Smads and insect hemimetabolan metamorphosis

Egfr signaling promotes juvenile hormone biosynthesis in the German cockroach

BACKGROUND

In insects, an interplay between the activities of distinct hormones, such as juvenile hormone (JH) and 20-hydroxyecdysone (20E), regulates the progression through numerous life history hallmarks. As a crucial endocrine factor, JH is mainly synthesized in the corpora allata (CA) to regulate multiple physiological and developmental processes, including molting, metamorphosis, and reproduction. During the last century, significant progress has been achieved in elucidating the JH signal transduction pathway, while less progress has been made in dissecting the regulatory mechanism of JH biosynthesis. Previous work has shown that receptor tyrosine kinase (RTK) signaling regulates hormone biosynthesis in both insects and mammals. Here, we performed a systematic RNA interference (RNAi) screening to identify RTKs involved in regulating JH biosynthesis in the CA of adult Blattella germanica females.

RESULTS

We found that the epidermal growth factor receptor (Egfr) is required for promoting JH biosynthesis in the CA of adult females. The Egf ligands Vein and Spitz activate Egfr, followed by Ras/Raf/ERK signaling, and finally activation of the downstream transcription factor Pointed (Pnt). Importantly, Pnt induces the transcriptional expression of two key enzyme-encoding genes in the JH biosynthesis pathway: juvenile hormone acid methyltransferase (JHAMT) and methyl farnesoate epoxidase (CYP15A1). Dual-luciferase reporter assay shows that Pnt is able to activate a promoter region of Jhamt. In addition, electrophoretic mobility shift assay confirms that Pnt directly binds to the - 941~ - 886 nt region of the Jhamt promoter.

CONCLUSIONS

This study reveals the detailed molecular mechanism of Egfr signaling in promoting JH biosynthesis in the German cockroach, shedding light on the intricate regulation of JH biosynthesis during insect development.

Requirement of Myoglianin for metamorphosis in the beetle Henosepilachna vigintioctopunctata

Henosepilachna vigintioctopunctata is a serious defoliating beetle attacking Solanaceae and Cucurbitaceae plants in many Asian countries. In the present paper, we identified a putative myoglianin (myo) gene. Hvmyo was actively transcribed throughout development, from embryo to adult. RNA interference (RNAi)-aided knockdown of Hvmyo delayed larval development by more than 2 days, reduced larval body size, inhibited the growth of antennae, wings and legs and disturbed gut purge. Knockdown of Hvmyo impaired the larval-pupal transition. All the Hvmyo RNAi larvae arrested at the larval stage or formed misshapen pupae or adults. The deformed pupae and adults were partially wrapped with exuviae, bearing separated wings. Moreover, the expression levels of five ecdysteroidogenesis genes (Hvspo, Hvphm, Hvdib, Hvsad and Hvshd), a prothocicotropic hormone (PTTH)/Torso pathway gene (Hvtorso), two 20E receptor genes (HvEcR and HvUSP), and two 20E signalling genes (HvE93 and HvFTZ-F1) were as a result of HvMyo RNAi significantly lowered. Conversely, the expression of a JH biosynthesis gene (Hvjhamt), a JH receptor gene HvMet and a JH signalling gene HvKr-h1 was greatly enhanced. Although ingestion of 20E and Hal rescued the 20E signal, it could not alleviate larval performance and defective phenotypes. Our results suggest that Myo exerts four distinctive roles in ecdysteroidogenesis, JH production, organ growth and larva-pupa-adult transformation in H. vigintioctopunctata.

