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

Knickkopf (LmKnk) is required for chitin organization in the foregut of Locusta migratoria

The foregut, located at the front of the digestive tract, serves a vital role in insects by storing and grinding food into small particles. The innermost layer of the foregut known as the chitinous intima, comes into direct contact with the food and acts as a protective barrier against abrasive particles. Knickkopf (Knk) is required for chitin organization in the chitinous exoskeleton, tracheae and wings. Despite its significance, little is known about the biological function of Knk in the foregut. In this study, we found that LmKnk was stably expressed in the foregut, and highly expressed before molting in Locusta migratoria. To ascertain the biological function of LmKnk in the foregut, we synthesized specific double-stranded LmKnk (dsLmKnk) and injected it into locusts. Our findings showed a significant decrease in the foregut size, along with reduced food intake and accumulation of residues in the foregut after dsLmKnk injection. Morphological observations revealed that newly formed intima became thinner and lacked chitin lamella. Furthermore, fluorescence immunohistochemistry revealed that LmKnk was located in the apical region of new intima and epithelial cells. Taken together, this study provides insights into the biological function of LmKnk in the foregut, and identifies the potential target gene for exploring biological pest management strategies.

Identification and Functional Insights of Knickkopf Genes in the Larval Cuticle of Leptinotarsa decemlineata

The Colorado potato beetle (Leptinotarsa decemlineata) is a major pest of potato crops. While Knickkopf (Knk) genes are essential for insect cuticle formation, their roles in pests like L. decemlineata remain unclear. This study aims to identify and characterize Knk genes in L. decemlineata and explore their functions in larval development and cuticle integrity. We used genomic and transcriptomic databases to identify LdKnk-family genes, validated through RT-PCR and RACE. Gene expression was analyzed at various developmental stages and tissues using qRT-PCR. RNA interference (RNAi) and Transmission electron microscopy (TEM) were applied to determine the functional roles of these genes. Four LdKnk-family genes were identified. Spatio-temporal expression analysis indicated significant gene expression during larval molting and pupal stages, especially in the epidermis. RNAi experiments showed that silencing LdKnk and LdKnk3-5' led to reduced larval weight, cuticle thinning, and increased mortality, while LdKnk3-FL knockdown caused abnormal cuticle thickening and molting disruptions. LdKnk2 knockdown increased epicuticle and endocuticle thickness without visible phenotypic changes. The study highlights the essential roles of LdKnk-family genes in maintaining cuticle structure and integrity, suggesting their potential as targets for RNAi-based pest control.

Dynamics of cuticle-associated transcript profiles during moulting of the bed bug Cimexlectularius

The bed bug Cimex lectularius is a worldwide human pest. The sequenced genome allows molecular analyses of all aspects of bed bug biology. The present work was conducted to contribute to bed bug cuticle biology. As in other insect species, the C. lectularius cuticle consists of the three horizontal layers procuticle, epicuticle and envelope. To analyse the genes needed for the establishment of the stratified cuticle, we studied the expression pattern of 42 key cuticle-related genes at the transition of the penultimate nymphal stage to adult animals when a new cuticle is formed. Based on gene expression dynamics, in simplified model, we distinguish two key events during cuticle renewal in C. lectularius. First, upon blood feeding, modulation of ecdysone signalling culminates in the transcriptional activation of the transcription factor Clec-Ftz-F1 that possibly controls the expression of 32 of the 42 genes tested. Second, timed expression of Clec-Ftz-F1 seems to depend also on the insulin signalling pathway as RNA interference against transcripts of the insulin receptor delays Clec-Ftz-F1 expression and stage transition. An important observation of our transcript survey is that genes needed for the construction of the three cuticle layers are largely expressed simultaneously. Based on these data, we hypothesise a considerable synchronous mechanism of layer formation rather than a strictly sequential one. Together, this work provides a basis for functional analyses of cuticle formation in C. lectularius.

