pH control in the midgut of Aedesaegypti under different nutritional conditions

Bacterial communities of Aedes aegypti mosquitoes differ between crop and midgut tissues

Microbiota studies of Aedes aegypti and other mosquitoes generally focus on the bacterial communities found in adult female midguts. However, other compartments of the digestive tract maintain communities of bacteria which remain almost entirely unstudied. For example, the Dipteran crop is a food storage organ, but few studies have looked at the microbiome of crops in mosquitoes, and only a single previous study has investigated the crop in Ae. aegypti. In this study, we used both culture-dependent and culture-independent methods to compare the bacterial communities in midguts and crops of laboratory reared Ae. aegypti. Both methods revealed a trend towards higher abundance, but also higher variability, of bacteria in the midgut than the crop. When present, bacteria from the genus Elizabethkingia (family Weeksellaceae) dominated midgut bacterial communities. In crops, we found a higher diversity of bacteria, and these communities were generally dominated by acetic acid bacteria (family Acetobacteriaceae) from the genera Tanticharoenia and Asaia. These three taxa drove significant community structure differences between the tissues. We used FAPROTAX to predict the metabolic functions of these communities and found that crop bacterial communities were significantly more likely to contain bacteria capable of methanol oxidation and methylotrophy. Both the presence of acetic acid bacteria (which commonly catabolize sugar to produce acetic acid) and the functional profile that includes methanol oxidation (which is correlated with bacteria found with natural sources like nectar) may relate to the presence of sugar, which is stored in the mosquito crop. A better understanding of what bacteria are present in the digestive tract of mosquitoes and how these communities assemble will inform how the microbiota impacts mosquito physiology and the full spectrum of functions provided by the microbiota. It may also facilitate better methods of engineering the mosquito microbiome for vector control or prevention of disease transmission.

Deciphering the interaction mechanism of natural actives against larval proteins of Aedes aegypti to identify potential larvicides: a computational biology analysis

Aedes aegypti is the target for repellents to curb incidences of vector-borne diseases. Stopping breeding of this mosquito species at its larval stages helps in controlling spread of insect-borne diseases. Therefore, the present study focused on deciphering the mechanism of interaction of selected natural actives against larval proteins of A. aegypti to identify potential natural alternative larvicides. 65 larval proteins were identified from literature, whose structures were modelled and validated using RaptorX and ProCheck. 11 natural actives were selected for predicting their ligand properties and toxicities via Toxicity Estimation Software Tool and ProTox-II. Molecular docking studies were carried out using POAP followed by 100 ns molecular dynamic simulation studies for top three best docked complexes to better understand the robustness of docking, complex stabilities and molecular mechanisms of interactions. Toxicity predictions revealed that 6 molecules belonged to toxicity class 4, and five to toxicity class 5, implying that they were all safe to use. Complexes goniothalamin-translation elongation factor (-10 kcal/mol), andrographolide-acetyl-CoA C-myristoyltransferase (-9.2 kcal/mol) and capillin-translation elongation factor (-8.4 kcal/mol) showed best binding energies. When simulated, capillin-translation elongation factor showed most stability, while the remaining two also evidenced robust docking. Evolutionary studies for top two larval proteins disclosed 100 other insect species in which these proteins can be targeted using various larvicides. Protein-protein interaction network analysis revealed several protein pathways that might be affected due to aforesaid naturals. Therefore, this study provides computational insights into the molecular interaction of naturals against larval proteins, acting as potential natural larvicides.Communicated by Ramaswamy H. Sarma.

