The morphology and fine structure of the larval midgut of a moth (Manduca sexta) in relation to active ion transport

An enteric ultrastructural surface atlas of the model insect Manducasexta

The tobacco hornworm is a laboratory model that is particularly suitable for analyzing gut inflammation, but a physiological reference standard is currently unavailable. Here, we present a surface atlas of the healthy hornworm gut generated by scanning electron microscopy and nano-computed tomography. This comprehensive overview of the gut surface reveals morphological differences between the anterior, middle, and posterior midgut, allowing the screening of aberrant gut phenotypes while accommodating normal physiological variations. We estimated a total resorptive midgut surface of 0.42 m2 for L5d6 larvae, revealing its remarkable size. Our data will support allometric scaling and dose conversion from Manduca sexta to mammals in preclinical research, embracing the 3R principles. We also observed non-uniform gut colonization by enterococci, characterized by dense biofilms in the pyloric cone and downstream of the pylorus associated with pore and spine structures in the hindgut intima, indicating a putative immunosurveillance function in the lepidopteran hindgut.

A quantitative micro-tomographic gut atlas of the lepidopteran model insect Manduca sexta

The tobacco hornworm is used extensively as a model system for ecotoxicology, immunology and gut physiology. Here, we established a micro-computed tomography approach based on the oral application of the clinical contrast agent iodixanol, allowing for a high-resolution quantitative analysis of the Manduca sexta gut. This technique permitted the identification of previously unknown and understudied structures, such as the crop or gastric ceca, and revealed the underlying complexity of the hindgut folding pattern, which is involved in fecal pellet formation. The acquired data enabled the volume rendering of all gut parts, the reliable calculation of their volumes, and the virtual endoscopy of the entire alimentary tract. It can provide information for accurate orientation in histology uses, enable quantitative anatomical phenotyping in three dimensions, and allow the calculation of locally effective midgut concentrations of applied chemicals. This atlas will provide critical insights into the evolution of the alimentary tract in lepidopterans.

Expression analysis of peptidergic enteroendocrine cells in the silkworm Bombyx mori

Enteroendocrine cells (ECs) in the insect midgut respond to physiological changes in the intestine by releasing multiple peptides to control food intake, gastrointestinal activity and systemic metabolism. Here, we performed a comprehensive mapping of ECs producing different regulatory peptides in the larval midgut of Bombyx mori. In total, we identified 20 peptide genes expressed in different ECs in specific regions of the midgut. Transcript-specific in situ hybridisation combined with antibody staining revealed approximately 30 subsets of ECs, each producing a unique peptide or a combination of several different peptides. Functional significance of this diversity and specific roles of different enteroendocrine peptides are largely unknown. Results of this study highlight the importance of the midgut as a major endocrine/paracrine source of regulatory molecules in insects and provide important information to clarify functions of ECs during larval feeding and development.

The Lepidopteran KAAT1 and CAATCH1: Orthologs to Understand Structure-Function Relationships in Mammalian SLC6 Transporters

To the SLC6 family belong 20 human transporters that utilize the sodium electrochemical gradient to move biogenic amines, osmolytes, amino acids and related compounds into cells. They are classified into two functional groups, the Neurotransmitter transporters (NTT) and Nutrient amino acid transporters (NAT). Here we summarize how since their first cloning in 1998, the insect (Lepidopteran) Orthologs of the SLC6 family transporters have represented very important tools for investigating functional–structural relationships, mechanism of transport, ion and pH dependence and substate interaction of the mammalian (and human) counterparts.

Chlorantraniliprole degenerates microvilli goblet cells of the Anticarsia gemmatalis (Lepidoptera: Noctuidae) midgut

Anticarsia gemmatalis Hübner (Lepidoptera: Noctuidae) is mainly controlled with synthetic insecticides such as chlorantraniliprole. However, these compounds may affect non-target organs of insect metabolism. The objective of this study was to evaluate the toxic effect in the midgut goblet cells of A. gemmatalis caterpillars exposed to chlorantraniliprole. The midgut of these caterpillars, which ingested the insecticide in medium-lethal dose (LD₅₀), was dissected and evaluated by transmission electron microscopy. The goblet cells microvilli, after exposure to the insecticide, were disorganized and degenerated. This can compromise ionic homeostasis and nutrient absorption, impair physiological mechanisms of detoxification, and reduce the movement of food boluses throughout the insect midgut.

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

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

Gelsolin role in microapocrine secretion

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

The discovery of a novel antagonist - Manduca sexta allatotropin analogue - as an insect midgut active ion transport inhibitor

BACKGROUND

The midgut is an important site for both nutrient absorption and ionic regulation in lepidopteran larvae, major pests in agriculture. The larval lepidopteran midgut has become a potent insecticide target over the past few decades. Recent studies have shown that an insect neuropeptide, Manduca sexta allatotropin (Manse-AT), exhibits inhibition of active ion transport (AIT) across the larval midgut epithelium. The full characteristic of the AIT inhibition capacity of Manse-AT is essential to assay. In this study, AIT inhibition across the M. sexta midgut by Manse-AT and its analogues in a range of concentrations was assayed. The structure-activity relationship of Manse-AT was also studied by truncated and alanine-replacement strategies.

RESULTS

Our results identified three residues, Thr4, Arg6 and Phe8, as the most important components for activity on the midgut. Replacement of Glu1, Met2 and Met3 reduced the potency of the analogues. The conservative substitution of Gly7 with alanine had little effect on the potency of the analogues. We demonstrated for the first time that Manse-AT (10-13) behaves as a potent antagonist in vitro on active ion transport across the epithelium of the posterior midgut in M. sexta.

CONCLUSION

Structure-activity studies of Manse-AT are useful in developing lead compounds for the design and testing of synthetic antagonists, ultimately to develop potent and specific pest control strategies. Manse-AT (10-13) has been discovered as the first Manse-AT antagonist, with a significant effect and a short sequence compared with other insect neuropeptides. It may be a new potential pest control agent in the future. © 2016 Society of Chemical Industry.

