Salmonella pathogenesis reveals that BMP signaling regulates blood cell homeostasis and immune responses in Drosophila

Effect of genetic variants in the SMAD1 and SMAD5 genes promoter on growth and beef quality traits in cattle

The SMAD1 and SMAD5 genes belong to mothers against decapentaplegic proteins family, which participate in the BMP pathway to control skeletal myogenesis and growth. In the present study, we analyzed the associations between polymorphisms of SMAD1 and SMAD5 genes promoter and important economical traits in Qinchuan cattle. Four SNPs in the SMAD1 gene promoter and three SNPs in the SMAD5 promoter were identified by sequencing of 448 Qinchuan cattles. Allelic and frequency analyses of these SNPs resulted in eight haplotypes both in the promoters of the two genes promoter and identified potential cis-regulatory transcription factor (TF) components. In addition, correlation analysis showed that cattle SMAD1 promoter activity of individuals with Hap4 (P < 0.01) was stronger than that of individuals with Hap2. while the transcriptional activity of individuals with Hap3 within SMAD5 gene promoter was significantly (P < 0.01) higher followed by H2. Uniformly, diplotypes H4-H6 of SMAD1 gene and H1-H3 of SMAD5 gene performed significant (P < 0.01) associations with body measurement and improved carcass quality traits. All these results have indicated that polymorphisms in SMAD1 and SMAD5 genes promoter could impact the transcriptional regulation and then affect muscle content in beef cattle. Moreover, both the SMAD1 and SMAD5 genes were expressed ubiquitously in 10 tissues and had higher expression in the longissimus thoracis tissue from 6-month-old and 12-month-old cattle than in cattle of other ages. We can conclude that SMAD1 and SMAD5 genes may play an important role in muscle growth and development, and the variants mapped within SMAD1 and SMAD5 genes can be utilized in molecular marker-assisted selection for cattle carcass quality and body measurement traits in breed improvement programs of Qinchuan cattle.

Differential Expression of Immune Genes between Two Closely Related Beetle Species with Different Immunocompetence following Attack by Asecodes parviclava

Endoparasitoid wasps are important natural enemies of many insect species and are major selective forces on the host immune system. Despite increased interest in insect antiparasitoid immunity, there is sparse information on the evolutionary dynamics of biological pathways and gene regulation involved in host immune defense outside Drosophila species. We de novo assembled transcriptomes from two beetle species and used time-course differential expression analysis to investigate gene expression differences in closely related species Galerucella pusilla and G. calmariensis that are, respectively, resistant and susceptible against parasitoid infection by Asecodes parviclava parasitoids. Approximately 271 million and 224 million paired-ended reads were assembled and filtered to form 52,563 and 59,781 transcripts for G. pusilla and G. calmariensis, respectively. In the whole-transcriptome level, an enrichment of functional categories related to energy production, biosynthetic process, and metabolic process was exhibited in both species. The main difference between species appears to be immune response and wound healing process mounted by G. pusilla larvae. Using reciprocal BLAST against the Drosophila melanogaster proteome, 120 and 121 immune-related genes were identified in G. pusilla and G. calmariensis, respectively. More immune genes were differentially expressed in G. pusilla than in G. calmariensis, in particular genes involved in signaling, hematopoiesis, and melanization. In contrast, only one gene was differentially expressed in G. calmariensis. Our study characterizes important genes and pathways involved in different immune functions after parasitoid infection and supports the role of signaling and hematopoiesis genes as key players in host immunity in Galerucella against parasitoid wasps.

