Extension of cell life-span and telomere length in animals cloned from senescent somatic cells

Partial cellular reprogramming: A deep dive into an emerging rejuvenation technology

Aging and age-associated disease are a major medical and societal burden in need of effective treatments. Cellular reprogramming is a biological process capable of modulating cell fate and cellular age. Harnessing the rejuvenating benefits without altering cell identity via partial cellular reprogramming has emerged as a novel translational strategy with therapeutic potential and strong commercial interests. Here, we explore the aging-related benefits of partial cellular reprogramming while examining limitations and future directions for the field.

Evaluation of postnatal growth, hematology, telomere length and semen attributes of multiple clones and re-clone of superior buffalo breeding bulls

The present study comprehensively evaluates the postnatal growth, hematology, telomere length, and semen attributes of multiple clones and re-clone derived from superior buffalo breeding bulls. To the best of our knowledge, we successfully produced multiple clones and a re-clone of an earlier cloned buffalo bull from an embryo developed from an adult bull's skin-derived cell for the first time. The cloned bulls' growth, blood hematology, plasma biochemistry, and telomere length were all shown to be normal at various stages of development. The bulls were used for semen production after being screened for testicular growth and training. Semen characteristics such as volume, concentration, and initial motility of fresh sperm as well as motility and kinetics characteristics such as straightness (STR), average lateral head displacement (ALH), and beat cross frequency (BCF) of frozen-thawed sperms of the cloned bulls were found to be similar to those of non-cloned bulls, including the donor bulls. Additionally, it was found that cloned bulls' functional sperm attributes, including acrosome intactness, mitochondrial membrane potential, and superoxide anion status, were comparable to those of non-cloned bulls. These characteristics are necessary for sperm to pass through the female reproductive system, penetrate the oocyte, and efficiently fertilize. Finally, this study adds to our understanding of the postnatal development, hematology, telomere length, and sperm characteristics of superior buffalo breeding bulls that have been cloned and re-cloned. The findings provide the groundwork for improving cloning practices, refining reproductive procedures, and optimizing the use of cloned genetic material in animal breeding and conservation.

Telomere Dynamics in Livestock

Telomeres are repeated sequences of nucleotides at the end of chromosomes. They deteriorate across mitotic divisions of a cell. In Homo sapiens this process of lifetime reduction has been shown to correspond with aspects of organismal aging and exposure to stress or other insults. The early impetus to characterize telomere dynamics in livestock related to the concern that aged donor DNA would result in earlier cell senescence and overall aging in cloned animals. Telomere length investigations in dairy cows included breed effects, estimates of additive genetic control (heritability 0.12 to 0.46), and effects of external stressors on telomere degradation across animal life. Evaluation of telomeres with respect to aging has also been conducted in pigs and horses, and there are fewer reports of telomere biology in beef cattle, sheep, and goats. There were minimal associations of telomere length with animal productivity measures. Most, but not all, work in livestock has documented an inverse relationship between peripheral blood cell telomere length and age; that is, a longer telomere length was associated with younger age. Because livestock longevity affects productivity and profitability, the role of tissue-specific telomere attrition in aging may present alternative improvement strategies for genetic improvement while also providing translational biomedical knowledge.

Ginsenoside F1-Mediated Telomere Preservation Delays Cellular Senescence

Telomeres play pivotal roles in processes closely related to somatic senescence and aging, making them a compelling target for interventions aimed at combating aging and age-related pathologies. Ginsenoside, a natural compound, has emerged as a potential remedy for promoting healthy aging, yet how it protects telomeres remains incompletely understood. Here, we show that treatment of F1 can effectively restore the level of TRF2, thereby preserving telomere integrity. This restoration leads to inhibition of the DNA damage response and improvements in mitochondrial function and, ultimately, delays in cellular senescence. Conversely, depletion of TRF2 causes mitochondrial dysfunction, accompanied by increased oxidative stress, autophagy inhibition, insufficient energy metabolism, and the onset of cellular senescence. These observations underscore the critical role of TRF2 in maintaining telomere integrity and direct association with the initiation of cellular senescence. We conduct a further analysis, suggesting F1 could bind in proximity to the TRF2 heterodimer interface, potentially enhancing dimerization stability. These findings suggest that F1 may be a promising natural remedy for anti-aging, and restoring TRF2 could potentially prevent telomere-dependent diseases commonly associated with the aging process.

Mitochondria-targeted antioxidant MitoQ ameliorates ROS production and improves cell viability in cryopreserved buffalo fibroblasts

Cryopreservation commonly decreases the cellular functionality and post-thaw viability of cells. Reactive oxygen species (ROS) generated during cryopreservation degrade mitochondrial activity and promote the release of cytochrome C which activates caspases required for apoptosis. Antioxidants have the potential to improve the recovery efficiency of cells by reducing ROS production and maintaining mitochondrial membrane potential (MMP). The present study was conducted to explore the role of MitoQ, a derivative of coenzyme Q10 on cryopreserved fibroblasts derived from buffalo skin. To achieve our goal, buffalo skin fibroblasts were treated with varying concentrations of MitoQ (0, 0.1, 0.5, 1, 2, and 10 μM) for 24, 48, and 72 h. The MMP, ROS generation, cell viability was measured by flow cytometry. Furthermore, expression of genes related to mitochondrial oxidative stress (NRF2, GPX, and SOD), apoptosis (BAK and caspase 3) and cell proliferation (AKT) were also assessed. The results showed that over a period of 72 h lower concentrations of MitoQ (0.1-0.5 μM) decrease the ROS production, improves MMP and cell viability whilst the high concentration of MitoQ (2-10 μM) increased the oxidative damage to the cells. Taken together, our study provide important insights into the novel role of MitoQ in cryopreserved buffalo skin fibroblasts. In conclusion, we demonstrated the dose-dependent functional role of MitoQ on cryopreserved fibroblasts for improving post-thaw cell viability and cellular function.

