The nervous system is one of the earliest organ systemsthat differentiate from the blastula stage embryo. In thehuman, the primitive neural tube forms by approximately thefourth week of gestation and neurogenesis has commencedby the fifth week. Development is quite prolonged and pro-ceeds throughout embryogenesis and myelination is notcompleted till late postnatal stages. As neural developmentproceeds, the initially formed tube undergoes differentialexpansion and regionalization to form identifiable rostrocau-dal regions that will generate the future subdivisions of thebrain. The anterior neural tube undergoes a dramatic expan-sion and can be delineated into three primary vesicles, theforebrain (prosencephalon), the midbrain (mesencephalon)and the hindbrain (rhombencephalon). Differential growth
and further segregation leads to additional delineation of theprosencephalon into the telencephalon and diencephalon andthe rhombencephalon into the metencephalon and myelen-cephalon. The caudal neural tube does not undergo a similarexpansion but does increase in size to parallel the growth ofthe embryo and undergoes further differentiation to form thespinal cord (Moskovkin et al., 1978; Cowan, 1979; Hersch-kowitz, 1988)
While the central nervous system (CNS) is undergoingregionalization, neural crest cells, which have differentiatedfrom the developing neural tube, have migrated to specificregions and begun to form peripheral nervous system (PNS)cells including glia. Additionally ectoderm in the craniofa-cial region is differentiating to form placodes which will giverise to additional PNS derivatives including glia (Schoenwolfand Nichols, 1984). Thus CNS and PNS glial cells haveclearly distinct lineages from very early stages of develop-ment (Figure 1). CNS precursors likely retain their ability togenerate PNS cells for at least a short time period in vivo andafter prolonged periods in culture (Keirstead et al., 1999;Mujtaba et al., 1999; Tsai and McKay, 2000), althoughwhether this is an important aspect of normal developmentremains unclear.
* To whom correspondence should be addressed. Tel: 410-558-8204;Fax: 410-558-8249.
In the developing CNS, glial differentiation parallels andoverlaps neuronal differentiation and can be examined inseparable stages. Like neuronal cells, glial cells are formedfrom the induced neuroectoderm and the initially formedglial precursors generated in the ventricular or subventricularzone migrate to their final destinations using cues similar tothose used by neurons. Precursor cells undergo maturationin-situ, aggregate with newly formed neurons to form spe-cific brain regions, and undergo selective apoptosis such thatan appropriate neuron to glia ratio of approximately 1:10 ismaintained. Glial progenitors are present throughout the pa-renchyma of the brain and likely divide at a slow rate in-situto provide cellular replacement of glial populations through-out life (Figure 2). Overall glial number is tightly regulatedand a constant proportion of neurons to support elements is
maintained. Regulatory processes include regulation of pro-liferation and apoptosis (Sommer and Rao, 2002).
Two major classes of glial cells oligodendrocytes andastrocytes that subserve distinct functions have been identi-fied. Astrocytes make up 20 to 50% of the volume of mostbrain areas and appear to be a more heterogeneous class ofcells that likely have many different roles. The degree ofastrocyte diversity in the CNS is unclear although at least twokinds of astrocytes can be distinguished based on morphol-ogy (fibrous and protoplasmic). A characteristic shared by allclasses of astrocytes is their expression of glial fibrillaryacidic protein (GFAP, Bignami et al., 1972) and/or the ex-pression by a significant proportion of S-100ß. Oligodendro-cytes are the second major population of glia and function toform the myelin sheaths that insulate axons and enable salta-tory conduction allowing myelinated nerve fibers to transmitimpulses up to ten times faster than non-myelinated fibers ofthe same diameter (Pellegrino et al., 1984). Oligodendro-cytes can be distinguished from astrocytes by morphology,antigenic expression and response to growth factors andcytokines. In addition to astrocytes and oligodendrocytes,several specialized glial cells have been identified. Theseinclude radial glia, Bergman glia, Muller glia, pituitary glia,olfactory ensheathing cells, etc. These cells originate fromdifferent regions of the brain and can be distinguished fromthe relatively more abundant astrocytes and oligodendro-cytes. These cells collectively have been termed aldynogliaon the basis of their overall similarity and specialized func-tions (Gudino-Cabrera and Nieto-Sampedro, 1999). Al-dynoglia share characteristics with both astrocytes and oligo-dendrocytes and overall resemble Schwann cells (Table 1).Radial glial cells are prototypic aldynoglial cells that providea scaffold for guiding neurons to their ultimate destinationsand are the first glial population that can be distinguishedfrom the proliferating neuroepithelium.
2. Multiple types of intermediate glial precursors
Glial cells must ultimately be derived from the neuroepi-thelial cells that formed the neural tube. Indeed, it has beenshown that the neural tube is comprised of multipotent stem
Fig. 1. Schematic illustration of neural differentiation in rodents.After the neural tube undergoes expansion and regionalization, differenttypes of cells of the CNS and PNS are formed.
Fig. 2. CNS glial differentiation.Glial progenitors are derived from ventricular zone (VZ) and subventricularzone (SVZ) stem cells. As they migrate, they start to differentiate andmature. A small population of progenitors continue to exist until adulthoodand may continue to mature into glia.
Table 1Aldynoglia in the central nervous system
Name CommentsRadial glia Function mainly in scaffoldingTanycytes and ependymal cells Supporting cells lining the
ventricular cavityPituitary glia Located in the posterior lobe of
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cells that can generate neurons, radial glia, astrocytes andoligodendrocytes in vitro and in vivo (reviewed in Rao,1999). What has become clear is that multipotent cells do notgenerate fully differentiated cells directly, rather they gener-ate intermediate or more restricted precursor cells that un-dergo progressive maturation to give rise to postmitotic ma-ture cells. This model of differentiation is conceptuallysimilar to the model of hematopoietic stem cell differentia-tion (reviewed in Rao, 1999) and predicts a direct lineagerelationship between multipotent ventricular zone neuroepi-thelial stem cells and/or subventricular zone (SVZ)-derivedneurosphere forming stem cells and more restricted precur-sor cell populations. The maturation process is likely a con-tinuum of events but stages can be distinguished based onmarker expression, restriction in differentiation potential,ability to divide and growth factor dependence. Multiplesuch restricted precursors have been identified in rodent and
human tissue (Figure 3, Mayer-Proschel et al., 1997; Rao,1999) and their existence substantiated by retroviral lineagestudies (Levison and Goldman, 1993; Levison and Goldman,1997)
We have shown for example that the neuroepithelial cellsthat comprise the early neural tube can differentiate into atleast three types of restricted precursors: a neuronal restrictedprecursor that generates mainly neurons; a glial restrictedprecursor, which generates mainly oligodendrocytes and as-trocytes; and a neural crest stem cell that generates peripheralnervous system (PNS) neurons, Schwann cells and craniofa-cial mesenchyme. Multiple additional intermediate precur-sors likely exist (reviewed in Lee et al., 2000; Rowitch et al.,2002) and several precursors with a restricted potential orbias for glial differentiation have been identified, includingoligodendrocyte and type-2 astrocyte precursors (O2A cells),
Fig. 3. Different types of glial precursors.Stem cells differentiate into oligodendrocytes and astrocytes via intermediate precursors. GRP cells, initially characterized by A2B5 immunoreactivity, acquirePDGFRa and NG2 expression later. APC and ARP are two glial precursors that are restricted to astrocyte lineage. Abbreviations: GRP, glial restricted precursor;O2A, oligodendrocyte type-2 astrocyte progenitors; MNOP, motoneuron-oligodendrocyte precursor; WMPC, white matter progenitor cell; APC, astrocyterestricted precursor; ARP, astrocyte restricted precursor. *, under certain conditions.
