Determination of juvenile hormone titers by means of LC-MS/MS/MS and a juvenile hormone-responsive Gal4/UAS system in Aedes aegypti mosquitoes

Rickettsia symbionts spread via mixed mode transmission, increasing female fecundity and sex ratio shift by host hormone modulating

Heritable symbionts are common among animals in nature, but the molecular mechanisms underpinning symbiont invasions of host populations have been elusive. In this study, we demonstrate the spread of Rickettsia in an invasive agricultural pest, the whitefly Bemisia tabaci Mediterranean (MED), across northeastern China from 2018 to 2023. Here, we show that the beneficial symbiont Rickettsia spreads by manipulating host hormone signals. Our analyses suggest that Rickettsia have been horizontally acquired by B. tabaci MED from another invasive whitefly B. tabaci Middle East-Asia Minor 1 during periods of coexistence. Rickettsia is transmitted maternally and horizontally from female B. tabaci MED individuals. Rickettsia infection enhances fecundity and results in female bias among whiteflies. Our findings reveal that Rickettsia infection stimulates juvenile hormone (JH) synthesis, in turn enhancing fecundity, copulation events, and the female ratio of the offspring. Consequently, Rickettsia infection results in increased whitefly fecundity and female bias by modulating the JH pathway. More female progeny facilitates the transmission of Rickettsia. This study illustrates that the spread of Rickettsia among invasive whiteflies in northeastern China is propelled by host hormone regulation. Such symbiont invasions lead to rapid physiological and molecular evolution in the host, influencing the biology and ecology of an invasive species.

The effects of female age on blood-feeding, insemination, sperm storage, and fertility in the dengue vector mosquito Aedes aegypti (Diptera: Culicidae)

Mating induces behavioral and physiological changes in female insects-collectively referred to as the female post-mating response (PMR)-that facilitate the production of progeny. PMRs are elicited by transfer of male-derived seminal components during mating, but are altered by other factors, including adult age. Increased female age is often accompanied by declines in fertility. However, mating shortly after emergence also impacts fertility in the insect model Drosophila melanogaster. Here, we determined the age post-emergence when females of the vector mosquito Aedes aegypti can be inseminated and blood-feed. We next examined fecundity, fertility, and the storage of sperm in the female reproductive tract in "young" (30-41 hours-old) and "old" (2- and 3-week-old) females, finding that blood-feeding began at 14 hours, and mating at ∼24 hours post-emergence. Although young females consumed smaller blood quantities and stored fewer sperm, they were similarly fertile to 4-day-old controls. Old females, however, suffered significant declines in fecundity by 2 weeks of age. Our results show that female Ae. aegypti start to become sexually receptive 1 day after their emergence, but can ingest blood much sooner, suggesting that mating is not a prerequisite to blood-feeding, and that females can ingest an arbovirus infected blood-meal shortly after emergence.

Prostaglandin E2 mediates chorion formation of the Asian tiger mosquito, Aedes albopictus, at late oogenesis

Chorion-i.e., the eggshell-is formed during the late stage of oogenesis by follicular epithelium in the ovary. Although the endocrine signal(s) driving choriogenesis remain unclear in mosquitoes, this process in other insects has been suspected to involve the mediation of prostaglandins (PGs). This study tested the role of PG in the choriogenesis of the Asian tiger mosquito, Aedes albopictus, and its influence on controlling the expressions of genes associated with chorion formation by a transcriptome analysis. An immunofluorescence assay showed that PGE2 is localised in follicular epithelium. With the treatment of aspirin, an inhibitor of PG biosynthesis, at mid oogenesis, the PGE2 signal disappeared in the follicular epithelium led to significantly inhibited chorion formation along with a malformed eggshell. Ovary transcriptomes were assessed by RNASeq at the mid and late ovarian developmental stages. Differentially expressed genes (DEGs) exhibiting more than twofold changes in expression levels included 297 genes at mid stage and 500 genes at late stage. These DEGs at these two developmental stages commonly included genes associated with egg and chorion proteins of Ae. albopictus. Most chorion-associated genes were clustered in the 168 Mb region on a chromosome and exhibited significantly induced expressions at both ovarian developmental stages. The inhibition of PG biosynthesis significantly suppressed the expression of the chorion-associated genes while the addition of PGE2 rescued the gene expressions and led to recovery of choriogenesis. These results suggest that PGE2 mediates the choriogenesis of Ae. albopictus.

