Radial glial origin of the adult neural stem cells in the subventricular zone
Introduction
The central nervous system (CNS) of mammals is a highly complex structure made up of a huge number of neurons, glial cells, and synapses, all linked by extremely heterogeneous anatomical and functional relationships (Chklovskii et al., 2004). Its elaborate architecture is the result of subsequent cell divisions and precise cell–cell and cell–substrate interactions starting from a small amount of undifferentiated cells in the neural tube, then assembling throughout development, including a relatively short postnatal period (Bayer and Altman, 2004). In parallel with important regional differences under morphological and gene expression pattern profiles, a general rule involves peri-ventricular germinative layers harbouring neural stem cells as a source of most neuronal and glial cell precursors. The stem cell progeny herein generated populate the growing CNS parenchyma by a combination of centrifugal radial and to a lesser extent, tangential cell migration (Rakic, 1990, Marin and Rubenstein, 2003), later differentiating into specific neuronal and glial fates according to a precise spatial and temporal pattern, finally establishing appropriate connections.
After CNS assembly, besides the functional specificity of its structure and hardwiring, neural plasticity, namely the ability to make adaptive changes related to the architecture and function of the nervous system, remains a complementary attribute (Zilles, 1992). In addition to widespread structural changes reshaping adult neuronal circuits through multiple microscopic modifications mainly affecting synaptic contacts, neurogenesis does persist in the CNS of adult mammals, albeit in restricted domains (Gage, 2000, Gross, 2000). In the brain of adult rodents two consistently active germinative layers are present: the subventricular zone (SVZ), associated with the anterior part of the forebrain lateral ventricles, and the subgranular zone (SGZ), corresponding to the inner layer of the dentate gyrus, within the hippocampal formation (Peretto et al., 1999, Gage, 2000, Alvarez-Buylla and Garcìa-Verdugo, 2002; Fig. 1). The SVZ more evidently retains embryonic features of primitive germinal layers. Firstly, it maintains direct contact with the ventricles, whereas the SGZ loses such a contact after the ‘rollin in’ of the hippocampus during development (Smart, 1961). Furthermore, SVZ neuronal precursors undergo long-distance migration to reach their final site of destination in the olfactory bulb (Lois and Alvarez-Buylla, 1994), whereas those generated within the dentate gyrus differentiate locally (Zhao et al., 2006).
Although harbouring all subsequent steps of cell differentiation from stem cell division to cell replacement, persisting neurogenic sites do not faithfully recapitulate development. Indeed, they change pre- and post-natally under their morphological, cellular, and molecular profile in order to adapt to the non-permissive environment of the mature nervous tissue (Alves et al., 2002, Tramontin et al., 2003, Peretto et al., 2005). Apart from anatomical changes relating to a different conformation of cerebral ventricles (Bonfanti and Ponti, in press), the most evident differences concern SVZ glial cell types and their distribution (Fig. 1A). On the other hand, notwithstanding differences in the cytoarchitecture, a common pattern can be found under morphological and functional profiles in the cell composition of different neurogenic sites and at different developmental stages.
The primitive germinal layers consist of a primary proliferative zone called ventricular zone (VZ) containing direct descendants of the primitive neuroectoderm (neuroepithelium), and a secondary proliferative zone called subventricular zone (SVZ), which later emerges from the VZ. The SVZ contains rapidly proliferating cells and expands greatly during the last third of prenatal development, in parallel with progressive reduction of the VZ (for review see Levison and Goldman, 2006). This trend continues early postnatally (first postnatal week in rodents), followed by the disappearance of the VZ and the persistence of the SVZ exclusively within the forebrain.
The embryonic/perinatal SVZ contains cell types that are heterogeneous under the profile of their morphology and molecular expression, and whose exact origin has not yet been fully understood. Different regional specializations such as the dorsolateral neuroepithelium of the telencephalon, the medial (MGE) and lateral (LGE) ganglionic eminences do contribute cells to the forebrain SVZ (Levison and Goldman, 2006). In this context it is likely that cells with different potentials, ranging from stem cells to cell progenitors endowed with various differentiative capabilities, can coexist along the entire SVZ extension, their mutual relationships being modulated at different developmental stages. Beside the first migrating neural cell precursors, one of the earliest cells to differentiate in the developing CNS is radial glia. Radial glial cells are bipolar elements oriented orthogonally to the growing tissue, with a soma in the VZ sending a short cellular process to make contact with the ventricular surface (basal foot), and a long radial fiber reaching the pial surface with two or more enlarged endfeet (glia limitans) (Fig. 1, Fig. 2). These cells can be marked by several antigens (Hartfuss et al., 2001), such as RC1, RC2 (Misson et al., 1988), vimentin (Pixley and De Vellis, 1984), nestin (Hockfield and McKay, 1985), the calcium-binding protein S-100β, the brain lipid-binding protein BLBP (Feng and Heintz, 1995), the glutamate transporter GLAST (Shibata et al., 1997), and tenascin-C (Peretto et al., 2005), most of which can also be found in astrocytes. Some of these antigens, particularly vimentin and nestin, are not specific for radial glia since they are intermediate filament proteins present in a wide range of cell progenitor cells and immature glia. In addition, the expression and time of appearance of most antigens vary depending on the species and the developmental stages (e.g., in primates radial glial cells express GFAP very early, whereas in rodents they are GFAP-negative until the completion of corticogenesis; Rakic, 2003).
