Phylogeny and cloning of ion transporters in mosquitoes

Water fluxes and nutrient absorption along the midgut of three hemipterans, Mahanarva fimbriolata, Dysdercus peruvianus, and Rhodnius prolixus

Hemiptera Order comprises insect species adapted to different diets regarding water and nutrient content and availability, thus suggesting different combinations of proteins to ensure their absorption. To find out whether hemipterans use the same or distinct set of proteins and whether these differences are related to the phylogeny or the diet, RNAseq analyses were conducted in gut sections of three hemipterans, M. fimbriolata, D. peruvianus, and R. prolixus, with remarkable distinct diet. Since only a few of the selected proteins were functionally characterized, the coded putative proteins were manually curated by bioinformatics to infer their physiological function. The results suggest a relationship between gene expression patterns and water and nutrient dietary content and availability. In contrast, putative gene expansions and deletions are related to phylogeny, corresponding to evolutionary adaptations of ancestral forms to feed on xylem, cotton seeds, and blood, resulting in more resemblances between D. peruvianus and R. prolixus than M. fimbriolata. M. fimbriolata absorbs water through aquaporins Drip and Prip in the filtration chamber by passive diffusion, with a higher contribution of water-selective Drip. D. peruvianus water absorption involves Drip and Prip, but Prip contribution appears to be higher, and they probably cooperate with water-ion cotransporters in the posterior midgut. R. prolixus absorbs water in the anterior midgut involving a sodium transporter and a putative water-urea Prip. Sugars, amino acids, and lipids might be absorbed along the midgut in the three species, with a higher contribution of the posterior midgut for amino acid and lipid absorption in M. fimbriolata and D. peruvianus and the middle midgut in R. prolixus.

Proton-driven sodium secretion in a saline water animal

Aquatic animals residing in saline habitats either allow extracellular sodium concentration to conform to environmental values or regulate sodium to lower levels. The latter strategy requires an energy-driven process to move sodium against a large concentration gradient to eliminate excess sodium that diffuses into the animal. Previous studies of invertebrate and vertebrate species indicate a sodium pump, Na+/K+ ATPase, powers sodium secretion. We provide the first functional evidence of a saline-water animal, Aedes taeniorhynchus mosquito larva, utilizing a proton pump to power this process. Vacuolar-type H+ ATPase (VHA) protein is highly expressed on the apical membrane of the posterior rectal cells, and in situ sodium flux across this epithelium increases significantly in larvae held in higher salinity and is sensitive to Bafilomycin A1, an inhibitor of VHA. We also report the first evidence of splice variants of the sodium/proton exchanger, NHE3, with both high and low molecular weight variants highly expressed on the apical membrane of the posterior rectal cells. Evidence of NHE3 function was indicated with in situ sodium transport significantly inhibited by a NHE3 antagonist, S3226. We propose that the outward proton pumping by VHA establishes a favourable electromotive gradient to drive sodium secretion via NHE3 thus producing a hyperosmotic, sodium-rich urine. This H+- driven Na+ secretion process is the primary mechanism of ion regulation in salt-tolerant culicine mosquito species and was first investigated over 80 years ago.

Salinity-induced ionoregulatory changes in the gill proteome of the mayfly, Neocloeon triangulifer

Ecologists have observed declines in the biodiversity of sensitive freshwater organisms in response to increasing concentrations of major ions (salinization). Yet, how changing salinities physiologically challenge aquatic organisms, such as mayflies, remains remarkably understudied. Moreover, it is not well understood the degree to which species respond and acclimate to salinity changes. Our lab is developing the Baetid mayfly, N. triangulifer, as a model organism for physiological research. We have previously described acclimatory changes in both ion flux rates and altered mRNA transcript levels in response to chronic exposures to elevated major ion concentrations at the whole animal level. In the present study, we use shotgun proteomics to identify the specific proteins associated with apical ion transport and how their abundance changes in response to chronic salinity exposures in gills. Gills were isolated from the penultimate nymphal stage of N. triangulifer reared under control culture conditions, elevated NaCl (157 mg L^-1 Na), elevated CaCl₂ (121 mg L^-1 Ca), elevated Ca/MgSO₄ (735 mg L^-1 SO₄). These conditions mirrored those from previously published physiological work. We also acutely exposed nymphs to dilute (50% dilution of culture water with deionized water) to explore proteomic changes in the gills in response to dilute conditions. We report 710 unique peptide sequences among treatment groups, including important apical ion transporters such as Ca-ATPase, Na/K ATPase, and V-ATPase. Treatment with elevated NaCl and Ca/MgSO₄ appeared to cause more significant differential protein expression (452 and 345, respectively) compared to CaCl₂ and dilute groups (134 and 17, respectively). Finally, we demonstrated the breadth of physiological functions in gills by exploring non-transport related pathways found in our dataset, including ATP synthesis, calcium signaling, and oxidative stress response. We discuss our results in the context of freshwater salinization and the challenges of working with non-model species without fully sequenced and annotated genomes.

Sequence analysis and function of mosquito aeCCC2 and Drosophila Ncc83 orthologs

Dipteran insects have genes that code for two different Na⁺-dependent cation-chloride cotransporter (CCC) paralogs. Aedes aegypti aeNKCC1 is an ortholog of Drosophila melanogaster Ncc69, a bumetanide-sensitive Na⁺-K⁺-2Cl^- cotransporter (NKCC). Aedes aegypti aeCCC2 and aeCCC3 are orthologs of Drosophila Ncc83. Prior work suggests that the transport properties of aeCCC2 differ from canonical NKCCs. In particular, Xenopus oocytes expressing aeCCC2 have increased Na⁺-dependent membrane currents compared to controls, whereas NKCCs are electroneutral. Here, we further evaluated the function and localization of aeCCC2 and Ncc83. In oocytes expressing aeCCC2 or Ncc83, membrane potential (V_m) hyperpolarized upon Na⁺ removal; following hypotonic exposure the change in V_m was greater than it was in controls. In voltage-clamp experiments, membrane currents were concentration dependent on Na⁺ with an apparent affinity (K_m) of approximately 4.6 mM. In Malpighian tubules of larval and adult mosquitoes, aeCCC2 was localized along the basolateral aspect of principal cells. Sequence comparisons among transporters from Drosophila, Aedes, Anopheles, and Culex revealed 33 residues within the transmembrane domains (TMDs) that are fully conserved within paralogs but that differ between orthologs of NKCC1 and orthologs of aeCCC2/Ncc83. These residues are distributed across all 12 TMDs. Our results provide a foundation for further exploration of the structural basis for functional differences between insect Na⁺-dependent CCCs.

