Vertebrate Neural Stem Cells: Development, Plasticity, and Regeneration

Abstract

Natural recovery from disease and damage in the adult mammalian central nervous system (CNS) is limited compared with that in lower vertebrate species, including fish and salamanders. Species-specific differences in the plasticity of the CNS reflect these differences in regenerative capacity. Despite numerous extensive studies in the field of CNS regeneration, our understanding of the molecular mechanisms determining the regenerative capacity of the CNS is still relatively poor. The discovery of adult neural stem cells (aNSCs) in mammals, including humans, in the early 1990s has opened up new possibilities for the treatment of CNS disorders via self-regeneration through the mobilization of these cells. However, we now know that aNSCs in mammals are not plastic enough to induce significant regeneration. In contrast, aNSCs in some regenerative species have been found to be as highly plastic as early embryonic neural stem cells (NSCs). We must expand our knowledge of NSCs and of regenerative processes in lower vertebrates in an effort to develop effective regenerative treatments for damaged CNS in humans.

Introduction

In vertebrate CNS development, NSCs generate various types of neurons and glia in spatially and temporally regulated patterns to form complex networks that control complex behavioral patterns. Moreover, NSCs exist in the ventricular neuroaxis throughout the life of vertebrates and continue to generate new neurons constitutively in particular CNS regions.^1,2,3 Adult neurogenesis is involved in continuous brain growth or neuronal turnover in different functional contexts in various species.⁴ Adult NSCs, including latent NSCs in non-neurogenic regions, can also be mobilized for regeneration in response to CNS damage.^4,5 In particular, many non-mammalian vertebrates are able to effectively regenerate damaged CNS via mobilization of NSCs and progenitors.⁴ In contrast, adult mammals have limited regenerative capacity, although some NSCs and progenitors are mobilized in response to several types of damage.⁵ The species-specific differences in regenerative ability and broadening of neurogenic regions within vertebrates appear to correlate with phylogenetic interrelations.³ These differences partly may be caused by differences in the developmental potential of adult NSCs. In mammals, the developmental potential of NSCs is likely to be temporally and spatially restricted during development.^6,7 For instance, many projection neurons, such as motor neurons (MNs) and dopaminergic (DA) neurons (DANs), which are generated in the early embryonic stage, are hardly generated at all by adult NSCs located in the corresponding ventricular neuroaxis where these neurons were born. In contrast, motor neuron and DA neuron regeneration can be induced in teleost fish SC and newt midbrain, respectively, after specific lesions.^8,9

In humans, restoration of CNS functions that are lost in neurological disorders and injuries is very hard to achieve because of poor plasticity of the mature CNS. The identification of embryonic and adult NSCs in mammals, including humans, has elicited high expectations for their clinical application, especially in cell replacement therapies, to treat many types of CNS disorders. Indeed, a number of studies in animal models of various neurological disorders have investigated the transplantation of in vitro-expanded neural progenitor cells (NPCs), including NSCs and their differentiated progenies, and the mobilization of latent NPCs in vivo.¹⁰ Some enhancement of functional recovery was observed in many of these studies. The creation of induced pluripotent stem cells^11,12 and the direct reprogramming of non-neural somatic cells into NPCs and neurons¹³ appear to have resolved many of the divisive ethical issues associated with the sourcing of highly plastic NSCs. Before these new techniques were introduced, NSCs in the early developmental state and early-born neurons such as MNs and DANs were obtainable only from embryos and embryonic stem cells (ESCs). However, there are still potential problems with the quality of neural cells derived via reprogramming, which often induce aberrant epigenomic reprogramming even in the absence of genomic integration of reprogramming factors, raising safety issues in their clinical use.¹⁴ Furthermore, transplantation of large numbers of specific types of neurons into specific brain regions may result in unusual structural changes, thereby causing unexpected side effects. Indeed, some Parkinson’s disease patients who underwent transplantation into striatum of DANs derived from the human embryonic midbrain developed involuntary movements called dyskinesias as a side effect.¹⁵ Thus, ideal and ultimate cures for damaged CNS should be based on self-regeneration of lost neurons and glia via mobilization of endogenous NPCs and rewiring of functional neural circuits, processes that are often observed in certain lower vertebrates.

In this review, I discuss the ways in which mammalian NSCs develop and lose plasticity and summarize evolutional differences in the regenerative capacity of adult CNS and the plasticity of adult NSCs. Progress in these fields is requisite for the development of an ultimate cure for damaged CNS via self-regeneration.

Temporal Specification of NSCs

During vertebrate ontogeny, the first irreversible specification of NSCs is regional specification, which is regulated by various inductive signals emanating from certain places in the embryo with the right timing to generate regionally specific neuronal subtypes¹⁶ (Fig. 1). NSCs can first be detected during the period of neural plate formation¹⁷ and are at this stage already partially regionalized along the anterior–posterior (A-P) axis that is defined until the end of gastrulation¹⁶ (Fig. 1A). At this time, NSCs can give rise to both the neural crest lineage and the CNS lineage, depending on the concentration of neural crest inductive signals, including bone morphogenic proteins (BMPs), Wnt family proteins, and fibroblast growth factors (FGFs). The neural plate with proliferating cells then invaginates and detaches from the epidermis to form the neural tube. During this period, the regional identity of NSCs can be determined more precisely by their position along the dorsal–ventral (D-V) and A-P axes (Fig. 1B-D).

Fig. 1

Regional specification in the developing CNS.

(A) Neuroectoderm is partially regionalized along the A-P axis by gradients of various inductive signals including Wnt, FGFs, and retinoic acid secreted from the node and Wnt inhibitors/antagonists and nodal antagonists secreted from the anterior visceral endoderm at the neural plate stage. (B, C) Regionalization of neuroepithelium occurs along the D-V axis as a result of gradients of Shh from the NC and FP and gradients of BMPs and Wnts from the roof plate (RP) during neural tube closure. Neural crest cells arise and delaminate from the RP of the neural tube shortly after closure and migrate through the periphery. (D) The regional specification of NSCs regulated by various inductive signals during development is summarized. A, anterior; P, posterior; D, dorsal; V, ventral; TEL, telencephalon; DI, diencephalon; ME, mesencephalon; RH, rhombomere; ZLI, zona limitans intrathalamica; IS, isthmus.

After neural tube closure, NSCs as neuroepithelial cells in the ventricular zone initially symmetrically divide to expand the NSC pool and transform into radial glia (RG); they then start asymmetric division to generate neurons. In the developing mammalian cortex, neurons generated by RG in the VZ and subventricular zone (SVZ) migrate radially to the cortical plate to form six distinct layers containing specific types of excitatory neurons in each layer (Fig. 2A). The layer-specific neurons are sequentially generated by RG, and later-born neurons migrate past the earlier-born neurons to their final laminar location in an “inside-out” fashion based on their birth order. Clonal lineage analyses in vitro and in vivo have indicated that the sequential generation of preplate (ppl), deep-layer (DL), and upper-layer (UL) neurons and then glia is encoded, at least in part, within individual RG ^18,19,20 (Fig. 2A). In contrast, Franco et al. provided evidence that supports a model in which multiple types of progenitors differentiate into the layer-specific neurons at different times under control of intrinsic programs and/or extrinsic signals²¹ (Fig. 2B). By lineage tracing analyses using cut-like homeobox 2 (Cux2)-Cre and -CreERT2 driver mouse lines, they found that Cux2-expressing VZ progenitors that exist during the period of DL neuron production are already fate-restricted to give rise to UL neurons. However, using the same driver and reporter lines, Guo et al. later demonstrated that Cux2⁺ RG generate both DL and UL neurons.¹⁹ There is no clear explanation for this discrepancy.

Fig. 2

Temporal regulation of cortical cytogenesis and layer formation in mammals.

(A) The schematic shows a competence model for the temporally regulated generation of various types of neurons for formation of the six layers of the cerebral cortex and glia. Several in vivo and in vitro clonal analyses have indicated that individual progenitors, including stem cells, are temporally specified to sequentially produce different types of neurons that migrate along RG processes to the superficial layers in an “inside-out” fashion. RG in the VZ start production of neurons after transformation from neuroepithelial cells. They first produce ppl neurons, including CR cells, which primarily migrate into layer I, and subplate neurons, followed by DL and UL neurons. Intermediate progenitors and basal RG (bRG) in the SVZ are born from RG to increase neuronal production. RG and bRG acquire gliogenic competence to respond to gliogenic signals during midgestation and finally differentiate into glia after neurogenesis is complete. (B) A lineage model for the temporally regulated generation of DL and UL neurons. Two types of progenitors, Cux2^–/Fezf2⁺ progenitors and Cux2⁺/Fezf2⁺ or − progenitors are born from Cux2^–/Fezf2⁺ progenitors at the same time but differentiate into DL (TBR1⁺ and Ctip2⁺/Fezf2⁺) neurons and UL (Cux2⁺/Satb2⁺) neurons, respectively, at different time points.

