Ecdysteroid promotes cell cycle progression in the Bombyx wing disc through activation of c-Myc

Ecdysone and gene expressions for chromatin remodeling, histone modification, and Broad Complex in relation to pupal commitment in Bombyx mori

In the present study, we tried to clarify when and how pupal commitment (PT) better to use PC occurs and what is involved in the PT of Bombyx mori. To clarify this, we examined the responsiveness of a wing disc to ecdysone, referring to metamorphosis-related BR-C, development-related Myc and Wnt, and chromatin remodeling-related genes at around the predicted PT stage of the Bombyx wing disc. Wing disc responsiveness to juvenile hormone (JH) and ecdysone was examined using Methoprene and 20-hydroxyecdysone (20E) in vitro. The body weight of B. mori increased after the last larval ecdysis, peaked at Day 5 of the fifth larval instar (D5L5), and then decreased. The responsiveness of the wing disc to JH decreased after the last larval ecdysis up to D3L5. Bmbr-c (the Broad Complex of B. mori) showed enhanced expression in D4L5 wing discs with 20E treatment. Some chromatin remodeler and histone modifier genes (Bmsnr1, Bmutx, and Bmtip60) showed upregulation after being cultured with 20E in D4L5 wing discs. A low concentration of 20E is suggested to induce responsiveness to 20E in D4L5 wing discs. Bmbr-c, Bmsnr1, Bmutx, and Bmtip60 were upregulated after being cultured with a low concentration of 20E in D4L5 wing discs. The expression of Bmmyc and Bmwnt1 did not show a change after being cultured with or without 20E in D4L5 wing discs, while enhanced expression was observed with 20E in D5L5 wing discs. From the present results, we concluded that PT of the wing disc of B. mori occurred beginning on D4L5 with the secretion of low concentrations of ecdysteroids. Bmsnr1, Bmutx, Bmtip60, and BR-C are also involved.

Proliferative and preparative cell divisions in wing discs of the last larval instar are regulated by different hormones and determine the size and differentiation of the wing of Bombyx mori

Through investigating the two different enhanced cell division stages, we tried to clarify the switch from the growth to differentiation in the wing disc of the last larval instar of Bombyx mori. The response to insulin and 20E in vitro was stage specific. Bmmyc expression in V1 wing discs showed differences after being cultured with and without insulin. Bmmyc expression in V5 wing discs also showed differences after being cultured with and without 20E. Cell cycle-related genes, BmE2F1 and BmcycE, were upregulated with insulin or 20E in cultured wing discs of V1 or V5, respectively. Bmwnt1 and Bmras1 showed upregulation with 20E in cultured wing discs. Bmwnt1 showed upregulation with insulin in cultured wing discs, but Bmras1 did not show clear upregulation with insulin treatment. In contrast, Bmdpp showed upregulation with insulin, but did not show clear upregulation with 20E. The addition of PI3K or TOR inhibitors inhibited the upregulation of Bmmyc expression that was upregulated with insulin or 20E. The upregulation of Bmmyc and Bmwnt1 with insulin or 20E was inhibited with the addition of Myc or Wnt inhibitors, respectively. Genes related to matrix metalloprotease showed upregulation with 20E, and the upregulation was inhibited by the addition of Myc or Wnt inhibitors. From the present results, we concluded that cell division during the feeding stage occurred through PI3K/TOR cascade, and that at the wandering stage occurred through ecdysone and PI3K/TOR cascade; the former is for growth and the latter for differentiation.

