Integrative physiology and functional genomics of epithelial function in a genetic model organism

Hsp27 over expression protect against cadmium induced nephrotoxicity in Drosophila melanogaster

Cadmium (Cd) exposure to the animals including humans is reported as nephrotoxic compounds i.e., disturbing redox status (increase oxidative stress), mitochondrial dysfunction, renal cell death and altered transporters in the renal system. Hsp27 (a small heat shock protein) has been shown as one of the modulators in the renal dysfunction and increased against the Cd induced toxicity. However, no studies are reported on the genetic modulation of stress protein against the Cd-induced nephrotoxicity. The current study aimed to examine the protective role of hsp27 overexpression against the Cd-induced nephrotoxicity using Drosophila melanogaster as an animal model. D. melanogaster renal system includes nephrocytes and Malpighian tubules (MTs) that show the functional similarity with mammalian kidney nephron. Overexpression of the hsp27 was found to reduce the Cd induced oxidative stress, rescue cell death in MTs of Cd exposed D. melanogaster larvae. The rescued GSH level, NADPH level and glucose 6 phosphate dehydrogenase (G6PD) activity were also observed in the MTs of the Cd exposed organism. Function (efflux activity and fluid secretion rate) of the MTs was restored in Cd exposed hsp27 overexpressed larvae. Further, results were confirmed by restored brush border microvilli density and reduced uric acid level. Tissue specific knockdown of hsp27 developed Cd like phenotypes in MTs and the phenotypes enhanced in Cd exposed condition. The present study clearly shows the role of hsp27 overexpression in restoration of the MTs function and protection against the Cd induced renal toxicity.

Functional characterization of a diuretic hormone receptor associated with desiccation, starvation and temperature tolerance in gypsy moth, Lymantria dispar

Variety of diuretic hormone neuropeptides is known to regulate water and ion balance in invertebrates. By activating their specific neuropeptide, diuretic hormone receptor (DHR) transmits extracellular signals into the cell, and then produces functional cell activity, which plays an important role in regulating physiology and behavior. However, little is known about the function of DHR gene in Lymantria dispar. DHR gene was firstly identified in L. dispar and its physiological functions were investigated using RNA interference (RNAi) technology. The results showed that except for the 6^th instar larvae, the expression levels of DHR gene in the larval stages are higher than that in the egg, pupal and adult stages. The DHR gene is highly expressed in hindgut and midgut tissues. The L. dispar larvae significantly increased their water content and high temperature tolerance after the DHR was silenced, while decreasing excretion and feeding behavior. The physiological function of DHR is associated with desiccation, high temperature and starvation resistance. DHR could contribute to future development of novel insecticide to manage this global forest pest population.

Vacuolar ATPase subunit F is critical for larval survival in Henosepilachna vigintioctopunctata

Vacuolar ATPase (vATPase) is an important proton pump in insect tissues including gut and Malpighian tubule. Subunit F, one of the 16 subunits of the vATPase holoenzyme, is not well characterized. Here, we found that two HvvATPaseF isoforms were highly expressed in the hindgut and Malpighian tubules (MT) in the 28-spotted lady-beetle Henosepilachna vigintioctopunctata, an agricultural pest that feeds on Solanaceae and Cucurbitaceae. Knockdown of both HvvATPaseF variants by RNA interference (RNAi) delayed larval growth and negatively affected ecdysis and adult emergence. In the midgut, RNAi treatment resulted in the disappearance of peritrophic membrane, the reduction in the size and the impaired integrity of the gut, which was associated with sparse principle cells and an increase in TUNEL- and EdU-positive cells. Whereas the MT were opaque and the tubule lumens were full of urine in dsegfp-fed larvae, the tubules were clear and the tubule lumens were empty in the dsvATPaseF-fed larvae. HvvATPaseF knockdown was also associated with a decrease in the abundance of the fat body and the levels of glucose, trehalose, triglyceride, total soluble amino acids and proteins, and an increase in glycogen. Consistent with the known effects of sugars on chitin formation, both the expression level of a chitin biosynthesis gene and the thickness of the head capsule cuticle were reduced in the HvvATPaseF-depleted beetles. Our results demonstrated that subunit F plays an essential role in H. vigintioctopunctata development.

Knockdown of Vacuolar ATPase Subunit G Gene Affects Larval Survival and Impaired Pupation and Adult Emergence in Henosepilachna vigintioctopunctata

Simple Summary

Vacuolar ATPase (vATPase), a proton pump driven by ATP hydrolysis, acts as a membrane energizer to motivate the movement of ions and nutrients across the cellular membrane in insect guts and Malpighian tubules, among others. The vATPase holoenzyme contains 16 subunits. Out of these subunits, mammalian G subunit includes three isoforms (G1-G3) which are encoded by three distinctive genes. The physiological role of a specific G isoform can be compensated by others. Thus, current experimental evidence on the in vivo function of G is rather limited among eight V₁ subunits. In the present paper, particular attention was paid to an insect model, Henosepilachna vigintioctopunctata ladybird, a serious defoliator of Solanaceae and Cucurbitaceae plants in many Asian countries. Given that the beetle is sensitive to RNA interference (RNAi), HvvATPaseG gene was knocked down by ingestion of its corresponding dsRNA at the fourth-instar larval stage. Silence of HvvATPaseG affected larval growth and survival, impaired pupation and adult emergence. Our results provide a basis for further functional research on the vATPase G subunit in insects and suggest new ideas for the management of H. vigintioctopunctata.

