Hydrogen sulfide protects the retina from light-induced degeneration by the modulation of Ca2+ influx

H2S and polysulfide metabolism: Conventional and unconventional pathways

It is now well established that hydrogen sulfide (H₂S) is an effector of a wide variety of physiological processes. It is also clear that many of the effects of H₂S are mediated through reactions with cysteine sulfur on regulatory proteins and most of these are not mediated directly by H₂S but require prior oxidation of H₂S and the formation of per- and polysulfides (H₂S_n, n = 2-8). Attendant with understanding the regulatory functions of H₂S and H₂S_n is an appreciation of the mechanisms that control, i.e., both increase and decrease, their production and catabolism. Although a number of standard "conventional" pathways have been described and well characterized, novel "unconventional" pathways are continuously being identified. This review summarizes our current knowledge of both the conventional and unconventional.

Regulation of Aqueous Humor Dynamics by Hydrogen Sulfide: Potential Role in Glaucoma Pharmacotherapy

Hydrogen sulfide (H₂S) is a gaseous transmitter with well-known biological actions in a wide variety of tissues and organs. The potential involvement of this gas in physiological and pathological processes in the eye has led to several in vitro, ex vivo, and in vivo studies to understand its pharmacological role in some mammalian species. Evidence from literature demonstrates that 4 enzymes responsible for the biosynthesis of this gas (cystathionine β-synthase, CBS; cystathionine γ-lyase, CSE; 3-mercaptopyruvate sulfurtransferase, 3MST; and d-amino acid oxidase) are present in the cornea, iris, ciliary body, lens, and retina. Studies of the pharmacological actions of H₂S (using several compounds as fast- and slow-releasing gas donors) on anterior uveal tissues reveal an effect on sympathetic neurotransmission and the ability of the gas to relax precontracted iris and ocular vascular smooth muscles, responses that were blocked by inhibitors of CSE, CBS, and K_ATP channels. In the retina, there is evidence that H₂S can inhibit excitatory amino acid neurotransmission and can also protect this tissue from a wide variety of insults. Furthermore, exogenous application of H₂S-releasing compounds was reported to increase aqueous humor outflow facility in an ex vivo model of the porcine ocular anterior segment and lowered intraocular pressure (IOP) in both normotensive and glaucomatous rabbits. Taken together, the finding that H₂S-releasing compounds can lower IOP and can serve a neuroprotective role in the retina suggests that H₂S prodrugs could be used as tools or therapeutic agents in diseases such as glaucoma.

Diurnal Fluctuations in Plasma Hydrogen Sulfide of the Mice

Circadian rhythms are essential in a myriad of physiological processes to maintain homeostasis, especially the redox homeostasis. However, little is known about whether plasma H₂S exhibits the physiological diurnal variation. The present study was performed to investigate the diurnal fluctuations of plasma H₂S and explore the potential mechanisms. We found that the plasma H₂S of the C57BL/6J mice was significantly higher at 19 o’clock than those at 7 o’clock which was not affected by the blood-collecting sequence and the concentrations of plasma cysteine (a precursor of H₂S). No significant differences in mRNA or protein expression of the CSE, CBS, or MPST were observed between 7: 00 and 19: 00. There were also no significant differences in the CSE and CBS activities, while the activities of MPST in tissues were significantly higher at 19 o’clock. After treatment with AOAA (a CBS inhibitor) or PPG (a CSE inhibitor) for 14 days, plasma H₂S concentrations at 19 o’clock were still significantly higher than those at 7 o’clock, although they were both significantly decreased as compared with controls. Identical findings were also observed in CSE KO mice. We also found the plasma H₂O₂ concentrations were significantly higher at 19 o’clock than those at 7 o’clock. However, H₂O₂ concentrations were significantly decreased at 19 o’clock than those at 7 o’clock when mice were exposed to continuous light for 24 h. Meanwhile, the diurnal fluctuations of plasma H₂S levels and MPST activities in tissues were disappeared. After treatment with DTT for 14 days, there was no significant difference in plasma H₂O₂ concentrations between 7 o’clock and 19 o’clock. Meanwhile, the diurnal fluctuations of plasma H₂S levels and MPST activities in tissues were disappeared. Identical findings were also observed in SOD^2+/- mice. When heart tissues were incubated with increasing concentrations of H₂O₂in vitro, H₂O₂ could dose-dependently increase the activity of MPST within a certain concentration range. In conclusion, our studies revealed that plasma H₂S concentration and tissue MPST activity exhibited diurnal fluctuations. Modulated by plasma H₂O₂ concentration, changes of MPST activity probably led to the diurnal fluctuations of plasma H₂S.

Beyond a Gasotransmitter: Hydrogen Sulfide and Polysulfide in Cardiovascular Health and Immune Response

SIGNIFICANCE

Hydrogen sulfide (H₂S) metabolism leads to the formation of oxidized sulfide species, including polysulfide, persulfide, and others. Evidence is emerging that many biological effects of H₂S may indeed be due to polysulfide and persulfide activation of signaling pathways and reactivity with discrete small molecules. Recent Advances: Exogenous oxidized sulfide species, including polysulfides, are more reactive than H₂S with a wide range of molecules. Importantly, endogenous polysulfide and persulfide formation has been reported to occur via transsulfuration enzymes, cystathionine γ-lyase (CSE) and cystathionine β-synthase (CBS).

CRITICAL ISSUES

In light of the recent understanding of oxidized sulfide metabolite formation and reactivity, comparatively few studies have been reported comparing cellular biological and in vivo effects of H₂S donors versus polysulfide and persulfide donors. Likewise, it is equally unclear when, how, and to what extent persulfide and polysulfide formation occurs in vivo under pathophysiological conditions.

FUTURE DIRECTIONS

Additional studies regarding persulfide and polysulfide formation and molecular reactions are needed in nearly all aspects of biology to better understand how sulfide metabolites contribute to key chemical biology reactions involved in cardiovascular health and immune responses. Antioxid. Redox Signal. 27, 634-653.

Role of Hydrogen Sulfide in Retinal Diseases

As the third gasotransmitter, hydrogen sulfide (H₂S) plays a crucial role in the physiology and pathophysiology of many systems in the body, such as the nervous, cardiovascular, respiratory, and gastrointestinal systems. The mechanisms for its effects, including inhibiting ischemic injury, reducing oxidative stress damage, regulating apoptosis, and reducing the inflammation reaction in different systems, have not been fully understood. Recently, H₂S and its endogenous synthesis pathway were found in the mammalian retina. This review describes the production and the metabolism of H₂S and the evidence of a role of H₂S in the retina physiology and in the different retinal diseases, including retinal degenerative diseases and vascular diseases. In the retina, H₂S is generated in the presence of cystathionine-β-synthase, cystathionine-γ-lyase, and 3-mercaptopyruvate sulfurtransferase from L-cysteine. The role of endogenous H₂S and its physiologic effect in the retina are still elusive. However, strong evidence shows that retina-derived H₂S might play protective or deleterious role in the pathogenesis of retinal diseases. For example, by regulating Ca²⁺ influx, H₂S can protect retinal neurons against light-induced degeneration. H₂S preconditioning can mediate the anti-apoptotic effect of retinal ganglion cells in retinal ischemia/reperfusion injury. Treatment with H₂S in rats relieves diabetic retinopathy by suppressing oxidative stress and reducing inflammation. Further studies would greatly improve our understanding of the pathophysiologic mechanisms responsible for retinal diseases and the potential for the H₂S-related therapy of the retinal diseases as well.

