(2R,1'S,2'R,3'S)-2-(2'-Carboxy-3'-phenylcyclopropyl)glycine (PCCG-13), the first potent and selective competitive antagonist of phospholipase D-coupled metabotropic glutamate receptors: asymmetric synthesis and preliminary biological properties

Prokineticins are neuroprotective in models of cerebral ischemia and ischemic tolerance in vitro

Bv8/prokineticin 2 (PK2) is a member of a bioactive family of peptides that regulate multiple functions in the CNS including hyperalgesia, neurogenesis, neuronal survival and inflammation. Recent studies have associated PK2 and prokineticin receptors (PKR) with human diseases, but because their role in neuropathology is still debated we examined whether prokineticins exert a protective or deleterious role in models of cerebral ischemia and ischemic tolerance in vitro. In order to mimic cerebral ischemia, we exposed primary murine cortical cell cultures or rat organotypic hippocampal slices to appropriate periods of oxygen-glucose deprivation (OGD), which leads to neuronal damage 24 h later. Ischemic tolerance was induced by exposing hippocampal slices to a preconditioning subtoxic pharmacological stimulus (3 μM NMDA for 1 h) 24 h before the exposure to OGD. Bv8 (10-100 nM) attenuated OGD injury in cortical cultures and hippocampal slices, and the effect was prevented by the PKR antagonist PC7. The development of OGD tolerance was associated with an increase in the expression of PK2, PKR1 and PKR2 mRNA and proteins and was prevented by addition of the antagonist PC7 into the medium during preconditioning. Both Bv8 at protective concentrations and the NMDA preconditioning stimulus promoted the phosphorylation of ERK1/2 and Akt. These findings indicate that the prokineticin system can be up-regulated by a defensive preconditioning subtoxic NMDA stimulus and that PK2 may act as an endogenous neuroprotective factor through the activation of the ERK1/2 and Akt transduction pathways.

Fear potentiated startle increases phospholipase D (PLD) expression/activity and PLD-linked metabotropic glutamate receptor mediated post-tetanic potentiation in rat amygdala

Long-term memory (LTM) of fear stores activity dependent modifications that include changes in amygdala signaling. Previously, we identified an enhanced probability of release of glutamate mediated signaling to be important in rat fear potentiated startle (FPS), a well-established translational behavioral measure of fear. Here, we investigated short- and long-term synaptic plasticity in FPS involving metabotropic glutamate receptors (mGluRs) and associated downstream proteomic changes in the thalamic-lateral amygdala pathway (Th-LA). Aldolase A, an inhibitor of phospholipase D (PLD), expression was reduced, concurrent with significantly elevated PLD protein expression. Blocking the PLD-mGluR signaling significantly reduced PLD activity. While transmitter release probability increased in FPS, PLD-mGluR agonist and antagonist actions were occluded. In the unpaired group (UNP), blocking the PLD-mGluR increased while activating the receptor decreased transmitter release probability, consistent with decreased synaptic potentials during tetanic stimulation. FPS Post-tetanic potentiation (PTP) immediately following long-term potentiation (LTP) induction was significantly increased. Blocking PLD-mGluR signaling prevented PTP and reduced cumulative PTP probability but not LTP maintenance in both groups. These effects are similar to those mediated through mGluR7, which is co-immunoprecipitated with PLD in FPS. Lastly, blocking mGluR-PLD in the rat amygdala was sufficient to prevent behavioral expression of fear memory. Thus, our study in the Th-LA pathway provides the first evidence for PLD as an important target of mGluR signaling in amygdala fear-associated memory. Importantly, the PLD-mGluR provides a novel therapeutic target for treating maladaptive fear memories in posttraumatic stress and anxiety disorders.

