The insect plasma membrane H+ V-ATPase: intra-, inter-, and supramolecular aspects

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.

Comprehensive Assessment of Reference Gene Expression within the Whitefly Dialeurodes citri Using RT-qPCR

The citrus whitefly, Dialeurodes citri, is a destructive pest that infests citrus plants. It is a major vector in transmitting plant viruses such as citrus yellow vein clearing virus (CYVCV), which has caused severe economic losses worldwide, and therefore efficient control of this pest is economically important. However, the scope of genetic studies primarily focused on D. citri is restricted, something that has potentially limited further study of efficient control options. To explore the functionalities of D. citri target genes, screening for specific reference genes using RT-qPCR under different experimental conditions is essential for the furtherance of biological studies concerning D. citri. The eight candidate reference genes were evaluated by dedicated algorithms (geNorm, Normfinder, BestKeeper and ΔCt method) under five specific experimental conditions (developmental stage, sex, tissue, population and temperature). In addition, the RefFinder software, a comprehensive evaluation platform integrating all of the above algorithms, ranked the expression stability of eight candidate reference genes. The results showed that the best reference genes under different experimental settings were V-ATP-A and RPS18 at different developmental stages; α-tubulin, 18S and V-ATP-A in both sexes; EF1A and α-tubulin in different tissues; Actin and Argk under different populations; and RPS18 and RPL13 in different temperatures. The validation of selected reference genes was further identified using heat shock protein (Hsp) 70 as a reporter gene. Our study, for the first time, provides a detailed compilation of internal reference genes for D. citri that are suitable for RT-qPCR analysis, which is robust groundwork for comprehensive investigation of the functional target genes of D. citri.

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.

Clathrin-dependent endocytosis plays a critical role in larval and pupal development, and female oocyte production in the red flour beetle (Tribolium castaneum)

BACKGROUND

Clathrin-dependent endocytosis is a vesicular transport process by which cells take macromolecules from the extracellular space to the intracellular space. It plays important roles in various cellular functions, but its biological significance in insect development and reproduction has not been well studied.

RESULTS

We characterized and functionally analyzed four major clathrin-dependent endocytic pathway genes (TcChc, TcAP50, TcVhaSFD, TcRab7) in Tribolium castaneum. RNA interference (RNAi) by injecting double-stranded RNA (dsRNA) targeting each gene at three doses (50, 100, or 200 ng per insect) in 20-day-old larvae led to 100% larval mortality. When the expressions of TcChc, TcVhaSFD, and TcRab7 were suppressed by injecting their respective dsRNAs at each dose in 1-day-old pupae, the adults that emerged from the dsRNA-injected pupae were deformed, with the absence of wing development. The deformed adults died within 2 days after eclosion. When the expression of TcAP50 was suppressed by injecting its dsRNA into 1-day-old pupae, although no apparent deformed adults were observed, all the adults died within 35 days after eclosion. In addition, when the expressions of TcChc and TcVhaSFD were suppressed by injecting their respective dsRNAs at a reduced dose (10 ng per insect) in 5-day-old pupae, the ovarian development and oocyte production in the resultant females were completely inhibited.

CONCLUSION

Our results indicate that clathrin-dependent endocytosis is essential for insect development and reproduction. The results from this study can help researchers identify potential molecular targets for developing novel strategies for insect pest management. © 2023 Society of Chemical Industry.

V-ATPase subunit a is required for survival and midgut development of Locusta migratoria

The vacuolar-type H⁺ -ATPase (V-ATPase) is an ATP-dependent proton pump, which regulates various cellular processes. To date, most functional studies on V-ATPases of insects have focused on subunits of the V1 complex, and there is little information on the VO genes. In this study, two cDNA sequences of LmV-ATPase a were identified in Locusta migratoria. RT-qPCR analysis revealed that LmV-ATPase a1 and LmV-ATPase a2 are differentially expressed in various tissues and developmental stages. Injection of dsRNA for the common region of LmV-ATPase a1 and LmV-ATPase a2 into third-instar nymphs resulted in a significant suppression of LmV-ATPase a. The injected nymphs ceased feeding, lost body weight and finally died at a mortality of 98.6%. Furthermore, aberrations of midgut epithelial cells, the accumulation of electron-lucent vesicles in the cytoplasm, and a partially damaged brush border were observed in dsLmV-ATPase a-injected nymphs using transmission electron microscopy. Especially, the mRNA level of wingles, and notch genes were dramatically down-regulated in the dsLmV-ATPase a-injected nymphs. Taken together, our results suggest that LmV-ATPase a is required for survival and midgut development of L. migratoria. Hence, this gene could be a good target for RNAi-based control against locusts.

