Neural stem cells and neurospheres--re-evaluating the relationship

For most of the past century, the prospect of replacing lost or damaged cells in the central nervous system (CNS) was hampered by the opinion that the adult mammalian CNS was incapable of generating new nerve cells. This belief, like most dogmas, was essentially founded on a lack of experimental evidence to the contrary. The overturning of this 'no new neuron' hypothesis began midway through the twentieth century with a series of reports documenting neurogenesis in the postnatal and adult brain, continued with the isolation and in vitro culture of neurogenic cells from the adult mammalian brain, and culminated in the discovery of a population of multipotent, self-renewing cells in the adult CNS (that is, bona fide neural stem cells). Although a variety of techniques were initially used, the neurosphere assay (NSA) rapidly emerged as the assay of choice and has since become a valuable tool for isolating, and understanding the biology of, embryonic and adult CNS stem cells. Like all technologies, it is not without its limitations. In this article we will highlight several shortcomings of the assay related to its application and interpretation that we believe have led to a significant body of research whose conclusions may well be misleading.     

Mechanism of stem cells in the central nervous system

Injury to the central nervous system (CNS) can result in severe functional impairment. The brain and spinal cord, which constitute the CNS, have been viewed for decades as having a very limited capacity for regeneration. However, over the last several years, the body of evidence supporting the concept of regeneration and continuous renewal of neurons in specific regions of the CNS has increased. This evidence has significantly altered our perception of the CNS and has offered new hope for possible cell therapy strategies to repair lost function. Transplantation of stem cells or the recruitment of endogenous stem cells to repair specific regions of the brain or spinal cord is the next exciting research challenge. However, our understanding of the existing stem cell pool in the adult CNS remains limited. This review will discuss the identification and characterization of CNS stem cells in the adult brain and spinal cord.     

Cell replacement therapies for nervous system regeneration

The adult brain was thought to be a slowly decaying organ, a sophisticated but flawed machine condemned to inevitable decline. Today we know that the brain is more plastic than previously assumed, as most prominently demonstrated by the constitutive birth of new neurons that occurs in selected regions of the adult brain, even in humans. However, the overall modest capacity for endogenous repair of the central nervous system (CNS) has sparked interest in understanding the barriers to neuronal regeneration and in developing novel approaches to enable neuronal and circuit repair for therapeutic benefit in neurodegenerative disorders and traumatic injuries. Scientists recently assembled in Baeza, a picturesque town in the south of Spain, to discuss aspects of CNS regeneration. The picture that emerged shows how an integrated view of developmental and adult neurogenesis may inform the manipulation of neural progenitors, differentiated cells, and pluripotent stem cells for therapeutic benefit and foster new understanding of the inner limits of brain plasticity.     

Orchestrating brain-cell renewal: the role of immune cells in adult neurogenesis in health and disease

Immune cells and immune molecules have recently been shown to support neurogenesis from neural stem and progenitor cells in the adult brain. This non-classical immune activity takes place constantly under normal physiological conditions and is extended under acute pathological conditions to include the attraction of progenitor cells and induction of neurogenesis in regions of the adult central nervous system (CNS) in which formation of new neurons does not normally occur. We suggest that the immune system should be viewed as a novel player in the adult neural stem cell niche and a coordinator of cell renewal processes after injury. We discuss these notions in light of the well-known facts that both immune-cell activity and cell renewal are inherently limited in the adult CNS and that immune and stem cells provide the body's mechanisms of repair.     

Adult neurogenesis and cellular brain repair with neural progenitors, precursors and stem cells

Recent work in neuroscience has shown that the adult central nervous system (CNS) contains neural progenitors, precursors and stem cells that are capable of generating new neurons, astrocytes and oligodendrocytes. While challenging the previous dogma that no new neurons are born in the adult mammalian CNS, these findings bring with them the future possibilities for development of novel neural repair strategies. The purpose of this review is to present the current knowledge about constitutively occurring adult mammalian neurogenesis, highlight the critical differences between 'neurogenic' and 'non-neurogenic' regions in the adult brain, and describe the cardinal features of two well-described neurogenic regions-the subventricular zone/olfactory bulb system and the dentate gyrus of the hippocampus. We also provide an overview of presently used models for studying neural precursors in vitro, mention some precursor transplantation models and emphasize that, in this rapidly growing field of neuroscience, one must be cautious with respect to a variety of methodological considerations for studying neural precursor cells both in vitro and in vivo. The possibility of repairing neural circuitry by manipulating neurogenesis is an intriguing one, and, therefore, we also review recent efforts to understand the conditions under which neurogenesis can be induced in non-neurogenic regions of the adult CNS. This work aims towards molecular and cellular manipulation of endogenous neural precursors in situ, without transplantation. We conclude this review with a discussion of what might be the function of newly generated neurons in the adult brain, and provide a summary of present thinking about the consequences of disturbed adult neurogenesis and the reaction of neurogenic regions to disease.     

