EcR isoforms in Drosophila: testing tissue-specific requirements by targeted blockade and rescue

Stem cell-specific ecdysone signaling regulates the development of dorsal fan-shaped body neurons and sleep homeostasis

Complex behaviors arise from neural circuits that assemble from diverse cell types. Sleep is a conserved behavior essential for survival, yet little is known about how the nervous system generates neuron types of a sleep-wake circuit. Here, we focus on the specification of Drosophila 23E10-labeled dorsal fan-shaped body (dFB) long-field tangential input neurons that project to the dorsal layers of the fan-shaped body neuropil in the central complex. We use lineage analysis and genetic birth dating to identify two bilateral type II neural stem cells (NSCs) that generate 23E10 dFB neurons. We show that adult 23E10 dFB neurons express ecdysone-induced protein 93 (E93) and that loss of ecdysone signaling or E93 in type II NSCs results in their misspecification. Finally, we show that E93 knockdown in type II NSCs impairs adult sleep behavior. Our results provide insight into how extrinsic hormonal signaling acts on NSCs to generate the neuronal diversity required for adult sleep behavior. These findings suggest that some adult sleep disorders might derive from defects in stem cell-specific temporal neurodevelopmental programs.

The nutrient sensor CRTC and Sarcalumenin/thinman represent an alternate pathway in cardiac hypertrophy

CREB-regulated transcription co-activator (CRTC) is activated by Calcineurin (CaN) to regulate gluconeogenic genes. CaN also has roles in cardiac hypertrophy. Here, we explore a cardiac-autonomous role for CRTC in cardiac hypertrophy. In Drosophila, CRTC mutants exhibit severe cardiac restriction, myofibrillar disorganization, fibrosis, and tachycardia. Cardiac-specific CRTC knockdown (KD) phenocopies mutants, and cardiac overexpression causes hypertrophy. CaN-induced hypertrophy in Drosophila is reduced in CRTC mutants, suggesting that CRTC mediates the effects. RNA sequencing (RNA-seq) of CRTC-KD and -overexpressing hearts reveals contraregulation of metabolic genes. Genes with conserved CREB sites include the fly ortholog of Sarcalumenin, a Ca2+-binding protein. Cardiac manipulation of this gene recapitulates the CRTC-KD and -overexpression phenotypes. CRTC KD in zebrafish also causes cardiac restriction, and CRTC KD in human induced cardiomyocytes causes a reduction in Srl expression and increased action potential duration. Our data from three model systems suggest that CaN-CRTC-Sarcalumenin signaling represents an alternate, conserved pathway underlying cardiac function and hypertrophy.

Steroid signaling controls sex-specific development in an invertebrate

In vertebrate sexual development, two important steroid hormones, testosterone and estrogen, regulate the sex-specific development of many tissues. In contrast, invertebrates utilize a single steroid hormone, ecdysone, to regulate developmental timing in both sexes. However, here we show that in Drosophila melanogaster, sex-specific ecdysone (E) signaling controls important aspects of gonad sexual dimorphism. Rather than being regulated at the level of hormone production, hormone activity is regulated cell-autonomously through sex-specific hormone reception. Ecdysone receptor (EcR) expression is restricted to the developing ovary and is repressed in the testis at a time when ecdysone initiates ovary morphogenesis. Interestingly, EcR expression is regulated downstream of the sex determination factor Doublesex (Dsx), the founding member of the Dsx/Mab3 Related Transcription Factor (DMRT) family that regulates gonad development in all animals. E signaling is required for normal ovary development1,2, and ectopic activation of E signaling in the testis antagonized stem cell niche identity and feminized somatic support cells, which were transformed into follicle-like cells. This work demonstrates that invertebrates can also use steroid hormone signaling to control sex-specific development. Further, it may help explain recent work showing that vertebrate sexual development is surprisingly cell-autonomous. For example, chickens utilize testosterone and estrogen to control sex-specific development, but when they have a mixture of cells with male and female genotypes, the male cells develop as male and the female cells develop as female despite exposure to the same circulating hormones3. Sex-specific regulation of steroid hormone signaling may well underly such cell-autonomous sexual fate choices in vertebrates as it does in Drosophila.

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.

Inter-organ steroid hormone signaling promotes myoblast fusion via direct transcriptional regulation of a single key effector gene

Steroid hormones regulate tissue development and physiology by modulating the transcription of a broad spectrum of genes. In insects, the principal steroid hormones, ecdysteroids, trigger the expression of thousands of genes through a cascade of transcription factors (TFs) to coordinate developmental transitions such as larval molting and metamorphosis. However, whether ecdysteroid signaling can bypass transcriptional hierarchies to exert its function in individual developmental processes is unclear. Here, we report that a single non-TF effector gene mediates the transcriptional output of ecdysteroid signaling in Drosophila myoblast fusion, a critical step in muscle development and differentiation. Specifically, we show that the 20-hydroxyecdysone (commonly referred to as "ecdysone") secreted from an extraembryonic tissue, amnioserosa, acts on embryonic muscle cells to directly activate the expression of antisocial (ants), which encodes an essential scaffold protein enriched at the fusogenic synapse. Not only is ants transcription directly regulated by the heterodimeric ecdysone receptor complex composed of ecdysone receptor (EcR) and ultraspiracle (USP) via ecdysone-response elements but also more strikingly, expression of ants alone is sufficient to rescue the myoblast fusion defect in ecdysone signaling-deficient mutants. We further show that EcR/USP and a muscle-specific TF Twist synergistically activate ants expression in vitro and in vivo. Taken together, our study provides the first example of a steroid hormone directly activating the expression of a single key non-TF effector gene to regulate a developmental process via inter-organ signaling and provides a new paradigm for understanding steroid hormone signaling in other developmental and physiological processes.

Visualization of the Association of Dimeric Protein Complexes on Specific Enhancers in the Salivary Gland Nuclei of Drosophila Larva

Transcription factors (TFs) regulate gene expression by recognizing specific target enhancers in the genome. The DNA-binding and regulatory activity of TFs depend on the presence of additional protein partners, leading to the formation of versatile and dynamic multimeric protein complexes. Visualizing these protein-protein interactions (PPIs) in the nucleus is key for decrypting the molecular cues underlying TF specificity in vivo. Over the last few years, Bimolecular Fluorescence Complementation (BiFC) has been developed in several model systems and applied in the analysis of different types of PPIs. In particular, BiFC has been applied when analyzing PPIs with hundreds of TFs in the nucleus of live Drosophila embryos. However, the visualization of PPIs at the level of specific target enhancers or genomic regions of interest awaits the advent of DNA-labelling methods that can be coupled with BiFC. Here, we present a novel experimental strategy that we have called BiFOR and that is based on the coupling of BiFC with the bacterial ANCHOR DNA-labelling system. We demonstrate that BiFOR enables the precise quantification of the enrichment of specific dimeric protein complexes on target enhancers in Drosophila salivary gland nuclei. Given its versatility and sensitivity, BiFOR could be applied more widely to other tissues during Drosophila development. Our work sets up the experimental basis for future applications of this strategy.

Ectopic expression in commonly used transgenic Drosophila GAL4 driver lines

Transgenic tools such as the GAL4/UAS system in Drosophila have been used extensively to induce spatiotemporally controlled changes in gene expression and tissue-specific expression of a range of transgenes. We previously discovered unexpected expression of the commonly used dilp2-GAL4 line in tracheal tissue which significantly impacted growth phenotypes. We realized that few GAL4 lines have been thoroughly characterized, particularly when considering transient activity that may have significant impact on phenotypic readouts. Here, we characterized a further subset of 12 reportedly tissue-specific GAL4 lines commonly used in genetic studies of development, growth, endocrine regulation, and metabolism. Ten out of 12 GAL4 lines exhibited ectopic activity in other larval tissues, with seven being active in the larval trachea. Since this ectopic activity may result in phenotypes that do not depend on the manipulation in the intended target tissue, it is recommended to carefully analyze the outcome while taking this aspect into consideration.

