Coefficient of Variation of Lifespan Across the Tree of Life: Is It a Signature of Programmed Aging?

Measurements of variation are of great importance for studying the stability of pathological phenomena and processes. For the biology of aging, it is very important not only to determine average mortality, but also to study its stability in time and the size of fluctuations that are indicated by the variation coefficient of lifespan (CV_LS). It is believed that a relatively small (~20%) value of CV_LS in humans, comparable to the coefficients of variation of other events programmed in ontogenesis (for example, menarche and menopause), indicates a relatively rigid determinism (N. S. Gavrilova et al. (2012) Biochemistry (Moscow), 77, 754-760). To assess the prevalence of this phenomenon, we studied the magnitude of CV_LS, as well as the coefficients of skewness and kurtosis in diverse representatives of the animal kingdom using data provided by the Institute for Demographic Research (O. R. Jones et al. (2014) Nature, 505, 169-173). We found that, unlike humans and laboratory animals, in most examined species the values of CV_LS are rather high, indicating heterogeneity of the lifespan in the cohorts studied. This is probably due to the large influence of background mortality, as well as the non-monotonicity of total mortality in the wild, especially at the earliest ages. One way to account for this influence is to "truncate" the data (removing the earliest and latest ages from consideration). To reveal the effect of this procedure, we proposed a new indicator, the stability coefficient of mortality dynamics, which indicates how quickly CV_LS is reduced to values that characterize a relatively homogeneous population (33%) when the data are "truncated". Such indicators facilitate the use of the parameters of survival curves for analysis of the effects of geroprotectors, lifestyle, and other factors on lifespan, and for the quantification of relative contributions of genetic and environmental factors to the dynamics of aging in human and animal populations, including those living in the wild.

Protein Poly(ADP-ribosyl)ation System: Changes in Development and Aging as well as due to Restriction of Cell Proliferation

It is well known that the number of dividing cells in an organism decreases with age. The average rate of cell division in tissues and organs of a mature organism sharply decreases, which is probably a trigger for accumulation of damage leading to disturbance of genome integrity. This can be a cause for the development of many age-related diseases and appearance of phenotypic and physiological signs of aging. In this connection, the protein poly(ADP-ribosyl)ation system, which is activated in response to appearance of various DNA damage, attracts great interest. This review summarizes and analyzes data on changes in the poly(ADP-ribosyl)ation system during development and aging in vivo and in vitro, and due to restriction of cell proliferation. Special attention is given to methodological aspects of determination of activity of poly(ADP-ribose) polymerases (PARPs). Analysis of relevant publications and our own data has led us to the conclusion that PARP activity upon the addition of free DNA ends (in this review referred to as stimulated PARP activity) is steadily decreasing with age. However, the dynamics of PARP activity measured without additional activation of the enzyme (in this review referred to as unstimulated activity) does not have such a clear trend: in many studies, the presented differences are statistically non-significant, although it is well known that the number of unrepaired DNA lesions steadily increases with aging. Apparently, the cell has additional regulatory systems that limit its own capability of reacting to DNA damage. Special attention is given to the influence of the cell proliferative status on PARP activity. We have systematized and analyzed data on changes in PARP activity during development and aging of an organism, as well as data on differences in the dynamics of this activity in the presence/absence of additional stimulation and on cellular processes that are associated with activation of these enzymes. Moreover, data obtained in different models of cellular aging are compared.

Is It Possible to Prove the Existence of an Aging Program by Quantitative Analysis of Mortality Dynamics?

Accumulation of various types of lesions in the course of aging increases an organism's vulnerability and results in a monotonous elevation of mortality rate, irrespective of the position of a species on the evolutionary tree. Stroustrup et al. (Nature, 530, 103-107) [1] showed in 2016 that in the nematode Caenorhabditis elegans, longevity-altering factors (e.g. oxidative stress, temperature, or diet) do not change the shape of the survival curve, but either stretch or shrink it along the time axis, which the authors attributed to the existence of an "aging program". Modification of the accelerated failure time model by Stroustrup et al. uses temporal scaling as a basic approach for distinguishing between quantitative and qualitative changes in aging dynamics. Thus we analyzed data on the effects of various longevity-increasing genetic manipulations in flies, worms, and mice and used several models to choose a theory that would best fit the experimental results. The possibility to identify the moment of switch from a mortality-governing pathway to some other pathways might be useful for testing geroprotective drugs. In this work, we discuss this and other aspects of temporal scaling.

