The apical plasma membrane of chitin-synthesizing epithelia

Dynamic microvilli sculpt bristles at nanometric scale

Organisms generate shapes across size scales. Whereas patterning and morphogenesis of macroscopic tissues has been extensively studied, the principles underlying the formation of micrometric and submicrometric structures remain largely enigmatic. Individual cells of polychaete annelids, so-called chaetoblasts, are associated with the generation of chitinous bristles of highly stereotypic geometry. Here we show that bristle formation requires a chitin-producing enzyme specifically expressed in the chaetoblasts. Chaetoblasts exhibit dynamic cell surfaces with stereotypical patterns of actin-rich microvilli. These microvilli can be matched with internal and external structures of bristles reconstructed from serial block-face electron micrographs. Individual chitin teeth are deposited by microvilli in an extension-disassembly cycle resembling a biological 3D printer. Consistently, pharmacological interference with actin dynamics leads to defects in tooth formation. Our study reveals that both material and shape of bristles are encoded by the same cell, and that microvilli play a role in micro- to submicrometric sculpting of biomaterials.

Pri peptides temporally coordinate transcriptional programs during epidermal differentiation

To achieve a highly differentiated state, cells undergo multiple transcriptional processes whose coordination and timing are not well understood. In Drosophila embryonic epidermal cells, polished-rice (Pri) smORF peptides act as temporal mediators of ecdysone to activate a transcriptional program leading to cell shape remodeling. Here, we show that the ecdysone/Pri axis concomitantly represses the transcription of a large subset of cuticle genes to ensure proper differentiation of the insect exoskeleton. The repression relies on the transcription factor Ken and persists for several days throughout early larval stages, during which a soft cuticle allows larval crawling. The onset of these cuticle genes normally awaits the end of larval stages when the rigid pupal case assembles, and their premature expression triggers abnormal sclerotization of the larval cuticle. These results uncovered a temporal switch to set up distinct structures of cuticles adapted to the animal lifestyle and which might be involved in the evolutionary history of insects.

Live Detection of Intracellular Chitin in Butterfly Wing Epithelial Cells In Vivo Using Fluorescent Brightener 28: Implications for the Development of Scales and Color Patterns

Chitin is the major component of the extracellular cuticle and plays multiple roles in insects. In butterflies, chitin builds wing scales for structural colors. Here, we show that intracellular chitin in live cells can be detected in vivo with fluorescent brightener 28 (FB28), focusing on wing epithelial cells of the small lycaenid butterfly Zizeeria maha immediately after pupation. A relatively small number of cells at the apical surface of the epithelium were strongly FB28-positive in the cytosol and seemed to have extensive ER-Golgi networks, which may be specialized chitin-secreting cells. Some cells had FB28-positive tadpole-tail-like or rod-like structures relative to the nucleus. We detected FB28-positive hexagonal intracellular objects and their associated structures extending toward the apical end of the cell, which may be developing scale bases and shafts. We also observed FB28-positive fibrous intracellular structures extending toward the basal end. Many cells were FB28-negative in the cytosol, which contained FB28-positive dots or discs. The present data are crucial to understanding the differentiation of the butterfly wing epithelium, including scale formation and color pattern determination. The use of FB28 in probing intracellular chitin in live cells may be applicable to other insect systems.

A dynamic interplay between chitin synthase and the proteins Expansion/Rebuf reveals that chitin polymerisation and translocation are uncoupled in Drosophila

Chitin is a highly abundant polymer in nature and a principal component of apical extracellular matrices in insects. In addition, chitin has proved to be an excellent biomaterial with multiple applications. In spite of its importance, the molecular mechanisms of chitin biosynthesis and chitin structural diversity are not fully elucidated yet. To investigate these issues, we use Drosophila as a model. We previously showed that chitin deposition in ectodermal tissues requires the concomitant activities of the chitin synthase enzyme Kkv and the functionally interchangeable proteins Exp and Reb. Exp/Reb are conserved proteins, but their mechanism of activity during chitin deposition has not been elucidated yet. Here, we carry out a cellular and molecular analysis of chitin deposition, and we show that chitin polymerisation and chitin translocation to the extracellular space are uncoupled. We find that Kkv activity in chitin translocation, but not in polymerisation, requires the activity of Exp/Reb, and in particular of its conserved Nα-MH2 domain. The activity of Kkv in chitin polymerisation and translocation correlate with Kkv subcellular localisation, and in absence of Kkv-mediated extracellular chitin deposition, chitin accumulates intracellularly as membrane-less punctae. Unexpectedly, we find that although Kkv and Exp/Reb display largely complementary patterns at the apical domain, Exp/Reb activity nonetheless regulates the topological distribution of Kkv at the apical membrane. We propose a model in which Exp/Reb regulate the organisation of Kkv complexes at the apical membrane, which, in turn, regulates the function of Kkv in extracellular chitin translocation.

