Novel aspects of the transport of organic anions by the malpighian tubules of Drosophila melanogaster

The Drosophila Malpighian tubule as a model for mammalian tubule function

PURPOSE OF REVIEW

Studies of the genetic model organism, Drosophila melanogaster, have unraveled molecular pathways relevant to human physiology and disease. The Malpighian tubule, the Drosophila renal epithelium, is described here, including tools available to study transport; conserved transporters, channels, and the signaling pathways regulating them; and fly models of kidney stone disease.

RECENT FINDINGS

Tools to measure Malpighian tubule transport continue to advance, including use of a transgenic sensor to quantify intracellular pH and proton fluxes. A recent study generated an RNA-sequencing-based atlas of tissue-specific gene expression, with resulting insights into Malpighian tubule gene expression of transporters and channels. Advances have been made in understanding the molecular physiology of the With No Lysine kinase-Ste20-related proline/alanine rich kinase/oxidative stress response kinase cascade that regulates epithelial ion transport in flies and mammals. New studies in Drosophila kidney stone models have characterized zinc transporters and used Malpighian tubules to study the efficacy of a plant metabolite in decreasing stone burden.

SUMMARY

Study of the Drosophila Malpighian tubule affords opportunities to better characterize the molecular physiology of epithelial transport mechanisms relevant to mammalian renal physiology.

The rectal complex and Malpighian tubules of the cabbage looper (Trichoplusia ni): regional variations in Na+ and K+ transport and cation reabsorption by secondary cells

In larvae of most Lepidoptera the distal ends of the Malpighian tubules are closely applied to the rectal epithelia and are ensheathed within the perinephric membrane, thus forming the rectal complex. The cryptonephric Malpighian tubules within the rectal complex are bathed in fluid within a functional compartment, the perinephric space, which is separate from the haemolymph. In this study, the scanning ion-selective electrode technique (SIET) was used to measure transport of Na+ and K+ across the rectal complex and across multiple regions of the Malpighian tubules of larvae of the cabbage looper Trichoplusia ni. Measurements were made in an intact preparation in which connections of the tubules upstream to the rectal complex and downstream to the urinary bladder and gut remained intact. SIET measurements revealed reabsorption of Na+ and K+ across the intact rectal complex and into the bath (haemolymph), with K+ fluxes approximately twice as large as those of Na+. Analyses of fluxes in larvae with empty guts, found in recently moulted larvae, versus those with full guts highlighted differences in the rates of K+ or Na+ transport within tubule regions that appeared morphologically homogeneous, such as the rectal lead. The distal rectal lead of larvae with empty guts reabsorbed K+, whereas the same region secreted K+ in tubules of larvae with full guts. SIET measurements of the ileac plexus also indicated a novel role for secondary (type II) cells in cation reabsorption. Secondary cells reabsorb K+, whereas the adjacent principal (type I) cells secrete K+. Na+ is reabsorbed by both principal and secondary cells, but the rate of reabsorption by the secondary cells is approximately twice the rate in the adjacent principal cells.

Oatp58Dc contributes to blood-brain barrier function by excluding organic anions from the Drosophila brain

The blood-brain barrier (BBB) physiologically isolates the brain from the blood and, thus, plays a vital role in brain homeostasis. Ion transporters play a critical role in this process by effectively regulating access of chemicals to the brain. Organic anion-transporting polypeptides (Oatps) transport a wide range of amphipathic substrates and are involved in efflux of chemicals across the vertebrate BBB. The anatomic complexity of the vascularized vertebrate BBB, however, creates challenges for experimental analysis of these processes. The less complex structure of the Drosophila BBB facilitates measurement of solute transport. Here we investigate a physiological function for Oatp58Dc in transporting small organic anions across the BBB. We used genetic manipulation, immunocytochemistry, and molecular techniques to supplement a whole animal approach to study the BBB. For this whole animal approach, the traceable small organic anion fluorescein was injected into the hemolymph. This research shows that Oatp58Dc is involved in maintaining a chemical barrier against fluorescein permeation into the brain. Oatp58Dc expression was found in the perineurial and subperineurial glia, as well as in postmitotic neurons. We specifically targeted knockdown of Oatp58Dc expression in the perineurial and subperineurial glia to reveal that Oatp58Dc expression in the perineurial glia is necessary to maintain the barrier against fluorescein influx into the brain. Our results show that Oatp58Dc contributes to maintenance of a functional barrier against fluorescein influx past the BBB into the brain.

