Neurovascular dysfunction in glaucoma

Retinal ganglion cells, the neurons that die in glaucoma, are endowed with a high metabolism requiring optimal provision of oxygen and nutrients to sustain their activity. The timely regulation of blood flow is, therefore, essential to supply firing neurons in active areas with the oxygen and glucose they need for energy. Many glaucoma patients suffer from vascular deficits including reduced blood flow, impaired autoregulation, neurovascular coupling dysfunction, and blood-retina/brain-barrier breakdown. These processes are tightly regulated by a community of cells known as the neurovascular unit comprising neurons, endothelial cells, pericytes, Müller cells, astrocytes, and microglia. In this review, the neurovascular unit takes center stage as we examine the ability of its members to regulate neurovascular interactions and how their function might be altered during glaucomatous stress. Pericytes receive special attention based on recent data demonstrating their key role in the regulation of neurovascular coupling in physiological and pathological conditions. Of particular interest is the discovery and characterization of tunneling nanotubes, thin actin-based conduits that connect distal pericytes, which play essential roles in the complex spatial and temporal distribution of blood within the retinal capillary network. We discuss cellular and molecular mechanisms of neurovascular interactions and their pathophysiological implications, while highlighting opportunities to develop strategies for vascular protection and regeneration to improve functional outcomes in glaucoma.

Neuroprotection of Rodent and Human Retinal Ganglion Cells In Vitro/Ex Vivo by the Hybrid Small Molecule SA-2

The mechanisms underlying the neuroprotective effects of the hybrid antioxidant-nitric oxide donating compound SA-2 in retinal ganglion cell (RGC) degeneration models were evaluated. The in vitro trophic factor (TF) deprivation model in primary rat RGCs and ex vivo human retinal explants were used to mimic glaucomatous neurodegeneration. Cell survival was assessed after treatment with vehicle or SA-2. In separate experiments, tert-Butyl hydroperoxide (TBHP) and endothelin-3 (ET-3) were used in ex vivo rat retinal explants and primary rat RGCs, respectively, to induce oxidative damage. Mitochondrial and intracellular reactive oxygen species (ROS) were assessed following treatments. In the TF deprivation model, SA-2 treatment produced a significant decrease in apoptotic and dead cell counts in primary RGCs and a significant increase in RGC survival in ex vivo human retinal explants. In the oxidative stress-induced models, a significant decrease in the production of ROS was observed in the SA-2-treated group compared to the vehicle-treated group. Compound SA-2 was neuroprotective against various glaucomatous insults in the rat and human RGCs by reducing apoptosis and decreasing ROS levels. Amelioration of mitochondrial and cellular oxidative stress by SA-2 may be a potential therapeutic strategy for preventing neurodegeneration in glaucomatous RGCs.

Elevated endothelin-1 levels as risk factor for an impaired ocular blood flow measured by OCT-A in glaucoma

The purpose of this study was to ascertain whether a correlation exists between glaucoma-associated alteration of ocular vascular haemodynamics and endothelin-1 (ET-1) levels exist. Eyes of patients with cataract (n = 30) or glaucoma (n = 68) were examined with optical coherence tomography (OCT) and OCT-angiography (OCT-A; AngioVue™-RTVue-XR; Optovue, Fremont, California, USA). The peripapillary and the macular vessel density (VD) values were measured. Inferior and superior retinal nerve fibre layer (RNFL) thickness loss was used for further OCT staging. Aqueous humour of the examined eye and plasma were sampled during cataract or glaucoma surgery and analysed by means of ELISA to determine their ET-1 level. Glaucoma eyes are characterised by reductions in RNFL thickness and VD that correlate significantly with the OCT GSS score. Peripheral and ocular ET-1 level were significantly elevated in patients with glaucoma and correlate positively with the OCT-GSS score of the entire study population. Peripapillary and macula VD of glaucoma patients correlates negatively with plasma ET-1 levels. Multivariable analysis showed a subordinate role of intraocular pressure predictive factor for impaired retinal blood flow compared with plasma ET-1 level in glaucoma. Peripheral ET-1 level serves as risk factor for detection of ocular blood flow changes in the optic nerve head region of glaucomatous eyes.

The Piezo1 ion channel in glaucoma: a new perspective on mechanical stress

Glaucomatous optic nerve damage caused by pathological intraocular pressure elevation is irreversible, and its course is often difficult to control. This group of eye diseases is closely related to biomechanics, and the correlation between glaucoma pathogenesis and mechanical stimulation has been studied in recent decades. The nonselective cation channel Piezo1, the most important known mechanical stress sensor, is a transmembrane protein widely expressed in various cell types. Piezo1 has been detected throughout the eye, and the close relationship between Piezo1 and glaucoma is being confirmed. Pathological changes in glaucoma occur in both the anterior and posterior segments of the eye, and it is of great interest for researchers to determine whether Piezo1 plays a role in these changes and how it functions. The elucidation of the mechanisms of Piezo1 action in nonocular tissues and the reported roles of similar mechanically activated ion channels in glaucoma will provide an appropriate basis for further investigation. From a new perspective, this review provides a detailed description of the current progress in elucidating the role of Piezo1 in glaucoma, including relevant questions and assumptions, the remaining challenging research directions and mechanism-related therapeutic potential.

The Association among Blood Pressure, Blood Pressure Medications, and Glaucoma in a Nationwide Electronic Health Records Database

PURPOSE

To measure the association among blood pressure (BP), BP medications, and glaucoma using the All of Us Research Program database.

