Transglutaminase-catalyzed protein-protein cross-linking suppresses the activity of the NF-κB-like transcription factor relish

Translocation of gut bacteria promotes tumor-associated mortality by inducing immune-activated renal damage

Paraneoplastic syndrome represents severe and complex systemic clinical symptoms manifesting in multiple organs of cancer patients, but its cause and cellular underpinnings remain little explored. In this study, establishing a Drosophila model of paraneoplastic syndrome triggered by tumor transplantation, we found that the innate immune response, initiated by translocated commensal bacteria from a compromised intestine, significantly contributes to reduced lifespan in tumor-bearing hosts. Our data identify the renal system as a central hub of this paraneoplastic syndrome model, wherein the pericardial nephrocytes undergo severe damage due to an elevated immune response triggered by gut dysbiosis and bacterial translocation. This innate immune response-induced nephrocyte damage is a major contributor to reduced longevity in tumor-bearing hosts, as blocking the NF-kB/Imd pathway in nephrocytes or removing gut bacteria via germ-free derivation or antibiotic treatment ameliorates nephrocyte deterioration and extends the lifespan of tumor-bearing flies. Consistently, treatment with a detoxifying drug also extended the lifespan of the tumor hosts. Our findings highlight a critical role of the gut-kidney axis in the paraneoplastic complications observed in cancer-bearing flies, suggesting potential therapeutic targets for mitigating similar complications in cancer patients.

Effects of two transglutaminases on innate immune responses in the oriental armyworm, Mythimna separata

Transglutaminase (TGase) is a key enzyme that mediates hemolymph coagulation and is thought to contribute to the elimination of pathogenic microorganisms in invertebrates. The objective of this study was to elucidate the involvement of TGase in insect immune responses via functional analysis of this enzyme in the oriental armyworm, Mythimna separata, using recombinant proteins and RNA interference technique. We identified two TGase genes, mystgase1 and mystgase2, in Mythimna separata and found that both genes are expressed in all surveyed tissues in M. separata larvae. Significant changes were induced in hemocytes following Escherichia coli injection. Injection of Gram-positive bacteria (Micrococcus luteus) and Gram-negative bacteria (Escherichia coli and Serratia marcescens) into larvae triggered a time-specific induction of both mystgase1 and mystgase2 in hemocytes. Recombinant MysTGase1 and MysTGase2 proteins bound to both E. coli and M. luteus, localizing within bacterial clusters and resulting in agglutination in a Ca2+-dependent manner. The hemocytes of larvae injected with recombinant MysTGase1 or MysTGase2 exhibited enhanced phagocytic ability against E. coli, improved in vivo bacterial clearance, and increased resistance to S. marcescens, decreasing larval mortality rate. Conversely, RNA interference targeting mystgase1 or mystgase2 significantly reduced hemocyte phagocytic capability, decreased bacterial clearance, and increased susceptibility to S. marcescens infection, thereby increasing larval mortality rate. The findings of this study are anticipated to expand our understanding of the function of TGases within insect immune responses and may contribute to developing new pest control strategies.

Activation of the immune deficiency pathway (IMD) reduces the mosquito heart rate via a nitric oxide-based mechanism

The immune deficiency pathway (IMD) is an important component of the antibacterial, antimalarial and antiviral response in mosquitoes. The IMD pathway also drives the infection induced migration of hemocytes to the heart. During an infection, periostial hemocytes kill pathogens in areas of high hemolymph flow and produce nitric oxide that reduces the heart rate. Here, we investigated the consequences of repressing the IMD pathway by silencing the transcription factor, rel2, or activating the pathway by silencing the negative regulator, caspar, in Anopheles gambiae. In uninfected mosquitoes, repression of the IMD pathway does not affect the circulatory system. However, activating the IMD pathway decreases the heart rate, and this correlates with increased transcription and activity of nitric oxide synthase (NOS), but not increased transcription of the lysozymes, LysC1 or LysC2. In infected mosquitoes, however, activation of the IMD pathway does not affect the heart rate but repression of the pathway decreases the heart rate. This latter phenotype correlates with increased transcription and activity of nitric oxide synthase, which is likely due to an increase in infection intensity. In conclusion, we demonstrate that a major immune signaling pathway that regulates periostial hemocyte aggregation, the IMD pathway, reduces the heart rate via a nitric oxide-based mechanism.

Effects of osteoblast-derived extracellular vesicles on aggressiveness, redox status and mitochondrial bioenergetics of MNNG/HOS osteosarcoma cells

Osteosarcoma is the most common primary bone malignancy. The crosstalk between osteosarcoma and the surrounding tumour microenvironment (TME) drives key events that lead to metastasization, one of the main obstacles for definitive cure of most malignancies. Extracellular vesicles (EVs), lipid bilayer nanoparticles used by cells for intercellular communication, are emerging as critical biological mediators that permit the interplay between neoplasms and the tumour microenvironment, modulating re-wiring of energy metabolism and redox homeostatic processes. We previously showed that EVs derived from the human osteosarcoma cells influence bone cells, including osteoblasts. We here investigated whether the opposite could also be true, studying how osteoblast-derived EVs (OB-EVs) could alter tumour phenotype, mitochondrial energy metabolism, redox status and oxidative damage in MNNG/HOS osteosarcoma cells.These were treated with EVs obtained from mouse primary osteoblasts, and the following endpoints were investigated: i) cell viability and proliferation; ii) apoptosis; iii) migration and invasive capacity; iv) stemness features; v) mitochondrial function and energy metabolism; vi) redox status, antioxidant capacity and oxidative molecular damage. OB-EVs decreased MNNG/HOS metabolic activity and viability, which however was not accompanied by impaired proliferation nor by increased apoptosis, with respect to control. In addition, OB-EV-treated cells exhibited a significant reduction of motility and in vitro invasion as compared to untreated cells. Although the antioxidant N-acetyl-L-cysteine reverted the cytotoxic effect of OB-EVs, no evidence of oxidative stress was observed in treated cells. However, the redox balance of glutathione was significantly shifted towards a pro-oxidant state, even though the major antioxidant enzymatic protection did not respond to the pro-oxidant challenge. We did not find strong evidence of mitochondrial involvement or major energy metabolic switches induced by OB-EVs, but a trend of reduction in seahorse assay basal respiration was observed, suggesting that OB-EVs could represent a mild metabolic challenge for osteosarcoma cells. In summary, our findings suggest that OB-EVs could serve as important means through which TME and osteosarcoma core cross-communicate. For the first time, we proved that OB-EVs reduced osteosarcoma cells' aggressiveness and viability through redox-dependent signalling pathways, even though mitochondrial dynamics and energy metabolism did not appear as processes critically needed to respond to OB-EVs.

