Ectopic expression of bovine type 5 phosphodiesterase confers a renal phenotype in Drosophila

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.

The Drosophila NKCC Ncc69 is required for normal renal tubule function

Epithelial ion transport is essential to renal homeostatic function, and it is dysregulated in several diseases, such as hypertension. An understanding of the insect renal (Malpighian) tubule yields insights into conserved epithelial ion transport processes in higher organisms and also has implications for the control of insect infectious disease vectors. Here, we examine the role of the Na(+)-K(+)-2Cl(-) (NKCC) cotransporter Ncc69 in Drosophila tubule function. Ncc69 mutant tubules have decreased rates of fluid secretion and K(+) flux, and these phenotypes were rescued by expression of wild-type Ncc69 in the principal cells of the tubule. Na(+) flux was unaltered in Ncc69 mutants, suggesting Na(+) recycling across the basolateral membrane. In unstimulated tubules, the principal role of the Na(+)-K(+)-ATPase is to generate a favorable electrochemical gradient for Ncc69 activity: while the Na(+)-K(+)-ATPase inhibitor ouabain decreased K(+) flux in wild-type tubules, it had no effect in Ncc69 mutant tubules. However, in the presence of cAMP, which stimulates diuresis, additional Na(+)-K(+)-ATPase-dependent K(+) transport pathways are recruited. In studying the effects of capa-1 on wild-type and Ncc69 mutant tubules, we found a novel antidiuretic role for this hormone that is dependent on intact Ncc69, as it was abolished in Ncc69 mutant tubules. Thus, Ncc69 plays an important role in transepithelial ion and fluid transport in the fly renal tubule and is a target for regulation in antidiuretic states.

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.

Mechanism and function of Drosophila capa GPCR: a desiccation stress-responsive receptor with functional homology to human neuromedinU receptor

The capa peptide receptor, capaR (CG14575), is a G-protein coupled receptor (GPCR) for the D. melanogaster capa neuropeptides, Drm-capa-1 and -2 (capa-1 and -2). To date, the capa peptide family constitutes the only known nitridergic peptides in insects, so the mechanisms and physiological function of ligand-receptor signalling of this peptide family are of interest. Capa peptide induces calcium signaling via capaR with EC₅₀ values for capa-1 = 3.06 nM and capa-2 = 4.32 nM. capaR undergoes rapid desensitization, with internalization via a b-arrestin-2 mediated mechanism but is rapidly re-sensitized in the absence of capa-1. Drosophila capa peptides have a C-terminal -FPRXamide motif and insect-PRXamide peptides are evolutionarily related to vertebrate peptide neuromedinU (NMU). Potential agonist effects of human NMU-25 and the insect -PRLamides [Drosophila pyrokinins Drm-PK-1 (capa-3), Drm-PK-2 and hugin-gamma [hugg]] against capaR were investigated. NMU-25, but not hugg nor Drm-PK-2, increases intracellular calcium ([Ca²⁺]i) levels via capaR. In vivo, NMU-25 increases [Ca²⁺]i and fluid transport by the Drosophila Malpighian (renal) tubule. Ectopic expression of human NMU receptor 2 in tubules of transgenic flies results in increased [Ca²⁺]i and fluid transport. Finally, anti-porcine NMU-8 staining of larval CNS shows that the most highly immunoreactive cells are capa-producing neurons. These structural and functional data suggest that vertebrate NMU is a putative functional homolog of Drm-capa-1 and -2. capaR is almost exclusively expressed in tubule principal cells; cell-specific targeted capaR RNAi significantly reduces capa-1 stimulated [Ca²⁺]i and fluid transport. Adult capaR RNAi transgenic flies also display resistance to desiccation. Thus, capaR acts in the key fluid-transporting tissue to regulate responses to desiccation stress in the fly.

Drosophila gustatory preference behaviors require the atypical soluble guanylyl cyclases

The intracellular messenger cGMP has been suggested to play a role in taste signal transduction in both vertebrates and invertebrates. In the present study, we have examined the role of the Drosophila atypical soluble guanylyl cyclases (sGCs), Gyc-89Da and Gyc-89Db, in larval and adult gustatory preference behaviors. We showed that in larvae, sucrose attraction requires Gyc-89Db and caffeine avoidance requires Gyc-89Da. In adult flies, sucrose attraction is unaffected by mutations in either gene whereas avoidance of low concentrations of caffeine is eliminated by loss of either gene. Similar defective behaviors were observed when cGMP increases were prevented by the expression of a cGMP-specific phosphodiesterase. We also showed that both genes were expressed in gustatory receptor neurons (GRNs) in larval and adult gustatory organs, primarily in a non-overlapping pattern, with the exception of a small group of cells in the adult labellum. In addition, in adults, several cells co-expressed the bitter taste receptor, Gr66a, with either Gyc-89Da or Gyc-89Db. We also showed that the electrophysiological responses of a GRN to caffeine were significantly reduced in flies mutant for the atypical sGCs, suggesting that at least part of the adult behavioral defects were due to a reduced ability to detect caffeine.

