The search for the fountain of youth did not end with Ponce de Leon. Rather, the quest has picked up again lately as researchers have begun to understand the biology of aging and its link to lifespan and disease. Two papers this week deal with distinct mechanisms of anti-aging that have been implicated in Alzheimer disease and other neurodegenerative disorders. In yesterday’s Science, Akiko Taguchi, Lynn Wartschow, and Morris White at Harvard Medical School show that reducing insulin signaling specifically in the brain extends the life of mice. Their results suggest that forces which lower insulin levels—diet, exercise, and weight control—might help us live longer by protecting our brains. That protection might extend to AD as well, where the same factors of diet, inactivity, and overweight raise the risk of disease (see ARF related news story), and derangements in insulin levels or signaling may contribute to AD pathology (see ARF meeting report).

In the second paper, this one in the July 19 Nature, the anti-aging effect comes from a different pathway. Manuel Serrano and colleagues at the Spanish National Cancer Research Center in Madrid demonstrate that boosting levels of the tumor suppressor p53 and its partner Arf slows aging in mice by elevating antioxidant defenses. In neurodegenerative disease, p53 has been implicated as the executioner, mediating the neuronal response to toxic insults like APP products or huntingtin protein (see ARF related news story and ARF news story). The new results raise the possibility that during normal aging, the Arf/p53 pathway could have a protective effect.

First, to insulin and aging. Lower signaling through the insulin pathway causes lifespan extension in worms and flies, and changes in insulin levels are one possible explanation for the life-prolonging effects of calorie restriction in higher animals. White and colleagues messed with insulin signaling in mice by knocking out a key substrate for the insulin receptor tyrosine kinase, insulin receptor substrate 2 (Irs2). Previously, they showed that Irs2-null mice suffer metabolic derangements, diabetes, and early death. The current work analyzes heterozygotes that appear metabolically normal early in life. As the Irs2+/- mice age, they become overweight, but remain sensitive to insulin, a characteristic associated with longevity. Indeed, the Irs2+/- mice lived 17 percent longer than wild-type animals.

In flies and worms, reduced insulin signaling in neurons increases lifespan, so the researchers asked if this was also true in mice by creating a brain-specific Irs2 knockout. These mice ate more, were overweight, and displayed insulin resistance, two metabolic changes that should shorten their lives. Despite this, the heterozygous knockouts lived 18 percent (5 months) longer, and the homozygous knockouts survived 14 percent longer than control mice. The mice show some positive metabolic effects—they were more active, and their glucose utilization and antioxidant response to eating resembled young mice more than control aged mice. The authors conclude that, just as in worms and flies, the results “point to the brain as the site where reduced insulin-like signaling can have a consistent effect to extend mammalian life span.”

Besides insulin, the body’s defenses against cancer may come into play against aging, too, according to the second paper, in which the Spanish group reports that mice with increased but normally regulated levels of the tumor suppressor protein p53 and its stabilizer Arf seem to age more slowly. The mice show both strong cancer resistance and decreased levels of aging-associated oxidative damage.

The p53 protein is a cell’s alarm system, sensing stress and triggering the death of heavily damaged cells. The investigators wondered if p53 might also play a role in response to the low-level, chronic stress that accompanies aging. To probe this question, first authors Ander Matheu and Antonio Maraver generated mice with an extra copy of the p53 and Arf genes under the control of their endogenous promoters. Mice with one extra copy of either gene were resistant to cancer, and adding both together gave even higher protection. While either gene alone did not significantly affect lifespan, the two together produced a 16 percent extension of median lifespan. This is similar to the effect seen with calorie restriction, but unlike calorie restriction, Arf/p53 augmentation did not increase the maximum lifespan. The changes in death rates were not due to differences in the incidence of cancer, since comparison of cancer-free mice from both groups produced a similar shift in median lifespan.

The results suggest that the Arf/p53 pathway can inhibit at least some part of the aging process, and the researchers went on to show that the Arf/p53-enhanced mice also showed changes in biochemical markers of aging, including neuromuscular coordination, hair growth, and the accumulation of phosphorylated histone lesions. The differences in aging correlated with decreased levels of reactive oxygen species in the mice, and the researchers demonstrated that Arf/p53 drove expression of a panel of presumably protective antioxidant genes. The protective effect of these genes was dramatically demonstrated by an increase in the resistance of Arf/p53 mice to a lethal dose of paraquat, a strong oxidative agent.

