Metabotropic Glutamate Receptors mGlu1 Research Articles

GRID1/GluD1 homozygous variants linked to intellectual disability and spastic paraplegia impair mGlu1/5 receptor signaling and excitatory synapses

The ionotropic glutamate delta receptor GluD1, encoded by the GRID1 gene, is involved in synapse formation, function, and plasticity. GluD1 does not bind glutamate, but instead cerebellin and D-serine, which allow the formation of trans-synaptic bridges, and trigger transmembrane signaling. Despite wide expression in the nervous system, pathogenic GRID1 variants have not been characterized in humans so far. We report homozygous missense GRID1 variants in five individuals from two unrelated consanguineous families presenting with intellectual disability and spastic paraplegia, without (p.Thr752Met) or with (p.Arg161His) diagnosis of glaucoma, a threefold phenotypic association whose genetic bases had not been elucidated previously. Molecular modeling and electrophysiological recordings indicated that Arg161His and Thr752Met mutations alter the hinge between GluD1 cerebellin and D-serine binding domains and the function of this latter domain, respectively. Expression, trafficking, physical interaction with metabotropic glutamate receptor mGlu1, and cerebellin binding of GluD1 mutants were not conspicuously altered. Conversely, upon expression in neurons of dissociated or organotypic slice cultures, we found that both GluD1 mutants hampered metabotropic glutamate receptor mGlu1/5 signaling via Ca2+ and the ERK pathway and impaired dendrite morphology and excitatory synapse density. These results show that the clinical phenotypes are distinct entities segregating in the families as an autosomal recessive trait, and caused by pathophysiological effects of GluD1 mutants involving metabotropic glutamate receptor signaling and neuronal connectivity. Our findings unravel the importance of GluD1 receptor signaling in sensory, cognitive and motor functions of the human nervous system.

Less means more: The magnitude of synaptic plasticity along the hippocampal dorso-ventral axis is inversely related to the expression levels of plasticity-related neurotransmitter receptors.

The dorsoventral axis of the hippocampus exhibits functional differentiations with regard to (spatial Vs emotional) learning and information retention (rapid encoding Vs long‐term storage), as well as its sensitivity to neuromodulation and information received from extrahippocampal structures. The mechanisms that underlie these differentiations remain unclear. Here, we explored neurotransmitter receptor expression along the dorsoventral hippocampal axis and compared hippocampal synaptic plasticity in the CA1 region of the dorsal (DH), intermediate (IH) and ventral hippocampi (VH). We observed a very distinct gradient of expression of the N‐methyl‐D‐aspartate receptor GluN2B subunit in the Stratum radiatum (DH< IH< VH). A similar distribution gradient (DH< IH< VH) was evident in the hippocampus for GluN1, the metabotropic glutamate receptors mGlu1 and mGlu2/3, GABAB and the dopamine‐D1 receptor. GABAA exhibited the opposite expression relationship (DH > IH > VH). Neurotransmitter release probability was lowest in DH. Surprisingly, identical afferent stimulation conditions resulted in hippocampal synaptic plasticity that was the most robust in the DH, compared with IH and VH. These data suggest that differences in hippocampal information processing and synaptic plasticity along the dorsoventral axis may relate to specific differences in the expression of plasticity‐related neurotransmitter receptors. This gradient may support the fine‐tuning and specificity of hippocampal synaptic encoding.

Nuclear localization of functional metabotropic glutamate receptor mGlu1 in HEK293 cells and cortical neurons: role in nuclear calcium mobilization and development