Behavioral responses to hypoxia in Drosophila larvae are mediated by atypical soluble guanylyl cyclases

The three Drosophila atypical soluble guanylyl cyclases, Gyc-89Da, Gyc-89Db, and Gyc-88E, have been proposed to act as oxygen detectors mediating behavioral responses to hypoxia. Drosophila larvae mutant in any of these subunits were defective in their hypoxia escape response-a rapid cessation of feeding and withdrawal from their food. This response required cGMP and the cyclic nucleotide-gated ion channel, cng, but did not appear to be dependent on either of the cGMP-dependent protein kinases, dg1 and dg2. Specific activation of the Gyc-89Da neurons using channel rhodopsin showed that activation of these neurons was sufficient to trigger the escape behavior. The hypoxia escape response was restored by reintroducing either Gyc-89Da or Gyc-89Db into either Gyc-89Da or Gyc-89Db neurons in either mutation. This suggests that neurons that co-express both Gyc-89Da and Gyc-89Db subunits are primarily responsible for activating this behavior. These include sensory neurons that innervate the terminal sensory cones. Although the roles of Gyc-89Da and Gyc-89Db in the hypoxia escape behavior appeared to be identical, we also showed that changes in larval crawling behavior in response to either hypoxia or hyperoxia differed in their requirements for these two atypical sGCs, with responses to 15% oxygen requiring Gyc-89Da and responses to 19 and 25% requiring Gyc-89Db. For this behavior, the identity of the neurons appeared to be critical in determining the ability to respond appropriately.

Cell-specific inositol 1,4,5 trisphosphate 3-kinase mediates epithelial cell apoptosis in response to oxidative stress in Drosophila

Organismal stress responses to oxidative stress are relevant to ageing and disease and involve key cell-/tissue-specific signal transduction mechanisms. Using Drosophila, an established in vivo model for stress studies, we show that cell-specific inositol phosphate signalling specifically via inositol 1,4,5 trisphosphate 3-kinase (InsP(3) 3-K, IP(3)K), negatively regulates organismal responses to oxidative stress. We demonstrate that the Drosophila Malpighian tubule (equivalent to vertebrate kidney and liver) is a key epithelial sensor for organismal oxidative stress responses: precise targeting of either gain-of-function constructs of Drosophila IP(3)Ks (IP(3)K-1 and IP(3)K-2), or loss-of-function (RNAi) constructs to only one cell type in tubule reversibly modulates survival of stress-challenged adult flies. In vivo, targeted IP(3)K-1 directly increases H(2)O(2) production, pro-apoptotic caspase-9 activity and mitochondrial membrane potential. The mitochondrial calcium load in tubule principal cells-assessed by luminescent and fluorescent genetically-encoded mitochondrial calcium reporters-is significantly increased by IP(3)K-1 under oxidative stress conditions, leading to apoptosis. The Drosophila orthologues of human apoptotic bcl-2 genes include debcl and buffy. Oxidative stress challenge does not modulate gene expression of either debcl or buffy in tubules; and altered debcl expression does not influence survival rates under oxidative stress challenge. Finally, targeted over-expression of either debcl or buffy to tubule principal cells does not impact on tubule caspase-9 activity. Thus, IP(3)K-1 modulates epithelial cell apoptosis without involvement of bcl-2-type proteins.

Mislocalization of mitochondria and compromised renal function and oxidative stress resistance in Drosophila SesB mutants

Mitochondria accumulate at sites of intense metabolic activity within cells, but the adaptive value of this placement is not clear. In Drosophila, sesB encodes the ubiquitous isoform of adenine nucleotide translocase (ANT, the mitochondrial inner membrane ATP/ADP exchanger); null alleles are lethal, whereas hypomorphic alleles display sensitivity to a range of stressors. In the adult renal tubule, which is densely packed with mitochondria and hence enriched for sesB, both hypomorphic alleles and RNA interference knockdowns cause the mitochondria to lose their highly polarized distribution in the tissue and to become rounded. Basal cytoplasmic and mitochondrial calcium levels are both increased, and neuropeptide calcium response compromised, with concomitant defects in fluid secretion. The remaining mitochondria in sesB mutants are overactive and maintain depleted cellular ATP levels while generating higher levels of hydrogen peroxide than normal. When sesB expression is knocked down in just tubule principal cells, the survival of the whole organism upon oxidative stress is reduced, implying a limiting role for the tubule in homeostatic response to stressors. The physiological impacts of defective ANT expression are thus widespread and diverse.