Teratogenic impacts of Antiepileptic drugs on development, behavior and reproduction in Drosophila melanogaster

Clobazam (CLB) and Vigabatrin (VGB) are the two widely used Antiepileptic drugs, which may have teratogenic potentiality and it has been evaluated in the fruit fly Drosophila melanogaster. These different concentrations of CLB (0.156, 0.25, and 0.312 μg/ml) and VGB (17.6, 22, and 44 μg/ml) were used to evaluate the life-history parameters, developmental, and behavioral abnormalities. The results revealed that life-history parameters (fecundity, fertility, larval and pupal mortality) were significantly affected along with varied developmental duration, and pupal and adult deformities in flies on exposure of CLB and VGB in concentration dependent manner. The present study demonstrated that the prenatal treatment of CLB and VGB has displayed clear teratogenic potentiality with various deformities in the fruit fly. The findings could be correlated with the various abnormalities in human caused by the use of AEDs.

Chitinase 6 is required for procuticle thickening and organ shape in Drosophila wing

The polysaccharide chitin is a major scaffolding molecule in the insect cuticle. In order to be functional, both chitin amounts and chitin organization have been shown to be important parameters. Despite great advances in the past decade, the molecular mechanisms of chitin synthesis and organization are not fully understood. Here, we have characterized the function of the Chitinase 6 (Cht6) in the formation of the wing, which is a simple flat cuticle organ, in the fruit fly Drosophila melanogaster. Reduction of Cht6 function by RNA interference during wing development does not affect chitin organization, but entails a thinner cuticle suggesting reduced chitin amounts. This phenotype is opposed to the one reported recently to be caused by reduction of Cht10 expression. Probably as a consequence, cuticle permeability to xenobiotics is enhanced in Cht6-less wings. We also observed massive deformation of these wings. In addition, the shape of the abdomen is markedly changed upon abdominal suppression of Cht6. Finally, we found that suppression of Cht6 transcript levels influences the expression of genes coding for enzymes of the chitin biosynthesis pathway. This finding indicates that wing epidermal cells respond to activity changes of Cht6 probably trying to adjust chitin amounts. Together, in a working model, we propose that Cht6-introduced modifications of chitin are needed for chitin synthesis to proceed correctly. Cuticle thickness, according to our hypothesis, is in turn required for correct organ or body part shape. The molecular mechanisms of this processes shall be characterized in the future.

Fatty acid binding protein is required for chitin biosynthesis in the wing of Drosophila melanogaster

Chitin, the major structural polysaccharide in arthropods such as insects and mites, is a linear polymer of N-acetylglucosamine units. The growth and development of insects are intimately coupled with chitin biosynthesis. The membrane-bound β-glycosyltransferase chitin synthase is known to catalyze the key polymerization step of N-acetylglucosamine. However, the additional proteins that might assist chitin synthase during chitin biosynthesis are not well understood. Recently, fatty acid binding protein (Fabp) has been suggested as a candidate that interacts with the chitin synthase Krotzkopf verkehrt (Kkv) in Drosophila melanogaster. Here, using split-ubiquitin membrane yeast two-hybrid and pull-down assays, we have demonstrated that the Fabp-B splice variant physically interacts with Kkv in vitro. The global knockdown of Fabp in D. melanogaster using RNA interference (RNAi) induced lethality at the larval stage. Moreover, in tissue-specific RNAi experiments, silenced Fabp expression in the epidermis and tracheal system caused a lethal larval phenotype. Fabp knockdown in the wings resulted in an abnormal wing development and uneven cuticular surface. In addition to reducing the chitin content in the first longitudinal vein of wings, Fabp silencing also caused the loss of procuticle laminate structures. This study revealed that Fabp plays an important role in chitin synthesis and contributes to a comprehensive understanding of the complex insect chitin biosynthesis.