Abstract

The vATPase holoenzyme consists of two functional subcomplexes, the cytoplasmic (peripheral) V₁ and the membrane-embedded V₀. Both V₁ and V₀ sectors contain eight subunits, with stoichiometry of A₃B₃CDE₃FG₃H in V₁ and ac₈c’c”def(Voa1p) in V₀ respectively. However, the function of G subunit has not been characterized in any non-Drosophilid insect species. In the present paper, we uncovered that HvvATPaseG was actively transcribed from embryo to adult in a Coleopteran pest Henosepilachna vigintioctopunctata. Its mRNA levels peaked in larval hindgut and Malpighian tubules. RNA interference (RNAi)-mediated knockdown of HvvATPaseG significantly reduced larval feeding, affected chitin biosynthesis, destroyed midgut integrity, damaged midgut peritrophic membrane, and retarded larval growth. The function of Malpighian tubules was damaged, the contents of glucose, trehalose, lipid, total soluble amino acids and protein were lowered and the fat bodies were lessened in the HvvATPaseG RNAi larvae, compared with those in the PBS- and dsegfp-fed beetles. In contrast, the amount of glycogen was dramatically increased in the HvvATPaseG depletion ladybirds. As a result, the development was arrested, pupation was inhibited and adult emergence was impaired in the HvvATPaseG hypomorphs. Our results demonstrated that G subunit plays a critical role during larval development in H. vigintioctopunctata.

Transporters and tubule crystals in the insect Malpighian tubule

The insect renal (Malpighian) tubules are functionally homologous to the mammalian kidney. Accumulating evidence indicates that renal tubule crystals form in a manner similar to mammalian kidney stones. In Drosophila melanogaster, crystals can be induced by diet, toxic substances, or genetic mutations that reflect circumstances influencing or eliciting kidney stones in mammals. Incredibly, many mammalian proteins have distinct homologs in Drosophila, and the function of most homologs have been demonstrated to recapitulate their mammalian and human counterparts. Here, we discuss the present literature establishing Drosophila as a nephrolithiasis model. This insect model may be used to investigate and understand the etiology of kidney stone diseases, especially with regard to calcium oxalate, calcium phosphate and xanthine or urate crystallization.

Larval density affects phenotype and surrounding bacterial community without altering gut microbiota in Drosophila melanogaster

Larval crowding represents a complex stressful situation arising from inter-individual competition for time- and space-limited resources. The foraging of a large number of individuals may alter the chemical and bacterial composition of food and in turn affect individual's traits. Here we used Drosophila melanogaster to explore these assumptions. First, we used a wide larval density gradient to investigate the impact of crowding on phenotypical traits. We confirmed that high densities increased development time and pupation height, and decreased viability and body mass. Next, we measured concentrations of common metabolic wastes (ammonia, uric acid) and characterized bacterial communities, both in food and in larvae, for three contrasting larval densities (low, medium and high). Ammonia concentration increased in food from medium and high larval densities, but remained low in larvae regardless of the larval density. Uric acid did not accumulate in food but was detected in larvae. Surprisingly, bacterial composition remained stable in guts of larvae whatever their rearing density, although it drastically changed in the food. Overall, these results indicate that crowding deeply affects individuals, and also their abiotic and biotic surroundings. Environmental bacterial communities likely adapt to altered nutritional situations resulting from crowding, putatively acting as scavengers of larval metabolic wastes.

Maintenance of hindgut reabsorption during cold exposure is a key adaptation for Drosophila cold tolerance

Maintaining extracellular osmotic and ionic homeostasis is crucial for organismal function. In insects, hemolymph volume and ion content is regulated by the secretory Malpighian tubules and reabsorptive hindgut. When exposed to stressful cold, homeostasis is gradually disrupted, characterized by a debilitating increase in extracellular K+ concentration (hyperkalemia). Accordingly, studies have found a strong link between species-specific cold tolerance and the ability to maintain ion and water homeostasis at low temperature. This is also true for drosophilids where inter- and intra-specific differences in cold tolerance are linked to the secretory capacity of Malpighian tubules. There is, however, little information on the reabsorptive capacity of the hindgut in Drosophila To address this, we developed a novel method that permits continuous measurements of hindgut ion and fluid reabsorption in Drosophila We demonstrate that this assay is temporally stable (∼2 h) and responsive to cAMP stimulation and pharmacological intervention in accordance with the current insect hindgut reabsorption model. We then investigated how cold acclimation or cold adaptation affected hindgut reabsorption at benign (24°C) and low temperature (3°C). Cold-tolerant Drosophila species and cold-acclimated D. melanogaster maintain superior fluid and Na+ reabsorption at low temperature. Furthermore, cold adaptation and acclimation caused a relative reduction in K+ reabsorption at low temperature. These characteristic responses of cold adaptation/acclimation will promote maintenance of ion and water homeostasis at low temperature. Our study of hindgut function therefore provides evidence that adaptations in the osmoregulatory capacity of insects are critical for their ability to tolerate cold.

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.

Recent advances in neuropeptide signaling in Drosophila, from genes to physiology and behavior

This review focuses on neuropeptides and peptide hormones, the largest and most diverse class of neuroactive substances, known in Drosophila and other animals to play roles in almost all aspects of daily life, as w;1;ell as in developmental processes. We provide an update on novel neuropeptides and receptors identified in the last decade, and highlight progress in analysis of neuropeptide signaling in Drosophila. Especially exciting is the huge amount of work published on novel functions of neuropeptides and peptide hormones in Drosophila, largely due to the rapid developments of powerful genetic methods, imaging techniques and innovative assays. We critically discuss the roles of peptides in olfaction, taste, foraging, feeding, clock function/sleep, aggression, mating/reproduction, learning and other behaviors, as well as in regulation of development, growth, metabolic and water homeostasis, stress responses, fecundity, and lifespan. We furthermore provide novel information on neuropeptide distribution and organization of peptidergic systems, as well as the phylogenetic relations between Drosophila neuropeptides and those of other phyla, including mammals. As will be shown, neuropeptide signaling is phylogenetically ancient, and not only are the structures of the peptides, precursors and receptors conserved over evolution, but also many functions of neuropeptide signaling in physiology and behavior.

