Swatting flies: modelling wound healing and inflammation in Drosophila

Wound Repair of the Cell Membrane: Lessons from Dictyostelium Cells

The cell membrane is frequently subjected to damage, either through physical or chemical means. The swift restoration of the cell membrane's integrity is crucial to prevent the leakage of intracellular materials and the uncontrolled influx of extracellular ions. Consequently, wound repair plays a vital role in cell survival, akin to the importance of DNA repair. The mechanisms involved in wound repair encompass a series of events, including ion influx, membrane patch formation, endocytosis, exocytosis, recruitment of the actin cytoskeleton, and the elimination of damaged membrane sections. Despite the absence of a universally accepted general model, diverse molecular models have been proposed for wound repair in different organisms. Traditional wound methods not only damage the cell membrane but also impact intracellular structures, including the underlying cortical actin networks, microtubules, and organelles. In contrast, the more recent improved laserporation selectively targets the cell membrane. Studies on Dictyostelium cells utilizing this method have introduced a novel perspective on the wound repair mechanism. This review commences by detailing methods for inducing wounds and subsequently reviews recent developments in the field.

Integrative biology of injury in animals

Mechanical injury is a prevalent challenge in the lives of animals with myriad potential consequences for organisms, including reduced fitness and death. Research on animal injury has focused on many aspects, including the frequency and severity of wounding in wild populations, the short- and long-term consequences of injury at different biological scales, and the variation in the response to injury within or among individuals, species, ontogenies, and environmental contexts. However, relevant research is scattered across diverse biological subdisciplines, and the study of the effects of injury has lacked synthesis and coherence. Furthermore, the depth of knowledge across injury biology is highly uneven in terms of scope and taxonomic coverage: much injury research is biomedical in focus, using mammalian model systems and investigating cellular and molecular processes, while research at organismal and higher scales, research that is explicitly comparative, and research on invertebrate and non-mammalian vertebrate species is less common and often less well integrated into the core body of knowledge about injury. The current state of injury research presents an opportunity to unify conceptually work focusing on a range of relevant questions, to synthesize progress to date, and to identify fruitful avenues for future research. The central aim of this review is to synthesize research concerning the broad range of effects of mechanical injury in animals. We organize reviewed work by four broad and loosely defined levels of biological organization: molecular and cellular effects, physiological and organismal effects, behavioural effects, and ecological and evolutionary effects of injury. Throughout, we highlight the diversity of injury consequences within and among taxonomic groups while emphasizing the gaps in taxonomic coverage, causal understanding, and biological endpoints considered. We additionally discuss the importance of integrating knowledge within and across biological levels, including how initial, localized responses to injury can lead to long-term consequences at the scale of the individual animal and beyond. We also suggest important avenues for future injury biology research, including distinguishing better between related yet distinct injury phenomena, expanding the subjects of injury research to include a greater variety of species, and testing how intrinsic and extrinsic conditions affect the scope and sensitivity of injury responses. It is our hope that this review will not only strengthen understanding of animal injury but will contribute to building a foundation for a more cohesive field of 'injury biology'.

Fluctuations of cell geometry and their nonequilibrium thermodynamics in living epithelial tissue

We measure different contributions to entropy production in a living functional epithelial tissue. We do this by extracting the functional dynamics of development while at the same time quantifying fluctuations. Using the translucent Drosophila melanogaster pupal epithelium as an ideal tissue for high-resolution live imaging, we measure the entropy associated with the stochastic geometry of cells in the epithelium. This is done using a detailed analysis of the dynamics of the shape and orientation of individual cells which enables separation of local and global aspects of the tissue behavior. Intriguingly, we find that we can observe irreversible dynamics in the cell geometries but without a change in the entropy associated with those degrees of freedom, showing that there is a flow of energy into those degrees of freedom. Hence, the living system is controlling how the entropy is being produced and partitioned into its different parts.

On the origin of the functional versatility of macrophages

Macrophages represent the most functionally versatile cells in the animal body. In addition to recognizing and destroying pathogens, macrophages remove senescent and exhausted cells, promote wound healing, and govern tissue and metabolic homeostasis. In addition, many specialized populations of tissue-resident macrophages exhibit highly specialized functions essential for the function of specific organs. Sometimes, however, macrophages cease to perform their protective function and their seemingly incomprehensible response to certain stimuli leads to pathology. In this study, we address the question of the origin of the functional versatility of macrophages. To this end, we have searched for the evolutionary origin of macrophages themselves and for the emergence of their characteristic properties. We hypothesize that many of the characteristic features of proinflammatory macrophages evolved in the unicellular ancestors of animals, and that the functional repertoire of macrophage-like amoebocytes further expanded with the evolution of multicellularity and the increasing complexity of tissues and organ systems. We suggest that the entire repertoire of macrophage functions evolved by repurposing and diversification of basic functions that evolved early in the evolution of metazoans under conditions barely comparable to that in tissues of multicellular organisms. We believe that by applying this perspective, we may find an explanation for the otherwise counterintuitive behavior of macrophages in many human pathologies.

Dynamics of Actin Cytoskeleton and Their Signaling Pathways during Cellular Wound Repair

The repair of wounded cell membranes is essential for cell survival. Upon wounding, actin transiently accumulates at the wound site. The loss of actin accumulation leads to cell death. The mechanism by which actin accumulates at the wound site, the types of actin-related proteins participating in the actin remodeling, and their signaling pathways are unclear. We firstly examined how actin accumulates at a wound site in Dictyostelium cells. Actin assembled de novo at the wound site, independent of cortical flow. Next, we searched for actin- and signal-related proteins targeting the wound site. Fourteen of the examined proteins transiently accumulated at different times. Thirdly, we performed functional analyses using gene knockout mutants or specific inhibitors. Rac, WASP, formin, the Arp2/3 complex, profilin, and coronin contribute to the actin dynamics. Finally, we found that multiple signaling pathways related to TORC2, the Elmo/Doc complex, PIP2-derived products, PLA2, and calmodulin are involved in the actin dynamics for wound repair.

