Drosophila as a model for the two myeloid blood cell systems in vertebrates

Glucocerebrosidase deficiency leads to neuropathology via cellular immune activation

Mutations in GBA (glucosylceramidase beta), which encodes the lysosomal enzyme glucocerebrosidase (GCase), are the strongest genetic risk factor for the neurodegenerative disorders Parkinson's disease (PD) and Lewy body dementia. Recent work has suggested that neuroinflammation may be an important factor in the risk conferred by GBA mutations. We therefore systematically tested the contributions of immune-related genes to neuropathology in a Drosophila model of GCase deficiency. We identified target immune factors via RNA-Seq and proteomics on heads from GCase-deficient flies, which revealed both increased abundance of humoral factors and increased macrophage activation. We then manipulated the identified immune factors and measured their effect on head protein aggregates, a hallmark of neurodegenerative disease. Genetic ablation of humoral (secreted) immune factors did not suppress the development of protein aggregation. By contrast, re-expressing Gba1b in activated macrophages suppressed head protein aggregation in Gba1b mutants and rescued their lifespan and behavioral deficits. Moreover, reducing the GCase substrate glucosylceramide in activated macrophages also ameliorated Gba1b mutant phenotypes. Taken together, our findings show that glucosylceramide accumulation due to GCase deficiency leads to macrophage activation, which in turn promotes the development of neuropathology.

The NF-κB Factor Relish maintains blood progenitor homeostasis in the developing Drosophila lymph gland

Post-larval hematopoiesis in Drosophila largely depends upon the stockpile of progenitors present in the blood-forming organ/lymph gland of the larvae. During larval stages, the lymph gland progenitors gradually accumulate reactive oxygen species (ROS), which is essential to prime them for differentiation. Studies have shown that ROS triggers the activation of JNK (c-Jun Kinase), which upregulates fatty acid oxidation (FAO) to facilitate progenitor differentiation. Intriguingly, despite having ROS, the entire progenitor pool does not differentiate simultaneously in the late larval stages. Using expression analyses, genetic manipulation and pharmacological approaches, we found that the Drosophila NF-κB transcription factor Relish (Rel) shields the progenitor pool from the metabolic pathway that inducts them into the differentiation program by curtailing the activation of JNK. Although ROS serves as the metabolic signal for progenitor differentiation, the input from ROS is monitored by the developmental signal TAK1, which is regulated by Relish. This developmental circuit ensures that the stockpile of ROS-primed progenitors is not exhausted entirely. Our study sheds light on how, during development, integrating NF-κB-like factors with metabolic pathways seem crucial to regulating cell fate transition during development.

Drosophila immune cells transport oxygen through PPO2 protein phase transition

Insect respiration has long been thought to be solely dependent on an elaborate tracheal system without assistance from the circulatory system or immune cells1,2. Here we describe that Drosophila crystal cells-myeloid-like immune cells called haemocytes-control respiration by oxygenating Prophenoloxidase 2 (PPO2) proteins. Crystal cells direct the movement of haemocytes between the trachea of the larval body wall and the circulation to collect oxygen. Aided by copper and a neutral pH, oxygen is trapped in the crystalline structures of PPO2 in crystal cells. Conversely, PPO2 crystals can be dissolved when carbonic anhydrase lowers the intracellular pH and then reassembled into crystals in cellulo by adhering to the trachea. Physiologically, larvae lacking crystal cells or PPO2, or those expressing a copper-binding mutant of PPO2, display hypoxic responses under normoxic conditions and are susceptible to hypoxia. These hypoxic phenotypes can be rescued by hyperoxia, expression of arthropod haemocyanin or prevention of larval burrowing activity to expose their respiratory organs. Thus, we propose that insect immune cells collaborate with the tracheal system to reserve and transport oxygen through the phase transition of PPO2 crystals, facilitating internal oxygen homeostasis in a process that is comparable to vertebrate respiration.

S-nitrosylation-triggered unfolded protein response maintains hematopoietic progenitors in Drosophila

The Drosophila lymph gland houses blood progenitors that give rise to myeloid-like blood cells. Initially, blood progenitors proliferate, but later, they become quiescent to maintain multipotency before differentiation. Despite the identification of various factors involved in multipotency maintenance, the cellular mechanism controlling blood progenitor quiescence remains elusive. Here, we identify the expression of nitric oxide synthase in blood progenitors, generating nitric oxide for post-translational S-nitrosylation of protein cysteine residues. S-nitrosylation activates the Ire1-Xbp1-mediated unfolded protein response, leading to G2 cell-cycle arrest. Specifically, we identify the epidermal growth factor receptor as a target of S-nitrosylation, resulting in its retention within the endoplasmic reticulum and blockade of its receptor function. Overall, our findings highlight developmentally programmed S-nitrosylation as a critical mechanism that induces protein quality control in blood progenitors, maintaining their undifferentiated state by inhibiting cell-cycle progression and rendering them unresponsive to paracrine factors.

Numerous Serine/Threonine Kinases Affect Blood Cell Homeostasis in Drosophila melanogaster

Blood cells in Drosophila serve primarily innate immune responses. Various stressors influence blood cell homeostasis regarding both numbers and the proportion of blood cell types. The principle molecular mechanisms governing hematopoiesis are conserved amongst species and involve major signaling pathways like Notch, Toll, JNK, JAK/Stat or RTK. Albeit signaling pathways generally rely on the activity of protein kinases, their specific contribution to hematopoiesis remains understudied. Here, we assess the role of Serine/Threonine kinases with the potential to phosphorylate the transcription factor Su(H) in crystal cell homeostasis. Su(H) is central to Notch signal transduction, and its inhibition by phosphorylation impedes crystal cell formation. Overall, nearly twenty percent of all Drosophila Serine/Threonine kinases were studied in two assays, global and hemocyte-specific overexpression and downregulation, respectively. Unexpectedly, the majority of kinases influenced crystal cell numbers, albeit only a few were related to hematopoiesis so far. Four kinases appeared essential for crystal cell formation, whereas most kinases restrained crystal cell development. This group comprises all kinase classes, indicative of the complex regulatory network underlying blood cell homeostasis. The rather indiscriminative response we observed opens the possibility that blood cells measure their overall phospho-status as a proxy for stress-signals, and activate an adaptive immune response accordingly.

