Molecular insights into the heterogeneity of telomere reprogramming in induced pluripotent stem cells

Telomeres as hallmarks of iPSC aging: a review on telomere dynamics during stemness and cellular reprogramming

Telomeres, the protective ends of chromosome, are key to tissue repair and regeneration. Telomere shortening is linked to aging and age-related disorders, while excessive telomerase activity may support tissue regeneration or transformation. Some of the functions of telomeres and telomerase may be mediated by its important role in the process of stemness. Active telomerase, and subsequent telomerase-dependent telomere extension, supports stem-cells self-renewal and pluripotency - essential for tissue healing. During cellular reprogramming, differentiated cells are converted into induced pluripotent stem cells (iPSCs), which resemble embryonic stem cells. During iPSC derivation, telomere length is reset, enhancing iPSCs' regenerative potential. During this process, incomplete telomerase activation and telomere extension can lead to genomic instability and/or haltered cell functionality. Understanding the intricate relation of telomeres, telomerase and stemness may be critical when designing novel cell-based therapies targeting degenerative diseases or to unlock strategies to delay aging. Here, we explore the recent bibliography linking these areas, raising awareness of their important when designing novel breakthroughs in health and longevity.

Molecular basis of cell fate plasticity - insights from the privileged cells

In the post-Yamanaka era, the rolling balls on Waddington's hilly landscape not only roll downward, but also go upward or sideways. This new-found mobility implies that the tantalizing somatic cell plasticity fueling regeneration, once only known to planarians and newts, might be sparking in the cells of mice and humans, if only we knew how to fully unlock it. The hope for ultimate regeneration was made even more tangible by the observations that partial reprogramming by the Yamanaka factors reverses many hallmarks of aging [76], even though the underlying mechanism remains unclear. We intend to revisit the milestones in the evolving understanding of cell fate plasticity and glean molecular insights from an unusual somatic cell state, the privileged cell state that reprograms in a manner defying the stochastic model. We synthesize our view of the molecular underpinning of cell fate plasticity, from which we speculate how to harness it for regeneration and rejuvenation. We propose that senescence, aging and malignancy represent distinct cell states with definable biochemical and biophysical parameters.

Telomere Length as Both Cause and Consequence in Type 1 Diabetes: Evidence from Bidirectional Mendelian Randomization

Background/Objectives: Diabetes is the most prevalent metabolic disease globally, characterized by dysregulated glucose control and accompanied by multiple refractory complications. As a critical marker of cellular homeostasis, telomere length (TL) may be associated with the progression of diabetes. However, the causal relationship between diabetes and TL remains unclear, particularly whether cellular homeostasis imbalance acts as a consequence of diabetic complications or a precipitating factor in disease development. Methods: We performed a bidirectional Mendelian randomization (MR) analysis using genome-wide association study (GWAS) data. Following the three core assumptions of MR analysis, we conducted quality control on all instrumental variables to ensure methodological rigor. The inverse variance weighted (IVW) method served as the primary analytical method, supplemented by additional MR methods to evaluate the significance of the results. Furthermore, we performed sensitivity analyses to ensure the reliability and robustness of the findings. Results: Forward analysis revealed that shortened TL significantly increases the risk of broadly defined Type 1 diabetes (T1D) and unspecified types of diabetes (p < 0.05). Additionally, we identified a positive causal relationship between TL and several diabetes-related complications, including co-morbidities, diabetic nephropathy, and diabetic ketoacidosis (p < 0.05). Interestingly, the reverse analysis demonstrated a positive causal effect of T1D and its complications on TL (p < 0.05); however, this effect disappeared after adjusting for insulin use (p > 0.05). Conclusions: Bidirectional MR analyses revealed a complex relationship between TL and T1D, where shortened telomeres increase T1D risk while T1D itself may trigger compensatory mechanisms affecting telomere maintenance, with insulin playing a crucial regulatory role in this relationship. These findings suggest telomere biology may be fundamentally involved in T1D pathogenesis and could inform future therapeutic approaches.

An Unbiased Approach to Identifying Cellular Reprogramming-Inducible Enhancers

Cellular reprogramming of somatic cells towards induced pluripotency is a multistep stochastic process mediated by the transcription factors Oct4, Sox2, Klf4 and c-Myc (OSKM), which orchestrate global epigenetic and transcriptional changes. We performed a large-scale analysis of integrated ChIP-seq, ATAC-seq and RNA-seq data and revealed the spatiotemporal highly dynamic pattern of OSKM DNA binding during reprogramming. We found that OSKM show distinct temporal patterns of binding to different classes of pluripotency-related enhancers. Genes involved in reprogramming are regulated by the coordinated activity of multiple enhancers, which are sequentially bound by OSKM for strict transcriptional control. Based on these findings, we developed an unbiased approach to identify Reprogramming-Inducible Enhancers (RIEs), constructed enhancer-traps and isolated cells undergoing reprogramming in real time. We used a representative RIE taken from the Upp1 gene fused to Gfp and isolated cells at different time-points during reprogramming and found that they have unique developmental capacities as they are reprogrammed with high efficiency due to their distinct molecular signatures. In conclusion, our experiments have led to the development of an unbiased method to identify and isolate reprogrammable cells in real time by exploiting the functional dynamics of OSKM, which can be used as efficient reprogramming biomarkers.

Telomeric DNA breaks in human induced pluripotent stem cells trigger ATR-mediated arrest and telomerase-independent telomere damage repair

Although mechanisms of telomere protection are well-defined in differentiated cells, how stem cells sense and respond to telomere dysfunction, in particular telomeric double-strand breaks (DSBs), is poorly characterized. Here, we report the DNA damage signaling, cell cycle, and transcriptome changes in human induced pluripotent stem cells (iPSCs) in response to telomere-internal DSBs. We engineer human iPSCs with an inducible TRF1-FokI fusion protein to acutely induce DSBs at telomeres. Using this model, we demonstrate that TRF1-FokI DSBs activate an ATR-dependent DNA damage response, which leads to p53-independent cell cycle arrest in G2. Using CRISPR-Cas9 to cripple the catalytic domain of telomerase reverse transcriptase, we show that telomerase is largely dispensable for survival and lengthening of TRF1-FokI-cleaved telomeres, which instead are effectively repaired by robust homologous recombination (HR). In contrast to HR-based telomere maintenance in mouse embryonic stem cells, where HR causes ZSCAN4-dependent extension of telomeres beyond their initial lengths, HR-based repair of telomeric breaks is sufficient to maintain iPSC telomeres at a normal length, which is compatible with sustained survival of the cells over several days of TRF1-FokI induction. Our findings suggest a previously unappreciated role for HR in telomere maintenance in telomerase-positive iPSCs and reveal distinct iPSC-specific responses to targeted telomeric DNA damage.

A Youthful Touch: Reversal of Aging Hallmarks by Cell Reprogramming

BACKGROUND

With the elderly population projected to double by 2050, there is an urgent need to address the increasing prevalence of age-related debilitating diseases and ultimately minimize discrepancies between the rising lifespan and stagnant health span. Cellular reprogramming by overexpression of Oct3/4, Klf4, Sox2, and cMyc (OKSM) transcription factors is gaining attention in this context thanks to demonstrated rejuvenating effects in human cell cultures and live mice, many of which can be uncoupled from dedifferentiation and loss of cell identity.

SUMMARY

Here, we review current evidence of the impact of cell reprogramming on established aging hallmarks and the underlying mechanisms that mediate these effects. We also provide a critical assessment of the challenges in translating these findings and, overall, cell reprogramming technologies into clinically translatable antiaging interventions.

