Association study of the 5' flanking regions of endothelial-nitric oxide synthase and endothelin-1 genes in familial primary open-angle glaucoma

Glaucoma: from pathogenic mechanisms to retinal glial cell response to damage

Glaucoma is a neurodegenerative disease of the retina characterized by the irreversible loss of retinal ganglion cells (RGCs) leading to visual loss. Degeneration of RGCs and loss of their axons, as well as damage and remodeling of the lamina cribrosa are the main events in the pathogenesis of glaucoma. Different molecular pathways are involved in RGC death, which are triggered and exacerbated as a consequence of a number of risk factors such as elevated intraocular pressure (IOP), age, ocular biomechanics, or low ocular perfusion pressure. Increased IOP is one of the most important risk factors associated with this pathology and the only one for which treatment is currently available, nevertheless, on many cases the progression of the disease continues, despite IOP control. Thus, the IOP elevation is not the only trigger of glaucomatous damage, showing the evidence that other factors can induce RGCs death in this pathology, would be involved in the advance of glaucomatous neurodegeneration. The underlying mechanisms driving the neurodegenerative process in glaucoma include ischemia/hypoxia, mitochondrial dysfunction, oxidative stress and neuroinflammation. In glaucoma, like as other neurodegenerative disorders, the immune system is involved and immunoregulation is conducted mainly by glial cells, microglia, astrocytes, and Müller cells. The increase in IOP produces the activation of glial cells in the retinal tissue. Chronic activation of glial cells in glaucoma may provoke a proinflammatory state at the retinal level inducing blood retinal barrier disruption and RGCs death. The modulation of the immune response in glaucoma as well as the activation of glial cells constitute an interesting new approach in the treatment of glaucoma.

Energy Metabolism in the Inner Retina in Health and Glaucoma

Glaucoma, the leading cause of irreversible blindness, is a heterogeneous group of diseases characterized by progressive loss of retinal ganglion cells (RGCs) and their axons and leads to visual loss and blindness. Risk factors for the onset and progression of glaucoma include systemic and ocular factors such as older age, lower ocular perfusion pressure, and intraocular pressure (IOP). Early signs of RGC damage comprise impairment of axonal transport, downregulation of specific genes and metabolic changes. The brain is often cited to be the highest energy-demanding tissue of the human body. The retina is estimated to have equally high demands. RGCs are particularly active in metabolism and vulnerable to energy insufficiency. Understanding the energy metabolism of the inner retina, especially of the RGCs, is pivotal for understanding glaucoma’s pathophysiology. Here we review the key contributors to the high energy demands in the retina and the distinguishing features of energy metabolism of the inner retina. The major features of glaucoma include progressive cell death of retinal ganglions and optic nerve damage. Therefore, this review focuses on the energetic budget of the retinal ganglion cells, optic nerve and the relevant cells that surround them.

Shear Stress in Schlemm's Canal as a Sensor of Intraocular Pressure

Elevated intraocular pressure (IOP) narrows Schlemm’s canal (SC), theoretically increasing luminal shear stress. Using engineered adenoviruses containing a functional fragment of the shear-responsive endothelial nitric oxide synthase (eNOS) promoter, we tested effects of shear stress and elevated flow rate on reporter expression in vitro and ex vivo. Cultured human umbilical vein endothelial cells (HUVECs) and SC cells were transduced with adenovirus containing eNOS promoter driving secreted alkaline phosphatase (SEAP) or green fluorescent protein (GFP) and subjected to shear stress. In parallel, human anterior segments were perfused under controlled flow. After delivering adenoviruses to the SC lumen by retroperfusion, the flow rate in one anterior segment of pair was increased to double pressure. In response to high shear stress, HUVECs and SC cells expressed more SEAP and GFP than control. Similarly, human anterior segments perfused at higher flow rates released significantly more nitrites and SEAP into perfusion effluent, and SC cells expressed increased GFP near collector channel ostia compared to control. These data establish that engineered adenoviruses have the capacity to quantify and localize shear stress experienced by endothelial cells. This is the first in situ demonstration of shear-mediated SC mechanobiology as a key IOP-sensing mechanism necessary for IOP homeostasis.

Association of endothelial nitric oxide synthase (NOS3) gene polymorphisms with primary open-angle glaucoma in a Saudi cohort

Aim

To investigate the association of endothelial nitric oxide synthase (NOS3) gene polymorphisms in patients with primary open-angle glaucoma (POAG) of Saudi origin.

Methods

This case-control study included 173 patients with POAG (94 men and 79 women) and 171 controls (98 men and 73 women). Genotyping of rs2070744 (T-786C) and rs1799983 (G894T) variants of the NOS3 gene was performed using TaqMan^® assay.

Results

Rs1799983 genotypes showed a significant association with POAG but did not survive Bonferroni correction (p_correction = 0.01). The minor ‘T’ allele was significantly associated with the risk of POAG among men (p = 0.025, odds ratio (OR) = 1.77, 95% confidence interval (CI) = 1.07–2.94). Likewise, the genotypes were significantly associated with POAG among men in dominant (p = 0.030, OR = 1.92, 95% CI = 1.06–3.48) and log-additive models (p = 0.022, OR = 1.82, 95% CI = 1.08–3.07), and after adjustment for age and smoking. Genotype and allele frequencies of rs2070744 were not significantly different between POAG cases and controls, and after sex stratification. CG haplotype was significantly protective (p = 0.011, OR = 0.52, 95% CI = 0.32–0.87) and CT haplotype conferred significantly increased risk of POAG (p = 0.016, OR = 2.60, 95% CI = 1.16–5.82) among men. Rs1799983 showed trend (p = 0.054) towards risk of POAG independent of age, gender, smoking, and rs2070744 polymorphism in logistic regression analysis. Both the polymorphisms showed no association with POAG phenotypes such as intraocular pressure and cup/disc ratio.

