Nitric oxide and cyclic guanosine monophosphate signaling in the eye

Therapeutic Potential of Hydrogen Sulfide in Reproductive System Disorders

Hydrogen sulfide (H2S), previously regarded as a toxic exhaust and atmospheric pollutant, has emerged as the third gaseous signaling molecule following nitric oxide (NO) and carbon monoxide (CO). Recent research has revealed significant biological effects of H2S in a variety of systems, such as the nervous, cardiovascular, and digestive systems. Additionally, H2S has been found to impact reproductive system function and may have therapeutic implications for reproductive disorders. This paper explores the relationship between H2S and male reproductive disorders, specifically erectile dysfunction, prostate cancer, male infertility, and testicular damage. Additionally, it examines the impact of H2S regulation on the pathophysiology of the female reproductive system, including improvements in preterm birth, endometriosis, pre-eclampsia, fetal growth restriction, unexplained recurrent spontaneous abortion, placental oxidative damage, embryo implantation, recovery of myometrium post-delivery, and ovulation. The study delves into the regulatory functions of H2S within the reproductive systems of both genders, including its impact on the NO/cGMP pathway, the activation of K+ channels, and the relaxation mechanism of the spongy smooth muscle through the ROCK pathway, aiming to broaden the scope of potential therapeutic strategies for treating reproductive system disorders in clinical settings.

Potential Effects of NO-Induced Hypoxia-Inducible Factor-1α on Yak Meat Tenderness during Post-Mortem Aging

The objective of this study was to investigate the mechanism underlying nitric oxide (NO)-induced hypoxia-inducible factor-1α (HIF-1α) and its impact on yak muscle tenderness during post-mortem aging. The Longissimus thoracis et lumborum (LTL) muscle of yak were incubated at 4 °C for 0, 3, 6, 9, 12, 24, and 72 h after treatment with 0.9% saline, NO activator, or a combination of the NO activator and an HIF-1α inhibitor. Results indicated that elevated NO levels could increase HIF-1α transcription to achieve stable expression of HIF-1α protein (P < 0.05). Additionally, elevated NO triggered HIF-1α S-nitrosylation, which further upregulated the activity of key glycolytic enzymes, increased glycogen consumption, accelerated lactic acid accumulation, and decreased pH (P < 0.05). These processes eventually improved the tenderness of yak muscle during post-mortem aging (P < 0.05). The results demonstrated that NO-induced activation of HIF-1α S-nitrosylation enhanced glycolysis during post-mortem aging and provided a possible pathway for improving meat tenderness.

Dental Pulp Inflammation Initiates the Occurrence of Mast Cells Expressing the α1 and β1 Subunits of Soluble Guanylyl Cyclase

The binding of nitric oxide (NO) to heme in the β₁ subunit of soluble guanylyl cyclase (sGC) activates both the heterodimeric α₁β₁ and α₂β₁ isoforms of the enzyme, leading to the increased production of cGMP from GTP. In cultured human mast cells, exogenous NO is able to inhibit mast cell degranulation via NO-cGMP signaling. However, under inflammatory oxidative or nitrosative stress, sGC becomes insensitive to NO. The occurrence of mast cells in healthy and inflamed human tissues and the in vivo expression of the α₁ and β₁ subunits of sGC in human mast cells during inflammation remain largely unresolved and were investigated here. Using peroxidase and double immunohistochemical incubations, no mast cells were found in healthy dental pulp, whereas the inflammation of dental pulp initiated the occurrence of several mast cells expressing the α₁ and β₁ subunits of sGC. Since inflammation-induced oxidative and nitrosative stress oxidizes Fe²⁺ to Fe³⁺ in the β₁ subunit of sGC, leading to the desensitization of sGC to NO, we hypothesize that the NO- and heme-independent pharmacological activation of sGC in mast cells may be considered as a regulatory strategy for mast cell functions in inflamed human dental pulp.