Fluorescent Microscopy-Based Detection of Chitin in Intact Drosophila melanogaster

Chitin is the major scaffolding component of the insect cuticle. Ultrastructural analyses revealed that chitin adopts a quasi-crystalline structure building sheets of parallel running microfibrils. These sheets called laminae are stacked either helicoidally or with a preferred orientation of the microfibrils. Precise control of chitin synthesis is mandatory to ensure the correct chitin assembly and in turn proper function of cuticular structures. Thus, evaluation of chitin-metabolism deficient phenotypes is a key to our understanding of the function of the proteins and enzymes involved in cuticle architecture and more generally in cuticle biology in insects. Usually, these phenotypes have been assessed using electron microscopy, which is time-consuming and labor intensive. This stresses the need for rapid and straightforward histological methods to visualize chitin at the whole tissue level. Here, we propose a simple method of chitin staining using the common polysaccharide marker Fluorescent brightener 28 (FB28) in whole-mount Drosophila melanogaster. To overcome the physical barrier of FB28 penetration into the cuticle, staining is performed at 65°C without affecting intactness. We quantify FB28 fluorescence in three functionally different cuticular structures namely wings, dorsal abdomens and forelegs by fluorescence microscopy. We find that, as expected, cuticle pigmentation may interfere with FB28 staining. Down-regulation of critical genes involved in chitin metabolism, including those coding for chitin synthase or chitinases, show that FB28 fluorescence reflects chitin content in these organs. We think that this simple method could be easily applied to a large variety of intact insects.

The DOMON domain protein LmKnk contributes to correct chitin content, pore canal formation and lipid deposition in the cuticle of Locusta migratoria during moulting

Insects prevent uncontrolled penetration of water and xenobiotics by producing an impermeable cuticle. The major component of the cuticle is chitin that adopts a crystalline structure thereby contributing to cuticle stability. Our understanding of the contribution of chitin to the cuticle barrier function is limited. Here, we studied the role of the DOMON domain protein Knickkopf (LmKnk) that is involved in chitin organization and cuticle permeability in the migratory locust Locusta migratoria. We show that LmKnk localizes to the chitin layer in the newly produced cuticle. Injection of double-stranded RNA targeting LmKnk (dsLmKnk) in locust nymphs caused failure of moulting to the next stage. Histological experiments revealed that apolysis, i.e., the detachment of the old cuticle from the body surface, was normal; however, the newly synthesized cuticle was thinner than the cuticle of the control insects. Indeed, chitin content dropped after suppression of LmKnk expression. As seen by transmission electron microscopy, crystalline chitin organization was lost in dsLmKnk-treated insects. In addition, the structure of pore canals, which are lipid transporting routes in the cuticle, was abnormal. Consistently, their content was reduced and, probably by consequence, lipid deposition on the cuticle was decreased after injection of dsLmKnk. Suppression of LmKnk transcript levels rendered L. migratoria more susceptible to each of four selected insecticides including malathion, chlorpyrifos, carbaryl and deltamethrin. Overall, our data show that LmKnk is needed for correct chitin amounts and organization, and their changes ultimately affect cuticular permeability in L. migratoria.

Improving Polysaccharide-Based Chitin/Chitosan-Aerogel Materials by Learning from Genetics and Molecular Biology

Improved wound healing of burnt skin and skin lesions, as well as medical implants and replacement products, requires the support of synthetical matrices. Yet, producing synthetic biocompatible matrices that exhibit specialized flexibility, stability, and biodegradability is challenging. Synthetic chitin/chitosan matrices may provide the desired advantages for producing specialized grafts but must be modified to improve their properties. Synthetic chitin/chitosan hydrogel and aerogel techniques provide the advantages for improvement with a bioinspired view adapted from the natural molecular toolbox. To this end, animal genetics provide deep knowledge into which molecular key factors decisively influence the properties of natural chitin matrices. The genetically identified proteins and enzymes control chitin matrix assembly, architecture, and degradation. Combining synthetic chitin matrices with critical biological factors may point to the future direction with engineering materials of specific properties for biomedical applications such as burned skin or skin blistering and extensive lesions due to genetic diseases.