DNA damage signaling in Drosophila macrophages modulates systemic cytokine levels in response to oxidative stress

Environmental factors, infection, or injury can cause oxidative stress in diverse tissues and loss of tissue homeostasis. Effective stress response cascades, conserved from invertebrates to mammals, ensure reestablishment of homeostasis and tissue repair. Hemocytes, the Drosophila blood-like cells, rapidly respond to oxidative stress by immune activation. However, the precise signals how they sense oxidative stress and integrate these signals to modulate and balance the response to oxidative stress in the adult fly are ill-defined. Furthermore, hemocyte diversification was not explored yet on oxidative stress. Here, we employed high-throughput single nuclei RNA-sequencing to explore hemocytes and other cell types, such as fat body, during oxidative stress in the adult fly. We identified distinct cellular responder states in plasmatocytes, the Drosophila macrophages, associated with immune response and metabolic activation upon oxidative stress. We further define oxidative stress-induced DNA damage signaling as a key sensor and a rate-limiting step in immune-activated plasmatocytes controlling JNK-mediated release of the pro-inflammatory cytokine unpaired-3. We subsequently tested the role of this specific immune activated cell stage during oxidative stress and found that inhibition of DNA damage signaling in plasmatocytes, as well as JNK or upd3 overactivation, result in a higher susceptibility to oxidative stress. Our findings uncover that a balanced composition and response of hemocyte subclusters is essential for the survival of adult Drosophila on oxidative stress by regulating systemic cytokine levels and cross-talk to other organs, such as the fat body, to control energy mobilization.

Hemocyte Nuclei Isolation from Adult Drosophila melanogaster for snRNA-seq

In adult Drosophila, most of the hemocytes are macrophage-like cells (so called plasmatocytes), which serve various functions in organ homeostasis and immune defense. Ontogeny and functions are largely conserved between vertebrate and invertebrate macrophages. Hence, Drosophila offers a powerful genetic toolbox to study macrophage function and genetically modulate these cells. Technological advances in high-throughput sequencing approaches allowed to give an in-depth characterization of vertebrate macrophage populations and their heterogenous composition within different organs as well as changes in disease. Embryonic and larval hemocytes in Drosophila have been recently analyzed in single-cell RNA-sequencing (scRNA-seq) approaches during infection and steady state. These analyses revealed anatomical and functional Drosophila hemocyte subtypes dedicated to specific tasks. Only recently, the Fly Cell Atlas provided a whole transcriptomic single-cell atlas via single-nuclei RNA-sequencing (snRNA-seq) of adult Drosophila including many different tissues and cell types where hemocytes were also included. Yet, a specific protocol to isolate nuclei from adult hemocytes for snRNA-seq and study these cells in different experimental conditions was not available. In this chapter, we give a detailed protocol to purify hemocyte nuclei from adult Drosophila, which can be used in subsequent analyses such as snRNA-seq.

Glucocerebrosidase deficiency leads to neuropathology via cellular immune activation

Mutations in GBA (glucosylceramidase beta), which encodes the lysosomal enzyme glucocerebrosidase (GCase), are the strongest genetic risk factor for the neurodegenerative disorders Parkinson's disease (PD) and Lewy body dementia. Recent work has suggested that neuroinflammation may be an important factor in the risk conferred by GBA mutations. We therefore systematically tested the contributions of immune-related genes to neuropathology in a Drosophila model of GCase deficiency. We identified target immune factors via RNA-Seq and proteomics on heads from GCase-deficient flies, which revealed both increased abundance of humoral factors and increased macrophage activation. We then manipulated the identified immune factors and measured their effect on head protein aggregates, a hallmark of neurodegenerative disease. Genetic ablation of humoral (secreted) immune factors did not suppress the development of protein aggregation. By contrast, re-expressing Gba1b in activated macrophages suppressed head protein aggregation in Gba1b mutants and rescued their lifespan and behavioral deficits. Moreover, reducing the GCase substrate glucosylceramide in activated macrophages also ameliorated Gba1b mutant phenotypes. Taken together, our findings show that glucosylceramide accumulation due to GCase deficiency leads to macrophage activation, which in turn promotes the development of neuropathology.

Conserved mechanisms of NuRD function in hematopoetic gene expression

The Nucleosome Remodeling and Deacetylating Complex (NuRD) is ubiquitously expressed in all metazoans. It combines nucleosome remodeling and histone deacetylating activities to generate inaccessible chromatin structures and to repress gene transcription. NuRD is involved in the generation and maintenance of a wide variety of lineage-specific gene expression programs during differentiation and in differentiated cells. A close cooperation with a large number of lineage-specific transcription factors is key to allow NuRD to function in many distinct differentiation contexts. The molecular nature of this interplay between transcription factors and NuRD is complex and not well understood. This review uses hematopoiesis as a paradigm to highlight recent advances in our understanding of how transcription factors and NuRD cooperate at the molecular level during differentiation. A comparison of vertebrate and invertebrate systems serves to identify the conserved and fundamental concepts guiding functional interactions between transcription factors and NuRD. We also discuss how the transcription factor-NuRD axis constitutes a potential therapeutic target for the treatment of hemoglobinopathies.

Metabolic strategy of macrophages under homeostasis or immune stress in Drosophila

Macrophages are well known for their phagocytic functions in innate immunity across species. In mammals, they rapidly consume a large amount of energy by shifting their metabolism from mitochondrial oxidative phosphorylation toward aerobic glycolysis, to perform the effective bactericidal function upon infection. Meanwhile, they strive for sufficient energy resources by restricting systemic metabolism. In contrast, under nutrient deprivation, the macrophage population is down-regulated to save energy for survival. Drosophila melanogaster possesses a highly conserved and comparatively simple innate immune system. Intriguingly, recent studies have shown that Drosophila plasmatocytes, the macrophage-like blood cells, adopt comparable metabolic remodeling and signaling pathways to achieve energy reassignment when challenged by pathogens, indicating the conservation of such metabolic strategies between insects and mammals. Here, focusing on Drosophila macrophages (plasmatocytes), we review recent advances regarding their comprehensive roles in local or systemic metabolism under homeostasis or stress, emphasizing macrophages as critical players in the crosstalk between the immune system and organic metabolism from a Drosophila perspective.