A spatiotemporal atlas of the lepidopteran pest Helicoverpa armigera midgut provides insights into nutrient processing and pH regulation

Background

Caterpillars from the insect order Lepidoptera are some of the most widespread and destructive agricultural pests. Most of their impact is at the larval stage, where the midgut epithelium mediates the digestion and absorption of an astonishing amount of food. Although this tissue has been the subject of frequent investigation in Lepidoptera, a comprehensive expression atlas has yet to be generated.

Results

Here, we perform RNA-sequencing and proteomics on the gut of the polyphagous pest Helicoverpa armigera across, life stages, diet types, and compartments of the anterior-posterior axis. A striking relationship between the structural homology and expression pattern of a group of sugar transporters was observed in the early larval stages. Further comparisons were made among the spatial compartments of the midgut, which suggested a putative role for vATPases and SLC9 transporters in the generation of alkaline conditions in the H. armigera midgut.

Conclusions

This comprehensive resource will aid the scientific community in understanding lepidopteran gut physiology in unprecedented resolution. It is hoped that this study advances the understanding of the lepidopteran midgut and also facilitates functional work in this field.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08274-x.

The Inhibitory Effect of Celangulin V on the ATP Hydrolytic Activity of the Complex of V-ATPase Subunits A and B in the Midgut of Mythimna separata

Celangulin V (CV) is a compound isolated from Celastrus angulatus Max that has a toxic activity against agricultural insect pests. CV can bind to subunits a, H, and B of the vacuolar ATPase (V-ATPase) in the midgut epithelial cells of insects. However, the mechanism of action of CV is still unclear. In this study, the soluble complex of the V-ATPase A subunit mutant TSCA which avoids the feedback inhibition by the hydrolysate ADP and V-ATPase B subunit were obtained and then purified using affinity chromatography. The H⁺K⁺-ATPase activity of the complex and the inhibitory activity of CV on ATP hydrolysis were determined. The results suggest that CV inhibits the ATP hydrolysis, resulting in an insecticidal effect. Additionally, the homology modeling of the AB complex and molecular docking results indicate that CV can competitively bind to the AB complex at the ATP binding site, which inhibits ATP hydrolysis. These findings suggest that the AB subunits complex is one of the potential targets for CV and is important for understanding the mechanism of interaction between CV and V-ATPase.

Comparative Proteomic Analysis of the Effect of Periplocoside P from Periploca sepium on Brush Border Membrane Vesicles in Midgut Epithelium of Mythimna separata Larvae

Periplocoside P (PSP), a novel compound isolated from Periploca sepium Bunge, possesses insecticidal activity against some lepidopterans, such as Mythimna separata. In M. separata, the brush border membrane vesicles of the midgut epithelium are the initial site of action of periplocosides. We conducted two-dimensional gel electrophoresis and matrix-assisted laser desorption/ionization time of flight/time of flight mass spectrometry analysis to analyze differentially expressed proteins (DEPs) from periplocoside P (PSP)-treated M. separata. We successfully isolated seven up-regulated and three down-regulated DEPs that have been previously identified, as well as a novel DEP. The DEPs are implicated in protein degradation, transporter, folding, and synthesis, and in juvenile hormone biosynthesis. DEPs involved in the oxidative phosphorylation energy metabolism pathway are enriched. Through real-time polymerase chain reaction assay, we confirmed that vma1 expression is significantly up-regulated expression levels in PSP-treated M. separata larvae. Enzymology validation further indicated that PSP can significantly inhibit V-type ATPase activity in a concentration-dependent manner. Given these results, we speculate that in M. separata, the V-type ATPase A subunit in the midgut epithelium is the putative target binding site of periplocosides. This finding provides preliminary evidence for the mode of action of periplocosides.

