Development of non-defective recombinant densovirus vectors for microRNA delivery in the invasive vector mosquito, Aedes albopictus

Challenges in developing a split drive targeting dsx for the genetic control of the invasive malaria vector Anopheles stephensi

BACKGROUND

Anopheles stephensi is a competent malaria vector mainly present in southern Asia and the Arabian Peninsula. Since 2012, it has invaded several countries of eastern Africa, creating an emerging risk of urban transmission. Urgent efforts are required to develop novel and more efficient strategies for targeted vector control. CRISPR/Cas9-based homing gene drives have been proposed as attractive alternative strategies. Gene drives have the potential to spread a desired trait through a population at higher rates than via normal Mendelian inheritance, even in the presence of a fitness cost. Several target genes have been suggested and tested in different mosquito vector species such as Anopheles gambiae and Aedes aegypti. Several promising suppression drives have been developed in An. gambiae that target the sex determination gene doublesex (dsx).

METHODS

In this study, a geographically confineable gene drive system targeting dsx was developed (dsxgRNA). Here, a transgenic line which expresses Cas9 under the control of the endogenous zpg promoter was generated. Separately a transgenic line which expresses a gRNA targeting the female specific exon of dsx was inserted into that same target site. The reproductive fitness of males and females heterozygous and homozygous for this element was determined. A series of experimental crosses was performed to combine the two elements and assess the homing rate of the dsx element in a split drive system.

RESULTS

The drive was able to home in a super-Mendelian rate comparable to those obtained by an autonomous drive in this species. Although inheritance rates as high as 99.8% were observed, potentially providing very potent gene drive, dominant effects on male and female fertility were observed, which would be sufficient to hinder spread of such a drive. Molecular analysis indicated that the gRNA expressing insertion disrupted normal splicing of dsx.

CONCLUSIONS

These results should be considered when proposing the viability of dsx as a target gene for a population suppression gene drives in Anopheles stephensi. Although high homing rates were observed, the fitness defects found in both males and females carrying the transgene would likely prohibit this drive from functioning in the field.

Generating prophylactic immunity against arboviruses in vertebrates and invertebrates

The World Health Organization recently declared a global initiative to control arboviral diseases. These are mainly caused by pathogenic flaviviruses (such as dengue, yellow fever and Zika viruses) and alphaviruses (such as chikungunya and Venezuelan equine encephalitis viruses). Vaccines represent key interventions for these viruses, with licensed human and/or veterinary vaccines being available for several members of both genera. However, a hurdle for the licensing of new vaccines is the epidemic nature of many arboviruses, which presents logistical challenges for phase III efficacy trials. Furthermore, our ability to predict or measure the post-vaccination immune responses that are sufficient for subclinical outcomes post-infection is limited. Given that arboviruses are also subject to control by the immune system of their insect vectors, several approaches are now emerging that aim to augment antiviral immunity in mosquitoes, including Wolbachia infection, transgenic mosquitoes, insect-specific viruses and paratransgenesis. In this Review, we discuss recent advances, current challenges and future prospects in exploiting both vertebrate and invertebrate immune systems for the control of flaviviral and alphaviral diseases.

Going Viral: Virus-Based Biological Control Agents for Plant Protection

The most economically important biotic stresses in crop production are caused by fungi, oomycetes, insects, viruses, and bacteria. Often chemical control is still the most commonly used method to manage them. However, the development of resistance in the different pathogens/pests, the putative damage on the natural ecosystem, the toxic residues in the field, and, thus, the contamination of the environment have stimulated the search for saferalternatives such as the use of biological control agents (BCAs). Among BCAs, viruses, a major driver for controlling host populations and evolution, are somewhat underused, mostly because of regulatory hurdles that make the cost of registration of such host-specific BCAs not affordable in comparison with the limited potential market. Here, we provide a comprehensive overview of the state of the art of virus-based BCAs against fungi, bacteria, viruses, and insects, with a specific focus on new approaches that rely on not only the direct biocidal virus component but also the complex ecological interactions between viruses and their hosts that do not necessarily result in direct damage to the host.