Drosophila as a Model to Study Cellular Communication Between the Hematopoietic Niche and Blood Progenitors Under Homeostatic Conditions and in Response to an Immune Stress

In adult mammals, blood cells are formed from hematopoietic stem progenitor cells, which are controlled by a complex cellular microenvironment called "niche". Drosophila melanogaster is a powerful model organism to decipher the mechanisms controlling hematopoiesis, due both to its limited number of blood cell lineages and to the conservation of genes and signaling pathways throughout bilaterian evolution. Insect blood cells or hemocytes are similar to the mammalian myeloid lineage that ensures innate immunity functions. Like in vertebrates, two waves of hematopoiesis occur in Drosophila. The first wave takes place during embryogenesis. The second wave occurs at larval stages, where two distinct hematopoietic sites are identified: subcuticular hematopoietic pockets and a specialized hematopoietic organ called the lymph gland. In both sites, hematopoiesis is regulated by distinct niches. In hematopoietic pockets, sensory neurons of the peripheral nervous system provide a microenvironment that promotes embryonic hemocyte expansion and differentiation. In the lymph gland blood cells are produced from hematopoietic progenitors. A small cluster of cells called Posterior Signaling Centre (PSC) and the vascular system, along which the lymph gland develops, act collectively as a niche, under homeostatic conditions, to control the balance between maintenance and differentiation of lymph gland progenitors. In response to an immune stress such as wasp parasitism, lymph gland hematopoiesis is drastically modified and shifts towards emergency hematopoiesis, leading to increased progenitor proliferation and their differentiation into lamellocyte, a specific blood cell type which will neutralize the parasite. The PSC is essential to control this emergency response. In this review, we summarize Drosophila cellular and molecular mechanisms involved in the communication between the niche and hematopoietic progenitors, both under homeostatic and stress conditions. Finally, we discuss similarities between mechanisms by which niches regulate hematopoietic stem/progenitor cells in Drosophila and mammals.

Relish plays a dynamic role in the niche to modulate Drosophila blood progenitor homeostasis in development and infection

Immune challenges demand the gearing up of basal hematopoiesis to combat infection. Little is known about how during development, this switch is achieved to take care of the insult. Here, we show that the hematopoietic niche of the larval lymph gland of Drosophila senses immune challenge and reacts to it quickly through the nuclear factor-κB (NF-κB), Relish, a component of the immune deficiency (Imd) pathway. During development, Relish is triggered by ecdysone signaling in the hematopoietic niche to maintain the blood progenitors. Loss of Relish causes an alteration in the cytoskeletal architecture of the niche cells in a Jun Kinase-dependent manner, resulting in the trapping of Hh implicated in progenitor maintenance. Notably, during infection, downregulation of Relish in the niche tilts the maintenance program toward precocious differentiation, thereby bolstering the cellular arm of the immune response.

Proteomics of purified lamellocytes from Drosophila melanogaster HopTum-l identifies new membrane proteins and networks involved in their functions

In healthy Drosophila melanogaster larvae, plasmatocytes and crystal cells account for 95% and 5% of the hemocytes, respectively. A third type of hemocytes, lamellocytes, are rare, but their number increases after oviposition by parasitoid wasps. The lamellocytes form successive layers around the parasitoid egg, leading to its encapsulation and melanization, and finally the death of this intruder. However, the total number of lamellocytes per larva remains quite low even after parasitoid infestation, making direct biochemical studies difficult. Here, we used the Hop^Tum-l mutant strain that constitutively produces large numbers of lamellocytes to set up a purification method and analyzed their major proteins by 2D gel electrophoresis and their plasma membrane surface proteins by 1D SDS-PAGE after affinity purification. Mass spectrometry identified 430 proteins from 2D spots and 344 affinity-purified proteins from 1D bands, for a total of 639 unique proteins. Known lamellocyte markers such as PPO3 and the myospheroid integrin were among the components identified with specific chaperone proteins. Affinity purification detected other integrins, as well as a wide range of integrin-associated proteins involved in the formation and function of cell-cell junctions. Overall, the newly identified proteins indicate that these cells are highly adapted to the encapsulation process (recognition, motility, adhesion, signaling), but may also have several other physiological functions (such as secretion and internalization of vesicles) under different signaling pathways. These results provide the basis for further in vivo and in vitro studies of lamellocytes, including the development of new markers to identify coexisting populations and their respective origins and functions in Drosophila immunity.

Macrophages and Their Organ Locations Shape Each Other in Development and Homeostasis - A Drosophila Perspective

Across the animal kingdom, macrophages are known for their functions in innate immunity, but they also play key roles in development and homeostasis. Recent insights from single cell profiling and other approaches in the invertebrate model organism Drosophila melanogaster reveal substantial diversity among Drosophila macrophages (plasmatocytes). Together with vertebrate studies that show genuine expression signatures of macrophages based on their organ microenvironments, it is expected that Drosophila macrophage functional diversity is shaped by their anatomical locations and systemic conditions. In vivo evidence for diverse macrophage functions has already been well established by Drosophila genetics: Drosophila macrophages play key roles in various aspects of development and organogenesis, including embryogenesis and development of the nervous, digestive, and reproductive systems. Macrophages further maintain homeostasis in various organ systems and promote regeneration following organ damage and injury. The interdependence and interplay of tissues and their local macrophage populations in Drosophila have implications for understanding principles of organ development and homeostasis in a wide range of species.

Toward a Consensus in the Repertoire of Hemocytes Identified in Drosophila

The catalog of the Drosophila immune cells was until recently limited to three major cell types, based on morphology, function and few molecular markers. Three recent single cell studies highlight the presence of several subgroups, revealing a large diversity in the molecular signature of the larval immune cells. Since these studies rely on somewhat different experimental and analytical approaches, we here compare the datasets and identify eight common, robust subgroups associated to distinct functions such as proliferation, immune response, phagocytosis or secretion. Similar comparative analyses with datasets from different stages and tissues disclose the presence of larval immune cells resembling embryonic hemocyte progenitors and the expression of specific properties in larval immune cells associated with peripheral tissues.

