Differentiated astrocytes acquire sensitivity to hydrogen sulfide that is diminished by the transformation into reactive astrocytes

HB-EGF activates EGFR to induce reactive neural stem cells in the mouse hippocampus after seizures

Hippocampal seizures mimicking mesial temporal lobe epilepsy cause a profound disruption of the adult neurogenic niche in mice. Seizures provoke neural stem cells to switch to a reactive phenotype (reactive neural stem cells, React-NSCs) characterized by multibranched hypertrophic morphology, massive activation to enter mitosis, symmetric division, and final differentiation into reactive astrocytes. As a result, neurogenesis is chronically impaired. Here, using a mouse model of mesial temporal lobe epilepsy, we show that the epidermal growth factor receptor (EGFR) signaling pathway is key for the induction of React-NSCs and that its inhibition exerts a beneficial effect on the neurogenic niche. We show that during the initial days after the induction of seizures by a single intrahippocampal injection of kainic acid, a strong release of zinc and heparin-binding epidermal growth factor, both activators of the EGFR signaling pathway in neural stem cells, is produced. Administration of the EGFR inhibitor gefitinib, a chemotherapeutic in clinical phase IV, prevents the induction of React-NSCs and preserves neurogenesis.

Hydrogen sulfide and polysulfides induce GABA/glutamate/D-serine release, facilitate hippocampal LTP, and regulate behavioral hyperactivity

Hydrogen sulfide (H2S) and polysulfides (H2Sn, n ≥ 2) are signaling molecules produced by 3-mercaptopyruvate sulfurtransferase (3MST) that play various physiological roles, including the induction of hippocampal long-term potentiation (LTP), a synaptic model of memory formation, by enhancing N-methyl-D-aspartate (NMDA) receptor activity. However, the presynaptic action of H2S/H2Sn on neurotransmitter release, regulation of LTP induction, and animal behavior are poorly understood. Here, we showed that H2S/H2S2 applied to the rat hippocampus by in vivo microdialysis induces the release of GABA, glutamate, and D-serine, a co-agonist of NMDA receptors. Animals with genetically knocked-out 3MST and the target of H2S2, transient receptor potential ankyrin 1 (TRPA1) channels, revealed that H2S/H2S2, 3MST, and TRPA1 activation play a critical role in LTP induction, and the lack of 3MST causes behavioral hypersensitivity to NMDA receptor antagonism, as in schizophrenia. H2S/H2Sn, 3MST, and TRPA1 channels have therapeutic potential for psychiatric diseases and cognitive deficits.

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.

Inflammation-associated regulation of RGS in astrocytes and putative implication in neuropathic pain

Background

Regulators of G-protein signaling (RGS) are major physiological modulators of G-protein-coupled receptors (GPCR) signaling. Several GPCRs expressed in both neurons and astrocytes participate in the central control of pain processing, and the reduced efficacy of analgesics in neuropathic pain conditions may rely on alterations in RGS function. The expression and the regulation of RGS in astrocytes is poorly documented, and we herein hypothesized that neuroinflammation which is commonly observed in neuropathic pain could influence RGS expression in astrocytes.

Methods

In a validated model of neuropathic pain, the spared nerve injury (SNI), the regulation of RGS2, RGS3, RGS4, and RGS7 messenger RNA (mRNA) was examined up to 3 weeks after the lesion. Changes in the expression of the same RGS were also studied in cultured astrocytes exposed to defined activation protocols or to inflammatory cytokines.

Results

We evidenced a differential regulation of these RGS in the lumbar spinal cord of animals undergoing SNI. In particular, RGS3 appeared upregulated at early stages after the lesion whereas expression of RGS2 and RGS4 was decreased at later stages. Decrease in RGS7 expression was already observed after 3 days and outlasted until 21 days after the lesion. In cultured astrocytes, we observed that changes in the culture conditions distinctly influenced the constitutive expression of these RGS. Also, brief exposures (4 to 8 h) to either interleukin-1β, interleukin-6, or tumor necrosis factor α caused rapid changes in the mRNA levels of the RGS, which however did not strictly recapitulate the regulations observed in the spinal cord of lesioned animals. Longer exposure (48 h) to inflammatory cytokines barely influenced RGS expression, confirming the rapid but transient regulation of these cell signaling modulators.