KEY MESSAGES

Cellular reprogramming has the potential to reverse at least partially some key hallmarks of aging. However, further research is necessary to determine the biological significance and duration of such changes and to ensure the safety of cell reprogramming as a rejuvenation approach. With this review, we hope to stimulate new research directions in the quest to extend health span effectively.

Telomere dynamics and reproduction

The oocyte, a long-lived, postmitotic cell, is the locus of reproductive aging in women. Female germ cells replicate only during fetal life and age throughout reproductive life. Mechanisms of oocyte aging include the accumulation of oxidative damage, mitochondrial dysfunction, and disruption of proteins, including cohesion. Nobel Laureate Bob Edwards also discovered a "production line" during oogonial replication in the mouse, wherein the last oocytes to ovulate in the adult-derived from the last oogonia to exit mitotic replication in the fetus. On the basis of this, we proposed a two-hit "telomere theory of reproductive aging" to integrate the myriad features of oocyte aging. The first hit was that oocytes remaining in older women traversed more cell cycles during fetal oogenesis. The second hit was that oocytes accumulated more environmental and endogenous oxidative damage throughout the life of the woman. Telomeres (Ts) could mediate both of these aspects of oocyte aging. Telomeres provide a "mitotic clock," with T attrition an inevitable consequence of cell division because of the end replication problem. Telomere's guanine-rich sequence renders them especially sensitive to oxidative damage, even in postmitotic cells. Telomerase, the reverse transcriptase that restores Ts, is better at maintaining than elongating T. Moreover, telomerase remains inactive during much of oogenesis and early development. Oocytes are left with short Ts, on the brink of viability. In support of this theory, mice with induced T attrition and women with naturally occurring telomeropathy suffer diminished ovarian reserve, abnormal embryo development, and infertility. In contrast, sperm are produced throughout the life of the male by a telomerase-active progenitor, spermatogonia, resulting in the longest Ts in the body. In mice, cleavage-stage embryos elongate Ts via "alternative lengthening of telomeres," a recombination-based mechanism rarely encountered outside of telomerase-deficient cancers. Many questions about Ts and reproduction are raised by these findings: does the "normal" T attrition observed in human oocytes contribute to their extraordinarily high rate of meiotic nondisjunction? Does recombination-based T elongation render embryos susceptible to mitotic nondisjunction (and mosaicism)? Can some features of Ts serve as markers of oocyte quality?

Human and Pig Pluripotent Stem Cells: From Cellular Products to Organogenesis and Beyond

Pluripotent stem cells (PSCs) are important for studying development and hold great promise in regenerative medicine due to their ability to differentiate into various cell types. In this review, we comprehensively discuss the potential applications of both human and pig PSCs and provide an overview of the current progress and challenges in this field. In addition to exploring the therapeutic uses of PSC-derived cellular products, we also shed light on their significance in the study of interspecies chimeras, which has led to the creation of transplantable human or humanized pig organs. Moreover, we emphasize the importance of pig PSCs as an ideal cell source for genetic engineering, facilitating the development of genetically modified pigs for pig-to-human xenotransplantation. Despite the achievements that have been made, further investigations and refinement of PSC technologies are necessary to unlock their full potential in regenerative medicine and effectively address critical healthcare challenges.

Cellular rejuvenation: molecular mechanisms and potential therapeutic interventions for diseases

The ageing process is a systemic decline from cellular dysfunction to organ degeneration, with more predisposition to deteriorated disorders. Rejuvenation refers to giving aged cells or organisms more youthful characteristics through various techniques, such as cellular reprogramming and epigenetic regulation. The great leaps in cellular rejuvenation prove that ageing is not a one-way street, and many rejuvenative interventions have emerged to delay and even reverse the ageing process. Defining the mechanism by which roadblocks and signaling inputs influence complex ageing programs is essential for understanding and developing rejuvenative strategies. Here, we discuss the intrinsic and extrinsic factors that counteract cell rejuvenation, and the targeted cells and core mechanisms involved in this process. Then, we critically summarize the latest advances in state-of-art strategies of cellular rejuvenation. Various rejuvenation methods also provide insights for treating specific ageing-related diseases, including cellular reprogramming, the removal of senescence cells (SCs) and suppression of senescence-associated secretory phenotype (SASP), metabolic manipulation, stem cells-associated therapy, dietary restriction, immune rejuvenation and heterochronic transplantation, etc. The potential applications of rejuvenation therapy also extend to cancer treatment. Finally, we analyze in detail the therapeutic opportunities and challenges of rejuvenation technology. Deciphering rejuvenation interventions will provide further insights into anti-ageing and ageing-related disease treatment in clinical settings.

Accelerated neuronal aging in vitro ∼melting watch ∼

In developed countries, the aging of the population and the associated increase in age-related diseases are causing major unresolved medical, social, and environmental matters. Therefore, research on aging has become one of the most important and urgent issues in life sciences. If the molecular mechanisms of the onset and progression of neurodegenerative diseases are elucidated, we can expect to develop disease-modifying methods to prevent neurodegeneration itself. Since the discovery of induced pluripotent stem cells (iPSCs), there has been an explosion of disease models using disease-specific iPSCs derived from patient-derived somatic cells. By inducing the differentiation of iPSCs into neurons, disease models that reflect the patient-derived pathology can be reproduced in culture dishes, and are playing an active role in elucidating new pathological mechanisms and as a platform for new drug discovery. At the same time, however, we are faced with a new problem: how to recapitulate aging in culture dishes. It has been pointed out that cells differentiated from pluripotent stem cells are juvenile, retain embryonic traits, and may not be fully mature. Therefore, attempts are being made to induce cell maturation, senescence, and stress signals through culture conditions. It has also been reported that direct conversion of fibroblasts into neurons can reproduce human neurons with an aged phenotype. Here, we outline some state-of-the-art insights into models of neuronal aging in vitro. New frontiers in which stem cells and methods for inducing differentiation of tissue regeneration can be applied to aging research are just now approaching, and we need to keep a close eye on them. These models are forefront and intended to advance our knowledge of the molecular mechanisms of aging and contribute to the development of novel therapies for human neurodegenerative diseases associated with aging.

Effects of Recloning on the Telomere Lengths of Mouse Terc+/- Nuclear Transfer-Derived Embryonic Stem Cells

Haploinsufficiency of genes that participate in the telomere elongation and maintenance processes, such as Terc and Tert, often lead to premature aging related diseases such as dyskeratosis congenita and aplastic anemia. Previously we reported that when mouse Terc+/- tail tip fibroblasts (TTFs) were used as the donor cells for somatic cell nuclear transfer (SCNT, also known as "cloning"), the derivative embryonic stem cells (ntESCs) had elongated telomeres. In the present work, we are interested to know if an additional round of SCNT, or recloning, could bring further elongation of the telomeres. Terc+/- TTFs were used to derive the first generation (G1) ntESCs, followed by a second round SCNT using G1-Terc+/- ntESCs as donor cells to derive G2-Tert+/- ntESCs. Multiple lines of G1- and G2-Terc+/- ntESCs were efficiently established, and all expressed major pluripotent markers and supported efficient chondrocyte differentiation in vitro. Comparing to the donor TTFs, telomere lengths of G1-ntESCs were elongated to the level comparable to that in wildtype ntESCs. Interestingly, recloning did not further elongate telomere lengths of the Terc+/- ntESCs. Together, our work demonstrates that while a single round of SCNT is a viable means to reprogram Terc haploinsufficient cells to the ESC state, and to elongate these cells' telomere lengths, a second round of SCNT does not necessarily further elongate the telomeres.

