Retroviral vector silencing during iPS cell induction: an epigenetic beacon that signals distinct pluripotent states

Modeling inherited retinal diseases using human induced pluripotent stem cell derived photoreceptor cells and retinal pigment epithelial cells

Since the discovery of induced pluripotent stem cell (iPSC) technology, there have been many attempts to create cellular models of inherited retinal diseases (IRDs) for investigation of pathogenic processes to facilitate target discovery and validation activities. Consistency remains key in determining the utility of these findings. Despite the importance of consistency, quality control metrics are still not widely used. In this review, a toolkit for harnessing iPSC technology to generate photoreceptor, retinal pigment epithelial cell, and organoid disease models is provided. Considerations while developing iPSC-derived IRD models such as iPSC origin, reprogramming methods, quality control metrics, control strategies, and differentiation protocols are discussed. Various iPSC IRD models are dissected and the scientific hurdles of iPSC-based disease modeling are discussed to provide an overview of current methods and future directions in this field.

Role of small molecules as drug candidates for reprogramming somatic cells into induced pluripotent stem cells: A comprehensive review

With the use of specific genetic factors and recent developments in cellular reprogramming, it is now possible to generate lineage-committed cells or induced pluripotent stem cells (iPSCs) from readily available and common somatic cell types. However, there are still significant doubts regarding the safety and effectiveness of the current genetic methods for reprogramming cells, as well as the conventional culture methods for maintaining stem cells. Small molecules that target specific epigenetic processes, signaling pathways, and other cellular processes can be used as a complementary approach to manipulate cell fate to achieve a desired objective. It has been discovered that a growing number of small molecules can support lineage differentiation, maintain stem cell self-renewal potential, and facilitate reprogramming by either increasing the efficiency of reprogramming or acting as a genetic reprogramming factor substitute. However, ongoing challenges include improving reprogramming efficiency, ensuring the safety of small molecules, and addressing issues with incomplete epigenetic resetting. Small molecule iPSCs have significant clinical applications in regenerative medicine and personalized therapies. This review emphasizes the versatility and potential safety benefits of small molecules in overcoming challenges associated with the iPSCs reprogramming process.

Advances in Genetic Reprogramming: Prospects from Developmental Biology to Regenerative Medicine

The foundations of cell reprogramming were laid by Yamanaka and co-workers, who showed that somatic cells can be reprogrammed into pluripotent cells (induced pluripotency). Since this discovery, the field of regenerative medicine has seen advancements. For example, because they can differentiate into multiple cell types, pluripotent stem cells are considered vital components in regenerative medicine aimed at the functional restoration of damaged tissue. Despite years of research, both replacement and restoration of failed organs/ tissues have remained elusive scientific feats. However, with the inception of cell engineering and nuclear reprogramming, useful solutions have been identified to counter the need for compatible and sustainable organs. By combining the science underlying genetic engineering and nuclear reprogramming with regenerative medicine, scientists have engineered cells to make gene and stem cell therapies applicable and effective. These approaches have enabled the targeting of various pathways to reprogramme cells, i.e., make them behave in beneficial ways in a patient-specific manner. Technological advancements have clearly supported the concept and realization of regenerative medicine. Genetic engineering is used for tissue engineering and nuclear reprogramming and has led to advances in regenerative medicine. Targeted therapies and replacement of traumatized , damaged, or aged organs can be realized through genetic engineering. Furthermore, the success of these therapies has been validated through thousands of clinical trials. Scientists are currently evaluating induced tissue-specific stem cells (iTSCs), which may lead to tumour-free applications of pluripotency induction. In this review, we present state-of-the-art genetic engineering that has been used in regenerative medicine. We also focus on ways that genetic engineering and nuclear reprogramming have transformed regenerative medicine and have become unique therapeutic niches.

Hiding from allogeneic NK cells and macrophages by a synthetic receptor

Immune attack by natural killer (NK) cells is a major hurdle for allogeneic off-the-shelf cell therapy, especially when HLA molecules are removed. Gravina et al.1 utilized a membrane-anchored single-chain antibody (scFv) as a synthetic receptor, named "synthetic immune checkpoint engager," to prevent attack from NK cells and macrophages.

CRISPR/Cas System and Factors Affecting Its Precision and Efficiency

The diverse applications of genetically modified cells and organisms require more precise and efficient genome-editing tool such as clustered regularly interspaced short palindromic repeats/CRISPR-associated protein (CRISPR/Cas). The CRISPR/Cas system was originally discovered in bacteria as a part of adaptive-immune system with multiple types. Its engineered versions involve multiple host DNA-repair pathways in order to perform genome editing in host cells. However, it is still challenging to get maximum genome-editing efficiency with fewer or no off-targets. Here, we focused on factors affecting the genome-editing efficiency and precision of CRISPR/Cas system along with its defense-mechanism, orthologues, and applications.

Exogenous LIN28 Is Required for the Maintenance of Self-Renewal and Pluripotency in Presumptive Porcine-Induced Pluripotent Stem Cells

Porcine species have been used in preclinical transplantation models for assessing the efficiency and safety of transplants before their application in human trials. Porcine-induced pluripotent stem cells (piPSCs) are traditionally established using four transcription factors (4TF): OCT4, SOX2, KLF4, and C-MYC. However, the inefficiencies in the reprogramming of piPSCs and the maintenance of their self-renewal and pluripotency remain challenges to be resolved. LIN28 was demonstrated to play a vital role in the induction of pluripotency in humans. To investigate whether this factor is similarly required by piPSCs, the effects of adding LIN28 to the 4TF induction method (5F approach) on the efficiency of piPSC reprogramming and maintenance of self-renewal and pluripotency were examined. Using a retroviral vector, porcine fetal fibroblasts were transfected with human OCT4, SOX2, KLF4, and C-MYC with or without LIN28. The colony morphology and chromosomal stability of these piPSC lines were examined and their pluripotency properties were characterized by investigating both their expression of pluripotency-associated genes and proteins and in vitro and in vivo differentiation capabilities. Alkaline phosphatase assay revealed the reprogramming efficiencies to be 0.33 and 0.17% for the 4TF and 5TF approaches, respectively, but the maintenance of self-renewal and pluripotency until passage 40 was 6.67 and 100%, respectively. Most of the 4TF-piPSC colonies were flat in shape, showed weak positivity for alkaline phosphatase, and expressed a significantly high level of SSEA-4 protein, except for one cell line (VSMUi001-A) whose properties were similar to those of the 5TF-piPSCs; that is, tightly packed and dome-like in shape, markedly positive for alkaline phosphatase, and expressing endogenous pluripotency genes (pOCT4, pSOX2, pNANOG, and pLIN28), significantly high levels of pluripotent proteins (OCT4, SOX2, NANOG, LIN28, and SSEA-1), and a significantly low level of SSEA-4 protein. VSMUi001-A and all 5F-piPSC lines formed embryoid bodies, underwent spontaneous cardiogenic differentiation with cardiac beating, expressed cardiomyocyte markers, and developed teratomas. In conclusion, in addition to the 4TF, LIN28 is required for the effective induction of piPSCs and the maintenance of their long-term self-renewal and pluripotency toward the development of all germ layers. These piPSCs have the potential applicability for veterinary science.

