The Toll/NF-κB signaling pathway is required for epidermal wound repair in Drosophila

An amphibian-derived cathelicidin accelerates cutaneous wound healing through its main regulatory effect on phagocytes

Cathelicidins are an important family of antimicrobial peptides (AMPs) involved in the innate immunity in vertebrates. The mammalian cathelicidins have been well characterized, but the relationship between structure and function in amphibian cathelicidins is still not well understood. In this study, a novel 29-residue cathelicidin antimicrobial peptide (BugaCATH) was identified from the skin of Bufo gargarizans. Unlike other AMPs, BugaCATH does not display any direct antimicrobial effects in vitro. However, it effectively promotes full-thickness wound repair in mice. Following injury, BugaCATH initiates and expedites the inflammatory stage by recruiting neutrophils and macrophages to the wound site. BugaCATH not only regulates neutrophil phagocytic activity but also stimulates the generation of cytokines (TNF-α, IL-6, and IL-1β) and chemokines (CXCL1, CXCL2, CCL2, and CCL3) in macrophages and in mice. Furthermore, it promotes macrophage M2 polarization that facilitates the conversion from a pro-inflammatory macrophage-dominated wound environment to an anti-inflammatory one during the mid to late stages, which is crucial for reducing inflammation and effective wound repair. The MAPK (ERK, JNK, and p38) and NF-κB-NLRP3 signaling pathways are involved in the activity. Moreover, BugaCATH directly enhances the migration of keratinocytes and vascular endothelial cells without affecting their proliferation. Notably, BugaCATH significantly improves the proliferation of keratinocytes and endothelial cells in the presence of macrophages. The current study revealed that in addition to proliferation of keratinocytes and endothelial cells, BugaCATH possesses the ability to modulate inflammatory processes during skin injury through its regulatory effect on phagocytes. The combination of these capabilities makes BugaCATH a potent candidate for skin wound therapy.

Toll-like receptor signalling via IRAK4 affects epithelial integrity and tightness through regulation of junctional tension

Toll-like receptors (TLRs) in mammalian systems are well known for their role in innate immunity. In addition, TLRs also fulfil crucial functions outside immunity, including the dorsoventral patterning function of the original Toll receptor in Drosophila and neurogenesis in mice. Recent discoveries in flies suggested key roles for TLRs in epithelial cells in patterning of junctional cytoskeletal activity. Here, we address the function of TLRs and the downstream key signal transduction component IRAK4 in human epithelial cells. Using differentiated human Caco-2 cells as a model for the intestinal epithelium, we show that these cells exhibit baseline TLR signalling, as revealed by p-IRAK4, and that blocking IRAK4 function leads to a loss of epithelial tightness involving key changes at tight and adherens junctions, such as a loss of epithelial tension and changes in junctional actomyosin. Changes upon IRAK-4 inhibition are conserved in human bronchial epithelial cells. Knockdown of IRAK4 and certain TLRs phenocopies the inhibitor treatment. These data suggest a model whereby TLR receptors near epithelial junctions might be involved in a continuous sensing of the epithelial state to promote epithelial tightness and integrity.

Striped Expression of Leucine-Rich Repeat Proteins Coordinates Cell Intercalation and Compartment Boundary Formation in the Early Drosophila Embryo

Planar polarity is a commonly observed phenomenon in which proteins display a consistent asymmetry in their subcellular localization or activity across the plane of a tissue. During animal development, planar polarity is a fundamental mechanism for coordinating the behaviors of groups of cells to achieve anisotropic tissue remodeling, growth, and organization. Therefore, a primary focus of developmental biology research has been to understand the molecular mechanisms underlying planar polarity in a variety of systems to identify conserved principles of tissue organization. In the early Drosophila embryo, the germband neuroectoderm epithelium rapidly doubles in length along the anterior-posterior axis through a process known as convergent extension (CE); it also becomes subdivided into tandem tissue compartments through the formation of compartment boundaries (CBs). Both processes are dependent on the planar polarity of proteins involved in cellular tension and adhesion. The enrichment of actomyosin-based tension and adherens junction-based adhesion at specific cell-cell contacts is required for coordinated cell intercalation, which drives CE, and the creation of highly stable cell-cell contacts at CBs. Recent studies have revealed a system for rapid cellular polarization triggered by the expression of leucine-rich-repeat (LRR) cell-surface proteins in striped patterns. In particular, the non-uniform expression of Toll-2, Toll-6, Toll-8, and Tartan generates local cellular asymmetries that allow cells to distinguish between cell-cell contacts oriented parallel or perpendicular to the anterior-posterior axis. In this review, we discuss (1) the biomechanical underpinnings of CE and CB formation, (2) how the initial symmetry-breaking events of anterior-posterior patterning culminate in planar polarity, and (3) recent advances in understanding the molecular mechanisms downstream of LRR receptors that lead to planar polarized tension and junctional adhesion.

