Neuroendocrine regulation of Drosophila metamorphosis requires TGFbeta/Activin signaling

Evolution of insect metamorphosis - an update

Metamorphosis endowed the insects with properties that enabled them to conquer the Earth. It is a hormonally controlled morphogenetic process that transforms the larva into the adult. Metamorphosis appeared with the origin of wings and flight. The sesquiterpenoid juvenile hormone (JH) suppresses wing morphogenesis and ensures that metamorphosis takes place at the right ontogenetic time. This review explores the origin of insect metamorphosis and the ancestral function of JH. Fossil record shows that the first Paleozoic winged insects had (hemimetabolous) metamorphosis, and their larvae were likely aquatic. In the primitive wingless silverfish that lacks metamorphosis, JH is essential for late embryogenesis and reproduction. JH production after the embryo dorsal closure promotes hatching and terminal tissue maturation.

Early attainment of 20-hydroxyecdysone threshold shapes mosquito sexual dimorphism in developmental timing

In holometabolous insects, critical weight (CW) attainment triggers pupation and metamorphosis, but its mechanism remains unclear in non-model organisms like mosquitoes. Here, we investigate the role of 20-hydroxyecdysone (20E) in CW assessment and pupation timing in Aedes albopictus and Ae. aegypti, vectors of arboviruses including dengue and Zika. Our results show that the attainment of CW is contingent upon surpassing a critical 20E threshold, which results in entrance into a constant 22 h interval and the subsequent 20E pulse responsible for larval-pupal ecdysis. Sexual dimorphism in pupation time arises from higher basal 20E levels in males, enabling earlier CW attainment. Administering 20E at 50% of L3/L4 molt, when most of males but not females pass the pulse, results in female-specific lethality. These findings highlight the pivotal role of 20E thresholds in CW, pupation timing, and sexual dimorphism, suggesting that manipulating 20E levels can skew populations male, offering a potential mosquito sex separation strategy.

Glycogen homeostasis and mitochondrial DNA expression require motor neuron to muscle TGF-β/Activin signaling in Drosophila

Maintaining metabolic homeostasis requires coordinated nutrient utilization between intracellular organelles and across multiple organ systems. Many organs rely heavily on mitochondria to generate (ATP) from glucose, or stored glycogen. Proteins required for ATP generation are encoded in both nuclear and mitochondrial DNA (mtDNA). We show that motoneuron to muscle signaling by the TGFβ/Activin family member Actβ positively regulates glycogen levels during Drosophila development. Remarkably, we find that levels of stored glycogen are unaffected by altering cytoplasmic glucose catabolism. Instead, loss of Actβ reduces levels of nuclearly encoded genes required for mtDNA replication, transcription, and translation and mtDNA levels. Direct RNAi knockdown of nuclearly encoded mtDNA expression factors in muscle also results in decreased glycogen stores. Lastly, expressing an activated form of the type I receptor Baboon in muscle restores both glycogen and mtDNA levels in actβ mutants, thereby confirming a direct link between Actβ signaling, glycogen homeostasis, and mtDNA expression factors.

Activin β Is Critical for Larval-Pupal Transition in the 28 Spotted Lady Beetle Henosepilachna vigintioctopunctata

The activin cascade is activated when a pair of extracellular ligand (Myoglianin, Myo; Activin β, Actβ; Dawdle, Daw) binds to two pairs of transforming growth factor β (TGF) serine-threonine receptor kinases, TGF-β type I (Baboon, Babo) and II receptors. However, the roles of activin way have not well been explored in non-Drosophilid insects. In the present paper, we compared the functions of Activin β (Actβ) ligand and receptor isoform BaboB in post-embryonic development in a defoliating ladybird Henosepilachna vigintioctopunctata. RNA interference (RNAi) for Hvactβ but not Hvbabob upregulated juvenile hormone signal, reduced ecdysone pathway and impaired larval-pupal transformation. The arresting Hvactβ RNAi larvae formed pupa-specific black markings below the larval exuviae. Thus, the impairment of metamorphosis may be caused by failing to complete ecdysis behavior due to nonfunctional muscles. Consistently, larval body sizes were smaller and adult appendages were shorter in the Hvactβ RNAi larvae, in contrast to those in the Hvbabob depleted beetles. Conversely, knockdown of Hvbabob but not Hvactβ changed the pigmentation of adult elytra. Our results suggest that Actβ exerts regulative roles in JH production, ecdysteroidogenesis and organ remodeling, thus contributing to modulate the larva-pupa-adult transformation, through a BaboB independent way in H. vigintioctopunctata.

A critical role for the nuclear protein Akirin in larval development in Henosepilachna vigintioctopunctata

Akirin is a nuclear protein that controls development in vertebrates and invertebrates. The function of Akirin has not been assessed in any Coleopteran insects. We found that high levels of akirin transcripts in Henosepilachna vigintioctopunctata, a serious Coleopteran potato defoliator (hereafter Hvakirin), were present at prepupal, pupal and adult stages, especially in larval foregut and fat body. RNA interference (RNAi) targeting Hvakirin impaired larval development. The Hvakirin RNAi larvae arrested development at the final larval instar stage. They remained as stunted larvae, gradually blackened and finally died. Moreover, the remodelling of gut and fat body was inhibited in the Hvakirin depleted larvae. Two layers of cuticles, old and newly formed, were noted in the dsegfp-injected animals. In contrast, only a layer of cuticle was found in the dsakirin-injected beetles, indicating the arrest of larval development. Furthermore, the expression of three transforming growth factor-β cascade genes (Hvsmox, Hvmyo and Hvbabo), a 20-hydroxyecdysone (20E) receptor gene (HvEcR) and six 20E response genes (HvHR3, HvHR4, HvE75, HvBrC, HvE93 and Hvftz-f1) was significantly repressed, consistent with decreased 20E signalling. Conversely, the transcription of a juvenile hormone (JH) biosynthesis gene (Hvjhamt), a JH receptor gene (HvMet) and two JH response genes (HvKr-h1 and HvHairy) was greatly enhanced. Our findings suggest a critical role of Akirin in larval development in H. vigintioctopunctata.

Developmental analysis of Spalt function in the Drosophila prothoracic gland

The Spalt transcriptional regulators participate in a variety of cell fate specification processes during development, regulating transcription through interactions with DNA AT-rich regions. Spalt proteins also bind to heterochromatic regions, and some of their effects require interactions with the NuRD chromatin remodeling and deacetylase complex. Most of the biological roles of Spalt proteins have been characterized in diploid cells engaged in cell proliferation. Here, we address the function of Drosophila Spalt genes in the development of a larval tissue formed by polyploid cells, the prothoracic gland, the cells of which undergo several rounds of DNA replication without mitosis during larval development. We show that prothoracic glands depleted of Spalt expression display severe changes in the size of the nucleolus, the morphology of the nuclear envelope and the disposition of the chromatin within the nucleus, leading to a failure in the synthesis of ecdysone. We propose that loss of ecdysone production in the prothoracic gland of Spalt mutants is primarily caused by defects in nuclear pore complex function that occur as a consequence of faulty interactions between heterochromatic regions and the nuclear envelope.

