The activated β-integrin (CgβV) enhances RGD-binding and phagocytic capabilities of hemocytes in Crassostrea gigas

Integrins are an important family of cell receptors that can bind foreign particles and promote cell phagocytosis after they are activated. In the present study, a novel β integrin was identified from pacific oyster Crassostrea gigas with an extracellular domain, a single transmembrane segment, and a short cytoplasmic domain. It was phylogenetically clustered with phagocytosis-related insecta βV, and designated as CgβV. CgβV shared a conserved NPX[Y/F] motif related to integrin activation with other phagocytosis-related β integrins. The mRNA transcripts of CgβV were widely detected in oyster tissues including hemocytes, gonad, adductor muscle, mantle, gill, and hepatopancreas, and the expression level in hemocytes was significantly up-regulated at 12 h after lipopolysaccharide (LPS) stimulation (p < 0.05), which was 2.29-fold higher than that in the control group. CgβV proteins were mainly observed on the hemocytes surface. The oyster hemocytes were found to bind fluorescein isothiocyanate (FITC)-labeled Arg-Gly-Asp-containing peptides (RGDCPs), and the binding capability was significantly up-regulated with the peak percentage of 37.6% at 12 h post LPS stimulation, which was higher than that in the control group (8.8%, p < 0.05), suggesting the activation of RGD-binding integrins on oyster hemocytes surface. The label-free RGDCPs and anti-CgβV antibody inhibited the binding capability of hemocytes towards FITC-labeled RGDCPs, which were significant lower in RGD blocking group (7.4%, p < 0.05) and anti-CgβV blocking group (22.1%, p < 0.05) than that in the control group (37.6%), indicating that CgβV could be a RGD-binding integrin. Phagocytosis assay demonstrated that LPS could enhance the phagocytosis of hemocytes towards Escherichia coli and fluorescent beads with the phagocytic rate (PR) of 18.3% and 17.4%, and phagocytic index (PI) of 5.29 and 37.71, respectively, which were significant higher than that in the control group (11.0% and 3.65 for E. coli, 9.8% and 29.26 for fluorescent beads, respectively, p < 0.05). In addition, both the label-free RGDCPs and anti-CgβV antibody significantly hindered the phagocytosis of hemocytes towards E. coli and fluorescent beads. After the E. coli and fluorescent beads were opsonized by oyster serum, the label-free RGDCPs still inhibited the phagocytosis of hemocytes towards them, while the anti-CgβV antibody could only inhibit the phagocytosis of hemocytes towards E. coli, suggesting that only the activated CgβV was involved in the enhancing phagocytosis for bacteria in oyster. Moreover, the key components of conserved integrin-mediated phagocytosis pathway including GTPases, talin proteins, Ca²⁺ and cAMP were all induced by LPS in hemocytes of oyster. All these results suggested that the activated CgβV enhanced RGD-binding and phagocytic capabilities of hemocytes, shedding lights on the mechanisms of integrin-mediated phagocytosis in mollusks.