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

Proton-driven sodium secretion in a saline water animal

Aquatic animals residing in saline habitats either allow extracellular sodium concentration to conform to environmental values or regulate sodium to lower levels. The latter strategy requires an energy-driven process to move sodium against a large concentration gradient to eliminate excess sodium that diffuses into the animal. Previous studies of invertebrate and vertebrate species indicate a sodium pump, Na+/K+ ATPase, powers sodium secretion. We provide the first functional evidence of a saline-water animal, Aedes taeniorhynchus mosquito larva, utilizing a proton pump to power this process. Vacuolar-type H+ ATPase (VHA) protein is highly expressed on the apical membrane of the posterior rectal cells, and in situ sodium flux across this epithelium increases significantly in larvae held in higher salinity and is sensitive to Bafilomycin A1, an inhibitor of VHA. We also report the first evidence of splice variants of the sodium/proton exchanger, NHE3, with both high and low molecular weight variants highly expressed on the apical membrane of the posterior rectal cells. Evidence of NHE3 function was indicated with in situ sodium transport significantly inhibited by a NHE3 antagonist, S3226. We propose that the outward proton pumping by VHA establishes a favourable electromotive gradient to drive sodium secretion via NHE3 thus producing a hyperosmotic, sodium-rich urine. This H+- driven Na+ secretion process is the primary mechanism of ion regulation in salt-tolerant culicine mosquito species and was first investigated over 80 years ago.

Effects of dietary calcium (Ca2+) and blood feeding on the immunochemical expression of the plasma membrane Ca2+-ATPase (PMCA) in Malpighian tubules of adult female mosquitoes (Aedes aegypti)

Insect Malpighian tubules contribute to Ca2+ homeostasis via Ca2+ storage in intracellular compartments, Ca2+ secretion into the tubule lumen, and Ca2+ reabsorption into the hemolymph. A plasma membrane Ca2+-ATPase (PMCA) is hypothesized to be a Ca2+-transporter involved in renal Ca2+ transport of insects, however few studies have investigated its immunochemical expression in Malpighian tubules. Here we characterized the abundance and localization of PMCA-like immunoreactivity in Malpighian tubules of adult female mosquitoes Aedes aegypti using an antibody against Drosophila melanogaster PMCA. Western blotting revealed expression of a relatively abundant 109 kDa isoform and a relatively sparse 115 kDa isoform. Feeding mosquitoes 10% sucrose with 50 mM CaCl2 for 7 days did not affect PMCA immunoreactivity. However, at 24, 48, and 96 h post-blood feeding (PBF), the relative abundance of the 109 kDa isoform decreased while that of the 115 kDa isoform increased. Immunolabeling of Malpighian tubules revealed PMCA-like immunoreactivity in both principal and stellate cells; principal cell labeling was intracellular, whereas stellate cell labeling was along the basal membrane. Blood feeding enhanced immunolabeling of PMCA in stellate cells but weakened that in principal cells. Moreover, a unique apicolateral pattern of PMCA-like immunolabeling occurred in principal cells of the proximal segment at 24 h PBF, suggesting potential trafficking to septate junctions. Our results suggest PMCA isoforms are differentially expressed and localized in mosquito Malpighian tubules where they contribute to redistributing tubule Ca2+ during blood meal processing.

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

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

Protein localization of aquaporins in the adult female disease vector mosquito, Aedes aegypti

The female Aedes aegypti mosquito is a vector for several arboviral diseases, due to their blood feeding behavior and their association with urban communities. While ion transport in Ae. aegypti has been studied, much less is known about mechanisms of water transport. Rapid water and ion excretion occurs in the adult female mosquito post blood meal and involves a set of organs including the midgut, Malpighian tubules (MTs), and hindgut. The MTs are responsible for the formation of primary urine and are considered the most important site for active transport of ions. Within the cells of the MTs, along with various ion transporters, there are aquaporin water channels that aid in the transport of water across the tubule cell membrane. Six aquaporin genes have been molecularly identified in Ae. aegypti (AQP1-6) and found to be responsible for the transport of water and in some cases, small solutes such as glycerol. In this study, we used immunohistochemistry to localize AaAQP1, 2, 4, 5, and 6 in the adult female Ae. aegypti, in non-blood fed and post blood feeding (0.5 and 24hr) conditions. We further examined the main water transporting aquaporin, AaAQP1, using western blotting to determine protein abundance changes in isolated MTs pre- and post-blood feeding. Using fluorescence in situ hybridization, aqp1 mRNA was found exclusively in the principal cells of female MTs. Finally, we used immunogold staining with transmission electron microscopy to determine subcellular localization of AaAQP1 in the Malpighian tubules under non-blood fed conditions. Interestingly, AaAQP1 was found to be predominantly in the principal cells of the MTs, dispersed throughout the brush border; however, there was also evidence of some AaAQP1 localization in the stellate cells of the MTs.

Voltage-gated ion channels are expressed in the Malpighian tubules and anal papillae of the yellow fever mosquito (Aedes aegypti), and may regulate ion transport during salt and water imbalance

Vectors of infectious disease include several species of Aedes mosquitoes. The life cycle of Aedes aegypti, the yellow fever mosquito, consists of a terrestrial adult and an aquatic larval life stage. Developing in coastal waters can expose larvae to fluctuating salinity, causing salt and water imbalance, which is addressed by two prime osmoregulatory organs - the Malpighian tubules (MTs) and anal papillae (AP). Voltage-gated ion channels (VGICs) have recently been implicated in the regulation of ion transport in the osmoregulatory epithelia of insects. In the current study, we: (i) generated MT transcriptomes of freshwater-acclimated and brackish water-exposed larvae of Ae. aegypti, (ii) detected expression of several voltage-gated Ca2+, K+, Na+ and non-ion-selective ion channels in the MTs and AP using transcriptomics, PCR and gel electrophoresis, (iii) demonstrated that mRNA abundance of many altered significantly following brackish water exposure, and (iv) immunolocalized CaV1, NALCN, TRP/Painless and KCNH8 in the MTs and AP of larvae using custom-made antibodies. We found CaV1 to be expressed in the apical membrane of MTs of both larvae and adults, and its inhibition to alter membrane potentials of this osmoregulatory epithelium. Our data demonstrate that multiple VGICs are expressed in osmoregulatory epithelia of Ae. aegypti and may play an important role in the autonomous regulation of ion transport.

Developmental and Nutritional Dynamics of Malpighian Tubule Autofluorescence in the Asian Tiger Mosquito Aedes albopictus

Malpighian tubules (MTs) are arthropod excretory organs crucial for the osmoregulation, detoxification and excretion of xenobiotics and metabolic wastes, which include tryptophan degradation products along the kynurenine (KYN) pathway. Specifically, the toxic intermediate 3-hydroxy kynurenine (3-HK) is metabolized through transamination to xanthurenic acid or in the synthesis of ommochrome pigments. Early investigations in Drosophila larval fat bodies revealed an intracellular autofluorescence (AF) that depended on tryptophan administration. Subsequent observations documented AF changes in the MTs of Drosophila eye-color mutants genetically affecting the conversion of tryptophan to KYN or 3-HK and the intracellular availability of zinc ions. In the present study, the AF properties of the MTs in the Asian tiger mosquito, Aedes albopictus, were characterized in different stages of the insect's life cycle, tryptophan-administered larvae and blood-fed adult females. Confocal imaging and microspectroscopy showed AF changes in the distribution of intracellular, brilliant granules and in the emission spectral shape and amplitude between the proximal and distal segments of MTs across the different samples. The findings suggest AF can serve as a promising marker for investigating the functional status of MTs in response to metabolic alterations, contributing to the use of MTs as a potential research model in biomedicine.

