The control of Malpighian tubule secretion in a predacious hemipteran insect, the spined soldier bug Podisus maculiventris (Heteroptera, Pentatomidae)

CAPA neuropeptides and their receptor form an anti-diuretic hormone signaling system in the human disease vector, Aedes aegypti

Insect CAPA neuropeptides are homologs of mammalian neuromedin U and are known to influence ion and water balance by regulating the activity of the Malpighian 'renal' tubules (MTs). Several diuretic hormones are known to increase primary fluid and ion secretion by insect MTs and, in adult female mosquitoes, a calcitonin-related peptide (DH31) called mosquito natriuretic peptide, increases sodium secretion to compensate for the excess salt load acquired during blood-feeding. An endogenous mosquito anti-diuretic hormone was recently described, having potent inhibitory activity against select diuretic hormones, including DH31. Herein, we functionally deorphanized, both in vitro and in vivo, a mosquito anti-diuretic hormone receptor (AedaeADHr) with expression analysis indicating highest enrichment in the MTs where it is localized within principal cells. Characterization using a heterologous in vitro system demonstrated the receptor was highly sensitive to mosquito CAPA neuropeptides while in vivo, AedaeADHr knockdown abolished CAPA-induced anti-diuretic control of DH31-stimulated MTs. CAPA neuropeptides are produced within a pair of neurosecretory cells in each of the abdominal ganglia, whose axonal projections innervate the abdominal neurohaemal organs, where these neurohormones are released into circulation. Lastly, pharmacological inhibition of nitric oxide synthase (NOS) and protein kinase G (PKG) signaling eliminated anti-diuretic activity of CAPA, highlighting the role of the second messenger cGMP and NOS/PKG in this anti-diuretic signaling pathway.

Carbohydrases in the digestive system of the spined soldier bug, Podisus maculiventris (Say) (Hemiptera: Pentatomidae)

The spined soldier bug, Podisus maculiventris, is a generalist predator of insects and has been used in biological control. However, information on the digestion of food in this insect is lacking. Therefore, we have studied the digestive system in P. maculiventris, and further characterized carbohydrases in the digestive tract. The midgut of all developmental stages was composed of anterior, median, and posterior regions. The volumes of the anterior midgut decreased and the median midgut increased in older instars and adults, suggesting a more important role of the median midgut in food digestion. However, carbohydrase activities were predominant in the anterior midgut. In comparing the specific activity of carbohydrases, α-amylase activity was more in the salivary glands (with two distinct activity bands in zymograms), and glucosidase and galactosidase activities were more in the midgut. Salivary α-amylases were detected in the prey hemolymph, demonstrating the role of these enzymes in extra-oral digestion. However, the catalytic efficiency of midgut α-amylase activity was approximately twofold more than that of the salivary gland enzymes, and was more efficient in digesting soluble starch than glycogen. Midgut α-amylases were developmentally regulated, as one isoform was found in first instar compared to three isoforms in fifth instar nymphs. Starvation significantly affected carbohydrase activities in the midgut, and acarbose inhibited α-amylases from both the salivary glands and midgut in vitro and in vivo. The structural diversity and developmental regulation of carbohydrases in the digestive system of P. maculiventris demonstrate the importance of these enzymes in extra-oral and intra-tract digestion, and may explain the capability of the hemipteran to utilize diverse food sources.

Isolation and functional characterization of calcitonin-like diuretic hormone receptors in Rhodnius prolixus