The larval haematopoietic organs of Manduca sexta (Insecta, Lepidoptera): An insight into plasmatocyte development and larval haematopoiesis

Haematopoietic organs (HOs) in Lepidoptera are widely recognised as the source for at least two haemocyte types. With new specific markers for oenocytoids and spherule cells and a method to identify prohaemocytes, the haemocytes formed in and released by the HOs of Manduca sexta are characterised. Differentiation of HO-cells to haemocytes other than plasmatocytes and prohaemocytes neither occurs in the organ itself nor in cells released in vitro by the HOs. Differential labelling patterns evidence the existence of plasmatocyte subpopulations and prohaemocytes, which might represent a gradual differentiation of haemocytes within the organs. Prohaemocytes can be identified by PNA-labelling of the cell membrane. These prohaemocytes are found in circulation and in the HOs and are released by the organs. Circulating prohaemocytes possess characteristics for granular cells, plasmatocytes or oenocytoids while HO derived prohaemocytes share characteristics only with plasmatocytes. Ablation of the HOs diminishes the plasmatocyte and prohaemocyte number, indicating a true larval haematopoietic function.

A functional genomics screen identifying blood cell development genes in Drosophila by undergraduates participating in a course-based research experience

Undergraduate students participating in the UCLA Undergraduate Research Consortium for Functional Genomics (URCFG) have conducted a two-phased screen using RNA interference (RNAi) in combination with fluorescent reporter proteins to identify genes important for hematopoiesis in Drosophila. This screen disrupted the function of approximately 3500 genes and identified 137 candidate genes for which loss of function leads to observable changes in the hematopoietic development. Targeting RNAi to maturing, progenitor, and regulatory cell types identified key subsets that either limit or promote blood cell maturation. Bioinformatic analysis reveals gene enrichment in several previously uncharacterized areas, including RNA processing and export and vesicular trafficking. Lastly, the participation of students in this course-based undergraduate research experience (CURE) correlated with increased learning gains across several areas, as well as increased STEM retention, indicating that authentic, student-driven research in the form of a CURE represents an impactful and enriching pedagogical approach.

A novel site of haematopoiesis and appearance and dispersal of distinct haemocyte types in the Manduca sexta embryo (Insecta, Lepidoptera)

With a set of haemocyte specific markers novel findings on haematopoiesis in the Manduca sexta embryo are presented. We identify a hitherto unknown paired haematopoietic cluster, the abdominal haemocyte cluster in abdominal segment 7 (A7-HCC). These clusters are localised at distinct positions and are established at around katatrepsis. Later in embryogenesis, the A7-HCCs disintegrate, thereby releasing numerous embryonic plasmatocytes which disperse both anteriorly and posteriorly. These cells follow stereotypic migration routes projecting anteriorly. The thoracic larval haematopoietic organs are established at around midembryogenesis. We identify embryonic oenocytoids in the M. sexta embryo for the first time. They appear in the head region roughly at the same time as the A7-HCCs occur and successively disperse in the body cavity during development. Localisation of the prophenoloxidase (proPO) mRNA and of the proPO protein are identical. Morphological, cytometric and antigenic traits show three independently generated haemocyte types during embryogenesis.

Single-cell transcriptome maps of myeloid blood cell lineages in Drosophila

The Drosophila lymph gland, the larval hematopoietic organ comprised of prohemocytes and mature hemocytes, has been a valuable model for understanding mechanisms underlying hematopoiesis and immunity. Three types of mature hemocytes have been characterized in the lymph gland: plasmatocytes, lamellocytes, and crystal cells, which are analogous to vertebrate myeloid cells, yet molecular underpinnings of the lymph gland hemocytes have been less investigated. Here, we use single-cell RNA sequencing to comprehensively analyze heterogeneity of developing hemocytes in the lymph gland, and discover previously undescribed hemocyte types including adipohemocytes, stem-like prohemocytes, and intermediate prohemocytes. Additionally, we identify the developmental trajectory of hemocytes during normal development as well as the emergence of the lamellocyte lineage following active cellular immunity caused by wasp infestation. Finally, we establish similarities and differences between embryonically derived- and larval lymph gland hemocytes. Altogether, our study provides detailed insights into the hemocyte development and cellular immune responses at single-cell resolution.

How the Drosophila lymph gland hemocytes develop and are regulated at a single-cell level is unclear. Here, the authors use single-cell RNA sequencing to show heterogeneity of developing hemocytes in the lymph gland and how they react to wasp infestation, and compare hemocytes from two independent origins.

Basement membrane damage by ROS- and JNK-mediated Mmp2 activation drives macrophage recruitment to overgrown tissue

Macrophages are a major immune cell type infiltrating tumors and promoting tumor growth and metastasis. To elucidate the mechanism of macrophage recruitment, we utilize an overgrowth tumor model ("undead" model) in larval Drosophila imaginal discs that are attached by numerous macrophages. Here we report that changes to the microenvironment of the overgrown tissue are important for recruiting macrophages. First, we describe a correlation between generation of reactive oxygen species (ROS) and damage of the basement membrane (BM) in all neoplastic, but not hyperplastic, models examined. ROS and the stress kinase JNK mediate the accumulation of matrix metalloproteinase 2 (Mmp2), damaging the BM, which recruits macrophages to the tissue. We propose a model where macrophage recruitment to and activation at overgrowing tissue is a multi-step process requiring ROS- and JNK-mediated Mmp2 upregulation and BM damage. These findings have implications for understanding the role of the tumor microenvironment for macrophage activation.

Fatty acid β-oxidation is required for the differentiation of larval hematopoietic progenitors in Drosophila

Cell-intrinsic and extrinsic signals regulate the state and fate of stem and progenitor cells. Recent advances in metabolomics illustrate that various metabolic pathways are also important in regulating stem cell fate. However, our understanding of the metabolic control of the state and fate of progenitor cells is in its infancy. Using Drosophila hematopoietic organ: lymph gland, we demonstrate that Fatty Acid Oxidation (FAO) is essential for the differentiation of blood cell progenitors. In the absence of FAO, the progenitors are unable to differentiate and exhibit altered histone acetylation. Interestingly, acetate supplementation rescues both histone acetylation and the differentiation defects. We further show that the CPT1/whd (withered), the rate-limiting enzyme of FAO, is transcriptionally regulated by Jun-Kinase (JNK), which has been previously implicated in progenitor differentiation. Our study thus reveals how the cellular signaling machinery integrates with the metabolic cue to facilitate the differentiation program.

