Examination of the roles of CgTH in tyrosine metabolism and pigmentation of Pacific oyster (Crassostrea gigas)

The visually appealing shell colours and intricate patterns of shellfish not only attract consumers but also enhance their economic value. Understanding the inheritance pattern of shell colour and the molecular mechanism of pigmentation is highly important for enhancing breeding efficiency. This study involved the identification of a tyrosine hydroxylase gene (CgTH) and the investigation of its function in the metabolism of tyrosine and pigmentation in Pacific oysters (Crassostrea gigas). CgTH contains a conserved Biopterin_H domain that consists of a binding site for iron atoms and a binding site for tetrahydrobiopterin (BH4). Quantitative PCR (qPCR) and immunofluorescence analysis revealed that the expression of CgTH was higher in the mantle of black shell oysters in comparison to the white strain. The in vitro and in vivo experiments confirmed that CgTH is involved in tyrosine metabolism through the catalysis of the conversion of L-tyrosine to L-DOPA. In order to examine its functions in melanin synthesis in oysters, 3-Iodo-L-tyrosine (3-IT) treatment, RNA interference (RNAi), and L-DOPA treatment assays were conducted. The results of our study showed that 3-IT effectively inhibited CgTH activity, leading to a suppression of tyrosine metabolism. Knockdown of CgTH hindered the cAMP-CREM-MITF axis and decreased pigmentation. On the other hand, L-DOPA stimulated melanin production through cAMP signaling and counteracted the inhibitory effects of 3-IT on the metabolism of tyrosine. These data indicated that there may be a feedback loop involving CgTH and the cAMP-CREM-MITF axis that regulates tyrosine metabolism and melanogenesis in Pacific oysters. This study enhances our understanding of the molecular mechanism that governs oyster pigmentation and offers a possible molecular indicator for the targeted breeding of particular strains of Pacific oysters.