Pathophysiological role of H2S, H2Sn and NO
The overproduction of H2S suppresses cytochrome c oxidase that is likely to be involved in the pathogenesis of Down’s syndrome and ethylmalonic encephalopathy (Marechal et al., 2019; Panagaki et al., 2019; Tiranti et al., 2009; Fernandez Cardoso et al., 2017). In contrast, the lack of H2S production may cause the pathology of Parkinson’s, Huntington’s, and Alzheimer’s diseases (Chung et al., 2004; Vandiver et al., 2013; Xie et al., 2013; Paul et al., 2014; Sbodio et al., 2016; Wright et al., 2016; He et al., 2016; Cao et al., 2018; Sbodio et al., 2018). The detrimental effects caused by the excess production of H2S and H2Sn (Ide et al., 2019) as well as those by lacking in the neuroprotective effect of these molecules (Xiong et al., 2018; Topcuoglu et al., 2017; Unal et al., 2018; Koike et al., 2016) have been reported for the pathogenesis of schizophrenia.
  1. Toxicity of H2S and H2Sn
  2. Down’s syndrome
Down’s syndrome (DS) is characterized by impaired brain growth and maturation, which causes mental retardation, has a trisomy of chromosome 21. CBS is encoded on chromosome 21 (21q22.3) and the expression of CBS mRNA is 12 times greater in myeloblasts of DS children than those of normal individuals (Taub et al., 1999). We found that CBS protein levels in DS brains are approximately 3 times greater than those in the normal brains that is twice greater than those expected from the trisomy (Ichinohe et al., 2005). In addition, elderly adults of DS are associated with an Alzheimer’s type of dementia where CBS is localized to astrocytes that surround senile plaques in the brain (Ichinohe et al., 2005). The greater levels of thiosulfate, a metabolite of H2S, were detected in urine of DS patients compared to that of normal individuals (Kamoun et al., 2003). The increased production of H2S by overexpressed CBS may be the cause of the neurological impairments in DS patients (Fig. 4). In a mouse model of DS, three copies of CBS gene are necessary to cause the DS-related recognition memory deficit (Marechal et al., 2019). Panagaki et al (2019) showed that the levels of CBS and H2S are markedly elevated in DS fibroblast cells, and the mitochondrial electron transport, oxygen consumption and ATP generation are profoundly suppressed. They suggested the therapeutic potential of CBS inhibitors.
Ethylmalonyl encephalopathy
Ethylmalonyl encephalopathy (EE) is an autosomal recessive early-onset and defective in cytochrome c oxidase in muscle and brain and excrete ethylmalonic acid in urine. In this disease ETHE1, a gene encoding a beta-lactamase-like iron-coordinating metalloprotein or sulfur dioxygenase, is deficient. A great amount of thiosulfate, a metabolite of H2S, is excreted in urine of ETHE1 knockout mice and patients of this disease (Tiranti et al., 2009). H2S is mainly metabolized by mitochondrial enzymes, SQR, sulfur dioxygenase, and sulfur transferase. Deficiency of sulfur dioxygenase (ETHE1) increases in the brain and skeletal muscle the basal levels of H2S which suppress the cytochrome c oxidase that may lead to progressive neurological failure (Tiranti et al., 2009) (Fig. 4). In addition, acyl-protein thioesterase, which hydrolyses fatty acids bound to cysteine, and glutathione transferases are suppressed in ETHE1 knockout mice probably due to the increased levels of H2S or persulfides (Hildebrandt et al., 2013). The administration of metronidazole, an antibiotic, or N-acetylcysteine prolonged the lifespan of ETHE1 knockout mice and marked clinical improvement in patients with EE (Viscomi et al., 2010). Metronidazole may be expected to decrease the levels of H2S incorporated into blood from intestinal bacteria. N-acetylcystein is metabolized in cells to cysteine, which is a precursor of gluthatione being predicted to buffer H2S, while it is also a substrate to produce H2S. In this instance the former may be a dominant mechanism of N-acetylcysteine for the improvement of the disease. H2S decreased the activities of citrate synthase, aconitase and creatine kinase in the brain of mouse model of EE (Fernandez Cardoso et al., 2017). H2S also suppressed mitochondrial respiration, decreased mitochondrial membrane potential, and induced swelling caused by calcium in brain mitochondria. Changes in mitochondrial membrane potential and the swelling caused by H2S may be due to opening of mitochondrial permeability transition pore (Fernandez Cardoso et al., 2017). Bioenergetics disturbance, lipid peroxidation, and mitochondrial permeability transition pore opening mediated by H2S may be involved in the pathophysiology of brain damage observed in this disease (Fernandez Cardoso et al., 2017).
