Peritrophic membrane structure and formation in the larva of a moth, Heliothis

Effect of fluorescent brighteners on the insecticidal activity of Bacillus thuringiensis var. kurstaki and LdMNPV on Lymantria dispar asiatica in Korea

Stilbene-based fluorescent brighteners (FBs) have been demonstrated to improve the insecticidal activities of entomopathogenic viruses; however, there is limited information regarding their effect on entomopathogenic bacteria. We conducted this study to investigate the effect of two FBs (FB 28 and FB 71) on the insecticidal activities of Bacillus thuringiensis var. kurstaki (Btk) and Lymantria dispar multiple nuclear polyhedrosis virus (LdMNPV) on Lymantria dispar asiatica. FB 28 and Btk combination at low concentration (1.6 × 102  IU/mL) increased the mortality, whereas FB 71 and Btk combination at intermediate and high concentrations (1.6 × 103 and 1.6 × 104  IU/mL) slightly reduced the mortality compared with that with Btk alone. The lethal time was also shorter with combinations of Btk and FB 28 than with FB 71. Both FB 28 and FB 71 increased the mortality in combination with LdMNPV at all concentrations (3 × 102 , 3 × 104 , and 3 × 106  polyhedral occlusion bodies/mL compared with that with LdMNPV alone. Our results suggest that FBs improve the insecticidal activities of Btk and LdMNPV, and their activities depend on their interactions with the midgut structures of the host insect species.

Impact of weathered multi-walled carbon nanotubes on the epithelial cells of the intestinal tract in the freshwater grazers Lymnaea stagnalis and Rhithrogena semicolorata

Freshwater grazers are suitable organisms to investigate the fate of environmental pollutants, such as weathered multi-walled carbon nanotubes (wMWCNTs). One key process is the uptake of ingested materials into digestive or absorptive cells. To address this, we investigated the localization of wMWCNTs in the intestinal tracts of the mud snail Lymnaea stagnalis (L. stagnalis) and the mayfly Rhithrogena semicolorata (R. semicolorata). In L. stagnalis, bundles of wMWCNTs could be detected in the midgut lumen, whereas only single wMWCNTs could be detected in the lumina of the digestive gland. Intracellular uptake of wMWCNTs was detected by transmission electron microscopy (TEM) but was restricted to the cells of the digestive gland. In larvae of R. semicolorata, irritations of the microvilli and damages in the apical parts of the epithelial gut cells were detected after feeding with 1 to 10 mg/L wMWCNTs. In both models, we detected fibrillar structures in close association with the epithelial cells that formed peritrophic membranes (PMs). The PM may cause a reduced transmission of wMWCNT bundles into the epithelium by forming a filter barrier and potentially protecting the cells from the wMWCNTs. As a result, the uptake of wMWCNTs into cells is rare in mud snails and may not occur at all in mayfly larvae. In addition, we monitor physiological markers such as levels of glycogen or triglycerides and the RNA/DNA ratio. This ratio was significantly affected in L. stagnalis after 24 days with 10 mg/L wMWCNTs, but not in R. semicolorata after 28 days and 10 mg/L wMWCNTs. However, significant effects on the energy status of R. semicolorata were analysed after 28 days of exposure to 1 mg/L wMWCNTs. Furthermore, we observed a significant reduction of phagosomes per enterocyte cell in mayfly larvae at a concentration of 10 mg/L wMWCNTs (p < 0.01).

Proteomics and ultrastructural analysis of Hermetia illucens (Diptera: Stratiomyidae) larval peritrophic matrix

BACKGROUND

The black soldier fly (Hermetia illucens) has significant economic potential. The larvae can be used in financially viable waste management systems, as they are voracious feeders able to efficiently convert low-quality waste into valuable biomass. However, most studies on H. illucens in recent decades have focused on optimizing their breeding and bioconversion conditions, while information on their biology is limited.

METHODS

About 200 fifth instar well-fed larvae were sacrificed in this work. The liquid chromatography-tandem mass spectrometry and scanning electron microscopy were employed in this study to perform a proteomic and ultrastructural analysis of the peritrophic matrix (PM) of H. illucens larvae.

