The Physiology of Aging

Cross-Sectional Design

The usual approach to the foregoing difficulty is a cross-sectional design in which investigators study cohorts with several different age ranges (e.g., 20- to 29-year-olds, 30- to 39-year-olds) over a brief period (e.g., a calendar year). However, this design suffers from two serious potential confounders. One is the cohort effect; that is, different cohorts have had different environmental experiences. For example, in studies of the effects of aging on cognition, a confounding factor could be that younger cohorts have had the benefit of a relatively higher level of education. If aware of a potential confounder, the investigator may be able to modify the study's design to avoid the confounder.

The second potential confounder is selective mortality —individuals with risk factors for diseases that cause death at a relatively young age are underrepresented in older age groups. For example, in a study on the effect of age on plasma lipoproteins, mortality at a young age from cardiovascular disease would preferentially eliminate individuals with the highest low-density lipoprotein levels.

Longitudinal Design

To circumvent the confounders encountered in cross-sectional designs, investigators can repeatedly study a subject over a significant portion of his or her lifetime. However, this longitudinal design has other problems. Long-term longitudinal studies require a special organizational structure that can outlive an individual investigator and ensure completion of the study. Even shorter longitudinal studies are very costly. Some problems are inherent in the time course of longitudinal studies, including the effect of repeated measurements on the function being assessed, changes in subjects' lifestyle (e.g., diet), dropout of subjects from the study, and changes in professional personnel and technology.

Reactive Oxygen Species

As illustrated in Figure 62-3 A , reactive oxygen species (ROS) include molecules such as hydrogen peroxide (H ₂ O ₂ ), neutral free radicals such as the hydroxyl radical ( ^. OH), and anionic radicals such as the superoxide anion radical ( ). Free radicals have an unpaired electron in the outer orbital, shown in red in Figure 62-3 A . These free radicals are extremely unstable because they react with a target molecule to capture an electron, so that they become a stable molecule with only paired electrons in the outer shell. However, the target molecule left behind becomes a free radical, which initiates a chain reaction that continues until two free radicals meet to create a product with a covalent bond. ROS—particularly ^. OH, which is the most reactive of them all—have the potential to damage important biological molecules, such as proteins, lipids, and DNA. However, ROS also play important physiological roles in the oxidation of iodide anions by thyroid peroxidase in the formation of thyroid hormone (see pp. 1006–1010 ), as well as in the destruction of certain bacteria by reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and myeloperoxidase in phagocytic cells. N62-6 Finally, the highly reactive signaling molecule nitric oxide (see p. 66 ) is a free radical (see Fig. 62-3 A ). N62-7

Quantitatively, the most important source of ROS is the mitochondrial electron transport chain (see p. 118 ). Complex I and complex III of the electron transport chain generate as byproducts (see Fig. 62-3 B ). The enzyme superoxide dismutase (SOD) converts to hydrogen peroxide, which in turn can yield the highly reactive ^. OH.

Only a small fraction of the oxygen used in aerobic metabolism (<1%) generates ROS. However, even that amount would be lethal in the absence of protective mechanisms. Fortunately, organisms have two potent antioxidant defenses. The major defense is enzymatic, specifically SODs, catalase, and glutathione peroxidase ( Fig. 62-4 ). In addition, low-molecular-weight antioxidants, such as vitamins C and E, play a minor role in the defense against the metabolically produced radicals.

Because these antioxidant defense mechanisms are not fully protective, the dominant concept of the oxidative stress theory is that an imbalance between the production and removal of ROS by antioxidant defenses is the major cause of aging. Nevertheless, recent studies using genetically engineered mice—with either deficient or overexpressed antioxidant enzymes—do not support this theory.

Glycation and Glycoxidation

Glycation refers to nonenzymatic reactions between the carbonyl groups of reducing sugars (e.g., glucose) and the amino groups of macromolecules (e.g., proteins, DNA) to form advanced glycation end products (AGEs). Figure 62-5 shows an interaction of open-chain d -glucose with a lysine residue on a protein, yielding a Schiff base and water. The Schiff base undergoes an intramolecular rearrangement to form an open-chain Amadori compound that undergoes the Amadori rearrangement to form a ring structure called an Amadori product. In cooking, Amadori products undergo a series of further reactions to produce polymers and copolymers called melanoidins, which give a brown color to cooked food. N62-8 In humans, the Amadori product can undergo a series of intramolecular and intermolecular rearrangements that include oxidation— glycoxidation —to form AGE molecules. For example, the Amadori product in Figure 62-5 can either form carboxymethyllysine or react with an arginine residue on the same or a different protein to form a cross-link called pentosidine.

