Overview of chitin metabolism enzymes in Manduca sexta: Identification, domain organization, phylogenetic analysis and gene expression

LmCDA1 organizes the cuticle by chitin deacetylation in Locusta migratoria

Cells produce an extracellular matrix (ECM) with a stereotypic organization that is important for tissue function. The insect cuticle is a layered ECM that mainly consists of the polysaccharide chitin and associated proteins adopting a quasi-crystalline structure. Our understanding of the molecular mechanisms deployed during construction of the highly ordered protein-chitin ECM so far is limited. In this study, we report on the role of the chitin deacetylase 1 (LmCDA1) in the organization of the protein-chitin ECM in the migratory locust Locusta migratoria, and LmCDA1 localizes predominantly to the apical tier of the protein-chitin ECM, but it is also found in lower regions. Reduction of LmCDA1 function correlates with lower amounts of chitin and impedes conversion of chitin to chitosan by deacetylation. Establishment of the quasi-crystalline architecture of the protein-chitin ECM is, however, independent of LmCDA1 activity, but it is dependent on another chitin deacetylase, LmCDA2, which has no detectable effects on chitin deacetylation and, as shown previously, no influence on chitin content. Our data reveal that LmCDA1 and LmCDA2 act in parallel and independently from each other in defining the dimensions of the cuticle. Both enzymes are non-uniformly distributed within the protein-chitin matrix, suggesting a site-autonomous function.

Biosynthesis, modifications and degradation of chitin in the formation and turnover of peritrophic matrix in insects

The peritrophic matrix (PM) is an extracellular, semi-permeable biocomposite that lines the midgut of most insects. The PM serves as the first defense in the midgut to resist microorganisms such as viruses, bacteria and other pathogens, and to protect epithelial cells from mechanical damage. The PM also separates the midgut lumen into different compartments, which play important roles in nutrient ingestion and digestion. The PM is a highly dynamic structure that consists mainly of chitin fibers cross-linked by proteins, glycoproteins, and proteoglycans. The PM is continuously biosynthesized, assembled, and degraded in response to feeding and development. Chitin chains are synthesized by several enzymes and organized in several hierarchical levels, in which various PM-associated proteins appear to be essential for maintaining the structural integrity and physiological function of the PM. This review summarizes research advances on molecular components of the PM and their functions, as well as related proteins and enzymes that contribute to PM formation and modification. Crucial gaps in our current understanding of the PM are also addressed.

Structural and biochemical insights into the catalytic mechanisms of two insect chitin deacetylases of the carbohydrate esterase 4 family

Insect chitin deacetylases (CDAs) catalyze the removal of acetyl groups from chitin and modify this polymer during its synthesis and reorganization. CDAs are essential for insect survival and therefore represent promising targets for insecticide development. However, the structural and biochemical characteristics of insect CDAs have remained elusive. Here, we report the crystal structures of two insect CDAs from the silk moth Bombyx mori: BmCDA1, which may function in cuticle modification, and BmCDA8, which may act in modifying peritrophic membranes in the midgut. Both enzymes belong to the carbohydrate esterase 4 (CE4) family. Comparing their overall structures at 1.98–2.4 Å resolution with those from well-studied microbial CDAs, we found that two unique loop regions in BmCDA1 and BmCDA8 contribute to the distinct architecture of their substrate-binding clefts. These comparisons revealed that both BmCDA1 and BmCDA8 possess a much longer and wider substrate-binding cleft with a very open active site in the center than the microbial CDAs, including VcCDA from Vibrio cholerae and ArCE4A from Arthrobacter species AW19M34-1. Biochemical analyses indicated that BmCDA8 is an active enzyme that requires its substrates to occupy subsites 0, +1, and +2 for catalysis. In contrast, BmCDA1 also required accessory proteins for catalysis. To the best of our knowledge, our work is the first to unveil the structural and biochemical features of insect proteins belonging to the CE4 family.

Biochemical characterization of three midgut chitin deacetylases of the Lepidopteran insect Bombyx mori

Peritrophic membrane (PM) is a chitin and protein-containing extracellular matrix that lines the midgut in most insect species, functioning as a barrier to exogenous toxins and pathogens. Midgut chitin deacetylases (CDAs) are chitin-modifying enzymes known to alter the mechanical property and permeability of PM. However, biochemical properties and specific roles of these enzymes remain elusive. In this study, the midgut-expressed CDAs (BmCDA6, BmCDA7 and BmCDA8) from Bombyx mori were cloned, recombinantly expressed and purified and their enzymatic activities toward PM chitin were determined. Of the three enzymes, BmCDA7 exhibited the highest activity (0.284 μmol/min/μmol), while BmCDA8 showed lower activity of 0.061 μmol/min/μmol. BmCDA6 was inactive towards PM chitin. Gene expression patterns indicated that although all three CDA genes were specifically expressed in the anterior midgut, they differed in their temporal expression patterns. BmCDA6 was expressed almost exclusively at the mid-molt stage, the stage when the PM was thick and with multiple chitin layers. Unlike BmCDA6, high expression levels of BmCDA7 and BmCDA8 were observed only at the feeding stage, the stage when the PM is thin and with fewer chitin layers. The different gene expression patterns and biochemical characteristics provide new information about the functional specialization among BmCDA6, BmCDA7 and BmCDA8 proteins.

