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

Effect of C-type lectin 16 on dengue virus infection in Aedes aegypti salivary glands

C-type lectins (CTLs) are a family of carbohydrate-binding proteins and an important component of mosquito saliva. Although CTLs play key roles in immune activation and viral pathogenesis, little is known about their role in regulating dengue virus (DENV) infection and transmission. In this study, we established a homozygous CTL16 knockout Aedes aegypti mutant line using CRISPR/Cas9 to study the interaction between CTL16 and viruses in mosquito vectors. Furthermore, mouse experiments were conducted to confirm the transmission of DENV by CTL16-/- A. aegypti mutants. We found that CTL16 was mainly expressed in the medial lobe of the salivary glands (SGs) in female A. aegypti. CTL16 knockout increased DENV replication and accumulation in the SGs of female A. aegypti, suggesting that CTL16 plays an important role in DENV transmission. We also found a reduced expression of immunodeficiency and Janus kinase/signal transducer and activator of transcription pathway components correlated with increased DENV viral titer, infection rate, and transmission efficiency in the CTL16 mutant strain. The findings of this study provide insights not only for guiding future investigations on the influence of CTLs on immune responses in mosquitoes but also for developing novel mutants that can be used as vector control tools.

Mosquito Trilogy: Microbiota, Immunity and Pathogens, and Their Implications for the Control of Disease Transmission

In mosquitoes, the interaction between the gut microbiota, the immune system, and the pathogens that these insects transmit to humans and animals is regarded as a key component toward the development of control strategies, aimed at reducing the burden of severe diseases, such as malaria and dengue fever. Indeed, different microorganisms from the mosquito microbiota have been investigated for their ability to affect important traits of the biology of the host insect, related with its survival, development and reproduction. Furthermore, some microorganisms have been shown to modulate the immune response of mosquito females, significantly shaping their vector competence. Here, we will review current knowledge in this field, focusing on i) the complex interaction between the intestinal microbiota and mosquito females defenses, both in the gut and at humoral level; ii) how knowledge on these issues contributes to the development of novel and targeted strategies for the control of mosquito-borne diseases such as the use of paratransgenesis or taking advantage of the relationship between Wolbachia and mosquito hosts. We conclude by providing a brief overview of available knowledge on microbiota-immune system interplay in major insect vectors.

Inhibition of defensin A and cecropin A responses to dengue virus 1 infection in Aedes aegypti

Introduction

It is essential to determine the interactions between viruses and mosquitoes to diminish dengue viral transmission. These interactions constitute a very complex system of highly regulated pathways known as the innate immune system of the mosquito, which produces antimicrobial peptides that act as effector molecules against bacterial and fungal infections. There is less information about such effects on virus infections.

Objective

To determine the expression of two antimicrobial peptide genes, defensin A and cecropin A, in Aedes aegypti mosquitoes infected with DENV-1.

Materials and methods

We used the F₁ generation of mosquitoes orally infected with DENV-1 and real-time PCR analysis to determine whether the defensin A and cecropin A genes played a role in controlling DENV-1 replication in Ae. aegypti. As a reference, we conducted similar experiments with the bacteria Escherichia coli.

Results

Basal levels of defensin A and cecropin A mRNA were expressed in uninfected mosquitoes at different times post-blood feeding. The infected mosquitoes experienced reduced expression of these mRNA by at least eightfold when compared to uninfected control mosquitoes at all times post-infection. In contrast with the behavior of DENV-1, results showed that bacterial infection produced up-regulation of defensin and cecropin genes; however, the induction of transcripts occurred at later times (15 days). Conclusion: DENV-1 virus inhibited the expression of defensin A and cecropin A genes in a wild Ae. aegypti population from Venezuela.

Fas-associated death domain (FADD) in sea cucumber (Apostichopus japonicus): Molecular cloning, characterization and pro-apoptotic function analysis

Fas-associated death domain (FADD) is an adaptor protein that functions in transferring the apoptotic signals regulated by the death receptors. In this study, a full-length cDNA of FADD homologue in sea cucumber Apostichopus japonicas (AjFADD) was cloned and characterized, and its functional roles in apoptosis investigated. In healthy sea cucumbers, AjFADD was expressed in all detected tissues, with higher levels in coelomocytes and intestine. AjFADD mRNA and protein levels were significantly expressed in coelomocytes after exposed with LPS or poly (I:C) in vitro, and challenged with Vibrio splendidus in vivo. Moreover, siRNA-mediated AjFADD knockdown in coelomocyte much decreased AjFADD mRNA and protein levels as well as the coelomocytes apoptosis levels. Furthermore, over-expression of the expression plasmid pcDNA3.1 encoding AjFADD (pcAjFADD) significantly increased the apoptosis levels in HEK293 cells. Taken together, our results support that AjFADD is a novel pro-apoptotic protein that might play key roles in defensing the bacterial and virus invasion in sea cucumber.

