Protein kinase C transiently activated heteromeric N-methyl-D-aspartate receptor channels independent of the phosphorylatable C-terminal splice domain and of consensus phosphorylation sites

Nobiletin regulates intracellular Ca2+ levels via IP3R and ameliorates neuroinflammation in Aβ42-induced astrocytes

Astrocytes are the major glial cells in the human brain and provide crucial metabolic and trophic support to neurons. The amyloid-β peptide (Aβ) alter the morphological and functional properties of astrocytes and induce inflammation and calcium dysregulation, contributing to Alzheimer's disease (AD) pathology. Recent studies highlight the role of Toll-like receptor (TLR) 4/nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling in inflammation. Reactive oxygen species (ROS) generated due to Aβ, induce apoptosis in the brain cells worsening AD progression. Astrocytic cell surface receptors, such as purinergic receptors (P2Y1 and P2Y2), metabotropic glutamate receptor (mGLUR)5, α7 nicotinic acetylcholine receptor (α7nAChR), and N-methyl-d-aspartate receptors (NMDARs), have been suggested to interact with inositol trisphosphate receptor (IP3R) on the endoplasmic reticulum (ER) to induce Ca2+ movement from ER to cytoplasm, causing Ca2+ dysregulation. We found that the citrus flavonoid nobiletin (NOB) protected primary astrocytes from Aβ42-induced cytotoxicity and inhibited TLR4/NF-κB signaling in Aβ42-induced primary rat astrocytes. NOB was found to regulate Aβ42-induced ROS levels through Keap1-Nrf2 pathway. The receptors P2Y1, P2Y2, mGLUR5, α7nAChR, and NMDARs induced intracellular Ca2+ levels by activating IP3R and NOB regulated them, thereby regulating intracellular Ca2+ levels. Molecular docking analysis revealed a possible interaction between NOB and IP3R in IP3R regulation. Furthermore, RNA sequencing revealed various NOB-mediated biological signaling pathways, such as the AD-presenilin, AD-amyloid secretase, and Wnt signaling pathway, suggesting possible neuroprotective roles of NOB. To conclude, NOB is a promising therapeutic agent for AD and works by modulating AD pathology at various levels in Aβ42-induced primary rat astrocytes.

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

N-methyl-d-aspartate receptors (NMDARs) have multiple prominent roles in CNS function but their excessive or insufficient activity contributes to neuropathological/psychiatric disorders. Consequently, a variety of positive and negative allosteric modulators (PAMs and NAMs, respectively) have recently been developed. Although these modulators bind to extracellular domains, in the present report we find that the NMDAR's intracellular C-terminal domains (CTDs) significantly influence PAM/NAM activity. GluN2 CTD deletion robustly affected NAM and PAM activity with both enhancing and inhibiting effects that were compound-specific and NMDAR subunit-specific. In three cases, individual PAMs became NAMs at specific GluN2-truncated receptors. In contrast to GluN2, GluN1 CTD removal only reduced PAM activity of UBP684 and CIQ, and did not affect NAM activity. Consistent with these findings, agents altering phosphorylation state or intracellular calcium levels displayed receptor-specific and compound-specific effects on PAM activity. It is possible that the GluN2's M4 domain transmits intracellular modulatory signals from the CTD to the M1/M4 channel gating machinery and that this site is a point of convergence in the direct or indirect actions of several PAMs/NAMs thus rendering them sensitive to CTD status. Thus, allosteric modulators are likely to have a marked and varied sensitivity to post-translational modifications, protein-protein associations, and intracellular ions. The interaction between PAM activity and NMDAR CTDs appears reciprocal. GluN1 CTD-deletion eliminated UBP684, but not pregnenolone sulfate (PS), PAM activity. And, in the absence of agonists, UBP684, but not PS, was able to promote movement of fluorescently-tagged GluN1-CTDs. Thus, it may be possible to pharmacologically target NMDAR metabotropic activity in the absence of channel activation.

The Regulation of GluN2A by Endogenous and Exogenous Regulators in the Central Nervous System

The NMDA receptor is the most widely studied ionotropic glutamate receptor, and it is central to many physiological and pathophysiological processes in the central nervous system. GluN2A is one of the two main types of GluN2 NMDA receptor subunits in the forebrain. The proper activity of GluN2A is important to brain function, as the abnormal regulation of GluN2A may induce some neuropsychiatric disorders. This review will examine the regulation of GluN2A by endogenous and exogenous regulators in the central nervous system.

