Hydrogen sulfide: from brain to gut

H2S and its role in redox signaling

Hydrogen sulfide (H2S) has emerged as an important gaseous signaling molecule that is produced endogenously by enzymes in the sulfur metabolic network. H2S exerts its effects on multiple physiological processes important under both normal and pathological conditions. These functions include neuromodulation, regulation of blood pressure and cardiac function, inflammation, cellular energetics and apoptosis. Despite the recognition of its biological importance and its beneficial effects, the mechanism of H2S action and the regulation of its tissue levels remain unclear in part owing to its chemical and physical properties that render handling and analysis challenging. Furthermore, the multitude of potential H2S effects has made it difficult to dissect its signaling mechanism and to identify specific targets. In this review, we focus on H2S metabolism and provide an overview of the recent literature that sheds some light on its mechanism of action in cellular redox signaling in health and disease. This article is part of a Special Issue entitled: Thiol-Based Redox Processes.

Endogenous hydrogen sulfide in the rostral ventrolateral medulla/Bötzinger complex downregulates ventilatory responses to hypoxia

Hydrogen sulfide (H2S) is now recognized as a new gaseous transmitter involved in several brain-mediated responses. The rostral ventrolateral medulla (RVLM)/Bötzinger complex is a region in the brainstem that is involved in cardiovascular and respiratory functions. Recently, it has been shown that exogenous H2S in the RVLM modulates autonomic function and thus blood pressure. In the present study, we investigated whether H2S, endogenously produced in the RVLM/Bötzinger complex, plays a role in the control of hypoxia-induced hyperventilation. Ventilation (VE) was measured before and after bilateral microinjection of Na2S (H2S donor, 0.04, 1 and 2 pmol/100 nl) or aminooxyacetate (AOA, 0.2, 1 and 2 pmol/100 nl, a cystathionine β-synthase, CBS, inhibitor) into the RVLM/Bötzinger complex followed by a 60-min period of hypoxia (7% inspired O2) or normoxia exposure. Control rats received microinjection of vehicle. Microinjection of vehicle, AOA or Na2S did not change VE in normoxic conditions. Exposure to hypoxia evoked a typical increase in VE. Microinjection of Na2S (2 pmol) followed by hypoxia exposure attenuated the hyperventilation. Conversely, microinjection of AOA (2 pmol) into the RVLM/Bötzinger complex caused an increase in the hypoxia-induced hyperventilation. Thus, endogenous H2S in the RVLM/Bötzinger complex seems to play no role in the maintenance of basal pulmonary ventilation during normoxia whereas during hypoxia H2S has a downmodulatory function. Homogenates of RVLM/Bötzinger complex of animals previously exposed to hypoxia for 60 min exhibited a decreased rate of H2S production. Our data are consistent with the notion that the gaseous messenger H2S synthesis is downregulated in the RVLM/Bötzinger complex during hypoxia favoring hyperventilation.

Hydrogen sulfide and nitric oxide interactions in inflammation

Together with carbon monoxide (CO), nitric oxide (NO) and hydrogen sulfide (H2S) form a group of physiologically important gaseous transmitters, sometimes referred to as the "gaseous triumvirate". The three molecules share a wide range of physical and physiological properties: they are small gaseous molecules, able to freely penetrate cellular membranes; they are all produced endogenously in the body and they seem to exert similar biological functions. In the cardiovascular system, for example, they are all vasodilators, promote angiogenesis and protect tissues against damage (e.g. ischemia-reperfusion injury). In addition, they have complex roles in inflammation, with both pro- and anti-inflammatory effects reported. Researchers have focused their efforts in understanding and describing the roles of each of these molecules in different physiological systems, and in the past years attention has also been given to the gases interaction or "cross-talk". This review will focus on the role of NO and H2S in inflammation and will give an overview of the evidence collected so far suggesting the importance of their cross-talk in inflammatory processes.

A systematic review of neuroprotective strategies after cardiac arrest: from bench to bedside (part II-comprehensive protection)

Neurocognitive deficits remain a significant source of morbidity in survivors of cardiac arrest. We conducted a literature review of treatment protocols designed to evaluate neurologic outcome and survival following global cerebral ischemia associated with cardiac arrest. The search was limited to investigational therapies that were implemented either during cardiopulmonary resuscitation or after return of spontaneous circulation in studies that included assessment of impact on neurologic outcome. Given that complex pathophysiology underlies global brain hypoxic ischemia following cardiac arrest, neuroprotective strategies targeting multiple stages of neuropathologic cascades should promise to improve survival and neurologic outcomes in cardiac arrest victims. In Part II of this review, we discuss several approaches that can provide comprehensive protection against global brain injury associated with cardiac arrest, by modulating multiple targets of neuropathologic cascades. Pharmaceutical approaches include adenosine and growth factors/hormones including brain-derived neurotrophic factor, insulin-like growth factor-1 and glycine-proline-glutamate, granulocyte colony stimulating factor and estrogen. Preclinical studies of these showed some benefit but were inconclusive in models of global brain injury involving systemic ischemia. Several medical gases that can mediate neuroprotection have been evaluated in experimental settings. These include hydrogen sulfide, hyperbaric oxygen and molecular hydrogen. Hyperbaric oxygen and molecular hydrogen showed promising results; however, further investigation is required prior to clinical application of these agents in cardiac arrest patients.

Detection of hydrogen sulfide through photoluminescence quenching of penicillamine-copper nanocluster aggregates

We have developed a one-pot, inexpensive, simple and rapid method to synthesize photoluminescent copper nanocluster (Cu NC) aggregates from Cu(2+) ions in 65% (v v(-1)) dimethylformamide aqueous solution containing penicillamine (PA) as a capping and reducing agent. As-prepared PA-Cu NC aggregates emit at 580 nm when excited at 326 nm, with a quantum yield of 2.0%. The photoluminescence of PA-Cu NC aggregates originate from ligand-to-metal charge transfer, which is supported by a long lifetime (126.5 ns) and a large Stokes shift (254 nm). As-prepared PA-Cu NC aggregates have different emission wavelengths with the same excitation wavelength in various organic-aqueous solutions. The PA-Cu NC aggregates are highly selective and sensitive to the detection of hydrogen sulfide (H₂S), based on analyte-induced photoluminescence quenching through the formation of CuS nanoparticles. The probe allows the detection of H₂S, with a linear range of 1-100 μM and a limit of detection (signal-to-noise ratio = 3) of 500 nM. The practicality of this probe has been validated through the analysis of hot spring water samples.