Glycogen homeostasis and mtDNA expression require motor neuron to muscle TGFβ/Activin Signaling in Drosophila

Maintaining metabolic homeostasis requires coordinated nutrient utilization between intracellular organelles and across multiple organ systems. Many organs rely heavily on mitochondria to generate (ATP) from glucose, or stored glycogen. Proteins required for ATP generation are encoded in both nuclear and mitochondrial DNA (mtDNA). We show that motoneuron to muscle signaling by the TGFβ/Activin family member Actβ positively regulates glycogen levels during Drosophila development. Remarkably, we find that levels of stored glycogen are unaffected by altering cytoplasmic glucose catabolism. Instead, Actβ loss reduces levels of mtDNA and nuclearly encoded genes required for mtDNA replication, transcription and translation. Direct RNAi mediated knockdown of these same nuclearly encoded mtDNA expression factors also results in decreased glycogen stores. Lastly, we find that expressing an activated form of the type I receptor Baboon in muscle restores both glycogen and mtDNA levels in actβ mutants, thereby confirming a direct link between Actβ signaling, glycogen homeostasis and mtDNA expression.

Multiple isoforms of the Activin-like receptor baboon differentially regulate proliferation and conversion behaviors of neuroblasts and neuroepithelial cells in the Drosophila larval brain

In Drosophila coordinated proliferation of two neural stem cells, neuroblasts (NB) and neuroepithelial (NE) cells, is pivotal for proper larval brain growth that ultimately determines the final size and performance of an adult brain. The larval brain growth displays two phases based on behaviors of NB and NEs: the first one in early larval stages, influenced by nutritional status and the second one in the last larval stage, promoted by ecdysone signaling after critical weight checkpoint. Mutations of the baboon (babo) gene that produces three isoforms (BaboA-C), all acting as type-I receptors of Activin-type transforming growth factor β (TGF-β) signaling, cause a small brain phenotype due to severely reduced proliferation of the neural stem cells. In this study we show that loss of babo function severely affects proliferation of NBs and NEs as well as conversion of NEs from both phases. By analyzing babo-null and newly generated isoform-specific mutants by CRISPR mutagenesis as well as isoform-specific RNAi knockdowns in a cell- and stage-specific manner, our data support differential contributions of the isoforms for these cellular events with BaboA playing the major role. Stage-specific expression of EcR-B1 in the brain is also regulated primarily by BaboA along with function of the other isoforms. Blocking EcR function in both neural stem cells results in a small brain phenotype that is more severe than baboA-knockdown alone. In summary, our study proposes that the Babo-mediated signaling promotes proper behaviors of the neural stem cells in both phases and achieves this by acting upstream of EcR-B1 expression in the second phase.

Timed receptor tyrosine kinase signaling couples the central and a peripheral circadian clock in Drosophila

Circadian clocks impose daily periodicities to behavior, physiology, and metabolism. This control is mediated by a central clock and by peripheral clocks, which are synchronized to provide the organism with a unified time through mechanisms that are not fully understood. Here, we characterized in Drosophila the cellular and molecular mechanisms involved in coupling the central clock and the peripheral clock located in the prothoracic gland (PG), which together control the circadian rhythm of emergence of adult flies. The time signal from central clock neurons is transmitted via small neuropeptide F (sNPF) to neurons that produce the neuropeptide Prothoracicotropic Hormone (PTTH), which is then translated into daily oscillations of Ca2+ concentration and PTTH levels. PTTH signaling is required at the end of metamorphosis and transmits time information to the PG through changes in the expression of the PTTH receptor tyrosine kinase (RTK), TORSO, and of ERK phosphorylation, a key component of PTTH transduction. In addition to PTTH, we demonstrate that signaling mediated by other RTKs contributes to the rhythmicity of emergence. Interestingly, the ligand to one of these receptors (Pvf2) plays an autocrine role in the PG, which may explain why both central brain and PG clocks are required for the circadian gating of emergence. Our findings show that the coupling between the central and the PG clock is unexpectedly complex and involves several RTKs that act in concert and could serve as a paradigm to understand how circadian clocks are coordinated.

Dpp-mediated TGF-β signaling regulates vitellogenesis through 20-hydroxyecdysone signaling in the cabbage beetle, Colaphellus bowringi

The Dpp signaling, as one of the branches within the TGF-β superfamily, plays a crucial role in regulating various biological processes in insects. However, its impact on female reproduction through vitellogenesis remains unclear. In this study, the expression profiles implied that the Dpp signaling genes, including Dpp, Punt, Mad, and Medea, were up-regulated during reproductive development in the ovary of Colaphellus bowringi. Knockdown of these five Dpp signaling genes revealed significant effects of Dpp, Tkv, Mad, and Medea on ovarian development through vitellogenesis in the fat body. Our finding further indicated that Dpp signaling influences the expression of 20-hydroxyecdysone (20E) receptor and responsive genes in the fat body. Additionally, knockdown of 20E receptor EcR resulted in similar phenotypes as observed in the Dpp pathway genes knockdown, implying a regulatory role for Dpp signaling via EcR in vitellogenesis. Furthermore, knocking down Dpp, Tkv, and EcR in female adults led to a reduction in total dry weight and protein content, as well as the expression of mTOR, a factor linked to protein intake. These results suggest that the Dpp signaling pathway modulates vitellogenesis by impacting the AA/TOR-mediated 20E pathway in the fat body, providing novel insights into the network governing insect reproduction and offering potential targets for controlling female pest reproduction.

TGFß/activin-dependent activation of Torso controls the timing of the metamorphic transition in the red flour beetle Tribolium castaneum

Understanding the mechanisms governing body size attainment during animal development is of paramount importance in biology. In insects, a crucial phase in determining body size occurs at the larva-pupa transition, marking the end of the larval growth period. Central to this process is the attainment of the threshold size (TS), a critical developmental checkpoint that must be reached before the larva can undergo metamorphosis. However, the intricate molecular mechanisms by which the TS orchestrates this transition remain poor understood. In this study, we investigate the role of the interaction between the Torso and TGFß/activin signaling pathways in regulating metamorphic timing in the red flour beetle, Tribolium castaneum. Our results show that Torso signaling is required specifically during the last larval instar and that its activation is mediated not only by the prothoracicotropic hormone (Tc-Ptth) but also by Trunk (Tc-Trk), another ligand of the Tc-Torso receptor. Interestingly, we show that while Tc-Torso activation by Tc-Ptth determines the onset of metamorphosis, Tc-Trk promotes growth during the last larval stage. In addition, we found that the expression of Tc-torso correlates with the attainment of the TS and the decay of juvenile hormone (JH) levels, at the onset of the last larval instar. Notably, our data reveal that activation of TGFß/activin signaling pathway at the TS is responsible for repressing the JH synthesis and inducing Tc-torso expression, initiating metamorphosis. Altogether, these findings shed light on the pivotal involvement of the Ptth/Trunk/Torso and TGFß/activin signaling pathways as critical regulatory components orchestrating the TS-driven metamorphic initiation, offering valuable insights into the mechanisms underlying body size determination in insects.

The Drosophila ecdysone receptor promotes or suppresses proliferation according to ligand level

The steroid hormone 20-hydroxy-ecdysone (20E) promotes proliferation in Drosophila wing precursors at low titer but triggers proliferation arrest at high doses. Remarkably, wing precursors proliferate normally in the complete absence of the 20E receptor, suggesting that low-level 20E promotes proliferation by overriding the default anti-proliferative activity of the receptor. By contrast, 20E needs its receptor to arrest proliferation. Dose-response RNA sequencing (RNA-seq) analysis of ex vivo cultured wing precursors identifies genes that are quantitatively activated by 20E across the physiological range, likely comprising positive modulators of proliferation and other genes that are only activated at high doses. We suggest that some of these "high-threshold" genes dominantly suppress the activity of the pro-proliferation genes. We then show mathematically and with synthetic reporters that combinations of basic regulatory elements can recapitulate the behavior of both types of target genes. Thus, a relatively simple genetic circuit can account for the bimodal activity of this hormone.