Modulating mechanosensory afferent excitability by an atypical mGluR

Mechanotransduction by proprioceptive sensory organs is poorly understood. Evidence was recently shown that muscle spindle and hair follicle primary afferents (lanceolates) constantly release glutamate from synaptic-like vesicles (SLVs) within the terminals. The secreted glutamate activates a highly unusual metabotropic glutamate receptor (mGluR) to modulate the firing rate (spindles) and SLV recycling (lanceolates). This receptor has yet to be isolated and sequenced. To further investigate this receptor's pharmacology, ligands selective for classical mGluRs have been recently characterised for their ability to alter stretch-evoked spindle firing and SLV endocytosis in these different endings. Here, it is described how the results of these screens facilitated the development of novel compounds to be used in the process of isolating and sequencing of this non-canonical mGluR. This study shows how the compounds were tested for their ability to alter stretch-evoked afferent firing in muscle spindles and SLV endocytosis in the lanceolate endings of hair follicles to ensure they maintained their ability to bind to the receptor. For the development of novel compounds, kainate was chosen as the parent ligand due to its potency and ease of chemical modification. Novel kainate derivatives were then synthesised and tested to find potent analogues suitable for 'click-chemistry', an established technique for relatively quick, cheap, stereospecific and high-yield chemical modifications (Angewandte Chemie (International ed. in English), 40, 2001, pp2004). Of the novel kainate analogues developed, unfortunately ZCZ49 and ZCZ50 lost the ability to produce a significant change in spindle stretch-evoked firing. However, ZCZ90 was as potent as kainate, increasing firing by a similar margin at 1 μm (n = 8; P < 0.001). The addition of either a biotin or a fluorescein side group to ZCZ90, using the click-chemistry technique, did not affect the potency and hence these compounds will be used in further studies of the receptor. As well as the development of these compounds, the study found not only many similarities, but also some key differences between the two types of primary mechanosensory endings investigated. These differences must be taken into account in further study. However, they also present an intriguing opportunity for these receptors to be targeted selectively to modulate ending sensitivity as treatments for muscle spasm in multiple sclerosis and spinal cord injury, and possibly even baroreceptor firing to treat hypertension.

Mechanotransduction in the muscle spindle

The focus of this review is on the principal sensory ending of the mammalian muscle spindle, known as the primary ending. The process of mechanosensory transduction in the primary ending is examined under five headings: (i) action potential responses to defined mechanical stimuli—representing the ending's input–output properties; (ii) the receptor potential—including the currents giving rise to it; (iii) sensory-terminal deformation—measurable changes in the shape of the primary-ending terminals correlated with intrafusal sarcomere length, and what may cause them; (iv) putative stretch-sensitive channels—pharmacological and immunocytochemical clues to their identity; and (v) synaptic-like vesicles—the physiology and pharmacology of an intrinsic glutamatergic system in the primary and other mechanosensory endings, with some thoughts on the possible role of the system. Thus, the review highlights spindle stretch-evoked output is the product of multi-ionic receptor currents plus complex and sophisticated regulatory gain controls, both positive and negative in nature, as befits its status as the most complex sensory organ after the special senses.

Synthesis of a novel series of 2,3,4-trisubstituted oxazolidines designed by isosteric replacement or rigidification of the structure and cytotoxic evaluation

Rigidification of the structure of 2,3,4-trisubstituted oxazolidines enhances the activity against LNCaP cells without affecting normal cell proliferation.

Dopamine-induced plasticity, phospholipase D (PLD) activity and cocaine-cue behavior depend on PLD-linked metabotropic glutamate receptors in amygdala