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glial restricted precursors (GRPs), polydendrocytes, moto-neuron-oligodendrocyte precursors (MNOPs), astrocyte pre-cursor cells (APCs), and white matter progenitors (WMPCs).Their characteristics are briefly described below.
2.1. Glial restricted precursor (GRP)
The earliest developing glial precursor identified so far isthe GRP cell. Initially isolated from the developing rat caudalneural tube, this precursor has been shown to exist through-out the rostrocaudal axis (Rao et al., 1998). GRPs cells can befirst isolated around E12 to E14 (Rao et al., 1998) and can bedistinguished from neuroepithelial cells (NEP) by the acqui-sition of A2B5 immunoreactivity. GRP cells are similar toO2As, in their ability to differentiate into oligodendrocytesand in their expression of the A2B5 epitope. GRP cells differfrom O2As in their ability to differentiate into astrocytes invitro and in vivo, their initial lack of NG2 and platelet derivedgrowth factor receptor a (PDGFRa) expression, and theirlimited adhesion to laminin and preference for fibronectin(Rao and Mayer-Proschel, 1997; Rao, 1999). GRP cells canbe localized to the ventral half of the developing neural tubebased on immunocytochemistry and microdissection experi-ments (Orentas and Miller, 1996; Rao et al., 1998; Liu et al.,2002). GRP cells therefore represent an early intermediateprogenitor committed to glial lineage between NEP cells andfully differentiated glia and may be one of the earliest bornglial precursors.
2.2. O2A
The O2A progenitor is one of the first and possibly thebest-characterized intermediate glial precursor (Noble et al.,1988). O2As were first isolated from the perinatal rat opticnerve (Raff et al., 1983). Later studies also identified O2As inthe neonatal rat cerebellar cortex (Levi et al., 1987), thecerebral cortex (Behar et al., 1988) and the spinal cord(Fok-Seang and Miller, 1994). O2A cells are A2B5 immu-noreactive, express PDGFRa and differentiate into oligoden-drocytes under the influence of thyroid hormone, retinoicacid and ciliary neurotrophic factor (CNTF). An intrinsictiming mechanism (clock) appears to regulate the number oftimes O2A cells divide before they stop proliferation andterminally differentiate (Bogler and Noble, 1994; Gao et al.,1997; Yakovlev et al., 1998). The clock can be dysregulatedby cytokine application, although after withdrawal of thecytokine, cells continue to differentiate into oligodendro-cytes (by default) in culture and following transplantation(Noble and Wolswijk, 1992; Groves et al., 1993). Morerecently it has been suggested that O2A cells can transforminto stem cells when exposed to appropriate environmentalsignals. Equally important, A2B5+ O2A cells appear to be-come immortal in culture under appropriate growth factorstimulus (Kondo and Raff, 2000). When grown in medium
supplemented with platelet derived growth factor (PDGF)and basic fibroblast growth factor (bFGF), they can indefi-nitely divide without undergoing senescence (Bogler andNoble, 1994), as is typical of most somatic cell populations.
In addition to oligodendrocytes, O2A cells can also differ-entiate into type-2 astrocytes in vitro, but have not beenshown to differentiate into neurons under any conditionstested. Type-2 astrocytes differ from the more commontype-1 astrocytes in their expression of A2B5 epitope and intheir absence of Ran-2 immunoreactivity and fibroblastgrowth factor receptor (FGFR) expression (Rao and Mayer-Proschel, 1997). Thus, O2A cells are bipotential in vitro andunipotential in vivo. Indeed, O2A cells were renamed oligo-dendroblasts as they did not appear to make astrocytes whentransplanted in vivo (Espinosa de los Monteros et al., 1993).Adult O2A cells have been isolated from optic nerves thatdisplay similar properties as their neonatal counterparts(Noble and Wolswijk, 1992); on the other hand, they differfrom the fetal O2A cells in that they divide much moreslowly. It has also been reported that fetal O2A might be ableto give rise to adult O2A. Cortical O2A-like cells have beenisolated from neonates and adults (Behar et al., 1988; Nobleet al., 1992), and a lineage relationship of GRP cells givingrise to O2A cells has been established (see below and Gregoriet al., 2002).
2.3. The Polydendrocyte
NG2, a proteoglycan, has been shown to identify a poten-tial glial precursor population (Nishiyama et al., 1997; Nish-iyama et al., 2002). NG2 proteoglycan and PDGFRa areco-expressed by glial cells and NG2 modulates the responseof progenitor cells to PDGFR activation (Grako et al., 1999).In vitro and in the developing brain in vivo, NG2 andPDGFRa are expressed on oligodendrocyte progenitor cellsbut are downregulated as the progenitor cells differentiateinto mature oligodendrocytes. In the mature CNS, numerousNG2+/PDGFRa+ cells with extensive arborization of theircell processes are found ubiquitously long after oligodendro-cytes are generated. NG2+ cells in the mature CNS do notexpress antigens specific to mature oligodendrocytes, astro-cytes, microglia, or neurons, suggesting that they are a glialprogenitor cell population. NG2+ cells in the adult CNS havebeen shown to undergo proliferation and morphologicalchanges in response to a variety of stimuli, such as demyeli-nation and inflammation. More recently it has been shownthat high levels of NG2 and PDGFRa are expressed onoligodendroglioma cells (Fleming et al., 1992; Shoshan etal., 1999), consistent with the possibility that theNG2+/PDGFRa+ cells in the mature CNS are initially glialprogenitors that can undergo neoplastic transformation. Howthe NG2+ polydendrocyte is related to the GRP and O2A cellremains unclear. Whether this cell is identical to the adultO2A cells or whether adult O2A cells are a subset of thispopulation needs further study. We have noted that NG2
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expression appears in a domain that overlaps the precedingexpression of A2B5 (Liu et al., 2002) and is coincident withthe acquisition of immunoreactivity for PDGFRa both invitro and in vivo, and that loss of glial progenitors in theOlig2, Nkx2.2 and Ngn3 null mice results in alterations inNG2 immunoreactivity (Lee et al., 2003; Liu and Rao, 2004).Thus, polydendrocytes may represent a stage in the con-tinuum of stem cells to oligodendrocyte differentiation al-though the exact stage of maturation represented by thepolydendrocytes remains to be determined. Recently, how-ever, NG2+ cells isolated from postnatal hippocampus havebeen shown to be multipotent and can differentiate intoneurons (Belachew et al., 2003). This indicates that NG2 as amarker may characterize distinct group of cells depending onthe developmental stage assessed.
2.4. The MNOP
More recently a novel intermediate precursor has beenproposed based on the results of the loss-of-function andgain-of-function of the transcription factors Olig1 and Olig2.Olig1/2 double knockouts affect the development of moto-neurons and oligodendrocytes, but not other types of neuralcells (Lu et al., 2002; Zhou and Anderson, 2002; Sun et al.,2003) and some laboratories have proposed the existence of amotoneuron-oligodendrocyte precursor (MNOP, Lu et al.,2001a; Zhou et al., 2001; Lu et al., 2002; Rowitch et al.,2002; Zhou and Anderson, 2002), which gives rise to botholigodendrocytes and a subtype of neurons – motoneurons inthe spinal cord. Investigators have proposed that a similarprecursor is likely to exist in the cortex where it generatesoligodendrocytes and neurons although data on such aneuron-oligodendrocyte precursor (NOP) is sparse (Rowitchet al., 2002).
