Uptake, flow, and digestion of casein and green fluorescent protein in the digestive system of Lygus hesperus Knight

Bioactive peptide release and the absorption tracking of casein in the gastrointestinal digestion of rats

Bovine casein is considered as an important source of many bioactive peptides (BAPs), which can also be produced via in vitro simulated gastrointestinal hydrolysis. To perform their physiological functions, some active peptides need to pass through the intestinal epithelial barrier and keep their structural integrity after oral administration. Owing to the complexity of in vivo digestion and absorption, there have been few studies in this area. In this study, casein was labeled with FITC to trace its digestion and absorption in Sprague Dawley (SD) rats. Gastric juice, intestinal fluid, blood, and intestinal tissue samples were collected at different time-points for preservation and analysis after intragastric administration. The results showed that CN-FITC exhibited good labeling stability in the gastrointestinal digestive juice both in vivo and in vitro, suggesting its potential to be used for the detection and tracking of casein hydrolysate. After the intra-gastric administration of FITC, the diffusion rates of fluorescent substances in serum were much higher than in the CN-FITC group. The maximum peptide content in the CN-FITC group during intestinal digestion was achieved 2 h after administration, and electrophoretic analysis of the hydrolysate composition suggested that the molecular weights of the peptides were mainly concentrated in the range of 3.4-10 kDa. The hydrolyzed peptides from CN-FITC could be absorbed into the blood just 1 h after administration. Frozen sections of rat duodenal tissue were observed under a confocal laser scanning microscope, and they showed that the CN-FITC digested products were absorbed from villi to mucosa in the rat intestines, and the casein-hydrolyzed polypeptides were accumulated significantly in tissue samples 2 h after administration. The peptides were mainly absorbed in the duodenum on the basis of absorption experiments using an everted gut sac. After intestinal digestion for 2 h, peptides with weights less than 5 kDa were enriched and identified using LC-MS-MS, and they were found to be mainly derived from β-casein, containing potential angiotensin-I-converting enzyme inhibitory, antioxidant, dipeptidyl peptidase IV inhibitory, and morphine-like peptides. The peptides from casein hydrolysate were tracked entering the blood through the intestinal epithelial barrier in the form of complete fragments, and they might exert potential physiological activity in vivo.

Chilling induces unidirectional solute leak through the locust gut epithelia

Chill-susceptible insects, like the migratory locust, often die when exposed to low temperatures from an accumulation of tissue damage that is unrelated to freezing (chilling injury). Chilling injury is often associated with a loss of ion balance across the gut epithelia. It has recently been suggested that this imbalance is at least partly caused by a cold-induced disruption of epithelial barrier function. Here, we aimed to test this hypothesis in the migratory locust (Locustamigratoria). First, chill tolerance was quantified by exposing locusts to -2°C and recording chill coma recovery time and survival 24 h post-cold exposure. Longer exposure times significantly increased recovery time and caused injury and death. Ion-selective microelectrodes were also used to test for a loss of ion balance in the cold. We found a significant increase of haemolymph K+ and decrease of haemolymph Na+ concentration over time. Next, barrier failure along the gut was tested by monitoring the movement of an epithelial barrier marker (FITC-dextran) across the gut epithelia during exposure to -2°C. We found a significant increase in haemolymph FITC-dextran concentration over time in the cold when assayed in the mucosal to serosal direction. However, when tested in the serosal to mucosal direction, we saw minimal marker movement across the gut epithelia. This suggests that while cold-induced barrier disruption is present, it is apparently unidirectional. It is important to note that these data reveal only the phenomenon itself. The location of this leak as well as the underlying mechanisms remain unclear and require further investigation.