Effects and Progress of Photo-Crosslinking Hydrogels in Wound Healing Improvement

Wound healing is a dynamic physiological process, including three stages: inflammation, tissue formation, and remodeling. The quality of wound healing is affected by many topical and systemic factors, while any small factor may affect the process. Therefore, improving the quality of wound healing is a complex and arduous challenge. Photo-crosslinking reaction using visible light irradiation is a novel method for hydrogel preparation. Photo-crosslinking hydrogels can be controlled in time and space, and are not interfered by temperature conditions, which have been widely used in the fields of medicine and engineering. This review aims to summarize the application of photo-crosslinking hydrogels in improving the quality of wound healing, mainly including the material design, application mechanism, and effect of photo-crosslinking hydrogels applied in wound healing, followed by the applicable animal models for experimental research. Finally, this review analyzes the clinical application prospects of photo-crosslinking hydrogels in the field of wound healing.

What good is maths in studies of wound healing?

Wound healing is an aspect of normal physiology that we all take for granted until it goes wrong, such as, for example, the scarring that results from a severe burn, or those patients who suffer from debilitating chronic wounds that fail to heal. Ever since wound repair research began as a discipline, clinicians and basic scientists have collaborated to try and understand the cell and molecular mechanisms that underpin healthy repair in the hope that this will reveal clues for the therapeutic treatment of pathological healing. In recent decades mathematicians and physicists have begun to join in with this important challenge. Here we describe examples of how mathematical modeling married to biological experimentation has provided insights that biology alone could not fathom. To date, these studies have largely focused on wound re-epithelialization and inflammation, but we also discuss other components of wound healing that might be ripe for similar interdisciplinary approaches.

SUMOylation of Jun fine-tunes the Drosophila gut immune response

Post-translational modification by the small ubiquitin-like modifier, SUMO can modulate the activity of its conjugated proteins in a plethora of cellular contexts. The effect of SUMO conjugation of proteins during an immune response is poorly understood in Drosophila. We have previously identified that the transcription factor Jra, the Drosophila Jun ortholog and a member of the AP-1 complex is one such SUMO target. Here, we find that Jra is a regulator of the Pseudomonas entomophila induced gut immune gene regulatory network, modulating the expression of a few thousand genes, as measured by quantitative RNA sequencing. Decrease in Jra in gut enterocytes is protective, suggesting that reduction of Jra signaling favors the host over the pathogen. In Jra, lysines 29 and 190 are SUMO conjugation targets, with the Jra^K29R+K190R double mutant being SUMO conjugation resistant (SCR). Interestingly, a Jra^SCR fly line, generated by CRISPR/Cas9 based genome editing, is more sensitive to infection, with adults showing a weakened host response and increased proliferation of Pseudomonas. Transcriptome analysis of the guts of Jra^SCR and Jra^WT flies suggests that lack of SUMOylation of Jra significantly changes core elements of the immune gene regulatory network, which include antimicrobial agents, secreted ligands, feedback regulators, and transcription factors. Mechanistically, SUMOylation attenuates Jra activity, with the TFs, forkhead, anterior open, activating transcription factor 3 and the master immune regulator Relish being important transcriptional targets. Our study implicates Jra as a major immune regulator, with dynamic SUMO conjugation/deconjugation of Jra modulating the kinetics of the gut immune response.

The intestine has a resident population of commensal microorganisms against which the immune machinery is tuned to show low or no reactivity. In contrast, when pathogenic microorganisms are ingested, the gut responds by activating signaling cascades that lead to the killing and clearance of the pathogen. In this study, we examine the role played by the well-known transcription factor Jun in regulating the immune response in the Drosophila gut. We find that loss of Jun leads to the change in intensity and kinetics of the gut immune transcriptome. The transcriptional profile indicates a stronger response when Jun activity is reduced. Also, animals infected with Pseudomonas entomophila live longer when Jun signaling is reduced. Further, we find that Jun is post-translationally modified on Lys29 and Lys190 by SUMO. To understand the effect of SUMO-conjugation of Jun, we create by state-of-the-art CRISPR/Cas9 genome editing a Drosophila line where Jun is resistant to SUMOylation. This line is more sensitive to infection, with a weaker host-defense response. Our data suggest that Jun Signaling favors the pathogen by dampening the immune response. SUMO conjugation of Jun reverses the dampening and strengthens the immune response in favor of the host. Dynamic SUMOylation of Jun thus fine-tunes the gut immune response to pathogens.

Forced into shape: Mechanical forces in Drosophila development and homeostasis

Mechanical forces play a central role in shaping tissues during development and maintaining epithelial integrity in homeostasis. In this review, we discuss the roles of mechanical forces in Drosophila development and homeostasis, starting from the interplay of mechanics with cell growth and division. We then discuss several examples of morphogenetic processes where complex 3D structures are shaped by mechanical forces, followed by a closer look at patterning processes. We also review the role of forces in homeostatic processes, including cell elimination and wound healing. Finally, we look at the interplay of mechanics and developmental robustness and discuss open questions in the field, as well as novel approaches that will help tackle them in the future.

Proto-pyroptosis: An Ancestral Origin for Mammalian Inflammatory Cell Death Mechanism in Drosophila melanogaster

Pyroptosis has been described in mammalian systems to be a form of programmed cell death that is important in immune function through the subsequent release of cytokines and immune effectors upon cell bursting. This form of cell death has been increasingly well-characterized in mammals and can occur using alternative routes however, across phyla, there has been little evidence for the existence of pyroptosis. Here we provide evidence for an ancient origin of pyroptosis in an in vivo immune scenario in Drosophila melanogaster. Crystal cells, a type of insect blood cell, were recruited to wounds and ruptured subsequently releasing their cytosolic content in a caspase-dependent manner. This inflammatory-based programmed cell death mechanism fits the features of pyroptosis, never before described in an in vivo immune scenario in insects and relies on ancient apoptotic machinery to induce proto-pyroptosis. Further, we unveil key players upstream in the activation of cell death in these cells including the apoptosome which may play an alternative role akin to the inflammasome in proto-pyroptosis. Thus, Drosophila may be a suitable model for studying the functional significance of pyroptosis in the innate immune system.

