Functional characterization of melanocortin-3 receptor in rainbow trout (Oncorhynchus mykiss).

The melanocortin-3 receptor (MC3R) is an important regulator of energy homeostasis and inflammation in mammals. However, its function in teleost fish needs to be further explored. In this study, we characterized rainbow trout MC3R (rtMC3R), which encoded a putative protein of 331 amino acids. Phylogenetic and chromosomal synteny analyses showed that rtMC3R was closely related to bony fishes. Quantitative PCR (qPCR) revealed that the transcripts of rtMC3R were highly expressed in the brain and muscle. The cellular function of rtMC3R was further verified by the signal-pathway-specific luciferase reporter assays. Four agonists such as α-MSH, ß-MSH, ACTH (1-24), and NDP-MSH can active rtMC3R, increasing the production of intracellular cAMP and upregulating MAPK/ERK signals. Moreover, we found that rtMC3R stimulated with α-MSH and NDP-MSH can significantly inhibit the NF-κB signaling pathway. This research will be helpful for further studies on the function of MC3R in rainbow trout, especially the role of energy metabolism and immune regulation.

Molecular Cloning, Tissue Distribution, and Pharmacological Characterization of GPR84 in Grass Carp (Ctenopharyngodon Idella).

The G-protein-coupled receptor GPR84, activated by medium-chain fatty acids, primarily expressed in macrophages and microglia, is involved in inflammatory responses and retinal development in mammals and amphibians. However, our understanding of its structure, function, tissue expression, and signaling pathways in fish is limited. In this study, we cloned and characterized the coding sequence of GPR84 (ciGPR84) in grass carp. A phylogenetic analysis revealed its close relationship with bony fishes. High expression levels of GPR84 were observed in the liver and spleen. The transfection of HEK293T cells with ciGPR84 demonstrated its responsiveness to medium-chain fatty acids and diindolylmethane (DIM). Capric acid, undecanoic acid, and lauric acid activated ERK and inhibited cAMP signaling. Lauric acid showed the highest efficiency in activating the ERK pathway, while capric acid was the most effective in inhibiting cAMP signaling. Notably, DIM did not activate GPR84 in grass carp, unlike in mammals. These findings provide valuable insights for mitigating chronic inflammation in grass carp farming and warrant further exploration of the role of medium-chain fatty acids in inflammation regulation in this species.