Variability of polyteny of giant chromosomes in Drosophila melanogaster salivary glands

Polyteny is an effective mechanism for accelerating growth and enhancing gene expression in eukaryotes. The purpose of investigation was to study the genetic variability of polyteny degree of giant chromosomes in the salivary glands of Drosophila melanogaster Meig. in relation to the differential fitness of different genotypes. 16 strains, lines and hybrids of fruit flies were studied. This study demonstrates the significant influence of hereditary factors on the level of polytenization of giant chromosomes in Drosophila. This is manifested in the differences between strains and lines, the effect of inbreeding, chromosome isogenization, hybridization, adaptively significant selection, sexual differences, and varying degrees of individual variability of a trait in different strains, lines, and hybrids. The genetic component in the variability of the degree of chromosome polyteny in Drosophila salivary glands was 45.3%, the effect of sex was 9.5%. It has been shown that genetic distances during inbreeding, outbreeding or hybridization, which largely determine the selective value of different genotypes, also affect polyteny patterns. Genetic, humoral, and epigenetic aspects of endocycle regulation, which may underlie the variations in the degree of chromosome polyteny, as well as the biological significance of the phenomenon of endopolyploidy, are discussed.

Transcriptional Regulation of Reproductive Diapause in the Convergent Lady Beetle, Hippodamia convergens

Simple Summary

Diapause is a dormant period typically controlled by daylength that ensures an insect’s survival through harsh environmental conditions. The convergent lady beetle, Hippodamia convergens, undergoes a reproductive diapause in winter, where female ovaries remain immature and no eggs are laid. This species is an important biological control agent, but during diapause, beetles are less likely to eat pest insects. Thus, knowledge of diapause mechanisms may facilitate manipulation thereof to improve biological control. Further, molecular studies of adult diapause and diapause in Coleoptera are relatively lacking. Here, we assembled and annotated a transcriptome for this species and quantified transcript expression changes during diapause. Female beetles were sampled at three times in diapause (early, mid, and late diapause), which allowed us to characterize the molecular processes occurring at distinct transitions throughout diapause. We found that transcripts involved in flight were consistently upregulated during diapause, which is consistent with dispersal flights at this stage, while transcripts involved in ovarian development were downregulated, which is consistent with the shutdown of reproduction in diapausing females. These findings identify key regulators of diapause in H. convergens and contribute to a growing body of literature on the molecular mechanisms of diapause across the insect phylogeny.

Abstract

Diapause is an alternate development program that synchronizes an insect’s life cycle with seasonally abundant resources and ensures survival in unfavorable conditions. The physiological basis of diapause has been well characterized, but the molecular mechanisms regulating it are still being elucidated. Here, we present a de novo transcriptome and quantify transcript expression during diapause in the convergent lady beetle Hippodamia convergens. H. convergens is used as an augmentative biocontrol agent, and adult females undergo reproductive diapause that is regulated by photoperiod. We sampled females at three stages (early, mid, and late diapause) and compared transcript expression to non-diapausing individuals. Based on principle component analysis, the transcriptomes of diapausing beetles were distinct from non-diapausing beetles, and the three diapausing points tended to cluster together. However, there were still classes of transcripts that differed in expression across distinct phases of diapause. In general, transcripts involved in muscle function and flight were upregulated during diapause, likely to support dispersal flights that occur during diapause, while transcripts involved in ovarian development were downregulated. This information could be used to improve biological control by manipulating diapause. Additionally, our data contribute to a growing understanding of the genetic regulation of diapause across diverse insects.

Enhanced Myc Expression in Silkworm Silk Gland Promotes DNA Replication and Silk Production

Simple Summary

Based on a transgenic approach, enhancing Myc expression in the silkworm posterior silk gland (PSG), which was driven by the promoter of the fibroin heavy chain (FibH) gene, was performed for investigating the biological functions of Myc in silk gland. Enhanced Myc expression elevated the cocoon size. This elevation might be resulted from the increasing of FibH expression and DNA content in the PSG cells by promoting the transcription of the genes that are involved in DNA replication.