Modulation of epithelial innate immunity by autocrine production of nitric oxide

Mechanisms of innate immunity especially with relevance to epithelial tissue, are currently the focus of intense research, as epithelial immunity greatly impacts on health and disease. However, many findings regarding innate immunity signalling pathways in vertebrates stems from research using the genetic model Drosophila melanogaster. Here we discuss the central importance of epithelial tissues in innate immunity in Drosophila; the modulation of the Imd pathway via autocrine production of nitric oxide (NO); and the central importance of the Malpighian (renal) tubule in immune function of the whole animal.

Regulation of a Drosophila melanogaster cGMP-specific phosphodiesterase by prenylation and interaction with a prenyl-binding protein

Post-translational modification by isoprenylation is a pivotal process for the correct functioning of many signalling proteins. The Drosophila melanogaster cGMP-PDE (cGMP-specific phosphodiesterase) DmPDE5/6 possesses a CaaX-box prenylation signal motif, as do several novel cGMP-PDEs from insect and echinoid species (in CaaX, C is cysteine, a is an aliphatic amino acid and X is 'any' amino acid). DmPDE5/6 is prenylated in vivo at Cys(1128) and is localized to the plasma membrane when expressed in Drosophila S2 cells. Site-directed mutagenesis of the prenylated cysteine residue (C1128S-DmPDE5/6), pharmacological inhibition of prenylation or co-expression of DmPrBP (Drosophila prenyl-binding protein)/delta each alters the subcellular localization of DmPDE5/6. Thus prenylation constitutes a critical post-translational modification of DmPDE5/6 for membrane targeting. Co-immunoprecipitation and subcellular-fractionation experiments have shown that DmPDE5/6 interacts with DmPrBP/delta in Drosophila S2 cells. Transgenic lines allow targeted expression of tagged prenylation-deficient C1128S-DmPDE5/6 in Type I (principal) cells in Drosophila Malpighian tubules, an in vivo model for DmPDE5/6 function. In contrast with wild-type DmPDE5/6, which was exclusively associated with the apical membrane, the C1128S-DmPDE5/6 mutant form was located primarily in the cytosol, although some residual association occurred at the apical membrane. Despite the profound change in intracellular localization of C1128S-DmPDE5/6, active transport of cGMP is affected in the same way as it is by DmPDE5/6. This suggests that, in addition to prenylation and interaction with DmPrBP/delta, further functional membrane-targeting signals exist within DmPDE5/6.

Synaptic transmission in neurons that express the Drosophila atypical soluble guanylyl cyclases, Gyc-89Da and Gyc-89Db, is necessary for the successful completion of larval and adult ecdysis

Insect ecdysis is a precisely coordinated series of behavioral and hormonal events that occur at the end of each molt. A great deal is known about the hormonal events that underlie this process, although less is known about the neuronal circuitry involved. In this study we identified two populations of neurons that are required for larval and adult ecdyses in the fruit fly, Drosophila melanogaster (Meigen). These neurons were identified by using the upstream region of two genes that code for atypical soluble guanylyl cyclases to drive tetanus toxin in the neurons that express these cyclases to block their synaptic activity. Expression of tetanus toxin in neurons that express Gyc-89Da blocked adult eclosion whereas expression of tetanus toxin in neurons that express Gyc-89Db prevented the initiation of the first larval ecdysis. Expression of tetanus toxin in the Gyc-89Da neurons also resulted in about 50% lethality just prior to pupariation; however, this was probably due to suffocation in the food as lethality was prevented by stopping the larvae from burrowing deep within the food. This result is consistent with our model that the atypical soluble guanylyl cyclases can act as molecular oxygen detectors. The expression pattern of these cyclases did not overlap with any of the neurons containing peptides known to regulate ecdysis and eclosion behaviors. By using the conditional expression of tetanus toxin we were also able to demonstrate that synaptic activity in the Gyc-89Da and Gyc-89Db neurons is required during early adult development for adult eclosion.