The hypothesis that a MNOP exists needs additional veri-fication by clonal analysis and retroviral tracing but is con-sistent with the knockout data and pattern of gene expressionreported (Lu et al., 2002; Zhou and Anderson, 2002). It hasbeen difficult to assess the properties of this cell, as investi-gators have suggested that it is a labile population that doesnot maintain its differentiation characteristics in culture orafter perturbation in vivo. Indeed, Anderson and colleagues(Gabay et al., 2003) have reported that the MNOP dediffer-entiates into a multipotent stem cell when maintained in FGFin culture (in contrast to the more stable characteristics ofother glial precursors) and in knockouts in vivo can generateastrocytes, a fate not normally expected of this cell (Gabay etal., 2003). A further difficulty in examining this population ispresented by the fact that Olig1/2 expression which charac-terizes this cell is not unique to this population but persists inmore mature glial populations (Liu and Rao, 2004). The laterexpression of Olig1 and 2 does not identify a MNOP or aNOP but appears to be restricted to a glial progenitor cell thatmakes oligodendrocytes (and possibly astrocytes). Distin-guishing between these glial restricted cells and MNOPs is
difficult, as few clear-cut markers that define a MNOP exist.While like all other glia generating cells, MNOPs must arisefrom neuroepithelium, how this cell relates to other glialprogenitors is undetermined. It is clear, however, that severalreports have shown that the expression of Olig2 persists inthe adult and is elevated in gliomas (Lu et al., 2001b; Marie etal., 2001; Hoang-Xuan et al., 2002; Bouvier et al., 2003;Ohnishi et al., 2003), and in later stages of development,Olig2 expression may be restricted to glial progenitors.
2.5. Other types of glial precursors
Several additional types of glial precursors have beenreported. One isolated from the neonatal rat brain and fromneurospheres derived from the adult striatum (Avellana-Adalid et al., 1996; Zhang et al., 1999) was named an oligo-dendrocyte precursor, although its progeny includes astro-cytes as well. Another type of glial precursor called precursorto the oligodendrocyte-type-2 astrocyte has been describedby Grinspan and colleagues (1990) and Hardy and Reynolds(1991). This cell differs from the optic nerve O2A cell in itsexpression of A2B5. It responds to (PDGF) and differentiatesinto O2As under appropriate conditions and further differen-tiates into oligodendrocytes and type-2 astrocytes. A whitematter progenitor cell (WMPC) has been identified fromtissue in adult human brains (Roy et al., 1999; Nunes et al.,2003). WMPCs are A2B5 immunoreactive small bipolarcells, are capable of proliferation, and appear to retain theability to differentiate into neurons under certain cell cultureconditions and after in utero transplantation, which distin-guishes them from other glial precursors (Roy et al., 1999;Nunes et al., 2003). A recent report also showed that WMPCsfrom human brain white matter are multipotent, can formneurospheres and differentiate into neurons, oligodendro-cytes and astrocytes (Nunes et al., 2003). This behavior isreminiscent of the behavior of O2A cells which appear toalter their differentiation ability when exposed to the appro-priate environmental conditions, and contrasts with the be-havior of GRP cells which do not appear to generate neuronseven in clonal culture. While the ability of WMPCs to gener-ate neurons is not in doubt, it is important to note thatWMPCs or other glial precursors present in the adult do notmake neurons after injury or when transplanted at normaldensity. Special culture conditions or manipulations are re-quired to reveal their ability to generate neurons. This is incontrast with stem cells or neuronal precursors which willreadily generate neurons in vitro or in vivo. We would sug-gest therefore that WMPCs still be regarded as biased pro-genitors.
2.6. Astrocyte precursors
The in vivo origin of astrocytes in the developing brainand spinal cord is not yet clear. Several reports suggested
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that, in contrast to the ventral origin of oligodendrocytes,astrocytes might be generated from the dorsal neural tube(Noll and Miller, 1993; Fok-Seang and Miller, 1994; Pringleet al., 1998) . Several laboratories have reported the identifi-cation of astrocyte precursors. For example, Fok-Seang andMiller (1992) have reported an astrocyte precursor isolatedfrom neonatal spinal cord. This astrocyte precursor, whichalso expresses an epitope recognized by the A2B5 antibody,is highly migratory, proliferates in response to serum andPDGF, and can differentiate into astrocytes in culture. Mi andBarres (1999) isolated an astrocyte precursor from the E17rat optic nerve, which is characterized by Pax2 and A2B5immunoreactivity and absence of GFAP expression. In addi-tion, Seidman et al. (1997) have described an astrocyte cellline derived from immortalized glial precursor cells isolatedfrom E16 mouse cerebellum. Whittemore and colleaguesalso have shown that multipotent stem cells may becomerestricted to generating only astrocytes after prolonged cul-ture (Quinn et al., 1999). Data from these studies and resultsfrom our laboratory have suggested that astrocytes could begenerated from NEP, radial glia or GRP (Noble andWolswijk, 1992; Rao et al., 1998; Cai et al., 2002). However,it is not clear whether these cells differentiate into astrocytesmainly through the precursors described above and whetherthese precursors themselves are also lineally related. A com-plete set of data on the characteristics of these astrocyteprecursors is not currently available. Presumably, these astro-cyte precursors are distinct from other precursor populationsby their potential to differentiate only into astrocytes, but notoligodendrocytes or neurons, and it is likely that specific cellsurface markers will distinguish astrocyte precursors fromother types of glial precursors and stem cells. Recently, anextracellular matrix transmembrane protein, CD44, has beenreported to co-label with the astrocyte marker (GFAP). CD44expression is detected at an earlier stage than GFAP expres-
sion in the developing chick spinal cord (Alfei et al., 1999).CD44 expression also appears to be specific to the astrocytelineage, since early oligodendrocytes or radial glia are CD44negative (Moretto et al., 1993; Alfei et al., 1999). We havecharacterized CD44 as a marker for astrocyte precursors andsuccessfully purified this new type of astrocyte precursorcells from rat developing spinal cord using a CD44 mono-clonal antibody. By using clonal analysis and transplantationexperiments we here show that CD44+ astrocyte precursorsare derived from A2B5+ GRP cells, which are generated byNEP stem cells (Liu and Rao, unpublished results). More-over, a CD44 overexpression mouse model, CNP-CD44transgenic mouse strain, which was generated by cloningCD44 cDNA under the control of the mouse 2’,3’-cyclicnucleotide-3’-phosphodiesterase (CNP) promoter, showed aclear-cut phenotype of an increase of GFAP expression(Tuohy et al., 2004). This result indicates that CD44, not onlymay serve as an astrocyte precursor marker, but also mayplay a role in the differentiation of astrocytes.