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.

Functional characterisation of Holothuria leucospilota Fas-associated death domain in the innate immune-related signalling pathways

In this study, the functions of Holothuria leucospilota Fas-associated death domain (HLFADD) in the innate immune-related signalling pathways were investigated. The results showed that over-expression of HLFADD in HEK293T cells could activate the transcription factors NF-κB and activator protein-1 (AP-1), and induce the secretion of downstream pro-inflammatory cytokines IL-6, IL-8 and IL-18, suggesting the involvement of the sea cucumber FADD in activating the NF-κB and c-Jun NH₂-terminal kinase-dependent pathways. On the other hand, HLFADD could down-regulate the activations of NF-κB and AP-1 that induced by over-expression of H. leucospilota myeloid differentiation factor 88 (HLMyD88), which is supposed to be mediated through its interaction with HLMyD88 to keep the MyD88-dependent TLR signalling at a proper magnitude. The interaction of HLFADD and HLMyD88 were further supported by a co-immunoprecipitation assay. Moreover, HLFADD could activate transcription factor IFN regulatory factor-3 and induced the secretion of downstream IFN-α and IFN-β, indicating that the sea cucumber FADD may also activate the antiviral IFN signalling pathway. In summary, our study may give new insights on the functions of sea cucumber FADD in the innate immune-related signalling pathways.

Curious entanglements: interactions between mosquitoes, their microbiota, and arboviruses

Mosquitoes naturally harbor a diverse community of microorganisms that play a crucial role in their biology. Mosquito-microbiota interactions are abundant and complex. They can dramatically alter the mosquito immune response, and impede or enhance a mosquito's ability to transmit medically important arboviral pathogens. Yet critically, given the massive public health impact of arboviral disease, few such interactions have been well characterized. In this review, we describe the current state of knowledge of the role of microorganisms in mosquito biology, how microbial-induced changes to mosquito immunity moderate infection with arboviruses, cases of mosquito-microbial-virus interactions with a defined mechanism, and the molecular interactions that underlie the endosymbiotic bacterium Wolbachia's ability to block virus infection in mosquitoes.

Immune response-related genes associated to blocking midgut dengue virus infection in Aedes aegypti strains that differ in susceptibility

Aedes (Stegomyia) aegypti, the principal global vector of dengue viruses, has differences in its susceptibility to dengue virus infection. We compared the global expression of genes in the midguts of Colombian Ae. aegypti dengue-susceptible (Cali-S) and dengue-refractory (Cali-MIB) field derived strains after ingesting either a sugarmeal, a bloodmeal, or a bloodmeal containing dengue virus serotype 2 (DENV-2). Microarray-based transcriptome analysis among treatments indicated a total of 4725 transcripts with differential expression between the two strains. Eleven genes were selected from different functional groups based on their significant up or down expression levels as well as reports in the literature suggesting they are associated with dengue virus elimination. We measured mRNA abundance of these 11 genes at 0, 8, 24, and 36 h postinfection using quantitative real time PCR (qPCR) to confirm the microarray results and assess any temporal patterns. Four genes were selected (Gram-negative binding protein-GNBP [AAEL009176], Niemann Pick Type-C2-NPC2 [AAEL015136], Keratinocyte lectin [AAEL009842], and Cathepsin-b [AAEL007585]) for knockdown experiments using RNA interference (RNAi) methodology to determine the phenotype (DENV-2 susceptible or refractory). Silencing GNBP, Cathepsin-b and Keratinocyte lectin reduced the percentage of mosquitoes with disseminated virus in the Cali-S strain to 8%, 20%, and 12% respectively compared with 96% in the controls. Silencing of NPC2 increased the percentage of mosquitos with disseminated virus infections in Cali-MIB to 66% compared with 35% in the controls. This study provides insight into genes that may contribute to the Cali-S susceptible and Cali-MIB refractory phenotypes in Ae. aegypti.