In recent years, many factors involved in the specification of laminar fates have been identified^18,22,23,24 (Fig. 3). However, how temporal identity of the developing NSCs is determined is poorly understood. Forebrain embryonic zinc finger protein 2 (Fezf2), which is transiently expressed in early cortical RG and is maintained in early-born DL V–VI neurons, has been shown to promote the specification of DL projection neurons.²⁴ Fezf2 represses the expression of chromatin-remodeling protein special AT-rich sequence-binding protein 2 (Satb2), which promotes UL callosal-projection neuron identity by repressing expression of COUP transcription factor (TF)-interacting protein 2 (Ctip2).²⁴ However, it is not clear whether Fezf2 expression in RG is essential for the specification of DL neurons. Both Satb2 and Ctip2 act in postmitotic neurons to promote UL- and DL-specific phenotypes, respectively,²⁴ suggesting that Fezf2 plays a critical role in postmitotic neurons, rather than in progenitors, in terms of the acquisition of DL neuron identities. Indeed, forced expression of Fezf2 in postmitotic UL neurons convert their identities to that of DL neurons.^25,26 Foxg1 is a member of the forkhead box family of TF that is highly expressed in early cortical progenitors, DL neurons, and UL neurons but not in ppl Cajal-Retzius (CR) cells. Foxg1 is required for the transition from the production of the earliest CR cells to DL neurons.^22,23 This transition requires, at least in part, the repression of T-box brain 1 (Tbr1) TF which represses Fezf2 expression by Foxg1,^27,28 which represses Fezf2 expression²⁸. Since Tbr1 is detectable only in postmitotic neurons, Foxg1 may also act in postmitotic neurons. COUP-TF1 and COUP-TFII nuclear receptors are also essential for the switch from early-born to late-born neurons not only in the cerebral cortex but also in the basal ganglia.²⁹ However, it is also unclear whether these factors act in NSCs or in committed neuronal lineages. In contrast, Ikaros, a mammalian homolog of Drosophila hunchback (Hb, a temporal identity factor specifying the first-born fate in the neuroblasts that sequentially generate several types of neurons in the Drosophila embryonic CNS³⁰), may also act as a temporal identity factor for NSCs in the developing mammalian cortex.³¹ In the developing mouse cortex, Ikaros is specifically expressed in progenitors, and its expression level dramatically decreases with development.³¹ Forced and sustained expression of Ikaros, specifically in cortical progenitors from an early stage, prolonged production of DL neurons, whereas overexpression of Ikaros at a later stage after the natural decrease of Ikaros did not rejuvenate progenitors producing UL neurons back to DL neuron production mode.³¹ Because COUP-TFI/II are mammalian homologs of Drosophila Seven-up, which is transiently expressed in neuroblasts to regulate the timing of expressions of temporal identity factors including Hb,³⁰ it would be intriguing to investigate the genetic interaction between COUP-TFI/II and Ikaros in the regulation of laminar fate determination.

Fig. 3

Current model for the specification of layer-specific neuronal subtypes by TF in the developing cortex.

The final fate of cortical projection neurons are likely determined postmitotically by a network of TF. In ppl and layer VI neurons, Tbr1 and its upstream factor Sox5, both of which repress Fezf2 expression, are key determinants. In layer V neurons, Foxg1 and Ctip2, both of which repress Tbr1 expression, and Fezf2, which represses Satb2 expression, are essential for the layer V phenotype. In UL (layers II–IV) neurons, Foxg1 and Satb2, which represses Ctip2 expression, are key factors. Since ectopic expression of Fezf2 in UL neurons converts their identity into layer V neurons, repression of Fezf2 expression by unknown mechanisms may be essential for the specification of UL neurons. In the progenitor level, high expression of Ikaros in the early stages is essential for the specification of DL neurons. Transient increases of COUP-TFI/II at the later stage may be required for the specification of UL neurons. The specific function of Fezf2 in progenitors is not shown.

In addition to that in the cerebral cortex, the regulation of sequential generation of distinct types of neurons by TFs has also been found in the developing midbrain and hindbrain. In the ventral midbrain, the sequential generation of ocular MNs and red nucleus neurons from progenitors located lateral to the DA neuron progenitor domain is controlled by the LIM homeodomain TF Lmx1b and the homeodomain TF Phox2a.³² In the ventral hindbrain, a complex network formed by several TF, including Nkx2.2, Foxa2, and Phox2b, is likely to control the sequential generation of MNs, serotonergic neurons, and oligodendrocyte progenitors (OLPs).³³ To elucidate the regulatory mechanisms of temporal specification of NSCs for the sequential generation of distinct types of neurons during development, future studies should address specific functions and regulations of expression of known factors involved in this process in NSCs, in addition to the identification of additional temporal identity factors. Finally, it will be essential to understand how these factors are integrated into the transcription network in NSCs and their progenies for the regulation of temporal specification.

In the developing vertebrate CNS, the first gliogenesis (which is actually oligodendrogliogenesis) starts in the ventral regions after or during neurogenesis, depending on the A-P neuroaxis³⁴ (Fig. 4A). Despite extensive studies by numerous investigators, the mechanism of oligodendrocytic differentiation of NSCs is still enigmatic. It is even unclear whether oligodendrocytes or OLPs can be generated stochastically from NSCs (stochastic model), or are generated deterministically through the differentiation of NSCs into bipotent glial-restricted progenitors (GRPs) (deterministic model), or both (Fig. 4B). The lack of definitive NSC markers to distinguish stem cells from committed intermediate progenitor cells makes it difficult to visualize how NSCs differentiate into neurons or glia. In particular, there is no reliable method to distinguish GRPs and astrocyte precursors from stem cells prospectively, whereas neuron-restricted progenitors can be identified by prominent expressions of several markers, including the basic helix-loop-helix TF Neurogs and Tbr2 in the developing cortex.³⁵ The first acquisition of OLPs in the embryonic SC requires sonic hedgehog (Shh), which is initially secreted from the floor plate (FP) and notochord (NC) for D-V patterning of the progenitor domain; this induces pMN domain generating MNs, which later become a site of OLP origin.³⁴ However, the detailed molecular mechanisms by which Shh actions are transduced remain to be determined. Progenitors in the pMN domain do not generate OLPs during motor neuron generation, despite the existence of the Shh signal. Moreover, the existence of a Shh-independent pathway for OLP specification has been demonstrated.^36,37 Consequently, it is not clear how NSCs differentiate into OLPs. Nonetheless, factors that are required for the acquisition of OLPs have been identified. Deletion of Sox9, a member of the SOX family of high mobility group TF, resulted in a prolonged period of motoneurogenesis in the developing spinal cord (SC), coupled with a delay in the onset of oligodendrogliogenesis.³⁸ The knockdown of COUP-TFI/II also resulted in prolonged neurogenesis and inhibition of OLP acquisition in the developing mouse ventral forebrain, as well as in cultured NPCs derived from mouse ESCs.²⁹ The important thing is that these factors are essential also for the initiation of astrogliogenesis.^29,39

Fig. 4

Mechanisms of glial development.

(A) The timing of oligodendrocyte progenitor (OLP) generation in the developing mouse CNS. In the developing SC, OLPs are first generated from a progenitor domain, termed the pMN domain, located in the ventral portion of the VZ after the completion of motor neuron production. In the forebrain, OLP production starts during the period of interneuron generation in the VZ of the medial ganglionic eminence (MGE) and spreads into the ventral part of the lateral ganglionic eminence (vLGE). (B) Two models for the differentiation of NSCs. In the stochastic model, NSCs can differentiate stochastically into the neuron (N), astrocyte (A), or oligodendrocyte (O) lineages. In the deterministic model, astrocytes and oligodendrocytes are differentiated via glial restricted progenitors (GRPs). Environmental signals bias the fate determination in both cases. (C) Intrinsic and extrinsic factors for the regulation of gliogenesis during development in the mouse. Time-dependent increases in the expression levels of MAPK14, NFIA/B, and Sox9 in NSCs are essential for the acquisition of gliogenic competence to respond to gliogenic signals such as CT-1, Notch signaling, BMPs, and TGF-β1. The expression of MAPK14 is inhibited by members of the miR-17/106 family, which are down-regulated by transient increases of COUP-TFI/II. Time-dependent increases of gliogenic signals, including CT-1 and Notch ligands from neurons, facilitate differentiation of astrocytes after the acquisition of gliogenic competence. MEK1/2 which mediate the action of TGF-β1 as a gliogenic signal may be required for the increase of COUP-TFII. eNSC, early NSC; lNSC, late NSC. (D) Two models for the neurogenesis-to-gliogenesis switch. In the competence model, NSCs acquire gliogenic competence at midgestation, and the fate determination largely depends on environmental signals in the later stages. NSCs may be able to differentiate into astrocytes or oligodendrocytes directly without a GRP stage. In the differentiation model, the frequency of stochastic differentiation of NSCs into GRPs increases with development. This frequency may be regulated by a balance between Myc activity and p53 activity. In this case, only GRPs can respond to environmental signals to differentiate into astrocytes or oligodendrocytes.

