Carbon Monoxide (CO), Nitric Oxide, and Hydrogen Sulfide Signaling During Acute CO Poisoning

Major toxic effects of acute carbon monoxide (CO) poisoning result from increases in reactive oxygen species (ROS) and reactive nitrogen species (RNS) producing oxidative stress. The importance of altered nitric oxide (NO) signaling in evoking increases in RNS during CO poisoning has been established. Although there is extensive literature describing NO and hydrogen sulfide (H2S) signaling in different types of cells under normal conditions, how CO poisoning-evoked deregulation of additional NO signaling pathways and H2S signaling pathways could result in cell injury has not been previously considered in detail. The goal of this article was to do this. The approach was to use published data to describe signaling pathways driven by CO bonding to different ferroproteins and then to collate data that describe NO and H2S signaling pathways that could interact with CO signaling pathways and be important during CO poisoning. Arteriolar smooth muscle cells—endothelial cells located in the coronary and some cerebral circulations—were used as a model to illustrate major signaling pathways driven by CO bonding to different ferroproteins. The results were consistent with the concept that multiple deregulated and interacting NO and H2S signaling pathways can be involved in producing cell injury evoked during acute CO poisoning and that these pathways interact with CO signaling pathways.


INTRODUCTION
Recent reviews of the pathogenesis of the toxicity resulting from acute carbon monoxide (CO) poisoning have emphasized increases in reactive oxygen species (ROS) and reactive nitrogen species (RNS) that evoke oxidative stress (Piantadosi, 2008;Akyol et al., 2014;Roderique et al., 2015). Other articles describe roles of ion channels (Peers, 2011), impaired metabolism-blood flow coupling (M-BFC) and alveolar ventilation-blood flow coupling (Coburn, 2020), tissue hypoxia (Darling and Roughton, 1944), increased carboxymyoglobin % saturation ([COMb]) (Sangali and Bidanset, 1990), and increased free heme (Cronje et al., 2004). The goal of the present article is to consider using previously published data if nitric oxide (NO) and hydrogen sulfide (H 2 S) signaling pathways could interact with CO signaling pathways during acute CO poisoning. The importance of altered NO signaling in evoking increases in reactive nitrogen species (RNS) during CO poisoning has been established (Ischiropoulos et al., 1996;Thom et al., 1997). Roderique et al. (2015) emphasize the importance of NO signaling during CO poisoning. However, it has not been previously considered in detail if additional NO signaling could be a factor in cell injuries that occur under this condition. In addition, the possible roles of H 2 S signaling during CO poisoning have not been addressed in previous CO poisoning reviews. There is extensive literature, some of which are cited later in this article, describing normal CO, NO, and H 2 S signaling in different types of cells and the cross-talk signaling that occurs between the three gaso-transmitters. The reader is referred to publications by Treuer and Gonzalez (2015) and Radi (2018) who reviewed some general properties of NO signaling mechanisms and by Li et al. (2011), Paul and Snyder (2014), Kabil et al. (2014), and Giuffre and Vincent (2018) who reviewed the properties of normal H 2 S signaling. An important review has been published of relationships between CO and NO signaling and interactions between the heme oxygenase and nitric oxide synthase systems oriented to normal signaling (Wu and Wang, 2005).
For this analysis, a vascular model was selected to illustrate major signaling pathways driven by CO bonding to different ferroproteins (FPs). Smooth muscle cells (SMCs) with neighboring endothelial cells (ECs) located in arterioles in the coronary and some cerebral circulations were chosen for this model because there is some available information regarding the FPs that reside within these cells (Coburn, 2020). In addition, there is evidence that NO and H 2 S are involved when M-BFC in these circulations is impaired during CO poisoning (Coburn, 2020). With the model, NO formed in ECs is transported into SMCs. This transport may not be explained by simple diffusion. In some small arteries, monomeric hemoglobin (Hb) a expressed in endothelial cells and enriched at the myoendothelial junction can regulate eNOS and NO transport into SMCs (Straub et al., 2012). A major function of the arteriolar SMCs and ECs is to affect M-BFC, also called metabolic vasodilation, which is defined as coupling between O 2 consumption and blood flow. This mechanism promotes PO 2 uniformity within tissues, that is, prevents regional hypoxia, at least partially by controlling capillary recruitment and capillary densities. O 2 sensors located within both arteriolar SMCs and partner ECs sense low PO 2 in tissue areas that have high O 2 uptake/blood flow ratios which evokes SMC relaxations producing vasodilation (Coburn, 2020). M-BFC impairment in these circulations during CO poisoning was considered to be due to the loss of arteriolar SMCs and EC O 2 sensing, unregulated endothelial NO formation, effects on ion channels, and increases in ROS signaling (Coburn, 2020).
