Silencing of the expression of pluripotent driven-reporter genes stably transfected into human pluripotent cells

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.

Lent-On-Plus Lentiviral vectors for conditional expression in human stem cells

Conditional transgene expression in human stem cells has been difficult to achieve due to the low efficiency of existing delivery methods, the strong silencing of the transgenes and the toxicity of the regulators. Most of the existing technologies are based on stem cells clones expressing appropriate levels of tTA or rtTA transactivators (based on the TetR-VP16 chimeras). In the present study, we aim the generation of Tet-On all-in-one lentiviral vectors (LVs) that tightly regulate transgene expression in human stem cells using the original TetR repressor. By using appropriate promoter combinations and shielding the LVs with the Is2 insulator, we have constructed the Lent-On-Plus Tet-On system that achieved efficient transgene regulation in human multipotent and pluripotent stem cells. The generation of inducible stem cell lines with the Lent-ON-Plus LVs did not require selection or cloning, and transgene regulation was maintained after long-term cultured and upon differentiation toward different lineages. To our knowledge, Lent-On-Plus is the first all-in-one vector system that tightly regulates transgene expression in bulk populations of human pluripotent stem cells and its progeny.

Wnt/β-catenin signaling promotes self-renewal and inhibits the primed state transition in naïve human embryonic stem cells

In both mice and humans, pluripotent stem cells (PSCs) exist in at least two distinct states of pluripotency, known as the naïve and primed states. Our understanding of the intrinsic and extrinsic factors that enable PSCs to self-renew and to transition between different pluripotent states is important for understanding early development. In mouse embryonic stem cells (mESCs), Wnt proteins stimulate mESC self-renewal and support the naïve state. In human embryonic stem cells (hESCs), Wnt/β-catenin signaling is active in naïve-state hESCs and is reduced or absent in primed-state hESCs. However, the role of Wnt/β-catenin signaling in naïve hESCs remains largely unknown. Here, we demonstrate that inhibition of the secretion of Wnts or inhibition of the stabilization of β-catenin in naïve hESCs reduces cell proliferation and colony formation. Moreover, we show that addition of recombinant Wnt3a partially rescues cell proliferation in naïve hESCs caused by inhibition of Wnt secretion. Notably, inhibition of Wnt/β-catenin signaling in naïve hESCs did not cause differentiation. Instead, it induced primed hESC-like proteomic and metabolic profiles. Thus, our results suggest that naïve hESCs secrete Wnts that activate autocrine or paracrine Wnt/β-catenin signaling to promote efficient self-renewal and inhibit the transition to the primed state.

Using Zinc Finger Nuclease Technology to Generate CRX-Reporter Human Embryonic Stem Cells as a Tool to Identify and Study the Emergence of Photoreceptors Precursors During Pluripotent Stem Cell Differentiation

The purpose of this study was to generate human embryonic stem cell (hESC) lines harboring the green fluorescent protein (GFP) reporter at the endogenous loci of the Cone-Rod Homeobox (CRX) gene, a key transcription factor in retinal development. Zinc finger nucleases (ZFNs) designed to cleave in the 3' UTR of CRX were transfected into hESCs along with a donor construct containing homology to the target region, eGFP reporter, and a puromycin selection cassette. Following selection, polymerase chain reaction (PCR) and sequencing analysis of antibiotic resistant clones indicated targeted integration of the reporter cassette at the 3' of the CRX gene, generating a CRX-GFP fusion. Further analysis of a clone exhibiting homozygote integration of the GFP reporter was conducted suggesting genomic stability was preserved and no other copies of the targeting cassette were inserted elsewhere within the genome. This clone was selected for differentiation towards the retinal lineage. Immunocytochemistry of sections obtained from embryoid bodies and quantitative reverse transcriptase PCR of GFP positive and negative subpopulations purified by fluorescence activated cell sorting during the differentiation indicated a significant correlation between GFP and endogenous CRX expression. Furthermore, GFP expression was found in photoreceptor precursors emerging during hESC differentiation, but not in the retinal pigmented epithelium, retinal ganglion cells, or neurons of the developing inner nuclear layer. Together our data demonstrate the successful application of ZFN technology to generate CRX-GFP labeled hESC lines, which can be used to study and isolate photoreceptor precursors during hESC differentiation.

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.

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.

