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Melanocortin-4 receptor (MC4R) plays important roles in regulation of multiple physiological processes, and interaction of MC4R and melanocortin receptor accessory protein 2 (MRAP2) is suggested to play pivotal role in energy balance of vertebrates. Topmouth culter (Culter alburnus) is an economically important freshwater fish in China. Herein we cloned culter mc4r, mrap2a, and mrap2b. Culter mc4r consisted of a 981 bp open reading frame encoding a protein of 326 amino acids. qRT-PCR revealed that mc4r, mrap2a, and mrap2b were primarily expressed in the central nervous system. In the periphery, mc4r and mrap2b were expressed more widely in the male, while mrap2a was expressed more widely in the female. Culter MC4R could bind to four peptide agonists and increase intracellular cAMP production dose dependently. Culter MC4R was constitutively active in both cAMP and ERK1/2 pathways, which was differentially regulated by culter MRAP2a and MRAP2b. Culter MRAP2a significantly increased B max and decreased agonist-stimulated cAMP, while MRAP2b increased cell surface and total expression but did not affect B max and agonist-stimulated cAMP. These results will aid the investigation of the potential physiological processes that MC4R might be involved in topmouth culter.

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The study was conducted to investigate the effects of dietary tea polyphenols (TP) on growth performance, biochemical and antioxidants responses, fatty acid composition, and lipid metabolism-related gene expressions of large yellow croaker (Larimichthys crocea). Four diets were formulated with different levels of TP (0.00%, 0.01%, 0.02% and 0.05%). Results showed that growth performance of L. croceawere not different among dietary treatments. Compared with the control group, fish in 0.02% TP group had lower body and hepatic lipid content and lower total cholesterol content. The minimum content of triglycerides and low-density lipoproteincholesterol were found in 0.05% TP group. Hepatic n-6 PUFA and n-3 PUFA were significantly higher in TP supplementation groups. Malondialdehyde content was lower in TP supplementation groups, and superoxide dismutase activity was higher in 0.01% TP group than the control group. The mRNA expressions of carnitine palmitoyltransferase1, acyl-CoA oxidase and peroxisome proliferators-activated receptor a were up-regulated in 0.01% and 0.02% TP groups, while lipoprotein lipase expression was down-regulated in TP supplementation groups than the control group. Results suggested that 0.01%-0.02% TP supplementation could reduce the deposition of liver lipid of L. crocea caused by high-lipid diet, which might be due to the increase in lipid oxidation related gene expressions.

Melanocortin-3 receptor (MC3R) is a regulator of energy homeostasis, and interaction of MC3R and melanocortin-2 receptor accessory protein 2 (MRAP2) plays a critical role in MC3R signaling of mammals. However, the physiological roles of MC3R in teleosts are not well understood. In this study, qRT-PCR was used to measure gene expression. Radioligand binding assay was used to study the binding properties of topmouth culter MC3R (caMC3R). Intracellular cAMP generation was determined by radioimmunoassay and caMC3R expression was quantified with flow cytometry. We showed that culter mc3r had higher expression in the central nervous system. All agonists could bind and stimulate caMC3R to increase dose-dependently intracellular cAMP accumulation. Compared to hMC3R, culter MC3R showed higher constitutive activity, higher efficacies and Rmax to α-MSH, des-α-MSH, and ACTH. Both caMRAP2a and caMRAP2b markedly decreased caMC3R basal cAMP production. However, only caMRAP2a significantly decreased cell surface expression, Bmax and Rmax of caMC3R. Expression analysis suggested that MRAP2a and MRAP2b might be more important in regulating MC3R/MC4R signaling during larval period, and reduced mc3r, mc4r, and pomc expression might be primarily involved in modulation of MC3R/MC4R in adults. These data indicated that the cloned caMC3R was a functional receptor. MRAP2a and MRAP2a had different effects on expression and signaling of caMC3R. In addition, expression analysis suggested that MRAP2s, receptors, and hormone might play different roles in regulating culter development and growth.

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A 10-weeks feeding trial was conducted to investigate effects of dietary curcumin on growth, antioxidant responses, fatty acid composition, and expression of lipid metabolism-related genes of large yellow croaker fed high-fat diet. Four diets (lipid level at 18%) were formulated with different levels of curcumin (0%, 0.02%, 0.04% and 0.06%). Results showed that the best growth performance was found in 0.04% curcumin group, with the body and hepatic lipid levels lower than the control group (0% curcumin). The content of triglycerides, total cholesterol and low-density lipoprotein-cholesterol was the least in 0.06% curcumin group. The lowest malondialdehyde and the highest superoxide dismutase, catalase activity and total antioxidant capacity were observed in 0.04% curcumin group. In 0.04% curcumin group, expression of fatty acyl desaturase 6, elongases very long-chain fatty acids 5, and elongases very long-chain fatty acids 4 was upregulated in liver; moreover, the ratio of hepatic n-6 polyunsaturated fatty acid (PUFA) and n-3 PUFA was higher. Expression of peroxisome proliferators-activated receptor α, carnitine palmitoyltransferase I, and acyl-CoA oxidase was significantly increased, while expression of sterol-regulatory element-binding protein 1 and fatty acid synthase was dramatically decreased in curcumin groups compared with the control group. Overall, 0.04% curcumin supplementation could mitigate the negative effects caused by high-fat diet, and promote growth via reducing hepatic lipid deposition, improving antioxidant activity and increasing PUFA of large yellow croaker. Therefore, it is deduced that reduced abnormal hepatic lipid deposition was probably due to increased fatty acid oxidation and reduced de novo synthesis of fatty acids.

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