Abstract

Silkworm is an economically important insect that synthetizes silk proteins for silk production in silk gland, and silk gland cells undergo endoreplication during larval period. Transcription factor Myc is essential for cell growth and proliferation. Although silkworm Myc gene has been identified previously, its biological functions in silkworm silk gland are still largely unknown. In this study, we examined whether enhanced Myc expression in silk gland could facilitate cell growth and silk production. Based on a transgenic approach, Myc was driven by the promoter of the fibroin heavy chain (FibH) gene to be successfully overexpressed in posterior silk gland. Enhanced Myc expression in the PSG elevated FibH expression by about 20% compared to the control, and also increased the weight and shell rate of the cocoon shell. Further investigation confirmed that Myc overexpression increased nucleus size and DNA content of the PSG cells by promoting the transcription of the genes involved in DNA replication. Therefore, we conclude that enhanced Myc expression promotes DNA replication and silk protein expression in endoreplicating silk gland cells, which subsequently raises silk yield.

BmFoxO Gene Regulation of the Cell Cycle Induced by 20-Hydroxyecdysone in BmN-SWU1 Cells

Simple Summary

Ecdysteroid titer determines the state of the cell cycle in silkworm (Bombyx mori) metamorphosis. However, the mechanism of this process is unclear. In this study, we reported that 20-Hydroxyecdysone (20E) can promote BmFoxO (Bombyx mori Forkhead box protein O) gene expression and induce BmFoxO nuclear translocation in BmN-SWU1 cells. Overexpression of the BmFoxO gene affects cell cycle progression, which results in cell cycle arrest in the G0/G1 phase as well as inhibition of DNA replication. Further investigations showed that the effect of 20E was attenuated after BmFoxO gene knockdown. The findings of this study confirmed that BmFoxO is a key mediator in the cell cycle regulation pathway induced by 20E. This suggests a novel pathway for ecdysteroid-induced cell cycle regulation in the process of silkworm metamorphosis, and it is likely to be conserved between Lepidoptera insects.

Abstract

Ecdysteroid titer determines the state of the cell cycle in silkworm (Bombyxmori) metamorphosis. However, the mechanism of this process is unclear. In this study, we demonstrated that the BmFoxO gene participates in the regulation of the cell cycle induced by 20-Hydroxyecdysone (20E) in BmN-SWU1 cells. The 20E blocks the cell cycle in the G2/M phase through the ecdysone receptor (EcR) and inhibits DNA replication. The 20E can promote BmFoxO gene expression. Immunofluorescence and Western blot results indicated that 20E can induce BmFoxO nuclear translocation in BmN-SWU1 cells. Overexpression of the BmFoxO gene affects cell cycle progression, which results in cell cycle arrest in the G0/G1 phase as well as inhibition of DNA replication. Knockdown of the BmFoxO gene led to cell accumulation at the G2/M phase. The effect of 20E was attenuated after BmFoxO gene knockdown. These findings increase our understanding of the function of 20E in the regulation of the cell cycle in B. mori.

Juvenile hormone acts through FoxO to promote Cdc2 and Orc5 transcription for polyploidy-dependent vitellogenesis

Vitellogenin (Vg) is a prerequisite for egg production and embryonic development after ovipositioning in oviparous animals. In many insects, juvenile hormone (JH) promotes fat body cell polyploidization for the massive Vg synthesis required for the maturation of multiple oocytes, but the underlying mechanisms remain poorly understood. Using the migratory locust Locusta migratoria as a model system, we report here that JH induces the dephosphorylation of Forkhead box O transcription factor (FoxO) through a signaling cascade including leucine carboxyl methyltransferase 1 (LCMT1) and protein phosphatase 2A (PP2A). JH promotes PP2A activity via LCMT1-mediated methylation, consequently triggering FoxO dephosphorylation. Dephosphorylated FoxO binds to the upstream region of two endocycle-related genes, cell-division-cycle 2 (Cdc2) and origin-recognition-complex subunit 5 (Orc5), and activates their transcription. Depletion of FoxO, Cdc2 or Orc5 results in blocked polyploidization of fat body cells, accompanied by markedly reduced Vg expression, impaired oocyte maturation and arrested ovarian development. The results suggest that JH acts via LCMT1-PP2A-FoxO to regulate Cdc2 and Orc5 expression, and to enhance ploidy of fat body cells in preparation for the large-scale Vg synthesis required for synchronous maturation of multiple eggs.

