Symposium 32 Co-sponsored by Scientific Research on Innovation Areas "Interplay of developmental clock and extracellular environment in brain formation"
Mechanisms of neuronal production and differentiation in the cerebral cortex
シンポジウム32
大脳皮質におけるニューロンの産生と分化のメカニズム |
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| SY32-1
Dynamic transcriptional control of neural stem cells
神経幹細胞のダイナミックな転写制御
Kageyama Ryoichiro（影山 龍一郎）
Institute for Frontier Life and Medical Sciences, Kyoto University, Kyoto, Japan
During brain development, neural stem cells proliferate intensively while they change their competency over time, giving rise to various types of neurons and glial cells sequentially. It is therefore very important to maintain neural stem cells until the final stage of development to generate a sufficient number of cells and a full diversity of cell types. We previously found that cell fate determination factors such as Hes1 and Ascl1/Mash1 are expressed in an oscillatory manner in neural stem cells but exhibit sustained expression during cell fate determination and differentiation, and that Hes1 and Ascl1 oscillations are important for proliferation of neural stem cells. The expression of the Notch ligand Delta-like1 (Dll1), which is controlled by Hes1 and Ascl1, is also oscillatory in neural stem cells, but when Dll1 oscillations are dampened, Hes1 oscillations are also dampened. Under this condition, proliferation of neural stem cells is impaired, which causes microcephaly. These results suggest that oscillatory expression is important for proliferation of active neural stem cells in the embryonic brain. By contrast, in quiescent neural stem cells, Hes1 expression is sustained, and Ascl1 expression is repressed. Furthermore, sustained Hes1 expression is sufficient to inhibit neurogenesis and maintain quiescent neural stem cells in the adult brain. These results suggest that oscillatory versus sustained Hes1 expression may be important for active versus quiescent neural stem cells. | | SY32-2
Revising the view of radial glia division; regenerative plasticity ensures radial glia proliferation during the early mouse brain development
増殖期神経幹細胞の構造的再生能: 対称分裂メカニズムの見直し
Matsuzaki Fumio（松崎 文雄），藤田 生水，楠本 史也，下向 篤紀，間瀬 俊，末次 妙子，今野 大治郎
RIKWN Ceter for Biosystems Dynamics Research, Kobe Japan
In the mammalian brain development, self-renewing progenitors, radial glia (RG), initially proliferate by symmetric divisions, and subsequently produce neurons or neuronally committed progenitors by asymmetric divisions. In the neurogenic phase, perturbation of normal spindle orientations in neurogenic RGs does not significantly affect self-renewal vs differentiation of the daughter cells, but induces RGs to translocate out of the ventricular zone by the frequent loss of the apical attachment On the other hand, the consensus for the proliferative stage has been that planar division orientations are critical for RGs to produce two self-renewing daughter cells, and that abnormal spindle orientations cause premature neurogenesis. Here we show that this accepted mechanism for proliferative divisions needs to be revised. We found that randomized divisions by the loss of LGN did not affect the daughter cell fate of RGs (with no premature neurogenesis) at the proliferative stage; instead, RGs that lose the apical or basal process by oblique divisions regenerate the missing process to recover the entire epithelial structure so that RGs generate two daughter RGs. This ability is, however, gradually diminished once neurogenesis begins in the mouse. This is also the case in the ferret that forms a gyrencephalic brain, suggesting that during mammalian evolution, RG’s ability to regenerate the epithelial organization was lost at the neurogenic stage. This would be an important event in the evolution of gyrencephalic brains, because the absence of the epithelial regeneration activity of RGs is a prerequisite for the formation of the outer/basal RGs from RGs in the ventricular zone. | | SY32-3
Growth cone subtype-specific molecular machinery controlling circuit development and diversity: “Subcellular RNA-proteome mapping”
Macklis Jeffrey D.
Max and Anne Wien Professor of Life Sciences, Dept. of Stem Cell and Regenerative Biology and Center for Brain Science, Harvard University, USA
Formation and function of circuits throughout the nervous system, within and from cerebral cortex in particular, relies critically on molecular machinery localized in growth cones (GCs) at tips of growing axons. Previously unknown subsets of neurons’ transcriptomes and proteomes localize to GCs to implement growth of axons toward specific targets, driving brain circuit development, function, likely maintenance, disease, failed regeneration. Previously, these subcellular and subtype-specific RNA/protein networks were not experimentally accessible directly from brain.
We developed subtype-specific GC purification and "subcellular RNA-proteome mapping" as a generalizable approach to investigate this molecular machinery. Applied to long-range axons of callosal projection neurons connecting the cortical hemispheres (also applied to other neurons), we identify that 1) native GCs possess remarkably rich molecular constituents for local protein synthesis, folding, turnover, suggesting function as "mini-cellular"/~autonomous units; 2) each subtype contains both distinct, subtype-specific machinery plus shared machinery; 3) hundreds of proteins and RNAs are enriched orders of magnitude in GCs compared to their own parent somata, indicating subcellular polarization of functions; 4) targeting motifs direct GC localization. We identify, e.g., that the hub mTOR pathway regulating cell growth is highly enriched in extending GCs compared with their own cell bodies, and that mRNA classes distribute within developing neurons based on a 5'TOP motif.
Beyond enabling identification of specific molecular substrates of circuit development, function, and mis-wiring causing neuronal circuit pathology, subcellular organization might also have important implications for degenerative diseases and regenerative strategies. It enables direct molecular investigation of subtype-specific GCs compared with GCs from mutant, regenerative, non-regenerative, or reprogrammed neurons to discover molecular mechanisms of circuit development, mis-wiring, lack of circuit/synaptic maintenance, and regeneration. The ability to directly compare multiple distinct GC-soma subtypes using multi-color sorting makes this approach broadly applicable to future studies in the fields of axon guidance, neurodevelopmental disorders, neurodegenerative disease, regeneration, and neuron reprogramming. | | SY32-4
Subtype Specification and Reprogramming of Cortical Neurons in Progenitor Cells and Postmitotic Immature Neurons
前駆細胞および分裂後の幼若ニューロンにおける大脳皮質ニューロンサブタイプ決定機構
Oishi Koji（大石 康二），仲嶋 一範
Department of Anatomy, Keio University School of Medicine
Although the mammalian cerebral cortex consists of hundreds of different types of neurons, the precise mechanisms that control generation of this diversity remain elusive. Given that different cortical subtypes are generated in a sequential manner, a long-standing hypothesis to account for this temporal fate specification process is that neural progenitor cells change their differentiation potential over time. However, it is also conceivable that cortical lamination or appropriate cell positioning in the cortical plate is required for correct differentiation of neurons. According to the latter hypothesis, we recently discovered that ‘future (normally destined to become)’ layer 4 neurons were able to acquire layer 2/3 characteristics when they were forced to locate in layer 2/3 (Oishi et al., 2016, eLIFE; Oishi et al., 2016, PNAS; Oishi and Nakajima, 2018, Neurochem Res). In the current study, we tried to extend the idea of subtype specification in postmitotic neurons by their environment to other subtypes. We found that disruption of the positioning of future layer 6 neurons dysregulated the fate specification process including upregulation of layer 5 markers such as Fezf2, a critical regulator for layer 5 specification. These results suggest that the precise control of extracellular environment surrounding neurons is required for correct differentiation of cortical neurons.I will also discuss the temporal specification process of neural progenitor cells, by which a cortical neuron subtype is controlled by multiple locking mechanisms involving both transcriptional and epigenetic processes. | |
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