An 11-point time course midgut transcriptome across 72 h after bloodfeeding provides detailed temporal resolution of transcript expression in the arbovirus vector, Aedes aegypti

As the major vector for dengue, Zika, yellow fever, and chikungunya viruses, the mosquito Aedes aegypti is one of the most important insects in public health. These viruses are transmitted by bloodfeeding, which is also necessary for the reproduction of the mosquito. Thus, the midgut plays an essential role in mosquito physiology as the center for bloodmeal digestion and as an organ that serves as the first line of defense against viruses. Despite its importance, transcriptomic dynamics with fine temporal resolution across the entire digestion cycle have not yet been reported. To fill this gap, we conducted a transcriptomic analysis of A. aegypti female midguts across a 72-h bloodmeal digestion cycle for 11 time points, with a particular focus on the first 24 h. PCA analysis confirmed that 72 h is indeed a complete digestion cycle. Cluster and GO enrichment analysis showed the orchestrated modulation of thousands of genes to accomplish the midgut's role as the center for digestion, as well as nutrient transport with a clear progression with sequential emphasis on transcription, translation, energy production, nutrient metabolism, transport, and finally, autophagy by 24-36 h. We further determined that many serine proteases are robustly expressed as if to prepare for unexpected physiological challenges. This study provides a powerful resource for the analysis of genomic features that coordinate the rapid and complex transcriptional program induced by mosquito bloodfeeding.

Rho 1 participates in parasitoid wasp eggs maturation and host cellular immunity inhibition

Endoparasitoid wasps introduce venom into their host insects during the egg-laying stage. Venom proteins play various roles in the host physiology, development, immunity, and behavior manipulation and regulation. In this study, we identified a venom protein, MmRho1, a small guanine nucleotide-binding protein derived from ovary in the endoparasitoid wasp Microplitis mediator and found that knockdown of its expression by RNA interference caused down-regulation of vitellogenin and juvenile hormone, egg production, and cocoons formation in the female wasps. We demonstrated that MmRho1 entered the cotton bollworm's (host) hemocytes and suppressed cellular immune responses after parasitism using immunofluorescence staining. Furthermore, wasp MmRho1 interacted with the cotton bollworm's actin cytoskeleton rearrangement regulator diaphanous by yeast 2-hybrid and glutathione s-transferase pull-down. In conclusion, this study indicates that MmRho1 plays dual roles in wasp development and the suppression of the host insect cellular immune responses.

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

Anautogenous mosquitoes must blood feed on a vertebrate host to produce eggs. Each gonadotrophic cycle is subdivided into a sugar-feeding previtellogenic phase that produces primary follicles and a blood meal-activated vitellogenic phase in which large numbers of eggs synchronously mature and are laid. Multiple endocrine factors including juvenile hormone (JH), insulin-like peptides (ILPs), ovary ecdysteroidogenic hormone (OEH), and 20-hydroxyecdysone (20E) coordinate each gonadotrophic cycle. Egg formation also requires nutrients from feeding that are stored in the fat body. Regulation of egg formation is best understood in Aedes aegypti but the role different endocrine factors play in regulating nutrient mobilization and storage remains unclear. In this study, we report that adult female Ae. aegypti maintained triacylglycerol (TAG) stores during the previtellogenic phase of the first gonadotrophic cycle while glycogen stores declined. In contrast, TAG and glycogen stores were rapidly mobilized during the vitellogenic phase and then replenishment. Several genes encoding enzymes with functions in TAG and glycogen metabolism were differentially expressed in the fat body, which suggested regulation was mediated in part at the transcriptional level. Gain of function assays indicated that stored nutrients were primarily mobilized by adipokinetic hormone (AKH) while juvenoids and OEH regulated replenishment. ILP3 further showed evidence of negatively regulating certain lipolytic enzymes. Loss of function assays indicated AKH depends on the AKH receptor (AKHR) for function. Altogether, our results indicate that the opposing activities of different hormones regulate nutrient stores during a gonadotrophic cycle in Ae. aegypti.

miR-309a is a regulator of ovarian development in the oriental fruit fly Bactrocera dorsalis

Fecundity is arguably one of the most important life history traits, as it is closely tied to fitness. Most arthropods are recognized for their extreme reproductive capacity. For example, a single female of the oriental fruit fly Bactrocera dorsalis, a highly invasive species that is one of the most destructive agricultural pests worldwide, can lay more than 3000 eggs during its life span. The ovary is crucial for insect reproduction and its development requires further investigation at the molecular level. We report here that miR-309a is a regulator of ovarian development in B. dorsalis. Our bioinformatics and molecular studies have revealed that miR-309a binds the transcription factor pannier (GATA-binding factor A/pnr), and this activates yolk vitellogenin 2 (Vg 2) and vitellogenin receptor (VgR) advancing ovarian development. We further show that miR-309a is under the control of juvenile hormone (JH) and independent from 20-hydroxyecdysone. Thus, we identified a JH-controlled miR-309a/pnr axis that regulates Vg2 and VgR to control the ovarian development. This study has further enhanced our understanding of molecular mechanisms governing ovarian development and insect reproduction. It provides a background for identifying targets for controlling important Dipteran pests.

