**1. Introduction**

The apical Na+/H+-*exchanger* (NHE) can release ammonia nitrogen from the cytoplasm into the environment, so it has attracted the attention of researchers. In the human kidney, the proximal tubules secrete ammonia equivalent to 75–120% of total urinary ammonia, suggesting that NHE-3 plays a greater role in NH4<sup>+</sup> excretion than Rh C Glycoprotein [1,2]. The study found that the NHE-3 may be the primary mechanism by which NH4<sup>+</sup> is secreted from the apical plasma membrane of proximal tubules. NHE-3 is a member of the Na+/H+ *exchanger* family. NHE-3-mediated ammonia secretion may be due to the substitution of NH4+ for H+ at the cytosolic H+ binding site, activating Na+/NH4+ exchange activity. It has been shown that the NH4+ in the cytoplasm competes with the H<sup>+</sup> in the cytoplasm to exchange with Na+ in the lumen, hence the Na+/NH4+ exchange [3,4].

Aquaculture systems are adversely affected by ammonia nitrogen, a significant pollutant. Research has been conducted to understand the mechanisms by which ammonia negatively affects aquatic animals [5,6]. The research on the mechanism of ammonia excretion of crustaceans found that inhibition of NHE activity in *Carcinus maenas*, *Cancer pagurus*,

**Citation:** Li, Y.; Jiang, S.; Fan, H.; Yang, Q.; Jiang, S.; Huang, J.; Yang, L.; Zhang, W.; Chen, X.; Zhou, F. A Na+/H+-Exchanger Gene from *Penaeus monodon*: Molecular Characterization and Expression Analysis under Ammonia Nitrogen Stress. *J. Mar. Sci. Eng.* **2022**, *10*, 1897. https://doi.org/10.3390/jmse10121897

Academic Editor: Ka Hou Chu

Received: 26 October 2022 Accepted: 2 December 2022 Published: 5 December 2022

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and *Astacus leptodactylus* with the nonspecific inhibitor amiloride blocked ammonia nitrogen excretion from gills [7,8]. Moreover, Towle et al. identified the NHE transport mode on the gills of the *Carcinus maenas* at the molecular level and believed that it may belong to a transport mode cation or proton binding side [9]. Furthermore, it showed that the expression level of NHE mRNA in the gills of *Eriocheir sinensis* and *Portunus trituberculatus* was decreased after 2 days of exposure to ammonia nitrogen.

Most studies focus on the acid–base balance and salinity stress [10–13]. Little research has been conducted on whether NHE responds to nitrogen stress induced by ammonia and its role in the aquaculture water environment with high ammonia nitrogen concentration. Therefore, in this study, the NHE gene of *P. monodon* was cloned for the first time by race technology, bioinformatics analysis and tissue expression analysis were carried out, and the mRNA expression trend of NHE after 96 h acute ammonia nitrogen stress was detected. Moreover, RNA interference technology was employed to explore the death curve of shrimp under high-concentration ammonia nitrogen stress after NHE silencing.