The initiation of astrogliogenesis by NSCs requires two steps: the acquisition of gliogenic competence and the increase of inductive gliogenic signals such as cardiotrophin-1 (CT-1), with development^40,41 (Fig. 4C). The most important known gliogenic signaling pathways that promote astrocytic differentiation [defined by expression of astrocyte-specific proteins such as glial fibrillary acid protein (GFAP) and S100β] are the interleukin-6 (IL-6)/Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway, the BMP2/4/mothers against decapentaplegic homolog (SMAD) pathway, and the Notch signaling pathway.^40,41 IL-6 family cytokines [e.g., ciliary neurotrophic factor (CNTF)], leukemia inhibitory factor, and CT-1 bind to receptors that share a common co-receptor (glycoprotein 130: gp130), thereby triggering activation of the JAK family of non-receptor tyrosine kinases, which in turn activate STAT1 and STAT3.⁴² BMP2/4 bind to a tetrameric complex of type I and type II serine/threonine kinase receptors that phosphorylate and activate SMAD TFs.⁴³ In canonical Notch signaling, Notch ligands such as Delta and Jagged bind to Notch receptors at the cell surface, leading to nuclear translocation of the Notch intercellular domain (NICD) after proteolytic cleavage of Notch. NICD subsequently forms a transcriptional complex with the coactivator Mastermind and the DNA-binding protein recombination signal binding protein for immunoglobulin kappa J region (RBPJ)/CSL to activate target genes.⁴⁴ It has been shown that BMP signals and gp130-mediated cytokine signals synergistically facilitate astrocytic differentiation of NPCs via the cooperative activation of SMAD1 and STAT3 to induce astrocyte-specific genes.^40,41 However, the responsiveness of NSCs to these signals changes with development. NSCs in the early neurogenic phase cannot differentiate into astrocytes even in the presence of BMPs and IL-6 family cytokines.^{29,45,46,47,48} In fact, BMPs facilitate neuronal differentiation of NSCs at this developmental phase.^49,50 NSCs acquire gliogenic competence to respond to these signals at midgestation in mammals. Sox9 and COUP-TFI/II are involved in this competence change in the developing NSCs. An increase of SOX9 expression at midgestation induces expression of the TF nuclear factor I(NFI)A, which acts as a key regulator for the initiation of gliogenesis.^39,51,52 SOX9 forms a complex with NFIA to induce a subset of glial-specific genes.³⁹ Moreover, NFIA is required for Notch signaling-induced demethylation of Gfap gene promoter in NSCs.⁵³ Developmental changes in the epigenetic status of astrocyte-specific genes critically determine the responsiveness of NSCs to gliogenic signals.⁴⁷ The transient increase of COUP-TFI/II expression in NSCs at midgestation is also essential for the acquisition of gliogenic competence and alters the epigenetic status of the Gfap promoter to respond to gliogenic signals.²⁹ Moreover, members of the microRNA-17/106 (miR-17/106) family were found to play a critical role in regulation of the neurogenesis-to-gliogenesis switch downstream of COUP-TFI/II in NSCs.⁵⁴ The downregulation of miR-17/106 in NSCs during development leads to increased levels of one of their targets, mitogen-activated protein kinase 14 (MAPK14, also known as p38α), which is essential for the acquisition of gliogenic competence independent of epigenetic regulation of the Gfap gene. Intriguingly, overexpression of miR-17/106 or the loss of function of MAPK14 forced stage-progressed NSCs, which are normally highly gliogenic, to regain neurogenic competence.⁵⁴ Because the epigenetic status of the promoter of neurogenic gene Neurog1 and its enhancer element (which transcribes a long non-coding RNA designated as utNgn1, which regulates Neurog1 transcription in cortical NPCs) becomes silent during the acquisition of gliogenic competence,^55,56 it would be interesting to know how the miR-17/106-MAPK14 axis regulates neurogenic competence, including the epigenetic status of neurogenic genes. The extracellular signal-regulated kinase kinases (MEK)/extracellular signal-regulated kinase pathway also plays a critical role in the acquisition of gliogenic competence. Loss of MEK1 and 2 in NSCs leads to attenuated gliogenesis and prolonged neurogenesis, whereas forced expression of constitutively active MEK1 robustly enhances astrogliogenesis in the developing mouse cortex.⁵⁷ More importantly, NSCs lacking MEK1/2 cannot respond to the astrogliogenic signal CNTF in vitro. Intriguingly, the expression level of COUP-TFII is significantly decreased in the brains of Mek1/2-deleted mice, as is that of the Ets transcription family member Erm, which is a downstream effector of the MEK-mediated pathway for the acquisition of gliogenic competence.⁵⁷ The transforming growth factor (TGF)-β signal may be a candidate upstream factor for the activation of MEK1/2, because inhibition of MEK activity attenuates astrocytic differentiation stimulated by TGF-β1 in vitro.⁵⁸ MAPK14 is also activated by the TGF-β signal in other cellular contexts.⁵⁹ Moreover, the TGF-β signal has been shown to control the timing of oligodendrogliogenesis in the developing hindbrain and SC.⁶⁰ Therefore, the neurogenic-to-gliogenic competence transition of NSCs is probably governed by a multi-layered system composed of complex gene networks (Fig. 4C).

Despite recent advances in the identification of factors that regulate the neurogenic-to-gliogenic competence transition, as described above, it is still not clear how NSCs differentiate into glia because NSCs and GRPs are currently indistinguishable, as mentioned above. Therefore, we do not even know whether astrogliogenic signals act on NSCs, GRPs, or both to effect astrocytic differentiation. If they can induce astrocytic differentiation of NSCs directly, “gliogenic competence” must be the competence to respond to instructive gliogenic signals for a stochastic differentiation of NSCs into astrocytes (see the competence model in Fig. 4D). Otherwise, there can be no acquisition of “gliogenic competence”; instead, an increased frequency of differentiation of NSCs into the glial lineage must be initiated by an intrinsic timer mechanism that involves regulations operated by the factors described above (see the differentiation model in Fig. 4D). Work by Nagao et al. (2008) may support the latter possibility.⁶¹ They proposed a linkage between the self-renewal capacity and neurogenic competence in NSCs under control of the opposing actions of Myc TF and the tumor suppressor p19^ARF-p53 TF pathway during CNS development. In their model, the Myc-dominant status resulting from low expression of p19^ARF (which activates p53) links the high self-renewal capacity in NSCs with a high neurogenic propensity at early neurogenic stages. A time-dependent increase in p19^ARF expression attenuates self-renewal and neurogenesis while facilitating gliogenesis via the actions of p53. In short, increased activation of p53 (which inhibits Myc functions) may increase the frequency of differentiation of NSCs into GRPs, resulting in reductions of self-renewal of NSCs and neurogenesis. In this case, astrogliogenic signals merely induce astrocytic differentiation of GRPs and/or terminal differentiation of astrocyte precursors. Intriguingly, p38 has also been shown to activate p53 directly in other cellular contexts.⁵⁹ Future study should include identification of markers to distinguish NSCs from to draw firm conclusions about what constitutes “gliogenic competence.”

Timing of Differentiation by Environmental Factors

The timing of terminal and phenotypic differentiation of neural cells in the developing CNS often depends on time-dependent changes in environmental signals (Fig. 4B). In the developing mouse cortex, a time-dependent increase in the local concentration of CT-1 secreted by newly born neurons is critical for the expression of GFAP and CD44, another astrocyte marker,⁶² which means that increased activation of the JAK/STAT pathway during the progression of neurogenesis is essential for the timing of astrocyte differentiation. Thus, astrocyte markers normally become detectable only after the end of neurogenesis. Similarly, the activation of Notch signaling in RG likely increases along with an increase in the number of neurons that express high levels of Notch ligands, such as Delta,⁵³ the expression of which is positively regulated by the JAK/STAT pathway.⁶³ Non-cell autonomous cooperation of the JAK/STAT and Notch signaling pathways may be important for the progression of astrogliogenesis.

