シンポジウム（Symposium）

Symposium Understanding the homeostatic maintenance mechanisms and logistics of cellular community in the brain "Brain Infrastructure" シンポジウム脳細胞社会の恒常性維持機構と物流システムの解明＜脳内インフラストラクチャー＞

7月25日（木）14:40～15:00　第4会場（朱鷺メッセ　3F　301） 1S04a-1 神経の発達と変性に関わる脳内免疫システム Shogo Tanabe（田辺章悟）国立精神・神経セ神経研神経薬理 Brain homeostasis is maintained by various biological systems such as vascular system, glymphatic system and immune system. The disruption of these systems results in the breakdown of brain homeostasis, leading to the neurological disorders. In particular, recent studies demonstrated that immune system in the brain, called neuroimmune system, is a key player in neurodevelopment and neurodegeneration. In this study, we examined the contribution of neuroimmune system, especially lymphocytes, to brain development and neurological disorders. In the developing brain, development of neural cells is rigorously regulated by several factors including immune system. Immune-related molecules and microglia, resident immune cells in central nervous system, are known to contribute to brain development. However, it remains unclear whether peripheral lymphocytes mediate brain development. We investigated the localization and subtype of lymphocytes in the developing brain and found that that large amounts of B cells are present in meningeal space and choroid plexus of neonatal brain. Gene expression analysis showed the subtype of B cells in neonatal brain is B-1a cells. Functional blocking experiment demonstrated that B-1a cells are involved in oligodendrogenesis by promoting the proliferation of oligodendrocyte progenitors. These results suggest that unique immune system is formed in the neonatal brain and associated with brain development. These findings prompt us to investigate whether immune system in neonatal brain is also involved in the pathogenesis of neurodevelopmental disorders. To resolve this, we examined the proportion and subtype of immune cells in pathological condition of neonatal brain and investigate the role of immune cells in the pathogenesis by blocking experiments. In this symposium, we will introduce the contribution of neuroimmune system to brain development and degeneration.		7月25日（木）15:00～15:20　第4会場（朱鷺メッセ　3F　301） 1S04a-2 ミクログリアとアストロサイトの貪食作用の補完性 Hiroyuki Konishi（小西博之）¹，Katsuaki Sato（佐藤克明）²，Hiroshi Kiyama（木山博資）¹ ¹名古屋大院医機能組織 ²宮崎大医免疫 Microglia are considered the main phagocytes in the CNS. We recently established microglia ablation model, in which gene encoding diphtheria toxin receptor is knocked into microglia-specific gene locus Siglech in mice. In this model, microglial debris completely disappeared even under the absence of functional microglia, raising a question how the debris were cleared. Other CNS-related mononuclear phagocytes, such as perivascular macrophages and circulating monocytes, were alive in this ablation model. However, those mononuclear cells did not participate in the clearance, suggesting that non-professional phagocytes in the brain play roles. We found that astrocytes became activated with upregulation of GFAP upon microglial ablation. Confocal and electron microscopy demonstrated that the hypertrophic astrocytes extended their processes to microglial debris, and phagocytosed the debris. Co-culture of astrocytes with apoptotic microglia showed that microglial debris were engulfed by astrocytes via TAM family of phagocytic receptors. Finally, we explored physiological roles of astrocytic phagocytic activity, and found that astrocytes engulfed natural apoptotic cells in some mutant mice with phagocytosis-deficient microglia, while microglia phagocytosed the apoptotic cells in wild-type mice. Collectively, astrocytes compensate for the dysfunction of microglial phagocytic activity. The complementarity may contribute to the maintenance of brain homeostasis.

