Novel Inhibitory Site Revealed by Mode of Action on the Metabotropic Glutamate 7 Receptor Venus Flytrap Domain

Metabotropic glutamate (mGlu) receptors play a key role in modulating most synapses in the brain. The mGlu7 receptors inhibit presynaptic neurotransmitter release and offer therapeutic possibilities for post-traumatic stress disorders or epilepsy. Screening campaigns provided mGlu7-specific allosteric modulators as the inhibitor XAP044 (Gee et al. J. Biol. Chem. 2014). In contrast to other mGlu receptor allosteric modulators, XAP044 does not bind in the transmembrane domain but to the extracellular domain of the mGlu7 receptor and not at the orthosteric site. Here, we identified the mode of action of XAP044, combining synthesis of derivatives, modeling and docking experiments, and mutagenesis. We propose a unique mode of action of these inhibitors, preventing the closure of the Venus flytrap agonist binding domain. While acting as a noncompetitive antagonist of L-AP4, XAP044 and derivatives act as apparent competitive antagonists of LSP4-2022. These data revealed more potent XAP044 analogues and new possibilities to target mGluRs.

Activation of the mu-delta opioid receptor heteromers blocks morphine rewarding effects

BACKGROUND

Evidence has accumulated demonstrating the existence of opioid receptor heteromers and recent data suggest that targeting these heteromers could reduce opioid side effects while retaining therapeutic effects. Indeed, CYM51010 characterised as a MOR (mu opioid receptor)/DOR (delta opioid receptor) heteromers preferring agonist promoted antinociception comparable to morphine but with less tolerance. In the perspective of developing these new classes of pharmacological agents, data on their putative side effects are mandatory.

METHODS

Therefore, in this study, we investigated the effects of CYM51010 in different models related to drug addiction in mice including behavioural sensitization, conditioned place preference and withdrawal.

RESULTS

We found that, like morphine, CYM51010 promoted acute locomotor activity as well as psychomotor sensitization and rewarding effect. However, it induced less physical dependence than morphine. We also investigated the ability of CYM51010 to modulate some morphine-induced behaviour. Whereas CYM51010 was unable to block morphine-induced physical dependence, it blocked reinstatement of an extinguished morphine induced-conditioned place preference.

CONCLUSIONS

Altogether, our results reveal that targeting MOR-DOR heteromers could represent a promising strategy to block morphine reward.

LSP2-9166, an orthosteric mGlu4 and mGlu7 receptor agonist, reduces cocaine self-administration under a progressive ratio schedule in rats

Cocaine addiction is a serious health issue in Western countries. Despite the regular increase in cocaine consumption across the population, there is no specific treatment for cocaine addiction. Critical roles for glutamate neurotransmission in the rewarding effects of psychostimulants as well as relapse have been suggested and accumulating evidence indicates that targeting mGlu group III receptors could represent a promising strategy to develop therapeutic compounds to treat addiction. In this context, the aim of our study was to examine the effect of LSP2-9166, a mGlu4/mGlu7 receptor orthosteric agonist, on the motivation for cocaine intake. We used an intravenous self-administration paradigm in male Wistar rats as a reliable model of voluntary drug intake. We first evaluated the direct impact of cocaine on Grm4 and Grm7 gene expression. Voluntary cocaine intake under a fixed ratio schedule of injections induced an increase of both mGlu4 and mGlu7 receptor transcripts in nucleus accumbens and hippocampus. We then evaluated the ability of LSP2-9166 to affect cocaine self-administration under a progressive ratio schedule of reinforcement. We found that this compound inhibits the motivation to obtain the drug, although it induced a hypolocomotor effect which could biais motivation index. Our findings demonstrate that mGlu group III receptors represent new targets for decreasing motivation to self-administer cocaine.

Amino Acids Bearing Aromatic or Heteroaromatic Substituents as a New Class of Ligands for the Lysosomal Sialic Acid Transporter Sialin

Sialin, encoded by the SLC17A5 gene, is a lysosomal sialic acid transporter defective in Salla disease, a rare inherited leukodystrophy. It also enables metabolic incorporation of exogenous sialic acids, leading to autoantibodies against N-glycolylneuraminic acid in humans. Here, we identified a novel class of human sialin ligands by virtual screening and structure-activity relationship studies. The ligand scaffold is characterized by an amino acid backbone with a free carboxylate, an N-linked aromatic or heteroaromatic substituent, and a hydrophobic side chain. The most potent compound, 45 (LSP12-3129), inhibited N-acetylneuraminic acid 1 (Neu5Ac) transport in a non-competitive manner with IC₅₀ ≈ 2.5 μM, a value 400-fold lower than the K_M for Neu5Ac. In vitro and molecular docking studies attributed the non-competitive character to selective inhibitor binding to the Neu5Ac site in a cytosol-facing conformation. Moreover, compound 45 rescued the trafficking defect of the pathogenic mutant (R39C) causing Salla disease. This new class of cell-permeant inhibitors provides tools to investigate the physiological roles of sialin and help develop pharmacological chaperones for Salla disease.

