Strategies for regulation of hemolymph pH in acidic and alkaline water by the larval mosquito Aedes aegypti (L.) (Diptera; Culicidae)

Ammonia transport in the excretory system of mosquito larvae (Aedes aegypti): Rh protein expression and the transcriptome of the rectum

The role of the mosquito excretory organs (Malpighian tubules, MT and hindgut, HG) in ammonia transport as well as expression and function of the Rhesus (Rh protein) ammonia transporters within these organs was examined in Aedes aegypti larvae and adult females. Immunohistological examination revealed that the Rh proteins are co-localized with V-type H+-ATPase (VA) to the apical membranes of MT and HG epithelia of both larvae and adult females. Of the two Rh transporter genes present in A. aegypti, AeRh50-1 and AeRh50-2, we show using quantitative real-time PCR (qPCR) and an RNA in-situ hybridization (ISH) assay that AeRh50-1 is the predominant Rh protein expressed in the excretory organs of larvae and adult females. Further assessment of AeRh50-1 function in larvae and adults using RNAi (i.e. dsRNA-mediated knockdown) revealed significantly decreased [NH4+] (mmol l-1) levels in the secreted fluid of larval MT which does not affect overall NH4+ transport rates, as well as significantly decreased NH4+ flux rates across the HG (haemolymph to lumen) of adult females. We also used RNA sequencing to identify the expression of ion transporters and enzymes within the rectum of larvae, of which limited information currently exists for this important osmoregulatory organ. Of the ammonia transporters in A. aegypti, AeRh50-1 transcript is most abundant in the rectum thus validating our immunohistochemical and RNA ISH findings. In addition to enriched VA transcript (subunits A and d1) in the rectum, we also identified high Na+-K+-ATPase transcript (α subunit) expression which becomes significantly elevated in response to HEA, and we also found enriched carbonic anhydrase 9, inwardly rectifying K+ channel Kir2a, and Na+-coupled cation-chloride (Cl-) co-transporter CCC2 transcripts. Finally, the modulation in excretory organ function and/or Rh protein expression was examined in relation to high ammonia challenge, specifically high environmental ammonia (HEA) rearing of larvae. NH4+ flux measurements using the scanning-ion selective electrode (SIET) technique revealed no significant differences in NH4+ transport across organs comprising the alimentary canal of larvae reared in HEA vs freshwater. Further, significantly increased VA activity, but not NKA, was observed in the MT of HEA-reared larvae. Relatively high Rh protein immunostaining persists within the hindgut epithelium, as well as the ovary, of females at 24-48 h post blood meal corresponding with previously demonstrated peak levels of ammonia formation. These data provide new insight into the role of the excretory organs in ammonia transport physiology and the contribution of Rh proteins in mediating ammonia movement across the epithelia of the MT and HG, and the first comprehensive examination of ion transporter and channel expression in the mosquito rectum.

Impact of SumiLarv® 2MR on Aedes aegypti larvae: a multicenter study in Brazil

BACKGROUND

Aedes aegypti is associated with dengue, Zika, and chikungunya transmission. These arboviruses are responsible for national outbreaks with severe public health implications. Vector control is one of the tools used to prevent mosquito proliferation, and SumiLarv® 2MR is an alternative commercial product based on pyriproxyfen for larval/pupal control. In this study, the residual effectiveness of SumiLarv® 2MR in different regions of Brazil was evaluated in simulated field conditions.

METHODS

We conducted a multicenter study across four Brazilian states-Amapá, Pernambuco, Rio de Janeiro, and São Paulo-given the importance to the country's climatic variances in the north, northeast, and southeast regions and their influence on product efficiency. The populations of Ae. aegypti from each location were held in an insectary. Third-instar larvae (L3) were added every 2 weeks to water containers with SumiLarv® 2MR discs in 250-, 500- and 1000-l containers in Amapá and Rio de Janeiro, and 100-l containers in Pernambuco and São Paulo, using concentrations of 0.04, 0.08, and 0.16 mg/l.

RESULTS

Adult emergence inhibition over 420 days was observed in all tests conducted at a concentration of 0.16 mg/l; inhibition for 308-420 days was observed for 0.08 mg/l, and 224-420 days for 0.04 mg/l.

CONCLUSIONS

Sumilarv® 2MR residual activity demonstrated in this study suggests that this new pyriproxyfen formulation is a promising alternative for Aedes control, regardless of climatic variations and ideal concentration, since the SumiLarv® 2MR showed adult emergence inhibition of over 80% and residual activity greater than 6 months, a period longer than that recommended by the Ministry of Health of Brazil between product re-application in larval breeding sites.

