Lin Pei

Dual Regulation of NMDA Receptor Functions by Direct Protein-Protein Interactions with the Dopamine D1 Receptor

Dopamine D1-like receptors, composed of D1 and D5 receptors, have been documented to modulate glutamate-mediated fast excitatory synaptic neurotransmission. Here, we report that dopamine D1 receptors modulate NMDA glutamate receptor-mediated functions through direct protein-protein interactions. Two regions in the D1 receptor carboxyl tail can directly and selectively couple to NMDA glutamate receptor subunits NR1-1a and NR2A. While one interaction is involved in the inhibition of NMDA receptor-gated currents, the other is implicated in the attenuation of NMDA receptor-mediated excitotoxicity through a PI-3 kinase-dependent pathway.

Activation of PI3-kinase is required for AMPA receptor insertion during LTP of mEPSCs in cultured hippocampal neurons.

Hippocampal CA1 homosynaptic long-term potentiation (LTP) is expressed specifically at activated synapses. Increased insertion of postsynaptic alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid receptors (AMPARs) appears to be crucial for CA1 LTP. However, the mechanism underlying AMPAR insertion during LTP remains largely unknown. We now report that phosphatidylinositol 3-kinase (PI3K) is complexed with AMPARs at synapses and activated by selective stimulation of synaptic N-methyl-D-aspartate (NMDA) receptors. Activation of the AMPAR-associated PI3K is required for the increased cell surface expression of AMPARs and LTP. Thus, our results strongly suggest that the AMPAR-PI3K complex may constitute a critical molecular signal responsible for AMPAR insertion at activated CA1 synapses during LTP, and consequently, this lipid kinase may serve to determine the polarity of NMDA receptor-dependent synaptic plasticity.

Control of Synaptic Strength, a Novel Function of Akt

Akt (also known as PKB), a serine/threonine kinase involved in diverse signal-transduction pathways, is highly expressed in the brain. Akt is known to have a strong antiapoptotic action and thereby to be critically involved in neuronal survival, but its potential role in the dynamic modulation of synaptic transmission is unknown. Here we report that Akt phosphorylates, both in vitro and in vivo, the type A gamma-aminobutyric acid receptor (GABA(A)R), the principal receptor mediating fast inhibitory synaptic transmission in the mammalian brain. Akt-mediated phosphorylation increases the number of GABA(A)Rs on the plasma membrane surface, thereby increasing the receptor-mediated synaptic transmission in neurons. These results identify the GABA(A)R as a novel substrate of Akt, thereby linking Akt to the regulation of synaptic strength. This work also provides evidence for the rapid regulation of neurotransmitter receptor numbers in the postsynaptic domain by direct receptor phosphorylation as an important means of producing synaptic plasticity.

Regulation of Dopamine D1 Receptor Function by Physical Interaction with the NMDA Receptors

Functional interactions between dopamine D1-like receptors and NMDA subtype glutamate receptors have been implicated in the maintenance of normal brain activity and neurological dysfunction. Although modulation of NMDA receptor functions by D1 receptor activation has been the subject of extensive investigation, little is known as to how the activation of NMDA receptors alters D1 function. Here we report that NMDA receptors regulate D1 receptor function via a direct protein–protein interaction mediated by the carboxyl tail regions of both receptors. In both cotransfected cells and cultured hippocampal neurons the activation of NMDA receptors increases the number of D1 receptors on the plasma membrane surface and enhances D1 receptor-mediated cAMP accumulation via a SNARE-dependent mechanism. Furthermore, overexpression of mini-genes encoding either NR1 or D1 carboxyl tail fragments disrupts the D1–NR1 direct protein–protein interaction and abolishes NMDA-induced changes in both D1 cell surface expression and D1-mediated cAMP accumulation. Our results demonstrate that the D1–NR1 physical interaction enables NMDA receptors to increase plasma membrane insertion of D1 receptors and provides a novel mechanism by which the activation of NMDA receptors upregulates D1 receptor function. Understanding the molecular mechanisms by which D1 and NMDA receptors functionally interact may provide insight toward elucidating the molecular neurobiological mechanisms involved in many neuropsychiatric illnesses, such as schizophrenia.