The PAX-SIX-EYA-DACH network modulates GATA-FOG function in fly hematopoiesis and human erythropoiesis

The GATA and PAX-SIX-EYA-DACH transcriptional networks (PSEDNs) are essential for proper development across taxa. Here, we demonstrate novel PSEDN roles in vivo in Drosophila hematopoiesis and in human erythropoiesis in vitro Using Drosophila genetics, we show that PSEDN members function with GATA to block lamellocyte differentiation and maintain the prohemocyte pool. Overexpression of human SIX1 stimulated erythroid differentiation of human erythroleukemia TF1 cells and primary hematopoietic stem-progenitor cells. Conversely, SIX1 knockout impaired erythropoiesis in both cell types. SIX1 stimulation of erythropoiesis required GATA1, as SIX1 overexpression failed to drive erythroid phenotypes and gene expression patterns in GATA1 knockout cells. SIX1 can associate with GATA1 and stimulate GATA1-mediated gene transcription, suggesting that SIX1-GATA1 physical interactions contribute to the observed functional interactions. In addition, both fly and human SIX proteins regulated GATA protein levels. Collectively, our findings demonstrate that SIX proteins enhance GATA function at multiple levels, and reveal evolutionarily conserved cooperation between the GATA and PSEDN networks that may regulate developmental processes beyond hematopoiesis.

TGFβ/BMP immune signaling affects abundance and function of C. elegans gut commensals

The gut microbiota contributes to host health and fitness, and imbalances in its composition are associated with pathology. However, what shapes microbiota composition is not clear, in particular the role of genetic factors. Previous work in Caenorhabditis elegans defined a characteristic worm gut microbiota significantly influenced by host genetics. The current work explores the role of central regulators of host immunity and stress resistance, employing qPCR and CFU counts to measure abundance of core microbiota taxa in mutants raised on synthetic communities of previously-isolated worm gut commensals. This revealed a bloom, specifically of Enterobacter species, in immune-compromised TGFβ/BMP mutants. Imaging of fluorescently labeled Enterobacter showed that TGFβ/BMP-exerted control operated primarily in the anterior gut and depended on multi-tissue contributions. Enterobacter commensals are common in the worm gut, contributing to infection resistance. However, disruption of TGFβ/BMP signaling turned a normally beneficial Enterobacter commensal to pathogenic. These results demonstrate specificity in gene-microbe interactions underlying gut microbial homeostasis and highlight the pathogenic potential of their disruption.

Hedgehog signaling from the Posterior Signaling Center maintains U-shaped expression and a prohemocyte population in Drosophila

Hematopoietic progenitor choice between multipotency and differentiation is tightly regulated by intrinsic factors and extrinsic signals from the surrounding microenvironment. The Drosophila melanogaster hematopoietic lymph gland has emerged as a powerful tool to investigate mechanisms that regulate hematopoietic progenitor choice in vivo. The lymph gland contains progenitor cells, which share key characteristics with mammalian hematopoietic progenitors such as quiescence, multipotency and niche-dependence. The lymph gland is zonally arranged, with progenitors located in medullary zone, differentiating cells in the cortical zone, and the stem cell niche or Posterior Signaling Center (PSC) residing at the base of the medullary zone (MZ). This arrangement facilitates investigations into how signaling from the microenvironment controls progenitor choice. The Drosophila Friend of GATA transcriptional regulator, U-shaped, is a conserved hematopoietic regulator. To identify additional novel intrinsic and extrinsic regulators that interface with U-shaped to control hematopoiesis, we conducted an in vivo screen for factors that genetically interact with u-shaped. Smoothened, a downstream effector of Hedgehog signaling, was one of the factors identified in the screen. Here we report our studies that characterized the relationship between Smoothened and U-shaped. We showed that the PSC and Hedgehog signaling are required for U-shaped expression and that U-shaped is an important intrinsic progenitor regulator. These observations identify a potential link between the progenitor regulatory machinery and extrinsic signals from the PSC. Furthermore, we showed that both Hedgehog signaling and the PSC are required to maintain a subpopulation of progenitors. This led to a delineation of PSC-dependent versus PSC-independent progenitors and provided further evidence that the MZ progenitor population is heterogeneous. Overall, we have identified a connection between a conserved hematopoietic master regulator and a putative stem cell niche, which adds to our understanding of how signals from the microenvironment regulate progenitor multipotency.

