Evidence for a relationship between longevity of mammalian species and life spans of normal fibroblasts in vitro and erythrocytes in vivo

Does Effective Population Size Govern Evolutionary Differences in Telomere Length?

Lengths of telomeres vary by an order of magnitude across mammalian species. Similarly, age- and sex-standardized telomere lengths differ by up to 1 kb (14%) across human populations. How to explain these differences? Telomeres play a central role in senescence and aging, and genes that affect telomere length are likely under weak selection (i.e. telomere length is a trait that is subject to nearly neutral evolution). Importantly, natural selection is more effective in large populations than in small populations. Here, we propose that observed differences in telomere length across species and populations are largely due to differences in effective population sizes. In this perspective, we present preliminary evolutionary genetic evidence supporting this hypothesis and highlight the need for more data.

The spontaneous immortalization probability of mammalian cell culture strains, as their proliferative capacity, correlates with species body mass, not longevity

BACKGROUND

The Peto's paradox consists in the observation that individuals from long-lived and large animal species do not experience a higher cancer incidence, despite being exposed for longer time to the possibility of accumulating mutations and having more target cells exposed to the phenomenon. The existence of this paradox has been recently confirmed (Vincze et al., 2022). Concurrently, robust evidence has been published that longevity involves a convergent evolution of cellular mechanisms that prevent the accumulation of mutations (Cagan et al., 2022). It remains unclear which cellular mechanisms are critical to allow the evolution of a large body mass while keeping cancer at bay.

METHODS

Adding to existing data linking cellular replicative potential and species body mass (Lorenzini et al., 2005), we have grown a total of 84 skin fibroblast cell strains from 40 donors of 17 mammalian species and analyzed their Hayflick's limit, i.e., their senescent plateau, and eventual spontaneous immortalization escape. The correlation of immortalization and replicative capacity of the species with their longevity, body mass and metabolism has been assessed through phylogenetic multiple linear regression (MLR).

RESULTS

The immortalization probability is negatively related to species body mass. The new evaluation and additional data about replicative potential strengthen our previous observation, confirming that stable and extended proliferation is strongly correlated with the evolution of a large body mass rather than lifespan.

CONCLUSION

The relation between immortalization and body mass suggests a need to evolve stringent mechanisms that control genetic stability during the evolution of a large body mass.

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.

Taking flight: An ecological, evolutionary and genomic perspective on bat telomeres

Over 20% of all living mammals are bats (order Chiroptera). Bats possess extraordinary adaptations including powered flight, laryngeal echolocation and a unique immune system that enables them to tolerate a diversity of viral infections without presenting clinical disease symptoms. They occupy multiple trophic niches and environments globally. Significant physiological and ecological diversity occurs across the order. Bats also exhibit extreme longevity given their body size with many species showing few signs of ageing. The molecular basis of this extended longevity has recently attracted attention. Telomere maintenance potentially underpins bats' extended healthspan, although functional studies are still required to validate the causative mechanisms. In this review, we detail the current knowledge on bat telomeres, telomerase expression, and how these relate to ecology, longevity and life-history strategies. Patterns of telomere shortening and telomerase expression vary across species, and comparative genomic analyses suggest that alternative telomere maintenance mechanisms evolved in the longest-lived bats. We discuss the unique challenges faced when working with populations of wild bats and highlight ways to advance the field including expanding long-term monitoring across species that display contrasting life-histories and occupy different environmental niches. We further review how new high quality, chromosome-level genome assemblies can enable us to uncover the molecular mechanisms governing telomere dynamics and how phylogenomic analyses can reveal the adaptive significance of telomere maintenance and variation in bats.

Colon fibroblasts from Pirc rats (F344/NTac-Apcam1137 ) exhibit a proliferative and inflammatory phenotype that could support early stages of colon carcinogenesis

The role of fibroblast APC mutation in carcinogenesis is not clear. Apc^+/− colon fibroblasts have been previously characterized: however, little is known about their behavior at very early‐stage of colon carcinogenesis. We cultured colon mucosa fibroblasts (PCF, Apc^+/−) of Pirc rats (F344/NTac‐Apc^am1137) at an early stage of tumorigenesis, in absence of preneoplastic lesions, and of age‐matched wt (WCF): DNA damage levels, inflammatory phenotype and the expression of known markers of CAFs were analyzed. The latter were also assessed by microarray analysis on colon normal mucosa of Pirc and wt animals. PCF exhibited higher proliferative rates (P < .001) and delayed replicative senescence onset (P < .05) compared to WCF, along with a lower level of oxidative DNA damage (P < .05). Furthermore, a constitutively higher expression of COX‐2 and sensitivity to inflammatory stimuli was found in PCF compared to WCF (P < .05), accompanied by higher invasive capability (P < .05) and presence of cytoplasmic chromatin foci (cytoplasmic chromatin foci, P < .05). However, they neither expressed CAFs markers (α‐SMA, IL‐6) nor responded to CAFs activating stimuli (TGF‐β). Accordingly, CAFs markers and activating stimuli resulted down‐regulated in Pirc normal mucosa compared to wt, whereas DNA damage response and tolerance pathways were overexpressed. These data show for the first time that a proliferative and inflammatory phenotype characterizes Apc^+/− colon fibroblasts since very early stages of colon tumorigenesis, and indicate a role of Apc mutation in driving fibroblast phenotypic alterations that could support the establishment of a protumorigenic environment. Early pharmacological targeting of these dysfunctions might impact on tumor prevention in FAP patients.

What's new?

Heterozygous mutations in APC represent the earliest event in sporadic colorectal carcinogenesis onset and cause familial adenomatous polyposis syndrome. However, the role of APC‐mutated fibroblasts remains unclear. Here, Apc^+/‐ fibroblasts isolated from apparently‐normal colon tissue of Pirc rats showed proliferative, inflammatory features and resistance to oxidative DNA damage, although they did not show cancer‐associated fibroblast features. These data suggest that, at the very early stages of colon tumourigenesis, Apc‐mutated colon fibroblasts favour the establishment of a pro‐tumourigenic environment for pre‐neoplastic lesion development. Early pharmacological targeting of these dysfunctions might be valuable for tumour prevention in familial adenomatous polyposis patients.