Expansive and Diverse Phenotypic Landscape of Field Aedes aegypti (Diptera: Culicidae) Larvae with Differential Susceptibility to Temephos: Beyond Metabolic Detoxification

Arboviruses including dengue, Zika, and chikungunya are amongst the most significant public health concerns worldwide. Arbovirus control relies on the use of insecticides to control the vector mosquito Aedes aegypti (Linnaeus), the success of which is threatened by widespread insecticide resistance. The work presented here profiled the gene expression of Ae. aegypti larvae from field populations of Ae. aegypti with differential susceptibility to temephos originating from two Colombian urban locations, Bello and Cúcuta, previously reported to have distinctive disease incidence, socioeconomics, and climate. We demonstrated that an exclusive field-to-lab (Ae. aegypti strain New Orleans) comparison generates an over estimation of differential gene expression (DGE) and that the inclusion of a geographically relevant field control yields a more discrete, and likely, more specific set of genes. The composition of the obtained DGE profiles is varied, with commonly reported resistance associated genes including detoxifying enzymes having only a small representation. We identify cuticle biosynthesis, ion exchange homeostasis, an extensive number of long noncoding RNAs, and chromatin modelling among the differentially expressed genes in field resistant Ae. aegypti larvae. It was also shown that temephos resistant larvae undertake further gene expression responses when temporarily exposed to temephos. The results from the sampling triangulation approach here contribute a discrete DGE profiling with reduced noise that permitted the observation of a greater gene diversity, increasing the number of potential targets for the control of insecticide resistant mosquitoes and widening our knowledge base on the complex phenotypic network of the Ae. aegypti response to insecticides.

Fluid Secretion by Malpighian Tubules of Rhodnius prolixus: Neuroendocrine Control With New Insights From a Transcriptome Analysis

Rhodnius prolixus (the kissing bug and a major vector of Chagas disease) is an obligate blood feeder that in the case of the fifth instar consumes up to 10 times its unfed body weight in a single 20-minute feed. A post-prandial diuresis is initiated, within minutes of the start of gorging, in order to lower the mass and concentrate the nutrients of the meal. Thus, R. prolixus rapidly excretes a fluid that is high in NaCl content and hypo-osmotic to the hemolymph, thereby eliminating 50% of the volume of the blood meal within 3 hours of gorging. In R. prolixus, as with other insects, the Malpighian tubules play a critical role in diuresis. Malpighian tubules are not innervated, and their fine control comes under the influence of the neuroendocrine system that releases amines and neuropeptides as diuretic or antidiuretic hormones. These hormones act upon the Malpighian tubules via a variety of G protein-coupled receptors linked to second messenger systems that influence ion transporters and aquaporins; thereby regulating fluid secretion. Much has been discovered about the control of diuresis in R. prolixus, and other model insects, using classical endocrinological studies. The post-genomic era, however, has brought new insights, identifying novel diuretic and antidiuretic hormone-signaling pathways whilst also validating many of the classical discoveries. This paper will focus on recent discoveries into the neuroendocrine control of the rapid post-prandial diuresis in R. prolixus, in order to emphasize new insights from a transcriptome analysis of Malpighian tubules taken from unfed and fed bugs.

Gene transcription changes in a locust model of noise-induced deafness

Locusts have auditory structures called Müller’s organs attached to tympanic membranes on either side of the abdomen. We measured the normalized abundances of 500 different mRNA transcripts in 320 Müller’s organs obtained from 160 locusts (Schistocerca gregaria) that had been subjected to a loud continuous 3-kHz tone for 24 h. Abundance ratios were then measured relative to transcripts from 360 control organs. A histogram of the number of observed transcripts versus their abundance ratios (noise exposed/control) was well fitted by a Cauchy distribution with median value near one. Transcripts below 5% and above 95% of the cumulative distribution function of the fitted Cauchy distribution were selected as putatively different from the expected values of an untreated preparation. This yielded eight transcripts with ratios increased by noise exposure (ratios 1.689–3.038) and 18 transcripts with reduced ratios (0.069–0.457). Most of the transcripts with increased abundance represented genes responsible for cuticular construction, suggesting extensive remodeling of some or all the cuticular components of the auditory structure, whereas the reduced abundance transcripts were mostly involved in lipid and protein storage and metabolism, suggesting a profound reduction in metabolic activity in response to the overstimulation.

NEW & NOTEWORTHY Locust ears have functional and genetic similarities to human ears, including loss of hearing from age or noise exposure. We measured transcript abundances in transcriptomes of noise-exposed and control locust ears. The data indicate remodeling of the ear tympanum and profound reductions in metabolism that may explain reduced sound transduction. These findings advance our understanding of this useful model and suggest further experiments to elucidate mechanisms that ears use to cope with excessive stimulation.