Conclusion

Our results suggest that the polymorphism rs1799983 and the haplotypes of rs20707440 and rs1799983 in the NOS3 gene may significantly modulate the risk of POAG in Saudi’s, particularly among men. Further larger studies are needed to confirm these findings.

Microvascular endothelial function and primary open angle glaucoma

Purpose

To determine the role of microvascular endothelial dysfunction as risk factor for primary open angle glaucoma.

Methods

A cross-sectional study was conducted involving 114 Malay patients with POAG seen at the eye clinic of Hospital Universiti Sains Malaysia. Patients aged between 40 and 80 years who were diagnosed with other types of glaucoma, previous glaucoma filtering surgery or other surgeries except uncomplicated cataract surgery and pterygium surgery were excluded. A total of 101 patients who were followed up for dry eyes, age-related cataracts or post cataracts extraction surgery were recruited as control subjects. Those with family history of glaucoma or glaucoma suspect were excluded. Microvascular endothelial function was assessed using laser Doppler fluximetry and the process of iontophoresis. Iontophoresis with acetylcholine (ACh) and sodium nitroprusside (SNP) was used to measure microvascular endothelium-dependent and endothelium-independent vasodilatations, respectively.

Results

In general, POAG patients demonstrated lower ACh% and ACh_max values compared with controls. There was significant difference in microvascular endothelial function [ACh%: mean, 95% confidence interval = 503.1 (378.0, 628.3), and ACh_max: mean, 95% confidence interval = 36.8 (30.2, 43.5)] between primary open angle glaucoma cases (p < 0.001) and controls [ACh%: mean, 95% confidence interval = 1378.4 (1245.4, 1511.3), and ACh_max: mean, 95% confidence interval = 79.2 (72.1, 86.2)]; this difference remained significant even after controlling for potential confounders. Similar difference was also found in SNP% and SNP_max between POAG and controls (p < 0.001). Age and diastolic blood pressure were inversely correlated with microvascular endothelial function.

Conclusion

There was an impairment of microvascular endothelial function and endothelial-independent vasodilatation in POAG patients. Microvascular endothelial function is a potential risk factor for POAG.

Increased bioavailability of cyclic guanylate monophosphate prevents retinal ganglion cell degeneration

The nitric oxide - guanylyl cyclase-1 - cyclic guanylate monophosphate (NO-GC-1-cGMP) pathway has emerged as a potential pathogenic mechanism for glaucoma, a common intraocular pressure (IOP)-related optic neuropathy characterized by the degeneration of retinal ganglion cells (RGCs) and their axons in the optic nerve. NO activates GC-1 to increase cGMP levels, which are lowered by cGMP-specific phosphodiesterase (PDE) activity. This pathway appears to play a role in both the regulation of IOP, where reduced cGMP levels in mice leads to elevated IOP and subsequent RGC degeneration. Here, we investigated whether potentiation of cGMP signaling could protect RGCs from glaucomatous degeneration. We administered the PDE5 inhibitor tadalafil orally (10 mg/kg/day) in murine models of two forms of glaucoma - primary open angle glaucoma (POAG; GC-1^-/- mice) and primary angle-closure glaucoma (PACG; Microbead Occlusion Model) - and measured RGC viability at both the soma and axon level. To determine the direct effect of increased cGMP on RGCs in vitro, we treated axotomized whole retina and primary RGC cultures with the cGMP analogue 8-Br-cGMP. Tadalafil treatment increased plasma cGMP levels in both models, but did not alter IOP or mean arterial pressure. Nonetheless, tadalafil treatment prevented degeneration of RGC soma and axons in both disease models. Treatment of whole, axotomized retina and primary RGC cultures with 8-Br-cGMP markedly attenuated both necrotic and apoptotic cell death pathways in RGCs. Our findings suggest that enhancement of the NO-GC-1-cGMP pathway protects the RGC body and axon in murine models of POAG and PACG, and that enhanced signaling through this pathway may serve as a novel glaucoma treatment, acting independently of IOP.

The nitric oxide-guanylate cyclase pathway and glaucoma

Glaucoma is a prevalent optic neuropathy characterized by the progressive dysfunction and loss of retinal ganglion cells (RGCs) and their optic nerve axons, which leads to irreversible visual field loss. Multiple risk factors for the disease have been identified, but elevated intraocular pressure (IOP) remains the primary risk factor amenable to treatment. Reducing IOP however does not always prevent glaucomatous neurodegeneration, and many patients progress with the disease despite having IOP in the normal range. There is increasing evidence that nitric oxide (NO) is a direct regulator of IOP and that dysfunction of the NO-Guanylate Cyclase (GC) pathway is associated with glaucoma incidence. NO has shown promise as a novel therapeutic with targeted effects that: 1) lower IOP; 2) increase ocular blood flow; and 3) confer neuroprotection. The various effects of NO in the eye appear to be mediated through the activation of the GC- guanosine 3:5'-cyclic monophosphate (cGMP) pathway and its effect on downstream targets, such as protein kinases and Ca²⁺ channels. Although NO-donor compounds are promising as therapeutics for IOP regulation, they may not be ideal to harness the neuroprotective potential of NO signaling. Here we review evidence that supports direct targeting of GC as a novel pleiotrophic treatment for the disease, without the need for direct NO application. The identification and targeting of other factors that contribute to glaucoma would be beneficial to patients, particularly those that do not respond well to IOP-dependent interventions.

The Relation Between Endothelial Nitric Oxide Synthase Polymorphisms and Normal Tension Glaucoma

PURPOSE

We investigated whether polymorphisms of the endothelial NO synthase (eNOS) gene are associated with normal tension glaucoma (NTG). We also investigated whether the eNOS polymorphisms are associated with NTG subgroups [NTG with and without optic disc hemorrhage (DH)].

METHODS

A total of 251 patients with NTG and 245 healthy volunteers were enrolled in this study. DNA from peripheral blood leukocytes was extracted, and the genotypes of 4 polymorphisms (rs2070744, rs1549758, rs1799983, and rs2566514) in the eNOS gene were determined using restriction fragment length polymorphism and the SNaPshot method. The primary outcome was to investigate the relation between eNOS polymorphisms and NTG. The secondary outcome was to compare the frequencies of the polymorphic genotypes among the NTG subgroups. Bonferroni correction was used to adjust for type I error.