The formation of AGEs is especially important for long-lived proteins and appears to play a role in the long-term complications of diabetes. The similarity between the aging phenotype and that of the diabetic patient led Anthony Cerami to propose the glycation hypothesis of aging. Although glucose is not the only reducing sugar involved in glycation, it is an important one. Thus, the level of glycemia is a major factor in glycation, and periods of hyperglycemia are probably the reason glycation—including the glycation of hemoglobin (see Box 29-1 )—is enhanced in patients with diabetes. Proteins containing AGEs exhibit altered structural and functional properties. For example, AGE formation in lens proteins of the eye probably contributes to age-associated opacification. Moreover, with advancing age, the increased stiffness of collagen in connective tissues (e.g., blood vessels; see pp. 458–459 ) may also, in part, be due to AGE-mediated collagen cross-links. AGE-induced DNA damage may lead to alterations in genomic function.

Mitochondrial Damage

Because mitochondria are the major source of ROS, they are also likely to be a major target of oxidative damage. Damage to mitochondrial DNA (mtDNA) increases greatly with age because, unlike genomic DNA, mtDNA is not protected by histones (see pp. 75–76 ). According to the mitochondrial theory of aging, the damage to mtDNA reduces the ability of the mitochondria to generate ATP, and this decreased production of ATP results in the loss of cell function and hence aging.

Somatic Mutations

Damage to genomic and mitochondrial DNA can occur as the result of radiation and other environmental agents, such as toxic chemicals. In recent years, oxidative stress has been recognized as a major source of DNA damage. Cells can repair much of the damage to DNA, and the level of damage is in a steady state between damaging and repair processes. According to the DNA damage theory of aging, accumulated DNA damage interferes with DNA replication and transcription, thereby impairing the ability of cells to function and causing aging. Moreover, this loss of function increases as the steady-state level of DNA damage increases with advancing age. However, it is not clear that DNA damage and mutations in somatic cells are sufficient to cause the organismic functional deterioration that characterizes the aging phenotype.

DNA Repair

As noted above, the steady-state level of damaged DNA depends on the balance between damage and repair processes. The DNA repair theory of aging proposes that DNA repair declines with advancing age, which causes a rise in the steady-state level of damaged DNA and thereby compromises the integrity of the genome. Because DNA repair is a complex process, it is difficult to measure in vivo. Moreover, not only the rate but also the accuracy of the repair processes could contribute to aging. Particularly in stem cells, unrepaired or inappropriately repaired DNA may play a major role in aging.

Protein Homeostasis

In addition to oxidative stress and nonenzymatic glycation, a host of other processes—including deamidation, racemization, and isomerization—may lead to deterioration of proteins, resulting in changes in the secondary and tertiary structures as well as aggregation and fragmentation. Protecting the organism from an excessive accumulation of altered proteins are proteolytic degradation and replacement— protein turnover.

As noted beginning on page 33 , cells have mechanisms for monitoring and maintaining the quality of their proteome. A major role of these mechanisms is the maintenance of the appropriate conformation of the proteins in the face of factors tending to unfold and misfold them, thereby abolishing function and often converting the protein into a toxic agent. An age-associated decrease in chaperone proteins—involved in protein folding and refolding—may be a factor in the accumulation of protein oligomers and aggregates. Small oligomers, not mature amyloid fibers, are believed to be the most toxic species in some age-associated diseases, such as Alzheimer disease. The ubiquitin/proteasome system (see pp. 33–34 ) degrades many of the proteins not refolded by the chaperones. The catalytic activity of the proteasome may decrease with age.

The rate of total-body protein turnover in humans decreases with age. Thus, the average lifetime of most but not all protein species increases with age. Long-lived proteins in the extracellular matrix, particularly collagen and elastin, undergo age-associated changes such as oxidation, glycation, and cross-linking. These changes, for cells embedded in the matrix, probably alter proliferation, migration, and the response to extracellular signals.