Neural progenitor diversity and their therapeutic potential for spinal cord repair

Abstract

Development of the central nervous system (CNS) requires progressive differentiation of neural stem cells, which generate a variety of neural progenitors with distinct properties and differentiation potentials in a spatiotemporally restricted manner. The underlying mechanisms of neural progenitor diversification during development started to be unraveled over the past years. We have addressed these questions by v-myc immortalization method and generation of neural progenitor clones. These clones are served as in vitro models of neural differentiation and cellular tools for transplantation in animal models of neurological disorders including spinal cord injury. In this review, we will discuss features of two neural progenitor types (radial glia and GABAergic interneuron progenitor) and diversification even within each progenitor type. We will also discuss pathophysiology of spinal cord injury and our ongoing research to address both motor and sensory malfunctions by transplantation of these neural progenitors.

Keywords

neural progenitors / diversity / radial glia / interneuron progenitor / spinal cord injury / cell transplantation

Neural progenitor diversity

Stem cells, neural stem cells and neural progenitors

Stem cells are defined by their ability to self-renew and to produce differentiated progeny by cell division. The first proof of stem cell existence arose from the work of Till and Becker, who were studying hematopoietic stem cells derived from bone marrow (Till and McCULLOCH, 1961; Becker et al., 1963). Pluripotent embryonic stem cells (ESCs) were first isolated from the inner cell mass (ICM) of the mouse blastocyst (Evans and Kaufman, 1981; Martin, 1981). For a short time window during development, ICM cells possess the capacity to differentiate into every cell type of the adult body. Recent work has demonstrated a broad variety of potential sources and culture methods for preparing stem cells, including cultured stem cells from early embryos (Kim et al., 2002; Ying et al., 2003; Wu et al., 2010), bone marrow (Woodbury et al., 2000; Spiropoulos et al., 2010), umbilical cord blood (Lee et al., 2010), and adult tissues (Shihabuddin et al., 2000).

Neural stem cells (NSCs), a simple type of cells, give rise to complicate and yet precisely organized neural tissues such as the mammalian brain. This process and the underlying mechanisms are always attractive to neurobiologists. NSCs are multipotent giving rise to most mature CNS cell types including neurons, astrocytes and oligodendrocytes. As development proceeds, NSCs acquire additional features (caused by their different gene expression profiles) and become regionally diversified. These cells still proliferate, but their differentiation potential may be limited, capable of giving rise to fewer cell types as they divide. This group of dividing cells at any CNS developmental stages is often referred as “neural progenitor pool”.

Lineage restriction hypothesis indicates that NSCs undergo progressive loss of differentiation potential (i.e. the number of cell types they can give rise to) as neural development proceeds. The dividing progenitors with limited potentials are called lineage-restricted precursors, which include at least two major populations. One population, called neuronal restricted precursors (NRPs) and expressing polysialylated neural cell adhesion molecule (PSA-NCAM), can be fractioned from embryonic spinal cord using monoclonal antibody 5A5 (Mayer-Proschel et al., 1997). A different population, called glial restricted precursors (GRPs), can be fractioned using the A2B5 antibody, which recognizes a carbohydrate antigen different from 5A5 (Mayer-Proschel et al., 1997). In developing spinal cord, the distributions of cells expressing 5A5 and A2B5 are non-overlapping (Liu et al., 2002). It has been suggested that most of the precursor cells in the ventricular zone (VZ) of E14.5 rat spinal cord are either A2B5⁺ or 5A5⁺, with less than 10% of the cells lacking either marker (Cai et al., 2002). Lineage restriction is not the only mechanism of NSC development. One exception in spinal cord is that ventrally derived motor neurons and oligodendrocytes originate from an Olig2⁺ common progenitor (MNOP) in a sequential manner (Lu et al., 2002; Noble et al., 2004).

Radial glia and its diversity

Radial glia (RG), a fascinating cell type with unique bipolar radial morphology, has drawn a great deal of attention over the past years. Rakic named these cells “radial glia” based both on their morphology and characteristics they share with astroglia (Rakic, 1971). RG cells are located in most of the developing CNS where laminated structures are found. These include developing cortex, cerebellum, retina and spinal cord. The cell bodies of RG are confined to the VZ, but their processes span the width of the neural tube. Consistent with their morphology, one major function of RG is to serve as guides for radial migration of neural cells from proliferative zones to their postmitotic destinations (Rakic, 1990). RG has long been recognized as glial progenitors, but years ago, several laboratories presented strong evidence that they are neurogenic (Malatesta et al., 2000; Miyata et al., 2001; Noctor et al., 2001). Moreover, RG cells are likely to be the major neuronal precursors during development of the rodent cortex (Noctor et al., 2002; Hansen et al., 2010). Several cellular markers have been used to identify RG including RC1, RC2 and antibodies against brain lipid-binding protein (BLBP), glutamate transporter GLAST, glial fibrillary acidic protein (GFAP), vimentin and nestin (Pinto and Götz, 2007).

The heterogeneity of RG has been reported between CNS regions and developmental stages (Kriegstein and Götz, 2003; Pinto and Götz, 2007). RG cells differ in their differentiation potentials, e.g. neurogenic and gliogenic, and their gene expression, particularly the transcription factors. For example, dorsally derived RG cells contain the transcription factor Pax-6 (Götz et al., 1998), while ventrally derived RG cells contain Gsh2 and Olig2 (Malatesta et al., 2003). Pax-6 has been implicated in the neurogenic property of RG. The loss of Pax-6 reduced the neurogenic RG and resulted in the reduction of neurons in a region-specific manner (Heins et al., 2002; Larsen et al., 2010). Overexpression of Pax-6 can even convert young astrocytes into TuJ1⁺ neurons (Heins et al., 2002). The Olig2⁺ RG cells in the ventral part of the forebrain are likely to be the source of oligodendrocytes in that region. Notch activation promotes RG identity both in vivo and in vitro (Gaiano et al., 2000; Li et al., 2008a; Sibbe et al., 2009), and differential Notch signaling readout by the expression of its downstream target CBF1 distinguishes neural progenitor subtypes along the VZ in the developing forebrain (Mizutani et al., 2007).

Studies on isolated NSCs indicate that RG cells are more neurogenic at early stages and more gliogenic at later stages (Temple, 2001). During neurogenesis, RG cells in cortex are the major, if not the only, neuronal precursors in VZ (Noctor et al., 2002). RG cells in ganglionic eminance (GE) region generate mainly glial cell types (Malatesta et al., 2003). Consistent with these observations, we described the spatiotemporal heterogeneity of RG during CNS development in terms of their expressions of markers for restricted precursors (RPs) (Li et al., 2004). At early times during development (E12.5–13.5), there is little expression of RPs markers on BLBP⁺ cells, and later nearly all BLBP⁺ RG cells in embryonic forebrain co-expressed either GRP marker A2B5 or NRP marker 5A5. BLBP expression overlapped more with 5A5 in the E15 cortex than in the GE, and BLBP overlapped more with A2B5 in the GE than in the cortex. The results suggested that sub-populations of RG acquire markers for either neuronal or glial precursors in both a temporally and spatially restricted manner (Li et al., 2004).