Differential expression of putative sodium-dependent cation-chloride cotransporters in Aedes aegypti

The yellow fever mosquito, Aedes aegypti, has three genes that code for proteins with sequence similarity to vertebrate Na⁺-K⁺-Cl^- cotransporters (NKCCs) of the solute-linked carrier 12 superfamily of cation-chloride cotransporters (CCCs). We hypothesized that these mosquito NKCC orthologues have diverged to perform distinct roles in salt secretion and absorption. In phylogenetic analyses, one protein (aeNKCC1) groups with a Drosophila melanogaster NKCC that mediates salt secretion whereas two others (aeCCC2 and aeCCC3) group with a Drosophila transporter that is not functionally characterized. The aeCCC2 and aeCCC3 genes probably result from a tandem gene duplication in the mosquito lineage; they have similar exon structures and are consecutive in genomic DNA. Predicted aeCCC2 and aeCCC3 proteins differ from aeNKCC1 and vertebrate NKCCs in residues from the third transmembrane domain known to influence ion and inhibitor binding. Quantitative PCR revealed that aeNKCC1 and aeCCC2 were approximately equally expressed in larvae and adults, whereas aeCCC3 was approximately 100-fold more abundant in larvae than in adults. In larval tissues, aeCCC2 was approximately 2-fold more abundant in Malpighian tubules compared to anal papillae. In contrast, aeCCC3 was nearly 100-fold more abundant in larval anal papillae compared to Malpighian tubules, suggesting a role in absorption. Western blots with polyclonal antibodies against isoform-specific peptides revealed stronger aeCCC2 immunoreactivity in adults versus larvae, whereas aeCCC3 immunoreactivity was stronger in larvae versus adults. The differential expression pattern of aeCCC2 and aeCCC3, and their sequence divergence in transmembrane domains, suggests that they may have different roles in transepithelial salt transport.

Solute carriers affect Anopheles stephensi survival and Plasmodium berghei infection in the salivary glands

Malaria is caused by mosquito-borne Plasmodium spp. parasites that must infect and survive within mosquito salivary glands (SGs) prior to host transmission. Recent advances in transcriptomics and the complete genome sequencing of mosquito vectors have increased our knowledge of the SG genes and proteins involved in pathogen infection and transmission. Membrane solute carriers are key proteins involved in drug transport and are useful in the development of new interventions for transmission blocking. Herein, we applied transcriptomics analysis to compare SGs mRNA levels in Anopheles stephensi fed on non-infected and P. berghei-infected mice. The A. stephensi solute carriers prestinA and NDAE1 were up-regulated in response to infection. These molecules are predicted to interact with each other, and are reportedly involved in the maintenance of cell homeostasis. To further evaluate their functions in mosquito survival and parasite infection, these genes were knocked down by RNA interference. Knockdown of prestinA and NDAE1 resulted in reduction of the number of sporozoites in mosquito SGs. Moreover, NDAE1 knockdown strongly impacted mosquito survival, resulting in the death of half of the treated mosquitoes. Overall, our findings indicate the importance of prestinA and NDAE1 in interactions between mosquito SGs and Plasmodium, and suggest the need for further research.

Toxicological perspective on the osmoregulation and ionoregulation physiology of major ions by freshwater animals: Teleost fish, crustacea, aquatic insects, and Mollusca

Anthropogenic sources increase freshwater salinity and produce differences in constituent ions compared with natural waters. Moreover, ions differ in physiological roles and concentrations in intracellular and extracellular fluids. Four freshwater taxa groups are compared, to investigate similarities and differences in ion transport processes and what ion transport mechanisms suggest about the toxicity of these or other ions in freshwater. Although differences exist, many ion transporters are functionally similar and may belong to evolutionarily conserved protein families. For example, the Na+ /H+ -exchanger in teleost fish differs from the H+ /2Na+ (or Ca2+ )-exchanger in crustaceans. In osmoregulation, Na+ and Cl- predominate. Stenohaline freshwater animals hyperregulate until they are no longer able to maintain hypertonic extracellular Na+ and Cl- concentrations with increasing salinity and become isotonic. Toxic effects of K+ are related to ionoregulation and volume regulation. The ionic balance between intracellular and extracellular fluids is maintained by Na+ /K+ -adenosine triphosphatase (ATPase), but details are lacking on apical K+ transporters. Elevated H+ affects the maintenance of internal Na+ by Na+ /H+ exchange; elevated HCO3- inhibits Cl- uptake. The uptake of Mg2+ occurs by the gills or intestine, but details are lacking on Mg2+ transporters. In unionid gills, SO42- is actively transported, but most epithelia are generally impermeant to SO42- . Transporters of Ca2+ maintain homeostasis of dissolved Ca2+ . More integration of physiology with toxicology is needed to fully understand freshwater ion effects. Environ Toxicol Chem 2017;36:576-600. Published 2016 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

PGE2 MEDIATES OENOCYTOID CELL LYSIS VIA A SODIUM-POTASSIUM-CHLORIDE COTRANSPORTER

Prostaglandin E2 (PGE2 ) mediates immune responses of the beet armyworm, Spodoptera exigua, including oenocytoid cell lysis (a class of lepidopteran hemocytes: OCL) via its specific membrane receptor to release inactive prophenoloxidase (PPO) into hemolymph. PPO is activated into phenoloxidase in the plasma to play crucial roles in the immune responses of S. exigua. The mechanism of OCL has not been elucidated, however we posed the hypothesis that a rapid accumulation of sodium ions within the oenocytoids allows a massive influx of water by the ion gradient, which leads to the cell lysis. It remains unclear which sodium channel is responsible for the OCL in response to PGE2 . This study identified a specific sodium channel called sodium-potassium-chloride cotransporter 1 (Se-NKCC1) expressed in hemocytes of S. exigua and analyzed its function in the OCL in response to PGE2 . Se-NKCC1 encodes a basic membrane protein (pI value = 8.445) of 1,066 amino acid residues, which contains 12 putative transmembrane domains. Se-NKCC1 was expressed in all developmental stages and tissues. qPCR showed that bacterial challenge significantly induced its expression. A specific inhibitor of NKCC, bumetanide, prevented the OCL in a dose-dependent manner. When RNA interference (RNAi) using double-stranded RNA specific to Se-NKCC1 suppressed its expression, the OCL and PPO activation were significantly inhibited in response to PGE2 . The RNAi treatment also reduced nodule formation to bacterial challenge. These results suggest that Se-NKCC1 is associated with OCL by facilitating inward transport of ions in response to PGE2 .

