Energizing epithelial transport with the vacuolar H(+)-ATPase

Mechanisms of Na+ uptake from freshwater habitats in animals

Life in fresh water is osmotically and energetically challenging for living organisms, requiring increases in ion uptake from dilute environments. However, mechanisms of ion uptake from freshwater environments are still poorly understood and controversial, especially in arthropods, for which several hypothetical models have been proposed based on incomplete data. One compelling model involves the proton pump V-type H⁺ ATPase (VHA), which energizes the apical membrane, enabling the uptake of Na⁺ (and other cations) via an unknown Na⁺ transporter (referred to as the "Wieczorek Exchanger" in insects). What evidence exists for this model of ion uptake and what is this mystery exchanger or channel that cooperates with VHA? We present results from studies that explore this question in crustaceans, insects, and teleost fish. We argue that the Na⁺/H⁺ antiporter (NHA) is a likely candidate for the Wieczorek Exchanger in many crustaceans and insects; although, there is no evidence that this is the case for fish. NHA was discovered relatively recently in animals and its functions have not been well characterized. Teleost fish exhibit redundancy of Na⁺ uptake pathways at the gill level, performed by different ion transporter paralogs in diverse cell types, apparently enabling tolerance of low environmental salinity and various pH levels. We argue that much more research is needed on overall mechanisms of ion uptake from freshwater habitats, especially on NHA and other potential Wieczorek Exchangers. Such insights gained would contribute greatly to our general understanding of ionic regulation in diverse species across habitats.

Na+ K+ ATPase isoform switching in zebrafish during transition to dilute freshwater habitats

Na⁺ K⁺ ATPase (NKA) is crucial to branchial ion transport as it uses the energy from ATP to move Na⁺ against its electrochemical gradient. When fish encounter extremely dilute environments the energy available from ATP hydrolysis may not be sufficient to overcome thermodynamic constraints on ion transport. Yet many fish species-including zebrafish-are capable of surviving in dilute environments. Despite much study, the physiological mechanisms by which this occurs remain poorly understood. Here, we demonstrate that zebrafish acclimated to less than 10 µM Na⁺ water exhibit upregulation of a specific NKA α subunit ( zatp1a1a.5) that, unlike most NKA heterotrimers, would result in transfer of only a single Na⁺ and K⁺ per ATP hydrolysis reaction. Thermodynamic models demonstrate that this change is sufficient to reduce the activation energy of NKA, allowing it to overcome the adverse electrochemical gradient imposed by dilute freshwater. Importantly, upregulation of zatp1a1a.5 also coincides with the recovery of whole body Na⁺ post-transfer, which occurs within 24 h. While these structural modifications are crucial for allowing zebrafish to survive in ion-poor environments, phylogenetic and structural analysis of available α subunits from a range of teleosts suggests this adaptation is not widely distributed.

The septate junction protein Tetraspanin 2A is critical to the structure and function of Malpighian tubules in Drosophila melanogaster

Tetraspanin-2A (Tsp2A) is an integral membrane protein of smooth septate junctions in Drosophila melanogaster. To elucidate its structural and functional roles in Malpighian tubules, we used the c42-GAL4/UAS system to selectively knock down Tsp2A in principal cells of the tubule. Tsp2A localizes to smooth septate junctions (sSJ) in Malpighian tubules in a complex shared with partner proteins Snakeskin (Ssk), Mesh, and Discs large (Dlg). Knockdown of Tsp2A led to the intracellular retention of Tsp2A, Ssk, Mesh, and Dlg, gaps and widening spaces in remaining sSJ, and tumorous and cystic tubules. Elevated protein levels together with diminished V-type H⁺-ATPase activity in Tsp2A knockdown tubules are consistent with cell proliferation and reduced transport activity. Indeed, Malpighian tubules isolated from Tsp2A knockdown flies failed to secrete fluid in vitro. The absence of significant transepithelial voltages and resistances manifests an extremely leaky epithelium that allows secreted solutes and water to leak back to the peritubular side. The tubular failure to excrete fluid leads to extracellular volume expansion in the fly and to death within the first week of adult life. Expression of the c42-GAL4 driver begins in Malpighian tubules in the late embryo and progresses upstream to distal tubules in third instar larvae, which can explain why larvae survive Tsp2A knockdown and adults do not. Uncontrolled cell proliferation upon Tsp2A knockdown confirms the role of Tsp2A as tumor suppressor in addition to its role in sSJ structure and transepithelial transport.

Synthesis and insecticidal activity in vitro and vivo of novel benzenesulfonyl derivatives based on potent target subunit H of V-ATPase

Two lead compounds with benzenesulfonamide were found through virtual screening based on the 3D structure of the subunit H of V-ATPase in previous study. 74 benzenesulfonyl derivatives were synthesized and their insecticidal activities were evaluated. The derivatives with propargyl substituents exhibit excellent insecticidal activities against Mythimna separata Walker. The LD₅₀ values of compounds A5.7 (28.0 μg·g^-1) and B5.7 (36.4 μg·g^-1) were significantly less than that of Celangulin V (344.0 μg·g^-1). Furthermore, Isothermal Titration Calorimetry (ITC) data indicate there is a strong binding affinity between A5.7 and V-ATPase Subunit H. These results demonstrate that it is a practical way to develop pesticides targeting at H subunit of V-ATPase.

