Telomere loss: mitotic clock or genetic time bomb?

Adaptive and Maladaptive Clonal Hematopoiesis in Telomere Biology Disorders

PURPOSE OF REVIEW

Telomere biology disorders (TBDs) are germline-inherited conditions characterized by reduction in telomerase function, accelerated shortening of telomeres, predisposition to organ-failure syndromes, and increased risk of neoplasms, especially myeloid malignancies. In normal cells, critically short telomeres trigger apoptosis and/or cellular senescence. However, the evolutionary mechanism by which TBD-related telomerase-deficient cells can overcome this fitness constraint remains elusive.

RECENT FINDINGS

Preliminary data suggests the existence of adaptive somatic mosaic states characterized by variants in TBD-related genes and maladaptive somatic mosaic states that attempt to overcome hematopoietic fitness constraints by alternative methods leading to clonal hematopoiesis. TBDs are both rare and highly heterogeneous in presentation, and the association of TBD with malignant transformation is unclear. Understanding the clonal complexity and mechanisms behind TBD-associated molecular signatures that lead to somatic adaptation in the setting of defective hematopoiesis will help inform therapy and treatment for this set of diseases.

2.7 Å cryo-EM structure of human telomerase H/ACA ribonucleoprotein

Telomerase is a ribonucleoprotein (RNP) enzyme that extends telomeric repeats at eukaryotic chromosome ends to counterbalance telomere loss caused by incomplete genome replication. Human telomerase is comprised of two distinct functional lobes tethered by telomerase RNA (hTR): a catalytic core, responsible for DNA extension; and a Hinge and ACA (H/ACA) box RNP, responsible for telomerase biogenesis. H/ACA RNPs also have a general role in pseudouridylation of spliceosomal and ribosomal RNAs, which is critical for the biogenesis of the spliceosome and ribosome. Much of our structural understanding of eukaryotic H/ACA RNPs comes from structures of the human telomerase H/ACA RNP. Here we report a 2.7 Å cryo-electron microscopy structure of the telomerase H/ACA RNP. The significant improvement in resolution over previous 3.3 Å to 8.2 Å structures allows us to uncover new molecular interactions within the H/ACA RNP. Many disease mutations are mapped to these interaction sites. The structure also reveals unprecedented insights into a region critical for pseudouridylation in canonical H/ACA RNPs. Together, our work advances understanding of telomerase-related disease mutations and the mechanism of pseudouridylation by eukaryotic H/ACA RNPs.

Telomere C-Strand Fill-In Machinery: New Insights into the Human CST-DNA Polymerase Alpha-Primase Structures and Functions

Telomeres at the end of eukaryotic chromosomes are extended by a specialized set of enzymes and telomere-associated proteins, collectively termed here the telomere "replisome." The telomere replisome acts on a unique replicon at each chromosomal end of the telomeres, the 3' DNA overhang. This telomere replication process is distinct from the replisome mechanism deployed to duplicate the human genome. The G-rich overhang is first extended before the complementary C-strand is filled in. This overhang is extended by telomerase, a specialized ribonucleoprotein and reverse transcriptase. The overhang extension process is terminated when telomerase is displaced by CTC1-STN1-TEN1 (CST), a single-stranded DNA-binding protein complex. CST then recruits DNA polymerase α-primase to complete the telomere replication process by filling in the complementary C-strand. In this chapter, the recent structure-function insights into the human telomere C-strand fill-in machinery (DNA polymerase α-primase and CST) will be discussed.

Biomimetic Cardiac Tissue Models for In Vitro Arrhythmia Studies

Cardiac arrhythmias are a major cause of cardiovascular mortality worldwide. Many arrhythmias are caused by reentry, a phenomenon where excitation waves circulate in the heart. Optical mapping techniques have revealed the role of reentry in arrhythmia initiation and fibrillation transition, but the underlying biophysical mechanisms are still difficult to investigate in intact hearts. Tissue engineering models of cardiac tissue can mimic the structure and function of native cardiac tissue and enable interactive observation of reentry formation and wave propagation. This review will present various approaches to constructing cardiac tissue models for reentry studies, using the authors' work as examples. The review will highlight the evolution of tissue engineering designs based on different substrates, cell types, and structural parameters. A new approach using polymer materials and cellular reprogramming to create biomimetic cardiac tissues will be introduced. The review will also show how computational modeling of cardiac tissue can complement experimental data and how such models can be applied in the biomimetics of cardiac tissue.

The impact of the cardiovascular component and somatic mutations on ageing

Mechanistic insight into ageing may empower prolonging the lifespan of humans; however, a complete understanding of this process is still lacking despite a plethora of ageing theories. In order to address this, we investigated the association of lifespan with eight phenotypic traits, that is, litter size, body mass, female and male sexual maturity, somatic mutation, heart, respiratory, and metabolic rate. In support of the somatic mutation theory, we analysed 15 mammalian species and their whole-genome sequencing deriving somatic mutation rate, which displayed the strongest negative correlation with lifespan. All remaining phenotypic traits showed almost equivalent strong associations across this mammalian cohort, however, resting heart rate explained additional variance in lifespan. Integrating somatic mutation and resting heart rate boosted the prediction of lifespan, thus highlighting that resting heart rate may either directly influence lifespan, or represents an epiphenomenon for additional lower-level mechanisms, for example, metabolic rate, that are associated with lifespan.

Low Antioxidant Glutathione Levels Lead to Longer Telomeres: A Sex-Specific Link to Longevity?

Telomeres are repetitive DNA sequences at the end of chromosomes that protect them from degradation. They have been the focus of intense research because short telomeres would predict accelerated ageing and reduced longevity in vertebrates. Oxidative stress is considered a physiological driver of the telomere shortening and, consequently, short lifespan. Among molecules fighting against oxidative stress, glutathione is involved in many antioxidant pathways. Literature supports that oxidative stress may trigger a compensatory "hormetic" response increasing glutathione levels and telomere length. Here, we tested the link between total glutathione concentration and telomere length in captive birds (zebra finches; Taeniopygia guttata). Total glutathione levels were experimentally decreased during birds' growth using a specific inhibitor of glutathione synthesis (buthionine sulfoximine; BSO). We monitored the birds' reproductive performance in an outdoor aviary during the first month of life, and their longevity for almost 9 years. Among control individuals, erythrocyte glutathione levels during development positively predicted erythrocyte telomere length in adulthood. However, BSO-treated females, but not males, showed longer telomeres than control females in adulthood. This counterintuitive finding suggests that females mounted a compensatory response. Such compensation agrees with precedent findings in the same population where the BSO treatment increased growth and adult body mass in females but not males. BSO did not influence longevity or reproductive output in any sex. However, early glutathione levels and adult telomere length interactively predicted longevity only among control females. Those females with "naturally" low (non-manipulated) glutathione levels at the nestling age but capable of producing longer telomeres in adulthood seem to live longer. The results suggest that the capability to mount a hormetic response triggered by low early glutathione levels can improve fitness via telomere length. Overall, the results may indicate a sex-specific link between glutathione and telomere values. Telomerase activity and sexual steroids (estrogens) are good candidates to explain the sex-biased mechanism underlying the early-life impact of oxidative stress on adult telomere length.

