The analysis of single-gene mutations in flies and nematode worms has begun to yield important clues to the molecular basis of aging and genetic control of longevity in invertebrates.
At present there are four examples of single gene mutations that extend longevity in mammals.
The best documented of these is the Ames dwarf mutation, now known as Prop-1df, which in homozygous form has been shown to extend longevity by 50% in both males and females.
Homozygous df/df mice show defects in embryonic development of the anterior pituitary that lead to an absence of cells responsible for the production of growth hormone (GH), thyroid-stimulating hormone, and prolactin (PRL).
The small body size of these mice is apparent within the first 3 weeks of age, and young adults are approximately one-third of the size of 1/df or 1/1 littermates, which are themselves phenotypically indistinguishable from one another.
Extended longevity (about 20%) and small body size also are seen in transgenic mice that express high brain levels of urokinase-type plasminogen activator; in this case the phenotypes are thought to reflect a loss of appetite and diminished food intake similar to that seen in genetically normal mice and rats subjected to involuntary food restriction.
In a third instance, targeted deletion of the p66shc signal transduction protein has been shown to lead to increased lifespan presumably mediated by increased cellular resistance to apoptosis.
The fourth example, the Snell dwarf mutation Pit1dw and the coallelic mutation Pit1dwJ are the topic of this report.
The Pit1 (pituitary-specific transcription factor 1) locus on mouse chromosome 16 encodes a transcription factor, detectable by day 14 of embryonic life that is required for normal development of the anterior pituitary.
Induction of Pit1 expression requires prior activation of the Prop1 pathway deficient in the Ames dwarf mouse, and the developmental and hormonal properties of the two dwarf mutants are thus very similar.
The two spontaneous mutations Pit1dw and Pit1dwJ are non complementing, and compound heterozygotes resemble dw/dw and dwJ/dwJ homozygotes in growth rate and adult body size.
A previous publication has shown the preliminary results of an experiment comparing survival of (C3HyHeJ 3 DW/J) F1-dwydwJ to control mice.
At the time of this preliminary report, 60% of dwarf mice were still alive at an age, 1,200 days, at which all controls had succumbed, showing that Snell dwarf mice, like Ames dwarfs, were remarkably long-lived.
Alterations in longevity alone, however, provide only a limited index of altered aging, because mutations that extend longevity might do so through an effect on specific lethal illnesses without a more general retardation of senescent processes.
The report of Silberberg that Snell dwarf mice exhibit diminished osteoarthritis of the knee joint provides preliminary evidence that the Pit1dw mutation might delay aspects of biological senescence, as does a previous report that this mutation prevents age-dependent splenomegaly and impaired splenic T cell proliferation in theDW/J stock.
The current paper documents extended survival in two independent colonies of Snell dwarf mice, on two background genotypes, shows decelerated changes in age-sensitive assays of connective tissue and immune system status, explores the effects of altered PRL and GH axes on the longevity outcome, and contrasts the late-life obesity of the dw/dwJ mice to the lifelong leanness of long-lived calorically restricted (CR) mice.
Thus these observations show that a single genetic difference can retard multiple indices of senescence as well as increasing longevity in a mammal and suggest that alterations in GH-dependent pathways play a critical role in the antigeric effects of the dwarfing mutations.
The remarkable increase in mean and maximal longevity seen in the Snell dwarf mice is consistent with published data on the phenotypically similar Ames dwarf model.
Extended longevity in Pit1 and Prop1 mutant mice has now been documented in three laboratories (Southern Illinois, University of Michigan, The Jackson Laboratory) and on three genetic backgrounds Ames stock, DW/J, and the (C3H 3 DW) F1 hybrid among which the F1 hybrid provides excellent survival even in nonmutant controls.
It thus seems reasonable to conclude that the effect of these dwarfing mutations does not reflect merely the correction of some hypothetical life-shortening abnormality present in a specific control mouse stock.
The demonstration that a specific genetic or environmental intervention extends lifespan provides prima facie evidence, but not compelling proof, that the effect represents alteration in aging rate per se.
The gradual realization that caloric restriction not only extended lifespan but also decelerated age-dependent changes in multiple organs, cell types, and intracellular and extracellular processes was a critical step in the widespread adoption of the CR rodent as a model for decelerated aging.
Delayed senescence in Snell dwarf mice has been reported previously for knee joint pathology and age-related splenomegaly and T cell proliferative responses.
Here we have shown that Snell dwarf mice show slower than normal change in tail tendon collagen cross-linking, an index of aging in extracellular macromolecules, and in six age-sensitive indices of immunological development and function.
These data thus suggest that the ability of the dwarfing mutations to postpone fatal diseases and extend lifespan is accompanied by, and probably caused by, a generalized deceleration or delay in a wide range of age-dependent processes.