Sox2 modulation increases naïve pluripotency plasticity

Induced pluripotency provides a tool to explore mechanisms underlying establishment, maintenance, and differentiation of naive pluripotent stem cells (nPSCs). Here, we report that self-renewal of nPSCs requires minimal Sox2 expression (Sox2-low). Sox2-low nPSCs do not show impaired neuroectoderm specification and differentiate efficiently in vitro into all embryonic germ lineages. Strikingly, upon the removal of self-renewing cues Sox2-low nPSCs differentiate into both embryonic and extraembryonic cell fates in vitro and in vivo. This differs from previous studies which only identified conditions that allowed cells to differentiate to one fate or the other. At the single-cell level self-renewing Sox2-low nPSCs exhibit a naive molecular signature. However, they display a nearer trophoblast identity than controls and decreased ability of Oct4 to bind naïve-associated regulatory sequences. In sum, this work defines wild-type levels of Sox2 as a restrictor of developmental potential and suggests perturbation of naive network as a mechanism to increase cell plasticity.

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Low Sox2 expression is sufficient for naive pluripotent stem cell self-renewal

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Low Sox2 expression does not impair neurectoderm differentiation in vitro

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Low Sox2 expression impairs Oct4 genomic occupancy

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Low Sox2 increases naive pluripotent stem cell plasticity in vitro and in vivo

From Hair to iPSCs-A Guide on How to Reprogram Keratinocytes and Why

Keratinocytes, as a primary somatic cell source, offer exceptional advantages compared to fibroblasts, which are commonly used for reprogramming. Keratinocytes can beat fibroblasts in reprogramming efficiency and reprogramming time and, in addition, can be easily and non-invasively harvested from human hair roots. However, there is still much to know about acquiring keratinocytes and maintaining them in cell culture. In this article, we want to offer readers the profound knowledge that we have gained since our initial use of keratinocytes for reprogramming more than 10 years ago. Here, all hints and tricks, from plucking the hair roots to growing and maintaining keratinocytes, are described in detail. Additionally, an overview of the currently used reprogramming methods, viral and non-viral, is included, with a special focus on their applicability to keratinocytes. This overview is intended to provide a brief but comprehensive insight into the field of keratinocytes and their use for reprogramming into induced pluripotent stem cells (iPSCs). © 2020 The Authors.

Reprogramming and transdifferentiation - two key processes for regenerative medicine

Regenerative medicine based on transplants obtained from donors or foetal and new-born mesenchymal stem cells, encounter important obstacles such as limited availability of organs, ethical issues and immune rejection. The growing demand for therapeutic methods for patients being treated after serious accidents, severe organ dysfunction and an increasing number of cancer surgeries, exceeds the possibilities of the therapies that are currently available. Reprogramming and transdifferentiation provide powerful bioengineering tools. Both procedures are based on the somatic differentiated cells, which are easily and unlimitedly available, like for example: fibroblasts. During the reprogramming procedure mature cells are converted into pluripotent cells - which are capable to differentiate into almost any kind of desired cells. Transdifferentiation directly converts differentiated cells of one type into another differentiated cells type. Both procedures allow to obtained patient's dedicated cells for therapeutic purpose in regenerative medicine. In combination with biomaterials, it is possible to obtain even whole anatomical structures. Those patient's dedicated structures may serve for them upon serious accidents with massive tissue damage but also upon cancer surgeries as a replacement of damaged organ. Detailed information about reprogramming and transdifferentiation procedures as well as the current state of the art are presented in our review.

Efficient and Reproducible Multigene Expression after Single-Step Transfection Using Improved BAC Transgenesis and Engineering Toolkit

Achieving stable expression of a single transgene in mammalian cells remains challenging; even more challenging is obtaining simultaneous stable expression of multiple transgenes at reproducible, relative expression levels. Previously, we attained copy-number-dependent, chromosome-position-independent expression of reporter minigenes by embedding them within a BAC "scaffold" containing the mouse Msh3-Dhfr locus (DHFR BAC). Here, we extend this "BAC TG-EMBED" approach. First, we report a toolkit of endogenous promoters capable of driving transgene expression over a 0.01- to 5-fold expression range relative to the CMV promoter, allowing fine-tuning of relative expression levels of multiple reporter genes. Second, we demonstrate little variation in expression level and long-term expression stability of a reporter gene embedded in BACs containing either transcriptionally active or inactive genomic regions, making the choice of BAC scaffolds more flexible. Third, we present a novel BAC assembly scheme, "BAC-MAGIC", for inserting multiple transgenes into BAC scaffolds, which is much more time-efficient than traditional galK-based methods. As a proof-of-principle for our improved BAC TG-EMBED toolkit, we simultaneously fluorescently labeled three nuclear compartments at reproducible, relative intensity levels in 94% of stable clones after a single transfection using a DHFR BAC scaffold containing 4 transgenes assembled with BAC-MAGIC. Our extended BAC TG-EMBED toolkit and BAC-MAGIC method provide an efficient, versatile platform for stable simultaneous expression of multiple transgenes at reproducible, relative levels.

Efficient Generation and Correction of Mutations in Human iPS Cells Utilizing mRNAs of CRISPR Base Editors and Prime Editors

In contrast to CRISPR/Cas9 nucleases, CRISPR base editors (BE) and prime editors (PE) enable predefined nucleotide exchanges in genomic sequences without generating DNA double strand breaks. Here, we employed BE and PE mRNAs in conjunction with chemically synthesized sgRNAs and pegRNAs for efficient editing of human induced pluripotent stem cells (iPSC). Whereas we were unable to correct a disease-causing mutation in patient derived iPSCs using a CRISPR/Cas9 nuclease approach, we corrected the mutation back to wild type with high efficiency utilizing an adenine BE. We also used adenine and cytosine BEs to introduce nine different cancer associated TP53 mutations into human iPSCs with up to 90% efficiency, generating a panel of cell lines to investigate the biology of these mutations in an isogenic background. Finally, we pioneered the use of prime editing in human iPSCs, opening this important cell type for the precise modification of nucleotides not addressable by BEs and to multiple nucleotide exchanges. These approaches eliminate the necessity of deriving disease specific iPSCs from human donors and allows the comparison of different disease-causing mutations in isogenic genetic backgrounds.

Efficient Generation and Correction of Mutations in Human iPS Cells Utilizing mRNAs of CRISPR Base Editors and Prime Editors

In contrast to CRISPR/Cas9 nucleases, CRISPR base editors (BE) and prime editors (PE) enable predefined nucleotide exchanges in genomic sequences without generating DNA double strand breaks. Here, we employed BE and PE mRNAs in conjunction with chemically synthesized sgRNAs and pegRNAs for efficient editing of human induced pluripotent stem cells (iPSC). Whereas we were unable to correct a disease-causing mutation in patient derived iPSCs using a CRISPR/Cas9 nuclease approach, we corrected the mutation back to wild type with high efficiency utilizing an adenine BE. We also used adenine and cytosine BEs to introduce nine different cancer associated TP53 mutations into human iPSCs with up to 90% efficiency, generating a panel of cell lines to investigate the biology of these mutations in an isogenic background. Finally, we pioneered the use of prime editing in human iPSCs, opening this important cell type for the precise modification of nucleotides not addressable by BEs and to multiple nucleotide exchanges. These approaches eliminate the necessity of deriving disease specific iPSCs from human donors and allows the comparison of different disease-causing mutations in isogenic genetic backgrounds.

