HIF-1 and SKN-1 coordinate the transcriptional response to hydrogen sulfide in Caenorhabditis elegans

Rhodoquinone-dependent electron transport chain is essential for Caenorhabditis elegans survival in hydrogen sulfide environments

Hydrogen sulfide (H2S) has traditionally been considered an environmental toxin for animal lineages; yet, it plays a signaling role in various processes at low concentrations. Mechanisms controlling H2S in animals, especially in sulfide-rich environments, are not fully understood. The main detoxification pathway involves the conversion of H2S into less harmful forms, through a mitochondrial oxidation pathway. The first step of this pathway oxidizes sulfide and reduces ubiquinone (UQ) through sulfide-quinone oxidoreductase (SQRD/SQOR). Because H2S inhibits cytochrome oxidase and hence UQ regeneration, this pathway becomes compromised at high H2S concentrations. The free-living nematode Caenorhabditis elegans feeds on bacteria and can face high sulfide concentrations in its natural environment. This organism has an alternative ETC that uses rhodoquinone (RQ) as the lipidic electron transporter and fumarate as the final electron acceptor. In this study, we demonstrate that RQ is essential for survival in sulfide. RQ-less animals (kynu-1 and coq-2e KO) cannot survive high H2S concentrations, while UQ-less animals (clk-1 and coq-2a KO) exhibit recovery, even when provided with a UQ-deficient diet. Our findings highlight that sqrd-1 uses both benzoquinones and that RQ-dependent ETC confers a key advantage (RQ regeneration) over UQ in sulfide-rich conditions. C. elegans also faces cyanide, another cytochrome oxidase inhibitor, whose detoxification leads to H2S production, via cysl-2. Our study reveals that RQ delays killing by the HCN-producing bacteria Pseudomonas aeruginosa PAO1. These results underscore the fundamental role that RQ-dependent ETC serves as a biochemical adaptation to H2S environments, and to pathogenic bacteria producing cyanide and H2S toxins.

Whole genome profiling of short-term hypoxia induced genes and identification of HIF-1 binding sites provide insights into HIF-1 function in Caenorhabditis elegans

Oxygen is essential to all the aerobic organisms. However, during normal development, disease and homeostasis, organisms are often challenged by hypoxia (oxygen deprivation). Hypoxia-inducible transcription factors (HIFs) are master regulators of hypoxia response and are evolutionarily conserved in metazoans. The homolog of HIF in the genetic model organism C. elegans is HIF-1. In this study, we aimed to understand short-term hypoxia response to identify HIF-1 downstream genes and identify HIF-1 direct targets in C. elegans. The central research questions were: (1) which genes are differentially expressed in response to short-term hypoxia? (2) Which of these changes in gene expression are dependent upon HIF-1 function? (3) Are any of these hif-1-dependent genes essential to survival in hypoxia? (4) Which genes are the direct targets of HIF-1? We combine whole genome gene expression analyses and chromatin immunoprecipitation sequencing (ChIP-seq) experiments to address these questions. In agreement with other published studies, we report that HIF-1-dependent hypoxia-responsive genes are involved in metabolism and stress response. Some HIF-1-dependent hypoxia-responsive genes like efk-1 and phy-2 dramatically impact survival in hypoxic conditions. Genes regulated by HIF-1 and hypoxia overlap with genes responsive to hydrogen sulfide, also overlap with genes regulated by DAF-16. The genomic regions that co-immunoprecipitate with HIF-1 are strongly enriched for genes involved in stress response. Further, some of these potential HIF-1 direct targets are differentially expressed under short-term hypoxia or are differentially regulated by mutations that enhance HIF-1 activity.

Transcriptome analyses describe the consequences of persistent HIF-1 over-activation in Caenorhabditis elegans

Metazoan animals rely on oxygen for survival, but during normal development and homeostasis, animals are often challenged by hypoxia (low oxygen). In metazoans, many of the critical hypoxia responses are mediated by the evolutionarily conserved hypoxia-inducible transcription factors (HIFs). The stability and activity of HIF complexes are strictly regulated. In the model organism C. elegans, HIF-1 stability and activity are negatively regulated by VHL-1, EGL-9, RHY-1 and SWAN-1. Importantly, C. elegans mutants carrying strong loss-of-function mutations in these genes are viable, and this provides opportunities to interrogate the molecular consequences of persistent HIF-1 over-activation. We find that the genome-wide gene expression patterns are compellingly similar in these mutants, supporting models in which RHY-1, VHL-1 and EGL-9 function in common pathway(s) to regulate HIF-1 activity. These studies illuminate the diversified biological roles played by HIF-1, including metabolism and stress response. Genes regulated by persistent HIF-1 over-activation overlap with genes responsive to pathogens, and they overlap with genes regulated by DAF-16. As crucial stress regulators, HIF-1 and DAF-16 converge on key stress-responsive genes and function synergistically to enable hypoxia survival.

Hypoxia-inducible factor induces cysteine dioxygenase and promotes cysteine homeostasis in Caenorhabditis elegans

Dedicated genetic pathways regulate cysteine homeostasis. For example, high levels of cysteine activate cysteine dioxygenase, a key enzyme in cysteine catabolism in most animal and many fungal species. The mechanism by which cysteine dioxygenase is regulated is largely unknown. In an unbiased genetic screen for mutations that activate cysteine dioxygenase (cdo-1) in the nematode Caenorhabditis elegans, we isolated loss-of-function mutations in rhy-1 and egl-9, which encode proteins that negatively regulate the stability or activity of the oxygen-sensing hypoxia inducible transcription factor (hif-1). EGL-9 and HIF-1 are core members of the conserved eukaryotic hypoxia response. However, we demonstrate that the mechanism of HIF-1-mediated induction of cdo-1 is largely independent of EGL-9 prolyl hydroxylase activity and the von Hippel-Lindau E3 ubiquitin ligase, the classical hypoxia signaling pathway components. We demonstrate that C. elegans cdo-1 is transcriptionally activated by high levels of cysteine and hif-1. hif-1-dependent activation of cdo-1 occurs downstream of an H2S-sensing pathway that includes rhy-1, cysl-1, and egl-9. cdo-1 transcription is primarily activated in the hypodermis where it is also sufficient to drive sulfur amino acid metabolism. Thus, the regulation of cdo-1 by hif-1 reveals a negative feedback loop that maintains cysteine homeostasis. High levels of cysteine stimulate the production of an H2S signal. H2S then acts through the rhy-1/cysl-1/egl-9 signaling pathway to increase HIF-1-mediated transcription of cdo-1, promoting degradation of cysteine via CDO-1.