A Comprehensive Review on the Role of ZSCAN4 in Embryonic Development, Stem Cells, and Cancer

ZSCAN4 is a transcription factor that plays a pivotal role during early embryonic development. It is a unique gene expressed specifically during the first tide of de novo transcription during the zygotic genome activation. Moreover, it is reported to regulate telomere length in embryonic stem cells and induced pluripotent stem cells. Interestingly, ZSCAN4 is expressed in approximately 5% of the embryonic stem cells in culture at any given time, which points to the fact that it has a tight regulatory system. Furthermore, ZSCAN4, if included in the reprogramming cocktail along with core reprogramming factors, increases the reprogramming efficiency and results in better quality, genetically stable induced pluripotent stem cells. Also, it is reported to have a role in promoting cancer stem cell phenotype and can prospectively be used as a marker for the same. In this review, the multifaceted role of ZSCAN4 in embryonic development, embryonic stem cells, induced pluripotent stem cells, cancer, and germ cells are discussed comprehensively.

Differentiation Capacity of Human Urine-Derived Stem Cells to Retain Telomerase Activity

Telomerase activity is essential for the self-renewal and potential of embryonic, induced pluripotent, and cancer stem cells, as well as a few somatic stem cells, such as human urine-derived stem cells (USCs). However, it remains unclear how telomerase activity affects the regeneration potential of somatic stem cells. The objective of this study was to determine the regenerative significance of telomerase activity, particularly to retain cell surface marker expression, multipotent differentiation capability, chromosomal stability, and in vivo tumorigenic transformation, in each clonal population of human primary USCs. In total, 117 USC specimens from 10 healthy male adults (25–57 years of age) were obtained. Polymerase chain reaction amplification of a telomeric repeat was used to detect USCs with positive telomerase activity (USCs^TA+). A total of 80 USCs^TA+ (70.2%) were identified from 117 USC clones, but they were not detected in the paired normal bladder smooth muscle cell and bone marrow stromal cell specimens. In the 20–40 years age group, approximately 75% of USC clones displayed positive telomerase activity, whereas in the 50 years age group, 59.2% of the USC clones expressed positive telomerase activity. USCs^TA+ extended to passage 16, underwent 62.0 ± 4.8 population doublings, produced more cells, and were superior for osteogenic, myogenic, and uroepithelial differentiation compared to USCs^TA−. Importantly, USCs displayed normal chromosome and no oncological transformation after being implanted in vivo. Overall, as a safe cell source, telomerase-positive USCs have a robust regenerative potential in cell proliferation and multipotent differentiation capacity.

Tumorigenicity Risk of iPSCs in vivo: Nip it in the Bud

In 2006, Takahashi and Yamanaka first created induced pluripotent stem cells from mouse fibroblasts via the retroviral introduction of genes encoding the transcription factors Oct3/4, Sox2, Klf44, and c-Myc. Since then, the future clinical application of somatic cell reprogramming technology has become an attractive research topic in the field of regenerative medicine. Of note, considerable interest has been placed in circumventing ethical issues linked to embryonic stem cell research. However, tumorigenicity, immunogenicity, and heterogeneity may hamper attempts to deploy this technology therapeutically. This review highlights the progress aimed at reducing induced pluripotent stem cells tumorigenicity risk and how to assess the safety of induced pluripotent stem cells cell therapy products.

Zscan4 Contributes to Telomere Maintenance in Telomerase-Deficient Late Generation Mouse ESCs and Human ALT Cancer Cells

Proper telomere length is essential for indefinite self-renewal of embryonic stem (ES) cells and cancer cells. Telomerase-deficient late generation mouse ES cells and human ALT cancer cells are able to propagate for numerous passages, suggesting telomerase-independent mechanisms responding for telomere maintenance. However, the underlying mechanisms ensuring the telomere length maintenance are unclear. Here, using late generation telomerase KO (G4 Terc^-/-) ESCs as a model, we show that Zscan4, highly upregulated in G4 Terc^-/- ESCs, is responsible for the prolonged culture of these cells with stably short telomeres. Mechanistically, G4 Terc^-/- ESCs showed reduced levels of DNA methylation and H3K9me3 at Zscan4 promoter and subtelomeres, which relieved the expression of Zscan4. Similarly, human ZSCAN4 was also derepressed by reduced H3K9me3 at its promoter in ALT U2 OS cells, and depletion of ZSCAN4 significantly shortened telomeres. Our results define a similar conserved pathway contributing to the telomere maintenance in telomerase-deficient late generation mESCs and human ALT U2OS cancer cells.

A ride through the epigenetic landscape: aging reversal by reprogramming

Aging has become one of the fastest-growing research topics in biology. However, exactly how the aging process occurs remains unknown. Epigenetics plays a significant role, and several epigenetic interventions can modulate lifespan. This review will explore the interplay between epigenetics and aging, and how epigenetic reprogramming can be harnessed for age reversal. In vivo partial reprogramming holds great promise as a possible therapy, but several limitations remain. Rejuvenation by reprogramming is a young but rapidly expanding subfield in the biology of aging.

L-carnitine Extends the Telomere Length of the Cardiac Differentiated CD117+- Expressing Stem Cells

Stem cell-based therapy has emerged as an attractive method for regenerating and repairing the lost heart organ. On other hand, poor survival and maintenance of the cells transferred into the damaged heart tissue are broadly accepted as serious barriers to enhance the efficacy of the regenerative therapy. For this reason, external factors, such as antioxidants are used as a favorite strategy by the investigators to improve the cell survival and retention properties. Therefore, the present study was conducted to investigate the In -vitro effect of L-carnitine (LC) on the telomere length and human telomerase reverse transcriptase (hTERT) gene expression in the cardiac differentiated bone marrow resident CD117+ stem cells through Wnt3/β-catenin and ERK1/2 pathways. To do this, bone marrow resident CD117+ stem cells were enriched by the magnetic-activated cell sorting (MACS) method, and were differentiated to the cardiac cells in the absence (-LC) and presence of the LC (+LC). Also, characterization of the enriched c-kit+ cells was performed using the flow cytometry and immunocytochemistry. At the end of the treatment period, the cells were subjected to the real-time PCR technique along with western blotting assay for measurement of the telomere length and assessment of mRNA and protein, respectively. The results showed that 0.2 mM LC caused the elongation of the telomere length and increased the hTERT gene expression in the cardiac differentiated CD117+ stem cells. In addition, a significant increase was observed in the mRNA and protein expression of Wnt3, β-catenin and ERK1/2 as key components of these pathways. It can be concluded that the LC can increase the telomere length as an effective factor in increasing the cell survival and maintenance of the cardiac differentiated bone marrow resident CD117+ stem cells via Wnt3/β-catenin and ERK1/2 signaling pathway components.

Role of CD133 in human embryonic stem cell proliferation and teratoma formation

Background

Pluripotent stem cells (PSCs), including human embryonic stem cells (hESCs), hold great potential for regenerative medicine and cell therapy. One of the major hurdles hindering the clinical development of PSC-based therapy is the potential risk of tumorigenesis. CD133 (Prominin 1, PROM1) is a transmembrane protein whose mRNA and glycosylated forms are highly expressed in many human cancer cell types. CD133 also serves as a cancer stem cell (CSC) marker associated with cancer progression and patient outcome. Interestingly, CD133 is highly expressed in hESCs as well as in human preimplantation embryos, but its function in hESCs has remained largely unknown.

