Multiple endocrine factors regulate nutrient mobilization and storage in Aedes aegypti during a gonadotrophic cycle

Morin hydrate reduces survival and fertility, delays development and weakens lipid reserves in Aedes aegypti

The Aedes aegypti mosquito is generally associated with arboviruses that cause yellow fever, dengue, zika and chikungunya. The most efficient way to control their populations is through application in breeding sites of highly toxic insecticides that can also impact non-target organisms and generate resistant populations. Therefore, the use of compounds is desirable. Morin hydrate has broad pharmacological applications based on its antioxidant potential, in addition to not having negative effects on mammals. Therefore, the objective of the present study was to investigate the effects of morin hydrate on A. aegypti survival, pupation rate, egg laying, triacylglycerol reserves and expression of proteins related to lipid metabolism 24 h after exposure of larvae. For this, rearing media containing A. aegypti larvae with different concentrations of morin hydrate were formulated to evaluate the lethal concentration. Calculation of the expected lethal concentrations showed LC25 of 52.692 μM, LC40 of 111.121 μM, LC50 of 174.775 μM, LC75 of 575.083 μM and LC90 of 1685.936 μM. Twenty-four hours after treatment with morin hydrate, surviving larvae were transferred to morin-free water with food, and their pupation rate and fertility were evaluated. We observed that an increase in the concentration of morin hydrate induced a dose-dependent reduction in survival, doubled pupation time in survivors and reduced the number of eggs laid by treated females during the larval stage by approximately 30% at concentrations exceeding 100 μM. From this, the impact of 24 h on the triacylglycerol (TAG) stock was evaluated, in addition to evaluating the expression of proteins involved in lipid metabolism. Larvae 24 h after treatment with 100 μM morin showed a reduction in TAG reserves of approximately 17%, while at 175 μM, there was a reduction of more than 33% in stocks, and at 500 μM there was a reduction of 61%. Furthermore, the lipolytic proteins TAGL1 and HSL were upregulated, while the lipogenic proteins FAS1, DGAT1 and GPAT1 were downregulated. Insulin-like receptors were also downregulated, unlike AKHr, which was also upregulated. These data demonstrate that morin hydrate reduces the survival and fertility of A. aegypti by affecting its lipid metabolism. Morin hydrate did not exhibit toxicity toward non-target organisms, demonstrating interesting potential for the control of mosquito populations.

Juvenile hormone induces phosphorylation of insulin/insulin-like growth factor signaling proteins in previtellogenic Aedes aegypti mosquitoes

Juvenile hormone (JH) plays a pivotal role in regulating post-emergence development and metabolism in previtellogenic female Aedes aegypti mosquitoes. In contrast, yolk protein precursor production and egg maturation after a blood meal are regulated by the steroid hormone 20-hydroxyecdysone, the insulin-like growth factor (IGF)/insulin signaling (IIS) pathway, and the mammalian target of rapamycin (mTOR) pathway. The role of IIS/mTOR signaling in female adults prior to blood feeding has not been thoroughly investigated. In this study, we identified a significant increase in the phosphorylation of key effector proteins in the IIS/mTOR signaling pathway, including eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1), ribosomal protein S6 kinase (S6K) and forkhead box protein O1 (FoxO1), in previtellogenic females. In vitro fat body culture experiments suggest that JH induces these phosphorylations through rapid nongenomic signaling mediated by the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt)/mTOR network. RNA interference experiments demonstrated that activation of IIS/mTOR signaling in previtellogenic females modulate metabolic gene expression, promoting the accumulation of energy reserves (glycogen and triglycerides), which influence mosquito fecundity. Additionally, depletion of either the insulin receptor (InR) or the JH receptor Methoprene-tolerant (Met) in adult mosquitoes abolished the phosphorylation of these proteins, indicating that both receptors are involved in JH-induced membrane-initiated signal transduction. Although the precise mechanisms remain unclear, this study uncovers a novel function of the IIS/mTOR pathway in adult mosquitoes before blood feeding, as well as a new mode of JH action through its crosstalk with the IIS pathway.

Maternal lipid mobilization is essential for embryonic development in the malaria vector Anopheles gambiae

Lipid metabolism is an essential component in reproductive physiology. While lipid mobilization has been implicated in the growth of Plasmodium falciparum malaria parasites in their Anopheles vectors, the role of this process in the reproductive biology of these mosquitoes remains elusive. Here, we show that impairing lipolysis in Anopheles gambiae, the major malaria vector, leads to embryonic lethality. Embryos derived from females in which we silenced the triglyceride lipase AgTL2 or the lipid storage droplet AgLSD1 develop normally during early embryogenesis but fail to hatch due to severely impaired metabolism. Embryonic lethality is efficiently recapitulated by exposing adult females to broad-spectrum lipase inhibitors prior to blood feeding, unveiling lipolysis as a potential target for inducing mosquito sterility. Our findings provide mechanistic insights into the importance of maternal lipid mobilization in embryonic health that may inform studies on human reproduction.

