A review and appraisal of the DNA damage theory of ageing

Given the central role of DNA in life, and how ageing can be seen as the gradual and irreversible breakdown of living systems, the idea that damage to the DNA is the crucial cause of ageing remains a powerful one. DNA damage and mutations of different types clearly accumulate with age in mammalian tissues. Human progeroid syndromes resulting in what appears to be accelerated ageing have been linked to defects in DNA repair or processing, suggesting that elevated levels of DNA damage can accelerate physiological decline and the development of age-related diseases not limited to cancer. Higher DNA damage may trigger cellular signalling pathways, such as apoptosis, that result in a faster depletion of stem cells, which in turn contributes to accelerated ageing. Genetic manipulations of DNA repair pathways in mice further strengthen this view and also indicate that disruption of specific pathways, such as nucleotide excision repair and non-homologous end joining, is more strongly associated with premature ageing phenotypes. Delaying ageing in mice by decreasing levels of DNA damage, however, has not been achieved yet, perhaps due to the complexity inherent to DNA repair and DNA damage response pathways. Another open question is whether DNA repair optimization is involved in the evolution of species longevity, and we suggest that the way cells from different organisms respond to DNA damage may be crucial in species differences in ageing. Taken together, the data suggest a major role of DNA damage in the modulation of longevity, possibly through effects on cell dysfunction and loss, although understanding how to modify DNA damage repair and response systems to delay ageing remains a crucial challenge.     

Tendon stem/progenitor cell ageing: Modulation and rejuvenation

Tendon ageing is a complicated process caused by multifaceted pathways and ageing plays a critical role in the occurrence and severity of tendon injury. The role of tendon stem/progenitor cells (TSPCs) in tendon maintenance and regeneration has received increasing attention in recent years. The decreased capacity of TSPCs in seniors contributes to impaired tendon functions and raises questions as to what extent these cells either affect, or cause ageing, and whether these age-related cellular alterations are caused by intrinsic factors or the cellular environment. In this review, recent discoveries concerning the biological characteristics of TSPCs and age-related changes in TSPCs, including the effects of cellular epigenetic alterations and the mechanisms involved in the ageing process, are analyzed. During the ageing process, TSPCs ageing might occur as a natural part of the tendon ageing, but could also result from decreased levels of growth factor, hormone deficits and changes in other related factors. Here, we discuss methods that might induce the rejuvenation of TSPC functions that are impaired during ageing, including moderate exercise, cell extracellular matrix condition, growth factors and hormones; these methods aim to rejuvenate the features of youthfulness with the ultimate goal of improving human health during ageing.     

Ageing and the Regulation of Cell Activities during Exposure to Cold

The inability to maintain body temperature and a selective pattern of changes in the regulation of cell activities were revealed by briefly exposing ageing C57B1/6J male mice to cold (10°C). The induction of liver tyrosine aminotransferase (TAT) during exposure to cold (a gene-dependent process) was markedly delayed in senescent mice (26 months old) as compared with younger mice (3–16 months old); after the delay, the rate of increase of TAT was similar to that prevailing in younger mice. Direct challenge of the liver with injections of corticosterone or insulin elicited the induction of TAT on an identical time course in young and senescent mice. These experiments provide an example of an age change in a gene-dependent cell process (the delayed induction of TAT in senescent mice during exposure to cold) which is not due to a change in the potential of the genome for responding when exogenous stimulae are supplied (injection of hormones). In contrast to the age-related change in liver cell activities, no significant changes were found in the secretion of corticosterone during exposure to cold. Although the seat of these selective age-related changes in the regulation of cell activities remains unclear, it is argued that generalized damage to the genome of cells throughout the body is not involved. The results of this and other studies showing the selective effect of age on cell activities are considered in terms of the concept that many cellular age changes represent the response of cells to primary age-related changes in humoral factors in the internal environment of the body.     

Modelling age-related metabolic disorders in the mouse

Ageing can be characterised by a general decline in cellular function, which affects whole-body homoeostasis with metabolic dysfunction—a common hallmark of ageing. The identification and characterisation of the genetic pathways involved are paramount to the understanding of how we age and the development of therapeutic strategies for combating age-related disease. Furthermore, in addition to understanding the ageing process itself, we must understand the interactions ageing has with genetic variation that results in disease phenotypes. The use of model systems such as the mouse, which has a relatively short lifespan, rapid reproduction (resulting in a large number of offspring), well-characterised biology, a fully sequenced genome, and the availability of tools for genetic manipulation is essential for such studies. Here we review the relationship between ageing and metabolism and highlight the need for modelling these processes.     

