Sulphonylurea sensitivity and enriched expression implicate inward rectifier K+ channels in Drosophila melanogaster renal function

Voltage-gated ion channels as novel regulators of epithelial ion transport in the osmoregulatory organs of insects

Voltage-gated ion channels (VGICs) respond to changes in membrane potential (Vm) and typically exhibit fast kinetic properties. They play an important role in signal detection and propagation in excitable tissues. In contrast, the role of VGICs in non-excitable tissues like epithelia is less studied and less clear. Studies in epithelia of vertebrates and invertebrates demonstrate wide expression of VGICs in epithelia of animals. Recently, VGICs have emerged as regulators of ion transport in the Malpighian tubules (MTs) and other osmoregulatory organs of insects. This mini-review aims to concisely summarize which VGICs have been implicated in the regulation of ion transport in the osmoregulatory epithelia of insects to date, and highlight select groups for further study. We have also speculated on the roles VGICs may potentially play in regulating processes connected directly to ion transport in insects (e.g., acid-base balance, desiccation, thermal tolerance). This review is not meant to be exhaustive but should rather serve as a thought-provoking collection of select existing highlights on VGICs, and to emphasize how understudied this mechanism of ion transport regulation is in insect epithelia.

Cloning, functional expression, and pharmacological characterization of inwardly rectifying potassium channels (Kir) from Apis mellifera

Potassium channels belong to the super family of ion channels and play a fundamental role in cell excitability. Kir channels are potassium channels with an inwardly rectifying property. They play a role in setting the resting membrane potential of many excitable cells including neurons. Although putative Kir channel family genes can be found in the Apis mellifera genome, their functional expression, biophysical properties, and sensitivity to small molecules with insecticidal activity remain to be investigated. We cloned six Kir channel isoforms from Apis mellifera that derive from two Kir genes, AmKir1 and AmKir2, which are present in the Apis mellifera genome. We studied the tissue distribution, the electrophysiological and pharmacological characteristics of three isoforms that expressed functional currents (AmKir1.1, AmKir2.2, and AmKir2.3). AmKir1.1, AmKir2.2, and AmKir2.3 isoforms exhibited distinct characteristics when expressed in Xenopus oocytes. AmKir1.1 exhibited the largest potassium currents and was impermeable to cesium whereas AmKir2.2 and AmKir2.3 exhibited smaller currents but allowed cesium to permeate. AmKir1 exhibited faster opening kinetics than AmKir2. Pharmacological experiments revealed that both AmKir1.1 and AmKir2.2 are blocked by the divalent ion barium, with IC50 values of 10-5 and 10-6 M, respectively. The concentrations of VU041, a small molecule with insecticidal properties required to achieve a 50% current blockade for all three channels were higher than those needed to block Kir channels in other arthropods, such as the aphid Aphis gossypii and the mosquito Aedes aegypti. From this, we conclude that Apis mellifera AmKir channels exhibit lower sensitivity to VU041.

Mechanisms of Systemic Osmoregulation in Insects

Water is essential to life. Terrestrial insects lose water by evaporation from the body surface and respiratory surfaces, as well as in the excretory products, posing a challenge made more acute by their high surface-to-volume ratio. These losses must be kept to a minimum and be offset by water gained from other sources. By contrast, insects such as the blood-sucking bug Rhodnius prolixus consume up to 10 times their body weight in a single blood meal, necessitating rapid expulsion of excess water and ions. How do insects manage their ion and water budgets? A century of study has revealed a great deal about the organ systems that insects use to maintain their ion and water balance and their regulation. Traditionally, a taxonomically wide range of species were studied, whereas more recent research has focused on model organisms to leverage the power of the molecular genetic approach. Key advances in new technologies have become available for a wider range of species in the past decade. We document how these approaches have already begun to inform our understanding of the diversity and conservation of insect systemic osmoregulation. We advocate that these technologies be combined with traditional approaches to study a broader range of nonmodel species to gain a comprehensive overview of the mechanism underpinning systemic osmoregulation in the most species-rich group of animals on earth, the insects.

Gliotransmission of D-serine promotes thirst-directed behaviors in Drosophila

Thirst emerges from a range of cellular changes that ultimately motivate an animal to consume water. Although thirst-responsive neuronal signals have been reported, the full complement of brain responses is unclear. Here, we identify molecular and cellular adaptations in the brain using single-cell sequencing of water-deprived Drosophila. Water deficiency primarily altered the glial transcriptome. Screening the regulated genes revealed astrocytic expression of the astray-encoded phosphoserine phosphatase to bi-directionally regulate water consumption. Astray synthesizes the gliotransmitter D-serine, and vesicular release from astrocytes is required for drinking. Moreover, dietary D-serine rescues aay-dependent drinking deficits while facilitating water consumption and expression of water-seeking memory. D-serine action requires binding to neuronal NMDA-type glutamate receptors. Fly astrocytes contribute processes to tripartite synapses, and the proportion of astrocytes that are themselves activated by glutamate increases with water deprivation. We propose that thirst elevates astrocytic D-serine release, which awakens quiescent glutamatergic circuits to enhance water procurement.

Early patterning followed by tissue growth establishes distal identity in Drosophila Malpighian tubules

Specification and elaboration of proximo-distal (P-D) axes for structures or tissues within a body occurs secondarily from that of the main axes of the body. Our understanding of the mechanism(s) that pattern P-D axes is limited to a few examples such as vertebrate and invertebrate limbs. Drosophila Malpighian/renal tubules (MpTs) are simple epithelial tubules, with a defined P-D axis. How this axis is patterned is not known, and provides an ideal context to understand patterning mechanisms of a secondary axis. Furthermore, epithelial tubules are widespread, and their patterning is not well understood. Here, we describe the mechanism that establishes distal tubule and show this is a radically different mechanism to that patterning the proximal MpT. The distal domain is patterned in two steps: distal identity is specified in a small group of cells very early in MpT development through Wingless/Wnt signalling. Subsequently, this population is expanded by proliferation to generate the distal MpT domain. This mechanism enables distal identity to be established in the tubule in a domain of cells much greater than the effective range of Wingless.

The Molecular Physiology and Toxicology of Inward Rectifier Potassium Channels in Insects

Inward rectifier K⁺ (Kir) channels have been studied extensively in mammals, where they play critical roles in health and disease. In insects, Kir channels have recently been found to be key regulators of diverse physiological processes in several tissues. The importance of Kir channels in insects has positioned them to serve as emerging targets for the development of insecticides with novel modes of action. In this article, we provide the first comprehensive review of insect Kir channels, highlighting the rapid progress made in understanding their molecular biology, physiological roles, pharmacology, and toxicology. In addition, we highlight key gaps in our knowledge and suggest directions for future research to advance our understanding of Kir channels and their roles in insect physiology. Further knowledge of their functional roles will also facilitate their exploitation as targets for controlling arthropod pests and vectors of economic, medical, and/or veterinary relevance.

Nonapoptotic role of caspase-3 in regulating Rho1GTPase-mediated morphogenesis of epithelial tubes of Drosophila renal system

BACKGROUND

Cells trigger caspase-mediated apoptosis to eliminate themselves from the system when tissue needs to be sculptured, or they detect any abnormality within them, thus preventing irreparable damage to the host. However, nonapoptotic activities of caspases are also involved in many cellular functions. Interestingly, Drosophila Malpighian tubules (MTs) express apoptotic proteins, without succumbing to cell death.

RESULTS

We show apoptosis-independent role of executioner caspase-3, Drice, in MT morphogenesis. Drice is required for precise cytoskeleton organization and convergent extension, failing which morphology, size, cell number, and arrangement get affected. Furthermore, characteristic stellate cell shape transformation in MTs is also governed by Drice. Genetic interaction study shows that Drice mediates its action by regulating Rho1GTPase functionally, and localization of polarity protein Disc large. Subsequently, downregulation of Rho1GTPase in Drice mutants significantly rescues the cystic MTs phenotype. The study shows a mechanism by which Drice governs tubulogenesis via Rho1GTPase-mediated coordinated organization of actin cytoskeleton and membrane stabilization.

