ACCELERATED AGING: ANIMAL MODELS

Animal models have been used to study accelerated aging, accelerated senescence, premature aging, premature senescence, and progeria-like syndromes. These models may be grouped into four classes: (1) experimentally induced models, (2) gene-modified models, (3) selection models, and (4) spontaneous models. There has been much debate over the connection between accelerated aging and disease status in animal models. Investigators interested in the basic mechanisms of normal aging have had to be prudent in their choice of animal models because early diseases leading to reduced life spans usually result from certain defects unrelated to mechanisms associated with normal aging, as suggested by David E. Harrison. In actuality, however, it is fairly difficult to discriminate between accelerated aging due to acceleration of the normal aging process and that due to the manifestation of diseases or pathologies. Thus, it might be important to check the pathologies or diseases an animal model manifests throughout its lifetime. In this context, species such as the nematode and fruit fly have disadvantages, in spite of their usefulness for genetic studies, because of a scarcity of information on diseases and pathologies.

Experimentally induced models

Adult-thymectomized lab mice. When male mice are thymectomized (removal of the thymus), their mean life span is reduced, though they don't show any specific pathologies. The reduced life span is thought to be due to accelerated aging of the immune system; that is, an accelerated decline in spleen cell responsiveness to T-cell mitogens such as phytohemagglutinin and staphylococcal enterotoxin.

Dihydrotachysterol-treated rats. In laboratory tests, when young rats are given dihydrotachysterol (reduced form of tachysterol generated from irradiated ultraviolet) orally, their life span is greatly reduced. They show pathologies such as loss of body weight; atrophy of the liver, kidney, thymicolymphatic apparatus, and fat and connective tissue; generalized arteriosclerosis with calcification; and loss of elasticity of the skin. Hans Selye, et al. designated the changes induced by dihydrotachysterol a progeria-like syndrome (progeria is a condition characterized by retardation of growth, wrinkled skin, cataracts, osteoporosis, and premature senile manifestations, among other symptoms). However, cataracts are rarely observed, and osteosclerosis, not osteoporosis, occurs in the bone in dihydrotachysterol-treated rats. So, the progeria-like syndrome induced by dihydrotachysterol in rats cannot be considered an exact replica of premature senility in humans.

Radiation model. Beginning with the experiments of S. Russ and G. M. Scott, who reported an increased death rate in chronically irradiated rats, there has been repeated confirmation of the fact that sublethal total-body irradiation from an external source can increase the mortality rate or reduce the life span of mammals. In some of the experiments, the irradiated animals died prematurely with roughly the same diseases as those associated with death in the control groups. However, the diseases did not necessarily have the same incidence or order, and sometimes there were increases or decreases in the absolute incidence of certain diseases. Some diseases occurred in irradiated groups that were not observed in the control groups. A single exposure of mice to sublethal irradiation shortened life span at some doses by increasing the initial mortality rate (IMR)—the age-independent mortality rate calculated at the age of maturation—but without further accelerating the age-dependent mortality rate. The general increase of IMR without increase in the acceleration of mortality suggests that the rate of organismic senescence is not accelerated by life-shortening radiation, as discussed by Caleb E. Finch.

Gene-modified models

Mev-1 (kn 1). A methyl viologen–sensitive mutant of the nematode Caenorhabditis elegans, named mev-1 (kn 1), was isolated after ethyl methanesulfonate mutagenesis. The herbicide methyl viologen, or paraquat, is a potent free-radical generator. The mutant C. elegans strain is about four times more sensitive to methyl viologen than the wild type (N2), and is also hypersensitive to oxygen. The mean life span of the mutant is remarkably reduced. The activity of superoxide dismutase, a scavenging enzyme for superoxide anion (O₂), is reduced in mev-1 (kn 1) by about half, compared to N2. (O₂is also a free-radical generator.) In 1998, Naoaki Ishii et al. reported that mev-1 encodes a subunit of the enzyme succinate dehydrogenase cytochrome b, which is a component of complex II of the mitochondrial respiratory chain, and that the ability of complex II to catalyze electron transport from succinate to ubiquinone is compromised in the mutant. This defect may cause an indirect increase in superoxide levels, which in turn leads to oxygen hypersensitivity and accelerated aging.

D. melanogaster cSOD ⁿ¹⁰⁸. The role of copper/zinc-containing superoxide dismutase (cSOD) in metabolic defense against O₂toxicity in the fruit fly, Drosophila melanogaster, has been examined through the properties of a mutant strain of D. melanogaster carrying cSOD-null mutation. The mutant, termed cSOD ⁿ¹⁰⁸confers recessive sensitivity to the paraquat. This indicates that the cSOD-null condition in fact leads to impaired O₂>metabolism. The mean adult life span of the mutant (11.8 days) is remarkably decreased compared to that of the control parental stocks (55.4–57.8 days). Furthermore, male mutants are completely sterile, and females are nearly so. Thus, the primary biological consequence of the reduced O₂dismutation capacity of cSODⁿ¹⁰⁸is infertility and a reduction in life span.

