Telomerase and cellular lifespan: ending the debate?

Functional T cells are capable of supernumerary cell division and longevity

Differentiated somatic mammalian cells putatively exhibit species-specific division limits that impede cancer but may constrain lifespans^1-3. To provide immunity, transiently stimulated CD8⁺ T cells undergo unusually rapid bursts of numerous cell divisions, and then form quiescent long-lived memory cells that remain poised to reproliferate following subsequent immunological challenges. Here we addressed whether T cells are intrinsically constrained by chronological or cell-division limits. We activated mouse T cells in vivo using acute heterologous prime-boost-boost vaccinations⁴, transferred expanded cells to new mice, and then repeated this process iteratively. Over 10 years (greatly exceeding the mouse lifespan)⁵ and 51 successive immunizations, T cells remained competent to respond to vaccination. Cells required sufficient rest between stimulation events. Despite demonstrating the potential to expand the starting population at least 10⁴⁰-fold, cells did not show loss of proliferation control and results were not due to contamination with young cells. Persistent stimulation by chronic infections or cancer can cause T cell proliferative senescence, functional exhaustion and death⁶. We found that although iterative acute stimulations also induced sustained expression and epigenetic remodelling of common exhaustion markers (including PD1, which is also known as PDCD1, and TOX) in the cells, they could still proliferate, execute antimicrobial functions and form quiescent memory cells. These observations provide a model to better understand memory cell differentiation, exhaustion, cancer and ageing, and show that functionally competent T cells can retain the potential for extraordinary population expansion and longevity well beyond their organismal lifespan.

Proliferation of adult human bronchial epithelial cells without a telomere maintenance mechanism for over 200 population doublings

To date, there is no direct evidence of telomerase activity in adult lung epithelial cells, but typical culture conditions only support cell proliferation for 30-40 population doublings (PD), a point at which telomeres remain relatively long. Here we report that in in vitro low stress culture conditions consisting of a fibroblast feeder layer, rho-associated coiled coil protein kinase inhibitor (ROCKi), and low oxygen (2%), normal human bronchial epithelial basal progenitor cells (HBECs) divide for over 200 PD without engaging a telomere maintenance mechanism (almost four times the "Hayflick limit"). HBECs exhibit critically short telomeres at 200 PD and the population of cells start to undergo replicative senescence. Subcloning these late passage cells to clonal density, to mimic lung injury in vivo, selects for rare subsets of HBECs that activate low levels of telomerase activity to maintain short telomeres. CRISPR/Cas9 knockout of human telomerase reverse transcriptase or treatment with the telomerase-mediated telomere targeting agent 6-thio-2'deoxyguanosine abrogates colony growth in these late passage cultures (>200 PD) but not in early passage cultures (<200 PD). To our knowledge, this is the first study to report such long-term growth of HBECs without a telomere maintenance mechanism. This report also provides direct evidence of telomerase activation in HBECs near senescence when telomeres are critically short. This novel cell culture system provides an experimental model to understand how telomerase is regulated in normal adult tissues.

Telomeres and telomerase: three decades of progress

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

Accelerated in vivo epidermal telomere loss in Werner syndrome

Many data pertaining to the accelerated telomere loss in cultured cells derived from Werner syndrome (WS), a representative premature aging syndrome, have been accumulated. However, there have been no definitive data on in vivo telomere shortening in WS patients. In the present study, we measured terminal restriction fragment (TRF) lengths of 10 skin samples collected from extremities of 8 WS patients aged between 30 and 61 years that had been surgically amputated because of skin ulceration, and estimated the annual telomere loss. Whereas the values of TRF length in younger WS patients (in their thirties) were within the normal range, those in older WS patients were markedly shorter relative to non-WS controls. Regression analyses indicated that the TRF length in WS was significantly shorter than that in controls (p < 0.001). Furthermore, we found that TRF lengths in muscle adjacent to the examined epidermis were also significantly shorter than those of controls (p = 0.047). These data demonstrate for the first time that in vivo telomere loss is accelerated in systemic organs of WS patients, suggesting that abnormal telomere erosion is one of the major causes of early onset of age-related symptoms and a predisposition to sarcoma and carcinoma in WS.