Chitinous Structures as Potential Targets for Insect Pest Control

Chitinous structures are physiologically fundamental in insects. They form the insect exoskeleton, play important roles in physiological systems and provide physical, chemical and biological protections in insects. As critically important structures in insects, chitinous structures are attractive target sites for the development of new insect-pest-control strategies. Chitinous structures in insects are complex and their formation and maintenance are dynamically regulated with the growth and development of insects. In the past few decades, studies on insect chitinous structures have shed lights on the physiological functions, compositions, structural formation, and regulation of the chitinous structures. Current understanding of the chitinous structures has indicated opportunities for exploring new target sites for insect control. Mechanisms to disrupt chitinous structures in insects have been studied and strategies for the potential development of new means of insect control by targeting chitinous structures have been proposed and are practically to be explored.

Chitin in Arthropods: Biosynthesis, Modification, and Metabolism

Chitin is a structural constituent of extracellular matrices including the cuticle of the exoskeleton and the peritrophic matrix (PM) of the midgut in arthropods. Chitin chains are synthesized through multiple biochemical reactions, organized in several hierarchical levels and associated with various proteins that give their unique physicochemical characteristics of the cuticle and PM. Because, arthropod growth and morphogenesis are dependent on the capability of remodeling chitin-containing structures, chitin biosynthesis and degradation are highly regulated, allowing ecdysis and regeneration of the cuticle and PM. Over the past 20 years, much progress has been made in understanding the physiological functions of chitinous matrices. In this chapter, we mainly discussed the biochemical processes of chitin biosynthesis, modification and degradation, and various enzymes involved in these processes. We also discussed cuticular proteins and PM proteins, which largely determine the physicochemical properties of the cuticle and PM. Although rapid advances in genomics, proteomics, RNA interference, and other technologies have considerably facilitated our research in chitin biosynthesis, modification, and metabolism in recent years, many aspects of these processes are still partially understood. Further research is needed in understanding how the structural organization of chitin synthase in plasma membrane accommodate chitin biosynthesis, transport of chitin chain across the plasma membrane, and release of the chitin chain from the enzyme. Other research is also needed in elucidating the roles of chitin deacetylases in chitin organization and the mechanism controlling the formation of different types of chitin in arthropods.

LmCht5-1 promotes pro-nymphal molting during locust embryonic development

Chitinases, key enzymes involved in degradation of chitin, have been repeatedly shown to play an indispensable role during insect post-embryonic molting processes at stage transitions. However, how chitinases affect insect embryonic development remains to be analyzed. In this study, we investigated the role of chitinase 5-1 (LmCht5-1) during embryonic development of the hemimetabolous insect Locusta migratoria. LmCht5-1 transcript levels were high in pro-nymphs during late embryogenesis. The respective protein localized to both the pro-nymphal and, to a much lesser extent, the newly formed nymphal cuticle. After injection of double stranded RNA against LmCht5-1 into 8 days old embryos, LmCht5-1 transcripts were strongly reduced. Most of dsLmCht5-1-injected pro-nymphs failed to develop to first-instar nymphs and died at or before hatching. Histological analyzes showed that degradation of the pro-nymph cuticle was blocked in these animals. At the ultra-structural level, we found that LmCht5-1 was needed for the degradation of the lamellar procuticle, while the separation of the procuticle from the epicuticle and epidermis (apolysis) was independent of LmCht5-1 function. Taken together, our results indicate that LmCht5-1 and other yet unknown degrading enzymes act in parallel at distinct positions of the cuticle during molting of the pro-nymph to the first-instar nymph during locust embryogenesis.

RNA-seq analysis of gene expression changes during pupariation in Bactrocera dorsalis (Hendel) (Diptera: Tephritidae)

Background

The oriental fruit fly, Bactrocera dorsalis (Hendel) has been considered to be one of the most important agricultural pest around the world. As a holometabolous insect, larvae must go through a metamorphosis process with dramatic morphological and structural changes to complete their development. To better understand the molecular mechanisms of these changes, RNA-seq of B. dorsalis from wandering stage (WS), late wandering stage (LWS) and white puparium stage (WPS) were performed.

Results

In total, 11,721 transcripts were obtained, out of which 1914 genes (578 up-regulated and 1336 down-regulated) and 2047 genes (655 up-regulated and 1392 down-regulated) were found to be differentially expressed between WS and LWS, as well as between WS and WPS, respectively. Of these DEGs, 1862 and 1996 genes were successfully annotated in various databases. The analysis of RNA-seq data together with qRT-PCR validation indicated that during this transition, the genes in the oxidative phosphorylation pathway, and genes encoding P450s, serine protease inhibitor, and cuticular proteins were down-regulated, while the serine protease genes were up-regulated. Moreover, we found some 20-hydroxyecdysone (20E) biosynthesis and signaling pathway genes had a higher expression in the WS, while the genes responsible for juvenile hormone (JH) synthesis, degradation, signaling and transporter pathways were down-regulated, suggesting these genes might be involved in the process of larval pupariation in B. dorsalis. For the chitinolytic enzymes, the genes encoding chitinases (chitinase 2, chitinase 5, chitinase 8, and chitinase 10) and chitin deacetylase might play the crucial role in the degradation of insect chitin with their expressions significantly increased during the transition. Here, we also found that chitin synthase 1A might be involved in the chitin synthesis of cuticles during the metamorphosis in B. dorsalis.

