Molecular cloning, tissue distribution, and pharmacologic function of melanocortin-3 receptor in common carp (Cyprinus carpio)

Cloning, distribution, and effects of growth regulation of MC3R and MC4R in red crucian carp (Carassius auratus red var.)

Background

Melanocortin-3 and -4 receptors (MC3R and MC4R), G protein-coupled receptors, play vital roles in the regulation of energy homeostasis. To understand the functions of mc3r and mc4r in the energy homeostasis of red crucian carp (Carassius auratus red var., RCC), we cloned mc3r and mc4r, analyzed the tissue expression and localization of the genes, and investigated the effects of knockout of mc3r (mc3r +/-) and mc4r (mc4r +/-) in RCC.

Results

The full-length cDNAs of RCC mc3r and mc4r were 1459 base pairs (bp) and 1894 bp, respectively. qRT-PCR indicated that mc3r and mc4r were profusely expressed in the brain, but lower expressed in the periphery tissues. ISH revealed that mc3r and mc4r were located in NPP, NPO, NAPv, NSC, NAT, NRL, NLTl, and NLTp of the brain, suggesting that mc3r and mc4r might regulate many physiological and behavioral aspects in RCC. To further verify the roles of mc3r and mc4r in energy homeostasis, the mc3r+/- and mc4r+/- fish were obtained by the CRISPR/Cas9 system. The average body weights, total lengths, body depths, and food intake of mc4r+/- fish were significantly higher than those of mc3r+/- and the normal wild-type (WT) fish, but there was no difference between the mc3r+/- and WT fish, indicating that the RCC phenotype and food intake were mainly influenced by mc4r but not mc3r. Interestingly, mc4r+/- fish displayed more visceral fat mass than mc3r+/- and WT fish, and mc3r+/- fish also exhibited slightly more visceral fat mass compared to WT. RNA-seq of the liver and muscle revealed that a large number of differentially expressed genes (DEGs) differed in WT vs. mc3r+/-, WT vs. mc4r+/-, and mc3r+/- vs. mc4r+/-, mainly related to lipid, glucose, and energy metabolism. The KEGG enrichment analysis revealed that DEGs were mainly enriched in pathways such as steroid biosynthesis, fatty acid metabolism, fatty acid biosynthesis, glycolysis/gluconeogenesis, wnt signaling pathway, PPAR signaling pathway, and MAPK signaling pathway, thereby affecting lipid accumulation and growth.

Conclusion

In conclusion, these results will assist in the further investigation of the molecular mechanisms in which MC3R and MC4R were involved in the regulation of energy homeostasis in fish.

Molecular cloning and functional characterization of melanocortin-3 receptor in grass carp (Ctenopharyngodon idella)

The melanocortin-3-receptor (MC3R) plays an important role in mammals' food intake and energy homeostasis. However, its physiological role in bony fishes, such as grass carp, has not been well understood. This study reports the molecular cloning, tissue distribution, and pharmacological characterization of grass carp melanocortin-3-receptor (ciMC3R). Phylogenetic and chromosomal synteny analyses indicated that ciMC3R was closest to cyprinid fishes in evolution. Quantitative PCR experiments revealed that the mRNA of ciMC3R was highly expressed in the brain of grass carp. The cytological function of ciMC3R was investigated by the co-transfection of pcDNA3.1-ciMC3R and the signal-pathway-specific luciferase into the HEK293T cells. Results revealed that the four agonists, α-MSH, β-MSH, ACTH, and NDP-MSH, potentiate the activation of ciMC3R and further increase the production of cAMP and upregulate the MAPK/ERK signaling, respectively. Our study will provide basic data for exploring the physiological functions of grass carp MC3R, especially in energy homeostasis and food intake.