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The cystine/glutamate antiporter system Xc- (SXc-) mediates the exchange of intracellular L-glutamate (L-Glu) with extracellular L-cystine (L-Cys2). Both the import of L-Cys2 and the export of L-Glu take on added significance in CNS cells, especially astrocytes. When the relative activity of SXc- overwhelms the regulatory capacity of the EAATs, the efflux of L-Glu through the antiporter can be significant enough to trigger excitotoxic pathology, as is thought to occur in glioblastoma. This has prompted considerable interest in the pharmacological specificity of SXc- and the development of inhibitors. The present study explores a series of analogues that are structurally related to sulfasalazine, a widely employed inhibitor of SXc-. We identify a number of novel aryl-substituted amino-naphthylsulfonate analogues that inhibit SXc- more potently than sulfasalazine. Interestingly, the inhibitors switch from a competitive to noncompetitive mechanism with increased length and lipophilic substitutions, a structure-activity relationship that was previously observed with aryl-substituted isoxazole. These results suggest that the two classes of inhibitors may interact with some of the same domains on the antiporter protein and that the substrate and inhibitor binding sites may be in close proximity to one another. Molecular modeling is used to explore this possibility.

The vesicular glutamate transporter (VGLUT) facilitates the uptake of glutamate (Glu) into neuronal vesicles. VGLUT has not yet been fully characterized pharmacologically but a body of work established that certain azo-dyes bearing two Glu isosteres via a linker were potent inhibitors. However, the distance between the isostere groups that convey potent inhibition has not been delineated. This report describes the synthesis and pharmacologic assessment of Congo Red analogs that contain one or two glutamate isostere or mimic groups; the latter varied in the interatomic distance and spacer properties to probe strategic binding interactions within VGLUT. The more potent inhibitors had two glutamate isosteres symmetrically linked to a central aromatic group and showed IC(50) values ~ 0.3 – 2.0 μM at VGLUT. These compounds contained phenyl, diphenyl ether (PhOPh) or 1,2-diphenylethane as the linker connecting 4-aminonaphthalene sulfonic acid groups. A homology model for VGLUT2 using D-galactonate transporter (DgoT) to dock and identify R88, H199 and F219 as key protein interactions with Trypan Blue, Congo Red and selected potent analogs prepared and tested in this report.

The system xc(-) antiporter is a plasma membrane transporter that mediates the exchange of extracellular l-cystine with intracellular l-glutamate. This exchange is significant within the context of the CNS because the import of l-cystine is required for the synthesis of the antioxidant glutathione, while the efflux of l-glutamate has the potential to contribute to either excitatory signaling or excitotoxic pathology. Changes in the activity of the transport system have been linked to the underlying pathological mechanisms of a variety of CNS disorders, one of the most prominent of which is its highly enriched expression in glial brain tumors. In an effort to produce more potent system xc(-) blockers, we have been using amino-3-carboxy-5-methylisoxazole propionic acid (ACPA) as a scaffold for inhibitor development. We previously demonstrated that the addition of lipophilic aryl groups to either the #4 or #5 position on the isoxazole ring markedly increased the inhibitory activity at system xc(-). In the present work a novel series of analogues has been prepared in which aryl groups have been introduced at both the #4 and #5 positions. In contrast to the competitive action of the mono-substituted analogues, kinetic analyses indicate that the di-substituted isoxazoles block system xc(-)-mediated uptake of (3)H-l-glutamate into SNB-19 cells by a noncompetitive mechanism. These new analogues appear to be the first noncompetitive inhibitors identified for this transport system, as well as being among the most potent blockers identified to date. These diaryl-isoxazoles should be of value in assessing the physiological roles and molecular pharmacology of system xc(-).

Microwave accelerated reaction system (MARS) technology provided a good method to obtain selective and open isoxazole ligands that bind to and inhibit the Sx c - antiporter. The MARS provided numerous advantages, including: shorter time, better yield and higher purity of the product. Of the newly synthesized series of isoxazoles the salicyl hydrazide 6 exhibited the highest level of inhibitory activity in the transport assay. A homology model has been developed to summarize the SAR results to date, and provide a working hypothesis for future studies.

System x(c)(-) represents an intriguing target in attempts to understand the pathological states of the central nervous system. Also called a cystine-glutamate antiporter, system x(c)(-) typically functions by exchanging one molecule of extracellular cystine for one molecule of intracellular glutamate. Nonvesicular glutamate released during cystine-glutamate exchange activates extrasynaptic glutamate receptors in a manner that shapes synaptic activity and plasticity. These findings contribute to the intriguing possibility that extracellular glutamate is regulated by a complex network of release and reuptake mechanisms, many of which are unique to glutamate and rarely depicted in models of excitatory signaling. Because system x(c)(-) is often expressed on non-neuronal cells, the study of cystine-glutamate exchange may advance the emerging viewpoint that glia are active contributors to information processing in the brain. It is noteworthy that system x(c)(-) is at the interface between excitatory signaling and oxidative stress, because the uptake of cystine that results from cystine-glutamate exchange is critical in maintaining the levels of glutathione, a critical antioxidant. As a result of these dual functions, system x(c)(-) has been implicated in a wide array of central nervous system diseases ranging from addiction to neurodegenerative disorders to schizophrenia. In the current review, we briefly discuss the major cellular components that regulate glutamate homeostasis, including glutamate release by system x(c)(-). This is followed by an in-depth discussion of system x(c)(-) as it relates to glutamate release, cystine transport, and glutathione synthesis. Finally, the role of system x(c)(-) is surveyed across a number of psychiatric and neurodegenerative disorders.

