The role of exoskeleton in insect defense

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Insect exoskeleton has often been considered to be a barrier physically protecting internal tissues from the aggression of microbes and parasitoids and from mechanical injury. We previously demonstrated that insect exoskeleton (integument, including cuticle and epidermal cells) engaged in defense not only by physical means but also by ective synthesis of antibacterial peptide under stress conditions such as infection of microbes at injured site (Proc.Natl.Acad.Sci.U.S.A.90,6275-6279(1993)). The observation changed our classical view on the function of integument in insect defense. In the present project we further investigated the active engagement of the insect integument in insect defense. The major results are as follows : a) Prophenol oxidase cascade similar to that in hemolymph is present in body wall cuticle and tracheal cuticle. ; b) Cuticular prophenol oxidase is synthesized in hemocytes and transported to cuticle, where it is localized in a specific orderly arrayd pattern. c) Cuticular prophenol oxidase had retention time different from that of hemolymph on reversed phase ODS column chromatography, indicating that prophenol oxidase in hemolymph is modified in the process of its transport from hemolymph to cuticle. We are currently investigating the structural difference between prophenol oxidases from hemolymph and from cuticle. d) Prophenol oxidase a activating enzyme was purified from cuticles and its primary structure was deduced from base sequence of prophenol oxidase activating enzyme cDNA.e) Cecropin synthesis is induced in epidermal cells after abrasion of body wall cuticle and the application of lipopolysaccharide at the abraded part. Very small amount of lipopolysaccharide (a few picogram) could induce the synthesis when it was applied at abraded cuticle of 5th instar silkworm.