Effects of genetic knock-down of organic anion transporter genes on secretion of fluorescent organic ions by Malpighian tubules of Drosophila melanogaster

An earlier study has shown that RNAi knock-down of a single organic anion transporter (OAT) gene in the principal cells of Drosophila Malpighian tubules is associated with reductions in the expression of multiple, functionally related genes. In this study, we measured the rates of secretion of four fluorescent ions by tubules isolated from flies expressing targeted RNAi knock-down of specific OAT genes. Droplets secreted by isolated tubules set up in the Ramsay assay were collected in optically flat capillary tubes and the concentrations of fluorescent ions were determined by confocal laser scanning microscopy. Reductions in the expression of organic anion (OA) transporting polypeptide 58Dc (OATP; CG3380) were associated with reduced secretion of the OAs fluorescein and Texas Red. Reduction in the expression of Drosophila multidrug resistance associated protein (dMRP; CG6214) was correlated with reduced secretion of the P-glycoprotein substrate daunorubicin. Secretion of the organic cation quinacrine was unaffected by reduced expression of OATP, dMRP, or a multidrug efflux transporter (MET; CG30344). The results highlight the difficulties of assigning a rate-limiting role in transport of a specific OA to a single membrane transporter.

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.

Genetic knockdown of a single organic anion transporter alters the expression of functionally related genes in Malpighian tubules of Drosophila melanogaster

Insects excrete a wide variety of toxins via the Malpighian (renal) tubules. Previous studies have implicated three transporters in the secretion of the organic anion (OA) methotrexate (MTX) by the Drosophila Malpighian tubule: Drosophila multidrug resistance-associated protein (dMRP, CG6214), a multidrug efflux transporter (MET, CG30344), and an organic anion transporting polypeptide 58Dc (OATP58Dc, CG3380). RNA interference (RNAi) knockdown and P-element insertion mutation of single OA transporter genes were used to evaluate the importance of these three putative transporters in the secretion of MTX by the Malpighian tubules of Drosophila melanogaster. A major finding is that genetic knockdown of a single OA transporter gene leads to reductions in the expression of at least one other OA transporter gene and in secretion of MTX by Malpighian tubules isolated from flies reared on a standard diet. The pattern of changes indicates that decreases in MTX secretion do not correspond to decreases in dMRP expression in all of the RNAi lines. Genetic knockdown of a single OA transporter gene also alters the extent of upregulation of multiple OA transporter genes in the tubules in response to dietary MTX. Knockdown of dMRP is associated with a decrease in MET expression but an increase in OATP expression when flies are reared on MTX-enriched diet. Our results indicate that dMRP and MET are not the dominant MTX transporters in the tubules when flies are reared on MTX-enriched diets. At least one additional transporter, and possibly OATP, are required for MTX secretion. The implications of our results for studies using genetic knockdown techniques to identify OA transporters in whole tissues such as Malpighian tubules are discussed.

First evidence of epithelial transport in tardigrades: a comparative investigation of organic anion transport

We investigated transport of the organic anion Chlorophenol Red (CPR) in the tardigrade Halobiotus crispae using a new method for quantifying non-fluorescent dyes. We compared the results acquired from the tardigrade with CPR transport data obtained from Malpighian tubules of the desert locust Schistocerca gregaria. CPR accumulated in the midgut lumen of H. crispae, indicating that organic anion transport takes place here. Our results show that CPR transport is inhibited by the mitochondrial un-coupler DNP (1 mmol l–1; 81% reduction), the Na+/K+-ATPase inhibitor ouabain (10 mmol l–1; 21% reduction) and the vacuolar H+-ATPase inhibitor bafilomycin (5 μmol l–1; 21% reduction), and by the organic anions PAH (10 mmol l–1; 44% reduction) and probenecid (10 mmol l–1; 61% reduction, concentration-dependent inhibition). Transport by locust Malpighian tubules exhibits a similar pharmacological profile, albeit with markedly higher concentrations of CPR being reached in S. gregaria. Immunolocalization of the Na+/K+-ATPase α-subunit in S. gregaria revealed that this transporter is abundantly expressed and localized to the basal cell membranes. Immunolocalization data could not be obtained from H. crispae. Our results indicate that organic anion secretion by the tardigrade midgut is transporter mediated with likely candidates for the basolateral entry step being members of the Oat and/or Oatp transporter families. From our results, we cautiously suggest that apical H+ and possibly basal Na+/K+ pumps provide the driving force for the transport; the exact coupling between electrochemical gradients generated by the pumps and transport of ions, as well as the nature of the apical exit step, are unknown. This study is, to our knowledge, the first to show active epithelial transport in tardigrades.