  1. H2S and H2Snare beneficial
  2. Parkinson’s disease
Parkinson’s disease (PD) is a neurodegenerative disorder mainly causing motor dysfunction. Parkin and α-synuclein are associated with pathophysiology of PD. Αlpha-synuclein is a major component of Lewy bodies associated with rare cases of PD, while parkin, which is a E3 ubiquitin ligase that ubiquitinates diverse substrates and responsible for the clearance of misfolded proteins including α-synuclein, and its mutations cause autosomal recessive PD (Choi et al., 2001; Jesko et al., 2019). Parkin is S-nitrosylated and suppressed its activity and thereby decreases its protective role in the brains of PD patients (Chung et al., 2004). In contrast, in the brains of normal individuals it is active and S-sulfurated. Cys95, Cys 59 and Cys182 are S-sulfurated in normal individuals, while the same cysteine residues are S-nitrosylated in PD brains (Vandiver et al., 2013) (Fig. 5). H2S may be involved in the conversion of S-nitrosylated cysteine to S-sulfurated one in parkin to improve its function (Yao et al., 2004; Chung et al., 2004; Vandiver et al., 2013). Systemic administration of sodium salt of H2S, NaHS, dramatically reversed the progression of movement dysfunction and loss of dopaminergic neurons in the PD model rats (Hu et al., 2010). A similar effect was observed with a H2S-releasing L-dopa derivative compound (Xie et al., 2013). Overexpression of CBS increases the endogenous levels of H2S, reversed the behavior induced in PD model rats, decreased apoptotic neuronal loss of the nigral dopaminergic neurons (Xie et al., 2013). The application of H2S and the regulation of CBS activity have a therapeutic potential (Yin et al., 2017).
Huntington’s disease
Huntington’s disease (HD) is a devastating neurodegenerative disorder characterized by the progressive development of involuntary movements, neuropsychiatric symptoms and cognitive impairment (Cepeda and Tong, 2018). HD belongs to triplet repeat diseases, in which elongated polyglutamine stretches affect the protein product, and the mutant huntingtin disrupts a number of vital cellular processes, including neuronal transmission, metabolism and gene transcription (Chaganti et al., 2017). Although the levels of CSE are very low in the brain compared to other tissues (Ishii et al., 2004), those are even much lower in mouse models of HD and human HD brain compared to a control mice and normal individuals, respectively (Paul et al., 2014). It is caused by the suppression of the transcription factor specificity protein 1 (SP1), which regulates the transcription of CSE, by mutant huntingtin (Ishii et al., 2004; Paul et al., 2014) (Fig. 5). The expression of CSE is also regulated by transcription factor 4 (ATF4), which is dysfunctional in HD, under ER stress or amino acid deficiency (Sbodio et al., 2016). Striatal cell lines derived from HD model mice have decreased levels of cystine/glutamate antiporter xCT mRNA and protein expression, leading to the lower basal levels of GSH and higher basal levels of reactive oxygen species (ROS) (Wright et al., 2016) (Fig.5). Administration of N-acetylcysteine ameliorates HD pathology in this model.
Alzheimer’s disease
Alzheimer’s disease (AD) is a chronic neurodegenerative disease that affects cognitive functions and memory formation. The cause of the disease is mostly sporadic with much less familial. In familial cases of AD the mutations in genes encoding amyloid precursor protein (APP), presenilin (PS) 1 and 2, which constitute the catalytic subunits of γ-secretase, have been identified (Selkoe and Hardy, 2016). In the process of metabolism in normal individual, APP is digested by α-secretase following by γ-secretase, leading to the product which is easily eliminated, while in AD brain the metabolism by β-secretase (BACE) and γ-secretase generates amyloid β-protein (Aβ1-42), which aggregates and exerts neurotoxicity. Phosphorylated tau protein also causes deposition as neurofibrillary tangles (Reddy and Oliver, 2019). Neuropolypeptide h3, also known as hippocampal cholinergic neurostimulating peptide, upregulates the levels of choline acetyltransferase whose activity is decreased in patients with AD in cholinergic neurons (Reed et al., 2009; Ojika et al., 1998). Nitration of tyrosine in this peptide decreases its neurotropic activity on cholinergic neurons and may lead to the decline in cognitive function (Reed et al., 2009). The levels of BACE1, PS1 and pp38 mitogen-activated protein kinase (MAPK) are increased while those disintegrin and metalloproteinase domain-containing protein 17 (ADAM17) are decreased in the APP/PS1 transgenic mice (He et al., 2016). The administration of NaHS into the transgenic mice restores the changes in these factors characterized in AD, suggesting that H2S inhibits the expression of BACE1 and PS1 by activating phosphoinositide 3-kinase (PI3K)/Akt pathway in AD (He et al., 2016). H2S inhibited exogenous ATP-induced inflammatory responses through reducing pro-inflammatory cytokines, ROS, and activation of NF-kB pathway. H2S also suppressed the production of Aβ1-42, which was induced by exogenous ATP probably due to the augmented production of amyloid precursor protein and the activation of β- and γ-secretase (Cao et al., 2018) (Fig. 5). As a mechanism for H2S reducing inflammation and the production of Aβ1-42, they suggested that H2S suppresses the activities of signal transducer and activator of transcription 3 (STAT3) by inhibiting ATP-induced phosphorylation and decreases the activity of cathepsin S through S-sulfuration of this enzyme (Cao et al., 2018) (Fig. 5).