Osmoregulatory responses to cadmium in reference and historically metal contaminated Gammarus fossarum (Crustacea, Amphipoda) populations

In order to better understand the variable sensitivities of crustaceans to metals, we investigated the impact of cadmium exposure in 3 populations of Gammarus fossarum from different rivers of France. The first population lives in a Cd-contaminated river from a geochemical background, while the others inhabit Cd-free sites. Osmoregulation, a relevant biomarker to evaluate crustacean health following metal contamination, was used as a proxy to evaluate the intra- and inter-populationnal sensitivities to Cd. Specimens from each population were experimentally exposed to 9 μg Cd²⁺/L Cd for 7 days and hemolymph osmolality (HO) was then individually measured. In exposed populations, high inter-individual variations in HO values were noted, resulting in their separation into non-impacted and slightly or highly Cd-impacted (with lower HO) animals. In gills of impacted organisms, deep histopathological alterations and protein overexpression of Na⁺/K⁺-ATPase and V-H⁺-ATPase were observed through histology and immunolocalization, while non-impacted animals showed profiles comparable to controls. Moreover, the osmoregulatory processes in the population living in the Cd-contaminated site were impacted by acute Cd exposure in the laboratory as much as for one of the two populations originating from Cd-free sites. The observed changes did not reveal any obvious adaptive osmoregulatory phenomena at the population scale, but they may be due to differences in fitness between individuals and between populations in relation to the features of their respective environments, unrelated with the presence of the metal.

A de novo transcriptome of the Malpighian tubules in non-blood-fed and blood-fed Asian tiger mosquitoes Aedes albopictus: insights into diuresis, detoxification, and blood meal processing

Background. In adult female mosquitoes, the renal (Malpighian) tubules play an important role in the post-prandial diuresis, which removes excess ions and water from the hemolymph of mosquitoes following a blood meal. After the post-prandial diuresis, the roles that Malpighian tubules play in the processing of blood meals are not well described.

Methods. We used a combination of next-generation sequencing (paired-end RNA sequencing) and physiological/biochemical assays in adult female Asian tiger mosquitoes (Aedes albopictus) to generate molecular and functional insights into the Malpighian tubules and how they may contribute to blood meal processing (3–24 h after blood ingestion).

Results/Discussion. Using RNA sequencing, we sequenced and assembled the first de novo transcriptome of Malpighian tubules from non-blood-fed (NBF) and blood-fed (BF) mosquitoes. We identified a total of 8,232 non-redundant transcripts. The Malpighian tubules of NBF mosquitoes were characterized by the expression of transcripts associated with active transepithelial fluid secretion/diuresis (e.g., ion transporters, water channels, V-type H⁺-ATPase subunits), xenobiotic detoxification (e.g., cytochrome P450 monoxygenases, glutathione S-transferases, ATP-binding cassette transporters), and purine metabolism (e.g., xanthine dehydrogenase). We also detected the expression of transcripts encoding sodium calcium exchangers, G protein coupled-receptors, and septate junctional proteins not previously described in mosquito Malpighian tubules. Within 24 h after a blood meal, transcripts associated with active transepithelial fluid secretion/diuresis exhibited a general downregulation, whereas those associated with xenobiotic detoxification and purine catabolism exhibited a general upregulation, suggesting a reinvestment of the Malpighian tubules’ molecular resources from diuresis to detoxification. Physiological and biochemical assays were conducted in mosquitoes and isolated Malpighian tubules, respectively, to confirm that the transcriptomic changes were associated with functional consequences. In particular, in vivo diuresis assays demonstrated that adult female mosquitoes have a reduced diuretic capacity within 24 h after a blood meal. Moreover, biochemical assays in isolated Malpighian tubules showed an increase in glutathione S-transferase activity and the accumulation of uric acid (an end product of purine catabolism) within 24 h after a blood meal. Our data provide new insights into the molecular physiology of Malpighian tubules in culicine mosquitoes and reveal potentially important molecular targets for the development of chemical and/or gene-silencing insecticides that would disrupt renal function in mosquitoes.

Evolutionary mechanisms of habitat invasions, using the copepod Eurytemora affinis as a model system

The study of the copepod Eurytemora affinis has provided unprecedented insights into mechanisms of invasive success. In this invited review, I summarize a subset of work from my laboratory to highlight key insights gained from studying E. affinis as a model system. Invasive species with brackish origins are overrepresented in freshwater habitats. The copepod E. affinis is an example of such a brackish invader, and has invaded freshwater habitats multiple times independently in recent years. These invasions were accompanied by the evolution of physiological tolerance and plasticity, increased body fluid regulation, and evolutionary shifts in ion transporter (V‐type H⁺ATPase, Na⁺, K⁺‐ATPase) activity and expression. These evolutionary changes occurred in parallel across independent invasions in nature and in laboratory selection experiments. Selection appears to act on standing genetic variation during invasions, and maintenance of this variation is likely facilitated through ‘beneficial reversal of dominance’ in salinity tolerance across habitats. Expression of critical ion transporters is localized in newly discovered Crusalis leg organs. Increased freshwater tolerance is accompanied by costs to development time and greater requirements for food. High‐food concentration increases low‐salinity tolerance, allowing saline populations to invade freshwater habitats. Mechanisms observed here likely have relevance for other taxa undergoing fundamental niche expansions.