Conclusions

Significant changes at transcriptional level were identified during the larval pupariation of B. dorsalis. Importantly, we also obtained a vast quantity of RNA-seq data and identified metamorphosis associated genes, which would all help us to better understand the molecular mechanism of metamorphosis process in B. dorsalis.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5077-z) contains supplementary material, which is available to authorized users.

A massive incorporation of microbial genes into the genome of Tetranychus urticae, a polyphagous arthropod herbivore

A number of horizontal gene transfers (HGTs) have been identified in the spider mite Tetranychus urticae, a chelicerate herbivore. However, the genome of this mite species has at present not been thoroughly mined for the presence of HGT genes. Here, we performed a systematic screen for HGT genes in the T. urticae genome using the h-index metric. Our results not only validated previously identified HGT genes but also uncovered 25 novel HGT genes. In addition to HGT genes with a predicted biochemical function in carbohydrate, lipid and folate metabolism, we also identified the horizontal transfer of a ketopantoate hydroxymethyltransferase and a pantoate β-alanine ligase gene. In plants and bacteria, both genes are essential for vitamin B5 biosynthesis and their presence in the mite genome strongly suggests that spider mites, similar to Bemisia tabaci and nematodes, can synthesize their own vitamin B5. We further show that HGT genes were physically embedded within the mite genome and were expressed in different life stages. By screening chelicerate genomes and transcriptomes, we were able to estimate the evolutionary histories of these HGTs during chelicerate evolution. Our study suggests that HGT has made a significant and underestimated impact on the metabolic repertoire of plant-feeding spider mites.

Genome-wide identification of chitinase and chitin deacetylase gene families in the oriental fruit fly, Bactrocera dorsalis (Hendel)

Chitinases (Chts) and chitin deacetylases (CDAs) are important enzymes required for chitin metabolism in insects. In this study, 12 Cht-related genes (including seven Cht genes and five imaginal disc growth factor genes) and 6 CDA genes (encoding seven proteins) were identified in Bactrocera dorsalis using genome-wide searching and transcript profiling. Based on the conserved sequences and phylogenetic relationships, 12 Cht-related proteins were clustered into eight groups (group I-V and VII-IX). Further domain architecture analysis showed that all contained at least one chitinase catalytic domain, however, only four (BdCht5, BdCht7, BdCht8 and BdCht10) possessed chitin-binding domains. The subsequent phylogenetic analysis revealed that seven CDAs were clustered into five groups (group I-V), and all had one chitin deacetylase catalytic domain. However, only six exhibited chitin-binding domains. Finally, the development- and tissue-specific expression profiling showed that transcript levels of the 12 Cht-related genes and 6 CDA genes varied considerably among eggs, larvae, pupae and adults, as well as among different tissues of larvae and adults. Our findings illustrate the structural differences and expression patterns of Cht and CDA genes in B. dorsalis, and provide important information for the development of new pest control strategies based on these vital enzymes.

Immune function against bacteria of chitin deacetylase 1 (EcCDA1) from Exopalaemon carinicauda

Chitin deacetylase (CDA, EC 3.5.1.41), belonging to a family of extracellular chitin-modifying enzymes, can catalyze the deacetylation of chitin. In this study, the full-length cDNA sequence encoding chitin deacetylase 1 (EcCDA1) was obtained fromExopalaemon carinicauda. The complete nucleotide sequence of EcCDA1 contained a 1611 bp open reading frame (ORF) encoding EcCDA1 precursor of 536 amino acids. The domain architecture of the deduced EcCDA1 protein contained a signal peptide, a chitin-binding peritrophin-A domain (ChtBD2), a low-density lipoprotein receptor class A domain (LDLa) and a Polysacc_deac_1 domain. EcCDA1 mRNA was predominantly expressed in the gills. The expression of EcCDA1 in the prawns challenged with Vibrio parahaemolyticus and Aeromonas hydrophila changed in a time-dependent manner. The expression of EcCDA1 in the prawns challenged with V. parahaemolyticus was up-regulated at 12 h (p < 0.05), and significantly up-regulated at 24 h and 48 h (p < 0.01), and then returned to the control levels at 96 h post-challenge (p > 0.05). At the same time, the expression in Aeromonas-challenged group was significantly up-regulated at 12, 24 and 48 h (p < 0.01) and returned to the control levels at 120 h post-challenge (p > 0.05). Then, EcCDA1 was recombinantly expressed in Pichia pastoris and the purified recombinant EcCDA1 could not inhibit the growth of V. parahaemolyticus or A. hydrophila, which indicated that the CDA1 may play its biological activity in immune defense by deacetylation from chitin.