The ovary is a very critical organ for insect reproduction. Especially, many insect pests are famous for their large reproductive capacity. Therefore, understanding the molecular mechanisms involved in ovarian development could significantly contribute in the development of new insect pest control strategies. In this study, we report that miR-309a regulates the development of the ovary in an important dipteran pest, B. dorsalis, through a transcriptional factor, pannier (GATA-binding factor A/pnr), which in turn directly mediates the expression of yolk vitellogenin 2 (Vg 2) and vitellogenin receptor (VgR). Moreover, miR-309a is under the upstream control of juvenile hormone (JH). Here, in Dipterans, a novel JH-miR-309a-pnr-Vg-related genes regulatory pathway was found in ovarian development. This finding advances our understanding of a mechanism regulating insect ovarian development and provides new insights for potential targets to control dipteran pests through the reproductive strategy.

A transcriptomic atlas of Aedes aegypti reveals detailed functional organization of major body parts and gut regional specializations in sugar-fed and blood-fed adult females

Mosquitoes transmit numerous pathogens, but large gaps remain in our understanding of their physiology. To facilitate explorations of mosquito biology, we have created Aegypti-Atlas (http://aegyptiatlas.buchonlab.com/), an online resource hosting RNAseq profiles of Ae. aegypti body parts (head, thorax, abdomen, gut, Malpighian tubules, ovaries), gut regions (crop, proventriculus, anterior and posterior midgut, hindgut), and a gut time course of blood meal digestion. Using Aegypti-Atlas, we provide insights into regionalization of gut function, blood feeding response, and immune defenses. We find that the anterior and posterior midgut possess digestive specializations which are preserved in the blood-fed state. Blood feeding initiates the sequential induction and repression/depletion of multiple cohorts of peptidases. With respect to defense, immune signaling components, but not recognition or effector molecules, show enrichment in ovaries. Basal expression of antimicrobial peptides is dominated by holotricin and gambicin, which are expressed in carcass and digestive tissues, respectively, in a mutually exclusive manner. In the midgut, gambicin and other effectors are almost exclusively expressed in the anterior regions, while the posterior midgut exhibits hallmarks of immune tolerance. Finally, in a cross-species comparison between Ae. aegypti and Anopheles gambiae midguts, we observe that regional digestive and immune specializations are conserved, indicating that our dataset may be broadly relevant to multiple mosquito species. We demonstrate that the expression of orthologous genes is highly correlated, with the exception of a ‘species signature’ comprising a few highly/disparately expressed genes. With this work, we show the potential of Aegypti-Atlas to unlock a more complete understanding of mosquito biology.

Helicoverpa armigera miR-2055 regulates lipid metabolism via fatty acid synthase expression

Insect hormones and microRNAs regulate lipid metabolism, but the mechanisms are not fully elucidated. Here, we found that cotton bollworm larvae feeding on Arabidopsis thaliana (AT) leaves had a lower triacylglycerol (TAG) level and more delayed development than individuals feeding on artificial diet (AD). Association analysis of small RNA and mRNA revealed that the level of miR-2055, a microRNA related to lipid metabolism, was significantly higher in larvae feeding on AT. Dual-luciferase reporter assays demonstrated miR-2055 binding to 3' UTR of fatty acid synthase (FAS) mRNA to suppress its expression. Elevating the level of miR-2055 in larvae by agomir injection decreased FAS mRNA and protein levels, which resulted in reduction of free fatty acid (FFA) and TAG in fat body. Interestingly, in vitro assays illustrated that juvenile hormone (JH) increased miR-2055 accumulation in a dosage-dependent manner, whereas knockdown of Methoprene tolerant (Met) or Kruppel homologue 1 (Kr-h1) decreased the miR-2055 level. This implied that JH induces the expression of miR-2055 via a Met-Kr-h1 signal. These findings demonstrate that JH and miRNA cooperate to modulate lipid synthesis, which provides new insights into the regulatory mechanisms of metabolism in insects.