3. Possible lineage relationship between precursors
Accumulated data have shown that most glial cells, in-cluding oligodendrocytes and astrocytes, arise from gliallineage restricted populations, while neurons are derivedfrom neuronal precursors. Both glial and neuronal precursorsare generated from multipotent stem cells. This is supportedby in vitro clonal analysis results and in vivo transplantationdata (Rao et al., 1998; Gregori et al., 2002). However, theintermediate stages of precursor cell differentiation remain tobe defined and many models relating precursor populationscan be constructed that explain the current observationsmade. A hypothetical relationship is schematized in Figure 4
Fig. 4. Lineage relationships of glial precursors.Glial restricted precursor (GRP) cell might be the earliest glial precursor identified so far. Polydendrocyte, O2A, and astrocyte precursor are highly likely derivedfrom GRP.A, astrocyte; MNOP, motoneuron-oligodendrocyte precursor; N, neuron; NOP, neuron-oligodendrocyte precursor; O, oligodendrocyte; ?, not determinated
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and data on some lineage relationships are summarized be-low. A lineage relationship between multipotent stem cellsand more restricted precursors been demonstrated. Workdone by Rao and colleagues have clearly demarcated thelineage relationship between multipotent NEP stem cells andmany of their more restricted progeny (Mayer-Proschel et al.,1997; Rao and Mayer-Proschel, 1997; Mujtaba et al., 1998;Rao et al., 1998). They have shown a direct lineage relation-ship between FGF-dependent NEP cells and GRP cells(Mayer-Proschel et al., 1997; Rao et al., 1998). The authorsgrew NEP stem cells in culture and then differentiated theminto A2B5 immunoreactive cells. When these A2B5+ cellswere isolated the authors found that NEP-derived A2B5immunoreactive cells differentiated into astrocytes and oli-godendrocytes but not into neurons. NEP-derived glial cellsappear morphologically and antigenically similar to GRPcells directly isolated from E13.5 neural tubes, demonstrat-ing a direct lineage relationship between multipotent NEPstem cells and GRP cells. This demonstration of a transitionfrom a NEP cell to a GRP cell provides the first evidence thatrestricted precursors are an intermediate stage between pluri-potent stem cells and fully differentiated, postmitotic cells.
In analogous experiments, a similar lineage relationshipbetween EGF-dependent neurosphere forming stem cells andoligodendroglial precursors (oligospheres) has been estab-lished. Duncan and colleagues showed that a glial-restrictedprecursor could be isolated from canine neurospheres or ratneurospheres by manipulating culture conditions (Zhang etal., 1998; Zhang et al., 1999). The authors reported that,unlike GRP cells, these oligospheres were initially A2B5immunonegative and subsequently acquired A2B5 immu-noreactivity. Cells could be maintained in culture for severalmonths and oligodendroglial progenitors underwent self-renewal and could generate astrocytes and oligodendrocytes.Transplanted oligospheres myelinated axons in vivo (Zhanget al., 1998), indicating that cultured precursor cells werefunctionally competent. Hence multipotent stem cells likelygenerate oligodendrocytes and astrocytes via a more re-stricted progenitor cell.
Mayer-Proschel and colleagues (Gregori et al., 2002) haveestablished a further lineage relationship between oligoden-droblasts and glial restricted progenitor cells. They haveshown that the tripotential GRP cell can generate a morerestricted precursor that generates oligodendrocytes but nottype-1 astrocytes. We have established that astrocyte precur-sor cells can be generated from GRP cells as well and can bedistinguished from GRP cells by the expression of CD44(Liu and Rao, unpublished results). Induction of the astrocyteprecursor fate is regulated by Notch/delta signaling and bybone morphogenetic protein (BMP) and leukemia inhibitoryfactor (CiF)/CNTF BMP and LIF/CNTF. Other more re-stricted glial precursors have been described (see above), butwhether they can be generated from cultured stem cellsremains to be proven.
This model has the advantage that it provides a frameworkthat integrates a large body of disparate data and offers
readily testable hypotheses. For example, one can testwhether radial glial cells arise from a GRP cell or whetherCD44 expression labels an astrocyte precursor cell. Suchexperiments are in progress and a clearer picture shouldemerge soon. It is important to note that while this model isappealing, it is likely not complete and does not explain allthe current data available.
The origin of astrocytes is still unclear. While a commonorigin of astrocytes from a GRP cell is consistent with the invitro and in vivo data, other compelling data clearly docu-ment the maturation of astrocytes from radial glial cells.Radial glia arise from neural stem cells and have differenti-ated at around the same time as the earliest identifiable glialprogenitor. Radial glial cells can be readily distinguishedfrom GRP cells in the developing brain and these cells can beshown to generate astrocytes in vitro and in vivo. The acqui-sition of GFAP expression appears to coincide with thetiming of radial glia differentiation and suggests that mostradial glia mature into astrocytes by birth. However, it isunlikely that all astrocytes arise from radial glia as a popula-tion of S-100ß+ cells is seen to arise from the ventricularzone that does not appear to express oligodendrocytic orneuronal or radial glial markers. This population of dividingcells is initially GFAP- and more predominant in the graymatter (Liu et al., 2002). Pulse labeling suggests that thispopulation matures to form GFAP+ cells present throughoutthe brain. The S-100ß+ cells are CD44+ and likely representa second source of astrocytes during development.
The model proposed here also does not explain how theMNOP/NOP is related to the other glial precursors proposed(see above). Perhaps multiple pathways to generating oligo-dendrocytes exist and the MNOP represents one such path-way. Indeed, this concept of multiple origins of glial precur-sors has been proposed earlier based on work done by Zalcand colleagues (Spassky et al., 1998). We note that much ofthe interpretation relies on work done in the caudal neuraltube and it is possible that cortical development is different. Itis also possible that the interpretation of the transgenic andknockout results has been confounded by the relative promis-cuity of Olig2 expression and the apparent discrepancy be-tween in vitro and in vivo results (Lu et al., 2002; Rowitch etal., 2002; Zhou and Anderson, 2002; Liu and Rao, 2003; Liuand Rao, 2004; Noble et al., 2004). We note that this popula-tion appears relatively labile and in our hands we note co-expression for Olig2 with stem cell markers in vivo, suggest-ing that Olig2 is required at multiple stages in development.It is possible that the knockouts reveal the role ofOlig1/2 only at specific stages. We note that retroviral lineagestudies in the spinal cord do not show the existence of aMNOP (Leber et al., 1990; Leber and Sanes, 1991; Levisonand Goldman, 1993; Levison and Goldman, 1997; Levison etal., 1999).
While clearly much remains to be ascertained, the currentmodels do offer a framework in which to test the effect offactors and knockout on glial differentiation.
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Overall the results suggest that two populations of stemcells present in the nervous system can generate astrocytesand oligodendrocytes. Both populations of stem cells likelygenerate differentiated progeny via the generation of pro-gressively more restricted precursor cells and several suchprecursor cells have been identified. A lineage relationshipbetween some precursor cells has been established which isconsistent with the temporal appearance of these precursorsin vivo.
4. Glial precursors and response to injury
Brain injury secondary to traumatic, ischemic or otheracute insults leads to a characteristic sequela probably bestcharacterized in traumatic injury models (Figure 5). Theprimary injury leads to a secondary injury response that ismediated via multiple molecular mechanisms including al-terations in the blood brain barriers (BBB), brain edema,release of cytokines and cell death regulators, activation ofthe immune system and activation of microglia. The acuteand secondary responses are followed by a prolonged inflam-matory response that can aid and antagonize neurodegenera-tion both locally and distal to the site of injury. This phase
overlaps with the endogenous repair and regenerative pro-cess that attempts to limit the damage, clear debris andregenerate to some limited extent (reviewed in Horner andGage, 2002; Kipnis et al., 2002; Ray et al., 2002; Schwartzand Hauben, 2002; Stoll et al., 2002 and references therein ).