Dopamine transporter cell surface localization facilitated by a direct interaction with the dopamine D2 receptor.

Altered synaptic dopamine levels have been implicated in several neurological/neuropsychiatric disorders, including drug addiction and schizophrenia. However, it is unclear what precipitates these changes in synaptic dopamine levels. One of the key presynaptic components involved in regulating dopaminergic tone is the dopamine transporter (DAT). Here, we report that the DAT is also regulated by the dopamine D2 receptor through a direct protein–protein interaction involving the DAT amino‐terminus and the third intracellular loop of the D2 receptor. This physical coupling facilitates the recruitment of intracellular DAT to the plasma membrane and leads to enhanced dopamine reuptake. Moreover, mice injected with peptides that disrupt D2–DAT interaction exhibit decreased synaptosomal dopamine uptake and significantly increased locomotor activity, reminiscent of DAT knockout mice. Our data highlight a novel mechanism through which neurotransmitter receptors can functionally modulate neurotransmitter transporters, an interaction that can affect the synaptic neurotransmitter levels in the brain.

Uncoupling the dopamine D1-D2 receptor complex exerts antidepressant-like effects

We report that coupling between dopamine D1 and D2 receptors was markedly increased in postmortem brain of subjects suffering from major depression. Biochemical analyses revealed that D1 and D2 receptors form heterodimers via a direct protein-protein interaction. Administration of an interfering peptide that disrupts the D1-D2 receptor complex substantially reduced immobility in the forced swim test (FST) without affecting locomotor activity, and decreased escape failures in learned helplessness tests in rats.

Protein-Protein Coupling/Uncoupling Enables Dopamine D2 Receptor Regulation of AMPA Receptor-Mediated Excitotoxicity

There is considerable evidence that dopamine D2 receptors can modulate AMPA receptor-mediated neurotoxicity. However, the molecular mechanism underlying this process remains essentially unclear. Here we report that D2 receptors inhibit AMPA-mediated neurotoxicity through two pathways: the activation of phosphoinositide-3 kinase (PI-3K) and downregulation of AMPA receptor plasma membrane expression, both involving a series of protein-protein coupling/uncoupling events. Agonist stimulation of D2 receptors promotes the formation of the direct protein-protein interaction between the third intracellular loop of the D2 receptor and the ATPase N-ethylmaleimide-sensitive factor (NSF) while uncoupling the NSF interaction with the carboxyl tail (CT) of the glutamate receptor GluR2 subunit of AMPA receptors. Previous studies have shown that full-length NSF directly couples to the GluR2CT and facilitates AMPA receptor plasma membrane expression. Furthermore, the CT region of GluR2 subunit is also responsible for several other intracellular protein couplings, including p85 subunit of PI-3K. Therefore, the direct coupling of D2-NSF and concomitant decrease in the NSF-GluR2 interaction results in a decrease of AMPA receptor membrane expression and an increase in the interaction between GluR2 and the p85 and subsequent activation of PI-3K. Disruption of the D2-NSF interaction abolished the ability of D2 receptor to attenuate AMPA-mediated neurotoxicity by blocking the D2 activation-induced changes in PI-3K activity and AMPA receptor plasma membrane expression. Furthermore, the D2-NSF-GluR2-p85 interactions are also responsible for the D2 inhibition of ischemia-induced cell death. These data may provide a new avenue to identify specific targets for therapeutics to modulate glutamate receptor-governed diseases, such as stroke.

