Mosquito transgenesis for malaria control

A novel broad-spectrum antibacterial and anti-malarial Anopheles gambiae Cecropin promotes microbial clearance during pupation

Anophelinae mosquitoes are exposed to a variety of microbes including Plasmodium parasites that cause malaria. When infected, mosquitoes mount versatile immune responses, including the production of antimicrobial peptides. Cecropins are one of the most widely distributed families of antimicrobial peptides in insects and all previously studied Anopheles members are playing roles in adult mosquito immunity. We have identified and characterized a novel member of the Anopheles gambiae cecropin family, cecropin D (CecD), that is uniquely expressed and immune-responsive at late larval stages to promote microbial clearance through its broad-spectrum antibacterial activity during larval-pupal developmental transition. Interestingly, Cecropin D also exhibited highly potent activity against Plasmodium falciparum sporozoites, the malaria parasite stage that is transmitted from mosquitoes and infects humans and thereby holds promise as a malaria transmission-blocking agent. Finally, we have defined unequivocal cecropin-specific molecular signatures to systematically organize the diversity of the cecropin family in malaria vectors.

Distinct developmental patterns in Anopheles stephensi organ systems

Anatomical profiles of insects inform vector biology, comparative development and evolutionary studies with applications in forensics, agriculture and disease control. This study presents a comprehensive, high-resolution developmental profile of Anopheles stephensi, encompassing larval, pupal, and adult stages, obtained through microCT scanning. The results indicate in situ anatomical changes in most organ systems, including the central nervous system, eyes, musculature, alimentary canal, salivary glands, and ovaries, among other organ systems, except for the developing heart. We find significant differences in the mosquito gut, body-wall, and flight muscle development during metamorphosis from other dipterans like Drosophila. Specifically, indirect flight muscle specification and growth can be traced back at least to the 4th instar A. stephensi larvae, as opposed to post-puparial development in other Dipterans like Drosophila and Calliphora. Further, while Drosophila larval body-wall muscles and gut undergo histolysis, changes to these organs during mosquito metamorphosis are less pronounced. These observations, and raw data therein may serve as a reference for studies on the development and the genetics of mosquitoes. Overall, the detailed developmental profile of A. stephensi presented here illuminates the unique anatomy and developmental processes of Culicidae, with important implications for vector biology, disease control, and comparative evolutionary studies.

mosGILT controls innate immunity and germ cell development in Anopheles gambiae

Gene-edited mosquitoes lacking a gamma-interferon-inducible lysosomal thiol reductase-like protein, namely (mosGILTnull) have lower Plasmodium infection, which is linked to impaired ovarian development and immune activation. The transcriptome of mosGILTnull Anopheles gambiae was therefore compared to wild type (WT) mosquitoes by RNA-sequencing to delineate mosGILT-dependent pathways. Compared to WT mosquitoes, mosGILTnull A. gambiae demonstrated altered expression of genes related to oogenesis, 20-hydroxyecdysone synthesis, as well as immune-related genes. Serendipitously, the zero population growth gene, zpg, an essential regulator of germ cell development was found to be one of the most downregulated genes in mosGILTnull mosquitoes. These results provide a crucial missing link between two previous studies on the role of zpg and mosGILT in ovarian development. This study further demonstrates that mosGILT has the potential to serve as a target for the biological control of mosquito vectors and to influence the Plasmodium life cycle within the vector.

Aedes aegypti Beta-1,3-Glucan-Binding Protein Inhibits Dengue and ZIKA Virus Replication

GNBPB6, a beta-1,3-glucan-binding protein, was identified in the transcriptome of Aedes aegypti (A. aegypti) with dengue (DENV), Zika (ZIKV), and chikungunya viruses (CHIKV). In this study, we not only clarified that DENV2 and ZIKV regulate the changes in GNBPB6 expression but also identified the relationship of this gene with viral infections. The changes in GNBPB6 expression were quantified and showed a decrease in A. aegypti cells (Aag2 cells) at 2 dpi and 3 dpi and an increase at 4 dpi and 5 dpi (p < 0.05). A significant increase was observed only at 5 dpi after DENV2 infection. Subsequently, a GNBPB6 knockout (KO) cell line was constructed using the CRISPR/Cas9 system, and the DENV2 and ZIKV RNA copies, along with cell densities, were quantified and compared between the KO and wild type (WT) cells at different dpi. The result showed that DENV2 and ZIKV RNA copies were significantly increased in the KO cell line with no significant change in cell growth. Finally, DENV2 copies decreased after GNBPB6 was complemented in the KO. In conclusion, GNBPB6 knockout and complementation in Aag2 cells revealed that GNBPB6 can inhibit the replication of both DENV2 and ZIKV. These results contribute to subsequent research on mosquito-virus interactions.