RESULTS

A total of 565 proteins were identified in the PM samples of H. illucen, of which 177 proteins were predicted to contain signal peptides, bioinformatics analysis and manual curation determined 88 proteins may be associated with the PM, with functions in digestion, immunity, PM modulation, and others. The ultrastructure of the H. illucens larval PM observed by scanning electron microscopy shows a unique diamond-shaped chitin grid texture.

CONCLUSIONS

It is the first and most comprehensive proteomics research about the PM of H. illucens larvae to date. All the proteins identified in this work has been discussed in details, except several unnamed or uncharacterized proteins, which should not be ignored and need further study. A comparison of the ultrastructure between H. illucens larval PM and those of other insects as observed by SEM indicates that the PM displays diverse textures on an ultra-micro scale and we suscept a unique diamond-shaped chitin grid texture may help H. illucens larval to hold more food. This work deepens our understanding of the molecular architecture and ultrastructure of the H. illucens larval PM.

Toxicity and cytopathology mediated by Bacillus thuringiensis in the midgut of Anticarsia gemmatalis (Lepidoptera: Noctuidae)

Bioinsecticides and transgenic plants, based on Bacillus thuringiensis (Bt) toxins are important when managing Anticarsia gemmatalis Hübner (Lepidoptera: Noctuidae), a soybean defoliator pest. The interaction of these toxins with the caterpillar’s midgut cells determines their efficacy as an insecticide. The objective was to evaluate the toxicity of B. thuringiensis, subsp. kurstaki strain HD-1 and cytopathological changes mediated by these bacterial toxins in the midgut of A. gemmatalis caterpillars. Insecticidal efficacy was determined by calculating lethal concentration values (LC₂₅, LC₅₀, LC₇₅, LC₉₀ and LC₉₉) in the laboratory. Midgut fragments from A. gemmatalis were extracted after bacterial ingestion and evaluated by light, transmission electron and confocal microscopy. The Bt median lethal concentrations showed toxicity [LC₅₀ = 0.46 (0.43–0.49) mg mL⁻¹] to fourth instar A. gemmatalis caterpillars after 108 hours. Bt induces severe cytotoxicity to A. gemmatalis midgut epithelial cells with increasing exposure over time, causing cellular disorganization, microvillus degeneration, cell fragmentation and protrusion, peritrophic membrane rupture, and cell vacuolization. The cell nuclei presented condensed chromatin and an increase in lysosome numbers. Apoptosis occurred in the midgut cells of caterpillars exposed to Bt. A regenerative response in A. gemmatalis caterpillars was observed 8 hours after exposure to Bt, however this response was not continuous. Toxins produced by Bt are harmful to A. gemmatalis at median concentration with structural damage and death of the midgut epithelial cells of this insect.

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.

Influence of Ephestia kuehniella stage larvae on the potency of Bacillus thuringiensis Cry1Aa delta-endotoxin

The economically important crop pest Ephestia kuehniella was tested at two stages of larval development for susceptibility to Bacillus thuringiensis Cry1Aa toxin. Bioassays showed that toxicity decreased during the development of larvae stage. In fact, Cry1Aa toxins from BNS3-Cry^- (pHT-cry1Aa) showed low toxicity against the first-instar larvae (L1) with a LC₅₀ value of about 421.02μg/g of diet and was not toxic against the fifth-instar (L5), comparing to the BLB1 toxins used as positive control which represent a LC₅₀ value of about 56.96 and 84.21μg/g of diet against L1 and L5 instars larvae, respectively. Effects of Cry1Aa toxins were reflected in histopathological observations by the weak destruction of midgut epithelium, slight hypertrophy of epithelial cells, and minor alteration of brush border membrane (BBM) detected mainly in L1 larvae stage comparing to the more extensive damage caused by BLB1 toxins. Interestingly, in vitro proteolysis of Cry1Aa toxins was found to correlate with the difference of toxicity during larval stage development. In fact, the weak proteinase activity detected inside the L1 midgut has led to the persistence of the Cry1Aa active forms (65 and 58kDa) during prolonged incubations, causing the alterations described previously. Three subfamilies of aminopeptidase (APN) receptors were detected in both larvae instars with different intensities and molecular weights (150kDa and 55kDa for APN1, and 90kDa for APN2 and APN4). Remarkably, binding assay using Cry1Aa toxin seems to have no direct correlation with larval stages toxicity differences, since same putative receptors were detected. Understanding the reasons for the clear differences in the effectiveness of Cry1Aa toxins during larval development stages of E. kuehniella is very important for the design of future improvement insecticidal approaches and for the accomplishment of resistance prevention strategies.