Identification and functional characterization of muscle satellite cells in Drosophila

Work on genetic model systems such as Drosophila and mouse has shown that the fundamental mechanisms of myogenesis are remarkably similar in vertebrates and invertebrates. Strikingly, however, satellite cells, the adult muscle stem cells that are essential for the regeneration of damaged muscles in vertebrates, have not been reported in invertebrates. In this study, we show that lineal descendants of muscle stem cells are present in adult muscle of Drosophila as small, unfused cells observed at the surface and in close proximity to the mature muscle fibers. Normally quiescent, following muscle fiber injury, we show that these cells express Zfh1 and engage in Notch-Delta-dependent proliferative activity and generate lineal descendant populations, which fuse with the injured muscle fiber. In view of strikingly similar morphological and functional features, we consider these novel cells to be the Drosophila equivalent of vertebrate muscle satellite cells.

Functional Conservation of the Glide/Gcm Regulatory Network Controlling Glia, Hemocyte, and Tendon Cell Differentiation in Drosophila

High-throughput screens allow us to understand how transcription factors trigger developmental processes, including cell specification. A major challenge is identification of their binding sites because feedback loops and homeostatic interactions may mask the direct impact of those factors in transcriptome analyses. Moreover, this approach dissects the downstream signaling cascades and facilitates identification of conserved transcriptional programs. Here we show the results and the validation of a DNA adenine methyltransferase identification (DamID) genome-wide screen that identifies the direct targets of Glide/Gcm, a potent transcription factor that controls glia, hemocyte, and tendon cell differentiation in Drosophila. The screen identifies many genes that had not been previously associated with Glide/Gcm and highlights three major signaling pathways interacting with Glide/Gcm: Notch, Hedgehog, and JAK/STAT, which all involve feedback loops. Furthermore, the screen identifies effector molecules that are necessary for cell-cell interactions during late developmental processes and/or in ontogeny. Typically, immunoglobulin (Ig) domain-containing proteins control cell adhesion and axonal navigation. This shows that early and transiently expressed fate determinants not only control other transcription factors that, in turn, implement a specific developmental program but also directly affect late developmental events and cell function. Finally, while the mammalian genome contains two orthologous Gcm genes, their function has been demonstrated in vertebrate-specific tissues, placenta, and parathyroid glands, begging questions on the evolutionary conservation of the Gcm cascade in higher organisms. Here we provide the first evidence for the conservation of Gcm direct targets in humans. In sum, this work uncovers novel aspects of cell specification and sets the basis for further understanding of the role of conserved Gcm gene regulatory cascades.

MANF silencing, immunity induction or autophagy trigger an unusual cell type in metamorphosing Drosophila brain

Glia are abundant cells in the brain of animals ranging from flies to humans. They perform conserved functions not only in neural development and wiring, but also in brain homeostasis. Here we show that by manipulating gene expression in glia, a previously unidentified cell type appears in the Drosophila brain during metamorphosis. More specifically, this cell type appears in three contexts: (1) after the induction of either immunity, or (2) autophagy, or (3) by silencing of neurotrophic factor DmMANF in glial cells. We call these cells MANF immunoreactive Cells (MiCs). MiCs are migratory based on their shape, appearance in brain areas where no cell bodies exist and the nuclear localization of dSTAT. They are labeled with a unique set of molecular markers including the conserved neurotrophic factor DmMANF and the transcription factor Zfh1. They possess the nuclearly localized protein Relish, which is the hallmark of immune response activation. They also express the conserved engulfment receptor Draper, therefore indicating that they are potentially phagocytic. Surprisingly, they do not express any of the common glial and neuronal markers. In addition, ultrastructural studies show that MiCs are extremely rich in lysosomes. Our findings reveal critical molecular and functional components of an unusual cell type in the Drosophila brain. We suggest that MiCs resemble macrophages/hemocytes and vertebrate microglia based on their appearance in the brain upon genetically challenged conditions and the expression of molecular markers. Interestingly, macrophages/hemocytes or microglia-like cells have not been reported in the fly nervous system before.

