Dual Effects of TARP γ-2 on Glutamate Efficacy Can Account for AMPA Receptor Autoinactivation

Abstract

SummaryFast excitatory transmission in the CNS is mediated mainly by AMPA-type glutamate receptors (AMPARs) associated with transmembrane AMPAR regulatory proteins (TARPs). At the high glutamate concentrations typically seen during synaptic transmission, TARPs slow receptor desensitization and enhance mean channel conductance. However, their influence on channels gated by low glutamate concentrations, as encountered during delayed transmitter clearance or synaptic spillover, is poorly understood. We report here that TARP γ-2 reduces the ability of low glutamate concentrations to cause AMPAR desensitization and enhances channel gating at low glutamate occupancy. Simulations show that, by shifting the balance between AMPAR activation and desensitization, TARPs can markedly facilitate the transduction of spillover-mediated synaptic signaling. Furthermore, the dual effects of TARPs can account for biphasic steady-state glutamate concentration-response curves—a phenomenon termed “autoinactivation,” previously thought to reflect desensitization-mediated AMPAR/TARP dissociation.

Mechanism of modulation of AMPA receptors by TARP-γ8

“…A number of investigations have focused on the structure and mechanism of modulation of AMPA receptors with the TARP stargazin, also known as γ2 (Cho et al, 2007;Tomita et al, 2005;MacLean and Bowie, 2011;MacLean et al, 2014;Cais et al, 2014;Zhang et al, 2014;Carbone and Plested, 2016;Shaikh et al, 2016;Twomey et al, 2016;Zhao et al, 2016;Coombs et al, 2017;Ben-Yaacov et al, 2017;Twomey et al, 2017;Baranovic and Plested, 2018). These studies show that γ2 interacts with the lower lobe of the ligand-binding domain (LBD) and the preM1 linker via an electrostatic patch, which may stabilize closed cleft states of the LBD, enhancing agonist efficacy.…”

Homomeric Q/R edited AMPA receptors conduct when desensitized

“…Heterologous expression. We expressed recombinant AMPAR subunits and TARPs (plus EGFP) in HEK293 cells maintained under standard protocols, as described previously 25 . AMPAR subunit cDNAs (rat) were 'flip' splice variants and the GluA2 forms were additionally R/G edited.…”

“…We next considered whether the presence of conducting desensitized receptors could account quantitatively for our observations. To examine this, we incorporated such states into a modified version of a kinetic scheme we used previously to describe AMPAR/TARP concentration-dependent behaviors 25 (Scheme 1; Fig. 5a), and attempted to mimic the macroscopic kinetics and NSFA of GluA2(R)/ -2 by varying the rate constants and conductances.…”

“…These auxiliary proteins determine many biophysical and pharmacological properties of AMPARs and influence their desensitization 23,24 . The prototypical TARP -2 markedly slows the rate of AMPAR desensitization and accelerates recovery from desensitization 25,26 , while TARP -8 and GSG1L slow both the entry into and the recovery from desensitization 21,22,27 . The structures of desensitized complexes, composed of homomeric GluA2 AMPARs with either -2 or GSG1L, have recently been determined at~8 Å resolution by cryo-electron microscopy (cryo-EM) 16,19 .…”

Slow AMPA receptors in hippocampal principal cells

“…Auxiliary subunits can slow AMPA receptor kinetics in heterologous expression (Priel et al, 2005) and reduce their tendency to desensitize whilst boosting activation (Coombs et al, 2017). Evidence for desensitization-resistant AMPA receptors in the brain is so far limited to high-frequency activity at certain connections in the cerebellum (Lu et al, 2017;DiGregorio et al, 2007), where hints of a distinct pharmacology were reported (Devi et al, 2016).…”