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

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.

Hematopoietic aging: Cellular, molecular, and related mechanisms

ABSTRACT

Aging is accompanied by significant inhibition of hematopoietic and immune system function and disruption of bone marrow structure. Aging-related alterations in the inflammatory response, immunity, and stem cell niches are at the root of hematopoietic aging. Understanding the molecular mechanisms underlying hematopoietic and bone marrow aging can aid the clinical treatment of aging-related diseases. In particular, it is unknown how the niche reprograms hematopoietic stem cells (HSCs) in an age-dependent manner to maintain normal hematopoiesis in elderly individuals. Recently, specific inhibitors and blood exchange methods have been shown to reshape the hematopoietic niche and reverse hematopoietic aging. Here, we present the latest scientific discoveries related to hematopoietic aging and hematopoietic system rejuvenation, discuss the relationships between hematopoietic niche aging and HSC aging, and describe related studies on stem cell-mediated regulation of hematopoietic aging, aiming to provide new ideas for further study.

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.

Molecular Mechanisms of Drosophila Hematopoiesis

As a model organism, the fruit fly (Drosophila melanogaster) has assumed a leading position in modern biological research. The Drosophila genetic system has a number of advantages making it a key model in investigating the molecular mechanisms of metazoan developmental processes. Over the past two decades, significant progress has been made in understanding the molecular mechanisms regulating Drosophila hematopoiesis. This review discusses the major advances in investigating the molecular mechanisms involved in maintaining the population of multipotent progenitor cells and their differentiation into mature hemocytes in the hematopoietic organ of the Drosophila larva. The use of the Drosophila hematopoietic organ as a model system for hematopoiesis has allowed to characterize the complex interactions between signaling pathways and transcription factors in regulating the maintenance and differentiation of progenitor cells through the signals from the hematopoietic niche, autocrine and paracrine signals, and the signals emanated by differentiated cells.

Towards an integrated understanding of inflammatory pathway influence on hematopoietic stem and progenitor cell differentiation

Recent research highlights that inflammatory signaling pathways such as pattern recognition receptor (PRR) signaling and inflammatory cytokine signaling play an important role in both on-demand hematopoiesis as well as steady-state hematopoiesis. Knockout studies have demonstrated the necessity of several distinct pathways in these processes, but often lack information about the contribution of specific cell types to the phenotypes in question. Transplantation studies have increased the resolution to the level of specific cell types by testing the necessity of inflammatory pathways specifically in donor hematopoietic stem and progenitor cells (HSPCs) or in recipient niche cells. Here, we argue that for an integrated understanding of how these processes occur in vivo and to inform the development of therapies that modulate hematopoietic responses, we need studies that knockout inflammatory signaling receptors in a cell-specific manner and compare the phenotypes caused by knockout in individual niche cells versus HSPCs.

Maintenance of hematopoietic stem cell niche homeostasis requires gap junction-mediated calcium signaling

Regulation of stem cells requires coordination of the cells that make up the stem cell niche. Here, we describe a mechanism that allows communication between niche cells to coordinate their activity and shape the signaling environment surrounding resident stem cells. Using the Drosophila hematopoietic organ, the lymph gland, we show that cells of the hematopoietic niche, the posterior signaling center (PSC), communicate using gap junctions (GJs) and form a signaling network. This network allows PSC cells to exchange Ca2+ signals repetitively which regulate the hematopoietic niche. Disruption of Ca2+ signaling in the PSC or the GJ-mediated network connecting niche cells causes dysregulation of the PSC and blood progenitor differentiation. Analysis of PSC-derived cell signaling shows that the Hedgehog pathway acts downstream of GJ-mediated Ca2+ signaling to modulate the niche microenvironment. These data show that GJ-mediated communication between hematopoietic niche cells maintains their homeostasis and consequently controls blood progenitor behavior.

A mechanosensitive vascular niche for Drosophila hematopoiesis

Hematopoietic stem and progenitor cells maintain blood cell homeostasis by integrating various cues provided by specialized microenvironments or niches. Biomechanical forces are emerging as key regulators of hematopoiesis. Here, we report that mechanical stimuli provided by blood flow in the vascular niche control Drosophila hematopoiesis. In vascular niche cells, the mechanosensitive channel Piezo transduces mechanical forces through intracellular calcium upregulation, leading to Notch activation and repression of FGF ligand transcription, known to regulate hematopoietic progenitor maintenance. Our results provide insight into how the vascular niche integrates mechanical stimuli to regulate hematopoiesis.

