Aedes cadherin receptor that mediates Bacillus thuringiensis Cry11A toxicity is essential for mosquito development

CRISPR-Cas9 knockout of membrane-bound alkaline phosphatase or cadherin does not confer resistance to Cry toxins in Aedes aegypti

The Aedes aegypti cadherin-like protein (Aae-Cad) and the membrane-bound alkaline phosphatase (Aae-mALP) are membrane proteins identified as putative receptors for the larvicidal Cry toxins produced by Bacillus thuringiensis subsp. israelensis bacteria. Cry toxins are the most used toxins in the control of different agricultural pest and mosquitos. Despite the relevance of Aae-Cad and Aae-mALP as possible toxin-receptors in mosquitoes, previous efforts to establish a clear functional connection among them and Cry toxins activity have been relatively limited. In this study, we used CRISPR-Cas9 to generate knockout (KO) mutations of Aae-Cad and Aae-mALP. The Aae-mALP KO was successfully generated, in contrast to the Aae-Cad KO which was obtained only in females. The female-linked genotype was due to the proximity of aae-cad gene to the sex-determining loci (M:m). Both A. aegypti KO mutant populations were viable and their insect-development was not affected, although a tendency on lower egg hatching rate was observed. Bioassays were performed to assess the effects of these KO mutations on the susceptibility of A. aegypti to Cry toxins, showing that the Aae-Cad female KO or Aae-mALP KO mutations did not significantly alter the susceptibility of A. aegypti larvae to the mosquitocidal Cry toxins, including Cry11Aa, Cry11Ba, Cry4Ba, and Cry4Aa. These findings suggest that besides the potential participation of Aae-Cad and Aae-mALP as Cry toxin receptors in A. aegypti, additional midgut membrane proteins are involved in the mode of action of these insecticidal toxins.

Exposure to low-concentration fipronil impairs survival, behavior, midgut morphology and physiology of Aedes aegypti larvae

The Aedes aegypti mosquito is a vector for various arboviruses, including dengue and yellow fever. Insecticides, such as pyrethroids and organophosphates, are widely used to manage and control these insects. However, mosquitoes have developed resistance to these chemicals. Therefore, this study aimed to investigate the effects of the commercial formulation of fipronil (Tuit® Florestal; 80% purity) on the survival, behavior, morphology, and proteins related to signaling pathways of the midgut in A. aegypti larvae under controlled laboratory conditions. Significant reductions in immature survival were observed in all concentrations of fipronil tested. Low insecticide concentration (0.5 ppb) led to decreased locomotor activity in the larvae and caused disorganization of the epithelial tissue in the midgut. Moreover, exposure to the insecticide decreased the activity of detoxifying enzymes such as catalase, superoxide dismutase, and glutathione-S-transferase. On the other hand, the insecticide increased protein oxidation and nitric oxide levels. The detection of LC3, caspase-3, and JNK proteins, related to autophagy and apoptosis, increased after exposure. However, there was a decrease in the positive cells for ERK 1/2. Furthermore, the treatment with fipronil decreased the number of positive cells for the proteins FMRF, Prospero, PH3, Wg, Armadillo, Notch, and Delta, which are related to cell proliferation and differentiation. These findings demonstrate that even at low concentrations, fipronil exerts larvicidal effects on A. aegypti by affecting behavior and enzymatic detoxification, inducing protein oxidation, free radical generation, midgut damage and cell death, and inhibiting cell proliferation and differentiation. Thus, this insecticide may represent a viable alternative for controlling the spread of this vector.

Identification and Functional Characterization of CYP4D2 Putatively Associated with β-Cypermethrin Detoxification in Phortica okadai