RESULTS

In all subjects, the genotype distribution was in accordance with Hardy-Weinberg equilibrium. None of the 4 polymorphisms showed any significant difference in the frequencies of alleles or genotypes between the NTG patients and controls. In the further analysis comparing the genotypic frequencies between NTG with DH and normal controls, the CC/CT genotype of rs2070744 was significantly associated with DH in NTG patients (genotypic association test, P-value=0.0041). On the multiple logistic regression analysis adjusted for covariates such as sex and age, the NTG with DH was associated with polymorphic genotypes of rs2070744 with a borderline significance (additive genetic model, P=0.0070).

CONCLUSIONS

Our results indicates that eNOS rs2070744 can be associated with NTG patients with DH. This finding suggests that the eNOS polymorphism may be a genetic risk factor in the development of DH in NTG patients.

The Role of Nitric Oxide in the Intraocular Pressure Lowering Efficacy of Latanoprostene Bunod: Review of Nonclinical Studies

Latanoprostene bunod (LBN) is a topical ophthalmic therapeutic for the reduction of intraocular pressure (IOP) in patients with open-angle glaucoma or ocular hypertension (OHT). LBN is composed of latanoprost acid (LA) linked to a nitric oxide (NO)-donating moiety and is the first NO-releasing prostaglandin analog to be submitted for marketing authorization in the United States. The role of latanoprost in increasing uveoscleral outflow of aqueous humor (AqH) is well established. Herein, we review findings from nonclinical studies, which evaluated the role of NO in the IOP-lowering efficacy of LBN. Pharmacokinetic studies in rabbits and corneal homogenates indicate that LBN is rapidly metabolized to LA and butanediol mononitrate (BDMN). NO is subsequently released by BDMN as shown by increased cyclic guanosine monophosphate (cGMP) levels in (1) the AqH and iris-ciliary body after administration of LBN in rabbits and in (2) human trabecular meshwork (TM) cells after incubation with LBN. LBN reduced myosin light chain phosphorylation, induced cytoskeletal rearrangement, and decreased resistance to current flow to a greater extent than latanoprost in TM cells, indicating that NO released from LBN elicited TM cell relaxation. LBN also lowered IOP to a greater extent than latanoprost in FP receptor knockout mice, rabbits with transient OHT, glaucomatous dogs, and primates with OHT. Along with results from a Phase 2 clinical study in which treatment with LBN 0.024% resulted in greater IOP-lowering efficacy than latanoprost 0.005%, these data indicate that LBN has a dual mechanism of action, increasing AqH outflow through both the uveoscleral (using LA) and TM/Schlemm's canal (using NO) pathways.

Association of Dietary Nitrate Intake With Primary Open-Angle Glaucoma: A Prospective Analysis From the Nurses' Health Study and Health Professionals Follow-up Study

IMPORTANCE

Nitric oxide signaling alterations in outflow facility and retinal blood flow autoregulation are implicated in primary open-angle glaucoma (POAG). Nitric oxide donation has emerged as a POAG therapeutic target. An exogenous source of nitric oxide is dietary nitrates.

OBJECTIVE

To evaluate the association between dietary nitrate intake, derived mainly from green leafy vegetables, and POAG.

DESIGN, SETTING, AND PARTICIPANTS

We followed up participants biennially in the prospective cohorts of the Nurses' Health Study (63 893 women; 1984-2012) and the Health Professionals Follow-up Study (41 094 men; 1986-2012) at each 2-year risk period. Eligible participants were 40 years or older, were free of POAG, and reported eye examinations.

EXPOSURES

The primary exposure was dietary nitrate intake. Information on diet and potential confounders was updated with validated questionnaires.

MAIN OUTCOMES AND MEASURES

The main outcome was the incidence of POAG and POAG subtypes; 1483 cases were confirmed with medical records and classified into subtypes defined by intraocular pressure (IOP) (≥22 or <22 mm Hg) or by visual field (VF) loss pattern at diagnosis (peripheral loss only or early paracentral loss). Cohort-specific and pooled multivariable rate ratios (MVRRs) and 95% CIs were estimated.

RESULTS

During 1 678 713 person-years of follow-up, 1483 incident cases of POAG were identified. The mean (SD) age for the 1483 cases was 66.8 (8.3). Compared with the lowest quintile of dietary nitrate intake (quintile 1: approximately 80 mg/d), the pooled MVRR for the highest quintile (quintile 5: approximately 240 mg/d) was 0.79 (95% CI, 0.66-0.93; P for trend = .02). The dose response was stronger (P for heterogeneity = .01) for POAG with early paracentral VF loss (433 cases; quintile 5 vs quintile 1 MVRR = 0.56; 95% CI, 0.40-0.79; P for trend < .001) than for POAG with peripheral VF loss only (835 cases; quintile 5 vs quintile 1 MVRR = 0.85; 95% CI, 0.68-1.06; P for trend = .50). The association did not differ (P for heterogeneity = .75) by POAG subtypes defined by IOP (997 case patients with IOP ≥22 mm Hg: quintile 5 vs quintile 1 MVRR = 0.82; 95% CI, 0.67-1.01; P for trend = .11; 486 case patients with IOP <22 mm Hg: quintile 5 vs quintile 1 MVRR = 0.71; 95% CI, 0.53-0.96; P for trend = .12). Green leafy vegetables accounted for 56.7% of nitrate intake variation. Compared with consuming 0.31 servings per day, the MVRR for consuming 1.45 or more servings per day was 0.82 for all POAG (95% CI, 0.69-0.97; P for trend = .02) and 0.52 for POAG with paracentral VF loss (95% CI, 0.29-0.96; P for trend < .001).

CONCLUSIONS AND RELEVANCE

Higher dietary nitrate and green leafy vegetable intake was associated with a lower POAG risk, particularly POAG with early paracentral VF loss at diagnosis.

