A baculovirus enhancin alters the permeability of a mucosal midgut peritrophic matrix from lepidopteran larvae

Peritrophins are involved in the defense against Bacillus thuringiensis and nucleopolyhedrovirus formulations in Spodoptera littoralis (Lepidoptera: Noctuidae)

The peritrophic matrix (or peritrophic membrane, PM) is present in most insects where it acts as a barrier to mechanical insults and pathogens, as well as a facilitator of digestive processes. The PM is formed by the binding of structural PM proteins, referred to as peritrophins, to chitin fibrils and spans the entire midgut in lepidopterans. To investigate the role of peritrophins in a highly polyphagous lepidopteran pest, namely the cotton leafworm (Spodoptera littoralis), we generated Insect Intestinal Mucin (IIM-) and non-mucin Peritrophin (PER-) mutant strains via CRISPR/Cas9 mutagenesis. Both strains exhibited deformed PMs and retarded developmental rates. Bioassays conducted with Bacillus thuringiensis (Bt) and nucleopolyhedrovirus (SpliNPV) formulations showed that both the IIM- and PER- mutant larvae were more susceptible to these bioinsecticides compared to the wild-type (WT) larvae with intact PM. Interestingly, the provision of chitin-binding agent Calcofluor (CF) in the diet lowered the toxicity of Bt formulations in both WT and IIM- larvae and the protective effect of CF was significantly lower in PER- larvae. This suggested that the interaction of CF with PER is responsible for Bt resistance mediated by CF. In contrast, the provision of CF caused increased susceptibility to SpliNPV in both mutants and WT larvae. The study showed the importance of peritrophins in the defense against pathogens in S. littoralis and revealed novel insights into CF-mediated resistance to Cry toxin.

Insights into midgut cell types and their crucial role in antiviral immunity in the lepidopteran model Bombyx mori

The midgut, a vital component of the digestive system in arthropods, serves as an interface between ingested food and the insect's physiology, playing a pivotal role in nutrient absorption and immune defense mechanisms. Distinct cell types, including columnar, enteroendocrine, goblet and regenerative cells, comprise the midgut in insects and contribute to its robust immune response. Enterocytes/columnar cells, the primary absorptive cells, facilitate the immune response through enzyme secretions, while regenerative cells play a crucial role in maintaining midgut integrity by continuously replenishing damaged cells and maintaining the continuity of the immune defense. The peritrophic membrane is vital to the insect's innate immunity, shielding the midgut from pathogens and abrasive food particles. Midgut juice, a mixture of digestive enzymes and antimicrobial factors, further contributes to the insect's immune defense, helping the insect to combat invading pathogens and regulate the midgut microbial community. The cutting-edge single-cell transcriptomics also unveiled previously unrecognized subpopulations within the insect midgut cells and elucidated the striking similarities between the gastrointestinal tracts of insects and higher mammals. Understanding the intricate interplay between midgut cell types provides valuable insights into insect immunity. This review provides a solid foundation for unraveling the complex roles of the midgut, not only in digestion but also in immunity. Moreover, this review will discuss the novel immune strategies led by the midgut employed by insects to combat invading pathogens, ultimately contributing to the broader understanding of insect physiology and defense mechanisms.

The effect of group IV chitinase, HaCHT4, on the chitin content of the peritrophic matrix (PM) during larval growth and development of Helicoverpa armigera

BACKGROUND

Extensive research has been conducted on insect chitinases. However, little is known about the function of chitinase in the regulation of the surface structure of the peritrophic matrix (PM) in larval midguts. The aim of this study was to analyze the effect of HaCHT4 on the chitin content and surface structure of the PM during larval growth and development of Helicoverpa armigera.

RESULTS

The expression level of HaCHT4 was lower and the chitin content was higher in the early stages of fourth to sixth instar larvae, but they were reversed in the corresponding late stages. The correlation coefficient between the expression level of HaCHT4 and the chitin content was -0.585 (P < 0.05), with a higher negative correlation of -0.934 for the fourth instar (P < 0.01). Scanning electron microscopy (SEM) showed that the surface structure of PM was multi-laminated with small pores in the early stages of fourth to sixth instar larvae, and more and bigger pores in the late stages. Low expression of HaCHT4 caused by RNA interference (RNAi) resulted in the increase of chitin content in the PM, and the surface structure of PM became multilayered with smaller pore size in the late stage of fourth instar larvae. Also, induction of HaCHT4 by application of 2-tridecanone (2-TD), decreased the chitin content of PM, caused larger pores to form and lots of food bolus to attach to the PM surface, and also increased the larval susceptibility to chlorantraniliprole.

CONCLUSION

These results provided strong evidence that HaCHT4 plays an important role by regulating the chitin content of the PM and its surface structure, thereby affecting the sensitivity of H. armigera to chlorantraniliprole.

Bacmid Expression of Granulovirus Enhancin En3 Accumulates in Cell Soluble Fraction to Potentiate Nucleopolyhedrovirus Infection

Enhancins are metalloproteinases that facilitate baculovirus infection in the insect midgut. They are more prevalent in granuloviruses (GVs), constituting up to 5% of the proteins of viral occlusion bodies (OBs). In nucleopolyhedroviruses (NPVs), in contrast, they are present in the envelope of the occlusion-derived virions (ODV). In the present study, we constructed a recombinant Autographa californica NPV (AcMNPV) that expressed the Trichoplusia ni GV (TnGV) enhancin 3 (En3), with the aim of increasing the presence of enhancin in the OBs or ODVs. En3 was successfully produced but did not localize to the OBs or the ODVs and accumulated in the soluble fraction of infected cells. As a result, increased OB pathogenicity was observed when OBs were administered in mixtures with the soluble fraction of infected cells. The mixture of OBs and the soluble fraction of Sf9 cells infected with BacPhEn3 recombinant virus was ~3- and ~4.7-fold more pathogenic than BacPh control OBs in the second and fourth instars of Spodoptera exigua, respectively. In contrast, when purified, recombinant BacPhEn3 OBs were as pathogenic as control BacPh OBs. The expression of En3 in the soluble fraction of insect cells may find applications in the development of virus-based insecticides with increased efficacy.