The first cloned sea cucumber FADD from Holothuria leucospilota: Molecular characterization, inducible expression and involvement of apoptosis

In this study, a sea cucumber Fas-associated death domain (FADD) named HLFADD was first cloned from Holothuria leucospilota. The full-length cDNA of HLFADD is 2137 bp in size, containing a 116-bp 5'-untranslated region (UTR), a 1334-bp 3'-UTR and a 687-bp open reading frame (ORF) encoding a protein of 228 amino acids with a deduced molecular weight of 26.42 kDa. HLFADD protein contains a conserved death effector domain at its N-terminal and a conserved death domain at its C-terminal, structurally similar to its counterparts in vertebrates. The over-expressed HLFADD protein could induce apoptosis in HEK293 cells, suggesting a possible death receptor-mediated apoptosis pathway in echinoderms adapted with FADD. Moreover, HLFADD mRNA is ubiquitously expressed in all examined tissues, with the highest transcript level in the coelomocytes, followed by intestine. In vitro experiments performed in the H. leucospilota coelomocytes, the expression of HLFADD mRNA was significantly up-regulated by lipopolysaccharides (LPS) or polyriboinosinic-polyribocytidylic acid [poly (I:C)] challenge, suggesting that HLFADD might play important roles in the innate immune defense of sea cucumber against the invasion of bacteria and viruses.

Molecular cloning and characterization of FADD from the manila clam Ruditapes philippinarum

Fas-associated protein with death domain (FADD) is an essential element in cell death, and also implicates in cell cycle progression, inflammation and innate immunity. In the study, an FADD (designated as RpFADD) was identified and characterized from manila clam, Ruditapes philippinarum. Multiple alignments and phylogenetic analysis strongly suggested that RpFADD was a new member of the FADD family. The RpFADD transcripts were constitutively expressed in a wide range of tissues, and dominantly expressed in hemocytes. After challenged with Vibrio anguillarum or Micrococcus luteus, the expression level of RpFADD transcripts was significantly induced and reached the maximum level at 72 h and 48 h, respectively. Knockdown of RpFADD down-regulated the transcript levels of RpIKK, RpTAK1 and RpNF-κB with the exception of RpIκB. Moreover, RpFADD primarily localized in the cell cytoplasm, and its over-expression promoted the apoptosis of HeLa cells. These results revealed that RpFADD perhaps regulated the NF-κB signaling pathways positively, which provided a better understanding of RpFADD in innate immunity.

The RNAi pathway plays a small part in Wolbachia-mediated blocking of dengue virus in mosquito cells

Wolbachia pipientis is an insect endosymbiont known to limit the replication of viruses including dengue and Zika in their primary mosquito vector, Aedes aegypti. Wolbachia is being released into mosquito populations globally in a bid to control the diseases caused by these viruses. It is theorized that Wolbachia’s priming of the insect immune system may confer protection against subsequent viral infection. Other hypotheses posit a role for competition between Wolbachia and viruses for host cellular resources. Using an A. aegypti cell line infected with Wolbachia, we tested the effects of targeting siRNAs against the major innate immune pathways on dengue virus loads. We show that while Wolbachia infection induces genes in the Toll, JAK/STAT and RNAi pathways, only reduced expression of RNAi leads to a rebound of dengue virus loads in Wolbachia-infected cells. The magnitude of the effect explained less than 10% of the total DENV load, demonstrating that blocking must be dependent on other factors in addition to the expression of RNAi. The findings bode well for the long-term stability of blocking given that immunity gene expression would likely be highly plastic and susceptible to rapid evolution.

Identification of Aadnr1, a novel gene related to innate immunity and apoptosis in Aedes albopictus

Innate immunity and apoptosis play critical roles in defending pathogens in insects. In Drosophila, Dnr1 was reported as a negative regulator of apoptosis and immune deficiency (Imd) pathway which belongs to innate immunity. Aedes albopictus is an important kind of arbovirus vector and becoming a significant threat to public health due to its rapid global expansion. Here we identified an ortholog of dnr1 from A. albopictus, named as Aadnr1. Aadnr1 encoded a putative protein containing an N-terminal FERM domain and a C-terminal RING domain. AaDnr1 shared high identity with dipteran insects Dnr1 orthologs. Phylogenetic analyses showed that the closest relative of AaDnr1 was Aedes aegypti Dnr1. Real-time PCR proved that Aadnr1 mRNA was expressed ubiquitously during developmental and adult stages. Transcriptional levels of Aadnr1 were decreased drastically in C6/36 cells underwent apoptosis induced by Actinomycin D (Act D) treatment. Partial silence of Aadnr1 enhanced Act D-induced caspase activity. When challenged by heat-inactivated E. coli, transcriptional level of Aadnr1 was also decreased dramatically in C6/36 cells. While when C6/36 cells were infected with Sindbis virus TE/GFP, transcriptional level of Aadnr1 was reduced and recovered repeatedly, with an overall decreasing trend. It was also shown in this study that similar to Drosophila Dnr1, RING domain destabilized AaDnr1 protein. Taken together, the study identified an innate immunity and apoptosis related gene Aadnr1 in A. albopictus.