Transcriptome analyses describe the consequences of persistent HIF-1 over-activation in Caenorhabditis elegans

Metazoan animals rely on oxygen for survival, but during normal development and homeostasis, animals are often challenged by hypoxia (low oxygen). In metazoans, many of the critical hypoxia responses are mediated by the evolutionarily conserved hypoxia-inducible transcription factors (HIFs). The stability and activity of HIF complexes are strictly regulated. In the model organism C. elegans, HIF-1 stability and activity are negatively regulated by VHL-1, EGL-9, RHY-1 and SWAN-1. Importantly, C. elegans mutants carrying strong loss-of-function mutations in these genes are viable, and this provides opportunities to interrogate the molecular consequences of persistent HIF-1 over-activation. We find that the genome-wide gene expression patterns are compellingly similar in these mutants, supporting models in which RHY-1, SWAN-1 and EGL-9 function in common pathway(s) to regulate HIF-1 activity. These studies illuminate the diversified biological roles played by HIF-1, including metabolism, hypoxia and other stress responses, reproduction and development. Genes regulated by persistent HIF-1 over-activation overlap with genes responsive to pathogens, and they overlap with genes regulated by DAF-16. As crucial stress regulators, HIF-1 and DAF-16 converge on key stress-responsive genes and function synergistically to enable hypoxia survival.

Whole genome profiling of short-term hypoxia induced genes and identification of HIF-1 binding sites provide insights into HIF-1 function in Caenorhabditis elegans

Oxygen is essential to all the aerobic organisms. However, during normal development, disease and homeostasis, organisms are often challenged by hypoxia (oxygen deprivation). Hypoxia-inducible transcription factors (HIFs) are master regulators of hypoxia response and are evolutionarily conserved in metazoans. The homolog of HIF in the genetic model organism C. elegans is HIF-1. In this study, we aimed to understand short-term hypoxia response and to identify HIF-1 direct targets in C. elegans. The central research questions were: (1) which genes are differentially expressed in response to short-term hypoxia? (2) Which of these changes in gene expression are dependent upon HIF-1 function? (3) How do HIF-1-dependent hypoxia-responsive genes affect hypoxia adaptation? (4) Which genes are the direct targets of HIF-1? We combine whole genome gene expression analyses and chromatin immunoprecipitation sequencing (ChIP-seq) experiments to address these questions. In agreement with other published studies, we report that HIF-1-dependent hypoxia-responsive genes are involved in metabolism, oxidation-reduction process, and stress response. Some HIF-1-dependent hypoxia-responsive genes like efk-1 andphy-2 dramatically impact survival in hypoxic conditions. HIF-1 co-immunoprecipitates with genomic regions proximal genes involved in stress response, protein processing in endoplasmic reticulum, and cell recognition. Further, some of these potential HIF-1 direct targets are differentially expressed under short-term hypoxia or are differentially regulated by mutations that enhance HIF-1 activity.

Thiol reductive stress activates the hypoxia response pathway

Owing to their capability to disrupt the oxidative protein folding environment in the endoplasmic reticulum (ER), thiol antioxidants, such as dithiothreitol (DTT), are used as ER-specific stressors. We recently showed that thiol antioxidants modulate the methionine-homocysteine cycle by upregulating an S-adenosylmethionine-dependent methyltransferase, rips-1, in Caenorhabditis elegans. However, the changes in cellular physiology induced by thiol stress that modulate the methionine-homocysteine cycle remain uncharacterized. Here, using forward genetic screens in C. elegans, we discover that thiol stress enhances rips-1 expression via the hypoxia response pathway. We demonstrate that thiol stress activates the hypoxia response pathway. The activation of the hypoxia response pathway by thiol stress is conserved in human cells. The hypoxia response pathway enhances thiol toxicity via rips-1 expression and confers protection against thiol toxicity via rips-1-independent mechanisms. Finally, we show that DTT might activate the hypoxia response pathway by producing hydrogen sulfide. Our studies reveal an intriguing interaction between thiol-mediated reductive stress and the hypoxia response pathway and challenge the current model that thiol antioxidant DTT disrupts only the ER milieu in the cell.

Hypoxia-inducible factor induces cysteine dioxygenase and promotes cysteine homeostasis in Caenorhabditis elegans

Dedicated genetic pathways regulate cysteine homeostasis. For example, high levels of cysteine activate cysteine dioxygenase, a key enzyme in cysteine catabolism in most animal and many fungal species. The mechanism by which cysteine dioxygenase is regulated is largely unknown. In an unbiased genetic screen for mutations that activate cysteine dioxygenase (cdo-1) in the nematode C. elegans, we isolated loss-of-function mutations in rhy-1 and egl-9, which encode proteins that negatively regulate the stability or activity of the oxygen-sensing hypoxia inducible transcription factor (hif-1). EGL-9 and HIF-1 are core members of the conserved eukaryotic hypoxia response. However, we demonstrate that the mechanism of HIF-1-mediated induction of cdo-1 is largely independent of EGL-9 prolyl hydroxylase activity and the von Hippel-Lindau E3 ubiquitin ligase, the classical hypoxia signaling pathway components. We demonstrate that C. elegans cdo-1 is transcriptionally activated by high levels of cysteine and hif-1. hif-1-dependent activation of cdo-1 occurs downstream of an H2S-sensing pathway that includes rhy-1, cysl-1, and egl-9. cdo-1 transcription is primarily activated in the hypodermis where it is also sufficient to drive sulfur amino acid metabolism. Thus, the regulation of cdo-1 by hif-1 reveals a negative feedback loop that maintains cysteine homeostasis. High levels of cysteine stimulate the production of an H2S signal. H2S then acts through the rhy-1/cysl-1/egl-9 signaling pathway to increase HIF-1-mediated transcription of cdo-1, promoting degradation of cysteine via CDO-1.

Mitochondrial sulfide promotes life span and health span through distinct mechanisms in developing versus adult treated Caenorhabditis elegans

Living longer without simultaneously extending years spent in good health ("health span") is an increasing societal burden, demanding new therapeutic strategies. Hydrogen sulfide (H2S) can correct disease-related mitochondrial metabolic deficiencies, and supraphysiological H2S concentrations can pro health span. However, the efficacy and mechanisms of mitochondrion-targeted sulfide delivery molecules (mtH2S) administered across the adult life course are unknown. Using a Caenorhabditis elegans aging model, we compared untargeted H2S (NaGYY4137, 100 µM and 100 nM) and mtH2S (AP39, 100 nM) donor effects on life span, neuromuscular health span, and mitochondrial integrity. H2S donors were administered from birth or in young/middle-aged animals (day 0, 2, or 4 postadulthood). RNAi pharmacogenetic interventions and transcriptomics/network analysis explored molecular events governing mtH2S donor-mediated health span. Developmentally administered mtH2S (100 nM) improved life/health span vs. equivalent untargeted H2S doses. mtH2S preserved aging mitochondrial structure, content (citrate synthase activity) and neuromuscular strength. Knockdown of H2S metabolism enzymes and FoxO/daf-16 prevented the positive health span effects of mtH2S, whereas DCAF11/wdr-23 - Nrf2/skn-1 oxidative stress protection pathways were dispensable. Health span, but not life span, increased with all adult-onset mtH2S treatments. Adult mtH2S treatment also rejuvenated aging transcriptomes by minimizing expression declines of mitochondria and cytoskeletal components, and peroxisome metabolism hub components, under mechanistic control by the elt-6/elt-3 transcription factor circuit. H2S health span extension likely acts at the mitochondrial level, the mechanisms of which dissociate from life span across adult vs. developmental treatment timings. The small mtH2S doses required for health span extension, combined with efficacy in adult animals, suggest mtH2S is a potential healthy aging therapeutic.

