Telomere Dysfunctional Mice Research Articles

C/EBP homologous protein deficiency enhances hematopoietic stem cell function via reducing ATF3/ROS-induced cell apoptosis.

Hematopoietic stem cells (HSCs) reside in a quiescent niche to reserve their capacity of self‐renewal. Upon hematopoietic injuries, HSCs enter the cell cycle and encounter protein homeostasis problems caused by accumulation of misfolded proteins. However, the mechanism by which protein homeostasis influences HSC function and maintenance remains poorly understood. Here, we show that C/EBP homologous protein (CHOP), demonstrated previously to induces cell death upon unfolded protein response (UPR), plays an important role in HSCs regeneration. CHOP−/− mice showed normal hematopoietic stem and progenitor cell frequencies in steady state. However, when treated with 5‐FU, CHOP deficiency resulted in higher survival rates, associated with an increased number of HSCs and reduced level of apoptosis. In serial competitive transplantation experiments, CHOP−/− HSCs showed a dramatic enhancement of repopulation ability and a reduction of protein aggresomes. Mechanistically, CHOP deletion causes reduced ATF3 expression and further leads to decreased protein aggregation and ROS. In addition, CHOP−/− HSCs exhibited an increased resistance to IR‐induced DNA damage and improved HSCs homeostasis and function in telomere dysfunctional (G3Terc −/−) mice. In summary, these findings disclose a new role of CHOP in the regulation of the HSCs function and homeostasis through reducing ATF3 and ROS signaling.

Age-induced NLRP3 Inflammasome Over-activation Increases Lethality of SARS-CoV-2 Pneumonia in Elderly Patients.

Age is one of the most important prognostic factors associated to lethality in SARS-CoV-2 infection. In multivariate analysis, advanced age was an independent risk factor for death. Recent studies suggest a role for the nucleotide-binding domain and leucine rich repeat containing family, pyrin domain containing 3 (NLRP3) inflammasome activation in lung inflammation and fibrosis in SARS-CoV and SARS-CoV-2 infections. Increased NLRP3/ apoptosis-associated speck-like protein (ASC) mRNA expression and increased caspase-1 activity, have been observed in aged lung, provoking increased and heightened expression levels of mature Interleukin (IL)-1β and IL-18 in aged individuals. Aged individuals have a basal predisposition to over-react to infection, displaying an increased hyper-inflammatory cascade, that seems not to be fully physiologically controlled. NLRP3 inflammasome is over-activated in aged individuals, through deficient mitochondrial functioning, increased mitochondrial Reactive Oxigen Species (mtROS) and/or mitochondrial (mt)DNA, leading to a hyper-response of classically activated macrophages and subsequent increases in IL-1 β. This NLRP3 over-activated status in elderly individuals, is also observed in telomere dysfunctional mice models. In our opinion, the NLRP3 inflammasome plays a central role in the increased lethality observed in elderly patients infected by COVID-19. Strategies blocking inflammasome would deserve to be studied.

Correction for: Sod2 haploinsufficiency does not accelerate aging of telomere dysfunctional mice.

Correction for: Sod2 haploinsufficiency does not accelerate aging of telomere dysfunctional mice.

Analyses of Selenotranscriptomes and Selenium Concentrations in Response to Dietary Selenium Deficiency and Age Reveal Common and Distinct Patterns by Tissue and Sex in Telomere-Dysfunctional Mice

