Molecular mechanism underlying juvenile hormone-mediated repression of precocious larval-adult metamorphosis

Juvenile hormone signal transducer hairy inhibits Krüppel homolog1 expression

Hairy and Krüppel homolog 1 (Kr-h1) are transcriptional repressors that act synergistically to mediate the gene-repressive action of juvenile hormone (JH). However, whether a regulatory relationship exists between Hairy and Kr-h1 remains unclear. In this study, an inhibitory effect of Hairy on Kr-h1 expression was found. Genetic studies in Drosophila have shown that the simultaneous overexpression of Hairy and Kr-h1 can rescue the defective phenotypes caused by the overexpression of a single factor. Reduced expression of Kr-h1 was observed in Hairy-overexpressing flies and cells, whereas the expression levels of Hairy were unaffected in cells with ectopic expression of Kr-h1. The inhibitory effect of Hairy on Kr-h1 expression was found to occur at the transcriptional level, as Hairy bound directly to the B-box within the Kr-h1 promoter via the bHLH motif and recruited the corepressors C-terminal binding protein (CtBP) and Groucho (Gro) through the PLSLV and WRPW motifs, respectively. Our findings revealed a regulatory relationship between two JH response factors, which advances our understanding of the molecular mechanism of JH signaling.

Schlank orchestrates insect developmental transition by switching H3K27 acetylation to trimethylation in the prothoracic gland

Insect developmental transitions are precisely coordinated by ecdysone and juvenile hormone (JH). We previously revealed that accumulated H3K27 trimethylation (H3K27me3) at the locus encoding JH signal transducer Hairy is involved in the larval-pupal transition in insects, but the underlying mechanism remains to be fully defined. Here, we show in Drosophila and Bombyx that Rpd3-mediated H3K27 deacetylation in the prothoracic gland during the last larval instar promotes ecdysone biosynthesis and the larval-pupal transition by enabling H3K27me3 accumulation at the Hairy locus to induce its transcriptional repression. Importantly, we find that the homeodomain transcription factor Schlank acts to switch active H3K27 acetylation (H3K27ac) to repressive H3K27me3 at the Hairy locus by directly binding to the Hairy promoter and then recruiting the histone deacetylase Rpd3 and the histone methyltransferase PRC2 component Su(z)12 through physical interactions. Moreover, Schlank inhibits Hairy transcription to facilitate the larval-pupal transition, and the Schlank signaling cascade is suppressed by JH but regulated in a positive feedback manner by ecdysone. Together, our data uncover that Schlank mediates epigenetic reprogramming of H3K27 modifications in hormone actions during insect developmental transition.

Transcriptome and proteome analyses reveal genes and signaling pathways involved in the response to two insect hormones in the insect-fungal pathogen Hirsutella satumaensis

The insect hormones ecdysone (20E) and juvenile hormone III (JH) have been demonstrated to stimulate the secretion of conidia mucilage and pigments in Hirsutella satumaensis. However, the underlying mechanisms remain elusive. Here, comparative transcriptome and proteome analyses were performed to identify the fungal genes and proteins of H. satumaensis that are up- or downregulated in response to insect hormones. A total of 17,407 unigenes and 1,016 proteins in conidia mucilage were identified. The genes involved in response to the hormones were classified into four functional groups: (1) stress response-related genes that are required for the removal of reactive oxygen species (glutathione synthetase, c7144) and genes involved in the response to osmotic stress in the hemocoel, such as those encoding proteins involved in the G, mTOR, and MAPK signaling pathways (2); insect hormone metabolic genes, including genes encoding ecdysteroid UDP-glucosyltransferase, ecdysteroid-22-kinase, and a key aldehyde dehydrogenase in a juvenile hormone synthesis pathway (3); secretory proteins that share homology with those of the host Bombyx mori, including fibrohexamerin, sericin 1, metalloprotease 1 protein, and silk gum protein, which were revealed by the omics data; and (4) proteins related to amino sugar metabolism and oxidative phosphorylation that were specifically expressed in mucilage in response to 20E and JH, respectively. These findings revealed that H. satumaensis can mount effective responses by modulating the expression of genes involved in the detoxification, adaptation, and evasion of insect hormone-mediated immune responses, providing fresh insights into fungal pathogen-host insect interactions.IMPORTANCEInsect hormones are highly important for the regulation of insect growth, development, and immune system function. Thus, the expansion of entomopathogenic fungi (EPF) could be affected by these hormones when they inhabit the host hemocoel. However, the molecular basis of EPF in response to insect hormones has yet to be determined. Our results revealed that EPF are impacted by 20E and JH, both of which act as signals, as these hormones lead to changes in metabolic pathways of the fungus, thus demonstrating a direct relationship between the fungus and the hormones. Furthermore, adaptive strategies, such as the use of ecdysone-inactivating enzymes and secreted filamentous proteins in H. satumaensis, which strongly resemble those of the host insect, have been discovered, thus illustrating the importance of adaptation to insect hormones for a better understanding of the interaction between insects and EPF.

Fat body-derived juvenile hormone acid methyltransferase functions to maintain iron homeostasis in Drosophila melanogaster

Iron homeostasis is of critical importance to living organisms. Drosophila melanogaster has emerged as an excellent model to study iron homeostasis, while the regulatory mechanism of iron metabolism remains poorly understood. Herein, we accidently found that knockdown of juvenile hormone (JH) acid methyltransferase (Jhamt) specifically in the fat body, a key rate-limiting enzyme for JH synthesis, led to iron accumulation locally, resulting in serious loss and dysfunction of fat body. Jhamt knockdown-induced phenotypes were mitigated by iron deprivation, antioxidant and Ferrostatin-1, a well-known inhibitor of ferroptosis, suggesting ferroptosis was involved in Jhamt knockdown-induced defects in the fat body. Further study demonstrated that upregulation of Tsf1 and Malvolio (Mvl, homolog of mammalian DMT1), two iron importers, accounted for Jhamt knockdown-induced iron accumulation and dysfunction of the fat body. Mechanistically, Kr-h1, a key transcription factor of JH, acts downstream of Jhamt inhibiting Tsf1 and Mvl transcriptionally. In summary, the findings indicated that fat body-derived Jhamt is required for the development of Drosophila by maintaining iron homeostasis in the fat body, providing unique insight into the regulatory mechanisms of iron metabolism in Drosophila.

Juvenile hormone-induced microRNA miR-iab-8 regulates lipid homeostasis and metamorphosis in Drosophila melanogaster

Metamorphosis plays an important role in the evolutionary success of insects. Accumulating evidence indicated that microRNAs (miRNAs) are involved in the regulation of processes associated with insect metamorphosis. However, the miRNAs coordinated with juvenile hormone (JH)-regulated metamorphosis remain poorly reported. In the present study, using high-throughput miRNA sequencing combined with Drosophila genetic approaches, we demonstrated that miR-iab-8, which primarily targets homeotic genes to modulate haltere-wing transformation and sterility was up-regulated by JH and involved in JH-mediated metamorphosis. Overexpression of miR-iab-8 in the fat body resulted in delayed development and failure of larval-pupal transition. Furthermore, metabolomic analysis results revealed that overexpression of miR-iab-8 caused severe energy metabolism defects especially the lipid metabolism, resulting in significantly reduced triacylglycerol (TG) content and glycerophospholipids but enhanced accumulation of phosphatidylcholine (PC) and phosphatidylethanolamine (PE). In line with this, Nile red staining demonstrated that during the third larval development, the TG content in the miR-iab-8 overexpression larvae was continuously decreased, which is opposite to the control. Additionally, the transcription levels of genes committed to TG synthesis and breakdown were found to be significantly increased and the expression of genes responsible for glycerophospholipids metabolism were also altered. Overall, we proposed that JH induced miR-iab-8 expression to perturb the lipid metabolism homeostasis especially the TG storage in the fat body, which in turn affected larval growth and metamorphosis.

The genetic determination of alternate stages in polyphenic insects

Molt-based transitions in form are a central feature of insect life that have enabled adaptation to diverse and changing environments. The endocrine regulation of these transitions is well established, but an understanding of their genetic regulation has only recently emerged from insect models. The pupal and adult stages of metamorphosing insects are determined by the stage specifying transcription factors broad-complex (br) and Ecdysone inducible protein 93 (E93), respectively. A probable larval determinant, chronologically inappropriate metamorphosis (chinmo), has just recently been characterized. Expression of these three transcription factors in the metamorphosing insects is regulated by juvenile hormone with ecdysteroid hormones, and by mutual repression between the stage-specific transcription factors. This review explores the hypothesis that variations in the onset, duration, and tissue-specific expression of chinmo, br, and E93 underlie other polyphenisms that have arisen throughout insects, including the castes of social insects, aquatic stages of mayflies, and the neoteny of endoparasites. The mechanisms that constrain how chinmo, br, and E93 expression may vary will also constrain the ways that insect life history may evolve. I find that four types of expression changes are associated with novel insect forms: (1) heterochronic shift in the turnover of expression, (2) expansion or contraction of expression, (3) tissue-specific expression, and (4) redeployment of stage-specific expression. While there is more to be learned about chinmo, br, and E93 function in diverse insect taxa, the studies outlined here show that insect stages are modular units in developmental time and a substrate for evolutionary forces to act upon.