B10. F mice. A congenic line of mice, called B10. F, was produced by introducing a segment of chromosome 17 (H-2 ⁿ) from the strain F/St on the background of the C57BL/10 of H-2 ^b by back-crossing. The congenic B10. F mice gray early, suffer severe weight loss, and have skin that becomes thin and fragile. The mean life span of the B10. F mice is reduced, with 90 percent of the mice dying within seventeen months. The survival curve also shows significantly accelerated aging in the B10. F mice. At twelve to fourteen months these mice have a capacity to produce plaques to red-blood cells of sheep (an antigen for immunization) that is 12 percent of the capacity at four months, as assessed as one aspect of the immune competence of the B10. F mice. Since the strain is a congenic line, the genetic difference (and possibly the basis for the degenerative signs and reduced life span) is amenable to analysis.

Klotho mice. The insertion of a mutated transgene in mice has been found to disrupt a new gene locus, termed klotho, resulting in the manifestation of various phenotypes resembling those in patients with premature-aging syndromes. Homozygous klotho mice show growth retardation and have a short life span—up to three to four weeks of age—and the mice grow normally but show growth retardation thereafter, gradually becoming inactive and marasmic (emaciated), and dying prematurely at eight to nine weeks of age. The average life span of klotho mice is 60.7 days. The pathological phenotypes are infertility, hypokinesis (decreased function of the left ventricle), atrophy of genital organs and the thymus, arteriosclerosis, ectopic calcification, osteoporosis, skin atrophy, emphysema, and abnormalities in the pituitary gland. The klotho gene encodes a membrane protein that shares a sequence similarity with the β-glucosidase enzymes and expresses mainly in kidney and brain. It is hoped that the study of klotho mice with a single gene mutation will help to clarify the molecular-genetic mechanisms of both premature aging and accelerated aging.

Selection models

L line. High (H) and low (L) antibody responder lines of mice have been separated by selective breeding, presenting a maximal interline difference in antibody response to sheep red-blood cells, which was reached after fifteen successive generations of selective breeding. The life span of the H line and L line is 612 days and 346 days, respectively. Interpopulation correlation between life span and antibody response has shown that the life span is correlated positively with 2-mercaptoethanol (a reducing agent)– resistant agglutinin response and negatively with 2-mercaptoethanol-sensitive agglutinin response. Pathological examinations have revealed that chronic nephritis and malignant lymphoma are cardinal phenotypes observed at death in both the H and L lines. Mortality due to an early incidence of chronic nephritis, with a reducedsized kidney and an irregular scarring of the cortex contribute to the shorter life span of the L line. The L responder mice also show a significant increase in malignant lymphomas, compared to the H responder mice. Thus, chronic nephritis and malignant lymphomas should be considered as the two main diseases accounting for the reduced life span of the lower antibody responder L line.

Senescence-accelerated mouse. The senescence-accelerated mouse (SAM), which consists of fourteen senescence-prone inbred strains (SAMP) and four senescence-resistant inbred strains (SAMR), has been under development since 1970. The manifestation of senescence in SAMP does not occur in the developmental stage, but it occurs in an accelerated manner following normal development, though there is no evidence of growth retardation, malformation, limb palsy, or other neurological signs, such as tremors and convulsions. The life span of SAMP is about 40 percent shorter than that of SAMR. Thus, accelerated senescence is considered to be a characteristic feature common to all SAMP mice. Both SAMP and SAMR strains manifest various pathobiological phenotypes, which are often characteristic enough to differentiate the strains. These phenotypes include senile amyloidosis, impaired immune response, hyperinflation of the lungs, hearing impairment, deficits in learning and memory, cataracts, alveolar bone loss, degenerative joint disease, abnormality of circadian rhythms, emotional disorders, and brain atrophy.

Studies suggest that a hyperoxidative status due to mitochondrial dysfunction plays a pivotal role in the manifestation of accelerated senescence, as well as pathologic phenotypes, in SAMP. Genetic studies to identify the genes for accelerated senescence of SAMP mice and for pathological phenotypes such as senile osteoporosis of SAMP6 mice are underway.

S strain. By selection and inbreeding of Wistar rats (an ordinary strain of rats) for sensitivity to the cataractogenic effect of a galactose-rich diet, a sensitive (S) and a resistant (R) rat strain were developed. A heritable increase in cellular hexose uptake has been associated with an increased intracellular generation of hydroxy radicals, increased endogenous lipid peroxidation, mitochondrial dysfunction, numerous DNA rearrangements, and membrane fragility in the S rats. Age-related degenerative diseases such as emphysema, cataracts, myocardial alterations, spinal column deformations, and impaired retention of long-term memory also manifest in the S rats. The life span of S rats is more than 50 percent shorter than that of R rats. Because of high embryonic mortality, fertility is also lower in the S rats. It is reasonable to conclude that continuous oxidative damage results ultimately in premature aging and in early death of the S rats.

Spontaneous models

Belgian hares. A certain family of Belgian hares, part of a large breeding colony organized for the study of constitutional problems, developed premature senescence as a hereditary entity. From observations made on individuals representing twenty generations with the condition, two principal forms of the degenerative syndrome were recognized: acute and chronic (the essential difference between the two forms being the degree and rate of deterioration). However, there have been no reports on the aging characteristics and pathological findings, including histopathology, of the various degenerative lesions.

Toshio Takeda

See also Genetics; Longevity: Selection; Molecular Biology of Aging.

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