Integrative Analyses of Human Reprogramming Reveal Dynamic Nature of Induced Pluripotency.

Induced pluripotency is a promising avenue for disease modeling and therapy, but the molecular principles underlying this process, particularly in human cells, remain poorly understood due to donor-to-donor variability and intercellular heterogeneity. Here, we constructed and characterized a clonal, inducible human reprogramming system that provides a reliable source of cells at any stage of the process. This system enabled integrative transcriptional and epigenomic analysis across the human reprogramming timeline at high resolution. We observed distinct waves of gene network activation, including the ordered re-activation of broad developmental regulators followed by early embryonic patterning genes and culminating in the emergence of a signature reminiscent of pre-implantation stages. Moreover, complementary functional analyses allowed us to identify and validate novel regulators of the reprogramming process. Altogether, this study sheds light on the molecular underpinnings of induced pluripotency in human cells and provides a robust cell platform for further studies. PAPERCLIP.

DHODH modulates transcriptional elongation in the neural crest and melanoma.

Melanoma is a tumour of transformed melanocytes, which are originally derived from the embryonic neural crest. It is unknown to what extent the programs that regulate neural crest development interact with mutations in the BRAF oncogene, which is the most commonly mutated gene in human melanoma. We have used zebrafish embryos to identify the initiating transcriptional events that occur on activation of human BRAF(V600E) (which encodes an amino acid substitution mutant of BRAF) in the neural crest lineage. Zebrafish embryos that are transgenic for mitfa:BRAF(V600E) and lack p53 (also known as tp53) have a gene signature that is enriched for markers of multipotent neural crest cells, and neural crest progenitors from these embryos fail to terminally differentiate. To determine whether these early transcriptional events are important for melanoma pathogenesis, we performed a chemical genetic screen to identify small-molecule suppressors of the neural crest lineage, which were then tested for their effects on melanoma. One class of compound, inhibitors of dihydroorotate dehydrogenase (DHODH), for example leflunomide, led to an almost complete abrogation of neural crest development in zebrafish and to a reduction in the self-renewal of mammalian neural crest stem cells. Leflunomide exerts these effects by inhibiting the transcriptional elongation of genes that are required for neural crest development and melanoma growth. When used alone or in combination with a specific inhibitor of the BRAF(V600E) oncogene, DHODH inhibition led to a marked decrease in melanoma growth both in vitro and in mouse xenograft studies. Taken together, these studies highlight developmental pathways in neural crest cells that have a direct bearing on melanoma formation.

The LIN28B/TGF-ß/TGFBI feedback loop promotes cell migration and tumour initiation potential in cholangiocarcinoma.

Cholangiocarcinoma (CCA), a lethal malignancy of the biliary epithelium, is the second most common primary liver cancer. The poor prognosis of CCA is due to the high rate of tumour invasion and distant metastasis. We found that the RNA-binding protein LIN28B, a known regulator of microRNA biogenesis, stem cell maintenance, and oncogenesis, is expressed in a subpopulation of CCA patients. To further investigate the potential role of LIN28B in CCA pathogenesis, we studied the effect of LIN28B overexpression in the cholangiocyte cell line MMNK-1 and cholangiocarcinoma cell lines HuCCT-1 and KKU-214. Here, we show that enhanced LIN28B expression promoted cancer stem cell-like properties in CCA, including enhanced cell migration, epithelial-to-mesenchymal transition (EMT), increased cell proliferation and spheroid formation. Proteomic analysis revealed TGF-ß-induced protein (TGFBI) as a novel LIN28B target gene, and further analysis showed upregulation of other components of the TGF-ß signalling pathway, including TGF-ß receptor type I (TGFBRI) expression and cytokine TGFB-I, II and III secretion. Importantly, the small molecule TGF-ß inhibitor SB431542 negated the effects of LIN28B on both cell migration and clonogenic potential. Overexpression of TGFBI alone promoted cholangiocarcinoma cell migration and EMT changes, but not spheroid formation, suggesting that TGFBI partially contributes to LIN28B-mediated aggressive cell behaviour. These observations are consistent with a model in which TGF-ß and LIN28B work together to form a positive feedback loop during cholangiocarcinoma metastasis and provide a therapeutic intervention opportunity.