Conclusion

Changes in the environment of astrocytes mimicking the inflammation observed in the model of neuropathic pain can affect RGS expression. Considering the role of astrocytes in the onset and progression of neuropathic pain, we propose that the inflammation-mediated modulation of RGS in astrocytes constitutes an adaptive mechanism in a context of neuroinflammation and may participate in the regulation of nociception.

A novel method for culturing stellate astrocytes reveals spatially distinct Ca2+ signaling and vesicle recycling in astrocytic processes

Communication between astrocytes and neurons has been difficult to study because cultured astrocytes do not resemble those in vivo. Wolfes et al. develop a stellate astrocyte monoculture with physiological characteristics and find that VAMP2 and SYT7 mark distinct vesicle populations in astrocytes.

Interactions between astrocytes and neurons rely on the release and uptake of glial and neuronal molecules. But whether astrocytic vesicles exist and exocytose in a regulated or constitutive fashion is under debate. The majority of studies have relied on indirect methods or on astrocyte cultures that do not resemble stellate astrocytes found in vivo. Here, to investigate vesicle-associated proteins and exocytosis in stellate astrocytes specifically, we developed a simple, fast, and economical method for growing stellate astrocyte monocultures. This method is superior to other monocultures in terms of astrocyte morphology, mRNA expression profile, protein expression of cell maturity markers, and Ca²⁺ fluctuations: In astrocytes transduced with GFAP promoter–driven Lck-GCaMP3, spontaneous Ca²⁺ events in distinct domains (somata, branchlets, and microdomains) are similar to those in astrocytes co-cultured with other glia and neurons but unlike Ca²⁺ events in astrocytes prepared using the McCarthy and de Vellis (MD) method and immunopanned (IP) astrocytes. We identify two distinct populations of constitutively recycling vesicles (harboring either VAMP2 or SYT7) specifically in branchlets of cultured stellate astrocytes. SYT7 is developmentally regulated in these astrocytes, and we observe significantly fewer synapses in wild-type mouse neurons grown on Syt7^−/− astrocytes. SYT7 may thus be involved in trafficking or releasing synaptogenic factors. In summary, our novel method yields stellate astrocyte monocultures that can be used to study Ca²⁺ signaling and vesicle recycling and dynamics in astrocytic processes.

Hydrogen Sulfide Promotes Surface Insertion of Hippocampal AMPA Receptor GluR1 Subunit via Phosphorylating at Serine-831/Serine-845 Sites Through a Sulfhydration-Dependent Mechanism

AIMS

Hydrogen sulfide (H2 S) has been widely accepted as a gas neuromodulator to regulate synaptic function. Herein, we set out to determine the effect of H2 S on α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR) and its mechanism.

METHODS

BS(3) protein cross-linking, Western blot, patch clamp, and biotin-switch assay.

RESULTS

Bath application of H2 S donor NaHS (50 and 100 μM) rapidly promoted surface insertion of hippocampal AMPAR GluR1 subunit. This effect can be abolished by dithiothreitol (DTT) and mimicked by Na2 S4 , indicating that a sulfhydration-dependent mechanism may be involved. NaHS increased APMAR-mediated EPSC and led to an elevation of GluR2-lacking AMPAR content. Notably, NaHS did not increase the sulfhydration of AMPAR subunits, but it significantly increased the phosphorylation of GluR1 at serine-831 and serine-845 sites. Postsynaptic signal pathways that control GluR1 phosphorylation, such as protein kinase A (PKA), protein kinase C, and calcium/calmodulin-dependent protein kinases II (CaMKII), were sulfhydrated, activated by NaHS, and these effects can be occluded by DTT. H2 S increased S-sulfhydration of protein phosphatase type 2A (PP2A), which may be partially involved in the activation of signal pathways.

