A granulocyte highly-expressed α2 adrenergic receptor promotes the expression of IL-17 and TNFs in the immune response of oyster Magallana gigas

Catecholamine, along with adrenergic receptor (ARs), plays a pivotal role in mediating the immune function of vertebrates under stressful environment. However, the immunomodulatory role of adrenergic receptors in invertebrates remains largely unknown, hindering the development of effective stress management strategies in invertebrate aquaculture. In the present study, an α2 type adrenergic receptor, designated as Mgα2AR, was identified in an economically important species, the Pacific oyster Magallana gigas. The Mgα2AR was consisted of seven transmembrane domains and one short carboxyl terminus. The mRNA transcript of Mgα2AR was expressed in haemocytes and all the examined tissues, with the highest expression level in haemocytes, particularly in granulocyte (9.28-fold of that in agranulocyte, p < 0.05). By immunofluorescence analysis, Mgα2AR was found primarily localized in the cytoplasmic membrane of granulocytes. The mRNA expression of Mgα2AR in haemocytes increased significantly at 6 h, and peaked at 12 h (4.71-fold of that in control group, p < 0.01) after LPS stimulation in vivo. After transfection and expression of Mgα2AR in HEK293T cells, a significant decrease in cAMP concentration, rather than in Ca²⁺ concentration, was observed upon treatment with the agonist NE (p < 0.05), while the administration of the α2 adrenergic receptor antagonist (Ida) yielded opposite results. The expressions of MgIL17–5, MgTNF-1, and MgTNF-2 in oyster haemocytes were also downregulated following treatment with α2AR antagonist Ida, which was 0.44-fold, 0.54-fold, and 0.32-fold of that in the NE+ LPS group (p < 0.01), respectively. After Mgα2AR was knockdown by RNAi, the transcripts of MgIL17–5, MgTNF-1, and MgTNF-2 significantly decreased. These results collectively suggested that the granulocytes highly-expressed Mgα2AR could induce the expressions of TNFs and IL-17 by reducing the intracellular cAMP level in the immune response of oysters. These findings provide a novel insight into the immunomodulatory role of adrenergic receptors in mollusks, and also offer valuable information for the development of effective stress management strategies in aquaculture.