DESIGN

A retrospective, longitudinal cohort study leveraging a national electronic health record (EHR) database administered by the National Institutes of Health.

PARTICIPANTS

Eye patients in the All of Us Research Program database with at least 15 months of follow-up and 1 BP measurement.

METHODS

Univariable and multivariable Cox regression models predicted the risk of developing incident open-angle glaucoma (OAG). Mean arterial pressure (MAP) and the number of BP medication classes were entered as time-varying predictors to account for changes over time.

MAIN OUTCOME MEASURES

The risk of developing incident OAG, as defined by billing diagnosis codes.

RESULTS

Of 20 815 eligible eye patients who qualified for this study, 462 developed OAG. Low BP (MAP < 83.0 mmHg) was associated with increased risk of developing OAG (hazard ratio [HR], 1.32; 95% confidence interval [CI], 1.04-1.67). High BP (MAP > 101.3 mmHg) and the number of BP medication classes were not associated with OAG after adjustment for covariates. Other risk factors associated with OAG included being Black (HR, 3.31, 95% CI, 2.63-4.17), Hispanic or Latino (HR, 2.53, 95% CI, 1.94-3.28), Asian (HR, 2.22, 95% CI, 1.24-3.97), older in age (80+ years, HR, 20.1, 95% CI, 9.10-44.5), and diabetic (HR, 1.32, 95% CI, 1.04-1.67). Female gender was associated with decreased hazard of developing OAG (HR, 0.66, 95% CI, 0.55-0.80). No significant interaction was observed between MAP and the number of BP medications on the risk of developing OAG.

CONCLUSIONS

We found that low BP is associated with increased risk of developing OAG in a national longitudinal EHR database. We did not find evidence supporting a differential effect of medically treated and untreated low BP. This study adds to the body of literature implicating vascular dysregulation as a potential etiology for the development of OAG, particularly emphasizing the lack of influence of BP medications on this relationship.

Damage-Associated Molecular Patterns (DAMPs) in Retinal Disorders

Damage-associated molecular patterns (DAMPs) are endogenous danger molecules released from the extracellular and intracellular space of damaged tissue or dead cells. Recent evidence indicates that DAMPs are associated with the sterile inflammation caused by aging, increased ocular pressure, high glucose, oxidative stress, ischemia, mechanical trauma, stress, or environmental conditions, in retinal diseases. DAMPs activate the innate immune system, suggesting their role to be protective, but may promote pathological inflammation and angiogenesis in response to the chronic insult or injury. DAMPs are recognized by specialized innate immune receptors, such as receptors for advanced glycation end products (RAGE), toll-like receptors (TLRs) and the NOD-like receptor family (NLRs), and purine receptor 7 (P2X7), in systemic diseases. However, studies describing the role of DAMPs in retinal disorders are meager. Here, we extensively reviewed the role of DAMPs in retinal disorders, including endophthalmitis, uveitis, glaucoma, ocular cancer, ischemic retinopathies, diabetic retinopathy, age-related macular degeneration, rhegmatogenous retinal detachment, proliferative vitreoretinopathy, and inherited retinal disorders. Finally, we discussed DAMPs as biomarkers, therapeutic targets, and therapeutic agents for retinal disorders.

Involvement of c-Jun N-terminal kinase 2 (JNK2) in Endothelin-1 (ET-1) Mediated Neurodegeneration of Retinal Ganglion Cells

Purpose

The goal of this study was to determine whether JNK2 played a causative role in endothelin-mediated loss of RGCs in mice.

Methods

JNK2^−/− and wild type (C57BL/6) mice were intravitreally injected in one eye with 1 nmole of ET-1, whereas the contralateral eye was injected with the vehicle. At two time points (two hours and 24 hours) after the intravitreal injections, mice were euthanized, and phosphorylated c-Jun was assessed in retinal sections. In a separate set of experiments, JNK2^−/− and wild type mice were intravitreally injected with either 1 nmole of ET-1 or its vehicle and euthanized seven days after injection. Retinal flat mounts were stained with antibodies to the RGC marker, Brn3a, and surviving RGCs were quantified. Axonal degeneration was assessed in paraphenylenediamine stained optic nerve sections.

Results

Intravitreal ET-1 administration produced a significant increase in immunostaining for phospho c-Jun in wild type mice, which was appreciably lower in the JNK2 ^−/− mice. A significant (P < 0.05) 26% loss of RGCs was found in wild type mice, seven days after injection with ET-1. JNK2^−/− mice showed a significant protection from RGC loss following ET-1 administration, compared to wild type mice injected with ET-1. A significant decrease in axonal counts and an increase in the collapsed axons was found in ET-1 injected wild type mice eyes.

Conclusions

JNK2 appears to play a major role in ET-1 mediated loss of RGCs in mice. Neuroprotective effects in JNK2^−/− mice following ET-1 administration occur mainly in the soma and not in the axons of RGCs.

[Vasculat treatment concepts in glaucoma patients]

BACKGROUND

Approximately 40% of all open-angle glaucomas do not show high intraocular pressure (IOP). Vascular risk factors play an important role in the pathogeneses of normal pressure glaucoma but high pressure glaucoma is also often accompanied by significant vascular components.

OBJECTIVE

What are the practice relevant possibilities of vascular glaucoma treatment?

MATERIAL AND METHODS

An evaluation of scientific articles from PubMed dealing with vascular glaucoma was carried out.

RESULTS

The treatment of vascular risk factors in glaucoma patients requires a thorough medical history regarding vascular symptoms (peripheral vasospasm, tinnitus, migraine etc.) and information on the presence of systemic diseases. Furthermore, a 24h blood pressure profile and the determination of the fat metabolism status represent important and simple examinations.