Background: The hierarchies of tissue selenium distribution and selenotranscriptomes are thought to critically affect healthspan and longevity.Objective: We determined selenium status and selenotranscriptomes in response to long-term dietary selenium deficiency and age in tissues of male and female mice.Methods: Weanling telomerase RNA component knockout C57BL/6 mice were fed a selenium-deficient (0.03 mg Se/kg) Torula yeast-based AIN-93G diet or a diet supplemented with sodium selenate (0.15 mg Se/kg) until age 18 or 24 mo. Plasma, hearts, kidneys, livers, and testes were collected to assay for selenotranscriptomes, selected selenoproteins, and tissue selenium concentrations. Data were analyzed with the use of 2-factor ANOVA (diet × age) in both sexes.Results: Dietary selenium deficiency decreased (P ≤ 0.05) selenium concentrations (65-72%) and glutathione peroxidase (GPX) 3 (82-94%) and selenoprotein P (SELENOP) (17-41%) levels in the plasma of both sexes of mice and mRNA levels (9-68%) of 4, 4, and 12 selenoproteins in the heart, kidney, and liver of males, respectively, and 5, 16, and 14 selenoproteins, respectively, in females. Age increased selenium concentrations and SELENOP levels (27% and 30%, respectively; P ≤ 0.05) in the plasma of males only but decreased (12-46%; P < 0.05) mRNA levels of 1, 5, and 13 selenoproteins in the heart, kidney, and liver of males, respectively, and 6, 5, and 0 selenoproteins, respectively, in females. Among these mRNAs, selenoprotein H (Selenoh), selenoprotein M (Selenom), selenoprotein W (Selenow), methionine-R-sulfoxide reductase 1 (MsrB1), Gpx1, Gpx3, thioredoxin reductase 1 (Txnrd1), Txnrd2, selenoprotein S (Selenos), selenoprotein F (Selenof), and selenoprotein O (Selenoo) responded in parallel to dietary selenium deficiency and age in ≥1 tissue or sex, or both. Dietary selenium deficiency upregulated (40-160%; P ≤ 0.05) iodothyronine deiodinase 2 (Dio2) and selenoprotein N (Selenon) in the kidneys of males. Age upregulated (11-44%; P < 0.05) Selenon in the kidneys of males, selenoprotein K (Selenok) and selenoprotein I (Selenoi) in the kidneys of females, and Selenof and Selenok in the testes.Conclusions: These results illustrate tissue-specific sexual dimorphisms of selenium status and selenotranscriptomes because of dietary selenium deficiency and age.

Dietary Selenium Deprivation Oppositely Impacts Longevity and Healthspan in Telomere Dysfunctional Mice

Selenium (Se), a trace metalloid essential for life, plays nutritional and physiological roles mainly through selenoproteins. While telomere attrition provokes DNA damage response and, subsequently, replicative senescence, such an aging process is restricted in mice due to their intrinsically long telomeres. To circumvent this limitation and employ a mouse model with humanized telomeres, weanling third generation telomerase RNA component knockout mice carrying short telomeres were fed a Se‐deficient, Torula‐yeast basal diet or the diet supplemented with 0.15 ppm Se as sodium selenate throughout their life. Dietary Se deprivation delayed wound healing and accelerated a wide range of age‐related degeneration including osteoporosis, grey hair, alopecia, cataract, and hyperglycemia. Surprisingly, long‐term dietary Se deprivation paradoxically promoted longevity. Plasma microRNA profiling revealed a circulating signature of Se deprivation and aging, and subsequent ontological analyses predicted dominant changes in metabolism. Dietary Se deprivation accelerated age‐dependent declines in glucose tolerance, glucose‐stimulated insulin production and insulin sensitivity, as well as DNA damage and senescence responses in the pancreas. Selenotranscriptomic and metagenomic analyses identified key selenoproteins and gut bacteria in the response to dietary Se deprivation in the mice at 18 and 24 months of age. Altogether, these results suggest a novel model of aging with conceptual advances, whereby Se at low levels may be considered a hormetic chemical and decouple healthspan and longevity.

Opposing impacts on healthspan and longevity by limiting dietary selenium in telomere dysfunctional mice.

SummarySelenium (Se) is a trace metalloid essential for life, but its nutritional and physiological roles during the aging process remain elusive. While telomere attrition contributes to replicative senescence mainly through persistent DNA damage response, such an aging process is mitigated in mice with inherently long telomeres. Here, weanling third generation telomerase RNA component knockout mice carrying short telomeres were fed a Se‐deficient basal diet or the diet supplemented with 0.15 ppm Se as sodium selenate to be nutritionally sufficient throughout their life. Dietary Se deprivation delayed wound healing and accelerated incidence of osteoporosis, gray hair, alopecia, and cataract, but surprisingly promoted longevity. Plasma microRNA profiling revealed a circulating signature of Se deprivation, and subsequent ontological analyses predicted dominant changes in metabolism. Consistent with this observation, dietary Se deprivation accelerated age‐dependent declines in glucose tolerance, insulin sensitivity, and glucose‐stimulated insulin production in the mice. Moreover, DNA damage and senescence responses were enhanced and Pdx1 and MafA mRNA expression were reduced in pancreas of the Se‐deficient mice. Altogether, these results suggest a novel model of aging with conceptual advances, whereby Se at low levels may be considered a hormetic chemical and decouple healthspan and longevity.

