OCT4 and SOX2 transcription factors

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Revision as of 20:53, 16 June 2020

Introduction

Oct4 and Sox2 are two transcription factors (TFs) involved in various roles in murine and primate cells, mainly related to the maintenance of pluripotency and self-renewal properties in embryonic stem cells. These two factors, encoded by the POU5F1 (POU Class 5 Homeobox 1) and SOX2 (SRY-Box Transcription Factor 2) genes, respectively, serve as repprogramming TFs and occupy the same target genes in vivo ^[1][2], forming the complex OCT4-SOX2, which is the main way in which they act, although they are not obligate heterodimers in solution.

OCT4

The OCT4 transcription factor (octamer-binding transcription factor 4), also known as OCT-3, OCT3 / 4, OTF3 or NF-A3, was discovered almost three decades ago, where its use relationship with pluripotent CTE in primate and rodent species (Zeineddine et al., 2014). This protein is encoded by the POU5F1 gene, which is located on chromosome 6 in humans and 17 in rats, and belongs to the POU family (Pit, October, Unc) of DNA-binding proteins, which regulate the expression of target genes (Zeineddine et al., 2014; Malakootian et al., 2017). In humans, through alternative splicing, POU5F1 generates less than eight distinct RNA transcripts, these being OCT4A, OCT4B-190, OCT4B-265, OCT4B-164, OCT4B1 and more recently reported as OCT4C, OCT4C1 and OCT4B4 variants [16; 17] In addition to the generated isoforms, many studies have been carried out mainly with respect to the functions of the OCT4A isoform. Studies targeting the OCT4B isoforms (190, 265 and 164) that are not able to support an automatic restoration of the CTE, but they can respond to cellular stress, whereas the functions of OCT4B1, OCT4C and OCT4C1 have not yet been clarified (Wang and Dai, 2010). OCT4A is normally expressed in the early stages of embryonic development and represents one of the main regulatory factors for pluripotency and self-review of embryonic stem cells, being considered a marker of pluripotency (Da SILVA et al., 2017). A further differentiation of CTE into cells used for different tissues depends on rapid and rapid expression of OCT4A, and these cells are differentiated remain with the OCT4A factor silenced (Villodre et al., 2016; Atlasi et al, 2008; Hatefi et al., 2012). However, we have already documented an open expression of transcription factors such as OCT4, SOX2 and NANOG, together or controlled, lead to tumors, metastases and the greatest recurrence after use, in different types of cancer (Zeineddine et al., 2014).

SOX2

The SOX/Sox (SRY homology box) family of proteins comprises 20 individual members in man and mouse ^[3], which SOX2 is the most explored. SOX proteins are principally defined by a conserved DNA-binding element, the so-called high mobility group (HMG) that relates to a transcriptional master regulator of virility (i.e., SEX determining factor Y, SRY) and thus functionally qualifies SOX/Sox proteins as DNA-binders ^[4][5]. While Sox proteins contribute to various cellular functionalities, reprogramming capacity is largely confined to members of the SoxB1 group (i.e., Sox1, Sox2, and Sox3)^[6]. SOX2 significantly often imposes transcription modulatory in conjunction with co-factors, such as Oct3/4. OCT3/4 in ES cells

Murine Expression

Human Expression

Function as a gatekeeper for Embryonic Stem Cell Pluripotency

Intersection between Transcriptional Core and Lif Signalling

The OCT4-SOX2 mechanism

The nucleosome is the chromatin basic unit, composed of a 147 pb DNA segment wrapped around 8 histone proteins. It is a convention that the sites in which a DNA major groove is pointed to the nucleosome core are called "superhelix location" (SHL). The SHL are enumerated from 0 to ±7, having 0 as the nucleosome main axis, known as "dyad". The OCT4-SOX2 binds in the SHL-6 site (Fig 1) and both of them act in the DNA removal from the core histones [ref1]. OCT4 has a bipartite DNA binding domain (DBD) comprised of a POU-specific (POUS) and POU-homeo-domain (POUHD) separated by 17-residues (Fig 2) and SOX2 has a high-mobility group (HMG) domain (Fig 2) [refs 1, 8]. The OCT4-POUS and SOX2-HMG DBDs engage major and minor grooves, respectively [ref1]. The DNA remains attached and straightened around the OCT4 site but is detached around the SOX2 motif [ref1].

OCT4 recognizes a partial motif, engaging DNA with its POUS domain, whereas the POUHD is not engaged. On free DNA, both POU domains engage the major groove over 8bp on opposite sides of the DNA [ref1]. SOX2 competes with histones for DNA binding and kinks DNA by ~90° at SHL-6.5 away from the histones [ref. 8]. This is accomplished by intercalation of the SOX2 Phe48 and Met49 ‘wedge’ at the TT base step [ref 8]. SOX2 kinks the DNA and synergistically with OCT4 releases the DNA from the core histones [movie1].

Figure 1 - Extracted from the article Mechanisms of OCT4-SOX2 motif readout on nucleosomes (Alicia K. Michael et al., 2020). (B) OCT4-SOX2-NCPSHL+6 model. (C) Details of SOX2-induced DNA kink.

Figure 2 - Extracted from the article Mechanisms of OCT4-SOX2 motif readout on nucleosomes (Alicia K. Michael et al., 2020). (A) Domain schematic of OCT4 and SOX2 constructs.

Movie 1 - Extracted from the Supplementary Materials for the article Mechanisms of OCT4-SOX2 motif readout on nucleosomes (Alicia K. Michael et al., 2020). OCT4-SOX2 binding at SHL-6 removes DNA from the histone core. A morph video modelling the structural change induced in the nucleosome upon OCT4-SOX2 binding at SHL-6. Morph is between the DNA of the NCP-SHL-6 and OCT4-SOX2-NCP-SHL-6 models.