Antioxidants maintain E-cadherin levels to limit Drosophila prohemocyte differentiation

Mitochondrial reactive oxygen species (ROS) regulate a variety of biological processes by networking with signal transduction pathways to maintain homeostasis and support adaptation to stress. In this capacity, ROS have been shown to promote the differentiation of progenitor cells, including mammalian embryonic and hematopoietic stem cells and Drosophila hematopoietic progenitors (prohemocytes). However, many questions remain about how ROS alter the regulatory machinery to promote progenitor differentiation. Here, we provide evidence for the hypothesis that ROS reduce E-cadherin levels to promote Drosophila prohemocyte differentiation. Specifically, we show that knockdown of the antioxidants, Superoxide dismutatase 2 and Catalase reduce E-cadherin protein levels prior to the loss of Odd-skipped-expressing prohemocytes. Additionally, over-expression of E-cadherin limits prohemocyte differentiation resulting from paraquat-induced oxidative stress. Furthermore, two established targets of ROS, Enhancer of Polycomb and FOS, control the level of E-cadherin protein expression. Finally, we show that knockdown of either Superoxide dismutatase 2 or Catalase leads to an increase in the E-cadherin repressor, Serpent. As a result, antioxidants and targets of ROS can control E-cadherin protein levels, and over-expression of E-cadherin can ameliorate the prohemocyte response to oxidative stress. Collectively, these data strongly suggest that ROS promote differentiation by reducing E-cadherin levels. In mammalian systems, ROS promote embryonic stem cell differentiation, whereas E-cadherin blocks differentiation. However, it is not known if elevated ROS reduce E-cadherin to promote embryonic stem cell differentiation. Thus, our findings may have identified an important mechanism by which ROS promote stem/progenitor cell differentiation.

Drosophila E-cadherin functions in hematopoietic progenitors to maintain multipotency and block differentiation

A fundamental question in stem cell biology concerns the regulatory strategies that control the choice between multipotency and differentiation. Drosophila blood progenitors or prohemocytes exhibit key stem cell characteristics, including multipotency, quiescence, and niche dependence. As a result, studies of Drosophila hematopoiesis have provided important insights into the molecular mechanisms that control these processes. Here, we show that E-cadherin is an important regulator of prohemocyte fate choice, maintaining prohemocyte multipotency and blocking differentiation. These functions are reminiscent of the role of E-cadherin in mammalian embryonic stem cells. We also show that mis-expression of E-cadherin in differentiating hemocytes disrupts the boundary between these cells and undifferentiated prohemocytes. Additionally, upregulation of E-cadherin in differentiating hemocytes increases the number of intermediate cell types expressing the prohemocyte marker, Patched. Furthermore, our studies indicate that the Drosophila GATA transcriptional co-factor, U-shaped, is required for E-cadherin expression. Consequently, E-cadherin is a downstream target of U-shaped in the maintenance of prohemocyte multipotency. In contrast, we showed that forced expression of the U-shaped GATA-binding partner, Serpent, repressed E-cadherin expression and promoted lamellocyte differentiation. Thus, U-shaped may maintain E-cadherin expression by blocking the inhibitory activity of Serpent. Collectively, these observations suggest that GATA:FOG complex formation regulates E-cadherin levels and, thereby, the choice between multipotency and differentiation. The work presented in this report further defines the molecular basis of prohemocyte cell fate choice, which will provide important insights into the mechanisms that govern stem cell biology.

Characterization of the TGF-β1 signaling abnormalities in the Gata1low mouse model of myelofibrosis