Steroid hormone signaling synchronizes cell migration machinery, adhesion and polarity to direct collective movement

Migratory cells - either individually or in cohesive groups - are critical for spatiotemporally regulated processes such as embryonic development and wound healing. Their dysregulation is the underlying cause of formidable health problems such as congenital abnormalities and metastatic cancers. Border cell behavior during Drosophila oogenesis provides an effective model to study temporally regulated, collective cell migration in vivo. Developmental timing in flies is primarily controlled by the steroid hormone ecdysone, which acts through a well-conserved, nuclear hormone receptor complex. Ecdysone signaling determines the timing of border cell migration, but the molecular mechanisms governing this remain obscure. We found that border cell clusters expressing a dominant-negative form of ecdysone receptor extended ineffective protrusions. Additionally, these clusters had aberrant spatial distributions of E-cadherin (E-cad), apical domain markers and activated myosin that did not overlap. Remediating their expression or activity individually in clusters mutant for ecdysone signaling did not restore proper migration. We propose that ecdysone signaling synchronizes the functional distribution of E-cadherin, atypical protein kinase C (aPKC), Discs large (Dlg1) and activated myosin post-transcriptionally to coordinate adhesion, polarity and contractility and temporally control collective cell migration.

Timing Drosophila development through steroid hormone action

Specifically timed pulses of the moulting hormone ecdysone are necessary for developmental progression in insects, guiding development through important milestones such as larval moults, pupation and metamorphosis. It also coordinates the acquisition of cell identities, known as cell patterning, and growth in a tissue-specific manner. In the absence of ecdysone, the ecdysone receptor heterodimer Ecdysone Receptor and Ultraspiracle represses expression of target primary response genes, which become de-repressed as the ecdysone titre rises. However, ecdysone signalling elicits both repressive and activating responses in a temporal and tissue-specific manner. To understand how ecdysone achieves such specificity, this review explores the layers of gene regulation involved in stage-appropriate ecdysone responses in Drosophila fruit flies.

Macrophage subpopulation identity in Drosophila is modulated by apoptotic cell clearance and related signalling pathways

In Drosophila blood, plasmatocytes of the haemocyte lineage represent the functional equivalent of vertebrate macrophages and have become an established in vivo model with which to study macrophage function and behaviour. However, the use of plasmatocytes as a macrophage model has been limited by a historical perspective that plasmatocytes represent a homogenous population of cells, in contrast to the high levels of heterogeneity of vertebrate macrophages. Recently, a number of groups have reported transcriptomic approaches which suggest the existence of plasmatocyte heterogeneity, while we identified enhancer elements that identify subpopulations of plasmatocytes which exhibit potentially pro-inflammatory behaviours, suggesting conservation of plasmatocyte heterogeneity in Drosophila. These plasmatocyte subpopulations exhibit enhanced responses to wounds and decreased rates of efferocytosis when compared to the overall plasmatocyte population. Interestingly, increasing the phagocytic requirement placed upon plasmatocytes is sufficient to decrease the size of these plasmatocyte subpopulations in the embryo. However, the mechanistic basis for this response was unclear. Here, we examine how plasmatocyte subpopulations are modulated by apoptotic cell clearance (efferocytosis) demands and associated signalling pathways. We show that loss of the phosphatidylserine receptor Simu prevents an increased phagocytic burden from modulating specific subpopulation cells, while blocking other apoptotic cell receptors revealed no such rescue. This suggests that Simu-dependent efferocytosis is specifically involved in determining fate of particular subpopulations. Supportive of our original finding, mutations in amo (the Drosophila homolog of PKD2), a calcium-permeable channel which operates downstream of Simu, phenocopy simu mutants. Furthermore, we show that Amo is involved in the acidification of the apoptotic cell-containing phagosomes, suggesting that this reduction in pH may be associated with macrophage reprogramming. Additionally, our results also identify Ecdysone receptor signalling, a pathway related to control of cell death during developmental transitions, as a controller of plasmatocyte subpopulation identity. Overall, these results identify fundamental pathways involved in the specification of plasmatocyte subpopulations and so further validate Drosophila plasmatocytes as a heterogeneous population of macrophage-like cells within this important developmental and immune model.

Eclosion muscles secrete ecdysteroids to initiate asymmetric intestinal stem cell division in Drosophila

During organ development, tissue stem cells first expand via symmetric divisions and then switch to asymmetric divisions to minimize the time to obtain a mature tissue. In the Drosophila midgut, intestinal stem cells switch their divisions from symmetric to asymmetric at midpupal development to produce enteroendocrine cells. However, the signals that initiate this switch are unknown. Here, we identify the signal as ecdysteroids. In the presence of ecdysone, EcR and Usp promote the expression of E93 to suppress Br expression, resulting in asymmetric divisions. Surprisingly, the primary source of pupal ecdysone is not from the prothoracic gland but from dorsal internal oblique muscles (DIOMs), a group of transient skeletal muscles that are required for eclosion. Genetic analysis shows that DIOMs secrete ecdysteroids during mTOR-mediated muscle remodeling. Our findings identify sequential endocrine and mechanical roles for skeletal muscle, which ensure the timely asymmetric divisions of intestinal stem cells.

Finishing the egg

Gamete development is a fundamental process that is highly conserved from early eukaryotes to mammals. As germ cells develop, they must coordinate a dynamic series of cellular processes that support growth, cell specification, patterning, the loading of maternal factors (RNAs, proteins, and nutrients), differentiation of structures to enable fertilization and ensure embryonic survival, and other processes that make a functional oocyte. To achieve these goals, germ cells integrate a complex milieu of environmental and developmental signals to produce fertilizable eggs. Over the past 50 years, Drosophila oogenesis has risen to the forefront as a system to interrogate the sophisticated mechanisms that drive oocyte development. Studies in Drosophila have defined mechanisms in germ cells that control meiosis, protect genome integrity, facilitate mRNA trafficking, and support the maternal loading of nutrients. Work in this system has provided key insights into the mechanisms that establish egg chamber polarity and patterning as well as the mechanisms that drive ovulation and egg activation. Using the power of Drosophila genetics, the field has begun to define the molecular mechanisms that coordinate environmental stresses and nutrient availability with oocyte development. Importantly, the majority of these reproductive mechanisms are highly conserved throughout evolution, and many play critical roles in the development of somatic tissues as well. In this chapter, we summarize the recent progress in several key areas that impact egg chamber development and ovulation. First, we discuss the mechanisms that drive nutrient storage and trafficking during oocyte maturation and vitellogenesis. Second, we examine the processes that regulate follicle cell patterning and how that patterning impacts the construction of the egg shell and the establishment of embryonic polarity. Finally, we examine regulatory factors that control ovulation, egg activation, and successful fertilization.

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.

Stem cell-specific ecdysone signaling regulates the development and function of a Drosophila sleep homeostat

Complex behaviors arise from neural circuits that are assembled from diverse cell types. Sleep is a conserved and essential behavior, yet little is known regarding how the nervous system generates neuron types of the sleep-wake circuit. Here, we focus on the specification of Drosophila sleep-promoting neurons-long-field tangential input neurons that project to the dorsal layers of the fan-shaped body neuropil in the central complex (CX). We use lineage analysis and genetic birth dating to identify two bilateral Type II neural stem cells that generate these dorsal fan-shaped body (dFB) neurons. We show that adult dFB neurons express Ecdysone-induced protein E93, and loss of Ecdysone signaling or E93 in Type II NSCs results in the misspecification of the adult dFB neurons. Finally, we show that E93 knockdown in Type II NSCs affects adult sleep behavior. Our results provide insight into how extrinsic hormonal signaling acts on NSCs to generate neuronal diversity required for adult sleep behavior. These findings suggest that some adult sleep disorders might derive from defects in stem cell-specific temporal neurodevelopmental programs.