The spirotetramat inhibits growth and reproduction of silkworm by interfering with the fatty acid metabolism

Spirotetramat is a novel insecticide and acaricide that can effectively control many species of piercing-sucking pests by inhibiting lipid synthesis. The silkworm is an economically important insect and a model organism for genetics and biochemical research. However, the toxic effect on their development and reproduction remain unclear. In this study, we demonstrated the negative effects of spirotetramat on the development, vitality, silk protein synthesis, and fecundity of silkworm. We also compared expression changes of silkworm genes using digital gene expression (DGE). A total of 1567 differentially expressed genes (DEGs) were detected, of which 874 genes were downregulated and 693 genes were upregulated. Gene Ontology (GO) enrichment analysis showed that the DEGs were enriched in the oxidation-reduction process, oxidoreductase activity, and fatty-acyl-CoA reductase activity. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that the DEGs were mainly enriched in fatty acid metabolism and lysosome pathways. We detected the relative expression of silkworm genes related to fatty acid synthesis and decomposition pathways and the degradation pathway of juvenile hormone by quantitative real-time PCR. The expression levels of Acetyl CoA carboxylase (ACC), fatty acyl-CoA reductase (FACR), Enoyl-CoA hydratase (ECH), very-long-chain (3R)-3-hydroxyacyl-CoA dehydratase (LCHAD), juvenile hormone epoxide hydrolase (JHEH), and phytanoyl-CoA dioxygenase (PCD) genes were downregulated. These data demonstrate the effects of spirotetramat on silkworm and its effects on genes involved in fatty acid metabolism.

Silencing uridine diphosphate N-acetylglucosamine pyrophosphorylase gene impairs larval development in Henosepilachna vigintioctopunctata

BACKGROUND

Uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc) diphosphorylase (UAP) catalyzes the formation of UDP-GlcNAc, the precursor for the production of chitin in ectodermally derived epidermal cells and midgut, for GlcNAcylation of proteins and for generation of glycosyl-phosphatidyl-inositol anchors in all tissues in Drosophila melanogaster.

RESULTS

Here, we identified a putative HvUAP gene in Henosepilachna vigintioctopunctata. Knockdown of HvUAP at the second-, third- and fourth-instar stages impaired larval development. Most resultant HvUAP hypomorphs showed arrested development at the third-, fourth-instar larval or prepupal stages, and became paralyzed, depending on the age when treated. Some HvUAP-silenced larvae had weak and soft scoli. A portion of HvUAP-depleted beetles formed misshapen pupae. No HvUAP RNA interference pupae successfully emerged as adults. Dissection and microscopic observation revealed that knockdown of HvUAP affected gut growth and food ingestion, reduced cuticle thickness, and negatively affected the formation of newly generated cuticle layers during ecdysis. Furthermore, HvUAP deficiency inhibited development of the tracheal respiratory system and thinned tracheal taenidia.

CONCLUSION

The phenotypical defects in HvUAP hypomorphs suggest that HvUAP is involved in the production of chitin. Moreover, our findings will enable the development of a double-stranded RNA-based pesticide to control H. vigintioctopunctata. © 2021 Society of Chemical Industry.

SERCA interacts with chitin synthase and participates in cuticular chitin biogenesis in Drosophila

The biogenesis of chitin, a major structural polysaccharide found in the cuticle and peritrophic matrix, is crucial for insect growth and development. Chitin synthase, a membrane-integral β-glycosyltransferase, has been identified as the core of the chitin biogenesis machinery. However, a yet unknown number of auxiliary proteins appear to assist in chitin biosynthesis, whose precise function remains elusive. Here, we identified a sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA), in the fruit fly Drosophila melanogaster, as a chitin biogenesis-associated protein. The physical interaction between DmSERCA and epidermal chitin synthase (Krotzkopf verkehrt, Kkv) was demonstrated and analyzed using split-ubiquitin membrane yeast two-hybrid, bimolecular fluorescent complementation, pull-down, and immunoprecipitation assays. The interaction involves N-terminal regions (aa 48-81 and aa 247-33) and C-terminal regions (aa 743-783 and aa 824-859) of DmSERCA and two N-terminal regions (aa 121-179 and aa 369-539) of Kkv, all of which are predicted be transmembrane helices. While tissue-specific knock-down of DmSERCA in the epidermis caused larval and pupal lethality, the knock-down of DmSERCA in wings resulted in smaller and crinkled wings, a significant decrease in chitin deposition, and the loss of chitin lamellar structure. Although DmSERCA is well-known for its role in muscular contraction, this study reveals a novel role in chitin synthesis, contributing to our knowledge on the machinery of chitin biogenesis.

