Comparison of ecdysteroid production in Drosophila and Manduca: pharmacology and cross-species neural reactivity

Transcriptome Analysis of Drosophila melanogaster Third Instar Larval Ring Glands Points to Novel Functions and Uncovers a Cytochrome p450 Required for Development

In Drosophila melanogaster larvae, the ring gland (RG) is a control center that orchestrates major developmental transitions. It is a composite organ, consisting of the prothoracic gland, the corpus allatum, and the corpora cardiaca, each of which synthesizes and secretes a different hormone. Until now, the RG’s broader developmental roles beyond endocrine secretion have not been explored. RNA sequencing and analysis of a new transcriptome resource from D. melanogaster wandering third instar larval RGs has provided a fascinating insight into the diversity of developmental signaling in this organ. We have found strong enrichment of expression of two gene pathways not previously associated with the RG: immune response and fatty acid metabolism. We have also uncovered strong expression for many uncharacterized genes. Additionally, RNA interference against RG-enriched cytochrome p450s Cyp6u1 and Cyp6g2 produced a lethal ecdysone deficiency and a juvenile hormone deficiency, respectively, flagging a critical role for these genes in hormone synthesis. This transcriptome provides a valuable new resource for investigation of roles played by the RG in governing insect development.

How clocks and hormones act in concert to control the timing of insect development

During the last century, insect model systems have provided fascinating insights into the endocrinology and developmental biology of all animals. During the insect life cycle, molts and metamorphosis delineate transitions from one developmental stage to the next. In most insects, pulses of the steroid hormone ecdysone drive these developmental transitions by activating signaling cascades in target tissues. In holometabolous insects, ecdysone triggers metamorphosis, the remarkable remodeling of an immature larva into a sexually mature adult. The input from another developmental hormone, juvenile hormone (JH), is required to repress metamorphosis by promoting juvenile fates until the larva has acquired sufficient nutrients to survive metamorphosis. Ecdysone and JH act together as key endocrine timers to precisely control the onset of developmental transitions such as the molts, pupation, or eclosion. In this review, we will focus on the role of the endocrine system and the circadian clock, both individually and together, in temporally regulating insect development. Since this is not a coherent field, we will review recent developments that serve as examples to illuminate this complex topic. First, we will consider studies conducted in Rhodnius that revealed how circadian pathways exert temporal control over the production and release of ecdysone. We will then take a look at molecular and genetic data that revealed the presence of two circadian clocks, located in the brain and the prothoracic gland, that regulate eclosion rhythms in Drosophila. In this context, we will also review recent developments that examined how the ecdysone hierarchy delays the differentiation of the crustacean cardioactive peptide (CCAP) neurons, an event that is critical for the timing of ecdysis and eclosion. Finally, we will discuss some recent findings that transformed our understanding of JH function.

Neuronal influence on peripheral circadian oscillators in pupal Drosophila prothoracic glands

Rhythmic expression of period (per) and timeless (tim) genes in central circadian pacemaker neurons and prothoracic gland cells, part of the peripheral circadian oscillators in flies, may synergistically control eclosion rhythms, but their oscillatory profiles remain unclear. Here we show differences and interactions between peripheral and central oscillators using per-luciferase and cytosolic Ca²⁺ reporter (yellow cameleon) imaging in organotypic prothoracic gland cultures with or without the associated central nervous system. Isolated prothoracic gland cells exhibit light-insensitive synchronous per-transcriptional rhythms. In prothoracic gland cells associated with the central nervous system, however, per transcription is markedly amplified following 12-h light exposure, resulting in the manifestation of day–night rhythms in nuclear PER immunostaining levels and spontaneous Ca²⁺ spiking. Unlike PER expression, nuclear TIM expression is associated with day–night cycles that are independent of the central nervous system. These results demonstrate that photoreception and synaptic signal transduction in/from the central nervous system coordinate molecular 'gears' in endocrine oscillators to generate physiological rhythms.

In the fruit fly Drosophila, changes in expression of circadian clock genes are believed to control eclosion. Morioka and colleagues show that transcriptional oscillations of the clock gene, period, in prothoracic gland cells are amplified by photic inputs from the central nervous system.

Prothoracicotropic hormone regulates developmental timing and body size in Drosophila

In insects, control of body size is intimately linked to nutritional quality as well as environmental and genetic cues that regulate the timing of developmental transitions. Prothoracicotropic hormone (PTTH) has been proposed to play an essential role in regulating the production and/or release of ecdysone, a steroid hormone that stimulates molting and metamorphosis. In this report, we examine the consequences on Drosophila development of ablating the PTTH-producing neurons. Surprisingly, PTTH production is not essential for molting or metamorphosis. Instead, loss of PTTH results in delayed larval development and eclosion of larger flies with more cells. Prolonged feeding, without changing the rate of growth, causes the overgrowth and is a consequence of low ecdysteroid titers. These results indicate that final body size in insects is determined by a balance between growth-rate regulators such as insulin and developmental timing cues such as PTTH that set the duration of the feeding interval.