Magnetic Resonance Imaging of Alimentary Tract Development in Manduca sexta

Non-invasive 3D magnetic resonance imaging techniques were used to investigate metamorphosis of the alimentary tract of Manduca sexta from the larval to the adult stage. The larval midgut contracts in volume immediately following cessation of feeding and then greatly enlarges during the late pharate pupal period. Magnetic resonance imaging revealed that the foregut and hindgut of the pharate pupa undergo ecdysis considerably earlier than the external exoskeleton. Expansion of air sacs in the early pupa and development of flight muscles several days later appear to orient the midgut into its adult position in the abdomen. The crop, an adult auxiliary storage organ, begins development as a dorsal outgrowth of the foregut. This coincides with a reported increase in pupal ecdysteroid titers. An outgrowth of the hindgut, the rectal sac, appears several days later and continues to expand until it nearly fills the dorsal half of the abdominal cavity. This development correlates with a second rise in pupal ecdysteroid titers. In the pharate pupa, the presence of paramagnetic species renders the silk glands hyperintense.

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.

Two essential peritrophic matrix proteins mediate matrix barrier functions in the insect midgut

The peritrophic matrix (PM) in the midgut of insects consists primarily of chitin and proteins and is thought to support digestion and provide protection from abrasive food particles and enteric pathogens. We examined the physiological roles of 11 putative peritrophic matrix protein (PMP) genes of the red flour beetle, Tribolium castaneum (TcPMPs). TcPMP genes are differentially expressed along the length of the midgut epithelium of feeding larvae. RNAi of individual PMP genes revealed no abnormal developmental phenotypes for 9 of the 11 TcPMPs. However, RNAi for two PMP genes, TcPMP3 and TcPMP5-B, resulted in depletion of the fat body, growth arrest, molting defects and mortality. In situ permeability assays after oral administration of different-sized FITC-dextran beads demonstrated that the exclusion size of the larval peritrophic matrix (PM) decreases progressively from >2 MDa to <4 kDa from the anterior to the most posterior regions of the midgut. In the median midguts of control larvae, 2 MDa dextrans were completely retained within the PM lumen, whereas after RNAi for TcPMP3 and TcPMP5-B, these dextrans penetrated the epithelium of the median midgut, indicating loss of structural integrity and barrier function of the larval PM. In contrast, RNAi for TcPMP5-B, but not RNAi for TcPMP3, resulted in breakdown of impermeability to 4 and 40 kDa dextrans in the PM of the posterior midgut. These results suggest that specific PMPs are involved in the regulation of PM permeability, and that a gradient of barrier function is essential for survival and fat body maintenance.

Book gill development in embryos and first and second instars of the horseshoe crab Limulus polyphemus L. (Chelicerata, Xiphosura)

The scanning electron microscope (SEM) was used to study the development of the opisthosomal appendages and book gills of the horseshoe crab, Limulus polyphemus. Later embryonic stages were examined as well as the first and second instars. The observations are compared with a much earlier light microscopic description of book gill development in the horseshoe crab and with book lung development in scorpion embryos and first and second instars in a recent study with SEM. After the third embryonic molt in the horseshoe crab, the opisthosomal appendages are of sufficient size so they could be fractured or dissected open so internal cells and other structures could be examined. The opisthosomal appendages and book gill lamellae of first and second instars were also opened. The observations support the earlier histological report that the gill lamellae are a hypodermal outgrowth from the posterior surface of the preceding branchial appendages. The genital operculum, branchial appendages and gill lamellae are very thin and consist of external cuticle, hypodermis and space holders. The latter help hold the cuticle walls in place so hemolymph can flow through the narrow channels. The space holders are formed from cell processes that extend into the lumen from the hypodermis just inside the external cuticle. In the recent SEM study in scorpion embryos and in some histological investigations in spider embryos, the book lung lamellae are formed by alignment of cells from an invaginated sac or mass of cells. This clearly differs from the mode of formation of gill lamellae as observed in this and earlier investigations. These reports of differences in embryology refine but do not preclude hypotheses about book gill/book lung homology since addition, deletion or modification of ancestral features often occur for the benefit of the embryos and larvae.

