TARP Auxiliary Subunits Switch AMPA Receptor Antagonists into Partial Agonists

Abstract

Quinoxalinedione compounds such as 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) are the most commonly used alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonists. However, we find that in the presence of transmembrane AMPA receptor regulatory proteins (TARPs), which are AMPA receptor auxiliary subunits, CNQX acts as a partial agonist. CNQX induced small depolarizing currents in neurons of the central nervous system, and reconstitution of this agonist activity required coexpression of TARPs. A crystal structure of CNQX bound to the TARP-less AMPA receptor ligand-binding domain showed that, although CNQX induces partial domain closure, this movement is not transduced into linker separation, suggesting that TARPs may increase agonist efficacy by strengthening the coupling between domain closure and channel opening. Our results demonstrate that the presence of an auxiliary subunit can determine whether a compound functions as an agonist or antagonist.

Functional comparison of the effects of TARPs and cornichons on AMPA receptor trafficking and gating

“…Rather, coexpression of CNIH2 and GluA1γ-8 results in channels exhibiting both deactivation kinetics consistent with CNIH association and kainate efficacy consistent with TARP association, demonstrating that these proteins can modulate AMPARs simultaneously. We further tested whether CNIH2 shares with TARPs the ability to convert the competitive antagonist CNQX into a partial agonist (18) and found that CNIH2 has no effect on the action of CNQX (Fig. S2C).…”

“…CNIH2 also increases AMPAR single channel conductance similarly to TARPs. However, CNIHs differ from TARPs in that they only minimally increase the efficacy of the partial agonist kainate (7,9) and fail to convert the competitive antagonist CNQX into a partial agonist (18,22).…”

“…This is in contrast to the consistent and profound effect that TARPs have on AMPAR trafficking, pharmacology, and kinetics in neurons. For example, we have previously demonstrated that diverse neuronal cell types in the brain exhibit high kainate efficacy and depolarize in response to CNQX application (18), suggesting that the vast majority of surface-expressed AMPARs in the brain are associated with TARPs. In summary, the longstanding enigma of the inability of AMPARs expressed in heterologous cell systems to mimic the properties of neuronal AMPARs was resolved by the discovery of TARPs.…”

AMPA receptor/TARP stoichiometry visualized by single-molecule subunit counting

“…TARPs regulate AMPA-R function by several mechanisms. They mediate the efficient cell surface expression of AMPA-Rs, modulate gating, affect agonist efficacy, and even attenuate intracellular polyamine block of calciumpermeable AMPA-Rs (13)(14)(15)(16)(17)(18)(19)(20)(21)(22). The effect of stargazin (γ-2) and other TARPs on GluA1 is not neuron specific and can be accurately mimicked in nonneuronal mammalian cells (18,(23)(24)(25), as well as Xenopus oocytes (13,14,17,26,27), suggesting that the basic mechanisms for TARP/AMPA-R interaction and TARPmediated regulation of AMPA-R trafficking are preserved in nonneuronal cells.…”

Regulation of AMPA receptors and synaptic plasticity

“…Moreover, in the presence of TARPs, the AMPARs competitive antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) acts as a partial agonist (Menuz et al, 2007). These results, together with the resolution of the crystal structure of CNQX bound to the TARP-less AMPAR LBD (Menuz et al, 2007), led the authors to propose a model where TARPs either strengthen the coupling between agonist-induced domain closure and channel opening, perhaps by promoting linker separation, or directly enhance the degree of domain closure induced by CNQX.…”