Neuron-like differentiation and selective ablation of undifferentiated embryonic stem cells containing suicide gene with Oct-4 promoter

Dysregulation of Immune Tolerance to Autologous iPSCs and Their Differentiated Derivatives

Induced pluripotent stem cells (iPSCs), capable of differentiating into any cell type, are a promising tool for solving the problem of donor organ shortage. In addition, reprogramming technology makes it possible to obtain a personalized, i.e., patient-specific, cell product transplantation of which should not cause problems related to histocompatibility of the transplanted tissues and organs. At the same time, inconsistent information about the main advantage of autologous iPSC-derivatives - lack of immunogenicity - still casts doubt on the possibility of using such cells beyond immunosuppressive therapy protocols. This review is devoted to immunogenic properties of the syngeneic and autologous iPSCs and their derivatives, as well as to the reasons for dysregulation of their immune tolerance.

Small-Caliber Tissue-Engineered Vascular Grafts Based on Human-Induced Pluripotent Stem Cells: Progress and Challenges

Small-caliber tissue-engineered vascular grafts (TEVGs, luminal diameter <6 mm) are promising therapies for coronary or peripheral artery bypassing surgeries or emergency treatments of vascular trauma, and a robust seed cell source is required for scalable manufacturing of small-caliber TEVGs with robust mechanical strength and bioactive endothelium in future. Human-induced pluripotent stem cells (hiPSCs) could serve as a robust cell source to derive functional vascular seed cells and potentially lead to generation of immunocompatible engineered vascular tissues. Up to date, this rising field of small-caliber hiPSC-derived TEVG (hiPSC-TEVG) research has received increasing attention and achieved significant progress. Implantable, small-caliber, hiPSC-TEVGs have been generated. These hiPSC-TEVGs displayed rupture pressure and suture retention strength approaching to those of human native saphenous veins, with vessel wall decellularized and luminal surface endothelialized with monolayer of hiPSC-endothelial cells. Meanwhile, a series of challenges remain in this field, including functional maturity of hiPSC-derived vascular cells, poor elastogenesis, suboptimal efficiency of obtaining hiPSC-derived seed cells, and relative low ready availability of hiPSC-TEVGs, which are waiting to be addressed. This review is conceived to introduce representative achievements and challenges in small-caliber TEVG generation using hiPSCs, and encapsulate the potential solution and future directions.

Engaging stemness improves cancer immunotherapy

Intra-tumoral immune cells promote the stemness of cancer stem cells (CSCs) in the tumor microenvironment (TME). CSCs promote tumor progression, relapse, and resistance to immunotherapy. Cancer stemness induces the expression of neoantigens and neo-properties in CSCs, creating an opportunity for targeted immunotherapies. Isolation of stem-like T cells or retaining stemness in T clonotypes strategies produces exhaustion-resistance T cells with superior re-expansion capacity and long-lasting responses after adoptive cell therapies. Stem cells-derived NK cells may be the next generation of NK cell products for immunotherapy. Here, we have reviewed mechanisms by which stemness factors modulated the immunoediting of the TME and summarized the potentials of CSCs in the development of immunotherapy regimens, including CAR-T cells, CAR-NK cells, cancer vaccines, and monoclonal antibodies. We have discussed the natural or genetically engineered stem-like T cells and stem cell-derived NK cells with increased cytotoxicity to tumor cells. Finally, we have provided a perspective on approaches that may improve the therapeutic efficacy of these novel adoptive cell-based products in targeting immunosuppressive TME.

Strategies to Improve the Safety of iPSC-Derived β Cells for β Cell Replacement in Diabetes

Allogeneic islet transplantation allows for the re-establishment of glycemic control with the possibility of insulin independence, but is severely limited by the scarcity of organ donors. However, a new source of insulin-producing cells could enable the widespread use of cell therapy for diabetes treatment. Recent breakthroughs in stem cell biology, particularly pluripotent stem cell (PSC) techniques, have highlighted the therapeutic potential of stem cells in regenerative medicine. An understanding of the stages that regulate β cell development has led to the establishment of protocols for PSC differentiation into β cells, and PSC-derived β cells are appearing in the first pioneering clinical trials. However, the safety of the final product prior to implantation remains crucial. Although PSC differentiate into functional β cells in vitro, not all cells complete differentiation, and a fraction remain undifferentiated and at risk of teratoma formation upon transplantation. A single case of stem cell-derived tumors may set the field back years. Thus, this review discusses four approaches to increase the safety of PSC-derived β cells: reprogramming of somatic cells into induced PSC, selection of pure differentiated pancreatic cells, depletion of contaminant PSC in the final cell product, and control or destruction of tumorigenic cells with engineered suicide genes.