Stem cells are special precursor cells, found in all animals from flies to humans, that can give rise to all the mature cell types in the body. Their job is to generate supplies of new cells wherever these are needed. This is important because it allows damaged or worn-out tissues to be repaired and replaced by fresh, healthy cells.

As part of this renewal process, stem cells generate pools of more specialized cells, called progenitor cells. These can be thought of as half-way to maturation and can only develop in a more restricted number of ways. For example, so-called myeloid progenitor cells from humans can only develop into a specific group of blood cell types, collectively termed the myeloid lineage.

Fruit flies, like many other animals, also have several different types of blood cells. The fly’s repertoire of blood cells is very similar to the human myeloid lineage, and these cells also develop from the fly equivalent of myeloid progenitor cells. These progenitors are found in a specialized organ in fruit fly larvae called the lymph gland, where the blood forms. These similarities between fruit flies and humans mean that flies are a good model to study how myeloid progenitor cells mature.

A lot is already known about the molecules that signal to progenitor cells how and when to mature. However, the role of metabolism – the chemical reactions that process nutrients and provide energy inside cells – is still poorly understood. Tiwari et al. set out to identify which metabolic reactions myeloid progenitor cells require and how these reactions might shape the progenitors’ development into mature blood cells.

The experiments in this study used fruit fly larvae that had been genetically altered so that they could no longer perform key chemical reactions needed for the breakdown of fats. In these mutant larvae, the progenitors within the lymph gland could not give rise to mature blood cells. This showed that myeloid progenitor cells need to be able to break down fats in order to develop properly.

These results highlight a previously unappreciated role for metabolism in controlling the development of progenitor cells. If this effect also occurs in humans, this knowledge could one day help medical researchers engineer replacement tissues in the lab, or even increase our own bodies’ ability to regenerate blood, and potentially other organs.

Adult Drosophila Lack Hematopoiesis but Rely on a Blood Cell Reservoir at the Respiratory Epithelia to Relay Infection Signals to Surrounding Tissues

The use of adult Drosophila melanogaster as a model for hematopoiesis or organismal immunity has been debated. Addressing this question, we identify an extensive reservoir of blood cells (hemocytes) at the respiratory epithelia (tracheal air sacs) of the thorax and head. Lineage tracing and functional analyses demonstrate that the majority of adult hemocytes are phagocytic macrophages (plasmatocytes) from the embryonic lineage that parallels vertebrate tissue macrophages. Surprisingly, we find no sign of adult hemocyte expansion. Instead, hemocytes play a role in relaying an innate immune response to the blood cell reservoir: through Imd signaling and the Jak/Stat pathway ligand Upd3, hemocytes act as sentinels of bacterial infection, inducing expression of the antimicrobial peptide Drosocin in respiratory epithelia and colocalizing fat body domains. Drosocin expression in turn promotes animal survival after infection. Our work identifies a multi-signal relay of organismal humoral immunity, establishing adult Drosophila as model for inter-organ immunity.

Putative stem cells in the hemolymph and in the intestinal submucosa of the solitary ascidian Styela plicata

Background

In various ascidian species, circulating stem cells have been documented to be involved in asexual reproduction and whole-body regeneration. Studies of these cell population(s) are mainly restricted to colonial species. Here, we investigate the occurrence of circulating stem cells in the solitary Styela plicata, a member of the Styelidae, a family with at least two independent origins of coloniality.

Results

Using flow cytometry, we characterized a population of circulating putative stem cells (CPSCs) in S. plicata and determined two gates likely enriched with CPSCs based on morphology and aldehyde dehydrogenase (ALDH) activity. We found an ALDH + cell population with low granularity, suggesting a stem-like state. In an attempt to uncover putative CPSCs niches in S. plicata, we performed a histological survey for hemoblast-like cells, followed by immunohistochemistry with stem cell and proliferation markers. The intestinal submucosa (IS) showed high cellular proliferation levels and high frequency of undifferentiated cells and histological and ultrastructural analyses revealed the presence of hemoblast aggregations in the IS suggesting a possible niche. Finally, we document the first ontogenetic appearance of distinct metamorphic circulatory mesenchyme cells, which precedes the emergence of juvenile hemocytes.

Conclusions

We find CPSCs in the hemolymph of the solitary ascidian Styela plicata, presumably involved in the regenerative capacity of this species. The presence of proliferating and undifferentiated mesenchymal cells suggests IS as a possible niche.

Chemical depletion of phagocytic immune cells in Anopheles gambiae reveals dual roles of mosquito hemocytes in anti-Plasmodium immunity

Mosquito immunity is composed of both cellular and humoral factors that provide protection from invading pathogens. Immune cells known as hemocytes, have been intricately associated with phagocytosis and innate immune signaling. However, the lack of genetic tools has limited hemocyte study despite their importance in mosquito anti-Plasmodium immunity. To address these limitations, we employ the use of a chemical-based treatment to deplete phagocytic immune cells in Anopheles gambiae, demonstrating the role of phagocytes in complement recognition and prophenoloxidase production that limit the ookinete and oocyst stages of malaria parasite development, respectively. Through these experiments, we also define specific subtypes of phagocytic immune cells in An. gambiae, providing insights beyond the morphological characteristics that traditionally define mosquito hemocyte populations. Together, this study represents a significant advancement in our understanding of the roles of mosquito phagocytes in mosquito vector competence and demonstrates the utility of clodronate liposomes as an important tool in the study of invertebrate immunity.

