Removal of reprogramming transgenes improves the tissue reconstitution potential of keratinocytes generated from human induced pluripotent stem cells

A Systematic Review of Stem Cell Differentiation into Keratinocytes for Regenerative Applications

To improve wound healing or treatment of other skin diseases, and provide model cells for skin biology studies, in vitro differentiation of stem cells into keratinocyte-like cells (KLCs) is very desirable in regenerative medicine. This study examined the most recent advancements in in vitro differentiation of stem cells into KLCs, the effect of biofactors, procedures, and preparation for upcoming clinical cases. A range of stem cells with different origins could be differentiated into KLCs under appropriate conditions. The most effective ways of stem cell differentiation into keratinocytes were found to include the co-culture with primary epithelial cells and keratinocytes, and a cocktail of growth factors, cytokines, and small molecules. KLCs should also be supported by biomaterials for the extracellular matrix (ECM), which replicate the composition and functionality of the in vivo extracellular matrix (ECM) and, thus, support their phenotypic and functional characteristics. The detailed efficient characterization of different factors, and their combinations, could make it possible to find the significant inducers for stem cell differentiation into epidermal lineage. Moreover, it allows the development of chemically known media for directing multi-step differentiation procedures.In conclusion, the differentiation of stem cells to KLCs is feasible and KLCs were used in experimental, preclinical, and clinical trials. However, the translation of KLCs from in vitro investigational system to clinically valuable cells is challenging and extremely slow.

Elimination of Reprogramming Transgenes Facilitates the Differentiation of Induced Pluripotent Stem Cells into Hepatocyte-like Cells and Hepatic Organoids

Hepatocytes and hepatic organoids (HOs) derived from human induced pluripotent stem cells (hiPSCs) are promising cell-based therapies for liver diseases. The removal of reprogramming transgenes can affect hiPSC differentiation potential into the three germ layers but not into hepatocytes and hepatic organoids in the late developmental stage. Herein, we generated hiPSCs from normal human fibroblasts using an excisable polycistronic lentiviral vector based on the Cre recombinase-mediated removal of the loxP-flanked reprogramming cassette. Comparing the properties of transgene-carrying and transgene-free hiPSCs with the same genetic background, the pluripotent states of all hiPSCs were quite similar, as indicated by the expression of pluripotent markers, embryonic body formation, and tri-lineage differentiation in vitro. However, after in vitro differentiation into hepatocytes, transgene-free hiPSCs were superior to the transgene-residual hiPSCs. Interestingly, the generation and hepatic differentiation of human hepatic organoids (hHOs) were significantly enhanced by transgene elimination from hiPSCs, as observed by the upregulated fetal liver (CK19, SOX9, and ITGA6) and functional hepatocyte (albumin, ASGR1, HNF4α, CYP1A2, CYP3A4, and AAT) markers upon culture in differentiation media. Thus, the elimination of reprogramming transgenes facilitates hiPSC differentiation into hepatocyte-like cells and hepatic organoids with properties of liver progenitor cells. Our findings thus provide significant insights into the characteristics of iPSC-derived hepatic organoids.

Potential of transposon-mediated cellular reprogramming towards cell-based therapies

Induced pluripotent stem (iPS) cells present a seminal discovery in cell biology and promise to support innovative treatments of so far incurable diseases. To translate iPS technology into clinical trials, the safety and stability of these reprogrammed cells needs to be shown. In recent years, different non-viral transposon systems have been developed for the induction of cellular pluripotency, and for the directed differentiation into desired cell types. In this review, we summarize the current state of the art of different transposon systems in iPS-based cell therapies.

Error-free and error-prone DNA repair gene expression data through reprogramming and passage in human iPS cells

We recently found that DNA repair-related gene expression could be altered by reprogramming as well as the increased expression of genes that accurately convey genomic information, such as homologous recombination (HR) and mismatch repair (MMR), and the decreased expression of error-prone translesion synthesis (TLS) polymerase. Here, we confirmed this change in expression in another cell-line and found that such alteration was maintained by overlapping passages as well as OCT3/4 and NANOG. Our findings suggest that changes in the expression of DNA repair-related genes associated with reprogramming and their maintenance can be novel indicators of the quality control of the cells exhibiting pluripotency.

