CLK2 inhibition ameliorates autistic features associated with SHANK3 deficiency

SH3 and multiple ankyrin repeat domains 3 (SHANK3) haploinsufficiency is causative for the neurological features of Phelan-McDermid syndrome (PMDS), including a high risk of autism spectrum disorder (ASD). We used unbiased, quantitative proteomics to identify changes in the phosphoproteome of Shank3-deficient neurons. Down-regulation of protein kinase B (PKB/Akt)-mammalian target of rapamycin complex 1 (mTORC1) signaling resulted from enhanced phosphorylation and activation of serine/threonine protein phosphatase 2A (PP2A) regulatory subunit, B56β, due to increased steady-state levels of its kinase, Cdc2-like kinase 2 (CLK2). Pharmacological and genetic activation of Akt or inhibition of CLK2 relieved synaptic deficits in Shank3-deficient and PMDS patient-derived neurons. CLK2 inhibition also restored normal sociability in a Shank3-deficient mouse model. Our study thereby provides a novel mechanistic and potentially therapeutic understanding of deregulated signaling downstream of Shank3 deficiency.

Atg4b-dependent autophagic flux alleviates Huntington's disease progression

The accumulation of aggregated mutant huntingtin (mHtt) inclusion bodies is involved in Huntigton’s disease (HD) progression. Medium sized-spiny neurons (MSNs) in the corpus striatum are highly vulnerable to mHtt aggregate accumulation and degeneration, but the mechanisms and pathways involved remain elusive. Here we have developed a new model to study MSNs degeneration in the context of HD. We produced organotypic cortico-striatal slice cultures (CStS) from HD transgenic mice mimicking specific features of HD progression. We then show that induction of autophagy using catalytic inhibitors of mTOR prevents MSNs degeneration in HD CStS. Furthermore, disrupting autophagic flux by overexpressing Atg4b in neurons and slice cultures, accelerated mHtt aggregation and neuronal death, suggesting that Atg4b-dependent autophagic flux influences HD progression. Under these circumstances induction of autophagy using catalytic inhibitors of mTOR was inefficient and did not affect mHtt aggregate accumulation and toxicity, indicating that mTOR inhibition alleviates HD progression by inducing Atg4b-dependent autophagic flux. These results establish modulators of Atg4b-dependent autophagic flux as new potential targets in the treatment of HD.

A selective metabotropic glutamate receptor 7 agonist: activation of receptor signaling via an allosteric site modulates stress parameters in vivo

Metabotropic glutamate receptor (mGluR) subtypes (mGluR1 to mGluR8) act as important pre- and postsynaptic regulators of neurotransmission in the CNS. These receptors consist of two domains, an extracellular region containing the orthosteric agonist site and a transmembrane heptahelical domain involved in G protein activation and recognition of several recently synthesized pharmacological modulators. The presynaptic receptor mGluR7 shows the highest evolutionary conservation within the family, but no selective pharmacological tool was known. Here we characterize an mGluR7-selective agonist, N,N'-dibenzhydrylethane-1,2-diamine dihydrochloride (AMN082), which directly activates receptor signaling via an allosteric site in the transmembrane domain. At transfected mammalian cells expressing mGluR7, AMN082 potently inhibits cAMP accumulation and stimulates GTPgammaS binding (EC50-values, 64-290 nM) with agonist efficacies comparable with those of L-2-amino-4-phosphonobutyrate (L-AP4) and superior to those of L-glutamate. AMN082 (< or = 10 microM) failed to show appreciable activating or inhibitory effects at other mGluR subtypes and selected ionotropic GluRs. Chimeric receptor studies position the binding site of AMN082 in the transmembrane region of mGluR7, and we demonstrate that this allosteric agonist has little, if any, effect on the potency of orthosteric ligands. Here we provide evidence for full agonist activity mediated by the heptahelical domain of family 3 G protein-coupled receptors (which have mGluR-like structure) that may lead to drug development opportunities. Further, AMN082 is orally active, penetrates the blood-brain barrier, and elevates the plasma stress hormones corticosterone and corticotropin in an mGluR7-dependent fashion. Therefore, AMN082 is a valuable tool for unraveling the role of mGluR7 in stress-related CNS disorders.

