Ancestry and progeny of nutrient amino acid transporters

Characterization and functional analysis of Cshsp19.0 encoding a small heat shock protein in Chilo suppressalis (Walker)

Small heat shock proteins (sHSPs) function as ATP-independent chaperones that preserve cellular proteostasis under stressful conditions. In this study, Cshsp19.0, which encodes a new small heat shock protein, was isolated and characterized from Chilo suppressalis (Walker) to better understand the contribution of sHSPs to insect development and stress tolerance. The full-length Cshsp19.0 cDNA was 697 bp and encoded a 19.0 kDa protein with an isoelectric point of 5.95. Phylogenetic analysis and amino acid alignments indicated that Cshsp19.0 is a member of the sHSP family. Cshsp19.0 was expressed at maximal levels in foreguts and showed the least amount of expression in fat bodies. Expression analysis in different developmental stages of C. suppressalis revealed that Cshsp19.0 was most highly expressed in 1st instar larvae. Furthermore, Cshsp19.0 was upregulated when insects were exposed to heat and cold stress for a 2-h period. There were significant differences in the male and female pupae in response to humidity; Cshsp19.0 expression increased in male pupae as RH increased, whereas the inverse pattern was observed in female pupae. Larvae exhibited a lower rate of survival when Cshsp19.0 was silenced by a nanomaterial-promoted RNAi method. The results confirm that Cshsp19.0 functions to increase environmental stress tolerance and regulates physiological activities in C. suppressalis.

The Lepidopteran KAAT1 and CAATCH1: Orthologs to Understand Structure-Function Relationships in Mammalian SLC6 Transporters

To the SLC6 family belong 20 human transporters that utilize the sodium electrochemical gradient to move biogenic amines, osmolytes, amino acids and related compounds into cells. They are classified into two functional groups, the Neurotransmitter transporters (NTT) and Nutrient amino acid transporters (NAT). Here we summarize how since their first cloning in 1998, the insect (Lepidopteran) Orthologs of the SLC6 family transporters have represented very important tools for investigating functional–structural relationships, mechanism of transport, ion and pH dependence and substate interaction of the mammalian (and human) counterparts.

Characterization of the Domoic Acid Uptake Mechanism of the Mussel (Mytilus galloprovincialis) Digestive Gland

Cultures of the mussel Mytilus galloprovincialis are frequently affected by accumulation of the amnesic shellfish poisoning toxin domoic acid (DA). This species is characterized by a fast uptake and release of the toxin. In this work, the main characteristics of the uptake mechanism have been studied by incubation of digestive gland thin slices in media with different composition and DA concentration. DA uptake seems to follow Michaelis-Menten kinetics, with a very high estimated KM (1722 µg DA mL-1) and a Vmax of 71.9 µg DA g-1 h-1, which is similar to those found for other amino acids in invertebrates. Replacement of NaCl from the incubation media by Cl-choline (Na+-free medium) did not significantly reduce the uptake, but replacement by sorbitol (Na+-free and Cl--depleted medium) did. A new experiment replacing all chlorides with their equivalent gluconates (Na+- and Cl--free medium) showed an important reduction in the uptake that should be attributed to the absence of chloride, pointing to a Na+-independent, Cl- (or anion-) dependent transporter. In media with Na+ and Cl-, neither decreasing the pH nor adding cyanide (a metabolic inhibitor) had significant effect on DA uptake, suggesting that the transport mechanism is not H+- or ATP-dependent. In a chloride depleted medium, lowering pH or adding CN increased the uptake, suggesting that other anions could, at least partially, substitute chloride.

A SLC6 transporter cloned from the lion's mane jellyfish (Cnidaria, Scyphozoa) is expressed in neurons

In the course of recent comparative genomic studies conducted on nervous systems across the phylogeny, current thinking is leaning in favor of more heterogeneity among nervous systems than what was initially expected. The isolation and characterization of molecular components that constitute the cnidarian neuron is not only of interest to the physiologist but also, on a larger scale, to those who study the evolution of nervous systems. Understanding the function of those ancient neurons involves the identification of neurotransmitters and their precursors, the description of nutrients used by neurons for metabolic purposes and the identification of integral membrane proteins that bind to those compounds. Using a molecular cloning strategy targeting membrane proteins that are known to be present in all forms of life, we isolated a member of the solute carrier family 6 from the scyphozoan jellyfish Cyanea capillata. The phylogenetic analysis suggested that the new transporter sequence belongs to an ancestral group of the nutrient amino acid transporter subfamily and is part of a cluster of cnidarian sequences which may translocate the same substrate. We found that the jellyfish transporter is expressed in neurons of the motor nerve net of the animal. To this end, we established an in situ hybridization protocol for the tissues of C. capillata and developed a specific antibody to the jellyfish transporter. Finally, we showed that the gene that codes for the jellyfish transporter also expresses a long non-coding RNA. We hope that this research will contribute to studies that seek to understand what constitutes a neuron in species that belong to an ancient phylum.

Methionine as a safe and effective novel biorational mosquito larvicide

BACKGROUND

Mosquito larvicides provide a source-reduction strategy to diminish adult females that bite and potentially spread pathogens. Demands are mounting for new and innovative effective biorational larvicides, due to the development of resistance to some currently utilized mosquito larvicides, undesirable non-target effects, and US Environmental Protection Agency (EPA) restrictions. Methionine is a human nutrient essential amino acid that unexpectedly has been shown to be a valuable safe pest management tool against select insect pests that possess alkaline gut physiology. The present study evaluated larvicidal toxicity of methionine in several pestiferous mosquito (Diptera: Culicidae) genera.

RESULTS

Concentration-dependent DL-methionine kinetics assays of survival and pupation were conducted in larvae of Aedes albopictus Skuse, Anopheles quadrimaculatus Say, and Culex tarsalis Coquillett in glass jars. High concentrations of DL-methionine yielded 100% mortality for all test species and prevented pupation at a rate equivalent to Bacillus thuringiensis israelensis (Bti) treatments. Concentration kinetics indicated that An. quadrimaculatus was 10-fold more sensitive to DL-methionine than Ae. albopictus and Cx. tarsalis.

CONCLUSIONS

EPA regulations currently exempt methionine in pesticide formulations applied to agricultural crops. This study demonstrates that methionine is a highly effective mosquito larvicide that can provide a beneficial new biorational, environmentally sustainable tool to control pestiferous mosquitoes. © 2018 Society of Chemical Industry.

Genome-Wide Identification, Characterization and Expression Analysis of the Solute Carrier 6 Gene Family in Silkworm (Bombyx mori)

The solute carrier 6 (SLC6) gene family, initially known as the neurotransmitter transporters, plays vital roles in the regulation of neurotransmitter signaling, nutrient absorption and motor behavior. In this study, a total of 16 candidate genes were identified as SLC6 family gene homologs in the silkworm (Bombyx mori) genome. Spatio-temporal expression patterns of silkworm SLC6 gene transcripts indicated that these genes were highly and specifically expressed in midgut, brain and gonads; moreover, these genes were expressed primarily at the feeding stage or adult stage. Levels of expression for most midgut-specific and midgut-enriched gene transcripts were down-regulated after starvation but up-regulated after re-feeding. In addition, we observed that expression levels of these genes except for BmSLC6-15 and BmGT1 were markedly up-regulated by a juvenile hormone analog. Moreover, brain-enriched genes showed differential expression patterns during wandering and mating processes, suggesting that these genes may be involved in modulating wandering and mating behaviors. Our results improve our understanding of the expression patterns and potential physiological functions of the SLC6 gene family, and provide valuable information for the comprehensive functional analysis of the SLC6 gene family.