A chitinase with two catalytic domains is required for organization of the cuticular extracellular matrix of a beetle

Insect cuticle or exoskeleton is an extracellular matrix formed primarily from two different structural biopolymers, chitin and protein. During each molt cycle, a new cuticle is deposited simultaneously with degradation of the inner part of the chitinous procuticle of the overlying old exoskeleton by molting fluid enzymes including epidermal chitinases. In this study we report a novel role for an epidermal endochitinase containing two catalytic domains, TcCHT7, from the red flour beetle, Tribolium castaneum, in organizing chitin in the newly forming cuticle rather than in degrading chitin present in the prior one. Recombinant TcCHT7 expressed in insect cells is membrane-bound and capable of hydrolyzing an extracellular chitin substrate, whereas in vivo, this enzyme is also released from the plasma membrane and co-localizes with chitin in the entire procuticle. RNAi of TcCHT7 reveals that this enzyme is nonessential for any type of molt or degradation of the chitinous matrix in the old cuticle. In contrast, TcCHT7 is required for maintaining the integrity of the cuticle as a compact structure of alternating electron-dense and electron-lucent laminae. There is a reduction in thickness of elytral and leg cuticles after RNAi for TcCHT7. TcCHT7 is also required for formation of properly oriented long chitin fibers inside pore canals that are vertically oriented columnar structures, which contribute to the mechanical strength of a light-weight, yet rigid, adult cuticle. The conservation of CHT7-like proteins harboring such a unique domain configuration among many insect and other arthropod species indicates a critical role for the group III class of chitinases in the higher ordered organization of chitin fibers for development of the structural integrity of many invertebrate exoskeletons.

Insect cuticle or exoskeleton is an extracellular matrix consisting of three major morphologically distinct layers, the water-proofing envelope, the protein-rich epicuticle and the chitin/protein-rich procuticle. To accommodate growth, insects must periodically replace their cuticles in a process called “molting or ecdysis”. During each molt cycle a new cuticle is deposited simultaneously with degradation of the inner part of the chitinous procuticle of the old one by molting fluid enzymes including epidermal chitinases. We show that a chitinase, CHT7, from the red flour beetle, Tribolium castaneum, belonging to a subfamily (group III) of chitinases that have two catalytic domains, is necessary for organization of chitin-containing structures in nascent cuticle, which contributes to the rigidity of the extracellular matrix. This unexpected function is distinct from that of other groups of epidermal chitinases that catalyze the turnover of chitin in old cuticle during the molting process. Because group III chitinases are highly conserved among insect and other arthropod species, we propose that these enzymes have a novel function in processing nascent chitin chains during cuticle assembly and organization into higher order structures that include horizontally stacked laminae and vertically oriented pore canals of many invertebrate cuticular extracellular matrices.

Group I chitin deacetylases are essential for higher order organization of chitin fibers in beetle cuticle

Roles in the organization of the cuticle (exoskeleton) of two chitin deacetylases (CDAs) belonging to group I, TcCDA1 and TcCDA2, as well as two alternatively spliced forms of the latter, TcCDA2a and TcCDA2b, from the red flour beetle, Tribolium castaneum, were examined in different body parts using transmission EM and RNAi. Even though all TcCDAs are co-expressed in cuticle-forming cells from the hardened forewing (elytron) and ventral abdomen, as well as in the softer hindwing and dorsal abdomen, there are significant differences in the tissue specificity of expression of the alternatively spliced transcripts. Loss of either TcCDA1 or TcCDA2 protein by RNAi causes abnormalities in organization of chitinous horizontal laminae and vertical pore canals in all regions of the procuticle of both the hard and soft cuticles. Simultaneous RNAi for TcCDA1 and TcCDA2 produces the most serious abnormalities. RNAi of either TcCDA2a or TcCDA2b affects cuticle integrity to some extent. Following RNAi, there is accumulation of smaller disorganized fibers in both the horizontal laminae and pore canals, indicating that TcCDAs play a critical role in elongation/organization of smaller nanofibers into longer fibers, which is essential for structural integrity of both hard/thick and soft/thin cuticles. Immunolocalization of TcCDA1 and TcCDA2 proteins and effects of RNAi on their accumulation indicate that these two proteins function in concert exclusively in the assembly zone in a step involving the higher order organization of the procuticle.

Structural analysis of group II chitinase (ChtII) catalysis completes the puzzle of chitin hydrolysis in insects

Chitin is a linear homopolymer of N-acetyl-β-d-glucosamines and a major structural component of insect cuticles. Chitin hydrolysis involves glycoside hydrolase family 18 (GH18) chitinases. In insects, chitin hydrolysis is essential for periodic shedding of the old cuticle ecdysis and proceeds via a pathway different from that in the well studied bacterial chitinolytic system. Group II chitinase (ChtII) is a widespread chitinolytic enzyme in insects and contains the greatest number of catalytic domains and chitin-binding domains among chitinases. In Lepidopterans, ChtII and two other chitinases, ChtI and Chi-h, are essential for chitin hydrolysis. Although ChtI and Chi-h have been well studied, the role of ChtII remains elusive. Here, we investigated the structure and enzymology of OfChtII, a ChtII derived from the insect pest Ostrinia furnacalis We present the crystal structures of two catalytically active domains of OfChtII, OfChtII-C1 and OfChtII-C2, both in unliganded form and complexed with chitooligosaccharide substrates. We found that OfChtII-C1 and OfChtII-C2 both possess long, deep substrate-binding clefts with endochitinase activities. OfChtII exhibited structural characteristics within the substrate-binding cleft similar to those in OfChi-h and OfChtI. However, OfChtII lacked structural elements favoring substrate binding beyond the active sites, including an extra wall structure present in OfChi-h. Nevertheless, the numerous domains in OfChtII may compensate for this difference; a truncation containing one catalytic domain and three chitin-binding modules (OfChtII-B4C1) displayed activity toward insoluble polymeric substrates that was higher than those of OfChi-h and OfChtI. Our observations provide the last piece of the puzzle of chitin hydrolysis in insects.

