Rhodnius prolixus Malpighian tubules and control of diuresis by neurohormones

Corticotropin-releasing factor-like diuretic hormone acts as a gonad-inhibiting hormone in adult female, Rhodnius prolixus

Within insects, corticotropin-releasing factor/diuretic hormones (CRF/DHs) are responsible for the modulation of a range of physiological and behavioural processes such as feeding, diuresis, and reproduction. Rhopr-CRF/DH plays a key role in feeding and diuresis in Rhodnius prolixus, a blood-gorging insect and a vector for human Chagas disease. Here, we extend our understanding on the role of this neurohormone in reproduction in adult female R. prolixus. Double-label immunohistochemistry displays co-localized staining of CRF-like and the glycoprotein hormone (GPA2/GPB5) subunit GPB5-like immunoreactivity in the same neurosecretory cells (NSCs) in the mesothoracic ganglionic mass (MTGM) and in their neurohemal sites in adult female R. prolixus, suggesting these peptides could work together to regulate physiological processes. qPCR analysis reveals that the transcript for Rhopr-CRF/DH receptor 2 (Rhopr-CRF/DH-R2) is expressed in reproductive tissues and fat body (FB) in adult female R. prolixus, and its expression increases post blood meal (PBM), a stimulus that triggers diuresis and reproduction. Using RNA interference, transcript expression of Rhopr-CRF/DH-R2 was knocked down, and egg production monitored by examining the major yolk protein, vitellogenin (Vg), the number and quality of eggs laid, and their hatching ratio. Injection of dsCRFR2 into adult females reduces Rhopr-CRF/DH-R2 transcript expression, accelerates oogenesis, increases the number of eggs produced, and reduces hatching rate in female R. prolixus. Downregulation of Rhopr-CRF/DH-R2 leads to an increase in the transcript expression of RhoprVg1 in the fat body and ovaries, and increases the transcript level for the Vg receptor, RhoprVgR, in the ovaries. A significant increase in Vg content in the fat body and in the hemolymph is also observed. Incubation of isolated tissues with Rhopr-CRF/DH leads to a significant decrease in transcript expression of RhoprVg1 in the fat body and RhoprVg1 in the ovaries. In addition, Rhopr-CRF/DH reduces transcript expression of the ecdysteroid biosynthetic enzymes and reduces ecdysteroid titer in the culture medium containing isolated ovaries. These results suggest the involvement of the CRF-signaling pathway in reproduction, and that Rhopr-CRF/DH acts as a gonad-inhibiting hormone in the adult female R. prolixus, as previously shown for the colocalized glycoprotein, GPA2/GPB5.

cAMP: A second messenger involved in the mechanism of midgut alkalinization in Lutzomyia longipalpis

The sand fly Lutzomyia longipalpis is the main vector of Leishmania infantum in the Americas. Female sand flies ingest sugar-rich solutions and blood, which are digested in the midgut. Digestion of nutrients is an essential function performed by digestive enzymes, which require appropriate physiological conditions. One of the main aspects that influence enzymatic activity is the gut pH, which must be tightly controlled. Considering second messengers are frequently involved in the coordination of tightly regulated physiological events, we investigated if the second messenger cAMP would participate in the process of alkalinization in the abdominal midgut of female L. longipalpis. In midguts containing the indicator dye bromothymol-blue, cAMP stimulated the alkalinization of the midgut lumen. Through another technique based on the use of fluorescein as a pH indicator, we propose that cAMP is involved in the alkalinization of the midgut by activating HCO3^- transport from the enterocyte's cytoplasm to the lumen. The results strongly suggested that the carrier responsible for this process would be a HCO3^- /Cl^- antiporter located in the enterocytes' apical membrane. Hematophagy promotes the release of alkalinizing hormones in the hemolymph; however, when the enzyme adenylyl cyclase, responsible for cAMP production, was inhibited, we observed that the hemolymph from blood-fed L. longipalpis' females did not stimulate midgut alkalinization. This result indicated that hormone-stimulated alkalinization is mediated by cAMP. In the present study, we provide evidences that cAMP has a key role in the control of intestinal pH.

