Characterization of three alternatively spliced isoforms of the Rel/NF-kappa B transcription factor Relish from the mosquito Aedes aegypti

Maintenance of persistent transmission of a plant arbovirus in its insect vector mediated by the Toll-Dorsal immune pathway

Throughout evolution, arboviruses have developed various strategies to counteract the host's innate immune defenses to maintain persistent transmission. Recent studies have shown that, in addition to bacteria and fungi, the innate Toll-Dorsal immune system also plays an essential role in preventing viral infections in invertebrates. However, whether the classical Toll immune pathway is involved in maintaining the homeostatic process to ensure the persistent and propagative transmission of arboviruses in insect vectors remain unclear. In this study, we revealed that the transcription factor Dorsal is actively involved in the antiviral defense of an insect vector (Laodelphax striatellus) by regulating the target gene, zinc finger protein 708 (LsZN708), which mediates downstream immune-related effectors against infection with the plant virus (Rice stripe virus, RSV). In contrast, an antidefense strategy involving the use of the nonstructural-protein (NS4) to antagonize host antiviral defense through competitive binding to Dorsal from the MSK2 kinase was employed by RSV; this competitive binding inhibited Dorsal phosphorylation and reduced the antiviral response of the host insect. Our study revealed the molecular mechanism through which Toll-Dorsal-ZN708 mediates the maintenance of an arbovirus homeostasis in insect vectors. Specifically, ZN708 is a newly documented zinc finger protein targeted by Dorsal that mediates the downstream antiviral response. This study will contribute to our understanding of the successful transmission and spread of arboviruses in plant or invertebrate hosts.

The localization of Toll and Imd pathway and complement system components and their response to Vibrio infection in the nemertean Lineus ruber

BACKGROUND

Innate immunity is the first line of defense against pathogens. In animals, the Toll pathway, the Imd pathway, the complement system, and lectins are well-known mechanisms involved in innate immunity. Although these pathways and systems are well understood in vertebrates and arthropods, they are understudied in other invertebrates.

RESULTS

To shed light on immunity in the nemertean Lineus ruber, we performed a transcriptomic survey and identified the main components of the Toll pathway (e.g., myD88, dorsal/dif/NFκB-p65), the Imd pathway (e.g., imd, relish/NFκB-p105/100), the complement system (e.g., C3, cfb), and some lectins (FreD-Cs and C-lectins). In situ hybridization showed that TLRβ1, TLRβ2, and imd are expressed in the nervous system; the complement gene C3-1 is expressed in the gut; and the lectins are expressed in the nervous system, the blood, and the gut. To reveal their potential role in defense mechanisms, we performed immune challenge experiments, in which Lineus ruber specimens were exposed to the gram-negative bacteria Vibrio diazotrophicus. Our results show the upregulation of specific components of the Toll pathway (TLRα3, TLRβ1, and TLRβ2), the complement system (C3-1), and lectins (c-lectin2 and fred-c5).

CONCLUSIONS

Therefore, similarly to what occurs in other invertebrates, our study shows that components of the Toll pathway, the complement system, and lectins are involved in the immune response in the nemertean Lineus ruber. The presence of these pathways and systems in Lineus ruber, but also in other spiralians; in ecdysozoans; and in deuterostomes suggests that these pathways and systems were involved in the immune response in the stem species of Bilateria.

Entomopathogenic fungi and Schinus molle essential oil: The combination of two eco-friendly agents against Aedes aegypti larvae

Aedes aegypti transmits arbovirus, which is a public health concern. Certain filamentous fungi have the potential to control the disease. Here, the effects of Metarhizium anisopliae s.l. CG 153, Beauveria bassiana s.l. CG 206 and Schinus molle L. were investigated against Aedes aegypti larvae. In addition, the effect of essential oil on fungal development was analyzed. Fungal germination was assessed after combination with essential oil at 0.0025 %, 0.0075 %, 0.005 %, or 0.01 %; all of the oil concentrations affected germination except 0.0025 % (v/v). Larvae were exposed to 0.0025 %, 0.0075 %, 0.005 %, or 0.01 % of the essential oil or Tween 80 at 0.01 %; however, only the essential oil at 0.0025 % achieved similar results as the control. Larvae were exposed to fungi at 10⁷ conidia mL^-1 alone or in combination with the essential oil at 0.0025 %. Regardless of the combination, M. anisopliae reduced the median survival time of mosquitoes more than B. bassiana. The cumulative survival of mosquitoes exposed to M. anisopliae alone or in combination with essential oil was 7.5 % and 2 %, respectively, and for B. bassiana, it was 75 % and 71 %, respectively. M. anisopliae + essential oil had a synergistic effect against larvae, whereas B. bassiana + essential oil was antagonistic. Scanning and transmission electron microscopy, and histopathology confirmed that the interaction of M. anisopliae was through the gut and hemocoel. In contrast, the mosquito's gut was the main route for invasion by B. bassiana. Results from gas chromatography studies demonstrated sabinene and bicyclogermacrene as the main compounds of S. molle, and the in-silico investigation found evidence that both compounds affect a wide range of biological activity. For the first time, we demonstrated the potential of S. molle and its interaction with both fungal strains against A. aegypti larvae. Moreover, for the first time, we reported that S. molle might be responsible for significant changes in larval physiology. This study provides new insights into host-pathogen interplay and contributes to a better understanding of pathogenesis in mosquitoes, which have significant consequences for biological control strategies.

The IMD and Toll canonical immune pathways of Triatoma pallidipennis are preferentially activated by Gram-negative and Gram-positive bacteria, respectively, but cross-activation also occurs

BACKGROUND

Antimicrobial peptides (AMPs) participate in the humoral immune response of insects eliminating invasive microorganisms. The immune deficiency pathway (IMD) and Toll are the main pathways by which the synthesis of these molecules is regulated in response to Gram-negative (IMD pathway) or Gram-positive (Toll pathway) bacteria. Various pattern-recognition receptors (PRRs) participate in the recognition of microorganisms, such as pgrp-lc and toll, which trigger signaling cascades and activate NF-κB family transcription factors, such as relish, that translocate to the cell nucleus, mainly in the fat body, inducing AMP gene transcription.

METHODS

T. pallidipennis inhibited in Tppgrp-lc, Tptoll, and Tprelish were challenged with E. coli and M. luteus to analyze the expression of AMPs transcripts in the fat body and to execute survival assays.

RESULTS

In this work we investigated the participation of the pgrp-lc and toll receptor genes and the relish transcription factor (designated as Tppgrp-lc, Tptoll, and Tprelish), in the transcriptional regulation of defensin B, prolixicin, and lysozyme B in Triatoma pallidipennis, one of the main vectors of Chagas disease. AMP transcript abundance was higher in the fat body of blood-fed than non-fed bugs. Challenge with Escherichia coli or Micrococcus luteus induced differential increases in AMP transcripts. Additionally, silencing of Tppgrp-lc, Tptoll, and Tprelish resulted in reduced AMP transcription and survival of bugs after a bacterial challenge.

CONCLUSIONS

Our findings demonstrated that the IMD and Toll pathways in T. pallidipennis preferentially respond to Gram-negative and Gram-positive bacteria, respectively, by increasing the expression of AMP transcripts, but cross-induction also occurs.

Two NF-κB subunits are associated with antimicrobial immunity in Hyriopsis cumingii

The NF-κB pathway activated by bacteria and viruses produces a series of antimicrobial peptides that participate in the innate immune response. In this study, two NF-κB subunits were cloned and identified from Hyriopsis cumingii (named Hcp65 and Hcp105) using RT-PCR and RACE. The predicted Hcp65 protein possessed a N-terminal Rel homology domain (RHD) and an Ig-like/plexins/transcription factors domain (IPT); the Hcp105 contained an RHD, an IPT domain, 6 ankyrin (ANK) domain and a death domain. Quantitative reverse transcription PCR (qRT-PCR) showed that Hcp65 and Hcp105 were constitutively expressed in the detected tissues, and were significantly up-regulated in hemocytes, hepatopancreas and gill of mussels challenged with lipopolysaccharide (LPS), peptidoglycan (PGN) and polyinosinic-polycytidylic acid (poly I: C). The dsRNA-mediated silencing of Hcp65 and Hcp105 caused significant reduction of immune genes such as lysozyme (HcLyso), theromacin (Hcther), whey acid protein (HcWAP), LPS-binding protein/bactericidal permeability protein (HcLBP/BPI) 1 and 2. In addition, subcellular localization experiments showed that Hcp65 and Hcp105 proteins were expressed in both the nucleus and cytoplasm of HEK-293T cells, and Hcp50 proteins (mature peptide of Hcp105) were mainly localized in the nucleus. The recombinant Hcp65 and Hcp50 protein could form homodimer and heterodimer and bind κB site in vitro. These results provide useful information for understanding the role of NF-κB in mollusks.