20-hydroxyecdysone Upregulates Ecdysone Receptor (ECR) Gene to Promote Pupation in the Honeybee, Apis mellifera Ligustica

A heterodimeric complex of two nuclear receptors, the ecdysone receptor (ECR) and ultraspiracle (USP), transduces 20-hydroxyecdysone (20E) signaling to modulate insect growth and development. Here, we aimed to determine the relationship between ECR and 20E during larval metamorphosis and also the specific roles of ECR during larval-adult transition in Apis mellifera. We found that ECR gene expression peaked in the 7-day-old larvae, then decreased gradually from the pupae stage. 20E slowly reduced food consumption and then induced starvation, resulting in small-sized adults. In addition, 20E induced ECR expression to regulate larval development time. Double-stranded RNAs (dsRNAs) were prepared using common dsECR as templates. After dsECR injection, larval transition to the pupal stage was delayed, and 80% of the larvae showed prolonged pupation beyond 18 h. Moreover, the mRNA levels of shd, sro, nvd, and spo, and ecdysteroid titers were significantly decreased in ECR RNAi larvae compared with those in GFP RNAi control larvae. ECR RNAi disrupted 20E signaling during larval metamorphosis. We performed rescuing experiments by injecting 20E in ECR RNAi larvae and found that the mRNA levels of ECR, USP, E75, E93, and Br-c were not restored. 20E induced apoptosis in the fat body during larval pupation, while RNAi knockdown of ECR genes reduced apoptosis. We concluded that 20E induced ECR to modulate 20E signaling to promote honeybee pupation. These results assist our understanding of the complicated molecular mechanisms of insect metamorphosis.

Sterol Regulation of Development and 20-Hydroxyecdysone Biosynthetic and Signaling Genes in Drosophila melanogaster

Ecdysteroids are crucial in regulating the growth and development of insects. In the fruit fly Drosophila melanogaster, both C27 and C28 ecdysteroids have been identified. While the biosynthetic pathway of the C27 ecdysteroid 20-hydroxyecdysone (20E) from cholesterol is relatively well understood, the biosynthetic pathway of C28 ecdysteroids from C28 or C29 dietary sterols remains unknown. In this study, we found that different dietary sterols (including the C27 sterols cholesterol and 7-dehydrocholesterol, the C28 sterols brassicasterol, campesterol, and ergosterol, and the C29 sterols β-sitosterol, α-spinasterol, and stigmasterol) differentially affected the expression of 20E biosynthetic genes to varying degrees, but similarly activated 20E primary response gene expression in D. melanogaster Kc cells. We also found that a single dietary sterol was sufficient to support D. melanogaster growth and development. Furthermore, the expression levels of some 20E biosynthetic genes were significantly altered, whereas the expression of 20E signaling primary response genes remained unaffected when flies were reared on lipid-depleted diets supplemented with single sterol types. Overall, our study provided preliminary clues to suggest that the same enzymatic system responsible for the classical C27 ecdysteroid 20E biosynthetic pathway also participated in the conversion of C28 and C29 dietary sterols into C28 ecdysteroids.

Rbf/E2F1 control growth and endoreplication via steroid-independent Ecdysone Receptor signalling in Drosophila prostate-like secondary cells

In prostate cancer, loss of the tumour suppressor gene, Retinoblastoma (Rb), and consequent activation of transcription factor E2F1 typically occurs at a late-stage of tumour progression. It appears to regulate a switch to an androgen-independent form of cancer, castration-resistant prostate cancer (CRPC), which frequently still requires androgen receptor (AR) signalling. We have previously shown that upon mating, binucleate secondary cells (SCs) of the Drosophila melanogaster male accessory gland (AG), which share some similarities with prostate epithelial cells, switch their growth regulation from a steroid-dependent to a steroid-independent form of Ecdysone Receptor (EcR) control. This physiological change induces genome endoreplication and allows SCs to rapidly replenish their secretory compartments, even when ecdysone levels are low because the male has not previously been exposed to females. Here, we test whether the Drosophila Rb homologue, Rbf, and E2F1 regulate this switch. Surprisingly, we find that excess Rbf activity reversibly suppresses binucleation in adult SCs. We also demonstrate that Rbf, E2F1 and the cell cycle regulators, Cyclin D (CycD) and Cyclin E (CycE), are key regulators of mating-dependent SC endoreplication, as well as SC growth in both virgin and mated males. Importantly, we show that the CycD/Rbf/E2F1 axis requires the EcR, but not ecdysone, to trigger CycE-dependent endoreplication and endoreplication-associated growth in SCs, mirroring changes seen in CRPC. Furthermore, Bone Morphogenetic Protein (BMP) signalling, mediated by the BMP ligand Decapentaplegic (Dpp), intersects with CycD/Rbf/E2F1 signalling to drive endoreplication in these fly cells. Overall, our work reveals a signalling switch, which permits rapid growth of SCs and increased secretion after mating, independently of previous exposure to females. The changes observed share mechanistic parallels with the pathological switch to hormone-independent AR signalling seen in CRPC, suggesting that the latter may reflect the dysregulation of a currently unidentified physiological process.

Ecdysone signaling determines lateral polarity and remodels neurites to form Drosophila's left-right brain asymmetry

Left-right (LR) asymmetry of the brain is fundamental to its higher-order functions. The Drosophila brain's asymmetrical body (AB) consists of a structural pair arborized from AB neurons and is larger on the right side than the left. We find that the AB initially forms LR symmetrically and then develops LR asymmetrically by neurite remodeling that is specific to the left AB and is dynamin dependent. Additionally, neuronal ecdysone signaling inhibition randomizes AB laterality, suggesting that ecdysone signaling determines AB's LR polarity. Given that AB's LR asymmetry relates to memory formation, our research establishes AB as a valuable model for studying LR asymmetry and higher-order brain function relationships.

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.

Teflon promotes chromosomal recruitment of homolog conjunction proteins during Drosophila male meiosis

Meiosis in males of higher dipterans is achiasmate. In their spermatocytes, pairing of homologs into bivalent chromosomes does not include synaptonemal complex and crossover formation. While crossovers preserve homolog conjunction until anaphase I during canonical meiosis, an alternative system is used in dipteran males. Mutant screening in Drosophila melanogaster has identified teflon (tef) as being required specifically for alternative homolog conjunction (AHC) of autosomal bivalents. The additional known AHC genes, snm, uno and mnm, are needed for the conjunction of autosomal homologs and of sex chromosomes. Here, we have analyzed the pattern of TEF protein expression. TEF is present in early spermatocytes but cannot be detected on bivalents at the onset of the first meiotic division, in contrast to SNM, UNO and MNM (SUM). TEF binds to polytene chromosomes in larval salivary glands, recruits MNM by direct interaction and thereby, indirectly, also SNM and UNO. However, chromosomal SUM association is not entirely dependent on TEF, and residual autosome conjunction occurs in tef null mutant spermatocytes. The higher tef requirement for autosomal conjunction is likely linked to the quantitative difference in the amount of SUM protein that provides conjunction of autosomes and sex chromosomes, respectively. During normal meiosis, SUM proteins are far more abundant on sex chromosomes compared to autosomes. Beyond promoting SUM recruitment, TEF has a stabilizing effect on SUM proteins. Increased SUM causes excess conjunction and consequential chromosome missegregation during meiosis I after co-overexpression. Similarly, expression of SUM without TEF, and even more potently with TEF, interferes with chromosome segregation during anaphase of mitotic divisions in somatic cells, suggesting that the known AHC proteins are sufficient for establishment of ectopic chromosome conjunction. Overall, our findings suggest that TEF promotes alternative homolog conjunction during male meiosis without being part of the final physical linkage between chromosomes.

