Identification of Embryonic Neural Plate Border Stem Cells and Their Generation by Direct Reprogramming from Adult Human Blood Cells.

We report the direct reprogramming of both adult human fibroblasts and blood cells into induced neural plate border stem cells (iNBSCs) by ectopic expression of four neural transcription factors. Self-renewing, clonal iNBSCs can be robustly expanded in defined media while retaining multilineage differentiation potential. They generate functional cell types of neural crest and CNS lineages and could be used to model a human pain syndrome via gene editing of SCN9A in iNBSCs. NBSCs can also be derived from human pluripotent stem cells and share functional and molecular features with NBSCs isolated from embryonic day 8.5 (E8.5) mouse neural folds. Single-cell RNA sequencing identified the anterior hindbrain as the origin of mouse NBSCs, with human iNBSCs sharing a similar regional identity. In summary, we identify embryonic NBSCs and report their generation by direct reprogramming in human, which may facilitate insights into neural development and provide a neural stem cell source for applications in regenerative medicine.

Myc Depletion Induces a Pluripotent Dormant State Mimicking Diapause.

Mouse embryonic stem cells (ESCs) are maintained in a naive ground state of pluripotency in the presence of MEK and GSK3 inhibitors. Here, we show that ground-state ESCs express low Myc levels. Deletion of both c-myc and N-myc (dKO) or pharmacological inhibition of Myc activity strongly decreases transcription, splicing, and protein synthesis, leading to proliferation arrest. This process is reversible and occurs without affecting pluripotency, suggesting that Myc-depleted stem cells enter a state of dormancy similar to embryonic diapause. Indeed, c-Myc is depleted in diapaused blastocysts, and the differential expression signatures of dKO ESCs and diapaused epiblasts are remarkably similar. Following Myc inhibition, pre-implantation blastocysts enter biosynthetic dormancy but can progress through their normal developmental program after transfer into pseudo-pregnant recipients. Our study shows that Myc controls the biosynthetic machinery of stem cells without affecting their potency, thus regulating their entry and exit from the dormant state.

Nanog induces suppression of senescence through downregulation of p27KIP1 expression.

A comprehensive analysis of the molecular network of cellular factors establishing and maintaining pluripotency as well as self renewal of pluripotent stem cells is key for further progress in understanding basic stem cell biology. Nanog is necessary for the natural induction of pluripotency in early mammalian development but dispensable for both its maintenance and its artificial induction. To gain further insight into the molecular activity of Nanog, we analyzed the outcomes of Nanog gain-of-function in various cell models employing a recently developed biologically active recombinant cell-permeant protein, Nanog-TAT. We found that Nanog enhances the proliferation of both NIH 3T3 and primary fibroblast cells. Nanog transduction into primary fibroblasts results in suppression of senescence-associated ß-galactosidase activity. Investigation of cell cycle factors revealed that transient activation of Nanog correlates with consistent downregulation of the cell cycle inhibitor p27(KIP1) (also known as CDKN1B). By performing chromatin immunoprecipitation analysis, we confirmed bona fide Nanog-binding sites upstream of the p27(KIP1) gene, establishing a direct link between physical occupancy and functional regulation. Our data demonstrates that Nanog enhances proliferation of fibroblasts through transcriptional regulation of cell cycle inhibitor p27 gene.

Partial Dedifferentiation of Murine Radial Glia-Type Neural Stem Cells by Brn2 and c-Myc Yields Early Neuroepithelial Progenitors.

Direct cell conversion developed into an important paradigm for generating cells with enhanced differentiation capability. We combined a transcription-factor-based cell fate conversion strategy with the use of pharmacological compounds to derive early neuroepithelial progenitor cells from developmentally more restricted radial glia-type neural stem cells. By combining the small molecules CHIR99021, Tranylcypromine, SB431542 and valproic acid with viral transduction of the transcription factor c-Myc and the POU domain transcription factor Brn2, we dedifferentiated radial glia-type neural stem cells into an early neuroepithelial progenitor cell state within 6 days. Reverse transcription PCR analyses showed a rapid down-regulation of the radial glia markers Olig2 and Vimentin during conversion, whereas the neuroepithelial markers Dach1 and Sox1 were fastly up-regulated. Furthermore, a switch from N-Cadherin to E-Cadherin indicates a mesenchymal-to-epithelial transition. The differentiation of cells converted by Brn2/c-Myc yielded smooth muscle actin- and Peripherin-positive cells in addition to the neuronal marker TUJ1 and cells that are positive for the glial marker GFAP. This differentiation potential suggests that the applied reprogramming strategy induced an early neuroepithelial cell population, which might resemble cells of the neural border or even more primitive neuroepithelial cells.