Acute ethanol exposure prevents PMA-mediated augmentation of N-methyl-D-aspartate receptor function in primary cultured cerebellar granule cells

Many intracellular proteins and signaling cascades contribute to the ethanol sensitivity of native N-methyl-D-aspartate receptors (NMDARs). One putative protein is the serine/threonine kinase, protein kinase C (PKC). The purpose of this study was to assess if PKC modulates the ethanol sensitivity of native NMDARs expressed in primary cultured cerebellar granule cells (CGCs). With the whole-cell patch-clamp technique, we assessed if ethanol inhibition of NMDA-induced currents (I(NMDA)) (100 μM NMDA plus 10 μM glycine) were altered in CGCs in which the novel and classical PKC isoforms were activated by phorbol-12-myristate-13-acetate (PMA). Percent inhibition by 10, 50, or 100 mM ethanol of NMDA-induced steady-state current amplitudes (I(SS)) or peak current amplitudes (I(Pk)) of NMDARs expressed in CGCs in which PKC was activated by a 12.5 min, 100 nM PMA exposure at 37°C did not differ from currents obtained from receptors contained in control cells. However, PMA-mediated augmentation of I(Pk) in the absence of ethanol was abolished after brief applications of 10 or 1 mM ethanol coapplied with agonists, and this suppression of enhanced receptor function was observed for up to 8 min post-ethanol exposure. Because we had previously shown that PMA-mediated augmentation of I(NMDA) of NMDARs expressed in these cells is by activation of PKCα, we assessed the effect of ethanol (1, 10, 50, and 100 mM) on PKCα activity. Ethanol decreased PKCα activity by 18% for 1 mM ethanol and activity decreased with increasing ethanol concentrations with a 50% inhibition observed with 100 mM ethanol. The data suggest that ethanol disruption of PMA-mediated augmentation of I(NMDA) may be due to a decrease in PKCα activity by ethanol. However, given the incomplete blockade of PKCα activity and the low concentration of ethanol at which this phenomenon is observed, other ethanol-sensitive signaling cascades must also be involved.

Phorbol 12-myristate 13-acetate potentiation of N-methyl-D-aspartate-induced currents in primary cultured cerebellar granule cells is mediated by protein kinase C alpha

We have previously reported that activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate (PMA) results in potentiation of N-methyl-D-aspartate-induced currents (I(NMDA))of receptors contained in primary cultured cerebellar granule cells (CGCs). The purpose of this study was to identify which PKC isoform(s) was responsible for this effect by using the whole-cell patch-clamp technique. Experiments were conducted on CGCs that expressed both the NR2A and NR2B NMDA receptor subunits as well as the PMA-sensitive PKC isoforms alpha, betaI, betaII, delta, epsilon, gamma, and . As observed previously, N-methyl-D-aspartate-induced peak currents (I(Pk)) were enhanced by a 12.5-min, 100 nM PMA exposure at 37 degrees C under normal recording conditions. Potentiation of receptor function was not observed when extracellular Ca(2+) was removed and 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid was present inside the cell. PMA-induced potentiation of I(Pk) did not occur when PKCalpha-specific antibody was introduced into the cell via the recording electrode. However, in similar experiments with antibodies specific for PKCbetaII, delta, epsilon, gamma, and , PMA potentiation of I(Pk) was observed. Down-regulation of PMA-sensitive PKC isoforms by an overnight exposure of 100 nM PMA resulted in lack of potentiation by PMA that was rescued when catalytically active PKCalpha was introduced into the cell via the patch electrode. PMA potentiation of I(Pk) was not recovered when catalytically active PKCbetaI, PKCbetaII, or PKCgamma was introduced into the cell via the patch electrode. Collectively, our data provide strong evidence that PMA-enhanced function of native NMDA receptors expressed in primary cultured CGCs is mediated by activation of PKCalpha.

Regulation of NMDA receptors by phosphorylation

N-methyl-D-aspartate (NMDA) receptors are critical for neuronal development and synaptic plasticity. The molecular mechanisms underlying the synaptic localization and functional regulation of NMDA receptors have been the subject of extensive studies. In particular, phosphorylation has emerged as a fundamental mechanism that regulates NMDA receptor trafficking and can alter the channel properties of NMDA receptors. Here we summarize recent advances in the characterization of NMDA receptor phosphorylation, emphasizing subunit-specific phosphorylation, which differentially controls the trafficking and surface expression of NMDA receptors.