An analysis of 67 RNA-seq datasets from various tissues at different stages of a model insect, Manduca sexta

Background

Manduca sexta is a large lepidopteran insect widely used as a model to study biochemistry of insect physiological processes. As a part of its genome project, over 50 cDNA libraries have been analyzed to profile gene expression in different tissues and life stages. While the RNA-seq data were used to study genes related to cuticle structure, chitin metabolism and immunity, a vast amount of the information has not yet been mined for understanding the basic molecular biology of this model insect. In fact, the basic features of these data, such as composition of the RNA-seq reads and lists of library-correlated genes, are unclear. From an extended view of all insects, clear-cut tempospatial expression data are rarely seen in the largest group of animals including Drosophila and mosquitoes, mainly due to their small sizes.

Results

We obtained the transcriptome data, analyzed the raw reads in relation to the assembled genome, and generated heatmaps for clustered genes. Library characteristics (tissues, stages), number of mapped bases, and sequencing methods affected the observed percentages of genome transcription. While up to 40% of the reads were not mapped to the genome in the initial Cufflinks gene modeling, we identified the causes for the mapping failure and reduced the number of non-mappable reads to <8%. Similarities between libraries, measured based on library-correlated genes, clearly identified differences among tissues or life stages. We calculated gene expression levels, analyzed the most abundantly expressed genes in the libraries. Furthermore, we analyzed tissue-specific gene expression and identified 18 groups of genes with distinct expression patterns.

Conclusion

We performed a thorough analysis of the 67 RNA-seq datasets to characterize new genomic features of M. sexta. Integrated knowledge of gene functions and expression features will facilitate future functional studies in this biochemical model insect.

Electronic supplementary material

The online version of this article doi: (10.1186/s12864-017-4147-y) contains supplementary material, which is available to authorized users.

Combined Venom Gland Transcriptomic and Venom Peptidomic Analysis of the Predatory Ant Odontomachus monticola

Ants (hymenoptera: Formicidae) have adapted to many different environments and have become some of the most prolific and successful insects. To date, 13,258 ant species have been reported. They have been classified into 333 genera and 17 subfamilies. Except for a few Formicinae, Dolichoderinae, and members of other subfamilies, most ant species have a sting with venom. The venoms are composed of formic acid, alkaloids, hydrocarbons, amines, peptides, and proteins. Unlike the venoms of other animals such as snakes and spiders, ant venoms have seldom been analyzed comprehensively, and their compositions are not yet completely known. In this study, we used both transcriptomic and peptidomic analyses to study the composition of the venom produced by the predatory ant species Odontomachus monticola. The transcriptome analysis yielded 49,639 contigs, of which 92 encoded toxin-like peptides and proteins with 18,106,338 mapped reads. We identified six pilosulin-like peptides by transcriptomic analysis in the venom gland. Further, we found intact pilosulin-like peptide 1 and truncated pilosulin-like peptides 2 and 3 by peptidomic analysis in the venom. Our findings related to ant venom peptides and proteins may lead the way towards development and application of novel pharmaceutical and biopesticidal resources.

Human Chitotriosidase: Catalytic Domain or Carbohydrate Binding Module, Who's Leading HCHT's Biological Function

Chitin is an important structural component of numerous fungal pathogens and parasitic nematodes. The human macrophage chitotriosidase (HCHT) is a chitinase that hydrolyses glycosidic bonds between the N-acetyl-D-glucosamine units of this biopolymer. HCHT belongs to the Glycoside Hydrolase (GH) superfamily and contains a well-characterized catalytic domain appended to a chitin-binding domain (ChBD_CHIT1). Although its precise biological function remains unclear, HCHT has been described to be involved in innate immunity. In this study, the molecular basis for interaction with insoluble chitin as well as with soluble chito-oligosaccharides has been determined. The results suggest a new mechanism as a common binding mode for many Carbohydrate Binding Modules (CBMs). Furthermore, using a phylogenetic approach, we have analysed the modularity of HCHT and investigated the evolutionary paths of its catalytic and chitin binding domains. The phylogenetic analyses indicate that the ChBD_CHIT1 domain dictates the biological function of HCHT and not its appended catalytic domain. This observation may also be a general feature of GHs. Altogether, our data have led us to postulate and discuss that HCHT acts as an immune catalyser.

Cloning, Site-Directed Mutagenesis, and Functional Analysis of Active Residues in Lymantria dispar Chitinase

Chitinases are glycosyl hydrolases that catalyze the hydrolysis of β-(1,4)-glycosidic bonds in chitin, the major structural polysaccharide presented in the cuticle and gut peritrophic matrix of insects. Two aspartate residues (D143, D145) and one tryptophan (W146) in the Lymantria dispar chitinase are highly conserved residues observed within the second conserved motif of the family 18 chitinase catalytic region. In this study, a chitinase cDNA, LdCht5, was cloned from L. dispar, and the roles of the three residues were investigated using site-directed mutagenesis and substituting them with three other amino acids. Seven mutant proteins, D143E, D145E, W146G, D143E/D145E, D143E/W146G, D145E/W146G, and D143E/D145E/W146G, as well as the wild-type enzyme, were produced using the baculovirus-insect cell line expression system. The enzymatic and kinetic properties of these mutant enzymes were measured using the oligosaccharide substrate MU-(GlcNAc)₃. Among the seven mutants, the D145E, D143E/D145E, and D145E/W146G mutations kept some extant catalytic activity toward MU-(GlcNAc)₃, while the D143E, W146G, D143E/W146G, and D143E/D145E/W146G mutant enzymes were inactivated. Compared with the mutant enzymes, the wild-type enzyme had higher values of k _cat and k _cat / K _m . A study of the multiple point mutations in the second conserved catalytic region would help to elucidate the role of the critical residues and their relationships.