Juvenile hormone regulates photoperiod-mediated male reproductive diapause via the methoprene-tolerant gene in the ladybeetle Harmonia axyridis

Juvenile hormone (JH) absence induces photoperiod-mediated reproductive diapause, which is characterized by reproductive cessation. Although the role of methoprene-tolerant (Met)-mediated JH signaling in photoperiod-mediated female reproduction has been well documented, its role in male reproduction remains unclear. In this study, we investigated the role of JH in regulating photoperiod-mediated development of the male internal reproductive system (IRS) in the predatory ladybeetle Harmonia axyridis (Pallas). In a previous study, we found that adult male H. axyridis reared under either a short-day (SD) or long-day (LD) photoperiod had obvious differences in IRS development, but we were unable to identify the regulators of male reproductive diapause. In this study, we found that beetles reared under an SD photoperiod had significantly lower JH titer and a relatively undeveloped male IRS compared with those reared under an LD photoperiod. Additionally, application of the JH analog (JHA) methoprene promoted IRS development. Furthermore, Met knockdown strongly blocked JH signaling in males reared under the LD photoperiod, thereby slowing IRS development. Moreover, exogenous JHA did not reverse the suppressed development of the male IRS caused by Met knockdown. These results indicate that photoperiod regulates male IRS development in H. axyridis through a conserved Met-dependent JH signaling pathway.

E93 confers steroid hormone responsiveness of digestive enzymes to promote blood meal digestion in the midgut of the mosquito Aedes aegypti

Anautogenous female mosquitoes obtain the nutrients needed for egg development from vertebrate blood, and consequently they transmit numerous pathogens of devastating human diseases. Digestion of blood proteins into amino acids that are used for energy production, egg maturation and replenishment of maternal reserves is an essential part of the female mosquito reproductive cycle. However, the regulatory mechanisms underlying this process remain largely unknown. Here, we report that the transcription factor E93 is a critical factor promoting blood meal digestion in adult females of the major arboviral vector Aedes aegypti in response to the steroid hormone 20-hydroxyecdysone (20E). E93 was upregulated in the female mosquito midgut after a blood meal, and RNA interference (RNAi)-mediated knockdown of E93 inhibited midgut blood digestion. E93 RNAi depletion repressed late trypsin (LT), serine protease I (SPI), SPVI and SPVII, and activated early trypsin (ET) expression in the female mosquito midgut after a blood meal. Injection of 20E activated E93, LT, SPI, SPVI and SPVII, and repressed ET expression, whereas RNAi knockdown of the ecdysone receptor (EcR) repressed E93, LT, SPI, SPVI and SPVII, and activated ET expression in the midgut. Furthermore, E93 depletion resulted in a complete loss of 20E responsiveness of LT, SPVI and SPVII. Our findings reveal important mechanisms regulating blood meal digestion in disease-transmitting mosquitoes.

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.

Steroid hormone ecdysone deficiency stimulates preparation for photoperiodic reproductive diapause

Diapause, a programmed developmental arrest primarily induced by seasonal environmental changes, is very common in the animal kingdom, and found in vertebrates and invertebrates alike. Diapause provides an adaptive advantage to animals, as it increases the odds of surviving adverse conditions. In insects, individuals perceive photoperiodic cues and modify endocrine signaling to direct reproductive diapause traits, such as ovary arrest and increased fat accumulation. However, it remains unclear as to which endocrine factors are involved in this process and how they regulate the onset of reproductive diapause. Here, we found that the long day-mediated drop in the concentration of the steroid hormone ecdysone is essential for the preparation of photoperiodic reproductive diapause in Colaphellus bowringi, an economically important cabbage beetle. The diapause-inducing long-day condition reduced the expression of ecdysone biosynthetic genes, explaining the drop in the titer of 20-hydroxyecdysone (20E, the active form of ecdysone) in female adults. Application of exogenous 20E induced vitellogenesis and ovarian development but reduced fat accumulation in the diapause-destined females. Knocking down the ecdysone receptor (EcR) in females destined for reproduction blocked reproductive development and induced diapause traits. RNA-seq and hormone measurements indicated that 20E stimulates the production of juvenile hormone (JH), a key endocrine factor in reproductive diapause. To verify this, we depleted three ecdysone biosynthetic enzymes via RNAi, which confirmed that 20E is critical for JH biosynthesis and reproductive diapause. Importantly, impairing Met function, a component of the JH intracellular receptor, partially blocked the 20E-regulated reproductive diapause preparation, indicating that 20E regulates reproductive diapause in both JH-dependent and -independent manners. Finally, we found that 20E deficiency decreased ecdysis-triggering hormone signaling and reduced JH production, thereby inducing diapause. Together, these results suggest that 20E signaling is a pivotal regulator that coordinates reproductive plasticity in response to environmental inputs.