Human stem cells harboring a suicide gene improve the safety and standardisation of neural transplants in Parkinsonian rats

Despite advancements in human pluripotent stem cells (hPSCs) differentiation protocols to generate appropriate neuronal progenitors suitable for transplantation in Parkinson's disease, resultant grafts contain low proportions of dopamine neurons. Added to this is the tumorigenic risk associated with the potential presence of incompletely patterned, proliferative cells within grafts. Here, we utilised a hPSC line carrying a FailSafe^TM suicide gene (thymidine kinase linked to cyclinD1) to selectively ablate proliferative cells in order to improve safety and purity of neural transplantation in a Parkinsonian model. The engineered FailSafe^TM hPSCs demonstrated robust ventral midbrain specification in vitro, capable of forming neural grafts upon transplantation. Activation of the suicide gene within weeks after transplantation, by ganciclovir administration, resulted in significantly smaller grafts without affecting the total yield of dopamine neurons, their capacity to innervate the host brain or reverse motor deficits at six months in a rat Parkinsonian model. Within ganciclovir-treated grafts, other neuronal, glial and non-neural populations (including proliferative cells), were significantly reduced-cell types that may pose adverse or unknown influences on graft and host function. These findings demonstrate the capacity of a suicide gene-based system to improve both the standardisation and safety of hPSC-derived grafts in a rat model of Parkinsonism.

Strategies for Genetically Engineering Hypoimmunogenic Universal Pluripotent Stem Cells

Despite progress in developing cell therapies, such as T cell or stem cell therapies to treat diseases, immunoincompatibility remains a major barrier to clinical application. Given the fact that a host's immune system may reject allogeneic transplanted cells, methods have been developed to genetically modify patients' primary cells. To advance beyond this time-consuming and costly approach, recent research efforts focus on generating universal pluripotent stem cells to benefit a broader spectrum of patients. In this review, we first summarize current achievements to harness immunosuppressive mechanisms in cells to reduce immunogenicity. Then, we discuss several recent studies demonstrating the feasibility of genetically modifying pluripotent stem cells to escape immune attack and summarize the methods to evaluate hypoimmunogenicity. Although challenges remain, progress to develop genetically engineered universal pluripotent stem cells holds the promise of expediting their use in future gene and cell therapeutics and regenerative medicine.

Application of Human Induced Pluripotent Stem Cells in Generating Tissue-Engineered Blood Vessels as Vascular Grafts

In pace with the advancement of tissue engineering during recent decades, tissue-engineered blood vessels (TEBVs) have been generated using primary seed cells, and their impressive success in clinical trials have demonstrated the great potential of these TEBVs as implantable vascular grafts in human regenerative medicine. However, the production, therapeutic efficacy, and readiness in emergencies of current TEBVs could be hindered by the accessibility, expandability, and donor-donor variation of patient-specific primary seed cells. Alternatively, using human induced pluripotent stem cells (hiPSCs) to derive seed vascular cells for vascular tissue engineering could fundamentally address this current dilemma in TEBV production. As an emerging research field with a promising future, the generation of hiPSC-based TEBVs has been reported recently with significant progress. Simultaneously, to further promote hiPSC-based TEBVs into vascular grafts for clinical use, several challenges related to the safety, readiness, and structural integrity of vascular tissue need to be addressed. Herein, this review will focus on the evolution and role of hiPSCs in vascular tissue engineering technology and summarize the current progress, challenges, and future directions of research on hiPSC-based TEBVs.

