Telomeres acquire embryonic stem cell characteristics in induced pluripotent stem cells

SIRT1 is necessary for proficient telomere elongation and genomic stability of induced pluripotent stem cells

The NAD-dependent deacetylase SIRT1 is involved in chromatin silencing and genome stability. Elevated SIRT1 levels in embryonic stem cells also suggest a role for SIRT1 in pluripotency. Murine SIRT1 attenuates telomere attrition in vivo and is recruited at telomeres in induced pluripotent stem cells (iPSCs). Because telomere elongation is an iPSC hallmark, we set out to study the role of SIRT1 in pluripotency in the setting of murine embryonic fibroblasts reprogramming into iPSCs. We find that SIRT1 is required for efficient postreprogramming telomere elongation, and that this effect is mediated by a c-MYC-dependent regulation of the mTert gene. We further demonstrate that SIRT1-deficient iPSCs accumulate chromosomal aberrations and show a derepression of telomeric heterochromatin. Finally, SIRT1-deficient iPSCs form larger teratomas that are poorly differentiated, highlighting a role for SIRT1 in exit from pluripotency. In summary, this work demonstrates a role for SIRT1 in the maintenance of pluripotency and modulation of differentiation.

Telomerase reverse transcriptase has an extratelomeric function in somatic cell reprogramming

Reactivation of the endogenous telomerase reverse transcriptase (TERT) catalytic subunit and telomere elongation occur during the reprogramming of somatic cells to induced pluripotent stem (iPS) cells. However, the role of TERT in the reprogramming process is unclear. To clarify its function, the reprogramming process was examined in TERT-KO somatic cells. To exclude the effect of telomere elongation, tail-tip fibroblasts (TTFs) from first generation TERT-KO mice were used. Although iPS cells were successfully generated from TERT-KO TTFs, the efficiency of reprogramming these cells was markedly lower than that of WT TTFs. The gene expression profiles of iPS cells induced from TERT-KO TTFs were similar to those of WT iPS cells and ES cells, and TERT-KO iPS cells formed teratomas that differentiated into all three germ layers. These data indicate that TERT plays an extratelomeric role in the reprogramming process, but its function is dispensable. However, TERT-KO iPS cells showed transient defects in growth and teratoma formation during continuous growth. In addition, TERT-KO iPS cells developed chromosome fusions that accumulated with increasing passage numbers, consistent with the fact that TERT is essential for the maintenance of genome structure and stability in iPS cells. In a rescue experiment, an enzymatically inactive mutant of TERT (D702A) had a positive effect on somatic cell reprogramming of TERT-KO TTFs, which confirmed the extratelomeric role of TERT in this process.

Quality control: Genome maintenance in pluripotent stem cells

Pluripotent stem cells (PSCs) must maintain their proper genomic content in order to preserve appropriate self-renewal and differentiation capacities. However, their prolonged in vitro propagation, as well as the environmental culture conditions, present serious challenges to genome maintenance. Recent work has been focused on potential means to alleviate the genomic insults experienced by PSCs, and to detect them as soon as they arise, in order to prevent the detrimental consequences of these genomic aberrations on PSC application in basic research and regenerative medicine.

Breaking new ground: digging into TERRA function

Despite the fact that telomeres carry chromatin marks typically associated with silent heterochromatin, they are actively transcribed into TElomeric Repeat containing RNA (TERRA). TERRA transcription is conserved from yeast to man, initiates in the subtelomeric region and proceeds through the telomeric tract of presumably each individual telomere. TERRA levels are increased in yeast survivors and in cancer cells employing ALT as a telomere maintenance mechanism (TMM). Thus, TERRA may be a promising biomarker and potential target in anti-cancer therapy. Interestingly, several recent publications implicate TERRA in regulatory processes including telomere end protection and the establishment of the heterochromatic state at telomeres. A picture is emerging whereby TERRA acts as a regulator of telomere length and hence the associated onset of replicative senescence in a cell. In this review we will summarize the latest results regarding TERRA transcription, localization and related function. A special focus will be set on the potential role of TERRA in the regulation of telomere length and replicative senescence. Possible implications of increased TERRA levels in yeast survivors and in ALT cancer cells will be discussed.

Phases of reprogramming

Despite advances in the field of somatic cell reprogramming, an understanding and exploration of the underlying mechanisms governing this process are only recently emerging. It is now increasingly apparent that key sequential events correlate with the reprogramming process; a process previously thought to be random and unpredictable is now looking, to a greater extent, defined and controlled. Herein, we will review the key cellular and molecular events associated with the reprogramming process, giving an integrative and conciliatory view of the different studies addressing the mechanism of nuclear reprogramming.

Telomerase protects werner syndrome lineage-specific stem cells from premature aging

Werner syndrome (WS) patients exhibit premature aging predominantly in mesenchyme-derived tissues, but not in neural lineages, a consequence of telomere dysfunction and accelerated senescence. The cause of this lineage-specific aging remains unknown. Here, we document that reprogramming of WS fibroblasts to pluripotency elongated telomere length and prevented telomere dysfunction. To obtain mechanistic insight into the origin of tissue-specific aging, we differentiated iPSCs to mesenchymal stem cells (MSCs) and neural stem/progenitor cells (NPCs). We observed recurrence of premature senescence associated with accelerated telomere attrition and defective synthesis of the lagging strand telomeres in MSCs, but not in NPCs. We postulate this “aging” discrepancy is regulated by telomerase. Expression of hTERT or p53 knockdown ameliorated the accelerated aging phenotypein MSC, whereas inhibition of telomerase sensitized NPCs to DNA damage. Our findings unveil a role for telomerase in the protection of accelerated aging in a specific lineage of stem cells.

