DmMyD88 controls dorsoventral patterning of the Drosophila embryo

Genetic characterization of immune adaptor molecule MyD88 in Culex pipiens complex (Diptera: Culicidae) mosquitoes from China

Mosquitoes of the Culex (Cx.) pipiens complex are vectors of severe diseases including West Nile fever by West Nile virus, Japanese encephalitis by Japanese encephalitis virus, and Lymphatic filariasis by filarial nematode Wuchereria bancrofti. As a major portion of mosquito immune system, the Toll pathway implicates in response against infections of mosquito-borne pathogens and biocontrol agents. The genetic diversity of immune-related molecules is expected to be a feasible and effective introduction to expand our knowledge of the mosquito-microbe interplay. However, a comprehensive description is currently lacking regarding the genetic characteristic of the Toll pathway molecules in Cx. pipiens complex mosquitoes. In the present study, genetic changes in Cx. pipiens complex MyD88 (Myeloid differentiation primary response protein 88) were analyzed as a precedent for the Toll pathway molecules in this taxon. MyD88 is a critical adaptor of the pathway transducing signals from TIR-containing receptors to downstream death domain-containing molecules. Our results revealed that adaptive selection has influenced the genetic changes of the molecule, giving rise to acceleration of diversity at a number of amino acid sites. The adaptively selected sites lie in the death domain, intermediate domain, and C-terminal extension. The characteristics of the genetic changes shed insights into the prominent molecular-level structural basis and the involvement strategy of the adaptor in the arms race against exogenous challenges. This finding would be beneficial for further exploration and deeper understanding of the mosquitoes' vectorial capacity and facilitating the effectiveness and sustainability of the biocontrol agents.

Identification and functional analysis of myeloid differentiation factor 88 (MyD88) in early development of Haliotis diversicolor

Myeloid differentiation factor 88 (MyD88) is a universal adaptor protein and plays an important role in the signal transduction of Toll like receptors (TLR) family. In this study, the MyD88 gene from the Haliotis diversicolor (hdMyD88) was identified. The full-length cDNA of hdMyD88 has a 1927 base pairs (bp), with an open reading frame of 1314 bp encoding 437 amino acids including a death domain (DD) at the N-terminus and TIR domain at the C-terminus which are typical features of MyD88 family proteins. Three conserved boxes are also found in the hdMyD88, which are similar to MyD88 in vertebrates. The expression levels of hdMyD88 mRNA at different early embryonic developmental stages of abalone were measured by qPCR revealed that their constitutive expression at all developmental stages analyzed with the considerably highest values at 8 cell stage and the lowest level at the trochosphere stage. Additionally, the mRNA expression of hdMyD88 decreased significantly (P < 0.05) after MyD88-dsRNA soak in the stage of trochosphere and veliger than EGFP-dsRNA group and blank control group. Whole embryo in situ hybridization showed that the positive signals of hdMyD88 were in visceral mass of trochophore larvae and veliger larvae. These results indicate hdMyD88 may could respond to pathogenic infection and may play an important role in early innate immunity in the process of abalone larval development.

Molecular characterization of MyD88 as a potential biomarker for pesticide-induced stress in Bombyx mori

The widespread use of pesticides hampers the immune system of non-target organisms, however, there is a lack of common biomarkers to detect such effects. Myeloid differentiation primary response factor 88 (MyD88) is a crucial junction protein in the Toll-like receptor signaling pathway, which plays an important role in the inflammatory response. In this study, we investigated MyD88 as a potential biomarker for pesticide-induced stress. Phylogenetic analysis revealed that MyD88 was a conserved protein in the evolution of vertebrates and invertebrates. MyD88s usually have death domain (DD) and Toll/interleukin-1 receptor (TIR) domain. Bombyx mori (B. mori) is an important economic insect that is sensitive to toxic substances. We found microbial pesticides enhanced the expression level of MyD88 in B. mori. Transcriptome analysis demonstrated that MyD88 expression level was increased in the fatbody after dinotefuran exposure, a third-generation neonicotinoid pesticide. Moreover, the expression of MyD88 was upregulated in fatbody and midgut by imidacloprid, a first-generation neonicotinoid pesticide. Additionally, insect growth regulator (IGR) pesticides, such as methoprene and fenoxycarb, could induce MyD88 expression in the fatbody of B. mori. These results indicated that MyD88 is a potential biomarker for pesticide-induced stress in B. mori. This study provides novel insights into screening common biomarkers for multiple pesticide stresses and important implications for the development of more sustainable pest management strategies.