Another cell type appearing in late embryonic germinal layers is ependyma. In mice, most ependymal cells are generated from radial glia between embryonic days 14 and 16 (Spassky et al., 2005), a process that has been proposed might occur both through radial glia division and by its direct transformation into ependymal cells. These cells differentiate later, as revealed by the appearance of cilia during the first postnatal week.
In most CNS regions the germinal layers disappear soon after birth, leaving a non-germinal epithelium composed of multiciliated cells: the ependymal monolayer (Boulder Committee, 1970). On the lateral wall of the lateral ventricle the embryonic SVZ corresponding to the dorsolateral neuroepithelium, MGE, LGE, is more prominent (Anderson et al., 1999, Wichterle et al., 2001, Métin et al., 2006) then persisting throughout adulthood as a forebrain neurogenic site.
The VZ is thought to disappear in the postnatal and adult brain, replaced by an actively proliferating SVZ which continues to harbour neural stem cells. The anatomical arrangement of adult forebrain SVZ, as well as its extension, depends on the species and their cerebral ventricle conformation (Bonfanti and Ponti, in press). In laboratory rodents, due to the postnatal closure of the olfactory ventricle, only the middle/posterior part of the SVZ remains in contact with the ventricles, the anterior part forming a rostral extension or rostral migratory stream (RMS) through the olfactory peduncle and bulb axes (Peretto et al., 1999). Nevertheless, apart from the absence of the ependymal monolayer in the RMS of these species, the SVZ cytoarchitecture is substantially similar at all levels (Doetsch et al., 1997, Peretto et al., 1999, Alvarez-Buylla and Garcìa-Verdugo, 2002, Levison and Goldman, 2006). Two main cell compartments are detectable: (i) newly generated neuroblasts, which migrate in the form of tangentially-oriented chains (Lois et al., 1996, Doetsch and Alvarez-Buylla, 1996), and (ii) astrocytes forming a dense meshwork of intermingled cell bodies and processes throughout the SVZ area, thus delineating longitudinally-oriented channels called glial tubes (Jankovski and Sotelo, 1996, Lois et al., 1996, Peretto et al., 1997). Neuroblasts (also referred to as type A cells; Lois et al., 1996) are bipolar cells with a large nucleus and a thin rim of electrondense cytoplasm mostly co-expressing β-tubulin, doublecortin, and PSA-NCAM (Bonfanti and Theodosis, 1994, Menezes and Luskin, 1994, Rousselot et al., 1995, Nacher et al., 2001). Astrocytes (type B cells) are ramified cells with electronlucent, watery cytoplasm, characterized by immunocytochemical staining for GFAP (Jankovski and Sotelo, 1996, Lois et al., 1996, Peretto et al., 1997; Fig. 3G and H). These glial cells have some cytological and molecular features of radial glia, since they contain glycogen granules, the intermediate filaments vimentin and nestin (Jankovski and Sotelo, 1996, Peretto et al., 1997, Peretto et al., 1999, Doetsch, 2003a, Doetsch, 2003b), and glutamate transporters (Bolteus and Bordey, 2004). Furthermore, some of them retain a thin cellular process protruding in the ventricle through the ependymal monolayer (Doetsch et al., 1997). In addition to A and B cell types, a third element with intermediate ultrastructural features and high proliferative capacity has been identified as type C cells (Doetsch et al., 1997). Type C cells are considered to function as ‘transit amplifying’ cells in the neural stem cell niche, namely a bridge between the slow proliferating stem cells and their progeny of neuronal precursors (Doetsch, 2003a; see below). Indeed, the SVZ harbours a population of multipotent neural progenitor cells that can be isolated and expanded in culture under the effect of trophic factors such as the epidermal growth factor (EGF), the fibroblast growth factor 2 (FGF-2) or a combination of them, thus giving rise to self-renewing and multipotent neurospheres that can differentiate into neurons, astrocytes and oligodendrocytes (Reynolds and Weiss, 1992, Gage, 2000). Although stem cells can be isolated throughout the SVZ extension (Gritti et al., 2002), transit amplifying cells are rare in the RMS, thus suggesting a prevalent activity of the neural stem cell niche at the ventricular level, wherein ependyma represents a fourth cell type.
