The evolutionary theory of aging, particularly antagonistic pleiotropy (AP), provides an account of the ultimate origins of aging. What remains unclear is the nature of the proximate mechanisms by which AP gives rise to diseases of aging, like cardiovascular disease and Alzheimer’s disease. Damage-centric theories focusing on loss of genetic and cellular function have been proposed, as well as programmatic theories focusing on unwanted gene and cellular function. The latter include the hyperfunction and developmental theory that view aging as the futile continuation, or run-on, of growth and developmental programmes into later life. Yet neither type of theory has performed well in explaining late-life disease aetiology, particularly with respect to disease onset, presentation and progression. What is proposed here in this review is a new programmatic theory of aging. We argue that the emergence of many specific diseases may involve quasi-programmes that are not the result of run-on, but rather are triggered by other factors in late life. Such triggers may be non-programmatic (e.g. infection, mechanical injury) or programmatic. Moreover, the consequent pre-pathological and pathological changes may in some cases trigger further changes, leading to futile and destructive cascades of quasi-programmes and pathology. The origins of triggered quasi-programmes can be traced to biological constraint i.e. the inability of organisms to optimise all functions at once. And, to some extent, the new theory presented here revises the understanding of AP. That is, because any gene can be triggered in an erroneous manner, every gene is potentially an AP gene that risks pathology, though level of risk varies according to constraint. To help validate the theory, we test it against several complex diseases of aging. The new model in this review attempts to provide a blueprint understanding that, to a certain extent, closes the gap in the causal chain of events between evolutionary causes of aging and the aetiology of age-related diseases. It also helps to explain why certain disorders mimic accelerated aging and how interventions, such as the suppression of IIS and mTOR retard many aspects of aging; notably, though, unlike prior programmatic theories, the new theory is not mTOR-centric. Finally, it provides new perspectives on possible treatment of aging.

One of the most striking findings in biogerontology in the 2010s was the demonstration that elimination of senescent cells delays many late-life diseases and extends lifespan in mice. This implied that accumulation of senescent cells promotes late-life diseases, particularly through action of senescent cell secretions (the senescence-associated secretory phenotype or SASP). But what exactly is a senescent cell? Subsequent to the initial characterization of cellular senescence it became clear that, prior to aging, this phenomenon is in fact adaptive. It supports tissue remodeling functions in a variety of contexts, including embryogenesis, parturition and acute inflammatory processes that restore normal tissue architecture and function, such as wound healing, tissue repair after infection, and amphibian limb regeneration. In these contexts such cells are normal and healthy, and not in any way senescent in the true sense of the word, as originally meant by Hayflick. Thus, it is misleading to refer to them as “senescent”. Similarly, the common assertion that senescent cells accumulate with age due to stress and DNA damage is no longer safe, particularly given their role in inflammation - a process that becomes persistent in later life. We therefore suggest that it would be useful to update some terminology, to bring it into line with contemporary understanding, and to avoid future confusion. To open a discussion of this issue, we propose replacing the term cellular senescence with remodeling activation, and SASP with RASP (remodeling-associated secretory phenotype).

Aging rate differs greatly between species, indicating that the process of senescence is largely genetically determined. Senescence evolves in part due to antagonistic pleiotropy (AP), where selection favors gene variants that increase fitness earlier in life but promote pathology later. Identifying the biological mechanisms by which AP causes senescence is key to understanding the endogenous causes of aging and its attendant diseases. Here we argue that the frequent occurrence of AP as a property of genes reflects the presence of constraint in the biological systems that they specify. This arises particularly because the functionally interconnected nature of biological systems constrains the simultaneous optimization of coupled traits (interconnection constraints), or because individual traits cannot evolve (impossibility constraints). We present an account of aging that integrates AP and biological constraint with recent programmatic aging concepts, including costly programs, quasi-programs, hyperfunction and hypofunction. We argue that AP mechanisms of costly programs and triggered quasi-programs are consequences of constraint, in which costs resulting from hyperfunction or hypofunction cause senescent pathology. Impossibility constraint can also cause hypofunction independently of AP. We also describe how AP corresponds to Stephen Jay Gould’s constraint-based concept of evolutionary spandrels, and argue that pathologies arising from AP are bad spandrels. Biological constraint is a missing link between ultimate and proximate causes of senescence, including diseases of aging. That this was not realized previously may reflect a combination of hyperadaptationism among evolutionary biologists, and the erroneous assumption by biogerontologists that molecular damage accumulation is the principal primary cause of aging.

In some species of salmon, reproductive maturity triggers the development of massive pathology resulting from reproductive effort, leading to rapid post-reproductive death. Such reproductive death, which occurs in many semelparous organisms (with a single bout of reproduction), can be prevented by blocking reproductive maturation, and this can increase lifespan dramatically. Reproductive death is often viewed as distinct from senescence in iteroparous organisms (with multiple bouts of reproduction) such as humans. Here we review the evidence that reproductive death occurs in C. elegans and discuss what this means for its use as a model organism to study aging. Inhibiting insulin/IGF-1 signaling and germline removal suppresses reproductive death and greatly extends lifespan in C. elegans, but can also extend lifespan to a small extent in iteroparous organisms. We argue that mechanisms of senescence operative in reproductive death exist in a less catastrophic form in iteroparous organisms, particularly those involving costly resource reallocation, and exhibiting endocrine-regulated plasticity. Thus, mechanisms of senescence in semelparous organisms (including plants) and iteroparous ones form an etiological continuum. Therefore understanding mechanisms of reproductive death in C. elegans can teach us about some mechanisms of senescence that are operative in iteroparous organisms.

Liposome-mediated delivery is a possible means to overcome several shortcomings with C. elegans as a model for identifying and testing drugs that retard aging. These include interactions between drugs and the nematodes’ bacterial food source, and failure of drugs to be taken up into nematode tissues. To explore this, we have tested liposome-mediated delivery of a range of fluorescent dyes and drugs in C. elegans. Liposome encapsulation led to enhanced effects on lifespan, using smaller quantities of compound, and enhanced uptake of three dyes into the gut lumen. However, one dye (Texas red) did not cross into nematode tissues, indicating that liposomes cannot ensure uptake of any compound. Of six compounds previously reported to extend lifespan (vitamin C, N-acetylcysteine, glutathione (GSH), trimethadione, thioflavin T (ThT) and rapamycin), this effect was reproduced for the latter four in a condition-dependent manner. For GSH and ThT, antibiotics abrogated life extension, implying a bacterially-mediated effect. With GSH, this was attributable to reduced early death from pharyngeal infection, and associated with alterations of mitochondrial morphology in a manner suggesting a possible innate immune training effect. By contrast, ThT exhibited antibiotic effects. For rapamycin, significant increases in lifespan were only seen when bacterial proliferation was prevented. These results document the utility and limitations of liposome-mediated drug delivery for C. elegans. They also show how nematode-bacteria interactions can determine the effects of compounds on C. elegans lifespan in a variety of ways.