Synaptic plasticity and phosphorylation

A number of neuronal functions, including synaptic plasticity, depend on proper regulation of synaptic proteins, many of which can be rapidly regulated by phosphorylation. Neuronal activity controls the function of these synaptic proteins by exquisitely regulating the balance of various protein kinase and protein phosphatase activity. Recent understanding of synaptic plasticity mechanisms underscores important roles that these synaptic phosphoproteins play in regulating both pre- and post-synaptic functions. This review will focus on key postsynaptic phosphoproteins that have been implicated to play a role in synaptic plasticity.

Identification of critical residues in the amino terminal domain of the human NR2B subunit involved in the RO 25-6981 binding pocket

N-Methyl-d-aspartate (NMDA) receptors play key roles in both physiological processes, particularly synaptic plasticity, and in neuropathological states such as epilepsy and acute neurodegeneration. R-(R*,S*)-alpha-(4-Hydroxyphenyl)-beta-methyl-4-(phenyl-methyl)-1-piperidine propanol (RO 25-6981), is a high-affinity and selective blocker of NMDA receptors containing the NR2B subunit. Using site-directed mutagenesis, [3H]RO 25-6981 binding, Xenopus oocyte voltage-clamp recordings, and molecular modeling, we have identified several critical residues involved in the RO 25-6981 binding site within the N-terminal LIVBP-like domain of the human NR2B subunit. Two mutations, NR2B(D101A) and NR2B(F176A), resulted in a complete loss of [3H]RO 25-6981 binding and also abolished the high-affinity RO 25-6981-mediated inhibition of NMDA-induced currents. The mutation NR2B(T233A) led to a marked reduction in binding affinity by 13-fold. Mutations F182A, D104A, or K234A had a more moderate influence on the binding affinity (KD values increased by 8-, 7-, and 6-fold, respectively). In a three-dimensional model of the NR2B LIVBP-like domain based on the X-ray crystal structure of the amino-terminal domain of the mGlu1 receptor, the critical residues are located in the central cleft where interaction with RO 25-6981 may stabilize the closed structure of the domain. Our results suggest that the three amino acids Asp-101, Phe-176, and Thr-233 are important molecular determinants for the high-affinity binding of RO 25-6981 to the LIVBP-like domain of human NR2B. A possible binding mode for RO 25-6981 is proposed.

Modulation of 5-HT3 receptor-mediated response and trafficking by activation of protein kinase C

Modulation of neurotransmitter-gated membrane ion channels by protein kinase C (PKC) has been the subject of a number of studies. However, less is known about PKC modulation of the serotonin type 3 (5-HT3) receptor, a ligand-gated membrane ion channel that can mediate fast synaptic transmission in the central and peripheral nervous system. Here, we show that PKC potentiated 5-HT3 receptor-mediated current in Xenopus oocytes expressing 5-HT3A receptors and mouse N1E-115 neuroblastoma cells. In addition, using a specific antibody directed to the extracellular N-terminal domain of the 5-HT3A receptor, treatment with the PKC activator, 4 beta-phorbol 12-myristate 13-acetate (PMA), significantly increased surface immunofluorescence. PKC also increased the amount of 5-HT3A receptor protein in the cell membrane without affecting the amount receptor protein in the total cell extract. The magnitude of PMA potentiation of 5-HT3A receptor-mediated responses is correlated with the magnitude of PMA enhancement of the receptor abundance in the cell surface membrane. PMA potentiation is unlikely to occur via direct phosphorylation of the 5-HT3A receptor protein since the potentiation was not affected by point mutation of each of the putative sites for PKC phosphorylation. However, preapplication of phalloidin, which stabilizes the actin polymerization, significantly inhibited PMA potentiation of 5-HT-activated responses in both N1E-115 cells and oocytes expressing 5-HT3A receptors. On the other hand, latrunculin-A, which destabilizes actin cytoskeleton, enhanced the PMA potentiation of 5-HT3A receptors. The observations suggest that PKC can modulate 5-HT3A receptor function and trafficking through an F-actin-dependent mechanism.