Pericardin, a Drosophila collagen, facilitates accumulation of hemocytes at the heart

Hematopoietic cell lineages support organismal needs by responding to positional and systemic signals that balance proliferative and differentiation events. Drosophila provides an excellent genetic model to dissect these signals, where the activity of cues in the hemolymph or substrate can be traced to determination and differentiation events of well characterized hemocyte types. Plasmatocytes in third instar larvae increase in number in response to infection and in anticipation of metamorphosis. Here we characterize hemocyte clustering, proliferation and transdifferentiation on the heart or dorsal vessel. Hemocytes accumulate on the inner foldings of the heart basement membrane, where they move with heart contraction, and are in proximity to the heart ostia and pericardial nephrocytes. The numbers of hemocytes vary, but increase transiently before pupariation, and decrease by 4 h before pupa formation. During their accumulation at the heart, plasmatocytes can proliferate and can transdifferentiate into crystal cells. Serrate expressing cells as well as lamellocyte-like, Atilla expressing ensheathing cells are associated with some, but not all hemocyte clusters. Hemocyte aggregation is enhanced by the presence of a heart specific Collagen, Pericardin, but not the associated pericardial cells. The varied and transient number of hemocytes in the pericardial compartment suggests that this is not a hematopoietic hub, but a niche supporting differentiation and rapid dispersal in response to systemic signals.

Lime is a new protein linking immunity and metabolism in Drosophila

The proliferation, differentiation and function of immune cells in vertebrates, as well as in the invertebrates, is regulated by distinct signalling pathways and crosstalk with systemic and cellular metabolism. We have identified the Lime gene (Linking Immunity and Metabolism, CG18446) as one such connecting factor, linking hemocyte development with systemic metabolism in Drosophila. Lime is expressed in larval plasmatocytes and the fat body and regulates immune cell type and number by influencing the size of hemocyte progenitor populations in the lymph gland and in circulation. Lime mutant larvae exhibit low levels of glycogen and trehalose energy reserves and they develop low number of hemocytes. The low number of hemocytes in Lime mutants can be rescued by Lime overexpression in the fat body. It is well known that immune cell metabolism is tightly regulated with the progress of infection and it must be supported by systemic metabolic changes. Here we demonstrate that Lime mutants fails to induce such systemic metabolic changes essential for the larval immune response. Indeed, Lime mutants are not able to sustain high numbers of circulating hemocytes and are compromised in the number of lamellocytes produced during immune system challenge, using a parasitic wasp infection model. We therefore propose the Lime gene as a novel functional link between systemic metabolism and Drosophila immunity.

Inflammatory production of reactive oxygen species by Drosophila hemocytes activates cellular immune defenses

The production of reactive oxygen species (ROS) is a prominent response to infection among innate immune cells such as macrophages and neutrophils. To better understand the relationship between antimicrobial and regulatory functions of blood cell ROS, we have characterized the ROS response to infection in Drosophila hemocytes. Using fluorescent probes, we find a biphasic hemocyte ROS response to bacterial infection. In the first hour, virtually all hemocytes generate a transient ROS signal, with nonphagocytic cells including prohemocytes and crystal cells displaying exceptionally strong responses. A distinct, and more delayed ROS response starting at 90 min is primarily within cells that have engulfed bacteria, and is sustained for several hours. The early response has a clear regulatory function, as dampening or intensifying the intracellular ROS level has profound effects on plasmatocyte activation. In addition, ROS are necessary and sufficient to activate JNK signalling in crystal cells, and to promote JNK-dependent crystal cell rupture. These findings indicate that Drosophila will be a promising model in which to dissect the mechanisms of ROS stimulation of immune activation.

Embryonic hematopoiesis modulates the inflammatory response and larval hematopoiesis in Drosophila

Recent lineage tracing analyses have significantly improved our understanding of immune system development and highlighted the importance of the different hematopoietic waves. The current challenge is to understand whether these waves interact and whether this affects the function of the immune system. Here we report a molecular pathway regulating the immune response and involving the communication between embryonic and larval hematopoietic waves in Drosophila. Down-regulating the transcription factor Gcm specific to embryonic hematopoiesis enhances the larval phenotypes induced by over-expressing the pro-inflammatory Jak/Stat pathway or by wasp infestation. Gcm works by modulating the transduction of the Upd cytokines to the site of larval hematopoiesis and hence the response to chronic (Jak/Stat over-expression) and acute (wasp infestation) immune challenges. Thus, homeostatic interactions control the function of the immune system in physiology and pathology. Our data also indicate that a transiently expressed developmental pathway has a long-lasting effect on the immune response.

Systemic control of immune cell development by integrated carbon dioxide and hypoxia chemosensation in Drosophila

Drosophila hemocytes are akin to mammalian myeloid blood cells that function in stress and innate immune-related responses. A multi-potent progenitor population responds to local signals and to systemic stress by expanding the number of functional blood cells. Here we show mechanisms that demonstrate an integration of environmental carbon dioxide (CO2) and oxygen (O2) inputs that initiate a cascade of signaling events, involving multiple organs, as a stress response when the levels of these two important respiratory gases fall below a threshold. The CO2 and hypoxia-sensing neurons interact at the synaptic level in the brain sending a systemic signal via the fat body to modulate differentiation of a specific class of immune cells. Our findings establish a link between environmental gas sensation and myeloid cell development in Drosophila. A similar relationship exists in humans, but the underlying mechanisms remain to be established.

The human Smoothened inhibitor PF-04449913 induces exit from quiescence and loss of multipotent Drosophila hematopoietic progenitor cells

The efficient treatment of hematological malignancies as Acute Myeloid Leukemia, myelofibrosis and Chronic Myeloid Leukemia, requires the elimination of cancer-initiating cells and the prevention of disease relapse through targeting pathways that stimulate generation and maintenance of these cells. In mammals, inhibition of Smoothened, the key mediator of the Hedgehog signaling pathway, reduces Chronic Myeloid Leukemia progression and propagation. These findings make Smo a candidate target to inhibit maintenance of leukemia-initiating cells. In Drosophila melanogaster the same pathway maintains the hematopoietic precursor cells of the lymph gland, the hematopoietic organ that develops in the larva. Using Drosophila as an in vivo model, we investigated the mode of action of PF-04449913, a small-molecule inhibitor of the human Smo protein. Drosophila larvae fed with PF-04449913 showed traits of altered hematopoietic homeostasis. These include the development of melanotic nodules, increase of circulating hemocytes, the size increase of the lymph gland and accelerated differentiation of blood cells likely due to the exit of multi-potent precursors from quiescence. Importantly, the Smo inhibition can lead to the complete loss of hematopoietic precursors. We conclude that PF-04449913 inhibits Drosophila Smo blocking the Hh signaling pathway and causing the loss of hematopoietic precursor cells. Interestingly, this is the effect expected in patients treated with PF-04449913: number decrease of cancer initiating cells in the bone marrow to reduce the risk of leukemia relapse. Altogether our results indicate that Drosophila comprises a model system for the in vivo study of molecules that target evolutionary conserved pathways implicated in human hematological malignancies.

