Cast Call for Players: Healthy Aging Through Lean Living
Leigh Ann Henricksen and Howard J. Federoff led this live discussion on 17 August 2004. Readers are invited to submit additional comments by using our Comments form at the bottom of the page.
View Transcript of Live Discussion — Posted 23 August 2006
By Leigh Ann Henricksen and Howard J. Federoff
University of Rochester School of Medicine & Dentistry, Dept. of Neurology, Center for Aging and Developmental Biology, 601 Elmwood Ave., Rochester, NY 14624, USA.
For many, growing old seems inevitable. Aging is the result of a lifetime of accumulated use and attendant damage. To a great extent our genomes direct the longevity cinema, casting the molecular characters, their interactions, and orchestrated response to extrinsic stress. Those fortunate enough to have inherited a good endowment would appear to have a higher probability of attaining a functional long life. Yet this analysis does not suggest that we should be resigned to simply let it play out as written. Rather, our important task is to modulate the genomic direction to improve on an otherwise preordained outcome. How can this be achieved and through what means? Expanding research on the phenomenon of caloric restriction and its molecular underpinnings is pointing the way. Our growing understanding portends novel strategies to address age-related disorders.
A breakthrough in aging research was the observation that caloric restriction extends life span. It is well documented that a nutritional diet with a reduced total caloric intake will increase the life span of organisms as diverse as yeast, worms, fruit flies and mice (Koubova and Guarente, 2003). The evolutionary conservation of the effects of caloric restriction (CR) suggests that these genetically tractable organisms may provide the models necessary to define the mechanisms and molecules required to understand and define aging in higher eukaryotes. In worms and flies, CR leads to a reduction in body fat, temperature, fecundity and an increased sensitivity to insulin (Clancy et al., 2001; Dillin et al., 2002). In mammals, the levels of insulin and insulin growth factor (IGF) also are reduced, illustrating the importance of insulin and IGF-1 on aging. Molecular genetics in both C. elegans and Drosophila have proven that a reduction in insulin and IGF-like peptides leads to an increased life span (Clancy et al., 2001; Tatar et al., 2001; Tu et al., 2002).
In yeast, screens for genetic determinants of aging uncovered that mutation of the NAD+-dependent protein deacetylase SIR2 (herein called SIRT1), results in a reduction in life span (Anderson et al., 2003; Lin and Guarente, 2003). Overexpression of SIRT1 in both yeast and worms leads to an increase in life span. Further, it has been shown that manipulation of nuclear NAD+ levels also contribute to longevity in a SIRT1-dependent manner (Rongvaux et al., 2003). Finally, several types of low-level stress also lead to increased longevity, potentially through either insulin-like signaling, SIRT1, and other stress-induced pathways (Tissenbaum and Guarente, 2002). These data underscore the importance of these proteins and their downstream effects, and suggest possible prospects for therapeutics development. The current excitement in our understanding of the molecular mechanism of aging arises from the realization that these pathways are interconnected and may ultimately converge.
The importance of insulin signaling on life expectancy is well established, as loss of either the insulin receptor or its substrate slows aging. Even so, it was not known if this was the direct or indirect result of insulin-like peptides. In worms, stimulation of the insulin receptor leads to the activation of a kinase cascade leading to the phosphorylation of members of the forkhead (FOXO) transcription family (Tissenbaum and Guarente, 2002). When phosphorylated, FOXO is sequestered within the cytoplasm. Translocation of FOXO to the nucleus through a reduction in the insulin-like signaling, or over-expression of FOXO within adipose tissue or neurons, leads to an increase in life expectancy.
Two recent reports (Giannakou et al., 2004; Hwangbo et al., 2004) show that the ability of Drosophila FOXO (dFOXO) to modulate life span is conserved in Drosophila. Constitutive over-expression of FOXO in a number of specific tissues is lethal in the fly. However, controlled expression of dFOXO within fat bodies, particularly those in the brain, result in a long life span. Interestingly, increased expression of dFOXO within neurons failed to increase life span. However, dFOXO expression in the head fat body altered the levels of insulin-like peptide within the adult brain, suggesting that insulin signaling within a limited number of cells may have a paracrine effect on other cells within the CNS. In addition, the expression of dFOXO within fat bodies provides cytoprotection against the oxidative stress caused by the drug paraquat.
