Hydrogen sulfide induced disruption of Na+ homeostasis in the cortex

The Ying and Yang of Hydrogen Sulfide as a Paracrine/Autocrine Agent in Neurodegeneration: Focus on Amyotrophic Lateral Sclerosis

Ever since its presence was reported in the brain, the nature and role of hydrogen sulfide (H2S) in the Central Nervous System (CNS) have changed. Consequently, H2S has been elected as the third gas transmitter, along with carbon monoxide and nitric oxide, and a number of studies have focused on its neuromodulatory and protectant functions in physiological conditions. The research on H2S has highlighted its many facets in the periphery and in the CNS, and its role as a double-faced compound, switching from protective to toxic depending on its concentration. In this review, we will focus on the bell-shaped nature of H2S as an angiogenic factor and as a molecule released by glial cells (mainly astrocytes) and non-neuronal cells acting on the surrounding environment (paracrine) or on the releasing cells themselves (autocrine). Finally, we will discuss its role in Amyotrophic Lateral Sclerosis, a paradigm of a neurodegenerative disease.

Thermal stress induces a positive phenotypic and molecular feedback loop in zebrafish embryos

Aquatic organisms must cope with both rising and rapidly changing temperatures. These thermal changes can affect numerous traits, from molecular to ecological scales. Biotic stressors are already known to induce the release of chemical cues which trigger behavioural responses in other individuals. In this study, we infer whether fluctuating temperature, as an abiotic stressor, may similarly induce stress-like responses in individuals not directly exposed to the stressor. To test this hypothesis, zebrafish (Danio rerio) embryos were exposed for 24 h to fluctuating thermal stress, to medium in which another embryo was thermally stressed before ("stress medium"), and to a combination of these. Growth, behaviour, expression of molecular markers, and of whole-embryo cortisol were used to characterise the thermal stress response and its propagation between embryos. Both fluctuating high temperature and stress medium significantly accelerated development, by shifting stressed embryos from segmentation to pharyngula stages, and altered embryonic activity. Importantly, we found that the expression of sulfide:quinone oxidoreductase (SQOR), the antioxidant gene SOD1, and of interleukin-1β (IL-1β) were significantly altered by stress medium. This study illustrates the existence of positive thermal stress feedback loops in zebrafish embryos where heat stress can induce stress-like responses in conspecifics, but which might operate via different molecular pathways. If similar effects also occur under less severe heat stress regimes, this mechanism may be relevant in natural settings as well.

Construction of ratiometric hydrogen sulfide probe with two reaction sites and its applications in solution and in live cells

Hydrogen sulfide (H₂S), as the third multifunctional signaling biomolecule, it acts as a neuromodulator in the human brain and is recognized as an important gas transmitter in human physiology. The abnormal concentrations of H₂S in human cells can result in several common diseases. Therefore, accurate, fast, and reliable methodologies are required for measuring the in vitro and in vivo concentrations of H₂S to further investigate its function. In this study, a novel DR-SO₂N₃ fluorescent probe containing the fluorophore Disperse Red 277 and a sulfonyl azide group was developed and exploited based on the structural characteristic of Disperse Red 277 that contains the active site easily can be attacked by HS^-. Therefore, this probe featured two reaction sites that involved the reduction and Michael addition of H₂S and exhibited rapid ratiometric fluorescence changes and high selectivity towards H₂S with a 619-fold enhancement factor. Further, the density functional theory (DFT)/time-dependent density functional theory (TDDFT) studies are conducted to understand the photophysical properties of DR-SO₂N₃ and the final product DRHS-SO₂NH₂, which makes the proposed mechanism more reasonable. Furthermore, the probe was successfully applied for the ratiometric fluorescence imaging of exogenous H₂S in living cells.

