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.

A novel chitin-binding protein from the vestimentiferan Riftia pachyptila interacts specifically with beta-chitin. Cloning, expression, and characterization

A cDNA from Riftia pachyptila was cloned. It encodes a novel 21.3-kDa protein from the worm protective tube, named RCBP (for Riftia chitin-binding protein). On the basis of partial tube-peptide sequences previously obtained, experiments using reverse transcriptase-mediated polymerase chain reaction and rapid amplification of cDNA ends led to the complete cDNA sequence. Analysis of its deduced amino acid sequence shows the presence of two chitin-binding domains. These domains are closely related to type 2 chitin-binding domains that are restricted to the animal kingdom. We showed by affinity assay and immunogold labeling that RCBP is the first protein so far known that binds specifically beta-chitin and that is unable to bind the most common alpha-form found in chitin secreting animals. The RCBP mRNA was found to be present in specific epidermal cells from the worm body wall, but never in the chitin-secreting gland cells. This unexpected result clearly indicates that this tube protein is synthesized in specialized areas of the outer epithelium and that at least two different tissues are involved in this exoskeleton synthesis.

Characterization of a cDNA encoding RP43, a CUB-domain-containing protein from the tube of Riftia pachyptila (Vestimentifera), and distribution of its transcript

A major 43 kDa protein from the protective tube of Riftia pachyptila (Vestimentifera), named RP43, was partly microsequenced after isolation by SDS/PAGE from the protein fraction of tubes collected around the hydrothermal vents at the East Pacific Rise. On the basis of the partial peptide sequences obtained, experiments using reverse-transcriptase-mediated PCR and rapid amplification of cDNA ends led to the complete cDNA sequence. Analysis of deduced amino acid sequence of RP43 showed the presence of CUB domains (100-110-residue-spanning domains first reported in the complement subcomponents C1r/C1s, epidermal-growth-factor-related sea urchin protein and bone morphogenetic protein 1) that seem to be involved in protein-protein and glycosaminoglycan-protein interactions. This peculiarity strongly suggests that RP43 might have a crucial role in tightening the different elements of the worm tube. However, the absence of chitin-binding motifs inclines us to favour a role in protein-protein interactions during assembly of the tube components. The RP43 mRNA was found to be present in specific epidermal cells from the worm body wall but never in the chitin-synthesizing gland cells. This unexpected result clearly indicates that the major tube protein is synthesized in specialized areas of the outer epithelium and that at least two different tissues are involved in the synthesis of the exoskeleton.

Molecular cloning and expression of Tenebrio molitor ultraspiracle during metamorphosis and in vivo induction of its phosphorylation by 20-hydroxyecdysone

Using a RT-PCR approach, the Tenebrio molitor homologue of Drosophila Ultraspiracle (TmUSP) was characterized. Its DNA binding domain shows a degree of identity with those of the other insect USPs. However, the ligand binding domain is closer to those of retinoid X receptors. Using an antibody raised against DmUSP, Western blot analysis of proteins from epidermis and other tissues revealed five immunoreactive bands, corresponding to different phosphorylated forms of a unique polypeptide, as shown by lambda-phosphatase treatment. The nuclear form of TmUSP seems unphosphorylated. An in vivo 20-hydroxyecdysone treatment increases considerably and rapidly the phosphorylated forms of TmUSP. This post-translational modification may play a role in the 20-hydroxyecdysone response.

Cloning and characterization of new orphan nuclear receptors and their developmental profiles during Tenebrio metamorphosis

Five PCR fragments corresponding to a part of the DNA-binding domain of different hormone nuclear receptors were isolated from Tenebrio molitor mRNAs. The sequence identity of three of them with known Drosophila nuclear receptors strongly suggests that they are the Tenebrio orthologs of seven-up, DHR3 and beta-FTZ-F1, and thus named Tmsvp, TmHR3 and TmFTZ-F1. The full-length sequences of the other two were established. TmHR78 is either a new receptor of the DHR78 family or the same gene which has evolved rapidly, particularly in the E domain. TmGRF belongs to the GCNF1 family and its in vitro translated product binds to the extended half site TCAAGGTCA with high affinity. The periods of expression of the corresponding transcripts in epidermal cells during Tenebrio metamorphosis were analyzed as a function of 20-hydroxyecdysone titers measured in the hemolymph of the animals taken for RNA extraction. Comparison of the expression profiles of these nuclear receptors with those observed during Drosophila metamorphosis revealed similar temporal correlations as a function of ecdysteroid variations, which further supported the sequence identity data for TmSVP, TmHR3, TmFTZ-F1 and TmHR78.

