Crosstalk between genetic and epigenetic components plays a crucial role in determining cell fate of neural stem cells during cortical neurogenesis in the developing brain, according to new research from Byoung-San Moon, Wange Lu and colleagues.
During neurogenesis, NSCs are derived from neuroepithelial cells (NECs), which first divide symmetrically to expand the population and then undergo a series of asymmetric cell divisions to produce neural progenitor cells (NPCs), lineage-restricted precursor cells (RPCs), and mature neural cells. NSC fate determination is tightly regulated by intrinsic and extrinsic factors.
Suppressor Of Mek Null
Suppressor of Mek null (Smek), an evolutionarily conserved protein family, consists of two isoforms, Smek1 (PP4R3A) and Smek2 (PP4R3B), first reported as playing a role in the formation of a functional phosphatase group with PP4c, PP4R1, and PP4R2 complex. Smek was initially identified in Dictyostelium discoideum as a playing a role in cell polarity, chemotaxis, and gene expression.
Smek also has several functions in lower eukaryotes, such as Caenorhabditis elegans, including roles in longevity by modulating DAF-16/FOXO3a transcriptional activity, DNA repair through dephosphorylation of phosphorylated H2AX (g-H2AX) during DNA replication, and glucose metabolism by controlling cAMP-response element binding protein (CREB)-regulated, transcriptional coactivator 2 (CRTC2)-dependent gene expression.
Notably, Smek also plays a critical role in cell-fate determination in higher eukaryotes.
In Drosophila neuroblasts, PP4R3/Falafel (Flfl), which is an orthologous of Smek and is conserved throughout eukaryotic evolution, regulates asymmetric cell division by controlling localization of Miranda. In mice, which express orthologous Smek 1 and 2, both Smek proteins suppress brachyury expression in embryonic stem cells (ESCs), and Smek1, especially, promotes NSC neuronal differentiation by negatively regulating Par3.
Although it has been shown that the Smek isoform Smek1 promotes NSC neuronal differentiation, signaling pathways required for that activity remain unclear.
Methyl-CpG–binding domain protein 3 (Mbd3), a core component of the repressive nucleosome remodeling and deacetylase (NuRD) complex, possesses a conserved methyl-CpG–binding domain (Mbd). Unlike other family members, which recognize 5′-methyl-cytosine (5′-mC)-modified DNA, Mbd3 specifically recognizes 5′-hydroxymethyl-cytosine (5′-hmC), an epigenetic marker highly enriched in NSCs.
Mbd3 plays an important role in brain development. Mbd3 expression is reported to be predominant in cortical NECs of the embryonic forebrain. Mice lacking Mbd3 die in utero before neurogenesis is completed.
Conditional knockout of Mbd3 in neural progenitor cells leads to defects of differentiation of appropriate cell types during neurogenesis. Despite emerging evidence that Mbd3 has a critical function in the CNS, little is known about its regulatory mechanism in NSCs.
Neural Progenitor Cell Differentiation
To understand Smek protein function during mammalian CNS neurogenesis, Byoung-San Moon’s team screened for novel Smek-binding proteins that regulate NPC neuronal differentiation and identified Mbd3, a potent epigenetic regulator, as a Smek-interacting protein. They found that Mbd3 is highly expressed in NPC populations in the ventricular zone, and it was predominantly expressed in the nucleus.
Smek interacted directly with the Mbd3’s Mbd domain, destabilizing Mbd3 protein and its interaction with NuRD components, and sequentially, preventing accumulation of the Mbd3/NuRD complex on target gene loci functioning in neurogenesis. Such dissociation of Mbd3/NuRD complex promotes NPC neuronal differentiation.
Moreover, overexpression of Mbd3 significantly inhibited neuronal differentiation of wild-type NPCs, while Mbd3 depletion rescued neurogenesis defects seen in Smek knockout mice.
The work identifies a novel pathway of Smek and Mbd3/NuRD complex in brain development and could encourage discovery of novel epigenetic regulators governing neuronal differentiation.
© 2017 Moon et al. Republished Creative Commons Attribution License. Top Image: pbio.2001220