CONCLUSION

Collectively our findings suggest a nonapoptotic function of caspase-3 in fine-tuning of cellular rearrangement during tubule development, and these results will add to the growing understanding of diverse roles of caspases during its evolution in metazoans.

Updates on ion and water transport by the Malpighian tubule

The Malpighian (renal) tubule is capable of transporting fluid at remarkable rates. This review will focus on recent insights into the mechanisms by which these high rates are achieved and controlled, with particular reference to the tubules of Drosophila melanogaster, in which the combination of physiology and genetics has led to particularly rapid progress. Like many vertebrate epithelia, the Drosophila tubule has specialized cell types, with active cation transport confined to a large, metabolically active principal cell; whereas the smaller intercalated stellate cell controls chloride and water shunts to achieve net fluid secretion. Recently, the genes underlying many of these processes have been identified, functionally validated and localized within the tubule. The imminent arrival of new types of post-genomic data (notably single cell sequencing) will herald an exciting era of new discovery.

High sucrose diet induces morphological, structural and functional impairments in the renal tubules of Drosophila melanogaster: A model for studying type-2 diabetes mediated renal tubular dysfunction

Continuous feeding of high dietary sugar is strongly associated with type 2 diabetes (T2D) and its secondary complications. Diabetic nephropathy (DN) is a major secondary complication that leads to glomerular and renal tubular dysfunction. The present study is aimed to investigate the effects of chronic exposure of high sugar diet (HSD) on renal tubules. Malpighian tubules (MTs), a renal organ of Drosophila, were used as a model in the study. Feeding of HSD develops T2D condition in Drosophila. The MTs showed structural abnormalities in 20 days of HSD fed flies. Impaired insulin signaling, oxidative stress, enhanced levels of AGE-RAGE and induction of apoptosis were observed in the MTs of these flies. Further, altered expression of transporters, enhanced uric acid level and reduced fluid secretion rate confirmed the impaired function of MTs in these flies. RNA-seq and RT-PCR analyses in the MTs of HSD fed-and control-flies revealed the altered expression of candidate genes that regulate several important pathways including extracellular matrix (ECM), advanced glycation end products-receptor for advanced glycation end products (AGE-RAGE), transforming growth factor β (TGF-β), galactose, starch and sucrose metabolism that are well known mediators of renal tubular dysfunction in DN patients. Disruption of insulin signaling in the MTs also causes renal tubular dysfunction similar to HSD fed flies. Overall, the study suggests that phenotypes observed in the MTs of HSD fed flies recapitulate several hallmarks of renal tubular dysfunction in DN patients. Therefore, we conclude that MTs of HSD fed flies may be used for deciphering the underlying mechanisms of T2D mediated renal tubular dysfunction.

Genome-wide transcriptional effects of deletions of sulphur metabolism genes in Drosophila melanogaster

In recent years, the gasotransmitter hydrogen sulphide (H₂S), produced by the transsulphuration pathway, has been recognized as a biological mediator playing an important role under normal conditions and in various pathologies in both eukaryotes and prokaryotes. The transsulphuration pathway (TSP) includes the conversion of homocysteine to cysteine following the breakdown of methionine. In Drosophila melanogaster and other eukaryotes, H₂S is produced by cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE), and 3-mercaptopyruvate sulphurtransferase (MST). In the experiments performed in this study, we were able to explore the CRISPR/Cas9 technique to obtain single and double deletions in homozygotes of these three major genes responsible for H₂S production in Drosophila melanogaster. In most cases, the deletion of one studied gene does not result in the compensatory induction of two other genes responsible for H₂S production. Transcriptomic studies demonstrated that the deletions of the above CBS and CSE genes alter genome expression to different degrees, with a more pronounced effect being exerted by deletion of the CBS gene. Furthermore, the double deletion of both CBS and CSE resulted in a cumulative effect on transcription in the resulting strains. Overall, we found that the obtained deletions affect numerous genes involved in various biological pathways. Specifically, genes involved in the oxidative reduction process, stress-response genes, housekeeping genes, and genes participating in olfactory and reproduction are among the most strongly affected. Furthermore, characteristic differences in the response to the deletions of the studied genes are apparently organ-specific and have clear-cut sex-specific characteristics. Single and double deletions of the three genes responsible for the production of H₂S helped to elucidate new aspects of the biological significance of this vital physiological mediator.

Identification and Expression of Inward-Rectifying Potassium Channel Subunits in Plutella xylostella

In insects, inward-rectifying potassium (Kir) channels regulate vital physiological functions, such as feeding behavior, silk secretion, renal excretion, and immune function. Therefore, they offer promising potential as targets for insecticides. Three types of Kir subunits have been identified in Diptera and Hemiptera, but the Kir subunits of Lepidoptera still remain unclear. This study identified five Kir subunit genes (pxkir1, pxkir2, pxkir3A, pxkir3B, and pxkir4) in the transcriptome of Plutella xylostella. Phylogenetic analysis identified pxkir1, pxkir2, pxkir3A, and pxkir3B as orthologous genes of kir1–3 in other insects. Interestingly, pxkir4 may be encoding a new class of Kir subunit in Lepidoptera that has not been reported to date. To identify further Kir channel subunits of P. xylostella, the gene expression profiles of five pxkir genes were studied by quantitative real-time PCR. These pxkir genes are expressed throughout the development of P. xylostella. pxkir1 and pxkir2 were highly expressed in thoraxes and legs, while pxkir3 (3A and 3B) and pxkir4 had high expression levels in the midgut and Malpighian tubules. This study identified the composition and distribution of Kir subunits in P. xylostella for the first time, and provides useful information for the further study of Kir channel subunits in Lepidoptera.

Small-Molecule Inhibitors of Inward Rectifier Potassium (Kir) Channels Reduce Bloodmeal Feeding and Have Insecticidal Activity Against the Horn Fly (Diptera: Muscidae)

Bloodmeal feeding by the horn fly, Haematobia irritans (L.), is associated with reduced milk production and blood loss that ultimately prevents weight gain of calves and yearlings. Thus, blood feeding by H. irritans causes significant economic losses in several continents. As with other arthropods, resistance to the majority of commercialized insecticides reduces the efficacy of current control programs. Thus, innovative technologies and novel biochemical targets for horn fly control are needed. Salivary gland and Malpighian tubule function are critical for H. irritans survivorship as they drive bloodmeal acquisition and maintain ion- and fluid homeostasis during bloodmeal processing, respectively. Experiments were conducted to test the hypothesis that pharmacological modulation of H. irritans inward rectifier potassium (Kir) channels would preclude blood feeding and induce mortality by reducing the secretory activity of the salivary gland while simultaneously inducing Malpighian tubule failure. Experimental results clearly indicate structurally diverse Kir channel modulators reduce the secretory activity of the salivary gland by up to fivefold when compared to control and the reduced saliva secretion was highly correlated to a reduction in bloodmeal acquisition in adult flies. Furthermore, adult feeding on blood treated with Kir channel modulators resulted in significant mortality. In addition to validating the Kir channels of H. irritans as putative insecticide targets, the knowledge gained from this study could be applied to develop novel therapeutic technologies targeting salivary gland or Malpighian tubule function to reduce the economic burden of horn fly ectoparasitism on cattle health and production.

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.