Deciphering Molecular Mechanisms Governing the Reproductive Molt of Macrobrachium nipponense: A Transcriptome Analysis of Ovaries across Various Molting Stages

The relationship between molting and reproduction has received more attention in economically important crustacean decapods. Molting and reproduction are synergistic events in Macrobrachium nipponense, but the molecular regulatory mechanisms behind them are unclear. In the current study, we performed Illumina sequencing for the ovaries of M. nipponense during the molt cycle (pre-molting, Prm; mid-molting, Mm; and post-molting, Pom). A total of 66.57 Gb of transcriptome data were generated through sequencing, resulting in the identification of 105,149 unigenes whose alignment ratio with the reference genome exceeded 87.57%. Differentially expressed genes (DEGs) were annotated through the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases for gene classification and pathway analysis. A total of twenty-six molt-related DEGs were found, and their expression patterns were examined across various molting stages. The KEGG enrichment analysis revealed that the key pathways involved in regulating the molting process of M. nipponense primarily include the mTOR, insect hormone biosynthesis, TGF-beta, and Wnt signaling pathways. Our transcriptomic data suggest that these pathways crosstalk with each other to regulate the synthesis and degradation of ecdysone throughout the molt cycle. The current study has deepened our understanding of the molecular mechanisms of crustacean molting and will serve as a basis for future studies of crustaceans and other molting animals.

Drosophila postembryonic nervous system development: a model for the endocrine control of development

During postembryonic life, hormones, including ecdysteroids, juvenile hormones, insulin-like peptides, and activin/TGFβ ligands act to transform the larval nervous system into an adult version, which is a fine-grained mosaic of recycled larval neurons and adult-specific neurons. Hormones provide both instructional signals that make cells competent to undergo developmental change and timing cues to evoke these changes across the nervous system. While touching on all the above hormones, our emphasis is on the ecdysteroids, ecdysone and 20-hydroxyecdysone (20E). These are the prime movers of insect molting and metamorphosis and are involved in all phases of nervous system development, including neurogenesis, pruning, arbor outgrowth, and cell death. Ecdysteroids appear as a series of steroid peaks that coordinate the larval molts and the different phases of metamorphosis. Each peak directs a stereotyped cascade of transcription factor expression. The cascade components then direct temporal programs of effector gene expression, but the latter vary markedly according to tissue and life stage. The neurons read the ecdysteroid titer through various isoforms of the ecdysone receptor, a nuclear hormone receptor. For example, at metamorphosis the pruning of larval neurons is mediated through the B isoforms, which have strong activation functions, whereas subsequent outgrowth is mediated through the A isoform through which ecdysteroids play a permissive role to allow local tissue interactions to direct outgrowth. The major circulating ecdysteroid can also change through development. During adult development ecdysone promotes early adult patterning and differentiation while its metabolite, 20E, later evokes terminal adult differentiation.

Mammalian puberty: a fly perspective

Mammalian puberty and Drosophila metamorphosis, despite their evolutionary distance, exhibit similar design principles and conservation of molecular components. In this Viewpoint, we review recent advances in this area and the similarities between both processes in terms of the signaling pathways and neuroendocrine circuits involved. We argue that the detection and uptake of peripheral fat by Drosophila prothoracic endocrine cells induces endomembrane remodeling and ribosomal maturation, leading to the acquisition of high biosynthetic and secretory capacity. The absence of this fat-neuroendocrine interorgan communication leads to giant, obese, non-pupating larvae. Importantly, human leptin is capable of signaling the pupariation process in Drosophila, and its expression prevents obesity and triggers maturation in mutants that do not pupate. This implies that insect metamorphosis can be used to address issues related to the biology of leptin and puberty.

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.

Su(var)2-10- and Su(var)205-dependent upregulation of the heterochromatic gene neverland is required for developmental transition in Drosophila

Animals develop from juveniles to sexually mature adults through the action of steroid hormones. In insect metamorphosis, a surge of the steroid hormone ecdysone prompts the transition from the larval to the adult stage. Ecdysone is synthesized by a series of biosynthetic enzymes that are specifically expressed in an endocrine organ, the prothoracic gland. At the late larval stage, the expression levels of ecdysone biosynthetic enzymes are upregulated through the action of numerous transcription factors, thus initiating metamorphosis. In contrast, the mechanism by which chromatin regulators support the expression of ecdysone biosynthetic genes is largely unknown. Here, we demonstrate that Su(var)2-10 and Su(var)205, suppressor of variegation [Su(var)] genes encoding a chromatin regulator Su(var)2-10 and nonhistone heterochromatic protein 1a, respectively, regulate the transcription of one of the heterochromatic ecdysone biosynthetic genes, neverland, in Drosophila melanogaster. Knockdown of Su(var)2-10 and Su(var)205 in the prothoracic gland caused a decrease in neverland expression, resulting in a defect in larval-to-prepupal transition. Furthermore, overexpression of neverland and administration of 7-dehydrocholesterol, a biosynthetic precursor of ecdysone produced by Neverland, rescued developmental defects in Su(var)2-10 and Su(var)205 knockdown animals. These results indicate that Su(var)2-10- and Su(var)205-mediated proper expression of neverland is required for the initiation of metamorphosis. Given that Su(var)2-10-positive puncta are juxtaposed with the pericentromeric heterochromatic region, we propose that Su(var)2-10- and Su(var)205-dependent regulation of inherent heterochromatin structure at the neverland gene locus is essential for its transcriptional activation.

The TGF-β Receptor Gene Saxophone Influences Larval-Pupal-Adult Development in Tribolium castaneum

The transforming growth factor-β (TGF-β) superfamily encodes a large group of proteins, including TGF-β isoforms, bone morphogenetic proteins and activins that act through conserved cell-surface receptors and signaling co-receptors. TGF-β signaling in insects controls physiological events, including growth, development, diapause, caste determination and metamorphosis. In this study, we used the red flour beetle, Tribolium castaneum, as a model species to investigate the role of the type I TGF-β receptor, saxophone (Sax), in mediating development. Developmental and tissue-specific expression profiles indicated Sax is constitutively expressed during development with lower expression in 19- and 20-day (6th instar) larvae. RNAi knockdown of Sax in 19-day larvae prolonged developmental duration from larvae to pupae and significantly decreased pupation and adult eclosion in a dose-dependent manner. At 50 ng dsSax/larva, Sax knockdown led to an 84.4% pupation rate and 46.3% adult emergence rate. At 100 ng and 200 ng dsSax/larva, pupation was down to 75.6% and 50%, respectively, with 0% adult emergence following treatments with both doses. These phenotypes were similar to those following knockdowns of 20-hydroxyecdysone (20E) receptor genes, ecdysone receptor (EcR) or ultraspiracle protein (USP). Expression of 20E biosynthesis genes disembodied and spookier, 20E receptor genes EcR and USP, and 20E downstream genes BrC and E75, were suppressed after the Sax knockdown. Topical application of 20E on larvae treated with dsSax partially rescued the dsSax-driven defects. We can infer that the TGF-β receptor gene Sax influences larval-pupal-adult development via 20E signaling in T. castaneum.

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.