Successful Derivation of an Induced Pluripotent Stem Cell Line from a Genetically Nonpermissive Enhanced Green Fluorescent Protein-Transgenic FVB/N Mouse Strain

The embryonic stem cell line derivation from nonpermissive mouse strains is a challenging and highly inefficient process. The cellular reprogramming strategy provides an alternative route for generating pluripotent stem cell (PSC) lines from such strains. In this study, we successfully derived an enhanced green fluorescent protein (EGFP)-transgenic "N9" induced pluripotent stem cell (iPS cell, iPSC) line from the FVB/N strain-derived mouse embryonic fibroblasts (MEFs). The exposure of MEFs to human OCT4, SOX2, KLF4, and c-MYC (OSKM) transgenes via lentiviral transduction resulted in complete reprogramming. The N9 iPS cell line demonstrated all the criteria of a typical mouse PSC line, including normal colony morphology and karyotype (40,XY), high replication and propagation efficiencies, expression of the pluripotency-associated genes, spontaneous differentiation to three germ lineage-derived cell types, and robust potential of chimeric blastocyst formation. Taken together, using human OSKM genes for transduction, we report, for the first time, the successful derivation of an EGFP-expressing iPS cell line from a genetically nonpermissive transgenic FVB/N mouse. This cell line could provide opportunities for designing protocols for efficient derivation of PSC lines from other nonpermissive strains and developing mouse models of human diseases.

Inducing Pluripotency in the Domestic Cat (Felis catus)

Domestic cats suffer from a range of inherited genetic diseases, many of which display similarities with equivalent human conditions. Developing cellular models for these inherited diseases would enable drug discovery, benefiting feline health and welfare as well as enhancing the potential of cats as relevant animal models for translation to human medicine. Advances in our understanding of these diseases at the cellular level have come from the use of induced pluripotent stem cells (iPSCs). iPSCs can differentiate into virtually any cell type and can be derived from adult somatic cells, therefore overcoming the ethical implications of destroying embryos to obtain embryonic stem cells. No studies, however, report the generation of iPSCs from domestic cats [feline iPSCs (fiPSCs)]. Feline adipose-derived fibroblasts were infected with amphotropic retrovirus containing the coding sequences for human Oct4, Sox2, Klf4, cMyc, and Nanog. Isolated iPSC clones were expanded on inactivated mouse embryonic fibroblasts in the presence of feline leukemia inhibitory factor (fLIF). Retroviral delivery of human pluripotent genes gave rise to putative fiPSC colonies within 5-7 days. These iPS-like cells required fetal bovine serum and fLIF for maintenance. Colonies were domed with refractile edges, similar to mouse iPSCs. Immunocytochemistry demonstrated positive staining for stem cell markers: alkaline phosphatase, Oct4, Sox2, Nanog, and SSEA1. Cells were negative for SSEA4. Expression of endogenous feline Nanog was confirmed by quantitative polymerase chain reaction. The cells were able to differentiate in vitro into cells representative of the three germ layers. These results confirm the first generation of induced pluripotent stem cells from domestic cats. These cells will provide valuable models to study genetic diseases and explore novel therapeutic strategies.

Exploring induced pluripotency in human fibroblasts via construction, validation, and application of a gene regulatory network

Reprogramming of somatic cells to induced pluripotent stem cells, by overexpressing certain factors referred to as the reprogramming factors, can revolutionize regenerative medicine. To provide a coherent description of induced pluripotency from the gene regulation perspective, we use 35 microarray datasets to construct a reprogramming gene regulatory network. Comprising 276 nodes and 4471 links, the resulting network is, to the best of our knowledge, the largest gene regulatory network constructed for human fibroblast reprogramming and it is the only one built using a large number of experimental datasets. To build the network, a model that relates the expression profiles of the initial (fibroblast) and final (induced pluripotent stem cell) states is proposed and the model parameters (link strengths) are fitted using the experimental data. Twenty nine additional experimental datasets are collectively used to test the model/network, and good agreement between experimental and predicted gene expression profiles is found. We show that the model in conjunction with the constructed network can make useful predictions. For example, we demonstrate that our approach can incorporate the effect of reprogramming factor stoichiometry and that its predictions are consistent with the experimentally observed trends in reprogramming efficiency when the stoichiometric ratios vary. Using our model/network, we also suggest new (not used in training of the model) candidate sets of reprogramming factors, many of which have already been experimentally verified. These results suggest our model/network can potentially be used in devising new recipes for induced pluripotency with higher efficiencies. Additionally, we classify the links of the network into three classes of different importance, prioritizing them for experimental verification. We show that many of the links in the top ranked class are experimentally known to be important in reprogramming. Finally, comparing with other methods, we show that using our model is advantageous.

Nanoinjection system for precise direct delivery of biomolecules into single cells

Intracellular delivery of functional molecules such as proteins, transcription factors and DNA is effective and promising in cell biology. However, existing transfection methods are often unsuitable to deliver big molecules into cells or require carriers such as viruses and peptides specific to the target molecules. In addition, the nature of bulk processing does not generally provide accurate dose control of individual cells. The concept of single-cell-based material injection based on electrokinetic pumping through nanocapillaries could overcome these problems, yet the fabrication and operation of nanoscale 3-dimensional structures have remained unsolved. In this research, a hybrid (PDMS/glass) microfluidic chip with a true 3-dimensional nanoinjection structure (called "nanoinjection system") is presented. The nanoinjection structure was fabricated by femtosecond-laser (fs-laser) ablation in a single solid glass, which showed very successful delivery of red fluorescent protein (RFP) and expression of plasmid DNA in several different types of cells. This system is promising in that the amount of molecules to be delivered is controllable and the processed cells are systematically separated into a harvesting chamber, which can radically improve the purity of the processed cells. In addition, it was confirmed that the cells were healthy even after the molecule injection for a few seconds, indicating that the injection time can be significantly elongated, further improving the delivery efficiency of biomolecules without affecting the cell viability. We envision that the nanoinjection system having the major features of being carrier-free and dose-controllable, having an unlimited injection period, and ease of harvesting will greatly contribute to the next-generation research studies in the fields of cell biology and cell therapeutics.