Phortica okadai, a polyphagous pest, serves as a vector for Thelazia callipaeda in China. Currently, there are no effective control strategies for this vector. Agricultural pest control may cause P. okadai to become a threat due to the development of pesticide resistance. Cytochrome P450s (CYP450) plays a significant role in detoxifying xenobiotics in insects. In this study, we performed RNA sequencing of P. okadai exposed to β-cypermethrin for 0 and 1 h and then gene cloning of the five up-regulated CYP450 genes. Three CYP450 genes were successfully cloned, and their expression patterns in different developmental stages and in different tissues were analyzed by RT-qPCR. Pocyp4d2 was observed to have the highest expression in the midgut (fold change 2.82 for Pocyp4d2, 2.62 for Pocyp49a1, and 1.77 for Pocyp28d2). Functional analysis was carried out according to overexpression in S2 cells from the pfastbac1 vector and RNAi with siRNA. The results of the CCK8 assay indicated that the overexpression of the recombinant protein PoCYP4D2 suppressed the decrease in S2 cell viability due to β-cypermethrin. The expression levels of PoCYP4D2 decreased significantly, and the mortality rates increased from 6.25% to 15.0% at 3 h and from 15.0% to 27.5% at 6 h after Pocyp4d2-siRNA injection. These results suggest that Pocyp4d2 may be an essential key gene in the metabolism of β-cypermethrin in P. okadai. This study constitutes a foundation to explore further the functions of P. okadai CYP450 genes in insecticide metabolism.

Assessing Aedes aegypti candidate genes during viral infection and Wolbachia-mediated pathogen blocking

One approach to control dengue virus transmission is the symbiont Wolbachia, which limits viral infection in mosquitoes. Despite plans for its widespread use in Aedes aegypti, Wolbachia's mode of action remains poorly understood. Many studies suggest that the mechanism is likely multifaceted, involving aspects of immunity, cellular stress and nutritional competition. A previous study from our group used artificial selection to identify a new mosquito candidate gene related to viral blocking; alpha-mannosidase-2a (alpha-Mann-2a) with a predicted role in protein glycosylation. Protein glycosylation pathways tend to be involved in complex host-viral interactions; however, the function of alpha-mannosidases has not been described in mosquito-virus interactions. We examined alpha-Mann-2a expression in response to virus and Wolbachia infections and whether reduced gene expression, caused by RNA interference, affected viral loads. We show that dengue virus (DENV) infection affects the expression of alpha-Mann-2a in a tissue- and time-dependent manner, whereas Wolbachia infection had no effect. In the midgut, DENV prevalence increased following knockdown of alpha-Mann-2a expression in Wolbachia-free mosquitoes, suggesting that alpha-Mann-2a interferes with infection. Expression knockdown had the same effect on the togavirus chikungunya virus, indicating that alpha-Mann-2a may have broad antivirus effects in the midgut. Interestingly, we were unable to knockdown the expression in Wolbachia-infected mosquitoes. We also provide evidence that alpha-Mann-2a may affect the transcriptional level of another gene predicted to be involved in viral blocking and cell adhesion; cadherin87a. These data support the hypothesis that glycosylation and adhesion pathways may broadly be involved in viral infection in Ae. aegypti.

Role of Lectin in the Response of Aedes aegypti Against Bt Toxin

Aedes aegypti is one of the world’s most dangerous mosquitoes, and a vector of diseases such as dengue fever, chikungunya virus, yellow fever, and Zika virus disease. Currently, a major global challenge is the scarcity of antiviral medicine and vaccine for arboviruses. Bacillus thuringiensis var israelensis (Bti) toxins are used as biological mosquito control agents. Endotoxins, including Cry4Aa, Cry4Ba, Cry10Aa, Cry11Aa, and Cyt1Aa, are toxic to mosquitoes. Insect eradication by Cry toxin relies primarily on the interaction of cry toxins with key toxin receptors, such as aminopeptidase (APN), alkaline phosphatase (ALP), cadherin (CAD), and ATP-binding cassette transporters. The carbohydrate recognition domains (CRDs) of lectins and domains II and III of Cry toxins share similar structural folds, suggesting that midgut proteins, such as C-type lectins (CTLs), may interfere with interactions among Cry toxins and receptors by binding to both and alter Cry toxicity. In the present review, we summarize the functional role of C-type lectins in Ae. aegypti mosquitoes and the mechanism underlying the alteration of Cry toxin activity by CTLs. Furthermore, we outline future research directions on elucidating the Bti resistance mechanism. This study provides a basis for understanding Bti resistance, which can be used to develop novel insecticides.

