Retinoic acid and neurotrophins collaborate to regulate neurogenesis in adult‐derived neural stem cell cultures

The adult rat hippocampus contains fibroblast growth factor 2–responsive stem cells that are self‐renewing and have the ability to generate both neurons and glia in vitro, but little is known about the molecular events that regulate stem cell differentiation. Hippocampus‐derived stem cell clones were used to examine the effects of retinoic acid (RA) on neuronal differentiation. Exposure to RA caused an immediate up‐regulation of NeuroD, increased p21 expression, and concurrent exit from cell cycle. These changes were accompanied by a threefold increase in the number of cells differentiating into immature neurons. An accompanying effect of RA was to sustain or up‐regulate trkA, trkB, trkC, and p75NGFR expression. Without RA treatment, cells were minimally responsive to neurotrophins (NTs), whereas the sequential application of RA followed by brain‐derived neurotrophic factor or NT‐3 led to a significant increase in neurons displaying mature γ‐a‐minobutyric acid, acetylcholinesterase, tyrosine hydroxylase, or calbindin phenotypes. Although NTs promoted maturation, they had little effect on the total number of neurons generated, suggesting that RA and neurotrophins acted at distinct stages in neurogenesis. RA first promoted the acquisition of a neuronal fate, and NTs subsequently enhanced maturation by way of RA‐dependent expression of the Trk receptors. In combination, these sequential effects were sufficient to stimulate stem cell–derived progenitors to differentiate into neurons displaying a variety of transmitter phenotypes. © 1999 John Wiley & Sons, Inc. J Neurobiol 38: 65–81, 1999     

p57 controls adult neural stem cell quiescence and modulates the pace of lifelong neurogenesis

Throughout life, neural stem cells (NSCs) in the adult hippocampus persistently generate new neurons that modify the neural circuitry. Adult NSCs constitute a relatively quiescent cell population but can be activated by extrinsic neurogenic stimuli. However, the molecular mechanism that controls such reversible quiescence and its physiological significance have remained unknown. Here, we show that the cyclin‐dependent kinase inhibitor p57^kip2 (p57) is required for NSC quiescence. In addition, our results suggest that reduction of p57 protein in NSCs contributes to the abrogation of NSC quiescence triggered by extrinsic neurogenic stimuli such as running. Moreover, deletion of p57 in NSCs initially resulted in increased neurogenesis in young adult and aged mice. Long‐term p57 deletion, on the contrary, led to NSC exhaustion and impaired neurogenesis in aged mice. The regulation of NSC quiescence by p57 might thus have important implications for the short‐term (extrinsic stimuli‐dependent) and long‐term (age‐related) modulation of neurogenesis.     

Cellular organization of adult neurogenesis in the Common Marmoset

Adult neurogenesis within the subgranular zone (SGZ) of the hippocampal dentate gyrus and the subventricular zone (SVZ) of the lateral ventricle (LV) has been most intensely studied within the brains of rodents such as mice and rats. However, little is known about the cell types and processes involved in adult neurogenesis within primates such as the common marmoset (Callithrix jacchus). Moreover, substantial differences seem to exist between the neurogenic niche of the LV between rodents and humans. Here, we set out to use immunohistochemical and autogradiographic analysis to characterize the anatomy of the neurogenic niches and the expression of cell type-specific markers in those niches in the adult common marmoset brain. Moreover, we demonstrate significant differences in the activity of neurogenesis in the adult marmoset brain compared to the adult mouse brain. Finally, we provide evidence for ongoing proliferation of neuroblasts within both the SGZ and SVZ of the adult brain and further show that the age-dependent decline of neurogenesis in the hippocampus is associated with a decrease in neuroblast cells.     

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。     

A novel flow cytometry-based technique to measure adult neurogenesis in the brain

The stimulation of neurogenesis is an exciting novel therapeutic option for diseases of the central nervous system, ranging from depression to neurodegeneration. One major bottleneck in screening approaches for neurogenesis-inducing compounds is the very demanding in vivo quantification of newborn neurons based on stereological techniques. To effectively develop compounds in this area, novel fast and reliable techniques for quantification of in vivo neurogenesis are needed. In this study, we introduce a flow cytometry-based method for quantifying newly generated neurons in the brain based on the counting of cell nuclei from dissected brain regions. Important steps involve density sedimentation of the cell nuclei, and staining for the proliferation marker bromodeoxy uridine and nuclear cell type markers such as NeuN. We demonstrate the ability of the technique to detect increased neurogenesis in the hippocampus of animals which underwent physical exercise and received fluoxetine treatment.     