Dissection, Fixation, and Immunostaining of the Drosophila Midgut

The Drosophila midgut is mainly composed of highly polarized epithelial cells called enterocytes that establish their apical-basal polarity in a fundamentally different way from other Drosophila epithelia. The roles of polarity factors in the midgut can be studied by generating clones of homozygous mutant cells in the background of wild-type tissue. In this chapter, we will introduce and discuss the procedures for producing positively marked mutant clones in the midgut and describe specific protocols for dissecting, fixing, and immunostaining this tissue.

RNAi for chitin synthase 1 rather than 2 causes growth delay and molting defect in Henosepilachna vigintioctopunctata

Chitin synthase (CHS) plays a critical role in chitin synthesis and excretion. In most insects, CHSs have been segregated into 1 and 2 classes. CHS1 is responsible for chitin production in the ectodermally-derived epidermal cells. CHS2 is dedicated to chitin biosynthesis in the midgut peritrophic matrix (PM). Henosepilachna vigintioctopunctata is a serious pest of Solanaceae and Cucurbitaceae plants. In this study, we identified HvCHS1 and HvCHS2. We found that HvCHS1 was abundantly transcribed in the larval tracheae and epidermis, whereas HvCHS2 was mainly expressed in the guts. Escherichia coli HT115 expressed double stranded RNAs targeting HvCHS1 and HvCHS2 (dsCHS1 and dsCHS2) were used to immerse potato foliage and the treated leaves were provided to the newly-molted fourth- and third-instar larvae. Ingestion of dsCHS1 by the fourth-instar larvae significantly diminished the target mRNA level and had slight influence on the expression of HvCHS2. In contrast, consumption of dsCHS2 significantly lowered the target mRNA level but triggered the transcription of HvCHS1. Knockdown of HvCHS1, rather than HvCHS2, arrested larval development and impaired larva-pupa-adult transition. A large proportion of HvCHS1 hypomorphs became stunting prepupae, deformed pupae or misshapen adults. Moreover, knockdown of HvCHS1 damaged gut integrity, decreased cuticle thickness, and delayed the formation of newly-generated cuticle layer during ecdysis. Furthermore, depletion of HvCHS1 inhibited the development of trachea system and thinned tracheal taenidia. Ingestion of dsCHS1 at the third-instar stage caused similar but severe negative effects. Our results demonstrated that HvCHS1 is responsible for chitin biosynthesis during ecdysis. Moreover, HvCHS1 is a potential amenable target gene and young larvae are more susceptible to dsRNA.

Understanding the structural diversity of chitins as a versatile biomaterial

Chitin is one of the most abundant biopolymers, and it has adopted many different structural conformations using a combination of different natural processes like biopolymerization, crystallization and non-equilibrium self-assembly. This leads to a number of striking physical effects like complex light scattering and polarization as well as unique mechanical properties. In doing so, chitin uses a fine balance between the highly ordered chain conformations in the nanofibrils and random disordered structures. In this opinion piece, we discuss the structural hierarchy of chitin, its crystalline states and the natural biosynthesis processes to create such specific structures and diversity. Among the examples we explored, the unified question arises from the generation of completely different bioarchitectures like the Christmas tree-like nanostructures, gyroids or helicoidal geometries using similar dynamic non-equilibrium growth processes. Understanding the in vivo development of such structures from gene expressions, enzymatic activities as well as the chemical matrix employed in different stages of the biosynthesis will allow us to shift the material design paradigms. Certainly, the complexity of the biology requires a collaborative and multi-disciplinary research effort. For the future's advanced technologies, using chitin will ultimately drive many innovations and alternatives using biomimicry in materials science. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 1)'.

Mechanobiology: Forging a strong matrix at tendons

Nature faces the challenge of stably attaching soft muscles to a stiff skeleton. A new study combines live imaging and fly genetics to reveal that mechanical tension and a putative intracellular chaperone assist in assembling the gigantic extracellular matrix protein Dumpy at fly tendon-skeleton interfaces.

DIA-based proteome reveals the involvement of cuticular proteins and lipids in the wing structure construction in the silkworm