Intron-derived small RNAs for silencing viral RNAs in mosquito cells

Aedes aegypti and Ae. albopictus are the main vectors of mosquito-borne viruses of medical and veterinary significance. Many of these viruses have RNA genomes. Exogenously provided, e.g. transgene encoded, small RNAs could be used to inhibit virus replication, breaking the transmission cycle. We tested, in Ae. aegypti and Ae. albopictus cell lines, reporter-based strategies for assessing the ability of two types of small RNAs to inhibit a chikungunya virus (CHIKV) derived target. Both types of small RNAs use a Drosophila melanogaster pre-miRNA-1 based hairpin for their expression, either with perfect base-pairing in the stem region (shRNA-like) or containing two mismatches (miRNA-like). The pre-miRNA-1 stem loop structure was encoded within an intron; this allows co-expression of one or more proteins, e.g. a fluorescent protein marker tracking the temporal and spatial expression of the small RNAs in vivo. Three reporter-based systems were used to assess the relative silencing efficiency of ten shRNA-like siRNAs and corresponding miRNA-like designs. Two systems used a luciferase reporter RNA with CHIKV RNA inserted either in the coding sequence or within the 3’ UTR. A third reporter used a CHIKV derived split replication system. All three reporters demonstrated that while silencing could be achieved with both miRNA-like and shRNA-like designs, the latter were substantially more effective. Dcr-2 was required for the shRNA-like siRNAs as demonstrated by loss of inhibition of the reporters in Dcr-2 deficient cell lines. These positive results in cell culture are encouraging for the potential use of this pre-miRNA-1-based system in transgenic mosquitoes.

Mosquitoes are important globally, spreading viral diseases worldwide. Chikungunya virus causes epidemics of disease in people. Here we have investigated using two types of small RNAs and pathways inherent in Aedes aegypti mosquitoes to target a piece of the chikungunya virus’s genome, potentially preventing viral replication. We express these small RNAs using a pre-miRNA-1 based system, inserted into the intron within a commonly used promoter. We have used reporter systems in cell lines which can give preliminary indications of how these systems might work in mosquitoes. Our results indicate that short-hairpin-like designs are more effective than micro-RNA-like designs at knocking down expression of their targets. This knock-down requires Dcr-2 indicating that the short-hairpin-like RNAs are likely using the endo-siRNA pathway to degrade mRNA which contains their complementary RNA.

Armigeres subalbatus is a potential vector for Zika virus but not dengue virus

BACKGROUND

Zika virus (ZIKV) and dengue virus (DENV) are closely related flaviviruses primarily transmitted by Aedes mosquitoes. Armigeres subalbatus is an emerging and widely distributed mosquito, and ZIKV has been detected and isolated from it. However, it is not clear whether Ar. subalbatus could be a vector for ZIKV and DENV or not. In this study, we investigated the infection and transmission of Ar. subalbatus to ZIKV and DENV.

METHODS

A line of Ar. subalbatus was isolated from Guangdong, China, and further identified by the mitochondrial cytochrome oxidase subunit 1 (COI) gene. The adults of Ar. subalbatus were fed with blood meal containing ZIKV or DENV-2. At 4, 7, 10, 14, and 21 days post-inoculation (dpi), the infections of ZIKV or DENV-2 in the midguts, ovaries and salivary glands were detected and quantified by RT-PCR and RT-qPCR. To assess the transmissibility, suckling mice were exposed to bites of ZIKV-infected mosquitoes, and ZIKV was detected in brain tissue by RT-qPCR and plaque assays. Furthermore, the larvae of Ar. subalbatus were reared in artificial urine containing ZIKV or DENV-2. The infection rates and viral titers of larvae and adults were analyzed by RT-PCR and RT-qPCR, and the viral distribution in larval tissues was observed by immunohistochemistry. Chi-square test and one-way ANOVA analysis were used for assessing the infection rate and viral titer in varied tissues and different time points, respectively.