Corneal cell therapy: with iPSCs, it is no more a far-sight

Human-induced pluripotent stem cells (hiPSCs) provide a personalized approach to study conditions and diseases including those of the eye that lack appropriate animal models to facilitate the development of novel therapeutics. Corneal disease is one of the most common causes of blindness. Hence, significant efforts are made to develop novel therapeutic approaches including stem cell-derived strategies to replace the diseased or damaged corneal tissues, thus restoring the vision. The use of adult limbal stem cells in the management of corneal conditions has been clinically successful. However, its limited availability and phenotypic plasticity necessitate the need for alternative stem cell sources to manage corneal conditions. Mesenchymal and embryonic stem cell-based approaches are being explored; nevertheless, their limited differentiation potential and ethical concerns have posed a significant hurdle in its clinical use. hiPSCs have emerged to fill these technical and ethical gaps to render clinical utility. In this review, we discuss and summarize protocols that have been devised so far to direct differentiation of human pluripotent stem cells (hPSCs) to different corneal cell phenotypes. With the summarization, our review intends to facilitate an understanding which would allow developing efficient and robust protocols to obtain specific corneal cell phenotype from hPSCs for corneal disease modeling and for the clinics to treat corneal diseases and injury.

Generation of Footprint-Free Canine Induced Pluripotent Stem Cells Using Auto-Erasable Sendai Virus Vector

Canine induced pluripotent stem cells (ciPSCs) can be used in regenerative medicine. However, there are no reports on the generation of genome integration-free and completely exogenous gene-silenced (footprint free) ciPSCs that are tolerant to enzymatic single-cell passage. In this study, we reprogrammed canine embryonic fibroblasts using the auto-erasable replication-defective and persistent Sendai virus vector, SeVdp(KOSM)302L, and generated two ciPSC lines. The ciPSCs were positive for pluripotent markers, including alkaline phosphatase activity as well as OCT3/4, SOX2, and NANOG transcripts, and NANOG, stage-specific embryonic antigen-1, and partial TRA-1-60 protein expression, even after SeVdp(KOSM)302L removal. The ciPSCs were induced to differentiate into all the three germ layers as embryoid bodies in vitro and as teratomas in vivo. Furthermore, SeVdp(KOSM)302L-free ciPSCs maintained a normal karyotype even after repeated enzymatic single-cell passaging. Therefore, to our knowledge, for the first time, we demonstrated the generation of footprint-free and high-quality ciPSCs that can be passaged at the single-cell stage using enzymatic methods. Our method for generation of ciPSCs is a good step toward the development of clinical application of ciPSCs.

Therapeutic gene editing in CD34+ hematopoietic progenitors from Fanconi anemia patients

Gene targeting constitutes a new step in the development of gene therapy for inherited diseases. Although previous studies have shown the feasibility of editing fibroblasts from Fanconi anemia (FA) patients, here we aimed at conducting therapeutic gene editing in clinically relevant cells, such as hematopoietic stem cells (HSCs). In our first experiments, we showed that zinc finger nuclease (ZFN)‐mediated insertion of a non‐therapeutic EGFP‐reporter donor in the AAVS1 “safe harbor” locus of FA‐A lymphoblastic cell lines (LCLs), indicating that FANCA is not essential for the editing of human cells. When the same approach was conducted with therapeutic FANCA donors, an efficient phenotypic correction of FA‐A LCLs was obtained. Using primary cord blood CD34⁺ cells from healthy donors, gene targeting was confirmed not only in in vitro cultured cells, but also in hematopoietic precursors responsible for the repopulation of primary and secondary immunodeficient mice. Moreover, when similar experiments were conducted with mobilized peripheral blood CD34⁺ cells from FA‐A patients, we could demonstrate for the first time that gene targeting in primary hematopoietic precursors from FA patients is feasible and compatible with the phenotypic correction of these clinically relevant cells.

Reprogramming of Mouse Calvarial Osteoblasts into Induced Pluripotent Stem Cells

Previous studies have demonstrated the ability of reprogramming endochondral bone into induced pluripotent stem (iPS) cells, but whether similar phenomenon occurs in intramembranous bone remains to be determined. Here we adopted fluorescence-activated cell sorting-based strategy to isolate homogenous population of intramembranous calvarial osteoblasts from newborn transgenic mice carrying both Osx1-GFP::Cre and Oct4-EGFP transgenes. Following retroviral transduction of Yamanaka factors (Oct4, Sox2, Klf4, and c-Myc), enriched population of osteoblasts underwent silencing of Osx1-GFP::Cre expression at early stage of reprogramming followed by late activation of Oct4-EGFP expression in the resulting iPS cells. These osteoblast-derived iPS cells exhibited gene expression profiles akin to embryonic stem cells and were pluripotent as demonstrated by their ability to form teratomas comprising tissues from all germ layers and also contribute to tail tissue in chimera embryos. These data demonstrate that iPS cells can be generated from intramembranous osteoblasts.

The CpG-sites of the CBX3 ubiquitous chromatin opening element are critical structural determinants for the anti-silencing function

Suppression of therapeutic transgene expression from retroviral gene therapy vectors by epigenetic defence mechanisms represents a problem that is particularly encountered in pluripotent stem cells (PSCs) and their differentiated progeny. Transgene expression in these cells, however, can be stabilised by CpG-rich ubiquitous chromatin opening elements (UCOEs). In this context we recently demonstrated profound anti-silencing properties for the small (679 bp) CBX3-UCO element and we now confirmed this observation in the context of the defined murine chromosomal loci ROSA26 and TIGRE. Moreover, since the structural basis for the anti-silencing activity of UCOEs has remained poorly defined, we interrogated various CBX3 subfragments in the context of lentiviral vectors and murine PSCs. We demonstrated marked though distinct anti-silencing activity in the pluripotent state and during PSC-differentiation for several of the CBX3 subfragments. This activity was significantly correlated with CpG content as well as endogenous transcriptional activity. Interestingly, also a scrambled CBX3 version with preserved CpG-sites retained the anti-silencing activity despite the lack of endogenous promoter activity. Our data therefore highlight the importance of CpG-sites and transcriptional activity for UCOE functionality and suggest contributions from different mechanisms to the overall anti-silencing function of the CBX3 element.

Characterization of companion animal pluripotent stem cells

Pluripotent stem cells have the capacity to grow indefinitely in culture and differentiate into derivatives of the three germ layers. These properties underpin their potential to be used in regenerative medicine. Originally derived from early embryos, pluripotent stem cells can now be derived by reprogramming an adult cell back to a pluripotent state. Companion animals such as horses, dogs, and cats suffer from many injuries and diseases for which regenerative medicine may offer new treatments. As many of the injuries and diseases are similar to conditions in humans the use of companion animals for the experimental and clinical testing of stem cell and regenerative medicine products would provide relevant animal models for the translation of therapies to the human field. In order to fully utilize companion animal pluripotent stem cells robust, standardized methods of characterization must be developed to ensure that safe and effective treatments can be delivered. In this review we discuss the methods that are available for characterizing pluripotent stem cells and the techniques that have been applied in cells from companion animals. We describe characteristics which have been described consistently across reports as well as highlighting discrepant results. Significant steps have been made to define the in vitro culture requirements and drive lineage specific differentiation of pluripotent stem cells in companion animal species. However, additional basic research to compare pluripotent stem cell types and define characteristics of pluripotency in companion animal species is still required. © 2017 International Society for Advancement of Cytometry.

