Unveiling Hydrogen Sulfide: A New Frontier in Neuroprotection and Neuromodulation

Abstract

Hydrogen sulfide (H₂S), once recognized as a harmful gas, is now emerging as a very significant biological substance with great emphasis on neuroprotection and neuromodulation. It has several functions within the nervous system, placing its physiological activities, biochemical characteristics as well as therapeutic possibilities to their proper perspective. Endogenously produced by cystathione beta-synthase (CBS), cystathione gamma-lyase (CSE), and 3-mercaptopyruvate sulfurtransferase (3-MST) enzymes, H₂S is a unique naturally occurring substance that contains multiple biological activities; it is anti-inflammatory, antioxidant, and anti-apoptotic. Such activity allows H₂S to reduce oxidative stress, which limits mechanisms of cell death and prevents disruption of mitochondria and thus neuronal injury. H₂S can also be regarded as a neuromodulator because it interacts with and affects the glutamatergic, dopaminergic and GABAergic systems at the synaptic level as well as on neurotransmitter systems and synapse dynamics. It modulates synaptic transmission and its plasticity which is essential for cognitive as well as motor activities and exhibits anti-inflammatory effects which are helpful in the progress of the neurodegenerative condition. Recently gathered some evidence emphasizes as well the possible use of H₂S in therapeutic interventions in Alzheimer’s disease, Parkinson’s disease and stroke. In the models of Alzheimer’s disease, H₂S is able to reduce the toxicity of amyloid-beta peptides and improve cognitive performance. In Parkinson’s disease, it protects dopaminergic neurons and reduces the severity of motor deficits. Moreover, H₂S provides protection in ischemic stroke models through decrease of reactive oxygen species and inflammation. Clinical practice with H₂S-based therapies seems to have certain hurdles even if preclinical results are promising. Aside from stabilizing H₂S in a biologically active form, developing a delivery system for H₂S appears a challenge as well. This review will attempt to summarize the existing studies on H₂S as neuroprotective and neuromodulatory agents, and their avenues of future use and development.

Introduction

Hydrogen sulfide (H₂S), once viewed primarily as a paralytic or noxious contributor, is now regarded as an important factor in molecular biology. It is a gas known for its rotten egg smell which is extremely toxic to many tissues in the human body [1, 2]. For a very long time, the toxic H₂S was considered only as a harmful gas and its prospective applicability was not taken in account. The benefits of H₂S have recently been re-evaluated due to the effectiveness of other gasotransmitters such as NO and CO which is known to have therapeutic effects as well. NO and H2S on inhalation ‘expand’ blood vessels which in turn helps restore normal blood flow to the affected areas. The possibility of using inhaled NO as a treatment for cardio-cerebrovascular diseases has been published, although it has demonstrated limited outcomes so far in treating traumatic brain injuries (TBI). H₂S acts in a similar manner to NO by regulating its signalling and modulating the autonomic nervous system. Clinical studies suggest that both NO and H₂S are effective therapeutic agents specifically when administered via inhalation in treating cardio-cerebrovascular vascular defects after trauma [3], after nitric oxide and carbon monoxide, H₂S is the third gas signal molecule that is present in mammals. It exerts effects on the cardiovascular system that include anti-inflammatory and antioxidant stress. It has been known for some time now that H₂S has an anti-vascular calcification effect, and that vascular calcification can be reduced by supplementing with H₂S and its providers [4].

H₂S was previously linked to industrial emissions and environmental risks. Nonetheless, a paradigm change in science has revealed its physiological roles and endogenous production in the human body. Because of its weak intermolecular interactions, H₂S, a sulphur analogue of water, exists as a colourless gas with a distinct, pungent odor [5]. At pH 7.4, around one-fifth of H₂S is still undissociated in the mammalian body, with the remaining portion existing as sulphide (S2−) and hydrosulfide anions (HS−). H₂S can readily pass across cell plasma membranes in its undissociated form due to its high lipid solubility [6], albeit it is yet unknown what physiological significance this form has [7]. Recent investigations have elucidated that H₂S modulates the secretion of neurotransmitters including gamma-aminobutyric acid (GABA), glutamate, and D-serine, which serves as a co-agonist for N-methyl-D-aspartate (NMDA) receptors. H₂S promotes the facilitation of long-term potentiation in the hippocampus, a synaptic mechanism pertinent to memory consolidation, by augmenting the functionality of NMDA receptors.