Interaction of Viruses with the Insect Intestine

In nature, insects face a constant threat of infection by numerous exogeneous viruses, and their intestinal tracts are the predominant ports of entry. Insects can acquire these viruses orally during either blood feeding by hematophagous insects or sap sucking and foliage feeding by insect herbivores. However, the insect intestinal tract forms several physical and immunological barriers to defend against viral invasion, including cell intrinsic antiviral immunity, the peritrophic matrix and the mucin layer, and local symbiotic microorganisms. Whether an infection can be successfully established in the intestinal tract depends on the complex interactions between viruses and those barriers. In this review, we summarize recent progress on virus-intestinal tract interplay in insects, in which various underlying mechanisms derived from nutritional status, dynamics of symbiotic microorganisms, and virus-encoded components play intricate roles in the regulation of virus invasion in the intestinal tract, either directly or indirectly. Expected final online publication date for the Annual Review of Virology, Volume 8 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

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.

Complex Relationships at the Intersection of Insect Gut Microbiomes and Plant Defenses

Insect herbivores have ubiquitous associations with microorganisms that have major effects on how host insects may interact in their environment. Recently, increased attention has been given to how insect gut microbiomes mediate interactions with plants. In this paper, I discuss the ecology and physiology of gut bacteria associated with insect herbivores and how they may shape interactions between insects and their various host plants. I first establish how microbial associations vary between insects with different feeding styles, and how the insect host physiology and ecology can shape stable or transient relationships with gut bacteria. Then, I describe how these relationships factor in with plant nutrition and plant defenses. Within this framework, I suggest that many of the interactions between plants, insects, and the gut microbiome are context-dependent and shaped by the type of defense and the isolates present in the environment. Relationships between insects and plants are not pairwise, but instead highly multipartite, and the interweaving of complex microbial interactions is needed to fully explore the context-dependent aspects of the gut microbiome in many of these systems. I conclude the review by suggesting studies that would help reduce the unsureness of microbial interactions with less-defined herbivore systems and identify how each could provide a path to more robust roles and traits.

The peritrophic membrane as a target of proteins that play important roles in plant defense and microbial attack

The peritrophic membrane (or peritrophic matrix: PM) is a thin membranous structure that lies along the midgut epithelium in the midgut lumen and consists of chitin and proteins. PM exists between ingested food material and midgut epithelium cells and it is on the frontline of insect-plant and insect-microbe interactions. Therefore, proteins that play major roles in plant defense against herbivorous insects and in microbial attack on insects should penetrate, destroy or modify the PM to accomplish their roles. Recently, it has become clear that some proteins crucial to plant defense or microbial attack have the PM as their primary target. In addition, several plant defense proteins have been reported to affect the PM, although it is still unclear whether the PM is their primary target. This review introduces several of these proteins: fusolin and enhancin, two proteins produced by insect viruses that greatly enhance infection of the viruses by disrupting the PM; the MLX56 family proteins found in mulberry latex as defense proteins against insect herbivores, which modify the PM to a thick structure that inhibits digestive processes; Mir1-CP, a defense cysteine protease from maize that inhibits the growth of insects at very low concentrations and degrades the PM structures; and chitinases and lectins. The importance, necessary characteristics, and modes of action of PM-targeting proteins are then discussed from a strategic point of view, by spotlighting the importance of selective permeability of the PM. Finally, the review discusses the possibility of applying PM-targeting proteins for the control of pest insects.

Role of the peritrophic matrix in insect-pathogen interactions

The peritrophic matrix (PM) is an acellular chitin and glycoprotein layer that lines the invertebrate midgut. The PM has long been considered a physical as well as a biochemical barrier, protecting the midgut epithelium from abrasive food particles, digestive enzymes and pathogens infectious per os. This short review will focus on the latter function, as a barrier to pathogens infectious per os. We focus on the evidence confirming the role of the PM as protective barrier against pathogenic microorganisms of insects, mainly bacteria and viruses, as well as the evolution of a variety of mechanisms used by pathogens to overcome the PM barrier.

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.

A Gut Commensal Bacterium Promotes Mosquito Permissiveness to Arboviruses

Mosquitoes are hematophagous vectors that can acquire human viruses in their intestinal tract. Here, we define a mosquito gut commensal bacterium that promotes permissiveness to arboviruses. Antibiotic depletion of gut bacteria impaired arboviral infection of a lab-adapted Aedes aegypti mosquito strain. Reconstitution of individual cultivable gut bacteria in antibiotic-treated mosquitoes identified Serratia marcescens as a commensal bacterium critical for efficient arboviral acquisition. S. marcescens facilitates arboviral infection through a secreted protein named SmEnhancin, which digests membrane-bound mucins on the mosquito gut epithelia, thereby enhancing viral dissemination. Field Aedes mosquitoes positive for S. marcescens were more permissive to dengue virus infection than those free of S. marcescens. Oral introduction of S. marcescens into field mosquitoes that lack this bacterium rendered these mosquitoes highly susceptible to arboviruses. This study defines a commensal-driven mechanism that contributes to vector competence, and extends our understanding of multipartite interactions among hosts, the gut microbiome, and viruses.