Effect of substrate salinity and pH on life history traits of the bluetongue virus vector Culicoides peregrinus

Habitat selection of Culicoides spp. (Diptera: Ceratopogonidae) is influenced by the physicochemical factors such as temperature, pH, salinity, moisture, conductivity, organic and inorganic compounds of substrates. These factors determine the life history traits of the vectors. We studied the influence of substrate salinity (0-40 parts per thousand, ppt) and pH (pH 1-13) on oviposition, egg hatching, larval survivability, and adult emergence of Culicoides peregrinus Kieffer under laboratory conditions. Most eggs (80.74%) were laid in 0 ppt and 95% in pH 7 but lowered with increased salinity and pH levels. It was observed that the females did not lay eggs in 30 ppt to 40 ppt salinity; pH 1 and pH 13 but interestingly up to 95% of the eggs were retained within the abdomen. Little effect of salinity and pH on egg hatching was observed up to 5 ppt and 10 ppt except at the extreme values of 40 ppt and pH 1, pH 13. Pupation did not occur in rearing plates with high salinities, 30 ppt and 40 ppt, although the few eggs hatched when exposed to such salinity. In low salinity (0 to 2 ppt), occurrence of adult emergence was more and then decreased with increasing salinity. Maximum emergence was seen when the rearing media was alkaline. This study deals with the suitability of breeding substrate of C. peregrinus when exposed to salinity and pH ranges. Our study suggests the ambient salinity and pH ranges to be maintained during laboratory rearing of this vector species.

D. R, Lakshmi M. I, Isvaran K, Ghosh SK, Ishtiaq F. Determinants of Aedes mosquito larval ecology in a heterogeneous urban environment- a longitudinal study in Bengaluru, India

BACKGROUND

Aedes-borne disease risk is associated with contemporary urbanization practices where city developing structures function as a catalyst for creating mosquito breeding habitats. We lack better understanding on how the links between landscape ecology and urban geography contribute to the prevalence and abundance of mosquito and pathogen spread.

METHODS

An outdoor longitudinal study in Bengaluru (Karnataka, India) was conducted between February 2021 and June 2022 to examine the effects of macrohabitat types on the diversity and distribution of larval habitats, mosquito species composition, and body size to quantify the risk of dengue outbreak in the landscape context.

FINDINGS

A total of 8,717 container breeding sites were inspected, of these 1,316 were wet breeding habitats. A total of 1,619 mosquito larvae representing 16 species from six macrohabitats and nine microhabitats were collected. Aedes aegypti and Aedes albopictus were the dominant species and significantly higher in artificial habitats than in natural habitats. Breeding preference ratio for Aedes species was high in grinding stones and storage containers. The Aedes infestation indices were higher than the WHO threshold and showed significant linear increase from Barren habitat to High density areas. We found Ae. albopictus breeding in sympatry with Ae. aegypti had shorter wing length.

CONCLUSIONS

A large proportion of larval habitats were man-made artificial containers. Landscape ecology drives mosquito diversity and abundance even at a small spatial scale which could be affecting the localized outbreaks. Our findings showed that sampling strategies for mosquito surveillance must include urban environments with non-residential locations and dengue transmission reduction programmes should focus on 'neighbourhood surveillance' as well to prevent and control the rising threat of Aedes-borne diseases.

Common predators and factors influencing their abundance in Anopheles funestus aquatic habitats in rural south-eastern Tanzania

BACKGROUND

The role of larval predators in regulating the Anopheles funestus population in various malaria-endemic countries remains relatively unknown. This study aimed to investigate the common predators that co-exist with Anopheles funestus group larvae and evaluate factors that influence their abundance in rural south-eastern Tanzania.

METHODS

Mosquito larvae and predators were sampled concurrently using standard dipper (350 ml) or 10 L bucket in previously identified aquatic habitats in selected villages in southern Tanzania. Predators and mosquito larvae were identified using standard identification keys. All positive habitats were geo-located and their physical features characterized. Water physicochemical parameters such as dissolved oxygen (DO), pH, electrical conductivity (EC), total dissolved solids (TDS) and temperature were also recorded.

