Affiliation: University of California Santa Barbara
Keywords: a. cytoskeleton; a. cytoskeleton
The Rho family of small GTPases, Rac, Rho, and Cdc42 represent central nodes in the cytoskeletal and signaling networks that drive cell migration and engulfment. How the cells utilize the same RhoGTPase networks to promote different processes in different contexts is not well understood. We address this question in the Drosophila ovary, which contains ~850 somatic follicle cells and 16 much larger germline cells. Previously we reported that local and transient activation of Rac using a photoactivatable Rac (PA-Rac) induced protrusions and motility in border cells. However, persistent expression of a constitutively active form of Rac (Rac-CA) in just a subset of follicle cells, using a cell-type-specific enhancer (slbo-Gal4), destroyed the entire egg chamber, a phenotype that is significantly suppressed by mutation of a single engulfment receptor, Draper. Additionally, border follicle cells expressing Rac-CA by slbo-Gal4 engulf adjacent polar cells in early-stage 8 egg chambers. To further assess the effects of Rac-CA mediated engulfment, we generated FlpoutGal4 driven clones to express Rac-CA in a subset of follicle cells. These clones displayed a non-autonomous expression of Draper in follicle cells. Therefore, the expression of Rac-CA in a few cells destroys the whole tissue likely by stimulating engulfment.
To understand the mechanisms underlying Rac-CA mediated tissue destruction, we performed fixed tissue imaging with cell death markers. We observed that expression of Rac-CA caused caspase activation in follicle cells non-autonomously and increased acidification of germline nurse cells evident by lysotracker staining. Live imaging of follicle cells expressing Rac-CA by slbo-Gal4 unveiled rapid and synchronous death of germline nurse cells. It was initiated by biting the nurse cell membrane by border follicle cells. Expression of Rac-CA by stretch follicle cell-specific PG150-Gal4 also results in egg chamber death suggesting that the Rac-induced egg chamber death shares some features with late-stage developmental programmed cell death.
These results indicate that focal and transient activation of Rac promotes protrusion and motility, whereas high and sustained Rac activation promotes phagocytosis, initially of living cells, ultimately destroying the whole tissue. These results offer a novel explanation for a previously mysterious human immunodeficiency.