Sergey Pshenichkin

Transcriptional and Posttranscriptional Mechanisms Involved in the Interleukin‐1, Steroid, and Protein Kinase C Regulation of Nerve Growth Factor in Cortical Astrocytes

Abstract: Neonatal rat cortical astrocytes in primary culture synthesize and secrete nerve growth factor (NGF). Treatment of astrocytes with interleukin‐1β (IL‐1) or the protein kinase C (PKC) activator 12‐O‐tetradecanoylphorbol 13‐acetate (TPA) increased NGF mRNA content by six‐ to 10‐fold, followed in time by increases in cell content and cell secretion of NGF. Dexamethasone potently inhibited the effects of IL‐1 and TPA on astroglial cell NGF expression. The action of IL‐1 was not mediated by PKC because treatment of cells with maximal concentrations of both IL‐1 and TPA gave an additive increase in NGF mRNA content and NGF secretion, and because down‐regulating PKC activity failed to inhibit the stimulatory effects of IL‐1. Moreover, both agents increased NGF gene transcription in nuclear run‐on assays, but only IL‐1 significantly stabilized the NGF mRNA. An analysis of the effects of IL‐1 and TPA on immediate early gene expression indicated that IL‐1 preferentially induced c‐jun gene expression, whereas TPA greatly increased c‐fos and zif/268 gene expression. These results suggest that IL‐1 activates c‐jun and NGF gene expression, and NGF mRNA stabilization in astrocytes by a distinct PKC‐independent signaling pathway.

The Protective Signaling of Metabotropic Glutamate Receptor 1 Is Mediated by Sustained, β-Arrestin-1-dependent ERK Phosphorylation

Metabotropic glutamate receptor 1 (mGlu1) is a G protein-coupled receptor that enhances the hydrolysis of membrane phosphoinositides. In addition to its role in synaptic transmission and plasticity, mGlu1 has been shown to be involved in neuroprotection and neurodegeneration. In this capacity, we have reported previously that in neuronal cells, mGlu1a exhibits the properties of a dependence receptor, inducing apoptosis in the absence of glutamate, while promoting neuronal survival in its presence (Pshenichkin, S., Dolińska, M., Klauzińska, M., Luchenko, V., Grajkowska, E., and Wroblewski, J. T. (2008) Neuropharmacology 55, 500–508). Here, using CHO cells expressing mGlu1a receptors, we show that the protective effect of glutamate does not rely on the classical mGlu1 signal transduction. Instead, mGlu1a protective signaling is mediated by a novel, G protein-independent, pathway which involves the activation of the MAPK pathway and a sustained phosphorylation of ERK, which is distinct from the G protein-mediated transient ERK phosphorylation. Moreover, the sustained phosphorylation of ERK and protective signaling through mGlu1a receptors require expression of β-arrestin-1, suggesting a possible role for receptor internalization in this process. Our data reveal the existence of a novel, noncanonical signaling pathway associated with mGlu1a receptors, which mediates glutamate-induced protective signaling.

Ligand Bias at Metabotropic Glutamate 1a Receptors: Molecular Determinants That Distinguish β-Arrestin-Mediated from G Protein-Mediated Signaling

The metabotropic glutamate 1a (mGlu1a) receptor is a G protein-coupled receptor linked with phosphoinositide (PI) hydrolysis and with β-arrestin-1-mediated sustained extracellular signal-regulated kinase (ERK) phosphorylation and cytoprotective signaling. Previously, we reported the existence of ligand bias at this receptor, inasmuch as glutamate induced both effects, whereas quisqualate induced only PI hydrolysis. In the current study, we showed that mGlu1 receptor agonists such as glutamate, aspartate, and l-cysteate were unbiased and activated both signaling pathways, whereas quisqualate and (S)-3,5-dihydroxyphenylglycine stimulated only PI hydrolysis. Competitive antagonists inhibited only PI hydrolysis and not the β-arrestin-dependent pathway, whereas a noncompetitive mGlu1 receptor antagonist blocked both pathways. Mutational analysis of the ligand binding domain of the mGlu1a receptor revealed that Thr188 residues were essential for PI hydrolysis but not for protective signaling, whereas Arg323 and Lys409 residues were required for β-arrestin-1-mediated sustained ERK phosphorylation and cytoprotective signaling but not for PI hydrolysis. Therefore, the mechanism of ligand bias appears to involve different modes of agonist interactions with the receptor ligand binding domain. Although some mGlu1a receptor agonists are biased toward PI hydrolysis, we identified two endogenous compounds, glutaric acid and succinic acid, as new mGlu1 receptor agonists that are fully biased toward β-arrestin-mediated protective signaling. Pharmacological studies indicated that, in producing the two effects, glutamate interacted in two distinct ways with mGlu1 receptors, inasmuch as competitive mGlu1 receptor antagonists that blocked PI hydrolysis did not inhibit cytoprotective signaling. Quisqualate, which is biased toward PI hydrolysis, failed to inhibit glutamate-induced protection, and glutaric acid, which is biased toward protection, did not interfere with glutamate-induced PI hydrolysis. Taken together, these data indicate that ligand bias at mGlu1 receptors is attributable to different modes of receptor-glutamate interactions, which are differentially coupled to PI hydrolysis and β-arrestin-mediated cytoprotective signaling, and they reveal the existence of new endogenous agonists acting at mGlu1 receptors.

