Adult neurogenesis and neural stem cells of the central nervous system in mammals (2002)

Neural stem cells (NSCs) are the self-renewing, multipotent cells that generate neurons, astrocytes, and oligodendrocytes in the nervous system (Fig. 1a). Over the past decades, the confirmation that neurogenesis occurs in discrete areas of the adult brain and that NSCs reside in the adult brain has overturned the long-held dogma that we are born with a certain number of nerve cells and that the brain cannot generate new neurons and renew itself. In this article, we review the evidence that neurogenesis occurs in the adult mammalian central nervous system (CNS) and that the adult CNS contains NSCs. Neurogenesis and neural stem cells of the adult mammal central nervous system. a: Neural stem cells (NSCs) are self-renewing, multipotent cells that generate cells of the neuronal lineage: neurons, astrocytes and oligodendrocytes. b: Sagittal representation of the two neurogenic areas of the adult CNS: the olfactory bulb (OB) and the dentate gyrus (DG) of the hippocampus. The new neuronal cells in the OB are generated from NSCs of the subventricular zone (SVZ). The SVZ is a narrow zone of tissue in the wall of the lateral ventricle in the forebrain. The NSCs of the SVZ migrate to the OB via the rostro-migratory stream (RMS), where they differentiate into interneurons of the OB. c: There are two conflicting theories regarding the cellular origin of the NSCs of the adult SVZ (c). In one, the NSCs of the SVZ are differentiated ependymal cells. Ependymal cells are specialized, ciliated cells that line the ventricle wall. In the other, they are astrocyte-like cells expressing glial fibrillary acidic protein in the SVZ. d: The new neuronal cells in the adult DG are generated from NSCs of the subgranular zone (SGZ) of the hippocampus and differentiate into neural and glial cells in the granular layer of the DG. The first evidence that neurogenesis occurs in certain regions of the adult mammalian brain came from [3H]-thymidine labeling studies conducted by Altman and Das (1965). In the 1990s, new methods for labeling dividing cells, such as retroviral virus and bromodeoxyuridine (BrdU), a marker of the S-phase of the cell cycle, were introduced to study neurogenesis in the adult central nervous system (CNS) (Corotto et al., 1993; Luskin, 1993; Seki and Arai, 1993). These studies helped to confirm that neurogenesis occurs in the adult mammalian brain and that new neurons are generated continuously in some regions of the adult CNS. Neurogenesis has been shown to occur throughout adulthood in two neurogenic areas of the adult mammalian CNS: the olfactory bulb (OB) and the dentate gyrus (DG) of the hippocampus (Fig. 1b). Low levels of neurogenesis have also been reported in the Ammon's horn of the adult mouse (Rietze et al., 2000). Previous studies have shown that neurogenesis occurs in the OB of adult rodents (Altman, 1969; Bayer, 1983; Kaplan et al., 1985; Kishi, 1987; Corotto et al., 1993; Lois and Alvarez-Buylla, 1994) and non-human primates (Kornack and Rakic, 2001a,b). The new neuronal cells in the adult mammalian OB are generated from neural progenitor cells (NPCs) in the anterior part of the subventricular zone (SVZ) (Fig. 1c) (Luskin, 1993; Type C cells, Doetsch et al., 1997). The SVZ is a narrow zone of tissue in the wall of the lateral ventricle in the forebrain. The NPCs of the SVZ migrate to the OB via the rostro-migratory stream (RMS; Type A cells, Doetsch et al., 1997), where they differentiate into interneurons of the OB: granule cells and periglomerular cells (Altman, 1969; Corotto et al., 1993; Luskin, 1993; Lois and Alvarez-Buylla, 1994). Neurogenesis also occurs in the adult DG of the hippocampus of rodents (Altman and Das, 1965; Kaplan and Hinds, 1977; Bayer et al., 1982; Kaplan and Bell, 1983,1984; Seki and Arai, 1993; Cameron et al., 1993a; Kuhn et al., 1996; Kempermann et al., 1997a), humans (Eriksson et al., 1998) and non-human primates (Kornack and Rakic, 1999; Gould et al., 2001). The NPCs of the adult DG are generated in the subgranular zone (SGZ) of the DG and differentiate into neuronal and glial cells in the granular layer of the DG (Fig. 1d; Cameron et al., 1993). Retrograde tracing studies have shown that the newly generated neuronal cells extend axons into the CA3 region of the hippocampus, (Stanfield and Trice, 1988; Markakis and Gage, 1999) as soon as 4–10 days after mitosis (Hastings and Gould, 1999). They receive synaptic input (Kaplan and Bell, 1983; Markakis and Gage, 1999) and project functional connections in vitro (Song et al., 2002) and in vivo to the CA3 region (van Praag et al., 2002). The evidence that neurogenesis occurs in the adult mammalian forebrain raised the question of the existence of NSCs in the adult CNS. The fact that a cell can be labeled in vivo by administration of [3H]-thymidine or BrdU, or by retroviral labeling, does not mean that it is a stem cell; thus NSC research has focused on identifying the NSCs of the adult CNS. The first cells from the adult CNS characterized as capable of generating the three main phenotypes of the CNS in vitro were isolated from mouse striatal tissue (Reynolds and Weiss, 1992). These putative NSCs were called NPCs because their stem cell properties had yet to be demonstrated. The NPCs were found to be immunoreactive for the intermediate filament protein nestin and to give rise to neuronal and glial cells, astrocytes and oligodendrocytes in vitro. Nestin has been characterized as a marker for neuroepithelial and CNS stem cells in vitro and in vivo (Frederiksen and McKay, 1988). NPCs have since been isolated from diverse areas of the adult CNS: in mouse brain (Richards et al., 1992); in the SVZ of mouse (Lois and Alvarez-Buylla, 1993; Morshead et al., 1994), rat (Palmer et al., 1995) and human (Kirschenbaum et al., 1994; Pincus et al., 1998; Kukekov et al., 1999; Arsenijevic et al., 2001); in rat (Gage et al., 1995; Palmer et al., 1995, 1999) and human (Kukekov et al., 1999; Roy et al., 2000a; Arsenijevic et al., 2001) hippocampus; in rat septum and striatum (Palmer et al., 1995); in human cortex (Arsenijevic et al., 2001); in human (Pagano et al., 2000) and mouse (Gritti et al., 2002) OB; in the rostral extension of the mouse SVZ (Gritti et al., 2002); and in different levels of the spinal cord (cervical, thoracic, lumbar, and sacral) of the mouse (Weiss et al., 1996) and rat (Shihabuddin et al., 1997). In the spinal cord, NPCs can be isolated from the periventricular area and the parenchyma (Yamamoto et al., 2001). NPCs have also been isolated and cultured from adult postmortem brain tissues. NPCs have been isolated and cultured after postmortem intervals of up to 140 hr from adult mouse SVZ and spinal cord (Laywell et al., 1999), adult human OB (Roisen et al., 2001), and human hippocampus and SVZ (Palmer et al, 2001). The demonstration that multipotent, self-renewing progenitor cells of neurons and glial cells can be cultured from NPCs from these adult brain regions shows that NPC cultures contain some NSCs, and demonstrating that NPCs are multipotent relies on evidence that neurons, astrocytes and oligodendrocytes, the three main phenotypes of the CNS, can be generated from single cells. The demonstration that NPCs can self-renew relies on showing that NPCs maintain their multipotentiality over time. Although these criteria are well accepted to show that a single cell is a NSC in vitro, they are not absolute. The main criticism resides in the number of subcloning steps that one must show to qualify a cell as self-renewing in vitro (Reynolds and Weiss, 1996). The first evidence that NSCs could be isolated from the CNS in vitro was reported by Gritti et al. (1996), who characterized multipotent NSCs from adult mouse striatal tissue. Adult-derived NSCs have now been characterized from other brain regions, such as the hippocampus, the spinal cord and the SVZ, and from different species, including rodents and humans (Weiss et al., 1996; Palmer et al., 1997; Gritti et al., 1999; Johansson et al., 1999; Shihabuddin et al., 2000; Taupin et al., 2000; Yamamoto et al., 2001). Taken together, these studies confirm the existence of