CONCLUSION

Besides optimizing systemic blood pressure, reducing an increased central retinal venous pressure, treatment with statins, calcium channel blockers, Ginkgo biloba extract, increased physical exercise and fluid replacement are options to ameliorate vascular conditions. An interdisciplinary cooperation with general practitioners and internists is an important component of holistic treatment.

Membrane dynamics simulation and virtual screening reveals potential dual natural inhibitors of endothelin receptors for targeting glaucomatous condition

Glaucoma is the second leading cause of blindness in the world and is characterized by the loss of retinal ganglion cells (RGC) over a period of time, leading to complete blindness. Recently, endothelin has been identified as an important factor that influences intraocular pressure IOP, OBF, and direct RGC damage. Targeting the endothelin receptor signaling pathway in glaucoma is considered to be highly beneficial, as it can effectively modulate IOP, OBF, and RGC damage, the key factors which are essential to modulate the disease progression holistically. Currently, synthetic drugs like Bosentan, BQ-123, and prostaglandin analogues are available as endothelin receptor antagonists, which are extensively used in the treatment of cardiovascular and other conditions like systemic hypertension. However, the usage of these drugs in glaucoma is limited due to toxicity and poor bioavailability in the ocular milieu. Thus, there is a need for potential natural compounds as endothelin receptor antagonists that acts as dual inhibitors by targeting both ETA and ETB and are highly efficient with the least toxicity. Hence, this study is intended to prioritize endothelin receptor antagonists by structural bioinformatics approaches involving molecular modeling, molecular dynamics, and molecular docking studies. Subsequently, High throughput virtual screening (HTVS) vs. Natural compound databases targeting the optimal binding sites of both ETA and ETB. Following this, the common hits were subjected to binding free energy calculations (MMGBSA) and ADMETox analysis. Finally, the most potential hits were analyzed for MD based binding stability analysis and binding free energy. Similarly, the known synthetic inhibitors were also docked to the receptors and the results were analyzed. From this study, it was inferred that among the natural compounds dataset (8929 compounds), only 4 common compounds were identified as hits. Among these, only one compound ST075640 surpassed all the prioritization criteria including MMGBSA, ADMETox prediction, dual inhibitory potential (ETA & ETB), and also in structural comparative analysis with bosentan it showed similar efficiency. Thus, the validated hit shall prove to be effective in modulating endothelin mediated IOP, OBF, and RGC damage in glaucomatous condition.

Endothelin-1 Mediated Decrease in Mitochondrial Gene Expression and Bioenergetics Contribute to Neurodegeneration of Retinal Ganglion Cells

Endothelin-1 (ET-1) is a vasoactive peptide that is elevated in aqueous humor as well as circulation of primary open angle glaucoma (POAG) patients. ET-1 has been shown to promote degeneration of optic nerve axons and apoptosis of retinal ganglion cells (RGCs), however, the precise mechanisms are still largely unknown. In this study, RNA-seq analysis was used to assess changes in ET-1 mediated gene expression in primary RGCs, which revealed that 23 out of 156 differentially expressed genes (DEGs) had known or predicted mitochondrial function, of which oxidative phosphorylation emerged as the top-most enriched pathway. ET-1 treatment significantly decreased protein expression of key mitochondrial genes including cytochrome C oxidase copper chaperone (COX17) and ATP Synthase, H⁺ transporting, Mitochondrial Fo Complex (ATP5H) in primary RGCs and in vivo following intravitreal ET-1 injection in rats. A Seahorse ATP rate assay revealed a significant decrease in the rate of mitochondrial ATP production following ET-1 treatment. IOP elevation in Brown Norway rats showed a trend towards decreased expression of ATP5H. Our results demonstrate that ET-1 produced a decrease in expression of vital components of mitochondrial electron transport chain, which compromise bioenergetics and suggest a mechanism by which ET-1 promotes neurodegeneration of RGCs in glaucoma.

Quantitative analysis of neural tissues around the optic disc after panretinal photocoagulation in patients with diabetic retinopathy

In this retrospective cross-sectional study, we quantitatively analyzed the tomographic features in the neural tissues around the optic disc in patients with diabetic retinopathy with and without panretinal photocoagulation. We analyzed 206 eyes, comprising 33 normal eyes in subjects without diabetes (group I), 30 eyes without diabetic retinopathy (group II), 66 eyes with non-proliferative diabetic retinopathy (group III), 45 eyes with panretinal photocoagulation (group IV), and 32 eyes with normal tension glaucoma (group V). Sequential images acquired using swept-source optical coherence tomography in three-dimensional mode were used to measure peripapillary retinal nerve fiber layer thickness, neuro-retinal rim thickness, anterior lamina cribrosa depth, prelaminar thickness, and thickness of the lamina cribrosa. The peripapillary retinal nerve fiber layer thickness and lamina cribrosa thickness were significantly thinner in group IV than in group III (p = 0.019 and p < 0.001). However, there was no significant difference in rim thickness, anterior lamina cribrosa depth, or prelaminar thickness between groups III and IV (p = 0.307, p = 0.877, and p = 0.212). Multivariate analysis revealed that time since panretinal photocoagulation and thickness of the lamina cribrosa had a significant effect on peripapillary retinal nerve fiber layer thickness (p < 0.001 and p = 0.014). In group IV, there was a negative correlation between time elapsed since panretinal photocoagulation and peripapillary retinal nerve fiber layer thickness, rim thickness, and thickness of the lamina cribrosa (r = -0.765, r = -0.490, and r = -0.419), but no correlation with prelaminar thickness or anterior lamina cribrosa depth (r = 0.104 and r = -0.171). Panretinal photocoagulation may be related to thinning of the peripapillary retinal nerve fiber layer, rim thickness, and lamina cribrosa, but not prelaminar thickness or anterior lamina cribrosa depth. These features are different from the peripapillary features of eyes with typical normal tension glaucoma.