Developmental Expression of Ecdysone-Related Genes Associated With Metamorphic Changes During Midgut Remodeling of Silkworm Bombyx mori (Lepidoptera:Bombycidae)

Steroid hormone 20-hydroxyecdysone is known as the systemic regulators of insect cells; however, how to impact the fate and function of mature and stem cells is unclear. For the first time, we report ecdysone regulatory cascades in both mature midgut cell and stem cell fractions related to developmental events by using histological, immunohistochemical, biochemical and gene expression analysis methods. Ecdysone receptor-B1 (EcR-B1) and ultraspiracle 1 (USP-1) mRNAs were detected mainly in mature cells during programmed cell death (PCD). Lowered E75A and probably BR-C Z4 in mature cells appear to provide a signal to the initiation of PCD. E74B, E75B and BR-C Z2 seem to be early response genes which are involved in preparatory phase of cell death. It is likely that βFTZ-F1, E74A and BR-C Z1 are probably associated with execution of death. EcR-A and USP2 mRNAs were found in stem cells during remodeling processes but EcR-B1, USP1 and E74B genes imply an important role during initial phase of metamorphic events in stem cells. BHR3 mRNAs were determined abundantly in stem cells suggesting its primary role in differentiation. All of these results showed the determination the cell fate in Bombyx mori (Linnaeus) midgut depends on type of ecdysone receptor isoforms and ecdysone-related transcription factors.

Magnetic Resonance Imaging of Alimentary Tract Development in Manduca sexta

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

Participation of D-serine in the development and reproduction of the silkworm Bombyx mori

The silkworm Bombyx mori contains high concentrations of free D-serine, an optical isomer of L-serine. To elucidate its function, we first investigated the localization of D-serine in various organs of silkworm larvae, pupae, and adult moths. Using immunohistochemical analysis with an anti-D-serine antibody, we found D-serine in the microvilli of midgut goblet and cylindrical cells and in peripheral matrix components of testicular and ovarian cells. By spectrophotometric analysis, D-serine was also found in the hemolymph and fat body. D-Alanine was not detected in the various organs by immunohistochemistry. Serine racemase, which catalyzes the inter-conversion of L- and D-serine, was found to co-localize with D-serine, and D-serine production from L-serine by intrinsic serine racemase was suggested. O-Phospho-L-serine is an inhibitor of serine racemase, and it was administered to the larvae to reduce the D-serine level. This reagent decreased the midgut caspase-3 level and caused a delay in spermatogenesis and oogenesis. The reagent also decreased mature sperm and egg numbers, suggesting D-serine participation in these processes. D-Serine administration induced an increase in pyruvate levels in testis, midgut, and fat body, indicating conversion of D-serine to pyruvate. On the basis of these results, together with our previous investigation of ATP biosynthesis in testis, we consider the possible involvement of D-serine in ATP synthesis for metamorphosis and reproduction.

Surface-Associated Lipoproteins Link Enterococcus faecalis Virulence to Colitogenic Activity in IL-10-Deficient Mice Independent of Their Expression Levels

The commensal Enterococcus faecalis is among the most common causes of nosocomial infections. Recent findings regarding increased abundance of enterococci in the intestinal microbiota of patients with inflammatory bowel diseases and induction of colitis in IL-10-deficient (IL-10-/-) mice put a new perspective on the contribution of E. faecalis to chronic intestinal inflammation. Based on the expression of virulence-related genes in the inflammatory milieu of IL-10-/- mice using RNA-sequencing analysis, we characterized the colitogenic role of two bacterial structures that substantially impact on E. faecalis virulence by different mechanisms: the enterococcal polysaccharide antigen and cell surface-associated lipoproteins. Germ-free wild type and IL-10-/- mice were monoassociated with E. faecalis wild type OG1RF or the respective isogenic mutants for 16 weeks. Intestinal tissue and mesenteric lymph nodes (MLN) were collected to characterize tissue pathology, loss of intestinal barrier function, bacterial adhesion to intestinal epithelium and immune cell activation. Bone marrow-derived dendritic cells (BMDC) were stimulated with bacterial lysates and E. faecalis virulence was additionally investigated in three invertebrate models. Colitogenic activity of wild type E. faecalis (OG1RF score: 7.2±1.2) in monoassociated IL-10-/- mice was partially impaired in E. faecalis lacking enterococcal polysaccharide antigen (ΔepaB score: 4.7±2.3; p<0.05) and was almost completely abrogated in E. faecalis deficient for lipoproteins (Δlgt score: 2.3±2.3; p<0.0001). Consistently both E. faecalis mutants showed significantly impaired virulence in Galleria mellonella and Caenorhabditis elegans. Loss of E-cadherin in the epithelium was shown for all bacterial strains in inflamed IL-10-/- but not wild type mice. Inactivation of epaB in E. faecalis reduced microcolony and biofilm formation in vitro, altered bacterial adhesion to intestinal epithelium of germ-free Manduca sexta larvae and impaired penetration into the colonic mucus layer of IL-10-/- mice. Lipoprotein-deficient E. faecalis exhibited an impaired TLR2-mediated activation of BMDCs in vitro despite their ability to fully reactivate MLN cells as well as MLN-derived colitogenic T cells ex vivo. E. faecalis virulence factors accounting for bacterial adhesion to mucosal surfaces as well as intestinal barrier disruption partially contribute to colitogenic activity of E. faecalis. Beyond their well-known role in infections, cell surface-associated lipoproteins are essential structures for colitogenic activity of E. faecalis by mediating innate immune cell activation.

Enterococcus faecalis is a commensal of the human intestinal core microbiota harboring several putative virulence factors, which highlight its role as opportunistic pathogen. This dualistic character is supported by recent evidence linking Enterococcus spp. to the pathogenesis of inflammatory bowel diseases (IBD). Although several studies suggest a crucial role for opportunistic pathogens in IBD pathogenesis targeting genetically susceptible individuals, the dynamic relationship between disease-relevant host compartments and specific bacterial structures able to trigger intestinal inflammation remain unclear. Here, we report that cell surface-associated lipoproteins and the enterococcal polysaccharide antigen, which are relevant for E. faecalis virulence in invertebrate infection models, but whose expression is minimally affected by the intestinal inflammatory milieu, exhibit colitogenic activity in a mouse model susceptible for chronic colitis. Bacterial lipoproteins trigger innate immune cell activation and are a critical prerequisite for E. faecalis-induced colitis. The enterococcal polysaccharide antigen mediates bacterial mucus penetration and adhesion to mucosal surfaces, promotes the formation of biofilm and contributes to E. faecalis colitogenic activity. Using E. faecalis as a model organism, we demonstrate that colitogenic activity of opportunistic pathogens can be assigned to specific bacterial structures, a finding that may help to identify the most essential steps in IBD-related microbe-host interactions.