Osmotic/ionic status of body fluids in the euryhaline cephalopod suggest possible parallel evolution of osmoregulation

Acclimation from marine to dilute environments constitutes among the dramatic evolutionary transitions in the history of life. Such adaptations have evolved in multiple lineages, but studies of the blood/hemolymph homeostasis mechanisms are limited to those using evolutionarily advanced Deuterostome (chordates) and Ecdysozoa (crustaceans). Here, we examined hemolymph homeostasis in the advanced Lophotrochozoa/mollusc, the other unexplored taxa, and its possible regulation by the vasopressin/oxytocin superfamily peptides known to be implicated in fluid homeostasis in Chordata and Arthropoda. The hemolymph osmotic and ionic status in the euryhaline cephalopod (Octopus ocellatus) following transfer from 30-ppt normal seawater to 20 ppt salinity indicate hyperosmo- and hyperionoregulatory abilities for more than 1 week, as in crustaceans and teleost fish. While ventilation frequency decreased by 1 day, Na⁺/K⁺-ATPase activity, which has been generally implicated in ion transport, was induced in two of the eight posterior gills after 1 week. In addition, the octopuses were intravenously injected with 1 or 100 ng/g octopressin or cephalotocin, which are Octopus vasopressin/oxytocin orthologs. After 1 day, octopressin, but not cephalotocin, decreased the hemolymph osmolality and Ca concentrations, as well as urinary Na concentrations. These data provide evidence for possible parallel evolution in hyperionoregulatory mechanisms and coordination by conserved peptides.

Targeting renal epithelial channels for the control of insect vectors

Three small molecules were identified in high throughput screens that 1) block renal inward rectifier potassium (Kir) channels of Aedes aegypti expressed in HEK cells and Xenopus oocytes, 2) inhibit the secretion of KCl but not NaCl in isolated Malpighian tubules, and after injection into the hemolymph, 3) inhibit KCl excretion in vivo, and 4) render mosquitoes flightless or dead within 24h. Some mosquitoes had swollen abdomens at death consistent with renal failure. VU625, the most potent and promising small molecule for development as mosquitocide, inhibits AeKir1-mediated currents with an IC50 less than 100 nM. It is highly selective for AeKir1 over mammalian Kir channels, and it affects only 3 of 68 mammalian membrane proteins. These results document 1) renal failure as a new mode-of-action for mosquitocide development, 2) renal Kir channels as molecular target for inducing renal failure, and 3) the promise of the discovery and development of new species-specific insecticides.

Sustained activation of Akt elicits mitochondrial dysfunction to block Plasmodium falciparum infection in the mosquito host

The overexpression of activated, myristoylated Akt in the midgut of female transgenic Anopheles stephensi results in resistance to infection with the human malaria parasite Plasmodium falciparum but also decreased lifespan. In the present study, the understanding of mitochondria-dependent midgut homeostasis has been expanded to explain this apparent paradox in an insect of major medical importance. Given that Akt signaling is essential for cell growth and survival, we hypothesized that sustained Akt activation in the mosquito midgut would alter the balance of critical pathways that control mitochondrial dynamics to enhance parasite killing at some cost to survivorship. Toxic reactive oxygen and nitrogen species (RNOS) rise to high levels in the midgut after blood feeding, due to a combination of high NO production and a decline in FOXO-dependent antioxidants. Despite an apparent increase in mitochondrial biogenesis in young females (3 d), energy deficiencies were apparent as decreased oxidative phosphorylation and increased [AMP]/[ATP] ratios. In addition, mitochondrial mass was lower and accompanied by the presence of stalled autophagosomes in the posterior midgut, a critical site for blood digestion and stem cell-mediated epithelial maintenance and repair, and by functional degradation of the epithelial barrier. By 18 d, the age at which An. stephensi would transmit P. falciparum to human hosts, mitochondrial dysfunction coupled to Akt-mediated repression of autophagy/mitophagy was more evident and midgut epithelial structure was markedly compromised. Inhibition of RNOS by co-feeding of the nitric-oxide synthase inhibitor L-NAME at infection abrogated Akt-dependent killing of P. falciparum that begins within 18 h of infection in 3–5 d old mosquitoes. Hence, Akt-induced changes in mitochondrial dynamics perturb midgut homeostasis to enhance parasite resistance and decrease mosquito infective lifespan. Further, quality control of mitochondrial function in the midgut is necessary for the maintenance of midgut health as reflected in energy homeostasis and tissue repair and renewal.

Malaria is a major public health problem in the world and various strategies are under development for control, including vaccines and transgenic mosquitoes that block parasite transmission. We previously reported that overexpression of the major signaling protein Akt in the midgut of female Anopheles stephensi mosquitoes could impart resistance to infection with the most important human malaria parasite and also reduce the duration of mosquito infectivity to human hosts. However, to use this strategy for malaria transmission control in endemic areas, we must understand the mechanism by which parasites are killed to ensure that transmission of other human pathogens (e.g., viruses, nematodes) is not unexpectedly enhanced and to allow the design of rational, preventive interventions. Here, we report that overexpression of a constitutively active Akt in the mosquito midgut alters important cellular, and in particular, mitochondrial processes – in a manner similar to Akt control of these processes in mammalian cells – to generate high levels of toxic compounds that kill parasites within hours after infection. However, the same alterations in mitochondrial processes that result in parasite killing ultimately reduce mosquito infective lifespan for transmission, indicating that mitochondrial dynamics in the mosquito midgut could be targeted for multi-faceted genetic control of mosquito biology to reduce malaria transmission.