Evidence for direct protein kinase-C mediated modulation of N-methyl-D-aspartate receptor current

Protein kinase-C (PKC) activation differentially affects currents from N-methyl-D-aspartate (NMDA) type glutamate receptors depending upon their subunit composition. Experiments using chimeras initially indicated that the cytoplasmic C-terminal tails of NR2B (responsive to PKC) and NR2C (unresponsive to PKC) subunits contain the amino acid residues responsible for the observed disparity of PKC effects. However, truncation and point mutation experiments have suggested that PKC action on NMDA receptors may be entirely indirect, working via the phosphorylation of associated proteins. Here we suggest that PKC does, in fact, affect NR2B/NR1-011 NMDA currents by direct phosphorylation of the NR2B tail at residues S1303 and S1323. Replacement of either of these residues with Ala severely reduces PKC potentiation. To verify that S1303 and S1323 are sites of direct phosphorylation by PKC, synthetic peptides from the regions surrounding these sites were used as substrates for in vitro assays with purified rat brain PKC. These results indicate that PKC can directly phosphorylate S1303 and S1323 in the NR2B C terminus, leading to enhanced currents through NMDA receptor channels. The direct action of PKC on certain NMDA receptor subtypes may be important in any physiological or pathological process where PKC and NR2B/NR1 receptors interact.

Role of glutamatergic and GABAergic systems in alcoholism

The pharmacological effects of ethanol are complex and widespread without a well-defined target. Since glutamatergic and GABAergic innervation are both dense and diffuse and account for more than 80% of the neuronal circuitry in the human brain, alterations in glutamatergic and GABAergic function could affect the function of all neurotransmitter systems. Here, we review recent progress in glutamatergic and GABAergic systems with a special focus on their roles in alcohol dependence and alcohol withdrawal-induced seizures. In particular, NMDA-receptors appear to play a central role in alcohol dependence and alcohol-induced neurological disorders. Hence, NMDA receptor antagonists may have multiple functions in treating alcoholism and other addictions and they may become important therapeutics for numerous disorders including epilepsy, Parkinson's disease, amyotrophic lateral sclerosis, Huntington's chorea, anxiety, neurotoxicity, ischemic stroke, and chronic pain. One of the new family of NMDA receptor antagonists, such as DETC-MESO, which regulate the redox site of NMDA receptors, may prove to be the drug of choice for treating alcoholism as well as many neurological diseases.

Protein kinase C potentiation of N-methyl-D-aspartate receptor activity is not mediated by phosphorylation of N-methyl-D-aspartate receptor subunits

N-methyl-D-aspartate receptors (NMDARs) are Ca(2+)-permeable glutamate-gated ion channels whose physiological properties in neurons are modulated by protein kinase C (PKC). The present study was undertaken to determine the role in PKC-induced potentiation of the NR1 and NR2A C-terminal tails, which serve as targets of PKC phosphorylation [Tingley, W. G., Ehlers, M. D., Kameyama, K., Doherty, C., Ptak, J. B., Riley, C. T. & Huganir, R. L. (1997) J. Biol. Chem. 272, 5157-5166]. Serine residue 890 in the C1 cassette is a primary target of PKC phosphorylation and a critical residue in receptor clustering at the membrane. We report herein that the presence of the C1 cassette reduces PKC potentiation and that mutation of Ser-890 significantly restores PKC potentiation. Splicing out or deletion of other C-terminal cassettes singly or in combination had little or no effect on PKC potentiation. Moreover, experiments involving truncation mutants reveal the unexpected finding that NMDARs assembled from subunits lacking all known sites of PKC phosphorylation can show PKC potentiation. These results indicate that PKC-induced potentiation of NMDAR activity does not occur by direct phosphorylation of the receptor protein but rather of associated targeting, anchoring, or signaling protein(s). PKC potentiation of NMDAR function is likely to be an important mode of NMDAR regulation in vivo and may play a role in NMDA-dependent long-term potentiation.

Regulation of NMDA-stimulated [14C]GABA and [3H]acetylcholine release by striatal glutamate and dopamine receptors

Striatal function is heavily influenced by glutamatergic and dopaminergic afferent input. To ultimately better understand how the N-methyl-D-aspartate (NMDA) antagonist, phencyclidine (PCP), alters striatal function, we sought to determine how NMDA receptor function is influenced by activation of other glutamatergic receptors and by dopaminergic receptors. To this end, we used NMDA-stimulated efflux of [14C]GABA and [3H]acetylcholine (ACh) from striatal slices to assess the influence of these receptors on NMDA function. NMDA-stimulated [14C]GABA release was more sensitive to NMDA and glycine antagonists than was [3H]ACh release, suggesting that different NMDA receptors regulate the release of these neurotransmitters. Furthermore, NMDA-stimulated [3H]ACh release was inhibited by a D2 receptor mechanism whereas NMDA-stimulated [14C]GABA release was enhanced by D1 receptor activation. NMDA and (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid hydrobromide (AMPA) interact additively to evoke [3H]ACh release, and synergistically to evoke [14C]GABA release. An additive effect of NMDA and kainate (KA) was found on [14C]GABA release, but NMDA and KA acted in a less than additive manner in evoking [3H]ACh release. KA-stimulated [3H]ACh release was largely blocked by NMDA antagonists, suggesting mediation through activation of NMDA receptors, probably secondary to KA-induced glutamate release. A selective group II metabotropic receptor agonist inhibited NMDA-stimulated [14C]GABA and [3H]ACh release. On the other hand, NMDA-stimulated [14C]GABA release was potentiated by activation of group I metabotropic receptors. Thus, in addition to the differential modulation by D1- and D2-like receptors, the release of striatal neurotransmitters by NMDA receptor activation depends on the extent to which the other glutamate receptors, both ionotropic and metabotropic, are activated.

