Liver metabolomic analysis and expression under hypotonicity in American eel (Anguilla rostrata)

Proper freshwater acclimation is crucial in aquaculture settings to maximize survival. The present study examined the physiological and molecular responses of eels following transfer from seawater (SW, 35‰) to either SW or freshwater (FW, 0‰). In SW, plasma ion concentrations and osmolality remained stable throughout the experimental period. In contrast, FW exposure induced a significant decline in plasma sodium, chloride, and osmolality between 12 and 72 h, after which levels stabilized by 96 h. Metabolic analysis revealed significant responses of FOXO and mTOR signaling pathways, as well as pathways related to membrane transporters, including aquaporins (AQPs), in FW-acclimated eels. Among AQP family members, AQP5 showed the highest hepatic expression. To explore upstream regulatory mechanisms, we quantified mRNA and protein expression of key components of the SRC-RAS-RAF-MEK-MAPK cascade. Transcript levels of all examined genes were significantly upregulated in FW compared to SW. Consistently, protein expression of SRC, MEK, and AQP5 was elevated in FW. These findings demonstrate that eels effectively acclimate to low-salinity environments by rapidly restoring ionic homeostasis within hours, accompanied by transcriptional and translational responses of MAPK signaling and AQP5 in the liver.