20-hydroxyecdysone enhances the expression of the chitinase 5 via Broad-Complex Zinc-Finger 4 during metamorphosis in silkworm, Bombyx mori

Insect chitinases are hydrolytic enzymes required for the degradation of chitin. They are essential for insect moulting and metamorphosis. In this study, the regulation mechanism of a chitinase gene, Bombyx mori chitinase 5 (BmCHT5), was studied. Quantitative reverse transcription PCR (qRT-PCR) analysis showed that BmCHT5 was up-regulated during the larval-larval and larval-pupa transitions and notably induced by 20-hydroxyecdysone (20E). Analysis of the BmCHT5 promoter revealed the presence of one Bombyx mori Broad-Complex Zinc-Finger Isoform 4 (BR-C Z4), two BR-C Z2 and two ecdysone-induced protein 74A (E74A) cis-regulatory elements (CREs) that are related to 20E. qRT-PCR showed that the expression of both BmBR-C Z4 and BmBR-C Z2 during metamorphosis, and when induced by 20E, was anastomotic with the variations in BmCHT5 mRNA level. In contrast, BmE74A did not follow this trend. An electrophoretic mobility shift assay did not retrieve a binding partner for the two BR-C Z2 CREs in the BmN cell line nuclear extract, whereas BR-C Z4 CRE specifically bound to BmBR-C Z4. Besides, luciferase activity analysis confirmed that BmBR-C Z4 could enhance the activity of the BmCHT5 promoter with BR-C Z4 CRE and could not enhance the promoter activity by mutating BR-C Z4 CRE. Taken together, these data suggest that the transcription factor BmBR-C Z4 enhances the expression of BmCHT5 during metamorphosis.

Structure, Catalysis, and Inhibition of OfChi-h, the Lepidoptera-exclusive Insect Chitinase

Chitinase-h (Chi-h) is of special interest among insect chitinases due to its exclusive distribution in lepidopteran insects and high sequence identity with bacterial and baculovirus homologs. Here OfChi-h, a Chi-h from Ostrinia furnacalis, was investigated. Crystal structures of both OfChi-h and its complex with chitoheptaose ((GlcN)₇) reveal that OfChi-h possesses a long and asymmetric substrate binding cleft, which is a typical characteristics of a processive exo-chitinase. The structural comparison between OfChi-h and its bacterial homolog SmChiA uncovered two phenylalanine-to-tryptophan site variants in OfChi-h at subsites +2 and possibly -7. The F232W/F396W double mutant endowed SmChiA with higher hydrolytic activities toward insoluble substrates, such as insect cuticle, α-chitin, and chitin nanowhisker. An enzymatic assay demonstrated that OfChi-h outperformed OfChtI, an insect endo-chitinase, toward the insoluble substrates, but showed lower activity toward the soluble substrate ethylene glycol chitin. Furthermore, OfChi-h was found to be inhibited by N,N',N″-trimethylglucosamine-N,N',N″,N″'-tetraacetylchitotetraose (TMG-(GlcNAc)₄), a substrate analog which can be degraded into TMG-(GlcNAc)_1-2 Injection of TMG-(GlcNAc)₄ into 5th-instar O. furnacalis larvae led to severe defects in pupation. This work provides insights into a molting-indispensable insect chitinase that is phylogenetically closer to bacterial chitinases than insect chitinases.

Identification of the chitinase genes from the diamondback moth, Plutella xylostella

Chitinases have an indispensable function in chitin metabolism and are well characterized in numerous insect species. Although the diamondback moth (DBM) Plutella xylostella, which has a high reproductive potential, short generation time, and characteristic adaptation to adverse environments, has become one of the most serious pests of cruciferous plants worldwide, the information on the chitinases of the moth is presently limited. In the present study, using degenerated polymerase chain reaction (PCR) and rapid amplification of cDNA ends-PCR strategies, four chitinase genes of P. xylostella were cloned, and an exhaustive search was conducted for chitinase-like sequences from the P. xylostella genome and transcriptomic database. Based on the domain analysis of the deduced amino acid sequences and the phylogenetic analysis of the catalytic domain sequences, we identified 15 chitinase genes from P. xylostella. Two of the gut-specific chitinases did not cluster with any of the known phylogenetic groups of chitinases and might be in a new group of the chitinase family. Moreover, in our study, group VIII chitinase was not identified. The structures, classifications and expression patterns of the chitinases of P. xylostella were further delineated, and with this information, further investigations on the functions of chitinase genes in DBM could be facilitated.

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.