Ex vivo Generation of Genetically Modified Macrophages from Human Induced Pluripotent Stem Cells

BACKGROUND

Pluripotent stem cells, including induced pluripotent stem cells (iPSCs), have the capacity to differentiate towards all three germ layers and have been highlighted as an attractive cell source for the field of regenerative medicine. Thus, stable expression of therapeutic transgenes in iPSCs, as well as thereof derived progeny of hematopoietic lineage, may lay the foundation for innovative cell replacement therapies.

METHODS

We have utilized human iPSC lines genetically modified by lentiviral vector technology or targeted integration of reporter genes to evaluate transgene expression during hematopoietic specification and differentiation towards macrophages.

RESULTS

Use of lentiviral vectors equipped with an ubiquitous chromatin opening element (CBX3-UCOE) as well as zinc finger nuclease-mediated targeting of an expression cassette into the human adeno-associated virus integration site 1 (AAVS1) safe harbor resulted in stable transgene expression in iPSCs. When iPSCs were differentiated along the myeloid pathway into macrophages, both strategies yielded sustained transgene expression during the hematopoietic specification process including mature CD14+ and CD11b+ macrophages.

CONCLUSION

Combination of human iPSC technology with either lentiviral vector technology or designer nuclease-based genome editing allows for the generation of transgenic iPSC-derived macrophages with stable transgene expression which may be useful for novel cell and gene replacement therapies.

Transdifferentiation and reprogramming: Overview of the processes, their similarities and differences

Reprogramming, or generation of induced pluripotent stem (iPS) cells (functionally similar to embryonic stem cells or ES cells) by the use of transcription factors (typically: Oct3/4, Sox2, c-Myc, Klf4) called "Yamanaka factors" (OSKM), has revolutionized regenerative medicine. However, factors used to induce stemness are also overexpressed in cancer. Both, ES cells and iPS cells cause teratoma formation when injected to tissues. This raises a safety concern for therapies based on iPS derivates. Transdifferentiation (lineage reprogramming, or -conversion), is a process in which one mature, specialized cell type changes into another without entering a pluripotent state. This process involves an ectopic expression of transcription factors and/or other stimuli. Unlike in the case of reprogramming, tissues obtained by this method do not carry the risk of subsequent teratomagenesis.

Systematic evaluation of markers used for the identification of human induced pluripotent stem cells

Low efficiency of somatic cell reprogramming and heterogeneity among human induced pluripotent stem cells (hiPSCs) demand extensive characterization of isolated clones before their use in downstream applications. By monitoring human fibroblasts undergoing reprogramming for their morphological changes and expression of fibroblast (CD13), pluripotency markers (SSEA-4 and TRA-1-60) and a retrovirally expressed red fluorescent protein (RV-RFP), we compared the efficiency of these features to identify bona fide hiPSC colonies. The co-expression kinetics of fibroblast and pluripotency markers in the cells being reprogrammed and the emerging colonies revealed the heterogeneity within SSEA-4⁺ and TRA-1-60⁺ cells, and the inadequacy of these commonly used pluripotency markers for the identification of bona fide hiPSC colonies. The characteristic morphological changes in the emerging hiPSC colonies derived from fibroblasts expressing RV-RFP showed a good correlation between hiPSC morphology acquisition and silencing of RV-RFP and facilitated the easy identification of hiPSCs. The kinetics of retroviral silencing and pluripotency marker expression in emerging colonies suggested that combining both these markers could demarcate the stages of reprogramming with better precision than with pluripotency markers alone. Our results clearly demonstrate that the pluripotency markers that are routinely analyzed for the characterization of established iPSC colonies are not suitable for the isolation of pluripotent cells in the early stages of reprogramming, and silencing of retrovirally expressed reporter genes helps in the identification of colonies that have attained a pluripotent state and the morphology of human embryonic stem cells (hESCs).

Summary: The use of hESC-like morphology, retroviral transgene silencing and temporal expression of pluripotency markers are compared as methods to aid in the identification of hiPSC clones.

Tβ4-overexpression based on the piggyBac transposon system in cashmere goats alters hair fiber characteristics

Increasing cashmere yield is one of the vital aims of cashmere goats breeding. Compared to traditional breeding methods, transgenic technology is more efficient and the piggyBac (PB) transposon system has been widely applied to generate transgenic animals. For the present study, donor fibroblasts were stably transfected via a PB donor vector containing the coding sequence of cashmere goat thymosin beta-4 (Tβ4) and driven by a hair follicle-specific promoter, the keratin-associated protein 6.1 (KAP6.1) promoter. To obtain genetically modified cells as nuclear donors, we co-transfected donor vectors into fetal fibroblasts of cashmere goats. Five transgenic cashmere goats were generated following somatic cell nuclear transfer (SCNT). Via determination of the copy numbers and integration sites, the Tβ4 gene was successfully inserted into the goat genome. Histological examination of skin tissue revealed that Tβ4-overexpressing, transgenic goats had a higher secondary to primary hair follicle (S/P) ratio compared to wild type goats. This indicates that Tβ4-overexpressing goats possess increased numbers of secondary hair follicles (SHF). Our results indicate that Tβ4-overexpression in cashmere goats could be a feasible strategy to increase cashmere yield.

Rationale and Methodology of Reprogramming for Generation of Induced Pluripotent Stem Cells and Induced Neural Progenitor Cells

Great progress has been made regarding the capabilities to modify somatic cell fate ever since the technology for generation of induced pluripotent stem cells (iPSCs) was discovered in 2006. Later, induced neural progenitor cells (iNPCs) were generated from mouse and human cells, bypassing some of the concerns and risks of using iPSCs in neuroscience applications. To overcome the limitation of viral vector induced reprogramming, bioactive small molecules (SM) have been explored to enhance the efficiency of reprogramming or even replace transcription factors (TFs), making the reprogrammed cells more amenable to clinical application. The chemical induced reprogramming process is a simple process from a technical perspective, but the choice of SM at each step is vital during the procedure. The mechanisms underlying cell transdifferentiation are still poorly understood, although, several experimental data and insights have indicated the rationale of cell reprogramming. The process begins with the forced expression of specific TFs or activation/inhibition of cell signaling pathways by bioactive chemicals in defined culture condition, which initiates the further reactivation of endogenous gene program and an optimal stoichiometric expression of the endogenous pluri- or multi-potency genes, and finally leads to the birth of reprogrammed cells such as iPSCs and iNPCs. In this review, we first outline the rationale and discuss the methodology of iPSCs and iNPCs in a stepwise manner; and then we also discuss the chemical-based reprogramming of iPSCs and iNPCs.

Pluripotent stem cells and livestock genetic engineering

The unlimited proliferative ability and capacity to contribute to germline chimeras make pluripotent embryonic stem cells (ESCs) perfect candidates for complex genetic engineering. The utility of ESCs is best exemplified by the numerous genetic models that have been developed in mice, for which such cells are readily available. However, the traditional systems for mouse genetic engineering may not be practical for livestock species, as it requires several generations of mating and selection in order to establish homozygous founders. Nevertheless, the self-renewal and pluripotent characteristics of ESCs could provide advantages for livestock genetic engineering such as ease of genetic manipulation and improved efficiency of cloning by nuclear transplantation. These advantages have resulted in many attempts to isolate livestock ESCs, yet it has been generally concluded that the culture conditions tested so far are not supportive of livestock ESCs self-renewal and proliferation. In contrast, there are numerous reports of derivation of livestock induced pluripotent stem cells (iPSCs), with demonstrated capacity for long term proliferation and in vivo pluripotency, as indicated by teratoma formation assay. However, to what extent these iPSCs represent fully reprogrammed PSCs remains controversial, as most livestock iPSCs depend on continuous expression of reprogramming factors. Moreover, germline chimerism has not been robustly demonstrated, with only one successful report with very low efficiency. Therefore, even 34 years after derivation of mouse ESCs and their extensive use in the generation of genetic models, the livestock genetic engineering field can stand to gain enormously from continued investigations into the derivation and application of ESCs and iPSCs.

