Apoptosis in mosquito salivary glands: Sindbis virus-associated and tissue homeostasis

Immunity responses as checkpoints for efficient transmission of begomoviruses by whiteflies

Begomoviruses are a group of single stranded DNA plant viruses exclusively transmitted by the sweet potato whitefly Bemisia tabaci in a persistent, circulative manner. After acquisition from plant phloem, this group of viruses circulate and are retained within the whitefly, interacting with tissues, cells and molecular pathways for maintaining the safety of the infective intact virions, by exploiting cellular mechanisms and avoiding degradation by the insect immune responses. During retention, the virions are internalized in the midgut cells, exit and spend hours-days in the hemolymph and cross into salivary gland cells, before transmission. Destroying this group of viruses by the insect immune system seems inefficient for the most part, by examining their very efficient transmission. Thus, within the various sites along the transmission pathway especially in the midgut, it is thought that the immune system with its various layers is activated for avoiding the damage caused by the viruses on one hand, and for ensuring their safe circulation and transmission on the other hand. Begomoviruses have evolved mechanisms for counteracting and exploiting the activated immune system for their safe translocation within the whitefly. In this review, we discuss the various levels of immunity activated against begomoviruses in B. tabaci, taking other pathogen-vector systems as examples and reflecting relevant components on the interactions between B. tabaci and Begomoviruses.

Cellular takeover: How new world alphaviruses impact host organelle function

Alphavirus replication is dependent on host cell organelles to facilitate multiple steps of the viral life cycle. New world alphaviruses (NWA) consisting of eastern, western and Venezuelan equine encephalitis viruses are a subgroup of alphaviruses associated with central nervous system disease. Despite differing morbidity and mortality amongst these viruses, all are important human pathogens due to their transmission through viral aerosolization and mosquito transmission. In this review, we summarize the utilization of host organelles for NWA replication and the subversion of the host innate immune responses. The impact of viral proteins and replication processes on organelle function is also discussed. Literature involving old world alphaviruses (OWA), such as chikungunya virus and Sindbis virus, is included to compare and contrast between OWA and NWA and highlight gaps in knowledge for NWA. Finally, potential targets for therapeutics or vaccine candidates are highlighted with a focus on host-directed therapeutics.

F-actin and heparan sulfate proteoglycan distribution in female mosquito salivary glands and ducts

Directly involved in the "suck-and-spit" physiology, female mosquito salivary glands (SGs) primarily imbibe blood for egg development and release anticoagulants to keep blood flowing. Indirectly involved, mosquitoes can uptake arboviruses during blood feeding from a viremic host. This research examined the presence of the filamentous cytoplasmic contractile protein (F-actin) and heparan sulfate proteoglycan (HSPG), in the female mosquito SGs. Immunofluorescent antibody labeling of actin molecules or HSPG combined with anatomy suggests that F-actin forms a network in the SG lobe parenchymal cells attached to intralobar ducts by HSPG. In addition, F-actin twists around intralobar SG ducts in a beaded manner, altogether involved in the expulsion of SG secretions. This arrangement in female Aedes aegypti SGs, suggests that F-actin structures are integrally involved in transmitting infectious agents into hosts.

Susceptibility of Aedes albopictus, Ae. aegypti and human populations to Ross River virus in Kuala Lumpur, Malaysia

BACKGROUND

Emerging arboviruses such as chikungunya and Zika viruses have unexpectedly caused widespread outbreaks in tropical and subtropical regions recently. Ross River virus (RRV) is endemic in Australia and has epidemic potential. In Malaysia, Aedes mosquitoes are abundant and drive dengue and chikungunya outbreaks. We assessed risk of an RRV outbreak in Kuala Lumpur, Malaysia by determining vector competence of local Aedes mosquitoes and local seroprevalence as a proxy of human population susceptibility.

METHODOLOGY/PRINCIPAL FINDINGS

We assessed oral susceptibility of Malaysian Ae. aegypti and Ae. albopictus by real-time PCR to an Australian RRV strain SW2089. Replication kinetics in midgut, head and saliva were determined at 3 and 10 days post-infection (dpi). With a 3 log10 PFU/ml blood meal, infection rate was higher in Ae. albopictus (60%) than Ae. aegypti (15%; p<0.05). Despite similar infection rates at 5 and 7 log10 PFU/ml blood meals, Ae. albopictus had significantly higher viral loads and required a significantly lower median oral infectious dose (2.7 log10 PFU/ml) than Ae. aegypti (4.2 log10 PFU/ml). Ae. albopictus showed higher vector competence, with higher viral loads in heads and saliva, and higher transmission rate (RRV present in saliva) of 100% at 10 dpi, than Ae. aegypti (41%). Ae. aegypti demonstrated greater barriers at either midgut escape or salivary gland infection, and salivary gland escape. We then assessed seropositivity against RRV among 240 Kuala Lumpur inpatients using plaque reduction neutralization, and found a low rate of 0.8%.