The cytochrome P450 Cyp6t3 is not required for ecdysone biosynthesis in Drosophila melanogaster

The steroid hormone 20-hydroxyecdysone (20E) is essential for proper development and the timing of intermediary stage transitions in insects. As a result, there is intense interest in identifying and defining the roles of the enzymes and signaling pathways that regulate 20E production in the prothoracic gland (PG), the major endocrine organ of juvenile insect phases. Transcriptomics is one powerful tool that has been used to identify novel genes that are up- or down-regulated in the PG which may contribute to 20E regulation. Additional functional characterization of putative regulatory candidate genes typically involves qRT-PCR and/or RNAi mediated knockdown of the candidate mRNA in the PG to assess whether the gene's expression shows temporal regulation in the PG and whether its expression is essential for proper 20E production and the correct timing of developmental transitions. While these methods have proved fruitful for identifying novel regulators of 20E production, characterizing the null phenotype of putative regulatory genes is the gold standard for assigning gene function since RNAi is known to generate various types of "off target" effects. Here we describe the genetic null mutant phenotype of the Drosophila melanogaster Cyp6t3 gene . Cyp6t3 was originally identified as a differentially regulated gene in a PG microarray screen and assigned a place in the "Black Box" step of the E biosynthetic pathway based on RNAi mediated knockdown phenotypes and rescue experiments involving feeding of various intermediate compounds of the E biosynthetic pathway. In contrast, we find that Crispr generated null mutations in Cyp6t3 are viable and have normal developmental timing. Therefore, we conclude that Cyp6t3 is not required for E production under typical lab growth conditions and therefore is not an obligate enzymatic component of the Black Box.

Pea aphid winged and wingless males exhibit reproductive, gene expression, and lipid metabolism differences

Alternative, intraspecific phenotypes offer an opportunity to identify the mechanistic basis of differences associated with distinctive life history strategies. Wing dimorphic insects, in which both flight-capable and flight-incapable individuals occur in the same population, are particularly well-studied in terms of why and how the morphs trade off flight for reproduction. Yet despite a wealth of studies examining the differences between female morphs, little is known about male differences, which could arise from different causes than those acting on females. Here we examined reproductive, gene expression, and biochemical differences between pea aphid (Acyrthosiphon pisum) winged and wingless males. We find that winged males are competitively superior in one-on-one mating circumstances, but wingless males reach reproductive maturity faster and have larger testes. We suggest that males tradeoff increased local matings with concurrent possible inbreeding for outbreeding and increased ability to find mates. At the mechanistic level, differential gene expression between the morphs revealed a possible role for activin and insulin signaling in morph differences; it also highlighted genes not previously identified as being functionally important in wing polymorphism, such as genes likely involved in sperm production. Further, we find that winged males have higher lipid levels, consistent with their use as flight fuel, but we find no consistent patterns of different levels of activity among five enzymes associated with lipid biosynthesis. Overall, our analyses provide evidence that winged versus wingless males exhibit differences at the reproductive, gene expression, and biochemical levels, expanding the field's understanding of the functional aspects of morph differences.

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.

Transcriptional Regulators of Ecdysteroid Biosynthetic Enzymes and Their Roles in Insect Development

Steroid hormones are responsible for coordinating many aspects of biological processes in most multicellular organisms, including insects. Ecdysteroid, the principal insect steroid hormone, is biosynthesized from dietary cholesterol or plant sterols. In the last 20 years, a number of ecdysteroidogenic enzymes, including Noppera-bo, Neverland, Shroud, Spook/Spookier, Cyp6t3, Phantom, Disembodied, Shadow, and Shade, have been identified and characterized in molecular genetic studies using the fruit fly Drosophila melanogaster. These enzymes are encoded by genes collectively called the Halloween genes. The transcriptional regulatory network, governed by multiple regulators of transcription, chromatin remodeling, and endoreplication, has been shown to be essential for the spatiotemporal expression control of Halloween genes in D. melanogaster. In this review, we summarize the latest information on transcriptional regulators that are crucial for controlling the expression of ecdysteroid biosynthetic enzymes and their roles in insect development.

Body-fat sensor triggers ribosome maturation in the steroidogenic gland to initiate sexual maturation in Drosophila

Fat stores are critical for reproductive success and may govern maturation initiation. Here, we report that signaling and sensing fat sufficiency for sexual maturation commitment requires the lipid carrier apolipophorin in fat cells and Sema1a in the neuroendocrine prothoracic gland (PG). Larvae lacking apolpp or Sema1a fail to initiate maturation despite accruing sufficient fat stores, and they continue gaining weight until death. Mechanistically, sensing peripheral body-fat levels via the apolipophorin/Sema1a axis regulates endocytosis, endoplasmic reticulum remodeling, and ribosomal maturation for the acquisition of the PG cells' high biosynthetic and secretory capacity. Downstream of apolipophorin/Sema1a, leptin-like upd2 triggers the cessation of feeding and initiates sexual maturation. Human Leptin in the insect PG substitutes for upd2, preventing obesity and triggering maturation downstream of Sema1a. These data show how peripheral fat levels regulate the control of the maturation decision-making process via remodeling of endomembranes and ribosomal biogenesis in gland cells.

Activation of EGFR signaling by Tc-Vein and Tc-Spitz regulates the metamorphic transition in the red flour beetle Tribolium castaneum

Animal development relies on a sequence of specific stages that allow the formation of adult structures with a determined size. In general, juvenile stages are dedicated mainly to growth, whereas last stages are devoted predominantly to the maturation of adult structures. In holometabolous insects, metamorphosis marks the end of the growth period as the animals stops feeding and initiate the final differentiation of the tissues. This transition is controlled by the steroid hormone ecdysone produced in the prothoracic gland. In Drosophila melanogaster different signals have been shown to regulate the production of ecdysone, such as PTTH/Torso, TGFß and Egfr signaling. However, to which extent the roles of these signals are conserved remains unknown. Here, we study the role of Egfr signaling in post-embryonic development of the basal holometabolous beetle Tribolium castaneum. We show that Tc-Egfr and Tc-pointed are required to induced a proper larval-pupal transition through the control of the expression of ecdysone biosynthetic genes. Furthermore, we identified an additional Tc-Egfr ligand in the Tribolium genome, the neuregulin-like protein Tc-Vein (Tc-Vn), which contributes to induce larval-pupal transition together with Tc-Spitz (Tc-Spi). Interestingly, we found that in addition to the redundant role in the control of pupa formation, each ligand possesses different functions in organ morphogenesis. Whereas Tc-Spi acts as the main ligand in urogomphi and gin traps, Tc-Vn is required in wings and elytra. Altogether, our findings show that in Tribolium, post-embryonic Tc-Egfr signaling activation depends on the presence of two ligands and that its role in metamorphic transition is conserved in holometabolous insects.

Stop codon readthrough alters the activity of a POU/Oct transcription factor during Drosophila development

Background

A number of cellular processes have evolved in metazoans that increase the proteome repertoire in relation to the genome, such as alternative splicing and translation recoding. Another such process, translational stop codon readthrough (SCR), generates C-terminally extended protein isoforms in many eukaryotes, including yeast, plants, insects, and humans. While comparative genome analyses have predicted the existence of programmed SCR in many species including humans, experimental proof of its functional consequences are scarce.

Results

We show that SCR of the Drosophila POU/Oct transcription factor Ventral veins lacking/Drifter (Vvl/Dfr) mRNA is prevalent in certain tissues in vivo, reaching a rate of 50% in the larval prothoracic gland. Phylogenetically, the C-terminal extension is conserved and harbors intrinsically disordered regions and amino acid stretches implied in transcriptional activation. Elimination of Vvl/Dfr translational readthrough by CRISPR/Cas9 mutagenesis changed the expression of a large number of downstream genes involved in processes such as chromatin regulation, neurogenesis, development, and immune response. As a proof-of-principle, we demonstrate that the C-terminal extension of Vvl/Dfr is necessary for correct timing of pupariation, by increasing the capacity to regulate its target genes. The extended Vvl/Dfr isoform acts in synergy with the transcription factor Molting defective (Mld) to increase the expression and biosynthesis of the steroid hormone ecdysone, thereby advancing pupariation. Consequently, late-stage larval development was prolonged and metamorphosis delayed in vvl/dfr readthrough mutants.

