Extreme Ammonia Tolerance Mechanisms: A Model Vertebrate

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DESCRIPTION (provided by applicant): Hepatic Encephalopathy (HE), and resultant elevated blood and tissue ammonia concentrations (i.e., hyperammonemia, HA), has profound central nervous system (CNS) effects, and can have environmental causes. In particular, liver damage due to exposure to toxicants such as carbon tetrachloride, toluene, DDT, heptachlor, etc., as well as chronic alcoholism and direct exposure to environmental ammonia, can elicit symptoms of HE/HA. However, there are such a wide variety of CNS effects produced in the disease in humans, and in rodent experimental models, that it is difficult to determine which disease biomarkers are the most critical indicators of disease progression. Furthermore, characteristics of the rodent model present several weaknesses in the study of HE/HA. Because of this gap in our knowledge, no practical and effective clinical intervention strategies are available to prevent or reverse biomarkers or symptoms of the disease. Recently, we have identified a vertebrate model, the gulf toadfish (Opsanus beta), which is both extremely tolerant of ammonia insult, and which, by virtue of its aquatic lifestyle, enables a line of experimentation not practical in mammalian models, namely rapid "ammonia washout" protocols. Therefore, we propose to test several hypotheses aimed at exploiting these and other characteristics of this new model to address the lack of biomarkers and intervention strategies for HE/HA. In particular, we will: (1) test the hypothesis that there are reversible vs. irreversible biomarkers of HE/HA, and that these can be readily identified and distinguished in an aquatic model like the toadfish; (2) test the hypotheses that extreme ammonia tolerance in the toadfish, relative to mammals, is due to an unusual aspect of its physiology, in particular, either to a more robust ammonia detoxification system in the brain, or to an inherent insensitivity of brain mitochondrial metabolism to ammonia insult. As a further test of this second hypothesis, we will also explore the possibility that the toadfish has higher levels of naturally occurring ammonia protectant compounds (e.g., carnitine, trimethylamine oxide, etc.) in its brain tissues than do mammals. In sum, these experiments will lead to information which is not readily obtainable from humans and existing mammalian models concerning the mechanisms of action of ammonia and cellular capacity for tolerance and recovery, and thus to a better understanding of the causes and mechanisms underlying HE/HA that could lead to therapeutic strategies.