The larval alimentary canal of the Antarctic insect, Belgica antarctica

On the Antarctica continent the wingless midge, Belgica antarctica (Diptera, Chironomidae) occurs further south than any other insect. The digestive tract of the larval stage of Belgica that inhabits this extreme environment and feeds in detritus of penguin rookeries has been described for the first time. Ingested food passes through a foregut lumen and into a stomodeal valve representing an intussusception of the foregut into the midgut. A sharp discontinuity in microvillar length occurs at an interface separating relatively long microvilli of the stomodeal midgut region, the site where peritrophic membrane originates, from the midgut epithelium lying posterior to this stomodeal region. Although shapes of cells along the length of this non-stomodeal midgut epithelium are similar, the lengths of their microvilli increase over two orders of magnitude from anterior midgut to posterior midgut. Infoldings of the basal membranes also account for a greatly expanded interface between midgut cells and the hemocoel. The epithelial cells of the hindgut seem to be specialized for exchange of water with their environment, with the anterior two-thirds of the hindgut showing highly convoluted luminal membranes and the posterior third having a highly convoluted basal surface. The lumen of the middle third of the hindgut has a dense population of resident bacteria. Regenerative cells are scattered throughout the larval midgut epithelium. These presumably represent stem cells for the adult midgut, while a ring of cells, marked by a discontinuity in nuclear size at the midgut-hindgut interface, presumably represents stem cells for the adult hindgut.

The intestinal barrier in lepidopteran larvae: permeability of the peritrophic membrane and of the midgut epithelium to two biologically active peptides

Endogenous peptide regulators of insect physiology and development are presently being considered as potential biopesticides, but their efficacy by oral delivery cannot be easily anticipated because of the limited information on how the insect gut barrier handles these kind of molecules. We investigated, in Bombyx mori larvae, the permeability properties of the two components of the intestinal barrier, the peritrophic membrane (PM) and the midgut epithelium, separately isolated and perfused in conventional Ussing chambers. The PM discriminated compounds of different dimensions but was easily crossed by two small peptides recently proposed as bioinsecticides, the neuropeptide proctolin and Aedes aegypti Trypsin Modulating Oostatic Factor (Aea-TMOF), although their flux values indicated that the permeability was highly affected by their steric conformation. To date, there is very little functional data available on how peptides cross the insect intestinal epithelium, but it has been speculated that peptides could reach the haemocoel through the paracellular pathway. We characterized the permeability properties of this route to a number of organic molecules, showing that B. mori septate junction was highly selective to both the dimension and the charge of the permeant compound. Confocal images of whole-mount midguts incubated with rhodamine(rh)-proctolin or fluorescein isothiocyanate (FITC)-Aea-TMOF added to the mucosal side of the epithelium, revealed that rh-proctolin did not enter the cell and crossed the midgut only by the paracellular pathway, while FITC-Aea-TMOF did cross the cell apical membrane, permeating also through the transcellular route.

New insights into peritrophic matrix synthesis, architecture, and function

The peritrophic matrix (PM) is a chitin and glycoprotein layer that lines the invertebrate midgut. Although structurally different, it is functionally similar to the mucous secretions of the vertebrate digestive tract. The PM is a physical barrier, protecting the midgut epithelium from abrasive food particles, digestive enzymes, and pathogens infectious per os. It is also a biochemical barrier, sequestering and, in some cases, inactivating ingested toxins. Finally, the PM compartmentalizes digestive processes, allowing for efficient nutrient acquisition and reuse of hydrolytic enzymes. The PM consists of an organized lattice of chitin fibrils held together by chitin binding proteins. Glycans fill the interstitial spaces, creating a molecular sieve, the properties of which are dependent on the immediate ion content and pH. In this review, we have integrated recent structural and functional information to create a holistic model for the PM. We also show how this information may generate novel technologies for use in insect pest management.