Comparative Genomics and Description of Putative Virulence Factors of Melissococcus plutonius, the Causative Agent of European Foulbrood Disease in Honey Bees

In Europe, approximately 84% of cultivated crop species depend on insect pollinators, mainly bees. Apis mellifera (the Western honey bee) is the most important commercial pollinator worldwide. The Gram-positive bacterium Melissococcus plutonius is the causative agent of European foulbrood (EFB), a global honey bee brood disease. In order to detect putative virulence factors, we sequenced and analyzed the genomes of 14 M. plutonius strains, including two reference isolates. The isolates do not show a high diversity in genome size or number of predicted protein-encoding genes, ranging from 2.021 to 2.101 Mbp and 1589 to 1686, respectively. Comparative genomics detected genes that might play a role in EFB pathogenesis and ultimately in the death of the honey bee larvae. These include bacteriocins, bacteria cell surface- and host cell adhesion-associated proteins, an enterococcal polysaccharide antigen, an epsilon toxin, proteolytic enzymes, and capsule-associated proteins. In vivo expression of three putative virulence factors (endo-alpha-N-acetylgalactosaminidase, enhancin and epsilon toxin) was verified using naturally infected larvae. With our strain collection, we show for the first time that genomic differences exist between non-virulent and virulent typical strains, as well as a highly virulent atypical strain, that may contribute to the virulence of M. plutonius. Finally, we also detected a high number of conserved pseudogenes (75 to 156) per genome, which indicates genomic reduction during evolutionary host adaptation.

Variable Membrane Protein A of Flavescence Dorée Phytoplasma Binds the Midgut Perimicrovillar Membrane of Euscelidius variegatus and Promotes Adhesion to Its Epithelial Cells

Phytoplasmas are uncultivated plant pathogens and cell wall-less bacteria and are transmitted from plant to plant by hemipteran insects. The phytoplasma's circulative propagative cycle in insects requires the crossing of the midgut and salivary glands, and primary adhesion to cells is an initial step toward the invasion process. The flavescence dorée (FD) phytoplasma possesses a set of variable membrane proteins (Vmps) exposed on its surface, and this pathogen is suspected to interact with insect cells. The results showed that VmpA is expressed by the flavescence dorée phytoplasma present in the midgut and salivary glands. Phytoplasmas cannot be cultivated at present, and no mutant can be produced to investigate the putative role of Vmps in the adhesion of phytoplasma to insect cells. To overcome this difficulty, we engineered the Spiroplasma citri mutant G/6, which lacks the ScARP adhesins, for VmpA expression and used VmpA-coated fluorescent beads to determine if VmpA acts as an adhesin in ex vivo adhesion assays and in vivo ingestion assays. VmpA specifically interacted with Euscelidius variegatus insect cells in culture and promoted the retention of VmpA-coated beads to the midgut of E. variegatus In this latest case, VmpA-coated fluorescent beads were localized and embedded in the perimicrovillar membrane of the insect midgut. Thus, VmpA functions as an adhesin that could be essential in the colonization of the insect by the FD phytoplasmas.IMPORTANCE Phytoplasmas infect a wide variety of plants, ranging from wild plants to cultivated species, and are transmitted by different leafhoppers, planthoppers, and psyllids. The specificity of the phytoplasma-insect vector interaction has a major impact on the phytoplasma plant host range. As entry into insect cells is an obligate process for phytoplasma transmission, the bacterial adhesion to insect cells is a key step. Thus, studying surface-exposed proteins of phytoplasma will help to identify the adhesins implicated in the specific recognition of insect vectors. In this study, it is shown that the membrane protein VmpA of the flavescence dorée (FD) phytoplasma acts as an adhesin that is able to interact with cells of Euscelidius variegatus, the experimental vector of the FD phytoplasma.

A Novel Betabaculovirus Isolated from the Monocot Pest Mocis latipes (Lepidoptera: Noctuidae) and the Evolution of Multiple-Copy Genes

In this report, we described the genome of a novel baculovirus isolated from the monocot insect pest Mocis latipes, the striped grass looper. The genome has 134,272 bp in length with a G + C content of 38.3%. Based on the concatenated sequence of the 38 baculovirus core genes, we found that the virus is a betabaculovirus closely related to the noctuid-infecting betabaculoviruses including Pseudaletia unipuncta granulovirus (PsunGV), Trichoplusia ni granulovirus (TnGV), Helicoverpa armigera granulovirus (HearGV), and Xestia c-nigrum granulovirus (XecnGV). The virus may constitute a new Betabaculovirus species tentatively named Mocis latipes granulovirus (MolaGV). After gene content analysis, five open reading frames (ORFs) were found to be unique to MolaGV and several auxiliary genes were found including iap-3, iap-5, bro-a, bro-b, and three enhancins. The virus genome lacked both chitinase and cathepsin. We then looked at the evolutionary history of the enhancin gene and found that betabaculovirus acquired this gene from an alphabaculovirus followed by several duplication events. Gene duplication also happened to an endonuclease-like gene. Genomic and gene content analyses revealed both a strict collinearity and gene expansion into the genome of the MolaGV-related species. We also characterized the granulin gene using a recombinant Autographa californica multiple nucleopolyhedrovirus (AcMNPV) and found that occlusion bodies were produced into the nucleus of infected cells and presented a polyhedral shape and no occluded virions within. Overall, betabaculovirus genome sequencing is of importance to the field as few genomes are publicly accessible. Mocislatipes is a secondary pest of maize, rice, and wheat crops in Brazil. Certainly, both the discovery and description of novel baculoviruses may lead to development of greener and safer pesticides in order to counteract and effectively control crop damage-causing insect populations

Genomic analysis of a Trichoplusia ni Betabaculovirus (TnGV) with three different viral enhancing factors and two unique genes

The complete genome of a Trichoplusia ni granulovirus (TnGV) is described and analyzed. The genome contains 175,360 bp (KU752557), becoming the third largest genome within the genus Betabaculovirus, smaller only than the Xestia c-nigrum GV (XecnGV) (178,733 pb) and the Pseudaletia unipuncta GV (PsunGV) (176,677 pb) genomes. The TnGV genome has a 39.81% C+G content and a total of 180 ORFs were identified, 96 of them in the granulin gene direction and 84 in the opposite direction. A total of 94.38% of the ORFs showed high identity with those of ClanGV, HaGV, and SlGV. Eight homologous regions (hrs) were identified as well as one apoptosis inhibitor (IAP-3). Interestingly, three viral enhancing factors (VEFs) were located in TnGV genome: VEF-1 (orf153), VEF-3 (orf155), and VEF-4 (orf164), additional to another metalloprotease (orf37). Two ORFs were unique to TnGV (orf100 and orf101) and another one was shared by only TnGV and AgseGV (orf2). Eleven of the deduced proteins showed high identity with proteins from nucleopolyhedroviruses, three with proteins from ascoviruses, and one with an entomopoxvirus protein. The largest deduced protein contains 1,213 amino acids (orf43) and the smallest deduced protein contains only 50 amino acids (orf143). Sequence identity and phylogenetic analyses showed that the closest related genomes to TnGV are, to date, those of PsunGV and XecnGV. This genome analysis may contribute to functional research on TnGV, and may form the bases for the utilization of this betabaculovirus as a pest control agent.