Methods

CD133 knockout hESC WA26 cell line was generated with CRISPR/Cas9. CD133 knockout and wide type hESC lines were subjected to pluripotency, proliferation, telomere biology, and teratoma tests; the related global changes and underlying mechanisms were further systemically analyzed by RNA-seq.

Results

CD133 deficiency did not affect hESC pluripotency or in vivo differentiation into three germ layers but significantly decreased cell proliferation. RNA-seq revealed that CD133 deficiency dysregulated the p53, PI3K-Akt, AMPK, and Wnt signaling pathways. Alterations in these pathways have been implicated in tumor proliferation and apoptotic escape.

Conclusions

Our data imply that CD133 could be an additional target and used as a selective marker to sort and eliminate undifferentiated cells in reducing potential teratoma formation risk of hESCs in regenerative medicine.

Alternative Lengthening of Telomeres (ALT) in Tumors and Pluripotent Stem Cells

A telomere consists of repeated DNA sequences (TTAGGG)n as part of a nucleoprotein structure at the end of the linear chromosome, and their progressive shortening induces DNA damage response (DDR) that triggers cellular senescence. The telomere can be maintained by telomerase activity (TA) in the majority of cancer cells (particularly cancer stem cells) and pluripotent stem cells (PSCs), which exhibit unlimited self-proliferation. However, some cells, such as telomerase-deficient cancer cells, can add telomeric repeats by an alternative lengthening of the telomeres (ALT) pathway, showing telomere length heterogeneity. In this review, we focus on the mechanisms of the ALT pathway and potential clinical implications. We also discuss the characteristics of telomeres in PSCs, thereby shedding light on the therapeutic significance of telomere length regulation in age-related diseases and regenerative medicine.

Impelling force and current challenges by chemicals in somatic cell reprogramming and expansion beyond hepatocytes

In the field of regenerative medicine, generating numerous transplantable functional cells in the laboratory setting on a large scale is a major challenge. However, the in vitro maintenance and expansion of terminally differentiated cells are challenging because of the lack of specific environmental and intercellular signal stimulations, markedly hindering their therapeutic application. Remarkably, the generation of stem/progenitor cells or functional cells with effective proliferative potential is markedly in demand for disease modeling, cell-based transplantation, and drug discovery. Despite the potent genetic manipulation of transcription factors, integration-free chemically defined approaches for the conversion of somatic cell fate have garnered considerable attention in recent years. This review aims to summarize the progress thus far and discuss the advantages, limitations, and challenges of the impact of full chemicals on the stepwise reprogramming of pluripotency, direct lineage conversion, and direct lineage expansion on somatic cells. Owing to the current chemical-mediated induction, reprogrammed pluripotent stem cells with reproducibility difficulties, and direct lineage converted cells with marked functional deficiency, it is imperative to generate the desired cell types directly by chemically inducing their potent proliferation ability through a lineage-committed progenitor state, while upholding the maturation and engraftment capacity posttransplantation in vivo. Together with the comprehensive understanding of the mechanism of chemical drives, as well as the elucidation of specificity and commonalities, the precise manipulation of the expansion for diverse functional cell types could broaden the available cell sources and enhance the cellular function for clinical application in future.

Telomere dynamics and hematopoietic differentiation of human DKC1-mutant induced pluripotent stem cells

Telomeropathies are a group of phenotypically heterogeneous diseases molecularly unified by pathogenic mutations in telomere-maintenance genes causing critically short telomeres. X-linked dyskeratosis congenita (DC), the prototypical telomere disease, manifested with ectodermal dysplasia, cancer predisposition, and severe bone marrow failure, is caused by mutations in DKC1, encoding a protein responsible for telomerase holoenzyme complex stability. To investigate the effects of pathogenic DKC1 mutations on telomere repair and hematopoietic development, we derived induced pluripotent stem cells (iPSCs) from fibroblasts of a DC patient carrying the most frequent mutation: DKC1 p.A353V. Telomeres eroded immediately after reprogramming in DKC1-mutant iPSCs but stabilized in later passages. The telomerase activity of mutant iPSCs was comparable to that observed in human embryonic stem cells, and no evidence of alternative lengthening of telomere pathways was detected. Hematopoietic differentiation was carried out in DKC1-mutant iPSC clones that resulted in increased capacity to generate hematopoietic colony-forming units compared to controls. Our study indicates that telomerase-dependent telomere maintenance is defective in pluripotent stem cells harboring DKC1 mutation and unable to elongate telomeres, but sufficient to maintain cell proliferation and self-renewal, as well as to support the primitive hematopoiesis, the program that is recapitulated with our differentiation protocol.

Dysfunction telomeres in embryonic fibroblasts and cultured in vitro pluripotent stem cells of Rattus norvegicus (Rodentia, Muridae)

We studied the level of spontaneous telomere dysfunction in Rattus norvegicus (Berkenhout, 1769) (Rodentia, Muridae) embryonic fibroblasts (rEFs) and in cultured in vitro rat pluripotent stem cells (rPSCs), embryonic stem cells (rESCs) and induced pluripotent stem cells (riPSCs), on early passages and after prolonged cultivation. Among studied cell lines, rESCs showed the lowest level of telomere dysfunction, while the riPSCs demonstrated an elevated level on early passages of cultivation. In cultivation, the frequency of dysfunctional telomeres has increased in all studied cell lines; this is particularly true for dysfunctional telomeres occurring in G1 stage in riPSCs. The obtained data are mainly discussed in the connection with the specific structure of the telomere regions and their influence on the differential DNA damage response in them.

The role of telomere-binding modulators in pluripotent stem cells

Pluripotent stem cells (PSCs) such as embryonic stem cells (ESCs), ESCs derived by somatic cell nuclear transfer (ntESCs), and induced pluripotent stem cells (iPSCs) have unlimited capacity for self-renewal and pluripotency and can give rise to all types of somatic cells. In order to maintain their self-renewal and pluripotency, PSCs need to preserve their telomere length and homeostasis. In recent years, increasing studies have shown that telomere reprogramming is essential for stem cell pluripotency maintenance and its induced pluripotency process. Telomere-associated proteins are not only required for telomere maintenance in both stem cells, their extra-telomeric functions have also been found to be critical as well. Here, we will discuss how telomeres and telomere-associated factors participate and regulate the maintenance of stem cell pluripotency.

Metabolomic and Transcriptional Analyses Reveal Atmospheric Oxygen During Human Induced Pluripotent Stem Cell Generation Impairs Metabolic Reprogramming

The transition to pluripotency invokes profound metabolic restructuring; however, reprogramming is accompanied by the retention of somatic cell metabolic and epigenetic memory. Modulation of metabolism during reprogramming has been shown to improve reprogramming efficiency, yet it is not known how metabolite availability during reprogramming affects the physiology of resultant induced pluripotent stem cells (iPSCs). Metabolic analyses of iPSCs generated under either physiological (5%; P-iPSC) or atmospheric (20%; A-iPSC) oxygen conditions revealed that they retained aspects of somatic cell metabolic memory and failed to regulate carbohydrate metabolism with A-iPSC acquiring different metabolic characteristics. A-iPSC exhibited a higher mitochondrial membrane potential and were unable to modulate oxidative metabolism in response to oxygen challenge, contrasting with P-iPSC. RNA-seq analysis highlighted that A-iPSC displayed transcriptomic instability and a reduction in telomere length. Consequently, inappropriate modulation of metabolism by atmospheric oxygen during reprogramming significantly impacts the resultant A-iPSC metabolic and transcriptional landscape. Furthermore, retention of partial somatic metabolic memory in P-iPSC derived under physiological oxygen suggests that metabolic reprogramming remains incomplete. As the metabolome is a regulator of the epigenome, these observed perturbations of iPSC metabolism will plausibly have downstream effects on cellular function and physiology, both during and following differentiation, and highlight the need to optimize nutrient availability during the reprogramming process. Stem Cells 2019;37:1042-1056.

