Pharmacology and Structural Analysis of Ligand Binding to the Orthosteric Site of Glutamate-Like GluD2 Receptors.

The GluD2 receptor is a fundamental component of postsynaptic sites in Purkinje neurons, and is required for normal cerebellar function. GluD2 and the closely related GluD1 are classified as members of the ionotropic glutamate receptor (iGluR) superfamily on the basis of sequence similarity, but do not bind l-glutamate. The amino acid neurotransmitter D-Ser is a GluD2 receptor ligand, and endogenous D-Ser signaling through GluD2 has recently been shown to regulate endocytosis of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type iGluRs during synaptic plasticity in the cerebellum, such as long-term depression. Here, we investigate the pharmacology of the orthosteric binding site in GluD2 by examining the activity of analogs of D-Ser and GluN1 glycine site competitive antagonists at GluD2 receptors containing the lurcher mutation (GluD2(LC)), which promotes spontaneous channel activation. We identify several compounds that modulate GluD2(LC), including a halogenated alanine analog as well as the kynurenic acid analog 7-chloro-4-oxo-1H-quinoline-2-carboxylic acid (7-chlorokynurenic acid; 7-CKA). By correlating thermodynamic and structural data for 7-CKA binding to the isolated GluD2 ligand binding domain (GluD2-LBD), we find that binding 7-CKA to GluD2-LBD differs from D-Ser by inducing an intermediate cleft closure of the clamshell-shaped LBD. The GluD2 ligands identified here can potentially serve as a starting point for development of GluD2-selective ligands useful as tools in studies of the signaling role of the GluD2 receptor in the brain.

Origins, distribution and expression of the Duarte-2 (D2) allele of galactose-1-phosphate uridylyltransferase.

Duarte galactosemia is a mild to asymptomatic condition that results from partial impairment of galactose-1-phosphate uridylyltransferase (GALT). Patients with Duarte galactosemia demonstrate reduced GALT activity and carry one profoundly impaired GALT allele (G) along with a second, partially impaired GALT allele (Duarte-2, D2). Molecular studies reveal at least five sequence changes on D2 alleles: a p.N314D missense substitution, three intronic base changes and a 4 bp deletion in the 5' proximal sequence. The four non-coding sequence changes are unique to D2. The p.N314D substitution, however, is not; it is found together with a silent polymorphism, p.L218(TTA), on functionally normal Duarte-1 alleles (D1, also called Los Angeles or LA alleles). The HapMap database reveals that p.N314D is a common human variant, and cross-species comparisons implicate D314 as the ancestral allele. The p.N314D substitution is also functionally neutral in mammalian cell and yeast expression studies. In contrast, the 4 bp 5' deletion characteristic of D2 alleles appears to be functionally impaired in reporter gene transfection studies. Here we present allele-specific qRT-PCR evidence that D2 alleles express less mRNA in vivo than their wild-type counterparts; the difference is small but statistically significant. Furthermore, we characterize the prevalence of the 4 bp deletion in GG, NN and DG populations; the deletion appears exclusive to D2 alleles. Combined, these data strongly implicate the 4 bp 5' deletion as a causal mutation in Duarte galactosemia and suggest that direct tests for this deletion, as proposed here, could enhance or supplant current tests, which define D2 alleles on the basis of the presence and absence of linked coding sequence polymorphisms.

Modulation of the dimer interface at ionotropic glutamate-like receptor delta2 by D-serine and extracellular calcium.

GluRdelta2 is a member of the iGluR family, but despite a prominent role in cerebellar synaptic plasticity, this receptor does not appear to function as an ion channel. Endogenous ligands that modulate the activity of native GluRdelta2 in the cerebellum have not been identified, but two candidate modulators are d-serine and extracellular calcium. Taking advantage of known crystal structures and spontaneously active GluRdelta2 receptors containing the lurcher mutation (GluRdelta2(Lc)), we investigated the mechanism by which calcium and d-serine regulate the activity of GluRdelta2(Lc). Our data suggest that calcium binding stabilizes the dimer interface formed between two agonist-binding domains and increases GluRdelta2(Lc) currents. The data further suggest that d-serine binding induces rearrangements at the dimer interface to diminish GluRdelta2(Lc) currents by a mechanism that resembles desensitization at AMPA and kainate receptors. Thus, we propose that calcium and d-serine binding have opposing effects on the stability of the dimer interface. Furthermore, the effects of calcium are observed at concentrations that are within the physiological range, suggesting that the ability of native GluRdelta2 to respond to ligand binding may be modulated by extracellular calcium. These findings place GluRdelta2 among AMPA and kainate receptors, where the dimer interface is not only a biologically important site for functional regulation, but also an important target for exogenous and endogenous ligands that modulate receptor function.