The variable associations between PFASs and biological aging by sex and reproductive stage in NHANES 1999-2018

BACKGROUND

Per- and polyfluoroalkyl substances (PFASs) are endocrine disrupting chemicals that have myriad effects on human physiology. Estrogenic PFASs may influence biological aging by mimicking the activity of endogenous estrogens, which can decrease inflammation and oxidative stress and enhance telomerase activity. We hypothesized that PFAS exposure would be differentially associated with measures of biological aging based on biological sex and reproductive stage.

METHODS

We analyzed associations between serum PFAS levels and measures of biological aging for pre- and postmenopausal women and men (n = 3193) using data from the 2003 to 2018 waves of the National Health and Nutrition Examination Survey. Examining PFASs both individually and in mixture models, we investigated four measures of clinical aging (Homeostatic Dysregulation, the Klemera-Doubal Method, Phenotypic Age Acceleration, and Allostatic Load), oxidative stress, and telomere length.

RESULTS

PFOA and PFOS were negatively associated with Phenotypic Age Acceleration (e.g. decelerated aging) for men B = -0.22, 95% CI: -0.32, -0.12; B = -0.04, 95% CI: -0.06, -0.03) , premenopausal women (B = -0.58, 95% CI: -0.83, -0.32; B = -0.15, 95% CI: -0.20, -0.09), and postmenopausal women (B= -0.22, 95% CI: -0.43, -0.01; B = -0.05, 95% CI: -0.08, -0.02). In mixture models, we found net negative effects for Phenotypic Age Acceleration and Allostatic Load for men, premenopausal women, and postmenopausal women. We also found significant mixture effects for the antioxidants bilirubin and albumin among the three sample groups. We found no evidence to support effects on telomere length.

DISCUSSION

Our findings suggest that PFAS exposure may be inversely associated with some measures of biological aging at the relatively low levels of exposure in this sample, regardless of reproductive stage and sex, which does not support our hypothesis. This research provides insights into how PFAS exposure may variably influence aging measures depending on the physiological process investigated.

Telomere Length Changes in Cancer: Insights on Carcinogenesis and Potential for Non-Invasive Diagnostic Strategies

Telomere dynamics play a crucial role in the maintenance of chromosome integrity; changes in telomere length may thus contribute to the development of various diseases including cancer. Understanding the role of telomeric DNA in carcinogenesis and detecting the presence of cell-free telomeric DNA (cf-telDNA) in body fluids offer a potential biomarker for novel cancer screening and diagnostic strategies. Liquid biopsy is becoming increasingly popular due to its undeniable benefits over conventional invasive methods. However, the organization and function of cf-telDNA in the extracellular milieu are understudied. This paper provides a review based on 3,398,017 cancer patients, patients with other conditions, and control individuals with the aim to shed more light on the inconsistent nature of telomere lengthening/shortening in oncological contexts. To gain a better understanding of biological factors (e.g., telomerase activation, alternative lengthening of telomeres) affecting telomere homeostasis across different types of cancer, we summarize mechanisms responsible for telomere length maintenance. In conclusion, we compare tissue- and liquid biopsy-based approaches in cancer assessment and provide a brief outlook on the methodology used for telomere length evaluation, highlighting the advances of state-of-the-art approaches in the field.

Ligand-Based Design on the Dog-Bone-Shaped BIBR1532 Pharmacophoric Features and Synthesis of Novel Analogues as Promising Telomerase Inhibitors with In Vitro and In Vivo Evaluations

Telomerase is an outstanding biological target for cancer treatment. BIBR1532 is a non-nucleoside selective telomerase inhibitor; however, it experiences ineligible pharmacokinetics. Herein, we aimed to design new BIBR1532-based analogues as promising telomerase inhibitors. Therefore, two novel series of pyridazine-linked to cyclopenta[b]thiophene (8a-f) and tetrahydro-1-benzothiophene (9a-f) were synthesized. A quantitative real-time polymerase chain reaction was utilized to investigate the telomerase inhibitory activity of candidates. Notably, 8e and 9e exhibited the best inhibition profiles. Moreover, 8e showed strong antitumor effects against both MCF-7 and A549 cancer cell lines. The effects of 8e on the cell cycle and apoptosis were measured. Besides, 8e was evaluated for its in vivo antitumor activity using solid Ehrlich carcinoma. The reduction in both the tumor weight and volume was greater than doxorubicin. Also, molecular docking and ADME studies were performed. Finally, a SAR study was conducted to gain further insights into the different telomerase inhibition potentials upon variable structural modifications.

Targeting the "hallmarks of aging" to slow aging and treat age-related disease: fact or fiction?

Aging is a major risk factor for a number of chronic diseases, including neurodegenerative and cerebrovascular disorders. Aging processes have therefore been discussed as potential targets for the development of novel and broadly effective preventatives or therapeutics for age-related diseases, including those affecting the brain. Mechanisms thought to contribute to aging have been summarized under the term the "hallmarks of aging" and include a loss of proteostasis, mitochondrial dysfunction, altered nutrient sensing, telomere attrition, genomic instability, cellular senescence, stem cell exhaustion, epigenetic alterations and altered intercellular communication. We here examine key claims about the "hallmarks of aging". Our analysis reveals important weaknesses that preclude strong and definitive conclusions concerning a possible role of these processes in shaping organismal aging rate. Significant ambiguity arises from the overreliance on lifespan as a proxy marker for aging, the use of models with unclear relevance for organismal aging, and the use of study designs that do not allow to properly estimate intervention effects on aging rate. We also discuss future research directions that should be taken to clarify if and to what extent putative aging regulators do in fact interact with aging. These include multidimensional analytical frameworks as well as designs that facilitate the proper assessment of intervention effects on aging rate.

Telomere length is heritable and genetically correlated with lifespan in a wild bird

Telomeres are protective caps at the end of eukaryotic chromosomes that shorten with age and in response to stressful or resource-demanding conditions. Their length predicts individual health and lifespan across a wide range of animals, but whether the observed positive association between telomere length and lifespan is environmentally induced, or set at conception due to a shared genetic basis, has not been tested in wild animals. We applied quantitative genetic "animal models" to longitudinal telomere measurements collected over a 10-year period from individuals of a wild seabird (common tern; Sterna hirundo) with known pedigree. We found no variation in telomere shortening with age among individuals at the phenotypic and genetic level, and only a small permanent environmental effect on adult telomere length. Instead, we found telomere length to be highly heritable and strongly positively genetically correlated with lifespan. Such heritable differences between individuals that are set at conception may present a hitherto underappreciated component of variation in somatic state.

The Therapeutic Potential of the Endocannabinoid System in Age-Related Diseases

The endocannabinoid system (ECS) dynamically regulates many aspects of mammalian physiology. ECS has gained substantial interest since growing evidence suggests that it also plays a major role in several pathophysiological conditions due to its ability to modulate various underlying mechanisms. Furthermore, cannabinoids, as components of the cannabinoid system (CS), have proven beneficial effects such as anti-inflammatory, immunomodulatory, neuromodulatory, antioxidative, and cardioprotective effects. In this comprehensive review, we aimed to describe the complex interaction between CS and most common age-related diseases such as neuro-degenerative, oncological, skeletal, and cardiovascular disorders, together with the potential of various cannabinoids to ameliorate the progression of these disorders. Since chronic inflammation is postulated as the pillar of all the above-mentioned medical conditions, we also discuss in this paper the potential of CS to ameliorate aging-associated immune system dysregulation.