Primary myelofibrosis (PMF) is characterized by fibrosis, ineffective hematopoiesis in marrow, and hematopoiesis in extramedullary sites and is associated with abnormal megakaryocyte (MK) development and increased transforming growth factor (TGF)-β1 release. To clarify the role of TGF-β1 in the pathogenesis of this disease, the TGF-β1 signaling pathway of marrow and spleen of the Gata1(low) mouse model of myelofibrosis (MF) was profiled and the consequences of inhibition of TGF-β1 signaling on disease manifestations determined. The expression of 20 genes in marrow and 36 genes in spleen of Gata1(low) mice was altered. David-pathway analyses identified alterations of TGF-β1, Hedgehog, and p53 signaling in marrow and spleen and of mammalian target of rapamycin (mTOR) in spleen only and predicted that these alterations would induce consequences consistent with the Gata1(low) phenotype (increased apoptosis and G1 arrest both in marrow and spleen and increased osteoblast differentiation and reduced ubiquitin-mediated proteolysis in marrow only). Inhibition of TGF-β1 signaling normalized the expression of p53-related genes, restoring hematopoiesis and MK development and reducing fibrosis, neovascularization, and osteogenesis in marrow. It also normalized p53/mTOR/Hedgehog-related genes in spleen, reducing extramedullary hematopoiesis. These data identify altered expression signatures of TGF-β1 signaling that may be responsible for MF in Gata1(low) mice and may represent additional targets for therapeutic intervention in PMF.

Signal transduction pathways, intrinsic regulators, and the control of cell fate choice

BACKGROUND

Information regarding changes in organismal status is transmitted to the stem cell regulatory machinery by a limited number of signal transduction pathways. Consequently, these pathways derive their functional specificity through interactions with stem cell intrinsic master regulators, notably transcription factors. Identifying the molecular underpinnings of these interactions is critical to understanding stem cell function.

SCOPE OF REVIEW

This review focuses on studies in Drosophila that identify the gene regulatory basis for interactions between three different signal transduction pathways and an intrinsic master transcriptional regulator in the context of hematopoietic stem-like cell fate choice. Specifically, the interface between the GATA:FOG regulatory complex and the JAK/STAT, BMP, and Hedgehog pathways is examined.

MAJOR CONCLUSIONS

The GATA:FOG complex coordinates information transmitted by at least three different signal transduction pathways as a means to control stem-like cell fate choice. This illustrates emerging principles concerning regulation of stem cell function and describes a gene regulatory link between changes in organismal status and stem cell response.

GENERAL SIGNIFICANCE

The Drosophila model system offers a powerful approach to identify the molecular basis of how stem cells receive, interpret, and then respond to changes in organismal status. This article is part of a Special Issue entitled: Biochemistry of Stem Cells.

Size control of the Drosophila hematopoietic niche by bone morphogenetic protein signaling reveals parallels with mammals

The Drosophila melanogaster larval hematopoietic organ, the lymph gland, is a model to study in vivo the function of the hematopoietic niche. A small cluster of cells in the lymph gland, the posterior signaling center (PSC), maintains the balance between hematopoietic progenitors (prohemocytes) and their differentiation into specialized blood cells (hemocytes). Here, we show that Decapentaplegic/bone morphogenetic protein (Dpp/BMP) signaling activity in PSC cells controls niche size. In the absence of BMP signaling, the number of PSC cells increases. Correlatively, no hemocytes differentiate. Controlling PSC size is, thus, essential for normal blood cell homeostasis. Activation of BMP signaling in the PSC requires expression of the Dally-like heparan-sulfate proteoglycan, under the control of the Collier/early B-cell factor (EBF) transcription factor. A Dpp > dpp autoregulatory loop maintains BMP signaling, which limits PSC cell proliferation by repressing the protooncogene dmyc. Dpp antagonizes activity of wingless (Wg)/Wnt signaling, which positively regulates the number of PSC cells via the control of Dmyc expression. Together, our data show that Collier controls hemocyte homeostasis via coordinate regulation of PSC cell number and PSC signaling to prohemocytes. In mouse, EBF2, BMP, and Wnt signaling in osteoblasts is required for the proper number of niche and hematopoietic stem cells. Our findings bring insights to niche size control and draw parallels between Drosophila and mammalian hematopoiesis.

