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

The role of neuropeptide prothoracicotropic hormone (PTTH) - Torso in pyriproxyfen-induced larval-pupal abnormal metamorphosis in silkworms

The neuropeptide prothoracicotropic hormone (PTTH) plays a key role in regulating ecdysone synthesis and promoting insect metamorphosis. Pyriproxyfen is a juvenile hormone analogue. We previously reported that pyriproxyfen disrupts ecdysone secretion and inhibits larval-pupal metamorphosis in silkworms. However, the specific molecular mechanisms by which pyriproxyfen interferes with ecdysone signaling remain to be elucidated. Herein, the RNA-seq analysis on the ecdysone-secretion organ prothoracic gland (PG) was conducted following pyriproxyfen exposure. A total of 3774 differentially expressed genes (DEGs) were identified, with 1667 up-regulated and 2107 down-regulated. KEGG analysis showed that DEGs were enriched in the MAPK signaling pathway, a conserved pathway activated by PTTH binding to Torso, which regulates the ecdysone synthesis. qRT-PCR results indicated a significant up-regulation in PTTH transcription level, while the transcription levels of torso and downstream MAPK pathway genes, Ras2, Raf and ERK, were down-regulated 24 h post-pyriproxyfen treatment. Consistent with these transcriptional changes, PTTH titers in the brain also increased following pyriproxyfen treatment. These results suggest that pyriproxyfen induces abnormal metamorphosis in silkworms by impairing PTTH-Torso signaling. This study enhances our understanding of the molecular mechanisms of pyriproxyfen-induced larval-pupal abnormal metamorphosis in silkworms, and also provides insights for developing detoxification strategies for juvenile hormone analog pesticides to non-target organisms.

α-Solanine and α-Tomatine Affect the Retrocerebral Complexes of Tenebrio molitor and Zophobas atratus Beetles

Presently, the European Commission is moving forward with a plan to reduce the use of chemical pesticides and increase the percentage of organic farming by 2030; this plan demands increased availability of biopesticides for organic farming. Glycoalkaloids are a class of molecules found in certain plants, including tomatoes and potatoes, which contain sugar and an alkaloid group. These compounds are known to have insecticidal properties, as they can act as natural defenses against insect pests. This study examined how a biological substance, glycoalkaloids, affects mortality and the retrocerebral complex of two beetle species, Tenebrio molitor and Zophobas atratus. More specifically, we focused on two glycoalkaloids, α-solanine and α-tomatine. Although these two insects belong to the same taxonomic family, they were not equally susceptible to these substances. In the mortality test, we observed that the T. molitor beetle seemed more susceptible, whereas the changes in the retrocerebral complex were more significant in the case of Z. atratus. In general, the effects vary between substances and insects, and changes in the retrocerebral complex may impair the reproduction of insects.

Endocrine factors modulating vitellogenesis and oogenesis in insects: An update

The endocrine system plays a pivotal role in shaping the mechanisms that ensure successful reproduction. With over a million known insect species, understanding the endocrine control of reproduction has become increasingly complex. Some of the key players include the classic insect lipid hormones juvenile hormone (JH) and ecdysteroids, and neuropeptides such as insulin-like peptides (ILPs). Individual endocrine factors not only modulate their own target tissue but also play crucial roles in crosstalk among themselves, ensuring successful vitellogenesis and oogenesis. Recent advances in omics, gene silencing, and genome editing approaches have accelerated research, offering both fundamental insights and practical applications for studying in-depth endocrine signaling pathways. This review provides an updated and integrated view of endocrine factors modulating vitellogenesis and oogenesis in insect females.

Cyp6g2 is the major P450 epoxidase responsible for juvenile hormone biosynthesis in Drosophila melanogaster

BACKGROUND

Juvenile hormones (JH) play crucial role in regulating development and reproduction in insects. The most common form of JH is JH III, derived from MF through epoxidation by CYP15 enzymes. However, in the higher dipterans, such as the fruitfly, Drosophila melanogaster, a bis-epoxide form of JHB3, accounted most of the JH detected. Moreover, these higher dipterans have lost the CYP15 gene from their genomes. As a result, the identity of the P450 epoxidase in the JH biosynthesis pathway in higher dipterans remains unknown.