Population Density-Dependent Developmental Regulation in Migratory Locust

Insect development is intricately governed by hormonal signaling pathways, yet the pivotal upstream regulator that potentiates hormone activation remains largely elusive. The migratory locust, Locusta migratoria, exhibits population density-dependent phenotypic plasticity, encompassing traits such as flight capability, body coloration, and behavior. In this study, we elucidated a negative correlation between population density and ontogenetic development during the nymphal stage of locusts. We found that the level of density influences the developmental trajectory by modulating transcript abundance within the ecdysone signaling pathway, with knockdown of the prothoracicotropic hormone (PTTH) resulting in developmental delay. Transcriptomic analysis of locust brains across solitary and gregarious phases revealed significant differential expression of genes involved in various pathways, including protein synthesis, energy metabolism, hormonal regulation, and immunity. Notably, knockdown experiments targeting two energy regulators, adipokinetic hormone (AKH) and insulin-like polypeptide 1 (ilp1), failed to elicit changes in the developmental process in solitary locusts. However, knockdown of immunoglobulin (IG) significantly shortened the developmental time in higher-density populations. Collectively, our findings underscore the regulatory role of population density in determining developmental duration and suggest that an immune-related gene contributes to the observed differences in developmental patterns.

N(alpha)-acetyltransferase 40-mediated histone acetylation plays an important role in ecdysone regulation of metamorphosis in the red flour beetle, Tribolium castaneum

Histone acetylation, a crucial epigenetic modification, is governed by histone acetyltransferases (HATs), that regulate many biological processes. Functions of HATs in insects are not well understood. We identified 27 HATs and determined their functions using RNA interference (RNAi) in the model insect, Tribolium castaneum. Among HATs studied, N-alpha-acetyltransferase 40 (NAA40) knockdown caused a severe phenotype of arrested larval development. The steroid hormone, ecdysone induced NAA40 expression through its receptor, EcR (ecdysone receptor). Interestingly, ecdysone-induced NAA40 regulates EcR expression. NAA40 acetylates histone H4 protein, associated with the promoters of ecdysone response genes: EcR, E74, E75, and HR3, and causes an increase in their expression. In the absence of ecdysone and NAA40, histone H4 methylation by arginine methyltransferase 1 (ART1) suppressed the above genes. However, elevated ecdysone levels at the end of the larval period induced NAA40, promoting histone H4 acetylation and increasing the expression of ecdysone response genes. NAA40 is also required for EcR, and steroid-receptor co-activator (SRC) mediated induction of E74, E75, and HR3. These findings highlight the key role of ecdysone-induced NAA40-mediated histone acetylation in the regulation of metamorphosis.

Photoperiodism of Diapause Induction in the Silkworm, Bombyx mori

The silkworm Bombyx mori exhibits a photoperiodic response (PR) for embryonic diapause induction. This article provides a comprehensive review of literature on the silkworm PR, starting from early works on population to recent studies uncovering the molecular mechanism. Makita Kogure (1933) conducted extensive research on the PR, presenting a pioneering paper on insect photoperiodism. In the 1970s and 80s, artificial diets were developed, and the influence of nutrition on PR was well documented. The photoperiodic photoreceptor has been investigated from organ to molecular level in the silkworm. Culture experiments demonstrated that the photoperiodic induction can be programmed in an isolated brain (Br)-subesophageal ganglion (SG) complex with corpora cardiaca (CC)-corpora allata (CA). The requirement of dietary vitamin A for PR suggests the involvement of opsin pigment in the photoperiodic reception, and a cDNA encoding an opsin (Boceropsin) was cloned from the brain. The effector system concerning the production and secretion of diapause hormone (DH) has also been extensively investigated in the silkworm. DH is produced in a pair of posterior cells of SG, transported to CC by nervi corporis cardiaci, and ultimately released into the hemolymph. Possible involvement of GABAergic and corazonin (Crz) signal pathways was suggested in the control of DH secretion. Knockout (KO) experiments of GABA transporter (GAT) and circadian clock genes demonstrated that GAT plays a crucial role in PR through circadian control. A model outlining the PR mechanism, from maternal photoperiodic light reception to DH secretion, has been proposed.

Juvenile hormone inhibits adult cuticle formation in Drosophila melanogaster through Kr-h1/Dnmt2-mediated DNA methylation of Acp65A promoter

Differentiation of imaginal epidermal cells of Drosophila melanogaster to form adult cuticles occurs at approximately 40-93 h after puparium formation. Juvenile hormone (JH) given at pupariation results in formation of a second pupal cuticle in the abdomen instead of the adult cuticle. Although the adult cuticle gene Acp65A has been reported to be down-regulated following JH treatment, the regulatory mechanism remains unclear. Here, we found that the JH primary response gene Krüppel homologue 1 (Kr-h1) plays a vital role in the repression of adult cuticle formation through the mediation of JH action. Overexpression of Kr-h1 mimicked-while knocking down of Kr-h1 attenuated-the inhibitory action of JH on the formation of the adult abdominal cuticle. Further, we found that Kr-h1 inhibited the transcription of Acp65A by directly binding to the consensus Kr-h1 binding site (KBS) within the Acp65A promoter region. Moreover, the DNA methyltransferase Dnmt2 was shown to interact with Kr-h1, combined with the KBS to promote the DNA methylation of sequences around the KBS, in turn inhibiting the transcription of Acp65A. This study advances our understanding of the molecular basis of the "status quo" action of JH on the Drosophila adult metamorphosis.

Kruppel homolog 1 modulates ROS production and antimicrobial peptides expression in shrimp hemocytes during infection by the Vibrio parahaemolyticus strain that causes AHPND

Shrimp aquaculture has been seriously affected by acute hepatopancreatic necrosis disease (AHPND), caused by a strain of Vibrio parahaemolyticus that carries the Pir toxin plasmids (V. parahaemolyticus (AHPND)). In this study, the transcription factor, Kruppel homolog 1-like of Peneaus vannamei (PvKr-h1), was significantly induced in shrimp hemocytes after V. parahaemolyticus (AHPND) challenge, suggesting that PvKr-h1 is involved in shrimp immune response. Knockdown of PvKr-h1 followed by V. parahaemolyticus (AHPND) challenge increased bacterial abundance in shrimp hemolymph coupled with high shrimp mortality. Moreover, transcriptome and immunofluorescence analyses revealed that PvKr-h1 silencing followed by V. parahaemolyticus (AHPND) challenge dysregulated the expression of several antioxidant-related enzyme genes, such as Cu-Zu SOD, GPX, and GST, and antimicrobial peptide genes, i.e., CRUs and PENs, and reduced ROS activity and nuclear translocation of Relish. These data reveal that PvKr-h1 regulates shrimps' immune response to V. parahaemolyticus (AHPND) infection by suppressing antioxidant-related enzymes, enhancing ROS production and promoting nuclei import of PvRelish to stimulate antimicrobial peptide genes expression.

Insect growth-regulating activity of 1-benzyl-2-methylbenzimidazole derivatives on silkworms

Derivatives of 1-benzyl-2-methylbenzimidazoles (BMBIs) were synthesized to evaluate their biological activities against Bombyx mori, a lepidopteran model insect. Synthesized BMBIs exhibited two different biological activities: inhibition of development and acute lethality. From a structural perspective, the activity varied with the position of the substitutions on the 1-benzyl moiety; BMBIs with substitutions on the 2 and/or 4 positions had comparatively high activity in comparison with those with substitutions on the 3-position. There was more activity for the inhibition of development with low doses, and more for acute lethality with high doses. The activity was also affected by the applied stage, that is, application in the 4th instar mostly interfered the larval molting or pupation, whereas that in the 3rd instar caused more acute mortality. Taken together, these results suggest that BMBIs have multiple modes of action.