Sensing wounds by spilling cellular guts: Damage-associated protease release can initiate tissue repair

Epithelia have an innate yet mysterious capacity to rapidly sense and respond to tissue damage. In this issue of Developmental Cell, O'Connor et al. exploit the genetics of Drosophila to reveal that protease release as a result of tissue injury activates insect cytokines to initiate immediate epithelial repair responses.

Anti-Fibrotic Activity of an Antimicrobial Peptide in a Drosophila Model

Fibrotic lesions accompany several pathological conditions, including tumors. We show that expression of a dominant-active form of the Ras oncogene in Drosophila salivary glands (SGs) leads to redistribution of components of the basement membrane (BM) and fibrotic lesions. Similar to several types of mammalian fibrosis, the disturbed BM attracts clot components, including insect transglutaminase and phenoloxidase. SG epithelial cells show reduced apicobasal polarity accompanied by a loss of secretory activity. Both the fibrotic lesions and the reduced cell polarity are alleviated by ectopic expression of the antimicrobial peptide drosomycin (Drs), which also restores the secretory activity of the SGs. In addition to extracellular matrix components, both Drs and F-actin localize to fibrotic lesions.

Investigation of Skin Wound Healing Using a Mouse Model

Wound repair is a fundamental physiological process to keep the integrity of the skin, and its dysregulation results in diseases, such as chronic nonhealing wounds or excessive scarring. To study the underlying cellular and molecular mechanisms and identify new therapeutic targets, animal models are often used in the wound healing research. In this chapter, we describe an easy step-by-step protocol to generate skin wounds in a mouse model. Briefly, two full-thickness wounds extending through the panniculus carnosus are made on the dorsum on each side of the midline of a mouse, which is followed by monitoring and quantifying the wound closure. Moreover, the biopsy tissues of skin and wound-edges are collected at different time points for subsequent histology and gene expression analysis.

Proteome of the Triatomine Digestive Tract: From Catalytic to Immune Pathways; Focusing on Annexin Expression

Rhodnius prolixus, Panstrongylus megistus, Triatoma infestans, and Dipetalogaster maxima are all triatomines and potential vectors of the protozoan Trypanosoma cruzi responsible for human Chagas' disease. Considering that the T. cruzi's cycle occurs inside the triatomine digestive tract (TDT), the analysis of the TDT protein profile is an essential step to understand TDT physiology during T. cruzi infection. To characterize the protein profile of TDT of D. maxima, P. megistus, R. prolixus, and T. infestans, a shotgun liquid chromatography-tandem mass spectrometry (LC-MS/MS) approach was applied in this report. Most proteins were found to be closely related to metabolic pathways such as gluconeogenesis/glycolysis, citrate cycle, fatty acid metabolism, oxidative phosphorylation, but also to the immune system. We annotated this new proteome contribution gathering it with those previously published in accordance with Gene Ontology and KEGG. Enzymes were classified in terms of class, acceptor, and function, while the proteins from the immune system were annotated by reference to the pathways of humoral response, cell cycle regulation, Toll, IMD, JNK, Jak-STAT, and MAPK, as available from the Insect Innate Immunity Database (IIID). These pathways were further subclassified in recognition, signaling, response, coagulation, melanization and none. Finally, phylogenetic affinities and gene expression of annexins were investigated for understanding their role in the protection and homeostasis of intestinal epithelial cells against the inflammation.

Wound healing across the animal kingdom: Crosstalk between the immune system and the extracellular matrix

Tissue regeneration is widespread in the animal kingdom. To date, key roles for different molecular and cellular programs in regeneration have been described, but the ultimate blueprint for this talent remains elusive. In animals capable of tissue regeneration, one of the most crucial stages is wound healing, whose main goal is to close the wound and prevent infection. In this stage, it is necessary to avoid scar formation to facilitate the activation of the immune system and remodeling of the extracellular matrix, key factors in promoting tissue regeneration. In this review, we will discuss the current state of knowledge regarding the role of the immune system and the interplay with the extracellular matrix to trigger a regenerative response.

Methanolic extract of Ephedra ciliata promotes wound healing and arrests inflammatory cascade in vivo through downregulation of TNF-α

Chronic wounds may lead to the development of various pathological conditions such as diabetic foot ulcers and pressure sores. The current study evaluated wound healing and anti-inflammatory potentials of methanolic extract of Ephedra ciliata using series of in vivo models. Methanolic extract of Ephedra ciliata was prepared by maceration (Ec.Me). Qualitative and quantitative (HPLC) phytochemical and metal analyses were conducted to explore the chemical and metal profiles of Ec.Me. Safety profile (behavioural) and, antimicrobial, antioxidant, wound healing, anti-inflammatory and anti-angiogenic potentials of Ec.Me were evaluated using well-established in vitro and in vivo models. ELISA assay was performed to estimate the effects of Ec.Me treatment on serum levels of TNF-α. HPLC analysis identified quercetin as one of the major compounds in Ec.Me. Safety study data showed that Ec.Me was safe up to the dose of 2000 mg/kg. Antimicrobial assay data showed that Ec.Me was active against bacterial (Staphylococcus aureus) as well as fungal (Candida albicans and Aspergillus niger) strains. Ec.Me showed modertate antioxidant potential in in vitro and in vivo models. Data of excision and burn wound healing models showed that Ec.Me, promoted wound closure in a dose and time-dependent manner. Treatment with 20% Ec.Me cream and heparin showed almost the same effects with no statistical differences (p > 0.05). Ec.Me also showed time-dependent anti-inflammatory activities in both acute and chronic models. In carrageenan model, treatment with 200 mg/kg of Ec.Me showed comparable anti-inflammatory effects (p > 0.05) with quercetin and indomethacin throughout the study. In cotton pellet granuloma model treatment with 200 mg/kg of Ec.Me and indomethacin inhibited granuloma formation significantly better (p < 0.05) as compared with the rest of the treatment groups. Histopathological examination of skin samples showed marked improvement in architecture with minimal infiltration of inflammatory cells. Data of in vivo angiogenesis assay showed marked improvement in vessels length, density, branching points, total segments and total nets after treatment with Ec.Me, indicating no toxic effects towards vasculature development. Significant (p < 0.05) downregulation of TNF-α was observed in serum samples of animals treated with Ec.Me. Based on data of the current study, it is concluded that quercetin-rich extract of Ephedra ciliata has wound healing and anti-inflammatory potentials via downregulation of TNF-α. Moreover, it is suggested that the antimicrobial activity of Ec.Me prevented microbial invasion, thus promoted natural wound healing mechanisms as well.

