Interactions of multiple gas-transducing systems: hallmarks and uncertainties of CO, NO, and H2S gas biology

CO metabolism, sensing, and signaling

CO is a colorless and odorless gas produced by the incomplete combustion of hydrocarbons, both of natural and anthropogenic origin. Several microorganisms, including aerobic and anaerobic bacteria and anaerobic archaea, use exogenous CO as a source of carbon and energy for growth. On the other hand, eukaryotic organisms use endogenous CO, produced during heme degradation, as a neurotransmitter and as a signal molecule. CO sensors act as signal transducers by coupling a "regulatory" heme-binding domain to a "functional" signal transmitter. Although high CO concentrations inhibit generally heme-protein actions, low CO levels can influence several signaling pathways, including those regulated by soluble guanylate cyclase and/or mitogen-activated protein kinases. This review summarizes recent insights into CO metabolism, sensing, and signaling.

The vertebrate homolog of sulfide-quinone reductase is expressed in mitochondria of neuronal tissues

Hydrogen sulfide (H₂S) can be consumed by both invertebrates and vertebrates as an inorganic substrate. The pathway metabolizing H₂S probably involves three mitochondrial enzymes, one of which is sulfide-quinone oxidoreductase (SQR), known as sulfide-quinone reductase-like protein (SQRDL) in vertebrates. Evidence from fission yeast suggests that SQR might have a role in regulating sulfide levels in the cell. Regulation might be essential for H₂S to act as a gaseous transmitter (gasotransmitter). The brain is an organ with high activity of gasotransmitters, like nitric oxide (NO) and H₂S, which are known to affect synaptic transmission. In this study, we provide evidence that SQRDL is expressed in the mammalian brain. Real-time polymerase chain reaction (PCR) showed an increase in the number of Sqrdl transcripts in the brain with increasing age. Cellular fractionation and subsequent analysis by Western blotting indicated that the protein is located in mitochondria, which is the site of sulfide consumption in the cell. With an immunohistochemical approach, we demonstrated that the SQRDL protein is expressed in neurons, oligodendrocytes, and endothelial cells. Taken together, our data suggest that brain tissue harbors the machinery required for local regulation of sulfide levels.

Mitochondrial dysfunction and biogenesis: do ICU patients die from mitochondrial failure?

Mitochondrial functions include production of energy, activation of programmed cell death, and a number of cell specific tasks, e.g., cell signaling, control of Ca²⁺ metabolism, and synthesis of a number of important biomolecules. As proper mitochondrial function is critical for normal performance and survival of cells, mitochondrial dysfunction often leads to pathological conditions resulting in various human diseases. Recently mitochondrial dysfunction has been linked to multiple organ failure (MOF) often leading to the death of critical care patients. However, there are two main reasons why this insight did not generate an adequate resonance in clinical settings. First, most data regarding mitochondrial dysfunction in organs susceptible to failure in critical care diseases (liver, kidney, heart, lung, intestine, brain) were collected using animal models. Second, there is no clear therapeutic strategy how acquired mitochondrial dysfunction can be improved. Only the benefit of such therapies will confirm the critical role of mitochondrial dysfunction in clinical settings. Here we summarized data on mitochondrial dysfunction obtained in diverse experimental systems, which are related to conditions seen in intensive care unit (ICU) patients. Particular attention is given to mechanisms that cause cell death and organ dysfunction and to prospective therapeutic strategies, directed to recover mitochondrial function. Collectively the data discussed in this review suggest that appropriate diagnosis and specific treatment of mitochondrial dysfunction in ICU patients may significantly improve the clinical outcome.

Reversible heme-dependent regulation of human cystathionine β-synthase by a flavoprotein oxidoreductase

Human CBS is a PLP-dependent enzyme that clears homocysteine, gates the flow of sulfur into glutathione, and contributes to the biogenesis of H(2)S. The presence of a heme cofactor in CBS is enigmatic, and its conversion from the ferric- to ferrous-CO state inhibits enzyme activity. The low heme redox potential (-350 mV) has raised questions about the feasibility of the ferrous-CO state forming under physiological conditions. Herein, we provide the first evidence of reversible inhibition of CBS by CO in the presence of a human flavoprotein and NADPH. These data provide a mechanism for cross talk between two gas-signaling systems, CO and H(2)S, via heme-mediated allosteric regulation of CBS.