New Technologies Based on Stem Cell-Therapies in Regenerative Medicine and Reproductive Biology

Stem cells seem to hold major promise for contemporary medicine, one which could almost be more significant than a discovery of DNA and ultimate its relevance for organismal integration in the past century [...].

Dexmedetomidine reduces enteric glial cell injury induced by intestinal ischaemia-reperfusion injury through mitochondrial localization of TERT

This study was performed to uncover the effects of dexmedetomidine on oxidative stress injury induced by mitochondrial localization of telomerase reverse transcriptase (TERT) in enteric glial cells (EGCs) following intestinal ischaemia-reperfusion injury (IRI) in rat models. Following establishment of intestinal IRI models by superior mesenteric artery occlusion in Wistar rats, the expression and distribution patterns of TERT were detected. The IRI rats were subsequently treated with low or high doses of dexmedetomidine, followed by detection of ROS, MDA and GSH levels. Calcein cobalt and rhodamine 123 staining were also carried out to detect mitochondrial permeability transition pore (MPTP) and the mitochondrial membrane potential (MMP), respectively. Moreover, oxidative injury of mtDNA was determined, in addition to analyses of EGC viability and apoptosis. Intestinal tissues and mitochondria of EGCs were badly damaged in the intestinal IRI group. In addition, there was a reduction in mitochondrial localization of TERT, oxidative stress, whilst apoptosis of EGCs was increased and proliferation was decreased. On the other hand, administration of dexmedetomidine was associated with promotion of mitochondrial localization of TERT, whilst oxidative stress, MPTP and mtDNA in EGCs, and EGC apoptosis were all inhibited, and the MMP and EGC viability were both increased. A positive correlation was observed between different doses of dexmedetomidine and protective effects. Collectively, our findings highlighted the antioxidative effects of dexmedetomidine on EGCs following intestinal IRI, as dexmedetomidine alleviated mitochondrial damage by enhancing the mitochondrial localization of TERT.

Epigenetic Inheritance From Normal Origin Cells Can Determine the Aggressive Biology of Tumor-Initiating Cells and Tumor Heterogeneity

The acquisition of genetic- and epigenetic-abnormalities during transformation has been recognized as the two fundamental factors that lead to tumorigenesis and determine the aggressive biology of tumor cells. However, there is a regularity that tumors derived from less-differentiated normal origin cells (NOCs) usually have a higher risk of vascular involvement, lymphatic and distant metastasis, which can be observed in both lymphohematopoietic malignancies and somatic cancers. Obviously, the hypothesis of genetic- and epigenetic-abnormalities is not sufficient to explain how the linear relationship between the cellular origin and the biological behavior of tumors is formed, because the cell origin of tumor is an independent factor related to tumor biology. In a given system, tumors can originate from multiple cell types, and tumor-initiating cells (TICs) can be mapped to different differentiation hierarchies of normal stem cells, suggesting that the heterogeneity of the origin of TICs is not completely chaotic. TIC’s epigenome includes not only genetic- and epigenetic-abnormalities, but also established epigenetic status of genes inherited from NOCs. In reviewing previous studies, we found much evidence supporting that the status of many tumor-related “epigenetic abnormalities” in TICs is consistent with that of the corresponding NOC of the same differentiation hierarchy, suggesting that they may not be true epigenetic abnormalities. So, we speculate that the established statuses of genes that control NOC’s migration, adhesion and colonization capabilities, cell-cycle quiescence, expression of drug transporters, induction of mesenchymal formation, overexpression of telomerase, and preference for glycolysis can be inherited to TICs through epigenetic memory and be manifested as their aggressive biology. TICs of different origins can maintain different degrees of innate stemness from NOC, which may explain why malignancies with stem cell phenotypes are usually more aggressive.

Technical, Biological and Molecular Aspects of Somatic Cell Nuclear Transfer – A Review

Since the announcement of the birth of the first cloned mammal in 1997, Dolly the sheep, 24 animal species including laboratory, farm, and wild animals have been cloned. The technique for somatic cloning involves transfer of the donor nucleus of a somatic cell into an enucleated oocyte at the metaphase II (MII) stage for the generation of a new individual, genetically identical to the somatic cell donor. There is increasing interest in animal cloning for different purposes such as rescue of endangered animals, replication of superior farm animals, production of genetically engineered animals, creation of biomedical models, and basic research. However, the efficiency of cloning remains relatively low. High abortion, embryonic, and fetal mortality rates are frequently observed. Moreover, aberrant developmental patterns during or after birth are reported. Researchers attribute these abnormal phenotypes mainly to incomplete nuclear remodeling, resulting in incomplete reprogramming. Nevertheless, multiple factors influence the success of each step of the somatic cloning process. Various strategies have been used to improve the efficiency of nuclear transfer and most of the phenotypically normal born clones can survive, grow, and reproduce. This paper will present some technical, biological, and molecular aspects of somatic cloning, along with remarkable achievements and current improvements.

Telomere length in dromedary camels (Camelus dromedarius) produced by somatic cell nuclear transfer (SCNT) and their age-matched naturally produced counterparts

There are controversial reports on the restoration of eroded telomere length in offspring produced by somatic cell nuclear transfer (SCNT) in different animal species. To the best of our knowledge, no earlier studies report the telomere length in naturally produced or cloned animals in any of the camelid species. Therefore, the present study was conducted to estimate the telomere length in dromedary camels produced by SCNT, the donor cells, and their age-matched naturally produced counterparts by Terminal Restriction Fragment (TRF) length analysis and real-time Q PCR T/S ratio methods. Genomic DNA was extracted from venous blood collected from 6 cloned animals and their age-matched counterparts. Using the southern blot technique, digested DNA was blotted onto a positively charged nylon membrane, and its hybridization was carried out using telomere (TTAGGG)n specific, DIG-labeled hybridization probe (Roche Diagnostics, Germany) at 42 °C for 4 h. Stringent washes were carried out at the same temperature, followed by a chemiluminescence reaction. The signals were captured using the Azure Biosystems C600 gel documentation system. A TeloTool program from MATLAB software with a built-in probe intensity correction algorithm was used for TRF analysis. The experiment was replicated three times, and the data, presented as mean ± SEM, were analyzed using a two-sample t-test (MINITAB statistical software, Minitab ltd, CV3 2 TE, UK). No difference was found in the mean telomere length of cloned camels when compared to their naturally produced age-matched counterparts. However, the telomere length was more (P < 0.05) than that of the somatic cells used for producing the SCNT embryos. A moderate positive Pearson correlation coefficient (r = 0.6446) was observed between the telomere lengths estimated by TRF and Q PCR T/S ratio method. In conclusion, this is the first study wherein we are reporting telomere length in naturally produced and cloned dromedary camels produced by somatic cell nuclear transfer. We found that telomere lengths in cloned camels were similar to their age-matched naturally produced counterparts, suggesting that the camel cytoplast reprograms the somatic cell nucleus and restores the telomere length to its totipotency stage.