Conversely, a signal mediated by ErbB2 and ErbB4 receptors suppresses astrogliogenesis during the neurogenic period in the developing mouse cortex.⁴⁰ ErbB4 activation in RG by ligands such as neuregulin-1 leads to the cleavage of ErbB4 and translocation of its intracellular domain to form a complex of ErbB4/TGF-β-activated kinase-binding protein 2 (TAB2)/nuclear receptor co-repressor (N-CoR) to repress the transcription of Gfap and S100β.⁶⁴ Because JAK/STAT signaling leads to the translocation of N-CoR to the cytoplasm,⁶⁵ which may inhibit formation of the ErbB4 complex, a balance between the JAK/STAT pathway and ErbB4-mediated signaling may also be important for determining the timing of astrocyte differentiation.

The phenotypic specification of UL neurons in the developing mouse cortex is also likely to depend on time-dependent changes in the environment. Toma et al. demonstrated that the acquisition of UL competence in neurons or their precursor cells after DL neurogenesis requires a feedback signal from postmitotic DL neurons.²⁸ Specific ablation of postmitotic DL neurons prolonged DL neurogenesis at the expense of UL neurogenesis.

Adult Stem Cells and Regeneration

The regenerative capacity of the CNS largely depends on the number and developmental potential of NSCs. In mammals, as the developmental potential and neurogenic competence of NSCs decrease during development in most regions of the CNS (Figs. 2–4), so also does the degree of functional and structural recovery from CNS damage(Fig. 5).⁶⁶ Finally, in the adult CNS, although a limited number of NSCs are present throughout the ventricular neuroaxis, most are dormant; the exceptions are those NSCs localized in two regions (ventricular–subventricular zone (V-SVZ) of the lateral ventricles and subgranular zone (SGZ) of the hippocampus) that continuously generate new neurons that functionally integrate into specific neural circuits² (Fig. 6). NSCs in the V-SVZ give rise to GABAergic neurons that migrate into olfactory bulbs (OB) throughout life from midgestation. The SGZ generates granule neurons in the dentate gyrus. In the adult brain, both latent and active NPCs, including stem cells, can be mobilized to generate new neurons and glia in response to brain lesions, such as those resulting from ischemia, traumatic injury, and demyelination⁵ (Fig. 6). Transient forebrain ischemia in the rat induces selective degeneration of hippocampal CAI pyramidal neurons followed by their limited regeneration by mobilization of latent progenitor cells located around the posterior periventricle (pPV) adjacent to the hippocampus.⁶⁷ A middle cerebral artery occlusion model of stroke in rodents induces enhanced neurogenesis in the V-SVZ and subsequent migration of new neurons toward the ischemic cortex and striatum.⁶⁸ Neurogenesis in the SGZ is facilitated in several models of brain injury, including hypoxia–ischemia, stroke, and the controlled cortical impact model of traumatic brain injury.⁵ Although such injury-induced neurogenesis is insufficient to repair significant brain damage, further enhancement of neurogenesis and neuronal migration by administration of appropriate growth factors and cytokines (such as FGF2, epidermal growth factor/TGF-α, brain-derived neurotrophic factor, vascular endothelial growth factor, erythropoietin, stem cell factor, and granulocyte-colony stimulating factor) often results in substantial and functional tissue regeneration.^68,69,70 Factors that are known to enhance neurogenesis or neuronal migration after various brain injuries are summarized in Table 1.^{67,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108} However, there is no clear evidence for the regeneration of early-born neurons, such as midbrain DA neurons (DANs), MNs, and forebrain cholinergic neurons (CNs), by NPCs in the adult mammalian CNS. Moreover, regeneration of cortical projection neurons has not been reported. A major cause of this limited regenerative capacity in the mammalian CNS is likely the developmental decrease in the plasticity of NSCs and/or the depletion of juvenile NPCs that can generate early-born neurons.

Fig. 5

Regulation of gfap gene expression by environmental signals.

The transcription of Gfap is repressed by recruitment of the ErbB4 intracellular domain/TAB2/NCoR complex into the nucleus and its association with RBPJ on the Gfap promoter by the activation of ErbB4 receptor by ligands such as NRG1 in RG during the neurogenic period. Activation of JAK/STAT signaling via gp130/LIFR, caused by increases in CT-1 secretion from neurons, leads to the formation of the STAT3/SMAD/p300 complex on the Gfap promoter and the translocation of N-CoR to the cytoplasm, resulting in derepression and activation of Gfap promoter in the differentiation of astrocytes. NFIA, levels of which also increase in RG during development, facilitates Gfap transcription. The white region in the center of the two cell types indicates nuclear processes, whereas the outer colored region represents cytoplasmic processes.

Fig. 6

Neurogenic niches and mobilization of NPCs in the adult rodent brain. NSCs located in the V-SVZ and SGZ generate GABAergic neurons that migrate into OB via a rostral migratory stream (RMS) and granule neurons (G) in the dentate gyrus, respectively. These NSCs are further mobilized to generate new neurons in response to brain injury such as ischemia. Latent stem and/or progenitor cells (S/P) located around the pPV near the hippocampus are also mobilized to regenerate CAI pyramidal neurons in response to ischemic injury. A, type A cell (neuroblast); B, type B cell (stem cell); C, type C cell (transit amplifying progenitor); E, ependymal cell; O, oligodendrocyte; S, stem cell; I, intermediate progenitor.

Table 1 Growth factors and cytokines that facilitate CNS regeneration via mobilization of NPCs

Factor	Reference	Function
FGF	67, 71–73	Proliferation of progenitors
EGF/TGF-a	67, 74–79	Proliferation of progenitors
BMP7	80	Proliferation of progenitors
Gal-1	81	Proliferation of progenitors
GDNF	82, 83	Proliferation of progenitors
SCF	84, 85	Proliferation of progenitors
G-CSF	84–88	Proliferation of progenitors, neuronal differentiation
HB-EGF	89, 90	Proliferation of progenitors, neuronal differentiation
HGF	91	Proliferation of progenitors, neuronal differentiation
IGF1	82, 92–95	Proliferation of progenitors, neuronal differentiation
TGF-β	96	Proliferation of progenitors, neuronal differentiation
VEGF	97–99	Proliferation of progenitors, neuronal differentiation
EPO	76, 100–103	Proliferation of progenitors, neuronal differentiation
BDNF	104–106	Neuronal differentiation
ANG1	107	Cell migration
Meteorin	108	Cell migration
SDF-1	107	Cell migration

EGF, epidermal growth factor; Gal-1, galectin-1; GDNF, glial cell line-derived neurotrophic factor; SCF, stem cell factor; G-CSF, granulocyte colony stimulating factor; HB-EGF, heparin-binding EGF; HGF, hepatocyte growth factor; IGF1, insulin-like growth factor 1; VEGF, vascular endothelial growth factor; EPO, erythropoietin; BDNF, brain-derived neurotrophic factor; ANG1, angiopoietin 1; SDF-1, stromal cell-derived factor-1.

In contrast, some lower vertebrates, such as urodele amphibians and teleost fish, have much higher regenerative capacity in the adult CNS than mammals do. In these animals, adult neurogenesis is much more widespread, and this may be linked to the overall regenerative capacity of their CNSs (Table 2).^3,4 Among all vertebrates studied, teleost fish exhibit the most active and widespread adult neurogenesis, with a remarkable regenerative capacity after severe injuries, such as ablation of a large portion of the CNS, along the whole rostro-caudal neuroaxis.⁸ In zebrafish, 16 different neurogenic niches have been identified along the entire neuroaxis, including the SC.¹⁰⁹ In these regions, many different neuronal subtypes are constitutively generated throughout adult life. Other than those in teleost fish, neurogenic regions in the adult CNS are restricted to forebrain in most species, except for the axolotl, which has proliferating NPCs in more caudal regions, including the SC to a limited extent.^3,4,110,111 Brain lesions often enhance proliferation of NPCs in these neurogenic regions. However, specific types of neurons can also be regenerated outside of adult neurogenesis (Fig. 7). For instance, midbrain DANs and basal forebrain cholinergic neurons (BFCNs) can be regenerated after their selective ablation in the adult red-spotted newt.^9,112 In DA neuron regeneration, quiescent stem-like cells called ependymoglia in midbrain are activated to generate DANs in response to the loss of DANs, leading to complete recovery.^9,113 The selective ablation of BFCNs increases neurogenesis by both active and relatively quiescent populations of ependymoglia, although the origin of new cholinergic neurons has not been determied.¹¹¹

Table 2 Neurogenic niches and regenerative capacity in the adult vertebrate CNS

Region	Fish		Urodeles		Anurans		Reptiles		Birds		Mammals
	N	R	N	R	N	R	N	R	N	R	N	R
FB	+	+	+	+	+	?	+	+	+	+/−	+	+/−
MB	+	?	+/−	+	+/−	?	-	?	-	?	-	-
HB	+	+	+	?	+	?	+/−	?	-	-	-	-
SC	+	+	+	+	-	-	-	+/−	-	-	-	-

N, neurogenesis; R, regeneration; FB, forebrain; MB, midbrain; HB, hindbrain; SC, spinal cord; ?, Lack of sufficient information; +, reported in multiple species; +/−in N, rare case; +/−in R, a limited recovery or rare case.