ROS-directed activation of Toll/NF-κB in the hematopoietic niche triggers benzene-induced emergency hematopoiesis

Hematopoietic stem cells/progenitor cells (HSC/HPCs) orchestrate the hematopoietic process, effectively regulated by the hematopoietic niche under normal and stressed conditions. The hematopoietic niche provides various soluble factors which influence the differentiation and self-renewal of HSC/HSPs. Unceasing differentiation/proliferation/high metabolic activity of HSC/HPCs makes them susceptible to damage by environmental toxicants like benzene. Oxidative stress, epigenetic modifications, and DNA damage in the HSC/HPCs are the key factors of benzene-induced hematopoietic injury. However, the role of the hematopoietic niche in benzene-induced hematopoietic injury/response is still void. Therefore, the current study aims to unravel the role of the hematopoietic niche in benzene-induced hematotoxicity using a genetically tractable model, Drosophila melanogaster. The lymph gland is a dedicated hematopoietic organ in Drosophila larvae. A group of 30-45 cells called the posterior signaling center (PSC) in the lymph gland acts as a niche that regulates Drosophila HSC/HPCs maintenance. Benzene exposure to Drosophila larvae (48 h) resulted in aberrant hemocyte production, especially hyper-differentiation of lamellocytes followed by premature lymph gland dispersal and reduced adult emergence upon developmental exposure. Subsequent genetic experiments revealed that benzene-induced lamellocyte production and premature lymph gland dispersal were PSC mediated. The genetic experiments further showed that benzene generates Dual oxidase (Duox)-dependent Reactive Oxygen Species (ROS) in the PSC, activating Toll/NF-κB signaling, which is essential for the aberrant hemocyte production, lymph gland dispersal, and larval survival. Together, the study establishes a functional perspective of the hematopoietic niche in a benzene-induced hematopoietic emergency in a genetic model, Drosophila, which might be relevant to higher organisms.

Drosophila hemocytes recognize lymph gland tumors of mxc mutants and activate the innate immune pathway in a reactive oxygen species-dependent manner

Mechanisms of cancer cell recognition and elimination by the innate immune system remains unclear. The immune signaling pathways are activated in the fat body to suppress the tumor growth in mxcmbn1 hematopoietic tumor mutants in Drosophila by inducing antimicrobial peptides (AMP). Here, we investigated the regulatory mechanism underlying the activation in the mutant. Firstly, we found that reactive oxygen species (ROS) accumulated in the hemocytes due to induction of dual oxidase and one of its activators. This was required for the AMP induction and the tumor growth suppression. Next, more hemocytes transplanted from normal larvae were associated with the mutant tumor than normal lymph glands (LGs). Matrix metalloproteinase 1 and 2 (MMP2) were highly expressed in the tumors. The basement membrane components in the tumors were reduced and ultimately lost inside. Depletion of the MMP2 rather than MMP1 resulted in a significantly reduced AMP expression in the mutant larvae. The hemocytes may recognize the disassembly of basement membrane in the tumors and activate the ROS production. Our findings highlight the mechanism via which macrophage-like hemocytes recognize tumor cells and subsequently convey the information to induce AMPs in the fat body. They contribute to uncover the role of innate immune system against cancer.

Hematopoietic plasticity mapped in Drosophila and other insects

Hemocytes, similar to vertebrate blood cells, play important roles in insect development and immunity, but it is not well understood how they perform their tasks. New technology, in particular single-cell transcriptomic analysis in combination with Drosophila genetics, may now change this picture. This review aims to make sense of recently published data, focusing on Drosophila melanogaster and comparing to data from other drosophilids, the malaria mosquito, Anopheles gambiae, and the silkworm, Bombyx mori. Basically, the new data support the presence of a few major classes of hemocytes: (1) a highly heterogenous and plastic class of professional phagocytes with many functions, called plasmatocytes in Drosophila and granular cells in other insects. (2) A conserved class of cells that control melanin deposition around parasites and wounds, called crystal cells in D. melanogaster, and oenocytoids in other insects. (3) A new class of cells, the primocytes, so far only identified in D. melanogaster. They are related to cells of the so-called posterior signaling center of the larval hematopoietic organ, which controls the hematopoiesis of other hemocytes. (4) Different kinds of specialized cells, like the lamellocytes in D. melanogaster, for the encapsulation of parasites. These cells undergo rapid evolution, and the homology relationships between such cells in different insects are uncertain. Lists of genes expressed in the different hemocyte classes now provide a solid ground for further investigation of function.