Use of UTF1 genetic control elements as iPSC reporter

The reprogramming of adult somatic cells into an embryonic stem cell (ESC) state by various means has opened a new chapter in basic and applied life science. While this technology will create great opportunities for regenerative medicine, the more immediate impact is likely to be found in human disease modeling and drug testing/development. An important aspect in the latter contexts is the ability to reliably monitor the pluripotent stem cell state, in particular with respect to human cell reprogramming using patient-specific somatic cells and high-throughput screens. Undifferentiated transcription factor 1 (UTF1) belongs to the core transcriptional network characterizing pluripotency. UTF1 is involved in ESC-specific chromatin organization, and its expression pattern during cell reprogramming and subsequent differentiation appears to be tightly connected with the pluripotent stem cell state. Here, we capitalized on these features and generated a reliable reporter system that was used to monitor induced pluripotent stem cell (iPSC) formation and subsequent differentiation. Our reporter cassette comprises less than 2.3 kb and remains functional during many cell passages after genomic integration. The fact that the human UTF1 genetic control elements work in a mouse background and the demonstrated functionality of the reporter in an epigenetic state further qualifies this system as a versatile new tool for iPSC research.

Differential localization and high expression of SURVIVIN splice variants in human embryonic stem cells but not in differentiated cells implicate a role for SURVIVIN in pluripotency

The BIRC5 gene encodes the oncofetal protein SURVIVIN, as well as four additional splice variants (ΔEx3, 2B, 3B and 2α). SURVIVIN, an inhibitor of apoptosis, is also a chromosomal passenger protein (CPP). Previous results have demonstrated that SURVIVIN is expressed at high levels in embryonic stem cells and inhibition of SURVIVIN function results in apoptosis, however these studies have not investigated the other four splice variants. In this study, we demonstrate that all variants are expressed at significantly higher levels in human embryonic stem (hES) cells than in differentiated cells. We examined the subcellular localization of the three most highly expressed variants. SURVIVIN displayed canonical CPP localization in mitotic cells and cytoplasmic localization in interphase cells. In contrast, SURVIVIN-ΔEx3 and SURVIVIN-2B did not localize as a CPP; SURVIVIN-ΔEx3 was found constitutively in the nucleus while SURVIVIN-2B was distributed along the chromosomes during mitosis and also to the mitotic spindle poles. We used inducible shRNA against SURVIVIN to inhibit expression in a titratable fashion. Using this system, we reduced the mRNA levels of these three variants to approx. 40%, resulting in a concomitant reduction of OCT4 and NANOG mRNA, suggesting a role for the SURVIVIN variants in pluripotency.

Human-induced pluripotent stem cells: in quest of clinical applications

In the field of regenerative medicine, the development of induced pluripotent stem (iPS) cells may represent a potential strategy to overcome the limitations of human embryonic stem cells (ESCs). iPS cells have the potential to mimic human disease, since they carry the genome of the donor. Hypothetically, with iPS cell technology it is possible to screen patients for a genetic cause of disease (genetic mutation), develop cell lines, reprogram them back to iPS cells, finally differentiate them into one or more cell types that develop the disease. Although the creation of multiple lineages with iPS cells can seem limitless, a number of challenges need to be addressed in order to effectively use these cell lines for disease modeling. These include the low efficiency of iPS cell generation without genetic alterations, the possibility of tumor formation in vivo, the random integration of retroviral-based delivery vectors into the genome, and unregulated growth of the remaining cells that are partially reprogrammed and refractory to differentiation. The establishment of protein or RNA-based reprogramming strategies will help generate human iPS cells without permanent genetic alterations. Finally, direct reprogramming strategies can provide rapid production of models of human "diseases in a dish", without first passing the cells through a pluripotent state, so avoiding the challenges of time-consumming and labor-intensive iPS cell line generation. This review will overview methods to develop iPS cells, current strategies for direct reprogramming, and main applications of iPS cells as human disease model, focusing on human cardiovascular diseases, with the aim to be a potential information resource for biomedical scientists and clinicians who exploit or intend to exploit iPS cell technology in a range of applications.

Differential responses to retinoic acid and endocrine disruptor compounds of subpopulations within human embryonic stem cell lines

