The early gene E74B isoform is a transcriptional activator of the ecdysteroid regulatory hierarchy in mosquito vitellogenesis

Effect of nanoformulation Azadirachta indica on some factors associated with the vectorial capacity and competence of Anopheles aquasalis experimentally infected with Plasmodium vivax

Malaria remains a highly prevalent infectious disease worldwide, particularly in tropical and subtropical regions. Effectively controlling of mosquitoes transmitting of Plasmodium spp. is crucial in to control this disease. A promising strategy involves utilizing plant-derived products, such as the Neem tree (Azadirachta indica), known for its secondary metabolites with biological activity against various insect groups of agricultural and public health importance. This study investigated the effects of a nanoformulation prototype Neem on factors linked to the vector competence of Anopheles aquasalis, a malaria vector in Latin America. Different concentrations of the nanoformulation were supplied through sugar solution and blood feeding, assessing impacts on longevity, fecundity, fertility, and transgenerational survival from larvae to adults. Additionally, the effects of the Neem nanoformulation and NeemAZAL® formulation on the sporogonic cycle of P. vivax were evaluated. Overall, significant impacts were observed at 100 ppm and 1,000 ppm concentrations on adult survival patterns and on survival of the F1 generation. A trend of reduced oviposition and hatching rates was also noted in nanoformulation-consuming groups, with fertility and fecundity declining proportionally to the concentration. Additionally, a significant decrease in the infection rate and intensity of P. vivax was observed in the 1,000 ppm group, with a mean of 3 oocysts per female compared to the control's 27 oocysts per female. In the commercial formulation, the highest tested concentration of 3 ppm yielded 5.36 oocysts per female. Concerning sporozoite numbers, there was a reduction of 52 % and 87 % at the highest concentrations compared to the control group. In conclusion, these findings suggest that the A. indica nanoformulation is a potential as a tool for malaria control through reduction in the vector longevity and reproductive capacity, possibly leading to decreased vector population densities. Moreover, the nanoformulation interfered with the sporogonic development of P. vivax. However, further basic research on Neem formulations, their effects, and mechanisms of action is imperative to gain a more specific perspective for safe field implementation.

The miR184-3p targets neuron-specific ecdysone inducible protein 78 to promote rice black streaked dwarf virus propagation in its planthopper vector

BACKGROUND

MicroRNAs (miRNAs) are non-coding RNAs that play a pivotal role in antiviral infection. The miR184-3p has been identified to promote rice black streaked dwarf virus (RBSDV) infection in vector Laodelphax striatellus, whether it targets other genes of L. striatellus to modulate RBSDV propagation remains unknown.

RESULTS

We first analyzed the expression profiles of miR184-3p and its role in regulating RBSDV infection in L. striatellus. Then the candidate genes expression of miR184-3p were systemically analyzed with gain and loss function of miR184-3p, and the interaction of candidate gene, ecdysone inducible protein 78 (Eip78) with miR184-3p was verified by dual luciferase reporter assay. We found Eip78 is evolutionary conserved among agricultural pests and predominantly expressed in the central nervous system (CNS) of L. striatellus. Knockdown of Eip78 effectively increased RBSDV propagation and transmission. Blockade with Eip78 antibody or injection with Eip78 protein could significantly regulate RBSDV infection. Further analysis revealed that knockdown of Eip78 specifically suppresses RBSDV infection in the head part but not in the body part of L. striatellus. Besides, knockdown of ecdysone receptor (EcR) notably restricted Eip78 expression and increased RBSDV accumulation in L. striatellus.

CONCLUSIONS

Taken together, we identified a novel target gene of miR184-3p, Eip78, a member of the ecdysone signaling pathway, and revealed the anti-RBSDV role of Eip78 in the CNS of L. striatellus. These results shed light on the interaction mechanisms of miRNAs, virus and ecdysone signaling pathway in insect vector. © 2024 Society of Chemical Industry.

