirable local or systemic side effects, frequency of dosing, lack of sustained IOP lowering, and lack of prevention of diurnal IOP spikes are among the greatest challenges. The very recent discovery, characterization, and publication of 2 novel IOP-lowering agents that pertain to the renin-angiotensin and kallikrein-kinin axes potentially offer novel means to treat and control ocular hypertension (OHT). Here, some contextual introductory information is provided first, followed by more detailed discussion of the properties and actions of diminazene aceturate (DIZE; a novel angiotensin-converting enzyme-2 activator) and FR-190997 (a nonpeptide bradykinin receptor-2 agonist) in relation to their anti-OHT activities in rodent and cynomolgus monkey eyes, respectively. It is anticipated that these compounds will pave the way for future discovery, development, and marketing of novel drugs to treat glaucoma and thus help save sight for millions of people afflicted with this slow progressive optic neuropathy.

Role of angiotensin-converting enzyme 2 (ACE2) in diabetic cardiovascular complications

Diabetes mellitus results in severe cardiovascular complications, and heart disease and failure remain the major causes of death in patients with diabetes. Given the increasing global tide of obesity and diabetes, the clinical burden of diabetes-induced cardiovascular disease is reaching epidemic proportions. Therefore urgent actions are needed to stem the tide of diabetes which entails new prevention and treatment tools. Clinical and pharmacological studies have demonstrated that AngII (angiotensin II), the major effector peptide of the RAS (renin–angiotensin system), is a critical promoter of insulin resistance and diabetes mellitus. The role of RAS and AngII has been implicated in the progression of diabetic cardiovascular complications and AT1R (AngII type 1 receptor) blockers and ACE (angiotensin-converting enzyme) inhibitors have shown clinical benefits. ACE2, the recently discovered homologue of ACE, is a monocarboxypeptidase which converts AngII into Ang-(1–7) [angiotensin-(1–7)] which, by virtue of its actions on the MasR (Mas receptor), opposes the effects of AngII. In animal models of diabetes, an early increase in ACE2 expression and activity occurs, whereas ACE2 mRNA and protein levels have been found to decrease in older STZ (streptozotocin)-induced diabetic rats. Using the Akita mouse model of Type 1 diabetes, we have recently shown that loss of ACE2 disrupts the balance of the RAS in a diabetic state and leads to AngII/AT1R-dependent systolic dysfunction and impaired vascular function. In the present review, we will discuss the role of the RAS in the pathophysiology and treatment of diabetes and its complications with particular emphasis on potential benefits of the ACE2/Ang-(1–7)/MasR axis activation.

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.

Antiglaucomatous effects of the activation of intrinsic Angiotensin-converting enzyme 2

PURPOSE

To evaluate the effects of the activation of endogenous angiotensin-converting enzyme 2 (ACE2) using the compound diminazene aceturate (DIZE) in an experimental model of glaucoma in Wistar rats.

METHODS

DIZE (1 mg/kg) was administered daily, either systemically or topically, and the IOP was measured weekly. To examine the role of the Mas receptor in the effects of DIZE, the Ang-(1-7) antagonist A-779 was co-administered. Drainage of the aqueous humor was evaluated by using scintigraphy. The analysis of ACE2 expression by immunohistochemistry and the counting of retinal ganglion cells (RGCs) were performed in histologic sections. Additionally, the nerve fiber structure was evaluated by transmission electron microscopy.

RESULTS

The systemic administration and topical administration (in the form of eye drops) of DIZE increased the ACE2 expression in the eyes and significantly decreased the IOP of glaucomatous rats without changing the blood pressure. Importantly, this IOP-lowering action of DIZE was similar to the effects of dorzolamide. The antiglaucomatous effects of DIZE were blocked by A-779. Histologic analysis revealed that the reduction in the number of RGCs and the increase in the expression of caspase-3 in the RGC layer in glaucomatous animals were prevented by DIZE. This compound also prevented alterations in the cytoplasm of axons in glaucomatous rats. In addition to these neuroprotective effects, DIZE facilitated the drainage of the aqueous humor.