cDNA cloning of an adult male putative lipocalin specific to tergal gland aphrodisiac secretion in an insect (Leucophaea maderae)

Lma-P22 is a cuticular surface protein specific to the tergal gland secretion of Leucophaea maderae adult males which is ingested by females just before copulation. The complete Lma-P22 cDNA sequence was determined by RT-PCR using primers based on Edman degradation fragments. The recombinant protein expressed in Escherichia coli was recognized by an anti-Lma-P22 antibody. Northern blot analysis indicates that the corresponding mRNA is transcribed only in the epidermis of male tergites. Sequence analysis indicated that Lma-P22 deduced protein belongs to the lipocalin family. Lipocalins are extracellular proteins which carry hydrophobic compounds and some of them can bind sexual pheromone in vertebrates. Lma-P22 is the first example of a lipocalin-like protein involved in insect sexual behavior.

Is tubulin the sole antigen recognized by a putative anti-bursicon antibody?

A 56-kDa polypeptide suspected to be the tanning hormone 'bursicon' was analyzed using the monoclonal antibody (mAb) 01C10 of Song and Ma. We studied the beetle Tenebrio molitor, for which data on bursicon have been recently published. After purification by two-dimensional gel electrophoresis of brain proteins, the immunoreactive 56-kDa polypeptide was trypsinated and microsequenced. The obtained sequences revealed a high homology with alpha- and beta-tubulins. In a complementary study, immunoreactive clones were isolated, using the 01C10 mAb, from a library in expression vector obtained from Drosophila melanogaster head cDNAs. Again, the isolated clones were found, after cDNA sequencing, to correspond to tubulin. Our results suggest that, although the 01C10 mAb could possibly still have a great affinity for a polypeptide present in very low quantities in a few brain neurosecretory cells, it also proved to have an artefactual affinity for a 56-kDa polypeptide, identified as tubulin, which is not involved in tanning control.

Structure, organization and expression of two clustered cuticle protein genes during the metamorphosis of an insect, Tenebrio molitor

A 4-kb DNA segment of Tenebrio molitor (Insecta, Coleoptera) genomic DNA containing two larval-pupal cuticular genes has been cloned and sequenced. These genes, transcribed in opposite directions, are related in DNA sequence and the proteins encoded are very similar. Each of them contains a single intron located inside the sequence encoding the signal peptide, and a conserved sequence at -200 bp from the mRNA start position. These similarities in sequence suggest that these genes have evolved by duplication followed by diversification and that they are members of a family of genes with a common ancestry. They are the first example of clustered genes in Tenebrio molitor.

Characterization of two new cuticular genes specifically expressed during the post-ecdysial molting period in Tenebrio molitor

In a previous study, we have isolated a cDNA, TM-ACP17, coding for a post-ecdysial adult protein of Tenebrio molitor. After screening of a genomic library with TM-ACP17, we report isolation and sequencing of TM-ACP17 gene and a new gene, TM-LPCP29, coding for a larval-pupal protein. These two genes exhibit a common sequence of 15 nucleotides and a characteristic of most cuticular protein genes so far described: an intron interrupting the signal peptide. The deduced aa sequence of TM-LPCP29 exhibits a high percentage of Ala (26.5%) and Val (17.5%) and is highly hydrophobic. In the N-terminal part, the motif VAAPV is repeated ten times. Numerous histidine residues are present in the C- and N-terminal regions. A comparison is made with other cuticle protein sequences. Northern hybridization analysis showed that TM-LPCP29 is present during larval and mainly pupal post-ecdysial cuticle secretion. In-situ hybridization revealed that TM-LPCP29 mRNA is expressed in epidermis and not in muscles or fat body.