New insights into the hemolymph coagulation cascade of horseshoe crabs initiated by autocatalytic activation of a lipopolysaccharide-sensitive zymogen

The concept of a chain reaction of proteolytic activation of multiple protease zymogens was first proposed to explain the blood clotting system in mammals as an enzyme cascade. In multicellular organisms, similar enzyme cascades are widely present in signal transduction and amplification systems. The initiation step of the blood coagulation cascade often consists of autocatalytic activation of the corresponding zymogens located on the surfaces of host- or foreign-derived substances at injured sites. However, the molecular mechanism underlying the concept of autocatalytic activation remains speculative. In this review, we will focus on the autocatalytic activation of prochelicerase C on the surface of lipopolysaccharide as a potential initiator of hemolymph coagulation in horseshoe crabs. Prochelicerase C is presumed to have evolved from a common complement factor in Chelicerata; thus, evolutionary insights into the hemolymph coagulation and complement systems in horseshoe crabs will also be discussed.

Drosophila as a Model Organism to Study Basic Mechanisms of Longevity

The spatio-temporal regulation of gene expression determines the fate and function of various cells and tissues and, as a consequence, the correct development and functioning of complex organisms. Certain mechanisms of gene activity regulation provide adequate cell responses to changes in environmental factors. Aside from gene expression disorders that lead to various pathologies, alterations of expression of particular genes were shown to significantly decrease or increase the lifespan in a wide range of organisms from yeast to human. Drosophila fruit fly is an ideal model system to explore mechanisms of longevity and aging due to low cost, easy handling and maintenance, large number of progeny per adult, short life cycle and lifespan, relatively low number of paralogous genes, high evolutionary conservation of epigenetic mechanisms and signalling pathways, and availability of a wide range of tools to modulate gene expression in vivo. Here, we focus on the organization of the evolutionarily conserved signaling pathways whose components significantly influence the aging process and on the interconnections of these pathways with gene expression regulation.

Dynamic Regulation of NF-κB Response in Innate Immunity: The Case of the IMD Pathway in Drosophila

Metazoans have developed strategies to protect themselves from pathogenic attack. These preserved mechanisms constitute the immune system, composed of innate and adaptive responses. Among the two kinds, the innate immune system involves the activation of a fast response. NF-κB signaling pathways are activated during infections and lead to the expression of timely-controlled immune response genes. However, activation of NF-κB pathways can be deleterious when uncontrolled. Their regulation is necessary to prevent the development of inflammatory diseases or cancers. The similarity of the NF-κB pathways mediating immune mechanisms in insects and mammals makes Drosophila melanogaster a suitable model for studying the innate immune response and learning general mechanisms that are also relevant for humans. In this review, we summarize what is known about the dynamic regulation of the central NF-κB-pathways and go into detail on the molecular level of the IMD pathway. We report on the role of the nuclear protein Akirin in the regulation of the NF-κB Relish immune response. The use of the Drosophila model allows the understanding of the fine-tuned regulation of this central NF-κB pathway.

The immune deficiency and c-Jun N-terminal kinase pathways drive the functional integration of the immune and circulatory systems of mosquitoes

The immune and circulatory systems of animals are functionally integrated. In mammals, the spleen and lymph nodes filter and destroy microbes circulating in the blood and lymph, respectively. In insects, immune cells that surround the heart valves (ostia), called periostial haemocytes, destroy pathogens in the areas of the body that experience the swiftest haemolymph (blood) flow. An infection recruits additional periostial haemocytes, amplifying heart-associated immune responses. Although the structural mechanics of periostial haemocyte aggregation have been defined, the genetic factors that regulate this process remain less understood. Here, we conducted RNA sequencing in the African malaria mosquito, Anopheles gambiae, and discovered that an infection upregulates multiple components of the immune deficiency (IMD) and c-Jun N-terminal kinase (JNK) pathways in the heart with periostial haemocytes. This upregulation is greater in the heart with periostial haemocytes than in the circulating haemocytes or the entire abdomen. RNA interference-based knockdown then showed that the IMD and JNK pathways drive periostial haemocyte aggregation and alter phagocytosis and melanization on the heart, thereby demonstrating that these pathways regulate the functional integration between the immune and circulatory systems. Understanding how insects fight infection lays the foundation for novel strategies that could protect beneficial insects and harm detrimental ones.