A non-canonical Raf function is required for dorsal-ventral patterning during Drosophila embryogenesis

Proper embryonic development requires directional axes to pattern cells into embryonic structures. In Drosophila, spatially discrete expression of transcription factors determines the anterior to posterior organization of the early embryo, while the Toll and TGFβ signalling pathways determine the early dorsal to ventral pattern. Embryonic MAPK/ERK signaling contributes to both anterior to posterior patterning in the terminal regions and to dorsal to ventral patterning during oogenesis and embryonic stages. Here we describe a novel loss of function mutation in the Raf kinase gene, which leads to loss of ventral cell fates as seen through the loss of the ventral furrow, the absence of Dorsal/NFκB nuclear localization, the absence of mesoderm determinants Twist and Snail, and the expansion of TGFβ. Gene expression analysis showed cells adopting ectodermal fates much like loss of Toll signaling. Our results combine novel mutants, live imaging, optogenetics and transcriptomics to establish a novel role for Raf, that appears to be independent of the MAPK cascade, in embryonic patterning.

Synthetically engineered Medea gene drive system in the worldwide crop pest Drosophila suzukii

Synthetic gene drive systems possess enormous potential to replace, alter, or suppress wild populations of significant disease vectors and crop pests; however, their utility in diverse populations remains to be demonstrated. Here, we report the creation of a synthetic Medea gene drive system in a major worldwide crop pest, Drosophila suzukii We demonstrate that this drive system, based on an engineered maternal "toxin" coupled with a linked embryonic "antidote," is capable of biasing Mendelian inheritance rates with up to 100% efficiency. However, we find that drive resistance, resulting from naturally occurring genetic variation and associated fitness costs, can be selected for and hinder the spread of such a drive. Despite this, our results suggest that this gene drive could maintain itself at high frequencies in a wild population and spread to fixation if either its fitness costs or toxin resistance were reduced, providing a clear path forward for developing future such systems in this pest.

Synthetically engineered Medea gene drive system in the worldwide crop pest Drosophila suzukii

Here we describe a fully functional gene drive system constructed in a major worldwide crop pest, Drosophila suzukii. This system is composed of a synthetic Medea drive with a maternal miRNA “toxin” and a zygotic “antidote,” and we demonstrate that it can bias inheritance with 100% efficiency and can persist in a population given high release frequencies. We discuss how such a system may be used to suppress D. suzukii populations or render them harmless to target crops.

Synthetic gene drive systems possess enormous potential to replace, alter, or suppress wild populations of significant disease vectors and crop pests; however, their utility in diverse populations remains to be demonstrated. Here, we report the creation of a synthetic Medea gene drive system in a major worldwide crop pest, Drosophila suzukii. We demonstrate that this drive system, based on an engineered maternal “toxin” coupled with a linked embryonic “antidote,” is capable of biasing Mendelian inheritance rates with up to 100% efficiency. However, we find that drive resistance, resulting from naturally occurring genetic variation and associated fitness costs, can be selected for and hinder the spread of such a drive. Despite this, our results suggest that this gene drive could maintain itself at high frequencies in a wild population and spread to fixation if either its fitness costs or toxin resistance were reduced, providing a clear path forward for developing future such systems in this pest.

A novel myeloid differentiation factor 88 homolog, SpMyD88, exhibiting SpToll-binding activity in the mud crab Scylla paramamosain