RESULTS

Following oral inoculation, ZIKV but not DENV-2 could be detected in Ar. subalbatus midguts at 4 dpi, ovaries at 7 dpi and salivary glands at 10 dpi. The highest infection rate (IR) of ZIKV was 27.8% in midgut at 7 dpi, 9.7% in ovary and 5.6% in salivary gland at 21 dpi. Eight days after being bitten by ZIKV-positive mosquitoes, ZIKV was detected in three brain tissues out of four suckling mice exposed to bites. ZIKV could be detected in the larvae reared in artificial urine contained ZIKV at a high concentration of 105 pfu/ml and various tissues of adults with a low infection rate (0.70-1.35%). ZIKV could be observed in anal papillae and midgut of larvae at 4 dpi under laboratory conditions.

CONCLUSIONS

ZIKV but not DENV-2 can infect Ar. subalbatus by blood meal and artificial urine, and the infected mosquitoes can transmit ZIKV to suckling mice by bite. From these findings, we can conclude that the Ar. subalbatus isolated from Guangdong province, China, is a potential vector for ZIKV and should therefore be considered in vector control programs to prevent and control of Zika virus disease.

Overview of paratransgenesis as a strategy to control pathogen transmission by insect vectors

This article presents an overview of paratransgenesis as a strategy to control pathogen transmission by insect vectors. It first briefly summarises some of the disease-causing pathogens vectored by insects and emphasises the need for innovative control methods to counter the threat of resistance by both the vector insect to pesticides and the pathogens to therapeutic drugs. Subsequently, the state of art of paratransgenesis is described, which is a particularly ingenious method currently under development in many important vector insects that could provide an additional powerful tool for use in integrated pest control programmes. The requirements and recent advances of the paratransgenesis technique are detailed and an overview is given of the microorganisms selected for genetic modification, the effector molecules to be expressed and the environmental spread of the transgenic bacteria into wild insect populations. The results of experimental models of paratransgenesis developed with triatomines, mosquitoes, sandflies and tsetse flies are analysed. Finally, the regulatory and safety rules to be satisfied for the successful environmental release of the genetically engineered organisms produced in paratransgenesis are considered.

Densovirus Oil Suspension Significantly Improves the Efficacy and Duration of Larvicidal Activity against Aedes albopictus

Aedes albopictus is the sole vector for various mosquito-borne viruses, including dengue, chikungunya, and Zika. Ecofriendly biological agents are required to reduce the spread of these mosquito-borne infections. Mosquito densoviruses (MDVs) are entomopathogenic mosquito-specific viruses, which can reduce the capacity of isolated vectors and decrease mosquito-borne viral disease transmission. However, their variable pathogenicity restricts their commercial use. In the present study, we developed a series of novel larvicide oil suspensions (denoted Bacillus thuringiensis (Bti) oil, Ae. albopictus densovirus (AalDV-5) oil, and a mixture of AalDV-5+Bti oil), which were tested against Ae. albopictus larvae under experimental semi-field and open-field conditions. The effect of AalDV-5 on non-target species was also evaluated. The combined effect of AalDV-5+Bti was greater than that of individual toxins and was longer lasting and more persistent compared with the laboratory AalDV-5 virus strain. The virus was quantified on a weekly basis by quantitative polymerase chain reaction (qPCR) and was persistently detected in rearing water as well as in dead larvae. Wildtype densovirus is not pathogenic to non-target organisms. The present findings confirm the improved effect of a mixed microbial suspension (AalDV-5+Bti oil) larvicide against Ae. albopictus. The development and testing of these products will enable better control of the vector mosquitoes.