Elucidating the in vivo targets of photorhabdus toxins in real-time using Drosophila embryos

The outcome of any bacterial infection, whether it is clearance of the infecting pathogen, establishment of a persistent infection, or even death of the host, is as dependent on the host as on the pathogen (Finlay and Falkow 1989). To infect a susceptible host bacterial pathogens express virulence factors, which alter host cell physiology and allow the pathogen to establish a nutrient-rich niche for growth and avoid clearance by the host immune response. However survival within the host often results in tissue damage, which to some cases accounts for the disease-specific pathology. For many bacterial pathogens the principal determinants of virulence and elicitors of host tissue damage are soluble exotoxins, which allow bacteria to penetrate into deeper tissue or pass through a host epithelial or endothelial barrier. Therefore, exploring the complex interplay between host tissue and bacterial toxins can help us to understand infectious disease and define the contributions of the host immune system to bacterial virulence. In this chapter, we describe a new model, the Drosophila embryo, for addressing a fundamental issue in bacterial pathogenesis, the elucidation of the in vivo targets of bacterial toxins and the monitoring of the first moments of the infection process in real-time. To develop this model, we used the insect and emerging human pathogen Photorhabdus asymbiotica and more specifically we characterised the initial cross-talk between the secreted cytotoxin Mcf1 and the embryonic hemocytes. Mcf1 is a potent cytotoxin which has been detected in all Photorhabdus strains isolated so far, which can rapidly kill insects upon injection. Despite several in vitro tissue culture studies, the biology of Mcf1 in vivo is not well understood. Furthermore, despite the identification of many Photorhabdus toxins using recombinant expression in E. coli (Waterfield et al. 2008), very few studies address the molecular mechanism of action of these toxins in relation to specific immune responses in vivo in the insect model.

Odd-skipped maintains prohemocyte potency and blocks blood cell development in Drosophila

Studies using Drosophila have contributed significantly to our understanding of regulatory mechanisms that control stem cell fate choice. The Drosophila blood cell progenitor or prohemocyte shares important characteristics with mammalian hematopoietic stem cells, including quiescence, niche dependence, and the capacity to form all three fly blood cell types. This report extends our understanding of prohemocyte fate choice by showing that the zinc-finger protein Odd-skipped promotes multipotency and blocks differentiation. Odd-skipped was expressed in prohemocytes and downregulated in terminally differentiated plasmatocytes. Furthermore, Odd-skipped maintained the prohemocyte population and blocked differentiation of plasmatocytes and lamellocytes but not crystal cells. A previous study showed that Odd-skipped expression is downregulated by Decapentaplegic signaling. This report provides a functional basis for this regulator/target pair by suggesting that Decapentaplegic signaling limits Odd-skipped expression to promote prohemocyte differentiation. Overall, these studies are the basis for a gene regulatory model of prohemocyte cell fate choice.

Lineage tracing of lamellocytes demonstrates Drosophila macrophage plasticity

Leukocyte-like cells called hemocytes have key functions in Drosophila innate immunity. Three hemocyte types occur: plasmatocytes, crystal cells, and lamellocytes. In the absence of qimmune challenge, plasmatocytes are the predominant hemocyte type detected, while crystal cells and lamellocytes are rare. However, upon infestation by parasitic wasps, or in melanotic mutant strains, large numbers of lamellocytes differentiate and encapsulate material recognized as "non-self". Current models speculate that lamellocytes, plasmatocytes and crystal cells are distinct lineages that arise from a common prohemocyte progenitor. We show here that over-expression of the CoREST-interacting transcription factor Chn in plasmatocytes induces lamellocyte differentiation, both in circulation and in lymph glands. Lamellocyte increases are accompanied by the extinction of plasmatocyte markers suggesting that plasmatocytes are transformed into lamellocytes. Consistent with this, timed induction of Chn over-expression induces rapid lamellocyte differentiation within 18 hours. We detect double-positive intermediates between plasmatocytes and lamellocytes, and show that isolated plasmatocytes can be triggered to differentiate into lamellocytes in vitro, either in response to Chn over-expression, or following activation of the JAK/STAT pathway. Finally, we have marked plasmatocytes and show by lineage tracing that these differentiate into lamellocytes in response to the Drosophila parasite model Leptopilina boulardi. Taken together, our data suggest that lamellocytes arise from plasmatocytes and that plasmatocytes may be inherently plastic, possessing the ability to differentiate further into lamellocytes upon appropriate challenge.