RESULTS

In this study, we show that Cyp6g2 serves as the major JH epoxidase responsible for the biosynthesis of JHB3 and JH III in D. melanogaster. The Cyp6g2 is predominantly expressed in the corpus allatum (CA), concurring with the expression pattern of jhamt, another well-studied gene that is crucial in the last steps of JH biosynthesis. Mutation in Cyp6g2 leads to severe disruptions in larval-pupal metamorphosis and exhibits reproductive deficiencies, exceeding those seen in jhamt mutants. Notably, Cyp6g2-/-::jhamt2 double mutants all died at the pupal stage but could be rescued through the topical application of JH analogs. JH titer analyses revealed that both Cyp6g2-/- mutant and jhamt2 mutant lacking JHB3 and JH III, while overexpression of Cyp6g2 or jhamt caused a significant increase in JHB3 and JH III titer.

CONCLUSIONS

These findings collectively established that Cyp6g2 as the major JH epoxidase in the higher dipterans and laid the groundwork for the further understanding of JH biosynthesis. Moreover, these findings pave the way for developing specific Cyp6g2 inhibitors as insect growth regulators or insecticides.

A short neuropeptide F analog (sNPF), III-2 may particularly regulate juvenile hormone III to influence Spodoptera frugiperda metamorphosis and development

Allatostatin (AS) or Allatotropin (AT) is a class of insect short neuropeptide F (sNPF) that affects insect growth and development by inhibiting or promote the synthesis of juvenile hormone (JH) in different insects. III-2 is a novel sNPF analog derived from a group of nitroaromatic groups connected by different amino acids. In this study, we found that III-2 showed high insecticidal activity against S. frugiperda larvae with a LC50 of 18.7 mg L-1. As demonstrated by ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS), III-2 particularly facilitated JH III and hindered 20E synthesis in S. frugiperda. The results of RNA-Seq and quantitative real-time polymerase chain reaction (qPCR) showed that III-2 treatment promoted the expression of key genes such as SfCYP15C1 in JH synthesis pathway and inhibited the expression of SfCYP314A1 and other genes in the 20E synthetic pathway. Significant differences were also observed in the expression of the genes related to cuticle formation. We report for the first time that sNPF compounds specifically interfere with the synthesis and secretion of a certain JH in insects, thus affecting the ecdysis and growth of insects, and leading to death. This study may provide a new plant conservation concept for us to seek the targeted control of certain insects based on specific interference with different JH.

Nacα protects the larval fat body from cell death by maintaining cellular proteostasis in Drosophila

Protein homeostasis (proteostasis) is crucial for the maintenance of cellular homeostasis. Impairment of proteostasis activates proteotoxic and unfolded protein response pathways to resolve cellular stress or induce apoptosis in damaged cells. However, the responses of individual tissues to proteotoxic stress and evoking cell death program have not been extensively explored in vivo. Here, we show that a reduction in Nascent polypeptide-associated complex protein alpha subunit (Nacα) specifically and progressively induces cell death in Drosophila fat body cells. Nacα mutants disrupt both ER integrity and the proteasomal degradation system, resulting in caspase activation through JNK and p53. Although forced activation of the JNK and p53 pathways was insufficient to induce cell death in the fat body, the reduction of Nacα sensitized fat body cells to intrinsic and environmental stresses. Reducing overall protein synthesis by mTor inhibition or Minute mutants alleviated the cell death phenotype in Nacα mutant fat body cells. Our work revealed that Nacα is crucial for protecting the fat body from cell death by maintaining cellular proteostasis, thus demonstrating the coexistence of a unique vulnerability and cell death resistance in the fat body.