Opposing dorsalventral gradients of morphogenetic factors in the CNS play critical roles in cell division and specification during development (Panchision and McKay, 2002). Neural progenitors respond to these gradients by expressing unique combinations of transcription factors (Zhou and Anderson, 2002; Sugimori et al., 2007), which in turn instruct these progenitors to go down certain paths of differentiation. For example, ventral expression of sonic hedgehog (SHH) controls local patterns of transcription factors including Olig-1 and Olig-2 that are critical for oligodendrocyte specification and differentiation (Fig. 1) (Lu et al., 2000). Dorsal expression of bone morphogenic proteins (BMPs) is thought to control differentiation of neurons in the dorsal cortex early (about E12) while suppressing oligodendroglial differentiation, and later (about E18) to suppress neurogenesis and promote gliogenesis (Mehler, 2002; Marchal et al., 2009). These morphogenetic factors also affect RG heterogeneity. Our lab presented data indicating that in both primary and clonal RG cultures, leukemia inhibitory factor (LIF)/ciliary neurotrophic factor (CNTF) up-regulates, whereas BMP2 down-regulates GRP antigens recognized by monoclonal antibodies 4D4/A2B5 (Fig. 1). The regulation of the GRP markers 4D4/A2B5 by LIF/CNTF is mediated through the Janus kinase-signal transducers and activators of transcription (JAK-STAT) pathway. BMP2 promotes expression of NRP antigen (PSA-NCAM) on RG cells most likely by regulating NCAM gene expression itself, as well as at least one polysialyl transferase responsible for synthesis of polysialic acid on NCAM protein (Li and Grumet, 2007).

GABAergic interneuron progenitor and its diversity

GABAergic interneurons constitute about 25% of neuronal population in the cerebral cortex. Cortical GABAergic interneurons are inhibitory neurons that form synapses with glutamatergic projection neurons and modulate their functions. Mature GABAergic interneurons exhibit a remarkable diversity of subtypes, which can be classified by their morphologies (Woo and Lu, 2006) and their expression of calcium-binding proteins, e.g. calretinin (CR), calbindin (CB) and parvalbumin (PV), and neuropeptides, e.g. neuropeptide Y (NPY), somatostatin (SST), neurotensin and vasoactive intestinal peptide (Xu et al., 2004; Tanaka et al., 2010). A standardized nomenclature of interneuron properties was proposed by The Petilla Interneuron Nomenclature Group (PING), which provided a set of terms to describe the anatomical, physiological and molecular features of GABAergic interneurons of the cerebral cortex (Ascoli et al., 2008). The most common classification might be based on their largely non-overlapping expression of neurochemical markers: PV, SST and CR (Gonchar and Burkhalter, 1997). Notably, no single calcium-binding protein or neuropeptide corresponds to a distinct morphological or electrophysiological type of interneurons.

Although progenitors in the dorsal cortex have been shown to have the potential to give rise to interneurons under the influence of dorsoventral morphogens (Gulacsi and Lillien, 2003), interneurons, at least in rodents, are believed to originate mainly from the GEs of the developing forebrain and migrate tangentially into the developing cortex (Xu et al., 2004). Diversity of interneuron subtypes is also developmentally regulated in a spatiotemporal fashion. Different interneuron subtypes originate from sub-domains of the GE during embryogenesis. Most of them are fate-determined at progenitor stage by combinatory expression of specific transcription factors in response to gradients of early expressed morphogens (Corbin et al., 2008). For example, there exists a strong fate bias for SST⁺ and NPY⁺ interneurons being generated by progenitors in the dorsal medial ganglionic eminence (MGE) and PV⁺ interneurons from the ventral MGE, while CR⁺ interneurons are primarily generated in the dorsal caudal ganglionic eminence (CGE) (Xu et al., 2004; Wonders and Anderson, 2006; Wonders et al., 2008). On the other hand, MGE-derived interneurons also show subtype difference in a time-dependent manner. Using an MGE-predominant transcription factor Olig2-cre reporter mouse line, Miyoshi G and colleagues reported that early-born (E9.5) interneurons from MGE are mostly SST⁺/CR^-, and this number decreased during development and were absent from late-born (E15.5) interneurons. Contrastingly, VIP⁺/CR^- MGE-derived interneurons were born only at late stage (E15.5) (Miyoshi et al., 2007). Furthermore, there appears to be a timely shift of electrophysiological properties in the MGE-derived interneurons (Miyoshi et al., 2007).

Ventrally derived interneurons and their progenitors express DLX transcription factors, which have been shown to be essential for their migration into the dorsal cortex (Anderson et al., 1997; Cobos et al., 2007). In the cortical culture of Dlx1/2 mutants, NPY⁺ and SST⁺ interneurons are reduced, while CR⁺ bipolar interneurons are nearly absent, suggesting a key role of Dlx1/2 in interneuron subtype determination (Xu et al., 2004). Nkx2.1 is expressed in MGE and absent from LGE and CGE, indicating its specific role in regulating MGE-derived cells. The removal of this gene at different time points during neurogenesis results in a switch in the subtypes of neurons observed at more mature stages (Butt et al., 2008). During CNS neurogenesis, Nkx2.1 is maintained by SHH signaling, which prevents the conversion of some MGE progenitors to a CGE-like CR⁺ neuronal fate, and promotes the generation of the SST⁺ interneurons at the expense of PV⁺ cells when expressed at a higher level (Xu et al., 2010).