The V-type H+-ATPase is targeted in antidiuretic hormone control of the Malpighian "renal" tubules

Like other insects, secretion by mosquito Malpighian tubules (MTs) is driven by the V-type H+-ATPase (VA) localized in the apical membrane of principal cells. In Aedes aegypti, the antidiuretic neurohormone CAPA inhibits secretion by MTs stimulated by select diuretic hormones; however, the cellular effectors of this inhibitory signaling cascade remain unclear. Herein, we demonstrate that the VA inhibitor bafilomycin selectively inhibits serotonin (5HT)- and calcitonin-related diuretic hormone (DH31)-stimulated secretion. VA activity increases in DH31-treated MTs, whereas CAPA abolishes this increase through a NOS/cGMP/PKG signaling pathway. A critical feature of VA activation involves the reversible association of the cytosolic (V1) and membrane (Vo) complexes. Indeed, higher V1 protein abundance was found in membrane fractions of DH31-treated MTs, whereas CAPA significantly decreased V1 abundance in membrane fractions while increasing it in cytosolic fractions. V1 immunolocalization was observed strictly in the apical membrane of DH31-treated MTs, whereas immunoreactivity was dispersed following CAPA treatment. VA complexes colocalized apically in female MTs shortly after a blood meal consistent with the peak and postpeak phases of diuresis. Comparatively, V1 immunoreactivity in MTs was more dispersed and did not colocalize with the Vo complex in the apical membrane at 3 h post blood meal, representing a time point after the late phase of diuresis has concluded. Therefore, CAPA inhibition of MTs involves reducing VA activity and promotes complex dissociation hindering secretion. Collectively, these findings reveal a key target in hormone-mediated inhibition of MTs countering diuresis that provides a deeper understanding of this critical physiological process necessary for hydromineral balance.

The role of intercellular junction proteins in the penetration resistance of Drosophila larvae to avermectin

Insecticide resistance has become an increasingly serious challenge for agriculture in the world. To reveal the mechanisms of insecticide resistance, majority of studies have been carried out on the insensitivity of insecticide targets and the metabolism of insecticides. However, the mechanism of the insecticide penetration resistance in insects remains unclear. This study aimed to reveal the mechanism underlying the penetration resistance of Drosophila larvae to insecticide avermectin (AVM). Levels of intercellular junction proteins (IJPs) in the larvae were determined by Western blotting analysis and immunofluorescence assay. The result showed that the expression of IJPs septate junction and adherens junction proteins increased in the AVM-resistant insects compared with those in the AVM-susceptible ones, and the upregulation of the IJPs was mediated by the activation of protein kinase C (PKC) pathway. That AVM induced the activation of PKC was found not only in the Drosophila larvae but also in Drosophila S2 cells. These findings revealed that AVM could activate PKC pathway in Drosophila larvae, which mediated the upregulation of the IJPs and then led to the resistance to AVM, suggesting that the chemicals that can disrupt PKC activation may potentially be used to circumvent the resistance to AVM in insects.

Exploratory phosphoproteomics profiling of Aedes aegypti Malpighian tubules during blood meal processing reveals dramatic transition in function

Malpighian tubules, the renal organs of mosquitoes, facilitate the rapid dehydration of blood meals through aquaporin-mediated osmosis. We performed phosphoproteomics analysis of three Malpighian tubule protein-libraries (1000 tubules/sample) from unfed female mosquitoes as well as one and 24 hours after a blood meal. We identified 4663 putative phosphorylation sites in 1955 different proteins. Our exploratory dataset reveals blood meal-induced changes in phosphorylation patterns in many subunits of V-ATPase, proteins of the target of rapamycin signaling pathway, vesicle-mediated protein transport proteins, proteins involved in monocarboxylate transport, and aquaporins. Our phosphoproteomics data suggest the involvement of a variety of new pathways including nutrient-signaling, membrane protein shuttling, and paracellular water flow in the regulation of urine excretion. Our results support a model in which aquaporin channels translocate from intracellular vesicles to the cell membrane of stellate cells and the brush border membrane of principal cells upon blood feeding.

Gene expression profiles in Malpighian tubules of the vector leafhopper Psammotettix striatus (L.) revealed regional functional diversity and heterogeneity

Background

Many leafhoppers are known as pests and disease vectors of economically important plants. Previous studies of the physiological functions of vector leafhoppers have mainly focused on the salivary glands and the alimentary tract that are deemed to be associated with digestion, host defense and phytoplasma and/or virus transmission. By contrast, the significance of Malpighian tubules (MTs) is less studied. To clarify the physiological function of MTs of the vector leafhopper Psammotettix striatus that transmits phytoplasma triggering the wheat blue dwarf disease, we performed a transcriptome study on P. striatus MTs and compared gene expression profiles among different anatomical regions in the tubules (i.e., MT1+2, the anterior segment together with the sub-anterior segment; MT3, the inflated segment; and MT4, the distal segment).

Results

Transcriptome of P. striatus MTs generate a total of 42,815 high-quality unigenes, among which highly expressed unigenes are mainly involved in organic solute transport, detoxification and immunity in addition to osmoregulation. Region-specific comparative analyses reveal that all these MT regions have functions in osmoregulation, organic solute transport and detoxification, but each region targets different substrates. Differential expression and regional enrichment of immunity-related effector activities and molecules involved in phagocytosis and the biosynthesis of antimicrobial peptides among different regions indicate that MT1+2 and MT4 have the ability to eliminate the invading pathogens. However, in MT3 which secrets brochosomes to the integument and eggs as physical barriers, disulfide-isomerase, acidic ribosomal protein P and many other unigenes were highly expressed, which can be attractive candidate genes for future studies of the biosynthesis and the origin of brochosomes.

Conclusions

Psammotettix striatus MTs perform multiple physiological functions as versatile organs than just excretory organs with osmoregulatory function. Heterogeneity of physiological functions among different MT regions is related to organic solute transport, detoxification, immunity and brochosome biosynthesis in addition to osmoregulation, and each region targets different substrates. These functions may be helpful for P. striatus to resist pathogens from habitats and to utilize a wider range of host plants, which may assist the transmission and spread of phytoplasmas. The results provide potential molecular targets for the exploit of chemical and/or gene-silencing insecticides.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08300-6.

Novel inhibitors of the renal inward rectifier potassium channel of the mosquito vector Aedes aegypti

The mosquito continues to be the most lethal animal to humans due to the devastating diseases that it carries and transmits. Controlling mosquito-borne diseases relies heavily on vector management using neurotoxic insecticides with limited modes of action. This has led to the emergence of resistance to pyrethroids and other neurotoxic insecticides in mosquitoes, which has reduced the efficacy of chemical control agents. Moreover, many neurotoxic insecticides are not selective for mosquitoes and negatively impact beneficial insects such as honeybees. Developing new mosquitocides with novel mechanisms of action is a clear unmet medical need; this review covers the efforts made toward this end by targeting the renal inward rectifier potassium channel (Kir) of the mosquito.