Characterization of hemocytes and hematopoietic cells of a freshwater crayfish based on single-cell transcriptome analysis

Crustaceans constitute a species-rich and ecologically important animal group, and their circulating blood cells (hemocytes) are of critical importance in immunity as key players in pathogen recognition, phagocytosis, melanization, and antimicrobial defense. To gain a better understanding of the immune responses to different pathogens, it is crucial that we identify different hemocyte subpopulations with different functions and gain a better understanding of how these cells are formed. Here, we performed single-cell RNA sequencing of isolated hematopoietic tissue (HPT) cells and hemocytes from the crayfish Pacifastacus leniusculus to identify hitherto undescribed hemocyte types in the circulation and show that the circulating cells are more diversified than previously recognized. In addition, we discovered cell populations in the HPT with clear precursor characteristics as well as cells involved in iron homeostasis, representing a previously undiscovered cell type. These findings may improve our understanding of hematopoietic stem cell regulation in crustaceans and other animals.

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Single-cell RNA sequencing of hematopoietic cell types reveals new cell types

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One cell type contains iron homeostasis-associated transcripts

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Hemocytes and hematopoietic cells differ in their transcript profiles

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Prophenoloxidase is only expressed in hemocytes

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 Innate Immunity Involves Multiple Signaling Pathways and Coordinated Communication Between Different Tissues

The innate immune response provides the first line of defense against invading pathogens, and immune disorders cause a variety of diseases. The fruit fly Drosophila melanogaster employs multiple innate immune reactions to resist infection. First, epithelial tissues function as physical barriers to prevent pathogen invasion. In addition, macrophage-like plasmatocytes eliminate intruders through phagocytosis, and lamellocytes encapsulate large particles, such as wasp eggs, that cannot be phagocytosed. Regarding humoral immune responses, the fat body, equivalent to the mammalian liver, secretes antimicrobial peptides into hemolymph, killing bacteria and fungi. Drosophila has been shown to be a powerful in vivo model for studying the mechanism of innate immunity and host-pathogen interactions because Drosophila and higher organisms share conserved signaling pathways and factors. Moreover, the ease with which Drosophila genetic and physiological characteristics can be manipulated prevents interference by adaptive immunity. In this review, we discuss the signaling pathways activated in Drosophila innate immunity, namely, the Toll, Imd, JNK, JAK/STAT pathways, and other factors, as well as relevant regulatory networks. We also review the mechanisms by which different tissues, including hemocytes, the fat body, the lymph gland, muscles, the gut and the brain coordinate innate immune responses. Furthermore, the latest studies in this field are outlined in this review. In summary, understanding the mechanism underlying innate immunity orchestration in Drosophila will help us better study human innate immunity-related diseases.

Molecular mechanism and potential application of bacterial infection in the silkworm, Bombyx mori

As a representative species of Lepidoptera, Bombyx mori has been widely studied and applied. However, bacterial infection has always been an important pathogen threatening the growth of silkworms. Bombyx mori can resist various pathogenic bacteria through their own physical barrier and innate immune system. However, compared with other insects, such as Drosophila melanogaster, research on the antibacterial mechanism of silkworms is still in its infancy. This review systematically summarized the routes of bacterial infection in silkworms, the antibacterial mechanism of silkworms after ingestion or wounding infection, and the intestinal bacteria and infection of silkworms. Finally, we will discuss silkworms as a model animal for studying bacterial infectious diseases and screening antibacterial drugs.

Identification of Bipotential Blood Cell/Nephrocyte Progenitors in Drosophila: Another Route for Generating Blood Progenitors

The Drosophila lymph gland is the larval hematopoietic organ and is aligned along the anterior part of the cardiovascular system, composed of cardiac cells, that form the cardiac tube and its associated pericardial cells or nephrocytes. By the end of embryogenesis the lymph gland is composed of a single pair of lobes. Two additional pairs of posterior lobes develop during larval development to contribute to the mature lymph gland. In this study we describe the ontogeny of lymph gland posterior lobes during larval development and identify the genetic basis of the process. By lineage tracing we show here that each posterior lobe originates from three embryonic pericardial cells, thus establishing a bivalent blood cell/nephrocyte potential for a subset of embryonic pericardial cells. The posterior lobes of L3 larvae posterior lobes are composed of heterogeneous blood progenitors and their diversity is progressively built during larval development. We further establish that in larvae, homeotic genes and the transcription factor Klf15 regulate the choice between blood cell and nephrocyte fates. Our data underline the sequential production of blood cell progenitors during larval development.