Several families of diuretic hormones exist in insects, one of which is the calcitonin-like diuretic hormone (CT/DH) family. CT/DH mediates its effects by binding to family B G-protein coupled receptors (GPCRs). Here we isolate and functionally characterize two R. prolixusCT/DH receptor paralogs (Rhopr-CT/DH-R1 and Rhopr-CT/DH-R2) using a novel heterologous assay utilizing a modified human embryonic kidney 293 (HEK293) cell line. Rhopr-CT/DH-R1 is orthologous to the previously characterized D. melanogasterCT/DH receptor (CG17415) while Rhopr-CT/DH-R2 is orthologous to the D. melanogaster receptor (CG4395), an orphan receptor whose ligand was unknown until now. We determine the cDNA sequences of three splice variants encoding Rhopr-CT/DH-R1 (Rhopr-CT/DH-R1-A, Rhopr-CT/DH-R1-B and Rhopr-CT/DH-R1-C) and two splice variants encoding Rhopr-CT/DH-R2 (Rhopr-CT/DH-R2-A and Rhopr-CT/DH-R2-B). Rhopr-CT/DH-R1-A and Rhopr-CT/DH-R2-A encode truncated receptors that lack six and seven of the characteristic seven transmembrane domains, respectively. Rhopr-CT/DH-R1-B and Rhopr-CT/DH-R1-C, which only differ by 2 amino acids in their C-terminal domain, can both be activated by Rhopr-CT/DH at equal sensitivities (EC50 = 200-300 nM). Interestingly, Rhopr-CT/DH-R2-B is much more sensitive to Rhopr-CT/DH (EC50 = 15 nM) compared to Rhopr-CT/DH-R1-B/C and also yields a much greater response (amplitude) in our heterologous assay. This is the first study to reveal that insects possess at least two CT/DH receptors, which may be functionally different. Quantitative PCR demonstrates that Rhopr-CT/DH-R1 and Rhopr-CT/DH-R2 have distinct expression patterns, with both receptors expressed centrally and peripherally. Moreover, the expression analysis also identified novel target tissues for this neuropeptide, including testes, ovaries and prothoracic glands, suggesting a possible role for Rhopr-CT/DH in reproductive physiology and development.

Evaluation of insect CAP2b analogs with either an (E)-alkene, trans- or a (Z)-alkene, cis-Pro isostere identifies the Pro orientation for antidiuretic activity in the stink bug

The CAP2b neuropeptide family plays an important role in the regulation of the processes of diuresis and/or antidiuresis in a variety of insects. While Manse-CAP2b (pELYAFPRV-NH2) and native CAP2bs elicit diuretic activity in a number of species of flies, native CAP2b sequences have been shown to elicit antidiuretic activity in the kissing bug Rhodnius prolixus and the green stink bug Acrosternum hilare, the latter being an important pest of cotton and soybean in the southern United States. Analogs of CAP2b containing either a (Z)-alkene, cis-Pro or an (E)-alkene, trans-Pro isosteric component were synthesized and evaluated in an in vitro stink bug diuretic assay, which involved measurement of fluid secretion by Malpighian tubules isolated from A. hilare. The conformationally constrained trans-Pro analog demonstrated statistically significant antidiuretic activity, whereas the cis-Pro analog failed to elicit activity. The results are consistent with the adoption of a trans orientation for the Pro in CAP2b neuropeptides during interaction with receptors associated with the antidiuretic process in the stink bug. In addition, the results are further consistent with a theory of ligand-receptor coevolution between the CAP2b and pyrokinin/PBAN neuropeptide classes, both members of the '-PRXamide' superfamily. This work further identifies a scaffold with which to design mimetic CAP2b analogs as potential leads in the development of environmentally favorable pest management agents capable of disrupting CAP2b-regulated diuretic/antidiuretic functions.

CAPA-gene products in the haematophagous sandfly Phlebotomus papatasi (Scopoli)--vector for leishmaniasis disease

Sandflies (Phlebotominae, Nematocera, Diptera) are responsible for transmission of leishmaniasis and other protozoan-borne diseases in humans, and these insects depend on the regulation of water balance to cope with the sudden and enormous intake of blood over a very short time period. The sandfly inventory of neuropeptides, including those that regulate diuretic processes, is completely unknown. Direct MALDI-TOF/TOF mass spectrometric analysis of dissected ganglia of Phlebotomus papatasi, combined with a data-mining of sandfly genome 'contigs', was used to identify native CAPA-peptides, a peptide class associated with the regulation of diuresis in other hematophagous insects. The CAPA-peptides identified in this study include two CAPA-PVKs, differentially processed CAPA-PK, and an additional CAPA precursor peptide. The mass spectrometric analysis of different parts of the neuroendocrine system of the sandfly indicate that it represents the first insect which accumulates CAPA-PVKs exclusively in hormone release sites of abdominal ganglia and CAPA-PK (nearly) exclusively in the corpora cardiaca. Additionally, sandflies feature the smallest abdominal ganglia (~35 μm) where CAPA-peptides could be detected so far. The small size of the abdominal ganglia does not appear to affect the development of the median neurosecretory system as it obviously does in another comparably small insect species, Nasonia vitripennis, in which no capa-gene expression was found. Rather, immunocytochemical analyses confirm that the general architecture in sandflies appears identical to that of much larger mosquitoes.

