Distinct, developmentally regulated brain mRNAs direct the synthesis of neurotransmitter transporters

GABA transporters in the mammalian cerebral cortex: localization, development and pathological implications

The extracellular levels of gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the mammalian cerebral cortex, are regulated by specific high-affinity, Na+/Cl- dependent transporters. Four distinct genes encoding GABA transporters (GATs), named GAT-1, GAT-2, GAT-3, and BGT-1 have been identified using molecular cloning. Of these, GAT-1 and -3 are expressed in the cerebral cortex. Studies of the cortical distribution, cellular localization, ontogeny and relationships of GATs with GABA-releasing elements using a variety of light and electron microscopic immunocytochemical techniques have shown that: (i) a fraction of GATs is strategically placed to mediate GABA uptake at fast inhibitory synapses, terminating GABA's action and shaping inhibitory postsynaptic responses; (ii) another fraction may participate in functions such as the regulation of GABA's diffusion to neighboring synapses and of GABA levels in cerebrospinal fluid; (iii) GATs may play a role in the complex processes regulating cortical maturation; and (iv) GATs may contribute to the dysregulation of neuronal excitability that accompanies at least two major human diseases: epilepsy and ischemia.

Na+-dependent high-affinity uptake ofl-glutamate in cultured fibroblasts

Uptake of 1 microM [3H]L-glutamate by cultured 3T3 fibroblasts was strongly dependent on extracellular Na+; it was reduced by elevated concentrations of K+ (60 mM) but it was not influenced by variations in the concentration of Ca2+ (0-9.6 mM). D- and L-Asparate, D- and L-threo-3-hydroxyaspartate DL-threo-3-methylaspartate and a few other glutamate derivatives and analogues inhibited the uptake but several close analogues of L-glutamate (including D-glutamate) had no effect, implying that the uptake system is highly structurally selective. The recently identified inhibitor of glutamate uptake in synaptosomal preparations, L-trans-pyrrolidine-2,4-dicarboxylate, was also among the inhibitors. Apparent Km of the uptake was found to be less than 10 microM. The present observations indicate that Na(+)-dependent 'high-affinity' uptake of L-glutamate may appear in structures which are apparently unrelated to glutamatergic synaptic transmission in the CNS.

Molecular changes in glutamatergic synapses induced by Pb2+: association with deficits of LTP and spatial learning

What are the molecular bases for the neurotoxicity that occurs after developmental exposure to low levels of Pb2+, and are these effects persistent and detrimental in adults? Our inability to understand specific mechanisms behind Pb2+ neurotoxicity has long been one of many problem areas of this preventable childhood disease. The sensitivity of the developing brain to Pb2+-induced neurotoxicity is an outcome of the many unique characteristics that comprise the developing central nervous system. The developing brain can be exposed to significant concentrations of Pb2+ during vulnerable periods of development such as synapse formation, gene and protein expression, and other diverse molecular changes associated with these processes. Recently, changes in NMDA receptor subunits were identified in animals that showed cognitive deficits induced by exposure to Pb2+. This molecular association is important because it provides new evidence in the characterization of developmental Pb2+ neurotoxicity that supports physiological findings of impairments in synaptic plasticity and behavior. This review updates information from molecular studies that can be directly associated with impairments of behavior and synaptic plasticity, and outlines the functional consequences of molecular differences in Pb2+-exposed animals that illuminate potential mechanisms of Pb2+-induced neurotoxicity.

Analysis of the transmembrane topology and membrane assembly of the GAT-1 gamma-aminobutyric acid transporter