Effect of Omega-3 Fatty Acids on Telomeres-Are They the Elixir of Youth?

Telomeres are complexes consisting of tandem repeat DNA combined with associated proteins that play a key role in protecting the ends of chromosomes and maintaining genome stability. They are considered a biological clock, as they shorten in parallel with aging. Furthermore, short telomeres are associated with several age-related diseases. However, the variability in telomere shortening independent of chronological age suggests that it is a modifiable factor. In fact, it is regulated inter alia by genetic damage, cell division, aging, oxidative stress, and inflammation. A key question remains: how can we prevent accelerated telomere attrition and subsequent premature replicative senescence? A number of studies have explored the possible impact of omega-3 fatty acids on telomere shortening. This review summarizes published cross-sectional studies, randomized controlled trials, and rodent studies investigating the role of omega-3 fatty acids in telomere biology. It also covers a broad overview of the mechanism, currently favored in the field, that explains the impact of omega-3 fatty acids on telomeres—the food compound’s ability to modulate oxidative stress and inflammation. Although the results of the studies performed to date are not consistent, the vast majority indicate a beneficial effect of omega-3 fatty acids on telomere length.

New Genetic Bomb Trigger: Design, Synthesis, Molecular Dynamics Simulation, and Biological Evaluation of Novel BIBR1532-Related Analogs Targeting Telomerase against Non-Small Cell Lung Cancer

Telomeres serve a critical function in cell replication and proliferation at every stage of the cell cycle. Telomerase is a ribonucleoprotein, responsible for maintaining the telomere length and chromosomal integrity of frequently dividing cells. Although it is silenced in most human somatic cells, telomere restoration occurs in cancer cells because of telomerase activation or alternative telomere lengthening. The telomerase enzyme is a universal anticancer target that is expressed in 85–95% of cancers. BIBR1532 is a selective non-nucleoside potent telomerase inhibitor that acts by direct noncompetitive inhibition. Relying on its structural features, three different series were designed, and 30 novel compounds were synthesized and biologically evaluated as telomerase inhibitors using a telomeric repeat amplification protocol (TRAP) assay. Target compounds 29a, 36b, and 39b reported the greatest inhibitory effect on telomerase enzyme with IC₅₀ values of 1.7, 0.3, and 2.0 μM, respectively, while BIBR1532 displayed IC₅₀ = 0.2 μM. Compounds 29a, 36b, and 39b were subsequently tested using a living-cell TRAP assay and were able to penetrate the cell membrane and inhibit telomerase inside living cancer cells. Compound 36b was tested for cytotoxicity against 60 cancer cell lines using the NCI (USA) procedure, and the % growth was minimally impacted, indicating telomerase enzyme selectivity. To investigate the interaction of compound 36b with the telomerase allosteric binding site, molecular docking and molecular dynamics simulations were used.

Vegetables and Their Bioactive Compounds as Anti-Aging Drugs

Aging is a continuous process over time that is mainly related to natural alterations in mechanical–biological processes. This phenomenon is due to several factors, including the time and energy of biological processes. Aging can be attributed to biological factors such as oxidative stress, cell longevity, and stem cell senescence. Currently, aging is associated with several diseases, such as neurodegenerative diseases, cancer, and other diseases related to oxidative stress. In addition, certain natural molecules, including those derived from vegetables, have shown the ability to delay the aging process. Their effects are linked to different mechanisms of action, such as tissue regeneration and the activation of longevity and anti-senescence genes. The present work discusses the impact of vegetables, and bioactive compounds isolated from vegetables, against the physiological and pathological aging process and accompanying human diseases.

Exosomes and Micro-RNAs in Aging Process

Exosomes are the main actors of intercellular communications and have gained great interest in the new cell-free regenerative medicine. These nanoparticles are secreted by almost all cell types and contain lipids, cytokines, growth factors, messenger RNA, and different non-coding RNA, especially micro-RNAs (mi-RNAs). Exosomes' cargo is released in the neighboring microenvironment but is also expected to act on distant tissues or organs. Different biological processes such as cell development, growth and repair, senescence, migration, immunomodulation, and aging, among others, are mediated by exosomes and principally exosome-derived mi-RNAs. Moreover, their therapeutic potential has been proved and reinforced by their use as biomarkers for disease diagnostics and progression. Evidence has increasingly shown that exosome-derived mi-RNAs are key regulators of age-related diseases, and their involvement in longevity is becoming a promising issue. For instance, mi-RNAs such as mi-RNA-21, mi-RNA-29, and mi-RNA-34 modulate tissue functionality and regeneration by targeting different tissues and involving different pathways but might also interfere with long life expectancy. Human mi-RNAs profiling is effectively related to the biological fluids that are reported differently between young and old individuals. However, their underlying mechanisms modulating cell senescence and aging are still not fully understood, and little was reported on the involvement of mi-RNAs in cell or tissue longevity. In this review, we summarize exosome biogenesis and mi-RNA synthesis and loading mechanism into exosomes' cargo. Additionally, we highlight the molecular mechanisms of exosomes and exosome-derived mi-RNA regulation in the different aging processes.

TRIM28 inhibits alternative lengthening of telomere phenotypes by protecting SETDB1 from degradation

Background

About 10–15% of tumor cells extend telomeres through the alternative lengthening of telomeres (ALT) mechanism, which is a recombination-dependent replication pathway. It is generally believed that ALT cells are related to the chromatin modification of telomeres. However, the mechanism of ALT needs to be further explored.

Results

Here we found that TRIM28/KAP1 is preferentially located on the telomeres of ALT cells and interacts with telomeric shelterin/telosome complex. Knocking down TRIM28 in ALT cells delayed cell growth, decreased the level of C-circle which is one kind of extrachromosomal circular telomeric DNA, increased the frequency of ALT-associated promyelocytic leukemia bodies (APBs), led to telomere prolongation and increased the telomere sister chromatid exchange in ALT cells. Mechanistically, TRIM28 protects telomere histone methyltransferase SETDB1 from degradation, thus maintaining the H3K9me3 heterochromatin state of telomere DNA.

Conclusions

Our work provides a model that TRIM28 inhibits alternative lengthening of telomere phenotypes by protecting SETDB1 from degradation. In general, our results reveal the mechanism of telomere heterochromatin maintenance and its effect on ALT, and TRIM28 may serve as a target for the treatment of ALT tumor cells.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00660-y.

Shorter Leukocyte Telomere Length Is Associated with Worse Survival of Patients with Bladder Cancer and Renal Cell Carcinoma

Simple Summary

Intrinsic telomere shortening promotes tumorigenesis in cells with impaired DNA damage repair mechanisms, as dysfunctional telomeres lead to chromosomal instability. More recent data show that the telomere length of peripheral blood leukocyte (PBL) cells can be a prognostic marker for survival of patients with solid tumors. However, reports on bladder cancer (BC) and renal cell carcinoma (RCC) are not consistent and partly contradictory. Our results show, first, that telomere length is shorter in patients with BC or RCC compared to patients without malignant disease. More importantly, the relative telomere length (RTL) of PBL cells is associated with survival of patients with BC and RCC. Thus, telomere length in PBL cells could be an auxiliary prognostic marker in BC and RCC.