Defining the NG2-expressing cell of the adult CNS

The NG2 proteoglycan is believed to be an in vivomarker for oligodendrocyte progenitors found in the developing brain. The prevalence of NG2-expressing cells that remain in the adult CNS following the end of gliogenesis is significant. Current research is focused on how this cell participates in the normal function of the adult CNS and whether it may be activated by injury and/or contribute to repair. Despite substantial evidence for a sub-population of NG2-expressing cells playing a glial progenitor role in the adult CNS, there is much to be learned. Specifically, the heterogeneity of this population has not been adequately addressed for the adult CNS and while NG2 cells continue to divide in the adult CNS it is not clear what function they serve once myelination is complete. Future studies should elucidate the functional importance of NG2 in a variety of cell functions and shed light on the role NG2-expressing cells play in the intact and diseasedCNS.     

Regeneration of the adult zebrafish brain from neurogenic radial glia-type progenitors

Severe traumatic injury to the adult mammalian CNS leads to life-long loss of function. By contrast, several non-mammalian vertebrate species, including adult zebrafish, have a remarkable ability to regenerate injured organs, including the CNS. However, the cellular and molecular mechanisms that enable or prevent CNS regeneration are largely unknown. To study brain regeneration mechanisms in adult zebrafish, we developed a traumatic lesion assay, analyzed cellular reactions to injury and show that adult zebrafish can efficiently regenerate brain lesions and lack permanent glial scarring. Using Cre-loxP-based genetic lineage-tracing, we demonstrate that her4.1-positive ventricular radial glia progenitor cells react to injury, proliferate and generate neuroblasts that migrate to the lesion site. The newly generated neurons survive for more than 3 months, are decorated with synaptic contacts and express mature neuronal markers. Thus, regeneration after traumatic lesion of the adult zebrafish brain occurs efficiently from radial glia-type stem/progenitor cells.     

Contact inhibition in the failure of mammalian CNS axonal regeneration

Anamniote animals, such as fish and amphibians, are able to regenerate damaged CNS nerves following injury, but regeneration in the mammalian CNS tracts, such as the optic nerve, does not occur. However, severed adult mammalian retinal axons can regenerate into peripheral nerve segments grafted into the brain and this finding has emphasized the importance of the environment in explaining regenerative failure in the adult mammalian CNS. Following lesions, regenerating axons encounter the glial cells, oligodendrocytes and astro-cytes, and their derivatives, respectively myelin and the astrocytic scar. Experiments to investigate the influence of these components on axon growth in culture have revealed cell-surface and extracellular matrix molecules that inhibit axon extension and growth cone motility. Structural and functional characterization of these ligands and their receptors is underway, and may solve the interesting neurobiological conundrum posed by the failure of mammalian CNS regeneration. Simultaneously, this might allow new possibilities for treatment of the severe clinical disabilities resulting from injury to the brain and spinal cord.     

Radiation-induced apoptosis in the adult central nervous system is p53-dependent

Oligodendrocytes and subependymal cells in the adult CNS have been shown to undergo radiation-induced apoptosis. Here, we examined the role of p53 in radiation-induced apoptosis in the adult mouse CNS. In the spinal cord of p53+/+ mice, apoptotic glial cells were observed within 24 h after irradiation, and the apoptotic response peaked at 8 h. These apoptotic cells demonstrated the immunohistochemical phenotype of oligodendrocytes, and decreased oligodendrocyte density was observed at 24 h after 22 Gy. A similar time course of radiation-induced apoptosis was seen in subependymal cells in the adult mouse brain. Radiation-induced apoptosis was preceded by an increase in nuclear p53 expression in glial cells of the spinal cord and subependymal cells of the brain. There was no evidence of radiation-induced apoptosis in the spinal cord and subependymal region of p53-/- animals. We conclude that the p53 pathway may be a mechanism through which DNA damage induces apoptosis in the adult CNS.     