Hydrogen Sulfide Contributes to Retinal Neovascularization in Ischemia-Induced Retinopathy

PURPOSE

Hydrogen sulfide (H2S) is an endogenous gaseous signaling molecule with significant pathophysiological importance, but its role in retinal neovascular diseases is unknown. Hydrogen sulfide is generated from L-cysteine by cystathionine-β-synthase (CBS), cystathionine-γ-lyase (CSE), and/or 3-mercaptopyruvate sulfurtransferase (3-MST). The aim of this study was to investigate the role of H2S in retinal neovascularization (NV) in ischemia-induced retinopathy.

METHODS

Studies were performed in a murine model of oxygen-induced retinopathy (OIR). Hydrogen sulfide was detected with a fluorescent assay. Western blots and immunohistochemistry were used to assess the changes of H2S-producing enzymes. Gene deletion and pharmacologic inhibition were used to investigate the role of H2S in retinal NV.

RESULTS

Hydrogen sulfide production was markedly increased in retinas from OIR mice compared with those from room air (RA) controls. Cystathionine-β-synthase and CSE were significantly increased in OIR retinas, whereas 3-MST was not changed. Cystathionine-β-synthase was expressed throughout the neuronal retina and upregulated in neurons and glia during OIR. Cystathionine-γ-lyase was also localized to multiple retinal layers. Its immunoreactivity was prominently increased in neovascular tufts in OIR. Pharmacologic inhibition of CBS/CSE or genetic deletion of CSE significantly reduced retinal NV in OIR.

CONCLUSIONS

Our data indicate that the H2S-generating enzymes/H2S contributes to retinal NV in ischemia-induced retinopathy and suggest that blocking this pathway may provide novel therapeutic approaches for the treatment of proliferative retinopathy.

Hydrogen sulfide inhibits mitochondrial fission in neuroblastoma N2a cells through the Drp1/ERK1/2 signaling pathway

Hydrogen sulfide (H2S) has been demonstrated to have various effects on mitochondrial function. The aim of the present study was to investigate the effects of H2S on mitochondrial fission and the potential underlying mechanisms of these effects. Transmission electron microscopy analysis demonstrated that sodium hydrosulfide (NaHS, a donor of H2S) inhibited mitochondrial fission in a dose‑ and time‑dependent manner. Treating neuro‑2a (N2a) mouse neuroblastoma cells with 400 µM NaHS for 16 h significantly increased the % of elongated mitochondria and reduced the number of mitochondria per cell compared with untreated cells. In addition, the viability and ATP generation of N2a cells that were treated with various concentrations of NaHS was examined. The results demonstrated that treatment with 400 and 600 µM NaHS increased cell viability and ATP generation compared with untreated cells. To further understand the effects of H2S on mitochondrial morphology, the protein and mRNA expression levels of dynamin 1 like (Dnm1l, also known as Drp1) were examined, and the results demonstrated that NaHS dose‑dependently reduced Drp1 mRNA and protein levels, consistent with the mitochondrial morphology changes. To determine whether H2S affects mitochondrial morphology through Drp1 expression, Drp1 was overexpressed in N2a cells using a lentivirus encoding the Drp1 cDNA. It was observed that Drp1 overexpression reversed the effects of NaHS. Furthermore, NaHS promoted the phosphorylation of extracellular signal‑regulated kinase (ERK) 1/2, and the effects of NaHS on Drp1 expression were abolished by an ERK1/2 inhibitor (PD98059). The results of the present study indicate that the H2S‑induced decrease in Drp1 mRNA and protein levels and mitochondrial fission may involve the ERK1/2 signaling pathway. The present study suggests that H2S may be used in the future as a potential therapeutic for diseases that may be mediated by abnormal mitochondria fragmentation, such as Alzheimer's disease.

RS9, a novel Nrf2 activator, attenuates light-induced death of cells of photoreceptor cells and Müller glia cells

The retina is highly sensitive to oxidative stress because of its high consumption of oxygen associated with the phototransductional processes. Recent findings have suggested that oxidative stress is involved in the pathology of age-related macular degeneration, a progressive degeneration of the central retina. A well-known environmental risk factor is light exposure, as excessive and continuous light exposure can damage photoreceptors. Nuclear factor-erythroid 2-related factor 2 (Nrf2) is a transcriptional factor that controls antioxidative responses and phase 2 enzymes. Thus, we hypothesized that RS9, a specific activator of Nrf2, decreases light-induced retinal cell death in vivo and in vitro. Nrf2 was detected in the nucleus of the 661W cells exposed to RS9 and also after light exposure, and the Nrf2-antioxidant response element binding was increased in 661W cells after exposure to RS9. Consequentially, the expression of the phase 2 enzyme's mRNAs of Ho-1, Nqo-1, and Gclm genes was increased in 661W cells after exposure to RS9. Furthermore, RS9 decreased the light-induced death of 661W cells (2500 lux, 24 h), and also reduced the functional damages and the histological degeneration of the nuclei in the outer nuclear layer or the retina in the in vivo studies (8000 lux, 3 h). Heme oxygenase-1 was increased after light exposure, and Nrf2 was translocated into the nucleus after light exposure in vivo. Silencing of Ho-1 reduced the protective effects of RS9 against light-induced death of 661W cells. These findings indicate that RS9 has therapeutic potential for retinal diseases that are aggravated by light exposure.

Effects of Hydrogen Sulfide-Releasing Compounds on Aqueous Humor Outflow Facility in Porcine Ocular Anterior Segments, Ex Vivo

PURPOSE

To investigate the pharmacological actions of hydrogen sulfide (H₂S)-releasing compounds l-cysteine and sodium hydrosulfide (NaHS) on aqueous humor (AH) outflow facility in porcine ocular anterior segment.

METHODS

Porcine ocular anterior segments were perfused with Dulbecco's modified Eagle's medium at a constant pressure of 7.35 mmHg. After stable outflow baseline, explants were exposed to NaHS or l-cysteine. The increase in outflow generated by the H₂S-releasing compounds was measured in the absence and presence of inhibitor of H₂S biosynthesis (aminooxyacetic acid; AOAA), blocker of K_ATP channels (glibenclamide), and inhibitor of adenylyl cyclase (SQ 22536). Hematoxylin and eosin (H&E) staining was used to assess trabecular meshwork (TM) morphology.