Direct electrochemistry of silver nanoparticles-decorated metal-organic frameworks for telomerase activity sensing via allosteric activation of an aptamer hairpin

A direct electrochemistry of silver nanoparticles (AgNPs)-anchored metal-organic frameworks (MOFs) is developed for detection of telomerase activity based on allosteric activation of an aptamer hairpin. AgNPs in situ decorated on PCN-224 (AgNPs/PCN-224) constituted the direct electrochemical labels that were further biofunctionalized by recognition moiety of streptavidin (SA). To achieve the target biosensing, an allosteric hairpin-structured DNA was elaborately designed for signal transduction. The presence of telomerase elongated its primer in the hairpin to displace partial stem strand, thus resulted in the formation of SA aptamer-open structure. Through the specific interaction with aptamer, SA-biofunctionalized AgNPs/PCN-224 probe was attached onto the electrode surface, generating electrochemical signal at + 0.072 V of AgNPs centralized by MOF structure. The direct electrochemical biosensor showed target activity-dependent response from 1.0 × 10^-7 to 1.0 × 10^-1 IU L^-1 with a detection limit of 5.4 × 10^-8 IU L^-1. Moreover, the sensor was applied in evaluation of telomerase activity in living cancer cells. The established electrochemical detection approach in this work avoids the critical deoxygenation conditions and additional electrocatalytic reagents, which opens a novel biosensing perspective for direct electrochemistry of MOF-based nanocomposites.

Cellular reprogramming and epigenetic rejuvenation

Ageing is an inevitable condition that afflicts all humans. Recent achievements, such as the generation of induced pluripotent stem cells, have delivered preliminary evidence that slowing down and reversing the ageing process might be possible. However, these techniques usually involve complete dedifferentiation, i.e. somatic cell identity is lost as cells are converted to a pluripotent state. Separating the rejuvenative properties of reprogramming from dedifferentiation is a promising prospect, termed epigenetic rejuvenation. Reprogramming-induced rejuvenation strategies currently involve using Yamanaka factors (typically transiently expressed to prevent full dedifferentiation) and are promising candidates to safely reduce biological age. Here, we review the development and potential of reprogramming-induced rejuvenation as an anti-ageing strategy.

Interphase Chromosomes in Replicative Senescence: Chromosome Positioning as a Senescence Biomarker and the Lack of Nuclear Motor-Driven Chromosome Repositioning in Senescent Cells

This study demonstrates, and confirms, that chromosome territory positioning is altered in primary senescent human dermal fibroblasts (HDFs). The chromosome territory positioning pattern is very similar to that found in HDFs made quiescent either by serum starvation or confluence; but not completely. A few chromosomes are found in different locations. One chromosome in particular stands out, chromosome 10, which is located in an intermediate location in young proliferating HDFs, but is found at the nuclear periphery in quiescent cells and in an opposing location of the nuclear interior in senescent HDFs. We have previously demonstrated that individual chromosome territories can be actively and rapidly relocated, with 15 min, after removal of serum from the culture media. These chromosome relocations require nuclear motor activity through the presence of nuclear myosin 1β (NM1β). We now also demonstrate rapid chromosome movement in HDFs after heat-shock at 42°C. Others have shown that heat shock genes are actively relocated using nuclear motor protein activity via actin or NM1β (Khanna et al., 2014; Pradhan et al., 2020). However, this current study reveals, that in senescent HDFs, chromosomes can no longer be relocated to expected nuclear locations upon these two types of stimuli. This coincides with a entirely different organisation and distribution of NM1β within senescent HDFs.

No Time to Age: Uncoupling Aging from Chronological Time

Multicellular life evolved from simple unicellular organisms that could replicate indefinitely, being essentially ageless. At this point, life split into two fundamentally different cell types: the immortal germline representing an unbroken lineage of cell division with no intrinsic endpoint and the mortal soma, which ages and dies. In this review, we describe the germline as clock-free and the soma as clock-bound and discuss aging with respect to three DNA-based cellular clocks (telomeric, DNA methylation, and transposable element). The ticking of these clocks corresponds to the stepwise progressive limitation of growth and regeneration of somatic cells that we term somatic restriction. Somatic restriction acts in opposition to strategies that ensure continued germline replication and regeneration. We thus consider the plasticity of aging as a process not fixed to the pace of chronological time but one that can speed up or slow down depending on the rate of intrinsic cellular clocks. We further describe how germline factor reprogramming might be used to slow the rate of aging and potentially reverse it by causing the clocks to tick backward. Therefore, reprogramming may eventually lead to therapeutic strategies to treat degenerative diseases by altering aging itself, the one condition common to us all.