The functional microscopic neuroanatomy of the human subthalamic nucleus

The subthalamic nucleus (STN) is successfully used as a surgical target for deep brain stimulation in the treatment of movement disorders. Interestingly, the internal structure of the STN is still incompletely understood. The objective of the present study was to investigate three-dimensional (3D) immunoreactivity patterns for 12 individual protein markers for GABA-ergic, serotonergic, dopaminergic as well as glutamatergic signaling. We analyzed the immunoreactivity using optical densities and created a 3D reconstruction of seven postmortem human STNs. Quantitative modeling of the reconstructed 3D immunoreactivity patterns revealed that the applied protein markers show a gradient distribution in the STN. These gradients were predominantly organized along the ventromedial to dorsolateral axis of the STN. The results are of particular interest in view of the theoretical underpinning for surgical targeting, which is based on a tripartite distribution of cognitive, limbic and motor function in the STN.

Alpha-Herpesvirus Thymidine Kinase Genes Mediate Viral Virulence and Are Potential Therapeutic Targets

Alpha-herpesvirus thymidine kinase (TK) genes are virulence-related genes and are nonessential for viral replication; they are often preferred target genes for the construction of gene-deleted attenuated vaccines and genetically engineered vectors for inserting and expressing foreign genes. The enzymes encoded by TK genes are key kinases in the nucleoside salvage pathway and have significant substrate diversity, especially the herpes simplex virus 1 (HSV-1) TK enzyme, which phosphorylates four nucleosides and various nucleoside analogues. Hence, the HSV-1 TK gene is exploited for the treatment of viral infections, as a suicide gene in antitumor therapy, and even for the regulation of stem cell transplantation and treatment of parasitic infection. This review introduces the effects of α-herpesvirus TK genes on viral virulence and infection in the host and classifies and summarizes the current main application domains and potential uses of these genes. In particular, mechanisms of action, clinical limitations, and antiviral and antitumor therapy development strategies are discussed.

A Double Fail-Safe Approach to Prevent Tumorigenesis and Select Pancreatic β Cells from Human Embryonic Stem Cells

The transplantation of human embryonic stem cell (hESC)-derived insulin-producing β cells for the treatment of diabetes is finally approaching the clinical stage. However, even with state-of-the-art differentiation protocols, a significant percentage of undefined non-endocrine cell types are still generated. Most importantly, there is the potential for carry-over of non-differentiated cell types that may produce teratomas. We sought to modify hESCs so that their differentiated progeny could be selectively devoid of tumorigenic cells and enriched for cells of the desired phenotype (in this case, β cells). Here we report the generation of a modified hESC line harboring two suicide gene cassettes, whose expression results in cell death in the presence of specific pro-drugs. We show the efficacy of this system at enriching for β cells and eliminating tumorigenic ones both in vitro and in vivo. Our approach is innovative inasmuch as it allows for the preservation of the desired cells while eliminating those with the potential to develop teratomas.

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hESCs were engineered with suicide genes for safety and differentiation efficiency

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One cassette is exclusively expressed in teratogenic cells (safety)

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Another is selectively excised out in hESC-derived pancreatic β cells (selectivity)

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Our strategy allows for hESC-derived tumors to be prevented or ablated in vivo

Targeted Elimination of Tumorigenic Human Pluripotent Stem Cells Using Suicide-Inducing Virus-like Particles

Sensitization to prodrugs via transgenic expression of suicide genes is a leading strategy for the selective elimination of potentially tumorigenic human pluripotent stem cells (hPSCs) in regenerative medicine, but transgenic modification poses safety risks such as deleterious mutagenesis. We describe here an alternative method of delivering suicide-inducing molecules explicitly to hPSCs using virus-like particles (VLPs) and demonstrate its use in eliminating undifferentiated hPSCs in vitro. VLPs were engineered from Qβ bacteriophage capsids to contain enhanced green fluorescent protein (EGFP) or cytosine deaminase (CD) and to simultaneously display multiple IgG-binding ZZ domains. After labeling with antibodies against the hPSC-specific surface glycan SSEA-5, EGFP-containing particles were shown to specifically bind undifferentiated cells in culture, and CD-containing particles were able to eliminate undifferentiated hPSCs with virtually no cytotoxicity to differentiated cells upon treatment with the prodrug 5-fluorocytosine.

Technical approaches to induce selective cell death of pluripotent stem cells

Despite the recent promising results of clinical trials using human pluripotent stem cell (hPSC)-based cell therapies for age-related macular degeneration (AMD), the risk of teratoma formation resulting from residual undifferentiated hPSCs remains a serious and critical hurdle for broader clinical implementation. To mitigate the tumorigenic risk of hPSC-based cell therapy, a variety of approaches have been examined to ablate the undifferentiated hPSCs based on the unique molecular properties of hPSCs. In the present review, we offer a brief overview of recent attempts at selective elimination of undifferentiated hPSCs to decrease the risk of teratoma formation in hPSC-based cell therapy.