RESULTS

l-cysteine elicited a concentration-dependent increase in AH outflow facility, reaching maximal effect at 100 nM (150.6% ± 17.2% of basal level). This increase in outflow induced by l-cysteine was significantly (P < 0.001) antagonized by AOAA (30 μM) and glibenclamide (100 μM). AOAA and glibenclamide had no significant action on baseline outflow, whereas SQ 22536 (100 μM) increased outflow for only an hour. In addition, NaHS produced a concentration-dependent increase in AH outflow, with a maximal effect at 10 μM (151.4% ± 22.9% of basal level). Likewise, the increase in outflow caused by NaHS was significantly (P < 0.04) blocked by glibenclamide and SQ 22536. H&E staining revealed that l-cysteine or NaHS did not alter TM conformation.

CONCLUSION

H₂S-releasing compounds can increase outflow facility in porcine ocular anterior segment. The stimulatory action of these compounds on outflow is mediated, in part by endogenously produced H₂S, K_ATP channels, and adenylyl cyclase.

Role of Hydrogen Sulfide on Autophagy in Liver Injuries Induced by Selenium Deficiency in Chickens

Selenium (Se) is an indispensable trace mineral that was associated with liver injuries in animal models. Hydrogen sulfide (H₂S) is involved in many liver diseases, and autophagy can maintain liver homeostasis with a stress stimulation. However, little is known about the correlation between H₂S and autophagy in the liver injury chicken models induced by Se deficiency. In this study, we aimed to investigate the correlation between H₂S and autophagy in the liver injury chicken models. We randomly divided 120 1-day-old chickens into two equal groups. The control group was fed with complete food with a Se content of 0.15 mg/kg, and the Se-deficiency group (lab group) was fed with a Se-deficient diet with a Se content of 0.033 mg/kg. When the time comes to 15, 25, and 35 days, the chickens were sacrificed (20 each). The liver tissues were gathered and examined for pathological observations, the mRNA and protein levels of H₂S synthases (CSE, CBS, and 3-MST) and the mRNA and protein levels of autophagy-related genes. The results showed that the expression of CSE, CBS, and 3-MST and H₂S production were higher in the lab group than in the control group. Swellings, fractures, and vacuolizations were visible in the mitochondria cristae in the livers of the lab group and autophagosomes were found as well. In addition, the expression of autophagy-related genes (ATG5, LC3-I, LC3-II, Beclin1, and Dynein) was higher in the lab group than in the control group (p < 0.05) while TOR decreased significantly in the lab group (p < 0.05). The results showed that H₂S and autophagy were involved in the liver injury chicken models, and H₂S was correlated with autophagy.

Hydrogen sulfide from a NaHS source attenuates dextran sulfate sodium (DSS)-induced inflammation via inhibiting nuclear factor-κB

This study investigated the alleviating effects of hydrogen sulfide (H2S), derived from sodium hydrosulfide (NaHS), on inflammation induced by dextran sulfate sodium (DSS) in both in vivo and in vitro models. We found that NaHS injection markedly decreased rectal bleeding, diarrhea, and histological injury in DSS-challenged mice. NaHS (20 μmol/L) reversed DSS-induced inhibition in cell viability in Caco-2 cells and alleviated pro-inflammation cytokine expression in vivo and in vitro, indicating an anti-inflammatory function for H2S. It was also found that H2S may regulate cytokine expression by inhibiting the nuclear factor-κB (NF-κB) signaling pathway. In conclusion, our results demonstrated that H2S alleviated DSS-induced inflammation in vivo and in vitro and that the signal mechanism might be associated with the NF-κB signaling pathway.

Relevant variations and neuroprotecive effect of hydrogen sulfide in a rat glaucoma model

Glaucoma is an irreversible and blinding neurodegenerative disease of the eye, and is characterized by progressive loss of retinal ganglion cells (RGCs). Since endogenous hydrogen sulfide (H₂S) was reported to be involved in neurodegeneration in the central nervous system, the authors aimed to develop a chronic ocular hypertension (COH) rat model simulating glaucoma and therein test the H₂S level together with the retinal protein expressions of related synthases, and further investigated the effect of exogenous H₂S supplement on RGC survival. COH rat model was induced by cross-linking hydrogel injection into anterior chamber, and the performance of the model was assessed by intraocular pressure (IOP) measurement, RGC counting and retinal morphological analysis. Endogenous H₂S level was detected along with the retinal protein expressions of H₂S-related synthases cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE) and 3-mercaptopyruvate sulfurtransferase (3-MST) in the COH rats. Retinal H₂S level and RGC survival were evaluated again after NaHS (a H₂S donor) treatment in the COH rats. The results showed that the COH model succeeded in simulating glaucoma features, and retinal H₂S level decreased significantly when the retinal protein expressions of CBS, CSE and 3-MST were downregulated generally in the COH rats. Furthermore, the decrease of retinal H₂S level and loss of RGCs were both improved by NaHS treatment in experimental glaucoma, without obvious variation of IOP. Our study revealed that the intracameral injection of cross-linking hydrogel worked efficiently in modeling glaucoma, and H₂S had protective effect on RGCs and might be involved in the pathological mechanism of glaucomatous neuropathy.

Essential Roles of Natural Products and Gaseous Mediators on Neuronal Cell Death or Survival

Although precise cellular and molecular mechanisms underlying neurodegeneration still remain enigmatic, key factors associated with degenerative disorders, such as glutamate toxicity and oxidative stress, have been recently identified. Accordingly, there has been growing interest in examining the effects of exogenous and endogenous molecules on neuroprotection and neurodegeneration. In this paper, we review recent studies on neuroprotective and/or neurodegenerative effects of natural products, such as caffeic acid and chlorogenic acid, and gaseous mediators, including hydrogen sulfide and nitric oxide. Furthermore, possible molecular mechanisms of these molecules in relation to glutamate signals are discussed. Insight into the pathophysiological role of these molecules will make progress in our understanding of molecular mechanisms underlying neurodegenerative diseases, and is expected to lead to potential therapeutic approaches.

Hydrogen Sulfide as a "Double-Faced" Compound: One with Pro- and Antioxidant Effect

Hydrogen sulfide (H₂S), like other gasotransmitters such as nitric oxide (NO^•) and carbon monoxide (CO), acts as a signaling molecule in various biological systems. It may also regulate the oxidative stress observed in several diseases sometimes associated with changes of H₂S concentration. This chapter describes the "double face" of hydrogen sulfide as both an antioxidant and a prooxidant in biological systems. One proposed mechanism by which H₂S exerts its antioxidative effects is its ability to modulate the concentration of glutathione, which is a very important physiological antioxidant. This chapter discusses the interactions of H₂S with various reactive oxygen species and reactive nitrogen species, including the superoxide radical anion [Formula: see text] , hydrogen peroxide (H₂O₂), and peroxynitrite anion (ONOO^-), which is produced in a rapid reaction between [Formula: see text] and NO^•.

