Recent progress in understanding subtype specific regulation of NMDA receptors by G Protein Coupled Receptors (GPCRs)

Regulation of Src family kinases by muscarinic acetylcholine receptors in heterologous cells and neurons

Five muscarinic acetylcholine (mACh) receptor subtypes are divided into two classes: the M1 class (M1, M3, and M5) and the M2 class (M2 and M4). The former is coupled to Gq proteins, while the latter is coupled to Gi/o proteins. Accumulating evidence indicates that mACh receptors play a significant role in the regulation of the Src family kinase (SFK), a subfamily of non-receptor tyrosine kinases. mACh receptors exert their roles in a subtype-dependent fashion and preferentially target Src and Fyn, two members of SFKs that are expressed in the brain and enriched at synaptic sites. While the M1 receptor positively modulates SFK activity, the M4 receptor inhibits it. By modulating SFKs, mACh receptors are actively involved in the regulation of expression and function of a variety of receptors, structural proteins, and signaling molecules. In particular, the M4 receptor and the dopamine D1 receptor are coexpressed in striatonigral projection neurons of the striatum. Gi/o-coupled M4 and Gq-coupled D1 receptors antagonistically regulate SFK activity, thereby forming a dynamic balance controlling glutamate receptor activity, excitability of neurons, and synaptic plasticity. In summary, mACh receptors play a crucial role in regulating SFK activity in heterologous cells and neurons.

Target cell-specific plasticity rules of NMDA receptor-mediated synaptic transmission in the hippocampus

Long-term potentiation and depression of NMDA receptor-mediated synaptic transmission (NMDAR LTP/LTD) can significantly impact synapse function and information transfer in several brain areas. However, the mechanisms that determine the direction of NMDAR plasticity are poorly understood. Here, using physiologically relevant patterns of presynaptic and postsynaptic burst activities, whole-cell patch clamp recordings, 2-photon laser calcium imaging in acute rat hippocampal slices and immunoelectron microscopy, we tested whether distinct calcium dynamics and group I metabotropic glutamate receptor (I-mGluR) subtypes control the sign of NMDAR plasticity. We found that postsynaptic calcium transients (CaTs) in response to hippocampal MF stimulation were significantly larger during the induction of NMDAR-LTP compared to NMDAR-LTD at the MF-to-CA3 pyramidal cell (MF-CA3) synapse. This difference was abolished by pharmacological blockade of mGluR5 and was significantly reduced by depletion of intracellular calcium stores, whereas blocking mGluR1 had no effect on these CaTs. In addition, we discovered that MF to hilar mossy cell (MF-MC) synapses, which share several structural and functional commonalities with MF-CA3 synapses, also undergoes NMDAR plasticity. To our surprise, however, we found that the postsynaptic distribution of I-mGluR subtypes at these two synapses differ, and the same induction protocol that induces NMDAR-LTD at MF-CA3 synapses, only triggered NMDAR-LTP at MF-MC synapses, despite a comparable calcium dynamics. Thus, postsynaptic calcium dynamics alone cannot predict the sign of NMDAR plasticity, indicating that both postsynaptic calcium rise and the relative contribution of I-mGluR subtypes likely determine the learning rules of NMDAR plasticity.

Autoantibodies targeting G protein-coupled receptors: An evolving history in autoimmunity. Report of the 4th international symposium

G protein-coupled receptors (GPCR) are involved in various physiological and pathophysiological processes. Functional autoantibodies targeting GPCRs have been associated with multiple disease manifestations in this context. Here we summarize and discuss the relevant findings and concepts presented in the biennial International Meeting on autoantibodies targeting GPCRs (the 4th Symposium), held in Lübeck, Germany, 15-16 September 2022. The symposium focused on the current knowledge of these autoantibodies' role in various diseases, such as cardiovascular, renal, infectious (COVID-19), and autoimmune diseases (e.g., systemic sclerosis and systemic lupus erythematosus). Beyond their association with disease phenotypes, intense research related to the mechanistic action of these autoantibodies on immune regulation and pathogenesis has been developed, underscoring the role of autoantibodies targeting GPCRs on disease outcomes and etiopathogenesis. The observation repeatedly highlighted that autoantibodies targeting GPCRs could also be present in healthy individuals, suggesting that anti-GPCR autoantibodies play a physiologic role in modeling the course of diseases. Since numerous therapies targeting GPCRs have been developed, including small molecules and monoclonal antibodies designed for treating cancer, infections, metabolic disorders, or inflammatory conditions, anti-GPCR autoantibodies themselves can serve as therapeutic targets to reduce patients' morbidity and mortality, representing a new area for the development of novel therapeutic interventions.

