Research |
Our research focuses on the physiological adaptations of marine and freshwater organisms to naturally-occurring environmental stressors, such as hypoxia, hydrogen sulfide, temperature and caloric restriction, and the effects of contaminants on these adaptations. We utilize a wide variety of techniques (ranging from biophysics, biochemistry and molecular biology, to whole-animal metabolic measurements and field studies) to understand these adaptations among many animals and habitats. We focus primarily on marine invertebrates, but we also use "model" systems, such as yeast and mammalian cell cultures, when we think those systems will help us get closer to the answers.
We currently have two major projects in our lab:
- Cellular and mitochondrial adaptations of marine invertebrates to hydrogen sulfide and free radicals.
- Mitochondrial responses to free radicals and aging in mammalian tissues
(with Christiaan Leeuwenburgh).
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We are particularly interested in physiological adaptations of marine animals to the metabolic poison hydrogen sulfide. This gas (and weak acid) is a natural byproduct of bacterial reduction in environments that are both high in nutrients and low in oxygen. These two conditions commonly coexist in the sediments of mudflats, estuaries, rivers and lakes. Hydrogen sulfide is also present at high concentrations at certain locations on the sea floor, such as at hydrothermal vents and cold seeps.
We've been focusing recently on the polychaete bloodworm Glycera dibranchiata and the hard clam Mercenaria mercenaria, the latter in studies led primarily by Joanna Joyner-Matos.
In the past, one of our favorite animals to study the adaptations of animals to hydrogen sulfide was the echiuran worm Urechis caupo. Much of this work was done with Alissa Arp. More information on U. caupo is on our Urechis page, where you can even find some videos of U. caupo's feeding behavior. We have also worked with Charles Fisher on deep sea tubeworms.
References:
Joyner-Matos J, Chapman LJ, Downs CA, Hofer T, Leeuwenburgh C and Julian D. Stress response of a freshwater clam along an abiotic gradient: Too much oxygen may limit distribution. Functional Ecology, in press.
Joyner-Matos JL, Downs CA and Julian D. Increased expression of stress proteins in the surf clam Donax variabilis following hydrogen sulfide exposure. Comparative Biochemistry and Physiology, Part A, 145: 245-257. |
The most common method by which animals protect themselves from the sulfide toxicity is oxidation of hydrogen sulfide to less toxic endproducts, especially thiosulfate. In general, this oxidation is catalyzed by metal-containing molecules within cells, such as hemoglobin, hematin and mitochondrial cytochromes. In some cases, transition metals are concentrated within electron-dense, intracellular organelles that are commonly called "sulfide oxidizing bodies". One of our current projects is to understand the origin and function of these organelles. In our lab, Stephanie Wohlgemuth, working with Alissa Arp, has shown that these organelles increase in number in the epithelial tissues within hours after an polychaete is exposed to sulfide in the lab, and furthermore that these organelles then disappear almost as quickly. We are currently investigating the hypothesis that these structures represent autophagosomes. |
Reference: Wohlgemuth SE, Arp AJ, Bergquist DC and Julian D. Rapid induction and disappearance of electron-dense organelles following sulfide exposure in the marine annelid Branchioasychis americana. Invertebrate Biology, in press. |
Even in animals with sulfide detoxification mechanisms, at least some tissues (especially respiratory epithelia and blood cells) are exposed to potentially toxic sulfide levels. Sulfide is well known as a reversible poison of mitochondrial oxidative phosphorylation via inhibition of cytochrome c oxidase (COX). However, we have recently shown that sulfide also causes direct and apparently irreversible mitochondrial injury, and that this even occurs in cells of sulfide-adapted animals. The injury is manifested in mitochondrial depolarization (i.e., loss of the mitochondrial proton motive force), as shown in the figure at right. The mechanism of toxicity is likely not via inhibition of COX, since other COX inhibitors and other mitochondrial electron transport chain inhibitors do not produce similar effects. Furthermore, pharmacological inhibition of the mitochondrial permeability transition (PT) pore fails to prevent sulfide-induced depolarization. Finally, increased oxidation of the free radical indicators H2DCFDA and MitoSOXTM in erythrocytes exposed to sulfide suggests that sulfide oxidation increased oxidative stress and superoxide production, respectively. Together, these results indicate that sulfide exposure causes mitochondrial depolarization in cells of a sulfide-tolerant annelid, and that this effect, which differs from the actions of other COX inhibitors, may be via increased free radical damage.
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| Reference: Julian D, April KL, Patel S, Stein JR and Wohlgemuth SE (2005). Mitochondrial depolarization following hydrogen sulfide exposure in erythrocytes from a sulfide-tolerant marine invertebrate. Journal of Experimental Biology 208, 4109-4122 |
Hydrogen sulfide at concentrations of 0.05 to 1 mmol l‑1 appears to function as a gasotransmitter in vertebrates, analogous to nitric oxide (NO) and carbon monoxide, but the actions of H2S in invertebrate tissue are largely unknown. We recently investigated the role of H2S in modulating body wall muscle tone in Urechis caupo (reference 2 below). We first determined that U. caupo body wall homogenates produce H2S upon addition of L-cysteine and pyridoxal-5′-phosphate (PLP), and that the rate is increased by addition of 2-mercaptoethanol, suggesting the presence of an activated L-serine sulfhydrase pathway, as we had shown previously in a clam and annelid (reference 1 below). We then measured the contractile response of U. caupo body wall circular muscle strips to NaHS (which produces H2S in solution) and the NO donor sodium nitroprusside (SNP), both with and without subsequent application of acetylcholine (ACh). We found that NaHS alone stimulated contraction in muscle strips equivalent to about one-third the force of ACh alone, whereas SNP alone had no effect on muscle tone (see figure at right). However, simultaneous addition of NaHS with SNP elicited a much stronger contraction, reaching more than twice that of ACh alone, which could be increased further by subsequent application of ACh.
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References:
Julian D, Statile JL, Wohlgemuth SE, Arp AJ (2002). Enzymatic hydrogen sulfide production in marine invertebrate tissues. Comp. Biochem. Physiol. A. Mol. Integr. Physiol. 133:105-115.
Julian D, Statile J, Roepke T and Arp AJ (2005). Sodium nitroprusside potentiates H2S-induced contractions in body wall muscle from a marine worm. Biological Bulletin 209: 6-10. |
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