The Complete Genome Sequence of a Second Distinct Betabaculovirus from the True Armyworm, Mythimna unipuncta

The betabaculovirus originally called Pseudaletia (Mythimna) sp. granulovirus #8 (MyspGV#8) was examined by electron microscopy, host barcoding PCR, and determination of the nucleotide sequence of its genome. Scanning and transmission electron microscopy revealed that the occlusion bodies of MyspGV#8 possessed the characteristic size range and morphology of betabaculovirus granules. Barcoding PCR using cytochrome oxidase I primers with DNA from the MyspGV#8 collection sample confirmed that it had been isolated from the true armyworm, Mythimna unipuncta (Lepidoptera: Noctuidae) and therefore was renamed MyunGV#8. The MyunGV#8 genome was found to be 144,673 bp in size with a nucleotide distribution of 49.9% G+C, which was significantly smaller and more GC-rich than the genome of Pseudaletia unipuncta granulovirus H (PsunGV-H), another M. unipuncta betabaculovirus. A phylogeny based on concatenated baculovirus core gene amino acid sequence alignments placed MyunGV#8 in clade a of genus Betabaculovirus. Kimura-2-parameter nucleotide distances suggested that MyunGV#8 represents a virus species different and distinct from other species of Betabaculovirus. Among the 153 ORFs annotated in the MyunGV#8 genome, four ORFs appeared to have been obtained from or donated to the alphabaculovirus lineage represented by Leucania separata nucleopolyhedrovirus AH1 (LeseNPV-AH1) during co-infection of Mythimna sp. larvae. A set of 33 ORFs was identified that appears only in other clade a betabaculovirus isolates. This clade a-specific set includes an ORF that encodes a polypeptide sequence containing a CIDE_N domain, which is found in caspase-activated DNAse/DNA fragmentation factor (CAD/DFF) proteins. CAD/DFF proteins are involved in digesting DNA during apoptosis.

Silencing gut genes associated with the peritrophic matrix of Reticulitermes flavipes (Blattodea: Rhinotermitidae) increases susceptibility to termiticides

The peritrophic matrix (PM) is a noncellular structure that lines the gut of most insects. Because of its close involvement in digestive processes and its role as a barrier against pathogens and toxins, the PM is an attractive target for pest management strategies. The objectives of this study were to (1) reduce the expression of a chitin synthase gene (Reticulitermes flavipes chitin synthase B, RfCHSB), a putative peritrophin [R. flavipes Protein with Peritrophin-A domain 1, (RfPPAD1)] and a confirmed peritrophin [R. flavipes Protein with Peritrophin-A domain 2 (RfPPAD2)] in R. flavipes by means of RNA interference, and (2) to evaluate the susceptibility of R. flavipes to termiticides and a bacterial pathogen, after silencing the target genes. Force feeding termites with 55 and 100 ng of long double-stranded RNAs (dsRNAs), targeting RfCHSB and RfPPAD2, respectively, resulted in the highest levels of transcript suppression. RfCHSB expression was reduced by 70%, whereas the transcript level of RfPPAD2 was decreased by 90%. Force feeding 100 ng/termite of a long RfPPAD1 dsRNA reduced the expression of the transcript by 30%. Challenging termites with imidacloprid, chlorantraniliprole and noviflumuron, after silencing RfCHSB, significantly increased termite mortality. Force feeding termites a dsRNA cocktail, targeting RfCHSB, RfPPAD1 and RfPPAD2, caused the highest significant increase in termite mortality after challenging the insects with imidacloprid. These results demonstrate the viability of the R. flavipes PM as a target in termite pest management.

Gut immunity in Lepidopteran insects

Lepidopteran insects constitute one of the largest fractions of animals on earth, but are considered pests in their relationship with man. Key to the success of this order of insects is its ability to digest food and absorb nutrition, which takes place in the midgut. Because environmental microorganisms can easily enter Lepidopteran guts during feeding, the innate immune response guards against pathogenic bacteria, virus and microsporidia that can be devoured with food. Gut immune responses are complicated by both resident gut microbiota and the surrounding peritrophic membrane and are distinct from immune responses in the body cavity, which depend on the function of the fat body and hemocytes. Due to their relevance to agricultural production, studies of Lepidopteran insect midgut and immunity are receiving more attention, and here we summarize gut structures and functions, and discuss how these confer immunity against different microorganisms. It is expected that increased knowledge of Lepidopteran gut immunity may be utilized for pest biological control in the future.

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.

The gene expression profile of resistant and susceptible Bombyx mori strains reveals cypovirus-associated variations in host gene transcript levels

High-throughput paired-end RNA sequencing (RNA-Seq) was performed to investigate the gene expression profile of a susceptible Bombyx mori strain, Lan5, and a resistant B. mori strain, Ou17, which were both orally infected with B. mori cypovirus (BmCPV) in the midgut. There were 330 and 218 up-regulated genes, while there were 147 and 260 down-regulated genes in the Lan5 and Ou17 strains, respectively. Gene ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment for differentially expressed genes (DEGs) were carried out. Moreover, gene interaction network (STRING) analyses were performed to analyze the relationships among the shared DEGs. Some of these genes were related and formed a large network, in which the genes for B. mori cuticular protein RR-2 motif 123 (BmCPR123) and the gene for B. mori DNA replication licensing factor Mcm2-like (BmMCM2) were key genes among the common up-regulated DEGs, whereas the gene for B. mori heat shock protein 20.1 (Bmhsp20.1) was the central gene among the shared down-regulated DEGs between Lan5 vs Lan5-CPV and Ou17 vs Ou17-CPV. These findings established a comprehensive database of genes that are differentially expressed in response to BmCPV infection between silkworm strains that differed in resistance to BmCPV and implied that these DEGs might be involved in B. mori immune responses against BmCPV infection.