The heterogeneous nature of stem cells is an important issue in both research and therapeutic use in terms of directing cell lineage differentiation pathways, as well as self-renewal properties. Using flow cytometry we have identified two distinct subpopulations by size, large and small, within cultures of human embryonic stem (hES) cell lines. These two cell populations respond differentially to retinoic acid (RA) differentiation and several endocrine disruptor compounds (EDC). The large cell population responds to retinoic acid differentiation with greater than a 50% reduction in cell number and loss of Oct-4 expression, whereas the number of the small cell population does not change and Oct-4 protein expression is maintained. In addition, four estrogenic compounds altered SSEA-3 expression differentially between the two cell subpopulations changing their ratios relative to each other. Both populations express stem cell markers Oct-4, Nanog, Tra-1-60, Tra-1-80 and SSEA-4, but express low levels of differentiation markers common to the three germ layers. Cloning studies indicate that both populations can revive the parental population. Furthermore, whole genome microarray identified approximately 400 genes with significantly different expression between the two populations (p<0.01). We propose the differential response to RA in these populations is due to differential gene expression of Notch signaling members, CoupTF1 and CoupTF2, chromatin remodeling and histone modifying genes that render the small population resistant to RA differentiation. The findings that hES cells exist as heterogeneous populations with distinct responses to differentiation signals and environmental stimuli will be relevant for their use for drug discovery and disease therapy.

Specific marking of hESCs-derived hematopoietic lineage by WAS-promoter driven lentiviral vectors

Genetic manipulation of human embryonic stem cells (hESCs) is instrumental for tracing lineage commitment and to studying human development. Here we used hematopoietic-specific Wiskott-Aldrich syndrome gene (WAS)-promoter driven lentiviral vectors (LVs) to achieve highly specific gene expression in hESCs-derived hematopoietic cells. We first demonstrated that endogenous WAS gene was not expressed in undifferentiated hESCs but was evident in hemogenic progenitors (CD45⁻CD31⁺CD34⁺) and hematopoietic cells (CD45⁺). Accordingly, WAS-promoter driven LVs were unable to express the eGFP transgene in undifferentiated hESCs. eGFP⁺ cells only appeared after embryoid body (EB) hematopoietic differentiation. The phenotypic analysis of the eGFP⁺ cells showed marking of different subpopulations at different days of differentiation. At days 10–15, AWE LVs tag hemogenic and hematopoietic progenitors cells (CD45⁻CD31⁺CD34^dim and CD45⁺CD31⁺CD34^dim) emerging from hESCs and at day 22 its expression became restricted to mature hematopoietic cells (CD45⁺CD33⁺). Surprisingly, at day 10 of differentiation, the AWE vector also marked CD45⁻CD31^low/−CD34⁻ cells, a population that disappeared at later stages of differentiation. We showed that the eGFP⁺CD45⁻CD31⁺ population generate 5 times more CD45⁺ cells than the eGFP⁻CD45⁻CD31⁺ indicating that the AWE vector was identifying a subpopulation inside the CD45⁻CD31⁺ cells with higher hemogenic capacity. We also showed generation of CD45⁺ cells from the eGFP⁺CD45⁻CD31^low/−CD34⁻ population but not from the eGFP⁻CD45⁻CD31^low/−CD34⁻ cells. This is, to our knowledge, the first report of a gene transfer vector which specifically labels hemogenic progenitors and hematopoietic cells emerging from hESCs. We propose the use of WAS-promoter driven LVs as a novel tool to studying human hematopoietic development.

Concise review: putting a finger on stem cell biology: zinc finger nuclease-driven targeted genetic editing in human pluripotent stem cells

Human pluripotent stem cells (hPSCs) encompassing human embryonic stem cells and human induced pluripotent stem cells (hiPSCs) have a wide appeal for numerous basic biology studies and for therapeutic applications because of their potential to give rise to almost any cell type in the human body and immense ability to self-renew. Much attention in the stem cell field is focused toward the study of gene-based anomalies relating to the causative affects of human disease and their correction with the potential for patient-specific therapies using gene corrected hiPSCs. Therefore, the genetic manipulation of stem cells is clearly important for the development of future medicine. Although successful targeted genetic engineering in hPSCs has been reported, these cases are surprisingly few because of inherent technical limitations with the methods used. The development of more robust and efficient means by which to achieve specific genomic modifications in hPSCs has far reaching implications for stem cell research and its applications. Recent proof-of-principle reports have shown that genetic alterations with minimal toxicity are now possible through the use of zinc finger nucleases (ZFNs) and the inherent DNA repair mechanisms within the cell. In light of recent comprehensive reviews that highlight the applications, methodologies, and prospects of ZFNs, this article focuses on the application of ZFNs to stem cell biology, discussing the published work to date, potential problems, and future uses for this technology both experimentally and therapeutically.

