A dual role for the βPS integrin myospheroid in mediating Drosophila embryonic macrophage migration

Evolution of insect innate immunity through domestication of bacterial toxins

Toxin cargo genes are often horizontally transferred by phages between bacterial species and are known to play an important role in the evolution of bacterial pathogenesis. Here, we show how these same genes have been horizontally transferred from phage or bacteria to animals and have resulted in novel adaptations. We discovered that two widespread bacterial genes encoding toxins of animal cells, cytolethal distending toxin subunit B (cdtB) and apoptosis-inducing protein of 56 kDa (aip56), were captured by insect genomes through horizontal gene transfer from bacteria or phages. To study the function of these genes in insects, we focused on Drosophila ananassae as a model. In the D. ananassae subgroup species, cdtB and aip56 are present as singular (cdtB) or fused copies (cdtB::aip56) on the second chromosome. We found that cdtB and aip56 genes and encoded proteins were expressed by immune cells, some proteins were localized to the wasp embryo's serosa, and their expression increased following parasitoid wasp infection. Species of the ananassae subgroup are highly resistant to parasitoid wasps, and we observed that D. ananassae lines carrying null mutations in cdtB and aip56 toxin genes were more susceptible to parasitoids than the wild type. We conclude that toxin cargo genes were captured by these insects millions of years ago and integrated as novel modules into their innate immune system. These modules now represent components of a heretofore undescribed defense response and are important for resistance to parasitoid wasps. Phage or bacterially derived eukaryotic toxin genes serve as macromutations that can spur the instantaneous evolution of novelty in animals.

Focal adhesion-mediated cell anchoring and migration: from in vitro to in vivo

Cell-extracellular matrix interactions have been studied extensively using cells cultured in vitro. These studies indicate that focal adhesion (FA)-based cell-extracellular matrix interactions are essential for cell anchoring and cell migration. Whether FAs play a similarly important role in vivo is less clear. Here, we summarize the formation and function of FAs in cultured cells and review how FAs transmit and sense force in vitro. Using examples from animal studies, we also describe the role of FAs in cell anchoring during morphogenetic movements and cell migration in vivo. Finally, we conclude by discussing similarities and differences in how FAs function in vitro and in vivo.

Cell division in tissues enables macrophage infiltration

Cells migrate through crowded microenvironments within tissues during normal development, immune response, and cancer metastasis. Although migration through pores and tracks in the extracellular matrix (ECM) has been well studied, little is known about cellular traversal into confining cell-dense tissues. We find that embryonic tissue invasion by Drosophila macrophages requires division of an epithelial ectodermal cell at the site of entry. Dividing ectodermal cells disassemble ECM attachment formed by integrin-mediated focal adhesions next to mesodermal cells, allowing macrophages to move their nuclei ahead and invade between two immediately adjacent tissues. Invasion efficiency depends on division frequency, but reduction of adhesion strength allows macrophage entry independently of division. This work demonstrates that tissue dynamics can regulate cellular infiltration.

Macrophage network dynamics depend on haptokinesis for optimal local surveillance

Macrophages are key immune cells with important roles for tissue surveillance in almost all mammalian organs. Cellular networks made up of many individual macrophages allow for optimal removal of dead cell material and pathogens in tissues. However, the critical determinants that underlie these population responses have not been systematically studied. Here, we investigated how cell shape and the motility of individual cells influences macrophage network responses in 3D culture settings and in mouse tissues. We show that surveying macrophage populations can tolerate lowered actomyosin contractility, but cannot easily compensate for a lack of integrin-mediated adhesion. Although integrins were dispensable for macrophage chemotactic responses, they were crucial to control cell movement and protrusiveness for optimal surveillance by a macrophage population. Our study reveals that β1 integrins are important for maintaining macrophage shape and network sampling efficiency in mammalian tissues, and sets macrophage motility strategies apart from the integrin-independent 3D migration modes of many other immune cell subsets.

Macrophages are immune cells in the body that remove dying cells and debris from tissues. They live in almost all the body’s organs, surveilling for signs of infection and destroying microbes. They also migrate to wound sites, where they can eliminate foreign particles and stop microbes from entering the body.

To perform their surveillance role, macrophages need to work together as a team. They form a network, coordinating their movements to optimise the removal of particles and dead cells. How this happens is something of a mystery. As individuals, cells travel through tissues using a balance of several activities: they change their shape, they contract and relax, and they grab hold of their surroundings using proteins called integrins. It is thought that the choice between these types of movement may affect the rest of the network.