Epithelial Function in the Drosophila Malpighian Tubule: An In Vivo Renal Model

The insect renal (Malpighian) tubule has long been a model system for the study of fluid secretion and its neurohormonal control, as well as studies on ion transport mechanisms. To extend these studies beyond the boundaries of classical physiology, a molecular genetic approach together with the 'omics technologies is required. To achieve this in any vertebrate transporting epithelium remains a daunting task, as the genetic tools available are still relatively unsophisticated. Drosophila melanogaster, however, is an outstanding model organism for molecular genetics. Here we describe a technique for fluid secretion assays in the D. melanogaster equivalent of the kidney nephron. The development of this first physiological assay for a Drosophila epithelium, allowing combined approaches of integrative physiology and functional genomics, has now provided biologists with an entirely new model system, the Drosophila Malpighian tubule, which is utilized in multiple fields as diverse as kidney disease research and development of new modes of pest insect control.

Anti-diuretic action of a CAPA neuropeptide against a subset of diuretic hormones in the disease vector Aedes aegypti

The mosquito Aedes aegypti is a vector responsible for transmitting various pathogens to humans, and their prominence as chief vectors of human disease is largely due to their anthropophilic blood feeding behaviour. Larval stage mosquitoes must deal with the potential dilution of their haemolymph in freshwater, whereas the haematophagus A. aegypti female faces the challenge of excess ion and water intake after a blood meal. The excretory system, composed of the Malpighian tubules (MTs) and hindgut, is strictly controlled by neuroendocrine factors, responsible for the regulation of diuresis across all developmental stages. The highly studied insect MTs are influenced by a variety of diuretic hormones and, in some insects, anti-diuretic factors. In the present study, we investigated the effects of AedaeCAPA-1 neuropeptide on larval and adult female A. aegypti MTs stimulated with various diuretic factors including serotonin (5-HT), a corticotropin-related factor (CRF) diuretic peptide, a calcitonin-related diuretic hormone (DH₃₁) and a kinin-related diuretic peptide. Overall, our findings establish that AedaeCAPA-1 specifically inhibits secretion of larval and adult MTs stimulated by 5-HT and DH₃₁, whilst having no activity on MTs stimulated by other diuretic factors. Furthermore, although AedaeCAPA-1 acts as an anti-diuretic, it does not influence the relative proportions of cations transported by adult MTs, thus maintaining the kaliuretic activity of 5-HT and natriuretic activity of DH₃₁ In addition, we tested the effects of the second messenger cGMP in adult MTs. We established that cGMP has similar effects to AedaeCAPA-1, strongly inhibiting 5-HT- and DH₃₁-stimulated fluid secretion, but with only minor effects on CRF-stimulated diuresis. Interestingly, although AedaeCAPA-1 has no inhibitory activity on kinin-stimulated fluid secretion, cGMP strongly inhibited fluid secretion by this diuretic hormone, which targets stellate cells specifically. Collectively, these results support that AedaeCAPA-1 inhibits select diuretic factors acting on the principal cells and this probably involves cGMP as a second messenger. Kinin-stimulated diuresis, which targets stellate cells, is also inhibited by cGMP, suggesting that another anti-diuretic factor in addition to AedaeCAPA-1 exists and may utilize cGMP as a second messenger.

A genomic investigation of ecological differentiation between free-living and Drosophila-associated bacteria

Various bacterial taxa have been identified both in association with animals and in the external environment, but the extent to which related bacteria from the two habitat types are ecologically and evolutionarily distinct is largely unknown. This study investigated the scale and pattern of genetic differentiation between bacteria of the family Acetobacteraceae isolated from the guts of Drosophila fruit flies, plant material and industrial fermentations. Genome-scale analysis of the phylogenetic relationships and predicted functions was conducted on 44 Acetobacteraceae isolates, including newly sequenced genomes from 18 isolates from wild and laboratory Drosophila. Isolates from the external environment and Drosophila could not be assigned to distinct phylogenetic groups, nor are their genomes enriched for any different sets of genes or category of predicted gene functions. In contrast, analysis of bacteria from laboratory Drosophila showed they were genetically distinct in their universal capacity to degrade uric acid (a major nitrogenous waste product of Drosophila) and absence of flagellar motility, while these traits vary among wild Drosophila isolates. Analysis of the competitive fitness of Acetobacter discordant for these traits revealed a significant fitness deficit for bacteria that cannot degrade uric acid in culture with Drosophila. We propose that, for wild populations, frequent cycling of Acetobacter between Drosophila and the external environment prevents genetic differentiation by maintaining selection for traits adaptive in both the gut and external habitats. However, laboratory isolates bear the signs of adaptation to persistent association with the Drosophila host under tightly defined environmental conditions.

Drosophila Malpighian Tubules: A Model for Understanding Kidney Development, Function, and Disease

The Malpighian tubules of insects are structurally simple but functionally important organs, and their integrity is important for the normal excretory process. They are functional analogs of human kidneys which are important physiological organs as they maintain water and electrolyte balance in the blood and simultaneously help the body to get rid of waste and toxic products after various metabolic activities. In addition, it receives early indications of insults to the body such as immune challenge and other toxic components and is essential for sustaining life. According to National Vital Statistics Reports 2016, renal dysfunction has been ranked as the ninth most abundant cause of death in the USA. This chapter provides detailed descriptions of Drosophila Malpighian tubule development, physiology, immune function and also presents evidences that Malpighian tubules can be used as a model organ system to address the fundamental questions in developmental and functional disorders of the kidney.