Mechanical phenotyping of mouse embryonic stem cells: increase in stiffness with differentiation

Atomic force microscopy (AFM) has emerged as a promising tool to characterize the mechanical properties of biological materials and cells. In our studies, undifferentiated and early differentiating mouse embryonic stem cells (mESCs) were assessed individually using an AFM system to determine if we could detect changes in their mechanical properties by surface probing. Probes with pyramidal and spherical tips were assessed, as were different analytical models for evaluating the data. The combination of AFM probing with a spherical tip and analysis using the Hertz model provided the best fit to the experimental data obtained and thus provided the best approximation of the elastic modulus. Our results showed that after only 6 days of differentiation, individual cell stiffness increased significantly with early differentiating mESCs having an elastic modulus two- to threefold higher than undifferentiated mESCs, regardless of cell line (R1 or D3 mESCs) or treatment. Single-touch (indentation) probing of individual cells is minimally invasive compared to other techniques. Therefore, this method of mechanical phenotyping should prove to be a valuable tool in the development of improved methods of identification and targeted cellular differentiation of embryonic, adult, and induced-pluripotent stem cells for therapeutic and diagnostic purposes.

Mechanical phenotyping of stem cells

Elasticity and visco-elasticity are mechanical properties of cells which directly reflect cellular composition, internal structure (cytoskeleton), and external interactions (cell-cell and/or cell-surface). A variety of techniques involving probing, pulling, or deforming cells have been used to characterize these mechanical properties. With continuing advances in the technology, it may be possible to establish mechanical phenotypes that can be used to identify cells at specific points of differentiation and dedifferentiation with direct applications to regenerative medicine, therapeutics, and diagnostics.

Intracoronary administration of cardiac progenitor cells alleviates left ventricular dysfunction in rats with a 30-day-old infarction

BACKGROUND

Administration of cardiac progenitor cells (CPCs) 4 hours after reperfusion ameliorates left ventricular function in rats with acute myocardial infarction (MI). Clinically, however, this approach is not feasible, because expansion of autologous CPCs after acute MI requires several weeks. Therefore, we sought to determine whether CPCs are beneficial in the more clinically relevant setting of an old MI (scar).

METHODS AND RESULTS

One month after coronary occlusion/reperfusion, rats received an intracoronary infusion of vehicle or enhanced green fluorescent protein-labeled CPCs. Thirty-five days later, CPC-treated rats exhibited more viable myocardium in the risk region, less fibrosis in the noninfarcted region, and improved left ventricular function. Cells that stained positive for enhanced green fluorescent protein that expressed cardiomyocyte, endothelial, and vascular smooth muscle cell markers were observed only in 7 of 17 treated rats and occupied only 2.6% and 1.1% of the risk and noninfarcted regions, respectively. Transplantation of CPCs was associated with increased proliferation and expression of cardiac proteins by endogenous CPCs.

CONCLUSIONS

Intracoronary administration of CPCs in the setting of an old MI produces beneficial structural and functional effects. Although exogenous CPCs can differentiate into new cardiac cells, this mechanism is not sufficient to explain the benefits, which suggests paracrine effects; among these, the present data identify activation of endogenous CPCs. This is the first report that CPCs are beneficial in the setting of an old MI when given by intracoronary infusion, the most widely applicable therapeutic approach in patients. Furthermore, this is the first evidence that exogenous CPC administration activates endogenous CPCs. These results open the door to new therapeutic applications for the use of autologous CPCs in patients with old MI and chronic ischemic cardiomyopathy.

Clonality of mouse and human cardiomyogenesis in vivo

An analysis of the clonality of cardiac progenitor cells (CPCs) and myocyte turnover in vivo requires genetic tagging of the undifferentiated cells so that the clonal marker of individual mother cells is traced in the specialized progeny. CPC niches in the atria and apex of the mouse heart were infected with a lentivirus carrying EGFP, and the destiny of the tagged cells was determined 1-5 months later. A common integration site was identified in isolated CPCs, cardiomyocytes, endothelial cells (ECs), and fibroblasts, documenting CPC self-renewal and multipotentiality and the clonal origin of the differentiated cell populations. Subsequently, the degree of EGFP-lentiviral infection of CPCs was evaluated 2-4 days after injection, and the number of myocytes expressing the reporter gene was measured 6 months later. A BrdU pulse-chasing protocol was also introduced as an additional assay for the analysis of myocyte turnover. Over a period of 6 months, each EGFP-positive CPC divided approximately eight times generating 230 cardiomyocytes; this value was consistent with the number of newly formed cells labeled by BrdU. To determine whether, human CPCs (hCPCs) are self-renewing and multipotent, these cells were transduced with the EGFP-lentivirus and injected after acute myocardial infarction in immunosuppressed rats. hCPCs, myocytes, ECs, and fibroblasts collected from the regenerated myocardium showed common viral integration sites in the human genome. Thus, our results indicate that the adult heart contains a pool of resident stem cells that regulate cardiac homeostasis and repair.