Plant-mediated alteration of the peritrophic matrix and baculovirus infection in lepidopteran larvae

The peritrophic matrix (PM) lines the midgut of most insects, providing protection to the midgut epithelial cells while permitting passage of nutrients and water. Herein, we provide evidence that plant-mediated alteration of the PM contributes to the well-documented inhibition of fatal infection by Autographa californica multiple nucleopolyhedrovirus (AcMNPV) of Heliothis virescens F. larvae fed cotton foliage. We examined the impact of the PM on pathogenesis using a viral construct expressing a reporter gene (AcMNPV-hsp70/lacZ) orally inoculated into larvae with either intact PMs or PMs disrupted by Trichoplusia ni granulovirus occlusion bodies containing enhancin, known to degrade insect intestinal mucin. Larvae possessing disrupted PMs displayed infection foci (lacZ signaling) earlier than those with intact PMs. We then examined PMs from larvae fed artificial diet or plant foliage using electron microscopy; foliage-fed larvae had significantly thicker PMs than diet-fed larvae. Moreover, mean PM width was inversely related to both the proportion of larvae with lacZ signaling at 18h post-inoculation and the final percentage mortality from virus. Thus, feeding on foliage altered PM structure, and these foliage-mediated changes reduced baculoviral efficacy. These data indicate that the PM is an important factor determining the success of an ingested pathogen in foliage-fed lepidopteran larvae.

Plant cyclotides disrupt epithelial cells in the midgut of lepidopteran larvae

Several members of the Rubiaceae and Violaceae plant families produce a series of cyclotides or macrocyclic peptides of 28-37 aa with an embedded cystine knot. The cyclic peptide backbone together with the knotted and strongly braced structure confers exceptional chemical and biological stability that has attracted attention for potential pharmaceutical applications. Cyclotides display a diverse range of biological activities, such as uterotonic action, anti-HIV activity, and neurotensin antagonism. In plants, their primary role is probably protection from insect attack. Ingestion of the cyclotide kalata B1 severely retards the growth of larvae from the Lepidopteran species Helicoverpa armigera. We examined the gut of these larvae after consumption of kalata B1 by light, scanning, and transmission electron microscopy. We established that kalata B1 induces disruption of the microvilli, blebbing, swelling, and ultimately rupture of the cells of the gut epithelium. The histology of this response is similar to the response of H. armigera larvae to the Bacillus thuringiensis delta-endotoxin, which is widely used to control these insect pests of crops such as cotton.

Enhancement of the biological activity of nucleopolyhedrovirus through disruption of the peritrophic matrix of insect larvae by chlorfluazuron

The enhancement of Spodoptera litura (F.) nucleopolyhedrovirus (SlNPV) activity using the chitin synthesis inhibitor chlorfluazuron was investigated. When tested against fifth-instar S. litura larvae, chlorfluazuron produced synergistic effects at doses of 0.05 and 0.025 microg per insect, and additive effects at doses of 0.1 and 0.2 microg. Furthermore, the time required for SlNPV to kill larvae was significantly reduced by chlorfluazuron at all doses tested. The activity and killing speed of Autographa californica (Spey) nucleopolyhedrovirus (AcMNPV) against third-instar Spodoptera exigua (Hübner) larvae were similarly improved by chlorfluazuron at a dose of 0.05 microg per larva. Furthermore, the growth of S. exigua was significantly retarded by chlorfluazuron. Environmental scanning electron microscopy (ESEM) showed that the peritrophic matrices (PMs) of S. litura exposed to chlorfluazuron alone, or the combination treatment, were markedly disrupted. Obvious ruptures on the outer surfaces of the PM were observed, which potentially facilitated the passage of virions through the matrix.

Peritrophic membrane of the penaeid shrimp Sicyonia ingentis: structure, formation, and permeability

Peritrophic membranes (PTMs) are secreted acellular layers that separate ingested materials from the gut epithelium in a variety of invertebrates. In insects and crustaceans, PTMs are produced in the midgut trunk (MGT, or intestine), but the MGT in decapod crustaceans, unlike that of insects, is not involved with digestion or absorption of food. We demonstrate that the PTM in the penaeid shrimp Sicyonia ingentis is similar to that in other crustaceans that have been studied and is primarily composed of chitin. The lectin WGA binds only to the PTM and glycocalyx along the microvilli of the midgut cells, which is consistent with the suggestion that the chitin is synthesized along the microvilli. The PTM is only permeable to inert particles smaller than 20 nm. We also describe the secretion of granules, which fill the apices of the epithelial cells, into the ectoperitrophic space. Although their function is not clear, they do not contribute to the PTM.

