Physiological Approach to Understand the Mechanisms of Hydrogen Sulfide Tolerance Using a Fish Study System

Hydrogen sulfide (H2S) is well known as a toxic gas produced by the decomposition of organic matter and geothermal sources and also produced endogenously by cysteine catabolism. Exposure to H2S drives hormetic effects including toxic inhibition of cytochrome c oxidase of the mitochondrial electron transport chain at high concentrations, and maintenance of normal vascular and neural functions at low concentrations. Abnormal elevation of cellular H2S, due to either environmental exposure or defective detoxification, is correlated with vascular and metabolic dysfunction in most aerobic organisms, however Poecilia mexicana thrives in H2S rich environments. The cellular mechanisms whereby organisms tolerate extreme H2S are not fully understood. Our central hypothesis is that sulfide tolerant fish have an enhanced H2S detoxification capacity and/or resistance to H2S toxicity following exposure, relative to non-tolerant fish. Specifically, we hypothesized that sulfide tolerant fish differentially express genes involved in maintaining H2S homeostasis. We found significant differences in gene expression patterns related to H2S detoxification between lab-reared sulfide tolerant and non-tolerant populations originating from the Tacotalpa drainage. Since mitochondria are both the site of H2S toxicity as well as enzymatic detoxification, we further hypothesized that tolerance is achieved by modifications to mitochondrial respiration. To test this, we compared mitochondrial function between 1) lab-reared and wild captured sulfide tolerant and non-tolerant populations originating from Tacotalpa drainage and 2) wild captured sulfide tolerant and non-tolerant populations originating from the Puyacatengo and Pichucalco drainages. We predicted that sulfide tolerant fish are able to maintain mitochondrial respiration in the presence of increasing concentrations of H2S relative to non-tolerant fish and that the sulfide tolerant population captured from Pichucalco drainage, which has the highest concentration of environmental H2S compared to other drainages tested, would exhibit the greatest degree of H2S tolerance compared to the sulfide tolerant populations from drainages with lower environmental H2S. We determined that mitochondria from sulfide tolerant fish have increased maximal and spare respiratory capacities following exposure to high concentrations of H2S, relative to non-tolerant fish, and that the population captured from Pichucalco exhibits the greatest degree of tolerance compared to the other two drainages.Integrative Biolog

Convergent evolution of conserved mitochondrial pathways underlies repeated adaptation to extreme environments

Extreme environments test the limits of life; yet, some organisms thrive in harsh conditions. Extremophile lineages inspire questions about how organisms can tolerate physiochemical stressors and whether the repeated colonization of extreme environments is facilitated by predictable and repeatable evolutionary innovations. We identified the mechanistic basis underlying convergent evolution of tolerance to hydrogen sulfide (H2S)-a toxicant that impairs mitochondrial function-across evolutionarily independent lineages of a fish (Poecilia mexicana, Poeciliidae) from H2S-rich springs. Using comparative biochemical and physiological analyses, we found that mitochondrial function is maintained in the presence of H2S in sulfide spring P. mexicana but not ancestral lineages from nonsulfidic habitats due to convergent adaptations in the primary toxicity target and a major detoxification enzyme. Genome-wide local ancestry analyses indicated that convergent evolution of increased H2S tolerance in different populations is likely caused by a combination of selection on standing genetic variation and de novo mutations. On a macroevolutionary scale, H2S tolerance in 10 independent lineages of sulfide spring fishes across multiple genera of Poeciliidae is correlated with the convergent modification and expression changes in genes associated with H2S toxicity and detoxification. Our results demonstrate that the modification of highly conserved physiological pathways associated with essential mitochondrial processes mediates tolerance to physiochemical stress. In addition, the same pathways, genes, and-in some instances-codons are implicated in H2S adaptation in lineages that span 40 million years of evolution

The roles of plasticity and evolutionary change in shaping gene expression variation in natural populations of extremophile fish