Cloning of two putative ecdysteroid receptor isoforms from Tenebrio molitor and their developmental expression in the epidermis during metamorphosis

Using the Drosophila EcR-B1 cDNA as a probe, we have cloned the putative ecdysteroid receptor from the mealworm Tenebrio molitor. We have isolated two cDNAs with different 5' termini that contain a complete open reading frame. These two cDNAs encode two proteins with distinct N-terminal regions corresponding to two isoforms. The coleopteran receptor is obviously related to the ecdysteroid receptor of other insects, but shares only 89% and 61% amino acid identities with the DNA-binding and ligand-binding domains of the Drosophila receptor, respectively. Its expression pattern has been examined in the epidermis during the last larval instar and pupal stage of T. molitor, in correlation with the hemolymph ecdysteroid titer. Hybridizations revealed two transcripts of 7 kb and 6.5 kb detected in most stages during metamorphosis and corresponding to the A and B1 isoforms. These two mRNAs are highly evident just before the rise of each ecdysteroid peak both in prepupae and in pupae. They show almost the same expression pattern in epidermis except for the second part of the pupal stage, during which only the A isoform is detected.

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.

Cloning and sequencing of a cDNA encoding a larval-pupal-specific cuticular protein in Tenebrio molitor (Insecta, Coleoptera). Developmental expression and effect of a juvenile hormone analogue

A cDNA clone encoding a larval-pupal cuticular protein, named TMLPCP-22, has been isolated by screening a library in expression vector with a monoclonal antibody made against pupal cuticular proteins of Tenebrio molitor. Northern-blot and in situ hybridization analyses showed that the expression of TMLPCP-22 is regulated in a stage-specific and tissue-specific manner; the transcript was present during the secretion of preecdysial larval and pupal cuticles and was restricted to epidermal cells. No expression was observed during adult cuticle deposition. In supernumerary pupae obtained after application of a juvenile hormone analogue, which is known to inhibit the adult programme, TMLPCP-22 mRNA was expressed again, confirming its larval-pupal specificity.

Identification, sequence and mRNA expression pattern during metamorphosis of a cDNA encoding a glycine-rich cuticular protein in Tenebrio molitor

The study of insect cuticular proteins and their sequences is of interest because they are involved in protein-protein and protein-chitin interactions which confer the mechanical properties and fine architecture of the cuticle. Moreover, in the coleopteran Tenebrio molitor there is a dramatic change in cuticular architecture between pre- and postecdysial secretion. We report the isolation, by differential screening, and the sequence characterization of a cDNA clone encoding a cuticular protein of T. molitor, ACP17. After insertion in the expression vector pEX1, the recognition of the fusion protein by an anti-cuticular monoclonal antibody confirmed the cuticular nature of ACP17. Northern hybridization analysis showed that ACP17 mRNA expression begins weakly 3 days before adult ecdysis and strongly increases during the secretion of postecdysial adult cuticle, with a maximum just after ecdysis. In situ hybridization revealed that the ACP17 mRNA is only present in the epidermis which secretes hard cuticle. The deduced amino acid (aa) composition exhibits a high content of Gly (28%) and Ala (20%) and, particularly, two poly(Gx) stretches separated by repetitive motifs with proline AAPVA. A comparison is made with other cuticle aa sequences.

Nucleotide sequence of an adult-specific cuticular protein gene from the beetle Tenebrio molitor: effects of 20-hydroxyecdysone on mRNA accumulation

The accumulation of transcripts from two adult-specific cuticular genes (ACP-20 and ACP-22) is shown to be modified after addition of exogenous 20-hydroxyecdysone. In the continuous presence of high levels of the hormone, the expression of ACP-20 gene is significantly weaker than that of untreated controls, while ACP-22 expression is 2.5-fold increased. During active synthesis of the ACP messages, a 0.5 microg 20-hydroxyecdysone injection causes a rapid 2-fold increase in ACP-22 mRNA and is not able to repress ACP-20 mRNA accumulation. We conclude that these genes whose transcripts appear in an almost coordinated manner in epidermal cells during the moulting cycle are regulated by ecdysteroids in a different way. In order to undertake a functional dissection of the promoter regions of ACP-22 gene, we have isolated and sequenced a genomic clone. The sequence similarities with other cuticular protein genes are discussed.