Evaluation of Chemical Strategies for Improving the Stability and Oral Toxicity of Insecticidal Peptides

Spider venoms are a rich source of insecticidal peptide toxins. Their development as bioinsecticides has, however, been hampered due to concerns about potential lack of stability and oral bioactivity. We therefore systematically evaluated several synthetic strategies to increase the stability and oral potency of the potent insecticidal spider-venom peptide ω-HXTX-Hv1a (Hv1a). Selective chemical replacement of disulfide bridges with diselenide bonds and N- to C-terminal cyclization were anticipated to improve Hv1a resistance to proteolytic digestion, and thereby its activity when delivered orally. We found that native Hv1a is orally active in blowflies, but 91-fold less potent than when administered by injection. Introduction of a single diselenide bond had no effect on the susceptibility to scrambling or the oral activity of Hv1a. N- to C-terminal cyclization of the peptide backbone did not significantly improve the potency of Hv1a when injected into blowflies and it led to a significant decrease in oral activity. We show that this is likely due to a dramatically reduced rate of translocation of cyclic Hv1a across the insect midgut, highlighting the importance of testing bioavailability in addition to toxin stability.

Dietary Safety Assessment of Flk1-Transgenic Fish

Genetic engineering, also called genetic modification, is facing with growing demands of aquaculture and aquatic products. Although various genetically modified (GM) aquatics have been generated, it is important to evaluate biosafety of GM organisms on the human health before entering into our food chain. For this purpose, we establish a zebrafish wild adult feeding Flk1-transgenic larvae model to examine the predatory fish's histology in multiple tissues, and the global gene expression profile in the liver. 180 days of feeding trial show that there are no significantly morphological changes in intestine, liver, kidney, and sex gonads between fish fed with Flk1 transgenic fish diet (TFD) and fish fed with regular food meal (RFM). However, a characteristic skin spot and autofluorescence increase in the theca of follicle are observed in F1 generation of TFD fish. Liver RNA-sequencing analyses demonstrate that 53 out of 56712 genes or isoforms are differentially transcribed, and mostly involved in proteolysis in extracellular region. According to GO enrichment terms these deregulated genes function in catalytic activity, steroid storing, lipid metabolic process and N-Glycan biosynthesis. These results suggest that a long term of Flk1-transgenic fish diet could alter certain metabolic pathways and possibly cause related tissue deformation. Compared to the previous reports, our feasible transgenic dietary assess system could evaluate subchronic and potential health impact of transgenic fish diet by combining multi-tissue histology and liver transcriptome analyses.

Uptake and bioaccumulation of Cry toxins by an aphidophagous predator

Uptake of Cry toxins by insect natural enemies has rarely been considered and bioaccumulation has not yet been demonstrated. Uptake can be demonstrated by the continued presence of Cry toxin after exposure has stopped and gut contents eliminated. Bioaccumulation can be demonstrated by showing uptake and that the concentration of Cry toxin in the natural enemy exceeds that in its food. We exposed larvae of the aphidophagous predator, Harmonia axyridis, to Cry1Ac and Cry1F through uniform and constant tritrophic exposure via an aphid, Myzus persicae, and looked for toxin presence in the pupae. We repeated the experiment using only Cry1F and tested newly emerged adults. Both Cry toxins were detected in pupae, and Cry1F was detected in recently emerged, unfed adults. Cry1Ac was present 2.05 times and Cry1F 3.09 times higher in predator pupae than in the aphid prey. Uptake and bioaccumulation in the third trophic level might increase the persistence of Cry toxins in the food web and mediate new exposure routes to natural enemies.

Fabrication and characterization of PCL/CaCO3 electrospun composite membrane for bone repair

CaCO₃/casein microspheres were entrapped in PCL membranes using electrospinning to mimic the hierarchical structure of ECM in bone. The composite membranes showed enhanced biomineralization property, proliferation and osteogenic differentiation potential of HMSCs.

Toxins for transgenic resistance to hemipteran pests

The sap sucking insects (Hemiptera), which include aphids, whiteflies, plant bugs and stink bugs, have emerged as major agricultural pests. The Hemiptera cause direct damage by feeding on crops, and in some cases indirect damage by transmission of plant viruses. Current management relies almost exclusively on application of classical chemical insecticides. While the development of transgenic crops expressing toxins derived from the bacterium Bacillus thuringiensis (Bt) has provided effective plant protection against some insect pests, Bt toxins exhibit little toxicity against sap sucking insects. Indeed, the pest status of some Hemiptera on Bt-transgenic plants has increased in the absence of pesticide application. The increased pest status of numerous hemipteran species, combined with increased prevalence of resistance to chemical insecticides, provides impetus for the development of biologically based, alternative management strategies. Here, we provide an overview of approaches toward transgenic resistance to hemipteran pests.