The Signaling Pathways Involved in the Anticonvulsive Effects of the Adenosine A1 Receptor

Adenosine acts as an endogenous anticonvulsant and seizure terminator in the brain. Many of its anticonvulsive effects are mediated through the activation of the adenosine A₁ receptor, a G protein-coupled receptor with a wide array of targets. Activating A₁ receptors is an effective approach to suppress seizures. This review gives an overview of the neuronal targets of the adenosine A₁ receptor focusing in particular on signaling pathways resulting in neuronal inhibition. These include direct interactions of G protein subunits, the adenyl cyclase pathway and the phospholipase C pathway, which all mediate neuronal hyperpolarization and suppression of synaptic transmission. Additionally, the contribution of the guanyl cyclase and mitogen-activated protein kinase cascades to the seizure-suppressing effects of A₁ receptor activation are discussed. This review ends with the cautionary note that chronic activation of the A₁ receptor might have detrimental effects, which will need to be avoided when pursuing A₁ receptor-based epilepsy therapies.

Regulator of G-Protein Signaling 5 Maintains Brain Endothelial Cell Function in Focal Cerebral Ischemia

Background

Regulator of G‐protein signaling 5 (RGS5) is a negative modulator of G‐protein–coupled receptors. The role of RGS5 in brain endothelial cells is not known. We hypothesized that RGS5 in brain microvascular endothelial cells may be an important mediator of blood‐brain barrier function and stroke severity after focal cerebral ischemia.

Methods and Results

Using a transient middle cerebral artery occlusion model, we found that mice with global and endothelial‐specific deletion of Rgs5 exhibited larger cerebral infarct size, greater neurological motor deficits, and increased brain edema. In our in vitro models, we observed increased G_q activity and elevated intracellular Ca²⁺ levels in brain endothelial cells. Furthermore, the loss of endothelial RGS5 leads to decreased endothelial NO synthase expression and phosphorylation, relocalization of endothelial tight junction proteins, and increased cell permeability. Indeed, RGS5 deficiency leads to increased Rho‐associated kinase and myosin light chain kinase activity, which were partially reversed in our in vitro model by pharmacological inhibition of G_q, metabotropic glutamate receptor 1, and ligand‐gated ionotropic glutamate receptor.

Conclusions

Our findings indicate that endothelial RGS5 plays a novel neuroprotective role in focal cerebral ischemia. Loss of endothelial RGS5 leads to hyperresponsiveness to glutamate signaling pathways, enhanced Rho‐associated kinase– and myosin light chain kinase–mediated actin‐cytoskeleton reorganization, endothelial dysfunction, tight junction protein relocalization, increased blood‐brain barrier permeability, and greater stroke severity. These findings suggest that preservation of endothelial RGS5 may be an important therapeutic strategy for maintaining blood‐brain barrier integrity and limiting the severity of ischemic stroke.

Modulation of NMDA Receptors by G-protein-coupled receptors: Role in Synaptic Transmission, Plasticity and Beyond

NMDA receptors (NMDARs) play a critical role in excitatory synaptic transmission, plasticity and in several forms of learning and memory. In addition, NMDAR dysfunction is believed to underlie a number of neuropsychiatric conditions. Growing evidence has demonstrated that NMDARs are tightly regulated by several G-protein-coupled receptors (GPCRs). Ligands that bind to GPCRs, such as neurotransmitters and neuromodulators, activate intracellular pathways that modulate NMDAR expression, subcellular localization and/or functional properties in a short- or a long-term manner across many synapses throughout the central nervous system. In this review article we summarize current knowledge on the molecular and cellular mechanisms underlying NMDAR modulation by GPCRs, and we discuss the implications of this modulation spanning from synaptic transmission and plasticity to circuit function and brain disease.

