LRFN (leucine-rich repeat and fibronectin type-III domain-containing protein) recognizes bacteria and promotes hemocytic phagocytosis in the Pacific oyster Crassostrea gigas.

In bivalve mollusks, circulating hemocyte mediated phagocytosis is one of the primary ways to eliminate invading microbes. Here, we have identified one CgLRFN (leucine-rich repeat and fibronectin type-III domain-containing protein) in the Crassostrea gigas as a novel transmembrane LRR (Leucine-rich repeat) domain containing protein in C. gigas, homologous to the jawless fish VLR protein, that plays an important role in recognizing bacteria and promoting hemocytic phagocytosis. Tissue distribution analysis of CgLRFN in Pacific oyster showed that it is widely expressed in various tissues like the gills, adductor muscles, digestive glands, gonads, heart and in the hemocytes. Furthermore, infection of Pacific oysters with two marine Vibrio strains V. alginolyticus and V. parahaemolyticus was found to significantly increase CgLRFN expression in the hemocytes. Analysis of subcellular localization showed that CgLRFN is primarily localized in the cell membrane. Additionally, CgLRFN was found to be able to bind both the bacterial strains, indicating its possible role as a cell surface receptor. Flow cytometry analysis revealed that CgLRFN coated bacteria was phagocytosed by oyster hemocytes at a significantly higher rate compared to the uncoated bacteria. Finally, RNAi mediated knockdown of CgLRFN in vivo resulted in reduced clearance of both the bacterial strains from the oyster hemolymph. Overall, our study demonstrates that CgLRFN acts as a pattern recognition receptor for Vibrio spp. and promotes hemocytic phagocytosis in the Pacific oyster, which is critical for understanding the mechanism of bacterial infection in lower invertebrates, and also contributes to disease management of this economically and ecologically important marine mollusk.