The disruption of H₂S signalling has been associated with a variety of pathological conditions, encompassing cognitive and psychiatric disorders. These observations highlight the intricate functions of H₂S within neurotransmission and its prospective involvement in the etiology of diseases [8]. H₂S is recognized for its neuroprotective properties against oxidative stress, its capacity to inhibit light-induced degeneration of photoreceptor cells in the retina, its function in modulating endoplasmic reticulum stress, and its protective role against ischemic reperfusion injury in renal tissues [9]. The significance of H₂S in the central nervous system (CNS) has garnered substantial scholarly interest in recent decades. H₂S is acknowledged for its contributions to neuromodulation and neuroprotection of neuronal cells [9], with its enzymatic pathways impacting a spectrum of disorders, including Parkinson’s disease (PD), Alzheimer’s disease (AD), Down syndrome, cerebral injuries, and Huntington’s disease [10, 11].

H₂S, in conjunction with NO and CO, is classified among a group of gaseous signalling molecules known as gasotransmitters, which exhibit a variety of essential biological functions. In mammalian tissues, H₂S is endogenously generated via enzymatic processes that involve cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE), and 3-mercaptopyruvate sulfurtransferase (3-MST) [7]. These enzymes catalyze the conversion of sulfur-containing amino acids, such as cysteine and homocysteine, into H₂S. The elucidation of these biosynthetic pathways has ignited significant interest in the physiological and pathophysiological implications of H₂S. Within the nervous system, H₂S has exhibited notable neuroprotective and neuromodulatory attributes. This review seeks to investigate the implications of H₂S in various diseases, as well as its participation in the degeneration and regeneration of peripheral nerves. By synthesizing findings in these domains, this review endeavours to delineate promising research avenues concerning H₂S in neural regeneration and the therapeutic intervention of neurodegenerative conditions, with the potential to facilitate the development of innovative therapeutic approaches that harness H₂S, while also aiming to elucidate the molecular mechanisms that govern these properties, the therapeutic viability of H₂S in neurological disorders, and the current landscape of clinical investigations in this specialized field of research.

Biosynthesis Pathways for Hydrogen Sulphide

H₂S is an endogenously synthesized gaseous signalling molecule that participates in a multitude of physiological processes. Its biosynthesis within mammalian tissues predominantly transpires via enzymatic pathways that involve three principal enzymes: cystathionine β-synthase (CBS), CSE, and 3-MST. The production of H₂S derives from methionine through a sequence of intricate biochemical reactions. This metabolic pathway incorporates intermediary compounds such as homocysteine and cystathionine, along with crucial enzymes including CBS and CSE. These enzymes enable the transformation of methionine into H₂S, which is essential for various physiological roles and signalling mechanisms (Fig. 1). These enzymes employ sulfur-containing amino acids in the generation of H₂S, thereby emphasizing the complex biochemical pathways that are integral to its production and regulatory mechanisms [12].

Fig. 1.

The above figure demonstrates hydrogen sulfide (H₂S) production from methionine through a series of biochemical reactions involving intermediary molecules and enzymes

Cystathionine β-Synthase (CBS)

Cystathionine β-synthase (CBS), primarily localized within the brain, liver, and kidneys, facilitates the initial reaction in the trans-sulfuration biochemical pathway. This enzyme utilizes pyridoxal 5’-phosphate (PLP) as a necessary cofactor to transform homocysteine and serine into cystathionine, which subsequently undergoes catabolic processes to yield cysteine, α-ketobutyrate, and ammonia [9]. Concurrently, during this biochemical reaction, CBS generates hydrogen sulphide (H₂S) as a secondary product. The enzymatic activity of CBS is modulated by S-adenosylmethionine (SAM), which effectively increases its substrate affinity, thereby influencing H₂S synthesis [12]. Within the central nervous system, H2S derived from CBS is integral to neurotransmission and neuroprotective mechanisms, affecting critical processes such as long-term potentiation (LTP) and synaptic plasticity. Additionally, it functions as a potent agent for mitochondrial-targeted sulphide delivery, which may offer a therapeutic avenue for mitigating the progression of Primary Mitochondrial Diseases (PMD) [13].