What has become clear over the recent years has been thatwhile endogenous repair is limited, it does occur (Chen et al.,2003; Han et al., 2004). In the SVZ of the anterior ventricleand hippocampus, where significant numbers of stem cellshave been identified and where limited neurogenesis occurs(Suhonen et al., 1996; Seaberg and van der Kooy, 2002),there appears to be a stem cell response. This includes theincrease in the number of cells that incorporate BrdU andincrease in the number of newborn neurons and an increase inthe number of neurons which survive and integrate followinga kainite or ischemic lesion (Liu et al., 1998; Arvidsson et al.,2001; Jin et al., 2001; Zhang et al., 2001; Jin et al., 2002).
In other brain regions however, limited numbers of stemcells exist and the predominant response that is seen is glial.Glial progenitor cells present in the parenchyma proliferateand participate in reforming the BBB, and remyelinatingdemyelinated tracts. Irrespective of the exact lineage of theadult glial progenitor, what has become clear in recent yearsis that the predominant proliferative response after injury in
Fig. 5. Response to traumatic injury.Under normal conditions, Sox2+ stem cells reside in the subventricular zone around ventricle and glial precursors are distributed sporadically throughout thegray and white matter. After injury, the CNS responds by proliferation, activation of macrophages and the development of a glial scar. Glial precursors, ascharacterized by NG2, Nkx2.2, Olig2 expression, proliferate in response to the insult, but in most regions of the brain, ventricular zone (VZ) and SVZ cells failto proliferate or migrate towards the site of injury. Abbreviations: V, ventricle; SVZ, subventricular zone.
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the CNS is that of glial progenitors that are resident in theparenchyma (Horner and Gage, 2002; Kipnis et al., 2002;Ray et al., 2002; Schwartz and Hauben, 2002; Stoll et al.,2002; Han et al., 2004). This response is robust and has beenobserved in multiple models of disease, suggesting that cuesexist to direct glial progenitors to proliferate, migrate to aninjured site and participate in the repair process (Han et al.,2004).
More recent work has begun to identify the characteristicsof the glial cells that respond in injury. Work by Whittemore,Fischer and their colleagues has suggested that the cells thatrespond after injury are characterized by being present in theparenchyma, expressing Olig2 and NG2 and acquiringNkx2.2 immunoreactivity (Han et al., 2004, Talbott andWhittemore, in press). This precursor cell is capable of sur-viving in the damaged environment and migrating into thelesioned site to participate in remyelination of damaged tis-sue. These observations raise the possibility that modulatingthe glial reparative response by either transplantation ormobilization of endogenous precursors may be a logicalintervention for obtaining therapeutic improvement. Whilepreliminary this observation that endogenous precursors ex-ist and participate in a repair (albeit abortive) process pro-vides an important underpinning to the idea that transplant-ing cells may be useful. These results show that cues to directappropriate differentiation of precursor cells exist and thatselecting the appropriate cell type and timing for transplantwill be critical to ensuring success.
Interestingly despite recent reports that at least some glialcells can function as stem cells (Kukekov et al., 1997; Lay-well et al., 2000; Sanai et al., 2004), rarely is there any newneurogenesis seen. Neither endogenous glial cells or trans-planted glial progenitors will differentiate into neurons (Hanet al., 2002; Nunes et al., 2003; Han et al., 2004; Sanai et al.,2004). Given that under appropriate conditions a glial pro-genitor can generate neurons (Roy et al., 1999; Laywell andSteindler, 2002; Gotz and Steindler, 2003; Nunes et al., 2003;Steindler and Laywell, 2003), it suggests that inhibitory cuesthat prevent stem cells or glial progenitors from differentiat-ing into neurons exist. Indeed we have shown that this inhi-bition is likely at the transition from stem cell to a neuronrestricted precursor (an early stage in the process) and that ifone bypasses that stage then neurons can survive and inte-grate even in white matter compartments. Aruga and col-leagues (Aruga et al., 2002) have suggested that Notch acti-vation may be responsible for this inhibition. We and othershave noted an upregulation of Notch and its ligands jaggedand delta in a variety of injury paradigms, suggesting that thismay be important in vivo as well (Talbott and Whittemore,personal communication). Activated Notch signaling hasbeen shown inhibit the maturation of oligodendrocytes (Johnet al., 2002; Qiu et al., 2002; Wu et al., 2003) and thisprovides a possible explanation for why remyelination isgenerally partial after injury. These results while preliminarysuggest that it may be possible to mobilize endogenous
precursors and promote glial repair although providing neu-ronal replacement may be much more difficult.
5. Summary
Several different types of glial precursor cells have beenidentified in the past few years (Grinspan et al., 1990; Raoand Mayer-Proschel, 1997; Seidman et al., 1997; Zhang etal., 1998; Mi and Barres, 1999). GRPs can generate type-1astrocytes, type-2 astrocytes and oligodendrocytes, whileO2A cells and APCs generate subsets of these populations.All three classes of glial precursors are present at differentstages of development and a lineage relationship betweenthem has been demonstrated. Other classes of glial precur-sors likely exist however, and the existence of the MNOP hasbeen hypothesized based on expression of Olig2 and knock-out data. How these additional precursors relate to otherpreviously described precursors remains unclear. Either mul-tiple parallel paths to oligodendrocyte differentiation exist orcurrent models need to be reinterpreted. Irrespective howeverof the type of glial precursor, it is clear that glial precursorcells have the ability to respond to injury and can participatein the repair process.
Acknowledgements
This research was supported by NIA, and CNS and, ALSawards to MSR. YL was supported by NIA. We gratefullyacknowledge the input of all members of our laboratoryprovided through discussions and constructive criticisms.MSR acknowledges the contributions of Dr. S. Rao that madeundertaking this project possible.
References
Alfei, L., Aita, M., Caronti, B., De Vita, R., Margotta, V., MedolagoAlbani, L., Valente, A.M., 1999. Hyaluronate receptor CD44 isexpressed by astrocytes in the adult chicken and in astrocyte cell precur-sors in early development of the chick spinal cord. Eur J Histochem 43,29–38.
Aruga, J., Tohmonda, T., Homma, S., Mikoshiba, K., 2002. Zic1 promotesthe expansion of dorsal neural progenitors in spinal cord by inhibitingneuronal differentiation. Dev Biol 244, 329–341.
Arvidsson, A., Kokaia, Z., Lindvall, O., 2001. N-methyl-D-aspartatereceptor-mediated increase of neurogenesis in adult rat dentate gyrusfollowing stroke. Eur J Neurosci 14, 10–18.
Avellana-Adalid, V., Nait-Oumesmar, B., Lachapelle, F., Baron-Van Ever-cooren, A., 1996. Expansion of rat oligodendrocyte progenitors intoproliferative “oligospheres” that retain differentiation potential. J Neu-rosci Res 45, 558–570.
Behar, T., McMorris, F.A., Novotny, E.A., Barker, J.L., Dubois-Dalcq, M.,1988. Growth and differentiation properties of O-2A progenitors puri-fied from rat cerebral hemispheres. J Neurosci Res 21, 168–180.