Recombinant Mosquito Densovirus with Bti Toxins Significantly Improves Pathogenicity against Aedes albopictus

Mosquito densoviruses (MDVs) are mosquito-specific viruses that are recommended as mosquito bio-control agents. The MDV Aedes aegypti densovirus (AeDNV) is a good candidate for controlling mosquitoes. However, the slow activity restricts their widespread use for vector control. In this study, we introduced the Bacillus thuringiensis (Bti) toxin Cry11Aa domain II loop α8 and Cyt1Aa loop β6-αE peptides into the AeDNV genome to improve its mosquitocidal efficiency; protein expression was confirmed using nanoscale liquid chromatography coupled to tandem mass spectrometry (nano LC-MS/MS). Recombinant plasmids were transfected into mosquito C6/36 cell lines, and the expression of specific peptides was detected through RT-PCR. A toxicity bioassay against the first instar Aedes albopictus larvae revealed that the pathogenic activity of recombinant AeDNV was significantly higher and faster than the wild-type (wt) viruses, and mortality increased in a dose-dependent manner. The recombinant viruses were genetically stable and displayed growth phenotype and virus proliferation ability, similar to wild-type AeDNV. Our novel results offer further insights by combining two mosquitocidal pathogens to improve viral toxicity for mosquito control.

Development of miRNA-Based Approaches to Explore the Interruption of Mosquito-Borne Disease Transmission

MicroRNA (miRNA or miR)-based approaches to interrupt the transmission of mosquito-borne diseases have been explored since 2005. A review of these studies and areas in which to proceed is needed. In this review, significant progress is reviewed at the level of individual miRNAs, and miRNA diversification and relevant confounders are described in detail. Current miRNA studies in mosquitoes include four steps, namely, identifying miRNAs, validating miRNA-pathogen interactions, exploring action mechanisms, and performing preapplication investigations. Notably, regarding the Plasmodium parasite, mosquito miRNAs generally bind to mosquito immunity- or development-related mRNAs, indirectly regulating Plasmodium infection; However, regarding arboviruses, mosquito miRNAs can bind to the viral genome, directly modifying viral replication. Thus, during explorations of miRNA-based approaches, researchers need select an ideal miRNA for investigation based on the mosquito species, tissue, and mosquito-borne pathogen of interest. Additionally, strategies for miRNA-based approaches differ for arboviruses and protozoan parasites.

Paratransgenic manipulation of a tsetse microRNA alters the physiological homeostasis of the fly's midgut environment

Tsetse flies are vectors of parasitic African trypanosomes, the etiological agents of human and animal African trypanosomoses. Current disease control methods include fly-repelling pesticides, fly trapping, and chemotherapeutic treatment of infected people and animals. Inhibiting tsetse's ability to transmit trypanosomes by strengthening the fly's natural barriers can serve as an alternative approach to reduce disease. The peritrophic matrix (PM) is a chitinous and proteinaceous barrier that lines the insect midgut and serves as a protective barrier that inhibits infection with pathogens. African trypanosomes must cross tsetse's PM in order to establish an infection in the fly, and PM structural integrity negatively correlates with trypanosome infection outcomes. Bloodstream form trypanosomes shed variant surface glycoproteins (VSG) into tsetse's gut lumen early during the infection establishment, and free VSG molecules are internalized by the fly's PM-producing cardia. This process results in a reduction in the expression of a tsetse microRNA (miR275) and a sequential molecular cascade that compromises PM integrity. miRNAs are small non-coding RNAs that are critical in regulating many physiological processes. In the present study, we investigated the role(s) of tsetse miR275 by developing a paratransgenic expression system that employs tsetse's facultative bacterial endosymbiont, Sodalis glossinidius, to express tandem antagomir-275 repeats (or miR275 sponges). This system induces a constitutive, 40% reduction in miR275 transcript abundance in the fly's midgut and results in obstructed blood digestion (gut weights increased by 52%), a significant increase (p-value < 0.0001) in fly survival following infection with an entomopathogenic bacteria, and a 78% increase in trypanosome infection prevalence. RNA sequencing of cardia and midgut tissues from paratransgenic tsetse confirmed that miR275 regulates processes related to the expression of PM-associated proteins and digestive enzymes as well as genes that encode abundant secretory proteins. Our study demonstrates that paratransgenesis can be employed to study microRNA regulated pathways in arthropods that house symbiotic bacteria.

