The family of AMPA receptors is encoded by four genes that are differentially spliced to result in the flip or flop versions of the four subunits GluR1 to GluR4. GluR2 is further modified at the so-called Q/R site by posttranscriptional RNA editing. Delivery of AMPA receptors to the plasma membrane and synaptic trafficking are controlled by transmembrane AMPA receptor regulatory proteins (TARPs). Additionally, TARPs influence essential electrophysiological properties of AMPA receptor channels such as desensitization and agonist efficacies. Here, we compare the influence of all known TARPs (␥2, ␥3, ␥4, and ␥8) on agonist-induced currents of the four AMPA receptor subunits, including flip and flop splice variants and editing variants. We show that, although agonist-induced currents of all homomeric AMPA receptor subunits as well as all heteromeric combinations tested are significantly potentiated when coexpressed with members of the TARP family in Xenopus laevis oocytes, the extent of TARP-mediated increase in agonist-induced responses is highly dependent on both the AMPA receptor subunit and the coexpressed TARP. Moreover, we demonstrate that the splice variant of the AMPA receptor plays a key role in determining the modulation of electrophysiological properties by associated TARPs. We furthermore present evidence that individual TARP-AMPA receptor interactions control the degree of desensitization of AMPA receptors. Consequently, because of their subunit-specific impact on the electrophysiological properties, TARPs play a major role as modulatory subunits of AMPA receptors and thus contribute to the functional diversity of AMPA receptors encountered in the CNS.

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AMPA receptors mediate the majority of the fast excitatory synaptic transmission in the brain. A family of recently described auxiliary proteins, the transmembrane AMPA receptor regulatory proteins (TARPs) ␥2, ␥3, ␥4, and ␥8, have been shown to modulate the trafficking of receptors to the plasma membrane as well as electrophysiological key properties. Most studies published to date focus exclusively on ␥2 (stargazin), neglecting the other three members of the TARP family. Here, we analyzed the modulation of electrophysiological properties of AMPA receptors by ␥4 and compare it with ␥2, using heterologous coexpression in human embryonic kidney 293 cells. We show for the first time that ␥4, a previously poorly examined TARP, modulates the desensitization properties of AMPA receptors significantly stronger than ␥2 does. In contrast, other properties such as kainate efficacy and current-voltage relationships are modulated in a similar way by both of these TARPs. From these TARP-specific effects, we propose an interaction mechanism between AMPA receptors and TARPs and address the physiological relevance of ␥4 and its regulatory effects, particularly on AMPA receptor desensitization properties, to developmental and regulatory processes in the brain.

The family of ionotropic glutamate receptors includes 2 subunits, delta1 and delta2, the physiological relevance of which remains poorly understood. Both are nonfunctional in heterologous expression systems, although the isolated, crystallized ligand binding domain (LBD) of delta2 is capable of binding D-serine. To investigate these seemingly contradictory observations we tested whether delta receptors can be ligand gated at all. We used a strategy that replaced the native LBD of delta2 by a proven glutamate-binding LBD. Test transplantations between ␣-amino-3-hydroxy-5-methylisoxazole propionate (AMPA) and kainate receptors (GluR1 and GluR6, respectively) showed that this approach can produce functional chimeras even if only one part of the bipartite LBD is swapped. Upon outfitting delta2 with the LBD of GluR6, the chimera formed glutamate-gated ion channels with low Ca 2؉ permeability and unique rectification properties. Ligandinduced conformational changes can thus gate delta2, suggesting that the LBD of this receptor works fundamentally differently from that of other ionotropic glutamate receptors.orphan receptor ͉ AMPA receptor ͉ kainate receptor ͉ chimeric receptors ͉ ligand binding domain

The ionotropic α-amino-3-hydroxy-5-methyl-4-isoxazole propionate glutamate receptors (AMPARs) have been implicated in the establishment of dendritic architecture. The transmembrane AMPA receptor regulatory proteins (TARPs) regulate AMPAR function and trafficking into synaptic membranes. In the current study, we employ type I and type II TARPs to modulate expression levels and function of endogenous AMPARs and investigate in organotypic cultures (OTCs) of rat occipital cortex whether this influences neuronal differentiation. Our results show that in early development [5-10 days in vitro (DIV)] only the type I TARP γ-8 promotes pyramidal cell dendritic growth by increasing spontaneous calcium amplitude and GluA2/3 expression in soma and dendrites. Later in development (10-15 DIV), the type I TARPs γ-2, γ-3 and γ-8 promote dendritic growth, whereas γ-4 reduced dendritic growth. The type II TARPs failed to alter dendritic morphology. The TARP-induced dendritic growth was restricted to the apical dendrites of pyramidal cells and it did not affect interneurons. Moreover, we studied the effects of short hairpin RNA-induced knockdown of endogenous γ-8 and showed a reduction of dendritic complexity and amplitudes of spontaneous calcium transients. In addition, the cytoplasmic tail (CT) of γ-8 was required for dendritic growth. Single-cell calcium imaging showed that the γ-8 CT domain increases amplitude but not frequency of calcium transients, suggesting a regulatory mechanism involving the γ-8 CT domain in the postsynaptic compartment. Indeed, the effect of γ-8 overexpression was reversed by APV, indicating a contribution of NMDA receptors. Our results suggest that selected type I TARPs influence activity-dependent dendritogenesis of immature pyramidal neurons.