cDNA cloning and deduced amino acid sequence of a major, glycine-rich cuticular protein from the coleopteran Tenebrio molitor. Temporal and spatial distribution of the transcript during metamorphosis

In Coleoptera, the elytra (forewings), with a very hard and thick cuticle, protect the membranous and delicate hindwings against mechanical stress. We have isolated and characterized a cDNA encoding a major cuticle protein in Tenebrio molitor, named ACP-20. The deduced amino acid sequence is roughly tripartite, with two terminal glycine-rich domains and a central region showing pronounced similarities with some other hard cuticle proteins. Northern blot and in situ hybridization analyses reveal that ACP-20 gene expression is developmentally regulated since transcript accumulation occurs only in epidermal regions synthesizing hard cuticle and is restricted to the period of preecdysial adult cuticle deposition. Moreover, application of a juvenile hormone analogue prevents the appearance of the transcript, indicating that juvenile hormone, a key molecule involved in the control of insect metamorphosis, negatively regulates the expression of the ACP-20 gene.

Developmental profiles of epidermal mRNAs during the pupal-adult molt of Tenebrio molitor and isolation of a cDNA clone encoding an adult cuticular protein: effects of a juvenile hormone analogue

Changes in translatable mRNAs from the wing epidermis of the Coleoptera Tenebrio molitor have been investigated during metamorphosis by analysis of in vitro translated products. Striking differences between the patterns obtained from mRNAs extracted during pupal and adult cuticle secretion indicated that a drastic change in gene expression occurs during the pupal-adult transition. In addition to these stage-specific modifications, the mRNA patterns changed within each cuticular synthesis program (pupal or adult), especially at ecdysis. After tritiated leucine incorporation, some of the major radiolabeled cuticular proteins showed similar changes suggesting that the sequential appearance of mRNAs corresponds to sequential deposition of cuticular proteins. In supernumerary pupae obtained after juvenile hormone analogue (JHA) application on newly ecdysed pupae, translatable mRNA were very similar to those of pharate pupae. The JHA seemed, therefore, to prevent the expression of the adult program. By immunoblotting in vitro translated products with a monoclonal antibody recognizing an adult-specific cuticular protein, the developmental profile of the corresponding mRNA was studied. This mRNA was detected in anterior wing epidermis during the first 80 hr of the pharate adult stage. Using the same antibody, a cDNA clone was isolated from epidermal mRNA. The hybrid selected mRNA coded for only one protein with an apparent MW of 22 kDa which was, furthermore, recognized by the antibody. The Northern blot analysis performed with the clone confirmed the Western blot analysis of the in vitro translation products. JHA application at the beginning of the pupal-adult reprograming prevented the appearance of this mRNA; however, this transcript was present during the following molting cycle. This reversibility of the JHA action was confirmed by immunogold labeling of the cuticles formed in treated animals.

A monoclonal antibody against an adult-specific cuticular protein of Tenebrio molitor (Insecta, Coleoptera)

To study the sequential expression of the epidermal program in the mealworm Tenebrio molitor, monoclonal antibodies were prepared against the water-soluble proteins from preecdysial adult cuticle. Among the 16 clones obtained, one of them (named K2F6) recognized a 20-kDa antigen, found only in adult extracts but not in the larval or pupal ones, as revealed by immunoblot analysis. Our results strongly suggest an epidermal origin for this protein. The monoclonal antibody K2F6 fails to react with water-soluble proteins from fat body and hemolymph taken during the deposition of the 20-kDa antigen. Electron microscopic immunogold localization of this antigen showed that it is secreted, just after epicuticle deposition, in the 30 first-deposited preecdysial lamellae of sternal and elytral cuticles only. The sclerotizing process, which modifies the physicochemical properties of these cuticles, does not prevent the immunoreaction. When the expression of the adult program was inhibited by application of a juvenile hormone analog (ZR 515), the water-soluble proteins from different pupal-adult intermediates were never recognized by the monoclonal antibody K2F6 using immunoblot analysis. These results support the conclusion that this 20-kDa antigen is a protein specific for the sclerotized cuticle of the adult stage.