Host and pathogen glycosaminoglycan-binding proteins modulate antimicrobial peptide responses in Drosophila melanogaster

During group B streptococcal infection, the alpha C protein (ACP) on the bacterial surface binds to host cell surface heparan sulfate proteoglycans (HSPGs) and facilitates entry of bacteria into human epithelial cells. Previous studies in a Drosophila melanogaster model showed that binding of ACP to the sulfated polysaccharide chains (glycosaminoglycans) of HSPGs promotes host death and is associated with higher bacterial burdens. We hypothesized that ACP-glycosaminoglycan binding might determine infection outcome by altering host responses to infection, such as expression of antimicrobial peptides. As glycosaminoglycans/HSPGs also interact with a number of endogenous secreted signaling molecules in Drosophila, we examined the effects of host and pathogen glycosaminoglycan/HSPG-binding structures in host survival of infection and antimicrobial peptide expression. Strikingly, host survival after infection with wild-type streptococci was enhanced among flies overexpressing the endogenous glycosaminoglycan/HSPG-binding morphogen Decapentaplegic-a transforming growth factor β-like Drosophila homolog of mammalian bone morphogenetic proteins-but not by flies overexpressing a mutant, non-glycosaminoglycan-binding Decapentaplegic, or the other endogenous glycosaminoglycan/HSPG-binding morphogens, Hedgehog and Wingless. While ACP-glycosaminoglycan binding was associated with enhanced transcription of peptidoglycan recognition proteins and antimicrobial peptides, Decapentaplegic overexpression suppressed transcription of these genes during streptococcal infection. Further, the glycosaminoglycan-binding domain of ACP competed with Decapentaplegic for binding to the soluble glycosaminoglycan heparin in an in vitro assay. These data suggest that, in addition to promoting bacterial entry into host cells, ACP competes with Decapentaplegic for binding to glycosaminoglycans/HSPGs during infection and that these bacterial and endogenous glycosaminoglycan-binding structures determine host survival and regulate antimicrobial peptide transcription.

An in vivo RNA interference screen identifies gene networks controlling Drosophila melanogaster blood cell homeostasis

BACKGROUND

In metazoans, the hematopoietic system plays a key role both in normal development and in defense of the organism. In Drosophila, the cellular immune response involves three types of blood cells: plasmatocytes, crystal cells and lamellocytes. This last cell type is barely present in healthy larvae, but its production is strongly induced upon wasp parasitization or in mutant contexts affecting larval blood cell homeostasis. Notably, several zygotic mutations leading to melanotic mass (or "tumor") formation in larvae have been associated to the deregulated differentiation of lamellocytes. To gain further insights into the gene regulatory network and the mechanisms controlling larval blood cell homeostasis, we conducted a tissue-specific loss of function screen using hemocyte-specific Gal4 drivers and UAS-dsRNA transgenic lines.

RESULTS

By targeting around 10% of the Drosophila genes, this in vivo RNA interference screen allowed us to recover 59 melanotic tumor suppressor genes. In line with previous studies, we show that melanotic tumor formation is associated with the precocious differentiation of stem-cell like blood progenitors in the larval hematopoietic organ (the lymph gland) and the spurious differentiation of lamellocytes. We also find that melanotic tumor formation can be elicited by defects either in the fat body, the embryo-derived hemocytes or the lymph gland. In addition, we provide a definitive confirmation that lymph gland is not the only source of lamellocytes as embryo-derived plasmatocytes can differentiate into lamellocytes either upon wasp infection or upon loss of function of the Friend of GATA cofactor U-shaped.

CONCLUSIONS

In this study, we identify 55 genes whose function had not been linked to blood cell development or function before in Drosophila. Moreover our analyses reveal an unanticipated plasticity of embryo-derived plasmatocytes, thereby shedding new light on blood cell lineage relationship, and pinpoint the Friend of GATA transcription cofactor U-shaped as a key regulator of the plasmatocyte to lamellocyte transformation.