Cocaine-cue associations induce synaptic plasticity with long lasting molecular and cellular changes in the amygdala, a site crucial for cue-associated memory mechanisms. The underlying neuroadaptations can include marked alterations in signaling via dopamine (DA) receptors (DRs) and metabotropic glutamate (Glu) receptors (mGluRs). Previously, we reported that DR antagonists blocked forms of synaptic plasticity in amygdala slices of Sprague-Dawley rats withdrawn from repeated cocaine administration. In the present study, we investigated synaptic plasticity induced by exogenous DA and its dependence on mGluR signaling and a potential role for phospholipase D (PLD) as a downstream element linked to mGluR and DR signaling. Utilizing a modified conditioned place preference (CPP) paradigm as a functional behavioral measure, we studied the neurophysiological effects after two-weeks to the last cocaine conditioning. We recorded, electrophysiologically, a DR-induced synaptic potentiation in the basolateral to lateral capsula central amygdala (BLA-lcCeA) synaptic pathway that was blocked by antagonists of group I mGluRs, particularly, the PLD-linked mGluR. In addition, we observed 2–2.5 fold increase in PLD expression and 3.7-fold increase in basal PLD enzyme activity. The enhanced PLD activity could be further stimulated (9.3 fold) by a DA D1-like (D1/5R) receptor agonist, and decreased to control levels by mGluR1 and PLD-linked mGluR antagonists. Diminished CPP was observed by infusion of a PLD-linked mGluR antagonist, PCCG-13, in the amygdala 15 minutes prior to testing, two weeks after the last cocaine injection. These results imply a functional interaction between D1/5Rs, group I mGluRs via PLD in the amygdala synaptic plasticity associated with cocaine-cues.

1-Aminocyclopentane-1,2,4-tricarboxylic acids screening on glutamatergic and serotonergic systems

Enantiopure constrained 1-aminocyclopentane-1,2,4-tricarboxylic acids containing the glutamic acid skeleton were prepared as two diastereomers characterized by having the carboxylic groups in position two and four cis-oriented to each other and trans with respect to 1-carboxylic group and all cis-oriented carboxylic groups, respectively. A biochemical screening of activity of the above amino acids was investigated on glutamate and 5-HT receptors to find a possible metabotropic agonist, acting on the serotoninergic system.

Contrasting Roles of Protein Kinase C in Induction Versus Suppression of Group I mGluR-Mediated Epileptogenesis In Vitro

Activation of group I metabotropic glutamate receptors (mGluRs) elicits persistent ictaform discharges in guinea pig hippocampal slices, providing an in vitro model of epileptogenesis. The induction of these persistent ictaform bursts is prevented by l-cysteine sulfinic acid (CSA), an agonist at phospholipase D (PLD)–coupled mGluRs. Studies described herein examined the role of protein kinase C (PKC) in both the group I mGluR–mediated induction and CSA-mediated suppression of this form of epileptogenesis. Intracellular recordings were performed from CA3 stratum pyramidale and synchronized burst length was monitored. In the presence of 50 μM picrotoxin, a γ-aminobutyric acid type A antagonist, 250- to 500-ms synchronized bursts were elicited. ( S)-3,5-Dihydroxyphenylglycine (DHPG, 50 μM), an agonist at group I mGluRs, increased the burst length to 1–3 s in duration, a change that persisted after agonist washout. This persistent change in burst length was elicited in the presence of 10 μM chelerythrine, a PKC inhibitor, indicating that DHPG-induced epileptogenesis is PKC independent. However, although PLD activation with CSA (100 μM) was highly effective at suppressing group I mGluR–mediated induction of burst prolongation, CSA application in the presence of chelerythrine was no longer effective and resulted in the expression of persistent ictaform bursts. These data suggest that CSA-mediated suppression of group I mGluR–induced epileptogenesis is PKC dependent. We propose that CSA mediates its effect by PLD-driven activation of PKC, which may desensitize the phospholipase C–linked group I mGluRs and thereby prevent group I mGluR–induced epileptogenesis.

Molecular cloning, expression, and sequence analysis of GPRC6A, a novel family C G-protein-coupled receptor