The notorious plasticity of gene expression responses and the complexity of environmental gradients complicate the identification of adaptive differences in gene regulation among populations. We combined transcriptome analyses in nature with common-garden and exposure experiments to establish cause-effect relationships between the presence of a physiochemical stressor and expression differences, as well as to test how evolutionary change and plasticity interact to shape gene expression variation in natural systems. We studied two evolutionarily independent population pairs of an extremophile fish (Poecilia mexicana) living in toxic, hydrogen sulphide (HS)-rich springs and adjacent nontoxic habitats and assessed genomewide expression patterns of wild-caught and common-garden-raised individuals exposed to different concentrations of HS. We found that 7.7% of genes that were differentially expressed between sulphidic and nonsulphidic ecotypes remained differentially expressed in the laboratory, indicating that sources of selection other than HS-or plastic responses to other environmental factors-contribute substantially to gene expression patterns observed in the wild. Concordantly differentially expressed genes in the wild and the laboratory were primarily associated with HS detoxification, sulphur processing and metabolic physiology. While shared, ancestral plasticity played a minor role in shaping gene expression variation observed in nature, we documented evidence for evolved population differences in the constitutive expression as well as the HS inducibility of candidate genes. Mechanisms underlying gene expression variation also varied substantially across the two ecotype pairs. These results provide a springboard for studying evolutionary modifications of gene regulatory mechanisms that underlie expression variation in locally adapted populations

The roles of plasticity and evolutionary change in shaping gene expression variation in natural populations of extremophile fish

The notorious plasticity of gene expression responses and the complexity of environmental gradients complicate the identification of adaptive differences in gene regulation among populations. We combined transcriptome analyses in nature with common-garden and exposure experiments to establish cause-effect relationships between the presence of a physiochemical stressor and expression differences, as well as to test how evolutionary change and plasticity interact to shape gene expression variation in natural systems. We studied two evolutionarily independent population pairs of an extremophile fish (Poecilia mexicana) living in toxic, hydrogen sulphide (HS)-rich springs and adjacent nontoxic habitats and assessed genomewide expression patterns of wild-caught and common-garden-raised individuals exposed to different concentrations of HS. We found that 7.7% of genes that were differentially expressed between sulphidic and nonsulphidic ecotypes remained differentially expressed in the laboratory, indicating that sources of selection other than HS-or plastic responses to other environmental factors-contribute substantially to gene expression patterns observed in the wild. Concordantly differentially expressed genes in the wild and the laboratory were primarily associated with HS detoxification, sulphur processing and metabolic physiology. While shared, ancestral plasticity played a minor role in shaping gene expression variation observed in nature, we documented evidence for evolved population differences in the constitutive expression as well as the HS inducibility of candidate genes. Mechanisms underlying gene expression variation also varied substantially across the two ecotype pairs. These results provide a springboard for studying evolutionary modifications of gene regulatory mechanisms that underlie expression variation in locally adapted populations

Convergent evolution of conserved mitochondrial pathways underlies repeated adaptation to extreme environments

Extreme environments test the limits of life; yet, some organisms thrive in harsh conditions. Extremophile lineages inspire questions about how organisms can tolerate physiochemical stressors and whether the repeated colonization of extreme environments is facilitated by predictable and repeatable evolutionary innovations. We identified the mechanistic basis underlying convergent evolution of tolerance to hydrogen sulfide (H2S)-a toxicant that impairs mitochondrial function-across evolutionarily independent lineages of a fish (Poecilia mexicana, Poeciliidae) from H2S-rich springs. Using comparative biochemical and physiological analyses, we found that mitochondrial function is maintained in the presence of H2S in sulfide spring P. mexicana but not ancestral lineages from nonsulfidic habitats due to convergent adaptations in the primary toxicity target and a major detoxification enzyme. Genome-wide local ancestry analyses indicated that convergent evolution of increased H2S tolerance in different populations is likely caused by a combination of selection on standing genetic variation and de novo mutations. On a macroevolutionary scale, H2S tolerance in 10 independent lineages of sulfide spring fishes across multiple genera of Poeciliidae is correlated with the convergent modification and expression changes in genes associated with H2S toxicity and detoxification. Our results demonstrate that the modification of highly conserved physiological pathways associated with essential mitochondrial processes mediates tolerance to physiochemical stress. In addition, the same pathways, genes, and-in some instances-codons are implicated in H2S adaptation in lineages that span 40 million years of evolution

Convergent evolution of conserved mitochondrial pathways underlies repeated adaptation to extreme environments