Ca2+-Calmodulin Dependent Wound Repair in Dictyostelium Cell Membrane

Wound repair of cell membrane is a vital physiological phenomenon. We examined wound repair in Dictyostelium cells by using a laserporation, which we recently invented. We examined the influx of fluorescent dyes from the external medium and monitored the cytosolic Ca²⁺ after wounding. The influx of Ca²⁺ through the wound pore was essential for wound repair. Annexin and ESCRT components accumulated at the wound site upon wounding as previously described in animal cells, but these were not essential for wound repair in Dictyostelium cells. We discovered that calmodulin accumulated at the wound site upon wounding, which was essential for wound repair. The membrane accumulated at the wound site to plug the wound pore by two-steps, depending on Ca²⁺ influx and calmodulin. From several lines of evidence, the membrane plug was derived from de novo generated vesicles at the wound site. Actin filaments also accumulated at the wound site, depending on Ca²⁺ influx and calmodulin. Actin accumulation was essential for wound repair, but microtubules were not essential. A molecular mechanism of wound repair will be discussed.

Model systems for regeneration: Drosophila

Drosophila melanogaster has historically been a workhorse model organism for studying developmental biology. In addition, Drosophila is an excellent model for studying how damaged tissues and organs can regenerate. Recently, new precision approaches that enable both highly targeted injury and genetic manipulation have accelerated progress in this field. Here, we highlight these techniques and review examples of recently discovered mechanisms that regulate regeneration in Drosophila larval and adult tissues. We also discuss how, by applying these powerful approaches, studies of Drosophila can continue to guide the future of regeneration research.

Injury Activates a Dynamic Cytoprotective Network to Confer Stress Resilience and Drive Repair

In healthy individuals, injured tissues rapidly repair themselves following damage. Within a healing skin wound, recruited inflammatory cells release a multitude of bacteriocidal factors, including reactive oxygen species (ROS), to eliminate invading pathogens. Paradoxically, while these highly reactive ROS confer resistance to infection, they are also toxic to host tissues and may ultimately delay repair. Repairing tissues have therefore evolved powerful cytoprotective "resilience" machinery to protect against and tolerate this collateral damage. Here, we use in vivo time-lapse imaging and genetic manipulation in Drosophila to dissect the molecular and cellular mechanisms that drive tissue resilience to wound-induced stress. We identify a dynamic, cross-regulatory network of stress-activated cytoprotective pathways, linking calcium, JNK, Nrf2, and Gadd45, that act to both "shield" tissues from oxidative damage and promote efficient damage repair. Ectopic activation of these pathways confers stress protection to naive tissue, while their inhibition leads to marked delays in wound closure. Strikingly, the induction of cytoprotection is tightly linked to the pathways that initiate the inflammatory response, suggesting evolution of a fail-safe mechanism for tissue protection each time inflammation is triggered. A better understanding of these resilience mechanisms-their identities and precise spatiotemporal regulation-is of major clinical importance for development of therapeutic interventions for all pathologies linked to oxidative stress, including debilitating chronic non-healing wounds.

Fat Body Biology in the Last Decade

The insect fat body is analogous to vertebrate adipose tissue and liver. In this review, the new and exciting advancements made in fat body biology in the last decade are summarized. Controlled by hormonal and nutritional signals, insect fat body cells undergo mitosis during embryogenesis, endoreplication during the larval stages, and remodeling during metamorphosis and regulate reproduction in adults. Fat body tissues are major sites for nutrient storage, energy metabolism, innate immunity, and detoxification. Recent studies have revealed that the fat body plays a central role in the integration of hormonal and nutritional signals to regulate larval growth, body size, circadian clock, pupal diapause, longevity, feeding behavior, and courtship behavior, partially by releasing fat body signals to remotely control the brain. In addition, the fat body has emerged as a fascinating model for studying metabolic disorders and immune diseases. Potential future directions for fat body biology are also proposed herein.

Epithelial cells release adenosine to promote local TNF production in response to polarity disruption

Disruption of epithelial integrity contributes to chronic inflammatory disorders through persistent activation of stress signalling. Here we uncover a mechanism whereby disruption of apico-basal polarity promotes stress signalling. We show that depletion of Scribbled (Scrib), a baso-lateral determinant, causes epithelial cells to release adenosine through equilibrative channels into the extracellular space. Autocrine activation of the adenosine receptor leads to transcriptional upregulation of TNF, which in turn boosts the activity of JNK signalling. Thus, disruption of cell polarity feeds into a well-established stress pathway through the intermediary of an adenosine signalling branch. Although this regulatory input could help ensuring an effective response to acute polarity stress, we suggest that it becomes deleterious in situations of low-grade chronic disruption by provoking a private inflammatory-like TNF-driven response within the polarity-deficient epithelium.

Drosophila immune cells extravasate from vessels to wounds using Tre1 GPCR and Rho signaling

In contrast to vertebrates, adult Drosophila melanogaster have an open cardiovascular system. However, Thuma et al. find that in late pupation, hemolymph flows through Drosophila wing veins, providing a unique genetic and live-imaging opportunity to investigate the mechanisms driving immune cell extravasation from vessels to wounds and reveal new roles for Tre1 and Rho signaling in this process.

