Man Ryul Lee

Hematopoietic stem/progenitor cells, generation of induced pluripotent stem cells, and isolation of endothelial progenitors from 21- to 23.5-year cryopreserved cord blood

Cryopreservation of hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) is crucial for cord blood (CB) banking and transplantation. We evaluated recovery of functional HPC cryopreserved as mononuclear or unseparated cells for up to 23.5 years compared with prefreeze values of the same CB units. Highly efficient recovery (80%-100%) was apparent for granulocyte-macrophage and multipotential hematopoietic progenitors, although some collections had reproducible low recovery. Proliferative potential, response to multiple cytokines, and replating of HPC colonies was extensive. CD34(+) cells isolated from CB cryopreserved for up to 21 years had long-term (≥ 6 month) engrafting capability in primary and secondary immunodeficient mice reflecting recovery of long-term repopulating, self-renewing HSCs. We recovered functionally responsive CD4(+) and CD8(+) T lymphocytes, generated induced pluripotent stem (iPS) cells with differentiation representing all 3 germ cell lineages in vitro and in vivo, and detected high proliferative endothelial colony forming cells, results of relevance to CB biology and banking.

SIRT1 Positively Regulates Autophagy and Mitochondria Function in Embryonic Stem Cells Under Oxidative Stress

SIRT1, an NAD‐dependent deacetylase, plays a role in regulation of autophagy. SIRT1 increases mitochondrial function and reduces oxidative stress, and has been linked to age‐related reactive oxygen species (ROS) generation, which is highly dependent on mitochondrial metabolism. H2O2 induces oxidative stress and autophagic cell death through interference with Beclin 1 and the mTOR signaling pathways. We evaluated connections between SIRT1 activity and induction of autophagy in murine (m) and human (h) embryonic stem cells (ESCs) upon ROS challenge. Exogenous H2O2 (1 mM) induced apoptosis and autophagy in wild‐type (WT) and Sirt1−/− mESCs. High concentrations of H2O2 (1 mM) induced more apoptosis in Sirt1−/−, than in WT mESCs. However, addition of 3‐methyladenine, a widely used autophagy inhibitor, in combination with H2O2 induced more cell death in WT than in Sirt1−/− mESCs. Decreased induction of autophagy in Sirt1−/− mESCs was demonstrated by decreased conversion of LC3‐I to LC3‐II, lowered expression of Beclin‐1, and decreased LC3 punctae and LysoTracker staining. H2O2 induced autophagy with loss of mitochondrial membrane potential and disruption of mitochondrial dynamics in Sirt1−/− mESCs. Increased phosphorylation of P70/85‐S6 kinase and ribosomal S6 was noted in Sirt1−/− mESCs, suggesting that SIRT1 regulates the mTOR pathway. Consistent with effects in mESCs, inhibition of SIRT1 using Lentivirus‐mediated SIRT1 shRNA in hESCs demonstrated that knockdown of SIRT1 decreased H2O2‐induced autophagy. This suggests a role for SIRT1 in regulating autophagy and mitochondria function in ESCs upon oxidative stress, effects mediated at least in part by the class III PI3K/Beclin 1 and mTOR pathways. Stem Cells 2014;32:1183–1194

Enhancing Hematopoietic Stem Cell Transplantation Efficacy by Mitigating Oxygen Shock

Hematopoietic stem cells (HSCs) reside in hypoxic niches within bone marrow and cord blood. Yet, essentially all HSC studies have been performed with cells isolated and processed in non-physiologic ambient air. By collecting and manipulating bone marrow and cord blood in native conditions of hypoxia, we demonstrate that brief exposure to ambient oxygen decreases recovery of long-term repopulating HSCs and increases progenitor cells, a phenomenon we term extraphysiologic oxygen shock/stress (EPHOSS). Thus, true numbers of HSCs in the bone marrow and cord blood are routinely underestimated. We linked ROS production and induction of the mitochondrial permeability transition pore (MPTP) via cyclophilin D and p53 as mechanisms of EPHOSS. The MPTP inhibitor cyclosporin A protects mouse bone marrow and human cord blood HSCs from EPHOSS during collection in air, resulting in increased recovery of transplantable HSCs. Mitigating EPHOSS during cell collection and processing by pharmacological means may be clinically advantageous for transplantation.