Neural progenitor clones

Our approach to study NSCs has been the stable expression of v-myc gene, which can yield clones of NSCs and neural progenitors that can be passed extensively while retaining many of their key properties (Fig. 2) (Villa et al., 2000). Non-transforming oncogenes such as myc and Large T antigen have been widely used to immortalize primary CNS progenitor cells from mouse (Frisa et al., 1994), rat (Frederiksen et al., 1988), and human (De Filippis et al., 2007). Many of these lines have been characterized in culture and after transplantation into adult tissue and shown to give rise to both neurons and glia. A murine clone C17.2 that exhibits differentiation in culture and in vivo also can influence the milleu in rodent models of injury and disease (Lu et al., 2003; Yang et al., 2003; Yan et al., 2004; Ryu et al., 2005). Thus, C17.2 cells have been studied extensively and have been proposed as a model for NSCs, but the generality of the conclusions using C17.2 has been questioned since they express much higher levels of certain key growth factors than nonimmortalized neurospheres and neural progenitors (Mi et al., 2005). In any case, we have applied the v-myc stabilization approach to neural stem and progenitor cells and derived progenitor clones including RG clones (L2.3 and RG3.6) that exhibit markers expressed in RG and NSCs, display morphological features of RG in vitro and in vivo, support migration of neurons, can be induced to differentiate into neurons and glia, and can be passed extensively in culture while retaining RG characteristics (Li et al., 2004; Hasegawa et al., 2005). Another neural progenitor clone L2.2 is a neuronal-restricted progenitor that differentiates exclusively into electrogenic neurons with a GABAergic phenotype in culture (Li et al., 2008b). A recent selection of immortalized neural progenitor clones from rat embryonic forebrains showed a variety of progenitors with distinct differentiation potentials (unpublished data). This approach of neural progenitor clones has been proven to be invaluable models to study mechanisms of neural progenitor diversity and provide powerful tools to test their therapeutic potentials in neurological disorders such as spinal cord injury (Li et al., 2004; Hasegawa et al., 2005; Li et al., 2008a).

Spinal cord injury and progenitor cell transplantation

Pathophysiology of spinal cord injury

Spinal cord injury (SCI) is a severe CNS injury often resulting in long-term disability. Pathophysiology of SCI can be classified into two phases: one is primary insult during acute phase, and the other is secondary injury during sub-acute to chronic phase (Norenberg et al., 2004; Beck et al., 2010). Immediately after injury, blood vessels rupture and blood flush into the gray matter, and spinal cord swells right away. Spinal cord quickly undergoes ischemia, which causes significant hypotension to the patients. Broken bones or vertebra compression mechanically depolarize neurons, followed by excess calcium influx and massive glutamate release, which initiates first wave of cell death among neurons (Braughler et al., 1985; Rothman and Olney, 1986; Young and Koreh, 1986).

Without any interventions, all these events induce secondary injury in the surrounding tissue. Local microglia are the first activated immune cells in response to the injury (Olson, 2010). Dying cells and activated microglia recruit other immune cells such as neutrophils, lymphocytes, and macrophages, which secret excess pro-inflammatory cytokines (Babcock et al., 2003; Pineau and Lacroix, 2007). The prolonged inflammation induces second wave of cell death in the neighboring cells including oligodendrocytes (Beattie et al., 2002). When myelin or oligodendrocytes are damaged, inhibitory factors are exposed such as Nogo and myelin associated proteins to inhibit future axonal regeneration (Schweigreiter and Bandtlow, 2006). Several weeks after contusive injury in humans as well as in rats (but not in mice), cystic cavities develop, surrounded by gliotic scars associated with extracellular matrix components including chondroitin sulfate proteoglycans (CSPG), which is not hospitable to axonal regeneration (Busch and Silver, 2007).

Current interventions for treating SCI

Current strategy for treating SCI includes neuroprotection and reducing secondary damage, promoting axonal regeneration, bridging injury site using cell transplants or biomaterials, replacing lost neurons and glia, encouraging remyelination with endogenous or transplanted myelin forming cells. There is no single “magic pill,” and given the complex nature of SCI pathology, combinatory therapies to target different destructive mechanisms will be the trend for a future cure. At the moment, the only approved therapy for treating SCI in clinic is to give patients high dose of methylprednisolone within 8 hours after the trauma. By doing this, about 20% of the motor function can be preserved (Hall, 2001; Bracken, 2002).

Endogenous NSCs in spinal cord are activated and proliferate upon injury. After contusion, spinal ependymal stem cells proliferate and about 2 million new cells are produced at the injury site within a month, peaking at 3–7 days post injury (Moreno-Manzano et al., 2009). Although endogenous NSCs have been shown to differentiate into multi-lineage of mature cell types in the spinal cord (Meletis et al., 2008), their capacity for repair is limited by the non-permissive environment and growth inhibitors at the injury site.

Advances in cell characterization and isolation are opening new opportunities for cell transplantation to repair tissue damage and restore lost function (Gage, 2000; Kumagai et al., 2009). Schwann cells and olfactory ensheathing glial cells have been transplanted in SCI to facilitate regrowth of axons as well as myelination (Bunge, 1994; Keirstead et al., 1999; Imaizumi et al., 2000; RamónCueto et al., 2000; Shields et al., 2000; Kohama et al., 2001). Implantation of other cells such as umbilical cord blood stem cells (Park et al., 2010), mesenchymal stromal cells (Pal et al., 2010), all have been shown to have somewhat beneficial effects in SCI.

ESCs and induced pluripotent stem cells (iPSCs) are thought to be the ideal source of cell transplantation in SCI. Pioneer studies by McDonald and colleagues showed that mouse ESCs survived, differentiated into neural lineage cells, and improved functional recovery without tumor formation when implanted in the injured spinal cord (McDonald et al., 1999). It was recently reported that iPSC-derived NSCs can differentiate into neurons and glia, and promote functional recovery in SCI rat. This report also raised the concerns of safety issues of using iPSCs and developed a screening protocol for safe human iPSC for potential transplantation in clinic (Tsuji et al., 2010).

Transplantation of neural progenitors in SCI

Directed differentiation of NSCs into certain mature cell types is critical in stem cell replacement therapy. Primitive NSCs have the ability to become multiple cell types, but their cell fate is heavily influenced by the environmental cues. In this regard, diversified neural progenitors are predetermined in their differentiation potential and maybe more advantageous in transplantation to replace lost cells in diseases or trauma. Neural progenitors with restricted differentiation potential have been attempted in SCI models. For example, Hill and colleagues transplanted GRPs into injured spinal cord and found reduced astrocytic scarring, differentiation of transplants into astrocytes and oligodendrocytes, and a morphology of corticospinal track fibers that was altered toward growth cones, suggesting improved tissue architecture and, potentially, remyelination (Hill et al., 2004). Transplantation of oligodendrocyte progenitor cells 7 days after SCI suggests that they differentiate within several weeks into oligodendrocytes in the injured spinal cord, generate myelin and improve the functional outcome (Hofstetter et al., 2005). NRPs appeared to differentiate upon transplantation, but differentiated poorly into mature neuronal cell types probably due to the “non-neurogenic zone” in the injured spinal cord (Cao et al., 2002).