CONCLUSION

Our data suggest that H2 S promotes surface insertion of AMPARs via phosphorylation of GluR1, which depends on a sulfhydration-mediated mechanism.

H2S- and NO-Signaling Pathways in Alzheimer's Amyloid Vasculopathy: Synergism or Antagonism?

Alzheimer's type of neurodegeneration dramatically affects H2S and NO synthesis and interactions in the brain, which results in dysregulated vasomotor function, brain tissue hypoperfusion and hypoxia, development of perivascular inflammation, promotion of Aβ deposition, and impairment of neurogenesis/angiogenesis. H2S- and NO-signaling pathways have been described to offer protection against Alzheimer's amyloid vasculopathy and neurodegeneration. This review describes recent developments of the increasing relevance of H2S and NO in Alzheimer's disease (AD). More studies are however needed to fully determine their potential use as therapeutic targets in Alzheimer's and other forms of vascular dementia.

Signaling molecules: hydrogen sulfide and polysulfide

SIGNIFICANCE

Hydrogen sulfide (H2S) has been recognized as a signaling molecule as well as a cytoprotectant. It modulates neurotransmission, regulates vascular tone, and protects various tissues and organs, including neurons, the heart, and kidneys, from oxidative stress and ischemia-reperfusion injury. H2S is produced from l-cysteine by cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE), and 3-mercaptopyruvate sulfurtransferase (3MST) along with cysteine aminotransferase.

RECENT ADVANCES

In addition to these enzymes, we recently identified a novel pathway to produce H2S from d-cysteine, which involves d-amino acid oxidase (DAO) along with 3MST. These enzymes are localized in the cytoplasm, mitochondria, and peroxisomes. However, some enzymes translocate to organelles under specific conditions. Moreover, H2S-derived potential signaling molecules such as polysulfides and HSNO have been identified.

CRITICAL ISSUES

The physiological stimulations, which trigger the production of H2S and its derivatives and maintain their local levels, remain unclear.

FUTURE DIRECTIONS

Understanding the regulation of the H2S production and H2S-derived signaling molecules and the specific stimuli that induce their release will provide new insights into the biology of H2S and therapeutic development in diseases involving these substances.

Role of hydrogen sulfide in brain synaptic remodeling

Synapses are the functional connection between neurons which are necessary for the transfer of electric activity or chemical activity from one cell to another. Synapses are formed by the pre- and postsynaptic membrane which communicates between pre- and postneurons while a neurochemical modulator is operated in this process. H2S has been known as a toxic gas with rotten eggs smell. However, increasing number of researches show that it regulate a variety of physiological and pathological processes in mammals. Hence, H2S is a physiologically important molecule and has been referred to as the third gaseous molecule alongside carbon monoxide and nitric oxide. The previous era has made an exponential development in the physiological and pathological significance of H2S. Specifically, in the central nervous system, H2S facilitates long-term potentiation and regulates intracellular calcium concentration in brain cells. We as well as others have also shown that H2S has antioxidant, antiapoptotic, and anti-inflammatory properties against various neurodegenerative disorders such as stroke, Alzheimer's disease, and vascular dementia. In this chapter, we highlight the current knowledge of H2S and its neuroprotective effects with a special emphasis on synaptic remodeling.

Hydrogen sulfide and polysulfides as signaling molecules

Hydrogen sulfide (H2S) is a familiar toxic gas that smells of rotten eggs. After the identification of endogenous H2S in the mammalian brain two decades ago, studies of this molecule uncovered physiological roles in processes such as neuromodulation, vascular tone regulation, cytoprotection against oxidative stress, angiogenesis, anti-inflammation, and oxygen sensing. Enzymes that produce H2S, such as cystathionine β-synthase, cystathionine γ-lyase, and 3-mercaptopyruvate sulfurtransferase have been studied intensively and well characterized. Polysulfides, which have a higher number of inner sulfur atoms than that in H2S, were recently identified as potential signaling molecules that can activate ion channels, transcription factors, and tumor suppressors with greater potency than that of H2S. This article focuses on our contribution to the discovery of these molecules and their metabolic pathways and mechanisms of action.

