Date of Award:
Doctor of Philosophy (PhD)
Alan H. Savitzky
Many plants and animals are defended by toxic compounds, and circumvention of those defenses often has involved the evolution of elaborate mechanisms for tolerance or resistance of the toxins. Toads synthesize potent cardiotonic steroids known as bufadienolides (BDs) from cholesterol and store those toxins in high concentrations in their cutaneous glands. Those toxins protect toads from the majority of predators, including most snakes that readily consume other species of frogs. BDs exert their effect by inhibiting ion transport by the Na+/K+-ATPase (NKA). This ubiquitous transmembrane enzyme consists of a catalytic alpha-subunit, which carries out the enzyme's functions, and a glycoprotein beta-subunit, which provides structural stability. Inhibition of the NKA causes highly elevated intracellular Ca2+ levels and results in often lethal increased cardiac contraction strength. Molecular resistance to bufadienolides in snakes is conferred by mutations in the alpha-subunit of the Na+/K+-ATPase. I have found that these mutations are more prevalent in snakes than previously suggested, and that many genetically resistant species do not feed on toads. This suggests that possession of the mutations alone does not carry substantial negative consequences, and that feeding on toads may have been an ancestral habit in some groups of snakes. I have further found evidence of tissue-specific variation in resistance to bufadienolides, and gene expression investigations revealed that the bufadienolide resistance-conferring mutations are not expressed equally among different organs. Variation in resistance among different tissues indicates that possession of the mutations does not protect all cells equally. Finally, by testing the physiological responses of resistant snakes to exposure to cardiotonic steroid, I have found that feeding on toads incurs negative consequences and that toad-specialized resistant snakes respond differently from nontoad-specialized resistant snakes. The presence of physiological consequences of toxin exposure may explain why feeding on toads has been lost in some lineages of snakes that retain resistance-conferring mutations. In summary, these findings indicate that genetic resistance of the Na+/K+-ATPase is necessary in order for snakes to survive acute toxicity of bufadienolides, but it is not sufficient to explain fully the physiological mechanisms involved in dealing with chronic exposure to the toxins.
Mohammadi, Shabnam, "Molecular and Physiological Mechanisms of Toxin Resistance in Toad-Eating Snakes" (2017). All Graduate Theses and Dissertations. 5856.
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