Hydrogen polysulfide (H2S n ) signaling along with hydrogen sulfide (H2S) and nitric oxide (NO)

Hydrogen sulfide (H₂S) is a physiological mediator with various roles, including neuro-modulation, vascular tone regulation, and cytoprotection against ischemia-reperfusion injury, angiogenesis, and oxygen sensing. Hydrogen polysulfide (H₂S _n ), which possesses a higher number of sulfur atoms than H₂S, recently emerged as a potential signaling molecule that regulates the activity of ion channels, a tumor suppressor, transcription factors, and protein kinases. Some of the previously reported effects of H₂S are now attributed to the more potent H₂S _n . H₂S _n is produced by 3-mercaptopyruvate sulfurtransferase (3MST) from 3-mercaptopyruvate (3MP) and is generated by the chemical interaction of H₂S with nitric oxide (NO). H₂S _n sulfhydrates (sulfurates) cysteine residues of target proteins and modifies their activity, whereas H₂S sulfurates oxidized cysteine residues as well as reduces cysteine disulfide bonds. This review focuses on the recent progress made in studies concerning the production and physiological roles of H₂S _n and H₂S.

Hydrogen sulfide improves colonic barrier integrity in DSS-induced inflammation in Caco-2 cells and mice

Intestinal barrier involves in the pathogeny of inflammatory bowel disease (IBD) and hydrogen sulfide (H2S) has been reported to improve intestinal barrier integrity. Thus, this study investigated the effects of GYY4137, a slow-release H2S donor, on DSS-induced inflammation and intestinal dysfunction. In vitro model, cellular permeability was significantly increased and expression of tight junctions (ZO-1, Cauldin4, and Occludin) was downregulated in Caco-2 cells. GYY4137 treatment markedly attenuated DSS-induced inflammation and barrier dysfunction. Cystathionine β-synthase (CBS)-siRNA transfection further demonstrated that endogenous H2S system involves in DSS-induced inflammation and mediates barrier function. In vivo model, DSS exposure caused colonic inflammation and injury in mice and GYY4137 injection alleviated inflammatory response and improved intestinal barrier via reducing intestinal permeability and upregulating of tight junctions. In conclusion, endogenous H2S system involves in DSS-induced inflammation and H2S addition alleviated inflammation and intestinal dysfunction in vitro and in vivo.

A novel pathway for the production of H2 S by DAO in rat jejunum

BACKGROUND

Hydrogen sulfide (H2 S) is endogenously generated from L-cysteine (L-Cys) by the enzymes cystathionine-β-synthase (CBS) and cystathionine-γ-Lyase (CSE). Hydrogen sulfide is also produced from D-cysteine (D-Cys) by D-Amino acid oxidase (DAO).

METHODS

The H2 S production was measured by the methylene blue assay. The expression of DAO was investigated by Western blotting and immunohistochemistry. The short-circuit current (Isc) was recorded using the Ussing chamber technique.

KEY RESULTS

The epithelium in rat jejunum possesses DAO, and generates H2 S. D-cysteine, originally used as a negative control for L-Cys, significantly increases the H2 S release, which is inhibited by I2CA, an inhibitor of DAO. In vitro study by Ussing chamber technique reveals that D-Cys decreases the Isc across the epithelium of the rat jejunum and enhances the Na(+) -coupled L-alanine transport.

CONCLUSIONS & INFERENCES

A novel pathway for the production of H2 S by DAO exists in rat jejunum.

Effect of Hydrogen Sulfide Donors on Intraocular Pressure in Rabbits

PURPOSE

In this study, we investigated the effect of a slow-releasing hydrogen sulfide (H2S) donor, GYY 4137, on intraocular pressure (IOP) in normotensive rabbits. Furthermore, we compared the IOP-lowering action of GYY 4137 with those elicited by other H2S-producing compounds, l-cysteine and ACS67 (a hybrid compound of latanoprost with an H2S-releasing moiety).

METHODS

IOP was measured in New Zealand normotensive male albino rabbits using a pneumatonometer (model 30 classic; Reichert Ophthalmic Instruments, Depew, NY). At 0 h, 50 μL of test compounds were applied topically to 1 eye of each animal, while the contralateral eye received the same quantity of vehicle (saline). IOP was measured hourly until baseline IOP readings were attained and animal eyes monitored for potential side effects (i.e., tearing, hyperemia).

RESULTS

GYY 4137 (0.1%-2%) produced a dose-dependent decrease in IOP reaching a maximum of 27.8% ± 3.14% (n = 5) after 6 h. Interestingly, a significant contralateral effect was observed in vehicle-treated controls eyes at all doses tested. l-cysteine (5%) and ACS67 (0.005%) also elicited a significant (P < 0.01) decrease in IOP that achieved a maximum of 28.84% ± 1.53% (n = 5) and 23.27% ± 0.51% (n = 5), respectively, after 3 h. All 3 H2S-producing compounds also caused a significant contralateral effect in vehicle-treated control eyes.

CONCLUSION

We conclude that GYY 4137 and other H2S-producing donors can reduce IOP in normotensive rabbits. However, the profile of IOP-lowering action of GYY 4137 was different from the other H2S donors affirming its ability to act as a slow-releasing gas donor.

Hydrogen sulfide: physiological properties and therapeutic potential in ischaemia

Hydrogen sulfide (H2 S) has become a molecule of high interest in recent years, and it is now recognized as the third gasotransmitter in addition to nitric oxide and carbon monoxide. In this review, we discuss the recent literature on the physiology of endogenous and exogenous H2 S, focusing upon the protective effects of hydrogen sulfide in models of hypoxia and ischaemia.

NaHS inhibits NF-κB signal against inflammation and oxidative stress in post-infectious irritable bowel syndrome

In this study, the alleviating role of hydrogen sulfide (H₂S) was investigated in a Post-Infectious Irritable Bowel Syndrome (PI-IBS) murine model and Caco-2 cells.

The Role of Hydrogen Sulfide in Evolution and the Evolution of Hydrogen Sulfide in Metabolism and Signaling

The chemical versatility of sulfur and its abundance in the prebiotic Earth as reduced sulfide (H2S) implicate this molecule in the origin of life 3.8 billion years ago and also as a major source of energy in the first seven-eighths of evolution. The tremendous increase in ambient oxygen ∼600 million years ago brought an end to H2S as an energy source, and H2S-dependent animals either became extinct, retreated to isolated sulfide niches, or adapted. The first 3 billion years of molecular tinkering were not lost, however, and much of this biochemical armamentarium easily adapted to an oxic environment where it contributes to metabolism and signaling even in humans. This review examines the role of H2S in evolution and the evolution of H2S metabolism and signaling.