Derivation of Adult Human Fibroblasts and their Direct Conversion into Expandable Neural Progenitor Cells.

Generation of induced pluripotent stem cell (iPSCs) from adult skin fibroblasts and subsequent differentiation into somatic cells provides fascinating prospects for the derivation of autologous transplants that circumvent histocompatibility barriers. However, progression through a pluripotent state and subsequent complete differentiation into desired lineages remains a roadblock for the clinical translation of iPSC technology because of the associated neoplastic potential and genomic instability. Recently, we and others showed that somatic cells cannot only be converted into iPSCs but also into different types of multipotent somatic stem cells by using defined factors, thereby circumventing progression through the pluripotent state. In particular, the direct conversion of human fibroblasts into induced neural progenitor cells (iNPCs) heralds the possibility of a novel autologous cell source for various applications such as cell replacement, disease modeling and drug screening. Here, we describe the isolation of adult human primary fibroblasts by skin biopsy and their efficient direct conversion into iNPCs by timely restricted expression of Oct4, Sox2, Klf4, as well as c-Myc. Sox2-positive neuroepithelial colonies appear after 17 days of induction and iNPC lines can be established efficiently by monoclonal isolation and expansion. Precise adjustment of viral multiplicity of infection and supplementation of leukemia inhibitory factor during the induction phase represent critical factors to achieve conversion efficiencies of up to 0.2%. Thus far, patient-specific iNPC lines could be expanded for more than 12 passages and uniformly display morphological and molecular features of neural stem/progenitor cells, such as the expression of Nestin and Sox2. The iNPC lines can be differentiated into neurons and astrocytes as judged by staining against TUJ1 and GFAP, respectively. In conclusion, we report a robust protocol for the derivation and direct conversion of human fibroblasts into stably expandable neural progenitor cells that might provide a cellular source for biomedical applications such as autologous neural cell replacement and disease modeling.

MicroRNAs are indispensable for reprogramming mouse embryonic fibroblasts into induced stem cell-like cells.

MicroRNAs play a pivotal role in cellular maintenance, proliferation, and differentiation. They have also been implicated to play a key role in disease pathogenesis, and more recently, cellular reprogramming. Certain microRNA clusters can enhance or even directly induce reprogramming, while repressing key proteins involved in microRNA processing decreases reprogramming efficiency. Although microRNAs clearly play important roles in cellular reprogramming, it remains unknown whether microRNAs are absolutely necessary. We endeavored to answer this fundamental question by attempting to reprogram Dicer-null mouse embryonic fibroblasts (MEFs) that lack almost all functional microRNAs using a defined set of transcription factors. Transduction of reprogramming factors using either lentiviral or piggyBac transposon vector into two, independently derived lines of Dicer-null MEFs failed to produce cells resembling embryonic stem cells (ESCs). However, expression of human Dicer in the Dicer-null MEFs restored their reprogramming potential. Our study demonstrates for the first time that microRNAs are indispensable for dedifferentiation reprogramming.

Cellular reprogramming employing recombinant sox2 protein.

Induced pluripotent stem (iPS) cells represent an attractive option for the derivation of patient-specific pluripotent cells for cell replacement therapies as well as disease modeling. To become clinically meaningful, safe iPS cells need to be generated exhibiting no permanent genetic modifications that are caused by viral integrations of the reprogramming transgenes. Recently, various experimental strategies have been applied to accomplish transgene-free derivation of iPS cells, including the use of nonintegrating viruses, episomal expression, or excision of transgenes after reprogramming by site-specific recombinases or transposases. A straightforward approach to induce reprogramming factors is the direct delivery of either synthetic mRNA or biologically active proteins. We previously reported the generation of cell-permeant versions of Oct4 (Oct4-TAT) and Sox2 (Sox2-TAT) proteins and showed that Oct4-TAT is reprogramming-competent, that is, it can substitute for Oct4-encoding virus. Here, we explore conditions for enhanced Sox2-TAT protein stabilization and functional delivery into somatic cells. We show that cell-permeant Sox2 protein can be stabilized by lipid-rich albumin supplements in serum replacement or low-serum-supplemented media. Employing optimized conditions for protein delivery, we demonstrate that Sox2-TAT protein is able to substitute for viral Sox2. Sox2-piPS cells express pluripotency-associated markers and differentiate into all three germ layers.

Direct conversion of fibroblasts into stably expandable neural stem cells.