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Prevention of premature senescence with corrected telomeres in reprogrammed WS iPSCs

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Recurrence of premature senescence and telomere dysfunction in WS iPSC-derived MSCs

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Rescue of premature senescence in WS MSCs by hTERT overexpression or p53 depletion

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Telomerase protects and prevents NPCs from DNA damage

Functional differentiation of midbrain neurons from human cord blood-derived induced pluripotent stem cells

Introduction

Human induced pluripotent stem cells (hiPSCs) offer great promise for regenerative therapies or in vitro modelling of neurodegenerative disorders like Parkinson’s disease. Currently, widely used cell sources for the generation of hiPSCs are somatic cells obtained from aged individuals. However, a critical issue concerning the potential clinical use of these iPSCs is mutations that accumulate over lifetime and are transferred onto iPSCs during reprogramming which may influence the functionality of cells differentiated from them. The aim of our study was to establish a differentiation strategy to efficiently generate neurons including dopaminergic cells from human cord blood-derived iPSCs (hCBiPSCs) as a juvenescent cell source and prove their functional maturation in vitro.

Methods

The differentiation of hCBiPSCs was initiated by inhibition of transforming growth factor-β and bone morphogenetic protein signaling using the small molecules dorsomorphin and SB 431542 before final maturation was carried out. hCBiPSCs and differentiated neurons were characterized by immunocytochemistry and quantitative real time-polymerase chain reaction. Since functional investigations of hCBiPSC-derived neurons are indispensable prior to clinical applications, we performed detailed analysis of essential ion channel properties using whole-cell patch-clamp recordings and calcium imaging.

Results

A Sox1 and Pax6 positive neuronal progenitor cell population was efficiently induced from hCBiPSCs using a newly established differentiation protocol. Neuronal progenitor cells could be further maturated into dopaminergic neurons expressing tyrosine hydroxylase, the dopamine transporter and engrailed 1. Differentiated hCBiPSCs exhibited voltage-gated ion currents, were able to fire action potentials and displayed synaptic activity indicating synapse formation. Application of the neurotransmitters GABA, glutamate and acetylcholine induced depolarizing calcium signal changes in neuronal cells providing evidence for the excitatory effects of these ligand-gated ion channels during maturation in vitro.

Conclusions

This study demonstrates for the first time that hCBiPSCs can be used as a juvenescent cell source to generate a large number of functional neurons including dopaminergic cells which may serve for the development of novel regenerative treatment strategies.

Telomere regulation in pluripotent stem cells

Pluripotent stem cells (PSCs) have the potential to produce any types of cells from all three basic germ layers and the capacity to self-renew and proliferate indefinitely in vitro. The two main types of PSCs, embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), share common features such as colony morphology, high expression of Oct4 and Nanog, and strong alkaline phosphatase activity. In recent years, increasing evidences suggest that telomere length represents another important internal factor in maintaining stem cell pluripotency. Telomere length homeostasis and its structural integrity help to protect chromosome ends from recombination, end fusion, and DNA damage responses, ensuring the divisional ability of mammalian cells. PSCs generally exhibit high telomerase activity to maintain their extremely long and stable telomeres, and emerging data indicate the alternative lengthening of telomeres (ALT) pathway may play an important role in telomere functions too. Such characteristics are likely key to their abilities to differentiate into diverse cell types in vivo. In this review, we will focus on the function and regulation of telomeres in ESCs and iPSCs, thereby shedding light on the importance of telomere length to pluripotency and the mechanisms that regulate telomeres in PSCs.

Dedifferentiation and reprogramming: origins of cancer stem cells

Regenerative medicine aims to replace the lost or damaged cells in the human body through a new source of healthy transplanted cells or by endogenous repair. Although human embryonic stem cells were first thought to be the ideal source for cell therapy and tissue repair in humans, the discovery by Yamanaka and colleagues revolutionized the field. Almost any differentiated cell can be sent back in time to a pluripotency state by expressing the appropriate transcription factors. The process of somatic reprogramming using Yamanaka factors, many of which are oncogenes, offers a glimpse into how cancer stem cells may originate. In this review we discuss the similarities between tumor dedifferentiation and somatic cell reprogramming and how this may pose a risk to the application of this new technology in regenerative medicine.

Telomeric noncoding RNA: telomeric repeat-containing RNA in telomere biology

Telomeres are nucleoprotein structures that cap the ends of eukaryotic chromosomes, protecting them from degradation and activation of DNA damage response. For this reason, functional telomeres are vital to genome stability. For years, telomeres were assumed to be transcriptionally silent, because of their heterochromatic state. It was only recently shown that, in several organisms, telomeres are transcribed, giving rise to a long noncoding RNA (lncRNA) called telomeric repeat-containing RNA (TERRA). Several lines of evidence now indicate that TERRA molecules play crucial roles in telomere homeostasis and telomere functions. Recent studies have shown that the expression and regulation of TERRA are dynamically controlled by each chromosome end. TERRA has been involved in the regulation of telomere length, telomerase activity, and heterochromatin formation at telomeres. The correct regulation of the telomeric transcripts may be essential to genome stability, and altered TERRA levels associate with tumorigenesis and cellular senescence. Thus, the study of the molecular mechanisms of TERRA biogenesis and function may advance the understanding of telomere-related diseases, including cancer and aging.

Aging of blood can be tracked by DNA methylation changes at just three CpG sites

Background

Human aging is associated with DNA methylation changes at specific sites in the genome. These epigenetic modifications may be used to track donor age for forensic analysis or to estimate biological age.

Results

We perform a comprehensive analysis of methylation profiles to narrow down 102 age-related CpG sites in blood. We demonstrate that most of these age-associated methylation changes are reversed in induced pluripotent stem cells (iPSCs). Methylation levels at three age-related CpGs - located in the genes ITGA2B, ASPA and PDE4C - were subsequently analyzed by bisulfite pyrosequencing of 151 blood samples. This epigenetic aging signature facilitates age predictions with a mean absolute deviation from chronological age of less than 5 years. This precision is higher than age predictions based on telomere length. Variation of age predictions correlates moderately with clinical and lifestyle parameters supporting the notion that age-associated methylation changes are associated more with biological age than with chronological age. Furthermore, patients with acquired aplastic anemia or dyskeratosis congenita - two diseases associated with progressive bone marrow failure and severe telomere attrition - are predicted to be prematurely aged.

