Molecular cloning and functional characterization of a GABA transporter from the CNS of the cabbage looper, Trichoplusia ni

Maternal GABAergic and GnRH/corazonin pathway modulates egg diapause phenotype of the silkworm Bombyx mori

Diapause represents a major developmental switch in insects and is a seasonal adaptation that evolved as a specific subtype of dormancy in most insect species to ensure survival under unfavorable environmental conditions and synchronize populations. However, the hierarchical relationship of the molecular mechanisms involved in the perception of environmental signals to integration in morphological, physiological, behavioral, and reproductive responses remains unclear. In the bivoltine strain of the silkworm Bombyx mori, embryonic diapause is induced transgenerationally as a maternal effect. Progeny diapause is determined by the environmental temperature during embryonic development of the mother. Here, we show that the hierarchical pathway consists of a γ-aminobutyric acid (GABA)ergic and corazonin signaling system modulating progeny diapause induction via diapause hormone release, which may be finely tuned by the temperature-dependent expression of plasma membrane GABA transporter. Furthermore, this signaling pathway possesses similar features to the gonadotropin-releasing hormone (GnRH) signaling system for seasonal reproductive plasticity in vertebrates.

Transcriptomic analysis of the Malpighian tubules of Trichoplusia ni: Clues to mechanisms for switching from ion secretion to ion reabsorption in the distal ileac plexus

Excretion of metabolic wastes and toxins in insect Malpighian tubules (MTs) is coupled to secretion of ions and fluid. Larval lepidopterans demonstrate a complex and regionalized MT morphology, and recent studies of larvae of the cabbage looper, Trichoplusia ni, have revealed several unusual aspects of ion transport in the MTs. Firstly, cations are reabsorbed via secondary cells (SCs) in T. ni, whereas in most insects SCs secrete ions. Secondly, SCs are coupled to neighbouring principal cells (PCs) via gap junctions to enable such ion reabsorption. Thirdly, PCs in the SC-containing distal ileac plexus (DIP) region of the tubule reverse from cation secretion to reabsorption in response to dietary ion loading. Lastly, antidiuresis is observed in response to a kinin neuropeptide, which targets both PCs and SCs, whereas in most insects kinins are diuretics that act exclusively via SCs. Recent studies have generated a basic model of ion transport in the DIP of the larval T. ni. RNAseq was used to elucidate previously uncharacterised aspects of ion transport and endocrine regulation in the DIP, with the aim of painting a composite picture of ion transport and identifying putative regulatory mechanisms of ion transport reversal in this tissue. Results indicated an overall expression of 9103 transcripts in the DIP, 993 and 382 of which were differentially expressed in the DIP of larvae fed high-K⁺ and high-Na⁺ diets respectively. Differentially expressed transcripts include ion-motive ATPases, ion channels and co-transporters, aquaporins, nutrient and xenobiotic transporters, cell adhesion and junction components, and endocrine receptors. Notably, several transcripts for voltage-gated ion channels and cell volume regulation-associated products were detected in the DIP and differentially expressed in larvae fed ion-rich diet. The study provides insights into the transport of solutes (sugars, amino acids, xenobiotics, phosphate and inorganic ions) by the DIP of lepidopterans. Our data suggest that this region of the MT in lepidopterans (as previously reported) transports cations, fluid, and xenobiotics/toxic metals. Besides this, the DIP expresses genes coding for the machinery involved in Na⁺- and H⁺-dependent reabsorption of solutes, chloride transport, and base recovery. Additionally, many of the transcripts expressed by the DIP a capacity of this region to respond to, process, and sometimes produce, neuropeptides, steroid hormones and neurotransmitters. Lastly, the DIP appears to possess an arsenal of septate junction components, differential expression of which may indicate junctional restructuring in the DIP of ion-loaded larvae.

Functional characterization of the GABA transporter GAT-1 from the deep-sea mussel Bathymodiolus septemdierum

Mammalian γ-aminobutyric acid (GABA) transporter subtype 1 (GAT-1) is a specific transporter for GABA, an inhibitory neurotransmitter in GABA-ergic neurons. GAT-1 belongs to the GAT group, in which five related transporters, GAT-2, GAT-3, GAT-4, CT1, and TAUT are known in mammals. By contrast, the deep-sea mussel, Bathymodiolus septemdierum has only two GAT group members, BsGAT-1 and BsTAUT, and their function in environmental adaptation is of interest to better understand the physiology of deep-sea organisms. Compared with BsTAUT, the function of BsGAT-1 is unknown. Here, we report the functional characterization of BsGAT-1. Analyses of BsGAT-1 expressed in Xenopus oocytes showed that it could transport GABA in a Na⁺- and Cl^--dependent manner, with Km and Vmax values of 0.58 μM and 1.97 pmol/oocyte/h, respectively. BsGAT-1 activity was blocked by the GAT-1 selective inhibitors SKF89976A and ACHC. Competition assays indicated that BsGAT-1 has no affinity for taurine and thiotaurine. These characteristics were common with those of mammalian GAT-1, suggesting its conserved function in the nervous system. However, BsGAT-1 showed a certain affinity for hypotaurine, which is involved in sulfide detoxification in hydrothermal vent-specific animals. This result suggests an additional role for BsGAT-1 in sulfide detoxification, which may be specific to the deep-sea mussel. In a tissue distribution analysis, BsGAT-1 mRNA expression was observed in various tissues. The expression in the adductor and byssus retractor muscles, labial palp, and foot, which possibly contain ganglia, suggested a function in the neural system, while BsGAT-1 expression in other tissues might be related to sulfide detoxification.

