A neurotransmitter transporter encoded by the Drosophila inebriated gene

Syntaxin 1 Ser14 phosphorylation is required for nonvesicular dopamine release

Amphetamine (AMPH) is a psychostimulant that is commonly abused. The stimulant properties of AMPH are associated with its ability to increase dopamine (DA) neurotransmission. This increase is promoted by nonvesicular DA release mediated by reversal of DA transporter (DAT) function. Syntaxin 1 (Stx1) is a SNARE protein that is phosphorylated at Ser14 by casein kinase II. We show that Stx1 phosphorylation is critical for AMPH-induced nonvesicular DA release and, in Drosophila melanogaster, regulates the expression of AMPH-induced preference and sexual motivation. Our molecular dynamics simulations of the DAT/Stx1 complex demonstrate that phosphorylation of these proteins is pivotal for DAT to dwell in a DA releasing state. This state is characterized by the breakdown of two key salt bridges within the DAT intracellular gate, causing the opening and hydration of the DAT intracellular vestibule, allowing DA to bind from the cytosol, a mechanism that we hypothesize underlies nonvesicular DA release.

Location and functions of Inebriated in the Drosophila eye

Histamine (HA) is a neurotransmitter in arthropod photoreceptors. It is recycled via conjugation to β-alanine to form β-alanylhistamine (carcinine). Conjugation occurs in epithelial glia that surround photoreceptor terminals in the first optic neuropil, and carcinine (CA) is then transported back to photoreceptors and cleaved to liberate HA and β-alanine. The gene Inebriated (Ine) encodes an Na⁺/Cl^--dependent SLC6 family transporter translated as two protein isoforms, long (P1) and short (P2). Photoreceptors specifically express Ine-P2 whereas Ine-P1 is expressed in non-neuronal cells. Both ine¹ and ine³ have significantly reduced head HA contents compared with wild type, and a smaller increase in head HA after drinking 1% CA. Similarly, uptake of 0.1% CA was reduced in ine¹ and ine³ mutant synaptosomes, but increased by 90% and 84% respectively for fractions incubated in 0.05% β-Ala, compared with wild type. Screening potential substrates in Ine expressing Xenopus oocytes revealed very little response to carcinine and β-Ala but increased conductance with glycine. Both ine¹ and ine³ mutant responses in light-dark phototaxis did not differ from wild-type. Collectively our results suggest that Inebriated functions in an adjunct role as a transporter to the previously reported carcinine transporter CarT.

The fruit fly Drosophila melanogaster as an innovative preclinical ADME model for solute carrier membrane transporters, with consequences for pharmacology and drug therapy

Solute carrier membrane transporters (SLCs) control cell exposure to small-molecule drugs, thereby contributing to drug efficacy and failure and/or adverse effects. Moreover, SLCs are genetically linked to various diseases. Hence, in-depth knowledge of SLC function is fundamental for a better understanding of disease pathophysiology and the drug development process. Given that the model organism Drosophila melanogaster (fruit fly) expresses SLCs, such as for the excretion of endogenous and toxic compounds by the hindgut and Malpighian tubules, equivalent to human intestine and kidney, this system appears to be a promising preclinical model to use to study human SLCs. Here, we systematically compare current knowledge of SLCs in Drosophila and humans and describe the Drosophila model as an innovative tool for drug development.