Exploring the role of glycoprotein hormone GPA2/GPB5 in the medically important insect, Rhodnius prolixus

Glycoprotein hormones are formed by the heterodimerization of alpha and beta subunits. In vertebrates, there are five glycoprotein hormones, four of which have a common alpha subunit (GPA1) bound to a specific beta subunit (GPB1, GPB2, GPB3, or GPB4), and the fifth, thyrostimulin, is formed by the dimerization of GPA2 and GPB5 subunits. These hormones mediate physiological events such as development, metabolism, and reproduction, although the functional role of thyrostimulin in vertebrates has not been fully elucidated. Recent reports in invertebrates, specifically in holometabolous insects, suggest that GPA2/GPB5 plays a critical role in development, diuresis, and reproduction. In this study, we clone and characterize the transcripts for the glycoprotein hormone GPA2/GPB5 and its receptor (LGR1) in fifth instar Rhodnius prolixus, a hemimetabolous insect vector of Chagas disease. Sequence analyses reveals considerable identity and similarity between GPA2/GPB5 and LGR1 and those reported in other arthropod species. Quantitative PCR (qPCR) shows that both subunit transcripts, GPA2 and GPB5, and LGR1 transcripts are present in a variety of tissues, with greatest expression of the subunits in the central nervous system (CNS) and highest LGR1 expression in the Malpighian tubules (MT). Results from temporal qPCR analyses reveal a decrease in transcript expression 24 h after feeding, followed by an increase as the days post-feeding advance. Using immunohistochemistry, we show that GPB5 is expressed throughout the CNS, and importantly is present in neurosecretory cells in the brain and abdominal neuromeres and their neurohemal organs, indicating a neurohormonal role for this signaling pathway. A reduction in LGR1 transcript expression (via RNA interference) led to a greater weight loss and mortality rate in unfed insects. In addition, when a blood meal is offered, the insects with reduced LGR1 consume a significantly smaller blood meal and have higher mortality rates as the days post-feeding advance. Overall, the results suggest that the GPA2/GPB5 signaling pathway may play roles during a prolonged unfed state and in feeding-related events.

Salivary and Intestinal Transcriptomes Reveal Differential Gene Expression in Starving, Fed and Trypanosoma cruzi-Infected Rhodnius neglectus

Rhodnius neglectus is a potential vector of Trypanosoma cruzi (Tc), the causative agent of Chagas disease. The salivary glands (SGs) and intestine (INT) are actively required during blood feeding. The saliva from SGs is injected into the vertebrate host, modulating immune responses and favoring feeding for INT digestion. Tc infection significantly alters the physiology of these tissues; however, studies that assess this are still scarce. This study aimed to gain a better understanding of the global transcriptional expression of genes in SGs and INT during fasting (FA), fed (FE), and fed in the presence of Tc (FE + Tc) conditions. In FA, the expression of transcripts related to homeostasis maintenance proteins during periods of stress was predominant. Therefore, the transcript levels of Tret1-like and Hsp70Ba proteins were increased. Blood appeared to be responsible for alterations found in the FE group, as most of the expressed transcripts, such as proteases and cathepsin D, were related to digestion. In FE + Tc group, there was a decreased expression of blood processing genes for insect metabolism (e.g., Antigen-5 precursor, Pr13a, and Obp), detoxification (Sult1) in INT and acid phosphatases in SG. We also found decreased transcriptional expression of lipocalins and nitrophorins in SG and two new proteins, pacifastin and diptericin, in INT. Several transcripts of unknown proteins with investigative potential were found in both tissues. Our results also show that the presence of Tc can change the expression in both tissues for a long or short period of time. While SG homeostasis seems to be re-established on day 9, changes in INT are still evident. The findings of this study may be used for future research on parasite-vector interactions and contribute to the understanding of food physiology and post-meal/infection in triatomines.