Binding and entry of a non-enveloped T=4 insect RNA virus is triggered by alkaline pH

Tetraviruses are small, non-enveloped, RNA viruses that exclusively infect lepidopteran insects. Their particles comprise 240 copies of a single capsid protein precursor (CP), which undergoes autoproteolytic cleavage during maturation. The molecular mechanisms of capsid assembly and maturation are well understood, but little is known about the viral infectious lifecycle due to a lack of tissue culture cell lines that are susceptible to tetravirus infection. We show here that binding and entry of the alphatetravirus, Helicoverpa armigera stunt virus (HaSV), is triggered by alkaline pH. At pH 9.0, wild-type HaSV virus particles undergo conformational changes that induce membrane-lytic activity and binding to Spodoptera frugiperda Sf9 cells. Binding is followed by entry and infection, with virus replication complexes detected by immunofluorescence microscopy within 2h post-infection and the CP after 12h. HaSV particles produced in S. frugiperda Sf9 cells are infectious. Helicoverpa armigera larval virus biofeed assays showed that pre-treatment with the V-ATPase inhibitor, Bafilomycin A1, resulted in a 50% decrease in larval mortality and stunting, while incubation of virus particles at pH 9.0 prior to infection restored infectivity. Together, these data show that HaSV, and likely other tetraviruses, requires the alkaline environment of the lepidopteran larval midgut for binding and entry into host cells.

Multifaceted biological insights from a draft genome sequence of the tobacco hornworm moth, Manduca sexta

Manduca sexta, known as the tobacco hornworm or Carolina sphinx moth, is a lepidopteran insect that is used extensively as a model system for research in insect biochemistry, physiology, neurobiology, development, and immunity. One important benefit of this species as an experimental model is its extremely large size, reaching more than 10 g in the larval stage. M. sexta larvae feed on solanaceous plants and thus must tolerate a substantial challenge from plant allelochemicals, including nicotine. We report the sequence and annotation of the M. sexta genome, and a survey of gene expression in various tissues and developmental stages. The Msex_1.0 genome assembly resulted in a total genome size of 419.4 Mbp. Repetitive sequences accounted for 25.8% of the assembled genome. The official gene set is comprised of 15,451 protein-coding genes, of which 2498 were manually curated. Extensive RNA-seq data from many tissues and developmental stages were used to improve gene models and for insights into gene expression patterns. Genome wide synteny analysis indicated a high level of macrosynteny in the Lepidoptera. Annotation and analyses were carried out for gene families involved in a wide spectrum of biological processes, including apoptosis, vacuole sorting, growth and development, structures of exoskeleton, egg shells, and muscle, vision, chemosensation, ion channels, signal transduction, neuropeptide signaling, neurotransmitter synthesis and transport, nicotine tolerance, lipid metabolism, and immunity. This genome sequence, annotation, and analysis provide an important new resource from a well-studied model insect species and will facilitate further biochemical and mechanistic experimental studies of many biological systems in insects.

De Novo Assembly of the Transcriptome for Oriental Armyworm Mythimna separata (Lepidoptera: Noctuidae) and Analysis on Insecticide Resistance-Related Genes

Mythimna separata walker (Lepidoptera: Noctuidae) is a polyphagous pest of nearly 100 families of more than 300 kinds of food and industrial crops. So far, both nucleotide and protein sequence information has been rarely available in database for M. separata, strictly limiting molecular biology research in this insect species. In this study, we carried out a transcriptome sequencing for M. separata The sequencing and subsequent bioinformatics analysis yielded 69,238 unigenes, among which 45,227 unigenes were annotated to corresponding functions by blasting with high homologous genes in database, giving annotation rate of 65.32%. Several lepidopteran insects gave best matches with the transcriptome data. To gain insight into the mechanism of insecticide resistance in M. separata, 15 families of genes encoding insecticide resistance-related proteins were investigated. Substantial numbers of unigenes in these families were identified in the transcriptome data, and 17 out of 21 selected unigenes were successfully amplified. Expressions of most of these genes were detected at larval stages and in gut tissue, as was consistent with their putative involvement in insecticide resistance. Our study provides most comprehensive transcriptome data for M. separata to date, and also provides reference sequence information for other Noctuidae family insects.

Separation of Binding Protein of Celangulin V from the Midgut of Mythimna separata Walker by Affinity Chromatography

Celangulin V, an insecticidal compound isolated from the root bark of Chinese bittersweet, can affect the digestive system of insects. However, the mechanism of how Celangulin V induces a series of symptoms is still unknown. In this study, affinity chromatography was conducted through coupling of Celangulin V-6-aminoacetic acid ester to the CNBr-activated Sepharose 4B. SDS-PAGE was used to analyze the collected fraction eluted by Celangulin V. Eight binding proteins (Zinc finger protein, Thioredoxin peroxidase (TPx), Glyceraldehyde 3-phosphate dehydrogenase (GAPDH), SUMO E3 ligase RanBP2, Transmembrane protein 1, Actin, APN and V-ATPase) were obtained and identified by LC/Q-TOF-MS from the midgut of Mythimna separata larvae. The potential of these proteins to serve as target proteins involved in the insecticidal activity of Celangulin V is discussed.

