Differential effects of silver nanoparticles on two types of mitochondrion-rich ionocytes in zebrafish embryos

Silver nanoparticles (AgNPs) are increasingly used in our daily life and have become a potential environmental hazard. However, the toxic effects of AgNPs on the early stages of fish are not fully understood, and little is known about their effects on specific types of ionocytes. Using zebrafish embryos as a model, this study examined the effects (changes in cell number, morphology, NH₄⁺ secretion and gene expression) of sublethal concentrations of AgNPs (0.1, 1, and 3 mg/L) on two major types of ionocytes: H⁺ pump-rich (HR) ionocytes, and Na⁺ pump-rich (NaR) ionocytes in the skin of embryos. After exposure to AgNPs for 96 h, the number of HR ionocytes significantly declined by 30% and 41% in the 1 and 3 mg/L AgNP groups, respectively. In addition, the apical opening of HR ionocytes became smaller, suggesting that AgNPs impaired the critical structure for ion transport. NH₄⁺ secretion by HR ionocytes of embryos also declined significantly after AgNP exposure. In contrast, the number of NaR ionocytes increased by 29% and 43% in the 1 and 3 mg/L AgNP groups, respectively, while these cells deformed their shape. AgNPs altered mRNA levels of several ion channel and transporter genes involved in the functions of HR ionocytes and NaR ionocytes, and influenced hormone genes involved in regulating calcium homeostasis. This study shows that AgNPs can cause differential adverse effects on two types of ionocytes and the effects can threaten fish survival.