The multifaceted activity of insect caspases

Caspases are frequently considered synonymous with apoptotic cell death. Increasing evidence demonstrates that these proteases may exert their activities in non-apoptotic functions. The non-apoptotic roles of caspases may include developmentally regulated autophagy during insect metamorphosis, as well as neuroblast self-renewal and the immune response. Here, we summarize the established knowledge and the recent advances in the multiple roles of insect caspases to highlight their relevance for physiological processes and survival.

Gene discovery and differential expression analysis of humoral immune response elements in female Culicoides sonorensis (Diptera: Ceratopogonidae)

BACKGROUND

Female Culicoides sonorensis midges (Diptera: Ceratopogonidae) are vectors of pathogens that impact livestock and wildlife in the United States. Little is known about their biology on a molecular-genetic level, including components of their immune system. Because the insect immune response is involved with important processes such as gut microbial homeostasis and vector competence, our aims were to identify components of the midge innate immune system and examine their expression profiles in response to diet across time.

METHODS

In our previous work, we de novo sequenced and analyzed the transcriptional landscape of female midges under several feeding states including teneral (unfed) and early and late time points after blood and sucrose. Here, those transcriptomes were further analyzed to identify insect innate immune orthologs, particularly humoral immune response elements. Additionally, we examined immune gene expression profiles in response to diet over time, on both a transcriptome-wide, whole-midge level and more specifically via qRTPCR analysis of antimicrobial peptide (AMP) expression in the alimentary canal.

RESULTS

We identified functional units comprising the immune deficiency (Imd), Toll and JAK/STAT pathways, including humoral factors, transmembrane receptors, signaling components, transcription factors/regulators and effectors such as AMPs. Feeding altered the expression of receptors, regulators, AMPs, prophenoloxidase and thioester-containing proteins, where blood had a greater effect than sucrose on the expression profiles of most innate immune components. qRTPCR of AMP genes showed that all five were significantly upregulated in the alimentary canal after blood feeding, possibly in response to proliferating populations of gut bacteria.

CONCLUSIONS

Identification and functional insight of humoral/innate immune components in female C. sonorensis updates our knowledge of the molecular biology of this important vector. Because diet alone influenced the expression of immune pathway components, including their effectors, subsequent study of the role of innate immunity in biological processes such as gut homeostasis and life history are being pursued. Furthermore, since the humoral response is a key contributor in gut immunity, manipulating immune gene expression will help in uncovering genetic components of vector competence, including midgut barriers to infection. The results of such studies will serve as a platform for designing novel transmission-blocking strategies.

The effects of temperature and innate immunity on transmission of Campylobacter jejuni (Campylobacterales: Campylobacteraceae) between life stages of Musca domestica (Diptera: Muscidae)

The house fly (Musca domestica L.) is a well-established vector of human pathogens, including Campylobacter spp., which can cause infection of broiler chicken flocks, and through contaminated broiler meat can cause outbreaks of campylobacteriosis in humans. We investigated whether Campylobacter jejuni (Jones) could be transferred between life stages of M. domestica (larvae-pupae-adults) and determined bacterial counts of C. jejuni at different time points after bacterial exposure. C. jejuni was transmitted from infected larvae to pupae, but not to the adult stage. Infected larvae maintained at 25 degrees C had mean bacterial numbers of 6.5 +/- 0.2 SE log10 (colony forming units [CFU]/g) that subsequently dropped to 3.6 +/- 0.3 SE log10 (CFU/g) 8 h after infection. Pupae originating from infected larvae contained mean bacterial numbers of 5.3 +/- 0.1 SE log10 (CFU/g), and these numbers dropped to 4.8 +/- 0.1 SE log10 (CFU/g) 24 h after pupation. The decline in C. jejuni numbers during pupal development coincided with increased expression of antimicrobial peptides, including cecropin, diptericin, attacin, and defensin, in the larva-pupa transition stage and a later second peak in older pupae (4 or 48 h). Conversely, there was a reduced expression of the digestive enzyme, lysozyme, in pupae and adults compared with larvae.