How insects overcome two-component plant chemical defence: plant β-glucosidases as the main target for herbivore adaptation

Insect herbivory is often restricted by glucosylated plant chemical defence compounds that are activated by plant β-glucosidases to release toxic aglucones upon plant tissue damage. Such two-component plant defences are widespread in the plant kingdom and examples of these classes of compounds are alkaloid, benzoxazinoid, cyanogenic and iridoid glucosides as well as glucosinolates and salicinoids. Conversely, many insects have evolved a diversity of counteradaptations to overcome this type of constitutive chemical defence. Here we discuss that such counter-adaptations occur at different time points, before and during feeding as well as during digestion, and at several levels such as the insects’ feeding behaviour, physiology and metabolism. Insect adaptations frequently circumvent or counteract the activity of the plant β-glucosidases, bioactivating enzymes that are a key element in the plant’s two-component chemical defence. These adaptations include host plant choice, non-disruptive feeding guilds and various physiological adaptations as well as metabolic enzymatic strategies of the insect’s digestive system. Furthermore, insect adaptations often act in combination, may exist in both generalists and specialists, and can act on different classes of defence compounds. We discuss how generalist and specialist insects appear to differ in their ability to use these different types of adaptations: in generalists, adaptations are often inducible, whereas in specialists they are often constitutive. Future studies are suggested to investigate in detail how insect adaptations act in combination to overcome plant chemical defences and to allow ecologically relevant conclusions.

Cloning, expression and purification of subunit H of vacuolar H⁺-ATPase from Mythimna separata Walker (Lepidoptera: Noctuidae)

The vacuolar (H+)-ATPase (V-ATPase) of insect, which is composed of membrane-bound V0 complex and peripheral V1 complex, participates in lots of important physiological process. Subunit H, as a subunit of V1 complex, plays a vital role in bridging the communication between V1 and V0 complexes and interaction with other proteins. Yeast subunit H has been successfully crystallized through expression in E. coli, but little is known about the structure of insect subunit H. In this study, we cloned, expressed and purified the subunit H from midgut of Mythimna separata Walker. Through RACE (rapidly amplification of cDNA ends) technique, we got 1807 bp full length of subunit H, and to keep the nature structure of subunit H, we constructed Baculovirus expression vector with His-tag in the C-terminal and expressed the recombinant protein in insect sf9 cells, thereafter, purified the recombinant protein by Ni-NTA columns. Results of SDS-PAGE, western blotting and mass spectrometry showed that the recombinant protein was successfully expressed. The method of expressing and purifying M. separata subunit H will provide a foundation for obtaining the crystal of subunit H and further study of the design of novel insecticides based on its structure and function.

Fascin confers resistance to Listeria infection in dendritic cells

Ag-presenting dendritic cells (DCs) must survive bacterial infection to present Ag information to naive T cells. The greater ability of DCs' host defense is evident from the report that DCs are more resistant to Listeria monocytogenes than macrophages. However, the molecular mechanism of this resistance is unclear. We found that Listeria replicate more slowly in wild-type DCs compared with fascin1 knockout DCs. This finding is significant because fascin1, an actin-bundling protein, is specifically and greatly induced upon maturation of dendritic cells, but not other blood cells, including macrophages and neutrophils. Infection by Listeria makes phagosomes more acidic in wild-type DCs than in fascin1 knockout DCs, suggesting that fascin1 facilitates phagolysosomal fusion for killing of phagocytosed Listeria. We further found that fascin1 binds to LC3, an autophagosome marker, both in vivo and in vitro. Listeria are associated with LC3 to a greater extent in wild-type DCs than in fascin1 knockout DCs, suggesting that fascin1 facilitates autophagy for eradication of cytoplasmic Listeria. Taken together, our results suggest that fascin1 plays critical roles in the survival of DCs during Listeria infection, allowing DCs to function in innate and adaptive immunity.