Age-related changes in the contractile and passive arterial properties of murine mesenteric small arteries are altered by caveolin-1 knockout

Caveolin-1, an integral protein of caveolae, is associated with multiple cardiovascular signalling pathways. Caveolin-1 knockout (KO) mice have a reduced lifespan. As changes in artery structure and function are associated with ageing we have investigated the role of caveolin-1 ablation on age-related changes of small artery contractility and passive mechanical properties. Mesenteric small arteries isolated from 3 and 12-month wild-type (WT) and caveolin-1 KO mice were mounted on a pressure myograph and changes in passive and functional arterial properties were continuously monitored. In WT mice ageing was associated with a reduction in arterial contractility to noradrenaline which was reversed by inhibition of nitric oxide synthase with L-NNA. Similarly, in 3-month-old mice, caveolin-1 KO reduced contractility to noradrenaline by an L-NNA-sensitive mechanism. However, ageing in caveolin-1 KO mice was not associated with any further change in contractility. In WT mice ageing was associated with an increased passive arterial diameter and cross-sectional area (CSA), consistent with outward remodelling of the arterial wall, and a reduced arterial distensibility. Caveolin-1 ablation at 3 months of age resulted in similar changes in passive arterial properties to those observed with ageing in WT animals. However, ageing in caveolin-1 KO mice resulted in a reduced arterial CSA indicating different effects on passive structural characteristics from that observed in WT mice. Thus, caveolin-1 mice show abnormalities of small mesenteric artery function and passive mechanical characteristics indicative of premature vascular ageing. Moreover, caveolin-1 ablation modulates the age-related changes usually observed in mesenteric arteries of WT mice.     

The p53 network: cellular and systemic DNA damage responses in aging and cancer

Genome instability contributes to cancer development and accelerates age-related pathologies as evidenced by a variety of congenital cancer susceptibility and progeroid syndromes that are caused by defects in genome maintenance mechanisms. DNA damage response (DDR) pathways that are mediated through the tumor suppressor p53 play an important role in the cell-intrinsic responses to genome instability, including a transient cell cycle arrest, senescence and apoptosis. Both senescence and apoptosis are powerful tumor-suppressive pathways preventing the uncontrolled proliferation of transformed cells. However, both pathways can potentially deplete stem and progenitor cell pools, thus promoting tissue degeneration and organ failure, which are both hallmarks of aging. p53 signaling is also involved in mediating non-cell-autonomous interactions with the innate immune system and in the systemic adjustments during the aging process. The network of p53 target genes thus functions as an important regulator of cancer prevention and aging.     

Cellular stress response pathways and ageing: intricate molecular relationships

Ageing is driven by the inexorable and stochastic accumulation of damage in biomolecules vital for proper cellular function. Although this process is fundamentally haphazard and uncontrollable, senescent decline and ageing is broadly influenced by genetic and extrinsic factors. Numerous gene mutations and treatments have been shown to extend the lifespan of diverse organisms ranging from the unicellular Saccharomyces cerevisiae to primates. It is becoming increasingly apparent that most such interventions ultimately interface with cellular stress response mechanisms, suggesting that longevity is intimately related to the ability of the organism to effectively cope with both intrinsic and extrinsic stress. Here, we survey the molecular mechanisms that link ageing to main stress response pathways, and mediate age-related changes in the effectiveness of the response to stress. We also discuss how each pathway contributes to modulate the ageing process. A better understanding of the dynamics and reciprocal interplay between stress responses and ageing is critical for the development of novel therapeutic strategies that exploit endogenous stress combat pathways against age-associated pathologies.     

Pharmacological correction of the apoptosis level in cortical neurons of ageing HER2/neu transgenic mice

Neurodegenerative changes and neuronal death underlie ageing of the nervous system. We investigated the mechanisms of apoptosis in sensorimotor cortical neurons of HER2/neu transgenic mice during ageing, as well as the functional changes in the cortex and the involvement of exogenous neurometabolites (cytoflavin, piracetam) in the regulation of neuronal death and locomotor and psycho-emotional status in mice. The level of apoptosis and expression of the apoptotic protein markers (TUNEL, immunohistochemistry, Western blotting) were detected in HER2/neu transgenic mice versus wild type mice (FBV strain). In ageing wild type mice, the basal activity decreases while the anxiety level increases correlating with the high level of neuronal apoptosis. We revealed specific behavioral features of HER2/neu transgenic mice—their low basal activity which remains intact during ageing. Previously, we have shown that in this mouse strain the level of apoptosis is low, with no age-related dynamics, due to the suppression primarily of the p53-dependent pathway by HER2 (tyrosine kinase receptor) overexpression. Here we show that cytoflavin and piracetam have a pronounced neuroprotective effect, preserving and restoring the nervous system functions (improving locomotion and psychological status) in both mouse strains. The effect of the tested neurometabolites on neuronal apoptosis is ambiguous. In case of low-level apoptosis, there occurs its moderate stimulation in HER2/neu transgenic mice that are characterized by a high level of carcinogenesis (via the extrinsic and p53-dependent pathways with caspase-3 activation) which probably prevents tumor development. By contrast, in aged wild-type mice there is a marked decrease in the level of age-related apoptosis (via the stimulation of antiapoptotic protein Mcl-1 expression) supposed to prevent neurodegeneration.     

