Glutamate Receptor δ2 Is Essential for Input Pathway-Dependent Regulation of Synaptic AMPAR Contents in Cerebellar Purkinje Cells

Abstract

The number of synaptic AMPA receptors (AMPARs) is the major determinant of synaptic strength and is differently regulated in input pathway-dependent and target cell type-dependent manners. In cerebellar Purkinje cells (PCs), the density of synaptic AMPARs is approximately five times lower at parallel fiber (PF) synapses than at climbing fiber (CF) synapses. However, molecular mechanisms underlying this biased synaptic distribution remain unclear. As a candidate molecule, we focused on glutamate receptor ␦2 (GluR␦2 or GluD2), which is known to be efficiently trafficked to and selectively expressed at PF synapses in PCs. We applied postembedding immunogold electron microscopy to GluR␦2 knock-out (KO) and control mice, and measured labeling density for GluA1-4 at three excitatory synapses in the cerebellar molecular layer. In both control and GluR␦2-KO mice, GluA1-3 were localized at PF and CF synapses in PCs, while GluA2-4 were at PF synapses in interneurons. In control mice, labeling density for each of GluA1-3 was four to six times lower at PF-PC synapses than at CF-PC synapses. In GluR␦2-KO mice, however, their labeling density displayed a three-to fivefold increase at PF synapses, but not at CF synapses, thus effectively eliminating input pathway-dependent disparity between the two PC synapses. Furthermore, we found an unexpected twofold increase in labeling density for GluA2 and GluA3, but not GluA4, at PF-interneuron synapses, where we identified low but significant expression of GluR␦2. These results suggest that GluR␦2 is involved in a common mechanism that restricts the number of synaptic AMPARs at PF synapses in PCs and molecular layer interneurons.

Glutamate Receptor δ2 Associates with Metabotropic Glutamate Receptor 1 (mGluR1), Protein Kinase Cγ, and Canonical Transient Receptor Potential 3 and Regulates mGluR1-Mediated Synaptic Transmission in Cerebellar Purkinje Neurons

“…3), which is consistent with a recent report (Yamasaki et al, 2011), we find no change in the input-output for fEPSCs in the GluR␦2 hypomorph. This observation may indicate homeostatic compensation of fEPSCs in GluR␦2-deficient/GluR␦2-hypomorphic mice, in which PF-PC synapse numbers are reduced and AMPA receptors are increased (Yamasaki et al, 2011). To record reliable sEPSCs, we applied a relatively strong stimulation, which evokes ϳ500 pA fEPSCs without spikes, for both wild-type and ho-4J slices (Fig.…”

“…An increase in the level of the major AMPA receptor subunit GluA2 was observed in ho-4J (Fig. 3A), which is consistent with a recent report (Yamasaki et al, 2011). Levels of PKC␥, TRPC3, GluR␦2, GluA2, PSD-95, and synaptophysin were not changed in the mGluR1-KO (Fig.…”

“…Indeed, synaptic responses mediated by AMPA receptors at PF-PC synapses are not changed (Kashiwabuchi et al, 1995) (Fig. 7) or may even be decreased , whereas AMPA receptor expression levels and the localization of AMPA receptors at PF-PC synapses are increased (Yamasaki et al, 2011) (Fig. 6).…”

Developmental Switching of Perisomatic Innervation from Climbing Fibers to Basket Cell Fibers in Cerebellar Purkinje Cells

“…Doublelabeling postembedding immunogold electron microscopy was performed by the two-step method as described previously (Yamasaki et al, 2011). In double labeling for receptors (or scaffolds) and terminal markers (see Figs.…”

“…In adulthood, glutamatergic CF synapses are virtually confined to dendritic spines expressing AMPA-type glutamate receptor GluA1-3 subunits (MasugiTokita et al, 2007;Yamasaki et al, 2011), while GABAergic BF synapses are on the flat somatic and dendritic surface, at which GABA A receptor ␣1 subunit (GABA A R␣1) is condensed Viltono et al, 2008). In GABA A R␣1 knock-out mice and tetrodotoxin-infused rats, heterologous GABAergic synapses are formed on to dendritic spines that express glutamate receptors GluA2/3 and GluD2 (Morando et al, 2001;Fritschy et al, 2006: Cesa et al, 2008.…”