Cyanide Detoxification by the Cobalamin Precursor Cobinamide

Cyanide is a highly toxic agent that inhibits mitochondrial cytochrome-c oxidase, thereby depleting cellular ATP. it contributes to smoke inhalation deaths in fires and could be used as a weapon of mass destruction. Cobalamin (vitamin B12) binds cyanide with a relatively high affinity and is used in Europe to treat smoke inhalation victims. Cobinamide, the penultimate compound in cobalamin biosynthesis, binds cyanide with about 1010 greater affinity than cobalamin, and we found It was several-fold more effective than cobalamin in (i) reversing cyanide inhibition of oxidative phosphorylation in mammalian cells; (ii) rescuing mammalian cells and Drosophila melanogaster from cyanide toxicity; and (iii) reducing cyanide inhibition of Drosophila Malpighian tubule secretion. Cobinamide could be delivered by oral ingestion, inhalation, or injection to Drosophila, and it was as effective when administered up to 5 mins post-cyanide exposure as when given preexposure. We conclude that cobinamide is an effective cyanide detoxifying agent that has potential use as a cyanide antidote, both in smoke inhalation victims and in persons exposed to cyanide used as a weapon of mass destruction.

Use of the Ramsay Assay to Measure Fluid Secretion and Ion Flux Rates in the Drosophila melanogaster Malpighian Tubule

Modulation of renal epithelial ion transport allows organisms to maintain ionic and osmotic homeostasis in the face of varying external conditions. The Drosophila melanogaster Malpighian (renal) tubule offers an unparalleled opportunity to study the molecular mechanisms of epithelial ion transport, due to the powerful genetics of this organism and the accessibility of its renal tubules to physiological study. Here, we describe the use of the Ramsay assay to measure fluid secretion rates from isolated fly renal tubules, with the use of ion-specific electrodes to measure sodium and potassium concentrations in the secreted fluid. This assay allows study of transepithelial fluid and ion fluxes of ~20 tubules at a time, without the need to transfer the secreted fluid to a separate apparatus to measure ion concentrations. Genetically distinct tubules can be analyzed to assess the role of specific genes in transport processes. Additionally, the bathing saline can be modified to examine the effects of its chemical characteristics, or drugs or hormones added. In summary, this technique allows the molecular characterization of basic mechanisms of epithelial ion transport in the Drosophila tubule, as well as regulation of these transport mechanisms.

Transport proteins NHA1 and NHA2 are essential for survival, but have distinct transport modalities

The cation/proton antiporter (CPA) family includes the well-known sodium/proton exchanger (NHE; SLC9A) family of Na(+)/H(+) exchangers, and the more recently discovered and less well understood CPA2s (SLC9B), found widely in living organisms. In Drosophila, as in humans, they are represented by two genes, Nha1 (Slc9b1) and Nha2 (Slc9b2), which are enriched and functionally significant in renal tubules. The importance of their role in organismal survival has not been investigated in animals, however. Here we show that single RNAi knockdowns of either Nha1 or Nha2 reduce survival and in combination are lethal. Knockdown of either gene alone results in up-regulation of the other, suggesting functional complementation of the two genes. Under salt stress, knockdown of either gene decreases survival, demonstrating a key role for the CPA2 family in ion homeostasis. This is specific to Na(+) stress; survival on K(+) intoxication is not affected by sodium/hydrogen antiporter (NHA) knockdown. A direct functional assay in Xenopus oocytes shows that Nha2 acts as a Na(+)/H(+) exchanger. In contrast, Nha1 expressed in Xenopus oocytes shows strong Cl(-) conductance and acts as a H(+)-Cl(-) cotransporter. The activity of Nha1 is inhibited by chloride-binding competitors 4,4'-diiso-thiocyano-2,2'-disulfonic acid stilbene and 4,4'-dibenzamido-2,2'-stilbenedisulphonate. Salt stress induces a massive up-regulation of NHA gene expression not in the major osmoregulatory tissues of the alimentary canal, but in the crop, cuticle, and associated tissues. Thus, it is necessary to revise the classical view of the coordination of different tissues in the coordination of the response to osmoregulatory stress.

Cell signalling mechanisms for insect stress tolerance

Insects successfully occupy most environmental niches and this success depends on surviving a broad range of environmental stressors including temperature, desiccation, xenobiotic, osmotic and infection stress. Epithelial tissues play key roles as barriers between the external and internal environments and therefore maintain homeostasis and organismal tolerance to multiple stressors. As such, the crucial role of epithelia in organismal stress tolerance cannot be underestimated. At a molecular level, multiple cell-specific signalling pathways including cyclic cAMP, cyclic cGMP and calcium modulate tissue, and hence, organismal responses to stress. Thus, epithelial cell-specific signal transduction can be usefully studied to determine the molecular mechanisms of organismal stress tolerance in vivo. This review will explore cell signalling modulation of stress tolerance in insects by focusing on cell signalling in a fluid transporting epithelium--the Malpighian tubule. Manipulation of specific genes and signalling pathways in only defined tubule cell types can influence the survival outcome in response to multiple environmental stressors including desiccation, immune, salt (ionic) and oxidative stress, suggesting that studies in the genetic model Drosophila melanogaster may reveal novel pathways required for stress tolerance.

Proteomic-based insight into Malpighian tubules of silkworm Bombyx mori

Malpighian tubules (MTs) are highly specific organs of arthropods (Insecta, Myriapoda and Arachnida) for excretion and osmoregulation. In order to highlight the important genes and pathways involved in multi-functions of MTs, we performed a systematic proteomic analysis of silkworm MTs in the present work. Totally, 1,367 proteins were identified by one-dimensional gel electrophoresis coupled with liquid chromatography-tandem mass spectrometry, and as well as by Trans Proteomic Pipeline (TPP) and Absolute protein expression (APEX) analyses. Forty-one proteins were further identified by two-dimensional gel electrophoresis. Some proteins were revealed to be significantly associated with various metabolic processes, organic solute transport, detoxification and innate immunity. Our results might lay a good foundation for future functional studies of MTs in silkworm and other lepidoptera.