H2S Synthesizing Enzymes: Biochemistry and Molecular Aspects

Hydrogen sulfide (H2S) is a biologically active gas that is synthesized naturally by three enzymes, cystathionine γ-lyase (CSE), cystathionine β-synthetase (CBS) and 3-mercaptopyruvate sulfurtransferase (3-MST). These enzymes are constitutively present in a wide array of biological cells and tissues and their expression can be induced by a number of disease states. It is becoming increasingly clear that H2S is an important mediator of a wide range of cell functions in health and in disease. This review therefore provides an overview of the biochemical and molecular regulation of H2S synthesizing enzymes both in physiological conditions and their modulation in disease states with particular focus on their regulation in asthma, atherosclerosis and diabetes. The importance of small molecule inhibitors in the study of molecular pathways, the current use of common H2S synthesizing enzyme inhibitors and the relevant characteristics of mice in which these enzymes have been genetically deleted will also be summarized. With a greater understanding of the molecular regulation of these enzymes in disease states, as well as the availability of novel small molecules with high specificity targeted towards H2S producing enzymes, the potential to regulate the biological functions of this intriguing gas H2S for therapeutic effect can perhaps be brought one step closer.

Hydrogen sulfide prodrugs-a review

Hydrogen sulfide (H₂S) is recognized as one of three gasotransmitters together with nitric oxide (NO) and carbon monoxide (CO). As a signaling molecule, H₂S plays an important role in physiology and shows great potential in pharmaceutical applications. Along this line, there is a need for the development of H₂S prodrugs for various reasons. In this review, we summarize different H₂S prodrugs, their chemical properties, and some of their potential therapeutic applications.

The Cardioprotective Effects of Hydrogen Sulfide in Heart Diseases: From Molecular Mechanisms to Therapeutic Potential

Hydrogen sulfide (H₂S) is now recognized as a third gaseous mediator along with nitric oxide (NO) and carbon monoxide (CO), though it was originally considered as a malodorous and toxic gas. H₂S is produced endogenously from cysteine by three enzymes in mammalian tissues. An increasing body of evidence suggests the involvement of H₂S in different physiological and pathological processes. Recent studies have shown that H₂S has the potential to protect the heart against myocardial infarction, arrhythmia, hypertrophy, fibrosis, ischemia-reperfusion injury, and heart failure. Some mechanisms, such as antioxidative action, preservation of mitochondrial function, reduction of apoptosis, anti-inflammatory responses, angiogenic actions, regulation of ion channel, and interaction with NO, could be responsible for the cardioprotective effect of H₂S. Although several mechanisms have been identified, there is a need for further research to identify the specific molecular mechanism of cardioprotection in different cardiac diseases. Therefore, insight into the molecular mechanisms underlying H₂S action in the heart may promote the understanding of pathophysiology of cardiac diseases and lead to new therapeutic targets based on modulation of H₂S production.

Elevated hydrogen sulfide levels in vitreous body and plasma in patients with proliferative diabetic retinopathy

PURPOSE

Hydrogen sulfide (H2S), a colorless gas, has been confirmed to be a gaseous messenger molecule and an endogenous stimulus for angiogenesis recently. This study was performed to investigate the role of H2S in diabetic retinopathy.

METHODS

Blood samples were collected from normal controls and patients with diabetes. Vitreous samples were collected from patients with proliferative diabetic retinopathy (PDR) and patients with rhegmatogenous retinal detachment. Patients were grouped into diabetic patients without diabetic retinopathy (non-DR), with nonproliferative DR, and with PDR. Concentrations of H2S and vascular endothelial growth factor in the plasma and vitreous body were detected using a spectrophotometer.

RESULTS

A decreased H2S level in the plasma was observed in non-DR group (41.89 ± 8.52 μM, P < 0.05), and an increased H2S level in the plasma was observed in PDR group (60.49 ± 11.14 μM, P < 0.001), when compared with that in normal controls (49.67 ± 9.72 μM). There was no difference in plasma H2S level between patients with nonproliferative DR (54.13 ± 8.61 μM) and normal controls. In the vitreous body, H2S levels in PDR group were significantly higher (76.80 ± 24.08 μM, P < 0.001) than that in rhegmatogenous retinal detachment group (24.37 ± 11.25 μM). Levels of vascular endothelial growth factor in plasma from patients with diabetes were significantly lower (P < 0.001) than that in normal controls. Vascular endothelial growth factor levels in the vitreous body from diabetic patients with PDR were significantly higher (885.61 ± 190.41 pg/mL, P < 0.001) than that from patients with rhegmatogenous retinal detachment (89.98 ± 19.56 pg/mL). Seven days after an intravitreal injection of ranibizumab, a significantly decreased H2S level (55.58 ± 7.20 μM, P < 0.05) was observed in the vitreous body in patients with PDR when compared with that (75.07 ± 12.95 μM) in the vitreous body collected just before intravitreal injection.

CONCLUSION

These results indicated that anti-vascular endothelial growth factor may downregulate the H2S level in the vitreous body, and H2S may play a role in the development of DR. Hydrogen sulfide may be a novel target for the therapy of DR.

Signaling molecules: hydrogen sulfide and polysulfide

SIGNIFICANCE

Hydrogen sulfide (H2S) has been recognized as a signaling molecule as well as a cytoprotectant. It modulates neurotransmission, regulates vascular tone, and protects various tissues and organs, including neurons, the heart, and kidneys, from oxidative stress and ischemia-reperfusion injury. H2S is produced from l-cysteine by cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE), and 3-mercaptopyruvate sulfurtransferase (3MST) along with cysteine aminotransferase.

RECENT ADVANCES

In addition to these enzymes, we recently identified a novel pathway to produce H2S from d-cysteine, which involves d-amino acid oxidase (DAO) along with 3MST. These enzymes are localized in the cytoplasm, mitochondria, and peroxisomes. However, some enzymes translocate to organelles under specific conditions. Moreover, H2S-derived potential signaling molecules such as polysulfides and HSNO have been identified.

CRITICAL ISSUES

The physiological stimulations, which trigger the production of H2S and its derivatives and maintain their local levels, remain unclear.

FUTURE DIRECTIONS

Understanding the regulation of the H2S production and H2S-derived signaling molecules and the specific stimuli that induce their release will provide new insights into the biology of H2S and therapeutic development in diseases involving these substances.

Hydrogen sulfide as an oxygen sensor

SIGNIFICANCE

Although oxygen (O2)-sensing cells and tissues have been known for decades, the identity of the O2-sensing mechanism has remained elusive. Evidence is accumulating that O2-dependent metabolism of hydrogen sulfide (H2S) is this enigmatic O2 sensor.

RECENT ADVANCES

The elucidation of biochemical pathways involved in H2S synthesis and metabolism have shown that reciprocal H2S/O2 interactions have been inexorably linked throughout eukaryotic evolution; there are multiple foci by which O2 controls H2S inactivation, and the effects of H2S on downstream signaling events are consistent with those activated by hypoxia. H2S-mediated O2 sensing has been demonstrated in a variety of O2-sensing tissues in vertebrate cardiovascular and respiratory systems, including smooth muscle in systemic and respiratory blood vessels and airways, carotid body, adrenal medulla, and other peripheral as well as central chemoreceptors.