Osmoregulatory capacity at low temperature is critical for insect cold tolerance

At low temperature many insects lose extracellular ion homeostasis and the capacity to mitigate homeostatic imbalance determines their cold tolerance. Extracellular homeostasis is ensured by the osmoregulatory organs and recent research has emphasized key roles for the Malpighian tubules and hindgut in modulating insect cold tolerance. Here, we review the effects of low temperature on transport capacity of osmoregulatory organs and outline physiological processes leading from cold exposure to disruption of ion homeostasis and cold-injury in insects. We show how cold adaptation and cold acclimation are associated with physiological modifications to transport capacity in Malpighian tubules and hindgut. These responses mitigate loss of homeostasis and we highlight how further study of molecular and cellular mechanisms are critical to fully appreciate the adaptations that facilitate insect cold tolerance.

Hormonal regulation and functional role of the "renal" tubules in the disease vector, Aedes aegypti

The Aedes aegypti mosquito is a vector responsible for transmitting various arboviruses including dengue and yellow fever. Their ability to regulate the ionic and water composition of their hemolymph is a major physiological phenomenon, allowing the mosquito to adapt to a range of ecological niches. Hematophagus insects, including the female A. aegypti, face the challenge of excess salt and water intake after a blood meal. Post-prandial diuresis is under rigorous control by neuroendocrine factors, acting on the Malpighian "renal" tubules (MTs), to regulate primary urine production. The MTs are made up of two cell types; mitochondria-rich principal cells, which facilitate active transport of Na⁺ and K⁺ cations across the membrane, and thin stellate cells, which allows for transepithelial Cl^- secretion. The active driving force responsible for ion transport is the apical V-type H⁺ ATPase, which creates a proton gradient allowing for Na⁺ and/or K⁺ cation exchange through cation/H⁺ antiporters. Additionally, the basolaterally localized Na⁺-K⁺-2Cl^- cotransporter (NKCC) is responsible for the transport of these ions from the hemolymph into the principal cells. Numerous studies have examined hormonal regulation of the mosquito MTs and identified several diuretics including serotonin (5HT), a calcitonin-related diuretic hormone 31 (DH₃₁), a corticotropin-related factor like diuretic peptide (DH₄₄), a kinin-related diuretic peptide, as well as anti-diuretic factors including CAPA peptides, all of which are known to regulate fluid and ion transport by the MTs. This review therefore focuses on the control of ionic homeostasis in A. aegypti mosquitoes, emphasizing the importance of the MTs, the channels and transporters involved in maintaining hydromineral balance, and the neuroendocrine regulation of both diuresis and anti-diuresis.

Arthropods Under Pressure: Stress Responses and Immunity at the Pathogen-Vector Interface

Understanding what influences the ability of some arthropods to harbor and transmit pathogens may be key for controlling the spread of vector-borne diseases. Arthropod immunity has a central role in dictating vector competence for pathogen acquisition and transmission. Microbial infection elicits immune responses and imparts stress on the host by causing physical damage and nutrient deprivation, which triggers evolutionarily conserved stress response pathways aimed at restoring cellular homeostasis. Recent studies increasingly recognize that eukaryotic stress responses and innate immunity are closely intertwined. Herein, we describe two well-characterized and evolutionarily conserved mechanisms, the Unfolded Protein Response (UPR) and the Integrated Stress Response (ISR), and examine evidence that these stress responses impact immune signaling. We then describe how multiple pathogens, including vector-borne microbes, interface with stress responses in mammals. Owing to the well-conserved nature of the UPR and ISR, we speculate that similar mechanisms may be occurring in arthropod vectors and ultimately impacting vector competence. We conclude this Perspective by positing that novel insights into vector competence will emerge when considering that stress-signaling pathways may be influencing the arthropod immune network.

Beat the heat: Culex quinquefasciatus regulates its body temperature during blood feeding

Mosquitoes are regarded as one of the most dangerous animals on earth. Because they are responsible for the spread of a wide range of both human and animal pathogens, research of the underlying mechanisms of their feeding behavior and physiology is critical. Among disease vector mosquitoes, Culex quinquefasciatus, a known carrier of West Nile virus and Western Equine Encephalitis, remains relatively understudied. As blood-sucking insects, adaptations (either at the molecular or physiological level) while feeding on warm blood are crucial to their survival, as overheating can result in death due to heat stress. Our research aims to determine how Cx. quinquefasciatus copes with the heat associated with warm blood meal ingestion and possibly uncover the adaptations this species uses to avoid thermal stress. Through the use of thermographic imaging, we analyzed the body temperature of Cx. quinquefasciatus while blood feeding. Infrared thermography has allowed us to identify a cooling strategy, evaporative cooling via the production of fluid droplets, and an overall low body temperature in comparison to the blood temperature during feeding. Understanding Cx. quinquefasciatus' adaptations and the strategies they employ to reduce their body temperature while blood feeding constitutes the first step towards discovering potential targets that could be used for their control.

The Phlebotomus papatasi systemic transcriptional response to trypanosomatid-contaminated blood does not differ from the non-infected blood meal

Background

Leishmaniasis, caused by parasites of the genus Leishmania, is a disease that affects up to 8 million people worldwide. Parasites are transmitted to human and animal hosts through the bite of an infected sand fly. Novel strategies for disease control require a better understanding of the key step for transmission, namely the establishment of infection inside the fly.

Methods

The aim of this work was to identify sand fly systemic transcriptomic signatures associated with Leishmania infection. We used next generation sequencing to describe the transcriptome of whole Phlebotomus papatasi sand flies when fed with blood alone (control) or with blood containing one of three trypanosomatids: Leishmania major, L. donovani and Herpetomonas muscarum, the latter being a parasite not transmitted to humans.

Results

Of the trypanosomatids studied, only L. major was able to successfully establish an infection in the host P. papatasi. However, the transcriptional signatures observed after each parasite-contaminated blood meal were not specific to success or failure of a specific infection and they did not differ from each other. The transcriptional signatures were also indistinguishable after a non-contaminated blood meal.

Conclusions

The results imply that sand flies perceive Leishmania as just one feature of their microbiome landscape and that any strategy to tackle transmission should focus on the response towards the blood meal rather than parasite establishment. Alternatively, Leishmania could suppress host responses. These results will generate new thinking around the concept of stopping transmission by controlling the parasite inside the insect.

Paracellular transport to the coral calcifying medium: effects of environmental parameters

Coral calcification relies on the transport of ions and molecules to the extracellular calcifying medium (ECM). Little is known about paracellular transport (via intercellular junctions) in corals and other marine calcifiers. Here, we investigated whether the permeability of the paracellular pathway varied in different environmental conditions in the coral Stylophora pistillata Using the fluorescent dye calcein, we characterised the dynamics of calcein influx from seawater to the ECM and showed that increases in paracellular permeability (leakiness) induced by hyperosmotic treatment could be detected by changes in calcein influx rates. We then used the calcein-imaging approach to investigate the effects of two environmental stressors on paracellular permeability: seawater acidification and temperature change. Under conditions of seawater acidification (pH 7.2) known to depress pH in the ECM and the calcifying cells of S. pistillata, we observed a decrease in half-times of calcein influx, indicating increased paracellular permeability. By contrast, high temperature (31°C) had no effect, whereas low temperature (20°C) caused decreases in paracellular permeability. Overall, our study establishes an approach to conduct further in vivo investigation of paracellular transport and suggests that changes in paracellular permeability could form an uncharacterised aspect of the physiological response of S. pistillata to seawater acidification.

