Molecular characterization and expression analysis of dopa decarboxylase involved in the antibacterial innate immunity of the freshwater crayfish, Procambarus clarkii.

Dopa decarboxylase (DDC) is responsible for the synthesis of dopamine, which acts as an important modulator in the nervous systems of vertebrates and invertebrates. Recent studies have indicated that DDC also plays crucial roles in the insect innate immune system. However, the functions of DDC in immunomodulation in crustaceans have not been thoroughly elucidated to date. In this study, a new full-length cDNA of the DDC protein was identified from red swamp crayfish, Procambarus clarkii (named Pc-ddc). The ORF of Pc-ddc encoded 474 amino acids, which possessed a 377-amino-acid domain. Pc-ddc was expressed at a relatively high level in the hemocytes and gills of crayfish. This protein was expressed at a relatively low level in the hepatopancreas and intestine. The expression level of Pc-ddc was clearly upregulated in hemocytes, hepatopancreas, gills, and intestine tissues after challenge with S. aureus or E. ictaluri. The results of the enzyme catalysis assay showed that the enzyme catalysis activity of rPc-DDC was 35 ± 2.8 ng h mg (n = 3). In addition, the results of the mimetic crayfish hemocytes encapsulation assay showed that the encapsulation rate of beads coated with rPc-DDC was clearly increased. The results of the bacterial binding assay showed that rPc-DDC strongly binds to S. aureus and E. ictaluri. Finally, when Pc-ddc was knocked down, the number of surviving crayfish clearly decreased after S. aureus or E. ictaluri was injected. All of these results indicate that Pc-DDC is an important immunomodulating enzyme in the neuroendocrine-immune (NEI) system of crayfish.