ATM regulates proteasome-dependent subnuclear localization of TRF1, which is important for telomere maintenance

Post-Transcriptional and Post-Translational Modifications in Telomerase Biogenesis and Recruitment to Telomeres

Telomere length is associated with the proliferative potential of cells. Telomerase is an enzyme that elongates telomeres throughout the entire lifespan of an organism in stem cells, germ cells, and cells of constantly renewed tissues. It is activated during cellular division, including regeneration and immune responses. The biogenesis of telomerase components and their assembly and functional localization to the telomere is a complex system regulated at multiple levels, where each step must be tuned to the cellular requirements. Any defect in the function or localization of the components of the telomerase biogenesis and functional system will affect the maintenance of telomere length, which is critical to the processes of regeneration, immune response, embryonic development, and cancer progression. An understanding of the regulatory mechanisms of telomerase biogenesis and activity is necessary for the development of approaches toward manipulating telomerase to influence these processes. The present review focuses on the molecular mechanisms involved in the major steps of telomerase regulation and the role of post-transcriptional and post-translational modifications in telomerase biogenesis and function in yeast and vertebrates.

Telomere Lengths and Serum Proteasome Concentrations in Patients with Type 1 Diabetes and Different Severities of Diabetic Retinopathy in Latvia and Lithuania

The aim of the study was to compare telomere lengths and circulating proteasome concentrations in patients with different stages of diabetic retinopathy and type 1 diabetes in Latvia and Lithuania. Methods. Patients with no diabetic retinopathy and with non-proliferative diabetic retinopathy were included in the NDR/NPDR group (n = 187). Patients with proliferative diabetic retinopathy and status post laser-photocoagulation were included int the PDR/LPC group (n = 119). Telomeres were evaluated by real-time quantitative polymerase chain reaction. Proteasome concentration was measured by ELISA. Results. Telomeres were longer in PDR/LPC (ΔCT 0.21 (0.12−0.28)) vs. NDR/NPDR (ΔCT 0.18 (0.1−0.28)), p = 0.036. In NDR/NPDR, telomeres were correlated negatively with age (R = −0.17, p = 0.019), BMI (R = −0.21, p = 0.004), waist/hip ratio (R = −0.21, p = 0.005), total cholesterol (R = −0.18, p = 0.021), and low-density cholesterol (R = −0.20, p = 0.010), and positively with estimated glomerular filtration rate (eGFR) (R = 0.28, p < 0.001). None of the above correlations were observed in PRD/LPC. Proteasome concentrations were lower in PDR/LPC (130 (90−210) ng/mL) vs. NDR/NPDR (150 (100−240) ng/mL), p = 0.024. This correlated negatively with eGFR (R = −0.17, p = 0.025) in the NDR/NPDR group and positively with age (R = 0.23, p = 0.014) and systolic blood pressure (R = 0.20, p = 0.032) in the PRD/LPC group. Telomere lengths did not correlate with proteasome concentrations. Conclusion. Longer telomeres and lower circulating proteasome concentrations are observed in patients with type 1 diabetes and advanced diabetic retinopathy.

Relationship between genetically determined telomere length and glioma risk

Background

Telomere maintenance is increasingly recognized as being fundamental to glioma oncogenesis with longer leukocyte telomere length (LTL) reported to increase risk of glioma. To gain further insight into the relationship between telomere genetics and risk of glioma, we conducted several complementary analyses, using genome-wide association studies data on LTL (78 592 individuals) and glioma (12 488 cases and 18 169 controls).

Methods

We performed both classical and summary Mendelian randomization (SMR), coupled with heterogeneity in dependent instruments tests, at genome-wide significant LTL loci to examine if an association was mediated by the same causal variant in glioma. To prioritize genes underscoring glioma-LTL associations, we analyzed gene expression and DNA methylation data.

Results

Genetically increased LTL was significantly associated with increased glioma risk, random-effects inverse variance weighted ORs per 1 SD unit increase in the putative risk factor (odds ratio [OR]_SD) 4.79 (95% confidence interval: 2.11-10.85; P = 1.76 × 10⁻⁴). SMR confirmed the previously reported LTL associations at 3q26.2 (TERC; P_SMR = 1.33 × 10⁻⁵), 5p15.33 (TERT; P_SMR = 9.80 × 10⁻²⁷), 10q24.33 (STN1 alias OBFC1; P_SMR = 4.31 × 10⁻⁵), and 20q13.3 (STMN3/RTEL1; P_SMR = 2.47 × 10⁻⁴) glioma risk loci. Our analysis implicates variation at 1q42.12 (P_SMR = 1.55 × 10⁻²), 6p21.3 (P_SMR = 9.76 × 10⁻³), 6p22.2 (P_SMR = 5.45 × 10⁻³), 7q31.33 (P_SMR = 6.52 × 10⁻³), and 11q22.3 (P_SMR = 8.89 × 10⁻⁴) as risk factors for glioma risk. While complicated by patterns of linkage disequilibrium, genetic variation involving PARP1, PRRC2A, CARMIL1, POT1, and ATM-NPAT1 was implicated in the etiology of glioma.