Recent research has identified an important role for a cystine--glutamate anti-porter (system Xc) in the biology of malignant brain tumors. This transporter is effectively inhibited by sulfasalazine, a drug widely used to treat a number of chronic inflammatory conditions such as Crohn’s disease. Preclinical data show that sulfasalazine is an effective inhibitor of tumor growth and tumor-associated seizures. These studies suggest that the cystine--glutamate anti-porter is a valuable drug target for which tumor-specific drugs can be generated. In the meantime, sulfasalazine may be considered as adjuvant treatment for malignant gliomas.

System xc- is an amino acid antiporter that typically mediates the exchange of extracellular l-cystine and intracellular l-glutamate across the cellular plasma membrane. Studied in a variety of cell types, the import of l-cystine through this transporter is critical to glutathione production and oxidative protection. The exchange-mediated export of l-glutamate takes on added significance within the CNS, as it represents a non-vesicular route of release through which this excitatory neurotransmitter can participate in either neuronal signalling or excitotoxic pathology. When both the import of l-cystine and the export of l-glutamate are taken into consideration, system xc- has now been linked to a wide range of CNS functions, including oxidative protection, the operation of the blood–brain barrier, neurotransmitter release, synaptic organization, viral pathology, drug addiction, chemosensitivity and chemoresistance, and brain tumour growth. The ability to selectively manipulate system xc-, delineate its function, probe its structure and evaluate it as a therapeutic target is closely linked to understanding its pharmacology and the subsequent development of selective inhibitors and substrates. Towards that goal, this review will examine the current status of our understanding of system xc- pharmacology and the structure–activity relationships that have guided the development of an initial pharmacophore model, including the presence of lipophilic domains adjacent to the substrate binding site. A special emphasis is placed on the roles of system xc- within the CNS, as it is these actions that are among the most exciting as potential long-range therapeutic targets.

Prion protein (PrP) is expressed on a wide variety of cells and plays an important role in the pathogenesis of transmissible spongiform encephalopathies. However, its normal function remains unclear. Mice that do not express PrP exhibit deficits in spatial memory and abnormalities in excitatory neurotransmission suggestive that PrP may function in the glutamatergic synapse. Here, we show that transport of D-aspartate, a nonmetabolized L-glutamate analog, through excitatory amino acid transporters (EAATs) was faster in astrocytes from PrP knockout (PrPKO) mice than in astrocytes from C57BL/10SnJ wild-type (WT) mice. Experiments using EAAT subtype-specific inhibitors demonstrated that in both WT and PrPKO astrocytes, the majority of transport was mediated by EAAT1. Furthermore, PrPKO astrocytes were more effective than WT astrocytes at alleviating L-glutamate-mediated excitotoxic damage in both WT and PrPKO neuronal cultures. Thus, in this in vitro model, PrPKO astrocytes exerted a functional influence on neuronal survival and may therefore influence regulation of glutamatergic neurotransmission in vivo.

A panel of amino acid analogs and conformationally-restricted amino acids bearing a sulfonic acid were synthesized and tested for their ability to preferentially inhibit the obligate cysteine–glutamate transporter system x c - versus the vesicular glutamate transporter (VGLUT). Several promising candidate molecules were identified: R / S -4-[4′-carboxyphenyl]-phenylglycine, a biphenyl substituted analog of 4-carboxyphenylglycine and 2-thiopheneglycine-5-sulfonic acid both of which reduced glutamate uptake at system x c - by 70–75% while having modest to no effect on glutamate uptake at VGLUT.

The excitatory amino acid transporters (EAATs), or sodium-dependent glutamate transporters, provide the primary mechanism for glutamate removal from the synaptic cleft. EAAT distribution has been determined in the rat brain, but it is only partially characterized in the spinal cord.The regional anatomic distribution of EAATs in spinal cord was assessed by radioligand autoradiography throughout cervical, thoracic, and lumbar cord levels in female Sprague-Dawley rats. EAAT subtype regional distribution was evaluated by inclusion of pharmacologic transport inhibitors in the autoradiography assays and by immunohistochemistry using subtype-specific polyclonal antibodies to rat GLT1 (EAAT2), GLAST (EAAT1), and EAAC1 (EAAT3) rat transporter subtypes.[3H]-D-Aspartate binding was distributed throughout gray matter at the 3 spinal cord levels, with negligible binding in white matter. Inclusion of pharmacologic transport inhibitors indicates that the EAAT2/ GLT1 subtype represents 21% to 40% of binding. Both EAAT1/GLAST and EAAT3/EAAC1 contributed the remainder of binding. Immunoreactivity to subtype-specific antibodies varied, depending on cord level, and was present in both gray and white matter. All 3 subtypes displayed prominent immunoreactivity in the dorsal horn. EAAT3/EAAC1 and to a lesser extent EAAT1/GLAST immunoreactivity also occurred in a punctate pattern in the ventral horn.The results indicate heterogeneity of EAAT distribution among spinal cord levels and regions. The presence of these transporters throughout rat spinal cord suggests the importance of their contributions to spinal cord function.