We have applied v-myc immortalization method to generate neural progenitor clones, which provide useful in vitro models to study mechanisms of neural differentiation (Fig. 2). Some of these progenitor clones were also and will be transplanted in rat SCI to evaluate their potential beneficial effects. One great advantage of v-myc progenitor clones is that they are highly proliferative in the presence of mitogens such as epidermal growth factor (EGF) and fibroblast growth factor 2 (FGF2). One can obtain millions of cells in a very short period of time. Although v-myc immortalized progenitors have higher tendency to be tumorigenic, some of these clones have been transplanted in adult CNS with great success. Our rationale is that progenitor clones are served as tools to test our hypothesis. Once these clones show expected outcomes in promoting recovery in SCI, we will isolate cells with similar properties from “safe” source for potential clinic applications. Below, we will discuss two types of neural progenitor clones we generated and our hypothesis on these clones in treating SCI.

Transplantation of radial glia

Radial glia forms scaffolds to support neuronal migration during CNS development. Their elongated bipolar cell morphology is an important physical characteristic for fulfilling this task. In addition, adhesion molecules (such as integrins) expressed on the cell membrane of RG may also help them interact with migrating neurons and promote cell migration (Anton et al., 1999; Loulier et al., 2009). Our hypothesis of using RG in SCI is that they may provide a physical bridge to cross the injury site upon transplantation, and their adhesive cell membrane may facilitate regenerating axons to pass the injury site (Fig. 3). RG3.6, derived from E14.5 forebrain of GFP-rat embryo, is a representative RG clone, which exhibits simple bipolar morphology and expresses RG marker BLBP and nestin (Hasegawa et al., 2005). In vitro analysis showed that RG3.6 cells support migration of granule neurons isolated from rat postnatal cerebellum.

Upon transplantation, RG3.6 cells migrated extensively in the injured rat spinal cord, spread out across the entire injury site, formed a physical bridge as a population, and improved the rat’s open field walking behavior following contusive SCI (Hasegawa et al., 2005). In addition, the transplanted RG3.6 cells partially protected the rat spinal cord against several aspects of secondary injury including loss of axons and myelin as well as accumulation of CSPG and macrophages (Hasegawa et al., 2005). This effect may be due to the fact that RG, as a type of NSCs, secretes beneficial factors to improve the local environment for better regeneration (Chang et al., 2009).

Notch signaling plays a critical role in maintaining RG identity. Gaiano and colleagues reported that activated Notch (NICD) promoted RG phenotype in developing mouse forebrains (Gaiano et al., 2000). With the same NICD construct, we demonstrated that activated Notch enhanced radial morphology of both RG clone and primary E13.5 cortical cells in culture and increased the expression of RG marker BLBP in both types of cells (Li et al., 2008a). In white matter of rat normal spinal cord, NICD-overexpressing RG cells migrated over greater distance than its parental cells 3 days after transplantation and exhibited much longer bipolar processes retaining prolonged expression of RG marker nestin (Li et al., 2008a). Our future question is whether or not Notch signaling can stabilize RG properties and make them a better tool in bridging SCI upon transplantation.

Transplantation of interneuron progenitors

Patients with SCI not only lose motor function below the injury site, but often develop debilitating neuropathic pain (allodynia) over time (Siddall et al., 1999). The mechanisms underlying allodynia may be very complex and many possible factors contribute to these symptoms (Hulsebosch, 2005). One direct factor is the loss or reduction of inhibitory tone in the spinal cord sensory processing that may be due to the loss of the g-aminobutyric acid (GABA)-immunoreactive neurons following spinal injury (Zhang et al., 1994; Meisner et al., 2010). Most of the GABAergic interneurons are located in the dorsal horn of the gray matter where they help process the afferent signals. Upon injury, even far away from the injury site, inhibitory neurons died preferentially for unclear reasons. Some projection neurons also became highly sensitized by upregulating the expression of sodium channels on their cell surface (Hains et al., 2003). Therefore, there is a need to replenish the lost inhibitory GABAergic neurons to restore normal somatosensory function. In support of this idea, behavioral changes (i.e. allodynia) produced by SCI can be induced by the intrathecal administration of GABA antagonists (Linderoth et al., 1994). Furthermore, intrathecal drugs (GABA_A or GABA_B agonist) reversed allodynia in animals with injury (Hao et al., 1992; Naik et al., 2008). Therapeutic strategies that prevent induction of allodynia, such as cell transplants that release anti-nociceptive substances, can be used to enhance the endogenous descending inhibitory neurotransmitter systems including GABA and serotonin (5-HT). Indeed, several reports demonstrated that cells with the ability to release GABA showed reduced allodynia upon transplantation (Eaton et al., 2007; Mukhida et al., 2007).

Most mature cells including oligodendrocytes and neurons exhibit little or no migration in adult CNS. In contrast, neural progenitors migrate over great distances (as far as 1 cm in 2 weeks) in white matter regions (Hasegawa et al., 2005). This property when combined with the ability to restrict the differentiation provides the attractive possibility of targeting cells to specific regions before they differentiate into desired phenotypes. Using v-myc immortalization, we have isolated cortical GABAergic interneuron progenitor clones and demonstrated their restricted interneuronal differentiation in culture. In future studies, we will test our hypothesis that these specialized progenitors will migrate in SCI after transplantation and differentiate into GABAergic interneurons that can reduce allodynia. The consensus in translational research including application of stem cells in SCI is that we need to examine not only cell survival and differentiation after transplantation, but also maturation and functional integration of implanted stem cells. Towards that, our preliminary results have shown that immortalized interneuron progenitors were able to acquire action potentials and form synapses in co-cultures with hippocampal neurons (manuscript in preparation).

Conclusions

Immortalized neural progenitors maintain certain original features including ability to support neuronal migration in RG and differentiation potentials. This approach allowed us to obtain greater cell number and dissect unique properties of each progenitor type. Furthermore, the immortalized progenitor clones also provide a system to screen for selection markers of different progenitors from tissue. Complicated mechanisms in SCI need to be addressed individually with specialized interventions. Neural progenitor diversity could be reflected in terms of their potential applications in treating SCI with their unique properties. Multipotent NSCs protect host tissue upon acute transplantation in SCI, but they differentiate mostly to astroglia, not to neurons (Cao et al., 2001; Hasegawa et al., 2005). Neural progenitors with restricted differentiation potential can be identified and isolated from developing CNS, and their intrinsic characteristics programmed by unique combination of transcription factors (codes) are less influenced by environmental cues. In contrast to undetermined NSCs, neural progenitors maybe more readily differentiate into desired cell types such as interneurons in the uncontrollable injury milieu. Little can be done to change the injured environment in the spinal cord, but manipulation of cell intrinsic properties by overexpressing transcription factors may also prove to be an efficient way to overcome unfavorable environment (Hofstetter et al., 2005).

