The FASEB Journal • Silver Anniversary Review

The biology of aging: 1985–2010and beyond

George M. Martin1

Departments of Pathology and Genome Sciences, University of Washington,Seattle, Washington, USA; and the Molecular Biology Institute, University ofCalifornia, Los Angeles, California, USA

ABSTRACT In this contribution to the series of re-flective essays celebrating the 25th anniversary of TheFASEB Journal, our task is to assess the growth ofresearch on the biology of aging during this period andto suggest where we might be heading during the next25 yr. A review of the literature suggests a healthyacceleration of progress during the past decade, per-haps largely due to progress on the genetics of longev-ity of model organisms. Progress on the genetics ofhealth span in these model organisms has lagged,however. Research on the genetic basis of the remark-able interspecific variations in life span has only re-cently begun to be seriously addressed. The spectacularadvances in genomics should greatly accelerate prog-ress. Research on environmental effects on life spanand health span needs to be accelerated. Stochasticvariations in gene expression in aging have only re-cently been addressed. These can lead to randomdepartures from homeostasis during aging.—Martin,G. M. The biology of aging: 1985–2010 and beyond.FASEB J. 25, 3756–3762 (2011). www.fasebj.org

Key Words: dietary restriction � evolutionary biology � epige-netic drift � geriatric pathology � health span � life span

Given current demographic trends and the associatedhealth care costs of our aging societies (1), it is hard toimagine a more compelling area of biomedical researchthan basic research on the biology and pathobiology ofaging. Intrinsic biological aging is the major risk factor forvirtually all of the major diseases of the developed societ-ies—including Alzheimer’s disease, Parkinson’s disease,Lewy body dementia, frontotempral dementias, strokes,peripheral neuropathies, age-related macular degener-ation, ocular cataracts, presbycusis, type 2 diabetesmellitus, osteoporosis, osteoarthritis, sarcopenia, allforms of arteriosclerosis, benign prostatic hyperplasia,and most types of cancer. As we celebrate the 25thanniversary of The FASEB Journal, we should assess whatthose years have taught us about the what, why, andhow of biological aging—especially the how question,as one requires detailed information on mechanisms inorder to invent rational interventions. We should alsomake some educated guesses as to where to put ourmoney to best use over the next 25 yr. These areformidable tasks, especially given the recent funding

crisis at the National Institute on Aging (2). Because ofspace constraints, one cannot possibly provide a com-prehensive analysis. I therefore apologize in advance tomy many colleagues whose important research I couldnot review in these pages.

A SAMPLING OF THE LITERATURE, 1985–2010

Figure 1 summarizes the results of an August 16, 2011search of PubMed for the major topic medical subjectheadings (MeSH) for the term “aging” and 5 majorgeneralist, high-impact journals (Cell, FASEB Journal,Nature, Proceedings of the National Academy of Sciences ofthe USA, and Science) for the years 1985–2010. Thiswas a very small sample indeed of all references onthis subject to all journals indexed in PubMed duringthat period. The trends over time proved to beinformative, however, as they are consistent with myprospective hypothesis—namely, that our field ofresearch did not significantly attract the attention ofthese leading generalist journals until recent years.What happened to get their attention? One impor-tant development was a surge of research on thegenetic modulation of longevity in tractable modelorganisms (Caenorhabditis elegans, Drosophila melano-gaster, Mus musculus domesticus), fueled, in part, by thevision of the late Joshua Lederberg and the generos-ity of Mr. Larry Ellison, who organized the EllisonMedical Foundation some 15 yr ago to fund basicresearch in aging, much of it with a genetic theme(http://www.ellisonfoundation.org/index.jsp).

Figure 1 also includes what might be considered as acontrol: a comparable PubMed search using the MajorTopic MeSH subject of “congenital abnormalities.” Astatistical comparison of the slopes of the two curvessuggests that our field of biological aging has beendeveloping more rapidly.

1 Correspondence: Rm. K-543 Health Sciences Bldg., Uni-versity of Washington, 1959 NE Pacific St., Seattle, WA98195-7470, USA. E-mail: gmmartin@uw.edu

doi: 10.1096/fj.11-1102.ufm

3756 0892-6638/11/0025-3756 © FASEB

THE DISCOVERY OF THE FIRST PUBLICMODULATION OF LONGEVITY

In the early 1980s, Tom Johnson set out to disproveconclusions made by his colleague Michael Klass. TheKlass experiments suggested that a single gene muta-tion could extend the life span of an organism (in thiscase, C. elegans). To his surprise, Johnson and hisundergraduate student (D. B. Freidman) managed tomap the first “longevity gene,” age-1 (3). The Ruvkunlaboratory cloned this gene in 1996; it coded a phos-phatidylinositol-3-OH kinase (4). The Kenyon labora-tory had previously cloned the receptor locus for thispathway (5). The laboratories of Cynthia Kenyon, GaryRuvkun, Pam Larson, Adam Antebi, Tom Johnson andothers elaborated on these gene actions, most notablyregarding a downstream transcription factor, daf16.

