Multifaceted control of adult SVZ neurogenesis by the vascular niche

The subventricular zone is one of the 2 germinal niches of the adult brain where neural stem cells (NSC) generate new neurons and glia throughout life. NSC behavior is controlled by the integration of intrinsic signals and extrinsic cues provided by the surrounding microenvironment, or niche. Within the niche, the vasculature has emerged as a critical compartment, to which both neural stem cells and transit-amplifying progenitors are closely associated. A key function of the vasculature is to deliver blood-borne and secreted factors that promote proliferation and lineage progression of committed neural progenitors. We recently found that, in contrast to the established role of soluble cues, juxtacrine signals on vascular endothelial cells maintain neural stem cells in a quiescent and undifferentiated state through direct cell-cell interactions. In this perspective, we discuss how, through these apparently opposing signals, the vascular niche might coordinate stem cell decisions between maintenance and proliferation.     

A novel stem cell type at the basal side of the subventricular zone maintains adult neurogenesis

According to the current consensus, murine neural stem cells (NSCs) apically contacting the lateral ventricle generate differentiated progenitors by rare asymmetric divisions or by relocating to the basal side of the ventricular–subventricular zone (V‐SVZ). Both processes will ultimately lead to the generation of adult‐born olfactory bulb (OB) interneurons. In contrast to this view, we here find that adult‐born OB interneurons largely derive from an additional NSC‐type resident in the basal V‐SVZ. Despite being both capable of self‐renewal and long‐term quiescence, apical and basal NSCs differ in Nestin expression, primary cilia extension and frequency of cell division. The expression of Notch‐related genes also differs between the two NSC groups, and Notch activation is greatest in apical NSCs. Apical downregulation of Notch‐effector Hes1 decreases Notch activation while increasing proliferation across the niche and neurogenesis from apical NSCs. Underscoring their different roles in neurogenesis, lactation‐dependent increase in neurogenesis is paralleled by extra activation of basal but not apical NSCs. Thus, basal NSCs support OB neurogenesis, whereas apical NSCs impart Notch‐mediated lateral inhibition across the V‐SVZ.     

Brain injury expands the numbers of neural stem cells and progenitors in the SVZ by enhancing their responsiveness to EGF

There is an increase in the numbers of neural precursors in the SVZ (subventricular zone) after moderate ischaemic injuries, but the extent of stem cell expansion and the resultant cell regeneration is modest. Therefore our studies have focused on understanding the signals that regulate these processes towards achieving a more robust amplification of the stem/progenitor cell pool. The goal of the present study was to evaluate the role of the EGFR [EGF (epidermal growth factor) receptor] in the regenerative response of the neonatal SVZ to hypoxic/ischaemic injury. We show that injury recruits quiescent cells in the SVZ to proliferate, that they divide more rapidly and that there is increased EGFR expression on both putative stem cells and progenitors. With the amplification of the precursors in the SVZ after injury there is enhanced sensitivity to EGF, but not to FGF (fibroblast growth factor)-2. EGF-dependent SVZ precursor expansion, as measured using the neurosphere assay, is lost when the EGFR is pharmacologically inhibited, and forced expression of a constitutively active EGFR is sufficient to recapitulate the exaggerated proliferation of the neural stem/progenitors that is induced by hypoxic/ischaemic brain injury. Cumulatively, our results reveal that increased EGFR signalling precedes that increase in the abundance of the putative neural stem cells and our studies implicate the EGFR as a key regulator of the expansion of SVZ precursors in response to brain injury. Thus modulating EGFR signalling represents a potential target for therapies to enhance brain repair from endogenous neural precursors following hypoxic/ischaemic and other brain injuries.     

Combinations of genetic mutations in the adult neural stem cell compartment determine brain tumour phenotypes

It has been suggested that intrinsic brain tumours originate from a neural stem/progenitor cell population in the subventricular zone of the post‐natal brain. However, the influence of the initial genetic mutation on the phenotype as well as the contribution of mature astrocytes to the formation of brain tumours is still not understood. We deleted Rb/p53, Rb/p53/PTEN or PTEN/p53 in adult subventricular stem cells; in ectopically neurografted stem cells; in mature parenchymal astrocytes and in transplanted astrocytes. We found that only stem cells, but not astrocytes, gave rise to brain tumours, independent of their location. This suggests a cell autonomous mechanism that enables stem cells to generate brain tumours, whereas mature astrocytes do not form brain tumours in adults. Recombination of PTEN/p53 gave rise to gliomas whereas deletion of Rb/p53 or Rb/p53/PTEN generated primitive neuroectodermal tumours (PNET), indicating an important role of an initial Rb loss in driving the PNET phenotype. Our study underlines an important role of stem cells and the relevance of initial genetic mutations in the pathogenesis and phenotype of brain tumours.     

