Abstract

Two isoforms of MC4R, namely caMC4R and caMC4R-like, have been identified in goldfish. • The knockdown of caMC4R and caMC4R-like promotes appetite and weight gain in goldfish. • Goldfish MC4R regulates muscle lipid accumulation via the pyruvate metabolic pathway. • Goldfish MC4R-like regulates hepatic lipid accumulation via PPARγ/SREBP-1c. Controlling abdominal fat accumulation and appetite have consistently been key research priorities in aquaculture. The melanocortin-4 receptor (MC4R) plays a crucial role in regulating appetite and lipid metabolism. In teleost fish, such as goldfish, there are two or more homologs of MC4R, yet the functions and significance of these homologs remain incompletely understood. To address this, we chose goldfish as a model organism and investigated the functions of caMC4R ( Carassius auratus MC4R) and caMC4R-like using molecular docking, RNA interference (RNAi) and RNA sequencing (RNA-seq). Our findings indicate that the third intracellular loop (ICL3) of caMC4R and caMC4R-like differs significantly, resulting in distinct activation of cAMP signaling downstream of the receptor. The knockdown of caMC4R/caMC4R-like promoted appetite and growth in goldfish. Interestingly, the knockdown of caMC4R enhanced muscle lipid accumulation by regulating the expression of pyruvate dehydrogenase (E1) component subunit beta (PDHB). The knockdown of caMC4R-like promoted hepatic lipid accumulation via the PPARγ and SREBP-1c pathways. Overall, the knockdown of caMC4R and caMC4R-like promoted lipid accumulation in the muscle and liver of goldfish, respectively. The results of this study will provide foundational data for investigating the functions of the MC4R subfamily in teleost fish and the significance of genome duplication in fish evolution. In addition, our findings offer actionable insights for enhancing growth performance and modulating body composition in cultured fish populations, offering tangible applications for both nutritional programmin