Multifaceted biological insights from a draft genome sequence of the tobacco hornworm moth, Manduca sexta

Manduca sexta, known as the tobacco hornworm or Carolina sphinx moth, is a lepidopteran insect that is used extensively as a model system for research in insect biochemistry, physiology, neurobiology, development, and immunity. One important benefit of this species as an experimental model is its extremely large size, reaching more than 10 g in the larval stage. M. sexta larvae feed on solanaceous plants and thus must tolerate a substantial challenge from plant allelochemicals, including nicotine. We report the sequence and annotation of the M. sexta genome, and a survey of gene expression in various tissues and developmental stages. The Msex_1.0 genome assembly resulted in a total genome size of 419.4 Mbp. Repetitive sequences accounted for 25.8% of the assembled genome. The official gene set is comprised of 15,451 protein-coding genes, of which 2498 were manually curated. Extensive RNA-seq data from many tissues and developmental stages were used to improve gene models and for insights into gene expression patterns. Genome wide synteny analysis indicated a high level of macrosynteny in the Lepidoptera. Annotation and analyses were carried out for gene families involved in a wide spectrum of biological processes, including apoptosis, vacuole sorting, growth and development, structures of exoskeleton, egg shells, and muscle, vision, chemosensation, ion channels, signal transduction, neuropeptide signaling, neurotransmitter synthesis and transport, nicotine tolerance, lipid metabolism, and immunity. This genome sequence, annotation, and analysis provide an important new resource from a well-studied model insect species and will facilitate further biochemical and mechanistic experimental studies of many biological systems in insects.

Improved activity and thermo-stability of the horse radish peroxidase with graphene quantum dots and its application in fluorometric detection of hydrogen peroxide

Graphene quantum dots (GQDs) have received extensive concern in many fields such as optical probe, bioimaging and biosensor. However, few reports refer on the influence of GQDs on enzyme performance. The paper reports two kinds of graphene quantum dots (termed as GO-GQDs and N,S-GQDs) that were prepared by cutting of graphene oxide and pyrolysis of citric acid and l-cysteine, and their use for the horse radish peroxidase (HRP) modification. The study reveals that GO-GQDs and N,S-GQDs exhibit an opposite effect on the HRP performance. Only HRP modified with GO-GQDs offers an enhanced activity (more than 1.9 times of pristine enzyme) and thermo-stability. This is because GO-GQDs offer a larger conjugate rigid plane and fewer hydrophilic groups compared to N,S-GQDs. The characteristics can make GO-GQDs induce a proper conformational change in the HRP for the catalytic performance, improving the enzyme activity and thermo-stability. The HRP modified with green luminescent GO-GQDs was also employed as a biocatalyst for sensing of H2O2 by a fluorometric sensor. The colorless tetramethylbenzidine (TMB) is oxidized into blue oxidized TMB in the presence of H2O2 by the assistance of HRP/GO-GQDs, leading to an obvious fluorescence quenching. The fluorescence intensity linearly decreases with the increase of H2O2 concentration in the range from 2×10-9 to 2×10-4M with the detection limit of 6.8×10-10M. The analytical method provides the advantage of sensitivity, stability and accuracy compared with present H2O2 sensors based on the pristine HRP. It has been successfully applied in the determination of H2O2 in real water samples. The study also opens a new avenue for modification of enzyme activity and stability that offers great promise in applications such as biological catalysis, biosensing and enzyme engineering.

Genome-wide analysis of chitinase genes and their varied functions in larval moult, pupation and eclosion in the rice striped stem borer, Chilo suppressalis

Some insect chitinases are required to degrade chitin and ensure successful metamorphosis. Although chitinase genes have been well characterized in several model insects, no reports exist for the rice striped stem borer, Chilo suppressalis, a highly destructive pest that causes huge yield losses in rice production. Here, we conducted a genome-level analysis of chitinase genes in C. suppressalis. After amplification of full-length transcripts with rapid amplification of cDNA ends, we identified 12 chitinase genes in C. suppressalis. All these genes had the conserved domains and motifs of glycoside hydrolase family 18 and grouped phylogenetically into five subgroups. C. suppressalis chitinase 1 (CsCht1) was highly expressed in late pupae, whereas CsCht3 was abundant in early pupae. Both CsCht2 and CsCht4 were highly expressed in larvae. CsCht2 was abundant specifically in the third-instar larvae and CsCht4 showed periodic high expression in 2- to 5-day-old larvae in each instar. Tissue specific expression analysis indicated that CsCht1 and CsCht3 were highly expressed in epidermis whereas CsCht2 and CsCht4 were specifically abundant in the midgut. Knockdown of CsCht1 resulted in adults with curled wings, indicating that CsCht1 might have an important role in wing expansion. Silencing of CsCht2 or CsCht4 arrested moulting, suggesting essential roles in larval development. When the expression of CsCht3 was interfered, defects in pupation occurred. Overall, we provide here the first catalogue of chitinase genes in the rice striped stem borer and have elucidated the functions of four chitinases in metamorphosis.