Cystathionine γ-Lyase (CSE)

Cystathionine γ-lyase (CSE) is an enzyme that relies on pyridoxal phosphate (PLP) for its functionality and is predominantly expressed within the cardiovascular system, as well as in the liver, kidneys, and various other tissues. CSE facilitates the biochemical transformation of cystathionine into cysteine, α-ketobutyrate, and ammonia, and it is also capable of directly converting homocysteine into H₂S, pyruvate, and ammonia [14]. Within the cardiovascular system, H₂S generated by CSE fulfils an integral role in promoting vasodilation, aiding in angiogenesis, and offering cytoprotection [8]. The modulation of CSE enzymatic activity is governed by multiple factors, encompassing calcium/calmodulin interactions and various post-translational modifications that can influence its catalytic efficiency and, in turn, the production of H₂S [15]. A comprehensive understanding of the regulatory mechanisms governing CSE is vital for elucidating its role in physiological processes and its potential therapeutic implications, particularly concerning cardiovascular health. By strategically targeting CSE along with its regulatory pathways, innovative approaches can be formulated to exploit the protective attributes of H₂S in the context of cardiovascular and other pathological conditions.

Mercaptopyruvate Sulfurtransferase (3-MST)

3-MST, in conjunction with its associated enzyme cysteine aminotransferase (CAT), forms an alternative metabolic route for the synthesis of H₂S. This metabolic route holds particular relevance within the cerebral and mitochondrial contexts. CAT catalyses the conversion of cysteine into 3-mercaptopyruvate, which subsequently undergoes transformation by 3-MST to yield H₂S and pyruvate [16]. In contrast to CBS and CSE, which are dependent on pyridoxal phosphate (PLP), 3-MST operates as a PLP-independent enzyme. The pathway involving 3-MST is integral to mitochondrial bioenergetics and the maintenance of redox equilibrium, thereby highlighting the critical role of H₂S in cellular metabolic processes and in the defence against oxidative damage [9].

Non-enzymatic Production

Apart from enzymatic pathways, H₂S is also non-enzymatically produced in the gut through the reduction of elementary sulphur and other sulphur-containing compounds, mainly by gut microbiota [17]. Non-enzymatic production contributes to systemic levels of H₂S and, in this way impacts a wide range of physiological and pathological processes. The role played by the gut microbiota in H₂S production underlines the significant importance of microbial interaction toward the maintenance of H₂S homeostasis and the influence of the same in health overall, showing how closely interwoven is the relationship between gut bacteria and host metabolic functions. Further understanding of such pathways underlines the complexity that involves the role of H₂S in the body.

Regulation and Pathophysiological Implications

H₂S production is highly regulated by substrate availability, enzyme activity, and other cofactors. The dysregulation of H₂S biosynthesis has considerable importance for health. Low H₂S levels are linked with hypertension, diabetes, and neurodegenerative diseases; thus suggesting its role in vascular health, glucose metabolism, and neural protection. High H₂S could also cause inflammation and cancer and may thus be involved in the pathological cell proliferation and immune response [18]. Elucidation of the complex mechanisms underlying H₂S biosynthesis and regulation will be critical to the development of targeted therapeutic strategies. In this way, it should be possible to treat or modulate these diverse disease states by manipulation of the levels of endogenous H₂S, building upon the protective and regulatory roles of H₂S in mammals. Biosynthesis in mammalian tissues is complex, either enzymatically or nonenzymatically, and CBS, CSE, and 3-MST are currently thought to be the key enzymes involved. The regulation of these pathways is critically important for the physiologic levels of H₂S and its beneficial effects on cellular function and protection.

Molecular Mechanisms of H₂S in Neuroprotection

These effects of H₂S on neuroprotection can be used through several mechanisms, the most crucial being in the modulation of oxidative stress and apoptosis and mitochondrial function. Oxidative stress is a key mediator in the induction of neuronal damage and neurodegeneration. A very strong antioxidant capable of directly scavenging ROS, among many other functions, endogenous antioxidant defences is regulated by H₂S. H₂S enhances the action of antioxidant enzymes, such as superoxide dismutase (SOD) and glutathione peroxidase (GPx), which prevents oxidative stress in neurons. Besides that, H₂S controls apoptosis pathways, therefore preventing programmed cell death of neuronal cells. It also interacts with other key pathways, including the phosphatidylinositol 3-kinase (PI3K)/Akt pathway, which is a survival mechanism against cell apoptosis. H₂S also preserves mitochondrial function and bioenergetics important for neuronal survival and function. It prevents mitochondrial membrane potential loss, inhibits ROS production via mitochondria, and improves ATP synthesis [19].