Neuron-glia interactions through the Heartless FGF receptor signaling pathway mediate morphogenesis of Drosophila astrocytes

Astrocytes are critically important for neuronal circuit assembly and function. Mammalian protoplasmic astrocytes develop a dense ramified meshwork of cellular processes to form intimate contacts with neuronal cell bodies, neurites, and synapses. This close neuron-glia morphological relationship is essential for astrocyte function, but it remains unclear how astrocytes establish their intricate morphology, organize spatial domains, and associate with neurons and synapses in vivo. Here we characterize a Drosophila glial subtype that shows striking morphological and functional similarities to mammalian astrocytes. We demonstrate that the Fibroblast growth factor (FGF) receptor Heartless autonomously controls astrocyte membrane growth, and the FGFs Pyramus and Thisbe direct astrocyte processes to ramify specifically in CNS synaptic regions. We further show that the shape and size of individual astrocytes are dynamically sculpted through inhibitory or competitive astrocyte-astrocyte interactions and Heartless FGF signaling. Our data identify FGF signaling through Heartless as a key regulator of astrocyte morphological elaboration in vivo.

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.

Localization of a GABA transporter to glial cells in the developing and adult olfactory pathway of the moth Manduca sexta

Glial cells have several critical roles in the developing and adult olfactory (antennal) lobe of the moth Manduca sexta. Early in development, glial cells occupy discrete regions of the developing olfactory pathway and processes of gamma-aminobutyric acid (GABA)ergic neurons extend into some of these regions. Because GABA is known to have developmental effects in a variety of systems, we explored the possibility that the glial cells express a GABA transporter that could regulate GABA levels to which olfactory neurons and glial cells are exposed. By using an antibody raised against a characterized high-affinity M. sexta GABA transporter with high sequence homology to known mammalian GABA transporters (Mbungu et al. [1995] Arch. Biochem. Biophys. 318:489-497; Umesh and Gill [2002] J. Comp. Neurol. 448:388-398), we found that the GABA transporter is localized to subsets of centrally derived glial cells during metamorphic adult development. The transporter persists into adulthood in a subset of the neuropil-associated glial cells, but its distribution pattern as determined by light-and electron-microscopic-level immunocytochemistry indicates that it could not serve to regulate GABA concentration in the synaptic cleft. Instead, its role is more likely to regulate extracellular GABA levels within the glomerular neuropil. Expression in the sorting zone glial cells disappears after the period of olfactory receptor axon ingrowth, but may be important during ingrowth if GABA regulates axon growth. Glial cells take up GABA, and that uptake can be blocked by L-2,4-diaminobutyric acid (DABA). This is the first molecular evidence that the central glial cell population in this pathway is heterogeneous.

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.

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.

GABAergic modulation of motor-driven behaviors in juvenileDrosophila and evidence for a nonbehavioral role for GABA transport

We have identified specific GABAergic-modulated behaviors in the juvenile stage of the fruit fly, Drosophila melanogaster via systemic treatment of second instar larvae with the potent GABA transport inhibitor DL-2,4-diaminobutyric acid (DABA). DABA significantly inhibited motor-controlled body wall and mouth hook contractions and impaired rollover activity and contractile responses to touch stimulation. The perturbations in locomotion and rollover activity were reminiscent of corresponding DABA-induced deficits in locomotion and the righting reflex observed in adult flies. The effects were specific to these motor-controlled behaviors, because DABA-treated larvae responded normally in olfaction and phototaxis assays. Recovery of these behaviors was achieved by cotreatment with the vertebrate GABA(A) receptor antagonist picrotoxin. Pharmacological studies performed in vitro with plasma membrane vesicles isolated from second instar larval tissues verified the presence of high-affinity, saturable GABA uptake mechanisms. GABA uptake was also detected in plasma membrane vesicles isolated from behaviorally quiescent stages. Competitive inhibition studies of [3H]-GABA uptake into plasma membrane vesicles from larval and pupal tissues with either unlabeled GABA or the transport inhibitors DABA, nipecotic acid, or valproic acid, revealed differences in affinities. GABAergic-modulation of motor behaviors is thus conserved between the larval and adult stages of Drosophila, as well as in mammals and other vertebrate species. The pharmacological studies reveal shared conservation of GABA transport mechanisms between Drosophila and mammals, and implicate the involvement of GABA and GABA transporters in regulating physiological processes distinct from neurotransmission during behaviorally quiescent stages of development.

