Parasitoid wasp venom SERCA regulates Drosophila calcium levels and inhibits cellular immunity

Abstract

Because parasite virulence factors target host immune responses, identification and functional characterization of these factors can provide insight into poorly understood host immune mechanisms. The fruit fly Drosophila melanogaster is a model system for understanding humoral innate immunity, but Drosophila cellular innate immune responses remain incompletely characterized. Fruit flies are regularly infected by parasitoid wasps in nature and, following infection, flies mount a cellular immune response culminating in the cellular encapsulation of the wasp egg. The mechanistic basis of this response is largely unknown, but wasps use a mixture of virulence proteins derived from the venom gland to suppress cellular encapsulation. To gain insight into the mechanisms underlying wasp virulence and fly cellular immunity, we used a joint transcriptomic/ proteomic approach to identify venom genes from Ganaspis sp.1 (G1), a previously uncharacterized Drosophila parasitoid species, and found that G1 venom contains a highly abundant sarco/endoplasmic reticulum calcium ATPase (SERCA) pump. Accordingly, we found that fly immune cells termed plasmatocytes normally undergo a cytoplasmic calcium burst following infection, and that this calcium burst is required for activation of the cellular immune response. We further found that the plasmatocyte calcium burst is suppressed by G1 venom in a SERCA-dependent manner, leading to the failure of plasmatocytes to become activated and migrate toward G1 eggs. Finally, by genetically manipulating plasmatocyte calcium levels, we were able to alter fly immune success against G1 and other parasitoid species. Our characterization of parasitoid wasp venom proteins led us to identify plasmatocyte cytoplasmic calcium bursts as an important aspect of fly cellular immunity.

Parasitoid wasp virulence

“…27,28 For our studies, we have chosen to specifically focus on the anti-immune mechanisms employed by the larval endoparasitoid wasp Ganaspis sp.1 to overcome the fly cellular encapsulation response. 18 We found that Ganaspis sp.1 was highly virulent and successfully parasitized nearly 100% of larvae from several wild type strains of D. melanogaster. 18 Additionally, Ganaspis sp.1 had a high parasitization success rate against several species within the melanogaster species subgroup, and against more distantly related flies across multiple species groups.…”

“…18 We found that Ganaspis sp.1 was highly virulent and successfully parasitized nearly 100% of larvae from several wild type strains of D. melanogaster. 18 Additionally, Ganaspis sp.1 had a high parasitization success rate against several species within the melanogaster species subgroup, and against more distantly related flies across multiple species groups. 18 This suggests that Ganaspis sp.1 can be classified as a generalist (broadly infects fly species of the genus Drosophila) rather than a specialist (infects a narrow range of Drosophila species) parasitoid wasp.…”

“…18 Additionally, Ganaspis sp.1 had a high parasitization success rate against several species within the melanogaster species subgroup, and against more distantly related flies across multiple species groups. 18 This suggests that Ganaspis sp.1 can be classified as a generalist (broadly infects fly species of the genus Drosophila) rather than a specialist (infects a narrow range of Drosophila species) parasitoid wasp. 13,18,29 Interestingly, one of the few Ganaspis sp.1 resistant species is D. paramelanica, a fly species that has previously been demonstrated to use a non-cellular encapsulation mediated antiwasp response.…”

“…[16][17][18] The plasmatocytes spread across the surface of the egg and form stable intercellular adhesions via the development of septate junctions, resulting in an uninterrupted plasmatocyte layer surrounding the wasp egg. 16,17,19,20 While the plasmatocyte layer is forming, wasp infection also induces the differentiation of specialized immune cells called lamellocytes.…”

“…This cellular immune response clearly provides a good model for the functional study of basic cell biological processes including cell differentiation, cell migration, cell-cell adhesion, signal transduction, and the regulation and function of post-translational protein modifications. [16][17][18][19][20]25,26 Not surprisingly, parasitoid wasps have evolved virulence mechanisms to overcome the fly cellular encapsulation response. Wasp virulence factors are expressed in the venom gland, secreted into the venom, and are co-injected with the egg during infection.…”