Differentiation of human pluripotent stem cells to cells similar to cord-blood endothelial colony–forming cells

The ability to differentiate human pluripotent stem cells into endothelial cells with properties of cord-blood endothelial colony–forming cells (CB-ECFCs) may enable the derivation of clinically relevant numbers of highly proliferative blood vessel–forming cells to restore endothelial function in patients with vascular disease. We describe a protocol to convert human induced pluripotent stem cells (hiPSCs) or embryonic stem cells (hESCs) into cells similar to CB-ECFCs at an efficiency of >108 ECFCs produced from each starting pluripotent stem cell. The CB-ECFC-like cells display a stable endothelial phenotype with high clonal proliferative potential and the capacity to form human vessels in mice and to repair the ischemic mouse retina and limb, and they lack teratoma formation potential. We identify Neuropilin-1 (NRP-1)-mediated activation of KDR signaling through VEGF165 as a critical mechanism for the emergence and maintenance of CB-ECFC-like cells.

Epigenetic Regulation of Nanog by MiR-302 Cluster-MBD2 Completes Induced Pluripotent Stem Cell Reprogramming

While most somatic cells undergoing induced pluripotent stem (iPS) cell reprogramming with Yamanaka factors accumulate at stable partially reprogrammed stages, the molecular mechanisms required to achieve full reprogramming are unknown. MicroRNAs (miRNAs) fine‐tune mRNA translation and are implicated in reprogramming, but miRNA functional targets critical for complete iPS cell reprogramming remain elusive. We identified methyl‐DNA binding domain protein 2 (MBD2) as an epigenetic suppressor, blocking full reprogramming of somatic to iPS cells through direct binding to NANOG promoter elements preventing transcriptional activation. When we overexpressed miR‐302 cluster we observed a significant increase in conversion of partial to fully reprogrammed iPS cells by suppressing MBD2 expression, thereby increasing NANOG expression. Thus, expression of exogenous miR‐302 cluster (without miR‐367) is efficient in attaining a fully reprogrammed iPS state in partially reprogrammed cells by relieving MBD2‐mediated inhibition of NANOG expression. Our studies provide a direct molecular mechanism involved in generating complete human iPS cell reprogramming to study disease pathogenesis, drug screening, and for potential cell‐based therapies. STEM CELLS 2013;31:666–681

5-Aminoimidazole-4-carboxyamide Ribonucleoside Induces G1/S Arrest and Nanog Downregulation via p53 and Enhances Erythroid Differentiation

Molecular mechanisms of how energy metabolism affects embryonic stem cell (ESC) pluripotency remain unclear. AMP‐activated protein kinase (AMPK), a key regulator for controlling energy metabolism, is activated in response to ATP‐exhausting stress. We investigated whether cellular energy homeostasis is associated with maintenance of self‐renewal and pluripotency in mouse ESCs (mESCs) by using 5‐aminoimidazole‐4‐carboxyamide ribonucleoside (AICAR) as an activator of AMPK. We demonstrate that AICAR treatment activates the p53/p21 pathway and markedly inhibits proliferation of R1 mESCs by inducing G1/S‐phase cell cycle arrest, without influencing apoptosis. Treatment with AICAR also significantly reduces pluripotent stem cell markers, Nanog and stage‐specific embryonic antigen‐1, in the presence of leukemia inhibitory factor, without affecting expression of Oct4. H9 human ESCs also responded to AICAR with induction of p53 activation and repression of Nanog expression. AICAR reduced Nanog mRNA levels in mESCs transiently, an effect not due to expression of miR‐134 which can suppress Nanog expression. AICAR induced Nanog degradation, an effect inhibited by MG132, a proteasome inhibitor. Although AICAR reduced embryoid body formation from mESCs, it increased expression levels of erythroid cell lineage markers (Ter119, GATA1, Klf1, Hbb‐b, and Hbb‐bh1). Although erythroid differentiation was enhanced by AICAR, endothelial lineage populations were remarkably reduced in AICAR‐treated cells. Our results suggest that energy metabolism regulated by AMPK activity may control the balance of self‐renewal and differentiation of ESCs. STEM CELLS 2012; 30:140–149.

MiR-31/SDHA Axis Regulates Reprogramming Efficiency through Mitochondrial Metabolism

Summary Metabolism is remodeled when somatic cells are reprogrammed into induced pluripotent stem cells (iPSCs), but the majority of iPSCs are not fully reprogrammed. In a shift essential for reprogramming, iPSCs use less mitochondrial respiration but increased anaerobic glycolysis for bioenergetics. We found that microRNA 31 (miR-31) suppressed succinate dehydrogenase complex subunit A (SDHA) expression, vital for mitochondrial electron transport chain (ETC) complex II. MiR-31 overexpression in partially reprogrammed iPSCs lowered SDHA expression levels and oxygen consumption rates to that of fully reprogrammed iPSCs, but did not increase the proportion of fully reprogrammed TRA1-60+ cells in colonies unless miR-31 was co-transduced with Yamanaka factors, which resulted in a 2.7-fold increase in full reprogramming. Thus switching from mitochondrial respiration to glycolytic metabolism through regulation of the miR-31/SDHA axis is critical for lowering the reprogramming threshold. This is supportive of multi-stage reprogramming whereby metabolic remodeling is fundamental.