Sexual reproduction depends on meiosis, a special cell division that generates haploid cells. Haploid cells have only one set of chromosomes in contrast to the diploid precursor cell, which has two sets. Haploid cells can differentiate into gametes. Fusion of two gametes during fertilization recreates the diploid state. Meiosis is distinct in males and females to produce two distinct types of compatible gametes, sperm and egg. In the fly Drosophila melanogaster, sex-specific differences are particularly pronounced. While pairing of homologous chromosomes into bivalents early in meiosis proceeds in a canonical manner in females, males use an alternative system. This system maintains homolog pairing, replacing crossovers that result from homologous recombination during canonical meiosis. Four genes (snm, uno, mnm and tef) are known to be required specifically for alternative homolog conjunction in males. Here, we demonstrate that the TEF protein binds directly to MNM. Thereby, TEF promotes the recruitment of MNM and consequentially SNM and UNO to chromosomes. However, while SNM, UNO and MNM remain on bivalent chromosomes until they are separated apart during the first meiotic division, TEF disappears prematurely, suggesting that it is not part of the final physical linkage between homologous chromosomes.

Developmental regulation of epithelial cell cuboidal-to-squamous transition in Drosophila follicle cells

Epithelial cells form continuous membranous structures for organ formation, and these cells are classified into three major morphological categories: cuboidal, columnar, and squamous. It is crucial that cells transition between these shapes during the morphogenetic events of organogenesis, yet this process remains poorly understood. All three epithelial cell shapes can be found in the follicular epithelium of Drosophila egg chamber during oogenesis. Squamous cells (SCs) are initially restricted to the anterior terminus in cuboidal shape. They then rapidly become flattened to assume squamous shape by stretching and expansion in 12 ​h during midoogenesis. Previously, we reported that Notch signaling activated a zinc-finger transcription factor Broad (Br) at the end of early oogenesis. Here we report that ecdysone and JAK/STAT pathways subsequently converge on Br to serve as an important spatiotemporal regulator of this dramatic morphological change of SCs. The early uniform pattern of Br in the follicular epithelium is directly established by Notch signaling at stage 5 of oogenesis. Later, ecdysone and JAK/STAT signaling activities synergize to suppress Br in SCs from stage 8 to 10a, contributing to proper SC squamous shape. During this process, ecdysone signaling is essential for SC stretching, while JAK/STAT regulates SC clustering and cell fate determination. This study reveals an inhibitory role of ecdysone signaling in suppressing Br in epithelial cell remodeling. In this study we also used single-cell RNA sequencing data to highlight the shift in gene expression which occurs as Br is suppressed and cells become flattened.

Pruning deficits of the developing Drosophila mushroom body result in mild impairment in associative odour learning and cause hyperactivity

The principles of how brain circuits establish themselves during development are largely conserved across animal species. Connections made during embryonic development that are appropriate for an early life stage are frequently remodelled later in ontogeny via pruning and subsequent regrowth to generate adult-specific connectivity. The mushroom body of the fruit fly Drosophila melanogaster is a well-established model circuit for examining the cellular mechanisms underlying neurite remodelling. This central brain circuit integrates sensory information with learned and innate valences to adaptively instruct behavioural decisions. Thereby, the mushroom body organizes adaptive behaviour, such as associative learning. However, little is known about the specific aspects of behaviour that require mushroom body remodelling. Here, we used genetic interventions to prevent the intrinsic neurons of the larval mushroom body (γ-type Kenyon cells) from remodelling. We asked to what degree remodelling deficits resulted in impaired behaviour. We found that deficits caused hyperactivity and mild impairment in differential aversive olfactory learning, but not appetitive learning. Maintenance of circadian rhythm and sleep were not affected. We conclude that neurite pruning and regrowth of γ-type Kenyon cells is not required for the establishment of circuits that mediate associative odour learning per se, but it does improve distinct learning tasks.

Ecdysone receptor isoform specific regulation of secretory granule acidification in the larval Drosophila salivary gland

Bulk production and release of glue containing secretory granules takes place in the larval salivary gland during Drosophila development in order to attach the metamorphosing animal to a dry surface. These granules undergo a maturation process to prepare glue for exocytosis, which includes homotypic fusions to increase the size of granules, vesicle acidification and ion uptake. The steroid hormone 20-hydroxyecdysone is known to be required for the first and last steps of this process: glue synthesis and secretion, respectively. Here we show that the B1 isoform of Ecdysone receptor (EcR), together with its binding partner Ultraspiracle, are also necessary for the maturation of glue granules by promoting their acidification via regulation of Vha55 expression, which encodes an essential subunit of the V-ATPase proton pump. This is antagonized by the EcR-A isoform, overexpression of which decreases EcR-B1 and Vha55 expression and glue granule acidification. Our data shed light on a previously unknown, ecdysone receptor isoform-specific regulation of glue granule maturation.

Steroid hormone signaling activates thermal nociception during Drosophila peripheral nervous system development

Sensory neurons enable animals to detect environmental changes and avoid harm. An intriguing open question concerns how the various attributes of sensory neurons arise in development. Drosophila melanogaster larvae undergo a behavioral transition by robustly activating a thermal nociceptive escape behavior during the second half of larval development (third instar). The Class IV dendritic arborization (C4da) neurons are multimodal sensors which tile the body wall of Drosophila larvae and detect nociceptive temperature, light, and mechanical force. In contrast to the increase in nociceptive behavior in the third instar, we find that ultraviolet light-induced Ca2+ activity in C4da neurons decreases during the same period of larval development. Loss of ecdysone receptor has previously been shown to reduce nociception in third instar larvae. We find that ligand-dependent activation of ecdysone signaling is sufficient to promote nociceptive responses in second instar larvae and suppress expression of subdued (encoding a TMEM16 channel). Reduction of subdued expression in second instar C4da neurons not only increases thermal nociception but also decreases the response to ultraviolet light. Thus, steroid hormone signaling suppresses subdued expression to facilitate the sensory switch of C4da neurons. This regulation of a developmental sensory switch through steroid hormone regulation of channel expression raises the possibility that ion channel homeostasis is a key target for tuning the development of sensory modalities.

A Drosophila toolkit for HA-tagged proteins unveils a block in autophagy flux in the last instar larval fat body

For in vivo functional analysis of a protein of interest (POI), multiple transgenic strains with a POI that harbors different tags are needed but generation of these strains is still labor-intensive work. To overcome this, we have developed a versatile Drosophila toolkit with a genetically encoded single-chain variable fragment for the HA epitope tag: ‘HA Frankenbody’. This system allows various analyses of HA-tagged POI in live tissues by simply crossing an HA Frankenbody fly with an HA-tagged POI fly. Strikingly, the GFP-mCherry tandem fluorescent-tagged HA Frankenbody revealed a block in autophagic flux and an accumulation of enlarged autolysosomes in the last instar larval and prepupal fat body. Mechanistically, lysosomal activity was downregulated at this stage, and endocytosis, but not autophagy, was indispensable for the swelling of lysosomes. Furthermore, forced activation of lysosomes by fat body-targeted overexpression of Mitf, the single MiTF/TFE family gene in Drosophila, suppressed the lysosomal swelling and resulted in pupal lethality. Collectively, we propose that downregulated lysosomal function in the fat body plays a role in the metamorphosis of Drosophila.

A global timing mechanism regulates cell-type-specific wiring programmes

The assembly of neural circuits is dependent on precise spatiotemporal expression of cell recognition molecules^1-5. Factors controlling cell type specificity have been identified^6-8, but how timing is determined remains unknown. Here we describe induction of a cascade of transcription factors by a steroid hormone (ecdysone) in all fly visual system neurons spanning target recognition and synaptogenesis. We demonstrate through single-cell sequencing that the ecdysone pathway regulates the expression of a common set of targets required for synaptic maturation and cell-type-specific targets enriched for cell-surface proteins regulating wiring specificity. Transcription factors in the cascade regulate the expression of the same wiring genes in complex ways, including activation in one cell type and repression in another. We show that disruption of the ecdysone pathway generates specific defects in dendritic and axonal processes and synaptic connectivity, with the order of transcription factor expression correlating with sequential steps in wiring. We also identify shared targets of a cell-type-specific transcription factor and the ecdysone pathway that regulate specificity. We propose that neurons integrate a global temporal transcriptional module with cell-type-specific transcription factors to generate different cell-type-specific patterns of cell recognition molecules regulating wiring.