Upregulation of the Drosophila Friend of GATA gene U-shaped by JAK/STAT signaling maintains lymph gland prohemocyte potency

Studies using Drosophila melanogaster have contributed significantly to our understanding of the interaction between stem cells and their protective microenvironments or stem cell niches. During lymph gland hematopoiesis, the Drosophila posterior signaling center functions as a stem cell niche to maintain prohemocyte multipotency through Hedgehog and JAK/STAT signaling. In this study, we provide evidence that the Friend of GATA protein U-shaped is an important regulator of lymph gland prohemocyte potency and differentiation. U-shaped expression was determined to be upregulated in third-instar lymph gland prohemocytes and downregulated in a subpopulation of differentiating blood cells. Genetic analyses indicated that U-shaped maintains the prohemocyte population by blocking differentiation. In addition, activated STAT directly regulated ush expression as evidenced by results from loss- and gain-of-function studies and from analyses of the u-shaped hematopoietic cis-regulatory module. Collectively, these findings identify U-shaped as a downstream effector of the posterior signaling center, establishing a novel link between the stem cell niche and the intrinsic regulation of potency and differentiation. Given the functional conservation of Friend of GATA proteins and the role that GATA factors play during cell fate choice, these factors may regulate essential functions of vertebrate hematopoietic stem cells, including processing signals from the stem cell niche.

Characterization of a lamellocyte transcriptional enhancer located within the misshapen gene of Drosophila melanogaster

Drosophila has emerged as an excellent model system in which to study cellular and genetic aspects of hematopoiesis. Under normal developmental conditions and in wild-type genetic backgrounds, Drosophila possesses two types of blood cells, crystal cells and plasmatocytes. Upon infestation by a parasitic wasp or in certain altered genetic backgrounds, a third hemocyte class called the lamellocyte becomes apparent. Herein we describe the characterization of a novel transcriptional regulatory module, a lamellocyte-active enhancer of the misshapen gene. This transcriptional control sequence appears to be inactive in all cell types of the wild-type larva, including crystal cells and plasmatocytes. However, in lamellocytes induced by wasp infestation or by particular genetic conditions, the enhancer is activated and it directs reporter GFP or DsRed expression exclusively in lamellocytes. The lamellocyte control region was delimited to a 140-bp intronic sequence that contains an essential DNA recognition element for the AP-1 transcription factor. Additionally, mutation of the kayak gene encoding the dFos subunit of AP-1 led to a strong suppression of lamellocyte production in tumorous larvae. As misshapen encodes a protein kinase within the Jun N-terminal kinase signaling pathway that functions to form an active AP-1 complex, the lamellocyte-active enhancer likely serves as a transcriptional target within a genetic auto-regulatory circuit that promotes the production of lamellocytes in immune-challenged or genetically- compromised animals.

Drosophila embryos as model systems for monitoring bacterial infection in real time

Drosophila embryos are well studied developmental microcosms that have been used extensively as models for early development and more recently wound repair. Here we extend this work by looking at embryos as model systems for following bacterial infection in real time. We examine the behaviour of injected pathogenic (Photorhabdus asymbiotica) and non-pathogenic (Escherichia coli) bacteria and their interaction with embryonic hemocytes using time-lapse confocal microscopy. We find that embryonic hemocytes both recognise and phagocytose injected wild type, non-pathogenic E. coli in a Dscam independent manner, proving that embryonic hemocytes are phagocytically competent. In contrast, injection of bacterial cells of the insect pathogen Photorhabdus leads to a rapid 'freezing' phenotype of the hemocytes associated with significant rearrangement of the actin cytoskeleton. This freezing phenotype can be phenocopied by either injection of the purified insecticidal toxin Makes Caterpillars Floppy 1 (Mcf1) or by recombinant E. coli expressing the mcf1 gene. Mcf1 mediated hemocyte freezing is shibire dependent, suggesting that endocytosis is required for Mcf1 toxicity and can be modulated by dominant negative or constitutively active Rac expression, suggesting early and unexpected effects of Mcf1 on the actin cytoskeleton. Together these data show how Drosophila embryos can be used to track bacterial infection in real time and how mutant analysis can be used to genetically dissect the effects of specific bacterial virulence factors.