Acute CO2-independent vasodilatation of penetrating and pre-capillary arterioles in mouse cerebral parenchyma upon hypoxia revealed by a thinned-skull window method

AIM

Investigating spatio-temporal relationship between regional metabolic changes and microvascular responses in hypoxic brain is critical for unravelling local O(2) -sensing mechanisms. However, no reliable method to examine the relationship has been available because of inherent disadvantages associated with use of a conventional cranial window preparation. We aimed to devise a method to solve the problem.

METHODS

Anaesthetized mice were equipped with either a conventional cranial window with craniotomy or a thinned-skull preparation. Mice were mechanically ventilated to avoid hypercapnia and exposed to systemic isobaric hypoxia for 30 min. Using two-photon laser scanning microscopy, nicotinamide adenine dinucleotide, reduced form (NADH) autofluorescence and diameter changes in penetrating and pre-capillary arterioles within the parenchyma were visualized to examine their temporal alterations.

RESULTS

With the conventional cranial window preparation, marked vertical displacement of the tissue occurred through oedema within 30 s after inducing hypoxia. With a thinned-skull preparation, however, such hypoxia-induced displacement was diminished, enabling us to examine acute spatio-temporal changes in diameters of penetrating and pre-capillary arterioles and NADH autofluorescence. Vasodilatation of these microvessels was evoked within 1 min after hypoxia, and sustained during the entire observation period despite the absence of hypercapnia. This event coincided with parenchymal NADH elevation, but the onset and peak dilatory responses of the penetrating arterioles preceded the local metabolic response of the parenchyma.

CONCLUSION

Observation of hypoxia-exposed brain by the thinned-skull preparation combined with two-photon intra-vital microscopy revealed rapid vasodilatory responses in penetrating arterioles preceding parenchymal NADH elevation, suggesting the presence of acute hypoxia-sensing mechanisms involving specific segments of cortical arterioles within the neurovascular unit.

Hydrogen sulfide and hemeproteins: knowledge and mysteries

Historically, hydrogen sulfide (H(2)S) has been regarded as a poisonous gas, with a wide spectrum of toxic effects. However, like ·NO and CO, H(2)S is now referred to as a signaling gas involved in numerous physiological processes. The list of reports highlighting the physiological effects of H(2)S is rapidly expanding and several drug candidates are now being developed. As with ·NO and CO, not a single H(2)S target responsible for all the biological effects has been found till now. Nevertheless, it has been suggested that H(2)S can bind to hemeproteins, inducing different responses that can mediate its effects. For instance, the interaction of H(2)S with cytochrome c oxidase has been associated with the activation of the ATP-sensitive potassium channels, regulating muscle relaxation. Inhibition of cytochrome c oxidase by H(2)S has also been related to inducing a hibernation-like state. Although H(2)S might induce these effects by interacting with hemeproteins, the mechanisms underlying these interactions are obscure. Therefore, in this review we discuss the current state of knowledge about the interaction of H(2)S with vertebrate and invertebrate hemeproteins and postulate a generalized mechanism. Our goal is to stimulate further research aimed at evaluating plausible mechanisms that explain H(2)S reactivity with hemeproteins.