Transcellular and paracellular pathways of transepithelial fluid secretion in Malpighian (renal) tubules of the yellow fever mosquito Aedes aegypti

Isolated Malpighian tubules of the yellow fever mosquito secrete NaCl and KCl from the peritubular bath to the tubule lumen via active transport of Na(+) and K(+) by principal cells. Lumen-positive transepithelial voltages are the result. The counter-ion Cl(-) follows passively by electrodiffusion through the paracellular pathway. Water follows by osmosis, but specific routes for water across the epithelium are unknown. Remarkably, the transepithelial secretion of NaCl, KCl and water is driven by a H(+) V-ATPase located in the apical brush border membrane of principal cells and not the canonical Na(+), K(+) -ATPase. A hypothetical cation/H(+) exchanger moves Na(+) and K(+) from the cytoplasm to the tubule lumen. Also remarkable is the dynamic regulation of the paracellular permeability with switch-like speed which mediates in part the post-blood-meal diuresis in mosquitoes. For example, the blood meal the female mosquito takes to nourish her eggs triggers the release of kinin diuretic peptides that (i) increases the Cl(-) conductance of the paracellular pathway and (ii) assembles V(1) and V(0) complexes to activate the H(+) V-ATPase and cation/H(+) exchange close by. Thus, transcellular and paracellular pathways are both stimulated to quickly rid the mosquito of the unwanted salts and water of the blood meal. Stellate cells of the tubule appear to serve a metabolic support role, exporting the HCO(3)(-) generated during stimulated transport activity. Septate junctions define the properties of the paracellular pathway in Malpighian tubules, but the proteins responsible for the permselectivity and barrier functions of the septate junction are unknown.

A SLC4-like anion exchanger from renal tubules of the mosquito (Aedes aegypti): evidence for a novel role of stellate cells in diuretic fluid secretion

Transepithelial fluid secretion across the renal (Malpighian) tubule epithelium of the mosquito (Aedes aegypti) is energized by the vacuolar-type (V-type) H(+)-ATPase and not the Na(+)-K(+)-ATPase. Located at the apical membrane of principal cells, the V-type H(+)-ATPase translocates protons from the cytoplasm to the tubule lumen. Secreted protons are likely to derive from metabolic H(2)CO(3), which raises questions about the handling of HCO(3)(-) by principal cells. Accordingly, we tested the hypothesis that a Cl/HCO(3) anion exchanger (AE) related to the solute-linked carrier 4 (SLC4) superfamily mediates the extrusion of HCO(3)(-) across the basal membrane of principal cells. We began by cloning from Aedes Malpighian tubules a full-length cDNA encoding an SLC4-like AE, termed AeAE. When expressed heterologously in Xenopus oocytes, AeAE is both N- and O-glycosylated and mediates Na(+)-independent intracellular pH changes that are sensitive to extracellular Cl(-) concentration and to DIDS. In Aedes Malpighian tubules, AeAE is expressed as two distinct forms: one is O-glycosylated, and the other is N-glycosylated. Significantly, AeAE immunoreactivity localizes to the basal regions of stellate cells but not principal cells. Concentrations of DIDS that inhibit AeAE activity in Xenopus oocytes have no effects on the unstimulated rates of fluid secretion mediated by Malpighian tubules as measured by the Ramsay assay. However, in Malpighian tubules stimulated with kinin or calcitonin-like diuretic peptides, DIDS reduces the diuretic rates of fluid secretion to basal levels. In conclusion, Aedes Malpighian tubules express AeAE in the basal region of stellate cells, where this transporter may participate in producing diuretic rates of transepithelial fluid secretion.

Vacuolar-type proton pumps in insect epithelia

Active transepithelial cation transport in insects was initially discovered in Malpighian tubules, and was subsequently also found in other epithelia such as salivary glands, labial glands, midgut and sensory sensilla. Today it appears to be established that the cation pump is a two-component system of a H(+)-transporting V-ATPase and a cation/nH(+) antiporter. After tracing the discovery of the V-ATPase as the energizer of K(+)/nH(+) antiport in the larval midgut of the tobacco hornworm Manduca sexta we show that research on the tobacco hornworm V-ATPase delivered important findings that emerged to be of general significance for our knowledge of V-ATPases, which are ubiquitous and highly conserved proton pumps. We then discuss the V-ATPase in Malpighian tubules of the fruitfly Drosophila melanogaster where the potential of post-genomic biology has been impressively illustrated. Finally we review an integrated physiological approach in Malpighian tubules of the yellow fever mosquito Aedes aegypti which shows that the V-ATPase delivers the energy for both transcellular and paracellular ion transport.

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.