Molecular Mechanisms of H₂S Through Oxidative Stress

H₂S displays significant neuroprotective properties, predominantly through inhibition of oxidative stress, an important factor in neuronal impairment and neurodegenerative disorders. Oxidative stress refers to the disequilibrium of the production of ROS over the antioxidant defence systems present within the body, whereby injury of the cell is ensued. H₂S does this by scavenging ROS directly and reducing oxidative damage. Furthermore, H₂S enhances the endogenous antioxidant defence mechanisms by activating the expression and activity of the major antioxidant enzymes, including SOD, GPx, and catalase; H₂S prevents macrophage pyroptosis through the inhibition of the pyroptosis signalling pathway and S‑sulfhydration of pro‑caspase‑1, thereby suppressing the activation of caspase-1 and activity of caspase-1 [19]. These enzymes play an important role in counteracting ROS and restoring redox balance within neurons. The molecule H₂S also modulates other signalling pathways involved in oxidative stress response, among them is the Nrf2 pathway that mediates antioxidant protein expression, protecting against damage through oxidative mechanisms [9]. Furthermore, H2S protects the function of mitochondria by maintaining mitochondrial membrane potential, mitochondrial ROS production, and ATP synthesis. By protecting mitochondria, H2S promotes energetic efficiency in the neurons and diminishes oxidative stress in the neurons [18]. H₂S has a pronounced effect on oxidative stress in the brain due to the induction of Nrf2, which is necessary for the expression of essential antioxidant enzymes like SOD1, GPx, CAT, and GST. This is crucial in maintaining cellular redox balance (Fig. 2). The neuroprotective effect of H₂S involves the impact on oxidative stress, providing a potential therapeutic for diseases in neurodegeneration linked to oxidative damage.

Fig. 2.

Hydrogen sulfide plays an important role in oxidative stress in brain. It induces the expression of the Nrf2 (nuclear factor erythroid 2-related factor 2), which is crucial for the synthesis of several key antioxidant enzymes such as superoxide dismutase 1 (SOD1), glutathione peroxidase (GPx), catalase (CAT), and glutathione S-transferases (GST)

Molecular Mechanisms of H₂S Through Apoptosis

H₂S has neuroprotective and neuromodulatory effects through multiple molecular mechanisms, mainly by modulating apoptosis, a process of programmed cell death that is crucial in neurodegenerative diseases. H₂S mainly inhibits apoptosis through interaction with key apoptotic pathways. It modulates the mitochondrial pathway for apoptosis by preserving the integrity and function of the mitochondria. H₂S preserves the mitochondrial membrane potential, so there will not be as much cytochrome c released into the cytosol-this is an important step in apoptosis [18]. Moreover, H₂S also induces anti-apoptotic proteins, like Bcl-2, and decreases pro-apoptotic proteins, including Bax, to inhibit the activation of the apoptosis cascade [9]. Another critical mechanism involved is the regulation of caspase activity. H₂S does not activate caspase-3 and caspase-9-the two major executing caspases of the apoptosis pathway-and therefore, does not allow any further progression of apoptosis in neurons [20].

This inhibition is partly mediated by the sulfhydration of caspases, a post-translational modification caused by H₂S that has an impact on these enzymes’ activities. H₂S also exerts anti-apoptotic effects through the regulation of oxidative stress. Acting as an antioxidant, H₂S reduces the levels of reactive oxygen species (ROS), known to induce apoptosis. The reduction in oxidative stress further protects neurons from apoptotic cell death [21]. H₂S appears to enhance antioxidant enzymes’ activities, such as superoxide dismutase (SOD) and glutathione peroxidase (GPx), possibly related to some neuroprotective activity [22]. Besides that, H₂S modulates several signalling pathways implicated in cell survival. It is documented that H₂S promotes the PI3K/Akt pathway, which in turn stimulates cell survival by dephosphorylation and thus inhibiting the components of the apoptotic machinery [23]. This pathway activation by H₂S further leads to expression of antioxidant protective and anti-apoptotic genes. All these together look very promising for therapeutic use of H₂S as a potential agent for neuroprotection as well as neuromodulation by inhibition of apoptosis and, thus, holds future promises in the treatment of neurodegenerative diseases.