Harnessing cellular aging in human stem cell models of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a relentlessly progressive neurodegenerative condition that is invariably fatal, usually within 3 to 5 years of diagnosis. The etiology of ALS remains unresolved and no effective treatments exist. There is therefore a desperate and unmet need for discovery of disease mechanisms to guide novel therapeutic strategies. The single major risk factor for ALS is aging, yet the molecular consequences of cell type‐specific aging remain understudied in this context. Induced pluripotent stem cells (iPSCs) have transformed the standard approach of examining human disease, generating unlimited numbers of disease‐relevant cells from patients, enabling analysis of disease mechanisms and drug screening. However, reprogramming patient cells to iPSCs reverses key hallmarks of cellular age. Therefore, although iPSC models recapitulate some disease hallmarks, a crucial challenge is to address the disparity between the advanced age of onset of neurodegenerative diseases and the fetal‐equivalent maturational state of iPSC‐derivatives. Increasing recognition of cell type‐specific aging paradigms underscores the importance of heterogeneity in ultimately tipping the balance from a state of compensated dysfunction (clinically pre‐symptomatic) to decompensation and progression (irreversible loss of neurological functions). In order to realize the true promise of iPSC technology in ALS, efforts need to prioritize faithfully recapitulating the clinical pathophysiological state, with proportionate emphasis on capturing the molecular sequelae of both cellular age and non‐cell‐autonomous disease mechanisms within this context.

In medio stat virtus: unanticipated consequences of telomere dysequilibrium

The integrity of chromosome ends, or telomeres, depends on myriad processes that must balance the need to compact and protect the telomeric, G-rich DNA from detection as a double-stranded DNA break, and yet still permit access to enzymes that process, replicate and maintain a sufficient reserve of telomeric DNA. When unable to maintain this equilibrium, erosion of telomeres leads to perturbations at or near the telomeres themselves, including loss of binding by the telomere protective complex, shelterin, and alterations in transcription and post-translational modifications of histones. Although the catastrophic consequences of full telomere de-protection are well described, recent evidence points to other, less obvious perturbations that arise when telomere length equilibrium is altered. For example, critically short telomeres also perturb DNA methylation and histone post-translational modifications at distal sites throughout the genome. In murine stem cells for example, this dysregulated chromatin leads to inappropriate suppression of pluripotency regulator factors such as Nanog. This review summarizes these recent findings, with an emphasis on how these genome-wide, telomere-induced perturbations can have profound consequences on cell function and fate.

This article is part of the theme issue ‘Understanding diversity in telomere dynamics’.

Feeders facilitate telomere maintenance and chromosomal stability of embryonic stem cells

Feeder cells like mouse embryonic fibroblasts (MEFs) have been widely applied for culture of pluripotent stem cells, but their roles remain elusive. Noticeably, ESCs cultured on the feeders display transcriptional heterogeneity. We investigated roles of feeder cells by examining the telomere maintenance. Here we show that telomere is longer in mESCs cultured with than without the feeders. mESC cultures without MEF feeders exhibit telomere loss, chromosomal fusion, and aneuploidy with increasing passages. Notably, feeders facilitate heterogeneous transcription of 2-cell genes including Zscan4 and telomere elongation. Moreover, feeders produce Fstl1 that together with BMP4 periodically activate Zscan4. Interestingly, Zscan4 is repressed in mESCs cultured in 2i (inhibitors of Mek and Gsk3β signaling) media, associated with shorter telomeres and increased chromosome instability. These data suggest the important role of feeders in maintaining telomeres for long-term stable self-renewal and developmental pluripotency of mESCs.

Feeder cells are widely used for the culture of embryonic stem cells (ESCs), but their specific effects are not well known. Here, the authors demonstrate that mouse ESCs exhibit telomere loss and chromosomal aberrations associated with reduced Zscan4 with increasing passages in the absence of feeders

Dynamics of Telomere Rejuvenation during Chemical Induction to Pluripotent Stem Cells

Chemically induced pluripotent stem cells (CiPSCs) may provide an alternative and attractive source for stem cell-based therapy. Sufficient telomere lengths are critical for unlimited self-renewal and genomic stability of pluripotent stem cells. Dynamics and mechanisms of telomere reprogramming of CiPSCs remain elusive. We show that CiPSCs acquire telomere lengthening with increasing passages after clonal formation. Both telomerase activity and recombination-based mechanisms are involved in the telomere elongation. Telomere lengths strongly indicate the degree of reprogramming, pluripotency, and differentiation capacity of CiPSCs. Nevertheless, telomere damage and shortening occur at a late stage of lengthy induction, limiting CiPSC formation. We find that histone crotonylation induced by crotonic acid can activate two-cell genes, including Zscan4; maintain telomeres; and promote CiPSC generation. Crotonylation decreases the abundance of heterochromatic H3K9me3 and HP1α at subtelomeres and Zscan4 loci. Taken together, telomere rejuvenation links to reprogramming and pluripotency of CiPSCs. Crotonylation facilitates telomere maintenance and enhances chemically induced reprogramming to pluripotency.

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CiPSCs acquire telomere elongation after clonal formation with increasing passages

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Both telomerase and recombination mechanisms are involved in the telomere elongation

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Telomere damage and shortening can occur during late stage of lengthy induction

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Crotonylation activates Zscan4 and promotes telomere elongation and CiPSC induction

Directed reprogramming of comprehensively characterized dental pulp stem cells extracted from natal tooth

The aim of this study was to extensively characterise natal dental pulp stem cells (nDPSC) and assess their efficiency to generate human induced pluripotent stem cells (hiPSC). A number of distinguishing features prompted us to choose nDPSC over normal adult DPSC, in that they differed in cell surface marker expression and initial doubling time. In addition, nDPSC expressed 17 out of 52 pluripotency genes we analysed, and the level of expression was comparable to human embryonic stem cells (hESC). Ours is the first group to report comprehensive characterization of nDPSC followed by directed reprogramming to a pluripotent stem cell state. nDPSC yielded hiPSC colonies upon transduction with Sendai virus expressing the pluripotency transcription factors POU5F1, SOX2, c-MYC and KLF4. nDPSC had higher reprogramming efficiency compared to human fibroblasts. nDPSC derived hiPSCs closely resembled hESC in terms of their morphology, expression of pluripotency markers and gene expression profiles. Furthermore, nDPSC derived hiPSCs differentiated into the three germ layers when cultured as embryoid bodies (EB) and by directed differentiation. Based on our findings, nDPSC present a unique marker expression profile compared with adult DPSC and possess higher reprogramming efficiency as compared with dermal fibroblasts thus proving to be more amenable for reprogramming.

