Kainate receptor pore‐forming and auxiliary subunits regulate channel block by a novel mechanism

Abstract

Key points Kainate receptor heteromerization and auxiliary subunits, Neto1 and Neto2, attenuate polyamine ion‐channel block by facilitating blocker permeation.Relief of polyamine block in GluK2/GluK5 heteromers results from a key proline residue that produces architectural changes in the channel pore α‐helical region.Auxiliary subunits exert an additive effect to heteromerization, and thus relief of polyamine block is due to a different mechanism.Our findings have broad implications for work on polyamine block of other cation‐selective ion channels. AbstractChannel block and permeation by cytoplasmic polyamines is a common feature of many cation‐selective ion channels. Although the channel block mechanism has been studied extensively, polyamine permeation has been considered less significant as it occurs at extreme positive membrane potentials. Here, we show that kainate receptor (KAR) heteromerization and association with auxiliary proteins, Neto1 and Neto2, attenuate polyamine block by enhancing blocker permeation. Consequently, polyamine permeation and unblock occur at more negative and physiologically relevant membrane potentials. In GluK2/GluK5 heteromers, enhanced permeation is due to a single proline residue in GluK5 that alters the dynamics of the α‐helical region of the selectivity filter. The effect of auxiliary proteins is additive, and therefore the structural basis of polyamine permeation and unblock is through a different mechanism. As native receptors are thought to assemble as heteromers in complex with auxiliary proteins, our data identify an unappreciated impact of polyamine permeation in shaping the signalling properties of neuronal KARs and point to a structural mechanism that may be shared amongst other cation‐selective ion channels.

Stargazin and cornichon-3 relieve polyamine block of AMPA receptors by enhancing blocker permeation

“…A similar mechanism also accounts for the relief of block in GluK2/GluK5 KAR heteromers ( Brown et al, 2016 ), suggesting that multiple structural pathways can affect channel block and permeation. Importantly, under both conditions, blocker permeation occurs at more negative and physiologically relevant membrane potentials ( Brown et al, 2016 ). The AMPAR auxiliary proteins stargazin (γ2) and cornichon-3 (CNIH-3) also attenuate polyamine block ( Soto et al, 2007 ; Coombs et al, 2012 ), but the underlying mechanism has yet to be determined.…”

“…However, appreciating that auxiliary proteins attenuate channel block in AMPARs and KARs ( Jackson and Nicoll, 2011 ), the potential contribution of blocker permeation to the overall block mechanism has recently been evaluated. For example, a study of homomeric GluK2 KARs revealed that auxiliary proteins Neto1 and Neto2 relieve polyamine channel block by enhancing blocker permeation rates ( Brown et al, 2016 ). A similar mechanism also accounts for the relief of block in GluK2/GluK5 KAR heteromers ( Brown et al, 2016 ), suggesting that multiple structural pathways can affect channel block and permeation.…”

“…For example, a study of homomeric GluK2 KARs revealed that auxiliary proteins Neto1 and Neto2 relieve polyamine channel block by enhancing blocker permeation rates ( Brown et al, 2016 ). A similar mechanism also accounts for the relief of block in GluK2/GluK5 KAR heteromers ( Brown et al, 2016 ), suggesting that multiple structural pathways can affect channel block and permeation. Importantly, under both conditions, blocker permeation occurs at more negative and physiologically relevant membrane potentials ( Brown et al, 2016 ).…”

Neto Auxiliary Subunits Regulate Interneuron Somatodendritic and Presynaptic Kainate Receptors to Control Network Inhibition

“…Recently, Neto1/Neto2 have emerged as auxiliary KAR subunits capable of regulating almost every parameter of receptor function (Copits and Swanson, 2012; Howe, 2015). Overexpression studies in heterologous cells or neurons have demonstrated that Netos regulate KAR desensitization and deactivation kinetics, channel open probability, ligand affinity, ion permeation, and subcellular localization (Brown et al, 2016; Copits et al, 2011; Fisher, 2015; Fisher and Mott, 2012, 2013; Griffith and Swanson, 2015; Orav et al, 2017; Palacios-Filardo et al, 2016; Zhang et al, 2014; Zhang et al, 2009). Consistent with these findings, studies at hippocampal mossy fiber to CA3 pyramidal cell (MF-CA3) synapses indicate that Neto1 regulates binding affinity, kinetics, and synaptic targeting of native GluK2/3-containing postsynaptic KARs (Straub et al, 2011a; Tang et al, 2011; Wyeth et al, 2014).…”

Uncorking AMPA receptors

“…In that study, the researchers observed that the KAR auxiliary proteins Neto1 and Neto2 relieve polyamine block by making it easier for polyamines to pass through the ion channel (5). Because AMPARs and KARs are closely related, Brown says, “it totally made sense to ask whether this could also be happening with AMPA receptors.”…”