Project description:In the context of most induced pluripotent stem (iPS) cell reprogramming methods, heterogeneous populations of nonproductive and staggered productive intermediates arise at different reprogramming time points1-11. Despite recent reports claiming substantially increased reprogramming efficiencies using genetically modified donor cells12,13 prospectively isolating distinct reprogramming intermediates remains an important goal to decipher reprogramming mechanisms. Previous attempts to identify surface markers of intermediate cell populations were based on the assumption that during reprogramming the cells progressively lose donor cell identity and gradually acquire iPS cell properties1,2,7,8,10. Here, we report that iPS cell and epithelial markers, such as SSEA1 and EpCAM, respectively, are not predictive of reprogramming during early phases. Instead, in a systematic functional surface marker screen we find that early reprogramming-prone cells express a unique set of surface markers, including CD73, CD49d and CD200 that are absent in fibroblasts and iPS cells. Single cell mass cytometry and prospective isolation show that these distinct intermediates are transient and bridge the gap between donor cell silencing and pluripotency marker acquisition during the early, presumably stochastic reprogramming phase2. Expression profiling revealed that the transcriptional regulators Nr0b1 and Etv5 are specifically expressed in this early reprogramming state, preceding activation of key pluripotency regulators such as Rex1, Dppa2, Nanog and Sox2. Both factors are required for the generation of the early intermediate state and fully reprogrammed iPS cells, and thus mark some of the earliest known regulators of iPS cell induction. Our study shows an ordered sequence of transitions during the earliest steps of iPS cell reprogramming that deconvolutes the first steps in a hierarchical series of events that lead to pluripotency acquisition. Samples for poised (CD73+ or CD49d+) and non-poised (CD73-) reprogramming samples were FACS sorted 6 and 9 days after induction of Klf4, Oct4, Sox2 and cMyc in Rosa-rtTA +/- mouse embryonic fibroblasts (MEFs). 'Total' populations are expression analyses for unsorted populations analyzed at the same time points. Control populations were also sampled: mouse embryonic fibroblasts (MEFs), partially reprogrammed cells (SC4) and mouse embryonic stem cell (ESC).

Project description:Factor induced reprogramming is a slow and inefficient process with only rare cells progressing towards induced pluripotent stem cells (iPSCs). Owing to these restraints, mechanistic studies have been limited to analyses of heterogeneous bulk populations undergoing reprogramming and partially reprogrammed cell lines. Here, by combining surface markers (Thy1, SSEA1) and an Oct4-GFP fluorescent reporter allele, we analyzed defined intermediate cell populations poised to becoming iPSCs at the transcriptional and epigenetic levels using genome-wide and single cell technologies. We found that factor-induced reprogramming elicits two discernible transcriptional waves that are characterized by the initial extinction of the somatic gene expression program and the concomitant acquisition of an ESC-like proliferative and metabolic state, followed by the activation of an embryonic pluripotent state primed for differentiation. The first wave is mostly driven by gene activation through c-Myc and gene repression by Klf4, whereas the second wave is a result of gradually activated Oct4/Sox2 targets in cooperation with Klf4 targets and other downstream regulators. While microRNA expression and enrichment for individual histone modifications (H3K4me3 or H3K27me3 enriched promoters) mirrored the observed biphasic transcriptional pattern, the establishment of bivalent domains (H3K4me3/H3K27me3 enriched promoters) occurred more gradually. In contrast, changes in DNA methylation took place predominantly at the end of reprogramming when cells assumed a stable pluripotent state. Cells that became refractory to reprogramming activated the first but failed to initiate the second transcriptional wave. However, introduction of additional copies of the reprogramming transgenes into these cells rescued their ability to form iPSCs, indicating that suboptimal transcription factor levels are a limiting factor for efficient iPSC formation. This integrative analysis allowed us to identify novel genes and microRNAs that enhance reprogramming and surface markers that further subdivide intermediate cell populations. Collectively, our data offer new mechanistic insights into the nature and sequence of molecular events inherent to cellular reprogramming and provide a valuable resource of molecules that may act as roadblocks during iPSC formation. Time series design with samples sorted into subpopulations according to surface markers. 18 Samples total

Project description:Factor induced reprogramming is a slow and inefficient process with only rare cells progressing towards induced pluripotent stem cells (iPSCs). Owing to these restraints, mechanistic studies have been limited to analyses of heterogeneous bulk populations undergoing reprogramming and partially reprogrammed cell lines. Here, by combining surface markers (Thy1, SSEA1) and an Oct4-GFP fluorescent reporter allele, we analyzed defined intermediate cell populations poised to becoming iPSCs at the transcriptional and epigenetic levels using genome-wide and single cell technologies. We found that factor-induced reprogramming elicits two discernible transcriptional waves that are characterized by the initial extinction of the somatic gene expression program and the concomitant acquisition of an ESC-like proliferative and metabolic state, followed by the activation of an embryonic pluripotent state primed for differentiation. The first wave is mostly driven by gene activation through c-Myc and gene repression by Klf4, whereas the second wave is a result of gradually activated Oct4/Sox2 targets in cooperation with Klf4 targets and other downstream regulators. While microRNA expression and enrichment for individual histone modifications (H3K4me3 or H3K27me3 enriched promoters) mirrored the observed biphasic transcriptional pattern, the establishment of bivalent domains (H3K4me3/H3K27me3 enriched promoters) occurred more gradually. In contrast, changes in DNA methylation took place predominantly at the end of reprogramming when cells assumed a stable pluripotent state. Cells that became refractory to reprogramming activated the first but failed to initiate the second transcriptional wave. However, introduction of additional copies of the reprogramming transgenes into these cells rescued their ability to form iPSCs, indicating that suboptimal transcription factor levels are a limiting factor for efficient iPSC formation. This integrative analysis allowed us to identify novel genes and microRNAs that enhance reprogramming and surface markers that further subdivide intermediate cell populations. Collectively, our data offer new mechanistic insights into the nature and sequence of molecular events inherent to cellular reprogramming and provide a valuable resource of molecules that may act as roadblocks during iPSC formation. Time series design with samples sorted into subpopulations according to surface markers.

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Single cell RNA-sequencing of induced Neural Plate Border Stem Cells

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