Smad on X is vital for larval-pupal transition in a herbivorous ladybird beetle

Insect development is regulated by a combination of juvenile hormone (JH) and 20-hydroxyecdysone (20E). Production of both JH and 20E is regulated by transforming growth factor-β (TGFβ) signaling. TGFβ can be classified into two branches, the Activin and Bone Morphogenetic Protein (BMP) pathways. In Drosophila melanogaster, BMP signaling is critical for JH synthesis, whereas Activin signal is required to generate the large pulse of 20E necessary for entering metamorphosis. However, to which extent the roles of these signals are conserved remains unknown. Here we studied the role of an Activin component Smad on X (Smox) in post-embryonic development in a defoliating ladybird Henosepilachna vigintioctopunctata. RNA interference (RNAi)-aided knockdown of Hvsmox inhibited larval growth, and impaired larval development. All Hvmyo RNAi larvae arrested at the fourth-instar larval stage. Moreover, knockdown of Hvsmox delayed gut and Malpighian tubules remodeling. Furthermore, the expression of a JH biosynthesis gene (Hvjhamt), a JH receptor gene HvMet and a JH response gene HvKr-h1 was greatly enhanced. Conversely, the expression levels of an ecdysteroidogenesis gene (Hvspo), a 20E receptor gene (HvEcR) and six 20E response genes (HvBrC, HvE74, HvE75, HvE93, HvHR3 and HvHR4) were significantly lowered. Knockdown of HvMet partially restored the negative phenotypes in the Hvsmox RNAi beetles. Our results suggest that Smox exerts regulative roles in JH production, ecdysteroidogenesis and organ remodeling, thus contributing to modulate the larva-pupa-adult transformation in H. vigintioctopunctata.

Mechanism of threshold size assessment: Metamorphosis is triggered by the TGF-beta/Activin ligand Myoglianin

Although the mechanisms that control growth are now well understood, the mechanism by which animals assess their body size remains one of the great puzzles in biology. The final larval instar of holometabolous insects, after which growth stops and metamorphosis begins, is specified by a threshold size. We investigated the mechanism of threshold size assessment in the tobacco hornworm, Manduca sexta. The threshold size was found to change depending on the amount of exposure to poor nutrient conditions whereas hypoxia treatment consistently led to a lower threshold size. Under these various conditions, the mass of the muscles plus integuments was correlated with the threshold size. Furthermore, the expression of myoglianin (myo) increased at the threshold size in both M. sexta and Tribolium castaneum. Knockdown of myo in T. castaneum led to larvae that underwent supernumerary larval molts and stayed in the larval stage permanently even after passing the threshold size. We propose that increasing levels of Myo produced by the growing tissues allow larvae to assess their body size and trigger metamorphosis at the threshold size.

RNAi-Mediated Knockdown of Transcription Factor E93 in Nymphs of the Desert Locust (Schistocerca gregaria) Inhibits Adult Morphogenesis and Results in Supernumerary Juvenile Stages

Postembryonic development of insects is coordinated by juvenile hormone (JH) together with ecdysteroids. Whereas the JH early response gene krüppel-homolog 1 (Kr-h1) plays a crucial role in the maintenance of juvenile characteristics during consecutive larval stages, the ecdysteroid-inducible early gene E93 appears to be a key factor promoting metamorphosis and adult morphogenesis. Here, we report on the developmental and molecular consequences of an RNAi-mediated knockdown of SgE93 in the desert locust, Schistocerca gregaria, a hemimetabolan species. Our experimental data show that injection of gregarious locust nymphs with a double-stranded RNA construct targeting the SgE93 transcript inhibited the process of metamorphosis and instead led to supernumerary nymphal stages. These supernumerary nymphal instars still displayed juvenile morphological features, such as a nymphal color scheme and body shape, while they reached the physical body size of the adult locusts, or even surpassed it after the next supernumerary molt. Interestingly, when compared to control locusts, the total duration of the fifth and normally final nymphal (N5) stage was shorter than normal. This appeared to correspond with temporal and quantitative changes in hemolymph ecdysteroid levels, as well as with altered expression of the rate-limiting Halloween gene, Spook (SgSpo). In addition, the levels of the ecdysone receptor (SgEcR) and retinoïd X receptor (SgRXR) transcripts were altered, indicating that silencing SgE93 affects both ecdysteroid synthesis and signaling. Upon knockdown of SgE93, a very potent upregulation of the SgKr-h1 transcript levels was observed in both head and fat body, while no significant changes were detected in the transcript levels of SgJHAMT and SgCYP15A1, the enzymes that catalyze the two final steps in JH biosynthesis. Moreover, the process of molting was disturbed in these supernumerary nymphs. While attempting ecdysis to the next stage, 50% of the N6 and all N7 nymphal instars eventually died. S. gregaria is a very harmful, swarm-forming pest species that destroys crops and threatens food security in many of the world's poorest countries. We believe that a better knowledge of the mechanisms of postembryonic development may contribute to the discovery of novel, more selective and sustainable strategies for controlling gregarious locust populations. In this context, identification of molecular target candidates that are capable of significantly reducing the fitness of this devastating swarming pest will be of crucial importance.