Inflammation of the Human Dental Pulp Induces Phosphorylation of eNOS at Thr495 in Blood Vessels

The activity of endothelial nitric oxide synthase (eNOS) in endothelial cells increased with the phosphorylation of the enzyme at Ser1177 and decreased at Thr495. The regulation of the phosphorylation sites of eNOS at Ser1177 and Thr495 in blood vessels of the healthy and inflamed human dental pulp is unknown. To investigate this, healthy and carious human third molars were immersion-fixed and decalcified. The localization of eNOS, Ser1177, and Thr495 in healthy and inflamed blood vessels was examined in consecutive cryo-sections using quantitative immunohistochemical methods. We found that the staining intensity of Ser1177 in healthy blood vessels decreased in inflamed blood vessels, whereas the weak staining intensity of Thr495 in healthy blood vessels strongly increased in inflamed blood vessels. In blood vessels of the healthy pulp, eNOS is active with phosphorylation of the enzyme at Ser1177. The phosphorylation of eNOS at Thr495 in inflamed blood vessels leads to a decrease in eNOS activity, contributing to eNOS uncoupling and giving evidence for a decrease in NO and an increase in O2− production. Since the formation of the tertiary dentin matrix depends on intact pulp circulation, eNOS uncoupling and phosphorylation of eNOS at Thr495 in the inflamed pulp blood vessels should be considered during caries therapy.

The role of nitric oxide in ocular surface physiology and pathophysiology

Nitric oxide (NO) has a wide array of biological functions including the regulation of vascular tone, neurotransmission, immunomodulation, stimulation of proinflammatory cytokine expression and antimicrobial action. These functions may depend on the type of isoform that is responsible for the synthesis of NO. NO is found in various ocular tissues playing a pivotal role in physiological mechanisms, namely regulating vascular tone in the uvea, retinal blood circulation, aqueous humor dynamics, neurotransmission and phototransduction in retinal layers. Unregulated production of NO in ocular tissues may result in production of toxic superoxide free radicals that participate in ocular diseases such as endotoxin-induced uveitis, ischemic proliferative retinopathy and neurotoxicity of optic nerve head in glaucoma. However, the role of NO on the ocular surface in mediating physiology and pathophysiological processes is not fully understood. Moreover, methods used to measure levels of NO in the biological samples of the ocular surface are not well established due to its rapid oxidation. The purpose of this review is to highlight the role of NO in the physiology and pathophysiology of ocular surface and propose suitable techniques to measure NO levels in ocular surface tissues and tears. This will improve the understanding of NO's role in ocular surface biology and the development of new NO-based therapies to treat various ocular surface diseases. Further, this review summarizes the biochemistry underpinning NO's antimicrobial action.

GAPDH delivers heme to soluble guanylyl cyclase

Soluble guanylyl cyclase (sGC) is a key component of NO-cGMP signaling in mammals. Although heme must bind in the sGC β1 subunit (sGCβ) for sGC to function, how heme is delivered to sGCβ remains unknown. Given that GAPDH displays properties of a heme chaperone for inducible NO synthase, here we investigated whether heme delivery to apo-sGCβ involves GAPDH. We utilized an sGCβ reporter construct, tetra-Cys sGCβ, whose heme insertion can be followed by fluorescence quenching in live cells, assessed how lowering cell GAPDH expression impacts heme delivery, and examined whether expressing WT GAPDH or a GAPDH variant defective in heme binding recovers heme delivery. We also studied interaction between GAPDH and sGCβ in cells and their complex formation and potential heme transfer using purified proteins. We found that heme delivery to apo-sGCβ correlates with cellular GAPDH expression levels and depends on the ability of GAPDH to bind intracellular heme, that apo-sGCβ associates with GAPDH in cells and dissociates when heme binds sGCβ, and that the purified GAPDH-heme complex binds to apo-sGCβ and transfers its heme to sGCβ. On the basis of these results, we propose a model where GAPDH obtains mitochondrial heme and then forms a complex with apo-sGCβ to accomplish heme delivery to sGCβ. Our findings illuminate a critical step in sGC maturation and uncover an additional mechanism that regulates its activity in health and disease.