Overexpression of BmJHBPd2 Repressed Silk Synthesis by Inhibiting the JH/Kr-h1 Signaling Pathway in Bombyx mori

The efficient production of silkworm silk is crucial to the silk industry. Silk protein synthesis is regulated by the juvenile hormone (JH) and 20-Hydroxyecdysone (20E). Therefore, the genetic regulation of silk production is a priority. JH binding protein (JHBP) transports JH from the hemolymph to target organs and cells and protects it. In a previous study, we identified 41 genes containing a JHBP domain in the Bombyx mori genome. Only one JHBP gene, BmJHBPd2, is highly expressed in the posterior silk gland (PSG), and its function remains unknown. In the present study, we investigated the expression levels of BmJHBPd2 and the major silk protein genes in the high-silk-producing practical strain 872 (S872) and the low-silk-producing local strain Dazao. We found that BmJHBPd2 was more highly expressed in S872 than in the Dazao strain, which is consistent with the expression pattern of fibroin genes. A subcellular localization assay indicated that BmJHBPd2 is located in the cytoplasm. In vitro hormone induction experiments showed that BmJHBPd2 was upregulated by juvenile hormone analogue (JHA) treatment. BmKr-h1 upregulation was significantly inhibited by the overexpression of BmJHBPd2 (BmJHBPd2OE) at the cell level when induced by JHA. However, overexpression of BmJHBPd2 in the PSG by transgenic methods led to the inhibition of silk fibroin gene expression, resulting in a reduction in silk yield. Further investigation showed that in the transgenic BmJHBPd2OE silkworm, the key transcription factor of the JH signaling pathway, Krüppel homolog 1 (Kr-h1), was inhibited, and 20E signaling pathway genes, such as broad complex (Brc), E74A, and ultraspiracle protein (USP), were upregulated. Our results indicate that BmJHBPd2 plays an important role in the JH signaling pathway and is important for silk protein synthesis. Furthermore, our findings help to elucidate the mechanisms by which JH regulates silk protein synthesis.

The Janus-Faced Role of Lipid Droplets in Aging: Insights from the Cellular Perspective

It is widely accepted that nine hallmarks-including mitochondrial dysfunction, epigenetic alterations, and loss of proteostasis-exist that describe the cellular aging process. Adding to this, a well-described cell organelle in the metabolic context, namely, lipid droplets, also accumulates with increasing age, which can be regarded as a further aging-associated process. Independently of their essential role as fat stores, lipid droplets are also able to control cell integrity by mitigating lipotoxic and proteotoxic insults. As we will show in this review, numerous longevity interventions (such as mTOR inhibition) also lead to strong accumulation of lipid droplets in Saccharomyces cerevisiae, Caenorhabditis elegans, Drosophila melanogaster, and mammalian cells, just to name a few examples. In mammals, due to the variety of different cell types and tissues, the role of lipid droplets during the aging process is much more complex. Using selected diseases associated with aging, such as Alzheimer's disease, Parkinson's disease, type II diabetes, and cardiovascular disease, we show that lipid droplets are "Janus"-faced. In an early phase of the disease, lipid droplets mitigate the toxicity of lipid peroxidation and protein aggregates, but in a later phase of the disease, a strong accumulation of lipid droplets can cause problems for cells and tissues.

Juvenile hormone suppresses sensory organ precursor determination to block Drosophila adult abdomen morphogenesis

Juvenile hormone (JH) has a classic "status quo" action at both the pupal and adult molts when administrated exogenously. In Drosophila, treatment with JH at pupariation inhibits the formation of abdominal bristles, which are derived from the histoblasts. However, the mechanism via which JH exerts this effect remains poorly understood. In this study, we analyzed the effect of JH on histoblast proliferation, migration, and differentiation. Our results indicated that whereas the proliferation and migration of histoblasts remained unaffected following treatment with a JH mimic (JHM), their differentiation, particularly the specification of sensor organ precursor (SOP) cells, was inhibited. This effect was attributable to downregulated proneural genes achaete (ac) and Scute (sc) expression levels, which prevented the specification of SOP cells in proneural clusters. Moreover, Kr-h1 was found to mediate this effect of JHM. Histoblast-specific overexpression or knockdown of Kr-h1, respectively mimicked or attenuated the effects exerted by JHM on abdominal bristle formation, SOP determination, and transcriptional regulation of ac and sc. These results indicated that the defective SOP determination was responsible for the inhibition of abdominal bristle formation by JHM, which, in turn, was mainly mediated via the transducing action of Kr-h1.

Exorista sorbillans (Diptera: Tachinidae) parasitism shortens host larvae growth duration by regulating ecdysone and juvenile hormone titers in Bombyx mori (Lepidoptera: Bombycidae)

The tachinid fly, Exorista sorbillans, is a notorious ovolarviparous endoparasitoid of the silkworm, Bombyx mori, causing severe damage to silkworm cocoon industry. Silkworm larvae show typically precocious wandering behavior after being parasitized by E. sorbillans; however, the underlying molecular mechanism remains unexplored. Herein, we investigated the changes in the levels of 20-hydroxyecdysone (20E) and juvenile hormone (JH) titer, and they both increased in the hemolymph of parasitized silkworms. Furthermore, we verified the expression patterns of related genes, which showed an upregulation of 20E signaling and biosynthesis genes but a significant downregulation of ecdysone oxidase (EO), a 20E inactivation enzyme, in parasitized silkworms. In addition, related genes of the JH signaling were activated in parasitized silkworms, while related genes of the JH degradation pathway were suppressed, resulting in an increase in JH titer. Notably, the precocious wandering behavior of parasitized silkworms was partly recoverable by silencing the transcriptions of BmCYP302A1 or BmCYP307A1 genes. Our findings suggest that the developmental duration of silkworm post parasitism could be shortened by regulation of 20E and JH titers, which may help silkworm to resist the E. sorbillans infestation. These findings provide a basis for deeper insight into the interplay between silkworms and E. sorbillans and may serve as a reference for the development of a novel approach to control silkworm myiasis.

CRISPR/Cas9-mediated methoprene-tolerant 1 knockout results in precocious metamorphosis of beet armyworm (Spodoptera exigua) only at the late larval stage

Juvenile hormone (JH) controls almost every aspect of an insect, especially metamorphosis. Since RNA interference works on transcripts and is often insufficient in Lepidoptera, how JH affects larval development in these insects is not well studied. Using the CRISPR/Cas9 technique, we knocked out Spodoptera exigua methoprene-tolerant 1 (SeMet1) gene of beet armyworm by modifying two sites in the coding region. However, SeMet1 knockout did not affect egg hatch rate or larval development at L1-L3 stages. In contrast to the consistent five larval instars of the control group, L4 SeMet1 mutants began to show signs of precocious metamorphosis, that is, small patches of pupal cuticle. Most L4 and all L5 SeMet1 mutants died for failing to shed their mosaic cuticles. RNA-seq indicated that most genes encoding pupal cuticle proteins and chitinase genes were altered in SeMet1 mutant L4 larvae. SeKr-h1, a key transcription factor in JH action was significantly down-regulated in L3-L5 larvae, while SeBR-C, a pupal indicator was only upregulated in L4-L5 larvae. These results suggested that S. exigua larvae may initially develop independently of JH, and involve SeMet1 in transducing JH signalling, leading to controlled larval metamorphosis at the late larval stage. We believe our findings will enhance better understanding of JH regulation of larval development.

Dimmed gene knockout shortens larval growth and reduces silk yield in the silkworm, Bombyx mori

The bHLH domain transcription factor, Bombyx mori-derived dimmed (Bmdimm), is directly regulated by the JH-BmMet/BmSRC-BmKr-h1 pathway and plays a key role in regulating the expression of FibH, which codes the main component of silk protein. However, the other roles of Bmdimm in silk protein synthesis remain unclear. Here, we established a Bmdimm knockout (KO) line containing a 7-bp deletion via CRISPR/Cas9 system, which led to the absence of the bHLH domain. The expression level of silk protein genes and silk yield decreased significantly in the Bmdimm KO line. Moreover, knocking out Bmdimm led to shortened larval stages and significant weight loss in larvae and adults. Bmdimm was found to be highly expressed in the silk gland, but it was also expressed in the fat body. The expression level of Bmkr-h1 in the fat body was significantly downregulated in the Bmdimm KO line. Exogenous JHA treatment upregulated Bmkr-h1 and rescued the phenotype of larval growth in the Bmdimm KO line. In conclusion, knocking out Bmdimm led to a shortened larval stage via the inhibition of Bmkr-h1 expression, then reduced silk yield. These findings help to elucidate the regulatory mechanism of fibroin synthesis and larval development in silkworms.