Simple Detection Methods for Senescent Cells: Opportunities and Challenges

Cellular senescence, the irreversible growth arrest of cells from conditional renewal populations combined with a radical shift in their phenotype, is a hallmark of ageing in some mammalian species. In the light of this, interest in the detection of senescent cells in different tissues and different species is increasing. However much of the prior work in this area is heavily slanted towards studies conducted in humans and rodents; and in these species most studies concern primary fibroblasts or cancer cell lines rendered senescent through exposure to a variety of stressors. Complex techniques are now available for the detailed analysis of senescence in these systems. But, rather than focussing on these methods this review instead examines techniques for the simple and reproducible detection of senescent cells. Intended primary for the non-specialist who wishes to quickly detect senescent cells in tissues or species which may lack a significant evidence base on the phenomenon it emphasises the power of the original techniques used to demonstrate the senescence of cells, their interrelationship with other markers and their potential to inform on the senescent state in new species and archival specimens.

On the evolution of cellular senescence

The idea that senescent cells are causally involved in aging has gained strong support from findings that the removal of such cells alleviates many age‐related diseases and extends the life span of mice. While efforts proceed to make therapeutic use of such discoveries, it is important to ask what evolutionary forces might have been behind the emergence of cellular senescence, in order better to understand the biology that we might seek to alter. Cellular senescence is often regarded as an anti‐cancer mechanism, since it limits the division potential of cells. However, many studies have shown that senescent cells often also have carcinogenic properties. This is difficult to reconcile with the simple idea of an anti‐cancer mechanism. Furthermore, other studies have shown that cellular senescence is involved in wound healing and tissue repair. Here, we bring these findings and ideas together and discuss the possibility that these functions might be the main reason for the evolution of cellular senescence. Furthermore, we discuss the idea that senescent cells might accumulate with age because the immune system had to strike a balance between false negatives (overlooking some senescent cells) and false positives (destroying healthy body cells).

Single-cell analyses of aging, inflammation and senescence

Single-cell gene expression (transcriptomics) data are becoming robust and abundant, and are increasingly used to track organisms along their life-course. This allows investigation into how aging affects cellular transcriptomes, and how changes in transcriptomes may underlie aging, including chronic inflammation (inflammaging), immunosenescence and cellular senescence. We compiled and tabulated aging-related single-cell datasets published to date, collected and discussed relevant findings, and inspected some of these datasets ourselves. We specifically note insights that cannot (or not easily) be based on bulk data. For example, in some datasets, the fraction of cells expressing p16 (CDKN2A), one of the most prominent markers of cellular senescence, was reported to increase, in addition to its upregulated mean expression over all cells. Moreover, we found evidence for inflammatory processes in most datasets, some of these driven by specific cells of the immune system. Further, single-cell data are specifically useful to investigate whether transcriptional heterogeneity (also called noise or variability) increases with age, and many (but not all) studies in our review report an increase in such heterogeneity. Finally, we demonstrate some stability of marker gene expression patterns across closely similar studies and suggest that single-cell experiments may hold the key to provide detailed insights whenever interventions (countering aging, inflammation, senescence, disease, etc.) are affecting cells depending on cell type.

Follow Me! A Tale of Avian Heart Development with Comparisons to Mammal Heart Development

Avian embryos have been used for centuries to study development due to the ease of access. Because the embryos are sheltered inside the eggshell, a small window in the shell is ideal for visualizing the embryos and performing different interventions. The window can then be covered, and the embryo returned to the incubator for the desired amount of time, and observed during further development. Up to about 4 days of chicken development (out of 21 days of incubation), when the egg is opened the embryo is on top of the yolk, and its heart is on top of its body. This allows easy imaging of heart formation and heart development using non-invasive techniques, including regular optical microscopy. After day 4, the embryo starts sinking into the yolk, but still imaging technologies, such as ultrasound, can tomographically image the embryo and its heart in vivo. Importantly, because like the human heart the avian heart develops into a four-chambered heart with valves, heart malformations and pathologies that human babies suffer can be replicated in avian embryos, allowing a unique developmental window into human congenital heart disease. Here, we review avian heart formation and provide comparisons to the mammalian heart.

Further assessment of stochastic proliferation and its potential application to hematopoietic scaling across species

Spleen colony-forming unit (CFU-s) growth in spleen colonies is a stochastic process in which CFU-s, with each cell division, can either self-renew or differentiate, but not both. The fundamental parameter governing this process is p, or the probability of CFU-s self-renewing. Previously, when CFU-s growth was modeled by Monte Carlo simulations, p was kept constant during the 20 cell cycles required for the modeling. However, it is known that CFU-s self-renewal undergoes decline with proliferation. In the present study, this was taken into consideration, such that p was forced to undergo a small decline with each cell division. These new Monte Carlo calculations give an improved fit to CFU-s cumulative growth curves as compared with those calculations using fixed p. This new model, referred to as the variable p model, offers an explanation as to how large mammals can amplify marrow output from stem cell compartments that are no larger than those found in small mammals. It is a model in which small changes in active stem cell aging generate disproportionally large increases in the size of active stem cell clones.

Rates of erythropoiesis in mammals and their relationship with lifespan and hematopoietic stem cells aging

Investigations on possible links between hematological parameters and longevity are nearly absent. We tested the hypothesis that a fast rate of erythropoiesis, causing an earlier aging of the hematopoietic stem cells pool, contributes to a shorter lifespan. With this aim, we employed a new quantity, daily produced red blood cells per gram of body mass, as a measure of mass-specific rate of erythropoiesis. We found that among mammals rate of erythropoiesis and maximum lifespan are significantly correlated, independently from mass residuals. This seems to be confirmed also by intra-species comparisons and, although with limited data, by the significant correlation of rate of erythropoiesis and rate of telomere shortening in leukocytes (a proxy for hematopoietic stem cell telomere shortening). In our view, this may give a link of causality between rate of erythropoiesis and maximum lifespan. Further studies could test a similar hypothesis also for other kinds of stem cells.