Longevity in the 21st Century

The main objective of this paper is to offer a detailed analysis of mortality change in the United Kingdom at the beginning the 21st century. Starting from an exploration of 20th century mortality trends, focusing in particular on the 1990s, underlying forces driving trends in longevity are discussed. These include the ‘cohort effect’ and the ‘ageing of mortality improvement’. International mortality statistics and trends are also analysed. The pace of medical advances is discussed, with specific focus on research into the ageing process and a potential treatment for cardiovascular disease. The paper also discusses the potential threat from infectious diseases.

The analysis of underlying trends suggests that life expectancy in retirement in the U.K. is likely to increase rapidly in the early part of the 21st century. Some scientists are also claiming that we will be seeing the fruits of anti-ageing research within just a few decades.

A core theme of the paper is that future projections should be grounded in as good an understanding of the past as possible. Different methods for projecting future rates of mortality are discussed, and it is noted that emphasis should be placed on the uncertainty surrounding projections.

The financial impact of using different assumptions for future mortality is explored. Significant differences in the cost of an annuity or pension arise from the use of the various projection bases.

Life assurance companies have already declared significant losses as a result of strengthening reserves on annuity portfolios. Taken together, future increases in life expectancy, increasing awareness of the risk of providing longevity insurance, changes in legislation and shortages in market capacity and capital, may well lead to worsening annuity rates.

It is difficult to assess the precise impact of future changes in life expectancy on final salary pension schemes. There is a lack of readily available information on the mortality assumptions being used in practice. It is therefore suggested that more disclosure in this area would be helpful. Employers sponsoring final salary schemes are making promises to their employees that extend up to 70 or 80 years into the future. Actuaries should be clear in spelling out to employers and trustees the nature of the risks behind the promises they are making. Future scheme design should reflect the possibility of substantial increases in life expectancy.

An over-riding implication of the anticipated increases in life expectancy is that people will remain in work for longer in the future. The age at which people retire will inevitably have to increase, and this trend will necessarily drive changes in all aspects of our society. As actuaries we have a vital role in helping to inform the wider debate.

Extended-Term Culture of Bone Cells in a Compartmentalized Bioreactor

A specialized bioreactor is used to grow mineralizing, collagenous tissue up to 150 microm thick from an inoculum of isolated murine (mouse calvaria MC3T3-E1, American Type Culture Collection (ATCC) CRL-2593) or human (hFOB 1.19 ATCC CRL-11372) fetal osteoblasts over uninterrupted culture periods longer than 120 days (4 months). Proliferation and phenotypic progression of an osteogenic-cell monolayer into a tissue consisting of 6 or more cell layers of mature osteoblasts in the bioreactor was compared with cell performance in conventional tissue-culture polystyrene (TCPS) controls. Cells in the bioreactor basically matched results obtained in TCPS over a 15-day culture interval, but loss of insoluble extracellular matrix and an approximate doubling of apoptosis rates in TCPS after 30 days indicated that progressive instability of cultures maintained in TCPS with periodic refeeding but without subculture. In contrast, stable cultures were maintained in the bioreactor for more than 120 days, suggesting that extended-term tissue maintenance is feasible with little or no special technique. Transmission electron microscopy ultramorphology of tissue derived from hFOB 1.19 recovered from the bioreactor after only 15 days of culture showed evidence of osteocytic-like processes and gap junctions between cells like those observed in vivo, in addition to elaboration of the usual osteoblastic markers such as alkaline phosphatase activity and mineralization (alizarin red). Thus, the bioreactor design based on the principle of simultaneous growth and dialysis was shown to create an extraordinarily stable peri-cellular environment that better simulates the in vivo condition than conventional tissue culture. The bioreactor shows promise as a tool for the in vitro study of osteogenesis and osteopathology.

An Improved Method for Determining Telomere Length and Its Use in Assessing Age in Blood and Saliva

BACKGROUND

It is disputed whether telomeres shorten with age.

OBJECTIVE

The proposition of this study was to answer this question.

METHODS

The question was answered by a simple method consisting of a color reaction combined with long-term electrophoresis and weak voltage.

RESULTS

The telomeres shorten and the shortening is much higher than previously assumed.

CONCLUSION

The telomere theory of ageing and cancer is corroborated and for the first time it is possible to estimate age in blood or saliva.