Receptor Characterization and Functional Activity of Pyrokinins on the Hindgut in the Adult Mosquito, Aedes aegypti

Pyrokinins are structurally related insect neuropeptides, characterized by their myotropic, pheromonotropic and melanotropic roles in some insects, but their function is unclear in blood-feeding arthropods. In the present study, we functionally characterized the pyrokinin-1 and pyrokinin-2 receptors (PK1-R and PK2-R, respectively), in the yellow fever mosquito, Aedes aegypti, using a heterologous cell system to characterize their selective and dose-responsive activation by members of two distinct pyrokinin subfamilies. We also assessed transcript-level expression of these receptors in adult organs and found the highest level of PK1-R transcript in the posterior hindgut (rectum) while PK2-R expression was enriched in the anterior hindgut (ileum) as well as in reproductive organs, suggesting these to be prominent target sites for their peptidergic ligands. In support of this, PRXa-like immunoreactivity (where X = V or L) was localized to innervation along the hindgut. Indeed, we identified a myoinhibitory role for a PK2 on the ileum where PK2-R transcript was enriched. However, although we found that PK1 did not influence myoactivity or Na⁺ transport in isolated recta, the PRXa-like immunolocalization terminating in close association to the rectal pads and the significant enrichment of PK1-R transcript in the rectum suggests this organ could be a target of PK1 signaling and may regulate the excretory system in this important disease vector species.

The Drosophila Malpighian tubule as a model for mammalian tubule function

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

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

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

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.

The septate junction protein Mesh is required for epithelial morphogenesis, ion transport, and paracellular permeability in the Drosophila Malpighian tubule

Septate junctions (SJs) are occluding cell-cell junctions that have roles in paracellular permeability and barrier function in the epithelia of invertebrates. Arthropods have two types of SJs, pleated SJs and smooth SJs (sSJs). In Drosophila melanogaster, sSJs are found in the midgut and Malpighian tubules, but the functions of sSJs and their protein components in the tubule epithelium are unknown. Here we examined the role of the previously identified integral sSJ component, Mesh, in the Malpighian tubule. We genetically manipulated mesh specifically in the principal cells of the tubule at different life stages. Tubules of flies with developmental mesh knockdown revealed defects in epithelial architecture, sSJ molecular and structural organization, and lack of urine production in basal and kinin-stimulated conditions, resulting in edema and early adult lethality. Knockdown of mesh during adulthood did not disrupt tubule epithelial and sSJ integrity but decreased the transepithelial potential, diminished transepithelial fluid and ion transport, and decreased paracellular permeability to 4-kDa dextran. Drosophila kinin decreased transepithelial potential and increased chloride permeability, and it stimulated fluid secretion in both control and adult mesh knockdown tubules but had no effect on 4-kDa dextran flux. Together, these data indicate roles for Mesh in the developmental maturation of the Drosophila Malpighian tubule and in ion and macromolecular transport in the adult tubule.

Mechanisms and regulation of chloride transport in the Malpighian tubules of the larval cabbage looper Trichoplusia ni

Malpighian tubules (MTs) and the hindgut together constitute the excretory system of insects. Larvae of lepidopterans (butterflies and moths) demonstrate the so-called cryptonephric arrangement, where the distal blind end of each MT is embedded into the rectal complex. The rest of the free tubule is modified into several distinct regions that differ greatly in the transport of cations and water. However, relatively little is known about the transport of counter-anions (e.g., Cl^- and HCO₃^-) by the MTs of lepidopteran larvae. In the current study we used ion-selective microelectrodes to characterize Cl^- transport in the distinct regions of the free MT of the larval Trichoplusia ni. Firstly, we note that Cl^- transport in the MTs is sensitive to the Cl^- concentration of the bathing saline, and several regions of the MTs are capable of either secreting or reabsorbing Cl^-. In the distal ileac plexus (DIP), a region previously characterized by cellular heterogeneity and its ability to switch between cation secretion and reabsorption, principal cells (PCs) toggled between Cl^- reabsorption (in high-Cl^- saline) and Cl^- secretion (in low-Cl^- saline). In contrast, secondary cells (SCs) in the DIP secreted Cl^- regardless of saline Cl^- concentration. Mechanistically, we have detected a number of 'leak' and ligand-gated Cl^- channels (ClC) and demonstrated that Cl^- channels are involved in Cl^- secretion. Additionally, we demonstrated that the lumen-positive transepithelial potential increased in response to glycine. Using the scanning ion-selective electrode technique we demonstrated that glycine stimulated Cl^- secretion by SCs, but not by PCs. In contrast, when MTs were deprived of glycine, a decrease in Cl^- secretion, coupled with a decrease in the TEP, was observed. In contrast to the effects of glycine, an active dose of helicokinin reduced Cl^- secretion by PCs, but not by SCs. Lastly, we detected expression of chloride-bicarbonate exchangers (CBE) in all regions of the free tubule. Scans of H⁺ transport across the tubule indicated that base equivalents are likely reabsorbed across the ileac plexus. Blocking ClC or CBE led to secretion of a more basic fluid, indicating lack of base reabsorption. We suggest that the transport of Cl^- in the MTs of larval lepidopterans (i) may be correlated with the reabsorption of base, (ii) may be sensitive to Cl^- concentration in the haemolymph, and (iii) could be regulated by helicokinin and glycine.

Helicokinin alters ion transport in the secondary cell-containing region of the Malpighian tubule of the larval cabbage looper Trichoplusia ni

Excretion in insects is accomplished by the combined actions of the Malpighian tubules (MTs) and hindgut, which together form the functional kidney. MTs of many insect groups consist of principal cells (PC) and secondary cells (SC). In most insect groups SCs are reported to secrete ions from haemolymph into the tubule lumen. Paradoxically, SCs in the MTs of the lepidopteran cabbage looper T. ni are used to reabsorb Na⁺ and K⁺ back into haemolymph. The current study was designed to investigate the effects and mode of action of the lepidopteran kinin, Helicokinin (HK), on ion transport in the SC-containing region of MT of T. ni. We identified a HK receptor (HK-R) homologue in T. ni and detected its expression in the SC-containing region of the MTs. The mRNA abundance of hk-r altered in response to changes in dietary K⁺ and Na⁺ content. HK-R immunolocalized to both PCs and SCs. Ramsay assays of preparations of the isolated distal ileac plexus (DIP) indicated that [HK] = 10^-8 M: (i) decreased fluid secretion rate in unstimulated and serotonin-stimulated preparations, and (ii) increased [Na⁺]/[K⁺] ratio in the secreted fluid. Scanning ion-selective electrode technique measurements revealed that HK reduced: (i) K⁺ secretion by the PCs, and (ii) Na⁺ reabsorption by the SCs in intact tubules. In vitro incubation of the DIP with HK resulted in reduced mRNA abundance of hk-r as well as Na⁺/K⁺-ATPase subunit α (NKAα), Na⁺/K⁺/Cl^- co-transporter (nkcc), Na⁺/H⁺ exchangers (nhe) 7 and 8, and aquaporin (aqp) 1. Taken together, results of the current study suggest that HK is capable of altering fluid secretion rate and [Na⁺]/[K⁺] ratio of the fluid, and that HK targets both PCs and SCs in the DIP of T. ni.