CONCLUSIONS

Our results evidence the pathophysiologic relevance of the ocular ACE2/Ang-(1-7)/Mas axis of the renin-angiotensin system and, importantly, indicate that the activation of intrinsic ACE2 is a potential therapeutic strategy to treat glaucoma.

The ACE2 gene: its potential as a functional candidate for cardiovascular disease

The RAS (renin–angiotensin system) plays an important role in the pathophysiology of CVD (cardiovascular disease), and RAS blockade is an important therapeutic strategy in the management of CVD. A new counterbalancing arm of the RAS is now known to exist in which ACE (angiotensin-converting enzyme) 2 degrades Ang (angiotensin) II, the main effector of the classic RAS, and generates Ang-(1–7). Altered ACE2 expression is associated with cardiac and vascular disease in experimental models of CVD, and ACE2 is increased in failing human hearts and atherosclerotic vessels. In man, circulating ACE2 activity increases with coronary heart disease, as well as heart failure, and a large proportion of the variation in plasma ACE2 levels has been attributed to hereditary factors. The ACE2 gene maps to chromosome Xp22 and this paper reviews the evidence associating ACE2 gene variation with CVD and considers clues to potential functional ACE2 variants that may alter gene expression or transcriptional activity. Studies to date have investigated ACE2 gene associations in hypertension, left ventricular hypertrophy and coronary artery disease, but the results have been inconsistent. The discrepancies may reflect the sample size of the studies, the gender or ethnicity of subjects, the cardiovascular phenotype or the ACE2 SNP investigated. The frequent observation of apparent sex-dependence might be of special importance, if confirmed. As yet, there are no studies to concurrently assess ACE2 gene polymorphisms and circulating ACE2 activity. Large-scale carefully conducted clinical studies are urgently needed to clarify more precisely the potential role of ACE2 in the CVD continuum.

Interaction of diabetes and ACE2 in the pathogenesis of cardiovascular disease in experimental diabetes

Local and systemic AngII (angiotensin II) levels are regulated by ACE2 (angiotensin-converting enzyme 2), which is reduced in diabetic tissues. In the present study, we examine the effect of ACE2 deficiency on the early cardiac and vascular changes associated with experimental diabetes. Streptozotocin diabetes was induced in male C57BL6 mice and Ace2-KO (knockout) mice, and markers of RAS (renin–angiotensin system) activity, cardiac function and injury were assessed after 10 weeks. In a second protocol, diabetes was induced in male ApoE (apolipoprotein E)-KO mice and ApoE/Ace2-double-KO mice, and plaque accumulation and markers of atherogenesis assessed after 20 weeks. The induction of diabetes in wild-type mice led to reduced ACE2 expression and activity in the heart, elevated circulating AngII levels and reduced cardiac Ang-(1–7) [angiotensin-(1–7)] levels. This was associated structurally with thinning of the LV (left ventricular) wall and mild ventricular dilatation, and histologically with increased cardiomyocyte apoptosis on TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling) staining and compensatory hypertrophy denoted by an increased cardiomyocyte cross-sectional area. By contrast Ace2-KO mice failed to increase circulating AngII concentration, experienced a paradoxical fall in cardiac AngII levels and no change in Ang-(1–7) following the onset of diabetes. At the same time the major phenotypic differences between Ace2-deficient and Ace2-replete mice with respect to BP (blood pressure) and cardiac hypertrophy were eliminated following the induction of diabetes. Consistent with findings in the heart, the accelerated atherosclerosis that was observed in diabetic ApoE-KO mice was not seen in diabetic ApoE/Ace2-KO mice, which experienced no further increase in plaque accumulation or expression in key adhesion molecules beyond that seen in ApoE/Ace2-KO mice. These results point to the potential role of ACE2 deficiency in regulating the tissue and circulating levels of AngII and their sequelae in the context of diabetes, as well as the preservation or augmentation of ACE2 expression or activity as a potential therapeutic target for the prevention of CVD (cardiovascular disease) in diabetes.

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.

Local ocular renin-angiotensin system - a target for glaucoma therapy?