References

[1] Anderson S A, Eisenstat D D, Shi L, Rubenstein J L (1997). Interneuron migration from basal forebrain to neocortex: dependence on Dlx genes. Science, 278(5337): 474–476

[2] Anton E S, Kreidberg J A, Rakic P (1999). Distinct functions of alpha3 and alpha(v) integrin receptors in neuronal migration and laminar organization of the cerebral cortex. Neuron, 22(2): 277–289

[3] Ascoli G A, Alonso-Nanclares L, Anderson S A, Barrionuevo G, Benavides-Piccione R, Burkhalter A, Buzsáki G, Cauli B, Defelipe J, Fairén A, Feldmeyer D, Fishell G, Fregnac Y, Freund TF, Gardner D, Gardner E P, Goldberg J H, Helmstaedter M, Hestrin S, Karube F, Kisvárday ZF, Lambolez B, Lewis D A, Marin O, Markram H, Muñoz A, Packer A, Petersen C C, Rockland K S, Rossier J, Rudy B, Somogyi P, Staiger J F, Tamas G, Thomson A M, Toledo-Rodriguez M, Wang Y, West D C, Yuste R (2008). Petilla terminology: nomenclature of features of GABAergic interneurons of the cerebral cortex. Nat Rev Neurosci, 9: 557–568

[4] Babcock A A, Kuziel W A, Rivest S, Owens T (2003). Chemokine expression by glial cells directs leukocytes to sites of axonal injury in the CNS. J Neurosci, 23(21): 7922–7930

[5] Beattie M S, Hermann G E, Rogers R C, Bresnahan J C (2002). Cell death in models of spinal cord injury. Prog Brain Res, 137: 37–47

[6] Beck K D, Nguyen H X, Galvan M D, Salazar D L, Woodruff T M, Anderson A J (2010). Quantitative analysis of cellular inflammation after traumatic spinal cord injury: evidence for a multiphasic inflammatory response in the acute to chronic environment. Brain, 133(Pt 2): 433–447

[7] Becker A J, McCULLOCH E A, Till J E (1963). Cytological demonstration of the clonal nature of spleen colonies derived from transplanted mouse marrow cells. Nature, 197: 452–454

[8] Bracken M B (2002). Steroids for acute spinal cord injury. Cochrane Database Syst Rev, (3): CD001046

[9] Braughler J M, Duncan L A, Chase R L (1985). Interaction of lipid peroxidation and calcium in the pathogenesis of neuronal injury. Cent Nerv Syst Trauma, 2(4): 269–283

[10] Bunge M B (1994). Transplantation of purified populations of Schwann cells into lesioned adult rat spinal cord. J Neurol, 242(1 Suppl 1): S36–S39

[11] Busch S A, Silver J (2007). The role of extracellular matrix in CNS regeneration. Curr Opin Neurobiol, 17(1): 120–127

[12] Butt S J, Sousa V H, Fuccillo M V, Hjerling-Leffler J, Miyoshi G, Kimura S, Fishell G (2008). The requirement of Nkx2-1 in the temporal specification of cortical interneuron subtypes. Neuron, 59(5): 722–732

[13] 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 multipotent neural stem cell (NEP cell). Dev Biol, 251(2): 221–240

[14] Cao Q L, Howard R M, Dennison J B, Whittemore S R (2002). Differentiation of engrafted neuronal-restricted precursor cells is inhibited in the traumatically injured spinal cord. Exp Neurol, 177(2): 349–359

[15] Cao Q L, Zhang Y P, Howard R M, Walters W M, Tsoulfas P, Whittemore S R (2001). Pluripotent stem cells engrafted into the normal or lesioned adult rat spinal cord are restricted to a glial lineage. Exp Neurol, 167(1): 48–58

[16] Chang Y W, Goff L A, Li H, Kane-Goldsmith N, Tzatzalos E, Hart R P, Young W, Grumet M (2009). Rapid induction of genes associated with tissue protection and neural development in contused adult spinal cord after radial glial cell transplantation. J Neurotrauma, 26(7): 979–993

[17] Cobos I, Borello U, Rubenstein J L (2007). Dlx transcription factors promote migration through repression of axon and dendrite growth. Neuron, 54(6): 873–888

[18] Corbin J G, Gaiano N, Juliano S L, Poluch S, Stancik E, Haydar T F (2008). Regulation of neural progenitor cell development in the nervous system. J Neurochem, 106(6): 2272–2287

[19] De Filippis L, Lamorte G, Snyder E Y, Malgaroli A, Vescovi A L (2007). A novel, immortal, and multipotent human neural stem cell line generating functional neurons and oligodendrocytes. Stem Cells, 25(9): 2312–2321

[20] Eaton M J, Wolfe S Q, Martinez M, Hernandez M, Furst C, Huang J, Frydel B R, Gómez-Marín O (2007). Subarachnoid transplant of a human neuronal cell line attenuates chronic allodynia and hyperalgesia after excitotoxic spinal cord injury in the rat. J Pain, 8(1): 33–50

[21] Evans M J, Kaufman M H (1981). Establishment in culture of pluripotential cells from mouse embryos. Nature, 292(5819): 154–156

[22] Frederiksen K, Jat P S, Valtz N, Levy D, McKay R (1988). Immortalization of precursor cells from the mammalian CNS. Neuron, 1(6): 439–448

[23] Frisa P S, Goodman M N, Smith G M, Silver J, Jacobberger J W (1994). Immortalization of immature and mature mouse astrocytes with SV40 T antigen. J Neurosci Res, 39(1): 47–56

[24] Gage F H (2000). Mammalian neural stem cells. Science, 287(5457): 1433–1438

[25] Gaiano N, Nye J S, Fishell G (2000). Radial glial identity is promoted by Notch1 signaling in the murine forebrain. Neuron, 26(2): 395–404

[26] Gonchar Y, Burkhalter A (1997). Three distinct families of GABAergic neurons in rat visual cortex. Cereb Cortex, 7: 347–358

[27] Götz M, Stoykova A, Gruss P (1998). Pax6 controls radial glia differentiation in the cerebral cortex. Neuron, 21(5): 1031–1044

[28] Gulacsi A, Lillien L (2003). Sonic hedgehog and bone morphogenetic protein regulate interneuron development from dorsal telencephalic progenitors in vitro. J Neurosci, 23(30): 9862–9872

[29] Hains B C, Klein J P, Saab C Y, Craner M J, Black J A, Waxman S G (2003). Upregulation of sodium channel Nav1.3 and functional involvement in neuronal hyperexcitability associated with central neuropathic pain after spinal cord injury. J Neurosci, 23(26): 8881–8892

[30] Hall E D (2001). Pharmacological treatment of acute spinal cord injury: how do we build on past success? J Spinal Cord Med, 24(3): 142–146