Inhibition of diuretic stimulation of an insect secretory epithelium by a cGMP-dependent protein kinase

The rate of urine secretion by insect Malpighian tubules (MTs) is regulated by multiple diuretic and antidiuretic hormones, often working either synergistically or antagonistically. In the Drosophila melanogaster MT, only diuretic factors have been reported. Two such agents are the biogenic amine tyramine (TA) and the peptide drosokinin (DK), both of which act on the stellate cells of the tubule to increase transepithelial chloride conductance. In the current study, TA and DK signaling was quantified by microelectrode recording of the transepithelial potential in isolated Drosophila MTs. Treatment of tubules with cGMP caused a significant reduction in the depolarizing responses to both TA and DK, while cAMP had no effect on these responses. To determine whether a specific cGMP-dependent protein kinase (PKG) was mediating this inhibition, PKG expression was knocked down by RNAi in either the principal cells or the stellate cells. Knockdown of Pkg21D in the stellate cells eliminated the modulation of TA and DK signaling. Knockdown of Pkg21D with a second RNAi construct also reduced the modulation of TA signaling. In contrast, knockdown of the expression of foraging or CG4839, which encodes a known and a putative PKG, respectively, had no effect. These data indicate that cGMP, acting through the Pkg21D gene product in the stellate cells, can inhibit signaling by the diuretic agents TA and DK. This represents a novel function for cGMP and PKG in the Drosophila MT and suggests the existence of an antidiuretic hormone in Drosophila.

Calcitonin-like diuretic hormones in insects

Insect neuropeptides control various biological processes including growth, development, homeostasis and reproduction. The calcitonin-like diuretic hormone (CT/DH) is one such neuropeptide that has been shown to affect salt and water transport by Malpighian tubules of several insects. With an increase in the number of sequenced insect genomes, CT/DHs have been predicted in several insect species, making it easier to characterize the gene encoding this hormone and determine its function in the species in question. This mini review summarizes the current knowledge on insect CT/DHs, focusing on mRNA and peptide structures, distribution patterns, physiological roles, and receptors in insects.

Anti-diuretic factors in insects: the role of CAPA peptides

Insects have adapted to live in a wide variety of habitats and utilize an array of feeding strategies that present challenges to their ability to maintain osmotic balance. Regardless of the feeding strategy, water and ion levels within the haemolymph (insect blood) are maintained within a narrow range. This homeostasis involves the action of a variety of tissues, but is often chiefly regulated by the excretory system. Until recently, most research on the hormonal control of the excretory tissues has focused on factors known to have diuretic activities. In this mini-review, the current state of knowledge on anti-diuretic factors in insects will be discussed with a particular emphasis on the CAPA peptides in the blood-feeding Chagas' disease vector, Rhodnius prolixus.

Natriuresis and diuretic hormone synergism in R. prolixus upper Malpighian tubules is inhibited by the anti-diuretic hormone, RhoprCAPA-α2

Insects contain an array of hormones that coordinate the actions of the excretory system to achieve osmotic and ionic balance. In the hematophagous insect, Rhodnius prolixus, two diuretic hormones have been identified, serotonin (5HT) and a corticotropin releasing factor-related peptide (RhoprDH), and both lead to an increase in fluid secretion by Malpighian tubules (MTs). However, only 5HT activates reabsorption by the lower MTs to recover K(+) and Cl(-). An anti-diuretic hormone (RhoprCAPA-α2) is believed to coordinate the cessation of the rapid diuresis following blood meal engorgement. However, the role of RhoprCAPA-α2 on fluid secretion by MTs stimulated by RhoprDH was previously unknown. Here we demonstrate that, unlike the inhibitory effect on 5HT-stimulated secretion by MTs, RhoprCAPA-α2 does not inhibit secretion stimulated by RhoprDH although it does abolish the synergism that occurs between the two diuretic hormones. In addition, we show that the natriuresis elicited by either diuretic hormone is reduced by RhoprCAPA-α2. Using electrophysiological tools, we investigate the possible mechanism by which this complex regulatory pathway is achieved. Analysis of the pH of secreted fluid as well as the triphasic response in transepithelial potential in MTs treated with diuretic hormones, suggests that RhoprCAPA-α2 does not inhibit the V-type H(+) ATPase. Taken together, these results indicate that RhoprCAPA-α2 functions to reduce the rapid diuresis following blood feeding, and in addition, it inhibits the natriuresis associated with diuretic hormone stimulated MTs. This may reflect an important regulatory mechanism related to the slow diuresis that occurs as the K(+)-rich blood cells are digested.