The D-amino acid transport by the invertebrate SLC6 transporters KAAT1 and CAATCH1 from Manduca sexta

The ability of the SLC6 family members, the insect neutral amino acid cotransporter KAAT1(K⁺‐coupled amino acid transporter 1) and its homologous CAATCH1(cation anion activated amino acid transporter/channel), to transport D‐amino acids has been investigated through heterologous expression in Xenopus laevis oocytes and electrophysiological techniques. In the presence of D‐isomers of leucine, serine, and proline, the msKAAT1 generates inward, transport‐associated, currents with variable relative potencies, depending on the driving ion Na⁺ or K⁺. Higher concentrations of D‐leucine (≥1 mmol/L) give rise to an anomalous response that suggests the existence of a second binding site with inhibitory action on the transport process. msCAATCH1 is also able to transport the D‐amino acids tested, including D‐leucine, whereas L‐leucine acts as a blocker. A similar behavior is exhibited by the KAAT1 mutant S308T, confirming the relevance of the residue in this position in L‐leucine binding and the different interaction of D‐leucine with residues involved in transport mechanism. D‐leucine and D‐serine on various vertebrate orthologs B⁰AT1 (SLC6A19) elicited only a very small current and singular behavior was not observed, indicating that it is specific of the insect neutral amino acid transporters. These findings highlight the relevance of D‐amino acid absorption in the insect nutrition and metabolism and may provide new evidences in the molecular transport mechanism of SLC6 family.

Knockdown of a nutrient amino acid transporter gene LdNAT1 reduces free neutral amino acid contents and impairs Leptinotarsa decemlineata pupation

A Leptinotarsa decemlineata SLC6 NAT gene (LdNAT1) was cloned. LdNAT1 was highly expressed in the larval alimentary canal especially midgut. LdNAT1 mRNA levels were high right after the molt and low just before the molt. JH and a JH analog pyriproxyfen activated LdNAT1 expression. RNAi of an allatostatin gene LdAS-C increased JH and upregulated LdNAT1 transcription. Conversely, silencing of a JH biosynthesis gene LdJHAMT decreased JH and reduced LdNAT1 expression. Moreover, 20E and an ecdysteroid agonist halofenozide repressed LdNAT1 expression, whereas a decrease in 20E by RNAi of an ecdysteroidogenesis gene LdSHD and disruption of 20E signaling by knockdown of LdE75 and LdFTZ-F1 activated LdNAT1 expression. Thus, LdNAT1 responded to both 20E and JH. Moreover, knockdown of LdNAT1 reduced the contents of cysteine, histidine, isoleucine, leucine, methionine, phenylalanine and serine in the larval bodies and increased the contents of these amino acids in the larval feces. Furthermore, RNAi of LdNAT1 inhibited insulin/target of rapamycin pathway, lowered 20E and JH titers, reduced 20E and JH signaling, retarded larval growth and impaired pupation. These data showed that LdNAT1 was involved in the absorption of several neutral amino acids critical for larval growth and metamorphosis.

Drosophila melanogaster as a genetic model system to study neurotransmitter transporters

The model genetic organism Drosophila melanogaster, commonly known as the fruit fly, uses many of the same neurotransmitters as mammals and very similar mechanisms of neurotransmitter storage, release and recycling. This system offers a variety of powerful molecular-genetic methods for the study of transporters, many of which would be difficult in mammalian models. We review here progress made using Drosophila to understand the function and regulation of neurotransmitter transporters and discuss future directions for its use.

A novel eukaryotic Na+ methionine selective symporter is essential for mosquito development

AeNAT5 (NCBI, ABZ81822), an orphan member of the insect-specific Nutrient Amino acid Transporter subfamily of SoLute Carrier family 6 (NAT-SLC6) and the first representative of a novel eukaryotic methionine-selective transport system (M), was cloned from cDNA of the vector mosquito, Aedes aegypti. It has orphan orthologs throughout several mosquito genomes, but not in Drosophila or outside Diptera. It shows the highest apparent affinity to L-Met (K(0.5) = 0.021 mM) and its metabolites Homocysteine and Cysteine (K(0.5) = 0.89 and 2.16 mM), but weakly interact with other substrates. It has a Na(+) - coupled mechanism (K(0.5) Na(+) ∼ 46 mM) with 1AA:1Na(+) stoichiometry that maintains ∼60% activity in Cl(-) - free media. In situ hybridization showed accumof AeNAT5 transcript in the absorptive and secretory epithelia, as well as in specific peripheral neurons and the central ganglia of mosquito larvae. The labeling pattern is distinct from that of the previously characterized AeNAT1. RNAi of AeNAT5 increases larval mortality during ecdysis and dramatically suppresses adult emergence. Our results showed that in addition to previously characterized broad spectra and aromatic amino acid selective transport systems, the mosquito NAT-SLC6 subfamily evolved a unique mechanism for selective absorption of sulfur-containing substrates. We demonstrated specific patterns of alimentary and neuronal transcription of AeNAT5 in mosquito larvae that is collateral with the indispensable function of this transporter in mosquito development.

Functional properties of a newly cloned fish ortholog of the neutral amino acid transporter B0AT1 (SLC6A19)

The functional properties of an ortholog of the B(0)AT1 (SLC6A19) amino acid transporter, cloned from the intestine of the sea bass Dicentrachus labrax, were investigated. The two-electrode voltage-clamp technique was applied to Xenopus laevis oocytes heterologously expressing the transporter in order to measure the currents associated with the transport process in different conditions. In particular the substrate specificity, the ionic requirements, and possible effects of pH were examined. Among the organic substrates, leucine, glycine, serine and valine generated the largest transport currents with apparent affinities in the lower millimolar range. The importance of Na(+) as the driver ion in the transport process is confirmed, although Li(+) is also capable to sustain transport, while K(+) is not. No evidence of a relevant role of Cl(-) in the transport activity was found. Concerning the other two kinds of currents commonly found in electrogenic transporters, very fast pre-steady-state currents were detected in the absence of organic substrate, while lithium-specific leak currents were not observed. The comparison of these properties with those of the mammalian and insect orthologs may give interesting indication for future structure-function studies in this transporter subfamily.

Tissue-specific ionomotive enzyme activity and K+ reabsorption reveal the rectum as an important ionoregulatory organ in larval Chironomus riparius exposed to varying salinity

A role for the rectum in the ionoregulatory homeostasis of larval Chironomus riparius was revealed by rearing animals in different saline environments and examining: (1) the spatial distribution and activity of keystone ionomotive enzymes Na(+)-K(+)-ATPase (NKA) and V-type H(+)-ATPase (VA) in the alimentary canal, and (2) rectal K(+) transport with the scanning ion-selective electrode technique (SIET). NKA and VA activity were measured in four distinct regions of the alimentary canal as follows: the combined foregut and anterior midgut, the posterior midgut, the Malpighian tubules and the hindgut. Both enzymes exhibited 10-20 times greater activity in the hindgut relative to all other areas. When larvae were reared in either ion-poor water (IPW) or freshwater (FW), no significant difference in hindgut enzyme activity was observed. However, in larvae reared in brackish water (BW), NKA and VA activity in the hindgut significantly decreased. Immunolocalization of NKA and VA in the hindgut revealed that the bulk of protein was located in the rectum. Therefore, K(+) transport across the rectum was examined using SIET. Measurement of K(+) flux along the rectum revealed a net K(+) reabsorption that was reduced fourfold in BW-reared larvae versus larvae reared in FW or IPW. Inhibition of NKA with ouabain, VA with bafilomycin and K(+) channels with charybdotoxin diminished rectal K(+) reabsorption in FW- and IPW-reared larvae, but not BW-reared larvae. Data suggest that the rectum of C. riparius plays an important role in allowing these larvae to cope with dilute as well as salinated environmental conditions.