Lipids as a key element of insect defense systems

The relationship between insect pathogenic fungi and their insect hosts is a classic example of a co-evolutionary arms race between pathogen and target host: parasites evolve towards mechanisms that increase their advantage over the host, and the host increasingly strengthens its defenses. The present review summarizes the literature data describing the direct and indirect role of lipids as an important defense mechanism during fungal infection. Insect defense mechanisms comprise anatomical and physiological barriers, and cellular and humoral response mechanisms. The entomopathogenic fungi have the unique ability to digest the insect cuticle by producing hydrolytic enzymes with chitin-, lipo- and proteolytic activity; besides the oral tract, cuticle pays the way for fungal entry within the host. The key factor in insect resistance to fungal infection is the presence of certain types of lipids (free fatty acids, waxes or hydrocarbons) which can promote or inhibit fungal attachment to cuticle, and might also have antifungal activity. Lipids are considered as an important source of energy, and as triglycerides are stored in the fat body, a structure analogous to the liver and adipose tissue in vertebrates. In addition, the fat body plays a key role in innate humoral immunity by producing a range of bactericidal proteins and polypeptides, one of which is lysozyme. Energy derived from lipid metabolism is used by hemocytes to migrate to the site of fungal infection, and for phagocytosis, nodulation and encapsulation. One polyunsaturated fatty acid, arachidonic acid, is used in the synthesis of eicosanoids, which play several crucial roles in insect physiology and immunology. Apolipoprotein III is important compound with antifungal activity, which can modulate insect cellular response and is considered as important signal molecule.

The genetic basis of wing spots in Pieris canidia butterflies

Spots in pierid butterflies and eyespots in nymphalid butterflies are likely non-homologous wing colour pattern elements, yet they share a few features in common. Both develop black scales that depend on the function of the gene spalt, and both might have central signalling cells. This suggests that both pattern elements may be sharing common genetic circuitry. Hundreds of genes have already been associated with the development of nymphalid butterfly eyespot patterns, but the genetic basis of the simpler spot patterns on the wings of pierid butterflies has not been investigated. To facilitate studies of pierid wing patterns, we report a high-quality draft genome assembly for Pieris canidia, the Indian cabbage white. We then conducted transcriptomic analyses of pupal wing tissues sampled from the spot and non-spot regions of P. canidia at 3-6 h post-pupation. A total of 1352 genes were differentially regulated between wing tissues with and without the black spot, including spalt, Krüppel-like factor 10, genes from the Toll, Notch, TGF-β, and FGFR signalling pathways, and several genes involved in the melanin biosynthetic pathway. We identified 14 genes that are up-regulated in both pierid spots and nymphalid eyespots and propose that spots and eyespots share regulatory modules despite their likely independent origins.

The epithelial polarity genes frazzled and GUK-holder adjust morphogen gradients to coordinate changes in cell position with cell fate specification

Morphogenetic gradients specify distinct cell populations within tissues. Originally, morphogens were conceived as substances that act on a static field of cells, yet cells usually move during development. Thus, the way cell fates are defined in moving cells remains a significant and largely unsolved problem. Here, we investigated this issue using spatial referencing of cells and 3D spatial statistics in the Drosophila blastoderm to reveal how cell density responds to morphogenetic activity. We show that the morphogen decapentaplegic (DPP) attracts cells towards its peak levels in the dorsal midline, whereas dorsal (DL) stalls them ventrally. We identified frazzled and GUK-holder as the downstream effectors regulated by these morphogens that constrict cells and provide the mechanical force necessary to draw cells dorsally. Surprisingly, GUKH and FRA modulate the DL and DPP gradient levels and this regulation creates a very precise mechanism of coordinating cell movement and fate specification.

Bioinspired gelatin based sticky hydrogel for diverse surfaces in burn wound care

Proper burn wound management considers patient’s compliance and provides an environment to accelerate wound closure. Sticky hydrogels are conducive to wound management. They can act as a preventive infection patch with controlled drug delivery and diverse surface adherence. A hypothesis-driven investigation explores a bioinspired polydopamine property in a gelatin-based hydrogel (GbH) where polyvinyl alcohol and starch function as hydrogel backbone. The GbH displayed promising physical properties with O–H group rich surface. The GbH was sticky onto dry surfaces (glass, plastic and aluminium) and wet surfaces (pork and chicken). The GbH demonstrated mathematical kinetics for a transdermal formulation, and the in vitro and in vivo toxicity of the GbH on test models confirmed the models’ healthy growth and biocompatibility. The quercetin-loaded GbH showed 45–50% wound contraction on day 4 for second-degree burn wounds in rat models that were equivalent to the silver sulfadiazine treatment group. The estimates for tensile strength, biochemicals, connective tissue markers and NF-κB were restored on day 21 in the GbH treated healed wounds to imitate the normal level of the skin. The bioinspired GbH promotes efficient wound healing of second-degree burn wounds in rat models, indicating its pre-clinical applicability.

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

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

Mechanical Forces Govern Interactions of Host Cells with Intracellular Bacterial Pathogens

To combat infectious diseases, it is important to understand how host cells interact with bacterial pathogens. Signals conveyed from pathogen to host, and vice versa, may be either chemical or mechanical. While the molecular and biochemical basis of host-pathogen interactions has been extensively explored, relatively less is known about mechanical signals and responses in the context of those interactions. Nevertheless, a wide variety of bacterial pathogens appear to have developed mechanisms to alter the cellular biomechanics of their hosts in order to promote their survival and dissemination, and in turn many host responses to infection rely on mechanical alterations in host cells and tissues to limit the spread of infection. In this review, we present recent findings on how mechanical forces generated by host cells can promote or obstruct the dissemination of intracellular bacterial pathogens. In addition, we discuss how in vivo extracellular mechanical signals influence interactions between host cells and intracellular bacterial pathogens. Examples of such signals include shear stresses caused by fluid flow over the surface of cells and variable stiffness of the extracellular matrix on which cells are anchored. We highlight bioengineering-inspired tools and techniques that can be used to measure host cell mechanics during infection. These allow for the interrogation of how mechanical signals can modulate infection alongside biochemical signals. We hope that this review will inspire the microbiology community to embrace those tools in future studies so that host cell biomechanics can be more readily explored in the context of infection studies.

