Impaired Parallel Fiber→Purkinje Cell Synapse Stabilization during Cerebellar Development of Mutant Mice Lacking the Glutamate Receptor δ2 Subunit

Abstract

The glutamate receptor delta2 subunit (GluRdelta2) is specifically expressed in cerebellar Purkinje cells (PCs) from early developmental stages and is selectively localized at dendritic spines forming synapses with parallel fibers (PFs). Targeted disruption of the GluRdelta2 gene leads to a significant reduction of PF-->PC synapses. To address its role in the synaptogenesis, the morphology and electrophysiology of PF-->PC synapses were comparatively examined in developing GluRdelta2 mutant and wild-type cerebella. PCs in GluRdelta2 mutant mice were normally produced, migrated, and formed spines, as did those in wild-type mice. At the end of the first postnatal week, 74-78% of PC spines in both mice formed immature synapses, which were characterized by small synaptic contact, few synaptic vesicles, and incomplete surrounding by astroglial processes, eliciting little electrophysiological response. During the second and third postnatal weeks when spines and terminals are actively generated, the percentage of PC spines forming synapses attained 98-99% in wild type but remained as low as 55-60% in mutants, and the rest were unattached to any nerve terminals. As a result, the number of PF synapses per single-mutant PCs was reduced to nearly a half-level of wild-type PCs. Parallelly, PF stimulation less effectively elicited EPSCs in mutant PCs than in wild-type PCs during and after the second postnatal week. These results suggest that the GluRdelta2 is involved in the stabilization and strengthening of synaptic connectivity between PFs and PCs, leading to the association of all PC spines with PF terminals to form functionally mature synapses.

Role of glutamate δ-2 receptors in activity-dependent competition between heterologous afferent fibers

“…On the other hand, in ␦-2-deficient mice, the number of PF to PC synapses is reduced to nearly half, although spine density is normal. Therefore, this receptor is involved in the stabilization and strengthening of this specific synaptic connection (21,22).…”

“…Thus, we suggest that ␦-2 expression is an intrinsic property of all PC spines, independent from the afferent type. Because ␦-2 receptors are involved in stabilizing and strengthening PF-PC synapses (21,22), the competitive advantage of the PFs over the CFs in the absence of activity is attributable to the activityindependent expression of these receptors by the PCs. The low or absent expression of ␦-2 receptors in primary dendritic spines under normal conditions is the result of a selective repression caused by activity at synapses other than that of PFs.…”

Postsynaptic P/Q-type Ca 2+ channel in Purkinje cell mediates synaptic competition and elimination in developing cerebellum

“…These results suggest that initial synapse formation by multiple CFs is almost normal, but the following elimination process from P7 is severely impaired in PC-Ca v 2.1 KO mice. This developmental period is before the robust formation of parallel fiber (PF) synapses (36)(37)(38)(39). The impairment of CF synapse elimination becomes evident clearly earlier than that of the knockout mice deficient in the mGluR1 signaling cascade in which the late phase is selectively impaired (17)(18)(19)(20).…”

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

“…At PF-PC synapses, GluR␦2 plays a key role in the formation and maintenance of PF-PC synapses (Kashiwabuchi et al, 1995;Kurihara et al, 1997;Lalouette et al, 2001;Takeuchi et al, 2005;Matsuda et al, 2010;Uemura et al, 2010). Furthermore, GluR␦2 regulates endocytosis of AMPARs (Hirai et al, 2003) and mediates long-term depression (LTD) at PF-PC synapses (Kashiwabuchi et al, 1995).…”