The peritrophic membrane of Spodoptera frugiperda: secretion of peritrophins and role in immobilization and recycling digestive enzymes

Impact of Three Thiazolidinone Compounds with Piperine Skeletons on Trehalase Activity and Development of Spodoptera frugiperda Larvae

Trehalases (TREs) are pivotal enzymes involved in insect development and reproduction, making them prime targets for pest control. We investigated the inhibitory effect of three thiazolidinones with piperine skeletons (6a, 7b, and 7e) on TRE activity and assessed their impact on the growth and development of the fall armyworm (FAW), Spodoptera frugiperda. The compounds were injected into FAW larvae, while the control group was treated with 2% DMSO solvent. All three compounds effectively inhibited TRE activity, resulting in a significant extension of the pupal development stage. Moreover, the treated larvae exhibited significantly decreased survival rates and a higher incidence of abnormal phenotypes related to growth and development compared to the control group. These results suggest that these TRE inhibitors affect the molting of larvae by regulating the chitin metabolism pathway, ultimately reducing their survival rates. Consequently, these compounds hold potential as environmentally friendly insecticides.

RNAi of Mannosidase-Ia in the Colorado potato beetle and changes in the midgut and peritrophic membrane

BACKGROUND

In addition to its role in the digestive system, the peritrophic membrane (PM) provides a physical barrier protecting the intestine from abrasion and against pathogens. Because of its sensitivity to RNA interference (RNAi), the notorious pest insect, the Colorado potato beetle (CPB, Leptinotarsa decemlineata), has become a model insect for functional studies. Previously, RNAi-mediated silencing of Mannosidase-Ia (ManIa), a key enzyme in the transition from high-mannose glycan moieties to paucimannose N-glycans, was shown to disrupt the transition from larva to pupa and the metamorphosis into adult beetles. While these effects at the organismal level were interesting in a pest control context, the effects at the organ or tissue level and also immune effects have not been investigated yet. To fill this knowledge gap, we performed an analysis of the midgut and PM in ManIa-silenced insects.

RESULTS

As marked phenotype, the ManIa^RNAi insects, the PM pore size was found to be decreased when compared to the control GFP^RNAi insects. These smaller pores are related to the observation of thinner microvilli (Mv) on the epithelial cells of the midgut of ManIa^RNAi insects. A midgut and PM proteome study and reverse transcription quantitative polymerase chain reaction (RT-qPCR) analysis with a selection of marker genes was performed to characterize the midgut cells and understand their response to the silencing of ManIa. In agreement with the loss of ManIa activity, an accumulation of high-mannose N-glycans was observed in the ManIa-silenced insects. As a pathogen-associated molecular pattern (PAMP), the presence of these glycan structures could trigger the activation of the immune pathways.

CONCLUSION

The observed decrease in PM pore size could be a response to prevent potential pathogens to access the midgut epithelium. This hypothesis is supported by the strong increase in transcription levels of the anti-fungal peptide drosomycin-like in ManIa^RNAi insects, although further research is required to elucidate this possibility. The potential immune response in the midgut and the smaller pore size in the PM shed a light on the function of the PM as a physical barrier and provide evidence for the relation between the Mv and PM. © 2022 Society of Chemical Industry.

Morphological and histological description of the midgut caeca in true crabs (Malacostraca: Decapoda: Brachyura): origin, development and potential role

Background

The decapods are a major group of crustaceans that includes shrimps, prawns, crayfishes, lobsters, and crabs. Several studies focused on the study of the digestive system of the decapods, constituted by the oesophagus, stomach, midgut tract, midgut gland, and hindgut. Nevertheless, in the midgut tract there are associated a set of organs called “midgut caeca”, which are among the most controversial and less studied digestive organs of this group. This work used the common spider crab Maja brachydactyla Balss, 1922 as a model to resolve the origin, development, and potential role of the midgut caeca. Such organs were studied in the larvae (zoea I, zoea II, megalopa), first juveniles, and adult phases, being employed traditional and modern techniques: dissection, micro-computed tomography (Micro-CT), and light and electron microscopical analyses (TEM and SEM).

Results

The common spider crab has a pair of anterior midgut caeca and a single posterior caecum that originate from the endoderm germ layer: they develop from the midgut tract, and their epithelium is composed by secretory cells while lacking a cuticle lining. The midgut caeca are small buds in the newly hatched larvae, enlarge linearly during the larval development, and then continue growing until became elongated and coiled blind-tubules in adults. The adult midgut caeca are internally folded to increase their inner surface. The electron microscopy observations showed that the midgut caeca are highly active organs with important macroapocrine and microapocrine secretory activity. Our results suggest that the role of the caeca might be related to the digestive enzyme secretion. The secretory activity should increase as the animal grows in size.

Conclusion

The present study resolves the embryonic origin of the midgut caeca (endoderm derived organs), development (general lengthening starting from small buds), and role (active secretory organs). The secretory activity of the midgut caeca should be incorporated in the current models of the digestive physiology in different decapod taxa.

A proteomic approach to identify digestive enzymes, their exocytic and microapocrine secretory routes and their compartmentalization in the midgut of Spodoptera frugiperda

A proteomic approach was used to identify the digestive enzymes secreted by exocytosis and by microapocrine vesicles and enzyme midgut compartmentalization in Spodoptera frugiperda larvae. For this, proteomic analyses were performed in isolated midgut enterocyte microvillar membrane, in a fraction enriched in microapocrine vesicles (separated in soluble and membrane fractions), in the washings of the peritrophic membrane to isolate its loosely- and tightly-bound proteins, and in the peritrophic membrane contents. PM washings correspond to proteins extracted from the mucus layer surrounding PM. Serine endopeptidases (trypsins, chymotrypsins and serine endopeptidase homologs that have substitutions in the catalytic residues) and lipases are mainly secreted by exocytosis. Aminopeptidases are mainly microvillar enzymes and some are secreted membrane-bound to microapocrine vesicles, whereas carboxypeptidase isoforms follow different secretory routes. The results also showed that most polymer hydrolases (such as amylase and endopeptidases) are not retained in the ectoperitrophic fluid (found in PM washings but absent from PM contents). On the contrary, most enzymes involved in intermediate digestion (exemplified by carboxypeptidase and aminopeptidase) do not pass through the peritrophic membrane. Finally, the data revealed that the protein composition of PM includes peritrophins classified as peritrophic membrane proteins, PMP, and chitin deacetylase.

Intestinal proteases profiling from Anticarsia gemmatalis and their binding to inhibitors

Although the importance of intestinal hydrolases is recognized, there is little information on the intestinal proteome of lepidopterans such as Anticarsia gemmatalis. Thus, we carried out the proteomic analysis of the A. gemmatalis intestine to characterize the proteases by LC/MS. We examined the interactions of proteins identified with protease inhibitors (PI) using molecular docking. We found 54 expressed antigens for intestinal protease, suggesting multiple important isoforms. The hydrolytic arsenal featured allows for a more comprehensive understanding of insect feeding. The docking analysis showed that the soybean PI (SKTI) could bind efficiently with the trypsin sequences and, therefore, insect resistance does not seem to involve changing the sequences of the PI binding site. In addition, a SERPIN was identified and the interaction analysis showed the inhibitor binding site is in contact with the catalytic site of trypsin, possibly acting as a regulator. In addition, this SERPIN and the identified PI sequences can be targets for the control of proteolytic activity in the caterpillar intestine and serve as a support for the rational design of a molecule with greater stability, less prone to cleavage by proteases and viable for the control of insect pests such as A. gemmatalis.