Ecdysone exerts biphasic control of regenerative signaling, coordinating the completion of regeneration with developmental progression

In Drosophila melanogaster, loss of regenerative capacity in wing imaginal discs coincides with an increase in systemic levels of the steroid hormone ecdysone, a key coordinator of their developmental progression. Regenerating discs release the relaxin hormone Dilp8 (Drosophila insulin-like peptide 8) to limit ecdysone synthesis and extend the regenerative period. Here, we describe how regenerating tissues produce a biphasic response to ecdysone levels: lower concentrations of ecdysone promote local and systemic regenerative signaling, whereas higher concentrations suppress regeneration through the expression of broad splice isoforms. Ecdysone also promotes the expression of wingless during both regeneration and normal development through a distinct regulatory pathway. This dual role for ecdysone explains how regeneration can still be completed successfully in dilp8- mutant larvae: higher ecdysone levels increase the regenerative activity of tissues, allowing regeneration to reach completion in a shorter time. From these observations, we propose that ecdysone hormone signaling functions to coordinate regeneration with developmental progression.

Mocs1 (Molybdenum cofactor synthesis 1) may contribute to lifespan extension in Drosophila

While evaluating the effect on lifespan of decreased ribosomal protein (Rp) expression in Drosophila, we discovered a potential function in the same process for the Molybdenum cofactor synthesis 1 (Mocs1) gene. We utilized the UAS-GAL4 inducible system, by crossing tissue-specific GAL4 drivers to the Harvard Drosophila Transgenic RNAi Project (TrIP) responder lines for Rp gene knockdown. We also employed a negative control that knocked down a gene unrelated to Drosophila (GAL4). Relative to the genetic background in which no driven transgenes were present, lifespan was significantly lengthened in females, both for Rp knockdown and the negative GAL4 control. We reasoned that the Mocs1 gene, located immediately downstream of the integration site on the third chromosome where all the TrIP responders are targeted might be responsible for the lifespan effects observed, due to the potential for upregulation using the UAS-GAL4 system. We repeated the lifespan experiment using an enhancer trap in the same location as the TrIP transgenes, and found that lifespan was significantly lengthened in females that possessed both the driver and responder, relative to controls, implicating Mocs1 in the biology of aging.

Drosophila Keap1 xenobiotic response factor regulates developmental transcription through binding to chromatin

The Keap1-Nrf2 complex is a central regulator that mediates transcriptional responses to xenobiotic stimuli and is highly related with multiple human diseases. The molecular mechanisms and biological functions of Keap1 and Nrf2 are not fully understood. The Drosophila Keap1 homolog (dKeap1) is conserved with mammalian Keap1 except that dKeap1 contains a 156 aa C-terminal tail (CTD). A dKeap1 truncation with the CTD removed (dKeap1-ΔCTD) shows abolished nuclear localization and chromatin-binding. Expression of dKeap1-ΔCTD in the dKeap1 null background significantly rescues this mutant to the adult stage, but the files showed partial lethality, sterility and defects in adipose tissue. In the rescued flies, expression levels of ecdysone-response genes, ecdysone-synthetic genes and adipogenesis genes were down-regulated in specific tissues, indicating that the chromatin-binding of dKeap1 mediates the activation of these developmental genes. At the same time, dKeap1-ΔCTD can still suppress the basal expression of detoxifying genes and mediate the activation of these genes in response to xenobiotic stimuli, suggesting that the chromatin-binding of dKeap1 is not required for the regulation of detoxifying genes. These results support a model in which dKeap1 on one hand functions as an inhibitor for the Nrf2-mediated transcription in the xenobiotic response pathway and on the other hand functions as a chromatin-binding transcription activator in the developmental pathway. Our study reveals a novel mechanism whereby Keap1-Nrf2 xenobiotic response signaling regulates development using a mechanism independent of redox signaling.

Characterizing a gene expression toolkit for eye- and photoreceptor-specific expression in Drosophila

Binary expression systems are a powerful tool for tissue- and cell-specific research. Many of the currently available Drosophila eye-specific drivers have not been systematically characterized for their expression level and cell-type specificity in the adult eye or during development. Here, we used a luciferase reporter to measure expression levels of different drivers in the adult Drosophila eye, and characterized the cell type-specificity of each driver using a fluorescent reporter in live 10-day-old adult males. We also further characterized the expression pattern of these drivers in various developmental stages. We compared several Gal4 drivers from the Bloomington Drosophila Stock Center (BDSC) including GMR-Gal4, longGMR-Gal4 and Rh1-Gal4 with newly developed Gal4 and QF2 drivers that are specific to different cell types in the adult eye. In addition, we generated drug-inducible Rh1-GSGal4 lines and compared their induced expression with an available GMR-GSGal4 line. Although both lines had significant induction of gene expression measured by luciferase activity, Rh1-GSGal4 was expressed at levels below the detection of the fluorescent reporter by confocal microscopy, while GMR-GSGal4 showed substantial reporter expression in the absence of drug by microscopy. Overall, our study systematically characterizes and compares a large toolkit of eye- and photoreceptor-specific drivers, while also uncovering some of the limitations of currently available expression systems in the adult eye.

Co-option of immune effectors by the hormonal signalling system triggering metamorphosis in Drosophila melanogaster

Insect metamorphosis is triggered by the production, secretion and degradation of 20-hydroxyecdysone (ecdysone). In addition to its role in developmental regulation, increasing evidence suggests that ecdysone is involved in innate immunity processes, such as phagocytosis and the induction of antimicrobial peptide (AMP) production. AMP regulation includes systemic responses as well as local responses at surface epithelia that contact with the external environment. At pupariation, Drosophila melanogaster increases dramatically the expression of three AMP genes, drosomycin (drs), drosomycin-like 2 (drsl2) and drosomycin-like 5 (drsl5). We show that the systemic action of drs at pupariation is dependent on ecdysone signalling in the fat body and operates via the ecdysone downstream target, Broad. In parallel, ecdysone also regulates local responses, specifically through the activation of drsl2 expression in the gut. Finally, we confirm the relevance of this ecdysone dependent AMP expression for the control of bacterial load by showing that flies lacking drs expression in the fat body have higher bacterial persistence over metamorphosis. In contrast, local responses may be redundant with the systemic effect of drs since reduction of ecdysone signalling or of drsl2 expression has no measurable negative effect on bacterial load control in the pupa. Together, our data emphasize the importance of the association between ecdysone signalling and immunity using in vivo studies and establish a new role for ecdysone at pupariation, which impacts developmental success by regulating the immune system in a stage-dependent manner. We speculate that this co-option of immune effectors by the hormonal system may constitute an anticipatory mechanism to control bacterial numbers in the pupa, at the core of metamorphosis evolution.

An Oatp transporter-mediated steroid sink promotes tumor-induced cachexia in Drosophila

Cancer cachexia is associated with many types of tumors and is characterized by a combination of anorexia, loss of body weight, catabolic alterations, and systemic inflammation. We developed a tumor model in Drosophila larvae that causies cachexia-like syndrome, and we found that cachectic larvae show reduced levels of the circulating steroid ecdysone (Ec). Artificially importing Ec in the tumor through the use of the EcI/Oatp74D importer aggravated cachexia, whereas feeding animals with Ec rescued cachectic defects. This suggests that a steroid sink induced by the tumor promotes catabolic alterations in healthy tissues. We found that Oatp33Eb, a member of the Oatp transporter family, is specifically induced in tumors promoting cachexia. The overexpression of Oatp33Eb in noncachectic tumors induced cachexia, whereas its inhibition in cachectic tumors restored circulating Ec and reversed cachectic alterations. Oatp transporters are induced in several types of hormone-dependent tumors, and this result suggests that a similar sink effect could modify hormonal balance in cachectic cancer patients.

