Function of Rho GTPases in embryonic blood cell migration in Drosophila

Circulating blood cells function as a surveillance system for damaged tissue in Drosophila larvae

Insects have an open circulatory system in which the heart pumps blood (hemolymph) into the body cavity, where it directly bathes the internal organs and epidermis. The blood contains free and tissue-bound immune cells that function in the inflammatory response. Here, we use live imaging of transgenic Drosophila larvae with fluorescently labeled blood cells (hemocytes) to investigate the circulatory dynamics of larval blood cells and their response to tissue injury. We find that, under normal conditions, the free cells rapidly circulate, whereas the tissue-bound cells are sessile. After epidermal wounding, tissue-bound cells around the wound site remain sessile and unresponsive, whereas circulating cells are rapidly recruited to the site of damage by adhesive capture. After capture, these cells distribute across the wound, appear phagocytically active, and are subsequently released back into circulation by the healing epidermis. The results demonstrate that circulating cells function as a surveillance system that monitors larval tissues for damage, and that adhesive capture, an important mechanism of recruitment of circulating cells to inflammatory sites in vertebrates, is shared by insects and vertebrates despite the vastly different architectures of their circulatory systems.

Identification of neural outgrowth genes using genome-wide RNAi

While genetic screens have identified many genes essential for neurite outgrowth, they have been limited in their ability to identify neural genes that also have earlier critical roles in the gastrula, or neural genes for which maternally contributed RNA compensates for gene mutations in the zygote. To address this, we developed methods to screen the Drosophila genome using RNA-interference (RNAi) on primary neural cells and present the results of the first full-genome RNAi screen in neurons. We used live-cell imaging and quantitative image analysis to characterize the morphological phenotypes of fluorescently labelled primary neurons and glia in response to RNAi-mediated gene knockdown. From the full genome screen, we focused our analysis on 104 evolutionarily conserved genes that when downregulated by RNAi, have morphological defects such as reduced axon extension, excessive branching, loss of fasciculation, and blebbing. To assist in the phenotypic analysis of the large data sets, we generated image analysis algorithms that could assess the statistical significance of the mutant phenotypes. The algorithms were essential for the analysis of the thousands of images generated by the screening process and will become a valuable tool for future genome-wide screens in primary neurons. Our analysis revealed unexpected, essential roles in neurite outgrowth for genes representing a wide range of functional categories including signalling molecules, enzymes, channels, receptors, and cytoskeletal proteins. We also found that genes known to be involved in protein and vesicle trafficking showed similar RNAi phenotypes. We confirmed phenotypes of the protein trafficking genes Sec61alpha and Ran GTPase using Drosophila embryo and mouse embryonic cerebral cortical neurons, respectively. Collectively, our results showed that RNAi phenotypes in primary neural culture can parallel in vivo phenotypes, and the screening technique can be used to identify many new genes that have important functions in the nervous system.

Development and function of the brain requires the coordinated action of thousands of genes, and currently we understand the roles of only a small fraction of them. Recent advances in genomics, such as the sequencing of entire genomes and the discovery of RNA-interference as a means of testing the effects of gene loss, have opened up the possibility to systematically analyze the function of all known and predicted genes in an organism. Until now, this type of functional genomics approach has not been applied to the study of very complex cells, such as the brain's neurons, on a full-genome scale. In this work, we developed techniques to test all genes, one by one in a rapid manner, for their potential role in neuronal development using neurons isolated from fruit fly embryos. These results yielded a global perspective of what types of genes are necessary for brain development; importantly, they show that a large variety of genes can be studied in this way.

Haemocyte-derived SPARC is required for collagen-IV-dependent stability of basal laminae in Drosophila embryos

SPARC is an evolutionarily conserved collagen-binding extracellular matrix (ECM) glycoprotein whose morphogenetic contribution(s) to embryonic development remain elusive despite decades of research. We have therefore used Drosophila genetics to gain insight into the role of SPARC during embryogenesis. In Drosophila embryos, high levels of SPARC and other basal lamina components (such as network-forming collagen IV, laminin and perlecan) are synthesized and secreted by haemocytes, and assembled into basal laminae. A SPARC mutant was generated by P-element mutagenesis that is embryonic lethal because of multiple developmental defects. Whereas no differences in collagen IV immunostaining were observed in haemocytes between wild-type and SPARC-mutant embryos, collagen IV was not visible in basal laminae of SPARC-mutant embryos. In addition, the laminin network of SPARC-mutant embryos appeared fragmented and discontinuous by late embryogenesis. Transgenic expression of SPARC protein by haemocytes in SPARC-mutant embryos restored collagen IV and laminin continuity in basal laminae. However, transgenic expression of SPARC by neural cells failed to rescue collagen IV in basal laminae, indicating that the presence of collagen IV deposition requires SPARC expression by haemocytes. Our previous finding that haemocyte-derived SPARC protein levels are reduced in collagen-IV-mutant embryos and the observation that collagen-IV-mutant embryos showed a striking phenotypic similarity to SPARC-mutant embryos suggests a mutual dependence between these major basal laminae components during embryogenesis. Patterning defects and impaired condensation of the ventral nerve cord also resulted from the loss SPARC expression prior to haemocyte migration. Hence, SPARC is required for basal lamina maturation and condensation of the ventral nerve cord during Drosophila embryogenesis.