Cell mechanics and cell-cell recognition controls by Toll-like receptors in tissue morphogenesis and homeostasis

Signal transduction by the Toll-like receptors (TLRs) is conserved and essential for innate immunity in metazoans. The founding member of the TLR family, Drosophila Toll-1, was initially identified for its role in dorsoventral axis formation in early embryogenesis. The Drosophila genome encodes nine TLRs that display dynamic expression patterns during development, suggesting their involvement in tissue morphogenesis and homeostasis. Recent progress on the developmental functions of TLRs beyond dorsoventral patterning has revealed not only their diverse functions in various biological processes, but also unprecedented molecular mechanisms in directly regulating cell mechanics and cell-cell recognition independent of the canonical signal transduction pathway involving transcriptional regulation of target genes. In this review, I feature and discuss the non-immune functions of TLRs in the control of epithelial tissue homeostasis, tissue morphogenesis, and cell-cell recognition between cell populations with different cell identities.

Mitochondria Lead the Way: Mitochondrial Dynamics and Function in Cellular Movements in Development and Disease

The dynamics, distribution and activity of subcellular organelles are integral to regulating cell shape changes during various physiological processes such as epithelial cell formation, cell migration and morphogenesis. Mitochondria are famously known as the powerhouse of the cell and play an important role in buffering calcium, releasing reactive oxygen species and key metabolites for various activities in a eukaryotic cell. Mitochondrial dynamics and morphology changes regulate these functions and their regulation is, in turn, crucial for various morphogenetic processes. In this review, we evaluate recent literature which highlights the role of mitochondrial morphology and activity during cell shape changes in epithelial cell formation, cell division, cell migration and tissue morphogenesis during organism development and in disease. In general, we find that mitochondrial shape is regulated for their distribution or translocation to the sites of active cell shape dynamics or morphogenesis. Often, key metabolites released locally and molecules buffered by mitochondria play crucial roles in regulating signaling pathways that motivate changes in cell shape, mitochondrial shape and mitochondrial activity. We conclude that mechanistic analysis of interactions between mitochondrial morphology, activity, signaling pathways and cell shape changes across the various cell and animal-based model systems holds the key to deciphering the common principles for this interaction.

Jumu is required for the activation of JAK/STAT in Drosophila lymph gland development and epidermal wounds

The regulatory mechanism of hematopoiesis and innate immunity in Drosophila is highly similar to that in mammals, and Drosophila has become a suitable model to understand vertebrate hematopoiesis and the immune response. JAK-STAT signaling pathway components are widely conserved during evolution, and contribute to hematopoiesis and multiple tissue damage and immune responses. Here, we demonstrate that Stat92E is widely expressed in the lymph gland, and the loss of jumu inhibits the maintenance of the JAK/STAT pathway in the CZ and MZ but not in the PSC of the lymph gland. Furthermore, we found that clean puncture wounding of the larval epidermis can lead to the activation of JAK/STAT signaling and the generation of lamellocytes, and Jumu is required for the activation of JAK/STAT in response to epidermal wounds.

Extracellular Vesicles (Secretomes) from Human Trophoblasts Promote the Regeneration of Skin Fibroblasts

To date, placental trophoblasts have been of interest in the fields of obstetrics and gynecology, mainly due to their involvement in the formation of a connection between the mother and fetus that aids in placental development and fetal survival. However, the regenerative capacities of trophoblasts for application in regenerative medicine and tissue engineering are poorly understood. Here, we aim to determine the skin regeneration and anti-aging capacities of trophoblast-derived conditioned medium (TB-CM) and exosomes (TB-Exos) using human normal dermal fibroblasts (HNDFs). TB-CM and TB-Exos treatments significantly elevated the migration and proliferation potencies of HNDF cells in a dose- and time-dependent manner. When RNA sequencing (RNA-seq) was used to investigate the mechanism underlying TB-CM-induced cell migration on scratch-wounded HNDFs, the increased expression of genes associated with C-X-C motif ligand (CXCL) chemokines, toll-like receptors, and nuclear factor-kappa B (NF-κB) signaling was observed. Furthermore, treatment of intrinsically/extrinsically senescent HNDFs with TB-CM resulted in an enhanced rejuvenation of HNDFs via both protection and restoration processes. Gene expression of extracellular matrix components in the skin dermis significantly increased in TB-CM- and TB-Exos-treated HNDFs. These components are involved in the TB-CM and Exo-mediated regeneration and anti-aging of HNDFs. Thus, this study demonstrated the regenerative and anti-aging efficacies of trophoblast-derived secretomes, suggesting their potential for use in interventions for skin protection and treatment.

Toll receptors remodel epithelia by directing planar-polarized Src and PI3K activity

Toll-like receptors are essential for animal development and survival, with conserved roles in innate immunity, tissue patterning, and cell behavior. The mechanisms by which Toll receptors signal to the nucleus are well characterized, but how Toll receptors generate rapid, localized signals at the cell membrane to produce acute changes in cell polarity and behavior is not known. We show that Drosophila Toll receptors direct epithelial convergent extension by inducing planar-polarized patterns of Src and PI3-kinase (PI3K) activity. Toll receptors target Src activity to specific sites at the membrane, and Src recruits PI3K to the Toll-2 complex through tyrosine phosphorylation of the Toll-2 cytoplasmic domain. Reducing Src or PI3K activity disrupts planar-polarized myosin assembly, cell intercalation, and convergent extension, whereas constitutive Src activity promotes ectopic PI3K and myosin cortical localization. These results demonstrate that Toll receptors direct cell polarity and behavior by locally mobilizing Src and PI3K activity.

