Maty Tzukerman

Identification and selection of cardiomyocytes during human embryonic stem cell differentiation

Human embryonic stem cells (hESC) are pluripotent lines that can differentiate in vitro into cell derivatives of all three germ layers, including cardiomy‐ocytes. Successful application of these unique cells in the areas of cardiovascular research and regenerative medicine has been hampered by difficulties in identifying and selecting specific cardiac progenitor cells from the mixed population of differentiating cells. We report the generation of stable transgenic hESC lines, using lentiviral vectors, and single‐cell clones that express a reporter gene (eGFP) under the transcriptional control of a cardiac‐specific promoter (the human myosin light chain‐2V promoter). Our results demonstrate the appearance of eGFP‐expressing cells during the differentiation of the hESC as embryoid bodies (EBs) that can be identified and sorted using FACS (purity>95%, viability>85%). The eGFP‐expressing cells were stained positively for cardiac‐specific proteins (>93%), expressed cardiac‐specific genes, displayed cardiac‐specific action‐potentials, and could form stable myocardial cell grafts following in vivo cell transplantation. The generation of these transgenic hESC lines may be used to identify and study early cardiac precursors for developmental studies, to robustly quantify the extent of cardiomyocyte differentiation, to label the cells for in vivo grafting, and to allow derivation of purified cell populations of cardiomyocytes for future myocardial cell therapy strategies.—Huber, I., Itzhaki, I., Caspi, O., Arbel, G., Tzukerman, M., Gepstein, A., Habib, M., Yankelson, L., Kehat, I., Gepstein, L. Identification and selection of cardiomy‐ocytes during human embryonic stem cell differentiation. FASEB J. 21, 2551–2563 (2007)

Multipotent Vasculogenic Pericytes From Human Pluripotent Stem Cells Promote Recovery of Murine Ischemic Limb

Background— Pericytes represent a unique subtype of microvessel-residing perivascular cells with diverse angiogenic functions and multilineage developmental features of mesenchymal stem cells. Although various protocols for derivation of endothelial and/or smooth muscle cells from human pluripotent stem cells (hPSC, either embryonic or induced) have been described, the emergence of pericytes in the course of hPSC maturation has not yet been elucidated. Methods and Results— We found that during hPSC development, spontaneously differentiating embryoid bodies give rise to CD105+CD90+CD73+CD31− multipotent clonogenic mesodermal precursors, which can be isolated and efficiently expanded. Isolated and propagated cells expressed characteristic pericytic markers, including CD146, NG2, and platelet-derived growth factor receptor &bgr;, but not the smooth muscle cell marker &agr;-smooth muscle actin. Coimplantation of hPSC-derived endothelial cells with pericytes resulted in functional and rapid anastomosis to the murine vasculature. Administration of pericytes into immunodeficient mice with limb ischemia promoted significant vascular and muscle regeneration. At day 21 after transplantation, recruited hPSC pericytes were found incorporated into recovered muscle and vasculature. Conclusions— Derivation of vasculogenic and multipotent pericytes from hPSC can be used for the development of vasculogenic models using multiple vasculogenic cell types for basic research and drug screening and can contribute to angiogenic regenerative medicine.

p53-dependent Down-regulation of Telomerase Is Mediated by p21waf1

Inactivation of p53 and activation of telomerase occur in the majority of human cancers, raising the possibility of a link between these two pathways. Overexpression of wild-type p53 down-regulates the enzymatic activity of telomerase in various cancer cell lines through transcriptional repression of its catalytic subunit, human telomerase reverse transcriptase (hTERT). In this study, we re-evaluated the role of p53 in telomerase regulation using isogenic cell lines expressing physiological levels of p53. We demonstrate that endogenous wild-type p53 was able to down-regulate telomerase activity, hTERT mRNA levels, and promoter activity; however, the ability to repress hTERT expression was found to be cell type-specific. The integrity of the DNA-binding core domain, the N-terminal transactivation domain, and the C-terminal oligomerization domains of p53 was essential for hTERT promoter repression, whereas the proline-rich domain and the extreme C terminus were not required. Southwestern and chromatin immunoprecipitation experiments demonstrated lack of p53 binding to the hTERT promoter, raising the possibility of an indirect repressive mechanism. The down-regulation of hTERT promoter activity was abolished by a dominant-negative E2F1 mutant. Mutational analysis identified a specific E2F site responsible for p53-mediated repression. Knockdown of the key p53 transcriptional target, p21, was sufficient to eliminate the p53-dependent repression of hTERT. Inactivation of the Rb family using either viral oncoproteins or RNA interference attenuated the repression. Inhibition of histone deacetylases also interfered with the repression of hTERT by p53. Therefore, our results suggest that repression of hTERT by endogenous p53 is mediated by p21 and E2F.

