Role of the Basolateral Na+/H+ Exchanger-2 (NHE2) in Ionocytes of Seawater- Acclimated Medaka (Oryzias latipes)

Abstract

Ionocytes in the skin and gills of seawater (SW) fishes are responsible for acid-base regulation and salt secretion. Na+/H+ exchangers (NHEs) are considered the major acid (H+)-secreting transporters in ionocytes of SW fishes. However, the subcellular localization and function of a specific NHE isoform (NHE2) have never clearly been revealed. In this study, we cloned and sequenced NHE2 from an SW-acclimated medaka (Oryzias latipes) and examined its functions in medaka embryos. A phylogenetic analysis showed that the evolutionary relationships of mammalian NHE2 and NHE4 are close to those of fish NHE2. A gene structure analysis showed that tetrapod NHE4 might be a tandem duplication of fish NHE2. Immunohistochemistry with a medaka-specific antibody localized NHE2 to the basolateral membrane of ionocytes. Lost-of-function experiments with photo-activated morpholino oligonucleotides showed that both H+ and Cl– secretion by ionocytes were suppressed in NHE2-knockdown embryos, suggesting that the basolateral NHE2 facilitates acid and salt secretion by ionocytes of medaka in seawater.

Expression and Functional Analysis of AMT1 Gene Responding to High Ammonia Stress in Razor Clam (Sinonovacula constricta)

“…The gill serves as an important organ that can perform ammonia excretion, as well as exchange of gas and ions between the internal fluids and external environment, and maintain the acid-base balance in aquatic organisms [37]. A variety of proteins participating in substances transport have been detected in gill, including AMT, Rh, NKA, NKCC, NHE, VHA, and carbonic anhydrase (CA) [38][39][40]. However, it is still unclear how AMT interacts with other proteins to regulate the excretion of ammonia in mollusks.…”

Genome-wide identification of the NHE gene family in Coilia nasus and its response to salinity challenge and ammonia stress

“…Thus, H + and HCO 3 − transport in fish gills plays a critical role in acid-base regulation [ 10 , 12 ]. In hypertonic environments, fish excrete metabolic acids through the apical NHE, which is generally believed to play a critical role in ionocytes [ 13 ]. NHE can also coordinate with carbonic anhydrase and bicarbonate transporter in Tribolodon hakonensis and medaka ( Oryzias latipes ) at acidic environment [ 14 , 15 ].…”