Ambient geothermal hydrogen sulfide exposure and peripheral neuropathy

The mechanism of toxicity of hydrogen sulfide (H₂S) gas is thought mainly to operate through effects on the nervous system. The gas has high acute toxicity, but whether chronic exposure causes effects, including peripheral neuropathy, is yet unclear. The city of Rotorua, New Zealand, sits on an active geothermal field and the population has some of the highest measured ambient H₂S exposures. A previous study in Rotorua provided evidence that H₂S is associated with peripheral neuropathy. Using clinical methods, the present study sought to investigate and possibly confirm this association in the Rotorua population. The study population comprised 1635 adult residents of Rotorua, aged 18-65. Collected data relevant to the peripheral neuropathy investigation included symptoms, ankle stretch reflex, vibration sensitivity, as measured by the timed-tuning fork test and a Bio-Thesiometer (Bio-Medical Instrument Co., Ohio), and light touch sensitivity measured by monofilaments. An exposure metric, estimating time-weighted H₂S exposure across the last 30 years was used. Principal components analysis was used to combine data across the various indicators of possible peripheral neuropathy. The main data analysis used linear regression to examine associations between the peripheral nerve function indicators and H₂S exposure. None of the peripheral nerve function indicators were associated with H₂S exposure, providing no evidence that H₂S exposure at levels found in Rotorua is a cause of peripheral neuropathy. The earlier association between H₂S exposure and peripheral neuropathy diagnoses may be attributable to the ecological study design used. The possibility that H₂S exposure misclassification could account for the lack of association found cannot be entirely excluded.

Transcriptional events co-regulated by hypoxia and cold stresses in Zebrafish larvae

BACKGROUND

Hypoxia and temperature stress are two major adverse environmental conditions often encountered by fishes. The interaction between hypoxia and temperature stresses has been well documented and oxygen is considered to be the limiting factor for the thermal tolerance of fish. Although both high and low temperature stresses can impair the cardiovascular function and the cross-resistance between hypoxia and heat stress has been found, it is not clear whether hypoxia acclimation can protect fish from cold injury.

RESULTS

Pre-acclimation of 96-hpf zebrafish larvae to mild hypoxia (5% O2) significantly improved their resistance to lethal hypoxia (2.5% O2) and increased the survival rate of zebrafish larvae after lethal cold (10°C) exposure. However, pre-acclimation of 96-hpf larvae to cold (18°C) decreased their tolerance to lethal hypoxia although their ability to endure lethal cold increased. RNA-seq analysis identified 132 up-regulated and 41 down-regulated genes upon mild hypoxia exposure. Gene ontology enrichment analyses revealed that genes up-regulated by hypoxia are primarily involved in oxygen transport, oxidation-reduction process, hemoglobin biosynthetic process, erythrocyte development and cellular iron ion homeostasis. Hypoxia-inhibited genes are enriched in inorganic anion transport, sodium ion transport, very long-chain fatty acid biosynthetic process and cytidine deamination. A comparison with the dataset of cold-regulated gene expression identified 23 genes co-induced by hypoxia and cold and these genes are mainly associated with oxidation-reduction process, oxygen transport, hemopoiesis, hemoglobin biosynthetic process and cellular iron ion homeostasis. The alleviation of lipid peroxidation damage by both cold- and hypoxia-acclimation upon lethal cold stress suggests the association of these genes with cold resistance. Furthermore, the alternative promoter of hmbsb gene specifically activated by hypoxia and cold was identified and confirmed.

CONCLUSIONS

Acclimation responses to mild hypoxia and cold stress were found in zebrafish larvae and pre-acclimation to hypoxia significantly improved the tolerance of larvae to lethal cold stress. RNA-seq and bioinformatics analyses revealed the biological processes associated with hypoxia acclimation. Transcriptional events co-induced by hypoxia and cold may represent the molecular basis underlying the protection of hypoxia-acclimation against cold injury.

Aggravation of seizure-like events by hydrogen sulfide: involvement of multiple targets that control neuronal excitability

AIMS

Epileptic seizures are well-known neurological complications following stroke, occurring in 3% of patients. However, the intrinsic correlation of seizures with stroke remains largely unknown. Hydrogen sulfide (H2 S) is a gas transmitter that may mediate cerebral ischemic injury. But the role of H2 S in seizures has not been understood yet. We examined the effect of H2 S on seizure-like events (SLEs) and underlying mechanisms.