An active local intraocular renin-angiotensin system (RAS) has recently been shown to exist in the human eye, and evidence is now accumulating that antihypertensive drugs acting on RAS can also lower intraocular pressure. They seem also to work as neuroprotective agents against retinal ganglion cell loss in vivo; though no compounds are in ophthalmological use at present. Classically, the highly vasoconstrictive angiotensin II (Ang II) is the key peptide in the circulatory RAS. However, the final effect of RAS activation at tissue level is more complex, being based not only on the biological activity of Ang II but also on the activities of other products of angiotensinogen metabolism, often exerting opposite effects to Ang II action.

ACE2 and Ang-(1-7) confer protection against development of diabetic retinopathy

Despite evidence that hyperactivity of the vasodeleterious axis (ACE/angiotensin II (Ang II)/AT1 receptor) of the renin-angiotensin system (RAS) is associated with the pathogenesis of diabetic retinopathy (DR) use of the inhibitors of this axis has met with limited success in the control of this pathophysiology. We investigated the hypothesis that enhancing the local activity of the recently established protective axis of the RAS, ACE2/Ang-(1-7), using adeno-associated virus (AAV)-mediated gene delivery of ACE2 or Ang-(1-7) would confer protection against diabetes-induced retinopathy. Genes expressing ACE2 and Ang-(1-7) were cloned in AAV vector. The effects of ocular AAV-ACE2/Ang-(1-7) gene transfer on DR in diabetic eNOS(-/-) mice and Sprague-Dawley (SD) rats were examined. Diabetes was associated with approximately tenfold and greater than threefold increases in the ratios of ACE/ACE2 and AT1R/Mas mRNA levels in the retina respectively. Intraocular administration of AAV-ACE2/Ang-(1-7) resulted in significant reduction in diabetes-induced retinal vascular leakage, acellular capillaries, infiltrating inflammatory cells and oxidative damage in both diabetic mice and rats. Our results demonstrate that DR is associated with impaired balance of retinal RAS. Increased expression of ACE2/Ang-(1-7) overcomes this imbalance and confers protection against DR. Thus, strategies enhancing the protective ACE2/Ang-(1-7) axis of RAS in the eye could serve as a novel therapeutic target for DR.

Diabetic retinopathy: pathogenic mechanisms and current treatments

Since diabetes is now a global epidemic, the incidence of retinopathy, a leading cause of blindness in patients aged 20-74 years, is also expected to rise to alarming levels. The risk of development and progression of diabetic retinopathy is closely associated with the type and duration of diabetes, blood glucose, blood pressure and possibly lipids. It is an unmet medical need that can lead to severe and irreversible loss of vision in people of working age worldwide. The aim of this review is to give an overview of the clinical and anatomical changes during the progression of retinopathy, the underlying pathogenic mechanisms that link hyperglycemia with retinal tissue damage, current treatments, and the emerging pharmacological therapies for this sight-threatening complication of diabetes.

The contribution of hypertension to diabetic nephropathy and retinopathy: the role of inflammation and oxidative stress

Diabetes and hypertension frequently coexist and constitute the most notorious combination for the pathogenesis of diabetic nephropathy and retinopathy. Large clinical trials have clearly demonstrated that tight control of glycemia and/or blood pressure significantly reduces the incidence and progression of diabetic retinopathy (DR) and nephropathy. However, the mechanism by which hypertension interacts with diabetes to induce and/or exacerbate nephropathy and retinopathy is very unclear. Substantial evidence implicates the involvement of chronic inflammation and oxidative stress in the pathogenesis of DR and nephropathy. In addition, hypertension causes oxidative stress and inflammation in the kidney and retina. In the present review, we summarized data obtained from our research along with those from other groups to better understand the role of hypertension in the pathogenesis of diabetic nephropathy and retinopathy. It is suggested that oxidative stress and inflammation may be common denominators of kidney and retinal damage in the concomitant presence of diabetes and hypertension.