Cotton plants expressing a hemipteran-active Bacillus thuringiensis crystal protein impact the development and survival of Lygus hesperus (Hemiptera: Miridae) nymphs

The plant bugs Lygus hesperus Knight (Hemiptera: Miridae) and L. lineolaris (Palisot de Beauvois) have emerged as economic pests of cotton in the United States. These hemipteran species are refractory to the insect control traits found in genetically modified commercial varieties of cotton. In this article, we report the isolation and characterization of a 35 kDa crystal protein from Bacillus thuringiensis, designated TIC807, which causes reduced mass gain and mortality of L. hesperus and L. lineolaris nymphs when presented in an artificial diet feeding assay. Cotton plants expressing the TIC807 protein were observed to impact the survival and development of L. hesperus nymphs in a concentration-dependent manner. These results, demonstrating in planta activity of a Lygus insecticidal protein, represent an important milestone in the development of cotton varieties protected from Lygus feeding damage.

Enhanced activity of an insecticidal protein, trypsin modulating oostatic factor (TMOF), through conjugation with aliphatic polyethylene glycol

BACKGROUND

Trypsin modulating oostatic factor (TMOF), a decapeptide (Tyr-Asp-Pro-Ala-Pro(6)) isolated from the ovaries of the adult yellow fever mosquito, Aedes aegypti, regulates trypsin biosynthesis. TMOF per os is insecticidal to larval mosquitoes and a good model for the development of technologies to enhance protein insecticide activity by reduced catabolism and/or enhanced delivery to the target.

RESULTS

TFA-TMOF-K (TFA = trifluoro acetyl) allowed the specific conjugation of monodispersed, aliphatic polyethylene glycol (PEG) to the amino group of lysine-producing TMOF-K-methyl(ethyleneglycol)(7)-O-propionyl (TMOF-K-PEG(7) P). The addition of lysine to TMOF reduced its per os larval mosquitocidal activity relative to the parent TMOF, but conjugation of TMOF-K with methyl(ethyleneglycol)(7)-O-propionyl increased its toxicity 5.8- and 10.1-fold above that of TMOF and TMOF-K for Ae. aegypti. Enhanced insecticidal activity was also found for larval Ae. albopictus and for neonates of Heliothis virescens and Heliocoverpa zea. Only TMOF-K was found by MS/MS in the hemolymph for H. virescens fed on TMOF-K-PEG(7) P. No TMOF, TMOF-K or PEGylated TMOF-K was detected in the hemolymph after topical applications.

CONCLUSIONS

This research suggests that aliphatic PEG polymers can be used as a new method for increasing the activity of insecticidal proteins.

Phenotypic variation and host interactions of Xenorhabdus bovienii SS-2004, the entomopathogenic symbiont of Steinernema jollieti nematodes

Xenorhabdus bovienii (SS-2004) bacteria reside in the intestine of the infective-juvenile (IJ) stage of the entomopathogenic nematode, Steinernema jollieti. The recent sequencing of the X. bovienii genome facilitates its use as a model to understand host - symbiont interactions. To provide a biological foundation for such studies, we characterized X. bovienii in vitro and host interaction phenotypes. Within the nematode host X. bovienii was contained within a membrane bound envelope that also enclosed the nematode-derived intravesicular structure. Steinernema jollieti nematodes cultivated on mixed lawns of X. bovienii expressing green or DsRed fluorescent proteins were predominantly colonized by one or the other strain, suggesting the colonizing population is founded by a few cells. Xenorhabdus bovienii exhibits phenotypic variation between orange-pigmented primary form and cream-pigmented secondary form. Each form can colonize IJ nematodes when cultured in vitro on agar. However, IJs did not develop or emerge from Galleria mellonella insects infected with secondary form. Unlike primary-form infected insects that were soft and flexible, secondary-form infected insects retained a rigid exoskeleton structure. Xenorhabdus bovienii primary and secondary form isolates are virulent towards Manduca sexta and several other insects. However, primary form stocks present attenuated virulence, suggesting that X. bovienii, like Xenorhabdus nematophila may undergo virulence modulation.