Molecular hydrogen as a preventive and therapeutic medical gas: initiation, development and potential of hydrogen medicine

Molecular hydrogen (H2) has been accepted to be an inert and nonfunctional molecule in our body. We have turned this concept by demonstrating that H2 reacts with strong oxidants such as hydroxyl radical in cells, and proposed its potential for preventive and therapeutic applications. H2 has a number of advantages exhibiting extensive effects: H2 rapidly diffuses into tissues and cells, and it is mild enough neither to disturb metabolic redox reactions nor to affect signaling reactive oxygen species; therefore, there should be no or little adverse effects of H2. There are several methods to ingest or consume H2; inhaling H2 gas, drinking H2-dissolved water (H2-water), injecting H2-dissolved saline (H2-saline), taking an H2 bath, or dropping H2-saline into the eyes. The numerous publications on its biological and medical benefits revealed that H2 reduces oxidative stress not only by direct reactions with strong oxidants, but also indirectly by regulating various gene expressions. Moreover, by regulating the gene expressions, H2 functions as an anti-inflammatory and anti-apoptotic, and stimulates energy metabolism. In addition to growing evidence obtained by model animal experiments, extensive clinical examinations were performed or are under investigation. Since most drugs specifically act to their targets, H2 seems to differ from conventional pharmaceutical drugs. Owing to its great efficacy and lack of adverse effects, H2 has promising potential for clinical use against many diseases.

Estrogens increase cystathionine-γ-lyase expression and decrease inflammation and oxidative stress in the myocardium of ovariectomized rats

OBJECTIVE

Hydrogen sulfide (H2S), generated in the myocardium predominantly via cystathionine-γ-lyase (CSE), is cardioprotective. The objectives of the present study were to investigate the effects of estrogens on CSE expression and H2S generation in the myocardium and to examine whether serum 17β-estradiol (E2) level is associated with CSE activity and H2S generation and whether H2S or E2 level is associated with proinflammatory cytokines and oxidative stress status.

METHODS

Ovariectomized Sprague-Dawley rats received subcutaneous E2 (30 μg/kg/d) or vehicle for 12 weeks. At the end of the 12-week treatment, CSE expression, H2S generation, reduced glutathione/oxidized glutathione (GSH/GSSG) ratio, total antioxidant capacity (T-AOC), superoxide dismutase (SOD) activity, catalase (CAT) activity, interleukin (IL)-6 concentration, and tumor necrosis factor-α (TNF-α) concentration in the left ventricle were determined.

RESULTS

E2 increased CSE expression and H2S generation in the myocardium of ovariectomized rats. H2S production rate and serum E2 were positively correlated. E2 increased GSH/GSSG ratio, T-AOC, CAT, and SOD activity but decreased IL-6 and TNF-α levels. Serum E2 level was positively correlated with GSH/GSSG ratio, T-AOC, CAT, and SOD activity, and inversely correlated with IL-6 and TNF-α levels. H2S generation rate was positively correlated with T-AOC and GSH/GSSG ratio, and inversely correlated with IL-6 and TNF-α levels.

CONCLUSIONS

E2 increases CSE expression and endogenous H2S generation in the myocardium. The effects of E2 are associated with decreased oxidative stress and inflammatory status. Our data suggest that estrogens might exert cardioprotective effects through up-regulation of CSE expression and H2S generation.

Hypoxia-inducible factors regulate human and rat cystathionine β-synthase gene expression

Increased catalytic activity of CBS (cystathionine β-synthase) was recently shown to mediate vasodilation of the cerebral microcirculation, which is initiated within minutes of the onset of acute hypoxia. To test whether chronic hypoxia was a stimulus for increased CBS expression, U87-MG human glioblastoma and PC12 rat phaeochromocytoma cells were exposed to 1% or 20% O2 for 24-72 h. CBS mRNA and protein expression were increased in hypoxic cells. Hypoxic induction of CBS expression was abrogated in cells transfected with vector encoding shRNA targeting HIF (hypoxia-inducible factor) 1α or 2α. Exposure of rats to hypobaric hypoxia (0.35 atm; 1 atm=101.325 kPa) for 3 days induced increased CBS mRNA, protein and catalytic activity in the cerebral cortex and cerebellum, which was blocked by administration of the HIF inhibitor digoxin. HIF-binding sites, located 0.8 and 1.2 kb 5' to the transcription start site of the human CBS and rat Cbs genes respectively, were identified by ChIP assays. A 49-bp human sequence, which encompassed an inverted repeat of the core HIF-binding site, functioned as a hypoxia-response element in luciferase reporter transcription assays. Thus HIFs mediate tissue-specific CBS expression, which may augment cerebral vasodilation as an adaptive response to chronic hypoxia.