Ras activity in the Drosophila prothoracic gland regulates body size and developmental rate via ecdysone release

BACKGROUND

In Drosophila, each of the three larval instars ends with a molt, triggered by release of steroid molting hormone ecdysone from the prothoracic gland (PG). Because all growth occurs during the larval stages, final body size depends on both the larval growth rate and the duration of each larval stage, which in turn might be regulated by the timing of ecdysone release.

RESULTS

Here, we show that the expression of activated Ras, PI3 kinase (PI3K), or Raf specifically in the PG reduces body size, whereas activated Ras or PI3K, but not Raf, increases PG cell size. In contrast, expression of either dominant-negative (dn) Ras, Raf, or PI3K increases body size and prolongs the larval stages, leading to delayed pupariation, whereas expression of dn-PI3K, but not of dn-Raf or dn-Ras, reduces PG cell size. To test the possibility that altered ecdysone release is responsible for these phenotypes, we measured larval ecdysone levels indirectly, via the transcriptional activation of two ecdysone targets, E74A and E74B. We found that the activation of Ras within the PG induces precocious ecdysone release, whereas expression of either dn-PI3K or dn-Raf in the PG greatly attenuates the [ecdysone] increase that causes growth cessation and pupariation onset.

CONCLUSIONS

We conclude that Ras activity in the PG regulates body size and the duration of each larval stage by regulating ecdysone release. We also suggest that ecdysone release is regulated in two ways: a PI3K-dependent growth-promoting effect on PG cells, and a Raf-dependent step that may involve the transcriptional regulation of ecdysone biosynthetic genes.

Control and biochemical nature of the ecdysteroidogenic pathway

Molting is elicited by a critical titer of ecdysteroids that includes the principal molting hormone, 20-hydroxyecdysone (20E), and ecdysone (E), which is the precursor of 20E but also has morphogenetic roles of its own. The prothoracic glands are the predominate source of ecdysteroids, and the rate of synthesis of these polyhydroxylated sterols is critical for molting and metamorphosis. This review concerns three aspects of ecdysteroidogenesis: (a) how the brain neuropeptide prothoracicotropic hormone (PTTH) initiates a transductory cascade in cells of the prothoracic gland, which results in an increased rate of ecdysteroid biosynthesis (upregulation); (b) how the concentrations of 20E in the hemolymph feed back on the prothoracic gland to decrease rates of ecdysteroidogenesis (downregulation); and (c) how the prothoracic gland cells convert cholesterol to the precursor of E and then 20E, a series of reactions only now being understood because of the use of a combination of classical biochemistry and molecular genetics.

Cyclic AMP-dependent and independent stimulations of ovarian steroidogenesis by brain factors in the blowfly, Phormia regina

The involvement of cyclic-AMP (cAMP) as a potential second messenger in the neurohormonal control of ovarian steroidogenesis was investigated in the adult female blowfly Phormia regina. Individual measurements of ovarian cAMP concentrations and of ovarian biosynthesis of ecdysteroids, stimulated after a protein meal, demonstrated that steroidogenesis is preceded by a peak of cAMP in the ovaries. In vitro, ovarian steroidogenesis was stimulated by cell-permeable analogues of cAMP and by forskolin. Crude brain extracts were also able to elicit a rise of cAMP in the ovaries in vitro and the secretion of ecdysteroids into the medium: such extracts were more active before than after the protein meal, suggesting a rapid release of neuroendocrine material after feeding. Extracts were then prepared from the dorso-medial part of the brain, containing the neurosecretory cells of the pars intercerebralis (PI): these extracts were again found to stimulate the ovarian ecdysteroid secretion, but surprisingly, they failed to trigger a rise of cAMP in the ovaries in vitro. However, extracts from the rest of the cephalic nervous mass, deprived of PI, were also steroidogenic and they increased ovarian cAMP. Experiments with Rp-cAMPS, a cAMP antagonist, were not able to prevent the ecdysteroid stimulation by PI extracts, but did so partly for the extracts deprived of PI. This study thus indicates that at least two different cephalic factors are able to stimulate ovarian steroidogenesis in the blowfly, one elaborated by PI and acting via a cAMP-independent mechanism, and the other elaborated outside PI and using cAMP as a second messenger.