Remarkably, long-lived mutants in homologous signal-ing pathways (IGF-1/insulin signaling) were found inD. melanogaster (6, 7) and in dwarf mice (reviewed inref. 8). More recently, variants in this pathway havebeen associated with unusual longevity in Homo sapiens.A particularly compelling observation has been theassociation with variants at a locus (FOXO3a) that codesfor a transcription factor comparable to daf16, as therehave been several confirmations (reviewed in ref. 9).Thus was established the first biochemical geneticpathway capable of modulating the life spans of diverseorganisms.

REPLICATIVE SENESCENCE AND THEBIOLOGY OF TELEMORES: AGING RESEARCHWINS ITS FIRST NOBEL PRIZE!

The pioneering research of Leonard Hayflick and PaulMoorhead on the limited replicative life spans of hu-man somatic cells (10) was revitalized by the demon-stration, by Woody Wright, Jerry Shay, and colleagues,that such cells could be essentially “immortalized” viatransfected telomerase (11). Interest in this area alsowas enhanced by Judy Campisi and colleagues, whodeveloped both theoretical and experimental lines ofevidence consistent with the view that the limited lifespan of human somatic cells had tumor suppressorfunctions, for which a price had to be paid, however,during aging. The accumulated postreplicative senes-cent cells were shown to secrete large amounts ofcytokines, mitogens, and enzymes capable of alteringextracellular matrix, thus, paradoxically, enhancing thedevelopment of neoplasia (reviewed in ref. 12). Therehas been increasing evidence that senescent cells accu-mulate in vivo, including assays of tissues in agingnonhuman primates (13).

Elizabeth Blackburn, Carol Greider, and Jack Szos-tak, the 2009 Nobel laureates, were quite aware of theimplications of their research on telomerase for thebiology of aging (14). Carol Greiger, in particular, hadbeen quite active in this field. This is perhaps also areason for the upturn in interest in the field that manyof us now refer to as biogerontology.

STEM CELLS AND REGENERATIVE MEDICINE:ON “WAKING UP” OUR AGINGENDOGENOUS STEM CELLS ANDREPROGRAMMING OUR TERMINALLYDIFFERENTIATED CELLS

Regenerative medicine and stem cell centers have beensprouting around the country like weeds, in part be-cause of the hope that progress in this area willexpedite clinical translations, certainly to include geri-atric patients with diverse degenerative disorders. Aparticularly promising line of research emerged fromthe laboratory of Tom Rando at Stanford University(Stanford, CA, USA), an institution that has taken a

Figure 1. Factor increase (normalized relative to 1985) ofPubMed publications with MeSH search word “aging” ineither selected high-impact journals (Science, Nature, Cell,Proceedings of the National Academy of Sciences of the USA, andFASEB Journal; solid red line) or in all PubMed journals(dashed red line). For comparison, the factor increase inPubMed publications (relative to 1985) is shown for theMeSH search words “congenital abnormalities” (CA) either inselected high-impact journals (above; solid black line) or inall PubMed journals (black dotted line). The rate of increasefor aging publications in the selected journals is greater thanthat for aging publications among all PubMed journals by ahighly significant factor of 3.3 (P�1E-10). The rate of in-crease of aging publications among the selected journals issignificantly different from that for CA publications in thesejournals (P�2E-10). After 1997, the rate of increase of CApublications in these selected journals is not significantlydifferent from 0 (P�0.27). The rate of increase in citationsfor “aging” among all PubMed journals is not significantlydifferent compared to the rate of increase of citations for CAin all PubMed journals (P�0.07). Methods: comparison ofrates of change in publication rates over time was donewith simple linear regression and a linear spline with onechange point at 1996. The lowess smoother was used toproduce smooth functions using the R statistical package(http://www.r-project.org).

3757BIOLOGY OF AGING

major leadership role in this field. The Rando teamused the old technique of parabiosis to demonstratethat factors derived from the circulating blood of ayoung mouse to its older partner could substantiallyameliorate or bypass a deficient response to injury ofskeletal muscle, a response that is mediated by satellitecells, the adult stem cells of skeletal muscle (15). Thereis, therefore, the potential for the development ofsmall molecular weight compounds that, when in-jected, would be capable of waking up particularfamilies of endogenous stem cells. Equally exciting ismore recent work from the laboratory of Helen Blau,also of Stanford University. These investigators haveshown that siRNA knockdowns of just two loci (Rband an alternative reading frame of Ink4a) wassufficient to induce the dedifferentiation of postrep-licative skeletal muscle myocytes to form mitoticallyactive myoblasts capable of once again making multi-nucleated skeletal muscle (16).

While the potential clinical applications to regener-ative medicine are enormous, of equal importance isthe potential to unravel the pathogenesis of a widerange of genetically modulated degenerative geriatricdisorders using induced pleuripotent stem cells fromindividual donors. In the short 4 yr since the Yamanakalaboratory opened up this field of research (17), therehave been numerous methodological enhancements, arecent example being a very exciting innovation fromthe laboratory of Derrick Rossi (Children’s Hospital,Boston, MA, USA) involving the use of engineeredRNA (18).

