Evidence that aging and amyloid promote microglial cell senescence

Advanced age and presence of intracerebral amyloid deposits are known to be major risk factors for development of neurodegeneration in Alzheimer's disease (AD), and both have been associated with microglial activation. However, the specific role of activated microglia in AD pathogenesis remains unresolved. Here we report that microglial cells exhibit significant telomere shortening and reduction of telomerase activity with normal aging in rats, and that in humans there is a tendency toward telomere shortening with presence of dementia. Human brains containing high amyloid loads demonstrate a significantly higher degree of microglial dystrophy than nondemented, amyloid-free control subjects. Collectively, these findings show that microglial cell senescence associated with telomere shortening and normal aging is exacerbated by the presence of amyloid. They suggest that degeneration of microglia is a factor in the pathogenesis of AD.     

Telomere biology and cardiovascular disease

Accumulation of cellular damage with advancing age leads to atherothrombosis and associated cardiovascular disease. Ageing is also characterized by shortening of the DNA component of telomeres, the specialized genetic segments located at the end of eukaryotic chromosomes that protect them from end-to-end fusions. By inducing genomic instability, replicative senescence and apoptosis, shortening of the telomeric DNA is thought to contribute to organismal ageing. In this Review, we discuss experimental and human studies that have linked telomeres and associated proteins to several factors which influence cardiovascular risk (eg, estrogens, oxidative stress, hypertension, diabetes, and psychological stress), as well as to neovascularization and the pathogenesis of atherosclerosis and heart disease. Two chief questions that remain unanswered are whether telomere shortening is cause or consequence of cardiovascular disease, and whether therapies targeting the telomere may find application in treating these disorders (eg, cell "telomerization" to engineer blood vessels of clinical value for bypass surgery, and to facilitate cell-based myocardial regeneration strategies). Given that most research to date has focused on the role of telomerase, it is also of up most importance to investigate whether alterations in additional telomere-associated proteins may contribute to the pathogenesis of cardiovascular disease.     

Determining the influence of telomere dysfunction and DNA damage on stem and progenitor cell aging – what markers can we use?

The decline in organ maintenance and function is one of the major problems limiting quality of life during aging. The accumulation of telomere dysfunction and DNA damage appears to be one of the underlying causes. Uncapping of chromosome ends in response to critical telomere shortening limits the proliferative capacity of human cells by activation of DNA damage checkpoints inducing senescence or apoptosis. Telomere shortening occurs in the vast majority of human tissues during aging and in chronic diseases that increase the rate of cell turnover. There is emerging evidence that telomere shortening can limit the maintenance and function of adult stem cells -- a cell type of utmost importance for organ maintenance and regeneration. In mouse models, telomere dysfunction leads to a depletion of adult stem cell compartments suggesting that stem cells are very sensitive to DNA damage. Both the rarity of stem and progenitor cells in adult organs and their removal in response to damage make it difficult to assess the impact of telomere dysfunction and DNA damage on stem and progenitor cell aging. Such approaches require the development of sensitive biomarkers recognizing low levels of telomere dysfunction and DNA damage in stem and progenitor cells. Here, we review experimental data on the prevalence of telomere dysfunction and DNA damage during aging and its possible impact on stem and progenitor cell aging.     

Model of human aging: Recent findings on Werner’s and Hutchinson-Gilford progeria syndromes

The molecular mechanisms involved in human aging are complicated. Two progeria syndromes, Werner’s syndrome (WS) and Hutchinson-Gilford progeria syndrome (HGPS), characterized by clinical features mimicking physiological aging at an early age, provide insights into the mechanisms of natural aging. Based on recent findings on WS and HGPS, we suggest a model of human aging. Human aging can be triggered by two main mechanisms, telomere shortening and DNA damage. In telomere-dependent aging, telomere shortening and dysfunction may lead to DNA damage responses which induce cellular senescence. In DNA damage-initiated aging, DNA damage accumulates, along with DNA repair deficiencies, resulting in genomic instability and accelerated cellular senescence. In addition, aging due to both mechanisms (DNA damage and telomere shortening) is strongly dependent on p53 status. These two mechanisms can also act cooperatively to increase the overall level of genomic instability, triggering the onset of human aging phenotypes.     