Conclusions

These observations extend the role of telomere-related genes in the development of glioma.

Telomere maintenance in interplay with DNA repair in pathogenesis and treatment of colorectal cancer

Colorectal cancer (CRC) continues to be one of the leading malignancies and causes of tumour-related deaths worldwide. Both impaired DNA repair mechanisms and disrupted telomere length homeostasis represent key culprits in CRC initiation, progression and prognosis. Mechanistically, altered DNA repair results in the accumulation of mutations in the genome and, ultimately, in genomic instability. DNA repair also determines the response to chemotherapeutics in CRC treatment, suggesting its utilisation in the prediction of therapy response and individual approach to patients. Telomere attrition resulting in replicative senescence, simultaneously by-passing cell cycle checkpoints, is a hallmark of malignant transformation of the cell. Telomerase is almost ubiquitous in advanced solid cancers, including CRC, and its expression is fundamental to cell immortalisation. Therefore, there is a persistent effort to develop therapeutics, which are telomerase-specific and gentle to non-malignant tissues. However, in practice, we are still at the level of clinical trials. The current state of knowledge and the route, which the research takes, gives us a positive perspective that the problem of molecular models of telomerase activation and telomere length stabilisation will finally be solved. We summarise the current literature herein, by pointing out the crosstalk between proteins involved in DNA repair and telomere length homeostasis in relation to CRC.

The Connection Between Cell Fate and Telomere

Abolition of telomerase activity results in telomere shortening, a process that eventually destabilizes the ends of chromosomes, leading to genomic instability and cell growth arrest or death. Telomere shortening leads to the attainment of the "Hayflick limit", and the transition of cells to state of senescence. If senescence is bypassed, cells undergo crisis through loss of checkpoints. This process causes massive cell death concomitant with further telomere shortening and spontaneous telomere fusions. In functional telomere of mammalian cells, DNA contains double-stranded tandem repeats of TTAGGG. The Shelterin complex, which is composed of six different proteins, is required for the regulation of telomere length and stability in cells. Telomere protection by telomeric repeat binding protein 2 (TRF2) is dependent on DNA damage response (DDR) inhibition via formation of T-loop structures. Many protein kinases contribute to the DDR activated cell cycle checkpoint pathways, and prevent DNA replication until damaged DNA is repaired. Thereby, the connection between cell fate and telomere length-associated telomerase activity is regulated by multiple protein kinase activities. Contrarily, inactivation of DNA damage checkpoint protein kinases in senescent cells can restore cell-cycle progression into S phase. Therefore, telomere-initiated senescence is a DNA damage checkpoint response that is activated with a direct contribution from dysfunctional telomeres. In this review, in addition to the above mentioned, the choice of main repair pathways, which comprise non-homologous end joining and homologous recombination in telomere uncapping telomere dysfunctions, are discussed.

Targeting telomerase for cancer therapy

Telomere maintenance via telomerase reactivation is a nearly universal hallmark of cancer cells which enables replicative immortality. In contrast, telomerase activity is silenced in most adult somatic cells. Thus, telomerase represents an attractive target for highly selective cancer therapeutics. However, development of telomerase inhibitors has been challenging and thus far there are no clinically approved strategies exploiting this cancer target. The discovery of prevalent mutations in the TERT promoter region in many cancers and recent advances in telomerase biology has led to a renewed interest in targeting this enzyme. Here we discuss recent efforts targeting telomerase, including immunotherapies and direct telomerase inhibitors, as well as emerging approaches such as targeting TERT gene expression driven by TERT promoter mutations. We also address some of the challenges to telomerase-directed therapies including potential therapeutic resistance and considerations for future therapeutic applications and translation into the clinical setting. Although much work remains to be done, effective strategies targeting telomerase will have a transformative impact for cancer therapy and the prospect of clinically effective drugs is boosted by recent advances in structural models of human telomerase.