Physiology, Development, and Disease Modeling in the Drosophila Excretory System

The insect excretory system contains two organ systems acting in concert: the Malpighian tubules and the hindgut perform essential roles in excretion and ionic and osmotic homeostasis. For over 350 years, these two organs have fascinated biologists as a model of organ structure and function. As part of a recent surge in interest, research on the Malpighian tubules and hindgut of Drosophila have uncovered important paradigms of organ physiology and development. Further, many human disease processes can be modeled in these organs. Here, focusing on discoveries in the past 10 years, we provide an overview of the anatomy and physiology of the Drosophila excretory system. We describe the major developmental events that build these organs during embryogenesis, remodel them during metamorphosis, and repair them following injury. Finally, we highlight the use of the Malpighian tubules and hindgut as accessible models of human disease biology. The Malpighian tubule is a particularly excellent model to study rapid fluid transport, neuroendocrine control of renal function, and modeling of numerous human renal conditions such as kidney stones, while the hindgut provides an outstanding model for processes such as the role of cell chirality in development, nonstem cell-based injury repair, cancer-promoting processes, and communication between the intestine and nervous system.

Specialized stellate cells offer a privileged route for rapid water flux in Drosophila renal tubule

Insects are highly successful, in part through an excellent ability to osmoregulate. The renal (Malpighian) tubules can secrete fluid faster on a per-cell basis than any other epithelium, but the route for these remarkable water fluxes has not been established. In Drosophila melanogaster, we show that 4 genes of the major intrinsic protein family are expressed at a very high level in the fly renal tissue: the aquaporins (AQPs) Drip and Prip and the aquaglyceroporins Eglp2 and Eglp4 As predicted from their structure, and by their transport function by expressing these proteins in Xenopus oocytes, Drip, Prip, and Eglp2 show significant and specific water permeability, whereas Eglp2 and Eglp4 show very high permeability to glycerol and urea. Knockdowns of any of these genes result in impaired hormone-induced fluid secretion. The Drosophila tubule has 2 main secretory cell types: active cation-transporting principal cells, wherein the aquaglyceroporins localize to opposite plasma membranes, and small stellate cells, the site of the chloride shunt conductance, with these AQPs localizing to opposite plasma membranes. This suggests a model in which osmotically obliged water flows through the stellate cells. Consistent with this model, fluorescently labeled dextran, an in vivo marker of membrane water permeability, is trapped in the basal infoldings of the stellate cells after kinin diuretic peptide stimulation, confirming that these cells provide the major route for transepithelial water flux. The spatial segregation of these components of epithelial water transport may help to explain the unique success of the higher insects in regulating their internal environments.

Pharmacological Inhibition of Inward Rectifier Potassium Channels Induces Lethality in Larval Aedes aegypti

The inward rectifier potassium (Kir) channels play key roles in the physiology of mosquitoes and other insects. Our group, among others, previously demonstrated that small molecule inhibitors of Kir channels are promising lead molecules for developing new insecticides to control adult female mosquitoes. However, the potential use of Kir channel inhibitors as larvicidal agents is unknown. Here we tested the hypothesis that pharmacological inhibition of Kir channels in the larvae of Aedes aegypti, the vector of several medically important arboviruses, induces lethality. We demonstrated that adding barium, a non-specific blocker of Kir channels, or VU041, a specific small-molecule inhibitor of mosquito Kir1 channels, to the rearing water (deionized H₂O) of first instar larvae killed them within 48 h. We further showed that the toxic efficacy of VU041 within 24 h was significantly enhanced by increasing the osmolality of the rearing water to 100 mOsm/kg H₂O with NaCl, KCl or mannitol; KCl provided the strongest enhancement compared to NaCl and mannitol. These data suggest: (1) the important role of Kir channels in the acclimation of larvae to elevated ambient osmolality and KCl concentrations; and (2) the disruption of osmoregulation as a potential mechanism of the toxic action of VU041. The present study provides the first evidence that inhibition of Kir channels is lethal to larval mosquitoes and broadens the potential applications of our existing arsenal of small molecule inhibitors of Kir channels, which have previously only been considered for developing adulticides.

Inward rectifier potassium (Kir) channels in the soybean aphid Aphis glycines: Functional characterization, pharmacology, and toxicology

Inward rectifier K+ (Kir) channels contribute to a variety of physiological processes in insects and are emerging targets for insecticide development. Previous studies on insect Kir channels have primarily focused on dipteran species (e.g., mosquitoes, fruit flies). Here we identify and functionally characterize Kir channel subunits in a hemipteran insect, the soybean aphid Aphis glycines, which is an economically important insect pest and vector of soybeans. From the transcriptome and genome of Ap. glycines we identified two cDNAs, ApKir1 and ApKir2, encoding Kir subunits that were orthologs of insect Kir1 and Kir2, respectively. Notably, a gene encoding a Kir3 subunit was absent from the transcriptome and genome of Ap. glycines, similar to the pea aphid Acyrthosiphon pisum. Heterologous expression of ApKir1 and ApKir2 in Xenopus laevis oocytes enhanced K+-currents in the plasma membrane; these currents were inhibited by barium and the small molecule VU041. Compared to ApKir2, ApKir1 mediated currents that were larger in magnitude, more sensitive to barium, and less inhibited by small molecule VU041. Moreover, ApKir1 exhibited stronger inward rectification compared to ApKir2. Topical application of VU041 in adult aphids resulted in dose-dependent mortality within 24 h that was more efficacious than flonicamid, an established insecticide also known to inhibit Kir channels. We conclude that despite the apparent loss of Kir3 genes in aphid evolution, Kir channels are important to aphid survival and represent a promising target for the development of new insecticides.

Molecular mechanisms of bi-directional ion transport in the Malpighian tubules of a lepidopteran crop pest, Trichoplusia ni

Classical studies have described in detail the complex and regionalized morphology of the Malpighian tubule (MT) in larval Lepidoptera. Recent studies revealed unusual aspects of ion transport in the Malpighian tubules of the larva of the cabbage looper, Trichoplusia ni. These included: cation reabsorption via secondary cells (SC); coupling of SCs to neighbouring PCs via gap junctions to enable reabsorption; and a reversal from cation secretion to reabsorption by the principal cells in the distal ileac plexus region of the in situ tubule in response to dietary ion loading. The current paper aimed to identify molecular components of ion transport in the MTs of T. ni and to describe their role in the recently reported reversal of ion transport in ion-loaded animals. Using a combination of molecular, immunohistochemical and electrophysiological techniques, we assigned roles to Na⁺/K⁺-ATPase (NKA), V-type H⁺-ATPase (VA), Na⁺/K⁺/Cl^- co-transporter (NKCC), K⁺/Cl^- co-transporter (KCC), inward-rectifying K⁺ channel (Kir), and Na⁺/H⁺ exchangers (NHE)-7 and -8 in the transport of Na⁺ and K⁺ by the distal ileac plexus of T. ni. The yellow region of the tubule lacked all of the above except VA, and the white region lacked all of the above transporters but expressed an amiloride-sensitive Na⁺ channel (NaC). Overall, the ion transport machinery in the distal ileac plexus of the T. ni tubule shows remarkable similarity to that in tubules of other groups of insects, yet this region transports ions very differently. Shutdown of secretory ATPases and utilisation of the same molecular machinery in the face of changing ion gradients may enable ion transport reversal in lepidopteran MTs. We propose that gap junction-based coupling of the two cell types likely aids in toggling between ion secretion and ion reabsorption in this segment.