287Y. Liu, M.S. Rao / Biology of the Cell 96 (2004) 279–290
Belachew, S., Chittajallu, R., Aguirre, A.A., Yuan, X., Kirby, M., Ander-son, S., Gallo, V., 2003. Postnatal NG2 proteoglycan-expressing pro-genitor cells are intrinsically multipotent and generate functional neu-rons. J Cell Biol 161, 169–186.
Bignami,A., Eng, L.F., Dahl, D., Uyeda, C.T., 1972. Localization of the glialfibrillary acidic protein in astrocytes by immunofluorescence. Brain Res43, 429–435.
Bogler, O., Noble, M., 1994. Measurement of time in oligodendrocyte-type-2 astrocyte (O-2A) progenitors is a cellular process distinct fromdifferentiation or division. Dev Biol 162, 525–538.
Bouvier, C., Bartoli, C., Aguirre-Cruz, L., Virard, I., Colin, C., Fernan-dez, C., Gouvernet, J., Figarella-Branger, D., 2003. Shared oligodendro-cyte lineage gene expression in gliomas and oligodendrocyte progenitorcells. J Neurosurg 99, 344–350.
Cai, J., Wu, Y., Mirua, T., Pierce, J.L., Lucero, M.T., Albertine, K.H.,Spangrude, G.J., Rao, M.S., 2002. Properties of a fetal multipotentneural stem cell (NEP cell). Dev Biol 251, 221–240.
Chen, S., Pickard, J.D., Harris, N.G., 2003. Time course of cellular pathol-ogy after controlled cortical impact injury. Exp Neurol 182, 87–102.
Cowan, W.M., 1979. The development of the brain. Sci Am 241, 113–133.
Espinosa de los Monteros, A., Zhang, M., De Vellis, J., 1993. O2A progeni-tor cells transplanted into the neonatal rat brain develop into oligoden-drocytes but not astrocytes. Proc Natl Acad Sci U S A 90, 50–54.
Fleming, T.P., Saxena, A., Clark, W.C., Robertson, J.T., Oldfield, E.H.,Aaronson, S.A., Ali, I.U., 1992. Amplification and/or overexpression ofplatelet-derived growth factor receptors and epidermal growth factorreceptor in human glial tumors. Cancer Res 52, 4550–4553.
Fok-Seang, J., Miller, R.H., 1992. Astrocyte precursors in neonatal rat spinalcord cultures. J Neurosci 12, 2751–2764.
Fok-Seang, J., Miller, R.H., 1994. Distribution and differentiation of A2B5+glial precursors in the developing rat spinal cord. J Neurosci Res 37,219–235.
Gabay, L., Lowell, S., Rubin, L.L., Anderson, D.J., 2003. Deregulation ofdorsoventral patterning by FGF confers trilineage differentiation capac-ity on CNS stem cells in vitro. Neuron 40, 485–499.
Gao, F.B., Durand, B., Raff, M., 1997. Oligodendrocyte precursor cellscount time but not cell divisions before differentiation. Curr Biol 7,152–155.
Gotz, M., Steindler, D., 2003. To be glial or not-how glial are the precursorsof neurons in development and adulthood? Glia 43, 1–3.
Grako, K.A., Ochiya, T., Barritt, D., Nishiyama, A., Stallcup, W.B., 1999.PDGF (alpha)-receptor is unresponsive to PDGF-AA in aortic smoothmuscle cells from the NG2 knockout mouse. J Cell Sci 112 (Pt 6),905–915.
Gregori, N., Proschel, C., Noble, M., Mayer-Proschel, M., 2002. The tripo-tential glial-restricted precursor (GRP) cell and glial development in thespinal cord: generation of bipotential oligodendrocyte-type-2 astrocyteprogenitor cells and dorsal-ventral differences in GRP cell function. JNeurosci 22, 248–256.
Grinspan, J.B., Stern, J.L., Pustilnik, S.M., Pleasure, D., 1990. Cerebralwhite matter contains PDGF-responsive precursors to O2A cells. JNeurosci 10, 1866–1873.
Groves, A.K., Barnett, S.C., Franklin, R.J., Crang, A.J., Mayer, M.,Blakemore, W.F., Noble, M., 1993. Repair of demyelinated lesions bytransplantation of purified O-2A progenitor cells. Nature 362, 453–455.
Gudino-Cabrera, G., Nieto-Sampedro, M., 1999. Estrogen receptor immu-noreactivity in Schwann-like brain macroglia. J Neurobiol 40, 458–470.
Han, S.S., Kang, D.Y., Mujtaba, T., Rao, M.S., Fischer, I., 2002. Graftedlineage-restricted precursors differentiate exclusively into neurons in theadult spinal cord. Exp Neurol 177, 360–375.
Han, S.S., Liu, Y., Tyler-Polsz, C., Rao, M.S., Fischer, I., 2004. Transplan-tation of glial-restricted precursor cells into the adult spinal cord: sur-vival, glial-specific differentiation, and preferential migration in whitematter. Glia 45, 1–16.
Hardy, R., Reynolds, R., 1991. Proliferation and differentiation potential ofrat forebrain oligodendroglial progenitors both in vitro and in vivo.Development 111, 1061–1080.
Herschkowitz, N., 1988. Brain development in the fetus, neonate and infant.Biol Neonate 54, 1–19.
Hoang-Xuan, K., Aguirre-Cruz, L., Mokhtari, K., Marie, Y., Sanson, M.,2002. OLIG-1 and 2 gene expression and oligodendroglial tumours.Neuropathol Appl Neurobiol 28, 89–94.
Horner, P.J., Gage, F.H., 2002. Regeneration in the adult and aging brain.Arch Neurol 59, 1717–1720.
Jin, K., Minami, M., Lan, J.Q., Mao, X.O., Batteur, S., Simon, R.P., Green-berg, D.A., 2001. Neurogenesis in dentate subgranular zone and rostralsubventricular zone after focal cerebral ischemia in the rat. Proc NatlAcad Sci U S A 98, 4710–4715.
Jin, K., Mao, X.O., Sun,Y., Xie, L., Greenberg, D.A., 2002. Stem cell factorstimulates neurogenesis in vitro and in vivo. J Clin Invest 110, 311–319.
John, G.R., Shankar, S.L., Shafit-Zagardo, B., Massimi, A., Lee, S.C.,Raine, C.S., Brosnan, C.F., 2002. Multiple sclerosis: re-expression of adevelopmental pathway that restricts oligodendrocyte maturation. NatMed 8, 1115–1121.
Keirstead, H.S., Ben-Hur, T., Rogister, B., O’Leary, M.T., Dubois-Dalcq, M., Blakemore, W.F., 1999. Polysialylated neural cell adhesionmolecule-positive CNS precursors generate both oligodendrocytes andSchwann cells to remyelinate the CNS after transplantation. J Neurosci19, 7529–7536.
Kipnis, J., Mizrahi, T., Hauben, E., Shaked, I., Shevach, E., Schwartz, M.,2002. Neuroprotective autoimmunity: naturally occurring CD4+CD25+regulatory T cells suppress the ability to withstand injury to the centralnervous system. Proc Natl Acad Sci U S A 99, 15620–15625.
Kondo, T., Raff, M., 2000. Oligodendrocyte precursor cells reprogrammedto become multipotential CNS stem cells. Science 289, 1754–1757.
Kukekov, V.G., Laywell, E.D., Thomas, L.B., Steindler, D.A., 1997. Anestin-negative precursor cell from the adult mouse brain gives rise toneurons and glia. Glia 21, 399–407.