Sequences of cDNAs and expression of genes encoding chitin synthase and chitinase in the midgut of Spodoptera frugiperda

The focus of this study was on the characterization and expression of genes encoding enzymes responsible for the synthesis and degradation of chitin, chitin synthase (SfCHSB) and chitinase (SfCHI), respectively, in the midgut of the fall armyworm, Spodoptera frugiperda. Sequences of cDNAs for SfCHSB and SfCHI were determined by amplification of overlapping PCR fragments and the expression patterns of these two genes were analyzed during insect development by RT-PCR. SfCHSB encodes a protein of 1523 amino acids containing several transmembrane segments, whereas SfCHI encodes a protein of 555 amino acids composed of a catalytic domain, a linker region and a chitin-binding domain. SfCHSB is expressed in the midgut during the feeding stages, whereas SfCHI is expressed during the wandering and pupal stages. Both genes are expressed along the whole midgut. Chitin staining revealed that this polysaccharide is present in the peritrophic membrane (PM) only when SfCHSB is expressed. There is little or no chitin in the midgut when SfCHI is expressed. These results support the hypothesis that SfCHSB is responsible for PM chitin synthesis during the larval feeding stages and SfCHI carries out PM chitin degradation during larval-pupal molting, suggesting mutually exclusive temporal patterns of expression of these genes.

AcMNPV ChiA protein disrupts the peritrophic membrane and alters midgut physiology of Bombyx mori larvae

Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) chitinase A (ChiA) is a protein which promotes the final liquefaction of infected host larvae. The potential of this viral molecule as a new tool for insect control is explored here. The ChiA gene was isolated from the AcMNPV genome by PCR and expressed in E. coli. The recombinant protein, purified by affinity chromatography, showed both exo- and endo-chitinase activities and produced perforations on the peritrophic membrane (PM) of Bombyx mori larvae which increased in number and in size, in a dose-dependent manner. This structural alteration resulted into a significant increase of PM permeability to methylene blue and to the small neuropeptide proctolin. When the fifth instar larvae of B. mori were fed on a artificial diet supplemented with the recombinant ChiA, 100% mortality was observed at a dose of 1 microg/g of larval body weight (LW), while at sub-lethal doses of 0.56 microg/g LW, a reduced larval growth was recorded. These results indicate that AcMNPV-ChiA may offer interesting new opportunities for pest control.

Roles of peritrophic membranes in protecting herbivorous insects from ingested plant allelochemicals

Four mechanisms by which peritrophic membranes (PMs) potentially protect herbivorous insects from ingested allelochemicals are reviewed: adsorption, ultrafiltration, polyanion exclusion, and the capacity of PMs to act as antioxidants. Most of the research on the protective roles of PMs against ingested allelochemicals has focused on their impermeability to tannins. Adsorption of tannins by the PMs in grasshoppers may limit their permeability, but ultrafiltration of tannin complexes in the caeca is an alternative explanation. Polyanion exclusion does not explain the impermeability of caterpillar PMs to tannins (polyphenolate anions). Ultrafiltration remains the most likely mechanism by which tannins, and other tested allelochemicals, are retained in the endoperitrophic space. Although the pores in PMs are too large to impede the passage of most free allelochemicals, large allelochemical complexes are retained. Such complexes form in the gut fluid of caterpillars between tannic acid, proteins, lipids, and polyvalent metal cations, and also in the gut fluid of grasshoppers (Melanoplus sanguinipes) between some amphiphilic allelochemicals (digitoxin) and surfactant micelles. Further work is needed to examine the role of PMs as antioxidants in vivo, such as their potential to bind catalytically-active metal ions.