Hazard Assessment of the Effects of Acute and Chronic Exposure to Permethrin, Copper Hydroxide, Acephate, and Validamycin Nanopesticides on the Physiology of Drosophila: Novel Insights into the Cellular Internalization and Biological Effects

New insights into the interactions between nanopesticides and edible plants are required in order to elucidate their impacts on human health and agriculture. Nanopesticides include formulations consisting of organic/inorganic nanoparticles. Drosophila melanogaster has become a powerful model in genetic research thanks to its genetic similarity to mammals. This project mainly aimed to generate new evidence for the toxic/genotoxic properties of different nanopesticides (a nanoemulsion (permethrin nanopesticides, 20 ± 5 nm), an inorganic nanoparticle as an active ingredient (copper(II) hydroxide [Cu(OH)₂] nanopesticides, 15 ± 6 nm), a polymer-based nanopesticide (acephate nanopesticides, 55 ± 25 nm), and an inorganic nanoparticle associated with an organic active ingredient (validamycin nanopesticides, 1177 ± 220 nm)) and their microparticulate forms (i.e., permethrin, copper(II) sulfate pentahydrate (CuSO₄·5H₂O), acephate, and validamycin) widely used against agricultural pests, while also showing the merits of using Drosophila—a non-target in vivo eukaryotic model organism—in nanogenotoxicology studies. Significant biological effects were noted at the highest doses of permethrin (0.06 and 0.1 mM), permethrin nanopesticides (1 and 2.5 mM), CuSO₄·5H₂O (1 and 5 mM), acephate and acephate nanopesticides (1 and 5 mM, respectively), and validamycin and validamycin nanopesticides (1 and 2.5 mM, respectively). The results demonstrating the toxic/genotoxic potential of these nanopesticides through their impact on cellular internalization and gene expression represent significant contributions to future nanogenotoxicology studies.

Current Status of Immune Deficiency Pathway in Tenebrio molitor Innate Immunity

Yellow mealworm (Tenebrio molitor) is a highly beneficial beetle that serves as an excellent source of edible protein as well as a practical study model. Therefore, studying its immune system is important. Like in other insects, the innate immune response effected through antimicrobial peptides production provides the most critical defense armory in T. molitor. Immune deficiency (Imd) signaling is one of the major pathways involved in the humoral innate immune response in this beetle. However, the nature of the molecules involved in the signaling cascade of the Imd pathway, from recognition to the production of final effectors, and their mechanism of action are yet to be elucidated in T. molitor model. In this review, we present a general overview of the current literature available on the Imd signaling pathway and its identified interaction partners in T. molitor.

Silencing Transglutaminase Genes TGase2 and TGase3 Has Infection-Dependent Effects on the Heart Rate of the Mosquito Anopheles gambiae

Transglutaminases are pleiotropic enzymes that in mosquitoes participate in the formation of the mating plug and the wound-induced antimalarial response. Moreover, one transglutaminase, TGase3, negatively regulates the infection-induced aggregation of hemocytes on the heart. Given that TGase3 is an inhibitor of periostial hemocyte aggregation, we used RNAi-based gene silencing followed by intravital video imaging to scrutinize whether any of the three transglutaminases encoded in the genome of the mosquito, Anopheles gambiae, play a role in modulating the heart rate of uninfected and infected mosquitoes. Initially, we confirmed that an infection decreases the heart rate. Then, we uncovered that silencing TGase1 does not impact heart physiology, but silencing TGase2 results in a constant heart rate regardless of infection status, eliminating the infection-induced decrease in the heart rate. Finally, silencing TGase3 decreases the heart rate in uninfected mosquitoes but increases the heart rate in infected mosquitoes. We conclude that TGase2 and TGase3 modulate heart physiology and demonstrate that factors not classically associated with insect circulatory physiology are involved in the functional integration of the immune and circulatory systems of mosquitoes.

Mutations of γCOP Gene Disturb Drosophila melanogaster Innate Immune Response to Pseudomonas aeruginosa

Drosophila melanogaster (the fruit fly) is a valuable experimental platform for modeling host–pathogen interactions. It is also commonly used to define innate immunity pathways and to understand the mechanisms of both host tolerance to commensal microbiota and response to pathogenic agents. Herein, we investigate how the host response to bacterial infection is mirrored in the expression of genes of Imd and Toll pathways when D. melanogaster strains with different γCOP genetic backgrounds are infected with Pseudomonas aeruginosa ATCC 27853. Using microarray technology, we have interrogated the whole-body transcriptome of infected versus uninfected fruit fly males with three specific genotypes, namely wild-type Oregon, γCOP^S057302/TM6B and γCOP^14a/γCOP^14a. While the expression of genes pertaining to Imd and Toll is not significantly modulated by P. aeruginosa infection in Oregon males, many of the components of these cascades are up- or downregulated in both infected and uninfected γCOP^S057302/TM6B and γCOP^14a/γCOP^14a males. Thus, our results suggest that a γCOP genetic background modulates the gene expression profiles of Imd and Toll cascades involved in the innate immune response of D. melanogaster, inducing the occurrence of immunological dysfunctions in γCOP mutants.

Rhodnius prolixus Hemolymph Immuno-Physiology: Deciphering the Systemic Immune Response Triggered by Trypanosoma cruzi Establishment in the Vector Using Quantitative Proteomics

Understanding the development of Trypanosoma cruzi within the triatomine vector at the molecular level should provide novel targets for interrupting parasitic life cycle and affect vectorial competence. The aim of the current study is to provide new insights into triatomines immunology through the characterization of the hemolymph proteome of Rhodnius prolixus, a major Chagas disease vector, in order to gain an overview of its immune physiology. Surprisingly, proteomics investigation of the immunomodulation of T. cruzi-infected blood reveals that the parasite triggers an early systemic response in the hemolymph. The analysis of the expression profiles of hemolymph proteins from 6 h to 24 h allowed the identification of a broad range of immune proteins expressed already in the early hours post-blood-feeding regardless of the presence of the parasite, ready to mount a rapid response exemplified by the significant phenol oxidase activation. Nevertheless, we have also observed a remarkable induction of the immune response triggered by an rpPGRP-LC and the overexpression of defensins 6 h post-T. cruzi infection. Moreover, we have identified novel proteins with immune properties such as the putative c1q-like protein and the immunoglobulin I-set domain-containing protein, which have never been described in triatomines and could play a role in T. cruzi recognition. Twelve proteins with unknown function are modulated by the presence of T. cruzi in the hemolymph. Determining the function of these parasite-induced proteins represents an exciting challenge for increasing our knowledge about the diversity of the immune response from the universal one studied in holometabolous insects. This will provide us with clear answers for misunderstood mechanisms in host-parasite interaction, leading to the development of new generation strategies to control vector populations and pathogen transmission.