Comparative transcriptome analyses of the Drosophila pupal eye

Tissue function is dependent on correct cellular organization and behavior. As a result, the identification and study of genes that contribute to tissue morphogenesis is of paramount importance to the fields of cell and developmental biology. Many of the genes required for tissue patterning and organization are highly conserved between phyla. This has led to the emergence of several model organisms and developmental systems that are used to study tissue morphogenesis. One such model is the Drosophila melanogaster pupal eye that has a highly stereotyped arrangement of cells. In addition, the pupal eye is postmitotic that allows for the study of tissue morphogenesis independent from any effects of proliferation. While the changes in cell morphology and organization that occur throughout pupal eye development are well documented, less is known about the corresponding transcriptional changes that choreograph these processes. To identify these transcriptional changes, we dissected wild-type Canton S pupal eyes and performed RNA-sequencing. Our analyses identified differential expression of many loci that are documented regulators of pupal eye morphogenesis and contribute to multiple biological processes including signaling, axon projection, adhesion, and cell survival. We also identified differential expression of genes not previously implicated in pupal eye morphogenesis such as components of the Toll pathway, several non-classical cadherins, and components of the muscle sarcomere, which could suggest these loci function as novel patterning factors.

Multiscale In Vivo Imaging of Collective Cell Migration in Drosophila Embryos

Coordinated cell movements drive embryonic development and tissue repair, and can also spread disease. Time-lapse microscopy is an integral part in the study of the cell biology of collective cell movements. Advances in imaging techniques enable monitoring dynamic cellular and molecular events in real time within living animals. Here, we demonstrate the use of spinning disk confocal microscopy to investigate coordinated cell movements and epithelial-to-mesenchymal-like transitions during embryonic wound closure in Drosophila. We describe image-based metrics to quantify the efficiency of collective cell migration. Finally, we show the application of super-resolution radial fluctuation microscopy to obtain multidimensional, super-resolution images of protrusive activity in collectively moving cells in vivo. Together, the methods presented here constitute a toolkit for the modern analysis of collective cell migration in living animals.

Drp1-mediated mitochondrial fission regulates calcium and F-actin dynamics during wound healing

Mitochondria adapt to cellular needs by changes in morphology through fusion and fission events, referred to as mitochondrial dynamics. Mitochondrial function and morphology are intimately connected and the dysregulation of mitochondrial dynamics is linked to several human diseases. In this work, we investigated the role of mitochondrial dynamics in wound healing in the Drosophila embryonic epidermis. Mutants for mitochondrial fusion and fission proteins fail to close their wounds, indicating that the regulation of mitochondrial dynamics is required for wound healing. By live-imaging, we found that loss of function of the mitochondrial fission protein Dynamin-related protein 1 (Drp1) compromises the increase of cytosolic and mitochondrial calcium upon wounding and leads to reduced reactive oxygen species (ROS) production and F-actin defects at the wound edge, culminating in wound healing impairment. Our results highlight a new role for mitochondrial dynamics in the regulation of calcium, ROS and F-actin during epithelial repair.

Cellular, molecular, and biophysical control of epithelial cell intercalation

Convergent extension is a conserved mechanism for elongating tissues. In the Drosophila embryo, convergent extension is driven by planar polarized cell intercalation and is a paradigm for understanding the cellular, molecular, and biophysical mechanisms that establish tissue structure. Studies of convergent extension in Drosophila have provided key insights into the force-generating molecules that promote convergent extension in epithelial tissues, as well as the global systems of spatial information that systematically organize these cell behaviors. A general framework has emerged in which asymmetrically localized proteins involved in cytoskeletal tension and cell adhesion direct oriented cell movements, and spatial signals provided by the Toll, Tartan, and Teneurin receptor families break planar symmetry to establish and coordinate planar cell polarity throughout the tissue. In this chapter, we describe the cellular, molecular, and biophysical mechanisms that regulate cell intercalation in the Drosophila embryo, and discuss how research in this system has revealed conserved biological principles that control the organization of multicellular tissues and animal body plans.

Casein kinase 1α decreases β-catenin levels at adherens junctions to facilitate wound closure in Drosophila larvae

Skin wound repair is essential to restore barrier function and prevent infection after tissue damage. Wound-edge epidermal cells migrate as a sheet to close the wound. However, it is still unclear how cell-cell junctions are regulated during wound closure (WC). To study this, we examined adherens junctions during WC in Drosophila larvae. β-Catenin is reduced at the lateral cell-cell junctions of wound-edge epidermal cells in the early healing stages. Destruction complex components, including Ck1α, GSK3β and β-TrCP, suppress β-catenin levels in the larval epidermis. Tissue-specific RNAi targeting these genes also caused severe WC defects. The Ck1αRNAi -induced WC defect is related to adherens junctions because loss of either β-catenin or E-cadherin significantly rescued this WC defect. In contrast, TCFRNAi does not rescue the Ck1αRNAi -induced WC defect, suggesting that Wnt signaling is not related to this defect. Direct overexpression of β-catenin recapitulates most of the features of Ck1α reduction during wounding. Finally, loss of Ck1α also blocked junctional E-cadherin reduction around the wound. Our results suggest that Ck1α and the destruction complex locally regulate cell adhesion to facilitate efficient wound repair.

