Molecular physiology of ion transport in lower vertebrates.

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The central objective of my research program is to elucidate the mechanisms of ion and acid-base regulation in aquatic vertebrates with the ultimate goal of understanding adaptation across evolutionary to life-cycle time scales. This involves investigating groups from the agnathans through to the amphibians. My research is integrative, incorporating molecular-genetic, biochemical and cellular techniques to whole animal studies. This includes a strong emphasis on the development and use of immunodetection techniques to characterize the expression of ion transporters, a focus which has served as the backbone of my methodological approach over the past decade. My first NSERC-DG will be an original and innovative exploration into the functional role of the hydrogen-potassium ATPase (HKA) in ion and acid-base regulation. This “pump” has multiple subunit isoforms with diverse evolutionary patterns of expression across taxa. The HKA is also central to gastric acidification, and plays a part in the intriguing story of stomach loss in vertebrate evolution that will be addressed. Homeostasis of extracellular [K+] is essential to control the excitability of nerve and muscle cells and small changes can have dramatic, adverse effects. Active K+ uptake has been observed in freshwater fishes, however, the mechanism has never been defined. I propose the HKA as a novel mechanism for K+ uptake and acid excretion in lower vertebrates. There are multiple HKA a subunits (HKa1, a1b, a2) that pair with HKß or NKß subunits to function as heterodimers. My program will address the physiological significance of the diversity of HKa subunits in different taxonomic groups. I will test the hypothesis that expression of multiple isoforms results in sub-functionalization and/or neofunctionalization of HKAs. In cases where only one isoform is expressed, I predict a dual function or loss of a function. The complex pattern of taxa-specific expression patterns suggests distinct functional patterns. Given the importance of a-ß pairings in determining transport properties, in vivo and ex vivo expression will be determined. The stomach is an important vertebrate innovation that is defined by acid-peptic digestion. Surprisingly, despite its contribution to digestive efficiency, stomach loss has occurred independently multiple times. My recent progress has established a clear pattern between loss of the gastric phenotype and loss of the responsible acid-peptic genes. In this proposal, the stomach loss pattern down to the family/genus level will be resolved in order to identify transition groups with loss of phenotype but with retention of gastric genes in either ion and/or acid-base regulation or recruitment for novel functions. The cost of gastric acid secretion will be estimated using a pharmacological approach in vivo and in vitro. I will test the hypotheses that high dietary buffer capacity, and environmental Cl- limitation are drivers for stomach loss by decreasing the efficacy of HKA acidification. In fish larvae, stomach development may be delayed for months and therefore heterochrony can result in paedomorphosis presenting an intriguing situation of loss of phenotype potentially preceding acid-peptic gene loss. The fate of these genes will be determined. My NSERC-DG will elucidate the role of the HKAs in an evolutionary context across a wide range of taxa and clarify its functional importance to adaptation to natural or anthropogenic changes in their environments. Insights will be gained into the intriguing story of stomach loss and the fate of gastric genes. The project will build upon established collaborations and create three new ones, while providing a strong training component for HQP including research opportunities for 6 graduate and +5 undergraduates.