Aquaporin expression in the Japanese medaka (Oryzias latipes, Temminck & Schlegel) in FW and SW: challenging the paradigm for intestinal water transport?

Abstract

Abstract We investigated the salinity dependent expression dynamics of 7 aquaporin paralogs (aqp1a, -3a, -7, -8ab, -10a, -10b and -11a) in several tissues of euryhaline Japanese medaka (Oryzias latipes). All paralogs except aqp7 and -10a had a broad tissue distribution and several were affected by salinity in both osmoregulatory and non-osmoregulatory tissues. In the intestine, aqp1a, -7, -8ab and -10a decreased upon seawater (SW)-acclimation, both when comparing long-term acclimated fish and during 1-3 days of the transition period (freshwater (FW)-to-SW and SW-to-FW). In the gill, aqp3a was lower and aqp10a higher in SW than in FW. In the kidney no aqps were affected by salinity. In the skin, aqp1a and -3a were lower in SW than in FW. In the liver aqp8ab and -10a were lower in SW than in FW. Further, 6 Na+,K+-ATPase α-subunit isoform transcripts were analyzed in the intestine but showed no consistent response to salinity, suggesting that water transport is not regulated at this level. In contrast, mRNA of the Na+,K+, 2Cl--cotransporter type-2 showed a strong increase in the intestine in SW compared to FW fish. Using custom made antibodies, we localized Aqp1a, -8ab and -10a proteins in the apical region of enterocytes of FW fish. Apical staining intensity strongly decreased, vanished or moved to sub apical regions, when fish were acclimated to SW, supporting the lower mRNA expression in SW. Western blots confirmed the decrease in Aqp1a and -10a protein in SW. The strong decrease in aquaporin expression in the intestine of SW fish is surprising and challenges the paradigm for transepithelial intestinal water absorption in SW fishes.