Comprehensive Molecular Interaction Studies to Construe the Repellent/Kill Activity of Geraniol During Binding Event Against Aedes aegypti Proteins

Aedes aegypti is an etiological agent for dengue, chikungunya, zika, and yellow fever viruses. With the advent of the use of natural alternatives as repellents, their precise mode of action during the event of binding is still unclear. Geraniol is one such bioactive natural that has been previously shown to have some insecticide properties. Thus, the present study aimed to understand the mechanism of the binding event of geraniol with the whole proteome of A. aegypti. Twenty protein target categories were shortlisted for the mosquito, wherein the proteins were downloaded with respect to the reference proteome. Conserved domain analysis was performed for the same using the CDD search tool to find the proteins that have common domains. 309 proteins were modeled using RaptorX standalone tool, and validated using Ramachandran plots from SAVES v6.0 from ProCheck. These modeled and validated proteins were then docked against geraniol, using POAP software, for understanding the binding energies. The top 3 best-docked complexes were then analyzed for their stabilities and event of binding via 100 ns simulation studies using DESMOND's Maestro environment. The docking results showed that the geraniol-voltage-gated sodium channel had the best energy of - 7.1 kcal/mol, followed by geraniol-glutathione-S-transferase (- 6.8 kcal/mol) and geraniol-alpha esterase (- 6.8 kcal/mol). The simulations for these 3 complexes revealed that several residues of the proteins interacted well with geraniol at a molecular level, and all three docked complexes were found to be stable when simulated (RMSD: 16-18 Å, 3.6-4.8 Å, 4.8-5.6 Å, respectively). Thus, the present study provides insights into the mechanism of the binding event of geraniol with the major A. aegypti targets, thereby, assisting the use of geraniol as a natural repellent.

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.

Na+/K+-ATPase Activation by cAMP in the Midgut of Lutzomyia longipalpis (Lutz & Neiva, 1912; Diptera: Psychodidae)

Lutzomyia longipalpis (Lutz & Neiva, 1912) females have been intensively studied regarding the regulation of midgut pH. The mechanisms involved in pH regulation are complex, and some aspects remain to be clarified. Here, we investigated the role of the Na+/K+-ATPase pump as an electrochemical potential generator and its modulation by the second messenger cAMP in the midgut of female L. longipalpis. Our results suggest that not only may Na+/K+-ATPase be the main generator of an electrochemical potential across membranes in the midgut of female L. longipalpis, but also its activity is positively regulated by cAMP. cAMP-mediated Na+/K+-ATPase pump activity might be necessary to maintain the transport of the nutrients produced during blood digestion.

Hot and sour: parasite adaptations to honeybee body temperature and pH

Host temperature and gut chemistry can shape resistance to parasite infection. Heat and acidity can limit trypanosomatid infection in warm-blooded hosts and could shape infection resistance in insects as well. The colony-level endothermy and acidic guts of social bees provide unique opportunities to study how temperature and acidity shape insect-parasite associations. We compared temperature and pH tolerance between three trypanosomatid parasites from social bees and a related trypanosomatid from poikilothermic mosquitoes, which have alkaline guts. Relative to the mosquito parasites, all three bee parasites had higher heat tolerance that reflected body temperatures of hosts. Heat tolerance of the honeybee parasite Crithidia mellificae was exceptional for its genus, implicating honeybee endothermy as a plausible filter of parasite establishment. The lesser heat tolerance of the emerging Lotmaria passim suggests possible spillover from a less endothermic host. Whereas both honeybee parasites tolerated the acidic pH found in bee intestines, mosquito parasites tolerated the alkaline conditions found in mosquito midguts, suggesting that both gut pH and temperature could structure host-parasite specificity. Elucidating how host temperature and gut pH affect infection-and corresponding parasite adaptations to these factors-could help explain trypanosomatids' distribution among insects and invasion of mammals.

Comparison of the hemolysis machinery in two evolutionarily distant blood-feeding arthropod vectors of human diseases

Host blood protein digestion plays a pivotal role in the ontogeny and reproduction of hematophagous vectors. The gut of hematophagous arthropods stores and slowly digests host blood and represents the primary gateway for transmitted pathogens. The initial step in blood degradation is induced lysis of host red blood cells (hemolysis), which releases hemoglobin for subsequent processing by digestive proteolytic enzymes. The activity cycles and characteristics of hemolysis in vectors are poorly understood. Hence, we investigated hemolysis in two evolutionarily distant blood-feeding arthropods: The mosquito Culex pipiens and the soft tick Argas persicus, both of which are important human and veterinary disease vectors. Hemolysis in both species was cyclical after blood meal ingestion. Maximum digestion occurs under slightly alkaline conditions in females. Hemolytic activity appears to be of lipoid origin in C. pipiens and enzymatic activity (proteolytic) in A. persicus. We have assessed the effect of pH, incubation time, and temperature on hemolytic activity and the hemolysin. The susceptibility of red blood cells from different hosts to the hemolysin and the effect of metabolic inhibition of hemolytic activity were assessed. We conclude that in C. pipiens and A. persicus midgut hemolysins control the amplitude of blood lysis step to guarantee an efficient blood digestion.