Abstract

Background: Telomeres are protein–DNA complexes at the tips of linear chromosomes. They protect the DNA from end-to-end fusion and exonucleolytic degradation. Shortening of telomeric DNA during aging can generate dysfunctional telomeres, promoting tumorigenesis. More recent data indicate that both short and long telomeres of peripheral blood leukocyte (PBL) cells can serve as prognostic biomarkers for cancer risk and may be associated with survival of patients with solid cancers. Telomere length in PBL cells could also be a potential prognostic biomarker for survival in bladder cancer (BC) or renal cell carcinoma (RCC). Methods: The relative telomere length (RTL) of PBL cells was assessed in patients with BC (n = 144) and RCC (n = 144) by using qPCR. A control population of patients without malignant disease (NC, n = 73) was included for comparison. The correlation and association of RTL with histopathological parameters and overall survival (OS) were evaluated. Results: Patients with BC and RCC had significantly shorter telomeres compared to patients without malignant disease. Within the cancer cohorts, multivariate analysis revealed that short RTL is an independent predictor of worse survival in BC (p = 0.039) and RCC (p = 0.041). Conclusion: Patients with BC and RCC had significantly shorter telomeres compared to the normal population. Shorter RTL in BC and RCC was an independent predictor of reduced survival.

Are Biological Consequences of Childhood Exposures Detectable in Telomere Length Decades Later?

Negative early-life exposures have been linked to a host of poor adult health outcomes, but are such early exposures associated with cellular senescence decades later? This study uses data from the Health and Retirement Study to examine the association between six childhood exposure domains (eg, socioeconomic disadvantage, risky parental behavior) and a biomarker of aging, telomere length, among 4,935 respondents. Telomere length is obtained from DNA of cells found in saliva and is measured as the telomere repeat copy number to single gene copy number ratio (T/S). Men who as children were exposed to risky parental behaviors or who reported risky adolescent behaviors have shorter telomeres (b = -0.031, p = .052; b = -0.041, p = .045, respectively); however, these relationships are attenuated after adjusting for adult risks and resources. Among women, parental substance abuse is associated with shorter telomeres even after adjusting for adult risks and resources (b = -0.041, p = .005). In addition, men and women whose mother lived at least until the age of 85 have longer telomeres than those without a long-lived mother (b = 0.021, p = .045; b = 0.032, p = .005, respectively). Taken together, the ways in which early-life exposures are associated with adult telomeres vary for men and women.

External environmental agents influence telomere length and telomerase activity by modulating internal cellular processes: Implications in human aging

External environment affects cellular physiological processes and impact the stability of our genome. The most important structural components of our linear chromosomes which endure the impact by these agents, are the chromosomal ends called telomeres. Telomeres preserve the integrity of our genome by preventing end to end fusions and telomeric loss through by inhibiting DNA damage response (DDR) activation. This is accomplished by the presence of a six membered shelterin complex at telomeres. Further, telomeres cannot be replicated by normal DNA polymerase and require a special enzyme called telomerase which is expressed only in stem cells, few immune cells and germ cells. Telomeres are rich in guanine content and thus become extremely prone to damage arising due to physiological processes like oxidative stress and inflammation. External environmental factors which includes various physical, biological and chemical agents also affect telomere homeostasis by increasing oxidative stress and inflammation. In the present review, we highlight the effect of these external factors on telomerase activity and telomere length. We also discuss how the external agents affect the physiological processes, thus modulating telomere stability. Further, we describe its implication in the development of aging and its related pathologies.

Telomere Dynamics and Telomerase in the Biology of Hair Follicles and their Stem Cells as a Model for Aging Research

In this review, we propose that telomere length dynamics play an important but underinvestigated role in the biology of the hair follicle (HF), a prototypic, cyclically remodeled miniorgan that shows an intriguing aging pattern in humans. Whereas the HF pigmentary unit ages quickly, its epithelial stem cell (ESC) component and regenerative capacity are surprisingly aging resistant. Telomerase-deficient mice with short telomeres display an aging phenotype of hair graying and hair loss that is attributed to impaired HF ESC mobilization. Yet, it remains unclear whether the function of telomerase and telomeres in murine HF biology translate to the human system. Therefore, we propose new directions for future telomere research of the human HF. Such research may guide the development of novel treatments for selected disorders of human hair growth or pigmentation (e.g., chemotherapy-induced alopecia, telogen effluvium, androgenetic alopecia, cicatricial alopecia, graying). It might also increase the understanding of the global role of telomeres in aging-related human disease.

Single-cell analyses of aging, inflammation and senescence

Single-cell gene expression (transcriptomics) data are becoming robust and abundant, and are increasingly used to track organisms along their life-course. This allows investigation into how aging affects cellular transcriptomes, and how changes in transcriptomes may underlie aging, including chronic inflammation (inflammaging), immunosenescence and cellular senescence. We compiled and tabulated aging-related single-cell datasets published to date, collected and discussed relevant findings, and inspected some of these datasets ourselves. We specifically note insights that cannot (or not easily) be based on bulk data. For example, in some datasets, the fraction of cells expressing p16 (CDKN2A), one of the most prominent markers of cellular senescence, was reported to increase, in addition to its upregulated mean expression over all cells. Moreover, we found evidence for inflammatory processes in most datasets, some of these driven by specific cells of the immune system. Further, single-cell data are specifically useful to investigate whether transcriptional heterogeneity (also called noise or variability) increases with age, and many (but not all) studies in our review report an increase in such heterogeneity. Finally, we demonstrate some stability of marker gene expression patterns across closely similar studies and suggest that single-cell experiments may hold the key to provide detailed insights whenever interventions (countering aging, inflammation, senescence, disease, etc.) are affecting cells depending on cell type.

Molecular Mechanisms to Target Cellular Senescence in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) has emerged as a major cause of cancer-related death and is the most common type of liver cancer. Due to the current paucity of drugs for HCC therapy there is a pressing need to develop new therapeutic concepts. In recent years, the role of Serum Response Factor (SRF) and its coactivators, Myocardin-Related Transcription Factors A and B (MRTF-A and -B), in HCC formation and progression has received considerable attention. Targeting MRTFs results in HCC growth arrest provoked by oncogene-induced senescence. The induction of senescence acts as a tumor-suppressive mechanism and therefore gains consideration for pharmacological interventions in cancer therapy. In this article, we describe the key features and the functional role of senescence in light of the development of novel drug targets for HCC therapy with a focus on MRTFs.

Telomere Length Inheritance and Shortening in Trisomy 21

Objective: Trisomy 21 is a genetic disorder that shows premature aging symptoms. As an aging marker, telomere length is therefore of importance in trisomy families. Methods: We included 63 maternally originated trisomy 21 and 77 control families with infants in the first data set; 48 trisomy 21 and 60 control children in the second set; and 14 paternally originated trisomy 21 families in the third data set. We used Southern blot to measure the telomere length. Results: (1) Offsprings' telomere length increased with parents' age (p < .0001). (2) Trisomy 21 infants had longer telomere than the controls (p < .0001). (3) Post-birth, the telomere attrition rate was higher in cases than in controls (58 bps/year vs. 38 bps/year). Conclusion: (1) Our data supports the older parents-longer gamete telomere hypothesis. (2) Trisomy 21 patients are born with longer telomeres, (3) with advancing trisomy 21 age, the telomere shortens more quickly than euploids.