Contribution of Quantitative Methods of Estimating Mortality Dynamics to Explaining Mechanisms of Aging

Accumulation of various types of unrepaired damage of the genome because of increasing production of reactive oxygen species and decreasing efficiency of the antioxidant defense system and repair systems can cause age-related diseases and emergence of phenotypic signs of senescence. This should lead to increasing vulnerability and to mortality monotonously increasing with age independently of the position of the species on the evolutionary tree. In this light, the survival, mortality, and fertility curves for 45 animal and plant species and one alga published by the Max Planck Institute for Demographic Research (Germany/Denmark) are of special interest (Jones, O. R., et al. (2014) Nature, 505, 169-173). We divided all species treated in that study into four groups according to the ratio of mortality at the terminal age (which corresponds to 5% survival) and average mortality during the entire studied period. For animals of group IV (long-lived and senescent), including humans, the Jones method makes it possible to trace mortality during the entire life cycle. The same applies to short-lived animals (e.g. nematodes or the tundra vole), whether they display the Gompertz type of senescence or not. However, in long-lived species with a less pronounced increase in mortality with age (e.g. the freshwater crocodile, hermit crab, or Scots pine), as well as in animals of average lifespan that reach the terminal age earlier than they could have enough time to become senescent, the Jones method is capable of characterizing only a small part of the life cycle and does not allow judging how senescence manifests itself at late stages of the life cycle. Thus, it is known that old trees display signs of biological senescence rather clearly; although Jones et al. consider them non-senescent organisms because less than 5% of sexually mature individuals survive to display the first manifestations of these characters. We have concluded that the classification proposed by Jones et al. makes it possible to approximately divide animals and plants only by their levels of the Gompertz type of senescence (i.e. actuarial senescence), whereas susceptibility to biological senescence can be estimated only when principally different models are applied.

The protein poly(ADP-ribosyl)ation system: its role in genome stability and lifespan determination

The processes that lead to violation of genome integrity are known to increase with age. This phenomenon is caused both by increased production of reactive oxygen species and a decline in the efficiency of antioxidant defense system as well as systems maintaining genome stability. Accumulation of different unrepairable genome damage with age may be the cause of many age-related diseases and the development of phenotypic and physiological signs of aging. It is also clear that there is a close connection between the mechanisms of the maintenance of genome stability, on one hand, and the processes of spontaneous tumor formation and lifespan, on the other. In this regard, the system of protein poly(ADP-ribosyl)ation activated in response to a variety of DNA damage seems to be of particular interest. Data accumulated to date suggest it to be a kind of focal point of cellular processes, guiding the path of cell survival or death depending on the degree of DNA damage. This review summarizes and analyzes data on the involvement of poly(ADP-ribosyl)ation in various mechanisms of DNA repair, its interaction with progeria proteins, and the possible role in the development of spontaneous tumors and lifespan determination. Special attention is given to the relationship between various polymorphisms of the human poly(ADP-ribose) polymerase-1 gene and longevity.

Ageing of trees: application of general ageing theories

The main questions posed in ageing theories are how ageing evolved and whether or not it is programmed. While these questions have not yet been clearly resolved, several groups of possible theories have been published on this topic. However, most of these theories do not consider plants, and the specific traits involved in their ageing mechanisms. The first trait covers clonality and sectoriality and the second concerns the lack of a differentiated germ line. The lack of a germ line prevents telomere shortening which can lead to the transfer of somatic mutations into sexual offspring, while sectoriality in trees causes isolation of potentially catastrophic events in one tree part, thus creating a population of more or less independent modules within one axis. The processes of population dynamics, including ageing, can act within the framework of an individual tree as well as in that of the population as a whole, although the processes involved differ and consequently result in different effects.