[31] Hansen D V, Lui J H, Parker P R, Kriegstein A R (2010). Neurogenic radial glia in the outer subventricular zone of human neocortex. Nature, 464(7288): 554–561

[32] Hao J X, Xu X J, Yu Y X, Seiger A, Wiesenfeld-Hallin Z (1992). Baclofen reverses the hypersensitivity of dorsal horn wide dynamic range neurons to mechanical stimulation after transient spinal cord ischemia; implications for a tonic GABAergic inhibitory control of myelinated fiber input. J Neurophysiol, 68(2): 392–396

[33] Hasegawa K, Chang Y W, Li H, Berlin Y, Ikeda O, Kane-Goldsmith N, Grumet M (2005). Embryonic radial glia bridge spinal cord lesions and promote functional recovery following spinal cord injury. Exp Neurol, 193(2): 394–410

[34] Heins N, Malatesta P, Cecconi F, Nakafuku M, Tucker K L, Hack M A, Chapouton P, Barde Y A, Götz M (2002). Glial cells generate neurons: the role of the transcription factor Pax6. Nat Neurosci, 5(4): 308–315

[35] Hill C E, Proschel C, Noble M, Mayer-Proschel M, Gensel J C, Beattie M S, Bresnahan J C (2004). Acute transplantation of glial-restricted precursor cells into spinal cord contusion injuries: survival, differentiation, and effects on lesion environment and axonal regeneration. Exp Neurol, 190(2): 289–310

[36] Hofstetter C P, Holmström N A, Lilja J A, Schweinhardt P, Hao J, Spenger C, Wiesenfeld-Hallin Z, Kurpad S N, Frisén J, Olson L (2005). Allodynia limits the usefulness of intraspinal neural stem cell grafts; directed differentiation improves outcome. Nat Neurosci, 8(3): 346–353

[37] Hulsebosch C E (2005). From discovery to clinical trials: treatment strategies for central neuropathic pain after spinal cord injury. Curr Pharm Des, 11(11): 1411–1420

[38] Imaizumi T, Lankford K L, Kocsis J D (2000). Transplantation of olfactory ensheathing cells or Schwann cells restores rapid and secure conduction across the transected spinal cord. Brain Res, 854(1–2): 70–78

[39] Keirstead H S, Morgan S V, Wilby M J, Fawcett J W (1999). Enhanced axonal regeneration following combined demyelination plus schwann cell transplantation therapy in the injured adult spinal cord. Exp Neurol, 159(1): 225–236

[40] Kim J H, Auerbach J M, Rodríguez-Gómez J A, Velasco I, Gavin D, Lumelsky N, Lee S H, Nguyen J, Sánchez-Pernaute R, Bankiewicz K, McKay R (2002). Dopamine neurons derived from embryonic stem cells function in an animal model of Parkinson’s disease. Nature, 418(6893): 50–56

[41] Kohama I, Lankford K L, Preiningerova J, White F A, Vollmer T L, Kocsis J D (2001). Transplantation of cryopreserved adult human Schwann cells enhances axonal conduction in demyelinated spinal cord. J Neurosci, 21(3): 944–950

[42] Kriegstein A R, Götz M (2003). Radial glia diversity: a matter of cell fate. Glia, 43(1): 37–43

[43] Kumagai G, Okada Y, Yamane J, Nagoshi N, Kitamura K, Mukaino M, Tsuji O, Fujiyoshi K, Katoh H, Okada S, Shibata S, Matsuzaki Y, Toh S, Toyama Y, Nakamura M, Okano H (2009). Roles of ES cell-derived gliogenic neural stem/progenitor cells in functional recovery after spinal cord injury. PLoS One, 4(11): e7706

[44] Larsen K B, Lutterodt M C, Laursen H, Graem N, Pakkenberg B, Møllgård K, Møller M (2010). Spatiotemporal distribution of PAX6 and MEIS2 expression and total cell numbers in the ganglionic eminence in the early developing human forebrain. Dev Neurosci, 32(2): 149–162

[45] Lee H J, Lee J K, Lee H, Shin J W, Carter J E, Sakamoto T, Jin H K, Bae J S (2010). The therapeutic potential of human umbilical cord blood-derived mesenchymal stem cells in Alzheimer’s disease. Neurosci Lett, 481(1): 30–35

[46] Li H, Babiarz J, Woodbury J, Kane-Goldsmith N, Grumet M (2004). Spatiotemporal heterogeneity of CNS radial glial cells and their transition to restricted precursors. Dev Biol, 271(2): 225–238

[47] Li H, Chang Y W, Mohan K, Su H W, Ricupero C L, Baridi A, Hart R P, Grumet M (2008a). Activated Notch1 maintains the phenotype of radial glial cells and promotes their adhesion to laminin by upregulating nidogen. Glia, 56(6): 646–658

[48] Li H, Grumet M (2007). BMP and LIF signaling coordinately regulate lineage restriction of radial glia in the developing forebrain. Glia, 55: 24–35

[49] Li H, Han Y R, Bi C, Davila J, Goff L A, Thompson K, Swerdel M, Camarillo C, Ricupero C L, Hart R P, Plummer M R, Grumet M (2008b). Functional differentiation of a clone resembling embryonic cortical interneuron progenitors. Dev Neurobiol, 68(14): 1549–1564

[50] Linderoth B, Stiller C O, Gunasekera L, O’Connor W T, Ungerstedt U, Brodin E (1994). Gamma-aminobutyric acid is released in the dorsal horn by electrical spinal cord stimulation: an in vivo microdialysis study in the rat. Neurosurgery, 34(3): 484–488, discussion 488–489

[51] 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 situ and immunohistological analysis during embryonic development. Glia, 40(1): 25–43

[52] Loulier K, Lathia J D, Marthiens V, Relucio J, Mughal M R, Tang S C, Coksaygan T, Hall P E, Chigurupati S, Patton B, Colognato H, Rao M S, Mattson M P, Haydar T F, Ffrench-Constant C (2009). beta1 integrin maintains integrity of the embryonic neocortical stem cell niche. PLoS Biol, 7(8): e1000176

[53] Lu P, Jones L L, Snyder E Y, Tuszynski M H (2003). Neural stem cells constitutively secrete neurotrophic factors and promote extensive host axonal growth after spinal cord injury. Exp Neurol, 181(2): 115–129

[54] 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 a motor neuron/oligodendrocyte connection. Cell, 109(1): 75–86

[55] Lu Q R, Yuk D, Alberta J A, Zhu Z, Pawlitzky I, Chan J, McMahon A P, Stiles C D, Rowitch D H (2000). Sonic hedgehog—regulated oligodendrocyte lineage genes encoding bHLH proteins in the mammalian central nervous system. Neuron, 25(2): 317–329