Optical Quantification of Intracellular pH in Drosophila melanogaster Malpighian Tubule Epithelia with a Fluorescent Genetically-encoded pH Indicator

Epithelial ion transport is vital to systemic ion homeostasis as well as maintenance of essential cellular electrochemical gradients. Intracellular pH (pHi) is influenced by many ion transporters and thus monitoring pHi is a useful tool for assessing transporter activity. Modern Genetically Encoded pH-Indicators (GEpHIs) provide optical quantification of pHi in intact cells on a cellular and subcellular scale. This protocol describes real-time quantification of cellular pHi regulation in Malpighian Tubules (MTs) of Drosophila melanogaster through ex vivo live-imaging of pHerry, a pseudo-ratiometric GEpHI with a pKa well-suited to track pH changes in the cytosol. Extracted adult fly MTs are composed of morphologically and functionally distinct sections of single-cell layer epithelia, and can serve as an accessible and genetically tractable model for investigation of epithelial transport. GEpHIs offer several advantages over conventional pH-sensitive fluorescent dyes and ion-selective electrodes. GEpHIs can label distinct cell populations provided appropriate promoter elements are available. This labeling is particularly useful in ex vivo, in vivo, and in situ preparations, which are inherently heterogeneous. GEpHIs also permit quantification of pHi in intact tissues over time without need for repeated dye treatment or tissue externalization. The primary drawback of current GEpHIs is the tendency to aggregate in cytosolic inclusions in response to tissue damage and construct over-expression. These shortcomings, their solutions, and the inherent advantages of GEpHIs are demonstrated in this protocol through assessment of basolateral proton (H⁺) transport in functionally distinct principal and stellate cells of extracted fly MTs. The techniques and analysis described are readily adaptable to a wide variety of vertebrate and invertebrate preparations, and the sophistication of the assay can be scaled from teaching labs to intricate determination of ion flux via specific transporters.

Colony Collapse Disorder (CCD) and bee age impact honey bee pathophysiology

Honey bee (Apis mellifera) colonies continue to experience high annual losses that remain poorly explained. Numerous interacting factors have been linked to colony declines. Understanding the pathways linking pathophysiology with symptoms is an important step in understanding the mechanisms of disease. In this study we examined the specific pathologies associated with honey bees collected from colonies suffering from Colony Collapse Disorder (CCD) and compared these with bees collected from apparently healthy colonies. We identified a set of pathological physical characteristics that occurred at different rates in CCD diagnosed colonies prior to their collapse: rectum distension, Malpighian tubule iridescence, fecal matter consistency, rectal enteroliths (hard concretions), and venom sac color. The multiple differences in rectum symptomology in bees from CCD apiaries and colonies suggest effected bees had trouble regulating water. To ensure that pathologies we found associated with CCD were indeed pathologies and not due to normal changes in physical appearances that occur as an adult bee ages (CCD colonies are assumed to be composed mostly of young bees), we documented the changes in bees of different ages taken from healthy colonies. We found that young bees had much greater incidences of white nodules than older cohorts. Prevalent in newly-emerged bees, these white nodules or cellular encapsulations indicate an active immune response. Comparing the two sets of characteristics, we determined a subset of pathologies that reliably predict CCD status rather than bee age (fecal matter consistency, rectal distension size, rectal enteroliths and Malpighian tubule iridescence) and that may serve as biomarkers for colony health. In addition, these pathologies suggest that CCD bees are experiencing disrupted excretory physiology. Our identification of these symptoms is an important first step in understanding the physiological pathways that underlie CCD and factors impacting bee health.

Dynamic expression of genes encoding subunits of inward rectifier potassium (Kir) channels in the yellow fever mosquito Aedes aegypti

Inward rectifier potassium (Kir) channels play fundamental roles in neuromuscular, epithelial, and endocrine function in mammals. Recent research in insects suggests that Kir channels play critical roles in the development, immune function, and excretory physiology of fruit flies and/or mosquitoes. Moreover, our group has demonstrated that mosquito Kir channels may serve as valuable targets for the development of novel insecticides. Here we characterize the molecular expression of 5 mRNAs encoding Kir channel subunits in the yellow fever mosquito, Aedes aegypti: Kir1, Kir2A-c, Kir2B, Kir2B', and Kir3. We demonstrate that 1) Kir mRNA expression is dynamic in whole mosquitoes, Malpighian tubules, and the midgut during development from 4th instar larvae to adult females, 2) Kir2B and Kir3 mRNA levels are reduced in 4th instar larvae when reared in water containing an elevated concentration (50mM) of KCl, but not NaCl, and 3) Kir mRNAs are differentially expressed in the Malpighian tubules, midgut, and ovaries within 24h after blood feeding. Furthermore, we provide the first characterization of Kir mRNA expression in the anal papillae of 4th instar larval mosquitoes, which indicates that Kir2A-c is the most abundant. Altogether, the data provide the first comprehensive characterization of Kir mRNA expression in Ae. aegypti and offer insights into the putative physiological roles of Kir subunits in this important disease vector.

Localization and role of inward rectifier K(+) channels in Malpighian tubules of the yellow fever mosquito Aedes aegypti

Malpighian tubules of adult female yellow fever mosquitoes Aedes aegypti express three inward rectifier K(+) (Kir) channel subunits: AeKir1, AeKir2B and AeKir3. Here we 1) elucidate the cellular and membrane localization of these three channels in the Malpighian tubules, and 2) characterize the effects of small molecule inhibitors of AeKir1 and AeKir2B channels (VU compounds) on the transepithelial secretion of fluid and electrolytes and the electrophysiology of isolated Malpighian tubules. Using subunit-specific antibodies, we found that AeKir1 and AeKir2B localize exclusively to the basolateral membranes of stellate cells and principal cells, respectively; AeKir3 localizes within intracellular compartments of both principal and stellate cells. In isolated tubules bathed in a Ringer solution containing 34 mM K(+), the peritubular application of VU590 (10 μM), a selective inhibitor of AeKir1, inhibited transepithelial fluid secretion 120 min later. The inhibition brings rates of transepithelial KCl and fluid secretion to 54% of the control without a change in transepithelial NaCl secretion. VU590 had no effect on the basolateral membrane voltage (Vbl) of principal cells, but it significantly reduced the cell input conductance (gin) to values 63% of the control within ∼90 min. In contrast, the peritubular application of VU625 (10 μM), an inhibitor of both AeKir1 and AeKir2B, started to inhibit transepithelial fluid secretion as early as 60 min later. At 120 min after treatment, VU625 was more efficacious than VU590, inhibiting transepithelial KCl and fluid secretion to ∼35% of the control without a change in transepithelial NaCl secretion. Moreover, VU625 caused the Vbl and gin of principal cells to respectively drop to values 62% and 56% of the control values within only ∼30 min. Comparing the effects of VU590 with those of VU625 allowed us to estimate that AeKir1 and AeKir2B respectively contribute to 46% and 20% of the transepithelial K(+) secretion when the tubules are bathed in a Ringer solution containing 34 mM K(+). Thus, we uncover an important role of AeKir1 and stellate cells in transepithelial K(+) transport under conditions of peritubular K(+) challenge. The physiological role of AeKir3 in intracellular membranes of both stellate and principal cells remains to be determined.

Two inwardly rectifying potassium channels, Irk1 and Irk2, play redundant roles in Drosophila renal tubule function

Inwardly rectifying potassium channels play essential roles in renal physiology across phyla. Barium-sensitive K(+) conductances are found on the basolateral membrane of a variety of insect Malpighian (renal) tubules, including Drosophila melanogaster. We found that barium decreases the lumen-positive transepithelial potential difference in isolated perfused Drosophila tubules and decreases fluid secretion and transepithelial K(+) flux. In those insect species in which it has been studied, transcripts from multiple genes encoding inwardly rectifying K(+) channels are expressed in the renal (Malpighian) tubule. In Drosophila melanogaster, this includes transcripts of the Irk1, Irk2, and Irk3 genes. The role of each of these gene products in renal tubule function is unknown. We found that simultaneous knockdown of Irk1 and Irk2 in the principal cell of the fly tubule decreases transepithelial K(+) flux, with no additive effect of Irk3 knockdown, and decreases barium sensitivity of transepithelial K(+) flux by ∼50%. Knockdown of any of the three inwardly rectifying K(+) channels individually has no effect, nor does knocking down Irk3 simultaneously with Irk1 or Irk2. Irk1/Irk2 principal cell double-knockdown tubules remain sensitive to the kaliuretic effect of cAMP. Inhibition of the Na(+)/K(+)-ATPase with ouabain and Irk1/Irk2 double knockdown have additive effects on K(+) flux, and 75% of transepithelial K(+) transport is due to Irk1/Irk2 or ouabain-sensitive pathways. In conclusion, Irk1 and Irk2 play redundant roles in transepithelial ion transport in the Drosophila melanogaster renal tubule and are additive to Na(+)/K(+)-ATPase-dependent pathways.

