Tissue-specific gene signatures of gilthead sea bream (Sparus aurata L.) after hyper- and hypo-osmotic challenges

Euryhaline teleosts can live in a wide range of environmental salinities. Transcriptomic analysis in important osmoregulatory and non-osmoregulatory tissues (liver, gills and hypothalamus) by means of a specific oligo-microarray revealed new aspects related to the osmoregulatory processes mediated by different canonical pathways after hypo- or hyper-omotic challenges in gilthead sea bream (Sparus aurata) juveniles. Liver was the most responsive tissue to salinity changes. In addition, around 24 %, 8 % and 12 % of the total genes differentially expressed were affected and regulated in the same way (up- or down-regulated) by both hyper- and hypo-osmotic challenges in liver, gills and hypothalamus, respectively. In liver and gills, functional analysis of the differentially expressed genes established 2 major clusters of canonical pathways related to i) Energy Metabolism and ii) Oxidative Stress: Damage and Repair, whereas for hypothalamus only a main clusters linked to Oxidative Stress: Damage and Repair was identified. Further, this common cluster presents different sub-nodes in all analyzed tissues, with a clear dualism between liver and gills in those pathways involved in nitrogenous turnover, as well as between gills and hypothalamus in cell stress processes. In addition, several sub-nodes related to cell and tissue architecture was only found in hypothalamus. Moreover, only genes involved in xenobiotic metabolism signaling canonical pathway were differentially regulated by both hypo- and/or hyper-salinity transfer in the three tissues considered. Our results indicated that hepatic, branchial and hypothalamic transcriptome of the sea bream clearly responded after 7 days in different environmental salinities, when mainly metabolic and different cell functions are activated to reach homeostasis, or even return to their basal levels. Immature males of gilthead sea bream (Sparus aurata L., 80-100 g body mass, n=36) were acclimated for 10 days to experimental control sea water (SW, 38 ‰ salinity, 1049 mOsm·kg−1 H2O osmolality). Fish were randomly distributed in three 400-L tanks in an open system circuit (~ 2.5 kg·m−3 density), under natural photoperiod and constant temperature (18-19 °C). After the acclimation period, the animals were directly transferred, in duplicate, to the following environmental salinities: seawater (SW, 38 ‰ salinity, control group), low salinity water (LSW, 5 ‰ salinity, 139 mOsm·kg−1 H2O osmolality, hypoosmotic environment), and high salinity water (HSW, 55 ‰ salinity, 1439 mOsm·kg−1 H2O osmolality, hyperosmotic environment). On day 7 post-transfer, fish were anesthetized with 2-phenoxyethanol (1 mL·L−1 at the specific salinity water). After blood extraction, hypothalamic lobules and representative samples of liver and gills were dissected.