Revisiting the cellular mechanisms of strong luminal alkalinization in the anterior midgut of larval mosquitoes

Here we critically review two recent hypotheses about the mechanism of strong alkalinization by the anterior midgut of mosquito larvae and our tests of these hypotheses. We present experimental evidence against the major components of transport models proposed in these hypotheses. Measurements of the transapical and transbasal proton electrochemical gradients provide an indication of driving forces faced by and generated by the transport mechanisms of the tissue. These measurements confirmed that basal V-ATPase energizes alkalinization. Serotonin stimulates the V-ATPase, as indicated by the ensuing increase in proton-motive force across the basal membrane. Moreover, the neurohormone resulted in a surprisingly large increase in the intracellular pH. The results of inhibitor studies indicate that, contrary to previous proposals, carbonic anhydrase is apparently not involved in supplying acid-base equivalents to the respective transporters. Furthermore, any apical processes proposed to be involved in alkali secretion or acid absorption must be Cl(-) independent and insensitive to DIDS, amiloride, Zn(2+) and ouabain. These results argue against the involvement of putative apical Cl(-)/HCO (-)(3) exchangers, apical H(+) channels, apical cation/proton exchangers and the importance of the apical Na(+)/K(+) pump. The studies analyzed here thus provide both a limitation and direction for further studies of the mechanism of strong alkalinization in this system.

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.

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.

Cationic pathway of pH regulation in larvae of Anopheles gambiae

Anopheles gambiae larvae (Diptera: Culicidae) live in freshwater with low Na(+) concentrations yet they use Na(+) for alkalinization of the alimentary canal, for electrophoretic amino acid uptake and for nerve function. The metabolic pathway by which larvae accomplish these functions has anionic and cationic components that interact and allow the larva to conserve Na(+) while excreting H(+) and HCO(3)(-). The anionic pathway consists of a metabolic CO(2) diffusion process, carbonic anhydrase and Cl(-)/HCO(3)(-) exchangers; it provides weak HCO(3)(-) and weaker CO(3)(2-) anions to the lumen. The cationic pathway consists of H(+) V-ATPases and Na(+)/H(+) antiporters (NHAs), Na(+)/K(+) P-ATPases and Na(+)/H(+) exchangers (NHEs) along with several (Na(+) or K(+)):amino acid(+/-) symporters, a.k.a. nutrient amino acid transporters (NATs). This paper considers the cationic pathway, which provides the strong Na(+) or K(+) cations that alkalinize the lumen in anterior midgut then removes them and restores a lower pH in posterior midgut. A key member of the cationic pathway is a Na(+)/H(+) antiporter, which was cloned recently from Anopheles gambiae larvae, localized strategically in plasma membranes of the alimentary canal and named AgNHA1 based upon its phylogeny. A phylogenetic comparison of all cloned NHAs and NHEs revealed that AgNHA1 is the first metazoan NHA to be cloned and localized and that it is in the same clade as electrophoretic prokaryotic NHAs that are driven by the electrogenic H(+) F-ATPase. Like prokaryotic NHAs, AgNHA1 is thought to be electrophoretic and to be driven by the electrogenic H(+) V-ATPase. Both AgNHA1 and alkalophilic bacterial NHAs face highly alkaline environments; to alkalinize the larva mosquito midgut lumen, AgNHA1, like the bacterial NHAs, would have to move nH(+) inwardly and Na(+) outwardly. Perhaps the alkaline environment that led to the evolution of electrophoretic prokaryotic NHAs also led to the evolution of an electrophoretic AgNHA1 in mosquito larvae. In support of this hypothesis, antibodies to both AgNHA1 and H(+) V-ATPase label the same membranes in An. gambiae larvae. The localization of H(+) V-ATPase together with (Na(+) or K(+)):amino acid(+/-) symporter, AgNAT8, on the same apical membrane in posterior midgut cells constitutes the functional equivalent of an NHE that lowers the pH in the posterior midgut lumen. All NATs characterized to date are Na(+) or K(+) symporters so the deduction is likely to have wide application. The deduced colocalization of H(+) V-ATPase, AgNHA1 and AgNAT8, on this membrane forms a pathway for local cycling of H(+) and Na(+) in posterior midgut. The local H(+) cycle would prevent unchecked acidification of the lumen while the local Na(+) cycle would regulate pH and support Na(+):amino acid(+/-) symport. Meanwhile, a long-range Na(+) cycle first transfers Na(+) from the blood to gastric caeca and anterior midgut lumen where it initiates alkalinization and then returns Na(+) from the rectal lumen to the blood, where it prevents loss of Na(+) during H(+) and HCO(3)(-) excretion. The localization of H(+) V-ATPase and Na(+)/K(+)-ATPase in An. gambiae larvae parallels that reported for Aedes aegypti larvae. The deduced colocalization of the two ATPases along with NHA and NAT in the alimentary canal constitutes a cationic pathway for Na(+)-conserving midgut alkalinization and de-alkalinization which has never been reported before.