The excitatory amino acid transporters (EAATs) play key roles in the regulation of CNS l -glutamate, especially related to synthesis, signal termination, synaptic spillover, and excitotoxic protection. Inhibitors available to delineate EAAT pharmacology and function are essentially limited to those that non-selectively block all EAATs or those that exhibit a substantial preference for EAAT2. Thus, it is difficult to selectively study the other subtypes, particularly EAAT1 and EAAT3. Structure activity studies on a series of β-substituted aspartate analogues identify l -β-benzyl-aspartate ( l -β-BA) as among the first blockers that potently and preferentially inhibits the neuronal EAAT3 subtype. Kinetic analysis of d -[3H]aspartate uptake into C17.2 cells expressing the hEAATs demonstrate that l -β-threo-BA is the more potent diastereomer, acts competitively, and exhibits a 10-fold preference for EAAT3 compared to EAAT1 and EAAT2. Electrophysiological recordings of EAAT-mediated currents in Xenopus oocytes identify l -β-BA as a non-substrate inhibitor. Analyzing l -β-threo-BA within the context of a novel EAAT2 pharmacophore model suggests: (1) a highly conserved positioning of the electrostatic carboxyl and amino groups; (2) nearby regions that accommodate select structural modifications (cyclopropyl rings, methyl groups, oxygen atoms); and (3) a unique region l -β-threo-BA occupied by the benzyl moieties of l -TBOA, l -β-threo-BA and related analogues. It is plausible that the preference of l -β-threo-BA and l -TBOA for EAAT3 and EAAT2, respectively, could reside in the latter two pharmacophore regions.

L-glutamate serves as the primary excitatory neurotransmitter in the mammalian CNS, where it can contribute to either neuronal communication or neuropathological damage through the activation of a wide variety of excitatory amino acid (EAA) receptors. By regulating the levels of extracellular L-glutamate that have access to these receptors, glutamate uptake systems hold the potential to effect both normal synaptic signaling and the abnormal over-activation of the receptors that can trigger excitotoxic pathology. Among the various membrane transporters that are capable of translocating this dicarboxylic amino acid, the majority of glutamate transport in the CNS, particularly as related to excitatory transmission, is mediated by the high-affinity, sodium-dependent, excitatory amino acid transporters (EAATs). At least 5 subtypes of EAATs have been identified, each of which exhibits a distinct distribution and pharmacology. Our growing appreciation for the functional significance of the EAATs is closely linked to our understanding of their pharmacology and the consequent development of inhibitors and substrates with which to delineate their activity. As was the case with EAA receptors, conformationally constrained glutamate mimics have been especially valuable in this effort. The success of these compounds is based upon the concept that restricting the spatial positions that can be occupied by required functional groups can serve to enhance both the potency and selectivity of the analogues. In the instance of the transporters, useful pharmacological probes have emerged through the introduction of additional functional groups (e.g., methyl, hydroxyl, benzyloxy) onto the acyclic backbone of glutamate and aspartate, as well as through the exploitation of novel ring systems (e.g., pyrrolidine-, cyclopropyl-, azole-, oxazole-, and oxazoline-based analogues) to conformationally lock the position of the amino and carboxyl groups. The focus of the present review is on the pharmacology of the EAATs and, in particular, the potential to identify those chemical properties that differentiate the processes of binding and translocation (i.e., substrates from non-substrate inhibitors), as well as strategies to develop glutamate analogues that act selectively among the various EAAT subtypes.

The vesicular glutamate transporter (VGLUT) is responsible for the uptake of the excitatory amino acid, L-glutamate, into synaptic vesicles. VGLUT activity is coupled to an electrochemical gradient driven by a vacuolar ATPase and stimulated by low Cl-. VGLUT has relatively low affinity (K(m) = 1-3 mM) for glutamate and is pharmacologically and structurally distinct from the Na+-dependent, excitatory amino acid transporters (EAATs) found on the plasma membrane. Because glutamatergic neurotransmission begins with vesicular release, compounds that block the uptake of glutamate into the vesicle may reduce excitotoxic events. Several classes of competitive VGLUT inhibitors have emerged including amino acids and amino acid analogs, fatty acids, azo dyes, quinolines and alkaloids. The potency with which these agents inhibit VGLUT varies from millimolar (amino acids) to nanomolar (azo dyes) concentrations. These inhibitors represent highly diverse structures and have collectively begun to reveal key pharmacophore elements that may elucidate the key interactions important to binding VGLUT. Using known inhibitor structures and preliminary molecular modeling, a VGLUT pharmacophore is presented that will aid in the design of new, highly potent and selective agents.