Prolonged presynaptic posttetanic cyclic GMP signaling in Drosophila motoneurons

Ca(2+) can stimulate cyclic nucleotide synthesis, but it is not known whether this signaling occurs in nerve terminals in response to activity. Here, in vivo imaging of Drosophila motoneuron terminals shows that activity rapidly induces a long-lasting signal from a transgenically expressed optical indicator based on the epac1 (exchange protein directly activated by cAMP 1) cAMP-binding domain. The epac1-cAMP sensor (camps) response in synaptic boutons depends on extracellular Ca(2+) and ryanodine receptor-mediated Ca(2+)-induced Ca(2+) release from the endoplasmic reticulum. However, mutations that inhibit rutabaga Ca(2+)-stimulated adenylyl cyclase and dunce cAMP-specific phosphodiesterase (PDE) have no effect. Instead, the activity-dependent presynaptic epac1-camps signal reflects elevation of cGMP in response to nitric oxide-activated guanylyl cyclase. Posttetanic presynaptic cGMP is long-lived because of limited PDE activity. Thus, nerve terminal biochemical signaling induced by brief bouts of activity temporally summates on a time scale orders of magnitude longer than fast transmission.

Identification of two partners from the bacterial Kef exchanger family for the apical plasma membrane V-ATPase of Metazoa

The vital task of vectorial solute transport is often energised by a plasma membrane, proton-motive V-ATPase. However, its proposed partner, an apical alkali-metal/proton exchanger, has remained elusive. Here, both FlyAtlas microarray data and in situ analyses demonstrate that the bacterial kefB and kefC (members of the CPA2 family) homologues in Drosophila, CG10806 and CG31052, respectively, are both co-expressed with V-ATPase genes in transporting epithelia. Immunocytochemistry localises endogenous CG10806 and CG31052 to the apical plasma membrane of the Malpighian (renal) tubule. YFP-tagged CG10806 and CG31052 both localise to the plasma membrane of Drosophila S2 cells, and when driven in principal cells of the Malpighian tubule, they localise specifically to the apical plasma membrane. V-ATPase-energised fluid secretion is affected by overexpression of CG10806, but not CG31052; in the former case, overexpression causes higher basal rates, but lower stimulated rates, of fluid secretion compared with parental controls. Overexpression also impacts levels of secreted Na+ and K+. Both genes rescue exchanger-deficient (nha1 nhx1) yeast, but act differently; CG10806 is driven predominantly to the plasma membrane and confers protection against excess K+, whereas CG31052 is expressed predominantly on the vacuolar membrane and protects against excess Na+. Thus, both CG10806 and CG31052 are functionally members of the CPA2 gene family, colocalise to the same apical membrane as the plasma membrane V-ATPase and show distinct ion specificities, as expected for the Wieczorek exchanger.

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.

Model organisms and molecular genetics for endocrinology

Molecular endocrinology has revolutionised our understanding of neuroendocrine signalling. However, reductionist technologies emphasise the molecule at the expense of the organism. Can modern, post-genomic technologies also help us understand neuroendocrine signalling in the whole organism? The judicious use of genetic model organisms, and their associated genetic tools, can help this integrative approach. This theme is illustrated with the study of fluid balance in the fruit fly, Drosophila melanogaster. This organism is a member of the Diptera, a large Order of the largest Class of life on earth (Insecta); one with a huge impact on human health and wealth. Advanced genetic technologies allow the modulation of function of individual genes in specific cells of otherwise normal tissues in a normal animal. This organotypic context is a prerequisite for integrative physiology. Such technologies have been exploited to generate the first animals transgenic for both cytoplasmic and mitochondrial calcium reporters, and to generate a transgenic toolkit that allows cAMP, cGMP or calcium to be manipulated in any cell or tissue of choice. The results have provided new insights into neuroendocrine control in Drosophila and other organisms.

A Drosophila systems approach to xenobiotic metabolism

Insecticide resistance is a major problem for both medicine and agriculture and is frequently associated with overexpression of metabolic enzymes that catalyze the breakdown of pesticides, leading to broad-spectrum resistance. However, the insect tissues within which these metabolic enzymes normally reside remain unclear. Microarray analysis of nine adult tissues from Drosophila melanogaster reveals that cytochrome P-450s and glutathione-S-transferases show highly tissue-specific expression patterns; most were confined to one or more epithelial tissues, and half showed dominant expression in a single tissue. The particular detoxifying enzymes encountered by a xenobiotic thus depend critically on the route of administration. In particular, known insecticide metabolism genes are highly enriched in insect Malpighian (renal) tubules, implicating them in xenobiotic metabolism. The tubules thus display, with the fat body, roles analogous to the vertebrate liver and immune system, as well as its acknowledged renal function. To illustrate this, when levels of a single gene, Cyp6g1, were manipulated in just the Malpighian tubules of adult Drosophila, the survival of the whole insect after 1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane (DDT) challenge was altered, whereas corresponding manipulations in the nervous system or the fat body were without effect. This shows that, although detoxification enzymes are widely distributed, baseline protection against DDT resides primarily in the insect excretory system, corresponding to less than 0.1% of the mass of the organism.