Hydrogen sulfide and polysulfides as biological mediators

Hydrogen sulfide (H2S) is recognized as a biological mediator with various roles such as neuromodulation, regulation of the vascular tone, cytoprotection, anti-inflammation, oxygen sensing, angiogenesis, and generation of mitochondrial energy. It is produced by cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE), and 3-mercaptopyruvate sulfurtransferase (3MST). The activity of CBS is enhanced by S-adenosyl methionine (SAM) and glutathionylation, while it is inhibited by nitric oxide (NO) and carbon monoxide (CO). The activity of CSE and cysteine aminotransferase (CAT), which produces the 3MST substrate 3-mercaptopyruvate (3MP), is regulated by Ca2+. H2S is oxidized to thiosulfate in mitochondria through the sequential action of sulfide quinone oxidoreductase (SQR), sulfur dioxygenase, and rhodanese. The rates of the production and clearance of H2S determine its cellular concentration. Polysulfides (H2Sn) have been found to occur in the brain and activate transient receptor potential ankyrin 1 (TRPA1) channels, facilitate the translocation of nuclear factor erythroid 2-related factor 2 (Nrf2) to the nucleus, and suppress the activity of phosphatase and tensin homolog (PTEN) by sulfurating (sulfhydrating) the target cysteine residues. A cross talk between H2S and NO also plays an important role in cardioprotection as well as regulation of the vascular tone. H2S, polysulfides, and their cross talk with NO may mediate various physiological and pathophysiological responses.

Polysulfides are possible H2S-derived signaling molecules in rat brain

Accumulating evidence shows that hydrogen sulfide (H2S) has a variety of physiological functions. H2S is produced from cysteine by 3 sulfurtransferases. H2S, in turn, generates polysulfides, the functions of which are not well understood. H2S induces Ca(2+) influx in astrocytes, a type of glia. However, the receptor that mediates the response has not been identified. Here, we have shown that polysulfides induce Ca(2+) influx by activating transient receptor potential (TRP)A1 channels in rat astrocytes (EC50 91 nM, Hill coefficient value 1.77±0.26) and that the maximum response was induced at 0.5 μM, which is 1/320 of the concentration of H2S required to achieve a response of similar magnitude (160 μM, EC50 116 μM). TRPA1-selective agonists, allyl isothiocyanate and cinnamaldehyde, induced Ca(2+) influx, and responses to polysulfides were suppressed by TRPA1-selective inhibitors, HC-030031 and AP-18, as well as by siRNAs selective to TRPA1. The present study suggests that polysulfides are possible H2S-derived signaling molecules that stimulate TRP channels in the brain.

A mechanism of retinal protection from light-induced degeneration by hydrogen sulfide

Since our initial demonstrations that hydrogen sulfide (H(2)S) may function as a neuromodulator in the brain and a smooth muscle relaxant in the vascular system, accumulating evidence shows that H(2)S may function as a signaling molecule. We and others also found that H(2)S has a cytoprotective effect. Because H(2)S is well-known toxic gas, a cytoprotective role has been overlooked. H(2)S protects neurons from oxidative stress. It also protects cardiac muscle from ischemia-reperfusion injury. The finding led to the application of H(2)S to the bypass surgery patients in Phase II clinical trial. Cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE) are well known as H(2)S-producing enzymes. We recently demonstrated that the other H(2)S-producing enzyme, 3-mercaptopyruvate sulfurtransferase (3MST) along with cysteine aminotransferase (CAT) is localized to neurons in the brain and to the vascular endothelium. However, the regulation of H(2)S production by 3MST/CAT pathway had not been well understood. The present study shows that H(2)S production by 3MST/CAT pathway is regulated by Ca(2+) and that H(2)S protects retinal photoreceptor cells from light induced degeneration by suppressing excessive Ca(2+) influx caused by intense light.