Inflammation is pivotal to fight infection, clear debris, and orchestrate repair of injured tissues. Although Drosophila melanogaster have proven invaluable for studying extravascular recruitment of innate immune cells (hemocytes) to wounds, they have been somewhat neglected as viable models to investigate a key rate-limiting component of inflammation—that of immune cell extravasation across vessel walls—due to their open circulation. We have now identified a period during pupal development when wing hearts pulse hemolymph, including circulating hemocytes, through developing wing veins. Wounding near these vessels triggers local immune cell extravasation, enabling live imaging and correlative light-electron microscopy of these events in vivo. We show that RNAi knockdown of immune cell integrin blocks diapedesis, just as in vertebrates, and we uncover a novel role for Rho-like signaling through the GPCR Tre1, a gene previously implicated in the trans-epithelial migration of germ cells. We believe this new Drosophila model complements current murine models and provides new mechanistic insight into immune cell extravasation.

Long-term In Vivo Tracking of Inflammatory Cell Dynamics Within Drosophila Pupae

During the rapid inflammatory response to tissue damage, cells of the innate immune system are quickly recruited to the injury site. Once at the wound, innate immune cells perform a number of essential functions, such as fighting infection, clearing necrotic debris, and stimulating matrix deposition. In order to fully understand the diverse signaling events that regulate this immune response, it is crucial to observe the complex behaviors of (and interactions that occur between) multiple cell lineages in vivo, and in real-time, with the high spatio-temporal resolution. The optical translucency and the genetic tractability of Drosophila embryos have established Drosophila as an invaluable model to live-image and dissect fundamental aspects of inflammatory cell behavior, including mechanisms of developmental dispersal, clearance of apoptotic corpses and/or microbial pathogens, and recruitment to wounds. However, more recent work has now demonstrated that employing a much later stage in the Drosophila lifecycle - the Drosophila pupa - offers a number of distinct advantages, including improved RNAi efficiency, longer imaging periods, and significantly greater immune cell numbers. Here we describe a protocol for imaging wound repair and the associated inflammatory response at the high spatio-temporal resolution in live Drosophila pupae. To follow the dynamics of both re-epithelialization and inflammation, we use a number of specific in vivo fluorescent markers for both the epithelium and innate immune cells. We also demonstrate the effectiveness of photo-convertible fluorophores, such as Kaede, for following the specific immune cell subsets, to track their behavior as they migrate to, and resolve from, the injury site.

Learning on the Fly: The Interplay between Caspases and Cancer

The ease of genetic manipulation, as well as the evolutionary conservation of gene function, has placed Drosophila melanogaster as one of the leading model organisms used to understand the implication of many proteins with disease development, including caspases and their relation to cancer. The family of proteases referred to as caspases have been studied over the years as the major regulators of apoptosis: the most common cellular mechanism involved in eliminating unwanted or defective cells, such as cancerous cells. Indeed, the evasion of the apoptotic programme resulting from caspase downregulation is considered one of the hallmarks of cancer. Recent investigations have also shown an instrumental role for caspases in non-lethal biological processes, such as cell proliferation, cell differentiation, intercellular communication, and cell migration. Importantly, malfunction of these essential biological tasks can deeply impact the initiation and progression of cancer. Here, we provide an extensive review of the literature surrounding caspase biology and its interplay with many aspects of cancer, emphasising some of the key findings obtained from Drosophila studies. We also briefly describe the therapeutic potential of caspase modulation in relation to cancer, highlighting shortcomings and hopeful promises.

Thioester-Containing Proteins 2 and 4 Affect the Metabolic Activity and Inflammation Response in Drosophila

Drosophila melanogaster is an outstanding model for studying host antipathogen defense. Although substantial progress has been made in understanding how metabolism and immunity are interrelated in flies, little information has been obtained on the molecular players that regulate metabolism and inflammation in Drosophila during pathogenic infection. Recently, we reported that the inactivation of thioester-containing protein 2 (Tep2) and Tep4 promotes survival and decreases the bacterial burden in flies upon infection with the virulent pathogens Photorhabdus luminescens and Photorhabdus asymbiotica Here, we investigated physiological and pathological defects in tep mutant flies in response to Photorhabdus challenge. We find that tep2 and tep4 loss-of-function mutant flies contain increased levels of carbohydrates and triglycerides in the presence or absence of Photorhabdus infection. We also report that Photorhabdus infection leads to higher levels of nitric oxide and reduced transcript levels of the apical caspase-encoding gene Dronc in tep2 and tep4 mutants. We show that Tep2 and Tep4 are upregulated mainly in the fat body rather than the gut in Photorhabdus-infected wild-type flies and that tep mutants contain decreased numbers of Photorhabdus bacteria in both tissue types. We propose that the inactivation of Tep2 or Tep4 in adult Drosophila flies results in lower levels of inflammation and increased energy reserves in response to Photorhabdus, which could confer a survival-protective effect during the initial hours of infection.

Epithelial-to-mesenchymal transition in cutaneous wound healing: Where we are and where we are heading

Cutaneous wound healing occurs in distinct yet overlapping steps with the end goal of reforming a stratified epithelium to restore epidermal barrier function. A key component of this process is re-epithelialization, which involves the proliferation and migration of epidermal keratinocytes surrounding the wound. This spatiotemporally controlled process resembles aspects of the epithelial-to-mesenchymal transition (EMT) process and is thus proposed to involve a partial EMT. Here, we review current literature on the cellular and molecular changes that occur during, and the known or potential regulatory factors of cutaneous wound re-epithelialization and EMT to highlight their similarities and differences. We also discuss possible future directions toward a better understanding of the underlying regulatory mechanisms with implications for developing new therapeutics to improve wound repair in humans. Developmental Dynamics 247:473-480, 2018. © 2017 Wiley Periodicals, Inc.