Conclusions

Our epigenetic aging signature provides a simple biomarker to estimate the state of aging in blood. Age-associated DNA methylation changes are counteracted in iPSCs. On the other hand, over-estimation of chronological age in bone marrow failure syndromes is indicative for exhaustion of the hematopoietic cell pool. Thus, epigenetic changes upon aging seem to reflect biological aging of blood.

The aging signature: a hallmark of induced pluripotent stem cells?

The discovery that somatic cells can be induced into a pluripotent state by the expression of reprogramming factors has enormous potential for therapeutics and human disease modeling. With regard to aging and rejuvenation, the reprogramming process resets an aged, somatic cell to a more youthful state, elongating telomeres, rearranging the mitochondrial network, reducing oxidative stress, restoring pluripotency, and making numerous other alterations. The extent to which induced pluripotent stem cell (iPSC)s mime embryonic stem cells is controversial, however, as iPSCs have been shown to harbor an epigenetic memory characteristic of their tissue of origin which may impact their differentiation potential. Furthermore, there are contentious data regarding the extent to which telomeres are elongated, telomerase activity is reconstituted, and mitochondria are reorganized in iPSCs. Although several groups have reported that reprogramming efficiency declines with age and is inhibited by genes upregulated with age, others have successfully generated iPSCs from senescent and centenarian cells. Mixed findings have also been published regarding whether somatic cells generated from iPSCs are subject to premature senescence. Defects such as these would hinder the clinical application of iPSCs, and as such, more comprehensive testing of iPSCs and their potential aging signature should be conducted.

Telomere biology in stem cells and reprogramming

Telomerase expression in humans is restricted to different populations of stem and progenitor cells, being silenced in most somatic tissues. Efficient telomere homeostasis is essential for embryonic and adult stem cell function and therefore essential for tissue homeostasis throughout organismal life. Accordingly, the mutations in telomerase culminate in reduced stem cell function both in vivo and in vitro and have been associated with tissue dysfunction in human patients. Despite the importance of telomerase for stem cell biology, the mechanisms behind telomerase regulation during development are still poorly understood, mostly due to difficulties in acquiring and maintaining pluripotent stem cell populations in culture. In this chapter, we will analyze recent developments in this field, including the importance of efficient telomere homeostasis in different stem cell types and the role of telomerase in different techniques used for cellular reprogramming.

Human iPSC-based modeling of late-onset disease via progerin-induced aging

Reprogramming somatic cells to induced pluripotent stem cells (iPSCs) resets their identity back to an embryonic age and, thus, presents a significant hurdle for modeling late-onset disorders. In this study, we describe a strategy for inducing aging-related features in human iPSC-derived lineages and apply it to the modeling of Parkinson's disease (PD). Our approach involves expression of progerin, a truncated form of lamin A associated with premature aging. We found that expression of progerin in iPSC-derived fibroblasts and neurons induces multiple aging-related markers and characteristics, including dopamine-specific phenotypes such as neuromelanin accumulation. Induced aging in PD iPSC-derived dopamine neurons revealed disease phenotypes that require both aging and genetic susceptibility, such as pronounced dendrite degeneration, progressive loss of tyrosine hydroxylase (TH) expression, and enlarged mitochondria or Lewy-body-precursor inclusions. Thus, our study suggests that progerin-induced aging can be used to reveal late-onset age-related disease features in hiPSC-based disease models.

Enhanced telomere rejuvenation in pluripotent cells reprogrammed via nuclear transfer relative to induced pluripotent stem cells

Although somatic cell nuclear transfer (SCNT) and induction of pluripotency (to form iPSCs) are both recognized reprogramming methods, there has been relatively little comparative analysis of the resulting pluripotent cells. Here, we examine the capacity of these two reprogramming approaches to rejuvenate telomeres using late-generation telomerase-deficient (Terc(-/-)) mice that exhibit telomere dysfunction and premature aging. We found that embryonic stem cells established from Terc(-/-) SCNT embryos (Terc(-/-) ntESCs) have greater differentiation potential and self-renewal capacity than Terc(-/-) iPSCs. Remarkably, SCNT results in extensive telomere lengthening in cloned embryos and improved telomere capping function in the established Terc(-/-) ntESCs. In addition, mitochondrial function is severely impaired in Terc(-/-) iPSCs and their differentiated derivatives but significantly improved in Terc(-/-) ntESCs. Thus, our results suggest that SCNT-mediated reprogramming mitigates telomere dysfunction and mitochondrial defects to a greater extent than iPSC-based reprogramming. Understanding the basis of this differential could help optimize reprogramming strategies.

Understanding telomere diseases through analysis of patient-derived iPS cells

A unique characteristic of tissue stem cells is the ability to self-renew, a process that enables the life-long maintenance of many organs. Stem cell self-renewal is dependent in part on the synthesis of telomere repeats by the enzyme telomerase. Defects in telomerase and in genes in the telomere maintenance pathway result in diverse disease states, including dyskeratosis congenita, pulmonary fibrosis, aplastic anemia, liver cirrhosis and cancer. Many of these disease states share a tissue failure phenotype, such as loss of bone marrow cells or failure of pulmonary epithelium, suggesting that stem cell dysfunction is a common pathophysiological mechanism underlying these telomere diseases. Studies of telomere diseases in undifferentiated iPS cells have provided a quantitative relationship between the magnitude of biochemical defects in the telomerase pathway and disease severity in patients, thereby establishing a clear correlation between genotype and phenotype in telomere disease states. Modeling telomere diseases in iPS cells has also revealed diverse underlying disease mechanisms, including reduced telomerase catalytic activity, diminished assembly of the telomerase holoenzyme and impaired trafficking of the enzyme within the nucleus. These studies highlight the need for therapies tailored to the underlying biochemical defect in each class of patients.