A growth chamber for chronic exposure of mammalian cells to H2S

H2S is a redox-active signaling molecule that exerts an array of cellular and physiological effects. While intracellular H2S concentrations are estimated to be in the low nanomolar range, intestinal luminal concentrations can be significantly higher due to microbial metabolism. Studies assessing H2S effects are typically conducted with a bolus treatment with sulfide salts or slow releasing sulfide donors, which are limited by the volatility of H2S, and by potential off-target effects of the donor molecules. To address these limitations, we describe the design and performance of a mammalian cell culture incubator for sustained exposure to 20-500 ppm H2S (corresponding to a dissolved sulfide concentrations of ∼4-120 μM in the cell culture medium). We report that colorectal adenocarcinoma HT29 cells tolerate prolonged exposure to H2S with no effect on cell viability after 24 h although ≥50 ppm H2S (∼10 μM) restricts cell proliferation. Even the lowest concentration of H2S used in this study (i.e. ∼4 μM) significantly enhanced glucose consumption and lactate production, revealing a much lower threshold for impacting cellular energy metabolism and activating aerobic glycolysis than has been previously appreciated from studies with bolus H2S treatment regimens.

Dietary-derived vitamin B12 protects Caenorhabditis elegans from thiol-reducing agents

BACKGROUND

One-carbon metabolism, which includes the folate and methionine cycles, involves the transfer of methyl groups which are then utilised as a part of multiple physiological processes including redox defence. During the methionine cycle, the vitamin B12-dependent enzyme methionine synthetase converts homocysteine to methionine. The enzyme S-adenosylmethionine (SAM) synthetase then uses methionine in the production of the reactive methyl carrier SAM. SAM-binding methyltransferases then utilise SAM as a cofactor to methylate proteins, small molecules, lipids, and nucleic acids.

RESULTS

We describe a novel SAM methyltransferase, RIPS-1, which was the single gene identified from forward genetic screens in Caenorhabditis elegans looking for resistance to lethal concentrations of the thiol-reducing agent dithiothreitol (DTT). As well as RIPS-1 mutation, we show that in wild-type worms, DTT toxicity can be overcome by modulating vitamin B12 levels, either by using growth media and/or bacterial food that provide higher levels of vitamin B12 or by vitamin B12 supplementation. We show that active methionine synthetase is required for vitamin B12-mediated DTT resistance in wild types but is not required for resistance resulting from RIPS-1 mutation and that susceptibility to DTT is partially suppressed by methionine supplementation. A targeted RNAi modifier screen identified the mitochondrial enzyme methylmalonyl-CoA epimerase as a strong genetic enhancer of DTT resistance in a RIPS-1 mutant. We show that RIPS-1 is expressed in the intestinal and hypodermal tissues of the nematode and that treating with DTT, β-mercaptoethanol, or hydrogen sulfide induces RIPS-1 expression. We demonstrate that RIPS-1 expression is controlled by the hypoxia-inducible factor pathway and that homologues of RIPS-1 are found in a small subset of eukaryotes and bacteria, many of which can adapt to fluctuations in environmental oxygen levels.

CONCLUSIONS

This work highlights the central importance of dietary vitamin B12 in normal metabolic processes in C. elegans, defines a new role for this vitamin in countering reductive stress, and identifies RIPS-1 as a novel methyltransferase in the methionine cycle.

Assessment of the effects of organic vs. inorganic arsenic and mercury in Caenorhabditis elegans

Exposures to mercury and arsenic are known to pose significant threats to human health. Effects specific to organic vs. inorganic forms of these toxic elements are less understood however, especially for organic dimethylarsinic acid (DMA), which has recently been detected in pups of rodent dams orally exposed to inorganic sodium (meta)arsenite (NaAsO2). Caenorhabditis elegans is a small animal alternative toxicity model. To fill data gaps on the effects of DMA relative to NaAsO2, C. elegans were exposed to these two compounds alongside more thoroughly researched inorganic mercury chloride (HgCl2) and organic methylmercury chloride (meHgCl). For timing of developmental milestone acquisition in C. elegans, meHgCl was 2 to 4-fold more toxic than HgCl2, and NaAsO2 was 20-fold more toxic than DMA, ranking the four compounds meHgCl > HgCl2 > NaAsO2 ≫ DMA for developmental toxicity. Methylmercury induced significant decreases in population locomotor activity levels in developing C. elegans. DMA was also associated with developmental hypoactivity, but at >100-fold higher concentrations than meHgCl. Transcriptional alterations in native genes were observed in wild type C. elegans adults exposed to concentrations equitoxic for developmental delay in juveniles. Both forms of arsenic induced genes involved in immune defense and oxidative stress response, while the two mercury species induced proportionally more genes involved in transcriptional regulation. A transgenic bioreporter for activation of conserved proteosome specific unfolded protein response was strongly activated by NaAsO2, but not DMA at tested concentrations. HgCl2 and meHgCl had opposite effects on a bioreporter for unfolded protein response in the endoplasmic reticulum. Presented experiments indicating low toxicity for DMA in C. elegans are consistent with human epidemiologic data correlating higher arsenic methylation capacity with resistance to arsenic toxicity. This work contributes to the understanding of the accuracy and fit-for-use categories for C. elegans toxicity screening and its usefulness to prioritize compounds of concern for further testing.

Physiological roles of hydrogen sulfide in mammalian cells, tissues and organs

H2S belongs to the class of molecules known as gasotransmitters, which also includes nitric oxide (NO) and carbon monoxide (CO). Three enzymes are recognized as endogenous sources of H2S in various cells and tissues: cystathionine g-lyase (CSE), cystathionine β-synthase (CBS) and 3-mercaptopyruvate sulfurtransferase (3-MST). The current article reviews the regulation of these enzymes as well as the pathways of their enzymatic and non-enzymatic degradation and elimination. The multiple interactions of H2S with other labile endogenous molecules (e.g. NO) and reactive oxygen species are also outlined. The various biological targets and signaling pathways are discussed, with special reference to H2S and oxidative posttranscriptional modification of proteins, the effect of H2S on channels and intracellular second messenger pathways, the regulation of gene transcription and translation and the regulation of cellular bioenergetics and metabolism. The pharmacological and molecular tools currently available to study H2S physiology are also reviewed, including their utility and limitations. In subsequent sections, the role of H2S in the regulation of various physiological and cellular functions is reviewed. The physiological role of H2S in various cell types and organ systems are overviewed. Finally, the role of H2S in the regulation of various organ functions is discussed as well as the characteristic bell-shaped biphasic effects of H2S. In addition, key pathophysiological aspects, debated areas, and future research and translational areas are identified A wide array of significant roles of H2S in the physiological regulation of all organ functions emerges from this review.