Protein digestion in cereal aphids (Sitobion avenae) as a target for plant defence by endogenous proteinase inhibitors

Gut extracts from cereal aphids (Sitobion avenae) showed significant levels of proteolytic activity, which was inhibited by reagents specific for cysteine proteases and chymotrypsin-like proteases. Gut tissue contained cDNAs encoding cathepsin B-like cysteine proteinases, similar to those identified in the closely related pea aphid (Acyrthosiphon pisum). Analysis of honeydew (liquid excreta) from cereal aphids fed on diet containing ovalbumin showed that digestion of ingested proteins occurred in vivo. Protein could partially substitute for free amino acids in diet, although it could not support complete development. Recombinant wheat proteinase inhibitors (PIs) fed in diet were antimetabolic to cereal aphids, even when normal levels of free amino acids were present. PIs inhibited proteolysis by aphid gut extracts in vitro, and digestion of protein fed to aphids in vivo. Wheat subtilisin/chymotrypsin inhibitor, which was found to inhibit serine and cysteine proteinases, was more effective in both inhibitory and antimetabolic activity than wheat cystatin, which inhibited cysteine proteases only. Digestion of ingested protein is unlikely to contribute significantly to nutritional requirements when aphids are feeding on phloem, and the antimetabolic activity of dietary proteinase inhibitors is suggested to result from effects on proteinases involved in degradation of endogenous proteins.

The fate of vicilins, 7S storage globulins, in larvae and adult Callosobruchus maculatus (Coleoptera: Chrysomelidae: Bruchinae)

The fate of vicilins ingested by Callosobruchus maculatus and the physiological importance of these proteins in larvae and adults were investigated. Vicilins were quantified by ELISA in the haemolymph and fat body during larval development (2nd to 4th instars), in pupae and adults, as well as in ovaries and eggs. Western blot analysis demonstrated that the majority of absorbed vicilins were degraded in the fat body. Tracing the fate of vicilins using FITC revealed that the FITC-vicilin complex was present inside cells of the fat body of the larvae and in the fat bodies of both male and female adult C. maculatus. Labelled vicilin was also detected in ovocytes and eggs. Based on the results presented here, we propose that following absorption, vicilins accumulate in the fat body, where they are partially degraded. These peptides are retained throughout the development of the insects and eventually are sequestered by the eggs. It is possible that accumulation in the eggs is a defensive strategy against pathogen attack as these peptides are known to have antimicrobial activity. Quantifications performed on internal organs from larvae of C. maculatus exposed to extremely dry seeds demonstrated that the vicilin concentration in the haemolymph and fat body was significantly higher when compared to larvae fed on control seeds. These results suggest that absorbed vicilins may also be involved in the survival of larvae in dry environments.

A megalin-like receptor is involved in protein endocytosis in the midgut of an insect (Bombyx mori, Lepidoptera)

The mechanism responsible for fluorescein isothiocyanate (FITC)-albumin internalization by columnar cells in culture obtained from the midgut of Bombyx mori larvae was examined by confocal laser scanning microscopy. Protein uptake changed over time, and it appeared to be energy dependent, since it was strongly reduced by both low temperatures and metabolic inhibitors. Labeled albumin uptake as a function of increasing protein concentration showed a saturation kinetics with a Michaelis constant value of 2.0 +/- 0.6 microM. These data are compatible with the occurrence of receptor-mediated endocytosis. RT-PCR analysis and colocalization experiments with an anti-megalin primary antibody indicated that the receptor involved was a putative homolog of megalin, the multiligand endocytic receptor belonging to the low-density lipoprotein receptor family, responsible for the uptake of various molecules, albumin included, in many epithelial cells of mammals. This insect receptor, like the mammalian counterpart, required Ca(2+) for albumin internalization and was inhibited by gentamicin. FITC-albumin internalization was clathrin mediated, since two inhibitors of this process caused a significant reduction of the uptake, and clathrin and albumin colocalized in the intermicrovillar areas of the apical plasma membrane. The integrity of actin and microtubule organization was essential for the correct functioning of the endocytic machinery.