In vitro and in vivo characterization of MPEP, an allosteric modulator of the metabotropic glutamate receptor subtype 5: review article

There is a need to identify subtype-specific ligands for mGlu receptors to elucidate the potential of these receptors for the treatment of nervous system disorders. To date, most mGlu receptor antagonists are amino acid-like compounds acting as competitive antagonists at the glutamate binding site located in the large extracellular N-terminal domain. We have characterized novel subtype-selective mGlu(5) receptor antagonists which are structurally unrelated to competitive mGlu receptor ligands. Using a series of chimeric receptors and point mutations we demonstrate that these antagonists act as inverse agonists with a novel allosteric binding site in the seven-transmembrane domain. Recent studies in animal models implicate mGlu(5) receptors as a potentially important therapeutic target particularly for the treatment of pain and anxiety.

(-)-PHCCC, a positive allosteric modulator of mGluR4: characterization, mechanism of action, and neuroprotection

Group-III metabotropic glutamate receptors (mGluR4, -6, -7, and -8) modulate neurotoxicity of excitatory amino acids and beta-amyloid-peptide (betaAP), as well as epileptic convulsions, most likely via presynaptic inhibition of glutamatergic neurotransmission. Due to the lack of subtype-selective ligands for group-III receptors, we previously utilized knock-out mice to identify mGluR4 as the primary receptor mediating neuroprotection of unselective group-III agonists such as L-AP(4) or (+)-PPG, whereas mGluR7 is critical for anticonvulsive effects. In a recent effort to find group-III subtype-selective drugs we identified (+/-)-PHCCC as a positive allosteric modulator for mGluR4. This compound increases agonist potency and markedly enhances maximum efficacy and, at higher concentrations, directly activates mGluR4 with low efficacy. All the activity of (+/-)-PHCCC resides in the (-)-enantiomer, which is inactive at mGluR2, -3, -5a, -6, -7b and -8a, but shows partial antagonist activity at mGluR1b (30% maximum antagonist efficacy). Chimeric receptor studies showed that the binding site of (-)-PHCCC is localized in the transmembrane region.Finally, (-)-PHCCC showed neuroprotection against betaAP- and NMDA-toxicity in mixed cultures of mouse cortical neurons. This neuroprotection was additive to that induced by the highly efficacious mGluR1 antagonist CPCCOEt and was blocked by MSOP, a group-III mGluR antagonist. Our data provide evidence for a novel pharmacological site on mGluR4, which may be used as a target-site for therapeutics.

[(3)H]-M-MPEP, a potent, subtype-selective radioligand for the metabotropic glutamate receptor subtype 5

The synthesis of a new potent, subtype-selective radioligand [(3)H]-M-MPEP (2-methyl-6-((3-methoxyphenyl)ethynyl)-pyridine) and its in vitro pharmacological characteristics are described. Science Ltd.

Discovery and characterization of non-competitive antagonists of group I metabotropic glutamate receptors

We have investigated the mechanism of inhibition of the new group I mGluR antagonists CPCCOEt and MPEP and determined that both compounds have a non-competitive mode of inhibition. Furthermore using chimeric/mutated receptors constructs we have found that these antagonists act at a novel pharmacological site located in the trans-membrane (TM). Specific non-conserved amino acid residues in the TM domain have been identified which are necessary for the inhibition by CPCCOEt and MPEP of the mGlul and mGlu5 receptors, respectively. Using molecular modeling a model of the TM domain was built for both mGlu1 and mGlu5 receptor subtypes. Docking of CPCCOEt and MPEP into their respective model allowed the modelisation of the novel binding site.

