J Physiol 591.7 (2013) p 1589 1589

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Rebuttal from David M. MacLean

David M. MacLeanCenter for Membrane Biology, Departmentof Biochemistry and Molecular Biology,University of Texas Health Science Center,Houston, TX 77030, USA

Email: david.m.maclean@uth.tmc.edu

The main effects of the transmembraneAMPA receptor (AMPAR) auxiliaryproteins (TARPs) on AMPAR gating areslowing of deactivation and desensitization,increasing peak Popen and speeding ofrecovery from desensitization (Prielet al. 2005; Tomita et al. 2005). All ofthese observations can be reproducedby increasing the rate of recovery fromdesensitization, γ, and by one of twoadditional means: either increasing theopen rate, β (as proposed by Dr Howe,2013), or reducing the cleft openingrate, CO, and desensitization rate, δ (assuggested by myself, MacLean, 2013).Moreover, increasing the channel openingrate or decreasing the cleft open rate bothresult in the reported substantial left-shiftin steady-state glutamate EC50 valuesproduced by TARPs (Priel et al. 2005;Tomita et al. 2005; Kott et al. 2007) as wellas the smaller left-shift in peak EC50 values(Morimoto-Tomita et al. 2009). Therefore,such concentration–response data are nothelpful in distinguishing an effect on gatingtransitions from an effect on pre-gatingtransitions such as stabilizing closed-cleftstates.

However, peak inhibition curves usingNBQX and CNQX may provide somesupport for the hypothesis that TARPsaffect pre-gating transitions by promoting

states with closed ligand-binding domains(LBDs). The LBDs of AMPARs exist inmultiple closed-cleft conformations, bothwhen the agonist is bound and in the apostate (Landes et al. 2011). If the presence ofTARPs causes AMPAR LBDs to adopt moreclosed conformations, then even in the apostate the binding cleft would be, on average,slightly more shut. Consequently, largerligands such as NBQX or CNQX wouldhave slower binding rates since there wouldbe fewer favourable collisions betweenthese compounds and the more closedLBDs. Such slowed binding rates of largerantagonists would result in right-shifts ofpeak inhibition curves and this is pre-cisely what is observed experimentally.The peak response IC50 values obtainedusing rapid perfusion on outside-outpatches for both CNQX and NBQXare right-shifted approximately 2-fold bystargazin (MacLean & Bowie, 2011). Whilethis observation is consistent with TARPsstabilizing more closed LBD conformations,more compelling evidence is needed.Ultimately, to discriminate between thesemechanisms, direct measurements of boththe degree of LBD closure and couplingbetween LBD closure and channel openingwill be required.

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References

Howe JR (2013). CrossTalk proposal: TARPsmodulate AMPA receptor gating transitions.J Physiol 591, 1581–1583.

Kott S, Werner M, Korber C & Hollmann M(2007). Electrophysiological properties ofAMPA receptors are differentially modulateddepending on the associated member of theTARP family. J Neurosci 27, 3780–3789.

Landes CF, Rambhadran A, Taylor JN, Salatan F& Jayaraman V (2011). Structural landscapeof isolated agonist-binding domains fromsingle AMPA receptors. Nat Chem Biol 7,168–173.

MacLean DM (2013). CrossTalk opposing view:TARPs modulate AMPA receptorconformations before the gating transitions.J Physiol 591, 1585–1586.

MacLean DM & Bowie D (2011).Transmembrane AMPA receptor regulatoryprotein regulation of competitive antagonism:a problem of interpretation. J Physiol 589,5383–5390.

Morimoto-Tomita M, Zhang W, Straub C, ChoCH, Kim KS, Howe JR & Tomita S (2009).Autoinactivation of neuronal AMPA receptorsvia glutamate-regulated TARP interaction.Neuron 61, 101–112.

Priel A, Kolleker A, Ayalon G, Gillor M, Osten P& Stern-Bach Y (2005). Stargazin reducesdesensitization and slows deactivation of theAMPA-type glutamate receptors. J Neurosci25, 2682–2686.

Tomita S, Adensik H, Sekiguchi M, Zhang W,Wada K, Howe JR, Nicoll RA & Bredt DS(2005). Stargazin modulates AMPA receptorgating and trafficking by distinct domains.Nature 435, 1052–1058.

Acknowledgements

This work was supported by an American HeartAssociation grant, number 11GRNT7890004, toDr Vasanthi Jayaraman.

C© 2013 The Author. The Journal of Physiology C© 2013 The Physiological Society DOI: 10.1113/jphysiol.2013.251520