Transepithelial flux of an allatostatin and analogs across the anterior midgut of Manduca sexta larvae in vitro

The transepithelial flux of cydiastatin 4 and analogs across flat sheet preparations of the anterior midgut of larvae of the tobacco hawkmoth moth, Manduca sexta, was investigated using a combination of reversed-phase high-performance liquid chromatography (RP-HPLC), enzyme-linked immunosorbent assay (ELISA) and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). The lumen to hemolymph (L-H) flux of cydiastatin 4 was dose and time-dependent, with a maximum rate of flux of c. 178 pmol/cm2/h) measured after a 60-min incubation with 100 micromol/l of peptide in the lumen bathing fluid. The rates of flux, L-H and H-L, across the isolated gut preparations were not significantly different. These data suggest that uptake across the anterior midgut of larval M. sexta is via a paracellular route. Cydiastatin 4 was modified to incorporate a hexanoic acid (Hex) moiety at the N-terminus, the N-terminus extended with 5 P residues and/or the substitution of G7 with Fmoc-1-amino-cyclopropylcarboxylic acid (Acpc). The incorporation of hexanoic acid enhanced the uptake of these amphiphilic analogs compared to the native peptide. Analogs were also more resistant to enzymes in hemolymph and gut preparations from larval M. sexta. A modified N-terminus gave protection against aminopeptidase-like activity and incorporation of Acpc inhibited endopeptidase-like activity. Although analogs were stable in the hemolymph, they were susceptible to amidase-like activity in the gut, which appears to convert the C-terminal amide group to a free carboxylic acid, identified by an increase in 1 mass unit of the peptide analog.

The movement of proteins across the insect and tick digestive system

The movement of intact proteins across the digestive system was shown in a number of different blood-feeding and non-blood-feeding insects in the orders Blattaria, Coleoptera, Diptera, Hemiptera, Lepidoptera, Orthoptera, Neuroptera and Siphonaptera, as well as in two tick families Ixodidae and Argasidae. Protein movement was observed for both normal dietary and xenobiotic proteins, which suggest that the mechanism for transfer is not substrate specific. The number of studies on the mechanism of movement is limited. The research so far suggests that movement can occur by either a transcellular or an intercellular pathway in the ventriculus with most of the research describing the former. Transfer is by continuous diffusion with no evidence of pinocytosis or vesicular transport common in mammalian systems. Proteins can move across the digestive system without modification of their primary or multimeric structure and with retention of their functional characteristics. Accumulation in the hemolymph is the result of the protein degradation rate in the gut and hemolymph and transfer rate across the digestive system and can be highly variable depending on species. Research on the development of delivery systems to enhance protein movement across the insect digestive system is in its infancy. The approaches so far considered with some success include the use of lipophilic-polyethylene glycol (PEG) polymers, the development of fusion proteins with lectins, reduced gut protease activity and the development of amphiphilic peptidic analogs. Additional research on understanding the basic mechanisms of protein delivery across the insect digestive system, the importance of structure activity in this transfer and the development of technology to improve movement across the gut could be highly significant to the future of protein and nucleic acid-based insecticide development as well as traditional chemical insecticidal technologies.

Metabolism of cydiastatin 4 and analogues by enzymes associated with the midgut and haemolymph of Manduca sexta larvae

