成年哺乳动物神经发生的研究进展

神经干细胞是一类具有自我更新能力和多向分化潜能的干细胞。在特定条件下,神经干细胞可分化为神经元、少突胶质细胞和星形胶质细胞从而参与神经功能的修复过程,该过程称为神经发生。一直以来,人们认为神经发生主要发生在哺乳动物胚胎时期,而成体是不存在神经发生的。然而近年的研究表明,成体神经发生在哺乳动物中枢神经系统中是终生存在的,且通过多种信号通路来调控。现就成年哺乳动物神经发生的研究进展展开论述。     

Noncanonical Sites of Adult Neurogenesis in the Mammalian Brain

Two decades after the discovery that neural stem cells (NSCs) populate some regions of the mammalian central nervous system (CNS), deep knowledge has been accumulated on their capacity to generate new neurons in the adult brain. This constitutive adult neurogenesis occurs throughout life primarily within remnants of the embryonic germinal layers known as “neurogenic sites.” Nevertheless, some processes of neurogliogenesis also occur in the CNS parenchyma commonly considered as “nonneurogenic.” This “noncanonical” cell genesis has been the object of many claims, some of which turned out to be not true. Indeed, it is often an “incomplete” process as to its final outcome, heterogeneous by several measures, including regional location, progenitor identity, and fate of the progeny. These aspects also strictly depend on the animal species, suggesting that persistent neurogenic processes have uniquely adapted to the brain anatomy of different mammals. Whereas some examples of noncanonical neurogenesis are strictly parenchymal, others also show stem cell niche-like features and a strong link with the ventricular cavities. This work will review results obtained in a research field that expanded from classic neurogenesis studies involving a variety of areas of the CNS outside of the subventricular zone (SVZ) and subgranular zone (SGZ). It will be highlighted how knowledge concerning noncanonical neurogenic areas is still incomplete owing to its regional and species-specific heterogeneity, and to objective difficulties still hampering its full identification and characterization.The central nervous system (CNS) of adult mammals is assembled during developmental neurogenesis, and its architectural specificity is maintained through a vast cohort of membrane-bound and extracellular matrix molecules (Gumbiner 1996; Bonfanti 2006). Although CNS structure is sculpted by experience-dependent synaptic plasticity at different postnatal developmental stages (critical periods) (see Sale et al. 2009) and, to a lesser extent, during adulthood (Holtmaat and Svoboda 2009), the neural networks are rather stabilized in the “mature” nervous tissue (Spolidoro et al. 2009). The differentiated cellular elements forming adult neural circuitries remain substantially unchanged in terms of their number and types, because cell renewal/addition in the CNS is very low. This situation is intuitive because connectional, neurochemical, and functional specificities are fundamental features of the mature CNS in highly complex brains, allowing specific cell types to be connected and to act in a relatively invariant way (Frotscher 1992).Since the discovery of neural stem cells (NSCs) (Reynolds and Weiss 1992), we realized that the aforementioned rules of CNS stability have a main exception in two brain regions: the forebrain subventricular zone (SVZ) (Lois and Alvarez-Buylla 1994) and the hippocampal subgranular zone (SGZ) (Gage 2000). These “adult neurogenic sites” are remnants of the embryonic germinal layers (although indirectly for the SGZ, which forms ectopically from the embryonic germinative matrix), which retain stem/progenitor cells within a special microenvironment, a “niche,” allowing and regulating NSC activity (Kriegstein and Alvarez-Buylla 2009). In addition, the areas of destination (olfactory bulb and dentate gyrus) reached by neuroblasts generated within these neurogenic sites harbor specific, not fully identified yet, environmental signals allowing the integration of young, newborn neurons. These two “canonical” sites of adult neurogenesis have been found in all animal species studied so far, including humans (reviewed in Lindsey and Tropepe 2006; Bonfanti and Ponti 2008; Kempermann 2012; Grandel and Brand 2013). Although in several classes of vertebrates including fish, amphibians, and reptiles, adult neurogenesis is widespread in many areas of the CNS (Zupanc 2006; Chapouton et al. 2007; Grandel and Brand 2013), in mammals, the vast majority of the brain and spinal cord regions out of the germinal-layer-derived neurogenic sites are commonly referred to as “nonneurogenic parenchyma” (Sohur et al. 2006; Bonfanti and Peretto 2011; Bonfanti and Nacher 2012). However, this viewpoint has changed during the last few years. New examples of cell genesis, involving both neurogenesis and gliogenesis, have been shown to occur in the so-called nonneurogenic regions of the mammalian CNS (Horner et al. 2000; Dayer et al. 2005; Kokoeva et al. 2005; Luzzati et al. 2006; Ponti et al. 2008; reviewed in Butt et al. 2005; Nishiyama et al. 2009; Migaud et al. 2010; Bonfanti and Peretto 2011), suggesting that structural plasticity involving de novo neural cell genesis could be more widespread than previously thought. Apart from their temporal persistence (some of them represent examples of delayed developmental neurogenesis, which persist postnatally; see below), neurogliogenic processes vary as to their regional localization, origin, and final outcome. In this review, “noncanonical” neurogenic processes occurring in adult mammals will be reviewed by underlining their heterogeneity across the species and their differences in intensity and outcome with respect to canonical neurogenic sites.