CRITICAL ISSUES

Information is now needed on the intracellular location and stoichometry of these signaling processes and how and which downstream effectors are activated by H2S and its metabolites.

FUTURE DIRECTIONS

Development of specific inhibitors of H2S metabolism and effector activation as well as cellular organelle-targeted compounds that release H2S in a time- or environmentally controlled way will not only enhance our understanding of this signaling process but also provide direction for future therapeutic applications.

NaHS induces relaxation response in prostaglandin F(2α) precontracted bovine retinal arteries partially via K(v) and K(ir) channels

Hydrogen sulphide (H2S) is known to be produced endogenously in ocular tissues with the highest levels in the retina and cornea. However, it is yet unclear whether it can modulate retinal arterial tone. Herein, we aimed to investigate the effectiveness and the mechanism of the action of H2S in the isolated bovine retinal arteries. For this purpose, the probable vasorelaxant and inhibitory effects of H2S on vascular reactivity were tested comparatively in the retinal arteries by using the donor, sodium hydrosulphide (NaHS). Thereafter, in relation to the mechanism of action of H2S, the role of nitric oxide (NO) and endothelial vasodilators of cyclooxygenase pathway as well as ATP-sensitive potassium channel (KATP), voltage-dependent potassium channel (Kv), calcium-activated potassium channel (KCa(++)), inwardly rectifying potassium channel (Kir), L-type voltage-dependent calcium channel and adenylate cyclase pathway were evaluated. NaHS (1μM-3mM) displayed prominent relaxations over the concentrations of 300 μM in both PGF2α and K(+) precontracted retinal arteries. Comparatively, in the presence of NaHS (3 mM) pretreatment, the maximum contractile responses and pEC50 values to PGF2α and K(+) were significantly reduced as well. Neither the presence of the known inhibitors of NO synthase, guanylate cyclase, cyclooxygenase, adenylate cyclase, KATP and KCa(++) type K(+) channels, and L-type voltage-dependent calcium channels nor the removal of endothelium, modified the relaxation response to NaHS in retinal arteries. However, a remarkable decrease was observed in the presence of the inhibitors of Kv or Kir type K(+) channels. In addition, administration of l-cysteine (1μM-3mM), the precursor of H2S, induced a modest relaxation response in PGF2α precontracted retinal arteries, which was significantly decreased in the presence of cystathionine-β-synthase (CBS) inhibitor, aminooxyacetic acid, but was unmodified in the presence of the cystathionine-γ-lyase (CSE) inhibitor, dl-propargylglycine or the deendothelization of retinal arteries. Our findings suggested that H2S might play a substantial role in the regulation of retinal arterial tone possibly by acting on Kv and Kir channels.

Physiological Roles of Hydrogen Sulfide and Polysulfides

Hydrogen sulfide (H2S) has been recognized as a signaling molecule as well as a cytoprotective molecule. H2S modulates neurotransmission, regulates vascular tone, protects various tissues and organs, regulates inflammation, induces angiogenesis, and detects cellular oxygen levels. H2S is produced from L-cysteine by cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE), and 3-mercaptopyruvate sulfurtransferase (3MST) together with cysteine aminotransferase (CAT). Recently, a novel pathway for the production of H2S from D-cysteine was identified, involving D-amino acid oxidase (DAO) together with 3MST. Sulfuration (also called sulfhydration), which adds sulfur atoms to the cysteine residues of target proteins to modify protein activity, has been extensively studied as a mode of H2S action. Recently, hydrogen polysulfides (H2Sn, where n=3-7; n=2 is termed as persulfide) have been found to sulfurate target proteins in the brain, including transient receptor potential ankyrin 1 (TRPA1) channels, Kelch-like ECH-associating protein 1 (Keap1), and phosphatase and tensin homolog (PTEN), much more potently than H2S. The physiological stimuli that trigger the production of H2S and polysulfides, and the mechanisms maintaining their local levels, remain unknown. Understanding the regulation of H2Sn (including H2S) production, and the specific stimuli that induce their release, will provide new insight into the biology of H2S and will provide novel avenues for therapeutic development in diseases involving H2S-related substances.

Brain, Learning, and Memory: Role of H2S in Neurodegenerative Diseases

For more than 300 years, the toxicity of hydrogen sulfide (H2S) has been known to mankind. However, this point of view is changing as an increased interest was observed in H2S biology in the last two decades. The scientific community has succeeded to unravel many important physiological and pathological effects of H2S on mammalian body systems. Thus, H2S is now referred to as a third endogenous gaseous mediator along with nitric oxide and carbon monoxide. Acting as a neuromodulator, H2S facilitates long-term potentiation and regulates intracellular calcium levels, which are important processes in learning and memory. Aberrant endogenous production and metabolism of H2S are implicated in pathogenesis of neurodegenerative diseases including Alzheimer's disease (AD) and Parkinson's disease (PD). Various H2S donors have shown beneficial therapeutic effects in neurodegenerative disease models by targeting hallmark pathological events (e.g., amyloid-β production in AD and neuroinflammation in PD). The results obtained from many in vivo studies clearly show that H2S not only prevents neuronal and synaptic deterioration but also improves deficits in memory, cognition, and learning. The anti-inflammatory, antioxidant, and anti-apoptotic effects of H2S underlie its neuroprotective properties. In this chapter, we will overview the current understanding of H2S in context of neurodegenerative diseases, with special emphasis on its corrective effects on impaired learning, memory, and cognition.

Hydrogen sulfide and polysulfides as signaling molecules

Hydrogen sulfide (H2S) is a familiar toxic gas that smells of rotten eggs. After the identification of endogenous H2S in the mammalian brain two decades ago, studies of this molecule uncovered physiological roles in processes such as neuromodulation, vascular tone regulation, cytoprotection against oxidative stress, angiogenesis, anti-inflammation, and oxygen sensing. Enzymes that produce H2S, such as cystathionine β-synthase, cystathionine γ-lyase, and 3-mercaptopyruvate sulfurtransferase have been studied intensively and well characterized. Polysulfides, which have a higher number of inner sulfur atoms than that in H2S, were recently identified as potential signaling molecules that can activate ion channels, transcription factors, and tumor suppressors with greater potency than that of H2S. This article focuses on our contribution to the discovery of these molecules and their metabolic pathways and mechanisms of action.

Correlation of lower concentrations of hydrogen sulfide with atherosclerosis in chronic hemodialysis patients with diabetic nephropathy

BACKGROUND/AIMS

To explore the relationship between hydrogen sulfide (H2S) and uremic accelerated atherosclerosis (UAAS) in chronic hemodialysis patients with diabetic nephropathy (CHD/DN).