Telomere Dysfunction-Induced DNA Damage Drives Hematopoietic Stem Cell Fate

Accumulating evidence supports the view that DNA damage checkpoints activated by telomere erosion can drive hematopoietic stem cell (HSC) decline, thereby compromising HSC self-renewal, repopulating capacity, and differentiation. However, the precise mechanisms underlying telomere dysfunction-related HSC defects are still largely unknown. In this study, we employed the inducible telomerase deficient mice TERTER/ER to molecularly define the adverse effects of wide-spread endogenous telomere dysfunction-induced DNA damage signaling on stem cell function in vivo.The HSC compartment of 3-month-old telomere dysfunctional mice (G4/G5 TERTER/ER) showed an increased expansion in the steady-state absolute number of long-term HSCs (LT-HSC) and short-term HSCs with a concomitant decrease of multipotent progenitor cells. Accordingly, telomere dysfunctional LT-HSC showed a significant decrease of the quiescence state (p=0.018) associated with an increase of cells in the G1/G2-M phase of the cell cycle (p=0.038), although the preferential accumulation of phospho-H2AX foci (p=7x10-4). Furthermore, peripheral blood analysis revealed that the total CD45.2-derived reconstitution was significantly compromised in mice competitively transplanted with G4/G5 TERTER/ER LT-HSC, which shows that they have a finite potential for self-renewal under regenerative stress. Overall, these findings suggest the existence of a telomere dysfunction-induced differentiation checkpoint, which occurs at the level of LT-HSC and is responsible for their premature exhaustion. Correspondingly, aged telomere-dysfunctional mice (n=20) showed a significant decrease in the absolute number of LT-HSC in comparison to aged mice with intact telomeres (n=10) (p=0.04). On the contrary, leukemic transformation which occurred in about 5% of G4/G5 TERTER/ER mice both in homeostatic conditions and in the setting of competitive transplantation induced a significant expansion of the HSC pool, suggesting the existence of secondary events able to overcome the decline of telomere dysfunction-induced HSC self-renewal capability.One way in which cells can balance renewal with differentiation is through the control of asymmetric and symmetric division. During asymmetric division, one daughter cell remains a stem cell, while the other becomes a committed progenitor cell. In contrast, during symmetric divisions, a stem cell divides to become two HSCs (symmetric self-renewal) or two committed cells (symmetric commitment). Asymmetric cell division involves the polarized distribution of determinants, such as Numb, within the mother cell and their unequal inheritance by each daughter cell; in contrast, symmetric division allows both daughter cells to adopt equivalent fates.To determine if telomere dysfunction-induced DNA damage was directly responsible for HSC exhaustion by altering the mechanism of HSC self-renewal versus differentiation cell fate decisions, we evaluated Numb inheritance and expression in sorted telomere dysfunctional LT-HSC (n=310 LT-HSC isolated from 12 mice) in comparison to LT-HSC with intact telomeres (n=273 LT-HSC, isolated from 7 mice) induced to proliferate in culture. Specifically, we found that the frequency of symmetric self-renewal divisions was approximately 1.5-fold lower in telomere dysfunctional LT-HSC compared with those with intact telomeres (p=0.02), with a concomitant 2-fold increase in the frequency of symmetric commitment (p=0.006). Thus, telomere dysfunction-induced DNA damage is associated with a cell-intrinsic skewing toward symmetric commitment, which leads to compromised self-renewal capability. In contrast, and consistent with our in vivo data, LT-HSC isolated from G4/G5 TERTER/ER mice in leukemic transformation preferentially underwent symmetric self-renewal divisions. Next, we performed unbiased RNA sequencing on sorted G4/G5 TERTER/ER LT-HSC induced to proliferate in vitro, which underwent to preferential symmetric commitment or symmetric self-renewal divisions. Results of these analyses will provide insights into the mechanistic basis of how telomere dysfunction-induced DNA damage drives aberrant commitment of HSC, which results in their exhaustion, whereas leukemic transformation leads to deregulated and enhanced self-renewal, which results in their expansion and suppression of normal hematopoiesis. DisclosuresNo relevant conflicts of interest to declare.

