Annotation and expression profiling of apoptosis-related genes in the yellow fever mosquito, Aedes aegypti

Expressing the Pro-Apoptotic Reaper Protein via Insertion into the Structural Open Reading Frame of Sindbis Virus Reduces the Ability to Infect Aedes aegypti Mosquitoes

Arboviruses continue to threaten a significant portion of the human population, and a better understanding is needed of the determinants of successful arbovirus infection of arthropod vectors. Avoiding apoptosis has been shown to be one such determinant. Previous work showed that a Sindbis virus (SINV) construct called MRE/rpr that expresses the Drosophila pro-apoptotic protein Reaper via a duplicated subgenomic promoter had a reduced ability to orally infect Aedes aegypti mosquitoes at 3 days post-blood meal (PBM), but this difference diminished over time as virus variants containing deletions in the inserted reaper gene rapidly predominated. In order to further clarify the effect of midgut apoptosis on disseminated infection in Ae. aegypti, we constructed MRE/rprORF, a version of SINV containing reaper inserted into the structural open reading frame (ORF) as an in-frame fusion. MRE/rprORF successfully expressed Reaper, replicated similarly to MRE/rpr in cell lines, induced apoptosis in cultured cells, and caused increased effector caspase activity in mosquito midgut tissue. Mosquitoes that fed on blood containing MRE/rprORF developed significantly less midgut and disseminated infection when compared to MRE/rpr or a control virus up to at least 7 days PBM, when less than 50% of mosquitoes that ingested MRE/rprORF had detectable disseminated infection, compared with around 80% or more of mosquitoes fed with MRE/rpr or control virus. However, virus titer in the minority of mosquitoes that became infected with MRE/rprORF was not significantly different from control virus. Deep sequencing of virus populations from ten mosquitoes infected with MRE/rprORF indicated that the reaper insert was stable, with only a small number of point mutations and no deletions being observed at frequencies greater than 1%. Our results indicate that expression of Reaper by this method significantly reduces infection prevalence, but if infection is established then Reaper expression has limited ability to continue to suppress replication.

Defining the mechanisms of action and mosquito larva midgut response to a yeast-encapsulated orange oil larvicide

BACKGROUND

Yeast-encapsulated orange oil (YEOO) is a novel, ingestible larvicide that combines the benefits of a low-cost essential oil with yeast, an attractive food source for mosquito larvae. In this work, we investigated the underlying mechanisms of action associated with YEOO ingestion by Aedes aegypti larvae.

METHODS

Aedes aegypti third-stage larvae (L3) were treated with sublethal or lethal concentrations of YEOO. Genes associated with apoptosis, autophagy and innate immune responses were investigated by RT-qPCR in guts and carcasses dissected from treated and control larvae. Differential expression of cytochrome P450 genes in the CYP6 and CYP9 families were also investigated. Confocal and transmission electron microscopy were used to assess damage caused by YEOO throughout the larval alimentary canal. TUNEL was used to assess apoptosis via DNA fragmentation.

RESULTS

The apoptosis genes IAP1 and IAP2 in larvae displayed opposing effects following exposure to lethal doses of YEOO, with a 26-fold induction of IAP1 at 8 h post YEOO ingestion. The effector caspase CASPS8 displayed a 6.7-fold induction in the gut and concomitant 70-fold induction in the carcass at 8 h post YEOO ingestion. The midgut epithelia regenerator, Vein, had an 11-fold induction in the gut after 4 h and was repressed 7.6-fold in the carcass at 24 h. Sublethal concentrations (< LC₅₀) led to significant differential expression of CYP6 and CYP9 genes. Midgut epithelial damage was highlighted by the destruction of microvilli, vacuolization of midgut cells and damage to cell junctions and basal lamina as early as 30 min. Larval type 2 peritrophic matrix structural integrity and porosity remain unchanged.

CONCLUSION

Our results strongly suggest that the robust larvicidal activity of YEOO is due to a generalized broad-acting mechanism combining epithelial damage and apoptosis, with concomitant expression of multiple innate response genes involved in epithelial regeneration and detoxification. YEOO's amenability for use as part of an integrated vector management program makes this novel larvicide a practical approach for mosquito larval control in the future.

Evolutionary novelty in the apoptotic pathway of aphids

Apoptotic processes play an important role in the development and physiology of almost all metazoan clades. In the highly diverse group of insects, apoptotic pathways have been characterized in only a few dipteran and lepidopteran species, which may not be representative of all insect species. Here, we report the first complete annotation of the apoptotic pathway in a hemipteran insect, the pea aphid Acyrthosiphon pisum. We showed that its apoptotic pathway is rewired compared to other insects, with a significant increase in the number of inhibitors of apoptosis (IAPs) and evidence for functional diversification and structural modularity of this protein family. These novelties are widespread in the aphid lineage, suggesting a yet not understood novel aphid-specific function of IAPs.