Telomere reprogramming and maintenance in porcine iPS cells

Telomere reprogramming and silencing of exogenous genes have been demonstrated in mouse and human induced pluripotent stem cells (iPS cells). Pigs have the potential to provide xenotransplant for humans, and to model and test human diseases. We investigated the telomere length and maintenance in porcine iPS cells generated and cultured under various conditions. Telomere lengths vary among different porcine iPS cell lines, some with telomere elongation and maintenance, and others telomere shortening. Porcine iPS cells with sufficient telomere length maintenance show the ability to differentiate in vivo by teratoma formation test. IPS cells with short or dysfunctional telomeres exhibit reduced ability to form teratomas. Moreover, insufficient telomerase and incomplete telomere reprogramming and/or maintenance link to sustained activation of exogenous genes in porcine iPS cells. In contrast, porcine iPS cells with reduced expression of exogenous genes or partial exogene silencing exhibit insufficient activation of endogenous pluripotent genes and telomerase genes, accompanied by telomere shortening with increasing passages. Moreover, telomere doublets, telomere sister chromatid exchanges and t-circles that presumably are involved in telomere lengthening by recombination also are found in porcine iPS cells. These data suggest that both telomerase-dependent and telomerase-independent mechanisms are involved in telomere reprogramming during induction and passages of porcine iPS cells, but these are insufficient, resulting in increased telomere damage and shortening, and chromosomal instability. Active exogenes might compensate for insufficient activation of endogenous genes and incomplete telomere reprogramming and maintenance of porcine iPS cells. Further understanding of telomere reprogramming and maintenance may help improve the quality of porcine iPS cells.

Is aging a barrier to reprogramming? Lessons from induced pluripotent stem cells

The discovery of induced pluripotent stem cells (iPSCs) has the potential to revolutionize the field of regenerative medicine. In the past few years, iPSCs have been the subject of intensive research towards their application in disease modeling and drug screening. In the future, these cells may be applied in cell therapy to replace or regenerate tissues by autologous transplantation. However, two major hurdles need to be resolved in order to reach the later goal: the low reprogramming efficiency and the safety risks, such as the integration of foreign DNA into the genome of the cells and the tumor formation potential arising from transplantation of residual undifferentiated cells. Recently, aging emerged as one of the barriers that accounts, at least in part, for the low reprogramming efficiency of bona fide iPSCs. Here, we review the molecular pathways linking aging and reprogramming along with the unanswered questions in the field. We discuss whether reprogramming rejuvenates the molecular and cellular features associated with age, and present the recent advances with iPSC-based models, contributing to our understanding of physiological and premature aging.

Human pluripotent stem cell-derived cardiomyocytes for heart regeneration, drug discovery and disease modeling: from the genetic, epigenetic, and tissue modeling perspectives

Heart diseases remain a major cause of mortality and morbidity worldwide. However, terminally differentiated human adult cardiomyocytes (CMs) possess a very limited innate ability to regenerate. Directed differentiation of human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) into CMs has enabled clinicians and researchers to pursue the novel therapeutic paradigm of cell-based cardiac regeneration. In addition to tissue engineering and transplantation studies, the need for functional CMs has also prompted researchers to explore molecular pathways and develop strategies to improve the quality, purity and quantity of hESC-derived and iPSC-derived CMs. In this review, we describe various approaches in directed CM differentiation and driven maturation, and discuss potential limitations associated with hESCs and iPSCs, with an emphasis on the role of epigenetic regulation and chromatin remodeling, in the context of the potential and challenges of using hESC-CMs and iPSC-CMs for drug discovery and toxicity screening, disease modeling, and clinical applications.

Concealing cellular defects in pluripotent stem cells

Inherent and acquired defects in gene expression, protein homeostasis, metabolic pathways, and organelle function are linked to aging and a wide range of human diseases. Although concealed or dormant in the embryonic stage, they often manifest later in life. We review and discuss recent observations on how somatic cells bearing specific phenotypic defects can be reprogrammed into a pluripotent state where most phenotypic abnormalities can be reset or tolerated. Gaining insights into the tolerance of cellular defects in pluripotent stem cells will facilitate our understanding of the properties of reprogrammed cells and may provide theoretical guidance for induced pluripotent stem cell based disease modeling and clinical therapies.

Stem cells and neurodegenerative diseases: where is it all going?

Over the last few years there have been a number of major breakthroughs in the development of stem cells for diseases of the CNS. One of these has been in the ability to reprogram adult somatic cells to a more pluripotent state as well as directly to neurons and, by so doing, use patient-derived cells to study disease. In addition, the capacity to engineer embryonic stem cells to defined neuronal fates in the absence of proliferative contaminant cells is now feasible, which opens up the possibility of using these cells for cell transplantation. In this review, we will discuss how these developments have come about, particularly in the context of Parkinson's disease, and what this means for the future of this whole field over the next few years.

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.