“We propose that the spectra of genes activated by p53 under normal physiological conditions have a global antioxidant effect, thus decreased aging-associated oxidative damage,” the authors conclude. Reactive oxygen species have been linked to various neurodegenerative diseases including Alzheimer’s and Parkinson’s, so the work opens the possibility that Arf/p53 might play a protective role not only for cancer, but in normal aging and in age-related neurodegenerative diseases.—Pat McCaffrey.

References:
Taguchi A, Wartschow LM, White MF. Brain IRS2 signaling coordinates life span and nutrient homeostasis. Science. 2007 July 20; 317:369-372. Abstract

Matheu A, Maraver A, Klatt P, Flores I, Garcia-Cao I, Borras C, Flores JM, Vina J, Blasco MA, Serrano M. Delayed ageing through damage protection by the Arf/p53 pathway. Nature. 2007 Jul 19;448(7151):375-379. Abstract

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  1. Taguchi and colleagues’ recent studies suggest that reducing insulin signaling through deletion of insulin receptor substrate 2 in brain (bIRS2) increases longevity even though it induces increased peripheral insulin resistance and hyperinsulinemia. Neuronal IRS2 is known to mediate critical aspects of systemic energy homeostasis (1); thus, the peripheral metabolic derangement caused by genetic deletion of bIRS2 is not surprising. It is of interest, however, that the typical life-shortening effects of peripheral insulin resistance can be reversed by inactivation of bIRS2-mediated insulin signaling. The authors make a good case that retention of youthful fat and carbohydrate metabolism and postprandial superoxide dismutase response may contribute to the resilience of bIRS2 -/- and +/- animals.

    There are mixed implications of these results for understanding the potential contribution of insulin signaling abnormalities to Alzheimer disease (AD) pathogenesis. The finding that brain size is reduced in the bIRS2 -/- both supports the role of IRS2 in brain development and raises the question of potential behavioral deficits in these animals. Further behavioral testing with this model will provide useful data in that regard. There are additional direct implications of this work for Aβ regulation, given suggestions that IRS2 is required for insulin/IGF1-mediated switching from TrkA to the p75NTR neurotrophin receptor, which then stabilizes BACE1 and increases Aβ generation (2). Reducing IRS2-mediated insulin signaling may thus conceivably reduce brain amyloid levels.

    One final caveat is needed regarding the extreme plasticity of the ancient and complex insulin signaling pathway. Knockout models of insulin/IGF1 signaling are highly susceptible to effects of reorganization that may not always be easy to discern and control. Replicative findings with conditional knockout models or techniques such as siRNA will help solidify the relevance of these interesting results for human aging and AD.

    References:

    . Distribution of insulin receptor substrate-2 in brain areas involved in energy homeostasis. Brain Res. 2006 Sep 27;1112(1):169-78. PubMed.

    . An aging pathway controls the TrkA to p75NTR receptor switch and amyloid beta-peptide generation. EMBO J. 2006 May 3;25(9):1997-2006. PubMed.

  2. Comment by Kazuhiro Nakamura and Kun Ping Lu

    Aging, Cancer, and Neurodegeneration
    Matheu et al. (1) have made the important discovery that overexpression of both Arf and p53 under normal regulation can confer cancer resistance, reduce aging-associated damage, and delay normal aging in mice. These results are especially interesting because these authors have previously shown that overexpression of either Arf or p53 alone can confer cancer resistance, but not longevity (2,3), highlighting the tight regulation of the aging process. The authors have provided a rationale for the co-evolution of cancer resistance and longevity, suggesting that it may be possible to live longer without worrying about cancer.

    These results have a general impact on many age-related disorders, including neurodegeneration, which also result from age-related cellular damage in the nervous system. Interestingly, p53-mediated cell death has been associated with the progressive neuronal death in Huntington disease, Parkinson disease, Alzheimer disease, and amyotrophic lateral sclerosis (4). For instance, p53 mediates cellular dysfunction and behavioral abnormalities in Huntington disease (5). In addition, increased expression of Arf and p16ink4a has been shown to decrease brain stem/progenitor cells and neurogenesis during aging and in Bmi1-deficient mice (6-9). Reduced stem/progenitor cell function may be a major cause of the decline in regenerative capacity and contribute to degenerative diseases observed in aging tissues. Thus, it would be of interest to examine whether this Arf/p53 longevity pathway would affect the development of stem/progenitor and progression of neurodegeneration.