Histomorphology of the Malpighian Tubules and the Chemical Composition of the Spherocrystals in the Tubule Epithelial Cells of Adult Leptophyes albovittata (Kollar, 1833) (Orthoptera, Tettigoniidae)

In insects, the number, cytological and histological structures, and the spherocrystals of the Malpighian tubules (MTs) can vary considerably in different insect groups. These differences are considered important because they can be used as taxonomic characters. For this purpose, the ultrastructure of the MT epithelial cells in Leptophyes albovittata (Kollar, 1833) (Orthoptera, Tettigoniidae) was examined by light microscopy, scanning electron microscopy, and transmission electron microscopy. The wall of each tubule consists of a single layer of cells. These cells have round-shaped nuclei. Two different cell types were demonstrated in the tubule cell. These are cells that have electron-dense cytoplasm and electron-lucent cytoplasm. It was observed that the cytoplasm of these cells has many spherocrystals. The chemical composition of the spherocrystals was found to be high in carbon, phosphorus, and manganese in tubule cells.

SIFamide Influences Feeding in the Chagas Disease Vector, Rhodnius prolixus

SIFamides are a family of highly conserved neuropeptides in arthropods, and in insects are mainly expressed in four medial neurons in the pars intercerebralis of the brain. Although SIFamide has been shown to influence sexual behavior, feeding, and sleep regulation in holometabolous insects such as Drosophila melanogaster, little is known about its role in hemimetabolous insects, including the blood-sucking bug, Rhodnius prolixus. In this study, we confirm the nucleotide sequence for R. prolixus SIFamide (Rhopr-SIFa) and find characteristic phenotypic expression of SIFamide in four cells of the pars intercerebralis in the brain. In addition to extensive SIFa projections throughout the entire central nervous system, SIFamidergic processes also enter into the corpus cardiacum, and project along the dorsal vessel, suggestive of Rhopr-SIFa acting as a neurohormone. Physiologically, Rhopr-SIFamide induces dose-dependent increases in heartbeat frequency in vitro suggesting the presence of peripheral receptors, and thereby indicating Rhopr-SIFa is released to act upon peripheral targets. We also explore the function of Rhopr-SIFa in R. prolixus, specifically in relation to feeding, since R. prolixus is a blood-gorging insect and a vector for Chagas disease. The intensity of SIFamide-like staining in the neurons in the brain is diminished 2 h following feeding, and restocking of those cells is finished 24 h later, indicating Rhopr-SIFa may be released at feeding. The results of temporal qPCR analysis were consistent with the immunohistochemical findings, showing an increase in Rhopr-SIFa transcript expression in the brain 2 h after feeding. We also observed enhanced feeding (size of meal) in insects injected with Rhopr-SIFa whereas insects with RNAi-mediated knockdown of the Rhopr-SIFa transcript consumed a significantly smaller blood meal relative to controls. These data suggest that the four SIFamidergic neurons and associated arborizations may play an important function in the neuronal circuitry controlling R. prolixus feeding, with Rhopr-SIFa acting as a central and peripheral neuromodulator/neurohormone.

The involvement of Rhopr-CRF/DH in feeding and reproduction in the blood-gorging insect Rhodnius prolixus

Rhodnius prolixus is a blood-gorging insect and a vector for human Chagas disease. The insect transmits the disease following feeding, when it excretes urine and feces contaminated with the Trypanosoma cruzi parasite. A corticotropin-releasing factor-like peptide acts as a diuretic hormone in R. prolixus (Rhopr-CRF/DH); however, its distribution throughout the insect's central nervous system (CNS) and the expression of its receptor in feeding-related tissue as well as the female reproductive system suggests a multifaceted role for the hormone beyond that of diuresis. Here we investigate the involvement of Rhopr-CRF/DH in feeding and reproduction in R. prolixus. Immunohistochemistry of the CNS showed diminished CRF-like staining in neurosecretory cells (NSCs) of the mesothoracic ganglionic mass (MTGM) immediately following feeding, and partial restocking of those same cells two hours later, indicating Rhopr-CRF/DH stores in this regions are involved in feeding. The results of the temporal qPCR analysis were consistent with the immunohistochemical findings, showing an increase in Rhopr-CRF/DH transcript expression in the MTGM immediately after feeding, presumably capturing the restocking of Rhopr-CRF/DH in the lateral NSCs following release of the peptide during feeding. Elevating haemolymph Rhopr-CRF/DH titres by injection of Rhopr-CRF/DH prior to feeding resulted in the intake of a significantly smaller blood meal in 5th instars and adults without an apparent effect on the rate of short-term diuresis. When adult females were injected with Rhopr-CRF/DH, they also produced and laid significantly fewer eggs. Finally, in vitro oviduct contraction assays illustrate that Rhopr-CRF/DH inhibits the amplitude of contractions of the lateral oviducts, highlighting a potential mechanism via which the hormone diminishes reproductive capacity. To conclude, the study of the Rhopr-CRF/DH pathway, its components and mechanisms of action, has implications for vector control by highlighting targets to alter feeding, diuresis, and reproduction of this disease vector.