Schizophrenia, amphetamine-induced sensitized state and acute amphetamine exposure all show a common alteration: increased dopamine D2 receptor dimerization

BackgroundAll antipsychotics work via dopamine D2 receptors (D2Rs), suggesting a critical role for D2Rs in psychosis; however, there is little evidence for a change in receptor number or pharmacological nature of D2Rs. Recent data suggest that D2Rs form dimers in-vitro and in-vivo, and we hypothesized that schizophrenia, as well as preclinical models of schizophrenia, would demonstrate altered dimerization of D2Rs, even though the overall number of D2Rs was unaltered.MethodsWe measured the expression of D2Rs dimers and monomers in patients with schizophrenia using Western blots, and then in striatal tissue from rats exhibiting the amphetamine-induced sensitized state (AISS). We further examined the interaction between D2Rs and the dopamine transporter (DAT) by co-immunoprecipitation, and measured the expression of dopamine D2High receptors with ligand binding assays in rat striatum slices with or without acute amphetamine pre-treatment.ResultsWe observed significantly enhanced expression of D2Rs dimers (277.7 ± 33.6%) and decreased expression of D2Rs monomers in post-mortem striatal tissue of schizophrenia patients. We found that amphetamine facilitated D2Rs dimerization in both the striatum of AISS rats and in rat striatal neurons. Furthermore, amphetamine-induced D2Rs dimerization may be associated with the D2R-DAT protein-protein interaction as an interfering peptide that disrupts the D2R-DAT coupling, blocked amphetamine-induced up-regulation of D2Rs dimerization.ConclusionsGiven the fact that amphetamine induces psychosis and that the AISS rat is a widely accepted animal model of psychosis, our data suggest that D2R dimerization may be important in the pathophysiology of schizophrenia and may be a promising new target for novel antipsychotic drugs.

Direct interaction between GluR2 and GAPDH regulates AMPAR-mediated excitotoxicity

Over-activation of AMPARs (α−amino-3-hydroxy-5-methylisoxazole-4-propionic acid subtype glutamate receptors) is implicated in excitotoxic neuronal death associated with acute brain insults, such as ischemic stroke. However, the specific molecular mechanism by which AMPARs, especially the calcium-impermeable AMPARs, induce neuronal death remains poorly understood. Here we report the identification of a previously unrecognized molecular pathway involving a direct protein-protein interaction that underlies GluR2-containing AMPAR-mediated excitotoxicity. Agonist stimulation of AMPARs promotes GluR2/GAPDH (glyceraldehyde-3-phosphate dehydrogenase) complex formation and subsequent internalization. Disruption of GluR2/GAPDH interaction by administration of an interfering peptide prevents AMPAR-mediated excitotoxicity and protects against damage induced by oxygen-glucose deprivation (OGD), an in vitro model of brain ischemia.

Extracellular dopamine induces the oxidative toxicity of SH‐SY5Y cells

Dopamine‐induced neuronal cytotoxicity has been proposed as a leading pathological mechanism underlying many neuronal degenerative disorders including Parkinson disease. Various hypotheses have been proposed including oxidative stress and dopamine (DA)‐induced intracellular signal disorder via DA D1 and D2 receptors. The exact mechanism involved in this process is far from clear. In this study, employing a neuronal blastoma cell line, SH‐SY5Y, we tried to elucidate the roles of these different suggested mechanisms in this pathological process. The results showed that DA induced cell toxicity in a dose‐ and time‐dependent way. Selective D1 and D2 DA receptor antagonist could not block the cytotoxic effects, whereas reductive reagent ascorbic acid but not GSH could effectively rescue the cell death, suggesting that DA‐induced cell toxicity was caused by an extracellular oxidative stress. This was further supported by the enhancing effects of DA transporter blocker, GBR, which could increase the cell death when pretreated. Finally, ascorbic acid could also protect SY5Y cells from DA‐induced cellular apoptotic signal changes including PARP and P53. Our studies suggested that DA exerted its cytotoxic effects via an extracellular metabolism, whereas intracellular transportation could reduce its oxidative stress. Cytotoxicity effects induced by extracellular DA could be protected by reductive agents as ascorbic acid. These results help to broaden our understanding of the mechanisms of DA‐induced cell death and may provide potentially therapeutical alternative for the neurodegenerative disorders. Synapse 62:797–803, 2008.