Akt regulates TPP1 homodimerization and telomere protection

A Novel Role of Prostate-Specific Membrane Antigen in Telomere Stability in Prostate Cancer Cells

Prostate-specific membrane antigen (PSMA) expression increases with prostate cancer grade and progression; however, the role of PSMA in prostate cancer progression remains poorly understood. Telomere stability is essential for the survival and genome stability of cancer cells. We found massive telomere DNA damage in PSMA-negative prostate cancer cells (PC-3 and DU145) compared with PSMA-positive prostate cancer (LNCaP) cells. The ectopic expression of PSMA suppressed telomere DNA damage in PC3 cells. PSMA inhibitor, 2-PMPA, and PSMA knockdown induced telomere DNA damage in PSMA-positive LNCaP cells but not in PSMA-negative PC-3 cells, suggesting that PSMA plays a critical role in telomere stability in prostate cancer cells. In addition, we observed that inhibition of PSMA or inhibition of glutamate receptor, which mediates PSMA-dependent activation of AKT, suppressed AKT phosphorylation, and caused telomere DNA damage. Furthermore, 2-PMPA-induced telomere DNA damage in LNCaP cells was associated with telomere aberrations, such as telomere-telomere fusions, sister-chromatid telomere fusions, and telomere breakages. AKT is reported to promote cell growth by stabilizing telomere association with telomere-binding proteins TRF1 and TPP1. We observed that TRF1 and TPP1 transfection of LNCaP cells attenuated the inhibitory effect of 2-PMPA on cell growth and telomere DNA damage. Together, these observations indicate that PSMA role in maintaining telomere stability in prostate cancer cells is mediated by AKT. Thus, these studies reveal an important role of PSMA in maintaining telomere stability that can promote cell survival and, thereby, prostate cancer progression.

IMPLICATIONS

Role of PSMA in telomere stability suggests a strong correlation between PSMA expression and prostate cancer progression.

The regulatory role of PI3K in ageing-related diseases

Ageing is a physiological/pathological process accompanied by the progressive damage of cell function, triggering various ageing-related disorders. Phosphatidylinositol 3-kinase (PI3K), which serves as one of the central regulators of ageing, is closely associated with cellular characteristics or molecular features, such as genome instability, telomere erosion, epigenetic alterations, and mitochondrial dysfunction. In this review, the PI3K signalling pathway was firstly thoroughly explained. The link between ageing pathogenesis and the PI3K signalling pathway was then summarized. Finally, the key regulatory roles of PI3K in ageing-related illnesses were investigated and stressed. In summary, we revealed that drug development and clinical application targeting PI3K is one of the focal points for delaying ageing and treating ageing-related diseases in the future.

TPP1 Inhibits DNA Damage Response and Chemosensitivity in Esophageal Cancer

TPP1, as one of the telomere-protective protein complex, functions to maintain telomere stability. In this study, we found that TPP1 was significantly upregulated in esophageal cancer (EC). We found that the proliferation and migration ability were significantly inhibited, while the results of flow cytometry assay indicated that the growth was hindered in the G1 phase after TPP1 knockdown. However, the proliferative viability and migratory ability were reversed after TPP1 overexpression in EC cells. Then, we found a significant increase in β-galactosidase positivity following TPP1 knockdown and the opposite following TPP1 overexpression in EC cells. Furthermore, TPP1 knockdown increased DNA damage and upregulated expression of the γ-H2AXS139 in the cell nucleus. Correspondingly, DNA damage was reversed after TPP1 overexpression in EC cells. Similarly, we found that the expression of ATM/ATR pathway proteins were upregulated after TPP1 knockdown, while the expression of the above proteins was downregulated after TPP1 overexpression in EC cells. TPP1 knockdown significantly inhibited the growth of transplanted tumors and upregulated the expression of ATM/ATR pathway proteins in transplanted tissues, whereas TPP1 overexpression significantly promoted their proliferation and downregulated the expression of the above proteins in vivo. Strikingly, we found that TPP1 could reduce the chemosensitivity of EC cells to cisplatin, which may have a potential link to clinical chemoresistance. In conclusion, TPP1 regulates the DNA damage response through the ATM/ATR-p53 signaling pathway and chemoresistance and may be a new target for improving the efficacy of chemotherapy in the treatment of EC.

