The immunomodulatory function of invertebrate specific neuropeptide FMRFamide in oyster Crassostrea gigas.

Abstract As one of the most important neuropeptides identified only in invertebrates of Mollusca, Annelida and Arthropoda, FMRFamide (Phe-Met-Arg-Phe-NH 2) involves in multiple physiological processes, such as mediating cardiac frequency and contraction of somatic and visceral muscles. However, its modulatory role in the immune defense has not been well understood. In the present study, an FMRFamide precursor (designed as Cg FMRFamide) was identified in oyster Crassostrea gigas , which could be processed into nineteen FMRFamide peptides. Phylogenetic analysis revealed that Cg FMRFamide shared high similarity with other identified FMRFamides in mollusks. The mRNA of Cg FMRFamide was mainly concentrated in the tissues of visceral ganglia, hepatopancreas and hemocytes, and a consistent distribution of FMRFamide peptide was confirmed by immunohistochemistry and immunocytochemistry assays. The mRNA expression level of Cg FMRFamide in hemocytes was significantly up-regulated after immune stimulation with lipopolysaccharide (LPS). After the concentration of FMRFamide was increased by exogenous injection, the in vivo expressions of pro-inflammatory cytokine Cg IL17-5, as well as the apoptosis-related Cg Caspase-1 and Cg Caspase-3 in hemocytes were promptly increased (p < 0.05), but the concentration of signal molecule nitric oxide (NO) was significantly down-regulated (p < 0.05). Meanwhile, an increased phosphorylation of p38 MAP kinase in hemocytes was also detected after the FMRFamide injection. These results collectively demonstrated that the conserved FMRFamide could not only respond to immune stimulation, but also regulate the expression of immune effectors and apoptosis-related genes, which might be mediated by p38 MAP kinase pathway, thereby effectively involved in clearing pathogens and maintaining homeostasis in oysters. Highlights • Highlight: • An invertebrate specific FMRFamide precursor was identified from oyster Crassostrea gigas. • The expression of Cg FMRFamide in hemocytes was significantly upregulated in response to immune stimulation. • Cg FMRFamide could regulate the expression of nitric oxide, Cg IL-17-5 and apoptosis-related genes. • Cg FMRFamide could induce the phosphorylation of p38 MAP kinase in hemocytes. [ABSTRACT FROM AUTHOR]