The in vitro development of the pupal integument and the effects of ecdysteroids in Tenebrio molitor (Insecta, Coleoptera)

Pupal Diapause Termination and Transcriptional Response of Antheraea pernyi (Lepidoptera: Saturniidae) Triggered by 20-Hydroxyecdysone

The pupal diapause of univoltine Antheraea pernyi hampers sericultural and biotechnological applications, which requires a high eclosion incidence after artificial diapause termination to ensure production of enough eggs. The effect of pupal diapause termination using 20-hydroxyecdysone (20E) on the eclosion incidence has not been well-documented in A. pernyi. Here, the dosage of injected 20E was optimized to efficiently terminate pupal diapause of A. pernyi, showing that inappropriate dosage of 20E can cause pupal lethality and a low eclosion incidence. The optimal ratio of 20E to 1-month-old pupae was determined as 6 μg/g. Morphological changes showed visible tissue dissociation at 3 days post-injection (dpi) and eye pigmentation at 5 dpi. Comprehensive transcriptome analysis identified 1,355/1,592, 494/203, 584/297, and 1,238/1,404 upregulated and downregulated genes at 1, 3, 6, and 9 dpi, respectively. The 117 genes enriched in the information processing pathways of “signal transduction” and “signaling molecules and interaction” were upregulated at 1 and 3 dpi, including the genes involved in FOXO signaling pathway. One chitinase, three trehalase, and five cathepsin genes related to energy metabolism and tissue dissociation showed high expression levels at the early stage, which were different from the upregulated expression of four other chitinase genes at the later stage. Simultaneously, the expression of several genes involved in molting hormone biosynthesis was also activated between 1 and 3 dpi. qRT-PCR further verified the expression patterns of two ecdysone receptor genes (EcRB1 and USP) and four downstream response genes (E93, Br-C, βFTZ-F1, and cathepsin L) at the pupal and pharate stages, respectively. Taken together, these genes serve as a resource for unraveling the mechanism underlying pupal-adult transition; these findings facilitate rearing of larvae more than once a year and biotechnological development through efficient termination of pupal diapause in A. pernyi in approximately half a month.

The regulation of expression of insect cuticle protein genes

The exoskeleton of insects (cuticle) is an assembly of chitin and cuticle proteins. Its physical properties are determined largely by the proteins it contains, and vary widely with developmental stages and body regions. The genes encoding cuticle proteins are therefore good models to study the molecular mechanisms of signalling by ecdysteroids and juvenile hormones, which regulate molting and metamorphosis in insects. This review summarizes the studies of hormonal regulation of insect cuticle protein genes, and the recent progress in the analysis of the regulatory sequences and transcription factors important for their expression.

A functional analysis of ACP-20, an adult-specific cuticular protein gene from the beetle Tenebrio: role of an intronic sequence in transcriptional activation during the late metamorphic period

A gene encoding the adult cuticular protein ACP-20 was isolated in Tenebrio. It consists of three exons interspersed by two introns, intron 1 interrupting the signal peptide. To understand the regulatory mechanisms of ACP-20 expression, ACP-20 promoter-luciferase reporter gene constructs were transfected into cultured pharate adult wing epidermis. Transfection assays needed the presence of 20-hydroxyecdysone, confirming that ACP-20 is up-regulated by ecdysteroids. Analysis of 5' deletion constructs revealed that three regions are necessary for high levels of transcription. Interaction experiments between intronic fragments and epidermal nuclear proteins confirmed the importance of intron 1 in ACP-20 transcriptional control, which results from the combined activity of regulatory cis-acting elements of the promoter and those of intron 1.