Myeloid differentiation factor 88 (MyD88) is an essential regulator in the Toll or Toll-like receptor (TLR) signaling pathway. In the current study, we characterized a novel crustacean MyD88 homolog, SpMyD88, and analyzed its binding activity with SpToll. The full-length cDNA sequence of SpMyD88 is 2933 bp, with a 1419 bp open reading frame encoding a 472-amino acid protein. No signal peptide was predicted. A death domain (residues 19-103), a Toll/interleukin-1 receptor (TIR) domain (residues 156-297), and a C-terminal extension (CTE) domain (residues 298-472) were also found. In a phylogenetic tree constructed with MyD88 homologs from both invertebrates and vertebrates, arthropod MyD88s including SpMyD88 formed a cluster containing a unique CTE domain. SpToll shared the highest identity with human TLR4. These two receptors were grouped into a cluster of a tree constructed based on the conserved TIR domains. SpToll also had a close relationship with other shrimp TLRs that possess potential antibacterial activity. SpMyD88 was highly expressed in the hemocytes, gills, hepatopancreas, and eye stalks. Upon challenge with Vibrio harveyi, both SpMyD88 and SpToll were significantly increased in the hemocytes, whereas only SpMyD88 was elevated by Staphylococcus aureus. In addition, a pull-down assay demonstrated that SpMyD88 showed a binding activity with SpToll. These results suggest that SpMyD88 and SpToll are involved in the defense system of mud crabs against Gram-negative bacteria.

A synthetic gene drive system for local, reversible modification and suppression of insect populations

Replacement of wild insect populations with genetically modified individuals unable to transmit disease provides a self-perpetuating method of disease prevention but requires a gene drive mechanism to spread these traits to high frequency. Drive mechanisms requiring that transgenes exceed a threshold frequency in order to spread are attractive because they bring about local but not global replacement, and transgenes can be eliminated through dilution of the population with wild-type individuals. These features are likely to be important in many social and regulatory contexts. Here we describe the first creation of a synthetic threshold-dependent gene drive system, designated maternal-effect lethal underdominance (UD(MEL)), in which two maternally expressed toxins, located on separate chromosomes, are each linked with a zygotic antidote able to rescue maternal-effect lethality of the other toxin. We demonstrate threshold-dependent replacement in single- and two-locus configurations in Drosophila. Models suggest that transgene spread can often be limited to local environments. They also show that in a population in which single-locus UD(MEL) has been carried out, repeated release of wild-type males can result in population suppression, a novel method of genetic population manipulation.

Toll signaling in flies and mammals: two sorts of MyD88

The mammalian MyD88 signaling molecule participates in Toll receptor signaling within the cytoplasm. In this issue of Immunity, Marek and Kagan (2012) report that Drosophila (d)MyD88 acts instead at the plasma membrane to sort the signaling adaptor Tube.

Phosphoinositide binding by the Toll adaptor dMyD88 controls antibacterial responses in Drosophila

The cell biological principles that govern innate immune responses in Drosophila are unknown. Here, we report that Toll signaling in flies was dictated by the subcellular localization of the adaptor protein dMyD88. dMyD88 was located at the plasma membrane by a process dependent on a C-terminal phosphoinositide-binding domain. In vivo analysis revealed that lipid binding by dMyD88 was necessary for its antimicrobial and developmental functions as well as for the recruitment of the downstream cytosolic adaptor Tube to the cell surface. These data are reminiscent of the interactions between the mammalian Toll adaptors MyD88 and TIRAP with one major exception. In the mammalian system, MyD88 is the cytosolic adaptor that depends on the phosphoinositide-binding protein TIRAP for its recruitment to the cell surface. We therefore propose that dMyD88 is the functional homolog of TIRAP and that both proteins function as sorting adaptors to recruit downstream signaling adaptors to activated receptors.

Toll-8/Tollo negatively regulates antimicrobial response in the Drosophila respiratory epithelium

Barrier epithelia that are persistently exposed to microbes have evolved potent immune tools to eliminate such pathogens. If mechanisms that control Drosophila systemic responses are well-characterized, the epithelial immune responses remain poorly understood. Here, we performed a genetic dissection of the cascades activated during the immune response of the Drosophila airway epithelium i.e. trachea. We present evidence that bacteria induced-antimicrobial peptide (AMP) production in the trachea is controlled by two signalling cascades. AMP gene transcription is activated by the inducible IMD pathway that acts non-cell autonomously in trachea. This IMD-dependent AMP activation is antagonized by a constitutively active signalling module involving the receptor Toll-8/Tollo, the ligand Spätzle2/DNT1 and Ect-4, the Drosophila ortholog of the human Sterile alpha and HEAT/ARMadillo motif (SARM). Our data show that, in addition to Toll-1 whose function is essential during the systemic immune response, Drosophila relies on another Toll family member to control the immune response in the respiratory epithelium.