Effects of acute or chronic exposure to dietary organic anions on secretion of methotrexate and salicylate by Malpighian tubules of Drosophila melanogaster larvae

The effects of dietary exposure to organic anions on the physiology of isolated Malpighian tubules and on tubule gene expression were examined using larvae of Drosophila melanogaster. Acute (24 h) or chronic (7 d) exposure to type I organic anions (fluorescein or salicylate) was associated with increased fluid secretion rates and increased fluxes of both salicylate and the type II organic anion methotrexate. By contrast, chronic exposure to dietary methotrexate was associated with increased fluid secretion rate and increased flux of methotrexate, but not salicylate. Exposure to methotrexate in the diet resulted in increases in the expression of a multidrug efflux transporter gene (MET; CG30344) in the Malpighian tubules. There were also increases in expression of genes for either a Drosophila multidrug resistance-associated protein (dMRP; CG6214) or an organic anion transporting polypeptide (OATP; CG3380), depending on the concentration of methotrexate in the diet. Exposure to salicylate in the diet was associated with an increase in expression of dMRP and with decreases of MET and OATP. Exposure to dietary salicylate or methotrexate was also associated with different patterns of expression of heat shock protein genes. The results suggest that exposure to specific type I or type II organic anions has multiple effects and results not only in increased organic anion transport but also in increased rates of inorganic ion transport, which drives osmotically-obliged fluid secretion. Increased fluid secretion may enhance secretion of organic anions by eliminating diffusive backflux from the tubule lumen to the hemolymph.

Physiological and molecular characterization of methotrexate transport by Malpighian tubules of adult Drosophila melanogaster

A radioisotope tracer technique and quantitative PCR were used to study the mechanisms and regulation of transepithelial transport of the type II organic anion methotrexate (MTX) by the Malpighian tubules of Drosophila melanogaster. Transport of MTX was saturable and Na(+)-independent; the kinetic parameters J(max) and K(t) were 437fmolmin(-1) and 23.5microM, respectively. The transport of MTX was competitively inhibited by phenol red and probenecid; non-competitively inhibited by salicylate, verapamil and MK-571; and uncompetitively inhibited by Texas Red. Dietary exposure to 0.1mM MTX led to dramatic increases in gene expression for several members of the ABC family of transporters in both the Malpighian tubules and the gut. Our results suggest that multiple transporters are upregulated in response to dietary exposure to MTX. Increased levels of the protein products which may result from expression of these genes may enhance elimination of toxic compounds such as MTX or its metabolites.

Too much of a good thing: how insects cope with excess ions or toxins in the diet

Much of our understanding of the ionoregulatory and excretory physiology of blood-feeding insects can be traced to a series of papers by Simon Maddrell and colleagues in the 1970s and 1980s. These studies of the Malpighian (renal)tubules of Rhodnius prolixus revealed a number of physiological adaptations to the short-term and long-term stresses associated with blood feeding. More recent electrophysiological studies using voltage- and ion-selective microelectrodes have extended our understanding of the mechanisms and control of ion transport by the secretory and reabsorptive segments of the Rhodnius Malpighian tubule. The discovery that the rates of transport of organic anions, urates and Ca2+ are synchronized to coincide with the appearance of the products of blood meal digestion in the haemolymph of Rhodnius has stimulated parallel studies in Drosophila. This recent research has examined how excretory mechanisms for organic cations and organic anions are altered by exposure to such compounds in the diet. These studies also show that the Drosophila Malpighian tubule provides a useful model for analysis of the roles of transporters such as P-glycoproteins and multidrug resistance-associated proteins in the excretion of toxins.