Fig. 7

Regeneration of early-born neurons in the adult newt brain and zebrafish SC. (A) Vertical lines on the lateral view of the newt brain correspond to the transverse sections shown in B and C. (B) Regeneration of forebrain CNs after their selective ablation. CNs can be derived from ependymoglia located in the proliferative hot spot (red nucleus) and/or non-hot spot (orange nucleus). (C) Regeneration of midbrain DANs after their selective ablation. Quiescent ependymoglia are mobilized to generate DANs. (D) In the adult zebrafish, regeneration of SC neurons, including MNs, from ependymoradial glia takes place after complete spinal transection. DP, dorsal pallium; LP, lateral pallium; BNST, bed nucleus of the stria terminalis; TC, tectum; TM, midbrain tegmentum.

MNs in the adult SC can also be regenerated in urodele amphibians and teleost fish. In the fully adult zebrafish, substantial numbers of new MNs are generated by relatively quiescent stem-like cells (called ependymoradial glia) for functional SC regeneration after complete spinal transection.¹¹⁴ In axolotl, cultured NPCs derived from adult SC can reconstitute all SC cell types, including MNs, after tail amputation.¹¹⁵ Why can these latent progenitors in the adult CNS generate early-born projection neurons (such as DA and MNs) in urodeles and fish? Are NSCs with early developmental potential retained or are aged NSCs rejuvenated and/or respecified in response to stimulation after injury? In the case of DA neuron regeneration in the newt, increased levels of Shh, which are essential for the specification of NSCs to generate DANs in the developing midbrain, are also essential for DA neurogenesis by ependymoglia in response to DA neuron ablation.¹¹³ The inhibition of Shh signaling reduced DA neuron regeneration but not the proliferation of ependymoglia after lesions were induced, indicating that ependymoglia in the midbrain of the adult newt are at least plastic enough to respond to Shh signaling to generate DANs. Similarly, Shh signaling regulates D-V patterning of SC progenitors during regeneration in the larval axolotl.¹¹⁶ Although the gene expression pattern along the D-V axis in the larval SC resembles that in the embryo, the regional identity can be respecified in response to injury.¹¹⁷ Thus, although it is not clear whether adult NSCs in these animals are rejuvenated in response to CNS lesions, such cells are plastic enough to be respecified to generate appropriate types of neurons, including early-born neurons, for complete CNS regeneration.

Conclusions

A primary objective of neural stem cell research is to answer the fundamental question “Can we regenerate our brain?” Currently, we do not have a clear answer to this question. Moreover, we do not have an efficient treatment for any neurodegenerative disease via the regenerative approach, including cell replacement therapy. Because the regenerative potential of our brain is quite limited, replacement of lost cells by transplantation to achieve functional recovery has extensively been studied.¹⁰ Recent developments in cell reprogramming technology, such as iPSC and direct reprogramming to induce NSCs, neurons, and glia from other somatic cells^11,12,13 (which resolve the ethical problems associated with cell sourcing), should further facilitate the development of transplantation therapeutics. However, the induction of self-regeneration is still the ultimate goal. To achieve self-regeneration, we have to overcome several barriers. First, we have to definitively answer the question “do we have plastic populations of NSCs in our brain?” If not, then we need to find ways to rejuvenate and/or respecify adult NSCs in vivo. To do this, it is of primary importance to elucidate the underlying mechanisms by which NSCs lose plasticity and are specified to generate specific types of neurons and glia during development. Simultaneously, we need to find ways to create a permissive environment for the differentiation of appropriate cells and the reconstruction of proper neural circuits. This is also important in transplantation therapeutics. In the mammalian brain, NPCs are difficult to differentiate into neurons outside of neurogenic regions.¹¹⁸ Although some lesions, such as ischemic lesions, may yield a permissive environment for neuronal differentiation, at least in a restricted time frame,⁵ it is not clear whether that is sufficient for the functional regeneration. Moreover, in addition to the fact that a large number of glia express several axon growth inhibitors that prevent axonal extensions and the fact that the ability of injured axons to regenerate declines with age,¹¹⁹ adult brain may not have adequate guidance cues for the projection of new neurons to their proper target sites. To help address these issues, we can learn much from comparative analyses of lower vertebrates with respect to the plasticity of NSCs and the environment in the adult CNS. We still do not know why NSCs in adult lower vertebrates are so plastic. I hope that we can make our brains more plastic in the future.

Acknowledgments

The author was supported by a Grant-in-Aid for Scientific Research on Innovative Areas entitled “Neural Diversity and Neocortical Organization” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.