Wing discs of Bombyx mori (B. mori) are transformed into wings during metamorphosis via dramatic morphological and structural changes. Mutations in genes related to the wings cause the adults to have altered wing shapes or abnormal wing colour. At present, there are more than 20 wing mutants recorded in the silkworm. However, the key factors that influence B. mori wing development are still unclear. Here, we used the strains +^Wes/+^Wes and Wes/+^Wes that are typical for the normal wing and shriveled wing phenotypes, respectively, to identify differentially expressed proteins by label-free data-independent acquisition (DIA). Ten enriched GO terms and 9 KEGG pathways were identified based on the 3993 proteins in the wings. Among the identified and quantified proteins, 370 differentially expressed proteins (DEPs) were detected (P-value <0.01, |log2FC| > 0.58). Mapping of the DEPs to the reference canonical pathways in KEGG showed that the top 20% of the pathways were related to fatty acid, cutin, suberin and wax biosynthesis, protein processing in endoplasmic reticulum, protein export, etc. Of the 370 DEPs, 238 were down-regulated, and 132 were up-regulated of Wes/+^Wes compared with +^Wes/+^Wes. Numerous cuticular proteins were down-regulated, and fatty metabolism enzymes were up-regulated, in Wes/+^Wes compared with +^Wes/+^Wes. SIGNIFICANCE: The comparative analysis of proteomes suggested that cuticular proteins and fatty metabolism enzymes are the main abnormally expressed proteins in the pupal wings of Wes/+^Wes, leading to curly and shrunken wings after moth transformation. Our results also identify the substances affecting the development of silkworm wings from the perspective of proteins. The information from this study is important for further research on the molecular mechanisms of wing development in lepidopteran insects, and these differentially expressed genes may be targets for Lepidoptera pest control.

Drosophila, Chitin and Insect Pest Management

Insects are a great menace in agriculture and vectors of human diseases. Hence, controlling insect populations is an important issue worldwide. A common strategy to control insects is the application of insecticides. However, insecticides entail three major problems. First, insecticides are chemicals that stress ecosystems and may even be harmful to humans. Second, insecticides are often unspecific and also eradicate beneficial insect species like the honeybee. Third, insects are able to develop resistance to insecticides. Therefore, the efficient generation of new potent insecticides and their intelligent delivery are the major tasks in agriculture. In addition, acceptance or refusal in society is a major issue that has to be considered in the application of a pest management strategy. In this paper, we unify two issues: 1) we illustrate that our molecular knowledge of the chitin synthesis and organization pathways may offer new opportunities to design novel insecticides that are environmentally harmless at the same time being specific to a pest species; and 2) we advocate that the fruit fly Drosophila melanogaster may serve as an excellent model of insect to study the effects of insecticides at the genetic, molecular and histology level in order to better understand their mode of action and to optimize their impact. Especially, chitin synthesis and organization proteins and enzymes are excellently dissected in the fruit fly, providing a rich source for new insecticide targets. Thus, D. melanogaster offers a cheap, efficient and fast assay system to address agricultural questions, as has been demonstrated to be the case in bio-medical research areas.

Knockdown of β-N-acetylglucosaminidase 2 Impairs Molting and Wing Development in Lasioderma serricorne (Fabricius)

β-N-acetylglucosaminidases (NAGs) are carbohydrate enzymes that degrade chitin oligosaccharides into N-acetylglucosamine monomers. This process is important for chitin degradation during insect development and metamorphosis. We identified and evaluated a β-N-acetylglucosaminidase 2 gene (LsNAG2) from the cigarette beetle, Lasioderma serricorne (Fabricius). The full-length open reading frame of LsNAG2 was 1776 bp and encoded a 591 amino acid protein. The glycoside hydrolase family 20 (GH20) catalytic domain and an additional GH20b domain of the LsNAG2 protein were highly conserved. Phylogenetic analysis revealed that LsNAG2 clustered with the group II NAGs. Quantitative real-time PCR analyses showed that LsNAG2 was expressed in all developmental stages and was most highly expressed in the late larval and late pupal stages. In the larval stage, LsNAG2 was predominantly expressed in the integument. Knockdown of LsNAG2 in fifth instar larvae disrupted larval-pupal molting and reduced the expression of four chitin synthesis genes (trehalase 1 (LsTRE1), UDP-N-acetylglucosamine pyrophosphorylase 1 and 2 (LsUAP1 and LsUAP2), and chitin synthase 1 (LsCHS1)). In late pupae, LsNAG2 depletion resulted in abnormal adult eclosion and wing deformities. The expression of five wing development-related genes (teashirt (LsTSH), vestigial (LsVG), wingless (LsWG), ventral veins lacking (LsVVL), and distal-less (LsDLL)) significantly declined in the LsNAG2-depleted beetles. These findings suggest that LsNAG2 is important for successful molting and wing development of L. serricorne.

Ultrastructural differentiation of plasma membrane and cell junctions in the hindgut cells is synchronized with key developmental transitions in Porcellio scaber

Differentiation of transporting epithelial cells during development of animal organisms includes remodelling of apical and basal plasma membranes to increase the available surface for transport and formation of occluding junctions, which maintain a paracellular diffusion barrier. This study provides a detailed ultrastructural analysis of apical and basal plasma membrane remodelling and cell junction formation in hindgut cells during late embryonic and early postembryonic development of the crustacean Porcellio scaber. Hindgut cells in late-stage embryos are columnar with flat apical and basal plasma membranes. In early-stage marsupial mancae the hindgut cells begin to acquire their characteristic dome shape, the first apical membrane folding is evident and the septate junctions expand considerably, all changes being probably associated with the onset of active feeding. In postmarsupial mancae the apical labyrinth is further elaborated and the septate junctions are expanded. This coincides with the transition to an external environment and food sources. First basal infoldings appear in the anterior chamber of early-stage marsupial mancae, but in the papillate region they are mostly formed in postmarsupial mancae. In molting late-stage marsupial mancae, the plasma membrane acquires a topology characteristic of cuticle-producing arthropod epithelia and the septate junctions are considerably reduced.