Deviating from the well travelled path: precursor cell migration in the pathological adult mammalian brain

The presence of both neural and glial precursor cells in the adult central nervous system (CNS) and the capacity of these cells to migrate through this mature structure to areas of pathological damage and injury raises hope for the development of new therapeutic strategies to treat brain injury and disease. Although at present time, the compensatory neurogenesis described after various types of brain pathologies appears to be modest, the development of a strategy promoting the directed mobilization and phenotypic induction of endogenous precursor cells to areas of neural cell loss remains of high interest. The development of such a strategy however is currently thwarted by a limited understanding of the process and factors influencing precursor cell migration. In this review, we will discuss the current knowledge around precursor cell migration in the pathological adult brain with particular focus on the response and fate of precursor sub-populations to neural cell loss and the role of the inflammatory system in mediating precursor cell migration. Through this discussion we will identify particular areas in which further detailed research is required in order to expand our current understanding and aid in the eventual development of a novel therapeutic application.     

Longterm quiescent cells in the aged human subventricular neurogenic system specifically express GFAP‐δ

A main neurogenic niche in the adult human brain is the subventricular zone (SVZ). Recent data suggest that the progenitors that are born in the human SVZ migrate via the rostral migratory stream (RMS) towards the olfactory bulb (OB), similar to what has been observed in other mammals. A subpopulation of astrocytes in the SVZ specifically expresses an assembly‐compromised isoform of the intermediate filament protein glial fibrillary acidic protein (GFAP‐δ). To further define the phenotype of these GFAP‐δ expressing cells and to determine whether these cells are present throughout the human subventricular neurogenic system, we analysed SVZ, RMS and OB sections of 14 aged brain donors (ages 74‐93). GFAP‐δ was expressed in the SVZ along the ventricle, in the RMS and in the OB. The GFAP‐δ cells in the SVZ co‐expressed the neural stem cell (NSC) marker nestin and the cell proliferation markers proliferating cell nuclear antigen (PCNA) and Mcm2. Furthermore, BrdU retention was found in GFAP‐δ positive cells in the SVZ. In the RMS, GFAP‐δ was expressed in the glial net surrounding the neuroblasts. In the OB, GFAP‐δ positive cells co‐expressed PCNA. We also showed that GFAP‐δ cells are present in neurosphere cultures that were derived from SVZ precursors, isolated postmortem from four brain donors (ages 63‐91). Taken together, our findings show that GFAP‐δ is expressed in an astrocytic subpopulation in the SVZ, the RMS and the OB. Importantly, we provide the first evidence that GFAP‐δ is specifically expressed in longterm quiescent cells in the human SVZ, which are reminiscent of NSCs.     

Neuronal precursors of the adult rat subependymal zone persist into senescence,with no decline in spatial extent or response to BDNF

The adult mammalian brain continues to harbor ependymal/subependymal zone (SZ) precursor cells, which can give rise to neurons in vitro. In adult rats, explants of the rostral 6–7 mm of the SZ give rise to neurons in vitro, and over this entire expanse, neuronal survival is supported specifically by brain-derived neurotrophic factor (BDNF). We asked whether either the (a) spatial distribution, (b) abundance, or (c) BDNF responsiveness of the neuronal precursor population was affected by age. Explants of three rostrocaudally defined regions were taken from both young and old rats (3 and 20 months old, respectively), and cultured in 2% fetal bovine serum-containing media with or without added BDNF (20 ng/mL). The extent of neuronal production by these explants varied only minimally with their level of derivation, such that substantial outgrowth was observed at each level tested. Neuronal outgrowth was marginally higher and more rapid in achieving its maximal extent in the 3-month-old rats compared with their aged counterparts, but neuronal outgrowth was robust at each age tested. The duration of survival of SZ-derived neurons did not differ between the young and old rats. At both ages, BDNF supported the survival of these new adult neurons. The extent of BDNF's influence was independent of both the age of the donor rat and the rostrocaudal level at which the parent SZ explant was taken. Thus, the neuronal precursors of the rat brain persist into senescence; the size of the precursor pool attenuates minimally with age, and its spatial extent remains constant. The neurons generated from these precursors can respond to BDNF throughout life. © 1997 John Wiley & Sons, Inc. J Neurobiol 32: 554–566, 1997     

GalR3 activation promotes adult neural stem cell survival in response to a diabetic milieu

