Distal Extension of Climbing Fiber Territory and Multiple Innervation Caused by Aberrant Wiring to Adjacent Spiny Branchlets in Cerebellar Purkinje Cells Lacking Glutamate Receptor δ2

Abstract

Organized synapse formation on to Purkinje cell (PC) dendrites by parallel fibers (PFs) and climbing fibers (CFs) is crucial for cerebellar function. In PCs lacking glutamate receptor delta2 (GluRdelta2), PF synapses are reduced in number, numerous free spines emerge, and multiple CF innervation persists to adulthood. In the present study, we conducted anterograde and immunohistochemical labelings to investigate how CFs innervate PC dendrites under weakened synaptogenesis by PFs. In the GluRdelta2 knock-out mouse, CFs were distributed in the molecular layer more closely to the pial surface compared with the wild-type mouse. Serial electron microscopy demonstrated that CFs in the knock-out mouse innervated all spines protruding from proximal dendrites of PCs, as did those in the wild-type mouse. In the knock-out mouse, however, CF innervation extended distally to spiny branchlets, where nearly half of the spines were free of innervation in contrast to complete synapse formation by PFs in the wild-type mouse. Furthermore, from the end point of innervation, CFs aberrantly jumped to form ectopic synapses on adjacent spiny branchlets, whose proximal portions were often innervated by different CFs. Without GluRdelta2, CFs are thus able to expand their territory along and beyond dendritic trees of the target PC, resulting in persistent surplus CFs by innervating the distal dendritic segment. We conclude that GluRdelta2 is essential to restrict CF innervation to the proximal dendritic segment, by which territorized innervation by PFs and CFs is properly structured and the formation of excess CF wiring to adjacent PCs is suppressed.

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

“…Dextran Alexa 594 (DA-594; Invitrogen) was injected into the inferior olive to label CFs as described (46). After 4 d of survival, mice were fixed by transcardial perfusion and parasagittal cerebellar sections (50 μm in thickness) were cut (11,15,25). Sections were either doubleimmunofluorescent labeled for calbindin and VGluT2 or triple-immunofluorescent labeled for calbindin, VGluT2, and GLT-1.…”

“…CF elimination consists of two distinct phases in mice, the early phase from P7 and the late phase from around P12 (12), which involve distinct mechanisms (9,12). Whereas several factors crucial for the late phase of CF synapse elimination have been disclosed (13)(14)(15)(16)(17)(18)(19)(20), mechanisms of biased strengthening of single CFs and the early phase of CF synapse elimination are unknown. Because synapse refinement critically depends on neuronal activity (1,4,5,16,21,22), we studied whether the P/Q-type voltage-dependent Ca 2+ channel (VDCC), the major high-threshold VDCC in PCs (23), is involved in the development of CF to PC synapses.…”

Type-1 metabotropic glutamate receptor in cerebellar Purkinje cells: a key molecule responsible for long-term depression, endocannabinoid signalling and synapse elimination

“…In the adult cerebellum, parallel fibres and a climbing fibre normally make synapses on distal and proximal dendrites, respectively. In the GluRd2 knockout mice, climbing fibres invade into the distal dendrites and form ectopic synapses there (Hashimoto et al 2001a;Ichikawa et al 2002). These ectopic climbing fibre synapses appear around P10 when parallel fibre synapse formation and Purkinje cell dendritic arborization occur most vigorously.…”

Glial Glutamate Transporters Maintain One-to-One Relationship at the Climbing Fiber-Purkinje Cell Synapse by Preventing Glutamate Spillover

“…Such a distinct electrotonic length from soma to the site of the synapse is one of the reasons for the slower 10 -90% rise time of PF-mediated EPSCs (PF-EPSCs) compared with those of CF-EPSCs (Llano et al, 1991;Aiba et al, 1994;Kano et al, 1995Kano et al, , 1997. In fact, in PCs lacking glutamate receptor ␦2 (GluR␦2), additional CF synapses at distal dendrites cause atypical CF-EPSCs with slow rise time (Hashimoto et al, 2001;Ichikawa et al, 2002). In addition, the passive membrane properties would affect the rise time of EPSCs (Llano et al, 1991).…”