Differential expression of apoptosis related genes in selected strains of Aedes aegypti with different susceptibilities to dengue virus

Aedes aegypti is the principal vector of Dengue viruses worldwide. We identified field collected insects with differential susceptibility to Dengue-2 virus (DENv-2) and used isofemale selection to establish susceptible and refractory strains based on midgut infection barriers. Previous experiments had identified higher expression of apoptosis-related genes in the refractory strain. To identify potential molecular mechanisms associated with DENv susceptibility, we evaluated the differential expression of Caspase-16, Aedronc, Aedredd, Inhibitor of apoptosis (AeIAP1) and one member of the RNAi pathway, Argonaute-2 in the midguts and fat body tissues of the selected strains at specific times post blood feeding or infection with DENv-2. In the refractory strain there was significantly increased expression of caspases in midgut and fatbody tissues in the presence of DENv-2, compared to exposure to blood alone, and significantly higher caspase expression in the refractory strain compared with the susceptible strain at timepoints when DENv was establishing in these tissues. We used RNAi to knockdown gene expression; knockdown of AeIAP1 was lethal to the insects. In the refractory strain, knockdown of the pro-apoptotic gene Aedronc increased the susceptibility of refractory insects to DENv-2 from 53% to 78% suggesting a contributing role of this gene in the innate immune response of the refractory strain.

RNA interference is induced in the glassy winged sharpshooter Homalodisca vitripennis by actin dsRNA

BACKGROUND

The glassy winged sharpshooter, Homalodisca vitripennis, is an unusually robust and efficient leafhopper vector of Xylella fastidiosa. X. fastidiosa is the causative agent of Pierce's disease, almond scorch, citrus variegated chlorosis and other serious plant diseases. The present study was conducted to establish whether RNA interference (RNAi) was induced in nymphal H. vitripennis that were injected with actin dsRNAs and other dsRNAs.

RESULTS

A dramatic reduction in target H. vitripennis actin mRNAs and the formation of small interfering RNAs (siRNAs), hallmarks of RNAi, were found following the injection of actin dsRNAs. Quantitative reverse transcription PCR indicated an 80% reduction in actin mRNA levels by 5 days post-injection. Western blot analysis showed a dramatic drop in actin protein levels by 3 days post-injection. Biological effects such as incomplete nymphal-adult ecdysis and > 95% mortality were also found following the injection of fifth-instar nymphs with actin dsRNA. Dramatic reductions in target mRNA levels were also found following the injection of other dsRNAs into fifth-instar H. vitripennis.

CONCLUSION

The findings indicate that RNAi is induced in post-embryonic leafhoppers by dsRNA. The present system can be used to screen potential gene-silencing targets that can be used for reducing the vector competence of H. vitripennis and other leafhoppers.

Cysteine proteases from bloodfeeding arthropod ectoparasites

Cysteine proteases have been discovered in various bloodfeeding ectoparasites. Here, we assemble the available information about the function of these peptidases and reveal their role in hematophagy and parasite development. While most of the data shed light on key proteolytic events that play a role in arthropod physiology, we also report on the association of cysteine proteases with arthropod vectorial capacity. With emphasis on ticks, specifically Ixodes ricinus, we finally propose a model about the contribution of cysteine peptidases to blood digestion and how their concerted action with other tick midgut proteases leads to the absorbance of nutrients by the midgut epithelial cells.

Quantitative analysis of replication and tropisms of Dengue virus type 2 in Aedes albopictus

Dengue virus serotype 2 (DENV-2) RNA replication profiles and tropisms were studied by using quantitative RT-PCR (q-RTPCR) in intrathoracically infected Aedes albopictus. The virus RNA replication profiles were diverse in mosquito organs. In fat body, brain, salivary gland, and malpighian tubes, it peaked at 8, 23, 23, and 27 days post-infection, respectively, and then, all declined. In midgut, it increased all the time and had no trend of decline. In ovary, it had no apparent increase. Subsequent Western blotting of DENV-2 E protein had similar results. Using ribosomal protein 7 (rpS7) as an internal control, we found that, in salivary gland, brain, fat body, and midgut, the average DENV-2 RNA levels (DENV-2 RNA/rpS7 mRNA) were 1,028, 464, 5.6, and 6.2, respectively; in malpighian tubes, it was 1, and in ovary, it was far less than 1. These results suggest that infection profiles and tropism of DENV-2 RNA in Ae. albopictus organs are significantly different.