Table 1.

Main sites of noncanonical neurogenesis in the mammalian brain

Stab Wound Injury of the Zebrafish Adult Telencephalon: A Method to Investigate Vertebrate Brain Neurogenesis and Regeneration

Adult zebrafish have an amazing capacity to regenerate their central nervous system after injury. To investigate the cellular response and the molecular mechanisms involved in zebrafish adult central nervous system (CNS) regeneration and repair, we developed a zebrafish model of adult telencephalic injury.In this approach, we manually generate an injury by pushing an insulin syringe needle into the zebrafish adult telencephalon. At different post injury days, fish are sacrificed, their brains are dissected out and stained by immunohistochemistry and/or in situ hybridization (ISH) with appropriate markers to observe cell proliferation, gliogenesis, and neurogenesis. The contralateral unlesioned hemisphere serves as an internal control. This method combined for example with RNA deep sequencing can help to screen for new genes with a role in zebrafish adult telencephalon neurogenesis, regeneration, and repair.     

成年海马中神经发生及影响因素

动物成年后在其中枢神经系统内仍有神经发生。成年神经发生的主要区域是海马齿状回的颗粒下层和脑室下区的侧脑室外侧壁。目前认为成年后的海马神经发生参与记忆的形成,尤其对癫痫和神经退行性疾病的缓解和治疗具有重要意义。成年海马的神经发生受多种生理、病理因素的调控。我们就近年来成年海马神经发生的影响因素及其可能机制进行综述。     

成年哺乳动物皮质区神经发生的研究进展

神经发生是神经干细胞在适当的条件下分化成功能性整合神经元的过程,主要包括细胞的增殖、迁移、分化和存活。成年神经发生区以前脑室管膜下区(Subventricular zones,SVZ)和海马齿状回颗粒层下区(Subgranular zones,SGZ)为主,但皮质作为神经元和神经胶质细胞数量最多、分布最广,同时也是哺乳动物高度发展的脑区,是否有成年神经元新生,已成为近年来神经科学领域的研究热点[1,2]。现本文就未成熟神经元在皮质区的研究方法、分布、来源与转归、病理生理功能影响等方面探讨成年哺乳动物皮质神经发生现象。     

Differences in Mitotic Spindle Architecture in Mammalian Neural Stem Cells Influence Mitotic Accuracy during Brain Development

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成年鸟脑神经更替研究进展

成年脊椎动物脑中仍有自发神经更替现象的存在.新生神经元的类型,神经更替的过程及其影响因素,都与中枢神经系统的可塑性有密切的关系.成体神经发生、神经更替和脑的干细胞生物学将使分子生物学和临床医学得到长足的发展.不用通过外源细胞的移植,就能达到良好的治疗效果,尤其是某些神经退化病和中枢神经系统损伤.主要对以鸣禽鸣唱系统为模型的成年鸟脑神经发生及神经更替进行了综述.     

脑髓——成脑中多能性神经干细胞存在的区域

近来大量研究表明成年哺乳动物的脑中存在着具有增殖和分化潜能的多能性神经干细胞.含有这些神经干细胞的脑组织,由于它与造血的骨髓有许多共性,而称之为“脑髓”.研究成体脑中神经细胞的新生是神经科学中十分重要的领域.深入的研究将会促进利用成体脑中增殖神经元的迁移以及胚胎干细胞的移植来治疗神经退行性疾病.     

Neurogenesis in the adult Drosophila brain

Neurodegenerative diseases such as Alzheimer’s and Parkinson’s currently affect ∼25 million people worldwide. The global incidence of traumatic brain injury (TBI) is estimated at ∼70 million/year. Both neurodegenerative diseases and TBI remain without effective treatments. We are utilizing adult Drosophila melanogaster to investigate the mechanisms of brain regeneration with the long-term goal of identifying targets for neural regenerative therapies. We specifically focused on neurogenesis, i.e., the generation of new cells, as opposed to the regrowth of specific subcellular structures such as axons. Like mammals, Drosophila have few proliferating cells in the adult brain. Nonetheless, within 24 hours of a penetrating traumatic brain injury (PTBI) to the central brain, there is a significant increase in the number of proliferating cells. We subsequently detect both new glia and new neurons and the formation of new axon tracts that target appropriate brain regions. Glial cells divide rapidly upon injury to give rise to new glial cells. Other cells near the injury site upregulate neural progenitor genes including asense and deadpan and later give rise to the new neurons. Locomotor abnormalities observed after PTBI are reversed within 2 weeks of injury, supporting the idea that there is functional recovery. Together, these data indicate that adult Drosophila brains are capable of neuronal repair. We anticipate that this paradigm will facilitate the dissection of the mechanisms of neural regeneration and that these processes will be relevant to human brain repair.     