Molecular Mechanisms of H₂S Through Mitochondrial Function

H₂S causes significant neuroprotective responses through the regulation of mitochondria functions thereby greatly impacting the health of the neurons and preventing neurodegenerative disorders. The protection of the mitochondrial membrane potential is one of the major mechanisms by which H₂S protects neurons as it plays a significant role in the ATP-generating function of the cells besides general cellular energy metabolism [21]. In this manner, H₂S increases mitochondrial bioenergetics by activating an enzyme that catalyses the production of ATP-ATP synthase. This therefore ensures efficient energy supply for neuronal functions, significantly under stress conditions. Furthermore, H₂S can be seen as an antioxidant molecule, as it decreases the production of ROS in the mitochondrion and, consequently, diminishes oxidative stress. Oxidative stress is a common trigger for mitochondrial dysfunction and neuronal apoptosis, as shown by [24]. H₂S further activates the process known as activation of mitochondrial biogenesis, a process through which new mitochondria are developed in the cells. This is due to the upregulation of other regulatory proteins such as PGC-1α (peroxisome proliferator-activated receptor gamma coactivator 1-alpha), which activates mitochondrial functions and resistance [7]. H₂S has also been proven to lead to inhibition of mPTP opening. This pore is one of the critical steps to cell death. Prevention of opening of mPTP by H₂S protects neurons against apoptosis and necrosis and, therefore, plays a role in neuroprotection [25]. H₂S supports neuroprotection by multiple mechanisms based on the maintenance of mitochondrial function, reduction of oxidative stress, enhancement of bioenergetics, promotion of biogenesis of mitochondria, and prevention of apoptosis.

Neuromodulatory Roles of H₂S

Thus, original neuroprotective per H₂S is focused with several neuromodulatory roles affecting the processes, essential to cognition and motor controls. These roles are mostly exerted through regulation of synaptic plasticity, neurotransmitters release, receptors activation and neuroinflammation. H₂S improves the expression of synapse plasticity that is crucial for the learning and retention of memory. It influences the tone of neurotransmitter release and operation of their receptors; thus, impacting on synaptic transfer and plasticity. In particular, it has demonstrated that H₂S improves N-methyl-D-aspartate (NMDA) receptor-activated responses. The NMDA receptor is integral to synaptic plasticity in combination with memory formation activity and; its activation causes influx of calcium that is important to LTP [9]. LTP is a long-term potentiation of the synaptic transmitted signal between neurons and represents a model of learning and memory. As a result of regulating NMDA receptor activated events, H₂S contributes to the formation and stabilization of LTP leading to improved learning abilities. Besides, H₂S affect and is affected by other neurotransmitter systems such glutamatergic, GABAergic, and dopaminergic systems. These interactions are indispensable to setting the right tone of activation and suppression of relayed signals in the brain. H₂S interacts with transmission in glutamatergic neurons which is a modified release of glutamate, the main excitatory neurotransmitter. It also affects the GABAergic system that controls the operations of gamma-aminobutyric acid (GABA) receptors that are the chief neurotransmitters bearing inhibitory functions in the brain [19]. This balance between the excitatory and the inhibitory signals is rather necessary for the normal cognitive and motoric actions. H₂S also affects dopaminergic system which plays a role in mood regulation, the reward system as well as motor control. There are evidences which confirmed that H₂S affect the release and activation of dopamine and its receptors and as a result it has been proposed to play beneficial part in treatment of such diseases as Parkinson’s disease which is caused by the Dopaminergic system malfunctions [18].

Apart from its action on neurotransmitter systems H₂S has potent anti-inflammatory actions. Neuroinflammation is an independent pathological event often observed in various NDs including the AD and the PD, among others. H₂S suppresses microglial and astrotic activation, the two most prominent immunologically active cells in the brain. As soon as these glial cells get activated it synthesizes pro-inflammatory cytokines that have a destructive impact on neurons and also promotes bankrupting of the disease [22]. Since H₂S suppress the activation of microglia and astrocytes, it prevents cytokines that always cause neuroinflammation and related neurodegenerative effects to be released. The neurotrophic effects of H₂S are mediated through different molecular signalling pathways. H₂S stimulates the Nrf2 pathway which in turn modulates antioxidant and cytoprotective genes and adds to its anti-inflammatory properties [7]. Besides, it also influence the signal transduction pathways including PI3K/Akt pathway that protect cells from apoptosis [26]. These pathways are relevant in healthy neuronal function and tissue homeostasis especially under stress or injury conditions. As well, H₂S also contributes to post translational modifications of proteins by a mechanism known as sulfhydration, which covalently attaches a sulfur atom corresponding to target proteins with cysteine residues. This modification can change the activity, stability and function of these proteins which in turn regulate many cellular events [15]. Ion channel and receptor sulfhydration can alter their conductance and signal transduction and may underlie synaptic transmission and plasticity.