The genome of pest Rhynchophorus ferrugineus reveals gene families important at the plant-beetle interface

The red palm weevil, Rhynchophorus ferrugineus, infests palm plantations, leading to large financial losses and soil erosion. Pest-host interactions are poorly understood in R. ferrugineus, but the analysis of genetic diversity and pest origins will help advance efforts to eradicate this pest. We sequenced the genome of R. ferrugineus using a combination of paired-end Illumina sequencing (150 bp), Oxford Nanopore long reads, 10X Genomics and synteny analysis to produce an assembly with a scaffold N50 of ~60 Mb. Structural variations showed duplication of detoxifying and insecticide resistance genes (e.g., glutathione S-transferase, P450, Rdl). Furthermore, the evolution of gene families identified those under positive selection including one glycosyl hydrolase (GH16) gene family, which appears to result from horizontal gene transfer. This genome will be a valuable resource to understand insect evolution and behavior and to allow the genetic modification of key genes that will help control this pest.

Regulatory Mechanisms of Cell Polyploidy in Insects

Polyploidy cells undergo the endocycle to generate DNA amplification without cell division and have important biological functions in growth, development, reproduction, immune response, nutrient support, and conferring resistance to DNA damage in animals. In this paper, we have specially summarized current research progresses in the regulatory mechanisms of cell polyploidy in insects. First, insect hormones including juvenile hormone and 20-hydroxyecdysone regulate the endocycle of variant cells in diverse insect species. Second, cells skip mitotic division in response to developmental programming and conditional stimuli such as wound healing, regeneration, and aging. Third, the reported regulatory pathways of mitotic to endocycle switch (MES), including Notch, Hippo, and JNK signaling pathways, are summarized and constructed into genetic network. Thus, we think that the studies in crosstalk of hormones and their effects on canonical pathways will shed light on the mechanism of cell polyploidy and elucidate the evolutionary adaptions of MES through diverse insect species.

Juvenile hormone promotes locust fat body cell polyploidization and vitellogenesis by activating the transcription of Cdk6 and E2f1

Juvenile hormone (JH) is known to promote cell polyploidization for insect vitellogenesis and egg production, but the underlying mechanisms remain poorly understood. Using the migratory locust Locusta migratoria as a model system, we report here that the expression of cyclin-dependent kinase 6 (Cdk6) and adenovirus E2 factor-1 (E2f1), the core mediators in cell cycle progression is regulated by JH and its receptor Methoprene-tolerant (Met). JH acts through its receptor complex comprised of Met and Taiman to directly activate the transcription of Cdk6 and E2f1. Depletion of Cdk6 or E2f1 results in significantly decreased ploidy, precocious mitotic entry and increased cell numbers in the fat body, accompanied by substantial reduction of Vitellogenin gene expression, blocked ovarian growth and arrested oocyte maturation. These findings indicate a crucial role of Cdk6 and E2f1 in JH-regulated polyploidization and vitellogenesis as well as a novel regulatory machinery for endocycling in insects.

Overexpression of c-Myc enhances recombinant protein production in High Five cells after baculovirus infection

Due to their numerous advantages, baculovirus expression vector systems (BEVS) have been widely used to express recombinant proteins for different purposes. Different strategies have been adopted to increase recombinant protein production. In this study, we transiently or stably expressed mouse c-Myc in High Five cells using a commercial pIB/V5 vector. Under the control of the OpIE2 promoter, this vector could enhance recombinant protein production. We found that transient expression of c-Myc in High Five cells improved recombinant protein production. Furthermore, we established two stable cell lines, High Five-c-Myc #1 and High Five-c-Myc #2, that stably expressed mouse c-Myc. We further found that the expression level of the recombinant protein was increased in these stable cell lines compared to control cell lines. These data indicate that overexpressing c-Myc in cells is a promising way to improve recombinant protein production in BEVS.