GOOD NEWS FROM RED WINE AND FROMEASTER ISLAND

Our red wine story begins with experiments with bud-ding yeast, starting with research in the MassachusettsInstitute of Technology (Cambridge, MA, USA) labora-tory of Leonard Guarente (19–22). Sirtuins are a familyof NAD�-dependent protein deacetylases. Their impor-tance in the modulation of longevity was establishedwith deletion and overexpression experiments in yeastand in C. elegans (23). Resveratrol (3,5,4�-trihydroxystil-bene), a compound present in red wine, initially con-sidered to activate a sirtuin, was shown to substantiallyameliorate the fatty livers and life spans of obese,diabetic mice (24). It did not, however, enhance thelongevity of mice in an exceptionally well designedstudy carried out by three independent laboratoriesusing genetically heterogeneous mice and two differentdoses of resveratrol (25). It remains to be seen, how-ever, whether more potent related compounds will beshown to enhance the life spans and health spans ofmammals. We shall also require more information onthe mechanisms of action of such compounds, asrecent work has failed to confirm that they activateSIRT1, a putative substrate (26).

An even more exciting line of research had its origins inEaster Island (Rapa Nui), where a species of Streptomycin

in a soil sample led to the discovery of a macrolide,rapamycin, a drug commonly used as an immunosuppres-sant in connection with kidney transplantations. In 2006,rapamycin was shown to enhance the replicative life spanof yeast cells via inhibition of the target of rapamycin(TOR) signal transduction pathway (27). In 2009, thegroup of three laboratories mentioned above for theirinvestigation of resveratrol (and for a confirmatory studyusing rapamycin) initiated a study of the effects of rapa-mycin on the life span of mice. By the time this groupcould invent a suitable vehicle for oral administration ofthe agent, their mouse cohorts were already 20 mo old,equivalent to �60 yr old humans. Fortunately, they pro-ceeded to administer the agent at those late ages anddiscovered a significant increase in maximum life span(28). Neoplasms, particularly lymphomas, are the majorcauses of death, even in these 4-way-cross mice. Given theevidence that rapamycin has antiprolferative properties(e.g., (29), it is conceivable that the life span extension wascaused by a slowing of the rate of growth of neoplasms inthese aging mice.

DIETARY RESTRICTION AND ENHANCEDLONGEVITY: ON THE POSSIBLE ROLE OFSNIFFING ONE’S FOOD

Caloric restriction has been shown to enhance the lifespans of an astonishing range of animal species, fromyeast to mammals (30). Since dietary restriction ofmethionine has also been shown to enhance life span(31), it is prudent to refer to this phenomenon asdietary restriction rather than caloric restriction. Likevirtually everything else in life, however, one has toconsider the relationship of the response to the dose ofthe intervention. This has been well established in thecase of fruit flies (32). The standard paradigm ofdietary restriction for laboratory mice, however, has formany years been to decrease the amount of foodprovided to the experimental group to 40% of thateaten by control animals fed ad libitum. Only recentlyhas a systematic study been initiated to determine howvarious murine genotypes would fare under such aregime. To the surprise of many investigators, a remark-able range of variable responses, from decreased toincreased life spans, was observed among a series ofrecombinant inbred strains (33). Those genotypes withenhanced life spans were better at retaining adiposetissue (34). It will be of interest to discover whichspecific types of fat depots were involved. Some evolu-tionary biologists have argued that dietary restriction,apart from avoiding obesity, is unlikely to be an effica-cious modality of life-span extension in human beings(35). While a complete life table of the results of dietaryrestriction in a primate species is not yet available, themost recent publication on this subject, involving Rhe-sus monkeys, does support both enhanced life spansand health spans when these animals are restricted by30% (36). (The animals could not tolerate a 40%restriction of food.) While the published photograph of

3758 Vol. 25 November 2011 MARTINThe FASEB Journal � www.fasebj.org

a lean, restricted animal did suggest a more youthfulphenotype, the animal looked a lot meaner than thewell-fed control.

Remarkably, research with D. melanogaster and C.elegans points to a major role for the perception of food,via olfactory clues, in the regulation of the response todietary restriction (37–41). Our friend, the mTORpathway, is among the biochemical genetic downstreamsensors involved in these complex responses (40).Activation of a FOXO-daf16 transcription factor alsoappears to be involved (42).