Characteristics of the Peritrophic Matrix of the Silkworm, Bombyx mori and Factors Influencing Its Formation

Simple Summary

The insect midgut is an important digestive organ with the peritrophic matrix (PM) being a semi-permeable membrane secreted by the midgut cells. The PM plays an important role in improving midgut digestion efficiency and protecting the midgut from food particles and exogenous pathogens. The silkworm, Bombyx mori, is an economically important insect. Understanding the structure of the PM is necessary for studying its function, but characteristics of PM in B. mori have been rarely reported. In this study, we conducted a comprehensive study on the PM structure of the PM in silkworms and found its thickness increased gradually during growth, but there was no difference in the thickness comparing the anterior, middle, and posterior regions. Permeability of the PM gradually decreased from the anterior to posterior regions. In addition, we found the formation of the PM was influenced by food ingestion and the gut microbiota.

Abstract

The peritrophic matrix (PM) secreted by the midgut cells of insects is formed by the binding of PM proteins to chitin fibrils. The PM envelops the food bolus, serving as a barrier between the content of the midgut lumen and its epithelium, and plays a protective role for epithelial cells against mechanical damage, pathogens, toxins, and other harmful substances. However, few studies have investigated the characteristics and synthesis factors of the PM in the silkworm, Bombyx mori. Here, we examined the characteristics of the PM in the silkworms. The PM thickness of the silkworms increased gradually during growth, while there was no significant difference in thickness along the entire PM region. Permeability of the PM decreased gradually from the anterior to posterior PM. We also found that PM synthesis was affected by food ingestion and the gut microbiota. Our results are beneficial for future studies regarding the function of the PM in silkworms.

Chitin binding protein from the kuruma shrimp Marsupenaeus japonicus facilitates the clearance of Vibrio anguillarum

Peritrophic membrane (PM) refers to a vital physical barrier enabling shrimp to resist pathogen invasion. It primarily consists of chitin and proteins, mostly chitin-binding protein (CBP). CBPs have been identified from microorganisms to higher organisms. In the present study, a CBP, designated MjCBP, was reported from Marsupenaeus japonicus. The open reading frame of MjCBP was 1854 bp, encoding a protein with 618 amino acids (MH544098). To be specific, the theoretical pI and molecular mass of mature MjCBP reached 5.43 and 66064.00 Da, respectively. MjCBP consisted of seven type Ⅱ chitin-binding domains (ChtB D2), which was up-regulated after being challenged with Vibrio anguillarum and then agglutinating several bacteria. In addition, MjCBP and the first chitin-binding domain (CBD1) could bind to several Gram-positive and Gram-negative bacteria via the binding process to lipopolysaccharides and peptidoglycans, whereas CBD1 was not capable of agglutinating bacteria. Moreover, the anterior and posterior segments of CBD1 were synthesized in vitro, and the posterior segment could bind to lipopolysaccharides. However, both segments fail to agglutinate bacteria. Furthermore, MjCBP and CBD1 facilitated the clearance of V. anguillarum in vivo, and the silencing of MjCBP via RNA interference reduced the ability of bacterial clearance. As revealed from the mentioned results, MjCBP acts as an opsonin or pattern recognition receptor to achieve antibacterial immune response in shrimp.

Functional redundancy of structural proteins of the peritrophic membrane in Trichoplusia ni

The peritrophic membrane (or peritrophic matrix) (PM) in insects is formed by binding of PM proteins with multiple chitin binding domains (CBDs) to chitin fibrils. Multi-CBD chitin binding proteins (CBPs) and the insect intestinal mucin (IIM) are major PM structural proteins. To understand the biochemical and physiological role of IIM in structural formation and physiological function of the PM, Trichoplusia ni mutant strains lacking IIM were generated by CRISPR/Cas9 mutagenesis. The mutant T. ni larvae were confirmed to lack IIM, but PM formation was observed as in wild type larvae and lacking IIM in the PM did not result in changes of protease activities in the larval midgut. Larval growth and development of the mutant strains were similar to the wild type strain on artificial diet and cabbage leaves, but had a decreased survival in the 5th instar. The larvae of the mutant strains with the PM formed without IIM did not have a change of susceptibility to the infection of the baculovirus AcMNPV and the Bacillus thuringiensis (Bt) formulation Dipel, to the toxicity of the Bt toxins Cry1Ac and Cry2Ab and the chemical insecticide sodium aluminofluoride. Treatment of the mutant T. ni larvae with Calcofluor reduced the larval susceptibility to the toxicity of Bt Cry1Ac, as similarly observed in the wild type larvae. Overall, in the mutant T. ni larvae, the PM was formed without IIM and the lacking of IIM in the PM did not drastically impact the performance of larvae on diet or cabbage leaves under the laboratory conditions.

Effects of a Reserve Protein on Spodoptera frugiperda Development: A Biochemical and Molecular Approach to the Entomotoxic Mechanism

Talisin is a storage protein from Talisia esculenta seeds that presents lectin-like and peptidase inhibitor properties. These characteristics suggest that talisin plays a role in the plant defense process, making it a multifunctional protein. This work aimed to investigate the effects of chronic intake of talisin on fifth instar larvae of Spodoptera frugiperda, considered the main insect pest of maize and the cause of substantial economic losses in several other crops. The chronic intake of talisin presented antinutritional effects on the larvae, reducing their weight and prolonging the total development time of the insects. In addition, talisin-fed larvae also showed a significant reduction in the activity of trypsin-like enzymes. Midgut histology analysis of talisin-fed larvae showed alterations in the intestinal epithelium and rupture of the peritrophic membrane, possibly causing an increase of aminopeptidase activity in the midgut lumen. Talisin also proved to be resistant to degradation by the digestive enzymes of S. frugiperda. The transcription profile of trypsin, chymotrypsin and aminopeptidase genes was also analyzed through qPCR technique. Talisin intake resulted in differential expression of at least two genes from each of these classes of enzymes. Molecular docking studies indicated a higher affinity of talisin for the less expressed enzymes.