Downregulation of the α1- and β1-subunit of sGC in Arterial Smooth Muscle Cells of OPSCC Is HPV-Independent

The nitric oxide (NO)-sensitive soluble guanylyl cyclase (sGC) is a heterodimeric enzyme with an α and β subunit. NO binds to heme of the β₁-subunit of sGC, activates the enzyme in the reduced heme iron state in vascular smooth muscle cells (VSMCs), and generates cGMP-inducing vasodilatation and suppression of VSMC proliferation. In the complex tumor milieu with higher levels of reactive oxygen species (ROS), sGC heme iron may become oxidized and insensitive to NO. To change sGC from an NO-insensitive to NO-sensitive state or NO-independent manner, protein expression of sGC in VSMC is required. Whether sGCα₁β₁ exists at the protein level in arterial VSMCs of oropharyngeal squamous cell carcinoma (OPSCC) is unknown. In addition, whether differences in the genetic profile between human papillomavirus (HPV)-positive and HPV-negative OPSCC contributes to the regulation of sGCα₁β₁ is unclear. Therefore, we compared the effects of HPV-positive and HPV-negative OPSCC on the expression of sGCα₁β₁ in arterial VSMCs from tumor-free and tumor-containing regions of human tissue sections using quantitative immunohistochemistry. In comparison to the tumor-free region, we found a decrease in expression of both α₁- and β₁-subunits in the arterial VSMC layer of the tumor-containing areas. The OPSCC-induced significant downregulation of the α₁- and β₁-subunits of sGC in arterial VSMC was HPV-independent. We conclude that the response of sGC to NO in tumor arterial VSMCs may be impaired by oxidation of the heme of the β₁-subunit, and thus, α₁- and β₁-subunits of sGC could be targeted to degradation under oxidative stress in OPSCC in an HPV-independent manner. The degradation of sGCα₁β₁ in VSMCs may result in increased proliferation of VSMCs, promoting tumor arteriogenesis in OPSCC. This can be interrupted by preserving the active heterodimer sGCα₁β₁ in arterial VSMCs.

Contribution of nitrergic nerve in canine gingival reactive hyperemia

Reactive hyperemia reflects a compensatory vasodilation response of the local vasculature in ischemic tissue. The purpose of this study is to clarify the mechanism of regulation of this response in gingival circulation by using pharmacological analysis of reactive hyperemia and histochemical analysis of gingival tissue. Application of pressure to the gingiva was used to create temporary ischemia, and gingival blood flow was measured after pressure release. Reactive hyperemia increased in proportion to the duration of pressure. Systemic hemodynamics remained unaffected by the stimulus; therefore, the gingival reactive hyperemia reflected a local adjustment in circulation. Gingival reactive hyperemia was significantly suppressed by nitric oxide (NO) synthase inhibitors, especially the neural NO synthase-selective antagonist 7-nitroindazole, but not by anticholinergic drugs, β-blockers, or antihistaminergic drugs. Moreover, immunohistochemical staining for neural NO synthase and histochemical staining for NADPH diaphorase activity were both positive in the gingival perivascular region. These histochemical and pharmacological analyses show that reactive hyperemia following pressure release is mediated by NO-induced vasodilation. Furthermore, histochemical analysis strongly suggests that NO originates from nitrergic nerves. Therefore, NO may play an important role in the neural regulation of local circulation in gingival tissue ischemia.

The diabetic vasculature: physiological mechanisms of dysfunction and influence of aerobic exercise training in animal models

Diabetes mellitus (DM) is associated with a number of complications of which chronic vascular complications are undoubtedly the most complex and significant consequence. With a significant impact on health care, 50-80% of people with diabetes die of cardiovascular disease (including coronary artery disease, stroke, peripheral vascular disease and other vascular disease), making it the major cause of morbidity and mortality in diabetic patients. A healthy lifestyle is essential in the management of DM, especially the inclusion of aerobic exercise, which has been shown effective in reducing the deleterious effects in vasculature. Interest in exercise studies has increased significantly with promising results that demonstrate a future for investigation. Considering the importance of this emerging field, the aim of this mini-review is to summarize and integrate animal studies investigating physiological mechanisms of vascular dysfunction and remodeling in type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM) and how these are influenced by chronic aerobic exercise training.