To investigate, Paterson and Lämmermann genetically engineered mouse macrophages grown in the laboratory so they would not produce working integrins. These macrophages were able to contract and relax, but they could not attach to the proteins in the structures they were exploring.

Paterson and Lämmermann then placed these macrophages in gels studded with proteins that mimic a biological matrix to observe their behaviour. When these macrophages were exposed to the chemicals that indicate the presence of a wound, they moved normally, changing shape and contracting and relaxing. Paterson and Lämmermann confirmed this normal behaviour for macrophages moving to sites of injuries in the tissue of living mice. However, when it came to surveillance, the macrophages’ abilities were seriously diminished, and they were unable to form an effective network to take up particles and dead cells.

This work sheds light on how the movement of individual cells affects the entire immune surveillance network. A deeper understanding could lead to new insights into how to prevent inflammation. The next step is to map macrophage networks in healthy and diseased tissues to understand how cell movement affects surveillance under different conditions.

Identification of functionally distinct macrophage subpopulations in Drosophila

Vertebrate macrophages are a highly heterogeneous cell population, but while Drosophila blood is dominated by a macrophage-like lineage (plasmatocytes), until very recently these cells were considered to represent a homogeneous population. Here, we present our identification of enhancer elements labelling plasmatocyte subpopulations, which vary in abundance across development. These subpopulations exhibit functional differences compared to the overall population, including more potent injury responses and differential localisation and dynamics in pupae and adults. Our enhancer analysis identified candidate genes regulating plasmatocyte behaviour: pan-plasmatocyte expression of one such gene (Calnexin14D) improves wound responses, causing the overall population to resemble more closely the subpopulation marked by the Calnexin14D-associated enhancer. Finally, we show that exposure to increased levels of apoptotic cell death modulates subpopulation cell numbers. Taken together this demonstrates macrophage heterogeneity in Drosophila, identifies mechanisms involved in subpopulation specification and function and facilitates the use of Drosophila to study macrophage heterogeneity in vivo.

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 development of body and organ shape

Background

Organisms show an incredibly diverse array of body and organ shapes that are both unique to their taxon and important for adapting to their environment. Achieving these specific shapes involves coordinating the many processes that transform single cells into complex organs, and regulating their growth so that they can function within a fully-formed body.

Main text

Conceptually, body and organ shape can be separated in two categories, although in practice these categories need not be mutually exclusive. Body shape results from the extent to which organs, or parts of organs, grow relative to each other. The patterns of relative organ size are characterized using allometry. Organ shape, on the other hand, is defined as the geometric features of an organ’s component parts excluding its size. Characterization of organ shape is frequently described by the relative position of homologous features, known as landmarks, distributed throughout the organ. These descriptions fall into the domain of geometric morphometrics.

Conclusion

In this review, we discuss the methods of characterizing body and organ shape, the developmental programs thought to underlie each, highlight when and how the mechanisms regulating body and organ shape might overlap, and provide our perspective on future avenues of research.

Simu-dependent clearance of dying cells regulates macrophage function and inflammation resolution

Macrophages encounter and clear apoptotic cells during normal development and homeostasis, including at numerous sites of pathology. Clearance of apoptotic cells has been intensively studied, but the effects of macrophage-apoptotic cell interactions on macrophage behaviour are poorly understood. Using Drosophila embryos, we have exploited the ease of manipulating cell death and apoptotic cell clearance in this model to identify that the loss of the apoptotic cell clearance receptor Six-microns-under (Simu) leads to perturbation of macrophage migration and inflammatory responses via pathological levels of apoptotic cells. Removal of apoptosis ameliorates these phenotypes, while acute induction of apoptosis phenocopies these defects and reveals that phagocytosis of apoptotic cells is not necessary for their anti-inflammatory action. Furthermore, Simu is necessary for clearance of necrotic debris and retention of macrophages at wounds. Thus, Simu is a general detector of damaged self and represents a novel molecular player regulating macrophages during resolution of inflammation.

An in vitro method for studying subcellular rearrangements during cell polarization in Drosophila melanogaster hemocytes

Thanks to the power of Drosophila genetics, this animal model has been a precious tool for scientists to uncover key processes associated to innate immunity. The fly immune system relies on a population of macrophage-like cells, also referred to as hemocytes, which are highly migratory and phagocytic, and can easily be followed in vivo. These cells have shown to play important roles in fly development, both at the embryonic and pupal stages. However, there is no robust assay for the study of hemocyte migration in vitro, which limits our understanding of the molecular mechanisms involved. Here, we contribute to fill this gap by showing that hemocytes adopt a polarized morphology upon ecdysone stimulation, allowing the study of the cytoskeleton rearrangements and organelle reorganization that take place during the first step of cell locomotion.