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

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

Ultrastructural and functional analysis of secretory goblet cells in the midgut of the lepidopteran Anticarsia gemmatalis

Defoliation caused by Anticarsia gemmatalis larvae affects the commercial production of the soybean. Although regulation of the digestion of soybean components has become part of the suggested strategy to overcome problems caused by Anticarsia larvae, few studies have focused on the morphological and cellular aspects of Anticarsia intestinal tissue. We have therefore further analyzed the morphology and ultrastructure of the midgut of 5th instar larvae of A. gemmatalis. Dissected midgut was subjected to chemical or cryo-fixation and then to several descriptive and analytical techniques associated with both light and electron microscopy in order to correlate anatomical and physiological aspects of this organ. Histological analysis revealed typical anatomy composed of a cell layer limited by a peritrophic membrane. The identified lepidoptera-specific goblet cells were shown to contain several mitochondria inside microvilli of the goblet cell cavity and a vacuolar H(+)-ATPase possibly coupled to a K(+)-pumping system. Columnar cells were present and exhibited microvilli dispersed along the apical region that also presented secretory characteristics. We additionally found evidence for the secretion of polyphosphate (PolyP) into the midgut, a result corroborating previous reports suggesting an excretion route from the goblet cell cavity toward the luminal space. Thus, our results suggest that the Anticarsia midgut not only possesses several typical lepidopteran features but also presents some unique aspects such as the presence of a tubular network and PolyP-containing apocrine secretions, plus an apparent route for the release of cellular debris by the goblet cells.

Midgut proteins released by microapocrine secretion in Spodoptera frugiperda

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

Stem cells and lineages of the intestine: a developmental and evolutionary perspective

The intestine consists of epithelial cells that secrete digestive enzymes and mucus (gland cells), absorb food particles (enterocytes), and produce hormones (endocrine cells). Intestinal cells are rapidly turned over and need to be replaced. In cnidarians, mitosis of differentiated intestinal cells accounts for much of the replacement; in addition, migratory, multipotent stem cells (interstitial cells) contribute to the production of intestinal cells. In other phyla, intestinal cell replacement is solely the function of stem cells entering the gut from the outside (such as in case of the neoblasts of platyhelminths) or intestinal stem cells located within the midgut epithelium (as in both vertebrates or arthropods). We will attempt in the following to review important aspects of midgut stem cells in different animal groups: where are they located, what types of lineages do they produce, and how do they develop. We will start out with a comparative survey of midgut cell types found across the animal kingdom; then briefly look at the specification of these cells during embryonic development; and finally focus on the stem cells that regenerate midgut cells during adult life. In a number of model systems, including mouse, zebrafish and Drosophila, the molecular pathways controlling intestinal stem cells proliferation and the specification of intestinal cell types are under intensive investigation. We will highlight findings of the recent literature, focusing on aspects that are shared between the different models and that point at evolutionary ancient mechanisms of intestinal cell formation.

Histopathology and the lethal effect of Cry proteins and strains of Bacillus thuringiensis Berliner in Spodoptera frugiperda J.E. Smith Caterpillars (Lepidoptera, Noctuidae)

Among the phytophagous insects which attack crops, the fall armyworm, Spodoptera frugiperda (J.E. Smith, 1797) (Lepidoptera, Noctuidae) is particularly harmful in the initial growth phase of rice plants. As a potential means of controlling this pest, and considering that the entomopathogen Bacillus thuringiensis Berliner demonstrates toxicity due to synthesis of the Cry protein, the present study was undertaken to evaluate this toxic effect of B. thuringiensis thuringiensis 407 (pH 408) and B. thuringiensis kurstaki HD-73 on S. frugiperda. The following method was used. Both bacterial strains were evaluated in vitro in 1st instar S. frugiperda caterpillars, by means of histopathological assays. The Cry1Ab and Cry1Ac proteins, codified by the respective strains of B. thuringiensis, were evaluated in vivo by bioassays of 1st instar S. frugiperda caterpillars in order to determine the Mean Lethal Concentration (LC50). The results of the histopathological analysis of the midget of S. frugiperda caterpillars demonstrate that treatment with the B. thuringiensis thuringiensis strain was more efficient, because the degradations of the microvilosities started 9 hours after treatment application (HAT), while in the B. thuringiensis kurstaki the same effect was noticed only after 12 HAT. Toxicity data of the Cry1Ab and Cry1Ac proteins presented for the target-species LC50 levels of 9.29 and 1.79 μg.cm-2 respectively. The strains and proteins synthesised by B. thuringiensis thuringiensis and B. thuringiensis kurstaki are effective in controlling S. frugiperda, and may be used to produce new biopesticides or the genes may be utilised in the genetic transformation of Oryza sativa L.

The effect of azadirachtin on the midgut histology of the locusts, Schistocerca gregaria and Locusta migratoria

The effects of azadirachtin on midgut cells of adult Schistocerca gregaria and Locusta migratoria were investigated by both light and electron microscopy. Pathological effects were seen in both species which could be related to dose and time after treatment. The midgut cells showed a slow necrosis, a rounding up of the cell and swelling of organelles; the nidi were reduced in size and number, the connective tissue beneath the epidermal layer became enlarged with many invading cells/nuclei and the circular and longitudinal muscles become rounded and swollen. The pathological effects were closely linked to a loss of feeding, with injections of 5, 10 and 15 microg/g azadirachtin causing an increasingly rapid onset of the effects associated with an increasingly reduced food intake. At the highest dose, full histopathological effects were seen at 8 hr and death occurred after 3 days. At 1 microg/g, however, the insects showed midgut disruption and reduced feeding by day 3 post-treatment but both feeding and gut pathology had recovered by day 7. At all times, histopathological effects were different from those in starved insects.

Sequencing of Spodoptera frugiperda midgut trehalases and demonstration of secretion of soluble trehalase by midgut columnar cells

Both soluble (SfTre1) and membrane-bound (SfTre2) trehalases occur along the midgut of Spodoptera frugiperda larvae. Released SfTre2 was purified as a 67 kDa protein. Its K(m) (1.6 mM) and thermal stability (half life 10 min at 62 degrees C) are different from the previously isolated soluble trehalase (K(m)= 0.47 mM; 100% stable at 62 degrees C). Two cDNAs coding for S. frugiperda trehalases have been cloned using primers based on consensus sequences of trehalases and having as templates a cDNA library prepared from total polyA-containing RNA extracted from midguts. One cDNA codes for a trehalase that has a predicted transmembrane sequence and was defined as SfTre2. The other, after being cloned and expressed, results in a recombinant trehalase with a K(m) value and thermal stability like those of native soluble trehalase. This enzyme was defined as SfTre1 and, after it was used to generate antibodies, it was immunolocalized at the secretory vesicles and at the glycocalyx of columnar cells. Escherichia coli trehalase 3D structure and sequence alignment with SfTre1 support a proposal regarding the residue modulating the pKa value of the proton donor.