Challenges in realizing selectivity for nanoparticle biodistribution and clearance: lessons from gold nanoparticles

The field of nanomedicine has received much attention for its potential to allow for targeted identification and treatment of tumors, while sparing healthy tissue. This promise has yet to be clinically realized; instead nanomedicine has translated into clinical benefit via formulations that improve the pharmacokinetics and toxicity profiles of toxic chemotherapeutic agents. In this perspective, we highlight that several of the defining strategies for using nanoparticles intravenously to target solid tumors have limited supporting data in animal studies. Namely, it does not appear that reducing macrophage (and other cell type) uptake in vitro leads to better biodistribution in vivo, nor does increasing blood circulation time nor active targeting. We suggest instead that the coming decade will primarily see nanoparticles impact immunotherapy and local/pseudolocal cancer therapy.

The role of cellular senescence in aging through the prism of Koch-like criteria

Since Hayflick's discovery of cellular senescence (CS), a great volume of knowledge in the field has been accumulated and intensively discussed. Here, we attempted to organize the evidence "for" and "against" the hypothesized causal role of CS in aging. For that purpose, we utilized robust Koch-like logical criteria, based on the assumption that some quantitative relationships between the accumulation of senescent cells and aging rate should exist. If so, it could be expected that (i) the "CS load" would be greater in the premature aging phenotype and lesser in longevity phenotype; (ii) CS would promote age-related diseases, and (iii) the interventions that modulate the levels of senescent cells should also modulate health/lifespan. The analysis shows that CS can be considered a causal factor of aging and an important player in various age-related diseases, though its contribution may greatly vary across species. While the relative impact of senescent cells to aging could overall be rather limited and their elimination is hardly expected to be the "fountain of youth", the potential benefits of the senolytic strategy seems a promising option in combating age-related diseases and extending healthspan.

Convergent adaptation of cellular machineries in the evolution of large body masses and long life spans

In evolutionary terms, life on the planet has taken the form of independently living cells for the majority of time. In comparison, the mammalian radiation is a relatively recent event. The common mammalian ancestor was probably small and short-lived. The “recent” acquisition of an extended longevity and large body mass of some species of mammals present on the earth today suggests the possibility that similar cellular mechanisms have been influenced by the forces of natural selection to create a convergent evolution of longevity. Many cellular mechanisms are potentially relevant for extending longevity; in this assay, we review the literature focusing primarily on two cellular features: (1) the capacity for extensive cellular proliferation of differentiated cells, while maintaining genome stability; and (2) the capacity to detect DNA damage. We have observed that longevity and body mass are both positively linked to these cellular mechanisms and then used statistical tools to evaluate their relative importance. Our analysis suggest that the capacity for extensive cellular proliferation while maintaining sufficient genome stability, correlates to species body mass while the capacity to correctly identify the presence of DNA damage seems more an attribute of long-lived species. Finally, our data are in support of the idea that a slower development, allowing for better DNA damage detection and handling, should associate with longer life span.

Cell culture-based profiling across mammals reveals DNA repair and metabolism as determinants of species longevity

Mammalian lifespan differs by >100 fold, but the mechanisms associated with such longevity differences are not understood. Here, we conducted a study on primary skin fibroblasts isolated from 16 species of mammals and maintained under identical cell culture conditions. We developed a pipeline for obtaining species-specific ortholog sequences, profiled gene expression by RNA-seq and small molecules by metabolite profiling, and identified genes and metabolites correlating with species longevity. Cells from longer lived species up-regulated genes involved in DNA repair and glucose metabolism, down-regulated proteolysis and protein transport, and showed high levels of amino acids but low levels of lysophosphatidylcholine and lysophosphatidylethanolamine. The amino acid patterns were recapitulated by further analyses of primate and bird fibroblasts. The study suggests that fibroblast profiling captures differences in longevity across mammals at the level of global gene expression and metabolite levels and reveals pathways that define these differences.

Multiple facets of p53 in senescence induction and maintenance

Cellular senescence is a state of durable cell cycle arrest with metabolic activities distinct from those of the proliferative state. Since senescence was originally reported to be induced by various genotoxic stressors, such as telomere erosion and oncogenic signaling, it has been proposed to play a pivotal role in aging‐related changes and as an antitumorigenic barrier in vivo. However, the mechanisms underlying its induction and maintenance remain entirely elusive. We have recently found that abrupt activation of p53 at G₂ results in a cell skipping mitosis and subsequently undergoing senescence. Surprisingly, we have also found that downregulation of p53 by SCF^Fbxo22 is crucial for the induction of a senescence‐associated phenotype. In this review, we provide an overview of recent advances in understanding the mechanisms underlying the timing and magnitude of activation of p53 during senescence.

Cellular Senescence as the Causal Nexus of Aging

In this paper we present cellular senescence as the ultimate driver of the aging process, as a "causal nexus" that bridges microscopic subcellular damage with the phenotypic, macroscopic effect of aging. It is important to understand how the various types of subcellular damage correlated with the aging process lead to the larger, visible effects of anatomical aging. While it has always been assumed that subcellular damage (cause) results in macroscopic aging (effect), the bridging link between the two has been hard to define. Here, we propose that this bridge, which we term the "causal nexus", is in fact cellular senescence. The subcellular damage itself does not directly cause the visible signs of aging, but rather, as the damage accumulates and reaches a critical mass, cells cease to proliferate and acquire the deleterious "senescence-associated secretory phenotype" (SASP) which then leads to the macroscopic consequences of tissue breakdown to create the physiologically aged phenotype. Thus senescence is a precondition for anatomical aging, and this explains why aging is a gradual process that remains largely invisible during most of its progression. The subcellular damage includes shortening of telomeres, damage to mitochondria, aneuploidy, and DNA double-strand breaks triggered by various genetic, epigenetic, and environmental factors. Damage pathways acting in isolation or in concert converge at the causal nexus of cellular senescence. In each species some types of damage can be more causative than in others and operate at a variable pace; for example, telomere erosion appears to be a primary cause in human cells, whereas activation of tumor suppressor genes is more causative in rodents. Such species-specific mechanisms indicate that despite different initial causes, most of aging is traced to a single convergent causal nexus: senescence. The exception is in some invertebrate species that escape senescence, and in non-dividing cells such as neurons, where senescence still occurs, but results in the SASP rather than loss of proliferation plus SASP. Aging currently remains an inevitable endpoint for most biological organisms, but the field of cellular senescence is primed for a renaissance and as our understanding of aging is refined, strategies capable of decelerating the aging process will emerge.