The Evolution, Gene Expression Profile, and Secretion of Digestive Peptidases in Lepidoptera Species

Serine peptidases (SPs) are responsible for most primary protein digestion in Lepidoptera species. An expansion of the number of genes encoding trypsin and chymotrypsin enzymes and the ability to upregulate the expression of some of these genes in response to peptidase inhibitor (PI) ingestion have been associated with the adaptation of Noctuidae moths to herbivory. To investigate whether these gene family expansion events are common to other Lepidoptera groups, we searched for all genes encoding putative trypsin and chymotrypsin enzymes in 23 publicly available genomes from this taxon. Phylogenetic analysis showed that several gene family expansion events may have occurred in the taxon’s evolutionary history and that these events gave rise to a very diverse group of enzymes, including proteins lacking the canonical SP catalytic triad. The expression profile of these enzymes along the midgut and the secretion mechanisms by which these enzymes enter the luminal content were also analyzed in Spodoptera frugiperda larvae using RNA-seq and proteomics. These results support the proposal of a midgut countercurrent flux responsible for the direction of these proteins to the anterior portion of the midgut and show that these enzymes reach the midgut lumen via both exocytosis and microapocrine secretion mechanisms.

Morphology and ultrastructure of the midgut gland ("hepatopancreas") during ontogeny in the common spider crab Maja brachydactyla Balss, 1922 (Brachyura, Majidae)

We studied the anatomy and cytology of the midgut gland (MGl) of the common spider crab Maja brachydactyla Balss, 1922 at several life stages (zoea, megalopa, first juvenile, and adult) using dissection, histology, electron microscopy, computed tomography, and micro-computed tomography (micro-CT). In newly hatched larvae, 14 blind-end tubules form the MGl. The length of the tubules increases during the larval development. In the late megalopa, the number of tubules also increases. In adults, 35,000 to 60,000 blind-ending tubules comprise the MGl. In all life stages, a square-net network of muscle fibers surround the tubules. We describe five cell types in the MGl in all larval stages, which have a similar location, histology, and ultrastructure in larvae and adults: embryonary (E-) cells, resorptive (R-) cells, fibrillar (F-) cells, blister-like (B-) cells, and midget (M-) cells. Major difference between larval and adult cells is the larger size of the adult cells. Microapocrine secretion occurs from the microvilli of the B-cells. No ultrastructural changes were observed during larval development, which suggests that the function of each cell type might be similar in all life stages. The role of each epithelial cell type in larvae and adults is discussed.

Functional expression of a peritrophin A-like SfPER protein is required for larval development in Spodoptera frugiperda (Lepidoptera: Noctuidae)

Peritrophins are associated with structural and functional integrity of peritrophic membranes (PM), structures composed of chitin and proteins. PM lines the insect midgut and has roles in digestion and protection from toxins. We report the full-length cDNA cloning, molecular characterization and functional analysis of SfPER, a novel PM peritrophin A protein, in Spodoptera frugiperda. The predicted amino acid sequence indicated SfPER's domain structure as a CMCMC-type, consisting of a signal peptide and three chitin-binding (C) domains with two intervening mucin-like (M) domains. Phylogenetic analysis determined a close relationship between SfPER and another S. frugiperda PM peritrophin partial sequence. SfPER transcripts were found in larvae and adults but were absent from eggs and pupae. Chitin affinity studies with a recombinant SfPER-C1 peritrophin A-type domain fused to SUMO/His-tag confirmed that SfPER binds to chitin. Western blots of S. frugiperda larval proteins detected different sized variants of SfPER along the PM, with larger variants found towards the posterior PM. In vivo suppression of SfPER expression did not affect susceptibility of larvae to Bacillus thuringiensis toxin, but significantly decreased pupal weight and adult emergence, possibly due to PM structural alterations impairing digestion. Our results suggest SfPER could be a novel target for insect control.

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.

Enhancement of insect susceptibility and larvicidal efficacy of Cry4Ba toxin by calcofluor

Background

Mosquitoes transmit many vector-borne infectious diseases including malaria, dengue, chikungunya, yellow fever, filariasis, and Japanese encephalitis. The insecticidal δ-endotoxins Cry4, Cry11, and Cyt produced from Bacillus thuringiensis have been used for bio-control of mosquito larvae. Cry δ-endotoxins are synthesised as inactive protoxins in the form of crystalline inclusions in which they are processed to active toxins in larval midgut lumen. Previously, we demonstrated that the activated Cry4Ba toxin has to alter the permeability of the peritrophic membrane (PM), allowing toxin passage across PM to reach specific receptors on microvilli of larval midgut epithelial cells, where the toxin undergoes conformational changes, followed by membrane insertion and pore formation, resulting in larval death. A peritrophic membrane (PM)-binding calcofluor has been proposed to inhibit chitin formation and enhance baculovirus infection of lepidopteran Trichoplusia ni.

Methods

In this study, Aedes aegypti larvae were fed with the calcofluor and Cry4Ba toxin to investigate the effect of this agent on the toxicity of the Cry4Ba toxin.

Results

Calcofluor displayed an enhancing effect when co-fed with the Cry4Ba wild-type toxin. The agent could restore the killing activity of the partially active Cry4Ba mutant E417A/Y455A toward Ae. aegypti larvae. PM destruction was observed after larval challenge with calcofluor together with the toxin. Interestingly, calcofluor increased Cry4Ba toxin susceptibility toward semi-susceptible Culex quinquefasciatus larvae. However, calcofluor alone or in combination with the toxin showed no mortality effect on non-susceptible fresh-water fleas, Moina macrocopa.

Conclusions

Our results suggest that PM may contribute to the resistance of the mosquito larvae to Cry4Ba toxin. The PM-permeability alternating calcofluor might be a promising candidate for enhancing insect susceptibility, which will consequently improve Cry4Ba efficacy in field settings in the future.

The physiological differentiation along the midgut of Bombyx mori - inspirations from proteomics and gene expression patterns of the secreted proteins in the ectoperitrophic space

The ectoperitrophic space (EcPS) between the insect midgut epithelial cells and the peritrophic matrix is an unexplored, clean resource for concentrated proteins secreted by the midgut epithelial cells, which offers an ideal opportunity to uncover the midgut functions. In this study, we used Bombyx mori as a model organism and performed comparative proteomic analyses of the secreted proteins in the EcPS at the feeding and wandering stages. A total of 372 proteins were identified from both stages and 70 proteins were predicted to be secreted. Amongst these proteins, 17 secreted digestive proteins were identified and their temporal and spatial transcriptional expression patterns demonstrated that all these proteins were up-regulated at the feeding stage and differentially expressed in different parts of the midgut. Proteins with nutrient reservoir activity and defence activity were found to be up-regulated at the wandering stage. This work is the first to show the presence of digestive enzymes in the EcPS of the insect midgut using a proteomic approach, which provides evidence that suggests a physiological functional differentiation of the insect midgut. It is very clear that the EcPS undergoes dynamic changes in its composition of proteins in response to the changing needs of the insect at different developmental stages.