Crosstalk between P53 and DNA damage response in ageing

Ageing is a sophisticated process, accompanied by reduction in general physiological capacity and increase in mortality and death, stemming from damage accumulation over time. Various signaling pathways are known to be involved in the functional decrease in various organs in ageing humans. One of the most prominent pathways is DNA damage response (DDR), which is responsible for maintenance of the genomic integrity and stability. Insufficient or dysfunctional DDR signaling and the subsequent accumulation of potential DNA lesions are associated with the initiation/progression of various human pathologies including ageing. As a tumor suppressor gene, with critical functions in the ageing process, p53 is considered as a DDR centerpiece. In this review, we aim to discuss the interactions between p53 and DDR signaling and their contributions in ageing.     

Impact of ROS on ageing of two fungal model systems: Saccharomyces cerevisiae and Podospora anserina

To provide a foundation for the development of effective interventions to counteract various age-related diseases in humans, ageing processes have been extensively studied in various model organisms and systems. However, the mechanisms underlying ageing are still not unravelled in detail in any system including rather simple organisms. In this article, we review some of the molecular mechanisms that were found to affect ageing in two fungal models, the unicellular ascomycete Saccharomyces cerevisiae and the filamentous ascomycete Podospora anserina. A selection of issues like retrograde response, genomic instability, caloric restriction, mtDNA reorganisation and apoptosis is presented and discussed with special emphasis on the role reactive oxygen species (ROS) play in these diverse molecular pathways.     

Impact of ROS on ageing of two fungal model systems: Saccharomyces cerevisiae and Podospora anserina

To provide a foundation for the development of effective interventions to counteract various age-related diseases in humans, ageing processes have been extensively studied in various model organisms and systems. However, the mechanisms underlying ageing are still not unravelled in detail in any system including rather simple organisms. In this article, we review some of the molecular mechanisms that were found to affect ageing in two fungal models, the unicellular ascomycete Saccharomyces cerevisiae and the filamentous ascomycete Podospora anserina. A selection of issues like retrograde response, genomic instability, caloric restriction, mtDNA reorganisation and apoptosis is presented and discussed with special emphasis on the role reactive oxygen species (ROS) play in these diverse molecular pathways.     

Endocrine regulation of ageing

Over the past 15 years it has become clear that mutations in genes that regulate endocrine signalling pathways can prolong lifespan. Lifespan can be increased by altered endocrine signalling in a group of cells or a single tissue, which indicates that crosstalk between tissues functions to coordinate ageing of the organism. These endocrine pathways might serve as targets for the manipulation of the ageing process and prevention of age-related diseases.     

Programmed ageing: decline of stem cell renewal,immunosenescence, and Alzheimer's disease

The characteristic maximum lifespan varies enormously across animal species from a few hours to hundreds of years. This argues that maximum lifespan, and the ageing process that itself dictates lifespan, are to a large extent genetically determined. Although controversial, this is supported by firm evidence that semelparous species display evolutionarily programmed ageing in response to reproductive and environmental cues. Parabiosis experiments reveal that ageing is orchestrated systemically through the circulation, accompanied by programmed changes in hormone levels across a lifetime. This implies that, like the circadian and circannual clocks, there is a master ‘clock of age’ (circavital clock) located in the limbic brain of mammals that modulates systemic changes in growth factor and hormone secretion over the lifespan, as well as systemic alterations in gene expression as revealed by genomic methylation analysis. Studies on accelerated ageing in mice, as well as human longevity genes, converge on evolutionarily conserved fibroblast growth factors (FGFs) and their receptors, including KLOTHO, as well as insulin-like growth factors (IGFs) and steroid hormones, as key players mediating the systemic effects of ageing. Age-related changes in these and multiple other factors are inferred to cause a progressive decline in tissue maintenance through failure of stem cell replenishment. This most severely affects the immune system, which requires constant renewal from bone marrow stem cells. Age-related immune decline increases risk of infection whereas lifespan can be extended in germfree animals. This and other evidence suggests that infection is the major cause of death in higher organisms. Immune decline is also associated with age-related diseases. Taking the example of Alzheimer's disease (AD), we assess the evidence that AD is caused by immunosenescence and infection. The signature protein of AD brain, Aβ, is now known to be an antimicrobial peptide, and Aβ deposits in AD brain may be a response to infection rather than a cause of disease. Because some cognitively normal elderly individuals show extensive neuropathology, we argue that the location of the pathology is crucial – specifically, lesions to limbic brain are likely to accentuate immunosenescence, and could thus underlie a vicious cycle of accelerated immune decline and microbial proliferation that culminates in AD. This general model may extend to other age-related diseases, and we propose a general paradigm of organismal senescence in which declining stem cell proliferation leads to programmed immunosenescence and mortality.     