Imd pathway-specific immune assays reveal NF-κB stimulation by viral RNA PAMPs in Aedes aegypti Aag2 cells

BACKGROUND

The mosquito Aedes aegypti is a major vector for the arthropod-borne viruses (arboviruses) chikungunya, dengue, yellow fever and Zika viruses. Vector immune responses pose a major barrier to arboviral transmission, and transgenic insects with altered immunity have been proposed as tools for reducing the global public health impact of arboviral diseases. However, a better understanding of virus-immune interactions is needed to progress the development of such transgenic insects. Although the NF-κB-regulated Toll and 'immunodeficiency' (Imd) pathways are increasingly thought to be antiviral, relevant pattern recognition receptors (PRRs) and pathogen-associated molecular patterns (PAMPs) remain poorly characterised in A. aegypti.

METHODOLOGY/PRINCIPLE FINDINGS

We developed novel RT-qPCR and luciferase reporter assays to measure induction of the Toll and Imd pathways in the commonly used A. aegypti-derived Aag2 cell line. We thus determined that the Toll pathway is not inducible by exogenous stimulation with bacterial, viral or fungal stimuli in Aag2 cells under our experimental conditions. We used our Imd pathway-specific assays to demonstrate that the viral dsRNA mimic poly(I:C) is sensed by the Imd pathway, likely through intracellular and extracellular PRRs. The Imd pathway was also induced during infection with the model insect-specific virus cricket paralysis virus (CrPV).

CONCLUSIONS/SIGNIFICANCE

Our demonstration that a general PAMP shared by many arboviruses is sensed by the Imd pathway paves the way for future studies to determine how viral RNA is sensed by mosquito PRRs at a molecular level. Our data also suggest that studies measuring inducible immune pathway activation through antimicrobial peptide (AMP) expression in Aag2 cells should be interpreted cautiously given that the Toll pathway is not responsive under all experimental conditions. With no antiviral therapies and few effective vaccines available to treat arboviral diseases, our findings provide new insights relevant to the development of transgenic mosquitoes as a means of reducing arbovirus transmission.

Aedes aegypti Immune Response and Its Potential Impact on Dengue Virus Transmission

Dengue virus (DENV) transmission to human populations requires infection of vector mosquitoes as an essential component of the transmission process. DENV transmission leads to infections that range from asymptomatic to life-threatening pathologies, such as dengue hemorrhagic fever and dengue shock syndrome. Aedes aegypti is the principal vector of DENV, and its vector competence consists of the intrinsic factors, genes, molecules, and pathways that allow infection, replication, and dissemination of this virus throughout the cells of mosquito tissues. In the search for mosquito molecular targets to block DENV transmission, the effect of DENV infection on mosquitoes has been an important focus of research. In this study, we review the findings of research on the effect of DENV infection on mosquito tissue cells and the immunity pathways and molecules that are involved in this infection. We emphasize the relevance of recent findings to understand the relationship between Ae. aegypti immune response, vector competence, and DENV transmission to human hosts.

The Mosquito Immune System and the Life of Dengue Virus: What We Know and Do Not Know

Flaviviruses are largely transmitted to humans by their arthropod vectors such as mosquitoes or ticks. The dengue virus (DENV) is one of the members of the family Flaviviridae and is the causative agent of dengue fever. In the mosquito vector, DENV enters through viremic blood meal and replicates in the mid-gut. Newly formed virion particles circulate to various mosquito organs and get transmitted to the next host in subsequent bites. Aedes aegypti and Aedes albopictus have intricate immune control to allow DENV production at a sub-pathogenic level. In the mosquito, antimicrobial peptides (AMP) and RNA inference (RNAi) are the two main antiviral strategies used against DENV. Apart from innate immunity, mosquito resident microbes play a significant role in modulating DENV replication. In this review, we discuss different immune mechanisms and preventive strategies that act against DENV in two of its vectors: Aedes aegypti and Aedes albopictus.

An in vitro study of NF-κB factors cooperatively in regulation of Drosophila melanogaster antimicrobial peptide genes

An important innate immune response in Drosophila melanogaster is the production of antimicrobial peptides (AMPs). Expression of AMP genes is mediated by the Toll and immune deficiency (IMD) pathways via NF-κB transcription factors Dorsal, DIF and Relish. Dorsal and DIF act downstream of the Toll pathway, whereas Relish acts in the IMD pathway. Dorsal and DIF are held inactive in the cytoplasm by the IκB protein Cactus, while Relish contains an IκB-like inhibitory domain at the C-terminus. NF-κB factors normally form homodimers and heterodimers to regulate gene expression, but formation of heterodimers between Relish and DIF or Dorsal and the specificity and activity of the three NF-κB homodimers and heterodimers are not well understood. In this study, we compared the activity of Rel homology domains (RHDs) of Dorsal, DIF and Relish in activation of Drosophila AMP gene promoters, demonstrated that Relish-RHD (Rel-RHD) interacted with both Dorsal-RHD and DIF-RHD, Relish-N interacted with DIF and Dorsal, and overexpression of individual RHD and co-expression of any two RHDs activated the activity of AMP gene promoters to various levels, suggesting formation of homodimers and heterodimers among Dorsal, DIF and Relish. Rel-RHD homodimers were stronger activators than heterodimers of Rel-RHD with either DIF-RHD or Dorsal-RHD, while DIF-RHD-Dorsal-RHD heterodimers were stronger activators than either DIF-RHD or Dorsal-RHD homodimers in activation of AMP gene promoters. We also identified the nucleotides at the 6th and 8th positions of the 3' half-sites of the κB motifs that are important for the specificity and activity of NF-κB transcription factors.

Effect of Clostridium butyricum in different forms on growth performance, disease resistance, expression of genes involved in immune responses and mTOR signaling pathway of Litopenaeus vannamai

A 42-day feeding trial was conducted to evaluate the effects of diet supplemented with various additives from Clostridium butyricum (fermentation supernatant, FS; live cells, LC; cell-free extract, CE; spray-dried spores, DS; mixture of live cells and supernatant, LCS) on the growth, intestinal morphology, disease resistance, immune gene expression and mTOR signaling-related gene expression in Litopenaeus vannamai. The feeding trial showed that the final weight and specific growth rate of the shrimp were improved significantly while the feed conversion ratio were reduced significantly in LC, CE, DS and LCS groups compared to the control. The villus height and intestinal wall thickness of shrimp's mid-intestine in LC, DS and LCS group increased significantly. After challenge test to Vibrio parahaemolyticus, the cumulative mortalities of the shrimp in LC, CE, DS and LCS groups were significantly lower than that of the control. As compared to the control, the relative expression levels of superoxide dismutase, lysozyme, prophenoloxidase (proPO), Toll, Immune deficiency (Imd), Relish, TOR, 4E-BP, eIF4E1α and eIF4E2 genes in the shrimp of DS and LCS groups enhanced significantly, whereas the relative expression levels of proPO, SOD, Toll, Imd, Relish, elF4E1α and elF4E2 genes were statistically the same between FS group and the control. These results suggested that the spray-dried spores and mixture of live cells and supernatant of C. butyricum exerted better probiotic benefits in modulating immune responses of shrimp. In addition, single supernatant could not be helpful to shrimp while mixture of live cells and supernatant could better improve the immune responses of shrimp in comparison to single live cells. The integration of C. butyricum and their metabolites supplemented into feed could significantly improve growth performance, intestinal morphology, immunity capacity and resistance against V. parahaemolyticus of L. vannamei.