Extranuclear Inheritance of Mitochondrial Genome and Epigenetic Reprogrammability of Chromosomal Telomeres in Somatic Cell Cloning of Mammals

The effectiveness of somatic cell nuclear transfer (SCNT) in mammals seems to be still characterized by the disappointingly low rates of cloned embryos, fetuses, and progeny generated. These rates are measured in relation to the numbers of nuclear-transferred oocytes and can vary depending on the technique applied to the reconstruction of enucleated oocytes. The SCNT efficiency is also largely affected by the capability of donor nuclei to be epigenetically reprogrammed in a cytoplasm of reconstructed oocytes. The epigenetic reprogrammability of donor nuclei in SCNT-derived embryos appears to be biased, to a great extent, by the extranuclear (cytoplasmic) inheritance of mitochondrial DNA (mtDNA) fractions originating from donor cells. A high frequency of mtDNA heteroplasmy occurrence can lead to disturbances in the intergenomic crosstalk between mitochondrial and nuclear compartments during the early embryogenesis of SCNT-derived embryos. These disturbances can give rise to incorrect and incomplete epigenetic reprogramming of donor nuclei in mammalian cloned embryos. The dwindling reprogrammability of donor nuclei in the blastomeres of SCNT-derived embryos can also be impacted by impaired epigenetic rearrangements within terminal ends of donor cell-descended chromosomes (i.e., telomeres). Therefore, dysfunctions in epigenetic reprogramming of donor nuclei can contribute to the enhanced attrition of telomeres. This accelerates the processes of epigenomic aging and replicative senescence in the cells forming various tissues and organs of cloned fetuses and progeny. For all the above-mentioned reasons, the current paper aims to overview the state of the art in not only molecular mechanisms underlying intergenomic communication between nuclear and mtDNA molecules in cloned embryos but also intrinsic determinants affecting unfaithful epigenetic reprogrammability of telomeres. The latter is related to their abrasion within somatic cell-inherited chromosomes.

Dark Matter of Primate Genomes: Satellite DNA Repeats and Their Evolutionary Dynamics

A substantial portion of the primate genome is composed of non-coding regions, so-called "dark matter", which includes an abundance of tandemly repeated sequences called satellite DNA. Collectively known as the satellitome, this genomic component offers exciting evolutionary insights into aspects of primate genome biology that raise new questions and challenge existing paradigms. A complete human reference genome was recently reported with telomere-to-telomere human X chromosome assembly that resolved hundreds of dark regions, encompassing a 3.1 Mb centromeric satellite array that had not been identified previously. With the recent exponential increase in the availability of primate genomes, and the development of modern genomic and bioinformatics tools, extensive growth in our knowledge concerning the structure, function, and evolution of satellite elements is expected. The current state of knowledge on this topic is summarized, highlighting various types of primate-specific satellite repeats to compare their proportions across diverse lineages. Inter- and intraspecific variation of satellite repeats in the primate genome are reviewed. The functional significance of these sequences is discussed by describing how the transcriptional activity of satellite repeats can affect gene expression during different cellular processes. Sex-linked satellites are outlined, together with their respective genomic organization. Mechanisms are proposed whereby satellite repeats might have emerged as novel sequences during different evolutionary phases. Finally, the main challenges that hinder the detection of satellite DNA are outlined and an overview of the latest methodologies to address technological limitations is presented.

Livestock pluripotency is finally captured in vitro

Pluripotent stem cells (PSCs) have demonstrated great utility in improving our understanding of mammalian development and continue to revolutionise regenerative medicine. Thanks to the improved understanding of pluripotency in mice and humans, it has recently become feasible to generate stable livestock PSCs. Although it is unlikely that livestock PSCs will be used for similar applications as their murine and human counterparts, new exciting applications that could greatly advance animal agriculture are being developed, including the use of PSCs for complex genome editing, cellular agriculture, gamete generation and invitro breeding schemes.

High Satellite Repeat Turnover in Great Apes Studied with Short- and Long-Read Technologies

Satellite repeats are a structural component of centromeres and telomeres, and in some instances, their divergence is known to drive speciation. Due to their highly repetitive nature, satellite sequences have been understudied and underrepresented in genome assemblies. To investigate their turnover in great apes, we studied satellite repeats of unit sizes up to 50 bp in human, chimpanzee, bonobo, gorilla, and Sumatran and Bornean orangutans, using unassembled short and long sequencing reads. The density of satellite repeats, as identified from accurate short reads (Illumina), varied greatly among great ape genomes. These were dominated by a handful of abundant repeated motifs, frequently shared among species, which formed two groups: 1) the (AATGG)n repeat (critical for heat shock response) and its derivatives; and 2) subtelomeric 32-mers involved in telomeric metabolism. Using the densities of abundant repeats, individuals could be classified into species. However, clustering did not reproduce the accepted species phylogeny, suggesting rapid repeat evolution. Several abundant repeats were enriched in males versus females; using Y chromosome assemblies or Fluorescent In Situ Hybridization, we validated their location on the Y. Finally, applying a novel computational tool, we identified many satellite repeats completely embedded within long Oxford Nanopore and Pacific Biosciences reads. Such repeats were up to 59 kb in length and consisted of perfect repeats interspersed with other similar sequences. Our results based on sequencing reads generated with three different technologies provide the first detailed characterization of great ape satellite repeats, and open new avenues for exploring their functions.

The Genetic Architecture of Bovine Telomere Length in Early Life and Association With Animal Fitness

Health and survival are key goals for selective breeding in farm animals. Progress, however, is often limited by the low heritability of these animal fitness traits in addition to measurement difficulties. In this respect, relevant early-life biomarkers may be useful for breeding purposes. Telomere length (TL), measured in leukocytes, is a good candidate biomarker since TL has been associated with health, ageing, and stress in humans and other species. However, telomere studies are very limited in farm animals. Here, we examined the genetic background, genomic architecture, and factors affecting bovine TL measurements in early life, and the association of the latter with animal fitness traits expressed later in life associated with survival, longevity, health, and reproduction. We studied two TL measurements, one at birth (TLB) and another during the first lactation (TLFL) of a cow. We performed a genome-wide association study of dairy cattle TL, the first in a non-human species, and found that TLB and TLFL are complex, polygenic, moderately heritable, and highly correlated traits. However, genomic associations with distinct chromosomal regions were identified for the two traits suggesting that their genomic architecture is not identical. This is reflected in changes in TL throughout an individual’s life. TLB had a significant association with survival, length of productive life and future health status of the animal, and could be potentially used as an early-life biomarker for disease predisposition and longevity in dairy cattle.