Voltages and resistances of the anterior Malpighian tubule of Drosophila melanogaster

The small size of Malpighian tubules in the fruit fly Drosophila melanogaster has discouraged measurements of the transepithelial electrical resistance. The present study introduces two methods for measuring the transepithelial resistance in isolated D . melanogaster Malpighian tubules using conventional microelectrodes and PClamp hardware and software. The first method uses three microelectrodes to measure the specific transepithelial resistance normalized to tubule length or luminal surface area for comparison with resistances of other epithelia. The second method uses only two microelectrodes to measure the relative resistance for comparing before and after effects in a single Malpighian tubule. Knowledge of the specific transepithelial resistance allows the first electrical model of electrolyte secretion by the main segment of the anterior Malpighian tubule of D . melanogaster The electrical model is remarkably similar to that of the distal Malpighian tubule of Aedes aegypti when tubules of Drosophila and Aedes are studied in vitro under the same experimental conditions. Thus, despite 189 millions of years of evolution separating these two genera, the electrophysiological properties of their Malpighian tubules remains remarkably conserved.

secCl is a cys-loop ion channel necessary for the chloride conductance that mediates hormone-induced fluid secretion in Drosophila

Organisms use circulating diuretic hormones to control water balance (osmolarity), thereby avoiding dehydration and managing excretion of waste products. The hormones act through G-protein-coupled receptors to activate second messenger systems that in turn control the permeability of secretory epithelia to ions like chloride. In insects, the chloride channel mediating the effects of diuretic hormones was unknown. Surprisingly, we find a pentameric, cys-loop chloride channel, a type of channel normally associated with neurotransmission, mediating hormone-induced transepithelial chloride conductance. This discovery is important because: 1) it describes an unexpected role for pentameric receptors in the membrane permeability of secretory epithelial cells, and 2) it suggests that neurotransmitter-gated ion channels may have evolved from channels involved in secretion.

Heterologous Expression of Aedes aegypti Cation Chloride Cotransporter 2 (aeCCC2) in Xenopus laevis Oocytes Induces an Enigmatic Na⁺/Li⁺ Conductance

The yellow fever mosquito Aedes aegypti possesses three genes encoding putative Na⁺-coupled cation chloride cotransporters (CCCs): aeNKCC1, aeCCC2, and aeCCC3. To date, none of the aeCCCs have been functionally characterized. Here we expressed aeCCC2 heterologously in Xenopus oocytes and measured the uptake of Li⁺ (a tracer for Na⁺) and Rb⁺ (a tracer for K⁺). Compared to control (H₂O-injected) oocytes, the aeCCC2-expressing oocytes exhibited significantly greater uptake of Li⁺, but not Rb⁺. However, the uptake of Li⁺ was neither Cl^--dependent nor inhibited by thiazide, loop diuretics, or amiloride, suggesting unconventional CCC activity. To determine if the Li⁺-uptake was mediated by a conductive pathway, we performed two-electrode voltage clamping (TEVC) on the oocytes. The aeCCC2 oocytes were characterized by an enhanced conductance for Li⁺ and Na⁺, but not K⁺, compared to control oocytes. It remains to be determined whether aeCCC2 directly mediates the Na⁺/Li⁺ conductance or whether heterologous expression of aeCCC2 stimulates an endogenous cation channel in the oocyte plasma membrane.

Transcriptomic analysis of the Malpighian tubules of Trichoplusia ni: Clues to mechanisms for switching from ion secretion to ion reabsorption in the distal ileac plexus

Excretion of metabolic wastes and toxins in insect Malpighian tubules (MTs) is coupled to secretion of ions and fluid. Larval lepidopterans demonstrate a complex and regionalized MT morphology, and recent studies of larvae of the cabbage looper, Trichoplusia ni, have revealed several unusual aspects of ion transport in the MTs. Firstly, cations are reabsorbed via secondary cells (SCs) in T. ni, whereas in most insects SCs secrete ions. Secondly, SCs are coupled to neighbouring principal cells (PCs) via gap junctions to enable such ion reabsorption. Thirdly, PCs in the SC-containing distal ileac plexus (DIP) region of the tubule reverse from cation secretion to reabsorption in response to dietary ion loading. Lastly, antidiuresis is observed in response to a kinin neuropeptide, which targets both PCs and SCs, whereas in most insects kinins are diuretics that act exclusively via SCs. Recent studies have generated a basic model of ion transport in the DIP of the larval T. ni. RNAseq was used to elucidate previously uncharacterised aspects of ion transport and endocrine regulation in the DIP, with the aim of painting a composite picture of ion transport and identifying putative regulatory mechanisms of ion transport reversal in this tissue. Results indicated an overall expression of 9103 transcripts in the DIP, 993 and 382 of which were differentially expressed in the DIP of larvae fed high-K⁺ and high-Na⁺ diets respectively. Differentially expressed transcripts include ion-motive ATPases, ion channels and co-transporters, aquaporins, nutrient and xenobiotic transporters, cell adhesion and junction components, and endocrine receptors. Notably, several transcripts for voltage-gated ion channels and cell volume regulation-associated products were detected in the DIP and differentially expressed in larvae fed ion-rich diet. The study provides insights into the transport of solutes (sugars, amino acids, xenobiotics, phosphate and inorganic ions) by the DIP of lepidopterans. Our data suggest that this region of the MT in lepidopterans (as previously reported) transports cations, fluid, and xenobiotics/toxic metals. Besides this, the DIP expresses genes coding for the machinery involved in Na⁺- and H⁺-dependent reabsorption of solutes, chloride transport, and base recovery. Additionally, many of the transcripts expressed by the DIP a capacity of this region to respond to, process, and sometimes produce, neuropeptides, steroid hormones and neurotransmitters. Lastly, the DIP appears to possess an arsenal of septate junction components, differential expression of which may indicate junctional restructuring in the DIP of ion-loaded larvae.

Septate junction in the distal ileac plexus of larval lepidopteran Trichoplusia ni: alterations in paracellular permeability during ion transport reversal

The Malpighian tubules (MTs) and hindgut together act as the functional kidney in insects. MTs of caterpillars are notably complex and consist of several regions that display prominent differences in ion transport. The distal ileac plexus (DIP) is a region of Malpighian tubule that is of particular interest because it switches from ion secretion to ion reabsorption in larvae fed on ion-rich diets. The pathways of solute transport in the DIP are not well understood, but one potential route is the paracellular pathway between epithelial cells. This pathway is regulated by the septate junctions (SJs) in invertebrates, and in this study, we found regional and cellular heterogeneity in expression of several integral SJ proteins. DIP of larvae fed ion-rich diets demonstrated a reduction in paracellular permeability, coupled with alterations in both SJ morphology and the abundance of its molecular components. Similarly, treatment in vitro with helicokinin (HK), an antidiuretic hormone identified by previous studies, altered mRNA abundance of many SJ proteins and reduced paracellular permeability. HK was also shown to target a secondary cell-specific SJ protein Tsp2A. Taken together, our data suggest that dietary ion loading, known to cause ion transport reversal in the DIP of larval T. ni, leads to alterations in the paracellular permeability, SJ morphology and its molecular component abundance. The results suggest that HK is an important endocrine factor that co-regulates ion transport, water transport and paracellular permeability in MTs of larval lepidopterans. We propose that co-regulation of all three components of the MT function in larval lepidopterans allows for safe toggling between ion secretion and reabsorption in the DIP in response to variations in dietary ion availability.

