Expression Analysis of Glycerol Synthesis–Related Liver Transcripts in Rainbow Smelt (Osmerus mordax) Exposed to a Controlled Decrease in Temperature

Abstract

Rainbow smelt (Osmerus mordax) accumulate high glycerol levels to avoid freezing at subzero temperatures. Glyceroneogenesis is activated by low temperature and occurs in liver via a branch in glycolysis and gluconeogenesis. In this study, carbohydrate and liver transcript levels of 21 genes potentially associated with glycerol production were assessed during a controlled warm to cold transition. Smelt were held at 8°C (warm smelt; non-glycerol accumulating) or subjected to a controlled decrease in water temperature from 8° to 0°C (cold smelt; glycerol accumulating) and sampled at the end of the temperature decrease and 1 mo later. In cold smelt compared with warm smelt, liver glycogen levels were lower and phosphoglucomutase transcript levels were higher. Plasma glycerol levels were higher and increased over time in cold smelt; in cold smelt, liver phosphofructokinase and pyruvate dehydrogenase kinase transcript levels increased over time. These findings imply that glycerol production is being fueled by glycogen degradation and inhibition of pyruvate oxidation serves to channel metabolic flux toward glycerol as opposed to complete glycolysis. Plasma glucose and liver glucose-6-phosphatase transcript levels were higher. Lipoprotein lipase transcript levels were higher, suggesting enhanced lipid breakdown to fuel energy metabolism. Glutamine synthetase transcript levels were higher, perhaps to store nitrogen for biosynthesis in spring.

Molecular basis of chill resistance adaptations in poikilothermic animals

“…Glycerol is by far the most common polyol identified in overwintering insects or in insect responses to cold conditioning (reviewed by Storey and Storey, 2005). Surprisingly, glycerol is also accumulated up to 500 mmol l -1 in arctic fish species, such as the rainbow smelt (Clow et al, 2008;Hall et al, 2012;Raymond, 1995), where it is believed to provide part of the necessary freeze resistance along with antifreeze proteins.However, in most instances it remains uncertain whether glycerol itself, or metabolites derived from glycerol, e.g. glycerol-3-phosphate (G-3-P), account for the enhanced stress tolerance.…”

Rainbow smelt: the unusual case of cryoprotection by sustained glycerol production in an aquatic animal

“…PDH kinase serves to deactivate PDH via phosphorylation. Rainbow smelt acclimated to 0 °C had 3-fold higher transcript levels of PDH kinase than animals held at 8 °C (Hall et al 2012). In liver cells isolated from 8 °C acclimated fish, transcript level increased after 24 h of incubation at 0.4 °C and by 72 h was 20-fold higher in cells incubated at 0.4 °C than at 8 °C (Fig.…”

“…Activity levels of phosphofructokinase (PFK) and aldolase were significantly higher in liver of rainbow smelt than capelin (Table 1) (Treberg et al 2002a). Furthermore, there were significantly higher PFK transcript levels in smelt held at 0 °C than animals maintained at 8 °C, but no difference in glyceraldehyde 3-phosphate dehydrogenase (GAPDH) transcript levels between groups (Hall et al 2012). Also, the transcript level of aldolase shows a substantial seasonal increase, with levels in February being about 13-fold higher than in October (Richards et al 2010).…”

“…6). Although considerably more evidence would be required to prove that PDH is inhibited during glycerol production, the findings to date are most compelling and indeed are the most clear cut, substantive and consistent of detailed functional genomic studies involving microarray and suppression subtractive hybridization analysis followed by QPCR (Richards et al 2008(Richards et al , 2010Hall et al 2011Hall et al , 2012. …”

“…It is more likely that urea is produced from dietary arginine (Raymond 1998) leaving the question of the fate of NH 4 + resulting from amino acid utilization. NH 4 + could be simply excreted; however, one of the major findings of the genomic studies is that glutamine synthetase expression in liver follows exactly the same pattern as that of PDH kinase in both whole animal and isolated cell experiments (Hall et al , 2012. Glutamine synthetase catalyzes the conversion of glutamate plus NH 4 + to glutamine.…”