Lin28 paralogs regulate lung branching morphogenesis.

The molecular mechanisms that govern the choreographed timing of organ development remain poorly understood. Our investigation of the role of the Lin28a and Lin28b paralogs during the developmental process of branching morphogenesis establishes that dysregulation of Lin28a/b leads to abnormal branching morphogenesis in the lung and other tissues. Additionally, we find that the Lin28 paralogs, which regulate post-transcriptional processing of both mRNAs and microRNAs (miRNAs), predominantly control mRNAs during the initial phases of lung organogenesis. Target mRNAs include Sox2, Sox9, and Etv5, which coordinate lung development and differentiation. Moreover, we find that functional interactions between Lin28a and Sox9 are capable of bypassing branching defects in Lin28a/b mutant lungs. Here, we identify Lin28a and Lin28b as regulators of early embryonic lung development, highlighting the importance of the timing of post-transcriptional regulation of both miRNAs and mRNAs at distinct stages of organogenesis.

Stem Cells in the Intestine: Possible Roles in Pathogenesis of Irritable Bowel Syndrome.

Irritable bowel syndrome is one of the most common functional gastrointestinal (GI) disorders that significantly impair quality of life in patients. Current available treatments are still not effective and the pathophysiology of this condition remains unclearly defined. Recently, research on intestinal stem cells has greatly advanced our understanding of various GI disorders. Alterations in conserved stem cell regulatory pathways such as Notch, Wnt, and bone morphogenic protein/TGF- ß have been well documented in diseases such as inflammatory bowel diseases and cancer. Interaction between intestinal stem cells and various signals from their environment is important for the control of stem cell self-renewal, regulation of number and function of specific intestinal cell types, and maintenance of the mucosal barrier. Besides their roles in stem cell regulation, these signals are also known to have potent effects on immune cells, enteric nervous system and secretory cells in the gut, and may be responsible for various aspects of pathogenesis of functional GI disorders, including visceral hypersensitivity, altered gut motility and low grade gut inflammation. In this article, we briefly summarize the components of these signaling pathways, how they can be modified by extrinsic factors and novel treatments, and provide evidenced support of their roles in the inflammation processes. Furthermore, we propose how changes in these signals may contribute to the symptom development and pathogenesis of irritable bowel syndrome.

LIN28 Regulates Stem Cell Metabolism and Conversion to Primed Pluripotency.

The RNA-binding proteins LIN28A and LIN28B play critical roles in embryonic development, tumorigenesis, and pluripotency, but their exact functions are poorly understood. Here, we show that, like LIN28A, LIN28B can function effectively with NANOG, OCT4, and SOX2 in reprogramming to pluripotency and that reactivation of both endogenous LIN28A and LIN28B loci are required for maximal reprogramming efficiency. In human fibroblasts, LIN28B is activated early during reprogramming, while LIN28A is activated later during the transition to bona fide induced pluripotent stem cells (iPSCs). In murine cells, LIN28A and LIN28B facilitate conversion from naive to primed pluripotency. Proteomic and metabolomic analysis highlighted roles for LIN28 in maintaining the low mitochondrial function associated with primed pluripotency and in regulating one-carbon metabolism, nucleotide metabolism, and histone methylation. LIN28 binds to mRNAs of proteins important for oxidative phosphorylation and modulates protein abundance. Thus, LIN28A and LIN28B play cooperative roles in regulating reprogramming, naive/primed pluripotency, and stem cell metabolism.

The epithelial-mesenchymal transition factor SNAIL paradoxically enhances reprogramming.

