Ischemic Insults Direct Glutamate Receptor Subunit 2-Lacking AMPA Receptors to Synaptic Sites

Abstract

Regulated AMPA receptor (AMPAR) trafficking at excitatory synapses is a mechanism critical to activity-dependent alterations in synaptic efficacy. The role of regulated AMPAR trafficking in insult-induced synaptic remodeling and/or cell death is, however, as yet unclear. Here we show that brief oxygen-glucose deprivation (OGD), an in vitro model of brain ischemia, promotes redistribution of AMPARs at synapses of hippocampal neurons, leading to a switch in AMPAR subunit composition. Ischemic insults promote internalization of glutamate receptor subunit 2 (GluR2)-containing AMPARs from synaptic sites via clathrin-dependent endocytosis and facilitate delivery of GluR2-lacking AMPARs to synaptic sites via soluble N-ethylmaleimide-sensitive factor attachment protein receptordependent exocytosis, evident at early times after insult. The OGD-induced switch in receptor subunit composition requires PKC activation, dissociation of GluR2 from AMPA receptor-binding protein, and association with protein interacting with C kinase-1. We further show that AMPARs at synapses of insulted neurons exhibit functional properties of GluR2-lacking AMPARs. AMPAR-mediated miniature EPSCs exhibit increased amplitudes and enhanced sensitivity to subunit-specific blockers of GluR2-lacking AMPARs, evident at 24 h after ischemia. The OGD-induced alterations in synaptic AMPA currents require clathrin-mediated receptor endocytosis and PKC activation. Thus, ischemic insults promote targeting of GluR2-lacking AMPARs to synapses of hippocampal neurons, mechanisms that may be relevant to ischemia-induced synaptic remodeling and/or neuronal death.

PICK1-mediated GluR2 endocytosis contributes to cellular injury after neuronal trauma

“…We last investigated whether a traumatically injured hippocampus showed an increased expression of calciumpermeable AMPARs, as these receptors contribute to progressive excitotoxic cell death and dysfunction. [6][7][8][9][10]21,22 We found during recordings from FPI rats that CA1 population spikes exhibited a naspm-induced rundown to 58.9 ± 1.7% of baseline, a significantly greater inhibition than sham animals (78.9 ± 0.79%, Po0.05, Figure 7g). However, injecting animals intravenously with TAT-QSAV (3 mg/kg) after the traumatic injury occluded naspm-induced rundown of CA1 population spike amplitude (88.2 ± 5.6%, Figure 7h).…”

“…First, ischemic incorporation of GluR2-lacking AMPARs and association of GluR2 with PICK1 was reported in cultured hippocampal neurons. 6 In that study, internalization of GluR2 was associated with a similar polyamine-sensitive increase in mEPSC amplitude. Subsequently, our lab reported PKC-dependent GluR2 internalization after mild stress injury in cerebellar Purkinje cells.…”

“…homomers of GluR1) exhibit two-to three-fold increases in single-channel conductance, increased calcium and zinc permeability and have inwardly rectifying current-voltage relationships. 5 These attributes increase synaptic strength, but are also known to exacerbate neuronal hyperactivity, acute cell swelling and delayed Ca 2 þ -mediated toxicity in CNS pathologies including ischemia, 6,7 epilepsy 8 and trauma. 9,10 Investigation into the intracellular regulation of the AMPAR GluR2 content under pathological conditions is therefore warranted, and may help in the design of targeted therapeutics aimed at reducing delayed Ca 2 þ overload or neuronal hyperexcitability after an initial trauma.…”

Role of the ubiquitin–proteasome system in brain ischemia: Friend or foe?

“…Exposure of hippocampal neurons to a brief period of OGD promotes a redistribution of AMPAR at the synapse, with internalization of synaptic GluA2-containing AMPAR and synaptic delivery of AMPAR lacking GluA2 subunits, which are permeable to calcium and zinc (Liu et al, 2006). In fact, activation of AMPAR was also shown to induce calpain activation (Araujo et al, 2004), and AMPAR antagonists provided neuroprotection in a model of in vivo ischemia (Sheardown et al, 1990;Noh et al, 2005).…”

Regulation of AMPA receptors and synaptic plasticity

“…Deregulation of AMPAR activity is also involved in pathology [e.g. (Kwak and Weiss, 2006;Liu et al, 2006)]. This review will concentrate on the molecular mechanisms of regulation of AMPARs, and their implications in synaptic plasticity.…”