Antidiuretic effects of a factor in brain/corpora cardiaca/corpora allata extract on fluid reabsorption across the cryptonephric complex of Manduca sexta

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.

Malpighian tubules of caterpillars: blending RNAseq and physiology to reveal regional functional diversity and novel epithelial ion transport control mechanisms

Malpighian tubules (MTs) and hindgut constitute the functional kidney of insects. MTs are outpouches of the gut and in most insects demonstrate proximodistal heterogeneity in function. In most insects, such heterogeneity is confined to ion/fluid secretion in the distal portion and ion/fluid reabsorption in the proximal portion. In contrast, MTs of larval Lepidoptera (caterpillars of butterflies and moths), are comprised of five regions that differ in their association with the gut, their structure, and ion/fluid transport function. Recent studies have shown that several regions can rapidly and reversibly switch between ion secretion and reabsorption. The current study employed RNAseq, pharmacology and electrophysiology to characterize four distinct regions of the MT in larval Trichoplusia ni. Luminal microelectrode measurements indicate changes in [K+], [Na+] and pH as fluid passes through different regions of the tubule. In addition, the regions examined differ in gene ontology enrichment, and demonstrate robust gradients in expression of ion transporters and endocrine ligand receptors. Lastly, the study provides evidence for direct involvement of voltage-gated and ligand-gated ion channels in epithelial ion transport of insect MTs.

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

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

Secretion of Na(+), K(+) and fluid by the Malpighian (renal) tubule of the larval cabbage looper Trichoplusia ni (Lepidoptera: Noctuidae)

The Malpighian (renal) tubules play important roles in ionic and osmotic homeostasis in insects. In Lepidoptera, the Malpighian tubules are structurally regionalized and the concentration of Na(+) and K(+) in the secreted fluid varies depending on the segment of tubule analyzed. In this work, we have characterized fluid and ion (Na(+), K(+), H(+)) transport by tubules of the larval stage of the cabbage looper Trichoplusia ni; we have also evaluated the effects of fluid secretion inhibitors and stimulants on fluid and ion transport. Ramsay assays showed that fluid was secreted by the iliac plexus but not by the yellow and white regions of the tubule. K(+) and Na(+) were secreted by the distal iliac plexus (DIP) and K(+) was reabsorbed in downstream regions. The fluid secretion rate decreased>50% after 25μM bafilomycin A1, 500μM amiloride or 50μM bumetanide was added to the bath. The concentration of K(+) in the secreted fluid did not change, whereas the concentration of Na(+) in the secreted fluid decreased significantly when tubules were exposed to bafilomycin A1 or amiloride. Addition of 500μM cAMP or 1μM 5-HT to the bath stimulated fluid secretion and resulted in a decrease in K(+) concentration in the secreted fluid. An increase in Na(+) concentration in the secreted fluid was observed only in cAMP-stimulated tubules. Secreted fluid pH and the transepithelial electrical potential (TEP) did not change when tubules were stimulated. Taken together, our results show that the secretion of fluid is carried out by the upper regions (DIP) in T. ni Malpighian tubules. Upper regions of the tubules secrete K(+), whereas lower regions reabsorb it. Stimulation of fluid secretion is correlated with a decrease in the K(+)/Na(+) ratio.

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

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

Serotonin triggers cAMP and PKA-mediated intracellular calcium waves in Malpighian tubules of Rhodnius prolixus

Rhodnius prolixus is a hematophagous insect vector of Chagas disease capable of ingesting up to 10 times its unfed body weight in blood in a single meal. The excess water and ions ingested with the meal are expelled through a rapid postprandial diuresis driven by the Malpighian tubules. Diuresis is triggered by at least two diuretic hormones, a CRF-related peptide and serotonin, which were traditionally believed to trigger cAMP as an intracellular second messenger. Recently, calcium has been suggested to act as a second messenger in serotonin-stimulated Malpighian tubules. Thus, we tested the role of calcium in serotonin-stimulated Malpighian tubules from R. prolixus. Our results show that serotonin triggers cAMP-mediated intracellular Ca(2+) waves that were blocked by incubation in Ca(2+)-free saline containing the cell membrane-permeant Ca(2+) chelator BAPTA-AM, or the PKA blocker H-89. Treatment with 8-Br-cAMP triggered Ca(2+) waves that were blocked by H-89 and BAPTA-AM. Analysis of the secreted fluid in BAPTA-AM-treated tubules showed a 75% reduction in fluid secretion rate with increased K(+) concentration, reduced Na(+) concentration. Taken together, the results indicate that serotonin triggers cAMP and PKA-mediated Ca(2+) waves that are required for maximal ion transport rate.

Two water-specific aquaporins at the apical and basal plasma membranes of insect epithelia: molecular basis for water recycling through the cryptonephric rectal complex of lepidopteran larvae