Bioelectric patterning during oogenesis: stage-specific distribution of membrane potentials, intracellular pH and ion-transport mechanisms in Drosophila ovarian follicles

Background

Bioelectric phenomena have been found to exert influence on various developmental and regenerative processes. Little is known about their possible functions and the cellular mechanisms by which they might act during Drosophila oogenesis. In developing follicles, characteristic extracellular current patterns and membrane-potential changes in oocyte and nurse cells have been observed that partly depend on the exchange of protons, potassium ions and sodium ions. These bioelectric properties have been supposed to be related to various processes during oogenesis, e. g. pH-regulation, osmoregulation, cell communication, cell migration, cell proliferation, cell death, vitellogenesis and follicle growth. Analysing in detail the spatial distribution and activity of the relevant ion-transport mechanisms is expected to elucidate the roles that bioelectric phenomena play during oogenesis.

Results

To obtain an overview of bioelectric patterning along the longitudinal and transversal axes of the developing follicle, the spatial distributions of membrane potentials (V_mem), intracellular pH (pH_i) and various membrane-channel proteins were studied systematically using fluorescent indicators, fluorescent inhibitors and antisera. During mid-vitellogenic stages 9 to 10B, characteristic, stage-specific V_mem-patterns in the follicle-cell epithelium as well as anteroposterior pH_i-gradients in follicle cells and nurse cells were observed. Corresponding distribution patterns of proton pumps (V-ATPases), voltage-dependent L-type Ca²⁺-channels, amiloride-sensitive Na⁺-channels and Na⁺,H⁺-exchangers (NHE) and gap-junction proteins (innexin 3) were detected. In particular, six morphologically distinguishable follicle-cell types are characterized on the bioelectric level by differences concerning V_mem and pH_i as well as specific compositions of ion channels and carriers. Striking similarities between V_mem-patterns and activity patterns of voltage-dependent Ca²⁺-channels were found, suggesting a mechanism for transducing bioelectric signals into cellular responses. Moreover, gradients of electrical potential and pH were observed within single cells.

Conclusions

Our data suggest that spatial patterning of V_mem, pH_i and specific membrane-channel proteins results in bioelectric signals that are supposed to play important roles during oogenesis, e. g. by influencing spatial coordinates, regulating migration processes or modifying the cytoskeletal organization. Characteristic stage-specific changes of bioelectric activity in specialized cell types are correlated with various developmental processes.

Investigations of the signaling cascade involved in diuretic hormone stimulation of Malpighian tubule fluid secretion in Rhodnius prolixus

In insects, the excretory system is comprised of the Malpighian tubules (MTs) and the hindgut, which collectively function to maintain ionic and osmotic balance of the haemolymph and rid the organism of toxic compounds or elements in excess. Secretion by the Malpighian tubules of insects is regulated by a variety of hormones including peptidergic factors as well as biogenic amines. In Rhodnius prolixus, two endogenous diuretic hormones have been identified; the biogenic amine serotonin (5-hydroxytryptamine, 5-HT) and the corticotropin releasing factor-related peptide, RhoprCRF. Both factors significantly increase secretion by MTs and are known to elevate intracellular levels of cAMP. Interestingly, applying sub-maximal doses of these two diuretic factors in combination on isolated MTs in vitro reveals synergistic effects as rates of fluid secretion are significantly higher than would be expected if rates of secretion from MTs treated with each factor alone were summed. This observed synergism suggests that different downstream targets may be activated by the two diuretic factors, but that some cellular elicitors may be shared since cAMP is elevated in response to either diuretic hormone. This study investigated the signaling cascade involved in the diuretic hormone regulation of Malpighian tubule fluid secretion. Bioassays were performed in physiological as well as modified salines (e.g. calcium-free) alone or in the presence of a variety of pharmacological compounds that interfere with prospective intracellular targets, such as the apical cation/H(+) exchanger. Intriguingly, only amiloride yielded differential effects on the two diuretics with 5HT-stimulated secretion being blocked, whereas in contrast, RhoprCRF-stimulated secretion was unaffected. In addition, experiments examining the role of extracellular and intracellular calcium on fluid secretion rate showed that both diuretics are dependent on intracellular calcium availability. Finally, fluid secretion stimulated by either diuretic hormone was also sensitive to inhibition of cAMP-dependent protein kinase A. Taken together, these results suggest that each diuretic hormone activates pathways dependent upon intracellular calcium and cAMP.

Intracellular Na+, K+ and Cl- activities in Acheta domesticus Malpighian tubules and the response to a diuretic kinin neuropeptide

The mechanism of primary urine production and the activity of a diuretic kinin, Achdo-KII, were investigated in malpighian tubules of Acheta domesticus by measuring intracellular Na(+), K(+) and Cl(-) activities, basolateral membrane voltage (V(b)), fluid secretion and transepithelial ion transport. Calculated electrochemical gradients for K(+) and Cl(-) across the basolateral membrane show they are actively transported into principal cells, and basolateral Ba(2+)-sensitive K(+) channels do not contribute to net transepithelial K(+) transport and fluid secretion. A basolateral Cl(-) conductance was revealed after the blockade of K(+) channels with Ba(2+), and a current carried by the passive outward movement of Cl(-) accounts for the hyperpolarization of V(b) in response to Ba(2+). Ion uptake via Na(+)/K(+)/2Cl(-) cotransport, driven by the inwardly directed Na(+) electrochemical gradient, is thermodynamically feasible, and is consistent with the actions of bumetanide, which reduces fluid secretion and both Na(+) and K(+) transport. The Na(+) gradient is maintained by its extrusion across the apical membrane and by a basolateral ouabain-sensitive Na(+)/K(+)-ATPase. Achdo-KII has no significant effect on the intracellular ion activities or V(b). Electrochemical gradients across the apical membrane were estimated from previously published values for the levels of Na(+), K(+) and Cl(-) in the secreted fluid. The electrochemical gradient for Cl(-) favours passive movement into the lumen, but falls towards zero after stimulation by Achdo-KII. This coincides with a twofold increase in Cl(-) transport, which is attributed to the opening of an apical Cl(-) conductance, which depolarises the apical membrane voltage.