CSB cooperates with SMARCAL1 to maintain telomere stability in ALT cells

Elevated replication stress is evident at telomeres of about 10-15% of cancer cells, which maintain their telomeres via a homologous recombination (HR)-based mechanism, referred to as alternative lengthening of telomeres (ALT). How ALT cells resolve replication stress to support their growth remains incompletely characterized. Here, we report that CSB (also known as ERCC6) promotes recruitment of HR repair proteins (MRN, BRCA1, BLM and RPA32) and POLD3 to ALT telomeres, a process that requires the ATPase activity of CSB and is controlled by ATM- and CDK2-dependent phosphorylation. Loss of CSB stimulates telomeric recruitment of MUS81 and SLX4, components of the structure-specific MUS81-EME1-SLX1-SLX4 (MUS-SLX) endonuclease complex, suggesting that CSB restricts MUS-SLX-mediated processing of stalled forks at ALT telomeres. Loss of CSB coupled with depletion of SMARCAL1, a chromatin remodeler implicated in catalyzing regression of stalled forks, synergistically promotes not only telomeric recruitment of MUS81 but also the formation of fragile telomeres, the latter of which is reported to arise from fork stalling. These results altogether suggest that CSB-mediated HR repair and SMARCAL1-mediated fork regression cooperate to prevent stalled forks from being processed into fragile telomeres in ALT cells.

Efficient UV repair requires disengagement of the CSB winged helix domain from the CSB ATPase domain

The ATP-dependent chromatin remodeler CSB is implicated in a variety of different DNA repair mechanisms, including transcription-coupled nucleotide excision repair (TC-NER), base excision repair and DNA double strand break (DSB) repair. However, how CSB is regulated in these various repair processes is not well understood. Here we report that the first 30 amino acids of CSB along with two phosphorylation events on S10 and S158, previously reported to be required for CSB function in homologous recombination (HR)-mediated repair, are dispensable for repairing UV-induced DNA damage, suggesting that the regulation of CSB in these two types of repair are carried out by distinct mechanisms. In addition, we show that although the central ATPase domain of CSB is engaged in interactions with both the N- and C-terminal regions, these interactions are disrupted following UV-induced DNA damage. The UV-induced disengagement of the C-terminal region of CSB from the ATPase domain requires two conserved amino acids W1486 and L1488, which are thought to contribute to the hydrophobic core formation of the winged helix domain (WHD) at its C-terminus. Failure to undergo UV-induced dissociation of the C-terminal region of CSB from the ATPase domain is associated with impairment in its UV-induced chromatin association, its UV-induced post-translational modification as well as cell survival. Collectively, these findings suggest that UV-induced dissociation of CSB domain interactions is a necessary step in repairing UV-induced DNA damage and that the WHD of CSB plays a key role in this dissociation.

Decreased expression of TERT correlated with postnatal cochlear development and proliferation reduction of cochlear progenitor cells

Cochlear progenitor cells are considered as one of the best candidates for hair cell regeneration, thus, the regulation of cochlear progenitor cell proliferation has become a focus in this field. Several genes expressed in the inner ear during postnatal development have been demonstrated to be involved in maintaining the proliferative potential of progenitor cells, but the mechanism for regulating the proliferation and differentiation of cochlear progenitor cells remains poorly understood. Telomerase reverse transcriptase (TERT) has rate limiting telomerase activity and the overexpression of TERT has been shown to promote cell proliferation in series of cell lines. The aim of the present study was to evaluate the expression of TERT in the postnatal development of the cochlea and progenitor cells. The results demonstrated that TERT was expressed in the basilar membranes during the first postnatal week. In vitro, TERT expression in progenitor cells reached a maximum at day 4 after culture and decreased as the culture time prolonged or the cell passage number increased. These results led us to hypothesize that TERT may be involved in the development of the cochlea and in maintaining the proliferation ability of progenitor cells.

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

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

ATM and CDK2 control chromatin remodeler CSB to inhibit RIF1 in DSB repair pathway choice

CSB, a member of the SWI2/SNF2 superfamily, is implicated in DNA double-strand break (DSB) repair. However, how it regulates this repair process is poorly understood. Here we uncover that CSB interacts via its newly identified winged helix domain with RIF1, an effector of 53BP1, and that this interaction mediates CSB recruitment to DSBs in S phase. At DSBs, CSB remodels chromatin by evicting histones, which limits RIF1 and its effector MAD2L2 but promotes BRCA1 accumulation. The chromatin remodeling activity of CSB requires not only damage-induced phosphorylation on S10 by ATM but also cell cycle-dependent phosphorylation on S158 by cyclin A-CDK2. Both modifications modulate the interaction of the CSB N-terminal region with its ATPase domain, the activity of which has been previously reported to be autorepressed by the N-terminal region. These results suggest that ATM and CDK2 control the chromatin remodeling activity of CSB in the regulation of DSB repair pathway choice.