Blockade of senescence-associated microRNA-195 in aged skeletal muscle cells facilitates reprogramming to produce induced pluripotent stem cells

The low reprogramming efficiency in cells from elderly patients is a challenge that must be overcome. Recently, it has been reported that senescence‐associated microRNA (miR)‐195 targets Sirtuin 1 (SIRT1) to advance cellular senescence. Thus, we hypothesized that a blockade of miR‐195 expression could improve reprogramming efficiency in old skeletal myoblasts (SkMs). We found that miR‐195 expression was significantly higher in old SkMs (24 months) isolated from C57BL/6 mice as compared to young SkMs (2 months, 2.3‐fold). Expression of SIRT1 and telomerase reverse transcriptase (TERT) was downregulated in old SkMs, and transduction of old SkMs with lentiviral miR‐195 inhibitor significantly restored their expression. Furthermore, quantitative in situ hybridization analysis demonstrated significant telomere elongation in old SkMs transduced with anti‐miR‐195 (1.7‐fold increase). It is important to note that blocking miR‐195 expression markedly increased the reprogramming efficiency of old SkMs as compared to scramble (2.2‐fold increase). Transduction of anti‐miR‐195 did not alter karyotype or pluripotency marker expression. Induced pluripotent stem cells (iPSCs) from old SkMs transduced with anti‐miR‐195 successfully formed embryoid bodies that spontaneously differentiated into three germ layers, indicating that deletion of miR‐195 does not affect pluripotency in transformed SkMs. In conclusion, this study provided novel evidence that the blockade of age‐induced miR‐195 is a promising approach for efficient iPSC generation from aging donor subjects, which has the potential for autologous transplantation of iPSCs in elderly patients.

OCT4 activity during conversion of human intermediately reprogrammed stem cells to iPSCs through mesenchymal-epithelial transition

To facilitate understanding the mechanisms of somatic reprogramming to human induced pluripotent stem cells (iPSCs), we have established intermediately reprogrammed stem cells (iRSCs), human mesenchymal cells that express exogenous Oct4, Sox2, Klf4 and c-Myc (OSKM) and endogenous SOX2 and NANOG. iRSCs can be stably maintained at low density. At high density, however, they are induced to enter mesenchymal-epithelial transition (MET), resulting in reprogramming to an iPSC state. Morphological changes through MET correlate with silencing of exogenous OSKM, and upregulation of endogenous OCT4. A CRISPR/Cas9-mediated GFP knock-in visualized the temporal regulation of endogenous OCT4 in cells converting from iRSC to iPSC state. OCT4 activation coincident with silencing of OSKM occurred prior to entering MET. Notably, OCT4 instability was frequently observed in cells of developing post-MET colonies until a late stage (>200 cells), demonstrating that OCT4-activated post-MET cells switched from asymmetric to symmetric cell division in late stage reprogramming.

Equine Induced Pluripotent Stem Cells have a Reduced Tendon Differentiation Capacity Compared to Embryonic Stem Cells

Tendon injuries occur commonly in horses and their repair through scar tissue formation predisposes horses to a high rate of re-injury. Pluripotent stem cells may provide a cell replacement therapy to improve tendon tissue regeneration and lower the frequency of re-injury. We have previously demonstrated that equine embryonic stem cells (ESCs) differentiate into the tendon cell lineage upon injection into the damaged horse tendon and can differentiate into functional tendon cells in vitro to generate artificial tendons. Induced pluripotent stem cells (iPSCs) have now been derived from horses but, to date, there are no reports on their ability to differentiate into tendon cells. As iPSCs can be produced from adult cell types, they provide a more accessible source of cells than ESCs, which require the use of horse embryos. The aim of this study was to compare tendon differentiation by ESCs and iPSCs produced through two independent methods. In two-dimensional differentiation assays, the iPSCs expressed tendon-associated genes and proteins, which were enhanced by the presence of transforming growth factor-β3. However, in three-dimensional (3D) differentiation assays, the iPSCs failed to differentiate into functional tendon cells and generate artificial tendons. These results demonstrate the utility of the 3D in vitro tendon assay for measuring tendon differentiation and the need for more detailed studies to be performed on equine iPSCs to identify and understand their epigenetic differences from pluripotent ESCs prior to their clinical application.

Understanding the Molecular Basis of Heterogeneity in Induced Pluripotent Stem Cells

Reprogramming of somatic cells to generate induced pluripotent stem cells (iPSCs) has considerable latency and generates epigenetically distinct partially and fully reprogrammed clones. To understand the molecular basis of reprogramming and to distinguish the partially reprogrammed iPSC clones (pre-iPSCs), we analyzed several of these clones for their molecular signatures. Using a combination of markers that are expressed at different stages of reprogramming, we found that the partially reprogrammed stable clones have significant morphological and molecular heterogeneity in their response to transition to the fully pluripotent state. The pre-iPSCs had significant levels of OCT4 expression but exhibited variable levels of mesenchymal-to-epithelial transition. These novel molecular signatures that we identified would help in using these cells to understand the molecular mechanisms in the late of stages of reprogramming. Although morphologically similar mouse iPSC clones showed significant heterogeneity, the human iPSC clones isolated initially on the basis of morphology were highly homogeneous with respect to the levels of pluripotency.

Mesenchymal and induced pluripotent stem cells: general insights and clinical perspectives

Mesenchymal stem cells have awakened a great deal of interest in regenerative medicine due to their plasticity, and immunomodulatory and anti-inflammatory properties. They are high-yield and can be acquired through noninvasive methods from adult tissues. Moreover, they are nontumorigenic and are the most widely studied. On the other hand, induced pluripotent stem (iPS) cells can be derived directly from adult cells through gene reprogramming. The new iPS technology avoids the embryo destruction or manipulation to generate pluripotent cells, therefore, are exempt from ethical implication surrounding embryonic stem cell use. The pre-differentiation of iPS cells ensures the safety of future approaches. Both mesenchymal stem cells and iPS cells can be used for autologous cell transplantations without the risk of immune rejection and represent a great opportunity for future alternative therapies. In this review we discussed the therapeutic perspectives using mesenchymal and iPS cells.