Conclusions

We demonstrate that translational recoding of a POU/Oct transcription factor takes place in a highly tissue-specific and temporally controlled manner. This dynamic and regulated recoding is necessary for normal expression of a large number of genes involved in many cellular and developmental processes. Loss of Vvl/Dfr translational readthrough negatively affects steroid hormone biosynthesis and delays larval development and progression into metamorphosis. Thus, this study demonstrates how SCR of a transcription factor can act as a developmental switch in a spatiotemporal manner, feeding into the timing of developmental transitions between different life-cycle stages.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01106-0.

Coordination among multiple receptor tyrosine kinase signals controls Drosophila developmental timing and body size

In holometabolous insects, metamorphic timing and body size are controlled by a neuroendocrine axis composed of the ecdysone-producing prothoracic gland (PG) and its presynaptic neurons (PGNs) producing PTTH. Although PTTH/Torso signaling is considered the primary mediator of metamorphic timing, recent studies indicate that other unidentified PGN-derived factors also affect timing. Here, we demonstrate that the receptor tyrosine kinases anaplastic lymphoma kinase (Alk) and PDGF and VEGF receptor-related (Pvr), function in coordination with PTTH/Torso signaling to regulate pupariation timing and body size. Both Alk and Pvr trigger Ras/Erk signaling in the PG to upregulate expression of ecdysone biosynthetic enzymes, while Alk also suppresses autophagy by activating phosphatidylinositol 3-kinase (PI3K)/Akt. The Alk ligand Jelly belly (Jeb) is produced by the PGNs and serves as a second PGN-derived tropic factor, while Pvr activation mainly relies on autocrine signaling by PG-derived Pvf2 and Pvf3. These findings illustrate that a combination of juxtacrine and autocrine signaling regulates metamorphic timing, the defining event of holometabolous development.

Signaling Pathways That Regulate the Crustacean Molting Gland

A pair of Y-organs (YOs) are the molting glands of decapod crustaceans. They synthesize and secrete steroid molting hormones (ecdysteroids) and their activity is controlled by external and internal signals. The YO transitions through four physiological states over the molt cycle, which are mediated by molt-inhibiting hormone (MIH; basal state), mechanistic Target of Rapamycin Complex 1 (mTORC1; activated state), Transforming Growth Factor-β (TGFβ)/Activin (committed state), and ecdysteroid (repressed state) signaling pathways. MIH, produced in the eyestalk X-organ/sinus gland complex, inhibits the synthesis of ecdysteroids. A model for MIH signaling is organized into a cAMP/Ca2+-dependent triggering phase and a nitric oxide/cGMP-dependent summation phase, which maintains the YO in the basal state during intermolt. A reduction in MIH release triggers YO activation, which requires mTORC1-dependent protein synthesis, followed by mTORC1-dependent gene expression. TGFβ/Activin signaling is required for YO commitment in mid-premolt. The YO transcriptome has 878 unique contigs assigned to 23 KEGG signaling pathways, 478 of which are differentially expressed over the molt cycle. Ninety-nine contigs encode G protein-coupled receptors (GPCRs), 65 of which bind a variety of neuropeptides and biogenic amines. Among these are putative receptors for MIH/crustacean hyperglycemic hormone neuropeptides, corazonin, relaxin, serotonin, octopamine, dopamine, allatostatins, Bursicon, ecdysis-triggering hormone (ETH), CCHamide, FMRFamide, and proctolin. Contigs encoding receptor tyrosine kinase insulin-like receptor, epidermal growth factor (EGF) receptor, and fibroblast growth factor (FGF) receptor and ligands EGF and FGF suggest that the YO is positively regulated by insulin-like peptides and growth factors. Future research should focus on the interactions of signaling pathways that integrate physiological status with environmental cues for molt control.

Function and Evolution of Nuclear Receptors in Environmental-Dependent Postembryonic Development

Nuclear receptors (NRs) fulfill key roles in the coordination of postembryonal developmental transitions in animal species. They control the metamorphosis and sexual maturation in virtually all animals and by that the two main environmental-dependent developmental decision points. Sexual maturation and metamorphosis are controlled by steroid receptors and thyroid receptors, respectively in vertebrates, while both processes are orchestrated by the ecdysone receptor (EcR) in insects. The regulation of these processes depends on environmental factors like nutrition, temperature, or photoperiods and by that NRs form evolutionary conserved mediators of phenotypic plasticity. While the mechanism of action for metamorphosis and sexual maturation are well studied in model organisms, the evolution of these systems is not entirely understood and requires further investigation. We here review the current knowledge of NR involvement in metamorphosis and sexual maturation across the animal tree of life with special attention to environmental integration and evolution of the signaling mechanism. Furthermore, we compare commonalities and differences of the different signaling systems. Finally, we identify key gaps in our knowledge of NR evolution, which, if sufficiently investigated, would lead to an importantly improved understanding of the evolution of complex signaling systems, the evolution of life history decision points, and, ultimately, speciation events in the metazoan kingdom.

Regulation of ecdysone production in Drosophila by neuropeptides and peptide hormones

In both mammals and insects, steroid hormones play a major role in directing the animal's progression through developmental stages. To maximize fitness outcomes, steroid hormone production is regulated by the environmental conditions experienced by the animal. In insects, the steroid hormone ecdysone mediates transitions between developmental stages and is regulated in response to environmental factors such as nutrition. These environmental signals are communicated to the ecdysone-producing gland via the action of neuropeptide and peptide hormone signalling pathways. While some of these pathways have been well characterized, there is evidence to suggest more signalling pathways than has previously been thought function to control ecdysone production, potentially in response to a greater range of environmental conditions. Here, we review the neuropeptide and peptide hormone signalling pathways known to regulate the production of ecdysone in the model genetic insect Drosophila melanogaster, as well as what is known regarding the environmental signals that trigger these pathways. Areas for future research are highlighted that can further contribute to our overall understanding of the complex orchestration of environmental, physiological and developmental cues that together produce a functioning adult organism.

Control of the insect metamorphic transition by ecdysteroid production and secretion

Ecdysteroids are a class of steroid hormones that controls molting and metamorphic transitions in Ecdysozoan species including insects, in which ecdysteroid biosynthesis and its regulation have been extensively studied. Insect ecdysteroids are produced from dietary sterols by a series of reduction-oxidation reactions in the prothoracic gland and in Drosophila they are released into the hemolymph via vesicle-mediated secretion at the time of metamorphosis. To initiate precisely controlled ecdysteroid pulses, the prothoracic gland functions as a central node integrating both intrinsic and extrinsic signals to control ecdysteroid biosynthesis and secretion. In this review, we outline recent progress in the characterization of ecdysone biosynthesis and steroid trafficking pathways and the discoveries of novel factors regulating prothoracic gland function.

Steroid hormones, dietary nutrients, and temporal progression of neurogenesis

Temporal patterning of neural progenitors, in which different factors are sequentially expressed, is an evolutionarily conserved strategy for generating neuronal diversity during development. In the Drosophila embryo, mechanisms that mediate temporal patterning of neural stem cells (neuroblasts) are largely cell-intrinsic. However, after embryogenesis, neuroblast temporal patterning relies on extrinsic cues as well, as freshly hatched larvae seek out nutrients and other key resources in varying natural environments. We recap current understanding of neuroblast-intrinsic temporal programs and discuss how neuroblast extrinsic cues integrate and coordinate with neuroblast intrinsic programs to control numbers and types of neurons produced. One key emerging extrinsic factor that impacts temporal patterning of neuroblasts and their daughters as well as termination of neurogenesis is the steroid hormone, ecdysone, a known regulator of large-scale developmental transitions in insects and arthropods. Lastly, we consider evolutionary conservation and discuss recent work on thyroid hormone signaling in early vertebrate brain development.