Developmental arrest pervades organismal development and physiology where it facilitates an enormous range of adaptive responses to stressful conditions. Many animals exhibit various forms of developmental arrest that ensures survival under the most adverse environments. Reproductive diapause occurs when adults temporarily suspend reproduction in response to environmental stress and has been documented for a variety of invertebrates, particularly insects. Endocrine signals play a central role in translating environmental cues such as photoperiod into reproductive diapause-related physiology and behavior. However, it has been an unresolved issue as to which endocrine factors can respond to photoperiodic inputs and regulate diapause outputs. In this study, we found that a decrease in ecdysone levels is critical for reproductive diapause to occur. Also, ecdysone could interact with juvenile hormone to regulate the occurrence of reproductive diapause in response to photoperiodic cues. Our findings provide new insight into endocrine mechanisms of photoperiodic reproductive diapause and an example of phenotypic plasticity in animals.

Regulation of antimicrobial peptides by juvenile hormone and its receptor, Methoprene-tolerant, in the mosquito Aedes aegypti

The trade-off between reproduction and immunity has been established for a number of insect species. However, the regulatory mechanisms governing this event is not well understood. In the mosquito Aedes aegypti, the vector of dangerous human arboviral diseases, juvenile hormone (JH) is required for the female post-eclosion development and reproductive maturation. In this study, we have revealed the JH negative effect on the expression of immunity-related genes, such as antimicrobial peptides (AMPs), during the post-eclosion phase of the female mosquito gonadotrophic reproductive cycle. Mosquitoes treated with JH became more sensitive to microbial infection. Mosquitoes subjected to the RNA interference knockdown (RNAi) of the JH receptor, Methoprene-tolerant (Met), showed increased expression of several AMP genes. Met binds to the E-box-like recognition motifs in the regulatory region of the diptericin (Dpt) gene, indicating that JH can suppress the Dpt gene expression through its receptor Met. Hence, JH is involved in the modulation of immune responses during the post-eclosion phase of reproduction. The RNAi knockdown of the peptidoglycan recognition protein (PGRP-LC) led to a significant reduction of the Dpt transcript level, indicating the PGRP-LC activating role on this AMP gene. Thus, Dpt appeared to be under the dual regulation of both the JH and the immune deficiency (IMD) signaling pathways. Our study provides a better understanding of how JH regulates insect immunity in adult mosquitoes.

Approaches and Tools to Study the Roles of Juvenile Hormones in Controlling Insect Biology

Simple Summary

The juvenile hormones (JHs) play critical roles during insect development and reproduction. The numerous effects of JHs have generated multiple basic scientific questions, as well as prospects for the development of insecticidal endocrine disruptors. There is an increasing need for methods to identify and quantify endogenous JHs. The low titers and difficulties in working with these lipophilic compounds have often hindered the study of JH biology. In this article, we review the existing information on the detection and quantification of JH from insect samples, the development of approaches to manipulate JH titers, and the use of next-generation tools to modulate JH homeostasis.

Abstract

The juvenile hormones (JHs) are a group of sesquiterpenoids synthesized by the corpora allata. They play critical roles during insect development and reproduction. To study processes that are controlled by JH, researchers need methods to identify and quantify endogenous JHs and tools that can be used to increase or decrease JH titers in vitro and in vivo. The lipophilic nature of JHs, coupled with the low endogenous titers, make handling and quantification challenging. JH titers in insects can easily be increased by the topical application of JH analogs, such as methoprene. On the other hand, experimentally reducing JH titers has been more difficult. New approaches to modulate JH homeostasis have been established based on advances in RNA interference and CRISPR/Cas9-based genome editing. This review will summarize current advances in: (1) the detection and quantification of JHs from insect samples; (2) approaches to manipulating JH titers; and (3) next-generation tools to modulate JH homeostasis.

Hemocyte-targeted gene expression in the female malaria mosquito using the hemolectin promoter from Drosophila

Hemocytes, the immune cells in mosquitoes, participate in immune defenses against pathogens including malaria parasites. Mosquito hemocytes can also be infected by arthropod-borne viruses but the pro- or anti-viral nature of this interaction is unknown. Although there has been progress on hemocyte characterization during pathogen infection in mosquitoes, the specific contribution of hemocytes to immune responses and the hemocyte-specific functions of immune genes and pathways remain unresolved due to the lack of genetic tools to manipulate gene expression in these cells specifically. Here, we used the Gal4-UAS system to characterize the activity of the Drosophila hemocyte-specific hemolectin promoter in the adults of Anopheles gambiae, the malaria mosquito. We established an hml-Gal4 driver line that we further crossed to a fluorescent UAS responder line, and examined the expression pattern in the adult progeny driven by the hml promoter. We show that the hml regulatory region drives hemocyte-specific transgene expression in a subset of hemocytes, and that transgene expression is triggered after a blood meal. The hml promoter drives transgene expression in differentiating prohemocytes as well as in differentiated granulocytes. Analysis of different immune markers in hemocytes in which the hml promoter drives transgene expression revealed that this regulatory region could be used to study phagocytosis as well as melanization. Finally, the hml promoter drives transgene expression in hemocytes in which o'nyong-nyong virus replicates. Altogether, the Drosophila hml promoter constitutes a good tool to drive transgene expression in hemocyte only and to analyze the function of these cells and the genes they express during pathogen infection in Anopheles gambiae.