Conformational pathway provides unique sensitivity to a synaptic mGluR

Metabotropic glutamate receptors (mGluRs) are dimeric G-protein-coupled receptors that operate at synapses. Macroscopic and single molecule FRET to monitor structural rearrangements in the ligand binding domain (LBD) of the mGluR7/7 homodimer revealed it to have an apparent affinity ~4000-fold lower than other mGluRs and a maximal activation of only ~10%, seemingly too low for activation at synapses. However, mGluR7 heterodimerizes, and we find it to associate with mGluR2 in the hippocampus. Strikingly, the mGluR2/7 heterodimer has high affinity and efficacy. mGluR2/7 shows cooperativity in which an unliganded subunit greatly enhances activation by agonist bound to its heteromeric partner, and a unique conformational pathway to activation, in which mGluR2/7 partially activates in the Apo state, even when its LBDs are held open by antagonist. High sensitivity and an unusually broad dynamic range should enable mGluR2/7 to respond to both glutamate transients from nearby release and spillover from distant synapses.

The mGlu7 receptor provides protective effects against epileptogenesis and epileptic seizures

Finding new targets to control or reduce seizure activity is essential to improve the management of epileptic patients. We hypothesized that activation of the pre-synaptic and inhibitory metabotropic glutamate receptor type 7 (mGlu7) reduces spontaneous seizures. We tested LSP2-9166, a recently developed mGlu7/4 agonist with unprecedented potency on mGlu7 receptors, in two paradigms of epileptogenesis. In a model of chemically induced epileptogenesis (pentylenetetrazole systemic injection), LSP2-9166 induces an anti-epileptogenic effect rarely observed in preclinical studies. In particular, we found a bidirectional modulation of seizure progression by mGlu4 and mGlu7 receptors, the latter preventing kindling. In the intra-hippocampal injection of kainic acid mouse model that mimics the human mesial temporal lobe epilepsy, we found that LSP2-9166 reduces seizure frequency and hippocampal sclerosis. LSP2-9166 also acts as an anti-seizure drug on established seizures in both models tested. Specific modulation of the mGlu7 receptor could represent a novel approach to reduce pathological network remodeling.

Synthesis and biological properties of new constrained CCK-B antagonists: discrimination of two affinity states of the CCK-B receptor on transfected CHO cells

To improve our knowledge of the bioactive conformation of CCK-B antagonists, we have developed a new series of constrained dipeptoids whose synthesis and biochemical properties are reported here. These compounds, of general structure N alpha-[(2-adamantyloxy)carbonyl]-alpha-methyltryptophanyl-(4 -X)-proline, were designed by introducing a cyclization in the structure of the previously described CCK-B/peptoid antagonist RB 210, N-[N-[(2-adamantyloxy)carbonyl]-DL-alpha-methyltryptophanyl] -N-(2-phenylethyl)glycine (Blommaert et al. J. Med. Chem. 1993, 36, 2868-2877), by means of a five-membered ring. Structure-affinity relationship studies showed that an R configuration of Trp-C alpha and a cis configuration of the pyrrolidine substituents were favorable for receptor recognition. The most potent compounds of this new series had similar affinities for the CCK-B receptor as RB 210 and proved to be far more efficient in inhibiting inositol phosphate production in CHO cells stably transfected with rat brain CCK-B receptor, with IC50 values approaching those of the commonly used antagonists L-365,260 and PD-134,308. Moreover, binding studies performed using transfected CHO cells showed that two affinity states of the CCK-B receptor can be discriminated by some of these compounds which also have different biological profiles and are therefore highly interesting tools for the biochemical and pharmacological characterization of CCK-B receptor heterogeneity.