Success Rate of Grafts With Multiple Renal Vessels in 3136 Kidney Transplants

OBJECTIVES

Multiple renal vessels are often detected in living and deceased organ donors. In the past, transplant with multiple renal vessel grafts has been a contraindication because of high vascular and urological complication rates. However, improvements in vascular reconstruction and anastomosis techniques have allowed graft function to be maintained for many years. Here, we retrospectively evaluated transplant of multiple renal vessel grafts and graft survival and postoperative vascular and urological complications.

MATERIALS AND METHODS

From November 1975 to July 2020, there were 3136 renal transplants (716 deceased donors, 2420 living donors) performed in our center. There were 2167 living donors and 643 deceased donors with single renal vessel grafts and 253 living donors and 73 deceased donors with multiple renal vessel grafts. For anastomoses, external iliac, internal iliac, common iliac, and inferior epigastric arteries and external iliac veins were used. Cold ischemia time, anastomosis time, postoperative vascular and urological complications, acute tubular necrosis, creatinine clearance, serum creatinine levels, graft rejection episodes, and graft and patient survival rates were evaluated.

RESULTS

With regard to creatinine clearance, cold ischemia and anastomosis time, acute tubular necrosis, rejection episodes, and 1-, 2-, and 5-year posttransplant serum creatinine levels, there were no significant differences between the groups. Graft survival rates in the single renal vessel group were 92.9% at 1 year posttransplant and 78.3% at 5 years posttransplant; rates in the multiple renal vessel group were 93.1% at 1 year and 79.7% at 5 years. The corresponding patient survival rates were 95.5% (1 year) and 92.9% (5 years) for the single renal vessel group and 96.9% (1 year) and 87.2% (5 years) for the multiple renal vessel group.

CONCLUSIONS

Improved anastomosis and recon struction techniques have allowed the safe transplant of multiple renal vessel grafts that may remain functional for many years.

Aberrant Telomere Length in Circulating Cell-Free DNA as Possible Blood Biomarker with High Diagnostic Performance in Endometrial Cancer

To investigate the diagnostic performance of relative telomere length (RTL) in cell-free DNA (cfDNA) for endometrioid endometrial cancer (EC). We measured RTL in cfDNA of 40 EC patients (65 ± 12 years) and 31 healthy controls (HC) (63 ± 13 years), excluding in both groups other oncologic and severe non-oncologic diseases to limit confounders. Circulating cfDNA was extracted from serum using the QIAamp DNA Blood Mini kit (Qiagen, Hilden, Germany). After the quantitative real-time polymerase chain reaction, telomere repeat copy number to single-gene copy number ratio was calculated. RTL in cfDNA was found to be significantly lower in EC patients than in HC (p < 0.0001). The diagnostic performance of cfDNA RTL was estimated with receiver operating characteristics (ROC) curve analysis, which showed a diagnostic accuracy for EC of 0.87 (95% CI: 0.79-0.95, p < 0.0001). The cutoff cfDNA RTL value of 2.505 (T/S copy ratio) reported a sensitivity of 80.0% (95% CI: 64.35-90.95) and a specificity of 80.65% (95% CI: 62.53-92.55). Significant differences of RTL among EC stages or grades (p = 0.85 and p = 0.89, respectively) were not observed. Our results suggest that cfDNA RTL analysis may be a diagnostic tool for EC detection since the early stage, whilst its diagnostic performance seems unsatisfactory for cancer progression, staging, and grading. However, further studies are needed to confirm these preliminary findings. In particular, future investigations should focus on high-risk patients (such as those with atypical endometrial hyperplasia) that may benefit from this tool, because TL shortening is not specific for EC and is influenced by other oncologic and non-oncologic diseases.

In silico design of telomerase inhibitors

Telomerase is a reverse transcriptase enzyme involved in DNA synthesis at the end of linear chromosomes. Unlike in most other cells, telomerase is reactivated most cancerous cells and, therefore, has become a promising new anticancer target. Despite extensive research, direct telomerase inhibitors have yet not been introduced to the clinics because of the complexity of this enzyme. Structures of this protein from simple organisms and human homology models are currently available and have been used in structure-based drug design efforts to find potential inhibitors. Different is silico strategies have been applied and different chemical groups have been explored. Here, we provide an overview of recent discoveries.

Telomeres and Telomerase in Heart Ontogenesis, Aging and Regeneration

The main purpose of the review article is to assess the contributions of telomere length and telomerase activity to the cardiac function at different stages of development and clarify their role in cardiac disorders. It has been shown that the telomerase complex and telomeres are of great importance in many periods of ontogenesis due to the regulation of the proliferative capacity of heart cells. The review article also discusses the problems of heart regeneration and the identification of possible causes of dysfunction of telomeres and telomerase.

Hydroxyl groups in cosolutes regulate the G-quadruplex topology of telomeric DNA

Telomeric G-quadruplex topology has the ability to regulate telomerase activity, which counteracts the shortening of telomere with successive cell divisions, thereby causing genomic longevity. However, the detailed mechanism of G-quadruplexes topologies formed by telomeric sequences requires further investigation. In this study, we quantitatively investigated the effect of cosolutes, particularly the varying number of hydroxyl groups, on the structural transition between hybrid type and parallel G-quadruplexes formed by telomeric DNA sequences. Cosolutes with one or no hydroxyl groups in the vicinal position more efficiently induced the transition to parallel G-quadruplex from hybrid G-quadruplex than those with more hydroxyl groups. We also examined the effect of cosolute structures on the hydration of G-quadruplex formation; the results indicated that cosolutes with fewer hydroxyl groups lead to the release of greater amount of water during G-quadruplex formation. Molecular dynamics results showed that the parallel G-quadruplex was more dehydrated than the hybrid type G-quadruplex. Generally, a dehydrated structure is favored under crowding condition. Thus, depending on the surrounding cosolutes, the G-quadruplex topology can be controlled by the G-quadruplex hydration state.

Cellular Senescence in Neurodegenerative Diseases

Cellular senescence is a homeostatic biological process characterized by a permanent state of cell cycle arrest that can contribute to the decline of the regenerative potential and function of tissues. The increased presence of senescent cells in different neurodegenerative diseases suggests the contribution of senescence in the pathophysiology of these disorders. Although several factors can induce senescence, DNA damage, oxidative stress, neuroinflammation, and altered proteostasis have been shown to play a role in its onset. Oxidative stress contributes to accelerated aging and cognitive dysfunction stages affecting neurogenesis, neuronal differentiation, connectivity, and survival. During later life stages, it is implicated in the progression of cognitive decline, synapse loss, and neuronal degeneration. Also, neuroinflammation exacerbates oxidative stress, synaptic dysfunction, and neuronal death through the harmful effects of pro-inflammatory cytokines on cell proliferation and maturation. Both oxidative stress and neuroinflammation can induce DNA damage and alterations in DNA repair that, in turn, can exacerbate them. Another important feature associated with senescence is altered proteostasis. Because of the disruption in the function and balance of the proteome, senescence can modify the proper synthesis, folding, quality control, and degradation rate of proteins producing, in some diseases, misfolded proteins or aggregation of abnormal proteins. There is an extensive body of literature that associates cellular senescence with several neurodegenerative disorders including Alzheimer’s disease (AD), Down syndrome (DS), and Parkinson’s disease (PD). This review summarizes the evidence of the shared neuropathological events in these neurodegenerative diseases and the implication of cellular senescence in their onset or aggravation. Understanding the role that cellular senescence plays in them could help to develop new therapeutic strategies.