Synergistic action of the transcription factors Krüppel homolog 1 and Hairy in juvenile hormone/Methoprene-tolerant-mediated gene-repression in the mosquito Aedes aegypti

Arthropod-specific juvenile hormones control numerous essential functions in development and reproduction. In the dengue-fever mosquito Aedes aegypti, in addition to its role in immature stages, juvenile hormone III (JH) governs post-eclosion (PE) development in adult females, a phase required for competence acquisition for blood feeding and subsequent egg maturation. During PE, JH through its receptor Methoprene-tolerant (Met) regulate the expression of many genes, causing either activation or repression. Met-mediated gene repression is indirect, requiring involvement of intermediate repressors. Hairy, which functions downstream of Met in the JH gene-repression hierarchy, is one such factor. Krüppel-homolog 1, a zinc-finger transcriptional factor, is directly regulated by Met and has been implicated in both activation and repression of JH-regulated genes. However, the interaction between Hairy and Kr-h1 in the JH-repression hierarchy is not well understood. Our RNAseq-based transcriptomic analysis of the Kr-h1-depleted mosquito fat body revealed that 92% of Kr-h1 repressed genes are also repressed by Met, supporting the existence of a hierarchy between Met and Kr-h1 as previously demonstrated in various insects. Notably, 130 genes are co-repressed by both Kr-h1 and Hairy, indicating regulatory complexity of the JH-mediated PE gene repression. A mosquito Kr-h1 binding site in genes co-regulated by this factor and Hairy was identified computationally. Moreover, this was validated using electrophoretic mobility shift assays. A complete phenocopy of the effect of Met RNAi depletion on target genes could only be observed after Kr-h1 and Hairy double RNAi knockdown, suggesting a synergistic action between these two factors in target gene repression. This was confirmed using a cell-culture-based luciferase reporter assay. Taken together, our results indicate that Hairy and Kr-h1 not only function as intermediate downstream factors, but also act together in a synergistic fashion in the JH/Met gene repression hierarchy.

Juvenile hormone (JH) plays an essential role in preparing Aedes aegypti female mosquitoes for blood feeding, egg development, and pathogen transmission. JH acting through its receptor Methoprene-tolerant (Met) regulates the expression of large gene cohorts. JH mediated gene repression, unlike activation that is directly mediated by Met, is indirect and requires intermediate transcriptional repressors Hairy and Krüppel-homolog 1 (Kr-h1). Here, we demonstrate that Hairy and Kr-h1 can act synergistically in the JH-Met gene repression pathway in Aedes female mosquitoes. These interact directly with regulatory regions of the genes that have both Hairy and Kr-h1 binding sites. Thus, this study has significantly advanced our understanding of the complexity of the JH-mediated gene expression pathway. This research yields valuable information about the JH control of reproductive development of the mosquito A. aegypti, one of the most important vectors of human diseases.

Blood feeding activates the vitellogenic stage of oogenesis in the mosquito Aedes aegypti through inhibition of glycogen synthase kinase 3 by the insulin and TOR pathways

Most mosquitoes, including Aedes aegypti, only produce eggs after blood feeding on a vertebrate host. Oogenesis in A. aegypti consists of a pre-vitellogenic stage before blood feeding and a vitellogenic stage after blood feeding. Primary egg chambers remain developmentally arrested during the pre-vitellogenic stage but complete oogenesis to form mature eggs during the vitellogenic stage. In contrast, the signaling factors that maintain primary egg chambers in pre-vitellogenic arrest or that activate vitellogenic growth are largely unclear. Prior studies showed that A. aegypti females release insulin-like peptide 3 (ILP3) and ovary ecdysteroidogenic hormone (OEH) from brain neurosecretory cells after blood feeding. Here, we report that primary egg chambers exit pre-vitellogenic arrest by 8 h post-blood meal as evidenced by proliferation of follicle cells, endoreplication of nurse cells, and formation of cytoophidia. Ex vivo assays showed that ILP3 and OEH stimulate primary egg chambers to exit pre-vitellogenic arrest in the presence of nutrients but not in their absence. Characterization of associated pathways indicated that activation of insulin/insulin growth factor signaling (IIS) by ILP3 or OEH inactivated glycogen synthase kinase 3 (GSK3) via phosphorylation by phosphorylated Akt. GSK3 inactivation correlated with accumulation of the basic helix-loop-helix transcription factor Max and primary egg chambers exiting pre-vitellogenic arrest. Direct inhibition of GSK3 by CHIR-99021 also stimulated Myc/Max accumulation and primary egg chambers exiting pre-vitellogenic arrest. Collectively, our results identify GSK3 as a key factor in regulating the pre- and vitellogenic stages of oogenesis in A. aegypti.