cGMP signalling in a transporting epithelium

The biochemical aspects of cGMP signalling are well known, although in vivo roles of cGMP have only been recently discovered through work in genetic model organisms. The Drosophila melanogaster Malpighian (renal) tubule has been used to address the roles of cGMP in epithelial function. Here, we describe some of this work and outline recent progress in understanding the organotypic function of novel phosphodiesterases encoded by the D. melanogaster genome.

TheSzAmutations of the B subunit of theDrosophilavacuolar H+ ATPase identify conserved residues essential for function in fly and yeast

V-ATPases play multiple roles in eukaryotes: in Drosophila, null mutations are recessive lethal. Here, mutations underlying five extant lethal alleles of vha55, encoding the B subunit, were identified, including a premature termination codon and two mutations very close to residues thought to participate in the catalytic site of the enzyme. Lethality of these alleles could be reverted by transformation of flies with a wild type vha55::GFP fusion, confirming that the lethal phenotype described for these alleles was due to defects in V-ATPase function. The chimeric protein was correctly localised to the apical domain of the Malpighian (renal) tubule, and restored fluid transport function to wild-type levels. No dominant-negative phenotype was apparent in heterozygotes. When the vha55::GFP fusion was driven ubiquitously, fluorescent protein was only detectable in tissues known to contain high levels of V-ATPase, suggesting that vha55 requires stoichometric co-expression of other subunits to be stable. Yeast (Saccharomyces cerevisiae) deleted for the corresponding gene (Δvma2) demonstrated a pH-sensitive growth phenotype that was rescued by the vha55::GFP construct. Δvma2 yeast could not be rescued with fly cDNAs encoding any of the mutant vha55 alleles, confirming the functional significance of the mutated residues. In yeast, bafilomycin-sensitive ATPase activity and growth rate correlated with the ability of different constructs to rescue the pH-sensitive conditional-lethal phenotype. These classical Drosophila mutants thus identify residues that are essential for function in organisms with wide phylogenetic separation.

Novel subcellular locations and functions for secretory pathway Ca2+/Mn2+-ATPases

Secretory pathway Ca2+/Mn2+-ATPases (SPCAs) are important for maintenance of cellular Ca2+and Mn2+homeostasis, and, to date, all SPCAs have been found to localize to the Golgi apparatus. The single Drosophila SPCA gene ( SPoCk) was identified by an in silico screen for novel Ca2+-ATPases. It encoded three SPoCk isoforms with novel, distinct subcellular specificities in the endoplasmic reticulum (ER) and peroxisomes in addition to the Golgi. Furthermore, expression of the peroxisome-associated SPoCk isoform was sexually dimorphic. Overexpression of organelle-specific SPoCk isoforms impacted on cytosolic Ca2+handling in both cultured Drosophila cells and a transporting epithelium, the Drosophila Malpighian (renal) tubule. Specifically, the ER isoform impacted on inositol ( 1 , 4 , 5 )-trisphosphate-mediated Ca2+signaling and the Golgi isoform impacted on diuresis, whereas the peroxisome isoform colocalized with Ca2+“spherites” and impacted on calcium storage and transport. Interfering RNA directed against the common exons of the three SPoCk isoforms resulted in aberrant Ca2+signaling and abolished neuropeptide-stimulated diuresis by the tubule. SPoCk thus contributed to both of the contrasting requirements for Ca2+in transporting epithelia: to transport or store Ca2+in bulk without compromising its use as a signal.

Differential gel electrophoresis and transgenic mitochondrial calcium reporters demonstrate spatiotemporal filtering in calcium control of mitochondria