Nuclear hormone receptor NHR-49 acts in parallel with HIF-1 to promote hypoxia adaptation in Caenorhabditis elegans

The response to insufficient oxygen (hypoxia) is orchestrated by the conserved hypoxia-inducible factor (HIF). However, HIF-independent hypoxia response pathways exist that act in parallel with HIF to mediate the physiological hypoxia response. Here, we describe a hypoxia response pathway controlled by Caenorhabditis elegans nuclear hormone receptor NHR-49, an orthologue of mammalian peroxisome proliferator-activated receptor alpha (PPARα). We show that nhr-49 is required for animal survival in hypoxia and is synthetic lethal with hif-1 in this context, demonstrating that these factors act in parallel. RNA-seq analysis shows that in hypoxia nhr-49 regulates a set of genes that are hif-1-independent, including autophagy genes that promote hypoxia survival. We further show that nuclear hormone receptor nhr-67 is a negative regulator and homeodomain-interacting protein kinase hpk-1 is a positive regulator of the NHR-49 pathway. Together, our experiments define a new, essential hypoxia response pathway that acts in parallel with the well-known HIF-mediated hypoxia response.

Crosstalk in oxygen homeostasis networks: SKN-1/NRF inhibits the HIF-1 hypoxia-inducible factor in Caenorhabditis elegans

During development, homeostasis, and disease, organisms must balance responses that allow adaptation to low oxygen (hypoxia) with those that protect cells from oxidative stress. The evolutionarily conserved hypoxia-inducible factors are central to these processes, as they orchestrate transcriptional responses to oxygen deprivation. Here, we employ genetic strategies in C. elegans to identify stress-responsive genes and pathways that modulate the HIF-1 hypoxia-inducible factor and facilitate oxygen homeostasis. Through a genome-wide RNAi screen, we show that RNAi-mediated mitochondrial or proteasomal dysfunction increases the expression of hypoxia-responsive reporter Pnhr-57::GFP in C. elegans. Interestingly, only a subset of these effects requires hif-1. Of particular importance, we found that skn-1 RNAi increases the expression of hypoxia-responsive reporter Pnhr-57::GFP and elevates HIF-1 protein levels. The SKN-1/NRF transcription factor has been shown to promote oxidative stress resistance. We present evidence that the crosstalk between HIF-1 and SKN-1 is mediated by EGL-9, the prolyl hydroxylase that targets HIF-1 for oxygen-dependent degradation. Treatment that induces SKN-1, such as heat or gsk-3 RNAi, increases expression of a Pegl-9::GFP reporter, and this effect requires skn-1 function and a putative SKN-1 binding site in egl-9 regulatory sequences. Collectively, these data support a model in which SKN-1 promotes egl-9 transcription, thereby inhibiting HIF-1. We propose that this interaction enables animals to adapt quickly to changes in cellular oxygenation and to better survive accompanying oxidative stress.

Hydrogen sulfide in longevity and pathologies: Inconsistency is malodorous

Hydrogen sulfide (H₂S) is one of the biologically active gases (gasotransmitters), which plays an important role in various physiological processes and aging. Its production in the course of methionine and cysteine catabolism and its degradation are finely balanced, and impairment of H₂S homeostasis is associated with various pathologies. Despite the strong geroprotective action of exogenous H₂S in C. elegans, there are controversial effects of hydrogen sulfide and its donors on longevity in other models, as well as on stress resistance, age-related pathologies and aging processes, including regulation of senescence-associated secretory phenotype (SASP) and senescent cell anti-apoptotic pathways (SCAPs). Here we discuss that the translation potential of H₂S as a geroprotective compound is influenced by a multiplicity of its molecular targets, pleiotropic biological effects, and the overlapping ranges of toxic and beneficial doses. We also consider the challenges of the targeted delivery of H₂S at the required dose. Along with this, the complexity of determining the natural levels of H₂S in animal and human organs and their ambiguous correlations with longevity are reviewed.

Hydrogen Sulfide Oxidation by Sulfide Quinone Oxidoreductase

Hydrogen sulfide (H2 S) is an environmental toxin and a heritage of ancient microbial metabolism that has stimulated new interest following its discovery as a neuromodulator. While many physiological responses have been attributed to low H2 S levels, higher levels inhibit complex IV in the electron transport chain. To prevent respiratory poisoning, a dedicated set of enzymes that make up the mitochondrial sulfide oxidation pathway exists to clear H2 S. The committed step in this pathway is catalyzed by sulfide quinone oxidoreductase (SQOR), which couples sulfide oxidation to coenzyme Q10 reduction in the electron transport chain. The SQOR reaction prevents H2 S accumulation and generates highly reactive persulfide species as products; these can be further oxidized or can modify cysteine residues in proteins by persulfidation. Here, we review the kinetic and structural characteristics of human SQOR, and how its unconventional redox cofactor configuration and substrate promiscuity lead to sulfide clearance and potentially expand the signaling potential of H2 S. This dual role of SQOR makes it a promising target for H2 S-based therapeutics.

Geroprotective potential of genetic and pharmacological interventions to endogenous hydrogen sulfide synthesis in Drosophila melanogaster

Endogenous hydrogen sulfide (H₂S) is a gasotransmitter with a wide range of physiological functions. Aging is accompanied by disruption of H₂S homeostasis, therefore, interventions to the processes of H₂S metabolism to maintain its balance may have geroprotective potential. Here we demonstrated the additive geroprotective effect of combined genetic and pharmacological interventions to the hydrogen sulfide biosynthesis system by overexpression of cystathionine-β-synthase and cystathionine-γ-lyase genes and treatment with precursors of H₂S synthesis cysteine (Cys) and N-acetyl-L-cysteine (NAC). The obtained results suggest that additive effects of genetic and pharmacological interventions to H₂S metabolism may be associated with the complex interaction between beneficial action of H₂S production and prevention of adverse effects of excess H₂S production by Cys and NAC treatment.

HIF-1 Has a Central Role in Caenorhabditis elegans Organismal Response to Selenium

Selenium is a trace element for most organisms; its deficiency and excess are detrimental. Selenium beneficial effects are mainly due to the role of the 21^st genetically encoded amino acid selenocysteine (Sec). Selenium also exerts Sec-independent beneficial effects. Its harmful effects are thought to be mainly due to non-specific incorporation in protein synthesis. Yet the selenium response in animals is poorly understood. In Caenorhabditis elegans, Sec is genetically incorporated into a single selenoprotein. Similar to mammals, a 20-fold excess of the optimal selenium requirement is harmful. Sodium selenite (Na₂SeO₃) excess causes development retardation, impaired growth, and neurodegeneration of motor neurons. To study the organismal response to selenium we performed a genetic screen for C. elegans mutants that are resistant to selenite. We isolated non-sense and missense egl-9/EGLN mutants that confer robust resistance to selenium. In contrast, hif-1/HIF null mutant was highly sensitive to selenium, establishing a role for this transcription factor in the selenium response. We showed that EGL-9 regulates HIF-1 activity through VHL-1, and identified CYSL-1 as a key sensor that transduces the selenium signal. Finally, we showed that the key enzymes involved in sulfide and sulfite stress (sulfide quinone oxidoreductase and sulfite oxidase) are not required for selenium resistance. In contrast, knockout strains in the persulfide dioxygenase ETHE-1 and the sulfurtransferase MPST-7 affect the organismal response to selenium. In sum, our results identified a transcriptional pathway as well as enzymes possibly involved in the organismal selenium response.