RESULTS

A total of 85 previously identified An. funestus aquatic habitats in nine villages were sampled for larvae and potential predators. A total of 8,295 predators were sampled. Of these Coenagrionidae 57.7% (n = 4785), Corixidae 12.8% (n = 1,060), Notonectidae 9.9% (n = 822), Aeshnidae 4.9% (n = 405), Amphibian 4.5% (n = 370), Dytiscidae 3.8% (n = 313) were common. A total of 5,260 mosquito larvae were sampled, whereby Anopheles funestus group were 60.3% (n = 3,170), Culex spp. 24.3% (n = 1,279), An. gambie s.l. 8.3% (n = 438) and other anophelines 7.1% (n = 373). Permanent and aquatic habitats larger than 100m2 were positively associated with An. funestus group larvae (P<0.05) and predator abundance (P<0.05). Habitats with submerged vegetation were negatively associated with An. funestus group larvae (P<0.05). Only dissolved oxygen (DO) was positively and significantly affect the abundance of An. funestus group larvae (P<0.05). While predators' abundance was not impacted by all physicochemical parameters.

CONCLUSION

Six potential predator families were common in aquatic habitats of An. funestus group larvae. Additional studies are needed to demonstrate the efficacy of different predators on larval density and adult fitness traits. Interventions leveraging the interaction between mosquitoes and predators can be established to disrupt the transmission potential and survival of the An. funestus mosquitoes.

Proteolytic Activation of Bacillus thuringiensis Cry3Aa Toxin in the Red Palm Weevil (Coleoptera: Curculionidae)

The red palm weevil (RPW), Rhynchophorus ferrugineus (Oliver) is an important pest of palms that causes significant damage by boring into and feeding within palm stem tissues. Here, we studied the proteolytic process of Cry3Aa in the RPW to understand the mechanism of Cry toxicity. The bioassays showed that Cry3Aa toxin is weakly toxic to the RPW. Proteolytic activation assays indicated the Cry3Aa protein is digested into smaller fragments than the 55-kDa activated fragments under different conditions. In particular, at higher mass ratios of gut protease and Cry3Aa protein (5:1, 2:1, and 1:1, respectively), and at 36.9°C for 16 h in a solution of pH 8.6, the Cry3Aa protoxin is over-digested by the gut proteases of weevil larvae. Moreover, the zymogram analysis of the gut proteases revealed the RPW larvae harbors intestinal digestive enzymes mainly composed of serine proteases. This study describes the proteolytic activation process of Cry3Aa in the midgut of RPW larvae.

Physiological responses of freshwater insects to salinity: molecular-, cellular- and organ-level studies

Salinization of freshwater is occurring throughout the world, affecting freshwater biota that inhabit rivers, streams, ponds, marshes and lakes. There are many freshwater insects, and these animals are important for ecosystem health. These insects have evolved physiological mechanisms to maintain their internal salt and water balance based on a freshwater environment that has comparatively little salt. In these habitats, insects must counter the loss of salts and dilution of their internal body fluids by sequestering salts and excreting water. Most of these insects can tolerate salinization of their habitats to a certain level; however, when exposed to salinization they often exhibit markers of stress and impaired development. An understanding of the physiological mechanisms for controlling salt and water balance in freshwater insects, and how these are affected by salinization, is needed to predict the consequences of salinization for freshwater ecosystems. Recent research in this area has addressed the whole-organism response, but the purpose of this Review is to summarize the effects of salinization on the osmoregulatory physiology of freshwater insects at the molecular to organ level. Research of this type is limited, and pursuing such lines of inquiry will improve our understanding of the effects of salinization on freshwater insects and the ecosystems they inhabit.

The Influence of the pH and Salinity of Water in Breeding Sites on the Occurrence and Community Composition of Immature Mosquitoes in the Green Belt of the City of São Paulo, Brazil

Simple Summary

Immature mosquitoes are found in natural and artificial aquatic habitats. Variations in physicochemical parameters of water, such as pH, salinity, conductivity, and total dissolved solids, in breeding habitats can influence larval occurrence and drive the proliferation of adult mosquitoes. Herein, we investigated the association between different values of physicochemical parameters in a variety of aquatic habitats and the occurrence and community composition of immature mosquito species in two environmentally protected areas in the city of São Paulo, Brazil. The aquatic habitats surveyed included epiphytic and ground bromeliads, bamboo internodes, ponds, tree hollows, lakes, and artificial containers. Our results revealed a statistically significant relationship between species occurrence and the variables of pH and salinity. The type of aquatic habitat also had a significant influence on mosquito species distribution. Investigating the interactions between immature mosquitoes and the environment in which they develop is important to elucidate the factors driving their occurrence and abundance, and could also be an important tool in planning and implementing immature mosquito control practices.