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

A peritrophin from the Spodoptera frugiperda peritrophic membrane (PM) and microvillar proteins from S. frugiperda anterior midgut cells were isolated and used to raise antibodies in a rabbit. These antibodies, as well as a Tenebrio molitor amylase antibody that cross-reacts with S. frugiperda amylases, and wheat-germ aglutinin were used in immunolocalization experiments performed with the aid of confocal fluorescence and immunogold techniques. The results showed that the peritrophin was secreted by anterior midgut columnar cells in vesicles pinched-off the microvilli (microapocrine secretion). The resulting double membrane vesicles become single membrane vesicles by membrane fusion, releasing peritrophin and part of the amylase and trypsin. The remaining membranes still containing microvillar proteins and membrane-bound amylase and trypsin are incorporated into a jelly-like material associated with PM. Calcofluor-treated larvae lacking a PM were shown to lose the decreasing gradient of trypsin and chymotrypsin observed along the midgut of control larvae. This gradient is thought to be formed by a countercurrent flux of fluid (in the space between PM and midgut cells) that powers enzyme recycling.

Lepidopteran peritrophic membranes and effects of dietary wheat germ agglutinin on their formation and structure

Peritrophic membrane (PM) structure and the effects of dietary wheat germ agglutinin (WGA) on PM formation were studied in larvae of the European corn borer (ECB), Ostrinia nubilalis, and the tobacco hornworm (THW), Manduca sexta. Growth of ECB was strongly inhibited by low amounts of WGA in the diet (0.05%), whereas THW was not affected by amounts of up to 2%. In ECB larvae, chitin microfibrils were secreted to form an orthogonal network within the apical region of the anterior midgut microvilli. The network then moved to the tips of the microvilli where proteinacious matrix was added prior to delamination of a single PM into the lumen to enclose the food bolus. Multiple PMs rapidly appeared as the food moved posteriorly and some of these became greatly thickened in the middle and posterior regions of the midgut. WGA in the diet caused hypersecretion of unorganized PM in the anterior midgut lumen, disintegration of microvilli, and cessation of feeding. It was also shown to bind to both the chitinous network and to several PM proteins, perhaps causing voids in the PM and sparse matrix material. This allowed the passage of food particles through a defective PM into the ectoperitrophic space and penetration into the microvillar brush border. Stimulation of PM secretion and cessation of feeding may have been a response to damage to the brush border. Unlike ECB, the chitinous network of THW is a randomly organized felt-like structure embedded in a proteinaceous matrix. This PM is secreted as a thin multilayered structure in the anterior region of the midgut, but multiple and thickened PMs occur in the middle and posterior lumens of the midgut. THW tolerated high amounts of WGA in its diet with no disruption of PM formation or inhibition of growth. WGA did accumulate as large masses embedded in the PM, but caused no voids that would allow the penetration of food particles and subsequent damage to the brush border. Therefore, differences in PM formation and structure between ECB and THW appeared to affect how WGA interacts with chitinous and proteinaceous components of the PM and subsequent effects on larval feeding and growth.

Chitin is only a minor component of the peritrophic matrix from larvae of Lucilia cuprina

The gut of most insects is lined with a peritrophic matrix that facilitates the digestive process and protects insects from invasion by micro-organisms and parasites. It is widely accepted that the matrix is composed of chitin, proteins and proteoglycans. Here we critically re-examine the chitin content of the typical type 2 peritrophic matrix from the larvae of the fly Lucilia cuprina using a range of techniques. Many of the histochemical and biochemical techniques indicate the presence of chitin, although they are often adversely influenced by the presence of highly glycosylated proteins, a principal component of the matrix. The alkali-stable fraction, which is used as an indicator of the maximum chitin content in a biological sample, is only 7.2% of the weight of the matrix. Larvae fed on the potent chitin synthase inhibitor polyoxin D or the chitin-binding agent Calcofluor White, showed strong concentration-dependent inhibition of larval weight and survival but no discernible effects on the matrix structure. A bacterial endochitinase fed to larvae had no effect on larval growth and no observable effect in vitro on the structure of isolated peritrophic matrix. RT-PCR did not detect a chitin synthase mRNA in cardia, the tissue from which PM originates. It is concluded that chitin is a minor structural component of the type 2 peritrophic matrix of this insect.

Peritrophic matrix proteins

The peritrophic matrix (or peritrophic membrane) lines the gut of most insects at one or more stages of the life cycle. It has important roles in the facilitation of the digestive processes in the gut and the protection of the insect from invasion by microorganisms and parasites. The traditional view of the peritrophic matrix as a relatively insert sieve, composed largely of proteins and glycosaminoglycans embedded in a chitinous matrix, is under revision as more is learned about the molecular characteristics of the peritrophic matrix proteins. This review summarizes emerging knowledge of the main protein constituents of the peritrophic matrix. The availability of the first sequences of integral peritrophic matrix proteins has coincided with the explosion of information in sequence databases. It is therefore possible to examine common structural themes in this family of proteins as well as in proteins of unknown location and function from a variety of other insects, nematodes and viruses. The review concludes with speculation about the biological functions of the proteins in this matrix.