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 prove that neurogenesis takes part even in the adult brain. The present review summarizes the data 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 dentate gyrus, neurogenesis appears to 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 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.     

Stem cell review series: role of neurogenesis in age-related memory disorders

Neuroplasticity is characterized by growth and branching of dendrites, remodeling of synaptic contacts, and neurogenesis, thus allowing the brain to adapt to changes over time. It is maintained in adulthood but strongly repressed during aging. An age-related decline in neurogenesis is particularly pronounced in the two adult neurogenic areas, the subventricular zone and the dentate gyrus. This age-related decline seems to be attributable mainly to limited proliferation, associated with an age-dependent increase in quiescence and/or a lengthening of the cell cycle, and is closely dependent on environmental changes. Indeed, when triggered by appropriate signals, neurogenesis can be reactivated in senescent brains, thus confirming the idea that the age-related decrease in new neuron production is not an irreversible, cell-intrinsic process. The coevolution of neurogenesis and age-related memory deficits – especially regarding spatial memory – during senescence supports the idea that new neurons in the adult brain participate in memory processing, and that a reduction in the ability to generate new neurons contributes to the appearance of memory deficits with advanced age. Furthermore, the age-related changes in hippocampal plasticity and function are under environmental influences that can favor successful or pathological aging. A better understanding of the mechanisms that regulate neurogenesis is necessary to develop new therapeutic tools to cure or prevent the development of memory disorders that may appear during the course of aging in some individuals.     

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.     

Dopamine and adult neurogenesis

Dopamine is an important neurotransmitter implicated in the regulation of mood, motivation and movement. We have reviewed here recent data suggesting that dopamine, in addition to being a neurotransmitter, also plays a role in the regulation of endogenous neurogenesis in the adult mammalian brain. In addition, we approach a highly controversial question: can the adult human brain use neurogenesis to replace the dopaminergic neurones in the substantia nigra that are lost in Parkinson's disease?     

Aging brain microenvironment decreases hippocampal neurogenesis through Wnt-mediated survivin signaling

Accumulating evidence suggests that adult hippocampal neurogenesis relies on the controlled and continued proliferation of neural progenitor cells (NPCs). With age, neurogenesis decreases through mechanisms that remain unclear but are believed to involve changes in the NPC microenvironment. Here, we provide evidence that NPC proliferation in the adult brain is in part regulated by astrocytes via Wnt signaling and that this cellular cross-talk is modified in the aging brain, leading to decreased proliferation of NPCs. Furthermore, we show that astrocytes regulate the NPC cell cycle by acting on the expression levels of survivin, a known mitotic regulator. Among cell cycle genes found down-regulated in aged NPCs, survivin was the only one that restored NPC proliferation in the aged brain. Our results provide a mechanism for the gradual loss of neurogenesis in the brain associated with aging and suggest that targeted modulation of survivin expression directly or through Wnt signaling could be used to stimulate adult neurogenesis.     

Quantitative analysis reveals dominance of gliogenesis over neurogenesis in an adult brainstem oscillator

Neural stem/progenitor cells in the neurogenic niches of the adult brain are widely assumed to give rise predominantly to neurons, rather than glia. Here, we performed a quantitative analysis of the resident neural progenitors and their progeny in the adult pacemaker nucleus (Pn) of the weakly electric fish Apteronotus leptorhynchus. Approximately 15% of all cells in this brainstem nucleus are radial glia‐like neural stem/progenitor cells. They are distributed uniformly within the tissue and are characterized by the expression of Sox2 and Meis 1/2/3. Approximately 2–3% of them are mitotically active, as indicated by expression of proliferating cell nuclear antigen. Labeling of proliferating cells with a single pulse of BrdU, followed by chases of up to 100 days, revealed that new cells are generated uniformly throughout the nucleus and do not undergo substantial migration. New cells differentiate into S100+ astrocytes and Hu C/D+ small interneurons at a ratio of 4:1, reflecting the proportions of the total glia and neurons in this brain region. The continuous addition of new cells leads to a diffuse growth of the Pn, which doubles in volume and total cell number over the first 2 years following sexual maturation of the fish. However, the number of pacemaker and relay cells, which constitute the oscillatory neural network, remains constant throughout adult life. We hypothesize that the dominance of gliogenesis is an adaptation to the high‐frequency firing of the oscillatory neurons in this nucleus. © 2014 Wiley Periodicals, Inc. Develop Neurobiol 74: 934–952, 2014     

Plasticity of hippocampal stem/progenitor cells to enhance neurogenesis in response to kainate-induced injury is lost by middle age