Ecdysone has an effect on the regeneration of midgut epithelial cells that is distinct from 20-hydroxyecdysone in the silkworm Bombyx mori

We investigated the effects of two ecdysteroids, ecdysone (E) and 20-hydroxyecdysone (20E), on silkworm larval development. Silkworm larvae, Bombyx mori, were fed an artificial diet supplemented with 20E during the fourth instar to promote premature molting. At the onset of the fifth instar, these precocious fifth-instar larvae were fed diets supplemented with either E or 20E to determine the effects of the two ecdysteroids on the morphology of midgut epithelial cells. Regeneration of midgut epithelial cells normally occurs only during the molting period. However, in larvae fed E, complete replacement of midgut epithelial cells was observed 24 h before the larvae entered apolysis. In larvae fed 20E, the morphology of midgut epithelial cells was disrupted, leading to death of the larvae during the fifth instar. We also observed similar differences in the effects of the two ecdysteroids in an in vitro experiment. These results suggest that E has a specific effect on the morphological change of midgut epithelial cells in precocious fifth-instar larvae that is distinct from 20E.

2-Deoxyecdysone is a circulating ecdysteroid in the beetle Zophobas atratus

A qualitative analysis of ecdysteroids has been performed during the post-embryonic development of the tenebrionid beetle, Zophobas atratus, by high performance liquid chromatography (HPLC) combined with enzyme immunoassay (EIA) using two different antibodies. Three HPLC peaks were found to be immunoreactive, in hemolymph extracts of both sexes. Moreover, these peaks had ecdysteroid-like UV spectra, determined using a photodiode array detector. The use of two different HPLC systems (reverse and normal phases), in combination with two different EIA antibodies, allowed us to identify 20-hydroxyecdysone (20E) and ecdysone (E), as the two main ecdysteroids, but also suggested the presence of 2-deoxyecdysone (2dE) as the third hemolymph component. Secretion of putative 2dE, together with E (but not 20E) was also demonstrated in vitro from incubations of prothoracic glands and of tegumental explants. In these experiments, either in vivo or in vitro, 3-dehydroecdysone was never observed. Our observations thus strongly suggest that 2dE is a circulating ecdysteroid in Z. atratus and may function as a prohormone during the development of some insects.

Ultrastructural Effects of a Non-Steroidal Ecdysone Agonist, RH-5992, on the Sixth Instar Larva of the Spruce Budworm, Choristoneura fumiferana

Force feeding of RH-5992 (Tebufenozide), a non-steroidal ecdysone agonist to newly moulted sixth instar larvae of the spruce budworm, Choristoneura fumiferana, (Lepidoptera: Tortricidae) initiates a precocious, incomplete moult. Within 6h post treatment (pt) the larva stops feeding and remains quiescent. Around 12hpt, the head capsule slips partially revealing an untanned new head capsule that appears wrinkled and poorly formed. By 24hrpt, the head capsule slippage is pronounced and there is a mid-dorsal split of the old cuticle in the thoracic region but there is no ecdysis. The larva remains moribund in this state and ultimately dies of starvation and desiccation. The temporal sequence of the external and internal changes of the integument were studied using both scanning and transmission electron microscopy. Within 3hpt, there is hypertrophy of the Golgi complex indicating synthetic activity and soon after, large, putative ecdysial droplets are seen. Within 24h, a new cuticle that lacks the endocuticular lamellae is formed. The formation of the various cuticular components, the degradation of the old cuticle and changes in the organelles of the epidermal cells of the mesothoracic tergite are described. The difference between the natural moult and the one induced by RH-5992 are explained on the basis of molecular events that take place during the moulting cycle. The persistence of this ecdysone agonist in the tissues permits the expression of all the genes that are up-regulated by the presence of the natural hormone but those that are turned on in the absence of the hormone are not expressed.

Up-regulation of an adult cuticular gene by 20-hydroxyecdysone in insect metamorphosing epidermis cultured in vitro

Pupal forewing epidermis of the Coleoptera, Tenebrio molitor, was used to develop an in vitro system to study the hormonal control of metamorphosis at the cellular and molecular levels. Exposure to 1 microM 20-hydroxyecdysone for 48 h caused the formation of a typical adult cuticle. Under these conditions the expression of ACP-20, an adult-specific cuticular gene, was fivefold higher than in absence of exogenous hormone. This stimulation was also observed when a higher level of 20-hydroxyecdysone was maintained, and prevented by protein inhibitors, indicating that 20-hydroxyecdysone does not act directly on this gene. Exposure to 20-hydroxyecdysone followed by exposure in hormone-free medium caused the cessation of this stimulation, showing the requirement of the 20-hydroxyecdysone continuous presence for stimulating ACP-20 gene expression. Thus, unlike the other cuticular protein genes so far studied, its expression is not repressed by 20-hydroxyecdysone, and does not need the decline in ecdysteroids titer.