An epigenome-wide association study of sex-specific chronological ageing

BACKGROUND

Advanced age is associated with cognitive and physical decline and is a major risk factor for a multitude of disorders. There is also a gap in life expectancy between males and females. DNA methylation differences have been shown to be associated with both age and sex. Here, we investigate age-by-sex differences in blood-based DNA methylation in an unrelated cohort of 2586 individuals between the ages of 18 and 87 years, with replication in a further 4450 individuals between the ages of 18 and 93 years.

METHODS

Linear regression models were applied, with stringent genome-wide significance thresholds (p < 3.6 × 10-8) used in both the discovery and replication data. A second, highly conservative mixed linear model method that better controls the false-positive rate was also applied, using the same genome-wide significance thresholds.

RESULTS

Using the linear regression method, 52 autosomal and 597 X-linked CpG sites, mapping to 251 unique genes, replicated with concordant effect size directions in the age-by-sex interaction analysis. The site with the greatest difference mapped to GAGE10, an X-linked gene. Here, DNA methylation levels remained stable across the male adult age range (DNA methylation by age r = 0.02) but decreased across female adult age range (DNA methylation by age r = - 0.61). One site (cg23722529) with a significant age-by-sex interaction also had a quantitative trait locus (rs17321482) that is a genome-wide significant variant for prostate cancer. The mixed linear model method identified 11 CpG sites associated with the age-by-sex interaction.

CONCLUSION

The majority of differences in age-associated DNA methylation trajectories between sexes are present on the X chromosome. Several of these differences occur within genes that have been implicated in sexually dimorphic traits.

Damage-Fitness Model: Evaluation and synthesis

Decades of research into stress responses have highlighted large variation among individuals, populations, and species, and the sources of this variation have been a center of research across disciplines. The most common measure of the vertebrate stress response is glucocorticoids. However, the predictive power of glucocorticoid responses to fitness is surprisingly low. This is partly because the hormone levels rapidly change in response to stressor exposure and elevated levels at one time point can indicate either that glucocorticoids are helping the organism cope with the stressor or that dysregulation of hormone release is harming the organism. Meaning, the fitness consequences of the stressor depends on how efficient the stress responses are at negating the harmful impacts of stressors to cells and tissues. To encompass the idea of the efficiency of stress responses and to integrate cellular and organismal stress responses, a new theoretical model called the Damage-Fitness Model was developed. The model focuses on the downstream effects of stress responses and predicts that the accumulation of damage in cells and tissues (e.g., persistent damage to proteins, lipids, and DNA) negatively impacts fitness components. In this mini-review, we examine evidence supporting the Damage-Fitness Model and explore new directions forward.

Telomeres and telomerase: three decades of progress

Many recent advances have emerged in the telomere and telomerase fields. This Timeline article highlights the key advances that have expanded our views on the mechanistic underpinnings of telomeres and telomerase and their roles in ageing and disease. Three decades ago, the classic view was that telomeres protected the natural ends of linear chromosomes and that telomerase was a specific telomere-terminal transferase necessary for the replication of chromosome ends in single-celled organisms. While this concept is still correct, many diverse fields associated with telomeres and telomerase have substantially matured. These areas include the discovery of most of the key molecular components of telomerase, implications for limits to cellular replication, identification and characterization of human genetic disorders that result in premature telomere shortening, the concept that inhibiting telomerase might be a successful therapeutic strategy and roles for telomeres in regulating gene expression. We discuss progress in these areas and conclude with challenges and unanswered questions in the field.

Longitudinal analysis of somatic and germ-cell telomere dynamics in outbred mice

Although telomere length (TL) shortens with age in most tissues, an age-related increase in length has been described in sperm through a mechanism that is not yet fully understood. Changes in TL with age in the same individual have not been explored. This longitudinal study examines TL dynamics in somatic tissue and gametes during an entire lifespan in an outbred mouse population (from 8 to up to 114 weeks of age). Our findings indicate a reduced life expectancy in males compared to females (84.75 ± 9.23 vs. 113.16 ± 0.20 weeks) and significant variability in TL dynamics between individuals. While with aging, a clear reduction in TL was produced in somatic cells and oocytes, telomeres in sperm cells significantly lengthened. Finally, we found evidence indicating that telomere elongation in sperm during aging may be dependent on different mechanisms, such as the survival of spermatogonia with longer telomeres and the alternative lengthening of telomeres mechanism in meiotic and postmeiotic spermatogenic cells.

Establishment and Characterization of an Immortalized Porcine Oral Mucosal Epithelial Cell Line as a Cytopathogenic Model for Porcine Circovirus 2 Infection

Porcine circovirus 2 (PCV2) is a major etiological agent for porcine circovirus-associated diseases and causes enormous economic losses in domestic and overseas swine production. However, there are currently no suitable cell models to study the cytopathic effects (CPE) of PCV2 in vitro, which severely restricts the study of PCV2 pathogenesis. In the present study, we established an immortalized porcine oral mucosal epithelial cell line (hTERT-POMEC) by introducing the hTERT gene into primary porcine oral mucosal epithelial cells (POMECs) derived from a neonatal, unsuckled piglet. The hTERT-POMEC cells have a homogeneous cobblestone-like morphology and retain the basic physiological properties of primary POMECs. No chromosome abnormality and tumorigenicity transformation was observed in immortalized hTERT-POMECs. Viral infection assays demonstrated that PCV2 propagated and caused CPE in hTERT-POMECs. We conclude that the immortalized cell line hTERT-POMEC is a crucial tool for further research into the pathogenesis of PCV2.

Association between prediagnostic leukocyte telomere length and breast cancer risk: the Singapore Chinese Health Study

BACKGROUND

Telomeres and telomerase play key roles in the chromosomal maintenance and stability. Recent epidemiological studies have shown that longer telomeres are associated with increased risk of several cancer types. However, epidemiological data for telomere length and risk of breast cancer are sparse.

METHODS

We prospectively studied the association between telomere length and risk of breast cancer in 14,305 middle-aged or older Chinese women of the Singapore Chinese Health Study including 442 incident breast cancer cases after 12.3 years of follow-up. Relative telomere length in peripheral blood leukocytes was quantified using a validated monochrome multiple quantitative polymerase chain reaction method. The Cox proportional hazard regression method was used to estimate hazard ratios (HRs) and the corresponding 95% confidence intervals (CIs) for breast cancer associated with longer telomeres after adjustment for potential confounders.

RESULTS

Longer telomeres were significantly associated with higher risk of breast cancer in a dose-dependent manner (P_trend = 0.006); the highest quartile of telomere length was associated with a statistically significant 47% higher risk of breast cancer compared with the lowest quartile of telomere length after the adjustment for age and other known risk factors for breast cancer (HR_{Q4 vs Q1} = 1.47, 95% CI = 1.11, 1.94).