Laywell, E.D., Rakic, P., Kukekov, V.G., Holland, E.C., Steindler, D.A.,2000. Identification of a multipotent astrocytic stem cell in the immatureand adult mouse brain. Proc Natl Acad Sci U S A 97, 13883–13888.
Laywell, E.D., Steindler, D.A., 2002. Glial stem-like cells: implications forontogeny, phylogeny, and CNS regeneration. Prog Brain Res 138, 435–450.
Leber, S.M., Breedlove, S.M., Sanes, J.R., 1990. Lineage, arrangement, anddeath of clonally related motoneurons in chick spinal cord. J Neurosci10, 2451–2462.
Leber, S.M., Sanes, J.R., 1991. Lineage analysis with a recombinantretrovirus: application to chick spinal motor neurons. Adv Neurol 56,27–36.
Lee, J., Wu, Y., Qi, Y., Xue, H., Liu, Y., Scheel, D., German, M., Qiu, M.,Guillemot, F., Rao, M., 2003. Neurogenin3 participates in gliogenesis inthe developing vertebrate spinal cord. Dev Biol 253, 84–98.
Lee, J.C., Mayer-Proschel, M., Rao, M.S., 2000. Gliogenesis in the centralnervous system. Glia 30, 105–121.
Levi, G., Aloisi, F., Wilkin, G.P., 1987. Differentiation of cerebellar bipoten-tial glial precursors into oligodendrocytes in primary culture: develop-mental profile of surface antigens and mitotic activity. J Neurosci Res 18,407–417.
Levison, S.W., Goldman, J.E., 1993. Both oligodendrocytes and astrocytesdevelop from progenitors in the subventricular zone of postnatal ratforebrain. Neuron 10, 201–212.
288 Y. Liu, M.S. Rao / Biology of the Cell 96 (2004) 279–290
Levison, S.W., Goldman, J.E., 1997. Multipotential and lineage restrictedprecursors coexist in the mammalian perinatal subventricular zone. JNeurosci Res 48, 83–94.
Levison, S.W.,Young, G.M., Goldman, J.E., 1999. Cycling cells in the adultrat neocortex preferentially generate oligodendroglia. J Neurosci Res 57,435–446.
Liu, J., Solway, K., Messing, R.O., Sharp, F.R., 1998. Increased neurogen-esis in the dentate gyrus after transient global ischemia in gerbils. JNeurosci 18, 7768–7778.
Liu, Y., Wu, Y., Lee, J.C., Xue, H., Pevny, L.H., Kaprielian, Z., Rao, M.S.,2002. Oligodendrocyte and astrocyte development in rodents: an in situand immunohistological analysis during embryonic development. Glia40, 25–43.
Liu, Y., Rao, M., 2003. Oligodendrocytes, GRPs and MNOPs. TrendsNeurosci 26, 410–412.
Liu, Y., Rao, M.S., 2004. Olig genes are expressed in a heterogeneouspopulation of precursor cells in the developing spinal cord. Glia 45,67–74.
Lu, Q.R., Cai, L., Rowitch, D., Cepko, C.L., Stiles, C.D., 2001a. Ectopicexpression of Olig1 promotes oligodendrocyte formation and reducesneuronal survival in developing mouse cortex. Nat Neurosci 4, 973–974.
Lu, Q.R., Park, J.K., Noll, E., Chan, J.A., Alberta, J., Yuk, D.,Alzamora, M.G., Louis, D.N., Stiles, C.D., Rowitch, D.H., Black, P.M.,2001b. Oligodendrocyte lineage genes (OLIG) as molecular markers forhuman glial brain tumors. Proc Natl Acad Sci U S A 98, 10851–10856.
Lu, Q.R., Sun, T., Zhu, Z., Ma, N., Garcia, M., Stiles, C.D., Rowitch, D.H.,2002. Common developmental requirement for Olig function indicates amotor neuron/oligodendrocyte connection. Cell 109, 75–86.
Marie, Y., Sanson, M., Mokhtari, K., Leuraud, P., Kujas, M., Delattre, J.Y.,Poirier, J., Zalc, B., Hoang-Xuan, K., 2001. OLIG2 as a specific markerof oligodendroglial tumour cells. Lancet 358, 298–300.
Mi, H., Barres, B.A., 1999. Purification and characterization of astrocyteprecursor cells in the developing rat optic nerve. J Neurosci 19, 1049–1061.
Moretto, G., Xu, R.Y., Kim, S.U., 1993. CD44 expression in human astro-cytes and oligodendrocytes in culture. J Neuropathol Exp Neurol 52,419–423.
Moskovkin, G.N., Fulop, Z., Hajos, F., 1978. Origin and proliferation ofastroglia in the immature rat cerebellar cortex.A double label autoradio-graphic study. Acta Morphol Acad Sci Hung 26, 101–106.
Mujtaba, T., Mayer-Proschel, M., Rao, M.S., 1998. A common neuralprogenitor for the CNS and PNS. Dev Biol 200, 1–15.
Mujtaba, T., Piper, D.R., Kalyani, A., Groves, A.K., Lucero, M.T.,Rao, M.S., 1999. Lineage-restricted neural precursors can be isolatedfrom both the mouse neural tube and cultured ES cells. Dev Biol 214,113–127.
Nishiyama, A.,Yu, M., Drazba, J.A., Tuohy, V.K., 1997. Normal and reactiveNG2+ glial cells are distinct from resting and activated microglia. JNeurosci Res 48, 299–312.
Nishiyama, A., Watanabe, M., Yang, Z., Bu, J., 2002. Identity, distribution,and development of polydendrocytes: NG2-expressing glial cells. J Neu-rocytol 31, 437–455.
Noble, M., Murray, K., Stroobant, P., Waterfield, M.D., Riddle, P., 1988.Platelet-derived growth factor promotes division and motility and inhib-its premature differentiation of the oligodendrocyte/type-2 astrocyteprogenitor cell. Nature 333, 560–562.
Noble, M., Wolswijk, G., 1992. Development and regeneration in the O-2Alineage: studies in vitro and in vivo. J Neuroimmunol 40, 287–293.
Noble, M., Wren, D., Wolswijk, G., 1992. The O-2A(adult) progenitor cell:a glial stem cell of the adult central nervous system. Semin Cell Biol 3,413–422.
Noble, M., Proschel, C., Mayer-Proschel, M., 2004. Getting a GR(i)P onoligodendrocyte development. Dev Biol 265, 33–52.
Noll, E., Miller, R.H., 1993. Oligodendrocyte precursors originate at theventral ventricular zone dorsal to the ventral midline region in theembryonic rat spinal cord. Development 118, 563–573.
Nunes, M.C., Roy, N.S., Keyoung, H.M., Goodman, R.R., McK-hann 2nd, G., Jiang, L., Kang, J., Nedergaard, M., Goldman, S.A., 2003.Identification and isolation of multipotential neural progenitor cells fromthe subcortical white matter of the adult human brain. Nat Med 9,439–447.
Ohnishi, A., Sawa, H., Tsuda, M., Sawamura, Y., Itoh, T., Iwasaki, Y.,Nagashima, K., 2003. Expression of the oligodendroglial lineage-associated markers Olig1 and Olig2 in different types of human gliomas.J Neuropathol Exp Neurol 62, 1052–1059.
Orentas, D.M., Miller, R.H., 1996. A novel form of migration of glialprecursors. Glia 16, 27–39.