Insights into the Malpighian tubule from functional genomics

Classical physiological study of the Malpighian tubule has led to a detailed understanding of fluid transport and its control across several species. With the sequencing of the Drosophila genome, and the concurrent development of post-genomic technologies such as microarrays,proteomics, metabolomics and systems biology, completely unexpected roles for the insect Malpighian tubule have emerged. As the insect body plan is simpler than that of mammals, tasks analogous to those performed by multiple mammalian organ systems must be shared out among insect tissues. As well as the classical roles in osmoregulation, the Malpighian tubule is highly specialized for organic solute transport, and for metabolism and detoxification. In Drosophila, the adult Malpighian tubule is the key tissue for defence against insecticides such as DDT; and it can also detect and mount an autonomous defence against bacterial invasion. While it is vital to continue to set insights obtained in Drosophila into the context of work in other species, the combination of post-genomic technologies and physiological validation can provide insights that might not otherwise have been apparent for many years.

Intracellular pH homeostasis and serotonin-induced pH changes inCalliphorasalivary glands: the contribution of V-ATPase and carbonic anhydrase

Blowfly salivary gland cells have a vacuolar-type H+-ATPase(V-ATPase) in their apical membrane that energizes secretion of a KCl-rich saliva upon stimulation with serotonin (5-hydroxytryptamine, 5-HT). We have used BCECF to study microfluometrically whether V-ATPase and carbonic anhydrase (CA) are involved in intracellular pH (pHi) regulation,and we have localized CA activity by histochemistry. We show: (1) mean pHi in salivary gland cells is 7.5±0.3 pH units(N=96), higher than that expected from passive H+distribution; (2) low 5-HT concentrations (0.3–3 nmol l–1) induce a dose-dependent acidification of up to 0.2 pH units, with 5-HT concentrations >10 nmol l–1, causing monophasic or multiphasic pH changes; (3) the acidifying effect of 5-HT is mimicked by bath application of cAMP, forskolin or IBMX; (4) salivary gland cells exhibit CA activity; (5) CA inhibition with acetazolamide and V-ATPase inhibition with concanamycin A lead to a slow acidification of steady-state pHi; (6) 5-HT stimuli in the presence of acetazolamide induce an alkalinization that can be decreased by simultaneous application of the V-ATPase inhibitor concanamycin A; (7) concanamycin A removes alkali-going components from multiphasic 5-HT-induced pH changes; (8) NHE activity and a Cl–-dependent process are involved in generating 5-HT-induced pH changes; (9) the salivary glands probably contain a Na+-driven amino acid transporter. We conclude that V-ATPase and CA contribute to steady-state pHi regulation and 5-HT-induced outward H+pumping does not cause an alkalinization of pHi because of cytosolic H+ accumulation attributable to stimulated cellular respiration and AE activity, masking the alkalizing effect of V-ATPase-mediated acid extrusion.

Tissue and developmental expression of a gene from Hessian fly encoding an ABC-active-transporter protein: implications for Malpighian tubule function during interactions with wheat

We report on the transcriptional patterns of a putative white (w) gene encoding an ABC-active-transporter protein during development in Hessian fly, Mayetiola destructor. The deduced amino acid sequence for the Hessian fly white showed 74-77% similarities to white/ATP-binding-cassette proteins and 52-57% similarities to scarlet/ATP-binding-cassette proteins from other dipterans. Conserved ATP-binding motifs and transmembrane alpha-helix segments were identified in the Hessian fly white protein further supporting its function as an ABC-active-transporter similar to the Drosophila white protein. Spatial analysis of transcript levels for white in larval Hessian fly tissues by quantitative real-time PCR revealed the greatest level of transcript in the Malpighian tubules, while analysis of temporal expression during development revealed the highest transcript levels in late 2nd- and early 3rd-instar larvae. Analysis of transcript levels for white in Hessian fly larvae feeding on susceptible and resistant wheat showed greater levels of the transcript in larvae feeding on resistant plants. We speculate the increased transcript level for white in larvae feeding on resistant wheat could be correlated with stress and increased Malpighian tubule activity associated with the metabolism and detoxification of toxic substrates generated either endogenously or encountered exogenously from the host plant.