A nanoparticle delivery vehicle for S-nitroso-N-acetyl cysteine: sustained vascular response

Interest in the development of nitric oxide (NO) based therapeutics has grown exponentially due to its well elucidated and established biological functions. In line with this surge, S-nitroso thiol (RSNO) therapeutics are also receiving more attention in recent years both as potential stable sources of NO as well as for their ability to serve as S-nitrosating agents; S-nitrosation of protein thiols is implicated in many physiological processes. We describe two hydrogel based RSNO containing nanoparticle platforms. In one platform the SNO groups are covalently attached to the particles (SNO-np) and the other contains S-nitroso-N-acetyl cysteine encapsulated within the particles (NAC-SNO-np). Both platforms function as vehicles for sustained activity as trans-S-nitrosating agents. NAC-SNO-np exhibited higher efficiency for generating GSNO from GSH and maintained higher levels of GSNO concentration for longer time (24 h) as compared to SNO-np as well as a previously characterized nitric oxide releasing platform, NO-np (nitric oxide releasing nanoparticles). In vivo, intravenous infusion of the NAC-SNO-np and NO-np resulted in sustained decreases in mean arterial pressure, though NAC-SNO-np induced longer vasodilatory effects as compared to the NO-np. Serum chemistries following infusion demonstrated no toxicity in both treatment groups. Together, these data suggest that the NAC-SNO-np represents a novel means to both study the biologic effects of nitrosothiols and effectively capitalize on its therapeutic potential.

Functional importance of blood flow dynamics and partial oxygen pressure in the anterior pituitary

The pulsatile release of hormone is obligatory for the control of a range of important body homeostatic functions. To generate these pulses, endocrine organs have developed finely regulated mechanisms to modulate blood flow both to meet the metabolic demand associated with intense endocrine cell activity and to ensure the temporally precise uptake of secreted hormone into the bloodstream. With a particular focus on the pituitary gland as a model system, we review here the importance of the interplay between blood flow regulation and oxygen tensions in the functioning of endocrine systems, and the known regulatory signals involved in the modification of flow patterns under both normal physiological and pathological conditions.

Carbon monoxide and the eye: Implications for glaucoma therapy

In the late 1990s, the scientific community witnessed a very peculiar phenomenon: the transformation of nitric oxide (NO) from a noxious gas into a key chemical messenger. The importance of NO in biology and medicine was highlighted in 1998 when the Nobel Prize was awarded in Physiology and Medicine to Robert Furchgott, Louis Ignarro and Ferid Murad for their pioneering work on the role of NO in the nervous, cardiovascular and immune systems. In this same time period, carbon monoxide (CO), another gas usually associated with environmental pollution, air poisoning and suicidal behavior, was also undergoing a similar change in image, although not as closely followed. It had been known for several decades that the human body generated CO upon the decomposition of hemoglobin, which was determined by the discovery that heme oxygenase (HO) is the enzymatic source of CO. However, CO's role as an endogenous neurotransmitter was established only in the early 1990s. Since then, many biological activities of CO have been demonstrated in studies using different tools, such as the pharmacological induction of HO by hemin, the direct administration of CO or the use of pro-drugs that generate CO. This review focuses on CO as a fine modulator of intraocular pressure and on its potential implications in glaucoma.

Morphine-induced ocular hypotension is modulated by nitric oxide and carbon monoxide: role of mu3 receptors

PURPOSE

Recent findings generated from our laboratory have demonstrated the involvement of nitric oxide (NO) in morphine-induced reduction of intraocular pressure (IOP). The present study was designed to investigate the possible involvement of carbon monoxide (CO) in morphine-induced reduction of IOP and the role of mu(3) opioid receptors.