Invertebrates solely rely on innate immune responses for defense against microbial infections. Taking advantage of its powerful genetics, the fly Drosophila melanogaster has been extensively used as a model system to dissect the molecular mechanisms that control innate immunity. This work led to the discovery of the essential role of the Toll-1 receptor in triggering the systemic immune response in flies, and paved the way for the discovery of the function of members of the Toll-like receptor (TLR) family in mammalian immunity. Whereas all TLRs are implicated in the mammalian immune response, Toll-1 was, so far, the only Drosophila Toll family member to be involved in the regulation of the immune response. In the present study, we show that another Toll family member, Toll-8 (Tollo), plays an important role in controlling the respiratory epithelium immune response. Our data indicate that, by antagonizing the IMD pathway, Tollo is preventing over-activation of the antibacterial response in the airway epithelium.

Computer simulation on disease vector population replacement driven by the maternal effect dominant embryonic arrest

In this chapter, we present a series of computer simulations on the genetic modification of disease vectors. We compared the effectiveness of two techniques of genetic modification, transposable elements and maternal effect dominant embryonic arrest (MEDEA). A gene drive mechanism based on MEDEA is introduced in the population to confer immunity to individuals. Experimental results suggested that the genetic maternal effects could be necessary for the effectiveness of a disease control strategy based on the genetic modification of vectors.

Engineering the genomes of wild insect populations: challenges, and opportunities provided by synthetic Medea selfish genetic elements

Advances in insect transgenesis and our knowledge of insect physiology and genomics are making it possible to create transgenic populations of beneficial or pest insects that express novel traits. There are contexts in which we may want the transgenes responsible for these traits to spread so that all individuals within a wild population carry them, a process known as population replacement. Transgenes of interest are unlikely to confer an overall fitness benefit on those who carry them. Therefore, an essential component of any population replacement strategy is the presence of a drive mechanism that will ensure the spread of linked transgenes. We discuss contexts in which population replacement might be desirable and the requirements a drive system must satisfy to be both effective and safe. We then describe the creation of synthetic Medea elements, the first selfish genetic elements synthesized de novo, with the capability of driving population replacement, in this case in Drosophila. The strategy used to create Drosophila Medea is applicable to a number of other insect species and the Medea system satisfies key requirements for scientific and social acceptance. Finally, we highlight several challenges to implementing population replacement in the wild.

Specificity and signaling in the Drosophila immune response

The Drosophila immune response is characterized by the rapid and robust production of a battery of antimicrobial peptides immediately following infection. The genes encoding these antimicrobial peptides are controlled by two NF-κB signaling pathways that respond to microbial infection. The IMD pathway is triggered by DAP-type peptidoglycan, from the cell wall of most Gram-negative and certain Gram-positive bacteria, and activates the NF-κB precursor protein Relish. The Toll pathway, on the other hand, is stimulated by lysine-type peptidoglycan from many Gram-positive bacteria, β 1,3 glucans from many fungi, as well as by microbial proteases. Toll signaling leads to the activation and nuclear translocation of DIF or Dorsal, two other NF-κB homologs. This review presents our current understanding of the molecular mechanisms involved in microbial recognition and signal transduction in these two innate immune pathways.

A synthetic maternal-effect selfish genetic element drives population replacement in Drosophila

One proposed strategy for controlling the transmission of insect-borne pathogens uses a drive mechanism to ensure the rapid spread of transgenes conferring disease refractoriness throughout wild populations. Here, we report the creation of maternal-effect selfish genetic elements in Drosophila that drive population replacement and are resistant to recombination-mediated dissociation of drive and disease refractoriness functions. These selfish elements use microRNA-mediated silencing of a maternally expressed gene essential for embryogenesis, which is coupled with early zygotic expression of a rescuing transgene.

Weckle is a zinc finger adaptor of the toll pathway in dorsoventral patterning of the Drosophila embryo

BACKGROUND

The Drosophila Toll pathway takes part in both establishment of the embryonic dorsoventral axis and induction of the innate immune response in adults. Upon activation by the cytokine Spätzle, Toll interacts with the adaptor proteins DmMyD88 and Tube and the kinase Pelle and triggers degradation of the inhibitor Cactus, thus allowing the nuclear translocation of the transcription factor Dorsal/Dif. weckle (wek) was previously identified as a new dorsal group gene that encodes a putative zinc finger transcription factor. However, its role in the Toll pathway was unknown.