Transepithelial transport of salicylate by the Malpighian tubules of insects from different orders

The organic anion salicylate is a plant secondary metabolite that protects plants against phytophagous insects. In this study, a combination of salicylate-selective microelectrodes and a radioisotope tracer technique was used to study the transepithelial transport of salicylate by the Malpighian tubules of 10 species of insects from five orders. Our results show that salicylate is transported into the lumen of the Malpighian tubules in all the species evaluated, except Rhodnius prolixus. The transepithelial transport of salicylate by the Malpighian tubules of Drosophila simulans, Drosophila erecta, Drosophila sechellia, and Acheta domesticus was saturable, Na(+)-dependent and inhibited by alpha-cyano-4-hydroxycinnamic acid. This transport system resembles that previously found in tubules of Drosophila melanogaster. In contrast, transepithelial transport of salicylate by Malpighian tubules of Tenebrio molitor, Plagiodera versicolora, Aedes aegypti, and Trichoplusia ni was unaffected by Na(+)-free bathing saline. The presence of both salicylate and salicylate metabolites in the secreted fluid samples from the Malpighian tubules of A. domesticus, R. prolixus, T. molitor, and T. ni indicates that insect Malpighian tubules may both transport and metabolize salicylate. The highest capacities to rid the hemolymph of salicylate were found in T. molitor, P. versicolora and Drosphila spp. Our results suggest that transport of salicylate by the Malpighian tubules might contribute to elimination of this organic anion from the hemolymph, particularly in some species that encounter high levels of organic anion in the diet.

The Malpighian tubule: rapid insights from post-genomic biology

Good osmoregulation is critical to the success of insects, and the Malpighian tubules play a key role in osmoregulation. Recently, the application of genetics and genomics to the Drosophila tubule has revealed far more extensive roles than ion and water transport. Microarray analysis shows that organic solute transporters dominate the tubule transcriptome. The tubule thus has the capability to excrete actively the broadest range of organic solutes and xenobiotics. Such transporters can produce unexpected, emergent roles for the whole tissue; e.g. the tubule is highly resistant to ouabain not because the Na+, K+ ATPase is unimportant, but because it co-localises with a potent alkaloid excretory mechanism. Reinforcing this role in excretion, the tubule expresses very high levels of a particular cytochrome P450s, glutathione-S-transferases and alcohol dehydrogenases which suggest that the tubule plays a major role in metabolism and detoxification of both endogenous solutes and xenobiotics, such as insecticides. Additionally, the tubule plays a significant role in immunity; tubules are capable of sensing bacterial challenge, and mounting an effective killing response by secretion of antimicrobial peptides, entirely independent of the fat body, the canonical immune tissue. The tubule has also proved to be a good model for some human renal disease, and to act as an organotypic 'testbed' for mammalian genes. The tubule can thus bask in a greatly enhanced reputation as a key tissue for an unexpectedly wide range of functions in the insect.

Transepithelial transport of fluorescent p-glycoprotein and MRP2 substrates by insect Malpighian tubules: confocal microscopic analysis of secreted fluid droplets

Transport of fluorescent substrates of p-glycoprotein (P-gp) and multidrug resistance-associated protein 2 (MRP2) by insect Malpighian tubules was examined using confocal laser scanning microscopy (CLSM). Isolated tubules of the cricket Teleogryllus commodus accumulated the MRP2 substrate Texas Red in the cells and lumen at concentrations up to 20 and 40 times,respectively, those in the bathing medium. Quantitative CLSM analysis of fluorochrome transport in some cricket tubules and all Drosophilatubules was not practical because of interfering effects of concretions in the cells and lumen. Samples of fluid secreted by tubules set up in Ramsay assays were therefore collected in hollow rectangle glass capillaries. Transepithelial dye flux was calculated as the product of fluid secretion rate(measured in the Ramsay assay) and dye concentration (measured by CLSM of the fluid samples). Dose–response curves for transport and the ratio of dye concentration in the secreted fluid to that in the bathing medium (S/M) were determined for Texas Red as well as for P-gp substrates (rhodamine 123,daunorubicin), the organic anion fluorescein and the organic cation quinacrine. Transepithelial transport of Texas Red was reduced by the MRP2 inhibitors MK571 and probenecid. Transport of daunorubicin was reduced by the P-gp inhibitors verapamil and quinacrine and also by the organic cation tetraethylammonium. The results indicate the presence of P-gp-like and MRP2-like transporters in the Malpighian tubules of both species.

