Differential Interactions Between Notch and ID Factors Control
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© 2017. Published by The Company of Biologists Ltd | Development (2017) 144, 3465-3474 doi:10.1242/dev.152520 STEM CELLS AND REGENERATION RESEARCH ARTICLE Differential interactions between Notch and ID factors control neurogenesis by modulating Hes factor autoregulation Marcelo Boareto1,2,*, Dagmar Iber1,2,* and Verdon Taylor3,* ABSTRACT neurogenesis is active in two defined regions: the ventricular- During embryonic and adult neurogenesis, neural stem cells (NSCs) subventricular zone (V-SVZ) of the lateral ventricle wall; and generate the correct number and types of neurons in a temporospatial the dentate gyrus of the hippocampus (Doetsch et al., 1999; fashion. Control of NSC activity and fate is crucial for brain formation Doetsch, 2003; Spalding et al., 2013; Ernst et al., 2014; Fuentealba and homeostasis. Neurogenesis in the embryonic and adult brain et al., 2015; Furutachi et al., 2015). In the adult brain the majority differ considerably, but Notch signaling and inhibitor of DNA-binding of the NSCs are mitotically inactive (qNSC) and infrequently enter (ID) factors are pivotal in both. Notch and ID factors regulate NSC cell cycle, becoming active NSCs (aNSCs) to generate neurons maintenance; however, it has been difficult to evaluate how these before returning to quiescence or differentiating into glial cells pathways potentially interact. Here, we combined mathematical (Fig. 1A) (Lois et al., 1996; Kirschenbaum et al., 1999; Encinas modeling with analysis of single-cell transcriptomic data to elucidate et al., 2011; Ihrie and Alvarez-Buylla, 2011; Shook et al., 2012; unforeseen interactions between the Notch and ID factor pathways. Giachino et al., 2014). Thus, although embryonic and adult During brain development, Notch signaling dominates and directly neurogenesis share some similarities, there are also fundamental regulates Id4 expression, preventing other ID factors from inducing differences and stem cell quiescence is one of them. Currently, it is NSC quiescence. Conversely, during adult neurogenesis, Notch not known why NSCs of the adult brain remain quiescent and the signaling and Id2/3 regulate neurogenesis in a complementary mechanisms that control the transition of NSC to activation are manner and ID factors can induce NSC maintenance and also unclear. However, the balance between activity and quiescence quiescence in the absence of Notch. Our analyses unveil key is crucial not only to maintain the NSC pool for later neuron molecular interactions underlying NSC maintenance and mechanistic production and regeneration but also to prevent overproliferation differences between embryonic and adult neurogenesis. Similar Notch and tumor formation (Lugert et al., 2010; Silva-Vargas et al., 2016). and ID factor interactions may be crucial in other stem cell systems. Thus, understanding the molecular mechanism that regulates maintenance and differentiation of NSCs is not only of theoretical KEY WORDS: Notch signaling, Id transcription factors, Neural stem interest but crucial for understanding disease mechanism and cells, Neurogenesis, Computational biology developing new therapeutic strategies (Lie et al., 2004; Lazarov et al., 2010). INTRODUCTION The processes of NSC maintenance and differentiation are Neurogenesis is the production of neurons from neural stem cells controlled by a core regulatory network of basic helix-loop-helix (NSCs). The correct balance between NSC proliferation and (bHLH) transcription factors (Lee, 1997; Ross et al., 2003; Heng differentiation is essential for embryonic formation of the brain and Guillemot, 2013; Imayoshi and Kageyama, 2014b). Members and to confer regenerative capacities in the adult brain (Doe, 2008). of the bHLH family have two conserved functional domains: a basic Any deviation from the regulated neurogenic program can lead to region for DNA binding and a helix-loop-helix (HLH) region for drastic problems during development, including microcephaly and dimerization. These transcript factors can act as repressors or cognitive impairment. During embryonic development of the activators of gene expression. The hairy and enhancer of split (Hes) central nervous system, NSCs divide frequently and produce proteins Hes1 and Hes5 are central repressors of NSC differentiation neurons either directly or via a committed intermediate progenitor during brain development (Ohtsuka et al., 1999; Kageyama et al., (IP) cell (Fig. 1A). In the peak neurogenic period, a few NSCs exit 2007, 2008), while bHLH factors including Ascl1 and Neurog2 are the cell cycle and become quiescent (qNSCs) (Furutachi et al., activators of neural differentiation and thus referred to as proneural 2013, 2015; Fuentealba et al., 2015). qNSCs are only reactivated in factors (Wilkinson et al., 2013; Imayoshi and Kageyama, 2014b). the adult neurogenic niches. In the adult brain, NSCs remain and Hes proteins in conjunction with TLE factors repress gene expression by binding to N-box and class-C sites in the promoters of target genes. Proneural factors activate gene expression by 1 Department of Biosystems Science and Engineering (D-BSSE), ETH Zurich, binding to E-box consensus sequences in the promoters of their Mattenstrasse 26, 4058 Basel, Switzerland. 