RESULTS

Here, we isolated new wek alleles and demonstrated that cactus is epistatic to wek, which in turn is epistatic to Toll. Consistent with this, Wek localizes to the plasma membrane of embryos, independently of Toll signaling. Wek homodimerizes and associates with Toll. Moreover, Wek binds to and localizes DmMyD88 to the plasma membrane. Thus, Wek acts as an adaptor to assemble/stabilize a Toll/Wek/DmMyD88/Tube complex. Remarkably, unlike the DmMyD88/tube/pelle/cactus gene cassette of the Toll pathway, wek plays a minimal role, if any, in the immune defense against Gram-positive bacteria and fungi.

CONCLUSIONS

We conclude that Wek is an adaptor to link Toll and DmMyD88 and is required for efficient recruitment of DmMyD88 to Toll. Unexpectedly, wek is dispensable for innate immune response, thus revealing differences in the Toll-mediated activation of Dorsal in the embryo and Dif in the fat body of adult flies.

Dorsoventral axis formation in the Drosophila embryo--shaping and transducing a morphogen gradient

The graded nuclear location of the transcription factor Dorsal along the dorsoventral axis of the early Drosophila embryo provides positional information for the determination of different cell fates. Nuclear uptake of Dorsal depends on a complex signalling pathway comprising two parts: an extracellular proteolytic cascade transmits the dorsoventral polarity of the egg chamber to the early embryo and generates a gradient of active Spätzle protein, the ligand of the receptor Toll; an intracellular cascade downstream of Toll relays this graded signal to embryonic nuclei. The slope of the Dorsal gradient is not determined by diffusion of extracellular or intracellular components from a local source, but results from self-organised patterning, in which positive and negative feedback is essential to create and maintain the ratio of key factors at different levels, thereby establishing and stabilising the graded spatial information for Dorsal nuclear uptake.

Drosophila WntD is a target and an inhibitor of the Dorsal/Twist/Snail network in the gastrulating embryo

The maternal Toll signaling pathway sets up a nuclear gradient of the transcription factor Dorsal in the early Drosophila embryo. Dorsal activates twist and snail, and the Dorsal/Twist/Snail network activates and represses other zygotic genes to form the correct expression patterns along the dorsoventral axis. An essential function of this patterning is to promote ventral cell invagination during mesoderm formation, but how the downstream genes regulate ventral invagination is not known. We show here that wntD is a novel member of the Wnt family. The expression of wntD is activated by Dorsal and Twist, but the expression is much reduced in the ventral cells through repression by Snail. Overexpression of WntD in the early embryo inhibits ventral invagination, suggesting that the de-repressed WntD in snail mutant embryos may contribute to inhibiting ventral invagination. The overexpressed WntD inhibits invagination by antagonizing Dorsal nuclear localization, as well as twist and snail expression. Consistent with the early expression of WntD at the poles in wild-type embryos, loss of WntD leads to posterior expansion of nuclear Dorsal and snail expression, demonstrating that physiological levels of WntD can also attenuate Dorsal nuclear localization. We also show that the de-repressed WntD in snail mutant embryos contributes to the premature loss of snail expression, probably by inhibiting Dorsal. Thus, these results together demonstrate that WntD is regulated by the Dorsal/Twist/Snail network, and is an inhibitor of Dorsal nuclear localization and function.

Sensing and signaling during infection in Drosophila

Most of the progress in dissecting the Drosophila antimicrobial response over the past decade has centered around intracellular signaling pathways in immune response tissues and expression of genes encoding antimicrobial peptide genes. The past few years, however, have witnessed significant advances in our understanding of the recognition of microbial invaders and subsequent activation of signaling cascades. In particular, the roles of peptidoglycan recognition proteins, which have known homologues in mammals, have been recognized and examined at the structural and functional levels.