The Steroid Hormone 20-Hydroxyecdysone Regulates the Conjugation of Autophagy-Related Proteins 12 and 5 in a Concentration and Time-Dependent Manner to Promote Insect Midgut Programmed Cell Death

Autophagy requires the conjugation of autophagy-related protein 12 (ATG12) to autophagy-related protein 5 (ATG5) through covalent attachment. However, the signals regulating ATG12-ATG5 conjugation are unclear. The larval midgut of lepidopteran insects performs autophagy and apoptosis sequentially during the transition of larvae to pupae under regulation by the steroid hormone 20-hydroxyecdysone (20E), thus representing a model to study steroid hormone regulation of ATG12-ATG5 conjugation. In the present study, using the lepidopteran insect Helicoverpa armigera as a model, we report that 20E regulates the conjugation of ATG12-ATG5 in a concentration and time-dependent manner. The ATG12-ATG5 conjugate was abundant in the epidermis, midgut, and fat body during metamorphosis from the larvae to the pupae; however, the ATG12-ATG5 conjugate level decreased at the time of pupation. At low concentrations (2-5 µM) over a short time course (1-48 h), 20E promoted the conjugation of ATG12-ATG5; however, at 10 µM and 72 h, 20E repressed the conjugation of ATG12-ATG5. ATG12 was localized in the larval midgut during metamorphosis. Knockdown of ATG12 in larvae caused death with delayed pupation, postponed the process of midgut programmed cell death (PCD), and repressed ATG8 (also called LC3-I) transformation to LC3-II and the cleavage of caspase-3; therefore, knockdown of ATG12 in larvae blocked both autophagy and apoptosis. Knockdown of ATG12 in H. armigera epidermis cell line cells also repressed 20E-induced autophagosome formation and caspase-3 activation. The results suggested that 20E plays key role in the regulation of ATG12-ATG5 conjugation in a concentration and time-dependent manner for autophagy or apoptosis, and that ATG12 is necessary by both autophagy and apoptosis during insect midgut PCD.

Differentiation of lepidoptera scale cells from epidermal stem cells followed by ecdysone-regulated DNA duplication and scale secreting

Integuments are the first line to protect insects from physical damage and pathogenic infection. In lepidopteran insects, they undergo distinct morphology changes such as scale formation during metamorphosis. However, we know little about integument development and scale formation during this stage. Here, we use the silkworm, Bombyx mori, as a model and show that stem cells in the integument of each segment, but not intersegmental membrane, divide into two scale precursor cells during the spinning stage. In young pupae, the scale precursor cell divides again. One of the daughter cells becomes a mature scale-secreting cell that undergoes several rounds of DNA duplication and the other daughter cell undergoes apoptosis later on. This scale precursor cell division is crucial to the development and differentiation of scale-secreting cells because scale production can be blocked after treatment with the cell division inhibitor paclitaxel. Subsequently, the growth of scale-secreting cells is under the control of 20-hydroxyecdysone but not juvenile hormone since injection of 20-hydroxyecdysone inhibited scale formation. Further work demonstrated that 20-hydroxyecdysone injection inhibits DNA duplication in scale-secreting cells while the expression of scale-forming gene ASH1 was down-regulated by BR-C Z2. Therefore, this research demonstrates that the scale cells of the silkworm develops through stem cell division prior to pupation and then another wave of cell division differentiates these cells into scale secreting cells soon after entrance into the pupal stage. Additionally, DNA duplication and scale production in the scale-secreting cells were found to be under the regulation of 20-hydroxyecdysone.