The transmembrane topology of the Na+- and Cl--dependent gamma-aminobutyric acid transporter GAT-1 has been studied using protein chimeras in Xenopus oocytes. A series of COOH-terminal truncations was generated to which a prolactin epitope was fused. Following expression of transporter-prolactin chimeras in Xenopus oocytes, the transmembrane orientation of each chimera was determined by testing for protease sensitivity in an oocyte membrane preparation. Data from protease protection assays with GAT-1-prolactin chimeras has shown that residues in the loops connecting hydrophobic domain (HD)3 and HD4 and HD7 and HD8 are accessible to protease in the cytoplasm and suggest the presence of pore loop structures which extend into the membrane from the extracellular face. Such pore loop structures may be involved in the formation of the substrate-binding pocket. Studies presented herein confirm that the NH2 and COOH termini are cytosolic and hydrophobic domains span the membrane in a manner consistent with the predicted hydropathy model for Na+- and Cl--dependent transporters. These data also provide insight into GAT-1 transmembrane assembly and suggest that a complex series of topogenic sequences directs this process. A potential pause-transfer sequence has been identified and may be responsible for the translocational pausing observed in this study.

Regional distribution and developmental variation of the glycine transporters GLYT1 and GLYT2 in the rat CNS

The high-affinity glycine transporter in neurons and glial cells is the primary means of inactivating synaptic glycine. Previous molecular cloning studies have indicated heterogeneity of glycine transporters in the CNS. Here the distribution of glycine transporter GLYT1 and GLYT2 transcripts and proteins in different regions and developmental stages of the rat brain were analysed by Northern, Western and in situ hybridization techniques. Sequence-specific riboprobes and two specific antibodies raised against fusion proteins were used, containing either 76 or 193 amino acids of the C or N terminus of the GLYT1 and GLYT2 transporters respectively. High levels of GLYT1 transcripts were found in the spinal cord, brainstem and cerebellum, and moderate levels in forebrain regions such as the cortex or hippocampus. GLYT2 transcripts are restricted to the spinal cord, brainstem and cerebellum. The onset of both GLYT1 and GLYT2 expression in the brainstem occurred in late fetal life, and full expression of these proteins was observed before weaning. There was a stepwise increase in the levels of mRNA and protein for these two transporters, reaching a maximum by the second postnatal week, followed by a slight decrease until adult values were reached by the fourth postnatal week. These data reveal interesting parallelism between the distribution of different glycine transporters and glycine receptor subunits, and suggest discrete roles for distinct glycine transporters.

Effects of L-trans-pyrrolidine-2,4-dicarboxylate and L-threo-3-hydroxyaspartate on the binding of [3H]L-aspartate, [3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), [3H]DL-(E)-2-amino-4-propyl-5-phosphono-3-pentenoate (CGP 39653), [3H]6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and [3H]kainate studied by autoradiography in rat forebrain

L-trans-Pyrrolidine-2,4-dicarboxylate (L-t-PDC) and L-threo-3- hydroxyaspartate (L-t-3OHA), compounds known to interact strongly with the Na(+)-dependent high affinity uptake of excitatory amino acids in central nervous tissue, were tested as potential inhibitors of binding to glutamate receptors and transport sites in frozen sections of rat brain. [3H] alpha-amino-3-hydroxy- 5-methyl-4-isoxazolepropionate (AMPA), [3H]6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and [3H] kainate were used as ligands for the binding sites on the "non-NMDA" classes of glutamate receptors and [3H]DL-(E)-2-amino-4-propyl-5-phosphono-3-pentenoate (CGP 39653) was used to label NMDA receptor binding sites. The Na(+)-dependent glutamate-uptake site was marked by [3H]L-aspartate. The autoradiograms, obtained by exposing 3H-sensitive film to sections of rat forebrain preincubated with 3H-labelled ligands, were scanned by laser beam and quantified. Distribution patterns of the receptor and transporter sites visualized by the 3H-labelled ligands were compatible with previously published results. [3H]CNQX binding, however, was found to be significantly decreased by Na+.L-t-3OHA was about an order of magnitude stronger than L-t-PDC as an inhibitor of [3H]L-aspartate binding. Neither of the compounds had any important effect at the "non-NMDA" receptor binding sites but L-t-3OHA was a weak inhibitor of [3H]CGP 39653 binding (< 40% at 100 microM). The results suggest that, at low nanomolar concentrations, both compounds are likely to be selective for Na(+)-dependent high affinity glutamate transporter sites. Moreover, L-t-3OHA seems to have a sufficiently high affinity for the site to be almost certainly useful, if available in a 3H-labelled form, as a ligand in autoradiographic studies.