Role of Jnk1 in development of neural precursors revealed by iPSC modeling

Jnk1-deficient mice manifest disrupted anterior commissure formation and loss of axonal and dendritic microtubule integrity. However, the mechanisms and the specific stages underlying the developmental defects remain to be elucidated. Here, we report the generation of Jnk1-deficient (Jnk1 KO) iPSCs from Jnk1 KO mouse tail-tip fibroblasts (TTFs) for modeling the neural disease development. The efficiency in the early induction of iPSCs was higher from Jnk1 KO fibroblasts than that of wild-type (WT) fibroblasts. These Jnk1 KO iPSCs exhibited pluripotent stem cell properties and had the ability of differentiation into general three embryonic germ layers in vitro and in vivo. However, Jnk1 KO iPSCs showed reduced capacity in neural differentiation in the spontaneous differentiation by embryoid body (EB) formation. Notably, by directed lineage differentiation, Jnk1 KO iPSCs specifically exhibited an impaired ability to differentiate into early stage neural precursors. Furthermore, the neuroepitheliums generated from Jnk1 KO iPSCs appeared smaller, indicative of neural stem cell developmental defects, as demonstrated by teratoma tests in vivo. These data suggest that Jnk1 deficiency inhibits the development of neural stem cells/precursors and provide insights to further understanding the complex pathogenic mechanisms of JNK1-related neural diseases.

Age Is Relative-Impact of Donor Age on Induced Pluripotent Stem Cell-Derived Cell Functionality

Induced pluripotent stem cells (iPSCs) avoid many of the restrictions that hamper the application of human embryonic stem cells: limited availability of source material due to legal restrictions in some countries, immunogenic rejection and ethical concerns. Also, the donor’s clinical phenotype is often known when working with iPSCs. Therefore, iPSCs seem ideal to tackle the two biggest tasks of regenerative medicine: degenerative diseases with genetic cause (e.g., Duchenne’s muscular dystrophy) and organ replacement in age-related diseases (e.g., end-stage heart or renal failure), especially in combination with recently developed gene-editing tools. In the setting of autologous transplantation in elderly patients, donor age becomes a potentially relevant factor that needs to be assessed. Here, we review and critically discuss available data pertinent to the questions: How does donor age influence the reprogramming process and iPSC functionality? Would it even be possible to reprogram senescent somatic cells? How does donor age affect iPSC differentiation into specialised cells and their functionality? We also identify research needs, which might help resolve current unknowns. Until recently, most hallmarks of ageing were attributed to an accumulation of DNA damage over time, and it was thus expected that DNA damage from a somatic cell would accumulate in iPSCs and the cells derived from them. In line with this, a decreased lifespan of cloned organisms compared with the donor was also observed in early cloning experiments. Therefore, it was questioned for a time whether iPSC derived from an old individual’s somatic cells would suffer from early senescence and, thus, may not be a viable option either for disease modelling nor future clinical applications. Instead, typical signs of cellular ageing are reverted in the process of iPSC reprogramming, and iPSCs from older donors do not show diminished differentiation potential nor do iPSC-derived cells from older donors suffer early senescence or show functional impairments when compared with those from younger donors. Thus, the data would suggest that donor age does not limit iPSC application for modelling genetic diseases nor regenerative therapies. However, open questions remain, e.g., regarding the potential tumourigenicity of iPSC-derived cells and the impact of epigenetic pattern retention.

Aurora Kinase B, a novel regulator of TERF1 binding and telomeric integrity

AURKB (Aurora Kinase B) is a serine/threonine kinase better known for its role at the mitotic kinetochore during chromosome segregation. Here, we demonstrate that AURKB localizes to the telomeres in mouse embryonic stem cells, where it interacts with the essential telomere protein TERF1. Loss of AURKB function affects TERF1 telomere binding and results in aberrant telomere structure. In vitro kinase experiments successfully identified Serine 404 on TERF1 as a putative AURKB target site. Importantly, in vivo overexpression of S404-TERF1 mutants results in fragile telomere formation. These findings demonstrate that AURKB is an important regulator of telomere structural integrity.

Telomere heterogeneity linked to metabolism and pluripotency state revealed by simultaneous analysis of telomere length and RNA-seq in the same human embryonic stem cell

Background

Telomere length heterogeneity has been detected in various cell types, including stem cells and cancer cells. Cell heterogeneity in pluripotent stem cells, such as embryonic stem cells (ESCs), is of particular interest; however, the implication and mechanisms underlying the heterogeneity remain to be understood. Single-cell analysis technology has recently been developed and effectively employed to investigate cell heterogeneity. Yet, methods that can simultaneously measure telomere length and analyze the global transcriptome in the same cell have not been available until now.

Results

We have established a robust method that can simultaneously measure telomere length coupled with RNA-sequencing analysis (scT&R-seq) in the same human ESC (hESC). Using this method, we show that telomere length varies with pluripotency state. Compared to those with long telomere, hESCs with short telomeres exhibit the lowest expressions of TERF1/TRF1, and ZFP42/REX1, PRDM14 and NANOG markers for pluripotency, suggesting that these hESCs are prone to exit from the pluripotent state. Interestingly, hESCs ubiquitously express NOP10 and DKC1, stabilizing components of telomerase complexes. Moreover, new candidate genes, such as MELK, MSH6, and UBQLN1, are highly expressed in the cluster of cells with long telomeres and higher expression of known pluripotency markers. Notably, short telomere hESCs exhibit higher oxidative phosphorylation primed for lineage differentiation, whereas long telomere hESCs show elevated glycolysis, another key feature for pluripotency.

Conclusions

Telomere length is a marker of the metabolic activity and pluripotency state of individual hESCs. Single cell analysis of telomeres and RNA-sequencing can be exploited to further understand the molecular mechanisms of telomere heterogeneity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-017-0453-8) contains supplementary material, which is available to authorized users.

A balance between elongation and trimming regulates telomere stability in stem cells

Telomere length maintenance ensures self-renewal of human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs), however the mechanisms governing telomere length homeostasis in these cell types are unclear. Here, we report that telomere length is determined by the balance between telomere elongation mediated by telomerase and telomere trimming, controlled by the homologous recombination proteins XRCC3 and Nbs1 that generate single-stranded C-rich telomeric DNA and double-stranded telomeric circular DNA (T-circles), respectively. We found that reprogramming of differentiated cells induces T-circle and single stranded C-rich telomeric DNA accumulation, indicating the activation of telomere trimming pathways that compensate telomerase dependent telomere elongation in hiPSCs. Excessive telomere elongation compromises telomere stability and promotes the formation of partially single-stranded telomeric DNA circles (C-circles) in hESCs, suggesting heightened sensitivity of stem cells to replication stress at overly long telomeres. Thus, tight control of telomere length homeostasis is essential to maintain telomere stability in hESCs.

Concise Review: Getting to the Core of Inherited Bone Marrow Failures

Bone marrow failure syndromes (BMFS) are a group of disorders with complex pathophysiology characterized by a common phenotype of peripheral cytopenia and/or hypoplastic bone marrow. Understanding genetic factors contributing to the pathophysiology of BMFS has enabled the identification of causative genes and development of diagnostic tests. To date more than 40 mutations in genes involved in maintenance of genomic stability, DNA repair, ribosome and telomere biology have been identified. In addition, pathophysiological studies have provided insights into several biological pathways leading to the characterization of genotype/phenotype correlations as well as the development of diagnostic approaches and management strategies. Recent developments in bone marrow transplant techniques and the choice of conditioning regimens have helped improve transplant outcomes. However, current morbidity and mortality remain unacceptable underlining the need for further research in this area. Studies in mice have largely been unable to mimic disease phenotype in humans due to difficulties in fully replicating the human mutations and the differences between mouse and human cells with regard to telomere length regulation, processing of reactive oxygen species and lifespan. Recent advances in induced pluripotency have provided novel insights into disease pathogenesis and have generated excellent platforms for identifying signaling pathways and functional mapping of haplo‐insufficient genes involved in large‐scale chromosomal deletions–associated disorders. In this review, we have summarized the current state of knowledge in the field of BMFS with specific focus on modeling the inherited forms and how to best utilize these models for the development of targeted therapies. Stem Cells2017;35:284–298