From DNA damage to functional changes of the trabecular meshwork in aging and glaucoma

Glaucoma is a degenerative disease of the eye. Both the anterior and posterior segments of the eye are affected, extensive damage being detectable in the trabecular meshwork and the inner retina-central visual pathway complex. Oxidative stress is claimed to be mainly responsible for molecular damage in the anterior chamber. Indeed, oxidation harms the trabecular meshwork, leading eventually to endothelial cell decay, tissue malfunction, subclinical inflammation, changes in the extracellular matrix and cytoskeleton, altered motility, reduced outflow facility and (ultimately) increased IOP. Moreover, free radicals are involved in aging and can be produced in the brain (as well as in the eye) as a result of ischemia, leading to oxidation of the surrounding neurons. Glaucoma-related cell death occurs by means of apoptosis, and apoptosis is triggered by oxidative stress via (a) mitochondrial damage, (b) inflammation, (c) endothelial dysregulation and dysfunction, and (d) hypoxia. The proteomics of the aqueous humor is significantly altered in glaucoma as a result of oxidation-induced trabecular damage. Those proteins whose aqueous humor levels are increased in glaucoma are biomarkers of trabecular meshwork impairment. Their diffusion from the anterior to the posterior segment of the eye may be relevant in the cascade of events triggering apoptosis in the inner retinal layers, including the ganglion cells.

Vascular and autonomic dysregulation in primary open-angle glaucoma

PURPOSE OF REVIEW

The purpose of this review is to discuss whether vascular dysfunction and autonomic dysfunction are related to primary open-angle glaucoma stratified by the intraocular pressure (IOP) level.

RECENT FINDINGS

Patients with primary open angle glaucoma (POAG) across the spectrum of IOP exhibit a variety of ocular and nonocular vascular abnormalities. Interestingly, common genetic variation in nitric oxide synthase 3 (NOS3) and the caveolin 1/caveolin 2 (CAV1/CAV2) genomic regions, which code for proteins involved in setting vascular tone, are associated with POAG. These markers seem to stratify with POAG subtypes stratified by sex or pattern of initial visual field loss and not by IOP level. Overall, it is clear that there is also cardiovascular autonomic dysfunction in high-tension glaucoma and normal-tension glaucoma but it is unclear if this dysfunction is more common in normal-tension glaucoma compared with high-tension glaucoma.

SUMMARY

Overall, POAG is likely a heterogeneous disease but stratifying cases by IOP level associated with initial optic nerve damage may be less useful than using other endophenotype approaches. Embracing the evidence suggesting systemic endothelial and autonomic dysfunction are operative in POAG will help us move beyond an IOP-centric view of the disease and facilitate 'tearing down the wall' that divides treating physicians and a better understanding of POAG pathogenesis.

Risk Factors for Normal and High-Tension Glaucoma in Poland in Connection with Polymorphisms of the Endothelial Nitric Oxide Synthase Gene

Aim

The purpose of this study was to evaluate the influence of polymorphisms of the eNOS gene on the clinical status of patients with normal and high tension glaucoma.

Methods

266 Polish Caucasian patients with primary open angle glaucoma were studied. Of the 266, 156 had normal tension glaucoma (NTG) and 110 high tension glaucoma (HTG). DNA material was isolated from peripheral venous blood using commercial kits. Real-time PCR reaction was used to amplify the promoter site of the endothelial nitric oxide synthase (eNOS) gene, including the single nucleotide polymorphism (SNP) site T-786C and part of the 7^th exon of eNOS, including G894T SNP. Genotypes were determined with TaqMan SNP Genotyping Assays.

Results

There were no significant differences in frequencies of the allelic variants of both polymorphisms. In G894T SNP, however, the wild GG form was more common in the HTG group. The SNP of the eNOS gene did not significantly influence the progression rate in either of the groups studied. There were no differences in variants of the eNOS gene regarding the necessity for and success of surgery and the progression of the disease. In the NTG group, no statistical correlation was observed between G894T, T786C polymorphism variants, and risk factors such as optic disc haemorrhages, optic disc notches, and peripapillary atrophy. Mean diastolic and systolic pressure during the day and night were lowest in NTG patients with the CC variant of the T786C polymorphism. No statistical correlation was observed between the G894T and T786C polymorphisms and capillaroscopic examination results.

Conclusions

Genotype frequencies are similar for both the eNOS G894T and T-786C polymorphisms in NTG and HTG patients. These polymorphisms do not correlate with risk factors and do not influence the state of the capillary system in NTG patients. Systolic blood pressure is lower in NTG patients with mutated alleles of both polymorphisms.

Is Estrogen a Therapeutic Target for Glaucoma?

This article's objective is to provide an overview of the association between estrogen and glaucoma. A literature synthesis was conducted of articles published in peer-reviewed journals screened through May 5, 2015, using the PubMed database. Keywords used were "estrogen and glaucoma," "reproductive factors and glaucoma," and "estrogen, nitric oxide and eye." Forty-three journal articles were included. Results indicated that markers for lifetime estrogen exposure have been measured by several studies and show that the age of menarche onset, oral contraceptive (OC) use, bilateral oophorectomy, age of menopause onset and duration between menarche to menopause are associated with primary open-angle glaucoma (POAG) risk. The Blue Mountain Eye Study found a significantly increased POAG risk with later (>13 years) compared with earlier (≤12 years) age of menarche. Nurses' Health Study (NHS) investigators found that OC use of greater than 5 years was associated with a 25% increased risk of POAG. The Mayo Clinic Cohort Study of Oophorectomy and Aging found that women who underwent bilateral oophorectomy before age 43 years had an increased risk of glaucoma. The Rotterdam Study found that women who went through menopause before reaching the age of 45 years had a higher risk of open-angle glaucoma (2.6-fold increased risk), while the NHS showed a reduced risk of POAG among women older than 65 who entered menopause after age ≥ 54 years. Increased estrogen states may confer a reduced risk of glaucoma or glaucoma-related traits such as reduced intraocular pressure (IOP). Pregnancy, a hyperestrogenemic state, is associated with decreased IOP during the third trimester. Though the role of postmenopausal hormone (PMH) use in the reduction of IOP is not fully conclusive, PMH use may reduce the risk of POAG. From a genetic epidemiologic perspective, estrogen metabolic pathway single nucleotide polymorphisms (SNPs) were associated with POAG in women and polymorphisms in endothelial nitric oxide synthase, a gene receptive to estrogen regulation, are associated with glaucoma. The study concluded that increasing evidence suggests that lifetime exposure to estrogen may alter the pathogenesis of glaucoma. Estrogen exposure may have a neuroprotective effect on the progression of POAG but further studies need to confirm this finding. The role of sex-specific preventive and therapeutic treatment may be on the horizon.