Promise and problems in relating cellular senescence in vitro to aging in vivo

According to the 'Hayflick limit', human fetal fibroblasts have a uniform, limited replicative lifespan of about 50 population doublings in cell culture. This concept was extrapolated to diverse cells in the body. It seemed to decrease with the age of the cell donor and, as a form of cell senescence, was thought to underlie the aging process. More discriminating analysis, however, showed that the fibroblasts decayed in a stochastic manner from the time of their explantation, at a rate that increased with the number of population doublings in culture. There was no consistent relation to the age of the donor. Despite the contradictory evidence, the original version of the Hayflick limit retained its general acceptance. Cell senescence was attributed to the absence of telomerase in the fibroblasts, which resulted in shortening of telomeres at each division until they fell below a critical length needed for further division. However, it is well established that stem cells in renewing tissues undergo many more than 50 divisions in a lifetime, without apparent senescence. Contrary to early findings of no telomerase in most tissues, their stem cells retain telomerase and presumably telomere length despite many divisions in vivo. Massive accumulation of lipofuscin granules occurs under stress in long term crowded cultures, but the granules dissipate on subculture or neoplastic transformation. The overall results indicate a critical disjunction between cell senescence in vitro and aging in vivo. By contrast, cell culture has been useful in showing a need for telomere capping in maintaining cell stability and viability. It may also provide information about the biochemical mechanism of lipofuscin production.

SCL interacts with VEGF to suppress apoptosis at the onset of hematopoiesis

During development, hematopoiesis initiates in the yolk sac through a process that depends on VEGF/Flk1 signaling and on the function of the SCL/Tal1 transcription factor. Here we show that VEGF modifies the developmental potential of primitive erythroid progenitors and prolongs their life span. Furthermore, the survival of yolk sac erythrocytes in vivo depends on the dose of VEGF. Interestingly, in Vegflo/lo embryos carrying a hypomorph allele, Flk1-positive cells reach the yolk sac at E8.5,but are severely compromised in their ability to generate primitive erythroid precursors. These observations indicate that during embryonic development,different thresholds of VEGF are required for the migration and clonal expansion of hematopoietic precursors. The near absence of primitive erythroid precursors in Vegflo/lo embryos correlates with low levels of Scl in the yolk sac. Strikingly, gain-of-function of SCL partially complements the hematopoietic defect caused by the hypomorph Vegflo allele, and re-establishes the survival of erythroid cells and the expression of erythroid genes (Gata1 andβH1). This indicates that SCL functions downstream of VEGF to ensure an expansion of the hematopoietic compartment.

Historical claims and current interpretations of replicative aging

Replicative aging is the process by which most normal human cells "count" the number of times they have divided, eventually undergoing a growth arrest termed cellular senescence. This process is dependent on the shortening of telomeres, repeated sequences at the ends of the chromosomes. The loss of telomeric sequences with each cell division eventually induces a growth arrest that has a similar phenotype to that of cells stressed by inadequate culture or other conditions. Experiments over the past several years have identified species in which replicative aging does not occur and many examples in which a failure to proliferate has been misinterpreted as replicative senescence. Insights from these studies now permit a reevaluation of much of the seemingly contradictory data concerning replicative aging. There are good theoretical reasons for believing a limited proliferative capacity contributes to declining tissue homeostasis with increasing age. Although the presence of telomere shortening provides strong circumstantial evidence that replicative aging is occurring in vivo, thus far there is only very limited direct evidence for actual physiological effects of replicative aging.

The disparity between human cell senescence in vitro and lifelong replication in vivo

Cultured human fibroblasts undergo senescence (a loss of replicative capacity) after a uniform, fixed number of approximately 50 population doublings, commonly termed the Hayflick limit. It has been long known from clonal and other quantitative studies, however, that cells decline in replicative capacity from the time of explantation and do so in a stochastic manner, with a half-life of only approximately 8 doublings. The apparent 50-cell doubling limit reflects the expansive propagation of the last surviving clone. The relevance of either figure to survival of cells in the body is questionable, given that stem cells in some renewing tissues undergo >1,000 divisions in a lifetime with no morphological sign of senescence. Oddly enough, these observations have had little if any effect on general acceptance of the Hayflick limit in its original form. The absence of telomerase in cultured human cells and the shortening of telomeres at each population doubling have suggested that telomere length acts as a mitotic clock that accounts for their limited lifespan. This concept assumed an iconic character with the report that ectopic expression of telomerase by a vector greatly extended the lifespan of human cells. That something similar might occur in vivo seemed consistent with initial reports that most human somatic tissues lack telomerase activity. More careful study, however, has revealed telomerase activity in stem cells and some dividing transit cells of many renewing tissues and even in dividing myocytes of repairing cardiac muscle. It now seems likely that telomerase is active in vivo where and when it is needed to maintain tissue integrity. Caution is recommended in applying telomerase inhibition to kill telomerase-expressing cancer cells, because it would probably damage stem cells in essential organs and even increase the likelihood of secondary cancers. The risk may be especially high in sun-exposed skin, where there are usually thousands of p53-mutant clones of keratinocytes predisposed to cancer.