Deleting ku80 improves the efficiency of targeted gene editing in Neospora caninum

CRISPR/Cas9 technology has been widely used for gene editing in organisms. Gene deletion of the ku80/ku70 complex can improve the efficiency of gene replacement in Arabidopsis thaliana, Cryptococcus neoformans, and Toxoplasma gondii, which remained elusive in Neospora caninum. Here, we knock out the ku80 gene in Nc1 strain by using CRISPR/Cas9, detect the growth rate and virulence of NcΔku80. Then we compare the efficiency of gene replacements between NcΔku80 and Nc1 strains by transfected with the same HA-tagged plasmids, and the percentage of HA-tagged parasites was investigated by IFA. The results showed that gene targeting efficiency was increased in the NcΔku80 strain via double crossover at several genetic loci, but its growth rate and virulence were unaffected. In conclusion, the NcΔku80 strain can be used as an effective strain for rapid gene editing of N. caninum.

Molecular insight of dyskeratosis congenita: Defects in telomere length homeostasis

BACKGROUND

Dyskeratosis congenita (DC) is a rare disease and is a heterogenous disorder, with its inheritance patterns as autosomal dominant, autosomal recessive, and X-linked recessive. This disorder occurs due to faulty maintenance of telomeres in stem cells. This congenital condition is diagnosed with three symptoms: oral leukoplakia, nail dystrophy, and abnormal skin pigmentation. However, because it has a wide range of symptoms, it may have phenotypes similar to other diseases. For this reason, it is necessary to use methods of measuring the Telomere Length (TL) and determining the shortness of the telomere in these patients so that it can be distinguished from other diseases. Today, the Next Generation Sequencing technique accurately detects mutations in the target genes.

AIM

This work aims to review and summarize how each of the DC genes is involved in TL, and how to diagnose and differentiate the disease using clinical signs and methods to measure TL. It also offers treatments for DC patients, such as Hematopoietic Stem Cell Transplantation and Androgen therapy.

RELEVANCE FOR PATIENTS

In DC patients, the genes involved in telomere homeostasis are mutated. Because these patients may have an overlapping phenotype with other diseases, it is best to perform whole-exome sequencing after genetics counseling to find the relevant mutation. As DC is a multi-systemic disease, we need to monitor patients frequently through annual lung function tests, ultrasounds, gynecological examinations, and skin examinations.

[Integrin α3β1 signaling in regulation of the SK-Mel-147 melanoma cell senescence]

Using a model of the human SK-Mel-147 melanoma cell line, it was shown that blocking the expression of integrin α3β1 by transduction of cells with α3-specific shRNA did not affect their proliferation, but sharply increased the proportion of SA-β-Gal-positive cells, a phenotypic feature of cell senescence. These findings were accompanied by a significant increase in the activity of the Akt and mTOR protein kinases and the expression of p53 and p21 oncosupressors. Pharmacological inhibition of mTORC1 reduced the number SA-β-Gal-positive cells in the SK-Mel-147 cell population depleted of α3β1. Based on our recent data on a non-canonical function of Akt isomers in the regulation of SK-Mel-147 cell senescence caused by deficiency of α2β1 receptor, we investigated the role of Akt isomers in senescence induced by the α3β1 knockdown. It appeared that in the cell population with downregulated α3β1, inhibition of Akt1 reduced the number SA-β-Gal positive cells to the level of control cell population, while inhibition of Akt2 had no visible effect. Our results demonstrate that the laminin-specific integrin α3β1, like the collagen-specific receptor α2β1, is involved in tumor cell protection from senescence, and senescence induced by α3β1 depletion, like that caused by α2β1 deficiency, is based on a signaling mechanism employing a non-canonical function of the Akt1 isoform.