Apoptosis, a conserved form of programmed cell death, shows interspecies differences that may reflect evolutionary diversification and adaptation, a notion that remains largely untested. Among insects, the most speciose animal group, the apoptotic pathway has only been fully characterized in Drosophila melanogaster, and apoptosis-related proteins have been studied in a few other dipteran and lepidopteran species. Here, we studied the apoptotic pathway in the aphid Acyrthosiphon pisum, an insect pest belonging to the Hemiptera, an earlier-diverging and distantly related order. We combined phylogenetic analyses and conserved domain identification to annotate the apoptotic pathway in A. pisum and found low caspase diversity and a large expansion of its inhibitory part, with 28 inhibitors of apoptosis (IAPs). We analyzed the spatiotemporal expression of a selected set of pea aphid IAPs and showed that they are differentially expressed in different life stages and tissues, suggesting functional diversification. Five IAPs are specifically induced in bacteriocytes, the specialized cells housing symbiotic bacteria, during their cell death. We demonstrated the antiapoptotic role of these five IAPs using heterologous expression in a tractable in vivo model, the Drosophila melanogaster developing eye. Interestingly, IAPs with the strongest antiapoptotic potential contain two BIR and two RING domains, a domain association that has not been observed in any other species. We finally analyzed all available aphid genomes and found that they all show large IAP expansion, with new combinations of protein domains, suggestive of evolutionarily novel aphid-specific functions.

Temperature Dramatically Shapes Mosquito Gene Expression With Consequences for Mosquito-Zika Virus Interactions

Vector-borne flaviviruses are emerging threats to human health. For successful transmission, the virus needs to efficiently enter mosquito cells and replicate within and escape several tissue barriers while mosquitoes elicit major transcriptional responses to flavivirus infection. This process will be affected not only by the specific mosquito-pathogen pairing but also by variation in key environmental variables such as temperature. Thus far, few studies have examined the molecular responses triggered by temperature and how these responses modify infection outcomes, despite substantial evidence showing strong relationships between temperature and transmission in a diversity of systems. To define the host transcriptional changes associated with temperature variation during the early infection process, we compared the transcriptome of mosquito midgut samples from mosquitoes exposed to Zika virus (ZIKV) and non-exposed mosquitoes housed at three different temperatures (20, 28, and 36°C). While the high-temperature samples did not show significant changes from those with standard rearing conditions (28°C) 48 h post-exposure, the transcriptome profile of mosquitoes housed at 20°C was dramatically different. The expression of genes most altered by the cooler temperature involved aspects of blood-meal digestion, ROS metabolism, and mosquito innate immunity. Further, we did not find significant differences in the viral RNA copy number between 24 and 48 h post-exposure at 20°C, suggesting that ZIKV replication is limited by cold-induced changes to the mosquito midgut environment. In ZIKV-exposed mosquitoes, vitellogenin, a lipid carrier protein, was most up-regulated at 20°C. Our results provide a deeper understanding of the temperature-triggered transcriptional changes in Aedes aegypti and can be used to further define the molecular mechanisms driven by environmental temperature variation.

Pectin Digestion in Herbivorous Beetles: Impact of Pseudoenzymes Exceeds That of Their Active Counterparts

Many protein families harbor pseudoenzymes that have lost the catalytic function of their enzymatically active counterparts. Assigning alternative function and importance to these proteins is challenging. Because the evolution toward pseudoenzymes is driven by gene duplication, they often accumulate in multigene families. Plant cell wall-degrading enzymes (PCWDEs) are prominent examples of expanded gene families. The pectolytic glycoside hydrolase family 28 (GH28) allows herbivorous insects to break down the PCW polysaccharide pectin. GH28 in the Phytophaga clade of beetles contains many active enzymes but also many inactive counterparts. Using functional characterization, gene silencing, global transcriptome analyses, and recordings of life history traits, we found that not only catalytically active but also inactive GH28 proteins are part of the same pectin-digesting pathway. The robustness and plasticity of this pathway and thus its importance for the beetle is supported by extremely high steady-state expression levels and counter-regulatory mechanisms. Unexpectedly, the impact of pseudoenzymes on the pectin-digesting pathway in Phytophaga beetles exceeds even the influence of their active counterparts, such as a lowered efficiency of food-to-energy conversion and a prolongation of the developmental period.

Mosquito Innate Immunity

Mosquitoes live under the endless threat of infections from different kinds of pathogens such as bacteria, parasites, and viruses. The mosquito defends itself by employing both physical and physiological barriers that resist the entry of the pathogen and the subsequent establishment of the pathogen within the mosquito. However, if the pathogen does gain entry into the insect, the insect mounts a vigorous innate cellular and humoral immune response against the pathogen, thereby limiting the pathogen's propagation to nonpathogenic levels. This happens through three major mechanisms: phagocytosis, melanization, and lysis. During these processes, various signaling pathways that engage intense mosquito⁻pathogen interactions are activated. A critical overview of the mosquito immune system and latest information about the interaction between mosquitoes and pathogens are provided in this review. The conserved, innate immune pathways and specific anti-pathogenic strategies in mosquito midgut, hemolymph, salivary gland, and neural tissues for the control of pathogen propagation are discussed in detail.