A Mouse Model of Telomere Dysfunction Recapitulates Hallmark Features of Human Myelodysplastic Syndrome

Myelodysplastic syndrome (MDS) is a heterogeneous group of hematopoietic neoplastic disorders that are characterized by ineffective myeloid differentiation and dysplasia as well as telomere shortening and accumulated DNA damage in progenitor cells. Less understood is whether DNA damage is the instigator of impaired progenitor cell differentiation and MDS development. Telomerase deficient mice have served as a model system to demonstrate the adverse effects of wide-spread endogenous DNA damage signaling on stem cell function in vivo. In recent studies, we sought to determine whether persistent physiological DNA damage can impair the function of specific hematopoietic lineages by employing the 4-hydroxytamoxifen (OHT)-inducible telomerase reverse transcriptase-estrogen receptor (TERTER) model. For the first time, we demonstrate that late generation TERTER/ER mice with dysfunctional telomeres exhibit hallmark features of MDS, including peripheral blood cytopenias, bone marrow (BM) hyper-cellularity, and an increased myeloid-to-erythroid progenitor ratio in the absence of increased apoptosis. Severe tri-lineage myelodysplasia, and an increase of immature, morphologically abnormal myeloid blasts frequently with pronounced monocytic differentiation were consistent with refractory anemia with excess of blasts (RAEB) or chronic myelo-monocytic leukemia (CMML), a specific sub-group of MDS that is characterized by a high propensity to develop acute myeloid leukemia (AML). Accordingly, approximately 5% of aged TERTER/ER mice progressed to AML, as demonstrated by a marked increase of BM myeloid blasts, and infiltration of myeloid precursors into the splenic white-red pulp architecture, resulting in myeloid sarcoma with the complete effacement of lymphoid follicles. Compared to control mice with intact telomeres, the progenitor compartment of telomere dysfunctional mice shows a significant increase in the number of granulocyte-macrophage progenitors (GMP) with a concomitant loss of the megakaryocyte-erythroid progenitors (MEP) and slight reduction in the number of common myeloid progenitors (CMP), which is consistent with the condition of skewed myeloid differentiation occurring in MDS patients with higher risk of leukemic transformation. Transplantation experiments of long-term hematopoietic stem cells isolated from telomere dysfunctional mice into wild type congenic recipients revealed that the level of donor-derived skewed myeloid differentiation was comparable to that observed at steady state in the same telomere dysfunctional mice before transplantation, suggesting that impaired progenitor differentiation occurred as a result of cell intrinsic defects of telomere dysfunctional hematopoietic cells. In the setting of telomere dysfunction, somatic in vivo and in vitro telomerase reactivation reduced DNA damage signaling and specifically reversed defective differentiation and MDS phenotypes. Unbiased transcriptomic network analyses of CMP with telomere dysfunction revealed profound down-regulation of genes in the mRNA splicing and processing pathways which was rescued by telomerase reactivation, indicating that telomere dysfunction-induced DNA damage response can impact on the expression of genes involved in splicing regulation. RNA-seq analysis of telomere dysfunctional CMP suggested altered splicing activity at the level of exon usage and identified aberrantly spliced variants of genes implicated in chromatin remodeling, and histone modifications. The prominence of aberrantly spliced epigenetic regulators prompted us to hypothesize that there was a link between impaired myeloid differentiation and aberrant splicing patterns as a result of telomere dysfunction-induced repression of splicing components. In conclusion, our studies have revealed an unanticipated link between telomere biology, RNA splicing, and MDS pathogenesis and support the development of strategies designed to modulate the downstream targets of splicing alterations in specific hematopoietic populations. DisclosuresNo relevant conflicts of interest to declare.

Glucose substitution prolongs maintenance of energy homeostasis and lifespan of telomere dysfunctional mice.