Retinal Macroglial Responses in Health and Disease

Due to their permanent and close proximity to neurons, glial cells perform essential tasks for the normal physiology of the retina. Astrocytes and Müller cells (retinal macroglia) provide physical support to neurons and supplement them with several metabolites and growth factors. Macroglia are involved in maintaining the homeostasis of extracellular ions and neurotransmitters, are essential for information processing in neural circuits, participate in retinal glucose metabolism and in removing metabolic waste products, regulate local blood flow, induce the blood-retinal barrier (BRB), play fundamental roles in local immune response, and protect neurons from oxidative damage. In response to polyetiological insults, glia cells react with a process called reactive gliosis, seeking to maintain retinal homeostasis. When malfunctioning, macroglial cells can become primary pathogenic elements. A reactive gliosis has been described in different retinal pathologies, including age-related macular degeneration (AMD), diabetes, glaucoma, retinal detachment, or retinitis pigmentosa. A better understanding of the dual, neuroprotective, or cytotoxic effect of macroglial involvement in retinal pathologies would help in treating the physiopathology of these diseases. The extensive participation of the macroglia in retinal diseases points to these cells as innovative targets for new drug therapies.

Risk Factors in Normal-Tension Glaucoma and High-Tension Glaucoma in relation to Polymorphisms of Endothelin-1 Gene and Endothelin-1 Receptor Type A Gene

The aim of the research is to analyse the influence of polymorphisms of endothelin-1 gene and endothelin-1 receptor type A gene on the clinical condition of patients with primary open angle glaucoma. Methods. 285 Polish patients took part in the research (160 normal-tension glaucoma and 125 high-tension glaucoma). DNA was isolated by standard methods and genotype distributions of four polymorphisms in genes encoding endothelin-1 (K198N) and endothelin-1 receptor type A polymorphisms (C1222T, C70G, and G231A) were determined. Genotype distributions were compared between NTG and HTG groups. The clinical condition of participants was examined for association with polymorphisms. Results. A similar frequency of occurrence of the polymorphic varieties of the studied genes was observed in patients with NTG and HTG. There is no relation between NTG risk factors and examined polymorphisms. NTG patients with TT genotype of K198N polymorphism presented with the lowest intraocular pressure in comparison to GG + GT genotype (p = 0.03). In NTG patients with CC genotype of C1222T polymorphism (p = 0.028) and GG of C70G polymorphism (p = 0.03) the lowest values of mean blood pressure were observed. Conclusions. The studied polymorphic varieties (K198N, C1222T) do have an influence on intraocular pressure as well as arterial blood pressure in NTG patients.

The role of astrocytes in optic nerve head fibrosis in glaucoma

Glaucoma is defined as a progressive optic neuropathy and is characterized by an irreversible loss of retinal ganglion cells. The main risk factor to develop glaucoma is an increased intraocular pressure (IOP). During the course of glaucoma structural changes in the optic nerve head (ONH) take place which lead to the characteristic excavation or cupping of the ONH. In this review we will focus on mechanisms and processes involved in structural alterations of the extracellular matrix in the lamina cribrosa (LC) of the ONH, which are associated with astrocytes. In glaucoma, a disordered deposition of elastic and collagen fibers and a typical pronounced thickening of the connective tissue septae surrounding the nerve fibers can be observed in the LC region. The remodeling process of the LC and the loss of ON axons are associated with a conversion of astrocytes from quiescent to a reactivated state. The extracellular matrix changes in the LC are thought to be due to a disturbed homeostatic balance of growth factors and the reactivated astrocytes are part of this process. Reactivated astrocytes, remodeling of the ECM within the LC and an elevated IOP are taking part in the retinal ganglion cell loss in glaucoma.

Using genetic mouse models to gain insight into glaucoma: Past results and future possibilities

While all forms of glaucoma are characterized by a specific pattern of retinal ganglion cell death, they are clinically divided into several distinct subclasses, including normal tension glaucoma, primary open angle glaucoma, congenital glaucoma, and secondary glaucoma. For each type of glaucoma there are likely numerous molecular pathways that control susceptibility to the disease. Given this complexity, a single animal model will never precisely model all aspects of all the different types of human glaucoma. Therefore, multiple animal models have been utilized to study glaucoma but more are needed. Because of the powerful genetic tools available to use in the laboratory mouse, it has proven to be a highly useful mammalian system for studying the pathophysiology of human disease. The similarity between human and mouse eyes coupled with the ability to use a combination of advanced cell biological and genetic tools in mice have led to a large increase in the number of studies using mice to model specific glaucoma phenotypes. Over the last decade, numerous new mouse models and genetic tools have emerged, providing important insight into the cell biology and genetics of glaucoma. In this review, we describe available mouse genetic models that can be used to study glaucoma-relevant disease/pathobiology. Furthermore, we discuss how these models have been used to gain insights into ocular hypertension (a major risk factor for glaucoma) and glaucomatous retinal ganglion cell death. Finally, the potential for developing new mouse models and using advanced genetic tools and resources for studying glaucoma are discussed.