2Swiss Institute of Bioinformatics, Mattenstrasse 26, 4058 Basel, Switzerland. 3Department of Biomedicine, University targets (Fig. 1B) (Imayoshi and Kageyama, 2014b). Furthermore, of Basel, Mattenstrasse 28, 4058 Basel, Switzerland. the binding affinity of proneural factors to E-boxes can be enhanced *Authors for correspondence ([email protected]; dagmar.iber@bsse. by the formation of heterodimers with other members of the bHLH ethz.ch; [email protected]) family: the E-proteins Tcf4 and Tcf3 (Massari and Murre, 2000; Bohrer et al., 2015). V.T., 0000-0003-3497-5976 During brain development, Notch signaling activates Hes gene This is an Open Access article distributed under the terms of the Creative Commons Attribution expression, which in turn inhibits NSC differentiation by repressing License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution and reproduction in any medium provided that the original work is properly attributed. proneural genes, including Ascl1 and Neurog2 (Lee, 1997; Heng and Guillemot, 2013). In addition, Hes proteins repress expression Received 23 March 2017; Accepted 14 August 2017 of their own genes, counteracting Notch and leading to oscillations DEVELOPMENT 3465 STEM CELLS AND REGENERATION Development (2017) 144, 3465-3474 doi:10.1242/dev.152520 Fig. 1. The NSC differentiation processes in the embryonic and adult brain and its regulatory network. (A) NSC fate in the embryonic and adult brain is dependent on the levels of proneural transcription factor expression. During embryonic neurogenesis, the majority of the NSCs are in a mitotically active state (aNSC) while a few will enter quiescence (qNSC) and remain inactive until adulthood. In the adult neurogenic niches, most NSCs are mitotically inactive (qNSC) and rarely transit to the mitotically active, neurogenic state (aNSC). In aNSCs, low levels of proneural activity drive cell cycle progression but is insufficient to induce differentiation. In the absence of proneural transcription factor activity, NSCs are quiescent (qNSC) and high proneural transcription factor activity drives neural differentiation (Diff). (B) The Notch-Hes-Proneural transcription factor interaction network. Notch signaling through the DNA-binding protein Rbpj activates expression of Hes genes. Hes protein homodimers repress proneural gene expression, including Ascl1 and Neurog2 via N-box and class-C sites, and their own expression by binding to N- box sites in their promotor regions. Proneural transcription factors activate cell cycle progression and differentiation via E-box sites. (C) The current known Notch-Hes-IDs-proneural interactions. IDs form heterodimers with Hes transcription factors, which are unable to bind to N-box sites but can bind to class-C sites, although with lower efficiency than Hes homodimers. IDs also form heterodimers with proneural factors that are unable to activate the differentiation and cell cycle progression genes. in their expression (Fig. 1B) (Hirata et al., 2002). This dynamic 2014b). Hence, IDs potentially regulate neurogenesis at multiple Hes gene activity has been suggested to result in low-level levels, including enhancing Hes expression and blockage of expression of proneural factors, including Ascl1, and this low proneural factor activity (Fig. 1C). expression drives cell cycle progression but is not sufficient to Owing to the complex and reciprocal interplay between Notch- induce NSC differentiation (Castro et al., 2011; Imayoshi et al., Hes and IDs, it has been challenging to access the consequences of 2013; Andersen et al., 2014). In contrast, high and sustained their interactions experimentally and their respective roles in the expression of Hes proteins drives complete repression of Ascl1, control of NSC activity. As a first step to address this problem, we leading to cell cycle exit and NSC entry into a quiescent state (Baek developed a specific theoretical framework that takes into account et al., 2006; Castro et al., 2011; Imayoshi et al., 2013; Andersen the interactions between Notch, IDs and the members of the bHLH et al., 2014). NSCs in the adult brain niches are predominantly family of transcriptional factors. Our theoretical framework is in line quiescent, a state not observed frequently in the developing brain.