Regulation of Motor Neuron Specification by Phosphorylation of Neurogenin 2

F.M. Kirby Neurobiology Center, Children's Hospital, and Departments of Neurology and Neurobiology, Harvard Medical School, 300 Longwood Avenue, Boston, MA 02115, USA.
Neuron (Impact Factor: 15.05). 05/2008; 58(1):65-77. DOI: 10.1016/j.neuron.2008.01.037
Source: PubMed


The mechanisms by which proneural basic helix-loop-helix (bHLH) factors control neurogenesis have been characterized, but it is not known how they specify neuronal cell-type identity. Here, we provide evidence that two conserved serine residues on the bHLH factor neurogenin 2 (Ngn2), S231 and S234, are phosphorylated during motor neuron differentiation. In knockin mice in which S231 and S234 of Ngn2 were mutated to alanines, neurogenesis occurs normally, but motor neuron specification is impaired. The phosphorylation of Ngn2 at S231 and S234 facilitates the interaction of Ngn2 with LIM homeodomain transcription factors to specify motor neuron identity. The phosphorylation-dependent cooperativity between Ngn2 and homeodomain transcription factors may be a general mechanism by which the activities of bHLH and homeodomain proteins are temporally and spatially integrated to generate the wide diversity of cell types that are a hallmark of the nervous system.

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    • "Part of the specificity of proneural gene function relates to the structure of the proteins. The bHLH domain is comprised of a basic domain that confers DNA binding and a HLH domain that mediates hetero-or homodimerization (Bertrand, Castro, & Guillemot, 2002; Powell & Jarman, 2008). The common heterodimerization partner of proneural proteins in Drosophila is daughterless (da), which encodes a ubiquitously expressed class I bHLH transcription factor (Cabrera & Alonso, 1991). "
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    ABSTRACT: Proneural genes encode evolutionarily conserved basic-helix-loop-helix transcription factors. In Drosophila, proneural genes are required and sufficient to confer a neural identity onto naïve ectodermal cells, inducing delamination and subsequent neuronal differentiation. In vertebrates, proneural genes are expressed in cells that already have a neural identity, but they are still required and sufficient to initiate neurogenesis. In all organisms, proneural genes control neurogenesis by regulating Notch-mediated lateral inhibition and initiating the expression of downstream differentiation genes. The general mode of proneural gene function has thus been elucidated. However, the regulatory mechanisms that spatially and temporally control proneural gene function are only beginning to be deciphered. Understanding how proneural gene function is regulated is essential, as aberrant proneural gene expression has recently been linked to a variety of human diseases-ranging from cancer to neuropsychiatric illnesses and diabetes. Recent insights into proneural gene function in development and disease are highlighted herein.
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    • "RESULTS Ascl1 is regulated by multisite phosphorylation Ascl1 is a core component of a variety of transcription factor protocols that have been used to drive transdifferentiation of mammalian fibroblasts directly into neurons (Vierbuchen et al., 2010; Caiazzo et al., 2011; Pang et al., 2011; Pfisterer et al., 2011; Yang et al., 2011; Torper et al., 2013), but the post-translational control of this protein is largely unknown. Phosphorylation of a number of conserved serineproline (SP) sites regulates the activity of Ngn2 (Neurog2 – Mouse Genome Informatics) and Olig2 basic helix-loop-helix (bHLH) proneural proteins (Ma et al., 2008; Ali et al., 2011; Gaber and Novitch, 2011). Ascl1 also contains multiple SP sites (supplementary material Fig. S1) that we hypothesized could be functionally modified through phosphorylation by proline-directed kinases. "
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    ABSTRACT: Generation of neurons from patient fibroblasts using a combination of developmentally defined transcription factors has great potential in disease modelling, as well as ultimately for use in regeneration and repair. However, generation of physiologically mature neurons in vitro remains problematic. Here we demonstrate the cell-cycle-dependent phosphorylation of a key reprogramming transcription factor, Ascl1, on multiple serine-proline sites. This multisite phosphorylation is a crucial regulator of the ability of Ascl1 to drive neuronal differentiation and maturation in vivo in the developing embryo; a phosphomutant form of Ascl1 shows substantially enhanced neuronal induction activity in Xenopus embryos. Mechanistically, we see that this un(der)phosphorylated Ascl1 is resistant to inhibition by both cyclin-dependent kinase activity and Notch signalling, both of which normally limit its neurogenic potential. Ascl1 is a central component of reprogramming transcription factor cocktails to generate neurons from human fibroblasts; the use of phosphomutant Ascl1 in place of the wild-type protein significantly promotes neuronal maturity after human fibroblast reprogramming in vitro. These results demonstrate that cell-cycle-dependent post-translational modification of proneural proteins directly regulates neuronal differentiation in vivo during development, and that this regulatory mechanism can be harnessed to promote maturation of neurons obtained by transdifferentiation of human cells in vitro.
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    • "Thus, further investigation may be warranted to determine whether a similar cooperative mechanism involving a yet to be identified Sox partner recruits Oct4 to genes that promote the mesodermal fate (Figure 1b). Finally, given that posttranslational modifications (e.g., phosphorylation) are also known to modulate the functional outcome of key transcription factors during neural tube development (Ma et al, 2008), it will be of interest to investigate whether such modifications can also affect the partnership of Oct4 with other lineage commitment factors. "
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    ABSTRACT: The transcription factor Oct4 plays a crucial role in the maintenance of the embryonic pluripotent state, but can also regulate early lineage commitment. In this issue of The EMBO Journal, Aksoy et al (2013) lend critical mechanistic insights into the ability of Oct4 to regulate and specify the primitive endodermal lineage. These regulatory actions are governed by alternative direct partnering of Oct4 with Sox17, instead of Sox2, that leads to global reprogramming of enhancer occupancy by Oct4 during primitive endoderm differentiation.
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