The function of stearoyl-CoA desaturase in unsaturated fatty acid biosynthesis and cold tolerance in Haliotis discus hannai

The Pacific abalone Haliotis discus hannai is a nutritionally and economically important mollusk species predominantly cultivated in northern China. In recent years, recurrent winter cold spells have severely impaired the survival and growth of abalone, thereby constraining the development of its aquaculture industry. Unsaturated fatty acids (UFAs) play a vital role in cold tolerance, with stearoyl-CoA desaturase (SCD) acting as the rate-limiting enzyme in de novo UFA synthesis. In the present study, both UFA level and SCD activity in the hepatopancreas of H. discus hannai were significantly elevated after low temperature exposure. An SCD homolog HdSCD1, containing a conserved FA_desaturase domain, was identified in H. discus hannai. HdSCD1 mRNA was predominantly expressed in the hepatopancreas of abalone and increased significantly after low temperature exposure (p < 0.001). Recombinant HdSCD1 protein can catalyze the conversion of saturated fatty acids (SFAs) to UFAs. Overexpression of HdSCD1 in HEK293T cells increased UFA content by 72%, whereas knockdown of HdSCD1 by RNA interference (RNAi) decreased UFA content by 32% in the abalone hepatopancreas. Under cold stress, HdSCD1 knockdown significantly suppressed UFA accumulation in the hepatopancreas and resulted in a 2.88-fold increase in abalone mortality compared to the EGFP group. Moreover, HdSCD1 knockdown impaired the antioxidant system in abalone, as evidenced by decreased superoxide dismutase (SOD) and catalase (CAT) activities (0.61- and 0.56-fold of the EGFP group, respectively), reduced total antioxidant capacity (T-AOC; 0.92-fold of the EGFP group) and elevated malondialdehyde (MDA) content (1.23-fold of the EGFP group). In addition, the mRNA expression levels of HdNRF2, HdSOD and HdCAT were significantly decreased, while that of HdKEAP1 was significantly increased. Concurrently, compared to the EGFP group, the mRNA expression levels of inflammation-related genes (HdTLR4, HdMyD88, HdTNF-α, HdNF-κB, HdIL-6 and HdIL-1β) and apoptosis-related genes (HdCaspase3, HdCaspase7 and HdBax) were significantly upregulated. These results demonstrated that HdSCD1 functions as a fatty acid desaturase involved in UFA biosynthesis and plays an essential role in cold tolerance in H. discus hannai.