Assessment of retinal ganglion cell damage in glaucomatous optic neuropathy: Axon transport, injury and soma loss

Glaucoma is a disease characterized by progressive axonal pathology and death of retinal ganglion cells (RGCs), which causes structural changes in the optic nerve head and irreversible vision loss. Several experimental models of glaucomatous optic neuropathy (GON) have been developed, primarily in non-human primates and, more recently and commonly, in rodents. These models provide important research tools to study the mechanisms underlying glaucomatous damage. Moreover, experimental GON provides the ability to quantify and monitor risk factors leading to RGC loss such as the level of intraocular pressure, axonal health and the RGC population. Using these experimental models we are able to gain a better understanding of GON, which allows for the development of potential neuroprotective strategies. Here we review the advantages and disadvantages of the relevant and most often utilized methods for evaluating axonal degeneration and RGC loss in GON. Axonal pathology in GON includes functional disruption of axonal transport (AT) and structural degeneration. Horseradish peroxidase (HRP), rhodamine-B-isothiocyanate (RITC) and cholera toxin-B (CTB) fluorescent conjugates have proven to be effective reporters of AT. Also, immunohistochemistry (IHC) for endogenous AT-associated proteins is often used as an indicator of AT function. Similarly, structural degeneration of axons in GON can be investigated via changes in the activity and expression of key axonal enzymes and structural proteins. Assessment of axonal degeneration can be measured by direct quantification of axons, qualitative grading, or a combination of both methods. RGC loss is the most frequently quantified variable in studies of experimental GON. Retrograde tracers can be used to quantify RGC populations in rodents via application to the superior colliculus (SC). In addition, in situ IHC for RGC-specific proteins is a common method of RGC quantification used in many studies. Recently, transgenic mouse models that express fluorescent proteins under the Thy-1 promoter have been examined for their potential to provide specific and selective labeling of RGCs for the study of GON. While these methods represent important advances in assessing the structural and functional integrity of RGCs, each has its advantages and disadvantages; together they provide an extensive toolbox for the study of GON.

Decreased endothelin-1 plasma levels in multiple sclerosis patients: a possible factor of vascular dysregulation?

BACKGROUND

Multiple sclerosis (MS) is a demyelinating autoimmune disease of the central nervous system with possible involvement of vascular dysregulation secondary to endothelial dysfunction caused by destruction of the vessel wall. Vascular dysregulation leads to excessive vasoconstriction or insufficient vasodilatation, resulting in vasospasm mediated by endothelin-1 (ET-1), the most potent and long-lasting mediator. Vascular dysregulation can play an important role in the pathogenesis of some eye disorders and it has been hypothesized that it is a vascular risk factor for glaucomatous optic neuropathy. The aim of this study was to estimate endothelin-1 (ET-1) plasma levels in patients with MS.

MATERIAL AND METHODS

The MS group consisted of 39 patients (9 males, 30 females), mean age: 38.8 ± 10.02 years, range: 22-62. The control group consisted of 27 healthy volunteers (3 males and 24 females), mean age: 37.4 ± 10.88 years, range: 20-62; clinically, in a non-active stage of the disease. ET-1 plasma levels were measured using the Endothelin-1 ELISA Kit (Immuno-Biological Laboratories Co., Japan). Statistical analysis was performed with the nonparametric Mann-Whitney U test for independent groups.

RESULTS

Endothelin-1 (ET-1) plasma levels were significantly lower in MS patients compared to healthy controls: mean value 0.55 ± 0.44 pg/ml (146.05 ± 118.27 fmol/ml) vs. 0.95 ± 0.48 pg/ml (252.83 ± 127.16 fmol/ml); P=0.012.

CONCLUSIONS

Significantly decreased ET-1 plasma levels in the MS patients could reflect the non-active disease at the time of ET-1 measurements or the effects of immunomodulatory treatment, but it cannot be excluded that decreased ET-1 plasma levels in these patients might result from vascular dysregulation.

Glial cell interactions and glaucoma

Purpose of review

The present review describes new advances in our understanding of the role of glial cells in the pathogenesis of glaucoma. It is becoming clear that retinal glia should not be studied in isolation in glaucoma because glia have dynamic and diverse interactions with a range of different cell types that could influence the disease process.

Recent findings

Microglial activity is modulated by signals from retinal ganglion cells and macroglia that influence RGC survival in various models of injury. New studies suggest that circulating monocytic populations may play a role in mediating the immune response to glaucoma. Astrocytes have been found to develop discrete localized processes that interact with a specific subset of retinal ganglion cells, possibly responding to the expression of phagocytic signals by stressed retinal ganglion cells.

Summary

Retinal glia constitute a highly versatile population that interacts with various cells to maintain homeostasis and limit disease. Defining the mechanisms that underlie glial communication could enable the development of more selective therapeutic targets, with great potential clinical applications.

Combinatorial targeting of early pathways profoundly inhibits neurodegeneration in a mouse model of glaucoma

The endothelin system is implicated in various human and animal glaucomas. Targeting the endothelin system has great promise as a treatment for human glaucoma, but the cell types involved and the exact mechanisms of action are not clearly elucidated. Here, we report a detailed characterization of the endothelin system in specific cell types of the optic nerve head (ONH) during glaucoma in DBA/2J mice. First, we show that key components of the endothelin system are expressed in multiple cell types. We discover that endothelin 2 (EDN2) is expressed in astrocytes as well as microglia/monocytes in the ONH. The endothelin receptor type A (Ednra) is expressed in vascular endothelial cells, while the endothelin receptor type B (Ednrb) receptor is expressed in ONH astrocytes. Second, we show that Macitentan treatment protects from glaucoma. Macitentan is a novel, orally administered, dual endothelin receptor antagonist with greater affinity, efficacy and safety than previous antagonists. Finally, we test the combinatorial effect of targeting both the endothelin and complement systems as a treatment for glaucoma. Similar to endothelin, the complement system is implicated in a variety of human and animal glaucomas, and has great promise as a treatment target. We discovered that combined targeting of the endothelin (Bosentan) and complement (C1qa mutation) systems is profoundly protective. Remarkably, 80% of DBA/2J eyes subjected to this combined inhibition developed no detectable glaucoma. This opens an exciting new avenue for neuroprotection in glaucoma.