Transglutaminase 3 negatively regulates immune responses on the heart of the mosquito, Anopheles gambiae

The immune and circulatory systems of insects are functionally integrated. Following infection, immune cells called hemocytes aggregate around the ostia (valves) of the heart. An earlier RNA sequencing project in the African malaria mosquito, Anopheles gambiae, revealed that the heart-associated hemocytes, called periostial hemocytes, express transglutaminases more highly than hemocytes elsewhere in the body. Here, we further queried the expression of these transglutaminase genes and examined whether they play a role in heart-associated immune responses. We found that, in the whole body, injury upregulates the expression of TGase2, whereas infection upregulates TGase1, TGase2 and TGase3. RNAi-based knockdown of TGase1 and TGase2 did not alter periostial hemocyte aggregation, but knockdown of TGase3 increased the number of periostial hemocytes during the early stages of infection and the sequestration of melanin by periostial hemocytes during the later stages of infection. In uninfected mosquitoes, knockdown of TGase3 also slightly reduced the number of sessile hemocytes outside of the periostial regions. Taken altogether, these data show that TGase3 negatively regulates periostial hemocyte aggregation, and we hypothesize that this occurs by negatively regulating the immune deficiency pathway and by altering hemocyte adhesion. In conclusion, TGase3 is involved in the functional integration between the immune and circulatory systems of mosquitoes.

Dual oxidase enables insect gut symbiosis by mediating respiratory network formation

Most animals harbor a gut microbiota that consists of potentially pathogenic, commensal, and mutualistic microorganisms. Dual oxidase (Duox) is a well described enzyme involved in gut mucosal immunity by the production of reactive oxygen species (ROS) that antagonizes pathogenic bacteria and maintains gut homeostasis in insects. However, despite its nonspecific harmful activity on microorganisms, little is known about the role of Duox in the maintenance of mutualistic gut symbionts. Here we show that, in the bean bug Riptortus pedestris, Duox-dependent ROS did not directly contribute to epithelial immunity in the midgut in response to its mutualistic gut symbiont, Burkholderia insecticola Instead, we found that the expression of Duox is tracheae-specific and its down-regulation by RNAi results in the loss of dityrosine cross-links in the tracheal protein matrix and a collapse of the respiratory system. We further demonstrated that the establishment of symbiosis is a strong oxygen sink triggering the formation of an extensive network of tracheae enveloping the midgut symbiotic organ as well as other organs, and that tracheal breakdown by Duox RNAi provokes a disruption of the gut symbiosis. Down-regulation of the hypoxia-responsive transcription factor Sima or the regulators of tracheae formation Trachealess and Branchless produces similar phenotypes. Thus, in addition to known roles in immunity and in the formation of dityrosine networks in diverse extracellular matrices, Duox is also a crucial enzyme for tracheal integrity, which is crucial to sustain mutualistic symbionts and gut homeostasis. We expect that this is a conserved function in insects.

Regulators and signalling in insect antimicrobial innate immunity: Functional molecules and cellular pathways

Insects possess an immune system that protects them from attacks by various pathogenic microorganisms that would otherwise threaten their survival. Immune mechanisms may deal directly with the pathogens by eliminating them from the host organism or disarm them by suppressing the synthesis of toxins and virulence factors that promote the invasion and destructive action of the intruder within the host. Insects have been established as outstanding models for studying immune system regulation because innate immunity can be explored as an integrated system at the level of the whole organism. Innate immunity in insects consists of basal immunity that controls the constitutive synthesis of effector molecules such as antimicrobial peptides, and inducible immunity that is activated after detection of a microbe or its product(s). Activation and coordination of innate immune defenses in insects involve evolutionary conserved immune factors. Previous research in insects has led to the identification and characterization of distinct immune signalling pathways that modulate the response to microbial infections. This work has not only advanced the field of insect immunology, but it has also rekindled interest in the innate immune system of mammals. Here we review the current knowledge on key molecular components of insect immunity and discuss the opportunities they present for confronting infectious diseases in humans.

Transglutaminase Type 2 regulates the Wnt/β-catenin pathway in vertebrates

TG2 is a multifunctional enzyme involved in several cellular processes and has emerging as a potential regulator of gene expression. In this regard, we have recently shown that TG2 is able to activate HSF1, the master transcriptional regulator of the stress-responsive genes; however, its effect on the overall gene expression remains unclear. To address this point, we analyzed, by RNA-seq, the effect of TG2 on the overall transcriptome as well as we characterized the TG2 interactome in the nucleus. The data obtained from these omics approaches reveal that TG2 markedly influences the overall cellular transcriptome profile and specifically the Wnt and HSF1 pathways. In particular, its ablation leads to a drastic downregulation of many key members of these pathways. Interestingly, we found that key components of the Wnt/β-catenin pathway are also downregulated in cells lacking HSF1, thus confirming that TG2 regulates the HSF1 and this axis controls the Wnt signaling. Mechanistic studies revealed that TG2 can regulate the Wnt pathway by physically interacts with β-catenin and its nuclear interactome includes several proteins known to be involved in the regulation of the Wnt signaling. In order to verify whether this effect is playing a role in vivo, we ablated TG2 in Danio rerio. Our data show that the zebrafish lacking TG2 cannot complete the development and their death is associated with an evident downregulation of the Wnt pathway and a defective heat-shock response. Our findings show for the first time that TG2 is essential for the correct embryonal development of lower vertebrates, and its action is mediated by the Wnt/HSF1 axis.

ppGpp functions as an alarmone in metazoa

Guanosine 3′,5′-bis(pyrophosphate) (ppGpp) functions as a second messenger in bacteria to adjust their physiology in response to environmental changes. In recent years, the ppGpp-specific hydrolase, metazoan SpoT homolog-1 (Mesh1), was shown to have important roles for growth under nutrient deficiency in Drosophila melanogaster. Curiously, however, ppGpp has never been detected in animal cells, and therefore the physiological relevance of this molecule, if any, in metazoans has not been established. Here, we report the detection of ppGpp in Drosophila and human cells and demonstrate that ppGpp accumulation induces metabolic changes, cell death, and eventually lethality in Drosophila. Our results provide the evidence of the existence and function of the ppGpp-dependent stringent response in animals.