GPR40 modulates epileptic seizure and NMDA receptor function

Epilepsy is a common neurological disease, and approximately 30% of patients do not respond adequately to antiepileptic drug treatment. Recent studies suggest that G protein-coupled receptor 40 (GPR40) is expressed in the central nervous system and is involved in the regulation of neurological function. However, the impact of GPR40 on epileptic seizures remains unclear. In this study, we first reported that GPR40 expression was increased in epileptic brains. In the kainic acid-induced epilepsy model, GPR40 activation after status epilepticus alleviated epileptic activity, whereas GPR40 inhibition showed the opposite effect. In the pentylenetetrazole-induced kindling model, susceptibility to epilepsy was reduced with GPR40 activation and increased with GPR40 inhibition. Whole-cell patch-clamp recordings demonstrated that GPR40 affected N-methyl-d-aspartate (NMDA) receptor-mediated synaptic transmission. Moreover, GPR40 regulated NR2A and NR2B expression on the surface of neurons. In addition, endocytosis of NMDA receptors and binding of GPR40 with NR2A and NR2B can be regulated by GPR40. Together, our findings indicate that GPR40 modulates epileptic seizures, providing a novel antiepileptic target.

Chemogenetic Activation of Prefrontal Cortex Rescues Synaptic and Behavioral Deficits in a Mouse Model of 16p11.2 Deletion Syndrome

Microdeletion of the human 16p11.2 gene locus has been linked to autism spectrum disorder (ASD) and intellectual disability and confers risk for a number of other neurodevelopmental deficits. Transgenic mice carrying 16p11.2 deletion (16p11^+/-) display phenotypes reminiscent of those in human patients with 16p11.2 deletion syndrome, but the molecular mechanisms and treatment strategies for these phenotypes remain unknown. In this study, we have found that both male and female 16p11^+/- mice exhibit deficient NMDA receptor (NMDAR) function in the medial prefrontal cortex (mPFC), a brain region critical for high-level "executive" functions. Elevating the activity of mPFC pyramidal neurons with a CaMKII-driven Gq-DREADD (Gq-coupled designer receptors exclusively activated by designer drugs) led to the significant increase of NR2B subunit phosphorylation and the restoration of NMDAR function, as well as the amelioration of cognitive and social impairments in 16p11^+/- mice. These results suggest that NMDAR hypofunction in PFC may contribute to the pathophysiology of 16p11.2 deletion syndrome and that restoring PFC activity is sufficient to rescue the behavioral deficits.SIGNIFICANCE STATEMENT The 16p11.2 deletion syndrome is strongly associated with autism spectrum disorder and intellectual disability. Using a mouse model carrying the 16p11.2 deletion, 16p11^+/-, we identified NMDA receptor hypofunction in the prefrontal cortex (PFC). Elevating the activity of PFC pyramidal neurons with a chemogenetic tool, Gq-DREADD, led to the restoration of NMDA receptor function and the amelioration of cognitive and social impairments in 16p11^+/- mice. These results have revealed a novel route for potential therapeutic intervention of 16p11.2 deletion syndrome.

Physiological activation of mGlu5 receptors supports the ion channel function of NMDA receptors in hippocampal LTD induction in vivo

Synaptic long-term depression (LTD) is believed to underlie critical mnemonic processes in the adult hippocampus. The roles of the metabotropic and ionotropic actions of glutamate in the induction of synaptic LTD by electrical low-frequency stimulation (LFS) in the living adult animal is poorly understood. Here we examined the requirement for metabotropic glutamate (mGlu) and NMDA receptors in LTD induction in anaesthetized adult rats. LTD induction was primarily dependent on NMDA receptors and required the involvement of both the ion channel function and GluN2B subunit of the receptor. Endogenous mGlu5 receptor activation necessitated the local application of relatively high doses of either competitive or non-competitive NMDA receptor antagonists to block LTD induction. Moreover, boosting endogenous glutamate activation of mGlu5 receptors with a positive allosteric modulator lowered the threshold for NMDA receptor-dependent LTD induction by weak LFS. The present data provide support in the living animal that NMDA receptor-dependent LTD is boosted by endogenously released glutamate activation of mGlu5 receptors. Given the predominant perisynaptic location of mGlu5 receptors, the present findings emphasize the need to further evaluate the contribution and mechanisms of these receptors in NMDA receptor-dependent synaptic plasticity in the adult hippocampus in vivo.

