Effects of acclimation salinity on the expression of selenoproteins in the tilapia, Oreochromis mossambicus

Abstract

Selenoproteins are ubiquitously expressed, act on a variety of physiological redox-related processes, and are mostly regulated by selenium levels in animals. To date, the expression of most selenoproteins has not been verified in euryhaline fish models. The Mozambique tilapia, Oreochromis mossambicus, a euryhaline cichlid fish, has a high tolerance for changes in salinity and survives in fresh water (FW) and seawater (SW) environments which differ greatly in selenium availability. In the present study, we searched EST databases for cichlid selenoprotein mRNAs and screened for their differential expression in FW and SW-acclimated tilapia. The expression of mRNAs encoding iodothyronine deiodinases 1, 2 and 3 (Dio1, Dio2, Dio3), Fep15, glutathione peroxidase 2, selenoproteins J, K, L, M, P, S, and W, was measured in the brain, eye, gill, kidney, liver, pituitary, muscle, and intraperitoneal white adipose tissue. Gene expression of selenophosphate synthetase 1, Secp43, and selenocysteine lyase, factors involved in selenoprotein synthesis or in selenium metabolism, were also measured. The highest variation in selenoprotein and synthesis factor mRNA expression between FW- and SW-acclimated fish was found in gill and kidney. While the branchial expression of Dio3 was increased upon transferring tilapia from SW to FW, the inverse effect was observed when fish were transferred from FW to SW. Protein content of Dio3 was higher in fish acclimated to FW than in those acclimated to SW. Together, these results outline tissue distribution of selenoproteins in FW and SW-acclimated tilapia, and indicate that at least Dio3 expression is regulated by environmental salinity.

Keywords: Acclimation salinity; Euryhaline; Fish; Selenium; Selenoprotein; Tilapia.

Figure 1

Expression of selenoproteins of male Mozambique tilapia acclimated to FW (open bars) or SW (solid bars). Selenoprotein mRNA levels are shown as the ratio of target genes to EF1α. The following genes were measured: Dio 1 (A), Dio2 (B), Dio3 (C), Fep15 (D), GPx2 (E), SelJ (F), SelK (G), Sel L (H), SelM (I), Sepp1a (J), SelS (K), and SelW (L). *, **, *** correspond to significant differences between FW and SW at P<0.05, P<0.01 and P<0.001, respectively, using unpaired Student’s t-test, n=6–36.

Figure 2

Expression of selenoprotein synthesis factors of male Mozambique tilapia acclimated to FW (open bars) or SW (solid bars). Selenoprotein synthesis factor mRNA levels are shown as the ratio of target genes to EF1α. The following genes were measured: SPS1 (A), Secp43 (B) and Scly (C).*, ** correspond to significant difference between FW and SW at P<0.05 and P<0.01, respectively, using unpaired Student’s t-test, n=6.

Figure 3

Expression of Dio3 in gill of Mozambique tilapia acclimated to either FW or SW. Dio3 protein intensity levels were normalized by the β-actin levels, and ratios are plotted as average ± SEM (n=6). White bar represents FW-acclimated fish; black bar represents SW-acclimated fish. *P<0.05 by analysis with Student’s t-test.

Figure 4

Changes in Dio3 mRNA expression after transfer of Mozambique tilapia from SW to FW and FW to SW. Bars represent branchial mRNA expression ± SEM (n=6–29). Gene expression is presented relative to the time 0 SW group (black bar). Differences among groups were evaluated by one-way ANOVA, followed by Fisher’s LSD test. Means not sharing the same letter are significantly different (P<0.05).