Two novel forms of ERG oscillation in Drosophila: age and activity dependence

Over an animal's lifespan, neuronal circuits and systems often decline in an inherently heterogeneous fashion. To compare the age-dependent progression of changes in visual behavior with alterations in retinal physiology, we examined phototaxis and electroretinograms (ERGs) in a wild-type D. melanogaster strain (Canton-S) across their lifespan. In aged flies (beyond 50% median lifespan), we found a marked decline in phototaxis, while motor coordination was less disrupted, as indicated by relatively stronger negative geotaxis. These aged flies displayed substantially reduced ERG transient amplitudes while the receptor potentials (RP) remained largely intact. Using a repetitive light flash protocol, we serendipitously discovered two forms of activity-dependent oscillation in the ERG waveforms of young flies: 'light-off' and 'light-on' oscillations. After repeated 500 ms light flashes, light-off oscillations appeared during the ERG off-transients (frequency: 50-120 Hz, amplitude: ∼1 mV). Light-on oscillations (100-200 Hz, ∼0.3 mV) were induced by a series of 50 ms flashes, and were evident during the ERG on-transients. Both forms of oscillation were observed in other strains of D. melanogaster (Oregon-R, Berlin), additional Drosophila species (D. funerbris, D. euronotus, D. hydei, D. americana), and were evoked by a variety of light sources. Both light-off and light-on oscillations were distinct from previously described ERG oscillations in the visual mutant rosA in terms of location within the waveform and frequency. However, within rosA mutants, light-off oscillations, but not light-on oscillations could be recruited by the repetitive light flash protocol. Importantly though, we found that both forms of oscillation were rarely observed in aged flies. Although the physiological bases of these oscillations remain to be elucidated, they may provide important clues to age-related changes in neuronal excitability and synaptic transmission.

Lithium-Responsive Seizure-Like Hyperexcitability Is Caused by a Mutation in the Drosophila Voltage-Gated Sodium Channel Gene paralytic

Shudderer (Shu) is an X-linked dominant mutation in Drosophila melanogaster identified more than 40 years ago. A previous study showed that Shu caused spontaneous tremors and defects in reactive climbing behavior, and that these phenotypes were significantly suppressed when mutants were fed food containing lithium, a mood stabilizer used in the treatment of bipolar disorder (Williamson, 1982). This unique observation suggested that the Shu mutation affects genes involved in lithium-responsive neurobiological processes. In the present study, we identified Shu as a novel mutant allele of the voltage-gated sodium (Na_v) channel gene paralytic (para). Given that hypomorphic para alleles and RNA interference-mediated para knockdown reduced the severity of Shu phenotypes, Shu was classified as a para hypermorphic allele. We also demonstrated that lithium could improve the behavioral abnormalities displayed by other Na_v mutants, including a fly model of the human generalized epilepsy with febrile seizures plus. Our electrophysiological analysis of Shu showed that lithium treatment did not acutely suppress Na_v channel activity, indicating that the rescue effect of lithium resulted from chronic physiological adjustments to this drug. Microarray analysis revealed that lithium significantly alters the expression of various genes in Shu, including those involved in innate immune responses, amino acid metabolism, and oxidation-reduction processes, raising the interesting possibility that lithium-induced modulation of these biological pathways may contribute to such adjustments. Overall, our findings demonstrate that Na_v channel mutants in Drosophila are valuable genetic tools for elucidating the effects of lithium on the nervous system in the context of neurophysiology and behavior.

Impacts of a newly identified behaviour-altering trematode on its host amphipod: from the level of gene expression to population

Changes to host behaviour induced by some trematode species, as a means of increased trophic transmission, represents one of the seminal examples of host manipulation by a parasite. The amphipod Echinogammarus marinus (Leach, 1815) is infected with a previously undescribed parasite, with infected individuals displaying positive phototaxic and negative geotaxic behaviour. This study reveals that the unknown parasite encysts in the brain, nerve cord and the body cavity of E. marinus, and belongs to the Microphallidae family. An 18 month population study revealed that host abundance significantly and negatively correlated with parasite prevalence. Investigation of the trematode's influence at the transcriptomic level revealed genes with putative neurological functions, such as serotonin receptor 1A, an inebriated-like neurotransmitter, tryptophan hydroxylase and amino acid decarboxylase, present consistent altered expression in infected animals. Therefore, this study provides one of the first transcriptomic insights into the neuronal gene pathways altered in amphipods infected with a trematode parasite associated with changes to its host's behaviour and population structure.