Genes expressed in the ring gland, the major endocrine organ of Drosophila melanogaster

We have used an enhancer-trap approach to begin characterizing the function of the Drosophila endocrine system during larval development. Five hundred and ten different lethal PZ element insertions were screened to identify those in which a reporter gene within the P element showed strong expression in part or all of the ring gland, the major site of production and release of developmental hormones, and which had a mutant phenotype consistent with an endocrine defect. Nine strong candidate genes were identified in this screen, and eight of these are expressed in the lateral cells of the ring gland that produce ecdysteroid molting hormone (EC). We have confirmed that the genes detected by these enhancer traps are expressed in patterns similar to those detected by the reporter gene. Two of the genes encode proteins, protein kinase A and calmodulin, that have previously been implicated in the signaling pathway leading to EC synthesis and release in other insects. A third gene product, the translational elongation factor EF-1alpha F1, could play a role in the translational regulation of EC production. The screen also identified the genes couch potato and tramtrack, previously known from their roles in peripheral nervous system development, as being expressed in the ring gland. One enhancer trap revealed expression of the gene encoding the C subunit of vacuolar ATPase (V-ATPase) in the medial cells of the ring gland, which produce the juvenile hormone that controls progression through developmental stages. This could reveal a function of V-ATPase in the response of this part of the ring gland to adenotropic neuropeptides. However, the gene identified by this enhancer trap is ubiquitously expressed, suggesting that the enhancer trap is detecting only a subset of its control elements. The results show that the enhancer trap approach can be a productive way of exploring tissue-specific genetic functions in Drosophila.

Ecdysteroidogenic action of Bombyx prothoracicotropic hormone and bombyxin on the prothoracic glands of Rhodnius prolixus in vitro

An in-vitro assay for ecdysteroid synthesis by the prothoracic glands (PGs) of fifth instar Rhodnius prolixus has been employed to evaluate the actions of prothoracicotropic neuropeptides from the silkmoth, Bombyx mori. Crude prothoracicotropic hormone (PTTH) extracts of recently emerged adult brain complexes of Bombyx induced a dose-dependent stimulation of ecdysteroid synthesis by Rhodnius PGs, which was similar to that obtained using crude Rhodnius PTTH. In both cases, maximum stimulation was obtained with one brain equivalent. Rhodnius PGs were then challenged with incremental doses of recombinant Bombyx PTTH and synthetic bombyxin-II. Dose-response curves for the action of both peptides on Rhodnius PGs were very similar to those obtained for their action on the pupal PGs of Bombyx in vitro. Bombyx PTTH stimulated the PGs of Rhodnius at concentrations comparable to those effective on Bombyx. The curve for Bombyx PTTH showed a steep ascending region from 3 to 8ng/ml and a sharp peak. For bombyxin, concentrations 40-fold higher were required to elicit the same amount of stimulation as obtained using Bombyx PTTH. Therefore, Rhodnius PGs possess recognition sites for both Bombyx PTTH and bombyxin. This is the first study of the ecdysteroidogenic properties of the Bombyx peptides on a heterologous species. It is suggested that the function and conformation of PTTH may be conserved between distantly related insect groups.

Disruption of the IP3 receptor gene of Drosophila affects larval metamorphosis and ecdysone release

BACKGROUND

The inositol 1,4,5-trisphosphate (IP3) receptor is an intracellular calcium channel that couples cell membrane receptors, via the second messenger IP3, to calcium signal transduction pathways within many types of cells. IP3 receptor function has been implicated in development, but the physiological processes affected by its function have yet to be elucidated. In order to identify these processes, we generated mutants in the IP3 receptor gene (itpr) of Drosophila and studied their phenotype during development.

RESULTS

All itpr mutant alleles were lethal. Lethality occurred primarily during the larval stages and was preceded by delayed moulting. Insect moulting occurs in response to the periodic release of the steroid hormone ecdysone which, in Drosophila, is synthesized and secreted by the ring gland. The observation of delayed moulting in the mutants, coupled with the expression of the IP3 receptor in the larval ring gland led us to examine the effect of the itpr alleles on ecdysone levels. On feeding ecdysone to mutant larvae, a partial rescue of the itpr phenotype was observed. In order to assess ecdysone levels at all larval stages, we examined transcripts of an ecdysone-inducible gene, E74; these transcripts were downregulated in larvae expressing each of the itpr alleles.

CONCLUSIONS

Our data show that disruption of the Drosophila IP3 receptor gene leads to lowered levels of ecdysone. Synthesis and release of ecdysone from the ring gland is thought to occur in response to a neurosecretory peptide hormone secreted by the brain. We propose that this peptide hormone requires an IP3 signalling pathway for ecdysone synthesis and release in Drosophila and other insects. This signal transduction mechanism which links neuropeptide hormones to steroid hormone secretion might be evolutionarily conserved.

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.