Molecular and Epigenetic Consequences of Short-Term Hypoxia Exposure in Rainbow Trout (Oncorhynchus mykiss)

Abstract

Environmental hypoxia affects freshwater systems, and acute hypoxic events are expected to increase in frequency with climate change. Rainbow trout (Oncorhynchus mykiss), a salmonid species of economic and cultural importance in Canada and introduced to freshwaters across most continents, is an active species that is sensitive to environmental hypoxia. In this thesis, I investigate the hypothesis that rainbow trout acutely exposed to mild (50% oxygen saturation) and severe (25% oxygen saturation) hypoxia exhibit rapid metabolic changes which are indicative of anerobic glycolytic and are, at least in part, mediated at the gene expression level and I hypothesize that these changes in gene expression are mediated by histone modifications. While glucose and lipid metabolite concentrations are not significantly affected by acute hypoxia exposure suggesting tight regulation to maintain steady-state, transcripts coding for key components of glucose, lactate, and lipid transcripts are differentially induced in a tissue-specific manner. Hepatic transcripts were most sensitive to hypoxia, favouring induction of transcripts related to glucose metabolism like in white muscle tissue. Under severe hypoxia exposure, coordinated expression of transcripts related to lipid metabolism was observed in liver, but also muscle and adipose tissue, possibly indicating a scavenging response of peroxided lipids rather than a use as fuel. When probing a role for the O2-sensitive epigenetic mark H3K4me3 in upstream prompter regions of induced hepatic genes mct1 and pck1, no clear relationship between O2-dependent regulation of this mark and gene expression was found, suggesting other transcriptional control mechanisms are involved. Together, this thesis provides novel insight into molecular underpinnings and thresholds of tissue-specific metabolic responses in rainbow trout and opens the field for future comparative investigation of epigenetic mechanisms of responses to hypoxia in fishes.