回路形成1
Circuit Formation 1
O2-7-5-1
stargazin (TARP γ-2)の欠損による小脳プルキンエ細胞でのカルシウムシグナルの減弱により登上線維刈り込みの後期過程が選択的に障害される
Reduced calcium signaling in cerebellar Purkinje cells by deletion of stargazin (TARP γ-2) leads to selective impairment of the late phase of climbing fiber synapse elimination

○川田慎也1, 橋本浩一1,2, 三國貴康1, 山崎真弥3, 宮崎太輔4, 山崎美和子4, 渡辺雅彦4, 崎村健司3, 狩野方伸1
○Shinya Kawata1, Kouichi Hashimoto1,2, Takayasu Mikuni1, Maya Yamazaki3, Taisuke Miyazaki4, Miwako Yamasaki4, Masahiko Watanabe4, Kenji Sakimura3, Masanobu Kano1
東京大学大学院 医学系研究科 神経生理学1, 広島大・医歯薬保健・神経生理2, 新潟大・脳研・細胞生物3, 北海道大・医・解剖4
Dept Neurophysiol, Univ of Tokyo, Tokyo1, Dept Neurophysiol, Grad Sch Biomed Sci, Hiroshima Univ, Hiroshima, Japan2, Dept Cell Neurobiol, Brain Res Inst, Niigata Univ, Niigata, Japan3, Dept Anat., Grad Sch Med, Hokkaido Univ, Sapporo, Japan4

In neonatal mouse cerebellum, the soma of each Purkinje cell (PC) is innervated by multiple climbing fibers (CFs) with similar synaptic strengths. During the first postnatal week, a single CF input becomes stronger relatively to other CF inputs in each PC (functional differentiation). Then, the strengthened CF translocates to PC's dendrites from P9 (CF translocation). In parallel, weaker CF synapses remaining on the soma are eliminated in two distinct phases, the early phase from P7 to P11 and the late phase from P12 to P16. In this study, we examined development of CF-PC synapses in mice with PC-selective deletion of stargazin (TARP γ-2), a major AMPA receptor auxiliary subunit in PC (γ-2 PC-KO mice). The absolute amplitudes of CF-mediated EPSCs of γ-2 PC-KO mice were about half of wild-type mice. Nevertheless, functional differentiation and the early phase of CF elimination were normal in γ-2 PC-KO mice. In contrast, CF translocation was slightly reduced and the late phase of CF elimination was significantly impaired. There were no changes in type 1 metabotropic glutamate receptor signaling and GABAergic inhibition to PCs, both of which are known to be crucial for the late phase of CF elimination. Therefore, the decrease of the absolute synaptic strength is thought to have affected the late phase of CF elimination. CF inputs are known to induce characteristic complex spikes and Ca2+ transients in PCs. We found that CF-induced Ca2+ transients were significantly smaller in γ-2 PC-KO mice than in wild-type mice. These results suggest that the reduced amplitudes of CF-mediated EPSCs in γ-2 PC-KO mice lead to smaller Ca2+ transients in PCs and eventually result in weaker activation of Ca2+-dependent signaling. We are now investigating whether Arc mediates Ca2+ signaling that is required for the late phase of CF elimination.
O2-7-5-2
Linxは内包を形成する神経束の“握手”相互作用を仲介する
Linx: a mediator of handshake axonal interactions forming the internal capsule

○萬代研二1, ドロシーライマールト2, デビットギンティー2
○Kenji Mandai1, Dorothy V Reimert2, David Ginty2
神戸大院・医・生化学分子生物学1, ジョンズホプキンス大学神経科学ハワードヒューズ医学研究所2
Dept Biochem and Mol Bio, Kobe Univ, Kobe1, Dept Neurosci, HHMI, Johns Hopkins Univ, Baltimore2

For many years, it has been a major goal of developmental neurobiology to identify the cellular and molecular mechanisms that coordinate the establishment of connectivity between the neocortex and subcortical structures. The main connections between neocortical and subcortical targets are carried via the ascending thalamocortical and descending corticofugal tracts that together form the internal capsule. According to the "handshake hypothesis" (reviewed in Molnar Z. et al., Eur. J. Neurosci. 35, 1573-1585, 2012), thalamocortical and corticofugal axons associate at the pallial-subpallial boundary (PSPB), and this association serves to guide their distal trajectories beyond the PSPB to appropriate cortical and subcortical targets. However, the identification of axonal proteins that control growth, guidance, and reciprocal interactions of thalamocortical and corticofugal axons has remained elusive. We show that the transmembrane protein Linx guides the extension of thalamocortical axons across the diencephalic-telencephalic boundary and subsequently at the PSPB. At the PSPB, Linx expressed on corticofugal axons binds to thalamocortical axons to mediate reciprocal interactions between these ascending and descending axons thus promoting their projections through the internal capsule. Thus, Linx is an essential mediator of the "handshake" between thalamocortical and corticofugal axons to form the internal capsule.
O2-7-5-3
細胞興奮性の制御は大脳皮質において神経細胞移動および樹状突起形成に重要である
Regulation of excitability is critical for the control of migration and dendrite formation in the developing cerebral cortex