It will be of great interest to determine if regulation of FOXO and its downstream targets will attenuate the oxidative damage thought to participate in neurodegenerative diseases such Alzheimer's and Parkinson's diseases. These data continue to illustrate that one's fate can be altered by modulating the balance of various cellular signals. Moderating insulin levels by diet or developing compounds that trigger the appropriate downstream event may lead to lifelong benefits.
All of these data indicate that CR alters longevity by ultimately regulating gene expression through the transcription factor FOXO. Yet, in yeast, SIRT1 also is clearly involved in mediating effects of CR. How does it fit in? Recent studies (Brunet et al., 2004; Motta et al., 2004) of SIRT1 in C. elegans provide the bridge linking the insulin pathway and SIRT1. The data showed that FOXO activity is required for SIRT1-mediated longevity and established that SIRT1 interacts and deacetylates FOXO, altering transcription of its targets. Further, deacetylation of FOXO appears to upregulate stress-resistance genes and downregulate proapoptotic targets. This led the authors to speculate that SIRT1, through FOXO, regulates the cell's response to favor stress resistance over death.
The ability of insulin signaling to affect life expectancy is documented not only in worms and flies, but also in mammals, as mice lacking the insulin receptor in adipose tissue are long-lived (Bluher et al., 2003; see ARF related news story). However, it has remained a question if SIRT1 would promote cytoprotection within mammals in response to CR. As reported most recently by David Sinclair's group, (Cohen et al., 2004; see ARF related news story), SIRT1 expression is increased in multiple tissues in response to CR in part due to a response mediated by insulin signaling. These authors further identify a new target for SIRT1, the DNA repair protein Ku70. When deacetylated, Ku70 forms a complex with the proapoptotic factor Bax, preventing Bax from entering the mitochondria and mediating cell death. Interestingly, sera from mice maintained on a CR diet were able to induce SIRT1 and protect against oxidative stress. Again, specific changes in gene expression are shown to provide cytoprotection and link SIRT1 action to apoptosis regulation.
Although understanding the multiple effects of CR on survival may seem daunting, these studies collectively illustrate a relationship between the insulin signaling cascade and SIRT1. What remains to be seen is how NAD+ metabolism will affect these pathways. SIRT1 and other DNA stress response enzymes require NAD+ as a co-substrate. Manipulation of the synthesis of NAD+ through the NAD+ salvage enzymes PNC1 and NPT1 increases the life span of yeast in a SIRT1 dependent manner (Anderson et al., 2003; Gallo et al., 2004). It is possible that CR also may alter NAD+ synthesis enzymes.
Evidence is growing that dietary habits may play a role in determining the risk for brain diseases. Alterations in glucose metabolism due to insulin resistance and hyperinsulinemia are observed in patients with affective disorders, depression, and AD (Rasgon and Jarvik, 2004). It has been proposed that the failure to properly utilize glucose in the brain may cause neuronal injury leading to neurodegeneration. There is growing concern within developed countries regarding the dangers associated with overeating and its link to disorders such as diabetes (Mattson, 2003). A high caloric intake also is correlated with an increased occurrence of Alzheimer's disease (Gustafson et al., 2003). Of interest is the protection that CR affords in models of these disorders (Prolla and Mattson, 2001). Animal models demonstrate that CR protects against neuronal damage in the hippocampus triggered by the toxin kainic acid, and that CR leads to a reduction in age-related neuronal loss in Alzheimer's disease. Further, animals maintained on CR are resistant to oxidative damage caused by neurotoxicants such MPTP. Thus, understanding the molecular mechanisms of CR may lead to new targets for therapeutics development for neurodegenerative diseases. Pharmaceuticals that mimic the positive effects of CR may also confer its neuroprotective benefits. In fact, one agonist of SIRT1, resveratrol , reduces BAX-mediated apoptosis in cell culture (Cohen et al., 2004).
