ABSTRACT: Several Sox family members, small molecules, and inhibitors of the TGF-β pathway can replace exogenous Sox2 during the generation of induced pluripotent stem cells (iPSCs), suggesting that SOX2 is dispensable for initiation of reprogramming. However, the time point at which the endogenous Sox2 locus is activated in these conditions was not thoroughly evaluated. To identify the Sox2-independent reprogramming steps, we excluded Sox2 from the reprogramming cocktail and used Sox2-deficient mouse embryonic fibroblasts. We show that reprogramming is initiated in the absence of SOX2 as shown by formation of pre-iPSC colonies that have undergone a mesenchymal-to-epithelial transition and that have activated early reprogramming markers. However, these iPSC-like intermediates cannot progress through the final stabilization phase of reprogramming in the absence of SOX2. Overall, our results provide a better understanding of the molecular events underlying the induction of pluripotency and of the role that SOX2 plays in the process. Both pLV-tetO-E1A-2A-Cherry and pLV-tetO-Cherry lentiviral constructs were cloned in house and defective-lentiviral particles were produced as previously described (Han et al. 2013). Next, 5x10 4 ESCs containing a tetracycline-inducible transactivator (irtTA-VP16-GBD, Anastassiadis et al. 2002) plus an Oct4-GFP reporter transgene (Yoshimizu et al. 1999) were transduced twice using 20 µl of replication-defective lentiviral particles plus polybrene (4 µg/ml, Sigma). Cells were washed 24 h after transduction and subsequently induced with doxycycline (1 µg/ml, Sigma) and dexamethasone (10-7 M, Sigma). Samples were harvested at several time-points, 0, 3, 6, 9, 12, 24, and 48 hours after induction, and used for microarray analysis as previously reported (Tapia et al. 2012). Cells transduced with pLV-tetO-Cherry and induced for 48 h were used as control. RNA wasprocessed using the RNeasy micro and mini kits with on-column DNase digestion of QIAGEN as per the manufacturer’s instructions. Integrity of RNA samples was quality checked using a 2100 Bioanalyzer (Agilent). When possible, 300 ng of total RNA per sample was used as starting material for linear amplification (Illumina TotalPrep RNA Amplification Kit, Ambion), which involved synthesis of T7-linked double-stranded cDNA and 14 hr of in-vitro transcription incorporating biotin-labeled nucleotides. Purified and labeled cRNA was then quality checked on a 2100 Bioanalyser and hybridized as biological or technical duplicates for 18 hr onto MouseRef-8 v2 gene expression BeadChips (Illumina), following the manufacturer's instructions. After being washed, the chips were stained with streptavidin-Cy3 (GE Healthcare) and scanned using an iScan reader and accompanying software. Bead intensities were mapped to gene information using BeadStudio 3.2 (Illumina). Background correction was performed using the Affymetrix Robust Multi-array Analysis (RMA) background correction model. Variance stabilization was performed using log2 scaling, and gene expression normalization was calculated with the quantile method implemented in the lumi package of R-Bioconductor. Data post-processing and graphics were performed with in-house developed functions in Matlab.