Novel gcm-dependent lineages in the postembryonic nervous system of Drosophila melanogaster

glial cells missing genes (gcm and gcm2) act as the glial fate determinants in the Drosophila embryo. However, their requirement in the adult central nervous system (CNS) is at present not known, except for their role in lamina glia. This is particularly important with respect to two recent sets of data. Adult glial subpopulations differentiate through embryonic glia proliferation. Also, gcm-gcm2 are required for the differentiation of specific adult neurons. We here show that gcm is expressed in precursors and postmitotic, migrating, cells of the medulla neuropile glia (mng) lineage. It is also expressed in a thoracic glial lineage and in neurons of the ventral nerve cord (VNC). Finally, while gcm is required for gliogenesis in medulla and VNC, it does not seem to be required for the generation of VNC neurons.

The Rap GTPase activator Drosophila PDZ-GEF regulates cell shape in epithelial migration and morphogenesis

Epithelial morphogenesis is characterized by an exquisite control of cell shape and position. Progression through dorsal closure in Drosophila gastrulation depends on the ability of Rap1 GTPase to signal through the adherens junctional multidomain protein Canoe. Here, we provide genetic evidence that epithelial Rap activation and Canoe effector usage are conferred by the Drosophila PDZ-GEF (dPDZ-GEF) exchange factor. We demonstrate that dPDZ-GEF/Rap/Canoe signaling modulates cell shape and apicolateral cell constriction in embryonic and wing disc epithelia. In dPDZ-GEF mutant embryos with strong dorsal closure defects, cells in the lateral ectoderm fail to properly elongate. Postembryonic dPDZ-GEF mutant cells generated in mosaic tissue display a striking extension of lateral cell perimeters in the proximity of junctional complexes, suggesting a loss of normal cell contractility. Furthermore, our data indicate that dPDZ-GEF signaling is linked to myosin II function. Both dPDZ-GEF and cno show strong genetic interactions with the myosin II-encoding gene, and myosin II distribution is severely perturbed in epithelia of both mutants. These findings provide the first insight into the molecular machinery targeted by Rap signaling to modulate epithelial plasticity. We propose that dPDZ-GEF-dependent signaling functions as a rheostat linking Rap activity to the regulation of cell shape in epithelial morphogenesis at different developmental stages.

Drosophila melanogaster embryonic haemocytes: masters of multitasking

Drosophila melanogaster haemocytes constitute the cellular arm of a robust innate immune system in flies. In the adult and larva, these cells operate as the first line of defence against invading microorganisms: they phagocytose pathogens and produce antimicrobial peptides. However, in the sterile environment of the embryo, these important immune functions are largely redundant. Instead, throughout development, embryonic haemocytes are occupied with other tasks: they undergo complex migrations and carry out several non-immune functions that are crucial for successful embryogenesis.

Drosophila hemopoiesis and cellular immunity

In Drosophila melanogaster larvae, three classes of circulating cellular immune surveillance cells (hemocytes) can be identified: plasmatocytes, crystal cells, and lamellocytes. Plasmatocytes are professional phagocytes most similar to the mammalian monocyte/macrophage lineage and make up approximately 95% of circulating hemocytes. The other approximately 5% of circulating hemocytes consists of crystal cells, which secrete components necessary for the melanization of invading organisms, as well as for wound repair. A third cell type known as lamellocytes are rarely seen in healthy larvae and are involved in the encapsulation of invading pathogens. There are no obvious mammalian counterparts for crystal cells or lamellocytes, and there is no equivalent to the lymphoid lineage in insects. In this review, I will discuss what is currently known about Drosophila hemopoiesis and the cellular immune response and where possible compare it to vertebrate mechanisms.

Reciprocal regulation of Rac1 and Rho1 inDrosophilacirculating immune surveillance cells

In many cell types it is evident that the small GTPases Rac and Rho regulate each other's activities. What is unclear is exactly how this regulation occurs. To further elucidate this interaction we examined the activities of Rac1 and Rho1 in Drosophila cellular immune surveillance cells. In larvae the cellular immune response involves circulating cells (hemocytes) that can be recruited from a hematopoietic organ located behind the brain, as well as a sessile population found just underneath the larval cuticle. We demonstrate for the first time that Rho-kinase activation requires both Rho1 and the Drosophila c-Jun N-terminal kinase (Basket). We also show that Rac1, via Basket, regulates Rho1 activity, possibly by inhibiting RhoGAPp190. In the reciprocal pathway, co-expression of dominant negative Rho-kinase and constitutive active Rho1 induces a Rac1-like phenotype. This induction requires the formin Diaphanous. Co-expression of dominant negative Rho-kinase and constitutive active Rho1 also induces filopodia formation, with Diaphanous enriched at the tips. The Rac1-like phenotypes, and filopodia formation, could be blocked by co-expression of dominant negative Rac1. Finally, though dominant negative Rac1 is able to block filopodia formation in the overexpression experiments, only Rac2 is necessary for filopodia formed by hemocytes after parasitization.