Toward a unified theory of aging and regeneration

Growing evidence supports the antagonistic pleiotropy theory of mammalian aging. Accordingly, changes in gene expression following the pluripotency transition, and subsequent transitions such as the embryonic–fetal transition, while providing tumor suppressive and antiviral survival benefits also result in a loss of regenerative potential leading to age-related fibrosis and degenerative diseases. However, reprogramming somatic cells to pluripotency demonstrates the possibility of restoring telomerase and embryonic regeneration pathways and thus reversing the age-related decline in regenerative capacity. A unified model of aging and loss of regenerative potential is emerging that may ultimately be translated into new therapeutic approaches for establishing induced tissue regeneration and modulation of the embryo-onco phenotype of cancer.

Factors and molecules that could impact cell differentiation in the embryo generated by nuclear transfer

Somatic cell nuclear transfer is a technique to create an embryo using an enucleated oocyte and a donor nucleus. Nucleus of somatic cells must be reprogrammed in order to participate in normal development within an enucleated egg. Reprogramming refers to the erasing and remodeling of cellular epigenetic marks to a lower differentiation state. Somatic nuclei must be reprogrammed by factors in the oocyte cytoplasm to a rather totipotent state since the reconstructed embryo must initiate embryo development from the one cell stage to term. In embryos reconstructed by nuclear transfer, the donor genetic material must respond to the cytoplasmic environment of the cytoplast and recapitulate this normal developmental process. Enucleation is critically important for cloning efficiency because may affect the ultrastructure of the remaining cytoplast, thus resulting in a decline or destruction of its cellular compartments. Nonetheless, the effects of in vitro culturing are yet to be fully understood. In vitro oocyte maturation can affect the abundance of specific transcripts and are likely to deplete the developmental competence. The epigenetic modifications established during cellular differentiation are a major factor determining this low efficiency as they act as epigenetic barriers restricting reprogramming of somatic nuclei. In this review we discuss some factors that could impact cell differentiation in embryo generated by nuclear transfer.

Cytoplasmic volume of recipient oocytes affects the nucleus reprogramming and the developmental competence of HMC buffalo (Bubalus bubalis) embryos

The present study was undertaken to examine the effects of cytoplasmic volume on nucleus reprogramming and developmental competence of buffalo handmade cloning (HMC) embryos. We found that both HMC embryos derived from ~150% cytoplasm or ~225% cytoplasm resulted in a higher blastocyst rate and total cell number of blastocyst in comparison with those from ~75% cytoplasm (25.4 ± 2.0, 27.9 ± 1.6% vs. 17.9 ± 3.1%; 150 ± 10, 169 ± 12 vs. 85 ± 6, P<0.05). Meanwhile, the proportions of nuclear envelope breakdown (NEBD) and premature chromosome condensation (PCC) were also increased in the embryos derived from ~150 or ~225% enucleated cytoplasm compared to those from ~75% cytoplasm. Moreover, HMC embryos derived from ~225% cytoplasm showed a decrease of global DNA methylation from the 2-cell to the 4-cell stage in comparison with those of ~75% cytoplasm (P<0.05). Furthermore, the expression of embryonic genome activation (EGA) relative genes (eIF1A and U2AF) in HMC embryos derived from ~225% cytoplasm at the 8-cell stages was also found to be enhanced compared with that of the ~75% cytoplasm. Two of seven recipients were confirmed to be pregnant following transfer of blastocysts derived from ~225% cytoplasm, and one healthy cloned calf was delivered at the end of the gestation period, whereas no recipients were pregnant after the transfer of blastocysts derived from ~75% cytoplasm. These results indicate that the cytoplasmic volume of recipient oocytes affects donor nucleus reprogramming, and then further accounted for the developmental ability of the reconstructed embryos.

Determination of Oocyte-Manipulation, Zygote-Manipulation, and Genome-Reprogramming Effects on the Transcriptomes of Bovine Blastocysts

Somatic cell nuclear transfer (scNT) embryos suffer from damage caused by micro-operation (manipulation) and inefficient genome reprograming that hinder their normal development at different levels and in distinct ways. These two effects are inseparable in the nature of the scNT embryo, although methods to separately measure them are needed to improve scNT technology and evaluate incoming reprogramming tools. As an attempt to meet these demands, we made bovine sham nuclear-transfer (shNT) blastocysts, special embryos made with a standard nuclear-transfer procedure at the zygote stage, while retaining an intact genome. We compared their transcriptomes with those of other blastocysts derived by in-vitro fertilization (IVF) or scNT. Correlation analysis revealed a singularity of shNT blastocysts as they separately gathered from the others. Analysis of developmentally important genes revealed that, in shNTs, the stemness-associated differentially expressed genes (DEGs), including OCT4, were mostly underrepresented. Overrepresented epi-driver genes were largely associated with heterochromatin establishment and maintenance. By multilateral comparisons of their transcriptomes, we classified DEGs into three groups: 561 manipulation-associated DEGs (MADs) common to shNTs and scNTs, 764 donor genome-associated DEGs (DADs) specific to scNTs, and 1743 zygote manipulation-associated DEGs (zMADs) specific to shNTs. GO enrichment analysis generated various terms involving “cell-cell adhesion,” “translation,” and “transcription” for MADs and “cell differentiation” and “embryo implantation” for DADs. Because of the transcriptomic specificity of shNTs, we studied zMADs in detail. GO enrichment analysis with the 854 zMADs underrepresented in shNTs yielded terms related to protein and mitochondria homeostasis, while GO enrichment analysis of 889 shNT-high zMADs yielded terms related to endoplasmic reticulum stress and protein transport. We summarized the DEGs, which, with further investigation, may help improve our understanding of molecular events occurring in cloned embryos and our ability to control clonal reprogramming.

Preventing aging with stem cell rejuvenation: Feasible or infeasible?