Laboratory Models for the Study of Normal and Pathologic Wound Healing

Current knowledge of wound healing is based on studies using various in vitro and in vivo wound models. In vitro models allow for biological examination of specific cell types involved in wound healing. In vivo models generally provide the full spectrum of biological responses required for wound healing, including inflammation and angiogenesis, and provide cell-cell interactions not seen in vitro. In this review, the authors aim to delineate the most relevant wound healing models currently available and to discuss their strengths and limitations in their approximation of the human wound healing processes to aid scientists in choosing the most appropriate wound healing models for designing, testing, and validating their experiments.

Pathogen-induced maternal effects result in enhanced immune responsiveness across generations

Parental investment theory postulates that adults can accurately perceive cues from their surroundings, anticipate the needs of future offspring based on those cues, and selectively allocate nongenetic resources to their progeny. Such context‐dependent parental contributions can result in phenotypically variable offspring. Consistent with these predictions, we show that bacterially exposed Manduca sexta mothers oviposited significantly more variable embryos (as measured by mass, volume, hatching time, and hatching success) relative to naïve and control mothers. By using an in vivo “clearance of infection” assay, we also show that challenged larvae born to heat‐killed‐ or live‐Serratia‐injected mothers, supported lower microbial loads and cleared the infection faster than progeny of control mothers. Our data support the notion that mothers can anticipate the future pathogenic risks and immunological needs of their unborn offspring, providing progeny with enhanced immune protection likely through transgenerational immune priming. Although the inclusion of live Serratia into oocytes does not appear to be the mechanism by which mothers confer protection to their young, other mechanisms, including epigenetic modifications in the progeny due to maternal pathogenic stress, may be at play. The adaptive nature of maternal effects in the face of pathogenic stress provides insights into parental investment, resource allocation, and life‐history theories and highlights the significant role that pathogen‐induced maternal effects play as generators and modulators of evolutionary change.

Toll pathway is required for wound-induced expression of barrier repair genes in the Drosophila epidermis

The epidermis serves as a protective barrier in animals. After epidermal injury, barrier repair requires activation of many wound response genes in epidermal cells surrounding wound sites. Two such genes in Drosophila encode the enzymes dopa decarboxylase (Ddc) and tyrosine hydroxylase (ple). In this paper we explore the involvement of the Toll/NF-κB pathway in the localized activation of wound repair genes around epidermal breaks. Robust activation of wound-induced transcription from ple and Ddc requires Toll pathway components ranging from the extracellular ligand Spätzle to the Dif transcription factor. Epistasis experiments indicate a requirement for Spätzle ligand downstream of hydrogen peroxide and protease function, both of which are known activators of wound-induced transcription. The localized activation of Toll a few cell diameters from wound edges is reminiscent of local activation of Toll in early embryonic ventral hypoderm, consistent with the hypothesis that the dorsal-ventral patterning function of Toll arose from the evolutionary cooption of a morphogen-responsive function in wound repair. Furthermore, the combinatorial activity of Toll and other signaling pathways in activating epidermal barrier repair genes can help explain why developmental activation of the Toll, ERK, or JNK pathways alone fail to activate wound repair loci.

Insulin and TOR signal in parallel through FOXO and S6K to promote epithelial wound healing

The TOR and Insulin/IGF signalling (IIS) network controls growth, metabolism and ageing. Although reducing TOR or insulin signalling can be beneficial for ageing, it can be detrimental for wound healing, but the reasons for this difference are unknown. Here we show that IIS is activated in the cells surrounding an epidermal wound in Drosophila melanogaster larvae, resulting in PI3K activation and redistribution of the transcription factor FOXO. Insulin and TOR signalling are independently necessary for normal wound healing, with FOXO and S6K as their respective effectors. IIS is specifically required in cells surrounding the wound, and the effect is independent of glycogen metabolism. Insulin signalling is needed for the efficient assembly of an actomyosin cable around the wound, and constitutively active myosin II regulatory light chain suppresses the effects of reduced IIS. These findings may have implications for the role of insulin signalling and FOXO activation in diabetic wound healing.

Systems Analysis of the Dynamic Inflammatory Response to Tissue Damage Reveals Spatiotemporal Properties of the Wound Attractant Gradient

In the acute inflammatory phase following tissue damage, cells of the innate immune system are rapidly recruited to sites of injury by pro-inflammatory mediators released at the wound site. Although advances in live imaging allow us to directly visualize this process in vivo, the precise identity and properties of the primary immune damage attractants remain unclear, as it is currently impossible to directly observe and accurately measure these signals in tissues. Here, we demonstrate that detailed information about the attractant signals can be extracted directly from the in vivo behavior of the responding immune cells. By applying inference-based computational approaches to analyze the in vivo dynamics of the Drosophila inflammatory response, we gain new detailed insight into the spatiotemporal properties of the attractant gradient. In particular, we show that the wound attractant is released by wound margin cells, rather than by the wounded tissue per se, and that it diffuses away from this source at rates far slower than those of previously implicated signals such as H₂O₂ and ATP, ruling out these fast mediators as the primary chemoattractant. We then predict, and experimentally test, how competing attractant signals might interact in space and time to regulate multi-step cell navigation in the complex environment of a healing wound, revealing a period of receptor desensitization after initial exposure to the damage attractant. Extending our analysis to model much larger wounds, we uncover a dynamic behavioral change in the responding immune cells in vivo that is prognostic of whether a wound will subsequently heal or not.