The roles of mucus-forming mucins, peritrophins and peritrophins with mucin domains in the insect midgut

Most insects have a gut lined with a peritrophic membrane (PM) consisting of chitin and proteins, mainly peritrophins that have chitin-binding domains. The PM is proposed to originate from mucus-forming mucins (Mf-mucins), which acquired a chitin-binding domain that interlocked with chitin, replacing mucus in function. We evaluated the expression of Mf-mucins and peritrophins by RNA-sequencing (RNA-seq) throughout the midgut of four distantly related insects. Mf-mucins were identified as proteins with high o-glycosylation and a series of uninterrupted Pro/Thr/Ser residues. The results demonstrate that the mucus layer is widespread in insects, and suggest that insect Mf-mucins are derived from those found in other animals by the loss of the cysteine knot and von Willebrand domains. The data also support a role of Mf-mucins in protecting the middle midgut of Musca domestica against acidic buffers. Mf-mucins may also produce a jelly-like material associated with the PM that immobilizes digestive enzymes in Spodoptera frugiperda. Peritrophins with a domain similar to Mf-mucins may be close to the ancestor of peritrophins. Expression data of peritrophins and chitin synthase genes throughout the midgut of M. domestica, S. frugiperda and Tenebrio molitor indicated that peritrophins were incorporated along the PM, according to their preferential sites of formation. Finally, the data support the view that mucus has functions distinct from the PM.

Pleiotropic and novel phenotypes in the Drosophila gut caused by mutation of drop-dead

Normal gut function is vital for animal survival, and deviations from such function can contribute to malnutrition, inflammation, increased susceptibility to pathogens, diabetes, neurodegenerative diseases, and cancer. In the fruit fly Drosophila melanogaster, mutation of the gene drop-dead (drd) results in defective gut function, as measured by enlargement of the crop and reduced food movement through the gut, and drd mutation also causes the unrelated phenotypes of neurodegeneration, early adult lethality and female sterility. In the current work, adult drd mutant flies are also shown to lack the peritrophic matrix (PM), an extracellular barrier that lines the lumen of the midgut and is found in many insects including flies, mosquitos and termites. The use of a drd-gal4 construct to drive a GFP reporter in late pupae and adults revealed drd expression in the anterior cardia, which is the site of PM synthesis in Drosophila. Moreover, the ability of drd knockdown or rescue with several gal4 drivers to recapitulate or rescue the gut phenotypes (lack of a PM, reduced defecation, and reduced adult survival 10-40 days post-eclosion) was correlated to the level of expression of each driver in the anterior cardia. Surprisingly, however, knocking down drd expression only in adult flies, which has previously been shown not to affect survival, eliminated the PM without reducing defecation rate. These results demonstrate that drd mutant flies have a novel phenotype, the absence of a PM, which is functionally separable from the previously described gut dysfunction observed in these flies. As the first mutant Drosophila strain reported to lack a PM, drd mutants will be a useful tool for studying the synthesis of this structure.

Non-glandular trichomes of Solanum carolinense deter feeding by Manduca sexta caterpillars and cause damage to the gut peritrophic matrix

Plant trichomes constitute a first line of defence against insect herbivores. The pre- and post-ingestive defensive functions of glandular trichomes are well documented and include direct toxicity, adhesion, antinutrition and defence gene induction. By contrast, the defensive functions of non-glandular trichomes are less well characterized, although these structures are thought to serve as physical barriers that impede herbivore feeding and movement. We experimentally varied the density of stellate non-glandular trichomes in several ways to explore their pre- and post-ingestive effects on herbivores. Larvae of Manduca sexta (Sphingidae) initiated feeding faster and gained more weight on Solanum carolinense (Solanaceae) leaves having lower trichome densities (or experimentally removed trichomes) than on leaves having higher trichome densities. Adding trichomes to artificial diet also deterred feeding and adversely affected caterpillar growth relative to controls. Scanning electron and light microscopy revealed that the ingestion of stellate trichomes by M. sexta caterpillars caused extensive damage to the peritrophic membrane, a gut lining that is essential to digestion and pathogen isolation. These findings suggest that, in addition to acting as a physical barrier to deter feeding, trichomes can inhibit caterpillar growth and development via post-ingestive effects.

Gelsolin role in microapocrine secretion

A role of gelsolin in opening the way along the microvilli for secretory vesicles during microapocrine secretion is proposed here. Data obtained with different techniques showed that many digestive enzymes are released by microapocrine secretion in insects. Proteins that might be involved in the machinery of microapocrine secretion were selected from our transcriptomes and literature searches. The proteins were annexin, Complex actin-related proteins 2 and 3 (ARP 2/3) cofilin, fimbrin, gelsolin 1, gelsolin 2, moesin, myosin 1, myosin 6, protein disulphide isomerase 1 (PDI 1), PDI 2 and profilin. The cDNAs coding for annexin, fimbrin, gelsolin 1, myosin 1, PDI 1 and PDI 2 were cloned and their sequences deposited in GenBank. Only gelsolin 1 and myosin 1 are expressed exclusively in the midgut (semiquantitative reverse transcriptase PCR). As myosin 1 may have a structural role in microvilli, gelsolin 1 is the best guess to be involved in the secretory machinery. A truncated recombinant gelsolin 1 was used to generate antibodies with which it was shown labelling inside and around midgut cell microvilli shown in an electron microscope, reinforcing a microvillar role for gelsolin 1. Suppression of gelsolin 1 synthesis by RNA interference prevents secretory vesicles from advancing inside the microvilli, in agreement with its putative role in severing the actin filaments to free the way for the vesicles.

GUT GENES ASSOCIATED WITH THE PERITROPHIC MATRIX IN Reticulitermes flavipes (Blattodea: Rhinotermitidae): IDENTIFICATION AND CHARACTERIZATION

The peritrophic matrix (PM) is an acellular structure that lines the gut of most insects. It is an attractive target for pest management strategies because of its close involvement in digestive processes and role as a barrier against pathogens and toxins. The purpose of this study was to identify and characterize the genes that translate for principal components of the Reticulitermes flavipes PM. Genes encoding a gut chitin synthase (CHS), two proteins with peritrophin-A domains, and a chitin deacetylase were identified from an R. flavipes symbiont-free gut cDNA library, a pyrosequencing study of termite lignocellulose digestion, and a metatranscriptomic analysis of R. flavipes fed on agricultural biomass. Quantitative expression analysis of the identified genes, in the termite digestive tract, revealed that the transcripts coding for a CHS (RfCHSB) and the proteins with peritrophin-A domains (RfPPAD1 and RfPPAD2) were predominantly expressed in the midgut, suggesting an association with the PM. The peritrophin identity of the RfPPAD2 gene was confirmed by immunodetection of its translated peptide in the midgut and PM. The discovery and characterization of PM components of R. flavipes provides a basis for further investigation of the viability of this structure as a target for candidate termiticides.