Ankyrin exposure induced by activated protein kinase C plays a potential role in erythrophagocytosis

BACKGROUND

In physiological and pathological conditions activated protein kinace C (PKC) has been observed in the erythrocytes. Externalization of ankyrin followed by Arg-Gly-Asp (RGD)/integrin recognition also triggers erythrophagocytosis. In the present study, to test whether activated PKC is associated with ankyrin exposure in erythrophagocytosis.

METHODS

Phorbol 12-myristate-13-acetate (PMA)-induced PKC activation and ankyrin phosphorylation were tested, and under different treatment conditions the subpopulation of erythrocytes with ankyrin exposure and the levels of intracellular calcium were analyzed by flow cytometry.

RESULTS

Results showed that treatment of erythrocytes with PMA in a calcium-containing buffer led to ankyrin exposure. In the absence of extracellular calcium, no ankyrin exposure was observed. PKC inhibition with calphostin C, a blocker of the PMA binding site, completely prevented the calcium entry, protein phosphorylation and ankyrin exposure. PKC inhibition with chelerythrine chloride, an inhibitor of the active site, diminished the level of ankyrin-exposing cells and ankyrin phosphorylation; however it even led to a higher percentage of cells with increased levels of calcium than with PMA treatment alone. Although PKC was activated and ankyrin phosphorylation occurred, no ankyrin exposure was observed in the absence of extracellular calcium.

CONCLUSION

Analyses of results suggested that PMA induces calcium influx into the erythrocytes, leading to the activation of calcium-dependent enzymes and the phosphorylation of membrane proteins, ultimately inducing ankyrin exposure and erythrophagocytosis. This study may provide insights into the molecular mechanisms of removing aged or diseased erythrocytes.

[Principles of biological aging]

The aging process is the substrate on which aging-associated diseases develop; therefore, the scientific discipline of gerontology aims at understanding this biological aging process, which refers to the progressive increase in the risk of death caused by a loss of body functions. Studies in simple model organisms demonstrate that pharmacological substances, genetic interventions and dietary restriction can slow down the process of aging. The cell culture model of cellular senescence gives researchers the opportunity to conduct studies in a system more closely related to the human organism; therefore, cells from different human tissues are cultured in vitro until they stop proliferating. This permanent growth arrest is called cellular senescence. Recent studies have demonstrated that senescent cells also accumulate in many tissues in vivo and contribute to age-related pathologies.

A role for SUV39H1-mediated H3K9 trimethylation in the control of genome stability and senescence in WI38 human diploid lung fibroblasts

Cellular senescence has been associated with the age-dependent decline in tissue repair and regeneration, the increasing deterioration of the immune system, and the age-dependent increase in the incidence of cancer. Here, we show that senescence of human lung fibroblast WI-38 cells is associated with extensive changes to the gene expression profile, including the differential expression of transcriptional and epigenetic regulators. Among those, SUV39H1 was downregulated in senescent cells, correlated with a decrease in global H3K9 trimethylation, reduced H3K9me3 levels in repetitive DNA sequence regions such as satellites and transposable elements, and increased transcription of these repetitive DNA sequences. This indicates that SUV39H1 plays a role in limiting genomic instability in dividing cells and suggests that SUV39H1 downregulation may contribute to the establishment of senescence by increasing genomic instability. Additionally, the manipulation of SUV39H1 expression levels resulted in altered cell cycle distribution, suggesting a causal role of SUV39H1 in the establishment of cellular senescence. Thus, based on our findings and the results from previous reports, we propose a model in which SUV39H1 downregulation promotes the establishment of cellular senescence.

WI-38 senescence is associated with global and site-specific hypomethylation

Cellular senescence plays an important role in the age-dependent functional decline of organs and organ systems, as well as in age-related pathologies, such as cancer. Therefore, a better understanding of its underlying molecular mechanisms is crucial in the search for intervening measures. In this study, we considered the role of DNA methylation in senescence. We found that senescence is associated with global DNA hypomethylation, but also involves site-specific DNA hypo- and hypermethylation. In some cases, this differential methylation may affect gene expression and thereby modulate functional processes within cells. However, the majority of the CpG sites that were differentially methylated did not correspond with altered gene expression, suggesting that DNA methylation affects senescence by other means also, such as, for instance, genome stability.

A new hypothesis of aging

INTRODUCTION AND AIMS

There are over 300 hypotheses of aging, but none of them has enough predictive power to explain most experiments and observations on this process. On the basis of a critical analysis of the most relevant data on aging, especially on the factors that influences its rhythm, we present a new hypothesis, as well as the way the hypothesis' predictions explain some of the phylogenetic implications of the aging process.

METHODS

The hypothesis starts from a new, biochemical view on evolution and the behavior of living matter. According to this view, life is a fabric of chemical reactions that sustain each other. Reactants and energy support are needed for these reactions to take place in a cell. Given this, aging stems from a leftward shift of the global equilibrium of some biochemical reactions involved in cell differentiation and repair, which take place at a high level during the organism's growth period. In time, for species with evident aging, some reactions lose their specificity, which affects cell division and differentiation. This, in turn, influences cell energy metabolism.

RESULTS

Cell and tissue degeneration appears when, while some specific reactions are absent, non-specific reactions such as those of cell proliferation receive additional energy support.