Epithelial ultrastructure and cellular mechanisms of acid and base transport in the Drosophila midgut

There is a resurgence of interest in the Drosophila midgut on account of its potential value in understanding the structure, development and function of digestive organs and related epithelia. The recent identification of regenerative or stem cells in the adult gut of Drosophila has opened up new avenues for understanding development and turnover of cells in insect and mammalian gastrointestinal tracts. Conversely, the physiology of the Drosophila gut is less well understood as it is a difficult epithelial preparation to study under controlled conditions. Recent progress in microperfusion of individual segments of the Drosophila midgut, in both larval and adult forms, has enabled ultrastructural and electrophysiological study and preliminary characterization of cellular transport processes in the epithelium. As larvae are more active feeders, the transport rates are higher than in adults. The larval midgut has at least three segments: an anterior neutral zone, a short and narrow acid-secreting middle segment and a long and wider posterior segment (which is the best studied) that secretes base (probably HCO(3)(-)) into the lumen. The posterior midgut has a lumen-negative transepithelial potential (35-45 mV) and a high resistance (800-1400 Omega.cm(2)) that correlates with little or no lateral intercellular volume. The primary transport system driving base secretion into the lumen appears to be a bafilomycin-A(1)-sensitive, electrogenic H(+) V-ATPase located on the basal membrane, which extrudes acid into the haemolymph, as inferred from the extracellular pH gradients detected adjacent to the basal membrane. The adult midgut is also segmented (as inferred from longitudinal gradients of pH dye-indicators in the lumen) into anterior, middle and posterior regions. The anterior segment is probably absorptive. The middle midgut secretes acid (pH<4.0), a process dependent on a carbonic-anhydrase-catalysed H(+) pool. Cells of the middle segment are alternately absorptive (apically amplified by approximately 9-fold, basally amplified by >90-fold) and secretory (apically amplified by >90-fold and basally by approximately 10-fold). Posterior segment cells have an extensively dilated basal extracellular labyrinth, with a volume larger than that of anterior segment cells, indicating more fluid reabsorption in the posterior segment. The luminal pH of anterior and posterior adult midgut is 7-9. These findings in the larval and adult midgut open up the possibility of determining the role of plasma membrane transporters and channels involved in driving not only H(+) fluxes but also secondary fluxes of other solutes and water in Drosophila.

Identification and expression profile of a putative basement membrane protein gene in the midgut of Helicoverpa armigera

BACKGROUND

The midgut undergoes histolysis and remodeling during the larval to adult transition in holometabolous insects, but the molecular mechanisms underlying this process are not well understood.

RESULTS

Using Suppression Subtractive Hybridization (SSH), we identified a 531 bp cDNA predicted to encode a 176 amino acid protein, which we call hmg176. Northern and western blot analysis suggested that high levels of hmg176 are expressed in the midgut during molting, but not during metamorphosis. HMG176 protein was detected by immunofluorescence within the membrane of fat bodies and the basement membrane of the midgut of both molting and feeding larvae, but not in metamorphically committed larvae. In situ hybridization revealed that hmg176 transcripts mainly localized to the columnar cells of the midgut. Interestingly, a non-steroidal ecdysone agonist, RH-2485, significantly upregulated expression of hmg176.

CONCLUSION

These observations suggest that hmg176 encodes a larval-specific protein that may participate in sustaining larval midgut during larval development, possibly in response to ecdysteroid in vivo. This study will enlighten our understanding of the molecular mechanisms of tissue histolysis during metamorphosis.

Calcium-activated potassium channel of the tobacco hornworm, Manduca sexta: molecular characterization and expression analysis

Large-conductance calcium- and voltage-gated potassium channels (BK or Slowpoke) serve as dynamic integrators linking electrical signaling and intracellular activity. These channels can mediate many different Ca2+-dependent physiological processes including the regulation of neuronal and neuroendocrine cell excitability and muscle contraction. To gain insights into the function of BK channels in vivo, we isolated a full-length cDNA encoding the alpha subunit of a Slowpoke channel from the tobacco hornworm, Manduca sexta (msslo). Amino acid sequence comparison of the deduced Manduca protein revealed at least 80% identity to the insect Slo channels. The five C-terminal alternative splice regions are conserved, but the cloned cDNA fragments contained some unique combinations of exons E, G and I. Our spatial profile revealed that transcript levels were highest in skeletal muscle when compared with the central nervous system (CNS) and visceral muscle. The temporal profile suggested that msslo expression is regulated developmentally in a tissue- and regional-specific pattern. The levels of msslo transcripts remain relatively constant throughout metamorphosis in the CNS, transiently decline in the heart and are barely detectable in the gut except in adults. A dramatic upregulation of msslo transcript levels occurs in thoracic but not abdominal dorsal longitudinal body wall muscles (DLM), suggesting that the msSlo current plays an important role in the excitation or contractile properties of the phasic flight muscle. Our developmental profile of msslo expression suggests that msSlo currents may contribute to the changes in neural circuits and muscle properties that produce stage-specific functions and behaviors.

Plasma membranes from insect midgut cells

Plasma membranes from insect midgut cells are separated into apical and basolateral domains. The apical domain is usually modified into microvilli with a molecular structure similar to other animals. Nevertheless, the microvillar structure should differ in some insects to permit the traffic inside them of secretory vesicles that may budd laterally or pinch-off from the tips of microvilli. Other microvillar modifications are associated with proton-pumping or with the interplay with an ensheathing lipid membrane (the perimicrovilllar membrane) observed in the midgut cells of hemipterans (aphids and bugs). The perimicrovillar membranes are thought to be involved in amino acid absorption from diluted diets. The microvillar and perimicrovillar membranes have densities (and protein content) that depend on the insect taxon. The role played by the microvillar and perimicrovillar proteins in insect midgut physiology is reviewed here trying to provide a coherent picture of data and highlighting further research areas.

Small RNA Viruses of Insects: Expression in Plants and RNA Silencing

Interest in insect small RNA viruses (SRVs) has grown slowly but steadily. A number of new viruses have been analyzed at the sequence level, adding to our knowledge of their diversity at the level of both individual virus species and families. In particular, a number of possible new virus families have emerged. This research has largely been driven by interest in their potential for pest control, as well as in their importance as the causal agents of disease in beneficial arthropods. At the same time, research into known viruses has made valuable contributions to our understanding of an emerging new field of central importance to molecular biology-the existence of RNA-based gene silencing, developmental control, and adaptive immune systems in eukaryotes. Subject to RNA-based adaptive immune responses in their hosts, viruses have evolved a variety of genes encoding proteins capable of suppressing the immune response. Such genes were first identified in plant viruses, but the first examples known from animal viruses were identified in insect RNA viruses. This chapter will address the diversity of insect SRVs, and attempts to harness their simplicity in the engineering of transgenic plants expressing viruses for resistance to insect pests. We also describe RNA interference and antiviral pathways identified in plants and animals, how they have led viruses to evolve genes capable of suppressing such adaptive immunity, and the problems presented by these pathways for the strategy of expressing viruses in transgenic plants. Approaches for countering these problems are also discussed.