By similarity searching of the human genome sequence using known family C G-protein-coupled receptors (GPCRs) as query sequences, we have identified a putative novel human gene product of unknown function (located on chromosome band 6q22.31). The transcript, entitled GPRC6A (isoform 1), was cloned from a human kidney cDNA (DNA complementary to RNA) library and shown to encode a protein of 926 amino acids (aa). Protein sequence analysis revealed the presence of a seven-transmembrane (7TM) domain and an unusually long amino-terminal domain (ATD) of 590 amino acids. These traits, along with a significant homology to the human calcium-sensing receptor (CaR, 34% aa sequence identity), the taste receptor 1 (T1R1, 28%), and the metabotropic glutamate receptor 1 (mGluR1, 24%), places GPRC6A in family C of the GPCRs. Interestingly, GPRC6A bears the highest resemblance with an odorant goldfish 5.24 receptor (45%) which suggests that GPRC6A is the human orthologue of this receptor. GPRC6A is widely expressed in brain and peripheral tissues with highest levels in kidney, skeletal muscle, testis, and leucocytes. All three isoforms are expressed in mammalian cells, but are poorly expressed on the cell surface. In this work, we report the existence of two additional GPRC6A isoforms (2 and 3) carrying in-frame deletions in the ATD. Except for the kidney, where isoforms 1 and 2 appear equally expressed, isoforms 2 and 3 are generally less abundant than isoform 1. Analysis of the intron-exon composition of the GPRC6A gene confirms that isoforms 2 and 3 are naturally occurring splice variants.

Evidence That Phospholipase D Activation Prevents Group I mGluR-Induced Persistent Prolongation of Epileptiform Bursts

Selective activation of group I metabotropic glutamate receptors (mGluRs) with ( S)-3,5-dihydroxyphenylglycine (DHPG) in guinea pig hippocampal slices converts 275- to 475-ms picrotoxin-induced interictal bursts into persistent seizure-length discharges typically over 1 s in duration. Here we report that l-cysteine sulfinic acid (CSA), a sulfur-containing amino acid, prevented the induction of this persistent group I mGluR-mediated epileptiform burst prolongation. However, CSA had no effect on baseline interictal bursting activity and failed to suppress the expression of the group I mGluR-induced persistent prolonged bursts once they were fully induced. (2 R,1′ S,2′ R,3′ S)-2-(2′-carboxy-3′-phenylcyclopropyl)glycine (PCCG-13), a selective antagonist at the phospholipase D (PLD)-coupled mGluR, had no effect of its own on DHPG-induced burst prolongation; however, CSA applied in the presence of PCCG-13 could no longer fully block the burst prolongation induced by DHPG, suggesting that CSA's antiepileptogenic effect is mediated by agonist action at this PLD-coupled receptor. These data parallel our previous data revealing that protein synthesis inhibitors prevent induction but not expression of group I mGluR-mediated persistent seizure-length discharges. Hence, PLD activation with CSA may prevent the synthesis of a protein critical for the induction of group I mGluR-mediated epileptogenesis.

(2S,1'S,2'S,3'R)-2-(2'-carboxy-3'-methylcyclopropyl) glycine is a potent and selective metabotropic group 2 receptor agonist with anxiolytic properties

The asymmetric synthesis and biological activity of (2S,1'S,2'S,3'R)-2-(2'-carboxy-3'-methylcyclopropyl) glycine 7 and its epimer at the C3' center 6 are described. Compound 7 is a highly potent and selective agonist for group 2 metabotropric glutamate receptors (mGluRs). It is also systemically 4 orders of magnitude more active in the fear-potentiated startle model of anxiety in rats than the rigid constrained bicyclic system LY354740. Therefore, we have shown that high molecular complexity of conformationally constrained bicyclic systems is not a requirement to achieve highly selective and potent group 2 mGluRs agonists.

Spiro[2.2]pentane as a dissymmetric scaffold for conformationally constrained analogues of glutamic acid: focus on racemic 1-aminospiro[2.2]pentyl-1,4-dicarboxylic acids

In search for novel conformationally constrained analogues of L-glutamic acid, a diastereodivergent synthesis of the four 1-aminospiro[2.2]pentyl-1,4-dicarboxylic acid racemic pairs is reported along with their stereochemical assignment, conformational analysis, and preliminary biological evaluation as potential glutamate (ionotropic and metabotropic) ligands.

