The heuristic value of redundancy models of aging

Exploring Patterns of Human Mortality and Aging: A Reliability Theory Viewpoint

The most important manifestation of aging is an increased risk of death with advancing age, a mortality pattern characterized by empirical regularities known as mortality laws. We highlight three significant ones: the Gompertz law, compensation effect of mortality (CEM), and late-life mortality deceleration and describe new developments in this area. It is predicted that CEM should result in declining relative variability of mortality at older ages. The quiescent phase hypothesis of negligible actuarial aging at younger adult ages is tested and refuted by analyzing mortality of the most recent birth cohorts. To comprehend the aging mechanisms, it is crucial to explain the observed empirical mortality patterns. As an illustrative example of data-directed modeling and the insights it provides, we briefly describe two different reliability models applied to human mortality patterns. The explanation of aging using a reliability theory approach aligns with evolutionary theories of aging, including idea of chronic phenoptosis. This alignment stems from their focus on elucidating the process of organismal deterioration itself, rather than addressing the reasons why organisms are not designed for perpetual existence. This article is a part of a special issue of the journal that commemorates the legacy of the eminent Russian scientist Vladimir Petrovich Skulachev (1935-2023) and his bold ideas about evolution of biological aging and phenoptosis.

Glucose tolerance predicts survival in old zebra finches

The capacity to deal with external and internal challenges is thought to affect fitness, and the age-linked impairment of this capacity defines the ageing process. Using a recently developed intra-peritoneal glucose tolerance test (GTT) in zebra finches, we tested for a link between the capacity to regulate glucose levels and survival. We also investigated for the effects of ambient factors, age, sex, and manipulated developmental and adult conditions (i.e. natal brood size and foraging cost, in a full factorial design) on glucose tolerance. Glucose tolerance was quantified using the incremental ‘area under the curve’ (AUC), with lower values indicating higher tolerance. Glucose tolerance predicted survival probability in old birds, above the median age, with individuals with higher glucose tolerance showing better survival than individuals with low or intermediate glucose tolerance. In young birds there was no association between glucose tolerance and survival. Experimentally induced adverse developmental conditions did not affect glucose tolerance, but low ambient temperature at sampling and hard foraging conditions during adulthood induced a fast return to baseline levels (i.e. high glucose tolerance). These findings can be interpreted as an efficient return to baseline glucose levels when energy requirements are high, with glucose presumably being used for energy metabolism or storage. Glucose tolerance was independent of sex. Our main finding that old birds with higher glucose tolerance had better survival supports the hypothesis that the capacity to efficiently cope with a physiological challenge predicts lifespan, at least in old birds.

Modeling aging and its impact on cellular function and organismal behavior

Aging is a complex phenomenon of functional decay in a biological organism. Although the effects of aging are readily recognizable in a wide range of organisms, the cause(s) of aging are ill defined and poorly understood. Experimental methods on model organisms have driven significant insight into aging as a process, but have not provided a complete model of aging. Computational biology offers a unique opportunity to resolve this gap in our knowledge by generating extensive and testable models that can help us understand the fundamental nature of aging, identify the presence and characteristics of unaccounted aging factor(s), demonstrate the mechanics of particular factor(s) in driving aging, and understand the secondary effects of aging on biological function. In this review, we will address each of the above roles for computational biology in aging research. Concurrently, we will explore the different applications of computational biology to aging in single-celled versus multicellular organisms. Given the long history of computational biogerontological research on lower eukaryotes, we emphasize the key future goals of gradually integrating prior models into a holistic map of aging and translating successful models to higher-complexity organisms.

Tissue Failure Propagation as Mediated by Circulatory Flow

Aging is driven by subcellular processes that are relatively well understood. However, the qualitative mechanisms and quantitative dynamics of how these micro-level failures cascade to a macro-level catastrophe in a tissue or organs remain largely unexplored. Here, we experimentally and theoretically study how cell failure propagates in an engineered tissue in the presence of advective flow. We argue that cells secrete cooperative factors, thereby forming a network of interdependence governed by diffusion and flow, which fails with a propagating front parallel to advective circulation.

Interdependence theory of tissue failure: bulk and boundary effects

The mortality rate of many complex multicellular organisms increases with age, which suggests that net ageing damage is accumulative, despite remodelling processes. But how exactly do these little mishaps in the cellular level accumulate and spread to become a systemic catastrophe? To address this question we present experiments with synthetic tissues, an analytical model consistent with experiments, and a number of implications that follow the analytical model. Our theoretical framework describes how shape, curvature and density influences the propagation of failure in a tissue subjected to oxidative damage. We propose that ageing is an emergent property governed by interaction between cells, and that intercellular processes play a role that is at least as important as intracellular ones.

Food availability affects adult survival trajectories depending on early developmental conditions

Food availability modulates survival in interaction with (for example) competition, disease and predators, but to what extent food availability in natural populations affects survival independent of these factors is not well known. We tested the effect of food availability on lifespan and actuarial senescence in a large population of captive zebra finches by increasing the effort required to obtain food, reflecting natural contrasts in food availability. Food availability may not affect all individuals equally and we therefore created heterogeneity in phenotypic quality by raising birds with different numbers of siblings. Low food availability had no effect on lifespan for individuals from benign developmental conditions (raised in small broods), but shortened lifespan for individuals from harsh developmental conditions. The lifespan difference arose through higher baseline mortality rate of individuals from harsh developmental conditions, despite a decrease in the rate of actuarial senescence. We found no evidence for sex-specific environmental sensitivity, but females lived shorter than males due to increased actuarial senescence. Thus, low food availability by itself shortens lifespan, but only in individuals from harsh developmental conditions. Our food availability manipulation resembles dietary restriction as applied to invertebrates, where it extends lifespan in model organisms and we discuss possible reasons for the contrasting results.

Lifespan divergence between social insect castes: challenges and opportunities for evolutionary theories of aging

The extraordinarily long lifespans of queens (and kings) in eusocial insects and the strikingly large differences in life expectancy between workers and queens challenge our understanding of the evolution of aging and provide unique opportunities for studying the causes underlying adaptive variation in lifespan within species. Here we review the major evolutionary theories of aging, focusing on their scope and limitations when applied to social insects. We show that reproductive division of labor, interactions between kin, caste-specific gene regulation networks, and the integration of colony-level trade-offs with individual-level trade-offs provide challenges to the classical theories We briefly indicate how these challenges could be met in future models of adaptive phenotypic plasticity in lifespan between and within different castes.