Reaching the melting point: Degradative enzymes and protease inhibitors involved in baculovirus infection and dissemination

Baculovirus infection of a host insect involves several steps, beginning with initiation of virus infection in the midgut, followed by dissemination of infection from the midgut to other tissues in the insect, and finally culminating in "melting" or liquefaction of the host, which allows for horizontal spread of infection to other insects. While all of the viral gene products are involved in ultimately reaching this dramatic infection endpoint, this review focuses on two particular types of baculovirus-encoded proteins: degradative enzymes and protease inhibitors. Neither of these types of proteins is commonly found in other virus families, but they both play important roles in baculovirus infection. The types of degradative enzymes and protease inhibitors encoded by baculoviruses are discussed, as are the roles of these proteins in the infection process.

The biochemistry behind biopesticide efficacy

Biopesticides have the potential to play an important role in sustainable, environmentally safe pest control. A diverse range of biopesticides employ chemical modes of action. This review explores three such biopesticides: a fungus used in weed control, beneficial bacteria controlling fungal and bacterial disease and a virus active against insect pests. Through these case studies, we demonstrate that biopesticides rely on both chemical and biochemical approaches and complementary biological modalities. Hence, biopesticides are more complex than synthetic pesticides. The latter typically utilize a single chemical with a single mode of action, while the former often have more complex or holistic modes of action. The success of current and future biopesticides could be enhanced through increased research focusing on the chemistry involved.

Pathogen growth in insect hosts: inferring the importance of different mechanisms using stochastic models and response-time data

Pathogen population dynamics within individual hosts can alter disease epidemics and pathogen evolution, but our understanding of the mechanisms driving within-host dynamics is weak. Mathematical models have provided useful insights, but existing models have only rarely been subjected to rigorous tests, and their reliability is therefore open to question. Most models assume that initial pathogen population sizes are so large that stochastic effects due to small population sizes, so-called demographic stochasticity, are negligible, but whether this assumption is reasonable is unknown. Most models also assume that the dynamic effects of a host's immune system strongly affect pathogen incubation times or "response times," but whether such effects are important in real host-pathogen interactions is likewise unknown. Here we use data for a baculovirus of the gypsy moth to test models of within-host pathogen growth. By using Bayesian statistical techniques and formal model-selection procedures, we are able to show that the response time of the gypsy moth virus is strongly affected by both demographic stochasticity and a dynamic response of the host immune system. Our results imply that not all response-time variability can be explained by host and pathogen variability, and that immune system responses to infection may have important effects on population-level disease dynamics.

Role of enhancin in Mamestra configurata nucleopolyhedrovirus virulence: selective degradation of host peritrophic matrix proteins

To infect per os, baculovirus virions cross the peritrophic matrix (PM) to reach the midgut epithelium. Insect intestinal mucins (IIMs) are PM proteins that protect the PM and aid passage of the food bolus through the gut. Some baculoviruses, including Mamestra configurata nucleopolyhedrovirus (MacoNPV-A), encode metalloproteases, known as enhancins, that facilitate infection by degrading IIMs. We examined the interaction between MacoNPV-A enhancin and M. configurata IIMs both in vivo and in vitro. Per os inoculation of M. configurata larvae with MacoNPV-A occlusion bodies (OBs) resulted in the degradation of McIIM4 within 4 h of OB ingestion, while McIIM2 was unaffected. The PM recovered by 8 h post-inoculation. To investigate whether enhancin was responsible for the degradation of IIM, a recombinant Autographa californica multiple nucleopolyhedrovirus expressing MacoNPV enhancin (AcMNPV-enMP2) was constructed. Enhancin was found to be a component of occlusion-derived virions in AcMNPV-enMP2 and MacoNPV-A. In in vitro assays, McIIM4 was degraded after MacoNPV-A and AcMNPV-enMP2 treatments. Degradation of McIIM4 was inhibited by EDTA, a metalloprotease inhibitor, indicating that the degradation was due to enhancin activity. Thus, MacoNPV-A enhancin is able to degrade major structural PM proteins, but exhibits target substrate specificity.

Proteases as insecticidal agents

Proteases from a variety of sources (viruses, bacteria, fungi, plants, and insects) have toxicity towards insects. Some of these insecticidal proteases evolved as venom components, herbivore resistance factors, or microbial pathogenicity factors, while other proteases play roles in insect development or digestion, but exert an insecticidal effect when over-expressed from genetically engineered plants or microbial pathogens. Many of these proteases are cysteine proteases, although insect-toxic metalloproteases and serine proteases have also been examined. The sites of protease toxic activity range from the insect midgut to the hemocoel (body cavity) to the cuticle. This review discusses these insecticidal proteases along with their evaluation and use as potential pesticides.

Host-range expansion of Spodoptera exigua multiple nucleopolyhedrovirus to Agrotis segetum larvae when the midgut is bypassed

Given the high similarity in genome content and organization between Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) and Agrotis segetum nucleopolyhedrovirus (AgseNPV), as well as the high percentages of similarity found between their 30 core genes, the specificity of these NPVs was analysed for the respective insect hosts, S. exigua and A. segetum. The LD(50) for AgseNPV in second-instar A. segetum larvae was 83 occlusion bodies per larva and the LT(50) was 8.1 days. AgseNPV was orally infectious for S. exigua, but the LD(50) was 10 000-fold higher than for SeMNPV. SeMNPV was not infectious for A. segetum larvae when administered orally, but an infection was established by injection into the haemocoel. Bypassing midgut entry by intrahaemocoelic inoculation suggested that the midgut is the major barrier in A. segetum larvae for infection by SeMNPV. Delayed-early genes of SeMNPV are expressed in the midgut of A. segetum larvae after oral infections, indicating that the virus is able to enter midgut epithelial cells and that it proceeds through the first phases of the infection process. The possible mechanisms of A. segetum resistance to SeMNPV in per os infections are discussed.