Acyclic and cyclopropyl analogues of adenosine bisphosphate antagonists of the P2Y1 receptor: structure-activity relationships and receptor docking

The activation of P2Y1 receptors in platelets contributes to platelet aggregation, and selective antagonists are sought as potential antithrombotic agents. We reported (Kim et al. J. Med. Chem. 2000, 43, 746-755) that acyclic analogues of adenine nucleotides, containing two phosphate groups on a symmetrically branched aliphatic chain, attached at the 9-position of adenine, are moderately potent P2Y1 receptor antagonists. In this study we have varied the chain structure, to include asymmetric substitution, olefinic, and cyclopropyl groups. These antagonists inhibited the stimulation of phospholipase C in turkey erythrocyte membranes induced by 30 nM 2-MeS-ADP in the micromolar range. In the series of symmetrically branched aliphatic groups substituted with two phosphate groups, the optimal antagonist potency occurred with the 2-methylpropyl group. A 2-chloro-N(6)-methyladenine derivative, 2-[2-(2-chloro-6-methylaminopurin-9-yl)methyl]propane-1,3-bisoxy(diammoniumphosphate) (7), was a full antagonist at the P2Y1 receptor with an IC(50) value of 0.48 microM. Esterification of one of the phosphate groups or substitution with O-acetyl greatly reduced the antagonist potency at the P2Y1 receptor. Removal of a methylene group of 7 or inclusion of an olefinic or cyclopropyl group also reduced potency. A pair of enantiomeric glycerol derivatives demonstrated a 5-fold stereoselectivity for the S-isomer. Stereoisomerically defined analogues of 7 containing a cyclopropyl group in place of the branched carbon were less potent than 7 as antagonists, with IC(50) values of 2-3 microM. No agonist activity was observed for these analogues. A new rhodopsin-based molecular model of the P2Y1 receptor indicated that the optimal docked orientation of the two monophosphate moieties relative to the adenine N(6) (compared to a rigid, bicyclic analogue) was consistent with the dependence of antagonist potency on chain length. The 3'-phosphate was predicted to occupy a restricted space, deeper in the binding cleft than the 5'-phosphate location. In summary, modification of the flexible spacer chain linking bisphosphate groups to the adenine moiety provided many moderately potent antagonists.

Sequence and expression pattern of a novel human orphan G-protein-coupled receptor, GPRC5B, a family C receptor with a short amino-terminal domain

Query of GenBank with the amino acid sequence of human metabotropic glutamate receptor subtype 2 (mGluR2) identified a predicted gene product of unknown function on BAC clone CIT987SK-A-69G12 (located on chromosome band 16p12) as a homologous protein. The transcript, entitled GPRC5B, was cloned from an expressed sequence tag clone that contained the entire open reading frame of the transcript encoding a protein of 395 amino acids. Analysis of the protein sequence reveal that GPRC5B contains a signal peptide and seven transmembrane alpha-helices, which is a hallmark of G-protein-coupled receptors (GPCRs). GPRC5B displays homology to retinoic acid-inducible gene 1 (RAIG1, 33% sequence identity) and to several family C (mGluR-like) GPCRs (20-25% sequence identity). Both RAIG1 and GPRC5B have short extracellular amino-terminal domains (ATDs) that contrast the very long ATDs characterizing the receptors currently assigned to family C. However, our results strongly indicate that RAIG1 and GPRC5B form a new subgroup of family C characterized by short ATDs. GPRC5B mRNA is widely expressed in peripheral and central tissues with highest abundance in kidney, pancreas, and testis. This mRNA expression pattern is markedly different from that of RAIG1, which shows a slightly more restricted expression pattern with highest abundance in lung tissue.

Metabotropic G-protein-coupled glutamate receptors as therapeutic targets

Metabotropic glutamate receptors have received considerable attention over the past decade in view of their relevance in multiple aspects of glutamatergic transmission. Recent advances in the molecular biology, pharmacology and medicinal chemistry of this family of G-protein-coupled receptors have led to therapeutic opportunities for subtype-selective modulators in brain disorders and diseases such as ischemia and schizophrenia.