Signaling by Drosophila capa neuropeptides

The capa peptide family, originally identified in the tobacco hawk moth, Manduca sexta, is now known to be present in many insect families, with increasing publications on capa neuropeptides each year. The physiological actions of capa peptides vary depending on the insect species but capa peptides have key myomodulatory and osmoregulatory functions, depending on insect lifestyle, and life stage. Capa peptide signaling is thus critical for fluid homeostasis and survival, making study of this neuropeptide family attractive for novel routes for insect control. In Dipteran species, including the genetically tractable Drosophila melanogaster, capa peptide action is diuretic; via elevation of nitric oxide, cGMP and calcium in the principal cells of the Malpighian tubules. The identification of the capa receptor (capaR) in several insect species has shown this to be a canonical GPCR. In D. melanogaster, ligand-activated capaR activity occurs in a dose-dependent manner between 10(-6) and 10(-12)M. Lower concentrations of capa peptide do not activate capaR, either in adult or larval Malpighian tubules. Use of transgenic flies in which capaR is knocked-down in only Malpighian tubule principal cells demonstrates that capaR modulates tubule fluid secretion rates and in doing so, sets the organismal response to desiccation. Thus, capa regulates a desiccation-responsive pathway in D. melanogaster, linking its role in osmoregulation and fluid homeostasis to environmental response and survival. The conservation of capa action between some Dipteran species suggests that capa's role in desiccation tolerance may not be confined to D. melanogaster.

The tiptop/teashirt genes regulate cell differentiation and renal physiology in Drosophila

The physiological activities of organs are underpinned by an interplay between the distinct cell types they contain. However, little is known about the genetic control of patterned cell differentiation during organ development. We show that the conserved Teashirt transcription factors are decisive for the differentiation of a subset of secretory cells, stellate cells, in Drosophila melanogaster renal tubules. Teashirt controls the expression of the water channel Drip, the chloride conductance channel CLC-a and the Leukokinin receptor (LKR), all of which characterise differentiated stellate cells and are required for primary urine production and responsiveness to diuretic stimuli. Teashirt also controls a dramatic transformation in cell morphology, from cuboidal to the eponymous stellate shape, during metamorphosis. teashirt interacts with cut, which encodes a transcription factor that underlies the differentiation of the primary, principal secretory cells, establishing a reciprocal negative-feedback loop that ensures the full differentiation of both cell types. Loss of teashirt leads to ineffective urine production, failure of homeostasis and premature lethality. Stellate cell-specific expression of the teashirt paralogue tiptop, which is not normally expressed in larval or adult stellate cells, almost completely rescues teashirt loss of expression from stellate cells. We demonstrate conservation in the expression of the family of tiptop/teashirt genes in lower insects and establish conservation in the targets of Teashirt transcription factors in mouse embryonic kidney.

A biogenic amine and a neuropeptide act identically: tyramine signals through calcium in Drosophila tubule stellate cells

Insect osmoregulation is subject to highly sophisticated endocrine control. In Drosophila, both Drosophila kinin and tyramine act on the Malpighian (renal) tubule stellate cell to activate chloride shunt conductance, and so increase the fluid production rate. Drosophila kinin is known to act through intracellular calcium, but the mode of action of tyramine is not known. Here, we used a transgenically encoded GFP::apoaequorin translational fusion, targeted to either principal or stellate cells under GAL4/UAS control, to demonstrate that tyramine indeed acts to raise calcium in stellate, but not principal cells. Furthermore, the EC(50) tyramine concentration for half-maximal activation of the intracellular calcium signal is the same as that calculated from previously published data on tyramine-induced increase in chloride flux. In addition, tyramine signalling to calcium is markedly reduced in mutants of NorpA (a phospholipase C) and itpr, the inositol trisphosphate receptor gene, which we have previously shown to be necessary for Drosophila kinin signalling. Therefore, tyramine and Drosophila kinin signals converge on phospholipase C, and thence on intracellular calcium; and both act to increase chloride shunt conductance by signalling through itpr. To test this model, we co-applied tyramine and Drosophila kinin, and showed that the calcium signals were neither additive nor synergistic. The two signalling pathways thus represent parallel, independent mechanisms for distinct tissues (nervous and epithelial) to control the same aspect of renal 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.

AedesCAPA-PVK-1 displays diuretic and dose dependent antidiuretic potential in the larval mosquito Aedes aegypti (Liverpool)

This study reveals that AedesCAPA-PVK-1 (GPTVGLFAFPRV-NH(2)) inhibits basal and serotonin stimulated fluid secretion in the Malpighian tubules of larval Aedes aegypti at femtomolar concentrations. Conversely 10(-4)moll(-1) of the peptide stimulated fluid secretion rates. The diuretic effects of 10(-4)moll(-1)AedesCAPA-PVK-1 and antidiuretic effects of 10(-15)moll(-1)AedesCAPA-PVK-1 were abolished by protein kinase A (PKA) and protein kinase G (PKG) inhibition, respectively. Similar to the peptide, 10(-3)moll(-1) cGMP stimulated fluid secretion but doses in the micromolar to nanomolar range inhibited fluid secretion of the Malpighian tubules. Stimulatory effects of cGMP were abolished by PKA inhibition and inhibitory effects of cGMP were abolished by PKG inhibition. Furthermore, the nitric oxide synthase inhibitor l-NAME attenuated the inhibitory effects of AedesCAPA-PVK-1 but did not affect inhibition by cGMP. Based on the results we propose that AedesCAPA-PVK-1 inhibits fluid secretion rates of larval Malpighian tubules via the NOS/cGMP/PKG pathway and that high doses of the peptide lead to diuresis through the cGMP mediated activation of PKA.