Endogenous and exogenous hydrogen sulfide facilitates T-type calcium channel currents in Cav3.2-expressing HEK293 cells

Hydrogen sulfide (H2S), a gasotransmitter, is formed from l-cysteine by multiple enzymes including cystathionine-γ-lyase (CSE). We have shown that an H2S donor, NaHS, causes hyperalgesia in rodents, an effect inhibited by knockdown of Cav3.2 T-type Ca(2+) channels (T-channels), and that NaHS facilitates T-channel-dependent currents (T-currents) in NG108-15 cells that naturally express Cav3.2. In the present study, we asked if endogenous and exogenous H2S participates in regulation of the channel functions in Cav3.2-transfected HEK293 (Cav3.2-HEK293) cells. dl-Propargylglycine (PPG), a CSE inhibitor, significantly decreased T-currents in Cav3.2-HEK293 cells, but not in NG108-15 cells. NaHS at 1.5mM did not affect T-currents in Cav3.2-HEK293 cells, but enhanced T-currents in NG108-15 cells. In the presence of PPG, NaHS at 1.5mM, but not 0.1-0.3mM, increased T-currents in Cav3.2-HEK293 cells. Similarly, Na2S, another H2S donor, at 0.1-0.3mM significantly increased T-currents in the presence, but not absence, of PPG in Cav3.2-HEK293 cells. Expression of CSE was detected at protein and mRNA levels in HEK293 cells. Intraplantar administration of Na2S, like NaHS, caused mechanical hyperalgesia, an effect blocked by NNC 55-0396, a T-channel inhibitor. The in vivo potency of Na2S was higher than NaHS. These results suggest that the function of Cav3.2 T-channels is tonically enhanced by endogenous H2S synthesized by CSE in Cav3.2-HEK293 cells, and that exogenous H2S is capable of enhancing Cav3.2 function when endogenous H2S production by CSE is inhibited. In addition, Na2S is considered a more potent H2S donor than NaHS in vitro as well as in vivo.

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.

Hydrogen sulfide offers neuroprotection on traumatic brain injury in parallel with reduced apoptosis and autophagy in mice

Hydrogen sulfide (H₂S), a novel gaseous mediator, has been recognized as an important neuromodulator and neuroprotective agent in the central nervous system. The present study was undertaken to study the effects of exogenous H₂S on traumatic brain injury (TBI) and the underlying mechanisms. The effects of exogenous H₂S on TBI were examined by using measurement of brain edema, behavior assessment, propidium iodide (PI) staining, and Western blotting, respectively. Compared to TBI groups, H₂S pretreatment had reduced brain edema, improved motor performance and ameliorated performance in Morris water maze test after TBI. Immunoblotting results showed that H₂S pretreatment reversed TBI-induced cleavage of caspase-3 and decline of Bcl-2, suppressed LC3-II, Beclin-1 and Vps34 activation and maintained p62 level in injured cortex and hippocampus post TBI. The results suggest a protective effect and therapeutic potential of H₂S in the treatment of brain injury and the protective effect against TBI may be associated with regulating apoptosis and autophagy.

Nitrite reductase activity and inhibition of H₂S biogenesis by human cystathionine ß-synthase

Nitrite was recognized as a potent vasodilator >130 years and has more recently emerged as an endogenous signaling molecule and modulator of gene expression. Understanding the molecular mechanisms that regulate nitrite metabolism is essential for its use as a potential diagnostic marker as well as therapeutic agent for cardiovascular diseases. In this study, we have identified human cystathionine ß-synthase (CBS) as a new player in nitrite reduction with implications for the nitrite-dependent control of H₂S production. This novel activity of CBS exploits the catalytic property of its unusual heme cofactor to reduce nitrite and generate NO. Evidence for the possible physiological relevance of this reaction is provided by the formation of ferrous-nitrosyl (Fe^II-NO) CBS in the presence of NADPH, the human diflavin methionine synthase reductase (MSR) and nitrite. Formation of Fe^II-NO CBS via its nitrite reductase activity inhibits CBS, providing an avenue for regulating biogenesis of H₂S and cysteine, the limiting reagent for synthesis of glutathione, a major antioxidant. Our results also suggest a possible role for CBS in intracellular NO biogenesis particularly under hypoxic conditions. The participation of a regulatory heme cofactor in CBS in nitrite reduction is unexpected and expands the repertoire of proteins that can liberate NO from the intracellular nitrite pool. Our results reveal a potential molecular mechanism for cross-talk between nitrite, NO and H₂S biology.

Development of a highly selective H2S fluorescent probe and its application to evaluate CSE inhibitors

C359, a novel fluorescent probe for highly selective detection of H₂S over biothiols, was applied to evaluate CSE inhibitors.

Effect of endogenous hydrogen sulfide on the transwall gradient of the mouse colon circular smooth muscle

A transwall gradient in resting membrane potential (RMP) exists across the circular muscle layer in the mouse colon. This gradient is dependent on endogenous generation of CO. H2S is also generated in muscle layers of the mouse colon. The effect of endogenously generated H2S on the transwall gradient is not known. The aim was to investigate the role of endogenous H2S. Our results showed that the CSE inhibitor dl-propargylglycine (PAG, 500 μm) had no effect on the transwall gradient. However, in preparations pretreated with the nitric oxide synthase inhibitor N-nitro-l-arginine (l-NNA, 200 μm) and in nNOS-knockout (KO) mouse preparations, PAG shifted the transwall gradient in the depolarizing direction. In CSE-KO-nNOS-KO mice, the gradient was shifted in the depolarizing direction. Endogenous generation of NO was significantly higher in muscle preparations of CSE-KO mice compared to wild-type (WT) mice. The amplitude of NO-mediated slow inhibitory junction potentials (S-IJPs) evoked by electric field stimulation was significantly higher in CSE-KO mouse preparations compared to the amplitude of S-IJPs in wild-type mouse preparations. CSE was present in all submucosal ganglion neurons and in almost all myenteric ganglion neurons. Eleven per cent of CSE positive neurons in the submucosal plexus and 50% of CSE positive neurons in the myenteric plexus also contained nNOS. Our results suggest that endogenously generated H2S acts as a stealth hyperpolarizing factor on smooth muscle cells to maintain the CO-dependent transwall gradient and inhibits NO production from nNOS.