An SLC6 transporter of the novel B(0,)- system aids in absorption and detection of nutrient amino acids in Caenorhabditis elegans

Nutrient amino acid transporters (NATs) of solute carrier family 6 (SLC6) mediate uptake of essential amino acids in mammals and insects. Phylogenomic analysis of the Caenorhabditis elegans (Ce) SLC6 family identifies five genes paralogous to an insect-specific NAT subfamily. Here we cloned and characterized the first representative of the identified nematode-specific transporters, SNF-5. SNF-5 mediates broad spectrum cation-coupled transport of neutral amino acids with submillimolar affinities and stoichiometry of 1 AA:1 Na(+), except for 1 l-Pro:2 Na(+). Unexpectedly, it transports acidic l-Glu(-) and l-Asp(-) (1 AA(-):3 Na(+)), revealing it to be the first member of a new B(0,-) system among characterized SLC6 transporters. This activity correlates with a unique positively charged His(+) 377 in the substrate-binding pocket. snf-5 promoter-driven enhanced green fluorescent protein labels intestinal cells INT1-9 and three pairs of amphid sensory neurons: ASI, ADF and ASK. These cells are intimately involved in control of dauer diapause, development, metabolism and longevity. The snf-5 deletion mutants do not show apparent morphological disorders, but increase dauer formation while reducing dauer maintenance upon starvation. Overall, the present study characterized the first nematode-specific NAT and revealed important structural and functional aspects of this transporter. In addition to the predictable role in alimentary amino acid absorption, our results indicate possible neuronal roles of SNF-5 as an amino acid provider to specific neuronal functions, including sensing of amino acid availability.

An inverse relationship links temperature and substrate apparent affinity in the ion-coupled cotransporters rGAT1 and KAAT1

The effects of temperature on the operation of two ion-coupled cotransporters of the SLC6A family, namely rat GAT1 (SLC6A1) and KAAT1 (SLC6A19) from Manduca sexta, have been studied by electrophysiological means in Xenopus laevis oocytes expressing these proteins. The maximal transport-associated current (I_max) and the apparent substrate affinity (K₀₅) were measured. In addition to the expected increase in transport rate (Q₁₀ = 3–6), both transporters showed greater K₀₅ values (i.e., a decrease in apparent affinity) at higher temperatures. The transport efficiency, estimated as I_max/K₀₅, increased at negative potentials in both transporters, but did not show statistically significant differences with temperature. The observation that the apparent substrate affinity is inversely related to the transport rate suggests a kinetic regulation of this parameter. Furthermore, the present results indicate that the affinities estimated at room temperature for mammalian cotransporters may not be simply extrapolated to their physiological operating conditions.

Molecular basis of essential amino acid transport from studies of insect nutrient amino acid transporters of the SLC6 family (NAT-SLC6)

Two protein families that represent major components of essential amino acid transport in insects have been identified. They are annotated as the SLC6 and SLC7 families of transporters according to phylogenetic proximity to characterized amino acid transporters (HUGO nomenclature). Members of these families have been identified as important apical and basolateral parts of transepithelial essential amino acid absorption in the metazoan alimentary canal. Synergistically, they play critical physiological roles as essential substrate providers to diverse metabolic processes, including generic protein synthesis. This review briefly clarifies the requirements for amino acid transport and a variety of amino acid transport mechanisms, including the aforementioned families. Further it focuses on the large group of Nutrient Amino acid Transporters (NATs), which comprise a recently identified subfamily of the Neurotransmitter Sodium Symporter family (NSS or SLC6). The first insect NAT, cloned from the caterpillar gut, has a broad substrate spectrum similar to mammalian B(0) transporters. Several new NAT-SLC6 members have been characterized in an effort to explore mechanisms for the essential amino acid absorption in model dipteran insects. The identification and functional characterization of new B(0)-like and narrow specificity transporters of essential amino acids in fruit fly and mosquitoes leads to a fundamentally important insight: that NATs evolved and act together as the integrated active core of a transport network that mediates active alimentary absorption and systemic distribution of essential amino acids. This role of NATs is projected from the most primitive prokaryotes to the most complex metazoan organisms, and represents an interesting platform for unraveling the molecular evolution of amino acid transport and modeling amino acid transport disorders. The comparative study of NATs elucidates important adaptive differences between essential amino acid transportomes of invertebrate and vertebrate organisms, outlining a new possibility for selective targeting of essential amino acid absorption mechanisms to control medically and economically important arthropods and other invertebrate organisms.

SLC7 amino acid transporters of the yellow fever mosquito Aedes aegypti and their role in fat body TOR signaling and reproduction

BACKGROUND

An important function of the fat body in adult female mosquitoes is the conversion of blood meal derived amino acids (AA) into massive amounts of yolk protein precursors. A highly efficient transport mechanism for AAs across the plasma membrane of the fat body trophocytes is essential in order to deliver building blocks for the rapid synthesis of large amounts of these proteins. This mechanism consists in part of AA transporter proteins from the solute carrier family. These transporters have dual function; they function as transporters and participate in the nutrient signal transduction pathway that is activated in the fat body after a blood meal. In this study we focused on the solute carrier 7 family (SLC7), a family of AA transporters present in all metazoans that includes members with strong substrate specificity for cationic AAs.

METHODOLOGY/PRINCIPAL FINDINGS

We identified 11 putative SLC7 transporters in the genome sequence of Aedes aegypti. Phylogenetic analysis puts five of these in the cationic AA transporter subfamily (CAT) and six in the heterodimeric AA transporter (HAT) subfamily. All 11 A. aegypti SLC7 genes are expressed in adult females. Expression profiles are dynamic after a blood meal. We knocked down six fat body-expressed SLC7 transporters using RNAi and found that these 'knockdowns' reduced AA-induced TOR signaling. We also determined the effect these knockdowns had on the number of eggs deposited following a blood meal.

CONCLUSIONS/SIGNIFICANCE

Our analysis stresses the importance of SLC7 transporters in TOR signaling pathway and mosquito reproduction.

Nerveless and gutsy: intestinal nutrient sensing from invertebrates to humans

The increasingly recognized role of gastrointestinal signals in the regulation of food intake, insulin production and peripheral nutrient storage has prompted a surge of interest in studying how the gastrointestinal tract senses and responds to nutritional information. Identification of metabolically important intestinal nutrient sensors could provide potential new drug targets for the treatment of diabetes, obesity and gastrointestinal disorders. From a more fundamental perspective, the study of intestinal chemosensation is revealing novel, non-neuronal modes of communication involving differentiated epithelial cells. It is also identifying signalling mechanisms downstream of not only canonical receptors but also nutrient transporters, thereby supporting a chemosensory role for “transceptors” in the intestine. This review describes known and proposed mechanisms of intestinal carbohydrate, protein and lipid sensing, best characterized in mammalian systems. It also highlights the potential of invertebrate model systems such as C. elegans and Drosophila melanogaster by summarizing known examples of molecular evolutionary conservation. Recently developed genetic tools in Drosophila, an emerging model system for the study of physiology and metabolism, allow the temporal, spatial and high-throughput manipulation of putative intestinal sensors. Hence, fruit flies may prove particularly suited to the study of the link between intestinal nutrient sensing and metabolic homeostasis.