Characterization of tetraethylammonium uptake across the basolateral membrane of theDrosophilaMalpighian (renal) tubule

Basolateral transport of the prototypical type I organic cation tetraethylammonium (TEA) by the Malpighian tubules of Drosophila melanogaster was studied using measurements of basolateral membrane potential (Vbl) and uptake of [14C]-labeled TEA. TEA uptake was metabolically dependent and saturable (maximal rate of mediated TEA uptake by all potential transport processes, reflecting the total transport capacity of the membrane, 0.87 pmol·tubule−1·min−1; concentration of TEA at 0.5 of the maximal rate of TEA uptake value, 24 μM). TEA uptake in Malpighian tubules was inhibited by a number of type I (e.g., cimetidine, quinine, and TEA) and type II (e.g., verapamil) organic cations and was dependent on Vbl. TEA uptake was reduced in response to conditions that depolarized Vbl(high-K+saline, Na+-free saline, NaCN) and increased in conditions that hyperpolarized Vbl(low-K+saline). Addition of TEA to the saline bathing Malpighian tubules rapidly depolarized the Vbl, indicating that TEA uptake was electrogenic. Blockade of K+channels with Ba2+did not block effects of TEA on Vblor TEA uptake indicating that TEA uptake does not occur through K+channels. This is the first study to provide physiological evidence for an electrogenic carrier-mediated basolateral organic cation transport mechanism in insect Malpighian tubules. Our results also suggest that the mechanism of basolateral TEA uptake by Malpighian tubules is distinct from that found in vertebrate renal tubules.

Specificity of the fluorescein transport process in Malpighian tubules of the cricketAcheta domesticus

We demonstrate the presence of an efficient, multispecific transport system for excretion of organic anions in the Malpighian tubules of the cricket Acheta domesticus using fluorescein (FL) as a model substrate. Malpighian tubules rapidly accumulated FL via a high affinity process(Km=7.75 μmol l–1); uptake was completely eliminated by the prototypical organic anion transport inhibitor probenecid (1 mmol l–1), but not by p-aminohippuric acid (3 mmol l–1). FL uptake was inhibited by monocarboxylic acids at a high concentration (3 mmol l–1), and inhibition was more effective with an increase in the carbon chain of the monocarboxylic acid (37% inhibition by 5-carbon valeric acid, and 89% inhibition by 7-carbon caprylic acid). Likewise, tests using a series of aliphatic glutathione conjugates indicated that only the compound with the longest side-chain(decyl-glutathione) significantly inhibited FL uptake (81% inhibition). FL uptake was inhibited by a number of xenobiotics, including a plant alkaloid(quinine), herbicides (2,4-dichlorophenoxyacetic acid and 4-(2,4-dichlorophenoxy)-butyric acid), and the insecticide metabolites malathion monocarboxylic acid (MMA) and 3-phenoxybenzoic acid (PBA),suggesting that this transport system plays an active role in excretion of xenobiotics from Acheta by Malpighian tubules. HPLC quantification of MMA and PBA accumulation into Malpighian tubules verified that MMA accumulation was via a mediated transport process, but suggested that PBA accumulation was by nonspecific binding. The presence of a transport system in Malpighian tubules that handles at least one pesticide metabolite(MMA) suggests that transport processes could be a mechanism conferring resistance to xenobiotic exposure in insects.

Ion-selective microelectrode analysis of salicylate transport by the Malpighian tubules and gut ofDrosophila melanogaster

Transport of the organic anion salicylate by the Malpighian tubules and gut of larval and adult fruit flies was studied using two salicylate-selective microelectrode methods. The first method combined the high selectivity of tridodecylmethylammonium-based electrodes for salicylate with the self-referencing ion-selective microelectrode technique for non-invasive spatial and temporal analysis of salicylate flux. Measurements with this technique revealed secretion of salicylate across the main and distal segments of the Malpighian tubule as well as the midgut, ileum and rectum. The second method used a salicylate-selective microelectrode to measure the concentration of salicylate in fluid droplets secreted by isolated DrosophilaMalpighian tubules set up in a Ramsay secretion assay. Transepithelial salicylate flux was calculated as the product of fluid secretion rate and secreted fluid salicylate concentration. Measurements with this method revealed that salicylate transport was active and saturable; the kinetic parameters Jmax and Kt were 2.72 pmol min-1 tubule-1 and 0.046 mmol l-1,respectively. Measurements of transepithelial salicylate flux determined by both microelectrode methods were in good agreement. Transepithelial flux measurements measured by microelectrodes were also validated by comparing them with measurements of radiolabelled salicylate levels in secreted droplets. Salicylate concentrations in haemolymph samples were measured with salicylate-selective microelectrodes after injection of salicylate into the haemocoel or after insects were fed salicylate-rich diets. The rate of salicylate secretion by Malpighian tubules in vitro was sufficient to account for the measured rate of decline of salicylate concentration in the haemolymph in vivo.