Replicative senescence in organ transplantation-mechanisms and significance

In the past two decades, transplantation has become a preferred modality of treatment of end-stage failure of vital organs. Currently, with the significant improvement in short-term graft survival rates, the main effort is concentrated on prolonging the functional life span of transplanted organs. One of the theories which were put forward to explain the progressive deterioration of transplant function was that of replicative senescence. Senescence of an organ or tissue results from age and/or environmental stress-dependant modification of cellular function. With time, the accumulation of cellular alterations may lead to deleterious effects in various organs and tissues and adversely affect transplants. In this article we are reviewing the candidate mechanisms of senescence such as telomere shortening, genetic regulation and environmental-'toxic' factors and are examining the implications of the theory of replicative senescence for organ allograft. We are also presenting our experiments with renal ischemia/reperfusion in rat serving as a model of kidney transplantation, where baseline kidney telomere length and novel marker of cellular senescence--senescence associated beta-Galactosidase (SA-Gal) expression in tissue served as markers. For the first time in vivo, we were able to show that with aging of the animals the amount of senescent cells in kidney tissue was increasing, while the average renal tissue telomere length was decreasing. The degree of tissue senescence, as determined by amount of SA-Gal positively stained cells, was inversely correlated with the recovery of the kidney function after ischemia/reperfusion injury. These results confirm the theory of replicative senescence in organ ischemia for the first time in vivo, and quantitatively validate the direct correlation between the amount of senescent cells in the organ and its susceptibility to ischemic injury. We conclude that recent advances in study of the cellular basis of senescence, in vitro and especially in vivo, may hold clues to the understanding of events which could be implicated in the damage or protection of organ allografts.

Telomerase, immortality and cancer

Replication of eukaryotic linear chromosomes is incomplete and leaves terminal gaps. The evolutionary widely distributed solution to this "end replication" is twofold: chromosome ends are capped with telomeres, bearing multiple copies of redundant telomeric sequences, and the telomerase enzyme can add (lost) telomeric repeats. Telomerase in humans, as in all mammals, is ubiquitous in all embryonic tissues. In adults, telomerase remains active in germs cells, and, although down-regulated in most somatic tissues, telomerase is active in regenerative tissues and notably, in tumor cells. Telomerase activity is linked to cellular proliferation, and its activation seems to be a mandatory step in carcinogenesis. In contrast to mammals, indeterminately growing multicellular organisms, like fish and crustaceae, maintain unlimited growth potential or 'immortality' in all somatic tissues throughout their entire life. Also this cell immortalization is brought about by maintaining telomerase expression. Disease prognosis for human tumors includes evaluation of cell proliferation, based on the detection of proliferation markers with monoclonal antibodies. The significance of the classical marker Ki-67, and of a novel marker repp-86 are compared with semiquantitative telomerase assays. For tumor therapy, telomerase inhibitors are attractive tools. Results with telomerase knock-out mice have revealed promise, but also risk of this approach. On the other side, telomerase stimulation is attractive for expanding the potential of cellular proliferation in vitro, with possible applications for transplantation of in vitro expanded human cells, for immortalizing primary human cells as improved tissue models, and for the isolation of otherwise intractable products, like genuine human monoclonal antibodies.

Putative telomere-independent mechanisms of replicative aging reflect inadequate growth conditions

Telomere shortening is the mechanism underlying replicative aging in fibroblasts. A variety of reports now claim that inactivation of the p16(INK4a)/pRB pathway is required in addition to telomere maintenance for the immortalization of cells such as skin keratinocytes and breast epithelial cells. We here show that the premature growth arrest of these cell types can be explained by an inadequate culture environment. Providing mesenchymal/epithelial interactions by cultivating the telomerase-expressing cells on feeder layers avoids the growth arrest associated with increased p16(INK4a). These results do not support a telomere-independent mechanism of replicative aging.