Nitric Oxide-Induced Calcium Release: Activation of Type 1 Ryanodine Receptor, a Calcium Release Channel, through Non-Enzymatic Post-Translational Modification by Nitric Oxide

Nitric oxide (NO) is a typical gaseous messenger involved in a wide range of biological processes. In our classical knowledge, effects of NO are largely achieved by activation of soluble guanylyl cyclase to form cyclic guanosine-3', 5'-monophosphate. However, emerging evidences have suggested another signaling mechanism mediated by NO: "S-nitrosylation" of target proteins. S-nitrosylation is a covalent addition of an NO group to a cysteine thiol/sulfhydryl (RSH), and categorized into non-enzymatic post-translational modification (PTM) of proteins, contrasted to enzymatic PTM of proteins, such as phosphorylation mediated by various protein kinases. Very recently, we found novel intracellular calcium (Ca(2+)) mobilizing mechanism, NO-induced Ca(2+) release (NICR) in cerebellar Purkinje cells. NICR is mediated by type 1 ryanodine receptor (RyR1), a Ca(2+) release channel expressed in endoplasmic-reticular membrane. Furthermore, NICR is indicated to be dependent on S-nitrosylation of RyR1, and involved in synaptic plasticity in the cerebellum. In this review, molecular mechanisms and functional significance of NICR, as well as non-enzymatic PTM of proteins by gaseous signals, are described.

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.

T-type calcium channels: functional regulation and implication in pain signaling

Low-voltage-activated T-type Ca(2+) channels (T-channels), especially Cav3.2 among the three isoforms (Cav3.1, Cav3.2, and Cav3.3), are now considered to play pivotal roles in processing of pain signals. Cav3.2 T-channels are functionally modulated by extracellular substances such as hydrogen sulfide and ascorbic acid, by intracellular signaling molecules including protein kinases, and by glycosylation. Cav3.2 T-channels are abundantly expressed in both peripheral and central endings of the primary afferent neurons, regulating neuronal excitability and release of excitatory neurotransmitters such as substance P and glutamate, respectively. Functional upregulation of Cav3.2 T-channels is involved in the pathophysiology of inflammatory, neuropathic, and visceral pain. Thus, Cav3.2 T-channels are considered to serve as novel targets for development of drugs for treatment of intractable pain resistant to currently available analgesics.

Gasotransmitters, poisons, and antimicrobials: it's a gas, gas, gas!

We review recent examples of the burgeoning literature on three gases that have major impacts in biology and microbiology. NO, CO and H2S are now co-classified as endogenous gasotransmitters with profound effects on mammalian physiology and, potentially, major implications in therapeutic applications. All are well known to be toxic yet, at tiny concentrations in human and cell biology, play key signalling and regulatory functions. All may also be endogenously generated in microbes. NO and H2S share the property of being biochemically detoxified, yet are beneficial in resisting the bactericidal properties of antibiotics. The mechanism underlying this protection is currently under debate. CO, in contrast, is not readily removed; mounting evidence shows that CO, and especially organic donor compounds that release the gas in biological environments, are themselves effective, novel antimicrobial agents.

L-Cysteine ethyl ester reverses the deleterious effects of morphine on, arterial blood-gas chemistry in tracheotomized rats

This study determined whether the membrane-permeable ventilatory stimulant, L-cysteine ethylester (L-CYSee), reversed the deleterious actions of morphine on arterial blood-gas chemistry in isoflurane-anesthetized rats. Morphine (2 mg/kg, i.v.) elicited sustained decreases in arterial blood pH, pO₂ and sO₂, and increases in pCO₂ (all responses indicative of hypoventilation) and alveolar-arterial gradient (indicative of ventilation-perfusion mismatch). Injections of L-CYSee (100 μmol/kg, i.v.) reversed the effects of morphine in tracheotomized rats but were minimally active in non-tracheotomized rats. L-cysteine or L-serine ethylester (100 μmol/kg, i.v.) were without effect. It is evident that L-CYSee can reverse the negative effects of morphine on arterial blood-gas chemistry and alveolar-arterial gradient but that this positive activity is negated by increases in upper-airway resistance. Since L-cysteine and L-serine ethylester were ineffective, it is evident that cell penetrability and the sulfur moiety of L-CYSee are essential for activity. Due to its ready penetrability into the lungs, chest wall muscle and brain, the effects of L-CYSee on morphine-induced changes in arterial blood-gas chemistry are likely to involve both central and peripheral sites of action.

Hydrogen sulfide deficiency and diabetic renal remodeling: role of matrix metalloproteinase-9

Matrix metalloproteinase-9 (MMP-9) causes adverse remodeling, whereas hydrogen sulfide (H2S) rescues organs in vascular diseases. The involvement of MMP-9 and H2S in diabetic renovascular remodeling is, however, not well characterized. We determined whether MMP-9 regulates H2S generation and whether H2S modulates connexin through N-methyl-d-aspartate receptor (NMDA-R)-mediated pathway in the diabetic kidney. Wild-type (WT, C57BL/6J), diabetic (Akita, C57BL/6J-Ins2(Akita)), MMP-9(-/-) (M9KO), double knockout (DKO) of Akita/MMP-9(-/-) mice and in vitro cell culture were used in our study. Hyperglycemic Akita mice exhibited increased level of MMP-9 and decreased production of H2S. H2S-synthesizing enzymes cystathionine-β-synthase and cystathionine-γ-lyase were also diminished. In addition, increased expressions of NMDA-R1 and connexin-40 and -43 were observed in diabetic kidney. As expected, MMP-9 mRNA was not detected in M9KO kidneys. However, very thin protein expression and activity were detected. No other changes were noticed in M9KO kidney. In DKO mice, all the above molecules showed a trend toward baseline despite hyperglycemia. In vitro, glomerular endothelial cells treated with high glucose showed induction of MMP-9, attenuated H2S production, NMDA-R1 induction, and dysregulated conexin-40 and -43 expressions. Silencing MMP-9 by siRNA or inhibition of NMDA-R1 by MK801 or H2S treatment preserved connexin-40 and -43. We conclude that in diabetic renovascular remodeling MMP-9 plays a major role and that H2S has therapeutic potential to prevent adverse diabetic renal remodeling.

Chemical aspects of hydrogen sulfide measurements in physiological samples

BACKGROUND

Owing to recent discoveries of many hydrogen sulfide-mediated physiological processes, sulfide biology is in the focus of scientific research. However, the promiscuous chemical properties of sulfide pose complications for biological studies, which led to accumulation of controversial observations in the literature.