7月25日（木）15:20～15:40　第4会場（朱鷺メッセ　3F　301） 1S04a-3 シナプス除去におけるグリア・神経相互作用 Ryuta Koyama（小山隆太）東京大院薬薬品作用 The brain-resident immune cells microglia could phagocytose synapses to prune them. Phagocytosis of synapses by microglia, which is regulated by find-me and eat-me signals, has been shown to be fundamental for the reorganization of neural circuits both in health and disease. The complement C1q has been highlighted to serve as both find-me and eat-me signals: less active synapses are opsonized by C1q secreted from more active synapses and microglia detect accumulated C1q on the synapses to engulf them. However, we found that, under certain circumstances such as neonatal seizures, C1q can be spread randomly in brain parenchyma. The finding led us to investigate the mechanisms how microglia decide which synapse to phagocytose. Here, we established a live imaging system of microglia-synapse interactions in vitro in which neuronal activity can be modulated. Microglia were prepared from CX3CR1GFP/+ mice and cultured with both neurons and astrocytes so that the real-time imaging of ramified morphology of microglia is possible. Additionally, neurons were transfected with synaptophysin-mCherry to label pre-synaptic structures and/or DREADD proteins to regulate their activity. Using this coculture system, we successfully captured an actual moment of synaptic pruning by microglia. We further found that microglia contacted neurons more frequently and engulfed more synaptic puncta when neurons were activated. Next, we used a mouse model of febrile seizures which develops epilepsy after hyperthermia-induced seizures in neonatal periods to examine the role of microglia and neuronal activity in the synapse excitatory/inhibitory imbalance in vivo. We found that C1q was robustly deposited on both the excitatory and inhibitory synapses in the dentate gyrus after hyperthermia-induced seizures, while mainly the inhibitory but not the excitatory synapses were pruned by microglia, resulting in the hyperactivity of dentate neural circuits. Using the DREADD system, we determined that increased activity of the dentate inhibitory neurons during hyperthermia-induced seizures resulted in the preferable interaction between microglia and inhibitory synapses. Thus, it is likely that microglia contact more active synapses and engulf them only when C1q is present. Overall, our findings suggest that C1q could serve as an eat-me signal but not a find-me signal and that increased neuronal activity itself serves as a find-me signal in the process of synaptic pruning by microglia.		7月25日（木）15:40～16:00　第4会場（朱鷺メッセ　3F　301） 1S04a-4 ミクログリアによるアミロイドβの認識・応答とその分子機構 Sho Takatori（高鳥翔）¹，Akihiro Iguchi（井口明優）¹，Shingo Kimura（木村新伍）¹，Junko Sasaki（佐々木純子）²，Takehiko Sasaki（佐々木雄彦）²，Toshiyuki Takai（高井俊行）³，Takashi Saito（斉藤貴志）⁴，Takaomi C Saido（西道隆臣）⁴，Taisuke Tomita（富田泰輔）¹ ¹東京大院薬機能病態学 ²東京医歯大難治研病態生理化学 ³東北大加齢研遺伝子導入研究分野 ⁴理研CBS 神経老化制御 Alzheimer disease (AD), the most common neurodegenerative disease with dementia, is primarily caused by an aberrant deposition of amyloid β peptide (Aβ) in the brain. However, the identification of AD risk loci encoding microglia-specific proteins such as TREM2 has revealed an unexpected role of microglia, which are crucially important in the maintenance of brain infrastructure. TREM2, together with an essential adaptor TYROBP, promotes a microglial clustering around Aβ plaques and regulates their response to Aβ. However, the downstream signaling mechanism of TREM2 and TYROBP in the microglia has remained unknown. INPP5D, another AD risk gene, encodes a lipid phosphatase and is expressed only in microglia in the central nervous system. INPP5D has inhibitory roles to several receptor functions by dephosphorylating a lipid second messenger, PtdIns(3,4,5)P₃, suggesting that INPP5D regulates microglial responses against Aβ. To test this, we crossed Inpp5d knockout mice with AD model mice. We found that heterozygous deletion of Inpp5d augmented the microglial clustering around Aβ plaques, indicating an inhibitory role of INPP5D in the proliferation/activation of microglia. We then analyzed the Inpp5d^+/-; Tyrobp^-/- AD model mice. Importantly, Inpp5d-haplodeficiency significantly rescued the association of microglia to Aβ plaques, which was diminished in Tyrobp-deficient mice. However, the dystrophic neurite formation in the vicinity of Aβ plaques, which was increased by deletion of Tyrobp, was not reverted in the double knockout mice. Thus, the microglial clustering upregulated by Inpp5d-deficiency was not sufficient for the protection of the surrounding neurons. These results raise an interesting possibility that the mechanisms underlying the microglial clustering and neuroprotective function around Aβ plaques depend on different molecular entities. Future studies would clarify what factors are required for the execution of the neuroprotective functions by microglia.