Modulators of hormonal response regulate temporal fate specification in the Drosophila brain

Neuronal diversity is at the core of the complex processing operated by the nervous system supporting fundamental functions such as sensory perception, motor control or memory formation. A small number of progenitors guarantee the production of this neuronal diversity, with each progenitor giving origin to different neuronal types over time. How a progenitor sequentially produces neurons of different fates and the impact of extrinsic signals conveying information about developmental progress or environmental conditions on this process represent key, but elusive questions. Each of the four progenitors of the Drosophila mushroom body (MB) sequentially gives rise to the MB neuron subtypes. The temporal fate determination pattern of MB neurons can be influenced by extrinsic cues, conveyed by the steroid hormone ecdysone. Here, we show that the activation of Transforming Growth Factor-β (TGF-β) signalling via glial-derived Myoglianin regulates the fate transition between the early-born α’β’ and the pioneer αβ MB neurons by promoting the expression of the ecdysone receptor B1 isoform (EcR-B1). While TGF-β signalling is required in MB neuronal progenitors to promote the expression of EcR-B1, ecdysone signalling acts postmitotically to consolidate theα’β’ MB fate. Indeed, we propose that if these signalling cascades are impaired α’β’ neurons lose their fate and convert to pioneer αβ. Conversely, an intrinsic signal conducted by the zinc finger transcription factor Krüppel-homolog 1 (Kr-h1) antagonises TGF-β signalling and acts as negative regulator of the response mediated by ecdysone in promoting α’β’ MB neuron fate consolidation. Taken together, the consolidation of α’β’ MB neuron fate requires the response of progenitors to local signalling to enable postmitotic neurons to sense a systemic signal.

Throughout the development of the central nervous system (CNS), a vast number of neuronal types are produced with striking precision. The unique identity of each neuronal cell type and the great cellular complexity in the CNS are established by intricate gene regulatory networks. Disruption of these identity programs leads to neurodevelopmental disorders and defects in cognition. Here, we report an important regulatory mechanism involved in consolidating neuronal fate. We show that during brain development local signalling, derived from interactions between glial cells and neuronal progenitors, is required to promote the expression of a hormone receptor in immature neurons. The perception of a systemic hormonal cue in those postmitotic neurons is fundamental for the consolidation of their neuronal fate. In this context, we additionally uncover an intrinsic regulatory mechanism that coordinates the hormone response to maintain the final neuronal fate.

Cell death during complete metamorphosis

In insects that undergo complete metamorphosis, cell death is essential for reshaping or removing larval tissues and organs, thus contributing to formation of the adult's body structure. In the last few decades, the study of metamorphosis in Lepidoptera and Diptera has provided broad information on the tissue remodelling processes that occur during larva-pupa-adult transition and made it possible to unravel the underlying regulatory pathways. This review summarizes recent knowledge on cell death mechanisms in Lepidoptera and other holometabolous insects, highlighting similarities and differences with Drosophila melanogaster, and discusses the role of apoptosis and autophagy in this developmental setting. This article is part of the theme issue 'The evolution of complete metamorphosis'.