THE EVOLUTIONARY BIOLOGICAL THEORYOF AGING: STILL THE BEST EXPLANATIONFOR WHY WE AGE

There is widespread agreement among my colleaguesthat aging occurs because of the decline in the force ofnatural selection after the peak of reproduction. Excel-lent expositions of this idea can be found in books byMichael R. Rose (43) and Steven N. Austad (44). Twodominant mechanisms are widely supported. The first,attributed to J. B. S. Haldane and Peter Medawar, hasbeen referred to as mutation accumulation. It shouldnot be confused with somatic mutation, as it refers toinherited or constitutional mutations that do not reachsome phenotypic level of expression until middle ageor beyond, when the force of natural selection hasbecome greatly attenuated. There is a very large poten-tial catalog of such mutant alleles, some highly pene-trant and others somewhat “leaky,” providing one gen-eral explanation for why clinicians see such strikingvariations for patterns of aging among genetically het-erogeneous species such as Homo sapiens. The secondgeneral mechanism has been referred to by the lateGeorge C. Williams as antagonistic pleiotropy. Likemost selected alleles, these are gene variants that en-hance reproductive fitness early in the life course.Some such alleles, however, exhibit deleterious effectslate in the life span. An oft-cited example, as notedabove, would be classes of gene action responsible forreplicative senescence. This is thought to provide pro-tection against cancer. Paradoxically, however, accumu-lations of replicatively senescent cells later in the lifecourse are thought to enhance carcinogenesis via thesecretion of mitogens, proinflammatory cytokines andsubstances that alter the structure of stromal tissues[the senescent associated secretory phenotype (SASP);ref. 12]. The evolutionary biological theory of why weage also provides clues to how we age; for a brief review,see ref. 45. A detailed analysis of the numerous theorieson how we age is beyond the scope of this essay, but inthe next section, we shall briefly address one of themost venerable of these theories: the free radical theoryof aging and its associated focus on mitochondrialstructure and function.

HOW THE MITOCHONDRIAL FREE RADICALTHEORY OF AGING HAS FARED OVER THEYEARS

When our ancient eukaryotic ancestors decided to takein bacterial boarders, it seemed like a pretty good deal,as they were good at stoking the furnaces. Alas, they didnot foresee the possibility that these creatures, whichbecame our mitochondria, might also create somehousehold problems. While the electron transport en-zymatic machinery contributed by both the mitochon-drial and nuclear genomes does an excellent job withthe quadrivalent reduction of oxygen, there is a smalldegree of univalent reduction, leading to reactive oxy-gen species, the most dangerous member of which isthe hydroxyl radical, created by the Fenton reaction. In1956, Denham Harman suggested that such “free rad-icals” could be responsible for biological aging (46).Since then, numerous papers have provided evidenceboth for and against this theory. To make a very longstory very short, the reader is directed to four recentindependent critical reviews that seriously question thevalidity of this theory (47–50). Despite the best effortsof evolution to optimize the stability of mitochondrialproteins (51), mitochondrial abnormalities nonethe-less appear to be playing important roles in a variety ofneurodegenerative disorders of aging (52). The loss ofprotein homeostasis in general (proteostasis) appearsto be a key feature of many neurodegenerative diseasesof late life (53).

PROGRESS ON HUMAN PROGEROIDSYNDROMES

The author’s interest in the biology of aging began witha study of the Werner syndrome, an autosomal recessivedisorder resulting in the prototypic human segmentalprogeroid syndrome (54). After many years of workcollecting pedigrees from around the world, the genewas shown to code for a member of the RecQ family ofhelicases (55). The helicase and exonuuclease func-tions of the WRN protein participate in many aspects ofDNA metabolism, including maintenance of telomerestructure and homology-dependent recombination(56). The gene for the Hutchinson-Gilford progeriasyndrome (an autosomal dominant mutation) wascloned in the laboratory of Francis Collins in 2003 (57).Virtually all cases are caused by a point mutation inthe C-terminal domain of the LMNA gene that activatesa cryptic splice donor site, resulting in an isoform witha 50-aa deletion known as progerin. Progerin retains atransient post-translational modification (farnesyla-tion), as it lacks a recognition site for the cleavage eventthat removes this modification. As a result, progerinmislocalizes within the nucleus, resulting in nucleardistortions and alterations in gene expression (56). Ofthe many phenotypic features of the Hutchinson-Gil-ford progeria syndrome, atherosclerosis is by far themost clinically significant. Reports that progerin accu-

3759BIOLOGY OF AGING

mulates in human tissues during normative aging (re-viewed in ref. 58) are therefore of great potentialimportance. Also of great interest is a recent reportpointing to a collaboration of progerin and telomeredysfunction in the genesis of the replicative senescenceof normal diploid human fibroblasts (58).

LOOKING AHEAD

The striking success of research on the genetic modu-lation of life spans in model organisms has not yet beenmatched by investigations of the maintenance of struc-ture and function in various organs (i.e., health span).The laboratory of Monica Driscoll has pioneered suchstudies by demonstrating marked alterations in theskeletal muscles of C. elegans as they age (59), but muchmore work is needed in worms, flies, and even in mice,where such studies are more advanced. We also requiremuch more sensitive and specific assays for a range ofhuman physiological functions during aging; it is now-adays hard to find professional physiologists with suchinterests (60).

There is considerable enthusiasm among my col-leagues for the potential power of comparative geron-tology to elucidate mechanisms that have evolved toslow intrinsic rates of biological aging in exceptionallylong lived species. For example, we now have the fullgenome sequences of our nearest relative, the chim-panzee, to compare with that of our own sequence. Arethere secrets to be revealed from these genomes as towhy we live about twice as long as chimps? Remarkablyprescient research by Mary Claire King and the lateAllan Wilson suggested, some 36 yr ago, that pheno-typic differences between these two species are likelydetermined more by variations in the regulation ofgene expression than by variations in the structure ofindividual proteins, which exhibited very small varia-tions among these two species for the subset examined(61). The field of neurobiology has been the first tobenefit from this line of research (62). Biogerontolo-gists, however, are currently focusing on the newlyavailable genome sequence of the naked mole rat. Thisrodent is about the same size as laboratory mouse, butlives �10 times as long (63). A large number ofnecropsies on old naked mole rats have been per-formed by Rochelle Buffenstein (University of TexasHealth Science Center, San Antonio, TX, USA) and hercolleagues, but not a single neoplasm has been ob-served. This is in stark contrast to the laboratory mouse,most of which die of cancers (predominately lympho-mas). We look forward to the results of genomic studiesusing appropriate phylogenetic comparisons.