The putative Na⁺/Cl⁻-dependent neurotransmitter/osmolyte transporter inebriated in the Drosophila hindgut is essential for the maintenance of systemic water homeostasis

Most organisms are able to maintain systemic water homeostasis over a wide range of external or dietary osmolarities. The excretory system, composed of the kidneys in mammals and the Malpighian tubules and hindgut in insects, can increase water conservation and absorption to maintain systemic water homeostasis, which enables organisms to tolerate external hypertonicity or desiccation. However, the mechanisms underlying the maintenance of systemic water homeostasis by the excretory system have not been fully characterized. In the present study, we found that the putative Na⁺/Cl⁻-dependent neurotransmitter/osmolyte transporter inebriated (ine) is expressed in the basolateral membrane of anterior hindgut epithelial cells. This was confirmed by comparison with a known basolateral localized protein, the α subunit of Na⁺-K⁺ ATPase (ATPα). Under external hypertonicity, loss of ine in the hindgut epithelium results in severe dehydration without damage to the hindgut epithelial cells, implicating a physiological failure of water conservation/absorption. We also found that hindgut expression of ine is required for water conservation under desiccating conditions. Importantly, specific expression of ine in the hindgut epithelium can completely restore disrupted systemic water homeostasis in ine mutants under both conditions. Therefore, ine in the Drosophila hindgut is essential for the maintenance of systemic water homeostasis.

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.

Activity and coexpression of Drosophila black with ebony in fly optic lobes reveals putative cooperative tasks in vision that evade electroretinographic detection

Drosophila mutants black and ebony show pigmentation defects in the adult cuticle, which disclose their cooperative activity in β-alanyl-dopamine formation. In visual signal transduction, Ebony conjugates β-alanine to histamine, forming β-alanyl-histamine or carcinine. Mutation of ebony disrupts signal transduction and reveals an electroretinogram (ERG) phenotype. In contrast to the corresponding cuticle phenotype of black and ebony, there is no ERG phenotype observed when black expression is disrupted. This discrepancy calls into question the longstanding assumption of Black and Ebony interaction. The purpose of this study was to investigate the role of Black and Ebony in fly optic lobes. We excluded a presynaptic histamine uptake pathway and confirmed histamine recycling via carcinine formation in glia. β-Alanine supply for this pathway is independent of enzymatic synthesis by Black and β-alanine synthase Pyd3. Two versions of Black are expressed in vivo. Black is a specific aspartate decarboxylase with no activity on glutamate. RNA in situ hybridization and anti-Black antisera localized Black expression in the head. Immunolabeling revealed expression in lamina glia, in large medulla glia, in glia of the ocellar ganglion, and in astrocyte-like glia below the ocellar ganglion. In these glia types, Black expression is strictly accompanied by Ebony expression. Activity, localization, and strict coexpression with Ebony strongly indicate a specific mode of functional interaction that, however, evades ERG detection.

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.

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.

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.

The functional organisation of glia in the adult brain of Drosophila and other insects

This review annotates and categorises the glia of adult Drosophila and other model insects and analyses the developmental origins of these in the Drosophila optic lobe. The functions of glia in the adult vary depending upon their sub-type and location in the brain. The task of annotating glia is essentially complete only for the glia of the fly's lamina, which comprise: two types of surface glia-the pseudocartridge and fenestrated glia; two types of cortex glia-the distal and proximal satellite glia; and two types of neuropile glia-the epithelial and marginal glia. We advocate that the term subretinal glia, as used to refer to both pseudocartridge and fenestrated glia, be abandoned. Other neuropiles contain similar glial subtypes, but other than the antennal lobes these have not been described in detail. Surface glia form the blood brain barrier, regulating the flow of substances into and out of the nervous system, both for the brain as a whole and the optic neuropiles in particular. Cortex glia provide a second level of barrier, wrapping axon fascicles and isolating neuronal cell bodies both from neighbouring brain regions and from their underlying neuropiles. Neuropile glia can be generated in the adult and a subtype, ensheathing glia, are responsible for cleaning up cellular debris during Wallerian degeneration. Both the neuropile ensheathing and astrocyte-like glia may be involved in clearing neurotransmitters from the extracellular space, thus modifying the levels of histamine, glutamate and possibly dopamine at the synapse to ultimately affect behaviour.