Comparative Analysis of the Bacterial and Fungal Communities in the Gut and the Crop of Aedes albopictus Mosquitoes: A Preliminary Study

The Asian tiger mosquito Aedes albopictus is a major pathogen vector and one of the world’s most invasive species. In recent years, the study of mosquito-associated microbiota has received growing interest for reducing transmission of mosquito-borne pathogens. Most of studies on mosquito microbiota mainly focused on the gut bacteria. However, microorganisms can also colonize other organs and are not restricted to bacteria. In mosquitoes, the crop is the primary storage organ for sugars from the nectar feeding before it is transferred into the midgut for digestion. No study has yet investigated whether this organ can harbor microorganisms in Ae. albopictus. By using high-throughput sequencing, this study is the first to describe the microbiota including both bacteria and fungi in sugar-fed Ae. albopictus males and females. The results showed the presence of diverse and rich bacterial and fungal communities in the crop of both sexes that did not strongly differ from the community composition and structure found in the gut. Altogether, our results provide a thorough description of the crop-associated microbiota in Ae. albopictus which can open new avenues for further studies on trophic interactions between the mosquito and its microbiota.

Elizabethkingia anophelis: Physiologic and Transcriptomic Responses to Iron Stress

In this study, we investigated the global gene expression responses of Elizabethkingia anophelis to iron fluxes in the midgut of female Anopheles stephensi mosquitoes fed sucrose or blood, and in iron-poor or iron-rich culture conditions. Of 3,686 transcripts revealed by RNAseq technology, 218 were upregulated while 112 were down-regulated under iron-poor conditions. Hemolysin gene expression was significantly repressed when cells were grown under iron-rich or high temperature (37°C) conditions. Furthermore, hemolysin gene expression was down-regulated after a blood meal, indicating that E. anophelis cells responded to excess iron and its associated physiological stress by limiting iron loading. By contrast, genes encoding respiratory chain proteins were up-regulated under iron-rich conditions, allowing these iron-containing proteins to chelate intracellular free iron. In vivo studies showed that growth of E. anophelis cells increased 3-fold in blood-fed mosquitoes over those in sucrose-fed ones. Deletion of siderophore synthesis genes led to impaired cell growth in both iron-rich and iron-poor media. Mutants showed more susceptibility to H2O2 toxicity and less biofilm formation than did wild-type cells. Mosquitoes with E. anophelis experimentally colonized in their guts produced more eggs than did those treated with erythromycin or left unmanipulated, as controls. Results reveal that E. anophelis bacteria respond to varying iron concentration in the mosquito gut, harvest iron while fending off iron-associated stress, contribute to lysis of red blood cells, and positively influence mosquito host fecundity.

Evolutionary trends of digestion and absorption in the major insect orders

The spatial organization of digestion, which corresponds to the steps by which the ingested food is hydrolyzed in the different regions of the gut, was described in insects from the major insect orders. The pattern of digestion and absorption in the midgut shows a strong phylogenetic influence, modulated by adaptation to particular feeding habits. Based on this, basic digestive patterns were recognized and were proposed to represent the major ancestors from which the different orders evolved. The putative ancestors chosen to represent different points in the evolution from basal Neoptera to more derived orders were: Neoptera, Polyneoptera, Hemiptera, Hymenoptera-Panorpoidea (Diptera-Lepidoptera), Lepidoptera, and Cyclorrhapha. The basic plan of Neoptera was supposed to be alike that of Polyneoptera, which was hypothesized from studies performed in grasshoppers, crickets and from stick insects. For Holometabola, the basic plan was initially proposed from studies carried out in beetles, bees, nematocerous flies, common flies and also from moths. This review updates the physiological data supporting the putative midgut basic patterns by discussing available data on insects pertaining to different taxa and details the evolutionary trends of midgut function among the major insect orders. Furthermore, by using recent genomic and transcriptome data, this review discusses the few insects for which the spatial organization of midgut absorption is known.