Telomere elongation during morula-to-blastocyst transition in cloned porcine embryos

Although telomeres are elongated during morula-to-blastocyst transition in cloned embryos, it is still unknown whether donor cell types have any effect on this elongation. In the present study, we examined the changes of telomere length during morula-to-blastocyst transition in cloned porcine embryos using different types of donor cells. Porcine embryonic stem-like cells (pESLCs), porcine cumulus cells (PCs), and porcine embryonic fibroblasts at passages 7 and 10 (PEF7s and PEF10s, respectively) were used as donor cells. Telomere lengths of pESLCs (35.8±1.5 kb), PCs (24.4±0.5 kb), PEF7s (18.7±0.6 kb), and PEF10s (17.2±0.1 kb) were significantly different. In contrast, telomere length in morulae derived from pESLCs (18.2±0.3 kb), PC (17.8±0.7 kb), PEF7 (18.5±0.3 kb), and PEF10 (18.4±0.4 kb) did not differ significantly. Likewise, telomeres in blastocysts derived from pESLCs (22.3±1.5 kb), PCs (23.5±2.6 kb), PEF7s (20.2±1.0 kb), and PEF10s (20.9±1.0 kb) had similar lengths. However, telomeres in blastocysts were significant longer (p<0.05) compared with morulae in each group. Relative telomerase activities of morulae derived from pESLCs (4.2±0.4), PCs (4.0±0.5), PEF7s (5.1±0.4), and PEF10s (4.9±0.4) were significantly lower (p<0.01) than those of blastocysts derived from pESLCs (8.2±1.1), PCs (8.6±0.6), PEF7s (12.5±2.9), and PEF10s (8.3±1.1). In conclusion, the telomere elongation in cloned pig embryos that occurred during morula-to-blastocyst transition may be related to the rise of telomerase activity. The telomere elongation may also be independent of the type and telomere length of the donor cell.

p21 both attenuates and drives senescence and aging in BubR1 progeroid mice

BubR1 insufficiency occurs with natural aging and induces progeroid phenotypes in both mice and children with mosaic variegated aneuploidy syndrome. In response to BubR1 insufficiency, skeletal muscle, fat, and lens tissue engage p19(Arf) to attenuate senescence and age-related deterioration. Here, we address how p19(Arf) exerts this caretaker role using BubR1 progeroid mice lacking p53 or its transcriptional target p21. We show that p53 delays functional decline of skeletal muscle and fat in a p21-dependent fashion by inhibiting p16(Ink4a)-mediated senescence of progenitor cells. Strikingly, p53 also attenuates the formation of cataractous lenses, but here its antiaging effect is p21 independent, as we found p21 to promote senescence of lens epithelial cells and cataract formation. Together, these results demonstrate that p53 counteracts tissue destruction in response to BubR1 insufficiency through diverse mechanisms and uncover a causal link between senescence of the progenitor cell compartment and age-related dysfunction.

Defective telomere elongation and hematopoiesis from telomerase-mutant aplastic anemia iPSCs

Critically short telomeres activate p53-mediated apoptosis, resulting in organ failure and leading to malignant transformation. Mutations in genes responsible for telomere maintenance are linked to a number of human diseases. We derived induced pluripotent stem cells (iPSCs) from 4 patients with aplastic anemia or hypocellular bone marrow carrying heterozygous mutations in the telomerase reverse transcriptase (TERT) or the telomerase RNA component (TERC) telomerase genes. Both mutant and control iPSCs upregulated TERT and TERC expression compared with parental fibroblasts, but mutant iPSCs elongated telomeres at a lower rate compared with healthy iPSCs, and the deficit correlated with the mutations' impact on telomerase activity. There was no evidence for alternative lengthening of telomere (ALT) pathway activation. Elongation varied among iPSC clones derived from the same patient and among clones from siblings harboring identical mutations. Clonal heterogeneity was linked to genetic and environmental factors, but was not influenced by residual expression of reprogramming transgenes. Hypoxia increased telomere extension in both mutant and normal iPSCs. Additionally, telomerase-mutant iPSCs showed defective hematopoietic differentiation in vitro, mirroring the clinical phenotype observed in patients and demonstrating that human telomere diseases can be modeled utilizing iPSCs. Our data support the necessity of studying multiple clones when using iPSCs to model disease.

Short telomeres in ESCs lead to unstable differentiation

Functional telomeres are critical for stem cell proliferation; however, whether they are equally important for the stability of stem cell differentiation is not known. We found that mouse embryonic stem cells (ESCs) with critically short telomeres (Tert(-/-) ESCs) initiated normal differentiation after leukemia inhibitory factor (LIF) withdrawal but, unlike control ESCs, failed to maintain stable differentiation when LIF was reintroduced to the growth medium. Tert(-/-) ESCs expressed higher levels of Nanog and, overall, had decreased genomic CpG methylation levels, which included the promoters of Oct4 and Nanog. This unstable differentiation phenotype could be rescued by telomere elongation via reintroduction of Tert, via suppression of Nanog by small hairpin RNA (shRNA) knockdown, or via enforced expression of the de novo DNA methyltransferase 3b. These results demonstrate an unexpected role of functional telomeres in the genome-wide epigenetic regulation of cell differentiation and suggest a potentially important role of telomere instability in cell fate during development or disease.

mRNA transfection-based, feeder-free, induced pluripotent stem cells derived from adipose tissue of a 50-year-old patient

Induced pluripotent stem cells (iPSC) have successfully been derived from somatic fibroblasts through transfection of synthetic modified mRNA encoding transcription factors. This technique obviates the use of recombinant DNA and viral vectors in cellular reprogramming. The present study derived iPSC from adipose-derived mesenchymal stem cells (of a 50-year-old female patient) by utilizing a similar technique, but with defined culture medium without feeder cells, during both reprogramming and propagation. Clonal selection was performed to yield 12 putative iPSC lines from individual colonies of nascent reprogrammed cells, starting from 150,000 cells. However, only seven lines maintained their undifferentiated state after 10 continuous serial passages. These seven lines were then subjected to a rigorous battery of analyses to confirm their identity as iPSC. These tests included immunostaining, flow cytometry, qRT-PCR, in vitro differentiation assay, and teratoma formation assay within SCID mice. Positive results were consistently observed in all analyses, thus verifying the cells as fully reprogrammed iPSC. While all 7 iPSC lines displayed normal karyogram up to passage 13, chromosomal anomalies occurred in 4 of 7 lines with extended in vitro culture beyond 24 serial passages. Only three lines retained normal karyotype of 46,XX. The remaining four lines displayed mosaicism of normal and abnormal karyotypes. Hence, this study successfully derived iPSC from abundant and easily accessible adipose tissues of a middle-aged patient; utilizing a mRNA-based integration-free technique under feeder-free conditions. This is a step forward in translating iPSC into personalized regenerative medicine within the clinic.