Cloning and functional characterizations of an apoptogenic Hid gene in the Scuttle Fly, Megaselia scalaris (Diptera; Phoridae)

Although the mechanisms of apoptotic cell death have been well studied in the fruit fly, Drosophila melanogaster, it is unclear whether such mechanisms are conserved in other distantly related species. Using degenerate primers and PCR, we cloned a proapoptotic gene homologous to Head involution defective (Hid) from the Scuttle fly, Megaselia scalaris (MsHid). MsHid cDNA encodes a 197-amino acid-long polypeptide, which so far is the smallest HID protein. PCR analyses revealed that the MsHid gene consists of four exons and three introns. Ectopic expression of MsHid in various peptidergic neurons and non-neuronal tissues in Drosophila effectively induced apoptosis of these cells. However, deletion of either conserved domain, N-terminal IBM or C-terminal MTS, abolished the apoptogenic activity of MsHID, indicating that these two domains are indispensable. Expression of MsHid was found in all life stages, but more prominently in embryos and pupae. MsHid is actively expressed in the central nervous system (CNS), indicating its important role in CNS development. Together MsHID is likely to be an important cell death inducer during embryonic and post-embryonic development in this species. In addition, we found 2-fold induction of MsHid expression in UV-irradiated embryos, indicating a possible role for MsHid in UV-induced apoptosis.

The Strica Homolog AaCASPS16 Is Involved in Apoptosis in the Yellow Fever Vector, Aedes albopictus

Caspases are a family of cysteine proteases playing essential roles during apoptosis. Seven caspases identified in Drosophila were Dronc, Dredd, Strica, Dcp-1, Decay, Drice and Damm. Among them, Strica is an insect-specific caspase containing a long serine- and threonine- rich prodomain, of which function is not yet well studied. Here we identified a homolog of strica from Aedes albopictus, named as Aacasps16. Aacasps16 encoded a protein containing a putative serine- and threonine-rich prodomain and a well conserved caspase catalytic domain. AaCASPS16 shared high identity with dipteran insects Strica homologs. Alignment showed that the closest relative of AaCASPS16 was Aedes aegypti AeCASPS16. The expression profiles of Aacasps16 during developmental and adult stages were analyzed. Purified recombinant AaCASPS16 exhibited the highest caspase activity to WEHD, which is the substrate preferred by human caspase-9. AaCASPS16 induced apoptosis when over-expressed in C6/36 cells. AaCASPS16 was processed during apoptosis induced by actinomycin D and ultraviolet irradiation treatment, whereas partial silencing of Aacasps16 reduced actinomycin D- and ultraviolet irradiation-triggered apoptosis in C6/36 cells. Taken together, our study identified AaCASPS16 as a novel apoptotic caspase in Aedes albopictus.

Infection pattern and transmission potential of chikungunya virus in two New World laboratory-adapted Aedes aegypti strains

Chikungunya virus (CHIKV) is an emerging mosquito-borne virus belonging to the Togaviridae, which is transmitted to humans by Aedes aegypti and Ae. albopictus. We describe the infection pattern of CHIKV in two New World Ae. aegypti strains, HWE and ORL. Both mosquito strains were susceptible to the virus but showed different infection patterns in midguts and salivary glands. Even though acquisition of a bloodmeal showed moderate levels of apoptosis in midgut tissue, there was no obvious additional CHIKV-induced apoptosis detectable during midgut infection. Analysis of expression of apoptosis-related genes suggested that CHIKV infection dampens rather than promotes apoptosis in the mosquito midgut. In both mosquito strains, the virus was present in saliva within two days post-oral infection. HWE and ORL mosquitoes exhibited no salivary gland infection barrier; however, only 60% (HWE) to 65% (ORL) of the females had released the virus in their saliva at one week post-oral acquisition, suggesting a salivary gland escape barrier. CHIKV induced an apoptotic response in salivary glands of HWE and ORL mosquitoes, demonstrating that the virus caused pathology in its natural vector.

Rice ragged stunt virus-induced apoptosis affects virus transmission from its insect vector, the brown planthopper to the rice plant

Most plant viruses that seriously damage agricultural crops are transmitted by insects. However, the mechanisms enabling virus transmission by insect vectors are poorly understood. The brown planthopper (Nilaparvata lugens) is one of the most serious rice pests, causing extensive damage to rice plants by sucking the phloem sap and transmitting viruses, including Rice ragged stunt virus (RRSV). In this study, we investigated the mechanisms of RRSV transmission from its insect vector to the rice plant in vivo using the terminal deoxynucleotidyl transferase dUTP nick-end labeling assay and RNA interference technology. RRSV induced apoptosis in the salivary gland cells of its insect vector, N. lugens. The RRSV-induced apoptosis was regulated through a caspase-dependent manner, and inhibition of the expression of N. lugens caspase-1 genes significantly interfered with virus transmission. Our findings establish a link between virus-associated apoptosis and virus transmission from the insect vector to the host plant.