METHODS

New Zealand rabbits were used in this study. They were pretreated with the nitric oxide synthase inhibitor Nomega-nitro-L-arginine methyl ester (L-NAME, 1%, 30 microL), or an inhibitor of heme oxygenase (HO), zinc protoporphyrin-IX (ZnPP-IX; 0.1 mg/kg; i.v.). The same animals were then treated with morphine (100 microg/30 microL) with or without NO or CO donors administration, sodium nitroprusside (SNP) and tricarbonylchloro(glycinato)ruthenium(II) (CORM-3), respectively. A separate set of animals were pretreated with the nonselective opioid receptor antagonist, naloxone (100 microg/30 microL), or the micro(3) opioid receptor inhibitor, L-glutathione (GSH, 1%, 30 microL), in the presence of SNP or CORM-3 followed by morphine administration. IOP measurements were taken at different times after monolateral instillation of morphine.

RESULTS

Morphine induced a significant decrease in IOP and pretreatment with ZnPP-IX or L-NAME significantly prevented this effect whereas administration of NO or CO donors amplified morphine-induced decrease in IOP. This effect was partially abrogated both by pretreatment with ZnPP-IX or L-NAME, and by pretreatment with naloxone and GSH suggesting that the decrease in IOP relies on exogenous NO and CO liberated from SNP and CORM-3, respectively.

CONCLUSIONS

We conclude that the endogenous NO/CO system and micro(3) receptors contribute to morphine-induced ocular hypotension and that the reduction of IOP elicited by morphine can be augmented by exogenous NO and CO.

The role of blood vessels, endothelial cells, and vascular pericytes in insulin secretion and peripheral insulin action

The pathogenesis of type 2 diabetes is intimately intertwined with the vasculature. Insulin must efficiently enter the bloodstream from pancreatic beta-cells, circulate throughout the body, and efficiently exit the bloodstream to reach target tissues and mediate its effects. Defects in the vasculature of pancreatic islets can lead to diabetic phenotypes. Similarly, insulin resistance is accompanied by defects in the vasculature of skeletal muscle, which ultimately reduce the ability of insulin and nutrients to reach myocytes. An underappreciated participant in these processes is the vascular pericyte. Pericytes, the smooth muscle-like cells lining the outsides of blood vessels throughout the body, have not been directly implicated in insulin secretion or peripheral insulin delivery. Here, we review the role of the vasculature in insulin secretion, islet function, and peripheral insulin delivery, and highlight a potential role for the vascular pericyte in these processes.

Role of soluble guanylyl cyclase-cyclic GMP signaling in tumor cell proliferation

Our previous studies demonstrate a differential expression of nitric oxide (NO) signaling components in ES cells and our recent study demonstrated an enhanced differentiation of ES cells into myocardial cells with NO donors and soluble guanylyl cyclase (sGC) activators. Since NO-cGMP pathway exhibits a diverse role in cancer, we were interested in evaluating the role of the NO-receptor sGC and other components of the pathway in regulation of the tumor cell proliferation. Our results demonstrate a differential expression of the sGC subunits, NOS-1 and PKG mRNA and protein levels in various human cancer models. In contrast to sGC alpha(1), robust levels of sGC beta(1) were observed in OVCAR-3 (ovarian) and MDA-MB-468 (breast) cancer cells which correlated well with the sGC activity and a marked increase in cGMP levels upon exposure to the combination of a NO donor and a sGC activator. NOC-18 (DETA NONOate; NO donor), BAY41-2272 (3-(4-amino-5-cyclopropylpyrimidin-2-yl)-1-(2-fluorobenzyl)-1H-pyrazolo[3,4-b]pyridine); sGC activator), NOC-18+BAY41-2272, IBMX (3-isobutyl-1-methylxanthine; phosphodiesterase inhibitor) and 8-bromo-cGMP (cGMP analog) caused growth inhibition and apoptosis in various cancer cell lines. To elucidate the molecular mechanisms involved in growth inhibition, we evaluated the effect of activators/inhibitors on ERK phosphorylation. Our studies indicate that BAY41-2272 or the combination NOC-18+BAY41-2272 caused inhibition of the basal ERK1/2 phosphorylation in OVCAR-3 (high sGC activity), SK-OV-3 and SK-Br-3 (low sGC activity) cell lines and in some cases the inhibition was rescued by the sGC inhibitor ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one). These studies suggest that the effects of activators/inhibitors of NO-sGC-cGMP in tumor cell proliferation is mediated by both cGMP-dependent and independent mechanisms.