Insect tissue culture systems: models for study of hormonal control of development

The regulation of growth and development of insects is under endocrine control and involves both juvenile hormones and ecdysteroids. Neuropeptides are master regulators which control the secretion of these hormones. Most experiments in insect endocrinology have been conducted in vivo, but tissue culture methodology is playing an increasing role due to the great interest in simpler model systems for the study of complex processes that occur in vivo. The availability of appropriate media has allowed the culture of a variety of insect organs and cell lines of defined origin which have kept certain properties of the parent tissues. Tissue culture approaches have been useful for studying hormonal control of morphogenetic processes. Cell lines are particularly suited to the study of hormonally regulated mechanisms of macromolecular biosynthesis and gene expression. Thus, the value of in vitro analysis in studies of regulation of hormone production is now recognized. Results obtained from tissue culture allow more precise definition of the hormonal requirements of insect cells and tissues for growth and differentiation and might make possible the discovery of new growth regulators.

Ecdysteroid titers and Y-organ activity during late anecdysis and proecdysis in the fiddler crab, Uca pugilator

The titer of ecdysone in hemolymph and the ratio of ecdysone to other radioimmunoassay(RIA)-active hemolymph ecdysteroids were compared to in vitro secretion of ecdysone in Y-organs removed from eyestalkless fiddler crabs at various times following eyestalk ablation. Using high-pressure liquid chromatography (HPLC) and RIA it was established that ecdysone, 20-hydroxyecdysone, and RIA-active metabolites are present in the hemolymph at the end of anecdysis and throughout proecdysis. There was little correlation between in vitro secretory activity and total ecdysone in circulation. Ratios of ecdysone and 20-hydroxyecdysone changed during proecdysis as did the percentage of total RIA activity attributable to both. Positive correlations were observed between in vitro Y-organ secretion rates and amounts of ecdysteroids extracted from hemolymph and added to incubation media.

Filipin-cholesterol complexes in plasma membranes and cell junctions of Tenebrio molitor epidermis

The polyene antibiotic filipin combines with cholesterol in membranes to form complexes that are readily identifiable in the electron microscope. The distribution of filipin-cholesterol (FC) complexes is most easily studied by freeze-fracture. Larval epidermis of Tenebrio molitor (Insecta, Coleoptera) was maintained in vitro for 48 hr, since the electrophysiological properties of the cells are best characterized under these conditions. The cells were fixed in buffered 3.0% glutaraldehyde at RT for 15 min, transferred to fresh fixative containing 1% DMSO and filipin (final concentration; 0.5 mg/ml) for 3 hr RT. Control cells were treated in fixative containing 1% DMSO only. In freeze fracture replicas, FC complexes appear on the plasma membrane as large circular protrusions measuring 26.5 +/- 6.8 nm (x +/- s.d.) n = 50, in diameter and 17.1 +/- 2.8 nm, n = 50, in height and 11.7 +/- 2.6 nm, n = 25, in depth. Protrusions are about two times more frequent on the E face while pits are several times more frequent on the P face. FC complexes are most abundant (greater than 50/mu m2) on the basal membrane surface of the cells but are excluded from regions of hemidesmosomal plaques that anchor the cells to the basal lamina. FC complexes are also abundant on the apical surfaces of the cells where cuticle secretion occurs. In the lateral regions below the junctional belt, FC complexes are less numerous but often appear to increase in frequency in a graded fashion away from the junctional region. The septate junctions are relatively free of FC complexes except in regions where they open to form islands. These islands often contain gap junctions but the FC complexes rarely invade the particle domains of the gap junctions. Single FC complexes were seen in three out of a total of 97 gap junctions. Exposure of the epidermis to 20-hydroxyecdysone for 24 hr in vitro did not induce the appearance of FC complexes within the cell junctions.