Rapid selection against arbovirus-induced apoptosis during infection of a mosquito vector

Millions of people are infected each year by arboviruses (arthropod-borne viruses) such as chikungunya, dengue, and West Nile viruses, yet for reasons that are largely unknown, only a relatively small number of mosquito species are able to transmit arboviruses. Understanding the complex factors that determine vector competence could facilitate strategies for controlling arbovirus infections. Apoptosis is a potential antiviral defense response that has been shown to be important in other virus-host systems. However, apoptosis is rarely seen in arbovirus-infected mosquito cells, raising questions about its importance as an antiviral defense in mosquitoes. We tested the effect of stimulating apoptosis during arbovirus infection by infecting Aedes aegypti mosquitoes with a Sindbis virus (SINV) clone called MRE/Rpr, in which the MRE-16 strain of SINV was engineered to express the proapoptotic gene reaper from Drosophila. MRE/Rpr exhibited an impaired infection phenotype that included delayed midgut infection, delayed virus replication, and reduced virus accumulation in saliva. Nucleotide sequencing of the reaper insert in virus populations isolated from individual mosquitoes revealed evidence of rapid and strong selection against maintenance of Reaper expression in MRE/Rpr-infected mosquitoes. The impaired phenotype of MRE/Rpr, coupled with the observed negative selection against Reaper expression, indicates that apoptosis is a powerful defense against arbovirus infection in mosquitoes and suggests that arboviruses have evolved mechanisms to avoid stimulating apoptosis in mosquitoes that serve as vectors.

In silico models for predicting vector control chemicals targeting Aedes aegypti

Human arboviral diseases have emerged or re-emerged in numerous countries worldwide due to a number of factors including the lack of progress in vaccine development, lack of drugs, insecticide resistance in mosquitoes, climate changes, societal behaviours, and economical constraints. Thus, Aedes aegypti is the main vector of the yellow fever and dengue fever flaviviruses and is also responsible for several recent outbreaks of the chikungunya alphavirus. As for the other mosquito species, the A. aegypti control relies heavily on the use of insecticides. However, because of increasing resistance to the different families of insecticides, reduction of Aedes populations is becoming increasingly difficult. Despite the unquestionable utility of insecticides in fighting mosquito populations, there are very few new insecticides developed and commercialized for vector control. This is because the high cost of the discovery of an insecticide is not counterbalanced by the 'low profitability' of the vector control market. Fortunately, the use of quantitative structure-activity relationship (QSAR) modelling allows the reduction of time and cost in the discovery of new chemical structures potentially active against mosquitoes. In this context, the goal of the present study was to review all the existing QSAR models on A. aegypti. The homology and pharmacophore models were also reviewed. Specific attention was paid to show the variety of targets investigated in Aedes in relation to the physiology and ecology of the mosquito as well as the diversity of the chemical structures which have been proposed, encompassing man-made and natural substances.

Dimorphic ovary differentiation in honeybee (Apis mellifera) larvae involves caste-specific expression of homologs of ark and buffy cell death genes

The establishment of the number of repeated structural units, the ovarioles, in the ovaries is one of the critical events that shape caste polyphenism in social insects. In early postembryonic development, honeybee (Apis mellifera) larvae have a pair of ovaries, each one consisting of almost two hundred ovariole primordia. While practically all these ovarioles continue developing in queen-destined larvae, they undergo massive programmed cell death (PCD) in worker-destined larvae. So as to gain insight into the molecular basis of this fundamental process in caste differentiation we used quantitative PCR (qPCR) and fluorescent in situ hybridization (FISH) to investigate the expression of the Amark and Ambuffy genes in the ovaries of the two honeybee castes throughout the fifth larval instar. These are the homologs of ark and buffy Drosophila melanogaster genes, respectively, involved in activating and inhibiting PCD. Caste-specific expression patterns were found during this time-window defining ovariole number. Amark transcript levels were increased when ovariole resorption was intensified in workers, but remained at low levels in queen ovaries. The transcripts were mainly localized at the apical end of all the worker ovarioles, but appeared in only a few queen ovarioles, thus strongly suggesting a function in mediating massive ovariolar cell death in worker larvae. Ambuffy was mainly expressed in the peritoneal sheath cells covering each ovariole. The levels of Ambuffy transcripts increased earlier in the developing ovaries of queens than in workers. Consistent with a protective role against cell death, Ambuffy transcripts were localized in practically all queen ovarioles, but only in few worker ovarioles. The results are indicative of a functional relationship between the expression of evolutionary conserved cell death genes and the morphological events leading to caste-specific ovary differentiation in a social insect.

Novel Genetic and Molecular Tools for the Investigation and Control of Dengue Virus Transmission by Mosquitoes

Aedes aegypti is the principal vector of dengue virus (DENV) throughout the tropical world. This anthropophilic mosquito species needs to be persistently infected with DENV before it can transmit the virus through its saliva to a new vertebrate host. In the mosquito, DENV is confronted with several innate immune pathways, among which RNA interference is considered the most important. The Ae. aegypti genome project opened the doors for advanced molecular studies on pathogen-vector interactions including genetic manipulation of the vector for basic research and vector control purposes. Thus, Ae. aegypti has become the primary model for studying vector competence for arboviruses at the molecular level. Here, we present recent findings regarding DENV-mosquito interactions, emphasizing how innate immune responses modulate DENV infections in Ae. aegypti. We also describe the latest advancements in genetic manipulation of Ae. aegypti and discuss how this technology can be used to investigate vector transmission of DENV at the molecular level and to control transmission of the virus in the field.