○阪東勇輝1, 入江克雅2, 串田祐輝1, 下村拓史2, 藤吉好則2, 平野丈夫1, 田川義晃1
○Yuki Bando1, Katsumasa Irie2, Yuki Kushida1, Takushi Shimomura2, Yoshinori Fujiyoshi2, Tomoo Hirano1, Yoshiaki Tagawa1
京大院・理・生物物理1, 名古屋大・細胞生理学研究センター2
Dept. Biophys., Grad. Sch. Sci., Kyoto Univ., Kyoto, Japan1, CeSPI, Nagoya Univ., Nagoya, Japan2

Regulation of neural excitability is crucial for both mature and developing neural circuit. Previous studies have shown that immature cortical neurons such as those during migration are less excitable, mainly due to low-level expression of sodium channels. It is assumed that hyperexcitability during development has adverse effects on the formation of cortical circuits; however, how increased cellular excitability affects developmental processes such as migration remains unknown.Here, we examined the effect of hyperexcitability on the development of layer 2/3 cortical neurons in vivo. First, we found that neural activity is higher in neurons whose migration were completed than that during migration, using calcium imaging. We then used prokaryotic voltage-gated sodium channel NaChBac as a tool to elevate neural activity; NaChBac was ectopically expressed with a fluorescent marker in mouse cortical layer 2/3 neurons using in utero electroporation. Patch clamp recordings from NaChBac-expressing neurons showed large voltage-dependent current, suggesting that ectopically expressed NaChBac is functional in mammalian cortical layer 2/3 neurons. Calcium imaging showed that NaChBac expression drastically increased neural activity during migration. NaChBac expressing neurons showed severe migration defects: many neurons were located in the intermediate zone/lower cortical layers at postnatal day 3 when almost all control neurons already reached the upper cortical layers. Interestingly, migrating neurons expressing NaChBac had dendritic branches, and those branches were not attached with rafial glial fibers. This result implies that NaChBac expressing neurons start dendrite formation before reaching layer 2/3. These results together suggest that regulation of cellular excitability is critical for neuronal migration and dendrite formation; for proper migration, it may be important to keep neural activity at a low level.
O2-7-5-4
局所的カルシウム流入は樹状突起の選択的な刈り込みのトリガーとなる
Compartmentalized calcium transients trigger dendrite pruning in Drosophila sensory neurons

○金森崇浩1, 榎本和生1
○Takahiro Kanamori1, Kazuo Emoto1
(公財)大阪バイオサイエンス研究所 ・神経細胞生物学部門1
Department of Cell Biology, Osaka Bioscience Institute1

Dendrite pruning is a general mechanism for functional maturation of neural circuits; neurons prune their dendrites, without loss of themselves, to eliminate unwanted connections that are initially formed during development. Thus, proper dendrite pruning critically depends on local activation of the elimination machinery in unwanted dendrites, but our understanding of locally acting mechanisms involved in this process remains incomplete.
During Drosophila metamorphosis, larval-specific dendrites of class IV sensory neurons are pruned away and replaced by adult-specific processes. In this study, we report that compartmentalized calcium transients in dendritic branches act as temporal and spatial cues to trigger pruning. By performing long-term in vivo imaging, we show that calcium transients occur in dendritic branches, but not in the soma or axon which exhibits no pruning, at about 3 hours prior to branch elimination. The compartmentalized calcium transients are induced in part by a local increase of dendritic excitability, which thereby activates calcium influx via voltage-gated calcium channels (VGCCs); blockade of VGCC activity impairs dendrite pruning. Further genetic analyses suggest that the calcium-activated protease calpain functions downstream of the calcium transients to promote dendrite pruning. Our findings reveal the importance of compartmentalized sub-dendritic calcium signaling in spatio-temporally selective elimination of dendritic branches.
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