Ito et al. succeed in detecting guanosine tetraphosphate (ppGpp) in measurable levels in metazoan, specifically in Drosophila. They further demonstrate that the ppGpp-specific hydrolase, metazoan SpoT homolog-1 (Mesh1), is necessary, at least in certain conditions, to maintain low ppGpp levels, hence providing insights into the role of Mesh1 as a ppGpp hydrolase in vivo.

Beyond Host Defense: Deregulation of Drosophila Immunity and Age-Dependent Neurodegeneration

Age-dependent neurodegenerative disorders are a set of diseases that affect millions of individuals worldwide. Apart from a small subset that are the result of well-defined inherited autosomal dominant gene mutations (e.g., those encoding the β-amyloid precursor protein and presenilins), our understanding of the genetic network that underscores their pathology, remains scarce. Genome-wide association studies (GWAS) especially in Alzheimer's disease patients and research in Parkinson's disease have implicated inflammation and the innate immune response as risk factors. However, even if GWAS etiology points toward innate immunity, untangling cause, and consequence is a challenging task. Specifically, it is not clear whether predisposition to de-regulated immunity causes an inadequate response to protein aggregation (such as amyloid or α-synuclein) or is the direct cause of this aggregation. Given the evolutionary conservation of the innate immune response in Drosophila and humans, unraveling whether hyperactive immune response in glia have a protective or pathological role in the brain could be a potential strategy in combating age-related neurological diseases.

Genome Damage Sensing Leads to Tissue Homeostasis in Drosophila

DNA repair is a critical cellular process required for the maintenance of genomic integrity. It is now well appreciated that cells employ several DNA repair pathways to take care of distinct types of DNA damage. It is also well known that a cascade of signals namely DNA damage response or DDR is activated in response to DNA damage which comprise cellular responses, such as cell cycle arrest, DNA repair and cell death, if the damage is irreparable. There is also emerging literature suggesting a cross-talk between DNA damage signaling and several signaling networks within a cell. Moreover, cell death players themselves are also well known to engage in processes outside their canonical function of apoptosis. This chapter attempts to build a link between DNA damage, DDR and signaling from the studies mainly conducted in mammals and Drosophila model systems, with a special emphasis on their relevance in overall tissue homeostasis and development.

Spen limits intestinal stem cell self-renewal

Precise regulation of stem cell self-renewal and differentiation properties is essential for tissue homeostasis. Using the adult Drosophila intestine to study molecular mechanisms controlling stem cell properties, we identify the gene split-ends (spen) in a genetic screen as a novel regulator of intestinal stem cell fate (ISC). Spen family genes encode conserved RNA recognition motif-containing proteins that are reported to have roles in RNA splicing and transcriptional regulation. We demonstrate that spen acts at multiple points in the ISC lineage with an ISC-intrinsic function in controlling early commitment events of the stem cells and functions in terminally differentiated cells to further limit the proliferation of ISCs. Using two-color cell sorting of stem cells and their daughters, we characterize spen-dependent changes in RNA abundance and exon usage and find potential key regulators downstream of spen. Our work identifies spen as an important regulator of adult stem cells in the Drosophila intestine, provides new insight to Spen-family protein functions, and may also shed light on Spen's mode of action in other developmental contexts.

Back to homeostasis: Negative regulation of NF-κB immune signaling in insects

Maintenance of homeostasis requires prompt activation and down-regulation of immune signaling pathways. This review attempts to summarize our current knowledge regarding the negative regulation of two NF-κB signaling pathways in insects, Toll and IMD pathway, which are mostly essential for host defense against bacteria and fungus. Various types of negative regulators and their mechanisms are discussed here with the emphasis on the prominent roles of ubiquitination. The counterbalance between these two pathways, the crosstalk with other physiological pathways, and the difference in their repertoires of negative regulators are also discussed.

Beyond immunity: The Imd pathway as a coordinator of host defense, organismal physiology and behavior

The humoral arm of host defense in Drosophila relies on two evolutionarily conserved NFκB signaling cascades, the Toll and the immune deficiency (Imd) pathways. The Imd signaling pathway senses and neutralizes Gram-negative bacteria. Its activity is tightly adjusted, allowing the host to simultaneously prevent infection by pathogenic bacteria and tolerate beneficial gut microbiota. Over-activation of Imd signaling is detrimental at least in part by causing gut dysbiosis that further exacerbates intestinal pathologies. Furthermore, it is increasingly recognized that the Imd pathway or its components also play non-immune roles. In this review, we summarize recent advances in Imd signal transduction, discuss the gut-microbiota interactions mediated by Imd signaling, and finally elaborate on its diverse physiological functions beyond immunity. Understanding the multifaceted physiological outputs of Imd activation will help integrate its immune role into the regulation of whole organismal physiology.

NF-κB Immunity in the Brain Determines Fly Lifespan in Healthy Aging and Age-Related Neurodegeneration

During aging, innate immunity progresses to a chronically active state. However, what distinguishes those that “age well” from those developing age-related neurological conditions is unclear. We used Drosophila to explore the cost of immunity in the aging brain. We show that mutations in intracellular negative regulators of the IMD/NF-κB pathway predisposed flies to toxic levels of antimicrobial peptides, resulting in early locomotor defects, extensive neurodegeneration, and reduced lifespan. These phenotypes were rescued when immunity was suppressed in glia. In healthy flies, suppressing immunity in glial cells resulted in increased adipokinetic hormonal signaling with high nutrient levels in later life and an extension of active lifespan. Thus, when levels of IMD/NF-κB deviate from normal, two mechanisms are at play: lower levels derepress an immune-endocrine axis, which mobilizes nutrients, leading to lifespan extension, whereas higher levels increase antimicrobial peptides, causing neurodegeneration. Immunity in the fly brain is therefore a key lifespan determinant.