Aspirin Inhibition of Prostaglandin Synthesis Impairs Mosquito Egg Development

Several endocrine signals mediate mosquito egg development, including 20-hydroxyecdysone (20E). This study reports on prostaglandin E₂ (PGE₂) as an additional, but core, mediator of oogenesis in a human disease-vectoring mosquito, Aedes albopictus. Injection of aspirin (an inhibitor of cyclooxygenase (COX)) after blood-feeding (BF) inhibited oogenesis by preventing nurse cell dumping into a growing oocyte. The inhibitory effect was rescued by PGE₂ addition. PGE₂ was found to be rich in nurse cells and follicular epithelium after BF. RNA interference (RNAi) treatments of PG biosynthetic genes, including PLA₂ and two COX-like peroxidases, prevented egg development. Interestingly, 20E treatment significantly increased the expressions of PG biosynthetic genes, while the RNAi of Shade (which is a 20E biosynthetic gene) expression prevented inducible expressions after BF. Furthermore, RNAi treatments of PGE₂ receptor genes suppressed egg production, even under PGE₂. These results suggest that a signaling pathway of BF-20E-PGE₂ is required for early vitellogenesis in the mosquito.

E74 knockdown represses larval development and chitin synthesis in Hyphantria cunea

E74 is a key transcription factor induced by 20E, which plays a broad role in many physiological events during insect growth and development, including vitellogenesis, organ remodeling and new tissue formation, programmed cell death and metamorphosis. However, whether it is involved in regulating insect chitin biosynthesis remains largely unclear. Here, the E74 gene was identified for the first time from Hyphantria cunea, a notorious defoliator of forestry. Thereafter, the role of HcE74 in regulating growth, development and chitin synthesis in H. cunea larvae was evaluated. Bioinformatics analysis showed that HcE74 shared the highest identity (95.53%) with E74A of Spodoptera litura, which belonged to Ets superfamily. The results of RNAi bioassay showed that the larval mortality on 6 d after HcE74 knockdown was up to 51.11 ± 6.94%. Meanwhile, a distinct developmental deformity phenotype was found when HcE74 was silenced. These results indicated that HcE74 plays an important role in the development and molting of H. cunea larvae. Moreover, HcE74 knockdown also significantly decreased the expression of four key genes related to chitin synthesis, including glucose-6-phosphate isomerase (HcG6PI), UDP-N-acetylglucosamine pyrophosphorylase (HcUAP), chitin synthetase A (HcCHSA), and chitin synthetase B (HcCHSB). As a result, the content of chitin in midgut and epidermis decreased by 0.54- and 0.08-fold, respectively. Taken together, these results demonstrated that HcE74 not only plays a critical role in the growth and molting of H. cunea larvae, but also probably participates in the transcriptional regulation of genes involved in chitin biosynthesis.

Ecdysone signaling mediates the trade-off between immunity and reproduction via suppression of amyloids in the mosquito Aedes aegypti

The balance between immunity and reproduction is essential for many key physiological functions. We report that to maintain an optimal fertility, 20-hydroxyecdysone (20E) and the ecdysone receptor (EcR) downregulate the immune deficiency (IMD) pathway during the post blood meal phase (PBM) of the Aedes aegypti reproductive cycle. RNA interference-mediated depletion of EcR elicited an increased expression of the IMD pathway components, and these mosquitoes were more resistant to infection by Gram-negative bacteria. Moreover, 20E and EcR recruit Pirk-like, the mosquito ortholog of Drosophila melanogaster Pirk. CRISPR-Cas9 knockout of Pirk-like has shown that it represses the IMD pathway by interfering with IMD-mediated formation of amyloid aggregates. 20E and EcR disruption of the amyloid formation is pivotal for maintaining normal yolk protein production and fertility. Additionally, 20E and its receptor EcR directly induce Pirk-like to interfere with cRHIM-mediated formation of amyloid. Our study highlights the vital role of 20E in governing the trade-off between immunity and reproduction. Pirk-like might be a potential target for new methods to control mosquito reproduction and pathogen transmission.