The Group I metabotropic glutamate receptor (mGlu1) plays an important role in neuromodulation, development, and synaptic plasticity. Using immunocytochemistry, subcellular fractionation, and western blot analysis, the present study shows that mGlu1a receptors are present on nuclear membranes in stably transfected human embryonic kidney 293 (HEK293) cells as well as being endogenously expressed on rat cortical nuclei. Both glutamate and the group I agonist, quisqualate, directly activate nuclear mGlu1 receptors leading to a characteristic oscillatory pattern of calcium flux in isolated HEK nuclei and a slow rise to plateau in isolated cortical nuclei. In either case calcium responses could be terminated upon application of the mGlu1-selective antagonist, 7-(hydroxyamino)cyclopropa[b]chromen-1a-carboxylate ethyl ester. Responses could also be blocked by ryanodine and inositol 1,4,5-triphosphate receptor inhibitors, demonstrating the involvement of these calcium channels. Agonist activation of intracellular receptors was driven by Na(+)-dependent and -independent processes in nuclei isolated from either HEK or cortical neurons. Finally, mGlu1 nuclear receptors were dramatically up-regulated in the course of post-natal development. Therefore, like the other Group I receptor, mGlu5, mGlu1 can function as an intracellular receptor, suggesting a more encompassing role for nuclear G protein-coupled receptors and downstream signaling elements in the regulation of nuclear events.

Induction of Bcl-2 by functional regulation of G-protein coupled receptors protects from oxidative glutamate toxicity by increasing glutathione

Glutamate treatment depletes hippocampal HT22 cells of glutathione, which renders the cells incapable to reduce reactive oxygen species and ultimately cumulates in cell death by oxidative stress. HT22 cells resistant to glutamate displayed increased phosphorylation of cAMP-response-element binding (CREB) and decreased ERK1/2 suggestive of differences in signal transmission. We investigated the amount of candidate G-protein-coupled receptors involved in this resistance and found an increase in mRNA for receptors activated by the vasoactive intestinal peptide VIP (VPAC2, 12.6-fold) and glutamate like the metabotropic glutamate receptor mGlu1 (5.3-fold). Treating cells with VIP and glutamate led to the same changes in protein phosphorylation observed in resistant cells and induced the proto-oncogene Bcl-2. Bcl-2 overexpression protected by increasing the amount of intracellular glutathione and Bcl-2 knockdown by small interfering RNAs (siRNA) increased glutamate susceptibility of resistant cells. Other receptors upregulated in this paradigm might represent useful targets in the treatment of neurological diseases associated with oxidative stress.

Role of Group I Metabotropic Glutamate Receptors mGlu1 and mGlu5 in Nociceptive Signalling

Distribution, pharmacological and knockdown studies point to a role of group I receptors in nociceptive processing. mGlu1 and mGlu5 receptors are expressed on peripheral sensory afferents, in dorsal root ganglion neurons, dorsal and ventral horn of spinal cord, rostral ventromedial medulla, periaqeductal gray, amygdala, and ventrobasal thalamus. These receptors are functional: sensory neurons in these regions respond to pain stimuli and / or to mGlu1 and mGlu5 agonists. Intrathecal administration of group I agonists evokes spontaneous pain behaviour. Whereas mGlu1 / 5 antagonists do not alter normal sensation, their effect on acute nociception is controversial. Activation of spinal and peripheral mGlu1 or mGlu5 receptors is sufficient to evoke hyperalgesia and allodynia. Blockade of spinal or peripheral mGlu1 or peripheral mGlu5 receptors was antihyperalgesic in animal models of inflammatory pain. Although the role of supraspinal, spinal and peripheral mGlu1 and mGlu5 receptors in the development and maintenance of neuropathic pain states is not fully elucidated, spinal mGlu1 receptor blockade is effective in all pain states tested. In the periaqueductal gray, a group I agonist attenuates nociceptive responses, indicating that mGlu1 and / or mGlu5 receptors in this structure can activate the descending pathway to dampen pain. The data indicate that mGlu1 and mGlu5 antagonists have therapeutic potential for the treatment of chronic inflammatory and neuropathic pain states. More potent and bioavailable mGlu1 versus mGlu5 antagonists are needed to provide proof of concept. Keywords: mglu1, mglu5, metabotropic glutamate, nociception, pain, hyperalgesia

Synaptic activation of the group I metabotropic glutamate receptor mGlu1 on the thalamocortical neurons of the rat dorsal Lateral Geniculate Nucleus in vitro