Burkholderia gut symbiont induces insect host fecundity by modulating Kr-h1 gene expression

Full-length cDNAs of the Broad-Complex (BR-C) from Riptortus pedestris were cloned. Moreover, Kr-h1 and BR-C expression levels in apo-symbiotic and symbiotic host insects were compared to verify whether they are modulated by Burkholderia gut symbionts. Interestingly, Kr-h1 expression level was significantly increased in symbiotic females. To determine how Kr-h1 affects fecundity in insects, the biosynthesis of two reproduction-associated proteins, hexamerin-α and vitellogenin, was investigated in R. pedestris females. Hexamerin-α and vitellogenin expression at the transcriptional and translational levels decreased in Kr-h1-suppressed symbiotic females, subsequently reduced egg production. These results suggest that Burkholderia gut symbiont modulates Kr-h1 expression to enhance ovarian development and egg production of R. pedestris by increasing the biosynthesis of the two proteins.

Noncoding RNA Regulation of Hormonal and Metabolic Systems in the Fruit Fly Drosophila

The importance of RNAs is commonly recognised thanks to protein-coding RNAs, whereas non-coding RNAs (ncRNAs) were conventionally regarded as 'junk'. In the last decade, ncRNAs' significance and roles are becoming noticeable in various biological activities, including those in hormonal and metabolic regulation. Among the ncRNAs: microRNA (miRNA) is a small RNA transcript with ~20 nucleotides in length; long non-coding RNA (lncRNA) is an RNA transcript with >200 nucleotides; and circular RNA (circRNA) is derived from back-splicing of pre-mRNA. These ncRNAs can regulate gene expression levels at epigenetic, transcriptional, and post-transcriptional levels through various mechanisms in insects. A better understanding of these crucial regulators is essential to both basic and applied entomology. In this review, we intend to summarise and discuss the current understanding and knowledge of miRNA, lncRNA, and circRNA in the best-studied insect model, the fruit fly Drosophila.

Involvement of Methoprene-tolerant and Krüppel homolog 1 in juvenile hormone-mediated vitellogenesis of female Liposcelis entomophila (End.) (Psocoptera: Liposcelididae)

Methoprene-tolerant (Met) as an intracellular receptor of juvenile hormone (JH) and the Krüppel-homolog 1 (Kr-h1) as a JH-inducible transcription factor had been proved to contribute to insect reproduction. Their functions vary in different insect orders, however, they are not clear in Psocoptera. In this study, LeMet and LeKr-h1 were identified and their roles in vitellogenesis and ovarian development were investigated in Liposcelis entomophila (Enderlein). Treatment with exogenous JH III significantly induced the expression of LeKr-h1, LeVg, and LeVgR. Furthermore, silencing LeMet and LeKr-h1 remarkably reduced the transcription of LeVg and LeVgR, disrupted the production of Vg in fat body and the uptake of Vg by oocytes, and ultimately led to a decline in fecundity. The results indicated that the JH signaling pathway was essential to the reproductive process of this species. Interestingly, knockdown of LeMet or LeKr-h1 also resulted in fluctuations in the expression of FoxO, indicating the complex regulatory interactions between different hormone factors. Besides, knockdown of both LeMet and LeKr-h1 significantly increased L. entomophila mortality. Our study provides initial insight into the roles of JH signaling in the female reproduction of psocids and provided evidence that RNAi-mediated knockdown of Met or Kr-h1 is a potential pest control strategy.

The inhibition of ecdysone signal pathway was the key of pyriproxyfen poisoning for silkworm, Bombyx mori

Pyriproxyfen is a juvenile hormone-like pesticide. Once intake occurs, it leads to a series of poisoning characters consequences in silkworm, Bombyx mori (ID: 7091, Lepidoptera), such as non- cocooning, non-pupation, production of low-active eggs, and extended stages. However, the poisoning mechanism is still unclear. Here, silkworms were fed mulberry leaves soaked with different pyriproxyfen concentrations, and the heads were dissected for transcriptome analysis, while the hemolymph was used for determinations of ecdysone and juvenile hormone titers. As a result, after conjoint analysis of 3 feeding groups and a control group, 555 differentially expressed genes (DEGs) were obtained, which were mainly involved in hormone metabolism, glycometabolism and protein metabolism. Meanwhile, 119 genes were significantly correlated with the pyriproxyfen concentrations, and they were mainly involved in drug metabolism and glycometabolism. The ecdysone titers in several feeding groups were significantly lower than those of the control group, while juvenile hormone was not detected in all groups, including the control and feeding groups. Correspondingly, due to activation of the juvenile hormone signaling pathway by pyriproxyfen, key genes in the ecdysone synthesis pathway were downregulated, and a large number of downstream genes were up- or downregulated. In addition, nearly all genes in the detoxification pathway were upregulated. These results suggested that, affected by the juvenile hormone signaling pathway, ecdysone titers decreased and further affected a series of downstream processes, and this was the key reason for pyriproxyfen poisoning in silkworm, B. mori, which could lay a foundation for the study of pyriproxyfen resistance in silkworm.

Genotoxic and mutational potential of monocyclic terpenoids (carvacrol, carvone and thymol) in Drosophila melanogaster

Genotoxicity and mutagenicity of monoterpene carvone along with the related monoterpene alcohols - carvacrol and thymol have been studied using Drosophila melanogaster as model system. The viability, pre-imaginal stage duration, level of dominant lethal mutations, unequal crossover in the Bar mutant of D. melanogaster and the influence of monocyclic terpenoids on the multiplication of the nuclear genome in salivary gland cells were investigated. The compounds tested after oral administration (0.02% in 1,2-propylene glycol) influence the degree of chromosome polyteny in salivary gland cells of D. melanogaster larvae. Among the terpenoids examined, carvacrol exhibited the most significant impact on imago lifespan, frequency of dominant lethal mutations, unequal crossover in the Bar mutant when added to the culture medium. Oral administration of terpenoids increases the average level of chromosome polyteny with the highest value for carvacrol - 1178 C compared to control (776 C). The conceivable mechanism of action for monocyclic terpenoids associated with the impact on juvenile hormone activity is debated.

Functional Analysis of a Juvenile Hormone Inducible Transcription Factor, Krüppel homolog 1, in the Bean Bug, Riptortus pedestris

Juvenile hormone (JH) has two major functions in insects, i.e., suppression of metamorphosis in the larval or nymphal stage and promotion of reproduction in the adult stage. Krüppel homolog 1 (Kr-h1), a C2H2 zinc-finger type transcription factor, is reported to act downstream of the JH receptor complex. In the present study, the function of Kr-h1 was examined in adults and nymphs of Riptortus pedestris by RNA interference (RNAi). After injection of adults with dsRNA of Kr-h1, the expression level of Kr-h1 was significantly decreased in the abdomen. Kr-h1 dsRNA-injection resulted in a lower proportion of individuals with developed ovaries, but the difference was not statistically significant. The transcript levels of cyanoprotein-α and vitellogenin-1, which are JH-inducible genes encoding yolk proteins, were not affected in the abdomen by Kr-h1 knockdown. Kr-h1 dsRNA-injection was effective for suppression of Kr-h1 expression in nymphs. Some Kr-h1 dsRNA-injected fifth (final) instar nymphs had morphological defects in the wing bud. Moreover, they had several adult morphological features, including ocelli in the head, connexivum in the abdomen, coloring of the dorsal abdomen, and genitals. The nymphs possessing adult features did not emerge as adults during 1 month. These results demonstrated that Kr-h1 is necessary for maintaining nymphal characters in R. pedestris. The function of Kr-h1 in ovarian development remains unclear in R. pedestris.

HDAC3 Knockdown Dysregulates Juvenile Hormone and Apoptosis-Related Genes in Helicoverpa armigera

Insect development requires genes to be expressed in strict spatiotemporal order. The dynamic regulation of genes involved in insect development is partly orchestrated by the histone acetylation-deacetylation via histone acetyltransferases (HATs) and histone deacetylases (HDACs). Although histone deacetylase 3 (HDAC3) is required for mice during early embryonic development, its functions in Helicoverpa armigera (H. armigera) and its potential to be used as a target of insecticides remain unclear. We treated H. armigera with HDAC3 siRNA and RGFP966, a specific inhibitor, examining how the HDAC3 loss-of-function affects growth and development. HDAC3 siRNA and RGFP966 treatment increased mortality at each growth stage and altered metamorphosis, hampering pupation and causing abnormal wing development, reduced egg production, and reduced hatching rate. We believe that the misregulation of key hormone-related genes leads to abnormal pupa development in HDAC3 knockout insects. RNA-seq analysis identified 2788 differentially expressed genes (≥two-fold change; p ≤ 0.05) between siHDAC3- and siNC-treated larvae. Krüppel homolog 1 (Kr-h1), was differentially expressed in HDAC3 knockdown larvae. Pathway-enrichment analysis revealed the significant enrichment of genes involved in the Hippo, MAPK, and Wnt signaling pathways following HDAC3 knockdown. Histone H3K9 acetylation was increased in H. armigera after siHDAC3 treatment. In conclusion, HDAC3 knockdown dysregulated juvenile hormone (JH)-related and apoptosis-related genes in H. armigera. The results showed that the HDAC3 gene is a potential target for fighting H. armigera.