The role of carcinine in signaling at the Drosophila photoreceptor synapse

The Drosophila melanogaster photoreceptor cell has long served as a model system for researchers focusing on how animal sensory neurons receive information from their surroundings and translate this information into chemical and electrical messages. Electroretinograph (ERG) analysis of Drosophila mutants has helped to elucidate some of the genes involved in the visual transduction pathway downstream of the photoreceptor cell, and it is now clear that photoreceptor cell signaling is dependent upon the proper release and recycling of the neurotransmitter histamine. While the neurotransmitter transporters responsible for clearing histamine, and its metabolite carcinine, from the synaptic cleft have remained unknown, a strong candidate for a transporter of either substrate is the uncharacterized inebriated protein. The inebriated gene (ine) encodes a putative neurotransmitter transporter that has been localized to photoreceptor cells in Drosophila and mutations in ine result in an abnormal ERG phenotype in Drosophila. Loss-of-function mutations in ebony, a gene required for the synthesis of carcinine in Drosophila, suppress components of the mutant ine ERG phenotype, while loss-of-function mutations in tan, a gene necessary for the hydrolysis of carcinine in Drosophila, have no effect on the ERG phenotype in ine mutants. We also show that by feeding wild-type flies carcinine, we can duplicate components of mutant ine ERGs. Finally, we demonstrate that treatment with H₃ receptor agonists or inverse agonists rescue several components of the mutant ine ERG phenotype. Here, we provide pharmacological and genetic epistatic evidence that ine encodes a carcinine neurotransmitter transporter. We also speculate that the oscillations observed in mutant ine ERG traces are the result of the aberrant activity of a putative H₃ receptor.

During signaling in the nervous system, individual nerve cells transfer information to one another by a complex process called synaptic transmission. This communication involves the release of a specific neurotransmitter into the synaptic cleft, which then triggers signaling in the downstream neuron by binding to and activating specific cell surface receptors. In order to terminate the neuronal signal, the neurotransmitter must be rapidly removed from the synaptic cleft. This is done by two mechanisms: the neurotransmitter can be degraded or modified, or the transmitter can be taken up by the presynaptic neuron and packaged into vesicles for reuse. In the compound eye of the fruitfly D. melanogaster, the photoreceptor cell responds to light and releases histamine into the synaptic cleft. This signal is terminated by the removal of histamine from the synapse and the enzymatic conversion of histamine to carcinine. We have shown that it is not sufficient just to modify the histamine neurotransmitter, but it is also important to remove carcinine from the photoreceptor synapse. The failure to adequately remove carcinine results in defects in the visual transduction process. Moreover, the work suggests that carcinine itself modulates vision by regulating histamine release into the synapse.

Phosphatidylinositol 3-kinase and Akt nonautonomously promote perineurial glial growth in Drosophila peripheral nerves

Drosophila peripheral nerves, structured similarly to their mammalian counterparts, comprise a layer of motor and sensory axons wrapped by an inner peripheral glia (analogous to the mammalian Schwann cell) and an outer perineurial glia (analogous to the mammalian perineurium). Growth and proliferation within mammalian peripheral nerves are increased by Ras pathway activation: loss-of-function mutations in Nf1, which encodes the Ras inhibitor neurofibromin, cause the human genetic disorder neurofibromatosis, which is characterized by formation of neurofibromas (tumors of peripheral nerves). However, the signaling pathways that control nerve growth downstream of Ras remain incompletely characterized. Here we show that expression specifically within the Drosophila peripheral glia of the constitutively active Ras(V12) increases perineurial glial thickness. Using chromosomal loss-of-function mutations and transgenes encoding dominant-negative and constitutively active proteins, we show that this nonautonomous effect of Ras(V12) is mediated by the Ras effector phosphatidylinositol 3-kinase (PI3K) and its downstream kinase Akt. We also show that the nonautonomous, growth-promoting effects of activated PI3K are suppressed by coexpression within the peripheral glia of FOXO+ (forkhead box O) a transcription factor inhibited by Akt-dependent phosphorylation. We suggest that Ras-PI3K-Akt activity in the peripheral glia promotes growth of the perineurial glia by inhibiting FOXO. In mammalian peripheral nerves, the Schwann cell releases several growth factors that affect the proliferative properties of neighbors. Some of these factors are oversecreted in Nf1 mutants. Our results raise the possibility that neurofibroma formation in individuals with neurofibromatosis might result in part from a Ras-PI3K-Akt-dependent inhibition of FOXO within Schwann cells.

