Gill tissue homeostasis remodeling and metabolic reprogramming: Key mechanisms underlying high-temperature stress resistance in selectively bred rainbow trout.

Frequent extreme high-temperature events have severely restricted the development of the cold-water rainbow trout (Oncorhynchus mykiss) aquaculture industry, yet the systemic tolerance mechanisms behind the selection of heat-tolerant strains remain elusive. In this study, a 28-day thermal stress challenge (14-28 °C) was performed on common rainbow trout (N trout) and family-selected heat-tolerant rainbow trout (T trout, F2 generation). Integrated biochemical, histopathological, and transcriptomic-metabolomic analyses were conducted. The results indicated that T trout effectively delayed the collapse of the antioxidant defense system under high temperatures (with significantly higher SOD, CAT, and GSH activities and significantly lower lipid peroxidation product MDA, P < 0.01). Gill cell apoptosis rates were significantly reduced (P < 0.05). At the extreme temperature of 28 °C, T trout exhibited prominent compensatory structural remodeling of the gills (P < 0.01) and sustained blood glucose mobilization capacity, thereby avoiding the metabolic failure seen in N trout. Multi-omics analysis further revealed that, distinct from the endoplasmic reticulum stress and cell membrane damage observed in N trout, T trout maintained protein homeostasis, likely through the upregulation of heat shock proteins (e.g., hspa1s, hsp90). Moreover, T trout achieved efficient thermal adaptation through metabolic reprogramming (restructuring carbohydrate metabolic to support the glutathione antioxidant system, upregulating gpx4a, and accumulating CHAC) and adaptive regulation of membrane lipids (e.g., arachidonic acid and sphingolipid pathways). This study systematically clarifies the key mechanisms of high-temperature tolerance in selectively bred rainbow trout, providing a vital theoretical basis and candidate markers (e.g., hspa1s and gpx4a) for molecular marker-assisted breeding of stress-resistant cold-water fish.