Overview
This lecture reviews major scientific theories on why and how aging occurs, highlighting key mechanisms and experimental evidence behind biological aging processes.
What is a Theory of Aging?
- Theories of aging are divided into "why" we age (broad, often competing) and "how" we age (mechanisms, not mutually exclusive).
- Research now focuses on identifying mechanisms to potentially slow or alter aging.
Modifying the Course of Aging
- Evidence suggests only a few fundamental processes drive aging, as shown by single-gene mutations and caloric restriction in animals.
- Caloric restriction extends lifespan and delays age-related diseases in lab animals.
- Animals with longevity mutations/dietary restrictions may have survival disadvantages outside the lab.
Aging is Not a Programmed Aspect of Development
- Aging is considered a deterioration of survival programs, not a genetically programmed process.
- Even genetically identical individuals age differently, disputing the idea of a strictly programmed genetic blueprint.
Theories and Hypotheses of Aging
Cross-Linking/Glycation Hypothesis
- Aging results from inappropriate cross-links in proteins and DNA, reducing their function.
- Glycation (sugar sticking to proteins) leads to advanced glycosylation endproducts (AGEs), causing cross-links.
- Cross-linking is implicated in wrinkled skin, cataracts, atherosclerosis, and possibly Alzheimer's disease.
Evolutionary Senescence Theory
- Natural selection acts mainly on reproductive success, not late-life traits (mutation accumulation theory).
- Antagonistic pleiotropy: Genes benefiting early life may harm late life (e.g., p53 gene).
- Disposable soma theory: Organisms balance resources between body maintenance and reproduction, influencing lifespan.
Genome Maintenance Hypothesis
- Daily DNA damage accumulates, causing cellular dysfunction and aging.
- Mitochondria accumulate mutations, reducing energy production and efficiency with age.
- DNA repair declines with age, contributing to aging and diseases like cancer and Werner's syndrome.
Neuroendocrine Hypothesis
- Aging is partly due to declining function in the neuroendocrine (hormone-regulating) system.
- The insulin/IGF-1 pathway influences lifespan; reducing IGF-1 can extend life in animals but may cause other defects.
Oxidative Damage/Free Radical Hypothesis
- Free radicals produced during metabolism damage cellular components.
- Oxidative damage accumulates with age, linked to diseases like cancer, heart disease, and Alzheimer’s.
- Mitochondria are major sources/targets of oxidative damage.
Rate of Living Theory
- Proposes lifespan is inversely related to metabolic rate; largely disproven, but oxidative stress remains a focus.
Replicative Senescence Hypothesis
- Cells have a division limit (Hayflick Limit), mainly determined by telomere shortening.
- When telomeres become too short, cells stop dividing and become senescent.
- Cellular senescence contributes to tissue aging and has complex links to cancer risk.
Key Terms & Definitions
- Caloric Restriction — Diet with 30–40% fewer calories, extends lifespan in lab animals.
- Cross-Linking — Formation of chemical bonds between molecules, impairs function.
- Glycation — Sugar attaching to proteins/DNA, leading to AGEs.
- Advanced Glycosylation Endproducts (AGEs) — Molecules formed from glycation, cause cross-linking.
- Mutation Accumulation Theory — Aging results from late-acting harmful mutations persisting due to weak selection.
- Antagonistic Pleiotropy — Genes provide early-life benefits but cause late-life harm.
- Disposable Soma Theory — Resource allocation trade-off between repair (maintenance) and reproduction.
- Telomeres — Protective ends of chromosomes that shorten with each cell division.
- Senescence — State where cells no longer divide, contributing to aging.
- Oxidative Damage/Free Radicals — Cell damage caused by reactive oxygen species from metabolism.
Action Items / Next Steps
- Review and compare the main aging theories highlighted.
- Study specific examples of animal experiments mentioned for deeper understanding.
- Explore mechanisms of caloric restriction and the insulin/IGF-1 pathway in detail.