The degradation of synthetic cydiastatin 4 (ARPYSFGL-amide) and cydiastatin 4 analogues cydiastatin 4alpha (PPPPPARPYSFGL-amide) and cydiastatin 4beta (PPPPPARPYSF[Acpc]L-amide) by enzymes associated with the midgut and/or haemolymph of the tobacco hawkmoth moth, Manduca sexta was investigated using reversed-phase high performance liquid chromatography (RP-HPLC) combined with matrix assisted laser desorption ionisation-time of flight mass spectrometry (MALDI-TOF MS). Cydiastatin 4 had an estimated half-life of c. 16.5min when incubated with midgut tissue in vitro and c. 2.5min with midgut lumen contents. Two degradation products were identified; cydiastatin(1-6), due to cleavage of the C-terminal di-peptide GL-amide, and cydiastatin(2-8), due to cleavage of the N-terminal A residue. Both cydiastatin 4alpha and cydiastatin 4beta had increased stability to gut and haemolymph enzymes, and full biological activity, but reduced potency compared to cydiastatin 4 when assayed on foregut peristalsis. The P-extended N-terminus of both analogues prevented hydrolysis by aminopeptidases and the replacement of the susceptible G residue with cyclopropylalanine ([Acpc]) counteracted carboxypeptidase activity. However, both analogues were susceptible to amidase-like activity giving an increase in one mass unit presumably due to the conversion of the C-terminal amide group to the free carboxylic acid. No metabolism of cydiastatin 4beta occurred when incubated with larval M. sexta haemolymph over a 90min period.

A novel aminopeptidase in the fat body of the moth Achaea janata as a receptor for Bacillus thuringiensis Cry toxins and its comparison with midgut aminopeptidase

Bacillus thuringiensis insecticidal crystal proteins bind to cell-surface receptors which represent a family of aminopeptidases [APN (aminopeptidase N)] present on the brush border membrane of insect midgut cells of susceptible insects leading to pore formation and death of the insect. We report here for the first time the presence of a novel APN in the fat body of the moth Achaea janata. Northern blotting detected at least one APN-specific transcript in the fat body, whereas two transcripts of different sizes were detected in the midgut. We have cloned two full-length APN cDNAs of 3015 bp and 2850 bp from fat body and midgut respectively, which encode proteins of 1004 and 950 amino acids. These two APNs share only 33% amino acid sequence identity, but both display the typical APN features, such as the N-terminal signal peptide, several putative glycosylation sites, C-terminal glycosylphosphatidylinositol anchor signal, the APN-specific zinc-binding/gluzincin motif HEXXHX(18)E and gluzincin motif GAMENWG. The fat body APN manifested a variation in its expression with respect to tissue and developmental stage. In spite of the abundance of the APN transcript in the fat body, fairly low APN activity was detected in this tissue. The fat-body- and midgut-specific APNs showed differential interaction with various Cry1A toxins. Besides, the level of toxicity of different Cry subtypes varied enormously with mode/site of delivery, such as intrahaemocoelic injections and feeding bioassays. These data indicate that the fat body might be a potential alternative Cry toxin target site in the moth.

Absorption of horseradish peroxidase in Bombyx mori larval midgut

Increasing experimental evidence indicates that ingested proteins can in part reach the haemocoel undegraded, but information on the mechanisms involved in protein transport across the insect gut is very limited, in spite of the implications that this may have on the development of novel delivery strategies of insecticide proteins targeting haemocoelic receptors. Here we contribute to this field of study, by focusing on horseradish peroxidase (HRP) transport through Bombyx mori larval midgut, isolated and perfused in vitro. The protein crossed the intestinal barrier in a time-dependent manner and the influx was linearly related to time between 30 and 90 min of incubation. HRP absorption was strongly affected by temperature and inhibition of cell metabolism: protein influx at 4 degrees C was reduced to 27% of that measured at 25 degrees C and was similarly inhibited by the metabolic inhibitor DNP. Transmission electron microscopy analysis of midgut columnar cells exposed to HRP showed the presence of the protein both in vesicular structures inside the cytoplasm and in the space between two adjacent absorptive cells, indicating the occurrence of both a transcellular and a paracellular permeation route. The analysis of HRP influx as a function of increasing protein concentration in the lumen supported this morphological indication. The J(max) relative to the HRP transcellular transport component was 121+/-24 pmol/cm(2)/h and the K(d) of the passage through the paracellular route was 1.9+/-0.3 microl/cm(2)/h. The paracellular electrical resistance decreased in midguts exposed to HRP, indicating that its passage through this pathway was likely due to an alteration exerted on the junctional complex by the protein itself. The role of the cytoskeleton in HRP transport was investigated by assessing the impact of drugs affecting microtubules and actin filaments.