Characterized by dysfunction of tissues, organs, organ systems and the whole organism, aging results from the reduced function of effective stem cell populations. Recent advances in aging research have demonstrated that old tissue stem cells can be rejuvenated for the purpose of maintaining the old-organ function by youthful re-calibration of the environment where stem cells reside. Biochemical cues regulating tissue stem cell function include molecular signaling pathways that interact between stem cells themselves and their niches. Historically, plasma fractions have been shown to contain factors capable of controlling age phenotypes; subsequently, signaling pathways involved in the aging process have been identified. Consequently, modulation of signaling pathways such as Notch/Delta, Wnt, transforming growth factor-β, JAK/STAT, mammalian target of rapamycin and p38 mitogen-activated protein kinase has demonstrated potential to rejuvenate stem cell function leading to organismic rejuvenation. Several synthetic agents and natural sources, such as phytochemicals and flavonoids, have been proposed to rejuvenate old stem cells by targeting these pathways. However, several concerns still remain to achieve effective organismic rejuvenation in clinical settings, such as possible carcinogenic actions; thus, further research is still required.

TERT promoter mutations and long telomere length predict poor survival and radiotherapy resistance in gliomas

Increasing evidences have implicated somatic gain-of-function mutations at the telomerase reverse transcriptase (TERT) promoter as one of the major mechanisms that promote transcriptional activation of TERT and subsequently maintain telomere length in human cancers including glioma. To investigate the prognostic value of these mutations and telomere length, individually and their coexistence, in gliomas, we analyzed two somatic mutations C228T and C250T in the TERT promoter, relative telomere length (RTL), IDH1 mutation and MGMT methylation in 389 glioma patients, and explored their associations with patient characteristics and clinical outcomes. Our data showed that C228T and C250T mutations were found in 17.0% (66 of 389) and 11.8% (46 of 389) of gliomas, respectively, and these two mutations were mutually exclusive in this cancer. Moreover, they were significantly associated with WHO grade. We also found that the RTL was significant longer in gliomas than in meningiomas and normal brain tissues (Median, 0.89 vs. 0.44 and 0.50; P < 0.001), and demonstrated that the RTL was strongly correlated with tumor recurrence. Importantly, TERT promoter mutations or long RTL caused a significantly poorer survival than TERT wild-type or short RTL. Coexisting TERT promoter mutations and long RTL were more commonly associated with poor patient survival than they were individually. Notably, the patients with TERT promoter mutations particularly C228T or long RTL were resistant to radiotherapy. Collectively, TERT promoter mutations and long RTL are not only prognostic factors for poor clinical outcomes, but also the predictors of radiotherapy resistance in gliomas.

Phenotype of Cloned Mice: Development, Behavior, and Physiology

Cloning technology has potential to be a valuable tool in basic research, clinical medicine, and agriculture. However, it is critical to determine the consequences of this technique in resulting offspring before widespread use of the technology. Mammalian cloning using somatic cells was first demonstrated in sheep in 1997 and since then has been extended to a number of other species. We examined development, behavior, physiology, and longevity in B6C3F1 female mice cloned from adult cumulus cells. Control mice were naturally fertilized embryos subjected to the same in vitro manipulation and culture conditions as clone embryos. Clones attained developmental milestones similar to controls. Activity level, motor ability and coordination, and learning and memory skills of cloned mice were comparable with controls. Interestingly, clones gained more body weight than controls during adulthood. Increased body weight was attributable to higher body fat and was associated with hyperleptinemia and hyperinsulinemia indicating that cloned mice are obese. Cloned mice were not hyperphagic as adults and had hypersensitive leptin and melanocortin signaling systems. Longevity of cloned mice was comparable with that reported by the National Institute on Aging and the causes of death were typical for this strain of mouse. These studies represent the first comprehensive set of data to characterize cloned mice and provide critical information about the long-term effects of somatic cell cloning.

Effects of donor cells' sex on nuclear transfer efficiency and telomere lengths of cloned goats

The aim of this study was to investigate the effects of donor cells' sex on nuclear transfer efficiency and telomere length of cloned goats from adult skin fibroblast cells. The telomere length of somatic cell cloned goats and their offspring was determined by measuring their mean terminal restriction fragment (TRF) length. The result showed that (i) reconstructed embryos with fibroblast cells from males Boer goats obtained significantly higher kids rate and rate of live kids than those of female embryos and (ii) the telomere lengths of four female cloned goats were shorter compared to their donor cells, but five male cloned goats had the same telomere length with their donor cells, mainly due to great variation existed among them. The offspring from female cloned goats had the same telomere length with their age-matched counterparts. In conclusion, the donor cells' sex had significant effects on nuclear transfer efficiency and telomere lengths of cloned goats.

Healthy ageing of cloned sheep

The health of cloned animals generated by somatic-cell nuclear transfer (SCNT) has been of concern since its inception; however, there are no detailed assessments of late-onset, non-communicable diseases. Here we report that SCNT has no obvious detrimental long-term health effects in a cohort of 13 cloned sheep. We perform musculoskeletal assessments, metabolic tests and blood pressure measurements in 13 aged (7–9 years old) cloned sheep, including four derived from the cell line that gave rise to Dolly. We also perform radiological examinations of all main joints, including the knees, the joint most affected by osteoarthritis in Dolly, and compare all health parameters to groups of 5-and 6-year-old sheep, and published reference ranges. Despite their advanced age, these clones are euglycaemic, insulin sensitive and normotensive. Importantly, we observe no clinical signs of degenerative joint disease apart from mild, or in one case moderate, osteoarthritis in some animals. Our study is the first to assess the long-term health outcomes of SCNT in large animals.

Since the birth of the first cloned animal, Dolly the sheep, concerns have been raised about potential long-term health consequences of cloning. Here the authors report on a cohort of 13 aged cloned sheep, including four created from the same cells as Dolly, and find they are healthy and seem to age normally.