Electron microscopic study of the anterior midgut in Cenocorixa bifida Hung. (Hemiptera: Corixidae) with reference to its secretory function

The epithelium of anterior midgut of adult Cenocorixa bifida was examined with light and electron microscopy. The folded epithelium is composed of tall columnar cells extending to the lumen, differentiating dark and light cells with interdigitating apices and regenerative basal cells in the nidi surrounded by villiform ridges that penetrate deeply into the epithelium. The columnar cells display microvilli at their luminal surface. Microvilli lined intercellular spaces and basal plasma membrane infoldings are associated with mitochondria. These ultrastructural features suggest their role in absorption of electrolytes and nutrients from the midgut lumen. The columnar cells contain large oval nuclei with prominent nucleoli. Their cytoplasm is rich in rough endoplasmic reticulum, Golgi complexes and electron-dense secretory granules indicating that they are also engaged in synthesis of digestive enzymes. The presence of secretory granules in close proximity of the apical plasma membrane suggests the release of secretion is by exocytosis. The presence of degenerating cells containing secretory granules at the luminal surface and the occurance of empty vesicles and cell fragments in the lumen are consistent with the holocrine secretion of digestive enzymes. Apical extrusions of columnar cells filled with fine granular material are most likely formed in response to the lack of food in the midgut. The presence of laminated concretions in the cytoplasm is indicative of storageexcretion of surplus minerals. The peritrophic membrane is absent from the midgut of C. bifida.

Regulation of chitin synthesis in the larval midgut of Manduca sexta

In insects, chitin is not only synthesized by ectodermal cells that form chitinous cuticles, but also by endodermal cells of the midgut that secrete a chitinous peritrophic matrix. Using anti-chitin synthase (CHS) antibodies, we previously demonstrated that in the midgut of Manduca sexta, CHS is expressed by two cell types, tracheal cells forming a basal tracheal network and columnar cells forming the apical brush border [Zimoch and Merzendorfer, 2002, Cell Tissue Res. 308, 287-297]. Now, we show that two different genes, MsCHS1 and MsCHS2, encode CHSs of midgut tracheae and columnar cells, respectively. To investigate MsCHS2 expression and activity in the course of the larval development, we monitored chitin synthesis, enzyme levels as well as mRNA amounts. All of the tested parameters were significantly reduced during molting and in the wandering stage when compared to the values obtained from intermolt feeding larvae. By contrast, MsCHS1 appeared to be inversely regulated because its mRNA was detectable only during the molt at the time when tracheal growth occurs at the basal site of the midgut. To further examine midgut chitin synthesis, we measured enzyme activity in crude midgut extracts and different membrane fractions. When we analysed trypsin-mediated proteolytic activation, a phenomenon previously reported for insect and fungal systems, we recognized that midgut chitin synthesis was only activated in crude extracts, but not in the 12,000 g membrane fraction. However, proteolytic activation by trypsin in the 12,000 g membrane fraction could be reconstituted by re-adding a soluble fraction, indicating that limited proteolysis affects an unknown soluble factor, a process that in turn activates chitin synthesis.

Comparison of the localization of Bacillus thuringiensis Cry1A δ-endotoxins and their binding proteins in larval midgut of tobacco hornworm, Manduca sexta

Tobacco hornworm, Manduca sexta, is a model insect for studying the action of Bacillus thuringiensis (Bt) Cry toxins on lepidopterans. The proteins, which bind Bt toxins to midgut epithelial cells, are key factors involved in the insecticidal functions of the toxins. Three Cry1A-binding proteins, viz., aminopeptidase N (APN), the cadherin-like Bt-R1, and membrane-type alkaline phosphatase (m-ALP), were localized, by immunohistochemistry, in sections from the anterior, middle, and posterior regions of the midgut from second instar M. sexta larvae. Both APN and m-ALP were distributed predominantly along microvilli in the posterior region and to a lesser extent on the apical tip of microvilli in the anterior and middle regions. Bt-R1 was localized at the base of microvilli in the anterior region, over the entire microvilli in the middle region, and at both the apex and base of microvilli in the posterior region. The localization of rhodamine-labeled Cry1Aa, Cry1Ab, and Cry1Ac binding was determined on sections from the same midgut regions. Cry1Aa and Cry1Ab bound to the apical tip of microvilli almost equally in all midgut regions. Binding of Cry1Ac was much stronger in the posterior region than in the anterior and middle regions. Thus, binding sites for Bt proteins and Cry1A toxins are co-localized on the microvilli of M. sexta midgut epithelial cells.

Chitin metabolism in insects: structure, function and regulation of chitin synthases and chitinases

Chitin is one of the most important biopolymers in nature. It is mainly produced by fungi, arthropods and nematodes. In insects, it functions as scaffold material, supporting the cuticles of the epidermis and trachea as well as the peritrophic matrices lining the gut epithelium. Insect growth and morphogenesis are strictly dependent on the capability to remodel chitin-containing structures. For this purpose, insects repeatedly produce chitin synthases and chitinolytic enzymes in different tissues. Coordination of chitin synthesis and its degradation requires strict control of the participating enzymes during development. In this review, we will summarize recent advances in understanding chitin synthesis and its degradation in insects.

A novel role for subunit C in mediating binding of the H+-V-ATPase to the actin cytoskeleton

Primary proton transport by V-ATPases is regulated via the reversible dissociation of the V(1)V(0) holoenzyme into its V(1) and V(0) subcomplexes. Laser scanning microscopy of different tissues from the tobacco hornworm revealed co-localization of the holoenzyme and F-actin close to the apical membranes of the epithelial cells. In midgut goblet cells, no co-localization was observed under conditions where the V(1) complex detaches from the apical membrane. Binding studies, however, demonstrated that both the V(1) complex and the holoenzyme interact with F-actin, the latter with an apparently higher affinity. To identify F-actin binding subunits, we performed overlay blots that revealed two V(1) subunits as binding partners, namely subunit B, resembling the situation in the osteoclast V-ATPase (Holliday, L. S., Lu, M., Lee, B. S., Nelson, R. D., Solivan, S., Zhang, L., and Gluck, S. L. (2000) J. Biol. Chem. 275, 32331-32337), but, in addition, subunit C, which gets released during reversible dissociation of the holoenzyme. Overlay blots and co-pelleting assays showed that the recombinant subunit C also binds to F-actin. When the V(1) complex was reconstituted with recombinant subunit C, enhanced binding to F-actin was observed. Thus, subunit C may function as an anchor protein regulating the linkage between V-ATPase and the actin-based cytoskeleton.

