Glutamate-receptor ion channels (iGluRs) mediate the vast majority of excitatory synaptic transmission in the mammalian central nervous system. A host of studies have shown that the biophysical and kinetic properties of iGluRs play a key role in determining the amplitude and time-course of postsynaptic currents, as well as postsynaptic responses to repetitive presynaptic stimulation. The biophysical and kinetic properties of iGluRs are best revealed in studies of single receptor molecules, something that is possible with single-channel patch-clamp recording. In addition to the core subunits required to form functional receptors, in the last ten years it has become clear that iGluRs also contain auxiliary subunits. For the AMPA subtype of iGluRs, one family of such auxiliary subunits is the Transmembrane AMPA-receptor Regulatory Proteins (TARPs). In addition to playing a key role in AMPA receptor trafficking, our lab and others have demonstrated that TARPs modulate several features of AMPA receptor gating kinetics. Single-channel work that we have done strongly suggests that TARPs promote modal gating, where the receptors switch (on a time-scale of hundreds of milliseconds) between low and high Popen behavior.
In Aim 1, we propose to further explore these initial findings and determine whether the effects differ for different TARPs. If the data support evidence of modal gating, we will test whether this behavior influences the character of synaptic responses to repetitive stimuli. We will also investigate the effect of agonist-induced uncoupling of TARP-receptor interactions on single-channel currents by comparing results from co-expression studies with those from experiments with tandem constructs where individual TARPs are fused directly to the receptors. In the prior funding period, we demonstrated that the kainate subtype of iGluRs also have auxiliary subunits (NETO1, NETO2) that modulate receptor kinetics. Again, our preliminary single-channel studies suggest that NETO2 strongly promotes modal gating of kainate receptors, an effect that has a dramatic impact on the shape and amplitude of receptor responses. We will extend these studies in Aim 2 of this proposal. We also have preliminary data showing that GluK1 receptors open hundreds of milliseconds after brief pulses of saturating concentrations of glutamate. These results suggest that kainate receptors recover from desensitization with glutamate still bound and that the rate-determining step in recovery is the rate at which the receptor re-sensitizes, rather than the rate at which glutamate dissociates from the desensitized receptor.
In Aim 2 we will extend and quantify these preliminary studies.
The kinetic properties of glutamate-gated ion channels (iGluRs) have a profound effect on the fidelity of excitatory synaptic transmission in the mammalian brain and spinal cord, and iGluRs have been implicated in a number of neurological and psychiatric disorders. Understanding the kinetic properties of iGluRs, and what regulates these properties, is fundamental to understanding the role of iGluRs in both normal and pathological brain function.
|Howe, James R (2015) Modulation of non-NMDA receptor gating by auxiliary subunits. J Physiol 593:61-72|
|Zhang, Wei; Devi, Suma Priya Sudarsana; Tomita, Susumu et al. (2014) Auxiliary proteins promote modal gating of AMPA- and kainate-type glutamate receptors. Eur J Neurosci 39:1138-47|
|Howe, James R (2013) CrossTalk proposal: TARPs modulate AMPA receptor gating transitions. J Physiol 591:1581-3; discussion 1589|