Exogenous nitric oxide-induced release of calcium from intracellular IP3 receptor-sensitive stores via S-nitrosylation in respiratory burst-dependent neutrophils

PMA-induced respiratory burst neutrophils were exposed to exogenous nitric oxide (NO) donor sodium nitroprusside (SNP) to study the effect of NO on calcium signaling. A sharp rise of cytosolic calcium concentration ([Ca(2+)](c)) was triggered by 1mM SNP with and without external calcium. We found that GF 109203X, a specific inhibitor of protein kinase C, DPI, a putative inhibitor of the respiratory burst-generating NADPH oxidase, and 2-DG, a non-metabolizable analog of glucose, completely inhibited the SNP-induced rise of [Ca(2+)](c) in PMA-activated respiratory burst neutrophils. Meanwhile, 2-APB and TMB-8, two potent IP(3) receptor inhibitors, prevented calcium increase respectively. Furthermore, N-ethylmaleimide (NEM), a specific cysteine alkylating agent, evidently abolished the [Ca(2+)](c) elevation. In contrast, the sGC inhibitor NS2028 had little effect on the rise of [Ca(2+)](c). Taken together, these results indicated that exogenous NO induced the release of calcium from intracellular IP(3) receptor-sensitive stores of neutrophils via S-nitrosylation in a respiratory burst-dependent manner.

Oxidative injury to blood vessels and glia of the pre-laminar optic nerve head in human glaucoma

Glaucoma is a leading cause of irreversible world blindness. Oxidative damage and vascular injury have been implicated in the pathogenesis of this disease. The purpose of this study was to determine in human primary open angle glaucoma whether oxidative injury occurs in pre-laminar optic nerve blood vessels and glial cells. Following IRB approval, sections from post-mortem primary open angle glaucoma eyes (n=5) with mean age of 77 +/- 9 yrs (+/-SD) were compared to normal control eyes (n=4) with mean age 70 +/- 9 yrs (Eye Bank of Canada). Immunostaining with nitrotyrosine, a footprint for peroxynitrite-mediated injury, was performed and sections were double-labeled with markers for vascular endothelial cells, perivascular smooth muscle cells, and astrocytes with CD34, smooth muscle actin (SMA), and glial fibrillary acidic protein (GFAP), respectively. Immunostaining was captured in a masked fashion using confocal microscopy, and defined regions of interest for blood vessels and glial tissue. Intensity measurements of supra-threshold area in pixels as percent of the total number of pixels were calculated using ImageJ (NIH) and compared using two-tailed Mann-Whitney nonparametric tests between glaucoma and control groups. Colocalization coefficients with cell-specific markers were determined and compared with random coefficients of correlation. Increased nitrotyrosine immunoreactivity was observed in pre-laminar optic nerve head blood vessels of primary open angle glaucoma eyes compared to controls and this difference was statistically significant (1.35 +/- 1.11% [+/-SD] vs. 0.01 +/- 0.01%, P=0.016). NT-immunoreactivity was also increased in the glial tissue surrounding the pre-laminar optic nerve head in the glaucoma group and compared to controls, and this difference was statistically significant (18.37 +/-12.80% vs. 0.08 +/- 0.04%, P=0.016). Colocalization studies demonstrated nitrotyrosine staining in vascular endothelial and smooth muscle cells, in addition to astrocytes. Correlation coefficients for CD34, SMA, and GFAP were 0.37, 0.52, and 0.64, respectively. Oxidative injury is present in blood vessels and astrocytes in the pre-laminar optic nerve head in human primary open angle glaucoma. Peroxynitrite-mediated oxidative injury, whether primary or secondary, may contribute to the pathobiology of glaucoma disease.