A Spätzle-processing enzyme required for toll signaling activation in Drosophila innate immunity

Proteolytic activation of pro-spätzle is required for the induced transcription of antimicrobial peptide genes in lepidopteran insects

Microbial infection leads to proteolytic activation of Drosophila spätzle, which binds to the toll receptor and induces the synthesis of immune proteins. To test whether or not this mechanism exists in lepidopteran insects, we cloned the cDNA of Bombyx mori spätzle-1 and overexpressed the full-length and truncated BmSpz1 cDNA in Escherichia coli. The insoluble fusion proteins were affinity-purified under denaturing condition. After the silkworm larvae were injected with renatured BmSpz1, mRNA levels of antimicrobial peptide genes greatly increased. Similar transcriptional up-regulation was also found in Manduca sexta. Injection of pro-BmSpz1 had no such effect. When pro-BmSpz1 and Micrococcus luteus were incubated with the plasma from M. sexta larvae, we detected proteolytic processing of pro-BmSpz1. These results suggest that active spätzle is required for the induced production of antimicrobial peptides in B. mori and M. sexta.

[Intra- and extracellular recognition of pathogens and activation of innate immunity]

One of the fundamental questions in innate immunity is how a large battery of invading pathogens is recognized by a limited number of germ line-encoding receptors. In Drosophila, peptidoglycan recognition protein (PGRP) family members have a crucial role in recognizing invading bacterial pathogens and in inducing immune reactions. PGRP-SA, -SD, and -SC1a are involved in recognizing gram-positive bacteria and in activating the Toll pathway to produce antimicrobial peptides. PGRP-LC and -LE recognize diaminopimelic acid (DAP)-containing peptidoglycans, which are cell wall components of many gram-negative bacteria and some gram-positive bacteria, and activate the imd pathway to produce antibacterial peptides. In addition to the extracellular function of PGRP-LE to activate immune reactions in the hemolymph, PGRP-LE acts as an intracellular receptor for monomeric DAP-type peptidoglycans. Moreover, a version of PGRP-LE containing only the PGRP domain functions extracellularly as a CD14-like accessory factor, capable of enhancing PGRP-LC-mediated peptidoglycan recognition. Subsequent intracellular signaling is transduced through the RHIM-like motif found in PGRP-LC and -LE.

Comparative analysis of serine protease-related genes in the honey bee genome: possible involvement in embryonic development and innate immunity

We have identified 44 serine protease (SP) and 13 serine protease homolog (SPH) genes in the genome of Apis mellifera. Most of these genes encode putative secreted proteins, but four SPs and three SPHs may associate with the plasma membrane via a transmembrane region. Clip domains represent the most abundant non-catalytic structural units in these SP-like proteins -12 SPs and six SPHs contain at least one clip domain. Some of the family members contain other modules for protein-protein interactions, including disulphide-stabilized structures (LDL(r)A, SRCR, frizzled, kringle, Sushi, Wonton and Pan/apple), carbohydrate-recognition domains (C-type lectin and chitin-binding), and other modules (such as zinc finger, CUB, coiled coil and Sina). Comparison of the sequences with those from Drosophila led to a proposed SP pathway for establishing the dorsoventral axis of honey bee embryos. Multiple sequence alignments revealed evolutionary relationships of honey bee SPs and SPHs with those in Drosophila melanogaster, Anopheles gambiae, and Manduca sexta. We identified homologs of D. melanogaster persephone, M. sexta HP14, PAP-1 and SPH-1. A. mellifera genome includes at least five genes for potential SP inhibitors (serpin-1 through -5) and three genes of SP putative substrates (prophenoloxidase, spätzle-1 and spätzle-2). Quantitative RT-PCR analyses showed an elevation in the mRNA levels of SP2, SP3, SP9, SP10, SPH41, SPH42, SP49, serpin-2, serpin-4, serpin-5, and spätzle-2 in adults after a microbial challenge. The SP41 and SP6 transcripts significantly increased after an injection of Paenibacillus larva, but there was no such increase after injection of saline or Escherichia coli. mRNA levels of most SPs and serpins significantly increased by 48 h after the pathogen infection in 1st instar larvae. On the contrary, SP1, SP3, SP19 and serpin-5 transcript levels reduced. These results, taken together, provide a framework for designing experimental studies of the roles of SPs and related proteins in embryonic development and immune responses of A. mellifera.