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.

Effects of manipulating apoptosis on Sindbis virus infection of Aedes aegypti mosquitoes

Improved control of vector-borne diseases requires an understanding of the molecular factors that determine vector competence. Apoptosis has been shown to play a role in defense against viruses in insects and mammals. Although some observations suggest a correlation between apoptosis and resistance to arboviruses in mosquitoes, there is no direct evidence tying apoptosis to arbovirus vector competence. To determine whether apoptosis can influence arbovirus replication in mosquitoes, we manipulated apoptosis in Aedes aegypti mosquitoes by silencing the expression of genes that either positively or negatively regulate apoptosis. Silencing of the A. aegypti anti-apoptotic gene iap1 (Aeiap1) caused apoptosis in midgut epithelium, alterations in midgut morphology, and 60 to 70% mosquito mortality. Mortality induced by Aeiap1 silencing was rescued by cosilencing the initiator caspase gene Aedronc, indicating that the mortality was due to apoptosis. When mosquitoes which had been injected with Aeiap1 double-stranded RNA (dsRNA) were orally infected with Sindbis virus (SINV), increased midgut infection and virus dissemination to other organs were observed. This increase in virus infection may have been due to the effects of widespread apoptosis on infection barriers or innate immunity. In contrast, silencing the expression of Aedronc, which would be expected to inhibit apoptosis, reduced SINV midgut infection and virus dissemination. Thus, our data suggest that some level of caspase activity and/or apoptosis may be necessary for efficient virus replication and dissemination in mosquitoes. This is the first study to directly test the roles of apoptosis and caspases in determining mosquito vector competence for arboviruses.

Global cross-talk of genes of the mosquito Aedes aegypti in response to dengue virus infection

Background

The mosquito Aedes aegypti is the primary vector of dengue virus (DENV) infection in humans, and DENV is the most important arbovirus across most of the subtropics and tropics worldwide. The early time periods after infection with DENV define critical cellular processes that determine ultimate success or failure of the virus to establish infection in the mosquito.

Methods and Results

To identify genes involved in these processes, we performed genome-wide transcriptome profiling between susceptible and refractory A. aegypti strains at two critical early periods after challenging them with DENV. Genes that responded coordinately to DENV infection in the susceptible strain were largely clustered in one specific expression module, whereas in the refractory strain they were distributed in four distinct modules. The susceptible response module in the global transcriptional network showed significant biased representation with genes related to energy metabolism and DNA replication, whereas the refractory response modules showed biased representation across different metabolism pathway genes including cytochrome P450 and DDT [1,1,1-Trichloro-2,2-bis(4-chlorophenyl) ethane] degradation genes, and genes associated with cell growth and death. A common core set of coordinately expressed genes was observed in both the susceptible and refractory mosquitoes and included genes related to the Wnt (Wnt: wingless [wg] and integration 1 [int1] pathway), MAPK (Mitogen-activated protein kinase), mTOR (mammalian target of rapamycin) and JAK-STAT (Janus Kinase - Signal Transducer and Activator of Transcription) pathways.

Conclusions

Our data revealed extensive transcriptional networks of mosquito genes that are expressed in modular manners in response to DENV infection, and indicated that successfully defending against viral infection requires more elaborate gene networks than hosting the virus. These likely play important roles in the global-cross talk among the mosquito host factors during the critical early DENV infection periods that trigger the appropriate host action in susceptible vs. refractory mosquitoes.

Dengue virus is primarily transmitted by Aedes aegypti mosquitoes. Control of the vector mosquito is the major practice to prevent dengue. However, it is not well known how the virus can infect some mosquito strains but fail to do so with other refractory strains. To address that question, we conducted whole genome microarray based gene expression studies between susceptible and refractory strains of A. aegypti to identify gene expression patterns following challenge with dengue virus. Our analysis of the early infection periods reveals that a large number of genes are involved in a highly coordinated manner either to host or defend against the virus. Genes responding to dengue infection were clustered in seven expression modules. Genes associated with susceptibility to infection were largely clustered in one expression module, while those associated with refractoriness were distributed in four distinct modules. A common core set of genes expressed in both susceptible and refractory individuals were clustered in two expression modules. We identified genes and specific pathways that potentially regulate compatible or non-compatible interactions between dengue virus and the mosquito, most notably energy metabolism and DNA replication in the susceptible response in contrast to cell growth and death in the refractory response.