Subunit- and site-specific pharmacology of the NMDA receptor channel

N-Methyl-D-aspartate (NMDA) receptor channels play important roles in various physiological functions such as synaptic plasticity and synapse formation underlying memory, learning and formation of neural networks during development. They are also important for a variety of pathological states including acute and chronic neurological disorders, psychiatric disorders, and neuropathic pain syndromes. cDNA cloning has revealed the molecular diversity of NMDA receptor channels. The identification of multiple subunits with distinct distributions, properties and regulation, implies that NMDA receptor channels are heterogeneous in their pharmacological properties, depending on the brain region and the developmental stage. Furthermore, mutation studies have revealed a critical role for specific amino acid residues in certain subunits in determining the pharmacological properties of NMDA receptor channels. The molecular heterogeneity of NMDA receptor channels as well as their dual role in physiological and pathological functions makes it necessary to develop subunit- and site-specific drugs for precise and selective therapeutic intervention. This review summarizes from a molecular perspective the recent advances in our understanding of the pharmacological properties of NMDA receptor channels with specific references to agonists binding sites, channel pore regions, allosteric modulation sites for protons, polyamines, redox agents, Zn2+ and protein kinases, phosphatases.

Src, a molecular switch governing gain control of synaptic transmission mediated by N-methyl-D-aspartate receptors

The N-methyl-D-aspartate (NMDA) receptor is a principal subtype of glutamate receptor mediating fast excitatory transmission at synapses in the dorsal horn of the spinal cord and other regions of the central nervous system. NMDA receptors are crucial for the lasting enhancement of synaptic transmission that occurs both physiologically and in pathological conditions such as chronic pain. Over the past several years, evidence has accumulated indicating that the activity of NMDA receptors is regulated by the protein tyrosine kinase, Src. Recently it has been discovered that, by means of up-regulating NMDA receptor function, activation of Src mediates the induction of the lasting enhancement of excitatory transmission known as long-term potentiation in the CA1 region of the hippocampus. Also, Src has been found to amplify the up-regulation of NMDA receptor function that is produced by raising the intracellular concentration of sodium. Sodium concentration increases in neuronal dendrites during high levels of firing activity, which is precisely when Src becomes activated. Therefore, we propose that the boost in NMDA receptor function produced by the coincidence of activating Src and raising intracellular sodium may be important in physiological and pathophysiological enhancement of excitatory transmission in the dorsal horn of the spinal cord and elsewhere in the central nervous system.

