Mechanism and properties of positive allosteric modulation of N -methyl- d -aspartate receptors by 6-alkyl 2-naphthoic acid derivatives

Abstract

The theory that N-methyl-D-aspartate receptor (NMDAR) hypofunction is responsible for the symptoms of schizophrenia is well supported by many pharmacological and genetic studies. Accordingly, positive allosteric modulators (PAMs) that augment NMDAR signaling may be useful for treating schizophrenia. Previously we have identified several NMDAR PAMs containing a carboxylic acid attached to naphthalene, phenanthrene, or coumarin ring systems. In this study, we describe several functional and mechanistic properties of UBP684, a 2-naphthoic acid derivative, which robustly potentiates agonist responses at each of the four GluN1a/GluN2 receptors and at neuronal NMDARs. UBP684 increases the maximal L-glutamate/glycine response while having minor subunit-specific effects on agonist potency. PAM binding is independent of agonist binding, and PAM activity is independent of membrane voltage, redox state, and the GluN1 exon 5 N-terminal insert. UBP684 activity is, however, markedly pH-dependent, with greater potentiation occurring at lower pHs and inhibitory activity at pH 8.4. UBP684 increases channel open probability (Po) and slows receptor deactivation time upon removal of L-glutamate, but not glycine. The structurally related PAM, UBP753, reproduced most of these findings, but did not prolong agonist removal deactivation time. Studies using cysteine mutants to lock the GluN1 and GluN2 ligand-binding domains (LBDs) in the agonist-bound states indicate that PAM potentiation requires GluN2 LBD conformational flexibility. Together, these findings suggest that UBP684 and UBP753 stabilize the GluN2 LBD in an active conformation and thereby increase Po. Thus, UBP684 and UBP753 may serve as lead compounds for developing agents to enhance NMDAR activity in disorders associated with NMDAR hypofunction.

Higher ambient synaptic glutamate at inhibitory versus excitatory neurons differentially impacts NMDA receptor activity

“…Other aspects of synaptic microenvironments could also vary between synapses, such as pH 68 , zinc concentration 69 , polyamine concentration, and so on, and therefore modulators with dependence on these aspects could also potentially exhibit synapse-specific effectiveness. For example, modulators with marked pH dependence, like UBP684 70 , or certain GluN2B antagonists 71 could be used to probe for pH differences between synaptic microenvironments.…”

The NMDA receptor intracellular C-terminal domains reciprocally interact with allosteric modulators

“…UBP684 and UBP792 were from our own laboratories [22]. 5-Pregnen-3β-ol-20-onesulphate (pregnenolone sulfate, PS) and 5β-pregnan-3α-ol-20-onesulphate (pregnanolone sulphate, PAS) were purchased from Steraloids, Inc. (Newport, RI, USA).…”

“…We have recently characterized the mechanism of action of the general NMDAR PAM, UBP684 [22,25]. As this PAM can potentiate the activity of all four GluN1/GluN2A-D receptors, we used this compound to compare the effect of CTD-deletion at the different NMDAR subunits.…”

“…The activity of another NAM, UBP792 [22], was similarly affected by GluN2 CTD-deletion in that the largest effects were to enhance inhibition at GluN1/GluN2A receptors. The GluN2A DCTD receptor was inhibited 64 ± 6 % (n = 3 oocytes) by 30 µM UBP792, while the WT GluN2A-containing receptor was only inhibited by 22 ± 8 % (n = 4 oocytes).…”

Biased modulators of NMDA receptors control channel opening and ion selectivity

“…NMDARs are tetrameric assemblies of two glycine-binding GluN1 and two glutamate-binding GluN2 subunits, of which there are four subtypes GluN2A-2D 1 . The recent development of subunit-selective NMDAR positive allosteric modulators should allow cell-type-or circuit-specific modulation due to spatiallydistinct GluN2 expression, without global NMDAR overactivation [13][14][15][16][17][18][19][20] . However, while positive allosteric modulators act only when agonist is present to enhance NMDAR function, there remains a risk for overactivation at some synapses.…”