Ca2+ -permeable AMPA receptors and their auxiliary subunits in synaptic plasticity and disease

Abstract

AMPA receptors are tetrameric glutamate-gated ion channels that mediate a majority of fast excitatory neurotransmission in the brain. They exist as calcium-impermeable (CI-) and calcium-permeable (CP-) subtypes, the latter of which lacks the GluA2 subunit. CP-AMPARs display an array of distinctive biophysical and pharmacological properties that allow them to be functionally identified. This has revealed that they play crucial roles in diverse forms of central synaptic plasticity. Here we summarise the functional hallmarks of CP-AMPARs and describe how these are modified by the presence of auxiliary subunits that have emerged as pivotal regulators of AMPARs. A lasting change in the prevalence of GluA2-containing AMPARs, and hence in the fraction of CP-AMPARs, is a feature in many maladaptive forms of synaptic plasticity and neurological disorders. These include modifications of glutamatergic transmission induced by inflammatory pain, fear conditioning, cocaine exposure, and anoxia-induced damage in neurons and glia. Furthermore, defective RNA editing of GluA2 can cause altered expression of CP-AMPARs and is implicated in motor neuron damage (amyotrophic lateral sclerosis) and the proliferation of cells in malignant gliomas. A number of the players involved in CP-AMPAR regulation have been identified, providing useful insight into interventions that may prevent the aberrant CP-AMPAR expression. Furthermore, recent molecular and pharmacological developments, particularly the discovery of TARP subtype-selective drugs, offer the exciting potential to modify some of the harmful effects of increased CP-AMPAR prevalence in a brain region-specific manner.

Keywords: AMPA receptors; CKAMP44; GSG1L; GluA2; TARPs; amyotrophic lateral sclerosis; anoxia; auxiliary subunits; calcium-permeable AMPA receptors; cocaine; cornichon; fear conditioning; ionotropic glutamate receptors; malignant glioma; neurological disorder; pain; stargazin; synaptic plasticity; synaptic transmission.

Figure 1. Architecture of AMPARs and key auxiliary subunits

A, structure of a native heterotetrameric GluA2/3 receptor (PDB 6NJM; Zhao et al. 2019) with auxiliary subunits removed, showing the three‐layer arrangement formed from the amino terminal‐, ligand binding‐ and transmembrane domains (ATD, LBD, TMD), with the classical overall ‘Y’‐shape. The GluA2 subunits (positions B and D) are shown in red and the GluA3 subunits (positions A and C) are shown in blue. B, positions of the subunits within the ATD, LBD and TMD layers when viewed from the top (extracellular surface) of the receptor, along the overall twofold axis of symmetry. Of note, the arrangement of core subunits in AMPARs is not as strict as seen in NMDARs (Greger et al. 2017), and for this native AMPAR the positioning of the GluA2 subunits differs from the A/C positions reported for the first recombinant heteromeric GluA2/3 structure (Herguedas et al. 2016). Nevertheless, the fourfold symmetry of the TMD layer is common to both. C, cartoon representation of the TMD layer arrangement for a Ca²⁺‐impermeable (CI‐) AMPAR containing Q/R edited GluA2 subunits and a GluA2‐lacking Ca²⁺‐permeable (CP‐) AMPAR. D, schematic illustrations of AMPAR key auxiliary subunits. TARPs and GSG1L belong to the claudin superfamily and have four transmembrane α‐helices (numbered) and similar overall structures. Type Ia (γ2, 3) and Type 1b (γ4, 8) TARPs have canonical TTPV PDZ binding motifs whereas Type II TARPs (γ5, 7) have atypical PDZ binding motifs (SSPC and TSPC). Note that because the transmembrane helices form a bundle within the membrane the TM2/TM3 linker (dotted) is shorter than shown. CNIHs also have four transmembrane α‐helices but both the N and C termini are extracellular (Nakagawa, 2019). CKAMPs have a single transmembrane α‐helix, an extracellular cysteine‐rich region (the cysteine knot) and a PDZ binding motif (EVTV).

Figure 2. Functional hallmarks of CP‐AMPARs lacking edited GluA2

Selected recordings of native (A) and recombinant (B–E) CP‐AMPARs. A, left, I‐V relationships of whole‐cell responses to bath‐applied AMPA (20 μm) recorded from untreated cerebellar granule cells (control) and from cells transfected with short interfering RNAs to disrupt GluA2 production (ΔGluA2). Knockdown of GluA2 promotes spermine‐dependent inward rectification. Right, representative responses from granule cell outside‐out patches to application of AMPA (1 mm, 100 ms, −60 mV). Long‐lived bursts of channel openings are present in the tail of the currents from GluA2 knockdown cells while in control patches only a few smaller and briefer openings are discernible (modified from Studniarczyk et al. 2013). B, left, representative glutamate‐evoked (100 ms, 10 mm) currents at +60 and −60 mV for homomeric GluA4 AMPARs in the absence or presence of γ2. Right, inwardly rectifying I‐V relationships for peak currents, showing reduced rectification in the presence of γ2 (modified from Soto et al. 2007). C, representative glutamate‐evoked currents (10 mm, 100 ms with 100 μm spermine) and normalized peak I‐V relationships showing increased rectification in the presence of GSG1L (modified from McGee et al. 2015). D, top, resolved single‐channel openings at −80 mV in the tail of macroscopic currents (truncated), recorded from homomeric GluA1 AMPARs expressed in the absence and presence of γ2, illustrating the increased single‐channel conductance, increased open probability and slowed kinetics in the presence of TARP (modified from Coombs & Cull‐Candy, 2009). Bottom, representative single‐channel currents recorded in outside‐out patches from tsA201 cells with GluA1 expressed alone or with CNIH3 (−80 mV; 10 mm glutamate) (modified from Coombs et al. 2012). E, resolved single‐channel openings at −80 mV in the tail of macroscopic currents (truncated) from homomeric unedited GluA2(Q) AMPARs, with all‐point amplitude histograms from individual channel events (modified from McGee et al. 2015).

Figure 3. An example of CP‐AMPAR plasticity

Application of the group 1 mGluR agonist (S)‐3,5‐dihydroxyphenylglycine (DHPG) induces a persistent synaptic depression and change in the rectification of EPSCs recorded from stellate cells in acute slices of mouse cerebellum. Left, averaged parallel fibre‐evoked EPSCs recorded at −60 and +40 mV before and after the application of 50 μm DHPG (10 min). Dashed lines indicate baseline and peak currents for control EPSCs. Right, corresponding I‐V relationships. Control EPSCs show inward rectification, with a rectification index (RI) of 0.39, for this example. Following DHPG application the I‐V relationship became linear (RI of 1.1), indicating a shift from CP‐ to CI‐AMPARs. A similar shift can be induced by synaptic activation of mGluRs. (modified from Kelly et al. 2009).