The therapeutic potential of hydrogen sulfide: separating hype from hope

Hydrogen sulfide (H(2)S) has become the hot new signaling molecule that seemingly affects all organ systems and biological processes in which it has been investigated. It has also been shown to have both proinflammatory and anti-inflammatory actions and proapoptotic and anti-apoptotic effects and has even been reported to induce a hypometabolic state (suspended animation) in a few vertebrates. The exuberance over potential clinical applications of natural and synthetic H(2)S-"donating" compounds is understandable and a number of these function-targeted drugs have been developed and show clinical promise. However, the concentration of H(2)S in tissues and blood, as well as the intrinsic factors that affect these levels, has not been resolved, and it is imperative to address these points to distinguish between the physiological, pharmacological, and toxicological effects of this molecule. This review will provide an overview of H(2)S metabolism, a summary of many of its reported "physiological" actions, and it will discuss the recent development of a number of H(2)S-donating drugs that show clinical potential. It will also examine some of the misconceptions of H(2)S chemistry that have appeared in the literature and attempt to realign the definition of "physiological" H(2)S concentrations upon which much of this exuberance has been established.

Hydrogen sulfide and erectile function: a novel therapeutic target

Hydrogen sulfide (H(2)S) is a gaseous transmitter involved in the control of vascular homeostasis. H(2)S is formed endogenously from L-cysteine or L-methionine by two enzymes, cystathionine beta-synthase (CBS) and cystathionine gamma-lyase (CSE), and normally circulates in blood. Studies from the past few years have demonstrated the involvement of H(2)S in erectile mechanisms in animal and human tissues. Exogenous H(2)S relaxes human and animal tissues in vitro and increases intracavernous pressure in experimental animal models. Electrical field stimulation studies on animal and human tissues have demonstrated that endogenous H(2)S is involved in the physiological control of penile tone. In humans, both CBS and CSE are widely expressed on trabecular muscle, implying that the smooth muscle component is the major source of H(2)S. Thus, the L-cysteine-H(2)S pathway may represent a promising target for development of new therapeutics for erectile dysfunction.

The role of heme oxygenase and carbon monoxide in inflammatory bowel disease

Inflammatory bowel disease (IBD), including ulcerative colitis (UC) and Crohn's disease, is a chronic and recurrent inflammatory disorder of the intestinal tract. Since the precise pathogenesis of IBD remains unclear, it is important to investigate the pathogenesis of IBD and to evaluate new anti-inflammatory strategies. Recent evidence suggests that heme oxygenase-1 (HO-1) plays a critical protective role during the development of intestinal inflammation. In fact, it has been demonstrated that the activation of HO-1 may act as an endogenous defensive mechanism to reduce inflammation and tissue injury in various animal intestinal injury models induced by ischemia-reperfusion, indomethacin, lipopolysaccharide-associated sepsis, trinitrobenzene sulfonic acid or dextran sulfate sodium. In addition, carbon monoxide (CO) derived from HO-1 has been shown to be involved in the regulation of intestinal inflammation. Furthermore, administration of a low concentration of exogenous CO has a protective effect against intestinal inflammation. These data suggest that HO-1 and CO may be novel therapeutic molecules for patients with gastrointestinal inflammatory diseases. In this review, we present what is currently known regarding the role of HO-1 and CO in intestinal inflammation.

Administration of hydrogen sulfide via extracorporeal membrane lung ventilation in sheep with partial cardiopulmonary bypass perfusion: a proof of concept study on metabolic and vasomotor effects

Introduction

Although inhalation of 80 parts per million (ppm) of hydrogen sulfide (H₂S) reduces metabolism in mice, doses higher than 200 ppm of H₂S were required to depress metabolism in rats. We therefore hypothesized that higher concentrations of H₂S are required to reduce metabolism in larger mammals and humans. To avoid the potential pulmonary toxicity of H₂S inhalation at high concentrations, we investigated whether administering H₂S via ventilation of an extracorporeal membrane lung (ECML) would provide means to manipulate the metabolic rate in sheep.

Methods

A partial venoarterial cardiopulmonary bypass was established in anesthetized, ventilated (fraction of inspired oxygen = 0.5) sheep. The ECML was alternately ventilated with air or air containing 100, 200, or 300 ppm H₂S for intervals of 1 hour. Metabolic rate was estimated on the basis of total CO₂ production (V˙CO2) and O₂ consumption (V˙O2). Continuous hemodynamic monitoring was performed via indwelling femoral and pulmonary artery catheters.