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.

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.

Effects of the microbial metabolite destruxin A on ion transport by the gut and renal epithelia of Drosophila melanogaster

Destruxins have been implicated in the infection process by entomopathogenic fungi and have been also found to be highly toxic when applied topically or ingested by different insect species. To gain insight into the mechanism of action of this toxin on insect internal organs, we have evaluated the effects of destruxin A on Drosophila melanogaster Malpighian tubules and gut tissues. Destruxin A was toxic when injected into adults; the calculated EC(50) was 0.11 mM. Destruxin A significantly inhibited fluid secretion rate by Malpighian tubules as well; the calculated IC(50) was 0.25 μM. The Na(+) concentration in the secreted fluid increased significantly when tubules were exposed to 0.25 μM destruxin A, whereas pH and the concentrations of Ca(2+) and K(+) did not change. In gut, there was no effect of destruxin on H(+) flux, but there was a significant decrease in K(+) and Ca(2+) absorption. The concentration of Ca(2+) and K(+) in the hemolymph of destruxin A-injected flies was not significantly different from those of control flies after 3 h. Taken together, these results show that destruxin A produces differential effects on ion transport by renal and gut tissues. © 2012 Wiley Periodicals, Inc.

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.

Immune and stress response 'cross-talk' in the Drosophila Malpighian tubule

The success of insects is in large part due to their ability to survive environmental stress, including heat, cold, and dehydration. Insects are also exposed to infection, osmotic or oxidative stress, and to xenobiotics or toxins. The molecular mechanisms of stress sensing and response have been widely investigated in mammalian cell lines, and the area of stress research is now so vast to be beyond the scope of a single review article. However, the mechanisms by which stress inputs to the organism are sensed and integrated at the tissue and cellular level are less well understood. Increasingly, common molecular events between immune and other stress responses are observed in vivo; and much of this work stems of efforts in insect molecular science and physiology. We describe here the current knowledge in the area of immune and stress signalling and response at the level of the organism, tissue and cell, focussing on a key epithelial tissue in insects, the Malpighian tubule, and drawing together the known pathways that modulate responses to different stress insults. The tubules are critical for insect survival and are increasingly implicated in responses to multiple and distinct stress inputs. Importantly, as tubule function is central to survival, they are potentially key targets for insect control, which will be facilitated by increased understanding of the complexities of stress signalling in the organism.

The versatile stellate cell - more than just a space-filler

Most epithelia contain multiple cell types that interact to perform the roles required of the tissue. In insect epithelia, the apical plasma membrane V-ATPase dominates ion-transport models, and (as in vertebrates) is usually found in specialized intercalated cell types or regions. The Malpighian tubules of several insect Orders contain not just a mitochondrion-rich principal cell expressing high levels of V-ATPase, but a smaller, intercalated "type II", "secondary" or "stellate" cell. Recent data show that this cell type plays a key role in control of chloride and water flux across the tissue, but also may play other, still unsuspected dynamic roles.

Salt stress alters fluid and ion transport by Malpighian tubules of Drosophila melanogaster: evidence for phenotypic plasticity

Drosophila are tolerant of high levels of dietary salt and can provide a useful model for studies of the physiology of salt stress. The effects of NaCl- and KCl-rich diets on haemolymph ionoregulation and Malpighian tubule (MT) fluid secretion, Na(+) and K(+) secretion and transepithelial potential were examined in larval and adult Drosophila melanogaster. K(+) concentrations in the haemolymph of adults reared on the KCl-rich (0.4 mol l(-1)) diet did not differ from the values for insects reared on the control diet. In the haemolymph of larvae reared on the K-rich diet, K(+) concentrations increased from 23 to 75 mmol l(-1) after 6 h, then returned to the control value within 48 h. Na(+) concentrations in the haemolymph of adults or larvae reared for 1-7 days on the NaCl-rich (0.4 mol l(-1)) diet increased by ~50% relative to values for insects reared on the control diet. Rates of secretion of fluid, Na(+) and K(+) by MTs isolated from larvae reared on the Na-rich diet for >6 h and bathed in control saline containing 20 mmol l(-1) K(+) did not differ from the values for tubules of larvae reared on the control diet. Evidence of phenotypic plasticity was seen in the response of MTs isolated from larvae reared on the K-rich diet for >6 h and bathed in saline containing 60 mmol l(-1) K(+); secretion of fluid and K(+) increased by >50% relative to the values for tubules of larvae reared on the control diet. Secretion of fluid, Na(+) and K(+) increased when tubules were bathed in haemolymph collected from larvae reared on the Na- or K-rich diets. Secretion was further increased by addition of exogenous cAMP but not by addition of thapsigargin to the haemolymph. The results show that haemolymph ionoregulation in larvae reared on salt-rich diets involves both alterations in the basal secretion rates of Na(+) and/or K(+) as well as stimulatory effects of diuretic factors present in the haemolymph. The results suggest that such factors stimulate tubule fluid and ion secretion through increases in intracellular Ca(2+) in response to salt stress.

Physiologic and anatomic characterization of the brain surface glia barrier of Drosophila

Central nervous system (CNS) physiology requires special chemical, metabolic, and cellular privileges for normal function, and blood-brain barrier (BBB) structures are the anatomic and physiologic constructs that arbitrate communication between the brain and body. In the vertebrate BBB, two primary cell types create CNS exclusion biology, a polarized vascular endothelium (VE), and a tightly associated single layer of astrocytic glia (AG). Examples of direct action by the BBB in CNS disease are constantly expanding, including key pathophysiologic roles in multiple sclerosis, stroke, and cancer. In addition, its role as a pharmacologic treatment obstacle to the brain is long standing; thus, molecular model systems that can parse BBB functions and understand the complex integration of sophisticated cellular anatomy and highly polarized chemical protection physiology are desperately needed. Compound barrier structures that use two primary cell types (i.e., functional bicellularity) are common to other humoral/CNS barrier structures. For example, invertebrates use two cell layers of glia, perineurial and subperineurial, to control chemical access to the brain, and analogous glial layers, fenestrated and pseudocartridge, to maintain the blood-eye barrier. In this article, we summarize our current understanding of brain-barrier glial anatomy in Drosophila, demonstrate the power of live imaging as a screening methodology for identifying physiologic characteristics of BBB glia, and compare the physiologies of Drosophila barrier layers to the VE/AG interface of vertebrates. We conclude that many unique BBB physiologies are conserved across phyla and suggest new methods for modeling CNS physiology and disease.