Plant hormone receptors: perception is everything

Despite the bewildering ability of higher plants to change their development with respect to the environment, there appear to be only a few hormones that function to organize growth and development. With the recent identification of three plant hormone receptors, the molecular identities of all the major plant receptors are now known. Some plant hormones such as cytokinins, ethylene, and brassinosteroids (BR) use well-characterized signaling modules such as those involving receptor kinases, but in the case of the ethylene and BR receptors, there appear to be additional functions aside from the hormone they perceive. Auxin and gibberellin perception require unique mechanisms where the receptors are components involved in ubiquitination-dependent proteolysis. With plant hormone receptors in hand, comparisons can now be made between plants and other kingdoms as to how hormones control growth and development.

Expression and regulation of Spätzle-processing enzyme in Drosophila

The Drosophila melanogaster Toll receptor controls embryonic dorsal-ventral axis formation and is crucial for the innate immune response. In both cases, Toll is activated by the enzymatically cleaved form of its ligand Spätzle (Spz). During axis formation, Spz is cleaved by the maternally provided serine protease Easter while the Spätzle-processing enzyme (SPE) activates Spz after infection. We confirm the role of SPE in immunity and show that it is a zygotic gene specifically expressed in immune tissues implying that the dual activation of Spz is achieved by differential spatiotemporal expression of two similar but distinct serine proteases.

Gene silencing of serine proteases affects melanization of Sephadex beads in Anopheles gambiae

Serine proteases play an important role in activation of prophenoloxidase (proPO), a critical enzyme in the production of melanin. We tested the effect of knockdown of gene expression for five clip domain serine proteases on melanization of abiotic targets in Anopheles gambiae. Knockdown of CLIPB4 resulted in a striking lack of melanization of Sephadex beads while knockdown of CLIPB8 caused a strong shift towards incompletely melanized beads. Knockdown of CLIPB1, B9 and B10 had lesser effects. CLIPB4 and CLIPB8 are strong candidates for activating enzymes in the proPO enzymatic cascade.

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.

Drosophila immunity: a large-scale in vivo RNAi screen identifies five serine proteases required for Toll activation

Unlike mammalian Toll-like Receptors, the Drosophila Toll receptor does not interact directly with microbial determinants but is rather activated upon binding a cleaved form of the cytokine-like molecule Spatzle (Spz). During the immune response, Spz is thought to be processed by secreted serine proteases (SPs) present in the hemolymph that are activated by the recognition of gram-positive bacteria or fungi . In the present study, we have used an in vivo RNAi strategy to inactivate 75 distinct Drosophila SP genes. We then screened this collection for SPs regulating the activation of the Toll pathway by gram-positive bacteria. Here, we report the identification of five novel SPs that function in an extracellular pathway linking the recognition proteins GNBP1 and PGRP-SA to Spz. Interestingly, four of these genes are also required for Toll activation by fungi, while one is specifically associated with signaling in response to gram-positive bacterial infections. These results demonstrate the existence of a common cascade of SPs upstream of Spz, integrating signals sent by various secreted recognition molecules via more specialized SPs.

Pathogen recognition and signalling in the Drosophila innate immune response

Genetic analysis of the innate immune response in Drosophila has provided important insights into the mechanism of microbial sensing and the subsequent host signalling events. The two major players following immune challenge are the Toll and Immune deficiency (IMD) pathways, which are essential for fruit flies to survive infection. These pathways are homologous to the mammalian Toll-like receptor and tumour necrosis factor pathways, respectively. Moreover, microbial pattern-recognition receptors upstream of Toll and IMD, such as the peptidoglycan recognition proteins, have been isolated and studied at the structural and functional level. In the present, we will review recent data pertaining to the genetic, genomic, RNAi and infection studies that have added new complexities to the system.

Metamorphosis and collagen-IV-fragments stimulate innate immune response in the greater wax moth, Galleria mellonella

A novel link between development and immunity in insects is introduced. Transiently enhanced expression of lysozyme, gallerimycin and the insect metalloproteinase inhibitor was discovered at the onset of metamorphosis of the greater wax moth, Galleria mellonella. Relative quantification of mRNAs encoding for these antimicrobial peptides using real-time PCR documents their induced expression during transformation of last instar larvae into prepupae and upon injection of either recombinant interstitial collagenase (MMP-1) or small-sized fragments of collagen type IV. The latter were also found to stimulate both nuclear import of c-Rel-proteins in the fat body, implicating activation of Toll or Imd-related signaling pathways, and subsequent synthesis of antimicrobial peptides. Obtained results implicate that degradation of collagen-IV by either microbial metalloproteinases associated with invading pathogens or endogenous matrix metalloproteinases contributing to degradation of extracellular matrix during metamorphosis stimulate innate immune responses.