Induction of puffs in polytene chromosomes of in vitro cultured salivary glands of Drosophila melanogaster by ecdysone and echysone analogues

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.

Mechanisms of transcriptional regulation of ecdysone response

The mechanisms of ecdysone-dependent expression have been studied for many decades. Initially, the activation of individual genes under the influence of ecdysone was studied on the model of polythene chromosomes from salivary glands of Drosophila melanogaster. These works helped to investigate the many aspects of the Drosophila development. They also revealed plenty of valuable information regarding the fundamental mechanisms controlling the genes’ work. Many years ago, a model describing the process of gene activation by ecdysone, named after the author – Ashburner model – was proposed. This model is still considered an excellent description of the ecdysone cascade, which is implemented in the salivary glands during the formation of the Drosophila pupa. However, these days there is an opinion that the response of cells to the hormone ecdysone can develop with significant differences, depending on the type of cells. The same genes can be activated or repressed under the influence of ecdysone in different tissues. Likely, certain DNA-binding transcription factors that are involved in the ecdysonedependent response together with the EcR/Usp heterodimer are responsible for cell-type specificity. A number of transcriptional regulators involved in the ecdysone response have been described. Among them are several complexes responsible for chromatin remodeling and modification. It has been shown by various methods that ecdysone-dependent activation/repression of gene transcription develops with significant structural changes of chromatin on regulatory elements. The description of the molecular mechanism of this process, in particular, the role of individual proteins in it, as well as structural interactions between various regulatory elements is a matter of the future. This review is aimed to discuss the available information regarding the main regulators that interact with the ecdysone receptor. We provide a brief description of the regulator’s participation in the ecdysone response and links to the corresponding study. We also discuss general aspects of the mechanism of ecdysone-dependent regulation and highlight the most promising points for further research.

Ecdysteroid signalling components in metamorphosis and development of the desert locust, Schistocerca gregaria

The arthropod-specific hormone family of ecdysteroids plays an important role in regulating diverse physiological processes, such as moulting and metamorphosis, reproduction, diapause and innate immunity. Ecdysteroids mediate their response by binding to a heterodimeric complex of two nuclear receptors, the ecdysone receptor (EcR) and the retinoid-X-receptor/ultraspiracle (RXR/USP). In this study we investigated the role of EcR and RXR in metamorphosis and development of the desert locust, Schistocerca gregaria. The desert locust is a voracious, phytophagous, swarming pest that can ruin crops and harvests in some of the world's poorest countries. A profound knowledge of the ecdysteroid signalling pathway can be used in the development of more target-specific insecticides to combat this harmful plague insect. Here we report an in-depth profiling study of the transcript levels of EcR and RXR, as well as its downstream response genes, in different tissues isolated throughout the last larval stage of a hemimetabolous insect, showing a clear correlation with circulating ecdysteroid titres. Using RNA interference (RNAi), the role of SgEcR/SgRXR in moulting and development was investigated. We have proven the importance of the receptor components for successful moulting of locust nymphs into the adult stage. Some SgEcR/SgRXR knockdown females were arrested in the last larval stage, and 65 % of them initiated vitellogenesis and oocyte maturation, which normally only occurs in adults. Furthermore, our results clearly indicate that at the peak of ecdysteroid synthesis, on day six of the last larval stage, knockdown of SgEcR/SgRXR is affecting the transcript levels of the Halloween genes, Spook, Shadow and Shade.

Genome-wide examination of the transcriptional response to ecdysteroids 20-hydroxyecdysone and ponasterone A in Drosophila melanogaster

Background

The 20-hydroxyecdysone (20E) hierarchy of gene activation serves as an attractive model system for studying the mode of steroid hormone regulated gene expression and development. Many structural analogs of 20E exist in nature and among them the plant-derived ponasterone A (PoA) is the most potent. PoA has a higher affinity for the 20E nuclear receptor, composed of the ecysone receptor (EcR) and Ultraspiracle proteins, than 20E and a comparison of the genes regulated by these hormones has not been performed. Furthermore, in Drosophila different cell types elicit different morphological responses to 20E yet the cell type specificity of the 20E transcriptional response has not been examined on a genome-wide scale. We aim to characterize the transcriptional response to 20E and PoA in Drosophila Kc cells and to 20E in salivary glands and provide a robust comparison of genes involved in each response.