Bacterial Toxins Active against Mosquitoes: Mode of Action and Resistance

Larvicides based on the bacteria Bacillus thuringiensis svar. israelensis (Bti) and Lysinibacillus sphaericus are effective and environmentally safe compounds for the control of dipteran insects of medical importance. They produce crystals that display specific and potent insecticidal activity against larvae. Bti crystals are composed of multiple protoxins: three from the three-domain Cry type family, which bind to different cell receptors in the midgut, and one cytolytic (Cyt1Aa) protoxin that can insert itself into the cell membrane and act as surrogate receptor of the Cry toxins. Together, those toxins display a complex mode of action that shows a low risk of resistance selection. L. sphaericus crystals contain one major binary toxin that display an outstanding persistence in field conditions, which is superior to Bti. However, the action of the Bin toxin based on its interaction with a single receptor is vulnerable for resistance selection in insects. In this review we present the most recent data on the mode of action and synergism of these toxins, resistance issues, and examples of their use worldwide. Data reported in recent years improved our understanding of the mechanism of action of these toxins, showed that their combined use can enhance their activity and counteract resistance, and reinforced their relevance for mosquito control programs in the future years.

Microorganisms Associated with Mosquito Oviposition Sites: Implications for Habitat Selection and Insect Life Histories

Mosquitoes are considered one of the most important threats worldwide due to their ability to vector pathogens. They are responsible for the transmission of major pathogens such as malaria, dengue, zika, or chikungunya. Due to the lack of treatments or prophylaxis against many of the transmitted pathogens and an increasing prevalence of mosquito resistance to insecticides and drugs available, alternative strategies are now being explored. Some of these involve the use of microorganisms as promising agent to limit the fitness of mosquitoes, attract or repel them, and decrease the replication and transmission of pathogenic agents. In recent years, the importance of microorganisms colonizing the habitat of mosquitoes has particularly been investigated since they appeared to play major roles in their development and diseases transmission. In this issue, we will synthesize researches investigating how microorganisms present within water habitats may influence breeding site selection and oviposition strategies of gravid mosquito females. We will also highlight the impact of such microbes on the fate of females' progeny during their immature stages with a specific focus on egg hatching, development rate, and larvae or pupae survival.

Anopheles coluzzii stearoyl-CoA desaturase is essential for adult female survival and reproduction upon blood feeding

Vitellogenesis and oocyte maturation require anautogenous female Anopheles mosquitoes to obtain a bloodmeal from a vertebrate host. The bloodmeal is rich in proteins that are readily broken down into amino acids in the midgut lumen and absorbed by the midgut epithelial cells where they are converted into lipids and then transported to other tissues including ovaries. The stearoyl-CoA desaturase (SCD) plays a pivotal role in this process by converting saturated (SFAs) to unsaturated (UFAs) fatty acids; the latter being essential for maintaining cell membrane fluidity amongst other housekeeping functions. Here, we report the functional and phenotypic characterization of SCD1 in the malaria vector mosquito Anopheles coluzzii. We show that RNA interference (RNAi) silencing of SCD1 and administration of sterculic acid (SA), a small molecule inhibitor of SCD1, significantly impact on the survival and reproduction of female mosquitoes following blood feeding. Microscopic observations reveal that the mosquito thorax is quickly filled with blood, a phenomenon likely caused by the collapse of midgut epithelial cell membranes, and that epithelial cells are depleted of lipid droplets and oocytes fail to mature. Transcriptional profiling shows that genes involved in protein, lipid and carbohydrate metabolism and immunity-related genes are the most affected by SCD1 knock down (KD) in blood-fed mosquitoes. Metabolic profiling reveals that these mosquitoes exhibit increased amounts of saturated fatty acids and TCA cycle intermediates, highlighting the biochemical framework by which the SCD1 KD phenotype manifests as a result of a detrimental metabolic syndrome. Accumulation of SFAs is also the likely cause of the potent immune response observed in the absence of infection, which resembles an auto-inflammatory condition. These data provide insights into mosquito bloodmeal metabolism and lipid homeostasis and could inform efforts to develop novel interventions against mosquito-borne diseases.

Female mosquitoes can become infected with malaria parasites upon ingestion of blood from an infected person and can transmit the disease when they bite another person some days later. The bloodmeal is rich in proteins which female mosquitoes use to develop their eggs after converting them first to saturated and then to unsaturated fatty acids inside their gut cells. Here, we present the characterization of the enzyme that mosquitoes use to convert saturated to unsaturated fatty acids and show that when this enzyme is eliminated or inhibited mosquitoes cannot produce eggs and die soon after they feed on blood. The mosquito death appears to be primarily associated with the collapse of their gut epithelial barrier due to the loss of cell membrane integrity, leading to their inner body cavity being filled with the ingested blood. These mosquitoes also suffer from an acute and detrimental auto-inflammatory condition due to mounting of a potent immune response in the absence of any infection. We conclude that this enzyme and the mechanism of converting blood-derived proteins to unsaturated fatty acids as a whole can be a good target of interventions aiming at limiting the mosquito abundance and blocking malaria transmission.