Implication of integrin α2β1 in senescence of SK-Mel-147 human melanoma cells

This investigation addressed the impact of integrin-initiated signaling pathways on senescence of tumor cells. In a model of human SK-Mel-147 melanoma cells, the silencing of integrin α2β1 strongly reduced cell proliferation and enhanced the percentage of SA-β-Gal-positive cells, a phenotypic feature of cellular senescence. These changes were accompanied by a significant increase in the activity of Akt and mTOR protein kinases and also in the expression of p53 and p21 oncosuppressors. Pharmacological inhibition of Akt and mTORC1 and genetic inhibition of p53 and p21 reduced the senescence of α2β1-depleted SK-Mel-147 cells to the level of control cells. Based on our earlier data on the non-canonical functions of Akt isomers in the invasion and anoikis of SK-Mel-147 cells, we investigated the role of Akt isomers in senescence induced by α2β1 suppression. The inhibition of Akt1 strongly reduced the percentage of SA-β-Gal-positive cells in the α2β1-depleted cell population, while the inhibition of Akt2 did not have a noticeable effect. Our data demonstrated for the first time that α2β1 is involved in the protection of tumor cells against senescence and that senescence, which is induced by the downregulation of α2β, is based on a signaling mechanism in which Akt1 performs a non-canonical function.

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.

TPP1 Enhances the Therapeutic Effects of Transplanted Aged Mesenchymal Stem Cells in Infarcted Hearts via the MRE11/AKT Pathway

Background

Poor cell survival after transplantation restricts the therapeutic potential of mesenchymal stem cell (MSC) transplantation into infarcted hearts, particularly in older individuals. TPP1, a component of the shelterin complex that is involved in telomere protection, is highly expressed in young MSCs but declines in aged ones. Here, we explore whether TPP1 overexpression in aged mouse MSCs improves cell viability in vivo and in vitro.

Methods

Aged mouse MSCs overexpressing TPP1 were injected into the peri-infarct area of the mouse heart after left anterior descending coronary artery ligation. In parallel, to evaluate cellular-level effects, H₂O₂ was applied to MSCs in vitro to mimic the microenvironment of myocardial injury.

Results

In vivo, the transplantation of aged MSCs overexpressing TPP1 resulted in improved cell survival, enhanced cardiac function, and reduced fibrosis compared to unmodified aged MSCs. In vitro, TPP1 overexpression protected aged MSCs from H₂O₂-induced apoptosis and enhanced DNA double-strand break (DSB) repair. In addition, the phosphorylation of AKT and the key DSB repair protein MRE11 were both significantly upregulated in aged MSCs that overexpressed TPP1.

Conclusions

Our results reveal that TPP1 can enhance DNA repair through the AKT/MRE11 pathway, thereby improving the therapeutic effects of aged MSC transplantation and offering significant potential for the clinical application of autologous transplantation in aged patients.

Internalization of the TAT-PBX1 fusion protein significantly enhances the proliferation of human hair follicle-derived mesenchymal stem cells and delays their senescence

OBJECTIVES

To express a TAT-PBX1 fusion protein using a prokaryotic expression system and to explore potential effects of TAT-PBX1 in the proliferation and senescence of human hair follicle-derived mesenchymal stem cells.

RESULTS

The TAT-PBX1 fusion was produced in inclusion bodies and heterogenously expressed in Rosetta (DE3) cells. Immunofluorescence staining showed that TAT-PBX1 fusion proteins were internalized by human hair follicle-derived mesenchymal stem cells. The growth rate of cells was increased after treatment with more than 5.0 μg/mL of TAT-PBX1. The rate of senescence-associated β-galactosidase positive cells was reduced in the 10.0 μg/mL TAT-PBX1 group (28%) than the 0 μg/mL control group (60%). Cells treated with the TAT-PBX1 fusion protein showed higher expression of p-AKT (1.22-fold that of the control), which indicates that TAT-PBX1 activated AKT pathway after cellular uptake.