Beta-N-oxalyl-L-alpha-beta-diaminopropionic acid (beta-L-ODAP) is an unusual amino acid present in seeds of plants from the Lathyrus genus that is generally accepted as the causative agent underlying the motor neuron degeneration and spastic paraparesis in human neurolathyrism. Much of the neuropathology produced by beta-L-ODAP appears to be a direct consequence of its structural similarities to the excitatory neurotransmitter L-glutamate and its ability to induce excitotoxicity as an agonist of non-NMDA receptors. Its actions within the CNS are, however, not limited to non-NMDA receptors, raising the likely possibility that the anatomical and cellular specificity of the neuronal damage observed in neurolathyrism may result from the cumulative activity of beta-L-ODAP at multiple sites. Accumulating evidence suggests that system xc-, a transporter that mediates the exchange of L-cystine and L-glutamate, is one such site. In the present work, two distinct approaches were used to define the interactions of beta-L-ODAP with system xc-: Traditional radiolabel-uptake assays were employed to quantify inhibitory activity, while fluorometrically coupled assays that follow the exchange-induced efflux of L-glutamate were used to assess substrate activity. In addition to confirming that beta-L-ODAP is an effective competitive inhibitor of system xc-, we report that the compound exhibits a substrate activity comparable to that of the endogenous substrate L-cystine. The ability of system xc- to transport and accumulate beta-L-ODAP identifies additional variables that could influence its toxicity within the CNS, including the ability to limit its access to EAA receptors by clearing the excitotoxin from the extracellular synaptic environment, as well as serving as a point of entry through which beta-L-ODAP could have increased access to intracellular targets.

The vesicular glutamate transport (VGLUT) system selectively mediates the uptake of L-glutamate into synaptic vesicles. Uptake is linked to an H+-ATPase that provides coupling among ATP hydrolysis, an electrochemical proton gradient, and glutamate transport. Substituted quinoline-2,4-dicarboxylic acids (QDCs), prepared by condensation of dimethyl ketoglutaconate (DKG) with substituted anilines and subsequent hydrolysis, were investigated as potential VGLUT inhibitors in synaptic vesicles. A brief panel of substituted QDCs was previously reported (Carrigan et al. Bioorg. Med. Chem. Lett. 1999, 9, 2607-2612)(1) and showed that certain substituents led to more potent competitive inhibitors of VGLUT. Using these compounds as leads, an expanded series of QDC analogues were prepared either by condensation of DKG with novel anilines or via aryl-coupling (Suzuki or Heck) to dimethyl 6-bromoquinolinedicarboxylate. From the panel of almost 50 substituted QDCs tested as inhibitors of the VGLUT system, the 6-PhCH=CH-QDC (K(i) = 167 microM), 6-PhCH2CH2-QDC (K(i) = 143 microM), 6-(4'-phenylstyryl)-QDC (K(i) = 64 microM), and 6-biphenyl-4-yl-QDC (K(i) = 41 microM) were found to be the most potent blockers. A preliminary assessment of the key elements needed for binding to the VGLUT protein based on the structure-activity relationships for the panel of substituted QDCs is discussed herein. The substituted QDCs represent the first synthetically derived VGLUT inhibitors and are promising templates for the development of selective transporter inhibitors.

Na 1 -dependent excitatory amino acid transporters (EAATs) normally function to remove extracellular glutamate from brain extracellular space, but EAATs can also increase extracellular glutamate by reversal of uptake. Effects of inhibitors on EAATs can be complex, depending on cell type, whether conditions favor glutamate uptake or uptake reversal and whether the inhibitor itself is a substrate for the transporters. The present study assessed EAAT inhibitors for their ability to inhibit glutamate uptake, act as transporter substrates and block uptake reversal in astrocyte and neuron cultures. L-threo-bhydroxyaspartate (L-TBHA), DL-threo-b-benzyloxyaspartate (DL-TBOA), L-trans-pyrrolidine-2,4-dicarboxylic acid (L-trans2,4-PDC) (1/‐)-cis-4-methy-trans-pyrrolidine-2,4-dicarboxylic acid (cis-4-methy-trans-2,4-PDC) and L-antiendo-3,4-methanopyrrolidine-2,4-dicarboxylic acid (L-antiendo-3,4-MPDC) inhibited L-[ 14 C]glutamate uptake in astrocytes with equilibrium binding constants ranging from 17 mM (DL-TBOA and L-TBHA) ‐4 3mM (cis-4-methy-trans-2,4-PDC). Transportability of inhibitors was assessed in astrocytes and neurons. While L-TBHA, L-trans-2,4-PDC, cis-4-methy-trans-2,4-PDC and L-antiendo3,4-MPDC displayed significant transporter substrate activities in neurons and astrocytes, DL-TBOA was a substrate only in astrocytes. This effect of DL-TBOA was concentrationdependent, leading to complex effects on glutamate uptake reversal. At concentrations low enough to produce minimal DL-TBOA uptake velocity (# 10 mM), DL-TBOA blocked uptake reversal in ATP-depleted astrocytes; this blockade was negated at concentrations that drove substantial DL-TBOA uptake (. 10 mM). These findings indicate that the net effects of EAAT inhibitors can vary with cell type and exposure