Aedes aegypti Argonaute 2 controls arbovirus infection and host mortality

Ae. aegypti mosquitoes transmit some of the most important human viral diseases that are responsible for a significant public health burden worldwide. The small interfering RNA (siRNA) pathway is considered the major antiviral defense system in insects. Here we show that siRNA pathway disruption by CRISPR/Cas9-based Ago2 knockout impaired the mosquitoes' ability to degrade arbovirus RNA leading to hyper-infection accompanied by cell lysis and tissue damage. Ago2 disruption impaired DNA repair mechanisms and the autophagy pathway by altering histone abundance. This compromised DNA repair and removal of damaged cellular organelles and dysfunctional aggregates promoted mosquito death. We also report that hyper-infection of Ago2 knockout mosquitoes stimulated a broad-spectrum antiviral immunity, including apoptosis, which may counteract infection. Taken together, our studies reveal novel roles for Ago2 in protecting mosquitoes from arbovirus infection and associated death.

Anopheles gambiae mosGILT regulates innate immune genes and zpg expression

Gene-edited mosquitoes lacking a g amma-interferon-inducible lysosomal thiol reductase-like protein, namely ( mosGILT null ) have lower Plasmodium infection, which is linked to impaired ovarian development and immune activation. The transcriptome of mosGILT null A. gambiae was therefore compared to wild type (WT) by RNA-sequencing to delineate mosGILT-dependent pathways. Compared to WT mosquitoes, mosGILT null A. gambiae demonstrated altered expression of genes related to oogenesis, 20-hydroxyecdysone synthesis, as well as immune-related genes. Serendipitously, the zero population growth gene, zpg , an essential regulator of germ cell development was found to be one of the most downregulated genes in mosGILT null mosquitoes. These results provide the crucial missing link between two previous studies on the role of zpg and mosGILT in ovarian development. This study further demonstrates that mosGILT has the potential to serve as a target for the biological control of mosquito vectors and to influence the Plasmodium life cycle within the vector.

RNAi-mediated CHS-2 silencing affects the synthesis of chitin and the formation of the peritrophic membrane in the midgut of Aedes albopictus larvae

BACKGROUND

Mosquitoes are an important vector of viral transmission, and due to the complexity of the pathogens they transmit, vector control may be the most effective strategy to control mosquito-borne diseases. Chitin is required for insect growth and development and is absent in higher animals and plants, so regulating the chitin synthesis pathway can serve as a potentially effective means to control vector insects. Most of the current research on the chitin synthase (CHS) gene is focused on chitin synthase-1 (CHS-1), while relatively little is known about chitin synthase-2 (CHS-2).

RESULTS

The CHS-2 gene of Ae. albopictus is highly conserved and closely related to that of Aedes aegypti. The expression of CHS-2 in the third-instar larvae and pupal stage of Ae. albopictus was relatively high, and CHS-2 expression in adult mosquitoes reached the highest value 24 h after blood-feeding. In the fourth-instar larvae of Ae. albopictus, CHS-2 expression was significantly higher in the midgut than in the epidermis. Silencing CHS-2 in Ae. albopictus larvae had no effect on larval survival and emergence. The expression of four genes related to chitin synthesis enzymes was significantly upregulated, the expression level of three genes was unchanged, and only the expression level of GFAT was significantly downregulated. The expression of chitin metabolism-related genes was also upregulated after silencing. The level of chitin in the midgut of Ae. albopictus larvae was significantly decreased, while the chitinase activity was unchanged. The epithelium of the midgut showed vacuolization, cell invagination and partial cell rupture, and the structure of the peritrophic membrane was destroyed or even absent.

METHODS

The expression of CHS-2 in different developmental stages and tissues of Aedes albopictus was detected by real-time fluorescence quantitative PCR (qPCR). After silencing CHS-2 of the fourth-instar larvae of Ae. albopictus by RNA interference (RNAi), the expression levels of genes related to chitin metabolism, chitin content and chitinase activity in the larvae were detected. The structure of peritrophic membrane in the midgut of the fourth-instar larvae after silencing was observed by paraffin section and hematoxylin-eosin (HE) staining.