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

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

A Novel Densovirus Isolated From the Asian Tiger Mosquito Displays Varied Pathogenicity Depending on Its Host Species

Mosquito-borne viral diseases (MBVDs) continue to pose a significant global public health burden. Mosquito control remains a core intervention strategy in integrated mosquito management (IMM) programs to reduce the transmission of MBVDs. Mosquito densoviruses (MDVs) are mosquito-specific entomopathogenic viruses, and their attractive biological and pathogenic characteristics make MDVs potential biological control agents as alternatives to traditional chemical pesticides. However, different viral strains vary greatly in their pathogenicity against different mosquito species, which poses an obstacle for the wide application of MDVs in vector control. In this study, a novel MDV, Aedes albopictus densovirus-7 (AalDV-7), was isolated from field-collected Aedes albopictus in the dengue-endemic area of Guangzhou, China. The complete 4,048 nt genome of AalDV-7 was cloned and sequenced, and the transcription and translation of three open reading frames (ORFs) were characterized. Phylogenetic analysis indicated that AalDV-7 clustered with MDVs mostly isolated from indigenous mosquitoes. The pathogenicity of AalDV-7 to A. albopictus, Aedes aegypti, and Culex quinquefasciatus larvae was completely different, and the median lethal dose (LD₅₀) of AalDV-7 in A. albopictus which was 10^9.48 genome equivalents per ml (geq/ml) was 12 and 46 times lower than those in A. aegypti (10^10.56 geq/ml) and C. quinquefasciatus (10^11.15 geq/ml). Furthermore, the median lethal time (LT₅₀) value in A. albopictus (7.72 days) was 25% and 26% shorter than those in A. aegypti (10.24 days) and C. quinquefasciatus (10.42 days) at a titer of 10¹¹ geq/ml. Furthermore, the mortality of AalDV-7-infected mosquitoes increased in a dose-dependent manner, and the highest mortality was found in A. albopictus larvae exposed to 10¹¹ geq/ml AalDV-7 (82.00%). Sublethal effects analysis also showed that AalDV-7 infection significantly decreased pupation and emergence rates. The 1st–2nd instar larvae of all three mosquito species showed a near 100% infection rate, and the highest relative vial titer (305.97 ± 67.57 geq/ng) was observed in the 1st–2nd instar larvae of C. quinquefasciatus. These pathogenic characteristics make AalDV-7 a potential MBVDs control agent in China, whereas its negligible pathogenicity and high infection rate and viral dose in vivo make it a good candidate for gene delivery vectors in C. quinquefasciatus gene function analysis. In conclusion, the continuous discovery and isolation of new MDVs enrich the pool of mosquito entomopathogenic viruses and provide a variety of choices for optimal MDVs or combinations of MDVs to target certain mosquitoes.

Development of large-scale mosquito densovirus production by in vivo methods

BACKGROUND

Mosquito-borne diseases (MBDs) cause a significant proportion of the global infectious disease burden. Vector control remains the primary strategy available to reduce the transmission of MBDs. However, long-term, wide-scale and large-scale traditional chemical pesticide application has caused significant and increased negative effects on ecosystems and broader emerging insecticide resistance in vectors; therefore, the development of a novel alternative approach is urgently needed. Mosquito densoviruses (MDVs) are entomopathogenic viruses that exhibit a narrow host range and multiple transmission patterns, making MDVs a great potential bioinsecticide. However, the application process has been relatively stagnant over the past three decades. The major obstacle has been that viruses must be produced in mosquito cell lines; therefore, the production process is both expensive and time-consuming.

METHODS

In our study, two wild-type (wt) MDVs, AaeDV and AalDV-3, and a recombinant rAaeDV-210 were used to infect the Aag2 and C6/36 mosquito cell lines and the 1st-2nd-instar and 3rd-4th-instar larvae of Ae. albopictus, Ae. aegypti and Cx. quinquefasciatus. Viral titers and yields in cells, media, larvae and rearing water and total viral yield were evaluated. Three kinds of virus displayed higher maximum virus titers in vivo than in vitro, and they displayed higher maximum viral yields in rearing water.

