Huber R. Warner

Superoxide dismutase, aging, and degenerative disease.

Over 15 years of research on correlations between superoxide dismutase (SOD) activity and aging or life span have failed to provide a consistent picture of the role of SOD in aging. While genetic manipulations that increase CuZn-SOD activity have only a slight, if any, effect on maximum life span in several species, they do increase resistance to oxidative stress. However, increasing both CuZn-SOD and catalase does significantly increase maximum life span. Decreased SOD expression in a variety of species increases their vulnerability to oxidative stress, and in the case of genetically altered CuZn-SOD, leads to premature death of motor neurons in humans. Little is known about the regulation of expression of SOD and other antioxidant defense enzymes in eukaryotes. The research summarized below collectively suggest that SOD plays an important role in longevity and degenerative disease, but much remains to be learned before manipulation of SOD expression can be considered for effective intervention in either process.

Nordihydroguaiaretic acid and aspirin increase lifespan of genetically heterogeneous male mice

The National Institute on Agings Interventions Testing Program was established to evaluate agents that are purported to increase lifespan and delay the appearance of age‐related disease in genetically heterogeneous mice. Up to five compounds are added to the study each year and each compound is tested at three test sites (The Jackson Laboratory, University of Michigan, and University of Texas Health Science Center at San Antonio). Mice in the first cohort were exposed to one of four agents: aspirin, nitroflurbiprofen, 4‐OH‐α‐phenyl‐N‐tert‐butyl nitrone, or nordihydroguaiaretic acid (NDGA). Sample size was sufficient to detect a 10% difference in lifespan in either sex, with 80% power, using data from two of the three sites. Pooling data from all three sites, a log‐rank test showed that both NDGA (p = 0.0006) and aspirin (p = 0.01) led to increased lifespan of male mice. Comparison of the proportion of live mice at the age of 90% mortality was used as a surrogate for measurement of maximum lifespan; neither NDGA (p = 0.12) nor aspirin (p = 0.16) had a significant effect in this test. Measures of blood levels of NDGA or aspirin and its salicylic acid metabolite suggest that the observed lack of effects of NDGA or aspirin on lifespan in females could be related to gender differences in drug disposition or metabolism. Further studies are warranted to find whether NDGA or aspirin, over a range of doses, might prove to postpone death and various age‐related outcomes reproducibly in mice.

An aging Interventions Testing Program: study design and interim report

The National Institute on Agings Interventions Testing Program (ITP) has developed a plan to evaluate agents that are considered plausible candidates for delaying rates of aging. Key features include: (i) use of genetically heterogeneous mice (a standardized four‐way cross), (ii) replication at three test sites (the Jackson Laboratory, TJL; University of Michigan, UM; and University of Texas, UT), (iii) sufficient statistical power to detect 10% changes in lifespan, (iv) tests for age‐dependent changes in T cell subsets and physical activity, and (v) an annual solicitation for collaborators who wish to suggest new interventions for evaluation. Mice in the first cohort were exposed to one of four agents: aspirin, nitroflurbiprofen (NFP), 4‐OH‐α‐phenyl‐N‐tert‐butyl nitrone (4‐OH‐PBN), or nordihydroguiaretic acid (NDGA). An interim analysis was conducted using survival data available on the date at which at least 50% of the male control mice had died at each test site. Survival of control males was significantly higher, at the interim time‐point, at UM than at UT or TJL; all three sites had similar survival of control females. Males in the NDGA group had significantly improved survival (P = 0.0004), with significant effects noted at TJL (P < 0.01) and UT (P < 0.04). None of the other agents altered survival, although there was a suggestion (P = 0.07) of a beneficial effect of aspirin in males. More data will be needed to determine if any of these compounds can extend maximal lifespan, but the current data show that NDGA reduces early life mortality risks in genetically heterogeneous mice at multiple test sites.

Program for testing biological interventions to promote healthy aging

The National Institute on Aging (NIA) sponsored a workshop on September, 1999 to discuss the feasibility of establishing a program to evaluate potential intervention strategies to decelerate the rate of aging in mammals. The ultimate goal is to identify promising interventions in animals that might lead to clinical trials in humans. The participants discussed various animal models, biological endpoints and possible structure of such a program. The ability to implement such a program will require a decision by NIA staff about whether the anticipated benefits to be derived from identification of effective interventions under well controlled conditions in an animal model, in this case the mouse, would justify the anticipated cost of the testing program.

Longevity genes: from primitive organisms to humans.