Shorter telomeres, accelerated ageing and increased lymphoma in DNA-PKcs-deficient mice

Non-homologous end joining (NHEJ) is the principal repair mechanism used by mammalian cells to cope with double-strand breaks (DSBs) that continually occur in the genome. One of the key components of the mammalian NHEJ machinery is the DNA-PK complex, formed by the Ku86/70 heterodimer and the DNA-PK catalytic subunit (DNA-PKcs). Here, we report on the detailed life-long follow-up of DNA-PKcs-defective mice. Apart from defining a role of DNA-PKcs in telomere length maintenance in the context of the ageing organism, we observed that DNA-PKcs-defective mice had a shorter life span and showed an earlier onset of ageing-related pathologies than the corresponding wild-type littermates. In addition, DNA-PKcs ablation was associated with a markedly higher incidence of T lymphomas and infections. In conclusion, these data link the dual role of DNA-PKcs in DNA repair and telomere length maintenance to organismal ageing and cancer.     

Death and dessert: nutrient signalling pathways and ageing

Reduction in nutrient intake without malnutrition can delay ageing and extend healthy life in diverse organisms from yeast to primates. This effect can be recapitulated by genetic or pharmacological dampening of the signal through nutrient signalling pathways, making them a promising target for intervention into human ageing and age-related diseases. Here we review the current knowledge of the interactions between nutrient signalling pathways and ageing, focusing on the findings emerged in the past few years.     

Ageing as a risk factor for disease

Age is the main risk factor for the prevalent diseases of?developed countries: cancer, cardiovascular disease and?neurodegeneration. The ageing process is deleterious for fitness, but can nonetheless evolve as a consequence of the declining force of natural selection at later ages, attributable to extrinsic hazards to survival: ageing can then occur as a side-effect of accumulation of mutations that lower fitness at later ages, or of natural selection in favour of mutations that increase fitness of the young but?at the cost of a higher subsequent rate of ageing. Once thought of as an inexorable, complex and lineage-specific process of accumulation of damage, ageing has turned out to be influenced by mechanisms that show strong evolutionary conservation. Lowered activity of the nutrient-sensing insulin/insulin-like growth factor/Target of Rapamycin signalling network can extend healthy lifespan in yeast, multicellular invertebrates, mice and, possibly, humans. Mitochondrial activity can also promote?ageing, while genome maintenance and autophagy?can protect against it. We discuss the relationship between evolutionarily conserved mechanisms of ageing and disease, and the associated scientific challenges and opportunities.     

Ageing,Neuronal Connectivity and Brain Disorders: An Unsolved Ripple Effect

Cognitive decline associated with ageing and age-related disorders emerges as one of the greatest health challenges in the next decades. To date, the molecular mechanisms underlying the onset of neuronal physiological changes in the central nervous system remain unclear. Functional MRI and PET studies have indicated the decline in working memory performance in older adults. Similarly, age-related disorders, such as Alzheimer’s disease, are associated with changes in the prefontral cortex and related neural circuitry, which underlines the decline of integrative function between different brain regions. This is mainly attributed to the loss of synaptic connectivity, which is a feature commonly observed in neurodegenerative disorders. In humans, the morphological and functional changes in neurons, such as reduction of spine numbers and synaptic dysfunction, precede the first signs of cognitive decline and likely contribute to pathology progression. Thus, a new scenario emerges in which apparently unrelated diseases present common features, such as the remodelling of neuronal circuitries promoted by ageing. For many years, ageing was considered a process of slow deterioration triggered by accidental environmental factors. Conversely, it is now evident that ageing is a biological process tightly controlled by evolutionary highly conserved signalling pathways. Importantly, genetic mutations that enhance longevity significantly delay the loss of synaptic connectivity and, therefore, the onset of age-related brain disorders. Accordingly, tweaking ageing might be an attractive approach to prevent cognitive decline caused by age-related synaptic dysfunction.