Discovery and characterization of a potent and selective inhibitor of Aedes aegypti inward rectifier potassium channels

Vector-borne diseases such as dengue fever and malaria, which are transmitted by infected female mosquitoes, affect nearly half of the world's population. The emergence of insecticide-resistant mosquito populations is reducing the effectiveness of conventional insecticides and threatening current vector control strategies, which has created an urgent need to identify new molecular targets against which novel classes of insecticides can be developed. We previously demonstrated that small molecule inhibitors of mammalian Kir channels represent promising chemicals for new mosquitocide development. In this study, high-throughput screening of approximately 30,000 chemically diverse small-molecules was employed to discover potent and selective inhibitors of Aedes aegypti Kir1 (AeKir1) channels heterologously expressed in HEK293 cells. Of 283 confirmed screening ‘hits’, the small-molecule inhibitor VU625 was selected for lead optimization and in vivo studies based on its potency and selectivity toward AeKir1, and tractability for medicinal chemistry. In patch clamp electrophysiology experiments of HEK293 cells, VU625 inhibits AeKir1 with an IC₅₀ value of 96.8 nM, making VU625 the most potent inhibitor of AeKir1 described to date. Furthermore, electrophysiology experiments in Xenopus oocytes revealed that VU625 is a weak inhibitor of AeKir2B. Surprisingly, injection of VU625 failed to elicit significant effects on mosquito behavior, urine excretion, or survival. However, when co-injected with probenecid, VU625 inhibited the excretory capacity of mosquitoes and was toxic, suggesting that the compound is a substrate of organic anion and/or ATP-binding cassette (ABC) transporters. The dose-toxicity relationship of VU625 (when co-injected with probenecid) is biphasic, which is consistent with the molecule inhibiting both AeKir1 and AeKir2B with different potencies. This study demonstrates proof-of-concept that potent and highly selective inhibitors of mosquito Kir channels can be developed using conventional drug discovery approaches. Furthermore, it reinforces the notion that the physical and chemical properties that determine a compound's bioavailability in vivo will be critical in determining the efficacy of Kir channel inhibitors as insecticides.

Molecular and functional characterization of Anopheles gambiae inward rectifier potassium (Kir1) channels: a novel role in egg production

Inward rectifier potassium (Kir) channels play essential roles in regulating diverse physiological processes. Although Kir channels are encoded in mosquito genomes, their functions remain largely unknown. In this study, we identified the members of the Anopheles gambiae Kir gene family and began to investigate their function. Notably, we sequenced the A. gambiae Kir1 (AgKir1) gene and showed that it encodes all the canonical features of a Kir channel: an ion pore that is composed of a pore helix and a selectivity filter, two transmembrane domains that flank the ion pore, and the so-called G-loop. Heterologous expression of AgKir1 in Xenopus oocytes revealed that this gene encodes a functional, barium-sensitive Kir channel. Quantitative RT-PCR experiments then showed that relative AgKir1 mRNA levels are highest in the pupal stage, and that AgKir1 mRNA is enriched in the adult ovaries. Gene silencing of AgKir1 by RNA interference did not affect the survival of female mosquitoes following a blood meal, but decreased their egg output. These data provide evidence for a new role of Kir channels in mosquito fecundity, and further validates them as promising molecular targets for the development of a new class of mosquitocides to be used in vector control.

Identification of life-stage and tissue-specific splice variants of an inward rectifying potassium (Kir) channel in the yellow fever mosquito Aedes aegypti

Inward-rectifier potassium (Kir) channels play key roles in nerve, muscle, and epithelial cells in mammals, but their physiological roles in insects remain to be determined. The yellow fever mosquito (Aedes aegypti) possesses five different genes encoding Kir channel subunits: Kir1, Kir2A, Kir2B, Kir2B', and Kir3. We have recently cloned and characterized the Kir1, Kir2B, and Kir3 cDNAs in the renal (Malpighian) tubules of adult female Ae. aegypti. Here we characterize the expression of the Kir2A gene in Ae. aegypti, which was not abundantly expressed in Malpighian tubules. We find that the 1) Kir2A gene is expressed primarily in the midgut and hindgut of adult female mosquitoes, and 2) Kir2A mRNAs are alternatively spliced into three distinct variants (Kir2A-a, -b, and -c). The deduced Kir2A proteins from these splice forms share a completely conserved transmembrane domain (a pore-forming domain flanked by two transmembrane-spanning segments), but possess novel NH2-terminal and/or COOH-terminal domains. Semi-quantitative RT-PCR analyses indicate that the splice variants exhibit both developmental- and tissue-specific expression. Lastly, we provide insights into the conservation of alternative splicing among the Kir2A genes of dipterans, which may add molecular diversity that compensates for the relatively limited number of Kir channel genes in insects compared to mammals.

Lowered insulin signalling ameliorates age-related sleep fragmentation in Drosophila

Reduced insulin signaling improves sleep quality in flies and is protective against age-related sleep deterioration.

Sleep fragmentation, particularly reduced and interrupted night sleep, impairs the quality of life of older people. Strikingly similar declines in sleep quality are seen during ageing in laboratory animals, including the fruit fly Drosophila. We investigated whether reduced activity of the nutrient- and stress-sensing insulin/insulin-like growth factor (IIS)/TOR signalling network, which ameliorates ageing in diverse organisms, could rescue the sleep fragmentation of ageing Drosophila. Lowered IIS/TOR network activity improved sleep quality, with increased night sleep and day activity and reduced sleep fragmentation. Reduced TOR activity, even when started for the first time late in life, improved sleep quality. The effects of reduced IIS/TOR network activity on day and night phenotypes were mediated through distinct mechanisms: Day activity was induced by adipokinetic hormone, dFOXO, and enhanced octopaminergic signalling. In contrast, night sleep duration and consolidation were dependent on reduced S6K and dopaminergic signalling. Our findings highlight the importance of different IIS/TOR components as potential therapeutic targets for pharmacological treatment of age-related sleep fragmentation in humans.

Sleep is essential for human health, but the quality of this fundamental physiological process declines with age and reduces quality of life. We therefore investigated the mechanisms by which ageing impairs sleep. We used the fruit fly Drosophila, whose sleep has many features in common with that of humans, including the age-related decline in quality. We examined the role of the insulin/IGF (IIS) and TOR signaling network, which has an evolutionarily conserved role in ageing. We found that flies with reduced IIS activity had improved sleep quality at night and higher activity levels by day. Importantly, day activity and night sleep were regulated through distinct mechanisms—day activity by the key IIS transcription factor dFOXO, adipokinetic hormone, and octopaminergic signalling—whereas night sleep was mediated through TOR and dopaminergic signalling. Surprisingly, acute inhibition of TOR, by rapamycin, even in old flies, improved sleep quality, suggesting that age-related sleep decline is reversible even after it has commenced. Given the high evolutionarily conservation of IIS and TOR function, our results implicate potential therapeutic targets to improve sleep quality in humans.