A rat cortical astrocyte preparation was used to investigate the effects of organotins on glutamate regulation by astrocytes. Exposure of astrocytes to low levels of organotins produced significant changes in two key components of glutamate homeostasis: glutamine synthetase (CS) activity and the high-affinity transport of L-glutamate. Trimethyltin (TMT), triethyltin (TET), and triphenyltin (TPT) exhibited differential abilities to reduce GS activity and glutamate uptake. Cultures incubated with 1 microM TET or TPT, but not TMT, exhibited a marked decrease in GS activity. Exposure to TET or TPT also produced a significant decrease in glutamate transport activity that was not observed with TMT. These declines in activity were not attributable to cell loss as measured by MTT reduction and lactate dehydrogenase (LDH) leakage. Since the loss of GS activity and transporter activity was not seen with acute organotin exposure, it is most likely attributable to a decreased presence of fully functioning protein. While the attenuation of GS and glutamate transporter activities by organotins does not match their pattern of neurotoxicity, the results indicate the potential for subtoxic concentrations of these compounds to increase extracellular glutamate and interact with other excitotoxic episodes.

Polycyclic and halogenated aromatic hydrocarbons (PAHs and HAHs) can enhance the generation of reactive oxygen species (ROS) by inducing cytochrome P450 1A (CYP 1A) in vivo and in vitro. While the brain is vulnerable to oxidative injury, whether or not CYP 1A induction in the brain can produce measurable levels of oxidative damage has not been reported. The objective of this study was to investigate the effect of this induction on oxidative damage to the CNS. Time course changes in rat brain CYP 1A activity were determined by measurement of ethoxyresorufin O‐deethylase (EROD) activity in whole brain homogenates. Three days after exposure of rats to five daily injections of 3‐methylcholanthrene (3‐MC) an approximately sevenfold increase in EROD activity was observed. Hepatic levels were increased 60–100 fold. This increase in CYP 1A activity was not accompanied by increased protein or lipid oxidation as measured by tryptophan fluorescence and TBAR formation, or decreased glutamine synthetase (GS) activity. These findings indicate that if increased CYP 1A activity in the brain following 3‐MC treatment leads to increased ROS generation, the increase is insufficient to overwhelm the endogenous antioxidant defense system, produce detectable oxidative damage, and alter glutamate homeostasis.

Substituted quinoline-2,4-dicarboxylates (QDCs) are conformationally-restricted mimics of glutamate that were previously reported to selectively block the glutamate vesicular transporters (VGLUTs). We find that expanding the QDC scaffold to benzoquinoline dicarboxylic acids (BQDC) and naphthoquinoline dicarboxylic acids (NQDCs) improves inhibitory activity with the NQDCs showing IC 50 ∼ 70 μM. Modeling overlay studies showed that the polycyclic QDCs resembled steroid structures and led to the identification and testing of estrone sulfate, pregnenolone sulfate and pregnanolone sulfate that blocked the uptake of l -Glu by 50%, 70% and 85% of control, respectively. Pregnanolone sulfate was further characterized by kinetic pharmacological determinations that demonstrated competitive inhibition and a K i of ≈20 μM.

Abstract: The acidic amino acid L-glutamate ucts as both a primary excitatory neurotransmitter and a potential neurotoxin within the mammalian central nervous system. Functionally juxtaposed between these neurophysiological and pathological actions are un assorted group of integral membrane transporter proteins that rapidly and efficiently sequester glutamate into cellular and subcellular compartments. While multiple systems exist that are capable of mediating the uptake or L-glutamate, the high-affinity, sodium-dependent transporters have emerged as the most prominent players in the CNS with respect to terminating the excitatory signal. recycling the transmitter, and regulating extracellular levels of glutamate below those which could induce excitotoxic pathology. The focus of the present review is on the pharmacological specificity of these sodium­ dependent transporters and, more specifically, on the competitive inhibitors that have been used to delineate the chemical requirements for binding and translocation. Anulogues of glutamate that are conformationally constrained as a consequence of either the addition of substituents to the carbon backbone of glutamate or aspartate (e.g., -hydroxyaspartate or methylglutamate derivatives) or the incorporation of ring systems (e.g., (carboxycyclopropyl)glycines, aminocyclobutane dicarboxylates, or pyrrolidine dicarboxylates), have been especially valuable in these efforts. In this review, a particular emphasis is placed on the identification of analogues that exhibit preferential activity among the recently cloned transporter subtypes and on the differentiation of substrates from non-transportable inhibitors.

