Physiological roles of H2S, H2Sn and NO
Neuronal Plasticity
When a neurotransmitter glutamate activates NMDA receptors at postsynapse, Ca2+ influx is induced and Ca2+/calmodulin-dependent neuronal NOS is subsequently activated to produce NO (Garthwaite et al., 1988). NO produced at postsynapse crosses synaptic cleft to presynapse as a retrograde transmitter, which modifies a release of neurotransmitter glutamate, leading to the facilitation of hippocampal long-term potentiation (LTP) (O’Dell et al., 1991). NO also modifies LTP formation to modifies the activity of postsynapse (Taqatqeh et al., 2009).
H2S enhances the activity of NMDA receptors at the active synapses by reducing the cysteine disulfide bond located at the hinge of a ligand binding domain of the receptors to facilitate LTP induction (Abe and Kimura, 1996; Aizenman et al., 1989). On the other hand, H2Sn activate TRPA1 channels in astrocytes surrounding the synapse to induce Ca2+influx, which triggers a release of a gliotransmitter D-serine to the synaptic cleft to enhance the activity of NMDA receptors (Nagai et al., 2004; Nagai et al., 2006; Oosumi et al., 2010; Kimura et al., 2013; Shigetomi et al., 2013). Genetic knockdown of CBS impairs LTP, while S-sulfuration of serine racemase, which generates D-serine from L-serine, restores LTP (Li et al., 2017).
Neuroprotective role
There are two forms of glutamate toxicity; excitotoxicity is caused by the immoderate excitation of NMDA receptors which transport excessive Ca2+ into neurons to death (Choi, 1988). In oxidative glutamate toxicity called oxytosis, which is identical to ferroptosis, high concentrations of glutamate suppress cystine/glutamate antiporter, leading to the decreased transport of cystine into cells that causes decreased production of glutathione, an intracellular major antioxidant, resulting in making cells vulnerable to oxidative stress (Murphy et al., 1989; Tan et al., 2001; Lewerenz et al., 2018). H2S protects embryonic neurons from oxidative glutamate toxicity (Kimura and Kimura, 2004). H2S enhances the activity of cystine/glutamate antiporter xCT to transport cystine, which is reduced to cysteine to be used for the production of glutathione. H2S also augments the activity of glutamate cysteine ligase (GCL), a rate limiting enzyme for glutathione production, also known as γ-glutamyl cysteine synthetase (γ-GCS) (Kimura and Kimura, 2004; Kimura et al., 2010). The activity of ATP-dependent K+ channels and cystic fibrosys transmembrane receptor (CFTR) Cl- channels are also activated by H2S to suppress the excess excitation of the neurons (Kimura et al., 2006). H2Sn S-sulfurate Kelch-like ECH-associated protein 1 (Keap1) to release Nuclear factor erythroid 2-related factor 2 (Nrf2) from Keap1/Nrf2 complex to nucleus where Nrf2 upregulates antioxidant genes, leading to the protection of neurons from oxidative stress (Koike et al., 2016).
Sulfite, which is produced by further oxidization of H2S, also protects embryonic neurons from oxidative stress (Kimura et al,. 2018). It reacts with cystine to produce cysteine, which is more efficiently transported into cells than cystine, leading to the effective production of glutathione (Clarke, 1932). A counterpart product of this reaction is S-cysteinesulfonate, agonist of NMDA receptors (Clarke, 1932; Kumar et al., 2018). Because matured neurons express NMDA receptors, of which the activity is enhanced by H2S and S-cysteinesulfonate (Choi, 1988; Murphy et al., 1989; Tan et al., 2001; Abe and Kimura, 1996; Kumar et al., 2018), the simultaneous application of inhibitors for NMDA receptors must be required for the protection of matured neurons.
Nagahara et al. have proposed that S-sulfuration has a role to protect proteins from oxidative stress. In subsequent oxidation of cysteine-SH to –SOH, -SO2H, -SO3H, only –SOH is reversible to –SH but further oxidized forms are irreversible. In contrast, the oxidation of persulfurated cysteine-SSO2H and -SSO3H can be converted to cysteine-SH by the reduced form of thioredoxin (Nagahara et al. 2012). These observations were reproduced and confirmed by another group (Doka et al., 2020).
NO protects neurons from excitotoxicity by decreasing the excessive influx of Ca2+ by suppressing the activity of NMDA receptors through S-nitrosylation (Jeffrey et al., 2001; Choi et al., 2000). NO also exerts cytoprotective effects by suppressing caspase activity through S-nitrosylation of its active site cysteine (Melino et al., 1997). Because the suppression of soluble guanylate cyclase showed a similar cell death effect to that by the deprivation of NO, cGMP may be involved in the cytoprotective effect of NO (Contestabile et al., 2004).
Mitochondrial energy formation
H2S is well-known toxic gas at high concentrations, and its toxicity attributed to the inhibition of mitochondrial cytochrome c oxidase by suppressing the binding of oxygen (Hill et al., 1984), though its effect is not so potent compared to that of azide (Umemura and Kimura, 2007). There has been the hypothesis that mitochondria originated from sulfide-oxidizing symbionts. Yong and Searcy (2001) demonstrated that chicken liver mitochondria consumed oxygen at an accelerated rate when supplied with low concentrations of H2S, and H2S oxidation is coupled to adenosine trisphosphate (ATP) generation. ATP synthesis requires less than 5 µM H2S, and maximum respiration is induced at 10 µM and less efficient up to 60 µM (Yong and Searcy, 2001).
The balance between as the electron donor and the inhibitor of cytochrome c oxidase is likely controlled by H2S and oxygen availability. Low concentrations of H2S preserve the respiratory rate, while high concentrations inhibit it (Abou-Hamdan et al., 2016). Because sulfide oxidation requires three times more oxygen than that of nicotinamide adenine dinucleotide (NADH) or flavin adenine dinucleotide (FADH2), sulfide may be a poor energy substrate (Lagoutte et al., 2010). However, Szabo et al. suggested that this low energy yield is balanced by unique properties of H2S. 1) H2S freely diffuses across membrane without the need of transporters, and 2) the affinity of H2S to sulfide oxidation unit including SQR is high and 100% is oxidized (Szabo et al., 2014). In the brain the expression of SQR is very low (Linden et al., 2012), it is predicted that a haemoprotein neuroglobin, which is primarily expressed in neurons, plays a role in H2S oxidation (Ruetz et al., 2017).