The Structures and Some Interactions of the Proteins Oct-4, Sox2, NANOG and Lin28 That Are Used to Induce the Pluripotent Stem (iPS) Cells

Comprehensive Review

May 2010

Please cite this article as Ursachi, CD. (2010) The structures and some interactions of the proteins Oct-4, Sox2, Nanog and Lin28 that are used to induced stem (iPS) cells.

It is a well-known fact that the stem cells and their use in basic biology, regenerative therapies, and drug discovery are currently hot topics in biology, medicine, and ethics. In this context, one of the most debated questions is the source of stem cells. So far, the major source of pluripotent stem cells (embryonic stem cells, ES) is the inner cell mass (ICM) of the blastocysts, but the potential clinical application of these cells is faced with many practical and ethical concerns. Recent breakthrough studies (started by Takahashi and Yamanaka, 2006), using two cocktails, each with a combination of four factors, to reprogram human somatic cells without using embryos or eggs, led to an important revolution in stem cell research. Oct-3/4, Sox2, c-Myc, and Klf4 or Oct-4, Sox2, NANOG, and Lin28 were the two combinations of factors used to induced pluripotent stem (iPS) cells. These reprogrammed cells stem (iPS) resembled ES in many of their proprieties such as self-renewal and the potential for differentiation, and these proprieties are regulated by a low number of transcription factors. Consequently, it is important to understand the roles of these proteins for solving the practical and ethical problems of using the stem cells (ES and iPS).
Our major aim is to review the roles of these proteins (Oct-4, Sox2, NANOG, and Lin28) by describing their structures and identifying their interactions with other proteins, DNA and RNA. Therefore, in this review, we start by describing the structure of the main factors involved in the molecular process of induced reprogramming cells: Oct-3/4, Sox2, NANOG, and Lin28. Also, we present some interaction between them and between them and DNA and miRNAs. Finally, we discuss the interactions between these factors and other proteins and miRNAs that are involved in regulation of stem cell pluripotency.

Discussion and conclusion
As we have seen, a small group of transcription factors (Oct-4, Sox2, and NANOG) maintains the ES state and their cellular concentration decrease progressively during cell differentiation. In addition, if Oct-4 and Sox2 are expressed (introduced) into somatic cells, they contribute to the induction of pluripotent stem (iPS) cells. Thus, these transcription factors are able to induce and maintain the pluripotent state from ''de novo'' or from the somatic cells. It is interesting to elucidate how these transcription factors can induce cell transformation so radical. So far, there are several attempts to explain the molecular mechanisms of induction of pluripotent stem cell by transcription factors mentioned. Firstly, these transcription factors have a dual role (the transcriptional network controlling pluripotency), that is to say, they contribute to stem cells pluripotency by repressing genes linked to lineage commitment (Pax6, Hand1, Atbf1, Esx1l) and activating genes involved in pluripotency (Stat3, Tcf3, Smarcad1, Rest, Rif1). Secondly, the three regulators (Oct-4, Sox2, and NANOG) together targeted the promoters of their own genes (autoregulatory loop), as well as those of each other. Third, the transcription factors mentioned interaction with various proteins (interaction protein network) such as components of chromatin remodeling (HDACs and NuRD remodeling complex), the corepression complexes (PRC1 polycomb complex), extracellular signaling factors and other transcription factors (Dax1, REST). Fourth, Lin28 facilitate the reprogramming process by stabilizing mRNA transcripts from genes of the pluripotent transcriptional network. In addition, prevents the processing of let-7 miRNAs that is known to stimulate differentiation.
By analyzing the structure of the three transcription factors and the protein Lin28, and in agreement with the articles mentioned, we can affirm that these proteins can generate these major cells transformations due to their structural complexity. Firstly, each of the proteins presented has several domains that show trans-activator function (e.g. POUS, POUH, AD1 and AD2 for Oct-4). Secondly, to control a set of target genes, these transcription factors assemble into homodimers (e.g. dimerization of NANOG monomers mediated through WR subregions). Third, that Lin28 plays a key function in the maintenance of pluripotency by promoting the expression of H2a gene (and perhaps also other replication dependent histone genes) and miRNA let-7 at the posttranscriptional level. Finally, the transcription factors mentioned interacts with other proteins and DNA (promoters and enhancers) forming protein complexes (e.g. Nanog-Oct-4-deacetylase - NODE) or DNA-proteins complexes (e.g. POU/HMG/DNA). For example, when POU-Oct-4 domain interacts with HMG-Sox2 domain and various DNA sequences to form the ternary complexes, the HMG domain can rotate toward POUS and  present another surface contact with the DNA sequence. In ternary complex POU/HMG/FGF-4, as a result of molecular interactions between Oct-4 and Sox2, two activation domains (Oct-4 AD2 and Sox2 R3) are functional. In agreement with previous information, we can speculate that HMG and POU activation domains have several surfaces contact with the DNA sequences, and they can change them by modifying the domains position into the protein complexes.
To summarize, these transcription factors are able to induce and maintain the pluripotent state due to their structure domains  which interact with many other molecules (proteins, DNA and RNA) by changing their structures.
Despite the complexity of the molecular mechanisms underlying the induction of iPS, there is no doubt, that these cells will be used as powerful tools in basic regenerative therapies and drug discovery.

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