Involvement of AP-1 and C/EBPβ in upregulation of endothelin B (ETB) receptor expression in a rodent model of glaucoma

Previous studies showed that the endothelin B receptor (ET_B) expression was upregulated and played a key role in neurodegeneration in rodent models of glaucoma. However, the mechanisms underlying upregulation of ET_B receptor expression remain largely unknown. Using promoter-reporter assays, the 1258 bp upstream the human ET_B promoter region was found to be essential for constitutive expression of ET_B receptor gene in human non-pigmented ciliary epithelial cells (HNPE). The −300 to −1 bp and −1258 to −600 bp upstream promoter regions of the ET_B receptor appeared to be the key binding regions for transcription factors. In addition, the crucial AP-1 binding site located at −615 to −624 bp upstream promoter was confirmed by luciferase assays and CHIP assays which were performed following overexpression of c-Jun in HNPE cells. Overexpression of either c-Jun or C/EBPβ enhanced the ET_B receptor promoter activity, which was reflected in increased mRNA and protein levels of ET_B receptor. Furthermore, knock-down of either c-Jun or C/EBPβ in HNPE cells was significantly correlated to decreased mRNA levels of both ET_B and ET_A receptor. These observations suggest that c-Jun and C/EBPβ are important for regulated expression of the ET_B receptor in HNPE cells. In separate experiments, intraocular pressure (IOP) was elevated in one eye of Brown Norway rats while the corresponding contralateral eye served as control. Two weeks of IOP elevation produced increased expression of c-Jun and C/EBPβ in the retinal ganglion cell (RGC) layer from IOP-elevated eyes. The mRNA levels of c-Jun, ET_A and ET_B receptor were upregulated by 2.2-, 3.1- and 4.4-fold in RGC layers obtained by laser capture microdissection from retinas of eyes with elevated IOP, compared to those from contralateral eyes. Taken together, these data suggest that transcription factor AP-1 plays a key role in elevation of ET_B receptor in a rodent model of ocular hypertension.

Intrinsic axonal degeneration pathways are critical for glaucomatous damage

Glaucoma is a neurodegenerative disease affecting 70million people worldwide. For some time, analysis of human glaucoma and animal models suggested that RGC axonal injury in the optic nerve head (where RGC axons exit the eye) is an important early event in glaucomatous neurodegeneration. During the last decade advances in molecular biology and genome manipulation have allowed this hypothesis to be tested more critically, at least in animal models. Data indicate that RGC axon degeneration precedes soma death. Preventing soma death using mouse models that are mutant for BAX, a proapoptotic gene, is not sufficient to prevent the degeneration of RGC axons. This indicates that different degeneration processes occur in different compartments of the RGC during glaucoma. Furthermore, the Wallerian degeneration slow allele (Wld(s)) slows or prevents RGC axon degeneration in rodent models of glaucoma. These experiments and many others, now strongly support the hypothesis that axon degeneration is a critical pathological event in glaucomatous neurodegeneration. However, the events that lead from a glaucomatous insult (e.g. elevated intraocular pressure) to axon damage in glaucoma are not well defined. For developing new therapies, it will be necessary to clearly define and order the molecular events that lead from glaucomatous insults to axon degeneration.

Molecular mechanisms of retinal ganglion cell degeneration in glaucoma and future prospects for cell body and axonal protection

Glaucoma, which affects more than 70 million people worldwide, is a heterogeneous group of disorders with a resultant common denominator; optic neuropathy, eventually leading to irreversible blindness. The clinical manifestations of primary open-angle glaucoma (POAG), the most common subtype of glaucoma, include excavation of the optic disc and progressive loss of visual field. Axonal degeneration of retinal ganglion cells (RGCs) and apoptotic death of their cell bodies are observed in glaucoma, in which the reduction of intraocular pressure (IOP) is known to slow progression of the disease. A pattern of localized retinal nerve fiber layer (RNFL) defects in glaucoma patients indicates that axonal degeneration may precede RGC body death in this condition. The mechanisms of degeneration of neuronal cell bodies and their axons may differ. In this review, we addressed the molecular mechanisms of cell body death and axonal degeneration in glaucoma and proposed axonal protection in addition to cell body protection. The concept of axonal protection may become a new therapeutic strategy to prevent further axonal degeneration or revive dying axons in patients with preperimetric glaucoma. Further study will be needed to clarify whether the combination therapy of axonal protection and cell body protection will have greater protective effects in early or progressive glaucomatous optic neuropathy (GON).