Data-mining the FlyAtlas online resource to identify core functional motifs across transporting epithelia

Background

Comparative analysis of tissue-specific transcriptomes is a powerful technique to uncover tissue functions. Our FlyAtlas.org provides authoritative gene expression levels for multiple tissues of Drosophila melanogaster (1). Although the main use of such resources is single gene lookup, there is the potential for powerful meta-analysis to address questions that could not easily be framed otherwise. Here, we illustrate the power of data-mining of FlyAtlas data by comparing epithelial transcriptomes to identify a core set of highly-expressed genes, across the four major epithelial tissues (salivary glands, Malpighian tubules, midgut and hindgut) of both adults and larvae.

Method

Parallel hypothesis-led and hypothesis-free approaches were adopted to identify core genes that underpin insect epithelial function. In the former, gene lists were created from transport processes identified in the literature, and their expression profiles mapped from the flyatlas.org online dataset. In the latter, gene enrichment lists were prepared for each epithelium, and genes (both transport related and unrelated) consistently enriched in transporting epithelia identified.

Results

A key set of transport genes, comprising V-ATPases, cation exchangers, aquaporins, potassium and chloride channels, and carbonic anhydrase, was found to be highly enriched across the epithelial tissues, compared with the whole fly. Additionally, a further set of genes that had not been predicted to have epithelial roles, were co-expressed with the core transporters, extending our view of what makes a transporting epithelium work. Further insights were obtained by studying the genes uniquely overexpressed in each epithelium; for example, the salivary gland expresses lipases, the midgut organic solute transporters, the tubules specialize for purine metabolism and the hindgut overexpresses still unknown genes.

Conclusion

Taken together, these data provide a unique insight into epithelial function in this key model insect, and a framework for comparison with other species. They also provide a methodology for function-led datamining of FlyAtlas.org and other multi-tissue expression datasets.

Molecular characterization of genes encoding inward rectifier potassium (Kir) channels in the bed bug (Cimex lectularius)

The molecular genetics of inward-rectifier potassium (Kir) channels in insects is poorly understood. To date, Kir channel genes have been characterized only from a few representative dipterans (i.e., fruit flies and mosquitoes). The goal of the present study was to characterize Kir channel cDNAs in a hemipteran, the bed bug (Cimex lectularius). Using our previously reported bed bug transcriptome (RNA-seq), we identified two cDNAs that encode putative Kir channels. One was a full-length cDNA that encodes a protein belonging to the insect 'Kir3' clade, which we designate as 'ClKir3'. The other was a partial cDNA that encodes a protein with similarity to both the insect 'Kir1' and 'Kir2' clades, which we designate as 'ClKir1/2'. Quantitative real-time PCR analysis revealed that ClKir1/2 and ClKir3 exhibited peak expression levels in late-instar nymphs and early-instar nymphs, respectively. Furthermore, ClKir3, but not ClKir1/2, showed tissue-specific expression in Malpighian tubules of adult bed bugs. Lastly, using an improved procedure for delivering double-stranded RNA (dsRNA) to male and female bed bugs (via the cervical membrane) we demonstrate rapid and systemic knockdown of ClKir3 transcripts. In conclusion, we demonstrate that the bed bug possesses at least two genes encoding Kir channels, and that RNAi is possible for at least Kir3, thereby offering a potential approach for elucidating the roles of Kir channel genes in bed bug physiology.

Cloning and functional characterization of inward-rectifying potassium (Kir) channels from Malpighian tubules of the mosquito Aedes aegypti

Inward-rectifying K(+) (Kir) channels play critical physiological roles in a variety of vertebrate cells/tissues, including the regulation of membrane potential in nerve and muscle, and the transepithelial transport of ions in osmoregulatory epithelia, such as kidneys and gills. It remains to be determined whether Kir channels play similar physiological roles in insects. In the present study, we sought to 1) clone the cDNAs of Kir channel subunits expressed in the renal (Malpighian) tubules of the mosquito Aedes aegypti, and 2) characterize the electrophysiological properties of the cloned Kir subunits when expressed heterologously in oocytes of Xenopus laevis. Here, we reveal that three Kir subunits are expressed abundantly in Aedes Malpighian tubules (AeKir1, AeKir2B, and AeKir3); each of their full-length cDNAs was cloned. Heterologous expression of the AeKir1 or the AeKir2B subunits in Xenopus oocytes elicits inward-rectifying K(+) currents that are blocked by barium. Relative to the AeKir2B-expressing oocytes, the AeKir1-expressing oocytes 1) produce larger macroscopic currents, and 2) exhibit a modulation of their conductive properties by extracellular Na(+). Attempts to functionally characterize the AeKir3 subunit in Xenopus oocytes were unsuccessful. Lastly, we show that in isolated Aedes Malpighian tubules, the cation permeability sequence of the basolateral membrane of principal cells (Tl(+) > K(+) > Rb(+) > NH(4)(+)) is consistent with the presence of functional Kir channels. We conclude that in Aedes Malpighian tubules, Kir channels contribute to the majority of the barium-sensitive transepithelial transport of K(+).

Shaping up for action: the path to physiological maturation in the renal tubules of Drosophila

The Malpighian tubule is the main organ for excretion and osmoregulation in most insects. During a short period of embryonic development the tubules of Drosophila are shaped, undergo differentiation and become precisely positioned in the body cavity, so they become fully functional at the time of larval hatching a few hours later. In this review I explore three developmental events on the path to physiological maturation. First, I examine the molecular and cellular mechanisms that generate organ shape, focusing on the process of cell intercalation that drives tubule elongation, the roles of the cytoskeleton, the extracellular matrix and how intercalation is coordinated at the tissue level. Second, I look at the genetic networks that control the physiological differentiation of tubule cells and consider how distinctive physiological domains in the tubule are patterned. Finally, I explore how the organ is positioned within the body cavity and consider the relationship between organ position and function.

Separate roles of PKA and EPAC in renal function unraveled by the optogenetic control of cAMP levels in vivo

Cyclic AMP (cAMP) is a ubiquitous second messenger that regulates a variety of essential processes in diverse cell types, functioning via cAMP-dependent effectors such as protein kinase A (PKA) and/or exchange proteins directly activated by cAMP (EPAC). In an intact tissue it is difficult to separate the contribution of each cAMP effector in a particular cell type using genetic or pharmacological approaches alone. We, therefore, utilized optogenetics to overcome the difficulties associated with examining a multicellular tissue. The transgenic photoactive adenylyl cyclase bPAC can be activated to rapidly and reversibly generate cAMP pulses in a cell-type-specific manner. This optogenetic approach to cAMP manipulation was validated in vivo using GAL4-driven UAS-bPAC in a simple epithelium, the Drosophila renal (Malpighian) tubules. As bPAC was expressed under the control of cell-type-specific promoters, each cAMP signal could be directed to either the stellate or principal cells, the two major cell types of the Drosophila renal tubule. By combining the bPAC transgene with genetic and pharmacological manipulation of either PKA or EPAC it was possible to investigate the functional impact of PKA and EPAC independently of each other. The results of this investigation suggest that both PKA and EPAC are involved in cAMP sensing, but are engaged in very different downstream physiological functions in each cell type: PKA is necessary for basal secretion in principal cells only, and for stimulated fluid secretion in stellate cells only. By contrast, EPAC is important in stimulated fluid secretion in both cell types. We propose that such optogenetic control of cellular cAMP levels can be applied to other systems, for example the heart or the central nervous system, to investigate the physiological impact of cAMP-dependent signaling pathways with unprecedented precision.

