Procuticle proteins and chitin-like material in the inner epicuticle of the Drosophila pupal cuticle

A Blueprint of Microstructures and Stage-Specific Transcriptome Dynamics of Cuticle Formation in Bombyx mori

Insect cuticle is critical for the environmental adaptability and insecticide resistance of insects. However, there is no clear understanding of the structure and protein components of the cuticle during each developmental stage of holometabolous insects, and knowledge about the protein components within each layer is vague. We conducted serial sectioning, cuticular structure analysis, and transcriptome sequencing of the larval, pupal, and adult cuticles of Bombyx mori. The deposition processes of epicuticle, exocuticle, and endocuticle during larval, pupal, and adult cuticle formation were similar. Transcriptome analysis showed that these cuticle formations share 74% of the expressed cuticular protein (CP) genes and 20 other structural protein genes, such as larval serum protein and prisilkin. There are seven, six, and eleven stage-specific expressed CP genes in larval, pupal, and adult cuticles, respectively. The types and levels of CP genes may be the key determinants of the properties of each cuticular layer. For example, the CPs of the RR-2 protein family with high contents of histidine (His) are more essential for the exocuticle. Functional analysis suggested that BmorCPAP1-H is involved in cuticle formation. This study not only offers an in-depth understanding of cuticle morphology and protein components but also facilitates the elucidation of molecular mechanisms underlying cuticle formation in future studies.

Tweedle proteins form extracellular two-dimensional structures defining body and cell shape in Drosophila melanogaster

Tissue function and shape rely on the organization of the extracellular matrix (ECM) produced by the respective cells. Our understanding of the underlying molecular mechanisms is limited. Here, we show that extracellular Tweedle (Twdl) proteins in the fruit fly Drosophila melanogaster form two adjacent two-dimensional sheets underneath the cuticle surface and above a distinct layer of dityrosinylated and probably elastic proteins enwrapping the whole body. Dominant mutations in twdl genes cause ectopic spherical aggregation of Twdl proteins that recruit dityrosinylated proteins at their periphery within lower cuticle regions. These aggregates perturb parallel ridges at the surface of epidermal cells that have been demonstrated to be crucial for body shaping. In one scenario, hence, this disorientation of epidermal ridges may explain the squatty phenotype of Twdl mutant larvae. In an alternative scenario, this phenotype may be due to the depletion of the dityrosinylated and elastic layer, and the consequent weakening of cuticle resistance against the internal hydrostatic pressure. According to Barlow's formula describing the distribution of internal pressure forces in pipes in dependence of pipe wall material properties, it follows that this reduction in turn causes lateral expansion at the expense of the antero-posterior elongation of the body.

Identification and expression of cuticular protein genes based on Locusta migratoria transcriptome

Many types of cuticular proteins are found in a single insect species, and their number and features are very diversified among insects. The cuticle matrix consists of many different proteins that confer the physical properties of the exoskeleton. However, the number and properties of cuticle proteins in Locusta migratoria remain unclear. In the present study, Illumina sequencing and de novo assembly were combined to characterize the transcriptome of L. migratoria. Eighty-one cuticular protein genes were identified and divided into five groups: the CPR family (51), Tweedle (2), CPF/CPFLs (9), CPAP family (9), and other genes (10). Based on the expression patterns in different tissues and stages, most of the genes as a test were distributed in the integument, pronotum and wings, and expressed in selected stages with different patterns. The results showed no obvious correlation between the expression patterns and the conservative motifs. Additionally, each cluster displayed a different expression pattern that may possess a different function in the cuticle. Furthermore, the complexity of the large variety of genes displayed differential expression during the molting cycle may be associated with cuticle formation and may provide insights into the gene networks related to cuticle formation.

Drosophilagenescutandminiatureare associated with the susceptibility to infection bySerratia marcescens

A mutant strain ofDrosophila melanogasterwith five markers on theX-chromosome was found to be more sensitive than the wild type when infected with an insect-pathogenic strain ofSerratia marcescens. Two of the five mutations in this fly strain,cutandminiature, were found to be responsible for this sensitivity. A double-mutant, with bothcutandminiature, was as sensitive toSerratiainfection as was the original sensitiveDrosophilastrain with all five mutations. Recombinant flies with other alleles ofcutandminiaturewere also sensitive. A revertant ofcutwas found to be less sensitive than the parental flies. Our insect pathogenic strain ofSerratiaproduces several proteases and a chitinase. A bacterial mutant, lacking proteases and chitinase, was found to be less virulent than wild-type bacteria. When pupal shells from resistant andcut-miniatureflies were incubated with a mixture of protease and chitinase there was a release ofN-acetyl glucosamine, and 50% more material was liberated from pupal shells of sensitive flies. Sensitive flies reared on sucrose infected withSerratiashowed bacteria in their hemolymph earlier than wild-type flies. We conclude thatDrosophilagenes forcutandminiatureare associated with the sensitivity toSerratiainfection, presumably because the gut peritrophic membrane is more susceptible to bacterial proteases and chitinase.