Regulation of ligand-gated ion channels by protein phosphorylation

The studies discussed in this review demonstrate that phosphorylation is an important mechanism for the regulation of ligand-gated ion channels. Structurally, ligand-gated ion channels are heteromeric proteins comprised of homologous subunits. For both the AChR and the GABA(A) receptor, each subunit has a large extracellular N-terminal domain, four transmembrane domains, a large intracellular loop between transmembrane domains M3 and M4, and an extracellular C-terminal domain (Fig. 1B). All the phosphorylation sites on these receptors have been mapped to the major intracellular loop between M3 and M4 (Table 1). In contrast, glutamate receptors appear to have a very large extracellular N-terminal domain, one membrane hairpin loop, three transmembrane domains, a large extracellular loop between transmembrane domains M3 and M4, and an intracellular C-terminal domain (Fig. 1C). Most phosphorylation sites on glutamate receptors have been shown to be on the intracellular C-terminal domain, although some have been suggested to be on the putative extracellular loop between M3 and M4 (Table 1). A variety of extracellular factors and intracellular signal transduction cascades are involved in regulating phosphorylation of these ligand-gated ion channels (Fig. 2). Once again, the AChR at the neuromuscular junction is the most fully understood system. Phosphorylation of the AChR by PKA is stimulated synaptically by the neuropeptide CGRP and in an autocrine fashion by adenosine released from the muscle in response to acetylcholine. In addition, acetylcholine, via calcium influx through the AChR, appears to activate calcium-dependent kinases including PKC to stimulate serine phosphorylation of the receptor. Presently, agrin is the only extracellular factor known to stimulate phosphorylation of the AChR on tyrosine residues. For glutamate receptors, non-NMDA receptor phosphorylation by PKA is stimulated by dopamine, while NMDA receptor phosphorylation by PKA and PKC can be induced via the activation of beta-adrenergic receptors, and metabotropic glutamate or opioid receptors, respectively. In addition, Ca2+ influx through the NMDA receptor has been shown to activate PKC. CaMKII, and calcineurin, resulting in phosphorylation of AMPA receptors (by CaMKII) and inactivation of NMDA receptors (at least in part through calcineurin). In contrast to the AChR and glutamate receptors, no information is presently available regarding the identities of the extracellular factors and intracellular signal transduction cascades that regulate phosphorylation of the GABA(A) receptor. Surely, future studies will be aimed at further clarifying the molecular mechanisms by which the central receptors are regulated. The presently understood functional effects of ligand-gated ion channel phosphorylation are diverse. At the neuromuscular junction, a regulation of the AChR desensitization rate by both serine and tyrosine phosphorylation has been demonstrated. In addition, tyrosine phosphorylation of the AChR or other synaptic components appears to play a role in AChR clustering during synaptogenesis. For the GABA(A) receptor, the data are complex. Both activation and inhibition of GABA(A) receptor currents as a result of PKA and PKC phosphorylation have been reported, while phosphorylation by PTK enhances function. The predominant effect of glutamate receptor phosphorylation by a variety of kinases is a potentiation of the peak current response. However, PKC also modulates clustering of NMDA receptors. This complexity in the regulation of ligand-gated ion channels by phosphorylation provides diverse mechanisms for mediating synaptic plasticity. In fact, accumulating evidence supports the involvement of protein phosphorylation and dephosphorylation of AMPA receptors in LTP and LTD respectively. There has been a dramatic increase in our understanding of the nature by which phosphorylation regulates ligand-gated ion channels. However, many questions remain unanswered. (AB

Activation of protein kinase C induces gamma-aminobutyric acid type A receptor internalization in Xenopus oocytes

The inhibition of gamma-aminobutyric acid (GABA)-gated chloride currents by the protein kinase C (PKC) activator 4beta-phorbol 12-myristate 13-acetate (PMA) was investigated using recombinant human GABAA receptors expressed in Xenopus oocytes. PMA (5 nM) reduced the GABA response in oocytes expressing the alpha1 beta2 gamma2L receptor construct, as measured by the two-electrode voltage-clamp method. GABA responses declined to approximately 25% of their pretreatment value within 45 min. GABA responses in oocytes expressing a receptor construct from which the known PKC phosphorylation sites were absent, alpha1 beta2(S410A), were comparably inhibited. Phorbol 12-monomyristate (PMM; 5 nM), which does not activate PKC, did not alter the GABA response in either construct, while the PKC inhibitor calphostin C (0.5 microM) prevented the PMA effect. To further investigate PMA inhibition of the GABA response, a GABAA receptor alpha1 subunit/green fluorescent protein (GFP) chimera (alpha1GFP) was used to visualize GABAA receptor distribution. Similar to the wild type constructs, PMA robustly decreased GABA responses in oocytes expressing alpha1GFP beta2 gamma2L and alpha1GFP beta2(S410A) receptor constructs. Following PMA treatment, GFP fluorescence in the oocyte plasma membrane was decreased to approximately 45% of the pretreatment values indicating GABAA receptor internalization. This effect of PMA was prevented by calphostin C and was not produced by PMM. Experiments with bd24, a monoclonal antibody which recognizes an extracellular epitope of the alpha1 subunit, were used to demonstrate that PMA, but not PMM, decreases alpha1 subunit immunoreactivity in the plasma membrane of intact oocytes expressing the alpha1 beta2 gamma2L construct, thus confirming the results obtained with the chimeric receptor. It is concluded that, in Xenopus oocytes, PMA induces an internalization of the GABAA receptor through PKC-mediated phosphorylation of an unidentified protein(s) and that this contributes to the decrease in electrophysiological responses to GABA following PKC activation.