Elimination of proliferating cells from CNS grafts using a Ki67 promoter-driven thymidine kinase

Pluripotent stem cell (PSC)-based cell therapy is an attractive concept for neurodegenerative diseases, but can lead to tumor formation. This is particularly relevant as proliferating neural precursors rather than postmitotic mature neurons need to be transplanted. Thus, safety mechanisms to eliminate proliferating cells are needed. Here, we propose a suicide gene approach, based on cell cycle-dependent promoter Ki67-driven expression of herpes simplex virus thymidine kinase (HSV-TK). We generated a PSC line expressing this construct and induced neural differentiation. In vitro, proliferating PSC and early neural precursor cells (NPC) were killed by exposure to ganciclovir. In vivo, transplantation of PSC led to tumor formation, which was prevented by early ganciclovir treatment. Transplanted NPC did not lead to tumor formation and their survival and neural maturation were not affected by ganciclovir. In conclusion, the cell cycle promoter-driven suicide gene approach described in this study allows killing of proliferating undifferentiated precursor cells without expression of the suicide gene in mature neurons. This approach could also be of use for other stem cell-based therapies where the final target consists of postmitotic cells.

Preventing Pluripotent Cell Teratoma in Regenerative Medicine Applied to Hematology Disorders

Iatrogenic tumorigenesis is a major limitation for the use of human induced pluripotent stem cells (hiPSCs) in hematology. The teratoma risk comes from the persistence of hiPSCs in differentiated cell populations. Our goal was to evaluate the best system to purge residual hiPSCs before graft without compromising hematopoietic repopulation capability. Teratoma risk after systemic injection of hiPSCs expressing the reporter gene luciferase was assessed for the first time. Teratoma formation in immune‐deficient mice was tracked by in vivo bioimaging. We observed that systemic injection of hiPSCs produced multisite teratoma as soon as 5 weeks after injection. To eliminate hiPSCs before grafting, we tested the embryonic‐specific expression of suicide genes under the control of the pmiR‐302/367 promoter. This promoter was highly active in hiPSCs but not in differentiated cells. The gene/prodrug inducible Caspase‐9 (iCaspase‐9)/AP20187 was more efficient and rapid than thymidine kinase/ganciclovir, fully specific, and without bystander effect. We observed that iCaspase‐9‐expressing hiPSCs died in a dose‐dependent manner with AP20187, without reaching full eradication in vitro. Unexpectedly, nonspecific toxicity of AP20187 on iCaspase‐9‐negative hiPSCs and on CD34⁺ cells was evidenced in vitro. This toxic effect strongly impaired CD34⁺‐derived human hematopoiesis in adoptive transfers. Survivin inhibition is an alternative to the suicide gene approach because hiPSCs fully rely on survivin for survival. Survivin inhibitor YM155 was more efficient than AP20187/iCaspase‐9 for killing hiPSCs, without toxicity on CD34⁺ cells, in vitro and in adoptive transfers. hiPSC purge by survivin inhibitor fully eradicated teratoma formation in immune‐deficient mice. This will be useful to improve the safety management for hiPSC‐based medicine. Stem Cells Translational Medicine2017;6:382–393

Full biological characterization of human pluripotent stem cells will open the door to translational research

Since the discovery of human embryonic stem cells (hESC) and human-induced pluripotent stem cells (hiPSC), great hopes were held for their therapeutic application including disease modeling, drug discovery screenings, toxicological screenings and regenerative therapy. hESC and hiPSC have the advantage of indefinite self-renewal, thereby generating an inexhaustible pool of cells with, e.g., specific genotype for developing putative treatments; they can differentiate into derivatives of all three germ layers enabling autologous transplantation, and via donor-selection they can express various genotypes of interest for better disease modeling. Furthermore, drug screenings and toxicological screenings in hESC and hiPSC are more pertinent to identify drugs or chemical compounds that are harmful for human, than a mouse model could predict. Despite continuing research in the wide field of therapeutic applications, further understanding of the underlying basic mechanisms of stem cell function is necessary. Here, we summarize current knowledge concerning pluripotency, self-renewal, apoptosis, motility, epithelial-to-mesenchymal transition and differentiation of pluripotent stem cells.