Reciprocal Alterations in Regulator of G Protein Signaling 4 and microRNA16 in Schizophrenia

N-methyl-d-aspartate receptor (NMDAR) hypofunction in the dorsolateral prefrontal cortex (DLPFC) has been implicated in the pathology of schizophrenia. NMDAR activity is negatively regulated by some G protein-coupled receptors (GPCRs). Signaling through these GPCRs is reduced by Regulator of G protein Signaling 4 (RGS4). Thus, lower levels of RGS4 would enhance GPCR-mediated reductions in NMDAR activity and could contribute to NMDAR hypofunction in schizophrenia. In this study, we quantified RGS4 mRNA and protein levels at several levels of resolution in the DLPFC from subjects with schizophrenia and matched healthy comparison subjects. To investigate molecular mechanisms that could contribute to altered RGS4 levels, we quantified levels of small noncoding RNAs, known as microRNAs (miRs), which regulate RGS4 mRNA integrity after transcription. RGS4 mRNA and protein levels were significantly lower in schizophrenia subjects and were positively correlated across all subjects. The RGS4 mRNA deficit was present in pyramidal neurons of DLPFC layers 3 and 5 of the schizophrenia subjects. In contrast, levels of miR16 were significantly higher in the DLPFC of schizophrenia subjects, and higher miR16 levels predicted lower RGS4 mRNA levels. These findings provide convergent evidence of lower RGS4 mRNA and protein levels in schizophrenia that may result from increased expression of miR16. Given the role of RGS4 in regulating GPCRs, and consequently the strength of NMDAR signaling, these findings could contribute to the molecular substrate for NMDAR hypofunction in DLPFC pyramidal cells in schizophrenia.

The Relative Contribution of NMDARs to Excitatory Postsynaptic Currents is Controlled by Ca(2+)-Induced Inactivation

NMDA receptors (NMDARs) are important mediators of excitatory synaptic transmission and plasticity. A hallmark of these channels is their high permeability to Ca(2+). At the same time, they are themselves inhibited by the elevation of intracellular Ca(2+) concentration. It is unclear however, whether the Ca(2+) entry associated with single NMDAR mediated synaptic events is sufficient to self-inhibit their activation. Such auto-regulation would have important effects on the dynamics of synaptic excitation in several central neuronal networks. Therefore, we studied NMDAR-mediated synaptic currents in mouse hippocampal CA1 pyramidal neurons. Postsynaptic responses to subthreshold Schaffer collateral stimulation depended strongly on the absence or presence of intracellular Ca(2+) buffers. Loading of pyramidal cells with exogenous Ca(2+) buffers increased the amplitude and decay time of NMDAR mediated EPSCs (EPSPs) and prolonged the time window for action potential (AP) generation. Our data indicate that the Ca(2+) influx mediated by unitary synaptic events is sufficient to produce detectable self-inhibition of NMDARs even at a physiological Mg(2+) concentration. Therefore, the contribution of NMDARs to synaptic excitation is strongly controlled by both previous synaptic activity as well as by the Ca(2+) buffer capacity of postsynaptic neurons.

The ON:OFF switch, σ1R-HINT1 protein, controls GPCR-NMDA receptor cross-regulation: implications in neurological disorders

In the brain, the histidine triad nucleotide-binding protein 1 (HINT1) and sigma 1 receptors (σ1Rs) coordinate the activity of certain G-protein coupled receptors (GPCRs) with that of glutamate N-methyl-D-aspartate receptors (NMDARs). To determine the role of HINT1-σ1R in the plasticity of GPCR-NMDAR interactions, substances acting at MOR, cannabinoid CB1 receptor, NMDAR and σ1R were injected into mice, and their effects were evaluated through in vivo, ex vivo, and in vitro assays. It was observed that HINT1 protein binds to GPCRs and NMDAR NR1 subunits in a calcium-independent manner, whereas σ1R binding to these proteins increases in the presence of calcium. In this scenario, σ1R agonists keep HINT1 at the GPCR and stimulate GPCR-NMDAR interaction, whereas σ1R antagonists transfer HINT1 to NR1 subunits and disengage both receptors. This regulation is lost in σ1R-/- mice, where HINT1 proteins mostly associate with NMDARs, and GPCRs are physically and functionally disconnected from NMDARs. In HINT1-/- mice, ischemia produces low NMDAR-mediated brain damage, suggesting that several different GPCRs enhance glutamate excitotoxicity via HINT1-σ1R. Thus, several GPCRs associate with NMDARs by a dynamic process under the physiological control of HINT1 proteins and σ1Rs. The NMDAR-HINT1-σ1R complex deserves attention because it offers new therapeutic opportunities.

