Nanog

NANOG is a transcription factor, an upstream regulator in early mammalian development and a key determinant to maintain the pluripotency of embryonic stem cells. It’s named in reference to the Irish Legend of Oisín and Niamh and the Land of Eternal Youth, beauty, health and joy - Tír Na nÓg. [1] NANOG regulates the expression of hundreds of genes by binding their promoter elements. The murine Nanog homeodomain (HD) is capable of binding to interact with DNA elements derived from the Tcf3 promoter. [2]

The human NANOG protein coded by the NANOG1 gene, consists of 305 amino acids, molecular weight of 34.6 kDa and possesses 3 functional domains: the N-terminal domain, which contains 94 amino acids, is rich in Ser and Thr and acidic residues found in typical transactivators. The C-terminal domain contains 151 amino acids, it has a tryptophan repeat (WR) domain in which every fifth residue is a tryptophan. Last but not least, the conserved homeodomain motif has 60 amino acids. The homeodomain region facilitates not only DNA binding, optimally to the DNA consensus sequence 5'-TAAT[GT][GT]-3' or 5'-[CG][GA][CG]C[GC]ATTAN[GC]-3' [3], but also gene regulation and interaction with other proteins. The human Nanog 1 gene is located on chromosome 12, and the mRNA contains a 915 bp open reading frame (ORF) with 4 exons and 3 introns. [4][1].

Structural highlights

The consists in 3 alpha helices linked to each other by loops. The so called alpha-helix1 and alpha-helix 3 play a role into binding of the protein to DNA. The alpha-helix1, localized in the N-terminal arm, binds to DNA through minor grooves, whereas alpha-helix3 binds major grooves in DNA and is considered the recognition helix, due to it´s extensive DNA contact sufarce. Alpha helices 1 and 2 are arrenged as antiparallel to each other in the terciary structure, while positioning alpha-Helix3 in a favorable way to bind to the major grooves of the DNA helical structure.

DNA binding

Helix α3 (the so-called recognition helix) is inserted into the major groove of the DNA and forms an extensive DNA contact interface. The region upstream of helix α1 (HD residues 1–10) is referred to as the N-terminal arm and contributes to DNA binding through minor groove contacts. It was shown that in the absence and presence of DNA, the helical core of the HD constitutes a largely rigid entity that is not subjected to major structural changes upon binding to DNA. Pronounced flexibility is restricted to the N-terminal arm that becomes ordered after binding to the DNA's minor groove.[11][12] The figure below by Allouba M. H. et al., 2015 shows more detail about the aminoacid residues participating in the binding between DNA and Nanog.

Function

The expression of Nanog is detectable at embryonic stem cells during early stages of the embryo development. It is expressed in the proximal epiblast in a region presumed to be the primitive streak, its expression extends distally as the streak elongates during gastrulation and remains restricted to epiblast. Nanog is vital to maintain pluripotency, it is very much needed, since nanog-deficient inner cell mass from the blastocyst could not generate epiblast and only produce parietal endoderm-like cells. Meanwhile, Nanog-deficient embryonic stem cells lost pluripotency and differentiated into extraembryonic endoderm lineage. [5] Nanog expression is closely correlated with self-renewal capability and cell growth of embryonic stem cells. It´s function as a transcription factor have to be inserted into a large network of pluripontency factors that together grant the cells a indifferentiated and pluripotent state. Nanog expression is inhibits differentiation, thus Nanog expression is rapidly supressed during differentiation, and studies found that a lot of important pathways involved in animal development, such as the Wnt Pathway, have negative regulators of Nanog. [10]

Disease

Nanog as many pluripotency factors can be disregulated and end up contributing to protumorigenic proporties. These transcription factors are frequently detected in malignant tumours, where there are Cancer Stem Cells (CSC´s), wich recapitulate the expression of Embrionic Stem Cell associated proteins. Nanog is detected in a wide variety of cancers alongside other very well known transcription factor ,such as the Yamanaka Factors, Oct4 and Sox2 [9]. Nanog expression in tumours has been correlated with a worse prognosis, metastatic properties, tissue invasiveness and chemotherapy resistance [6, 7, 8]. Because CSC´s are the most resistent, invasive and proliferative cells, they are also the most probable reason for tumour recurrance after cirurgical removal. A lot of studies have been done to unravel the network that pluripotency associated proteins maintain in cancer cells, and yet the link between Nanog and other transcription factors have been stablished, the origin of that expression in cancer cells is still poorly understood.