Neurogenesis in adult CNS: from denial to opportunities and challenges for therapy

The discovery of neurogenesis and neural stem cells (NSC) in the adult CNS has overturned a long-standing and deep-routed "dogma" in neuroscience, established at the beginning of the 20(th) century. This dogma lasted for almost 90 years and died hard when NSC were finally isolated from the adult mouse brain. The scepticism in accepting adult neurogenesis has now turned into a rush to find applications to alleviate or cure the devastating diseases that affect the CNS. Here we highlight a number of methodological, technical and conceptual drawbacks responsible for the historical denial of adult neurogenesis. Furthermore, we discuss old and new issues that need to be faced before NSC or endogenous neurogenesis can safely enter into the doctor's bag for therapies.     

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.     

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.     

Culturing Microglia from the Neonatal and Adult Central Nervous System

Microglia are the resident macrophage-like cells of the central nervous system (CNS) and, as such, have critically important roles in physiological and pathological processes such as CNS maturation in development, multiple sclerosis, and spinal cord injury. Microglia can be activated and recruited to action by neuronal injury or stimulation, such as axonal damage seen in MS or ischemic brain trauma resulting from stroke. These immunocompetent members of the CNS are also thought to have roles in synaptic plasticity under non-pathological conditions. We employ protocols for culturing microglia from the neonatal and adult tissues that are aimed to maximize the viable cell numbers while minimizing confounding variables, such as the presence of other CNS cell types and cell culture debris. We utilize large and easily discernable CNS components (e.g. cortex, spinal cord segments), which makes the entire process feasible and reproducible. The use of adult cells is a suitable alternative to the use of neonatal brain microglia, as many pathologies studied mainly affect the postnatal spinal cord. These culture systems are also useful for directly testing the effect of compounds that may either inhibit or promote microglial activation. Since microglial activation can shape the outcomes of disease in the adult CNS, there is a need for in vitro systems in which neonatal and adult microglia can be cultured and studied.     

Do neurons in the vertebrate CNS migrate on laminin?

In adult rat brain the extracellular matrix glycoprotein, laminin, is found only in basement membranes, but is transiently expressed by astrocytes after brain injury. Here, I show that laminin also appears in immature brain cells during CNS development, and that its presence coincides with phases of neuronal migration. In early embryos, laminin is seen throughout the whole thickness of the forming brain, and is apparently synthesized by the cells, as judged by its intracytoplasmic localization. As development proceeds, intracellular laminin becomes restricted to the periventricular regions while punctate deposits of laminin follow the course of vimentin-positive radial glial fibers. In most brain regions, the adult pattern of laminin expression is achieved by birth. In the post-natal rat cerebellum, however, laminin is detected in external granule cells, in Purkinje cells, and in punctate deposits along the radial Bergmann glial fibers. By day 24 after birth, when the migration of external granule cells is complete, all laminin immunoreactivity disappears from these structures. The transient expression of laminin in regions where neurons are migrating raises the possibility that laminin plays a role in neuronal migration during CNS development.     

Neurogenesis in the adult central nervous system

Contrary to the long-held dogma, neurogenesis occurs throughout adulthood, and neural stem cells reside in the adult central nervous system (CNS) in mammals. The developmental process of the brain may thus never end, and the brain may be amenable to repair. Neurogenesis is modulated in a wide variety of physiological and pathological conditions, and is involved in processes such as learning and memory and depression. However, the relative contribution of newly generated neuronal cells to these processes, as well as to CNS plasticity, remains to be determined. Thus, not only neurogenesis contributes to reshaping the adult brain, it will ultimately lead us to redefine our knowledge and understanding of the nervous system.     

Distribution of alcohol dehydrogenase mRNA in the rat central nervous system. Consequences for brain ethanol and retinoid metabolism.