The HIST1 Locus Escapes Reprogramming in Cloned Bovine Embryos

Epigenetic reprogramming is necessary in somatic cell nuclear transfer (SCNT) embryos in order to erase the differentiation-associated epigenetic marks of donor cells. However, such epigenetic memories often persist throughout the course of clonal development, thus decreasing cloning efficiency. Here, we explored reprogramming-refractory regions in bovine SCNT blastocyst transcriptomes. We observed that histone genes residing in the 1.5 Mb spanning the cow HIST1 cluster were coordinately downregulated in SCNT blastocysts. In contrast, both the nonhistone genes of this cluster, and histone genes elsewhere remained unaffected. This indicated that the downregulation was specific to HIST1 histone genes. We found that, after trichostatin A treatment, HIST1 histone genes were derepressed, and DNA methylation at their promoters was decreased to the level of in vitro fertilization embryos. Therefore, our results indicate that the reduced expression of HIST1 histone genes is a consequence of poor epigenetic reprogramming in SCNT blastocysts.

Reflections on the role of senescence during development and aging

New and stimulating results have challenged the concept that cellular senescence might not be synonymous with aging. It is indisputable that during aging, senescent cell accumulation has an impact on organismal health. Nevertheless, senescent cells are now known to display physiological roles during embryonic development, during wound healing repair and as a cellular response to stress. The fact that senescence has been found in cells that did not attain their maximal round of replications, nor have metabolic alterations or DNA damage, also challenges the paradigm that senescence is cellular aging, and it is in favor of the idea that cellular senescence is a phenomenon that has a function by itself. Therefore, in order to understand this phenomenon it is important to analyze the relationship between senescence and other cellular responses that have many features in common, such as apoptosis, cancer and autophagy, particularly highlighting their role during development and adulthood.

The germline/soma dichotomy: implications for aging and degenerative disease

Human somatic cells are mortal due in large part to telomere shortening associated with cell division. Limited proliferative capacity may, in turn, limit response to injury and may play an important role in the etiology of age-related pathology. Pluripotent stem cells cultured in vitro appear to maintain long telomere length through relatively high levels of telomerase activity. We propose that the induced reversal of cell aging by transcriptional reprogramming, or alternatively, human embryonic stem cells engineered to escape immune surveillance, are effective platforms for the industrial-scale manufacture of young cells for the treatment of age-related pathologies. Such cell-based regenerative therapies will require newer manufacturing and delivery technologies to insure highly pure, identified and potent pluripotency-based therapeutic formulations.

Programming and Reprogramming Cellular Age in the Era of Induced Pluripotency

The ability to reprogram adult somatic cells back to pluripotency presents a powerful tool for studying cell-fate identity and modeling human disease. However, the reversal of cellular age during reprogramming results in an embryonic-like state of induced pluripotent stem cells (iPSCs) and their derivatives, which presents specific challenges for modeling late onset disease. This age reset requires novel methods to mimic age-related changes but also offers opportunities for studying cellular rejuvenation in real time. Here, we discuss how iPSC research may transform studies of aging and enable the precise programming of cellular age in parallel to cell-fate specification.

Cumulative stress in research animals: Telomere attrition as a biomarker in a welfare context?

Progress in improving animal welfare is currently limited by the lack of objective methods for assessing lifetime experience. I propose that telomere attrition, a cellular biomarker of biological age, provides a molecular measure of cumulative experience that could be used to assess the welfare impact of husbandry regimes and/or experimental procedures on non-human animals. I review evidence from humans that telomere attrition is accelerated by negative experiences in a cumulative and dose-dependent manner, but that this attrition can be mitigated or even reversed by positive life-style interventions. Evidence from non-human animals suggests that despite some specific differences in telomere biology, stress-induced telomere attrition is a robust phenomenon, occurring in a range of species including mice and chickens. I conclude that telomere attrition apparently integrates positive and negative experience in an accessible common currency that translates readily to novel species--the Holy Grail of a cumulative welfare indicator.

Transcriptomic Features of Bovine Blastocysts Derived by Somatic Cell Nuclear Transfer

Reprogramming incompletely occurs in most somatic cell nuclear transfer (SCNT) embryos, which results in misregulation of developmentally important genes and subsequent embryonic malfunction and lethality. Here we examined transcriptome profiles in single bovine blastocysts derived by in vitro fertilization (IVF) and SCNT. Different types of donor cells, cumulus cell and ear-skin fibroblast, were used to derive cSCNT and fSCNT blastocysts, respectively. SCNT blastocysts expressed 13,606 genes on average, similar to IVF (13,542). Correlation analysis found that both cSCNT and fSCNT blastocyst groups had transcriptomic features distinctive from the IVF group, with the cSCNT transcriptomes closer to the IVF ones than the fSCNT. Gene expression analysis identified 56 underrepresented and 78 overrepresented differentially expressed genes in both SCNT groups. A 400-kb locus harboring zinc-finger protein family genes in chromosome 18 were found coordinately down-regulated in fSCNT blastocysts, showing a feature of reprogramming-resistant regions. Probing into different categories of genes important for blastocyst development revealed that genes involved in trophectoderm development frequently were underrepresented, and those encoding epigenetic modifiers tended to be overrepresented in SCNT blastocysts. Our effort to identify reprogramming-resistant, differentially expressed genes can help map reprogramming error-prone loci onto the genome and elucidate how to handle the stochastic events of reprogramming to improve cloning efficiency.

Factors affecting the development of somatic cell nuclear transfer embryos in Cattle

Nuclear transfer is a complex multistep procedure that includes oocyte maturation, cell cycle synchronization of donor cells, enucleation, cell fusion, oocyte activation and embryo culture. Therefore, many factors are believed to contribute to the success of embryo development following nuclear transfer. Numerous attempts to improve cloning efficiency have been conducted since the birth of the first sheep by somatic cell nuclear transfer. However, the efficiency of somatic cell cloning has remained low, and applications have been limited. In this review, we discuss some of the factors that affect the developmental ability of somatic cell nuclear transfer embryos in cattle.