In vivo Drosophilia genetic model for calcium oxalate nephrolithiasis

Nephrolithiasis is a major public health problem with a complex and varied etiology. Most stones are composed of calcium oxalate (CaOx), with dietary excess a risk factor. Because of complexity of mammalian system, the details of stone formation remain to be understood. Here we have developed a nephrolithiasis model using the genetic model Drosophila melanogaster, which has a simple, transparent kidney tubule. Drosophilia reliably develops CaOx stones upon dietary oxalate supplementation, and the nucleation and growth of microliths can be viewed in real time. The Slc26 anion transporter dPrestin (Slc26a5/6) is strongly expressed in Drosophilia kidney, and biophysical analysis shows that it is a potent oxalate transporter. When dPrestin is knocked down by RNAi in fly kidney, formation of microliths is reduced, identifying dPrestin as a key player in oxalate excretion. CaOx stone formation is an ancient conserved process across >400 My of divergent evolution (fly and human), and from this study we can conclude that the fly is a good genetic model of nephrolithiasis.

Functional correlates of positional and gender-specific renal asymmetry in Drosophila

BACKGROUND

In humans and other animals, the internal organs are positioned asymmetrically in the body cavity, and disruption of this body plan can be fatal in humans. The mechanisms by which internal asymmetry are established are presently the subject of intense study; however, the functional significance of internal asymmetry (outside the brain) is largely unexplored. Is internal asymmetry functionally significant, or merely an expedient way of packing organs into a cavity?

METHODOLOGY/PRINCIPAL FINDINGS

Like humans, Drosophila shows internal asymmetry, with the gut thrown into stereotyped folds. There is also renal asymmetry, with the rightmost pair of renal (Malpighian) tubules always ramifying anteriorly, and the leftmost pair always sitting posteriorly in the body cavity. Accordingly, transcriptomes of anterior-directed (right-side) and posterior-directed (left-side) Malpighian (renal) tubules were compared in both adult male and female Drosophila. Although genes encoding the basic functions of the tubules (transport, signalling) were uniformly expressed, some functions (like innate immunity) showed positional or gender differences in emphasis; others, like calcium handling or the generation of potentially toxic ammonia, were reserved for just the right-side or left-side tubules, respectively. These findings correlated with the distinct locations of each tubule pair within the body cavity. Well known developmental genes (like dorsocross, dachshund and doublesex) showed continuing, patterned expression in adult tubules, implying that somatic tissues maintain both left-right and gender identities throughout life. Gender asymmetry was also noted, both in defence and in male-specific expression of receptors for neuropeptide F and sex-peptide: NPF elevated calcium only in male tubules.

CONCLUSIONS/SIGNIFICANCE

Accordingly, the physical asymmetry of the tubules in the body cavity is directly adaptive. Now that the detailed machinery underlying internal asymmetry is starting to be delineated, our work invites the investigation, not just of tissues in isolation, but in the context of their unique physical locations and milieux.

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.

Accumulation and excretion of morphine by Calliphora stygia, an Australian blow fly species of forensic importance

This study examined the ability of the forensically important blow fly, Calliphora stygia to actively excrete morphine, thereby maintaining a low morphine level within its body when fed on a diet containing morphine at low (7pmolg(-1)) and high (17.5pmolg(-1)) concentrations. Morphine was accumulated within the bodies of maggots (≈70% within the tissues) at concentrations which were lower than that of the meat (3-24%). The morphine content of the initial developing stages (second and third instar maggots) maintained on the high morphine diet was higher than those on the low morphine diet. Morphine was cleared from the body with negatively exponential kinetics (High morphine group: Morphine (pmolg(-1) wet weight)=8425e(-0.014t). Low morphine group: Morphine (pmolg(-1) wet weight)=2180e(-0.010t)). Clearance constants for morphine by animals in both groups were similar and thus both groups had a similar ability to excrete morphine. The Malpighian tubules of maggots were able to actively secrete morphine using a transport mechanism that transports small type II organic cations, such as morphine and quinine. The rate of morphine secretion by the Malpighian tubules could explain the clearance of the drug by the maggots. As the morphine was transported across the Malpighian tubules cells, a significant proportion was metabolised into a compound that is yet to be fully characterised.

Insulin signaling, lifespan and stress resistance are modulated by metabotropic GABA receptors on insulin producing cells in the brain of Drosophila

Insulin-like peptides (ILPs) regulate growth, reproduction, metabolic homeostasis, life span and stress resistance in worms, flies and mammals. A set of insulin producing cells (IPCs) in the Drosophila brain that express three ILPs (DILP2, 3 and 5) have been the main focus of interest in hormonal DILP signaling. Little is, however, known about factors that regulate DILP production and release by these IPCs. Here we show that the IPCs express the metabotropic GABA_B receptor (GBR), but not the ionotropic GABA_A receptor subunit RDL. Diminishing the GBR expression on these cells by targeted RNA interference abbreviates life span, decreases metabolic stress resistance and alters carbohydrate and lipid metabolism at stress, but not growth in Drosophila. A direct effect of diminishing GBR on IPCs is an increase in DILP immunofluorescence in these cells, an effect that is accentuated at starvation. Knockdown of irk3, possibly part of a G protein-activated inwardly rectifying K⁺ channel that may link to GBRs, phenocopies GBR knockdown in starvation experiments. Our experiments suggest that the GBR is involved in inhibitory control of DILP production and release in adult flies at metabolic stress and that this receptor mediates a GABA signal from brain interneurons that may convey nutritional signals. This is the first demonstration of a neurotransmitter that inhibits insulin signaling in its regulation of metabolism, stress and life span in an invertebrate brain.

Drosophila provides rapid modeling of renal development, function, and disease

The evolution of specialized excretory cells is a cornerstone of the metazoan radiation, and the basic tasks performed by Drosophila and human renal systems are similar. The development of the Drosophila renal (Malpighian) tubule is a classic example of branched tubular morphogenesis, allowing study of mesenchymal-to-epithelial transitions, stem cell-mediated regeneration, and the evolution of a glomerular kidney. Tubule function employs conserved transport proteins, such as the Na(+), K(+)-ATPase and V-ATPase, aquaporins, inward rectifier K(+) channels, and organic solute transporters, regulated by cAMP, cGMP, nitric oxide, and calcium. In addition to generation and selective reabsorption of primary urine, the tubule plays roles in metabolism and excretion of xenobiotics, and in innate immunity. The gene expression resource FlyAtlas.org shows that the tubule is an ideal tissue for the modeling of renal diseases, such as nephrolithiasis and Bartter syndrome, or for inborn errors of metabolism. Studies are assisted by uniquely powerful genetic and transgenic resources, the widespread availability of mutant stocks, and low-cost, rapid deployment of new transgenics to allow manipulation of renal function in an organotypic context.

Identification of ion channel genes in the Acyrthosiphon pisum genome

Aphids are major pests of crops, causing hundreds of millions of dollars worth of damage annually. Ion channel proteins are often the targets of modern insecticides and mutations in ion channel genes can lead to resistance to many leading classes of insecticides. The sequencing of the pea aphid, Acyrthosiphon pisum, genome has now allowed detailed in silico analysis of the aphid ion channels. The study has revealed significant differences in the composition of the ion channel families between the aphid and other insects. For example A. pisum does not appear to contain a homologue of the nACh receptor alpha 5 gene whilst the calcium channel beta subunit has been duplicated. These variations could result in differences in function or sensitivity to insecticides. The genome sequence will allow the study of aphid ion channels to be accelerated, leading to a better understanding of the function of these economically important channels. The potential for identifying novel insecticide targets within the aphid is now a step closer.

The developmental, molecular, and transport biology of Malpighian tubules

Molecular biology is reaching new depths in our understanding of the development and physiology of Malpighian tubules. In Diptera, Malpighian tubules derive from ectodermal cells that evaginate from the primitive hindgut and subsequently undergo a sequence of orderly events that culminates in an active excretory organ by the time the larva takes its first meal. Thereafter, the tubules enlarge by cell growth. Just as modern experimental strategies have illuminated the development of tubules, genomic, transcriptomic, and proteomic studies have uncovered new tubule functions that serve immune defenses and the breakdown and renal clearance of toxic substances. Moreover, genes associated with specific diseases in humans are also found in flies, some of which, astonishingly, express similar pathophenotypes. However, classical experimental approaches continue to show their worth by distinguishing between -omic possibilities and physiological reality while providing further detail about the rapid regulation of the transport pathway through septate junctions and the reversible assembly of proton pumps.