As indicated before, the present article is organized around CO bonding to FPs (Caughey, 1970;Traylor and Berzinis, 1980;Coburn, 2021). The reason for this, as stated before, is that this bonding activates or inactivates signaling pathways. This approach also facilitated comparisons of signaling pathways driven by CO bonding to FPs, and NO and H 2 S signaling pathways. Approaching CO poisoning by considering CO bonding to FPs follows the "surprising" non-reactivity of CO in mammalian tissues (Piantadosi, 2008) and that CO bonding to FPs usually occurs in their Fe 2+ or Fe 3+ oxidative states. It is possible that limiting CO signaling pathways to those initiated by CO bonding with FPs is missing some signals operating during CO poisoning. FPs considered in the SMC-EC duet in this study include cytochrome C oxidase (COX), NADPH oxidase (NOX), myoglobin (Mb), endothelial NO synthase (eNOS), catalase and peroxidase, cystathionine ß synthase (CBS), prolyl hydroxylase-hypoxia-induced factor (PHD), heme oxygenase-2 (HO-2), the K + ion channels described below, and EC monomeric Hb a. These FPs include hemoproteins and proteins that require binding of a prosthetic heme for their functions, or like PHD contain non-heme iron. HO-2 has heme-binding sites (Wu and Wang, 2005). CBS activity is dependent on the presence of free hemes (Taoka and Banerjee, 2001;Banerjee, 2017). Heme is involved in CO activation-inactivation mechanisms in K ATP , BK Ca , and Kv channels (Horrigan et al., 2005;Jaggar et al., 2005;Hou et al., 2009;Sahoo et al., 2013;Burton et al., 2016). Evidence that free hemes can increase during CO poisoning is cited later. However, the importance of levels of free hemes in controlling the operation of these CO targets during CO toxicity scenarios is not known. It is possible that CO evokes changes in BK Ca channel activity by a mechanism independent of CO binding to its prosthetic heme (Hou et al., 2009).
Possible FP CO targets involved in evoking cell injury during acute CO poisoning considered in this article included some that were not, to the best of the author's knowledge, described in previous CO poisoning reviews. These include the HO-2 and PHD O 2 sensors, ion channels involved in M-BFC, and EC monomeric Hb a. There is kinetic evidence that HO-2 activity can be by-product-inhibited during CO poisoning (Matsui et al., 2010). In vivo evidence was obtained by showing large decreases in endogenously formed CO during Hb catabolism in dogs after [COHb] was increased via CO inhalation (Coburn et al., 1967). Evidence that the O 2 sensing function of PHD can be inhibited by CO was provided by Goldberg et al. (1988), Huang et al. (1999), Hagen et al. (2003), and Mbenza et al. (2021). Evidence that increases in P CO can alter the ion channel function and O 2 sensors that affect M-BFC has been published (Coburn, 2020). Of the arteriolar SMC-EC CO targets listed before, only mitochondrial and NOX-ROS formations are proven to evoke cell injury during CO poisoning. Mb, Hb, eNOS, K + channels, and catalase are likely important CO targets during CO poisoning scenarios. HO-2, PHD, CBS, and EC monomeric Hb a are possible targets.