Juvenile hormone mediates the positive effects of nitrogen fertilization on weight and reproduction in pea aphid

BACKGROUND

The positive effects of nitrogen fertilization on the performance of phytophagous insects have been reported extensively; the physiological and molecular basis involved, however, is largely unclear. Here, we test experimentally whether enhancement of juvenile hormone (JH) is responsible for the increased weight and fecundity of pea aphid (Acyrthosiphon pisum) under nitrogen fertilization.

RESULTS

Aphids fed on Medicago truncatula with nitrogen fertilization have a greater amino acid content, higher weight at the fourth instar and adult stage, and produce more offspring than those without nitrogen fertilization. Furthermore, nitrogen fertilization upregulates the transcripts of JH biosynthesis-related genes and increases JH titre at the fourth instar and adult stage, suggesting that JH is involved in the positive responses of aphids to nitrogen fertilization. Application of 100 ng JH increases adult weight and fecundity in aphids fed on M. truncatula without nitrogen fertilization. Conversely, impairing JH signalling by pharmacologically inhibiting the target of rapamycin pathway or by knocking down JH biosynthetic gene decreases adult weight and fecundity in aphids fed on M. truncatula with nitrogen fertilization, whereas application of JH rescued the phenotype.

CONCLUSION

The increased JH titre at the fourth instar and adult stage is required for the increases of weight and fecundity of A. pisum under nitrogen fertilization. © 2018 Society of Chemical Industry.

Juvenile hormone-regulated alternative splicing of the taiman gene primes the ecdysteroid response in adult mosquitoes

Juvenile hormone (JH) regulates many aspects of insect development and reproduction. In some processes, JH plays a critical role in defining the action of the steroid hormone 20-hydroxyecdysone (20E). In Aedes aegypti mosquitoes, JH prepares newly emerged female adults to become competent to synthesize vitellogenin in response to 20E after blood ingestion. The molecular basis of this competence is still not well understood. Here, we report that JH regulates pre-mRNA splicing of the taiman gene, which encodes a key transcriptional regulator required for both JH- and 20E-controlled gene expression. JH stimulated the production of the Taiman isoforms A/B, while reducing the levels of the isoforms C/D, in the fat body after adult eclosion. The appearance of the A/B isoforms in maturing mosquitoes was accompanied by acquisition of the competence to respond to 20E. Depletion of the A/B isoforms, by inhibiting the alternative splicing or by isoform-specific RNA interference, considerably diminished the 20E-induced gene expression after a blood meal and substantially impaired oocyte development. In accordance with this observation, further studies indicated that in the presence of 20E, the Taiman A/B isoforms showed much stronger interactions with the 20E receptor complex than the Taiman C/D isoforms. In contrast, all four isoforms displayed similar capabilities of forming active JH receptor complexes with the methoprene-tolerant protein (Met). This study suggested that JH confers the competence to newly emerged female mosquitoes by regulating mRNA splicing to generate the Taiman isoforms that are essential for the vitellogenic 20E response.

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.

MicroRNA-275 targets sarco/endoplasmic reticulum Ca2+ adenosine triphosphatase (SERCA) to control key functions in the mosquito gut

The yellow fever mosquito Aedes aegypti is the major vector of arboviruses, causing numerous devastating human diseases, such as dengue and yellow fevers, Chikungunya and Zika. Female mosquitoes need vertebrate blood for egg development, and repeated cycles of blood feeding are tightly linked to pathogen transmission. The mosquito’s posterior midgut (gut) is involved in blood digestion and also serves as an entry point for pathogens. Thus, the mosquito gut is an important tissue to investigate. The miRNA aae-miR-275 (miR-275) has been shown to be required for normal blood digestion in the female mosquito; however, the mechanism of its action has remained unknown. Here, we demonstrate that miR-275 directly targets and positively regulates sarco/endoplasmic reticulum Ca²⁺adenosine triphosphatase, which is implicated in active transport of Ca²⁺ from the cytosol to the sarco/endoplasmic reticulum. We utilized a combination of the gut-specific yeast transcription activator protein Gal4/upstream activating sequence (Gal4/UAS) system and miRNA Tough Decoy technology to deplete the endogenous level of miR-275 in guts of transgenic mosquitoes. This gut-specific reduction of miR-275 post blood meal decreased SERCA mRNA and protein levels of the digestive enzyme late trypsin. It also resulted in a significant reduction of gut microbiota. Moreover, the decrease of miR-275 and SERCA correlated with defects in the Notch signaling pathway and assembly of the gut actin cytoskeleton. The adverse phenotypes caused by miR-275 silencing were rescued by injections of miR-275 mimic. Thus, we have discovered that miR-275 directly targets SERCA, and the maintenance of its level is critical for multiple gut functions in mosquitoes.