Comparative transcriptomic analysis of larval and adult Malpighian tubules from the cotton bollworm Helicoverpa armigera

Malpighian tubules (MTs) are usually considered the key excretory and osmoregulatory organs of insects. However, increasing evidence has suggested that MTs perform many more functions than just osmoregulation. Until now, the molecular and physiological functions of MTs in the cotton bollworm (Helicoverpa armigera), a very important agricultural pest, are largely unknown. In this study, the transcriptomes of H. armigera MTs from larvae, male adults and female adults were sequenced using RNA-Seq technology, and comparative analyses of transcriptomes between two life stages (larval and adult) and between adult sexes were conducted. We generated a total of 84 643 high-quality unigenes, and identified a large number of abundant transcripts putatively encoding proteins involved in diuresis, detoxification, immunity, carbohydrate transport and metabolism, development and reproduction. We found that the expression pattern of unigenes was relatively similar between female and male adult MTs, but different between larval and adult MTs. Our data suggest that insect MTs may take multiple physiological functions as versatile organs. The extensive alterations in gene expression in MTs occurred from larvae to adults reflect an ecological adaptation to different feeding habits. Sexual dimorphism in the cotton bollworm is somewhat indicated by the transcriptional difference of genes related to carbohydrate metabolism, detoxification, immunity and reproduction in the MTs of male and female adults.

Shotgun proteomics deciphered age/division of labor-related functional specification of three honeybee (Apis mellifera L.) exocrine glands

The honeybee (Apis mellifera L.) uses various chemical signals produced by the worker exocrine glands to maintain the functioning of its colony. The roles of worker postcerebral glands (PcGs), thoracic glands (TGs), and mandibular glands (MGs) and the functional changes they undergo according to the division of labor from nursing to foraging are not as well studied. To comprehensively characterize the molecular roles of these glands in workers and their changes according to the division of labor of workers, we analyzed the proteomes of PcGs, TGs, and MGs from nurse bees and foragers using shotgun proteomics technology. We identified approximately 2000 proteins from each of the nurse bee or forager glands and highlighted the features of these glands at the molecular level by semiquantitative enrichment analyses of frequently detected, gland-selective, and labor-selective proteins. First, we found the high potential to produce lipids in PcGs and MGs, suggesting their relation to pheromone production. Second, we also found the proton pumps abundant in TGs and propose some transporters possibly related to the saliva production. Finally, our data unveiled candidate enzymes involved in labor-dependent acid production in MGs.

The effect of cold acclimation on active ion transport in cricket ionoregulatory tissues

Cold-acclimated insects defend ion and water transport function during cold exposure. We hypothesized that this is achieved via enhanced active transport. The Malpighian tubules and rectum are likely targets for such transport modifications, and recent transcriptomic studies indicate shifts in Na+-K+ ATPase (NKA) and V-ATPase expression in these tissues following cold acclimation. Here we quantify the effect of cold acclimation (one week at 12°C) on active transport in the ionoregulatory organs of adult Gryllus pennsylvanicus field crickets. We compared primary urine production of warm- and cold-acclimated crickets in excised Malpighian tubules via Ramsay assay at a range of temperatures between 4 and 25°C. We then compared NKA and V-ATPase activities in Malpighian tubule and rectal homogenates from warm- and cold-acclimated crickets via NADH-linked photometric assays. Malpighian tubules of cold-acclimated crickets excreted fluid at lower rates at all temperatures compared to warm-acclimated crickets. This reduction in Malpighian tubule excretion rates may be attributed to increased NKA activity that we observed for cold-acclimated crickets, but V-ATPase activity was unchanged. Cold acclimation had no effect on rectal NKA activity at either 21°C or 6°C, and did not modify rectal V-ATPase activity. Our results suggest that an overall reduction, rather than enhancement of active transport in the Malpighian tubules allows crickets to maintain hemolymph water balance during cold exposure, and increased Malpighian tubule NKA activity may help to defend and/or re-establish ion homeostasis.

Malpighian tubules of Trichoplusia ni: recycling ions via gap junctions and switching between secretion and reabsorption of Na+ and K+ in the distal ileac plexus

The functional kidney in insects consists of the Malpighian tubules and hindgut. Malpighian tubules secrete ions and fluid aiding in hydromineral homeostasis, acid-base balance, and metabolic waste excretion. In many insects, including lepidopterans, the Malpighian tubule epithelium consists of principal cells (PCs) and secondary cells (SCs). The SCs in the Malpighian tubules of larvae of the lepidopteran Trichoplusia ni have been shown to reabsorb K+, transporting it in a direction opposite to that in the neighbouring PCs that secrete K+. One of the mechanisms that could enable such an arrangement is a gap junction (GJ)-based coupling of the two cell types. In the current study, we have immunolocalised GJ protein Innexin-2 to the PC-PC and SC-PC cell-cell borders. We have demonstrated that GJs in the SC-containing region of the Malpighian tubules enable Na+ and K+ reabsorption by the SCs. We also demonstrated that in ion-loaded animals PCs switch from Na+/K+ secretion to reabsorption, resulting in an ion-transporting phenotype similar to that of tubules with pharmacologically blocked GJs. Concomitantly, mRNA abundance encoding GJ proteins was downregulated. Finally, we observed that such PC-based reabsorption was only present in the distal ileac plexus connected to the rectal complex. We propose that this plasticity in the PC function in the distal ileac plexus is likely to be aimed at providing ion supply for the SC function in this segment of the tubule.

Bugs battle stress from hot blood

A heat exchange mechanism in the head of kissing bugs helps to prevent stress and regulate their temperature while they feed on warm blood.

The essential roles of metal ions in insect homeostasis and physiology

Metal ions play distinct roles in living organisms, including insects. Some, like sodium and potassium, are central players in osmoregulation and 'blood and guts' transport physiology, and have been implicated in cold adaptation. Calcium is a key player as a second messenger, and as a structural element. Other metals, particularly those with multiple redox states, can be cofactors in many metalloenzymes, but can contribute to toxic oxidative stress on the organism in excess. This short review selects some examples where classical knowledge has been supplemented with recent advances, in order to emphasize the importance of metals as essential nutrients for insect survival.