The literature search was limited to posttranslational signaling. It did not cover downstream cellular toxicity mechanisms resulting from increased [ROS], [RNS], or hypoxia; CO activation of different kinases and soluble guanylyl cyclase; NO signaling to eicosanoids; or signaling resulting from effects of the three gaso-transmitters on oxidative phosphorylation. The analysis given in this article has not been extended to include the discussion of complex relationships between tissue hypoxia and increased tissue P CO in the pathogenesis of acute CO poisoning-evoked cell injury. signaling could be altered or deregulated during CO poisoning scenarios. Some characteristics of these FPs relevant to CO poisoning are described.
BK Ca , K ATP , Kv 1.5 , and L-type Ca 2+ ion channels: As stated before, there is evidence that these arteriolar SMC ion channels are involved in CO-evoked impairment of M-BFC in coronary and some cerebral circulations (Coburn, 2020). In the cerebral circulation, H 2 S-evoked opening of K ATP and opening or closing BK Ca channels may be involved (Hou et al., 2009;Li et al., 2011;Morikawa et al., 2012;Coburn, 2020). CO inhibitory effects on L-type Ca 2+ channels are indirect, mediated by increased [ROS] (Scragg et al., 2008). CO-evoked effects on other ion channels are not discussed in this article because they are not relevant to what we presently know about the arteriolar SMC-EC model.
Catalase and peroxidases: These enzymes which scavenge ROS are inhibited by CO, a result of CO binding to their prosthetic hemes. CO ligation mechanisms differ with catalases and peroxidases and occur at different iron oxidative states during catalysis (Carlsson et al., 2005). Catalases are likely more important as ROS scavengers because of their rapid reaction rates with H 2 O 2 (Carlsson et al., 2005). Important to the goals of this article, NO also inhibits catalase (Purwar et al., 2011). The ability of these proteins to scavenge ROS and exert some control of the cellular oxidant-antioxidant balance could be impaired during CO poisoning.
CBS: The effects of CO poisoning on H 2 S signaling could be a result of inhibition of CBS, which results in a decreased formation of H 2 S (Puranik et al., 2006;Paul and Snyder, 2015;Banerjee, 2017). The dependency of this reaction on the presence of free heme is described before. H 2 S can scavenge ROS and RNS and is a powerful antioxidant considered to be critical for the intracellular balance between oxidants and antioxidants (Shefa et al., 2018). As described before, H 2 S-evoked effects on K ATP channels are likely to be important in CO poisoning-evoked impairment of M-BFC. CBS is inhibited by NO (Taoka and Banerjee, 2001), a property relevant to NO signaling during CO poisoning.
COX, NOX, and xanthine oxidase: CO bonding to COX and resulting ROS generation within mitochondrial complexes I and III are currently thought to be the major cause of acute CO poisoning-evoked cell injury (Zuckerbraun et al., 2007;Akyol et al., 2014;Almeida et al., 2015;Roderique et al., 2015;Motterlini and Foresti, 2017). Increased P CO can evoke ROS formation via NOX that results in cell injury (Kietzmann and Gorlach, 2005;Manea, 2010;Hara et al., 2017). Xanthine oxidase can be involved in CO poisoning-evoked ROS formation (Thom, 1992).
eNOS: This hemoprotein, which normally catalyzes the formation of NO in ECs, can be inhibited during CO poisoning (Thorup et al., 1999;Hara et al., 2003;Chen and Meyrick, 2004;Choi and Kim, 2021). (This is contrasted with increases in the eNOS activity that occur during exposure to low [CO] (Thorup et al., 1999).) Increased NO formation during CO poisoning (Ischiropoulos et al., 1996;Thom et al., 1997) is likely due to CO displacement of NO bound to FPs (Thorup et al., 1999;Di Meo et al., 2016). Brain toxicity during CO poisoning can be mediated by NO (Meilin et al., 1996). The rapid reaction of NO with ROS results in formation of RNS, including peroxynitrite and other oxidative NO by-products, major causes of cell injury during CO poisoning (Radi, 2018). NO is a free radical that is a strong oxidant that can cause cell injury during CO poisoning. As indicated before, increased or deregulated [NO] can impair coronary and cerebral M-BFC during CO poisoning (Coburn, 2020). Increased [NO] can scavenge H 2 S (Taoka and Banerjee, 2001). NO can inhibit CBS, resulting in decreased H 2 S formation (Tang et al., 2013) which in turn can cause increased NO formation (Mazza et al., 2013;Olas, 2015). Uncoupling of eNOS can produce ROS (Karbach et al., 2014). NO can inhibit HO-2 (Ding et al., 1999;Wu and Wang, 2005). The relationship of EC monomeric Hb a and eNOS formed NO is cited later.