In comparison with research on mutagens, carcino-gens, and teratogens, there is very little research on“gerontogens”—environmental agents with the poten-tial to accelerate the ages of onset or rates of develop-ment of aging and associated components of the senes-cent phenotype (64). A cogent example is cigarettesmoking, which influences a large number of aging

phenotypes (65). Exposure to gerontogens might beparticularly significant during prenatal and neonataldevelopment.

Except for studies of somatic mutation (the fre-quency of which rises substantially during aging; ref.66), investigations of the role of stochastic events in thedetermination of health span and life span have beenneglected. This seems strange, given the fact thatgenerations of researchers have been very much awarethat, despite every effort to control the genotype andthe environment of their experimental organisms, ev-ery life-span experiment reveals marked differences inthe life spans of individual organisms. The best avail-able model for such studies is C. elegans. As a hermaph-rodite, every diploid locus is driven to homozygosity. Itcan also be grown in axenic medium with excellentcontrol of environmental parameters. Yet, even inlong-lived mutants, there are individuals with shorterlife spans than those of wild type parentals (67). Strongevidence that these variations are stochastic comesfrom experiments from the laboratory of Thomas John-son (68).

Epigenetic drifts in gene expression have beenshown to be among the reasons identical human twinsdiffer as they age (69). Increased variegations of geneexpression with age among individual cardiomyocyteshave also been established in the aging mouse heart(70). Are such epigenetic variegations mainly due toenvironmental influences? Do they results from ther-modynamic noise? Are the degrees of variegation un-der genetic control, and, if so, are the rates of variega-tion and drift related to different degrees of biologicalaging (71)? If the latter explanation is correct, a deeperknowledge of its mechanisms might lead to interven-tions that could moderate the degree of drift and,perhaps, associated alterations in physiological homeo-stasis. Such departures from homeostasis are at theheart of what we define as biological aging.

The author thanks Mr. Brian Park for PubMed searchesand Dr. Mary Emond (Department of Biostatistics, Universityof Washington, Seattle, WA, USA) for statistical analysis of thedata of Fig. 1.

REFERENCES

1. Olshansky, S. J., Goldman, D. P., Zheng, Y., and Rowe, J. W.(2009) Aging in America in the twenty-first century: demo-graphic forecasts from the MacArthur Foundation ResearchNetwork on an Aging Society. Milbank Q. 87, 842–862

2. Wadman, M. (2010) Funding crisis hits US ageing research.Nature 468, 148

3. Friedman, D. B., and Johnson, T. E. (1988) A mutation in theage-1 gene in Caenorhabditis elegans lengthens life and reduceshermaphrodite fertility. Genetics 118, 75–86

4. Morris, J. Z., Tissenbaum, H. A., and Ruvkun, G. (1996) Aphosphatidylinositol-3-OH kinase family member regulatinglongevity and diapause in Caenorhabditis elegans. Nature 382,536–539

5. Kenyon, C., Chang, J., Gensch, E., Rudner, A., and Tabtiang, R.(1993) A C-elegans mutant that lives twice as long as wild-type.Nature 366, 461–464

3760 Vol. 25 November 2011 MARTINThe FASEB Journal � www.fasebj.org

6. Clancy, D. J., Gems, D., Harshman, L. G., Oldham, S., Stocker,H., Hafen, E., Leevers, S. J., and Partridge, L. (2001) Extensionof life-span by loss of CHICO, a Drosophila insulin receptorsubstrate protein. Science 292, 104–106

7. Tatar, M., Kopelman, A., Epstein, D., Tu, M. P., Yin, C. M., andGarofalo, R. S. (2001) A mutant Drosophila insulin receptorhomolog that extends life-span and impairs neuroendocrinefunction. Science 292, 107–110

8. Bartke, A., and Brown-Borg, H. (2004) Life extension in thedwarf mouse. Curr. Top. Dev. Biol. 63, 189–225

9. Ziv, E., and Hu, D. (2011) Genetic variation in insulin/IGF-1signaling pathways and longevity. Ageing Res. Rev. 10, 201–204

10. Hayflick, L., and Moorhead, P. S. (1961) The serial cultivationof human diploid cell strains. Exp. Cell Res. 25, 585–621

11. Bodnar, A. G., Ouellette, M., Frolkis, M., Holt, S. E., Chiu, C. P.,Morin, G. B., Harley, C. B., Shay, J. W., Lichtsteiner, S., andWright, W. E. (1998) Extension of life-span by introduction oftelomerase into normal human cells. Science 279, 349–352