Drosophila glia use a conserved cotransporter mechanism to regulate extracellular volume

The nervous system is protected by blood barriers that use multiple systems to control extracellular solute composition, osmotic pressure, and fluid volume. In the human nervous system, misregulation of the extracellular volume poses serious health threats. Here, we show that the glial cells that form the Drosophila blood-nerve barrier have a conserved molecular mechanism that regulates extracellular volume: the Serine/Threonine kinase Fray, which we previously showed is an ortholog of mammalian PASK/SPAK; and the Na-K-Cl cotransporter Ncc69, which we show is an ortholog of human NKCC1. In mammals, PASK/SPAK binds to NKCC1 and regulates its activity. In Drosophila, larvae mutant for Ncc69 develop a peripheral neuropathy, where fluid accumulates between glia and axons. The accumulation of fluid has no detectable impact on action potential conduction, suggesting that the role of Ncc69 is to maintain volume or osmotic homeostasis. Drosophila Ncc69 has kinetics similar to human NKCC1, and NKCC1 can rescue Ncc69, suggesting that they function in a conserved physiological mechanism. We show that fray and Ncc69 are coexpressed in nerve glia, interact in a yeast-two-hybrid assay, and have an essentially identical bulging nerve phenotype. We propose that normally functioning nerves generate extracellular solutes that are removed by Ncc69 under the control of Fray. This mechanism may perform a similar role in humans, given that NKCC1 is expressed at the blood-brain barrier.

Voltage coupling of primary H+ V-ATPases to secondary Na+- or K+-dependent transporters

This review provides alternatives to two well established theories regarding membrane energization by H(+) V-ATPases. Firstly, we offer an alternative to the notion that the H(+) V-ATPase establishes a protonmotive force (pmf) across the membrane into which it is inserted. The term pmf, which was introduced by Peter Mitchell in 1961 in his chemiosmotic hypothesis for the synthesis of ATP by H(+) F-ATP synthases, has two parts, the electrical potential difference across the phosphorylating membrane, Deltapsi, and the pH difference between the bulk solutions on either side of the membrane, DeltapH. The DeltapH term implies three phases - a bulk fluid phase on the H(+) input side, the membrane phase and a bulk fluid phase on the H(+) output side. The Mitchell theory was applied to H(+) V-ATPases largely by analogy with H(+) F-ATP synthases operating in reverse as H(+) F-ATPases. We suggest an alternative, voltage coupling model. Our model for V-ATPases is based on Douglas B. Kell's 1979 'electrodic view' of ATP synthases in which two phases are added to the Mitchell model - an unstirred layer on the input side and another one on the output side of the membrane. In addition, we replace the notion that H(+) V-ATPases normally acidify the output bulk solution with the hypothesis, which we introduced in 1992, that the primary action of a H(+) V-ATPase is to charge the membrane capacitance and impose a Deltapsi across the membrane; the translocated hydrogen ions (H(+)s) are retained at the outer fluid-membrane interface by electrostatic attraction to the anions that were left behind. All subsequent events, including establishing pH differences in the outside bulk solution, are secondary. Using the surface of an electrode as a model, Kell's 'electrodic view' has five phases - the outer bulk fluid phase, an outer fluid-membrane interface, the membrane phase, an inner fluid-membrane interface and the inner bulk fluid phase. Light flash, H(+) releasing and binding experiments and other evidence provide convincing support for Kell's electrodic view yet Mitchell's chemiosmotic theory is the one that is accepted by most bioenergetics experts today. First we discuss the interaction between H(+) V-ATPase and the K(+)/2H(+) antiporter that forms the caterpillar K(+) pump, and use the Kell electrodic view to explain how the H(+)s at the outer fluid-membrane interface can drive two H(+) from lumen to cell and one K(+) from cell to lumen via the antiporter even though the pH in the bulk fluid of the lumen is highly alkaline. Exchange of outer bulk fluid K(+) (or Na(+)) with outer interface H(+) in conjunction with (K(+) or Na(+))/2H(+) antiport, transforms the hydrogen ion electrochemical potential difference, mu(H), to a K(+) electrochemical potential difference, mu(K) or a Na(+) electrochemical potential difference, mu(Na). The mu(K) or mu(Na) drives K(+)- or Na(+)-coupled nutrient amino acid transporters (NATs), such as KAAT1 (K(+) amino acid transporter 1), which moves Na(+) and an amino acid into the cell with no H(+)s involved. Examples in which the voltage coupling model is used to interpret ion and amino acid transport in caterpillar and larval mosquito midgut are discussed.

NHE8 is an intracellular cation/H+ exchanger in renal tubules of the yellow fever mosquito Aedes aegypti

The goal of this study was to identify and characterize the hypothesized apical cation/H(+) exchanger responsible for K(+) and/or Na(+) secretion in the renal (Malpighian) tubules of the yellow fever mosquito Aedes aegypti. From Aedes Malpighian tubules, we cloned "AeNHE8," a full-length cDNA encoding an ortholog of mammalian Na(+)/H(+) exchanger 8 (NHE8). The expression of AeNHE8 transcripts is ubiquitous among mosquito tissues and is not enriched in Malpighian tubules. Western blots of Malpighian tubules suggest that AeNHE8 is expressed primarily as an intracellular protein, which was confirmed by immunohistochemical localizations in Malpighian tubules. AeNHE8 immunoreactivity is expressed in principal cells of the secretory, distal segments, where it localizes to a subapical compartment (e.g., vesicles or endosomes), but not in the apical brush border. Furthermore, feeding mosquitoes a blood meal or treating isolated tubules with dibutyryl-cAMP, both of which stimulate a natriuresis by Malpighian tubules, do not influence the intracellular localization of AeNHE8 in principal cells. When expressed heterologously in Xenopus laevis oocytes, AeNHE8 mediates EIPA-sensitive Na/H exchange, in which Li(+) partially and K(+) poorly replace Na(+). The expression of AeNHE8 in Xenopus oocytes is associated with the development of a conductive pathway that closely resembles the known endogenous nonselective cation conductances of Xenopus oocytes. In conclusion, AeNHE8 does not mediate cation/H(+) exchange in the apical membrane of Aedes Malpighian tubules; it is more likely involved with an intracellular function.