Escape Mutations, Ganciclovir Resistance, and Teratoma Formation in Human iPSCs Expressing an HSVtk Suicide Gene

Human pluripotent stem cells (hPSCs) hold great promise for cell therapy. However, a major concern is the risk of tumor formation by residual undifferentiated cells contaminating the hPSC-derived cell product. Suicide genes could safeguard against such adverse events by enabling elimination of cells gone astray, but the efficacy of this approach has not yet been thoroughly tested. Here, we engineered a lentivirally encoded herpes simplex virus thymidine kinase (HSVtk) with expression restricted to undifferentiated hPSCs through regulation by the let7 family of miRNAs. We show that induced pluripotent stem cells (iPSCs) expressing a let7-regulated HSVtk transgene are selectively killed by ganciclovir (GCV), whereas differentiated cells are fully protected. However, in contrast to previous studies, we find that in vivo GCV administration results in longer latency but does not prevent teratoma formation by iPSCs expressing either a constitutive or a let7-regulated HSVtk, without evidence of silencing of the HSVtk. Clonal analyses of iPSCs expressing HSVtk revealed frequent emergence of GCV resistance which, at least in some cases, could be attributed to preexisting inactivating mutations in the HSVtk coding sequence, selected for upon GCV treatment. Our findings have important consequences for the future use of suicide genes in hPSC-based cell therapies.

'Above all, do no harm': safeguarding pluripotent stem cell therapy against iatrogenic tumorigenesis

Human pluripotent stem cells are the foundations of regenerative medicine. However, the worst possible complication of using pluripotent stem cells in therapy could be iatrogenic cancerogenesis. Nevertheless, despite the rapid progress in the development of new techniques for induction of pluripotency and for directed differentiation, risks of cancerogenic transformation of therapeutically implanted pluripotent stem cells still persist. 'Above all, do no harm', as quoted from the Hippocratic Oath, is our ultimate creed. Therefore, the primary goal in designing any therapeutic regimes involving stem cells should be the elimination of any possibilities of their neoplasmic transformation. I review here the basic strategies that have been designed to attain this goal: sorting out undifferentiated, pluripotent stem cells with antibodies targeting surface-displayed biomarkers; sorting in differentiating cells, which express recombinant proteins as reporters; killing undifferentiated stem cells with toxic antibodies or antibody-guided toxins; eliminating undifferentiated stem cells with cytotoxic drugs; making potentially tumorigenic stem cells sensitive to pro-drugs by transformation with suicide-inducing genes; eradication of differentiation-refractive stem cells by self-triggered transgenic expression of human recombinant DNases. Every pluripotent undifferentiated stem cell poses a risk of neoplasmic transformation. Therefore, the aforementioned or other novel strategies that would safeguard against iatrogenic transformation of these stem cells should be considered for incorporation into every stem cell therapy trial.

Targeting of herpes simplex virus 1 thymidine kinase gene sequences into the OCT4 locus of human induced pluripotent stem cells

The in vitro differentiation of human induced pluripotent stem cells (hiPSC) to generate specific types of cells is inefficient, and the remaining undifferentiated cells may form teratomas. This raises safety concerns for clinical applications of hiPSC-derived cellular products. To improve the safety of hiPSC, we attempted to site-specifically insert a herpes simplex virus 1 thymidine kinase (HSV1-TK) suicide gene at the endogenous OCT4 (POU5F1) locus of hiPSC. Since the endogenous OCT4 promoter is active in undifferentiated cells only, we speculated that the HSV1-TK suicide gene will be transcribed in undifferentiated cells only and that the remaining undifferentiated cells can be depleted by treating them with the prodrug ganciclovir (GCV) prior to transplantation. To insert the HSV1-TK gene at the OCT4 locus, we cotransfected hiPSC with a pair of plasmids encoding an OCT4-specific zinc finger nuclease (ZFN) and a donor plasmid harboring a promoter-less transgene cassette consisting of HSV1-TK and puromycin resistance gene sequences, flanked by OCT4 gene sequences. Puromycin resistant clones were established and characterized regarding their sensitivity to GCV and the site of integration of the HSV1-TK/puromycin resistance gene cassette. Of the nine puromycin-resistant iPSC clones analyzed, three contained the HSV1-TK transgene at the OCT4 locus, but they were not sensitive to GCV. The other six clones were GCV-sensitive, but the TK gene was located at off-target sites. These TK-expressing hiPSC clones remained GCV sensitive for up to 90 days, indicating that TK transgene expression was stable. Possible reasons for our failed attempt to selectively target the OCT4 locus are discussed.