Book lung development in juveniles and adults of the cobweb spider, Parasteatoda tepidariorum C. L. Koch, 1841 (Araneomorphae, Theridiidae)

Light and transmission electron microscopy were used to study the development of new book lung lamellae in juvenile and adult spiders (Parasteatoda tepidariorum). As hypothesized earlier in a study of embryos, mesenchyme cells dispersed throughout the opisthosoma (EMT) are a likely source of precursor epithelial cells (MET) for the new lamellae. The precursor cells in juveniles and adults continue many of the complex activities observed in embryos, e.g., migration, alignment, lumen formation, thinning, elongation, and secretion of the cuticle of air channel walls and trabeculae. The apicobasal polarity of precursor cells for new channels is apparently induced by the polarity pattern of precursor cells of channels produced earlier. Thus, new air and hemolymph channels extend and continue the alternating pattern of older channels. At sites more distant from the spiracle and atrium, new channels are usually produced by the mode II process (intracellular alignment and merging of vesicles). These air channels have bridging trabeculae and are quite stable in size throughout their length. At sites closer to the spiracle and atrium, new channels may be produced by mode I (coalescence of merocrine vesicle secretion). This raises the hypothesis that structural and functional differences in mode I and II channels and differing oxygen and fluid conditions with distance from the spiracle and atrium determine the mode of formation of new channels. Observations herein support an earlier hypothesis that there is some intercellular apical/apical and basal/basal affinity among the opposed surfaces of aligned precursor cells. This results in the alternating pattern of air channels at the apical and hemolymph channels at the basal cell surfaces.

Timed Knickkopf function is essential for wing cuticle formation in Drosophila melanogaster

The insect cuticle is an extracellular matrix that consists of the polysaccharide chitin, proteins, lipids and organic molecules that are arranged in distinct horizontal layers. In Drosophila melanogaster, these layers are not formed sequentially, but, at least partially, at the same time. Timing of the underlying molecular mechanisms is conceivably crucial for cuticle formation. To study this issue, we determined the time period during which the function of Knickkopf (Knk), a key factor of chitin organization, is required for wing cuticle differentiation in D. melanogaster. Although knk is expressed throughout metamorphosis, we demonstrate that its expression 30 h prior and 48 h after pupariation is essential for correct wing cuticle formation. In other words, expression beyond this period is futile. Importantly, manipulation of Knk expression during this time causes wing bending suggesting an effect of Knk amounts on the physical properties of the wing cuticle. Manipulation of Knk expression also interferes with the structure and function of the cuticle surface. First, we show that the shape of surface nano-structures depends on the expression levels of knk. Second, we find that cuticle impermeability is compromised in wings with reduced knk expression. In summary, despite the extended supply of Knk during metamorphosis, controlled amounts of Knk are important for correct wing cuticle differentiation and function in a concise period of time.

Drosophila Chitinase 2 is expressed in chitin producing organs for cuticle formation

The architecture of the outer body wall cuticle is fundamental to protect arthropods against invading pathogens and numerous other harmful stresses. Such robust cuticles are formed by parallel running chitin microfibrils. Molting and also local wounding leads to dynamic assembly and disassembly of the chitin-matrix throughout development. However, the underlying molecular mechanisms that organize proper chitin-matrix formation are poorly known. Recently we identified a key region for cuticle thickening at the apical cell surface, the cuticle assembly zone, where Obstructor-A (Obst-A) coordinates the formation of the chitin-matrix. Obst-A binds chitin and the deacetylase Serpentine (Serp) in a core complex, which is required for chitin-matrix maturation and preservation. Here we present evidence that Chitinase 2 (Cht2) could be essential for this molecular machinery. We show that Cht2 is expressed in the chitin-matrix of epidermis, trachea, and the digestive system. There, Cht2 is enriched at the apical cell surface and the dense chitin-matrix. We further show that in Cht2 knockdown larvae the assembly zone is rudimentary, preventing normal cuticle formation and pore canal organization. As sequence similarities of Cht2 and the core complex proteins indicate evolutionarily conserved molecular mechanisms, our findings suggest that Cht2 is involved in chitin formation also in other insects.