A novel biorational pesticide: efficacy of methionine against Heraclides (Papilio) cresphontes, a surrogate of the invasive Princeps (Papilio) demoleus (Lepidoptera: Papilionidae)

ABSTRACT The Southeast Asian citrus-feeding butterfly Princeps (Papilio) demoleus (L.) was recently introduced into the Americas, causing an imminent threat to citrus production and ornamental flora. The human nutrient amino acid methionine has been shown by us to disrupt aminoacid-modulated ion transport systems in caterpillars and other insect larvae that possess an alkaline midgut. Heraclides (Papilio) cresphontes was bioassayed as a United States Department of Agriculture permitted surrogate of the Florida quarantined P. demoleus to test the potential efficacy of methionine. Larvae were allowed to feed ad libitum on wild lime plants with leaves treated with methionine or proline. Methionine caused 100% mortality in first through fourth instars in a time- and dose-dependent manner, as determined by probit analysis whereas proline was not toxic. Wild lime host plants did not exhibit phytotoxicity with methionine treatments during a 14 d test period. It is concluded that methionine is an effective larvicide against H. cresphontes, and therefore may be a candidate environmentally safe biorational pesticide for use against invasive P. demoleus in the Americas.

Novel male-biased expression in paralogs of the aphid slimfast nutrient amino acid transporter expansion

Background

A major goal of molecular evolutionary biology is to understand the fate and consequences of duplicated genes. In this context, aphids are intriguing because the newly sequenced pea aphid genome harbors an extraordinary number of lineage-specific gene duplications relative to other insect genomes. Though many of their duplicated genes may be involved in their complex life cycle, duplications in nutrient amino acid transporters appear to be associated rather with their essential amino acid poor diet and the intracellular symbiosis aphids rely on to compensate for dietary deficits. Past work has shown that some duplicated amino acid transporters are highly expressed in the specialized cells housing the symbionts, including a paralog of an aphid-specific expansion homologous to the Drosophila gene slimfast. Previous data provide evidence that these bacteriocyte-expressed transporters mediate amino acid exchange between aphids and their symbionts.

Results

We report that some nutrient amino acid transporters show male-biased expression. Male-biased expression characterizes three paralogs in the aphid-specific slimfast expansion, and the male-biased expression is conserved across two aphid species for at least two paralogs. One of the male-biased paralogs has additionally experienced an accelerated rate of non-synonymous substitutions.

Conclusions

This is the first study to document male-biased slimfast expression. Our data suggest that the male-biased aphid slimfast paralogs diverged from their ancestral function to fill a functional role in males. Furthermore, our results provide evidence that members of the slimfast expansion are maintained in the aphid genome not only for the previously hypothesized role in mediating amino acid exchange between the symbiotic partners, but also for sex-specific roles.

AaCAT1 of the yellow fever mosquito, Aedes aegypti: a novel histidine-specific amino acid transporter from the SLC7 family

Insect yolk protein precursor gene expression is regulated by nutritional and endocrine signals. A surge of amino acids in the hemolymph of blood-fed female mosquitoes activates a nutrient signaling system in the fat bodies, which subsequently derepresses yolk protein precursor genes and makes them responsive to activation by steroid hormones. Orphan transporters of the SLC7 family were identified as essential upstream components of the nutrient signaling system in the fat body of fruit flies and the yellow fever mosquito, Aedes aegypti. However, the transport function of these proteins was unknown. We report expression and functional characterization of AaCAT1, cloned from the fat body of A. aegypti. Expression of AaCAT1 transcript and protein undergoes dynamic changes during postembryonic development of the mosquito. Transcript expression was especially high in the third and fourth larval stages; however, the AaCAT1 protein was detected only in pupa and adult stages. Functional expression and analysis of AaCAT1 in Xenopus oocytes revealed that it acts as a sodium-independent cationic amino acid transporter, with unique selectivity to L-histidine at neutral pH (K(0.5)(L-His) = 0.34 ± 0.07 mM, pH 7.2). Acidification to pH 6.2 dramatically increases AaCAT1-specific His(+)-induced current. RNAi-mediated silencing of AaCAT1 reduces egg yield of subsequent ovipositions. Our data show that AaCAT1 has notable differences in its transport mechanism when compared with related mammalian cationic amino acid transporters. It may execute histidine-specific transport and signaling in mosquito tissues.

Neurotransmitter Transporter-Like: a male germline-specific SLC6 transporter required for Drosophila spermiogenesis

The SLC6 class of membrane transporters, known primarily as neurotransmitter transporters, is increasingly appreciated for its roles in nutritional uptake of amino acids and other developmentally specific functions. A Drosophila SLC6 gene, Neurotransmitter transporter-like (Ntl), is expressed only in the male germline. Mobilization of a transposon inserted near the 3' end of the Ntl coding region yields male-sterile mutants defining a single complementation group. Germline transformation with Ntl cDNAs under control of male germline-specific control elements restores Ntl/Ntl homozygotes to normal fertility, indicating that Ntl is required only in the germ cells. In mutant males, sperm morphogenesis appears normal, with elongated, individualized and coiled spermiogenic cysts accumulating at the base of the testes. However, no sperm are transferred to the seminal vesicle. The level of polyglycylation of Ntl mutant sperm tubulin appears to be significantly lower than that of wild type controls. Glycine transporters are the most closely related SLC6 transporters to Ntl, suggesting that Ntl functions as a glycine transporter in developing sperm, where augmentation of the cytosolic pool of glycine may be required for the polyglycylation of the massive amounts of tubulin in the fly's giant sperm. The male-sterile phenotype of Ntl mutants may provide a powerful genetic system for studying the function of an SLC6 transporter family in a model organism.

Cellular mechanisms of acid secretion in the posterior midgut of the larval mosquito (Aedes aegypti)

The gut contents of larval mosquitoes are alkalinized by the anterior midgut and reacidified by the posterior midgut. In the present study the cellular mechanisms of reacidification were studied in isolated, perfused posterior midgut by measuring the transepithelial voltage (V(te)) and the rate of acid secretion as indicated by the color change of m-cresol purple during intervals of perfusion stop. The lumen-positive V(te) and reacidification were significantly increased by serotonin (0.2 mumol l(-1)). The V-type H(+)-ATPase inhibitor concanamycin A (10 mumol l(-1)) on the luminal side inhibited acidification and decreased V(te). On the hemolymph side the carbonic anhydrase (CA) inhibitor acetazolamide (1 mmol l(-1)) almost abolished V(te), but had no effect on acidification. Similarly, hemolymph-side DIDS (0.1 mmol l(-1)), DPC (0.5 mmol l(-1)), amiloride (1 mmol l(-1)) and ouabain (2.5 mmol l(-1)) significantly reduced V(te), whereas Ba(2+) (5 mmol l(-1)) was without effect. DPC and amiloride also reduced V(te) when applied to the luminal side of the epithelium. Unilateral substitution of gluconate for Cl(-) affected V(te) in a way consistent with a greater permeability for Cl(-) versus Na(+). Cl(-) replacement in the lumen decreased V(te), whereas replacement on the hemolymph side increased it. Bilateral replacement left the control voltage unaffected. Na(+) replacement on either side of the tissue reduced V(te) to different degrees. Omission of luminal amino acids was followed by a significant decrease in V(te). Except for concanamycin A, none of the above manipulations impaired acidification, indicating that acidification requires only the apical proton pump. However, the chemical source of secreted H(+) is still unknown and needs to be investigated.