Larval lepidopteran and coleopteran insects have evolved a specialised cryptonephric system in the hindgut in which water is constantly and rapidly taken up before defecation. In the silkworm, Bombyx mori, the movement of water through the epithelia within the cryptonephric rectal complex is likely facilitated by the two aquaporins, AQP-Bom1 and AQP-Bom3. Both are functionally water-specific and are predominantly expressed in the hindgut (colon and rectum). Phylogenetically, AQP-Bom1 and AQP-Bom3 belong to the DRIP (Drosophila integral protein) and PRIP (Pyrocoelia rufa integral protein) subfamilies, respectively, of the insect AQP clade. In immunoblot analyses using antipeptide antibodies for each Bombyx AQP, the predicted molecular mass for the respective AQPs were around 25 kDa, and further indicated that both tended to be oligomerised as a homotetramer (∼110 kDa). AQP-Bom1 [DRIP] was exclusively expressed at the apical plasma membrane of colonic and rectal epithelial cells, whereas AQP-Bom3 [PRIP] was expressed at the basal plasma membrane of these cells. This polarised localisation of DRIP/PRIP was also observed in the outer cryptonephric Malpighian tubules (outer cMT) and in the six tubules just outside the cryptonephric rectal complex (rectal lead MT). In the rectal epithelia, water is transported from the rectal lumen to the perinephric space and then deposited into the lumen of the outer cMT; the water then goes through the tubular lumen to exit the complex and is finally transported across the rectal lead MT. We conclude that rectal water retrieval into the haemocoele occurs at the very limited region of the water-permeable sites in MT epithelia after passing the rectal and cMT epithelia and that the high osmotic permeability is due to the presence of two distinct water-specific AQPs (DRIP and PRIP) in the epithelial cells of lepidopteran hindgut.

Alteration of carbohydrates metabolism and midgut glucose absorption in Gromphadorhina portentosa after subchronic exposure to imidacloprid and fenitrothion

This study was undertaken to test the hypothesis that following exposure to insecticides, changes take place in the metabolism of carbohydrates and absorption in the midgut of insects. The Madagascar hissing cockroach (Gromphadorhina portentosa) was chosen for the experiment as a model organism, due to it being easy to breed and its relatively large alimentary tract, which was important when preparing the microperfusion midgut bioassay. In each group of cockroaches treated with imidacloprid and fenitrothion, absorption of glucose, expressed as the area under the curve (AUC), was elevated compared to the control group. Glucose in the hemolymph of the examined insects was present in a vestigial amount, often below the threshold of determination, so the determinable carbohydrate indices were: hemolymph trehalose concentration and fat body glycogen content. The level of trehalose found in the hemolymph of insects when exposed to fenitrothion, and irrespective of the level of concentration mixed into food, were significantly lower when comparing to the control samples. Imidacloprid acted analogically with one exception at the concentration of 10 mg·kg(-1) dry food where trehalose concentration did not differ from the control values. Coupling with fat body glycogen concentration was less visible and appeared only at the concentrations of 5 and 10 mg imidacloprid·kg(-1) dry food. As described in this study changes in the sugar distribution and midgut glucose absorption indicate that insects cover the increased energy needs induced by insecticides; also at the gastrointestinal tract level. The result indicates that the midgut glucose absorption parameters could be considered as a non-specific biomarker of insecticide toxicity.

Aquaporin and aquaglyceroporin in silkworms, differently expressed in the hindgut and midgut of Bombyx mori

Two cDNAs similar to aquaporins (AQPs) from other insect species were identified and characterized from the silkworm larva, Bombyx mori. The first cDNA (AQP-Bom1) cloned from the anterior silk gland encodes a 25 900 Da protein similar to insect AQPs isolated from several liquid-feeding insects. The second cDNA (AQP-Bom2) cloned from the posterior midgut encodes a 27 694 Da protein. Northern blot analysis has revealed that the AQP-Bom1 mRNA (2.3 kb) is expressed predominantly in the hindgut (colon and rectum), and moderately or minimally in the silk gland, midgut and Malpighian tubules, while the AQP-Bom2 mRNA (1.3 kb) is mainly expressed in the posterior midgut and Malpighian tubules. Functional analysis in Xenopus oocytes microinjected with the cRNA of these AQPs revealed that the AQP-Bom1 mRNA encodes a water-specific aquaporin, likely involved in the water retrieval function of the hindgut, while the AQP-Bom2 mRNA encodes an aquaglyceroporin, increasing glycerol and urea uptake.

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.

Isolation, identification and localization of a second beetle antidiuretic peptide

We isolated from head extracts of Tenebrio molitor a peptide that inhibits fluid secretion by the Malpighian tubules of this insect. This second antidiuretic factor, ADFb, like the previously published ADFa, works through cyclic GMP as a second messenger. It has primary structure Tyr-Asp-Asp-Gly-Ser-Tyr-Lys-Pro-His-Ile-Tyr-Gly-Phe-OH with an EC(50) of approximately 240 pM in a fluid secretion assay. This peptide is now the second sequenced endogenous insect ADF which inhibits Malpighian tubule fluid secretion. Immunohistochemical techniques show that the peptide is localized in the brain; it appears to be produced mainly in two pairs of bilaterally symmetrical cells in the protocerebrum.

Identification of a potent antidiuretic factor acting on beetle Malpighian tubules

Beetles, like other insects, depend on diuretic and antidiuretic hormones to control water balance. We have isolated, using head extracts from the beetle Tenebrio molitor, a peptide that strongly inhibits fluid secretion by the Malpighian tubules of this insect. This antidiuretic factor (ADF) appears to elicit its effect via cGMP as a second messenger but does not stimulate NO production. It has primary structure: Val-Val-Asn-Thr-Pro-Gly-His-Ala-Val-Ser-Tyr-His-Val-Tyr-OH. The ADF inhibits tubule secretion with high potency: the EC(50) is around 10 fM. It bears no significant resemblance to other biologically active neuropeptides. To our knowledge this is the only endogenous insect ADF acting on Malpighian tubules to be sequenced, and the first coleopteran (beetle) antidiuretic factor fully characterized to date.