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.

Chitinase 10 controls chitin amounts and organization in the wing cuticle of Drosophila

Wings are essential for insect fitness. A number of proteins and enzymes have been identified to be involved in wing terminal differentiation, which is characterized by the formation of the wing cuticle. Here, we addressed the question whether chitinase 10 (Cht10) may play an important role in chitin organization in the wings of the fruit fly Drosophila melanogaster. Initially, we first found that Cht10 expression coincides with the expression of the chitin synthase coding gene kkv. This suggests that the respective proteins may cooperate during wing differentiation. In tissue-specific RNA interference experiments, we demonstrate that suppression of Cht10 causes an excess in chitin amounts in the wing cuticle. Chitin organization is severely disrupted in these wings. Based on these data, we hypothesize that Cht10 restricts chitin amounts produced by Kkv in order to ensure normal chitin organization and wing cuticle formation. In addition, we found by scanning electron microscopy that Cht10 suppression also affects the cuticle surface. In turn, cuticle inward permeability is enhanced in Cht10-less wings. Moreover, flies with reduced Cht10 function are unable to fly. In conclusion, Cht10 is essential for wing terminal differentiation and function.

Effect of RNAi-mediated silencing of two Knickkopf family genes (LmKnk2 and LmKnk3) on cuticle formation and insecticide susceptibility in Locusta migratoria

BACKGROUND

Knickkopf (Knk) proteins play crucial roles in the formation of insect cuticle. Recent studies in the holometabolous insect red flour beetle (Tribolium castaneum) have shown that three Knk genes encoding TcKnk, TcKnk2 and TcKnk3 play different but essential roles at different developmental stages and in different tissues. However, the functions of Knk genes had not been fully explored in hemimetabolous insects such as the migratory locust Locusta migratoria.

RESULTS

We identified three transcripts of LmKnk-like genes LmKnk2 and LmKnk3 with the full-length cDNA sequences, which were named as LmKnk2, LmKnk3-FL and LmKnk3-5'. These three transcripts were highly expressed before molting and mainly expressed in the integument. Among these genes, silencing only LmKnk3-5' by RNA interference (RNAi) caused molting defects and high mortality of the locusts. Injection of dsLmKnk3-5' dramatically decreased chitin content, but did not affect cuticle laminar ultra-structures in the integument. After the knockdown of LmKnk3-5' transcript, lipid deposition on the cuticle surface was impeded, and locusts exhibited increased susceptibility to each of four insecticides in three different classes. However, no visible phenotypic changes were observed after LmKnk2 or LmKnk3-FL was silenced by RNAi.

CONCLUSION

We demonstrate that LmKnk3-5' is essential for cuticle formation in L. migratoria. This contrasts the findings that the cognate protein in T. castaneum TcKnk3-5' is dispensable for cuticle formation and survival. Hence, we provide some evidence that the function of Knk-type proteins may be species-specific. We therefore think that LmKnk3-5' may be a good target for the application of RNAi-based technologies for species-specific insect pest management. © 2020 Society of Chemical Industry.

The cuticle inward barrier in Drosophila melanogaster is shaped by mitochondrial and nuclear genotypes and a sex-specific effect of diet

An important role of the insect cuticle is to prevent wetting (i.e., permeation of water) and also to prevent penetration of potentially harmful substances. This barrier function mainly depends on the hydrophobic cuticle surface composed of lipids including cuticular hydrocarbons (CHCs). We investigated to what extent the cuticle inward barrier function depends on the genotype, comprising mitochondrial and nuclear genes in the fruit fly Drosophila melanogaster, and investigated the contribution of interactions between mitochondrial and nuclear genotypes (mito-nuclear interactions) on this function. In addition, we assessed the effects of nutrition and sex on the cuticle barrier function. Based on a dye penetration assay, we find that cuticle barrier function varies across three fly lines that were captured from geographically separated regions in three continents. Testing different combinations of mito-nuclear genotypes, we show that the inward barrier efficiency is modulated by the nuclear and mitochondrial genomes independently. We also find an interaction between diet and sex. Our findings provide new insights into the regulation of cuticle inward barrier function in nature.

