Cyclosporin A Attenuates Zinc Stress-Induced Immune Dysregulation, Oxidative Stress, and Mitochondrial Damage in Gymnocypris eckloni via Calcineurin Inhibition

Calcineurin (CN), a pivotal calcium-/calmodulin-dependent serine/threonine phosphatase, is critically involved in diverse physiological processes such as immune response, synaptic plasticity, and cardiac hypertrophy. This study investigated the mechanism by which cyclosporine A (CsA) modulates the NFAT and NF-κB signaling pathways through CN in Gymnocypris eckloni , and evaluated its protective role against Zn²⁺-induced stress. After optimizing the CsA treatment condition (15 mg/L, 48 h), the expression of CN-related genes ( GeCNAα , GeCNB ) and downstream immune-related genes ( GeNFAT , GeNF-κB , GeTNF-α , GeIL-1β , GeMT , GeHsp90 ) was quantified in gill and kidney tissues. The combined effects of CsA and Zn²⁺ on cytokine profiles, antioxidant enzyme activities (SOD, CAT, GSH-Px), and oxidative stress indicators (ROS, MDA) were further examined. The results showed that CsA significantly suppressed the transcription of CN and its downstream targets. Although Zn²⁺ stress strongly upregulated the expression of GeTNF-α and GeIL-1β , CsA treatment effectively counteracted this upregulation. In addition, CsA attenuated Zn²⁺-induced increases in antioxidant enzyme activities and alleviated oxidative damage. Ultrastructural observations revealed that Zn²⁺ exposure led to mitochondrial fragmentation, membrane rupture, and cristae loss in gills, and pronounced cristae depletion in kidneys. CsA intervention, however, restored mitochondrial membrane integrity and reduced cristae damage. In summary, these findings demonstrate that CsA, by inhibiting CN, regulates the NFAT/NF-κB signaling axis, mitigates oxidative stress, and preserves mitochondrial ultrastructure, thereby highlighting its potential in ameliorating immune dysregulation and oxidative injury caused by zinc stress.