Insect Transcription Factors: A Landscape of Their Structures and Biological Functions in Drosophila and beyond

Transcription factors (TFs) play essential roles in the transcriptional regulation of functional genes, and are involved in diverse physiological processes in living organisms. The fruit fly Drosophila melanogaster, a simple and easily manipulated organismal model, has been extensively applied to study the biological functions of TFs and their related transcriptional regulation mechanisms. It is noteworthy that with the development of genetic tools such as CRISPR/Cas9 and the next-generation genome sequencing techniques in recent years, identification and dissection the complex genetic regulatory networks of TFs have also made great progress in other insects beyond Drosophila. However, unfortunately, there is no comprehensive review that systematically summarizes the structures and biological functions of TFs in both model and non-model insects. Here, we spend extensive effort in collecting vast related studies, and attempt to provide an impartial overview of the progress of the structure and biological functions of current documented TFs in insects, as well as the classical and emerging research methods for studying their regulatory functions. Consequently, considering the importance of versatile TFs in orchestrating diverse insect physiological processes, this review will assist a growing number of entomologists to interrogate this understudied field, and to propel the progress of their contributions to pest control and even human health.

The immune strategies of mosquito Aedes aegypti against microbial infection

Yellow fever mosquito Aedes aegypti transmits many devastating arthropod-borne viruses (arboviruses), such as dengue virus, yellow fever virus, Chikungunya virus, and Zika virus, which cause great concern to human health. Mosquito control is an effective method to block the spread of infectious diseases. Ae. aegypti uses its innate immune system to fight against arboviruses, parasites, and fungi. In this review, we briefly summarize the recent findings in the immune response of Ae. aegypti against arboviral and entomopathogenic infections. This review enriches our understanding of the mosquito immune system and provides evidence to support the development of novel mosquito control strategies.

NF-κB-Like Signaling Pathway REL2 in Immune Defenses of the Malaria Vector Anopheles gambiae

The blood feeding requirements of insects are often exploited by pathogens for their transmission. This is also the case of the protozoan parasites of genus Plasmodium, the causative agents of malaria. Every year malaria claims the lives of a half million people, making its vector, the Anopheles mosquito, the deadliest animal in the world. However, mosquitoes mount powerful immune responses that efficiently limit parasite proliferation. Among the immune signaling pathways identified in the main malaria vector Anopheles gambiae, the NF-κB-like signaling cascades REL2 and REL1 are essential for eliciting proper immune reactions, but only REL2 has been implicated in the responses against the human malaria parasite Plasmodium falciparum. Instead, constitutive activation of REL1 causes massive killing of rodent malaria parasites. In this review, we summarize our present knowledge on the REL2 pathway in Anopheles mosquitoes and its role in mosquito immune responses to diverse pathogens, with a focus on Plasmodium. Mosquito-parasite interactions are crucial for malaria transmission and, therefore, represent a potential target for malaria control strategies.

Relish2 mediates bursicon homodimer-induced prophylactic immunity in the mosquito Aedes aegypti

Bursicon is a neuropeptide hormone consisting of two cystine-knot proteins (burs α and burs β), responsible for cuticle tanning and other developmental processes in insects. Recent studies show that each bursicon subunit forms homodimers that induce prophylactic immunity in Drosophila melanogaster. Here, we investigated the hypothesis that bursicon homodimers act in prophylactic immunity in insects, and possibly arthropods, generally, using the mosquito, Aedes aegypti. We found that burs α and burs β are expressed in larvae, pupae and newly emerged adults. Treating newly emerged Ae. aegypti and D. melanogaster adults with recombinant bursicon (r-bursicon) heterodimer led to cuticle tanning in both species. Treating larvae and adults with r-bursicon homodimers led to up-regulation of five anti-microbial peptide (AMP) genes, noting the possibility that bursicon heterodimers also lead to up-regulation of these genes can not been excluded. The induced AMPs effectively suppressed the growth of bacteria in vitro. RNAi knock-down of the transcriptional factor Relish2 abolished the influence of r-bursicon homodimers on AMP production. We infer the bursicon homodimers induce expression of AMP genes via Relish2 in Ae. aegypti, as prophylactic immunity to protect mosquitoes during the vulnerable stages of each molt.

Role of NF-kβ factor Rel2 during Plasmodium falciparum and bacterial infection in Anopheles dirus

BACKGROUND

Anopheles mosquitoes transmit malaria which is one of the world's most threatening diseases. Anopheles dirus (sensu stricto) is among the main vectors of malaria in South East Asia. The mosquito innate immune response is the first line of defence against malaria parasites during its development. The immune deficiency (IMD) pathway, a conserved immune signaling pathway, influences anti-Plasmodium falciparum activity in Anopheles gambiae, An. stephensi and An. albimanus. The aim of the study was to determine the role of Rel2, an IMD pathway-controlled NF-kappaβ transcription factor, in An. dirus.

METHODS

RACE (Rapid amplification of cDNA ends) was performed on the Rel2 gene. Double-stranded Rel2 was constructed and injected into the thorax of female mosquitoes. The injected mosquitoes were fed on a P. falciparum gametocyte culture and dissected on day 7-9 post-feeding in order to count the oocysts. A survival analysis was conducted by exposing the dsRNA injected mosquitoes to Gram-positive and Gram-negative bacteria.

RESULTS

This study demonstrated that the Rel2 gene in An. dirus has two isoforms, short length and full length. RNA interference-mediated gene silencing of Rel2 showed that the latter is involved in protection against P. falciparum, Gram-positive bacteria (Micrococcus luteus) with Lys-type peptidoglycan and Gram-negative bacteria (Escherichia coli) with DAP-type peptidoglycan.

CONCLUSION

This study suggested that there are similarities in the splicing events and functionality of the Rel2 gene, between the Anopheles species. Among all the important anophelines, the immunity of only a few has been thoroughly investigated. In order to develop novel vector-based control strategies and restrict malaria transmission, the immune pathways of these important vectors should be thoroughly investigated.

Co-expression of Dorsal and Rel2 Negatively Regulates Antimicrobial Peptide Expression in the Tobacco Hornworm Manduca sexta

Nuclear factor κB (NF-κB) plays an essential role in regulation of innate immunity. In mammals, NF-κB factors can form homodimers and heterodimers to activate gene expression. In insects, three NF-κB factors, Dorsal, Dif and Relish, have been identified to activate antimicrobial peptide (AMP) gene expression. However, it is not clear whether Dorsal (or Dif) and Relish can form heterodimers. Here we report the identification and functional analysis of a Dorsal homologue (MsDorsal) and two Relish short isoforms (MsRel2A and MsRel2B) from the tobacco hornworm, Manduca sexta. Both MsRel2A and MsRel2B contain only a Rel homology domain (RHD) and lack the ankyrin-repeat inhibitory domain. Overexpression of the RHD domains of MsDorsal and MsRel2 in Drosophila melanogaster S2 and Spodoptera frugiperda Sf9 cells can activate AMP gene promoters from M. sexta and D. melanogaster. We for the first time confirmed the interaction between MsDorsal-RHD and MsRel2-RHD, and suggesting that Dorsal and Rel2 may form heterodimers. More importantly, co-expression of MsDorsal-RHD with MsRel2-RHD suppressed activation of several M. sexta AMP gene promoters. Our results suggest that the short MsRel2 isoforms may form heterodimers with MsDorsal as a novel mechanism to prevent over-activation of antimicrobial peptides.

Identification and molecular characterization of dorsal and dorsal-like genes in the cyclopoid copepod Paracyclopina nana

To date, knowledge of the immune system in aquatic invertebrates has been reported in only a few model organisms, even though all metazoans have an innate immune system. In particular, information on the copepod's immunity and the potential role of key genes in the innate immune systems is still unclear. In this study, we identified dorsal and dorsal-like genes in the cyclopoid copepod Paracyclopina nana. In silico analyses for identifying conserved domains and phylogenetic relationships supported their gene annotations. The transcriptional levels of both genes were slightly increased from the nauplius to copepodid stages, suggesting that these genes are putatively involved in copepodid development of P. nana. To examine the involvement of both genes in the innate immune response and under stressful conditions, the copepods were exposed to lipopolysaccharide (LPS), different culture densities, salinities, and temperatures. LPS significantly upregulated mRNA expressions of dorsal and dorsal-like genes, suggesting that both genes are transcriptionally sensitive in response to immune modulators. Exposure to unfavorable culture conditions also increased mRNA levels of dorsal and dorsal-like genes. These findings suggest that transcriptional regulation of the dorsal and dorsal-like genes would be associated with environmental changes in P. nana.