Defining the core apoptosis pathway in the mosquito disease vector Aedes aegypti: the roles of iap1, ark, dronc, and effector caspases

To date, our knowledge of apoptosis regulation in insects comes almost exclusively from the model organism Drosophila melanogaster. In contrast, despite the identification of numerous genes that are presumed to regulate apoptosis in other insects based on sequence homology, little has been done to examine the molecular pathways that regulate apoptosis in other insects, including medically important disease vectors. In D. melanogaster, the core apoptosis pathway consists of the caspase negative regulator DIAP1, IAP antagonists, the initiator caspase Dronc and its activating protein Ark, and the effector caspase DrICE. Here we have studied the functions of several genes from the mosquito disease vector Aedes aegypti that share homology with the core apoptosis genes in D. melanogaster. Silencing of the iap1 gene in the A. aegypti cell line Aag2 caused spontaneous apoptosis, indicating that IAP1 plays a role in cell survival similar to that of DIAP1. Silencing A. aegypti ark or dronc completely inhibited apoptosis triggered by several different apoptotic stimuli. However, individual silencing of the effector caspases CASPS7 or CASPS8, which are the closest relatives to DrICE, only partially inhibited apoptosis, and silencing both CASPS7 and CASPS8 together did not have a significant additional effect. Our results suggest that the core pathway that regulates apoptosis in A. aegypti is similar to that of D. melanogaster, but that more than one effector caspase is involved in apoptosis in A. aegypti. This is interesting in light of the fact that the caspase family has expanded in mosquitoes compared to D. melanogaster.

The role of IAP antagonist proteins in the core apoptosis pathway of the mosquito disease vector Aedes aegypti

While apoptosis regulation has been studied extensively in Drosophila melanogaster, similar studies in other insects, including disease vectors, lag far behind. In D. melanogaster, the inhibitor of apoptosis (IAP) protein DIAP1 is the major negative regulator of caspases, while IAP antagonists induce apoptosis, in part, by binding to DIAP1 and inhibiting its ability to regulate caspases. In this study, we characterized the roles of two IAP antagonists, Michelob_x (Mx) and IMP, in apoptosis in the yellow fever mosquito Aedes aegypti. Overexpression of Mx or IMP caused apoptosis in A. aegypti Aag2 cells, while silencing expression of mx or imp attenuated apoptosis. Addition of recombinant Mx or IMP, but not cytochrome c, to Aag2 cytosolic extract caused caspase activation. Consistent with this finding, AeIAP1 bound and inhibited both initiator and effector caspases from A. aegypti, and Mx and IMP competed with caspases for binding to AeIAP1. However, a difference was observed in the BIR domains responsible for Dronc binding by AeIAP1 versus DIAP1. These findings demonstrate that the mechanisms by which IAP antagonists regulate apoptosis are largely conserved between A. aegypti and D. melanogaster, although subtle differences exist.

A comprehensive characterization of the caspase gene family in insects from the order Lepidoptera

Background

The cell suicide pathway of apoptosis is a necessary event in the life of multicellular organisms. It is involved in many biological processes ranging from development to the immune response. Evolutionarily conserved proteases, called caspases, play a central role in regulating apoptosis. Reception of death stimuli triggers the activation of initiator caspases, which in turn activate the effector caspases. In Lepidoptera, apoptosis is crucial in processes such as metamorphosis or defending against baculovirus infection. The discovery of p35, a baculovirus protein inhibiting caspase activity, has led to the characterization of the first lepidopteran caspase, Sf-Caspase-1. Studies on Sf-Caspase-1 mode of activation suggested that apoptosis in Lepidoptera requires a cascade of caspase activation, as demonstrated in many other species.

Results

In order to get insights into this gene family in Lepidoptera, we performed an extensive survey of lepidopteran-derived EST datasets. We identified 66 sequences distributed among 27 species encoding putative caspases. Phylogenetic analyses showed that Lepidoptera possess at least 5 caspases, for which we propose a unified nomenclature. According to homology to their Drosophila counterparts and their primary structure, we determined that Lep-Caspase-1, -2 and -3 are putative effector caspases, whereas Lep-Caspase-5 and -6 are putative initiators. The likely function of Lep-Caspase-4 remains unclear. Lep-Caspase-2 is absent from the silkworm genome and appears to be noctuid-specific, and to have arisen from a tandem duplication of the Caspase-1 gene. In the tobacco hawkmoth, 3 distinct transcripts encoding putative Caspase-4 were identified, suggesting at least 2 duplication events in this species.

Conclusions

The basic repertoire of five major types of caspases shared among Lepidoptera seems to be smaller than for most other groups studied to date, but gene duplication still plays a role in lineage-specific increases in diversity, just as in Diptera and mammals.

Comparative genomic analysis of Drosophila melanogaster and vector mosquito developmental genes

Genome sequencing projects have presented the opportunity for analysis of developmental genes in three vector mosquito species: Aedes aegypti, Culex quinquefasciatus, and Anopheles gambiae. A comparative genomic analysis of developmental genes in Drosophila melanogaster and these three important vectors of human disease was performed in this investigation. While the study was comprehensive, special emphasis centered on genes that 1) are components of developmental signaling pathways, 2) regulate fundamental developmental processes, 3) are critical for the development of tissues of vector importance, 4) function in developmental processes known to have diverged within insects, and 5) encode microRNAs (miRNAs) that regulate developmental transcripts in Drosophila. While most fruit fly developmental genes are conserved in the three vector mosquito species, several genes known to be critical for Drosophila development were not identified in one or more mosquito genomes. In other cases, mosquito lineage-specific gene gains with respect to D. melanogaster were noted. Sequence analyses also revealed that numerous repetitive sequences are a common structural feature of Drosophila and mosquito developmental genes. Finally, analysis of predicted miRNA binding sites in fruit fly and mosquito developmental genes suggests that the repertoire of developmental genes targeted by miRNAs is species-specific. The results of this study provide insight into the evolution of developmental genes and processes in dipterans and other arthropods, serve as a resource for those pursuing analysis of mosquito development, and will promote the design and refinement of functional analysis experiments.