Type 2 diabetes impairs adult neurogenesis which could play a role in the CNS complications of this serious disease. The goal of this study was to determine the potential role of galanin in protecting adult neural stem cells (NSCs) from glucolipotoxicity and to analyze whether apoptosis and the unfolded protein response were involved in the galanin‐mediated effect. We also studied the regulation of galanin and its receptor subtypes under diabetes in NSCs in vitro and in the subventricular zone (SVZ) in vivo. The viability of mouse SVZ‐derived NSCs and the involvement of apoptosis (Bcl‐2, cleaved caspase‐3) and unfolded protein response [C/EBP homologous protein (CHOP) Glucose‐regulated protein 78/immunoglobulin heavy‐chain binding protein (GRP78/BiP), spliced X‐box binding protein 1 (XBP1), c‐Jun N‐terminal kinases (JNK) phosphorylation] were assessed in the presence of glucolipotoxic conditions after 24 h. The effect of diabetes on the regulation of galanin and its receptor subtypes was assessed on NSCs in vitro and in SVZ tissues isolated from normal and type 2 diabetes ob/ob mice. We show increased NSC viability following galanin receptor (GalR)3 activation. This protective effect correlated with decreased apoptosis and CHOP levels. We also report how galanin and its receptors are regulated by diabetes in vitro and in vivo. This study shows GalR3‐mediated neuroprotection, supporting a potential future therapeutic development, based on GalR3 activation, for the treatment of brain disorders.

     

An ependymal cell census identifies heterogeneous and ongoing cell maturation in the adult mouse spinal cord that changes dynamically on injury

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Age‐related changes in stem cell dynamics,neurogenesis, apoptosis,and gliosis in the adult brain: A novel teleost fish model of negligible senescence

Adult neurogenesis, the generation of new neurons in the adult central nervous system, is a reported feature of all examined vertebrate species. However, a dramatic decline in the rates of cell proliferation and neuronal differentiation occurs in mammals, typically starting near the onset of sexual maturation. In the present study, we examined possible age‐related changes associated with adult neurogenesis in the brain of brown ghost knifefish (Apteronotus leptorhynchus), a teleost fish distinguished by its enormous neurogenic potential. Contrary to the well‐established alterations in the mammalian brain during aging, in the brain of this teleostean species we could not find evidence for any significant age‐related decline in the absolute levels of stem/progenitor cell proliferation, neuronal and glial differentiation, or long‐term survival of newly generated cells. Moreover, there was no indication that the amount of glial fibrillary acidic protein or the number of apoptotic cells in the brain was altered significantly over the course of adult life. We hypothesize that this first demonstration of negligible cellular senescence in the vertebrate brain is related to the continued growth of this species and to the lack of reproductive senescence during adulthood. The establishment of the adult brain of this species as a novel model of negligible senescence provides new opportunities for the advancement of our understanding of the biology of aging and the fundamental mechanisms that underlie senescence in the brain. © 2013 Wiley Periodicals, Inc. Develop Neurobiol 74: 514–530, 2014     

Exendin-4对成年小鼠脑室下区神经干细胞分化作用的体外研究*

目的:研究艾塞那肽(Ex-4)对成年小鼠脑室下区(SVZ)神经干细胞(NSCs)分化的影响及机制。方法:提取5周龄C57BL/6J小鼠SVZ的NSCs,100 nmol/L Ex-4处理分化14 d观察细胞形态,用免疫荧光检测巢蛋白(nestin)和胰高糖素样肽-1受体(GLP-1R)的表达。用shRNA敲低GLP-1R,将研究分为四组:对照组,Ex-4组,GLP-1R敲低组,GLP-1R敲低+Ex-4组。100 nmol/L Ex-4处理14 d后免疫荧光标记β-微管蛋白3(β-tublin III)和胶质纤维酸性蛋白(GFAP)并统计β-tublin III阳性细胞比例,Western blot检测环磷腺苷效应元件结合蛋白(CREB)的活化。为进一步研究Ex-4对MAPK和PI3K通路的影响,分别以丝裂原活化蛋白激酶(MAPK)抑制剂U0126 0.07 μmol/L预处理细胞30 min、磷脂酰肌醇-3激酶(PI3K)抑制剂LY294002 50 μmol/预处理细胞2 h,将研究分为: 对照组,Ex-4组,U0126组,U0126+Ex-4组,LY294002组,LY294002+Ex-4组,Western blot检测各组CREB的活化,各组实验独立重复三次。结果:成功从C57BL/6J小鼠SVZ提取NSCs,免疫荧光提示NSCs中nestin以及GLP-1R阳性。相对于对照组,Ex-4组分化为神经元的比例更高。GLP-1R敲低+Ex-4组中神经元比例与对照组基本一致(P＜0.01),β-tublin III阳性的细胞显示出GLP-1R以及CREB活化阳性。Western blot显示Ex-4组中CREB显著活化,GLP-1R敲低+Ex-4组的CERB活化与对照组基本一致(P＜0.01)。U0126+Ex-4组与Ex-4组CERB活化水平一致,LY294002+Ex-4组与对照组CERB活化水平一致(P＜0.01)。 结论:Ex-4通过GLP-1R受体促进成年小鼠SVZ中NSCs分化为神经元,这一作用可能通过PI3K/ CREB通路来实现。     