Recent results indicate that the longevity of both invertebrates and vertebrates can be altered through genetic manipulation and pharmacological intervention. Most of these interventions involve alterations of one or more of the following: insulin/IGF-I signaling pathway, caloric intake, stress resistance and nuclear structure. How longevity regulation relates to aging per se is less clear, but longevity increases are usually accompanied by extended periods of good health. How these results will translate to primate aging and longevity remains to be shown.

Apoptosis: A Two-edged Sword in Aging

Here we summarize briefly what is known about both the positive and negative impacts of apoptosis during aging in mammalian systems and also update an earlier review. It is important to understand both of these impacts to devise useful interventions. Such interventions include both physiological and molecular approaches, including transgenic interventions. The critical roles of the mitochondria in both generating reactive oxygen species, and in initiating apoptosis are recognized, suggesting that maintaining mitochondrial function could be an important therapeutic goal, especially in post‐mitotic tissues. In contrast, the ability to eliminate unwanted, damaged and dysfunctional cells through apoptosis has anti‐aging implications in mitotic tissues.

Models of accelerated ageing can be informative about the molecular mechanisms of ageing and/or age-related pathology.

During the past ten years considerable progress has been made in discovering genes that regulate longevity by identifying single gene mutations that lead to increased longevity. The initial success in nematodes was quickly followed by comparable success in fruit flies and mice. In contrast, mutations that cause a decrease in longevity have been largely discounted as unlikely to be informative about aging mechanisms. However, the recent creation of several mutant mouse models that develop a variety of aging-like phenotypes and die prematurely, suggests that such models may be useful in understanding aging mechanisms, particularly as they relate to progressive tissue and organ dysfunction. A possible common feature of these models may be an imbalance between loss of cells by apoptosis and subsequent cell replacement, leading gradually to a net loss of cells in multiple tissues.

Does genotypic sex have a direct effect on longevity

Females of the human species live longer than males, and the longevity differential is probably not entirely explained by reasons which are presently obvious. Genotypic sex has long been suspected to affect longevity to the advantage of the female. Several recent findings about the X and Y chromosomes must be reckoned with in considering determinants of longevity which derive from genotypic sex. The advantages of having two X chromosomes are apparent, notwithstanding X-chromosome inactivation. Not only can some cells compensate for biosynthetic deficiencies of others, but also cell selection according to which X chromosome is active can occur during development according to cell viability and proliferative capacity. It has recently been observed that at least some genes on inactive X chromosomes are reactivated late in life. Details of the reactivation process must be studied to determine its significance and the effects of the process on late life survival. The recent mapping of the catalytic polypeptide of DNA-polymerase-alpha to the X chromosome calls attention to a new property of the genotype which could affect the basic ability of cells to proliferate. It is likely that this enzyme, perhaps in concert with DNA-polymerase-delta, is required for DNA replication, suggesting that two alleles for this enzyme and cell selection within the female phenotypic mosaic for DNA replication may provide a sex-linked determinant of cell proliferation which could be advantageous in late life. Much remains to be learned about the Y chromosome, although there are early results consistent with a determinant of longevity on that chromosome which operates to the male disadvantage and probably does not involve sex hormones. The genotype may be a significant determinant of longevity in humans even if it does not appear to be so in non-human animals, because causes of death are different. Determinants of longevity are based on susceptibility or vulnerability to the causes and diseases of mortality, and these differ in different species.

Bacteriophage T5 and Related Phages

Bacteriophages T5 and BF23 have been the two most extensively investigated of the T5 group, which also includes bacteriophages PB, BG3, and 29 alpha. Unusual features of T5, BF23, and PB include the existence of interrupted phosphodiester bonds (“nicks”) at specific locations in only one of the strands of their duplex DNA, a two-step mechanism for transfer of their DNA into host cells, and large terminal repetitions in their DNA.

Is cell death and replacement a factor in aging

The central theme of the 3rd International Conference on Functional Genomics of Ageing was tissue regeneration as a remedial strategy to address age-related cellular damage and the pathology that ensues. The conference included sessions on maintaining genome integrity and the potential of stem cells to restore function to damaged tissues. In addition to several human syndromes that appear to reflect accelerated ageing, there are now a number of mouse models that prematurely display phenotypes associated with ageing. The intent of this summary presented at the end of the conference was to: (1) discuss various human syndromes and mouse models of accelerated ageing; (2) evaluate whether the phenotypes displayed might result from an elevated rate of cell death coupled with an inability to adequately maintain cell number in various tissues with increasing age; and (3) discuss whether similar events may be occurring during normal ageing, albeit much more slowly.