Abstract
Ectothermic fish exposure to hypothermal stress requires adjusting their metabolic molecular machinery, which was investigated using Indian medaka (Oryzias dancena; 10 weeks old, 2.5 ± 0.5 cm) cultured in fresh water (FW) and seawater (SW; 35‱) at room temperature (28 ± 1 °C). The fish were fed twice a day, once in the morning and once in the evening, and the photoperiod was 12 h:12 h light: dark. In this study, we applied two hypothermal treatments to reveal the mechanisms of energy metabolism via pgc-1α regulation in the gills of Indian medaka; cold-stress (18 °C) and cold-tolerance (extreme cold; 15 °C). The branchial ATP content was significantly higher in the cold-stress group, but not in the cold-tolerance group. In FW- and SW-acclimated medaka, the expression of genes related to mitochondrial energy metabolism, including pgc-1α, prc, Nrf2, tfam, and nd5, was analyzed to illustrate differential responses of mitochondrial energy metabolism to cold-stress and cold-tolerance environments. When exposed to cold-stress, the relative mRNA expression of pgc-1α, prc, and Nrf2 increased from 2 h, whereas that of tfam and nd5 increased significantly from 168 h. When exposed to a cold-tolerant environment, prc was significantly upregulated at 2 h post-cooling in the FW and SW groups, and pgc-1α was significantly upregulated at 2 and 12 h post-cooling in the FW group, while tfam and nd5 were downregulated in both FW and SW fish. Hierarchical clustering revealed gene interactions in the cold-stress group, which promoted diverse mitochondrial energy adaptations, causing an increase in ATP production. However, the cold-tolerant group demonstrated limitations in enhancing ATP levels through mitochondrial regulation via the PGC-1α energy metabolism pathway. These findings suggest that ectothermic fish may develop varying degrees of thermal tolerance over time in response to climate change. This study provides insights into the complex ways in which fish adjust their metabolism when exposed to cold stress, contributing to our knowledge of how they adapt.