Generation of targeted homozygosity in the genome of human induced pluripotent stem cells

When loss of heterozygosity (LOH) is correlated with loss or gain of a disease phenotype, it is often necessary to identify which gene or genes are involved. Here, we developed a region-specific LOH-inducing system based on mitotic crossover in human induced pluripotent stem cells (hiPSCs). We first tested our system on chromosome 19. To detect homozygous clones generated by LOH, a positive selection cassette was inserted at the AASV1 locus of chromosome 19. LOHs were generated by the combination of allele-specific double-stranded DNA breaks introduced by CRISPR/Cas9 and suppression of Bloom syndrome (BLM) gene expression by the Tet-Off system. The BLM protein inhibitor ML216 exhibited a similar crossover efficiency and distribution of crossover sites. We next applied this system to the short arm of chromosome 6, where human leukocyte antigen (HLA) loci are located. Genotyping and flow cytometric analysis demonstrated that LOHs associated with chromosomal crossover occurred at the expected positions. Although careful examination of HLA-homozygous hiPSCs generated from parental cells is needed for cancer predisposition and effectiveness of differentiation, they may help to mitigate the current shortcoming of hiPSC-based transplantation related to the immunological differences between the donor and host.

An insight into non-integrative gene delivery approaches to generate transgene-free induced pluripotent stem cells

Over a decade ago, a landmark study that reported derivation of induced Pluripotent Stem Cells (iPSCs) by reprogramming fibroblasts has transformed stem cell research attracting the interest of the scientific community worldwide. These cells circumvent the ethical and immunological concerns associated with embryonic stem cells, and the limited self-renewal ability and restricted differentiation potential linked to adult stem cells. iPSCs hold great potential for understanding basic human biology, in vitro disease modeling, high-throughput drug testing and discovery, and personalized regenerative medicine. The conventional reprogramming methods involving retro- and lenti-viral vectors to deliver reprogramming factors in somatic cells to generate iPSCs nullify the clinical applicability of these cells. Although these gene delivery systems are efficient and robust, they carry an enormous risk of permanent genetic modifications and are potentially tumorigenic. To evade these safety concerns and derive iPSCs for human therapy, tremendous technological advancements have resulted in the development of non-integrating viral- and non-viral approaches. These gene delivery techniques curtail or eliminate the risk of any genomic alteration and enhance the prospects of iPSCs from bench-to-bedside. The present review provides a comprehensive overview of non-integrating viral (adenoviral vectors, adeno-associated viral vectors, and Sendai virus vectors) and DNA-based, non-viral (plasmid transfection, minicircle vectors, transposon vectors, episomal vectors, and liposomal magnetofection) approaches that have the potential to generate transgene-free iPSCs. The understanding of these techniques could pave the way for the use of iPSCs for various biomedical applications.

Development of a vaccine based on bacteria-mimicking tumor cells coated with novel engineered toll-like receptor 2 ligands

For a successful tumor vaccine, it is necessary to develop effective immuno-adjuvants and identify specific tumor antigens. Tumor cells obtained from surgical or biopsy tissues are a good source of tumor antigens but, unlike bacteria, they do not induce strong immune responses. Here, we designed 2 novel lipopeptides that coat tumor cell surfaces and mimic bacterial components. Tumor cells coated with these lipopeptides (called bacteria-mimicking tumor cells [BMTC]) were prepared and their efficacy as a tumor vaccine examined. Natural bacterial lipopeptides act as ligands for toll-like receptor 2 (TLR2) and activate dendritic cells (DC). To increase the affinity of the developed lipopeptides for the negatively charged plasma membrane, a cationic polypeptide was connected to Pam2Cys (P2C), which is the basic structure of the TLR2 ligand. This increased the non-specific binding affinity of the peptides for the cell surface. Two such lipopeptides, P2CSK11 (containing 1 serine and 11 lysine residues) and P2CSR11 (containing 1 serine and 11 arginine residues) bound to irradiated tumor cells via the long cationic polypeptides more efficiently than the natural lipopeptide MALP2 (P2C-GNNDESNISFKEK) or a synthetic lipopeptide P2CSK4 (a short cationic polypeptide containing 1 serine and 4 lysines). BMTC coated with P2CSR11 or P2CSK11 were efficiently phagocytosed by DC and induced antigen cross-presentation in vitro. They also induced effective tumor-specific cytotoxic T cell responses and inhibited tumor growth in in vivo mouse models. P2CSR11 activated DC but induced less inflammation-inducing cytokines/interferons than other lipopeptides. Thus, P2CSR11 is a strong candidate antigen-specific immuno-adjuvant, with few adverse effects.