DNA damage and telomere dysfunction shorten organismal lifespan. Here we show that oral glucose administration at advanced age increases health and lifespan of telomere dysfunctional mice. The study reveals that energy consumption increases in telomere dysfunctional cells resulting in enhanced glucose metabolism both in glycolysis and in the tricarboxylic acid cycle at organismal level. In ageing telomere dysfunctional mice, normal diet provides insufficient amounts of glucose thus leading to impaired energy homeostasis, catabolism, suppression of IGF-1/mTOR signalling, suppression of mitochondrial biogenesis and tissue atrophy. A glucose-enriched diet reverts these defects by activating glycolysis, mitochondrial biogenesis and oxidative glucose metabolism. The beneficial effects of glucose substitution on mitochondrial function and glucose metabolism are blocked by mTOR inhibition but mimicked by IGF-1 application. Together, these results provide the first experimental evidence that telomere dysfunction enhances the requirement of glucose substitution for the maintenance of energy homeostasis and IGF-1/mTOR-dependent mitochondrial biogenesis in ageing tissues.

Puma and p21 represent cooperating checkpoints limiting self-renewal and chromosomal instability of somatic stem cells in response to telomere dysfunction

The tumour suppressor p53 activates Puma-dependent apoptosis and p21-dependent cell-cycle arrest in response to DNA damage. Deletion of p21 improved stem-cell function and organ maintenance in progeroid mice with dysfunctional telomeres, but the function of Puma has not been investigated in this context. Here we show that deletion of Puma improves stem- and progenitor-cell function, organ maintenance and lifespan of telomere-dysfunctional mice. Puma deletion impairs the clearance of stem and progenitor cells that have accumulated DNA damage as a consequence of critically short telomeres. However, further accumulation of DNA damage in these rescued progenitor cells leads to increasing activation of p21. RNA interference experiments show that upregulation of p21 limits proliferation and evolution of chromosomal imbalances of Puma-deficient stem and progenitor cells with dysfunctional telomeres. These results provide experimental evidence that p53-dependent apoptosis and cell-cycle arrest act in cooperating checkpoints limiting tissue maintenance and evolution of chromosomal instability at stem- and progenitor-cell levels in response to telomere dysfunction. Selective inhibition of Puma-dependent apoptosis can result in temporary improvements in maintenance of telomere-dysfunctional organs.

Telomere Shortening Impairs Regeneration of the Olfactory Epithelium in Response to Injury but Not Under Homeostatic Conditions

Atrophy of the olfactory epithelium (OE) associated with impaired olfaction and dry nose represents one of the most common phenotypes of human aging. Impairment in regeneration of a functional olfactory epithelium can also occur in response to injury due to infection or nasal surgery. These complications occur more frequently in aged patients. Although age is the most unifying risk factor for atrophic changes and functional decline of the olfactory epithelium, little is known about molecular mechanisms that could influence maintenance and repair of the olfactory epithelium. Here, we analyzed the influence of telomere shortening (a basic mechanism of cellular aging) on homeostasis and regenerative reserve in response to chemical induced injury of the OE in late generation telomere knockout mice (G3 mTerc−/−) with short telomeres compared to wild type mice (mTerc+/+) with long telomeres. The study revealed no significant influence of telomere shortening on homeostatic maintenance of the OE during mouse aging. In contrast, the regenerative response to chemical induced injury of the OE was significantly impaired in G3 mTerc−/− mice compared to mTerc+/+ mice. Seven days after chemical induced damage, G3 mTerc−/− mice exhibited significantly enlarged areas of persisting atrophy compared to mTerc+/+ mice (p = 0.031). Telomere dysfunction was associated with impairments in cell proliferation in the regenerating epithelium. Deletion of the cell cycle inhibitor, Cdkn1a (p21) rescued defects in OE regeneration in telomere dysfunctional mice. Together, these data indicate that telomere shortening impairs the regenerative capacity of the OE by impairing cell cycle progression in a p21-dependent manner. These findings could be relevant for the impairment in OE function in elderly people.