Changes in gene expression in the permissive larval host lightbrown apple moth (Epiphyas postvittana, Tortricidae) in response to EppoNPV (Baculoviridae) infection

Host cell and virus gene expression were measured five days after per os inoculation of 3rd instar lightbrown apple moth (LBAM) larvae with the Epiphyas postvittana nucleopolyhedrovirus (EppoNPV). Microarray analysis identified 84 insect genes that were up-regulated and 18 genes that were down-regulated in virus-infected larvae compared with uninfected larvae. From the 134 viral open reading frames represented on the microarray, 81 genes showed strong expression. Of the 38 functionally identifiable regulated insect genes, 23 coded for proteins that have roles in one of five processes; regulation of transcription and translation, induction of apoptosis, and maintenance of both juvenility and actin cytoskeletal integrity. Of the 34 functionally identifiable viral genes that were most strongly expressed, 12 had functions associated with these five processes, as did a further seven viral genes which were expressed at slightly lower levels. A survey of the LBAM-expressed sequence tag library identified further genes involved in these processes. In total, 135 insect genes and 38 viral genes were analysed by quantitative polymerase chain reaction. Twenty-one insect genes were strongly up-regulated and 31 genes strongly down-regulated. All 38 viral genes examined were highly expressed. These data suggest that induction of apoptosis and regulation of juvenility are the major 'battlegrounds' between virus and insect, with the majority of changes observed representing viral control of insect gene expression. Transcription and translational effects seem to be exerted largely through modulation of mRNA and protein degradation. Examples of attempts by the insect to repel the infection via changes in gene expression within these same processes were, however, also noted. The data also showed the extent to which viral transcription dominated in the infected insects at five days post inoculation.

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.

Peritrophic membrane role in enhancing digestive efficiency. Theoretical and experimental models

The peritrophic membrane (PM) is an anatomical structure surrounding the food bolus in most insects. Rejecting the idea that PM has evolved from coating mucus to play the same protective role as it, novel functions were proposed and experimentally tested. The theoretical principles underlying the digestive enzyme recycling mechanism were described and used to develop an algorithm to calculate enzyme distributions along the midgut and to infer secretory and absorptive sites. The activity of a Spodoptera frugiperda microvillar aminopeptidase decreases by 50% if placed in the presence of midgut contents. S. frugiperda trypsin preparations placed into dialysis bags in stirred and unstirred media have activities of 210 and 160%, respectively, over the activities of samples in a test tube. The ectoperitrophic fluid (EF) present in the midgut caeca of Rhynchosciara americana may be collected. If the enzymes restricted to this fluid are assayed in the presence of PM contents (PMC) their activities decrease by at least 58%. The lack of PM caused by calcofluor feeding impairs growth due to an increase in the metabolic cost associated with the conversion of food into body mass. This probably results from an increase in digestive enzyme excretion and useless homeostatic attempt to reestablish destroyed midgut gradients. The experimental models support the view that PM enhances digestive efficiency by: (a) prevention of non-specific binding of undigested material onto cell surface; (b) prevention of excretion by allowing enzyme recycling powered by an ectoperitrophic counterflux of fluid; (c) removal from inside PM of the oligomeric molecules that may inhibit the enzymes involved in initial digestion; (d) restriction of oligomer hydrolases to ectoperitrophic space (ECS) to avoid probable partial inhibition by non-dispersed undigested food. Finally, PM functions are discussed regarding insects feeding on any diet.

The N-terminal region of an entomopoxvirus fusolin is essential for the enhancement of peroral infection, whereas the C-terminal region is eliminated in digestive juice

The spindles of Anomala cuprea entomopoxvirus (AncuEPV), which are composed of glycoprotein fusolin, are known to enhance the peroral infectivity of AncuEPV itself and of nucleopolyhedroviruses. This has been demonstrated to involve the disruption of intestinal peritrophic membrane (PM), composed of chitin matrix, glycosaminoglycans, and proteins. To identify essential and nonessential regions for this enhancement activity, AncuEPV fusolin and its deletion mutants were expressed in Sf21 cells using a baculovirus system, and their enhancement abilities were analyzed. The recombinant fusolin enhanced the peroral infectivity of Bombyx mori nucleopolyhedrovirus up to 320-fold and facilitated the infection of host insect with AncuEPV. Deletion mutagenesis revealed that the N-terminal region (amino acids 1 to 253), a possible chitin-binding domain, is essential for the enhancement of infection, whereas the C-terminal region is entirely dispensable. The glycosylation-defective mutants N191Q, whose Asn(191) is replaced with Gln, and DeltaSIG, whose signal peptide is deleted, showed considerably reduced and abolished enhancing activities, respectively, indicating that the carbohydrate chain is important in the enhancing activity. Interestingly, the C-terminal dispensable region was digested by a serine protease(s) in insect digestive juice. Moreover, both the N-terminal conserved region and the carbohydrate chain were necessary not only for chitin binding but also for stability in digestive juice. A triple amino acid replacement mutant, IHE (Ile-His-Glu(161) to Ala-Ala-Ala), was stable in digestive juice and had chitin-binding ability but did not retain its enhancing activity. These results suggest that the enhancement of infectivity involves more than the tolerance to digestive juice and chitin-binding ability.

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.