A Novel Mechanism To Prevent H2S Toxicity in Caenorhabditis elegans

Hydrogen sulfide (H2S) is an endogenously produced signaling molecule that can be cytoprotective, especially in conditions of ischemia/reperfusion injury. However, H2S is also toxic, and unregulated accumulation or exposure to environmental H2S can be lethal. In Caenorhabditis elegans, the hypoxia inducible factor (hif-1) coordinates the initial transcriptional response to H2S, and is essential to survive exposure to low concentrations of H2S. We performed a forward genetic screen to identify mutations that suppress the lethality of hif-1 mutant animals in H2S. The mutations we recovered are specific for H2S, as they do not suppress embryonic lethality or reproductive arrest of hif-1 mutant animals in hypoxia, nor can they prevent the death of hif-1 mutant animals exposed to hydrogen cyanide. The majority of hif-1 suppressor mutations we recovered activate the skn-1/Nrf2 transcription factor. Activation of SKN-1 by hif-1 suppressor mutations increased the expression of a subset of H2S-responsive genes, consistent with previous findings that skn-1 plays a role in the transcriptional response to H2S. Using transgenic rescue, we show that overexpression of a single gene, rhy-1, is sufficient to protect hif-1 mutant animals in H2S. The rhy-1 gene encodes a predicated O-acyltransferase enzyme that has previously been shown to negatively regulate HIF-1 activity. Our data indicate that RHY-1 has novel, hif-1 independent, function that promotes survival in H2S.

Fasting prevents hypoxia-induced defects of proteostasis in C. elegans

Low oxygen conditions (hypoxia) can impair essential physiological processes and cause cellular damage and death. We have shown that specific hypoxic conditions disrupt protein homeostasis in C. elegans, leading to protein aggregation and proteotoxicity. Here, we show that nutritional cues regulate this effect of hypoxia on proteostasis. Animals fasted prior to hypoxic exposure develop dramatically fewer polyglutamine protein aggregates compared to their fed counterparts, indicating that the effect of hypoxia is abrogated. Fasting also reduced the hypoxia-induced exaggeration of proteostasis defects in animals that express Aβ_1–42 and in animals with a temperature-sensitive mutation in dyn-1, suggesting that this effect was not specific to polyglutamine proteins. Our data also demonstrate that the nutritional environment experienced at the onset of hypoxia dictates at least some aspects of the physiological response to hypoxia. We further demonstrate that the insulin/IGF-like signaling pathway plays a role in mediating the protective effects of fasting in hypoxia. Animals with mutations in daf-2, the C. elegans insulin-like receptor, display wild-type levels of hypoxia-induced protein aggregation upon exposure to hypoxia when fed, but are not protected by fasting. DAF-2 acts independently of the FOXO transcription factor, DAF-16, to mediate the protective effects of fasting. These results suggest a non-canonical role for the insulin/IGF-like signaling pathway in coordinating the effects of hypoxia and nutritional state on proteostasis.

When blood flow to various parts of the body becomes restricted, those tissues suffer from a lack of oxygen, a condition called hypoxia. Hypoxia can cause cellular damage and death, as in stroke and cardiovascular disease. We have found that in the model organism C. elegans (a roundworm) specific concentrations of hypoxia cause aggregation of polyglutamine proteins–the same kind of proteins that are found in an aggregated state in the neurodegenerative disorder Huntington’s disease. Here, we show that that worms can be protected from hypoxia-induced protein aggregation if they are fasted (removed from their food source) prior to experiencing hypoxia. Furthermore, we show that the insulin receptor is required for this protection. The insulin receptor is responsible for detecting insulin, a hormone that is released after feeding. Worms with a nonfunctional version of the insulin receptor displayed hypoxia-induced protein aggregation despite being fasted before the hypoxic exposure. Our results highlight a new role for the insulin signaling pathway in coordinating the effects of both hypoxia and nutritional state on protein aggregation.

Hydrogen sulfide ameliorates aging-associated changes in the kidney

Aging is associated with replacement of normal kidney parenchyma by fibrosis. Because hydrogen sulfide (H₂S) ameliorates kidney fibrosis in disease models, we examined its status in the aging kidney. In the first study, we examined kidney cortical H₂S metabolism and signaling pathways related to synthesis of proteins including matrix proteins in young and old male C57BL/6 mice. In old mice, increase in renal cortical content of matrix protein involved in fibrosis was associated with decreased H₂S generation and AMPK activity, and activation of insulin receptor (IR)/IRS-2-Akt-mTORC1-mRNA translation signaling axis that can lead to increase in protein synthesis. In the second study, we randomized 18-19 month-old male C57BL/6 mice to receive 30 μmol/L sodium hydrosulfide (NaHS) in drinking water vs. water alone (control) for 5 months. Administration of NaHS increased plasma free sulfide levels. NaHS inhibited the increase in kidney cortical content of matrix proteins involved in fibrosis and ameliorated glomerulosclerosis. NaHS restored AMPK activity and inhibited activation of IR/IRS-2-Akt-mTORC1-mRNA translation axis. NaHS inhibited age-related increase in kidney cortical content of p21, IL-1β, and IL-6, components of the senescence-associated secretory phenotype. NaHS abolished increase in urinary albumin excretion seen in control mice and reduced serum cystatin C levels suggesting improved glomerular clearance function. We conclude that aging-induced changes in the kidney are associated with H₂S deficiency. Administration of H₂S ameliorates aging-induced kidney changes probably by inhibiting signaling pathways leading to matrix protein synthesis.

Caenorhabditis elegans HIF-1 Is Broadly Required for Survival in Hydrogen Sulfide

Hydrogen sulfide is common in the environment, and is also endogenously produced by animal cells. Although hydrogen sulfide is often toxic, exposure to low levels of hydrogen sulfide improves outcomes in a variety of mammalian models of ischemia-reperfusion injury. In Caenorhabditis elegans, the initial transcriptional response to hydrogen sulfide depends on the hif-1 transcription factor, and hif-1 mutant animals die when exposed to hydrogen sulfide. In this study, we use rescue experiments to identify tissues in which hif-1 is required to survive exposure to hydrogen sulfide. We find that expression of hif-1 from the unc-14 promoter is sufficient to survive hydrogen sulfide. Although unc-14 is generally considered to be a pan-neuronal promoter, we show that it is active in many nonneuronal cells as well. Using other promoters, we show that pan-neuronal expression of hif-1 is not sufficient to survive exposure to hydrogen sulfide. Our data suggest that hif-1 is required in many different tissues to direct the essential response to hydrogen sulfide.