References

• 1.  Weiss S,Dunne C,Hewson J,Wohl C,Wheatley M,Peterson AC,Reynolds BA: Multipotent CNS stem cells are present in the adult mammalian SC and ventricular neuroaxis. J Neurosci 1996; 16: 7599–7609.
• 2.  Kriegstein A,Alvarez-Buylla A: The glial nature of embryonic and adult neural stem cells. Annu Rev Neurosci 2009; 32: 149–184.
• 3.  Ferretti P: Is there a relationship between adult neurogenesis and neuron generation following injury across evolution? Eur J Neurosci 2011; 34: 951–962.
• 4.  Grandel H,Brand M: Comparative aspects of adult neural stem cell activity in vertebrates. Dev Genes Evol 2013; 223: 131–147.
• 5.  Christie KJ,Turnley AM: Regulation of endogenous neural stem/progenitor cells for neural repair – factors that promote neurogenesis and gliogenesis in the normal and damaged brain. Front Cell Neurosci 2013; 6: 70.
• 6.  Allen ND: Temporal and epigenetic regulation of neurodevelopmental plasticity. Philos Trans R Soc Lond B Biol Sci 2008; 363: 23–38.
• 7.  Hirabayashi Y,Gotoh Y: Epigenetic control of neural precursor cell fate during development. Nat Rev Neurosci 2010; 11: 377–388.
• 8.  Zupanc GK,Sîrbulescu RF: Adult neurogenesis and neuronal regeneration in the central nervous system of teleost fish. Eur J Neurosci 2011; 34: 917–929.
• 9.  Parish CL,Beljajeva A,Arenas E,Simon A: Midbrain dopaminergic neurogenesis and behavioural recovery in a salamander lesion-induced regeneration model. Development 2007; 134: 2881–2887.
• 10.  English D,Sharma NK,Sharma K,Anand A: Neural stem cells – trends and advances. J Cell Biochem 2013; 114: 764–772.
• 11.  Takahashi K,Yamanaka S: Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultures by defined factors. Cell 2006; 126: 663–676.
• 12.  Takahashi K,Tanabe K,Ohnuki M,Narita M,Ichisaka T,Tomoda K,Yamanaka S: Induction of pluripotent stem cells from adult human fibroblasts by defined factors. Cell 2007; 131: 861–872.
• 13.  Mirakhori F,Zeynali B,Salekdeh GH,Baharvand H: Induced neural lineage cells as repair kits: so close, yet so far away. J Cell Physiol 2014; 229: 728–742.
• 14.  Liang G,Zhang Y: Genetic and epigenetic variations in iPSCs: potential causes and implications for application. Cell Stem Cell 2013; 13: 149–159.
• 15.  Lindvall O: Developing dopaminergic cell therapy for Parkinson’s disease – give up or move forward? Mov Disord 2013; 28: 268–273.
• 16.  Vieira C,Pombero A,García-Lopez R,Gimeno L,Echevarria D,Martínez S: Molecular mechanisms controlling brain development: an overview of neuroepithelial secondary organizers. Int J Dev Biol 2010; 54: 7–20.
• 17.  Tropepe V,Sibilia M,Ciruna BG,Rossant J,Wagner EF,van der Kooy D: Distinct neural stem cells proliferate in response to EGF and FGF in the developing mouse telencephalon. Dev Biol 1999; 208: 166–188.
• 18.  Shen Q,Wang Y,Dimos JT,Fasano CA,Phoenix TN,Lemischka IR,Ivanova NB,Stifani S,Morrisey EE,Temple S: The timing of cortical neurogenesis is encoded within lineages of individual progenitor cells. Nat Neurosci 2006; 9: 743–751.
• 19.  Guo C,Eckler MJ,McKenna WL,McKinsey GL,Rubenstein JL,Chen B: Fezf2 expression identifies a multipotent progenitor for neocortical projection neurons, astrocytes, and oligodendrocytes. Neuron 2013; 80: 1167–1174.
• 20.  Gao P,Postiglione MP,Krieger TG,Hernandez L,Wang C,Han Z,Streicher C,Papusheva E,Insolera R,Chugh K,Kodish O,Huang K,Simons BD,Luo L,Hippenmeyer S,Shi SH: Deterministic progenitor behavior and unitary production of neurons in the neocortex. Cell 2014; 159: 775–788.
• 21.  Franco SJ,Gil-Sanz C,Martinez-Garay I,Espinosa A,Harkins-Perry SR,Ramos C,Müller U: Fate-restricted neural progenitors in the mammalian cerebral cortex. Science 2012; 337: 746–749.
• 22.  Hanashima C,Li SC,Shen L,Lai E,Fishell G: Foxg1 suppresses early cortical cell fate. Science 2004; 303: 56–59.
• 23.  Kumamoto T,Toma K,Gunadi, McKenna WL,Kasukawa T,Katzman S,Chen B,Hanashima C: Foxg1 coordinates the switch from nonradially to radially migrating glutamatergic subtypes in the neocortex through spatiotemporal repression. Cell Reports 2013; 3: 931–945.
• 24.  Greig LC,Woodworth MB,Galazo MJ,Padmanabhan H,Macklis JD: Molecular logic of neocortical projection neuron specification, development and diversity. Nat Rev Neurosci 2013; 14: 755–769.
• 25.  Rouaux C,Arlotta P: Direct lineage reprogramming of post-mitotic callosal neurons into corticofugal neurons In vivo. Nat Cell Biol 2013; 15: 214–221.
• 26.  De la Rossa A,Bellone C,Golding B,Vitali I,Moss J,Toni N,Lüscher C,Jabaudon D: In vivo reprogramming of circuit connectivity in postmitotic neocortical neurons. Nat Neurosci 2013; 16: 193–200.
• 27.  Han W,Kwan KY,Shim S,Lam MM,Shin Y,Xu X,Zhu Y,Li M,Sestan N: TBR1 directly represses Fezf2 to control the laminar origin and development of the corticospinal tract. Proc Natl Acad Sci USA 2011; 108: 3041–3046.
• 28.  Toma K,Kumamoto T,Hanashima C: The timing of upper-layer neurogenesis is conferred by sequential derepression and negative feedback from deep-layer neurons. J Neurosci 2014; 34: 13259–13276.
• 29.  Naka H,Nakamura S,Shimazaki T,Okano H: Requirement for COUP-TFI and II in the temporal specification of neural stem cells in CNS development. Nat Neurosci 2008; 11: 1014–1023.
• 30.  Kohwi M,Doe CQ: Temporal fate specification and neural progenitor competence during development. Nat Rev Neurosci 2013; 14: 823–838.
• 31.  Alsiö JM,Tarchini B,Cayouette M,Livesey FJ: Ikaros promotes early-born neuronal fates in the cerebral cortex. Proc Natl Acad Sci USA 2013; 110: E716–E725.
• 32.  Deng Q,Andersson E,Hedlund E,Alekseenko Z,Coppola E,Panman L,Millonig JH,Brunet JF,Ericson J,Perlmann T: Specific and integrated roles of Lmx1a, Lmx1b and Phox2a in ventral midbrain development. Development 2011; 138: 3399–3408.
• 33.  Deneris ES,Wyler SC: Serotonergic transcriptional networks and potential importance to mental health. Nat Neurosci 2012; 15: 519–527.
• 34.  Kessaris N,Pringle N,Richardson WD: Specification of CNS glia from neural stem cells in the embryonic neuroepithelium. Philos Trans R Soc Lond B Biol Sci 2008; 363: 71–85.
• 35.  Hevner RF: From radial glia to pyramidal-projection neuron: transcription factor cascades in cerebral cortex development. Mol Neurobiol 2006; 33: 33–50.
• 36.  Chandran S,Kato H,Gerreli D,Compston A,Svendsen CN,Allen ND: FGF-dependent generation of oligodendrocytes by a hedgehog-independent pathway. Development 2003; 130: 6599–6609.
• 37.  Cai J,Qi Y,Hu X,Tan M,Liu Z,Zhang J,Li Q,Sander M,Qiu M: Generation of oligodendrocyte precursor cells from mouse dorsal spinal cord independent of Nkx6 regulation and Shh signaling. Neuron 2005; 45: 41–53.
• 38.  Stolt CC,Lommes P,Sock E,Chaboissier MC,Schedl A,Wegner M: The Sox9 transcription factor determines glial fate choice in the developing spinal cord. Genes Dev 2003; 17: 1677–1689.
• 39.  Kang P,Lee HK,Glasgow SM,Finley M,Donti T,Gaber ZB,Graham BH,Foster AE,Novitch BG,Gronostajski RM,Deneen B: Sox9 and NFIA coordinate a transcriptional regulatory cascade during the initiation of gliogenesis. Neuron 2012; 74: 79–94.
• 40.  Miller FD,Gauthier AS: Timing is everything: making neurons versus glia in the developing cortex. Neuron 2007; 54: 357–369.
• 41.  Okano H,Temple S: Cell types to order: temporal specification of CNS stem cells. Curr Opin Neurobiol 2009; 19: 112–119.
• 42.  Heinrich PC,Behrmann I,Haan S,Hermanns HM,Müller-Newen G,Schaper F: Principles of interleukin (IL)-6-type cytokine signalling and its regulation. Biochem J 2003; 374: 1–20.
• 43.  Mueller TD,Nickel J: Promiscuity and specificity in BMP receptor activation. FEBS Lett 2012; 586: 1846–1859.
• 44.  Guruharsha KG,Kankel MW,Artavanis-Tsakonas S: The Notch signalling system: recent insights into the complexity of a conserved pathway. Nat Rev Genet 2012; 13: 654–666.
