The role of apoptosis in parasitism: the case of Aphidius ervi

Endoparasitoids insect in the order Hymenoptera are natural enemies of several insect pest species. During oviposition they inject several factors, including venom, into the host, ensuring the successful development of their progeny. Furthermore, venom (maternal factor), together with teratocytes (embryonic factors), is responsible for alteration of host physiology. Parasitism by the endophagous braconid Aphidius ervi (Hymenoptera, Braconidae) has a negative impact on the reproductive activity of its host, Acyrthosiphon pisum (Homoptera, Aphididae). In this host - parasitoid system the combined action of venom, inducing apoptosis of the host germarial cell and therefore the blocking of oogenesis, and of teratocytes, responsible for host tissues extra-oral digestion mediated by the enolase-plasminogen system, induces the host castration and thus the nutritional suitability for parasitoid progeny development. The apoptotic activity of venom on host reproductive apparatus is due to a venom protein identified as a gamma-glutamyltranspeptidasee (Ae-GGT). Both in A. ervi crude venom and in the corresponding chromatographic fraction containing Ae-GGT a measurable γ-GT activity was detected. γ-GTs are enzymes playing a key role in the metabolism of glutathione (GSH) and, as observed in most organisms, they are membrane-bound heterodimers formed by a large and a small subunit, which originate by post-translational processing of a single-chain precursor. The in vitro expression of Ae-GGT in insect cells confirmed the occurrence of the expected post-translational processing, and demonstrated that, unlike other γ-GTs, this protein, is secreted in the extracellular environment. The ability of this venom protein to induce apoptosis in the host ovarioles is probably due to an alteration of the GSH metabolism and a consequent oxidative stress.