Recombinant AMPA receptors with low Ca2+ permeability increase intracellular Ca2+ in HEK 293 cells

Case Report: A Novel De Novo Missense Mutation of the GRIA2 Gene in a Chinese Case of Neurodevelopmental Disorder With Language Impairment

Introduction: Neurodevelopmental disorders with language impairment and behavioral abnormalities (NEDLIB) are a disease caused by heterozygous variants in the glutamate ionotropic receptor AMPA type subunit 2 (GRIA2) gene, which manifest as impaired mental development or developmental delay, behavioral abnormalities including autistic characteristics, and language disorders. Currently, only a few mutations in the GRIA2 gene have been discovered.

Methods: A GRIA2 variation was detected in a patient by whole-exome sequencing, and the site was validated by Sanger sequencing from the family.

Results: We report a Chinese case of NEDLIB in a girl with language impairment and developmental delay through whole-exome sequencing (WES). Genetic analysis showed that there was a de novo missense mutation, c.1934T > G (p.Leu645Arg), in the GRIA2 gene (NM_001083619.1), which has never been reported before.

Conclusion: Our case shows the potential diagnostic role of WES in NEDLIB, expands the GRIA2 gene mutation spectrum, and further deepens the understanding of NEDLIB. Deepening the study of the genetic and clinical heterogeneity, treatment, and prognosis of the disease is still our future challenge and focus.

Vagal afferent stimulation activates astrocytes in the nucleus of the solitary tract via AMPA receptors: evidence of an atypical neural-glial interaction in the brainstem

The nucleus of the solitary tract (NST), located in the dorsomedial medulla, is the site of visceral sensory modulation of a variety of homeostatic reflexes. Given recent advancements in the understanding of active regulation of synaptic information flow by astrocytes, we sought to determine whether afferent sensory inputs to NST neurons also activates NST astrocytes. Using confocal, live-cell calcium imaging of brainstem slices, we investigated the possibility that stimulation of vagal sensory afferents, the major sensory input into the NST, activated NST astrocytes, as indicated by increases in astrocytic intracellular calcium concentrations ([Ca²⁺](i)). Astrocytes and neurons were preloaded with the calcium reporter dye Calcium Green, and astrocytes were selectively stained by sulforhodamine 101. Electrical stimulation of vagal afferent axons produced rapid increases in [Ca²⁺](i) in NST astrocytes as well as neurons. Surprisingly, this effect on astrocytes was blocked by the AMPA receptor antagonist NBQX and was unaffected by antagonism of NMDA and metabotropic glutamate receptors. Bath application of AMPA also activated astrocytes. This activation was dependent on extracellular Ca²⁺ influx through both typical AMPA receptors and calcium-permeable AMPA receptors. This AMPA-mediated Ca²⁺ influx was further amplified by actions of the ryanodine receptor by way of calcium-induced calcium release. Our immunohistochemical staining of NST cells further verified the presence of the AMPAR subunit GluR1 on astrocytes. These observations suggest that NST astrocytes may be active participants in the regulation of autonomic reflexes even in the normal, healthy state.

Excitotoxicity through Ca2+-permeable AMPA receptors requires Ca2+-dependent JNK activation

The GluA4-containing Ca(2+)-permeable α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors (Ca-AMPARs) were previously shown to mediate excitotoxicity through mechanisms involving the activator protein-1 (AP-1), a c-Jun N-terminal kinase (JNK) substrate. To further investigate JNK involvement in excitotoxic pathways coupled to Ca-AMPARs we used HEK293 cells expressing GluA4-containing Ca-AMPARs (HEK-GluA4). Cell death induced by overstimulation of Ca-AMPARs was mediated, at least in part, by JNK. Importantly, JNK activation downstream of these receptors was dependent on the extracellular Ca(2+) concentration. In our quest for a molecular link between Ca-AMPARs and the JNK pathway we found that the JNK interacting protein-1 (JIP-1) interacts with the GluA4 subunit of AMPARs through the N-terminal domain. In vivo, the excitotoxin kainate promoted the association between GluA4 and JIP-1 in the rat hippocampus. Taken together, our results show that the JNK pathway is activated by Ca-AMPARs upon excitotoxic stimulation and suggest that JIP-1 may contribute to the propagation of the excitotoxic signal.

Stimulation of lateral hypothalamic AMPA receptors may induce feeding in rats

Glutamate or its ionotropic receptor (iGluR) agonists, N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxale propionate (AMPA), and kainate (KA) elicit feeding when microinjected into the lateral hypothalamus (LH) of satiated rats. In the present study we investigated the contributions of AMPA and KA receptors (AMPARs and KARs) to feeding initiation. Intense feeding was elicited by LH injection of RS-AMPA (1 and 10 nmol) but not by the isolated, inactive R-AMPA enantiomer (1 and 10 nmol). Further, LH pretreatment with either the non-selective AMPAR/KAR antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 4 nmol) or the selective AMPAR antagonist, GYKI 52466 (10 nmol), suppressed AMPA-elicited food intake and, when combined, blocked AMPA-elicited food intake. These findings suggest that LH AMPARs mediate AMPA injection-elicited feeding with a possible contribution by KARs. In contrast, CNQX or GYKI 52466 injected into the LH at the onset of the nocturnal period or into fasted rats did not suppress the feeding produced by either condition. RS-AMPA injected into the LH of fasted or nocturnal feeding subjects elicited eating in both conditions; however, the magnitude of the increase was greater in fasted rats. These data suggest that selective stimulation of AMPAR in the LH is sufficient to elicit feeding. In contrast, the results did not provide evidence that AMPAR stimulation is necessary for deprivation-induced or nocturnal eating; however, they did suggest that modulatory interactions may exist between these receptors and these forms of naturally occurring eating behavior.