Interorgan communication in the control of metamorphosis

Steroid hormones control major developmental transitions such as metamorphosis in insects and puberty in mammals. The juvenile must attain a sufficient size before it begins maturation in order to give rise to a properly sized and reproductively fit adult. Studies in the insect Drosophila have begun to reveal a remarkable example of the complex interplay between different organs and the neuroendocrine system that controls the production of the steroid ecdysone, which triggers metamorphosis. This review discusses the inter-organ signals mediating this crosstalk, which allows the neuroendocrine system to assess nutrient availability and growth status of internal organs, ensuring that maturation is initiated at the appropriate time. We discuss how the neuroendocrine system integrates signals from different tissues to coordinate growth and maturation. These studies are still unraveling the organ-to-organ signaling networks that control the timing of metamorphosis, defining important principles underlying the logic of growth and maturation coordination in animals.

Programmed Cell Death 2-Like (Pdcd2l) Is Required for Mouse Embryonic Development

Globozoospermia is a rare form of male infertility where men produce round-headed sperm that are incapable of fertilizing an oocyte naturally. In a previous study where we undertook a whole exome screen to define novel genetic causes of globozoospermia, we identified homozygous mutations in the gene PDCD2L. Two brothers carried a p.(Leu225Val) variant predicted to introduce a novel splice donor site, thus presenting PDCD2L as a potential regulator of male fertility. In this study, we generated a Pdcd2l knockout mouse to test its role in male fertility. Contrary to the phenotype predicted from its testis-enriched expression pattern, Pdcd2l null mice died during embryogenesis. Specifically, we identified that Pdcd2l is essential for post-implantation embryonic development. Pdcd2l^−/− embryos were resorbed at embryonic days 12.5-17.5 and no knockout pups were born, while adult heterozygous Pdcd2l males had comparable fertility to wildtype males. To specifically investigate the role of PDCD2L in germ cells, we employed Drosophila melanogaster as a model system. Consistent with the mouse data, global knockdown of trus, the fly ortholog of PDCD2L, resulted in lethality in flies at the third instar larval stage. However, germ cell-specific knockdown with two germ cell drivers did not affect male fertility. Collectively, these data suggest that PDCD2L is not essential for male fertility. By contrast, our results demonstrate an evolutionarily conserved role of PDCD2L in development.

Effect of ethylparaben on the growth and development of Drosophila melanogaster on preadult

Parabens are esters of p-hydroxybenzoic acid, including methylparaben (MP), ethylparaben (EP), propylparaben (PP), and the like. This substance has estrogenic and antiandrogenic effects, and a putative role in promoting cancer through endocrine disruption. By exposing Drosophila melanogaster to different concentrations of EP (300 mg/L, 700 mg/L, and 1000 mg/L), we investigated the effect of EP on the growth and development of D. melanogaster before emergence. We found that EP prolonged the development cycle of D. melanogaster, and changed the relative expression levels of Met, Gce, EcR, Kr-h1, and Br. In addition, EP reduced the titer of juvenile hormone Ⅲ (JH Ⅲ) and 20-hydroxyecdysone (20E), and delayed the peak of hormone secretion. This study provided a more objective and thorough assessment of safety for the parabens.

Metabolism and growth adaptation to environmental conditions in Drosophila

Organisms adapt to changing environments by adjusting their development, metabolism, and behavior to improve their chances of survival and reproduction. To achieve such flexibility, organisms must be able to sense and respond to changes in external environmental conditions and their internal state. Metabolic adaptation in response to altered nutrient availability is key to maintaining energy homeostasis and sustaining developmental growth. Furthermore, environmental variables exert major influences on growth and final adult body size in animals. This developmental plasticity depends on adaptive responses to internal state and external cues that are essential for developmental processes. Genetic studies have shown that the fruit fly Drosophila, similarly to mammals, regulates its metabolism, growth, and behavior in response to the environment through several key hormones including insulin, peptides with glucagon-like function, and steroid hormones. Here we review emerging evidence showing that various environmental cues and internal conditions are sensed in different organs that, via inter-organ communication, relay information to neuroendocrine centers that control insulin and steroid signaling. This review focuses on endocrine regulation of development, metabolism, and behavior in Drosophila, highlighting recent advances in the role of the neuroendocrine system as a signaling hub that integrates environmental inputs and drives adaptive responses.

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.

Regulation of Body Size and Growth Control

The control of body and organ growth is essential for the development of adults with proper size and proportions, which is important for survival and reproduction. In animals, adult body size is determined by the rate and duration of juvenile growth, which are influenced by the environment. In nutrient-scarce environments in which more time is needed for growth, the juvenile growth period can be extended by delaying maturation, whereas juvenile development is rapidly completed in nutrient-rich conditions. This flexibility requires the integration of environmental cues with developmental signals that govern internal checkpoints to ensure that maturation does not begin until sufficient tissue growth has occurred to reach a proper adult size. The Target of Rapamycin (TOR) pathway is the primary cell-autonomous nutrient sensor, while circulating hormones such as steroids and insulin-like growth factors are the main systemic regulators of growth and maturation in animals. We discuss recent findings in Drosophila melanogaster showing that cell-autonomous environment and growth-sensing mechanisms, involving TOR and other growth-regulatory pathways, that converge on insulin and steroid relay centers are responsible for adjusting systemic growth, and development, in response to external and internal conditions. In addition to this, proper organ growth is also monitored and coordinated with whole-body growth and the timing of maturation through modulation of steroid signaling. This coordination involves interorgan communication mediated by Drosophila insulin-like peptide 8 in response to tissue growth status. Together, these multiple nutritional and developmental cues feed into neuroendocrine hubs controlling insulin and steroid signaling, serving as checkpoints at which developmental progression toward maturation can be delayed. This review focuses on these mechanisms by which external and internal conditions can modulate developmental growth and ensure proper adult body size, and highlights the conserved architecture of this system, which has made Drosophila a prime model for understanding the coordination of growth and maturation in animals.

Two-factor specification of apoptosis: TGF-β signaling acts cooperatively with ecdysone signaling to induce cell- and stage-specific apoptosis of larval neurons during metamorphosis in Drosophila melanogaster

Developmentally regulated programmed cell death (PCD) is one of the key cellular events for precise controlling of neuronal population during postembryonic development of the central nervous system. Previously we have shown that a group of corazonin-producing peptidergic neurons (vCrz) undergo apoptosis in response to ecdysone signaling via ecdysone receptor (EcR)-B isoforms and Ultraspiracle during early phase of metamorphosis. Further utilizing genetic, transgenic, and mosaic analyses, we have found that TGF-β signaling mediated by a glia-produced ligand, Myoglianin, type-I receptor Baboon (particularly Babo-A isoform) and dSmad2, is also required autonomously for PCD of the vCrz neurons. Our studies show that TGF-β signaling is not acting epistatically to EcR or vice versa. We also show that ectopic expression of a constitutively active phosphomimetic form of dSmad2 (dSmad2^PM) is capable of inducing premature death of vCrz neurons in larva but not other larval neurons. Intriguingly, the dSmad2^PM-mediated killing is completely suppressed by coexpression of a dominant-negative form of EcR (EcR^DN), suggesting that EcR function is required for the proapoptotic dSmad2^PM function. Based on these data, we suggest that TGF-β and ecdysone signaling pathways act cooperatively to induce vCrz neuronal PCD. We propose that this type of two-factor authentication is a key developmental strategy to ensure the timely PCD of specific larval neurons during metamorphosis.