CONCLUSION

CHS-2 can affect midgut chitin synthesis and breakdown by regulating chitin metabolic pathway-related genes and is involved in the formation of the midgut peritrophic membrane in Ae. albopictus, playing an important role in growth and development. It may be a potential target for enhancing other control methods.

CRISPR/Cas advancements for genome editing, diagnosis, therapeutics, and vaccine development for Plasmodium parasites, and genetic engineering of Anopheles mosquito vector

Malaria as vector-borne disease remains important health concern with over 200 million cases globally. Novel antimalarial medicines and more effective vaccines must be developed to eliminate and eradicate malaria. Appraisal of preceding genome editing approaches confirmed the CRISPR/Cas nuclease system as a novel proficient genome editing system and a tool for species-specific diagnosis, and drug resistance researches for Plasmodium species, and gene drive to control Anopheles population. CRISPR/Cas technology, as a handy tool for genome editing can be justified for the production of transgenic malaria parasites like Plasmodium transgenic lines expressing Cas9, chimeric Plasmodium transgenic lines, knockdown and knockout transgenic parasites, and transgenic parasites expressing alternative alleles, and also mutant strains of Anopheles such as only male mosquito populations, generation of wingless mosquitoes, and creation of knock-out/ knock-in mutants. Though, the incorporation of traditional methods and novel molecular techniques could noticeably enhance the quality of results. The striking development of a CRISPR/Cas-based diagnostic kit that can specifically diagnose the Plasmodium species or drug resistance markers is highly required in malaria settings with affordable cost and high-speed detection. Furthermore, the advancement of genome modifications by CRISPR/Cas technologies resolves contemporary restrictions to culturing, maintaining, and analyzing these parasites, and the aptitude to investigate parasite genome functions opens up new vistas in the better understanding of pathogenesis.

Immune gene expression in the mosquito vector Culex quinquefasciatus during an avian malaria infection

Plasmodium relictum is the most widespread avian malaria parasite in the world. It is listed as one of the 100 most dangerous invasive species, having been responsible for the extinction of several endemic bird species, and the near-demise of several others. Here we present the first transcriptomic study focused on the effect of P. relictum on the immune system of its vector (the mosquito Culex quinquefasciatus) at different times post-infection. We show that over 50% of immune genes identified as being part of the Toll pathway and 30%-40% of the immune genes identified within the Imd pathway are overexpressed during the critical period spanning the parasite's oocyst and sporozoite formation (8-12 days), revealing the crucial role played by both these pathways in this natural mosquito-Plasmodium combination. Comparison of infected mosquitoes with their uninfected counterparts also revealed some unexpected immune RNA expression patterns earlier and later in the infection: significant differences in expression of several immune effectors were observed as early as 30 min after ingestion of the infected blood meal. In addition, in the later stages of the infection (towards the end of the mosquito lifespan), we observed an unexpected increase in immune investment in uninfected, but not in infected, mosquitoes. In conclusion, our work extends the comparative transcriptomic analyses of malaria-infected mosquitoes beyond human and rodent parasites and provides insights into the degree of conservation of immune pathways and into the selective pressures exerted by Plasmodium parasites on their vectors.

Use of Insect Promoters in Genetic Engineering to Control Mosquito-Borne Diseases

Mosquito transgenesis and gene-drive technologies provide the basis for developing promising new tools for vector-borne disease prevention by either suppressing wild mosquito populations or reducing their capacity from transmitting pathogens. Many studies of the regulatory DNA and promoters of genes with robust sex-, tissue- and stage-specific expression profiles have supported the development of new tools and strategies that could bring mosquito-borne diseases under control. Although the list of regulatory elements available is significant, only a limited set of those can reliably drive spatial-temporal expression. Here, we review the advances in our ability to express beneficial and other genes in mosquitoes, and highlight the information needed for the development of new mosquito-control and anti-disease strategies.