Kr-h1, a Cornerstone Gene in Insect Life History

Insect life cycle is coordinated by hormones and their downstream effectors. Krüppel homolog1 (Kr-h1) is one of the crucial effectors which mediates the actions of the two critical hormones of insects, the juvenile hormone (JH) and 20-hydroxyecdysone (20E). It is a transcription factor with a DNA-binding motif of eight C₂H₂ zinc fingers which is found to be conserved among insect orders. The expression of Kr-h1 is fluctuant during insect development with high abundance in juvenile instars and lower levels in the final instar and pupal stage, and reappearance in adults, which is governed by the coordination of JH, 20E, and miRNAs. The dynamic expression pattern of Kr-h1 is closely linked to its function in the entire life of insects. Over the past several years, accumulating studies have advanced our understanding of the role of Kr-h1 during insect development. It acts as a universal antimetamorphic factor in both hemimetabolous and holometabolous species by directly inhibiting the transcription of 20E signaling genes Broad-Complex (Br-C) and Ecdysone induced protein 93F (E93), and steroidogenic enzyme genes involved in ecdysone biosynthesis. Meanwhile, it promotes vitellogenesis and ovarian development in the majority of studied insects. In addition, Kr-h1 regulates insect behavioral plasticity and caste identity, neuronal morphogenesis, maturation of sexual behavior, as well as embryogenesis and metabolic homeostasis. Hence, Kr-h1 acts as a cornerstone regulator in insect life.

Molecular action of larvicidal flavonoids on ecdysteroidogenic glutathione S-transferase Noppera-bo in Aedes aegypti

Background

Mosquito control is a crucial global issue for protecting the human community from mosquito-borne diseases. There is an urgent need for the development of selective and safe reagents for mosquito control. Flavonoids, a group of chemical substances with variable phenolic structures, such as daidzein, have been suggested as potential mosquito larvicides with less risk to the environment. However, the mode of mosquito larvicidal action of flavonoids has not been elucidated.

Results

Here, we report that several flavonoids, including daidzein, inhibit the activity of glutathione S-transferase Noppera-bo (Nobo), an enzyme used for the biosynthesis of the insect steroid hormone ecdysone, in the yellow fever mosquito Aedes aegypti. The crystal structure of the Nobo protein of Ae. aegypti (AeNobo) complexed with the flavonoids and its molecular dynamics simulation revealed that Glu113 forms a hydrogen bond with the flavonoid inhibitors. Consistent with this observation, substitution of Glu113 with Ala drastically reduced the inhibitory activity of the flavonoids against AeNobo. Among the identified flavonoid-type inhibitors, desmethylglycitein (4′,6,7-trihydroxyisoflavone) exhibited the highest inhibitory activity in vitro. Moreover, the inhibitory activities of the flavonoids correlated with the larvicidal activity, as desmethylglycitein suppressed Ae. aegypti larval development more efficiently than daidzein.

Conclusion

Our study demonstrates the mode of action of flavonoids on the Ae. aegypti Nobo protein at the atomic, enzymatic, and organismal levels.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01233-2.

Transcriptome analysis reveals TOR signalling-mediated plant flush shoots governing Diaphorina citri Kuwayama oviposition

Asian Citrus Psyllid (ACP), Diaphorina citri, is a key vector transmitting the causative agent of Huanglongbing (HLB) disease. Population growth of ACP is evident after feeding on plant flush shoots, as they only oviposit here. However, the underlying mechanism as to why flush shoots govern oviposition is unclear. This study compares the fecundity and ovarian morphology of ACP between young flush and mature leaves. Furthermore, the transcriptome of mated females infesting Murraya paniculata was analysed. Finally, the gene of the key Target of Rapamycin (TOR) signalling pathway was silenced by RNAi. Results indicated that flush shoot feeding activated the development of the psyllids ovary and therefore induced oviposition. A total of 126 and 2794 differentially expressed genes were detected at 1 and 5 days, respectively, after pest infestation of flush shoots compared to mature leaves. Many genes are involved in protein metabolism, Mitogen-Activated Protein Kinase (MAPK) signalling pathway, hormone synthesis, and TOR signalling pathway: all thought to activate reproduction. Silencing of the positive regulator gene DcRheb in the TOR pathway resulted in lower levels of ecdysone and juvenile hormone and decreased vitellogenin synthesis, further disrupting reproductive ability. This study enhances understanding of the molecular mechanism underlying ACP's reproductive strategy.

