Energy and nitrogenous waste from glutamate/glutamine catabolism facilitates acute osmotic adjustment in non-neuroectodermal branchial cells

Abstract

Maintenance of homeostasis is one of the most important physiological responses for animals upon osmotic perturbations. Ionocytes of branchial epithelia are the major cell types responsible for active ion transport, which is mediated by energy-consuming ion pumps (e.g., Na+-K+-ATPase, NKA) and secondary active transporters. Consequently, in addition to osmolyte adjustments, sufficient and immediate energy replenishment is essenttableial for acclimation to osmotic changes. In this study, we propose that glutamate/glutamine catabolism and trans-epithelial transport of nitrogenous waste may aid euryhaline teleosts Japanese medaka (Oryzias latipes) during acclimation to osmotic changes. Glutamate family amino acid contents in gills were increased by hyperosmotic challenge along an acclimation period of 72 hours. This change in amino acids was accompanied by a stimulation of putative glutamate/glutamine transporters (Eaats, Sat) and synthesis enzymes (Gls, Glul) that participate in regulating glutamate/glutamine cycling in branchial epithelia during acclimation to hyperosmotic conditions. In situ hybridization of glutaminase and glutamine synthetase in combination with immunocytochemistry demonstrate a partial colocalization of olgls1a and olgls2 but not olglul with Na+/K+-ATPase-rich ionocytes. Also for the glutamate and glutamine transporters colocalization with ionocytes was found for oleaat1, oleaat3, and olslc38a4, but not oleaat2. Morpholino knock-down of Sat decreased Na+ flux from the larval epithelium, demonstrating the importance of glutamate/glutamine transport in osmotic regulation. In addition to its role as an energy substrate, glutamate deamination produces NH4+, which may contribute to osmolyte production; genes encoding components of the urea production cycle, including carbamoyl phosphate synthetase (CPS) and ornithine transcarbamylase (OTC), were upregulated under hyperosmotic challenges. Based on these findings the present work demonstrates that the glutamate/glutamine cycle and subsequent transepithelial transport of nitrogenous waste in branchial epithelia represents an essential component for the maintenance of ionic homeostasis under a hyperosmotic challenge.

Fish Skin and Gill Mucus: A Source of Metabolites for Non-Invasive Health Monitoring and Research

“…Among the top 10 metabolites that differentiated gill and skin mucus, several amino acids such as alanine, glutamine and glutamic acid were more abundant in gill mucus as compared to skin mucus, which could be related to functions like osmoregu-lation and ammoniagenesis of gill tissue [23,24]. By using the japanese medaka (Oryzias latipes) as a model species, the important functions of the glutamic acid/glutamine cycle in controlling osmoregulation in gills has been reported [25]. The rapid accumulation of glutamic acid in gills was closely correlated both with the elevated levels of nitrogenous ammonia (NH 3 /NH 4 + ) and urea after salinity challenge, contributing to the energetics of well-developed osmoregulatory abilities in euryhaline teleosts [25].…”

“…By using the japanese medaka (Oryzias latipes) as a model species, the important functions of the glutamic acid/glutamine cycle in controlling osmoregulation in gills has been reported [25]. The rapid accumulation of glutamic acid in gills was closely correlated both with the elevated levels of nitrogenous ammonia (NH 3 /NH 4 + ) and urea after salinity challenge, contributing to the energetics of well-developed osmoregulatory abilities in euryhaline teleosts [25]. As an effective ammonia detoxification strategy reported in Chinese loach (Paramisgurnus dabryanus) followed by aerial exposure, glutamic acid was partially converted to alanine without releasing ammonia via transamination [26].…”

Teleostean fishes may have developed an efficient Na+ uptake for adaptation to the freshwater system

“…Maintaining homeostasis in the body fluids by developing iono/osmo-regulation mechanisms similar to those found in terrestrial mammals is also proved to be achieved and maintained by active uptake machineries via gill/skin (Hwang et al, 2011;Tseng et al, 2020). Besides, epithelial driving forced transports are high energy-consuming processes (Evans et al, 2005;Tseng et al, 2007;Tseng and Hwang, 2008;Hwang et al, 2011;Huang et al, 2020); the effects of environmental ionic perturbations on the metabolic provision in aquatic organisms…”

Osmoregulatory strategies of estuarine fish Scatophagus argus in response to environmental salinity changes

“…Ionocytes (also called mitochondrion-rich cells [MRCs]) in branchial epithelia are majorly responsible for adenosine triphosphate-(ATP-) dependent active ion transport [22,25]. This regulatory mechanism depends on a lot of ion channels, pumps and exchangers on the membranes of branchial MRCs [26,27]. Na + -K + -ATPase (NKA) and Na + -K + -2Cl − cotransporter (NKCC), explicitly localized to branchial MRCs, are responsible for maintaining ionic balance and are generally considered as sensitive molecular biomarkers to understand the osmotic stress status of euryhaline fish [28][29][30].…”