Therapeutic Potential

Due to its roles in modulating neural circuits, it should have great promise for the treatment of numerous neurological diseases. Present day research is trying to find out new H2S-releasing compounds as therapeutic agents, with an intention to exploit these neuromodulatory effects of H₂S and at the same time avoiding the toxic properties of the H2S gas. Due to its anti-inflammatory effect, it exhibits potential for eliminating neuroinflammation diseases such as multiple sclerosis and traumatic brain injury. In addition, the capacity of H₂S in controlling the dopaminergic pathways proposes its therapeutic usefulness in Parkinson and affective disorders. In AD, H2S has shown in preclinical studies that it has many advantages. H₂S has been proved to lessen the toxicity of amyloid-beta (Aβ), which is one of the main pathological features in AD. Aβ is generally understood to be neurotoxic and neuroinflammatory in the brains of individuals affected with AD. Sulphide treatment in animal models of AD has also been reported to alleviating cognition through reducing the toxicity of Aβ and remitting synaptic plasticity [27].

Furthermore, H₂S restrained the activation of microglia/RAGE/Chamberin signalling pathway to alleviate neuroinflammation of AD by diminishing the production of pro-inflammatory cytokines [28]. Previous experiments in PD have demonstrated that H₂S has neuroprotective features against the loss of dopaminergic neurons. The dopaminergic neurons are widely degenerated in PD, which manifests motor dysfunction. H₂S has been proven to apply a neuroprotective effect on dopaminergic neuron from the oxidative stress and apoptosis in model animals [29]. In various animal models of PD, H₂S decreases oxidative stress, ameliorates mitochondrial dysfunction, and prevents neuronal death consequently improving the motor performance [30]. In models of cerebral ischemia and stroke, which H₂S also shows neuroprotective effect? Ischemic stroke, due to reduced blood flow leads to oxidative stress inflammation and apoptosis culminating in neuronal damage. H₂S has been demonstrated to improve antioxidant protection by eliminating free radicals hence diminishing oxidative damage [31]. Moreover, H₂S exerts anti-apoptotic effects and reduces the release of cytokines and microglial activation that may contribute to the neuronal damage and dysfunction [32]. This is why, H₂S with its multiple neuroprotective actions may be considered as a potential therapeutic agent in neurological disorders, caused by oxidative stress, neuroinflammation and neuronal apoptosis. Experimental data confirm that it may be beneficial in AD by decreasing the toxicity of Aβ in both in vitro and in vivo model systems and augmenting cerebral function; in PD by shielding dopaminergic neurons from toxins and bettering the motor functions; and in cerebral ischemia and stroke, by lessening oxidative damage and reducing inflammation. The research agenda for H₂S should encompass future studies on the much more detailed signalling and functioning of H₂S in neurological diseases and trending these findings towards clinical relevance.

Challenges and Controversies on Hydrogen Sulphide

However, it had only come up in recent years as a phenomenal neuromodulatory and neuroprotective gas, which has changed the earlier perception of the toxic effect of this gas to a physiological regulator in several physiological processes, especially neuronal activities. New data from preclinical and clinical studies have been able to provide a more profound vision on the different aspects of H₂S’s functionality in protecting the brain, as an antioxidant, anti-inflammatory and anti-apoptotic molecule. There are also new studies in favour of using H₂S as a therapeutic agent, whereas some difficulties, and controversies are also there.

Opposing Studies and Controversies: (Table 1)

Table 1.

Summarizing the challenges and controversies related to Hydrogen Sulfide

Challenges and Controversies on Hydrogen Sulfide	Detailed Explanation	References
Contradictory Findings in Dosage and Toxicity	At low concentrations, H2S has neuroprotective effects, but at high concentrations, it may lead to oxidative stress, neurotoxicity, and inhibition of mitochondrial respiration. Determining the optimal dosage and timing of administration is still unclear	Gopalakrishnan et al. (2019) [33]
Challenges in Translating Preclinical Findings	Preclinical studies show potential, but translating these findings to clinical applications is difficult due to limitations in H2S-releasing compounds, such as instability, short half-life, and difficulty in controlling release rates	Corvino et al. (2021) [34]
Lack of Robust Clinical Data	Despite promising results in animal models, no large-scale clinical trials have definitively shown the efficacy of H2S-based therapies in humans. Current trials with H2S donors like sodium thiosulfate and GYY4137 have not yielded clear evidence of effectiveness in neurological disorders	Merz et al. (2022) [35]
Difficulty in Blood–Brain Barrier Penetration	H2S is not easily targeted to the central nervous system, limiting its ability to cross the blood–brain barrier. Encapsulation using nanoparticles or other modifications may help control release and enhance targeting	Merz et al. (2022) [35]
Oncogenic and Antimutagenic Functions	H2S has been shown to stimulate cancer cell proliferation and angiogenesis in gastrointestinal and colorectal cancers, raising concerns about its safety in chronic therapies, particularly in individuals with cancer susceptibility	Zhao et al. (2023) [36]
Complexity in Disease Progression and Patient Variability	Disease progression, genetics, and patient medical history add additional challenges in the development of H2S-based therapies. The dual role of H2S as both protective and potentially harmful requires careful consideration in clinical use	Zhao et al. (2023) [36]