The ecdysone-induced protein 93 is a key factor regulating gonadotrophic cycles in the adult female mosquito Aedes aegypti

Repeated blood feedings are required for adult female mosquitoes to maintain their gonadotrophic cycles, enabling them to be important pathogen carriers of human diseases. Elucidating the molecular mechanism underlying developmental switches between these mosquito gonadotrophic cycles will provide valuable insight into mosquito reproduction and could aid in the identification of targets to disrupt these cycles, thereby reducing disease transmission. We report here that the transcription factor ecdysone-induced protein 93 (E93), previously implicated in insect metamorphic transitions, plays a key role in determining the gonadotrophic cyclicity in adult females of the major arboviral vector Aedes aegypti Expression of the E93 gene in mosquitoes is down-regulated by juvenile hormone (JH) and up-regulated by 20-hydroxyecdysone (20E). We find that E93 controls Hormone Receptor 3 (HR3), the transcription factor linked to the termination of reproductive cycles. Moreover, knockdown of E93 expression via RNAi impaired fat body autophagy, suggesting that E93 governs autophagy-induced termination of vitellogenesis. E93 RNAi silencing prior to the first gonadotrophic cycle affected normal progression of the second cycle. Finally, transcriptomic analysis showed a considerable E93-dependent decline in the expression of genes involved in translation and metabolism at the end of a reproductive cycle. In conclusion, our data demonstrate that E93 acts as a crucial factor in regulating reproductive cycle switches in adult female mosquitoes.

20-Hydroxyecdysone (20E) signaling as a promising target for the chemical control of malaria vectors

With the rapid development and spread of resistance to insecticides among anopheline malaria vectors, the efficacy of current World Health Organization (WHO)-approved insecticides targeting these vectors is under threat. This has led to the development of novel interventions, including improved and enhanced insecticide formulations with new targets or synergists or with added sterilants and/or antimalarials, among others. To date, several studies in mosquitoes have revealed that the 20-hydroxyecdysone (20E) signaling pathway regulates both vector abundance and competence, two parameters that influence malaria transmission. Therefore, insecticides which target 20E signaling (e.g. methoxyfenozide and halofenozide) may be an asset for malaria vector control. While such insecticides are already commercially available for lepidopteran and coleopteran pests, they still need to be approved by the WHO for malaria vector control programs. Until recently, chemicals targeting 20E signaling were considered to be insect growth regulators, and their effect was mostly studied against immature mosquito stages. However, in the last few years, promising results have been obtained by applying methoxyfenozide or halofenozide (two compounds that boost 20E signaling) to Anopheles populations at different phases of their life-cycle. In addition, preliminary studies suggest that methoxyfenozide resistance is unstable, causing the insects substantial fitness costs, thereby potentially circumventing one of the biggest challenges faced by current vector control efforts. In this review, we first describe the 20E signaling pathway in mosquitoes and then summarize the mechanisms whereby 20E signaling regulates the physiological processes associated with vector competence and vector abundance. Finally, we discuss the potential of using chemicals targeting 20E signaling to control malaria vectors.

RNA interference-mediated knockdown of the transcription factor Krüppel homologue 1 suppresses vitellogenesis in Chilo suppressalis

Vitellogenesis in holometabolous insects involves the production and secretion of vitellogenin (Vg) and other yolk protein precursors in developing oocyte by the fat body, all of which is predominantly orchestrated by juvenile hormone (JH). Krüppel homologue 1 (Kr-h1) is a zinc finger transcription factor that has been demonstrated to be a JH-early inducible gene and to contribute to reproduction. However, the exact molecular function of Kr-h1 in insect reproduction is poorly understood. In the current study, we used the notorious pest Chilo suppressalis as a model system to investigate the role of Kr-h1 in female reproduction. Cloning and sequencing C. suppressalis Kr-h1 revealed that it shares high identity with its homologues from other lepidopteran insects. Moreover, RNA interference-mediated knockdown of CsKr-h1 substantially reduced the transcription of Vg in the fat body, dramatically decreased yolk protein deposition and also impaired oocyte maturation and ovarian development, indicating that Kr-h1 is indispensable for normal vitellogenesis in C. suppressalis. Based on these results, we conclude that Kr-h1 is crucial to reproduction in insects and that targeting this gene could potentially be a new way to suppress rice pests.

