Concise review: pursuing self-renewal and pluripotency with the stem cell factor Nanog

Abstract

Pluripotent embryonic stem cells and induced pluripotent stem cells hold great promise for future use in tissue replacement therapies due to their ability to self-renew indefinitely and to differentiate into all adult cell types. Harnessing this therapeutic potential efficiently requires a much deeper understanding of the molecular processes at work within the pluripotency network. The transcription factors Nanog, Oct4, and Sox2 reside at the core of this network, where they interact and regulate their own expression as well as that of numerous other pluripotency factors. Of these core factors, Nanog is critical for blocking the differentiation of pluripotent cells, and more importantly, for establishing the pluripotent ground state during somatic cell reprogramming. Both mouse and human Nanog are able to form dimers in vivo, allowing them to preferentially interact with certain factors and perform unique functions. Recent studies have identified an evolutionary functional conservation among vertebrate Nanog orthologs from chick, zebrafish, and the axolotl salamander, adding an additional layer of complexity to Nanog function. Here, we present a detailed overview of published work focusing on Nanog structure, function, dimerization, and regulation at the genetic and post-translational levels with regard to the establishment and maintenance of pluripotency. The full spectrum of Nanog function in pluripotent stem cells and in cancer is only beginning to be revealed. We therefore use this evidence to advocate for more comprehensive analysis of Nanog in the context of disease, development, and regeneration.

Keywords: Embryonic stem cells; Induced pluripotent stem cells; Nanog; Pluripotency; Reprogramming; Self-renewal.

Fig. 1

Mouse and human ESC survival pathways. (A) Mouse ESCs require LIF and BMP4 for maintenance. (B) Human ESCs and mouse EpiSCs require IGF/insulin and bFGF for maintenance. Human ESC-derived fibroblast-like cells and MEFs are also stimulated by bFGF in culture to secrete IGF (dashed arrows). In both cell types, Nanog, Oct4, and Sox2 form a positive auto-regulatory loop.

Fig. 2

Sequence alignment of mouse, human, chick, axolotl salamander, and zebrafish Nanog (top to bottom). All five orthologs contain conserved residues, as indicated by shaded regions (darker = more conserved). All orthologs contain a homeodomain (boxed in red), but only mouse and human Nanog contain WR domains (boxed in green). Alignment created with ClustalW2 and analyzed in Jalview.

Fig. 3

Vertebrate Nanog orthologs have conserved domains. Highest sequence identities relative to mouse Nanog reside in the homeodomain. Sequence identity percentages calculated in Jalview. (ND, N-terminal domain; HD, homeodomain; WR, tryptophan-rich domain; CD, C-terminal domain, Full CD = CD1 + WR + CD2).

Fig. 4

Nanog or its direct target Esrrb is required in the final stages of somatic cell reprogramming. Mouse embryonic fibroblasts (MEFs) transduced with the Yamanaka factors yield pre-iPSCs. Nanog is required in the pre-iPSC to ground state iPSC transition, as shown in red. Once the pluripotent ground state is established, Nanog is no longer required.