CONCLUSIONS

The TAT-PBX1 fusion protein increased the proliferation of hair follicle mesenchymal stem cells and delayed their senescence by activating the AKT pathway following internalization by cells.

Shelterin Complex at Telomeres: Implications in Ageing

Different factors influence the development‎ and control of ageing. It is well known that progressive telomere shorting is one of the molecular mechanisms underlying ageing. The shelterin complex consists of six telomere-specific proteins which are involved in the protection of chromosome ends. More particularly, this vital complex protects the telomeres from degradation, prevents from activation of unwanted repair systems, regulates the activity of telomerase, and has a crucial role in cellular senescent and ageing-related pathologies. This review explores the organization and function of telomeric DNA along with the mechanism of telomeres during ageing, followed by a discussion of the critical role of shelterin components and their changes during ageing.

NANOG Attenuates Hair Follicle-Derived Mesenchymal Stem Cell Senescence by Upregulating PBX1 and Activating AKT Signaling

Stem cells derived from elderly donors or harvested by repeated subculture exhibit a marked decrease in proliferative capacity and multipotency, which not only compromises their therapeutic potential but also raises safety concerns for regenerative medicine. NANOG-a well-known core transcription factor-plays an important role in maintaining the self-renewal and pluripotency of stem cells. Unfortunately, the mechanism that NANOG delays mesenchymal stem cell (MSC) senescence is not well-known until now. In our study, we showed that both ectopic NANOG expression and PBX1 overexpression (i) significantly upregulated phosphorylated AKT (p-AKT) and PARP1; (ii) promoted cell proliferation, cell cycle progression, and osteogenesis; (iii) reduced the number of senescence-associated-β-galactosidase- (SA-β-gal-) positive cells; and (iv) downregulated the expression of p16, p53, and p21. Western blotting and dual-luciferase activity assays showed that ectopic NANOG expression significantly upregulated PBX1 expression and increased PBX1 promoter activity. In contrast, PBX1 knockdown by RNA interference in hair follicle- (HF-) derived MSCs that were ectopically expressing NANOG resulted in the significant downregulation of p-AKT and the upregulation of p16 and p21. Moreover, blocking AKT with the PI3K/AKT inhibitor LY294002 or knocking down AKT via RNA interference significantly decreased PBX1 expression, while increasing p16 and p21 expression and the number of SA-β-gal-positive cells. In conclusion, our findings show that NANOG delays HF-MSC senescence by upregulating PBX1 and activating AKT signaling and that a feedback loop likely exists between PBX1 and AKT signaling.

Spatial Organization and Molecular Interactions of the Schizosaccharomyces pombe Ccq1-Tpz1-Poz1 Shelterin Complex

The shelterin complex is a macromolecular assembly of proteins that binds to and protects telomeric DNA, which composes the ends of all linear chromosomes. Shelterin proteins prevent chromosome ends from fusing together and from eliciting erroneous induction of DNA damage response pathways. In addition, shelterin proteins play key roles in regulating the recruitment and activation of telomerase, an enzyme that extends telomeric DNA. In fission yeast, Schizosaccharomyces pombe, interactions between the shelterin proteins Ccq1, Tpz1, and Poz1 are important for regulating telomerase-mediated telomere synthesis and thus telomere length homeostasis. Here, we used electron microscopy combined with genetic labeling to define the three-dimensional arrangement of the S. pombe Ccq1-Tpz1-Poz1 (CTP) complex. Crosslinking mass spectrometry was used to identify individual residues that are in proximity to the protein-protein interfaces of the assembled CTP complex. Together, our data provide a first glimpse into the architectural design of the CTP complex and reveals unique interactions that are important in maintaining the S. pombe telomere in a non-extendible state.