Differential expression of putative sodium-dependent cation-chloride cotransporters in Aedes aegypti

The yellow fever mosquito, Aedes aegypti, has three genes that code for proteins with sequence similarity to vertebrate Na⁺-K⁺-Cl^- cotransporters (NKCCs) of the solute-linked carrier 12 superfamily of cation-chloride cotransporters (CCCs). We hypothesized that these mosquito NKCC orthologues have diverged to perform distinct roles in salt secretion and absorption. In phylogenetic analyses, one protein (aeNKCC1) groups with a Drosophila melanogaster NKCC that mediates salt secretion whereas two others (aeCCC2 and aeCCC3) group with a Drosophila transporter that is not functionally characterized. The aeCCC2 and aeCCC3 genes probably result from a tandem gene duplication in the mosquito lineage; they have similar exon structures and are consecutive in genomic DNA. Predicted aeCCC2 and aeCCC3 proteins differ from aeNKCC1 and vertebrate NKCCs in residues from the third transmembrane domain known to influence ion and inhibitor binding. Quantitative PCR revealed that aeNKCC1 and aeCCC2 were approximately equally expressed in larvae and adults, whereas aeCCC3 was approximately 100-fold more abundant in larvae than in adults. In larval tissues, aeCCC2 was approximately 2-fold more abundant in Malpighian tubules compared to anal papillae. In contrast, aeCCC3 was nearly 100-fold more abundant in larval anal papillae compared to Malpighian tubules, suggesting a role in absorption. Western blots with polyclonal antibodies against isoform-specific peptides revealed stronger aeCCC2 immunoreactivity in adults versus larvae, whereas aeCCC3 immunoreactivity was stronger in larvae versus adults. The differential expression pattern of aeCCC2 and aeCCC3, and their sequence divergence in transmembrane domains, suggests that they may have different roles in transepithelial salt transport.

Thermal acclimation mitigates cold-induced paracellular leak from the Drosophila gut

Chill susceptible insects suffer tissue damage and die at low temperatures. The mechanisms that cause chilling injury are not well understood but a growing body of evidence suggests that a cold-induced loss of ion and water homeostasis leads to hemolymph hyperkalemia that depolarizes cells, leading to cell death. The apparent root of this cascade is the net leak of osmolytes down their concentration gradients in the cold. Many insects, however, are capable of adjusting their thermal physiology, and cold-acclimated Drosophila can maintain homeostasis and avoid injury better than warm-acclimated flies. Here, we test whether chilling causes a loss of epithelial barrier function in female adult Drosophila, and provide the first evidence of cold-induced epithelial barrier failure in an invertebrate. Flies had increased rates of paracellular leak through the gut epithelia at 0 °C, but cold acclimation reduced paracellular permeability and improved cold tolerance. Improved barrier function was associated with changes in the abundance of select septate junction proteins and the appearance of a tortuous ultrastructure in subapical intercellular regions of contact between adjacent midgut epithelial cells. Thus, cold causes paracellular leak in a chill susceptible insect and cold acclimation can mitigate this effect through changes in the composition and structure of transepithelial barriers.

The plasticity of extracellular fluid homeostasis in insects

In chemistry, the ratio of all dissolved solutes to the solution's volume yields the osmotic concentration. The present Review uses this chemical perspective to examine how insects deal with challenges to extracellular fluid (ECF) volume, solute content and osmotic concentration (pressure). Solute/volume plots of the ECF (hemolymph) reveal that insects tolerate large changes in all three of these ECF variables. Challenges beyond those tolerances may be 'corrected' or 'compensated'. While a correction simply reverses the challenge, compensation accommodates the challenge with changes in the other two variables. Most insects osmoregulate by keeping ECF volume and osmotic concentration within a wide range of tolerance. Other insects osmoconform, allowing the ECF osmotic concentration to match the ambient osmotic concentration. Aphids are unique in handling solute and volume loads largely outside the ECF, in the lumen of the gut. This strategy may be related to the apparent absence of Malpighian tubules in aphids. Other insects can suspend ECF homeostasis altogether in order to survive extreme temperatures. Thus, ECF homeostasis in insects is highly dynamic and plastic, which may partly explain why insects remain the most successful class of animals in terms of both species number and biomass.

RNA-Seq Comparison of Larval and Adult Malpighian Tubules of the Yellow Fever Mosquito Aedes aegypti Reveals Life Stage-Specific Changes in Renal Function

Introduction: The life history of Aedes aegypti presents diverse challenges to its diuretic system. During the larval and pupal life stages mosquitoes are aquatic. With the emergence of the adult they become terrestrial. This shifts the organism within minutes from an aquatic environment to a terrestrial environment where dehydration has to be avoided. In addition, female mosquitoes take large blood meals, which present an entirely new set of challenges to salt and water homeostasis.

Methods: To determine differences in gene expression associated with these different life stages, we performed an RNA-seq analysis of the main diuretic tissue in A. aegypti, the Malpighian tubules. We compared transcript abundance in 4th instar larvae to that of adult females and analyzed the data with a focus on transcripts that encode proteins potentially involved in diuresis, like water and solute channels as well as ion transporters. We compared our results against the model of potassium- and sodium chloride excretion in the Malpighian tubules proposed by Hine et al. (2014), which involves at least eight ion transporters and a proton-pump.

Results: We found 3,421 of a total number of 17,478 (19.6%) unique transcripts with a P < 0.05 and at least a 2.5 fold change in expression levels between the two groups. We identified two novel transporter genes that are highly expressed in the adult Malpighian tubules, which have not previously been part of the transport model in this species and may play important roles in diuresis. We also identified candidates for hypothesized sodium and chloride channels. Detoxification genes were generally higher expressed in larvae.

Significance: This study represents the first comparison of Malpighian tubule transcriptomes between larval and adult A. aegypti mosquitoes, highlighting key differences in their renal systems that arise as they transform from an aquatic filter-feeding larval stage to a terrestrial, blood-feeding adult stage.

The diapause program impacts renal excretion and molecular expression of aquaporins in the northern house mosquito, Culex pipiens

Adult females of the mosquito Culex pipiens entering diapause increase sugar water ingestion and reduce evaporative water loss, but how these attributes of the diapause program impact activity of the renal excretory system remains unknown. Here we compared the renal excretory capacity of diapausing and non-diapausing females, as well as the molecular expression of aquaporin (AQP) genes that encode channels involved in transporting water and/or small metabolites. Baseline urine excretion rates in diapausing mosquitoes were higher than in those of their non-diapausing counterparts, possibly a consequence of the intense sugar feeding associated with diapause. But, diapausing mosquitoes exhibited a much lower capacity for diuresis than non-diapausing mosquitoes. The suppressed diuretic capacity likely reflects reduced investment in the energetically-expensive post-prandial diuresis, an event not observed in diapausing mosquitoes. The mRNA expression levels of two genes encoding AQPs, Eglp1 and Aqp12L, in diapausing mosquitoes were down-regulated (on day 14) and up-regulated (on both days 3 and 14), respectively, in whole body samples. These changes were not evident in the excretory system (i.e., Malpighian tubules and hindgut), which showed no differential expression of AQPs as a function of diapause. Several AQP mRNAs were, however, differentially expressed in the midgut, ovaries, and abdominal body wall of diapausing mosquitoes, suggesting that AQPs in these tissues may be playing important non-excretory roles that are unique to diapause physiology.