Antiviral responses of arthropod vectors: an update on recent advances

Arthropod vectors, such as mosquitoes, ticks, biting midges and sand flies, transmit many viruses that can cause outbreaks of disease in humans and animals around the world. Arthropod vector species are invading new areas due to globalisation and environmental changes, and contact between exotic animal species, humans and arthropod vectors is increasing, bringing with it the regular emergence of new arboviruses. For future strategies to control arbovirus transmission, it is important to improve our understanding of virus-vector interactions. In the last decade knowledge of arthropod antiviral immunity has increased rapidly. RNAi has been proposed as the most important antiviral response in mosquitoes and it is likely to be the most important antiviral response in all arthropods. However, other newly-discovered antiviral strategies such as melanisation and the link between RNAi and the JAK/STAT pathway via the cytokine Vago have been characterised in the last few years. This review aims to summarise the most important and most recent advances made in arthropod antiviral immunity.

Transcriptional profiles of Rel/NF-κB, inhibitor of NF-κB (IκB), and lipopolysaccharide-induced TNF-α factor (LITAF) in the lipopolysaccharide (LPS) and two Vibrio sp.-exposed intertidal copepod, Tigriopus japonicus

The immune system and the role of immunity-related genes have rarely been studied in copepods, even though copepods have a primitive immune response system and also have a potential in pathogen transport higher trophic levels. In this study, we firstly cloned and characterized three core immune genes such as nuclear factor κB (NF-κB), inhibitor of NF-κB (IκB), and lipopolysaccharide-induced TNF-α factor (LITAF) genes in the intertidal copepod Tigriopus japonicus. Several in silico analyses based on conserved domains, motifs, and phylogenetic relationships were supporting their annotations. To investigate the immune-related role of three genes, we exposed lipopolysaccharide (LPS) and two Vibrio sp. to T. japonicus. After exposure of different concentrations of LPS and two Vibrio sp., transcripts of TJ-IκB and TJ-LITAF genes were significantly elevated during the time course in a dose-dependent manner, while TJ-NF-κB transcripts were not significantly changed during exposure. These findings demonstrated that the copepod T. japonicus has a conserved immunity against infection.

Validation of Aedes aegypti Aag-2 cells as a model for insect immune studies

Background

The understanding of mosquito immune responses can provide valuable tools for development of novel mosquito control strategies. Aiming the study at insect innate immunity, continuous insect cell lines have been established and used as research tools due to the fact that they constitute more homogeneous, sensitive, and reproducible systems than the insects from which they originated. More recently, Aag-2, an Aedes aegypti cell lineage, began to be frequently used as a model for studies of mosquito immunity. Nevertheless, to our knowledge, no study has systematically characterized the responses of Aag-2 cell line against different kinds of pathogens and compared its response to those exhibited by whole mosquitoes. For this reason, in this study we characterized gene expression profiles of the Aag-2 cell line in response to different kinds of immune challenges, such as Gram negative and positive bacteria, fungi and viruses, comparing the obtained results with the ones already described in the literature for whole mosquitoes.

Methods

Aedes aegypti Aag-2 cells were exposed to different immune stimuli (gram-positive and gram negative heat inactivated bacteria, zymosan or Sindbis virus) for 24 hours and the expression of selected marker genes from toll, IMD and Jak/STAT pathways was analyzed by qPCR. Also, cells were incubated with fluorescent latex beads for evaluation of its phagocytosis capacity.

Results

Aag-2 cells were stimulated with two concentrations of heat-killed Gram negative (Enterobacter cloacae) or Gram positive (Micrococcus luteus) bacteria, Zymosan or infected with Sindbis virus and the expression of key genes from the main immune related pathways, Toll, IMD and Jak/STAT, were investigated. Our results suggest that Toll and IMD pathways are activated in response to both Gram positive and negative bacteria and Zymosan in Aag-2 cells, displaying an immune profile similar to those described in the literature for whole mosquitoes. The same stimuli were also capable of activating Jak/STAT pathway in Aag-2 cells. Infection with Sindbis virus led to an up-regulation of the transcription factor STAT but was not able to induce the expression of any other gene from any of the pathways assayed. We also showed that this cell line is able to phagocytose latex beads in culture.

Conclusions

Our results characterize the expression profile of Aag-2 cells in response to different immune stimuli and demonstrate that this cell lineage is immune-competent and closely resembles the response described for whole Ae. aegypti mosquitoes. Hence, our findings support the use of Aag-2 as a tool to comprehend Ae. aegypti immune response both at cellular and humoral levels.

Transcriptome analysis of Aedes aegypti transgenic mosquitoes with altered immunity

The mosquito immune system is involved in pathogen-elicited defense responses. The NF-κB factors REL1 and REL2 are downstream transcription activators of Toll and IMD immune pathways, respectively. We have used genome-wide microarray analyses to characterize fat-body-specific gene transcript repertoires activated by either REL1 or REL2 in two transgenic strains of the mosquito Aedes aegypti. Vitellogenin gene promoter was used in each transgenic strain to ectopically express either REL1 (REL1+) or REL2 (REL2+) in a sex, tissue, and stage specific manner. There was a significant change in the transcript abundance of 297 (79 up- and 218 down-regulated) and 299 (123 up- and 176 down-regulated) genes in fat bodies of REL1+ and REL2+, respectively. Over half of the induced genes had predicted functions in immunity, and a large group of these was co-regulated by REL1 and REL2. By generating a hybrid transgenic strain, which ectopically expresses both REL1 and REL2, we have shown a synergistic action of these NF-κB factors in activating immune genes. The REL1+ immune transcriptome showed a significant overlap with that of cactus (RNAi)-depleted mosquitoes (50%). In contrast, the REL2+ -regulated transcriptome differed from the relatively small group of gene transcripts regulated by RNAi depletion of a putative inhibitor of the IMD pathway, caspar (35 up- and 140 down-regulated), suggesting that caspar contributes to regulation of a subset of IMD-pathway controlled genes. Infections of the wild type Ae. aegypti with Plasmodium gallinaceum elicited the transcription of a distinct subset of immune genes (76 up- and 25 down-regulated) relative to that observed in REL1+ and REL2+ mosquitoes. Considerable overlap was observed between the fat body transcriptome of Plasmodium-infected mosquitoes and that of mosquitoes with transiently depleted PIAS, an inhibitor of the JAK-STAT pathway. PIAS gene silencing reduced Plasmodium proliferation in Ae. aegypti, indicating the involvement of the JAK-STAT pathway in anti-Plasmodium defense in this infection model.

The fat body transcriptomes of the yellow fever mosquito Aedes aegypti, pre- and post- blood meal

BACKGROUND

The fat body is the main organ of intermediary metabolism in insects and the principal source of hemolymph proteins. As part of our ongoing efforts to understand mosquito fat body physiology and to identify novel targets for insect control, we have conducted a transcriptome analysis of the fat body of Aedes aegypti before and in response to blood feeding.

RESULTS

We created two fat body non-normalized EST libraries, one from mosquito fat bodies non-blood fed (NBF) and another from mosquitoes 24 hrs post-blood meal (PBM). 454 pyrosequencing of the non-normalized libraries resulted in 204,578 useable reads from the NBF sample and 323,474 useable reads from the PBM sample. Alignment of reads to the existing reference Ae. aegypti transcript libraries for analysis of differential expression between NBF and PBM samples revealed 116,912 and 115,051 matches, respectively. De novo assembly of the reads from the NBF sample resulted in 15,456 contigs, and assembly of the reads from the PBM sample resulted in 15,010 contigs. Collectively, 123 novel transcripts were identified within these contigs. Prominently expressed transcripts in the NBF fat body library were represented by transcripts encoding ribosomal proteins. Thirty-five point four percent of all reads in the PBM library were represented by transcripts that encode yolk proteins. The most highly expressed were transcripts encoding members of the cathepsin b, vitellogenin, vitellogenic carboxypeptidase, and vitelline membrane protein families.

CONCLUSION

The two fat body transcriptomes were considerably different from each other in terms of transcript expression in terms of abundances of transcripts and genes expressed. They reflect the physiological shift of the pre-feeding fat body from a resting state to vitellogenic gene expression after feeding.