Results

Our genome-wide microarray analysis of Kc167 cells treated with 20E or PoA revealed that far more genes are regulated by PoA than by 20E (256 vs 148 respectively) and that there is very little overlap between the transcriptional responses to each hormone. Interestingly, genes induced by 20E relative to PoA are enriched in functions related to development. We also find that many genes regulated by 20E in Kc167 cells are not regulated by 20E in salivary glands of wandering 3^rd instar larvae and we show that 20E-induced levels of EcR isoforms EcR-RA, ER-RC, and EcR-RD/E differ between Kc cells and salivary glands suggesting a possible cause for the observed differences in 20E-regulated gene transcription between the two cell types.

Conclusions

We report significant differences in the transcriptional responses of 20E and PoA, two steroid hormones that differ by only a single hydroxyl group. We also provide evidence that suggests that PoA induced death of non-adapted insects may be related to PoA regulating different set of genes when compared to 20E. In addition, we reveal large differences between Kc cells and salivary glands with regard to their genome-wide transcriptional response to 20E and show that the level of induction of certain EcR isoforms differ between Kc cells and salivary glands. We hypothesize that the differences in the transcriptional response may in part be due to differences in the EcR isoforms present in different cell types.

Chromatin-bound nuclear pore components regulate gene expression in higher eukaryotes

Nuclear pore complexes have recently been shown to play roles in gene activation; however their potential involvement in metazoan transcription remains unclear. Here we show that the nucleoporins Sec13, Nup98, and Nup88, as well as a group of FG-repeat nucleoporins, bind to the Drosophila genome at functionally distinct loci that often do not represent nuclear envelope contact sites. Whereas Nup88 localizes to silent loci, Sec13, Nup98, and a subset of FG-repeat nucleoporins bind to developmentally regulated genes undergoing transcription induction. Strikingly, RNAi-mediated knockdown of intranuclear Sec13 and Nup98 specifically inhibits transcription of their target genes and prevents efficient reactivation of transcription after heat shock, suggesting an essential role of NPC components in regulating complex gene expression programs of multicellular organisms.

Insights into the Malpighian tubule from functional genomics

Classical physiological study of the Malpighian tubule has led to a detailed understanding of fluid transport and its control across several species. With the sequencing of the Drosophila genome, and the concurrent development of post-genomic technologies such as microarrays,proteomics, metabolomics and systems biology, completely unexpected roles for the insect Malpighian tubule have emerged. As the insect body plan is simpler than that of mammals, tasks analogous to those performed by multiple mammalian organ systems must be shared out among insect tissues. As well as the classical roles in osmoregulation, the Malpighian tubule is highly specialized for organic solute transport, and for metabolism and detoxification. In Drosophila, the adult Malpighian tubule is the key tissue for defence against insecticides such as DDT; and it can also detect and mount an autonomous defence against bacterial invasion. While it is vital to continue to set insights obtained in Drosophila into the context of work in other species, the combination of post-genomic technologies and physiological validation can provide insights that might not otherwise have been apparent for many years.

The neuralized homology repeat 1 domain of Drosophila neuralized mediates nuclear envelope association and delta-dependent inhibition of nuclear import