    The concept of the co-evolution of cancer resistance and longevity is intriguing and significant. However, there are many other examples where longevity and cancer resistance may be antagonistic (10). For example, overexpression of truncated (constitutively active?) p53 leads to premature aging and cancer resistance in mice (11,12). Similarly, we and others have shown that ablation of the prolyl isomerase Pin1 in mice leads to many premature aging phenotypes including neurodegeneration, and also protects against cancer development even induced by overexpression of certain oncogenes or ablation of p53 (13-18). These results are relevant because Pin1 appears to regulate p53 function in response to genotoxic stress (19-21). Therefore, it would be extremely interesting and important to understand how and why cancer resistance and longevity are antagonistic under some conditions, but compatible under some other conditions, and also to investigate the relationship among aging, cancer, and neurodegeneration.

    References:

    . Delayed ageing through damage protection by the Arf/p53 pathway. Nature. 2007 Jul 19;448(7151):375-9. PubMed.

    . "Super p53" mice exhibit enhanced DNA damage response, are tumor resistant and age normally. EMBO J. 2002 Nov 15;21(22):6225-35. PubMed.

    . Increased gene dosage of Ink4a/Arf results in cancer resistance and normal aging. Genes Dev. 2004 Nov 15;18(22):2736-46. PubMed.

    . The p53 family in nervous system development and disease. J Neurochem. 2006 Jun;97(6):1571-84. PubMed.

    . p53 mediates cellular dysfunction and behavioral abnormalities in Huntington's disease. Neuron. 2005 Jul 7;47(1):29-41. PubMed.

    . Increasing p16INK4a expression decreases forebrain progenitors and neurogenesis during ageing. Nature. 2006 Sep 28;443(7110):448-52. PubMed.

    . Stem-cell ageing modified by the cyclin-dependent kinase inhibitor p16INK4a. Nature. 2006 Sep 28;443(7110):421-6. PubMed.

    . Bmi-1 promotes neural stem cell self-renewal and neural development but not mouse growth and survival by repressing the p16Ink4a and p19Arf senescence pathways. Genes Dev. 2005 Jun 15;19(12):1432-7. PubMed.

    . Bmi-1 dependence distinguishes neural stem cell self-renewal from progenitor proliferation. Nature. 2003 Oct 30;425(6961):962-7. PubMed.

    . Senescent cells, tumor suppression, and organismal aging: good citizens, bad neighbors. Cell. 2005 Feb 25;120(4):513-22. PubMed.

    . p53 mutant mice that display early ageing-associated phenotypes. Nature. 2002 Jan 3;415(6867):45-53. PubMed.

    . Modulation of mammalian life span by the short isoform of p53. Genes Dev. 2004 Feb 1;18(3):306-19. PubMed.

    . The prolyl isomerase PIN1: a pivotal new twist in phosphorylation signalling and disease. Nat Rev Mol Cell Biol. 2007 Nov;8(11):904-16. PubMed.

    . Loss of Pin1 function in the mouse causes phenotypes resembling cyclin D1-null phenotypes. Proc Natl Acad Sci U S A. 2002 Feb 5;99(3):1335-40. PubMed.

    . Role of the prolyl isomerase Pin1 in protecting against age-dependent neurodegeneration. Nature. 2003 Jul 31;424(6948):556-61. PubMed.

    . The prolyl isomerase Pin1 regulates amyloid precursor protein processing and amyloid-beta production. Nature. 2006 Mar 23;440(7083):528-34. PubMed.

    . Modeling breast cancer in vivo and ex vivo reveals an essential role of Pin1 in tumorigenesis. EMBO J. 2004 Aug 18;23(16):3397-407. PubMed.

    . Ablation of a peptidyl prolyl isomerase Pin1 from p53-null mice accelerated thymic hyperplasia by increasing the level of the intracellular form of Notch1. Oncogene. 2007 May 31;26(26):3835-45. PubMed.

    . Role of Pin1 in the regulation of p53 stability and p21 transactivation, and cell cycle checkpoints in response to DNA damage. J Biol Chem. 2002 Dec 13;277(50):47976-9. PubMed.