Hayflick, his limit, and cellular ageing

Almost 40 years ago, Leonard Hayflick discovered that cultured normal human cells have limited capacity to divide, after which they become senescent -- a phenomenon now known as the 'Hayflick limit'. Hayflick's findings were strongly challenged at the time, and continue to be questioned in a few circles, but his achievements have enabled others to make considerable progress towards understanding and manipulating the molecular mechanisms of ageing.

Telomere dynamics, end-to-end fusions and telomerase activation during the human fibroblast immortalization process

Loss of telomeric repeats during cell proliferation could play a role in senescence. It has been generally assumed that activation of telomerase prevents further telomere shortening and is essential for cell immortalization. In this study, we performed a detailed cytogenetic and molecular characterization of four SV40 transformed human fibroblastic cell lines by regularly monitoring the size distribution of terminal restriction fragments, telomerase activity and the associated chromosomal instability throughout immortalization. The mean TRF lengths progressively decreased in pre-crisis cells during the lifespan of the cultures. At crisis, telomeres reached a critical size, different among the cell lines, contributing to the peak of dicentric chromosomes, which resulted mostly from telomeric associations. We observed a direct correlation between short telomere length at crisis and chromosomal instability. In two immortal cell lines, although telomerase was detected, mean telomere length still continued to decrease whereas the number of dicentric chromosomes associated was stabilized. Thus telomerase could protect specifically telomeres which have reached a critical size against end-to-end dicentrics, while long telomeres continue to decrease, although at a slower rate as before crisis. This suggests a balance between elongation by telomerase and telomere shortening, towards a stabilized 'optimal' length.

Human longevity: an evolutionary approach

Humans are unique in having a considerably longer life span post female reproductive age as compared to other primates. It is proposed that transmission of knowledge from grandparents to progeny served as a driving force for extending human longevity. It is suggested that grandparents and grandchildren have evolved adaptive complementary behaviors that fit the educator/pupil roles. Older individuals specifically retain the ability to preserve their knowledge base that allows them to focus on transmitting to children knowledge accumulated over generations. A repair management model of aging is proposed, according to which inhibition of repair in older individuals at times of stress allows to focus resources on the task at hand. Such an adaptation will inevitably accelerate the rate of aging in animals and in humans and will appear as 'programmed aging'. It is further suggested that in humans, older individuals in early societies who are no longer useful could increase their reproductive success by activating this programmed aging mechanism, which would result in channeling resources to progeny. Decreased emotional support and mastery are mortality risk factors in the elderly, supporting this hypothesis of programmed death in humans, and providing a rationale for increasing longevity.

The telomerase challenge - an unusual problem in drug discovery

Telomerase is a ribonucleoprotein that is responsible for maintaining telomere length. The observation that telomerase activity is found in many types of tumors, but not in adjacent normal tissue, has led to the hypothesis that telomerase is a novel target for chemotherapy. Inhibitors of telomerase activity are essential to validate this hypothesis, and their design presents special opportunities and challenges.

Cellular senescence and cancer

The proliferative lifespan of normal mammalian cells is limited by intrinsic controls, which desensitize the cell-cycle machinery to extrinsic stimulation after a given number of cell divisions. One underlying clock driving this process of 'replicative senescence' is the progressive erosion of chromosome telomeres, which occurs with each round of DNA replication. This appears to trigger growth inhibition via activation of the tumour suppressor gene (TSG) product, p53, and the consequent up-regulation of the cell-cycle inhibitor p21WAF1. Other inhibitory pathways are also activated (possibly by additional clocks), including the TSG p16INK4a and the less well-defined complementation group genes. Loss of one pathway can be compensated, after a limited extension of lifespan, by further up-regulation of the others, so that to escape mortality a developing tumour must overcome multiple 'proliferative lifespan barriers' (PLBs) by successive genetic events, each conferring a new wave of clonal expansion. This provides one explanation for the existence of multiple genetic abnormalities in human cancers; furthermore, the diversity in the nature and timing of these PLBs between different cell types may explain the variation in the spectrum of abnormalities observed between the corresponding cancers. Even if all senescence pathways are inactivated, immortalization can only be achieved if erosion of telomeres is halted, before their end-protecting function is lost. This usually requires either activation of telomerase during tumour development, if the cell of origin is telomerase-negative, or up-regulation if the normal cell already has some activity, but not enough to prevent erosion. In either case, cancers often maintain near-critical telomere lengths; hence pharmacological inhibition of telomerase remains an attractive approach to the selective killing of tumour cells.