Malpighian Tubules as Novel Targets for Mosquito Control

The Malpighian tubules and hindgut are the renal excretory tissues of mosquitoes; they are essential to maintaining hemolymph water and solute homeostasis. Moreover, they make important contributions to detoxifying metabolic wastes and xenobiotics in the hemolymph. We have focused on elucidating the molecular mechanisms of Malpighian tubule function in adult female mosquitoes and developing chemical tools as prototypes for next-generation mosquitocides that would act via a novel mechanism of action (i.e., renal failure). To date, we have targeted inward rectifier potassium (Kir) channels expressed in the Malpighian tubules of the yellow fever mosquito Aedes aegypti and malaria mosquito Anopheles gambiae. Inhibition of these channels with small molecules inhibits transepithelial K⁺ and fluid secretion in Malpighian tubules, leading to a disruption of hemolymph K⁺ and fluid homeostasis in adult female mosquitoes. In addition, we have used next-generation sequencing to characterize the transcriptome of Malpighian tubules in the Asian tiger mosquito Aedes albopictus, before and after blood meals, to reveal new molecular targets for potentially disrupting Malpighian tubule function. Within 24 h after a blood meal, the Malpighian tubules enhance the mRNA expression of genes encoding mechanisms involved with the detoxification of metabolic wastes produced during blood digestion (e.g., heme, NH₃, reactive oxygen species). The development of chemical tools targeting these molecular mechanisms in Malpighian tubules may offer a promising avenue for the development of mosquitocides that are highly-selective against hematophagous females, which are the only life stage that transmits pathogens.

A mosquito entomoglyceroporin, Aedes aegypti AQP5 participates in water transport across the Malpighian tubules of larvae

The mosquito, Aedes aegypti, is the primary vector for arboviral diseases such as Zika fever, dengue fever, chikungunya, and yellow fever. The larvae reside in hypo-osmotic freshwater habitats, where they face dilution of their body fluids from osmotic influx of water. The Malpighian tubules help maintain ionic and osmotic homeostasis by removing excess water from the hemolymph, but the transcellular pathway for this movement remains unresolved. Aquaporins are transmembrane channels thought to permit transcellular transport of water from the hemolymph into the Malpighian tubule lumen. Immunolocalization of Aedes aegypti aquaporin 5 (AaAQP5) revealed expression by Malpighian tubule principal cells of the larvae, with localization to both the apical and basolateral membranes. Knockdown of AaAQP5 with double stranded RNA decreased larval survival, reduced rates of fluid, K+, and Na+ secretion by the Malpighian tubules and reduced Cl− concentrations in the hemolymph. These findings indicate that AaAQP5 participates in transcellular water transport across the Malpighian tubules of larval Aedes aegypti where global AaAQP5 expression is important for larval survival.

Cold tolerance of Drosophila species is tightly linked to epithelial K+ transport capacity of the Malpighian tubules and rectal pads

Insect chill tolerance is strongly associated with the ability to maintain ion and water homeostasis during cold exposure. Maintenance of K+ balance is particularly important due to its role in setting the cell membrane potential that is involved in many aspects of cellular function and viability. In most insects, K+ balance is maintained through secretion at the Malpighian tubules balancing reabsorption from the hindgut and passive leak arising from the gut lumen. Here, we used a scanning ion-selective electrode technique (SIET) system at benign (23°C) and low (6°C) temperature to examine K+ flux across the Malpighian tubules and the rectal pads in the hindgut in five Drosophila species that differ in cold tolerance. We found that chill tolerant species were better at maintaining K+ secretion and supressing reabsorption during cold exposure. In contrast, chill susceptible species exhibited large reductions in secretion with no change, or a paradoxical increase, in K+ reabsorption. Using an assay to measure paracellular leak we found that chill susceptible species experience a large increase in leak during cold exposure, which could explain the increased K+ reabsorption found in these species. Our data therefore strongly support the hypothesis that cold tolerant Drosophila species are better at maintaining K+ homeostasis through an increased ability to maintain K+ secretion rates and through reduced leakage of K+ towards the hemolymph. These adaptations are manifested both at the Malpighian tubule and at the rectal pads in the hindgut and ensure that cold tolerant species experience less perturbation of K+ homeostasis during cold stress.

Salinity alters snakeskin and mesh transcript abundance and permeability in midgut and Malpighian tubules of larval mosquito, Aedes aegypti

This study examined the distribution and localization of the septate junction (SJ) proteins snakeskin (Ssk) and mesh in osmoregulatory organs of larval mosquito (Aedes aegypti), as well as their response to altered environmental salt levels. Ssk and mesh transcripts and immunoreactivity were detected in tissues of endodermal origin such as the midgut and Malpighian tubules of A. aegypti larvae, but not in ectodermally derived hindgut and anal papillae. Immunolocalization of Ssk and mesh in the midgut and Malpighian tubules indicated that both proteins are concentrated at regions of cell-cell contact between epithelial cells. Transcript abundance of ssk and mesh was higher in the midgut and Malpighian tubules of brackish water (BW, 30% SW) reared A. aegypti larvae when compared with freshwater (FW) reared animals. Therefore, [³H]polyethylene glycol (MW 400Da, PEG-400) flux was examined across isolated midgut and Malpighian tubule preparations as a measure of their paracellular permeability. It was found that PEG-400 flux was greater across the midgut of BW versus FW larvae while the Malpighian tubules of BW-reared larvae had reduced PEG-400 permeability in conjunction with increased Cl^- secretion compared to FW animals. Taken together, data suggest that Ssk and mesh are found in smooth SJs (sSJs) of larval A. aegypti and that their abundance alters in association with changes in epithelial permeability when larvae reside in water of differing salt content. This latter observation suggests that Ssk and mesh play a role in the homeostatic control of salt and water balance in larval A. aegypti.

The orphan pentameric ligand-gated ion channel pHCl-2 is gated by pH and regulates fluid secretion in Drosophila Malpighian tubules

Pentameric ligand-gated ion channels (pLGICs) constitute a large protein superfamily in metazoa whose role as neurotransmitter receptors mediating rapid, ionotropic synaptic transmission has been extensively studied. Although the vast majority of pLGICs appear to be neurotransmitter receptors, the identification of pLGICs in non-neuronal tissues and homologous pLGIC-like proteins in prokaryotes points to biological functions, possibly ancestral, that are independent of neuronal signalling. Here, we report the molecular and physiological characterization of a highly divergent, orphan pLGIC subunit encoded by the pHCl-2 (CG11340) gene, in Drosophila melanogaster We show that pHCl-2 forms a channel that is insensitive to a wide array of neurotransmitters, but is instead gated by changes in extracellular pH. pHCl-2 is expressed in the Malpighian tubules, which are non-innervated renal-type secretory tissues. We demonstrate that pHCl-2 is localized to the apical membrane of the epithelial principal cells of the tubules and that loss of pHCl-2 reduces urine production during diuresis. Our data implicate pHCl-2 as an important source of chloride conductance required for proper urine production, highlighting a novel role for pLGICs in epithelial tissues regulating fluid secretion and osmotic homeostasis.

Mosquito Peptide Hormones: Diversity, Production, and Function

Mosquitoes, like other insects, produce a diversity of peptide hormones that are processed from different precursor proteins and have a range of activities. Early studies relied on purification of bioactive peptides for hormone identification, but more recently genomic data have provided the information needed to more comprehensively identify peptide hormone genes and associated receptors. The first part of this chapter summarizes the known or predicted peptide hormones that are produced by mosquitoes. The second part of this chapter discusses the sources of these molecules and their functions.