Human Hair Reconstruction: Close, But Yet So Far

Billions of dollars are annually invested in pharmaceutical industry and cosmetic sector with intent to develop new drugs and treatment strategies for alopecia. Because the hair looks an important characteristic of humans-an effective appendage in perception, expression of beauty, and preservation of self-esteem-the global market for hair loss treatment products is exponentially increasing. However, current methods to treat hair loss endure yet multiple challenges, such as unfavorable outcomes, nonpermanent and patient-dependent results, as well as unpredictable impacts, which limit their application. Over recent years, remarkable advances in the fields of regenerative medicine and hair tissue engineering have raised new hopes for introducing novel cell-based approaches to treat hair loss. Through cell-based approaches, it is possible to produce hair-like structures in the laboratory setting or manipulate cells in their native niche (in vivo lineage reprogramming) to reconstruct the hair follicle. However, challenging issues still exist with the functionality of cultured human hair cells, the proper selection of nonhair cell sources in cases of shortage of donor hair, and the development of defined culture conditions. Moreover, in the case of in vivo lineage reprogramming, selecting appropriate induction factors and their efficient delivery to guide resident cells into a hair fate-with the aim of reconstructing functional hair-still needs further explorations. In this study, we highlight recent advances and current challenges in hair loss treatment using cell-based approaches and provide novel insights for crucial steps, which must be taken into account to develop reproducible, safe, and efficient cell-based treatment.

DNA Transposition at Work

DNA transposons are defined segments of DNA that are able to move from one genomic location to another. Movement is facilitated by one or more proteins, called the transposase, typically encoded by the mobile element itself. Here, we first provide an overview of the classification of such mobile elements in a variety of organisms. From a mechanistic perspective, we have focused on one particular group of DNA transposons that encode a transposase with a DD(E/D) catalytic domain that is topologically similar to RNase H. For these, a number of three-dimensional structures of transpososomes (transposase-nucleic acid complexes) are available, and we use these to describe the basics of their mechanisms. The DD(E/D) group, in addition to being the largest and most common among all DNA transposases, is the one whose members have been used for a wide variety of genomic applications. Therefore, a second focus of the article is to provide a nonexhaustive overview of transposon applications. Although several non-transposon-based approaches to site-directed genome modifications have emerged in the past decade, transposon-based applications are highly relevant when integration specificity is not sought. In fact, for many applications, the almost-perfect randomness and high frequency of integration make transposon-based approaches indispensable.

Mesenchymal to Epithelial Transition Induced by Reprogramming Factors Attenuates the Malignancy of Cancer Cells

Epithelial to mesenchymal transition (EMT) is a biological process of metastatic cancer. However, an effective anticancer therapy that directly targets the EMT program has not yet been discovered. Recent studies have indicated that mesenchymal to epithelial transition (MET), the reverse phenomenon of EMT, is observed in fibroblasts during the generation of induced pluripotent stem cells. In the present study, we investigated the effects of reprogramming factors (RFs) on squamous cell carcinoma (SCC) cells. RFs-introduced cancer cells (RICs) demonstrated the enhanced epithelial characteristics in morphology with altered expression of mRNA and microRNAs. The motility and invasive activities of RICs in vitro were significantly reduced. Furthermore, xenografts of RICs exhibited no lymph node metastasis, whereas metastasis was detected in parental SCC-inoculated mice. Thus, we concluded that RICs regained epithelial properties through MET and showed reduced cancer malignancy in vitro and in vivo. Therefore, the understanding of the MET process in cancer cells by introduction of RFs may lead to the designing of a novel anticancer strategy.

Inducible Transgene Expression in Human iPS Cells Using Versatile All-in-One piggyBac Transposons

Transgenics is a mainstay of functional genomics. Conditionally overexpressing genes of interest (GOIs) helps to reveal their roles in the control of complex biological processes. Complemented by findings in classic animal model systems, recent advances in human embryonic stem cell (hESC) and patient-specific induced pluripotent stem cell (hiPSC) differentiation have led to sophisticated in vitro models of human development and disease. Yet, as transgenic elements encoding inducible systems must be introduced de novo into each genetically unique human stem cell line, robust and straightforward solutions to gene delivery are required. Transposons are a family of mobile DNA elements that have been adapted as experimental tools for stable genomic integration of transgenes. The piggyBac (PB) transposon from Trichoplusia ni presents a number of benefits over classic viral or BAC transgenesis: ease of application, simple integration-site mapping, and the unique capacity for traceless excision. Moreover, their large capacity permits the consolidation of multiple transgene components in a single vector system. In this chapter, we outline the features of a panel of "All-in-One" PB transposons designed for drug-inducible gene expression and provide guidelines to establish and validate populations or clones of transgenic hiPSCs.