Requirement of Leptinotarsa decemlineata gene within the 74EF puff for larval-pupal metamorphosis and appendage growth

Two Drosophila melanogaster E-twenty-six domain transcription factor isoforms (E74A and E74B) act differentially at the start of the 20-hydroxyecdysone (20E) signalling cascade to regulate larval-pupal metamorphosis. In the present paper, we identified the two isoforms (LdE74A and LdE74B) in Leptinotarsa decemlineata. During the larval development stage, the mRNA transcript levels of the two LdE74 isoforms were correlated with circulating 20E titres. In vitro midgut culture and in vivo dietary supplementation with 20E revealed that the presence of 20E induced expression peaks of both LdE74A and LdE74B, with similar patterns observed for the two isoforms. Moreover, the mRNA transcript levels of both LdE74A and LdE74B isoforms were significantly downregulated in the L. decemlineata ecdysone receptor RNA interference (RNAi) specimens, but not in the LdE75 RNAi beetles. Ingestion of 20E reduced the larval fresh weights and shortened the larval development period, irrespective of knockdown of LdE74 or not. RNAi of LdE74 did not affect 20E-induced expression of the Ecdysone induced protein 75-hormone receptor 3-fushi tarazu factor 1 (E75-HR3-FTZ-F1) transcriptional cascade. Thus, it seems that LdE74 mediates 20E signalling independent of the E75-HR3-FTZ-F1 transcriptional cascade. Furthermore, silencing of both LdE74 isoforms caused failure of ecdysis. Most of the LdE74 RNAi beetles remained as prepupae. The LdE74 RNAi prepupae exhibited adult character-like forms underneath after removal of the apolysed larval cuticle. Their appendages such as antennae, legs and wings were shorter than those of control larvae. Only a few LdE74 RNAi larvae finally became deformed pupae, with shortened antennae and legs. Therefore, LdE74 is required for larval-pupal metamorphosis and appendage growth in L. decemlineata.

Regulatory Pathways Controlling Female Insect Reproduction

The synthesis of vitellogenin and its uptake by maturing oocytes during egg maturation are essential for successful female reproduction. These events are regulated by the juvenile hormones and ecdysteroids and by the nutritional signaling pathway regulated by neuropeptides. Juvenile hormones act as gonadotropins, regulating vitellogenesis in most insects, but ecdysteroids control this process in Diptera and some Hymenoptera and Lepidoptera. The complex crosstalk between the juvenile hormones, ecdysteroids, and nutritional signaling pathways differs distinctly depending on the reproductive strategies adopted by various insects. Molecular studies within the past decade have revealed much about the relationships among, and the role of, these pathways with respect to regulation of insect reproduction. Here, we review the role of juvenile hormones, ecdysteroids, and nutritional signaling, along with that of microRNAs, in regulating female insect reproduction at the molecular level.

Four-way regulation of mosquito yolk protein precursor genes by juvenile hormone-, ecdysone-, nutrient-, and insulin-like peptide signaling pathways

Anautogenous mosquito females require a meal of vertebrate blood in order to initiate the production of yolk protein precursors by the fat body. Yolk protein precursor gene expression is tightly repressed in a state-of-arrest before blood meal-related signals activate it and expression levels rise rapidly. The best understood example of yolk protein precursor gene regulation is the vitellogenin-A gene (vg) of the yellow fever mosquito Aedes aegypti. Vg-A is regulated by (1) juvenile hormone signaling, (2) the ecdysone-signaling cascade, (3) the nutrient sensitive target-of-rapamycin signaling pathway, and (4) the insulin-like peptide (ILP) signaling pathway. A plethora of new studies have refined our understanding of the regulation of yolk protein precursor genes since the last review on this topic in 2005 (Attardo et al., 2005). This review summarizes the role of these four signaling pathways in the regulation of vg-A and focuses upon new findings regarding the interplay between them on an organismal level.