Implementation of a fluorescence-based screening assay identifies histamine H3 receptor antagonists clobenpropit and iodophenpropit as subunit-selective N-methyl-D-aspartate receptor antagonists.

N-Methyl-D-aspartate (NMDA) receptors are ligand-gated ion channels that mediate a slow, Ca(2+)-permeable component of excitatory synaptic transmission in the central nervous system and play a pivotal role in synaptic plasticity, neuronal development, and several neurological diseases. We describe a fluorescence-based assay that measures NMDA receptor-mediated changes in intracellular calcium in a BHK-21 cell line stably expressing NMDA receptor NR2D with NR1 under the control of a tetracycline-inducible promoter (Tet-On). The assay selectively identifies allosteric modulators by using supramaximal concentrations of glutamate and glycine to minimize detection of competitive antagonists. The assay is validated by successfully identifying known noncompetitive, but not competitive NMDA receptor antagonists among 1800 screened compounds from two small focused libraries, including the commercially available library of pharmacologically active compounds. Hits from the primary screen are validated through a secondary screen that used two-electrode voltage-clamp recordings on recombinant NMDA receptors expressed in Xenopus laevis oocytes. This strategy identified several novel modulators of NMDA receptor function, including the histamine H3 receptor antagonists clobenpropit and iodophenpropit, as well as the vanilloid receptor transient receptor potential cation channel, subfamily V, member 1 (TRPV1) antagonist capsazepine. These compounds are noncompetitive antagonists and the histamine H3 receptor ligand showed submicromolar potency at NR1/NR2B NMDA receptors, which raises the possibility that compounds can be developed that act with high potency on both glutamate and histamine receptor systems simultaneously. Furthermore, it is possible that some actions attributed to histamine H3 receptor inhibition in vivo may also involve NMDA receptor antagonism.

Design, synthesis, and structure-activity relationship of a novel series of GluN2C-selective potentiators.

NMDA receptors are tetrameric complexes composed of GluN1 and GluN2A-D subunits that mediate a slow Ca(2+)-permeable component of excitatory synaptic transmission. NMDA receptors have been implicated in a wide range of neurological diseases and thus represent an important therapeutic target. We herein describe a novel series of pyrrolidinones that selectively potentiate only NMDA receptors that contain the GluN2C subunit. The most active analogues tested were over 100-fold selective for recombinant GluN2C-containing receptors over GluN2A/B/D-containing NMDA receptors as well as AMPA and kainate receptors. This series represents the first class of allosteric potentiators that are selective for diheteromeric GluN2C-containing NMDA receptors.

A subunit-selective potentiator of NR2C- and NR2D-containing NMDA receptors.

NMDA receptors are tetrameric complexes of NR1 and NR2A-D subunits that mediate excitatory synaptic transmission and have a role in neurological disorders. In this article, we identify a novel subunit-selective potentiator of NMDA receptors containing the NR2C or NR2D subunit, which could allow selective modification of circuit function in regions expressing NR2C/D subunits. The substituted tetrahydroisoquinoline CIQ (3-chlorophenyl)(6,7-dimethoxy-1-((4-methoxyphenoxy)methyl)-3,4-dihydroisoquinolin-2(1H)-yl)methanone) enhances receptor responses two-fold with an EC(50) of 3 µM by increasing channel opening frequency without altering mean open time or EC(50) values for glutamate or glycine. The actions of CIQ depend on a single residue in the M1 region (NR2D Thr592) and on the linker between the N-terminal domain and agonist binding domain. CIQ potentiates native NR2D-containing NMDA receptor currents from subthalamic neurons. Our identification of a subunit-selective NMDA receptor modulator reveals a new class of pharmacological tools with which to probe the role of NR2C- and NR2D-containing NMDA receptors in brain function and disease.