Loss-of-function mutations in the melanocortin-2-receptor (mc2r) lead to skin hyperpigmentation in teleost fish

Melanocortins regulate pigmentation via melanocortin receptors (MCRs), which exhibit high conservation across zebrafish and humans. MC2R is distinctive among melanocortin receptors in that it is activated exclusively by ACTH and requires Mrap1 for optimal functionality, in contrast to other receptors that possess the capacity to bind diverse ligands. Although the melanocortin 1 receptor (MC1R) is well characterised for its role in pigmentation, the function of MC2R remains largely unknown. This study examines the influence of MC2R on pigmentation by generating MC2R knockout zebrafish using the CRISPR/Cas9 system. The results demonstrated that the mc2rKO mutants exhibited pronounced hyperpigmentation, characterised by an elevated number of melanophores and xanthophores, while preserving the distinctive striped pattern and dorsoventral countershading. Gene expression analyses show an increase in the expression of genes involved in melanosome synthesis and melanin production, contributing to a darker phenotype, as well as genes associated with lipid metabolism and carotenoid accumulation, which result in enhanced yellow coloration. These results suggest that mc2r plays a role in modulating the development, distribution and metabolic activity of pigment cells. RNA-seq profiling of wild-type and mc2r gene knockout zebrafish