The immunomodulation mediated by a delta-opioid receptor for [Met5]-enkephalin in oyster Crassostrea gigas

• A homologue of δ-opioid receptor for [Met5]-enkaphalin was cloned from oyster.
• The CgDOR transcripts in haemocytes could be induced by LPS or ENK stimulation.
• Concentration of Ca2+ and cAMP in HEK293T cells decreased significantly after ENK stimulation.
• The phagocytic and antibacterial activities of haemocytes increased after ENK stimulation.
• The phagocytic and antibacterial activities were repressed after DOR antagonist stimulation.

Opioid receptors (OR) are a group of G protein-coupled receptors with opioids as ligands, which play an important role in triggering the second messengers to modulate immune response in vertebrate immunocytes. In the present study, the full length cDNA of a homologue of δ-opioid receptor (DOR) for [Met5]-enkaphalin was cloned from oyster Crassostrea gigas (designated as CgDOR), which was 1104 bp encoding a peptide of 367 amino acids containing a conserved 7tm_1 domain. After the stimulation of [Met5]-enkephalin, the concentration of second messengers Ca2+ and cAMP in the HEK293T cells decreased significantly (p < 0.05) with the expression of CgDOR. However, this trend was reverted with the addition of DOR antagonist BNTX. The CgDOR transcripts were ubiquitously detected in the tested tissues including haemocytes, gonad, mantle, kidney, gill, adductor muscle and hepatopancreas, with the highest expression level in the hepatopancreas. After LPS stimulation, the expression level of CgDOR mRNA began to increase (4.05-fold, p < 0.05) at 6 h, and reached the highest level (5.00-fold, p < 0.05) at 12 h. Haemocyte phagocytic and antibacterial activities increased significantly after [Met5]-enkephalin stimulation, whereas the increase was repressed with the addition of DOR antagonist BNTX. These results collectively suggested that CgDOR for [Met5]-enkephalin could modulate the haemocyte phagocytic and antibacterial functions through the second messengers Ca2+ and cAMP, which might be requisite for pathogen elimination and homeostasis maintenance in oyster.