Genome Editing Gene Therapy for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a severe genetic disorder caused by loss of function of the dystrophin gene on the X chromosome. Gene augmentation of dystrophin is challenging due to the large size of the dystrophin cDNA. Emerging genome editing technologies, such as TALEN and CRISPR-Cas9 systems, open a new erain the restoration of functional dystrophin and are a hallmark of bona fide gene therapy. In this review, we summarize current genome editing approaches, properties of target cell types for ex vivo gene therapy, and perspectives of in vivo gene therapy including genome editing in human zygotes. Although technical challenges, such as efficacy, accuracy, and delivery of the genome editing components, remain to be further improved, yet genome editing technologies offer a new avenue for the gene therapy of DMD.

Derivation of Canine Induced Pluripotent Stem Cells

Dogs and humans have many inherited genetic diseases in common and conditions that are increasingly prevalent in humans also occur naturally in dogs. The use of dogs for the experimental and clinical testing of stem cell and regenerative medicine products would benefit canine health and welfare and provide relevant animal models for the translation of therapies to the human field. Induced pluripotent stem cells (iPSCs) have the capacity to turn into all cells of the body and therefore have the potential to provide cells for therapeutic use and for disease modelling. The objective of this study was to derive and characterize iPSCs from karyotypically abnormal adult canine cells. Aneuploid adipose-derived mesenchymal stromal cells (AdMSCs) from an adult female Weimeraner were re-programmed into iPSCs via overexpression of four human pluripotency factors (Oct 4, Sox2, Klf4 and c-myc) using retroviral vectors. The iPSCs showed similarity to human ESCs with regard to morphology, pluripotency marker expression and the ability to differentiate into derivatives of all three germ layers in vitro (endoderm, ectoderm and mesoderm). The iPSCs also demonstrated silencing of the viral transgenes and re-activation of the silent X chromosome, suggesting full reprogramming had occurred. The levels of aneuploidy observed in the AdMSCs were maintained in the iPSCs. This finding demonstrates the potential for generating canine induced pluripotent stem cells for use as disease models in addition to regenerative medicine and pharmaceutical testing.

Application of iPS cell technology to cancer epigenome study: uncovering the mechanism of cell status conversion for drug resistance in tumor

Recent studies imply that cancer cells possess the ability to reversibly change their properties between a drug sensitive state and a drug resistant state accompanied by epigenetic changes. This evidence indicates that better understanding of cancer epigenetics is important for efficient cancer therapies. Nevertheless, it had been difficult to deeply examine the epigenetic mechanisms because of lack of the tools to actively modify coordinated epigenetic events. In this stagnant situation, the reprogramming technology established by Yamanaka and coworkers have shed a new light. The novel reprogramming technology has made it possible for researchers to artificially introduce epigenetic remodeling into somatic cells. Accordingly, we might be able to use this technology as a tool to introduce the coordinated epigenetic reorganization. In this review, we introduce the idea of cell state interconversion in cancer cells that is attributable to altered epigenetic regulations. We then depict the epigenetic modifications observed during the process of somatic cell reprogramming and give some examples of the difficulty in cancer cell reprogramming. Finally, we discuss how we can translate this reprogramming refractoriness of cancer cells into uncovering unique epigenetic regulations in cancer cells, which might be applicable eventually to the development of novel cancer therapeutics against drug resistant cancer cells.

Sustained high level transgene expression in mammalian cells mediated by the optimized piggyBac transposon system

Sustained, high level transgene expression in mammalian cells is desired in many cases for studying gene functions. Traditionally, stable transgene expression has been accomplished by using retroviral or lentiviral vectors. However, such viral vector-mediated transgene expression is often at low levels and can be reduced over time due to low copy numbers and/or chromatin remodeling repression. The piggyBac transposon has emerged as a promising non-viral vector system for efficient gene transfer into mammalian cells. Despite its inherent advantages over lentiviral and retroviral systems, piggyBac system has not been widely used, at least in part due to their limited manipulation flexibilities. Here, we seek to optimize piggyBac-mediated transgene expression and generate a more efficient, user-friendly piggyBac system. By engineering a panel of versatile piggyBac vectors and constructing recombinant adenoviruses expressing piggyBac transposase (PBase), we demonstrate that adenovirus-mediated PBase expression significantly enhances the integration efficiency and expression level of transgenes in mesenchymal stem cells and osteosarcoma cells, compared to that obtained from co-transfection of the CMV-PBase plasmid. We further determine the drug selection timeline to achieve optimal stable transgene expression. Moreover, we demonstrate that the transgene copy number of piggyBac-mediated integration is approximately 10 times higher than that mediated by retroviral vectors. Using the engineered tandem expression vector, we show that three transgenes can be simultaneously expressed in a single vector with high efficiency. Thus, these results strongly suggest that the optimized piggyBac system is a valuable tool for making stable cell lines with sustained, high transgene expression.

Methods of induced pluripotent stem cells for clinical application

Reprograming somatic cells using exogenetic gene expression represents a groundbreaking step in regenerative medicine. Induced pluripotent stem cells (iPSCs) are expected to yield novel therapies with the potential to solve many issues involving incurable diseases. In particular, applying iPSCs clinically holds the promise of addressing the problems of immune rejection and ethics that have hampered the clinical applications of embryonic stem cells. However, as iPSC research has progressed, new problems have emerged that need to be solved before the routine clinical application of iPSCs can become established. In this review, we discuss the current technologies and future problems of human iPSC generation methods for clinical use.

A minimal ubiquitous chromatin opening element (UCOE) effectively prevents silencing of juxtaposed heterologous promoters by epigenetic remodeling in multipotent and pluripotent stem cells

Epigenetic silencing of transgene expression represents a major obstacle for the efficient genetic modification of multipotent and pluripotent stem cells. We and others have demonstrated that a 1.5 kb methylation-free CpG island from the human HNRPA2B1-CBX3 housekeeping genes (A2UCOE) effectively prevents transgene silencing and variegation in cell lines, multipotent and pluripotent stem cells, and their differentiated progeny. However, the bidirectional promoter activity of this element may disturb expression of neighboring genes. Furthermore, the epigenetic basis underlying the anti-silencing effect of the UCOE on juxtaposed promoters has been only partially explored. In this study we removed the HNRPA2B1 moiety from the A2UCOE and demonstrate efficient anti-silencing properties also for a minimal 0.7 kb element containing merely the CBX3 promoter. This DNA element largely prevents silencing of viral and tissue-specific promoters in multipotent and pluripotent stem cells. The protective activity of CBX3 was associated with reduced promoter CpG-methylation, decreased levels of repressive and increased levels of active histone marks. Moreover, the anti-silencing effect of CBX3 was locally restricted and when linked to tissue-specific promoters did not activate transcription in off target cells. Thus, CBX3 is a highly attractive element for sustained, tissue-specific and copy-number dependent transgene expression in vitro and in vivo.

C-MYC transcriptionally amplifies SOX2 target genes to regulate self-renewal in multipotent otic progenitor cells

Sensorineural hearing loss is caused by the loss of sensory hair cells and neurons of the inner ear. Once lost, these cell types are not replaced. Two genes expressed in the developing inner ear are c-Myc and Sox2. We created immortalized multipotent otic progenitor (iMOP) cells, a fate-restricted cell type, by transient expression of C-MYC in SOX2-expressing otic progenitor cells. This activated the endogenous C-MYC and amplified existing SOX2-dependent transcripts to promote self-renewal. RNA-seq and ChIP-seq analyses revealed that C-MYC and SOX2 occupy over 85% of the same promoters. C-MYC and SOX2 target genes include cyclin-dependent kinases that regulate cell-cycle progression. iMOP cells continually divide but retain the ability to differentiate into functional hair cells and neurons. We propose that SOX2 and C-MYC regulate cell-cycle progression of these cells and that downregulation of C-MYC expression after growth factor withdrawal serves as a molecular switch for differentiation.