Bacterial infection of a model insect: Photorhabdus luminescens and Manduca sexta

Invertebrates, including insects, are being developed as model systems for the study of bacterial virulence. However, we understand little of the interaction between bacteria and specific invertebrate tissues or the immune system. To establish an infection model for Photorhabdus, which is released directly into the insect blood system by its nematode symbiont, we document the number and location of recoverable bacteria found during infection of Manduca sexta. After injection into the insect larva, P. luminescens multiplies in both the midgut and haemolymph, only later colonizing the fat body and the remaining tissues of the cadaver. Bacteria persist by suppressing haemocyte-mediated phagocytosis and culture supernatants grown in vitro, as well as plasma from infected insects, suppress phagocytosis of P. luminescens. Using GFP-labelled bacteria, we show that colonization of the gut begins at the anterior of the midgut and proceeds posteriorly. Within the midgut, P. luminescens occupies a specific niche between the extracellular matrix and basal membrane (lamina) of the folded midgut epithelium. Here, the bacteria express the gut-active Toxin complex A (Tca) and an RTX-like metalloprotease PrtA. This close association of the bacteria with the gut, and the production of toxins and protease, triggers a massive programmed cell death of the midgut epithelium.

Infection of its lepidopteran host by the Helicoverpa armigera stunt virus (Tetraviridae)

Techniques of microscopy and histopathology were employed to study the positive-sense, single-stranded RNA virus, the Helicoverpa armigera stunt virus (HaSV; omegatetravirus, Tetraviridae) infecting its caterpillar host. Infection of the virus per os during the first three instars of larval development is virulent and leads to rapid stunting and mortality. In contrast, no detectable symptoms occur in later larval development, signifying a high degree of developmental resistance. A quantitative study of cell populations in the host midgut during this time showed that increased cell numbers during development alone could not account for the increase in resistance. HaSV infection was restricted to the midgut and three of its four cell types. In younger larvae, the virus initiated its infection in closely situated foci that appeared to expand to link with others to cover larger areas of the midgut. The midgut cells of the infected larvae responded with an increased rate of sloughing to an extent rendering the midgut incapable of maintenance or recovery of normal function. In contrast, infection of older larvae by HaSV did not lead to overt pathology although foci of HaSV infection were detected in their midguts. However, the foci were more sparsely situated, failed to expand, and eventually disappeared, presumably due to cell sloughing. These observations indicate that cell sloughing is an immune response existing throughout larval development but midguts of older larvae have an additional mechanism to account for the increased resistance. This second mechanism results in midgut cells becoming more refractory to infection and, combined with cell sloughing, allows the midguts of older larvae to recover more readily from HaSV infection. These two mechanisms are similar to those seen with host responses to baculoviruses, which display developmental resistance to a lesser degree against more general infections. HaSV remaining in the midgut appears to amplify the degree of developmental resistance.

Absorption and storage of phosphorus by larval Manduca sexta

The role of phosphorus (P) in numerous important biological structures, coupled with the observation that P-content of many insect foods is disproportionately low, suggests that P may be a critical nutrient for growing insects - however, the few studies examining the effects of dietary P on insect performance have generally found only weak relationships. This mismatch may be reconciled by understanding the physiological mechanisms by which insects handle P. Here we describe P processing by larvae of Manduca sexta. When given un-manipulated leaves of a common host plant, Datura wrightii, fifth-instar larvae retained about 85% of P consumed; when given P-enriched leaves larvae retained only 25% of P consumed. Analysis of gut concentrations of P at four sites along the digestive tract, and in leaves and feces, indicates that the rectum is the primary site of P transport between the gut and body and that differences in P retention may be accounted for by differential rates of rectal P transport. Larvae given P-enriched leaves also showed an eightfold increase in the concentration of P in the hemolymph, primarily as alpha-glycerophosphate - but only a 12% increase in the concentration of P in body tissues, suggesting that hemolymph plays a central role in storage and buffering of P.

A potential dual apical pathway in polarized regenerative cells of the midgut of Manduca sexta

Two novel proteins with apparent molecular weight of 38 (Manduca sexta midgut MsM38) and 46kDa (MsM46) were isolated from midgut homogenates in wandering stage Manduca sexta larvae and both of them were found to be present exclusively in this tissue on Western blots. Immunocytochemical studies revealed that both proteins are expressed in the regenerative cells however, their distribution pattern is clearly different. MsM38 is localized in the cytoplasm of resting regenerative cells during the feeding period, and is accumulated in the calcospherits at the beginning of the wandering period. Along with the delamination of the larval epithelium, this protein is released apically from these vesicles. The antiserum labels an additional 76 kDa protein in the wandering larval midgut homogenates. The appearance of this 76 kDa protein coincides with the accumulation of the immunopositive material in the calcospherits. MsM46 is similarly distributed during the feeding period in the cytoplasm of regenerative cells. At the beginning of the wandering period it accumulates around the newly forming large apical vacuoles, that are released at the time of complete delamination of the larval epithelium. In parallel with this process MsM46, and another 40 kDa protein, that becomes labeled from this period on Western blots appeares on the apical microvillar projections. Thus both isolated proteins are directed apically from different compartments, that raises the possibility of a dual apical routing pathway in regenerative cells.

Regeneration of cultured midgut cells after exposure to sublethal doses of toxin from two strains of Bacillus thuringiensis

Toxin from two strains of Bacillus thuringiensis (Bt), AA 1-9 and HD-73, caused dose-dependent destruction of cultured midgut cells from Heliothis virescens larvae. HD-73 toxin was more effective although, at the doses used, not all cells were killed. After 2 days of exposure to 0.8 pg/µl AA 1-9 or 0.06 pg/µl HD-73, columnar and goblet cell numbers declined to ca 20% of controls. In contrast, stem and differentiating cells increased to 140-200% of controls. The dynamic of depletion and replacement depended on toxin type and concentration. Two days after toxin was washed out, ratios of cell types returned to approximate control levels, suggesting rapid population corrections in vitro. Regulation of the ratio of cell types in each population, and the rate of proliferation and differentiation of stem cells was induced by the cultured midgut cells themselves. Controls and cells treated with toxin from Bt strain AA 1-9 were stained using a polyclonal antibody to Lepidopteran midgut differentiation factor 1 (MDF1). With Bt toxin, 1.5 times more cells stained for MDF1, suggesting increased synthesis of this differentiation factor during increased stem cell differentiation. The response of cultured midgut cells to Bt toxin injury is similar to injured vertebrate tissues dependent on stem cells for replacement and healing.