Generation of rejuvenated antigen-specific T cells by reprogramming to pluripotency and redifferentiation

Adoptive immunotherapy with functional T cells is potentially an effective therapeutic strategy for combating many types of cancer and viral infection. However, exhaustion of antigen-specific T cells represents a major challenge to this type of approach. In an effort to overcome this problem, we reprogrammed clonally expanded antigen-specific CD8(+) T cells from an HIV-1-infected patient to pluripotency. The T cell-derived induced pluripotent stem cells were then redifferentiated into CD8(+) T cells that had a high proliferative capacity and elongated telomeres. These "rejuvenated" cells possessed antigen-specific killing activity and exhibited T cell receptor gene-rearrangement patterns identical to those of the original T cell clone from the patient. We also found that this method can be effective for generating specific T cells for other pathology-associated antigens. Thus, this type of approach may have broad applications in the field of adoptive immunotherapy.

Safeguarding clinical translation of pluripotent stem cells with suicide genes

The generation of human induced pluripotent stem cells (hiPSCs) opens a new avenue in regenerative medicine. However, transplantation of hiPSC-derived cells carries a risk of tumor formation by residual pluripotent stem cells. Numerous adaptive strategies have been developed to prevent or minimize adverse events and control the in vivo behavior of transplanted stem cells and their progeny. Among them, the application of suicide gene modifications, which is conceptually similar to cancer gene therapy, is considered an ideal means to control wayward stem cell progeny in vivo. In this review, the choices of vectors, promoters, and genes for use in suicide gene approaches for improving the safety of hiPSCs-based cell therapy are introduced and possible new strategies for improvements are discussed. Safety-enhancing strategies that can selectively ablate undifferentiated cells without inducing virus infection or insertional mutations may greatly aid in translating human pluripotent stem cells into cell therapies in the future.

Suicide gene-mediated ablation of tumor-initiating mouse pluripotent stem cells

Pluripotent stem cells, including embryonic stem (ES) and induced pluripotent stem (iPS) cells, serve as unlimited resources for cell replacement therapy and tissue engineering because such cells are capable of extensive proliferation in vitro and can give rise to lineages that represent any of the three embryonic germ layers. However, in the context of the in vivo behavior of cell transplants, key challenges need to be addressed and essential strategies should be developed before stem cells can be used in clinical practice. In the present study, we modified mouse ES/iPS cells to contain a suicide gene, deltaTK or CodA, under the transcriptional control of the EF1α or Nanog promoter. The suicide gene was introduced via lentivirus transduction without interfering with their self-renewal and pluripotency characteristics. We found that EF1α promoter-controlled deltaTK/CodA expression efficiently eliminated pluripotent stem cells and their derivatives both in vitro and in vivo. When the suicide gene was under the control of the Nanog promoter, tumor-initiating undifferentiated pluripotent stem cells were selectively ablated in vitro after prodrug treatment. These results indicate that modification of pluripotent stem cells with a suicide gene prior to transplantation offers a safe manner by which wayward stem cells, and their progeny, can be controlled in vivo. Our approach will render the clinical application of human pluripotent stem cells increasingly possible.

A scalable approach to prevent teratoma formation of human embryonic stem cells

As the renewable source of all cell types in the body, human embryonic stem cells (hESCs) hold great promise for human cell therapy. However, one major bottleneck that hinders the clinic application of hESCs is that hESCs remaining with their differentiated derivatives pose cancer risk by forming teratomas after transplantation. NANOG is a critical pluripotency factor specifically expressed in hESCs but rarely in their differentiated derivatives. By introducing a hyperactive variant of herpes simplex virus thymidine kinase gene into the 3'-untranslated region of the endogenous NANOG gene of hESCs through homologous recombination, we developed a safe and highly scalable approach to efficiently eliminate the teratoma risk associated with hESCs without apparent negative impact on their differentiated cell types. As thymidine kinase is widely used in human gene therapy trials and is the therapeutic target of U. S. Food and Drug Administration-approved drugs, our strategy could be effectively applied to the clinic development of hESC-based human cell therapy.