SCOPE OF REVIEW

We intend to provide an overview of fundamental thermodynamic and kinetic features of sulfide redox- and coordination-chemical reactions and protonation equilibria in relation to its biological functions. In light of these chemical properties we review the strengths and limitations of the most commonly used sulfide detection methods and recently developed fluorescent probes. We also give a personal perspective on blood and tissue sulfide measurements based on proposed biomolecule-sulfide interactions and point out important chemical aspects of handling sulfide reagent solutions.

MAJOR CONCLUSIONS

The diverse chemistries of sulfide detection methods resulted in orders of magnitude differences in measured physiological sulfide levels. Investigations that were aimed to dissect the underlying molecular reasons responsible for these controversies made the important recognition that there are large sulfide reserves in biological systems. These sulfide pools are tightly regulated in a dynamic manner and they are likely to play a major role in regulation of endogenous-sulfide-mediated biological functions and avoiding toxic side effects.

GENERAL SIGNIFICANCE

Working with sulfide is challenging, because it requires considerable amounts of chemical knowledge to adequately handle reagent sulfide solutions and interpret biological observations. Therefore, we propose that a rigorous chemical approach could aid the reconciliation of the increasing number of controversies in sulfide biology. This article is part of a Special Issue entitled Current methods to study reactive oxygen species - pros and cons and biophysics of membrane proteins. Guest Editor: Christine Winterbourn.

The transcriptomic response of mixed neuron-glial cell cultures to 1,25-dihydroxyvitamin d3 includes genes limiting the progression of neurodegenerative diseases

Seasonal or chronic vitamin D deficiency and/or insufficiency is highly prevalent in the human population. Receptors for 1,25-dihydroxyvitamin D3, the hormonal metabolite of vitamin D, are found throughout the brain. To provide further information on the role of this hormone on brain function, we analyzed the transcriptomic profiles of mixed neuron-glial cell cultures in response to 1,25-dihydroxyvitamin D3. 1,25-dihydroxyvitamin D3 treatment increases the mRNA levels of 27 genes by at least 1.9 fold. Among them, 17 genes were related to neurodegenerative and psychiatric diseases, or brain morphogenesis. Notably, 10 of these genes encode proteins potentially limiting the progression of Alzheimer's disease. These data provide support for a role of 1,25-dihydroxyvitamin D3 in brain disease prevention. The possible consequences of circannual or chronic vitamin D insufficiencies on a tissue with a low regenerative potential such as the brain should be considered.

cGMP-dependent protein kinase contributes to hydrogen sulfide-stimulated vasorelaxation

A growing body of evidence suggests that hydrogen sulfide (H₂S) is a signaling molecule in mammalian cells. In the cardiovascular system, H₂S enhances vasodilation and angiogenesis. H₂S-induced vasodilation is hypothesized to occur through ATP-sensitive potassium channels (K_ATP); however, we recently demonstrated that it also increases cGMP levels in tissues. Herein, we studied the involvement of cGMP-dependent protein kinase-I in H₂S-induced vasorelaxation. The effect of H₂S on vessel tone was studied in phenylephrine-contracted aortic rings with or without endothelium. cGMP levels were determined in cultured cells or isolated vessel by enzyme immunoassay. Pretreatment of aortic rings with sildenafil attenuated NaHS-induced relaxation, confirming previous findings that H₂S is a phosphodiesterase inhibitor. In addition, vascular tissue levels of cGMP in cystathionine gamma lyase knockouts were lower than those in wild-type control mice. Treatment of aortic rings with NaHS, a fast releasing H₂S donor, enhanced phosphorylation of vasodilator-stimulated phosphoprotein in a time-dependent manner, suggesting that cGMP-dependent protein kinase (PKG) is activated after exposure to H₂S. Incubation of aortic rings with a PKG-I inhibitor (DT-2) attenuated NaHS-stimulated relaxation. Interestingly, vasodilatory responses to a slowly releasing H₂S donor (GYY 4137) were unaffected by DT-2, suggesting that this donor dilates mouse aorta through PKG-independent pathways. Dilatory responses to NaHS and L-cysteine (a substrate for H₂S production) were reduced in vessels of PKG-I knockout mice (PKG-I−/−). Moreover, glibenclamide inhibited NaHS-induced vasorelaxation in vessels from wild-type animals, but not PKG-I−/−, suggesting that there is a cross-talk between K_ATP and PKG. Our results confirm the role of cGMP in the vascular responses to NaHS and demonstrate that genetic deletion of PKG-I attenuates NaHS and L-cysteine-stimulated vasodilation.

Dietary requirements of "nutritionally non-essential amino acids" by animals and humans

Amino acids are necessary for the survival, growth, development, reproduction and health of all organisms. They were traditionally classified as nutritionally essential or non-essential for mammals, birds and fish based on nitrogen balance or growth. It was assumed that all "non-essential amino acids (NEAA)" were synthesized sufficiently in the body to meet the needs for maximal growth and health. However, there has been no compelling experimental evidence to support this assumption over the past century. NEAA (e.g., glutamine, glutamate, proline, glycine and arginine) play important roles in regulating gene expression, cell signaling, antioxidative responses, neurotransmission, and immunity. Additionally, glutamate, glutamine and aspartate are major metabolic fuels for the small intestine to maintain its digestive function and protect its mucosal integrity. Therefore, based on new research findings, NEAA should be taken into consideration in revising the classical "ideal protein" concept and formulating balanced diets to improve protein accretion, food efficiency, and health in animals and humans.

Hydrogen sulfide as an endogenous modulator in mitochondria and mitochondria dysfunction

Hydrogen sulfide (H₂S) has historically been considered to be a toxic gas, an environmental and occupational hazard. However, with the discovery of its presence and enzymatic production through precursors of L-cysteine and homocysteine in mammalian tissues, H₂S has recently received much interest as a physiological signaling molecule. H₂S is a gaseous messenger molecule that has been implicated in various physiological and pathological processes in mammals, including vascular relaxation, angiogenesis, and the function of ion channels, ischemia/reperfusion (I/R), and heart injury. H₂S is an endogenous neuromodulator and present studies show that physiological concentrations of H₂S enhance NMDA receptor-mediated responses and aid in the induction of hippocampal long-term potentiation. Moreover, in the field of neuronal protection, physiological concentrations of H₂S in mitochondria have many favorable effects on cytoprotection.