Regulation of Drosophila hematopoietic sites by Activin-β from active sensory neurons

An outstanding question in animal development, tissue homeostasis and disease is how cell populations adapt to sensory inputs. During Drosophila larval development, hematopoietic sites are in direct contact with sensory neuron clusters of the peripheral nervous system (PNS), and blood cells (hemocytes) require the PNS for their survival and recruitment to these microenvironments, known as Hematopoietic Pockets. Here we report that Activin-β, a TGF-β family ligand, is expressed by sensory neurons of the PNS and regulates the proliferation and adhesion of hemocytes. These hemocyte responses depend on PNS activity, as shown by agonist treatment and transient silencing of sensory neurons. Activin-β has a key role in this regulation, which is apparent from reporter expression and mutant analyses. This mechanism of local sensory neurons controlling blood cell adaptation invites evolutionary parallels with vertebrate hematopoietic progenitors and the independent myeloid system of tissue macrophages, whose regulation by local microenvironments remain undefined.

The Immune Phenotype of Three Drosophila Leukemia Models

Many leukemia patients suffer from dysregulation of their immune system, making them more susceptible to infections and leading to general weakening (cachexia). Both adaptive and innate immunity are affected. The fruit fly Drosophila melanogaster has an innate immune system, including cells of the myeloid lineage (hemocytes). To study Drosophila immunity and physiology during leukemia, we established three models by driving expression of a dominant-active version of the Ras oncogene (Ras^V12 ) alone or combined with knockdowns of tumor suppressors in Drosophila hemocytes. Our results show that phagocytosis, hemocyte migration to wound sites, wound sealing, and survival upon bacterial infection of leukemic lines are similar to wild type. We find that in all leukemic models the two major immune pathways (Toll and Imd) are dysregulated. Toll-dependent signaling is activated to comparable extents as after wounding wild-type larvae, leading to a proinflammatory status. In contrast, Imd signaling is suppressed. Finally, we notice that adult tissue formation is blocked and degradation of cell masses during metamorphosis of leukemic lines, which is akin to the state of cancer-dependent cachexia. To further analyze the immune competence of leukemic lines, we used a natural infection model that involves insect-pathogenic nematodes. We identified two leukemic lines that were sensitive to nematode infections. Further characterization demonstrates that despite the absence of behavioral abnormalities at the larval stage, leukemic larvae show reduced locomotion in the presence of nematodes. Taken together, this work establishes new Drosophila models to study the physiological, immunological, and behavioral consequences of various forms of leukemia.

Macrophage Functions in Tissue Patterning and Disease: New Insights from the Fly

Macrophages are multifunctional innate immune cells that seed all tissues within the body and play disparate roles throughout development and in adult tissues, both in health and disease. Their complex developmental origins and many of their functions are being deciphered in mammalian tissues, but opportunities for live imaging and the genetic tractability of Drosophila are offering complementary insights into how these fascinating cells integrate a multitude of guidance cues to fulfill their many tasks and migrate to distant sites to either direct developmental patterning or raise an inflammatory response.

Advances in Myeloid-Like Cell Origins and Functions in the Model Organism Drosophila melanogaster

Drosophila has long served as a valuable model for deciphering many biological processes, including immune responses. Indeed, the genetic tractability of this organism is particularly suited for large-scale analyses. Studies performed during the last 3 decades have proven that the signaling pathways that regulate the innate immune response are conserved between Drosophila and mammals. This review summarizes the recent advances on Drosophila hematopoiesis and immune cellular responses, with a particular emphasis on phagocytosis.

Monocyte/Macrophage: NK Cell Cooperation-Old Tools for New Functions

Monocyte/macrophage and natural killer (NK) cells are partners from a phylogenetic standpoint of innate immune system development and its evolutionary progressive interaction with adaptive immunity. The equally conservative ways of development and differentiation of both invertebrate hemocytes and vertebrate macrophages are reviewed. Evolutionary conserved molecules occurring in macrophage receptors and effectors have been inherited by vertebrates after their common ancestor with invertebrates. Cytolytic functions of mammalian NK cells, which are rooted in immune cells of invertebrates, although certain NK cell receptors (NKRs) are mammalian new events, are characterized. Broad heterogeneity of macrophage and NK cell phenotypes that depends on surrounding microenvironment conditions and expression profiles of specific receptors and activation mechanisms of both cell types are discussed. The particular tissue specificity of macrophages and NK cells, as well as their plasticity and mechanisms of their polarization to different functional subtypes have been underlined. The chapter summarized studies revealing the specific molecular mechanisms and regulation of NK cells and macrophages that enable their highly specific cross-cooperation. Attention is given to the evolving role of human monocyte/macrophage and NK cell interaction in pathogenesis of hypersensitivity reaction-based disorders, including autoimmunity, as well as in cancer surveillance and progression.

RUNX in Invertebrates

Runx genes have been identified in all metazoans and considerable conservation of function observed across a wide range of phyla. Thus, insight gained from studying simple model organisms is invaluable in understanding RUNX biology in higher animals. Consequently, this chapter will focus on the Runx genes in the diploblasts, which includes sea anemones and sponges, as well as the lower triploblasts, including the sea urchin, nematode, planaria and insect. Due to the high degree of functional redundancy amongst vertebrate Runx genes, simpler model organisms with a solo Runx gene, like C. elegans, are invaluable systems in which to probe the molecular basis of RUNX function within a whole organism. Additionally, comparative analyses of Runx sequence and function allows for the development of novel evolutionary insights. Strikingly, recent data has emerged that reveals the presence of a Runx gene in a protist, demonstrating even more widespread occurrence of Runx genes than was previously thought. This review will summarize recent progress in using invertebrate organisms to investigate RUNX function during development and regeneration, highlighting emerging unifying themes.