Extrinsic and intrinsic mechanisms directing epithelial cell sheet replacement during Drosophila metamorphosis

The fusion of epithelial sheets is an essential morphogenetic event. Here, we study the development of the abdomen of Drosophila as a model of bounded epithelia expansion and uncover a complex multistep process for the generation of the adult epidermis from histoblasts, founder cells that replace the larval cells during metamorphosis. We find that histoblasts experience a biphasic cell cycle and emit apical projections that direct their invasive planar intercalation in between larval cells. Coordinately, the larval cells extrude from the epithelia by apical constriction of an actomyosin ring and as a consequence die by apoptosis and are removed by circulating haemocytes. We demonstrate that the proliferation of histoblasts and the death of larval cells are triggered by two independent extrinsic Ecdysone hormonal pulses. Finally, we show that histoblast spreading and the death of larval cells depend on a mutual exchange of signals and are non-autonomous processes.

The PDZ-GEF dizzy regulates cell shape of migrating macrophages via Rap1 and integrins in the Drosophila embryo

In Drosophila embryos, macrophages originate from the cephalic mesoderm and perform a complex migration throughout the entire embryo. The molecular mechanisms regulating this cell migration remain largely unknown. We identified the Drosophila PDZ G-nucleotide exchange factor (PDZ-GEF) Dizzy as a component essential for normal macrophage migration. In mutants lacking Dizzy, macrophages have smaller cellular protrusions, and their migration is slowed down significantly. This phenotype appears to be cell-autonomous, as it is also observed in embryos with a dsRNA-induced reduction of dizzy function in macrophages. In a complementary fashion, macrophages overexpressing Dizzy are vastly extended and form very long protrusions. These cell shape changes depend on the function of the small GTPase Rap1: in rap1 mutants, Dizzy is unable to induce the large protrusions. Furthermore, forced expression of a dominant-active form of Rap1, but not of the wild-type form, induces similar cell shape changes as Dizzy does overexpression. These findings suggest that Dizzy acts through Rap1. We propose that integrin-dependent adhesion is a Rap1-mediated target of Dizzy activity: in integrin mutants, neither Dizzy nor Rap1 can induce cell shape changes in macrophages. These data provide the first link between a PDZ-GEF, the corresponding small GTPase and integrin-dependent cell adhesion during cell migration in embryonic development.

Distinct mechanisms regulate hemocyte chemotaxis during development and wound healing in Drosophila melanogaster

Drosophila melanogaster hemocytes are highly motile macrophage-like cells that undergo a stereotypic pattern of migration to populate the whole embryo by late embryogenesis. We demonstrate that the migratory patterns of hemocytes at the embryonic ventral midline are orchestrated by chemotactic signals from the PDGF/VEGF ligands Pvf2 and -3 and that these directed migrations occur independently of phosphoinositide 3-kinase (PI3K) signaling. In contrast, using both laser ablation and a novel wounding assay that allows localized treatment with inhibitory drugs, we show that PI3K is essential for hemocyte chemotaxis toward wounds and that Pvf signals and PDGF/VEGF receptor expression are not required for this rapid chemotactic response. Our results demonstrate that at least two separate mechanisms operate in D. melanogaster embryos to direct hemocyte migration and show that although PI3K is crucial for hemocytes to sense a chemotactic gradient from a wound, it is not required to sense the growth factor signals that coordinate their developmental migrations along the ventral midline during embryogenesis.

Rac1 signalling in the Drosophila larval cellular immune response

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.

Drosophila melanogaster Rac2 is necessary for a proper cellular immune response

It has been reported that during Drosophila embryonic development, and in cell culture, that the Rac GTPases are redundant. To better elucidate Rac function in Drosophila, we decided to study the role of Rac2 in larval cellular defense reactions against the parasitiod Leptopilina boulardi. Here we show a dramatic effect in the context of cellular immunity where, unlike embryonic development, Rac2 appears to have a non-redundant function. When an invading parasitoid is recognized as foreign, circulating hemocytes (blood cells) should recognize and attach to the egg chorion. After attachment the hemocytes should then spread to form a multilayered capsule surrounding the invader. In Rac2 mutants this process is disrupted. Immune surveillance cells, known as plasmatocytes, adhere to the parasitoid egg but fail to spread, and septate junctions do not assemble, possibly due to mislocalization of the Protein 4.1 homolog Coracle. Finally, larger cells known as lamellocytes attach to the capsule but also fail to spread, and there is a lack of melanization. From these results it appears that Rac2 is necessary for the larval cellular immune response.