Research Article

Architecture of the heteromeric GluA1/2 AMPA receptor in complex with the auxiliary subunit TARP γ8

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Science  26 Apr 2019:
Vol. 364, Issue 6438, eaav9011
DOI: 10.1126/science.aav9011

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Regulating signals at synapses

At excitatory synapses in the brain, tetrameric cation channels called AMPA-type glutamate receptors (AMPARs) play a key role in the cellular processes that underlie learning and memory. AMPARs are heterotetramers comprising various compositions of the subunits GluA1 to 4. Herguedas et al. used cryo–electron microscopy to determine the structure of the most prevalent form of AMPAR in the hippocampus, the GluA1/2 heteromer in complex with its regulatory subunit TARPγ8. The structure shows the architecture of the complex and provides insight into how conductance is controlled and modulated by auxiliary subunits.

Science, this issue p. eaav9011

Structured Abstract

INTRODUCTION

Neuronal communication at excitatory synapses in the brain involves release of the neurotransmitter l-glutamate from the presynapse of one cell and its detection by postsynaptic ionotropic glutamate receptors (iGluRs) on another. A principal iGluR is the AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor (AMPAR). On activation, AMPARs induce depolarization of the postsynaptic membrane to mediate rapid synaptic signaling and therefore precise information transfer at synapses. Long-lasting changes in synaptic strength can occur through recruitment to, or removal of, AMPARs from the synapse in response to particular patterns of synaptic activity. These synaptic plasticity processes, such as long-term potentiation (LTP) or long-term depression (LTD), are considered a cellular basis for learning and memory.

AMPARs assemble into tetramers from four core subunits, GluA1 to GluA4, in various combinations. The GluA1/2 heteromer predominates throughout the forebrain and is selectively recruited during LTP at the intensely studied hippocampal CA3-CA1 synapse. Receptor function is further diversified by association with auxiliary subunits, such as the TARP (transmembrane AMPAR regulatory protein) family. TARP γ8, a potent AMPAR modulator and the target for recent therapeutics, is selectively enriched in the hippocampus, forming a major component of the GluA1/2 signaling complex.

Within the AMPAR tetramer, the core subunits arrange in two conformationally distinct pairs, termed AC and BD, which play different roles in channel opening and are differentially modulated by TARPs. Although the BD pair has dominant control of activation in iGluRs, the rules of subunit arrangement in AMPARs are unclear.

RATIONALE

To elucidate the architecture and subunit organization of heteromeric AMPARs, we determined the structure of the GluA1/2 receptor in complex with TARP γ8 by means of cryo–electron microscopy (cryo-EM). Because up to four TARPs can decorate an AMPAR tetramer, yet γ8 appears to preferentially associate in a two-TARP stoichiometry, we fused γ8 to the GluA2 subunit, which, when co-expressed with GluA1, allows production of GluA1/2 associated with two γ8 auxiliary subunits. In addition, we used targeted mutagenesis in electrophysiological assays to probe subunit arrangement, gating properties, and mechanisms of TARP-specific receptor modulation.

RESULTS

Functional assays demonstrate preferential positioning of the GluA1 subunits to the AC positions, giving the functionally critical GluA2 (BD pair) dominant control over gating. The receptor assembly adopts an overall “Y” shape, characteristic of homomeric GluA2 structures, with the two extracellular domain layers [the N-terminal domain (NTD) and ligand-binding domain (LBD)], forming a dimer-of-dimers arrangement. The arms of the Y shape are held in place through an interface between the GluA2 NTDs, dictated by their positioning to the BD sites. The γ8 subunits, associated through intramembrane interactions, locate beneath the LBD dimer-of-dimers interface, with their distinctly long extracellular loops selectively engaging the GluA2 LBD to modulate channel gating, whereas lipid-like cryo-EM densities are observed in cavities formed between γ8 and the GluA1 TMD sector. Side chain resolution of the ion selectivity filter at the heart of the channel reveals the atomic details of the calcium-restricting “Q/R editing site,” which critically determines the properties of heteromeric AMPAR assemblies throughout the brain.

CONCLUSION

This structural and functional characterization of the GluA1/2 TARP γ8 complex reveals the architecture of a prominent AMPAR heteromer, offering a blueprint for deciphering signaling mechanisms of synaptic AMPARs.

Organization of a GluA1/2 AMPA receptor heteromer associated with TARP γ8.

The GluA1/2 AMPA receptor with two TARP γ8 auxiliary subunits (green) is depicted with schematic association of its agonist, glutamate (left). The GluA1 (blue) and GluA2 (red) subunits preferentially arrange to nonequivalent positions in the tetramer (right), giving GluA2 greater control of channel gating. (Inset) Representative contributions to glutamate-gated ion flow. A γ8 loop (dashed line) interacts with the GluA2 ligand-binding domain to influence receptor gating.

Abstract

AMPA-type glutamate receptors (AMPARs) mediate excitatory neurotransmission and are central regulators of synaptic plasticity, a molecular mechanism underlying learning and memory. Although AMPARs act predominantly as heteromers, structural studies have focused on homomeric assemblies. Here, we present a cryo–electron microscopy structure of the heteromeric GluA1/2 receptor associated with two transmembrane AMPAR regulatory protein (TARP) γ8 auxiliary subunits, the principal AMPAR complex at hippocampal synapses. Within the receptor, the core subunits arrange to give the GluA2 subunit dominant control of gating. This structure reveals the geometry of the Q/R site that controls calcium flux, suggests association of TARP-stabilized lipids, and demonstrates that the extracellular loop of γ8 modulates gating by selectively interacting with the GluA2 ligand-binding domain. Collectively, this structure provides a blueprint for deciphering the signal transduction mechanisms of synaptic AMPARs.

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