Blockade of TrkB but not p75NTR activates a subpopulation of quiescent neural precursor cells and enhances neurogenesis in the adult mouse hippocampus

Brain‐derived neurotrophic factor (BDNF) signaling plays a major role in the regulation of hippocampal neurogenesis in the adult brain. While the majority of studies suggest that this is due to its effect on the survival and differentiation of newborn neurons, it remains unclear whether this signaling directly regulates neural precursor cell (NPC) activity and which of its two receptors, TrkB or the p75 neurotrophin receptor (p75^NTR) mediates this effect. Here, we examined both the RNA and protein expression of these receptors and found that TrkB but not p75^NTR receptors are expressed by hippocampal NPCs in the adult mouse brain. Using a clonal neurosphere assay, we demonstrate that pharmacological blockade of TrkB receptors directly activates a distinct subpopulation of NPCs. Moreover, we show that administration of ANA‐12, a TrkB‐selective antagonist, in vivo either by systemic intraperitoneal injection or by direct infusion within the hippocampus leads to an increase in the production of new neurons. In contrast, we found that NPC‐specific knockout of p75^NTR had no effect on the proliferation of NPCs and did not alter neurogenesis in the adult hippocampus. Collectively, these results demonstrate a novel role of TrkB receptors in directly regulating the activity of a subset of hippocampal NPCs and suggest that the transient blockade of these receptors could be used to enhance adult hippocampal neurogenesis.     

Regulation of adult neural precursor cell migration

Adult neural precursor cells (NPCs) are predominantly located in the subventricular zone (SVZ) of the lateral ventricles or in the subgranular zone of the dentate gyrus. These NPCs produce neuroblasts that normally migrate and integrate into the olfactory bulb and hippocampus, respectively. Following CNS damage due to disease or injury, NPCs can also migrate to the site of damage. Enhancement of NPC migration to sites of neural damage may increase their potential for repair but requires an understanding of processes that regulate basal and injury-induced migration so we can harness this potential. This review highlights the extrinsic factors and major intrinsic signalling pathways that regulate endogenous basal NPC migration to the olfactory bulb and the role of inflammatory mediators and chemokines in disease and injury-induced NPC migration.     

Cannabinoid receptor signaling in progenitor/stem cell proliferation and differentiation

Cannabinoids, the active components of cannabis (Cannabis sativa) extracts, have attracted the attention of human civilizations for centuries, much earlier than the discovery and characterization of their substrate of action, the endocannabinoid system (ECS). The latter is an ensemble of endogenous lipids, their receptors [in particular type-1 (CB₁) and type-2 (CB₂) cannabinoid receptors] and metabolic enzymes. Cannabinoid signaling regulates cell proliferation, differentiation and survival, with different outcomes depending on the molecular targets and cellular context involved. Cannabinoid receptors are expressed and functional from the very early developmental stages, when they regulate embryonic and trophoblast stem cell survival and differentiation, and thus may affect the formation of manifold adult specialized tissues derived from the three different germ layers (ectoderm, mesoderm and endoderm). In the ectoderm-derived nervous system, both CB₁ and CB₂ receptors are present in neural progenitor/stem cells and control their self-renewal, proliferation and differentiation. CB₁ and CB₂ show opposite patterns of expression, the former increasing and the latter decreasing along neuronal differentiation. Recently, endocannabinoid (eCB) signaling has also been shown to regulate proliferation and differentiation of mesoderm-derived hematopoietic and mesenchymal stem cells, with a key role in determining the formation of several cell types in peripheral tissues, including blood cells, adipocytes, osteoblasts/osteoclasts and epithelial cells. Here, we will review these new findings, which unveil the involvement of eCB signaling in the regulation of progenitor/stem cell fate in the nervous system and in the periphery. The developmental regulation of cannabinoid receptor expression and cellular/subcellular localization, together with their role in progenitor/stem cell biology, may have important implications in human health and disease.