Hydrogen sulfide is a signaling molecule and a cytoprotectant

SIGNIFICANCE

Accumulating evidence shows that hydrogen sulfide may function as a signaling molecule in processes such as neuromodulation in the brain and smooth muscle relaxation in the vascular system. It also has a cytoprotective effect, since it can protect neurons and cardiac muscle from oxidative stress and ischemia-reperfusion injury, respectively. Hydrogen sulfide can also modulate inflammation, insulin release, and angiogenesis.

RECENT ADVANCES

The regulation of the activity of 3-mercaptopyruvate sulfur transferase (3MST) along with cysteine aminotransferase (CAT), one of the H(2)S producing pathways, has been demonstrated. The production of H(2)S by the pathway, which is regulated by Ca(2+) and facilitated by thioredoxin and dihydrolipoic acid, is also involved in H(2)S signaling as well as cytoprotection. Sulfur hydration of proteins by H(2)S has been proposed to modulate protein functions. H(2)S-sensitive fluorescent probes, which enable us to measure the localization of H(2)S in real time, have been developed.

CRITICAL ISSUES

The basal concentrations of H(2)S have recently been measured and found to be much lower than those initially reported. However, the concentration of H(2)S reached in stimulated cells, as well as the regulation of H(2)S producing enzymes is not well understood. It has been proposed that some of the effects of H(2)S on the regulation of enzymes and receptors might be explained through the properties of sulfane sulfur (S(0)), another form of active sulfur.

FUTURE DIRECTIONS

The determination of H(2)S concentrations in activated cells using new methods including H(2)S-sensitive fluorescent probes, as well as the investigation of the effects of H(2)S using specific inhibitors, may provide better understanding of the physiological function of this molecule. Clarifying mechanisms of H(2)S activity may also facilitate the development of new therapeutic compounds.

Hydrogen sulfide protects the retina from light-induced degeneration by the modulation of Ca2+ influx

Hydrogen sulfide (H(2)S) has recently been recognized as a signaling molecule as well as a cytoprotectant. Cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE) are well-known as H(2)S-producing enzymes. We recently demonstrated that 3-mercaptopyruvate sulfurtransferase (3MST) along with cysteine aminotransferase (CAT) produces H(2)S in the brain and in vascular endothelium. However, the cellular distribution and regulation of these enzymes are not well understood. Here we show that 3MST and CAT are localized to retinal neurons and that the production of H(2)S is regulated by Ca(2+); H(2)S, in turn, regulates Ca(2+) influx into photoreceptor cells by activating vacuolar type H(+)-ATPase (V-ATPase). We also show that H(2)S protects retinal neurons from light-induced degeneration. The excessive levels of light exposure deteriorated photoreceptor cells and increased the number of TUNEL- and 8-hydroxy-2'-deoxyguanosine (8-OHdG)-positive cells. Degeneration was greatly suppressed in the retina of mice administered with NaHS, a donor of H(2)S. The present study provides a new insight into the regulation of H(2)S production and the modulation of the retinal transmission by H(2)S. It also shows a cytoprotective effect of H(2)S on retinal neurons and provides a basis for the therapeutic target for retinal degeneration.

Down-regulation of astrocytic GLAST by microglia-related inflammation is abrogated in dibutyryl cAMP-differentiated cultures

The influence of neuroinflammation on glutamate uptake by glial cells was examined after exposing primary cultures of rat astrocytes to conditioned culture medium from lipopolysaccharide-activated microglia. While such treatment triggered an inflammatory response in astrocytes, as revealed by the induction of cytokine expression, a significant decrease in GLAST expression and activity was observed after 72 h. This regulation of glutamate transporter was not observed with medium from naive microglia, but was mimicked by direct addition of tumor necrosis factor-alpha (TNF-alpha), a major cytokine released from activated microglia. Hence, on its own, TNF-alpha also triggered inflammation in astrocyte cultures, highlighting complex cross-talk between astrocytes and microglia in inflammatory conditions. This putatively detrimental regulation of GLAST in response to inflammation was also studied in cells exposed to dibutyryl cAMP, recognized as a model of astrocytes exhibiting a typical differentiated or activated phenotype. In this model, the conditioned culture medium from activated microglia, as well as TNF-alpha, were found to increase glutamate uptake capacity. Consistently, both of these treatments caused only modest induction of an inflammatory response in dibutyryl cAMP-matured astrocytes as compared to undifferentiated astrocytes. Together, these results suggest that differentiated/activated astrocytes are endowed with the capacity to confront inflammatory insults and that drugs influencing the astrocytes phenotype would deserve further consideration in the treatment of neurological disorders.