METHODS

A total of 36 CHD/DN and 32 chronic hemodialyzed non-diabetic patients with chronic glomerulonephritis (CHD/non-DN) were studied. Plasma H2S was measured with a sulfide sensitive electrode.

RESULTS

Plasma H2S in CHD/DN was significantly lower than that in CHD/non-DN patients. Plasma H2S was positively correlated with plasma TGF-β1, and negatively correlated with MMP-12 in CHD/DN patients. CHD/DN patients exhibited higher CCA-IMT, hsCRP, and lower H2S levels than in CHD/non-DN patients. Moreover, in CHD/DN patients, CCA-IMT was negatively correlated with plasma H2S, and positively correlated with hsCRP and LDL. On multiple regression analysis, H2S levels exhibited independent association with IMT in CHD/DN patients.

CONCLUSIONS

These findings suggest possible linkage between H2S metabolism and TGF-β/Smad signaling pathway modulation abnormalities that may contribute to the development of UAAS in CHD/DN patients.

Hydrogen sulfide in signaling pathways

For a long time hydrogen sulfide (H₂S) was considered a toxic compound, but recently H₂S (at low concentrations) has been found to play an important function in physiological processes. Hydrogen sulfide, like other well-known compounds - nitric oxide (NO) and carbon monoxide (CO) is a gaseous intracellular signal transducer. It regulates the cell cycle, apoptosis and the oxidative stress. Moreover, its functions include neuromodulation, regulation of cardiovascular system and inflammation. In this review, I focus on the metabolism of hydrogen sulfide (including enzymatic pathways of H₂S synthesis from l- and d-cysteine) and its signaling pathways in the cardiovascular system and the nervous system. I also describe how hydrogen sulfide may be used as therapeutic agent, i.e. in the cardiovascular diseases.

Hydrogen sulfide and polysulfides as biological mediators

Hydrogen sulfide (H2S) is recognized as a biological mediator with various roles such as neuromodulation, regulation of the vascular tone, cytoprotection, anti-inflammation, oxygen sensing, angiogenesis, and generation of mitochondrial energy. It is produced by cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE), and 3-mercaptopyruvate sulfurtransferase (3MST). The activity of CBS is enhanced by S-adenosyl methionine (SAM) and glutathionylation, while it is inhibited by nitric oxide (NO) and carbon monoxide (CO). The activity of CSE and cysteine aminotransferase (CAT), which produces the 3MST substrate 3-mercaptopyruvate (3MP), is regulated by Ca2+. H2S is oxidized to thiosulfate in mitochondria through the sequential action of sulfide quinone oxidoreductase (SQR), sulfur dioxygenase, and rhodanese. The rates of the production and clearance of H2S determine its cellular concentration. Polysulfides (H2Sn) have been found to occur in the brain and activate transient receptor potential ankyrin 1 (TRPA1) channels, facilitate the translocation of nuclear factor erythroid 2-related factor 2 (Nrf2) to the nucleus, and suppress the activity of phosphatase and tensin homolog (PTEN) by sulfurating (sulfhydrating) the target cysteine residues. A cross talk between H2S and NO also plays an important role in cardioprotection as well as regulation of the vascular tone. H2S, polysulfides, and their cross talk with NO may mediate various physiological and pathophysiological responses.

Treatment with hydrogen sulfide alleviates streptozotocin-induced diabetic retinopathy in rats

BACKGROUND AND PURPOSE

Retinopathy, as a common complication of diabetes, is a leading cause of reduced visual acuity and acquired blindness in the adult population. The aim of present study was to investigate the therapeutic effect of hydrogen sulfide on streptozotocin (STZ)-induced diabetic retinopathy in rats.

EXPERIMENTAL APPROACH

Rats were injected with a single i.p. injection of STZ (60 mg·kg⁻¹) to induce diabetic retinopathy. Two weeks later, the rats were treated with NaHS (i.p. injection of 0.1 mL·kg⁻¹·d⁻¹ of 0.28 mol·L⁻¹ NaHS, a donor of H₂S) for 14 weeks.

KEY RESULTS

Treatment with H₂S had no significant effect on blood glucose in STZ-induced diabetic rats. Treatment with exogenous H₂S enhanced H₂S levels in both plasma and retinas of STZ-induced diabetic rats. Treatment with H₂S in STZ-treated rats improved the retinal neuronal dysfunction marked by enhanced amplitudes of b-waves and oscillatory potentials and expression of synaptophysin and brain-derived neurotrophic factor, alleviated retinal vascular abnormalities marked by reduced retinal vascular permeability and acellular capillary formation, decreased vitreous VEGF content, down-regulated expressions of HIF-1α and VEGFR2, and enhanced occludin expression, and attenuated retinal thickening and suppressed expression of extracellular matrix molecules including laminin β1 and collagen IVα3 expression in retinas of STZ-induced diabetic rats. Treatment with H₂S in retinas of STZ-induced diabetic rats abated oxidative stress, alleviated mitochondrial dysfunction, suppressed NF-κB activation and attenuated inflammation.

CONCLUSIONS AND IMPLICATIONS

Treatment with H₂S alleviates STZ-induced diabetic retinopathy in rats possibly through abating oxidative stress and suppressing inflammation.

Hydrogen sulfide regulates Ca(2+) homeostasis mediated by concomitantly produced nitric oxide via a novel synergistic pathway in exocrine pancreas

AIM

The present study was designed to explore the effects of hydrogen sulfide (H2S) on Ca(2+) homeostasis in rat pancreatic acini.

RESULTS

Sodium hydrosulfide (NaHS; an H2S donor) induced a biphasic increase in the intracellular Ca(2+) concentration ([Ca(2+)]i) in a dose-dependent manner. The NaHS-induced [Ca(2+)]i elevation persisted with an EC50 of 73.3 μM in the absence of extracellular Ca(2+) but was abolished by thapsigargin, indicating that both Ca(2+) entry and Ca(2+) release contributed to the increase. The [Ca(2+)]i increase was markedly inhibited in the presence of NG-monomethyl L-arginine or 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), and diaminofluorescein-2/diaminofluorescein-2 triazole (DAF-2/DAF-2T) fluorometry demonstrated that nitric oxide (NO) was also produced by H2S in a dose-dependent manner with an EC50 of 64.8 μM, indicating that NO was involved in the H2S effect. The H2S-induced [Ca(2+)]i increase was inhibited by pretreatment with U73122, xestospongin C, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, KT5823, and GP2A, indicating that phospholipase C (PLC), the inositol 1,4,5-trisphosphate (IP3) receptor, soluble guanylate cyclase (sGC), protein kinase G (PKG), and Gq-protein play roles as intermediate components in the H2S-triggered intracellular signaling.

INNOVATION

To our knowledge, our study is the first one highlighting the effect of H2S on intracellular Ca(2+) dynamics in pancreatic acinar cells. Moreover, a novel cascade was presumed to function via the synergistic interaction between H2S and NO.