Metabolic restructuring during energy-limited states: insights from Artemia franciscana embryos and other animals

Many life history stages of animals that experience environmental insults enter developmental arrested states that are characterized by reduced cellular proliferation, with or without a concurrent reduction in overall metabolism. In the case of the most profound metabolic arrest reported in invertebrates, i.e., anaerobic quiescence in Artemia franciscana embryos, acidification of the intracellular milieu is a major factor governing catabolic and anabolic downregulation. Release of ions from intracellular compartments is the source for approximately 50% of the proton equivalents needed for the 1.5 unit acidification that is observed. Recovery from the metabolic arrest requires re-sequestration of the protons with a vacuolar-type ATPase (V-ATPase). The remarkable facet of this mechanism is the ability of embryonic cells to survive the dissipation of intracellular ion gradients. Across many diapause-like states, the metabolic reduction and subsequent matching of energy demand is accomplished by shifting energy metabolism from oxidative phosphorylation to aerobic glycolysis. Molecular pathways that are activated to induce these resilient hypometabolic states include stimulation of the AMP-activated protein kinase (AMPK) and insulin signaling via suite of daf (dauer formation) genes for diapause-like states in nematodes and insects. Contributing factors for other metabolically depressed states involve hypoxia-inducible factor-1 and downregulation of the pyruvate dehydrogenase complex. Metabolic similarities between natural states of stasis and some cancer phenotypes are noteworthy. Reduction of flux through oxidative phosphorylation helps prevent cell death in certain cancer types, similar to the way it increases viability of dauer stages in Caenorhabditis elegans. Mechanisms that underlie natural stasis are being used to pre-condition mammalian cells prior to cell biostabilization and storage.

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.

Chromatographic and electrophoretic resolution of proteins and protein complexes from the larval midgut microvilli of Manduca sexta

The microvillar proteome of Manduca sexta larval midguts was analyzed by subjecting brush border membrane vesicles (BBMV) to two different two-dimensional approaches: (i) Anion exchange chromatography followed by SDS-PAGE and (ii) Blue Native-PAGE followed by SDS-PAGE. The first technique was superior to conventional 2-D gel electrophoresis in resolving the most abundant proteins associated with the midgut microvilli. Twenty of them were successfully identified as digestive enzymes, binding targets of the insecticidal Cry1A toxins from Bacillus thuringiensis (Bt), and signal transduction proteins. A homolog of the chlorophyllide A binding protein from the silkworm and several aminopeptidases N represent the most abundant proteins associated with the BBMV. The second technique revealed protein oligomeric complexes associated with midgut microvilli in vivo. Two such complexes contained subunits of the vacuolar ATP synthase complex, and one was an oligomer of the chlorophyllide A binding protein. An additional complex consisted of homo- or hetero-tetramers of three different aminopeptidases N (APNs). As APNs are well-known binding partners of Cry1A toxins, their quaternary structure has implications for Bt toxin mode of action. Both techniques provide a useful complement to conventional 2-D gel electrophoresis in analyzing the complex proteome of the microvillar membrane fraction.

Signaling to the apical membrane and to the paracellular pathway: changes in the cytosolic proteome of Aedes Malpighian tubules

Using a proteomics approach, we examined the post-translational changes in cytosolic proteins when isolated Malpighian tubules of Aedes aegypti were stimulated for 1 min with the diuretic peptide aedeskinin-III (AK-III, 10(-7) mol l(-1)). The cytosols of control (C) and aedeskinin-treated (T) tubules were extracted from several thousand Malpighian tubules, subjected to 2-D electrophoresis and stained for total proteins and phosphoproteins. The comparison of C and T gels was performed by gel image analysis for the change of normalized spot volumes. Spots with volumes equal to or exceeding C/T ratios of +/-1.5 were robotically picked for in-gel digestion with trypsin and submitted for protein identification by nanoLC/MS/MS analysis. Identified proteins covered a wide range of biological activity. As kinin peptides are known to rapidly stimulate transepithelial secretion of electrolytes and water by Malpighian tubules, we focused on those proteins that might mediate the increase in transepithelial secretion. We found that AK-III reduces the cytosolic presence of subunits A and B of the V-type H(+) ATPase, endoplasmin, calreticulin, annexin, type II regulatory subunit of protein kinase A (PKA) and rab GDP dissociation inhibitor and increases the cytosolic presence of adducin, actin, Ca(2+)-binding protein regucalcin/SMP30 and actin-depolymerizing factor. Supporting the putative role of PKA in the AK-III-induced activation of the V-type H(+) ATPase is the effect of H89, an inhibitor of PKA, on fluid secretion. H89 reverses the stimulatory effect of AK-III on transepithelial fluid secretion in isolated Malpighian tubules. However, AK-III does not raise intracellular levels of cAMP, the usual activator of PKA, suggesting a cAMP-independent activation of PKA that removes subunits A and B from the cytoplasm in the assembly and activation of the V-type H(+) ATPase. Alternatively, protein kinase C could also mediate the activation of the proton pump. Ca(2+) remains the primary intracellular messenger of the aedeskinins that signals the remodeling of the paracellular complex apparently through protein kinase C, thereby increasing transepithelial anion secretion. The effects of AK-III on active transcellular and passive paracellular transport are additive, if not synergistic, to bring about the rapid diuresis.