CONCLUSIONS/SIGNIFICANCE

Both Ae. aegypti and Ae. albopictus are susceptible to RRV, but Ae. albopictus displays greater vector competence. Extensive travel links with Australia, abundant Aedes vectors, and low population immunity places Kuala Lumpur, Malaysia at risk of an imported RRV outbreak. Surveillance and increased diagnostic awareness and capacity are imperative to prevent establishment of new arboviruses in Malaysia.

Evaluating the mosquito host range of Getah virus and the vector competence of selected medically important mosquitoes in Getah virus transmission

BACKGROUND

The Getah virus (GETV) is a mosquito-borne Alphavirus (family Togaviridae) that is of significant importance in veterinary medicine. It has been associated with major polyarthritis outbreaks in animals, but there are insufficient data on its clinical symptoms in humans. Serological evidence of GETV exposure and the risk of zoonotic transmission makes GETV a potentially medically relevant arbovirus. However, minimal emphasis has been placed on investigating GETV vector transmission, which limits current knowledge of the factors facilitating the spread and outbreaks of GETV.

METHODS

To examine the range of the mosquito hosts of GETV, we selected medically important mosquitoes, assessed them in vitro and in vivo and determined their relative competence in virus transmission. The susceptibility and growth kinetics of GETVs in various mosquito-derived cell lines were also determined and quantified using plaque assays. Vector competency assays were also conducted, and quantitative reverse transcription-PCR and plaque assays were used to determine the susceptibility and transmission capacity of each mosquito species evaluated in this study.

RESULTS

GETV infection in all of the investigated mosquito cell lines resulted in detectable cytopathic effects. GETV reproduced the fastest in Culex tritaeniorhynchus- and Aedes albopictus-derived cell lines, as evidenced by the highest exponential titers we observed. Regarding viral RNA copy numbers, mosquito susceptibility to infection, spread, and transmission varied significantly between species. The highest vector competency indices for infection, dissemination and transmission were obtained for Cx. tritaeniorhynchus. This is the first study to investigate the ability of Ae. albopictus and Anopheles stephensi to transmit GETV, and the results emphasize the role and capacity of other mosquito species to transmit GETV upon exposure to GETV, in addition to the perceived vectors from which GETV has been isolated in nature.

CONCLUSIONS

This study highlights the importance of GETV vector competency studies to determine all possible transmission vectors, especially in endemic regions.

The Role of Mosquito Hemocytes in Viral Infections

Insect hemocytes are the only immune cells that can mount a humoral and cellular immune response. Despite the critical involvement of hemocytes in immune responses against bacteria, fungi, and parasites in mosquitoes, our understanding of their antiviral potential is still limited. It has been shown that hemocytes express humoral factors such as TEP1, PPO, and certain antimicrobial peptides that are known to restrict viral infections. Insect hemocytes also harbor the major immune pathways, such as JAK/STAT, TOLL, IMD, and RNAi, which are critical for the control of viral infection. Recent research has indicated a role for hemocytes in the regulation of viral infection through RNA interference and autophagy; however, the specific mechanism by which this regulation occurs remains uncharacterized. Conversely, some studies have suggested that hemocytes act as agonists of arboviral infection because they lack basal lamina and circulate throughout the whole mosquito, likely facilitating viral dissemination to other tissues such as salivary glands. In addition, hemocytes produce arbovirus agonist factors such as lectins, which enhance viral infection. Here, we summarize our current understanding of hemocytes’ involvement in viral infections.

Infection of Mammals and Mosquitoes by Alphaviruses: Involvement of Cell Death

Alphaviruses, such as the chikungunya virus, are emerging and re-emerging viruses that pose a global public health threat. They are transmitted by blood-feeding arthropods, mainly mosquitoes, to humans and animals. Although alphaviruses cause debilitating diseases in mammalian hosts, it appears that they have no pathological effect on the mosquito vector. Alphavirus/host interactions are increasingly studied at cellular and molecular levels. While it seems clear that apoptosis plays a key role in some human pathologies, the role of cell death in determining the outcome of infections in mosquitoes remains to be fully understood. Here, we review the current knowledge on alphavirus-induced regulated cell death in hosts and vectors and the possible role they play in determining tolerance or resistance of mosquitoes.