CONCLUSION

We conclude that H2S affects [Ca(2+)]i homeostasis that is mediated by H2S-evoked NO production via an endothelial nitric oxide synthase (eNOS)-NO-sGC-cyclic guanosine monophosphate-PKG-Gq-protein-PLC-IP3 pathway to induce Ca(2+) release, and this pathway is identical to the one we recently proposed for a sole effect of NO and the two gaseous molecules synergistically function to regulate Ca(2+) homeostasis.

Production and physiological effects of hydrogen sulfide

SIGNIFICANCE

Hydrogen sulfide (H2S) has been recognized as a physiological mediator with a variety of functions. It regulates synaptic transmission, vascular tone, inflammation, transcription, and angiogenesis; protects cells from oxidative stress and ischemia-reperfusion injury; and promotes healing of ulcers.

RECENT ADVANCES

In addition to cystathionine β-synthase and cystathionine γ-lyase, 3-mercaptopyruvate sulfurtransferase along with cysteine aminotransferase was recently demonstrated to produce H2S. Even in bacteria, H2S produced by these enzymes functions as a defense against antibiotics, suggesting that the cytoprotective effect of H2S is a universal defense mechanism in organisms from bacteria to mammals.

CRITICAL ISSUES

The functional form of H2S-undissociated H2S gas, dissociated HS ion, or some other form of sulfur-has not been identified.

FUTURE DIRECTIONS

The regulation of H2S production by three enzymes may lead to the identification of the physiological signals that are required to release H2S. The identification of the physiological functions of other forms of sulfur may also help understand the biological significance of H2S.

The physiological role of hydrogen sulfide and beyond

Hydrogen sulfide (H2S) has been considered to be a physiological mediator since the identification of endogenous sulfides in the mammalian brain. H2S is produced from L-cysteine by enzymes such as cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE), 3-mercaptopyruvate sulfurtransferase (3MST), and cysteine aminotransferase (CAT). CSE and CAT are regulated by Ca(2+). At steady-state low intracellular concentrations of Ca(2+), CSE and the 3MST/CAT pathway produce H2S. However, after intracellular concentrations of Ca(2+) increase in stimulated cells, the production of H2S by these enzymes decreases. We recently identified a fourth pathway, by which H2S is produced from D-cysteine by the enzymes D-amino acid oxidase (DAO) and 3MST. This pathway is mainly localized in the cerebellum and the kidney. The production of H2S from D-cysteine is 80 times more efficient than that from L-cysteine in the kidney, and the administration of D-cysteine to mice ameliorates renal ischemia-reperfusion injury more effectively than L-cysteine. These results suggest that D-cysteine might be used to treat renal diseases or even increase the success of kidney transplantation. We found that H2S-derived polysulfides exist in the brain and activate transient receptor potential ankyrin-1 (TRPA1) channels 300 times more potently than H2S. Although TRPA1 channels mediate sensory transduction and respond to a variety of stimuli, including cold temperature, pungent compounds and environmental irritants, their endogenous ligand(s) has not been identified. The sulfane sulfur of polysulfides is a reactive electrophile that is readily transferred to a nucleophilic protein thiolate to generate the protein persulfide or bound sulfane sulfur by sulfhydration (as referred to as sulfuration). The bound sulfane sulfur-producing activity of polysulfides is much greater than that of H2S. This review focuses on the physiological roles of H2S and H2S-derived polysulfides as signaling molecules.

Vacuolar ATPases and their role in vision

Vacuolar ATPases (v-ATPases) hydrolyze adenosine triphospate (ATP) to pump protons across cell membranes. Mutations in v-ATPase subunits are implicated in three human disorders: distal renal tubular acidosis, osteopetrosis, and cutis laxa type II. In the eye, the role of v-ATPases is only emerging. Mutations in v-ATPase subunits are not linked to human blindness, but altered proton pump function may underlie ocular pathologies. For example, inhibition of v-ATPase by A2E may accentuate age-related macular degeneration (AMD). In animal models, v-ATPase mutations perturb the retinal pigment epithelium (RPE) and photoreceptor outer segment (OS) phagocytosis, an event linked to retinal degeneration. As the RPE plays essential roles in eye development and vision, the study of v-ATPase-induced RPE dysfunction may improve our understanding of RPE diseases.

Role of hydrogen sulfide in secondary neuronal injury

In acute neuronal insult events, such as stroke, traumatic brain injury, and spinal cord injury, pathological processes of secondary neuronal injury play a key role in the severity of insult and clinical prognosis. Along with nitric oxide (NO) and carbon monoxide (CO), hydrogen sulfide (H2S) is regarded as the third gasotransmitter and endogenous neuromodulator and plays multiple roles in the central nervous system under physiological and pathological states, especially in secondary neuronal injury. The endogenous level of H2S in the brain is significantly higher than that in peripheral tissues, and is mainly formed by cystathionine β-synthase (CBS) in astrocytes and released in response to neuronal excitation. The mechanism of secondary neuronal injury exacerbating the damage caused by the initial insult includes microcirculation failure, glutamate-mediated excitotoxicity, oxidative stress, inflammatory responses, neuronal apoptosis and calcium overload. H2S dilates cerebral vessels by activating smooth muscle cell plasma membrane ATP-sensitive K channels (KATP channels). This modification occurs on specific cysteine residues of the KATP channel proteins which are S-sulfhydrated. H2S counteracts glutamate-mediated excitotoxicity by inducing astrocytes to intake more glutamate from the extracellular space and thus increasing glutathione in neurons. In addition, H2S protects neurons from secondary neuronal injury by functioning as an anti-oxidant, anti-inflammatory and anti-apoptotic mediator. However, there are still some reports suggest that H2S elevates neuronal Ca(2+) concentration and may contribute to the formation of calcium overload in secondary neuronal injury. H2S also elicits calcium waves in primary cultures of astrocytes and may mediate signals between neurons and glia. Consequently, further exploration of the molecular mechanisms of H2S in secondary neuronal injury will provide important insights into its potential therapeutic uses for the treatment of acute neuronal insult events.

Polysulfide exerts a protective effect against cytotoxicity caused by t-buthylhydroperoxide through Nrf2 signaling in neuroblastoma cells

Polysulfide is a bound sulfur species derived from endogenous H2S. When mouse neuroblastoma, Neuro2A cells were exposed to tert-butyl hydroperoxide after treatment with polysulfide, a significant decline in cell toxicity was observed. Rapid uptake of polysulfides induced translocation of Nrf2 into the nucleus, resulting in acceleration of GSH synthesis and HO-1 expression. We demonstrated that polysulfide reversibly modified Keap1 to form oxidized dimers and induced the translocation of Nrf2. Moreover, polysulfide treatment accelerated Akt phosphorylation, which is a known pathway of Nrf2 phosphorylation. Thus, polysulfide may mediate the activation of Nrf2 signaling, thereby exerting protective effects against oxidative damage in Neuro2A cells.