Hydrogen sulfide increases nitric oxide production with calcium-dependent activation of endothelial nitric oxide synthase in endothelial cells

Hydrogen sulfide (H(2)S) was recently discovered to be synthesized in mammalian tissues by several different enzymes. Numerous studies have shown that H(2)S has vasodilator and antihypertensive effects in the cardiovascular system. However, intracellular mechanisms of the H(2)S-induced vasodilation and its interactions with other endothelium-derived relaxing factors, such as nitric oxide (NO), remain unclear. We investigated whether H(2)S directly regulates endothelial NO synthase (eNOS) activity and NO production in endothelial cells. NaHS, a H(2)S donor, dose-dependently increased NO production in cultured endothelial cells. This effect was abolished by a calcium chelator (BAPTA-AM), but not by the absence of extracellular calcium. The NaHS-induced NO production was partially blocked by inhibitors of ryanodine receptor (dantrolene) or inositol 1,4,5-triphosphate receptor (xestospongin C). NaHS significantly increased intracellular calcium concentrations, and this effect was attenuated by dantrolene or xestospongin C. NaHS induced phosphorylation of eNOS at the activating phosphoserine residue 1179. The NaHS-induced eNOS phosphorylation and NO production were not affected by a PI3K/Akt inhibitor (wortmannin). The data of this study suggest that H(2)S directly acts on endothelial cells to induce eNOS activation and NO production by releasing calcium from the intracellular store in endoplasmic reticulum, which may explain one of mechanisms of its vasodilator function.

Hydrogen sulfide-induced mechanical hyperalgesia and allodynia require activation of both Cav3.2 and TRPA1 channels in mice

BACKGROUND AND PURPOSE

Hydrogen sulfide, a gasotransmitter, facilitates somatic pain signals via activation of Ca(v)3.2 T-type calcium channels in rats. Given evidence for the activation of transient receptor potential ankyrin-1 (TRPA1) channels by H(2)S, we asked whether TRPA1 channels, in addition to Ca(v)3.2 channels, contribute to the H(2)S-induced mechanical hyperalgesia and allodynia in mice.

EXPERIMENTAL APPROACH

Mechanical hyperalgesia and allodynia were evaluated by the von Frey test in mice. Ca(v)3.2 or TRPA1 channels in the sensory neurons were silenced by repeated intrathecal administration of antisense oligodeoxynucleotides in mice.

KEY RESULTS

Intraplantar administration of NaHS evoked hyperalgesia and allodynia in mice, an effect attenuated or abolished by NNC 55-0396 or mibefradil, T-type calcium channel blockers, and by ascorbic acid or zinc chloride, known to selectively inhibit Ca(v)3.2 channels, out of the three isoforms of T-type calcium channels. Silencing of Ca(v)3.2 channels in the sensory neurons also prevented the NaHS-induced hyperalgesia and allodynia in mice. The NaHS-induced hyperalgesia and allodynia in mice were significantly suppressed by AP18, a TRPA1 channel blocker, and by silencing of TRPA1 channels in the sensory neurons.

CONCLUSIONS AND IMPLICATIONS

Mechanical hyperalgesia and allodynia induced by NaHS/H(2)S required activation of both Ca(v)3.2 and TRPA1 channels in mice.

Biology and therapeutic potential of hydrogen sulfide and hydrogen sulfide-releasing chimeras

Hydrogen sulfide, H2S, is a colorless gas with a strong odor that until recently was only considered to be a toxic environmental pollutant with little or no physiological significance. However, the past few years have demonstrated its role in many biological systems and it is becoming increasingly clear that H2S is likely to join nitric oxide (NO) and carbon monoxide (CO) as a major player in mammalian biology. In this review, we have provided an overview of the chemistry and biology of H2S and have summarized the chemistry and biological activity of some natural and synthetic H2S-donating compounds. The naturally occurring compounds discussed include, garlic, sulforaphane, erucin, and iberin. The synthetic H2S donors reviewed include, GYY4137; cysteine analogs; S-propyl cysteine, S-allyl cysteine, S-propargyl cysteine, and N-acetyl cysteine. Dithiolethione and its NSAID and other chimeras such as, L-DOPA, sildenafil, aspirin, diclofenac, naproxen, ibuprofen, indomethacin, and mesalamine have also been reviewed in detail. The newly reported NOSH-aspirin that releases both NO and H2S has also been discussed.

TRPA1 has a key role in the somatic pro-nociceptive actions of hydrogen sulfide

Hydrogen sulfide (H₂S), which is produced endogenously from L-cysteine, is an irritant with pro-nociceptive actions. We have used measurements of intracellular calcium concentration, electrophysiology and behavioral measurements to show that the somatic pronociceptive actions of H₂S require TRPA1. A H₂S donor, NaHS, activated TRPA1 expressed in CHO cells and stimulated DRG neurons isolated from Trpa1^+/+ but not Trpa1^−/− mice. TRPA1 activation by NaHS was pH dependent with increased activity at acidic pH. The midpoint of the relationship between NaHS EC₅₀ values and external pH was pH 7.21, close to the expected dissociation constant for H₂S (pK_a 7.04). NaHS evoked single channel currents in inside-out and cell-attached membrane patches consistent with an intracellular site of action. In behavioral experiments, intraplantar administration of NaHS and L-cysteine evoked mechanical and cold hypersensitivities in Trpa1^+/+ but not in Trpa1^−/− mice. The sensitizing effects of L-cysteine in wild-type mice were inhibited by a cystathionine β-synthase inhibitor, D,L-propargylglycine (PAG), which inhibits H₂S formation. Mechanical hypersensitivity evoked by intraplantar injections of LPS was prevented by PAG and the TRPA1 antagonist AP-18 and was absent in Trpa1^−/− mice, indicating that H₂S mediated stimulation of TRPA1 is necessary for the local pronociceptive effects of LPS. The pro-nociceptive effects of intraplantar NaHS were retained in Trpv1^−/− mice ruling out TRPV1 as a molecular target. In behavioral studies, NaHS mediated sensitization was also inhibited by a T-type calcium channel inhibitor, mibefradil. In contrast to the effects of NaHS on somatic sensitivity, intracolonic NaHS administration evoked similar nociceptive effects in Trpa1^+/+ and Trpa1^−/− mice, suggesting that the visceral pro-nociceptive effects of H₂S are independent of TRPA1. In electrophysiological studies, the depolarizing actions of H₂S on isolated DRG neurons were inhibited by AP-18, but not by mibefradil indicating that the primary excitatory effect of H₂S on DRG neurons is TRPA1 mediated depolarization.