Mechanisms of defense against products of cysteine catabolism in the nematode Caenorhabditis elegans

Cysteine catabolism presents cells with a double-edged sword. On the one hand, cysteine degradation provides cells with essential molecules such as taurine and sulfide. The formation of sulfide in cells is thought to regulate important and diverse physiological processes including blood circulation, synaptic activity and inflammation. On the other hand, the catabolism of cysteine by gut microbiota can release high levels of sulfide that may underlie the development or relapse of ulcerative colitis, an inflammatory bowel disease affecting millions of people worldwide. Here, we have used the nematode C. elegans to explore how cells tolerate high levels of sulfide produced by cysteine degradation in bacteria. We have identified mutations in genes coding for thioredoxin family proteins, mitochondrial proteins, and collagens that confer tolerance to sulfide toxicity. Exposure to sulfide induces the unfolded protein response in the endoplasmic reticulum and mitochondria. Moreover, our results suggest that sulfide toxicity is mediated by reactive oxygen species (ROS). Indeed, pre-treatment of worms with antioxidants increases their tolerance to sulfide toxicity. Intriguingly, sub-toxic levels of the superoxide generator paraquat can also increase the tolerance of worms to sulfide. Therefore, it appears that activation of ROS detoxification pathway prior to the exposure to sulfide, can increase the tolerance to sulfide toxicity. Our results suggest that these detoxification pathways are mediated by the hypoxia inducible factor HIF-1. Finally, we show that sulfide resistance varies among wild C. elegans and other nematode species, suggesting that tolerance to sulfide was naturally selected in certain habitats.

The genetics of isoflurane-induced developmental neurotoxicity

INTRODUCTION

Neurotoxicity induced by early developmental exposure to volatile anesthetics is a characteristic of organisms across a wide range of species, extending from the nematode C. elegans to mammals. Prevention of anesthetic-induced neurotoxicity (AIN) will rely upon an understanding of its underlying mechanisms. However, no forward genetic screens have been undertaken to identify the critical pathways affected in AIN. By characterizing such pathways, we may identify mechanisms to eliminate isoflurane induced AIN in mammals.

METHODS

Chemotaxis in adult C. elegans after larval exposure to isoflurane was used to measure AIN. We initially compared changes in chemotaxis indices between classical mutants known to affect nervous system development adding mutants in response to data. Activation of specific genes was visualized using fluorescent markers. Animals were then treated with rapamycin or preconditioned with isoflurane to test effects on AIN.

RESULTS

Forty-four mutations, as well as pharmacologic manipulations, identified two pathways, highly conserved from invertebrates to humans, that regulate AIN in C. elegans. Activation of one stress-protective pathway (DAF-2 dependent) eliminates AIN, while activation of a second stress-responsive pathway (endoplasmic reticulum (ER) associated stress) causes AIN. Pharmacologic inhibition of the mechanistic Target of Rapamycin (mTOR) blocks ER-stress and AIN. Preconditioning with isoflurane prior to larval exposure also inhibited AIN.

DISCUSSION

Our data are best explained by a model in which isoflurane acutely inhibits mitochondrial function causing activation of responses that ultimately lead to ER-stress. The neurotoxic effect of isoflurane can be completely prevented by manipulations at multiple points in the pathways that control this response. Endogenous signaling pathways can be recruited to protect organisms from the neurotoxic effects of isoflurane.

Roles for ROS and hydrogen sulfide in the longevity response to germline loss in Caenorhabditis elegans

In Caenorhabditis elegans, removing germ cells slows aging and extends life. Here we show that transcription factors that extend life and confer protection to age-related protein-aggregation toxicity are activated early in adulthood in response to a burst of reactive oxygen species (ROS) and a shift in sulfur metabolism. Germline loss triggers H2S production, mitochondrial biogenesis, and a dynamic pattern of ROS in specific somatic tissues. A cytoskeletal protein, KRI-1, plays a key role in the generation of H2S and ROS. These kri-1-dependent redox species, in turn, promote life extension by activating SKN-1/Nrf2 and the mitochondrial unfolded-protein response, respectively. Both H2S and, remarkably, kri-1-dependent ROS are required for the life extension produced by low levels of the superoxide-generator paraquat and by a mutation that inhibits respiration. Together our findings link reproductive signaling to mitochondria and define an inducible, kri-1-dependent redox-signaling module that can be invoked in different contexts to extend life and counteract proteotoxicity.

The Role of Hydrogen Sulfide in Evolution and the Evolution of Hydrogen Sulfide in Metabolism and Signaling

The chemical versatility of sulfur and its abundance in the prebiotic Earth as reduced sulfide (H2S) implicate this molecule in the origin of life 3.8 billion years ago and also as a major source of energy in the first seven-eighths of evolution. The tremendous increase in ambient oxygen ∼600 million years ago brought an end to H2S as an energy source, and H2S-dependent animals either became extinct, retreated to isolated sulfide niches, or adapted. The first 3 billion years of molecular tinkering were not lost, however, and much of this biochemical armamentarium easily adapted to an oxic environment where it contributes to metabolism and signaling even in humans. This review examines the role of H2S in evolution and the evolution of H2S metabolism and signaling.

Mitochondrial Sulfide Quinone Oxidoreductase Prevents Activation of the Unfolded Protein Response in Hydrogen Sulfide

Hydrogen sulfide (H2S) is an endogenously produced gaseous molecule with important roles in cellular signaling. In mammals, exogenous H2S improves survival of ischemia/reperfusion. We have previously shown that exposure to H2S increases the lifespan and thermotolerance in Caenorhabditis elegans, and improves protein homeostasis in low oxygen. The mitochondrial SQRD-1 (sulfide quinone oxidoreductase) protein is a highly conserved enzyme involved in H2S metabolism. SQRD-1 is generally considered important to detoxify H2S. Here, we show that SQRD-1 is also required to maintain protein translation in H2S. In sqrd-1 mutant animals, exposure to H2S leads to phosphorylation of eIF2α and inhibition of protein synthesis. In contrast, global protein translation is not altered in wild-type animals exposed to lethally high H2S or in hif-1(ia04) mutants that die when exposed to low H2S. We demonstrate that both gcn-2 and pek-1 kinases are involved in the H2S-induced phosphorylation of eIF2α. Both ER and mitochondrial stress responses are activated in sqrd-1 mutant animals exposed to H2S, but not in wild-type animals. We speculate that SQRD-1 activity in H2S may coordinate proteostasis responses in multiple cellular compartments.