Contradictory Findings in Dosage and Toxicity

Since pre-clinical testing favours H₂S, counter studies that use dosing and methods of H₂S exposure have appeared. Whether the H₂S is a neuroprotective molecule or toxic to the neurons at high concentration still remains a controversy. High concentration of H₂S may intensify oxidative stress, lead to neurotoxicity and inhibit mitochondrial oxidations through inhibition of CcO, an essential element in the electron transport chain. Gopalakrishnan et al. noted that such a paradoxical effect at higher concentrations of H₂S donors was associated with the deterioration of neurological outcomes in stroke models. The study also highlighted the necessity of determining the specific notion of how much and when H₂S should be given to exert therapeutic value but minimize side effects [33].

Challenges in Translating Preclinical Findings

However, there are a lot of challenges in implementing such observations in animal models in clinical setting. A major issue still remains in the design of suitable and flexible H₂S-releasing groups that have pharmacokinetics properties able to release H₂S in a controlled manner for a long time. The current H₂S-releasing compounds have drawbacks including instability, short-lived half-life and high I.P at higher concentrations. Corvino et al. (2021) and other earlier researches carried out clinical trials pointing out that H₂S has potentials only theoretically but it is not easy to prove its therapeutic efficacy in human patients [34].

Lack of Robust Clinical Data

However, a certain number of drawbacks still persist about the use of H₂S based remedy for neurological sickness; first, no large scale study has compared and evaluated the efficacy of H₂S remedy in counteracting sickness impacting the human brain. Despite the fact that there are a vast number of protective effects of H₂S revealed in experimental animal models in vitro and in vivo, the corresponding positive effects in clinical cases remain unknown. H₂S donor trials are underway with sodium thiosulfate and GYY4137, but the outcomes have not provided definitive evidence of H₂S value in neurological disorders treatment [35]. Some limitations of these translations to clinic include; H₂S is a gas, the speed of which H₂S is released from molecular and it is not targeted to CNS and is therefore unable to successfully cross blood brain barrier. For the chemical modifications of the H₂S donor and nanoparticle formulations (both polymer and liposome based one) to encapsulate H₂S based compounds may hold the potential for protecting these compounds against rapid degradation and for controlling the rates of release, and for providing targeting capabilities to cross the blood brain barrier. Furthermore, the disease progression, genetics, and medical histories of patients will pose additional challenges to H₂S-based therapies, as well as genetic variation.

The Dual Role of H₂S in Disease Progression

One major issue for discussion is the oncogenic and antimutagenic functions of H₂S. While it can be seen that this is not a neurological problem, effects of H₂S on cancer cells brings in to question its safety in chronic therapies. Wang claims that H₂S provides protective effects through cell survival and inflammation reduction, but also stimulates cancer cell proliferation and angiogenesis particularly in gastrointestinal and colorectal cancers. This action is rather ominous in developing a pro-tumorigenic effect in patients with inherent cancer susceptibilities or previous tumor history. Still, it is unclear if similar things occur in the brain or if H₂S could act like the identified success factors of cancer stem cells on glial cells or other elements of the brain [36].

Clinical Research and Future Directions

As of now, research into H₂S donors in neurological disorders has primarily focused on exploring their potential therapeutic benefits across various conditions, including AD, PD, stroke, and neuropathic pain. Clinical trials investigating H₂S donors aim to assess their safety, efficacy, and mechanisms of action in treating these complex neurological conditions. On-going Clinical Trials includes several clinical trials evaluating the use of H₂S donors to mitigate neuroinflammation and oxidative stress, which are implicated in AD progression. These trials often include assessments of cognitive function, biomarkers of neurodegeneration (e.g., amyloid-beta and tau proteins), and safety profiles in AD patients, the neuroprotective roles of H₂S in AD and traumatic brain injury, is one the greatest risk factors for AD [29]. Clinical trials in PD focus on the neuroprotective effects of H₂S donors on dopaminergic neurons, aiming to improve motor symptoms and slow disease progression. These trials typically measure motor function outcomes, biomarkers of oxidative stress and inflammation, and overall quality of life in PD patients [37]. Research in stroke and cerebral ischemia explores the potential of H₂S donors to reduce neuronal damage, improve functional outcomes post-stroke, and enhance neuroplasticity. Clinical trials of H₂S assess neurological deficits, imaging outcomes (e.g., MRI, CT scans), and biomarkers of neuronal injury and inflammation [38]. To enhance the stability of H₂S donors and ensure targeted delivery, various delivery systems have been developed.