Abstract

The physicochemical parameters of water, such as pH, salinity, conductivity, and total dissolved solids, can influence mosquito larval development, survival, and abundance. Therefore, it is important to elucidate how these factors influence mosquito occurrence. We hypothesized that the occurrence and community composition of immature mosquito species are driven not only by the availability of suitable aquatic habitats, but also by the physicochemical factors of these habitats. The primary objective of this study was therefore to investigate the influence of the physicochemical parameters of water in different types of aquatic habitats on the occurrence of mosquito species in two remnants of Atlantic Forest in the city of São Paulo, Brazil. Collections of immature mosquitoes and assessment of the physicochemical characteristics of the water in the collection sites were carried out for twelve months. The variation in species composition and occurrence with the measured physicochemical parameters and the type of breeding site was assessed using constrained ordination methods. The results indicate that there was a statistically significant difference in species composition as a function of the different types of aquatic habitats, and that pH had an influence on species occurrence even when the variance explained by the type of aquatic habitat was removed from the analysis. There was a statistically significant association between mosquito species occurrence and pH and salinity, and the former had a significant influence on the mosquito species collected regardless of the type of aquatic habitat, showing that the pH of the breeding site water is an important factor in driving mosquito population dynamics and species distribution.

Transcriptome analysis of Chironomus sulfurosus larvae living in acidic environments: Insights into molecular mechanisms for acid tolerance

Larvae of chironomid Chironomus sulfurosus mainly live in acidic rivers near hot springs, suggesting that they naturally select acidic environments as preferred habitats. Here we showed that C. sulfurosus larvae moved toward acidic areas and stayed alive on agar gels with a pH gradient of H₂SO₄, and the body fluid pH of the homogenized larvae was near neutral even acclimated under the acidic conditions, indicating mechanisms for acid tolerance. In order to gain insights into this mechanism at the molecular level, de novo RNA-seq analysis was performed on C. sulfurosus larvae. As a result, 1,208 genes were found to be significantly up-regulated in larvae acclimated at pH 2.0 compared to controls at pH 7.0. Among the up-regulated genes, ones encoding cuticle proteins, peritrophic matrix proteins, mucus-forming proteins, F-type ATPase subunits, glutathione S transferases, β-1,3-D-glucan synthetase, hemoglobin, and cytochrome P450 were identified. This transcriptome analysis in conjunction with behavioral and biochemical assays expands our knowledge of gene expression in C. sulfurosus larvae living in acidic environments, which will provide a basis for further studies to elucidate the molecular mechanisms for acid tolerance employed by organisms in nature.

Natural Variation in Physicochemical Profiles and Bacterial Communities Associated with Aedes aegypti Breeding Sites and Larvae on Guadeloupe and French Guiana

Aedes aegypti develop in aquatic habitats in which mosquito larvae are exposed to physicochemical elements and microorganisms that may influence their life cycle and their ability to transmit arboviruses. Little is known about the natural bacterial communities associated with A. aegypti or their relation to the biotic and abiotic characteristics of their aquatic habitats. We characterized the physicochemical properties and bacterial microbiota of A. aegypti breeding sites and larvae on Guadeloupe and in French Guiana. In addition, we explored whether geographic location, the type of breeding site and physicochemical parameters influenced the microbiota associated with this mosquito species. We used large-scale 16S rRNA gene sequencing of 160 breeding sites and 147 pools of A. aegypti larvae and recorded 12 physicochemical parameters at the sampled breeding sites. Ordination plots and multiple linear regression were used to assess the influence of environmental factors on the bacterial microbiota of water and larvae. We found territory-specific differences in physicochemical properties (dissolved oxygen, conductivity) and the composition of bacterial communities in A. aegypti breeding sites that influenced the relative abundance of several bacteria genera (e.g., Methylobacterium, Roseoccocus) on the corresponding larvae. A significant fraction of the bacterial communities identified on larvae, dominated by Herbiconiux and Microvirga genera, were consistently enriched in mosquitoes regardless the location. In conclusion, territory-specific differences observed in the biotic and abiotic properties of A. aegypti breeding sites raise concern about the impact of these changes on pathogen transmission by different A. aegypti populations.