An annotated catalogue of salivary gland transcripts in the adult female mosquito, Aedes aegypti

Background

Saliva of blood-sucking arthropods contains a cocktail of antihemostatic agents and immunomodulators that help blood feeding. Mosquitoes additionally feed on sugar meals and have specialized regions of their glands containing glycosidases and antimicrobials that might help control bacterial growth in the ingested meals. To expand our knowledge on the salivary cocktail of Ædes ægypti, a vector of dengue and yellow fevers, we analyzed a set of 4,232 expressed sequence tags from cDNA libraries of adult female mosquitoes.

Results

A nonredundant catalogue of 614 transcripts (573 of which are novel) is described, including 136 coding for proteins of a putative secretory nature. Additionally, a two-dimensional gel electrophoresis of salivary gland (SG) homogenates followed by tryptic digestion of selected protein bands and MS/MS analysis revealed the expression of 24 proteins. Analysis of tissue-specific transcription of a subset of these genes revealed at least 31 genes whose expression is specific or enriched in female SG, whereas 24 additional genes were expressed in female SG and in males but not in other female tissues. Most of the 55 proteins coded by these SG transcripts have no known function and represent high-priority candidates for expression and functional analysis as antihemostatic or antimicrobial agents. An unexpected finding is the occurrence of four protein families specific to SG that were probably a product of horizontal transfer from prokaryotic organisms to mosquitoes.

Conclusion

Overall, this paper contributes to the novel identification of 573 new transcripts, or near 3% of the Æ. ægypti proteome assuming a 20,000-protein set, and to the best-described sialome of any blood-feeding insect.

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.

The paracellular pathway in the lepidopteran larval midgut: modulation by intracellular mediators

The features of the paracellular pathway, an important route for the transfer of ions and molecules in epithelia, are in insects still poorly investigated and it has not yet been elucidated how the septate junction (SJ) acts as a transepithelial barrier. In this study, some properties of the paracellular pathway of Bombyx mori larval midgut, isolated in Ussing chambers, were determined and the modulation of SJ permeability by intracellular events disclosed. Diffusion potentials evoked by transepithelial gradients of different salts indicated that the junction bore weak negative charges and that the paracellular pathway was selective with respect to ion charge and size. In standard conditions, the transepithelial resistance was 28.2+/-2.1 Omega cm(2), a value indicating that the midgut is a low resistance epithelium. The modulation of midgut SJ by typical enhancers of mammalian tight junction permeability known to act on the cytoskeleton was studied by measuring the shunt resistance and the lumen-to-haemolymph flux of sucrose. An increase of the intracellular level of cAMP and Ca(2+) caused a significant decrease of the shunt resistance and an increase of SJ permeability. The attenuation of Ca(2+) effect in the presence of the calcium channel blocker nifedipine indicated that the influx of external Ca(2+) into the cytoplasm was important for the opening of the SJ, as well as the release of Ca(2+) from the intracellular stores.

Structural features and nucleotide-binding capability of the C subunit are integral to the regulation of the eukaryotic V1Vo ATPases

V-ATPases (vacuolar ATPases) are responsible for acidification of intracellular compartments and, in certain cases, proton transport across the plasma membrane of eukaryotic cells. They are composed of a catalytic V1 sector, in which ATP hydrolysis takes place, and the Vo sector, which functions in proton conduction. The best established mechanism for regulating the V-ATPase activity in vivo involves reversible dissociation of the V1 and Vo domains, in which subunit C is intimately involved. In the last year, impressive progress has been made in elucidating the structure of the C subunit and its arrangement inside the V-ATPase. Nucleotide occupancy by subunit C, followed by conformational changes of this subunit has shed light on the mechanism of V-ATPase regulation.