Genes, pathways, and animal models in primary open-angle glaucoma

Glaucoma is an optic neuropathy characterized by loss of retinal ganglion cells (RGCs) and consequently visual field loss. It is a complex and heterogeneous disease in which both environmental and genetic factors play a role. With the advent of genome-wide association studies (GWASs), the number of loci associated with primary open-angle glaucoma (POAG) have increased greatly. There has also been major progress in understanding the genes determining the vertical cup-disc ratio (VCDR), disc area (DA), cup area (CA), intraocular pressure (IOP), and central corneal thickness (CCT). In this review, we will update and summarize the genetic loci associated so far with POAG, VCDR, DA, CA, IOP, and CCT. We will describe the pathways revealed and supported by genetic association studies, integrating current knowledge from human and experimental data. Finally, we will discuss approaches for functional genomics and clinical translation.

Shear stress-triggered nitric oxide release from Schlemm's canal cells

PURPOSE

Endothelial nitric oxide (NO) synthase is regulated by shear stress. At elevated intraocular pressures when the Schlemm's canal (SC) begins to collapse, shear stress is comparable with that in large arteries. We investigated the relationship between NO production and shear stress in cultured human SC cells.

METHODS

Schlemm's canal endothelial cells isolated from three normal and two glaucomatous human donors were seeded into Ibidi flow chambers at confluence, cultured for 7 days, and subjected to steady shear stress (0.1 or 10 dynes/cm(2)) for 6, 24, or 168 hours. Cell alignment with flow direction was monitored, and NO production was measured using 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM) and Griess reagents. Human trabecular meshwork (TM) and umbilical vein endothelial cells (HUVECs) were used as controls.

RESULTS

Normal SC strains aligned with the direction of flow by 7 days. Comparing 0.1 vs. 10 dynes/cm(2), NO levels increased by 82% at 24 hours and 8-fold after 7 days by DAF-FM, and similar results were obtained with Griess reagent. Shear responses by SC cells at 24 hours were comparable with HUVECs, and greater than TM cells, which appeared shear-insensitive. Nitric oxide production by SC cells was detectable as early as 6 hours and was inhibited by 100 μM nitro-L-arginine methyl ester. Two glaucomatous SC cell strains were either unresponsive or lifted from the plate in the face of shear.

CONCLUSIONS

Shear stress triggers NO production in human SC cells, similar to other vascular endothelia. Increased shear stress and NO production during SC collapse at elevated intraocular pressures may in part mediate IOP homeostasis.

Effect of brimonidine on retinal vascular autoregulation and short-term visual function in normal tension glaucoma

PURPOSE

To assess whether brimonidine 0.15% alters retinal vascular autoregulation and short-term visual function in normal tension glaucoma patients who demonstrate retinal vascular dysregulation.

DESIGN

Nonrandomized clinical trial.

METHODS

In this prospective study, 46 normal tension glaucoma patients not previously treated with brimonidine underwent retinal vascular autoregulation testing and visual function assessment using frequency doubling technology perimetry and equivalent noise motion sensitivity testing. We measured blood flow in a major temporal retinal artery with subjects seated and then while reclined for 30 minutes. Patients having a change in retinal blood flow with posture change outside the range previously found in healthy subjects were classified as having retinal vascular dysregulation. They were treated with brimonidine 0.15% for 8 weeks and designated for retesting.

RESULTS

Twenty-three patients demonstrated retinal vascular dysregulation at the initial visit. Younger age (P = .050) and diabetes (P = .055) were marginally significant risk factors for retinal vascular dysregulation. After the 8-week course with brimonidine, 14 of the 17 patients who completed the study showed a return of posture-induced retinal blood flow changes to levels consistent with normal retinal vascular autoregulation (P < .0001). We found no significant changes in frequency doubling technology perimetry or in motion detection parameters following treatment with brimondine (P > .09 for all tests performed).

CONCLUSIONS

Brimonidine significantly improved impaired retinal vascular autoregulation in normal tension glaucoma patients, but short-term alteration in visual function could not be demonstrated.

Vascular tone pathway polymorphisms in relation to primary open-angle glaucoma

AIMS

Vascular perfusion may be impaired in primary open-angle glaucoma (POAG); thus, we evaluated a panel of markers in vascular tone-regulating genes in relation to POAG.

METHODS

We used Illumina 660W-Quad array genotype data and pooled P-values from 3108 POAG cases and 3430 controls from the combined National Eye Institute Glaucoma Human Genetics Collaboration consortium and Glaucoma Genes and Environment studies. Using information from previous literature and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, we compiled single-nucleotide polymorphisms (SNPs) in 186 vascular tone-regulating genes. We used the 'Pathway Analysis by Randomization Incorporating Structure' analysis software, which performed 1000 permutations to compare the overall pathway and selected genes with comparable randomly generated pathways and genes in their association with POAG.

