The AMPAR subunit GluR2: still front and center-stage¹

Abstract

Abnormal influx of Ca²⁺ through AMPA-type glutamate receptors (AMPARs) is thought to contribute to the neuronal death associated with a number of brain disorders. AMPARs exist as both Ca²⁺-impermeable and Ca²⁺-permeable channels. AMPARs are encoded by four genes designated GluR1 (GluR-A) through GluR4 (GluR-D). The presence of the GluR2 subunit renders heteromeric AMPA receptor assemblies Ca²⁺-impermeable. Molecular diversity of AMPARs under physiological and pathological conditions is generated by differential spatio-temporal patterns of GluR expression, by alternative RNA splicing and editing and by targeting and trafficking of receptor subunits at dendritic spines. The GluR2 gene is under transcriptional control by the RE1 element specific transcription factor, a gene silencing factor which renders it neuron-specific. GluR2 transcripts are edited by ADAR2 (double-stranded RNA-specific editase 1). AMPAR targeting and trafficking to spines are regulated by synaptic activity and are critical to synaptic plasticity. Recent studies involving animal models of transient forebrain ischemia and epilepsy show that GluR2 mRNA and GluR2 subunit expression are downregulated in vulnerable neurons prior to cell death. Ca²⁺ imaging and electrical recording from individual pyramidal neurons in hippocampal slices reveal changes in AMPAR functional properties after ischemia. In slices from post-ischemia animals, CA1 neurons with robust action potentials exhibit greatly enhanced AMPA-elicited rises in intracellular Ca²⁺. Excitatory postsynaptic currents in post-ischemic CA1 exhibit an enhanced Ca²⁺-dependent component that appears to be mediated by Ca²⁺-permeable AMPARs. These studies provide evidence for Ca²⁺ influx through AMPARs in neurons destined to die. To examine whether acute GluR2 downregulation, even in the absence of a neurological insult, can induce neuronal death, we performed knockdown experiments in rats and gerbils with antisense oligonucleotides targeted to GluR2 mRNA. GluR2 antisense oligonucleotide induced neuronal cell death of pyramidal neurons and enhanced pathogenicity of brief ischemic episodes. These observations provide evidence for Ca²⁺ influx through AMPARs in neurons destined to die and implicate Ca²⁺-permeable AMPARs in the pathogenesis of ischemia-induced neuronal death.

Introduction

Glutamate receptors mediate excitatory neurotransmission and play a critical role in synaptogenesis and formation of neuronal circuitry, as well as in synaptic plasticity including long-term potentiation and long-term depression. Excessive activation of glutamate receptors is thought to contribute to the neurodegeneration following a wide range of neurological insults including ischemia, trauma, hypoglycemia and epileptic seizures. Chronic neurodegenerative disorders such as Alzheimer’s disease, Huntington’s chorea, AIDS encephalopathy, and amyotrophic lateral sclerosis may also involve glutamate-induced neuronal cell death (for reviews, see [28], [73]).

Considerable interest has focused on the molecular mechanisms underlying glutamate receptor-mediated neuronal death. Glutamate induces neuronal death by eliciting a rise in intracellular free Ca²⁺, which activates a number of proteases, phospholipases and endoncleases, by generation of free radicals that destroy cellular membranes by lipid peroxidations (for reviews, see [27], [29], [122], [135]) and by induction of apoptosis [29]. Possible mechanisms by which glutamate could elicit a rise in intracellular Ca²⁺ include: (1) activation of Ca²⁺-permeable AMPA (α-amino-3-hydroxy-5-methyl-4-isoazole-proprionic acid)-type glutamate receptors (AMPARs) [146], [148]; (2) activation of group 1 metabotropic glutamate receptors (mGluRs), which are positively linked to inositol phosphates; (3) activation of voltage-sensitive Ca²⁺ channels; and/or (4) de-activation of extrusion and/or sequestration systems [99].

Until recently, AMPARs were thought to be Ca²⁺-impermeable. It is now well established that the presence of the GluR2 subunit in heteromeric AMPAR assemblies governs the permeability of AMPARs to Ca²⁺ and Zn²⁺. In the adult mammalian central nervous system under physiological conditions, the vast majority of cells and tissues express GluR2-containing, Ca²⁺-impermeable AMPARs. Thus, a change in the level of GluR2 expression would be expected to have significant physiological consequences. The relative expression of GluR2 subunit mRNA and protein in neurons is not static but is regulated in a cell-specific manner during development [108] and may be remodeled after seizures [38], [113], [114] or ischemic insult [38], [44], [111], [114] and by administration of anti-psychotics [35] drugs of abuse [36], [102] or corticosteroids [93]. Ca²⁺-permeable AMPARs are implicated in synaptogenesis and formation of neuronal circuitry, particularly at times and in cells in which NMDAR expression is low. Ca²⁺ influx via Ca²⁺-permeable AMPARs is thought to play a critical role in growth cone movement and experience-dependent pruning of synaptic connections during early development [81].

This article reviews recent studies that address transcriptional and translation regulation and targeting and trafficking of the AMPAR subunit GluR2 under physiological and pathological conditions, with a particular emphasis on transcriptional regulation of GluR2 in ischemia and status epilepticus.