An epigenetic component of hematopoietic stem cell aging amenable to reprogramming into a young state

Aging of hematopoietic stem cells (HSCs) leads to several functional changes, including alterations affecting self-renewal and differentiation. Although it is well established that many of the age-induced changes are intrinsic to HSCs, less is known regarding the stability of this state. Here, we entertained the hypothesis that HSC aging is driven by the acquisition of permanent genetic mutations. To examine this issue at a functional level in vivo, we applied induced pluripotent stem (iPS) cell reprogramming of aged hematopoietic progenitors and allowed the resulting aged-derived iPS cells to reform hematopoiesis via blastocyst complementation. Next, we functionally characterized iPS-derived HSCs in primary chimeras and after the transplantation of re-differentiated HSCs into new hosts, the gold standard to assess HSC function. Our data demonstrate remarkably similar functional properties of iPS-derived and endogenous blastocyst-derived HSCs, despite the extensive chronological and proliferative age of the former. Our results, therefore, favor a model in which an underlying, but reversible, epigenetic component is a hallmark of HSC aging.

PML bodies provide an important platform for the maintenance of telomeric chromatin integrity in embryonic stem cells

We have previously shown that α-thalassemia mental retardation X-linked (ATRX) and histone H3.3 are key regulators of telomeric chromatin in mouse embryonic stem cells. The function of ATRX and H3.3 in the maintenance of telomere chromatin integrity is further demonstrated by recent studies that show the strong association of ATRX/H3.3 mutations with alternative lengthening of telomeres in telomerase-negative human cancer cells. Here, we demonstrate that ATRX and H3.3 co-localize with the telomeric DNA and associated proteins within the promyelocytic leukemia (PML) bodies in mouse ES cells. The assembly of these telomere-associated PML bodies is most prominent at S phase. RNA interference (RNAi)-mediated knockdown of PML expression induces the disassembly of these nuclear bodies and a telomere dysfunction phenotype in mouse ES cells. Loss of function of PML bodies in mouse ES cells also disrupts binding of ATRX/H3.3 and proper establishment of histone methylation pattern at the telomere. Our study demonstrates that PML bodies act as epigenetic regulators by serving as platforms for the assembly of the telomeric chromatin to ensure a faithful inheritance of epigenetic information at the telomere.

Mitochondrial fusion by pharmacological manipulation impedes somatic cell reprogramming to pluripotency: new insight into the role of mitophagy in cell stemness

Recent studies have suggested a pivotal role for autophagy in stem cell maintenance and differentiation. Reprogramming of somatic cells to induced pluripotent stem cells (iPSCs) has been also suggested to bio-energetically take advantage of mitochondrial autophagy (mitophagy). We have preliminary addressed how mitophagy might play a role in the regulation of induced pluripotency using mdivi-1 (for mitochondrial division inhibitor), a highly efficacious small molecule that selectively inhibits the self-assembly of DRP1, a member of the dynamin family of large GTPases that mediates mitochondrial fission. At mdivi-1 concentrations that rapidly induced the formation of mitochondrial net-like or collapsed perinuclear mitochondrial structures, we observed that the reprogramming efficiency of mouse embryonic fibroblasts transduced with the Yamanaka three-factor cocktail (OCT4, KLF4, and SOX2) is drastically reduced by more than 95%. Treatment of MEFs with mdivi-1 at the early stages of reprogramming before the appearance of iPSC colonies was sufficient to completely inhibit somatic cell reprogramming. Therefore, the observed effects on reprogramming efficiencies were due likely to the inhibition of the process of reprogramming itself and not to an impairment of iPSC colony survival or growth. Moreover, the typical morphology of established iPSC colonies with positive alkaline phosphatase staining was negatively affected by mdivi-1 exposure. In the presence of mdivi-1, the colony morphology of the iPSCs was lost, and they somewhat resembled fibroblasts. The alkaline phosphatase staining was also significantly reduced, a finding that is indicative of differentiation. Our current findings provide new insight into how mitochondrial division is integrated into the reprogramming factors-driven transcriptional network that specifies the unique pluripotency of stem cells.

Telomerase-mediated telomere elongation from human blastocysts to embryonic stem cells

High telomerase activity is a characteristic of human embryonic stem cells (hESCs), however the regulation and maintenance of correct telomere length in hESCs is unclear. In this study we investigated telomere elongation in hESCs in vitro and found that telomeres lengthened from derivation in blastocysts through early expansion, but telomere length stabilized at later passages. We report that the core unit of telomerase, hTERT, was highly expressed in hESCs in blastocysts and throughout long-term culture; furthermore, this was regulated in a Wnt–β-catenin signaling dependent manner. Our observations that the alternative lengthening of telomeres (ALT) pathway was suppressed in hESCs and that hTERT knockdown partially inhibited telomere elongation, demonstrated that high telomerase activity was required for telomere elongation. We observed that chromatin modification through trimethylation of H3K9 and H4K20 at telomeric regions decreased during early culture. This was concurrent with telomere elongation, suggesting that epigenetic regulation of telomeric chromatin may influence telomerase function. By measuring telomere length in 96 hESC lines, we were able to establish that telomere length remained relatively stable at 12.02±1.01 kb during later passages (15–95). In contrast, hESCs with genomic instability and hESC-derived teratomas displayed variations in telomere length. In summary, we propose that correct, stable telomere length may serve as a potential biomarker for genetically stable hESCs.

