Abstract
The ability of euryhaline Mozambique tilapia to tolerate extreme environmental salinities makes it an excellent model for investigating iono-regulation. This study aimed to characterize and fill important information gap of the expression levels of key ion transporters for Na+ and Cl− in the gill and esophageal-gastrointestinal tract of Mozambique tilapia acclimated to freshwater (0 ppt), seawater (30 ppt) and hypersaline (70 ppt) environments. Among the seven genes studied, it was found that nkcc2, nkcc1a, cftr, nka-α1 and nka-α3, were more responsive to salinity challenge than nkcc1b and ncc within the investigated tissues. The ncc expression was restricted to gills of freshwater-acclimated fish while nkcc2 expression was restricted to intestinal segments irrespective of salinity challenge. Among the tissues investigated, gill and posterior intestine were found to be highly responsive to salinity changes, followed by anterior and middle intestine. Both esophagus and stomach displayed significant up-regulation of nka-α1 and nka-α3, but not nkcc isoforms and cftr, in hypersaline-acclimated fish suggesting a response to hypersalinity challenge and involvement of other forms of transporters in iono-regulation. Changes in gene expression levels were partly corroborated by immunohistochemical localization of transport proteins. Apical expression of Ncc was found in Nka-immunoreactive cells in freshwater-acclimated gills while Nkcc co-localized with Nka-immunoreactive cells expressing Cftr apically in seawater- and hypersaline-acclimated gills. In the intestine, Nkcc-stained apical brush border was found in Nka-immunoreactive cells at greater levels under hypersaline conditions. These findings provided new insights into the responsiveness of these genes and tissues under hypersalinity challenge, specifically the posterior intestine being vital for salt absorption and iono-osmoregulation in the Mozambique tilapia; its ability to survive in hypersalinity may be in part related to its ability to up-regulate key ion transporters in the posterior intestine. The findings pave the way for future iono-regulatory studies on the Mozambique tilapia esophageal-gastrointestinal tract.
Highlights
The euryhaline teleost, Oreochromis mossambicus known as the Mozambique tilapia, can be acclimated to extreme environmental salinities ranging from freshwater (FW), seawater (SW) to hypersaline water (HSW) up to four-fold the salinity of SW [1,2]
The absolute mRNA expression levels of nkcc1a, nkcc1b, nkcc2, ncc, cftr, and nka-a1 and nka-a3 were determined in the gills and five different segments of EGI tract including the esophagus, stomach, anterior intestine (AI), middle intestine (MI), and posterior intestine (PI) in fish acclimated to FW, SW (30 ppt), and HSW (70 ppt) environments
We have successfully characterized the expression of key ion transporters for Na+ and Cl2 in the gill and EGI tract of the euryhaline Mozambique tilapia model acclimated to FW, SW, and HSW environments
Summary
The euryhaline teleost, Oreochromis mossambicus known as the Mozambique tilapia, can be acclimated to extreme environmental salinities ranging from freshwater (FW), seawater (SW) to hypersaline water (HSW) up to four-fold the salinity of SW [1,2]. In hyper-osmotic SW environments, osmotic water loss needs to be reduced if not replaced by ingestion of SW and the excess salt gain needs to be actively excreted from the body. These iono-osmoregulatory challenges escalate dramatically as environmental salinity increases beyond SW into hypersaline levels where only few teleost species, including several tilapias, have evolved the extraordinary capability to iono-osmoregulate in HSW environments [3,4,5]. There is little information with regards to the expression levels of these ion transporters in the tilapia gill under HSW environments and even much less is known in the esophageal-gastrointestinal (EGI) tract of tilapia acclimated to different environmental salinities. The quantitative changes in gene expression levels of these ion transporters in gills acclimated to HSW has yet to be determined, biochemical and physiological changes in gills had been investigated [10,11,12], while quantitative morphological changes of gill ionocytes in tilapia acclimated to HSW had been studied using ultrastructural [6] and immunohistochemical [9] approaches
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