Identification and validation of differentially expressed transcripts in a hepatocyte model of cold-induced glycerol production in rainbow smelt (Osmerus mordax)

Abstract

Rainbow smelt (Osmerus mordax) avoid freezing by producing antifreeze protein (AFP) and accumulating glycerol. Glyceroneogenesis occurs in liver via a branch in glycolysis and gluconeogenesis and is activated by low temperature. Hepatocytes were isolated from the livers of fish acclimated to 8°C. Cells were incubated at warm (8°C; nonglycerol accumulating) or cold (0.4°C; glycerol accumulating) temperature over a 72-h time course. Reciprocal suppression subtractive hybridization libraries enriched for cold-responsive transcripts were constructed at 72 h. Microarray analyses using a 16K salmonid cDNA array were performed at 24, 48, and 72 h. Expression of type II AFP and 21 carbohydrate, amino acid, or lipid metabolism-related transcripts were validated using quantitative RT-PCR. Type II AFP transcript levels were not directly temperature related. In cold cells, levels of the glucose synthesis transcript were transiently higher. Increased glycerol production was not associated with increased phosphofructokinase or cytosolic glycerol-3-phosphate dehydrogenase transcript levels. Levels of transcripts (phosphoenolpyruvate carboxykinase, mitochondrial malate dehydrogenase, alanine aminotransferase, glutamate dehydrogenase, and aquaglyceroporin 9) associated with mobilization of amino acids to fuel glycerol accumulation were all transiently higher, suggesting a common regulatory mechanism. In cold compared with warm cells, pyruvate dehydrogenase kinase [an inhibitor of pyruvate dehydrogenase (PDH)] transcript levels were 20-fold higher. Potent inhibition of PDH would direct pyruvate and oxaloacetate derived from amino acids to glycerol, as opposed to oxidation via the citric acid cycle. Levels of a transcript potentially encoding glycerol-3-phosphatase, an enzyme not yet characterized in any vertebrate species, were higher following cold incubation. Finally, this study also presents the novel finding of increased glutamine synthetase transcript levels in response to low temperature.