Distinct roles of isoforms of the heme-liganded nuclear receptor E75, an insect ortholog of the vertebrate Rev-erb, in mosquito reproduction

Mosquitoes are adapted to using vertebrate blood as a nutrient source to promote egg development and as a consequence serve as disease vectors. Blood-meal activated reproductive events in female mosquitoes are hormonally and nutritionally controlled with an insect steroid hormone 20-hydroxyecdysone (20E) playing a central role. The nuclear receptor E75 is an essential factor in the 20E genetic hierarchy, however functions of its three isoforms - E75A, E75B, and E75C - in mosquito reproduction are unclear. By means of specific RNA interference depletion of E75 isoforms, we identified their distinct roles in regulating the level and timing of expression of key genes involved in vitellogenesis in the fat body (an insect analog of vertebrate liver and adipose tissue) of the mosquito Aedes aegypti. Heme is required in a high level of expression of 20E-controlled genes in the fat body, and this heme action depends on E75. Thus, in mosquitoes, heme is an important signaling molecule, serving as a sensor of the availability of a protein meal for egg development. Disruption of this signaling pathway could be explored in the design of mosquito control approaches.

Downregulation of ultraspiracle gene expression delays pupal development in honeybees

Ecdysteroids regulate many aspects of insect physiology after binding to a heterodimer composed of the nuclear hormone receptor proteins ecdysone receptor (EcR) and ultraspiracle (Usp). Several lines of evidence have suggested that the latter also plays important roles in mediating the action of juvenile hormone (JH) and, thus, integrates signaling by the two morphogenetic hormones. By using an RNAi approach, we show here that Usp participates in the mechanism that regulates the progression of pupal development in Apis mellifera, as indicated by the observed pupal developmental delay in usp knocked-down bees. Knock-down experiments also suggest that the expression of regulatory genes such as ftz transcription factor 1 (ftz-f1) and juvenile hormone esterase (jhe) depend on Usp. Vitellogenin (vg), the gene coding the main yolk protein in honeybees, does not seem to be under Usp regulation, thus suggesting that the previously observed induction of vg expression by JH during the last stages of pupal development is mediated by yet unknown transcription factor complexes.

20-Hydroxyecdysone regulation of two isoforms of the Ets transcription factor E74 gene in programmed cell death in the silkworm anterior silk gland

Programmed cell death of larval-specific tissues in insects is under the control of 20-hydroxyecdysone (20E). The ecdysteroid-regulated early genes are conserved in the programmed cell death of anterior silk glands (ASGs) in Bombyx mori and salivary glands in Drosophila melanogaster. We identified and characterized two isoforms of the Ets transcription factor E74 gene in B. mori (BmE74). In ASGs of B. mori last instar larvae, the Bm74A mRNA level increased concomitantly with an increase in haemolymph ecdysteroid titre after gut purge. The optimal 20E concentration for stimulation of Bm74A in ASGs was 4 microM, a similar value to the peak haemolymph ecdysteroid concentration after gut purge. In contrast, BmE74B expression peaked on day 5 of the feeding period, after which it did not increase again. These findings suggest that the BmE74 isoforms play different roles in the regulation of programmed cell death in B. mori ASGs.

GATA factor translation is the final downstream step in the amino acid/target-of-rapamycin-mediated vitellogenin gene expression in the anautogenous mosquito Aedes aegypti

Ingestion of blood is required for vector mosquitoes to initiate reproductive cycles determining their role as vectors of devastating human diseases. Nutritional signaling plays a pivotal role in regulating mosquito reproduction. Transcription of yolk protein precursor genes is repressed until mosquitoes take blood. Previously, we have shown that to signal the presence of blood in the gut, mosquitoes utilize the target-of-rapamycin (TOR) pathway. The TOR signaling pathway transduces the amino acid signal activating the major yolk protein precursor gene, vitellogenin (Vg). Here we report the identification of a GATA factor (AaGATAa) that is synthesized after a blood meal and acts as a transcriptional activator of Vg. We showed that AaGATAa bound specifically to GATA-binding sites present in the proximal promoter region of the Vg gene and positively regulated Vg expression in transfection assays. RNA interference-mediated knock down of AaGATAa transcript resulted in a significant inhibition of Vg expression in both fat-body tissue culture and blood-fed mosquitoes. AaGATAa mRNA accumulated in the fat body prior to blood feeding. However, translation of GATA was activated by blood feeding because the GATA protein increased dramatically in the fat body of blood-fed mosquitoes. This increase was also reproduced in the fat-body culture stimulated with amino acids. GATA translation was inhibited by rapamycin and cycloheximide as well as by RNA interference-mediated knock down of S6 kinase. These experiments have revealed that the TOR signaling pathway induced by nutritional signaling regulates the translation of a GATA factor, which is the specific transcriptional activator of the Vg gene.