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Constitutive immunity predisposes to short lifespan with severe neurodegeneration

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Blocking constitutive immunity in glia rescues predisposed flies

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Suppression of immunity in glia of healthy flies triggers adipokinetic signaling

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This immune-endocrine axis mobilizes nutrients and extends active lifespan

Drosophila TG-A transglutaminase is secreted via an unconventional Golgi-independent mechanism involving exosomes and two types of fatty acylations

Transglutaminases (TGs) play essential intracellular and extracellular roles by covalently cross-linking many proteins. Drosophila TG is encoded by one gene and has two alternative splicing-derived isoforms, TG-A and TG-B, which contain distinct N-terminal 46- and 38-amino acid sequences, respectively. The TGs identified to date do not have a typical endoplasmic reticulum (ER)-signal peptide, and the molecular mechanisms of their secretion under physiologic conditions are unclear. Immunocytochemistry revealed that TG-A localizes to multivesicular-like structures, whereas TG-B localizes to the cytosol. We also found that TG-A, but not TG-B, was modified concomitantly by N-myristoylation and S-palmitoylation, and N-myristoylation was a pre-requisite for S-palmitoylation. Moreover, TG-A, but not TG-B, was secreted in response to calcium signaling induced by Ca²⁺ ionophores and uracil, a pathogenic bacteria-derived substance. Brefeldin A and monensin, inhibitors of the ER/Golgi-mediated conventional pathway, did not suppress TG-A secretion, whereas inhibition of S-palmitoylation by 2-bromopalmitate blocked TG-A secretion. Ultracentrifugation, electron microscopy analyses, and treatments with inhibitors of multivesicular body formation revealed that TG-A was secreted via exosomes together with co-transfected mammalian CD63, an exosomal marker, and the secreted TG-A was taken up by other cells. The 8-residue N-terminal fragment of TG-A containing the fatty acylation sites was both necessary and sufficient for the exosome-dependent secretion of TG-A. In conclusion, TG-A is secreted through an unconventional ER/Golgi-independent pathway involving two types of fatty acylations and exosomes.

Transglutaminase-catalyzed incorporation of polyamines masks the DNA-binding region of the transcription factor Relish

In Drosophila, the final immune deficiency (IMD) pathway-dependent signal is transmitted through proteolytic conversion of the nuclear factor-κB (NF-κB)-like transcription factor Relish to the active N-terminal fragment Relish-N. Relish-N is then translocated from the cytosol into the nucleus for the expression of IMD-controlled genes. We previously demonstrated that transglutaminase (TG) suppresses the IMD pathway by polymerizing Relish-N to inhibit its nuclear translocation. Conversely, we also demonstrated that orally ingested synthetic amines, such as monodansylcadaverine (DCA) and biotin-labeled pentylamine, are TG-dependently incorporated into Relish-N, causing the nuclear translocation of modified Relish-N in gut epithelial cells. It remains unclear, however, whether polyamine-containing Relish-N retains transcriptional activity. Here, we used mass spectrometry analysis of a recombinant Relish-N modified with DCA by TG activity after proteolytic digestion and show that the DCA-modified Gln residues are located in the DNA-binding region of Relish-N. TG-catalyzed DCA incorporation inhibited binding of Relish-N to the Rel-responsive element in the NF-κB-binding DNA sequence. Subcellular fractionation of TG-expressing Drosophila S2 cells indicated that TG was localized in both the cytosol and nucleus. Of note, natural polyamines, including spermidine and spermine, competitively inhibited TG-dependent DCA incorporation into Relish-N. Moreover, in vivo experiments demonstrated that Relish-N was modified by spermine and that this modification reduced transcription of IMD pathway-controlled cecropin A1 and diptericin genes. These findings suggest that intracellular TG regulates Relish-N-mediated transcriptional activity by incorporating polyamines into Relish-N and via protein-protein cross-linking.

Arthropod Innate Immune Systems and Vector-Borne Diseases

Arthropods, especially ticks and mosquitoes, are the vectors for a number of parasitic and viral human diseases, including malaria, sleeping sickness, Dengue, and Zika, yet arthropods show tremendous individual variation in their capacity to transmit disease. A key factor in this capacity is the group of genetically encoded immune factors that counteract infection by the pathogen. Arthropod-specific pattern recognition receptors and protease cascades detect and respond to infection. Proteins such as antimicrobial peptides, thioester-containing proteins, and transglutaminases effect responses such as lysis, phagocytosis, melanization, and agglutination. Effector responses are initiated by damage signals such as reactive oxygen species signaling from epithelial cells and recognized by cell surface receptors on hemocytes. Antiviral immunity is primarily mediated by siRNA pathways but coupled with interferon-like signaling, antimicrobial peptides, and thioester-containing proteins. Molecular mechanisms of immunity are closely linked to related traits of longevity and fertility, and arthropods have the capacity for innate immunological memory. Advances in understanding vector immunity can be leveraged to develop novel control strategies for reducing the rate of transmission of both ancient and emerging threats to global health.

Transglutaminase-2 is Involved in Cell Apoptosis of Osteosarcoma Cell Line U2OS Under Hypoxia Condition

Osteosarcoma is the most common type of solid bone cancer, which is the second leading cause of cancer-related death. Hypoxia is an ordinary phenomenon in solid tumor tissues and can induce cell apoptosis but the specific molecular mechanism remains unclear. In this study, we explored the effect and the molecular mechanism of Transglutaminase 2 (TG2) on cell apoptosis in osteosarcoma U2OS cells under hypoxia. We found the enzymatic activity of TG2 is significantly increased and the expression of TG2 is remarkably up-regulated under hypoxia condition. Cell apoptotic rate is markedly increased upon knockdown of TG2 by siRNA under hypoxia. We further investigated the mechanism of cell apoptosis and found Bax protein is significantly increased after depletion of TG2 under hypoxia. Moreover, our data also show that cytochrome C (Cyt C) is significantly increased in cytoplasm and markedly decreased in mitochondria of U2OS cells after depletion of TG2 under hypoxia. Our results suggest that TG2 can inhibit tumor cell apoptosis through down-regulation of Bax and prevention of release Cyt C from mitochondria into cytoplasm.