• 45.  Mehler MF,Mabie PC,Zhu G,Gokhan S,Kessler JA: Developmental changes in progenitor cell responsiveness to bone morphogenetic proteins differentially modulate progressive CNS lineage fate. Dev Neurosci 2000; 22: 74–85.
• 46.  Molné M,Studer L,Tabar V,Ting YT,Eiden MV,McKay RD: Early cortical precursors do not undergo LIF-mediated astrocytic differentiation. J Neurosci Res 2000; 59: 301–311.
• 47.  Takizawa T,Nakashima K,Namihira M,Ochiai W,Uemura A,Yanagisawa M,Fujita N,Nakao M,Taga T: DNA methylation is a critical cell-intrinsic determinant of astrocyte differentiation in the fetal brain. Dev Cell 2001; 1: 749–758.
• 48.  He F,Ge W,Martinowich K,Becker-Catania S,Coskun V,Zhu W,Wu H,Castro D,Guillemot F,Fan G,de Vellis J,Sun YE: A positive autoregulatory loop of Jak-STAT signaling controls the onset of astrogliogenesis. Nat Neurosci 2005; 8: 616–625.
• 49.  Li W,Cogswell CA,LoTurco JJ: Neuronal differentiation of precursors in the neocortical ventricular zone is triggered by BMP. J Neurosci 1998; 18: 8853–8862.
• 50.  Yung SY,Gokhan S,Jurcsak J,Molero AE,Abrajano JJ,Mehler MF: Differential modulation of BMP signaling promotes the elaboration of cerebral cortical GABAergic neurons or oligodendrocytes from a common sonic hedgehog-responsive ventral forebrain progenitor species. Proc Natl Acad Sci USA 2002; 99: 16273–16278.
• 51.  das Neves L,Duchala CS,Tolentino-Silva F,Haxhiu MA,Colmenares C,Macklin WB,Campbell CE,Butz KG,Gronostajski RM: Disruption of the murine nuclear factor I-A gene (Nfia) results in perinatal lethality, hydrocephalus, and agenesis of the corpus callosum. Proc Natl Acad Sci USA 1999; 96: 11946–11951.
• 52.  Deneen B,Ho R,Lukaszewicz A,Hochstim CJ,Gronostajski RM,Anderson DJ: The transcription factor NFIA controls the onset of gliogenesis in the developing spinal cord. Neuron 2006; 52: 953–968.
• 53.  Namihira M,Kohyama J,Semi K,Sanosaka T,Deneen B,Taga T,Nakashima K: Committed neuronal precursors confer astrocytic potential on residual neural precursor cells. Dev Cell 2009; 16: 245–255.
• 54.  Naka-Kaneda H,Nakamura S,Igarashi M,Aoi H,Kanki H,Tsuyama J,Tsutsumi S,Aburatani H,Shimazaki T,Okano H: The miR-17/106-p38 axis is a key regulator of the neurogenic-to-gliogenic transition in developing neural stem/progenitor cells. Proc Natl Acad Sci USA 2014; 111: 1604–1609.
• 55.  Hirabayashi Y,Suzki N,Tsuboi M,Endo TA,Toyoda T,Shinga J,Koseki H,Vidal M,Gotoh Y: Polycomb limits the neurogenic competence of neural precursor cells to promote astrogenic fate transition. Neuron 2009; 63: 600–613.
• 56.  Onoguchi M,Hirabayashi Y,Koseki H,Gotoh Y: A noncoding RNA regulates the neurogenin1 gene locus during mouse neocortical development. Proc Natl Acad Sci USA 2012; 109: 16939–16944.
• 57.  Li X,Newbern JM,Wu Y,Morgan-Smith M,Zhong J,Charron J,Snider WD: MEK is a key regulator of gliogenesis in the developing brain. Neuron 2012; 75: 1035–1050.
• 58.  Stipursky J, Francis D, Gomes FC: Activation of MAPK/PI3K/SMAD pathways by TGF-β(1) controls differentiation of radial glia into astrocytes in vitro. Dev Neurosci 2012; 34: 68–81.
• 59.  Mu Y,Gudey SK,Landström M: Non-Smad signaling pathways. Cell Tissue Res 2012; 347: 11–20.
• 60.  Dias JM,Alekseenko Z,Applequist JM,Ericson J: Tgfβ signaling regulates temporal neurogenesis and potency of neural stem cells in the CNS. Neuron 2014; 84: 927–939.
• 61.  Nagao M,Campbell K,Burns K,Kuan CY,Trumpp A,Nakafuku M: Coordinated control of self-renewal and differentiation of neural stem cells by Myc and the p19ARF-p53 pathway. J Cell Biol 2008; 183: 1243–1257.
• 62.  Barnabé-Heider F,Wasylnka JA,Fernandes KJ,Porsche C,Sendtner M,Kaplan DR,Miller FD: Evidence that embryonic neurons regulate the onset of cortical gliogenesis via cardiotrophin-1. Neuron 2005; 48: 253–265.
• 63.  Yoshimatsu T,Kawaguchi D,Oishi K,Takeda K,Akira S,Masuyama N,Gotoh Y: Non-cell-autonomous action of STAT3 in maintenance of neural precursor cells in the mouse neocortex. Development 2006; 133: 2553–2563.
• 64.  Sardi SP,Murtie J,Koirala S,Patten BA,Corfas G: Presenilin-dependent ErbB4 nuclear signaling regulates the timing of astrogenesis in the developing brain. Cell 2006; 127: 185–197.
• 65.  Hermanson O,Jepsen K,Rosenfeld MG: N-CoR controls differentiation of neural stem cells into astrocytes. Nature 2002; 419: 934–939.
• 66.  Kolb B,Mychasiuk R,Muhammad A,Gibb R: Brain plasticity in the developing brain. Prog Brain Res 2013; 207: 35–64.
• 67.  Nakatomi H,Kuriu T,Okabe S,Yamamoto S,Hatano O,Kawahara N,Tamura A,Kirino T,Nakafuku M: Regeneration of hippocampal pyramidal neurons after ischemic brain injury by recruitment of endogenous neural progenitors. Cell 2002; 110: 429–441.
• 68.  Dibajnia P,Morshead CM: Role of neural precursor cells in promoting repair following stroke. Acta Pharmacol Sin 2013; 34: 78–90.
• 69.  Lanfranconi S,Locatelli F,Corti S,Candelise L,Comi GP,Baron PL,Strazzer S,Bresolin N,Bersano A: Growth factors in ischemic stroke. J Cell Mol Med 2011; 15: 1645–1687.
• 70.  Zappaterra MW,Lehtinen MK: The cerebrospinal fluid: regulator of neurogenesis, behavior, and beyond. Cell Mol Life Sci 2012; 69: 2863–2878.
• 71.  Wada K,Sugimori H,Bhide PG,Moskowitz MA,Finklestein SP: Effect of basic fibroblast growth factor treatment on brain progenitor cells after permanent focal ischemia in rats. Stroke 2003; 34: 2722–2728.
• 72.  Leker RR,Soldner F,Velasco I,Gavin DK,Androutsellis-Theotokis A,McKay RD: Long-lasting regeneration after ischemia in the cerebral cortex. Stroke 2007; 38: 153–161.
• 73.  Sun D,Bullock MR,McGinn MJ,Zhou Z,Altememi N,Hagood S,Hamm R,Colello RJ: Basic fibroblast growth factor-enhanced neurogenesis contributes to cognitive recovery in rats following traumatic brain injury. Exp Neurol 2009; 216: 56–65.
• 74.  Teramoto T,Qiu J,Plumier JC,Moskowitz MA: EGF amplifies the replacement of parvalbumin-expressing striatal interneurons after ischemia. J Clin Invest 2003; 111: 1125–1132.
• 75.  Türeyen K,Vemuganti R,Bowen KK,Sailor KA,Dempsey RJ: EGF and FGF-2 infusion increases post-ischemic neural progenitor cell proliferation in the adult rat brain. Neurosurgery 2005; 57: 1254–1263, discussion 1254–1263.
• 76.  Kolb B,Morshead C,Gonzalez C,Kim M,Gregg C,Shingo T,Weiss S: Growth factor-stimulated generation of new cortical tissue and functional recovery after stroke damage to the motor cortex of rats. J Cereb Blood Flow Metab 2007; 27: 983–997.
• 77.  Guerra-Crespo M,Gleason D,Sistos A,Toosky T,Solaroglu I,Zhang JH,Bryant PJ,Fallon JH: Transforming growth factor-alpha induces neurogenesis and behavioral improvement in a chronic stroke model. Neuroscience 2009; 160: 470–483.
• 78.  Sun D,Bullock MR,Altememi N,Zhou Z,Hagood S,Rolfe A,McGinn MJ,Hamm R,Colello RJ: The effect of epidermal growth factor in the injured brain after trauma in rats. J Neurotrauma 2010; 27: 923–938.
• 79.  Cooke MJ,Wang Y,Morshead CM,Shoichet MS: Controlled epi-cortical delivery of epidermal growth factor for the stimulation of endogenous neural stem cell proliferation in stroke-injured brain. Biomaterials 2011; 32: 5688–5697.
• 80.  Chou J,Harvey BK,Chang CF,Shen H,Morales M,Wang Y: Neuroregenerative effects of BMP7 after stroke in rats. J Neurol Sci 2006; 240: 21–29.
• 81.  Ishibashi S,Kuroiwa T,Sakaguchi M,Sun L,Kadoya T,Okano H,Mizusawa H: Galectin-1 regulates neurogenesis in the subventricular zone and promotes functional recovery after stroke. Exp Neurol 2007; 207: 302–313.
• 82.  Dempsey RJ,Sailor KA,Bowen KK,Türeyen K,Vemuganti R: Stroke-induced progenitor cell proliferation in adult spontaneously hypertensive rat brain: effect of exogenous IGF-1 and GDNF. J Neurochem 2003; 87: 586–597.
• 83.  Kobayashi T,Ahlenius H,Thored P,Kobayashi R,Kokaia Z,Lindvall O: Intracerebral infusion of glial cell line-derived neurotrophic factor promotes striatal neurogenesis after stroke in adult rats. Stroke 2006; 37: 2361–2367.
