The activation of Group 1 metabotropic glutamate receptors, mGluR5 and mGluR1␣, triggers intracellular calcium release; however, mGluR5 activation is unique in that it elicits Ca 2؉ oscillations. A short region of the mGluR5 C terminus is the critical determinant and differs from the analogous region of mGluR1␣ by a single amino acid residue, Thr-840, which is an aspartic acid (Asp-854) in mGluR1␣. Previous studies show that mGluR5-elicited Ca 2؉ oscillations require protein kinase C (PKC)-dependent phosphorylation and identify Thr-840 as the phosphorylation site. However, direct phosphorylation of mGluR5 has not been studied in detail. We have used biochemical analyses to directly investigate the phosphorylation of the mGluR5 C terminus. We showed that Ser-839 on mGluR5 is directly phosphorylated by PKC, whereas Thr-840 plays a permissive role. Although Ser-839 is conserved in mGluR1␣ (Ser-853), it is not phosphorylated, as the adjacent residue (Asp-854) is not permissive; however, mutagenesis of Asp-854 to a permissive alanine residue allows phosphorylation of Ser-853 on mGluR1␣. We investigated the physiological consequences of mGluR5 Ser-839 phosphorylation using Ca 2؉ imaging. Mutations that eliminate Ser-839 phosphorylation prevent the characteristic mGluR5-dependent Ca 2؉ oscillations. However, mutation of Thr-840 to alanine, which prevents potential Thr-840 phosphorylation but is still permissive for Ser-839 phosphorylation, has no effect on Ca 2؉ oscillations. Thus, we showed that it is phosphorylation of Ser-839, not Thr-840, that is absolutely required for the unique Ca 2؉ oscillations produced by mGluR5 activation. The Thr-840 residue is important only in that it is permissive for the PKC-dependent phosphorylation of Ser-839. Metabotropic glutamate receptors (mGluRs)1 play important roles throughout the nervous system, including the activation of ion channels and the regulation of synaptic plasticity (1). In addition, they have been implicated in a variety of neurological diseases (2-5). There are eight different mGluRs, and these are subdivided into three groups based on sequence identity and pharmacological properties. Group 1 mGluRs (mGluR1 and mGluR5) are linked to phospholipase C, whereas Group 2 mGluRs (mGluR2 and mGluR3) and Group 3 mGluRs (mGluR4, mGluR6, mGluR7, and mGluR8) are negatively linked to adenylate cyclase. Activation of Group 1 mGluRs triggers phospholipase C, resulting in increases in IP 3 and diacylglycerol production and the concomitant release of intracellular Ca 2ϩ and activation of protein kinase C (PKC) (6). Group 1 mGluR-elicited Ca 2ϩ release stimulates PKC translocation to the plasma membrane and oscillations of both PKC activation and IP 3 production (7, 8).Although both mGluR1 and mGluR5 stimulate intracellular Ca 2ϩ release, they differ in that mGluR5 activation results in Ca 2ϩ oscillations, whereas mGluR1 activation results in a single Ca 2ϩ transient with or without subsequent low frequency oscillations (9, 10). A previous study (10) demonstrated that a short stret...

Neuronal kainate receptors are typically heteromeric complexes composed of GluR5-7 and KA1-2 subunits. Although GluR5-7 can exist as functional homomeric channels, the KA subunits cannot. KA2 is widely expressed in the CNS, and KA2/GluR6 heteromers are the most prevalent subunit composition in brain. Previous work has identified endoplasmic reticulum (ER)-retention motifs in the C terminus of KA2, which prevent surface expression of KA2 homomers. However, we find that, when these motifs are mutated, only a small fraction of KA2 is surface expressed. We now identify an additional ER retention motif in the intracellular loop region of KA2, which, when mutated together with the C-terminal motifs, significantly increases the level of KA2 surface expression. However, electrophysiological analysis of surface-expressed KA2 homomers indicates that they do not form functional ion channels. In heterologous cells, a large fraction of KA2 remains intracellular even when the trafficking motifs are mutated or when GluR6 is coexpressed. Therefore, we analyzed the trafficking of endogenous KA2 in vivo. We find that native KA2 surface expression is dramatically reduced in GluR6 knock-out mice compared with wild-type mice. In contrast, KA2 trafficking was unaffected in the GluR5 knock-out. Thus, our study demonstrates that trafficking motifs in both the intracellular loop and C terminus regulate KA2 surface expression; however, in neurons, GluR6 oligomerization is required for egress of KA2 from the ER and transport to the cell surface. The combination of these mechanisms likely prevents surface expression of nonfunctional KA2 homomers and ensures a high level of GluR6/KA2 heteromeric kainate receptors.

Hippocampal GABAergic interneurons are crucial for cortical network function and have been implicated in psychiatric disorders. We show here that Neuregulin 3 (Nrg3), a relatively little investigated low‐affinity ligand, is a functionally dominant interaction partner of ErbB4 in parvalbumin‐positive (PV) interneurons. Nrg3 and ErbB4 are located pre‐ and postsynaptically, respectively, in excitatory synapses on PV interneurons in vivo. Additionally, we show that ablation of Nrg3 results in a similar phenotype as the one described for ErbB4 ablation, including reduced excitatory synapse numbers on PV interneurons, altered short‐term plasticity, and disinhibition of the hippocampal network. In culture, presynaptic Nrg3 increases excitatory synapse numbers on ErbB4+ interneurons and affects short‐term plasticity. Nrg3 mutant neurons are poor donors of presynaptic terminals in the presence of competing neurons that produce recombinant Nrg3, and this bias requires postsynaptic ErbB4 but not ErbB4 kinase activity. Furthermore, when presented by non‐neuronal cells, Nrg3 induces postsynaptic membrane specialization. Our data indicate that Nrg3 provides adhesive cues that facilitate excitatory neurons to synapse onto ErbB4+ interneurons.