Linking Telomere Regulation to Stem Cell Pluripotency

Embryonic stem cells (ESCs), somatic cell nuclear transfer ESCs, and induced pluripotent stem cells (iPSCs) represent the most studied group of PSCs. Unlimited self-renewal without incurring chromosomal instability and pluripotency are essential for the potential use of PSCs in regenerative therapy. Telomere length maintenance is critical for the unlimited self-renewal, pluripotency, and chromosomal stability of PSCs. While telomerase has a primary role in telomere maintenance, alternative lengthening of telomere pathways involving recombination and epigenetic modifications are also required for telomere length regulation, notably in mouse PSCs. Telomere rejuvenation is part of epigenetic reprogramming to pluripotency. Insights into telomere reprogramming and maintenance in PSCs may have implications for understanding of aging and tumorigenesis. Here, I discuss the link between telomere elongation and homeostasis to the acquisition and maintenance of stem cell pluripotency, and their regulatory mechanisms by epigenetic modifications.

Concise Review: Recent Advances in the In Vitro Derivation of Blood Cell Populations

: Hematopoietic cell-based therapies are currently available treatment options for many hematological and nonhematological disorders. However, the scarcity of allogeneic donor-derived cells is a major hurdle in treating these disorders. Embryonic stem cell-based directed differentiation and direct reprogramming of somatic cells provide excellent tools for the potential generation of hematopoietic stem cells usable in the clinic for cellular therapies. In addition to blood stem cell transplantation, mature blood cells such as red blood cells, platelets, and engineered T cells have also been increasingly used to treat several diseases. Besides cellular therapies, induced blood progenitor cells generated from autologous sources (either induced pluripotent stem cells or somatic cells) can be useful for disease modeling of bone marrow failures and acquired blood disorders. However, although great progress has been made toward these goals, we are still far from the use of in vitro-derived blood products in the clinic. We review the current state of knowledge on the directed differentiation of embryonic stem cells and the reprogramming of somatic cells toward the generation of blood stem cells and derivatives.

SIGNIFICANCE

Hematopoietic cell-based therapies are currently available treatment options for many hematological and nonhematological disorders. However, the scarcity of allogeneic donor-derived cells is a major hurdle in treating these disorders. The current state of knowledge on the directed differentiation of embryonic stem cells and the reprogramming of somatic cells toward the generation of blood stem cells and derivatives is reviewed.

Telomere Length Defines the Cardiomyocyte Differentiation Potency of Mouse Induced Pluripotent Stem Cells

Induced pluripotent stem cells (iPSCs) can be differentiated in vitro and in vivo to all cardiovascular lineages and are therefore a promising cell source for cardiac regenerative therapy. However, iPSC lines do not all differentiate into cardiomyocytes (CMs) with the same efficiency. Here, we show that telomerase-competent iPSCs with relatively long telomeres and high expression of the shelterin-complex protein TRF1 (iPSC^highT ) differentiate sooner and more efficiently into CMs than those with relatively short telomeres and low TRF1 expression (iPSC^lowT ). Ascorbic acid, an enhancer of cardiomyocyte differentiation, further increases the cardiomyocyte yield from iPSC^highT but does not rescue the cardiomyogenic potential of iPSC^lowT . Interestingly, although iPSCs^lowT differentiate very poorly to the mesoderm and endoderm lineages, they differentiate very efficiently to the ectoderm lineage, indicating that cell fate can be determined by in vitro selection of iPSCs with different telomere content. Our findings highlight the importance of selecting iPSCs with ample telomere reserves in order to generate high numbers of CMs in a fast, reliable, and efficient way. Stem Cells 2017;35:362-373.

Tcstv1 and Tcstv3 elongate telomeres of mouse ES cells

Mouse embryonic stem cell (ESC) cultures exhibit a heterogeneous mixture of metastable cells sporadically entering the 2-cell (2C)-embryo-like state, critical for ESC potency. One of 2-cell genes, Zscan4, has been shown to be responsible for telomere maintenance, genomic stability and pluripotency of mouse ESCs. Functions of other 2C-genes in ESCs remain elusive. Here we show that 2C-genes Tcstv1 and Tcstv3 play a role in regulation of telomere lengths. Overexpression or knockdown Tcstv1 and Tcstv3 does not immediately affect proliferation, pluripotency and differentiation in vitro of ESCs. However, ectopic expression of Tcstv1 or Tcstv3 results in telomere elongation, whereas Tcstv1/3 knockdown shortens telomeres of ESCs. Overexpression of Tcstv1 or Tcstv3 does not alter telomere stability. Furthermore, Tcstv1 can increase Zscan4 protein levels and telomere recombination by telomere sister chromatid exchange (T-SCE). Depletion of Tcstv1/3 reduces Zscan4 protein levels. Together, Tcstv1 and Tcstv3 are involved in telomere maintenance that is required for long-term self-renewal of mouse ESCs. Our data also suggests that Tcstv1/3 may co-operate and stabilize Zscan4 protein but the molecular bases remain to be determined.

The role of telomeres and telomerase reverse transcriptase isoforms in pluripotency induction and maintenance

Telomeres are linear guanine-rich DNA structures at the ends of chromosomes. The length of telomeric DNA is actively regulated by a number of mechanisms in highly proliferative cells such as germ cells, cancer cells, and pluripotent stem cells. Telomeric DNA is synthesized by way of the ribonucleoprotein called telomerase containing a reverse transcriptase (TERT) subunit and RNA component (TERC). TERT is highly conserved across species and ubiquitously present in their respective pluripotent cells. Recent studies have uncovered intricate associations between telomeres and the self-renewal and differentiation properties of pluripotent stem cells. Interestingly, the past decade's work indicates that the TERT subunit also has the capacity to modulate mitochondrial function, to remodel chromatin structure, and to participate in key signaling pathways such as the Wnt/β-catenin pathway. Many of these non-canonical functions do not require TERT's catalytic activity, which hints at possible functions for the extensive number of alternatively spliced TERT isoforms that are highly expressed in pluripotent stem cells. In this review, some of the established and potential routes of pluripotency induction and maintenance are highlighted from the perspectives of telomere maintenance, known TERT isoform functions and their complex regulation.

Erk signaling is indispensable for genomic stability and self-renewal of mouse embryonic stem cells

Inhibition of Mek/Erk signaling by pharmacological Mek inhibitors promotes self-renewal and pluripotency of mouse embryonic stem cells (ESCs). Intriguingly, Erk signaling is essential for human ESC self-renewal. Here we demonstrate that Erk signaling is critical for mouse ESC self-renewal and genomic stability. Erk-depleted ESCs cannot be maintained. Lack of Erk leads to rapid telomere shortening and genomic instability, in association with misregulated expression of pluripotency genes, reduced cell proliferation, G1 cell-cycle arrest, and increased apoptosis. Erk signaling is also required for the activation of differentiation genes but not for the repression of pluripotency genes during ESC differentiation. Furthermore, we find an Erk-independent function of Mek, which may explain the diverse effects of Mek inhibition and Erk knockout on ESC self-renewal. Together, in contrast to the prevailing view, Erk signaling is required for telomere maintenance, genomic stability, and self-renewal of mouse ESCs.