Results

V˙CO2, V˙O2, and cardiac output ranged within normal physiological limits when the ECML was ventilated with air and did not change after administration of up to 300 ppm H₂S. Administration of 100, 200 and 300 ppm H₂S increased pulmonary vascular resistance by 46, 52 and 141 dyn·s/cm⁵, respectively (all P ≤ 0.05 for air vs. 100, 200 and 300 ppm H₂S, respectively), and mean pulmonary artery pressure by 4 mmHg (P ≤ 0.05), 3 mmHg (n.s.) and 11 mmHg (P ≤ 0.05), respectively, without changing pulmonary capillary wedge pressure or cardiac output. Exposure to 300 ppm H₂S decreased systemic vascular resistance from 1,561 ± 553 to 870 ± 138 dyn·s/cm⁵ (P ≤ 0.05) and mean arterial pressure from 121 ± 15 mmHg to 66 ± 11 mmHg (P ≤ 0.05). In addition, exposure to 300 ppm H₂S impaired arterial oxygenation (P_aO₂ 114 ± 36 mmHg with air vs. 83 ± 23 mmHg with H₂S; P ≤ 0.05).

Conclusions

Administration of up to 300 ppm H₂S via ventilation of an extracorporeal membrane lung does not reduce V˙CO2 and V˙O2, but causes dose-dependent pulmonary vasoconstriction and systemic vasodilation. These results suggest that administration of high concentrations of H₂S in venoarterial cardiopulmonary bypass circulation does not reduce metabolism in anesthetized sheep but confers systemic and pulmonary vasomotor effects.

Carbon monoxide stimulates global protein methylation via its inhibitory action on cystathionine β-synthase

Although carbon monoxide derived from heme oxygenase has been reported to exert diverse biological actions in mammals, macromolecules responsible for its direct reception and functional outcomes of the gas binding remain largely unknown. Based on our previous results in vivo suggesting carbon monoxide serves as an inhibitor of cystathionine β-synthase that rate-limits transsulfuration pathway for generation of hydrogen sulfide, we have herein hypothesized that the gas might serve as a regulator of protein methylation through accelerating turnover of remethylation cycle residing at the upstream of the enzyme. Metabolomic analysis in human monoblastic leukemia U937 cells in culture revealed that application of carbon monoxide-releasing molecules caused increases in methionine and S-adenosylmethionine and a decrease in cystathionine in the cells, suggesting the cystathionine β-synthase inhibition by carbon monoxide. Under these circumstances, the cells exhibited global protein arginine methylation: this event was also reproduced by the cell treatment with hemin, a heme oxygenase-1 inducer. The protein arginine methylation elicited by carbon monoxide was attenuated by knocking down cystathionine β-synthase with its small interfering RNA or by blocking S-adenosylhomocysteine hydrolase with adenosine dialdehyde, suggesting remethylation cycling is necessary to trigger the methylation processing. Furthermore, proteins undergoing the carbon monoxide-induced arginine methylation involved histone H3 proteins, suggesting chromatin modification by the gas. Collectively with our studies in vivo showing its inhibitory action on endogenous hydrogen sulfide production, the current results suggest that not only inhibition of transsulfuration pathway for H₂S generation but also activation of protein methylation accounts for notable biological actions of carbon monoxide via the cystathionine β-synthase inhibition.

Paradoxical ATP elevation in ischemic penumbra revealed by quantitative imaging mass spectrometry

Local responses of energy metabolism during brain ischemia are too heterogeneous to decipher redox distribution between anoxic core and adjacent salvageable regions such as penumbra. Imaging mass spectrometry combined by capillary electrophoresis/mass spectrometry providing quantitative metabolomics revealed spatio-temporal changes in adenylates and NADH in a mouse middle-cerebral artery occlusion model. Unlike the core where ATP decreased, the penumbra displayed paradoxical elevation of ATP despite the constrained blood supply. It is noteworthy that the NADH elevation in the ischemic region is clearly demarcated by the ATP-depleting core. Results suggest that metabolism in ischemic penumbra does not respond passively to compromised circulation, but actively compensates energy charges.