The biology of reactive sulfur species (RSS)

Sulfur is an essential and quantitatively important element for living organisms. Plants contain on average approximately 1 g S kg⁻¹ dry weight (for comparison plants contain approximately 15 g N kg⁻¹ dry weight). Sulfur is a constituent of many organic molecules, for example amino acids such as cysteine and methionine and the small tripeptide glutathione, but sulfur is also essential in the form of Fe-S clusters for the activity of many enzymes, particularly those involved in redox reactions. Sulfur chemistry is therefore important. In particular, sulfur in the form of thiol groups is central to manifold aspects of metabolism. Because thiol groups are oxidized and reduced easily and reversibly, the redox control of cellular metabolism has become an increasing focus of research. In the same way that oxygen and nitrogen have reactive species (ROS and RNS), sulfur too can form reactive molecular species (RSS), for example when a -SH group is oxidized. Indeed, several redox reactions occur via RSS intermediates. Several naturally occurring S-containing molecules are themselves RSS and because they are physiologically active they make up part of the intrinsic plant defence repertoire against herbivore and pathogen attack. Furthermore, RSS can also be used as redox-active pharmacological tools to study cell metabolism. The aim of this review is to familiarize the general reader with some of the chemical concepts, terminology and biology of selected RSS.

Therapeutic applications of organosulfur compounds as novel hydrogen sulfide donors and/or mediators

Hydrogen sulfide, once considered as toxic gas, is now recognized as an important biological mediator. The deficiency of hydrogen sulfide could lead to various pathological changes, such as arterial and pulmonary hypertension, Alzheimer's disease, gastric mucosal injury and liver cirrhosis. However, excessive production of hydrogen sulfide, by using inorganic hydrogen sulfide donors such as NaHS, may contribute to the pathogenesis of inflammatory diseases, septic shock, cerebral stroke and mental retardation in patients with Down syndrome. Therefore, an increasing interest in organic molecules that are capable of regulating the formation of hydrogen sulfide has extended in recent years. Allium vegetables are one natural source of organic sulfur-containing compounds and have been widely investigated regarding their therapeutic applications, and it has been proven that the ingredients of garlic, such as diallyl disulfide, diallyl trisulfide and S-ally cysteine act as hydrogen sulfide donors or mediators in pharmaceutical studies. In addition, S-propargyl cysteine (ZYZ-802) and S-propyl cysteine, two synthetic cysteine analogs, have been examined and could be used to treat ischemic heart disease via modulation of the hydrogen sulfide pathway. In addition, drugs containing hydrogen sulfide-releasing moieties have been synthesized and widely reported in recent years, such as S-nonsteroidal anti-inflammatory drugs and the derivative of Lawesson's reagents, which exhibit varied biological effects in experiments. As cystathionine β-synthase and cystathionine γ-lyase are the enzymes that are able to catalyze the production of endogenous hydrogen sulfide from cysteine, their inhibitors, such as dl-propylargylglycine and β-cyanoalanine, have been frequently used in studies on the biological mechanism of hydrogen sulfide. All these hydrogen sulfide donors, mediators and inhibitors have provided useful tools in the research of a variety of biological effects and are promising drug candidates of hydrogen sulfide.

Hydrogen sulfide in cell survival: a double-edged sword

Hydrogen sulfide (H(2)S) has been found to play an important role as a signal molecule in regulating cell survival. It appears paradoxical that, on one side, H(2)S acts as a physiological intercellular messenger to stimulate cell growth, and on the other side, it may display cytotoxic activity. This article summarizes the current body of evidence demonstrating the cytoprotective versus cytotoxic effects of H(2)S in mammalian cells and describes the janus-faced properties of this important gasotransmitter. This article will also provide a brief description of the current signaling mechanisms that have been demonstrated to be responsible for these different actions. The pharmacologic regulation of H(2)S production and the potential clinical significance of H(2)S are highlighted.

Hydrogen sulfide: role in ion channel and transporter modulation in the eye

Hydrogen sulfide (H₂S), a colorless gas with a characteristic smell of rotten eggs, has been portrayed for decades as a toxic environmental pollutant. Since evidence of its basal production in mammalian tissues a decade ago, H₂S has attracted substantial interest as a potential inorganic gaseous mediator with biological importance in cellular functions. Current research suggests that, next to its counterparts nitric oxide and carbon monoxide, H₂S is an important multifunctional signaling molecule with pivotal regulatory roles in various physiological and pathophysiological processes as diverse as learning and memory, modulation of synaptic activities, cell survival, inflammation, and maintenance of vascular tone in the central nervous and cardiovascular systems. In contrast, there are few reports of a regulatory role of H₂S in the eye. Accumulating reports on the pharmacological role of H₂S in ocular tissues indicate the existence of a functional trans-sulfuration pathway and a potential physiological role for H₂S as a gaseous neuromodulator in the eye. Thus, understanding the role of H₂S in vision-related processes is imperative to our expanding knowledge of this molecule as a gaseous mediator in ocular tissues. This review aims to provide a comprehensive and current understanding of the potential role of H₂S as a signaling molecule in the eye. This objective is achieved by discussing the involvement of H₂S in the regulation of (1) ion channels such as calcium (L-type, T-type, and intracellular stores), potassium (K_ATP and small conductance channels) and chloride channels, (2) glutamate transporters such as EAAT1/GLAST and the L-cystine/glutamate antiporter. The role of H₂S as an important mediator in cellular functions and physiological processes that are triggered by its interaction with ion channels/transporters in the eye will also be discussed.