Regeneration of the Exocrine Pancreas Is Delayed in Telomere-Dysfunctional Mice

IntroductionTelomere shortening is a cell-intrinsic mechanism that limits cell proliferation by induction of DNA damage responses resulting either in apoptosis or cellular senescence. Shortening of telomeres has been shown to occur during human aging and in chronic diseases that accelerate cell turnover, such as chronic hepatitis. Telomere shortening can limit organ homeostasis and regeneration in response to injury. Whether the same holds true for pancreas regeneration in response to injury is not known.MethodsIn the present study, pancreatic regeneration after acute cerulein-induced pancreatitis was studied in late generation telomerase knockout mice with short telomeres compared to telomerase wild-type mice with long telomeres.ResultsLate generation telomerase knockout mice exhibited impaired exocrine pancreatic regeneration after acute pancreatitis as seen by persistence of metaplastic acinar cells and markedly reduced proliferation. The expression levels of p53 and p21 were not significantly increased in regenerating pancreas of late generation telomerase knockout mice compared to wild-type mice.ConclusionOur results indicate that pancreatic regeneration is limited in the context of telomere dysfunction without evidence for p53 checkpoint activation.

CHK2‐independent induction of telomere dysfunction checkpoints in stem and progenitor cells

Telomere shortening limits the proliferation of primary human fibroblasts by the induction of senescence, which is mediated by ataxia telangiectasia mutated-dependent activation of p53. Here, we show that CHK2 deletion impairs the induction of senescence in mouse and human fibroblasts. By contrast, CHK2 deletion did not improve the stem-cell function, organ maintenance and lifespan of telomere dysfunctional mice and did not prevent the induction of p53/p21, apoptosis and cell-cycle arrest in telomere dysfunctional progenitor cells. Together, these results indicate that CHK2 mediates the induction of senescence in fibroblasts, but is dispensable for the induction of telomere dysfunction checkpoints at the stem and progenitor cell level in vivo.

Abstract 2972: Intestinal p53 deletion leads to accumulation of chromosomal instability without promoting tumor formation

Abstract Background: Colorectal cancer is after lung cancer the second biggest cancer threat for humans and the tumor burden is rising steadily during human ageing. One important issue during human ageing is the accumulation of DNA damage inducing a p53 dependent response leading to p21-dependent senescence or apoptosis. It has been shown that deletion of DNA repair proteins as well as other components of the DNA damage pathway leads to tumor formation. Methods: In this approach we investigated the role of the tumor suppressor p53 with and without telomere dysfunction in intestinal tumor formation. Late generation of iG4 mTerc−/−; p53lox/lox and iF1 mTerc+/− p53lox/lox mice were crossed to Villin-Cre-ERT2 transgenic mouse model. In 1 month old mice Tamoxifen was injected on 5 consecutive days leading to functional deletion of the Trp53 gene. Results: In aging IF1 Terc+/− p53 deletion did not induce an obvious intestinal phenotype including tumor formation until 24 months. In contrast to p21 deletion the deletion of p53 significantly shortened the lifespan of iG4 Terc−/− Trp53−/− animals compared to iG4 Terc−/− Trp53+/+. This reduction correlated with earlier appearance of crypt atrophy and weight loss but was not associated with tumor formation. Deletion of the Trp53 gene resulted in an abrogation of the p21-dependent senescence and accumulation of DNA damage in the intestinal epithelium of both p53 deleted cohorts. Moreover this correlated with an accumulation of intestinal stem cells carrying high levels of DNA damage. On contrary to p21 deletion the deletion of p53 impaired the depletion of chromosomal instable intestinal stem cells in aging telomere dysfunctional mice. Conclusion: These results provide first proof that deletion of p53 is not able to promote tumorigenesis in the intestinal epithelium of mice and that p53 induction has a protective role preventing accumulation of damaged intestinal stem cells in aging telomere dysfunctional mice. Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2972.

Alterations of the systemic environment are the primary cause of impaired B and T lymphopoiesis in telomere-dysfunctional mice

AbstractThere is growing evidence that telomere dysfunction can contribute to human aging. Telomere dysfunction limits lymphopoiesis in aging telomerase knockout (mTerc−/−) mice primarily by the induction of stem cell–extrinsic alterations. The relative contribution of alterations in the stem cell niche and the systemic environment to the impairment of lymphopoiesis in response to telomere dysfunction is currently unknown. This study reveals a minor impact of stem cell–intrinsic defects on the impairment of B and T lymphopoiesis in response to telomere dysfunction. The impairment in B and T lymphopoiesis in aging telomere-dysfunctional mice was mainly due to alterations of the systemic environment. Telomere dysfunction had no significant cell-autonomous effects impairing the function of thymic or bone marrow niches in supporting B and T lymphopoiesis. Moreover, age-related alterations in the cellular composition of the thymic epithelium in telomere-dysfunctional mice were rescued by transplantation of the thymus into a wild-type environment; these rejuvenated thymi supported normal T lymphopoiesis in recipient mice. Together, these data place alterations in the systemic environment on top of the hierarchy of events limiting lymphopoiesis in response to telomere dysfunction.