Parkinson's Disease in a Dish: What Patient Specific-Reprogrammed Somatic Cells Can Tell Us about Parkinson's Disease, If Anything?

Technologies allowing for the derivation of patient-specific neurons from somatic cells are emerging as powerful in vitro tools to investigate the intrinsic cellular pathological behaviours of the diseases that affect these patients. While the use of patient-derived neurons to model Parkinson's disease (PD) has only just begun, these approaches have allowed us to begin investigating disease pathogenesis in a unique way. In this paper, we discuss the advances made in the field of cellular reprogramming to model PD and discuss the pros and cons associated with the use of such cells.

Human ESC/iPSC-based "Omics" and Bioinformatics for Translational Research

The establishment of human embryonic stem cell lines (hESCs) created the basis for new approaches in regenerative medicine and drug discovery. Despite the potential of hESCs for cell based therapies, ethical controversies limit their use. These obstacles could be overcome by induced pluripotent stem cells (iPSCs) that are generated by reprogramming somatic cells. Before iPSCs can be used for clinical applications, however, they must be thoroughly analyzed for aberrations in the genome, epigenome, transcriptome, and proteome. Here, we review how 'omics' technologies can be employed for a quantitative and definitive assessment of these cells.

Aging and reprogramming: a two-way street

Aging is accompanied by the functional decline of cells, tissues, and organs, as well as a striking increase in a wide range of diseases. The reprogramming of somatic cells to induced pluripotent stem cells (iPSCs) opens new avenues for the aging field and has important applications for therapeutic treatments of age-related diseases. Here we review emerging studies on how aging and age-related pathways influence iPSC generation and property. We discuss the exciting possibility that reverting to a pluripotent stem cell stage erases several deficits associated with aging and offers new strategies for rejuvenation. Finally, we argue that reprogramming provides a unique opportunity to model aging and perhaps exceptional longevity.

Disease modelling using induced pluripotent stem cells: status and prospects

The ability to convert human somatic cells into induced pluripotent stem cells (iPSCs) is allowing the production of custom-tailored cells for drug discovery and for the study of disease phenotypes at the cellular and molecular level. IPSCs have been derived from patients suffering from a large variety of disorders with different severities. In many cases, disease related phenotypes have been observed in iPSCs or their lineage-specific progeny. Several proof of concept studies have demonstrated that these phenotypes can be reversed in vitro using approved drugs. However, several challenges must be overcome to take full advantage of this technology. Here, we highlight recent advances in the field and discuss the main challenges associated with this technology as it applies to disease modelling.

Zscan4 promotes genomic stability during reprogramming and dramatically improves the quality of iPS cells as demonstrated by tetraploid complementation

Induced pluripotent stem (iPS) cells generated using Yamanaka factors have great potential for use in autologous cell therapy. However, genomic abnormalities exist in human iPS cells, and most mouse iPS cells are not fully pluripotent, as evaluated by the tetraploid complementation assay (TCA); this is most likely associated with the DNA damage response (DDR) occurred in early reprogramming induced by Yamanaka factors. In contrast, nuclear transfer can faithfully reprogram somatic cells into embryonic stem (ES) cells that satisfy the TCA. We thus hypothesized that factors involved in oocyte-induced reprogramming may stabilize the somatic genome during reprogramming, and improve the quality of the resultant iPS cells. To test this hypothesis, we screened for factors that could decrease DDR signals during iPS cell induction. We determined that Zscan4, in combination with the Yamanaka factors, not only remarkably reduced the DDR but also markedly promoted the efficiency of iPS cell generation. The inclusion of Zscan4 stabilized the genomic DNA, resulting in p53 downregulation. Furthermore, Zscan4 also enhanced telomere lengthening as early as 3 days post-infection through a telomere recombination-based mechanism. As a result, iPS cells generated with addition of Zscan4 exhibited longer telomeres than classical iPS cells. Strikingly, more than 50% of iPS cell lines (11/19) produced via this "Zscan4 protocol" gave rise to live-borne all-iPS cell mice as determined by TCA, compared to 1/12 for lines produced using the classical Yamanaka factors. Our findings provide the first demonstration that maintaining genomic stability during reprogramming promotes the generation of high quality iPS cells.

Pluripotent stem cells escape from senescence-associated DNA methylation changes

Pluripotent stem cells evade replicative senescence, whereas other primary cells lose their proliferation and differentiation potential after a limited number of cell divisions, and this is accompanied by specific senescence-associated DNA methylation (SA-DNAm) changes. Here, we investigate SA-DNAm changes in mesenchymal stromal cells (MSC) upon long-term culture, irradiation-induced senescence, immortalization, and reprogramming into induced pluripotent stem cells (iPSC) using high-density HumanMethylation450 BeadChips. SA-DNAm changes are highly reproducible and they are enriched in intergenic and nonpromoter regions of developmental genes. Furthermore, SA-hypomethylation in particular appears to be associated with H3K9me3, H3K27me3, and Polycomb-group 2 target genes. We demonstrate that ionizing irradiation, although associated with a senescence phenotype, does not affect SA-DNAm. Furthermore, overexpression of the catalytic subunit of the human telomerase (TERT) or conditional immortalization with a doxycycline-inducible system (TERT and SV40-TAg) result in telomere extension, but do not prevent SA-DNAm. In contrast, we demonstrate that reprogramming into iPSC prevents almost the entire set of SA-DNAm changes. Our results indicate that long-term culture is associated with an epigenetically controlled process that stalls cells in a particular functional state, whereas irradiation-induced senescence and immortalization are not causally related to this process. Absence of SA-DNAm in pluripotent cells may play a central role for their escape from cellular senescence.

Does transcription factor induced pluripotency accurately mimic embryo derived pluripotency?