Endothelin B receptors contribute to retinal ganglion cell loss in a rat model of glaucoma

Glaucoma is an optic neuropathy, commonly associated with elevated intraocular pressure (IOP) characterized by optic nerve degeneration, cupping of the optic disc, and loss of retinal ganglion cells which could lead to loss of vision. Endothelin-1 (ET-1) is a 21-amino acid vasoactive peptide that plays a key role in the pathogenesis of glaucoma; however, the receptors mediating these effects have not been defined. In the current study, endothelin B (ET_B) receptor expression was assessed in vivo, in the Morrison's ocular hypertension model of glaucoma in rats. Elevation of IOP in Brown Norway rats produced increased expression of ET_B receptors in the retina, mainly in retinal ganglion cells (RGCs), nerve fiber layer (NFL), and also in the inner plexiform layer (IPL) and inner nuclear layer (INL). To determine the role of ET_B receptors in neurodegeneration, Wistar-Kyoto wild type (WT) and ET_B receptor-deficient (KO) rats were subjected to retrograde labeling with Fluoro-Gold (FG), following which IOP was elevated in one eye while the contralateral eye served as control. IOP elevation for 4 weeks in WT rats caused an appreciable loss of RGCs, which was significantly attenuated in KO rats. In addition, degenerative changes in the optic nerve were greatly reduced in KO rats compared to those in WT rats. Taken together, elevated intraocular pressure mediated increase in ET_B receptor expression and its activation may contribute to a decrease in RGC survival as seen in glaucoma. These findings raise the possibility of using endothelin receptor antagonists as neuroprotective agents for the treatment of glaucoma.

Endothelin-1 role in human eye: a review

Endothelin is a potent vasoactive peptide occurring in three isotypes, ET-1, ET-2, and ET-3. Through its two main receptors, endothelin A and endothelin B, it is responsible for a variety of physiological functions, primarily blood flow control. Recent evidence from both human and animal models shows involvement of endothelin in diabetes, retinal circulation, and optic neuropathies. Increased circulating levels of endothelin-1 (ET-1) have been found in patients with diabetes, and a positive correlation between plasma ET-1 levels and microangiopathy in patients with type-2 diabetes has been demonstrated. In addition to its direct vasoconstrictor effects, enhanced levels of ET-1 may contribute to endothelial dysfunction through inhibitory effects on nitric oxide (NO) production. Experimental studies have shown that chronic ET-1 administration to the optic nerve immediately behind the globe causes neuronal damage, activation of astrocytes, the major glial cell in the anterior optic nerve, and upregulation of endothelin B receptors. This paper outlines the ubiquitous role of endothelin and its potential involvement in ophthalmology.

Aqueous humor rapidly stimulates myocilin secretion from human trabecular meshwork cells

Myocilin, a protein associated with the development of glaucoma, is expressed in most eye tissues with highest expression observed in trabecular meshwork cells. In culture, primary human trabecular meshwork cells incubated in 10% fetal bovine serum have reduced myocilin expression compared to in vivo, but incubation in human aqueous humor, their normal in vivo nutrient source, restores myocilin expression to near in vivo levels. To investigate the mechanism by which human aqueous humor stimulates myocilin accumulation in conditioned media from normal human trabecular meshwork cells, three independent trabecular meshwork cell lines were cultured in Dulbecco's Modified Eagle's Medium (DMEM) containing various supplements: fetal bovine serum (10%), human serum (0.2%), porcine aqueous humor (50%), bovine serum albumin (0.1%), dexamethasone (10(-7)M), human aqueous humor (50%) or heat-inactivated human aqueous humor (50%). Conditioned media from cultured primary trabecular meshwork cells following incubation in human aqueous humor showed significant accumulation of myocilin in a time- (15 min) and dose-dependent manner (half maximal effective concentration ∼ 30%) while intracellular myocilin levels decreased. Minimal myocilin accumulation was observed in conditioned media isolated from trabecular meshwork cells cultured in DMEM containing fetal bovine or human serum, bovine serum albumin, porcine aqueous humor, dexamethasone or DMEM alone. Heat inactivation of human aqueous humor nearly eliminated human aqueous humor-stimulated myocilin secretion. Inhibitors of new protein synthesis, gene transcription, the endoplasmic reticulum/Golgi system and endocytic/exocytic secretory pathways failed to inhibit human aqueous humor-stimulated myocilin secretion. Using immunolabeling and transmission electron microscopy, myocilin was found associated with 70-90 nm vesicle-like structures within the cytoplasm of human aqueous humor treated trabecular meshwork cells. These studies suggest that myocilin secretion from trabecular meshwork cells occurs in a Golgi-independent manner following human aqueous humor treatment. Heat-labile factors in human aqueous humor are responsible for the time- and dose-dependent release of myocilin from vesicle-like structures within the cytoplasm of trabecular meshwork cells.

The complex interaction between ocular perfusion pressure and ocular blood flow - relevance for glaucoma

Glaucoma is an optic neuropathy of unknown origin. The most important risk factor for the disease is an increased intraocular pressure (IOP). Reducing IOP is associated with reduced progression in glaucoma. Several recent large scale trials have indicated that low ocular perfusion pressure (OPP) is a risk factor for the incidence, prevalence and progression of the disease. This is a strong indicator that vascular factors are involved in the pathogenesis of the disease, a hypothesis that was formulated 150 years ago. The relation between OPP and blood flow to the posterior pole of the eye is, however, complex, because of a phenomenon called autoregulation. Autoregulatory processes attempt to keep blood flow constant despite changes in OPP. Although autoregulation has been observed in many experiments in the ocular vasculature the mechanisms underlying the vasodilator and vasoconstrictor responses in face of changes in OPP remain largely unknown. There is, however, recent evidence that the human choroid regulates its blood flow better during changes in blood pressure induced by isometric exercise than during changes in IOP induced by a suction cup. This may have consequences for our understanding of glaucoma, because it indicates that blood flow regulation is strongly dependent not only on OPP, but also on the level of IOP itself. Indeed there is data indicating that reduction of IOP by pharmacological intervention improves optic nerve head blood flow regulation independently of an ocular vasodilator effect.