7月25日（木）16:00～16:20　第4会場（朱鷺メッセ　3F　301） 1S04a-5 グルタチオンによる脳細胞恒常性維持とアルツハイマー病について Shoko Hashimoto（橋本翔子），Yukio Matsuba（松葉由紀夫），Naoko Kamano（釜野直子），Takashi Saito（斉藤貴志），Takaomi C Saido（西道隆臣）理化学研究所　脳神経科学研究センター Oxidative stress is demonstrated to play an important role in the etiology of Alzheimer's disease (AD). In order to defense against oxidative stress, organisms possess glutathione as an important antioxidant. However, glutathione level is decreased with ageing and progression of diseases including AD. We found that glutathione level is also lowered in App-knockin (App-KI) mice, which we have generated as a next-generation AD mouse model, compared to wild-type. In this study, we elucidated the effect of glutathione reduction on AD progression. Glutamyl-Cysteine ligase (GCL), which consists of catalytic subunit (GCLC) and modifier subunit (GCLM), acts as a rate-limiting enzyme of glutathione synthesis. We first examined GCLC levels in App-KI and human AD samples. The GCLC expression levels were significantly decreased in App-KI and postmortem AD brain. Moreover, we elucidate the effect of GCLC decline on brain function, we analyzed the brain pathologies in brain specific-conditional knockout mouse of GCLC (GCLC ^flox/flox X CamKII-Cre). In 3-months old GCLC ^flox/flox X CamKII-Cre mice, we could see severe activation of astrocyte and microglia. In 8-months-old, we observed significant brain atrophy caused by caspase-3 mediated neuronal cell death. These results suggest that GCLC deficiency-induced neuroinflammation exerts cytotoxicity. From these findings, we concluded that neuroinflammation induced by amyloid pathology decreased glutathione level, and resulted in further activation of neuroinflammation and neuronal cell death. Further, we will also discuss the mechanistic interaction between GCLC-deficiency mediated neuroinflammation and neurodegeneration. In this session, we would like to discuss relationship between neudegeneration due to glutathione exhaustion and pathogenesis of Alzheimer's disease.		7月25日（木）16:20～16:40　第4会場（朱鷺メッセ　3F　301） 1S04a-6 脳内リンパ系を介したタウ・αシヌクレイン排泄の生体マルチスケールイメージング Hiroyuki Takuwa（田桑弘之）国立研究開発法人量子科学技術研究開発機構 Accumulations of tau protein and alpha-synuclein aggregates are mechanistically implicated in Alzheimer's disease and Parkinson's disease, respectively. Several researchers have recently demonstrated that tau proteins can transfer between neurons in a manner dependent on neural activations. Therefore, continuous removal of tau protein released from activated neurons is important for homeostatic protection of neurons against tau-induced toxicities. In the present study, we investigated mechanisms and pathways involved in the clearance of tau and alpha-synuclein assemblies from the brain using PBB3 and its analog as bimodal optical and radiological imaging agents for these aggregates. In optical imaging, neurons, glia cells and macrophages were also labeled with fluorescent protein expressed in these cells via an adeno-associated viral (AAV) vector. Longitudinal, multiscale measurements of the living mouse brains with macroscopic positron emission tomography (PET) and wide-field-of-view two-photon microscope (a. k. a. multiphoton mesoscope) have revealed that tau and alpha-synuclein fibrils injected into the brain parenchyma are transferred to the glymphatic system and subsequently to cerebral blood vessels, and this removal is mediated by astrocytes and immune cells such as perivascular macrophages. In addition, the animal PET and two-photon microscopic studies have indicated that mechanical blockade of the glymphatic flow by compression of the brain surface accelerates tau depositions in a tau transgenic model and induces neuronal alpha-synuclein accumulations in a wild-type mouse. These findings suggest that misfolded tau and alpha-synuclein species are excreted from neurons and are homeostatically cleared through the glymphatic and epidural lymphatic pathways in both normal physiological and diseased conditions, and deteriorations of these clearance processes lead to intraneuronal accumulations of fibrils composed of these proteins.