端粒与自身免疫性疾病

端粒是真核细胞染色体末端的特殊结构,其功能是保护染色体末端结构完整和功能稳定,染色体末端失去保护将出现染色体重组、融合、降解。端粒酶是维护端粒的核蛋白复合体,端粒酶缺乏会导致端粒长度随细胞分裂不断缩短而失去稳定性。端粒蛋白由端粒保护蛋白和端粒相关蛋白组成,是参与维护染色体结构稳定的蛋白复合体,在参与调节端粒长度、保护端粒结构和功能中发挥着重要作用。端粒的相关研究主要集中在肿瘤、遗传性疾病等领域,但越来越多的研究认为端粒与自身免疫性疾病的发病同样有着密切的关系。通过对端粒的研究可能对自身免疫性疾病的发病和治疗等提供新的方向。     

Proteins induced by telomere dysfunction and DNA damage represent biomarkers of human aging and disease

Telomere dysfunction limits the proliferative capacity of human cells by activation of DNA damage responses, inducing senescence or apoptosis. In humans, telomere shortening occurs in the vast majority of tissues during aging, and telomere shortening is accelerated in chronic diseases that increase the rate of cell turnover. Yet, the functional role of telomere dysfunction and DNA damage in human aging and diseases remains under debate. Here, we identified marker proteins (i.e., CRAMP, stathmin, EF-1alpha, and chitinase) that are secreted from telomere-dysfunctional bone-marrow cells of late generation telomerase knockout mice (G4mTerc(-/-)). The expression levels of these proteins increase in blood and in various tissues of aging G4mTerc(-/-) mice but not in aging mice with long telomere reserves. Orthologs of these proteins are up-regulated in late-passage presenescent human fibroblasts and in early passage human cells in response to gamma-irradiation. The study shows that the expression level of these marker proteins increases in the blood plasma of aging humans and shows a further increase in geriatric patients with aging-associated diseases. Moreover, there was a significant increase in the expression of the biomarkers in the blood plasma of patients with chronic diseases that are associated with increased rates of cell turnover and telomere shortening, such as cirrhosis and myelodysplastic syndromes (MDS). Analysis of blinded test samples validated the effectiveness of the biomarkers to discriminate between young and old, and between disease groups (MDS, cirrhosis) and healthy controls. These results support the concept that telomere dysfunction and DNA damage are interconnected pathways that are activated during human aging and disease.     

Cell intrinsic and extrinsic mechanisms of stem cell aging depend on telomere status

The function of adult stem cells declines during aging and chronic diseases. An understanding of the molecular mechanisms underlying these processes will help to identify targets for future therapies in order to improve regenerative reserve and organ maintenance. Telomere shortening represents a cell intrinsic mechanism inducing DNA damage in aging cells. Current studies in telomerase knockout mice have shown that telomere dysfunction induces cell intrinsic checkpoints and environmental alteration that limit stem cell function. While these phenotypes differ from wild-type mice with long telomere reserves, they appear to be relevant for human aging, which is associated with an accumulation of telomere dysfunction and DNA damage.     

Cellular senescence and tissue aging in vivo

A long-standing controversy concerns the relevance of cellular senescence, defined and observed as a cell culture phenomenon, to tissue aging in vivo. Here the evidence on this topic is reviewed. The main conclusions are as follows. First, telomere shortening, the principal known mediator of cellular senescence, occurs in many human tissues in aging. Second, it is not clear whether this results in cellular senescence or in some other cell fate (e.g., crisis). Third, rodents probably are not appropriate experimental models for these questions, because of important differences in telomere biology between rodent cells and cells from long-lived mammals (e.g., human or bovine cells). Fourth, better and more comprehensive observations on aging human tissues are needed to answer the question of the occurrence of senescent cells in tissues, and new experimental approaches are needed to elucidate the consequences of telomere shortening in tissues in aging.     

Telomeres and cardiovascular disease: does size matter?

Telomeres-the specialized DNA-protein structures at the ends of eukaryotic chromosomes-are essential for maintaining genome stability and integrity and for extended proliferative life span in both cultured cells and in the whole organism. Telomerase and additional telomere-associated proteins are necessary for preserving telomeric DNA length. Age-dependent telomere shortening in most somatic cells, including vascular endothelial cells, smooth muscle cells, and cardiomyocytes, is thought to impair cellular function and viability of the aged organism. Telomere dysfunction is emerging as an important factor in the pathogenesis of hypertension, atherosclerosis, and heart failure. In this Review, we discuss present studies on telomeres and telomere-associated proteins in cardiovascular pathobiology and their implications for therapeutics.     

Abrupt telomere shortening in normal human fibroblasts

Aging is one of the most basic properties of living organisms. Abundant evidence supports the idea that cell senescence underlies organismal aging in higher mammals. Therefore, examining the molecular mechanisms that control cell and replicative senescence is of great interest for biology and medicine. Several discoveries strongly support telomere shortening as the main molecular mechanism that limits the growth of normal cells. Although cultures gradually approach their growth limit, appearance of individual senescent cells is sudden and stochastic. A theoretical model of abrupt telomere shortening has been proposed in order to explain this phenomenon, but until now there was no reliable experimental evidence supporting this idea. Here, we have employed novel methodology to provide evidence for the generation of extrachromosomal circular telomeric DNA as a result of abrupt telomere shortening in normal human fibroblasts. This mechanism ensures heterogeneity in growth potential among individual cells, which is crucial for gradual progression of the aging process.     