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.

Hox Transcription Factor Ultrabithorax Ib Physically and Genetically Interacts with Disconnected Interacting Protein 1, a Double-stranded RNA-binding Protein

The Hox protein family consists of homeodomain-containing transcription factors that are primary determinants of cell fate during animal development. Specific Hox function appears to rely on protein-protein interactions; however, the partners involved in these interactions and their function are largely unknown. Disconnected Interacting Protein 1 (DIP1) was isolated in a yeast two-hybrid screen of a 0-12-h Drosophila embryo library designed to identify proteins that interact with Ultrabithorax (Ubx), a Drosophila Hox protein. The Ubx.DIP1 physical interaction was confirmed using phage display, immunoprecipitation, pull-down assays, and gel retardation analysis. Ectopic expression of DIP1 in wing and haltere imaginal discs malforms the adult structures and enhances a decreased Ubx expression phenotype, establishing a genetic interaction. Ubx can generate a ternary complex by simultaneously binding its target DNA and DIP1. A large region of Ubx, including the repression domain, is required for interaction with DIP1. These more variable sequences may be key to the differential Hox function observed in vivo. The Ubx.DIP1 interaction prevents transcriptional activation by Ubx in a modified yeast one-hybrid assay, suggesting that DIP1 may modulate transcriptional regulation by Ubx. The DIP1 sequence contains two dsRNA-binding domains, and DIP1 binds double-stranded RNA with a 1000-fold higher affinity than either single-stranded RNA or double-stranded DNA. The strong interaction of Ubx with an RNA-binding protein suggests a wider range of proteins may influence Ubx function than previously appreciated.

Mutation in slowmo causes defects in Drosophila larval locomotor behaviour

We have identified a mutant slowmotion phenotype in first instar larval peristaltic behaviour of Drosophila. By the end of embryogenesis and during early first instar phases, slowmo mutant animals show a marked decrease in locomotory behaviour, resulting from both a reduction in number and rate of peristaltic contractions. Inhibition of neurotransmitter release, using targeted expression of tetanus toxin light chain (TeTxLC), in the slowmo neurons marked by an enhancer-trap results in a similar phenotype of largely absent or uncoordinated contractions. Cloning of the slowmo gene identifies a product related to a family of proteins of unknown function. We show that Slowmo is associated with mitochondria, indicative of it being a mitochondrial protein, and that during embryogenesis and early larval development is restricted to the nervous system in a subset of cells. The enhancer-trap marks a cellular component of the CNS that is seemingly required to regulate peristaltic movement.

Characterization of a functional bacterial homologue of sodium-dependent neurotransmitter transporters

The tnaT gene of Symbiobacterium thermophilum encodes a protein homologous to sodium-dependent neurotransmitter transporters. Expression of the tnaT gene product in Escherichia coli conferred the ability to accumulate tryptophan from the medium and the ability to grow on tryptophan as a sole source of carbon. Transport was Na(+)-dependent and highly selective. The K(m) for tryptophan was approximately 145 nm, and tryptophan transport was unchanged in the presence of 100 microM concentrations of other amino acids. Tryptamine and serotonin were weak inhibitors with K(I) values of 200 and 440 microM, respectively. By using a T7 promoter-based system, TnaT with an N-terminal His(6) tag was expressed at high levels in the membrane and was purified to near-homogeneity in high yield.