Bacteria sensing mechanisms in Drosophila gut: Local and systemic consequences

All insects are colonized by microorganisms on their exoskeleton, their gut and even in some cases within their own somatic and germ line cells. This microbiota that can represent up to a few percent of the insect biomass may have a pervasive impact on many aspects of insect biology including physiology, nutrient acquisition, ageing, behaviour and resistance to infection. Mainly through ingestion of contaminated food, the mouth-gut axis represents the first and principal access of external bacteria to the host. Soon after ingestion, the feeding insect needs to rapidly and accurately identify the ingested microbes and decide whether to preserve them if beneficial or neutral, or to eliminate them if potentially harmful. We will review here the recent data acquired in Drosophila on the mechanisms that invertebrate enterocytes rely on to detect the presence of bacteria in the gut. We will compare these modes of bacteria sensing to those in other immune competent tissues and try to rationalize differences that may exist. We will also analyse the physiological consequences of bacteria detection not only locally for the gut itself but also for remote tissues. Finally, we will describe the physiological disorders that can occur due to inaccurate bacteria identification by the gut epithelium.

RNA Interference Directed against the Transglutaminase Gene Triggers Dysbiosis of Gut Microbiota in Drosophila

We recently reported that transglutaminase (TG) suppresses immune deficiency pathway-controlled antimicrobial peptides (IMD-AMPs), thereby conferring immune tolerance to gut microbes, and that RNAi of the TG gene in flies decreases the lifespan compared with non-TG-RNAi flies. Here, analysis of the bacterial composition of the Drosophila gut by next-generation sequencing revealed that gut microbiota comprising one dominant genus of Acetobacter in non-TG-RNAi flies was shifted to that comprising two dominant genera of Acetobacter and Providencia in TG-RNAi flies. Four bacterial strains, including Acetobacter persici SK1 and Acetobacter indonesiensis SK2, Lactobacillus pentosus SK3, and Providencia rettgeri SK4, were isolated from the midgut of TG-RNAi flies. SK1 exhibited the highest resistance to the IMD-AMPs Cecropin A1 and Diptericin among the isolated bacteria. In contrast, SK4 exhibited considerably lower resistance against Cecropin A1, whereas SK4 exhibited high resistance to hypochlorous acid. The resistance of strains SK1-4 against IMD-AMPs in in vitro assays could not explain the shift of the microbiota in the gut of TG-RNAi flies. The lifespan was reduced in gnotobiotic flies that ingested both SK4 and SK1, concomitant with the production of reactive oxygen species and apoptosis in the midgut, whereas the survival rate was not altered in gnotobiotic flies that mono-ingested either SK4 or SK1. Interestingly, significant amounts of reactive oxygen species were detected in the midgut of gnotobiotic flies that ingested SK4 and SK2, concomitant with no significant apoptosis in the midgut. In gnotobiotic flies that co-ingested SK4 and SK1, an additional unknown factor(s) may be required to cause midgut apoptosis.

YHV-responsive gene expression under the influence of PmRelish regulation

In animals, infection by Gram-negative bacteria and certain viruses activates the Imd signaling pathway wherein the a NF-κB transcription factor, Relish, is a key regulatory protein for the synthesis of antimicrobial proteins. Infection by yellow head virus (YHV) activates the Imd pathway. To investigate the expression of genes involved in YHV infection and under the influence of PmRelish regulation, RNA interference and suppression subtractive hybridization (SSH) are employed. The genes in forward library expressed in shrimp after YHV infection and under the activity of PmRelish were obtained by subtracting the cDNAs from YHV-infected and PmRelish-knockdown shrimp with cDNAs from YHV-infected shrimp. Opposite subtraction gave a reverse library whereby an alternative set of genes under YHV infection and no PmRelish expression were obtained. Nucleotide sequences of 252 and 99 cDNA clones from the forward and reverse libraries, respectively, were obtained and annotated through blast search against the GenBank sequences. Genes involved in defense and homeostasis were abundant in both libraries, 31% and 23% in the forward and reverse libraries, respectively. They were predominantly antimicrobial proteins, proteinases and proteinase inhibitors. The expression of antimicrobial protein genes, ALFPm3, crustinPm1, penaeidin3 and penaeidin5 were tested under PmRelish silencing and Gram-negative bacterium Vibrio harveyi infection. Together with the results using YHV infection previously reported, the expression of penaeidin5 and also penaeidin3 but not ALFPm3 and crustinPm1 were under the regulation of PmRelish in the Imd pathway.

Crosslinking of a Peritrophic Matrix Protein Protects Gut Epithelia from Bacterial Exotoxins

Transglutaminase (TG) catalyzes protein-protein crosslinking, which has important and diverse roles in vertebrates and invertebrates. Here we demonstrate that Drosophila TG crosslinks drosocrystallin, a peritrophic matrix protein, to form a stable fiber structure on the gut peritrophic matrix. RNA interference (RNAi) of the TG gene was highly lethal in flies and induced apoptosis of gut epithelial cells after oral infection with Pseudomonas entomophila. Moreover, AprA, a metalloprotease secreted by P. entomophila, digested non-crosslinked drosocrystallin fibers, but not drosocrystallin fibers crosslinked by TG. In vitro experiments using recombinant drosocrystallin and monalysin proteins demonstrated that monalysin, a pore-forming exotoxin of P. entomophila, was adsorbed on the crosslinked drosocrystallin fibers in the presence of P. entomophila culture supernatant. In addition, gut-specific TG-RNAi flies had a shorter lifespan than control flies after ingesting P. entomophila, whereas the lifespan after ingesting AprA-knockout P. entomophila was at control levels. We conclude that drosocrystallin fibers crosslinked by TG, but not non-crosslinked drosocrystallin fibers, form an important physical barrier against exotoxins of invading pathogenic microbes.