Gap junctions in Malpighian tubules ofAedes aegypti

We present electrical, physiological and molecular evidence for substantial electrical coupling of epithelial cells in Malpighian tubules via gap junctions. Current was injected into one principal cell of the isolated Malpighian tubule and membrane voltage deflections were measured in that cell and in two neighboring principal cells. By short-circuiting the transepithelial voltage with the diuretic peptide leucokinin-VIII we largely eliminated electrical coupling of principal cells through the tubule lumen,thereby allowing coupling through gap junctions to be analyzed. The analysis of an equivalent electrical circuit of the tubule yielded an average gap-junction resistance (Rgj) of 431 kΩ between two cells. This resistance would stem from 6190 open gap-junctional channels,assuming the high single gap-junction conductance of 375 pS found in vertebrate tissues. The addition of the calcium ionophore A23187 (2 μmol l–1) to the peritubular Ringer bath containing 1.7 mmol l–1 Ca2+ did not affect the gap-junction resistance, but metabolic inhibition of the tubule with dinitrophenol (0.5 mmol l–1) increased the gap-junction resistance 66-fold,suggesting the regulation of gap junctions by ATP. Lucifer Yellow injected into a principal cell did not appear in neighboring principal cells. Thus, gap junctions allow the passage of current but not Lucifer Yellow. Using RT-PCR we found evidence for the expression of innexins 1, 2, 3 and 7 (named after their homologues in Drosophila) in Malpighian tubules. The physiological demonstration of gap junctions and the molecular evidence for innexin in Malpighian tubules of Aedes aegypti call for the double cable model of the tubule, which will improve the measurement and the interpretation of electrophysiological data collected from Malpighian tubules.

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.

Electrochemical gradients for Na+, K+,Cl– and H+ across the apical membrane in Malpighian (renal) tubule cells ofRhodnius prolixus

Measurements of intracellular and luminal ion activities as well as membrane potential were used to calculate electrochemical gradients for Cl–, Na+, K+ and H+ across the apical membrane during fluid secretion by Malpighian tubules of Rhodnius prolixus. The results show that the contribution of Na+/H+ and/or K+/H+ exchangers to fluid secretion is feasible both in unstimulated and serotonin-stimulated tubules. Similarly, the electrochemical potential for Cl– is consistent with the passive movement of Cl– from cell to lumen through Cl– channels. The contribution of apical K+:Cl– cotransport and/or paracellular Cl– movement to net transepithelial ion transport is thermodynamically unfeasible. pH in the lumen (pH 6.08±0.1, N=6) was more acid than in the bath (pH 7.25±0.01, N=26) and serotonin stimulation produced a significant increase in lumen pH to 6.32±0.04 (N=5). Intracellular pH was 6.97±0.01 and 6.82±0.04 in unstimulated and serotonin-stimulated tubules, respectively. Lumen pH was altered whereas intracellular pH was tightly regulated during serotonin and bumetanide treatment. Furthermore, DIDS or amiloride treatment did not affect intracellular pH. However, intracellular pH shifted 0.25 pH units more acid in Na+-free saline, suggesting that a Na+-dependent pH regulatory mechanism is at play in steady state pH regulation during fluid secretion by Malpighian tubules of Rhodnius prolixus. The data are consistent with a role for a basolateral Na+/H+ exchanger in intracellular pH regulation during fluid secretion.

The V-type H+ ATPase: molecular structure and function, physiological roles and regulation

It was nearly 30 years before the V-type H+ ATPase was admitted to the small circle of bona fide transport ATPases alongside F-type and P-type ATPases. The V-type H+ ATPase is an ATP-driven enzyme that transforms the energy of ATP hydrolysis to electrochemical potential differences of protons across diverse biological membranes via the primary active transport of H+. In turn, the transmembrane electrochemical potential of H+ is used to drive a variety of (i) secondary active transport systems via H+-dependent symporters and antiporters and (ii) channel-mediated transport systems. For example, expression of Cl- channels or transporters next to the V-type H+ ATPase in vacuoles of plants and fungi and in lysosomes of animals brings about the acidification of the endosomal compartment, and the expression of the H+/neurotransmitter antiporter next to the V-type H+ ATPase concentrates neurotransmitters in synaptic vesicles. First found in association with endosomal membranes, the V-type H+ ATPase is now also found in increasing examples of plasma membranes where the proton pump energizes transport across cell membranes and entire epithelia. The molecular details reveal up to 14 protein subunits arranged in (i) a cytoplasmic V1 complex, which mediates the hydrolysis of ATP, and (ii) a membrane-embedded V0 complex, which translocates H+ across the membrane. Clever experiments have revealed the V-type H+ ATPase as a molecular motor akin to F-type ATPases. The hydrolysis of ATP turns a rotor consisting largely of one copy of subunits D and F of the V1 complex and a ring of six or more copies of subunit c of the V0 complex. The rotation of the ring is thought to deliver H+ from the cytoplasmic to the endosomal or extracellular side of the membrane, probably via channels formed by subunit a. The reversible dissociation of V1 and V0 complexes is one mechanism of physiological regulation that appears to be widely conserved from yeast to animal cells. Other mechanisms, such as subunit-subunit interactions or interactions of the V-type H+ ATPase with other proteins that serve physiological regulation, remain to be explored. Some diseases can now be attributed to genetic alterations of specific subunits of the V-type H+ ATPase.