The production and bioactivity of prodigiosin: quo vadis?

Prodigiosin (PG), a red tripyrrole pigment, belongs to a member of the prodiginine family and is normally secreted by various sources including Serratia marcescens and other Gram-negative bacteria. The studies of PG have received innovative devotion as a result of reported antimicrobial, larvicidal and anti-nematoid immunomodulation and antitumor properties, owing to its antibiotic and cytotoxic activities. This review provides a comprehensive summary of research undertaken toward the isolation and structural elucidation of the prodiginine family of natural products. Additionally, the current evidence-based understanding of the biological activities and medicinal potential of PG is employed to determine the efficacy, with some reports of information related to pharmacokinetics, pharmacodynamics and toxicology.

Blood Digestion by Trypsin-Like Serine Proteases in the Replete Lyme Disease Vector Tick, Ixodes scapularis

Ixodes scapularis is the major vector of Lyme disease in the Eastern United States. Each active life stage (larva, nymph, and adult) takes a blood meal either for developing and molting to the next stage (larvae and nymphs) or for oviposition (adult females). This protein-rich blood meal is the only food taken by Ixodes ticks and therefore efficient blood digestion is critical for survival. Studies in partially engorged ticks have shown that the initial stages of digestion are carried out by cathepsin proteases within acidic digestive cells. In this study, we investigated the potential role of serine proteases in blood digestion in replete ticks. RNA interference was used for functional analysis and a trypsin-benzoyl-D, L-arginine 4-nitoanilide assay was used to measure active trypsin levels. Hemoglobinolytic activity was determined in vitro, with or without a serine protease inhibitor. Our data suggest that trypsin levels increase significantly after repletion. Knockdown of serine proteases negatively impacted blood feeding, survival, fecundity, levels of active trypsin in the midgut, and resulted in lower hemoglobin degradation. Incubation of midgut extract with a trypsin inhibitor resulted in 65% lower hemoglobin degradation. We provide evidence of the serine proteases as digestive enzymes in fully engorged, replete females. Understanding the digestive profile of trypsin during blood meal digestion in I. scapularis improves our understanding of the basic biology of ticks and may lead to new methods for tick control.

The role of LuloPAT amino acid/proton symporters in midgut alkalinization in the sandfly Lutzomyia longipalpis (Diptera - Psychodidae)

In Lutzomyia longipalpis females, which are the main vectors of Leishmania infantum in the Americas, hematophagy is crucial for ovary development. The control of pH in the midgut during blood digestion is important to the functioning of the digestive enzymes, which release amino acids in the luminal compartment that are then transported through the enterocytes to the hemolymph for delivery to the ovary and other organs. In the present work, we investigated transport systems known as LuloPATs that are present in the midgut of L. longipalpis but not in other organs. These transporters achieve symport of amino acids with H⁺ ions, and one of them (LuloPAT1) is orthologous to a transporter described in Aedes aegypti. According to our results, the transcription levels of LuloPAT1 increased significantly immediately after a blood meal. Based on the variation of the fluorescence of fluorescein with the pH of the medium, we developed a technique that shows the acidification of the cytoplasm of gut cells when amino acids are cotransported with H⁺ from the lumen into the enterocytes. In our experiments, the midguts of the sandflies were dissected and opened longitudinally so that added amino acids could enter the enterocytes via the lumen (PAT carriers are apical). LuloPAT1 transporters are part of a complex of mechanisms that act synergistically to promote gut alkalinization as soon as blood intake by the vector occurs. In dissected but not longitudinally opened midguts, added amino acids could only enter through the basolateral region of enterocytes. However, alkalinization of the lumen was observed because the entry of some amino acids into the cytoplasm of enterocytes triggers a luminal alkalinization mechanism independent of LuloPATs. These findings provide new perspectives that will enable the characterization of the set of signaling pathways involved in pH regulation within the L. longipalpis midgut.