The interplay between intestinal bacteria and host metabolism in health and disease: lessons from Drosophila melanogaster

All higher organisms negotiate a truce with their commensal microbes and battle pathogenic microbes on a daily basis. Much attention has been given to the role of the innate immune system in controlling intestinal microbes and to the strategies used by intestinal microbes to overcome the host immune response. However, it is becoming increasingly clear that the metabolisms of intestinal microbes and their hosts are linked and that this interaction is equally important for host health and well-being. For instance, an individual's array of commensal microbes can influence their predisposition to chronic metabolic diseases such as diabetes and obesity. A better understanding of host-microbe metabolic interactions is important in defining the molecular bases of these disorders and could potentially lead to new therapeutic avenues. Key advances in this area have been made using Drosophila melanogaster. Here, we review studies that have explored the impact of both commensal and pathogenic intestinal microbes on Drosophila carbohydrate and lipid metabolism. These studies have helped to elucidate the metabolites produced by intestinal microbes, the intestinal receptors that sense these metabolites, and the signaling pathways through which these metabolites manipulate host metabolism. Furthermore, they suggest that targeting microbial metabolism could represent an effective therapeutic strategy for human metabolic diseases and intestinal infection.

RNA interference of chitin synthase genes inhibits chitin biosynthesis and affects larval performance in Leptinotarsa decemlineata (Say)

Dietary introduction of bacterially expressed double-stranded RNA (dsRNA) has great potential for management of Leptinotarsa decemlineata. Identification of the most attractive candidate genes for RNA interference (RNAi) is the first step. In the present paper, three complete chitin synthase cDNA sequences (LdChSAa, LdChSAb and LdChSB) were cloned. LdChSAa and LdChSAb, two splicing variants of LdChSA gene, were highly expressed in ectodermally-derived epidermal cells forming epidermis, trachea, foregut and hindgut, whereas LdChSB was mainly transcribed in midgut cells. Feeding bacterially expressed dsChSA (derived from a common fragment of LdChSAa and LdChSAb), dsChSAa, dsChSAb and dsChSB in the second- and fourth-instar larvae specifically knocked down their target mRNAs. RNAi of LdChSAa+LdChSAb and LdChSAa lowered chitin contents in whole body and integument samples, and thinned tracheal taenidia. The resulting larvae failed to ecdyse, pupate, or emerge as adults. Comparably, knockdown of LdChSAb mainly affected pupal-adult molting. The LdChSAb RNAi pupae did not completely shed the old larval exuviae, which caused failure of adult emergence. In contrast, silencing of LdChSB significantly reduced foliage consumption, decreased chitin content in midgut sample, damaged midgut peritrophic matrix, and retarded larval growth. As a result, the development of the LdChSB RNAi hypomorphs was arrested. Our data reveal that these LdChSs are among the effective candidate genes for an RNAi-based control strategy against L. decemlineata.

IDENTIFICATION AND HORMONE INDUCTION OF PUTATIVE CHITIN SYNTHASE GENES AND SPLICE VARIANTS IN Leptinotarsa decemlineata (SAY)

Chitin synthase (ChS) plays a critical role in chitin synthesis and excretion. In this study, two ChS genes (LdChSA and LdChSB) were identified in Leptinotarsa decemlineata. LdChSA contains two splicing variants, LdChSAa and LdChSAb. Within the first, second, and third larval instars, the mRNA levels of LdChSAa, LdChSAb, and LdChSB coincide with the peaks of circulating 20-hydroxyecdysone (20E) and juvenile hormone (JH). In vitro culture of midguts and an in vivo bioassay revealed that 20E and an ecdysteroid agonist halofenozide stimulated the expression of the three LdChSs. Conversely, a reduction of 20E by RNA interference (RNAi) of an ecdysteroidogenesis gene LdSHD repressed the expression of these LdChSs, and ingestion of halofenozide by LdSHD RNAi larvae rescued the repression. Moreover, disruption of 20E signaling by RNAi of LdEcR, LdE75, LdHR3, and LdFTZ-F1 reduced the expression levels of these genes. Similarly, in vitro culture and an in vivo bioassay showed that exogenous JH and a JH analog methoprene activated the expression of the three LdChSs, whereas a decrease in JH by RNAi of a JH biosynthesis gene LdJHAMT downregulated these LdChSs. It seems that JH upregulates LdChSs at the early stage of each instar, whereas a 20E pulse triggers the transcription of LdChSs during molting in L. decemlineata.

Origin, evolution and function of the hemipteran perimicrovillar membrane with emphasis on Reduviidae that transmit Chagas disease

The peritrophic matrix is a chitin-protein structure that envelops the food bolus in the midgut of the majority of insects, but is absent in some groups which have, instead, an unusual extra-cellular lipoprotein membrane named the perimicrovillar membrane. The presence of the perimicrovillar membrane (PMM) allows these insects to exploit restricted ecological niches during all life stages. It is found only in some members of the superorder Paraneoptera and many of these species are of medical and economic importance. In this review we present an overview of the midgut and the digestive system of insects with an emphasis on the order Paraneoptera and differences found across phylogenetic groups. We discuss the importance of the PMM in Hemiptera and the apparent conservation of this structure among hemipteran groups, suggesting that the basic mechanism of PMM production is the same for different hemipteran species. We propose that the PMM is intimately involved in the interaction with parasites and as such should be a target for biological and chemical control of hemipteran insects of economic and medical importance.