Effects of mating on female reproductive physiology in the insect model, Rhodnius prolixus, a vector of the causative parasite of Chagas disease

The blood-sucking hemipteran Rhodnius prolixus is one of the main vectors of Chagas disease, a neglected tropical disease that affects several million people worldwide. Consuming a blood meal and mating are events with a high epidemiological impact since after each meal, mated females can lay fertile eggs that result in hundreds of offspring. Thus, a better knowledge of the control of R. prolixus reproductive capacity may provide targets for developing novel strategies to control vector populations, thereby reducing vector-host contacts and disease transmission. Here, we have used a combination of gene transcript expression analysis, biochemical assays, hormone measurements and studies of locomotory activity to investigate how mating influences egg development and egg laying rates in R. prolixus females. The results demonstrate that a blood meal increases egg production capacity and leads to earlier egg laying in mated females compared to virgins. Virgin females, however, have increased survival rate over mated females. Circulating juvenile hormone (JH) and ecdysteroid titers are increased in mated females, a process mainly driven through an upregulation of the transcripts for their biosynthetic enzymes in the corpus allatum and ovaries, respectively. Mated females display weaker locomotory activity compared to virgin females, mainly during the photophase. In essence, this study shows how reproductive output and behaviour are profoundly influenced by mating, highlighting molecular, biochemical, endocrine and behavioral features differentially expressed in mated and virgin R. prolixus females.

One genome, multiple phenotypes: decoding the evolution and mechanisms of environmentally induced developmental plasticity in insects

Plasticity in developmental processes gives rise to remarkable environmentally induced phenotypes. Some of the most striking and well-studied examples of developmental plasticity are seen in insects. For example, beetle horn size responds to nutritional state, butterfly eyespots are enlarged in response to temperature and humidity, and environmental cues also give rise to the queen and worker castes of eusocial insects. These phenotypes arise from essentially identical genomes in response to an environmental cue during development. Developmental plasticity is taxonomically widespread, affects individual fitness, and may act as a rapid-response mechanism allowing individuals to adapt to changing environments. Despite the importance and prevalence of developmental plasticity, there remains scant mechanistic understanding of how it works or evolves. In this review, we use key examples to discuss what is known about developmental plasticity in insects and identify fundamental gaps in the current knowledge. We highlight the importance of working towards a fully integrated understanding of developmental plasticity in a diverse range of species. Furthermore, we advocate for the use of comparative studies in an evo-devo framework to address how developmental plasticity works and how it evolves.

Crosstalk between Nutrition, Insulin, Juvenile Hormone, and Ecdysteroid Signaling in the Classical Insect Model, Rhodnius prolixus

The rigorous balance of endocrine signals that control insect reproductive physiology is crucial for the success of egg production. Rhodnius prolixus, a blood-feeding insect and main vector of Chagas disease, has been used over the last century as a model to unravel aspects of insect metabolism and physiology. Our recent work has shown that nutrition, insulin signaling, and two main types of insect lipophilic hormones, juvenile hormone (JH) and ecdysteroids, are essential for successful reproduction in R. prolixus; however, the interplay behind these endocrine signals has not been established. We used a combination of hormone treatments, gene expression analyses, hormone measurements, and ex vivo experiments using the corpus allatum or the ovary, to investigate how the interaction of these endocrine signals might define the hormone environment for egg production. The results show that after a blood meal, circulating JH levels increase, a process mainly driven through insulin and allatoregulatory neuropeptides. In turn, JH feeds back to provide some control over its own biosynthesis by regulating the expression of critical biosynthetic enzymes in the corpus allatum. Interestingly, insulin also stimulates the synthesis and release of ecdysteroids from the ovary. This study highlights the complex network of endocrine signals that, together, coordinate a successful reproductive cycle.