Sodium uptake in different life stages of crustaceans: the water fleaDaphnia magnaStrauss

The concentration-dependent kinetics and main mechanisms of whole-body Na+ uptake were assessed in neonate and adult water flea Daphnia magna Strauss acclimated to moderately hard water (0.6 mmol l–1 NaCl, 1.0 mmol l–1 CaCO3 and 0.15 mmol l–1 MgSO4·7H2O; pH 8.2). Whole-body Na+ uptake is independent of the presence of Cl– in the external medium and kinetic parameters are dependent on the life stage. Adults have a lower maximum capacity of Na+ transport on a mass-specific basis but a higher affinity for Na+ when compared to neonates. Based on pharmacological analyses,mechanisms involved in whole-body Na+ uptake differ according to the life stage considered. In neonates, a proton pump-coupled Na+channel appears to play an important role in the whole-body Na+uptake at the apical membrane. However, they do not appear to contribute to whole-body Na+ uptake in adults, where only the Na+channel seems to be present, associated with the Na+/H+exchanger. In both cases, carbonic anhydrase contributes by providing H+ for the transporters. At the basolateral membrane of the salt-transporting epithelia of neonates, Na+ is pumped from the cells to the extracellular fluid by a Na+,K+-ATPase and a Na+/Cl– exchanger whereas K+ and Cl– move through specific channels. In adults, a Na+/K+/2Cl– cotransporter replaces the Na+/Cl– exchanger. Differential sensitivity of neonates and adults to iono- and osmoregulatory toxicants, such as metals, are discussed with respect to differences in whole-body Na+ uptake kinetics, as well as in the mechanisms of Na+ transport involved in the whole-body Na+ uptake in the two life stages.

Molecular cloning, phylogeny and localization of AgNHA1: the first Na+/H+ antiporter (NHA) from a metazoan,Anopheles gambiae

We have cloned a cDNA encoding a new ion transporter from the alimentary canal of larval African malaria mosquito, Anopheles gambiae Giles sensu stricto. Phylogenetic analysis revealed that the corresponding gene is in a group that has been designated NHA, and which includes(Na+ or K+)/H+ antiporters; so the novel transporter is called AgNHA1. The annotation of current insect genomes shows that both AgNHA1 and a close relative, AgNHA2, belong to the cation proton antiporter 2 (CPA2) subfamily and cluster in an exclusive clade of genes with high identity from Aedes aegypti, Drosophila melanogaster, D. pseudoobscura, Apis mellifera and Tribolium castaneum. Although NHA genes have been identified in all phyla for which genomes are available, no NHA other than AgNHA1 has previously been cloned,nor have the encoded proteins been localized or characterized.

The AgNHA1 transcript was localized in An. gambiae larvae by quantitative real-time PCR (qPCR) and in situ hybridization. AgNHA1 message was detected in gastric caeca and rectum, with much weaker transcription in other parts of the alimentary canal. Immunolabeling of whole mounts and longitudinal sections of isolated alimentary canal showed that AgNHA1 is expressed in the cardia, gastric caeca, anterior midgut, posterior midgut, proximal Malpighian tubules and rectum, as well as in the subesophageal and abdominal ganglia.

A phylogenetic analysis of NHAs and KHAs indicates that they are ubiquitous. A comparative molecular analysis of these antiporters suggests that they catalyze electrophoretic alkali metal ion/hydrogen ion exchanges that are driven by the voltage from electrogenic H+ V-ATPases. The tissue localization of AgNHA1 suggests that it plays a key role in maintaining the characteristic longitudinal pH gradient in the lumen of the alimentary canal of An. gambiae larvae.

Identification of midgut microvillar proteins from Tenebrio molitor and Spodoptera frugiperda by cDNA library screenings with antibodies

The objective of this study was to identify midgut microvillar proteins in insects appearing earlier (Coleoptera) and later (Lepidoptera) in evolution. For this, cytoskeleton-free midgut microvillar membrane from Spodoptera frugiperda (Lepidoptera) and Tenebrio molitor (Coleoptera) were used to raise antibodies. These were used for screening midgut cDNA expression libraries. Positive clones were sequenced, assembled and searched for similarities with gene/protein databases. The predicted midgut microvillar proteins from T. molitor were: cockroach allergens (unknown function), peritrophins (peritrophic membrane proteins), digestive enzymes (aminopeptidase, alpha-mannosidase) and unknown proteins. Predicted S. frugiperda midgut proteins may be grouped into six classes: (a) proteins involved in protection of midgut (thioredoxin peroxidase, aldehyde dehydrogenase, serpin and juvenile hormone epoxide hydrolase); (b) digestive enzymes (astacin, transporter-like amylase, aminopeptidase, and carboxypeptidase); (c) peritrophins; (d) proteins associated with microapocrine secretion (gelsolin, annexin); (e) membrane-tightly bound-cytoskeleton proteins (fimbrin, calmodulin) and (f) unidentified proteins. The novel approach is compared with others and microvillar function is discussed in the light of the predicted proteins.