Two Leptinotarsa uridine diphosphate N-acetylglucosamine pyrophosphorylases are specialized for chitin synthesis in larval epidermal cuticle and midgut peritrophic matrix

Uridine diphosphate-N-acetylglucosamine-pyrophosphorylase (UAP) is involved in the biosynthesis of chitin, an essential component of the epidermal cuticle and midgut peritrophic matrix (PM) in insects. In the present paper, two putative LdUAP genes were cloned in Leptinotarsa decemlineata. In vivo bioassay revealed that 20-hydroxyecdysone (20E) and an ecdysteroid agonist halofenozide activated the expression of the two LdUAPs, whereas a decrease in 20E by RNA interference (RNAi) of an ecdysteroidogenesis gene LdSHD and a 20E signaling gene LdFTZ-F1 repressed the expression. Juvenile hormone (JH), a JH analog pyriproxyfen and an increase in JH by RNAi of an allatostatin gene LdAS-C downregulated LdUAP1 but upregulated LdUAP2, whereas a decrease in JH by silencing of a JH biosynthesis gene LdJHAMT had converse effects. Thus, expression of LdUAPs responded to both 20E and JH. Moreover, knockdown of LdUAP1 reduced chitin contents in whole larvae and integument samples, thinned tracheal taenidia, impaired larval-larval molt, larval-pupal ecdysis and adult emergence. In contrast, silencing of LdUAP2 significantly reduced foliage consumption, decreased chitin content in midgut samples, damaged PM, and retarded larval growth. The resulting larvae had lighter fresh weights, smaller body sizes and depleted fat body. As a result, the development was arrested. Combined knockdown of LdUAP1 and LdUAP2 caused an additive negative effect. Our data suggest that LdUAP1 and LdUAP2 have specialized functions in biosynthesizing chitin in the epidermal cuticle and PM respectively in L. decemlineata.

Biosynthesis, Turnover, and Functions of Chitin in Insects

Chitin is a major component of the exoskeleton and the peritrophic matrix of insects. It forms complex structures in association with different assortments of cuticle and peritrophic matrix proteins to yield biocomposites with a wide range of physicochemical and mechanical properties. The growth and development of insects are intimately coupled with the biosynthesis, turnover, and modification of chitin. The genes encoding numerous enzymes of chitin metabolism and proteins that associate with and organize chitin have been uncovered by bioinformatics analyses. Many of these proteins are encoded by sets of large gene families. There is specialization among members within each family, which function in particular tissues or developmental stages. Chitin-containing matrices are dynamically modified at every developmental stage and are under developmental and/or physiological control. A thorough understanding of the diverse processes associated with the assembly and turnover of these chitinous matrices offers many strategies to achieve selective pest control.

ChtVis-Tomato, a genetic reporter for in vivo visualization of chitin deposition in Drosophila

Chitin is a polymer of N-acetylglucosamine that is abundant and widely found in the biological world. It is an important constituent of the cuticular exoskeleton that plays a key role in the insect life cycle. To date, the study of chitin deposition during cuticle formation has been limited by the lack of a method to detect it in living organisms. To overcome this limitation, we have developed ChtVis-Tomato, an in vivo reporter for chitin in Drosophila. ChtVis-Tomato encodes a fusion protein that contains an apical secretion signal, a chitin-binding domain (CBD), a fluorescent protein and a cleavage site to release it from the plasma membrane. The chitin reporter allowed us to study chitin deposition in time lapse experiments and by using it we have identified unexpected deposits of chitin fibers in Drosophila pupae. ChtVis-Tomato should facilitate future studies on chitin in Drosophila and other insects.

Elucidation of the serosal cuticle machinery in the beetle Tribolium by RNA sequencing and functional analysis of Knickkopf1, Retroactive and Laccase2