The localization of alcohol dehydrogenase (ADH) in brain regions would demonstrate active ethanol metabolism in brain during alcohol consumption, which would be a new basis to explain the effects of ethanol in the central nervous system. Tissue sections from several regions of adult rat brain were examined by in situ hybridization to detect the expression of genes encoding ADH1 and ADH4, enzymes highly active with ethanol and retinol. ADH1 mRNA was found in the granular and Purkinje cell layers of cerebellum, in the pyramidal and granule cells of the hippocampal formation and in some cell types of cerebral cortex. ADH4 expression was detected in the Purkinje cells, in the pyramidal and granule cells of the hippocampal formation and in the pyramidal cells of cerebral cortex. High levels of ADH1 and ADH4 mRNAs were detected in the CNS epithelial and vascular tissues: leptomeninges, choroid plexus, ependymocytes of ventricle walls, and endothelium of brain vessels. Histochemical methods detected ADH activity in rodent cerebellar slices, while Western-blot analysis showed ADH4 protein in homogenates from several brain regions. In consequence, small but significant levels of ethanol metabolism can take place in distinct areas of the CNS following alcohol consumption, which could be related to brain damage caused by a local accumulation of acetaldehyde. Moreover, the involvement of ADH in the synthesis of retinoic acid suggests a role for the enzyme in the regulation of adult brain functions. The impairment of retinol oxidation by competitive inhibition of ADH in the presence of ethanol may be an additional origin of CNS abnormalities caused by ethanol.     

Neural stem cells: involvement in adult neurogenesis and CNS repair

Recent advances in stem cell research, including the selective expansion of neural stem cells (NSCs) in vitro, the induction of particular neural cells from embryonic stem cells in vitro, the identification of NSCs or NSC-like cells in the adult brain and the detection of neurogenesis in the adult brain (adult neurogenesis), have laid the groundwork for the development of novel therapies aimed at inducing regeneration in the damaged central nervous system (CNS). There are two major strategies for inducing regeneration in the damaged CNS: (i) activation of the endogenous regenerative capacity and (ii) cell transplantation therapy. In this review, we summarize the recent findings from our group and others on NSCs, with respect to their role in insult-induced neurogenesis (activation of adult NSCs, proliferation of transit-amplifying cells, migration of neuroblasts and survival and maturation of the newborn neurons), and implications for therapeutic interventions, together with tactics for using cell transplantation therapy to treat the damaged CNS.     

Neurogenesis in the adult mammalian brain

The concept of the CNS cell composition stability has recently undergone significant changes. It was earlier believed that neurogenesis in the mammalian CNS took place only during embryonic and early postnatal development. New approaches make it possible to obtain new results overriding the dogma that neurogenesis is impossible in the adult brain. The present review summarizes the information about the neural stem cell. It has been demonstrated that new neurons are constantly formed in adult mammals, including man. In two brain zones, subventricular zone and denate gyrus, neurogenesis appears proceed throughout the entire life of mammals, including man. The newly arising neurons are essential for some important processes, such as memory and learning. Stem cells were found in the subependymal and/or ependymal layer. They express nestin, and have a low mitotic activity. During embryogenesis, the stem cell divides asymmetrically: one daughter cell resides as the stem cell in the ependymal layer and another migrates to the subventricular zone. There it gives rise very fast to a pool of dividing precursors, from which neural and glial cells differentiate and migrate to the sites of final localization. The epidermal and fibroblast growth factors act as mitogens for the neural stem cell. The neural stem cell gives rise to the cells of all germ layers in vitro and has a wide potential for differentiation in the adult organism. Hence, it can be used as a source of various cell types of the nervous tissue necessary for cellular transplantation therapy.     

对比不同类型干细胞对于缺血性脑卒中治疗的研究进展

成人中枢神经系统存在着一定量的神经干细胞,其具有两大关键能力;自我更新和多向分化潜能。缺血性脑卒中是一种由于由脑血流的缺失或减少引起的脑动脉闭塞,进而导致脑组织梗死的脑血管疾病。虽然对于脑损伤的药物治疗已经取得了一定的成果,但目前以干细胞为基础的治疗方法仍成为了研究热点。无论是内源性神经干细胞还是外源性神经干细胞移植均可在脑损伤后向远端损伤区迁移并分化成新的神经细胞,从而在中枢神经系统疾病尤其是脑梗死后进行组织修复和功能恢复。因此在这篇综述中,我们主要探讨不同类型的干细胞对脑梗死介导的脑损伤的应用潜能,对比不同类型干细胞对缺血性脑卒中的治疗优缺点。     

The incredible elastic brain: how neural stem cells expand our minds

Brain development was thought to be largely hardwired and accomplished by birth, and the brain was thought to have essentially no regenerative capacity. The remarkable discovery of adult neurogenesis and neural stem cells (NSCs) existing in the mature CNS changed that, allowing us to think optimistically about CNS repair. These discoveries helped to generate a robust field of neural progenitor cell biology, with relevance to CNS development, pathogenesis, the search for novel neurological therapies, as well as our understanding of how the brain works.