Excitotoxic mechanisms in the pathogenesis of dementia

Alzheimer disease and related dementias, in common with most major neurological diseases, are characterized by localized brain damage. An abundance of senile plaques and neurofibrillary tangles in certain brain areas is pathognomic of the disease: of the two, the density of tangles may correlate more closely with disease severity ante mortem. Clinical manifestation of the disease also results from a locally severe loss of neurones. This might be caused by over-stimulation by excitant amino acid transmitters such as glutamate, which would promote cell death. Mechanisms which might give rise to the localization of Alzheimer pathogenesis include hypersensitivity to damage because a cell carries a particular sub-set of post-synaptic receptors; local variations in the efficiency of excitatory amino acid transport; and, possibly, local exacerbation of toxicity by substances such as beta-amyloid. Elucidation of such mechanisms could lead to new pharmacotherapies of dementia.

High affinity [3H]glutamate uptake systems in normal and audiogenic seizure-susceptible mice

Two high affinity sodium-dependent and PDC-sensitive glutamate (GLU) uptake systems are present in a whole brain synaptosomal preparation from adult C57BL/10 SPS/SPS normal mice. System 1 has an apparent Km of 3.65 microM while that of System 2 is 46.8 microM. Glutamate uptake in the normal mice increases gradually during development, displaying a striking peak at postnatal day 15, and decreases rapidly between PN 16 and PN 20 until it reaches adult levels. The developmental pattern of GLU uptake System 1 and System 2 in audiogenic seizure susceptible mice is similar to that described in normal mice. However, there are differences between GLU uptake system 1 and 2 during ontogenesis: (1) System 1 could not be detected until PN 15 while being markedly diminished in adulthood; and (2) GLU uptake by System 2 is increased in adults. In addition, the Umax for System 2 is significantly greater than that of normal mice at PN 2 and PN 15.

Sodium ion-dependent transporters for neurotransmitters: a review of recent developments

Images

Reconstitution of transmitter secretion

Molecular mechanisms involved in the various stages of transmitter secretion have been studied by perturbing the composition of secretory cells using pharmacological and biochemical agents. An emerging approach is to reconstitute individual steps or the entire sequence of secretion mechanisms in non-secretory cells by loading the cell with presynaptic components or their mRNAs.

Amino acid neurotransmitter transporters: structure, function, and molecular diversity

Many biologically active compounds including neurotransmitters, metabolic precursors, and certain drugs are accumulated intracellularly by transporters that are coupled to the transmembrane Na+ gradient. Amino acid neurotransmitter transporters play a key role in the regulation of extracellular amino acid concentrations and termination of neurotransmission in the CNS section. Transporters for the major amino acid neurotransmitters glutamate, GABA, and glycine are found in both neurons and glial cells. Recent work has resulted in the identification of cDNAs encoding several amino acid neurotransmitter transport proteins, all of which belong to the Na(+)- and Cl(-)-dependent transporter gene family. The diversity of this family suggests a degree of transporter heterogeneity that is greater than that indicated by biochemical and pharmacological studies.

GABA transporter mRNA: in vitro expression and quantitation in neonatal rat and postmortem human brain

A previously isolated rat cDNA clone encoding the membrane transporter for the neurotransmitter gamma-aminobutyric acid was expressed in transfected COS cells. The resultant transporter protein was characterized kinetically and pharmacologically. The apparent Kt (6.1 microM) and the pharmacological profile of a neuronal-type transporter observed in these mammalian cells were consistent with previous data obtained in Xenopus laevis oocytes. Post-natal levels of gamma-aminobutyric acid transporter mRNA in rat cerebellum, cerebral cortex and striatum (as measured by nuclease protection assay) transiently exceeded levels present in the adult brain. Human gamma-aminobutyric acid transporter mRNA also was measured by nuclease protection assay using as probe a human transporter cDNA homolog obtained by polymerase chain reaction. These studies suggest that quantitation of rat and human gamma-aminobutyric acid transporter mRNAs may provide a useful index of transporter gene expression.