Insects have been extraordinary successful in colonizing terrestrial habitats and this success is partly due to a protective cuticle that mainly contains chitin and proteins. The cuticle has been well studied in larvae and adults, but little attention has been paid to the cuticle of the egg. This cuticle is secreted by the serosa, an extraembryonic epithelium that surrounds the yolk and embryo in all insect eggs, but was lost in the Schizophoran flies to which Drosophila belongs. We therefore set out to investigate serosal cuticle formation and function in a beetle (Tribolium castaneum) using RNAi-mediated knockdown of three candidate genes known to structure chitin in the adult cuticle, and we aimed to identify other serosal cuticle genes using RNA sequencing. Knockdown of Knickkopf (TcKnk-1) or Retroactive (TcRtv) affects the laminar structure of the serosal cuticle, as revealed by Transmission Electron Microscopy in knockdown eggs. In the absence of this laminar structure, significantly fewer eggs survive at low humidity compared to wild-type eggs. Survival in dry conditions is also adversely affected when cross-linking among proteins and chitin is prevented by Laccase2 (TcLac-2) RNAi. Finally, we compare the transcriptomes of wild-type eggs to serosa-less eggs and find serosa-biased expression of 21 cuticle-related genes including structural components, chitin deacetylases and chitinases. Our data indicate that the serosal cuticle utilizes the same machinery for structuring the cuticle as adults. We demonstrate that the structure of the cuticle is crucial for desiccation resistance, and we put forward the serosal cuticle of Tribolium as an excellent model to study the ecological properties of the insect cuticle.

Fruit flies in biomedical research

Many scientists complain that the current funding situation is dire. Indeed, there has been an overall decline in support in funding for research from the National Institutes of Health and the National Science Foundation. Within the Drosophila field, some of us question how long this funding crunch will last as it demotivates principal investigators and perhaps more importantly affects the long-term career choice of many young scientists. Yet numerous very interesting biological processes and avenues remain to be investigated in Drosophila, and probing questions can be answered fast and efficiently in flies to reveal new biological phenomena. Moreover, Drosophila is an excellent model organism for studies that have translational impact for genetic disease and for other medical implications such as vector-borne illnesses. We would like to promote a better collaboration between Drosophila geneticists/biologists and human geneticists/bioinformaticians/clinicians, as it would benefit both fields and significantly impact the research on human diseases.

Deciphering the genetic programme triggering timely and spatially-regulated chitin deposition

Organ and tissue formation requires a finely tuned temporal and spatial regulation of differentiation programmes. This is necessary to balance sufficient plasticity to undergo morphogenesis with the acquisition of the mature traits needed for physiological activity. Here we addressed this issue by analysing the deposition of the chitinous extracellular matrix of Drosophila, an essential element of the cuticle (skin) and respiratory system (tracheae) in this insect. Chitin deposition requires the activity of the chitin synthase Krotzkopf verkehrt (Kkv). Our data demonstrate that this process equally requires the activity of two other genes, namely expansion (exp) and rebuf (reb). We found that Exp and Reb have interchangeable functions, and in their absence no chitin is produced, in spite of the presence of Kkv. Conversely, when Kkv and Exp/Reb are co-expressed in the ectoderm, they promote chitin deposition, even in tissues normally devoid of this polysaccharide. Therefore, our results indicate that both functions are not only required but also sufficient to trigger chitin accumulation. We show that this mechanism is highly regulated in time and space, ensuring chitin accumulation in the correct tissues and developmental stages. Accordingly, we observed that unregulated chitin deposition disturbs morphogenesis, thus highlighting the need for tight regulation of this process. In summary, here we identify the genetic programme that triggers the timely and spatially regulated deposition of chitin and thus provide new insights into the extracellular matrix maturation required for physiological activity.

In this work we studied the maturation of the extracellular matrix during Drosophila embryogenesis. Drosophila deposit a chitin-rich extracellular matrix with key physiological functions, such as the control of organ size and shape, and cuticle formation. Chitin synthesis depends on chitin synthases, and in Drosophila the gene krotzkopf verkehrt (kkv) encodes the main enzyme of this family. Our observations indicate that Kkv alone is not sufficient to induce chitin formation. We have identified another function (which is exerted by the activity of two genes encoding MH2-domain proteins) that are equally required for chitin deposition. The most striking result of our analysis is that the presence of Kkv and the newly identified function is sufficient to trigger chitin deposition in ectodermally-derived tissues, even if they are normally devoid of this polysaccharide. Importantly, we also demonstrate that unregulated chitin deposition (absent, advanced, or ectopic) leads to severe defects in morphogenesis. We show that the temporal and spatial pattern of kkv and the other two genes perfectly recapitulates the deposition of chitin, thereby unveiling a highly co-ordinated mechanism for the acquisition of mature traits.

Peritrophic membrane origin in adult bees (Hymenoptera): immunolocalization

The midgut is a region of the digestive tract of bees with the lumen lined by a peritrophic membrane that is composed of chitin and proteins (peritrophins). The origin of the peritrophins in the midgut of adult bees is unknown. This study used an anti-peritrophin 55-kDa antibody to immunolocalize the sites of the peritrophic membrane synthesis in nine species of adult bees' representatives of different families and sociability levels. In all studied species the peritrophin-55 is produced by digestive cells in the entire midgut in the rough endoplasmic reticulum following transference to Golgi apparatus and released by secretory vesicles, which fuses with the plasma membrane and microvilli. Thus, in the representatives of different groups of bees, the PM is of type I.