Shelterin Telomere Protection Protein 1 Reduction Causes Telomere Attrition and Cellular Senescence via Sirtuin 1 Deacetylase in Chronic Obstructive Pulmonary Disease

Lung cellular senescence and inflammatory response are the key events in the pathogenesis of chronic obstructive pulmonary disease (COPD) when cigarette smoke (CS) is the main etiological factor. Telomere dysfunction is induced by either critical shortening or disruption of the shelterin complex, leading to cellular senescence. However, it remains unknown whether disruption of the shelterin complex is responsible for CS-induced lung cellular senescence. Here we show that telomere protection protein 1 (TPP1) levels are reduced on telomeres in lungs from mice with emphysema, as well as in lungs from smokers and from patients with COPD. This is associated with persistent telomeric DNA damage, leading to cellular senescence. CS disrupts the interaction of TPP1 with the Sirtuin 1 (Sirt1) complex, leading to increased TPP1 acetylation and degradation. Lung fibroblasts deficient in Sirt1 or treated with a selective Sirt1 inhibitor exhibit increased cellular senescence and decreased TPP1 levels, whereas Sirt1 overexpression and pharmacological activation protect against CS-induced TPP1 reduction and telomeric DNA damage. Our findings support an essential role of TPP1 in protecting CS-induced telomeric DNA damage and cellular senescence, and therefore provide a rationale for a potential therapy for COPD, on the basis of the shelterin complex, in attenuating cellular senescence.

Structural and functional consequences of a disease mutation in the telomere protein TPP1

Telomerase replicates chromosome ends to facilitate continued cell division. Mutations that compromise telomerase function result in stem cell failure diseases, such as dyskeratosis congenita (DC). One such mutation (K170Δ), residing in the telomerase-recruitment factor TPP1, provides an excellent opportunity to structurally, biochemically, and genetically dissect the mechanism of such diseases. We show through site-directed mutagenesis and X-ray crystallography that this TPP1 disease mutation deforms the conformation of two critical amino acids of the TEL [TPP1's glutamate (E) and leucine-rich (L)] patch, the surface of TPP1 that binds telomerase. Using CRISPR-Cas9 technology, we demonstrate that introduction of this mutation in a heterozygous manner is sufficient to shorten telomeres in human cells. Our findings rule out dominant-negative effects of the mutation. Instead, these findings implicate reduced TEL patch dosage in causing telomere shortening. Our studies provide mechanistic insight into telomerase-deficiency diseases and encourage the development of gene therapies to counter such diseases.

Shelterin proteins and cancer

The telomeric end structures of the DNA are known to contain tandem repeats of TTAGGG sequence bound with specialised protein complex called the "shelterin complex". It comprises six proteins, namely TRF1, TRF2, TIN2, POT1, TPP1 and RAP1. All of these assemble together to form a complex with double strand and single strand DNA repeats at the telomere. Such an association contributes to telomere stability and its protection from undesirable DNA damage control-specific responses. However, any alteration in the structure and function of any of these proteins may lead to undesirable DNA damage responses and thus cellular senescence and death. In our review, we throw light on how mutations in the proteins belonging to the shelterin complex may lead to various malfunctions and ultimately have a role in tumorigenesis and cancer progression.

CRISPR-Cas9 delivery to hard-to-transfect cells via membrane deformation

The CRISPR (clustered regularly interspaced short palindromic repeats)-Cas (CRISPR-associated) nuclease system represents an efficient tool for genome editing and gene function analysis. It consists of two components: single-guide RNA (sgRNA) and the enzyme Cas9. Typical sgRNA and Cas9 intracellular delivery techniques are limited by their reliance on cell type and exogenous materials as well as their toxic effects on cells (for example, electroporation). We introduce and optimize a microfluidic membrane deformation method to deliver sgRNA and Cas9 into different cell types and achieve successful genome editing. This approach uses rapid cell mechanical deformation to generate transient membrane holes to enable delivery of biomaterials in the medium. We achieved high delivery efficiency of different macromolecules into different cell types, including hard-to-transfect lymphoma cells and embryonic stem cells, while maintaining high cell viability. With the advantages of broad applicability across different cell types, particularly hard-to-transfect cells, and flexibility of application, this method could potentially enable new avenues of biomedical research and gene targeting therapy such as mutation correction of disease genes through combination of the CRISPR-Cas9-mediated knockin system.