Constraints and Opportunities for the Evolution of Metamorphic Organisms in a Changing Climate

We argue that developmental hormones facilitate the evolution of novel phenotypic innovations and timing of life history events by genetic accommodation. Within an individual’s life cycle, metamorphic hormones respond readily to environmental conditions and alter adult phenotypes. Across generations, the many effects of hormones can bias and at times constrain the evolution of traits during metamorphosis; yet, hormonal systems can overcome constraints through shifts in timing of, and acquisition of tissue specific responses to, endocrine regulation. Because of these actions of hormones, metamorphic hormones can shape the evolution of metamorphic organisms. We present a model called a developmental goblet, which provides a visual representation of how metamorphic organisms might evolve. In addition, because developmental hormones often respond to environmental changes, we discuss how endocrine regulation of postembryonic development may impact how organisms evolve in response to climate change. Thus, we propose that developmental hormones may provide a mechanistic link between climate change and organismal adaptation.

Pri smORF Peptides Are Wide Mediators of Ecdysone Signaling, Contributing to Shape Spatiotemporal Responses

There is growing evidence that peptides encoded by small open-reading frames (sORF or smORF) can fulfill various cellular functions and define a novel class regulatory molecules. To which extend transcripts encoding only smORF peptides compare with canonical protein-coding genes, yet remain poorly understood. In particular, little is known on whether and how smORF-encoding RNAs might need tightly regulated expression within a given tissue, at a given time during development. We addressed these questions through the analysis of Drosophila polished rice (pri, a.k.a. tarsal less or mille pattes), which encodes four smORF peptides (11-32 amino acids in length) required at several stages of development. Previous work has shown that the expression of pri during epidermal development is regulated in the response to ecdysone, the major steroid hormone in insects. Here, we show that pri transcription is strongly upregulated by ecdysone across a large panel of cell types, suggesting that pri is a core component of ecdysone response. Although pri is produced as an intron-less short transcript (1.5 kb), genetic assays reveal that the developmental functions of pri require an unexpectedly large array of enhancers (spanning over 50 kb), driving a variety of spatiotemporal patterns of pri expression across developing tissues. Furthermore, we found that separate pri enhancers are directly activated by the ecdysone nuclear receptor (EcR) and display distinct regulatory modes between developmental tissues and/or stages. Alike major developmental genes, the expression of pri in a given tissue often involves several enhancers driving apparently redundant (or shadow) expression, while individual pri enhancers can harbor pleiotropic functions across tissues. Taken together, these data reveal the broad role of Pri smORF peptides in ecdysone signaling and show that the cis-regulatory architecture of the pri gene contributes to shape distinct spatial and temporal patterns of ecdysone response throughout development.

20-hydroxyecdysone (20E) signaling regulates amnioserosa morphogenesis during Drosophila dorsal closure: EcR modulates gene expression in a complex with the AP-1 subunit, Jun

Steroid hormones influence diverse biological processes throughout the animal life cycle, including metabolism, stress resistance, reproduction, and lifespan. In insects, the steroid hormone, 20-hydroxyecdysone (20E), is the central hormone regulator of molting and metamorphosis, and plays roles in tissue morphogenesis. For example, amnioserosa contraction, which is a major driving force in Drosophila dorsal closure (DC), is defective in embryos mutant for 20E biosynthesis. Here, we show that 20E signaling modulates the transcription of several DC participants in the amnioserosa and other dorsal tissues during late embryonic development, including zipper, which encodes for non-muscle myosin. Canonical ecdysone signaling typically involves the binding of Ecdysone receptor (EcR) and Ultraspiracle heterodimers to ecdysone-response elements (EcREs) within the promoters of responsive genes to drive expression. During DC, however, we provide evidence that 20E signaling instead acts in parallel to the JNK cascade via a direct interaction between EcR and the AP-1 transcription factor subunit, Jun, which together binds to genomic regions containing AP-1 binding sites but no EcREs to control gene expression. Our work demonstrates a novel mode of action for 20E signaling in Drosophila that likely functions beyond DC, and may provide further insights into mammalian steroid hormone receptor interactions with AP-1.

Summary: During Drosophila dorsal closure, 20E signaling acts non-canonically through an interaction between EcR and the AP-1 subunit, Jun, to control gene expression at regions containing AP-1 motifs but no EcREs.

A novel function for Rab1 and Rab11 during secretory granule maturation

Regulated exocytosis is an essential process whereby specific cargo proteins are secreted in a stimulus-dependent manner. Cargo-containing secretory granules are synthesized in the trans-Golgi network (TGN); after budding from the TGN, granules undergo modifications, including an increase in size. These changes occur during a poorly understood process called secretory granule maturation. Here, we leverage the Drosophila larval salivary glands as a model to characterize a novel role for Rab GTPases during granule maturation. We find that secretory granules increase in size ∼300-fold between biogenesis and release, and loss of Rab1 or Rab11 reduces granule size. Surprisingly, we find that Rab1 and Rab11 localize to secretory granule membranes. Rab11 associates with granule membranes throughout maturation, and Rab11 recruits Rab1. In turn, Rab1 associates specifically with immature granules and drives granule growth. In addition to roles in granule growth, both Rab1 and Rab11 appear to have additional functions during exocytosis; Rab11 function is necessary for exocytosis, while the presence of Rab1 on immature granules may prevent precocious exocytosis. Overall, these results highlight a new role for Rab GTPases in secretory granule maturation.

polished rice mediates ecdysone-dependent control of Drosophila embryonic organogenesis

In many animals, progression of developmental stages is temporally controlled by steroid hormones. In Drosophila, the level of ecdysone titer oscillates and developmental stage transitions, such as larval molting and metamorphosis, are induced at each of ecdysone peaks. Ecdysone titer also peaks at the stage of mid-embryogenesis and the embryonic ecdysone is necessary for morphogenesis of several organs, although the regulatory mechanisms of embryonic organogenesis dependent on ecdysone signaling are still open questions. In this study, we find that absence or interruption of embryonic ecdysone signaling caused multiple defects in the tracheal system, including decrease in luminal protein deposition, uneven dilation of the dorsal trunk and loss of terminal branches. We also reveal that an ecdysone-inducible gene polished rice (pri) is essential for tip cell fate decision in dorsal branches. As over-expression of pri can restore the defects caused by disturbance of ecdysone biosynthesis, pri functions as one of the major mediators of embryonic ecdysone signal in tracheogenesis. These results demonstrate that ecdysone and its downstream target pri play essential roles in tracheal development by modulating cell fate decision.

Nuclear receptors linking physiology and germline stem cells in Drosophila

Maternal nutrition and physiology are intimately associated with reproductive success in diverse organisms. Despite decades of study, the molecular mechanisms linking maternal diet to the production and quality of oocytes remain poorly defined. Nuclear receptors (NRs) link nutritional signals to cellular responses and are essential for oocyte development. The fruit fly, Drosophila melanogaster, is an excellent genetically tractable model to study the relationship between NR signaling and oocyte production. In this review, we explore how NRs in Drosophila regulate the earliest stages of oocyte development. Long-recognized as an essential mediator of developmental transitions, we focus on the intrinsic roles of the Ecdysone Receptor and its ligand, ecdysone, in oogenesis. We also review recent studies suggesting broader roles for NRs as regulators of maternal physiology and their impact specifically on oocyte production. We propose that NRs form the molecular basis of a broad physiological surveillance network linking maternal diet with oocyte production. Given the functional conservation between Drosophila and humans, continued experimental investigation into the molecular mechanisms by which NRs promote oogenesis will likely aid our understanding of human fertility.

