**1. Introduction**

spinal projections and functional recovery after spinal cord injury in the neonatal rat.

[143] Chen Q, L Zhou and HD Shine. (2006). Expression of neurotrophin-3 promotes axo‐ nal plasticity in the acute but not chronic injured spinal cord. J Neurotrauma

[144] Hawryluk GW, AJ Mothe, M Chamankhah, J Wang, C Tator and MG Fehlings. (2010). In vitro characterization of trophic factor expression in neural precursor cells.

[145] Hawryluk GW, A Mothe, J Wang, S Wang, C Tator and MG Fehlings. (2012). An in vivo characterization of trophic factor production following neural precursor cell or bone marrow stromal cell transplantation for spinal cord injury. Stem Cells Dev

J Neurosci Res 81:457-468.

304 Trends in Cell Signaling Pathways in Neuronal Fate Decision

Stem Cells Dev 21:432-447.

23:1254-1260.

21:2222-2238.

During development, ESCs in the inner cell mass of the blastocyst undergo progressive fate restriction and sequentially give rise to tissue-specific multipotent progenitor cells [1-3]. In the nervous system, neural progenitor cells (NPCs) are defined as multipotent cells capable of differentiating into neurons and glia, the latter of which include astrocytes and oligoden‐ drocytes. NPCs undergo sequential fate restriction during nervous system development [4-6]. During early embryonic development, NPCs give rise exclusively to neuronal cell types, however at later stages they differentiate into glial cell types [7, 8]. Understanding how ESCs differentiate into neuronal or glial cell types, thus, constitutes a fundamental as‐ pect of nervous system development.

ESCs possess two cardinal cellular characteristics: self-renewal capacity and pluripotency [2, 3]. ESCs, when undifferentiated, maintain a unique gene expression profile which keeps themselves in a pluripotent state [3, 9-12]. When ESCs begin to differentiate, however, this profile is dramatically altered without changes in the DNA sequence, allowing initial cell lineage specification to take place [11, 13-15]. Studies on lineage choice and differentiation of stem cells suggest that the crosstalk between transcription factors and epigenetic mecha‐ nisms plays pivotal roles in maintaining the existing transcriptional profile or changing it in‐ to a new transcriptional profile [16, 17].

Chromatin in eukaryotes is organized into arrays of nucleosomes, which are comprised of DNA wrapped around a histone octamer containing 2 copies of individual core histones, H2A, H2B, H3, and H4 [18]. Chromatin can be tightly or loosely packed to alter its accessi‐ bility to transcription factors and RNA polymerases, thereby ultimately modulating the effi‐ ciency of gene transcription [19-23]. Epigenetic mechanisms act to change this accessibility both locally and globally through modifications of nucleosomes, and by remodeling of nu‐

© 2013 Shimomura and Hashino; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Shimomura and Hashino; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

cleosomes [21, 24, 25]. In addition to epigenetic modifications and chromatin remodeling, epigenetic regulators have been recently extended to non-coding RNAs (ncRNAs), because ncRNAs can affect chromatin structure and transcriptional activation by regulating expres‐ sion of key nucleosome modifiers [26]. These epigenetic controls appear to influence gene expression profiles, which are essential for self-renewal and differentiation capacities of ESCs. Thus, a clear understanding of the epigenetic mechanisms underlying gene expres‐ sion patterns will provide significant and novel insights into cell fate specification of ESCs directed to differentiate into neurons. Furthermore, the epigenetic mechanisms are believed to be capable of responding to extrinsic signals such as morphogens and cytokines [8]. Therefore, knowledge of the epigenetic mechanisms is also important for our understanding of neural differentiation by extrinsic factors. In this review, we will describe the major epige‐ netic processes that underlie the acquisition of the NPC fate from ESCs as well as the subse‐ quent neuronal subtype specification. The focus of this review is weighted on neuronal cell lineage specification, and not on glial cell specification.

acetylation in histones is an important mechanism of controlling gene expression because histone acetylation and deacetylation are linked to transcriptional activation and inactiva‐

Epigenetic Regulation of Neural Differentiation from Embryonic Stem Cells

http://dx.doi.org/10.5772/53650

307

DNA methylation is one of the major repressive epigenetic pathways. Methylation occurs at the cytosine residues followed by a guanine (CpG dinucleotides) in the DNA sequence. CpG DNA methylation of gene promoters is a well-known hallmark for transcriptionally inactive genes, and is generally associated with stable gene silencing, such as genomic imprinting and X chromosome inactivation [35, 36]. The DNA methylation state is established during embryogenesis by several DNA methyltransferases (DNMTs) [35]. In mammals, 2 types of DNMTs have been identified. DNMT3A and DNMT3B establish *de novo* DNA methylation, while DNMT1 maintains DNA methylation patterns during DNA replication [37]. These DNA methylation sites then recruit methyl-CpG-binding proteins, including methyl-CpGbinding domain (MBD) proteins [38, 39], which bind the histone deacetylase (HDAC)-con‐

Chromatin structure is not static, but subject to change in response to internal and external developmental signals [23]. Dynamic changes in the chromatin structure are regulated by ATP-dependent chromatin remodelers, which allow the transcriptional machinery to access its targets more or less effectively [42-44]. Using energy derived from ATP hydrolysis, ATPdependent chromatin remodelers relocate nucleosomes either by mobilizing or restructuring nucleosomes [45, 46]. Thus, ATP-dependent chromatin remodelers can function both in tran‐ scriptional activation and repression via their nucleosome remodeling activity. Nearly all ATP-dependent chromatin remodelers are multi-protein complexes that contain an ATPase subunit, which belongs to the sucrose non-fermenting 2 (SNF2) family of ATPases. Based on the homology between their ATPase domains, ATP-dependent chromatin remodeling com‐ plexes are divided into 4 groups: switch/sucrose non-fermenting (SWI/SNF), imitation switch (ISWI), chromo helicase DNA binding (CHD), and inositol auxotroph 80 (INO80)

Genetic and biochemical studies indicate that some ATP-dependent chromatin remodeling complexes contain epigenetic factors such as HDAC and MBD proteins. For example, nucle‐ osome-remodeling deacetylase (NuRD) is a multi-subunit complex that includes a SWI2/ SNF2 helicase/ATPase domain-containing Mi2 protein, HDAC1, HDAC2, and MBD3 [47]. The NuRD complexes promote the establishment of a specific chromatin structure at rRNA genes that are transcriptionally inactive but are poised for transcriptional activation and control transcription of these genes [48]. Thus, ATP-dependent chromatin complexes play essential roles in epigenetic regulation of transcription along with several histone-modifying

taining repressor complex, and consequently repress transcription [40, 41].

tion, respectively [25, 27].

**2.3. Chromatin remodeling**

enzymes and/or modified histone codes.

[42-44].

**2.2. DNA methylation**