Apoptosis in a Whitefly Vector Activated by a Begomovirus Enhances Viral Transmission

Of the approximately 1,100 known plant viruses, about one-third are DNA viruses that are vectored by insects. Plant virus infections often induce cellular and molecular responses in their insect vectors, which can, in many cases, affect the spread of viruses. However, the mechanisms underlying vector responses that affect virus accumulation and transmission are poorly understood. Here, we examined the role of virus-induced apoptosis in the transmission of begomoviruses, a group of single-stranded plant DNA viruses that are transmitted by whiteflies and cause extensive damage to many crops worldwide. We demonstrated that virus infection can induce apoptosis in the insect vector conferring protection to the virions from degradation, leading to enhanced viral accumulation and transmission to host plants. Our findings provide valuable clues for designing new strategies to block the transmission of insect-vectored plant viruses, particularly plant DNA viruses.

Apoptosis is generally considered the first line of defense against viral infection. However, the role of apoptosis in the interactions between plant viruses and their insect vectors has rarely been investigated. By studying plant DNA viruses of the genus Begomovirus within the family Geminiviridae, which are transmitted by whiteflies of the Bemisia tabaci species complex in a persistent manner, we revealed that virus-induced apoptosis in insect vectors can facilitate viral accumulation and transmission. We found that infection with tomato yellow leaf curl virus activated the apoptosis pathway in B. tabaci. Suppressing apoptosis by inhibitors or silencing caspase-3 significantly reduced viral accumulation, while the activation of apoptosis increased viral accumulation in vivo. Moreover, the positive effect of whitefly apoptosis on virus accumulation and transmission was not due to its cross talk with the autophagy pathway that suppresses begomovirus infection in whiteflies. We further showed that viral replication, rather than the viral coat protein, is likely the critical factor in the activation of apoptosis by the virus. These novel findings indicate that similarly to many animal and a few plant RNA viruses, plant DNA viruses may activate apoptosis in their insect vectors leading to enhanced viral accumulation and transmission.

IMPORTANCE Of the approximately 1,100 known plant viruses, about one-third are DNA viruses that are vectored by insects. Plant virus infections often induce cellular and molecular responses in their insect vectors, which can, in many cases, affect the spread of viruses. However, the mechanisms underlying vector responses that affect virus accumulation and transmission are poorly understood. Here, we examined the role of virus-induced apoptosis in the transmission of begomoviruses, a group of single-stranded plant DNA viruses that are transmitted by whiteflies and cause extensive damage to many crops worldwide. We demonstrated that virus infection can induce apoptosis in the insect vector conferring protection to the virions from degradation, leading to enhanced viral accumulation and transmission to host plants. Our findings provide valuable clues for designing new strategies to block the transmission of insect-vectored plant viruses, particularly plant DNA viruses.

A Hyperactive Kunjin Virus NS3 Helicase Mutant Demonstrates Increased Dissemination and Mortality in Mosquitoes

The unwinding of double-stranded RNA intermediates is critical for the replication and packaging of flavivirus RNA genomes. This unwinding activity is achieved by the ATP-dependent nonstructural protein 3 (NS3) helicase. In previous studies, we investigated the mechanism of energy transduction between the ATP and RNA binding pockets using molecular dynamics simulations and enzymatic characterization. Our data corroborated the hypothesis that motif V is a communication hub for this energy transduction. More specifically, mutations T407A and S411A in motif V exhibit a hyperactive helicase phenotype, leading to the regulation of translocation and unwinding during replication. However, the effect of these mutations on viral infection in cell culture and in vivo is not well understood. Here, we investigated the role of motif V in viral replication using West Nile virus (Kunjin subtype) T407A and S411A mutants (T407A and S411A Kunjin, respectively) in cell culture and in vivo We were able to recover S411A Kunjin but unable to recover T407A Kunjin. Our results indicated that S411A Kunjin decreased viral infection and increased cytopathogenicity in cell culture compared to wild-type (WT) Kunjin. Similarly, decreased infection rates in surviving S411A Kunjin-infected Culex quinquefasciatus mosquitoes were observed, but S411A Kunjin infection resulted in increased mortality compared to WT Kunjin infection. Additionally, S411A Kunjin infection increased viral dissemination and saliva positivity rates in surviving mosquitoes compared to WT Kunjin infection. These data suggest that S411A Kunjin increases viral pathogenesis in mosquitoes. Overall, these data indicate that NS3 motif V may play a role in the pathogenesis, dissemination, and transmission efficiency of Kunjin virus.IMPORTANCE Kunjin and West Nile viruses belong to the arthropod-borne flaviviruses, which can result in severe symptoms, including encephalitis, meningitis, and death. Flaviviruses have expanded into new populations and emerged as novel pathogens repeatedly in recent years, demonstrating that they remain a global threat. Currently, there are no approved antiviral therapeutics against either Kunjin or West Nile viruses. Thus, there is a pressing need for understanding the pathogenesis of these viruses in humans. In this study, we investigated the role of the Kunjin virus helicase on infection in cell culture and in vivo This work provides new insight into how flaviviruses control pathogenesis and mosquito transmission through the nonstructural protein 3 helicase.