Transcriptome analysis of the copepod Eurytemora affinis upon exposure to endocrine disruptor pesticides: Focus on reproduction and development

Copepods-which include freshwater and marine species-represent the most abundant group of aquatic invertebrates. Among them, the calanoid copepod Eurytemora affinis is widely represented in the northern hemisphere estuaries and has become a species of interest in ecotoxicology. Like other non-target organisms, E. affinis may be exposed to a wide range of chemicals such as endocrine disruptors (EDs). This study investigated the gene expression variation in E. affinis after exposure to ED pesticides-chosen as model EDs-in order to (i) improve the knowledge on their effects in crustaceans, and (ii) highlight relevant transcripts for further development of potential biomarkers of ED exposure/effect. The study focused on the reproduction function in response to ED. Copepods were exposed to sublethal concentrations of pyriproxyfen (PXF) and chlordecone (CLD) separately. After 48h, males and females (400 individuals each) were sorted for RNA extraction. Their transcriptome was pyrosequenced using the Illumina(®) technology. Contigs were blasted and functionally annotated using Blast2GO(®). The differential expression analysis between ED- and acetone-exposed organisms was performed according to sexes and contaminants. Half of the 19,721 contigs provided by pyrosequencing were annotated, mostly (80%) from arthropod sequences. Overall, 2,566 different genes were differentially expressed after ED exposures in comparison with controls. As many genes were differentially expressed after PXF exposure as after CLD exposure. In contrast, more genes were differentially expressed in males than in females after both exposures. Ninety-seven genes overlapped in all conditions. Finally, 31 transcripts involved in reproduction, growth and development, and changed in both chemical exposures were selected as potential candidates for future development of biomarkers.

Heterologous expression and characterization of two chitinase 5 enzymes from the migratory locust Locusta migratoria

Insect chitinases are involved in degradation of chitin from the exoskeleton or peritrophic metrix of midgut. In Locusta migratoria, two duplicated Cht5s (LmCht5-1 and LmCht5-2) have been shown to have distinct molecular characteristics and biological roles. To explore the protein properties of the two LmCht5s, we heterologously expressed both enzymes using baculovirus expression system in SF9 cells, and characterized kinetic and carbohydrate-binding properties of purified enzymes. LmCht5-1 and LmCht5-2 exhibited similar pH and temperature optimums. LmCht5-1 has lower Km value for the oligomeric substrate (4MU-(GlcNAc)3 ), and higher Km value for the longer substrate (CM-Chitin-RBV) compared with LmCht5-2. A comparison of amino acids and homology modeling of catalytic domain presented similar TIM barrel structures and differentiated amino acids between two proteins. LmCht5-1 has a chitin-binding domain (CBD) tightly bound to colloidal chitin, but LmCht5-2 does not have a CBD for binding to colloidal chitin. Our results suggested both LmCht5-1 and LmCht5-2, which have the critical glutamate residue in region II of catalytic domain, exhibited chitinolytic activity cleaving both polymeric and oligomeric substrates. LmCht5-1 had relatively higher activity against the oligomeric substrate, 4MU-(GlcNAc)3 , whereas LmCht5-2 exhibited higher activity toward the longer substrate, CM-Chitin-RBV. These findings are helpful for further research to clarify their different roles in insect growth and development.

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.

Analysis of chitin-binding proteins from Manduca sexta provides new insights into evolution of peritrophin A-type chitin-binding domains in insects

In insects, chitin is a major structural component of the cuticle and the peritrophic membrane (PM). In nature, chitin is always associated with proteins among which chitin-binding proteins (CBPs) are the most important for forming, maintaining and regulating the functions of these extracellular structures. In this study, a genome-wide search for genes encoding proteins with ChtBD2-type (peritrophin A-type) chitin-binding domains (CBDs) was conducted. A total of 53 genes encoding 56 CBPs were identified, including 15 CPAP1s (cuticular proteins analogous to peritrophins with 1 CBD), 11 CPAP3s (CPAPs with 3 CBDs) and 17 PMPs (PM proteins) with a variable number of CBDs, which are structural components of cuticle or of the PM. CBDs were also identified in enzymes of chitin metabolism including 6 chitinases and 7 chitin deacetylases encoded by 6 and 5 genes, respectively. RNA-seq analysis confirmed that PMP and CPAP genes have differential spatial expression patterns. The expression of PMP genes is midgut-specific, while CPAP genes are widely expressed in different cuticle forming tissues. Phylogenetic analysis of CBDs of proteins in insects belonging to different orders revealed that CPAP1s from different species constitute a separate family with 16 different groups, including 6 new groups identified in this study. The CPAP3s are clustered into a separate family of 7 groups present in all insect orders. Altogether, they reveal that duplication events of CBDs in CPAP1s and CPAP3s occurred prior to the evolutionary radiation of insect species. In contrast to the CPAPs, all CBDs from individual PMPs are generally clustered and distinct from other PMPs in the same species in phylogenetic analyses, indicating that the duplication of CBDs in each of these PMPs occurred after divergence of insect species. Phylogenetic analysis of these three CBP families showed that the CBDs in CPAP1s form a clearly separate family, while those found in PMPs and CPAP3s were clustered together in the phylogenetic tree. For chitinases and chitin deacetylases, most of phylogenetic analysis performed with the CBD sequences resulted in similar clustering to the one obtained by using catalytic domain sequences alone, suggesting that CBDs were incorporated into these enzymes and evolved in tandem with the catalytic domains before the diversification of different insect orders. Based on these results, the evolution of CBDs in insect CBPs is discussed to provide a new insight into the CBD sequence structure and diversity, and their evolution and expression in insects.