Regulation of myo-inositol biosynthesis in tilapia (Oreochromis mossambicus) brain during salinity stress

The induction of the myo-inositol biosynthesis (MIB) pathway in Mozambique tilapia (Oreochromis mossambicus) is an important component of the cellular response to hyperosmotic challenge. Compatible osmolytes, such as myo-inositol, are organic molecules that are accumulated in cells to offset the damaging effects of hyperosmotic stress. This dissertation examined the effects of acute and chronic salinity stress on the regulation of the MIB pathway in tilapia brain. First, acute and chronic salinity challenges were conducted in vivo to determine the association between changes in blood plasma chemistry and transcriptional induction of MIB enzymes and myo-inositol levels in brain tissue. Highly positive correlations between plasma osmolality, MIB enzyme mRNA abundance, and brain myo-inositol levels were found under both salinity regimens. Next, immortalized tilapia cell lines were established and characterized by their response to hyperosmotic stress, including MIB mRNA induction. Two newly derived brain- and lip-lines as well as one previously established bulbus arteriosus line were found to be highly osmotolerant to acute sodium chloride stress. The tilapia cell lines also possessed transcriptionally osmo-sensitive MIB enzymes, suggesting that they are ecophysiologically relevant and thus, represent valid systems for studying the MIB pathway. Finally, the last group of studies investigated the interactive effects of lithium, an environmental pollutant and inhibitor of one of the MIB enzymes, and salinity on MIB regulation in tilapia brain using in vivo and in vitro (brain cell line) approaches. MIB enzyme transcriptional induction was found to be suppressed in response to the combined stressors in vitro, but not in vivo, demonstrating the physiological complexity that exists in the whole organism and indicating that bioavailability/ lithium uptake may be an important parameter for its toxicity. Overall, this dissertation has highlighted the importance of the MIB pathway in tilapia brain using in vivo and in vitro approaches. In addition, this work has opened up new possibilities for in-depth mechanistic work taking advantage of these new cell culture tools and the robust salinity-dependent MIB pathway activation. Such work will significantly advance our understanding of the effects of environmental salinity and pollution (lithium) on fish.