Genetic evidence that Nkx2.2 and Pdgfra are major determinants of the timing of oligodendrocyte differentiation in the developing CNS

Abstract

In the central nervous system (CNS), oligodendrocyte maturation and axonal myelination occur on a predictable schedule, but the underlying timing mechanisms are largely unknown. In the present study, we demonstrate that Nkx2.2 homeodomain transcription factor is a key regulator for the timing of oligodendrocyte differentiation during development. Whereas induced expression of Nkx2.2 in early oligodendrocyte precursor cells (OPCs) causes precocious differentiation of oligodendrocytes, conditional ablation of Nkx2.2 temporally delays oligodendrocyte maturation. Moreover, Nkx2.2 can directly bind to the promoter of platelet-derived growth factor receptor alpha (Pdgfra) and repress its gene expression. Genetic ablation of Pdgfra mimics the effect of Nkx2.2 overexpression in accelerating OPC differentiation in the developing spinal cord. Together, our findings strongly suggest that Nkx2.2 functions as a major 'switch' to turn off Pdgfra signaling in OPCs and initiate the intrinsic program for oligodendrocyte differentiation. KEY WORDS: Spinal cord, Tet-on, Transcription factor, Mouse INTRODUCTIONA requisite component of nervous system development is the achievement of proper axonal myelination for rapid and accurate transmission of electric activities. In the central nervous system (CNS), myelin sheaths are elaborated by oligodendrocytes (OLs), and the myelination process is preceded by molecular and morphological differentiation of oligodendrocyte precursor cells (OPCs). It was observed that OPCs differentiate on a predictable schedule both in vivo and in vitro, but the molecular pathways that control the timing of OPC differentiation have not been clearly defined.It has been recently shown that multiple classes of transcription factors are involved in the regulation of the OL differentiation process. They include the negative differentiation regulators Id2, Id4 and Hes5 (Kondo and Raff, 2000; Liu et al., 2006;Wang et al., 2001), and positive regulators such as Olig1 (Lu et al., 2002), Mrf (Myrf -Mouse Genome Informatics) (Emery, 2010), Mash-1 (Ascl1 -Mouse Genome Informatics) (Sugimori et al., 2008), Sip1 (Gemin2 -Mouse Genome Informatics) (Weng et al., 2012), Nkx2.2 (Qi et al., 2001 and Sox10 (Soula et al., 2001). Among these transcription factors, Nkx2.2 (Nkx2-2 -Mouse Genome Informatics) is uniquely positioned as a candidate regulator for the timing of OL differentiation. In the developing mouse spinal cord, Nkx2.2 expression is upregulated in OPCs immediately before their differentiation but rapidly downregulated after OPC differentiation (Fu et al., 2002;Soula et al., 2001;Xu et al., 2000;Zhou et al., 2001). Thus, Nkx2.2 expression in differentiating OPCs correlates seamlessly with the onset of OL differentiation. Functional analyses revealed that Nkx2.2 plays an essential role in the terminal differentiation of OLs (Qi et al., 2001;Zhou et al., 2001). However, because of the neonatal lethality of Nkx2.2 mutants, it has remained unknown whether Nkx2.2 is absolutely required for OPC maturation or simp...

ASCL1 regulates proliferation of NG2‐glia in the embryonic and adult spinal cord

“…This finding is not surprising considering that key factors such as NKX2.2, OLIG1/2 and SOX10, which have been shown to be crucial for NG2-glia differentiation, are still present and are not affected by the loss of ASCL1. In contrast, Olig1 Cre conditional knock-out of Pdgfrα resulted in a drastic decrease in the number of OPC/NG2-glia in the spinal cord (Zhu et al, 2014), a phenotype that is similar but more severe than the Olig1 Cre ;Ascl1 -CKO observed in this study (Fig. 8).…”

“…The generation, development, and differentiation of NG2-glia in the CNS is a complex process across both space and time, and requires the precise regulation and function of multiple genes, particularly those encoding transcription factors ( Olig1/2, Nkx2.2, Ascl1, Sox10, Prox1, Smarca4/Brg1 , Myrf, and Sox17 ) (Hornig et al, 2013; Kato et al, 2015; Li, Lu, Smith, & Richardson, 2007; Lu et al, 2002; Stolt et al, 2004; Sugimori et al, 2007; Sugimori et al, 2008; Yu et al, 2013; Zhou, Choi, & Anderson, 2001; Zhou & Anderson, 2002; Zhu et al, 2014). Gain-of-function and loss-of-function studies in the last couple of decades have been informative in revealing the general mechanisms of how OLPs are specified by Sonic hedgehog (SHH) and the various transcription factors to give rise to NG2-glia and eventually myelinating OLs (Rowitch & Kriegstein, 2010).…”

TAPP1 inhibits the differentiation of oligodendrocyte precursor cells via suppressing the Mek/Erk pathway

“…To further confirm this, we examined the expression of this gene in Olig1 mutant and Nkx2.2 mutant spinal cords using TAPP1 ISH. It was previously shown that the differentiation of OLs is delayed and/or reduced in the spinal cords of these mutants [10,15] .…”

Interplay between exercise and dietary fat modulates myelinogenesis in the central nervous system

“…5 μm frozen sections were immunostained for Nkx2.2 (74.5A5, Developmental Studies Hybridoma Bank, University of Iowa, Iowa city, IA), or for anti-neural glial antigen-2 (NG2, AB5320, Millipore). Nkx2.2 is a homeodomain transcription factor expressed by OPCs that regulates differentiation (Cai et al 2010; Qi et al 2001; Zhu et al 2014). NG2 is a chondroitin sulfate proteoglycan that recognizes OPCs in the intact nervous system (Nishiyama et al 2009).…”