Knockdown of Ecdysone-Induced Protein 93F Causes Abnormal Pupae and Adults in the Eggplant Lady Beetle

Ecdysone-induced protein 93F (E93) plays triple roles during post-embryonic development in insects whose juvenile instars are more than four. However, it only acts as a specifier of adult structures in Drosophila flies whose larval instars are fixed at three. In this study, we determined the functions of E93 in the eggplant lady beetle (Henosepilachna vigintioctopunctata), which has four larval instars. We uncovered that E93 was abundantly expressed at the prepupal and pupal stages. A precocious inhibition of the juvenile hormone signal by RNA interference (RNAi) of HvKr-h1 or HvHairy, two vital downstream developmental effectors, at the penultimate instar larval stage increased the expression of E93, Conversely, ingestion of JH by the third-instar larvae stimulated the expression of HvKr-h1 but repressed the transcription of either HvE93X1 or HvE93X2. However, disturbance of the JH signal neither drove premature metamorphosis nor caused supernumerary instars. In contrast, depletion of E93 at the third- and fourth-instar larval and prepupal stages severely impaired pupation and caused a larval-pupal mixed phenotype: pupal spines and larval scoli were simultaneously presented on the cuticle. RNAi of E93 at the pupal stage affected adult eclosion. When the beetles had suffered from a dsE93 injection at the fourth-instar larval and pupal stages, a few resultant adults emerged, with separated elytra, abnormally folded hindwings, a small body size and short appendages. Taken together, our results suggest the larval instars are fixed in H. vigintioctopunctata; E93 serves as a repressor of larval characters and a specifier of adult structures during the larval–pupal–adult transition.

Conceptual framework for the insect metamorphosis from larvae to pupae by transcriptomic profiling, a case study of Helicoverpa armigera (Lepidoptera: Noctuidae)

BACKGROUND

Insect metamorphosis from larvae to pupae is one of the most important stages of insect life history. Relatively comprehensive information related to gene transcription profiles during lepidopteran metamorphosis is required to understand the molecular mechanism underlying this important stage. We conducted transcriptional profiling of the brain and fat body of the cotton bollworm Helicoverpa armigera (Lepidoptera: Noctuidae) during its transition from last instar larva into pupa to explore the physiological processes associated with different phases of metamorphosis.

RESULTS

During metamorphosis, the differences in gene expression patterns and the number of differentially expressed genes in the fat body were found to be greater than those in the brain. Each stage had a specific gene expression pattern, which contributed to different physiological changes. A decrease in juvenile hormone levels at the feeding stage is associated with increased expression levels of two genes (juvenile hormone esterase, juvenile hormone epoxide hydrolase). The expression levels of neuropeptides were highly expressed at the feeding stage and the initiation of the wandering stage and less expressed at the prepupal stage and the initiation of the pupal stage. The transcription levels of many hormone (or neuropeptide) receptors were specifically increased at the initiation of the wandering stage in comparison with other stages. The expression levels of many autophagy-related genes in the fat body were found to be gradually upregulated during metamorphosis. The activation of apoptosis was probably related to enhanced expression of many key genes (Apaf1, IAP-binding motif 1 like, cathepsins, caspases). Active proliferation might be associated with enhanced expression levels in several factors (JNK pathway: jun-D; TGF-β pathway: decapentaplegic, glass bottom boat; insulin pathway: insulin-like peptides from the fat body; Wnt pathway: wntless, TCF/Pangolin).

CONCLUSIONS

This study revealed several vital physiological processes and molecular events of metamorphosis and provided valuable information for illustrating the process of insect metamorphosis from larvae to pupae.

Phurealipids, produced by the entomopathogenic bacteria, Photorhabdus, mimic juvenile hormone to suppress insect immunity and immature development

Phurealipids (Photorhabdus urea lipids) are synthesized from Photorhabdus bacteria that are symbiotic to entomopathogenic nematodes. Their chemical structures are similar to that of juvenile hormone (JH) and have been suspected to mimic JH signaling in immunity and the development of insects. This study investigated the physiological roles of phurealipids with respect to their contribution to bacterial pathogenicity using four natural (HB13, HB69, HB416, and HB421) and one derivative (HB27) compound. First, phurealipids like JH suppressed insect immune responses. Overall, phurealipids showed JH like immunosuppressive behavior in a lepidopteran insect Spodoptera exigua larvae. More specifically, phurealipids significantly suppressed the hemocyte spreading behavior which is a key immune response upon immune challenge. Interestingly, the methyl urea derivatives (HB13, HB27, and HB69) were more potent than the unmethylated forms (HB416 and HB421). The inhibitory activity of phurealipids prevented the cellular immune response measured by hemocytic nodule formation in response to the bacterial challenge. Phurealipids also suppressed the expression of cecropin and gallerimycin, which are two highly inducible antimicrobial peptides, in S. exigua upon immune challenge. The immunosuppressive activity of the phurealipids significantly enhanced the bacterial pathogenicity of Bacillus thuringiensis against S. exigua. Second, phurealipids like JH prevented insect metamorphosis. Especially, the methylated urea derivatives of the phurealipids showed the JH-like function by inducing the expression of S. exigua Kr-h1, a transcriptional factor. At the pupal stage, exhibiting the lowest expression of Kr-h1, phurealipid treatments elevated the expression level of Kr-h1 and delayed the pupa-to-adult metamorphosis. These results suggest that phurealipids play crucial roles in Photorhabdus pathogenicity by suppressing host immune defenses and delaying host metamorphosis.

Sodium Benzoate Delays the Development of Drosophila melanogaster Larvae and Alters Commensal Microbiota in Adult Flies

Sodium benzoate (SB), the sodium salt of benzoic acid, is widely used as a preservative in foods and drinks. The toxicity of SB to the human body attracted people’s attention due to the excessive use of preservatives and the increased consumption of processed and fast foods in modern society. The SB can inhibit the growth of bacteria, fungi, and yeast. However, less is known of the effect of SB on host commensal microbial community compositions and their functions. In this study, we investigated the effect of SB on the growth and development of Drosophila melanogaster larvae and whether SB affects the commensal microbial compositions and functions. We also attempted to clarify the interaction between SB, commensal microbiota and host development by detecting the response of commensal microbiota after the intervention. The results show that SB significantly retarded the development of D. melanogaster larvae, shortened the life span, and changed the commensal microbial community. In addition, SB changed the transcription level of endocrine coding genes such as ERR and DmJHAMT. These results indicate that the slow down in D. melanogaster larvae developmental timing and shortened life span of adult flies caused by SB intake may result from the changes in endocrine hormone levels and commensal microbiota. This study provided experimental data that indicate SB could affect host growth and development of D. melanogaster through altering endocrine hormone levels and commensal microbial composition.

MicroRNA let-7 is required for hormonal regulation of metamorphosis in the silkworm, Bombyx mori

The heterochronic microRNA let-7, which was first identified in Caenorhabditis elegans, controls the timing of developmental programs, and let-7 triggers the onset of the juvenile-adult transition in bilaterians. The expression of let-7 is strongly induced during the last larval stage of C. elegans and is highly expressed in the late last instar larvae/nymphs of the fly Drosophila melanogaster and the cockroach Blattella germanica. In the silkworm Bombyx mori, the expression of let-7 remarkably increases in the corpus cardiacum-corpus allatum complex (CC-CA) at the beginning of the last larval instar and is maintained at high levels during this instar. To determine the biological function of let-7 in B. mori, we generated a let-7 knockout line and a transgenic UAS-let-7 line. The let-7 knockout larvae were developmentally arrested in the prepupal stage and became pupal-adult intermediates after apolysis. When let-7 was ubiquitously overexpressed under the transcriptional control of an Actin3-GAL4 driver, developmental timing and growth of larvae were severely impaired in the penultimate (L4) instar, and these larvae underwent precocious metamorphosis from L4. Furthermore, our results showed that reception and signaling of ecdysteroids and juvenile hormones (JHs) normally occurred in the absence of let-7, whereas the biosynthesis of ecdysone and JHs were affected by disruption and overexpression of let-7. Together, the present study demonstrates that let-7 is required for the coordination of the biosynthesis of ecdysone and JH to ensure the developmental transition during the metamorphosis of B. mori.