CYP303A1 has a conserved function in adult eclosion in Locusta migratoria and Drosophila melanogaster

Insect cytochrome P450 monooxygenases (CYPs) play essential roles in both xenobiotic metabolism and developmental processes. However, the exact physiological function of many CYP genes remains largely unknown. Screening the expression of the CYP genes from the CYP2 and mitochondrial CYP clans of Drosophila melanogaster revealed that Cyp303a1 is highly expressed in the pupal stage. Knockdown of CYP303A1 transcripts by RNAi using the Gal4/UAS system with a ubiquitous driver (tubulin-Gal4) in Drosophila or by dsRNA injection in the last nymph stage of Locusta migratoria resulted in severe defects in eclosion and lethality during and after adult emergence. In Drosophila, tissue-specific RNAi of Cyp303a1 with a wing-specific driver (MS1096-Gal4) revealed that Cyp303a1 was essential for wing extension. Stage-specific RNAi of Cyp303a1 using Gal80^ts for thermal-dependent-suppression found that the expression of Cyp303a1 at the middle pupal stage was absolutely required. Meanwhile, Cyp303a1 mutants exhibited more than 80% lethality at the late embryonic development stages. Embryonic lethality of the Cyp303a1 mutants was fully rescued by the ubiquitous overexpression of exogenous Cyp303a1. Taken together, we conclude that Cyp303a1 is indispensable for embryonic development and adult eclosion in D. melanogaster, the latter role being conserved over 400 million years of insect evolution.

The putative chitin deacetylases Serpentine and Vermiform have non-redundant functions during Drosophila wing development

The chitin modifying deacetylases (CDA) CDA1 and CDA2 have been reported to play partially redundant roles during insect cuticle formation and molting and tracheal morphogenesis in various insect species. In order to distinguish possible functional differences between these two enzymes, we analyzed their function during wing development in the fruit fly Drosophila melanogaster. In tissue-specific RNA interference experiments, we demonstrate that DmCDA1 (Serpentine, Serp) and DmCDA2 (Vermiform, Verm) have distinct functions during Drosophila adult wing cuticle differentiation. Chitosan staining revealed that Serp is the major enzyme responsible for chitin deacetylation during wing cuticle formation, while Verm does not seem to be needed for this process. Indeed, it is questionable whether Verm is a chitin deacetylase at all. Atomic force microscopy suggested that Serp and Verm have distinct roles in establishing the shape of nanoscale bumps at the wing surface. Moreover, our data indicate that Verm but not Serp is required for the laminar arrangement of chitin. Both enzymes participate in the establishment of the cuticular inward barrier against penetration of xenobiotics. Taken together, correct differentiation of the wing cuticle involves both Serp and Verm in parallel in largely non-overlapping functions.

Blimp-1 Mediates Tracheal Lumen Maturation in Drosophila melanogaster

The specification of tissue identity during embryonic development requires precise spatio-temporal coordination of gene expression. Many transcription factors required for the development of organs have been identified and their expression patterns are known; however, the mechanisms through which they coordinate gene expression in time remain poorly understood. Here, we show that hormone-induced transcription factor Blimp-1 participates in the temporal coordination of tubulogenesis in Drosophila melanogaster by regulating the expression of many genes involved in tube maturation. In particular, we demonstrate that Blimp-1 regulates the expression of genes involved in chitin deposition and F-actin organization. We show that Blimp-1 is involved in the temporal control of lumen maturation by regulating the beginning of chitin deposition. We also report that Blimp-1 represses a variety of genes involved in tracheal maturation. Finally, we reveal that the kinase Btk29A serves as a link between Blimp-1 transcriptional repression and apical extracellular matrix organization.