CONCLUSIONS

This hypothesis explains phylogenetic differences related to lifespan and longevity, and body-size differences between species and within the same species.

Population doublings of murine CD4(+) memory T cells during continuous antigen stimulation in vivo

We investigated the expansion rate of CD4(+) memory T cells using a newly developed in vivo system. Neonatal thymectomy abrogates the subsequent production of T cells and induces autoimmune gastritis (AIG) by the activation of CD4(+) T cells; this disease was transferred into athymic nude mice through the inoculation of splenic CD4(+) memory T cells. The transferred CD4(+) T cells increased logarithmically in number during the first 2months in the spleen of the recipients. The serial transfer of these splenocytes at two-month intervals revealed that the numbers of the AIG-transferable generations were inversely correlated with the age of the first AIG donors. The duration of the AIG-promoting capacity of CD4(+) T cells under continuous antigenic stimulation in vivo was approximately equivalent-one and a half years. These results indicate that there exists an intrinsic population doubling limit in memory CD4(+) T cells similar to that of self-renewing naïve ones.

Cellular metabolic rates from primary dermal fibroblast cells isolated from birds of different body masses

The rate of metabolism is the speed at which organisms use energy, an integration of energy transformations within the body; it governs biological processes that influence rates of growth and reproduction. Progress at understanding functional linkages between whole organism metabolic rate and underlying mechanisms that influence its magnitude has been slow despite the central role this issue plays in evolutionary and physiological ecology. Previous studies that have attempted to relate how cellular processes translate into whole-organism physiology have done so over a range of body masses of subjects. However, the data still remains controversial when observing metabolic rates at the cellular level. To bridge the gap between these ideas, we examined cellular metabolic rate of primary dermal fibroblasts isolated from 49 species of birds representing a 32,000-fold range in body masses to test the hypothesis that metabolic rate of cultured cells scales with body size. We used a Seahorse XF-96 Extracellular flux analyzer to measure cellular respiration in fibroblasts. Additionally, we measured fibroblast size and mitochondrial content. We found no significant correlation between cellular metabolic rate, cell size, or mitochondrial content and body mass. Additionally, there was a significant relationship between cellular basal metabolic rate and proton leak in these cells. We conclude that metabolic rate of cells isolated in culture does not scale with body mass, but cellular metabolic rate is correlated to growth rate in birds.

Senescence in cell oxidative status in two bird species with contrasting life expectancy

Oxidative stress occurs when the production of reactive oxygen species (ROS) by an organism exceeds its capacity to mitigate the damaging effects of the ROS. Consequently, oxidative stress hypotheses of ageing argue that a decline in fecundity and an increase in the likelihood of death with advancing age reported at the organism level are driven by gradual disruption of the oxidative balance at the cellular level. Here, we measured erythrocyte resistance to oxidative stress in the same individuals over several years in two free-living bird species with contrasting life expectancy, the great tit (known maximum life expectancy is 15.4 years) and the Alpine swift (26 years). In both species, we found evidence for senescence in cell resistance to oxidative stress, with patterns of senescence becoming apparent as subjects get older. In the Alpine swift, there was also evidence for positive selection on cell resistance to oxidative stress, the more resistant subjects being longer lived. The present findings of inter-individual selection and intra-individual deterioration in cell oxidative status at old age in free-living animals support a role for oxidative stress in the ageing of wild animals.

The mitogen-inducible gene-6 is involved in regulation of cellular senescence in normal diploid fibroblasts

BACKGROUND INFORMATION

The mitogen-inducible gene-6 (Mig-6) is a non-kinase scaffolding adaptor protein. It has been shown that Mig-6 may play important roles in regulating stress response, maintaining homeostasis and functioning as a tumour suppressor. In this study, we investigated the role of Mig-6 in cellular senescence.

RESULTS

Our results showed that Mig-6 is up-regulated during the senescence process. Functional analysis indicated that cells over-expressing Mig-6 have reduced DNA synthesis and showed the signs of senescence. Knockdown of Mig-6 delayed the initiation of Ras-induced cellular senescence. These results suggest that the increase of Mig-6 expression contributes to establishment of cellular senescence. Furthermore, our results showed that Mig-6 induction of senescence is related to its inhibition of EGF receptor (EGFR)/Erb B signalling. Subsequent analysis of the mechanism responsible for the up-regulation of its expression showed that FOXO3A transcriptionally up-regulates Mig-6 expression via directly binding to the FOXO response element in Mig-6 5'-flanking regulatory sequences.

CONCLUSIONS

Mig-6 induces premature senescence via functioning in regulation of cellular senescence in normal diploid fibroblasts.

FOXA1 mediates p16(INK4a) activation during cellular senescence

Mechanisms governing the transcription of p16(INK4a), one of the master regulators of cellular senescence, have been extensively studied. However, little is known about chromatin dynamics taking place at its promoter and distal enhancer. Here, we report that Forkhead box A1 protein (FOXA1) is significantly upregulated in both replicative and oncogene-induced senescence, and in turn activates transcription of p16(INK4a) through multiple mechanisms. In addition to acting as a classic sequence-specific transcriptional activator, FOXA1 binding leads to a decrease in nucleosome density at the p16(INK4a) promoter in senescent fibroblasts. Moreover, FOXA1, itself a direct target of Polycomb-mediated repression, antagonizes Polycomb function at the p16(INK4a) locus. Finally, a systematic survey of putative FOXA1 binding sites in the p16(INK4a) genomic region revealed an ∼150 kb distal element that could loop back to the promoter and potentiate p16(INK4a) expression. Overall, our findings establish several mechanisms by which FOXA1 controls p16(INK4a) expression during cellular senescence.