Recent advances have suggested that direct induction of neural stem cells (NSCs) could provide an alternative to derivation from somatic tissues or pluripotent cells. Here we show direct derivation of stably expandable NSCs from mouse fibroblasts through a curtailed version of reprogramming to pluripotency. By constitutively inducing Sox2, Klf4, and c-Myc while strictly limiting Oct4 activity to the initial phase of reprogramming, we generated neurosphere-like colonies that could be expanded for more than 50 passages and do not depend on sustained expression of the reprogramming factors. These induced neural stem cells (iNSCs) uniformly display morphological and molecular features of NSCs, such as the expression of Nestin, Pax6, and Olig2, and have a genome-wide transcriptional profile similar to that of brain-derived NSCs. Moreover, iNSCs can differentiate into neurons, astrocytes, and oligodendrocytes. Our results demonstrate that functional NSCs can be generated from somatic cells by factor-driven induction.

Automated selection and harvesting of pluripotent stem cell colonies.

The ability of pluripotent stem cells to differentiate into specialized cells of all three germ layers, their capability to self-renew, and their amenability to genetic modification provide fascinating prospects for the generation of cell lines for biomedical applications. Therefore, stem cells must increasingly suffice in terms of industrial standards, and automation of critical or time-consuming steps becomes a fundamental prerequisite for their routine application. Cumbersome manual picking of individual stem cell colonies still represents the most frequently used method for passaging or derivation of clonal stem cell lines. Here, we explore an automated harvesting system (CellCelector™) for detection, isolation, and propagation of human embryonic stem cells (hESCs) and murine induced pluripotent stem cells (iPSCs). Automatically transferred hESC colonies maintained their specific biological characteristics even after repeated passaging. We also selected and harvested primary iPSCs derived from mouse embryonic fibroblasts expressing the green fluorescent protein (GFP) under the control of the Oct4 promotor using either morphological criteria or GFP fluorescence. About 80% of the selected and harvested primary iPSC colonies gave rise to homogenously GFP-expressing iPSC lines. To validate the iPSC lines, we analyzed the expression of pluripotency-associated markers and multi-germ layer differentiation potential in vitro. Our data indicate that the CellCelector™ technology enables efficient identification and isolation of pluripotent stem cell colonies at the phase contrast or fluorescence level.

Exploring refined conditions for reprogramming cells by recombinant Oct4 protein.

The generation of human induced pluripotent stem (iPS) cells would represent an appealing option for the derivation of pluripotent patient-specific cells, as no embryos or oocytes are required. However, crucial safety issues have to be addressed in order to create human iPS cells that are clinically useful, as the classical iPS technique involves permanent genetic manipulation that may result in tumor formation. Various experimental strategies have been suggested to accomplish transgene-free derivation of iPS cells, including the use of non-integrating viruses, site specific recombinases to excise transgenes after reprogramming, or RNA transfection. Protein transduction, i.e. the direct delivery of biologically active proteins into cells, has been employed to generate iPS cells but has been found to have very low efficiency. In fact, success of protein transduction is limited by poor stability and solubility of recombinant factors, as well as their poor endosomal release. We recently reported the generation of cell-permeant versions of Oct4 and Sox2 and showed that both can be delivered intracellularly as biologically active proteins. Here we explore conditions for enhanced protein stabilization and delivery into somatic cells. Employing optimized conditions, we demonstrate that Oct4 protein delivery can substitute for Oct4 virus, yielding iPS derivation efficacy comparable to a four virus transduction protocol. The number of colonies is strictly dependent on the dose and duration of cell-permeant Oct4 exposure. We expect our transduction system to reach a thus far unattained level of control over reprogramming activity, turning it into a valuable tool for both the analysis of the reprogramming mechanism and the derivation of transgene-free iPS cells.

Deciphering the stem cell machinery as a basis for understanding the molecular mechanism underlying reprogramming.

Stem cells provide fascinating prospects for biomedical applications by combining the ability to renew themselves and to differentiate into specialized cell types. Since the first isolation of embryonic stem (ES) cells about 30 years ago, there has been a series of groundbreaking discoveries that have the potential to revolutionize modern life science. For a long time, embryos or germ cell-derived cells were thought to be the only source of pluripotency--a dogma that has been challenged during the last decade. Several findings revealed that cell differentiation from (stem) cells to mature cells is not in fact an irreversible process. The molecular mechanism underlying cellular reprogramming is poorly understood thus far. Identifying how pluripotency maintenance takes place in ES cells can help us to understand how pluripotency induction is regulated. Here, we review recent advances in the field of stem cell regulation focusing on key transcription factors and their functional interplay with non-coding RNAs.