Cyromazine Effects the Reproduction of Drosophila by Decreasing the Number of Germ Cells in the Female Adult Ovary

Simple Summary

Cyromazine, an insect growth regulator, is used to control the Dipteran pest population. Previous findings observed that treatment with cyromazine increased the larval mortality, by interfering with the ecdysone signaling. In addition, the application of exogenous 20E significantly reduced the mortality caused by cyromazine. Many studies have also supported the role of ecdysone signaling in the maintenance of germline stem cells (GSCs), where mutations in ecdysone signaling-related genes significantly decreased the number of GSCs. However, to date, no study has reported the effect of cyromazine on the GSCs of Drosophila melanogaster. In the present study, we observed that cyromazine significantly reduced the number of both GSCs and cystoblasts (CBs) in the ovary of adult female. To further understand the effect of cyromazine on germ cells, we selected some key genes related to the ecdysone signaling pathway and evaluated their expression through RT-qPCR. Additionally, we measured the ecdysone titer from the cyromazine-treated ovaries. Our results indicated a significant decrease in the expression of ecdysone signaling-related genes and also in the ecdysone titer. These results further supported our findings that cyromazine reduced the number of germ cells by interfering with the ecdysone signaling pathway.

Abstract

In the present study, we observed a 58% decrease in the fecundity of Drosophila melanogaster, after treatment with the cyromazine. To further elucidate the effects of cyromazine on reproduction, we counted the number of both germline stem cells (GSCs) and cystoblasts (CBs) in the ovary of a 3-day-old adult female. The results showed a significant decrease in the number of GSCs and CBs as compared to the control group. The mode of action of cyromazine is believed to be through the ecdysone signaling pathway. To further support this postulate, we observed the expression of key genes involved in the ecdysone signaling pathway and also determined the ecdysone titer from cyromazine-treated ovaries. Results indicated a significant decrease in the expression of ecdysone signaling-related genes as compared to the control group. Furthermore, the titer of the ecdysone hormone was also markedly reduced (90%) in cyromazine-treated adult ovaries, suggesting that ecdysone signaling was directly related to the decrease in the number of GSCs and CBs. However, further studies are required to understand the mechanism by which cyromazine affects the GSCs and CBs in female adult ovaries.

Roles of Krüppel Homolog 1 and Broad-Complex in the Development of Dendroctonus armandi (Coleoptera: Scolytinae)

In insects, metamorphosis is controlled by juvenile hormone (JH) and 20-hydroxyecdysone (20E). Krüppel homolog 1 (Kr-h1), a key JH-early inducible gene, is responsible for the suppression of metamorphosis and the regulation of the Broad-Complex (Br-C) gene, which is induced by 20E and functions as a “pupal specifier”. In this study, we identified and characterized the expression patterns and tissue distribution of DaKr-h1 and DaBr-C at various developmental stages of Dendroctonus armandi. The expression of the two genes was induced by JH analog (JHA) methoprene and 20E, and their functions were investigated by RNA interference. DaKr-h1 and DaBr-C were predominantly expressed in the heads of larvae and were significantly downregulated during the molting stage. In contrast, the DaKr-h1 transcript level was highest in the adult anterior midgut. DaBr-C was mainly expressed in female adults, with the highest transcript levels in the ovaries. In the larval and pupal stages, both JHA and 20E significantly induced DaKr-h1, but only 20E significantly induced DaBr-C, indicating the importance of hormones in metamorphosis. DaKr-h1 knockdown in larvae upregulated DaBr-C expression, resulting in precocious metamorphosis from larvae to pupae and the formation of miniature pupae. DaKr-h1 knockdown in pupae suppressed DaBr-C expression, increased emergence, caused abnormal morphology, and caused the formation of small-winged adults. These results suggest that DaKr-h1 is required for the metamorphosis of D. armandi. Our findings provide insight into the roles of DaKr-h1 and DaBr-C in JH-induced transcriptional repression and highlight DaKr-h1 as a potential target for metamorphosis suppression in D. armandi.

Silencing of transcription factor E93 inhibits adult morphogenesis and disrupts cuticle, wing and ovary development in Locusta migratoria

Ecdysone-induced protein 93F (E93) plays important roles during the metamorphosis process in insects. In this study, a cDNA of the LmE93 gene was identified from the transcriptome of Locusta migratoria, which consists of the 3378-nucleotide open-reading frame (ORF) and encodes 1125 amino acids with helix-turn-helix (HTH) motifs. Reverse transcription quantitative polymerase chain reaction analysis revealed that LmE93 was highest expressed in ovary. The LmE93 expression level was markedly low from the 3rd to 4th instar nymphs, and greatly increased in 1-day-old 5th instar nymphs with a peak on middle nymphal days, then declined in the late nymphal days. Moreover, injected dsLmE93 into 4th and 5th instar nymphs greatly reduced LmE93 transcripts, respectively, and prevented the process of metamorphosis, causing supernumerary nymphal stages. Hematoxylin-eosin staining of the integument showed that the apolysis occurred in advance in 4th instar nymphs, and old cuticle degradation was decreased in dsLmE93-injected locusts of 5th instar nymphs. Smaller and no fully developed wings with reduced columns between the anterior and posterior regions were found in N6 and N7 supernumerary nymphs. In addition, the development of the ovary in dsLmE93-injected locusts was severely blocked, the yolk was almost not formed and there was no development of ovarioles. The results indicated that LmE93 play key roles in the metamorphosis, cuticle, wing and ovarian development of locusts.

POU-M2 promotes juvenile hormone biosynthesis by directly activating the transcription of juvenile hormone synthetic enzyme genes in Bombyx mori

Juvenile hormone (JH) plays a key role in preventing larval precocious metamorphosis, maintaining larval state, controlling adult sexual development and promoting insect egg maturation. Genetic studies have shown that POU factor ventral veins lacking regulates JH synthesis to control the timing of insect metamorphosis. However, how POU factor regulates JH synthesis is largely unknown. Here, we found POU-M2 was highly expressed in corpora allata (CA) and specifically localized in the nucleus of CA. The overexpression of POU-M2 promoted the expression of JH synthase genes and kr-h1 and enhanced the activity of JH synthase genes promoter. Further, POU-M2 promoted the transcription of JH acid O-methyltransferase (JHAMT) by directly binding to the key cis-regulatory elements -207, -249 and -453 within the proximal regions of JHAMT promoter. Both the POU domain and homeodomain were vital for the activation of POU-M2 on JHAMT transcription. Our study reveals the mechanism by which POU-M2 regulates JHAMT transcription.

FKBP39 Controls the Larval Stage JH Activity and Development in Drosophila melanogaster

Simple Summary

Two endocrine hormones, ecdysone and juvenile hormone (JH), control insect development and reproduction. Some studies in the literature have suggested that FKBP39 functions as a transcriptional factor and regulates the JH pathway in Drosophila. However, the physiological roles of FKBP39 are still elusive. To determine the FKBP39 roles in vivo, we first developed an antibody to check the FKBP39 expression pattern and then detected JH activity-related phenotypes in fkbp39 mutants, such as pupariation, reproduction, and Kr-h1 expression. We found that FKBP39 expresses at a high level and controls JH activity at the larval stage. Moreover, we found that rp49, the most widely used reference gene for Real-time quantitative PCR (qRT-PCR), significantly decreased in the fkbp39 mutant. This work will provide valuable information for studies on JH activity and insect development.

Abstract

FK506-binding protein 39kD (FKBP39) localizes in the nucleus and contains multiple functional domains. Structural analysis suggests that FKBP39 might function as a transcriptional factor and control juvenile hormone (JH) activity. Here, we show that FKBP39 expresses at a high level and localizes in the nucleolus of fat body cells during the first two larval stages and early third larval stage. The fkbp39 mutant displays delayed larval-pupal transition and an increased expression of Kr-h1, the main mediator of the JH pathway, at the early third larval stage. Moreover, the fkbp39 mutant has a fertility defect that is independent of JH activity. Interestingly, the expression of rp49, the most widely used reference gene for qRT-PCR in Drosophila, significantly decreased in the fkbp39 mutant, suggesting that FKBP39 might regulate ribosome assembly. Taken together, our data demonstrate the expression pattern and physiological roles of FKBP39 in Drosophila.

Juvenile hormone-induced histone deacetylase 3 suppresses apoptosis to maintain larval midgut in the yellow fever mosquito

SignificanceJuvenile hormone (JH), a sesquiterpenoid, regulates many aspects of insect development, including maintenance of the larval stage by preventing metamorphosis. In contrast, ecdysteroids promote metamorphosis by inducing the E93 transcription factor, which triggers apoptosis of larval cells and remodeling of the larval midgut. We discovered that JH suppresses precocious larval midgut-remodeling by inducing an epigenetic modifier, histone deacetylase 3 (HDAC3). JH-induced HDAC3 deacetylates the histone H4 localized at the promoters of proapoptotic genes, resulting in the suppression of these genes. This eventually prevents programmed cell death of midgut cells and midgut-remodeling during larval stages. These studies identified a previously unknown mechanism of JH action in blocking premature remodeling of the midgut during larval feeding stages.