CONSTRUCTION OF SILKWORM MIDGUT cDNA LIBRARY FOR SCREEN AND SEQUENCE ANALYSIS OF PERITROPHIC MEMBRANE PROTEIN GENES

Silkworm is an important economic insect and the model species for Lepidoptera. The midgut of silkworm is an important physiological barrier, as its peritrophic membrane (PM) can resist pathogen invasion. In this study, a silkworm midgut cDNA library was constructed in order to identify silkworm PM genes. The capacity of the initial library was 6.92 × 10(6) pfu/ml, along with a recombination rate of 92.14% and a postamplification titer of 4.10 × 10(9) pfu/ml. Three silkworm PM protein genes were obtained by immunoscreening, two of which were chitin-binding protein (CBP) genes and one of which was a chitin deacetylase (CDA) gene as revealed by sequence analysis. Three genes were named BmCBP02, BmCBP13, and BmCDA17, and their ORF sizes are 678, 1,029, and 645 bp, respectively; all of them contain sequences of chitin-binding domains. Phylogenetic analysis indicated that BmCBP02 has the highest consensus with Mamestra configurata CBP at 61.0%; BmCBP13 has the highest consensus with Loxostege sticticalis PM CBP at 53.35%; BmCDA17 has the highest consensus with Helicoverpa armigera CDA5a at 70.83%. Tissue transcriptional analysis revealed that all three genes were specifically expressed in the midgut, and during the developmental process of fifth-instar silkworms, the transcription of all the genes showed an upward trend. This study laid a foundation for further studies on the functions of silkworm PM genes.

Midgut of the non-hematophagous mosquito Toxorhynchites theobaldi (Diptera, Culicidae)

In most mosquito species, the females require a blood-feeding for complete egg development. However, in Toxorhynchites mosquitoes, the eggs develop without blood-feeding, and both females and males exclusively feed on sugary diets. The midgut is a well-understood organ in blood-feeding mosquitoes, but little is known about it in non-blood-feeding ones. In the present study, the detailed morphology of the midgut of Toxorhynchites theobaldi were investigated using histochemical and ultrastructural methods. The midgut of female and male T. theobaldi adults consists of a long, slender anterior midgut (AMG), and a short, dilated posterior midgut (PMG). The AMG is subdivided into AMG1 (short, with folds) and AMG2 (long, without folds). Nerve branches and enteroendocrine cells are present in AMG and PMG, respectively. Compared with the PMG of blood-feeding female mosquitoes, the PMG of T. theobaldi is smaller; however, in both mosquitoes, PMG seems be the main region of food digestion and absorption, and protein secretion. The epithelial folds present in the AMG of T. theobaldi have not been reported in other mosquitoes; however, the midgut muscle organization and endocrine control of the digestion process are conserved in both T. theobaldi and blood-feeding mosquitoes.

Insect midgut carboxypeptidases with emphasis on S10 hemipteran and M14 lepidopteran carboxypeptidases

We compared the whole complement of midgut carboxypeptidases from 10 insects pertaining to five orders based on transcriptomes obtained by deep sequencing and biochemical data. Most of the carboxypeptidases were metallocarboxypeptidases from family M14, with carboxypeptidase A (CPA) predominating over carboxypeptidase B (CPB). They were found in all of the insects studied except for the hemipterans and a bruchid beetle. M14 carboxypeptidases were expressed only in the midgut of Spodoptera frugiperda (Lepidoptera). The most expressed CPA from this insect (SfCPA) was cloned, sequenced and expressed as a recombinant enzyme. This enzyme was used to generate antibodies used to demonstrate that SfCPA is secreted by an exocytic route. Serine carboxypeptidases from family S10 were found in all of the insects studied here. In S. frugiperda, they are expressed in all tissues besides the midgut, in accordance with their presumed lysosomal role. In the hemipteran Dysdercus peruvianus, S10 carboxypeptidases are expressed only in midgut, suggesting that they are digestive enzymes. This was confirmed by enzyme assays of midgut contents. Furthermore, the substrate specificity of D. peruvianus S10 carboxypeptidases are predicted to be one CPC (preferring hydrophobic residues) and one CPD (preferring basic residues), thus able to hydrolyse the peptides formed by their digestive cathepsin D and cathepsin L, respectively. The role of S10 carboxypeptidases in bruchid beetles are suggested to be the same as in hemipterans.

Identification of a new peritrophic membrane protein from larval Holotrichia parallela (Coleoptera: Motschulsky)

Peritrophic membranes (PMs) are composed of proteins, proteoglycans and chitin that play important roles in the structural formation and function of the PM. This study identified and characterized a new chitin binding protein named HpCBP45 by immunoscreening of the Holotrichia parallela larvae midgut expression library. The predicted amino acid sequence indicates that it contains eight tandem chitin binding domains belonging to the peritrophin-A family. The HpCBP45 protein was expressed as a recombinant protein in the yeast Pichia pastoris and chitin binding assay demonstrated that recombinant HpCBP45 protein could strongly bind to chitin. qRT-PCR analysis showed that HpCBP45 was mainly localized in the midgut, further confirming the H. parallela PM belongs to Type I PM. The discovery and characterization of the peritrophic membrane protein HpCBP45 provides a basis for the further investigation of its biochemical and physiological functions in H. parallela.

Spatial and temporal synthesis of Mamestra configurata peritrophic matrix through a larval stadium

The structure and synthesis of the Mamestra configurata peritrophic matrix (PM) was examined at various time points during a larval stadium. Bright field and confocal fluorescence microscopy revealed major differences between the PM of feeding and molting larvae. The PM from feeding larvae was thinner and composed of approximately 5-10 layers. In contrast, mid-molt larvae had a chitinaceaous PM composed of multiple thick layers which filled most of the midgut lumen. PM synthesis initiates in the anterior midgut, based on the expression of genes encoding chitin synthase-2 (CHS-2), coincident with the incorporation of the major structural PM proteins (McIIM1, McIIM2 and McPM1). This is followed by reinforcement with other PM proteins (McIIM3 and McIIM4) as it moves toward the posterior of the midgut. Chitin deacetylase (McCDA1) was associated only with the anterior PM. Collectively, these findings indicate that the structural properties of the PM differ along the length of the midgut. Genes encoding chitinolytic enzymes (McCHI and McNAG) were expressed and exochitinase activity was present when the PM had degraded (pre-molt) and when the new PM was forming (mid-molt), indicating that they are involved in either PM turnover and/or maintenance dependent upon the stage.

Identification and molecular characterization of a chitin-binding protein from the beet webworm, Loxostege sticticalis L

As the first crucial barrier in the midgut of insects, the peritrophic membrane (PM) plays an important role in preventing external invasion. PM proteins, as the major components of the PM, determine the structure and function of this membrane. A new PM protein, named LstiCBP, from the PM of Loxostege sticticalis larvae was identified using cDNA library screening. The full cDNA of LstiCBP is 2606 bp in length and contains a 2403 bp ORF that encodes an 808-amino acid preprotein with a 15-amino acid as signal peptide. The deduced protein sequence of the cDNA contains 8 cysteine-rich chitin-binding domains (CBDs). Recombinant LstiCBP was successfully expressed in BL21 cells using recombinant plasmid DNA and showed high chitin-binding activity. LstiCBP expression was detected in the midgut at both the transcriptional and translational levels; however, the biochemical and physiological functions of LstiCBP in L. sticticalis require further investigation.

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.