[56] Malatesta P, Hack M A, Hartfuss E, Kettenmann H, Klinkert W, Kirchhoff F, Götz M (2003). Neuronal or glial progeny: regional differences in radial glia fate. Neuron, 37(5): 751–764

[57] Malatesta P, Hartfuss E, Götz M (2000). Isolation of radial glial cells by fluorescent-activated cell sorting reveals a neuronal lineage. Development, 127(24): 5253–5263

[58] Marchal L, Luxardi G, Thomé V, Kodjabachian L (2009). BMP inhibition initiates neural induction via FGF signaling and Zic genes. Proc Natl Acad Sci U S A, 106(41): 17437–17442

[59] Martin G R (1981). Isolation of a pluripotent cell line from early mouse embryos cultured in medium conditioned by teratocarcinoma stem cells. Proc Natl Acad Sci U S A, 78(12): 7634–7638

[60] Mayer-Proschel M, Kalyani A J, Mujtaba T, Rao M S (1997). Isolation of lineage-restricted neuronal precursors from multipotent neuroepithelial stem cells. Neuron, 19(4): 773–785

[61] McDonald J W, Liu X Z, Qu Y, Liu S, Mickey S K, Turetsky D, Gottlieb D I, Choi D W (1999). Transplanted embryonic stem cells survive, differentiate and promote recovery in injured rat spinal cord. Nat Med, 5(12): 1410–1412

[62] Mehler M F (2002). Mechanisms regulating lineage diversity during mammalina cerebral cortical neurogenesis and gliogenesis. In: Hohmann C F, ed. Cortical Development, Berlin: Springer-Verlag. 27–52

[63] Meisner J G, Marsh A D, Marsh D R (2010). Loss of GABAergic interneurons in laminae I-III of the spinal cord dorsal horn contributes to reduced GABAergic tone and neuropathic pain after spinal cord injury. J Neurotrauma, 27(4): 729–737

[64] Meletis K, Barnabé-Heider F, Carlén M, Evergren E, Tomilin N, Shupliakov O, Frisén J (2008). Spinal cord injury reveals multilineage differentiation of ependymal cells. PLoS Biol, 6(7): e182

[65] Mi R, Luo Y, Cai J, Limke T L, Rao M S, Höke A (2005). Immortalized neural stem cells differ from nonimmortalized cortical neurospheres and cerebellar granule cell progenitors. Exp Neurol, 194(2): 301–319

[66] Miyata T, Kawaguchi A, Okano H, Ogawa M (2001). Asymmetric inheritance of radial glial fibers by cortical neurons. Neuron, 31(5): 727–741

[67] Miyoshi G, Butt S J, Takebayashi H, Fishell G (2007). Physiologically distinct temporal cohorts of cortical interneurons arise from telencephalic Olig2-expressing precursors. J Neurosci, 27(29): 7786–7798

[68] Mizutani K, Yoon K, Dang L, Tokunaga A, Gaiano N (2007). Differential Notch signalling distinguishes neural stem cells from intermediate progenitors. Nature, 449(7160): 351–355

[69] Moreno-Manzano V, Rodríguez-Jiménez F J, García-Roselló M, Laínez S, Erceg S, Calvo M T, Ronaghi M, Lloret M, Planells-Cases R, Sánchez-Puelles J M, Stojkovic M (2009). Activated spinal cord ependymal stem cells rescue neurological function. Stem Cells, 27(3): 733–743

[70] Mukhida K, Mendez I, McLeod M, Kobayashi N, Haughn C, Milne B, Baghbaderani B, Sen A, Behie L A, Hong M (2007). Spinal GABAergic transplants attenuate mechanical allodynia in a rat model of neuropathic pain. Stem Cells, 25(11): 2874–2885

[71] Naik A K, Pathirathna S, Jevtovic-Todorovic V (2008). GABAA receptor modulation in dorsal root ganglia in vivo affects chronic pain after nerve injury. Neuroscience, 154(4): 1539–1553

[72] Noble M, Pröschel C, Mayer-Pröschel M (2004). Getting a GR(i)P on oligodendrocyte development. Dev Biol, 265(1): 33–52

[73] Noctor S C, Flint A C, Weissman T A, Dammerman R S, Kriegstein A R (2001). Neurons derived from radial glial cells establish radial units in neocortex. Nature, 409(6821): 714–720

[74] Noctor S C, Flint A C, Weissman T A, Wong W S, Clinton B K, Kriegstein A R (2002). Dividing precursor cells of the embryonic cortical ventricular zone have morphological and molecular characteristics of radial glia. J Neurosci, 22(8): 3161–3173

[75] Norenberg M D, Smith J, Marcillo A (2004). The pathology of human spinal cord injury: defining the problems. J Neurotrauma, 21(4): 429–440

[76] Olson J K (2010). Immune response by microglia in the spinal cord. Ann N Y Acad Sci, 1198: 271–278

[77] Pal R, Gopinath C, Rao N M, Banerjee P, Krishnamoorthy V, Venkataramana N K, Totey S (2010) Functional recovery after transplantation of bone marrow-derived human mesenchymal stromal cells in a rat model of spinal cord injury. Cytotherapy, 2010<month>Jun</month><day>4</day>. [Epub ahead of print]

[78] Panchision D M, McKay R D (2002). The control of neural stem cells by morphogenic signals. Curr Opin Genet Dev, 12(4): 478–487

[79] Park D H, Lee J H, Borlongan C V, Sanberg P R, Chung Y G, Cho T H (2010). Transplantation of umbilical cord blood stem cells for treating spinal cord injury. Stem Cell Rev. DOI: 10.1007/s12015-010-9163-0

[80] Pineau I, Lacroix S (2007). Proinflammatory cytokine synthesis in the injured mouse spinal cord: multiphasic expression pattern and identification of the cell types involved. J Comp Neurol, 500(2): 267–285

[81] Pinto L, Götz M (2007). Radial glial cell heterogeneity—the source of diverse progeny in the CNS. Prog Neurobiol, 83(1): 2–23

[82] Rakic P (1990). Principles of neural cell migration. Experientia, 46(9): 882–891

[83] Rakic P J (1971). Neuron-glia relationship during granule cell migration in developing cerebellar cortex. A Golgi and electronmicroscopic study in Macacus Rhesus. J Comp Neurol, 141(3): 283–312

[84] Ramón-Cueto A, Cordero M I, Santos-Benito F F, Avila J (2000). Functional recovery of paraplegic rats and motor axon regeneration in their spinal cords by olfactory ensheathing glia. Neuron, 25(2): 425–435