Stage-specific effects of diflubenzuron on ecdysteroid titers during the development of Tenebrio molitor: evidence for a change in hormonal source

The effects of the insect growth regulator diflubenzuron (DFB) were observed on the larval-larval and larval-pupal moulting cycles of Tenebrio molitor, after treatment at ecdysis. In both cases, the first parts of the cycles, from ecdysis to apolysis, were apparently not affected, but the pharate periods were lengthened; treated animals were generally unable to perform ecdysis and died at this step. The ecdysteroid titers in the hemolymph of treated animals were measured with a radioimmunoassay and compared to controls. During larval-larval cycles, the single ecdysteroid increase was not affected by DFB treatment. On the contrary, during larval-pupal development, a significant modification was observed; whereas two ecdysteroid peaks occurred in controls, the second peak of treated animals was significantly reduced and slightly delayed; however, the first peak was not modified. Taking into account that previous observations demonstrated a complete inhibition of the ecdysteroid peak in Tenebrio pupae, these stage-specific differences could reveal either a change in the DFB sensitivity of a sole endocrine source (i.e., prothoracic gland) or a change in hormone origin during metamorphosis. Ligation experiments during the last larval stage, in combination or not with DFB applications, clearly demonstrated the change in the moulting hormone source at the end of larval development in Tenebrio.

Variability of ecdysteroid-induced cell cycle alterations in Drosophila Kc sublines

The cell cycle of two lines isolated from Drosophila Kc cells was followed by flow cytofluorometry and cell counting. The first line is the 8-9K clone which grew in a medium supplemented with 5% serum; the second, named subline KcO, grew in a serum-free medium. The stationary phase is characterized by a G2 cell accumulation: 73% in the 8-9K clone and 50% in the KcO subline. When the medium was supplemented with the steroid moulting hormone 20-hydroxyecdysone, more than 90% of 8-9K cells and 65% of KcO cells were progressively arrested in G2. In the continuous presence of 20-hydroxyecdysone, most of the 8-9K cells remain G2-arrested; no massive G2 release into M was observed and only a few cells were able to divide. When treated for only 3 or 7 days, a transient release into M and proliferation occurred after hormone-free medium renewal, largely masked by G2 cell death. These results are discussed in comparison with other reports on cell cycle alteration induced by ecdysteroids.

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

In order to study the pupal-adult metamorphosis of Tenebrio in vitro, pupal sternites of different ages were cultured in Landureau's medium and their development systematically observed by electron microscopy. In hormone-free medium, explants taken from young pupae do not secrete pupal postecdysial cuticle in vitro, and the epidermis spontaneously detaches from the pupal cuticle. On the contrary, explants taken from pharate adults continue to secrete adult preecdysial cuticle in vitro, and the epidermis never detaches from the cuticle. Ecdysterone in physiological concentrations (0.2 to 4 micrograms/ml) induces the secretion of a new cuticle in explants from young pupae but the epidermis remains undifferentiated. Ecdysone is necessary for the induction of some adult differentiation. Moreover, the quality of the cuticle secreted in vitro is increased by the addition of 2% foetal calf serum; the best results have thus far been obtained in a medium containing 0.2 microgram/ml ecdysone, 1 microgram/ml ecdysterone, and 2% foetal calf serum.

[Variations in the structure of the abdominal cuticle of Tenebrio molitor L]

The ultrastructure of the adult abdominal cuticle of Tenebrio is described and special attention is given to the intersegmental zone in which the cuticle presents several architectural types, i.e. helicoidal, preferred and 'plywood' cuticle (Neville's terminology). This architectural polymorphism of the adult cuticle contrasts with the uniformity of larval and pupal cuticle architecture, which is entirely helicoidal.