Nonselective cation permeation in an AMPA-type glutamate receptor

Abstract

Fast excitatory synaptic transmission in the central nervous system relies on the AMPA-type glutamate receptor (AMPAR). This receptor incorporates a nonselective cation channel, which is opened by the binding of glutamate. Although the open pore structure has recently became available from cryo-electron microscopy (Cryo-EM), the molecular mechanisms governing cation permeability in AMPA receptors are not understood. Here, we combined microsecond molecular dynamic (MD) simulations on a putative open-state structure of GluA2 with electrophysiology on cloned channels to elucidate ion permeation mechanisms. Na+, K+, and Cs+ permeated at physiological rates, consistent with a structure that represents a true open state. A single major ion binding site for Na+ and K+ in the pore represents the simplest selectivity filter (SF) structure for any tetrameric cation channel of known structure. The minimal SF comprised only Q586 and Q587, and other residues on the cytoplasmic side formed a water-filled cavity with a cone shape that lacked major interactions with ions. We observed that Cl− readily enters the upper pore, explaining anion permeation in the RNA-edited (Q586R) form of GluA2. A permissive architecture of the SF accommodated different alkali metals in distinct solvation states to allow rapid, nonselective cation permeation and copermeation by water. Simulations suggested Cs+ uses two equally populated ion binding sites in the filter, and we confirmed with electrophysiology of GluA2 that Cs+ is slightly more permeant than Na+, consistent with serial binding sites preferentially driving selectivity.

A Model for Predicting Cation Selectivity and Permeability in AMPA and NMDA Receptors Based on Receptor Subunit Composition

“…To validate the charge permeability equation, we tested its applicability in predicting Ca 2+ permeability of AMPARs ( Figure 5 ; for non-selective permeation of monovalent cations through AMPARs; see Biedermann et al, 2021 ). It is now well known that AMPARs assembled without the post-translationally modified GluA2 subunit are Ca 2+ permeable unlike those assembled with it ( Hollmann et al, 1991 ).…”

Opening of glutamate receptor channel to subconductance levels

“…Each successful count represents a passing of an ion through the boundary points defined at the M3 gate residues T625 at the entrance to the channel and an approximate centre of mass of the backbone atoms of the selectivity filter residues ( 586 QQGCD 590 ). Similar high voltage, high ion concentration computational electrophysiology approaches have been used in recent MD simulations of AMPA receptors and other channels to acquire statistically relevant ionic currents 46 , 52 , 53 . Only upward ion permeation (in the direction from the selectivity filter towards the gate) was observed in all simulations, as was also reported in simulations of AMPA receptors and K + -selective channels 87 , presumably as a consequence of a high applied voltage.…”

“…3b ), their selectivity filters are expected to permeate both ions and water. Indeed, molecular dynamics (MD) simulations of the transmembrane domain of the structure obtained at high Glu concentration (PDB ID: 5WEO ) demonstrated permeation of Na + and K + ions through the selectivity filter 46 . As the selectivity filter in all structures at 20 µM Glu was the same or slightly wider than in the 5WEO structure (Fig.…”

“…Second, we directly estimated ion conductance by performing non-equilibrium MD simulations of selected representative structures under an applied voltage (Extended Data Table 3 ). As ion channel permeation is rare on the MD time scale at physiological voltages and ion concentrations, it is common in simulations to use increased voltages (600 mV) and ion (K + ) concentrations (300 mM) 46 , 52 , 53 . Such MD simulations provide a semiquantitative estimate of the channel conductance because of non-physiological settings and short durations that result in low-count statistics and high fluctuations in the number of permeating ions (Fig.…”

Neuronal Swelling: A Non-osmotic Consequence of Spreading Depolarization

“…But the channel is non-selective, so Na + and K + counteract each other’s associated water flux. Indeed, in Biedermann et al [ 100 ], appended video simulations do not reveal directional water movement through the α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) receptor channel. Likewise, neuronal swelling in response to NMDA receptor activation involves the cotransporters NKCC1 and swelling-activated KCC2 [ 101 ].…”