The role of stem cells in midgut growth and regeneration

The Manduca sexta (L.) [Lepidoptera: Sphingidae] and Heliothis virescens (F.) [Lepidoptera: Noctuidae] midguts consist of a pseudostratified epithelium surrounded by striated muscle and tracheae. This epithelium contains goblet, columnar, and basal stem cells. The stem cells are critically important in that they are capable of massive proliferation and differentiation. This growth results in a fourfold enlargement of the midgut at each larval molt. The stem cells are also responsible for limited cell replacement during repair. While the characteristics of the stem cell population vary over the course of an instar, stem cells collected early in an instar and those collected late can start in vitro cultures. Cultures of larval stem, goblet, and columnar cells survive in vitro for several mo through proliferation and differentiation of the stem cells. One of the two polypeptide differentiation factors which have been identified and characterized from the culture medium has now been shown to be present in midgut in vivo. Thus the ability to examine lepidopteran midgut stem cell growth in vitro and in vivo is proving to be effective in determining the basic features of stem cell action and regulation.

Apocrine secretion of amylase and exocytosis of trypsin along the midgut of Tenebrio molitor larvae

Amylase and trypsin were purified from Tenebrio molitor midgut larvae and used to raise antibodies in a rabbit. A Western blot of T. molitor midgut homogenates, after sodium dodecyl sulfate-polyacrylamide gel electrophoresis using amylase and trypsin antisera, showed only bands co-migrating with the purified enzymes. The antisera were used to localize the enzymes by immunofluorescence and immunogold labeling. Amylase occurs in a few regularly disposed anterior midgut cells. Non-amylase-secreting anterior midgut cells are proposed to be water-absorbing cells based on morphology and dye movements. Amylase is found inside vesicles originating from Golgi areas that seem to fuse together before their release along with the now disorganized apical cytoplasm (apocrine secretion). Trypsin precursors are observed inside small vesicles near the apical plasma membrane of posterior midgut cells, suggesting an exocytic mechanism of secretion, followed by putative trypsin activation. Apocrine secretion is thought to be an adaptation to enhance the dispersion of secretory vesicle contents released from a water-absorbing epithelium, whereas exocytosis is an efficient secretory mechanism in a water-secreting epithelium.

Substrate specificity of the brush border K+-leucine symport of Bombyx mori larval midgut

L-leucine uptake into membrane vesicles from Bombyx mori larval midgut was tested for inhibition by 55 compounds, which included sugars, N-methylated, alpha-, beta-, gamma-, delta-, epsilon-amino acids, primary amines, alpha-amino alcohols, monocarboxylic organic acids and alpha-ketoacids. Based on cis-inhibition experiments performed at the high pH (10.8) characteristic of the midgut luminal content in vivo, we find that the carrier binding site interacts with molecules which possess a well-defined set of structural features. Amino acids are preferentially accepted as anions and the ideal inhibitor must have an hydrophobic region and a polar head constituted by a chiral carbon atom bearing two hydrophilic groups, a deprotonated amino-group and a dissociated carboxylic group. Binding is reduced if one of the two hydrophilic groups is removed. Lowering the pH to less alkaline value (8.8) only affects the affinity of delta- and epsilon-amino acids, which are excluded from binding because of their positively charged side-chain. Modifications of the potassium electrochemical gradient increased the affinity constant values of the molecules, but have little effect on the rank of specificity. Physiological implications of the data reported are discussed.

In vitro secretion of digestive phospholipase A(2) by midguts isolated from tobacco hornworms, manduca sexta

We report on secretion of phospholipase A(2) (PLA(2)) by in vitro preparations of midguts isolated from tobacco hornworms, Manduca sexta. This enzyme is responsible for hydrolysis of fatty acids from the sn-2 position of phospholipids, a necessary step in fatty acid absorption. The in vitro midgut preparations are competent to secrete PLA(2) into incubation buffer. Secretion began within the first 30 min of incubation and increased to a maximum at 8 h. We selected 2 h incubations because substantial loss of tissue integrity was observed after 8 h incubations. Using 2 h incubations, we recorded increased secretion of digestive PLA(2) from midguts incubated in buffer amended with diet or with yeast as a component of the diet. We also recorded small increases in secretion of PLA(2) from midguts incubated in buffer amended with a specific phospholipid, phosphatidylcholine. Midguts incubated in buffer amended with increased concentrations of phospholipid did not yield higher levels of PLA(2) activity. Lepidopteran midguts can be divided into three regions, and we recorded the highest secretion of PLA(2) from the middle region and lowest secretion from the anterior region. Because isolated midguts responded to food chemicals with increased secretion of digestive PLA(2), we suggest that secretion of digestive enzymes in tobacco hornworms is regulated by a prandial and/or paracrine mechanism, as suggested for digestive proteases in other insect species. Arch. Insect Biochem. Physiol. 42:179-187, 1999.Copyright 1999 Wiley-Liss, Inc.

Antibody to H(+) V-ATPase subunit E colocalizes with portasomes in alkaline larval midgut of a freshwater mosquito (Aedes aegypti)

The pH profile, gross structure, ultrastructure and immunolabeling of the mosquito (Aedes aegypti) larval midgut are described as a first step in analyzing the role of plasma membrane H(+)V-ATPase in the alkalization of the gut, nutrient uptake and ionic regulation. Binding of an antibody to H(+)V-ATPase subunit E colocalizes with ‘portasomes’ (approximately 10 nm in diameter), which are thought to correspond to the V(1) part of the H(+) V-ATPase. In gastric caeca (pH 8), both antibody-binding sites and portasomes are located apically; in the anterior midgut (pH 10–11), they are located basally; and in the posterior midgut (pH approximately equal to 8) they are again located apically. The hypothesis that the energization of alkalization is mediated by an H(+) V-ATPase is supported by the inability of larvae to maintain the high pH after 72 h in 10 (micro)M bafilomycin B1. Confirming earlier reports, the two principal epithelial cell types are designated as ‘columnar’ and ‘cuboidal’ cells. The apical plasma membranes (microvilli) of epithelial cells in the gastric caeca and basal infoldings of anterior midgut are invaded by mitochondria that lie within approximately 20 nm of the portasome-studded plasma membranes. The colocalization of V-ATPase-immunolabeling sites and portasomes to specific plasma membranes within so-called ‘mitochondria-rich’ cells of gastric caeca and anterior midgut suggests that midgut alkalization in mosquitoes is achieved by molecular mechanisms similar to those that have been described in caterpillars, even though the gross structure of the midgut and the localization of the V-ATPase are dissimilar in the two species. In caterpillars, the high alkalinity is thought to break down dietary tannins, which block nutrient absorption; it may play a similar role in plant-detritus-feeding mosquito larvae. The colocalization of immunolabeling sites and portasomes, together with the presence of long, ‘absorptive-type’ microvilli in the posterior midgut, suggest that the V-ATPase energizes nutrient uptake there.