When Takahashi and Yamanaka first demonstrated that just four transcription factors could reprogram a fibroblast to a pluripotent state, the first wave of data to emerge focused on how similar these induced pluripotent stem cells (iPSCs) were to embryo-derived pluripotent stem cells (ESCs) [1]. The next wave of data focused on determining the degree of difference between iPSCs and ESCs [2]. Now the focus is on tweaking the process to generate iPSCs that are more similar to ESCs [3,4]. Because transcription factor based reprogramming allows for nearly any type of cell to be created from any donor cell, there is obviously enormous interest in this technique as a tool for both basic developmental biology and for clinical applications. In this review, I will attempt to summarize the data that serve to distinguish these types of pluripotent stem cells and speculate on the ramifications of any differences.

Genomic stability in reprogramming

The genetic stability of induced pluripotent stem (iPS) cells has a significant impact on their potential use in regenerative medicine and basic research. Analysis of the genomic integrity of iPS cells suggests a tendency to develop aberrations ranging from whole chromosome trisomies to single nucleotide mutations. Furthermore, fluctuations in telomere elongation and changes in mitochondrial DNA are also observed. Some mutations may already exist in the founder cells or result from prolonged culturing, however, many of the mutations occur during the reprogramming event. Thus, great care should be given to the initial characterization and subsequent culturing of new iPS cell lines in order to avoid the use of potentially aberrant cells.

Defining stem cell types: understanding the therapeutic potential of ESCs, ASCs, and iPS cells

Embryonic, adult, artificially reprogrammed, and cancer…- there are various types of cells associated with stemness. Do they have something fundamental in common? Are we applying a common name to very different entities? In this review, we will revisit the characteristics that define 'pluripotency', the main property of stem cells (SCs). For each main type of physiological (embryonic and adult) or synthetic (induced pluripotent) SCs, markers and functional behavior in vitro and in vivo will be described. We will review the pioneering work that has led to obtaining human SC lines, together with the problems that have arisen, both in a biological context (DNA alterations, heterogeneity, tumors, and immunogenicity) and with regard to ethical concerns. Such problems have led to proposals for new operative procedures for growing human SCs of sufficiently high quality for use as models of disease and in human therapy. Finally, we will review the data from the first clinical trials to use various types of SCs.

The rate of increase of short telomeres predicts longevity in mammals

Aberrantly short telomeres result in decreased longevity in both humans and mice with defective telomere maintenance. Normal populations of humans and mice present high interindividual variation in telomere length, but it is unknown whether this is associated with their lifespan potential. To address this issue, we performed a longitudinal telomere length study along the lifespan of wild-type and transgenic telomerase reverse transcriptase mice. We found that mouse telomeres shorten ∼100 times faster than human telomeres. Importantly, the rate of increase in the percentage of short telomeres, rather than the rate of telomere shortening per month, was a significant predictor of lifespan in both mouse cohorts, and those individuals who showed a higher rate of increase in the percentage of short telomeres were also the ones with a shorter lifespan. These findings demonstrate that short telomeres have a direct impact on longevity in mammals, and they highlight the importance of performing longitudinal telomere studies to predict longevity.

Increased expression of importin13 in endometriosis and endometrial carcinoma

Background

Importin13 (IPO13) is a novel potential marker of corneal epithelial progenitor cells. We investigated the expression and localization of IPO13 in endometrial, endometriotic and endometrial carcinoma tissue.

Material/Methods

IPO13 expression in endometrial, endometriotic and endometrial carcinoma tissue was examined by immunohistochemistry, qPCR and Western blot.

Results

Immunohistochemistry studies showed that IPO13 protein was expressed mainly in cytoplasm of glandular epithelial cell and stromal cells. The rate of importin13-positive cells in proliferative phase endometrium was higher (by about 6-fold) than that in secretory endometrium (P<0.05) and the rate of importin13-positive cells in endometriosis and endometrial carcinoma was higher than that in normal secretory phase endometrial tissues (by about 4- and 9-fold, respectively). Immunofluorescence microscopy revealed co-localization of IPO13 with CD34, CD45, c-kit, telomerase, CD90 and CD146. QPCR revealed significantly increased IPO13 mRNA in endometriosis and endometrial carcinoma versus secretory phase endometrium (by about 2- and 10-fold, respectively). Western blot analysis showed that IPO13 protein is enhanced in endometriosis and endometrial carcinoma versus secretory phase endometrium (p<0.05).

Conclusions

These results demonstrate an increased expression of IPO13 in endometriosis and endometrial carcinoma, which could be involved in the pathogenesis of endometriosis and endometrial carcinoma; IPO13 can serve as an endometrial progenitor/stem cell marker.

Influences of lamin A levels on induction of pluripotent stem cells

Lamin A is an inner nuclear membrane protein that maintains nuclear structure integrity, is involved in transcription, DNA damage response and genomic stability, and also links to cell differentiation, senescence, premature aging and associated diseases. Induced pluripotent stem (iPS) cells have been successfully generated from various types of cells and used to model human diseases. It remains unclear whether levels of lamin A influence reprogramming of somatic cells to pluripotent states during iPS induction. Consistently, lamin A is expressed more in differentiated than in relatively undifferentiated somatic cells, and increases in expression levels with age. Somatic cells with various expression levels of lamin A differ in their dynamics and efficiency during iPS cell induction. Cells with higher levels of lamin A show slower reprogramming and decreased efficiency to iPS cells. Furthermore, depletion of lamin A by transient shRNA accelerates iPS cell induction from fibroblasts. Reduced levels of lamin A are associated with increased expression of pluripotent genes Oct4 and Nanog, and telomerase genes Tert and Terc. On the contrary, overexpression of lamin A retards somatic cell reprogramming to iPS-like colony formation. Our data suggest that levels of lamin A influence reprogramming of somatic cells to pluripotent stem cells and that artificial silencing of lamin A facilitates iPS cell induction. These findings may have implications in enhancing rejuvenation of senescent or older cells by iPS technology and manipulating lamin A levels.