JNK pathway restricts DENV2, ZIKV and CHIKV infection by activating complement and apoptosis in mosquito salivary glands

Arbovirus infection of Aedes aegypti salivary glands (SGs) determines transmission. However, there is a dearth of knowledge on SG immunity. Here, we characterized SG immune response to dengue, Zika and chikungunya viruses using high-throughput transcriptomics. We also describe a transcriptomic response associated to apoptosis, blood-feeding and lipid metabolism. The three viruses differentially regulate components of Toll, Immune deficiency (IMD) and c-Jun N- terminal Kinase (JNK) pathways. However, silencing of the Toll and IMD pathway components showed variable effects on SG infection by each virus. In contrast, regulation of the JNK pathway produced consistent responses in both SGs and midgut. Infection by the three viruses increased with depletion of the activator Kayak and decreased with depletion of the negative regulator Puckered. Virus-induced JNK pathway regulates the complement factor, Thioester containing protein-20 (TEP20), and the apoptosis activator, Dronc, in SGs. Individual and co-silencing of these genes demonstrate their antiviral effects and that both may function together. Co-silencing either TEP20 or Dronc with Puckered annihilates JNK pathway antiviral effect. Upon infection in SGs, TEP20 induces antimicrobial peptides (AMPs), while Dronc is required for apoptosis independently of TEP20. In conclusion, we revealed the broad antiviral function of JNK pathway in SGs and showed that it is mediated by a TEP20 complement and Dronc-induced apoptosis response. These results expand our understanding of the immune arsenal that blocks arbovirus transmission.

Arboviral diseases caused by dengue (DENV), Zika (ZIKV) and chikungunya (CHIKV) viruses are responsible for large number of death and debilitation around the world. These viruses are transmitted to humans by the mosquito vector, Aedes aegypti. During the bites, infected salivary glands (SGs) release saliva containing viruses, which initiate human infection. As the tissue where transmitted viruses are produced, SG infection is a key determinant of transmission. To bridge the knowledge gap in vector-virus molecular interactions in SGs, we describe the transcriptome after DENV, ZIKV and CHIKV infection using RNA-sequencing and characterized the immune response in this tissue. Our study reveals the broad antiviral function of c-Jun N-terminal kinase (JNK) pathway against DENV, ZIKV and CHIKV in SGs. We further show that it is mediated by the complement system and apoptosis, identifying the mechanism. Our study adds the JNK pathway to the immune arsenal that can be harnessed to engineer refractory vectors.

Developmental and comparative perspectives on mosquito immunity

Diseases spread by mosquitoes have killed more people than those spread by any other group of arthropod vectors and remain an important factor in determining global health and economic stability. The mosquito innate immune system can act to either modulate infection with human pathogens or fight off entomopathogens and increase the fitness and longevity of infected mosquitoes. While work remains towards understanding the larval immune system and the development of the mosquito immune system, it has recently become clearer that environmental factors heavily shape the developing mosquito immune system and continue to influence the adult immune system as well. The adult immune system has been well-studied and is known to involve multiple tissues and diverse molecular mechanisms. This review summarizes and synthesizes what is currently understood about the development of the mosquito immune system and includes comparisons of immune components unique to mosquitoes among the blood-feeding arthropods as well as important distinguishing factors between the anopheline and culicine mosquitoes. An explanation is included for how mosquito immunity factors into vector competence and vectorial capacity is presented along with a model for the interrelationships between nutrition, microbiome, pathogen interactions and behavior as they relate to mosquito development, immune status, adult female fitness and ultimately, vectorial capacity. Novel discoveries in the fields of mosquito ecoimmunology, neuroimmunology, and intracellular antiviral responses are highlighted.

Mosquito adaptations to hematophagia impact pathogen transmission

Mosquito-borne diseases such as Dengue fever, Chikungunya, and Malaria are critical threats to public health in many parts of the world. Female mosquitoes have evolved multiple adaptive mechanisms to hematophagy, including the ability to efficiently draw and digest blood, as well as the ability to eliminate excess fluids and toxic by-products of blood digestion. Pathogenic agents enter the mosquito digestive tract with the blood meal and need to travel through the midgut and into the hemocele in order to reach the salivary glands and infect a new host. Pathogens need to adjust to these hostile gut, hemocele, and salivary gland environments, and when possible influence the physiology and behavior of their hosts to enhance transmission.

Reproducibility and relevance in insect-arbovirus infection studies

Experimental infections of insects with arboviruses are performed to achieve a variety of objectives but principally to draw inferences about the potential role of field populations in transmission or to explore the molecular basis of vector-pathogen interactions. The design of such studies determines both their reproducibility and the extent to which their results can be extrapolated to natural environments, and is constrained by the resources available. We discuss recent findings regarding the effects of nutrition, the microbiome, co-infecting agents and feeding methods on the outcome of such experiments, and identify resource-efficient ways to increase their relevance and reproducibility, including the development of community standards for reporting such studies and better standards for cell line and colony authentication.

