Intestinal osmoregulatory mechanisms differ in Mediterranean and Atlantic European sea bass: A focus on hypersalinity

Abstract

European sea bass (Dicentrarchus labrax) migrate towards habitats where salinity can reach levels over 60‰, notably in Mediterranean lagoons. D. labrax are genetically subdivided in Atlantic and Mediterranean lineages and have evolved in slightly different salinities. We compared Atlantic and West-Mediterranean populations regarding their capacity to tolerate hypersalinity with a focus on the involvement of the intestine in solute-driven water reabsorption. Fish were analyzed following a two-week transfer from seawater (SW, 36‰) to either SW or hypersaline water (HW, 55‰). Differences among lineages were observed in posterior intestines of fish maintained in SW regarding NKA activities and mRNA expressions of nkaα1a, aqp8b, aqp1a and aqp1b with systematic higher levels in Mediterranean sea bass. High salinity transfer triggered similar responses in both lineages but at different magnitudes which may indicate slight different physiological strategies between lineages. High salinity transfer did not significantly affect the phenotypic traits measured in the anterior intestine. In the posterior intestine however, the size of enterocytes and NKA activity were higher in HW compared to SW. In this tissue, nka-α1a, nkcc2, aqp8ab and aqp8aa mRNA levels were higher in HW compared to SW as well as relative protein expression of AQP8ab. For aqp1a, 1b, 8aa and 8b, an opposite trend was observed. The subapical localization of AQP8ab in enterocytes suggests its role in transepithelial water reabsorption. Strong apical NKCC2/NCC staining indicates an increased Na+ and Cl-reuptake by enterocytes which could contribute to solute-coupled water reuptake in cells where AQP8ab is expressed.Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.

Hypo-Osmoregulatory Roles of Vasotocinergic and Isotocinergic Systems in the Intestines of Two European Sea Bass Lineages

“…PCA results were obtained from the posterior intestine for gene expression levels of avtr and itr that showed different expression levels between conditions ( avtrv1a2 , avtrv1a1 , avtrv2b1 , itr1 ) as well as Na + /K + -ATPase (NKA) activity and expressions of genes involved in ion transport and water balance previously analyzed in the same individuals by Cao et al [ 36 ] that showed different expression patterns between conditions (aquaporins: aqp1a , aqp1b , aqp8b , aqp8ab , aqp8aa , aqp10b , Na + /K + -ATPase α1a ( nkaα1a ) and Na + /K + /2Cl − cotransporter 2 ( nkcc2 )) ( Figure 5 ). Principal component 1 (PC1) described 43.9% of the original information whereas PC2 described 17.8%, making a cumulative percentage of 61.7%.…”

“…PCA analysis showed that there was a strong correlation between avtrv1a2 and the main ion transporters (NKA activity and expression of nka α1a , nkcc2 ) involved in ion uptake in the posterior intestine. We have previously shown that the expression of nka α1a , nkcc2 and NKA activity was high in Mediterranean and Atlantic sea bass challenged to HSW [ 36 ]. The aquaporins, aqp8ab and aqp8aa , were also correlated with avtrv1a2 and main ion transporters.…”

“…The aquaporins, aqp8ab and aqp8aa , were also correlated with avtrv1a2 and main ion transporters. Previous studies have shown that NKCC2 and AQP8ab were apically located in sea bass enterocytes, and AQP8 protein and gene expression increased upon hypersalinity transfer [ 36 ]. Altogether, avtrv1a2 seems to be a valid candidate for regulating a coordinated response that would lead to an increased solute-coupled water uptake upon hypersalinity.…”

“…The effect of hypersalinity on the structure and function of osmoregulatory organs in fish has been addressed by several authors [ 35 ]. In our previous study conducted on both D. labrax lineages, Na + /K + -ATPase (NKA) activities and mRNA expressions of nkaα1 , nkcc2 and several aquaporins ( aqp8b , aqp8aa , aqp8ab , aqp1a and aqp1b ) changed significantly according to hypersalinity acclimation in the posterior intestine [ 36 ]. Comparing posterior intestines of North Atlantic (A) and West Mediterranean (M) genetic lineages of D. labrax , we mainly observed differences in the expression patterns of these genes in seawater and much less in hypersaline water.…”

“…Comparing posterior intestines of North Atlantic (A) and West Mediterranean (M) genetic lineages of D. labrax , we mainly observed differences in the expression patterns of these genes in seawater and much less in hypersaline water. This indicates different physiological strategies of D. labrax genetic lineages in their native salinity regarding solute-regulated water transport [ 36 ].…”