3H-D-aspartate release from cerebellar granule neurons is differentially regulated by glutamate- and K+-stimulation

Neurotransmitter release in response to either 55 mM K+ or 25 microM glutamate as well as its dependency on Ca2+ from different sources was compared in cultured glutamatergic cerebellar granule cells from rat brain. The intracellular Ca2+ concentration was monitored at the single cell level in neurites as well as cell bodies employing the fluorescent Ca2+ indicator fura-2. Transmitter release was assayed using 3H-D-aspartate to label the exogenously accessible glutamate pools, which in these neurons is believed to also include the transmitter pool. In an attempt to distinguish whether transmitter release was dependent on an intact cytoskeleton or not, the colchicine-like drug Nocodazole, which also blocks transport of vesicles, was used. K(+)-stimulated transmitter release consisted for the major part (around 70%) of a Ca(2+)-dependent, Nocodazole sensitive release component and this K(+)-induced release appeared to be almost exclusively dependent on N-type Ca2+ channels. In contrast, 50% of the glutamate-induced Ca(2+)-dependent release was triggered by Ca2+ from a Dantrolene sensitive intracellular Ca2+ pool. Since these neurons undergo a pronounced maturational change in which neurotransmitter vesicles become increasingly prominent, the Ca2+ responses and transmitter release evoked by the two different stimuli were investigated as a function of the culture period. K+ and glutamate were found to increase intracellular [Ca2+] differentially. In 1-day-old cultures K+ elicited a small albeit significant increase in [Ca2+]i while glutamate was completely without effect. In 7-day-old neurons both agents induced a large increase in [Ca2+].(ABSTRACT TRUNCATED AT 250 WORDS)

L-glutamate transporter derived from mRNAs of primary glial cultures: expression in Xenopus laevis oocytes

A high-affinity sodium-dependent L-glutamate transporter was expressed in Xenopus oocytes after microinjection of poly(A)+ RNA from primary astrocyte cultures from rat brain cortex. mRNA-induced L-glutamate transport was saturable by substrate and shows kinetic features similar to those found in intact glial cell preparations. L-Glutamate accumulation was prevented by rising the external K+ concentration or by coincubation with L-, D-aspartate or D-glutamate. After fractionation by sucrose density gradient, the mRNA encoding for the expressed L-glutamate transporter from glial cells was found in fractions containing messages of 2.05-2.9 kilobases (kb) in length.

Functional expression and CNS distribution of a beta-alanine-sensitive neuronal GABA transporter

The synaptic action of gamma-aminobutyric acid (GABA) is terminated by high affinity, Na(+)-dependent transport processes in both neurons and glia. We have isolated a novel GABA transporter cDNA, GAT-B, which encodes a high affinity (Km = 2.3 microM), Na(+)- and Cl(-)-dependent GABA transport protein that is potently blocked by beta-alanine, a compound generally considered a selective inhibitor of glial transport. However, in situ hybridization studies indicate that GAT-B mRNA is expressed predominantly within neurons. These data indicate that the neuronal-glial distinction of GABA transporters based on inhibitor sensitivities must be reconsidered and suggest a greater diversity of GABA transporters than has been predicted by previous pharmacologic studies.

Calcium-Dependent Transmitter Secretion Reconstituted in Xenopus Oocytes: Requirement for Synaptophysin

Calcium-dependent glutamate secretion was reconstituted in Xenopus oocytes by injecting the oocyte with total rat cerebellar messenger RNA (mRNA). Co-injection of total mRNA with antisense oligonucleotides to synaptophysin message decreased the expression of synaptophysin in the oocyte and reduced the calcium-dependent secretion. A similar effect on secretion was observed for oocytes injected with total mRNA together with an antibody to rat synaptophysin. These results indicate that synaptophysin is necessary for transmitter secretion and that the oocyte expression system may be useful for dissecting the molecular events associated with the secretory process.