In vivo properties of the Drosophila inebriated-encoded neurotransmitter transporter

Altering neurotransmitter levels within the nervous system can cause profound changes in behavior and neuronal function. Neurotransmitter transporters play important roles in regulating neurotransmitter levels by performing neurotransmitter reuptake. It was previously shown that mutations in the Drosophila inebriated (ine)-encoded neurotransmitter transporter cause increased neuronal excitability. Here we report a further functional characterization of Ine. First we show that Ine functions in the short-term (time scale of minutes to a few hours) to regulate neuronal excitability. Second, we show that Ine is able to control excitability from either neurons or glia cells. Third, we show that overexpression of Ine reduces neuronal excitability. Overexpression phenotypes of ine include: delayed onset of long-term facilitation and increased failure rate of transmitter release at the larval neuromuscular junction, reduced amplitude of larval nerve compound action potentials, suppression of the leg-shaking behavior of mutants defective in the Shaker-encoded potassium channel, and temperature-sensitive paralysis. Each of these overexpression phenotypes closely resembles those of loss of function mutants in the para-encoded sodium channel. These data raise the possibility that Ine negatively regulates neuronal sodium channels, and thus that the substrate neurotransmitter of Ine positively regulates sodium channels.

In vivo properties of the Drosophila inebriated-encoded neurotransmitter transporter

Altering neurotransmitter levels within the nervous system can cause profound changes in behavior and neuronal function. Neurotransmitter transporters play important roles in regulating neurotransmitter levels by performing neurotransmitter reuptake. It was previously shown that mutations in the Drosophila inebriated (ine)-encoded neurotransmitter transporter cause increased neuronal excitability. Here we report a further functional characterization of Ine. First we show that Ine functions in the short-term (time scale of minutes to a few hours) to regulate neuronal excitability. Second, we show that Ine is able to control excitability from either neurons or glia cells. Third, we show that overexpression of Ine reduces neuronal excitability. Overexpression phenotypes of ine include: delayed onset of long-term facilitation and increased failure rate of transmitter release at the larval neuromuscular junction, reduced amplitude of larval nerve compound action potentials, suppression of the leg-shaking behavior of mutants defective in the Shaker-encoded potassium channel, and temperature-sensitive paralysis. Each of these overexpression phenotypes closely resembles those of loss of function mutants in the para-encoded sodium channel. These data raise the possibility that Ine negatively regulates neuronal sodium channels, and thus that the substrate neurotransmitter of Ine positively regulates sodium channels.

The Drosophila inebriated-encoded neurotransmitter/osmolyte transporter: dual roles in the control of neuronal excitability and the osmotic stress response

Water reabsorption by organs such as the mammalian kidney and insect Malpighian tubule/hindgut requires a region of hypertonicity within the organ. To balance the high extracellular osmolarity, cells within these regions accumulate small organic molecules called osmolytes. These osmolytes can accumulate to a high level without toxic effects on cellular processes. Here we provide evidence consistent with the possibility that the two protein isoforms encoded by the inebriated (ine) gene, which are members of the Na+/Cl--dependent neurotransmitter/osmolyte transporter family, perform osmolyte transport within the Malpighian tubule and hindgut. We show that ine mutants lacking both isoforms are hypersensitive to osmotic stress, which we assayed by maintaining flies on media containing NaCl, KCl, or sorbitol, and that this hypersensitivity is completely rescued by high-level ectopic expression of the ine-RB isoform. We provide evidence that this hypersensitivity represents a role for ine that is distinct from the increased neuronal excitability phenotype of ine mutants. Finally, we show that each ine genotype exhibits a "threshold" [NaCl]: long-term maintenance on NaCl-containing media above, but not below, the threshold causes lethality. Furthermore, this threshold value increases with the amount of ine activity. These data suggest that ine mutations confer osmotic stress sensitivity by preventing osmolyte accumulation within the Malpighian tubule and hindgut.