NHE8 mediates amiloride-sensitive Na+/H+ exchange across mosquito Malpighian tubules and catalyzes Na+ and K+ transport in reconstituted proteoliposomes

Following a blood meal, the mosquito Aedes aegypti will have acquired an enormous sodium load that must be rapidly excreted to restore ion homeostasis. It is a process that demands robust sodium and fluid transport capabilities. Even though the identities of the components involved in this ion transport across the mosquito Malpighian tubule epithelia have not been completely determined, electrophysiological studies suggest the contribution of a Na(+)/H(+) exchanger extruding cations into the lumen driven secondarily by the proton gradient created by the V-type H(+)-ATPase in the tubules' apical membrane. We have identified the putative exchanger and designated it AeNHE8. Immunolocalization studies demonstrated that AeNHE8 is expressed in the apical membranes of Malpighian tubules, gastric caecae, and rectum. When heterologously expressed in salt-sensitive yeast cells lacking Na(+) extrusion and Na(+)/H(+) exchange proteins, AeNHE8 rescues the salt-sensitive phenotype and restores the cells' ability to grow in high NaCl media. Furthermore, heterologous expression of AeNHE8 in NHE-deficient fibroblast cells results in an amiloride-sensitive (22)Na(+) uptake. To determine the exchanger's kinetic properties, we reconstituted membranes from yeast cells expressing the protein into lipid proteoliposomes and assayed for cation-dependent H(+) exchange by fluorimetric methods. Our results indicate that AeNHE8 mediates saturable exchange of Na(+) and K(+) for H(+). We propose that AeNHE8 may be coupled to the inward H(+) gradient across the Malpighian tubules and plays a role in the extrusion of excess sodium and potassium while maintaining steady intracellular pH in the principal cells.

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.

Molecular characterization of sodium/proton exchanger 3 (NHE3) from the yellow fever vector,Aedes aegypti

Transport across insect epithelia is thought to depend on the activity of a vacuolar-type proton ATPase (V-ATPase) that energizes ion transport through a secondary proton/cation exchanger. Although several of the subunits of the V-ATPase have been cloned, the molecular identity of the exchanger has not been elucidated. Here, we present the identification of sodium/proton exchanger isoform 3 (NHE3) from yellow fever mosquito, Aedes aegypti(AeNHE3). AeNHE3 localizes to the basal plasma membrane of Malpighian tubule, midgut and the ion-transporting sector of gastric caeca. Midgut expression of NHE3 shows a different pattern of enrichment between larval and adult stages, implicating it in the maintenance of regional pH in the midgut during the life cycle. In all tissues examined, NHE3 predominantly localizes to the basal membrane. In addition the limited expression in intracellular vesicles in the median Malpighian tubules may reflect a potential functional versatility of NHE3 in a tissue-specific manner. The localization of V-ATPase and NHE3, and exclusion of Na+/K+-ATPase from the distal ion-transporting sector of caeca, indicate that the role of NHE3 in ion and pH regulation is intricately associated with functions of V-ATPase. The AeNHE3 complements yeast mutants deficient in yeast NHEs, NHA1 and NHX1. To further examine the functional property of AeNHE3, we expressed it in NHE-deficient fibroblast cells. AeNHE3 expressing cells were capable of recovering intracellular pH following an acid load. The recovery was independent of the large cytoplasmic region of AeNHE3, implying this domain to be dispensable for NHE3 ion transport function. 22Na+uptake studies indicated that AeNHE3 is relatively insensitive to amiloride and EIPA and is capable of Na+ transport in the absence of the cytoplasmic tail. Thus, the core domain containing the transmembrane regions of NHE3 is sufficient for pH recovery and ion transport. The present data facilitate refinement of the prevailing models of insect epithelial transport by incorporating basal amiloride-insensitive NHE3 as a critical mediator of transepithelial ion and fluid transport and likely in the maintenance of intracellular pH.

The cation-chloride cotransporter, masBSC, is widely expressed in Manduca sexta tissues

Cation-chloride cotransporters, including the Na-K-Cl cotransporter, play an important role in epithelial ion transport in insects. We have determined the tissue distribution of Manduca sexta bumetanide sensitive cotransporter (masBSC), a putative Na-K-Cl cotransporter that was originally cloned from M. sexta Malpighian tubules. We developed a polyclonal antibody (M6) against a C-terminal fragment of masBSC. masBSC protein was detected by M6 at an apparent molecular mass of approximately 220kDa in M. sexta foregut, midgut, hindgut, Malpighian tubule, salivary gland, fat body, trachea, and nerve cord. Higher expression was observed in the foregut than in other tissues. M6 stained the apical membrane of midgut epithelial cells in cross-sections of third instar larvae. The transcript of masBSC was detected by RT-PCR in midgut, Malpighian tubule, hindgut, trachea, nerve cord, and fat bodies. Taken together, these findings demonstrate that masBSC is widely expressed in M. sexta. While the specific function of masBSC remains unknown, the wide distribution indicates a role of masBSC in a broad range of tissues.

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 isolated anterior stomach of larval mosquitoes (Aedes aegypti): Voltage-clamp measurements with a tubular epithelium

The anterior stomach of larval Aedes aegypti was isolated and perfused via two pipettes. For transepithelial voltage (V(te)) measurement, the inflow pipette and the bath were connected via agar bridges to calomel electrodes. For voltage-clamping, the lumen of the tissue contained an Ag/AgCl wire held by the outflow pipette, and the preparation was placed in a bath within a spiral of Ag/AgCl wire. After equilibrating the tissue in mosquito saline on both sides, a V(te) of -8+/-1 mV was measured (+/-S.E.M., N=32). Current-voltage curves (+/-100 mV) demonstrated ohmic behaviour of the epithelium. Short-circuiting resulted in a current (I(sc)) of 103+/-16 microA cm(-2) and a mean transepithelial conductance (G(te)) of 11.8+/-1.3 mS cm(-2) (+/-S.E.M., N=32). A Yonath-Civan plot of G(te) of individual preparations over the corresponding I(sc) resulted in a straight line (r(2)=0.8422), indicating that the difference in I(sc) of individual preparations is mainly based on different transcellular conductances (G(c)). This analysis allowed to estimate the mean leak conductance (G(l) approximately 3.9 mS cm(-2)) and the mean transcellular electromotive force (E(c) approximately 13 mV). After administering 0.2 micromol L(-1) serotonin, I(sc) and G(te) significantly increased, to 457+/-49 microA cm(-2) and to 21.3+/-2.3 mS cm(-2) (+/-S.E.M., N=31, P<0.05), respectively. The Yonath-Civan plot after serotonin resulted again in a straight line (r(2)=0.8219), indicating a mean G(l) of about 1 mS cm(-2) and a mean E(c) of about 22 mV. Dinitrophenol (2.5 mmol L(-1)) almost abolished I(sc) and significantly reduced G(te) (N=6). Concanamycin A (100 micromol L(-1)) reduced I(sc) by more than 90% without significantly affecting G(te).