Poly(A) Binding Protein Is Required for Nuclear Localization of the Ecdysteroidogenic Transcription Factor Molting Defective in the Prothoracic Gland of Drosophila melanogaster

Steroid hormone signaling contributes to the development of multicellular organisms. In insects, ecdysteroids, like ecdysone and the more biologically-active derivative 20-hydroxyecdysone (20E), promote molting and metamorphosis. Ecdysone is biosynthesized in the prothoracic gland (PG), via several steps catalyzed by ecdysteroidogenic enzymes that are encoded by Halloween genes. The spatio-temporal expression pattern of ecdysteroidogenic genes is strictly controlled, resulting in a proper fluctuation of the 20E titer during insect development. However, their transcriptional regulatory mechanism is still elusive. A previous study has found that the polyadenylated tail [poly(A)] deadenylation complex, called Carbon catabolite repressor 4-Negative on TATA (CCR4-NOT) regulates the expression of spookier (spok), which encodes one of the ecdysteroidogenic enzymes in the fruit fly Drosophila melanogaster. Based on this finding, we speculated whether any other poly(A)-related protein also regulates spok expression. In this study, we reported that poly(A) binding protein (Pabp) is involved in spok expression by regulating nuclear localization of the transcription factor molting defective (Mld). When pabp was knocked down specifically in the PG by transgenic RNAi, both spok mRNA and Spok protein levels were significantly reduced. In addition, the spok promoter-driven green fluorescence protein (GFP) signal was also reduced in the pabp-RNAi PG, suggesting that Pabp is involved in the transcriptional regulation of spok. We next examined which transcription factors are responsible for Pabp-dependent transcriptional regulation. Among the transcription factors acting in the PG, we primarily focused on the zinc-finger transcription factor Mld, as Mld is essential for spok transcription. Mld was localized in the nucleus of the control PG cells, while Mld abnormally accumulated in the cytoplasm of pabp-RNAi PG cells. In contrast, pabp-RNAi did not affect the nuclear localization of other transcription factors, including ventral vein lacking (Vvl) and POU domain motif 3 (Pdm3), in PG cells. From these results, we propose that Pabp regulates subcellular localization in the PG, specifically of the transcription factor Mld, in the context of ecdysone biosynthesis.

Muscle-derived Myoglianin regulates Drosophila imaginal disc growth

Organ growth and size are finely tuned by intrinsic and extrinsic signaling molecules. In Drosophila, the BMP family member Dpp is produced in a limited set of imaginal disc cells and functions as a classic morphogen to regulate pattern and growth by diffusing throughout imaginal discs. However, the role of TGFβ/Activin-like ligands in disc growth control remains ill-defined. Here, we demonstrate that Myoglianin (Myo), an Activin family member, and a close homolog of mammalian Myostatin (Mstn), is a muscle-derived extrinsic factor that uses canonical dSmad2-mediated signaling to regulate wing size. We propose that Myo is a myokine that helps mediate an allometric relationship between muscles and their associated appendages.

Developmental arrest of Drosophila larvae elicits presynaptic depression and enables prolonged studies of neurodegeneration

Synapses exhibit an astonishing degree of adaptive plasticity in healthy and disease states. We have investigated whether synapses also adjust to life stages imposed by novel developmental programs for which they were never molded by evolution. Under conditions in which Drosophila larvae are terminally arrested, we have characterized synaptic growth, structure and function at the neuromuscular junction (NMJ). Although wild-type larvae transition to pupae after 5 days, arrested third instar (ATI) larvae persist for 35 days, during which time NMJs exhibit extensive overgrowth in muscle size, presynaptic release sites and postsynaptic glutamate receptors. Remarkably, despite this exuberant growth, stable neurotransmission is maintained throughout the ATI lifespan through a potent homeostatic reduction in presynaptic neurotransmitter release. Arrest of the larval stage in stathmin mutants also reveals a degree of progressive instability and neurodegeneration that was not apparent during the typical larval period. Hence, an adaptive form of presynaptic depression stabilizes neurotransmission during an extended developmental period of unconstrained synaptic growth. More generally, the ATI manipulation provides a powerful system for studying neurodegeneration and plasticity across prolonged developmental timescales.

20-Hydroxyecdysone receptor-activated Bombyx mori CCAAT/enhancer-binding protein gamma regulates the expression of BmCBP and subsequent histone H3 lysine 27 acetylation in Bo. mori

Cyclic adenosine monophosphate (cAMP) response element binding protein (CREB)-binding protein (CBP or CREBBP) plays important roles in regulating gene transcription and animal development. However, the process by which CBP is up-regulated to impact insect development is unknown. In this study, the regulatory mechanism of Bombyx mori CBP (BmCBP) expression induced by 20-hydroxyecdysone (20E) was investigated. In the Bo. mori cell line, DZNU-Bm-12, 20E enhanced BmCBP transcription and histone H3K27 acetylation. BmCBP RNA interference (RNAi) resulted in decreased histone H3K27 acetylation. Additionally, the luciferase activity analysis revealed that the transcription factor, Bo. mori CCAAT/enhancer-binding protein gamma (BmC/EBPg), activated BmCBP transcription, which was suppressed by BmC/EBPg RNAi and promoted by BmC/EBPg overexpression. Electrophoretic mobility shift assay and chromatin immunoprecipitation results demonstrated that BmC/EBPg could bind to the C/EBP cis-regulatory elements in two positions of the BmCBP promoter. Moreover, BmC/EBPg transcription was enhanced by the 20E receptor (BmEcR), which bound to the BmC/EBPg promoter. BmEcR RNAi significantly inhibited the transcriptional levels of BmC/EBPg and BmCBP in the presence of 20E. Furthermore, the BmEcR-BmC/EBPg pathway regulated the acetylation levels of histone H3K27. Altogether, these results indicate that BmEcR enhances the expression of BmC/EBPg, which binds to the BmCBP promoter, activates BmCBP expression and leads to histone H3K27 acetylation.

Snail synchronizes endocycling in a TOR-dependent manner to coordinate entry and escape from endoreplication pausing during the Drosophila critical weight checkpoint

The final body size of any given individual underlies both genetic and environmental constraints. Both mammals and insects use target of rapamycin (TOR) and insulin signaling pathways to coordinate growth with nutrition. In holometabolous insects, the growth period is terminated through a cascade of peptide and steroid hormones that end larval feeding behavior and trigger metamorphosis, a nonfeeding stage during which the larval body plan is remodeled to produce an adult. This irreversible decision, termed the critical weight (CW) checkpoint, ensures that larvae have acquired sufficient nutrients to complete and survive development to adulthood. How insects assess body size via the CW checkpoint is still poorly understood on the molecular level. We show here that the Drosophila transcription factor Snail plays a key role in this process. Before and during the CW checkpoint, snail is highly expressed in the larval prothoracic gland (PG), an endocrine tissue undergoing endoreplication and primarily dedicated to the production of the steroid hormone ecdysone. We observed two Snail peaks in the PG, one before and one after the molt from the second to the third instar. Remarkably, these Snail peaks coincide with two peaks of PG cells entering S phase and a slowing of DNA synthesis between the peaks. Interestingly, the second Snail peak occurs at the exit of the CW checkpoint. Snail levels then decline continuously, and endoreplication becomes nonsynchronized in the PG after the CW checkpoint. This suggests that the synchronization of PG cells into S phase via Snail represents the mechanistic link used to terminate the CW checkpoint. Indeed, PG-specific loss of snail function prior to the CW checkpoint causes larval arrest due to a cessation of endoreplication in PG cells, whereas impairing snail after the CW checkpoint no longer affected endoreplication and further development. During the CW window, starvation or loss of TOR signaling disrupted the formation of Snail peaks and endocycle synchronization, whereas later starvation had no effect on snail expression. Taken together, our data demonstrate that insects use the TOR pathway to assess nutrient status during larval development to regulate Snail in ecdysone-producing cells as an effector protein to coordinate endoreplication and CW attainment.