An LRR Receptor-Teneurin System Directs Planar Polarity at Compartment Boundaries

Epithelial cells dynamically self-organize in response to extracellular spatial cues relayed by cell-surface receptors. During convergent extension in Drosophila, Toll-related receptors direct planar polarized cell rearrangements that elongate the head-to-tail axis. However, many cells establish polarity in the absence of Toll receptor activity, indicating the presence of additional spatial cues. Here we demonstrate that the leucine-rich-repeat receptor Tartan and the teneurin Ten-m provide critical polarity signals at epithelial compartment boundaries. The Tartan and Ten-m extracellular domains interact in vitro, and Tartan promotes Ten-m localization to compartment boundaries in vivo. We show that Tartan and Ten-m are necessary for the planar polarity and organization of compartment boundary cells. Moreover, ectopic stripes of Tartan and Ten-m are sufficient to induce myosin accumulation at stripe boundaries. These results demonstrate that the Tartan/Ten-m and Toll receptor systems together create a high-resolution network of spatial cues that guides cell behavior during convergent extension.

Forceful closure: cytoskeletal networks in embryonic wound repair

Embryonic tissues heal wounds rapidly and without scarring, in a process conserved across species and driven by collective cell movements. The mechanisms of coordinated cell movement during embryonic wound closure also drive tissue development and cancer metastasis; therefore, embryonic wound repair has received considerable attention as a model of collective cell migration. During wound closure, a supracellular actomyosin cable at the wound edge coordinates cells, while actin-based protrusions contribute to cell crawling and seamless wound healing. Other cytoskeletal networks are reorganized during wound repair: microtubules extend into protrusions and along cell-cell boundaries as cells stretch into damaged regions, septins accumulate at the wound margin, and intermediate filaments become polarized in the cells adjacent to the wound. Thus, diverse cytoskeletal networks work in concert to maintain tissue structure, while also driving and organizing cell movements to promote rapid repair. Understanding the signals that coordinate the dynamics of different cytoskeletal networks, and how adhesions between cells or with the extracellular matrix integrate forces across cells, will be important to elucidate the mechanisms of efficient embryonic wound healing and may have far-reaching implications for developmental and cancer cell biology.

Mechanical Force-Driven Adherens Junction Remodeling and Epithelial Dynamics

During epithelial tissue development, repair, and homeostasis, adherens junctions (AJs) ensure intercellular adhesion and tissue integrity while allowing for cell and tissue dynamics. Mechanical forces play critical roles in AJs' composition and dynamics. Recent findings highlight that beyond a well-established role in reinforcing cell-cell adhesion, AJ mechanosensitivity promotes junctional remodeling and polarization, thereby regulating critical processes such as cell intercalation, division, and collective migration. Here, we provide an integrated view of mechanosensing mechanisms that regulate cell-cell contact composition, geometry, and integrity under tension and highlight pivotal roles for mechanosensitive AJ remodeling in preserving epithelial integrity and sustaining tissue dynamics.

Principles of Actomyosin Regulation In Vivo

The actomyosin cytoskeleton is responsible for most force-driven processes in cells and tissues. How it assembles into the necessary structures at the right time and place is an important question. Here, we focus on molecular mechanisms of actomyosin regulation recently elucidated in animal models, and highlight several common principles that emerge. The architecture of the actomyosin network - an important determinant of its function - results from actin polymerization, crosslinking and turnover, localized myosin activation, and contractility-driven self-organization. Spatiotemporal regulation is achieved by tissue-specific expression and subcellular localization of Rho GTPase regulators. Subcellular anchor points of actomyosin structures control the outcome of their contraction, and molecular feedback mechanisms dictate whether they are transient, cyclic, or persistent.

Occluding junctions as novel regulators of tissue mechanics during wound repair

Simple epithelial repair is mediated by the contraction of an actomyosin cable and cellular rearrangements at the wound edge. Carvalho et al. show that occluding junctions are required for epithelial repair by regulating these cellular rearrangements and tissue mechanical properties.

In epithelial tissues, cells tightly connect to each other through cell–cell junctions, but they also present the remarkable capacity of reorganizing themselves without compromising tissue integrity. Upon injury, simple epithelia efficiently resolve small lesions through the action of actin cytoskeleton contractile structures at the wound edge and cellular rearrangements. However, the underlying mechanisms and how they cooperate are still poorly understood. In this study, we combine live imaging and theoretical modeling to reveal a novel and indispensable role for occluding junctions (OJs) in this process. We demonstrate that OJ loss of function leads to defects in wound-closure dynamics: instead of contracting, wounds dramatically increase their area. OJ mutants exhibit phenotypes in cell shape, cellular rearrangements, and mechanical properties as well as in actin cytoskeleton dynamics at the wound edge. We propose that OJs are essential for wound closure by impacting on epithelial mechanics at the tissue level, which in turn is crucial for correct regulation of the cellular events occurring at the wound edge.

The Imaginal Disc Growth Factors 2 and 3 participate in the Drosophila response to nematode infection

The Drosophila imaginal disc growth factors (IDGFs) induce the proliferation of imaginal disc cells and terminate cell proliferation at the end of larval development. However, the participation of Idgf-encoding genes in other physiological processes of Drosophila including the immune response to infection is not fully understood. Here, we show the contribution of Idgf2 and Idgf3 in the Drosophila response to infection with Steinernema carpocapsae nematodes carrying or lacking their mutualistic Xenorhabdus nematophila bacteria (symbiotic or axenic nematodes, respectively). We find that Idgf2 and Idgf3 are upregulated in Drosophila larvae infected with symbiotic or axenic Steinernema and inactivation of Idgf2 confers a survival advantage to Drosophila larvae against axenic nematodes. Inactivation of Idgf2 induces the Imd and Jak/Stat pathways, whereas inactivation of Idgf3 induces the Imd, Toll and Jak/Stat pathways. We also show that inactivation of the Imd pathway receptor PGRP-LE upregulates Idgf2 against Steinernema nematode infection. Finally, we demonstrate that inactivation of Idgf3 induces the recruitment of larval haemocytes in response to Steinernema. Our results indicate that Idgf2 and Idgf3 might be involved in different yet crucial immune functions in the Drosophila antinematode immune response. Similar findings will promote the development of new targets for species-specific pest control strategies.