A remarkable up-regulation of neurogenesis through increased proliferation of neural stem/progenitor cells (NSCs) is a well-known plasticity displayed by the young dentate gyrus (DG) following brain injury. To ascertain whether this plasticity is preserved during aging, we quantified DG neurogenesis in the young adult, middle-aged and aged F344 rats after kainic acid induced hippocampal injury. Measurement of new cells that are added to the dentate granule cell layer (GCL) between post-injury days 4 and 15 using 5'-bromodeoxyuridine labeling revealed an increased addition of new cells in the young DG but not in the middle-aged and aged DG. Quantification of newly born neurons using doublecortin immunostaining also demonstrated a similar trend. Furthermore, the extent of ectopic migration of new neurons into the dentate hilus was dramatically increased in the young DG but was unaltered in the middle-aged and aged DG. However, there was no change in neuronal fate-choice decision of newly born cells following injury in all age groups. Similarly, comparable fractions of new cells that are added to the GCL after injury exhibited 5-month survival and expressed the mature neuronal marker NeuN, regardless of age or injury at the time of their birth. Thus, hippocampal injury does not adequately stimulate NSCs in the middle-aged and aged DG, resulting in no changes in neurogenesis after injury. Interestingly, rates of both neuronal fate-choice decision and long-term survival of newly born cells remain stable with injury in all age groups. These results underscore that the ability of the DG to increase neurogenesis after injury is lost as early as middle age.     

室管膜下区神经干细胞与神经发生的研究进展

室管膜下区(subventricular zone,SVZ)存在着神经干细胞(nueral stem cells,NSCs),是成年哺乳动物脑内重要的神经发生区域。神经发生过程极为复杂,包括一系列的生物学事件。在病理状态下,SVZ区的细胞增殖,新生的神经细胞迁移到病灶处,取代或修复受损的细胞,起到保护脑组织的作用。该文就SVZ区的神经干细胞、神经发生过程及病理状态下神经发生的相关研究做一综述。     

NPY promotes chemokinesis and neurogenesis in the rat subventricular zone

The subventricular zone (SVZ) is a major reservoir for stem cells in the adult mammalian brain. Neural stem cells supply the olfactory bulb with new interneurons and provide cells that migrate towards lesioned brain areas. Neuropeptide Y (NPY), one of the most abundant neuropeptides in the brain, was previously shown to induce neuroproliferation on mice SVZ cells. In the present study, performed in rats, we demonstrate the endogenous synthesis of NPY by cells in the SVZ that suggests that NPY could act as an autocrine/paracrine factor within the SVZ area. We observed that NPY promotes SVZ cell proliferation as previously reported in mice, but does not affect self-renewal of SVZ stem cells. Additionally, this study provides the first direct evidence of a chemokinetic activity of NPY on SVZ cells. Using pharmacological approaches, we demonstrate that both the mitogenic and chemokinetic properties of NPY involve Y1 receptor-mediated activation of the ERK1/2 MAP kinase pathway. Altogether, our data establish that NPY through Y1 receptors activation controls chemokinetic activity and, as for mice, is a major neuroproliferative regulator of rat SVZ cells.     

Effect of aging on neurogenesis in the canine brain

An age-dependent decline in hippocampal neurogenesis has been reported in laboratory rodents. Environmental enrichment proved to be a strong trigger of neurogenesis in young and aged laboratory rodents, which are generally kept in facilities with a paucity of environmental stimuli. These data raise the question whether an age-dependent decline in hippocampal cell proliferation and neurogenesis can also be observed in individuals exposed to diversified and varying surroundings. Therefore, we determined rates of canine hippocampal neurogenesis using post-mortem tissue from 37 nonlaboratory dogs that were exposed to a variety of environmental conditions throughout their life. Expression of the neuronal progenitor cell marker doublecortin clearly correlated with age. The analysis of doublecortin-labeled cells in dogs aged > 133 months indicated a 96% drop in the aged canine brain as compared to young adults. Expression of the proliferation marker Ki-67 in the subgranular zone decreased until dogs were aged 85-132 months. In the aging canine brain amyloid-beta peptide deposits have been described that might resemble an early pathophysiological change in the course of human Alzheimer's disease. Comparison of Ki-67 and doublecortin expression in canine brain tissue with or without diffuse plaques revealed no differences. The data indicate that occurrence of diffuse plaques in the aging brain is not sufficient to trigger enhanced proliferation or enhanced neurogenesis such as described in human Alzheimer's disease. In addition, this study gives first proof that an age-dependent decline also dominates hippocampal neurogenesis rates in individuals living in diversified environments.