The ontogeny of the uptake systems for glutamate, GABA, and glycine in synaptic vesicles isolated from rat brain

The ontogeny of the uptake of glutamate, GABA and glycine into synaptic vesicles isolated from rat brain has been investigated. The vesicular uptake of the three amino acids increased with developmental age in parallel with synaptogenesis, indicating a functional role of uptake of the amino acids by synaptic vesicles in the nerve terminals. Uptake of the amino acids by plasma membrane particles (synaptosomes) in brain homogenate showed a somewhat different developmental profile. The uptake of glutamate increased markedly with developmental time, while the uptake of GABA showed only a slight increase. Uptake of glycine by plasma membrane particles was very low and therefore not registered. The observed developmental increase in uptake of glycine by synaptic vesicles isolated from brain, supports previous reports indicating that glycine can be taken up by vesicles from non-glycine terminals.

Expression of Size-Selected RNA Encoding Brain Serotonin Transporter in Xenopus laevis Oocytes

The mRNA that encodes a serotonin transporter was expressed using the Xenopus laevis oocyte expression system. Poly(A)+ RNA isolated from mouse brainstem was injected into Xenopus laevis oocytes, and the ability of oocytes to take up serotonin was measured 3 days postinjection. RNA-dependent serotonin uptake was sensitive to citalopram, a specific inhibitor of serotonin uptake, whereas background levels of serotonin uptake were not citalopram sensitive. Two RNA size fractions, 4.0 and 4.5 kb, were most efficient in stimulating uptake. Injection into Xenopus laevis oocytes of the 4.5-kb size fraction of mouse brainstem RNA resulted in threefold more serotonin uptake than did injection of unfractionated poly(A)+ RNA.

Heterogeneity of high affinity uptake of L-glutamate and L-aspartate in the mammalian central nervous system

Characteristics of high affinity uptake of L-glutamate are examined in order to evaluate the possible use of the uptake of [3H]L-glutamate, [3H]L-aspartate or any other suitable [3H]-labelled substrate as a marker for glutamatergic and aspartergic synapses in autoradiographic studies in the mammalian brain. Review of data on substrate specificity indicates the presence of at least two high affinity uptake systems specific for acidic amino acids in the central nervous tissue; one which takes up L-glutamate and L-aspartate and the other which is selective for L-glutamate only. Studies on ionic requirements, too, point to the existence of at least two distinct uptake systems with high affinity for L-glutamate. The Na(+)-dependent uptake system(s) handle(s) both L-glutamate and L-aspartate whereas the Na(+)-independent uptake system(s) show(s) selectivity for L-glutamate only. Available data do not favour the Na(+)-dependent binding of [3H]D-aspartate to thaw-mounted sections of frozen brain tissue as a suitable marker for glutamatergic/aspartergic synaptic nerve endings. However, there are reasons--such as the results of lesion studies and the existence of uptake sites which have a higher affinity for L-aspartate than for D-aspartate--to suggest that Na(+)-dependent binding of [3H]L-aspartate, rather than that of [3H]D-aspartate, should be further investigated as a possible marker for the glutamatergic/aspartergic synapses in the autoradiographic studies using sections of frozen brain.

The ontogeny of the uptake systems for glycine, GABA and glutamate in synaptic vesicles isolated from rat spinal cord-medulla

Synaptic vesicles have been isolated from rat spinal cord-medulla at different postnatal ages, and the ontogeny of the uptake of glycine, gamma-aminobutyric acid (GABA) and glutamate has been investigated. The accumulation of the 3 amino acids increased with increasing time after birth reaching adult level at about postnatal day 30. This developmental increase probably parallels the synaptogenesis and suggests a functional role of the uptake of the amino acids into synaptic vesicles in the nerve terminals. The developmental time course for these vesicular uptake systems was totally different from those of the corresponding plasma membrane uptakes.

Sodium-dependent neurotransmitter reuptake systems

The recent cloning of the gamma-aminobutyric acid and norepinephrine Na(+)-dependent neurotransmitter transporters has led to new approaches for understanding the structure and function of these important synaptic proteins and provides a conceptual model in which to consider recent advances in the physiology, pharmacology and biochemistry of this gene family.