Influence of water's physical and chemical parameters on mosquito (Diptera: Culicidae) assemblages in larval habitats in urban parks of São Paulo, Brazil

Water's physical and chemical characteristics are important constraints in aquatic ecosystems, acting on the development, survival, and adaptation of different organisms. Immature forms of mosquitoes develop in widely diverse aquatic environments and are mainly found in permanent or temporary freshwater bodies with little or no movement. The current study aimed to investigate whether variations in larval habitats' pH, salinity, dissolved oxygen, and water temperature influence the composition of Culicidae assemblages and the presence and abundance of Aedes albopictus and Ae. aegypti. From August 2012 to July 2013, captures of immature forms and measurement of water's physical and chemical profiles were performed monthly in natural and artificial breeding sites in four urban parks in the city of São Paulo, Brazil. Changes in species composition related to the parameters' variation were assessed by multivariate analysis. Regression trees were performed to evaluate the effect of breeding sites' physical and chemical variations on the presence and abundance of Ae. albopictus and Ae. aegypti. The observations suggest ranges of conditions for the measured variables in which most species tend to be found more frequently, and pH and salinity are the variables most closely associated with variations in mosquito composition. Ae. aegypti and Ae. albopictus were present in both natural and artificial breeding sites and were observed under significantly varying conditions of pH, salinity, dissolved oxygen, and temperature. For Ae. albopictus, larval habitat type and pH were the best predictors of incidence and abundance. For Ae. aegypti, pH and salinity were the best predictors of abundance, while dissolved oxygen and larval habitat type were better predictors of presence. This information broadens our understanding of the ecology and interaction of the investigated species with abiotic factors in the aquatic environments, providing useful data for studies that seek to elucidate the underlying mechanisms of selection and colonization of breeding sites by these mosquitoes. This study also reinforces previous observations indicating that Ae. albopictus and Ae. aegypti can colonize diverse types of larval habitats with widely varying physical and chemical conditions.

Evidence that Rh proteins in the anal papillae of the freshwater mosquito Aedes aegypti are involved in the regulation of acid-base balance in elevated salt and ammonia environments

Aedes aegypti commonly inhabit ammonia-rich sewage effluents in tropical regions of the world where the adults are responsible for the spread of disease. Studies have shown the importance of the anal papillae of A. aegypti in ion uptake and ammonia excretion. The anal papillae express ammonia transporters and Rhesus (Rh) proteins which are involved in ammonia excretion and studies have primarily focused on understanding these mechanisms in freshwater. In this study, effects of rearing larvae in salt (5 mmol l^-1 NaCl) or ammonia (5 mmol l^-1 NH₄Cl) on physiological endpoints of ammonia and ion regulation were assessed. In anal papillae of NaCl-reared larvae, Rh protein expression increased, NHE3 transcript abundance decreased and NH₄ ⁺ excretion increased, and this coincided with decreased hemolymph [NH₄ ⁺] and pH. We propose that under these conditions, larvae excrete more NH₄ ⁺ through Rh proteins as a means of eliminating acid from the hemolymph. In anal papillae of NH₄Cl-reared larvae, expression of an apical ammonia transporter and the Rh proteins decreased, the activities of NKA and VA decreased and increased, respectively, and this coincided with hemolymph acidification. The results present evidence for a role of Rh proteins in acid-base balance in response to elevated levels of salt, whereby ammonia is excreted as an acid equivalent.

Ammonia excretion in aquatic invertebrates: new insights and questions

Invertebrates employ a variety of ammonia excretion strategies to facilitate their survival in diverse aquatic environments, including freshwater, seawater and the water film surrounding soil particles. Various environmental properties set innate challenges for an organism's ammonia excretory capacity. These include the availability of NaCl and the respective ion-permeability of the organism's transport epithelia, and the buffering capacity of their immediate surrounding medium. To this end, some transporters seem to be conserved in the excretory process. This includes the Na+/K+(NH4+)-ATPase (NKA), the NH3/CO2 dual gas-channel Rhesus (Rh)-proteins and novel ammonia transporters (AMTs), which have been identified in several invertebrates but appear to be absent from vertebrates. In addition, recent evidence strongly suggests that the hyperpolarization-activated cyclic nucleotide-gated K+ channel (HCN) plays a significant role in ammonia excretion and is highly conserved throughout the animal kingdom. Furthermore, microtubule-dependent vesicular excretion pathways have been found in marine and soil-dwelling species, where, unlike freshwater systems, acid-trapping of excreted ammonia is difficult or absent owing to the high environmental buffering capacity of the surroundings. Finally, although ammonia is known to be a toxic nitrogenous waste product, certain marine species readily maintain potentially toxic hemolymph ammonia as a sort of ammonia homeostasis, which suggests that ammonia is involved in physiological processes and does not exist simply for excretion. Such findings are discussed within this Commentary and are hypothesized to be involved in acid–base regulation. We also describe excretory organs and processes that are dependent on environmental constraints and indicate gaps in the current knowledge in these topics.

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.