Control of Drosophila wing size by morphogen range and hormonal gating

The stereotyped dimensions of animal bodies and their component parts result from tight constraints on growth. Yet, the mechanisms that stop growth when organs reach the right size are unknown. Growth of the Drosophila wing-a classic paradigm-is governed by two morphogens, Decapentaplegic (Dpp, a BMP) and Wingless (Wg, a Wnt). Wing growth during larval life ceases when the primordium attains full size, concomitant with the larval-to-pupal molt orchestrated by the steroid hormone ecdysone. Here, we block the molt by genetically dampening ecdysone production, creating an experimental paradigm in which the wing stops growing at the correct size while the larva continues to feed and gain body mass. Under these conditions, we show that wing growth is limited by the ranges of Dpp and Wg, and by ecdysone, which regulates the cellular response to their signaling activities. Further, we present evidence that growth terminates because of the loss of two distinct modes of morphogen action: 1) maintenance of growth within the wing proper and 2) induced growth of surrounding "pre-wing" cells and their recruitment into the wing. Our results provide a precedent for the control of organ size by morphogen range and the hormonal gating of morphogen action.

The Evolution of Insect Metamorphosis

The evolution of insect metamorphosis is one of the most important sagas in animal history, transforming small, obscure soil arthropods into a dominant terrestrial group that has profoundly shaped the evolution of terrestrial life. The evolution of flight initiated the trajectory towards metamorphosis, favoring enhanced differences between juvenile and adult stages. The initial step modified postembryonic development, resulting in the nymph-adult differences characteristic of hemimetabolous species. The second step was to complete metamorphosis, holometaboly, and occurred by profoundly altering embryogenesis to produce a larval stage, the nymph becoming the pupa to accommodate the deferred development needed to make the adult. These changing life history patterns were intimately linked to two hormonal systems, the ecdysteroids and the juvenile hormones (JH), which function in both embryonic and postembryonic domains and control the stage-specifying genes Krüppel homolog 1 (Kr-h1), broad and E93. The ecdysteroids induce and direct molting through the ecdysone receptor (EcR), a nuclear hormone receptor with numerous targets including a conserved transcription factor network, the 'Ashburner cascade', which translates features of the ecdysteroid peak into the different phases of the molt. With the evolution of metamorphosis, ecdysteroids acquired a metamorphic function that exploited the repressor capacity of the unliganded EcR, making it a hormone-controlled gateway for the tissue development preceding metamorphosis. JH directs ecdysteroid action, controlling Kr-h1 expression which in turn regulates the other stage-specifying genes. JH appears in basal insect groups as their embryos shift from growth and patterning to differentiation. As a major portion of embryogenesis was deferred to postembryonic life with the evolution of holometaboly, JH also acquired a potent role in regulating postembryonic growth and development. Details of its involvement in broad expression and E93 suppression have been modified as life cycles became more complex and likely underlie some of the changes seen in the shift from incomplete to complete metamorphosis.

Ecdysone steroid hormone remote controls intestinal stem cell fate decisions via the PPARγ-homolog Eip75B in Drosophila

Developmental studies revealed fundamental principles on how organ size and function is achieved, but less is known about organ adaptation to new physiological demands. In fruit flies, juvenile hormone (JH) induces intestinal stem cell (ISC) driven absorptive epithelial expansion balancing energy uptake with increased energy demands of pregnancy. Here, we show 20-Hydroxy-Ecdysone (20HE)-signaling controlling organ homeostasis with physiological and pathological implications. Upon mating, 20HE titer in ovaries and hemolymph are increased and act on nearby midgut progenitors inducing Ecdysone-induced-protein-75B (Eip75B). Strikingly, the PPARγ-homologue Eip75B drives ISC daughter cells towards absorptive enterocyte lineage ensuring epithelial growth. To our knowledge, this is the first time a systemic hormone is shown to direct local stem cell fate decisions. Given the protective, but mechanistically unclear role of steroid hormones in female colorectal cancer patients, our findings suggest a tumor-suppressive role for steroidal signaling by promoting postmitotic fate when local signaling is deteriorated.

Vein patterning by tissue-specific auxin transport

Unlike in animals, in plants, vein patterning does not rely on direct cell-cell interaction and cell migration; instead, it depends on the transport of the plant hormone auxin, which in turn depends on the activity of the PIN-FORMED1 (PIN1) auxin transporter. The current hypotheses of vein patterning by auxin transport propose that, in the epidermis of the developing leaf, PIN1-mediated auxin transport converges to peaks of auxin level. From those convergence points of epidermal PIN1 polarity, auxin would be transported in the inner tissues where it would give rise to major veins. Here, we have tested predictions of this hypothesis and have found them unsupported: epidermal PIN1 expression is neither required nor sufficient for auxin transport-dependent vein patterning, whereas inner-tissue PIN1 expression turns out to be both required and sufficient for auxin transport-dependent vein patterning. Our results refute all vein patterning hypotheses based on auxin transport from the epidermis and suggest alternatives for future tests.

Temporal Coordination of Collective Migration and Lumen Formation by Antagonism between Two Nuclear Receptors

During development, cells undergo multiple, distinct morphogenetic processes to form a tissue or organ, but how their temporal order and time interval are determined remain poorly understood. Here we show that the nuclear receptors E75 and DHR3 regulate the temporal order and time interval between the collective migration and lumen formation of a coherent group of cells named border cells during Drosophila oogenesis. We show that E75, in response to ecdysone signaling, antagonizes the activity of DHR3 during border cell migration, and DHR3 is necessary and sufficient for the subsequent lumen formation that is critical for micropyle morphogenesis. DHR3's lumen-inducing function is mainly mediated through βFtz-f1, another nuclear receptor and transcription factor. Furthermore, both DHR3 and βFtz-f1 are required for chitin secretion into the lumen, whereas DHR3 is sufficient for chitin secretion. Lastly, DHR3 and βFtz-f1 suppress JNK signaling in the border cells to downregulate cell adhesion during lumen formation.

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E75 antagonizes DHR3's function in inducing lumen formation of border cells (BCs)

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E75 and DHR3 temporally coordinate collective migration and lumen formation of BCs

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DHR3 is required and sufficient for chitin secretion into the lumen

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DHR3 and βFtz-f1 downregulate JNK signaling and cell adhesion in the BCs

20-Hydroxyecdysone-responsive microRNAs of insects

20-Hydroxyecdysone (20-HE) plays essential roles in coordinating developmental transitions of insects through responsive protein-coding genes and microRNAs (miRNAs). The involvement of single miRNAs in the ecdysone-signalling pathways has been extensively explored, but the interplay between ecdysone and the majority of miRNAs still remains largely unknown. Here, by small RNA sequencing, we systematically investigated the genome-wide responses of miRNAs to 20-HE in the embryogenic cell lines of Bombyx mori and Drosophila melanogaster. Over 60 and 70 20-HE-responsive miRNAs were identified in the BmE cell line and S2 cell line, respectively. The response of miRNAs to ecdysone exhibited a time-dependent pattern, and the response intensity increased with extending exposure to 20-HE. The relationship between ecdysone and the miRNAs was further explored through knockdown of ecdysone-signalling pathway genes. Specifically, ecdysone regulated the cluster miR-275 and miR-305 through the coordination of BmEcR-B and downstream BmE75B, and the interaction between BmEcR and miR-275 cluster was strengthened by the feedback regulation of BmE75B. Ecdysone induced miR-275-3p and miR-305-5p through the ecdysone response effectors (EcREs) at the upstream of the pre-miR-275 cluster. Overall, the results might help us further understand the relationship between ecdysone signalling pathways and small RNAs in the development and metamorphosis of insects.