Genetics tools for corpora allata specific gene expression in Aedes aegypti mosquitoes

Juvenile hormone (JH) is synthesized by the corpora allata (CA) and controls development and reproduction in insects. Therefore, achieving tissue-specific expression of transgenes in the CA would be beneficial for mosquito research and control. Different CA promoters have been used to drive transgene expression in Drosophila, but mosquito CA-specific promoters have not been identified. Using the CRISPR/Cas9 system, we integrated transgenes encoding the reporter green fluorescent protein (GFP) close to the transcription start site of juvenile hormone acid methyl transferase (JHAMT), a locus encoding a JH biosynthetic enzyme, specifically and highly expressed in the CA of Aedes aegypti mosquitoes. Transgenic individuals showed specific GFP expression in the CA but failed to reproduce the full pattern of jhamt spatiotemporal expression. In addition, we created GeneSwitch driver and responder mosquito lines expressing an inducible fluorescent marker, enabling the temporal regulation of the transgene via the presence or absence of an inducer drug. The use of the GeneSwitch system has not previously been reported in mosquitoes and provides a new inducible binary system that can control transgene expression in Aedes aegypti.

Amazonian Anopheles with low numbers of oocysts transmit Plasmodium vivax sporozoites during a blood meal

Anopheles darlingi is the main malarial vector in the Brazilian Amazon region. An. nuneztovari s.l., An. triannulatus s.l., An. evansae, and An. benarrochi s.l. do not have a defined role as malarial vectors, although they have been found to be naturally infected with Plasmodium vivax, and some develop oocysts. In this study, we evaluated the importance of low numbers of oocysts in sporozoite salivary gland invasion and transmission. Field-collected mosquitoes were experimentally infected with P. vivax. The infection rates and oocyst and sporozoite infection intensities were evaluated and compared with those of An. aquasalis. We found the highest number of oocysts in An. darlingi (mean = 39.47) and the lowest in An. nuneztovari s.l. (mean = 2). The highest number of sporozoites was observed in An. darlingi (mean = 610) and lowest in An. benarrochi s.l. (mean = 30). Plasmodium vivax DNA was detected in the saliva of all mosquito species after a blood meal. Regardless of the number of oocysts, all species transmitted sporozoites during blood meals. Considering the abundance of these mosquitoes and transmission of sporozoites, it is logical to assume that An. nuneztovari s.l. and An. triannulatus s.l. are involved in the transmission of P. vivax.

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.

Metal ions in insect reproduction: a crosstalk between reproductive physiology and immunity

Most insects exhibit high reproductive capacity, which demands large amounts of energy, including macronutrients and micronutrients. Interestingly, many proteins involved in oogenesis depend on metals ions, in particular iron (Fe), zinc (Zn), and copper (Cu). Mechanisms by which metal ions influence reproduction have been described in Drosophila melanogaster, but remain poorly understood in hematophagous insects where blood meals include significant ingestion of metal ions. Moreover, there is evidence that some proteins involved in reproduction and immunity could have dual function in both processes. This review highlights the importance of metal ions in the reproduction of non-hematophagous and hematophagous insects. In addition, we discuss how insects optimize physiological processes using proteins involved in crosstalk between reproductive physiology and immunity, which is a double-edge sword in allocating their functions to protect the insect and ensure reproduction.

Modifying mosquitoes to suppress disease transmission: Is the long wait over?

For more than 50 years it has been a dream of medical entomologists and public health workers to control diseases like malaria and dengue fever by modifying, through genetics and other methods, the arthropods that transmit them to humans. A brief synopsis of the history of these efforts as applied to mosquitoes is presented; none proved to be effective in reducing disease prevalence. Only in the last few years have novel approaches been developed or proposed that indicate the long wait may be over. Three recent developments are particularly promising: CRISPR-Cas9 driven genetic modification, shifting naturally occurring allele frequencies, and microbe-based modifications. The last is the furthest along in implementation. Dengue fever incidence has been reduced between 40% and 96% in 4 different regions of the world where Wolbachia-infected Aedes aegypti have been established in the field. It is not yet clear how sustainable such control programs will prove to be, but there is good reason for optimism. In light of this, the time is ripe for reinvigorated research on vectors, especially genetics. Vector-borne diseases primarily affect under-developed countries and thus have not received the attention they deserve from wealthier countries with well-developed and funded biomedical research establishments.

New weapons to fight malaria transmission: A historical view

The stagnation of our fight against malaria in recent years, mainly due to the development of mosquito insecticide resistance, argues for the urgent development of new weapons. The dramatic evolution of molecular tools in the last few decades led to a better understanding of parasite-mosquito interactions and coalesced in the development of novel tools namely, mosquito transgenesis and paratransgenesis. Here we provide a historical view of the development of these new tools and point to some remaining challenges for their implementation in the field.

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.