Transmembrane AMPA receptor regulatory proteins and AMPA receptor function in the cerebellum

Abstract

Heterogeneity among AMPA receptor (AMPAR) subtypes is thought to be one of the key postsynaptic factors giving rise to diversity in excitatory synaptic signaling in the CNS. Recently, compelling evidence has emerged that ancillary AMPAR subunits-the so-called transmembrane AMPA receptor regulatory proteins (TARPs)-also play a vital role in influencing the variety of postsynaptic signaling. This TARP family of molecules controls both trafficking and functional properties of AMPARs at most, if not all, excitatory central synapses. Furthermore, individual TARPs differ in their effects on the biophysical and pharmacological properties of AMPARs. The critical importance of TARPs in synaptic transmission was first revealed in experiments on cerebellar granule cells from stargazer mice. These lack the prototypic TARP stargazin, present in granule cells from wild-type animals, and consequently lack synaptic transmission at the mossy fibre-to-granule cell synapse. Subsequent work has identified many other members of the stargazin family which act as functional TARPs. It has also provided valuable information about specific TARPs present in many central neurons. Because much of the initial work on TARPs was carried out on stargazer granule cells, the important functional properties of TARPs present throughout the cerebellum have received particular attention. Here we discuss some of these recent findings in relation to the main TARPs and the AMPAR subunits identified in cerebellar neurons and glia.

Fig. 1

Cell trafficking of the TARP-AMPAR assembly. AMPAR biogenesis is initiated in the ER, with formation of dimers (homo or heterodimers) of GluR subunits, followed by tetramerization. TARPs interact with the AMPARs in the ER (1). Within the ER, proteins such as BiP, interact with TARPs to facilitate trafficking of the TARP–AMPAR complex through the TGN (2). In the TGN, TARPs interact with proteins including nPIST, MAP1 LC2 and AP-4, which may assist vesicular trafficking to the cell surface (3). PKC/CaMKII phosphorylate multiple sites on the TARP protein to facilitate membrane mobilization (4) and hence increase the likelihood of AMPARs accumulating at the synapse. AMPARs are immobilized in the postsynaptic membrane via interaction with MAGUKs, including PSD-95 (5). Following endocytosis, potentially triggered by agonist dependent TARP-AMPAR dissociation, the subsequent fate of the AMPARs may be degradation (6) or possibly recycling to the cell surface by other AMPAR interacting proteins such as PICK1 or GRIP1 (7).

Fig. 2

Stargazin alters AMPAR properties. (a) (I) (ii), Currents evoked at −60 mV by rapid application of 10 mM glutamate (200 ms) to outside-out patches from cells expressing heteromeric GluR1/2 AMPARs in the absence and presence of stargazin (Stg). Black lines show means of 84 or 98 traces respectively; grey lines show representative traces. Inset shows a plot of variance versus mean current, obtained from non-stationary fluctuation analysis. Baseline variance is also displayed. For these cells the slope of the relationship gave a weighted mean single–channel conductance of 3.5 pS in the absence of Stg, and 6.5 pS in its presence. Thus, Stg associated AMPARs show an increased conductance and peak open probability (for methodology see Soto et al., 2007). (b) Current traces from a (i), (ii), super imposed and shown on an expanded time base (AMPAR currents with and without Stg are indicated), to illustrate the slower desensitization kinetics and larger steady-state open probability of receptors expressed with Stg. (c) Resolved single channel currents from homomeric GluR1 AMPARs expressed in the absence (upper traces) and presence (lower traces) of Stg, illustrating the increased single-channel conductance, slower kinetics and higher open probability of AMPAR expressed with a TARP. Each set of recordings shows three responses to fast glutamate application. Initial truncated peaks are followed by clear single-channel currents in the presence of Stg, and by openings that are barely discernible in its absence. (d) Dose–response curves of peak AMPAR current amplitude illustrating that that Stg increases the glutamate affinity of GluR1 (a–c unpublished observations; d, from Tomita et al., 2005b).

Fig. 3

TARP (γ−) subtype expression in cerebellar cell types. Main TARPs have been identified by rt-PCR, immunocytochemistry and in situ hybridization. Main AMPAR subunits (GluRs) are also indicated (see text). The differential distribution of TARPs, together with AMPAR subunit diversity, provides a rich variety of functionally distinct AMPAR subtypes in different cell types (Tomita et al., 2003; Fukaya et al., 2005; drawing of cerebellum after Sharpey-Schafer, 1929).