Synergistic action of E74B and ecdysteroid receptor in activating a 20-hydroxyecdysone effector gene

A number of insect effector genes activated by the steroid hormone 20-hydroxyecdysone (20E) are dually controlled by the ecdysteroid receptor (EcR/USP) and products of ecdysteroid early responsive genes (E74, E75, and Broad). However, the molecular mechanism of this dual action is poorly understood. Here we examined transcriptional activation of the vitellogenin (Vg) gene in the yellow fever mosquito, Aedes aegypti, by EcR/USP and E74 in response to an elevation of 20E titers. There are two isoforms of the Aedes E74 gene, AaE74A and AaE74B, which have a common C-terminal Ets DNA-binding domain and isoform-specific N termini in the female mosquito. Inhibiting expression of AaE74B but not AaE74A by RNA interference led to substantial reduction in the Vg gene expression. AaE74B and the ecdysteroid receptor synergistically enhanced 20E-induced transcription of the Vg promoter. This action required the E74-binding sites and the ecdysone response elements in the Vg 5' regulatory region. Two-hybrid assays and coimmunoprecipitation analyses demonstrated direct interaction between AaE74B and AaEcR/AaUSP. Moreover, disruption of this interaction by a dominant negative E74 mutant abolished the enhanced activation of Vg. Therefore, the cooperative interaction between AaE74B and the ecdysteroid receptor is required for high-level expression of the Vg gene in vivo. The synergistic activation is accomplished through their 20E-dependent protein-protein interaction on the gene promoter. This study reveals how the 20E direct-indirect regulation of an effector gene is achieved at the molecular level.

Nutritional regulation of vitellogenesis in mosquitoes: implications for anautogeny

Anautogeny is a successful reproductive strategy utilized by many mosquito species and other disease-transmitting arthropod vectors. Developing an understanding of the mechanisms underlying anautogeny in mosquitoes is very important because this reproductive strategy is the driving force behind the transmission of disease to millions of people. Information gained from mosquito studies may also be applicable to other blood feeding insect vectors. The conversion of protein from blood into yolk protein precursors for the developing oocytes is an essential part of the reproductive cycle, and understanding how this process is regulated could lead to safe, specific, and effective ways to block reproduction in blood feeding insects. Great gains have been made in elucidating the mechanisms that regulate vitellogenesis in mosquitoes, especially Ae. aegypti. However, a number of questions remain to be answered to make the picture more complete. In this review, we summarize what is currently known about the nutritional regulation of vitellogenesis in mosquitoes and the questions that remain to be answered about this important biological phenomenon.

Target of rapamycin-dependent activation of S6 kinase is a central step in the transduction of nutritional signals during egg development in a mosquito

Female mosquitoes are effective disease vectors, because they take blood from vertebrate hosts to obtain nutrients for egg development. Amino acid signaling via the target of rapamycin (TOR) pathway has been identified as a key requirement for the activation of egg development after a blood meal. We report the characterization of the TOR kinase and one of its major downstream targets, S6 kinase, of the yellow fever mosquito Aedes aegypti during egg development in adult females. Both TOR and S6K mRNA are expressed at high levels in the ovaries and in lower levels in fat body and other tissues. After a blood meal, the subcellular localization of TOR shifts from the cytoplasm to the plasma membrane of fat body cells. By detecting phosphothreonine 388 of mosquito S6 kinase, we show that TOR activity strongly increases in fat body and ovaries after a blood meal in vivo. Furthermore, phosphorylation of S6 kinase increases in in vitro cultured fat bodies after stimulation with amino acids. This increase is sensitive to the TOR inhibitor rapamycin in a concentration-dependent manner but not to the phosphatidylinositol 3-kinase/phosphatidylinositol 3-kinase-related kinase inhibitor LY294002, the MAPK inhibitor PD98059, or the translational inhibitor cycloheximide. RNA interference-mediated reduction of S6 kinase strongly inhibits the amino acid-induced up-regulation of the major yolk protein vitellogenin in vitro and effectively disrupts egg development after a blood meal in vivo. Our data show that TOR-dependent activation of S6 kinase is a central step in the transduction of nutritional information during egg development in mosquitoes.