Heterogeneity of CCK-B receptors involved in animal models of anxiety

The effects of the selective CCK-B agonists, BC 264 and BC 197, and the nonselective CCK agonist BDNL were investigated in the elevated plus-maze in rats. BDNL and BC 197 induced anxiogeniclike effects, in contrast to BC 264, which had no effect. The behavioral responses induced by BDNL were not significantly blocked by L-365,260, but were suppressed by CI-988, another selective CCK-B antagonist, and by high doses of L-364,718, a selective CCK-A antagonist. BC 197-induced effects were also blocked by CI-988. Competition experiments performed with [3H]pBC 264 using brain membranes of guinea pig, mouse, and rat were significantly better fitted when analyzed by a two site model than by a one site model with BC 197 but not with BC 264. Moreover, BC 264 produced anxiogeniclike effects when administered with increasing doses of L-365,260 and opposing effects with increasing doses of CI-988. Together these results give pharmacological and behavioral evidence for the existence of CCK-B receptor subtypes.

Preferred antagonist binding state of the NMDA receptor: synthesis, pharmacology, and computer modeling of (phosphonomethyl)phenylalanine derivatives

A series of substituted [phosphono-, sulfo-, carboxy-, and (N-hydroxycarbamoyl)methyl]phenylalanines were synthesized as probes for the investigation of the preferred antagonist state of the NMDA receptor antagonists. The potency of these compounds was evaluated by measuring electrophysiological responses induced by NMDA in cultured mouse cortical neurons. 3-(Phosphonomethyl)phenylalanine [1(m)] a formal AP7 analogue, has been shown to be the most potent antagonist in this study with an IC50 of around 5 microM. The isomeric 2-(phosphonomethyl)phenylalanine [1(o)] was about half as active as 1(m) and as active as compound 5(3), a derivative which is cis-hydrogenated on the phenyl ring of 1(m). Replacement of a phosphono by a sulfo group led to a large reduction in the ability of these compounds to antagonize NMDA responses, although the ortho and meta isomers retained some activity in their reduced forms. In both series the para isomers were almost completely inactive at 100 microM. Introduction of a carboxyl or a bidentate HONHCO group in place of the phosphono moiety in the 3-position results in compounds devoid of activity. The active and inactive compounds of this study were used in conjunction with the most potent linear and cyclic phosphono-containing NMDA antagonists reported to date to determine, via computer modeling techniques, a three-dimensional model corresponding to a antagonist preferring state of the NMDA binding site. This structure defines a pharmacophore which is characterized by (i) well-defined distances between the central atoms of the polar groups PO3H-, NHn+, (n = 2, 3), and COO- (P-N = 5.89 +/- 0.12 A, P-C = 6.66 +/- 0.08 A, and N-C = 2.28 +/- 0.01 A), (ii) a sterically allowed region between the C5 methylene and the PO3H- group, and (iii) a molecular electrostatic field in which the positive, neutral, and negative potential zones are self-contained--with the negative potential zone connecting the PO3H- and COO- groups as the largest. We have compared our results to a preliminary model of the NMDA antagonist site by Hutchison et al. and to a topological model of the NMDA-glycine receptor site by Cordi et al. Our proposed steric-electrostatic pharmacophore which refines, simplifies, and improves these models has now to be validated by the design of new NMDA antagonists.

Replacement of Tyr-SO3H by a p-carboxymethyl-phenylalanine in a CCK8-derivative preserves its high affinity for CCK-B receptor

The sulfated tyrosine present in the sequence of CCK8 Asp26-Tyr(SO3H)-Met-Gly-Trp-Met-Asp-PheNH2, seems to play a critical role in the recognition of CCK-A binding sites. In this work, we have investigated whether the presence of an anionic charge on the tyrosine moiety is strictly necessary and whether the sulfate moiety interacts with a divalent cation in the receptor subsite. For this purpose, the novel amino acids (L,D)Phe(p-CH2CO2H) and (L,D) Phe(p-CH2CONHOH), as well as their L-resolved forms were introduced into the sequence of Ac[X27, Nle28, Nle31]-CCK27-33 by solid phase method. The biological activities of these new derivatives were compared to two almost equiactive analogues of CCK8, Ac[Phe(p-CH2SO3H)27, Nle28, Nle31]-CCK27-33 and Boc[Nle28, Nle31]-CCK27-33 (BDNL) and to the nonsulfated analogue of the latter peptide (BDNL NS). All these new CCK-related analogues behave as agonists in stimulating pancreatic amylase release and display high affinity for brain binding sites (KI approximately 3-11 nM) but the only peptides which retain affinity for CCK-A receptors (KI approximately 20 nM) are those containing a p-carboxymethyl phenylalanine. Thus, introduction of this amino acid under an esterified form on the side chain, into specific and potent CCK-B agonists could allow compounds endowed with good bioavailabilities to be obtained.