Function of Rho GTPases in embryonic blood cell migration in Drosophila

Hemocyte development in the Drosophila embryo is a genetic model to study blood cell differentiation, cell migration and phagocytosis. Macrophages, which make up the majority of embryonic hemocytes, migrate extensively as individual cells on basement membrane-covered surfaces. The molecular mechanisms that contribute to this migration process are currently not well understood. We report the generation, by P element replacement, of two Gal4 lines that drive expression of UAS-controlled target genes during early (gcm-Gal4) or late (Coll-Gal4) stages of macrophage migration. gcm-Gal4 is used for live imaging analysis showing that macrophages extend large, dynamic lamellipodia as their main protrusions as well as filopodia. We use both Gal4 lines to express dominant-negative and constitutively active isoforms of the Rho GTPases Rac1, Cdc42, Rho1 and RhoL in macrophages, and complement these experiments by analyzing embryos mutant for Rho GTPases. Our findings suggest that Rac1 and Rac2 act redundantly in controlling migration and lamellipodia formation in Drosophila macrophages, and that the third Drosophila Rac gene, Mtl, makes no significant contribution to macrophage migration. Cdc42 appears not to be required within macrophages but in other tissues of the embryo to guide macrophages to the ventral trunk region. No evidence was found for a requirement of Rho1 or RhoL in macrophage migration. Finally, to estimate the number of genes whose zygotic expression is required for macrophage migration we analyzed 208 chromosomal deletions that cover most of the Drosophila genome. We find eight deletions that cause defects in macrophage migration suggesting the existence of approximately ten zygotic genes essential for macrophage migration.