Regulation of the renin expression in the retinal pigment epithelium by systemic stimuli

The retina expresses a local renin-angiotensin system (RAS). This study aimed to investigate the influence of systemic modulation of renin synthesis on the expression of renin in the retinal pigment epithelium (RPE), which forms part of the blood/retina barrier. Freshly isolated RPE cells showed expression of renin 1A, which is the secreted isoform of renin. Systemic administration of the angiotensin-converting enzyme inhibitor enalapril in mice increased the renin expression in both the kidney and the retina. Systemic infusion of ANG II led to a decrease in the renin expression in the kidney and in the retina and RPE. The ANG II-dependent down-regulation of renin expression in the RPE was prevented by systemic application of the AT(1) receptor blocker losartan. However, water deprivation lead to an increase of the renin expression in the kidney but unexpectedly to a decrease of the renin expression in the retina. In sections of the mouse retina, the ANG II receptor AT(1) was found in the RPE and localized at the blood side of the epithelium. Short-time cultured RPE cells showed increases in intracellular free Ca(2+) in response to stimulation by ANG II that were sensitive to losartan. In summary, we conclude that the renin expression in cells of the blood/retina barrier is influenced by the systemic RAS. ANG II circulating in the plasma is likely a mediator of this influence.

Plasma and aqueous humor angiotensin-converting enzyme levels in patients with diabetic retinopathy

PURPOSE

To assess angiotensin-converting enzyme (ACE) levels in aqueous humor and plasma of patients with nonproliferative diabetic retinopathy (NPDR) and proliferative diabetic retinopathy (PDR) as another potent ischemia-induced angiogenic factor.

METHODS

The clinical comparative study included 40 patients with diabetic retinopathy and 16 healthy subjects. For all patients, aqueous humors were collected during the cataract surgery or intravitreal injection of triamcinolone acetonide. ACE levels were measured using a solid-phase chemiluminescence immunoassay.

RESULTS

We observed significantly elevated ACE level in aqueous humor of patients with PDR compared with the patients with NPDR and normal subjects (P = 0.023), but no significant difference was detected between nonproliferative diabetics and control group (P = 0.239). There was no significant difference in plasma ACE levels among diabetic and control groups (P = 0.816).

CONCLUSION

Elevated ACE level may induce retinal angiogenesis and proliferative retinopathy in patients with DM. We consider that high levels of ACE in aqueous humor can reflect the association between retinal angiogenesis and DM serve as predictor in the progression of diabetic retinopathy.

The renin-angiotensin system in retinal health and disease: Its influence on neurons, glia and the vasculature

Renin-Angiotensin System is classically recognized for its role in the control of systemic blood pressure. However, the retina is recognized to have all the components necessary for angiotensin II formation, suggestive of a role for Angiotensin II in the retina that is independent of the systemic circulation. The most well described effects of Angiotensin II are on the retinal vasculature, with roles in vasoconstriction and angiogenesis. However, it is now emerging that Angiotensin II has roles in modulation of retinal function, possibly in regulating GABAergic amacrine cells. In addition, Angiotensin II is likely to have effects on glia. Angiotensin II has also been implicated in retinal vascular diseases such as Retinopathy of Prematurity and diabetic retinopathty, and more recently actions in choroidal neovascularizaiton and glaucoma have also emerged. The mechanisms by which Angiotensin II promotes angiogensis in retinal vascular diseases is indicative of the complexity of the RAS and the variety of cell types that it effects. Indeed, these diseases are not purely characterized by direct effects of Angiotensin II on the vasculature. In retinopathy of prematurity, for example, blockade of AT1 receptors prevents pathological angiogenesis, but also promotes revascularization of avascular regions of the retina. The primary site of action of Angiotensin II in this disease may be on retinal glia, rather than the vasculature. Indeed, blockade of AT1 receptors prevents glial loss and promotes the re-establishment of normal vessel growth. Blockade of RAS as a treatment for preventing the incidence and progression of diabetic retinopathy has also emerged based on a series of studies in animal models showing that blockade of the RAS prevents the development of a variety of vascular and neuronal deficits in this disease. Importantly these effects may be independent of actions on systemic blood pressure. This has culminated recently with the completion of several large multi-centre clinical trials that showed that blockade of the RAS may be of benefit in some at risk patients with diabetes. With the emergence of novel compounds targeting different aspects of the RAS even more effective ways of blocking the RAS may be possible in the future.