Src, N-methyl-D-aspartate (NMDA) receptors, and synaptic plasticity

The protein tyrosine kinase Src is expressed widely in the central nervous system and is abundant in neurons. Over the past several years, evidence has accumulated showing that one function of Src is to regulate the activity of N-methyl-D-aspartate (NMDA) receptors and other ion channels. NMDA receptors are a principal subtype of glutamate receptor that mediates fast excitatory transmission at most central synapses. Recently it has been discovered that, by means of up-regulating the function of NMDA receptors, Src mediates the induction of long-term potentiation (LTP) in the CA1 region of the hippocampus. This finding led to a new model for induction of LTP whereby tetanic stimulation produces a rapid activation of Src, causing enhanced NMDA receptor function. This enhanced NMDA receptor function boosts the entry of Ca2+, which may thereby trigger the downstream signalling cascade, ending in potentiation of non-NMDA receptors. This functional role for Src may be important in physiological and pathophysiological processes in the central nervous system.

State-dependent NMDA receptor antagonism by Ro 8-4304, a novel NR2B selective, non-competitive, voltage-independent antagonist

1. Subunit-selective blockade of N-methyl-D-aspartate (NMDA) receptors provides a potentially attractive strategy for neuroprotection in the absence of undesirable side effects. Here, we describe a novel NR2B-selective NMDA antagonist, 4-¿3-[4-(4-fluoro-phenyl)-3,6-dihydro-2H-pyridin-1-yl]-2-hydroxy-propoxy ¿-benzamide (Ro 8-4304), which exhibits >100 fold higher affinity for recombinant NR1(001)/NR2B than NR1(001)/NR2A receptors. 2. Ro 8-4304 is a voltage-independent, non-competitive antagonist of NMDA receptors in rat cultured cortical neurones and exhibits a state-dependent mode of action similar to that described for ifenprodil. 3. The apparent affinity of Ro 8-4304 for the NMDA receptor increased in an NMDA concentration-dependent manner so that Ro 8-4304 inhibited 10 and 100 microM NMDA responses with IC50s of 2.3 and 0.36 microM, respectively. Currents elicited by 1 microM NMDA were slightly potentiated in the presence of 10 microM Ro 8-4304, and Ro 8-4304 binding slowed the rate of glutamate dissociation from NMDA receptors. 4. These results were predicted by a reaction scheme in which Ro 8-4304 exhibits a 14 and 23 fold higher affinity for the activated and desensitized states of the NMDA receptor, respectively, relative to the agonist-unbound resting state. Additionally, Ro 8-4304 binding resulted in a 3 4 fold increase in receptor affinity for glutamate site agonists. 5. Surprisingly, whilst exhibiting a similar affinity for NR2B-containing NMDA receptors as ifenprodil, Ro 8-4304 exhibited markedly faster kinetics of binding and unbinding to the NMDA receptor. This spectrum of kinetic behaviour reveals a further important feature of this emerging class of NR2B-selective compounds.

Serotonin 5-HT2 receptor activation potentiates N-methyl-D-aspartate receptor-mediated ion currents by a protein kinase C-dependent mechanism

Modulation of N-methyl-D-aspartate (NMDA) receptor-mediated ion currents by serotonin was investigated with a two-electrode voltage clamp technique in Xenopus oocytes injected with rat brain RNA. After a 1-min application of 200 nM serotonin a transient potentiation of the NMDA receptor-mediated ion currents was observed. The serotonin-induced enhancement was mimicked by the protein kinase C activators 1-oleoyl-2-acetyl-sn-glycerol (100 microM) and phorbol 12-myristate 13-acetate (10 nM), whereas the inactive phorbol ester 4-alpha-phorbol 12-myristate 13-acetate (10 nM) had no effect. From these observations it was concluded that protein kinase C was involved in the enhancement of NMDA-induced currents. In agreement with this conclusion, it was found that the serotonin effect was inhibited by the protein kinase C inhibitors sphingosine (1 microM) or staurosporine (1 microM) added 20 min before NMDA application and by oocyte injection of protein kinase C (PKC)-inhibitor peptide (500 ng/oocyte) 1 hr prior to recordings. The serotonin receptor involved was identified as a 5-HT2 receptor subtype by the finding that 200 nM of the selective 5-HT2 receptor agonist alpha-methyl-5-hydroxytryptamine mimicked the potentiation of NMDA-induced ion currents by serotonin. Furthermore, the observed potentiation was significantly reduced by co-application of serotonin with 100 microM of the selective 5-HT2 receptor antagonist ketanserin. These results indicate that 5-HT2 receptors enhance NMDA receptor function via phosphoinositol hydrolysis and subsequent stimulation of PKC.