H2S and H2Snare beneficial or toxic
3-1) Schizophrenia
Schizophrenia is a chronic and severe mental disorder that affects a person’s thinking, feeling and behaviors. Symptoms, which typically come on gradually and begin in young adulthood, fall into three categories; positive symptoms (hallucinations, delusions, etc.), negative symptoms (flat affect, reduced feeling, etc.) and cognitive symptoms (poor executive function, trouble focusing, etc.) (American Psychiatric Association, 2013). Although the excessive NO production has been shown to be involved in the pathology of this disease (Pitsikas, 2016), the potential beneficial effects have been reported both NO donors and inhibitors on schizophrenia symptoms induced by amphetamine such as prepulse inhibition disruption and hyperlocomotion (Issy et al., 2018), suggesting the deviation of both decrease and increase of NO from the normal levels may be involved in the pathology of this disease. Both excess and deficiency of H2S and H2Sn have also been proposed to be involved in the pathogenesis of schizophrenia. Plasma H2S levels were significantly lower in patients with schizophrenia relative to healthy control subjects, and a positive association was observed between plasma H2S levels and working memory, visual memory or executive function in patients, suggesting that decreased H2S is involved in the psychopathology and cognitive deficits of this disease (Xiong et al., 2018) (Fig. 5). Untreated schizophrenia patients had significantly higher (disulfide/total thiol) and (disulfide/free thiol) ratio in blood and a significantly lower (free thiol/total thiol) ratio compared to those of healthy individuals (Fig. 5). Thiol homeostasis is disturbed by a shift to the disulfide bond formation (oxidized) in patients (Topcuoglu et al., 2017). Similar results were also obtained in schizophrenia patients using medication (Unal et al., 2018). Methylglyoxal, a highly reactive dicarbonyl compound, is a major precursor for advanced glycation end products (AGEs), of which the production is associated with various neurological disorders including schizophrenia (Toyoshima et al., 2019; Ohnishi et al., 2019). H2Sn protect differentiated human neuroblastoma SH-SY5Y cells from methylglyoxal-induced cytotoxicity, suggesting that H2Sn scavenge methylglyoxal and suppress the accumulation of AGEs and incidents induced by carbonyl stress (Koike et al., 2013; Koike et al., 2016). Excess production of H2S and H2Sn has also been proposed to be involved in the pathogenesis of schizophrenia. Yoshikawa and colleagues found that C57BL/6N (B6) strain mice exhibited greater scores of prepulse inhibition (PPI), which is the normal suppression of a startle response, than C3H/HeN (C3H) mice did (Watanabe et al., 2007). The impaired PPI is regarded as an endophenotype for schizophrenia (Braff et al., 2001). By proteomics analysis the same group found that the expression of MPST is increased in C3H mice compared to B6 (Ide et al., 2019). DNA methylation levels at MPST gene was highly enhanced in C3H mice, and the mean methylation levels of the sites were positively correlated with the expression levels of MPST. MPST levels in schizophrenia were positively correlated with symptom severity scores. In MPST -transgenic mice the expression of genes for energy formation was decreased and mitochondrial energy metabolism was impaired (Ide et al., 2019) (Fig. 4). Maternal immune activation model, which is induced by the injection of polyriboinosinic-polyribocytidilic acid to mother, shows perturbed early neural development via inflammation and oxidative insults (Meyer and Feldon, 2012; Giovanoli et al., 2013; Bundo et al., 2014). In this model the expression of MPST and other inflammatory and oxidative genes were elevated in the brain when pups grow to the adult (Ide et al., 2019). In human brains, catalase gene product was upregulated in schizophrenia samples compared to the controls (Ide et al., 2019). Inflammatory/oxidative insults in early brain development induce upregulation of H2S/polysulfides production excessively as an antioxidative response that suppresses cytocrhome c oxidase, leading to schizophrenia.PerspectiveNeurotransmitters, cytokines, and hormones diffuse to reach their targets to react with their receptors. An advantage of diffusion is that many targets localized in the area within its reach can be activated. After transmitting the signal, they are properly eliminated. For example, neurotransmitters are released from presynapse and diffuse the synaptic cleft to the receptors at postsynapse to exert their effects, and for cessation of responses the transmitters are recovered