Diverse cellular morphologies during lumen maturation in Anopheles gambiae larval salivary glands

Mosquitoes are the greatest animal threat to human health, causing hundreds of millions of infections and around 1 million deaths each year. All mosquito-borne pathogens must traverse the salivary glands (SGs) to be transmitted to the next host, making this organ an ideal target for interventions. The adult SG develops from precursor cells located in the larval SG duct bud. Characterization of the larval SG has been limited. We sought to better understand larval SG architecture, secretion and gene expression. We developed an optimized method for larval SG staining and surveyed hundreds of larval stage 4 (L4) SGs using fluorescence confocal microscopy. Remarkable variation in SG cell and chromatin organization differed among individuals and across the L4 stage. Lumen formation occurred during L4 stage through secretion likely involving a coincident cellular apical lipid enrichment and extracellular vesicle-like structures. Meta-analysis of microarray data showed that larval SG gene expression is divergent from adult SGs, more similar to larval gastric cecae, but different from other larval gut compartments. This work highlights the variable cell architecture of larval Anopheles gambiae SGs and provides candidate targets for genetic strategies aiming to disrupt SGs and transmission of mosquito-borne pathogens.

In vivo nanoscale analysis of the dynamic synergistic interaction of Bacillus thuringiensis Cry11Aa and Cyt1Aa toxins in Aedes aegypti

The insecticidal Cry11Aa and Cyt1Aa proteins are produced by Bacillus thuringiensis as crystal inclusions. They work synergistically inducing high toxicity against mosquito larvae. It was proposed that these crystal inclusions are rapidly solubilized and activated in the gut lumen, followed by pore formation in midgut cells killing the larvae. In addition, Cyt1Aa functions as a Cry11Aa binding receptor, inducing Cry11Aa oligomerization and membrane insertion. Here, we used fluorescent labeled crystals, protoxins or activated toxins for in vivo localization at nano-scale resolution. We show that after larvae were fed solubilized proteins, these proteins were not accumulated inside the gut and larvae were not killed. In contrast, if larvae were fed soluble non-toxic mutant proteins, these proteins were found inside the gut bound to gut-microvilli. Only feeding with crystal inclusions resulted in high larval mortality, suggesting that they have a role for an optimal intoxication process. At the macroscopic level, Cry11Aa completely degraded the gastric caeca structure and, in the presence of Cyt1Aa, this effect was observed at lower toxin-concentrations and at shorter periods. The labeled Cry11Aa crystal protein, after midgut processing, binds to the gastric caeca and posterior midgut regions, and also to anterior and medium regions where it is internalized in ordered "net like" structures, leading finally to cell break down. During synergism both Cry11Aa and Cyt1Aa toxins showed a dynamic layered array at the surface of apical microvilli, where Cry11Aa is localized in the lower layer closer to the cell cytoplasm, and Cyt1Aa is layered over Cry11Aa. This array depends on the pore formation activity of Cry11Aa, since the non-toxic mutant Cry11Aa-E97A, which is unable to oligomerize, inverted this array. Internalization of Cry11Aa was also observed during synergism. These data indicate that the mechanism of action of Cry11Aa is more complex than previously anticipated, and may involve additional steps besides pore-formation activity.

Antifungal immune responses in mosquitoes (Diptera: Culicidae): A review

Mosquitoes transmit many parasites and pathogens to humans that cause significant morbidity and mortality. As such, we are constantly looking for new methods to reduce mosquito populations, including the use of effective biological controls. Entomopathogenic fungi are excellent candidate biocontrol agents to control mosquitoes. Understanding the complex ecological, environmental, and molecular interactions between hosts and pathogens are essential to create novel, effective and safe biocontrol agents. Understanding how mosquitoes recognize and eliminate pathogens such as entomopathogenic fungi may allow us to create insect-order specific biocontrol agents to reduce pest populations. Here we summarize the current knowledge of fungal infection, colonization, development, and replication within mosquitoes and the innate immune responses of the mosquitoes towards the fungal pathogens, emphasizing those features required for an effective mosquito biocontrol agent.