Kidney-specific overexpression of Sirt1 protects against acute kidney injury by retaining peroxisome function

Sirt1, a NAD-dependent protein deacetylase, is reported to regulate intracellular metabolism and attenuate reactive oxidative species (ROS)-induced apoptosis leading to longevity and acute stress resistance. We created transgenic (TG) mice with kidney-specific overexpression of Sirt1 using the promoter sodium-phosphate cotransporter IIa (Npt2) driven specifically in proximal tubules and investigated the kidney-specific role of Sirt1 in the protection against acute kidney injury (AKI). We also elucidated the role of number or function of peroxisome and mitochondria in mediating the mechanisms for renal protective effects of Sirt1 in AKI. Cisplatin-induced AKI decreased the number and function of peroxisomes as well as mitochondria and led to increased local levels of ROS production and renal tubular apoptotic cells. TG mice treated with cisplatin mitigated AKI, local ROS, and renal tubular apoptotic tubular cells. Consistent with these results, TG mice treated with cisplatin also exhibited recovery of peroxisome number and function, as well as rescued mitochondrial function; however, mitochondrial number was not recovered. Immunoelectron microscopic findings consistently demonstrated that the decrease in peroxisome number by cisplatin in wild type mice was restored in transgenic mice. In HK-2 cells, a cultured proximal tubule cell line, overexpression of Sirt1 rescued the cisplatin-induced cell apoptosis through the restoration of peroxisome number, although the mitochondria number was not restored. These results indicate that Sirt1 overexpression in proximal tubules rescues cisplatin-induced AKI by maintaining peroxisomes number and function, concomitant up-regulation of catalase, and elimination of renal ROS levels. Renal Sirt1 can be a potential therapeutic target for the treatment of AKI.

An experimental platform for studying growth and invasiveness of tumor cells within teratomas derived from human embryonic stem cells

There is currently no available experimental system wherein human cancer cells can be grown in the context of a mixed population of normal differentiated human cells for testing biological aspects of cancer cell growth (e.g., tumor cell invasion and angiogenesis) or response to anti-cancer therapies. When implanted into immunocompromised mice, human embryonic stem cells develop teratomas containing complex structures comprising differentiated cell types representing the major germ line-derived lineages. We sought to determine whether human cancer cells would grow within such teratomas and display properties associated with malignancy, such as invasiveness and recruitment of blood vessels. HEY ovarian cancer cells stably expressing an H2A-GFP fusion protein (HEY-GFP) injected into mature teratomas developed into tumors, which allowed tracking of tumor cell invasion and recruitment of human teratoma-derived blood vessels. This provides a straightforward and powerful approach to studying the biological properties of cancer cells within the microenvironment of normal differentiated human cells.

Cell lineage analysis of acute leukemia relapse uncovers the role of replication-rate heterogeneity and microsatellite instability

Human cancers display substantial intratumoral genetic heterogeneity, which facilitates tumor survival under changing microenvironmental conditions. Tumor substructure and its effect on disease progression and relapse are incompletely understood. In the present study, a high-throughput method that uses neutral somatic mutations accumulated in individual cells to reconstruct cell lineage trees was applied to hundreds of cells of human acute leukemia harvested from multiple patients at diagnosis and at relapse. The reconstructed cell lineage trees of patients with acute myeloid leukemia showed that leukemia cells at relapse were shallow (divide rarely) compared with cells at diagnosis and were closely related to their stem cell subpopulation, implying that in these instances relapse might have originated from rarely dividing stem cells. In contrast, among patients with acute lymphoid leukemia, no differences in cell depth were observed between diagnosis and relapse. In one case of chronic myeloid leukemia, at blast crisis, most of the cells at relapse were mismatch-repair deficient. In almost all leukemia cases, > 1 lineage was observed at relapse, indicating that diverse mechanisms can promote relapse in the same patient. In conclusion, diverse relapse mechanisms can be observed by systematic reconstruction of cell lineage trees of patients with leukemia.

Reprogramming of telomeric regions during the generation of human induced pluripotent stem cells and subsequent differentiation into fibroblast-like derivatives

Human induced pluripotent stem (hiPS) cells provide therapeutic promises, as well as a potent in vitro model for studying biological processes which take place during human embryonic development and subsequent differentiation in normal and disease states. The epigenetic characteristics of iPS cells are reprogrammed to the embryonic state at which they acquire pluripotency. In addition, telomeres in hiPS cell must elongate sufficiently to provide the necessary replicative potential. Recent studies have demonstrated that the epigenetic characteristics of telomeric and subtelomeric regions are pivotal in regulating telomere length. Here we study telomere length, subtelomeric DNA methylation and telomeric-repeat-containing RNA (TERRA) expression in several hiPS cell clones derived from normal neonatal foreskin fibroblasts. We find that telomeres lengthen significantly in hiPS cells in comparison to the parental fibroblast source, and progressively shorten after differentiation back into fibroblast-like cells, concomitantly with telomerase activation and down-regulation, respectively. Subtelomeres in hiPS cells were found to be generally hypermethylated in comparison to the parental source. However bisulfite analysis revealed that at several subtelomeres examined, methylation levels differed between hiPS clones and that both de novo methylation and demethylation processes occurred during telomere reprogramming. Notably, although subtelomeres were in general very highly methylated, TERRA levels were elevated in hiPS cells, albeit to different degrees in the various clones. TERRA elevation may reflect enhanced stability or impaired degradation in hiPS cells, and/or alternatively, increased transcription from the hypomethylated subtelomeres. We suggest that TERRA may play a role in regulation of appropriate telomere function and length in hiPS cells.