Induction of reaper ortholog mx in mosquito midgut cells following baculovirus infection

Many vertebrate and insect viruses possess antiapoptotic genes that are required for their infectivity. This led to the hypothesis that apoptosis is an innate immunoresponse important for limiting virus infections. The role of apoptosis may be especially important in insect antiviral defense because of the lack of adaptive immunity. However, the cellular mechanism that elicits apoptosis in response to viral infection in insects has not been determined. Using an in vivo infection system with the mosquito baculovirus CuniNPV (Culex nigripalpus nucleopolyhedrovirus), we demonstrated that michelob_x (mx), the mosquito ortholog of Drosophila proapoptotic gene reaper, is specifically induced in larval midgut cells following viral infection. Interestingly, the dynamics of mx induction corresponds with the outcome of the infection. In the permissive mosquito C. quinquefasciatus, a slow induction of mx failed to induce prompt apoptosis, and the infected cells eventually undergo necrosis with heavy loads of encapsulated viruses. In contrast, in the refractory mosquito Aedes aegypti, a rapid induction of mx within 30  min p.i. is followed by apoptosis within 2-6  h p.i., suggesting a possible role for apoptosis in limiting viral infection. When the execution of apoptosis was delayed by caspase inhibitors, viral gene expression became detectable in the A. aegypti larvae.

Identification and expression of caspase-1 gene under heat stress in insecticide-susceptible and -resistant Plutella xylostella (Lepidoptera: Plutellidae)

A caspase gene in Plutella xylostella (DBM) was identified firstly and named Px-caspase-1. It had a full-length of 1172 bp and contained 900 bp open reading frame that encoded 300 amino acids with 33.6 kDa. The deduced amino acid of Px-caspase-1 had two domain profile including caspase_p20 (position 61-184) and caspase_p10 (position 203-298) (i.e. the big and small catalytic domains), and the highly conserved pentapeptide QACQG in caspase_p20 domain (the recognized catalytic site of caspases). Being highly homologous to effector caspase genes in other insect and mammalian species, Px-caspase-1 was thought to be an effector caspase gene. Heat stress could result in significant mortality increase on adult DBM. Px-caspase-1 mRNA expression and caspase-3 enzyme activity (a effector caspase) were elevated with age and heat treatment. And, heat stress facilitated the procession of Px-caspase-1 expression. Significantly higher mRNA transcription levels were found in a chlorpyrifos-resistant DBM strain, as compared to those in insecticide-susceptible DBM. The results indicated that high temperature could significantly promote apoptosis process resulting in an the increased DBM mortality rate, and that insecticide-susceptible DBM had a significantly higher physiological fitness at high temperatures than insecticide-resistant DBM.

The genomic underpinnings of apoptosis in the silkworm, Bombyx mori

Background

Apoptosis is regulated in an orderly fashion by a series of genes, and has a crucial role in important physiological processes such as growth development, immunological response and so on. Recently, substantial studies have been undertaken on apoptosis in model animals including humans, fruit flies, and the nematode. However, the lack of genomic data for silkworms limits their usefulness in apoptosis studies, despite the advantages of silkworm as a representative of Lepidoptera and an effective model system. Herein we have identified apoptosis-related genes in the silkworm Bombyx mori and compared them to those from insects, mammals, and nematodes.

Results

From the newly assembled genome databases, a genome-wide analysis of apoptosis-related genes in Bombyx mori was performed using both nucleotide and protein Blast searches. Fifty-two apoptosis-related candidate genes were identified, including five caspase family members, two tumor necrosis factor (TNF) superfamily members, one Bcl-2 family member, four baculovirus IAP (inhibitor of apoptosis) repeat (BIR) domain family members and 1 RHG (Reaper, Hid, Grim, and Sickle; Drosophila cell death activators) family member. Moreover, we identified a new caspase family member, BmCaspase-New, two splice variants of BmDronc, and Bm3585, a mammalian TNF superfamily member homolog. Twenty-three of these apoptosis-related genes were cloned and sequenced using cDNA templates isolated from BmE-SWU1 cells. Sequence analyses revealed that these genes could have key roles in apoptosis.

Conclusions

Bombyx mori possesses potential apoptosis-related genes. We hypothesized that the classic intrinsic and extrinsic apoptotic pathways potentially are active in Bombyx mori. These results lay the foundation for further apoptosis-related study in Bombyx mori.

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.