This rapidly growing body of data suggests that aging, at least at the cellular level, will be defined as the alteration and cross talk of different signaling pathways. CR not only increases life span, but also provides neuroprotection in a number of neurodegenerative models including Alzheimer's and Parkinson's disease.
However, is CR a viable lifestyle? Only those people with steely determination and self-control will adhere to such a strict dietary regime. Although CR offers protection during the mid-life of model organisms, it remains to be seen if humans will benefit. Also, for those patients diagnosed with a neurodegenerative disorders, will simply changing your diet be sufficient to soften the course of the disease? Unraveling how CR alters cellular physiology will enable the identification of novel cytoprotective targets. This knowledge might lead to the discovery of new classes of therapeutics. When targeted to the afflicted region of disease, these compounds may promote neuronal survival. Although these compounds will not prevent the final curtain call, they may rewrite the last act of life with increased intelligence and wit, and ultimately adulation by the cognescenti.
Let's discuss these questions during the discussion:
1. On willing mind but weaker flesh: Most people can't adhere to caloric restriction. Are there other ways of activating the pathway short of living on the edge of starvation?
2. For those considering trying CR: what are the side effects? How about fertility?
3. Could one design a diet that turns on CR signaling but leaves one feeling reasonably sated?
4. Short of CR, are there other dietary means to reduce insulin signaling?
5. What deleterious effects could activating these pathways have? They are, after all, a stress response of sorts.
6. What are the next steps scientifically to flesh out this hypothesis?
7. It's early days, but what might be some of the therapeutic strategies to exploit CR signaling? Gene therapy? Small molecule inhibitors? Against which targets?
8. Have AD mouse models been put on CR? Does it improve behavioral phenotype? Synaptic damage? Would this even be a good experiment?
9. NAD is a common and widely studied molecule. Are there NAD-based therapeutic approaches from other diseases that could be adapted to AD?
10. Does CR prolong an animals lifespan mainly by acting as a neuroprotectant? If so, could a neuroprotective drug mimic the effect of CR?
11. What new classes of therapeutics do we need? E.g., the biotech company Renovis has an oxygen radical scavenger that is in a phase 3 trial against stroke. Could such a drug, in the final outcome, have a similar effect as the signaling pathways triggered by CR?
Anderson RM, Bitterman KJ, Wood JG, Medvedik O, Sinclair DA (2003) Nicotinamide and PNC1 govern lifespan extension by calorie restriction in Saccharomyces cerevisiae. Nature 423:181-185. Abstract
Bluher M, Kahn BB, Kahn CR (2003) Extended longevity in mice lacking the insulin receptor in adipose tissue. Science 299:572-574. Abstract
Brunet A, Sweeney LB, Sturgill JF, Chua KF, Greer PL, Lin Y, Tran H, Ross SE, Mostoslavsky R, Cohen HY, Hu LS, Cheng HL, Jedrychowski MP, Gygi SP, Sinclair DA, Alt FW, Greenberg ME (2004) Stress-dependent regulation of FOXO transcription factors by the SIRT1 deacetylase. Science 303:2011-2015. Abstract
Clancy DJ, Gems D, Harshman LG, Oldham S, Stocker H, Hafen E, Leevers SJ, Partridge L (2001) Extension of life-span by loss of CHICO, a Drosophila insulin receptor substrate protein. Science 292:104-106. Abstract
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Gallo CM, Smith DL, Jr., Smith JS (2004) Nicotinamide clearance by Pnc1 directly regulates Sir2-mediated silencing and longevity. Mol Cell Biol 24:1301-1312. Abstract
Giannakou ME, Goss M, Junger MA, Hafen E, Leevers SJ, Partridge L (2004) Long-lived Drosophila with Overexpressed dFOXO in Adult Fat Body. Science. 2004 Jun 10 [Epub ahead of print] Abstract
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