Video Abstract

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Computational modeling of in vivo inflammatory response to tissue damage is applied

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The model infers novel spatiotemporal properties of the wound attractant gradient

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Wound signal is released from the wound edge for 30 min and diffuses at 200 μm²/min

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Modeling two competing wounds reveals a period of immune cell desensitization

Immune modulation by MANF promotes tissue repair and regenerative success in the retina

Regenerative therapies are limited by unfavorable environments in aging and diseased tissues. A promising strategy to improve success is to balance inflammatory and anti-inflammatory signals and enhance endogenous tissue repair mechanisms. Here, we identified a conserved immune modulatory mechanism that governs the interaction between damaged retinal cells and immune cells to promote tissue repair. In damaged retina of flies and mice, platelet-derived growth factor (PDGF)-like signaling induced mesencephalic astrocyte-derived neurotrophic factor (MANF) in innate immune cells. MANF promoted alternative activation of innate immune cells, enhanced neuroprotection and tissue repair, and improved the success of photoreceptor replacement therapies. Thus, immune modulation is required during tissue repair and regeneration. This approach may improve the efficacy of stem-cell-based regenerative therapies.

Wound-Induced Polyploidy Is Required for Tissue Repair

Significance: All organs suffer wounds to some extent during an animal's lifetime and to compensate for cell loss, tissues often rely on cell division. However, many organs are made up of differentiated cells with only a limited capacity to divide. It is not well understood how cells are replaced in the absence of cell division. Recent Advances: Recent studies in the model organism Drosophila melanogaster have proven that wound-induced polyploidy (WIP) is an essential mechanism to replace tissue mass and restore tissue integrity in the absence of cell division. In this repair mechanism, preexisting differentiated cells increase their DNA content and cell size by becoming polyploid. Critical Issues: Cells within mammalian organs such as the liver, heart, and cornea have also been observed to increase their DNA ploidy in response to injury, suggesting that WIP may be an evolutionarily conserved mechanism to compensate for cell loss. Future Directions: The Hippo signal transduction pathway is required for differentiated cells to initiate WIP in Drosophila. Continued studies in Drosophila will help to identify other signaling pathways required for WIP as well as the conserved mechanisms that polyploid cells may play during wound repair in all organisms.

Transition from inflammation to proliferation: a critical step during wound healing

The ability to rapidly restore the integrity of a broken skin barrier is critical and is the ultimate goal of therapies for hard-to-heal-ulcers. Unfortunately effective treatments to enhance healing and reduce scarring are still lacking. A deeper understanding of the physiology of normal repair and of the pathology of delayed healing is a prerequisite for the development of more effective therapeutic interventions. Transition from the inflammatory to the proliferative phase is a key step during healing and accumulating evidence associates a compromised transition with wound healing disorders. Thus, targeting factors that impact this phase transition may offer a rationale for therapeutic development. This review summarizes mechanisms regulating the inflammation-proliferation transition at cellular and molecular levels. We propose that identification of such mechanisms will reveal promising targets for development of more effective therapies.

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.

Transcriptomic response to injury sheds light on the physiological costs of reproduction in ant queens

The trade-off between reproduction and longevity is widespread among multicellular organisms. As an important exception, the reproductive females of perennial social insects (ants, honeybees, termites) are simultaneously highly fertile and very long-lived relative to their nonreproductive nestmates. The observation that increased fecundity is not coupled with decreased lifespan suggests that social insect queens do not have to reallocate resources between reproduction and self-maintenance. If queens have to compensate for the costs of reproduction on the level of the individual, the activation of other energy-demanding physiological processes might force them to reduce the production of eggs. To test this hypothesis in ant queens, we increased immunity costs by injury and measured the effect of this treatment on egg-laying rates and genomewide gene expression. Amputation of both middle legs led to a temporary decrease in egg-laying rates and affected the expression of 947 genes corresponding to 9% of the transcriptome. The changes comprised the upregulation of the immune and wound healing response on the one hand, and the downregulation of germ cell development, central nervous system development and learning ability on the other hand. Injury strongly influenced metabolism by inducing catabolism and repressing amino acid and nitrogen compound metabolism. By comparing our results to similar transcriptomic studies in insects, we found a highly consistent upregulation of immune genes due to sterile and septic wounding. The gene expression changes, complemented by the temporary decline of egg-laying rates, clearly reveal a trade-off between reproduction and the immune response in social insect queens.

Wound-Induced Polyploidization: Regulation by Hippo and JNK Signaling and Conservation in Mammals

Tissue integrity and homeostasis often rely on the proliferation of stem cells or differentiated cells to replace lost, aged, or damaged cells. Recently, we described an alternative source of cell replacement- the expansion of resident, non-dividing diploid cells by wound-induced polyploidization (WIP). Here we show that the magnitude of WIP is proportional to the extent of cell loss using a new semi-automated assay with single cell resolution. Hippo and JNK signaling regulate WIP; unexpectedly however, JNK signaling through AP-1 limits rather than stimulates the level of Yki activation and polyploidization in the Drosophila epidermis. We found that polyploidization also quantitatively compensates for cell loss in a mammalian tissue, mouse corneal endothelium, where increased cell death occurs with age in a mouse model of Fuchs Endothelial Corneal Dystrophy (FECD). Our results suggest that WIP is an evolutionarily conserved homeostatic mechanism that maintains the size and synthetic capacity of adult tissues.

FOXO links wing form polyphenism and wound healing in the brown planthopper, Nilaparvata lugens

Polyphenisms such as wing dimorphisms and caste determination are important in allowing animals to adapt to changing environments. The brown planthopper Nilaparvata lugens, one of the most serious insect agricultural pests, includes two wing forms, the long wing form (macropterous) and the short wing form (brachypterous). Long wings are specialized for migration, while short wings are found in individuals specialized for reproduction. While studying wing form polyphenism in the brown planthopper, we excised single wing pads from 4th instar nymphs in order to preserve transcriptional records to correlate with adult wing form. Surprisingly, we found that excision of one wing pad from a pair of the forewings changed the wing morph of the other wing after development to the adult, resulting in the short wing morph. Further experiments showed that not only excision or slicing of the wing pad, but also needle punctures in the abdomen all caused a significant increase in the proportion of nymphs developing into short winged adults. Thus wounding appears to cause a shift to short wing development. We then tested the transcriptional expression in N. lugens of the transcription factor FOXO, which has been shown to help mediate both wing polyphenism in brown planthoppers and wound healing in mice, after excision of the wing pad. Both NlFOXO and its downstream target Nl4EBP increased significantly after wing pad excision. These results indicate that FOXO mediates both wing development and wound healing in N. lugens, which results in an interesting linkage of these two physiological processes.