Molecular and physiological characterization of the chitin synthase B gene isolated from Culex pipiens pallens (Diptera: Culicidae)

BACKGROUND

The growth and development of insects is strictly dependent on the precise regulation of chitin synthase (CHS), which is absent in vertebrates and plants. Therefore, CHS represents an attractive target for insecticides. At present, the research on the CHS gene in mosquitoes, especially its biological functions, remains limited.

RESULTS

The full-length cDNA of the chitin synthase B gene in Culex pipiens pallens (CpCHSB) was prepared and consists of 5158 nucleotides with an open reading frame (ORF) of 4722 nucleotides encoding a protein of 1573 amino acid residues. Among different tissues, CpCHSB gene is mainly expressed in the midgut tissue with the highest expression in adult mosquitoes. Knockdown of CpCHSB in the larval stage significantly lowered the chitin content (16.5%) decreased body size (reduced by 25.6% in the larval stage and by 25.6% in the adult stage), and diminished reproduction (20%). Injecting siCHSB into adult mosquito mainly decreased reproduction (27%).

CONCLUSIONS

CpCHSB plays essential roles in growth and development, by severely reducing larval chitin content, midgut permeability, and reducing the number of female mosquito offspring. These results indicate that CHSB may serve as a potential novel target for exploring biosafe insecticides.

Physiochemical changes mediated by "Candidatus Liberibacter asiaticus" in Asian citrus psyllids

Plant pathogenic bacteria interact with their insect host(s)/vector(s) at the cellular and molecular levels. This interaction may alter the physiology of their insect vector, which may also promote the growth and transmission of the bacterium. Here we studied the effect of “Candidatus Liberibacter asiaticus” (“Ca. L. asiaticus”) on physiochemical conditions within its insect vector, the Asian citrus psyllid (ACP), and whether these changes were beneficial for the pathogen. The local microenvironments inside ACPs were quantified using microelectrodes. The average hemolymph pH was significantly higher in infected ACPs (8.13 ± 0.21) than in “Ca. L. asiaticus”-free ACPs (7.29 ± 0.15). The average hemolymph oxygen tension was higher in “Ca. L. asiaticus”-free ACPs than in infected ACPs (67.13% ± 2.11% vs. 35.61% ± 1.26%). Oxygen tension reduction and pH increase were accompanied by “Ca. L. asiaticus” infection. Thus, oxygen tension of the hemolymph is an indicator of infection status, with pH affected by the severity of the infection.

Nutrient depletion may trigger the Yersinia pestis OmpR-EnvZ regulatory system to promote flea-borne plague transmission

The flea's lumen gut is a poorly documented environment where the agent of flea-borne plague, Yersinia pestis, must replicate to produce a transmissible infection. Here, we report that both the acidic pH and osmolarity of the lumen's contents display simple harmonic oscillations with different periods. Since an acidic pH and osmolarity are two of three known stimuli of the OmpR-EnvZ two-component system in bacteria, we investigated the role and function of this Y. pestis system in fleas. By monitoring the in vivo expression pattern of three OmpR-EnvZ-regulated genes, we concluded that the flea gut environment triggers OmpR-EnvZ. This activation was not, however, correlated with changes in pH and osmolarity but matched the pattern of nutrient depletion (the third known stimulus for OmpR-EnvZ). Lastly, we found that the OmpR-EnvZ and the OmpF porin are needed to produce the biofilm that ultimately obstructs the flea's gut and thus hastens the flea-borne transmission of plague. Taken as a whole, our data suggest that the flea gut is a complex, fluctuating environment in which Y. pestis senses nutrient depletion via OmpR-EnvZ. Once activated, the latter triggers a molecular program (including at least OmpF) that produces the biofilm required for efficient plague transmission.