Biological potential study of metal complexes of sulphonylurea glibenclamide on the house fly, Musca domestica (Diptera-Muscidae): preparation, spectroscopic and thermal characterization

The ligatation behaviour of sulphonylurea glibenclamide drug is studied in order to give an idea about its potentiality towards some transition metals in vitro systems. Metal complexes of glibenclamide (GCA; H(3)L) drug are prepared and characterized based on elemental analyses, IR, diffused reflectance, magnetic moment, molar conductance and thermal analysis (TG and DTG) techniques. From the elemental analyses data, the complexes are proposed to have the general formulae [M(H(3)L)Cl(n)(H(2)O)(m)].yH(2)O (where M=Cr(III) (n=3, m=1, y=3); Mn(II) (n=2, m=0, y=1); Fe(III) (n=3, m=1, y=0), Co(II) (n=2, m=2, y=0); Ni(II) (n=2, m=2, y=3); Cu(II) (n=2, m=2, y=2) and Zn(II) (n=2, m=0, y=0). The molar conductance data reveal that all the metal chelates are non-electrolytes. IR spectra show that GCA is coordinated to the metal ions in a neutral bidentate manner with OO donor sites of the amide-O and sulphone-O. From the magnetic and solid reflectance spectra, it is found that the geometrical structures of these complexes are octahedral except Mn(II) and Zn(II) complexes which have tetrahedral structure. The thermal behaviour of these chelates is studied using thermogravimetric analysis (TG and DTG) technique. The activation thermodynamic parameters are calculated using Coats-Redfern method. The GCA drug, in comparison to its metal complexes also is screened for its biological activity against house fly, Musca domestica (Diptera-Muscidae). Dose of 5 microg/insect of GCA is topically applied against 3 days old larval instar of M. domestica. Survival of pupal and adult stages has been affected by the complexes of GCA more than larval instars. Morphogenic abnormalities of larvae, pupae and adults are studied. On the other hand pupation and adult emergence program is deteriorated by the effect of different chemicals.

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.

X-ray microanalysis of Rb+ entry into cricket Malpighian tubule cells via putative K+ channels

Elemental imaging by X-ray microanalysis of fully frozen-hydrated samples shows that when Malpighian tubules of the black field cricket(Teleogryllus oceanicus L.) are incubated in saline in which Rb+ has been substituted for K+, Rb+ replaces much of the cellular K+ in the main segment of control Malpighian tubules and this is prevented by incubation in saline containing Ba2+, a non-selective K+ channel blocker. Similarly the amount of cellular K+ is greatly reduced when tubules incubated in normal, i.e. K+ containing, saline are exposed to Ba2+. By considering the amounts of cellular K and Rb remaining in the main segments of tubules incubated in either K+ containing saline or Rb+ containing saline after Ba+ treatment, it is suggested that: (a) a major part (56%) of cellular K+ enters by Ba2+ sensitive K+ channels and that Rb+ can also enter by this route; (b) a smaller fraction (26%) of cellular K+ enters by a previously proposed Na+/K+/2Cl– co-transporter, which can also transport Rb+; (c) a previously proposed Na+/K+-ATPase is responsible for maintaining a K+ pool (18%) in the tubule cells that is not exchangeable by Rb+; and (d) entry by this Na+/K+-ATPase is not available to Rb+.

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.

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.

Salty dog, an SLC5 symporter, modulates Drosophila response to salt stress

To regulate their internal environments, organisms must adapt to varying ion levels in their diet. Adult Drosophila were exposed to dietary salt stress, and their physiological, survival, and gene expression responses monitored. Insects continued to feed on NaCl-elevated diet, although levels >4% wt/vol ultimately proved fatal. Affymetrix microarray analysis of flies fed on diet containing elevated NaCl showed a phased response: the earliest response was widespread upregulation of immune genes, followed by upregulation of carbohydrate metabolism as the immune response was downregulated, then finally a switch to amino acid catabolism and inhibition of genes associated with the reproductive axis. Significantly, the online transcriptomic resource FlyAtlas reports that most of the modulated genes are predominantly expressed in hindgut or Malpighian (renal) tubule, implicating these excretory tissues as the major responders to salt stress. Three genes were selected for further study: the SLC5 symporter CG2196, the GLUT transporter CG6484, and the transcription factor sugarbabe (previously implicated in starvation and stress responses). Expression profiles predicted by microarray were validated by quantitative PCR (qPCR); expression was mapped to the alimentary canal by in situ hybridization. CG2196::eYFP overexpression constructs were localized to the basolateral membrane of the Malpighian (renal) tubules, and RNAi against CG2196 improved survival on high-salt diet, even when driven specifically to just principal cells of the Malpighian tubule, confirming both this tissue and this transporter as major determinants of survival upon salt stress. Accordingly, CG2196 was renamed salty dog (salt).

A new role for a classical gene: white transports cyclic GMP

Guanosine 3'-5' cyclic monophosphate (cGMP) and adenosine 3'-5' cyclic monophosphate (cAMP) are important regulators of cell and tissue function. However, cGMP and cAMP transport have received relatively limited attention, especially in model organisms where such studies can be conducted in vivo. The Drosophila Malpighian (renal) tubule transports cGMP and cAMP and utilises these as signalling molecules. We show here via substrate competition and drug inhibition studies that cAMP transport - but not cGMP transport - requires the presence of di- or tri-carboxylates; and that transport of both cyclic nucleotides occurs via ATP binding cassette sub-family G2 (ABCG2), but not via ABC sub-family C (ABCC), transporters. In Drosophila, the white (w) gene is known for the classic eye colour mutation. However, gene expression data show that of all adult tissues, w is most highly expressed in Malpighian tubules. Furthermore, as White is a member of the ABCG2 transporter class, it is a potential candidate for a tubule cGMP transporter. Assay of cGMP transport in w(-) (mutant) tubules shows that w is required for cGMP transport but not cAMP transport. Targeted over-expression of w in w(-) tubule principal cells significantly increases cGMP transport compared with that in w(-) controls. Conversely, treatment of wild-type tubules with cGMP increases w mRNA expression levels, implying that cGMP is a physiologically relevant substrate for White. Immunocytochemical localisation reveals that White is expressed in intracellular vesicles in tubule principal cells, suggesting that White participates in vesicular transepithelial transport of cGMP.

Integrative physiology, functional genomics and the phenotype gap: a guide for comparative physiologists

Classical, curiosity-led comparative physiology finds itself at a crossroads. Major funding for classical physiology is becoming harder to find, as grant agencies focus on more molecular approaches or on science with more immediate strategic value to their respective countries. In turn, this shift in funding places Zoology and Animal Science departments under enormous stress: student numbers are buoyant, but how can research funding be maintained at high levels? Our research group has argued for the redefinition of integrative physiology as the investigation of gene function in an organotypic context in the intact animal. Implicit in this definition is the use of transgenics and reverse genetics to manipulate gene function in a cell-specific manner; this in turn implies the use of a genetically tractable 'model organism'. The significance of this definition is that it aligns integrative physiology with functional genomics. Again, functional genomics draws heavily on reverse genetics to elucidate the function of novel genes. The phenotype gap (the mismatch between what a genetic model organism's genome encodes and the reasons that it has historically been studied) emphasises the need to attract and empower functional biologists: can all 13,500 genes in Drosophila really be explained in terms of developmental biology? So, by embracing the integrative physiology manifesto, comparative physiologists can not only accelerate their own research, but their functional skills can make them indispensable in the post-genomic endeavour.