Renal clearance of absorbed intact GFP in the frog and rat intestine

Intestine absorption of intact green fluorescent protein (GFP) and its following accumulation in the renal proximal tubule cells after its intragastric administration have been established by confocal microscopy in the rat and frog. Reabsorbed GFP was revealed in the endosomes and lysosomes of the proximal tubule cells by the methods of GFP photooxidation and immunofluorescent microscopy. The GFP intestine absorption rate and GFP accumulation in the kidney were significantly higher in the frog than in the rat. No specific fluorescence was revealed in the liver and colon cells after the GFP intragastric administration. The data obtained indicate the ability of the small intestine in the frog and rat to absorb intact proteins and an important role of the kidney in exogenous protein metabolism.

In vitro transport of an allatostatin across the foregut of Manduca sexta larvae and metabolism by the gut and hemolymph

The degradation of synthetic cydiastatin 4 by enzymes of the foregut and hemolymph, and transport across the foregut of larvae of the tobacco hawkmoth moth, Manduca sexta, were investigated using reversed-phase high performance liquid chromatography (RP-HPLC) together with matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). In the hemolymph in vitro, cydiastatin 4 had a half-life of ca. 30 min. Two degradation products were identified; cydiastatin 4(1-6), due to cleavage of the C-terminal di-peptide GL-amide, and cydiastatin 4(2-8), due to cleavage of the N-terminal A residue. This hydrolysis could be inhibited by up to 93% by 1,10-phenanthroline. Other protease inhibitors had lesser effects (<21% inhibition of degradation) including the aminopeptidase inhibitors amastatin and bestatin, and the chelator EDTA. When incubated with foregut extract in vitro, cydiastatin 4 had a half-life of 23 min, and the hydrolysis products detected were also cydiastatin 4(1-6) and cydiastatin 4(2-8). Similarly, 1-10 phenanthroline inhibited foregut enzyme degradation of cydiastatin 4 by ca. 80%, whereas amastatin, bestatin, and EDTA had very little effect (<10% inhibition). Cydiastatin 4 was transported, intact, from the lumen to the hemolymph side of foregut tissues that were mounted as flat sheets in modified Ussing chambers. This trans-epithelial flux of peptide was dose and time-dependent, but was <3% of the amount of cydiastatin 4 present in the lumen bathing saline. In contrast, no trans-epithelial transport of peptide was apparent across everted foregut sac preparations.

The salivary gland and salivary enzymes of the giant waterbugs (Heteroptera; Belostomatidae)

The giant waterbugs are predators that utilize extra-oral digestion and are known to capture a wide variety of prey. Herein we describe the differences in salivary enzyme composition between large and small species of giant waterbug (Lethocerus uhleri, Lethocerinae and Belostoma lutarium, Belostomatinae, respectively). The saliva of L. uhleri contains 3 proteolytic enzymes and no amylase, while the salivary gland of B. lutarium produces 2 proteolytic enzymes and amylase. This fundamental difference in salivary enzyme composition correlates with the difference in diet preference between the Lethocerinae and Belostomatinae. Furthermore, we describe the ultrastructure of the salivary gland complex of B. lutarium and present data on the division of labor with respect to compartmentalization of enzyme production. Proteolytic enzymes are produced in the accessory salivary gland and amylase is produced in the main salivary gland lobe. This is the first reported evidence of protease production in the accessory salivary gland in the Heteroptera.