Signaling by the Notch (N) pathway is critical for many developmental processes and requires complex trafficking of both the N receptor and its transmembrane ligands, Delta (Dl) and Serrate. neuralized encodes an E3 ubiquitin ligase required for N ligand internalization. Neuralized (Neur) is conserved from worms to humans and comprises two Neur homology repeat (NHR) domains, NHR1 and NHR2, and a carboxyl-terminal RING domain. We have previously shown that the RING domain is required for ubiquitin ligase activity and that NHR1 mediates binding to Dl, a ubiquitination target. In Drosophila, Neur associates with the plasma membrane and hepatocyte responsive serum phosphoprotein-positive endosomes. Here we demonstrate that Neur also exhibits nuclear envelope localization. We have determined that Neur subcellular localization is regulated by nuclear trafficking and that inhibition of chromosome region maintenance 1, a nuclear export receptor, interferes with Neur nuclear export, trapping Neur in the nucleus. Moreover, we demonstrate that nuclear envelope localization is mediated by the Neur NHR1 domain. Interestingly, Dl expression in Schneider cells is sufficient to inhibit Neur nuclear import and inhibition occurs in an NHR1-dependent manner, suggesting that Neur nuclear localization occurs in contexts where Dl expression is either low or absent. Consistent with this, we found that Neur exhibits nuclear trafficking and associates with the nuclear envelope in the secretory cells of the larval salivary gland and that overexpression of Dl can reduce Neur localization to the nucleus. Altogether, our data demonstrate that Neur localizes to the nuclear envelope and that this localization can be negatively regulated by Dl, suggesting a possible nuclear function for Neur in Drosophila.

Towards selectively modulating mineralocorticoid receptor function: lessons from other systems

Although there is clinical utility in blocking mineralocorticoid receptor (MR) action, the usefulness of available MR antagonists is limited because of cross-reactivity with the androgen and progesterone receptors (spironolactone) or possibly by low affinity for MR (eplerenone). MR binds aldosterone and physiologic glucocorticoids, such as cortisol, which both can act as MR agonists in epithelial tissues. However, in preliminary studies aldosterone and cortisol appear to induce different conformations in non-epithelial tissues; in the cardiomyocyte, cortisol usually acts as an MR antagonist, whereas in vascular smooth muscle cortisol mimics aldosterone actions if it can access MR, just as it does in the kidney. Thus, there are needs for improved MR antagonists with higher selectivity and potency and, if possible, for compounds that lock MR into specific desirable conformations. Efforts are underway to modulate selectively the action of many nuclear receptors, and insights from one nuclear receptor may be applicable to others given the similarities in structure and function. We have used traditional approaches aided by X-ray crystallography to obtain several classes of selective ligands that modulate thyroid receptor (TR) action. We describe the properties of these selective TR modulators here, and discuss the possibility that similar approaches to ligand design may yield MR interacting compounds with improved specificity and, possibly, tissue specificity.

Ultrastructural changes of Drosophila larval and prepupal salivary glands cultured in vitro with ecdysone

Alterations in the ultrastructure of in vitro cultured larval salivary glands of Drosophila melanogaster in response to the steroid hormone ecdysone were studied in relation to complex changes in puffing patterns. We found that the changes in the fine structure of cultured glands reflected progression of the puffing pattern, and they paralleled those seen in vivo. We observed that glue secretion by exocytosis, the main function of salivary glands, took place between puff stage 5 (PS5) and PS7. Glue could not be expectorated under culture conditions but was slowly released from the lumen through a duct into the medium. After the cultured glands reached PS13/PS14, further progress of puffing and fine structural alterations required that the ecdysteroid titer be transiently extremely low or absent. Under in vitro conditions we did not observe the putative new secretory program(s) described for glands in vivo after PS12. However, ultrastructural changes which unambiguously indicated that an autohistolytic process had begun in vitro started to appear after PS17. Many salivary gland cells developed numerous features of progressive self-degradation between PS18 and PS21. Actual degradation of salivary glands in vivo seemed to be rapid, but in vitro degradation was never completed, probably due to a lack of exogenous factors from the hemolymph. Manipulations of ecdysone titer in vitro in the culture medium, known during the larval puffing cycle to cause premature induction of developmentally specific puffing patterns, did not affect the normal development of ultrastructural features of the cytoplasm and nucleus.