Subunit a of the yeast V-ATPase participates in binding of bafilomycin

Bafilomycin and concanamycin are potent and highly specific inhibitors of the vacuolar (H(+))-ATPases (V-ATPases), typically inhibiting at nanomolar concentrations. Previous studies have shown that subunit c of the integral V(0) domain participates in bafilomycin binding, and that this site resembles the oligomycin binding site of the F-ATPase (Bowman, B. J., and Bowman, E. J. (2002) J. Biol. Chem. 277, 3965-3972). Because mutations in F-ATPase subunit a also confer resistance to oligomycin, we investigated whether the a subunit of the V-ATPase might participate in binding bafilomycin. Twenty-eight subunit a mutations were constructed just N-terminal to the critical Arg(735) residue in transmembrane 7 required for proton transport, a region similar to that shown to participate in oligomycin binding by the F-ATPase. The mutants appeared to assemble normally and all but two showed normal growth at pH 7.5, whereas all but three had at least 25% of wild-type levels of proton transport and ATPase activity. Of the functional mutants, three displayed K(i) values for bafilomycin significantly different from wild-type (0.22 +/- 0.03 nm). These included E721K (K(i) 0.38 +/- 0.03 nm), L724A (0.40 +/- 0.02 nm), and N725F (0.54 +/- 0.06 nm). Only the N725F mutation displayed a K(i) for concanamycin (0.84 +/- 0.04 nm) that was slightly higher than wild-type (0.60 +/- 0.07 nm). These results suggest that subunit a of V-ATPase participates along with subunit c in binding bafilomycin.

V-ATPase inhibition prevents recovery from anoxia in Artemia franciscana embryos: quiescence signaling through dissipation of proton gradients

The metabolic downregulation critical for long-term survival of Artemia franciscana embryos under anoxia is mediated, in part, by a progressive intracellular acidification. However, very little is known about the mechanisms responsible for the pH transitions associated with exposure to, and recovery from, oxygen deprivation. In the present study, we demonstrate with 31P-NMR that incubation of intact embryos with the V-ATPase inhibitor bafilomycin A1 severely limits intracellular alkalinization during recovery from anoxia without affecting the restoration of cellular nucleotide triphosphate levels. Based on these data, it appears that oxidative phosphorylation and ATP resynthesis can only account for the first 0.3 pH unit alkalinization observed during aerobic recovery from the 1 pH unit acidification produced during 1 h of anoxia. The additional 0.7 pH unit increase requires proton pumping by the V-ATPase. Aerobic incubation with bafilomycin also suggests that V-ATPase inhibition alone is not enough to induce an acute dissipation of proton gradients under anoxia. In intact embryos, the dissipation of proton gradients and uncoupling of oxidative phosphorylation with carbonyl cyanide 3-chlorophenylhydrazone (CCCP) leads to an intracellular acidification similar to that seen after 1 h of anoxia. Subsequent exposure to anoxia, in the continued presence of CCCP, yields little additional acidification, suggesting that proton gradients are normally dissipated under anoxia. When combined with protons generated from net ATP hydrolysis, these data show that the dissipation of proton chemical gradients is sufficient to account for the reversible acidification associated with quiescence in these embryos.

Genome-wide survey of V-ATPase genes inDrosophilareveals a conserved renal phenotype for lethal alleles

V-ATPases are ubiquitous, vital proton pumps that play a multiplicity of roles in higher organisms. In many epithelia, they are the major energizer of cotransport processes and have been implicated in functions as diverse as fluid secretion and longevity. The first animal knockout of a V-ATPase was identified in Drosophila, and its recessive lethality demonstrated the essential nature of V-ATPases. This article surveys the entire V-ATPase gene family in Drosophila, both experimentally and in silico. Adult expression patterns of most of the genes are shown experimentally for the first time, using in situ hybridization or reporter gene expression, and these results are reconciled with published expression and microarray data. For each subunit, the single gene identified previously by microarray, as upregulated and abundant in tubules, is shown to be similarly abundant in other epithelia in which V-ATPases are known to be important; there thus appears to be a single dominant “plasma membrane” V-ATPase holoenzyme in Drosophila. This provides the most comprehensive view of V-ATPase expression yet in a multicellular organism. The transparent Malpighian tubule phenotype first identified in lethal alleles of vha55, the gene encoding the B-subunit, is shown to be general to those plasma membrane V-ATPase subunits for which lethal alleles are available, and to be caused by failure to accumulate uric acid crystals. These results coincide with the expression view of the gene family, in which 13 of the genes are specialized for epithelial roles, whereas others have spatially or temporally restricted patterns of expression.