Metabotropic glutamate receptor subtype 5 (mGlu5) and nociceptive function. I. Selective blockade of mGlu5 receptors in models of acute, persistent and chronic pain

The excitatory neurotransmitter, glutamate, is particularly important in the transmission of pain information in the nervous system through the activation of ionotropic and metabotropic glutamate receptors. A potent, subtype-selective antagonist of the metabotropic glutamate-5 (mGlu5) receptor, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), has now been discovered that has effective anti-hyperalgesic effects in models of inflammatory pain. MPEP did not affect rotarod locomotor performance, or normal responses to noxious mechanical or thermal stimulation in naïve rats. However, in models of inflammatory pain, systemic administration of MPEP produced effective reversal of mechanical hyperalgesia without affecting inflammatory oedema. In contrast to the non-steroidal anti-inflammatory drugs, indomethacin and diclofenac, the maximal anti-hyperalgesic effects of orally administered MPEP were observed without acute erosion of the gastric mucosa. In contrast to its effects in models of inflammatory pain, MPEP did not produce significant reversal of mechanical hyperalgesia in a rat model of neuropathic pain.

Chiral resolution, pharmacological characterization, and receptor docking of the noncompetitive mGlu1 receptor antagonist (+/-)-2-hydroxyimino- 1a, 2-dihydro-1H-7-oxacyclopropa[b]naphthalene-7a-carboxylic acid ethyl ester

Racemic CPCCOEt ((1aRS,7aRS)-2-hydroxyimino-1a, 2-dihydro-1H-7-oxacyclopropa[b]naphthalene-7a-carboxylic acid ethyl ester, (+/-)-1) derivatives have been shown to be subtype-selective metabotropic glutamate (mGlu) 1 receptor antagonists (Annoura et al. Bioorg. Med. Chem. Lett. 1996, 6, 763-766). The optical isomers of (+/-)-1 have been separated by chromatography on a chiral stationary phase. The absolute configuration at the C-1a and C-7a positions was determined using X-ray crystallography of an amide derivative with the methyl ester of L-phenylalanine (L-PheOMe) ((+)-6). In a phosphoinositol (PI) turnover assay at the cloned human mGlu1b receptor, (-)-1 and the new amide derivatives (-)-5 and (-)-6, all of which have (1aS,7aS)-stereochemistry on the chromane ring system, showed IC(50) values of 1.5, 0.43, and 0.93 microM, respectively. In contrast, (+)-1 and the new amide derivatives (+)-5 and (+)-6were found to be inactive up to a concentration of 30 microM indicating a selectivity for the (-)-enantiomers of at least 70-fold. In a previous study (Litschig et al. Mol. Pharmacol. 1999, 55, 453-461) we demonstrated using site-directed mutagenesis that the interaction site of (+/-)-1 is located in the transmembrane (TM) domain of hmGlu1b. To suggest a plausible binding mode of (-)-1, we have built a molecular mechanics model of the putative seven TM domain of hmGlu1 based on the alpha-carbon template of the TM helices of rhodopsin. A receptor docking hypothesis suggests that the OH of T815 (TMVII) comes in close contact with the oxime OH of (-)-1 and (-)-5, whereas no such close interactions could be demonstrated by docking of (+)-1.

The non-competitive antagonists 2-methyl-6-(phenylethynyl)pyridine and 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester interact with overlapping binding pockets in the transmembrane region of group I metabotropic glutamate receptors

We have investigated the mechanism of inhibition and site of action of the novel human metabotropic glutamate receptor 5 (hmGluR5) antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP), which is structurally unrelated to classical metabotropic glutamate receptor (mGluR) ligands. Schild analysis indicated that MPEP acts in a non-competitive manner. MPEP also inhibited to a large extent constitutive receptor activity in cells transiently overexpressing rat mGluR5, suggesting that MPEP acts as an inverse agonist. To investigate the molecular determinants that govern selective ligand binding, a mutagenesis study was performed using chimeras and single amino acid substitutions of hmGluR1 and hmGluR5. The mutants were tested for binding of the novel mGluR5 radioligand [(3)H]2-methyl-6-(3-methoxyphenyl)ethynyl pyridine (M-MPEP), a close analog of MPEP. Replacement of Ala-810 in transmembrane (TM) VII or Pro-655 and Ser-658 in TMIII with the homologous residues of hmGluR1 abolished radioligand binding. In contrast, the reciprocal hmGluR1 mutant bearing these three residues of hmGluR5 showed high affinity for [(3)H]M-MPEP. Radioligand binding to these mutants was also inhibited by 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester (CPCCOEt), a structurally unrelated non-competitive mGluR1 antagonist previously shown to interact with residues Thr-815 and Ala-818 in TMVII of hmGluR1. These results indicate that MPEP and CPCCOEt bind to overlapping binding pockets in the TM region of group I mGluRs but interact with different non-conserved residues.