Drosophila Embryonic Hemocytes Produce Laminins to Strengthen Migratory Response

The most prominent developmental function attributed to the extracellular matrix (ECM) is cell migration. While cells in culture can produce ECM to migrate, the role of ECM in regulating developmental cell migration is classically viewed as an exogenous matrix presented to the moving cells. In contrast to this view, we show here that Drosophila embryonic hemocytes deposit their own laminins in streak-like structures to migrate efficiently throughout the embryo. With the help of transplantation experiments, live microscopy, and image quantification, we demonstrate that autocrine-produced laminin regulates hemocyte migration by controlling lamellipodia dynamics, stability, and persistence. Proper laminin deposition is regulated by the RabGTPase Rab8, which is highly expressed and required in hemocytes for lamellipodia dynamics and migration. Our results thus support a model in which, during embryogenesis, the Rab8-regulated autocrine deposition of laminin reinforces directional and effective migration by stabilizing cellular protrusions and strengthening otherwise transient adhesion states.

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Drosophila embryonic hemocytes use autocrine-produced laminins for their migration

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Autocrine laminins regulate lamellipodia dynamics, stability, and persistence

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Rab8 regulates laminin deposition and lamellipodia dynamics in migrating hemocytes

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Laminins deposit in tracks around hemocytes and in a fibrillar mesh over the VNC

Gene expression boundary scaling and organ size regulation in the Drosophila embryo

How the shape and size of tissues and organs is regulated during development is a major question in developmental biology. Such regulation relies upon both intrinsic cues (such as signaling networks) and extrinsic inputs (such as from neighboring tissues). Here, we focus on pattern formation and organ development during Drosophila embryogenesis. In particular, we outline the importance of both biochemical and mechanical tissue-tissue interactions in size regulation. We describe how the Drosophila embryo can potentially provide novel insights into how shape and size are regulated during development. We focus on gene expression boundary scaling in the early embryo and how size is regulated in three organs (hindgut, trachea, and ventral nerve cord) later in development, with particular focus on the role of tissue-tissue interactions. Overall, we demonstrate that Drosophila embryogenesis provides a suitable model system for studying spatial and temporal scaling and size control in vivo.

Drosophila WASH is required for integrin-mediated cell adhesion, cell motility and lysosomal neutralization

The Wiskott-Aldrich syndrome protein and SCAR homolog (WASH; also known as Washout in flies) is a conserved actin-nucleation-promoting factor controlling Arp2/3 complex activity in endosomal sorting and recycling. Previous studies have identified WASH as an essential regulator in Drosophila development. Here, we show that homozygous wash mutant flies are viable and fertile. We demonstrate that Drosophila WASH has conserved functions in integrin receptor recycling and lysosome neutralization. WASH generates actin patches on endosomes and lysosomes, thereby mediating both aforementioned functions. Consistently, loss of WASH function results in cell spreading and cell migration defects of macrophages, and an increased lysosomal acidification that affects efficient phagocytic and autophagic clearance. WASH physically interacts with the vacuolar (V)-ATPase subunit Vha55 that is crucial to establish and maintain lysosome acidification. As a consequence, starved flies that lack WASH function show a dramatic increase in acidic autolysosomes, causing a reduced lifespan. Thus, our data highlight a conserved role for WASH in the endocytic sorting and recycling of membrane proteins, such as integrins and the V-ATPase, that increase the likelihood of survival under nutrient deprivation.

Mechanisms and in vivo functions of contact inhibition of locomotion

Contact inhibition of locomotion (CIL) is a process whereby a cell ceases motility or changes its trajectory upon collision with another cell. CIL was initially characterized more than half a century ago and became a widely studied model system to understand how cells migrate and dynamically interact. Although CIL fell from interest for several decades, the scientific community has recently rediscovered this process. We are now beginning to understand the precise steps of this complex behaviour and to elucidate its regulatory components, including receptors, polarity proteins and cytoskeletal elements. Furthermore, this process is no longer just in vitro phenomenology; we now know from several different in vivo models that CIL is essential for embryogenesis and in governing behaviours such as cell dispersion, boundary formation and collective cell migration. In addition, changes in CIL responses have been associated with other physiological processes, such as cancer cell dissemination during metastasis.