Expression and functional characterization of GABA transporters in crayfish neurosecretory cells

The effect of GABA on membrane potential and ionic currents of X-organ neurons isolated from the crayfish eyestalk was investigated. Under voltage-clamp conditions, GABA elicited an inward Na+ current followed by a sustained outward chloride current. Sodium current was partially blocked in a dose-dependent manner by antagonists of GABA plasma membrane transporters such as beta-alanine, nipecotic acid, 1-[2([(diphenylmethylene)imino]oxy)ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride (NO 711), and SKF89976-A at concentrations between 1 and 100 microm. This current was totally blocked by the combined application of NO 711 (5 microm) and beta-alanine (50 microm). We obtained an EC(50) of 5 microm and a Hill coefficient of 0.97 for the GABA transport mediated response. These results together with studies of immunolocalization using antibodies against neuronal vertebrate GABA transporters (GATs) indicate the presence of GAT-1- and GAT-3-like proteins in X-organ neurons. To isolate the sustained outward Cl- current, extracellular free sodium solution was used to minimize the contribution of GAT activity. We concluded that this current was caused by the activation of GABA(A)-like receptors with an EC50 of 10 microm and a Hill number of 1.7. To assign a functional role to the GATs in the X-organ sinus gland system, we determine the GABA concentration (0.46-0.15 microm) in hemolymph samples using HPLC. In summary, our results suggest that a sodium-dependent electrogenic GABA uptake mechanism has a direct influence on the excitability of the X-organ neurons, maintaining an excitatory tone that is dependent on the circulating GABA level.

Immunocytochemical localization of a Manduca sexta gamma-aminobutyric acid transporter

Gamma-aminobutyric acid (GABA) is a major inhibitory neurotransmitter in insect central and peripheral nervous systems. Although much work has focused on the downstream targets of GABA, signal termination at insect GABAergic synapses has received very little attention. One of the major mechanisms of terminating synaptic transmission involves transport of the neurotransmitter molecules into presynaptic neurons or surrounding glia. Here we report the immunolocalization of a GABA transporter in the tobacco hornworm, Manduca sexta (MasGAT), using an affinity-purified antibody developed to the C-terminus. This is the first demonstration of an insect neurotransmitter transporter immunolocalization study. Results showed strong staining in the neuropil regions of embryonic, larval, and pharate adult central nervous system. Expression pattern in the pharate adult brain mostly mimicked that observed for GABA, with staining in parts of the optic and antennal lobes, mushroom body, lateral protocerebrum, and central complex. Certain longitudinal and lateral connectives of ganglia were observed to have immunostained fibers representing axons. These data support the view that GABA is involved in visual and olfactory processing in the insect brain.

Pharmacological evidence for GABAergic regulation of specific behaviors in Drosophila melanogaster

We have identified several GABAergic-modulated behaviors in Drosophila melanogaster by employing a pharmacological approach to disrupt GABA transporter function in vivo. Systemic treatment of adult female flies with the GABA transport inhibitors DL-2,4-diaminobutyric acid (DABA) or R,S-nipecotic acid (NipA), resulted in diminished locomotor activity, deficits in geotaxis, and the induction of convulsive behaviors with a secondary loss of the righting reflex. Pharmacological evidence suggested that the observed behavioral phenotypes were specific to disruption of GABA transporter function and GABAergic activity. The effects of GABA reuptake inhibitors on locomotor activity were dose dependent, pharmacologically distinct, and paralleled their known effects in mammalian systems. Recovery of normal locomotor activity and the righting reflex in DABA- and NipA-treated flies was achieved by coadministration of bicuculline (BIC), a GABA receptor antagonist that supresses GABAergic activity in mammals. Recovery of these behaviors was also achieved by coadministration of gabapentin, an anticonvulsant agent that interacts with mammalian GABAergic systems. Finally, behavioral effects were selective because other specific behaviors such as feeding activity and female sexual receptivity were not affected. Related pharmacological analyses performed in vitro on isolated Drosophila synaptic plasma membrane vesicles demonstrated high affinity, saturable uptake mechanisms for [3H]-GABA; further competitive inhibition studies with DABA and NipA demonstrated their ability to inhibit [3H]-GABA transport. The existence of experimentally accessible GABA transporters in Drosophila that share conserved pharmacological properties with their mammalian counterparts has resulted in the identification of specific behaviors that are modulated by GABA.