Diverse Host and Restriction Factors Regulate Mosquito-Pathogen Interactions

Mosquitoes transmit diseases that seriously impact global human health. Despite extensive knowledge of the life cycles of mosquito-borne parasites and viruses within their hosts, control strategies have proven insufficient to halt their spread. An understanding of the relationships established between such pathogens and the host tissues they inhabit is therefore paramount for the development of new strategies that specifically target these interactions, to prevent the pathogens' maturation and transmission. Here we present an updated account of the antagonists and host factors that affect the development of Plasmodium, the parasite causing malaria, and mosquito-borne viruses, such as dengue virus and Zika virus, within their mosquito vectors, and we discuss the similarities and differences between Plasmodium and viral systems, looking toward the elucidation of new targets for disease control.

Mosquito Immunobiology: The Intersection of Vector Health and Vector Competence

As holometabolous insects that occupy distinct aquatic and terrestrial environments in larval and adult stages and utilize hematophagy for nutrient acquisition, mosquitoes are subjected to a wide variety of symbiotic interactions. Indeed, mosquitoes play host to endosymbiotic, entomopathogenic, and mosquito-borne organisms, including protozoa, viruses, bacteria, fungi, fungal-like organisms, and metazoans, all of which trigger and shape innate infection-response capacity. Depending on the infection or interaction, the mosquito may employ, for example, cellular and humoral immune effectors for septic infections in the hemocoel, humoral infection responses in the midgut lumen, and RNA interference and programmed cell death for intracellular pathogens. These responses often function in concert, regardless of the infection type, and provide a robust front to combat infection. Mosquito-borne pathogens and entomopathogens overcome these immune responses, employing avoidance or suppression strategies. Burgeoning methodologies are capitalizing on this concerted deployment of immune responses to control mosquito-borne disease.

Vector competence and innate immune responses to dengue virus infection in selected laboratory and field-collected Stegomyia aegypti (= Aedes aegypti)

Control of dengue virus (DenV) transmission, primarily based on strategies to reduce populations of the principle vector Stegomya aegypti (= Aedes aegypti) (Diptera: Culicidae), is difficult to sustain over time. Other potential strategies aim to manipulate characteristics such as vector competence (VC), the innate capacity of the vector to transmit the virus. Previous studies have identified genetic factors, including differential expression of apoptosis-related genes, associated with the refractory and susceptible phenotypes in selected strains of S. aegypti from Cali, Colombia. The present study was designed to evaluate the variability of VC in selected strains against different DenV serotypes and to determine whether field-collected mosquitoes respond similarly to selected laboratory strains in terms of enhanced or reduced expression of apoptosis-related genes. Vector competence differed between strains, but did not differ in response to different DenV serotypes. Differences in VC were observed among mosquitoes collected from different localities in Cali. The overexpression of the pro-apoptosis genes, caspase 16 and Aedronc, was conserved in field-collected refractory mosquitoes and the selected laboratory refractory strain. The results suggest that the apoptosis response is conserved among all refractory mosquitoes to inhibit the development of all DenV serotypes.

Filamentous Structures Induced by a Phytoreovirus Mediate Viral Release from Salivary Glands in Its Insect Vector

Numerous viral pathogens are persistently transmitted by insect vectors and cause agricultural or health problems. These viruses circulate in the vector body, enter the salivary gland, and then are released into the apical plasmalemma-lined cavities, where saliva is stored. The cavity plasmalemma of vector salivary glands thus represents the last membrane barrier for viral transmission. Here, we report a novel mechanism used by a persistent virus to overcome this essential barrier. We observed that the infection by rice gall dwarf virus (RGDV), a species of the genus Phytoreovirus in the family Reoviridae, induced the formation of virus-associated filaments constructed by viral nonstructural protein Pns11 within the salivary glands of its leafhopper vector, Recilia dorsalis Such filaments attached to actin-based apical plasmalemma and induced an exocytosis-like process for viral release into vector salivary gland cavities, through a direct interaction of Pns11 of RGDV and actin of R. dorsalis Failure of virus-induced filaments assembly by RNA interference with synthesized double-stranded RNA targeting the Pns11 gene inhibited the dissemination of RGDV into salivary cavities, preventing viral transmission by R. dorsalis For the first time, we show that a virus can exploit virus-induced inclusion as a vehicle to pass through the apical plasmalemma into vector salivary gland cavities, thus overcoming the last membrane barrier for viral transmission by insect vectors.IMPORTANCE Understanding how persistent viruses overcome multiple tissue and membrane barriers within the insect vectors until final transmission is the key for viral disease control. The apical plasmalemma of the cavities where saliva is stored in the salivary glands is the last barrier for viral transmission by insect vectors; however, the mechanism is still poorly understood. Here we show that a virus has evolved to exploit virus-induced filaments to perform an exocytosis-like process that enables viral passage through the apical plasmalemma into salivary cavities. This mechanism could be extensively exploited by other persistent viruses to overcome salivary gland release barriers in insect vectors, opening new perspectives for viral control.