RNAi-Mediated Knockdown of Acidic Ribosomal Stalk Protein P1 Arrests Egg Development in Adult Female Yellow Fever Mosquitoes, Aedes aegypti

After taking a blood meal, the fat body of the adult female yellow fever mosquito, Aedes aegypti, switches from a previtellogenic state of arrest to an active state of synthesizing large quantities of yolk protein precursors (YPPs) that are crucial for egg development. The synthesis of YPPs is regulated at both the transcriptional and translational levels. Previously, we identified the cytoplasmic protein general control nonderepressible 1 (GCN1) as a part of the translational regulatory pathway for YPP synthesis. In the current study, we used the C-terminal end of GCN1 to screen for protein-protein interactions and identified 60S acidic ribosomal protein P1 (P1). An expression analysis and RNAi-mediated knockdown of P1 was performed to further investigate the role of P1 in mosquito reproduction. We showed that in unfed (absence of a blood meal) adult A. aegypti mosquitoes, P1 was expressed ubiquitously in the mosquito organs and tissues tested. We also showed that the RNAi-mediated knockdown of P1 in unfed adult female mosquitoes resulted in a strong, transient knockdown with observable phenotypic changes in ovary length and egg deposition. Our results suggest that 60S acidic ribosomal protein P1 is necessary for mosquito reproduction and is a promising target for mosquito population control.

Insulin-like peptides and ovary ecdysteroidogenic hormone differentially stimulate physiological processes regulating egg formation in the mosquito Aedes aegypti

Mosquitoes including Aedes aegypti are human disease vectors because females must blood feed to produce and lay eggs. Blood feeding triggers insulin-insulin growth factor signaling (IIS) which regulates several physiological processes required for egg development. A. aegypti encodes 8 insulin-like peptides (ILPs) and one insulin-like receptor (IR) plus ovary ecdysteroidogenic hormone (OEH) that also activates IIS through the OEH receptor (OEHR). In this study, we assessed the expression of A. aegypti ILPs and OEH during a gonadotrophic cycle and produced each that were functionally characterized to further understand their roles in regulating egg formation. All A. aegypti ILPs and OEH were expressed during a gonadotrophic cycle. Five ILPs (1, 3, 4, 7, 8) and OEH were specifically expressed in the head, while antibodies to ILP3 and OEH indicated each was released after blood feeding from ventricular axons that terminate on the anterior midgut. A subset of ILP family members and OEH stimulated nutrient storage in previtellogenic females before blood feeding, whereas most IIS-dependent processes after blood feeding were activated by one or more of the brain-specific ILPs and/or OEH. ILPs and OEH with different biological activities also exhibited differences in IIS as measured by phosphorylation of the IR, phosphoinositide 3-kinase/Akt kinase (AKT) and mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK). Altogether, our results provide the first results that compare the functional activities of all ILP family members and OEH produced by an insect.

The mosquito Aedes aegypti requires a gut microbiota for normal fecundity, longevity and vector competence

Mosquitoes shift from detritus-feeding larvae to blood-feeding adults that can vector pathogens to humans and other vertebrates. The sugar and blood meals adults consume are rich in carbohydrates and protein but are deficient in other nutrients including B vitamins. Facultatively hematophagous insects like mosquitoes have been hypothesized to avoid B vitamin deficiencies by carryover of resources from the larval stage. However, prior experimental studies have also used adults with a gut microbiota that could provision B vitamins. Here, we used Aedes aegypti, which is the primary vector of dengue virus (DENV), to ask if carryover effects enable normal function in adults with no microbiota. We show that adults with no gut microbiota produce fewer eggs, live longer with lower metabolic rates, and exhibit reduced DENV vector competence but are rescued by provisioning B vitamins or recolonizing the gut with B vitamin autotrophs. We conclude carryover effects do not enable normal function.

Function Analysis of Cholesterol 7-Desaturase in Ovarian Maturation and Molting in Macrobrachium nipponense: Providing Evidence for Reproductive Molting Progress

The Cholesterol 7-desaturase gene plays an important role in insect ecdysone synthesis, but its role in ovarian development has not been reported. In this study, characteristics and the phylogenetic relationship of Cholesterol 7-desaturase were identified by bioinformatics. qPCR showed that the Mn-CH7D gene was highly expressed in the ovary, which was much higher than that in other tissues, and the expression level of Mn-CH7D reached the highest level at the third stage of the ovarian development stage (O-III). During embryonic development, the Mn-CH7D gene expression was highest in the zoea stage. The function of the Mn-CH7D gene was explored by RNA interference. The experimental group was injected with Mn-CH7D dsRNA through the pericardial cavity of M. nipponense, while the control group was injected with the same volume of dsGFP. Statistical analysis of gonadal development and GSI calculation showed that the silencing of Mn-CH7D resulted in the suppression of gonadal development. In addition, the molting frequency of the experimental group was significantly lower than that of the control group during the second molting cycle after silencing Mn-CH7D. On the seventh day after silencing, ecdysone content in the experimental group was significantly reduced. These results demonstrated that the Mn-CH7D gene played a dual role in ovarian maturation and molting of M. nipponense.

Targeting Aedes aegypti Metabolism with Next-Generation Insecticides

Aedes aegypti is the primary vector of dengue virus (DENV), zika virus (ZIKV), and other emerging infectious diseases of concern. A key disease mitigation strategy is vector control, which relies heavily on the use of insecticides. The development of insecticide resistance poses a major threat to public health worldwide. Unfortunately, there is a limited number of chemical compounds available for vector control, and these chemicals can have off-target effects that harm invertebrate and vertebrate species. Fundamental basic science research is needed to identify novel molecular targets that can be exploited for vector control. Next-generation insecticides will have unique mechanisms of action that can be used in combination to limit selection of insecticide resistance. Further, molecular targets will be species-specific and limit off-target effects. Studies have shown that mosquitoes rely on key nutrients during multiple life cycle stages. Targeting metabolic pathways is a promising direction that can deprive mosquitoes of nutrition and interfere with development. Metabolic pathways are also important for the virus life cycle. Here, we review studies that reveal the importance of dietary and stored nutrients during mosquito development and infection and suggest strategies to identify next-generation insecticides with a focus on trehalase inhibitors.