Insect resistance of transgenic tobacco expressing an insect chitinase gene

Chitinase expression in the insect gut normally occurs only during moulting, where the chitin of the peritrophic membrane is presumably degraded. Thus, insects feeding on plants that constitutively express an insect chitinase gene might be adversely affected, owing to an inappropriately timed exposure to chitinase. This hypothesis was tested by introducing a cDNA encoding a tobacco hornworm (Manduca sexta) chitinase (EC 3.2.1.14) into tobacco via Agrobacterium tumefaciens-mediated transformation. A truncated but enzymatically active chitinase was present in plants expressing the gene. Segregating progeny of high-expressing plants were compared for their ability to support growth of tobacco budworm (Heliothis virescens) larvae and for feeding damage. Both parameters were significantly reduced when budworms fed on transgenic tobacco plants expressing high levels of the chitinase gene. In contrast, hornworm larvae showed no significant growth reduction when fed on the chitinase-expressing transgenics. However, both budworm and hornworm larvae, when fed on chitinase-expressing transgenic plants coated with sublethal concentrations of a Bacillus thuringiensis toxin, were significantly stunted relative to larvae fed on toxin-treated non-transgenic controls. Foliar damage was also reduced. Plants expressing an insect chitinase gene may have agronomic potential for insect control.

Nucleopolyhedrovirus interactions with their insect hosts

It is clear from this brief review that our understanding of the molecular cross-talk between insects and their baculovirus pathogens is still very limited. Studies in cell culture have taught us a great deal about the basic baculovirus molecular machinery and how it is regulated, and in many cases this information has been predictive of what occurs in infected insects. Frequently, however, studies in cell culture do not adequately predict the infection process in insect hosts, as demonstrated by viral mutants (some of which were discussed in this review) that behave identically to wild-type virus in cell culture but differ markedly in larvae. More baculovirus studies, therefore, need to be conducted in vivo if we are to improve our understanding of the complex interactions between baculoviruses and their hosts. Conducting baculovirus studies in insects (or at least in primary cell culture) also offers the opportunity to address questions that reach beyond the baculovirus community in significance. For example, almost all of our knowledge of viral fusion mechanisms comes from infection of cells in culture where the pH is neutral or acidic and the temperature is constant at 27 degrees or 37 degrees C. An answer to the question of how the ODV envelope fuses with the microvillar membrane of columnar epithelial cells in the highly alkaline midgut environment at low temperatures will not only be important for an improved understanding of baculovirus infection in the natural world, but will also constitute a new chapter on viral entry mechanisms. Similarly, the answer to the question of how baculovirus nucleocapsids move basally within microvilli promises to involve factors and/or a mechanism not yet described by cell biologists, and so will constitute a valuable contribution to both baculovirology and cell biology. There are many more such examples of biological mechanisms that can be uniquely explored within the context of baculoviruses and their insect hosts, some of which have been highlighted in this review. As more and more young investigators realize the importance of combining a knowledge of virology, molecular technology, and insect biology, however, many of the outstanding mysteries will be solved.

Peritrophic matrix structure and function

Formed of proteins, glycoproteins, and chitin microfibrils in a proteoglycan matrix, the peritrophic matrix (PM) separates the food from the midgut epithelium in most but not all insects. A PM occurs in two forms. A type I PM is delaminated from the entire midgut epithelium and, in some cases, may only be formed in response to feeding and the type of meal ingested. A type II PM is produced by a specialized region of the anterior midgut called the cardia and forms a continuous sleeve (or sleeves) that is always present. As it is positioned between food and midgut epithelium, the PM plays key roles in the intestinal biology of the insect. The PM may protect the midgut epithelium from mechanical damage and insult from pathogens and toxins; it must act as a semipermeable membrane regulating passage of molecules between the different midgut compartments; and it may separate the midgut lumen into different, physiologically significant compartments.