Cysteine-mediated cross-linking indicates that subunit C of the V-ATPase is in close proximity to subunits E and G of the V1 domain and subunit a of the V0 domain

The vacuolar (H+)-ATPases (V-ATPases) are multisubunit complexes responsible for ATP-dependent proton transport across both intracellular and plasma membranes. The V-ATPases are composed of a peripheral domain (V1) that hydrolyzes ATP and an integral domain (V0) that conducts protons. Dissociation of V1 and V0 is an important mechanism of controlling V-ATPase activity in vivo. The crystal structure of subunit C of the V-ATPase reveals two globular domains connected by a flexible linker (Drory, O., Frolow, F., and Nelson, N. (2004) EMBO Rep. 5, 1-5). Subunit C is unique in being released from both V1 and V0 upon in vivo dissociation. To localize subunit C within the V-ATPase complex, unique cysteine residues were introduced into 25 structurally defined sites within the yeast C subunit and used as sites of attachment of the photoactivated sulfhydryl reagent 4-(N-maleimido)benzophenone (MBP). Analysis of photocross-linked products by Western blot reveals that subunit E (part of V1) is in close proximity to both the head domain (residues 166-263) and foot domain (residues 1-151 and 287-392) of subunit C. By contrast, subunit G (also part of V1) shows cross-linking to only the head domain whereas subunit a (part of V0) shows cross-linking to only the foot domain. The localization of subunit C to the interface of the V1 and V0 domains is consistent with a role for this subunit in controlling assembly of the V-ATPase complex.

H+-ATPase of crude homogenate of the outer mantle epithelium of Anodonta cygnea

The outer mantle epithelium of the freshwater bivalve, Anodonta cygnea, is responsible for the mineralisation of the shell. Under short circuit conditions, it generates a current that is due to the operation of an electrogenic proton pump located in the apical barrier of that epithelium. Bafilomycin A1 and Concanamycin A inhibited the short circuit current. The IC50 and maximum inhibition dose were 0.17 and 0.5 microM for Bafilomycin A1, and 0.7 and 5 microM for Concanamycin A. The present work was undertaken to further characterise V-type ATPase of the outer mantle cells. The V-ATPase enzymatic activity of crude homogenate, measured as the amount of inorganic phosphorous released, due to ATP hydrolysis, in the presence of Na2SO3 (200 mM) was found to be 4.6+/-1.1 micromol Pi/mg protein/h, at 27 degrees C, pH 7.0-7.4 and ATP 4.5-6.0 microM. Bafilomycin A1 and Concanamycin A inhibit the V-ATPase activity with an IC50 of 14 and 8 nmol mg(-1), respectively. Dicyclohexylcarbodiimide (DCCD; 100 mM) and NaNO3 (100 microM) inhibited the V-type ATPase in what it seems a non-specific manner and NEM (100 mM) was unable to do it. Bafilomycin A1 (10 microM) and Concanamycin A (10 microM), inhibited 50-60% of the total activity.

Analysis of Na+, Cl-, K+, H+and NH4+ concentration gradients adjacent to the surface of anal papillae of the mosquitoAedes aegypti: application of self-referencing ion-selective microelectrodes

Ion concentration gradients adjacent to the surface of the anal papillae of larvae of the mosquito Aedes aegypti were measured using self-referencing ion-selective microelectrodes. The gradients were used to calculate estimates of ion fluxes into and out of the papillae. There was a net influx of Na+, Cl- and K+ from the bathing medium and a net efflux of acid and NH4+. No Ca2+ gradients were detectable. Na+ and Cl-influx occurred against a concentration gradient suggesting active transport. Although Na+, Cl- and NH4+gradients were uniform along the length of the papillae, the proximal regions of the papillae in vivo revealed significantly higher H+and K+ gradients compared with distal regions. The calculated ion fluxes at the papillae are sufficient for complete Na+,K+ and Cl- haemolymph replacement in ∼4 h with external ion concentrations of 5 mmol l-1. Ion gradients were also detected adjacent to the surface of isolated papillae; however, Na+and H+ gradients were higher, and Cl- gradients were lower relative to papillae in vivo. The results support previous findings that the anal papillae of mosquito larvae are important structures for ion regulation, and suggest that these structures may be used for the excretion of nitrogenous waste.