Knock-down of REL2, but not defensin A, augments Aedes aegypti susceptibility to Bacillus subtilis and Escherichia coli

Some components of the Toll and Imd immune signaling pathways have remained conserved between Drosophila and mosquitoes, however, important differences in the way invading microorganisms activate these pathways in these organisms have started to be revealed. In the present study, we have attempted to silence the Aedes aegypti NF-kappaB-like factor REL2, which is analogous to Drosophila Relish, and analyze the effects on mosquito mortality upon infection with a Gram-negative and a Gram-positive bacterium, both containing a DAP-type peptidoglycan, and effects on embryo development. Moreover, we have silenced one of the antimicrobial peptides (AMPs) controlled by REL2, defensin A, a major AMP in A. aegypti, and compared the results on mosquito mortality upon bacterial infection to those obtained with REL2 silencing. Results show that REL2 is crucial for A. aegypti immunity upon infection with Escherichia coli and Bacillus subtilis, corroborating with previous studies on that REL2/Imd in mosquitoes is involved in a generalized antibacterial defense, differently than its analogous in Drosophila. However, defensin A silencing did not cause a significant increase in mortality in infected mosquitoes, indicating that this peptide is not essential in mosquito protection against the two bacteria and that other immune factors controlled by REL2 are playing this role. In regard to embryo development, REL2 knock-down did not cause any significant effect on: the number of laid eggs, number of developed pupae, percent of emerged adults, and ratio between emerged females versus males. A slight decrease in the number of hatched eggs (percent eclosion) was observed in REL2 knock-down mosquitoes, but these observations were inconclusive.

Cloning and characterization of Dorsal homologues in the hemipteran Rhodnius prolixus

Rhodnius prolixus is an ancient haematophagous hemipteran insect capable of mounting a powerful immune response. This response is transcriptionally regulated in part by transcription factors of the Rel/Nuclear Factor kappa B (Rel/NF-kappaB) family. We have cloned and characterized three members of this transcription factor family in this insect. Dorsal 1A is primarily expressed in early developmental stages. In contrast, dorsal 1B and 1C, both differentially spliced products of dorsal 1A, are expressed primarily in the adult fat body in response to septic injury, suggesting their exclusive role in immunity. Additionally, we identified putative kappaB binding sites in the 5' upstream regions of target genes known to be involved in the innate immune response of insects.

Identification of a novel relish homolog in Chinese shrimp Fenneropenaeus chinensis and its function in regulating the transcription of antimicrobial peptides

Penaeid shrimp, as an invertebrate, relies on the innate immunity to oppose the microbial invaders. Antimicrobial peptides (AMP) are an integral component of the innate immune system in most organisms and function as an early first line of defense against pathogens, but the knowledge about the pathways to regulate the shrimp AMP gene expression is still absent up to date. In the current study, a Relish homolog (FcRelish) was cloned from Chinese shrimp Fenneropenaeus chinensis. The full length cDNA of FcRelish consists of 2157 bp, including 1512 bp open reading frame, encoding 504 amino acids. The predicted molecular weight of FcRelish is 57 kDa, and the theoretical PI is 7.00. Spatial expression profiles showed that FcRelish had the highest expression levels in the hemocytes and lymphoid organ. Both Vibrio anguillarium and Micrococcus lysodeikticus stimulation to shrimp can affect the transcription profile of FcRelish. Silencing of FcRelish through DsRNA interference can greatly change the transcription profile of AMP. Therefore, we suggest that FcRelish identified in the present study is closely related to the transcription of AMP, and then we inferred that Imd pathway might exist in shrimp.

A novel system for the launch of alphavirus RNA synthesis reveals a role for the Imd pathway in arthropod antiviral response

Alphaviruses are RNA viruses transmitted between vertebrate hosts by arthropod vectors, primarily mosquitoes. How arthropods counteract alphaviruses or viruses per se is not very well understood. Drosophila melanogaster is a powerful model system for studying innate immunity against bacterial and fungal infections. In this study we report the use of a novel system to analyze replication of Sindbis virus (type species of the alphavirus genus) RNA following expression of a Sindbis virus replicon RNA from the fly genome. We demonstrate deficits in the immune deficiency (Imd) pathway enhance viral replication while mutations in the Toll pathway fail to affect replication. Similar results were observed with intrathoracic injections of whole virus and confirmed in cultured mosquito cells. These findings show that the Imd pathway mediates an antiviral response to Sindbis virus replication. To our knowledge, this is the first demonstration of an antiviral role for the Imd pathway in insects.

Identification and functional study of a shrimp Relish homologue

Rel/NF-kappaB transcription factors play central roles in induction and regulation of innate immune responses. Here we describe the identification and functional analysis of a Relish homologue, LvRelish and its shorter isoform sLvRelish, from the Pacific white shrimp, Litopenaeus vannamei. The LvRelish gene has 22 exons in approximately 15 kb genomic sequence. The full-length cDNA of LvRelish is 4071 bp with an open reading frame that encodes 1207 amino acids. LvRelish contains a conserved Rel homology domain (RHD), a nucleus localization signal, an IkappaB-like domain (six ankyrin repeats), and a death domain, suggesting that it belongs to the class I NF-kappaB. sLvRelish cDNA is 1051 bp encoding 317 amino acids. It shares the RHD region with LvRelish. RT-PCR analysis showed that LvRelish and sLvRelish mRNAs were expressed at different levels in tissues. Western blot analysis showed that recombinant intact LvRelish could be cleaved into two fragments in S2 cells, and immunofluorescence assay showed that the plasmid-expressed LvRelish protein was seen both in the cytoplasm and the nucleus. Electrophoretic mobility shift assay showed that recombinant RHD of LvRelish in S2 cells bound specifically with Drosophila melanogaster kappaB motifs in vitro. Both the LvRelish and its RHD domain transactivated the reporter gene controlled by the 5' flanking region of penaeidin 4, an antibacterial peptide of shrimp, suggesting that LvRelish can regulate the transcription of penaeidin 4 gene. Identification of LvRelish will help us better understand shrimp immunity and may help obtain more effective methods to prevent shrimp diseases.

The role of NF-kappaB factor REL2 in the Aedes aegypti immune response

Mosquitoes transmit numerous diseases that continue to be an enormous burden on public health worldwide. Transgenic mosquitoes impervious to vector-borne pathogens, in concert with vector control and drug and vaccine development, comprise an arsenal of means anticipated to defeat mosquito-spread diseases in the future. Mosquito transgenesis allows tissue-specific manipulation of their major immune pathways and enhances the ability to study mosquito-pathogen interactions. Here, we report the generation of two independent transgenic strains of Aedes aegypti overexpressing the NF-?B transcriptional factor REL2, a homologue of Drosophila Relish, which is shown to be under the control of the vitellogenin promoter in the mosquito fat body after a blood meal. We show that this REL2 overexpression in the fat body results in transcriptional activation of Defensins A, C, and D, and Cecropins A and N, as well as translation and secretion of Defensin A protein into the hemolymph. We also demonstrate that induction of REL2 results in the increased resistance of the mosquito to tested Gram-negative and Gram-positive bacteria. Importantly, induction of transgenic REL2 leads to the significant decrease in susceptibility of A. aegypti to Plasmodium gallinaceum infection. Consistently, RNAi knockdown of REL2 in wild-type mosquitoes results in a delay in Defensin A and Cecropin A expression in response to infection and in increased susceptibility to both bacteria and P. gallinaceum. Moreover, our transgenic assays demonstrate that the N-terminus of the mosquito REL2, which includes the His/Gln-rich and serine-rich regions, plays a role in its transactivation properties.

Aedes FADD: a novel death domain-containing protein required for antibacterial immunity in the yellow fever mosquito, Aedes aegypti

Microbial infections in insects activate a series of immune responses that culminate in the production of antimicrobial peptides (AMPs). In Drosophila, two signaling pathways, govern the challenge-dependent expression of AMPs; the Toll and IMD pathways. While AMPs have been the subject of much research in mosquitoes, the regulation of the pathways required for AMP expression remains largely unknown. We report here the identification of Aedes FADD (AeFADD), a death domain protein in Aedes aegypti. AeFadd is expressed in all immune-competent tissues and all developmental stages examined. At the transcriptional level, AeFadd transcripts increased when challenged with Escherichia coli but not Micrococcus luteus. In both cases, we observed the induction of two AMP genes; cecropin and defensin. Loss of AeFadd function by dsRNA interference impaired the inducible expression of both AMPs, and rendered adult mosquitoes susceptible to both types of bacteria. Identifying molecules that regulate mosquito immunity may help elucidate the factors that contribute to the vectorial capacity and provide insights into general mechanisms that regulate innate immunity.