The Role of the Phylogenetically Conserved Cochaperone Protein Droj2/DNAJA3 in NF-κB Signaling

The NF-κB pathway is a phylogenetically conserved signaling pathway with a central role in inflammatory and immune responses. Here we demonstrate that a cochaperone protein, Droj2/DNAJA3, is involved in the activation of canonical NF-κB signaling in flies and in human cultured cells. Overexpression of Droj2 induced the expression of an antimicrobial peptide in Drosophila. Conversely, Droj2 knockdown resulted in reduced expression of antimicrobial peptides and higher susceptibility to Gram-negative bacterial infection in flies. Similarly, Toll-like receptor-stimulated IκB phosphorylation and NF-κB activation were suppressed by DNAJA3 knockdown in HEK293 cells. IκB kinase overexpression-induced NF-κB phosphorylation was also compromised in DNAJA3 knockdown cells. Our study reveals a novel conserved regulator of the NF-κB pathway acting at the level of IκB phosphorylation.

What RNAi screens in model organisms revealed about microbicidal response in mammals?

The strategies evolved by pathogens to infect hosts and the mechanisms used by the host to eliminate intruders are highly complex. Because several biological pathways and processes are conserved across model organisms, these organisms have been used for many years to elucidate and understand the mechanisms of the host-pathogen relationship and particularly to unravel the molecular processes enacted by the host to kill pathogens. The emergence of RNA interference (RNAi) and the ability to apply it toward studies in model organisms have allowed a breakthrough in the elucidation of host-pathogen interactions. The aim of this mini-review is to highlight and describe recent breakthroughs in the field of host-pathogen interactions using RNAi screens of model organisms. We will focus specifically on the model organisms Drosophila melanogaster, Caenorhabditis elegans, and Danio rerio. Moreover, a recent study examining the immune system of planarian will be discussed.

Inhibition of transglutaminase exacerbates polyglutamine-induced neurotoxicity by increasing the aggregation of mutant ataxin-3 in an SCA3 Drosophila model

Transglutaminases (TGs) comprise a family of Ca(2+)-dependent enzymes that catalyze protein cross-linking, which include nine family members in humans but only a single homolog in Drosophila with three conserved domains. Drosophila Tg plays important roles in cuticle morphogenesis, hemolymph clotting, and innate immunity. Mammalian tissue TG (TG2) is involved in polyglutamine diseases (polyQ diseases), and TG6 has been identified as a causative gene of a novel spinocerebellar ataxia, SCA35. Using a well-established SCA3 fly model, we found that RNA interference-mediated suppression of Tg aggravated polyQ-induced neurodegenerative phenotypes. The administration of cystamine, a known effective Tg inhibitor, enhanced ommatidial degeneration in SCA3 flies. We also demonstrated that the aggregates of pathogenic ataxin-3 increased greatly, when the Tg activity was repressed. These findings indicate that Tg is crucial for polyQ-induced neurotoxicity because Tg ablation resulted in more severe neurodegeneration due to the elevated accumulation of insoluble ataxin-3 complexes in the SCA3 Drosophila model.

Dihydroisoxazole inhibitors of Anopheles gambiae seminal transglutaminase AgTG3

BACKGROUND

Current vector-based malaria control strategies are threatened by the rise of biochemical and behavioural resistance in mosquitoes. Researching mosquito traits of immunity and fertility is required to find potential targets for new vector control strategies. The seminal transglutaminase AgTG3 coagulates male Anopheles gambiae seminal fluids, forming a 'mating plug' that is required for male reproductive success. Inhibitors of AgTG3 can be useful both as chemical probes of A. gambiae reproductive biology and may further the development of new chemosterilants for mosquito population control.

METHODS

A targeted library of 3-bromo-4,5-dihydroxoisoxazole inhibitors were synthesized and screened for inhibition of AgTG3 in a fluorescent, plate-based assay. Positive hits were tested for in vitro activity using cross-linking and mass spectrometry, and in vivo efficacy in laboratory mating assays.

RESULTS

A targeted chemical library was screened for inhibition of AgTG3 in a fluorescent plate-based assay using its native substrate, plugin. Several inhibitors were identified with IC50 < 10 μM. Preliminary structure-activity relationships within the library support the stereo-specificity and preference for aromatic substituents in the chemical scaffold. Both inhibition of plugin cross-linking and covalent modification of the active site cysteine of AgTG3 were verified. Administration of an AgTG3 inhibitor to A. gambiae males by intrathoracic injection led to a 15% reduction in mating plug transfer in laboratory mating assays.

CONCLUSIONS

A targeted screen has identified chemical inhibitors of A. gambiae transglutaminase 3 (AgTG3). The most potent inhibitors are known inhibitors of human transglutaminase 2, suggesting a common binding pose may exist within the active site of both enzymes. Future efforts to develop additional inhibitors will provide chemical tools to address important biological questions regarding the role of the A. gambiae mating plug. A second use for transglutaminase inhibitors exists for the study of haemolymph coagulation and immune responses to wound healing in insects.

Host-microbe interactions in the gut of Drosophila melanogaster

Many insect species subsist on decaying and contaminated matter and are thus exposed to large quantities of microorganisms. To control beneficial commensals and combat infectious pathogens, insects must be armed with efficient systems for microbial recognition, signaling pathways, and effector molecules. The molecular mechanisms regulating these host-microbe interactions in insects have been largely clarified in Drosophila melanogaster with its powerful genetic and genomic tools. Here we review recent advances in this field, focusing mainly on the relationships between microbes and epithelial cells in the intestinal tract where the host exposure to the external environment is most frequent.