Programmed cell death during metamorphosis in the blow-fly Calliphora vomitoria

During metamorphosis, the salivary glands of the blow-fly undergo programmed cell death. Data is presented indicating that this programmed cell death does not in many respects emulate classical apoptosis. The cells are seen to vacuolate and swell rather than condense and shrink. There appears to be a transient enhancement in autophagy and an increase in acid phosphatase activity. This is followed by the characteristic appearance of ribosomal and extracisternal sources of the enzyme leading to autolysis. There appears to be no lysosomal leakage of acid phosphatase. As in apoptosis, the mitochondria persist until the cell fragments. The nucleus, however, does not show the distinct chromatin margination and blebbing that is typical of apoptosis. These changes are compared with necrotic changes induced by experimental anoxia. Overall the results show that a programmed cell death distinct from classical apoptosis is taking place.

Insect nuclear receptors: a developmental and comparative perspective

The appearance of puffs on the polytene chromosomes of insect salivary glands incubated with 20-hydroxyecdysone provided the first demonstration that steroids act directly at the gene transcriptional level to bring about subsequent cellular changes (Becker, 1959; Clever and Karlson, 1960). Despite that auspicious beginning, learning about the molecular mechanisms that underlie the hormonal regulation of insect development was impeded for many years by the difficulty associated with isolating and identifying rare regulatory factors from limited tissue sources. The advent of recombinant DNA methodology and powerful techniques such as the polymerase chain reaction (PCR) along with the recognition that many important endocrine factors are structurally conserved across a wide range of species has, however, all but eliminated the technical obstacles once facing the insect endocrinologist trying to isolate and study these regulatory molecules. This review will discuss recent progress and recall some earlier experiments concerning the molecular basis of hormonal action in insects focusing primarily on the members of the nuclear hormone receptor superfamily in Drosophila melanogaster. Two members of this family comprise the functional ecdysteroid receptor and at least a dozen other "orphans" have been identified in Drosophila for which no cognate ligand has yet been found. Many of these orphans are regulated by ecdysteroids. A discussion of juvenile hormone binding proteins that are not family members has been included because of their potential impact on nuclear receptor function. As receptor homologues have been identified in other insects, several general ideas concerning insect hormonal regulation have begun to emerge and these will be examined from a comparative point of view.

Gene expression during metamorphosis: an ideal model for post-embryonic development

The precocious induction in vivo and in culture of insect and amphibian metamorphosis by exogenous ecdysteroids and thyroid hormones, and its retardation or inhibition by juvenile hormone and prolactin, respectively, has allowed the analysis of such diverse processes of post-embryonic development as morphogenesis, tissue remodelling, functional reorganization, and programmed cell death. Metamorphosis in vertebrates also shares many similarities with mammalian development in the late foetal and perinatal period. This review describes the regulation of expression of some of the 'adult' gene products during metamorphosis in invertebrates and vertebrates. Recent studies on metamorphosis have revealed the important role played by auto-induction of hormone receptor genes, based on which a model will be presented to explain the activation of 'downstream' genes which give rise to the adult phenotype. It will also be argued that metamorphosis is an ideal model for analyzing some of the major mechanisms governing post-embryonic development.

Role of H+ ions in volume and voltage of epithelial cell nuclei

Condensation of chromatin depends upon the ion composition in the cell nucleus. We tested in isolated nuclei of Madin-Darby canine kidney cells the influence of various ions on nuclear volume (i. e. DNA packing) and intranuclear voltage. After isolation, nuclei were superfused with cytosolic solutions in which Na+, K+, Ca2+ and H+ ions were varied. With video-imaging and microelectrode techniques nuclear volume and intranuclear potential were measured in response to the various ions. In control cytosolic solution, isolated nuclei exhibited an intranuclear electrical potential of -6.5 +/- 0.5 mV (relative to a reference electrode in the cytosolic solution) corresponding to a nuclear volume of 250 +/- 10 fl (n = 104). Changing the Na+, K+ or free Ca2+ concentration in the superfusate in the physiological range resulted in minor changes of volume and intranuclear potential whereas pH altered both parameters dramatically. Nuclear swelling and intranuclear negative voltage increased with alkalinization and decreased when pH was reduced. An intact nuclear envelope was found to be no prerequisite for maintaining intranuclear negativity, indicating that the composition and functional state of nuclear chromatin rather than specific ion permeabilities of the nuclear envelope determine nuclear electrical potential. We present a model that explains nuclear volume and voltage on the basis of interaction between negatively charged DNA and positively charged histones of the nuclear chromatin.

