MeCP2 target genes and Rett syndrome 1569 Fig. 3. MeCP2 regulates differentiation through miR-137 and Ezh2. MeCP2 binds to methylated CpGs in a region upstream of miR-137 and, together with Sox2, represses miR-137 transcription. Ezh2, a target of miR-137, is not repressed and is able to establish H3K27me3 marks. This process enhances differentiation and inhibits proliferation of aNSCs. The absence of MeCP2 correlates with a gain of open chromatin marks (H3K9Ac and H3K4me3) in the area surrounding miR-137 and a loss of Sox2 in the regulatory region. Ezh2 is repressed by miR-137 that leads to a global loss of H3K27me3 marks; this enhances proliferation and inhibits differentiation of aNSCs (Szulwach et al., 2010). that Sox2 binding to DNA depends on its interaction with MeCP2. Accordingly, immunoprecipitation assays demonstrated Sox2–MeCP2 protein–protein interactions (Szulwach et al., 2010). The enhancer of Zeste homolog 2 (Ezh2), a H3K27 methyltrans- ferase, has been reported as a putative target for miR-137 (Szulwach et al., 2010). Ezh2 overexpression reduces cell proliferation and co- expression with miR-137 increases the number of differentiated neurons. Overexpression of miR-137 reduces Ezh2 protein levels; the same was observed in MeCP2-null mice where miR-137 is overex- pressed. This effect is speciﬁc as reducing miR-137 recovered normal Ezh2 in MeCP2) ⁄ y mice (Szulwach et al., 2010). Thus, the overexpression of miR-137 and the decrease in Ezh2 correlated with an overall reduction of H3K27me3 in mouse aNSCs (Fig. 3 and Table 2). These results point out a cross-talk between different epigenetic processes to regulate gene expression leading to correct neural development and differentiation. Once again, it would be interesting to determine whether adult neurogenesis is altered in patients with RTT. Methyl-binding domains and cell differentiation In addition to the necessary role of MeCP2 in mature neuronal and glial cells, it also has an important role during cell differentiation. MeCP2 promotes neuronal differentiation of neural stem cells while repressing astrocytic differentiation [induced by astrogenic factors like Leukemia inhibitory factor (Lif) and Bone morphogenetic protein 2 (BMP2); Tsujimura et al., 2009]. Interestingly, the truncated mutant of MeCP2 (R168X, lacking the complete transcriptional repressor domain), identiﬁed in patients with RTT (Amir et al., 1999), is unable to promote and suppress neuronal and astrocytic differentia- tion, respectively. This suggests a necessary role of this domain for the correct regulation by MeCP2. In addition to MeCP2, other MBDs have also been implicated during cell differentiation processes (Li et al., 2008; MacDonald et al., 2010). The ﬁbroblast growth factor 2 (FGF-2) is crucial for maintaining aNSCs in an undifferentiated state (Palmer et al., 1995; Zheng et al., 2004). Consequently, FGF-2 overexpression results in a signiﬁcant reduction of neuronal differentiation. FGF-2 expression is regulated negatively by MBD1 that binds to a speciﬁc CpG-rich region on the mouse and rat FGF-2 promoters (Li et al., 2008). Accordingly, Mbd1) ⁄ ) mice show increased levels of FGF-2 that correlate with impaired aNSC differentiation and adult neurogenesis. Depletion of Mbd1 using RNA interference results in a neuronal differentiation percentage that is signiﬁcantly reduced from aNSCs. These results corroborate the crucial role of MBD1 in neuronal differentiation. In the adult olfactory epithelium, the olfactory receptor neurons (ORNs) are in continuous replacement (Murdoch & Roskams, 2007). Mbd2 has two alternative splice variants: MBD2a and MBD2b (Hendrich & Bird, 1998). MBD2a is found predominantly in the mature ORNs, whereas MBD2b is found in progenitors and immature post-mitotic neurons. Consistently, the Mbd2-null mice exhibit an increase in progenitor cell proliferation and a reduced number of mature ORNs (MacDonald et al., 2010). In contrast, MeCP2 expression increases ORN maturation. MeCP2-null mice show an increased number of immature ORNs with no signiﬁcant change in the number of progenitors or mature ORNs. Speciﬁc Dnmts and HDACs are present in the different stages of ORN differentiation (MacDonald et al., 2005). Cycling progenitor cells express high levels of Dnmt3b and HDAC1, whereas they are downregulated in post-mitotic neurons, which express Dnmt3a and HDAC2. As neurons mature, Dnmt3a and HDAC2 expression levels are downregulated, whereas HDAC1 is expressed again. Therefore, in addition to MeCP2, MBD2 could regulate pivotal stages of ORN development via interaction with Dnmts, HDACs and MeCP2 as they are co-expressed at different stages of ORN maturation and have been reported to co-immunoprecipitate (MacDonald et al., 2005, 2010). ª 2011 The Authors. European Journal of Neuroscience ª 2011 Federation of European Neuroscience Societies and Blackwell Publishing Ltd European Journal of Neuroscience, 33, 1563–1574