Differential activation of the lectin and antimicrobial peptide genes in Sarcophaga peregrina (the flesh fly)

Sarcophaga lectin is an immune defense protein which is transcriptionally induced upon immune challenge in the flesh fly, Sarcophaga peregrina. So far, we have revealed that the Sarcophaga lectin gene has multiple NF-kappaB -binding motifs in its promoter. Here we showed that the nuclear extracts from Sarcophaga-derived culture cells, NIH-Sape-4, and larval fat bodies have binding activity to the multiple kappaB motifs in the lectin gene promoter, some of which were responsive to immune stimuli. We also compared the expression profiles of the lectin gene with those of the antibacterial peptide genes from the point of view of inducers, expression tissues and local induction in digestive tracts. In each case, the lectin gene was activated in different manners from other inducible defense genes. These results indicate the complex regulation of the lectin gene, possibly by NF-kappaB -related transcription factors.

Alternative splicing in the NF-kappaB signaling pathway

Activation of transcription factor NF-kappaB can affect the expression of several hundred genes, many of which are involved in inflammation and immunity. The proper NF-kappaB transcriptional response is primarily regulated by post-translational modification of NF-kappaB signaling constituents. Herein, we review the accumulating evidence suggesting that alternative splicing of NF-kappaB signaling components is another means of controlling NF-kappaB signaling. Several alternative splicing events in both the tumor necrosis factor and Toll/interleukin-1 NF-kappaB signaling pathways can inhibit the NF-kappaB response, whereas others enhance NF-kappaB signaling. Alternative splicing of mRNAs encoding some NF-kappaB signaling components can be induced by prolonged exposure to an NF-kappaB-activating signal, such as lipopolysaccharide, suggesting a mechanism for negative feedback to dampen excessive NF-kappaB signaling. Moreover, some NF-kappaB alternative splicing events appear to be specific for certain diseases, and could serve as therapeutic targets or biomarkers.

Elucidating the function of an ancient NF-kappaB p100 homologue, CrRelish, in antibacterial defense

The family of NF-kappaB transcription factors essentially regulates immune-related gene expression. Recently, we isolated and characterized the classical NF-kappaB/inhibitor kappaB (IkappaB) homologues from a "living fossil," the horseshoe crab, Carcinoscorpius rotundicauda. Interestingly, this ancient species also harbors another class I NF-kappaB p100 homologue, C. rotundicauda Relish (CrRelish). Similar to Drosophila Relish and the mammalian p100, CrRelish contains both the Rel-homology domains (RHD) and the IkappaB-like domain. In this study, we found that the RHD of CrRelish can recognize horseshoe crab and human kappaB response elements and activate the downstream reporter in vitro, thereby suggesting the evolutionary conservation of this molecule. Pseudomonas aeruginosa infection transcriptionally upregulates CrRelish, which exhibits a dynamic protein profile over the time course of infection. Surprisingly, secondary infection reinduced an upsurge in CrRelish protein expression to a level which overrode the protein degradation at 12 h postinfection. These observations strongly suggest the involvement of CrRelish in antibacterial defense. Secondary infection causes (i) the maintenance of a favorable expression-competent sequence context of the CrRelish gene and/or (ii) the derepression or stabilization of the CrRelish transcript resulting from the primary infection to enable the more rapid expression and accumulation of the CrRelish protein, reflecting apparent signal/immune priming in a repeated infection.

Identification and functional analysis of Relish homologs in the silkworm, Bombyx mori

Two cDNAs designated BmRelish1 and 2, that encode Relish homologs, were cloned from the silkworm, Bombyx mori. BmRelish1 had an IkappaB-like domain with 5 ankyrin repeats in addition to Rel homology domain (RHD), nuclear localization signal (NLS), and acidic and hydrophobic amino acids (AHAA) rich regions. On the other hand, BmRelish2 lacked the AHAA and ankyrin repeats (ANK). Knockdown of the BmRelish gene in transgenic silkworms resulted in failure of the activation of antimicrobial peptide genes by Escherichia coli, suggesting that BmRelish plays an important role in antimicrobial peptide gene expression. Functional analysis of BmRelish1 and 2 in mbn-2 cells showed that both Relish homologs do not activate promoters of B. mori antimicrobial peptide genes encoding cecropin B1, attacin, lebocin 3 and lebocin 4. However, a gene construct BmRelish1-d2 lacking the ANK strongly activated promoters of these genes. Another gene construct lacking AHAA and ANK failed to activate these genes, suggesting that BmRelish becomes active by removal of the ANK and that the AHAA-rich region is a transactivation domain. BmRelish2 was shown to repress activation of Cecropin B1 gene expression by BmRelish1-d2, suggesting that BmRelish2 plays a role as a dominant negative factor against the BmRelish1 active form. Necessity of kappaB sites of Cecropin B1, Attacin and Lebocin 4 genes for the full activation of these genes by BmRelish1-d2 was confirmed. The requirement of the mandatory kappaB sites for Lebocin 4 gene expression was different between BmRelish1 active form and BmRelA, suggesting differential roles for kappaB sites in antimicrobial peptide gene activation by different transcription factors. The binding of the RHD portion of BmRelish1 fusion protein to the kappaB sites of Cecropin B1 and Attacin genes was also confirmed.

A p38 MAP kinase regulates the expression of the Aedes aegypti defensin gene in mosquito cells

An Aedes aegypti p38 (Aap38) mitogen-activated protein kinase was isolated and characterized in this study. The 1761 bp long full-length Aap38 cDNA encodes an open reading frame of 358 amino acids, exhibiting characteristics of Thr/Tyr dual kinase specificities. We showed that bacteria activate both the kinase activity of Aap38 and the expression of the Aedes aegypti defensin A (AaDefA) gene, which is inhibited by a p38 kinase inhibitor SB203580 and dsRNA interference of Aap38. A similar result was obtained by a reporter construct containing the AaDefA regulatory region linked to Ds-Red. The lipopolysaccharide-activated reporter gene was inhibited by SB203580. In addition, Aap38 translocated to the nucleus after lipopolysaccharide induction. Our findings suggest that the p38 protein kinase pathway is involved in the antibacterial peptide synthesis in mosquitoes.

Innate immune responses regulate trypanosome parasite infection of the tsetse fly Glossina morsitans morsitans

Tsetse flies transmit the protozoan parasite African trypanosomes, the agents of human sleeping sickness in sub-Saharan Africa. Parasite transmission in the insect is restricted by a natural resistance phenomenon (refractoriness). Understanding the mechanism of parasite resistance is important as strengthening fly's response(s) via transgenic approaches can prevent parasite transmission and lead to the development of novel vector control strategies. Here, we investigated the role of one of the two major pathways regulating innate immunity in invertebrates, the immunodeficiency (Imd) pathway, for Glossina morsitans morsitans's natural defence against Trypanosoma brucei spp. infections. We determined the molecular structure of the Imd pathway transcriptional activator Relish (GmmRel), which shows high amino acid identity and structural similarity to its Drosophila homologue. Through a double-stranded RNA-based interference approach, we showed that the pathogen-induced expression profile of the antimicrobial peptides (AMPs) attacin and cecropin is under the regulation of GmmRel. Unexpectedly, the AMP diptericin appears to be constitutively expressed in tsetse independent of the presence of the Rel factor. Through GmmRel knock-down, we could successfully block the induction of attacin and cecropin expression in the immune responsive tissues fat body and proventriculus (cardia) following microbial challenge. The midgut and salivary gland trypanosome infection prevalence, as well as the intensity of midgut parasite infections were found to be significantly higher in flies when attacin and relish expression were knocked down. Our results provide the first direct evidence for the involvement of antimicrobial peptides in trypanosome transmission in tsetse.