Mitochondria must adjust both their intracellular location and their metabolism in order to balance their output to the needs of the cell. Here we show by the proteomic technique of time series difference gel electrophoresis that a major result of neuroendocrine stimulation of the Drosophila renal tubule is an extensive remodeling of the mitochondrial matrix. By generating Drosophila that were transgenic for both luminescent and fluorescent mitochondrial calcium reporters, it was shown that mitochondrial calcium tracked the slow (minutes) but not the rapid (<1 s) changes in cytoplasmic calcium and that this resulted in both increased mitochondrial membrane polarization and elevated cellular ATP levels. The selective V-ATPase inhibitor, bafilomycin, further enhanced ATP levels, suggesting that the apical plasma membrane V-ATPase is a major consumer of ATP. Both the mitochondrial calcium signal and the increase in ATP were abolished by the mitochondrial calcium uniporter blocker Ru360. By using both mitochondrial calcium imaging and the potential sensing dye JC-1, the apical mitochondria of principal cells were found to be selectively responsive to neuropeptide signaling. As the ultimate target is the V-ATPase in the apical plasma membrane, this selective activation of mitochondria is clearly adaptive. The results highlight the dynamic nature and both spatial and temporal heterogeneity of calcium signaling possible in differentiated, organotypic cells and provide a new model for neuroendocrine control of V-ATPase.

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.

Oxygen-sensitive guanylyl cyclases in insects and their potential roles in oxygen detection and in feeding behaviors

Responses to hypoxia and hyperoxia depend critically on the ability of the animal to detect changes in O2 levels. However, it has only been recently that an O2-sensing system has been identified in invertebrates. Evidence is accumulating that this molecular O2 sensor is, surprisingly, a class of soluble guanylyl cyclase (sGC) known as atypical sGCs. It has long been known that the conventional sGC alpha and beta subunits form heterodimeric enzymes that are potently activated by NO, but do not bind O2. By contrast, the Drosophila melanogaster atypical sGC subunits, Gyc-88E, Gyc-89Da and Gyc-89Db, are only slightly sensitive to NO, but are potently activated under hypoxic conditions. Here we review evidence that suggests that the atypical sGCs can function as molecular O2 sensors mediating behavioral responses to hypoxia. Sequence comparisons of other predicted O2-sensitive sGCs suggest that most, if not all, insects express two heterodimeric sGCs; an NO-sensitive isoform and a separate O2-sensitive isoform. Expression data and recent experiments that block the function of cells that express the atypical sGCs and experiments that reduce the cGMP levels in these cells also suggest a role in behavioral responses to sweet tastants.

A novel role for a Drosophila homologue of cGMP-specific phosphodiesterase in the active transport of cGMP

cGMP was first discovered in urine, demonstrating that kidney cells extrude this cyclic nucleotide. Drosophila Malpighian tubules provide a model renal system in which a homologue of mammalian PDE (phosphodiesterase) 6 is expressed. In humans, this cG-PDE (cGMP-specific PDE) is specifically expressed in the retinal system, where it controls visual signal transduction. In order to gain insight into the functional role of DmPDE6 (Drosophila PDE6-like enzyme) in epithelial function, we generated transgenic animals with targeted expression of DmPDE6 to tubule Type I (principal) cells. This revealed localization of DmPDE6 primarily at the apical membranes. As expected, overexpression of DmPDE6 resulted in elevated cG-PDE activity and decreased tubule cGMP content. However, such targeted overexpression of DmPDE6 creates a novel phenotype that manifests itself in inhibition of the active transport and efflux of cGMP by tubules. This effect is specific to DmPDE6 action, as no effect on cGMP transport is observed in tubules from a bovine PDE5 transgenic line which display reduced rates of fluid secretion, an effect not seen in DmPDE6 transgenic animals. Specific ablation of DmPDE6 in tubule principal cells, via expression of a targeted DmPDE6 RNAi (RNA interference) transgene, conferred increased active transport of cGMP, confirming a direct role for DmPDE6 in regulating cGMP transport in tubule principal cells. Pharmacological inhibition of DmPDE6 in wild-type tubules using the cG-PDE inhibitor, zaprinast, similarly results in stimulated cGMP transport. We provide the first demonstration of a novel role for a cG-PDE in modulating cGMP transport and efflux.

Signalling via cGMP: lessons from Drosophila

Guanosine 3', 5'-cyclic monophosphate (cGMP) signalling has received increasing attention over the last decade, since the discovery of the gaseous signalling molecule, nitric oxide, which activates cGMP synthesis. Furthermore, research into cGMP signalling has also been stimulated by the development of Viagra and pharmacologically active related compounds, which act to prevent cGMP breakdown. While much is known about the biochemical aspects of components of the cGMP signalling pathway, the precise in vivo roles of such components have only recently come to light through work in model organisms. This review outlines recent work utilising the genetic model organism Drosophila melanogaster in studies of organotypic cGMP signalling. While organisms such as Drosophila may not be the obvious choice for such studies, use of this model has proved that unique and detailed insights for cGMP signalling can be achieved.