CONCLUSIONS

The findings of the present study support the hypothesis that longer telomeres may be a risk factor for breast cancer. Telomere length in peripheral blood leukocytes may be developed as a biomarker for breast cancer risk prediction.

Telomeres and Telomerase in Cutaneous Squamous Cell Carcinoma

The role of telomere biology and telomerase activation in skin cancers has been investigated in melanoma and basal cell carcinoma but limited evidence is available for cutaneous squamous cell carcinoma (cSCC). We will review the current knowledge on the role of telomere and telomerase pathway in cSCC pathogenesis. At the somatic level, both long and short telomere lengths have been described in cSCC. This telomere dichotomy is probably related to two different mechanisms of tumour initiation which determines two tumour subtypes. Telomere shortening is observed during the invasive progression from in situ forms of cSCC, such as Bowen's disease (BD) and actinic keratosis (AK), to invasive cSCC. At the germline level, controversial results have been reported on the association between constitutive telomere length and risk of cSCC. Approximately 75⁻85% of cSCC tumours are characterized by a high level of telomerase activity. Telomerase activation has been also reported in AKs and BD and in sun-damaged skin, thus supporting the hypothesis that UV modulates telomerase activity in the skin. Activating TERT promoter mutations have been identified in 32⁻70% of cSCCs, with the majority showing the UV-signature. No significant correlation was observed between TERT promoter mutations and cSCC clinico-pathological features. However, TERT promoter mutations have been recently suggested to be independent predictors of an adverse outcome. The attention on telomere biology and telomerase activity in cSCC is increasing for the potential implications in the development of effective tools for prognostic assessment and of therapeutic strategies in patients with cutaneous cSCC.

Cell cycle arrest in replicative senescence is not an immediate consequence of telomere dysfunction

In replicative senescence, cells with critically-short telomeres activate a DNA-damage response leading to cell-cycle arrest, while those without telomere dysfunction would be expected to cycle normally. However, population growth declines more gradually than such a simple binary switch between cycling and non-cycling states would predict. We show here that late-passage cultures of human fibroblasts are not a simple mixture of cycling and non-cycling cells. Rather, although some cells had short cycle times comparable to those of younger cells, others continued to divide but with greatly extended cycle times, indicating a more-gradual approach to permanent arrest. Remarkably, in late passage cells, the majority showed prominent DNA-damage foci positive for 53BP1, yet many continued to divide. Evidently, the DNA-damage-response elicited by critically-short telomeres is not initially strong enough for complete cell-cycle arrest. A similar continuation of the cell cycle in the face of an active DNA-damage response was also seen in cells treated with a low dose of doxorubicin sufficient to produce multiple 53BP1 foci in all nuclei. Cell cycle checkpoint engagement in response to DNA damage is thus weaker than generally supposed, explaining why an accumulation of dysfunctional telomeres is needed before marked cell cycle elongation or permanent arrest is achieved.

Reflections on telomere dynamics and ageing-related diseases in humans

Epidemiological studies have principally relied on measurements of telomere length (TL) in leucocytes, which reflects TL in other somatic cells. Leucocyte TL (LTL) displays vast variation across individuals—a phenomenon already observed in newborns. It is highly heritable, longer in females than males and in individuals of African ancestry than European ancestry. LTL is also longer in offspring conceived by older men. The traditional view regards LTL as a passive biomarker of human ageing. However, new evidence suggests that a dynamic interplay between selective evolutionary forces and TL might result in trade-offs for specific health outcomes. From a biological perspective, an active role of TL in ageing-related human diseases could occur because short telomeres increase the risk of a category of diseases related to restricted cell proliferation and tissue degeneration, including cardiovascular disease, whereas long telomeres increase the risk of another category of diseases related to increased proliferative growth, including major cancers. To understand the role of telomere biology in ageing-related diseases, it is essential to expand telomere research to newborns and children and seek further insight into the underlying causes of the variation in TL due to ancestry and geographical location.

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

Anti-senescence effect and molecular mechanism of the major royal jelly proteins on human embryonic lung fibroblast (HFL-I) cell line

Royal jelly (RJ) from honeybee has been widely used as a health promotion supplement. The major royal jelly proteins (MRJPs) have been identified as the functional component of RJ. However, the question of whether MRJPs have anti-senescence activity for human cells remains. Human embryonic lung fibroblast (HFL-I) cells were cultured in media containing no MRJPs (A), MRJPs at 0.1 mg/ml (B), 0.2 mg/ml (C), or 0.3 mg/ml (D), or bovine serum albumin (BSA) at 0.2 mg/ml (E). The mean population doubling levels of cells in media B, C, D, and E were increased by 12.4%, 31.2%, 24.0%, and 10.4%, respectively, compared with that in medium A. The cells in medium C also exhibited the highest relative proliferation activity, the lowest senescence, and the longest telomeres. Moreover, MRJPs up-regulated the expression of superoxide dismutase-1 (SOD1) and down-regulated the expression of mammalian target of rapamycin (MTOR), catenin beta like-1 (CTNNB1), and tumor protein p53 (TP53). Raman spectra analysis showed that there were two unique bands related to DNA synthesis materials, amide carbonyl group vibrations and aromatic hydrogens. These results suggest that MRJPs possess anti-senescence activity for the HFL-I cell line, and provide new knowledge illustrating the molecular mechanism of MRJPs as anti-senescence factors.

Association of Telomere Length With Chromosomal Damage Among Chinese Workers Exposed to Vinyl Chloride Monomer

OBJECTIVE

To explore the relationship between relative telomere length (RTL) and chromosomal damage represented by micronucleus (MN) frequencies among vinyl chloride monomer (VCM) -exposed workers.

METHODS

A group of 126 VCM-exposed workers, 60 internal controls, and 25 external controls were examined for RTL by Quantitative polymerase chain reaction and MN frequencies by cytokinesis-block micronucleus test. Cumulative exposure dose was used to estimate the exposure of VCM-exposed workers.

RESULTS

The RTL were significantly shorter in exposed workers and internal controls than in external controls. The exposed workers had significantly increased MN frequencies than both control groups. Additionally, MN frequencies were negatively associated with RTL in VCM-exposed group.

CONCLUSIONS

VCM exposure may alter telomere length, which could be a potential biomarker of susceptibility to chromosomal damage.

Chemical screen identifies a geroprotective role of quercetin in premature aging

Aging increases the risk of various diseases. The main goal of aging research is to find therapies that attenuate aging and alleviate aging-related diseases. In this study, we screened a natural product library for geroprotective compounds using Werner syndrome (WS) human mesenchymal stem cells (hMSCs), a premature aging model that we recently established. Ten candidate compounds were identified and quercetin was investigated in detail due to its leading effects. Mechanistic studies revealed that quercetin alleviated senescence via the enhancement of cell proliferation and restoration of heterochromatin architecture in WS hMSCs. RNA-sequencing analysis revealed the transcriptional commonalities and differences in the geroprotective effects by quercetin and Vitamin C. Besides WS hMSCs, quercetin also attenuated cellular senescence in Hutchinson-Gilford progeria syndrome (HGPS) and physiological-aging hMSCs. Taken together, our study identifies quercetin as a geroprotective agent against accelerated and natural aging in hMSCs, providing a potential therapeutic intervention for treating age-associated disorders.