Control of Drosophila perineurial glial growth by interacting neurotransmitter-mediated signaling pathways

Drosophila peripheral nerves, similar structurally to the peripheral nerves of mammals, comprise a layer of axons and inner glia, surrounded by an outer perineurial glial layer. Although it is well established that intercellular communication occurs among cells within peripheral nerves, the signaling pathways used and the effects of this signaling on nerve structure and function remain incompletely understood. Here we demonstrate with genetic methods that the Drosophila peripheral nerve is a favorable system for the study of intercellular signaling. We show that growth of the perineurial glia is controlled by interactions among five genes: ine, which encodes a putative neurotransmitter transporter; eag, which encodes a potassium channel; push, which encodes a large, Zn(2+)-finger-containing protein; amn, which encodes a putative neuropeptide related to the pituitary adenylate cyclase activator peptide; and NF1, the Drosophila ortholog of the human gene responsible for type 1 neurofibromatosis. In other Drosophila systems, push and NF1 are required for signaling pathways mediated by Amn or the pituitary adenylate cyclase activator peptide. Our results support a model in which the Amn neuropeptide, acting through Push and NF1, inhibits perineurial glial growth, whereas the substrate neurotransmitter of Ine promotes perineurial glial growth. Defective intercellular signaling within peripheral nerves might underlie the formation of neurofibromas, the hallmark of neurofibromatosis.

The transporter-like protein inebriated mediates hyperosmotic stimuli through intracellular signaling

We cloned the inebriated homologue MasIne from Manduca sexta and expressed it in Xenopus laevis oocytes. MasIne is homologous to neurotransmitter transporters but no transport was observed with a number of putative substrates. Oocytes expressing MasIne respond to hyperosmotic stimulation by releasing intracellular Ca(2+), as revealed by activation of the endogenous Ca(2+)-activated Cl(−) current. This Ca(2+) release requires the N-terminal 108 amino acid residues of MasIne and occurs via the inositol trisphosphate pathway. Fusion of the N terminus to the rat gamma-aminobutyric acid transporter (rGAT1) also renders rGAT1 responsive to hyperosmotic stimulation. Immunohistochemical analyses show that MasIne and Drosophila Ine have similar tissue distribution patterns, suggesting functional identity. Inebriated is expressed in tissues and cells actively involved in K(+) transport, which suggests that it may have a role in ion transport, particularly of K(+). We propose that stimulation of MasIne releases intracellular Ca(2+) in native tissues, activating Ca(2+)-dependent K(+) channels, and leading to K(+) transport.

Amino acid transport in insects

Most insect cell membranes seem to contain uniporters that facilitate the diffusion of amino acids into and out of the cells. In addition to these passive diffusion systems, all but one of the insect tissues studied to date seem to contain at least one amino acid-cation symport system that allows their cells to accumulate certain amino acids from the extracellular medium. cDNAs encoding three such symporters have very recently been cloned and sequenced. The deduced amino acid sequence of each insect symporter was determined to be homologous to that of symporters mediating the transport of the same or related substrates in mammalian tissues.

An exploration of the sequence of a 2.9-Mb region of the genome of Drosophila melanogaster: the Adh region

A contiguous sequence of nearly 3 Mb from the genome of Drosophila melanogaster has been sequenced from a series of overlapping P1 and BAC clones. This region covers 69 chromosome polytene bands on chromosome arm 2L, including the genetically well-characterized "Adh region." A computational analysis of the sequence predicts 218 protein-coding genes, 11 tRNAs, and 17 transposable element sequences. At least 38 of the protein-coding genes are arranged in clusters of from 2 to 6 closely related genes, suggesting extensive tandem duplication. The gene density is one protein-coding gene every 13 kb; the transposable element density is one element every 171 kb. Of 73 genes in this region identified by genetic analysis, 49 have been located on the sequence; P-element insertions have been mapped to 43 genes. Ninety-five (44%) of the known and predicted genes match a Drosophila EST, and 144 (66%) have clear similarities to proteins in other organisms. Genes known to have mutant phenotypes are more likely to be represented in cDNA libraries, and far more likely to have products similar to proteins of other organisms, than are genes with no known mutant phenotype. Over 650 chromosome aberration breakpoints map to this chromosome region, and their nonrandom distribution on the genetic map reflects variation in gene spacing on the DNA. This is the first large-scale analysis of the genome of D. melanogaster at the sequence level. In addition to the direct results obtained, this analysis has allowed us to develop and test methods that will be needed to interpret the complete sequence of the genome of this species. Before beginning a Hunt, it is wise to ask someone what you are looking for before you begin looking for it. Milne 1926