Novel role for Grainy head in the regulation of cytoskeletal and junctional dynamics during epithelial repair

Tissue repair is critical for the maintenance of epithelial integrity and permeability. Simple epithelial repair relies on a combination of collective cell movements and the action of a contractile actomyosin cable at the wound edge that together promote the fast and efficient closure of tissue discontinuities. The Grainy head family of transcription factors (Grh in flies; GRHL1-GRHL3 in mammals) are essential proteins that have been implicated both in the development and repair of epithelia. However, the genes and the molecular mechanisms that it controls remain poorly understood. Here, we show that Grh knockdown disrupts actomyosin dynamics upon injury of the Drosophila pupa epithelial tissue. This leads to the formation of an ectopic actomyosin cable away from the wound edge and impaired wound closure. We also uncovered that E-Cadherin is downregulated in the Grh-depleted tissue around the wound, likely as a consequence of Dorsal (an NF-κB protein) misregulation, which also affects actomyosin cable formation. Our work highlights the importance of Grh as a stress response factor and its central role in the maintenance of epithelial characteristics necessary for tissue repair through regulating cytoskeleton and E-Cadherin dynamics.

NF‑κB is a key modulator in the signaling pathway of Borrelia burgdorferi BmpA‑induced inflammatory chemokines in murine microglia BV2 cells

Lyme disease, caused by the bacterial spirochete Borrelia burgdorferi, is a tick‑borne zoonosis. Lyme neuroborreliosis is a principal manifestation of Lyme disease and its pathogenesis remains incompletely understood. Recent studies have demonstrated that Borrelia burgdorferi lipoproteins caused similar inflammatory effects as exhibited in Lyme neuroborreliosis. Basic membrane protein A (BmpA) is one of the dominant lipoproteins in the Borrelia burgdorferi membrane. In addition, nuclear factor κ‑B (NF‑κB) modulates the regulation of gene transcription associated with immunity and inflammation; however, in unstimulated cells, NF‑κB is combined with the inhibitor of NF‑κB (IκB‑β). Therefore, it was hypothesized that NF‑κB may be associated with BmpA‑induced inflammation and the occurrence of Lyme neuroborreliosis. Therefore, the aim of the present study was to investigate the role that NF‑κB serves in the signaling pathway of rBmpA‑induced inflammatory chemokines. The present study measured the expression levels of NF‑κB, IκB‑β and inflammatory chemokines following recombinant BmpA (rBmpA) stimulation of murine microglia BV2 cells. Following stimulation with rBmpA, concentrations of pro‑inflammatory cytokines including C‑X‑C motif chemokine 2, C‑C motif chemokine (CCL) 5 and CCL22 were determined by ELISA analysis. Reverse transcription‑quantitative polymerase chain reaction and western blotting were used to detect the expression levels of NF‑κB p65 and IκB‑β. The data demonstrated that concentrations of these chemokines in cell supernatants increased significantly following rBmpA stimulation. NF‑κB was overexpressed, but IκB‑β expression was significantly decreased. In conclusion, these results suggested that NF‑κB serves an important stimulatory role in the signaling pathway of rBmpA‑induced inflammatory chemokines in BV2 cells.

The Toll pathway underlies host sexual dimorphism in resistance to both Gram-negative and Gram-positive bacteria in mated Drosophila

BACKGROUND

Host sexual dimorphism is being increasingly recognized to generate strong differences in the outcome of infectious disease, but the mechanisms underlying immunological differences between males and females remain poorly characterized. Here, we used Drosophila melanogaster to assess and dissect sexual dimorphism in the innate response to systemic bacterial infection.

RESULTS

We demonstrated sexual dimorphism in susceptibility to infection by a broad spectrum of Gram-positive and Gram-negative bacteria. We found that both virgin and mated females are more susceptible than mated males to most, but not all, infections. We investigated in more detail the lower resistance of females to infection with Providencia rettgeri, a Gram-negative bacterium that naturally infects D. melanogaster. We found that females have a higher number of phagocytes than males and that ablation of hemocytes does not eliminate the dimorphism in resistance to P. rettgeri, so the observed dimorphism does not stem from differences in the cellular response. The Imd pathway is critical for the production of antimicrobial peptides in response to Gram-negative bacteria, but mutants for Imd signaling continued to exhibit dimorphism even though both sexes showed strongly reduced resistance. Instead, we found that the Toll pathway is responsible for the dimorphism in resistance. The Toll pathway is dimorphic in genome-wide constitutive gene expression and in induced response to infection. Toll signaling is dimorphic in both constitutive signaling and in induced activation in response to P. rettgeri infection. The dimorphism in pathway activation can be specifically attributed to Persephone-mediated immune stimulation, by which the Toll pathway is triggered in response to pathogen-derived virulence factors. We additionally found that, in absence of Toll signaling, males become more susceptible than females to the Gram-positive Enterococcus faecalis. This reversal in susceptibility between male and female Toll pathway mutants compared to wildtype hosts highlights the key role of the Toll pathway in D. melanogaster sexual dimorphism in resistance to infection.