Reprogramming of fibroblasts to induced pluripotent stem cells (iPSCs) entails a mesenchymal to epithelial transition (MET). While attempting to dissect the mechanism of MET during reprogramming, we observed that knockdown (KD) of the epithelial-to-mesenchymal transition (EMT) factor SNAI1 (SNAIL) paradoxically reduced, while overexpression enhanced, reprogramming efficiency in human cells and in mouse cells, depending on strain. We observed nuclear localization of SNAI1 at an early stage of fibroblast reprogramming and using mouse fibroblasts expressing a knockin SNAI1-YFP reporter found cells expressing SNAI1 reprogrammed at higher efficiency. We further demonstrated that SNAI1 binds the let-7 promoter, which may play a role in reduced expression of let-7 microRNAs, enforced expression of which, early in the reprogramming process, compromises efficiency. Our data reveal an unexpected role for the EMT factor SNAI1 in reprogramming somatic cells to pluripotency.

Influence of threonine metabolism on S-adenosylmethionine and histone methylation.

Threonine is the only amino acid critically required for the pluripotency of mouse embryonic stem cells (mESCs), but the detailed mechanism remains unclear. We found that threonine and S-adenosylmethionine (SAM) metabolism are coupled in pluripotent stem cells, resulting in regulation of histone methylation. Isotope labeling of mESCs revealed that threonine provides a substantial fraction of both the cellular glycine and the acetyl-coenzyme A (CoA) needed for SAM synthesis. Depletion of threonine from the culture medium or threonine dehydrogenase (Tdh) from mESCs decreased accumulation of SAM and decreased trimethylation of histone H3 lysine 4 (H3K4me3), leading to slowed growth and increased differentiation. Thus, abundance of SAM appears to influence H3K4me3, providing a possible mechanism by which modulation of a metabolic pathway might influence stem cell fate.

Live-cell immunofluorescence staining of human pluripotent stem cells.

Antibodies are instrumental tools in stem cell identification, purification, and analysis. Most commonly, cell samples are either dissociated to obtain a single-cell suspension suitable for FACS analysis or cell sorting, or fixed in situ for immunostaining and fluorescence microscopy imaging. This unit describes an alternative method in which live adherent cells are stained and imaged in situ without the need for cell dissociation, fixation, or fluorescent reporter genes. This minimally invasive method is particularly useful for identification and distinction of fully and partially reprogrammed induced pluripotent stem cells (iPSCs). The unit also describes the use of mCD49e and hCD29 antibodies in live-cell (vital) imaging. mCD49e strongly stains mouse embryonic fibroblast (MEF) feeder cells in human pluripotent stem cell cultures, whereas hCD29 recognizes an antigen expressed on undifferentiated and many differentiated cells. A distinguishing feature of hCD29 in live-cell staining is that its antigen is precluded from detection wherever cells have formed tight epithelial junctions (e.g., in the center but not the periphery of pluripotent stem cell colonies) due to basolateral location. A non-fluorescent fixed-cell staining protocol is also provided for medium- to high-throughput quantification of stem cell experiments without an automated microscope. The discussion addresses technical limitations, pitfalls, troubleshooting, and potential applications, such as identification of emerging bona fide human iPSC colonies in reprogramming experiments.

Live cell imaging distinguishes bona fide human iPS cells from partially reprogrammed cells.

Somatic cells can be reprogrammed into induced pluripotent stem (iPS) cells by enforced expression of transcription factors. Using serial live imaging of human fibroblasts undergoing reprogramming, we identified distinct colony types that morphologically resemble embryonic stem (ES) cells yet differ in molecular phenotype and differentiation potential. By analyzing expression of pluripotency markers, methylation at the OCT4 and NANOG promoters and differentiation into teratomas, we determined that only one colony type represents true iPS cells, whereas the others represent reprogramming intermediates. Proviral silencing and expression of TRA-1-60, DNMT3B and REX1 can be used to distinguish the fully reprogrammed state, whereas alkaline phosphatase, SSEA-4, GDF3, hTERT and NANOG are insufficient as markers. We also show that reprogramming using chemically defined medium favors formation of fully reprogrammed over partially reprogrammed colonies. Our data define molecular markers of the fully reprogrammed state and highlight the need for rigorous characterization and standardization of putative iPS cells.