A Toll Receptor and a Cytokine, Toll5A and Spz1C, Are Involved in Toll Antifungal Immune Signaling in the Mosquito Aedes aegypti

The fungal-specific immune response in the mosquito Aedes aegypti involves the Toll immune pathway transduced through REL1, a homologue of the NF-kappaB transcription factor Drosophila Dorsal. The Toll receptor and its ligand, Spätzle (Spz), link extracellular immune signals to the Toll intracellular transduction pathway. Five homologues to the Drosophila Toll (Toll1) receptor (Toll1A, Toll1B, Toll5A, Toll5B, and Toll4) and three homologues to the Drosophila cytokine Spätzle (Spz1A, 1B, and 1C) were identified from genomic and cDNA sequence data bases. Toll1A, Toll5A, Toll5B, and Spz1A were specifically induced in the mosquito fat body following fungal challenge. This transcriptional up-regulation was mediated by REL1. Spz1C was constitutively expressed in the mosquito fat body, whereas Spz1B and Toll4 were primarily expressed in ovarian tissues of female mosquitoes. The transcripts of Toll1B were only detected in early stages of mosquito embryos. RNA interference knock down of Toll5A and Spz1C resulted in two phenotypes of Aedes Toll/REL1 pathway deficiency: decreased induction of Aedes Serpin-27A following fungal challenge and increased susceptibility to the entomopathogenic fungus Beauveria bassiana. These data suggest that Toll5A and Spz1C function as cytokine receptor systems specific to the Toll receptor-mediated immune response following fungal challenge in the mosquito fat body.

A novel association between clustered NF-kappaB and C/EBP binding sites is required for immune regulation of mosquito Defensin genes

A comparative analysis identified key cis-acting regulatory elements responsible for the temporal control of mosquito Defensin gene expression. The promoters of Anopheles gambiae Defensin 1 and two isoforms of Aedes aegypti Defensin A are up-regulated by immune challenge. This stimulated activity depends upon a cluster of three NF-kappaB binding sites and closely associated C/EBP-like motifs, which function as a unit for optimal promoter activity. Binding of NF-kappaB and C/EBP like transcription factors is confirmed by electrophoretic mobility shift assay, including supershifts with antibodies to C/EBP. KappaB-like motifs are abundant within antimicrobial peptide gene promoters and most are very closely associated with putative C/EBP binding sites. This novel association between NF-kappaB and C/EBP binding sites may, therefore, be of widespread significance.

Evidence for the ancient origin of the NF-kappaB/IkappaB cascade: its archaic role in pathogen infection and immunity

The evolutionary conservation of the NF-kappaB transcription factors, from Drosophila to humans, underscores its pivotal role in immune response. Unexpectedly, the canonical NF-kappaB signaling pathway is not functional in the immune system of Caenorhabditis elegans. Therefore, the ancient origin of the NF-kappaB signaling pathway is still unknown. Here, we report the discovery and characterization of a primitive and functional NF-kappaB/IkappaB pathway in the immune defense of a "living fossil," the horseshoe crab, Carcinoscorpius rotundicauda. The ancient NF-kappaB/IkappaB homologues, CrNFkappaB, CrRelish, and CrIkappaB, share numerous signature motifs with their vertebrate orthologues. CrNFkappaB recognizes both horseshoe crab and mammalian kappaB response elements. CrIkappaB interacts with CrNFkappaB and inhibits its nuclear translocation and DNA-binding activity. The activation of the CrNFkappaB is autoregulated by a feedback mechanism mediated by CrIkappaB, the natural inhibitor of CrNFkappaB. We further show that Gram-negative bacteria infection causes rapid degradation of CrIkappaB and nuclear translocation of CrNFkappaB. Infection also leads to an increase in the kappaB-binding activity and up-regulation of immune-related gene expression, like inducible nitric oxide synthase and Factor C, an LPS-activated serine protease. Altogether, our study shows that, although absent in C. elegans, the NF-kappaB/IkappaB signaling cascade remains well conserved from horseshoe crab to humans, playing an archaic but fundamental role in regulating the expression of critical immune defense molecules.

Variation in positive selection in termite GNBPs and Relish

Social insects are model organisms for investigating molecular evolution in the innate immune system. Their diversity affords comparative analysis among closely related species, and group living is likely to contribute to the pathogen stress imposed on the immune system. We used different models of nucleotide substitution at nonsynonymous (amino acid altering) and synonymous (silent) sites to compare the different levels and type of selection among three immunity genes in 13 Australian termite species (Nasutitermes). The immunity genes include two encoding pathogen recognition proteins (gram-negative bacterial-binding proteins) that duplicated and diverged before or soon after the evolution of the termites and a transcription factor (Relish), which induces the production of antimicrobial peptides. A comparison of evolutionary models that assign four unrestricted classes of dN/dS (the ratio of the nonsynonymous to synonymous substitution rate) to different Nasutitermes lineages revealed that the occurrence of positive selection (dN/dS > 1) varies among lineages and the three genes. Positive selection appears to have driven the evolution of all three genes in an ancestral lineage of three subterranean termites. It had previously been suggested that there was a transition along this ancestral lineage to termite morphology and ecology associated with a diet of decayed wood, a diet that may expose termites to elevated levels of fungal and bacterial pathogens. Relish appears to have experienced the highest levels of selective pressure for change among all three genes. Positively selected sites in the molecule are located in regions that are important for its activation, which suggests that amino acid substitutions at these sites are a counter response to pathogen mechanisms that disrupt the activation of Relish.

A novel Rel protein and shortened isoform that differentially regulate antibacterial peptide genes in the silkworm Bombyx mori

Two cDNAs encoding novel Rel proteins were cloned from the silkworm, Bombyx mori. These cDNA clones (BmRelA and BmRelB) showed identical nucleotide sequences except for the 5'-region. BmRelB cDNA derived probably from an alternatively spliced mRNA lacked 241 bp nucleotides at the 5'-region of the BmRelA cDNA, resulting in a loss of the first 52 amino acids. Expression of antibacterial peptide genes was strongly inhibited upon infection with Micrococcus luteus in transgenic silkworms in which BmRel gene expression was knocked down, suggesting that these two Rel proteins are involved in activation of antibacterial peptide genes. Co-transfection experiments indicated that BmRelB activated the Attacin gene strongly and other genes to a lesser extent, whereas BmRelA activated Lebocin 4 gene strongly and Attacin and Lebocin 3 genes very weakly. The Rel homology domain of BmRelA and BmRelB was shown to bind specifically to kappaB sites of antibacterial peptide genes. Proline-rich domains of the BmRels were necessary for activation of antibacterial peptide genes. These results illustrate that a minor structural change in Rel proteins can provoke a dramatic differential activation of antibacterial peptide genes, suggesting a novel regulatory mechanism for insect antibacterial peptide gene expression.

Immune signaling pathways regulating bacterial and malaria parasite infection of the mosquito Anopheles gambiae

We show that, in the malaria vector Anopheles gambiae, expression of Cecropin 1 is regulated by REL2, an NF-kappaB-like transcription factor orthologous to Drosophila Relish. Through alternative splicing, REL2 produces a full-length (REL2-F) and a shorter (REL2-S) protein isoform lacking the inhibitory ankyrin repeats and death domain. RNA interference experiments show that, in contrast to Drosophila Relish, which responds solely to Gram-negative bacteria, the Anopheles REL2-F and REL2-S isoforms are involved in defense against the Gram-positive Staphylococcus aureus and the Gram-negative Escherichia coli bacteria, respectively. REL2-F also regulates the intensity of mosquito infection with the malaria parasite, Plasmodium berghei. The adaptor IMD shares the same activities as REL2-F. Microarray analysis identified 10 additional genes regulated by REL2, including CEC3, GAM1, and LRIM1.

REL1, a homologue of Drosophila dorsal, regulates toll antifungal immune pathway in the female mosquito Aedes aegypti

Signaling by Drosophila Toll pathway activates two Rel/NF-kappaB transcription factors, Dorsal (Dl) and Dorsal-related immune factor (Dif). Dl plays a central role in the establishment of dorsoventral polarity during early embryogenesis, whereas Dif mediates the Toll receptor-dependent antifungal immune response in adult Drosophila. The absence of a Dif ortholog in mosquito genomes suggests that Dl may play its functional role in the mosquito Toll-mediated innate immune responses. We have cloned and molecularly characterized the gene homologous to Drosophila Dl and to Anopheles gambiae REL1 (Gambif1) from the yellow fever mosquito Aedes aegypti, named AaREL1. AaREL1 alternative transcripts encode two isoforms, AaREL1-A and AaREL1-B. Both transcripts are enriched during embryogenesis and are inducible by septic injury in larval and female mosquitoes. AaREL1 and AaREL2 (Aedes Relish) selectively bind to different kappaB motifs from insect immune gene promoters. Ectopic expression of AaREL1-A in both Drosophila mbn-2 cells and transgenic flies specifically activates Drosomycin and results in increased resistance against the fungus Beauveria bassiana. AaREL1-B acted cooperatively with AaREL1-A to enhance the immune gene activation in Aag-2 cells. The RNA interference knock-outs revealed that AaREL1 affected the expression of Aedes homologue of Drosophila Serpin-27A and mediated specific antifungal immune response against B. bassiana. These results indicate that the homologue of Dl, but not that of Dif, is a key regulator of the Toll antifungal immune pathway in A. aegypti female mosquitoes.

