Homeobox protein NANOG

Overview
NANOG (pron. nanOg) is a transcription factor critically involved with self-renewal of undifferentiated embryonic stem cells.

Embryonic stem cells
NANOG is a gene expressed in embryonic stem cells (ESCs) and is thought to be a key factor in maintaining pluripotency. NANOG thought to function in concert with other factors such as POU5F1 and SOX2 to establish ESC identity. These cells offer an important area of study because of their ability to maintain pluripotency. In other words, these cells have the ability to become virtually any cell of any of the three germ layers (endoderm, ectoderm, mesoderm). It is for this reason that understanding the mechanisms that maintain a cell's pluripotency is critical for researchers to understand how stem cells work; and may lead to future advances in treating degenerative diseases.

History
Dr Ian Chambers (currently of the Institute for Stem Cell Research, The University of Edinburgh, UK) who isolated the mouse Nanog gene said: "Nanog seems to be a master gene that makes embryonic stem cells grow in the laboratory. In effect this makes stem cells immortal. Being Scottish, I therefore chose the name after the Tir nan Og legend."

Genes that code for NANOG
Analysis of arrested embryos demonstrated that embryos express pluripotency marker genes such as OCT4, NANOG and REX1. Derived human ESC lines also expressed specific pluripotency markers: These markers allowed for the differentiation in vitro and in vivo conditions into derivatives of all three germ layers. POU5F1 (OCT4), TDGF1 (CRIPTO), SALL4, LECT1, and BUB1 are also related genes all responsible for self-renewal and pluripotent differentiation.
 * TRA-1-60
 * TRA-1-81
 * SSEA4
 * alkaline phosphatase
 * TERT
 * REX1

NANOG protein
Human NANOG protein (Accession number NP_079141) is a 305 amino acid protein with a conserved homeodomain motif that is localized to the nuclear component of cells. The homeodomain facilitates DNA binding.

There are N-terminal, homeodomain, and C-terminal region in human NANOG protein. Like murine NANOG, N-terminal region of human NANOG is rich in Ser, Thr and Pro residues and C-terminus contains W repeats. The homeodomain in hNANOG ranges from a.a.95 to a.a 155. The conserved sequence of homeodomain are a.a. 99-100, 102, 106-107, 110, 114, 119, 121, 127-128, 132, 134, 138-140, 142-145, 147, 149, and 151-152.

Molecular biology
Overexpression of Nanog in mouse embryonic stem cells causes them to self-renew in the absence of Leukemia inhibitory factor. In the absence of Nanog, mouse embryonic stem cells differentiate into visceral/parietal endoderm (Chambers et al, 2003 and Mitsui et al, 2003)

Loss of Nanog function causes differentiation of embryonic stem cells into other cell types (Lin et al, 2005).

NANOG overexpression in human embryonic stem cells enables their propagation for multiple passages during which the cells remain pluripotent. Gene knockdown of Nanog promotes differentiation, thereby demonstrating a role for these factors in human embryonic stem cell self-renewal.

It has been shown that the tumour suppressor p53 binds to the promoter of NANOG and suppresses its expression after DNA damage in mouse embryonic stem cells. p53 can thus induce differentiation of embronic stem cells into other cell types which undergo efficient p53-dependent cell-cycle arrest and apoptosis. (Lin et al, 2005)

Nanog transforms NIH3T3 cells. By using DNA microarray to find the transcription targets of Nanog, Nanog regulated genes have been identified. Some of these target genes explain the transformation of NIH3T3 cells. (Piestun et al, 2006)

GATA6 and Nanog have been linked due to the similar cellular differentiation of ES cells in their absence, which leads to the hypothesis that Nanog may prevent ectodermal growth via repressing GATA6.

Yamanaka et al., demonstrate induction of pluripotent stem cells from mouse embryonic or adult fibroblasts by introducing four factors, Oct3/4, Sox2, c-Myc, and Klf4, under ES cell culture conditions. Of these four factors it has been shown that Nanog was dispensable for such induction in this cell system.

Clinical medicine
NANOG may be useful in the immunohistochemical diagnosis of tumors. NANOG is expressed in germ cells of the fetus and in some germ cell tumors of the gonads and central nervous system (CNS). Among germ cell tumors, NANOG is expressed by seminoma and embryonal carcinoma but not by mature teratoma nor endodermal sinus tumor ; expression of NANOG by immature teratoma and choriocarcinoma is unknown. Among tumors usually found in the CNS, NANOG is expressed by germinoma (a germ cell tumor histologically identical to seminoma and dysgerminoma) but not by pineoblastoma, lymphoma, pituitary adenoma and gliomas; expression of NANOG by other germ cell tumors of the CNS is unknown.