Hence, a major difference between embryonic and adult SVZ cell composition concerns the glial compartment, whereby a meshwork of astrocytes replaces radial glial cells in the adult. On the other hand the neuronal precursors seem to change only in their mutual relationships, splitting from a homogeneous mass into discrete chains.
A consistent number of reports support the hypothesis that adult neural stem cells belong to the astrocytic lineage (Doetsch et al., 1999a, Doetsch et al., 1999b, Laywell et al., 2000, Skogh et al., 2001, Imura et al., 2003; reviewed in Alvarez-Buylla et al., 2001, Alvarez-Buylla et al., 2002, Doetsch, 2003b). Experiments carried out in vitro and in vivo on the mouse SVZ demonstrated that (i) astrocytes and transit amplifying cells (type B and type C cells) can produce neurospheres that contain multipotent progenitor cells (Doetsch et al., 2002); (ii) after targeting SVZ astrocytes in transgenic GFAP-TVA mice which express the receptor of the avian leukosis virus under the control of the GFAP promoter, marked neurons migrating to the olfactory bulb were detected (Doetsch et al., 1999a); (iii) after elimination of transit amplifying cells and neuroblasts with the arabinofuranoside Ara C, the remaining astrocytes within the SVZ were able to regenerate the entire system (Doetsch et al., 1999b).
In addition, GFAP-expressing astrocytes from non-neurogenic CNS regions such as cortex, cerebellum, and spinal cord can produce glial and neuronal cells in vitro only if isolated before the second postnatal week (Laywell et al., 2000). On the other hand, astrocytes of the forebrain SVZ do retain stem cell potential, as observed in cultures from transgenic mice expressing herpes simplex virus thymidine kinase from the GFAP promoter, in which dividing GFAP-expressing cells can be selectively eliminated by anti-viral agents (Imura et al., 2003). The same conclusion has been reached in vivo after observation of depletion of newly generated neurons in the olfactory bulb (Garcia et al., 2004). The first studies aimed at identifying in vivo a cell category coinciding with neural stem cells have led to the conclusion that some SVZ astrocytes undergo a low rate cell division giving rise to type-C cells that proliferate with remarkable frequency (Doetsch et al., 1999a, Doetsch et al., 1999b). Such a transition coincides with a high expression of the EGF receptor, thought to be implicated in the shift from a relatively quiescent cell to a ‘transit amplifying’ cell capable of expanding the progenitor population (Doetsch et al., 2002).
Thus, substantial evidence indicates GFAP-expressing progenitor cells as a predominant source of constitutive adult neurogenesis, suggesting that neural stem cells could retain cytological aspect and functions of highly specialized cells in the CNS in contrast with the common idea that stem cells are highly undifferentiated elements (Alvarez-Buylla et al., 2001).
Section snippets
Early and recent history of radial glia
The early history of radial glia starts between the end of the 18th and the beginning of the 19th century with a number of observations spanning from Giuseppe Magini to Ramòn y Cajal (quoted in Bentivoglio and Mazzarello, 1999; Fig. 2A), classifying them as a well defined glial cell morphological phenotype. With the advent of electron microscopy, their role as transient scaffolding for neuronal radial migration from the neuraxis toward the pial surface was confirmed (Rakic, 1971). Particularly
Conclusive remarks and future perspectives
An increasing number of data indicate glial cells as active players in neural plasticity in both the developing and adult nervous system. Subsets of astrocytes and radial glia-like cells persisting in the adult brain can be involved in structural modifications allowing a modulation in the number of synapses that control the neuronal activity under physiological stimulation, as it has been well demonstrated in non-neurogenic areas such as the hypothalamo-neurohypophysial system (Theodosis et
Acknowledgements
This work was supported by MURST (F.I.R.B.), Compagnia di San Paolo (Progetto NEUROTRANSPLANT), Regione Piemonte, and University of Turin. We thank G. Zanutto for expertise in graphics.
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2017, Neuroscience and Biobehavioral ReviewsCitation Excerpt :A-type cells account for 26% of dividing cells in the SVZ (Doetsch et al., 2002) showing a cell cycle similar to that of C cells (Ponti et al., 2013). They migrate tangentially (up to 5 mm in rodents and 20 mm in monkeys) through the rostral migratory stream (RSM) to reach the OB (Abrous et al., 2005; Bonfanti and Peretto, 2007) where they disperse from the main cell stream, migrate radially, and incorporate into the granular and periglomerular layers of the OB (Doetsch and Alvarez-Buylla, 1996; Alvarez-Buylla et al., 2002 Alvarez-Buylla and Garcia-Verdugo, 2002; Hagg, 2005). Approximately 10.000 new neurons are produced every day in the adult rodent OB (Lois and Alvarez-Buylla, 1994).