Densovirus crosses the insect midgut by transcytosis and disturbs the epithelial barrier function

Densoviruses are parvoviruses that can be lethal for insects of different orders at larval stages. Although the horizontal transmission mechanisms are poorly known, densoviral pathogenesis usually starts with the ingestion of contaminated food by the host. Depending on the virus, this leads to replication restricted to the midgut or excluding it. In both cases the success of infection depends on the virus capacity to enter the intestinal epithelium. Using the Junonia coenia densovirus (JcDNV) as the prototype virus and the lepidopteran host Spodoptera frugiperda as an interaction model, we focused on the early mechanisms of infection during which JcDNV crosses the intestinal epithelium to reach and replicate in underlying target tissues. We studied the kinetics of interaction of JcDNV with the midgut epithelium and the transport mechanisms involved. Using several approaches, in vivo, ex vivo, and in vitro, at molecular and cellular levels, we show that JcDNV is specifically internalized by endocytosis in absorptive cells and then crosses the epithelium by transcytosis. As a consequence, viral entry disturbs the midgut function. Finally, we showed that four mutations on the capsid of JcDNV affect specific recognition by the epithelial cells but not their binding.

Maize toxin degrades peritrophic matrix proteins and stimulates compensatory transcriptome responses in fall armyworm midgut

Understanding the molecular mechanisms underlying insect compensatory responses to plant defenses could lead to improved plant resistance to herbivores. The Mp708 inbred line of maize produces the maize insect resistant 1-cysteine protease (Mir1-CP) toxin. Reduced feeding and growth of fall armyworm larvae fed on Mp708 was previously linked to impairment of nutrient utilization and degradation of the midgut (MG) peritrophic matrix (PM) by Mir1-CP. Here we examine the biochemical and transcriptional responses of fall armyworm larvae to Mir1-CP. Insect Intestinal Mucin (IIM) was severely depleted from pure PMs treated in vitro with recombinant Mir1-CP. Larvae fed on Mp708 midwhorls excrete frass largely depleted of IIM. Cracks, fissures and increased porosity previously observed in the PM of larvae fed on Mp708 midwhorls could ensue when Mir1-CP degrades the IIM that cross-links chitin fibrils in the PM. Both targeted and global transcriptome analyses were performed to determine how complete dissolution of the structure and function of the PM is prevented, enabling larvae to continue growing in the presence of Mir1-CP. The MGs from fall armyworm fed on Mp708 upregulate expression of genes encoding proteins involved in PM production as an apparent compensation to replace the disrupted PM structure and restore appropriate counter-current MG gradients. Also, several families of digestive enzymes (endopeptidases, aminopeptidases, lipases, amylase) were more highly expressed in MGs from larvae fed on Mp708 than MGs from larvae fed on diets lacking Mir1-CP (artificial diet, midwhorls from Tx601 or B73 maize). Impaired growth of larvae fed on Mp708 probably results from metabolic costs associated with higher production of PM constituents and digestive enzymes in a compensatory attempt to maintain MG function.

Expression and distribution of cellulase, amylase and peptidase isoforms along the midgut of Morimus funereus L. (Coleoptera: Cerambycidae) larvae is dependent on nutrient substrate composition

The influence of diet composition--two substrates, wheat bran and sawdust--on isoform expression of digestive enzymes (cellulase, amylase and peptidase) in the midgut of Morimus funereus larvae was examined. Their impact on larval development was demonstrated by measuring the increase of larval weight during development and by analysis of digestive enzymes zymographic profiles, where the expression of cellulase isoforms from M. funereus larvae midgut has been examined for the first time in this study. Larvae reared on wheat bran had higher body weight between day 60 and day 100 than larvae reared on sawdust; however, both groups achieved similar body weight after day 110. Wheat bran as substrate induced different cellulase and amylase isoforms. Oak sawdust in substrate acted as inducer of peptidases. The highest cellulase activity and the greatest isoform variability were detected in the midgut extracts of larvae reared on wheat bran. From our results it can be assumed that M. funereus endocellulase, amylase and peptidase are secreted in the anterior midgut, and their concentration gradually decreases towards the hindgut.

In vitro and in vivo application of RNA interference for targeting genes involved in peritrophic matrix synthesis in a lepidopteran system

The midgut of most insects is lined with a semipermeable acellular tube, the peritrophic matrix (PM), composed of chitin and proteins. Although various genes encoding PM proteins have been characterized, our understanding of their roles in PM structure and function is very limited. One promising approach for obtaining functional information is RNA interference, which has been used to reduce the levels of specific mRNAs using double-stranded RNAs administered to larvae by either injection or feeding. Although this method is well documented in dipterans and coleopterans, reports of its success in lepidopterans are varied. In the current study, the silencing midgut genes encoding PM proteins (insect intestinal mucin 1, insect intestinal mucin 4, PM protein 1) and the chitin biosynthetic or modifying enzymes (chitin synthase-B and chitin deacetylase 1) in a noctuid lepidopteran, Mamestra configurata, was examined in vitro and in vivo. In vitro studies in primary midgut epithelial cell preparations revealed an acute and rapid silencing (by 24 h) for the gene encoding chitin deacetylase 1 and a slower rate of silencing (by 72 h) for the gene encoding PM protein 1. Genes encoding insect intestinal mucins were slightly silenced by 72 h, whereas no silencing was detected for the gene encoding chitin synthase-B. In vivo experiments focused on chitin deacetylase 1, as the gene was silenced to the greatest extent in vitro. Continuous feeding of neonates and fourth instar larvae with double-stranded RNA resulted in silencing of chitin deacetylase 1 by 24 and 36 h, respectively. Feeding a single dose to neonates also resulted in silencing by 24 h. The current study demonstrates that genes encoding PM proteins can be silenced and outlines conditions for RNA interference by per os feeding in lepidopterans.

Midgut proteins released by microapocrine secretion in Spodoptera frugiperda

Microapocrine vesicles bud from the lepidopteran midgut microvilli as double membrane vesicles. To identify the proteins secreted by this process, antibodies raised against isolated microapocrine vesicles from Spodoptera frugiperda were used for screening a midgut cDNA expression library. Positive clones were sequenced, assembled and N blasted against S. frugiperda sequences obtained by pyrosequencing midgut mRNA. This procedure led to the extension of microapocrine sequences that were annotated. A similar procedure was used to identify midgut microvillar proteins that necessarily are part of the microapocrine vesicle. Forty-eight proteins were associated with microvillar membranes. They pertain to 8 functional groups: digestive enzymes, peritrophic membrane, protection, transporters, receptors, secretory machinery, cytoskeleton and signaling, and unknown. Twenty-eight proteins are putatively secreted by microapocrine secretion. Most of them are digestive enzymes, but the list also includes proteins involved in protection and in peritrophic membrane formation. Among the identified digestive enzymes, aminopeptidases are typically microvillar and group into the classes 1, 2, 3, 5, and 6. There are two amylases secreted by microapocrine secretion: one is a digestive enzyme and the other is a transporter-like amylase with no clear function. One lipase has a predicted transmembrane loop, whereas the others are supposed to be secreted by microapocrine secretion and be digestive. Trypsin is membrane bound and is delivered by microapocrine secretion, but has no predicted features to bind membranes. It may remain bound through the signal peptide till be delivered into the midgut lumen. Proteins supposed to be involved in the microapocrine secretory machinery were: calmodulin, annexin, myosin 7a, and gelsolin 1. Their putative roles are discussed, but more research is necessary to settle this subject.