Melatonin suppresses doxorubicin-induced premature senescence of A549 lung cancer cells by ameliorating mitochondrial dysfunction

Melatonin is an indolamine that is synthesized in the pineal gland and shows a wide range of physiological functions. Although the anti-aging properties of melatonin have been reported in a senescence-accelerated mouse model, whether melatonin modulates cellular senescence has not been determined. In this study, we examined the effect of melatonin on anticancer drug-induced cellular premature senescence. We found that the doxorubicin (DOX)-induced senescence of A549 human lung cancer cells and IMR90 normal lung cells was substantially inhibited by cotreatment with melatonin in a dose-dependent manner. Mechanistically, the DOX-induced G2/M phase cell cycle arrest and the decrease in cyclinB and cdc2 expression were not affected by melatonin. However, the DOX-induced increase in intracellular levels of ROS, which is necessary for premature senescence, was completely abolished upon melatonin cotreatment. In addition, the reduction in mitochondrial membrane potential that occurs upon DOX treatment was inhibited by melatonin. An aberrant increase in mitochondrial respiration was also significantly suppressed by melatonin, indicating that melatonin ameliorates the mitochondrial dysfunction induced by DOX treatment. The treatment of A549 cells with luzindole, a potent inhibitor of melatonin receptors, failed to prevent the effects of melatonin treatment on mitochondrial functions and premature senescence in cells also treated with DOX; this suggests that melatonin suppresses DOX-induced senescence in a melatonin receptor-independent manner. Together, these results reveal that melatonin has an inhibitory effect of melatonin on premature senescence at the cellular level and that melatonin protects A549 cells from DOX-induced senescence. Thus, melatonin might have the therapeutic potential to prevent the side effects of anticancer drug therapy.

Protein oxidative damage at the crossroads of cellular senescence, aging, and age-related diseases

Protein damage mediated by oxidation, protein adducts formation with advanced glycated end products and with products of lipid peroxidation, has been implicated during aging and age-related diseases, such as neurodegenerative diseases. Increased protein modification has also been described upon replicative senescence of human fibroblasts, a valid model for studying aging in vitro. However, the mechanisms by which these modified proteins could impact on the development of the senescent phenotype and the pathogenesis of age-related diseases remain elusive. In this study, we performed in silico approaches to evidence molecular actors and cellular pathways affected by these damaged proteins. A database of proteins modified by carbonylation, glycation, and lipid peroxidation products during aging and age-related diseases was built and compared to those proteins identified during cellular replicative senescence in vitro. Common cellular pathways evidenced by enzymes involved in intermediate metabolism were found to be targeted by these modifications, although different tissues have been examined. These results underscore the potential effect of protein modification in the impairment of cellular metabolism during aging and age-related diseases.

Explaining differences in the lifespan and replicative capacity of cells: a general model and comparative analysis of vertebrates

A better understanding of the factors that govern individual cell lifespan and the replicative capacity of cells (i.e. Hayflick's limit) is important for addressing disease progression and ageing. Estimates of cell lifespan in vivo and the replicative capacity of cell lines in culture vary substantially both within and across species, but the underlying reasons for this variability remain unclear. Here, we address this issue by presenting a quantitative model of cell lifespan and cell replicative capacity. The model is based on the relationship between cell mortality and metabolic rate, which is supported with data for different cell types from ectotherms and endotherms. These data indicate that much of the observed variation in cell lifespan and cell replicative capacity is explained by differences in cellular metabolic rate, and thus by the three primary factors that control metabolic rate: organism size, organism temperature and cell size. Individual cell lifespan increases as a power law with both body mass and cell mass, and decreases exponentially with increasing temperature. The replicative capacity of cells also increases with body mass, but is independent of temperature. These results provide a point of departure for future comparative studies of cell lifespan and replicative capacity in the laboratory and in the field.

Aging in the mouse and perspectives of rejuvenation through induced pluripotent stem cells (iPSCs)

The mouse is a perfect model to study aging in mammals. It has a relatively short life span and genetic manipulations in this species are well established. Most interestingly, the mouse is a fantastic tool to produce stem cells. Forced expression of only four transcription factors (Oct3/4, Sox2, Klf4, and c-Myc) in murine and human somatic cells resets the expression of genes that are characteristic of differentiated cells and consequently induces the formation of pluripotent stem cells (iPSCs). This technology opens new and exciting possibilities in medical research, especially personalized cell therapies for treating human disease. To treat damaged tissues or repair organs in elderly patients, it will be necessary to establish iPSCs from their tissues. To determine the feasibility of using this technology with elderly patients, we asked whether it was indeed possible to establish iPSCs from the tissues of aged mice and to differentiate them to tissue cells. We succeeded in establishing iPSC clones using bone marrow (BM) from 21-month-old EGFP-C57BL/6 mice, which had been cultured for 4 days in the presence of granulocyte macrophage-colony stimulating factor (GM-CSF). Our iPSCs from aged mice (aged iPSCs) and those from mouse embryonic fibroblasts (MEFs) strongly expressed SSEA-1 and Pou5f1, and showed strong alkaline phosphatase (AP) activity. Our aged iPSCs made teratomas when injected into the back skin of syngeneic mice, and differentiated to tissue cells of three germ lines in vitro. Further experiments to make chimeric mice and germ line cells will determine whether the aged iPSCs possess the properties of much younger cells and are capable of regenerating aged mice.

Asthma in the elderly: Current understanding and future research needs--a report of a National Institute on Aging (NIA) workshop

Asthma in the elderly is underdiagnosed and undertreated, and there is a paucity of knowledge on the subject. The National Institute on Aging convened this workshop to identify what is known and what gaps in knowledge remain and suggest research directions needed to improve the understanding and care of asthma in the elderly. Asthma presenting at an advanced age often has similar clinical and physiologic consequences as seen with younger patients, but comorbid illnesses and the psychosocial effects of aging might affect the diagnosis, clinical presentation, and care of asthma in this population. At least 2 phenotypes exist among elderly patients with asthma; those with longstanding asthma have more severe airflow limitation and less complete reversibility than those with late-onset asthma. Many challenges exist in the recognition and treatment of asthma in the elderly. Furthermore, the pathophysiologic mechanisms of asthma in the elderly are likely to be different from those seen in young asthmatic patients, and these differences might influence the clinical course and outcomes of asthma in this population.