Cockayne syndrome group B protein (CSB) is a multifunctional chromatin remodeler involved in double-strand break repair. Here the authors investigate the molecular post-translational signals regulating CSB activity.

Telomere length, ATM mutation status and cancer risk in Ataxia-Telangiectasia families

Recent studies have linked constitutive telomere length (TL) to aging-related diseases including cancer at different sites. ATM participates in the signaling of telomere erosion, and inherited mutations in ATM have been associated with increased risk of cancer, particularly breast cancer. The goal of this study was to investigate whether carriage of an ATM mutation and TL interplay to modify cancer risk in ataxia-telangiectasia (A-T) families.The study population consisted of 284 heterozygous ATM mutation carriers (HetAT) and 174 non-carriers (non-HetAT) from 103 A-T families. Forty-eight HetAT and 14 non-HetAT individuals had cancer, among them 25 HetAT and 6 non-HetAT were diagnosed after blood sample collection. We measured mean TL using a quantitative PCR assay and genotyped seven single-nucleotide polymorphisms (SNPs) recurrently associated with TL in large population-based studies.HetAT individuals were at increased risk of cancer (OR = 2.3, 95%CI = 1.2-4.4, P = 0.01), and particularly of breast cancer for women (OR = 2.9, 95%CI = 1.2-7.1, P = 0.02), in comparison to their non-HetAT relatives. HetAT individuals had longer telomeres than non-HetAT individuals (P = 0.0008) but TL was not associated with cancer risk, and no significant interaction was observed between ATM mutation status and TL. Furthermore, rs9257445 (ZNF311) was associated with TL in HetAT subjects and rs6060627 (BCL2L1) modified cancer risk in HetAT and non-HetAT women.Our findings suggest that carriage of an ATM mutation impacts on the age-related TL shortening and that TL per se is not related to cancer risk in ATM carriers. TL measurement alone is not a good marker for predicting cancer risk in A-T families.

Nek7 Protects Telomeres from Oxidative DNA Damage by Phosphorylation and Stabilization of TRF1

Telomeric repeat binding factor 1 (TRF1) is essential to the maintenance of telomere chromatin structure and integrity. However, how telomere integrity is maintained, especially in response to damage, remains poorly understood. Here, we identify Nek7, a member of the Never in Mitosis Gene A (NIMA) kinase family, as a regulator of telomere integrity. Nek7 is recruited to telomeres and stabilizes TRF1 at telomeres after damage in an ATM activation-dependent manner. Nek7 deficiency leads to telomere aberrations, long-lasting γH2AX and 53BP1 foci, and augmented cell death upon oxidative telomeric DNA damage. Mechanistically, Nek7 interacts with and phosphorylates TRF1 on Ser114, which prevents TRF1 from binding to Fbx4, an Skp1-Cul1-F box E3 ligase subunit, thereby alleviating proteasomal degradation of TRF1, leading to a stable association of TRF1 with Tin2 to form a shelterin complex. Our data reveal a mechanism of efficient protection of telomeres from damage through Nek7-dependent stabilization of TRF1.

TRF1 phosphorylation on T271 modulates telomerase-dependent telomere length maintenance as well as the formation of ALT-associated PML bodies

TRF1, a component of the shelterin complex, plays a key role in both telomerase-dependent telomere maintenance and alternative lengthening of telomeres, the latter also known as ALT. Characteristics of ALT cells include C-circles and ALT-associated PML bodies, referred to as APBs. The function of TRF1 is tightly regulated by post-translational modification including phosphorylation, however TRF1 phosphorylation sites have yet to be fully characterized. Here we report a novel TRF1 phosphorylation site threonine 271. We show that a nonphosphorylatable mutation of T271A impairs TRF1 binding to telomeric DNA in vivo and renders TRF1 defective in inhibiting telomerase-dependent telomere elongation. On the other hand, TRF1 carrying a phosphomimic mutation of T271D is competent in not only binding to telomeric DNA but also inhibiting telomerase-mediated telomere lengthening. These results suggest that TRF1 phosphorylation on T271 negatively regulates telomerase-mediated telomere maintenance. We find that in telomerase-negative ALT cells, TRF1 carrying either a T271A or T271D mutation is able to promote C-circle production but fails to support APB formation. These results suggest that TRF1 phosphorylation on T271 is necessary for APB formation but dispensable for C-circle production. These results further imply that APB formation can be mechanistically separated from C-circle production.