P53 deletion impairs clearance of chromosomal-instable stem cells in aging telomere-dysfunctional mice

Telomere dysfunction limits the proliferative capacity of human cells and induces organismal aging by activation of p53 and p21. Although deletion of p21 elongates the lifespan of telomere-dysfunctional mice, a direct analysis of p53 in telomere-related aging has been hampered by early tumor formation in p53 knockout mice. Here we analyzed the functional consequences of conditional p53 deletion. Intestinal deletion of p53 shortened the lifespan of telomere-dysfunctional mice without inducing tumor formation. In contrast to p21 deletion, the deletion of p53 impaired the depletion of chromosomal-instable intestinal stem cells in aging telomere-dysfunctional mice. These instable stem cells contributed to epithelial regeneration leading to an accumulation of chromosomal instability, increased apoptosis, altered epithelial cell differentiation and premature intestinal failure. Together, these results provide the first experimental evidence for an organ system in which p53-dependent mechanisms prevent tissue destruction in response to telomere dysfunction by depleting genetically instable stem cells.

Telomere Dysfunction and DNA Damage Checkpoints in Diseases and Cancer of the Gastrointestinal Tract

Telomere Dysfunction and DNA Damage Checkpoints in Diseases and Cancer of the Gastrointestinal Tract

Keeping your senescent cells under control

Keeping your senescent cells under control

Sod2 haploinsufficiency does not accelerate aging of telomere dysfunctional mice

Telomere shortening represents a causal factor of cellular senescence. At the same time, several lines of evidence indicate a pivotal role of oxidative DNA damage for the aging process in vivo. A causal connection between the two observations was suggested by experiments showing accelerated telomere shorting under conditions of oxidative stress in cultured cells, but has never been studied in vivo. We therefore have analysed whether an increase in mitochondrial derived oxidative stress in response to heterozygous deletion of superoxide dismutase (Sod2(+/-)) would exacerbate aging phenotypes in telomere dysfunctional (mTerc(-/-)) mice. Heterozygous deletion of Sod2 resulted in reduced SOD2 protein levels and increased oxidative stress in aging telomere dysfunctional mice, but this did not lead to an increase in basal levels of oxidative nuclear DNA damage, an accumulation of nuclear DNA breaks, or an increased rate of telomere shortening in the mice. Moreover, heterozygous deletion of Sod2 did not accelerate the depletion of stem cells and the impairment in organ maintenance in aging mTerc(-/-) mice. In agreement with these observations, Sod2 haploinsufficiency did not lead to a further reduction in lifespan of mTerc(-/-) mice. Together, these results indicate that a decrease in SOD2-dependent antioxidant defence does not exacerbate aging in the context of telomere dysfunction.

Exonuclease-1 Deletion Impairs DNA Damage Signaling and Prolongs Lifespan of Telomere-Dysfunctional Mice

Exonuclease-1 (EXO1) mediates checkpoint induction in response to telomere dysfunction in yeast, but it is unknown whether EXO1 has similar functions in mammalian cells. Here we show that deletion of the nuclease domain of Exo1 reduces accumulation of DNA damage and DNA damage signal induction in telomere-dysfunctional mice. Exo1 deletion improved organ maintenance and lifespan of telomere-dysfunctional mice but did not increase chromosomal instability or cancer formation. Deletion of Exo1 also ameliorated the induction of DNA damage checkpoints in response to gamma-irradiation and conferred cellular resistance to 6-thioguanine-induced DNA damage. Exo1 deletion impaired upstream induction of DNA damage responses by reducing ssDNA formation and the recruitment of Replication Protein A (RPA) and ATR at DNA breaks. Together, these studies provide evidence that EXO1 contributes to DNA damage signal induction in mammalian cells, and deletion of Exo1 can prolong survival in the context of telomere dysfunction.