HYPOXIA-INDUCIBLE FACTOR AND TRANSCRIPTIONAL CONTROL OF OCTOPINE DEHYDROGENASE IN BAY SCALLOPS Argopecten irradians  

Hypoxia inducible factor, HIF, is the key transcription factor that regulates the metazoan response to low oxygen.  Recent studies show that HIF targets hundreds of genes, increasing oxygen supply and as well as the rate of glycolysis and anaerobic fermentation.  In vertebrates one known target of HIF regulation is lactate dehydrogenase, and Hif overexpression can be associated with Ldh overexpression and concomitant high rates of glycolytic flux.  In bay scallops, Argopecten irradians, as well as many other molluscs, anaerobic fermentation is catalyzed instead by octopine dehydrogenase, ODH, which condenses L-arginine and pyruvate to replenish NAD⁺ for glycolysis.  This activity sustains the high levels of glycolytic ATP needed for scallop burst swimming and recovery during the escape response.  We are interested using ODH as a model to understand transcriptional and translational regulation in scallops, and in particular we hypothesize that HIF regulates octopine dehydrogenase expression in a manner analogous to vertebrate Ldh.   Previous work in our lab identified putative binding sites for HIF in the promoter as well as in the 5' UTR of the octopine dehydrogenase gene, consistent with observations from other taxa suggesting that HIF pauses RNA polymerase II during transcription. We have successfully isolated and sequenced a portion of Hif from bay scallops. Using quantitative PCR, we are testing the hypothesis that Hif and Odh expression in muscle and gill tissues are associated in exercise-induced scallops, scallops exposed to environmental hypoxia, and in scallops experiencing both environmental hypoxia and exercise.  Additionally we are examining for the first time the relationship between Odh expression and ODH specific activity under these same experimental conditions.  Odh expression is upregulated by swimming and in scallops exposed to hypoxia and swimming, demonstrating that transcriptional control is, at least in part, responsible for changes in ODH enzyme activity.  Hif, however, appears to be expressed consitutively under all conditions tested.  Identifying the interactions between HIF and ODH will help us understand regulatory mechanisms responsible for cellular responses to physiological and environmental stress.  Future studies will mechanistically test the hypothesis for direct binding of HIF to scallop Odh.