Transport mechanisms of diuresis in Malpighian tubules of insects

We have studied Malpighian tubules of Aedes aegypti using a variety of methods: Ramsay fluid secretion assay, electron probe analysis of secreted fluid, in vitro microperfusion and two-electrode voltage clamp. Collectively, these methods have allowed us to elucidate transepithelial transport mechanisms under control conditions and in the presence of diuretic peptides. Mosquito natriuretic peptide (MNP), a corticotropin-releasing factor (CRF)-like diuretic peptide, selectively increases transepithelial secretion of NaCl and water, meeting the NaCl loads of the blood meal. The intracellular messenger of MNP is cAMP, which increases the Na+ conductance and activates the Na+/K+/2Cl- -cotransporter in the basolateral membrane of principal cells. Leucokinin non-selectively increases transepithelial NaCl and KCl secretion, which may deal with hemolymph volume expansions or reduce the flight pay load upon eclosion from the aquatic habitat. The non-selective NaCl and KCl diuresis stems from the increase in septate junctional Cl- conductance activated by leucokinin using Ca2+ as second messenger. Fundamental to diuretic mechanisms are powerful epithelial transport mechanisms in the distal segment of the Malpighian tubules, where transepithelial secretion rates can exceed the capacity of mammalian glomerular kidneys in the renal turnover of the extracellular fluid compartment. In conjunction with powerful epithelial transport mechanisms driven by the V-type H+-ATPase, diuretic hormones enable hematophagous and probably also phytophagous insects to deal with enormous dietary loads, thereby contributing to the evolutionary success of insects.

Mechanisms of K+ transport across basolateral membranes of principal cells in Malpighian tubules of the yellow fever mosquito, Aedes aegypti

The mechanisms of K+ entry from the hemolymph into principal cells of Malpighian tubules were investigated in the yellow fever mosquito, Aedes aegypti. The K+ channel blocker Ba2+ (5 mmol l–1) significantly decreased transepithelial (TEP) fluid secretion (Vs) from 0.84 nl min–1 to 0.37 nl min–1 and decreased the K+ concentration in secreted fluid from 119.0 mmol l–1 to 54.3 mmol l–1 with no change in the Cl– concentration. Even though the Na+ concentration increased significantly from 116.8 mmol l–1 to 144.6 mmol l–1, rates of TEP ion secretion significantly decreased for all three ions. In addition,Ba2+ had the following significant electrophysiological effects: it depolarized the TEP voltage (Vt) from 19.4 mV to 17.2 mV,increased the TEP resistance (Rt) from 6.4 kΩcm to 6.9 kΩcm, hyperpolarized the basolateral membrane voltage of principal cells (Vbl) from –75.2 mV to –88.2 mV and increased the cell input resistance from 363.7 kΩ to 516.3 kΩ. These effects of Ba2+ reflect the block of K+ channels that, apparently, are also permeable to Na+. Bumetanide (100μmol l–1) had no effect on TEP fluid secretion and electrical resistance but significantly decreased TEP K+ secretion,consistent with the inhibition of electroneutral Na+/K+/2Cl– cotransport. TEP Na+ secretion significantly increased because other Na+entry pathways remained active. Bumetanide plus Ba2+ completely inhibited TEP electrolyte and fluid secretion, with fast and slow kinetics reflecting the Ba2+ block of basolateral membrane K+channels and the inhibition of Na+/K+/2Cl– cotransport, respectively. The single and combined effects of Ba2+ and bumetanide suggest that(1) K+ channels and Na+/K+/2Cl– cotransport are the primary mechanisms for bringing K+ into cells, (2) K+ channels mediate a significant Na+ influx, (3) Na+ has as many as four entry pathways and (4) the mechanisms of TEP K+ and Na+ secretion are coupled such that complete block of TEP K+ renders the epithelium unable to secrete Na+.

Effects of leucokinin-VIII onAedesMalpighian tubule segments lacking stellate cells

The diuretic peptide leucokinin is known to increase fluid secretion in Malpighian tubules of the yellow fever mosquito Aedes aegypti by increasing a transepithelial Cl- conductance. The present study sought to examine whether stellate cells provided this transepithelial conductance in Aedes Malpighian tubules as they do in Drosophila Malpighian tubules. Aedes Malpighian tubule segments with and without stellate cells were perfused in vitro for measurements of the transepithelial voltage (Vt),resistance (Rt) and Cl- diffusion potentials(DPCl). In 11 tubule segments containing both principal cells and stellate cells, 1 μmol l-1 leucokinin-VIII added to the peritubular bath immediately and significantly decreased Vt from 39.3±14.3 mV to 2.3±0.7 mV,decreased Rt from 12.4±2.6 kΩcm to 2.4±0.3 kΩcm, and increased DPCl from 8.2±1.2 mV to 42.1±5.4 mV. These effects of leucokinin-VIII were qualitatively and quantitatively similar in six tubule segments containing no stellate cells; Vt decreased from 37.8±7.0 mV to 3.4±0.6 mV, Rt decreased from 8.8±2.1 kΩcm to 1.7±0.2 kΩcm, and DPClincreased from 5.8±2.6 mV to 50.0±2.1 mV. Thus, stellate cells are not required for signaling or mediating the effects of leucokinin in Malpighian tubules of Aedes aegypti. The results further support previous observations that principal cells signal the effects of leucokinin to increase the Cl- conductance of the paracellular pathway through septate (or tight) junctions.

