L-trans-2,3-pyrrolidine dicarboxylate: characterization of a novel excitotoxin.

This study investigated the in vitro and in vivo excitotoxic properties of a novel conformationally constrained analogue of L-glutamate, 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 h) exposure paradigms. This neurotoxicity was attenuated by co-addition of MK-801 (10 microM). 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 microM), 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 microM) of 3H-D-aspartate uptake into rat forebrain synaptosomes. In contrast to the action of a transported 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.

The synthesis of homoallylic amines utilizing a cuprate-based 1,2-metalate rearrangement.

Lithiation of the N-2,4,6-triisopropylbenzenesulfonyl-2-pyrroline (16) and treatment of the resulting cyclic vinyllithium reagent with R2CuCNLi2 produced an acyclic vinyl organometallic species that, when treated with an electrophile (H2O or RX), gave the homoallylic sulfonamides 18a-k in 37-93% yields and in > 95% diastereoselectivity. The deprotection of a representative homoallylic sulfonamide 18d was achieved in 83% yield by sonication in the presence of lithium wire and catalytic 4,4'-di-tert-butylbiphenyl (DBB). The efficacy of this general procedure for the production of homoallylic amine derivatives is demonstrated by the preparation of the diene amine 25, a key intermediate in the synthesis of a squalene synthetase inhibitor.

The design, synthesis, and biological evaluation of analogues of the serine-threonine protein phosphatase 1 and 2A selective inhibitor microcystin LA: rational modifications imparting PP1 selectivity.

Based on the results from previously reported molecular modeling analyses of the interactions between the inhibitor microcystin and the serine-threonine protein phosphatases 1 and 2A, we have designed analogues of microcystin LA with structural modifications intended to impart PP1 selectivity. The synthesis of several first generation analogues followed by inhibition assays revealed that all three are PP1-selective, as predicted. Although the observed selectivities are modest, one of the designed analogues is more selective for PP1 than any known small molecule inhibitor.

Methylation of the NMDA receptor agonist L-trans-2,3-pyrrolidine-dicarboxylate: enhanced excitotoxic potency and selectivity.

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.

Differentiation of substrate and nonsubstrate inhibitors of the high-affinity, sodium-dependent glutamate transporters.

Within the mammalian central nervous system, the efficient removal of L-glutamate from the extracellular space by excitatory amino acid transporters (EAATs) has been postulated to contribute to signal termination, the recycling of transmitter, and the maintenance of L-glutamate at concentrations below those that are excitotoxic. The development of potent and selective inhibitors of the EAATs has contributed greatly to the understanding of the functional roles of these transporters. In the present study, we use a library of conformationally constrained glutamate analogs to address two key issues: the differentiation of substrates from nontransportable inhibitors and the comparison of the pharmacological profile of synaptosomal uptake with those of the individual EAAT clones. We demonstrate that the process of transporter-mediated heteroexchange can be exploited in synaptosomes to rapidly distinguish transportable from nontransportable inhibitors. Using this approach, we demonstrate that 2,4-methanopyrrolidine-2,4-dicarboxylate, cis-1-aminocyclobutane-1,3-dicarboxylate, and L-trans-2, 4-pyrrolidine dicarboxylate act as substrates for the rat forebrain synaptosomal glutamate uptake system. In contrast, L-anti-endo-3, 4-methanopyrrolidine-3,4-dicarboxylate, L-trans-2,3-pyrrolidine dicarboxylate, and dihydrokainate proved to be competitive inhibitors of D-[(3)H]aspartate uptake that exhibited little or no activity as substrates. When these same compounds were characterized for substrate activity by recording currents in voltage-clamped Xenopus laevis oocytes expressing the human transporter clones EAAT1, EAAT2, or EAAT3, it was found that the pharmacological profile of the synaptosomal system exhibited the greatest similarity with the EAAT2 subtype, a transporter believed to be expressed primarily on glial cells.