Ichikawa Ryoichi, Hashimoto Kouichi, Miyazaki Taisuke, Uchigashima Motokazu, Yamasaki Miwako, Aiba Atsu, Kano Masanobu, Watanabe Masahiko
Department of Anatomy, Sapporo Medical University School of Medicine, Sapporo 060-8556, Japan;
Department of Neurophysiology, Graduate School of Biomedical & Health Sciences, Hiroshima University, Hiroshima 734-8551, Japan;
Proc Natl Acad Sci U S A. 2016 Feb 23;113(8):2282-7. doi: 10.1073/pnas.1511513113. Epub 2016 Feb 8.
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.
在小脑的浦肯野细胞（PCs）中，单一的“获胜”攀缘纤维（CF）独占近端树突，而成千上万的平行纤维（PFs）支配远端树突，并且CF和PF输入都支配一个狭窄的中间区域。目前尚不清楚这种分离的CF和PF神经支配是如何在PC树突上建立的。通过连续电子显微镜对树突神经支配进行重建，我们发现，在小鼠出生后第9至15天，由于重叠神经支配区域的扩大，CF和PF的神经支配区域都大力扩展。从出生后第15天起，这些区域开始分离，重叠的近端区域显著缩短。因此，异源输入的神经支配区域在出生后早期得到细化。有趣的是，在缺乏1型代谢型谷氨酸受体（mGluR1）或蛋白激酶Cγ（PKCγ）的突变小鼠中，这种转变被阻止，导致异常扩大的重叠区域持续存在。通过慢病毒介导将mGluR1α表达至mGluR1缺陷的PCs中，可以挽救这种停滞的区域细化。在挽救的PCs的近端树突处，PF突触被消除，取而代之的是出现游离棘突，而CF突触的数量和密度不变。由于mGluR1-PKCγ信号通路对于CF突触消除的后期阶段也至关重要，因此该信号通路促进了PCs中兴奋性突触连接的两个关键特征，即通过消除来自胞体的多余CF突触实现CF单神经支配，以及通过消除来自近端树突的竞争性PF突触实现CF和PF神经支配的分离区域。