CONCLUSION

Altogether, our data demonstrate that Toll pathway activity differs between male and female D. melanogaster in response to bacterial infection, thus identifying innate immune signaling as a determinant of sexual immune dimorphism.

In vivo imaging of epithelial wound healing in the cnidarian Clytia hemisphaerica demonstrates early evolution of purse string and cell crawling closure mechanisms

Background

All animals have mechanisms for healing damage to the epithelial sheets that cover the body and line internal cavities. Epithelial wounds heal either by cells crawling over the wound gap, by contraction of a super-cellular actin cable (“purse string”) that surrounds the wound, or some combination of the two mechanisms. Both cell crawling and purse string closure of epithelial wounds are widely observed across vertebrates and invertebrates, suggesting early evolution of these mechanisms. Cnidarians evolved ~600 million years ago and are considered a sister group to the Bilateria. They have been much studied for their tremendous regenerative potential, but epithelial wound healing has not been characterized in detail. Conserved elements of wound healing in bilaterians and cnidarians would suggest an evolutionary origin in a common ancestor. Here we test this idea by characterizing epithelial wound healing in live medusae of Clytia hemisphaerica.

Results

We identified cell crawling and purse string-mediated mechanisms of healing in Clytia epithelium that appear highly analogous of those seen in higher animals, suggesting that these mechanisms may have emerged in a common ancestor. Interestingly, we found that epithelial wound healing in Clytia is 75 to >600 times faster than in cultured cells or embryos of other animals previously studied, suggesting that Clytia may provide valuable clues about optimized healing efficiency. Finally, in Clytia, we show that damage to the basement membrane in a wound gap causes a rapid shift between the cell crawling and purse string mechanisms for wound closure. This is consistent with work in other systems showing that cells marginal to a wound choose between a super-cellular actin cable or lamellipodia formation to close wounds, and suggests a mechanism underlying this decision.

Conclusions

1. Cell crawling and purse string mechanisms of epithelial wound healing likely evolved before the divergence of Cnidaria from the bilaterian lineage ~ 600mya 2. In Clytia, the choice between cell crawling and purse string mechanisms of wound healing depends on interactions between the epithelial cells and the basement membrane.

Electronic supplementary material

The online version of this article (10.1186/s12861-017-0160-2) contains supplementary material, which is available to authorized users.

Wound healing, calcium signaling, and other novel pathways are associated with the formation of butterfly eyespots

Background

One hypothesis surrounding the origin of novel traits is that they originate from the co-option of pre-existing genes or larger gene regulatory networks into novel developmental contexts. Insights into a trait’s evolutionary origins can, thus, be gained via identification of the genes underlying trait development, and exploring whether those genes also function in other developmental contexts. Here we investigate the set of genes associated with the development of eyespot color patterns, a trait that originated once within the Nymphalid family of butterflies. Although several genes associated with eyespot development have been identified, the eyespot gene regulatory network remains largely unknown.

Results

In this study, next-generation sequencing and transcriptome analyses were used to identify a large set of genes associated with eyespot development of Bicyclus anynana butterflies, at 3-6 h after pupation, prior to the differentiation of the color rings. Eyespot-associated genes were identified by comparing the transcriptomes of homologous micro-dissected wing tissues that either develop or do not develop eyespots in wild-type and a mutant line of butterflies, Spotty, with extra eyespots. Overall, 186 genes were significantly up and down-regulated in wing tissues that develop eyespots compared to wing tissues that do not. Many of the differentially expressed genes have yet to be annotated. New signaling pathways, including the Toll, Fibroblast Growth Factor (FGF), extracellular signal–regulated kinase (ERK) and/or Jun N-terminal kinase (JNK) signaling pathways are associated for the first time with eyespot development. In addition, several genes involved in wound healing and calcium signaling were also found to be associated with eyespots.

Conclusions

Overall, this study provides the identity of many new genes and signaling pathways associated with eyespots, and suggests that the ancient wound healing gene regulatory network may have been co-opted to cells at the center of the pattern to aid in eyespot origins. New transcription factors that may be providing different identities to distinct wing sectors, and genes with sexually dimorphic expression in the eyespots were also identified.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4175-7) contains supplementary material, which is available to authorized users.

Trends in tissue repair and regeneration

The 6th EMBO conference on the Molecular and Cellular Basis of Regeneration and Tissue Repair took place in Paestum (Italy) on the 17th-21st September, 2016. The 160 scientists who attended discussed the importance of cellular and tissue plasticity, biophysical aspects of regeneration, the diverse roles of injury-induced immune responses, strategies to reactivate regeneration in mammals, links between regeneration and ageing, and the impact of non-mammalian models on regenerative medicine.

The Immune Phenotype of Three Drosophila Leukemia Models

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

Coordinating cell movements in vivo: junctional and cytoskeletal dynamics lead the way

Collective cell movements drive embryonic development and tissue repair, and can cause disease. However, the mechanisms that coordinate the migration of groups of cells in vivo are unclear. Cells generate, transmit and sense mechanical forces to align their movements. Therefore, the machinery used by cells to generate force (cytoskeleton) and to transmit and sense mechanical signals (cell-cell adhesion) is critical for collective movement. Here, we review the components and organization of the cytoskeletal and cell-cell adhesive machineries, and how they are organized to promote collective cell movements in living animals. We discuss the signals that orchestrate molecular rearrangements necessary for coordinated cell motility, and we provide specific examples of movements both in the plane of the tissue (wound healing) and perpendicular to that plane (apical constriction).