by uptake or degraded by enzymes to clear the synaptic cleft. NO barely dissolve in water (5.6 mg/100 ml at 20oC), while H2S does well (413 mg/100 ml at 20oC) (Kimura, 2015b). H2S dissociates to H+ and HS- (pK1 = 7.04), and further to S2- (pK2 = 11.96). Because H2S is also lipophilic, it readily passes through plasma membrane. These characteristics of H2S have an advantage to transmit signals across the membrane by diffusion. HS- channels or transporters, which have been identified in both bacteria and mammals (Czyzwski and Wang, 2012; Jennings, 2013), enable H2S to pass through plasma membrane even more efficiently and selectively. H2Sn, which can pass through the plasma membrane, diffuse to the targets on nearby cells (Nagai et al., 2006; Oosumi et al., 2010; Kimura et al., 2013; Greiner et al., 2013; Kimura et al., 2017). For example, H2Snactivate TRPA1 channels by passing through the plasma membrane to S-sulfurate the amino terminus located in the cytoplasm (Nagai et al., 2006; Oosumi et al., 2010; Kimura et al., 2013). Although 23 out of 31 cysteine residues are localized to the amino-terminus (Wang et al., 2012), only Cys422 and Cys634 are responsible for regulating the activity of TRPA1 channels (Hatakeyama et al, 2015; Kimura, 2015a). The remaining 21 cysteine residues were not involved in the activation of channels whether or not they are S-sulfurated. Signaling by diffusion may be less specific to target proteins than that by enzymatic modification. In this case, however, S-sulfuration of the two specific cysteine residues by sulfur transferases, if any, may not have any advantage on specificity to activate the channels compared to the diffusion of H2Sn. S-sulfuration is compared to phosphorylation, in which kinases incorporate phosphate to residues of serine, threonine and tyrosine (Kimura, 2020). H2S and H2Sn S-sulfurate cysteine residues even in the other cells but not specific ones. In contrast, MPST transfers sulfur from 3MP to specific cysteine residues, but the reaction is restricted only inside the cells (Shibuya et al., 2009; Shibuya et al., 2013; Kimura et al., 2017). H2S and H2Sn endogenously exist (Kimura et al,, 2015; Koike et al., 2017), and both their diffusion and enzyme mediated mechanisms play a role in S-sulfuration under physiological conditions. Bacteria use H2S produced by MPST to protect themselves from antibiotics (Shatalin et al., 2011). Because bacteria have channels specific to HS- (Czyzewski and Wang, 2012), signaling by H2S with other cells must have emerged in the early history of life on earth. Mammalian have developed HS-/Cl- transporter on erythrocytes to rapidly and selectively exchange HS- with the extracellular milieu (Jennings, 2013). Patients exposed to high concentrations of H2S are suffered from cognitive disability (Reiffenstain et al., 1992), and the levels of neurotransmitters in the brain were affected in animals exposed to H2S (Warenycia et al., 1989). The expression of SQR, which is the first step for the metabolism of H2S, is very low in the brain (Linden et al., 2012). These observations suggest the vulnerability of the brain to excessive H2S. In DS, EE, and schizophrenia, the high levels of H2S or its producing enzymes exert a toxic effect on cytochrome c oxidase, leading to neuronal dysfunction (Taub et al., 1999; Ichinohe et al., 2004; Panagaki et al., 2019; Tiranti et al., 2009; Viscomi et al., 2010; Linden et al., 2012; Ide et al., 2019). In addition, high concentrations of H2S metabolized by SQR must generate the toxic levels of polysulfides on neurons. The inhibitors of H2S producing enzymes, which may also decrease the levels of polyslufldes, have suggested to have therapeutic potential in these diseases. The accurate comparisons of the levels of H2S and polysulfides between patients and normal individuals are required to examine the efficiency of these inhibitors for therapeutic uses. S-sulfuration makes parkin active in the brains of normal individuals, while S-nitrosylation inactivates it in those of PD patients (Paul et al., 2014). It is intriguing to know whether or not exogenously applied H2S or the enhancement of H2S producing enzymes is able to convert S-nitrosylated cysteine to S-sulfurated one. The difference in the levels of H2S between patients and normal individuals is not well understood in PD. The concentrations of polysulfides, which must be accompanied by the changes of H2S levels, should also be clarified. Controlling the levels of these signaling molecules and the activities of enzymes related to S-nitrosylation and S-sulfuration may have a therapeutic benefit for diseases in the central nervous system.