Selectively enhanced expression of prophenoloxidase activating enzyme 1 (PPAE1) at a bacteria clearance site in the white shrimp, Litopenaeus vannamei

Here we report on a PPAE of white shrimp, Litopenaeus vannamei (lvPPAE1), which showed 94% similarity to PPAE1 of Penaeus monodon. We found that lvPPAE1 in fluid hemocytes was down regulated after challenge by Vibrio harveyi but was enhanced when shrimps were exposed to a bacteria-rich environment for long-term. In vivo gene silence of lvPPAE1 by RNAi can significantly reduce the phenoloxidase activity (PO) and increase the susceptibility of shrimps to V. harveyi. Although lvPPAE1 was down-regulated in fluid hemocytes by Vibrio challenge, its expression increased significantly in gill after bacteria injection, which is the primary bacteria-clearance tissue.Suppressed expression in fluid hemocytes and enhanced expression in gill indicates selectively enhanced expression at the bacterial clearance site. This is a novel feature for PPAE expression. The results will contribute to our understanding of the PO activating system in crustaceans.Innate immunity is of great importance to insects and crustaceans because they lack antibodies [1]. Innate immunity involves phagocytosis, encapsulation, hemocyte coagulation and activation of the prophenoloxidase (proPO) or melanization cascade [2,3]. Activation of proPO generates phenoloxidase (PO), which catalyzes the oxygenation of monophenols to o-diphenols and the oxidation of o-diphenols to the corresponding o-quinones [4]. These are reactive intermediates for melanin synthesis and other physiological processes such as cuticle sclerotization, wound healing and pathogen sequestration [5].In most cases, the proPO cascade is triggered by a small amount of microbe-derived molecules such as lipopolysaccharides (LPSs), β-1, 3-glucans, or peptidoglycan. Pattern-recognition proteins bind these molecules and initiate the proPO system through a quick proteolytic cascade, and many proteins involved in the proteolytic cascade are serine proteinases [6]. The final serine proteinase that converts the inactive proPO into its active form is ca