Territories of heterologous inputs onto Purkinje cell dendrites are segregated by mGluR1-dependent parallel fiber synapse elimination

Abstract

In Purkinje cells (PCs) of the cerebellum, a single "winner" climbing fiber (CF) monopolizes proximal dendrites, whereas hundreds of thousands of parallel fibers (PFs) innervate distal dendrites, and both CF and PF inputs innervate a narrow intermediate domain. It is unclear how this segregated CF and PF innervation is established on PC dendrites. Through reconstruction of dendritic innervation by serial electron microscopy, we show that from postnatal day 9-15 in mice, both CF and PF innervation territories vigorously expand because of an enlargement of the region of overlapping innervation. From postnatal day 15 onwards, segregation of these territories occurs with robust shortening of the overlapping proximal region. Thus, innervation territories by the heterologous inputs are refined during the early postnatal period. Intriguingly, this transition is arrested in mutant mice lacking the type 1 metabotropic glutamate receptor (mGluR1) or protein kinase Cγ (PKCγ), resulting in the persistence of an abnormally expanded overlapping region. This arrested territory refinement is rescued by lentivirus-mediated expression of mGluR1α into mGluR1-deficient PCs. At the proximal dendrite of rescued PCs, PF synapses are eliminated and free spines emerge instead, whereas the number and density of CF synapses are unchanged. Because the mGluR1-PKCγ signaling pathway is also essential for the late-phase of CF synapse elimination, this signaling pathway promotes the two key features of excitatory synaptic wiring in PCs, namely CF monoinnervation by eliminating redundant CF synapses from the soma, and segregated territories of CF and PF innervation by eliminating competing PF synapses from proximal dendrites.cerebellum | Purkinje cell | climbing fiber | parallel fiber synapse elimination

Glutamate transporter GLAST controls synaptic wrapping by Bergmann glia and ensures proper wiring of Purkinje cells

“…This wiring pattern is analogous to that observed in the developing cerebellum, where a single strengthened CF is not yet strong enough to eliminate the other weaker CFs from PC somata (7), and PF synapses cover the entire dendritic tree (45). To establish CF monoinnervation and segregated PF/CF territories, proper activations of P/Q-type Ca 2+ channels and the type 1 metabotropic glutamate receptor-Gαq-PLCβ4-protein kinase Cγ signaling pathway by glutamatergic inputs are crucial (8,(15)(16)(17). Therefore, one factor likely contributing to the impaired synaptic competition in GLAST-KO mice is the elevated [Glu] e , which may alter the balance of activation between glutamatergic synapses in favor of the inputs that should normally be eliminated over the ones that should remain or be further strengthened.…”

“…1 K-M and Fig. S1), when active synaptogenesis is occurring in rodents (17,30). Therefore, GLAST ablation impairs differentiation of fine BG processes and their wrapping of PC dendrites and synapses.…”

“…The late phase of CF synapse elimination critically depends on PF synapse formation, in which the GluD2-Cbln1-neurexin adhesion system ensures structural connectivity of PF-PC synapses (9-11), whereas the type 1 metabotropic glutamate receptor-protein kinase Cγ signaling pathway mediates the activity of PF-PC synapses for CF synapse elimination (12)(13)(14)(15)(16). This signaling pathway also regulates territorial segregation by eliminating PF synapses from proximal dendrites (17). Thus, activity-dependent [Ca 2+ ] i regulation and inputselective synaptic adhesion cooperate to sculpt PC circuitry properly.…”

GluD2 Endows Parallel Fiber–Purkinje Cell Synapses with a High Regenerative Capacity

“…Spiny branchlets in the distal compartment are innervated by 10 5 -10 6 parallel fibers (PFs), T-shaped granule cell axons running in the transverse plane, but each PF usually forms only one synapse onto a given PC (Palay and ChanPalay, 1974;Napper and Harvey, 1988;Kurihara et al, 1997). In comparison, a single climbing fiber (CF) originating from the inferior olivary nucleus forms hundreds of synapses onto a single PC by twisting around the dendritic shafts in the proximal compartment (Crepel et al, 1981;Rossi et al, 1993;Ichikawa et al, 2002). The segregated innervation territories of the two orthog-onal afferents and the mono-innervation by single CFs are the two distinguishing features of excitatory synaptic wiring on PCs, and are established in the first 3-4 postnatal weeks in rodents (Altman, 1972;Watanabe and Kano, 2011).…”

“…In comparison, a single climbing fiber (CF) originating from the inferior olivary nucleus forms hundreds of synapses onto a single PC by twisting around the dendritic shafts in the proximal compartment (Crepel et al, 1981;Rossi et al, 1993;Ichikawa et al, 2002). The segregated innervation territories of the two orthog-onal afferents and the mono-innervation by single CFs are the two distinguishing features of excitatory synaptic wiring on PCs, and are established in the first 3-4 postnatal weeks in rodents (Altman, 1972;Watanabe and Kano, 2011). These salient features are thought to be the neural basis for information processing and integration underlying cerebellar motor function and learning (Ito, 2012).…”