Transit through the flea vector induces a pretransmission innate immunity resistance phenotype in Yersinia pestis

Yersinia pestis, the agent of plague, is transmitted to mammals by infected fleas. Y. pestis exhibits a distinct life stage in the flea, where it grows in the form of a cohesive biofilm that promotes transmission. After transmission, the temperature shift to 37 degrees C induces many known virulence factors of Y. pestis that confer resistance to innate immunity. These factors are not produced in the low-temperature environment of the flea, however, suggesting that Y. pestis is vulnerable to the initial encounter with innate immune cells at the flea bite site. In this study, we used whole-genome microarrays to compare the Y. pestis in vivo transcriptome in infective fleas to in vitro transcriptomes in temperature-matched biofilm and planktonic cultures, and to the previously characterized in vivo gene expression profile in the rat bubo. In addition to genes involved in metabolic adaptation to the flea gut and biofilm formation, several genes with known or predicted roles in resistance to innate immunity and pathogenicity in the mammal were upregulated in the flea. Y. pestis from infected fleas were more resistant to phagocytosis by macrophages than in vitro-grown bacteria, in part attributable to a cluster of insecticidal-like toxin genes that were highly expressed only in the flea. Our results suggest that transit through the flea vector induces a phenotype that enhances survival and dissemination of Y. pestis after transmission to the mammalian host.

Leucine transport by the larval midgut of the parasitoid Aphidius ervi (Hymenoptera)

The larval midgut of the hymenopteran parasitoid Aphidius ervi accomplishes a large transport of nutrients from the lumen to the haemocoel, providing most of the organic molecules necessary for rapid insect development. l-amino acids in general, and leucine in particular, are efficiently accumulated in the larval body. We show here that the intact midgut of early 3rd instar larvae incubated in vitro can take up [(3)H]l-leucine from the basolateral side of the epithelium by transporters insensitive to the presence of monovalent cations. When the midgut is opened and the apical membrane of the absorbing epithelial cells is exposed to the medium containing radiolabelled leucine, a sodium-dependent uptake of the amino acid becomes apparent, disclosing the presence of a symport mechanism. Inhibition experiments of leucine uptake by a 100-fold excess of different amino acids, selected according to the properties of their side chain, revealed that this apical sodium-dependent mechanism is a broad spectrum transport system with a specialization for the absorption of aliphatic amino acids, that can also transfer glutamine and proline, but not phenylalanine, lysine and arginine. Altogether the experimental results obtained with intact- and open-gut preparations suggest that leucine transport across the basolateral membrane is mediated by both an uniporter and an obligatory amino acid exchange mechanism.

Characterization of a blood-meal-responsive proton-dependent amino acid transporter in the disease vector, Aedes aegypti

After anautogenous mosquitoes ingest the required blood meal, proteins in it are rapidly cleaved, yielding a large pool of amino acids. Transport of these amino acids into gut epithelial cells and their subsequent translocation into other tissues is critical for oogenesis and other physiological processes. We have identified a proton amino acid transporter (PAT) in Aedes aegypti (AaePAT1, AAEL007191) which facilitates this transport and is expressed in epithelial cell membranes of larval caecae and the adult midgut. AaePAT1 encodes a 475 amino acid protein showing high similarity to Anopheles gambiae AGAP009896, Culex pipiens CPIJ011438 and Drosophila melanogaster CG7888. When expressed in Xenopus oocytes the transport kinetics showed AaePAT1 is a low affinity transporter with low substrate specificity, having Km and Vmax values of about 7.2 mmol l(-1) and 69 pmol oocyte(-1) min(-1), respectively, for glutamine. A number of other amino acids are also transported by this PAT. In female adult midgut, AaePAT1 transcript levels were induced after ingestion of a blood meal.

Nutrient uptake by marine invertebrates: cloning and functional analysis of amino acid transporter genes in developing sea urchins (Strongylocentrotus purpuratus)

Transport of amino acids from low concentrations in seawater by marine invertebrates has been extensively studied, but few of the genes involved in this physiological process have been identified. We have characterized three amino acid transporter genes cloned from embryos of the sea urchin Strongylocentrotus purpuratus. These genes show phylogenetic proximity to classical amino acid transport systems, including Gly and B0+, and the inebriated gene (INE). Heterologous expression of these genes in frog oocytes induced a 40-fold increase in alanine transport above endogenous levels, demonstrating that these genes mediate alanine transport. Antibodies specific to one of these genes (Sp-AT1) inhibited alanine transport, confirming the physiological activity of this gene in larvae. Whole-mount antibody staining of larvae revealed expression of Sp-AT1 in the ectodermal tissues associated with amino acid transport, as independently demonstrated by autoradiographic localization of radioactive alanine. Maximum rates of alanine transport increased 6-fold during early development, from embryonic to larval stages. Analysis of gene expression during this developmental period revealed that Sp-AT1 transcript abundance remained nearly constant, while that of another transporter gene (Sp-AT2) increased 11-fold. The functional characterization of these genes establishes a molecular biological basis for amino acid transport by developmental stages of marine invertebrates.

Cloning and functional expression of the first eukaryotic Na+-tryptophan symporter, AgNAT6

The nutrient amino acid transporter (NAT) subfamily of the neurotransmitter sodium symporter family (NSS, also known as the solute carrier family 6, SLC6) represents transport mechanisms with putative synergistic roles in the absorption of essential and conditionally essential neutral amino acids. It includes a large paralogous expansion of insect-specific genes, with seven genes from the genome of the malaria mosquito, Anopheles gambiae. One of the An. gambiae NATs, AgNAT8, was cloned, functionally expressed and characterized in X. laevis oocytes as a cation-coupled symporter of aromatic amino acids, preferably l-phenylalanine, l-tyrosine and l-DOPA. To explore an evolutionary trend of NAT-SLC6 phenotypes, we have cloned and characterized AgNAT6, which represents a counterpart of AgNAT8 descending from a recent gene duplication (53.1% pairwise sequence identity). In contrast to AgNAT8, which preferably mediates the absorption of phenol-branched substrates, AgNAT6 mediates the absorption of indole-branched substrates with highest apparent affinity to tryptophan (K(0.5)(Trp)=1.3 micromol l(-1) vs K(0.5)(Phe)=430 micromol l(-1)) and [2 or 1 Na(+) or K(+)]:[aromatic substrate] stoichiometry. AgNAT6 is highly transcribed in absorptive and secretory regions of the alimentary canal and specific neuronal structures, including the neuropile of ventral ganglia and sensory afferents. The alignment of AgNATs and LeuT(Aa), a bacterial NAT with a resolved 3D structure, reveals three amino acid differences in the substrate-binding pocket that may be responsible for the indole- vs phenol-branch selectivity of AgNAT6 vs AgNAT8. The identification of transporters with a narrow selectivity for essential amino acids suggests that basal expansions in the SLC6 family involved duplication and retention of NATs, improving the absorption and distribution of under-represented essential amino acids and related metabolites. The identified physiological and expression profiles suggest unique roles of AgNAT6 in the active absorption of indole-branched substrates that are used in the synthesis of the neurotransmitter serotonin as well as the key circadian hormone and potent free-radical scavenger melatonin.

A putative amino acid transporter of the solute carrier 6 family is upregulated by lithium and is required for resistance to lithium toxicity in Drosophila

Lithium is an efficacious drug for the treatment of mood disorders, and its application is also considered a potential therapy for brain damage. However, the mechanisms underlying lithium's therapeutic action and toxic effects in the nervous system remain largely elusive. Here we report on the use of a versatile genetic model, the fruit fly Drosophila melanogaster, to discover novel molecular components involved in the lithium-responsive neurobiological process. We previously identified CG15088, which encodes a putative nutrient amino acid transporter of the solute carrier 6 (SLC6) family, as one of the genes most significantly upregulated in response to lithium treatment. This gene was the only SLC6 gene induced by lithium, and was thus designated as Lithium-inducible SLC6 transporter or List. Either RNA interference (RNAi)-mediated knockdown or complete deletion of List resulted in a remarkable increase in the susceptibility of adult flies to lithium's toxic effects, whereas transgenic expression of wild-type List significantly suppressed the lithium hypersensitive phenotype of List-deficient flies. Other ions such as sodium, potassium and chloride did not induce List upregulation, nor did they affect the viability of flies with suppressed List expression. These results indicate that lithium's biochemical or physical properties, rather than general osmotic responses, are responsible for the lithium-induced upregulation of List, as well as for the lithium-susceptible phenotype observed in List knockdown flies. Interestingly, flies became significantly more susceptible to lithium toxicity when List RNAi was specifically expressed in glia than when it was expressed in neurons or muscles, which is consistent with potential glial expression of List. These results show that the List transporter confers resistance to lithium toxicity, possibly as a consequence of its amino acid transporter activity in CNS glia. Our results have provided a new avenue of investigation toward a better understanding of the molecular and cellular mechanisms that underlie lithium-responsive neurobiological process.