Methods to Examine the Lymph Gland and Hemocytes in Drosophila Larvae

Many parallels exist between the Drosophila and mammalian hematopoietic systems, even though Drosophila lack the lymphoid lineage that characterize mammalian adaptive immunity. Drosophila and mammalian hematopoiesis occur in spatially and temporally distinct phases to produce several blood cell lineages. Both systems maintain reservoirs of blood cell progenitors with which to expand or replace mature lineages. The hematopoietic system allows Drosophila and mammals to respond to and to adapt to immune challenges. Importantly, the transcriptional regulators and signaling pathways that control the generation, maintenance, and function of the hematopoietic system are conserved from flies to mammals. These similarities allow Drosophila to be used to genetically model hematopoietic development and disease. Here we detail assays to examine the hematopoietic system of Drosophila larvae. In particular, we outline methods to measure blood cell numbers and concentration, visualize a specific mature lineage in vivo, and perform immunohistochemistry on blood cells in circulation and in the hematopoietic organ. These assays can reveal changes in gene expression and cellular processes including signaling, survival, proliferation, and differentiation and can be used to investigate a variety of questions concerning hematopoiesis. Combined with the genetic tools available in Drosophila, these assays can be used to evaluate the hematopoietic system upon defined genetic alterations. While not specifically outlined here, these assays can also be used to examine the effect of environmental alterations, such as infection or diet, on the hematopoietic system.

Drosophila hematopoiesis under normal conditions and in response to immune stress

The emergence of hematopoietic progenitors and their differentiation into various highly specialized blood cell types constitute a finely tuned process. Unveiling the genetic cascades that control blood cell progenitor fate and understanding how they are modulated in response to environmental changes are two major challenges in the field of hematopoiesis. In the last 20 years, many studies have established important functional analogies between blood cell development in vertebrates and in the fruit fly, Drosophila melanogaster. Thereby, Drosophila has emerged as a powerful genetic model for studying mechanisms that control hematopoiesis during normal development or in pathological situations. Moreover, recent advances in Drosophila have highlighted how intricate cell communication networks and microenvironmental cues regulate blood cell homeostasis. They have also revealed the striking plasticity of Drosophila mature blood cells and the presence of different sites of hematopoiesis in the larva. This review provides an overview of Drosophila hematopoiesis during development and summarizes our current knowledge on the molecular processes controlling larval hematopoiesis, both under normal conditions and in response to an immune challenge, such as wasp parasitism.

Reactive Oxygen Species Affect Transglutaminase Activity and Regulate Hematopoiesis in a Crustacean

Reactive oxygen species (ROS) serve as a prime signal in the commitment to hematopoiesis in both mammals and Drosophila In this study, the potential function of ROS during hematopoiesis in the crayfish Pacifastacus leniusculus was examined. The antioxidant N-acetylcysteine (NAC) was used to decrease ROS in both in vivo and in vitro experiments. An increase in ROS was observed in the anterior proliferation center (APC) after LPS injection. In the absence of NAC, the LPS-induced increase in ROS levels resulted in the rapid restoration of the circulating hemocyte number. In the presence of NAC, a delay in the recovery rate of the hemocyte number was observed. NAC treatment also blocked the spread of APC and other hematopoietic tissue (HPT) cells, maintaining these cells at an undifferentiated stage. Extracellular transglutaminase (TGase) has been shown previously to play a role in maintaining HPT cells in an undifferentiated form. In this study, we show that extracellular TGase activity increased when the ROS level in HPT or APC cells was reduced after NAC treatment. In addition, collagen, a major component of the extracellular matrix and a TGase substrate were co-localized on the HPT cell surface. Taken together, the results of this study show that ROS are involved in crayfish hematopoiesis, in which a low ROS level is required to maintain hematopoietic progenitor cells in the tissue and to reduce hemocyte release. The potential roles of TGase in this process are investigated and discussed.

Crustacean hematopoiesis

Crustacean hemocytes are important mediators of immune reactions, and the regulation of hemocyte homeostasis is of utmost importance for the health of these animals. This review discusses the current knowledge on the lineages, synthesis and differentiation of hemocytes in crustaceans. Hematopoietic tissues, their origins, and the regulation of hematopoiesis during molting, seasonal variation and infection are discussed. Furthermore, studies concerning the molecular regulation of hemocyte formation in crustaceans are also described, and the different lineages and their molecular markers are discussed and compared with several insect species. Signaling pathways and the regulation of hematopoiesis by transcription factors are typically conserved among these arthropods, whereas cytokines and growth factors are more variable and species specific. However, considering the great diversity among the crustaceans, one should be cautious in drawing general conclusions from studies of only a few species.

Cellular immune defenses of Drosophila melanogaster

Drosophila melanogaster is a widely used model for the characterization of blood cell development and function, with an array of protocols for the manipulation and visualization of fixed or live cells in vitro or in vivo. Researchers have deployed these techniques to reveal Drosophila hemocytes as a remarkably versatile cell type that engulfs apoptotic corpses; neutralizes invading parasites; seals epithelial wounds; and deposits extracellular matrix proteins. In this review, we will discuss the key features of Drosophila hemocyte development and function, and identify similarities with vertebrate counterparts.

Macrophages and cellular immunity in Drosophila melanogaster

The invertebrate Drosophila melanogaster has been a powerful model for understanding blood cell development and immunity. Drosophila is a holometabolous insect, which transitions through a series of life stages from embryo, larva and pupa to adulthood. In spite of this, remarkable parallels exist between Drosophila and vertebrate macrophages, both in terms of development and function. More than 90% of Drosophila blood cells (hemocytes) are macrophages (plasmatocytes), making this highly tractable genetic system attractive for studying a variety of questions in macrophage biology. In vertebrates, recent findings revealed that macrophages have two independent origins: self-renewing macrophages, which reside and proliferate in local microenvironments in a variety of tissues, and macrophages of the monocyte lineage, which derive from hematopoietic stem or progenitor cells. Like vertebrates, Drosophila possesses two macrophage lineages with a conserved dual ontogeny. These parallels allow us to take advantage of the Drosophila model when investigating macrophage lineage specification, maintenance and amplification, and the induction of macrophages and their progenitors by local microenvironments and systemic cues. Beyond macrophage development, Drosophila further serves as a paradigm for understanding the mechanisms underlying macrophage function and cellular immunity in infection, tissue homeostasis and cancer, throughout development and adult life.