The Influence of a Human Embryonic Stem Cell–Derived Microenvironment on Targeting of Human Solid Tumor Xenografts

The awareness of the important role that the surrounding tissue microenvironment and stromal response play in the process of tumorigenesis has grown as a result of in vivo models of tumor xenograft growth in immunocompromised mice. In the current study, we used human embryonic stem cells in order to study the interactions of tumor cells with the surrounding microenvironment of differentiated human cell tissues and structures. Several cancer cell types stably expressing an H2A-green fluorescence protein fusion protein, which allowed tracking of tumor cells, were injected into mature teratomas and developed into tumors. The salient findings were: (a) the observation of growth of tumor cells with high proliferative capacity within the differentiated microenvironment of the teratoma, (b) the identification of invasion by tumor cells into surrounding differentiated teratoma structures, and (c) the identification of blood vessels of human teratoma origin, growing adjacent to and within the cancer cell-derived tumor. Mouse embryonic stem cell-derived teratomas also supported cancer cell growth, but provided a less suitable model for human tumorigenesis studies. Anticancer immunotherapy treatment directed against A431 epidermoid carcinoma cell-related epitopes induced the complete regression of A431-derived tumor xenografts following direct i.m. injection in immunocompromised mice, as opposed to corresponding tumors growing within a human embryonic stem cell-derived microenvironment, wherein remnant foci of viable tumor cells were detected and resulted in tumor recurrence. We propose using this novel experimental model as a preclinical platform for investigating and manipulating the stromal response in tumor cell growth as an additional tool in cancer research.

Niche-Dependent Tumorigenic Capacity of Malignant Ovarian Ascites-Derived Cancer Cell Subpopulations

Purpose: In previous studies, we have used human embryonic stem cells (hESC) to generate a tissue microenvironment in immunocompromised mice as an experimental approach for studying human tumorigenesis. We now examine the attributes of such a cellular microenvironment in supporting the growth of human cancer cells freshly harvested from malignant ovarian ascites and to determine whether there are differences among subsets of ascites-derived cancer cells in terms of tumorigenic capacity in the conventional murine xenograft model and in the hESC-derived microenvironment. Experimental Design: Freshly harvested malignant ovarian ascites-derived cancer cells and six derivative ovarian cancer cell subpopulations (CCSP) were characterized for ovarian cancer–associated biomarker expression both in vitro and in vivo and for their capacity to generate tumors in the two models. Results: Ovarian cancer–associated biomarkers were detected in the ascites-derived cancer cells and in the six newly established CCSPs. Nevertheless, certain CCSPs that did not develop into tumors in a conventional murine xenograft model did generate tumors in the hESC-derived cellular microenvironment, indicating variable niche dependency for the tumorigenic capacity of the different CCSPs. The hESC-derived microenvironment provided an improved niche for supporting growth of certain tumor cell subpopulations. Conclusions: The results highlight the experimental utility of the hESC-derived cellular microenvironment to enable functional distinction of CCSPs, including the identification of cells that do not grow into a tumor in the conventional direct tumor xenograft platform, thereby rendering such cells accessible to characterization and testing of anticancer therapies.

Intratumoral Heterogeneity in the Self-Renewal and Tumorigenic Differentiation of Ovarian Cancer†‡§

Resistance to anticancer therapy has been attributed to interindividual differences in gene expression pathways among tumors, and to the existence within tumors of cancer stem cells with self‐renewal capacity. In previous studies, we have demonstrated that the human embryonic stem cell (hESC)‐derived cellular microenvironment in immunocompromised mice enables functional distinction of heterogeneous tumor cells, including cells that do not grow into a tumor in conventional direct tumor xenograft platform. In the current study, we use clonally expanded subpopulations derived from ovarian clear cell carcinoma of a single tumor, to demonstrate striking intratumoral phenotypic heterogeneity that is dynamically dependent on the tumor growth microenvironment. Each of six clonally expanded subpopulations displays a different level of morphologic and tumorigenic differentiation, wherein growth in the hESC‐derived microenvironment favors growth of CD44+ aldehyde dehydrogenase positive pockets of self‐renewing cells that sustain tumor growth through a process of tumorigenic differentiation into CD44− aldehyde dehydrogenase negative derivatives. Strikingly, these derivative cells display microenvironment‐dependent plasticity with the capacity to restore self‐renewal and CD44 expression. Such intratumoral heterogeneity and plasticity at the level of the key properties of self‐renewal and tumorigenic differentiation suggests that a paradigm shift is needed in the approach to anticancer therapy, with the aim of turning malignant growth into a chronic manageable disorder, based on continual monitoring of these tumor growth properties. The hESC‐based in vivo model renders intratumoral heterogeneity in the self‐renewal and tumorigenic differentiation amenable to biological analysis as well as anticancer therapy testing. STEM CELLS 2012;30:415–424