Higher DNA repair activity is related with longer replicative life span in mammalian embryonic fibroblast cells

Since the detailed comparison of DNA repair activities among mammalian embryonic fibroblast cells with different replicative life spans has not been investigated, we tested DNA repair activities in embryonic fibroblast cells derived from mammals including human, dog, rat, and mouse. The cell viability after treatment of four DNA damage agents appeared to be decreased in the order of human embryonic fibroblasts (HEFs) > dog embryonic fibroblasts (DEFs) > rat embryonic fibroblasts (REFs) > mouse embryonic fibroblasts (MEFs) although statistical significance was lacking. The amounts of strand breaks and AP (apurinic/apyrimidinic) sites also appear to be decreased in the order of HEFs > DEFs > REFs ≥ MEFs after treatment of DNA damage agents. The DNA repair activities and rates including base excision repair (BER), nucleotide excision repair (NER) and double-strand break repair (DSBR) including non-homologous end-joining (NHEJ) decreased again in the order of HEFs > DEFs > REFs ≥ MEFs. BER and NHEJ activities in 3% O(2) also decreased in the order of HEFs > DEFs > REFs > MEFs. This order in DNA repair activity appears to be coincident with that of replicative life span of fibroblasts and that of life span of mammals. These results indicate that higher DNA repair activity is related with longer replicative life span in embryonic fibroblast cells.

Improvement of a method to reproducibly immortalize human T cells by oncogene transfection

The method to immortalize human T cells efficiently and reproduciblyby oncogene transfection was improved. T cells were first grown selectively from peripheralblood lymphocytes population of healthy donors andatopic asthma patients, and from lymph nodelymphocytes population of lung cancer patients byactivating with mitogens (phytohemagglutinin andconcanavalin A) and recombinant human interleukin-2(rhIL-2) for five days. Plasmids expressingoncogenes, such as c-Ha-ras, c-myc,c-fos, v-myb and v-jun under the controlof human cytomegalovirus promoter, were then introducedinto these stimulated lymphocytes either separately orin various combinations by electropolation. Afterculturing these transfected lymphocytes for recoveryfor 1 day, they were fed every 3-4 days. Although all the control cells died within one month,oncogene-transfected lymphocytes continued toproliferate actively even for more than severalmonths, indicating that oncogene-transfectedlymphocytes were successfully immortalized. Flowcytometric analyses revealed that most of theimmortalized lymphocytes were T cells expressingCD3(+) surface antigen. The ratios of CD4(+)and CD8(+) subpopulations in immortalized T cellsderived from healthy donors varied, depending onthe kinds of oncogenes used. However, CD8(+)subpopulation in immortalized T cells derived fromcancer patients and atopic asthma patients weredominant, independent of the kinds of oncogenes. These immortalized T cells showed differentproliferative responses in the presence or absence ofexogenous human rhIL-2, depending on their origin ofdonors. Furthermore, immortalized T cells derivedfrom healthy donors showed stronger cytotoxicityagainst K562 cells, suggesting that MHC-nonrestrictedkiller T cells in T cell population were alsoimmortalized. Immortalized T cell lines, whichproliferate continuously without stimulation of amitogen or antigen in medium containing a lowconcentration of rhIL-2, have been maintained for morethan 2 years without any growth rate decrease.

Cell division and aging of the organism

The capacity to regenerate cell compartments through cell proliferation is an important characteristic of many developed metazoan tissues. Pre- and post-natal development proceeds through the modifications occurring during cell division. Experiments with cultivated cells showed that cell proliferation originates changes in cell functions and coordinations that contribute to aging and senescence. The implications of the finite cell proliferation to aging of the organism is not the accumulation of cells at the end of their life cycle, but rather the drift in cell function created by cell division. Comparative gerontology shows that the regulation of the length of telomeres has no implications for aging. On the other hand there are interspecies differences in regard to the somatic cell division potential that seem to be related with the "plasticity" of the genome and with longevity, which should be viewed independently of the aging phenomenon. Telomeres may play a role in this plasticity through the regulation of chromosome recombination, and via the latter also in development.

Biology and Evolution of Aging: Implications for Basic Gerontological Health Research

The maximum lifespan of different animal species is genetically determined. Many biochemical and physiological systems which influence longevity have apparently evolved to regulate growth and development in a way which maximizes fitness given the ecological niche and constraints on the species. The diversity of individual genetic effects on aging makes it unlikely that either extrinsic factors such as nutrition and medicine or genetic intervention will have dramatic effects on the maximum lifespan of a species, in spite of significant qualitative effects on individuals. However, understanding the fundamental genetic determinants of senescence may be of particular importance to the treatment and or prevention of age associated problems involving tissue degeneration and/or cancer. Considerable investment in basic biological research on both aging and developmental processes is needed before this understanding can be achieved.

Loss of Serum Response Factor Activity Is the Basis of Reduced C-FOS Expression in Aging Human Fibroblasts

Les fibroblastes diploïdes humains subissent un nombre limité de dédoublements de population in vitro et sont largement utilisés comme modèle de vieillissement cellulaire. Malgré l'évidence grandissante que le vieillissement cellulaire est dû à une modification de l'expression du gène, l'activité des facteurs de transcription des cellules âgées est encore mal connue. Ici, nous rapportons que la réduction dramatique de l'expression du facteur de transcription fos durant le vieillissement cellulaire semble due à l'incapacité d'un autre facteur de transcription, le facteur réponse de sérum (FRS), de se lier à son site de reconnaissance appelé élément de réponse du sérum (ERS). Ce site est situé en amont de plusieurs gènes comprenant le gène humain c-fos. À l'opposé, les activités des protéines liées à la boîte TATA de la polymérase ARN ainsi qu'à l'élément réponse AMPc sont conservées chez les fibroblastes humains vieillissants. Nous présentons l'évidence que l'hyperphosphorilation du FRS induit une baisse du pouvoir de liaison observée au cours des dernières divisions cellulaires comme ceci a été précédemment suggéré pour la protéine fos.