The recent advancements in a promising subset includes bio-molecular-based H₂S delive39ry systems, these systems integrate H₂S donors with bio-molecular vectors such as polysaccharides, peptides, and proteins. They explored their fundamental structures, construction strategies, and therapeutic applications, showcasing the inherent advantages provided by biomolecules, including biocompatibility, biodegradability, and expansibility []. Furthermore, the trials completed on H₂S have primarily focused on establishing the safety profiles and pharmacokinetics of various H₂S donors in human subjects. Studies provided foundational data on dosing regimens, adverse effects, and tolerability across different patient populations. Some completed trials have explored the efficacy of H₂S donors in modulating specific pathways relevant to neurodegenerative diseases, such as oxidative stress, inflammation, and neurotransmitter balance. These studies often include biomarker assessments and mechanistic investigations to elucidate how H₂S exerts its therapeutic effects. Given the controversies surrounding the dosage, delivery methods, and long-term effects of H₂S, future research should focus on developing optimized H₂S donors with improved stability and controlled release profiles. Additionally, more robust clinical trials are needed to confirm the safety and efficacy of H₂S-based therapies in humans, particularly in neurodegenerative diseases like Alzheimer’s, Parkinson’s, and stroke. A clearer understanding of the molecular mechanisms through which H₂S exerts its neuroprotective and neuromodulatory actions will further elucidate its clinical potential. While hydrogen sulfide holds promise as a novel therapeutic agent in neuroprotection and neuromodulation, critical evaluation of opposing studies highlights the challenges of translating preclinical success into clinical reality. The therapeutic potential of H₂S remains significant, but the path forward requires cautious exploration to ensure its safe and effective use in clinical practice [39].

Challenges and Opportunities

Translating promising preclinical findings of H₂S into clinical practice presents several challenges and opportunities for future research. One significant challenge is ensuring the safety and efficacy of H₂S donors in human subjects, as observed effects in animal models may not always replicate in clinical settings due to species differences and disease complexities, as H₂S can be toxic and even fatal at high concentrations, emerging evidence has revealed a significant neuroprotective role for lower doses, whether endogenously generated or exogenously administered. Unlike traditional neurotransmitters, H₂S is a gas and cannot be stored in vesicles for targeted delivery. Instead, it exerts its physiological effects through the persulfidation (or sulfhydration) of reactive cysteine residues on target proteins [29]. Moreover, optimizing the delivery methods and dosing regimens of H₂S donors to achieve therapeutic concentrations in target tissues while minimizing systemic effects remains a critical hurdle [40]. Future research directions should prioritize conducting rigorous clinical trials with well-defined patient populations to assess the long-term benefits and risks associated with H2S-based therapies. Mechanistic studies should aim to elucidate the specific molecular pathways through which H₂S exerts its neuroprotective and neuromodulatory effects, facilitating targeted therapies for neurological disorders [41]. Additionally, exploring novel H₂S donor compounds and combination therapies with existing treatments could enhance therapeutic efficacy and broaden the application spectrum of H₂S in clinical neurology [42]. Therefore, addressing these challenges through comprehensive clinical research and mechanistic investigations holds the potential to unlock the therapeutic promise of H₂S in neurological disorders, paving the way for innovative treatment strategies.

Conclusion

H₂S stands at the forefront of neurotherapeutics, offering diverse therapeutic avenues in neuroprotection and neuromodulation. Its capabilities in bolstering antioxidant defenses, thwarting apoptosis, safeguarding mitochondrial integrity, and modulating synaptic activity underline its potential in mitigating neurological disorders. However, to translate these promises into clinical reality, rigorous research and comprehensive clinical trials are imperative. These endeavours will not only elucidate the precise mechanisms of H₂S action but also refine its therapeutic applications across a spectrum of neurological conditions. By advancing our understanding and optimizing delivery strategies, H₂S holds promise as a ground-breaking therapeutic agent in neurology.

Author Contributions

All authors contributed equally to this work.

Funding

None.

Declarations

Competing interests

The authors reported no potential conflict of interests.