Characterization of digestive proteolytic activity in Lygus hesperus Knight (Hemiptera: Miridae)

The tarnished plant bug, Lygus hesperus Knight, is a pest that causes considerable economic losses to vegetables, cotton, canola, and alfalfa. Detailed knowledge of its digestive physiology will provide new opportunities for a sustainable pest management approach to control this insect. Little is known about the different protease class contributions to the overall digestion of a specific protein. To this end, the proteolytic activities in female adult L. hesperus salivary gland and midgut homogenates were quantified over a range of pH's and time points, and the contribution of different classes of proteases to the degradation of FITC-casein was determined. In the salivary gland, serine proteases were the predominant class responsible for caseinolytic activity, with the rate of activity increasing with increasing pH. In contrast, both aspartic and serine proteases contributed to caseinolytic activity in the midgut. Aspartic protease activity predominated at pH 5.0 and occurred immediately after incubation, whereas serine protease activity predominated at pH 7.5 after a 9h delay and was resistant to aprotinin. The salivary serine proteases were distinctly different from midgut serine proteases, based on the tissue-specific differential susceptibility to aprotinin and differing pH optima. Collectively, the caseinolytic activities complement one another, expanding the location and pH range over which digestion can occur.

Direct effects of snowdrop lectin (GNA) on larvae of three aphid predators and fate of GNA after ingestion

Plants genetically modified to express Galanthus nivalis agglutinin (GNA) have been found to confer partial resistance to homopteran pests. Laboratory experiments were conducted to investigate direct effects of GNA on larvae of three species of aphid predators that differ in their feeding and digestive physiology, i.e. Chrysoperla carnea, Adalia bipunctata and Coccinella septempunctata. Longevity of all three predator species was directly affected by GNA, when they were fed a sucrose solution containing 1% GNA. However, a difference in sensitivity towards GNA was observed when comparing the first and last larval stage of the three species. In vitro studies revealed that gut enzymes from none of the three species were able to break down GNA. In vivo feed-chase studies demonstrated accumulation of GNA in the larvae. After the larvae had been transferred to a diet devoid of GNA, the protein stayed present in the body of C. carnea, but decreased over time in both ladybirds. Binding studies showed that GNA binds to glycoproteins that can be found in the guts of larvae of all three predator species. Immunoassay by Western blotting of haemolymph samples only occasionally showed the presence of GNA. Fluorescence microscopy confirmed GNA accumulation in the midgut of C. carnea larvae. Implications of these findings for non-target risk assessment of GNA-transgenic crops are discussed.

Phloem sap proteins: their identities and potential roles in the interaction between plants and phloem-feeding insects

The phloem is a well-known target of sucking and piercing insects that utilize the transported fluid as their major nutrient source. In addition to small molecules like sugars and amino acids, phloem sap of higher land plants contains proteins that can accumulate up to high concentrations. Although the knowledge about the identities of these phloem sap proteins is increasing, the functions of most of them are still poorly understood. Since many phloem sap proteins have predicted roles in wound and defence responses, they constitute a class of compounds that can potentially influence plant-insect interactions. However, there are as yet no studies published that have examined direct effects of phloem sap proteins on insect feeding or vice versa. This review summarizes the current knowledge about the identities of phloem sap proteins, focused on polypeptides with probable functions in wound and defence reactions, and their potential impact on plant-insect interactions is discussed.

Absorption of albumin by the midgut of a lepidopteran larva

In the last decade, the study of peptide and protein absorption by the insect gut has received increasing attention because of the considerable impact this information may have on the development of new delivery strategies for insecticide macromolecules targeting haemocoelic receptors. Available experimental evidence in vivo suggests that, in insects, peptides and proteins can cross the intestinal barrier reaching the haemocoel, but the functional bases of this absorption pathway have not yet been thoroughly investigated. The current knowledge of the mechanisms involved in protein and polypeptide absorption in animals derives from the extensive studies performed in mammalian polarised epithelial cells, where the transcellular transport of proteins by transcytosis has been demonstrated. In this process, proteins are internalised at one pole of the cell and transported by cytoplasmic vesicular traffic to the opposite plasma membrane domain, where they are released with unchanged biological activity. Here we report data on albumin translocation across the isolated midgut of Bombyx mori caterpillars perfused in vitro. The functional properties of the transepithelial transport of this protein are described and, since absorption prevails over secretion, its lumen-to-haemolymph flux is characterised. Low-temperature incubations nearly abolish the transepithelial transport, while the peculiar physiological features of the larval midgut, i.e. the high lumen positive transepithelial voltage and the luminal alkaline pH, do not affect the flux. The obtained results indicate that albumin crosses B. mori larval midgut by transcytosis.