No abstract

In order to probe the interaction between an invading microorganism and its host, we have investigated differential gene expression in Atlantic salmon (Salmo salar) experimentally infected with the pathogen Aeromonas salmonicida, the causative agent of furunculosis. Subtractive cDNA libraries were constructed by suppression subtractive hybridization (SSH) from 3 immune-relevant tissues at 2 time points during the infection process. Both forward- and reverse-subtracted libraries were generated, and approximately 200 clones were sequenced from each library, giving a total of 1778 expressed sequence tags (ESTs), which were annotated according to functional categories and deposited in GenBank (BQ035314-BQ037059). Numerous genes involved in signal transduction, innate immunity, and other processes have been uncovered in the subtractive libraries. These include known acute-phase reactants, along with more novel genes encoding proteins such as tachylectin, hepcidin, precerebellin-like protein, O-methyltransferase, a putative saxitoxin-binding protein, and others. A subset of genes that were represented in the subtracted libraries was further analyzed by virtual Northern, or reverse transcription-polymerase chain reaction (RT-PCR) assays to verify their differential expression as a result of infection.

In winter, rainbow smelt (Osmerus mordax) accumulate glycerol and produce an antifreeze protein (AFP), which both contribute to freeze resistance. The role of differential gene expression in the seasonal pattern of these adaptations was investigated. First, cDNAs encoding smelt and Atlantic salmon (Salmo salar) phosphoenolpyruvate carboxykinase (PEPCK) and smelt glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were cloned so that all sequences required for expression analysis would be available. Using quantitative PCR, expression of beta actin in rainbow smelt liver was compared with that of GAPDH in order to determine its validity as a reference gene. Then, levels of glycerol-3-phosphate dehydrogenase (GPDH), PEPCK, and AFP relative to beta actin were measured in smelt liver over a fall-winter-spring interval. Levels of GPDH mRNA increased in the fall just before plasma glycerol accumulation, implying a driving role in glycerol synthesis. GPDH mRNA levels then declined during winter, well in advance of serum glycerol, suggesting the possibility of GPDH enzyme or glycerol conservation in smelt during the winter months. PEPCK mRNA levels rose in parallel with serum glycerol in the fall, consistent with an increasing requirement for amino acids as metabolic precursors, remained elevated for much of the winter, and then declined in advance of the decline in plasma glycerol. AFP mRNA was elevated at the onset of fall sampling in October and remained elevated until April, implying separate regulation from GPDH and PEPCK. Thus, winter freezing point depression in smelt appears to result from a seasonal cycle of GPDH gene expression, with an ensuing increase in the expression of PEPCK, and a similar but independent cycle of AFP gene expression.