Calorie restriction and methionine restriction in control of endogenous hydrogen sulfide production by the transsulfuration pathway

H2S is a gas easily identified by its distinctive odor. Although environmental exposure to H2S has been viewed alternately as therapeutic or toxic through the centuries, H2S has recently regained recognition for its numerous beneficial biological effects. Most experiments documenting such benefits, including improved glucose tolerance, increased stress resistance, and even lifespan extension, are based on exposure of experimental organisms to exogenous sources of H2S. However, appreciation is growing for the importance of H2S produced endogenously by the evolutionary conserved transsulfuration pathway (TSP) in health and longevity. Recent data implicate H2S produced by the TSP in pleiotropic benefits of dietary restriction (DR), or reduced nutrient/energy intake without malnutrition. DR, best known as the most reliable way to extend lifespan in a wide range of experimental organisms, includes various regimens aimed at either reducing overall calorie intake (calorie restriction, intermittent/every-other-day fasting) or reducing particular nutrients such as protein or the essential amino acid, methionine (methionine restriction), with overlapping functional benefits on stress resistance, metabolic fitness and lifespan. Here we will review the small but growing body of literature linking the TSP to the functional benefits of DR in part through the production of endogenous H2S, with an emphasis on regulation of the TSP and H2S production by diet and mechanisms of beneficial H2S action.

Hydrogen sulfide as an oxygen sensor

SIGNIFICANCE

Although oxygen (O2)-sensing cells and tissues have been known for decades, the identity of the O2-sensing mechanism has remained elusive. Evidence is accumulating that O2-dependent metabolism of hydrogen sulfide (H2S) is this enigmatic O2 sensor.

RECENT ADVANCES

The elucidation of biochemical pathways involved in H2S synthesis and metabolism have shown that reciprocal H2S/O2 interactions have been inexorably linked throughout eukaryotic evolution; there are multiple foci by which O2 controls H2S inactivation, and the effects of H2S on downstream signaling events are consistent with those activated by hypoxia. H2S-mediated O2 sensing has been demonstrated in a variety of O2-sensing tissues in vertebrate cardiovascular and respiratory systems, including smooth muscle in systemic and respiratory blood vessels and airways, carotid body, adrenal medulla, and other peripheral as well as central chemoreceptors.

CRITICAL ISSUES

Information is now needed on the intracellular location and stoichometry of these signaling processes and how and which downstream effectors are activated by H2S and its metabolites.

FUTURE DIRECTIONS

Development of specific inhibitors of H2S metabolism and effector activation as well as cellular organelle-targeted compounds that release H2S in a time- or environmentally controlled way will not only enhance our understanding of this signaling process but also provide direction for future therapeutic applications.

Hypoxia disrupts proteostasis in Caenorhabditis elegans

Oxygen is fundamentally important for cell metabolism, and as a consequence, O₂ deprivation (hypoxia) can impair many essential physiological processes. Here, we show that an active response to hypoxia disrupts cellular proteostasis – the coordination of protein synthesis, quality control, and degradation that maintains the functionality of the proteome. We have discovered that specific hypoxic conditions enhance the aggregation and toxicity of aggregation-prone proteins that are associated with neurodegenerative diseases. Our data indicate this is an active response to hypoxia, rather than a passive consequence of energy limitation. This response to hypoxia is partially antagonized by the conserved hypoxia-inducible transcription factor, hif-1. We further demonstrate that exposure to hydrogen sulfide (H₂S) protects animals from hypoxia-induced disruption of proteostasis. H₂S has been shown to protect against hypoxic damage in mammals and extends lifespan in nematodes. Remarkably, our data also show that H₂S can reverse detrimental effects of hypoxia on proteostasis. Our data indicate that the protective effects of H₂S in hypoxia are mechanistically distinct from the effect of H₂S to increase lifespan and thermotolerance, suggesting that control of proteostasis and aging can be dissociated. Together, our studies reveal a novel effect of the hypoxia response in animals and provide a foundation to understand how the integrated proteostasis network is integrated with this stress response pathway.

H2S: A New Approach to Lifespan Enhancement and Healthy Ageing?

Ageing, a progressive structural and functional decline, is considered to be a major risk factor for virtually all ageing-associated pathologies and disabilities, including Alzheimer's disease, Parkinson's disease, stroke, diabetes, atherosclerosis and certain cancers. Biogerontology research has now been largely directed towards finding novel drug targets to decelerate the ageing process and attain healthy ageing in order to delay the onset of all ageing-related diseases. H2S has been reported to exert vasodilatory, antioxidant, antiapoptotic and anti-inflammatory actions and has been shown to act as a signalling molecule, neuromodulator and cytoprotectant. Intriguingly, H2S has been reported to regulate cell cycle and survival in healthy cells which suggests that it may regulate cell fate and hence the ageing process. This chapter sets out to provide an overview of the current knowledge regarding the involvement of H2S in ageing, with a specific focus on the invertebrate model nematode C. elegans.

Toxic-selenium and low-selenium transcriptomes in Caenorhabditis elegans: toxic selenium up-regulates oxidoreductase and down-regulates cuticle-associated genes

Selenium (Se) is an element that in trace quantities is both essential in mammals but also toxic to bacteria, yeast, plants and animals, including C. elegans. Our previous studies showed that selenite was four times as toxic as selenate to C. elegans, but that deletion of thioredoxin reductase did not modulate Se toxicity. To characterize Se regulation of the full transcriptome, we conducted a microarray study in C. elegans cultured in axenic media supplemented with 0, 0.05, 0.1, 0.2, and 0.4 mM Se as selenite. C. elegans cultured in 0.2 and 0.4 mM Se displayed a significant delay in growth as compared to 0, 0.05, or 0.1 mM Se, indicating Se-induced toxicity, so worms were staged to mid-L4 larval stage for these studies. Relative to 0.1 mM Se treatment, culturing C. elegans at these Se concentrations resulted in 1.9, 9.7, 5.5, and 2.3%, respectively, of the transcriptome being altered by at least 2-fold. This toxicity altered the expression of 295 overlapping transcripts, which when filtered against gene sets for sulfur and cadmium toxicity, identified a dataset of 182 toxic-Se specific genes that were significantly enriched in functions related to oxidoreductase activity, and significantly depleted in genes related to structural components of collagen and the cuticle. Worms cultured in low Se (0 mM Se) exhibited no signs of deficiency, but low Se was accompanied by a transcriptional response of 59 genes changed ≥2-fold when compared to all other Se concentrations, perhaps due to decreases in Se-dependent TRXR-1 activity. Overall, these results suggest that Se toxicity in C. elegans causes an increase in ROS and stress responses, marked by increased expression of oxidoreductases and reduced expression of cuticle-associated genes, which together underlie the impaired growth observed in these studies.

Mitohormesis: Promoting Health and Lifespan by Increased Levels of Reactive Oxygen Species (ROS)

Increasing evidence indicates that reactive oxygen species (ROS), consisting of superoxide, hydrogen peroxide, and multiple others, do not only cause oxidative stress, but rather may function as signaling molecules that promote health by preventing or delaying a number of chronic diseases, and ultimately extend lifespan. While high levels of ROS are generally accepted to cause cellular damage and to promote aging, low levels of these may rather improve systemic defense mechanisms by inducing an adaptive response. This concept has been named mitochondrial hormesis or mitohormesis. We here evaluate and summarize more than 500 publications from current literature regarding such ROS-mediated low-dose signaling events, including calorie restriction, hypoxia, temperature stress, and physical activity, as well as signaling events downstream of insulin/IGF-1 receptors, AMP-dependent kinase (AMPK), target-of-rapamycin (TOR), and lastly sirtuins to culminate in control of proteostasis, unfolded protein response (UPR), stem cell maintenance and stress resistance. Additionally, consequences of interfering with such ROS signals by pharmacological or natural compounds are being discussed, concluding that particularly antioxidants are useless or even harmful.