2-Methyl-6-(phenylethynyl)-pyridine (MPEP), a potent, selective and systemically active mGlu5 receptor antagonist

In the present paper we describe 2-methyl-6-(phenylethynyl)-pyridine (MPEP) as a potent, selective and systemically active antagonist for the metabotropic glutamate receptor subtype 5 (mGlu5). At the human mGlu5a receptor expressed in recombinant cells, MPEP completely inhibited quisqualate-stimulated phosphoinositide (PI) hydrolysis with an IC50 value of 36 nM while having no agonist or antagonist activities at cells expressing the human mGlu1b receptor at concentrations up to 30 microM. When tested at group II and III receptors, MPEP did not show agonist or antagonist activity at 100 microM on human mGlu2, -3, -4a, -7b, and -8a receptors nor at 10 microM on the human mGlu6 receptor. Electrophysiological recordings in Xenopus laevis oocytes demonstrated no significant effect at 100 microM on human NMDA (NMDA1A/2A), rat AMPA (Glu3-(flop)) and human kainate (Glu6-(IYQ)) receptor subtypes nor at 10 microM on the human NMDA1A/2B receptor. In rat neonatal brain slices, MPEP inhibited DHPG-stimulated PI hydrolysis with a potency and selectivity similar to that observed on human mGlu receptors. Furthermore, in extracellular recordings in the CA1 area of the hippocampus in anesthetized rats, the microiontophoretic application of DHPG induced neuronal firing that was blocked when MPEP was administered by iontophoretic or intravenous routes. Excitations induced by microiontophoretic application of AMPA were not affected.

CPCCOEt, a noncompetitive metabotropic glutamate receptor 1 antagonist, inhibits receptor signaling without affecting glutamate binding

Metabotropic glutamate receptors (mGluRs) are a family of G protein-coupled receptors characterized by a large, extracellular N-terminal domain comprising the glutamate-binding site. In the current study, we examined the pharmacological profile and site of action of the non-amino-acid antagonist 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester (CPCCOEt). CPCCOEt selectively inhibited glutamate-induced increases in intracellular calcium at human mGluR1b (hmGluR1b) with an apparent IC50 of 6.5 microM while having no agonist or antagonist activity at hmGluR2, -4a, -5a, -7b, and -8a up to 100 microM. Schild analysis indicated that CPCCOEt acts in a noncompetitive manner by decreasing the efficacy of glutamate-stimulated phosphoinositide hydrolysis without affecting the EC50 value or Hill coefficient of glutamate. Similarly, CPCCOEt did not displace [3H]glutamate binding to membranes prepared from mGluR1a-expressing cells. To elucidate the site of action, we systematically exchanged segments and single amino acids between hmGluR1b and the related subtype, hmGluR5a. Substitution of Thr815 and Ala818, located at the extracellular surface of transmembrane segment VII, with the homologous amino acids of hmGluR5a eliminated CPCCOEt inhibition of hmGluR1b. In contrast, introduction of Thr815 and Ala818 at the homologous positions of hmGluR5a conferred complete inhibition by CPCCOEt (IC50 = 6.6 microM), i.e., a gain of function. These data suggest that CPCCOEt represents a novel class of G protein-coupled receptor antagonists inhibiting receptor signaling without affecting ligand binding. We propose that the interaction of CPCCOEt with Thr815 and Ala818 of mGluR1 disrupts receptor activation by inhibiting an intramolecular interaction between the agonist-bound extracellular domain and the transmembrane domain.