[85] Rothman S M, Olney J W (1986). Glutamate and the pathophysiology of hypoxic—ischemic brain damage. Ann Neurol, 19(2): 105–111

[86] Ryu M Y, Lee M A, Ahn Y H, Kim K S, Yoon S H, Snyder E Y, Cho K G, Kim S U (2005). Brain transplantation of neural stem cells cotransduced with tyrosine hydroxylase and GTP cyclohydrolase 1 in Parkinsonian rats. Cell Transplant, 14(4): 193–202

[87] Schweigreiter R, Bandtlow C E (2006). Nogo in the injured spinal cord. J Neurotrauma, 23(3–4): 384–396

[88] Shields S A, Blakemore W F, Franklin R J (2000). Schwann cell remyelination is restricted to astrocyte-deficient areas after transplantation into demyelinated adult rat brain. J Neurosci Res, 60(5): 571–578

[89] Shihabuddin L S, Horner P J, Ray J, Gage F H (2000). Adult spinal cord stem cells generate neurons after transplantation in the adult dentate gyrus. J Neurosci, 20(23): 8727–8735

[90] Sibbe M, Förster E, Basak O, Taylor V, Frotscher M (2009). Reelin and Notch1 cooperate in the development of the dentate gyrus. J Neurosci, 29(26): 8578–8585

[91] Siddall P J, Taylor D A, McClelland J M, Rutkowski S B, Cousins M J (1999). Pain report and the relationship of pain to physical factors in the first 6 months following spinal cord injury. Pain, 81(1–2): 187–197

[92] Spiropoulos A, Theodosaki M, Stefanaki K, Paterakis G, Tzetis M, Giannikou K, Petrakou E, Dimopoulou MN, Papassotiriou I, Roma ES, Kanavakis E, Graphakos S, Goussetis E (2010). Rapid clinical-scale propagation of mesenchymal stem cells using cultures initiated with immunoselected bone marrow CD105 cells. J Cell Mol Med. 2010<month>Aug</month><day>20</day>. [Epub ahead of print]

[93] Sugimori M, Nagao M, Bertrand N, Parras C M, Guillemot F, Nakafuku M (2007). Combinatorial actions of patterning and HLH transcription factors in the spatiotemporal control of neurogenesis and gliogenesis in the developing spinal cord. Development, 134(8): 1617–1629

[94] Tanaka D H, Mikami S, Nagasawa T, Miyazaki J I, Nakajima K, Murakami F (2010). CXCR4 is required for proper regional and laminar distribution of cortical somatostatin-, calretinin-, and neuropeptide y-expressing GABAergic interneurons. Cereb Cortex. 2010<month>Mar</month><day>3</day>. [Epub ahead of print]

[95] Temple S (2001). Stem cell plasticity—building the brain of our dreams. Nat Rev Neurosci, 2(7): 513–520

[96] Till J E, McCULLOCH E A (1961). A direct measurement of the radiation sensitivity of normal mouse bone marrow cells. Radiat Res, 14: 213–222

[97] Tsuji O, Miura K, Okada Y, Fujiyoshi K, Mukaino M, Nagoshi N, Kitamura K, Kumagai G, Nishino M, Tomisato S, Higashi H, Nagai T, Katoh H, Kohda K, Matsuzaki Y, Yuzaki M, Ikeda E, Toyama Y, Nakamura M, Yamanaka S, Okano H (2010). Therapeutic potential of appropriately evaluated safe-induced pluripotent stem cells for spinal cord injury. Proc Natl Acad Sci U S A, 107(28): 12704–12709

[98] Villa A, Snyder E Y, Vescovi A, Martínez-Serrano A (2000). Establishment and properties of a growth factor-dependent, perpetual neural stem cell line from the human CNS. Exp Neurol, 161(1): 67–84

[99] Wonders C P, Anderson S A (2006). The origin and specification of cortical interneurons. Nat Rev Neurosci, 7(9): 687–696

[100] Wonders C P, Taylor L, Welagen J, Mbata I C, Xiang J Z, Anderson S A (2008). A spatial bias for the origins of interneuron subgroups within the medial ganglionic eminence. Dev Biol, 314(1): 127–136

[101] Woo N H, Lu B (2006). Regulation of cortical interneurons by neurotrophins: from development to cognitive disorders. Neuroscientist, 12(1): 43–56

[102] Woodbury D, Schwarz E J, Prockop D J, Black I B (2000). Adult rat and human bone marrow stromal cells differentiate into neurons. J Neurosci Res, 61(4): 364–370

[103] Wu G, Gentile L, Do J T, Cantz T, Sutter J, Psathaki K, Arauzo-Bravo M J, Ortmeier C, Scholer H (2010). Efficient derivation of pluripotent stem cells from sirna-mediated cdx2-deficient mouse embryos. Stem Cells Dev. 2010<month>Jun</month><day>10</day>. [Epub ahead of print]

[104] Xu Q, Cobos I, De La Cruz E, Rubenstein J L, Anderson S A (2004). Origins of cortical interneuron subtypes. J Neurosci, 24(11): 2612–2622

[105] Xu Q, Guo L, Moore H, Waclaw R R, Campbell K, Anderson S A (2010). Sonic hedgehog signaling confers ventral telencephalic progenitors with distinct cortical interneuron fates. Neuron, 65(3): 328–340

[106] Yan J, Welsh A M, Bora S H, Snyder E Y, Koliatsos V E (2004). Differentiation and tropic/trophic effects of exogenous neural precursors in the adult spinal cord. J Comp Neurol, 480(1): 101–114

[107] Yang M, Donaldson A E, Jiang Y, Iacovitti L (2003). Factors influencing the differentiation of dopaminergic traits in transplanted neural stem cells. Cell Mol Neurobiol, 23(4–5): 851–864

[108] Ying Q L, Stavridis M, Griffiths D, Li M, Smith A (2003). Conversion of embryonic stem cells into neuroectodermal precursors in adherent monoculture. Nat Biotechnol, 21(2): 183–186

[109] Young W, Koreh I (1986). Potassium and calcium changes in injured spinal cords. Brain Res, 365(1): 42–53

[110] Zhang A L, Hao J X, Seiger A, Xu X J, Wiesenfeld-Hallin Z, Grant G, Aldskogius H (1994). Decreased GABA immunoreactivity in spinal cord dorsal horn neurons after transient spinal cord ischemia in the rat. Brain Res, 656(1): 187–190

[111] Zhou Q, Anderson D J (2002). The bHLH transcription factors OLIG2 and OLIG1 couple neuronal and glial subtype specification. Cell, 109(1): 61–73

RIGHTS & PERMISSIONS

Higher Education Press and Springer-Verlag Berlin Heidelberg