Protein kinase C delta phosphorylates ecdysone receptor B1 to promote gene expression and apoptosis under 20-hydroxyecdysone regulation

The nuclear receptor EcRB1, which is activated by the insect steroid hormone 20-hydroxyecdysone (20E), is reportedly phosphorylated by a protein kinase after 20E induction. However, the protein kinase has not been identified, and the significance of EcRB1 phosphorylation is unclear. In this study, we identified a protein kinase C δ (PKCδ) isoform (the E isoform) that phosphorylates EcRB1 in the lepidopteran Helicoverpa armigera, a serious agricultural pest worldwide, to promote apoptotic gene expression and apoptosis during metamorphosis. Through activation of the EcRB1/USP1 transcription complex by 20E, PKCδ expression was up-regulated in several tissues during the metamorphic stage. Knockdown of PKCδ caused failure to transition from larvae to pupae, prevented tissues from undergoing programmed cell death (PCD), and down-regulated the expression of the transcription factor Brz-7 and the apoptosis executors caspase-3 and caspase-6 The threonine residue at position 1343 of PKCδ was phosphorylated and was critical for its proapoptotic function. Overexpression of the PKCδ catalytic domain was localized to the nuclei in HaEpi cells, which increased caspase-3 activity and apoptosis. PKCδ directly phosphorylated a threonine residue at position 468 in the amino acid sequence of EcRB1. The phosphorylation of EcRB1 was critical for its heterodimeric interaction with the USP1 protein and for binding to the ecdysone response element. The data suggested that 20E up-regulates PKCδ expression to regulate EcRB1 phosphorylation for EcRB1/USP1 transcription complex formation, apoptotic gene transcription, and apoptosis.

Insulin-like growth factor (IGF)-like peptide and 20-hydroxyecdysone regulate the growth and development of the male genital disk through different mechanisms in the silkmoth, Bombyx mori

It is well established that ecdysteroids play pivotal roles in the regulation of insect molting and metamorphosis. However, the mechanisms by which ecdysteroids regulate the growth and development of adult organs after pupation are poorly understood. Recently, we have identified insulin-like growth factor (IGF)-like peptides (IGFLPs), which are secreted after pupation under the control of 20-hydroxyecdysone (20E). In the silkmoth, Bombyx mori, massive amounts of Bombyx-IGFLP (BIGFLP) are present in the hemolymph during pupal-adult development, suggesting its importance in the regulation of adult tissue growth. Thus, we hypothesized that the growth and development of adult tissues including imaginal disks are regulated by the combined effects of BIGFLP and 20E. In this study, we investigated the growth-promoting effects of BIGFLP and 20E using the male genital disks of B. mori cultured ex vivo, and further analyzed the cell signaling pathways mediating hormone actions. We demonstrate that 20E induces the elongation of genital disks, that both hormones stimulate protein synthesis in an additive manner, and that BIGFLP and 20E exert their effects through the insulin/IGF signaling pathway and mitogen-activated protein kinase pathway, respectively. These results show that the growth and development of the genital disk are coordinately regulated by both BIGFLP and 20E.

A Genome-Wide Identification and Analysis of the Basic Helix-Loop-Helix Transcription Factors in Brown Planthopper, Nilaparvata lugens

The basic helix-loop-helix (bHLH) transcription factors in insects play essential roles in multiple developmental processes including neurogenesis, sterol metabolism, circadian rhythms, organogenesis and formation of olfactory sensory neurons. The identification and function analysis of bHLH family members of the most destructive insect pest of rice, Nilaparvata lugens, may provide novel tools for pest management. Here, a genome-wide survey for bHLH sequences identified 60 bHLH sequences (NlbHLHs) encoded in the draft genome of N. lugens. Phylogenetic analysis of the bHLH domains successfully classified these genes into 40 bHLH families in group A (25), B (14), C (10), D (1), E (8) and F (2). The number of NlbHLHs with introns is higher than many other insect species, and the average intron length is shorter than those of Acyrthosiphon pisum. High number of ortholog families of NlbHLHs was found suggesting functional conversation for these proteins. Compared to other insect species studied, N. lugens has the highest number of bHLH members. Furthermore, gene duplication events of SREBP, Kn(col), Tap, Delilah, Sim, Ato and Crp were found in N. lugens. In addition, a putative full set of NlbHLH genes is defined and compared with another insect species. Thus, our classification of these NlbHLH members provides a platform for further investigations of bHLH protein functions in the regulation of N. lugens, and of insects in general.