The Culicoides sonorensis inhibitor of apoptosis 1 protein protects mammalian cells from apoptosis induced by infection with African horse sickness virus and bluetongue virus

African horse sickness virus (AHSV) and bluetongue virus (BTV) are arboviruses of the genus Orbivirus that are transmitted to their vertebrate hosts by Culicoides biting midges. These orbiviruses exhibit lytic infection (apoptosis) in mammalian cells, but cause persistent infection with no cytopathic effects in Culicoides sonorensis cells. Although regulation of apoptosis could thus be integral for establishing persistent virus infection in midge cells, nothing is known about the presence and function of apoptosis pathways in Culicoides midges and their derived cell lines. Here, we report the cloning and functional characterization of an inhibitor of apoptosis protein (IAP), designated CsIAP1, from C. sonorensis cells. The CsIAP1 protein contains two baculoviral IAP repeat (BIR) domains and a RING domain. Silencing of the Cs iap1 gene in C. sonorensis cells caused apoptosis, indicating that CsIAP1 plays a role in cell survival. Stable expression of the CsIAP1 protein in BSR mammalian cells suppressed apoptosis induced by AHSV-4 and BTV-10 infection, and biochemical data indicated that CsIAP1 is an inhibitor of mammalian caspase-9, an initiator caspase in the intrinsic apoptotic pathway. Mutagenesis studies indicated that the BIR2 and RING domains are required for the anti-apoptotic activity of CsIAP1. The results suggest that the mechanism by which CsIAP1 suppresses apoptosis in insect cells may involve inhibition of a Culicoides caspase-9 homologue through a mechanism that requires both the BIR2 and RING domains. This study provides the first evidence that the CsIAP1 protein is a key negative regulator of apoptosis in C. sonorensis cells.

Apoptosis-related genes control autophagy and influence DENV-2 infection in the mosquito vector, Aedes aegypti

The mosquito Aedes aegypti is the primary urban vector for dengue virus (DENV) worldwide. Insight into interactions occurring between host and pathogen is important in understanding what factors contribute to vector competence. However, many of the molecular mechanisms for vector competence remain unknown. Our previous global transcriptional analysis suggested that differential expression of apoptotic proteins is involved in determining refractoriness vs susceptibility to DENV-2 infection in Ae. aegypti females following a DENV-infected blood meal. To determine whether DENV-refractory Ae. aegypti showed more robust apoptosis upon infection, we compared numbers of apoptotic cells from midguts of refractory and susceptible strains and observed increased numbers of apoptotic cells in only the refractory strain upon DENV-2 infection. Thereafter, we manipulated apoptosis through dsRNA interference of the initiator caspase, Aedronc. Unexpectedly, dsAedronc-treated females showed both decreased frequency of disseminated infection and decreased virus titer in infected individuals. Insect caspases have also previously been identified as regulators of the cellular recycling process known as autophagy. We observed activation of autophagy in midgut and fat body tissues following a blood meal, as well as programmed activation of several apoptosis-related genes, including the effector caspase, Casps7. To determine whether autophagy was affected by caspase knockdown, we silenced Aedronc and Casps7, and observed reduced activation of autophagy upon silencing. Our results provide evidence that apoptosis-related genes are also involved in regulating autophagy, and that Aedronc may play an important role in DENV-2 infection success in Ae. aegypti, possibly through its regulation of autophagy.

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.

Arboviruses and apoptosis: the role of cell death in determining vector competence

A relatively small number of mosquito species transmit arboviruses such as dengue, yellow fever, chikungunya and West Nile viruses to hundreds of millions of people each year, yet we still lack a thorough understanding of the molecular factors that determine vector competence. Apoptosis has been shown to be an important factor in determining the outcome of virus infection for many viruses. However, until recently, it was not clear whether apoptosis plays a role in determining the outcome of arbovirus infections in mosquitoes. Recent work has begun to shed light on the roles of apoptosis in this important process.