Patchy accumulation of apical Na+ transporters allows cross talk between extracellular space and cell nucleus

Intracellular Na+ activities and local current densities were measured in fused Madin-Darby canine kidney cells using Na+ and voltage-sensing microelectrodes. Na+ that enters the cell across the apical plasma membrane accumulates initially in the nucleoplasm, several seconds ahead of its appearance in the cell cytoplasm. The spatial distribution of Na+ currents, produced by a local superfusion of the cell surface, indicates a nonuniform, patchy accumulation of apical Na+ transporters in the vicinity of the nucleus. Such pathways for direct Na+ flux between extracellular space and cell nucleus could be potentially important for gene activation.

The promoter of DNA puff gene II/9-1 of Sciara coprophila is inducible by ecdysone in late prepupal salivary glands of Drosophila melanogaster

DNA puffs occur in Sciarid salivary gland chromosomes; they are sites of DNA amplification and intense transcription and they appear to encode secreted structural proteins needed for pupation. In this report we have used P-element transformation of Drosophila to study regulation of a Sciara DNA puff gene. We found that a 718-bp promoter fragment of DNA puff gene II/9-1 from Sciara coprophila directs expression of the bacterial reporter gene CAT in late prepupal salivary glands of transgenic Drosophila melanogaster. The identical tissue and analogous stage specificity indicate that some aspects of the ecdysone response are evolutionarily conserved between Drosophila and Sciara. When transgenic salivary glands are cultured in vitro, CAT activity is rapidly induced by ecdysone, suggesting direct control of gene expression by the ecdysone receptor. Putative stage-specific factors limit expression of the chimeric Sciara-CAT gene in transgenic Drosophila to late prepupae but not to third instar larvae when ecdysone titers are also high.

26-[125I]iodoponasterone A is a potent ecdysone and a sensitive radioligand for ecdysone receptors

The effects of ecdysone, the steroid molting hormone of arthropods, are of considerable interest both to insect physiologists and to those studying steroid-regulated gene expression. Yet progress in understanding ecdysone receptors has been inhibited by the lack of a suitable highly radioactive hormone analog with high affinity for the receptor. Here we report that the synthetic ecdysteroid 26-iodoponasterone A is one of the most active ecdysones known, inducing half-maximal morphological transformation in Drosophila Kc167 cells when present at 0.5 nM. 26-[125I]Iodoponasterone A can be prepared at a specific activity of 2175 Ci/mmol (1 Ci = 37 GBq) by reaction of the precursor 26-mesylinokosterone with carrier-free Na125I. The radiolabeled material binds to Kc167 cell ecdysone receptors specifically and with affinity (Kd ca. 3.8 X 10(-10) M). Thus, 26-[125I]iodoponasterone A appears to be a superior radioligand for ecdysone receptors on grounds both of affinity and of specific activity. Its ready availability should greatly facilitate studies of these receptors.

The biosynthetic pathway of ecdysone: studies with vitellogenic ovaries of Locusta migratoria (Orthoptera)

Ovaries of adult females of Locusta migratoria synthesize impressive amounts of the steroid hormone ecdysone (and related ecdysteroids) during the late phases of vitellogenesis. The present study, aimed at elucidating the sequence of the biosynthetic steps that lead from cholesterol to ecdysone, has taken benefit of this remarkable biological model by using a double approach: (1) isolation and physico-chemical identification of endogenous biogenetic intermediates; (2) metabolic study of labelled putative precursor molecules. The data presented in this paper lead us to propose the following sequence of events: conversion of cholesterol to 3 beta-hydroxy-5 beta-cholest-7-en-6-one (via several intermediates not identified in this study) followed by 14 beta-hydroxylation to 3 beta, 14 alpha-dihydroxy-5 beta-cholest-7-en-6-one; hydroxylation on the side-chain at C-25 and C-22 (in this order) to 2-deoxyecdysone; hydroxylation at C-2 to ecdysone.

