The regulation of melanocyte-stimulating hormone on the pigment granule dispersion in the xanthophores and melanophores of the large yellow croaker (Larimichthys crocea)

The large yellow croaker (Larimichthys crocea) is one of the most valuable economic mariculture fish in China, and its yellowness in skin is considered as the most important traits that determine the market value. The fish skin colors present naturally silvery white during the daytime and golden yellow during the night-time. In the present study, we found that the xanthosomes of xanthophores aggregated during the daytime and dispersed during the night-time, while the melanosomes of melanophores always dispersed all day long in vivo. We further cloned three proopiomelanocortin (POMC) genes of the large yellow croaker. All these POMC genes were expressed in the pituitary, but only POMC-C mRNA was expressed in the skin. The results of single-cell RT-PCR showed that the POMC-C mRNA was expressed in isolated xanthophores but not in melanophores. The xanthophores, but not melanophores, were found in the ventral skin, where only one subtype of melanocortin receptor (MCR), i.e. MC5R, was expressed. MC1R, MC4R and MC5R mRNAs were detected in the dorsal skin, where there existed both the xanthophores and melanophores. The results of single-cell RT-PCR showed that only MC5R mRNA was expressed in the xanthophores, while only MC1R mRNA was detected in the melanophores. Thirty min after in vivo and in vitro treatment with melanocyte-stimulating hormone (MSH) peptides (alpha-MSH, Des-Ac-alpha-MSH, alpha-MSH-C and beta-MSH-C), dispersion degrees of the xanthosomes and melanosomes significantly increased, even under light exposure. Interestingly, when we immediately observed the dorsal scales that have been maintained under in vitro complete dark condition without MSH peptides treatment for at least 30 min, the melanosomes aggregated as expected, but the xanthosomes dispersed. Both alpha-MSH and alpha-MSH-C exhibited similar, but stronger effects than Des-Ac-alpha-MSH on dispersion of both the xanthosomes and melanosomes. Although the two MSH peptides (alpha-MSH-C and beta-MSH-C) are generated from POMC-C, the former showed stronger effect on xanthosome dispersion than the latter. Forskolin, an Adenylyl cyclase activator, dispersed pigment granules in both the xanthophores and melanophores. H 89 2HCl, a specific inhibitor of protein kinase A blocked alpha-MSH-induced pigment granule dispersion of xanthophores and melanophores. Taken together, our results indicated that the dispersion of melanosomes is stimulated by MSH, possibly secreted from pituitary, through endocrine pathway. The yellowness of the large yellow croaker skin was mainly due to the dispersion of xanthosomes. The dispersion of xanthosomes was regulated by MSH that is possibly released both from the pituitary through endocrine pathway and from the xanthophores through autocrine pathway.