Actin redistribution in mosquito malpighian tubules after a blood meal and cyclic AMP stimulation

Fluid secretion by mosquito Malpighian tubules is critical to maintaining fluid and electrolyte balance after a blood meal. Endogenous cAMP levels increase in Malpighian tubules after a blood meal. Here, we determined if corresponding changes in intracellular actin distribution occur after a blood meal or dibutyryl-cAMP (db-cAMP) stimulation and whether altering actin turnover inhibits secretion. In untreated Malpighian tubules, beta-actin immunostaining was more intense in the apical region of adult Malpighian tubules than in the cytoplasm. Stimulation by a blood meal or db-cAMP significantly decreased beta-actin immunostaining in the non-apical region of the cell. Db-cAMP had similar effects in larvae and pupae Malpighian tubules. In contrast, no detectable shift in F-actin distribution was detected; however, F-actin bundles within the cytoplasm increased in size after treatment with db-cAMP. Pretreatment of Malpighian tubules with agents perturbing actin fiber assembly and disassembly decreased basal secretion rates and inhibited the stimulatory effects of db-cAMP. Our results show (1) beta-actin redistributes toward the apical membrane after a blood meal and this correlates temporally with increase urine flow rate and intracellular cAMP levels, (2) Malpighian tubules from all developmental stages exhibit this same response to db-cAMP-stimulation, and (3) dynamic assembly and disassembly of beta-actin is required for db-cAMP-stimulated secretion.

Conservation of capa peptide-induced nitric oxide signalling in Diptera

In D. melanogaster Malpighian (renal) tubules, the capa peptides stimulate production of nitric oxide (NO) and guanosine 3', 5'-cyclic monophosphate (cGMP), resulting in increased fluid transport. The roles of NO synthase (NOS), NO and cGMP in capa peptide signalling were tested in several other insect species of medical relevance within the Diptera (Aedes aegypti, Anopheles stephensi and Glossina morsitans) and in one orthopteran out-group, Schistocerca gregaria. NOS immunoreactivity was detectable by immunocytochemistry in tubules from all species studied. D. melanogaster, A. aegypti and A. stephensi express NOS in only principal cells, whereas G. morsitans and S. gregaria show more general NOS expression in the tubule. Measurement of associated NOS activity (NADPH diaphorase) shows that both D. melanogaster capa-1 and the two capa peptides encoded in the A. gambiae genome, QGLVPFPRVamide (AngCAPA-QGL) and GPTVGLFAFPRVamide (AngCAPA-GPT), all stimulate NOS activity in D. melanogaster, A. aegypti, A. stephensi and G. morsitans tubules but not in S. gregaria. Furthermore, capa-stimulated NOS activity in all the Diptera was inhibited by the NOS inhibitor l-NAME. All capa peptides stimulate an increase in cGMP content across the dipteran species, but not in the orthopteran S. gregaria. Similarly, all capa peptides tested stimulate fluid secretion in D. melanogaster, A. aegypti, A. stephensi and G. morsitans tubules but are either without effect or are inhibitory on S. gregaria. Consistent with these results, the Drosophila capa receptor was shown to be expressed in Drosophila tubules, and its closest Anopheles homologue was shown to be expressed in Anopheles tubules. Thus, we provide the first demonstration of physiological roles for two putative A. gambiae neuropeptides. We also demonstrate neuropeptide modulation of fluid secretion in tsetse tubule for the first time. Finally, we show the generality of capa peptide action, to stimulate NO/cGMP signalling and increase fluid transport, across the Diptera, but not in the more primitive Orthoptera.

Basolateral ion transport mechanisms during fluid secretion byDrosophilaMalpighian tubules: Na+ recycling,Na+:K+:2Cl– cotransport and Cl– conductance

Mechanisms of ion transport during primary urine formation by the Malpighian tubule of Drosophila melanogaster were analyzed through measurements of fluid secretion rate, transepithelial ion flux, basolateral membrane potential (Vbl) and intracellular activities of K+ (aKi) and Cl–(aCli). Calculation of the electrochemical potentials for both ions permitted assessment of the possible contributions of K+ channels, Na+:K+:2Cl–cotransport, and K+:Cl– cotransport, to net transepithelial ion secretion across the basolateral membrane. The data show that passive movement of both K+ and Cl– from cell to bath is favoured across the basolateral membrane, indicating that both ions are actively transported into the cell. Contributions of basolateral K+ channels or K+:Cl– cotransporters to net transepithelial ion secretion can be ruled out. After prior exposure of tubules to ouabain, subsequent addition of bumetanide reduced fluid secretion rate, K+ flux and Na+ flux, indicating a role for a Na+:K+:2Cl– cotransporter in fluid secretion. Addition of the K+ channel blocker Ba2+ had no effect on aKi or aCli. Addition of Ba2+ unmasked a basolateral Cl– conductance and the hyperpolarization of Vbl in response to Ba2+ was Cl–-dependent. A new model for fluid secretion proposes that K+ and Cl– cross the basolateral membrane through a Na+-driven Na+:K+:2Cl–cotransporter and that most of the Na+ that enters the cells is returned to the bath through the Na+/K+-ATPase.