During Drosophila development, the time window when larvae assess their readiness for metamorphosis is marked by slowing of cell growth in the prothoracic gland that produces the molting hormone; cell growth (via DNA endoreplication) then increases, allowing the production of the amount of hormone required to trigger metamorphosis. This study shows that these processes depend on the transcription factor Snail.

Growth and Maturation in Development: A Fly's Perspective

In mammals like humans, adult fitness is improved due to resource allocation, investing energy in the developmental growth process during the juvenile period, and in reproduction at the adult stage. Therefore, the attainment of their target body height/size co-occurs with the acquisition of maturation, implying a need for coordination between mechanisms that regulate organismal growth and maturation timing. Insects like Drosophila melanogaster also define their adult body size by the end of the juvenile larval period. Recent studies in the fly have shown evolutionary conservation of the regulatory pathways controlling growth and maturation, suggesting the existence of common coordinator mechanisms between them. In this review, we will present an overview of the significant advancements in the coordination mechanisms ensuring developmental robustness in Drosophila. We will include (i) the characterization of feedback mechanisms between maturation and growth hormones, (ii) the recognition of a relaxin-like peptide Dilp8 as a central processor coordinating juvenile regeneration and time of maturation, and (iii) the identification of a novel coordinator mechanism involving the AstA/KISS system.

TGF-β Prodomain Alignments Reveal Unexpected Cysteine Conservation Consistent with Phylogenetic Predictions of Cross-Subfamily Heterodimerization

Evolutionary relationships between prodomains in the TGF-β family have gone unanalyzed due to a perceived lack of conservation. We developed a novel approach, identified these relationships, and suggest hypotheses for new regulatory mechanisms in TGF-β signaling. First, a quantitative analysis placed each family member from flies, mice, and nematodes into the Activin, BMP, or TGF-β subfamily. Second, we defined the prodomain and ligand via the consensus cleavage site. Third, we generated alignments and trees from the prodomain, ligand, and full-length sequences independently for each subfamily. Prodomain alignments revealed that six structural features of 17 are well conserved: three in the straitjacket and three in the arm. Alignments also revealed unexpected cysteine conservation in the "LTBP-Association region" upstream of the straitjacket and in β8 of the bowtie in 14 proteins from all three subfamilies. In prodomain trees, eight clusters across all three subfamilies were present that were not seen in the ligand or full-length trees, suggesting prodomain-mediated cross-subfamily heterodimerization. Consistency between cysteine conservation and prodomain clustering provides support for heterodimerization predictions. Overall, our analysis suggests that cross-subfamily interactions are more common than currently appreciated and our predictions generate numerous testable hypotheses about TGF-β function and evolution.

Body Size and Tissue-Scaling Is Regulated by Motoneuron-Derived Activinß in Drosophila melanogaster

Correct scaling of body and organ size is crucial for proper development, and the survival of all organisms. Perturbations in circulating hormones, including insulins and steroids, are largely responsible for changing body size in response to both genetic and environmental factors. Such perturbations typically produce adults whose organs and appendages scale proportionately with final size. The identity of additional factors that might contribute to scaling of organs and appendages with body size is unknown. Here, we report that loss-of-function mutations in DrosophilaActivinβ (Actβ), a member of the TGF-β superfamily, lead to the production of small larvae/pupae and undersized rare adult escapers. Morphometric measurements of escaper adult appendage size (wings and legs), as well as heads, thoraxes, and abdomens, reveal a disproportional reduction in abdominal size compared to other tissues. Similar size measurements of selected Actβ mutant larval tissues demonstrate that somatic muscle size is disproportionately smaller when compared to the fat body, salivary glands, prothoracic glands, imaginal discs, and brain. We also show that Actβ control of body size is dependent on canonical signaling through the transcription-factor dSmad2 and that it modulates the growth rate, but not feeding behavior, during the third-instar period. Tissue- and cell-specific knockdown, and overexpression studies, reveal that motoneuron-derived Actβ is essential for regulating proper body size and tissue scaling. These studies suggest that, unlike in vertebrates, where Myostatin and certain other Activin-like factors act as systemic negative regulators of muscle mass, in Drosophila, Actβ is a positive regulator of muscle mass that is directly delivered to muscles by motoneurons. We discuss the importance of these findings in coordinating proportional scaling of insect muscle mass to appendage size.

Superoxide dismutase down-regulation and the oxidative stress is required to initiate pupation in Bombyx mori

Perhaps, oxidative stress progresses pupation in some Lepidopteran insects; however, the reasons for this remain obscure. In our previous study, we clarified Bombyx mori SOD1 (BmSOD1) and B. mori SOD2 (BmSOD2) proteins respond in common to ultraviolet irradiation (UV) oxidative stress and metamorphosis. This result strongly suggested pupation initiates by oxidative stress and might mediate by down-regulation of expression of BmSOD1 and BmSOD2 proteins. Thus, we examined about these relationships in B. mori in this study. In the microarray data reanalysis, we found the Notch signaling pathways as the common pathways in pupation and UV oxidative stress in B. mori. Also, we showed a molting hormone, 20-hydroxyecdysone, leads not only generation of superoxide but also downregulation of the expression of BmSOD proteins during pupation in B. mori. Our findings can contribute to a deeper understanding of how biological defense systems work against environmental oxidative stress.

A Tissue- and Temporal-Specific Autophagic Switch Controls Drosophila Pre-metamorphic Nutritional Checkpoints

Properly timed production of steroid hormones by endocrine tissues regulates juvenile-to-adult transitions in both mammals (puberty) and holometabolous insects (metamorphosis). Nutritional conditions influence the temporal control of the transition, but the mechanisms responsible are ill defined. Here we demonstrate that autophagy acts as an endocrine organ-specific, nutritionally regulated gating mechanism to help ensure productive metamorphosis in Drosophila. Autophagy in the endocrine organ is specifically stimulated by nutrient restriction at the early, but not the late, third-instar larva stage. The timing of autophagy induction correlates with the nutritional checkpoints, which inhibit precocious metamorphosis during nutrient restriction in undersized larvae. Suppression of autophagy causes dysregulated pupariation of starved larvae, which leads to pupal lethality, whereas forced autophagy induction results in developmental delay/arrest in well-fed animals. Induction of autophagy disrupts production of the steroid hormone ecdysone at the time of pupariation not by destruction of hormone biosynthetic capacity but rather by limiting the availability of the steroid hormone precursor cholesterol in the endocrine cells via a lipophagy mechanism. Interestingly, autophagy in the endocrine organ functions by interacting with the endolysosome system, yet shows multiple features not fully consistent with a canonical autophagy process. Taken together, our findings demonstrate an autophagy mechanism in endocrine cells that helps shape the nutritional checkpoints and guarantee a successful juvenile-to-adult transition in animals confronting nutritional stress.