Marine Research Station, Institute of Cellular and organismic Biology, Academia Sinica, I-Lan County, Taiwan (ROC).
Institute of Physiology, Christian-Albrechts University Kiel, Kiel, Germany.
Sci Rep. 2020 Jun 11;10(1):9460. doi: 10.1038/s41598-020-65913-1.
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 NH, 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.
维持内环境稳态是动物应对渗透胁迫的最重要的生理反应之一。鳃上皮的离子细胞是负责主动离子转运的主要细胞类型，这种转运是由耗能的离子泵（例如，Na-K-ATPase，NKA）和次级主动转运体介导的。因此，除了渗透调节外，充足和即时的能量补充对于适应渗透变化至关重要。在这项研究中，我们提出谷氨酸/谷氨酰胺分解代谢和氮废物的跨上皮转运可能有助于广盐性硬骨鱼日本青鳉（Oryzias latipes）在适应渗透变化时维持离子稳态。在 72 小时的适应期内，高渗刺激导致鳃中谷氨酸族氨基酸含量增加。这种氨基酸的变化伴随着假定的谷氨酸/谷氨酰胺转运体（Eaats，Sat）和合成酶（Gls，Glul）的刺激，这些酶参与调节鳃上皮细胞中谷氨酸/谷氨酰胺循环，以适应高渗条件。谷氨酰胺酶和谷氨酰胺合成酶的原位杂交结合免疫细胞化学显示，olgls1a 和 olgls2 与 Na/K-ATPase 丰富的离子细胞部分共定位，但 olglul 不与 Na/K-ATPase 共定位。对于谷氨酸和谷氨酰胺转运体，oleaat1、oleaat3 和 olslc38a4 与离子细胞共定位，但 oleaat2 不与离子细胞共定位。Sat 的 morpholino 敲低降低了幼虫上皮的 Na 通量，证明了谷氨酸/谷氨酰胺转运在渗透调节中的重要性。除了作为能量底物的作用外，谷氨酸脱氨产生 NH，这可能有助于渗透调节物质的产生；尿素生成循环的基因，包括氨甲酰磷酸合成酶（CPS）和鸟氨酸转氨甲酰酶（OTC），在高渗刺激下上调。基于这些发现，本研究表明，谷氨酸/谷氨酰胺循环和随后的氮废物跨上皮转运是维持离子稳态的重要组成部分，特别是在高渗刺激下。
