Rac1 signalling in theDrosophilalarval cellular immune response

Abstract

The Drosophila larval cellular immune response involves cells (hemocytes) that can be recruited from a hematopoietic organ located behind the brain, as well as a sessile population of cells found just underneath the larval cuticle arranged in a segmental pattern. By using two Rac1 GTPase effector-loop mutants together with epistasis studies, we show that Rac1 requires the Drosophila melanogaster Jun N-terminal kinase Basket (Bsk), as well as stable actin formation to recruit the sessile hemocyte population. We show that actin stabilization is necessary for Rac1-induced hemocyte activation by lowering cofilin (encoded by the twinstar gene tsr) expression in blood cells. Removing Bsk by RNAi suppressed Rac1-induced release of sessile hemocytes. RNAi against Bsk also suppressed Rac1 induction of lamellocytes, a specialized population of hemocytes necessary for the encapsulation of invading pathogens. Furthermore, Rac1 and Bsk are involved in regulating the formation of actin- and focal adhesion kinase (FAK)-rich placodes in hemocytes. Lastly, Rac1 and Bsk are both required for the proper encapsulation of eggs from the parasitoid wasp Leptipolina boulardi. From these data we conclude that Rac1 induces Bsk activity and stable actin formation for cellular immune activation, leading to sessile hemocyte release and an increase in the number of circulating hemocytes.

Male killing Spiroplasma protects Drosophila melanogaster against two parasitoid wasps

“…In this study, some of the fly larvae in the Lb treatment exhibited melanized tissues at 72 and 144 h after attack. To be effective, however, melanotic encapsulation should kill the wasp before egg hatching, and it is typically completed by 24-40 h after attack (equivalent to B0 h in our study) (Russo et al, 1996;Williams et al, 2006). These observations suggest that Spiroplasma does not enhance the fly's ability to encapsulate wasp embryos, but improvement of other aspects of immunity cannot be ruled out (e.g., enhancement of cytotoxic products such as reactive oxygen species or intermediates of the melanization cascade; Lemaitre and Hoffmann, 2007).…”

Drosophila Hemopoiesis and Cellular Immunity

“…One family of proteins known to be centrally involved in the regulation of cell migration and cell shape change is the Rho GTPases (Rho, Rac, and Cdc42). Recently, two articles have been published showing that both Drosophila Rac GTPase (Rac1 and Rac2) are necessary for proper encapsulation eggs from the parasitoid wasp Leptopilina boulardi (64,65). In Rac2 mutants, the plasmatocytes and lamellocytes recognize and attach to the wasp egg but fail to spread around it.…”

“…This lack of spreading means septate junctions fail to form, and the capsule fails to melanize. Williams et al (65) showed that Rac1 and the Drosophila JNK (Basket) are also required for proper encapsulation, but the reason is still unknown. Interestingly, it has also been published that the Drosophila ␤ 2 -integrin (known as Myospheroid) is necessary for proper encapsulation of the wasp egg (66).…”

Extracellular Superoxide Dismutase in Insects

“…The venom of ISm females, a highly virulent strain against Drosophila melanogaster, contains a RhoGAP protein that is required for parasitism success (12). This factor targets Drosophila lamellocytes, the main capsule-forming hemocytes, and induces changes in their morphology by inactivating the RhoGTPases Rac1 and Rac2, both required for successful encapsulation (13)(14)(15)(16)(17). L. boulardi ISy females differ from ISm females in their ability to suppress immune defenses of both D. melanogaster and Drosophila yakuba hosts but depending on the host resistance genotype (11,18).…”