RESULTS

The three viruses displayed higher total maximum viral yields in C6/36 cells than in Aag2 cells. The three viruses displayed higher total maximum viral yields in Aedes mosquitoes than in Culex mosquitoes. Higher viral yields were produced by 1st-2nd-instar larvae compared to 3rd-4th-instar larvae. The recombinant viruses did not display significantly lower yields than wt viruses in nearly all samples. In summary, by using 100 1st-2nd-instar Aedes mosquito larvae in 200 ml of rearing water, more than 10¹³ genome equivalents (geq) MDV yield can be obtained.

CONCLUSIONS

Considering the lower production cost, this in vivo method has great potential for the large-scale production of MDVs. MDVs exhibit promising prospects and great potential for mosquito control in the future.

Densonucleosis viruses ('densoviruses') for mosquito and pathogen control

Mosquito specific viruses such as densonucleosis viruses ('densoviruses') have long been suggested as alternative mosquito control agents in the face of increasing insecticide resistance. Densoviruses are very species-specific and have been found to infect many important mosquito species. While some strains are highly pathogenic, other strains are more benign. Densoviruses have been proposed as a way to reduce mosquito populations through pathogenic interactions, but genetic strategies such as viral paratrangenesis offer new approaches. As small single-stranded DNA viruses, densoviruses can be easily genetically modified for the expression of genes or non-coding RNAs. A growing literature and variety of techniques have shown the potential for the use of densoviruses in the control of mosquitoes or mosquito-borne pathogens as well as the usefulness of densoviruses as molecular tools for understanding mosquito biology.

Use of a Recombinant Mosquito Densovirus As a Gene Delivery Vector for the Functional Analysis of Genes in Mosquito Larvae

In vivo microinjection is the most commonly used gene transfer technique for analyzing the gene functions in individual mosquitoes. However, this method requires a more technically demanding operation and involves complicated procedures, especially when used in larvae due to their small size, relatively thin and fragile cuticle, and high mortality, which limit its application. In contrast, viral vectors for gene delivery have been developed to surmount extracellular and intracellular barriers. These systems have the advantages of easy manipulation, high gene transduction efficiency, long-term maintenance of gene expression, and the ability to produce persistent effects in vivo. Mosquito densoviruses (MDVs) are mosquito-specific, small single-stranded DNA viruses that can effectively deliver foreign nucleic acids into mosquito cells; however, the replacement or insertion of foreign genes to create recombinant viruses typically causes a loss of packaging and/or replication abilities, which is a barrier to the development of these viruses as delivery vectors. Herein, we report using an artificial intronic small-RNA expression strategy to develop a non-defective recombinant Aedes aegypti densovirus (AaeDV) in vivo delivery system. Detailed procedures for the construction, packaging and quantitative analysis of the rAaeDV vectors, and for larval infection are described. This study demonstrates, for the first time, the feasibility of developing a non-defective recombinant MDV micro RNA (miRNA) expression system, and thus providing a powerful tool for the functional analysis of genes in mosquito and establishing a basis for the application of viral paratransgenesis for controlling mosquito-borne diseases. We demonstrated that Aedes albopictus 1^st instar larvae could be easily and effectively infected by introducing the virus into the water body of the larvae breeding site and that the developed rAaeDVs could be used to overexpress or knock down the expression of a specific target gene in larvae, providing a tool for the functional analysis of mosquito genes.

Microbial control of arthropod-borne disease

Arthropods harbor a diverse array of microbes that profoundly influence many aspects of host biology, including vector competence. Additionally, symbionts can be engineered to produce molecules that inhibit pathogens. Due to their intimate association with the host, microbes have developed strategies that facilitate their transmission, either horizontally or vertically, to conspecifics. These attributes make microbes attractive agents for applied strategies to control arthropod-borne disease. Here we discuss the recent advances in microbial control approaches to reduce the burden of pathogens such as Zika, Dengue and Chikungunya viruses, and Trypanosome and Plasmodium parasites. We also highlight where further investigation is warranted.