Tissue Barriers to Arbovirus Infection in Mosquitoes

Arthropod-borne viruses (arboviruses) circulate in nature between arthropod vectors and vertebrate hosts. Arboviruses often cause devastating diseases in vertebrate hosts, but they typically do not cause significant pathology in their arthropod vectors. Following oral acquisition of a viremic bloodmeal from a vertebrate host, the arbovirus disease cycle requires replication in the cellular environment of the arthropod vector. Once the vector has become systemically and persistently infected, the vector is able to transmit the virus to an uninfected vertebrate host. In order to systemically infect the vector, the virus must cope with innate immune responses and overcome several tissue barriers associated with the midgut and the salivary glands. In this review we describe, in detail, the typical arbovirus infection route in competent mosquito vectors. Based on what is known from the literature, we explain the nature of the tissue barriers that arboviruses are confronted with in a mosquito vector and how arboviruses might surmount these barriers. We also point out controversial findings to highlight particular areas that are not well understood and require further research efforts.

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.

Heparan sulfate proteoglycan: an arbovirus attachment factor integral to mosquito salivary gland ducts

Variants of the prototype Alphavirus, Sindbis (SINV), were used in per os infections of adult female mosquitoes to investigate arbovirus interaction with the salivary gland (SG). Infection of Aedine mosquitoes with AR339, a heparan sulfate proteoglycan (HSPG)-dependent variant, resulted in gross pathology in the SG lateral lobes while infection with TR339, a HSPG-independent variant, resulted in minimal SG pathology. HSPG was detected in the internal ducts of the SG lateral lobes by immunolabeling but not in the median lobe, or beyond the triad structure and external ducts. Reports that human lactoferrin interacts with HSPG, suggested an interference with virus attachment to receptors on vertebrate cells. Pre-incubation of Aedes albopictus cultured C7-10 cells with bovine lactoferrin (bLF) followed by adsorption of SINV resulted in earlier and greater intensity of cytopathic response to TR339 compared with AR339. Following pre-treatment of C7-10 cells with bLF, plaques from tissue culture-adapted high-titer SINVTaV-GFP-TC were observed at 48 h post-infection (p.i.), while plaques from low-titer SINVTaV-GFP-TC were not observed until 120 h p.i. Confocal optics detected this reporter virus at 30 days p.i. in the SG proximal lateral lobe, a region of HSPG-immunolocalization. Altogether these data suggest an association between SINV and HSPG in the host mosquito.

Eilat virus displays a narrow mosquito vector range

BACKGROUND

Most alphaviruses are arthropod-borne and utilize mosquitoes as vectors for transmission to susceptible vertebrate hosts. This ability to infect both mosquitoes and vertebrates is essential for maintenance of most alphaviruses in nature. A recently characterized alphavirus, Eilat virus (EILV), isolated from a pool of Anopheles coustani s.I. is unable to replicate in vertebrate cell lines. The EILV host range restriction occurs at both attachment/entry as well as genomic RNA replication levels. Here we investigated the mosquito vector range of EILV in species encompassing three genera that are responsible for maintenance of other alphaviruses in nature.

METHODS

Susceptibility studies were performed in four mosquito species: Aedes albopictus, A. aegypti, Anopheles gambiae, and Culex quinquefasciatus via intrathoracic and oral routes utilizing EILV and EILV expressing red fluorescent protein (-eRFP) clones. EILV-eRFP was injected at 10(7) PFU/mL to visualize replication in various mosquito organs at 7 days post-infection. Mosquitoes were also injected with EILV at 10(4)-10(1) PFU/mosquito and virus replication was measured via plaque assays at day 7 post-infection. Lastly, mosquitoes were provided bloodmeals containing EILV-eRFP at doses of 10(9), 10(7), 10(5) PFU/mL, and infection and dissemination rates were determined at 14 days post-infection.

RESULTS

All four species were susceptible via the intrathoracic route; however, replication was 10-100 fold less than typical for most alphaviruses, and infection was limited to midgut-associated muscle tissue and salivary glands. A. albopictus was refractory to oral infection, while A. gambiae and C. quinquefasciatus were susceptible only at 10(9) PFU/mL dose. In contrast, A. aegypti was susceptible at both 10(9) and 10(7) PFU/mL doses, with body infection rates of 78% and 63%, and dissemination rates of 26% and 8%, respectively.

CONCLUSIONS

The exclusion of vertebrates in its maintenance cycle may have facilitated the adaptation of EILV to a single mosquito host. As a consequence, EILV displays a narrow vector range in mosquito species responsible for the maintenance of other alphaviruses in nature.

Infection with dengue-2 virus alters proteins in naturally expectorated saliva of Aedes aegypti mosquitoes

Background

Dengue virus (DENV) is responsible for up to approximately 300 million infections and an increasing number of deaths related to severe manifestations each year in affected countries throughout the tropics. It is critical to understand the drivers of this emergence, including the role of vector-virus interactions. When a DENV-infected Aedes aegypti mosquito bites a vertebrate, the virus is deposited along with a complex mixture of salivary proteins. However, the influence of a DENV infection upon the expectorated salivary proteome of its vector has yet to be determined.