Slc4-like anion transporters of the larval mosquito alimentary canal

Mosquito larvae exhibit luminal pH extremes along the axial length of their alimentary canal that range from very alkaline (pH>10) in the anterior midgut to slightly acid in the hindgut. The principal buffer in the system is thought to be bicarbonate and/or carbonate, because the lumen is known to contain high levels of bicarbonate/carbonate and is surrounded by various epithelial cell types which express a variety of carbonic anhydrases. However, the precise mechanisms responsible for the transport of bicarbonate/carbonate into and out of the lumen are unclear. In the present study, we test the hypothesis that SLC4-like anion transporters play a role in bicarbonate/carbonate accumulation in the larval mosquito alimentary canal. Molecular, physiological and immnuohistochemical characterizations of Slc4-like transporters in the gut of larval mosquitoes (Aedes aegypti and Anopheles gambiae) demonstrate the presence of both a Na(+)-independent chloride/bicarbonate anion exchanger (AE) as well as a Na(+)-dependent anion exchanger (NDAE). Notably, immunolocalization experiments in Malpighian tubules show that the two proteins can be located in the same tissue, but to different cell types. Immunolabeling experiments in the gastric caecae show that the two proteins can be found in the same cells, but on opposite sides (basal vs. apical). In summary, our results indicate that the alimentary canal of larval mosquitoes exhibits robust expression of two SLC4-like transporters in locations that are consistent with a role in the regulation of luminal pH. The precise physiological contributions of each transporter remain to be determined.

Larvae of the midge Chironomus riparius possess two distinct mechanisms for ionoregulation in response to ion-poor conditions

This study examined the role of the anal papillae of the freshwater (FW) chironomid larva Chironomus riparius in ionoregulation under ion-poor conditions. The scanning ion-selective electrode technique (SIET) was utilized to characterize the species, direction, and rates of inorganic ion transport by the anal papillae following acute and long-term exposure to ion-poor water (IPW). The major inorganic ions in the hemolymph of larvae treated as above were measured using standard ion-selective microelectrodes. The anal papillae of C. riparius are sites of net NaCl uptake and H(+) secretion under FW and IPW conditions and are not likely to be a major contributor of K(+) exchange. Acute and long-term exposure to IPW increased total net transport of Na(+), Cl(-), and H(+) by the anal papillae, but the mechanisms underlying the increase under the two conditions were different. Acute IPW exposure increased the magnitude of net ion fluxes at sites along the anal papillae, while long-term IPW exposure resulted in increased size of the anal papillae with no change in the magnitude of net ion fluxes. The contribution of the anal papillae to observed alterations of hemolymph ion activities upon exposure to IPW is discussed. Inhibitors of the Na(+)/H(+) exchangers (EIPA) and carbonic anhydrase (methazolamide) provide evidence for Na(+)/H(+) and Cl(-)/HCO(3)(-) exchange mechanisms in the anal papillae. This study demonstrates that C. riparius larvae employ two different mechanisms to upregulate the total net transport of ions by the anal papillae, and these mechanisms are at least partially responsible for regulating hemolymph ion activity.

Effects of serotonergic agents on survival and hemolymph composition of the larval mosquito Aedes aegypti (Diptera: Culicidae, L.) in vivo: does serotonin regulate hemolymph acid-base homeostasis?

The role of serotonin in the regulation of larval Aedes aegypti hemolymph composition was investigated in vivo using two reuptake inhibitors (SSRIs), alaproclate HCl and 6-nitroquipazine maleate, and the receptor antagonist methiothepin mesylate. Larvae were placed in media differing in pH and salinity in the presence and absence of serotonergic agents. The toxicity of each agent was strongly influenced by ambient pH. For each agent, toxicity was negligible in acidic media, intermediate in neutral media and greatest in alkaline media. By contrast, toxicity of all agents was independent of salinity. No effects on mass-specific body water or hemolymph volume were observed whereas hemolymph osmotic pressure, Na(+) concentrations and pH differed significantly among treatments. 6-nitroquipazine caused a decrease in Na(+) from 115+/-1.7 to 103+/-0.9 mmol l(-1), and alaproclate caused alkalosis of the hemolymph from pH 7.55+/-0.026 to pH 7.72+/-0.044. Methiothepin decreased hemolymph osmotic pressure from 329+/-9.9 to 304+/-8.8 and showed the greatest overall toxicity. Control larvae excreted net base in pH 4 media (1.4 micromol g(-1) h(-1)) and net acid in pH 7 (1.2 micromol g(-1) h(-1)) and pH 11 (5.1 micromol g(-1) h(-1)) media. In pH 4 media, alaproclate and methiothepin caused a shift to net H(+) excretion (1.1 and 1.5 micromol g(-1) h(1), respectively) whereas these agents did not influence acid excretion rates in pH 7 or pH 11 media. The hypothesis that serotonin is involved in hemolymph acid-base balance is discussed.