Excitotoxicity mediated by Ca2+-permeable GluR4-containing AMPA receptors involves the AP-1 transcription factor

Cells preferentially expressing GluR4-containing alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors are particularly sensitive to excitotoxicity mediated through non-N-methyl-D-aspartate receptors. However, the excitotoxic signalling pathways associated with GluR4-containing AMPA receptors are not known. In this work, we investigated the downstream signals coupled to excitotoxicity mediated by Ca2+-permeable GluR4-containing AMPA receptors, using a HEK 293 cell line constitutively expressing the GluR4flip subunit of AMPA receptors (HEK-GluR4). Glutamate stimulation of GluR4-containing AMPA receptors decreased cell viability, in a calcium-dependent manner, when the receptor desensitisation was prevented with cyclothiazide. The excitotoxic stimulation mediated through GluR4-containing AMPA receptors increased activator protein-1 (AP-1) DNA-binding activity. Inhibition of the AP-1 activity by overexpression of a c-Jun dominant-negative form protected HEK-GluR4 cells against excitotoxic damage. Taken together, the results indicate that overactivation of Ca2+-permeable GluR4-containing AMPA receptors is coupled to a death pathway mediated, at least in part, by the AP-1 transcription factor.

The molecular pharmacology and cell biology of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) are of fundamental importance in the brain. They are responsible for the majority of fast excitatory synaptic transmission, and their overactivation is potently excitotoxic. Recent findings have implicated AMPARs in synapse formation and stabilization, and regulation of functional AMPARs is the principal mechanism underlying synaptic plasticity. Changes in AMPAR activity have been described in the pathology of numerous diseases, such as Alzheimer's disease, stroke, and epilepsy. Unsurprisingly, the developmental and activity-dependent changes in the functional synaptic expression of these receptors are under tight cellular regulation. The molecular and cellular mechanisms that control the postsynaptic insertion, arrangement, and lifetime of surface-expressed AMPARs are the subject of intense and widespread investigation. For example, there has been an explosion of information about proteins that interact with AMPAR subunits, and these interactors are beginning to provide real insight into the molecular and cellular mechanisms underlying the cell biology of AMPARs. As a result, there has been considerable progress in this field, and the aim of this review is to provide an account of the current state of knowledge.

Dual roles in feeding for AMPA/kainate receptors: receptor activation or inactivation within distinct hypothalamic regions elicits feeding behavior

We have previously shown that hypothalamic injections of glutamate, or agonists of its ionotropic receptors (iGluRs), elicit intense feeding responses in satiated rats [Brain Res. 613 (1993) 88, Brain Res. 630 (1993) 41]. While attempting to clarify the role of the AMPA and kainate (KA) receptor subtypes in glutamatergic feeding systems, we discovered that lateral hypothalamic (LH) injection of high doses of the competitive AMPA/KA receptor antagonist, NBQX (10 and 30 nmol), elicited a pronounced feeding response. We questioned whether this effect was due to inactivation of AMPA or possibly KA receptors. To determine whether other AMPA/KA antagonists can also elicit feeding, we tested whether injection of CNQX, another AMPA/KA receptor antagonist, also stimulates eating and whether these feeding stimulatory effects were due to antagonists' actions in the LH or in other hypothalamic sites. Here we report that NBQX and CNQX elicit feeding in a dose dependent manner and are most effective when injected into the perifornical hypothalamus (PFH), or into the paraventricular nucleus (PVN) and, to a lesser extent, into the LH of satiated rats. In contrast, AMPA was most effective in stimulating feeding when injected into the LH, confirming previous reports. These data suggest that either activation or inactivation of AMPA/KA receptors in distinct but overlapping hypothalamic sites may be sufficient to induce feeding behavior, indicating a broadened role for glutamate in hypothalamic feeding mechanisms.