Dietary protein-induced changes in excretory function: a general animal design feature

Mammals are ureotelic and respond to an increased protein intake with an increase in glomerular filtration rate and renal plasma flow. Birds and terrestrial insects are uricotelic and following a high protein intake increase tubular urate secretion by the kidney (birds) or Malpighian tubule (insects). Ureogenic fish given NH(4)Cl increase gill and renal clearance of urea and gill clearance of ammonia. Renal mass increases in mammals, birds and reptiles given a high protein intake. Thus, animals in general respond to an increase in protein intake with a change in excretory function such as to increase the clearance of the major nitrogenous end-products of protein metabolism. The components of this general animal excretory response include; a redistribution of regional perfusion with increased renal and gill blood flow, increased GFR and gill ammonia clearance, increased renal tubular urate clearance, changes in urea transport protein abundance and/or function and renal hypertrophy. Animal groups differ as to which components are accentuated. Amino acid catabolism with generation of ammonia appears to be a necessary prerequisite for this excretory response to occur. A hypothesis is put forward that ammonia itself is a regulatory molecule and an important signal communicating between amino acid catabolism following an increase in protein intake and the sequence of events leading to a change in excretory function.

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.

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.

Organic cation transport by Malpighian tubules ofDrosophila melanogaster: application of two novel electrophysiological methods

Transport of the prototypical organic cation tetraethylammonium (TEA) by the Malpighian tubules, ureters and gut of Drosophila melanogasterwas studied using two novel electrophysiological techniques. Both techniques exploited the high selectivity of the cation exchanger potassium tetra-p-chlorophenylborate for tetraalkylammonium compounds relative to inorganic cations such as K+. In the first technique, TEA fluxes were measured using a non-invasive self-referencing TEA-selective microelectrode positioned in the unstirred layer near the surface of each tissue. TEA fluxes from bath to lumen as large as 6 pmol cm–2s–1 were measured across the lower (reabsorptive) segment of the Malpighian tubule and the ureter bathed in saline containing 0.1 mmol l–1 TEA. Corresponding bath-to-lumen fluxes across the secretory main segment of the Malpighian tubule and the posterior midgut were∼1 pmol cm–2 s–1. TEA transport by the lower Malpighian tubule was enhanced by hyperpolarization of the basolateral membrane potential and was inhibited by cimetidine, quinidine, vinblastine and verapamil. In the second technique, TEA concentration was measured using a TEA-selective microelectrode positioned in droplets of fluid secreted by Malpighian tubules set up in saline droplets under oil in a Ramsay assay. Results from the Ramsay assay confirmed the dominant role of the lower Malpighian tubule in net transepithelial secretion of TEA and inhibition of TEA transport by cimetidine. Kinetic parameters (Jmax and Kt) were determined using both approaches.

Inorganic and organic anion transport by insect renal epithelia

Insect renal organs typically exhibit high rates of transport of inorganic and organic anions, and therefore provide useful models for the study of epithelial anion transport and its control. Isolated Malpighian tubules of some species secrete a volume of iso-osmotic fluid equal to their own volume in 10-15 s, which means that cellular Cl(-) content is exchanged every 3-5 s. Anion transport can also be achieved against extreme thermodynamic gradients. The concentration of K(+) and Cl(-) in the lumen of the Malpighian tubules of some desert beetles approaches or exceeds saturation. A basolateral Na(+):K(+):2Cl(-) cotransporter plays an important role in vectorial ion transport in Malpighian tubules of many species, but there is also evidence for coupling of Cl(-) transport to the movement of a single cationic species (Na(+) or K(+)). Although an apical vacuolar H(+)-ATPase plays a primary role in energizing transepithelial secretion of chloride via channels or cotransporters in the secretory segment of the Malpighian tubule, several different ATPases have been implicated in reabsorption of Cl(-) by the lower Malpighian tubule or hindgut. Chloride transport is known to be controlled by several neuropeptides, amines and intracellular second messengers. Insect renal epithelia are also important in excretion of potentially toxic organic anions, and the transporters involved may play a role in resistance to insecticides of natural or anthropogenic origin.