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.

Computational modeling of epithelial tissues

There is an extensive literature on the computational modeling of epithelial tissues at all levels from subcellular to whole tissue. This review concentrates on behavior at the individual cell to whole tissue level, and particularly on organizational aspects, and provides an indication of where information from other areas, such as the modeling of angiogenesis, is relevant. The skin, and the lining of all of the body cavities (lung, gut, cervix, bladder etc) are epithelial tissues, which in a topological sense are the boundary between inside and outside the body. They are thin sheets of cells (usually of the order of 0.5 mm thick) without extracellular matrix, have a relatively simple structure, and contain few types of cells. They have important barrier, secretory and transport functions, which are essential for the maintenance of life, so homeostasis and wound healing are important aspects of the behavior of epithelial tissues. Carcinomas originate in epithelial tissues.There are essentially two approaches to modeling tissues--to start at the level of the tissue (i.e., a length scale of the order of 1 mm) and develop generalized equations for behavior (a continuum approach); or to start at the level of the cell (i.e., a length scale of the order of 10 µm) and develop tissue behavior as an emergent property of cellular behavior (an individual-based approach). As will be seen, these are not mutually exclusive approaches, and they come in a variety of flavors.

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.

Meeting the challenges of on-host and off-host water balance in blood-feeding arthropods

In this review, we describe water balance requirements of blood-feeding arthropods, particularly contrasting dehydration tolerance during the unfed, off-host state and the challenges of excess water that accompany receipt of the bloodmeal. Most basic water balance characteristics during the off-host stage are applicable to other terrestrial arthropods, as well. A well-coordinated suite of responses enable arthropods to conserve water resources, enhance their desiccation tolerance, and increase their water supplies by employing a diverse array of molecular, structural and behavioral responses. Water loss rates during the off-host phase are particularly useful for generating a scheme to classify vectors according to their habitat requirements for water, thus providing a convenient tool with potential predictive power for defining suitable current and future vector habitats. Blood-feeding elicits an entirely different set of challenges as the vector responds to overhydration by quickly increasing its rate of cuticular water loss and elevating the rate of diuresis to void excess water and condense the bloodmeal. Immature stages that feed on blood normally have a net increase in water content at the end of a blood-feeding cycle, but in adults the water content reverts to the pre-feeding level when the cycle is completed. Common themes are evident in diverse arthropods that feed on blood, particularly the physiological mechanisms used to respond to the sudden influx of water as well as the mechanisms used to counter water shortfalls that are encountered during the non-feeding, off-host state.

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.

Drosophila TRPM channel is essential for the control of extracellular magnesium levels

The TRPM group of cation channels plays diverse roles ranging from sensory signaling to Mg²⁺ homeostasis. In most metazoan organisms the TRPM subfamily is comprised of multiple members, including eight in humans. However, the Drosophila TRPM subfamily is unusual in that it consists of a single member. Currently, the functional requirements for this channel have not been reported. Here, we found that the Drosophila TRPM protein was expressed in the fly counterpart of mammalian kidneys, the Malpighian tubules, which function in the removal of electrolytes and toxic components from the hemolymph. We generated mutations in trpm and found that this resulted in shortening of the Malpighian tubules. In contrast to all other Drosophila trp mutations, loss of trpm was essential for viability, as trpm mutations resulted in pupal lethality. Supplementation of the diet with a high concentration of Mg²⁺ exacerbated the phenotype, resulting in growth arrest during the larval period. Feeding high Mg²⁺ also resulted in elevated Mg²⁺ in the hemolymph, but had relatively little effect on cellular Mg²⁺. We conclude that loss of Drosophila trpm leads to hypermagnesemia due to a defect in removal of Mg²⁺ from the hemolymph. These data provide the first evidence for a role for a Drosophila TRP channel in Mg²⁺ homeostasis, and underscore a broad and evolutionarily conserved role for TRPM channels in Mg²⁺ homeostasis.

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.

Functional differences between two CRF-related diuretic hormone receptors in Drosophila

In Drosophila, two related G-protein-coupled receptors are members of the corticotropin releasing factor (CRF) receptor subfamily. We have previously reported that one of these receptors, encoded by CG8422 is a functional receptor for a diuretic hormone, DH(44). Here, we report that the other CRF receptor subfamily member, encoded by CG12370, is also a receptor for the DH(44) neuropeptide. The lines of evidence to support this identification include increases in cAMP levels due to CG12370 receptor activation and the recruitment of beta-arrestin-GFP to the plasma membrane in response to DH(44) application. We compared these features of the receptors DH44-R2 (encoded by CG12370) and DH44-R1 (encoded by CG8422) and found fundamental differences in signaling, association with the arrestins, and peptide sensitivity. We found that the sensitivity of DH44-R2 to the DH(44) peptide is lower than that of DH44-R1, specifically an estimated EC(50) of 7.98E-07 moll(-1) for DH(44) by DH44-R2 to an EC(50) of 5.12E-09 moll(-1) by DH44-R1 and found that previous reports on the sensitivity of the tubule to DH(44) is in agreement with our measurements of DH44-R2 sensitivity. We employed a specific RNAi construct to selectively knock-down DH44-R2 expression and this led to heightened sensitivity to osmotic challenges. The functional characterization of this diuretic hormone receptor in Drosophila demonstrates a high degree of conservation of CRF-like signaling.