Telomerase Regulation from Beginning to the End

The vast body of literature regarding human telomere maintenance is a true testament to the importance of understanding telomere regulation in both normal and diseased states. In this review, our goal was simple: tell the telomerase story from the biogenesis of its parts to its maturity as a complex and function at its site of action, emphasizing new developments and how they contribute to the foundational knowledge of telomerase and telomere biology.

Cdk-dependent phosphorylation regulates TRF1 recruitment to PML bodies and promotes C-circle production in ALT cells

TRF1, a duplex telomeric DNA binding protein, is implicated in homologous-recombination-based alternative lengthening of telomeres, known as ALT. However, how TRF1 promotes ALT activity has yet to be fully characterized. Here we report that Cdk-dependent TRF1 phosphorylation on T371 acts as a switch to create a pool of TRF1, referred to as (pT371)TRF1, which is recruited to ALT-associated PML bodies (APBs) in S and G2 phases independently of its binding to telomeric DNA. We find that phosphorylation of T371 is essential for APB formation and C-circle production, both of which are hallmarks of ALT. We show that the interaction of (pT371)TRF1 with APBs is dependent upon ATM and homologous-recombination-promoting factors Mre11 and BRCA1. In addition, (pT371)TRF1 interaction with APBs is sensitive to transcription inhibition, which also reduces DNA damage at telomeres. Furthermore, overexpression of RNaseH1 impairs (pT371)TRF1 recruitment to APBs in the presence of campothecin, an inhibitor that prevents topoisomerase I from resolving RNA-DNA hybrids. These results suggest that transcription-associated DNA damage, perhaps arising from processing RNA-DNA hybrids at telomeres, triggers (pT371)TRF1 recruitment to APBs to facilitate ALT activity.

ATM and ATR Signaling Regulate the Recruitment of Human Telomerase to Telomeres

The yeast homologs of the ATM and ATR DNA damage response kinases play key roles in telomerase-mediated telomere maintenance, but the role of ATM/ATR in the mammalian telomerase pathway has been less clear. Here, we demonstrate the requirement for ATM and ATR in the localization of telomerase to telomeres and telomere elongation in immortal human cells. Stalled replication forks increased telomerase recruitment in an ATR-dependent manner. Furthermore, increased telomerase recruitment was observed upon phosphorylation of the shelterin component TRF1 at an ATM/ATR target site (S367). This phosphorylation leads to loss of TRF1 from telomeres and may therefore increase replication fork stalling. ATM and ATR depletion reduced assembly of the telomerase complex, and ATM was required for telomere elongation in cells expressing POT1ΔOB, an allele of POT1 that disrupts telomere-length homeostasis. These data establish that human telomerase recruitment and telomere elongation are modulated by DNA-damage-transducing kinases.

Cockayne syndrome group B protein regulates DNA double-strand break repair and checkpoint activation

Mutations of CSB account for the majority of Cockayne syndrome (CS), a devastating hereditary disorder characterized by physical impairment, neurological degeneration and segmental premature aging. Here we report the generation of a human CSB-knockout cell line. We find that CSB facilitates HR and represses NHEJ. Loss of CSB or a CS-associated CSB mutation abrogating its ATPase activity impairs the recruitment of BRCA1, RPA and Rad51 proteins to damaged chromatin but promotes the formation of 53BP1-Rif1 damage foci in S and G2 cells. Depletion of 53BP1 rescues the formation of BRCA1 damage foci in CSB-knockout cells. In addition, knockout of CSB impairs the ATM- and Chk2-mediated DNA damage responses, promoting a premature entry into mitosis. Furthermore, we show that CSB accumulates at sites of DNA double-strand breaks (DSBs) in a transcription-dependent manner. The kinetics of DSB-induced chromatin association of CSB is distinct from that of its UV-induced chromatin association. These results reveal novel, important functions of CSB in regulating the DNA DSB repair pathway choice as well as G2/M checkpoint activation.