Pellegrino, R.G., Spencer, P.S., Ritchie, J.M., 1984. Sodium channels in theaxolemma of unmyelinated axons: a new estimate. Brain Res 305,357–360.
Pringle, N.P., Guthrie, S., Lumsden, A., Richardson, W.D., 1998. Dorsalspinal cord neuroepithelium generates astrocytes but not oligodendro-cytes. Neuron 20, 883–893.
Qiu, J., Cai, D., Dai, H., McAtee, M., Hoffman, P.N., Bregman, B.S.,Filbin, M.T., 2002. Spinal axon regeneration induced by elevation ofcyclic AMP. Neuron 34, 895–903.
Quinn, S.M., Walters, W.M., Vescovi, A.L., Whittemore, S.R., 1999. Lin-eage restriction of neuroepithelial precursor cells from fetal humanspinal cord. J Neurosci Res 57, 590–602.
Raff, M.C., Miller, R.H., Noble, M., 1983. A glial progenitor cell thatdevelops in vitro into an astrocyte or an oligodendrocyte depending onculture medium. Nature 303, 390–396.
Rao, M.S., Mayer-Proschel, M., 1997. Glial-restricted precursors arederived from multipotent neuroepithelial stem cells. Dev Biol 188,48–63.
Rao, M.S., Noble, M., Mayer-Proschel, M., 1998. A tripotential glial precur-sor cell is present in the developing spinal cord. Proc Natl Acad Sci U SA 95, 3996–4001.
Rao, M.S., 1999. Multipotent and restricted precursors in the central nervoussystem. Anat Rec 257, 137–148.
Ray, S.K., Dixon, C.E., Banik, N.L., 2002. Molecular mechanisms in thepathogenesis of traumatic brain injury. Histol Histopathol 17, 1137–1152.
Roy, N.S., Wang, S., Harrison-Restelli, C., Benraiss, A., Fraser, R.A.,Gravel, M., Braun, P.E., Goldman, S.A., 1999. Identification, isolation,and promoter-defined separation of mitotic oligodendrocyte progenitorcells from the adult human subcortical white matter. J Neurosci 19,9986–9995.
Sanai, N., Tramontin, A.D., Quinones-Hinojosa, A., Barbaro, N.M.,Gupta, N., Kunwar, S., Lawton, M.T., McDermott, M.W., Parsa, A.T.,Manuel-Garcia Verdugo, J., Berger, M.S., Alvarez-Buylla, A., 2004.Unique astrocyte ribbon in adult human brain contains neural stem cellsbut lacks chain migration. Nature 427, 740–744.
Schoenwolf, G.C., Nichols, D.H., 1984. Histological and ultrastructuralstudies on the origin of caudal neural crest cells in mouse embryos. JComp Neurol 222, 496–505.
Schwartz, M., Hauben, E., 2002. T cell-based therapeutic vaccination forspinal cord injury. Prog Brain Res 137, 401–406.
289Y. Liu, M.S. Rao / Biology of the Cell 96 (2004) 279–290
Seaberg, R.M., van der Kooy, D., 2002. Adult rodent neurogenic regions: theventricular subependyma contains neural stem cells, but the dentategyrus contains restricted progenitors. J Neurosci 22, 1784–1793.
Seidman, K.J., Teng, A.L., Rosenkopf, R., Spilotro, P., Weyhenmeyer, J.A.,1997. Isolation, cloning and characterization of a putative type-1 astro-cyte cell line. Brain Res 753, 18–26.
Shoshan, Y., Nishiyama, A., Chang, A., Mork, S., Barnett, G.H., Cow-ell, J.K., Trapp, B.D., Staugaitis, S.M., 1999. Expression of oligodendro-cyte progenitor cell antigens by gliomas: implications for the histogen-esis of brain tumors. Proc Natl Acad Sci U S A 96, 10361–10366.
Sommer, L., Rao, M., 2002. Neural stem cells and regulation of cell number.Prog Neurobiol 66, 1–18.
Spassky, N., Goujet-Zalc, C., Parmantier, E., Olivier, C., Martinez, S.,Ivanova, A., Ikenaka, K., Macklin, W., Cerruti, I., Zalc, B., Thomas, J.L.,1998. Multiple restricted origin of oligodendrocytes. J Neurosci 18,8331–8343.
Steindler, D.A., Laywell, E.D., 2003. Astrocytes as stem cells: nomencla-ture, phenotype, and translation. Glia 43, 62–69.
Stoll, G., Jander, S., Schroeter, M., Kipnis, J., Mizrahi, T., Hauben, E.,Shaked, I., Shevach, E., Schwartz, M., Horner, P.J., Gage, F.H., 2002.Detrimental and beneficial effects of injury-induced inflammation andcytokine expression in the nervous system Neuroprotectiveautoimmunity: naturally occurring CD4+CD25+ regulatory T cells sup-press the ability to withstand injury to the central nervous system Regen-eration in the adult and aging brain. Adv Exp Med Biol 513, 87–113.
Suhonen, J.O., Peterson, D.A., Ray, J., Gage, F.H., 1996. Differentiation ofadult hippocampus-derived progenitors into olfactory neurons in vivo.Nature 383, 624–627.
Sun, T., Dong, H., Wu, L., Kane, M., Rowitch, D.H., Stiles, C.D., 2003.Cross-repressive interaction of the Olig2 and Nkx2.2 transcription fac-tors in developing neural tube associated with formation of a specificphysical complex. J Neurosci 23, 9547–9556.
Tuohy, T.M.F., Wallingford, N., Liu, Y., Chan, F., Rizvi, T., Xing, R.,Bebo, B., Rao, M.S., Sherman, L.S., 2004. CD44 overexpression byoligodendrocytes: A novel mouse model of inflammation-independentdemyelination and dysmyelination. Glia In press.
Wu, Y., Liu, Y., Levine, E.M., Rao, M.S., 2003. Hes1 but not Hes5 regulatesan astrocyte versus oligodendrocyte fate choice in glial restricted precur-sors. Dev Dyn 226, 675–689.
Yakovlev,A.Y., Mayer-Proschel, M., Noble, M., 1998.A stochastic model ofbrain cell differentiation in tissue culture. J Math Biol 37, 49–60.
Zhang, R.L., Zhang, Z.G., Zhang, L., Chopp, M., 2001. Proliferation anddifferentiation of progenitor cells in the cortex and the subventricularzone in the adult rat after focal cerebral ischemia. Neuroscience 105,33–41.
Zhang, S.C., Lipsitz, D., Duncan, I.D., 1998. Self-renewing canine oligo-dendroglial progenitor expanded as oligospheres. J Neurosci Res 54,181–190.
Zhang, S.C., Ge, B., Duncan, I.D., 1999. Adult brain retains the potential togenerate oligodendroglial progenitors with extensive myelination capac-ity. Proc Natl Acad Sci U S A 96, 4089–4094.
Zhou, Q., Choi, G., Anderson, D.J., 2001. The bHLH transcription factorOlig2 promotes oligodendrocyte differentiation in collaboration withNkx2.2. Neuron 31, 791–807.
Zhou, Q., Anderson, D.J., 2002. The bHLH transcription factors OLIG2 andOLIG1 couple neuronal and glial subtype specification. Cell 109,61–73.
290 Y. Liu, M.S. Rao / Biology of the Cell 96 (2004) 279–290