METHODS AND RESULTS

Pentylenetetrazole (PTZ)- and pilocarpine-induced rat epileptic seizure models were tested. Low-Mg(2+) /high-K(+) - and 4-aminopyridine (4-AP)-induced epileptic seizure models were examined using patch-clamp recordings in brain slices. It was found that NaHS aggravated both PTZ- and pilocarpine-induced SLEs in rats, while both low-Mg(2+) /high-K(+) - and 4-AP-induced SLEs were also exacerbated by NaHS in brain slices, which may be due to its regulation on the voltage-gated sodium channel, N-methyl-D-aspartic acid receptor (NMDAR), and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) function. Furthermore, these effects were reversed by blocking voltage-gated sodium channel, NMDAR, and AMPAR.

CONCLUSIONS

These results suggest a pathological role of increased H2 S level in SLEs in vivo and in vitro. Enzymes that control H2 S biosynthesis could be interesting targets for antiepileptic strategies in poststroke epilepsy treatment.

Neuroprotection against hypoxia/ischemia: δ-opioid receptor-mediated cellular/molecular events

Hypoxic/ischemic injury remains the most dreaded cause of neurological disability and mortality. Despite the humbling experiences due to lack of promising therapy, our understanding of the complex cascades underlying the neuronal insult has led to advances in basic science research. One of the most noteworthy has been the effect of opioid receptors, especially the delta-opioid receptor (DOR), on hypoxic/ischemic neurons. Our recent studies, and those of others worldwide, present strong evidence that sheds light on DOR-mediated neuroprotection in the brain, especially in the cortex. The mechanisms of DOR neuroprotection are broadly categorized as: (1) stabilization of the ionic homeostasis, (2) inhibition of excitatory transmitter release, (3) attenuation of disrupted neuronal transmission, (4) increase in antioxidant capacity, (5) regulation of intracellular pathways-inhibition of apoptotic signals and activation of pro-survival signaling, (6) regulation of specific gene and protein expression, and (7) up-regulation of endogenous opioid release and/or DOR expression. Depending upon the severity and duration of hypoxic/ischemic insult, the release of endogenous opioids and DOR expression are regulated in response to the stress, and DOR signaling acts at multiple levels to confer neuronal tolerance to harmful insult. The phenomenon of DOR neuroprotection offers a potential clue for a promising target that may have significant clinical implications in our quest for neurotherapeutics.

DOR activation inhibits anoxic/ischemic Na+ influx through Na+ channels via PKC mechanisms in the cortex

Activation of delta-opioid receptors (DOR) is neuroprotective against hypoxic/ischemic injury in the cortex, which is at least partially related to its action against hypoxic/ischemic disruption of ionic homeostasis that triggers neuronal injury. Na(+) influx through TTX-sensitive voltage-gated Na(+) channels may be a main mechanism for hypoxia-induced disruption of K(+) homeostasis, with DOR activation attenuating the disruption of ionic homeostasis by targeting voltage-gated Na(+) channels. In the present study we examined the role of DOR in the regulation of Na(+) influx in anoxia and simulated ischemia (oxygen-glucose deprivation) as well as the effect of DOR activation on the Na(+) influx induced by a Na(+) channel opener without anoxic/ischemic stress and explored a potential PKC mechanism underlying the DOR action. We directly measured extracellular Na(+) activity in mouse cortical slices with Na(+) selective electrodes and found that (1) anoxia-induced Na(+) influx occurred mainly through TTX-sensitive Na(+) channels; (2) DOR activation inhibited the anoxia/ischemia-induced Na(+) influx; (3) veratridine, a Na(+) channel opener, enhanced the anoxia-induced Na(+) influx; this could be attenuated by DOR activation; (4) DOR activation did not reduce the anoxia-induced Na(+) influx in the presence of chelerythrine, a broad-spectrum PKC blocker; and (5) DOR effects were blocked by PKCβII peptide inhibitor, and PKCθ pseudosubstrate inhibitor, respectively. We conclude that DOR activation inhibits anoxia-induced Na(+) influx through Na(+) channels via PKC (especially PKCβII and PKCθ isoforms) dependent mechanisms in the cortex.