UC San Diego

Ionocytes are specialized epithelial ion-transporting cells that play major roles in maintaining blood ionic, ammonia, and acid-base homeostasis of internal fluids. In fish, these functions predominantly take place at the gills. While multiple studies have characterized various gill ionocyte types specialized for NaCl, H+, HCO3- or ammonia transport in elasmobranchs and freshwater teleosts, the putative presence of ionocyte types in marine teleosts remains unclear and it is not known whether all of these functions take place within the same cell. Here, I used immunohistochemistry (IHC) to investigate the presence and cellular localization of ionocytes and various ion-transporting proteins in gills of splitnose rockfish (Sebastes diploproa) and white seabass (Atractoscion nobilis), two marine teleost species with distinct environmental pressures and behavioral tendencies. Based on high expression of basolateral Na⁺/K⁺-ATPase (NKA), ionocytes were abundantly present in the gill filament and in the basal region of the lamellae of both rockfish and white seabass. In rockfish, every NKA-rich ionocyte also expressed cystic fibrosis transmembrane conductance regulator (CFTR), Na+-H+ antiporter 3 (NHE3), and a Rhesus channel (Rh) in their apical membrane which had the “pit” morphology typical described for marine teleost gill ionocytes. This suggests that rockfish gills have a single ionocyte type that performs NaCl, H+, and ammonia excretion. The gill NKA-rich ionocytes of white seabass also co-expressed NHE3 in its apical membrane which, interestingly, demonstrated abundant extended microvilli instead of a pit morphology. In addition, CFTR was present in cells that were not NKA-rich, and Rh channel was predominantly present in pavement cells. Finally, I investigated the presence and cellular localization of the G protein-coupled receptor GPR4, which is a sensor of extracellular pH in mammalian kidney cells. GPR4 gene orthologs were detected in gill transcriptomes from both species. In white seabass, in situ hybridization chain reaction (ISHCR) showed the presence of GPR4 mRNA in various gill cells including NKA-rich ionocytes. Furthermore, immunohistochemistry revealed localization of the protein on the apical membrane of NKA-rich ionocytes and pavement cells, suggesting a role in sensing pH of the water flowing over the gill epithelium. These findings improve our knowledge about general and species-specific gill cellular mechanisms used by marine teleosts to maintain blood ionic, ammonia, and acid-base homeostasis, which can help identified physiological differences that determine adaptation to different environments and lifestyles as well as responses to varying environmental conditions resulting from natural variability, anthropogenic impacts, and aquaculture.