Female mosquitoes transmit numerous devastating human diseases. The mosquito gut, in addition to its primary function as a site of blood digestion, represents the entry point for pathogen colonization in mosquito vectors. The conserved microRNA, miR-275, was shown to be required for blood digestion and egg development. In this study, we investigated the target of miR-275 contributing to the regulation of mosquito gut functions. We achieved spatiotemporal suppression of miR-275 using a transgenic Tough Decoy RNA approach in the A. aegypti female mosquito gut. Furthermore, we have uncovered that miR-275 targets sarco/endoplasmic reticulum Ca²⁺- adenosine triphosphatase (SERCA), affecting numerous gut functions including blood digestion, production of digestive proteases, and assembly of the gut actin cytoskeleton. SERCA is essential for maintenance of Ca²⁺ homeostasis, and its disturbance, in humans, leads to cardiac hypertrophy, heart failure and cancers. Therefore, the finding that the miRNA miR-275 targets SERCA not only contributes to the knowledge of mosquito gut regulation but also significantly adds to the general understanding of mechanisms governing this critical molecule.

Juvenile hormone and its receptor methoprene-tolerant promote ribosomal biogenesis and vitellogenesis in the Aedes aegypti mosquito

Juvenile hormone (JH) controls many biological activities in insects, including development, metamorphosis, and reproduction. In the Aedes aegypti mosquito, a vector of dengue, yellow fever, chikungunya, and zika viruses, the metabolic tissue (the fat body, which is an analogue of the vertebrate liver) produces yolk proteins for developing oocytes. JH is important for the fat body to acquire competence for yolk protein production. However, the molecular mechanisms of how JH promotes mosquito reproduction are not completely understood. In this study we show that stimulation of the JH receptor methoprene-tolerant (Met) activates expression of genes encoding the regulator of ribosome synthesis 1 (RRS1) and six ribosomal proteins (two ribosomal large subunit proteins, two ribosomal small subunit proteins, and two mitochondrial ribosomal proteins). Moreover, RNAi-mediated depletion of RRS1 decreased biosynthesis of the ribosomal protein L32 (RpL32). Depletion of Met, RRS1, or RpL32 led to retardation of ovarian growth and reduced mosquito fecundity, which may at least in part have resulted from decreased vitellogenin protein production in the fat body. In summary, our results indicate that JH is critical for inducing the expression of ribosomal protein genes and demonstrate that RRS1 mediates the JH signal to enhance both ribosomal biogenesis and vitellogenesis.

Hormone and receptor interplay in the regulation of mosquito lipid metabolism

Mosquitoes transmit devastating human diseases because they need vertebrate blood for egg development. Metabolism in female mosquitoes is tightly coupled with blood meal-mediated reproduction, which requires an extremely high level of energy consumption. Functional analysis has shown that major genes encoding for enzymes involved in lipid metabolism (LM) in the mosquito fat bodies are down-regulated at the end of the juvenile hormone (JH)-controlled posteclosion (PE) phase but exhibit significant elevation in their transcript levels during the post-blood meal phase (PBM), which is regulated mainly by 20-hydroxyecdysone (20E). Reductions in the transcript levels of genes encoding triacylglycerol (TAG) catabolism and β-oxidation enzymes were observed to correlate with a dramatic accumulation of lipids in the PE phase; in contrast, these transcripts were elevated significantly and lipid stores were diminished during the PBM phase. The RNAi depletion of Methoprene-tolerant (Met) and ecdysone receptor (EcR), receptors for JH and 20E, respectively, reversed the LM gene expression and the levels of lipid stores and metabolites, demonstrating the critical roles of these hormones in LM regulation. Hepatocyte nuclear factor 4 (HNF4) RNAi-silenced mosquitoes exhibited down-regulation of the gene transcripts encoding TAG catabolism and β-oxidation enzymes and an inability to use lipids effectively, as manifested by TAG accumulation. The luciferase reporter assay showed direct regulation of LM-related genes by HNF4. Moreover, HNF4 gene expression was down-regulated by Met and activated by EcR and Target of rapamycin, providing a link between nutritional and hormonal regulation of LM in female mosquitoes.