Characterization of a homologue of the Rel/NF-kappaB transcription factor from a beetle, Allomyrina dichotoma

A cDNA encoding a Rel/NF-kappaB homologue was cloned from a beetle, Allomyrina dichotoma, by reverse transcriptase-polymerase chain reactions (RT-PCR) taking advantage of the conserved Rel homology domain (RHD) to synthesize primers. The Rel/NF-kappaB homologue was designated A. dichotoma (A.d.) Rel A. The amino acid sequence of the A.d. Rel A RHD was compared with those of insect RHDs. The result showed that it has 70% identity with Tribolium castaneum Dorsal, 66% with Drosophila melanogaster Dorsal, 61% with Anopheles gambiae Gambif1, and 55% with D. melanogaster Dif. A putative phosphorylation site in the RHD, RRPS, and two putative nuclear localization signals were conserved in A.d. Rel A. A recombinant fusion protein containing the A.d. Rel A RHD was confirmed to bind specifically to the NF-kappaB site of a gene encoding A.d. coleoptericin A, an antibacterial peptide from A. dichotoma. The activity of A.d. Rel A in modulating a gene construct of the A.d. coleoptericin A promoter-luciferase reporter by expressing the A.d. coleoptericin A cDNA in a Bombyx mori cell line was analyzed. The result showed that A.d. Rel A strongly activates the A.d. coleoptericin A gene construct, whereas A.d. Rel A failed to activate the gene construct containing the mutated NF-kappaB site, suggesting the importance of the interaction between the NF-kappaB site and A.d. Rel A in the signal transduction for gene expression of antibacterial peptides in A. dichotoma.

Innate immunity in the malaria vector Anopheles gambiae:comparative and functional genomics

The resurgence of malaria is at least partly attributed to the absence of an effective vaccine, parasite resistance to antimalarial drugs and resistance to insecticides of the anopheline mosquito vectors. Novel strategies are needed to combat the disease on three fronts: protection (vaccines),prophylaxis/treatment (antimalarial drugs) and transmission blocking. The latter entails either killing the mosquitoes (insecticides), preventing mosquito biting (bednets and repellents), blocking parasite development in the vector (transmission blocking vaccines), genetic manipulation or chemical incapacitation of the vector. During the past decade, mosquito research has been energized by several breakthroughs, including the successful transformation of anopheline vectors, analysis of gene function by RNAi,genome-wide expression profiling using DNA microarrays and, most importantly,sequencing of the Anopheles gambiae genome. These breakthroughs helped unravel some of the mechanisms underlying the dynamic interactions between the parasite and the vector and shed light on the mosquito innate immune system as a set of potential targets to block parasite development. In this context, putative pattern recognition receptors of the mosquito that act as positive and negative regulators of parasite development have been identified recently. Characterizing these molecules and others of similar function, and identifying their ligands on the parasite surface, will provide clues on the nature of the interactions that define an efficient parasite–vector system and open up unprecedented opportunities to control the vectorial capacity of anopheline mosquitoes.

Transgenesis and reverse genetics of mosquito innate immunity

In recent years, mosquito molecular biology has been a scene of astounding achievements, namely the development of genetic transformation, characterization of inducible tissue-specific promoters, and acquirement of mosquito genome sequences. However, the lack of a complete genetic tool box for mosquitoes remains a serious obstacle in our ability to study essential mosquito-specific mechanisms. Unlike Drosophila, very few null mutations for mosquito genes exist. The development of reverse-genetic analyses based on RNAi and transgenic techniques will help to compensate for these deficiencies and aid in identification of critical genes in important regulatory pathways. The study of mosquito innate immunity is one example and described here. In this study, we combine mosquito transgenesis with reverse genetics. The advantage of transgenesis is the ability to establish genetically stable, dominant-negative and overexpression phenotypes. Using the blood-meal-activated vitellogenin gene (Vg) promoter, we have generated transgenic mosquitoes with blood-meal-activated, overexpressed antimicrobial peptides, Defensin A and Cecropin A. Moreover, we have recently generated a transgenic dominant-negative Relish mosquito strain, which after taking a blood meal, becomes immune-deficient to infection by Gram-negative bacteria. The latter accomplishment has opened the door to a reverse-genetic approach in mosquitoes based on transgenesis.

Alternative splicing of the human cyclin D-binding Myb-like protein (hDMP1) yields a truncated protein isoform that alters macrophage differentiation patterns

We have cloned two novel, alternatively spliced messages of human cyclin D-binding Myb-like protein (hDMP1). The known, full-length protein has been named hDMP1alpha and the new isoforms, hDMP1beta and hDMP1gamma. The hDMP1alpha, -beta, and -gamma splice variants have unique expression patterns in normal hematopoietic cells; hDMP1beta mRNA transcripts are strongly expressed in quiescent CD34+ cells and freshly isolated peripheral blood leukocytes, as compared with hDMP1alpha. In contrast, activated T-cells and developing myeloid cells, macrophages, and granulocytes express low levels of hDMP1beta transcripts, and hDMP1gamma is ubiquitously and weakly expressed. Mouse Dmp1 has been shown to activate CD13/aminopeptidase N (APN) and p19ARF gene expression via binding to canonical DNA recognition sites in the respective promoters. Assessment of CD13/APN promoter responsiveness demonstrated that hDMP1alpha but not hDMP1beta and -gamma, is a transcriptional activator. Furthermore, hDMP1beta was found to inhibit the CD13/APN promoter transactivation ability of hDMP1alpha. Stable, ectopic expression of hDMP1beta and, to a lesser extent hDMP1gamma, reduced endogenous cell surface levels of CD13/APN in U937 cells. Moreover, stable, ectopic expression of hDMP1beta altered phorbol 12-myristate 13-acetate-induced terminal differentiation of U937 cells to macrophages and resulted in maintenance of proliferation. These results demonstrate that hDMP1beta antagonizes hDMP1alpha activity and suggest that cellular functions of hDMP1 may be regulated by cellular hDMP1 isoform levels.

Involvement of the JNK-like protein of the Aedes albopictus mosquito cell line, C6/36, in phagocytosis, endocytosis and infection of West Nile virus

We recently cloned a c-Jun amino-terminal kinase (JNK) sequence from the C6/36 cell line, derived from the mosquito Aedes albopictus. We showed that SP600125, an inhibitor of JNK proteins, inhibits phagocytosis by C6/36 cells, suggesting that the JNK-like protein regulates phagocytosis. Here, we show that C6/36 cells constitutively express low levels of mRNA encoding the antibacterial peptides, cecropin and defensin, but that these mRNAs were up-regulated upon stimulation by lipopolysaccharide (LPS). Thus, the C6/36 cells have properties similar to those of mammalian macrophages. To characterize further the functional properties of C6/36 cells, we have assayed the role of the JNK-like protein in phagocytosis, endocytosis, and viral infection. C6/36 cells phagocytosed bacteria and artificial beads, and this was only slightly up-regulated following LPS stimulation, suggesting that newly stimulated JNK-like protein was not necessary for phagocytosis. SP600125 inhibited the acidification of intracellular compartments, including those involved in the endocytic pathway. Pretreatment of C6/36 cells with SP600125 or bafilomycin A1, but not cytochalasin D, inhibited the entry of West Nile virus (WNV), suggesting that WNV is internalized mainly by endocytosis, and that the JNK signalling pathway is important for endocytic entry. These findings indicate that the JNK-like protein regulates basic physiological functions, including phagocytosis and endocytosis and infection of WNV.