Regulation of cerebral cortex size and folding by expansion of basal progenitors

Size and folding of the cerebral cortex increased massively during mammalian evolution leading to the current diversity of brain morphologies. Various subtypes of neural stem and progenitor cells have been proposed to contribute differently in regulating thickness or folding of the cerebral cortex during development, but their specific roles have not been demonstrated. We report that the controlled expansion of unipotent basal progenitors in mouse embryos led to megalencephaly, with increased surface area of the cerebral cortex, but not to cortical folding. In contrast, expansion of multipotent basal progenitors in the naturally gyrencephalic ferret was sufficient to drive the formation of additional folds and fissures. In both models, changes occurred while preserving a structurally normal, six‐layered cortex. Our results are the first experimental demonstration of specific and distinct roles for basal progenitor subtypes in regulating cerebral cortex size and folding during development underlying the superior intellectual capability acquired by higher mammals during evolution.     

Adult Neurogenesis Is Sustained by Symmetric Self-Renewal and Differentiation

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Control of Cell Survival in Adult Mammalian Neurogenesis

The fact that continuous proliferation of stem cells and progenitors, as well as the production of new neurons, occurs in the adult mammalian central nervous system (CNS) raises several basic questions concerning the number of neurons required in a particular system. Can we observe continued growth of brain regions that sustain neurogenesis? Or does an elimination mechanism exist to maintain a constant number of cells? If so, are old neurons replaced, or are the new neurons competing for limited network access among each other? What signals support their survival and integration and what factors are responsible for their elimination? This review will address these and other questions regarding regulatory mechanisms that control cell-death and cell-survival mechanisms during neurogenesis in the intact adult mammalian brain.     

Mosaic Heterochrony in Neural Progenitors Sustains Accelerated Brain Growth and Neurogenesis in the Juvenile Killifish N. furzeri

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小鼠脑室内注射神经干细胞裂解液促进谷氨酸盐诱导的兴奋性神经元损伤的修复

先前的研究已经证实,阿魏酸钠诱导分化的PC12细胞裂解液的无细胞滤液具有改善抑郁症样模型大鼠的行为学障碍、上调其海马和大脑皮质神经生长因子(nerve growth factor,NGF)和脑源性神经营养因子(brain-derived neurotrophic factor,BDNF)的表达、增加海马神经干细胞(neural stem cells,NSCs)/神经前体细胞(neural progenitor cells)增殖的效果。该研究的目的在于探讨神经干细胞裂解液的无细胞滤液(cell-free filtrate ofneural stem cell lysates,FNSCL)脑室内注射促进谷氨酸盐诱导的成年小鼠兴奋性神经元损伤修复的可能性。成年小鼠谷氨酸单钠(monosodiumglutamate,MSG,2.0g/(kg·d))灌胃,连续10日,造成兴奋性神经元损伤模型。自孕15 d的昆明种小鼠取胎脑,分离、培养神经干细胞,免疫细胞化学法检测巢蛋白(nestin)抗原,制备神经干细胞裂解液的无细胞滤液。MSG+NSCs组动物在MSG灌胃后接收脑室内NSCs移植,MSG+FNSCL组动物在MSG灌胃...     

叶酸联合成体神经干细胞治疗创伤性脑损伤大鼠的实验

目的观察叶酸联合成体神经干细胞对创伤性脑损伤大鼠的治疗作用,探讨其可能作用机制。方法 120只Wistar大鼠随机分为6组,正常组,模型组,假手术组,叶酸注射组,成体神经干细胞移植组,成体神经干细胞移植＋叶酸注射组。倒置显微镜下观察神经干细胞形态学变化;流式细胞仪检测神经干细胞表面标记物CD105、CD45、CD44、CD29的表达;免疫荧光法检测神经元特异性烯醇酶（NSE成熟神经元的特异性标志）、胶质纤维酸性蛋白（GFAP胶质细胞的标记物）的表达;平衡木实验检测大鼠运动协调与整和能力;Morris水迷宫实验测试各组大鼠的学习记忆能力;HE染色及Brdu免疫组化实验观察脑组织形态学变化;酶联免疫吸附试验检测大鼠脑组中脑源性神经生长因子（BDNF）、神经生长因子（NGF）的表达;蛋白质印迹法检测脑组织中凋亡相关蛋白BCL-2、Bax、Caspase-3的表达。结果分离所得细胞能在体外传代培养,流式细胞仪检测发现细胞阳性表达CD44、CD29,阴性表达CD105、CD45,细胞经胎牛血清诱导分化后能形成NSE或GFAP阳性细胞。实验表明,叶酸与成体神经干细胞干预创伤性脑损伤大鼠模型后能显著改善其行为学变化,减轻脑组织的炎症反应,恢复受损神经细胞,增加脑组织内BDNF、NGF的含量,上调BCL-2的表达,下调Bax、Caspase-3的表达。结论叶酸联合成体神经干细胞干预创伤性脑损伤大鼠能显著改善中枢神经功能,对维持神经元微环境稳态具有重要的作用。