Insect renal tubules constitute a cell-autonomous immune system that protects the organism against bacterial infection

Innate immunity is a widespread and important defence against microbial attack, which in insects is thought to originate mainly in the fat body. Here we demonstrate that the fluid-transporting Malpighian (renal) tubule of Drosophila melanogaster constitutes an autonomous immune-sensing tissue utilising the nitric oxide (NO) signalling pathway. Reverse transcriptase PCR (RT-PCR) shows that tubules express those genes encoding components of the Imd pathway. Furthermore, isolated tubules bind and respond to lipopolysaccharide (LPS), by upregulating anti-microbial peptide (diptericin) gene expression and increased bacterial killing. Excised, LPS-challenged tubules, as well as tubules from LPS-infected flies, display increased NO synthase (NOS) activity upon immune challenge. Targetted expression of a Drosophila NOS (dNOS) transgene to only principal cells of the tubule main segment using the GAL4/UAS system increases diptericin expression. In live flies, such targetted over-expression of dNOS to tubule principal cells confers increased survival of the whole animal upon E. coli challenge. Thus, we describe a novel role of Malpighian tubules in immune sensing and insect survival.

Cyclic nucleotide phosphodiesterases in Drosophila melanogaster

Cyclic nucleotide PDEs (phosphodiesterases) are important enzymes that regulate intracellular levels of cAMP and cGMP. In the present study, we identify and characterize novel PDEs in the genetic model, Drosophila melanogaster. The Drosophila genome encodes five novel PDE genes in addition to dunce. Predicted PDE sequences of Drosophila show highly conserved critical domains when compared with human PDEs. Thus PDE-encoding genes of D. melanogaster are CG14940-PDE1C, CG8279-PDE6beta, CG5411-PDE8A, CG32648-PDE9 and CG10231-PDE11. Reverse transcriptase-PCRs of adult tissues reveal widespread expression of PDE genes. Drosophila Malpighian (renal) tubules express all the six PDEs: Drosophila PDE1, dunce (PDE4), PDE6, PDE8, PDE9 and PDE11. Antipeptide antibodies were raised against PDE1, PDE6, PDE9 and PDE11. Verification of antibody specificity by Western blotting of cloned and expressed PDE constructs allowed the immunoprecipitation studies of adult Drosophila lysates. Biochemical characterization of immunoprecipitated endogenous PDEs showed that PDE1 is a dual-specificity PDE (Michaelis constant Km for cGMP: 15.3+/-1 microM; Km cAMP: 20.5+/-1.5 microM), PDE6 is a cGMP-specific PDE (Km cGMP: 37+/-13 microM) and PDE11 is a dual-specificity PDE (Km cGMP: 6+/-2 microM; Km cAMP: 18.5+/-5.5 microM). Drosophila PDE1, PDE6 and PDE11 display sensitivity to vertebrate PDE inhibitors, zaprinast (IC50 was 71+/-39 microM for PDE1, 0.65+/-0.015 microM for PDE6 and 1.6+/-0.5 microM for PDE11) and sildenafil (IC50 was 1.3+/-0.9 microM for PDE1, 0.025+/-0.005 microM for PDE6 and 0.12+/-0.06 microM for PDE11). We provide the first characterization of a cGMP-specific PDE and two dual-specificity PDEs in Drosophila, and show a high degree of similarity in structure and function between human and Drosophila PDEs.

Conservation of capa peptide-induced nitric oxide signalling in Diptera

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

Cell-specific manipulation of second messengers; a toolbox for integrative physiology in Drosophila

Every living cell must detect, and respond appropriately to, external signals. The functions of intracellular second messengers, such as guanosine 3'5'-cyclic monophosphate (cGMP), adenosine 3'5'-cyclic monophosphate (cAMP), and intracellular calcium, are thus intensively studied. However, artifact-free manipulation of these messengers is problematic, and simple pharmacology may not allow selective intervention in distinct cell types in a real, complex tissue. We have devised a method by which second messenger levels can be manipulated in cells of choice using the GAL4/UAS system. By placing different receptors (rat atrial natriuretic peptide [ANP] receptor and Drosophila serotonin receptors [5HT(Dro7) and 5HT(Dro1A)]) under UAS control, they can be targeted to arbitrary defined populations of cells in any tissue of the fly, and second messenger levels can be manipulated simply by adding the natural ligand. The potential of the system is illustrated in the Drosophila renal (Malpighian) tubule, where each receptor was shown to stimulate fluid secretion, to act through its cognate second messenger, and to be blocked by appropriate pharmacological antagonists. The results uncovered a new role for cGMP signaling in tubule and also demonstrate the utility of the tubule as a possible in vivo test bed for novel receptors, ligands, or agonists/antagonists.

