Modulation of gene transcriptional level of different ion transporters in eel gill mitochondria-rich cells upon osmotic stress

Eels are euryhaline fishes that their life cycles involve the stay in fresh water and seawater. In order to compensate the osmotic challenges in fresh water (ion loss and water gain) and in seawater habitat (ion gain and water loss), eels have developed specific osmoregulatory mechanisms to acclimate rapidly and effectively in such two extreme aquatic environments. Fish gill is the outermost tissue that is in direct contact with the external media. In the gill tissue, two specific osmoregulatory cell types (pavement cells (PVCs) and mitochondria-rich/chloride cells (MRCs/CCs)) have been identified and characterized. It is generally believed that MRCs, which we will focus on, are the essential ion-regulatory cell type in the eel gill. With the recent advances in molecular biology, together with the immunohistochemical staining and electrophysiology studies, more transcriptional and translational profiling data are generated to unfold the molecular nature of the branchial osmoregulatory mechanisms. In this review, we will begin with the general description of eel gill structure and cell types (PVCs and MRCs), followed by the discussion on the differential mRNA expressions of different ion transporters in MRCs under hyper/ hypo- tonic stress conditions. Lastly, we will provide an opinion on the use of recent advanced genomic transcriptome data to form a public accessible database. To summarize, this chapter provides essential updated information on the expressions of ion channels/ transporters in eel gills acclimated in waters of different salinities, providing new in-sight on the future direction of eel gill studies in modern molecular perspectives.