Carbonic anhydrases and anion transport in mosquito midgut pH regulation

Mosquito larvae use a digestive strategy that is relatively rare in nature. The anterior half of the larval mosquito midgut has a luminal pH that ranges between 10.5 and 11.5. Most other organisms, both large and small, initiate digestion in an acid medium. The relative uniqueness of the highly alkaline digestive strategy has been a long-standing research focus in larval lepidopterans. More recently, the disease vector potential of mosquitoes has fueled specific interest in larval mosquito biology and the alkaline digestive environment in the midgut. The probable principle anion influencing the highly alkaline gut lumen is bicarbonate/carbonate. Bicarbonate/carbonate is regulated at least in part by the activity of carbonic anhydrases. Hence, we have focused attention on the carbonic anhydrases of the mosquito larva. Anopheles gambiae, the major malaria mosquito of Africa, is an organism with a published genome which has facilitated molecular analyses of the 12 carbonic anhydrase genes annotated for this mosquito. Microarray expression analyses, tissue-specific quantitative RT-PCR, and antibody localization have been used to generate a picture of carbonic anhydrase distribution in the larval mosquito. Cytoplasmic, GPI-linked extracellular membrane-bound and soluble extracellular carbonic anhydrases have been located in the midgut and hindgut. The distribution of the enzymes is consistent with an anion regulatory system in which carbonic anhydrases provide a continuous source of bicarbonate/carbonate from the intracellular compartments of certain epithelial cells to the ectoperitrophic space between the epithelial cells and the acellular membrane separating the food bolus from the gut cells and finally into the gut lumen. Carbonic anhydrase in specialized cells of the hindgut (rectum) probably plays a final role in excretion of bicarbonate/carbonate into the aquatic environment of the larva. Detection and characterization of classic anion exchangers of the SLC4A family in the midgut has been problematic. The distribution of carbonic anhydrases in the system may obviate the requirement for such transporters, making the system more dependent on simple carbon dioxide diffusion and ionization via the activity of the enzyme.

Larval anopheline mosquito recta exhibit a dramatic change in localization patterns of ion transport proteins in response to shifting salinity: a comparison between anopheline and culicine larvae

Mosquito larvae live in dynamic aqueous environments, which can fluctuate drastically in salinity due to environmental events such as rainfall and evaporation. Larval survival depends upon the ability to regulate hemolymph osmolarity by absorbing and excreting ions. A major organ involved in ion regulation is the rectum, the last region for modification of the primary urine before excretion. The ultrastructure and function of culicine larval recta have been studied extensively; however, very little published data exist on the recta of anopheline larvae. To gain insight into the structure and functions of this organ in anopheline species, we used immunohistochemistry to compare the localization of three proteins [carbonic anhydrase (CA9), Na+/K+ P-ATPase and H+ V-ATPase] in the recta of anopheline larvae reared in freshwater and saline water with the localization of the same proteins in culicine larvae reared under similar conditions. Based on the following key points, we concluded that anophelines differ from culicines in larval rectal structure and in regulation of protein expression: (1) despite the fact that obligate freshwater and saline-tolerant culicines have structurally distinct recta, all anophelines examined (regardless of saline-tolerance) have a structurally similar rectum consisting of distinct DAR (dorsal anterior rectal) cells and non-DAR cells; (2) anopheline larvae undergo a dramatic shift in rectal Na+/K+-ATPase localization when reared in freshwater vs saline water. This shift is not seen in any culicine larvae examined. Additionally, we use these immunohistochemical analyses to suggest possible functions for the DAR and non-DAR cells of anopheline larvae in freshwater and saline conditions.

Synergy and specificity of two Na+-aromatic amino acid symporters in the model alimentary canal of mosquito larvae