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.

Drosophila immune cell migration and adhesion during embryonic development and larval immune responses

The majority of immune cells in Drosophila melanogaster are plasmatocytes; they carry out similar functions to vertebrate macrophages, influencing development as well as protecting against infection and cancer. Plasmatocytes, sometimes referred to with the broader term of hemocytes, migrate widely during embryonic development and cycle in the larvae between sessile and circulating positions. Here we discuss the similarities of plasmatocyte developmental migration and its functions to that of vertebrate macrophages, considering the recent controversy regarding the functions of Drosophila PDGF/VEGF related ligands. We also examine recent findings on the significance of adhesion for plasmatocyte migration in the embryo, as well as proliferation, trans-differentiation, and tumor responses in the larva. We spotlight parallels throughout to vertebrate immune responses.

Drosophila blood cells and their role in immune responses

Drosophila melanogaster has been extensively used to study the humoral arm of innate immunity because of the developmental and functional parallels with mammalian innate immunity. However, the fly cellular response to infection is far less understood. Investigative work on Drosophila haemocytes, the immunosurveillance cells of the insect, has revealed that they fulfil roles similar to mammalian monocytes and macrophages. They respond to wound signals and orchestrate the coagulation response. In addition, they phagocytose and encapsulate invading pathogens, and clear up apoptotic bodies controlling inflammation. This review briefly describes the Drosophila haematopoietic system and discusses what is currently known about the contribution of haemocytes to the immune response upon infection and wounding, during all stages of development.

Drosophila PS2 and PS3 integrins play distinct roles in retinal photoreceptors-glia interactions

Cellular migration and differentiation are important developmental processes that require dynamic cellular adhesion. Integrins are heterodimeric transmembrane receptors that play key roles in adhesion plasticity. Here, we explore the developing visual system of Drosophila to study the roles of integrin heterodimers in glia development. Our data show that αPS2 is essential for retinal glia migration from the brain into the eye disc and that glial cells have a role in the maintenance of the fenestrated membrane (Laminin-rich ECM layer) in the disc. Interestingly, the absence of glial cells in the eye disc did not affect the targeting of retinal axons to the optic stalk. In contrast, αPS3 is not required for retinal glia migration, but together with Talin, it functions in glial cells to allow photoreceptor axons to target the optic stalk. Thus, we present evidence that αPS2 and αPS3 integrin have different and specific functions in the development of retinal glia.

Anchors and signals: the diverse roles of integrins in development

Integrins mediate cell adhesion by providing a link between the actin cytoskeleton and the extracellular matrix. As well as acting to anchor cells, integrin adhesions provide sensory input via mechanotransduction and synergism with signaling pathways, and provide the cell with the conditions necessary for differentiation in a permissive manner. In this review, we explore how integrins contribute to development, and what this tells us about how they work. From a signaling perspective, the influence of integrins on cell viability and fate is muted in a developmental context as compared to cell culture. Integrin phenotypes tend to arise from a failure of normally specified cells to create tissues properly, due to defective adhesion. The diversity of integrin functions in development shows how cell adhesion is continuously adjusted, both within and between animals, to fit developmental purpose.

Induction of focal adhesions and motility in Drosophila S2 cells

In this study, normally immotile S2 cells are engineered to induce the formation of focal adhesions and cell motility by the transfection of a single gene encoding an integrin subunit. It is demonstrated that the focal adhesions recruit expected components and exhibit mechanosensitive behavior on integrin-ligand substrates of different stiffnesses.

Focal adhesions are dynamic structures that interact with the extracellular matrix on the cell exterior and actin filaments on the cell interior, enabling cells to adhere and crawl along surfaces. We describe a system for inducing the formation of focal adhesions in normally non–ECM-adherent, nonmotile Drosophila S2 cells. These focal adhesions contain the expected molecular markers such as talin, vinculin, and p130Cas, and they require talin for their formation. The S2 cells with induced focal adhesions also display a nonpolarized form of motility on vitronectin-coated substrates. Consistent with findings in mammalian cells, the degree of motility can be tuned by changing the stiffness of the substrate and was increased after the depletion of PAK3, a p21-activated kinase. A subset of nonmotile, nonpolarized cells also exhibited focal adhesions that rapidly assembled and disassembled around the cell perimeter. Such cooperative and dynamic fluctuations of focal adhesions were decreased by RNA interference (RNAi) depletion of myosin II and focal adhesion kinase, suggesting that this behavior requires force and focal adhesion maturation. These results demonstrate that S2 cells, a cell line that is well studied for cytoskeletal dynamics and readily amenable to protein manipulation by RNAi, can be used to study the assembly and dynamics of focal adhesions and mechanosensitive cell motility.