Oxygen sensing strategies in mammals and bacteria

The ability to sense and adapt to changes in pO2 is crucial for basic metabolism in most organisms, leading to elaborate pathways for sensing hypoxia (low pO2). This review focuses on the mechanisms utilized by mammals and bacteria to sense hypoxia. While responses to acute hypoxia in mammalian tissues lead to altered vascular tension, the molecular mechanism of signal transduction is not well understood. In contrast, chronic hypoxia evokes cellular responses that lead to transcriptional changes mediated by the hypoxia inducible factor (HIF), which is directly controlled by post-translational hydroxylation of HIF by the non-heme Fe(II)/αKG-dependent enzymes FIH and PHD2. Research on PHD2 and FIH is focused on developing inhibitors and understanding the links between HIF binding and the O2 reaction in these enzymes. Sulfur speciation is a putative mechanism for acute O2-sensing, with special focus on the role of H2S. This sulfur-centered model is discussed, as are some of the directions for further refinement of this model. In contrast to mammals, bacterial O2-sensing relies on protein cofactors that either bind O2 or oxidatively decompose. The sensing modality for bacterial O2-sensors is either via altered DNA binding affinity of the sensory protein, or else due to the actions of a two-component signaling cascade. Emerging data suggests that proteins containing a hemerythrin-domain, such as FBXL5, may serve to connect iron sensing to O2-sensing in both bacteria and humans. As specific molecular machinery becomes identified, these hypoxia sensing pathways present therapeutic targets for diseases including ischemia, cancer, or bacterial infection.

C. elegans aging is modulated by hydrogen sulfide and the sulfhydrylase/cysteine synthase cysl-2

Exogenous hydrogen sulfide (H2S) administration and endogenous H2S metabolism were explored in the nematode C. elegans. Chronic treatment with a slow-releasing H2S donor, GYY4137, extended median survival by 17-23% and increased tolerance towards oxidative and endoplasmic reticulum (ER) stress. Also, cysl-2, a sulfhydrylase/cysteine synthase in C. elegans, was transcriptionally upregulated by GYY4137 treatment and the deletion of cysl-2 resulted in a significant reduction in lifespan which was partially recovered by the supplementation of GYY4137. Likewise, a mammalian cell culture system, GYY4137 was able to protect bovine aortic endothelial cells (BAECs) from oxidative stress and (H2O2)-induced cell death. Taken together, this provides further support that H2S exerts a protective function which is consistent with the longevity dividend theory. Overall, this study underlines the therapeutic potential of a slow-releasing H2S donor as regulators of the aging and cellular stress pathways.

Gene-environment and protein-degradation signatures characterize genomic and phenotypic diversity in wild Caenorhabditis elegans populations

Background

Analyzing and understanding the relationship between genotypes and phenotypes is at the heart of genetics. Research on the nematode Caenorhabditis elegans has been instrumental for unraveling genotype-phenotype relations, and has important implications for understanding the biology of mammals, but almost all studies, including forward and reverse genetic screens, are limited by investigations in only one canonical genotype. This hampers the detection and functional analysis of allelic variants, which play a key role in controlling many complex traits. It is therefore essential to explore the full potential of the natural genetic variation and evolutionary context of the genotype-phenotype map in wild C. elegans populations.

Results

We used multiple wild C. elegans populations freshly isolated from local sites to investigate gene sequence polymorphisms and a multitude of phenotypes including the transcriptome, fitness, and behavioral traits. The genotype, transcriptome, and a number of fitness traits showed a direct link with the original site of the strains. The separation between the isolation sites was prevalent on all chromosomes, but chromosome V was the largest contributor to this variation. These results were supported by a differential food preference of the wild isolates for naturally co-existing bacterial species. Comparing polymorphic genes between the populations with a set of genes extracted from 19 different studies on gene expression in C. elegans exposed to biotic and abiotic factors, such as bacteria, osmotic pressure, and temperature, revealed a significant enrichment for genes involved in gene-environment interactions and protein degradation.

Conclusions

We found that wild C. elegans populations are characterized by gene-environment signatures, and we have unlocked a wealth of genotype-phenotype relations for the first time. Studying natural isolates provides a treasure trove of evidence compared with that unearthed by the current research in C. elegans, which covers only a diminutive part of the myriad of genotype-phenotype relations that are present in the wild.

Cytochrome P450 drives a HIF-regulated behavioral response to reoxygenation by C. elegans

Oxygen deprivation followed by reoxygenation causes pathological responses in many disorders, including ischemic stroke, heart attacks, and reperfusion injury. Key aspects of ischemia-reperfusion can be modeled by a Caenorhabditis elegans behavior, the O2-ON response, which is suppressed by hypoxic preconditioning or inactivation of the O2-sensing HIF (hypoxia-inducible factor) hydroxylase EGL-9. From a genetic screen, we found that the cytochrome P450 oxygenase CYP-13A12 acts in response to the EGL-9-HIF-1 pathway to facilitate the O2-ON response. CYP-13A12 promotes oxidation of polyunsaturated fatty acids into eicosanoids, signaling molecules that can strongly affect inflammatory pain and ischemia-reperfusion injury responses in mammals. We propose that roles of the EGL-9-HIF-1 pathway and cytochrome P450 in controlling responses to reoxygenation after anoxia are evolutionarily conserved.

Interactions between oxygen homeostasis, food availability, and hydrogen sulfide signaling

The ability to sense and respond to stressful conditions is essential to maintain organismal homeostasis. It has long been recognized that stress response factors that improve survival in changing conditions can also influence longevity. In this review, we discuss different strategies used by animals in response to decreased O(2) (hypoxia) to maintain O(2) homeostasis, and consider interactions between hypoxia responses, nutritional status, and H(2)S signaling. O(2) is an essential environmental nutrient for almost all metazoans as it plays a fundamental role in development and cellular metabolism. However, the physiological response(s) to hypoxia depend greatly on the amount of O(2) available. Animals must sense declining O(2) availability to coordinate fundamental metabolic and signaling pathways. It is not surprising that factors involved in the response to hypoxia are also involved in responding to other key environmental signals, particularly food availability. Recent studies in mammals have also shown that the small gaseous signaling molecule hydrogen sulfide (H(2)S) protects against cellular damage and death in hypoxia. These results suggest that H(2)S signaling also integrates with hypoxia response(s). Many of the signaling pathways that mediate the effects of hypoxia, food deprivation, and H(2)S signaling have also been implicated in the control of lifespan. Understanding how these pathways are coordinated therefore has the potential to reveal new cellular and organismal homeostatic mechanisms that contribute to longevity assurance in animals.