Juvenile hormone membrane signaling phosphorylates USP and thus potentiates 20-hydroxyecdysone action in Drosophila

Juvenile hormone (JH) and 20-hydroxyecdysone (20E) coordinately regulate development and metamorphosis in insects. Two JH intracellular receptors, methoprene-tolerant (Met) and germ-cell expressed (Gce), have been identified in the fruit fly Drosophila melanogaster. To investigate JH membrane signaling pathway without the interference from JH intracellular signaling, we characterized phosphoproteome profiles of the Met gce double mutant in the absence or presence of JH in both chronic and acute phases. Functioning through a potential receptor tyrosine kinase and phospholipase C pathway, JH membrane signaling activated protein kinase C (PKC) which phosphorylated ultraspiracle (USP) at Ser35, the PKC phosphorylation site required for the maximal action of 20E through its nuclear receptor complex EcR-USP. The usp^S35A mutant, in which Ser was replaced with Ala at position 35 by genome editing, showed decreased expression of Halloween genes that are responsible for ecdysone biosynthesis and thus attenuated 20E signaling that delayed developmental timing. The usp^S35A mutant also showed lower Yorkie activity that reduced body size. Altogether, JH membrane signaling phosphorylates USP at Ser35 and thus potentiates 20E action that regulates the normal fly development. This study helps better understand the complex JH signaling network.

Cuticular protein genes showing peaks at different stages are probably regulated by different ecdysone responsive transcription factors during larval-pupal transformation

We aimed to explain the reason and function of the successive expression of ecdysone-responsive transcription factors (ERTFs) and related cuticular protein (CP) genes during transformation from larva to pupa. The regulation of the expression of CP genes by ERTFs was examined by in vitro wing disc culture and reporter assay using a gene gun transduction system. Two CP genes that showed expression peaks at different stages-BmorCPG12 at W3L and BmorCPH2 at P0 stage-were selected and examined. Reporter constructs conveying putative BHR3, ßFTZ-F1, BHR39, and E74A binding sites of BmorCPG12 and BmorCPH2 showed promoter activity when introduced into wing discs. In the present study, we showed the functioning of the putative BHR3 and E74A binding sites, together with putative ßFTZ-F1 binding sites, on the activation of CP genes, and different ERTF binding sites functioned in one CP gene. From these, we conclude that BHR3, ßFTZ-F1, and E74A that are successively expressed bring about the successive expression of CP genes, resulting in insect metamorphosis. In addition to this, reporter constructs conveying putative BHR39 binding sites of BmorCPG12 and BmorCPH2 showed negative regulation.

Impact of Cymbopogon flexuosus (Poaceae) essential oil and primary components on the eclosion and larval development of Aedes aegypti

The current study describes the effects of sub-lethal concentrations and constituent compounds (citral and geranyl acetate) of Cymbopogon flexuosus essential oil (EO) on the development of Aedes aegypti. We treated eggs with 6, 18, or 30 mg L^-1 and larvae with 3 or 6 mg L^-1 of EO and its major compounds (citral and geranyl acetate). Citral and geranyl acetate were evaluated at 18, 30, and 42 mg L^-1 and compared with commercial growth inhibitors (diflubenzuron and methoprene). We measured larval head diameter, siphon length, and larval length. Finally, we examined concentrations of molt hormone (MH) and juvenile hormone III (JH III) using high-performance liquid chromatography coupled to mass spectrometry. All geranyl acetate concentrations decreased egg hatching, while EO altered molting among larval instars and between larvae and pupae, with an increase in the larval length (3 mg L^-1: 6 ± 0.0 mm; 6 mg L^-1: 6 ± 0.7 mm) and head width (3 mg L^-1: 0.8 ± 0 mm; 6 mg L^-1: 0.8 ± 0.0 mm) compared with the control group. We did not detect chromatographic signals of MH and JH III in larvae treated with C. flexuosus EO or their major compounds. The sub-lethal concentrations C. flexuosus EO caused a similar effect to diflubenzuron, namely decreased hormone concentrations, an extended larval period, and death.

Genome-wide identification of target genes for transcription factor BR-C in the silkworm, Bombyx mori

Transcription factor Broad Complex (BR-C) is an ecdysone primary response gene in insects and participates in the regulation of insect growth and development. In this study, we performed a genome-wide identification of BR-C target genes in silkworm (Bombyx mori) using chromatin immunoprecipitation followed by high-throughput sequencing (ChIP-seq). As a result, a total of 1006 BR-C ChIP peaks were identified, and 15% of peaks were located in the promoter regions of 133 protein-coding genes. Functional annotation revealed that these ChIP peak-associated genes, as potential BR-C targets, were enriched in pathways related to biosynthetic process, metabolic process, and development. Transcriptome analysis and quantitative real-time polymerase chain reaction (PCR) examination revealed that developmental changes in expression patterns of a portion of potential BR-C targets, including HR96 and GC-α1, were similar to those of BR-C. ChIP-PCR examination confirmed that BR-C could directly bind to the promoters of potential targets. Further, dual luciferase assays demonstrated that HR96 promoter activity was significantly upregulated following BR-C overexpression, and this upregulation was abolished when the binding motif in the promoter was truncated. This study will be helpful for deciphering the regulatory roles of BR-C during insect growth and development.

Epoxidation of juvenile hormone was a key innovation improving insect reproductive fitness

Methyl farnesoate (MF) plays hormonal regulatory roles in crustaceans. An epoxidated form of MF, known as juvenile hormone (JH), controls metamorphosis and stimulates reproduction in insects. To address the evolutionary significance of MF epoxidation, we generated mosquitoes completely lacking either of the two enzymes that catalyze the last steps of MF/JH biosynthesis and epoxidation, respectively: the JH acid methyltransferase (JHAMT) and the P450 epoxidase CYP15 (EPOX). jhamt ^-/- larvae lacking both MF and JH died at the onset of metamorphosis. Strikingly, epox ^-/- mutants, which synthesized MF but no JH, completed the entire life cycle. While epox ^-/- adults were fertile, the reproductive performance of both sexes was dramatically reduced. Our results suggest that although MF can substitute for the absence of JH in mosquitoes, it is with a significant fitness cost. We propose that MF can fulfill most roles of JH, but its epoxidation to JH was a key innovation providing insects with a reproductive advantage.

Purification of an insect juvenile hormone receptor complex enables insights into its post-translational phosphorylation

Juvenile hormone (JH) plays vital roles in insect reproduction, development, and in many aspects of physiology. JH primarily acts at the gene-regulatory level through interaction with an intracellular receptor (JH receptor [JHR]), a ligand-activated complex of transcription factors consisting of the JH-binding protein methoprene-tolerant (MET) and its partner taiman (TAI). Initial studies indicated significance of post-transcriptional phosphorylation, subunit assembly, and nucleocytoplasmic transport of JHR in JH signaling. However, our knowledge of JHR regulation at the protein level remains rudimentary, partly because of the difficulty of obtaining purified and functional JHR proteins. Here, we present a method for high-yield expression and purification of JHR complexes from two insect species, the beetle T. castaneum and the mosquito Aedes aegypti. Recombinant JHR subunits from each species were coexpressed in an insect cell line using a baculovirus system. MET–TAI complexes were purified through affinity chromatography and anion exchange columns to yield proteins capable of binding both the hormonal ligand (JH III) and DNA bearing cognate JH-response elements. We further examined the beetle JHR complex in greater detail. Biochemical analyses and MS confirmed that T. castaneum JHR was a 1:1 heterodimer consisting of MET and Taiman proteins, stabilized by the JHR agonist ligand methoprene. Phosphoproteomics uncovered multiple phosphorylation sites in the MET protein, some of which were induced by methoprene treatment. Finally, we report a functional bipartite nuclear localization signal, straddled by phosphorylated residues, within the disordered C-terminal region of MET. Our present characterization of the recombinant JHR is an initial step toward understanding JHR structure and function.

Krüppel-homolog 1 exerts anti-metamorphic and vitellogenic functions in insects via phosphorylation-mediated recruitment of specific cofactors

Background

The zinc-finger transcription factor Krüppel-homolog 1 (Kr-h1) exerts a dual regulatory role during insect development by preventing precocious larval/nymphal metamorphosis and in stimulating aspects of adult reproduction such as vitellogenesis. However, how Kr-h1 functions both as a transcriptional repressor in juvenile metamorphosis and an activator in adult reproduction remains elusive. Here, we use the insect Locusta migratoria to dissect the molecular mechanism by which Kr-h1 functions as activator and repressor at these distinct developmental stages.