Effects of Scriptaid on the Histone Acetylation, DNA Methylation and Development of Buffalo Somatic Cell Nuclear Transfer Embryos

The present study was undertaken to examine the effect of Scriptaid treatment on histone acetylation, DNA methylation, expression of genes related to histone acetylation, and development of buffalo somatic cell nuclear transfer (SCNT) embryos. Treatment of buffalo SCNT embryos with 500 nM Scriptaid for 24 h resulted in a significant increase in the blastocyst formation rate (28.2% vs. 13.6%, p<0.05). Meanwhile, treatment of buffalo SCNT embryos with Scriptaid also resulted in higher acetylation levels of H3K18 and lower methylation levels of global DNA at the blastocyst stage, which was similar to fertilized counterparts. The expression levels of CBP, p300, HAT1, Dnmt1, and Dnmt3a in SCNT embryos treated with Scriptaid were significantly lower than the control group at the eight-cell stage (p<0.05), but the expression of HAT1 and Dnmt1a was higher than the control group at the blastocyst stage (p<0.05). When 96 blastocysts developed from Scriptaid-treated SCNT embryos were transferred into 48 recipients, 11 recipients (22.9%) became pregnant, whereas only one recipient (11.1%) became pregnant following transfer of 18 blastocysts developed from untreated SCNT embryos into nine recipients. These results indicate that treatment of buffalo SCNT embryos with Scriptaid can improve their developmental competence, and this action is mediated by resulting in a similar histone acetylation level and global DNA methylation level compared to in vitro-fertilized embryos through regulating the expression pattern of genes related to histone acetylation and DNA methylation.

Assessment of difference in gene expression profile between embryos of different derivations

Researchers have exerted sustained efforts to improve the viability of somatic cell nuclear transfer (SCNT) embryos, testing their experimental designs and probing the resultant embryos. However, the lack of a reliable method to estimate the efficacy of these experimental attempts is a chief hindrance to tackling the low-viability problem in SCNT. Here, we introduce a procedure that assesses the degree of difference in gene expression profiles (GEPs) of blastocysts from each other as a representative control of good quality. We first adapted a multiplex reverse transcription-polymerase chain reaction strategy to obtain GEPs for 15 reprogramming-related genes from single mouse blastocysts. GEPs of individual blastocysts displayed a broad range of variations, the extent of which was calculated using a weighted root mean square deviation (wRMSD). wRMSD-based quantitation of GEP difference (qGEP) found that GEP difference between in vivo-derived blastocysts (in vivo) and SCNT blastocysts was greater than the difference between in vivo blastocysts and in vitro-produced (IVP) blastocysts, demonstrating that the SCNT group was more distantly related to the in vivo group than the IVP group. Our qGEP approach for grading individual blastocysts would be useful for selecting a better protocol to derive embryos of better quality prior to field applications.

Telomere elongation facilitated by trichostatin a in cloned embryos and pigs by somatic cell nuclear transfer

Telomere attrition and genomic instability are associated with organism aging. Concerns still exist regarding telomere length resetting in cloned embryos and ntES cells, and possibilities of premature aging of cloned animals achieved by somatic cell nuclear transfer (SCNT). Trichostatin A (TSA), a histone deacetylase inhibitor, effectively improves the developmental competence of cloned embryos and animals, and recently contributes to successful generation of human ntES cells by SCNT. To test the function of TSA on resetting telomere length, we analyzed telomeres in cloned blastocysts and pigs following treatment of SCNT embryos with TSA. Here, we show that telomeres of cloned pigs generated by standard SCNT methods are not effectively restored, compared with those of donor cells, however TSA significantly increases telomere lengths in cloned pigs. Telomeres elongate in cloned porcine embryos during early cleavage from one-cell to four-cell stages. Notably, TSA facilitates telomere lengthening of cloned embryos mainly at morula-blastocyst stages. Knockdown of pTert by shRNA in donor cells reduces telomerase activity in cloned blastocysts but does not abrogate telomere elongation in the TSA-treated embryos (p > 0.05). However, genes associated with recombination or telomerase-independent mechanism of alternative lengthening of telomeres (ALT) Rad50 and BLM show increased expression in TSA-treated embryos. These data suggest that TSA may promote telomere elongation of cloned porcine embryos by ALT. Together, TSA can elongate telomeres in cloned embryos and piglets, and this could be one of the mechanisms underlying improved development of cloned embryos and animals treated with TSA.

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.

Stem cell aging: mechanisms, regulators and therapeutic opportunities

Aging tissues experience a progressive decline in homeostatic and regenerative capacities, which has been attributed to degenerative changes in tissue-specific stem cells, stem cell niches and systemic cues that regulate stem cell activity. Understanding the molecular pathways involved in this age-dependent deterioration of stem cell function will be critical for developing new therapies for diseases of aging that target the specific causes of age-related functional decline. Here we explore key molecular pathways that are commonly perturbed as tissues and stem cells age and degenerate. We further consider experimental evidence both supporting and refuting the notion that modulation of these pathways per se can reverse aging phenotypes. Finally, we ask whether stem cell aging establishes an epigenetic 'memory' that is indelibly written or one that can be reset.

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.

Studies that shed new light on aging

I will first discuss how all aging models that assume that the aged cell has irreversibly lost its youthful capabilities through such mechanisms as accumulated dysfunction, accumulated damage, and/or accumulation of toxic byproducts of metabolism have been shown to be incorrect. I will then briefly discuss models of aging and propose an experiment that would distinguish between those models and provide a basis for organismic rejuvenation.

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.

Detecting senescence: methods and approaches

The detection of senescent cells has become an important area of research in the aging field. Due to the complexity of the senescence program and the lack of a unique signature for senescence, the detection of these cells remains problematic. This is especially true for in vivo detection in aged or diseased tissue samples. This chapter outlines approaches for the detection of senescent cells based upon methods established for mesenchymal cells in culture. A stepwise approach to the detection of senescent cells using multiple techniques is provided.

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.