Renal immune-inflammatory responses of Gymnocypris eckloni under acute salinity stress revealed by histopathology and transcriptomics

As global climate change alters salinity patterns in freshwater habitats, the resulting environmental stress has become a major challenge for freshwater fish. Gymnocypris eckloni, an economically important high-altitude species inhabiting the upper and middle Yellow River and the Qaidam Basin, was selected for investigation because of salinity-adaptive traits documented in its close relatives. Fish were exposed to salinities of 0‰ (C), 7.5‰ (M), and 15.0‰ (H) for 12 and 24 h, followed by kidney sampling for histological, biochemical, and transcriptomic analyses to characterize stress-induced renal responses. Acute salinity exposure caused progressive renal deterioration, with glomerular atrophy in the M12 group advancing to glomerular disruption and widespread nephron necrosis in the H24 group. Biochemical analyses demonstrated that cortisol, TNF-α, and IL-1β levels increased markedly in both the M and H salinity groups as stress duration progressed. Antioxidant parameters (T-AOC, SOD, CAT) were significantly elevated in the M group, whereas in the H group they exhibited an initial increase followed by a pronounced decline, becoming significantly lower than those of the control group at 24 h. Transcriptomic analyses revealed that differentially expressed genes in the C vs M and C vs H comparisons were predominantly enriched in immune-related pathways, including the complement and coagulation cascades, IL-17 signaling pathway, hematopoietic cell lineage, and B cell receptor signaling pathway. Trend analysis showed that genes progressively upregulated with increasing salinity and exposure duration were enriched in the IL-17 signaling pathway, whereas genes downregulated at 24 h were significantly enriched in the complement and coagulation cascades. These patterns suggest that the IL-17 pathway may be involved in regulating immune defense and inflammation under salinity stress in Gymnocypris eckloni. However, at high salinity, some innate immune-related pathways, particularly complement and coagulation cascades, were transcriptionally suppressed. In summary, this study provides a systematic characterization of the impacts of salinity concentration and exposure duration on renal injury in G. eckloni, offering new insights into renal injury and early stress responses in freshwater fish exposed to acute salinity challenge.