Hemoglobin (Hb): In blood: Increases in [COHb] evoke oxyhemoglobin dissociation curve shifts which decrease mean capillary PO 2 (PcO 2 ) producing tissue hypoxia (Darling and Roughton, 1944). Increased [COHb]-evoked decreases in deoxyHb likely inhibit the Hb-nitrite reductase activity decreasing NO formation and may inhibit Hb NO scavenging (Kim-Shapiro et al., 2006). Endothelial monomeric Hb α: In some resistance arteries, endothelial monomeric ferrous Hb a-evoked increases in NO signaling to SMCs can be increased three fold by CO exposure (Straub et al. (2012). Thus, this mechanism could contribute to increase in [NO] that can occur during CO poisoning.
HO-2: As cited before, there is evidence that the HO-2 activity can be inhibited during CO poisoning. Effects relevant to CO poisoning include 1) that due to inhibition of its function as an O 2 sensor (Kemp, 2005) involved in CO-evoked impairment of M-BFC discussed before; 2) that due to inhibition of its function as a scavenger of free heme (Kumar and Bandyopadhyay, 2005;Chiabrando et al., 2014) following evidence that levels of free heme can increase markedly during severe CO poisoning (Cronje et al., 2004); and 3) that resulting from decreases in the formation of biliverdin/bilirubin which are important antioxidants and may scavenge ROS via biliverdin reductase cycling (Janson and Daiber, 2012). In addition to CO-evoked effects, the HO-2 O 2 sensor can be inhibited by H 2 S (Matsui et al., 2018). As cited before, NO can bind to the heme moiety of HO-2 and inhibit its function (Ding et al., 1999;Wu and Wang, 2005). In piglet cerebral microvessels, glutamatestimulated NOS produced NO that evoked CO formation via HO-2 (Leffler et al., 2005).

DISCUSSION
Using published data, the question is asked if NO and H 2 S signaling could interact with CO signaling pathways and be a factor in CO poisoning-evoked cellular injury. The approach of describing CO poisoning from the viewpoint of CO binding to FPs that exerts control of various signaling pathways illustrates the spectra of different CO targets considered in this article. For this analysis, a vascular model was used to illustrate major signaling pathways driven by CO bonding to different FPs. Other cells contain many of the same FPs. Thus, some of the possible NO and H 2 S signaling pathways and interactions with CO signaling pathways described using this model are projected to be valid for other cell types. As indicated in the introduction, the importance of CO binding to many of the FPs considered in this article as a function of different levels of CO poisoning is not known. In this article, the well-known effects of increased [NO] that occur during CO poisoning, described in pathways xv and xvii, were expanded to include NO-evoked inhibition of CBS-evoked H 2 S formation, impairment of M-BFC, inhibition of O 2 sensing by PHD, inhibition of HO-2 free heme and ROS scavenging, and inhibition of catalase. Several reactions are described that could exert positive feedback effects on NO formation during CO poisoning (pathways xvi, xxiii, and xxv) augmenting Pathway xv evoked increased [NO]. This article also emphasizes possible Cited data indicate interactions between H 2 S, NO, and ROS signaling. These include that both H 2 S and NO can be involved in mediating [ROS] or [RNS]. ROS targets considered in this article include PHD and multiple ion channels involved in M-BFC. Thus, it is suggested that alterations in NO and H 2 S signaling during CO poisoning scenarios may occur in tandem with deregulation of ROS signaling. Findings also suggest that normal NO and H 2 S signaling pathways, like normal endogenous CO signaling, may be deregulated during CO poisoning. This would not just be a result of changes in bioactivities of the gaso-transmitters described before. Precise signaling at various cellular and subcellular targets is likely lost as is well recognized to occur during increased P CO -evoked redox signaling. Whether deregulated NO and H 2 S signaling could contribute to cell injury occurring during different levels of CO poisoning is another issue. An argument supporting this hypothesis is that multiple deregulated and interacting NO and H 2 S signaling pathways are likely to be involved in producing or mediating cell injury evoked during CO poisoning because these pathways can potentially interact with CO signaling pathways that trigger cell