12. Coppe, J. P., Desprez, P. Y., Krtolica, A., and Campisi, J. (2010)The senescence-associated secretory phenotype: the dark side oftumor suppression. Annu. Rev. Pathol. 5, 99–118

13. Herbig, U., Ferreira, M., Condel, L., Carey, D., and Sedivy, J. M.(2006) Cellular senescence in aging primates. Science 311, 1257

14. Blackburn, E. H., Greider, C. W., and Szostak, J. W. (2006)Telomeres and telomerase: the path from maize, Tetrahy-mena and yeast to human cancer and aging. Nat. Med. 12,1133–1138

15. Conboy, I. M., Conboy, M. J., Wagers, A. J., Girma, E. R.,Weissman, I. L., and Rando, T. A. (2005) Rejuvenation of agedprogenitor cells by exposure to a young systemic environment.Nature 433, 760–764

16. Pajcini, K. V., Corbel, S. Y., Sage, J., Pomerantz, J. H., and Blau,H. M. (2010) Transient inactivation of Rb and ARF yieldsregenerative cells from postmitotic mammalian muscle. Cell StemCell 7, 198–213

17. Takahashi, K., and Yamanaka, S. (2006) Induction of pluripo-tent stem cells from mouse embryonic and adult fibroblastcultures by defined factors. Cell 126, 663–676

18. Warren, L., Manos, P. D., Ahfeldt, T., Loh, Y. H., Li, H., Lau, F.,Ebina, W., Mandal, P. K., Smith, Z. D., Meissner, A., Daley,G. Q., Brack, A. S., Collins, J. J., Cowan, C., Schlaeger, T. M., andRossi, D. J. (2010) Highly efficient reprogramming to pluripo-tency and directed differentiation of human cells with syntheticmodified mRNA. Cell Stem Cell 7, 618–630

19. Kennedy, B. K., Austriaco, N. R., Zhang, J. S., and Guarente, L.(1995) Mutation in the silencing gene Sir4 can delay aging inSaccharomyces-cerevisiae. Cell 80, 485–496

20. Kennedy, B. K., Gotta, M., Sinclair, D. A., Mills, K., McNabb,D. S., Murthy, M., Pak, S. M., Laroche, T., Gasser, S. M., andGuarente, L. (1997) Redistribution of silencing proteins fromtelomeres to the nucleolus is associated with extension of lifespan in S-cerevisiae. Cell 89, 381–391

21. Kaeberlein, M., McVey, M., and Guarente, L. (1999) TheSIR2/3/4 complex and SIR2 alone promote longevity in Saccha-romyces cerevisiae by two different mechanisms. Genes Dev. 13,2570–2580

22. Imai, S., Armstrong, C. M., Kaeberlein, M., and Guarente, L.(2000) Transcriptional silencing and longevity protein Sir2 is anNAD-dependent histone deacetylase. Nature 403, 795–800

23. Tissenbaum, H. A., and Guarente, L. (2001) Increased dosageof a sir-2 gene extends lifespan in Caenorhabditis elegans. Nature410, 227–230

24. Baur, J. A., Pearson, K. J., Price, N. L., Jamieson, H. A., Lerin, C.,Kalra, A., Prabhu, V. V., Allard, J. S., Lopez-Lluch, G., Lewis, K.,Pistell, P. J., Poosala, S., Becker, K. G., Boss, O., Gwinn, D.,Wang, M., Ramaswamy, S., Fishbein, K. W., Spencer, R. G.,Lakatta, E. G., Le Couteur, D., Shaw, R. J., Navas, P., Puigserver,P., Ingram, D. K., de Cabo, R., and Sinclair, D. A. (2006)Resveratrol improves health and survival of mice on a high-calorie diet. Nature 444, 337–342

25. Miller, R. A., Harrison, D. E., Astle, C. M., Baur, J. A., Boyd,A. R., de Cabo, R., Fernandez, E., Flurkey, K., Javors, M. A.,Nelson, J. F., Orihuela, C. J., Pletcher, S., Sharp, Z. D., Sinclair,D., Starnes, J. W., Wilkinson, J. E., Nadon, N. L., and Strong, R.(2011) Rapamycin, but not resveratrol or simvastatin, extendslife span of genetically heterogeneous mice. J. Gerontol. A Biol.Med. Sci. 66, 191–201

26. Pacholec, M., Bleasdale, J. E., Chrunyk, B., Cunningham, D.,Flynn, D., Garofalo, R. S., Griffith, D., Griffor, M., Loulakis, P.,Pabst, B., Qiu, X., Stockman, B., Thanabal, V., Varghese, A.,Ward, J., Withka, J., and Ahn, K. (2010) SRT1720, SRT2183,SRT1460, and resveratrol are not direct activators of SIRT1.J. Biol. Chem. 285, 8340–8351

27. Powers, R. W., 3rd, Kaeberlein, M., Caldwell, S. D., Kennedy,B. K., and Fields, S. (2006) Extension of chronological life spanin yeast by decreased TOR pathway signaling. Genes Dev. 20,174–184