The nutrient amino acid transporter (NAT) subfamily is the largest subdivision of the sodium neurotransmitter symporter family (SNF; also known as SLC6; HUGO). There are seven members of the NAT population in the African malaria mosquito Anopheles gambiae, two of which, AgNAT6 and AgNAT8, preferably transport indole- and phenyl-branched substrates, respectively. The relative expression and distribution of these aromatic NATs were examined with transporter-specific antibodies in Xenopus oocytes and mosquito larval alimentary canal, representing heterologous and tissue expression systems, respectively. NAT-specific aromatic-substrate-induced currents strongly corresponded with specific accumulation of both transporters in the plasma membrane of oocytes. Immunolabeling revealed elevated expressions of both transporters in specific regions of the larval alimentary canal, including salivary glands, cardia, gastric caeca, posterior midgut and Malpighian tubules. Differences in relative expression densities and spatial distribution of the transporters were prominent in virtually all of these regions, suggesting unique profiles of the aromatic amino acid absorption. For the first time reversal of the location of a transporter between apical and basal membranes was identified in posterior and anterior epithelial domains corresponding with secretory and absorptive epithelial functions, respectively. Both aromatic NATs formed putative homodimers in the larval gut whereas functional monomers were over-expressed heterologously in Xenopus oocytes. The results unequivocally suggest functional synergy between substrate-specific AgNAT6 and AgNAT8 in intracellular absorption of aromatic amino acids. More broadly, they suggest that the specific selectivity, regional expression and polarized membrane docking of NATs represent key adaptive traits shaping functional patterns of essential amino acid absorption in the metazoan alimentary canal and other tissues.

Cationic pathway of pH regulation in larvae of Anopheles gambiae

Anopheles gambiae larvae (Diptera: Culicidae) live in freshwater with low Na(+) concentrations yet they use Na(+) for alkalinization of the alimentary canal, for electrophoretic amino acid uptake and for nerve function. The metabolic pathway by which larvae accomplish these functions has anionic and cationic components that interact and allow the larva to conserve Na(+) while excreting H(+) and HCO(3)(-). The anionic pathway consists of a metabolic CO(2) diffusion process, carbonic anhydrase and Cl(-)/HCO(3)(-) exchangers; it provides weak HCO(3)(-) and weaker CO(3)(2-) anions to the lumen. The cationic pathway consists of H(+) V-ATPases and Na(+)/H(+) antiporters (NHAs), Na(+)/K(+) P-ATPases and Na(+)/H(+) exchangers (NHEs) along with several (Na(+) or K(+)):amino acid(+/-) symporters, a.k.a. nutrient amino acid transporters (NATs). This paper considers the cationic pathway, which provides the strong Na(+) or K(+) cations that alkalinize the lumen in anterior midgut then removes them and restores a lower pH in posterior midgut. A key member of the cationic pathway is a Na(+)/H(+) antiporter, which was cloned recently from Anopheles gambiae larvae, localized strategically in plasma membranes of the alimentary canal and named AgNHA1 based upon its phylogeny. A phylogenetic comparison of all cloned NHAs and NHEs revealed that AgNHA1 is the first metazoan NHA to be cloned and localized and that it is in the same clade as electrophoretic prokaryotic NHAs that are driven by the electrogenic H(+) F-ATPase. Like prokaryotic NHAs, AgNHA1 is thought to be electrophoretic and to be driven by the electrogenic H(+) V-ATPase. Both AgNHA1 and alkalophilic bacterial NHAs face highly alkaline environments; to alkalinize the larva mosquito midgut lumen, AgNHA1, like the bacterial NHAs, would have to move nH(+) inwardly and Na(+) outwardly. Perhaps the alkaline environment that led to the evolution of electrophoretic prokaryotic NHAs also led to the evolution of an electrophoretic AgNHA1 in mosquito larvae. In support of this hypothesis, antibodies to both AgNHA1 and H(+) V-ATPase label the same membranes in An. gambiae larvae. The localization of H(+) V-ATPase together with (Na(+) or K(+)):amino acid(+/-) symporter, AgNAT8, on the same apical membrane in posterior midgut cells constitutes the functional equivalent of an NHE that lowers the pH in the posterior midgut lumen. All NATs characterized to date are Na(+) or K(+) symporters so the deduction is likely to have wide application. The deduced colocalization of H(+) V-ATPase, AgNHA1 and AgNAT8, on this membrane forms a pathway for local cycling of H(+) and Na(+) in posterior midgut. The local H(+) cycle would prevent unchecked acidification of the lumen while the local Na(+) cycle would regulate pH and support Na(+):amino acid(+/-) symport. Meanwhile, a long-range Na(+) cycle first transfers Na(+) from the blood to gastric caeca and anterior midgut lumen where it initiates alkalinization and then returns Na(+) from the rectal lumen to the blood, where it prevents loss of Na(+) during H(+) and HCO(3)(-) excretion. The localization of H(+) V-ATPase and Na(+)/K(+)-ATPase in An. gambiae larvae parallels that reported for Aedes aegypti larvae. The deduced colocalization of the two ATPases along with NHA and NAT in the alimentary canal constitutes a cationic pathway for Na(+)-conserving midgut alkalinization and de-alkalinization which has never been reported before.