Optical brightener M2R destroys the peritrophic membrane of Spodoptera exigua (Lepidoptera: Noctuidae) larvae

Observations under an environmental scanning electron microscope showed that the peritrophic membrane (PM) of Spodoptera exigua (Hübner) was smooth without pores, but there were many pores and slits in the PM of larvae fed with 10 g L(-1) optical brightener Calcofluor white M2R. After incubation of S. exigua PM in vitro with 10 g L(-1) M2R, a significant amount of proteins was released from the PM structure. M2R disrupted the structure of larval PM, resulting in greatly increased permeability and increased larval susceptibility to Syngrapha falcifera multiple nucleopolyhedrovirus infection. Continuous feeding of larvae on a diet treated with a 10 g L(-1) M2R solution significantly retarded larval development and resulted in high mortality. The destructive effect of M2R on PM was transient and reversible, depending on the presence of M2R within the midgut.

Spindles of an entomopoxvirus facilitate its infection of the host insect by disrupting the peritrophic membrane

The mode of action by which entomopoxvirus (EPV) spindles, proteinaceous crystalline bodies produced by EPVs, enhance EPV infection has not been clarified. We fed Anomala cuprea EPV (AcEPV) spindles to host insects; subsequent scanning electron microscopy revealed the disruption of the peritrophic membranes (PMs) of these insects. The PM is reportedly a barrier against the infection of some insects by viruses. Quantitative PCR of AcEPV DNA in the ectoperitrophic area revealed that PM disruption facilitated the passage of EPVs through the PM toward the initial infection site, the midgut epithelium. These results indicate that EPV spindles enhance infection by EPVs by disrupting the PM in the host insects. Fusolin is almost exclusively the constituent protein of the spindles and is the enhancing factor of the infectivity of nucleopolyhedroviruses (NPVs) and possibly that of EPVs. Spheroid is another type of proteinaceous crystalline structure produced by EPVs. Pseudaletia separata EPV (PsEPV) spheroids reportedly contain considerable amounts of fusolin and enhance NPV infection. We assessed the ability of AcEPV spheroids to enhance EPV infectivity and their effect on the PM and carried out immunological experiments; these experiments showed that AcEPV spheroids contain little or no fusolin and are biologically inactive, in contrasts to the situation in PsEPV.

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.

Virus-derived genes for insect-resistant transgenic plants

Insect viruses have evolved to counter physiological barriers to infection presented by the host insect. For the Lepidoptera (butterflies and moths), these barriers include (1) the peritrophic membrane (PM) lining the gut, which presents a physical barrier to virus infection of the midgut epithelial cells, (2) the basement membrane (BM) that overlies the gut thereby restricting secondary infection of other tissues, and (3) the immune system of the host insect. Hence, insect viruses provide a resource for genes that disrupt host physiology in a specific manner, and these genes in turn serve as a resource both for the study of physiological processes, and for disruption of these processes for pest management purposes. There are several examples of the application of genes used by an insect virus to overcome the PM barrier for production of insect-resistant transgenic plants. There are other examples of intrahemocoelic effectors, such as BM-degrading proteases that can only be used with an appropriate system for delivery of the agent from the gut into the hemocoel (body cavity) of the insect pest. In this chapter, we describe (1) baculovirus- and entomopoxvirus-derived genes that alter the physiology of the host insect, (2) use of these and homologous genes for production of insect-resistant transgenic plants, (3) other viral genes that have potential for use in development of insect-resistant transgenic plants, and (4) the use of plant lectins for delivery of intrahemocoelic toxins from transgenic plants. Plant expression of polydnavirus-derived genes is described by Gill et al. (this volume, pp. 393-426).

Degradation of the S. frugiperda peritrophic matrix by an inducible maize cysteine protease

A unique 33-kDa cysteine protease (Mir1-CP) rapidly accumulates at the feeding site in the whorls of maize (Zea mays L.) lines that are resistant to herbivory by Spodoptera frugiperda and other lepidopteran species. When larvae were reared on resistant plants, larval growth was reduced due to impaired nutrient utilization. Scanning electron microscopy (SEM) indicated that the peritrophic matrix (PM) was damaged when larvae fed on resistant plants or transgenic maize callus expressing Mir1-CP. To directly determine the effects of Mir1-CP on the PM in vitro, dissected PMs were treated with purified, recombinant Mir1-CP and the movement of Blue Dextran 2000 across the PM was measured. Mir1-CP completely permeabilized the PM and the time required to reach full permeability was inversely proportional to the concentration of Mir1-CP. Inclusion of E64, a specific cysteine protease inhibitor prevented the damage. The lumen side of the PM was more vulnerable to Mir1-CP attack than the epithelial side. Mir1-CP damaged the PM at pH values as high as 8.5 and more actively permeabilized the PM than equivalent concentrations of the cysteine proteases papain, bromelain and ficin. The effect of Mir1-CP on the PMs of Helicoverpa zea, Danaus plexippus, Ostrinia nubilalis, Periplaneta americana and Tenebrio molitor also was tested, but the greatest effect was on the S. frugiperda PM. These results demonstrate that the insect-inducible Mir1-CP directly damages the PM in vitro and is critical to insect defense in maize.

The Lymantria dispar nucleopolyhedrovirus enhancins are components of occlusion-derived virus

Enhancins are metalloproteinases, first identified in granuloviruses, that can enhance nucleopolyhedrovirus (NPV) potency. We had previously identified two enhancin genes (E1 and E2) in the Lymantria dispar multinucleocapsid NPV (LdMNPV) and showed that both were functional. For this study, we have extended our analysis of LdMNPV enhancin genes through an immunocytochemical analysis of E1 and E2 expression and localization. E1 and E2 peptide antibodies recognized proteins of approximately 84 kDa and 90 kDa, respectively, on Western blots of extracts from L. dispar 652Y cells infected with wild-type virus. The 84- and 90-kDa proteins were first detected at 48 h postinfection (p.i.) and were present through 96 h p.i. E1 and E2 peptide antibodies detected E1 and E2 in polyhedron extracts, and the antibodies were shown to be specific for E1 and E2, respectively, through the use of recombinant virus strains lacking the enhancin genes. E1 and E2 were further localized to occlusion-derived virus (ODV). The enhancins were not found in budded virus. Immunoelectron microscopy indicated that E1 and E2 were present in ODV envelopes and possibly in nucleocapsids. Fractionation studies with several detergents suggested that the enhancins were present in ODV envelopes in association with nucleocapsids. In contrast, enhancins in granuloviruses are located within the granulin matrix. The presence of LdMNPV enhancins within ODV provides a position for the proteins to interact directly on the peritrophic membrane as ODV traverses this host defense barrier.