3-Mercaptopyruvate sulfurtransferase produces hydrogen sulfide and bound sulfane sulfur in the brain

Hydrogen sulfide (H(2)S) is a synaptic modulator as well as a neuroprotectant. Currently, pyridoxal-5'-phosphate (PLP)-dependent cystathionine beta-synthase (CBS) is thought to be the major H(2)S-producing enzyme in the brain. We recently found that brain homogenates of CBS-knockout mice, even in the absence of PLP, produce H(2)S at levels similar to those of wild-type mice, suggesting the presence of another H(2)S-producing enzyme. Here we show that 3-mercaptopyruvate sulfurtransferase (3MST) in combination with cysteine aminotransferase (CAT) produces H(2)S from cysteine. In addition, 3MST is localized to neurons, and the levels of bound sulfane sulfur, the precursor of H(2)S, are greatly increased in the cells expressing 3MST and CAT but not increased in cells expressing functionally defective mutant enzymes. These data present a new perspective on H(2)S production and storage in the brain.

Selective up-regulation of GLT-1 in cultured astrocytes exposed to soluble mediators released by activated microglia

Impaired glial glutamate uptake is commonly involved in neuronal damages observed in acute and chronic nervous disorders. As nervous insults are frequently associated with local inflammation involving microglia, this study aims at exploring the link between activated microglia and altered glutamate uptake in astrocytes. The regulation of the expression and activity of type 1 glutamate transporter (GLT-1) was examined after exposing cultures of rat astrocytes to conditioned medium from lipopolysaccharide-activated microglia cultures. Significant increases in GLT-1 mRNA expression and dihydrokainate sensitive uptake of aspartate were observed after 72h of treatment. These effects were reproduced by direct exposure of the astrocyte cultures to tumor necrosis factor alpha, a major cytokine released by activated microglia. The regulation of GLT-1 activity in response to inflammatory stimuli was also evidenced in cells exposed to dibutyryl cAMP, recognised as a model of reactive astrocytes in which the expression of this glutamate transporter is constitutively enhanced. Taken together, these results suggest that the GLT-1-dependent control of glutamate neurotransmission by either naive or chemically activated astrocytes is influenced by microglia-mediated inflammation.

A source of hydrogen sulfide and a mechanism of its release in the brain

Hydrogen sulfide (H2S) is recognized as a neuromodulator as well as neuroprotectant in the brain. H2S can be produced from cysteine by enzymes such as cystathionine beta-synthase. However, a mechanism for releasing H2S under physiologic conditions has not been identified. Here we show that H2S is released from bound sulfur, an intracellular store of sulfur, in neurons and astrocytes of mice and rats in the presence of physiologic concentrations of endogenous reducing substances glutathione and cysteine. The highest pH to release H2S from another sulfur store, acid-labile sulfur, which is localized mainly in mitochondria, is 5.4. Because mitochondria are not in the acidic condition, acid-labile sulfur may not be a physiologic source of H2S. Free H2S is immediately absorbed and stored as bound sulfur. Our novel method, using silver particles to measure free H2S, shows that free H2S is maintained at a low level in basal conditions. Alkalinization of the cytoplasm is required for effective release of H2S from bound sulfur, and this condition is achieved in astrocytes by the high concentrations of extracellular K+ that are normally present when nearby neurons are excited. These data present a new perspective on the regulation of H2S in the brain.