Snell and Ames dwarf mice show primary defects in at least three endocrine axes: thyroid-stimulating hormone stimulation of thyroid hormone production, GH stimulation of IGF-I, and PRL effects.
Our data suggest that among these, the changes in GH and IGF-I action may be particularly important.
The extended longevity of the lit/lit mice shows that alteration in the GH/IGF-I axis can be sufficient to lead to lifespan extension, and add a fifth member to the catalog of single gene mutations that extend lifespan in mammals.
We note that previous studies of longevity in colonies of lit/lit mice showed small and inconsistent effects.
Our current study of lit/lit mice made use of a diet containing only 4% fat content, which we speculate may have helped to extend longevity in the lit/lit animals by preventing the obesity seen at older ages in mice of this genotype on diets containing higher lipid levels (K.F., unpublished work).
Our data showing that pituitary transplantation does not diminish longevity in dw/dwJ dwarfs is consistent with the idea that lower circulating PRL levels are not a critical factor in this model; heterotopically transplanted pituitary glands continuously produce PRL at high physiologic levels, but GH is secreted only at very low levels, and there is no evidence for secretion of other hypophyseal hormones.
A preliminary report of extended longevity in knockout mice in which the GH receptor has been rendered nonfunctional lends additional support to the hypothesis that GH/IGF-I deficiency is the key element in the anti-aging effect of dw and df mutations.
It is also noteworthy that size disparities among breeds of dogs, which are very strongly associated (R2 5 56%) with interbreed differences in longevity, have in both of the studied instances been shown to reflect alterations in production of IGF-I, in each case smaller, long-lived breeds having lower IGF-I levels.
Genetically heterogeneous mouse stocks selected for differential growth rates in the first 10-56 days of life also differ greatly in mean and maximal lifespan, with smaller body size strongly associated (R2 5 48%, P , 0.004) with superior longevity, although in this instance it is not yet known to what extent the stocks differ in production of or response to GH and IGF-I.
The smaller impact of the lit mutation, with its relatively pure GH/IGF-I effect, compared with the effect size of the dw and df mutations, suggests that impaired thyroid hormone production also may play a role in extending lifespan in the later mutant, and studies in which GH and thyroid hormone levels are independently altered will be informative.
Thyroid hormone deficient animals are cold sensitive and might be expected to have diminished core body temperatures.
It therefore will be useful to study lifespan and age-sensitive phenotypes in dw/dw mice in which body temperature is kept within normal limits either by exogenous treatment with thyroxine or by housing them in a warmer room.
Snell dwarf mice share several characteristics with CR mice, including small body size and long lifespan.
Food consumption is also low in both models, although for different reasons: CR mice eat less because of extrinsic limitations of food supply, whereas Snell and Ames dwarf mice eat less because their small body size requires less fuel usage for maintenance of body temperature and metabolic activities.
Young adult Ames dwarf mice have been shown to consume more food calories per g body weight per day than sibling controls, consistent with the metabolic demands imposed by a high ratio of surface area to body mass.
These observations that aged dw/dwJ mice become obese and maintain high circulating leptin levels also clearly distinguish these mice from CR rodents, which remain exceptionally lean throughout life.
Compiling a catalog of similarities and differences between these and other models of decelerated aging in mice will help to narrow down the list of possible mechanisms for the longevity extension seen in these models.
This data also help address the controversial issue of whether the multiple degenerative processes seen in aging mammals are synchronized by some underlying control system.
Conventional wisdom, noting the very wide range of age-dependent changes in cellular, extracellular, and intercellular traits, views aging as a collection of independently regulated changes that work in parallel to diminish organismic homeostasis and increase vulnerability.
In contrast, the ability of caloric restriction to retard, in parallel, age-dependent changes in mitotic and nonmitotic cell types, extracellular macromolecules, and intercellular control pathways is difficult to reconcile with models based on multiple clocks, and thus supports models in which a very small number of fundamental timing processes acts to speed up or slow down age-dependent change in multiple domains.
Because age at death is changed only slightly by changes in the risk of individual forms of disease, it seems unlikely that single gene mutations that extend longevity do so by an influence on a single form of disease; extensive data on terminal pathology, not yet available for any genetic model of longevity, are needed to address this question explicitly.
Single gene mutations that extend lifespan thus provide prima facie evidence for the idea that multiple late-life processes including multiple forms of illness can be decelerated in parallel by some underlying control mechanism.
Our data, showing that the dw/dwJ genotype not only extends lifespan but also delays the effects of aging on T cell subsets and tail tendon collagen cross-linking, adds support to these "single clock" models of aging.
Further studies of these mutants, and of other mutants that affect the GH/IGF-I axis, seem likely to yield important clues to the molecular basis of aging and disease in mammals.