• 84.  Kawada H,Takizawa S,Takanashi T,Morita Y,Fujita J,Fukuda K,Takagi S,Okano H,Ando K,Hotta T: Administration of hematopoietic cytokines in the subacute phase after cerebral infarction is effective for functional recovery facilitating proliferation of intrinsic neural stem/progenitor cells and transition of bone marrow-derived neuronal cells. Circulation 2006; 113: 701–710.
• 85.  Zhao LR,Singhal S,Duan WM,Mehta J,Kessler JA: Brain repair by hematopoietic growth factors in a rat model of stroke. Stroke 2007; 38: 2584–2591.
• 86.  Sehara Y,Hayashi T,Deguchi K,Zhang H,Tsuchiya A,Yamashita T,Lukic V,Nagai M,Kamiya T,Abe K: G-CSF enhances stem cell proliferation in rat hippocampus after transient middle cerebral artery occlusion. Neurosci Lett 2007; 418: 248–252.
• 87.  Liu SP,Lee SD,Lee HT,Liu DD,Wang HJ,Liu RS,Lin SZ,Su CY,Li H,Shyu WC: Granulocyte colony-stimulating factor activating HIF-1alpha acts synergistically with erythropoietin to promote tissue plasticity. PLoS ONE 2010; 5: e10093.
• 88.  Popa-Wagner A,Stöcker K,Balseanu AT,Rogalewski A,Diederich K,Minnerup J,Margaritescu C,Schäbitz WR: Effects of granulocyte-colony stimulating factor after stroke in aged rats. Stroke 2010; 41: 1027–1031.
• 89.  Jin K,Sun Y,Xie L,Childs J,Mao XO,Greenberg DA: Post-ischemic administration of heparin-binding epidermal growth factor-like growth factor (HB-EGF) reduces infarct size and modifies neurogenesis after focal cerebral ischemia in the rat. J Cereb Blood Flow Metab 2004; 24: 399–408.
• 90.  Sugiura S,Kitagawa K,Tanaka S,Todo K,Omura-Matsuoka E,Sasaki T,Mabuchi T,Matsushita K,Yagita Y,Hori M: Adenovirus-mediated gene transfer of heparin-binding epidermal growth factor-like growth factor enhances neurogenesis and angiogenesis after focal cerebral ischemia in rats. Stroke 2005; 36: 859–864.
• 91.  Doeppner TR,Kaltwasser B,ElAli A,Zechariah A,Hermann DM,Bähr M: Acute hepatocyte growth factor treatment induces long-term neuroprotection and stroke recovery via mechanisms involving neural precursor cell proliferation and differentiation. J Cereb Blood Flow Metab 2011; 31: 1251–1262.
• 92.  Nakaguchi K,Jinnou H,Kaneko N,Sawada M,Hikita T,Saitoh S,Tabata Y,Sawamoto K: Growth factors released from gelatin hydrogel microspheres increase new neurons in the adult mouse brain. Stem Cells Int 2012; 2012: 915160.
• 93.  Zhu W,Fan Y,Frenzel T,Gasmi M,Bartus RT,Young WL,Yang GY,Chen Y: Insulin growth factor-1 gene transfer enhances neurovascular remodeling and improves long-term stroke outcome in mice. Stroke 2008; 39: 1254–1261.
• 94.  Zhu W,Fan Y,Hao Q,Shen F,Hashimoto T,Yang GY,Gasmi M,Bartus RT,Young WL,Chen Y: Postischemic IGF-1 gene transfer promotes neurovascular regeneration after experimental stroke. J Cereb Blood Flow Metab 2009; 29: 1528–1537.
• 95.  Carlson SW,Madathil SK,Sama DM,Gao X,Chen J,Saatman KE: Conditional overexpression of insulin-like growth factor-1 enhances hippocampal neurogenesis and restores immature neuron dendritic processes after traumatic brain injury. J Neuropathol Exp Neurol 2014; 73: 734–746.
• 96.  Ma M,Ma Y,Yi X,Guo R,Zhu W,Fan X,Xu G,Frey WH,Liu X: Intranasal delivery of transforming growth factor-beta1 in mice after stroke reduces infarct volume and increases neurogenesis in the subventricular zone. BMC Neurosci 2008; 9: 117.
• 97.  Sun Y,Jin K,Xie L,Childs J,Mao XO,Logvinova A,Greenberg DA: VEGF-induced neuroprotection, neurogenesis, and angiogenesis after focal cerebral ischemia. J Clin Invest 2003; 111: 1843–1851.
• 98.  Wang Y,Jin K,Mao XO,Xie L,Banwait S,Marti HH,Greenberg DA: VEGF-overexpressing transgenic mice show enhanced post-ischemic neurogenesis and neuromigration. J Neurosci Res 2007; 85: 740–747.
• 99.  Wang YQ,Guo X,Qiu MH,Feng XY,Sun FY: VEGF overexpression enhances striatal neurogenesis in brain of adult rat after a transient middle cerebral artery occlusion. J Neurosci Res 2007; 85: 73–82.
• 100.  Wang L,Zhang Z,Wang Y,Zhang R,Chopp M: Treatment of stroke with erythropoietin enhances neurogenesis and angiogenesis and improves neurological function in rats. Stroke 2004; 35: 1732–1737.
• 101.  Lu D,Mahmood A,Qu C,Goussev A,Schallert T,Chopp M: Erythropoietin enhances neurogenesis and restores spatial memory in rats after traumatic brain injury. J Neurotrauma 2005; 22: 1011–1017.
• 102.  Zhang L,Chopp M,Zhang RL,Wang L,Zhang J,Wang Y,Toh Y,Santra M,Lu M,Zhang ZG: Erythropoietin amplifies stroke-induced oligodendrogenesis in the rat. PLoS ONE 2010; 5: e11016.
• 103.  Wang Y,Cooke MJ,Morshead CM,Shoichet MS: Hydrogel delivery of erythropoietin to the brain for endogenous stem cell stimulation after stroke injury. Biomaterials 2012; 33: 2681–2692.
• 104.  Henry RA,Hughes SM,Connor B: AAV-mediated delivery of BDNF augments neurogenesis in the normal and quinolinic acid-lesioned adult rat brain. Eur J Neurosci 2007; 25: 3513–3525.
• 105.  Schäbitz WR,Steigleder T,Cooper-Kuhn CM,Schwab S,Sommer C,Schneider A,Kuhn HG: Intravenous brain-derived neurotrophic factor enhances poststroke sensorimotor recovery and stimulates neurogenesis. Stroke 2007; 38: 2165–2172.
• 106.  Han Q,Li B,Feng H,Xiao Z,Chen B,Zhao Y,Huang J,Dai J: The promotion of cerebral ischemia recovery in rats by laminin-binding BDNF. Biomaterials 2011; 32: 5077–5085.
• 107.  Ohab JJ,Fleming S,Blesch A,Carmichael ST: A neurovascular niche for neurogenesis after stroke. J Neurosci 2006; 26: 13007–13016.
• 108.  Wang Z,Andrade N,Torp M,Wattananit S,Arvidsson A,Kokaia Z,Jørgensen JR,Lindvall O: Meteorin is a chemokinetic factor in neuroblast migration and promotes stroke-induced striatal neurogenesis. J Cereb Blood Flow Metab 2012; 32: 387–398.
• 109.  Kizil C,Kaslin J,Kroehne V,Brand M: Adult neurogenesis and brain regeneration in zebrafish. Dev Neurobiol 2012; 72: 429–461.
• 110.  Holder N,Clarke JD,Stephens N,Wilson SW,Orsi C,Bloomer T,Tonge DA: Continuous growth of the motor system in the axolotl. J Comp Neurol 1991; 303: 534–550.
• 111.  Maden M,Manwell LA,Ormerod BK: Proliferation zones in the axolotl brain and regeneration of the telencephalon. Neural Dev 2013; 8: 1.
• 112.  Kirkham M,Hameed LS,Berg DA,Wang H,Simon A: Progenitor cell dynamics in the newt telencephalon during homeostasis and neuronal regeneration. Stem Cell Rep 2014; 2: 507–519.
• 113.  Berg DA,Kirkham M,Beljajeva A,Knapp D,Habermann B,Ryge J,Tanaka EM,Simon A: Efficient regeneration by activation of neurogenesis in homeostatically quiescent regions of the adult vertebrate brain. Development 2010; 137: 4127–4134.
• 114.  Reimer MM,Sörensen I,Kuscha V,Frank RE,Liu C,Becker CG,Becker T: Motor neuron regeneration in adult zebrafish. J Neurosci 2008; 28: 8510–8516.
• 115.  McHedlishvili L,Mazurov V,Grassme KS,Goehler K,Robl B,Tazaki A,Roensch K,Duemmler A,Tanaka EM: Reconstitution of the central and peripheral nervous system during salamander tail regeneration. Proc Natl Acad Sci USA 2012; 109: E2258–E2266.
• 116.  Schnapp E,Kragl M,Rubin L,Tanaka EM: Hedgehog signaling controls dorsoventral patterning, blastema cell proliferation and cartilage induction during axolotl tail regeneration. Development 2005; 132: 3243–3253.
• 117.  McHedlishvili L,Epperlein HH,Telzerow A,Tanaka EM: A clonal analysis of neural progenitors during axolotl spinal cord regeneration reveals evidence for both spatially restricted and multipotent progenitors. Development 2007; 134: 2083–2093.
• 118.  Fricker RA,Carpenter MK,Winkler C,Greco C,Gates MA,Björklund A: Site-specific migration and neuronal differentiation of human neural progenitor cells after transplantation in the adult rat brain. J Neurosci 1999; 19: 5990–6005.
• 119.  Murray AJ: Axon regeneration: what needs to be overcome? Methods Mol Biol 2014; 1162: 3–14.