Voltage coupling of primary H+ V-ATPases to secondary Na+- or K+-dependent transporters

This review provides alternatives to two well established theories regarding membrane energization by H(+) V-ATPases. Firstly, we offer an alternative to the notion that the H(+) V-ATPase establishes a protonmotive force (pmf) across the membrane into which it is inserted. The term pmf, which was introduced by Peter Mitchell in 1961 in his chemiosmotic hypothesis for the synthesis of ATP by H(+) F-ATP synthases, has two parts, the electrical potential difference across the phosphorylating membrane, Deltapsi, and the pH difference between the bulk solutions on either side of the membrane, DeltapH. The DeltapH term implies three phases - a bulk fluid phase on the H(+) input side, the membrane phase and a bulk fluid phase on the H(+) output side. The Mitchell theory was applied to H(+) V-ATPases largely by analogy with H(+) F-ATP synthases operating in reverse as H(+) F-ATPases. We suggest an alternative, voltage coupling model. Our model for V-ATPases is based on Douglas B. Kell's 1979 'electrodic view' of ATP synthases in which two phases are added to the Mitchell model - an unstirred layer on the input side and another one on the output side of the membrane. In addition, we replace the notion that H(+) V-ATPases normally acidify the output bulk solution with the hypothesis, which we introduced in 1992, that the primary action of a H(+) V-ATPase is to charge the membrane capacitance and impose a Deltapsi across the membrane; the translocated hydrogen ions (H(+)s) are retained at the outer fluid-membrane interface by electrostatic attraction to the anions that were left behind. All subsequent events, including establishing pH differences in the outside bulk solution, are secondary. Using the surface of an electrode as a model, Kell's 'electrodic view' has five phases - the outer bulk fluid phase, an outer fluid-membrane interface, the membrane phase, an inner fluid-membrane interface and the inner bulk fluid phase. Light flash, H(+) releasing and binding experiments and other evidence provide convincing support for Kell's electrodic view yet Mitchell's chemiosmotic theory is the one that is accepted by most bioenergetics experts today. First we discuss the interaction between H(+) V-ATPase and the K(+)/2H(+) antiporter that forms the caterpillar K(+) pump, and use the Kell electrodic view to explain how the H(+)s at the outer fluid-membrane interface can drive two H(+) from lumen to cell and one K(+) from cell to lumen via the antiporter even though the pH in the bulk fluid of the lumen is highly alkaline. Exchange of outer bulk fluid K(+) (or Na(+)) with outer interface H(+) in conjunction with (K(+) or Na(+))/2H(+) antiport, transforms the hydrogen ion electrochemical potential difference, mu(H), to a K(+) electrochemical potential difference, mu(K) or a Na(+) electrochemical potential difference, mu(Na). The mu(K) or mu(Na) drives K(+)- or Na(+)-coupled nutrient amino acid transporters (NATs), such as KAAT1 (K(+) amino acid transporter 1), which moves Na(+) and an amino acid into the cell with no H(+)s involved. Examples in which the voltage coupling model is used to interpret ion and amino acid transport in caterpillar and larval mosquito midgut are discussed.

NHE(VNAT): an H+ V-ATPase electrically coupled to a Na+:nutrient amino acid transporter (NAT) forms an Na+/H+ exchanger (NHE)

Glycolysis, the citric acid cycle and other metabolic pathways of living organisms generate potentially toxic acids within all cells. One ubiquitous mechanism for ridding cells of the acids is to expel H(+) in exchange for extracellular Na(+), mediated by electroneutral transporters called Na(+)/H(+) exchangers (NHEs) that are driven by Na(+) concentration gradients. The exchange must be important because the human genome contains 10 NHEs along with two Na(+)/H(+) antiporters (NHAs). By contrast, the genomes of two principal disease vector mosquitoes, Anopheles gambiae and Aedes aegypti, contain only three NHEs along with the two NHAs. This shortfall may be explained by the presence of seven nutrient amino acid transporters (NATs) in the mosquito genomes. NATs transport Na(+) stoichiometrically linked to an amino acid into the cells by a process called symport or co-transport. Three of the mosquito NATs and two caterpillar NATs have previously been investigated after heterologous expression in Xenopus laevis oocytes and were found to be voltage driven (electrophoretic). Moreover, the NATs are present in the same membrane as the H(+) V-ATPase, which generates membrane potentials as high as 120 mV. We review evidence that the H(+) V-ATPase moves H(+) out of the cells and the resulting membrane potential (V(m)) drives Na(+) linked to an amino acid into the cells via a NAT. The H(+) efflux by the V-ATPase and Na(+) influx by the NAT comprise the same ion exchange as that mediated by an NHE; so the V and NAT working together constitute an NHE that we call NHE(VNAT). As the H(+) V-ATPase is widely distributed in mosquito epithelial cells and there are seven NATs in the mosquito genomes, there are potentially seven NHE(VNAT)s that could replace the missing NHEs. We review published evidence in support of this hypothesis and speculate about broader functions of NHE(VNAT)s.

The invertebrate B(0) system transporter, D. melanogaster NAT1, has unique d-amino acid affinity and mediates gut and brain functions

The CG3252 gene product, DmNAT1, represents the first Nutrient Amino acid Transporter cloned from Drosophila. It absorbs a broader set of neutral amino acids versus earlier characterized insect NATs and mammalian NATs-B(0) system transporters from the Sodium Neurotransmitter symporter Family (SNF, a.k.a. solute carrier family 6, SLC6). In addition to B(0)-specific l-substrates, DmNAT1 equally or more effectively transports d-amino acids with sub-millimolar affinities and 1:1 sodium:amino acid transport stoichiometry. DmNAT1 is strongly transcribed in the absorptive and secretory regions of the larval alimentary canal and larval brain, revealing its roles in the primary absorption and redistribution of large neutral l-amino acids as well as corresponding d-isomers. The absorption of d-amino acids via DmNAT1 may benefit the acquisition of fermented and symbiotic products, and may support the unique capacity of fruit fly larvae to utilize a diet with substitution of essential amino acids by d-isomers. It also suggests a remarkable adaptive plasticity of NAT-SLC6 mechanisms via alterations of a few identifiable sites in the substrate-binding pocket. The strong transcription in the brain suggests roles for DmNAT1 in neuronal nutrition and clearance of l-neutral amino acids from the fly brain. In addition, neuronal DmNAT1 may absorb synaptic d-serine and modulate NMDA receptor-coupled signal transduction. The characterization of the first invertebrate B(0)-like transporter extends the biological roles of the SLC6 family, revealing adaptations for the absorption of d-isomers of the essential amino acids. These findings suggest that some members of the NAT-SLC6 subfamily are evolving specific properties which contribute to nutrient symbiotic relationships and neuronal functions.

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.