Cats, "rats," and bats: the comparative biology of aging in the 21st century

Laboratory models have suggested a link between metabolism and life span in vertebrates, and it is well known that the evolution of specific life histories can be driven by metabolic factors. However, little is known regarding how the adoption of specific life-history strategies can shape aging and life span in populations facing different energetic demands from either a theoretical or a mechanistic viewpoint but significant insight can be gained by using a comparative approach. Comparative biology plays several roles in our understanding of the virtually ubiquitous phenomenon of aging in animals. First, it provides a critical evaluation of broad hypotheses concerning the evolutionary forces underlying the modulation of aging rate. Second, it suggests mechanistic hypotheses about processes of aging. Third, it illuminates particularly informative species because of their exceptionally slow or rapid aging rates to be interrogated about potentially novel mechanisms of aging. Although comparative biology has played a significant role in research on aging for more than a century, the new comparative biology of aging is poised to dwarf those earlier contributions, because: (1) new cellular and molecular techniques for investigating novel species are in place and more are being continually generated, (2) molecular systematics has resolved the phylogenetic relationships among a wide range of species, which allow for the implementation of analytic tools specialized for comparative biology, and (3) in addition to facilitating the construction of accurate phylogenies, the dramatic acceleration in DNA-sequencing technology is providing us with new tools for a comparative genomic approach to understanding aging.

Telomere biology in Metazoa

In this review we present critical overview of some of the available literature on the fundamental biology of telomeres and telomerase in Metazoan. With the exception of Nematodes and Arthropods, the (TTAGGG)(n) sequence is conserved in most Metazoa. Available data show that telomerase-based end maintenance is a very ancient mechanism in unicellular and multicellular organisms. In invertebrates, fish, amphibian, and reptiles persistent telomerase activity in somatic tissues might allow the maintenance of the extensive regenerative potentials of these species. Telomerase repression among birds and many mammals suggests that, as humans, they may use replicative aging as a tumor protection mechanism.

Telomerase protects adult rodent olfactory ensheathing glia from early senescence

Adult olfactory bulb ensheathing glia (OB-OEG) promote the repair of acute, subacute, and chronic spinal cord injuries and autologous transplantation is a feasible approach. There are interspecies differences between adult rodent and primate OB-OEG related to their longevity in culture. Whereas primate OB-OEG exhibit a relatively long life span, under the same culture conditions rodent OB-OEG divide just three to four times, are sensitive to oxidative stress and become senescent after the third week in vitro. Telomerase is a "physiological key regulator" of the life span of normal somatic cells and also has extratelomeric functions such as increased resistance to oxidative stress. To elucidate whether telomerase has a role in the senescence of rodent OB-OEG, we have introduced the catalytic subunit of telomerase mTERT into cultures of these cells by retroviral infection. Native and modified adult rat OB-OEG behaved as telomerase-competent cells as they divided while expressing mTERT but entered senescence once the gene switched off. After ectopic expression of mTERT, OB-OEG resumed division at a nonsenescent rate, expressed p75 and other OEG markers, and exhibited the morphology of nonsenescent OB-OEG. The nonsenescent period of mTERT-OEG lasted 9weeks and then ectopic mTERT switched off and cells entered senescence again. Our results suggest a role of telomerase in early senescence of adult rodent OB-OEG cultures and a protection from oxidative damage. This article is part of a Special Issue entitled: Understanding olfactory ensheathing glia and their prospect for nervous system repair.

Vitamin D3 up-regulated protein 1(VDUP1) is regulated by FOXO3A and miR-17-5p at the transcriptional and post-transcriptional levels, respectively, in senescent fibroblasts

Vitamin D(3) up-regulated protein 1 (VDUP1) plays multifunctional roles in diverse cellular responses, particularly in its relation to proliferation, apoptosis, differentiation, and diseases such as cancer and stress-related diseases. In this study, we demonstrated that VDUP1 was up-regulated during the senescence process. Our results showed that overexpression of VDUP1 in young cells caused typical signs of senescence. We also found that VDUP1 knockdown delayed the onset of Ras-induced cellular senescence. Subsequently, we found that FOXO3A, whose activity increased in senescent cells, transcriptionally up-regulates VDUP1 expression and miR-17-5p, whose expression decreased in senescent cells, directly interacted with the 3'-untranslated region of VDUP1 transcripts, and destabilized VDUP1 mRNA in young cells. These results indicated that VDUP1 expression was regulated by FOXO3A at the transcriptional level and by miR-17-5p at the post-transcriptional levels during the senescence process.

Protein modification and replicative senescence of WI-38 human embryonic fibroblasts

Oxidized proteins as well as proteins modified by the lipid peroxidation product 4-hydroxy-2-nonenal (HNE) and by glycation (AGE) have been shown to accumulate with aging in vivo and during replicative senescence in vitro. To better understand the mechanisms by which these damaged proteins build up and potentially affect cellular function during replicative senescence of WI-38 fibroblasts, proteins targeted by these modifications have been identified using a bidimensional gel electrophoresis-based proteomic approach coupled with immunodetection of HNE-, AGE-modified and carbonylated proteins. Thirty-seven proteins targeted for either one of these modifications were identified by mass spectrometry and are involved in different cellular functions such as protein quality control, energy metabolism and cytoskeleton. Almost half of the identified proteins were found to be mitochondrial, which reflects a preferential accumulation of damaged proteins within the mitochondria during cellular senescence. Accumulation of AGE-modified proteins could be explained by the senescence-associated decreased activity of glyoxalase-I, the major enzyme involved in the detoxification of the glycating agents methylglyoxal and glyoxal, in both cytosol and mitochondria. This finding suggests a role of detoxification systems in the age-related build-up of damaged proteins. Moreover, the oxidized protein repair system methionine sulfoxide reductase was more affected in the mitochondria than in the cytosol during cellular senescence. Finally, in contrast to the proteasome, the activity of which is decreased in senescent fibroblasts, the mitochondrial matrix ATP-stimulated Lon-like proteolytic activity is increased in senescent cells but does not seem to be sufficient to cope with the increased load of modified mitochondrial proteins.