Synthesis and biological activity of cyclic analogues of MPPG and MCPG as metabotropic glutamate receptor antagonists

Abstract The synthesis of two rigidified phenylglycine analogues is disclosed. The cyclic analogue 1 of (R,S)-α-methyl-4-phosphonophenylglycine (MPPG) is shown to be a particularly interesting pharmacological tool, for it is a group II selective mGluR antagonist that possesses an inverse agonist-like-action.

Dual neurotoxic and neuroprotective role of metabotropic glutamate receptor 1 in conditions of trophic deprivation – Possible role as a dependence receptor

Group I metabotropic glutamate receptors have been often implicated in various models of neuronal toxicity, however, the role played by the individual receptors and their putative mechanisms of action contributing to neurotoxicity or neuroprotection remain unclear. Here, using primary cultures of rat cerebellar granule cells and mouse cortical neurons, we show that conditions of trophic deprivation increased mGlu1 expression which correlated with the developing cell death. The inhibition of mGlu1 expression by specific siRNA attenuated toxicity, while adenovirus-mediated overexpression of mGlu1 resulted in increased cell death, indicating a causal relationship between the level of receptor expression and neuronal survival. In pharmacological experiments selective mGlu1 antagonists failed to protect from mGlu1-induced cell death, instead, neuronal survival was promoted by glutamate acting at mGlu1 receptors. Such properties are characteristics of a novel heterogeneous family of dependence receptors which control neuronal apoptosis. Our findings indicate that increased expression of mGlu1 in neurons creates a state of cellular dependence on the presence of its endogenous agonist glutamate. We propose a new role and a new mechanism for mGlu1 action. This receptor may play a crucial role in determining the fate of individual neurons during the development of the nervous system.

Synthesis, molecular modeling, and biology of the 1-benzyl derivative of APDC - an apparent mGluR6 selective ligand

The synthesis of the 1-benzyl derivative of (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylic acid (1-benzyl-APDC) starting from cis-4-hydroxy-D-proline is disclosed together with a study of the activity of this compound at metabotropic glutamate receptors (mGluRs). The compound was found to display good mGluR6 selectivity, and may thus be a useful pharmacological research tool.

Synthesis and biology of the rigidified glutamate analogue, trans-2-carboxyazetidine-3-acetic acid (t-CAA)

Chemical approaches to the (-)- and (+)-trans-2-carboxyazetidine-3-acetic acids (-)-1 and (+)-1, and their homologues (-)-2 and (+)-2, compounds that represent rigidified analogues of glutamate (glu), are reported together with the complete biological characterization of(+)-1 (t-CAA) at the known glu recognition sites. t-CAA was found to be an inhibitor of Na+-dependent glu uptake and to act as a kainate receptor ligand. Copyright (C) 1996 Elsevier Science Ltd

Cyclothiazide selectively inhibits mGluR1 receptors interacting with a common allosteric site for non-competitive antagonists

Metabotropic glutamate receptors mGluR1 and mGluR5 stimulate phospholipase C, leading to an increased inositol trisphosphate level and to Ca(2+) release from intracellular stores. Cyclothiazide (CTZ), known as a blocker of AMPA receptor desensitization, produced a non-competitive inhibition of [Ca(2+)](i) increases induced by mGluR agonists in HEK 293 cells transfected with rat mGluR1a but had no effect on the [Ca(2+)](i) signals in cells expressing rat mGluR5a. In cells expressing mGluR1, CTZ also inhibited phosphoinositide hydrolysis, as well as cAMP accumulation and arachidonic acid release induced by mGluR1 agonists, indicating a direct inhibition of the receptor and not of a particular signal transduction system. However, CTZ failed to antagonize cAMP inhibition stimulated by rat mGluR2, -3, -4, -6, -7 and -8 receptors confirming its selectivity for mGluR1. The use of chimeric receptors with substituted N-terminal domains showed that CTZ did not interact with the N-terminal mGluR1a domain. Instead, mutation analysis revealed that CTZ interacts with the Thr-815 and Ala-818 residues, located at the 7th transmembrane domain, similarly as the mGluR1-selective antagonist CPCCOEt. In primary cultures of cerebellar granule neurons, expressing native metabotropic and ionotropic glutamate receptors, the final outcome of CTZ effects depended on its combined ability to potentiate AMPA receptors and inhibit mGluR1 receptors.

1-amino-APDC, a partial agonist of group II metabotropic glutamate receptors with neuroprotective properties.

The synthesis of the 1-amino derivative of (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylic acid (1-amino-APDC), a selective metabotropic glutamate ligand, is disclosed. This compound acts as a partial agonist of the group II mGluRs and shows pronounced neuroprotective properties in the NMDA model of cell toxicity.

α-Substituted quisqualic acid analogs : New metabotropic glutamate receptor group II selective antagonists.

Syntheses of both the alpha-methyl and benzyl analogs of quisqualic acid are described. Testing of these compounds for their activity at excitatory amino acid receptors revealed a striking change in activity in comparison to quisqualic acid. This structural modification results in the loss of quisqualates potent agonist action at both non-NMDA ionotropic glutamate receptors as well as at group I mGluRs, while allowing these analogs to acquire antagonist properties with relative selectivity for group II metabotropic glutamate receptors.