Endothelin-1 gene regulation

Over two decades of research have demonstrated that the peptide hormone endothelin-1 (ET-1) plays multiple, complex roles in cardiovascular, neural, pulmonary, reproductive, and renal physiology. Differential and tissue-specific production of ET-1 must be tightly regulated in order to preserve these biologically diverse actions. The primary mechanism thought to control ET-1 bioavailability is the rate of transcription from the ET-1 gene (edn1). Studies conducted on a variety of cell types have identified key transcription factors that govern edn1 expression. With few exceptions, the cis-acting elements bound by these factors have been mapped in the edn1 regulatory region. Recent evidence has revealed new roles for some factors originally believed to regulate edn1 in a tissue or hormone-specific manner. In addition, other mechanisms involved in epigenetic regulation and mRNA stability have emerged as important processes for regulated edn1 expression. The goal of this review is to provide a comprehensive overview of the specific factors and signaling systems that govern edn1 activity at the molecular level.

Blood pressure, perfusion pressure, and glaucoma

PURPOSE

To provide a critical review of the relationships between blood pressure, ocular blood flow, and glaucoma and the potential for glaucoma treatment through modulation of ocular perfusion.

DESIGN

Summaries of the pertinent literature and input from glaucoma researchers and specialists with relevant experience.

METHODS

Review and interpretation of selected literature and the results of a 1-day group discussion involving glaucoma researchers and specialists with expertise in epidemiology, blood flow measurements, and cardiovascular physiology.

RESULTS

Accurate, reproducible, and clinically relevant measurements of blood flow within the optic nerve head and associated capillary beds are not fully achievable with current methodology. Autoregulation of blood flow in the retina and optic nerve head occurs over a large range of intraocular pressures and blood pressures. Regulation of choroidal blood flow is provided by a mix of neurohumoral and local mechanisms. Vascular factors may be important in a subgroup of patients with primary open-angle glaucoma, and particularly in patients with normal-tension glaucoma and evidence of vasospasm. Low ocular perfusion pressure and low blood pressure are associated with an increased risk of glaucoma in population-based studies. The physiologic nocturnal dip in blood pressure is protective against systemic end-organ damage, but its effects on glaucoma are not well elaborated or understood. Large-scale longitudinal studies would be required to evaluate the risk of glaucomatous progression in non-dippers, dippers, and extreme nocturnal blood pressure dippers.

CONCLUSIONS

Decreases in perfusion pressure and blood pressure have been associated with glaucoma. However, there is no evidence to support the value of increasing a patient's blood pressure as therapy for glaucoma. Such recommendations are not currently warranted, since we lack crucial information about the microvascular beds in which perfusion is important in glaucoma, and the appropriate methods to evaluate their blood flow. There are also cardiovascular safety concerns associated with treatments designed to increase ocular perfusion pressure and blood flow by increasing blood pressure, especially in elderly patients. For these reasons and with present evidence it is unlikely that safe and effective glaucoma treatments based on altering optic nerve perfusion will soon be available.

Effects of endothelin-1 on the cytoskeleton protein F-actin of human trabecular meshwork cells in vitro

AIM

To observe the effect of endothelin-1 (ET-1) on the cytoskeleton protein F-actin of cultured human trabecular meshwork (HTM) cells.

METHODS

CULTURED HTM CELLS WERE RANDOMLY DIVIDED INTO FOUR GROUPS: control group, low-dose ET-1 (10(-9) mol/L) treatment group, middle-dose ET-1 (10(-8) mol/L) treatment group, and high-dose ET-1(10(-7) mol/L) treatment group. After treated with ET-1, the expression of cytoskeleton protein F-actin in trabecular meshwork was analyzed with Western-blot and the distribution of F-actin was detected with FITC-Phalloidin probe.

RESULTS

ET-1 dose-dependently and significantly increased F-actin in trabecular meshwork cells (P<0.05). The F-actin stress fiber and periphery actin fiber highly increased and manifested mild reorganization after treated with ET-1; and there were much more cell-to-cell and cell-to-extracellular matrix attachments formation in ET-1 treated HTM cells than that in the untreated HTM cells.

CONCLUSION

ET-1 promotes the expression of cytoske- leton protein F-actin and induced the trabecular meshwork actin cytoskeleton reorganization.

Endothelin receptors: what's new and what do we need to know?

Receptors are at the heart of how a molecule transmits a signal to a cell. Two receptor classes for endothelin (ET) are recognized, the ET(A) and ET(B) receptors. Intriguing questions have arisen in the field of ET receptor pharmacology, physiology, and function. For example, a host of pharmacological studies support the interaction of the ET(A) and ET(B) receptor in tissues (veins, arteries, bronchus, arterioles, esophagus), but yet few have been able to demonstrate direct ET(A)/ET(B) receptor interaction. Have we modeled this interaction wrong? Do we have a truly selective ET(A) receptor agonist such that we could selectively stimulate this important receptor? What can we learn from the recent phylogenic studies of the ET receptor family? Have we adequately addressed the number of biological molecules with which ET can interact to exert a biological effect? Recent mass spectrometry studies in our laboratory suggest that ET-1 interacts with other hereto unrecognized proteins. Biased ligands (ligands at the same receptor that elicit distinct signaling responses) have been discovered for other receptors. Do these exist for ET receptors and can we take advantage of this possibility in drug design? These and other questions will be posed in this minireview on topics on ET receptors.