Hypoxia alters gene expression in the gonads of zebrafish

The objectives of this study were to characterize gene expression responses to hypoxia in gonads of mature zebrafish (Danio rerio), and to start characterizing modes of action by which hypoxia could potentially alter reproduction. Adult male and female zebrafish were maintained under normoxia (7 mg O2/L), moderate hypoxia (3 mg O2/L) and severe hypoxia (1 mg O2/L) for 4 and 14 days (n = 5 per treatment) and changes in gene expression in gonadal tissues were evaluated using a commercial 21,000 gene zebrafish oligonucleotide microarray. Differentially expressed genes were determined using ANOVA (p < 0.01), and enriched gene ontology (GO) categories (p < 0.05) identified using GeneSpring GX software (Agilent, USA). Short-term (4 d) exposure to hypoxia affected expression of genes associated with the initial adaptive responses such as: metabolism of carbohydrates and proteins, nucleotide metabolism, haemoglobin synthesis, reactive oxygen species metabolism and locomotion. More prolonged (14 d) hypoxia affected a suite of genes belonging to different GO categories: lipid metabolism, reproduction (e.g., steroid hormone synthesis), and immune responses (cytokine synthesis). The results of the present study demonstrate that reproduction likely would be affected by hypoxia via multiple modes of action. These include previously hypothesized mechanisms such as inhibition of steroidogenesis via modulation of expression of steroidogenic genes, and downregulation of serotonergic pathway. But, in addition to these, we propose that there are multiple other points of disruption of reproductive system function linked, for example, to reorganization of lipid transport and other mechanisms involved in responding to hypoxia (e.g., hydroxysteroid dehydrogenase alterations, downregulation of contractile elements etc.). Keywords: time course and dose response Reproductively mature zebrafish (Danio rerio; ab wild-type strain; 7 months old) were obtained from an on-site culture unit. Mixed groups of four males and four females were randomly loaded into 10 L tanks, receiving a continuous flow of Lake Superior water. Zebrafish were exposed to hypoxia for duration of 4 and 14 days. There were three oxygen treatments: normoxia (6.5 mg dissolved O2/L), moderate hypoxia (3 mg dissolved O2/L) and severe hypoxia (1 mg dissolved O2/L). Each exposure tank contained four males and four females, and each time and oxygen content treatment combination was replicated twice (i.e., fish were held in two separate exposure tanks). The 4 and 14 d exposures were conducted concurrently. Aliquots of gonadal RNA from male and female fish exposed to hypoxia for 4 and 14 days (five biological replicates from each of the three oxygen treatment groups) were submitted to an Agilent certified facility, MOgene, LC (St. Louis, MO), for gene expression profiling.