RESULTS

The vascular tone pathway was not associated with POAG overall or POAG subtypes, defined by the type of visual field loss (early paracentral loss (n=224 cases) or only peripheral loss (n=993 cases)) (permuted P≥0.20). In gene-based analyses, eight were associated with POAG overall at permuted P<0.001: PRKAA1, CAV1, ITPR3, EDNRB, GNB2, DNM2, HFE, and MYL9. Notably, six of these eight (the first six listed) code for factors involved in the endothelial nitric oxide synthase activity, and three of these six (CAV1, ITPR3, and EDNRB) were also associated with early paracentral loss at P<0.001, whereas none of the six genes reached P<0.001 for peripheral loss only.

DISCUSSION

Although the assembled vascular tone SNP set was not associated with POAG, genes that code for local factors involved in setting vascular tone were associated with POAG.

Regulation of intraocular pressure by soluble and membrane guanylate cyclases and their role in glaucoma

Glaucoma is a progressive optic neuropathy characterized by visual field defects that ultimately lead to irreversible blindness (Alward, 2000; Anderson et al., 2006). By the year 2020, an estimated 80 million people will have glaucoma, 11 million of which will be bilaterally blind. Primary open-angle glaucoma (POAG) is the most common type of glaucoma. Elevated intraocular pressure (IOP) is currently the only risk factor amenable to treatment. How IOP is regulated and can be modulated remains a topic of active investigation. Available therapies, mostly geared toward lowering IOP, offer incomplete protection, and POAG often goes undetected until irreparable damage has been done, highlighting the need for novel therapeutic approaches, drug targets, and biomarkers (Heijl et al., 2002; Quigley, 2011). In this review, the role of soluble (nitric oxide (NO)-activated) and membrane-bound, natriuretic peptide (NP)-activated guanylate cyclases that generate the secondary signaling molecule cyclic guanosine monophosphate (cGMP) in the regulation of IOP and in the pathophysiology of POAG will be discussed.

Association of polymorphisms of endothelial nitric oxide synthase (eNOS) gene with the risk of primary open angle glaucoma in a Brazilian population

The present study aimed to investigate the association of endothelial nitric oxide synthase (eNOS) gene polymorphisms with primary open angle glaucoma (POAG). We conducted a case-control study that included 90 patients with POAG and 127 healthy controls whose blood samples were genotyped for the functional polymorphisms T-786C and Glu298Asp of the eNOS gene by Taqman fluorescent allelic discrimination assay. The T-786C polymorphism was significantly associated as a risk factor for POAG among women (OR: 2.28; 95% CI: 1.11 to 4.70, p=0.024) and marginally associated to the risk of POAG in the patients ≥52 years of age at diagnosis (OR: 2.11; 95% CI: 0.98 to 4.55, p=0,055). However, these results was not confirmed after adjustments for gender, age, self-declared skin color, tobacco smoking and eNOS genotypes by multivariate logistic regression model (OR: 2.08; 95% CI: 0.87 to 5.01, p=0.101 and OR: 2.20; 95% CI: 0.95 to 5.12, p=0.067, respectively). The haplotype CG of T-786C and Glu298Asp showed a borderline association with risk of POAG in the overall analysis (OR: 1.76; 95% CI: 0.98 to 3.14, p=0.055) and among women (OR: 2.02; 95% CI: 0.98 to 4.16, p=0.052). Furthermore, the CG haplotype was significantly associated with the development of POAG for the age at diagnosis group ≥52 years (OR: 3.48; 95% CI: 1.54 to 7.84, p=0.002).We suggested that haplotypes of the polymorphisms T-786C and Glu298Asp of eNOS may interact with gender and age in modulating the risk of POAG.

eNOS, a pressure-dependent regulator of intraocular pressure

PURPOSE

Pathology in the primary drainage pathway for aqueous humor in the eye is responsible for ocular hypertension, the only treatable risk factor in patients with glaucoma. Unfortunately, the mechanisms that regulate pressure-dependent drainage of aqueous humor and thus intraocular pressure (IOP) are unknown. To better understand one possible underlying molecular factor that regulates IOP, nitric oxide (NO), pressure-dependent drainage in transgenic mice overexpressing endothelial NO synthase (eNOS) was studied.

METHODS

IOP was measured by rebound tonometry in mice, and pressure versus flow data were measured by ex vivo perfusion at multiple pressures between 8 and 45 mm Hg, using mock AH ±100 μM L-NAME. A subset of eyes was examined histologically using standard techniques or was assayed for fusion protein expression by Western blot analysis.

RESULTS

IOP was lower (9.6 ± 2.7 vs. 11.4 ± 2.5 mm Hg; mean ± SD; P = 0.04) and pressure-dependent drainage was higher (0.0154 ± 0.006 vs. 0.0066 ± 0.0009 μL/min/mm Hg; P = 0.002) in the transgenic mice than in the wild-type animals; however, pressure-independent drainage was unaffected. The NOS inhibitor L-NAME normalized pressure-dependent drainage in transgenic animals. For IOP >35 mm Hg, the slope of the pressure-flow curve in wild-type mice increased to match that seen in transgenic mice. Shear stress in the pressure-dependent pathway at elevated pressures was calculated to be in a range known to affect eNOS expression and activity in vascular endothelia.

CONCLUSIONS

Endothelial NOS overexpression lowers IOP by increasing pressure-dependent drainage in the mouse eye. Data are consistent with NO's having a mechanoregulatory role in aqueous humor dynamics, with eNOS induction at elevated IOPs leading to increased pressure-dependent outflow.

Dysfunctional regulation of ocular blood flow: A risk factor for glaucoma?

Primary open angle glaucoma (OAG) is a multifactorial optic neuropathy characterized by progressive retinal ganglion cell death and associated visual field loss. OAG is an emerging disease with increasing costs and negative outcomes, yet its fundamental pathophysiology remains largely undetermined. A major treatable risk factor for glaucoma is elevated intraocular pressure (IOP). Despite the medical lowering of IOP, however, some glaucoma patients continue to experience disease progression and subsequent irreversible vision loss. The scientific community continues to accrue evidence suggesting that alterations in ocular blood flow play a prominent role in OAG disease processes. This article develops the thesis that dysfunctional regulation of ocular blood flow may contribute to glaucomatous optic neuropathy. Evidence suggests that impaired vascular autoregulation renders the optic nerve head susceptible to decreases in ocular perfusion pressure, increases in IOP, and/or increased local metabolic demands. Ischemic damage, which likely contributes to further impairment in autoregulation, results in changes to the optic nerve head consistent with glaucoma. Included in this review are discussions of conditions thought to contribute to vascular regulatory dysfunction in OAG, including atherosclerosis, vasospasm, and endothelial dysfunction.

