Dina A. Faddah

Single-cell expression analyses during cellular reprogramming reveal an early stochastic and a late hierarchic phase.

During cellular reprogramming, only a small fraction of cells become induced pluripotent stem cells (iPSCs). Previous analyses of gene expression during reprogramming were based on populations of cells, impeding single-cell level identification of reprogramming events. We utilized two gene expression technologies to profile 48 genes in single cells at various stages during the reprogramming process. Analysis of early stages revealed considerable variation in gene expression between cells in contrast to late stages. Expression of Esrrb, Utf1, Lin28, and Dppa2 is a better predictor for cells to progress into iPSCs than expression of the previously suggested reprogramming markers Fbxo15, Fgf4, and Oct4. Stochastic gene expression early in reprogramming is followed by a late hierarchical phase with Sox2 being the upstream factor in a gene expression hierarchy. Finally, downstream factors derived from the late phase, which do not include Oct4, Sox2, Klf4, c-Myc, and Nanog, can activate the pluripotency circuitry.

Systematic identification of culture conditions for induction and maintenance of naive human pluripotency.

Summary Embryonic stem cells (ESCs) of mice and humans have distinct molecular and biological characteristics, raising the question of whether an earlier, “naive” state of pluripotency may exist in humans. Here we took a systematic approach to identify small molecules that support self-renewal of naive human ESCs based on maintenance of endogenous OCT4 distal enhancer activity, a molecular signature of ground state pluripotency. Iterative chemical screening identified a combination of five kinase inhibitors that induces and maintains OCT4 distal enhancer activity when applied directly to conventional human ESCs. These inhibitors generate human pluripotent cells in which transcription factors associated with the ground state of pluripotency are highly upregulated and bivalent chromatin domains are depleted. Comparison with previously reported naive human ESCs indicates that our conditions capture a distinct pluripotent state in humans that closely resembles that of mouse ESCs. This study presents a framework for defining the culture requirements of naive human pluripotent cells.

Mechanisms and models of somatic cell reprogramming

Conversion of somatic cells to pluripotency by defined factors is a long and complex process that yields embryonic-stem-cell-like cells that vary in their developmental potential. To improve the quality of resulting induced pluripotent stem cells (iPSCs), which is important for potential therapeutic applications, and to address fundamental questions about control of cell identity, molecular mechanisms of the reprogramming process must be understood. Here we discuss recent discoveries regarding the role of reprogramming factors in remodelling the genome, including new insights into the function of MYC, and describe the different phases, markers and emerging models of reprogramming.

The Developmental Potential of iPSCs Is Greatly Influenced by Reprogramming Factor Selection

Induced pluripotent stem cells (iPSCs) are commonly generated by transduction of Oct4, Sox2, Klf4, and Myc (OSKM) into cells. Although iPSCs are pluripotent, they frequently exhibit high variation in terms of quality, as measured in mice by chimera contribution and tetraploid complementation. Reliably high-quality iPSCs will be needed for future therapeutic applications. Here, we show that one major determinant of iPSC quality is the combination of reprogramming factors used. Based on tetraploid complementation, we found that ectopic expression of Sall4, Nanog, Esrrb, and Lin28 (SNEL) in mouse embryonic fibroblasts (MEFs) generated high-quality iPSCs more efficiently than other combinations of factors including OSKM. Although differentially methylated regions, transcript number of master regulators, establishment of specific superenhancers, and global aneuploidy were comparable between high- and low-quality lines, aberrant gene expression, trisomy of chromosome 8, and abnormal H2A.X deposition were distinguishing features that could potentially also be applicable to human.

Single-cell analysis reveals that expression of nanog is biallelic and equally variable as that of other pluripotency factors in mouse ESCs.

The homeodomain transcription factor Nanog is a central part of the core pluripotency transcriptional network and plays a critical role in embryonic stem cell (ESC) self-renewal. Several reports have suggested that Nanog expression is allelically regulated and that transient downregulation of Nanog in a subset of pluripotent cells predisposes them toward differentiation. Using single-cell gene expression analyses combined with different reporters for the two alleles of Nanog, we show that Nanog is biallelically expressed in ESCs independently of culture condition. We also show that the overall variation in endogenous Nanog expression in ESCs is very similar to that of several other pluripotency markers. Our analysis suggests that reporter-based studies of gene expression in pluripotent cells can be significantly influenced by the gene-targeting strategy and genetic background employed.

Brief ReportSingle-Cell Analysis Reveals that Expression of Nanog Is Biallelic and Equally Variable as that of Other Pluripotency Factors in Mouse ESCs

The homeodomain transcription factor Nanog is a central part of the core pluripotency transcriptional network and plays a critical role in embryonic stem cell (ESC) self-renewal. Several reports have suggested that Nanog expression is allelically regulated and that transient downregulation of Nanog in a subset of pluripotent cells predisposes them toward differentiation. Using single-cell gene expression analyses combined with different reporters for the two alleles of Nanog, we show that Nanog is biallelically expressed in ESCs independently of culture condition. We also show that the overall variation in endogenous Nanog expression in ESCs is very similar to that of several other pluripotency markers. Our analysis suggests that reporter-based studies of gene expression in pluripotent cells can be significantly influenced by the gene-targeting strategy and genetic background employed.

Transcriptional profiling of cells sorted by RNA abundance

We have developed a quantitative technique for sorting cells on the basis of endogenous RNA abundance, with a molecular resolution of 10–20 transcripts. We demonstrate efficient and unbiased RNA extraction from transcriptionally sorted cells and report a high-fidelity transcriptome measurement of mouse induced pluripotent stem cells (iPSCs) isolated from a heterogeneous reprogramming culture. This method is broadly applicable to profiling transcriptionally distinct cellular states without requiring antibodies or transgenic fluorescent proteins.

Erratum: Systematic identification of culture conditions for induction and maintenance of naive human pluripotency (Cell Stem Cell (2014) 15 (471-487))

Thorold W. Theunissen, Benjamin E. Powell, Haoyi Wang, Maya Mitalipova, Dina A. Faddah, Jessica Reddy, Zi Peng Fan, Dorothea Maetzel, Kibibi Ganz, Linyu Shi, Tenzin Lungjangwa, Sumeth Imsoonthornruksa, Yonatan Stelzer, Sudharshan Rangarajan, Ana D’Alessio, Jianming Zhang, Qing Gao, Meelad M. Dawlaty, Richard A. Young, Nathanael S. Gray, and Rudolf Jaenisch* *Correspondence: jaenisch@wi.mit.edu http://dx.doi.org/10.1016/j.stem.2014.08.002