Telomere dysfunction and cell cycle checkpoints in hematopoietic stem cell aging

Stem cells are believed to be closely associated with tissue degeneration during aging. Studies of human genetic diseases and gene-targeted animal models have provided evidence that functional decline of telomeres and deregulation of cell cycle checkpoints contribute to the aging process of tissue stem cells. Telomere dysfunction can induce DNA damage response via key cell cycle checkpoints, leading to cellular senescence or apoptosis depending on the tissue type and developmental stage of a specific stem cell compartment. Telomerase mutation and telomere shortening have been observed in a variety of hematological disorders, such as dyskeratosis congenital, aplastic anemia, myelodysplastic syndromes and leukemia, in which the hematopoietic stem cells (HSC) are a major target during the pathogenesis. Moreover, telomere dysfunction is able to induce both cell-intrinsic checkpoints and environmental factors limiting the self-renewal capacity and differentiation potential of HSCs. Crucial components in the cascade of DNA damage response, including ataxia telangiectasia mutated, CHK2, p53, p21 and p16/p19^ARF, play important roles in HSC maintenance and self-renewal in the scenarios of both sufficient telomere reserve and dysfunctional telomere. Therefore, a further understanding of the molecular mechanisms underlying HSC aging may help identity new therapeutic targets for stem cell-based regenerative medicine.     

Vascular aging: insights from studies on cellular senescence, stem cell aging, and progeroid syndromes

Epidemiological studies have shown that age is the chief risk factor for atherosclerotic cardiovascular diseases, but the molecular mechanisms that underlie the increase in risk conferred by aging remain unclear. Evidence suggests that the cardiovascular repair system is impaired with advancing age, thereby inducing age-associated cardiovascular dysfunction. Such impairment could be attributable to senescence of cardiovascular tissues at the cellular level as a result of telomere shortening, DNA damage, and genomic instability. In fact, the replicative ability of cardiovascular cells, particularly stem cells and/or progenitor cells, has been shown to decline with age. Recently, considerable progress has been made in understanding the pathogenesis of human progeroid syndromes that feature cardiovascular aging. Most of the genes responsible have a role in DNA metabolism, and mutated forms of these genes result in alterations of the response to DNA damage and in decreased cell proliferation, which might be common features of a phenotype of aging. Here we review the cardiovascular research on cellular senescence, stem cell aging, and progeroid syndromes and discuss the potential role of cellular senescence in the mechanisms underlying both normal aging and premature aging syndromes.     

Is telomere length a biomarker of aging? A review

Telomeres, the DNA-protein structures located at the ends of chromosomes, have been proposed to act as a biomarker of aging. In this review, the human evidence that telomere length is a biomarker of aging is evaluated. Although telomere length is implicated in cellular aging, the evidence suggesting telomere length is a biomarker of aging in humans is equivocal. More studies examining the relationships between telomere length and mortality and with measures that decline with "normal" aging in community samples are required. These studies would benefit from longitudinal measures of both telomere length and aging-related parameters.     

端粒长度与心血管疾病危险因素关系的研究进展

端粒是位于染色体末端的DNA重复序列,其功能是阻止染色体末端不被断裂和降解,维持染色体结构稳定和遗传稳定性,且随着细胞分裂而不断缩短,所以端粒长度可以作为生物衰老过程中的一个重要标记物。与2型糖尿病、阿尔茨海默病和骨质疏松等疾病一样,冠心病（CHD）也是一种年龄相关性疾病。端粒长度和CHD与机体衰老、退化均有着密切联系,且已有相关研究证明,白细胞端粒长度与CHD患病风险之间可能存在相关性,提示端粒可能参与了CHD的发生、发展,但其因果关系尚不明确。因此我们猜测端粒是否通过影响CHD的传统危险因素,进而促进CHD的发生、发展。本文就白细胞端粒长度与CHD传统危险因素关系研究进展作一综述。     

Human Xp/Yp telomere analysis by Southern-STELA

Telomeres are specialized structures designed to protect the ends of linear chromosomes. They are dynamic structures such that in normal somatic cells they constantly shorten as cell division progresses. There is compelling evidence that telomere shortening leads to cell senescence, a process perceived as the main cause of aging in higher mammals. Therefore, the features of telomere shortening are of great importance in understanding cell senescence and aging in general. By identifying unique subtelomeric regions, large enough to produce strong chemiluminescent signals, we have provided a new tool for Southern blot analysis of individual human Xp/Yp telomeres. We extend these results with quantitative fluorescence in situ hybridization using peptide nucleic acid probe (PNA Q-FISH) analysis of telomeres on the Y chromosome. Our results demonstrates unequal shortening dynamics between the p and q telomeres.     

