In this work we have assessed the gene response of the three main mucosal surfaces of rainbow trout Oncorhynchus mykiss , skin, gills and gut, when subjected to a repeated hypoxia exposure. The aim of the work was to determine whether a hypoxia-sensitive fish species could habituate to repeated episodes of hypoxia, a circumstance that can occur more often provided under the situation of climate changes and global warming.
After the acclimation period, fish were randomly divided in 5 different treatment groups, 2 control groups, absolute control (AC) and manipulated control (MC), and 3 hypoxia groups: H1 that only received 1 hypoxic exposure, and H2 and H3 which received 2 and 3 hypoxic exposures respectively. Every exposure to hypoxia consisted in reducing the water oxygen level in the tanks from 8-9 mg/L to 2 mg/L by removing the aeration pumps and bubbling N2 into the system. After 1 hour hypoxia, fish were sampled at 1,6 and 24 post exposure. We analyzed a set of endocrine stress genes (CRH, GR1 and MR), immune-cytokines (IL1β, TNFα, IL10 and TGF β) and hypoxia-associated genes (HIF1, MYO).
The results showed that the response varied depending on the mucosa. Skin showed an alteration of both hormonal stress genes and cytokines that did not recover control levels, whereas hypoxia-related genes showed a ransient decrease followed by a recovery.
In gills, stress genes did not show relevant changes, cytokines showed a transient overexpression followed by a recovery for IL10 and TGF but not recovery for IL1 β and TNFα. Regarding hypoxia-related genes, MYO was overexpressed after 1 and 2 hypoxias and then recovered, whereas HIF1 was overexpressed only after 3 hypoxias.
In intestine, stress genes downregulated at H1 and H2 hypoxias and recoved at H3. Citokine genes showed a significant alteration at H1 but recovery at the subsequent H2 and H3 except for TNFα that was maintained downregulated and hypoxia-related genes showed an altered trend with a significant overexpression of HIF1 and a downregulation of MYO.
Overall the results show alterations in the gene expression after repeated hypoxia, with some degree of adaptation. Thus both hypoxia-related genes and cytokines show significant changes at H1 and H2, but also recovery at the third hypoxic repetition, whereas skin showed maintained downregulation in skin, but minor alterations in gills and intestine.