28. Harrison, D. E., Strong, R., Sharp, Z. D., Nelson, J. F., Astle,C. M., Flurkey, K., Nadon, N. L., Wilkinson, J. E., Frenkel, K.,Carter, C. S., Pahor, M., Javors, M. A., Fernandez, E., and Miller,R. A. (2009) Rapamycin fed late in life extends lifespan ingenetically heterogeneous mice. Nature 460, 392–395

29. Rao, R. D., Buckner, J. C., and Sarkaria, J. N. (2004) Mammaliantarget of rapamycin (mTOR) inhibitors as anti-cancer agents.Curr. Cancer Drug. Targets. 4, 621–635

30. Everitt, A. V., Rattan, S. I. S., Le Couteur, D. G., and de Cabo, R.,eds (2010) Calorie restriction, aging and longevity, Springer, NewYork

31. Miller, R. A., Buehner, G., Chang, Y., Harper, J. M., Sigler, R.,and Smith-Wheelock, M. (2005) Methionine-deficient diet ex-tends mouse lifespan, slows immune and lens aging, altersglucose, T4, IGF-I and insulin levels, and increases hepatocyteMIF levels and stress resistance. Aging Cell 4, 119–125

32. Partridge, L., Piper, M. D., and Mair, W. (2005) Dietary restric-tion in Drosophila. Mech. Ageing Dev. 126, 938–950

33. Liao, C. Y., Rikke, B. A., Johnson, T. E., Diaz, V., and Nelson,J. F. (2010) Genetic variation in the murine lifespan response todietary restriction: from life extension to life shortening. AgingCell 9, 92–95

34. Nelson, J. F., Liao, C. Y., Rikke, B. A., Johnson, T. E., Gelfond,J. A. L., and Diaz, V. (2011) Fat maintenance is a predictor ofthe murine lifespan response to dietary restriction. Aging Cell 10,629–639

35. Le Bourg, E. (2010) Predicting whether dietary restrictionwould increase longevity in species not tested so far. Ageing Res.Rev. 9, 289–297

36. Colman, R. J., Anderson, R. M., Johnson, S. C., Kastman, E. K.,Kosmatka, K. J., Beasley, T. M., Allison, D. B., Cruzen, C.,Simmons, H. A., Kemnitz, J. W., and Weindruch, R. (2009)Caloric restriction delays disease onset and mortality in rhesusmonkeys. Science 325, 201–204

37. Apfeld, J., and Kenyon, C. (1999) Regulation of lifespan bysensory perception in Caenorhabditis elegans. Nature 402, 804–809

38. Bishop, N. A., and Guarente, L. (2007) Two neurons mediatediet-restriction-induced longevity in C. elegans. Nature 447, 545–549

39. Libert, S., Zwiener, J., Chu, X., Vanvoorhies, W., Roman, G., andPletcher, S. D. (2007) Regulation of Drosophila life span byolfaction and food-derived odors. Science 315, 1133–1137

40. Poon, P. C., Kuo, T. H., Linford, N. J., Roman, G., and Pletcher,S. D. (2010) Carbon dioxide sensing modulates lifespan andphysiology in Drosophila. PLoS Biol. 8, e1000356

41. Smith, E. D., Kaeberlein, T. L., Lydum, B. T., Sager, J., Welton,K. L., Kennedy, B. K., and Kaeberlein, M. (2008) Age- andcalorie-independent life span extension from dietary restrictionby bacterial deprivation in Caenorhabditis elegans. BMC. Dev. Biol.8, 49

42. Greer, E. L., Dowlatshahi, D., Banko, M. R., Villen, J., Hoang, K.,Blanchard, D., Gygi, S. P., and Brunet, A. (2007) An AMPK-FOXO pathway mediates longevity induced by a novel methodof dietary restriction in C. elegans. Curr. Biol. 17, 1646–1656

43. Rose, M. R. (1991) Evolutionary biology of aging, Oxford Univer-sity Press, New York

44. Austad, S. N. (1997) Why we age : what science is discovering aboutthe body’s journey through life, John Wiley & Sons, New York

45. Martin, G. M. (2007) Modalities of gene action predicted by theclassical evolutionary biological theory of aging. Ann. N. Y. Acad.Sci. 1100, 14–20

46. Harman, D. (1956) Aging: a theory based on free radical andradiation chemistry. J. Gerontol. 11, 298–300

47. Alexeyev, M. F. (2009) Is there more to aging than mitochon-drial DNA and reactive oxygen species? FEBS J. 276, 5768–5787

3761BIOLOGY OF AGING

48. Hekimi, S., and Lapointe, J. (2010) When a theory of aging agesbadly. Cell. Mol. Life Sci. 67, 1–8

49. Pérez, V. I., Bokov, A., Van Remmen, H., Mele, J., Ran, Q. T.,Ikeno, Y., and Richardson, A. (2009) Is the oxidative stresstheory of aging dead? Biochim. Biophys. Acta 1790, 1005–1014

50. Ristow, M., and Schmeisser, S. (2011) Extending life span byincreasing oxidative stress. Free Radic. Biol. Med. 51, 327–336