Genome-wide identification of cuticular protein genes in the silkworm, Bombyx mori

Many kinds of cuticular proteins are found in a single insect species and their numbers and features are diversified among insects. Because there are so many cuticular proteins and so much sequence variation among them, an overview of cuticular protein gene is needed. Recently, a complete silkworm genome sequence was obtained through the integration of data from two whole genome sequence projects performed independently in 2004. To identify cuticular protein genes in the silkworm Bombyx mori exhaustively, we searched both the Bombyx whole genome sequence as well as various EST libraries, and found 220 putative cuticular protein genes. We also revised the annotation of the gene model, and named each identified cuticular protein based on its motif. The phylogenetic tree of cuticular protein genes among B. mori, Drosophila melanogaster, and Apis mellifera revealed that duplicate cuticular protein clusters have evolved independently among insects. Comparison of EST libraries and northern blot analyses showed that the tissue- and stage-specific expression of each gene was intricately regulated, even between adjacent genes in the same gene cluster. This study reveals many novel cuticular protein genes as well as insights into cuticular protein gene regulation.

CPF and CPFL, two related gene families encoding cuticular proteins of Anopheles gambiae and other insects

Cuticular proteins (CPs) are structural proteins of insects as well as other arthropods. Several CP families have been described, among them a small family defined by a 51 amino acid motif [Andersen, S.O., Rafn, K., Roepstorff, P., 1997. Sequence studies of proteins from larval and pupal cuticle of the yellow meal worm, Tenebrio molitor. Insect Biochem. Mol. Biol. 27, 121-131]. We identified four proteins of this family in Anopheles gambiae that we have named CPF. We have also identified CPFs from other insects by searching databases. Alignment of these CPF proteins showed that the conserved region is only 44 aa long and revealed another conserved motif at the C-terminus. A dendrogram divided the CPF proteins into four groups, one basal and three specialized. We also identified several proteins of another CP family, CPFL, which has similarities to CPFs. CPFs and CPFLs share some protein motifs. Expression studies with real-time qRT-PCR of the A. gambiae CPFs and CPFLs showed that the four CPFs and one CPFL gene are expressed just before pupal or adult ecdysis, suggesting that they are components of the outer layer of pupal and adult cuticles. The other CPFLs appear to contribute to larval cuticle. Recombinant CPF proteins did not bind to chitin in the assay we used.

Isolation and comparison of different ecdysone-responsive cuticle protein genes in wing discs of Bombyx mori

Microarray analysis was used to isolate an ecdysone up-regulated cuticle protein gene from wing discs of Bombyx mori. Transcripts of isolated cDNAs were identified by Northern blot analysis. Expression of the BMWCP10 gene was observed during the W0-W3 stages with the strongest signal being at the W2 stage. In contrast, expression of the BMWCP2 gene was observed at the W3and P0 stages. Expression of BMWCP10 was identified after exposure to 20E in vitro, while that of BMWCP2 was identified after exposure to 20E followed by its removal. Induction of BMWCP10 by 20E was observed in 30 min and was not inhibited by cycloheximide. Expression of BMWCP2 was observed in wing discs cultured for more than 18 h in a hormone-free medium after 20E removal. At least 4 h exposure to 20E was required before removal for induction of BMWCP2. Induction of BMWCP2 required protein synthesis. Thus, different ecdysone-responsive cuticle protein genes in wing discs of Bombyx mori were isolated.

Insect cuticular proteins

Insect cuticles are composite structural materials with mechanical properties optimal for their biological functions. The bulk properties of cuticles are to a large extent determined by the interactions between the various components, mainly the chitin filament system and the proteins. The various cuticular types show pronounced differences in mechanical properties, and it is suggested that these differences can be related to the properties of the individual proteins and to the degree of secondary stabilization (sclerotization). The amino acid sequences, which have been obtained for insect cuticular proteins either by direct sequencing of purified proteins or by deduction from corresponding DNA-sequences, are listed according to insect order and species. Extensive sequence similarity is observed among several cuticular proteins obtained from different insect orders. Other cuticular proteins are characterized by repeated occurrence of a few small motifs consisting mainly of hydrophobic residues. The latter group of proteins has so far only been reported from stiff cuticles. The possible relevance of the various motifs and repeats for protein interaction and the mechanical properties of cuticles is discussed.

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.