Roles for Histone Acetylation in Regulation of Telomere Elongation and Two-cell State in Mouse ES Cells

Mammalian telomeres and subtelomeres are marked by heterochromatic epigenetic modifications, including repressive DNA methylation and histone methylation (e.g., H3K9me3 and H4K20me3). Loss of these epigenetic marks results in increased rates of telomere recombination and elongation. Other than these repressive epigenetic marks, telomeric and subtelomeric H3 and H4 are underacetylated. Yet, whether histone acetylation also regulates telomere length has not been directly addressed. We thought to test the effects of histone acetylation levels on telomere length using histone deacetylase (HDAC) inhibitor (sodium butyrate, NaB) that mediates histone hyperacetylation and histone acetyltransferase (HAT) inhibitor (C646) that mediates histone hypoacetylation. We show that histone hyperacetylation dramatically elongates telomeres in wild-type ES cells, and only slightly elongates telomeres in Terc(-/-) ES cells, suggesting that Terc is involved in histone acetylation-induced telomere elongation. In contrast, histone hypoacetylation shortens telomeres in both wild-type and Terc(-/-) ES cells. Additionally, histone hyperacetylation activates 2-cell (2C) specific genes including Zscan4, which is involved in telomere recombination and elongation, whereas histone hypoacetylation represses Zscan4 and 2C genes. These data suggest that histone acetylation levels affect the heterochromatic state at telomeres and subtelomeres, and regulate gene expression at subtelomeres, linking histone acetylation to telomere length maintenance.

Current aging research in China

The mini-review stemmed from a recent meeting on national aging research strategies in China discusses the components and challenges of aging research in China. Highlighted are the major efforts of a number of research teams, funding situations and outstanding examples of recent major research achievements. Finally, authors discuss potential targets and strategies of aging research in China.

Werner Syndrome-specific induced pluripotent stem cells: recovery of telomere function by reprogramming

Werner syndrome (WS) is a rare human autosomal recessive premature aging disorder characterized by early onset of aging-associated diseases, chromosomal instability, and cancer predisposition. The function of the DNA helicase encoded by WRN, the gene responsible for WS, has been studied extensively. WRN helicase is involved in the maintenance of chromosome integrity through DNA replication, repair, and recombination by interacting with a variety of proteins associated with DNA repair and telomere maintenance. The accelerated aging associated with WS is reportedly caused by telomere dysfunction, and the underlying mechanism of the disease is yet to be elucidated. Although it was reported that the life expectancy for patients with WS has improved over the last two decades, definitive therapy for these patients has not seen much development. Severe symptoms of the disease, such as leg ulcers, cause a significant decline in the quality of life in patients with WS. Therefore, the establishment of new therapeutic strategies for the disease is of utmost importance. Induced pluripotent stem cells (iPSCs) can be established by the introduction of several pluripotency genes, including Oct3/4, Sox2, Klf4, and c-myc into differentiated cells. iPSCs have the potential to differentiate into a variety of cell types that constitute the human body, and possess infinite proliferative capacity. Recent studies have reported the generation of iPSCs from the cells of patients with WS, and they have concluded that reprogramming represses premature senescence phenotypes in these cells. In this review, we summarize the findings of WS patient-specific iPSCs (WS iPSCs) and focus on the roles of telomere and telomerase in the maintenance of these cells. Finally, we discuss the potential use of WS iPSCs for clinical applications.

Transient delivery of modified mRNA encoding TERT rapidly extends telomeres in human cells

Telomere extension has been proposed as a means to improve cell culture and tissue engineering and to treat disease. However, telomere extension by nonviral, nonintegrating methods remains inefficient. Here we report that delivery of modified mRNA encoding TERT to human fibroblasts and myoblasts increases telomerase activity transiently (24-48 h) and rapidly extends telomeres, after which telomeres resume shortening. Three successive transfections over a 4 d period extended telomeres up to 0.9 kb in a cell type-specific manner in fibroblasts and myoblasts and conferred an additional 28 ± 1.5 and 3.4 ± 0.4 population doublings (PDs), respectively. Proliferative capacity increased in a dose-dependent manner. The second and third transfections had less effect on proliferative capacity than the first, revealing a refractory period. However, the refractory period was transient as a later fourth transfection increased fibroblast proliferative capacity by an additional 15.2 ± 1.1 PDs, similar to the first transfection. Overall, these treatments led to an increase in absolute cell number of more than 10(12)-fold. Notably, unlike immortalized cells, all treated cell populations eventually stopped increasing in number and expressed senescence markers to the same extent as untreated cells. This rapid method of extending telomeres and increasing cell proliferative capacity without risk of insertional mutagenesis should have broad utility in disease modeling, drug screening, and regenerative medicine.

Defective repair of uracil causes telomere defects in mouse hematopoietic cells

Uracil in the genome can result from misincorporation of dUTP instead of dTTP during DNA synthesis, and is primarily removed by uracil DNA glycosylase (UNG) during base excision repair. Telomeres contain long arrays of TTAGGG repeats and may be susceptible to uracil misincorporation. Using model telomeric DNA substrates, we showed that the position and number of uracil substitutions of thymine in telomeric DNA decreased recognition by the telomere single-strand binding protein, POT1. In primary mouse hematopoietic cells, uracil was detectable at telomeres, and UNG deficiency further increased uracil loads and led to abnormal telomere lengthening. In UNG-deficient cells, the frequencies of sister chromatid exchange and fragility in telomeres also significantly increased in the absence of telomerase. Thus, accumulation of uracil and/or UNG deficiency interferes with telomere maintenance, thereby underscoring the necessity of UNG-initiated base excision repair for the preservation of telomere integrity.

Telomere elongation and naive pluripotent stem cells achieved from telomerase haplo-insufficient cells by somatic cell nuclear transfer

Haplo-insufficiency of telomerase genes in humans leads to telomere syndromes such as dyskeratosis congenital and idiopathic pulmonary fibrosis. Generation of pluripotent stem cells from telomerase haplo-insufficient donor cells would provide unique opportunities toward the realization of patient-specific stem cell therapies. Recently, pluripotent human embryonic stem cells (ntESCs) have been efficiently achieved by somatic cell nuclear transfer (SCNT). We tested the hypothesis that SCNT could effectively elongate shortening telomeres of telomerase haplo-insufficient cells in the ntESCs with relevant mouse models. Indeed, telomeres of telomerase haplo-insufficient (Terc(+/-)) mouse cells are elongated in ntESCs. Moreover, ntESCs derived from Terc(+/-) cells exhibit naive pluripotency as evidenced by generation of Terc(+/-) ntESC clone pups by tetraploid embryo complementation, the most stringent test of naive pluripotency. These data suggest that SCNT could offer a powerful tool to reprogram telomeres and to discover the factors for robust restoration of telomeres and pluripotency of telomerase haplo-insufficient somatic cells.

Telomere length maintenance, shortening, and lengthening

Telomeres maintain chromosome stability and cell replicative capacity. Telomere shortening occurs concomitant with aging. Short telomeres are associated with some diseases, such as dyskeratosis congenita, idiopathic pulmonary fibrosis, and aplastic anemia. Telomeres are longer in pluripotent stem cells than in somatic cells and lengthen significantly during preimplantation development. Furthermore, telomere elongation during somatic cell reprogramming is of great importance in the acquisition of authentic pluripotency. This review focuses primarily on regulatory mechanisms of telomere length maintenance in pluripotent cells, telomere length extension in early embryo development, and also telomere rejuvenation in somatic cell reprogramming. Telomere related diseases are also discussed in this review.