This study investigated the excitotoxic properties of a novel series of NMDA analogues in which a methyl group was introduced to the 5-position of the pyrrolidine ring of L-trans-2,3-PDC, a previously identified NMDA receptor agonist. While all of these compounds induced NMDA-receptor-mediated injury, methylation increased in vivo excitotoxic potency 1000-fold. Injections (1 mu 1) in rat dorsal hippocampus of cis- and trans-5-methyl-L-trans-2,3-PDC (0.1 nmol) induced 50-70% neuronal damage to areas CA1 and CA4, comparable to that induced by 100 nmol of L-trans-2,3-PDC. Further, cis- and trans-methylated analogues induced distinct patterns of hippocampal pathology consistent with differential excitotoxic vulnerability of neurons expressing NMDA receptors. Neuronal damage produced by the 5-methyl-L-trans-2,3-PDCs could be blocked by coadministration of MK-801 (3 mg/kg ip), but not NBQX (25 nmol). Biochemical and physiological assays confirmed the action of the analogues as NMDA agonists, but did not provide an explanation for differences in excitotoxic potency between the methylated and nonmethylated 2,3-PDCs. or example, the activity of the compounds as inhibitors of 3H-glutamate binding (IC50 values: 0.4, 1.4, and 1.2 microM for cis-5-methyl-,trans-5-methyl-, and L-trans-2,3-PDC, respectively), agonists at NR1A/NR2B receptors (EC50 values: 5, 49, and 16 microM for cis-5-methyl-,trans-5-methyl-, and L-trans-2,3-PDC, respectively), and in vitro excitotoxins in cortical cultures varied only two- to fivefold as a consequence of methylation. Potential roles of NMDA receptor subtypes and transport in these effects are discussed. As potent and selective NMDA excitotoxins, cis- and trans-5-methyl-L-trans-2,3-PDC will be of value studying excitotoxic mechanisms, MDA-receptor-mediated pathology, and NMDA receptor heterogeneity.

Molecular modeling was used evaluate conformational effects of side chain modifications to kainate and the pharmacological consequences of such modifications on binding to KA, NMDA, and AMPA receptors and to the high-affinity sodium-dependent glutamate transporter.

This study investigated the in vitro and in vivo excitotoxic properties of a novel conformationally constrained analogue of L-trans-2,3-pyrrolidine dicarboxylate (L-trans-2,3-PDC). When tested for excitotoxic activity in rat cortical cultures, L-trans-2,3-PDC mimicked the action of NMDA in both acute (30 min) and chronic (24 hr) exposure paradigms. This neurotoxicity was attenuated by co-addition of MK-801 (10 μM). Microinjections of L-trans-2,3-PDC into the dorsal hippocampus of male rats also induced a selective pattern of pathology indicative of an NMDA receptor excitotoxin. In contrast to the equipotency observed in vitro, 100 nmol of L-trans-2,3-PDC were needed to produce cellular damage comparable to that induced by 25 nmol of NMDA. Consistent with an action at NMDA receptors, L-trans-2,3-PDC-induced damage could be significantly reduced by co-administration of MK-801 (3 mg/kg i.p.), but not by NBQX (25 nmol). In radioligand binding assays L-trans-2,3-PDC inhibited the binding of 3H-L-glutamate to NMDA receptors (IC50 1 μM), although it also exhibited some cross reactivity with KA and AMPA receptors. L-trans-2,3-PDC was also identified as a competitive inhibitor (Ki = 33 μM) of 3H-D-aspartate uptake into rat forebrain synaptosomes. In contrast to the action of a transporter substrate, such as L-glutamate, L-trans-2,3-PDC did not exchange with 3H-D-aspartate that had been previously loaded into the synaptosomes. Copyright © 1996 Elsevier Science Ltd.

Using cultured cortical astrocytes we demonstrate differential activation of metabotropic signal transduction pathways with 1-aminocyuclopentane-trans-1S3R-dicarboxylic acid (1S3R-ACPD) and the glutamate transport inhibitor trans-2,4-pyrrolidine dicarboxylic acid (trans-2,4-PDC). Phosphoinositide hydrolysis was more potently stimulated by 1S3R-ACPD than by L -trans-2,4-PDC; however, L -trans-2,4-PDC was far more efficacious than 1S3R-ACPD at inhibiting cyclic AMP accumulation. The metabotropic receptor antagonist (+)-α-methyl-4-carboxyphenylglycine ((+)-MCPG) inhibited 1S3R-ACPD stimulation of phosphoinositide hydrolysis but not its ability to inhibit cyclic AMP accumulation thereby demonstrating a means to pharmacologically dissociate these two metabotropic signal transduction pathways in astrocytes. (+)-MCPG produced similar antagonism of the metabotropic agonist properties of L -trans-2,4-PDC. The metabotropic effects of L -trans-2,4-PDC could not be reduced with enzymatic treatment of the cultures to remove extracellular glutamate, suggesting that these effects are not secondary to the ability of this compound to inhibit glutamate uptake. Taken together the findings indicate the presence of multiple glutamatergic signal transduction pathways in astrocytes and suggest a similarity in the pharmacophores for metabotropic receptors and glutamate transporters.