GluN2B-containing NMDA receptors are upregulated in plasma membranes by the sphingosine-1-phosphate analog FTY720P

Sphingosine-1-phosphate (S1P) is a ceramide derivative serving not only as a regulator of immune properties but also as a modulator of brain functions. To better understand the mechanism underlying the effects of S1P on brain functions, we investigated the potential impact of S1P receptor (S1PR) activation on NMDA receptor subunits. We used acute rat hippocampal slices as a model system, and determined the effects of the active phosphorylated S1P analog, fingolimod (FTY720P) on various NMDA receptors. Treatment with FTY720P significantly increased phosphorylation of GluN2B-containing NMDA receptors at Tyr1472. This effect appears rather specific, as treatment with FTY720P did not modify GluN2B-Tyr1336, GluN2B-Ser1480, GluN2A-Tyr1325 or GluN1-Ser897 phosphorylation. Pre-treatment of hippocampal slices with the compounds W146 and PP1 indicated that FTY720P-induced GluN2B phosphorylation at Tyr1472 epitopes was dependent on activation of S1PR subunit 1 (S1PR1) and Src/Fyn kinase, respectively. Cell surface biotinylation experiments indicated that FTY720P-induced GluN2B phosphorylation at Tyr1472 was also associated with increased levels of GluN1 and GluN2B subunits on membrane surface, whereas no change was observed for GluN2A subunits. We finally demonstrate that FTY720P is inclined to favor Tau and Fyn accumulation on plasma membranes. These results suggest that activation of S1PR1 by FTY720P enhances GluN2B receptor phosphorylation in rat hippocampal slices, resulting in increased levels of GluN1 and GluN2B receptor subunits in neuronal membranes through a mechanism probably involving Fyn and Tau.

Astrocytes increase the activity of synaptic GluN2B NMDA receptors

Astrocytes regulate excitatory synapse formation and surface expression of glutamate AMPA receptors (AMPARs) during development. Less is known about glial modulation of glutamate NMDA receptors (NMDARs), which mediate synaptic plasticity and regulate neuronal survival in a subunit- and subcellular localization-dependent manner. Using primary hippocampal cultures with mature synapses, we found that the density of NMDA-evoked whole-cell currents was approximately twice as large in neurons cultured in the presence of glia compared to neurons cultured alone. The glial effect was mediated by (an) astrocyte-secreted soluble factor(s), was Mg(2+) and voltage independent, and could not be explained by a significant change in the synaptic density. Instead, we found that the peak amplitudes of total and NMDAR miniature excitatory postsynaptic currents (mEPSCs), but not AMPAR mEPSCs, were significantly larger in mixed than neuronal cultures, resulting in a decreased synaptic AMPAR/NMDAR ratio. Astrocytic modulation was restricted to synaptic NMDARs that contain the GluN2B subunit, did not involve an increase in the cell surface expression of NMDAR subunits, and was mediated by protein kinase C (PKC). Taken together, our findings indicate that astrocyte-secreted soluble factor(s) can fine-tune synaptic NMDAR activity through the PKC-mediated regulation of GluN2B NMDAR channels already localized at postsynaptic sites, presumably on a rapid time scale. Given that physiologic activation of synaptic NMDARs is neuroprotective and that an increase in the synaptic GluN2B current is associated with improved learning and memory, the astrocyte-induced potentiation of synaptic GluN2B receptor activity is likely to enhance cognitive function while simultaneously strengthening neuroprotective signaling pathways.

NMDA-type glutamate receptors participate in reduction of food intake following hindbrain melanocortin receptor activation

Hindbrain injection of a melanocortin-3/4 receptor agonist, MTII, reduces food intake primarily by reducing meal size. Our previously reported results indicate that N-methyl-D-aspartate-type glutamate receptors (NMDAR) in the nucleus of the solitary tract (NTS) play an important role in the control of meal size and food intake. Therefore, we hypothesized that activation of NTS NMDARs contribute to reduction of food intake in response to fourth ventricle or NTS injection of MTII. We found that coinjection of a competitive NMDAR antagonist (d-CPP-ene) with MTII into the fourth ventricle or directly into the NTS of adult male rats attenuated MTII-induced reduction of food intake. Hindbrain NMDAR antagonism also attenuated MTII-induced ERK1/2 phosphorylation in NTS neurons and prevented synapsin I phosphorylation in central vagal afferent endings, both of which are cellular mechanisms previously shown to participate in hindbrain melanocortinergic reduction of food intake. Together, our results indicate that NMDAR activation significantly contributes to reduction of food intake following hindbrain melanocortin receptor activation, and it participates in melanocortinergic signaling in NTS neural circuits that mediate reduction of food intake.