吳郭魚(Oreochromis mossambicus)適應不同環境鹽度時鰓上的滲透壓調節機制及其離子調節細胞功能之探討

Abstract

Gill is the major osmoregulatory organs in teleost and mitochondrion-rich (MR) cells in the gill epithelia are the main sites to regulate ion movements. Tilapia (Oreochromis mossambicus) is a euryhaline teleost with fresh water (FW) preference. Furthermore, previous studies observed tilapia died within 6 h after direct transfer from FW to full-strength seawater (SW). The process of pre-acclimation in hypertonic brackish water is necessary for tilapia to develop the appropriate osmoregulatory mechanisms before SW acclimation. In the present study, we found that when SW-acclimated tilapia were transferred back to FW for 96h (SW-FW-96h), the osmoregulatory status were similar to FW-acclimated tilapia with lower Na+/K+-ATPase (NKA) responses, higher expression of claudin 3- and 4-like proteins, and apical Na+/Cl- cotransporter (NCC) expressed in the MR cells without basolateral Na+/K+/2Cl- cotransporter (NKCC) and apical cystic fibrosis transmembrane conductance regulator (CFTR). Inaddition, SW-FW-96h tilapia could be directly transferred to SW with no mortality. Therefore, the aim of our study is to investigate the integrated oamoregulatory mechanisms in the gills of tilapia to illustrate the mechanisms that could be regulated efficiently by SW-FW-96h tilapia for successful acclimation of direct transfer to full-strength SW. The results of this study revealed that the ion secretion mechanisms of transporter protein-dependent (i.e., NKA, NKCC and CFTR) were activated at 24h post-transfer. In contrast, abundance of branchial claudin 3- and 4-like proteins declined evidently at 1h post-transfer and sustained to 24 h post-transfer. On the other hand, the scanning electron microscopic observation also showed that when SW-FW-96h tilapia were transferred to SW, the convex structure of apical membrane of MR cells was indentation to form the concave structure within 3h post-transfer and the deep-hole structure at 12h post-transfer. It might appear that tight junction proteins rather than transporter protein-dependent mechanisms were regulated efficiently to play a crucial role in reshaping the gill phenotype to leakier epithelium for promoting ion secretion in SW-FW-96h tilapia acutely exposed to SW. Furthermore, the mechanisms of molecular chaperones regulated protein quality control were investigated. Protein expression of gill heat shock protein 70 (HSP70) and HSP90 significantly elevated at 6h and the levels were sustained to 24 h after SW transfer. Meanwhile, the levels of aggregated protein were constant among different time points. Taken together, efficient activation of molecular chaperones regulated protein quality control might protect branchial cells from osmtic stress and lead to gill cells have the ability to regulate the mechanisms of paracellular permeability in SW-FW-96h tilapia transferred to SW. This study is the first to observe and investigate tilapia can survive on the experience of drastic changes in environmental salinity. Key words: tilapia, mitochondrion-rich cells, Na+/K+/2Cl− cotransporter, cystic fibrosis transmembrane conductance regulator, claudin and heat shock protein