Sulphide quinone reductase contributes to hydrogen sulphide metabolism in murine peripheral tissues but not in the CNS

BACKGROUND AND PURPOSE

Hydrogen sulphide (H(2) S) is gaining acceptance as a gaseous signal molecule. However, mechanisms regarding signal termination are not understood. We used stigmatellin and antimycin A, inhibitors of sulphide quinone reductase (SQR), to test the hypothesis that the catabolism of H(2) S involves SQR.

EXPERIMENTAL APPROACH

H(2) S production and consumption were determined in living and intact mouse brain, liver and colonic muscularis externa using gas chromatography and HPLC. Expressions of SQR, ethylmalonic encephalopathy 1 (Ethe1) and thiosulphate transferase (TST; rhodanese) were determined by RT-PCR and immunohistochemistry.

KEY RESULTS

In the colonic muscularis externa, H(2) (35) S was catabolized to [(35) S]-thiosulphate and [(35) S]-sulphate, and stigmatellin reduced both the consumption of H(2) (35) S and formation of [(35) S]-thiosulphate. Stigmatellin also enhanced H(2) S release by the colonic muscularis externa. In the brain, catabolism of H(2) (35) S to [(35) S]-thiosulphate and [(35) S]-sulphate, which was stigmatellin-insensitive, partially accounted for H(2) (35) S consumption, while the remainder was captured as unidentified (35) S that was probably bound to proteins. Levels of mRNA encoding SQR were higher in the colonic muscularis externa and the liver than in the brain.

CONCLUSIONS AND IMPLICATIONS

These data support the concept that termination of endogenous H(2) S signalling in the colonic muscularis externa occurs via catabolism to thiosulphate and sulphate partially via a mechanism involving SQR. In the brain, it appears that H(2) S signal termination occurs partially through protein sequestration and partially through catabolism not involving SQR. As H(2) S has beneficial effects in animal models of human disease, we suggest that selective inhibition of SQR is an attractive target for pharmaceutical development.

Physiological implications of hydrogen sulfide: a whiff exploration that blossomed

The important life-supporting role of hydrogen sulfide (H(2)S) has evolved from bacteria to plants, invertebrates, vertebrates, and finally to mammals. Over the centuries, however, H(2)S had only been known for its toxicity and environmental hazard. Physiological importance of H(2)S has been appreciated for about a decade. It started by the discovery of endogenous H(2)S production in mammalian cells and gained momentum by typifying this gasotransmitter with a variety of physiological functions. The H(2)S-catalyzing enzymes are differentially expressed in cardiovascular, neuronal, immune, renal, respiratory, gastrointestinal, reproductive, liver, and endocrine systems and affect the functions of these systems through the production of H(2)S. The physiological functions of H(2)S are mediated by different molecular targets, such as different ion channels and signaling proteins. Alternations of H(2)S metabolism lead to an array of pathological disturbances in the form of hypertension, atherosclerosis, heart failure, diabetes, cirrhosis, inflammation, sepsis, neurodegenerative disease, erectile dysfunction, and asthma, to name a few. Many new technologies have been developed to detect endogenous H(2)S production, and novel H(2)S-delivery compounds have been invented to aid therapeutic intervention of diseases related to abnormal H(2)S metabolism. While acknowledging the challenges ahead, research on H(2)S physiology and medicine is entering an exponential exploration era.

A practical look at the chemistry and biology of hydrogen sulfide

SIGNIFICANCE

Hydrogen sulfide (H(2)S) is garnering increasing interest as a biologically relevant signaling molecule. The effects of H(2)S have now been observed in virtually every organ system and numerous physiological processes.

RECENT ADVANCES

These studies have not only opened a new field of "gasotransmitter" biology, they have also led to the development of synthetic H(2)S "donating" compounds with the potential to be parlayed into a variety of therapeutic applications.

CRITICAL ISSUES

Often lost in the exuberance of this new field is a critical examination or understanding of practical aspects of H(2)S chemistry and biology. This is especially notable in the areas of handling and measuring H(2)S, evaluating biosynthetic and metabolic pathways, and separating physiological from pharmacological responses.

FUTURE DIRECTIONS

This brief review describes some of the pitfalls in H(2)S chemistry and biology that can lead or have already led to misleading or erroneous conclusions. The intent is to allow individuals entering or already in this burgeoning field to critically analyze the literature and to assist them in the design of future experiments.

Role of cystathionine γ-lyase/hydrogen sulfide pathway in cardiovascular disease: a novel therapeutic strategy?

SIGNIFICANCE

Hydrogen sulfide (H(2)S) has traditionally been considered a toxic environmental pollutant. In the late 1990s, the presumed solely harmful role of H(2)S has been challenged because H(2)S may also be involved in the maintenance and preservation of cardiovascular homeostasis.

RECENT ADVANCES

The production of endogenous H(2)S has been attributed to three key enzymes, cystathionine γ-lyase (CSE), cystathionine β-synthase, and 3-mercaptopyruvate sulfurtransferase. The recognition of H(2)S as the third gaseous signaling molecule has stimulated research on a multitude of pathophysiologic events in the cardiovascular system. In particular, important roles in cardiovascular disorder processes are ascribed to the CSE/H(2)S pathway, such as atherosclerosis, myocardial infarction, hypertension, and shock.

CRITICAL ISSUES

Many biological activities and molecular mechanisms of H(2)S in the cardiovascular system have been demonstrated in studies using different tools, such as the genetic overexpression of CSE, the direct administration of H(2)S donors, or the use of H(2)S-releasing pro-drugs. Unfortunately, the role of the CSE/H(2)S pathway in cardiovascular disease remains controversial in numerous areas, and many questions regarding the gaseous molecule still remain unanswered.

FUTURE DIRECTIONS

Advances in basic research indicate that the CSE/H(2)S pathway may provide potential therapeutic targets for treating cardiovascular disorders. But the molecular targets of H(2)S still need to be identified.