Impact of JH Signaling on Reproductive Physiology of the Classical Insect Model, Rhodnius prolixus

In adult females of several insect species, juvenile hormones (JHs) act as gonadotrophic hormones, regulating egg production. JH binds to its nuclear receptor, Methoprene tolerant (Met), triggering its dimerization with the protein Taiman (Tai). The resulting active complex induces transcription of JH response genes, such as Krüppel homolog 1 (Kr-h1). In this study we report for the first time the participation of the isoform JH III skipped bisepoxide (JHSB₃) and its signaling pathway in the reproductive fitness of the classical insect model Rhodnius prolixus. The topical application of synthetic JHSB₃ increases transcript and protein expression of yolk protein precursors (YPPs), mainly by the fat body but also by the ovaries, the second source of YPPs. These results are also confirmed by ex vivo assays. In contrast, when the JH signaling cascade is impaired via RNA interference by downregulating RhoprMet and RhoprTai mRNA, egg production is inhibited. Although RhoprKr-h1 transcript expression is highly dependent on JHSB₃ signaling, it is not involved in egg production but rather in successful hatching. This research contributes missing pieces of JH action in the insect model in which JH was first postulated almost 100 years ago.

Comparative Metabolomics and Lipidomics of Four Juvenoids Application to Scylla paramamosain Hepatopancreas: Implications of Lipid Metabolism During Ovarian Maturation

This study was the first to evaluate multiple hormonal manipulations to hepatopancreas over the ovarian development stages of the mud crab, Scylla paramamosain. A total of 1258 metabolites in 75 hepatopancreas explants from five female crabs were induced by juvenile hormone III (JH III), methyl farnesoate (MF), farnesoic acid (FA) and methoprene (Met), as identified from combined metabolomics and lipidomics (LC-MS/MS). 101 significant metabolites and 47 significant pathways were selected and compared for their comprehensive effects to ovarian maturation. While MF played an extensive role in lipid accumulation, JH III and Met shared similar effects, especially in the commonly and significantly elevated triglycerides and lysophospholipids (fold change≥2 and ≤0.5, VIP≥1). The significant upregulation of β-oxidation and key regulators in lipid degradation by FA (P ≤ 0.05) resulted in less lipid accumulation from this treatment, with a shift toward lipid export and energy consumption, unlike the effects of MF, JH III and Met. It was possible that MF and FA played their own unique roles and acted in synergy to modulate lipid metabolism during crab ovarian maturation. Our study yielded insights into the MF-related lipid metabolism in crustacean hepatopancreas for the overall regulation of ovarian maturation, and harbored the potential use of juvenoids to induce reproductive maturity of this economic crab species.

In silico identification and study of potential anti-mosquito juvenile hormone binding protein (MJHBP) compounds as candidates for dengue virus - Vector insecticides

Dengue has become a huge global health burden. It is currently recognized as the most rapidly spreading mosquito-borne viral disease. Yet, there are currently no licensed vaccines or specific therapeutics to manage the virus, thus, scaling up vector control approaches is important in controlling this viral spread. This study aimed to identify and study in silico, potential anti-mosquito compounds targeting Juvenile hormone (JH) mediated pathways via the Mosquito Juvenile Hormone Binding Protein (MJHBP). The study was implemented using series of computational methods. The query compounds included pyrethroids and those derived from ZINC and ANPDB databases using a simple pharmacophore model in Molecular Operating Environment (MOE). Molecular docking of selected compounds' library was implemented in MOE. The resultant high-score compounds were further validated by molecular dynamics simulation via Maestro 12.3 module and the respective Prime/Molecular Mechanics Generalized Born Surface Area (Prime/MM-GBSA) binding energies computed. The study identified compounds-pyrethroids, natural and synthetic - with high docking energy scores (ranging from 10.91-12.34 kcal/mol). On further analysis of the high-ranking (in terms of docking scores) compounds using MD simulation, the compounds - Ekeberin D4, Maesanin, Silafluofen and ZINC16919139- revealed very low binding energies (-122.99, -72.91 -104.50 and,-74.94 kcal/mol respectively), fairly stable complex and interesting interaction with JH-binding site amino acid residues on MJHBP. Further studies can explore these compounds in vitro/in vivo in the search for more efficient mosquito vector control.