Sensing infection in Drosophila: Toll and beyond

Drosophila has evolved a potent immune system that is somewhat adapted to the nature of infections through the selective activation of either one of two NF-kappa B-like signalling pathways, the Toll and IMD (Immune deficiency) pathways. In contrast to the mammalian system, the Toll receptor does not act as a pattern recognition receptor (PRR) but as a cytokine receptor. The sensing of microbial infections is achieved by at least four PRRs that belong to two distinct families: the peptidoglycan recognition proteins (PGRPs) and the Gram-negative binding proteins (GNBPs)/beta-glucan recognition proteins (beta GRPs).

The repressor function of snail is required for Drosophila gastrulation and is not replaceable by Escargot or Worniu

Mesoderm formation in the Drosophila embryo depends on the maternal Toll signaling pathway. The Toll pathway establishes the Dorsal nuclear gradient, which regulates many zygotic genes to establish the mesodermal fate and promote the invagination of ventral cells. An important target gene of Dorsal is snail, which is required for proper mesoderm invagination. The Snail protein contains five zinc fingers and is a transcriptional repressor. However, it is not clear whether repressing target genes is a requirement for Snail to control ventral invagination. To examine such requirement, we conducted a series of genetic rescue experiments in snail mutant embryos. Snail, Worniu, and Escargot are closely related zinc-finger proteins and have equal functions during neuroblast development. However, among these three proteins, only Snail can rescue the mesoderm invagination phenotype. Moreover, the ability of various Snail mutant constructs to repress gene expression correlates with their ability to control invagination. This unique property of Snail in mesoderm formation can be attributed mostly to the CtBP co-repressor interaction motifs in the N-terminus, not to the C-terminal DNA-binding zinc fingers. Ectopic expression of Snail outside the ventral domain is not sufficient to induce cell movement even though repression of target genes still occurs. Together, the results show that the repressor function of Snail is essential for gastrulation. The repression of target genes by Snail may permit other factors in the ventral cells to positively promote mesoderm invagination.

Biology of Toll receptors: lessons from insects and mammals

Toll receptors are type I transmembrane proteins that play important roles in development and immunity in animals. Comparison of the genomes of mouse and human on one side and of the fruitfly Drosophila and the mosquito Anopheles (two dipteran insects) on the other, revealed that the four species possess a similar number of Toll receptors (approximately 10). However, phylogenetic analyses indicate that the families of Toll receptors expanded independently in insects and mammals. We review recent results on these receptors, which point to differences in the activation and signaling between Tolls in insects and Toll-like receptors (TLRs) in mammals. Whereas mammalian TLRs appear to be solely dedicated to host-defense, insect Tolls may be predominantly linked to other functions, probably developmental.

Structural complementarity of Toll/interleukin-1 receptor domains in Toll-like receptors and the adaptors Mal and MyD88

The Toll/interleukin 1 receptor (TIR) domain is a region found in the cytoplasmic tails of members of the Toll-like receptor/interleukin-1 receptor superfamily. The domain is essential for signaling and is also found in the adaptor proteins Mal (MyD88 adaptor-like) and MyD88, which function to couple activation of the receptor to downstream signaling components. Experimental structures of two Toll/interleukin 1 receptor domains reveal a alpha-beta-fold similar to that of the bacterial chemotaxis protein CheY, and other evidence suggests that the adaptors can make heterotypic interactions with both the receptors and themselves. Here we show that the purified TIR domains of Mal and MyD88 can form stable heterodimers and also that Mal homodimers and oligomers are dissociated in the presence of ATP. To identify structural features that may contribute to the formation of signaling complexes, we produced models of the TIR domains from human Toll-like receptor 4 (TLR4), Mal, and MyD88. We found that although the overall fold is conserved the electrostatic surface potentials are quite distinct. Docking studies of the models suggest that Mal and MyD88 bind to different regions in TLRs 2 and 4, a finding consistent with a cooperative role of the two adaptors in signaling. Mal and MyD88 are predicted to interact at a third non-overlapping site, suggesting that the receptor and adaptors may form heterotetrameric complexes. The theoretical model of the interactions is supported by experimental data from glutathione S-transferase pull-downs and co-immunoprecipitations. Neither theoretical nor experimental data suggest a direct role for the conserved proline in the BB-loop in the association of TLR4, Mal, and MyD88. Finally we show a sequence relationship between the Drosophila protein Tube and Mal that may indicate a functional equivalence of these two adaptors in the Drosophila and vertebrate Toll pathways.