The influence of the telomere-telomerase system on diabetes mellitus and its vascular complications

INTRODUCTION

The telomere-telomerase system plays an important role in the pathogenesis and disease progression of diabetes mellitus as well as in its vascular complications. Recent studies suggest that telomere shortening and abnormal telomerase activity occur in patients with diabetes mellitus, and targeting the telomere-telomerase system has become a prospective treatment for diabetes mellitus and its vascular complications. This review highlights the significance of the telomere-telomerase system and supports its role as a possible therapeutic target for patients with diabetes mellitus and its vascular complications Areas covered: This review covers the advances in understanding the telomere-telomerase system over the last 30 years and its significance in diabetes mellitus. In addition, it provides knowledge regarding the significance of the telomere-telomerase system in diabetes mellitus and its vascular complications as well as its role and mechanisms in oxidative stress, cell therapy and antioxidant activity Expert opinion: The telomere-telomerase system may be a potential therapeutic target that can protect against DNA damage and apoptosis in patients with diabetes mellitus and its vascular complications. DNA damage and apoptosis are associated with oxidative stress and are involved in the dysfunction of pancreatic β cells, insulin resistance, and its vascular complications. Abnormalities in the telomere-telomerase system may be associated with diabetes mellitus and its vascular complications. Therapies targeting telomere-telomerase system, telomerase reverse transcriptase transfection and alterative telomere lengthening must be identified before gene therapy can commence.

Methylated TRF2 associates with the nuclear matrix and serves as a potential biomarker for cellular senescence

Methylation of N-terminal arginines of the shelterin component TRF2 is important for cellular proliferation. While TRF2 is found at telomeres, where it plays an essential role in maintaining telomere integrity, little is known about the cellular localization of methylated TRF2. Here we report that the majority of methylated TRF2 is resistant to extraction by high salt buffer and DNase I treatment, indicating that methylated TRF2 is tightly associated with the nuclear matrix. We show that methylated TRF2 drastically alters its nuclear staining as normal human primary fibroblast cells approach and enter replicative senescence. This altered nuclear staining, which is found to be overwhelmingly associated with misshapen nuclei and abnormal nuclear matrix folds, can be suppressed by hTERT and it is barely detectable in transformed and cancer cell lines. We find that dysfunctional telomeres and DNA damage, both of which are potent inducers of cellular senescence, promote the altered nuclear staining of methylated TRF2, which is dependent upon the ATM-mediated DNA damage response. Collectively, these results suggest that the altered nuclear staining of methylated TRF2 may represent ATM-mediated nuclear structural alteration associated with cellular senescence. Our data further imply that methylated TRF2 can serve as a potential biomarker for cellular senescence.

Ameliorating replicative senescence of human bone marrow stromal cells by PSMB5 overexpression

Multipotent human bone marrow stromal cells (hBMSCs) potentially serve as a source for cell-based therapy in regenerative medicine. However, in vitro expansion was inescapably accompanied with cell senescence, characterized by inhibited proliferation and compromised pluripotency. We have previously demonstrated that this aging process is closely associated with reduced 20S proteasomal activity, with down-regulation of rate-limiting catalytic β-subunits particularly evident. In the present study, we confirmed that proteasomal activity directly contributes to senescence of hBMSCs, which could be reversed by overexpression of the β5-subunit (PSMB5). Knocking down PSMB5 led to decreased proteasomal activity concurrent with reduced cell proliferation in early-stage hBMSCs, which is similar to the senescent phenotype observed in late-stage cells. In contrast, overexpressing PSMB5 in late-stage cells efficiently restored the normal activity of 20S proteasomes and promoted cell growth, possibly via upregulating the Cyclin D1/CDK4 complex. Additionally, PSMB5 could enhance cell resistance to oxidative stress, as evidenced by the increased cell survival upon exposing senescent hBMSCs to hydrogen peroxide. Furthermore, PSMB5 overexpression retained the pluripotency of late-stage hBMSCs by facilitating their neural differentiation both in vitro and in vivo. Collectively, our work reveals a critical role of PSMB5 in 20S proteasome-mediated protection against replicative senescence, pointing to a possible strategy for maintaining the integrity of culture-expanded hBMSCs by manipulating the expression of PSMB5.

Phosphorylated (pT371)TRF1 is recruited to sites of DNA damage to facilitate homologous recombination and checkpoint activation