Role of GluR2 expression in AMPA-induced toxicity in cultured murine cerebral cortical neurons

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA-R)-mediated neurotoxicity was studied in relation to subunit expression and the presence of Ca(2+)-permeable receptor channels. AMPA-mediated toxicity had two components: 1) a direct AMPA-R-mediated component, which was not due to Ca(2+) influx through voltage-gated Ca(2+) channels, reversal of the Na(+)/Ca(2+) exchanger or release of calcium from dantrolene-sensitive intracellular Ca(2+) stores, and 2) a minor, indirect component involving activation of NMDA receptor channels, because of glutamate release and removal of the Mg(2+) block of the NMDA receptor on AMPA-R stimulation. The involvement of Ca(2+) influx through AMPA-R was also examined. The number of neurons possessing Ca(2+)-permeable AMPA-R increased during culture development, concurrently with an increasing susceptibility for AMPA-induced toxicity during development. GluR2(R) levels also increased during development, and channel blockers of Ca(2+)-permeable AMPA-R lacking the GluR2(R) subunit (spermine and philanthotoxin) failed to prevent neurotoxicity or increases in [Ca(2+)](i). Thus, the direct AMPA-R-mediated toxicity may be explained by initiation of cell death by Ca(2+) fluxing through AMPA-R containing GluR2(R). The components of direct AMPA-R-mediated toxicity are proposed to be 1) toxicity mediated by GluR2(R)-lacking AMPA-R and 2) toxicity mediated by low-Ca(2+)-permeability AMPA-R containing GluR2(R).

Pharmacology of AMPA/kainate receptor ligands and their therapeutic potential in neurological and psychiatric disorders

It has been postulated, consistent with the ubiquitous presence of glutamatergic neurons in the brain, that defects in glutamatergic neurotransmission are associated with many human neurological and psychiatric disorders. This review evaluates the possible application of ligands acting on glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate (KA) receptors to minimise the pathology and/or symptoms of various diseases. Glutamate activation of AMPA receptors is thought to mediate most fast synaptic neurotransmission in the brain, while transmission via KA receptors contributes only a minor component. Variants of the protein subunits forming these receptors greatly extend the pharmacological and electrophysiological properties of AMPA/KA receptors. Disease and drug use can differentially affect the expression of the subunits and their variants. Ligands bind to AMPA receptors by competing with glutamate at the glutamate binding site, or non-competitively at other sites on the proteins (allosteric modulators). Ligands showing selective competitive antagonist actions at the AMPA/ KA class of glutamate receptors were first reported in 1988, and the systemically active antagonist 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX) was first shown to have useful therapeutic effects on animal models of neurological diseases in 1990. Since then, newer antagonists with increased potency, higher specificity, increased water solubility, and a longer duration of action in vivo have been developed. Negative allosteric modulators such as the prototype GYKI-52466 also block AMPA receptors but have little action at KA receptors. Positive allosteric modulators enhance glutamatergic neurotransmission at AMPA receptors. Polyamines and adamantane derivatives bind within the ion channel of calcium-permeable AMPA receptors. The latest developments include ligands selective for KA receptors containing Glu-R5 subunits. Evidence for advantages of AMPA receptor antagonists over N-methyl-D-aspartate (NMDA) receptor antagonists for symptomatic treatment of neurological and psychiatric conditions, and for minimising neuronal loss occurring after acute neurological diseases, such as physical trauma, ischaemia or status epilepticus, have been shown in animal models. However, as yet AMPA receptor antagonists have not been shown to be effective in clinical trials. On the other hand, a limited number of clinical trials have been reported for AMPA receptor ligands that enhance glutamatergic neurotransmission by extending the ion channel opening time (positive allosteric modulators). These acute studies demonstrate enhanced memory capability in both young and aged humans, without any apparent serious adverse effects. The use of these allosteric modulators as antipsychotic drugs is also possible. However, the long term use of both direct agonists and positive allosteric modulators must be approached with considerable caution because of potential adverse effects.

Dissociation between the Joro spider toxin sensitivity of recombinant alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and their ability to increase intracellular calcium

We compared the toxin sensitivity, Ca2+ flux response and rectification properties of recombinant alpha-amino-3-hydroxy-5-methyl-4-isoazolepropionic acid (AMPA) receptors obtained by transfecting human embryonic kidney (HEK) 293 cells with different ratios of GluR1 and GluR2 cDNAs (10:1 to 1:10). Simultaneous measurements of kainate-activated Ca2+ fluxes and inward currents, using fura-2 microfluorimetry under voltage clamp conditions, suggested the existence of GluR2 containing channels which are permeable to Ca2+ and insensitive to Joro spider toxin (JSTx). Imaging experiments showed that JSTx inhibition of the Ca2+ response induced by kainate was reduced by increasing the relative amount of GluR2. However, even at GluR1/GluR2(R) ratios of 1:1 and 1:4, cells were still able to flux Ca2+ when stimulated by kainate. GluR2 similarly inhibited the ability of JSTx to reduce kainate-evoked inward currents in whole cell patch-clamp experiments. Variations in the rectification properties of the AMPA currents, induced by changes in the cDNA ratio, were not always correlated with the changes in toxin sensitivity and [Ca2+]i response. Thus, cells with almost linear I-V relationships were partially blocked by JSTx and still Ca2+ permeable. Our results indicate a dissociation between the toxin sensitivity and Ca2+ flux through GluR2 containing AMPA receptors and suggest that receptors with diverse Ca2+ permeabilities are generated by the expression of variable amounts of GluR2.