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.

Towards a platform for the metabonomic profiling of different strains of Drosophila melanogaster using liquid chromatography-Fourier transform mass spectrometry

A platform based on hydrophilic interaction chromatography in combination with Fourier transform mass spectrometry was developed in order to carry out metabonomics of Drosophila melanogaster strains. The method was able to detect approximately 230 metabolites, mainly in the positive ion mode, after checking to eliminate false positives caused by isotope peaks, adducts and fragment ions. Two wild-type strains, Canton S and Oregon R, were studied, plus two mutant strains, Maroon Like and Chocolate. In order to observe the differential expression of metabolites, liquid chromatography-mass spectrometry analyses of the different strains were compared using sieve 1.2 software to extract metabolic differences. The output from sieve was searched against a metabolite database using an Excel-based macro written in-house. Metabolic differences were observed between the wild-type strains, and also between both Chocolate and Maroon Like compared with Oregon R. It was established that a metabonomic approach could produce results leading to the generation of new hypotheses. In addition, the structure of a new class of lipid with a histidine head group, found in all of the strains of flies, but lower in Maroon Like, was elucidated.

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+.

Insect ion transport peptides are derived from alternatively spliced genes and differentially expressed in the central and peripheral nervous system

Insect ionic and fluid homeostasis relies upon the Malpighian tubules (MT) and different hindgut compartments. Primary urine formed in MTs is finally modified by ion, solute and water reabsorptive processes primarily in the hindgut under the control of several large peptide hormones. One of these, the ion transport peptide (ITP), is a chloride transport-stimulating and acid secretion-inhibiting hormone similar to crustacean hyperglycaemic hormones (CHHs). In locusts, moths and fruit flies, ITP together with the slightly longer ITPL isoforms, inactive in hindgut bioassays, arise by alternative splicing from very similar itp genes. ITP and ITPL are differentially distributed in (1) pars lateralis/retrocerebral complex neurosecretory cells (NSCs) containing both splice forms, (2) interneurons with either one of the splice forms, (3) hindgut-innervating abdominal ITP neurons (in Drosophila only), and (4) intrinsic, putative sensory NSCs in peripheral neurohaemal perisympathetic/perivisceral organs or transverse nerves (usually containing ITPL). Both splice forms occur as hormones released into the haemolymph in response to feeding or stress stimuli. ITPL mainly released from the peripheral NSCs is discussed as a competitive inhibitor (as established in vitro) of ITP action on yet to be identified hindgut ITP receptors. Furthermore, some evidence has been provided for possible ecdysis-related functions of ITP and/or ITPL in moths. The comparative data on the highly similar gene, precursor and primary structures and similar differential distributions in insect and crustacean NSCs suggest that CHH/ITP and ITPL neuropeptide-producing cells and their gene products share common phylogenetic ancestry.

The role of aquaporins in excretion in insects

One of the aspects of insect osmoregulation that has most intrigued researchers is the ability of a simple tubular epithelium, such as the Malpighian tubule, to create both hypo- and hyperosmotic urine. Indeed,Ramsay's initial observation that isolated tubules could secrete a hypoosmotic urine led him to attribute the phenomenon to the active transport of water. In the ensuing decades several models for solute recycling have been proposed,but only in the last 15 years has it become clear that tubule water permeability is due to the presence of aquaporins (AQPs), the ubiquitous water transport proteins. There are 13 known human AQPs, and they are tissue and even membrane specific. It is now clear that the number and type of AQPs within a membrane are the major determinants of its water transport capacity. There are many gene homologs for the AQPs, so proof of function requires expression of the protein in a defined system. Within the insects, only seven AQPs have been functionally expressed and, of these, four directly or indirectly function in excretion. In this paper we review the basic structure and general function of AQPs and then examine the source, localization and functional attributes of those isolated from insects.

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).

Diuretic hormone 44 receptor in Malpighian tubules of the mosquito Aedes aegypti: evidence for transcriptional regulation paralleling urination

In the mosquito Aedes aegypti (L.), the molecular endocrine mechanisms underlying rapid water elimination upon eclosion and blood feeding are not fully understood. The genome contains a single predicted diuretic hormone 44 (DH44) gene, but two DH44 receptor genes. The identity of the DH44 receptor(s) in the Malpighian tubule is unknown in any mosquito species. We show that VectorBase gene ID AAEL008292 encodes the DH44 receptor (GPRDIH1) most highly expressed in Malpighian tubules. Sequence analysis and transcript localization indicate that AaegGPRDIH1 is the co-orthologue of the Drosophila melanogaster DH44 receptor (CG12370-PA). Time-course quantitative PCR analysis of Malpighian tubule cDNA revealed AaegGPRDIH1 expression changes paralleling periods of excretion. This suggests that target tissue receptor biology is linked to the known periods of release of diuretic hormones from the nervous system pointing to a common up-stream regulatory mechanism.

Metabolomic profiling of Drosophila using liquid chromatography Fourier transform mass spectrometry

Hydrophilic interaction chromatography (HILIC) interfaced with an Orbitrap Fourier transform mass spectrometer (FT-MS) was used to carry out metabolomic profiling of the classical Drosophila mutation, rosy (ry). This gene encodes a xanthine oxidase/dehydrogenase. In addition to validating the technology by detecting the same changes in xanthine, hypoxanthine, urate and allantoin that have been reported classically, completely unsuspected changes were detected in each of the tryptophan, arginine, pyrimidine and glycerophospholipid metabolism pathways. The rosy mutation thus ramifies far more widely than previously detected.

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.