Three-tier regulation of cell number plasticity by neurotrophins and Tolls in Drosophila

A three-tier mechanism involving distinct neurotrophin family ligand forms, different Toll receptors, and different adaptors regulates both cell survival and death. This rich mechanism confers cell number plasticity and could underlie structural plasticity in the nervous system and structural integrity, homeostasis, and regeneration in wider contexts.

Cell number plasticity is coupled to circuitry in the nervous system, adjusting cell mass to functional requirements. In mammals, this is achieved by neurotrophin (NT) ligands, which promote cell survival via their Trk and p75^NTR receptors and cell death via p75^NTR and Sortilin. Drosophila NTs (DNTs) bind Toll receptors instead to promote neuronal survival, but whether they can also regulate cell death is unknown. In this study, we show that DNTs and Tolls can switch from promoting cell survival to death in the central nervous system (CNS) via a three-tier mechanism. First, DNT cleavage patterns result in alternative signaling outcomes. Second, different Tolls can preferentially promote cell survival or death. Third, distinct adaptors downstream of Tolls can drive either apoptosis or cell survival. Toll-6 promotes cell survival via MyD88–NF-κB and cell death via Wek-Sarm-JNK. The distribution of adaptors changes in space and time and may segregate to distinct neural circuits. This novel mechanism for CNS cell plasticity may operate in wider contexts.

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.

MicroRNA-3570 Modulates the NF-κB Pathway in Teleost Fish by Targeting MyD88

The inflammatory response, a protective process to clear detrimental stimuli, constitutes the defense against infectious pathogens. However, excessive inflammation disrupts immune homeostasis, which may induce autoimmune and inflammatory diseases. In this study, we report that microRNA (miR)-3570 plays a negative role in the bacteria-induced inflammatory response of miiuy croaker. Upregulation of miR-3570 by Vibrio anguillarum and LPS inhibits LPS-induced inflammatory cytokine production, thus avoiding an excessive inflammation response. Evidence showed that miR-3570 targets MyD88 and posttranscriptionally downregulates its expression. Overexpression of miR-3570 in macrophages suppresses the expression of MyD88, as well as its downstream signaling of IL-1R-associated kinases 1 and 4 and TNFR-associated factor 6. These results suggest that miR-3570 plays a regulatory in the bacteria-induced inflammatory response through the MyD88-mediated NF-κB signaling pathway by targeting MyD88.

Impaired Epidermal to Dendritic T Cell Signaling Slows Wound Repair in Aged Skin

Aged skin heals wounds poorly, increasing susceptibility to infections. Restoring homeostasis after wounding requires the coordinated actions of epidermal and immune cells. Here we find that both intrinsic defects and communication with immune cells are impaired in aged keratinocytes, diminishing their efficiency in restoring the skin barrier after wounding. At the wound-edge, aged keratinocytes display reduced proliferation and migration. They also exhibit a dampened ability to transcriptionally activate epithelial-immune crosstalk regulators, including a failure to properly activate/maintain dendritic epithelial T cells (DETCs), which promote re-epithelialization following injury. Probing mechanism, we find that aged keratinocytes near the wound edge don't efficiently upregulate Skints or activate STAT3. Notably, when epidermal Stat3, Skints, or DETCs are silenced in young skin, re-epithelialization following wounding is perturbed. These findings underscore epithelial-immune crosstalk perturbations in general, and Skints in particular, as critical mediators in the age-related decline in wound-repair.

Polarized E-cadherin endocytosis directs actomyosin remodeling during embryonic wound repair

Clathrin, dynamin, and ARF6 accumulate around wounds in Drosophila embryos in a calcium- and actomyosin-dependent manner and drive polarized E-cadherin endocytosis, which is necessary for actomyosin remodeling during wound repair.

Embryonic epithelia have a remarkable ability to rapidly repair wounds. A supracellular actomyosin cable around the wound coordinates cellular movements and promotes wound closure. Actomyosin cable formation is accompanied by junctional rearrangements at the wound margin. We used in vivo time-lapse quantitative microscopy to show that clathrin, dynamin, and the ADP-ribosylation factor 6, three components of the endocytic machinery, accumulate around wounds in Drosophila melanogaster embryos in a process that requires calcium signaling and actomyosin contractility. Blocking endocytosis with pharmacological or genetic approaches disrupted wound repair. The defect in wound closure was accompanied by impaired removal of E-cadherin from the wound edge and defective actomyosin cable assembly. E-cadherin overexpression also resulted in reduced actin accumulation around wounds and slower wound closure. Reducing E-cadherin levels in embryos in which endocytosis was blocked rescued actin localization to the wound margin. Our results demonstrate a central role for endocytosis in wound healing and indicate that polarized E-cadherin endocytosis is necessary for actomyosin remodeling during embryonic wound repair.

Endocytosis-dependent coordination of multiple actin regulators is required for wound healing

The ability to heal wounds efficiently is essential for life. After wounding of an epithelium, the cells bordering the wound form dynamic actin protrusions and/or a contractile actomyosin cable, and these actin structures drive wound closure. Despite their importance in wound healing, the molecular mechanisms that regulate the assembly of these actin structures at wound edges are not well understood. In this paper, using Drosophila melanogaster embryos, we demonstrate that Diaphanous, SCAR, and WASp play distinct but overlapping roles in regulating actin assembly during wound healing. Moreover, we show that endocytosis is essential for wound edge actin assembly and wound closure. We identify adherens junctions (AJs) as a key target of endocytosis during wound healing and propose that endocytic remodeling of AJs is required to form "signaling centers" along the wound edge that control actin assembly. We conclude that coordination of actin assembly, AJ remodeling, and membrane traffic is required for the construction of a motile leading edge during wound healing.