The role of peritrophic membrane in the resistance of Anticarsia gemmatalis larvae (Lepidoptera: Noctuidae) during the infection by its nucleopolyhedrovirus (AgMNPV)

The aim of this study was to analyze morphologically the peritrophic membrane (PM) of Anticarsia gemmatalis larvae resistant (RL) and non-resistant (susceptible) (SL) to the A. gemmatalis multicapsid nucleopolyhedrovirus (AgMNPV), in the presence of viral infection. Also, in this investigation the results between SL and RL were compared to improve the understanding of the resistance mechanisms to the virus. The PM of SL of A. gemmatalis was less efficient as a barrier against the viral infection since it was found to be more fragile than the PM of RL. The lower chitin content as seen from weaker fluorescent staining in SL as well as the abundance of non-solubilized vesicular materials in the ectoperitrophic space, would cause the malformation of this membrane, facilitating the passage of the virus toward the epithelium of the midgut. On the other hand, in RL, the intensity of WGA (wheat germ agglutinin)-conjugated FITC (fluorescein) reaction of the PM was greater than in SL, making this insect more resistant to infection. We can conclude that the effectiveness of the PM in protecting against pathogens is dependent on the integrity of the epithelial cells of the midgut and of the structural preservation of the PM, being directly implicated in the resistance of A. gemmatalis larvae to AgMNPV.

Characterization of a β-1,3-glucanase active in the alkaline midgut of Spodoptera frugiperda larvae and its relation to β-glucan-binding proteins

Spodoptera frugiperda β-1,3-glucanase (SLam) was purified from larval midgut. It has a molecular mass of 37.5 kDa, an alkaline optimum pH of 9.0, is active against β-1,3-glucan (laminarin), but cannot hydrolyze yeast β-1,3-1,6-glucan or other polysaccharides. The enzyme is an endoglucanase with low processivity (0.4), and is not inhibited by high concentrations of substrate. In contrast to other digestive β-1,3-glucanases from insects, SLam is unable to lyse Saccharomyces cerevisae cells. The cDNA encoding SLam was cloned and sequenced, showing that the protein belongs to glycosyl hydrolase family 16 as other insect glucanases and glucan-binding proteins. Multiple sequence alignment of β-1,3-glucanases and β-glucan-binding protein supports the assumption that the β-1,3-glucanase gene duplicated in the ancestor of mollusks and arthropods. One copy originated the derived β-1,3-glucanases by the loss of an extended N-terminal region and the β-glucan-binding proteins by the loss of the catalytic residues. SLam homology modeling suggests that E228 may affect the ionization of the catalytic residues, thus displacing the enzyme pH optimum. SLam antiserum reacts with a single protein in the insect midgut. Immunocytolocalization shows that the enzyme is present in secretory vesicles and glycocalyx from columnar cells.

The effect of chitin metabolic effectors on the population increase of stored product mites

The study tested the effect of the chitin metabolic effectors, teflubenzuron, diflubenzuron, and calcofluor, and a combination of a chitinase and soybean trypsin inhibitor (STI) on the population growth of eight species of stored product mites under laboratory conditions. The compounds were incorporated into the diets of the mites in concentrations ranging from 0.01 to 50 mg g(-1). The final populations of mites were observed after 21 days of growth in controlled conditions. Diflubenzuron and calcofluor suppressed the growth of all the tested species more effectively than the other compounds. The doses required to suppress the mite populations to 50% (rc(50)) in comparison to the control ranged from 0.29 to 12.68 mg g(-1) for diflubenzuron and from 1.75 to 37.7 mg g(-1) for calcofluor, depending on the mite species. When tested at the highest concentration (10 mg g(-1)), teflubenzuron also suppressed all of the tested mite species in comparison to the control. The addition of chitinase/STI into the diet influenced population growth in several ways. When the highest concentration of chitinase in a cocktail of chitinase and STI (12.5 mg g(-1) of diet) was compared to the control, populations of Acarus siro, Aleuroglyphus ovatus and Aëroglyphus robustus decreased significantly, whereas populations of Tyroborus lini and Sancassania rodionovi increased significantly. The sensitivity of species to the tested compounds differed among species. The most tolerant species was S. rodionovi, the most sensitive was A. ovatus. The results confirmed that calcofluor and diflubenzuron have a toxic effect on stored product mites.

Identification and molecular characterization of a new member of the peritrophic membrane proteins from the meadow moth, loxostege sticticalis

The peritrophic membrane (PM) plays an important role in protecting insects. The PM proteins are important to determinate the formation and function of the PM. A new PM protein, named Lsti99, was identified from the PM of Loxostege sticticalis larvae by cDNA library screening. The full cDNA of Lsti99 is 1392 bp in length, contains an open reading frame (ORF) of 1245 bp that encodes a preprotein of 415 amino acid residues with a 17-amino acid signal peptide. The sequence of Lsti99 showed no homology to other known PM proteins. The recombinant Lsti99 was successfully expressed in insect cells (Sf9) using recombinant baculoviruses and was used to isolate the antibodies to Lsti99 from the polyclonal antiserum. Lsti99 was expressed mainly in the PM, but weaker bands could be detected in the head and integument as well. The Lsti99 protein could be separated from the PM complex by chitinase in vitro, but M2R did not show effect in vitro confirming the chitin-binding activity of Lsti99. The biochemical and physiological functions of Lsti99 in L. sticticalis require further investigation.

Exogenous and endogenous protease inhibitors in the gut of the fall armyworm larvae, Spodoptera frugiperda

A dose-dependent inhibition of endogenous trypsin and aminopeptidase occurs in the lumen of Spodoptera frugiperda after feeding L6 larvae exogenous inhibitors soybean trypsin inhibitor (SBTI), tosyl-L-lysine chloromethyl ketone-HCl (TLCK), or bestatin, respectively, for 3 days. TLCK inhibits trypsin in tissue extracts and in secretions more strongly than SBTI. The aminopeptidase released into the lumen (containing the peritrophic membrane) is strongly inhibited by bestatin, but the membrane-bound enzyme is not. A bound enzyme may be more resistant to an inhibitor than unbound. A cross-class elevation of aminopeptidase activity occurs in response to ingested trypsin inhibitor, but there was no cross-class effect of aminopeptidase inhibitor (bestatin) on trypsin activity. An endogenous trypsin and aminopeptidase inhibitor is present in the lumen and ventricular cells. The strength of the endogenous trypsin inhibition seems to be in the same range as that resulting from ingestion of the exogenous inhibitor SBTI. In some insect species, considerable trypsin secretion occurs in unfed as well as in fed animals, and endogenous protease inhibitors might function to protect the ventricular epithelium by inactivation of trypsin when less food is available.