Stem cell and biomaterials research in dental tissue engineering and regeneration

This review summarizes approaches used in tissue engineering and regenerative medicine, with a focus on dental applications. Dental caries and periodontal disease are the most common diseases resulting in tissue loss. To replace or regenerate new tissues, various sources of stem cells have been identified such as somatic stem cells from teeth and peridontium. Advances in biomaterial sciences including microfabrication, self-assembled biomimetic peptides, and 3-dimensional printing hold great promise for whole-organ or partial tissue regeneration to replace teeth and periodontium.

Mifepristone-inducible caspase-1 expression in mouse embryonic stem cells eliminates tumor formation but spares differentiated cells in vitro and in vivo

Embryonic stem cell (ESC)-based therapy is a promising treatment for neurodegenerative diseases. But there is always a risk of tumor formation that is due to contamination of undifferentiated ESCs. To reduce the risk and improve ESC-based therapy, we have established a novel strategy by which we can selectively eliminate tumor cells derived from undifferentiated ESCs but spare differentiated cells. In this study, we generated a caspase-1-ESC line transfected with a mifepristone-regulated caspase-1 expression system. Mifepristone induced caspase-1 overexpression both in differentiated and undifferentiated caspase-1-ESCs. All the undifferentiated caspase-1-ESCs were induced to death after mifepristone treatment. Tumors derived from undifferentiated caspase-1-ESCs were eliminated following 3 weeks of mifepristone treatment in vivo. However, differentiated caspase-1-ESCs survived well under the condition of mifepristone-induced caspase-1 overexpression. To examine in vivo the impact of mifepristone-induced caspase-1 activation on grafted cells, we transplanted wild-type ESCs or caspase-1-ESCs into nude mice brains. After 8 weeks of mifepristone treatment, we could not detect any tumor cells in the caspase-1-ESC grafts in the brains of mice. However, we found that donor dopamine neurons survived in the recipient brains. These data demonstrate that mifepristone-induced caspase-1 overexpression in ESCs can eliminate the potential tumor formation meanwhile spares the differentiated cells in the host brains. These results suggest that this novel ESC-based therapy can be used in Parkinson's disease and other related disorders without the risk of tumor formation.

Folate antagonist, methotrexate induces neuronal differentiation of human embryonic stem cells transplanted into nude mouse retina

Transplanted embryonic stem (ES) cells can be integrated into the retinas of adult mice as well-differentiated neuroretinal cells. However, the transplanted ES cells also have a tumorigenic activity as they have the ability for multipotent differentiation to various types of tissues. In the present study, human ES (hES) cells were transplanted into adult nude mouse retinas by intravitreal injections 20 h after intravitreal N-methyl-D-aspartate (NMDA) administration. After the transplantation of hES cells, the folate antagonist, methotrexate (MTX) was administrated in order to control the differentiation of the transplanted hES cells. Neuronal differentiation and teratogenic potential of hES cells were examined immunohistochemically 5 weeks after transplantation. The proliferative activity of transplanted cells was determined by both the mitotic index and the Ki-67 proliferative index. Disappearance of Oct-4-positive hES cells showing undifferentiated morphology was observed after intraperitoneal MTX treatment daily, for 15 days. Decreased mitotic and Ki-67 proliferative indices, and increased neuronal differentiation were detected in the surviving hES cells after the MTX treatment. These results suggest two important effects of intraperitoneal MTX treatment for hES cells transplanted into nude mouse retina: (1) MTX treatment following transplantation induces neuronal differentiation, and (2) MTX decreases proliferative activity and tumorigenic potential.

Genetic control of wayward pluripotent stem cells and their progeny after transplantation

The proliferative capacity of pluripotent stem cells and their progeny brings a unique aspect to therapeutics, in that once a transplant is initiated the therapist no longer has control of the therapy. In the context of the recent FDA approval of a human ESC trial and report of a neuronal-stem-cell-derived tumor in a human trial, strategies need to be developed to control wayward pluripotent stem cells. Here, we focus on one approach: direct genetic modification of the cells prior to transplantation with genes that can prevent the adverse events and/or eliminate the transplanted cells and their progeny.