    . The prolyl isomerase Pin1 is a regulator of p53 in genotoxic response. Nature. 2002 Oct 24;419(6909):849-53. PubMed.

    . The prolyl isomerase Pin1 reveals a mechanism to control p53 functions after genotoxic insults. Nature. 2002 Oct 24;419(6909):853-7. PubMed.

  3. It is well established that p53 is an oncogene, many mutations on which are responsible for the development of various types of cancer. It has been proposed that this pathology is due to the impairment of the ability of p53 to eliminate damaged cells. Aging also results from damaged cells which accumulate, and it is therefore tempting to postulate that p53 could be an endogenous “life prolongator.” Accordingly, in C. elegans, mutations which increase longevity also confer cancer resistance likely via p53 activation. Therefore, longevity and cancer resistance could have in common the potent ability of p53 to clear damaged cells.

    The paper by Matheu and colleagues very interestingly documents that in mice, overexpression of p53 and Arf (a p53 stabilizer) triggers cancer resistance and decreases age-associated damage. Therefore, the study gives support to the idea that the control of p53 could be central in aging and provides a rationale to the unexplained observation that long lifetime is apparently associated to increased cancer resistance.

    This very interesting and well-performed study raises a few questions. Particularly striking is the observation that the coexpression of both Arf and p53 slightly increases the median lifespan (by 16 percent) but that the shapes of the two survival/age curves are different (see Fig. 2a,b). Although the Arf/p53 mice begin to die later than wild-type mice, the decay in survival seems accelerated for the former mice and finally, both curves gather to give 100 percent mortality at about 150 weeks. In other words, there is not a subset of Arf/p53 mice which survives the group of wild-type mice. Whether the cooperative function of Arf and p53 is efficient at early stages of “old age” but remains ineffective at later stages is a matter of reflection.

    Has the work by Matheu implications for the understanding of neurodegenerative diseases? In Parkinson disease-affected brains, an increased p53-dependent cell death has been well documented. In both Parkinson and Alzheimer diseases, proteins responsible for familial cases, such as presenilins or α-synuclein, control p53-dependent cell death, and this function is increased by pathogenic mutations. Even if the demonstration that exacerbated p53-dependent apoptosis is causal in early death observed in genetic cases of AD or PD remains to be established, increased p53 expression accompanying early death would appear paradoxical with respect to Matheu’s work indicating that p53 overexpression triggers enhanced longevity. Close examination of data gives simple explanations. Thus, Matheu and colleagues show that the p53 or Arf overexpression alone did not mimic the phenotype triggered by coexpression of the two protein partners. No data are yet available concerning the levels of Arf in AD or PD brains and whether decreased levels of Arf could account for altered p53-dependent control of the longevity bestowed by p53. It should be noted that if this is the case, such alteration should be topologically restricted to cerebral areas lesioned in these pathologies (which are sometimes very narrow) in contrast to more widely distributed brain zones affected during normal aging. It is therefore more likely that the very nicely documented functional collaboration between Arf and p53 in the control of lifespan is not a central alteration responsible for neurodegenerative diseases associated with exacerbated apoptotic cell death.

References

News Citations

  1. Fat and Calories Mean Higher AD Risk
  2. Madrid: Highs and Lows of The Insulin Connection
  3. Linking APP to Apoptosis—p53 Does the Trick
  4. Friend or Foe? Tumor Suppressor p53 Enhances Huntingtin Toxicity

Paper Citations

  1. . Brain IRS2 signaling coordinates life span and nutrient homeostasis. Science. 2007 Jul 20;317(5836):369-72. PubMed.
  2. . Delayed ageing through damage protection by the Arf/p53 pathway. Nature. 2007 Jul 19;448(7151):375-9. PubMed.

Further Reading

Papers

  1. . Delayed ageing through damage protection by the Arf/p53 pathway. Nature. 2007 Jul 19;448(7151):375-9. PubMed.
  2. . Brain IRS2 signaling coordinates life span and nutrient homeostasis. Science. 2007 Jul 20;317(5836):369-72. PubMed.

Primary Papers

  1. . Delayed ageing through damage protection by the Arf/p53 pathway. Nature. 2007 Jul 19;448(7151):375-9. PubMed.
  2. . Brain IRS2 signaling coordinates life span and nutrient homeostasis. Science. 2007 Jul 20;317(5836):369-72. PubMed.