Results

We report that the kinase PKCα triggers Kr-h1 phosphorylation at the amino acid residue Ser¹⁵⁴, a step essential for its dual functions. During juvenile stage, phosphorylated Kr-h1 recruits a corepressor, C-terminal binding protein (CtBP). The complex of phosphorylated Kr-h1 and CtBP represses the transcription of Ecdysone induced protein 93F (E93) and consequently prevents the juvenile-to-adult transition. In adult insects, phosphorylated Kr-h1 recruits a coactivator, CREB-binding protein (CBP), and promotes vitellogenesis by inducing the expression of Ribosomal protein L36. Furthermore, Kr-h1 phosphorylation with the concomitant inhibition of E93 transcription is evolutionarily conserved across insect orders.

Conclusion

Our results suggest that Kr-h1 phosphorylation is indispensable for the recruitment of transcriptional cofactors, and for its anti-metamorphic and vitellogenic actions in insects. Our data shed new light on the understanding of Kr-h1 regulation and function in JH-regulated insect metamorphosis and reproduction.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01157-3.

Role of juvenile hormone receptor Methoprene-tolerant 1 in silkworm larval brain development and domestication

The insect brain is the central part of the neurosecretory system, which controls morphology, physiology, and behavior during the insect's lifecycle. Lepidoptera are holometabolous insects, and their brains develop during the larval period and metamorphosis into the adult form. As the only fully domesticated insect, the Lepidoptera silkworm Bombyx mori experienced changes in larval brain morphology and certain behaviors during the domestication process. Hormonal regulation in insects is a key factor in multiple processes. However, how juvenile hormone (JH) signals regulate brain development in Lepidoptera species, especially in the larval stage, remains elusive. We recently identified the JH receptor Methoprene tolerant 1 ( Met1) as a putative domestication gene. How artificial selection on Met1 impacts brain and behavioral domestication is another important issue addressing Darwin's theory on domestication. Here, CRISPR/Cas9-mediated knockout of Bombyx Met1 caused developmental retardation in the brain, unlike precocious pupation of the cuticle. At the whole transcriptome level, the ecdysteroid (20-hydroxyecdysone, 20E) signaling and downstream pathways were overactivated in the mutant cuticle but not in the brain. Pathways related to cell proliferation and specialization processes, such as extracellular matrix (ECM)-receptor interaction and tyrosine metabolism pathways, were suppressed in the brain. Molecular evolutionary analysis and in vitro assay identified an amino acid replacement located in a novel motif under positive selection in B. mori, which decreased transcriptional binding activity. The B. mori MET1 protein showed a changed structure and dynamic features, as well as a weakened co-expression gene network, compared with B. mandarina. Based on comparative transcriptomic analyses, we proposed a pathway downstream of JH signaling (i.e., tyrosine metabolism pathway) that likely contributed to silkworm larval brain development and domestication and highlighted the importance of the biogenic amine system in larval evolution during silkworm domestication.

Histone H3K27 methylation-mediated repression of Hairy regulates insect developmental transition by modulating ecdysone biosynthesis

Insect development is cooperatively orchestrated by the steroid hormone ecdysone and juvenile hormone (JH). The polycomb repressive complex 2 (PRC2)-mediated histone H3K27 trimethylation (H3K27me3) epigenetically silences gene transcription and is essential for a range of biological processes, but the functions of H3K27 methylation in insect hormone action are poorly understood. Here, we demonstrate that H3K27 methylation-mediated repression of Hairy transcription in the larval prothoracic gland (PG) is required for ecdysone biosynthesis in Bombyx and Drosophila H3K27me3 levels in the PG are dynamically increased during the last larval instar. H3K27me3 reduction induced by the down-regulation of PRC2 activity via inhibitor treatment in Bombyx or PG-specific knockdown of the PRC2 component Su(z)12 in Drosophila diminishes ecdysone biosynthesis and disturbs the larval-pupal transition. Mechanistically, H3K27 methylation targets the JH signal transducer Hairy to repress its transcription in the PG; PG-specific knockdown or overexpression of the Hairy gene disrupts ecdysone biosynthesis and developmental transition; and developmental defects caused by PG-specific Su(z)12 knockdown can be partially rescued by Hairy down-regulation. The application of JH mimic to the PG decreases both H3K27me3 levels and Su(z)12 expression. Altogether, our study reveals that PRC2-mediated H3K27 methylation at Hairy in the PG during the larval period is required for ecdysone biosynthesis and the larval-pupal transition and provides insights into epigenetic regulation of the crosstalk between JH and ecdysone during insect development.

Transcriptome Analysis Reveals Key Genes Involved in Weevil Resistance in the Hexaploid Sweetpotato

Because weevils are the most damaging pests of sweetpotato, the development of cultivars resistant to weevil species is considered the most important aspect in sweetpotato breeding. However, the genes and the underlying molecular mechanisms related to weevil resistance are yet to be elucidated. In this study, we performed an RNA sequencing-based transcriptome analysis using the resistant Kyushu No. 166 (K166) and susceptible Tamayutaka cultivars. The weevil resistance test showed a significant difference between the two cultivars at 30 days after the inoculation, specifically in the weevil growth stage and the suppressed weevil pupation that was only observed in K166. Differential expression and gene ontology analyses revealed that the genes upregulated after inoculation in K166 were related to phosphorylation, metabolic, and cellular processes. Because the weevil resistance was considered to be related to the suppression of larval pupation, we investigated the juvenile hormone (JH)-related genes involved in the inhibition of insect metamorphosis. We found that the expression of some terpenoid-related genes, which are classified as plant-derived JHs, was significantly increased in K166. This is the first study involving a comprehensive gene expression analysis that provides new insights about the genes and mechanisms associated with weevil resistance in sweetpotato.

Characterization of E93 in neometabolous thrips Frankliniella occidentalis and Haplothrips brevitubus

Insect metamorphosis into an adult occurs after the juvenile hormone (JH) titer decreases at the end of the juvenile stage. This generally coincides with decreased transcript levels of JH-response transcription factors Krüppel homolog 1 (Kr-h1) and broad (br), and increased transcript levels of the adult specifier E93. Thrips (Thysanoptera) develop through inactive and non-feeding stages referred to as “propupa” and “pupa”, and this type of distinctive metamorphosis is called neometaboly. To understand the mechanisms of hormonal regulation in thrips metamorphosis, we previously analyzed the transcript levels of Kr-h1 and br in two thrips species, Frankliniella occidentalis (Thripidae) and Haplothrips brevitubus (Phlaeothripidae). In both species, the transcript levels of Kr-h1 and br decreased in the “propupal” and “pupal” stages, and their transcription was upregulated by exogenous JH mimic treatment. Here we analyzed the developmental profiles of E93 in these two thrips species. Quantitative RT-PCR revealed that E93 expression started to increase at the end of the larval stage in F. occidentalis and in the “propupal” stage of H. brevitubus, as Kr-h1 and br mRNA levels decreased. Treatment with an exogenous JH mimic at the onset of metamorphosis prevented pupal-adult transition and caused repression of E93. These results indicated that E93 is involved in adult differentiation after JH titer decreases at the end of the larval stage of thrips. By comparing the expression profiles of Kr-h1, br, and E93 among insect species, we propose that the “propupal” and “pupal” stages of thrips have some similarities with the holometabolous prepupal and pupal stages, respectively.

Role of Endocrine System in the Regulation of Female Insect Reproduction

The proper synthesis and functioning of ecdysteroids and juvenile hormones (JHs) are very important for the regulation of vitellogenesis and oogenesis. However, their role and function contrast among different orders, and even in the same insect order. For example, the JH is the main hormone that regulates vitellogenesis in hemimetabolous insect orders, which include Orthoptera, Blattodea, and Hemiptera, while ecdysteroids regulate the vitellogenesis among the insect orders of Diptera, some Hymenoptera and Lepidoptera. These endocrine hormones also regulate each other. Even at some specific stage of insect life, they positively regulate each other, while at other stages of insect life, they negatively control each other. Such positive and negative interaction of 20-hydroxyecdysone (20E) and JH is also discussed in this review article to better understand the role of these hormones in regulating the reproduction. Therefore, the purpose of the present review is to deeply understand the complex interaction of endocrine hormones with each other and with the insulin signaling pathway. The role of microbiomes in the regulation of the insect endocrine system is also reviewed, as the endocrine hormones are significantly affected by the compounds produced by the microbiota.