TRF1, a duplex telomeric DNA-binding protein, plays an important role in telomere metabolism. We have previously reported that a fraction of endogenous TRF1 can stably exist free of telomere chromatin when it is phosphorylated at T371 by Cdk1; however, the role of this telomere-free (pT371)TRF1 has yet to be fully characterized. Here we show that phosphorylated (pT371)TRF1 is recruited to sites of DNA damage, forming damage-induced foci in response to ionizing radiation (IR), etoposide and camptothecin. We find that IR-induced (pT371)TRF1 foci formation is dependent on the ATM- and Mre11/Rad50/Nbs1-mediated DNA damage response. While loss of functional BRCA1 impairs the formation of IR-induced (pT371)TRF1 foci, depletion of either 53BP1 or Rif1 stimulates IR-induced (pT371)TRF1 foci formation. In addition, we show that TRF1 depletion or the lack of its phosphorylation at T371 impairs DNA end resection and repair of nontelomeric DNA double-strand breaks by homologous recombination. The lack of TRF1 phosphorylation at T371 also hampers the activation of the G2/M checkpoint and sensitizes cells to PARP inhibition, IR and camptothecin. Collectively, these results reveal a novel but important function of phosphorylated (pT371)TRF1 in facilitating DNA double-strand break repair and the maintenance of genome integrity.

DNA repair at telomeres: keeping the ends intact

The molecular era of telomere biology began with the discovery that telomeres usually consist of G-rich simple repeats and end with 3' single-stranded tails. Enormous progress has been made in identifying the mechanisms that maintain and replenish telomeric DNA and the proteins that protect them from degradation, fusions, and checkpoint activation. Although telomeres in different organisms (or even in the same organism under different conditions) are maintained by different mechanisms, the disparate processes have the common goals of repairing defects caused by semiconservative replication through G-rich DNA, countering the shortening caused by incomplete replication, and postreplication regeneration of G tails. In addition, standard DNA repair mechanisms must be suppressed or modified at telomeres to prevent their being recognized and processed as DNA double-strand breaks. Here, we discuss the players and processes that maintain and regenerate telomere structure.

Telomere, aging and age-related diseases

Aging is an inevitable biological process that affects most living organisms. The process of aging is regulated at the level of the organism, as well as at the level of tissues and cells. Despite the enormous consequences associated with the aging process, relatively little systematic effort has been expended on the scientific understanding of this important life process. Many theories have been proposed to explain the aging process, the centerpiece of which is molecular damage. Located at the ends of eukaryotic chromosomes and synthesized by telomerase, telomeres maintain the stabilization of chromosomes. Thus, the loss of telomeres may lead to DNA damage. The relationship between cellular senescence and telomere shortening is well established. Furthermore, telomere attrition occurs with age, and is proposed to be a fundamental factor in the aging process. Here, we review the contemporary literatures to explore the current views on the correlation of telomere loss and telomerase action with aging and age-related diseases.

ATM deficiency results in accumulation of DNA-topoisomerase I covalent intermediates in neural cells

Accumulation of peptide-linked DNA breaks contributes to neurodegeration in humans. This is typified by defects in tyrosyl DNA phosphodiesterase 1 (TDP1) and human hereditary ataxia. TDP1 primarily operates at single-strand breaks (SSBs) created by oxidative stress or by collision of transcription machinery with topoisomerase I intermediates (Top1-CCs). Cellular and cell-free studies have shown that Top1 at stalled Top1-CCs is first degraded to a small peptide resulting in Top1-SSBs, which are the primary substrates for TDP1. Here we established an assay to directly compare Top1-SSBs and Top1-CCs. We subsequently employed this assay to reveal an increased steady state level of Top1-CCs in neural cells lacking Atm; the protein mutated in ataxia telangiectasia. Our data suggest that the accumulation of endogenous Top1-CCs in Atm-/- neural cells is primarily due to elevated levels of reactive oxygen species. Biochemical purification of Top1-CCs from neural cell extract and the use of Top1 poisons further confirmed a role for Atm during the formation/resolution of Top1-CCs. Finally, we report that global transcription is reduced in Atm-/- neural cells and fails to recover to normal levels following Top1-mediated DNA damage. Together, these data identify a distinct role for ATM during the formation/resolution of neural Top1-CCs and suggest that their accumulation contributes to the neuropathology of ataxia telangiectasia.

Chromatin structure in telomere dynamics

The establishment of a specific nucleoprotein structure, the telomere, is required to ensure the protection of chromosome ends from being recognized as DNA damage sites. Telomere shortening below a critical length triggers a DNA damage response that leads to replicative senescence. In normal human somatic cells, characterized by telomere shortening with each cell division, telomere uncapping is a regulated process associated with cell turnover. Nevertheless, telomere dysfunction has also been associated with genomic instability, cell transformation, and cancer. Despite the essential role telomeres play in chromosome protection and in tumorigenesis, our knowledge of the chromatin structure involved in telomere maintenance is still limited. Here we review the recent findings on chromatin modifications associated with the dynamic changes of telomeres from protected to deprotected state and their role in telomere functions.