The V-type H(+)-ATPase in Malpighian tubules of Aedes aegypti: localization and activity

The V-type H(+)-ATPase is thought to provide the driving force for transepithelial electrolyte and fluid secretion in Malpighian tubules. To confirm the presence of this proton pump in Malpighian tubules of the yellow fever mosquito Aedes aegypti, we used several antibodies raised against the V-type H(+)-ATPase of Manduca sexta. Western blot analysis confirmed the presence of the V-type H(+)-ATPase in Malpighian tubules of Aedes aegypti. In situ immunostaining identified the V-type H(+)-ATPase at the apical membrane of the mitochondrion-rich brush border of principal cells. The V-type H(+)-ATPase was not found in stellate cells. Measurements of ATPase activity revealed that bafilomycin-sensitive and NO(3)(-)-sensitive ATPase activity accounted for 50-60% of total ATPase activity in crude extracts of Malpighian tubules. No significant ouabain- or vanadate-sensitive Na(+)/K(+)-ATPase activity was detected. These results support the conclusion reached previously in electrophysiological studies that the mechanisms for transepithelial electrolyte secretion in the Aedes Malpighian tubules rely on the V-type H(+)-ATPase as the principal energizer of epithelial transport. Measures of transepithelial Na(+) and K(+) secretion and estimates of the H(+) flux mediated by the V-type H(+)-ATPase suggest a 1:1 stoichiometry for Na(+)/H(+) and K(+)/H(+) exchange transport across the apical membrane.

Localization and characterization of phenamil-sensitive Na+ influx in isolated rainbow trout gill epithelial cells

Percoll density-gradient separation, combined with peanut lectin agglutinin (PNA) binding and magnetic bead separation, was used to separate dispersed fish gill cells into sub-populations. Functional characterization of each of the sub-populations was performed to determine which displayed acid-activated phenamil- and bafilomycin-sensitive Na(+) uptake. Analysis of the mechanism(s) of (22)Na(+) influx was performed in control and acid-activated (addition of 10 mmoll(-1) proprionic acid) cells using a variety of Na(+) transport inhibitors (ouabain, phenamil, HOE-694 and bumetanide) and a V-type ATPase inhibitor (bafilomycin). We found that cells migrating to a 1.03-1.05 g ml(-1) Percoll interface [pavement cells (PVCs)] possessed the lowest rates of Na(+) uptake and that influx was unchanged during either bafilomycin (10 nmoll(-1)) treatment or internal acidification with addition of proprionic acid (10 mmoll(-1)). Mitochondria-rich (MR) cells that migrated to the 1.05-1.09 g ml(-1) interface of the Percoll gradient demonstrated acidification-activated bafilomycin and phenamil-sensitive Na(+) influx. Further separation of the MR fraction into PNA(+) and PNA(-) fractions using magnetic separation demonstrated that only the PNA(-) cells (alpha-MR cells) demonstrated phenamil-and bafilomycin-sensitive acid-activated (22)Na(+) uptake. We confirm the coupling of a V-type H(+)-ATPase with phenamil-sensitive Na(+) uptake activity and conclude that high-density alpha-MR cells function in branchial Na(+) uptake in freshwater fish.

The dependence of electrical transport pathways in Malpighian tubules on ATP

The relationship between the intracellular ATP concentration [ATP](i) and the electrical properties of principal cells was investigated in Malpighian tubules of the yellow fever mosquito, Aedes aegypti. Under control conditions, [ATP](i) was 0.91 mmol l(-1), the input resistance of the principal cell (R(pc)) was 334.1 k Omega, and the basolateral membrane was marked by a large K(+)-conductance and a membrane voltage (V(bl)) of -75.8 mV. Peritubular cyanide (CN, 0.3 mmol l(-1)) reduced [ATP](i) to 0.08 mmol l(-1) in less than 2 min; however, V(bl) dropped to -8 mV and R(pc) increased to 3150.8 k Omega in 8 min, while the K(+)-conductance of the basolateral membrane disappeared. Upon washout of CN, V(bl) and R(pc) returned to control values within 2 min, and the basolateral membrane recovered its K(+)-conductance. The recovery of normal [ATP](i) took 15 min. Dose-dependence and EC(50) values for the CN-inhibition of V(bl) and the increase in R(pc) were strikingly similar (184.0 micromol l(-1) and 164.4 micromol l(-1)). Similar effects of metabolic inhibition were observed with dinitrophenol (DNP), but the EC(50) values were 50.3 micromol l(-1) and 71.7 micromol l(-1) for the effects on V(bl) and R(pc), respectively. Barium, a blocker of K(+)-channels, significantly hyperpolarized V(bl) to -89.1 mV and increased R(pc) to 769.4 k Omega under control conditions, but had no effects during metabolic inhibition. These results illustrate a temporal relationship between [ATP](i) and electrogenic and conductive transport pathways in principal cells that is consistent with the role of ATP as an integrator of transport steps at apical and basolateral membranes of the cell. When [ATP](i) drops to levels that are 10% of control, the V-type H(+)-ATPase is inhibited, preventing the extrusion of K(+) to the tubule lumen. At the same time, basolateral membrane K(+)-channels close, preventing the entry of K(+) from the hemolymph. Intracellular K(+) homeostasis is thus protected during metabolic inhibition, allowing the cell to re-establish K(+) transport when ATP is synthesized again.