The evolutionary history and tissue mapping of amino acid transporters belonging to solute carrier families SLC32, SLC36, and SLC38

Members of the solute carrier families (SLC) 32, 36, and 38, together also designated the beta-group of SLCs, are known to transport neutral amino acids. In this paper, we show that these three families were present before the split of the animal lineage and that they are likely to share a common decent. We also show that the APF transporters found in plants are most likely homologous to the mammalian beta-group, suggesting that this type of transporters arouse early in the evolution of eukaryotes. We performed detailed tissue expression analysis of all the members of the beta-group in rat and found several examples of highly specific expression patterns, with SLC38A7 being exclusively found in liver, SLC38A5 in blood, and SLC38A4 in muscle and liver. Moreover, we found that SLC38A10 is expressed in several endocrine organs. We also found that SLC38A1 is highly up regulated in the cortex from rats treated with diazepam and that SLC38A2 is significantly down regulated in the same tissue. In addition, we performed a detailed expression analysis of SLC38A1 and SLC38A6 in mouse brain using in situ hybridization, which showed that both these transporters are widely expressed in the brain.

Functional comparison of full-length and N-terminal-truncated octopamine transporters from Lepidoptera

We have cloned two new lepidopteran octopamine transporters (OATs), members of the solute-linked carrier family 6 (SLC6) of nutrient transporters, from the CNS of the European corn borer Ostrinia nubilalis and the cabbage white Pieris rapae. Comparison of these sequences with the previously cloned OAT from the cabbage looper Trichoplusia ni showed that the T. ni OAT sequence previously reported was truncated by 74 amino acids at the N-terminus. The cytoplasmic N-termini deduced here are considerably longer than the N-termini of other monoamine transporters in the SLC6 family and contain many more high-probability serine- and threonine-phosphorylation sites. Monoamine uptake and competitive inhibition studies on baculovirus-infected Sf9 cells expressing these three cloned OATs indicate that they are able to transport tyramine, octopamine and dopamine with high affinity (K(m) and K(i) range, 0.4 microM-2.7 microM) and capacity ((3)H-dopamine uptake by TrnOAT, 2.5 pmol/well/min). We aimed to examine the role of the N-terminus of OAT by comparing the properties of the full-length T. ni OAT with those of the previously reported N-truncated version. Results for the new full-length T. ni OAT showed no difference in the protein's affinity for octopamine or dopamine, although at low levels of viral infection it did show slightly higher transport activity ((3)H-dopamine uptake by truncated TrnOAT, 1.5 pmol/well/min). Treatment of Sf9 cells expressing full-length or truncated TrnOAT with a variety of protein kinase activators and inhibitors, however, did not change transporter activity. Neither an intact N-terminus, nor apparently a particular phosphorylation state of this extended N-terminus, is required for OAT to transport monoamines.

Identification of a chloride ion binding site in Na+/Cl -dependent transporters

The recent determination of the crystal structure of the leucine transporter from Aquifex aeolicus (aaLeuT) has provided significant insights into the function of neurotransmitter:sodium symporters. Transport by aaLeuT is Cl(-) independent, whereas many neurotransmitter:sodium symporters from higher organisms depend on Cl(-) ions. However, the only Cl(-) ion identified in the aaLeuT structure interacts with nonconserved residues in extracellular loops, and thus the relevance of this binding site is unclear. Here, we use calculations of pK(A)s and homology modeling to predict the location of a functionally important Cl(-) binding site in serotonin transporter and other Cl(-)-dependent transporters. We validate our model through the site-directed mutagenesis of residues predicted to coordinate the Cl(-) ion and through the observation of sequence conservation patterns in other Cl(-)-dependent transporters. The proposed site is located midway across the membrane and is formed by residues from transmembrane helices 2, 6, and 7. It is close to the Na1 sodium binding site, thus providing an explanation for the coupling of Cl(-) and Na(+) ions during transport. Other implications of the model are also discussed.

A screen for neurotransmitter transporters expressed in the visual system of Drosophila melanogaster identifies three novel genes

The fly eye provides an attractive substrate for genetic studies, and critical transport activities for synaptic transmission and pigment biogenesis in the insect visual system remain unknown. We therefore screened for transporters in Drosophila melanogaster that are down-regulated by genetically ablating the eye. Using a large panel of transporter specific probes on Northern blots, we identified three transcripts that are down-regulated in flies lacking eye tissue. Two of these, CG13794 and CG13795, are part of a previously unknown subfamily of putative solute carriers within the neurotransmitter transporter family. The third, CG4476, is a member of a related subfamily that includes characterized nutrient transporters expressed in the insect gut. Using imprecise excision of a nearby transposable P element, we have generated a series of deletions in the CG4476 gene. In fast phototaxis assays, CG4476 mutants show a decreased behavioral response to light, and the most severe mutant behaves as if it were blind. These data suggest an unforeseen role for the "nutrient amino acid transporter" subfamily in the nervous system, and suggest new models to study transport function using the fly eye.

Structural and functional basis of amino acid specificity in the invertebrate cotransporter KAAT1

The substrate specificity of KAAT1, a Na+- and K+-dependent neutral amino acid cotransporter cloned from the larva of the invertebrate Manduca sexta and belonging to the SLC6A gene family has been investigated using electrophysiological and radiotracer methods. The specificity of KAAT1 was compared to that of CAATCH1, a strictly related transporter with different amino acid selectivity. Competition experiments between different substrates indicate that both transporters bind leucine more strongly than threonine and proline, the difference between KAAT1 and CAATCH1 residing in the incapacity of the latter to complete the transport cycle in presence of leucine. The behaviour of CAATCH1 is mimicked by the S308T mutant form of KAAT1, constructed on the basis of the atomic structure of a leucine-transporting bacterial member of the family, which indicates the participation of this residue in the leucine-binding site. The reverse mutation T308S in CAATCH1 conferred to this transporter the ability to transport leucine in presence of K+. These results may be interpreted by a kinetic scheme in which, in presence of Na+, the leucine-bound state of the transporter is relatively stable, while in presence of K+ and at negative potentials the progression of the leucine-bound form along the cycle is favoured. In this context serine 308 appears to be important in allowing the change to the inward-facing conformation of the transporter following substrate binding, rather than in determining the binding specificity.

Ancestry of neuronal monoamine transporters in the Metazoa

Selective Na+-dependent re-uptake of biogenic monoamines at mammalian nerve synapses is accomplished by three types of solute-linked carrier family 6 (SLC6) membrane transporter with high affinity for serotonin(SERTs), dopamine (DATs) and norepinephrine (NETs). An additional SLC6 monoamine transporter (OAT), is responsible for the selective uptake of the phenolamines octopamine and tyramine by insect neurons. We have characterized a similar high-affinity phenoloamine transporter expressed in the CNS of the earthworm Lumbricus terrestris. Phylogenetic analysis of its protein sequence clusters it with both arthropod phenolamine and chordate catecholamine transporters. To clarify the relationships among metazoan monoamine transporters we identified representatives in the major branches of metazoan evolution by polymerase chain reaction (PCR)-amplifying conserved cDNA fragments from isolated nervous tissue and by analyzing available genomic data. Analysis of conserved motifs in the sequence data suggest that the presumed common ancestor of modern-day Bilateria expressed at least three functionally distinct monoamine transporters in its nervous system: a SERT currently found throughout bilaterian phyla, a DAT now restricted in distribution to protostome invertebrates and echinoderms and a third monoamine transporter (MAT), widely represented in contemporary Bilateria, that is selective for catecholamines and/or phenolamines. Chordate DATs, NETs,epinephrine transporters (ETs) and arthropod and annelid OATs all belong to the MAT clade. Contemporary invertebrate and chordate DATs belong to different SLC6 clades. Furthermore, the genes for dopamine and norepinephrine transporters of vertebrates are paralogous, apparently having arisen through duplication of an invertebrate MAT gene after the loss of an invertebrate-type DAT gene in a basal protochordate.