TeloPIN: a database of telomeric proteins interaction network in mammalian cells

Interaction network surrounding telomeres has been intensively studied during the past two decades. However, no specific resource by integrating telomere interaction information data is currently available. To facilitate the understanding of the molecular interaction network by which telomeres are associated with biological process and diseases, we have developed TeloPIN (Telomeric Proteins Interaction Network) database (http://songyanglab.sysu.edu.cn/telopin/), a novel database that points to provide comprehensive information on protein–protein, protein–DNA and protein–RNA interaction of telomeres. TeloPIN database contains four types of interaction data, including (i) protein–protein interaction (PPI) data, (ii) telomeric proteins ChIP-seq data, (iii) telomere-associated proteins data and (iv) telomeric repeat-containing RNAs (TERRA)-interacting proteins data. By analyzing these four types of interaction data, we found that 358 and 199 proteins have more than one type of interaction information in human and mouse cells, respectively. We also developed table browser and TeloChIP genome browser to help researchers with better integrated visualization of interaction data from different studies. The current release of TeloPIN database includes 1111 PPI, eight telomeric protein ChIP-seq data sets, 1391 telomere-associated proteins and 183 TERRA-interacting proteins from 92 independent studies in mammalian cells. The interaction information provided by TeloPIN database will greatly expand our knowledge of telomeric proteins interaction network.

Database URL: TeloPIN database address is http://songyanglab.sysu.edu.cn/telopin. TeloPIN database is freely available to non-commercial use.

Current aging research in China

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

Genetic and molecular identification of three human TPP1 functions in telomerase action: recruitment, activation, and homeostasis set point regulation

Telomere length homeostasis is essential for the long-term survival of stem cells, and its set point determines the proliferative capacity of differentiated cell lineages by restricting the reservoir of telomeric repeats. Knockdown and overexpression studies in human tumor cells showed that the shelterin subunit TPP1 recruits telomerase to telomeres through a region termed the TEL patch. However, these studies do not resolve whether the TPP1 TEL patch is the only mechanism for telomerase recruitment and whether telomerase regulation studied in tumor cells is representative of nontransformed cells such as stem cells. Using genome engineering of human embryonic stem cells, which have physiological telomere length homeostasis, we establish that the TPP1 TEL patch is genetically essential for telomere elongation and thus long-term cell viability. Furthermore, genetic bypass, protein fusion, and intragenic complementation assays define two distinct additional mechanisms of TPP1 involvement in telomerase action at telomeres. We demonstrate that TPP1 provides an essential step of telomerase activation as well as feedback regulation of telomerase by telomere length, which is necessary to determine the appropriate telomere length set point in human embryonic stem cells. These studies reveal and resolve multiple TPP1 roles in telomere elongation and stem cell telomere length homeostasis.

The telomeric protein Pot1 from Schizosaccharomyces pombe binds ssDNA in two modes with differing 3' end availability

Telomere protection and length regulation are important processes for aging, cancer and several other diseases. At the heart of these processes lies the single-stranded DNA (ssDNA)-binding protein Pot1, a component of the telomere maintenance complex shelterin, which is present in species ranging from fission yeast to humans. Pot1 contains a dual OB-fold DNA-binding domain (DBD) that fully confers its high affinity for telomeric ssDNA. Studies of S. pombe Pot1-DBD and its individual OB-fold domains revealed a complex non-additive behavior of the two OB-folds in the context of the complete Pot1 protein. This behavior includes the use of multiple distinct binding modes and an ability to form higher order complexes. Here we use NMR and biochemical techniques to investigate the structural features of the complete Pot1-DBD. These experiments reveal one binding mode characterized by only subtle alternations to the individual OB-fold subdomain structures, resulting in an inaccessible 3′ end of the ssDNA. The second binding mode, which has equivalent affinity, interacts differently with the 3′ end, rendering it available for interaction with other proteins. These findings suggest a structural switch that contributes to telomere end-protection and length regulation.