Intracellular recordings were made from thalamocortical neurons in slices of rat dorsal lateral geniculate nucleus in vitro, where ionotropic glutamate receptors and ionotropic and metabotropic GABA receptors had been blocked. The activation of specific metabotropic glutamate receptors by exogenous agonists and by the electrical stimulation of the corticothalamic pathway was then assessed using selective antagonists. The specific group I agonist ( S)-3,5-dihydoxyphenylglycine and the non-selective agonist (1 S,3 R)-1-aminocyclo-pentane-1,3-dicarboxylic acid both caused a concentration-dependent depolarization of membrane potential. These effects were associated with an increase in the apparent input resistance, and a more robust expression of both the depolarizing sag of the voltage response and the low-threshold Ca 2+ potential and an increase in thalamocortical neuron excitability. However, group I agonists selective for the mGlu5 receptor and agonists selective for group II and III receptors did not have these effects. Consequently, these data suggested that these actions were mediated specifically by the group I mGlu1 receptor. The activation of cortical fibres, with trains of 50 stimuli at 50 Hz, resulted in a two-component depolarizing response. The first part of this synaptic response and the agonist-induced depolarization of membrane potential were depressed by the novel group I receptor antagonists LY367366 and LY367385, which are active at mGlu1 receptors. However, they were not blocked by 6-methyl-2-(phenylethyl)-pyridine, a highly selective mGlu5 receptor antagonist. Thus, the membrane potential depolarization of thalamocortical neurons caused either by exogenous agonists or by the stimulation of cortical fibres resulted from the specific activation of mGlu1 but not mGlu5 receptors. This result is consistent with the location of this receptor type on the distal dendrites of thalamocortical neurons in the dorsal lateral geniculate nucleus of the thalamus.

Contributions of mGlu1 and mGlu5 receptors to interactions with N-methyl-d-aspartate receptor-mediated responses and nociceptive sensory responses of rat thalamic neurons

The nociceptive responses of rat ventrobasal thalamus neurons can be reduced by N-methyl-d-aspartate antagonists and by selective metabotropic glutamate receptor mGlu1 antagonists. The recent development of the mGlu5-selective antagonist 6-methyl-2-(phenylethynyl)-pyridine now allows the direct probing of the possible involvement of mGlu5 receptors in thalamic nociceptive responses. Extracellular recordings were made from single neurons in the ventrobasal thalamus and immediately overlying dorsal thalamic nuclei of adult urethane-anaesthetized rats using multi-barrel electrodes. Responses of neurons to iontophoretic applications of the mGlu5-selective agonist (R,S)-2-chloro-5-hydroxyphenylglycine were selectively reduced during continuous iontophoretic applications of 6-methyl-2-(phenylethynyl)-pyridine. Similar applications of 6-methyl-2-(phenylethynyl)-pyridine reduced neuronal responses to noxious thermal stimuli to 53±9.5% of control responses. Co- application by iontophoresis of N-methyl-d-aspartate and metabotropic glutamate receptor agonists resulted in a mutual potentiation of excitatory responses. This effect could be reduced by either 6-methyl-2-(phenylethynyl)-pyridine or the mGlu1 antagonist LY367385.These results, taken together with previous data, suggest that acute thalamic nociceptive responses are mediated by a combination of mGlu1, mGlu5 and N-methyl-d-aspartate receptor activation, and that co-activation of these receptors produces a synergistic excitatory effect. Thus blockade of any of these receptor types would have a profound effect on the overall nociceptive response.

Reversible and non-competitive antagonist profile of CPCCOEt at the human type 1 α metabotropic glutamate receptor

In transfected CHO cells expressing the human metabotropic glutamate receptor mGlu1 α, 7-(hydroxyimino)cyclopropan-[b]chromen-1a-carboxylic acid ethylester (CPCCOEt) was found to antagonize l-quisqualate-induced phosphoinositide hydrolysis in a non-competitive and reversible manner (apparent p K i value, 4.76±0.18; n=3). This suggests that CPCCOEt antagonizes type 1 α metabotropic glutamate receptor activation by interacting with a site distinct from the agonist binding site.