injury. Figure 1 and the previous discussion do not consider the relative importance of the different NO and H 2 S signaling pathways as effectors of cell injury during CO poisoning scenarios. It is not possible yet to describe a mechanistic continuum of the different pathways. It seems possible that cell injury resulting from different severities or time durations of CO poisoning involves different segments of a signaling network. However, because of a lack of NO and H 2 S binding and bioactivity information, it is not established that any of the NO pathways described before, other than those shown in pathways xv and xvii, nor any of the H 2 S pathways, operate in the SMC-EC model during CO poisoning. A weakness in the approach used in constructing the pathways shown in the Figure is that data taken from published results were obtained using different types of cells, and none of them include arteriolar SMC-ECs. In addition, the discussion of H 2 S pathways does not consider that metabolic pathways other than CBS can form H 2 S, or effects on H 2 S degradation during CO poisoning. There are also issues related to subcellular locations where H 2 S is formed. It is not clear how multiple downstream H 2 S reactions could influence [H 2 S]-dependent mechanisms described in this article. A major emphasis in this article is on H 2 S functions as an antioxidant and ROS scavenger during oxidative stress and its role in M-BFC. For NO, this article only considers reactions related to CO poisoning, on top of large literature describing multiple functions of NO signaling. However, even considering these caveats, it seems justified to conclude that listing and describing possible NO and H 2 S signaling pathways as described in this study is a step forward toward understanding how signaling of these gaso-transmitters could function during acute CO poisoning scenarios. More research is necessary to better delineate the importance and details of deregulated NO signaling and H 2 S signaling during CO poisoning-evoked cell injury. As stated before, more binding and NO and H 2 S bioactivity data are needed. The importance of specific FP CO targets in generating signaling also depends on their CO binding constants, and a challenge for future research is to determine these constants and the extent of CO bonding to different FPs at different P CO . There are calculated data that suggest that tissue P CO in resting humans could increase 50-to 60-fold as the [COHb] increases from normal to 40% saturation (Coburn, 2017). There also is evidence, so far limited to red skeletal muscle and heart muscle, that extravascular CO binding increases as [COHb] increases (Coburn, 2021).
Whether or not NO and H 2 S signaling could interact with CO signaling during CO poisoning-evoked tissue hypoxia has not been addressed in detail in this article. However, signaling pathways depicted in the Figure show multiple interactions that could occur in the arteriolar SMC-EC model. Increased P CO -evoked impairment of M-BFC can result in regional tissue hypoxia. Increased P CO -evoked impairment of PHD-[HIF-1α] can evoke regional or global hypoxia. Increased [COMb] can inhibit intracellular O 2 delivery, and increased [COHb] can evoke tissue hypoxia due to shifts in the oxyhemoglobin dissociation curve. Increased P CO -evoked impairment of alveolar-ventilation-blood flow coupling can cause decreases in PaO 2 and tissue hypoxia (Coburn, 2020), which is not shown in the Figure. Increased cellular P CO can evoke cellular and tissue hypoxia via several different mechanisms listed before, and cellular hypoxia can result in increased CO binding to FPs, emphasizing that multiple interactions between tissue hypoxia and increased P CO can occur during CO poisoning scenarios.

CONCLUSION
i) This study explores how multiple NO and H 2 S signaling pathways could interact with CO signaling pathways during acute CO poisoning scenarios. An arteriolar SMC-EC duet was used as a model in this analysis. Some of the signaling pathways operating in this model likely occur in other cells that have some of the same FPs. ii) Results are consistent with the hypothesis that during acute CO poisoning scenarios, multiple deregulated and interacting NO and H 2 S signaling pathways interact with deregulated CO and ROS signaling pathways.

AUTHOR CONTRIBUTIONS
RC is the only author and was responsible for all of the research and writing.