Modulation of amino acid-gated ion channels by protein phosphorylation

The major excitatory and inhibitory amino acid receptors in the mammalian central nervous system are considered to be glutamate, gamma-aminobutyric acid type A (GABAA), and glycine receptors. These receptors are widely acknowledged to participated in fast synaptic neurotransmission, which ultimately is responsible for the control of neuronal excitability. In addition to these receptors being regulated by endogenous factors, including the natural neurotransmitters, they also form target substrates for phosphorylation by a number of protein kinases, including serine/threonine and tyrosine kinases. The process of phosphorylation involves the transfer of a phosphate group(s) from adenosine triphosphate to one or more serine, threonine, or tyrosine residues, which are invariably found in an intracellular location within the receptor Phosphorylation is an important means of receptor regulation since it represents a covalent modification of the receptor structure, which can have important implications for ion channel function. This chapter reviews the current molecular and biochemical evidence regarding the sites of phosphorylation for both native neuronal and recombinant glutamate, GABAA and glycine receptors, and also reviews the functional electrophysiological implications of phosphorylation for receptor function.

Ammonia prevents activation of NMDA receptors by glutamate in rat cerebellar neuronal cultures

Acute ammonia toxicity is mediated by activation of NMDA receptors and is prevented by chronic moderate hyperammonaemia. The aim of this work was to assess whether the protective effect of chronic hyperammonaemia is due to impaired activation of the NMDA receptor. It is shown that chronic hyperammonaemia in rats decreases the binding of [3H]MK-801 to synaptosomal membranes from the hippocampus but not the amount of NMDAR1 receptor protein as determined by immunoblotting. In primary cultures of cerebellar neurons, long-term treatment with 1 mM ammonia also decreased significantly the binding of [3H]MK-801. These results suggest that ammonia impairs NMDA receptor activation. To confirm this possibility we tested the effect of long-term treatment of the cultured neurons with 1 mM ammonia on three well known events evoked by activation of the NMDA receptor: neuronal death induced by glutamate, increase in aspartate aminotransferase activity and increase in free intracellular [Ca2+]. Long-term treatment with ammonia prevented noticeably the effects of glutamate or NMDA on all these parameters. These results indicate that long-term treatment of neurons with 1 mM ammonia leads to impaired function of the NMDA receptor, which cannot be activated by glutamate or NMDA. Activation of protein kinase C by a phorbol ester restored the ability of the NMDA receptor to be activated in neurons treated with ammonia. This suggests that ammonia impairs NMDA receptor function by decreasing protein kinase C-dependent phosphorylation.

Insulin potentiates N-methyl-D-aspartate receptor activity in Xenopus oocytes and rat hippocampus

Growth factor signal transduction pathways have recently been shown to affect voltage-gated ion channel activity. In this study we report that insulin can modulate the activity of a ligand-gated ion channel, the N-methyl-D-aspartate (NMDA) receptor. In Xenopus oocytes, brief insulin exposure rapidly potentiated NR1a/NR2A and NR1a/NR2B receptor responses 2-3 fold and weakly potentiated NR1a/NR2C and NR1a/NR2D mediated-responses. Insulin potentiation of NR1a/NR2A receptor responses was significantly blocked by staurosporine, suggesting kinase involvement in insulin action. Insulin modulation of native NMDA receptors is suggested by the observation that insulin potentiated the NMDA receptor-mediated synaptic component in hippocampal slices. Regulation of NMDA receptor activity by growth factors may account for previous observations of growth factor modulation of central nervous system excitotoxicity.

Functional modulation of ligand-gated GABAA and NMDA receptor channels by phosphorylation

Ion channels are ubiquitous membrane proteins that may be organized into different families according to their predicted transmembrane topology. They are concerned with rapid signalling over plasma and intracellular membranes and are activated, depending on their type, by transmembrane voltage, intracellular second messengers or extracellular neurotransmitters. Intracellular activities of protein kinases and phosphatases act to modulate ion channel activity. The modulation of the function of ligand activated, neuronal ion channels, that are crucial for synaptic transmission, may be an important basis for a modulation of a synaptic efficiency. The following review concentrates, due to space limitations, on the postranslational modification, and on the modulation of the function by protein kinase C and protein kinase A, of ligand-gated GABAA channels and NMDA channels on a molecular level.