A caspase-like decoy molecule enhances the activity of a paralogous caspase in the yellow fever mosquito, Aedes aegypti

Caspases are cysteine proteases that play critical roles in apoptosis and other key cellular processes. A mechanism of caspase regulation that has been described in mammals and nematodes involves caspase-like decoy molecules, enzymatically inactive caspase homologs that have arisen by gene duplication and acquired the ability to regulate other caspases. Caspase-like decoy molecules are not found in Drosophila melanogaster, raising the question of whether this type of caspase regulation exists in insects. Phylogenomic analysis of caspase genes from twelve Drosophila and three mosquito species revealed several examples of duplicated caspase homologs lacking critical catalytic residues, making them candidate caspase-like decoy molecules. One of these, CASPS18 from the mosquito Aedes aegypti, is a homolog of the D. melanogaster caspase Decay and contains substitutions in two critical amino acid positions, including the catalytic cysteine residue. As expected, CASPS18 lacked caspase activity, but co-expression of CASPS18 with a paralogous caspase, CASPS19, in mosquito cells or co-incubation of CASPS18 and CASPS19 recombinant proteins resulted in greatly enhanced CASPS19 activity. The discovery of potential caspase-like decoy molecules in several insect species opens new avenues for investigating caspase regulation in insects, particularly in disease vectors such as mosquitoes.

Hyper-expansion of large DNA segments in the genome of kuruma shrimp, Marsupenaeus japonicus

Background

Higher crustaceans (class Malacostraca) represent the most species-rich and morphologically diverse group of non-insect arthropods and many of its members are commercially important. Although the crustacean DNA sequence information is growing exponentially, little is known about the genome organization of Malacostraca. Here, we constructed a bacterial artificial chromosome (BAC) library and performed BAC-end sequencing to provide genomic information for kuruma shrimp (Marsupenaeus japonicus), one of the most widely cultured species among crustaceans, and found the presence of a redundant sequence in the BAC library. We examined the BAC clone that includes the redundant sequence to further analyze its length, copy number and location in the kuruma shrimp genome.

Results

Mj024A04 BAC clone, which includes one redundant sequence, contained 27 putative genes and seemed to display a normal genomic DNA structure. Notably, of the putative genes, 3 genes encode homologous proteins to the inhibitor of apoptosis protein and 7 genes encode homologous proteins to white spot syndrome virus, a virulent pathogen known to affect crustaceans. Colony hybridization and PCR analysis of 381 BAC clones showed that almost half of the BAC clones maintain DNA segments whose sequences are homologous to the representative BAC clone Mj024A04. The Mj024A04 partial sequence was detected multiple times in the kuruma shrimp nuclear genome with a calculated copy number of at least 100. Microsatellites based BAC genotyping clearly showed that Mj024A04 homologous sequences were cloned from at least 48 different chromosomal loci. The absence of micro-syntenic relationships with the available genomic sequences of Daphnia and Drosophila suggests the uniqueness of these fragments in kuruma shrimp from current arthropod genome sequences.

Conclusions

Our results demonstrate that hyper-expansion of large DNA segments took place in the kuruma shrimp genome. Although we analyzed only a part of the duplicated DNA segments, our result suggested that it is difficult to analyze the shrimp genome following normal analytical methodology. Hence, it is necessary to avoid repetitive sequence (such as segmental duplications) when studying the other unique structures in the shrimp genome.

Differential gene expression from midguts of refractory and susceptible lines of the mosquito, Aedes aegypti, infected with Dengue-2 virus

Suppressive subtractive hybridization was used to evaluate the differential expression of midgut genes of feral populations of Aedes aegypti (Diptera: Culicidae) from Colombia that are naturally refractory or susceptible to Dengue-2 virus infection. A total of 165 differentially expressed sequence tags (ESTs) were identified in the subtracted libraries. The analysis showed a higher number of differentially expressed genes in the susceptible Ae. aegypti individuals than the refractory mosquitoes. The functional annotation of ESTs revealed a broad response in the susceptible library that included immune molecules, metabolic molecules and transcription factors. In the refractory strain, there was the presence of a trypsin inhibitor gene, which could play a role in the infection. These results serve as a template for more detailed studies aiming to characterize the genetic components of refractoriness, which in turn can be used to devise new approaches to combat transmission of dengue fever.

A lepidopteran orthologue of reaper reveals functional conservation and evolution of IAP antagonists

Genetic studies in Drosophila melanogaster have revealed that inhibitor of apoptosis (IAP) proteins and IAP antagonists such as reaper play a pivotal role in controlling cell death in insects. Interestingly, although the sequences and structures of IAPs are highly conserved, the sequence of IAP antagonists diverged very rapidly during evolution, making their identification difficult. Using a customized bioinformatics approach, we identified an IAP antagonist, IAP-binding motif 1 (Ibm1), from the genome of the silkworm Bombyx mori. This is the first reaper/grim orthologue identified in a nondipteran insect. Previous analysis indicated that both Reaper and Grim induce cell death through their N-terminal IBM as well as the Grim_helix3 (GH3) domain. Functional studies indicated that Ibm1 binds to an IAP protein from B. mori, BmIAP1, and induces apoptosis in insect cells via the IAP-binding motif, a seven amino acid sequence that is highly conserved in all IAP antagonists. Interestingly, Ibm1 also contains a region that is a statistically significant match to the GH3 domain. Mutational analysis indicated that the GH3-like motif in Ibm1 has an important supportive role in IAP-antagonist function and can trigger cell death under certain conditions.