Telomerase activity and telomere length distribution in vascular endothelial cells in a short-term culture under the presence of hydrogen peroxide

AIM

The aim of this study was to assess the biological effects of oxidative stress on human vascular endothelial cells.

METHODS

The telomeric changes and the alterations of the expression of telomere-associated proteins in human umbilical venous endothelial cells (HUVEC) cultured in the presence of hydrogen peroxide (H2 O2 ) were analyzed.

RESULTS

During the culture, the cell growth rate decreased, whereas the telomerase activity of the surviving cells increased. As the H2 O2 level increased, long telomeres decreased proportionally, thus resulting in a telomere length distribution that was rich in short telomeres. These observations suggested that H2 O2 -affected endothelial cells bear telomeric features similar to those of aged cells. In contrast, the expression of telomere-associated proteins, TRF1 and TRF2, showed different changes. TRF1 increased in relation to H2 O2 concentration, whereas TRF2 showed no significant change. The surviving cells exposed to H2 O2 showed a H2 O2 -dose dependent increase in telomerase activity, whereas the telomere protein and RNA components were only elevated in low concentrations of H2 O2 .

CONCLUSIONS

The increase in telomerase activity and TRF1 protein expression of vascular endothelial cell might show an aspect of cellular protective reaction against oxygen stress.

Cockayne Syndrome group B protein interacts with TRF2 and regulates telomere length and stability

The majority of Cockayne syndrome (CS) patients carry a mutation in Cockayne Syndrome group B (CSB), a large nuclear protein implicated in DNA repair, transcription and chromatin remodeling. However, whether CSB may play a role in telomere metabolism has not yet been characterized. Here, we report that CSB physically interacts with TRF2, a duplex telomeric DNA binding protein essential for telomere protection. We find that CSB localizes at a small subset of human telomeres and that it is required for preventing the formation of telomere dysfunction-induced foci (TIF) in CS cells. We find that CS cells or CSB knockdown cells accumulate telomere doublets, the suppression of which requires CSB. We find that overexpression of CSB in CS cells promotes telomerase-dependent telomere lengthening, a phenotype that is associated with a decrease in the amount of telomere-bound TRF1, a negative mediator of telomere length maintenance. Furthermore, we show that CS cells or CSB knockdown cells exhibit misregulation of TERRA, a large non-coding telomere repeat-containing RNA important for telomere maintenance. Taken together, these results suggest that CSB is required for maintaining the homeostatic level of TERRA, telomere length and integrity. These results further imply that CS patients carrying CSB mutations may be defective in telomere maintenance.

Post-translational modifications of TRF1 and TRF2 and their roles in telomere maintenance

Telomeres, heterochromatic structures, found at the ends of linear eukaryotic chromosomes, function to protect natural chromosome ends from nucleolytic attack. Human telomeric DNA is bound by a telomere-specific six-subunit protein complex, termed shelterin/telosome. The shelterin subunits TRF1 and TRF2 bind in a sequence-specific manner to double-stranded telomeric DNA, providing a vital platform for recruitment of additional shelterin proteins as well as non-shelterin factors crucial for the maintenance of telomere length and structure. Both TRF1 and TRF2 are engaged in multiple roles at telomeres including telomere protection, telomere replication, sister telomere resolution and telomere length maintenance. Regulation of TRF1 and TRF2 in these various processes is controlled by post-translational modifications, at times in a cell-cycle-dependent manner, affecting key functions such as DNA binding, dimerization, localization, degradation and interactions with other proteins. Here we review the post-translational modifications of TRF1 and TRF2 and discuss the mechanisms by which these modifications contribute to the function of these two proteins.

Posttranslational control of telomere maintenance and the telomere damage response

Telomeres help maintain genome integrity by protecting natural chromosome ends from being recognized as damaged DNA. When telomeres become dysfunctional, they limit replicative lifespan and prevent outgrowth of potentially cancerous cells by activating a DNA damage response that forces cells into senescence or apoptosis. On the other hand, chromosome ends devoid of proper telomere protection are subject to DNA repair activities that cause end-to-end fusions and, when cells divide, extensive genomic instability that can promote cancer. While telomeres represent unique chromatin structures with important roles in cancer and aging, we have limited understanding of the way telomeres and the response to their malfunction are controlled at the level of chromatin. Accumulating evidence indicates that different types of posttranslational modifications act in both telomere maintenance and the response to telomere uncapping. Here, we discuss the latest insights on posttranslational control of telomeric chromatin, with emphasis on ubiquitylation and SUMOylation events.