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The role of mitochondrial uncoupling in temperature responses in Atlantic killifish, Fundulus heteroclitus Bryant, Heather Jean


Environmental temperature can greatly impact the functioning of ectothermic organisms through effects on mitochondria, which are crucial to aerobic metabolism. Changes in temperature have the potential to influence mitochondrial ATP production and production of reactive oxygen species (ROS), both of which are influenced by the activity of the mitochondrial electron transport system, which generates the proton gradient necessary for mitochondrial ATP production. Thus, I hypothesized that ectothermic organisms have a mechanism for modulating the proton gradient in the face of changes in environmental temperature to maintain ATP production, and that this mechanism may act through uncoupling proteins (UCPs) which can cause a decrease in the proton gradient independent of the production of ATP. Here, I investigate changes in UCPs and mitochondrial function following thermal acclimation in two populations of the eurythermal Atlantic killifish, Fundulus heteroclitus. I show that UCP mRNA expression is tissue-specific, changes with thermal acclimation, and differs between two populations of killifish. However, these changes vary depending on the isoform, tissue, and population (Chapter 2). I also demonstrate that changes in UCP function are not necessarily consistent with changes in mRNA expression in isolated liver and brain mitochondria, but that UCP function may differ in liver between the two populations (Chapter 3). Cold-acclimated northern killifish increase liver mitochondrial capacity and coupling as indicated by increases in state III, respiratory control and ADP/O ratios (Chapter 3). Interestingly, I also observed increases in proton conductance in isolated liver mitochondria from cold-acclimated northern killifish as indicated by increased O₂ consumption rate at a common membrane potential (Chapter 3). Mitochondrial properties in southern killifish did not differ with thermal acclimation. Taken together, my data suggest that UCPs may play a role in thermal acclimation, although there is not a clear connection between UCP mRNA expression and function. Furthermore, my data indicate that northern killifish may have a greater capacity to respond to low temperature acclimation than southern killifish, suggesting a potential role for adaptive variation in mitochondrial responses to temperature.

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