Impaired parallel fiber-->Purkinje cell synapse stabilization during cerebellar development of mutant mice lacking the glutamate receptor delta2 subunit.

TLDR

GluRdelta2 is a Purkinje cell–specific receptor localized at parallel‑fiber synapses, and its loss markedly reduces these synapses. The study compared PF–PC synapse morphology and electrophysiology during development in GluRdelta2 mutant versus wild‑type cerebella to assess the receptor’s role in synaptogenesis. Synapse morphology and electrophysiology were examined across the first three postnatal weeks in mutant and wild‑type mice. Mutant PCs formed normal spines but had delayed and reduced PF synapse formation, with only 55–60 % of spines synapsing by week three versus 98–99 % in wild type, leading to roughly half the number of PF synapses per PC and weaker EPSCs.

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.