Inhibition of the SAPK/JNK pathway blocks the stimulatory effects of glutamine on fluid secretion by the Malpighian tubules of Rhodnius prolixus

Physiological levels of amino acids such as glutamine, glutamate, aspartate and proline increase the rates of fluid secretion and ion transport by serotonin-stimulated Malpighian tubules (MTs) of Rhodnius prolixus. Here, we examine the proposal that the effects of glutamine are mediated through activation of specific kinases to produce the observed increases in fluid secretion. The glutamine-dependent increase in MT fluid secretion rate was blocked by two chemically unrelated inhibitors of the stress activated protein kinase (SAPK) pathway, SP600125 and dicumarol. Inhibitors of phosphatidyl inositol-3 kinase, p38 mitogen activated protein kinase (MAPK), extracellular-signal regulated kinases and MAPK kinase did not block glutamine's effects on fluid secretion rate when applied at commonly used concentrations. Inhibitors of protein kinase A or C reduced fluid secretion rates of serotonin-stimulated MTs, but did not block the response to glutamine. The glutamine-dependent increase in fluid secretion was also insensitive to cytoskeletal disrupting agents and protein synthesis inhibitors. Results of this study are the first to suggest a role for the SAPK pathway in the control of fluid secretion rates by insect MTs.

Dual origin of the renal tubules in Drosophila: mesodermal cells integrate and polarize to establish secretory function

Organs are made up of cells from separate origins, whose development and differentiation must be integrated to produce a physiologically coherent structure. For example, during the development of the kidney, a series of interactions between the epithelial mesonephric duct and the surrounding metanephric mesenchyme leads to the formation of renal tubules. Cells of the metanephric mesenchyme first induce branching of the mesonephric duct to form the ureteric buds, and they then respond to signals derived from them. As a result, mesenchymal cells are recruited to the buds, where they undergo a mesenchymal-to-epithelial transition as they condense to form nephrons. In contrast, the simple renal tubules of invertebrates, such as insect Malpighian tubules (MpTs), have always been thought to arise from single tissue primordia, epithelial buds that grow by cell division and enlargement and from which a range of specialized subtypes differentiate. Here, we reveal unexpected parallels between the development of Drosophila MpTs and vertebrate nephrogenesis by showing that the MpTs also derive from two cell populations: ectodermal epithelial buds and the surrounding mesenchymal mesoderm. The mesenchymal cells are recruited to the growing tubules, where they undergo a mesenchymal-to-epithelial transition as they integrate and subsequently differentiate as a physiologically distinctive subset of tubule cells, the stellate cells. Strikingly, the normal incorporation of stellate cells and the later physiological activity of the mature tubules depend on the activity of hibris, an ortholog of mammalian NEPHRIN.

Membrane conductances of principal cells in Malpighian tubules of Aedes aegypti

Two-electrode voltage clamp (TEVC) methods were used to explore conductive transport pathways in principal cells, the dominant cell type in Malpighian tubules of the yellow fever mosquito. The basolateral membrane of principal cells had a voltage (V(bl)) of -85.1 mV in 49 principal cells under control conditions. Measures of the input resistance R(pc) together with membrane fractional resistance yielded estimates of the conductance of the basolateral membrane (g(bl) = 1.48 &mgr;S) and the apical membrane (g(a) = 3.13 &mgr;S). K(+) channels blocked by barium accounted for 0.94 &mgr;S of g(bl). Estimates of transference numbers yielded the basolateral membrane Na(+) conductance of 0.24 &mgr;S, leaving 0.30 &mgr;S (20%) of g(bl) unaccounted. The secretagogue db-cAMP (0.1 mM), a known activator of the basolateral membrane Na(+) conductance, significantly depolarized V(bl) to -65.0 mV and significantly increased g(bl) from 1.48 &mgr;S to 2.47 &mgr;S. The increase was blocked with amiloride (1 mM), a known blocker of epithelial Na(+) transport. The inhibition of metabolism with di-nitrophenol significantly depolarized V(bl) to -9.7 mV and significantly increased R(pc) from 391.6 kOmega to 2612.5 kOmega. Similar results were obtained with cyanide, but it remains unclear whether the large increases in R(pc) stem from the uncoupling of epithelial cells and/or the shutdown of conductive transport pathways in basolateral and apical membranes. Our results indicate that the apical membrane of principal cells is more than twice as conductive as the basolateral membrane. Partial ionic conductances suggest the rate-limiting step for transepithelial Na(+) secretion at the basolateral membrane.