Comparative sequence analysis and tissue localization of members of the SLC6 family of transporters in adult Drosophila melanogaster

The SLC6 family comprises proteins that move extracellular neurotransmitters, amino acids and osmolytes across the plasma membrane into the cytosol. In mammals, deletion of SLC6 family members has dramatic physiologic consequences, but in the model organism Drosophila melanogaster, little is known about this family of proteins. Therefore, in this study we carried out an initial analysis of 21 known or putative SLC6 family members from the Drosophila genome. Protein sequences from these genes segregated into either well-defined subfamilies, including the novel insect amino acid transporter subfamily, or into a group of weakly related sequences not affiliated with a recognized subfamily. Reverse transcription-polymerase chain reaction analysis and in situ hybridization showed that seven of these genes are expressed in the CNS. In situ hybridization revealed that two previously cloned SLC6 members, the serotonin and dopamine transporters, were localized to presumptive presynaptic neurons that previously immunolabelled for these transmitters. RNA for CG1732 (the putative GABA transporter) and CG15088 (a member of the novel insect amino acid transporter family) was localized in cells likely to be subtypes of glia, while RNA for CG5226, CG10804 (both members of the orphan neurotransmitter transporter subfamily) and CG5549 (a putative glycine transporter) were expressed broadly throughout the cellular cortex of the CNS. Eight of the 21 sequences were localized outside the CNS in the alimentary canal, Malpighian tubules and reproductive organs. Localization for six sequences was not found or not attempted in the adult fly. We used the Drosophila ortholog of the mammalian vesicular monoamine transporter 2, CG33528, to independently identify monoaminergic neurons in the adult fly. RNA for CG33528 was detected in a limited number of cells in the central brain and in a beaded stripe at the base of the photoreceptors in the position of glia, but not in the photoreceptors themselves. The SLC6 localization observations in conjunction with likely substrates based on phylogenetic inferences are a first step in defining the role of Na/Cl-dependent transporters in Drosophila physiology.

Identification of two cationic amino acid transporters required for nutritional signaling during mosquito reproduction

The defining characteristic of anautogenous mosquitoes is their requirement for a blood meal to initiate reproduction. The need for blood drives the association of vector and host, and is the primary reason why anautogenous mosquitoes are effective disease vectors. During mosquito vitellogenesis, a key process in reproduction, yolk protein precursor (YPP) gene expression is activated specifically in the fat body, the insect analogue of the vertebrate liver. We have demonstrated that blood meal derived amino acids (AAs) activate YPP genes via the target of rapamycin (TOR)-signal transduction pathway. Here we show, by stimulating fat bodies with balanced AA solutions lacking individual AAs, that specific cationic and branched AAs are essential for activation of the vitellogenin (vg) gene, the major YPP gene. Treatment of fat bodies with AA uptake inhibitors results in a strong inhibition of AA-induced vg gene expression proving that an active transport mechanism is necessary to transduce the AA signal. We identified two cationic AA transporters (CATs) in the fat body of Aedes aegypti females--Aa slimfast and iCAT2. RNAi knockdown of slimfast and iCAT2 results in a strong decrease in the response to AAs by the vg gene similar to that seen due to TOR inhibition. These data demonstrate that active uptake of specific AAs plays a key role in nutritional signaling during the onset of vitellogenic gene expression in mosquitoes and it is mediated by two cationic AA transporters.

Molecular characterization of the first aromatic nutrient transporter from the sodium neurotransmitter symporter family

Nutrient amino acid transporters (NATs, subfamily of sodium neurotransmitter symporter family SNF, a.k.a. SLC6) represent a set of phylogenetically and functionally related transport proteins, which perform intracellular absorption of neutral, predominantly essential amino acids. Functions of NATs appear to be critical for the development and survival in organisms. However, mechanisms of specific and synergetic action of various NAT members in the amino acid transport network are virtually unexplored. A new transporter, agNAT8, was cloned from the malaria vector mosquito Anopheles gambiae (SS). Upon heterologous expression in Xenopus oocytes it performs high-capacity, sodium-coupled (2:1)uptake of nutrients with a strong preference for aromatic catechol-branched substrates, especially phenylalanine and its derivatives tyrosine and L-DOPA,but not catecholamines. It represents a previously unknown SNF phenotype, and also appears to be the first sodium-dependent B0 type transporter with a narrow selectivity for essential precursors of catecholamine synthesis pathways. It is strongly and specifically transcribed in absorptive and secretory parts of the larval alimentary canal and specific populations of central and peripheral neurons of visual-, chemo- and mechano-sensory afferents. We have identified a new SNF transporter with previously unknown phenotype and showed its important role in the accumulation and redistribution of aromatic substrates. Our results strongly suggest that agNAT8 is an important, if not the major, provider of an essential catechol group in the synthesis of catecholamines for neurochemical signaling as well as ecdysozoan melanization and sclerotization pathways, which may include cuticle hardening/coloring, wound curing, oogenesis, immune responses and melanization of pathogens.

The orphan transporter v7-3 (slc6a15) is a Na+-dependent neutral amino acid transporter (B0AT2)

Transporters of the SLC6 (solute carrier 6) family play an important role in the removal of neurotransmitters in brain tissue and in amino acid transport in epithelial cells. In the present study, we demonstrate that mouse v7-3 (slc6a15) encodes a transporter for neutral amino acids. The transporter is functionally and sequence related to B(0)AT1 (slc6a19) and was hence named B(0)AT2. Leucine, isoleucine, valine, proline and methionine were recognized by the transporter, with values of K(0.5) (half-saturation constant) ranging from 40 to 200 microM. Alanine, glutamine and phenylalanine were low-affinity substrates of the transporter, with K(0.5) values in the millimolar range. Transport of neutral amino acids via B(0)AT2 was Na+-dependent, Cl--independent and electrogenic. Superfusion of mouse B(0)AT2-expressing oocytes with amino acid substrates generated robust inward currents. Na+-activation kinetics of proline transport and uptake under voltage clamp suggested a 1:1 Na+/amino acid co-transport stoichiometry. Susbtrate and co-substrate influenced each other's K(0.5) values, suggesting that they share the same binding site. A mouse B(0)AT2-like transport activity was detected in synaptosomes and cultured neurons. A potential role of B(0)AT2 in transporting neurotransmitter precursors and neuromodulators is proposed.

Characterization of a branched-chain amino-acid transporter SBAT1 (SLC6A15) that is expressed in human brain

The SLC6 gene family comprises membrane proteins that transport neurotransmitters, amino acids, or osmolytes. We report the first functional characterization of the human SLC6A15 gene, which codes for a sodium-coupled branched-chain amino-acid transporter 1 (SBAT1). SBAT1 expression is specific to the brain. When expressed in Xenopus oocytes, SBAT1 mediated Na+-coupled transport of hydrophobic, zwitterionic alpha-amino and imino acids. SBAT1 exhibited a strong preference for branched-chain amino acids (BCAA) and methionine (K0.5 80-160 microM). SBAT1 excluded aromatic or charged amino acids, beta-amino acids, glycine, and GABA. SBAT1-mediated transport of amino or imino acids was extremely temperature-dependent (Q10=9) and was inhibited at acidic pH. PKC activation reduced the plasma-membrane population of SBAT1 protein. SBAT1-mediated transport of BCAA, particularly leucine, may be an important source of amino nitrogen for neurotransmitter synthesis in glutamatergic and GABAergic neurons.