Spontaneously Differentiated GATA6-Positive Human Embryonic Stem Cells Represent an Important Cellular Step in Human Embryonic Development; They Are Not Just an Artifact of In Vitro Culture

In this study, we isolated and characterized spontaneously differentiated human embryonic stem cells (SD-hESCs) found in hESC colonies in comparison to the morphologically premature ESCs in the colonies to investigate the potential role of SD-hESCs in embryogenesis. SD-hESCs were distinguished from undifferentiated hESCs by their higher expression of GATA6, a marker for primitive endoderm and transthyretin, a marker visceral endoderm in embryoid bodies (EBs). SD-hESCs expressed OCT4 and NANOG, markers for pluripotent stem cells, at significantly lower levels than undifferentiated hESCs. EBs derived from isolated SD-hESCs were morphologically distinct from cells directly derived from the undifferentiated hESCs; they contained higher number of cysts compared to EBs from undifferentiated hESC-derived EBs (42% vs. 20%). Furthermore, the extracellular signal molecule, BMP2/4, induced a higher GATA4/6 expression and cystic EB formation than control and noggin-treated EBs. Since cystic formation in EBs play a role in primitive endoderm formation during embryogenesis, the SD-hESC may be a relevant cell type equipped to differentiate into primitive endoderm. Our results suggest that SD-ESCs generated during routine hESC culture are not just an artifact of in vitro culture and these cells could serve as a useful model to study the process of embryogenesis.

Generating autologous hematopoietic cells from human-induced pluripotent stem cells through ectopic expression of transcription factors

Purpose of review Hematopoietic cell transplantation (HCT) is a successful treatment modality for patients with malignant and nonmalignant disorders, usually when no other treatment option is available. The cells supporting long-term reconstitution after HCT are the hematopoietic stem cells (HSCs), which can be limited in numbers. Moreover, finding an appropriate human leukocyte antigen-matched donor can be problematic. If HSCs can be stably produced in large numbers from autologous or allogeneic cell sources, it would benefit HCT. Induced pluripotent stem cells (iPSCs) established from patients’ own somatic cells can be differentiated into hematopoietic cells in vitro. This review will highlight recent methods for regulating human (h) iPSC production of HSCs and more mature blood cells. Recent findings Advancements in transcription factor-mediated regulation of the developmental stages of in-vivo hematopoietic lineage commitment have begun to provide an understanding of the molecular mechanism of hematopoiesis. Such studies involve not only directed differentiation in which transcription factors, specifically expressed in hematopoietic lineage-specific cells, are overexpressed in iPSCs, but also direct conversion in which transcription factors are introduced into patient-derived somatic cells which are dedifferentiated to hematopoietic cells. As iPSCs derived from patients suffering from genetically mutated diseases would express the same mutated genetic information, CRISPR-Cas9 gene editing has been utilized to differentiate genetically corrected iPSCs into normal hematopoietic cells. Summary IPSCs provide a model for molecular understanding of disease, and also may function as a cell population for therapy. Efficient differentiation of patient-specific iPSCs into HSCs and progenitor cells is a potential means to overcome limitations of such cells for HCT, as well as for providing in-vitro drug screening templates as tissue-on-a-chip models.

Mechanotransduction of human pluripotent stem cells cultivated on tunable cell-derived extracellular matrix

Cell-derived matrices (CDM) are becoming an attractive alternative to conventional biological scaffolding platforms due to its unique ability to closely recapitulate a native extracellular matrix (ECM) de novo. Although cell-substrate interactions are recognized to be principal in regulating stem cell behavior, very few studies have documented the acclimation of human pluripotent stem cells (hPSCs) on pristine and altered cell-derived matrices. Here, we investigate crosslink-induced mechanotransduction of hPSCs cultivated on decellularized fibroblast-derived matrices (FDM) to explore cell adhesion, growth, migration, and pluripotency in various biological landscapes. The results showed either substrate-mediated induction or inhibition of the Epithelial-Mesenchymal-Transition (EMT) program, strongly suggesting that FDM stiffness can be a dominant factor in mediating hPSC plasticity. We further propose an optimal FDM substratum intended for long-term hPSC cultivation in a feeder-free niche-like microenvironment. This study carries significant implications for hPSC cultivation and encourages more in-depth studies towards the fundamentals of hPSC-CDM interactions.