Identification, functional characterization, and pharmacological profile of a serotonin type-2b receptor in the medically important insect, Rhodnius prolixus

In the Chagas disease vector, Rhodnius prolixus, two diuretic hormones act synergistically to dramatically increase fluid secretion by the Malpighian tubules (MTs) during the rapid diuresis that is initiated upon engorgement of vertebrate blood. One of these diuretic hormones is the biogenic amine, serotonin (5-hydroxytryptamine, 5-HT), which controls a variety of additional activities including cuticle plasticization, salivary gland secretion, anterior midgut absorption, cardioacceleratory activity, and myotropic activities on a number of visceral tissues. To better understand the regulatory mechanisms linked to these various physiological actions of serotonin, we have isolated and characterized a serotonin type 2b receptor in R. prolixus, Rhopr5HTR2b, which shares sequence similarity to the vertebrate serotonin type 2 receptors. Rhopr5HTR2b transcript is enriched in well-recognized physiological targets of serotonin, including the MTs, salivary glands and dorsal vessel (i.e., insect heart). Notably, Rhopr5HTR2b was not enriched in the anterior midgut where serotonin stimulates absorption and elicits myotropic control. Using a heterologous functional receptor assay, we examined Rhopr5HTR2b activation characteristics and its sensitivity to potential agonists, antagonists, and other biogenic amines. Rhopr5HTR2b is dose-dependently activated by serotonin with an EC50 in the nanomolar range. Rhopr5HTR2b is sensitive to alpha-methyl serotonin and is inhibited by a variety of serotonin receptor antagonists, including propranolol, spiperone, ketanserin, mianserin, and cyproheptadine. In contrast, the cardioacceleratory activity of serotonin revealed a unique pharmacological profile, with no significant response induced by alpha-methyl serotonin and insensitivity to ketanserin and mianserin. This distinct agonist/antagonist profile indicates that a separate serotonin receptor type may mediate cardiomodulatory effects controlled by serotonin in R. prolixus.

Morphology of reproductive accessory glands in eight species of blood-feeding Hemiptera (Hemiptera, Reduviidae) insect vectors of Chagas disease

This paper documents the morphology of previously undescribed adult reproductive accessory glands in eight species of blood-feeding Hemiptera, vectors of Chagas disease. These species are three Triatoma (T. dimidiata, T. klugi, T. sordida), three Rhodnius (R. brethesi, R. nasutus, R. pictipes), and one species each from Nesotriatoma (N. bruneri) and Panstrongylus (P. megistus). This survey shows that the male reproductive systems between species of four genera of Reduviidae adhere to the same general plan seen in previously described vectors of Chagas disease. This morphological similarity suggests that reproductive success of the male is contingent on the delivery of a vital set of male accessory gland secretions to the female in conjunction with material from the testes and seminal vesicle. However, variations were observed in the accessory glands of females, especially at the level of the genus. The spermathecae are morphologically distinct, and the posterior accessory glands are absent in some. The differences in spermathecae morphology likely reflect physiological adaptations associated with speciation driven by cryptic female choice in which the female determines which sperm are used for fertilization. Differences in the posterior reproductive accessory gland can be correlated with variations in ovipositioning behaviour. Since reproductive physiology is important for species success, this information also augments epidemiological studies by providing a comparison to R. prolixus, a Chagas disease vector for which the physiology is well known.