NSCs in the adult CNS and show that NSCs, like NPCs, can be isolated from neurogenic and non-neurogenic areas. Most of these studies have also investigated the expression of nestin by adult NPCs and NSCs, and have found that nestin is a marker for these cells in vitro and in vivo. Neurogenesis occurs constitutively throughout adulthood in the SVZ and the DG, but it has been reported that the rate of neurogenesis in the SVZ (Tropepe et al., 1997) and the DG (Seki and Arai, 1995; Kuhn et al., 1996) decreases with age in rodents. Studies have also demonstrated that NPCs can be isolated and cultured from aged rodent SVZ with the same efficiency as younger SVZ (Goldman et al., 1997; Tropepe et al., 1997), and that the decrease in neurogenesis observed in the DG of aged rodents can be reversed by reducing the corticosteroid levels by adrenalectomy (Cameron and McKay, 1999). Altogether, these data indicate that the NPC population in the adult CNS remains stable into old age and the decrease of neurogenesis with age likely reflects a progressive lengthening of the cell cycle time of the NPCs in vivo. They also emphasize the importance of adrenal steroid levels in the control of neurogenesis in the adult DG throughout adulthood and indicate the consequences of changes in levels of adrenal steroids on neurogenesis in the adult DG. Many of the newly generated NPCs in the adult rat DG die between the first and second week after they are born (Gould et al., 1999). The new neuronal cells that survive have been detected up to 781 days post-BrdU injection in the adult human DG (Eriksson et al., 1998) and up to 112 days post-BrdU injection in the adult mice OB (Corotto et al., 1993). Young adult rats seem to generate 9,000 new cells each day in the DG, or more than 250,000 per month. This neurogenesis contributes about 3.3% of the total granule cell population per month in the DG of the young adult, or about 0.1% per day (Kempermann et al., 1997b; Cameron and McKay, 2001). The precise magnitude of adult neurogenesis remains difficult to calculate, however, due to uncertainties in labeling efficiency. There are two conflicting theories regarding the cellular origin of the NSCs of the adult SVZ (Fig. 1c). One theory contends that the NSCs of the adult SVZ are differentiated ependymal cells that express the intermediate filament protein nestin (Johansson et al., 1999). The other theory identifies them as astrocyte-like cells expressing glial fibrillary acidic protein and nestin in the SVZ (Type B cells, Chiasson et al., 1999; Doetsch et al., 1999; Laywell et al., 2000) that would originate from a pool of slowly dividing cells (Morshead et al., 1994). Other studies from the developing cortex demonstrated that radial glial cells generate neurons and glial cells (Hartfuss et al., 2001; Miyata et al., 2001). These data show that multiple cell types with NSC capabilities have been characterized from the CNS; however, these data need to be reconciled, or one or the others proven to be correct, to determine the cellular origin of the NSCs in the adult CNS. These data, taken together, confirm the existence of NSCs in the adult CNS and show that nestin is a marker for immature progenitor cells; however, several important issues remain to be addressed: the identification of the cellular origin(s) of NSCs, the biochemical markers of the NSCs, and the functions of the newly generated neural cells in the adult CNS. Homogeneous populations of NSC/NPCs have been isolated using cell surface markers from human fetal spinal cord and brain tissues (Uchida et al., 2000), by promoter-targeted selection from adult rat (Wang et al., 2000) and human SVZ (Roy et al., 2000b), and from the lateral ventricle wall of adult mice by negative selection (Rietze et al., 2001). These studies, by providing homogeneous populations of NSCs, will allow further study into the origin and molecular identity of the NSCs. This work was supported by separate grants from the NINDS, NIA, and the LookOut Fund. P.T. was supported by the Pasarow Foundation. We thank M.L. Gage for helpful criticism of the article.