Ecdysone receptor isoforms play distinct roles in larval-pupal-adult transition in Leptinotarsa decemlineata

A heterodimer of two nuclear receptors, ecdysone receptor (EcR) and ultraspiracle, mediates 20-hydroxyecdysone (20E) signaling to modulate many aspects in insect life, such as molting and metamorphosis, reproduction, diapause and innate immunity. In the present paper, we intended to determine the isoform-specific roles of EcR during larval-pupal-adult transition in the Colorado potato beetle. Double-stranded RNAs (dsRNAs) were prepared using the common (dsEcR) or isoform-specific (dsEcRA, dsEcRB1) regions of EcR as templates. Ingestion of either dsEcR or dsEcRA, rather than dsEcRB1, by the penultimate (3rd) and final (4th) instar larvae caused failure of larval-pupal and pupal-adult ecdysis. The RNA interference (RNAi) larvae remained as prepupae, or became deformed pupae and adults. Determination of messenger RNA (mRNA) levels of EcR isoforms found that LdEcRA regulates the expression of LdEcRB1. Moreover, silencing the two EcR transcripts, LdEcRA or LdEcRB1 reduced the mRNA levels of Ldspo and Ldsad, and lowered 20E titer. In contrast, the expression levels of HR3, HR4, E74 and E75 were significantly decreased in the LdEcR or LdEcRA RNAi larvae, but not in LdEcRB1 depleted specimens. Dietary supplement with 20E did not restore the expression of five 20E signaling genes (USP, HR3, HR4, E74 and E75), and only partially alleviated the pupation defects in dsEcR- or dsEcRA-fed beetles. These data suggest that EcR plays isoform-specific roles in the regulation of ecdysteroidogenesis and the transduction of 20E signal in L. decemlineata.

Nuclear receptor Ftz-f1 promotes follicle maturation and ovulation partly via bHLH/PAS transcription factor Sim

The NR5A-family nuclear receptors are highly conserved and function within the somatic follicle cells of the ovary to regulate folliculogenesis and ovulation in mammals; however, their roles in Drosophila ovaries are largely unknown. Here, we discover that Ftz-f1, one of the NR5A nuclear receptors in Drosophila, is transiently induced in follicle cells in late stages of oogenesis via ecdysteroid signaling. Genetic disruption of Ftz-f1 expression prevents follicle cell differentiation into the final maturation stage, which leads to anovulation. In addition, we demonstrate that the bHLH/PAS transcription factor Single-minded (Sim) acts as a direct target of Ftz-f1 to promote follicle cell differentiation/maturation and that Ftz-f1’s role in regulating Sim expression and follicle cell differentiation can be replaced by its mouse homolog steroidogenic factor 1 (mSF-1). Our work provides new insight into the regulation of follicle maturation in Drosophila and the conserved role of NR5A nuclear receptors in regulating folliculogenesis and ovulation.

When animals reproduce, females release eggs from their ovaries which then get fertilized by sperm from males. Each egg needs to properly mature within a collection of cells known as follicle cells before it can be let go. As the egg matures, so do the follicle cells surrounding it, until both are primed and ready to discharge the egg from the ovary. Mammals rely on a protein called SF-1 to mature their follicle cells, but it is unclear how this process works.

Most animals – from humans to fruit flies – release their eggs in a very similar way, using many of the same proteins and genes. For example, the gene for SF-1 in mammals is similar to a gene in fruit flies which codes for another protein called Ftz-f1. Since it is more straightforward to study ovaries in fruit flies than in humans and other mammals, investigating this protein could shed light on how follicle cells mature. However, it remained unclear whether Ftz-f1 plays a similar role to its mammalian counterpart.

Here, Knapp et al. show that Ftz-f1 is present in the follicle cells of fruit flies and is required for them to properly mature. Ftz-f1 controlled this process by regulating the activity of another protein called Sim. Further experiments found that the gene that codes for the SF-1 protein in mice was able to compensate for the loss of Ftz-f1 and drive follicle cells to mature.

Studying how ovaries release eggs is an essential part of understanding female fertility. This work highlights the similarities between these processes in mammals and fruit flies and may help us understand how ovaries work in humans and other mammals. In the future, the findings of Knapp et al. may lead to new therapies for infertility in females and other disorders that affect ovaries.

Ultraspiracle-independent anti-apoptotic function of ecdysone receptors is required for the survival of larval peptidergic neurons via suppression of grim expression in Drosophila melanogaster

In Drosophila melanogaster a significant number of heterogenous larval neurons in the central nervous system undergo metamorphosis-associated programmed cell death, termed metamorphoptosis. Interestingly distinct groups of doomed larval neurons are eliminated at different metamorphic phases. Although ecdysone hormonal signaling via nuclear ecdysone receptors (EcRs) is known to orchestrate the neuronal metamorphoptosis, little is known about how this signaling controls such diverse neuronal responses. Crustacean cardioactive peptide (CCAP)-producing neurons in the ventral nerve cord are developmentally programmed to die shortly after adult emergence. In this study, we show that disruption of endogenous EcR function by ectopic expression of dominant negative forms of EcRs (EcR^DN) causes premature death of larval CCAP neurons in a caspase-dependent manner. This event is rescued by co-expression of individual EcR isoforms. Furthermore, larval CCAP neurons are largely normal in ecr mutants lacking either EcR-A or EcR-B isoforms, suggesting that EcR isoforms redundantly function to protect larval CCAP neurons. Of surprise, a role of Ultraspiracle (Usp), a canonical partner of EcR, is dispensable in the protection of CCAP neurons, whereas both EcR and Usp are required for inducing metamorphoptosis of vCrz neurons shortly after prepupal formation. As a downstream, grim is an essential cell death gene for the EcR^DN-mediated CCAP neuronal death, while either hid or rpr function is dispensable. Together, our results suggest that Usp-independent EcR actions protect CCAP neurons from their premature death by repressing grim expression until their normally scheduled apoptosis at post-emergence. Our studies highlight two opposite roles played by EcR function for metamorphoptosis of two different peptidergic neuronal groups, proapoptotic (vCrz) versus antiapoptotic (CCAP), and propose that distinct death timings of doomed larval neurons are determined by differential signaling mechanisms involving EcR.

Cell migration by swimming: Drosophila adipocytes as a new in vivo model of adhesion-independent motility

Several cell lineages migrate through the developing and adult tissues of our bodies utilising a variety of modes of motility to suit the different substrates and environments they encounter en route to their destinations. Here we describe a novel adhesion-independent mode of single cell locomotion utilised by Drosophila fat body cells - the equivalent of vertebrate adipocytes. Like their human counterpart, these large cells were previously presumed to be immotile. However, in the Drosophila pupa fat body cells appear to be motile and migrate in a directed way towards wounds by peristaltic swimming through the hemolymph. The propulsive force is generated from a wave of cortical actomyosin that travels rearwards along the length of the cell. We discuss how this swimming mode of motility overcomes the physical constraints of microscopic objects moving in fluids, how fat body cells switch on other "motility machinery" to plug the wound on arrival, and whether other cell lineages in Drosophila and other organisms may, under certain circumstances, also adopt swimming as an effective mode of migration.

Broad Promotes Neuroepithelial Stem Cell Differentiation in the Drosophila Optic Lobe

Brain development requires the generation of the right number, and type, of neurons and glial cells at the right time. The Drosophila optic lobe, like mammalian brains, develops from simple neuroepithelia; they first divide symmetrically to expand the progenitor pool and then differentiate into neuroblasts, which divide asymmetrically to generate neurons and glial cells. Here, we investigate the mechanisms that control neuroepithelial growth and differentiation in the optic lobe. We find that the Broad/Tramtrack/Bric a brac-zinc finger protein Broad, which is dynamically expressed in the optic lobe neuroepithelia, promotes the transition of neuroepithelial cells to medulla neuroblasts. Loss of Broad function causes neuroepithelial cells to remain highly proliferative and delays neuroepithelial cell differentiation into neuroblasts, which leads to defective lamina and medulla. Conversely, Broad overexpression induces neuroepithelial cells to prematurely transform into medulla neuroblasts. We find that the ecdysone receptor is required for neuroepithelial maintenance and growth, and that Broad expression in neuroepithelial cells is repressed by the ecdysone receptor. Our studies identify Broad as an important cell-intrinsic transcription factor that promotes the neuroepithelial-cell-to-neuroblast transition.