PGE2 MEDIATES OENOCYTOID CELL LYSIS VIA A SODIUM-POTASSIUM-CHLORIDE COTRANSPORTER

Prostaglandin E2 (PGE2 ) mediates immune responses of the beet armyworm, Spodoptera exigua, including oenocytoid cell lysis (a class of lepidopteran hemocytes: OCL) via its specific membrane receptor to release inactive prophenoloxidase (PPO) into hemolymph. PPO is activated into phenoloxidase in the plasma to play crucial roles in the immune responses of S. exigua. The mechanism of OCL has not been elucidated, however we posed the hypothesis that a rapid accumulation of sodium ions within the oenocytoids allows a massive influx of water by the ion gradient, which leads to the cell lysis. It remains unclear which sodium channel is responsible for the OCL in response to PGE2 . This study identified a specific sodium channel called sodium-potassium-chloride cotransporter 1 (Se-NKCC1) expressed in hemocytes of S. exigua and analyzed its function in the OCL in response to PGE2 . Se-NKCC1 encodes a basic membrane protein (pI value = 8.445) of 1,066 amino acid residues, which contains 12 putative transmembrane domains. Se-NKCC1 was expressed in all developmental stages and tissues. qPCR showed that bacterial challenge significantly induced its expression. A specific inhibitor of NKCC, bumetanide, prevented the OCL in a dose-dependent manner. When RNA interference (RNAi) using double-stranded RNA specific to Se-NKCC1 suppressed its expression, the OCL and PPO activation were significantly inhibited in response to PGE2 . The RNAi treatment also reduced nodule formation to bacterial challenge. These results suggest that Se-NKCC1 is associated with OCL by facilitating inward transport of ions in response to PGE2 .

Establishment of transgenic silkworms expressing GAL4 specifically in the haemocyte oenocytoid cells

Insect haemocytes play significant roles in innate immunity. The silkworm, a lepidopteran species, is often selected as the model for studies into the functions of haemocytes in immunity; however, our understanding of the role of haemocytes remains limited because the lack of haemocyte promoters for transgene expression makes genetic manipulations difficult. In the present study, we aimed to establish transgenic silkworm strains expressing GAL4 in their haemocytes. First, we identified three genes with strong expression in haemocytes, namely, lp44, Haemocyte Protease 1 (HP1) and hemocytin. Transgenic silkworms expressing GAL4 under the control of the putative promoters of these genes were then established and expression was examined. Although GAL4 expression was not detected in haemocytes of HP1-GAL4 or hemocytin-GAL4 strains, lp44-GAL4 exhibited a high level of GAL4 expression, particularly in oenocytoids. GAL4 expression was also detected in the midgut but in no other tissues, indicating that GAL4 expression in this strain is mostly oenocytoid-specific. Thus, we have identified a promoter that enables oenocytoid expression of genes of interest. Additionally, the lp44-GAL4 strain could also be used for other types of research, such as the functional analysis of genes in oenocytoids, which would facilitate advances in our understanding of insect immunity.