Extreme environments test the limits of life; yet, some organisms thrive in harsh conditions. Extremophile lineages inspire questions about how organisms can tolerate physiochemical stressors and whether the repeated colonization of extreme environments is facilitated by predictable and repeatable evolutionary innovations. We identified the mechanistic basis underlying convergent evolution of tolerance to hydrogen sulfide (H 2 S)—a toxicant that impairs mitochondrial function—across evolutionarily independent lineages of a fish ( Poecilia mexicana , Poeciliidae) from H 2 S-rich springs. Using comparative biochemical and physiological analyses, we found that mitochondrial function is maintained in the presence of H 2 S in sulfide spring P. mexicana but not ancestral lineages from nonsulfidic habitats due to convergent adaptations in the primary toxicity target and a major detoxification enzyme. Genome-wide local ancestry analyses indicated that convergent evolution of increased H 2 S tolerance in different populations is likely caused by a combination of selection on standing genetic variation and de novo mutations. On a macroevolutionary scale, H 2 S tolerance in 10 independent lineages of sulfide spring fishes across multiple genera of Poeciliidae is correlated with the convergent modification and expression changes in genes associated with H 2 S toxicity and detoxification. Our results demonstrate that the modification of highly conserved physiological pathways associated with essential mitochondrial processes mediates tolerance to physiochemical stress. In addition, the same pathways, genes, and—in some instances—codons are implicated in H 2 S adaptation in lineages that span 40 million years of evolution

Convergent evolution of conserved mitochondrial pathways underlies repeated adaptation to extreme environments.

Extreme environments test the limits of life; yet, some organisms thrive in harsh conditions. Extremophile lineages inspire questions about how organisms can tolerate physiochemical stressors and whether the repeated colonization of extreme environments is facilitated by predictable and repeatable evolutionary innovations. We identified the mechanistic basis underlying convergent evolution of tolerance to hydrogen sulfide (H2S)-a toxicant that impairs mitochondrial function-across evolutionarily independent lineages of a fish (Poecilia mexicana, Poeciliidae) from H2S-rich springs. Using comparative biochemical and physiological analyses, we found that mitochondrial function is maintained in the presence of H2S in sulfide spring P. mexicana but not ancestral lineages from nonsulfidic habitats due to convergent adaptations in the primary toxicity target and a major detoxification enzyme. Genome-wide local ancestry analyses indicated that convergent evolution of increased H2S tolerance in different populations is likely caused by a combination of selection on standing genetic variation and de novo mutations. On a macroevolutionary scale, H2S tolerance in 10 independent lineages of sulfide spring fishes across multiple genera of Poeciliidae is correlated with the convergent modification and expression changes in genes associated with H2S toxicity and detoxification. Our results demonstrate that the modification of highly conserved physiological pathways associated with essential mitochondrial processes mediates tolerance to physiochemical stress. In addition, the same pathways, genes, and-in some instances-codons are implicated in H2S adaptation in lineages that span 40 million years of evolution

Loss of DNA repair mechanisms in cardiac myocytes induce dilated cardiomyopathy.

Cardiomyopathy is a progressive disease of the myocardium leading to impaired contractility. Genotoxic cancer therapies are known to be potent drivers of cardiomyopathy, whereas causes of spontaneous disease remain unclear. To test the hypothesis that endogenous genotoxic stress contributes to cardiomyopathy, we deleted the DNA repair gene Ercc1 specifically in striated muscle using a floxed allele of Ercc1 and mice expressing Cre under control of the muscle-specific creatinine kinase (Ckmm) promoter or depleted systemically (Ercc1-/D mice). Ckmm-Cre+/- ;Ercc1-/fl mice expired suddenly of heart disease by 7 months of age. As young adults, the hearts of Ckmm-Cre+/- ;Ercc1-/fl mice were structurally and functionally normal, but by 6-months-of-age, there was significant ventricular dilation, wall thinning, interstitial fibrosis, and systolic dysfunction indicative of dilated cardiomyopathy. Cardiac tissue from the tissue-specific or systemic model showed increased apoptosis and cardiac myocytes from Ckmm-Cre+/- ;Ercc1-/fl mice were hypersensitive to genotoxins, resulting in apoptosis. p53 levels and target gene expression, including several antioxidants, were increased in cardiac tissue from Ckmm-Cre+/- ;Ercc1-/fl and Ercc1-/D mice. Despite this, cardiac tissue from older mutant mice showed evidence of increased oxidative stress. Genetic or pharmacologic inhibition of p53 attenuated apoptosis and improved disease markers. Similarly, overexpression of mitochondrial-targeted catalase improved disease markers. Together, these data support the conclusion that DNA damage produced endogenously can drive cardiac disease and does so mechanistically via chronic activation of p53 and increased oxidative stress, driving cardiac myocyte apoptosis, dilated cardiomyopathy, and sudden death