The Emerging Roles for Telomerase in the Central Nervous System

Telomerase, a specialized ribonucleoprotein enzyme complex, maintains telomere length at the 3′ end of chromosomes, and functions importantly in stem cells, cancer and aging. Telomerase exists in neural stem cells (NSCs) and neural progenitor cells (NPCs), at a high level in the developing and adult brains of humans and rodents. Increasing studies have demonstrated that telomerase in NSCs/NPCs plays important roles in cell proliferation, neuronal differentiation, neuronal survival and neuritogenesis. In addition, recent works have shown that telomerase reverse transcriptase (TERT) can protect newborn neurons from apoptosis and excitotoxicity. However, to date, the link between telomerase and diseases in the central nervous system (CNS) is not well reviewed. Here, we analyze the evidence and summarize the important roles of telomerase in the CNS. Understanding the roles of telomerase in the nervous system is not only important to gain further insight into the process of the neural cell life cycle but would also provide novel therapeutic applications in CNS diseases such as neurodegenerative condition, mood disorders, aging and other ailments.

Mechanisms of human telomerase reverse transcriptase (hTERT) regulation: clinical impacts in cancer

Background

Limitless self-renewal is one of the hallmarks of cancer and is attained by telomere maintenance, essentially through telomerase (hTERT) activation. Transcriptional regulation of hTERT is believed to play a major role in telomerase activation in human cancers.

Main body

The dominant interest in telomerase results from its role in cancer. The role of telomeres and telomere maintenance mechanisms is well established as a major driving force in generating chromosomal and genomic instability. Cancer cells have acquired the ability to overcome their fate of senescence via telomere length maintenance mechanisms, mainly by telomerase activation.

hTERT expression is up-regulated in tumors via multiple genetic and epigenetic mechanisms including hTERT amplifications, hTERT structural variants, hTERT promoter mutations and epigenetic modifications through hTERT promoter methylation. Genetic (hTERT promoter mutations) and epigenetic (hTERT promoter methylation and miRNAs) events were shown to have clinical implications in cancers that depend on hTERT activation. Knowing that telomeres are crucial for cellular self-renewal, the mechanisms responsible for telomere maintenance have a crucial role in cancer diseases and might be important oncological biomarkers. Thus, rather than quantifying TERT expression and its correlation with telomerase activation, the discovery and the assessment of the mechanisms responsible for TERT upregulation offers important information that may be used for diagnosis, prognosis, and treatment monitoring in oncology. Furthermore, a better understanding of these mechanisms may promote their translation into effective targeted cancer therapies.

Conclusion

Herein, we reviewed the underlying mechanisms of hTERT regulation, their role in oncogenesis, and the potential clinical applications in telomerase-dependent cancers.

GWAS of epigenetic aging rates in blood reveals a critical role for TERT

DNA methylation age is an accurate biomarker of chronological age and predicts lifespan, but its underlying molecular mechanisms are unknown. In this genome-wide association study of 9907 individuals, we find gene variants mapping to five loci associated with intrinsic epigenetic age acceleration (IEAA) and gene variants in three loci associated with extrinsic epigenetic age acceleration (EEAA). Mendelian randomization analysis suggests causal influences of menarche and menopause on IEAA and lipoproteins on IEAA and EEAA. Variants associated with longer leukocyte telomere length (LTL) in the telomerase reverse transcriptase gene (TERT) paradoxically confer higher IEAA (P < 2.7 × 10-11). Causal modeling indicates TERT-specific and independent effects on LTL and IEAA. Experimental hTERT-expression in primary human fibroblasts engenders a linear increase in DNA methylation age with cell population doubling number. Together, these findings indicate a critical role for hTERT in regulating the epigenetic clock, in addition to its established role of compensating for cell replication-dependent telomere shortening.

Telomeres: Implications for Cancer Development

Telomeres facilitate the protection of natural ends of chromosomes from constitutive exposure to the DNA damage response (DDR). This is most likely achieved by a lariat structure that hides the linear telomeric DNA through protein-protein and protein-DNA interactions. The telomere shortening associated with DNA replication in the absence of a compensatory mechanism culminates in unmasked telomeres. Then, the subsequent activation of the DDR will define the fate of cells according to the functionality of cell cycle checkpoints. Dysfunctional telomeres can suppress cancer development by engaging replicative senescence or apoptotic pathways, but they can also promote tumour initiation. Studies in telomere dynamics and karyotype analysis underpin telomere crisis as a key event driving genomic instability. Significant attainment of telomerase or alternative lengthening of telomeres (ALT)-pathway to maintain telomere length may be permissive and required for clonal evolution of genomically-unstable cells during progression to malignancy. We summarise current knowledge of the role of telomeres in the maintenance of chromosomal stability and carcinogenesis.

Agent-Based Modeling of Cancer Stem Cell Driven Solid Tumor Growth

Computational modeling of tumor growth has become an invaluable tool to simulate complex cell-cell interactions and emerging population-level dynamics. Agent-based models are commonly used to describe the behavior and interaction of individual cells in different environments. Behavioral rules can be informed and calibrated by in vitro assays, and emerging population-level dynamics may be validated with both in vitro and in vivo experiments. Here, we describe the design and implementation of a lattice-based agent-based model of cancer stem cell driven tumor growth.

Effect of cyclosporine, tacrolimus and sirolimus on cellular senescence in renal epithelial cells

INTRODUCTION

In transplantation medicine calcineurin inhibitors (CNI) still represent the backbone of immunosuppressive therapy. The nephrotoxic potential of the CNI Cyclosporine A (CsA) and Tacrolimus (FK506) is well recognized and CNI not only have been linked with toxicity, but also with cellular senescence which hinders parenchymal tissue regeneration and thus may prime kidneys for subsequent insults. To minimize pathological effects on kidney grafts, alternative immunosuppressive agents like mTOR inhibitors or the T-cell co-stimulation blocker Belatacept have been introduced.

METHODS

We compared the effects of CsA, FK506 and Sirolimus on the process of cellular senescence in different human renal tubule cell types (HK2, RPTEC). Telomere length (by real time PCR), DNA synthesis (by BrdU incorporation), cell viability (by Resazurin conversion), gene expression (by RT-PCR), protein (by western blotting), Immuncytochemistry and H₂O₂ production (by Amplex Red® conversion) were evaluated.

RESULTS

DNA synthesis was significantly reduced when cells were treated with cyclosporine but not with tacrolimus and sirolimus. Resazurin conversion was not altered by all three immunosuppressive agents. The gene expression as well as protein production of the cell cycle inhibitor p21 (CDKN1A) but not p16 (CDKN2A) was significantly induced by cyclosporine compared to the other two immunosuppressive agents when determined by western blotting an immuncytochemistry. Relative telomere length was reduced and hydrogen peroxide production increased after treatment with CsA but not with FK506 or sirolimus.

CONCLUSION

In summary, renal tubule cells exposed to CsA show clear signs of cellular senescence where on the contrary the second calcineurin inhibitor FK506 and the mTOR inhibitor sirolimus are not involved in such mechanisms. Chronic renal allograft dysfunction could be in part triggered by cellular senescence induced by immunosuppressive medication and the choice of drug could therefore influence long term outcome. Tacrolimus and Sirolimus are equally effective in avoiding cellular senescence compared to cyclosporine at least in parts due to a lack of induction of reactive oxygen species.