Genetic bases for glaucoma

Glaucoma is the leading cause of visual impairment and blindness throughout the world. Primary open angle glaucoma (POAG; MIM 137760) is the main type of glaucoma in most populations, and more than 20 genetic loci for POAG have been reported. Only three causative genes have been identified in these loci, viz. myocilin (MYOC), optineurin (OPTN), and WD repeat domain 36 (WDR36). However, mutations in these genes account for only a small percentage of the patients with POAG. Some of these glaucoma cases have a Mendelian inheritance pattern, and a considerable fraction of the cases result from a large number of variants in several genes each contributing small effects. Glaucoma is considered to be a common disease such as diabetes mellitus, coronary disease, Crohn disease, and several( )common cancers. The main technological approaches used to identify the genes associated with glaucoma are the candidate gene approach, linkage analysis, case-control association study, and genome-wide association study. Association studies have found about 27 genes related to POAG, but the glaucoma-causing effects of these genes need to be investigated in more detail. The current trend is to use case-control association studies or genome-wide association studies to map the genes associated with glaucoma. Such studies are expected to greatly advance our understanding of the genetic basis of glaucoma, and to provide information on the effectiveness of glaucoma therapy. This review gives an overview on the genetic aspects of glaucoma.

Molecular complexity of primary open angle glaucoma: current concepts

Glaucoma is a group of heterogeneous optic neuropathies with complex genetic basis. Among the three principle subtypes of glaucoma, primary open angle glaucoma (POAG) occurs most frequently. Till date, 25 loci have been found to be linked to POAG. However, only three underlying genes (Myocilin, Optineurin and WDR36) have been identified. In addition, at least 30 other genes have been reported to be associated with POAG. Despite strong genetic influence in POAG pathogenesis, only a small part of the disease can be explained in terms of genetic aberration. Current concepts of glaucoma pathogenesis suggest it to be a neurodegenerative disorder which is triggered by different factors including mechanical stress due to intra-ocular pressure, reduced blood flow to retina, reperfusion injury, oxidative stress, glutamate excitotoxicity, and aberrant immune response. Here we present a mechanistic overview of potential pathways and crosstalk between them operating in POAG pathogenesis.

Endothelial nitric oxide synthase gene variants and primary open-angle glaucoma: interactions with sex and postmenopausal hormone use

PURPOSE

To evaluate the association between the nitric oxide synthase gene (NOS3) variants and primary open-angle glaucoma (POAG).

METHODS

Two functional single-nucleotide polymorphisms (SNPs) (T-786C: rs2070744; Glu298Asp: rs1799983) and three tagging SNPs (rs7830, rs3918188, and rs1800779) were evaluated in a nested case-control study from the Nurses' Health Study (1980-2002) and the Health Professionals' Follow-up Study (1986-2002). Participants were aged >or=40 years and Caucasian. Included were 527 incident cases and 1543 controls, matched by cohort, age, and eye examination at the matched cases' diagnosis dates. Cohort-specific relative risks (RR) were estimated by using multivariable conditional logistic regression and were pooled with meta-analysis.

RESULTS

No NOS3 polymorphism was significantly associated with overall POAG. For high-tension POAG (HTPOAG), rs3918188 was significantly inversely associated among the women (AA versus CC genotype: RR = 0.48; 95% CI, 0.28-0.82) but not among the men (P-heterogeneity by sex = 0.02). The minor alleles of T -786C and rs1800779 showed positive association with high-tension POAG (P-trend < 0.02) in the women only, but P-heterogeneity was not significant. In the women, four of the five NOS3 SNPs showed significant interactions with postmenopausal hormone (PMH) use in relation to HTPOAG: for example, among the women with the TT genotype in T -786C, PMH use was inversely associated (RR = 0.41; 95% CI, 0.22-0.76), but among carriers of the minor allele, use of PMH was not associated.

CONCLUSIONS

Interactions were observed between NOS3 SNPs and female sex and postmenopausal hormone use in the women in relation to HTPOAG. These findings should be confirmed in different racial/ethnic groups.

Endothelial dysfunction in glaucoma

Glaucoma is a group of ocular diseases characterized by optic neuropathy associated with loss of the retinal nerve fibre layer and re-modelling of the optic nerve head, and a subsequent particular pattern of visual field loss. Increased intraocular pressure is the most important risk factor for the disease, but the pathogenesis of glaucoma is not monofactorial. Among other factors, ischaemia and vascular dysregulation have been implicated in the mechanisms underlying glaucoma. The vascular endothelium plays an important role in the regulation of ocular blood flow and pathological alterations of vascular endothelial cells may induce ischaemia and dysregulation. The present review summarizes our current evidence of endothelial dysfunction in glaucoma. This is of interest because endothelial dysfunction is a good prognostic factor for progression in several diseases. Although such data are lacking for glaucoma, endothelial dysfunction may provide an attractive target for therapeutic intervention in open-angle glaucoma and other vascular disorders of the eye.

The genetics of primary open-angle glaucoma: a review

Glaucoma is the major cause of irreversible blindness worldwide. Primary open-angle glaucoma (POAG), as the most prevalent form of glaucoma, is a complex inherited disorder and affects more than 2 million individuals in the United States. It has become increasingly clear that a host of genetic as well as environmental factors are likely to contribute to the phenotype. A number of chromosomal and genetic associations have been reported for POAG. This review examines what is currently known about the underlying genetic structure, what remains to be learned, and how this may affect our medical management of this major blinding disease.