TGFβ signaling related genes are involved in hormonal mediation during termite soldier differentiation

A working knowledge of the proximate factors intrinsic to sterile caste differentiation is necessary to understand the evolution of eusocial insects. Genomic and transcriptomic analyses in social hymenopteran insects have resulted in the hypothesis that sterile castes are generated by the novel function of co-opted or recruited universal gene networks found in solitary ancestors. However, transcriptome analysis during caste differentiation has not been tested in termites, and evolutionary processes associated with acquiring the caste are still unknown. Termites possess the soldier caste, which is regarded as the first acquired permanently sterile caste in the taxon. In this study, we performed a comparative transcriptome analysis in termite heads during 3 molting processes, i.e., worker, presoldier and soldier molts, under natural conditions in an incipient colony of the damp-wood termite Zootermopsis nevadensis. Although similar expression patterns were observed during each molting process, more than 50 genes were shown to be highly expressed before the presoldier (intermediate stage of soldier) molt. We then performed RNA interference (RNAi) of the candidate 13 genes, including transcription factors and uncharacterized protein genes, during presoldier differentiation induced by juvenile hormone (JH) analog treatment. Presoldiers induced after RNAi of two genes related to TGFβ (Transforming growth factor beta) signaling were extremely unusual and possessed soldier-like phenotypes. These individuals also displayed aggressive behaviors similar to natural soldiers when confronted with Formica ants as hypothetical enemies. These presoldiers never molted into the next instar, presumably due to the decreased expression levels of the molting hormone (20-hydroxyecdysone; 20E) signaling genes. These results suggest that TGFβ signaling was acquired for the novel function of regulating between JH and 20E signaling during soldier differentiation in termites.

Juvenile hormone signaling in short germ-band hemimetabolan embryos

The role of juvenile hormone (JH) in insect embryos is far from understood, especially in short germ-band hemimetabolan species. To shed light on this issue, we depleted the mRNA levels of Krüppel homolog 1, Methoprene-tolerant and JH acid O-methyltransferase, key elements of JH signaling, in embryos of the short germ-band hemimetabolan species Blattella germanica This precluded the formation of the germ-band anlage in a group of embryos. Hatchability was also reduced, which might have been caused by premature upregulation of laccase 2, a promoter of cuticle tanning. In other cases, development was interrupted in mid embryogenesis, involving defects related to dorsal closure and appendage formation. These phenotypes possibly result from the low levels of Broad-complex (BR-C) produced under JH-depleted conditions. This contrasts with holometabolan species, in which JH does not promote BR-C expression, which remains low during embryo development. Possibly, the stimulatory role of JH on BR-C expression and the morphogenetic functions of BR-C in hemimetabolan embryos were lost in holometabolan species. If so, this might have been a key driver for the evolution of holometabolan metamorphosis.

Regulatory Mechanisms of Metamorphic Neuronal Remodeling Revealed Through a Genome-Wide Modifier Screen in Drosophila melanogaster

During development, neuronal remodeling shapes neuronal connections to establish fully mature and functional nervous systems. Our previous studies have shown that the RNA-binding factor alan shepard (shep) is an important regulator of neuronal remodeling during metamorphosis in Drosophila melanogaster, and loss of shep leads to smaller soma size and fewer neurites in a stage-dependent manner. To shed light on the mechanisms by which shep regulates neuronal remodeling, we conducted a genetic modifier screen for suppressors of shep-dependent wing expansion defects and cellular morphological defects in a set of peptidergic neurons, the bursicon neurons, that promote posteclosion wing expansion. Out of 702 screened deficiencies that covered 86% of euchromatic genes, we isolated 24 deficiencies as candidate suppressors, and 12 of them at least partially suppressed morphological defects in shep mutant bursicon neurons. With RNA interference and mutant alleles of individual genes, we identified Daughters against dpp (Dad) and Olig family (Oli) as shep suppressor genes, and both of them restored the adult cellular morphology of shep-depleted bursicon neurons. Dad encodes an inhibitory Smad protein that inhibits bone morphogenetic protein (BMP) signaling, raising the possibility that shep interacted with BMP signaling through antagonism of Dad. By manipulating expression of the BMP receptor tkv, we found that activated BMP signaling was sufficient to rescue loss-of-shep phenotypes. These findings reveal mechanisms of shep regulation during neuronal development, and they highlight a novel genetic shep interaction with the BMP signaling pathway that controls morphogenesis in mature, terminally differentiated neurons during metamorphosis.