Isolation, cloning and expression of the Crustacean Cardioactive Peptide gene in the Chagas' disease vector, Rhodnius prolixus

The blood-gorging bug, Rhodnius prolixus, is a major vector of Chagas' disease in Central and South America. We have cloned and characterized the crustacean cardioactive peptide (CCAP) gene in R. prolixus. The RhoprCCAP gene contains five exons and four introns, and encodes a 129 amino acid prepropeptide, which following post-translation processing, produces CCAP. The predicted RhoprCCAP amino acid sequence is identical to CCAP of crustaceans and other insects, i.e. it is highly conserved. RhoprCCAP mRNA is observed in the central nervous system (CNS) using reverse transcriptase (RT) PCR, but not in the gut and salivary glands. In situ hybridization reveals that the expression of CCAP mRNA is localized to a small number of dorsally situated bilaterally paired neurons within the CNS.

Effects of the cyclopeptide mycotoxin destruxin A on the Malpighian tubules of Rhodnius prolixus (Stål)

The production of peptide toxins by entomopathogenic fungi during the infection process plays critical roles in pathogenesis. To gain insight into the mechanism of action of these mycotoxins on insect internal organs, we have evaluated the effects of destruxin A, a cyclic peptide produced by Metarhizium anispliae, on Rhodnius prolixus Malpighian tubules measuring fluid secretion rate, transepithelial electrical potential (TEP), pH and ion composition of secreted fluid, and ATP content. Destruxin A dramatically inhibited fluid secretion rate on tubules stimulated by 5-Hydroxytryptamine (5-HT) or cAMP. The calculated IC(50) for destruxin A on 5-HT-stimulated tubules was 3 x 10(-7) M. Fluid secretion rate by Malpighian tubules exposed for 20 min to 10(-6) M destruxin A recovered completely when tubules were washed with saline; however, when tubules were exposed to 5 x 10(-6) M destruxin A the fluid secretion rate only partially recovered upon wash off. The use of Ca(2+)-free saline or addition of the calcium channel blocker CoCl(2) to the bathing saline did not interfere with the effects of destruxin A, and neither did the modification of intracellular calcium by TMB-8. Measurement of TEP of tubules challenged with 5-HT after preincubation for 10 min in saline containing 10(-6) M destruxin A showed that the second and third phases of the typical triphasic response to 5-HT were disrupted. Likewise, the positive shift in TEP in response to 5-HT in chloride-free bathing saline was significantly reduced when tubules were preincubated for 10 min in 10(-6) M destruxin A. The pH of the secreted fluid, but not the Na(+) or K(+) concentration, increased significantly when 5-HT-stimulated tubules were exposed to 10(-6) M destruxin A. The ATP content was not significantly different when tubules stimulated with 5-HT were exposed to destruxin A. Taken together, these results show that destruxin A, without interfering with the intracellular ATP production, strongly inhibits fluid secretion rate by the Malpighian tubules of R. prolixus. Changes in properties of the TEP suggest that one of the target sites for this peptide toxin might be associated with inhibition of the apical V-type H(+) ATPase of tubule cells.

Functional anatomy of vagina muscles in the blood-feeding insect, Rhodnius prolixus

The physiology of the muscles associated with the vagina in the blood-feeding insect, Rhodnius prolixus Stal, was investigated with the use of Methylene Blue staining to visualize the anatomy, and a micro force transducer to record spontaneous and neurally-evoked contractions. The vagina is associated with a dorsal muscle and a set of paired lateral muscles. The dorsal muscle extends from the base of the common oviduct to apodemes located laterally on sternite VIII, the first genital segment. The lateral muscles extend from a medially-located apodeme on the posterior edge of sternite VI around each side of the common oviduct to travel posteriorly along the side of the vagina before inserting laterally on apodemes on sternite VIII. The vagina muscles display spontaneous and neurally-evoked contractions that are prolonged but transient. The response to evoked contractions shows that the muscles are innervated by both excitatory and inhibitory motor axons. The degree of tension generated by evoked contractions is dependent on the frequency of stimulation with maximal tension being generated at 20-30Hz. This tension, which often exceeds 400mg, is transient and returns to a baseline within 1 to 2min during continuous stimulation. These results, which are the first to describe this chamber in this well-studied insect, are discussed with respect to the act of egg laying.