Vasospasm in Glaucoma: Clinical and Laboratory Aspects

During the last decade, the presumed etiology of glaucoma has moved from a pure pressure concept to a combined mechanical and vascular theory. Evidence of a localized vascular insufficiency leading to perfusion deficits of ocular structures, including the optic nerve head, the retina, the choroid, and the retrobulbar vessels, is now clear. This article evaluates the role of vasospasm as the primary cause of such a vascular failure. The role of both ocular and systemic vasospasms and their clinical correlations are discussed. At a cellular level, the function of the modulating role of the vascular endothelium is reviewed. Evidence of abnormalities of the vascular endothelium and its vasoactive peptides as a conduit for vasospasm is mounting. Herein lies exciting prospects for potential pharmacologic targets in future glaucoma management.

Gene mapping for primary open angle glaucoma

Primary open angle glaucoma (POAG) is a leading cause of visual impairment and blindness worldwide. To date, at least 20 genetic loci for POAG have been reported. Only 3 causative genes are identified from these loci: myocilin (MYOC), optineurin (OPTN) and WD repeat domain 36 (WDR36), which together account for less than 10% of POAG. Only a portion of POAG follows Mendelian inheritance, and a considerable fraction results from a large number of variants in several genes, each contributing small effects. Over the past 10 years, there has been vigorous research on mapping the POAG genes. The main technological approaches are functional cloning, family linkage analysis, genome-wide scan, case-control association study, and microarray analysis. Association studies found 16 genes related to POAG, but reports on glaucoma-causing effects of these genes are conflicting. Ten microarray gene expression studies related to POAG have been published. A number of genes potentially related to POAG have been identified, and they provide a good resource to select candidate genes for mutation analysis in association studies. While linkage studies remain a mainstay, the current trend is to use genome-wide association studies to map genes for POAG. This review gives an overview of the efforts in the past decade to identify the POAG genes through linkage studies, genome-wide scans, case-control association studies and microarray studies. In the near future such comprehensive studies are expected to greatly advance our understanding of the genetic basis of POAG and provide information for effective glaucoma therapy.

Endothelin: is it a contributor to glaucoma pathophysiology?

Endothelin is a vasoactive peptide that has been shown to play an important role in vascular homeostasis. Recently, endothelin and its receptors have been found in ocular tissues where it appears to have a regulatory function. Endothelin is found in both the aqueous and vitreous humors and its concentration is elevated in glaucoma patients and in animal models of glaucoma. In the current review, the authors present information about the distribution of endothelin and endothelin receptors in the eye and the ocular actions of endothelins. Specifically, endothelin/aqueous humor dynamics, endothelin/nitric oxide interactions, endothelin and ischemia, and endothelin/optic nerve head effects. Observations concerning the potential role of endothelin in glaucoma pathophysiology is presented and discussed relative to its effects on the optic nerve head and in relation to glaucoma theories.

Relationship between Helicobacter pylori infection and glaucoma

OBJECTIVE

To determine the frequency of Helicobacter pylori (H. pylori) infection in glaucoma patients and in anemic control participants.

DESIGN

Prospective, nonrandomized, comparative study.

PARTICIPANTS

The authors investigated 32 patients with chronic open-angle glaucoma (COAG), 9 patients with pseudoexfoliation glaucoma (PEG), and 30 age-matched anemic control participants.

METHODS

Upper gastrointestinal endoscopy was performed to evaluate macroscopic abnormalities, and gastric mucosal biopsy specimens were obtained for the presence of H. pylori infection tested by rapid urease slide test (CLO test) and by Cresyl fast violet staining, Giemsa staining, or both. The presence of gastritis was classified in accordance with the Sydney system by using hematoxylin and eosin stain. In addition, intestinal metaplasia was evaluated with Alcian blue stain. Saliva samples were also tested by CLO. Serum was analyzed for the presence of H. pylori-specific IgG antibodies by enzyme-linked immunosorbent assay.

MAIN OUTCOME MEASURE

Histologic examination for the presence of H. pylori.

RESULTS

In 87.5% of the COAG patients, 88.9% of the PEG patients, and 46.7% of the anemic control participants, H. pylori infection was histologically confirmed (odds ratio, 8.00; chi-square, 11.81; P = 0.0006 and 9.14; chi-square, 5.01; P = 0.02, respectively). H. pylori was detected by urease test: (1) in the gastric mucosa in 71.9% of the COAG patients, in 77.8% of the PEG patients, and in 46.7% of the anemic control participants (P = 0.03 and P > 0.05, respectively); and (2) in the saliva in 37.5% of the COAG patients, in 55.6% of the PEG patients, and in 30% of the anemic control participants (P > 0.05). Sixty-eight percent of glaucoma patients and 30% of anemic control participants were seropositive for H. pylori (P = 0.002). When compared with anemic control participants, glaucoma patients exhibited less often endoscopic normal appearance of gastric mucosa (P = 0.01), and more often antral gastritis (P = 0.0004) or peptic ulcer disease (P = 0.01). Histologic grade 3 gastritis was observed only in the glaucoma patients (P = 0.03).

CONCLUSIONS

H. pylori infection seems more frequent in glaucoma patients. If confirmed, this may indicate either a common factor that causes susceptibilities to both glaucoma and H. pylori infection or that H. pylori may be a causal factor for developing glaucoma.

Potential role of nitric oxide and endothelin in the pathogenesis of glaucoma

Glaucoma is an optic nerve head neuropathy in which retinal ganglion cells are lost. A clear association exists between glaucoma and different risk factors, such as high intraocular pressure (IOP) or blood-flow dysregulation. Nitric oxide (NO) and endothelin, two recently identified cellular mediators, appear to be involved in the regulation of IOP as well as in the modulation of ocular blood flow. To some extent, NO is also involved in apoptosis, a mechanism of cell death that can lead to retinal ganglion cell loss in glaucoma. This article provides a short and simplified overview of the biochemistry of NO and endothelin and highlights the potential role of these two mediators in certain important aspects related to the pathogenesis of glaucoma.