The dg2 (for) gene confers a renal phenotype in Drosophila by modulation of cGMP-specific phosphodiesterase

Fluid transport in Drosophila melanogaster tubules is regulated by guanosine 3',5'-cyclic monophosphate (cGMP) signalling. Here we compare the functional effects on tubules of different alleles of the dg2 (foraging or for) gene encoding a cGMP-dependent protein kinase (cGK), and show that the fors allele confers an epithelial phenotype. This manifests itself as hypersensitivity of epithelial fluid transport to the nitridergic neuropeptide, capa-1, which acts through nitric oxide and cGMP. However, there was no significant difference in tubule cGK activity between fors and forR adults. Nonetheless, fors tubules contained higher levels of cGMP-specific phosphodiesterase (cG-PDE) activity compared to forR. This increase in cGMP-PDE activity sufficed to decrease cGMP content in fors tubules compared to forR. Challenge of tubules with capa-1 increases cGMP content in both fors and forR tubules, although the increase from resting cGMP levels is greater in fors tubules. Capa-1 stimulation of tubules reveals a potent inhibition of cG-PDE in both lines, although this is greater in fors; and is sufficient to explain the hypersensitive transport phenotype observed. Thus, polymorphisms at the dg2 locus do indeed confer a cGMP-dependent transport phenotype, but this can best be ascribed to an indirect modulation of cG-PDE activity, and thence cGMP homeostasis, rather than a direct effect on cGK levels.

Resolution of the insect ouabain paradox

Many insects are highly resistant to plant toxins, such as the cardiac glycoside ouabain. How can the epithelia that must handle such toxins, also be refractory to them? In Drosophila, the Malpighian (renal) tubule contains large amounts of Na(+),K(+) ATPase that is known biochemically to be exquisitely sensitive to ouabain, yet the intact tissue is almost unaffected by even extraordinary concentrations. The explanation is that the tubules are protected by an active ouabain transport system, colocated with the Na(+),K(+) ATPase, thus preventing ouabain from reaching inhibitory concentrations within the basolateral infoldings of principal cells. These data show that the Na(+),K(+) ATPase, previously thought to be unimportant, may be as vital in insect tissues as in vertebrates, but can be cryptic to conventional pharmacology.

Analysis of Drosophila cGMP-dependent Protein Kinases and Assessment of Their in Vivo Roles by Targeted Expression in a Renal Transporting Epithelium

cGMP-dependent protein kinase (cGK) forms encoded by the dg2 (for) gene are implicated in behavior and epithelial transport in Drosophila melanogaster. Here, we provide the first biochemical characterization and cellular localization of cGKs encoded by the major transcripts of dg2: dg2P1 and dg2P2. cGMP stimulates kinase activity of DG2P1 (EC(50): 0.13 +/- 0.039 microm) and DG2P2 (EC(50): 0.32 +/- 0.14 microm) in Malpighian tubule and S2 cell extracts. DG2P1 and DG2P2 are magnesium-requiring enzymes and were inhibited by 10 and 100 microm of a cGK inhibitor, 8-(4-chlorophenylthio)guanosine-3',5'-cyclic monophosphorothioate, Rp isomer; whereas DG1, the cGK encoded by the D. melanogaster dg1 gene, was unaffected. DG2P1 and DG2P2 were localized in the plasma membrane in S2 cells, whereas DG1 was localized in the cytosol. The D. melanogaster fluid-transporting Malpighian tubule was used as an organotypic model to analyze cGK localization and function in vivo. Targeted expression of DG2P2, DG2P1, and DG1 in tubule cells via the UAS/GAL4 system in transgenic flies revealed differential localization of all three cGKs in vivo: DG2P2 expression at the apical membrane; DG2P1 expression at both the apical and basolateral membranes; and DG1 expression at the basolateral membrane and in the cytosol. Transgenic tubules for all three cGKs displayed enhanced cGK activity compared with wild-type tubules. The physiological impact of targeted expression of individual cGKs in tubule principal cells was assessed by measuring basal and stimulated rates of fluid transport. DG1 expression greatly enhanced fluid transport by the tubule in response to exogenous cGMP, whereas DG2P2 expression significantly increased fluid transport in response to the nitridergic neuropeptide, capa-1. Thus, dg2-encoded proteins are bona fide cGKs, which have differential roles in epithelial fluid transport, as assessed by in vivo studies. Furthermore, a novel epithelial role is suggested for DG1, which is considerably more responsive to cGMP than to capa-1 stimulation.