Insect intestinal mucins and serine proteases associated with the peritrophic matrix from feeding, starved and moulting Mamestra configurata larvae

Insect intestinal mucins (McIIM2-4) expressed in the midgut of feeding, starved and moulting Mamestra configurata larvae were identified. McIIM2 and McIIM4 were associated with the peritrophic matrix (PM). PMs from feeding and starved larvae were translucent and contained organized chitin bundles perpendicular to their long axis, whereas PM from moulting larvae consisted of an inner opaque mass surrounded by an outer translucent sleeve. Serine protease genes (McSP1, McSP2, McSP25 and McSP29) were also expressed in these larvae and several serine proteases were associated with the PM. Serine protease activity was also detected in the midgut of feeding, starved and moulting larvae.

Genes encoding proteins with peritrophin A-type chitin-binding domains in Tribolium castaneum are grouped into three distinct families based on phylogeny, expression and function

This study is focused on the characterization and expression of genes in the red flour beetle, Tribolium castaneum, encoding proteins that possess one or more six-cysteine-containing chitin-binding domains related to the peritrophin A domain (ChtBD2). An exhaustive bioinformatics search of the genome of T. castaneum queried with ChtBD2 sequences yielded 13 previously characterized chitin metabolic enzymes and 29 additional proteins with signal peptides as well as one to 14 ChtBD2s. Using phylogenetic analyses, these additional 29 proteins were classified into three large families. The first family includes 11 proteins closely related to the peritrophins, each containing one to 14 ChtBD2s. These are midgut-specific and are expressed only during feeding stages. We propose the name "Peritrophic Matrix Proteins" (PMP) for this family. The second family contains eight proteins encoded by seven genes (one gene codes for 2 splice variants), which are closely related to gasp/obstructor-like proteins that contain 3 ChtBD2s each. The third family has ten proteins that are of diverse sizes and sequences with only one ChtBD2 each. The genes of the second and third families are expressed in non-midgut tissues throughout all stages of development. We propose the names "Cuticular Proteins Analogous to Peritophins 3" (CPAP3) for the second family that has three ChtBD2s and "Cuticular Proteins Analogous to Peritophins 1 (CPAP1) for the third family that has 1 ChtBD2. Even though proteins of both CPAP1 and CPAP3 families have the "peritrophin A" domain, they are expressed only in cuticle-forming tissues. We determined the exon-intron organization of the genes, encoding these 29 proteins as well as the domain organization of the encoded proteins with ChtBD2s. All 29 proteins have predicted cleavable signal peptides and ChtBD2s, suggesting that they interact with chitin in extracellular locations. Comparison of ChtBD2s-containing proteins in different insect species belonging to different orders suggests that ChtBD2s are ancient protein domains whose affinity for chitin in extracellular matrices has been exploited many times for a range of biological functions. The differences in the expression profiles of PMPs and CPAPs indicate that even though they share the peritrophin A motif for chitin binding, these three families of proteins have quite distinct biological functions.

Control of the release of digestive enzymes in the larvae of the fall armyworm, Spodoptera frugiperda

There is a basal level of enzyme activity for trypsin, aminopeptidase, amylase, and lipase in the gut of unfed larval (L6) Spodoptera frugiperda. Trypsin activity does not decrease with non-feeding, possibly because of the low protein levels in plants along with high amino acid requirements for growth and storage (for later reproduction in adults). Therefore, trypsin must always be present so that only a minimal protein loss via egestion occurs. Larvae, however, adjust amylase activity to carbohydrate ingestion, and indeed amylase activity is five-fold higher in fed larvae compared to unfed larvae. Gut lipase activity is low, typical of insects with a high carbohydrate diet. A flat-sheet preparation of the ventriculus was used to measure the release of enzymes in response to specific nutrients and known brain/gut hormones in S. frugiperda. Sugars greatly increase (>300%) amylase release, but starch has no effect. Proteins and amino acids have little or no effect on trypsin or aminopeptidase release. The control of enzyme release in response to food is likely mediated through neurohormones. Indeed, an allatostatin (Spofr-AS A5) inhibits amylase and trypsin, and allatotropin (Manse- AT) stimulates amylase and trypsin release. Spofr-AS A5 also inhibits ileum myoactivity and Manse-AT stimulates myoactivity. The epithelial secretion rate of amylase and trypsin was about 20% of the amount of enzyme present in the ventricular lumen, which, considering the efficient counter-current recycling of enzymes, suggests that the secretion rate is adequate to replace egested enzymes.

Identification of the proteome of the midgut of silkworm, Bombyx mori L., by multidimensional liquid chromatography (MDLC) LTQ-Orbitrap MS

The midgut is the digestive apparatus of the silkworm and its proteome was studied by using nano-LC (liquid chromatography) electrospray ionization MS/MS (tandem MS). MS data were analysed by using X!Tandem searching software using different parameters and validated by using the Poisson model. A total of 90 proteins were identified and 79 proteins were described for the first time. Among the new proteins, (i) 22 proteins were closely related to the digestive function of the midgut, including 11 proteins of digestive enzymes secreted by the epithelium, eight proteins of intestine wall muscle and mechanical digestion and three proteins of peritrophic membrane that could prevent the epithelium from being mechanically rubbed; (ii) 44 proteins were involved in metabolism of substance and energy; and (iii) 11 proteins were associated with signal transduction, substance transport and cell skeleton.

Digestive alpha-amylases of the flour moth Ephestia kuehniella--adaptation to alkaline environment and plant inhibitors

The digestive tract of lepidopteran insects is extremely alkaline. In the present work, molecular adaptation of amylolytic enzymes to this environment was investigated in the flour moth Ephestia kuehniella, an important stored-product pest. Three digestive alpha-amylases [Ephestia kuehniella alpha-amylase isoenzymes 1-3 (EkAmy1-3)] with an alkaline pH optimum were purified from larvae and biochemically characterized. These isoenzymes differ significantly in their sensitivity to alpha-amylase inhibitors of plant origin that are directed against herbivores as antifeedants. Such functional variability renders the amylolytic system less vulnerable to suppression by plant defensive molecules. Moreover, we found that expression of alpha-amylases is upregulated in larvae feeding on a diet enriched with an alpha-amylase inhibitor. The alpha-amylases are secreted into the larval midgut by an exocytotic mechanism, as revealed by immunogold microscopy. The cDNA sequence of EkAmy3 was determined, and a homology model of EkAmy3 was built in order to analyze the structural features responsible for adaptation to alkaline pH. First, the overall fold was found to be stabilized by remodeling of ion pairs. Second, molecular simulations supported by activity measurements showed that EkAmy3 does not bind a Cl(-), owing to an Arg-to-Gln mutation in a conserved binding site. The Cl(-)-binding residues are in contact with the catalytic residues, and this change might help to fine-tune the catalytic pK(a) values to an alkaline pH optimum. We conclude that lepidopteran alpha-amylases are evolutionarily adapted in terms of structure and expression dynamics for effective functioning in the digestive system.