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A single factor, C-MYC, induces self-renewal in SOX2-expressing otic progenitors

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C-MYC transcriptionally amplifies SOX2 target genes

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SOX2 modulates transcription of cell-cycle genes

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Immortalized multipotent otic progenitors can differentiate into otic cell types

Improved retroviral episome transfer of transcription factors enables sustained cell fate modification

Retroviral vectors are versatile gene transfer vehicles widely used in basic research and gene therapy. Mutation of retroviral integrase converts these vectors into transient, integration-deficient gene delivery vehicles associated with a high degree of biosafety. We explored the option to use integration-deficient retroviral vectors to achieve transient ectopic expression of transcription factors, which is considered an important tool for induced cell fate conversion. Stepwise optimization of the retroviral episome transfer as exemplified for the transcription factor Oct4 enabled to improve both expression magnitude and endurance. Long terminal repeat-driven γ-retroviral vectors were identified as the most suitable vector architecture. Episomal expression was enhanced by epigenetic modifiers, and Oct4 activity was increased following fusion to a minimal transactivation motif of herpes simplex virus VP16. Based on kinetic analyses, we identified optimal time intervals for repeated vector administration and established prolonged expression windows of choice. Providing proof-of-concept, episomal transfer of Oct4 was potent to mediate conversion of human fibroblasts stably expressing Klf4, Sox2 and c-Myc into induced pluripotent stem cells, which were mainly free of residual Oct4 vector integration. This study provides evidence for suitability of retroviral episome transfer of transcription factors for cell fate conversion, allowing the generation of distinct patient- or disease-specific cell types.

Vectorology and factor delivery in induced pluripotent stem cell reprogramming

Induced pluripotent stem cell (iPSC) reprogramming requires sustained expression of multiple reprogramming factors for a limited period of time (10-30 days). Conventional iPSC reprogramming was achieved using lentiviral or simple retroviral vectors. Retroviral reprogramming has flaws of insertional mutagenesis, uncontrolled silencing, residual expression and re-activation of transgenes, and immunogenicity. To overcome these issues, various technologies were explored, including adenoviral vectors, protein transduction, RNA transfection, minicircle DNA, excisable PiggyBac (PB) transposon, Cre-lox excision system, negative-sense RNA replicon, positive-sense RNA replicon, Epstein-Barr virus-based episomal plasmids, and repeated transfections of plasmids. This review provides summaries of the main vectorologies and factor delivery systems used in current reprogramming protocols.

Reprogramming sertoli cells into pluripotent stem cells

Induced pluripotent stem cells (iPSCs) have potential applications in the restoration of fertility, regenerative medicine, and animal biotechnology. In this study, we present the induction of iPSCs from mouse Sertoli cells (SCs) by introducing four factors--Oct4, Sox2, Klf4, and c-Myc. As early as day 3 after induction, expression of these factors was detected and typical embryonic stem-like cells began to form. On day 18, these exogenous genes were silenced and colonies were selected according to morphological characteristics. The iPSCs induced from SCs, termed SCiPSCs, strongly expressed pluripotent markers, showed a normal karyotype, and had proliferation and differentiation characteristics similar to those of embryonic stem cells (ESCs), both in vitro and in vivo. Furthermore, exposure of SCiPSCs to nitric oxide (NO) allowed them to maintain pluripotency through the activation of the pluripotent genes Oct4 and Sox2 and upregulation of Nanog expression. Moreover, NO prevented SCiPSCs from undergoing apoptosis by activating the antiapoptotic genes Bcl2 and Bcl2lll, downregulating the proapoptotic genes Bak1 and Casp7, and blocking the activation of the proapoptotic gene Bac. These effects were reversed by exposure to l-NG-monomethylarginine (l-NMMA), a NO inhibitor. These data demonstrate that iPSCs can be generated from SCs and that the self-renewal and pluripotency of SCiPS cells can be maintained in the presence of NO.

Characterization of constitutive promoters for piggyBac transposon-mediated stable transgene expression in mesenchymal stem cells (MSCs)

Multipotent mesenchymal stem cells (MSCs) can undergo self-renewal and give rise to multi-lineages under given differentiation cues. It is frequently desirable to achieve a stable and high level of transgene expression in MSCs in order to elucidate possible molecular mechanisms through which MSC self-renewal and lineage commitment are regulated. Retroviral or lentiviral vector-mediated gene expression in MSCs usually decreases over time. Here, we choose to use the piggyBac transposon system and conduct a systematic comparison of six commonly-used constitutive promoters for their abilities to drive RFP or firefly luciferase expression in somatic HEK-293 cells and MSC iMEF cells. The analyzed promoters include three viral promoters (CMV, CMV-IVS, and SV40), one housekeeping gene promoter (UbC), and two composite promoters of viral and housekeeping gene promoters (hEFH and CAG-hEFH). CMV-derived promoters are shown to drive the highest transgene expression in HEK-293 cells, which is however significantly reduced in MSCs. Conversely, the composite promoter hEFH exhibits the highest transgene expression in MSCs whereas its promoter activity is modest in HEK-293 cells. The reduced transgene expression driven by CMV promoters in MSCs may be at least in part caused by DNA methylation, or to a lesser extent histone deacetlyation. However, the hEFH promoter is not significantly affected by these epigenetic modifications. Taken together, our results demonstrate that the hEFH composite promoter may be an ideal promoter to drive long-term and high level transgene expression using the piggyBac transposon vector in progenitor cells such as MSCs.

Targeted gene therapy and cell reprogramming in Fanconi anemia

Gene targeting is progressively becoming a realistic therapeutic alternative in clinics. It is unknown, however, whether this technology will be suitable for the treatment of DNA repair deficiency syndromes such as Fanconi anemia (FA), with defects in homology-directed DNA repair. In this study, we used zinc finger nucleases and integrase-defective lentiviral vectors to demonstrate for the first time that FANCA can be efficiently and specifically targeted into the AAVS1 safe harbor locus in fibroblasts from FA-A patients. Strikingly, up to 40% of FA fibroblasts showed gene targeting 42 days after gene editing. Given the low number of hematopoietic precursors in the bone marrow of FA patients, gene-edited FA fibroblasts were then reprogrammed and re-differentiated toward the hematopoietic lineage. Analyses of gene-edited FA-iPSCs confirmed the specific integration of FANCA in the AAVS1 locus in all tested clones. Moreover, the hematopoietic differentiation of these iPSCs efficiently generated disease-free hematopoietic progenitors. Taken together, our results demonstrate for the first time the feasibility of correcting the phenotype of a DNA repair deficiency syndrome using gene-targeting and cell reprogramming strategies.