Conformationally restricted analogs of naturally-occurring amino acids can serve in a variety of ways as protein structure/function probes. A diastereo- and enantioselective synthesis of the four stereoisomers of 2,3-pyrrolidinedicarboxylic acid (an analog of aspartic acid) are described in this paper. The key step is a Rapoport-type aspartate alkylation that can be controlled to give good yields of either diastereomer as a function of enolate geometry. A novel type of chelation control is proposed to account for these results

Evidence was acquired prior to suggest that the vesicular glutamate transporter (VGLUT) but not other glutamate transporters were inhibited by structures containing a weakly basic α-amino group. To test this hypothesis, a series of analogs using a hydantoin (p K a ∼ 9.1) isostere were synthesized and analyzed as inhibitors of VGLUT and the obligate cystine–glutamate transporter (system x c - ). Of the hydantoin analogs tested, a thiophene-5-carboxaldehyde analog 2l and a bis-hydantoin 4b were relatively strong inhibitors of VGLUT reducing uptake to less than 6% of control at 5 mM but few inhibited system x c - greater than 50% of control. The benzene-2,4-disulfonic acid analog 2b and p -diaminobenzene analog 2e were also good hydantoin-based inhibitors of VGLUT reducing uptake by 11% and 23% of control, respectively, but neither analog was effective as a system x c - inhibitor. In sum, a hydantoin isostere adds the requisite chemical properties needed to produce selective inhibitors of VGLUT.

Recent studies have demonstrated that astrocytes have much greater abilities to produce and respond to signalling molecules in the CNS than had been previously estimated. We now report a dramatic enhancement in the ability of a glutamate metabotropic receptor agonist, 1-aminocyclopentane-trans-(+/-)-1,3-dicarboxylic acid (trans-(+/-)-ACPD, to stimulate phosphoinositide hydrolysis in astrocytes cultured in a serum-free defined medium compared with astrocytes cultured in conventional serum-containing medium (43.2 +/- 3.6 vs. 3.2 +/- 0.48-fold of basal, respectively). This enhancement was selective to trans-(+/-)-ACPD as little or no difference in the response to carbachol or norepinephrine was seen between the two culture conditions. These results indicate a great potential for the phosphoinositide pathway in astrocyte glutamatergic signal transduction.

A series of acidic amino acids has been prepared and evaluated in an effort to identify the structural features required for binding to and inhibiting the high affinity uptake system that clears L-glutamate from the synaptic cleft during excitatory amino acid-mediated neurotransmission in the mammalian CNS.

The system x(C)- (Sx(C)-) transporter functions to mediate the exchange of extracellular cystine (L-Cys(2)) and intracellular glutamate (L-Glu). Internalized L-Cys(2) serves as a rate-limiting precursor for the biosynthesis of glutathione (GSH), while the externalized L-Glu can contribute to either excitatory signaling or excitotoxicity. In the present study the influence of culture conditions (with and without dibutyryl-cAMP) and GSH levels on the expression of Sx(C)- were investigated in primary rat astrocyte cultures. Sx(C)- activity in dbcAMP-treated cells was nearly sevenfold greater than in untreated astrocytes and increased further (∼threefold) following the depletion of intracellular GSH with buthionine sulfoximine. This increase in Sx(C)- triggered by GSH depletion was only observed in the dbcAMP-treated phenotype and was distinct from the Nrf2-mediated response initiated by exposure to electrophiles. Changes in Sx(C)- activity correlated with increases in both protein and mRNA levels of the xCT subunit of the Sx(C)- heterodimer, an increase in the V(max) for L-Glu uptake and was linked temporally to GSH levels. This induction of Sx(C)- was not mimicked by hydrogen peroxide nor attenuated by nonspecific antioxidants but was partially prevented by the co-administration of the cell-permeant thiols GSH-ethyl ester and N-acetylcysteine. These findings demonstrate that the expression of Sx(C)- on astrocytes is dynamically regulated by intracellular GSH levels in a cell- and phenotype-dependent manner. The presence of this pathway likely reflects the inherent vulnerability of the CNS to oxidative damage and raises interesting questions as to the functional consequences of changes in Sx(C)- activity in CNS injury and disease.

The glial excitatory amino acid transporter 2 (EAAT2) mediates a majority of glutamate re-uptake in human CNS and, consequently, is associated with a variety of signaling and pathological processes. While our understanding of the function, mechanism and structure of this integral membrane protein is increasing, little if any mass spectrometric (MS) data is available for any of the EAATs specifically, and for only a few mammalian plasma membrane transporters in general. A protocol to express and purify functional EAAT2 in sufficient quantities to carry out MS-based peptide mapping as needed to study ligand–transporter interactions is described. A 6×HIS epitope was incorporated into the N-terminus of human EAAT2. The recombinant protein was expressed in high levels in mammalian HEK 293T cells, where it exhibited the pharmacological properties of the native transporter. EAAT2 was purified from isolated cell membranes in a single step using nickel affinity chromatography. In-gel and in-solution trypsin digestions were conducted on the isolated protein and then analyzed by MALDI-TOF and LC-MS/MS mass spectrometry. Overall, 89% sequence coverage of the protein was achieved with these methods. In particular, an 88 amino acid tryptic peptide covering the presumed substrate binding domains HP1, TMD7, HP2, and TMD8 domains of EAAT2 was also identified after N-deglycosylation. Beyond the specific applicability to EAAT2, this study provides an efficient, simple and scalable approach to express, purify, digest and characterize integral membrane transporter proteins by mass spectrometry.