Increased transmitter release and aberrant synapse morphology in a Drosophila calmodulin mutant

The ubiquitous calcium-binding protein calmodulin (CaM) has been implicated in the development and function of the nervous system in a variety of eukaryotic organisms. We have generated mutations in the single Drosophila Calmodulin (Cam) gene and examined the effects of these mutations on behavior, synaptic transmission at the larval neuromuscular junction, and structure of the larval motor nerve terminal. Flies hemizygous for Cam3c1, a mutation in the first Ca2+-binding site, exhibit behavioral, neurophysiological, and neuroanatomical abnormalities. In particular, adults exhibit defects in locomotion, coordination, and flight. Larvae exhibit increased neurotransmitter release from the motor nerve terminal at low [Ca2+] in the presence of the K+ channel-blocking drug quinidine. In addition, synaptic bouton structure at motor nerve terminals is altered. These effects are distinct from those produced by altering the activity of the CaM target enzymes CaM-activated kinase II (CaMKII) and CaM-activated adenylyl cyclase (CaMAC). Furthermore, previous in vitro studies of mutant Cam3c1 demonstrated that although its Ca2+ affinity is decreased, Cam3c1 protein can activate CaMKII, CaMAC, and CaM-activated phosphatase calcineurin in a manner similar to wild-type CaM. Thus, the Cam3c1 mutation might affect Ca2+ buffering or interfere with the activation or inhibition of a CaM target distinct from CaMKII or CaMAC.

Hormonally controlled chloride movement across Drosophila tubules is via ion channels in stellate cells

Anion conductance across the Drosophila melanogaster Malpighian (renal) tubule was investigated by a combination of physiological and transgenic techniques. Patch-clamp recordings identified clusters of 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-sensitive "maxi-chloride" channels in a small domain of the apical membrane. Fluid secretion assays demonstrated sensitivity to the chloride channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid, diphenylamine-2-carboxylate, anthracene-9-carboxylic acid, and niflumic acid. Electrophysiological analysis showed that the calcium-mediated increase in anion conductance was blocked by the same agents. Vibrating probe analysis revealed a small number of current density hot spots, coincident with "stellate" cells, that were abolished by low-chloride saline or the same chloride channel blockers. GAL-4-targeted expression of an aequorin transgene revealed that the neurohormone leucokinin elicits a rapid increase in intracellular calcium levels in stellate cells that precedes the fastest demonstrable physiological effect. Taken together, these data show that leucokinins act on stellate cells through intracellular calcium to increase transcellular chloride conductance through channels. As electrogenic cation conductance is confined to principal cells, the two pathways are spatially segregated in this tissue.

Drosophila rosA gene, which when mutant causes aberrant photoreceptor oscillation, encodes a novel neurotransmitter transporter homologue

The Drosophila receptor oscillation A (rosA) mutations, which cause electroretinogram (ERG) defects, including oscillations, were localized to the 24F4-25A2 region of chromosome 2L. Genomic fragments from this region, isolated from bacteriophage P1 clones, included those that detect transcriptional defects in rosA mutants in RNA blot experiments. One of these genomic fragments was used to screen a head cDNA library. The largest cDNA clone (3.6 kb) isolated was shown to rescue a rosA mutant in P element-germline transformation experiments. The ROSA protein deduced from the open reading frame in the 3.6 kb rosA cDNA is 943 amino acids long and is 36-41% identical to members of the superfamily of Na+/Cl(-)-dependent neurotransmitter transporters, with no indication of higher sequence identity to any one subgroup within the superfamily. RNA blot experiments revealed multiple transcripts in various developmental stages, the most abundant one being a 3.7 kb transcript, particularly in the adult head. Tissue in situ experiments identified the rosA transcript to be localized to many tissues, with higher levels of hybridization in the nervous system and digestive tract. The results demonstrate that the rosA gene encodes a novel Na+/Cl(-)-dependent transporter important for normal response properties of the photoreceptor.