ALTERED STEROID INDUCED PUFFING BY CHROMATIN BOUND ALUMINUM IN A POLYTENE CHROMOSOME OF THE BLACKFLY SIMULIUM VITTATUM

The neurotoxic element aluminum accumulates selectively upon nuclear chromatin of several cell types, including neurons and glial cells of experimental animals and man. However, no mechanism of toxic action has been identified. Histometric analyses of the puffing patterns induced by ecdysterone in polytene chromosomes in the dipteran, Simulium vittatum (Zetterstedt) showed that chromosomes of salivary gland cells exposed to aluminum in vitro exhibit significant alterations in their response to ecdysterone. Specifically, chromatin bound aluminum completely inhibits (p < 0.05) puffing at seven out of nine sites normally puffed by ecdysterone and partially inhibits (p < 0.05) puffing at one site. Aluminum induces a partial puff (p < 0.05) at one site previously inhibited when both aluminum and ecdysterone were present. These findings suggest that one mode of toxic action of chromatin bound aluminum may be related to changes in gene expression.

Acetylcholinesterase activity in an Aedes aegypti cell line

Considerable acetylcholinesterase (AChE) activity was detected in an Aedes aegypti established cell line. The enzyme is blocked by 10(-6) M eserine sulfate, displays excess substrate inhibition and slowly hydrolyzes butyrylthiocholine. A 2-fold stimulation of AChE activity was shown after 2 days exposure to 3 x 10(-7) M beta-ecdysone. AChE activity found in the fresh medium is the contribution of the fetal calf serum portion. A direct relationship between levels of serum and the AChE activity in the cultured cells was demonstrated.

Cell specific effect of ecdysone on RNA synthesis in the differentiated salivary gland of Acricotopus lucidus

The influence of the steroid hormone ecdysone on the RNA metabolism of 2 cell types (main and anterior lobe) of the Acricotopus salivary gland has been investigated. Ecdysone has been found to increase the synthesis of 4S RNA and poly(A)+ RNA in the main lobe, while the anterior lobe remained unaffected by the same treatement. The stimulated poly(A)+ RNA is characterized by its relatively low cytoplasmic turnover. No evidence has been found for the stimulation of ribosomal RNA synthesis in both cell types. Analysis of the metabolic stability of cytoplasmic poly(A)+ RNA after ecdysone administration revealed a different response of the main and anterior lobe cells. Only in the anterior lobe ecdysone causes a shortening of the lifetime of that part of poly(A)+ RNA which is peculiar in its early appearance and relatively low stability in the cytoplasm.

Evidence of hormonal control of the nucleolar activity in Sciara ocellaris

The nucleolus organizing region in the salivary gland of Sciara ocellaris becomes gradually less active as the larvae approach pupation. The cytological visible condensation of the nucleolar puff is paralleled by a decrease in the incorporation of [3H]-uridine into the ribosomal RNA. Alterations in the nucleolar behavior were brought about by extirpation of the neuroendocrine glands at six stages of development. There is a 'critical stage' during the 7th day of the 4th instar when the hormonal stimulus for pupation occurs. If the ligature is performed before this stage the NOR remains expanded and the larval-pupal ecdysis is inhibited; if done later on the NOR condenses at a normal rate and the ligated larvae mount to head-less pupae. These results suggest that inhibition of rRNA synthesis and consequent condensation of the NOR is under the control of the moulting hormone.

The ovary as a source of alpha-ecdysone in an adult mosquito

The ovaries of the mosquito Aedes aegypti cultured in vitro secrete material that behaves like ecdysone in a radioimmunoassay. The material was identified as alpha-ecdysone by high-resolution liquid and gas-liquid chromatography. Secretion reached a maximum 16 hr after a blood meal as shown by bioassay and direct determination. Ovariectomy reduced the concentration of ecdysone in the adult after a blood meal. Qualitative analysis of whole-body extracts indicated beta-ecdysone to be the principal species present. Thus the ovaries appear to secrete a prohormone, alpha-ecdysone, which is converted to beta-ecdysone. Beta-ecdysone plays a significant role in stimulating egg development in the adult mosquito and may have reproductive roles in other insects.