Telomere biology of the chicken: A model for aging research

Division-dependent telomere shortening correlating with age triggers senescence on a cellular level and telomere dysfunction can facilitate oncogenesis. Therefore, the study of telomere biology is critical to the understanding of aging and cancer. The domestic chicken, a classic model for the study of developmental biology, possesses a telomere genome with highly conserved aspects and distinctive features which make it uniquely suited for the study of telomere maintenance mechanisms, their function and dysfunction. The purpose of this review is to highlight the chicken as a model for aging research, specifically as a model for telomere and telomerase research, and to increase its utility as such by describing developments in the study of chicken telomeres and telomerase in the context of related research in human and mouse.     

A percentage analysis of the telomere length in Parkinson's disease patients

Telomeres are the repeated sequences at the chromosome ends which undergo shortening with cell division. The telomere shortening of the peripheral leukocytes is also facilitated by enhanced oxidative stress in various kinds of disease including ischemic heart disease, diabetes mellitus, apoplexy, and Alzheimer's disease. Telomere shortening in Parkinson's disease (PD) has not yet been reported. The pathogenesis for PD is also regarded to be associated with oxidative stress. We investigated 28 Japanese male PD patients ages 47-69. Although we could not find a statistical difference in the mean telomere length of peripheral leukocytes between the PD patients and the control participants, we found the mean telomere lengths to be shorter than 5 kb in only the PD patients and a significant PD-associated decrease in the telomeres with a length ranging from 23.1 to 9.4 kb in the patients in their 50s and 60s. These observations suggest that telomere shortening is accelerated in PD patients in comparison to the normal population.     

Telomere shortening and the population size-dependency of life span of human cell culture: further implication for two proliferation-restricting telomeres

Cultures of normal human cell can only undergo a finite number of population doublings. The proliferative life span of a culture is affected by population size, i.e., the number of cells a culture maintains. A 1000-fold transient reduction in population size can reduce the life span by as many as eight population doublings. The limited proliferative potential of human cells has been speculated to be a result of telomere shortening that occurs during DNA synthesis at each round of cell division. In this paper, I use computer simulation to test the telomere theory of cell aging against the population size-dependency of life span of human cell culture. It is found that telomere shortening well explains the above phenomenon. In addition, the results suggest that the proliferative potential of human cells might be limited by the shortening of only a few, most likely two, specific telomeres, providing further support to the same conclusion put forward in my previous paper (Tan, 1999).     

The natural history of telomeres: tools for aging animals and exploring the aging process

We have been exploring the use of telomere length as a technique to age animals. If telomere restriction fragments (TRFs) shorten predictably with age in a particular tissue, then measurement of TRFs will allow estimation of ages of animals when age cannot be measured directly. This would be particularly useful in population studies where tissue samples can be collected, but age of individuals or age structure of the population is otherwise unknown. We have demonstrated that rate of change in length of TRFs from blood cells can be used to estimate age in a number of avian species. Calibration of this telomere 'clock' using known-age individuals has led to new questions regarding the importance of TRF shortening in aging and its evolution in animals with differing life spans. Our current data show a tight correlation between telomere rate of change (TROC) and maximum life span in birds, with the longest living species having the slowest TROC. In contrast, absolute length of TRFs is not correlated with maximum life span. Very long-lived Leach's storm-petrels have telomeres that in fact lengthen with age! These data suggest that in the longest-lived organisms, cellular replicative life span may not be constrained by shortening telomeres. Published data show that TRFs shorten more slowly in long-lived mammals than in short-lived ones, although for birds and mammals of similar life span, telomere shortening is faster in mammals than in birds. This corresponds with the relatively greater longevity in birds than in mammals.     

Telomere shortening with aging in the human pancreas

We have conducted systematic studies to measure telomere length in human tissues of all types. Progressive telomere shortening with aging was studied in specimens of normal pancreas obtained at autopsy from 69 subjects aged 0 to 100 yr, and age-related shortening of telomere length at a rate of 36 base pairs (bp) per year was detected. Mean telomere length (+/-SD) was 13.9+/-1.4 kilobase pairs (kbp) in 16 neonates, as opposed to 8.4 kbp in 2 centenarians. Mean telomere length (+/-SD) in four age groups, 0-24, 25-49, 50-74, and 75-100 yr, was 13.5+/-1.5, 12.3+/-0.7, 11.3+/-2.5, and 10.7+/-1.8, respectively.