JAK/STAT signalling mediates cell survival in response to tissue stress

Tissue homeostasis relies on the ability of tissues to respond to stress. Tissue regeneration and tumour models in Drosophila have shown that JNK is a prominent stress-response pathway promoting injury-induced apoptosis and compensatory proliferation. A central question remaining unanswered is how both responses are balanced by activation of a single pathway. JAK/STAT signalling, a potential JNK target, is implicated in promoting compensatory proliferation. While we observe JAK/STAT activation in imaginal discs upon damage, our data demonstrates that JAK/STAT and its downstream effector Zfh2 promote survival of JNK-signalling cells instead. The JNK component fos and the pro-apoptotic gene hid are regulated in a JAK/STAT-dependent manner. This molecular pathway restrains JNK-induced apoptosis and spatial propagation of JNK-signalling, thereby limiting the extent of tissue damage, as well as facilitating systemic and proliferative responses to injury. We find that the pro-survival function of JAK/STAT also drives tumour growth under conditions of chronic stress. Our study defines JAK/STAT function in tissue stress and illustrates how crosstalk between conserved signalling pathways establishes an intricate equilibrium between proliferation, apoptosis and survival to restore tissue homeostasis.

The Drosophila Chitinase-Like Protein IDGF3 Is Involved in Protection against Nematodes and in Wound Healing

Chitinase-like proteins (CLPs) of the 18 glycosyl hydrolase family retain structural similarity to chitinases but lack enzymatic activity. Although CLPs are upregulated in several human disorders that affect regenerative and inflammatory processes, very little is known about their normal physiological function. We show that an insect CLP (Drosophila imaginal disc growth factor 3, IDGF3) plays an immune-protective role during entomopathogenic nematode (EPN) infections. During these infections, nematodes force their entry into the host via border tissues, thus creating wounds. Whole-genome transcriptional analysis of nematode-infected wild-type and Idgf3 mutant larvae have shown that, in addition to the regulation of genes related to immunity and wound closure, IDGF3 represses Jak/STAT and Wingless signaling. Further experiments have confirmed that IDGF3 has multiple roles in innate immunity. It serves as an essential component required for the formation of hemolymph clots that seal wounds, and Idgf3 mutants display an extended developmental delay during wound healing. Altogether, our findings indicate that vertebrate and invertebrate CLP proteins function in analogous settings and have a broad impact on inflammatory reactions and infections. This opens the way to further genetic analysis of Drosophila IDGF3 and will help to elucidate the exact molecular context of CLP function.

Cellular and molecular mechanisms of repair in acute and chronic wound healing

Summary

A considerable understanding of the fundamental cellular and molecular mechanisms underpinning healthy acute wound healing has been gleaned from studying various animal models, and we are now unravelling the mechanisms that lead to chronic wounds and pathological healing including fibrosis. A small cut will normally heal in days through tight orchestration of cell migration and appropriate levels of inflammation, innervation and angiogenesis. Major surgeries may take several weeks to heal and leave behind a noticeable scar. At the extreme end, chronic wounds – defined as a barrier defect that has not healed in 3 months – have become a major therapeutic challenge throughout the Western world and will only increase as our populations advance in age, and with the increasing incidence of diabetes, obesity and vascular disorders. Here we describe the clinical problems and how, through better dialogue between basic researchers and clinicians, we may extend our current knowledge to enable the development of novel potential therapeutic treatments.

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Wound repair and regeneration: mechanisms, signaling, and translation

The cellular and molecular mechanisms underpinning tissue repair and its failure to heal are still poorly understood, and current therapies are limited. Poor wound healing after trauma, surgery, acute illness, or chronic disease conditions affects millions of people worldwide each year and is the consequence of poorly regulated elements of the healthy tissue repair response, including inflammation, angiogenesis, matrix deposition, and cell recruitment. Failure of one or several of these cellular processes is generally linked to an underlying clinical condition, such as vascular disease, diabetes, or aging, which are all frequently associated with healing pathologies. The search for clinical strategies that might improve the body's natural repair mechanisms will need to be based on a thorough understanding of the basic biology of repair and regeneration. In this review, we highlight emerging concepts in tissue regeneration and repair, and provide some perspectives on how to translate current knowledge into viable clinical approaches for treating patients with wound-healing pathologies.

Cell death: a program to regenerate

Recent studies in Drosophila, Hydra, planarians, zebrafish, mice, indicate that cell death can open paths to regeneration in adult animals. Indeed injury can induce cell death, itself triggering regeneration following an immediate instructive mechanism, whereby the dying cells release signals that induce cellular responses over short and/or long-range distances. Cell death can also provoke a sustained derepressing response through the elimination of cells that suppress regeneration in homeostatic conditions. Whether common properties support what we name "regenerative cell death," is currently unclear. As key parameters, we review here the injury proapoptotic signals, the signals released by the dying cells, the cellular responses, and their respective timing. ROS appears as a common signal triggering cell death through MAPK and/or JNK pathway activation. But the modes of ROS production vary, from a brief pulse upon wounding, to repeated waves as observed in the zebrafish fin where ROS supports two peaks of cell death. Indeed regenerative cell death can be restricted to the injury phase, as in Hydra, Drosophila, or biphasic, immediate, and delayed, as in planarians and zebrafish. The dying cells release in a caspase-dependent manner a variety of signaling molecules, cytokines, growth factors, but also prostaglandins or ATP as recorded in Drosophila, Hydra, mice, and zebrafish, respectively. Interestingly, the ROS-producing cells often resist to cell death, implying a complex paracrine mode of signaling to launch regeneration, involving ROS-producing cells, ROS-sensing cells that release signaling molecules upon caspase activation, and effector cells that respond to these signals by proliferating, migrating, and/or differentiating.

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.