Animal plasma membrane energization by proton-motive V-ATPases

Proton-translocating, vacuolar-type ATPases, well known energizers of eukaryotic, vacuolar membranes, now emerge as energizers of many plasma membranes. Just as Na(+) gradients, imposed by Na(+)/K(+) ATPases, energize basolateral plasma membranes of epithelia, so voltage gradients, imposed by H(+) V-ATPases, energize apical plasma membranes. The energized membranes acidify or alkalinize compartments, absorb or secrete ions and fluids, and underwrite cellular homeostasis. V-ATPases acidify extracellular spaces of single cells such as phagocytes and osteoclasts and of polarized epithelia, such as vertebrate kidney and epididymis. They alkalinize extracellular spaces of lepidopteran midgut. V-ATPases energize fluid secretion by insect Malpighian tubules and fluid absorption by insect oocytes. They hyperpolarize external plasma membranes for Na(+) uptake by amphibian skin and fish gills. Indeed, it is likely that ion uptake by osmotically active membranes of all fresh water organisms is energized by V-ATPases. Awareness of plasma membrane energization by V-ATPases provides new perspectives for basic science and presents new opportunities for medicine and agriculture.

Acid/base transport across the midgut of the tobacco hornworm, Manduca sexta

Larval Lepidoptera generate a large pH gradient across the midgut epithelium. The in vitro rate of luminal alkalinization (J(OH-)) and hemolymph acidification (J(H+)) under nominally CO(2)-free conditions was measured in the three morphologically distinct regions of the tobacco hornworm midgut. Under open-circuit conditions, the highest J(OH-) and J(H+) was observed in the anterior section and the lowest was in the middle section. In all three sections the J(H+) was equal to J(OH-) indicating transepithelial movement of acid or base equivalents. Furthermore, the rate at which the midgut transported acid or base was the same under open- and short-circuit conditions, indicating that acid/base transport is an active process. Although the inhibitors, acetazolamide and ethoxyzolamide, inhibited the activity of carbonic anhydrase in tissue homogenates, they had no effect on J(OH-), J(H+), or transepithelial potential. Therefore, under the nominally CO(2)-free conditions of this study, it is unlikely that hydration of CO(2) and the formation of HCO(3)(-) is involved in luminal alkalinization.

Vacuolar and plasma membrane proton-adenosinetriphosphatases

The vacuolar H+-ATPase (V-ATPase) is one of the most fundamental enzymes in nature. It functions in almost every eukaryotic cell and energizes a wide variety of organelles and membranes. V-ATPases have similar structure and mechanism of action with F-ATPase and several of their subunits evolved from common ancestors. In eukaryotic cells, F-ATPases are confined to the semi-autonomous organelles, chloroplasts, and mitochondria, which contain their own genes that encode some of the F-ATPase subunits. In contrast to F-ATPases, whose primary function in eukaryotic cells is to form ATP at the expense of the proton-motive force (pmf), V-ATPases function exclusively as ATP-dependent proton pumps. The pmf generated by V-ATPases in organelles and membranes of eukaryotic cells is utilized as a driving force for numerous secondary transport processes. The mechanistic and structural relations between the two enzymes prompted us to suggest similar functional units in V-ATPase as was proposed to F-ATPase and to assign some of the V-ATPase subunit to one of four parts of a mechanochemical machine: a catalytic unit, a shaft, a hook, and a proton turbine. It was the yeast genetics that allowed the identification of special properties of individual subunits and the discovery of factors that are involved in the enzyme biogenesis and assembly. The V-ATPases play a major role as energizers of animal plasma membranes, especially apical plasma membranes of epithelial cells. This role was first recognized in plasma membranes of lepidopteran midgut and vertebrate kidney. The list of animals with plasma membranes that are energized by V-ATPases now includes members of most, if not all, animal phyla. This includes the classical Na+ absorption by frog skin, male fertility through acidification of the sperm acrosome and the male reproductive tract, bone resorption by mammalian osteoclasts, and regulation of eye pressure. V-ATPase may function in Na+ uptake by trout gills and energizes water secretion by contractile vacuoles in Dictyostelium. V-ATPase was first detected in organelles connected with the vacuolar system. It is the main if not the only primary energy source for numerous transport systems in these organelles. The driving force for the accumulation of neurotransmitters into synaptic vesicles is pmf generated by V-ATPase. The acidification of lysosomes, which are required for the proper function of most of their enzymes, is provided by V-ATPase. The enzyme is also vital for the proper function of endosomes and the Golgi apparatus. In contrast to yeast vacuoles that maintain an internal pH of approximately 5.5, it is believed that the vacuoles of lemon fruit may have a pH as low as 2. Similarly, some brown and red alga maintain internal pH as low as 0.1 in their vacuoles. One of the outstanding questions in the field is how such a conserved enzyme as the V-ATPase can fulfill such diverse functions.

Nature of the anchors of membrane-bound aminopeptidase, amylase, and trypsin and secretory mechanisms in Spodoptera frugiperda (Lepidoptera) midgut cells

Spodoptera frugiperda larvae have a microvillar aminopeptidase and both soluble and membrane-bound forms of amylase and trypsin. Membrane-bound aminopeptidase is solubilized by glycosyl phosphatidylinositol-specific phospholipase C (GPI-PLC) and detergents, suggesting it has a GPI anchor. Membrane-bound trypsin is not affected by GPI-PLC, although it is solubilized by papain and by different detergents. Membrane-bound amylase is similar to trypsin, although once solubilized in detergent it behaves as a hydrophilic protein. Musca domestica trypsin antiserum cross-reacts with only one polypeptide from S. frugiperda midgut. With this antiserum, trypsin was immunolocalized in the anterior midgut cells at the microvillar surface and on the membranes of secretory vesicles found in the apical cytoplasm and inside the microvilli. The data suggest that in this region trypsin is bound to the secretory vesicle membrane by a hydrophobic anchor. Vesicles migrate through the microvilli and are discharged into the lumen by a pinching-off process. Trypsin is then partly processed to a soluble form and partly, still bound to vesicle membranes, incorporated into the peritrophic membrane. In posterior midgut cells, trypsin immunolabelling is randomly distributed inside the secretory vesicles and at the microvilli surface, suggesting exocytosis. Amylase probably follows a route similar to that described for trypsin in anterior midgut, although membrane-bound forms (peptide anchor) solubilize apparently as a consequence of a pH increase inside the vesicles.