Methods

Therefore, we conducted a proteomic analysis using 2-D gel electrophoresis coupled with mass spectrometry based protein identification comparing the naturally expectorated saliva of Aedes aegypti infected with DENV-2 relative to that of uninfected Aedes aegypti.

Results

Several proteins were found to be differentially expressed in the saliva of DENV-2 infected mosquitoes, in particular proteins with anti-hemostatic and pain inhibitory functions were significantly reduced. Hypothetical consequences of these particular protein reductions include increased biting rates and transmission success, and lead to alteration of transmission potential as calculated in our vectorial capacity model.

Conclusions

We present our characterizations of these changes with regards to viral transmission and mosquito blood-feeding success. Further, we conclude that our proteomic analysis of Aedes aegypti saliva altered by DENV infection provides a unique opportunity to identify pro-viral impacts key to virus transmission.

Complement-related proteins control the flavivirus infection of Aedes aegypti by inducing antimicrobial peptides

The complement system functions during the early phase of infection and directly mediates pathogen elimination. The recent identification of complement-like factors in arthropods indicates that this system shares common ancestry in vertebrates and invertebrates as an immune defense mechanism. Thioester (TE)-containing proteins (TEPs), which show high similarity to mammalian complement C3, are thought to play a key role in innate immunity in arthropods. Herein, we report that a viral recognition cascade composed of two complement-related proteins limits the flaviviral infection of Aedes aegypti. An A. aegypti macroglobulin complement-related factor (AaMCR), belonging to the insect TEP family, is a crucial effector in opposing the flaviviral infection of A. aegypti. However, AaMCR does not directly interact with DENV, and its antiviral effect requires an A. aegypti homologue of scavenger receptor-C (AaSR-C), which interacts with DENV and AaMCR simultaneously in vitro and in vivo. Furthermore, recognition of DENV by the AaSR-C/AaMCR axis regulates the expression of antimicrobial peptides (AMPs), which exerts potent anti-DENV activity. Our results both demonstrate the existence of a viral recognition pathway that controls the flaviviral infection by inducing AMPs and offer insights into a previously unappreciated antiviral function of the complement-like system in arthropods.

Hosts are equipped with sophisticated machineries for detecting and eliminating invading viruses before they cause significant physiological damage. Unlike mammals which have both innate and adaptive immune systems, insects rely solely on the innate immune system to limit viral infection. Mosquitoes are natural vectors for many human pathogenic viruses, such as dengue virus (DENV) and yellow fever virus. Despite lacking an immunoglobulin-based humoral response, mosquitoes arm themselves with a functional complement-like system to ward off invading pathogens. Herein, we show that a system composed of complement-related factors recognizes and limits flaviviruses by inducing antimicrobial peptides expression. Understanding antiviral mechanisms in arthropods may provide novel strategies for limiting arboviral transmission in nature.

The role of innate immunity in conditioning mosquito susceptibility to West Nile virus

Arthropod-borne viruses (arboviruses) represent an emerging threat to human and livestock health globally. In particular, those transmitted by mosquitoes present the greatest challenges to disease control efforts. An understanding of the molecular basis for mosquito innate immunity to arbovirus infection is therefore critical to investigations regarding arbovirus evolution, virus-vector ecology, and mosquito vector competence. In this review, we discuss the current state of understanding regarding mosquito innate immunity to West Nile virus. We draw from the literature with respect to other virus-vector pairings to attempt to draw inferences to gaps in our knowledge about West Nile virus and relevant vectors.

Encapsidation of host-derived factors correlates with enhanced infectivity of Sindbis virus

The genus Alphavirus consists of a group of enveloped, single-stranded RNA viruses, many of which are transmitted by arthropods to a wide range of vertebrate host species. Here we report that Sindbis virus (SINV) produced from a representative mammalian cell line consists of at least two unique particle subpopulations, separable on the basis of virion density. In contrast, mosquito-derived SINV consists of a homogeneous population of particles. Our findings indicate that the denser particle subpopulation, SINV(Heavy), is more infectious on a per-particle basis than SINV(Light). SINV produced in mosquito cell lines (SINV(C6/36)) exhibited particle-to-PFU ratios similar to those observed for SINV(Heavy). In mammalian cells, viral RNA was synthesized and accumulated more rapidly following infection with SINV(Heavy) or SINV(C6/36) than following infection with SINV(Light), due partly to enhanced translation of viral genomic RNA early in infection. Analysis of the individual particle subpopulations indicated that SINV(Heavy) and SINV(C6/36) contain host-derived factors whose presence correlates with the enhanced translation, RNA synthesis, and infectivity observed for these particles.