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

Fish, mice, and humans rely on two coexisting myeloid blood cell systems. One is sustained by hematopoietic progenitor cells, which reside in specialized microenvironments (niches) in hematopoietic organs and give rise to cells of the monocyte lineage. The other system corresponds to the independent lineage of self-renewing tissue macrophages, which colonize organs during embryonic development and are maintained during later life by proliferation in local tissue microenvironments. However, little is known about the nature of these microenvironments and their regulation. Moreover, many vertebrate tissues contain a mix of both tissue-resident and monocyte-derived macrophages, posing a challenge to the study of lineage-specific regulatory mechanisms and function. This review highlights how research in the simple model organism Drosophila melanogaster can address many of these outstanding questions in the field. Drawing parallels between hematopoiesis in Drosophila and vertebrates, we illustrate the evolutionary conservation of the two myeloid systems across animal phyla. Much like vertebrates, Drosophila possesses a lineage of self-renewing tissue-resident macrophages, which we refer to as tissue hemocytes, as well as a "definitive" lineage of macrophages that derive from hematopoiesis in the progenitor-based lymph gland. We summarize key findings from Drosophila hematopoiesis that illustrate how local microenvironments, systemic signals, immune challenges, and nervous inputs regulate adaptive responses of tissue-resident macrophages and progenitor-based hematopoiesis to maximize fitness of the animal.

Drosophila blood cell chemotaxis

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Integrin-mediated adhesion used by Drosophila blood cells to migrate in vivo.

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SCAR/WAVE is required for lamellipodia but also for clearance of apoptotic cells.

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The formins Fhos and Diaphanous regulate Drosophila macrophage migration and morphology.

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Calcium waves drive hydrogen peroxide production to regulate inflammatory migrations.

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The steroid hormone Ecdysone controls the onset of immune competence.

Drosophila melanogaster contains a population of blood cells called hemocytes that represent the functional equivalent of vertebrate macrophages. These cells undergo directed migrations to disperse during development and reach sites of tissue damage or altered self. These chemotactic behaviors are controlled by the expression of PDGF/Vegf-related ligands in developing embryos and local production of hydrogen peroxide at wounds. Recent work reveals that many molecules important in vertebrate cell motility, including integrins, formins, Ena/VASP proteins and the SCAR/WAVE complex, have a conserved function in these innate immune cells. The use of this model organism has elucidated how damage signals are activated by calcium signaling during inflammation and that the steroid hormone ecdysone activates immune competence at key developmental stages.

Inducible defenses stay up late: temporal patterns of immune gene expression in Tenebrio molitor

The course of microbial infection in insects is shaped by a two-stage process of immune defense. Constitutive defenses, such as engulfment and melanization, act immediately and are followed by inducible defenses, archetypically the production of antimicrobial peptides, which eliminate or suppress the remaining microbes. By applying RNAseq across a 7-day time course, we sought to characterize the long-lasting immune response to bacterial challenge in the mealworm beetle Tenebrio molitor, a model for the biochemistry of insect immunity and persistent bacterial infection. By annotating a hybrid de novo assembly of RNAseq data, we were able to identify putative orthologs for the majority of components of the conserved insect immune system. Compared with Tribolium castaneum, the most closely related species with a reference genome sequence and a manually curated immune system annotation, the T. molitor immune gene count was lower, with lineage-specific expansions of genes encoding serine proteases and their countervailing inhibitors accounting for the majority of the deficit. Quantitative mapping of RNAseq reads to the reference assembly showed that expression of genes with predicted functions in cellular immunity, wound healing, melanization, and the production of reactive oxygen species was transiently induced immediately after immune challenge. In contrast, expression of genes encoding antimicrobial peptides or components of the Toll signaling pathway and iron sequestration response remained elevated for at least 7 days. Numerous genes involved in metabolism and nutrient storage were repressed, indicating a possible cost of immune induction. Strikingly, the expression of almost all antibacterial peptides followed the same pattern of long-lasting induction, regardless of their spectra of activity, signaling possible interactive roles in vivo.