Analogues of amino methylisoxazole propionic acid (AMPA), were prepared from a common intermediate 12, including lipophilic analogues using lateral metalation and electrophilic quenching, and were evaluated at System x c - . Both the 5-naphthylethyl-(16) and 5-naphthylmethoxymethyl-(17) analogues adopt an E-conformation in the solid state, yet while the former has robust binding at System x c - , the latter is virtually devoid of activity. The most potent analogues were amino acid naphthyl-ACPA 7g, and hydrazone carboxylic acid, 11e Y = Y′ = 3,5-(CF3)2, which both inhibited glutamate uptake by the System x c - transporter with comparable potency to the endogenous substrate cystine, whereas in contrast the closed isoxazolo[3,4-d] pyridazinones 13 have significantly lower activity. A preliminary pharmacophore model has been constructed to provide insight into the analogue structure–activity relationships.

The title compound, C26H23N5O8, was prepared and its structure investigated to further develop a working hypothesis for the essential binding pharmacophore for ligands of the System Xc- transporter [Patel et al. (2004). Neuropharmacology, 46, 273–284]. The hydrazone group displays an E geometry and the isoxazole double bond and C=N group of the hydrazone are in an s-cis relationship. The secondary amino NH group forms an intramolecular N—H...O hydrogen bond to a ring nitro group. There is a dihedral angle of 44.27 (5)° between the isoxazole plane and the hydrazone group plane.

β-N-Oxalyl- l -α,β-diaminopropionic acid (β- l -ODAP) is an excitatory amino acid agonist found in the seeds ofLathyrus sativus that is believed to be the major causative agent in the pathology of human lathyrism. We have found that in addition to its previously recognized neurotoxic properties, β- l -ODAP is also gliotoxic. When added to cultures of neonatal rat astrocytes, β- l -ODAP induced a series of morphological changes (e.g., extensive vacuole formation, pale and swollen nuclei with obvious nucleoli, and cellular swelling) that led to the eventual lysis of the glial cells. If the β- l -ODAP was removed prior to the lysis of the astrocytes, many of the early morphological changes appeared to be reversible. When quantitated by a loss of the lactate dehydrogenase activity, β- l -ODAP lysed the astrocytes with an LD50 of2.1 ± 0.2mM following 48 h of exposure. Lower concentrations of β- l -ODAP were found to be more toxic if the duration of the exposure was increased. The results suggest that the overall impact of the toxin on the CNS may represent the cumulative action of β- l -ODAP at a number of distinct points on both neurons and astrocytes. The potential that these multiple sites of action may affect the normal regulation of extracellular glutamate and, consequently, disturb the balance between its normal and pathological roles is discussed.

Olfactory receptor neurons are a CNS entry point for a wide variety of airborne substances. Therefore, it is probable that detoxification mechanisms are present in these neurons to neutralize such agents. Glutathione (GSH) is an essential component of several detoxification schemes, and this study we examined the distribution and levels of GSH in the olfactory epithelium, olfactory bulb, cortex, hippocampus and cerebellum in neonatal, weanling, adult and aged rats. We report that GSH is primarily localized to the olfactory receptor neurons and their axons within the olfactory epithelium. It is also localized within the glomerular neuropil and granule cells of the olfactory bulb. Levels of GSH in the olfactory epithelium and hippocampus do not change as a function of age, although GSH levels decrease in several brain regions, including the olfactory bulb, cerebellum and cortex.

A series of β-benzylaspartate derivatives were prepared from N -trityl- l -aspartate dimethyl ester and evaluated as inhibitors of neuronal glutamate transporter EAAT3. The result of the structure–activity studies suggests that the position occupied by the aromatic ring of β-benzylaspartate within the binding site of EAAT3 may be different from that occupied by comparable groups in previously identified inhibitors, such as l - threo -benzyloxy aspartate (TBOA). Further, halogen substitutions at the 3-postition of the aromatic ring of β-benzylaspartate can increase the potency with which the analogues inhibit EAAT3.

As the primary excitatory neurotransmitter in the mammalian CNS, l-glutamateparticipates not only in standard fast synaptic communication, but also contributes to higher order signalprocessing, as well as neuropathology. Given this variety of functional roles, interest has been growingas to how the extracellular concentrations of l-glutamate surroundingneurons are regulated by cellular transporter proteins. This review focuses on two prominent systems, eachof which appears capable of influencing both the signaling and pathological actions of l-glutamatewithin the CNS: the sodium-dependent excitatory amino acid transporters (EAATs) and the glutamate/cystineexchanger, system x c −(Sx c −). Whilethe family of EAAT subtypes limit access to glutamate receptors by rapidly and efficiently sequesteringl-glutamate in neurons and glia, Sxc −provides a route for the export of glutamate from cells into the extracellular environment. The primaryintent of this work is to provide an overview of the inhibitors and substrates that have been developedto delineate the pharmacological specificity of these transport systems, as well as be exploited as probeswith which to selectively investigate function. Particular attention is paid to the development of smallmolecule templates that mimic the structural properties of the endogenous substrates, l-glutamate,l-aspartate and l-cystine andhow strategic control of functional group position and/or the introduction of lipophilic R-groups can impactmultiple aspects of the transport process, including: subtype selectivity, inhibitory potency, and substrateactivity.