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.

Modeling the structure of the type I peritrophic matrix: characterization of a Mamestra configurata intestinal mucin and a novel peritrophin containing 19 chitin binding domains

Twelve to fourteen integral proteins were found to reside in the Type I peritrophic matrix (PM) of Mamestra configurata (bertha armyworm) larvae. Several methods were employed, including de novo peptide sequencing, the generation of a midgut-specific EST database and immunological screening, which led to the isolation of cDNAs encoding two integral PM proteins. McPM1, the largest PM protein described to date at 202 kDa, was comprised of a concatamer of 19 chitin binding domains (CBD), 12 of which resided within a central repetitive region consisting of six iterations of a two CBD module. The protein was found to reside within the PM primarily as several lower molecular weight, presumably proteolytically processed, forms. McMUC1 was similar in structure to other insect intestinal mucins (IIM) and was highly glycosylated. The expression of both proteins was restricted to the larval midgut. Lower molecular weight proteins that may represent non- and partially glycosylated forms of McMUC1 were also recognized by an anti-McMUC1 antiserum. These were preferentially degraded upon ingestion of M. configurata multi-capsid nucleopolyhedrovirus by larvae, possibly by a viral-encoded metalloprotease. A molecular model of PM structure is presented featuring the interaction of McPM1 with chitin inter-fibril junctions and McMUC1 with the extended chains in the internodal regions. The potential for interaction between PM proteins via intermolecular disulfide bond formation and through association of CBD with N-linked glycans is discussed.

Insect defenses against virus infection: the role of apoptosis

Insects, with their lack of an adaptive immune response, provide a unique animal model to examine the effects of apoptosis on viral infection. Several members of the baculovirus family of insect viruses have been shown to induce apoptosis during infection of cultured insect cells, and depending on the virus-host combination this apoptotic response can severely limit viral replication. In response to this evolutionary pressure, all baculoviruses studied to date carry antiapoptotic genes, including members of the p35 and IAP (inhibitor of apoptosis) gene families. Recent work has characterized the apoptotic response during infection of the host insect, and the results directly demonstrate the power of apoptosis as an antiviral response.

Characterization of an intestinal mucin from the peritrophic matrix of the diamondback moth, Plutella xylostella

The peritrophic matrix (PM) of Plutella xylostella larvae was found to contain twelve integral and eighteen loosely associated proteins. An antiserum against Mamestra configurata integral PM proteins cross-reacted with several P. xylostella PM proteins and was used to isolate a partial cDNA encoding an insect intestinal mucin (PxIIM). PxIIM was expressed primarily in the larval midgut. The deduced protein sequence of the partial cDNA contained three potentially glycosylated, mucin-like domains and six cysteine-rich chitin-binding domains (CBDs). An additional chitin-binding domain was proposed to reside at the amino terminus of the protein based on comparison with other IIM. The organization of mucin domains and CBDs exhibited features, including an internal triplet of regularly spaced CBDs and a carboxyl terminal CBD with two additional conserved cysteine residues, that were found to be common to other lepidopteran IIMs.

Disintegration of the peritrophic membrane of silkworm larvae due to spindles of an entomopoxvirus

Mode of action by which entomopoxvirus (EPV) spindles enhance nucleopolyhedrovirus (NPV) infection remains unclear. Spindles of Anomala cuprea entomopoxvirus (AcEPV), a coleoptran EPV, are known to enhance Bombyx mori NPV (BmNPV) infection in silkworm larvae. AcEPV spindles were orally administered to silkworm larvae with or without BmNPV polyhedra, and the peritrophic membranes (PMs) were observed using a binocular microscope. Soon after the larvae's access to spindles with or without the polyhedra had been terminated, some PMs disappeared wholly and some were observed in partial form. Some of the partial PMs observed were very fragile. The disintegration of the PM due to spindles also was observed by the histological sectioning of the midgut. However, a day after the larvae had terminated their access to the spindles, the PM regenerated partially or wholly. In contrast, the administration of AcEPV spheroids caused neither the disintegration of PMs nor the enhancement of BmNPV infection in silkworm larvae. These findings strongly suggest that the enhancement of NPV infection occurs due to that a greater number of NPV virions reaching the microvilli of midgut susceptible to NPV, since spindles lead to the disintegration of the PM as a barrier against NPV virions.

Insect feeding mobilizes a unique plant defense protease that disrupts the peritrophic matrix of caterpillars

Plants frequently respond to herbivorous insect attack by synthesizing defense proteins that deter insect feeding and prevent additional herbivory. Maize (Zea mays L.) lines, resistant to feeding by a number of lepidopteran species, rapidly mobilize a unique 33-kDa cysteine protease in response to caterpillar feeding. The accumulation of the 33-kDa cysteine protease in the maize mid-whorl was correlated with a significant reduction in caterpillar growth that resulted from impaired nutrient utilization. Black Mexican Sweetcorn callus transformed with mir1, the gene encoding the 33-kDa cysteine protease, expressed the protease and growth of caterpillars reared on the transgenic callus was reduced 60-80%. Scanning electron microscopy was used to examine the effect of plant material expressing the 33-kDa cysteine protease on the structure of the caterpillar peritrophic matrix. Because the peritrophic matrix surrounds the food bolus, assists in digestive processes, and protects the caterpillar midgut from physical and chemical damage, disruption of peritrophic matrix may reduce caterpillar growth. The results indicated that the peritrophic matrix was severely damaged when caterpillars fed on resistant maize plants or transgenic Black Mexican Sweetcorn. The accumulation of the 33-kDa cysteine protease in response to caterpillar feeding, and its ability to damage the insect peritrophic matrix, represents an unusual host-plant resistance mechanism that may have applications in agricultural biotechnology.