51. Kitazoe, Y., Kishino, H., Hasegawa, M., Matsui, A., Lane, N., andTanaka, M. (2011) Stability of mitochondrial membrane pro-teins in terrestrial vertebrates predicts aerobic capacity andlongevity. Genome Biol Evol

52. Przedborski, S., and Schon, E. A. (2011) Mitochondria: the next(neurode) generation. Neuron 70, 1033–1053

53. Douglas, P. M., and Dillin, A. (2010) Protein homeostasis andaging in neurodegeneration. J. Cell Biol. 190, 719–729

54. Epstein, C. J., Martin, G. M., Schultz, A. L., and Motulsky, A. G.(1966) Werner’s syndrome a review of its symptomatology,natural history, pathologic features, genetics and relationship tothe natural aging process. Medicine (Baltimore) 45, 177–221

55. Yu, C. E., Oshima, J., Fu, Y. H., Wijsman, E. M., Hisama, F.,Alisch, R., Matthews, S., Nakura, J., Miki, T., Ouais, S., Martin,G. M., Mulligan, J., and Schellenberg, G. D. (1996) Positionalcloning of the Werner’s syndrome gene. Science 272, 258–262

56. Kudlow, B. A., Kennedy, B. K., and Monnat, R. J., Jr. (2007) Wernerand Hutchinson-Gilford progeria syndromes: mechanistic basis ofhuman progeroid diseases. Nat. Rev. Mol. Cell. Biol. 8, 394–404

57. Eriksson, M., Brown, W. T., Gordon, L. B., Glynn, M. W., Singer,J., Scott, L., Erdos, M. R., Robbins, C. M., Moses, T. Y., Berglund,P., Dutra, A., Pak, E., Durkin, S., Csoka, A. B., Boehnke, M.,Glover, T. W., and Collins, F. S. (2003) Recurrent de novo pointmutations in lamin A cause Hutchinson-Gilford progeria syn-drome. Nature 423, 293–298

58. Cao, K., Blair, C. D., Faddah, D. A., Kieckhaefer, J. E., Olive, M., Erdos,M. R., Nabel, E. G., and Collins, F. S. (2011) Progerin and telomeredysfunction collaborate to trigger cellular senescence in normalhuman fibroblasts. J. Clin. Invest. 121, 2833–2844

59. Herndon, L. A., Schmeissner, P. J., Dudaronek, J. M., Brown,P. A., Listner, K. M., Sakano, Y., Paupard, M. C., Hall, D. H., andDriscoll, M. (2002) Stochastic and genetic factors influencetissue-specific decline in ageing C. elegans. Nature 419, 808–814

60. Martin, G. M. (2002) Help wanted: physiologists for research onaging. Sci. Aging Knowledge Environ. 2002, vp2

61. King, M. C., and Wilson, A. C. (1975) Evolution at 2 levels inhumans and chimpanzees. Science 188, 107–116

62. Haussler, D., Pollard, K. S., Salama, S. R., Lambert, N., Lambot,M. A., Coppens, S., Pedersen, J. S., Katzman, S., King, B.,Onodera, C., Siepel, A., Kern, A. D., Dehay, C., Igel, H., Ares,M., and Vanderhaeghen, P. (2006) An RNA gene expressedduring cortical development evolved rapidly in humans. Nature443, 167–172

63. Buffenstein, R., Edrey, Y. H., Hanes, M., Pinto, M., and Mele, J.(2011) Successful aging and sustained good health in the nakedmole rat: a long-lived mammalian model for biogerontology andbiomedical research. ILAR J. 52, 41–53

64. Martin, G. M. (1987) Interactions of aging and environmentalagents: the gerontological perspective. Prog. Clin. Biol. Res. 228,25–80

65. Bernhard, D., Moser, C., Backovic, A., and Wick, G. (2007)Cigarette smoke–an aging accelerator? Exp. Gerontol. 42, 160–165

66. Martin, G. M., Ogburn, C. E., Colgin, L. M., Gown, A. M.,Edland, S. D., and Monnat, R. J., Jr. (1996) Somatic mutationsare frequent and increase with age in human kidney epithelialcells. Hum. Mol. Genet. 5, 215–221

67. Kirkwood, T. B., Feder, M., Finch, C. E., Franceschi, C., Glober-son, A., Klingenberg, C. P., LaMarco, K., Omholt, S., andWestendorp, R. G. (2005) What accounts for the wide variationin life span of genetically identical organisms reared in aconstant environment? Mech. Ageing Dev. 126, 439–443

68. Rea, S. L., Wu, D., Cypser, J. R., Vaupel, J. W., and Johnson,T. E. (2005) A stress-sensitive reporter predicts longevity inisogenic populations of Caenorhabditis elegans. Nat. Genet. 37,894 – 898

69. Fraga, M. F., Ballestar, E., Paz, M. F., Ropero, S., Setien, F.,Ballestar, M. L., Heine-Suner, D., Cigudosa, J. C., Urioste, M.,Benitez, J., Boix-Chornet, M., Sanchez-Aguilera, A., Ling, C.,Carlsson, E., Poulsen, P., Vaag, A., Stephan, Z., Spector, T. D.,Wu, Y. Z., Plass, C., and Esteller, M. (2005) Epigenetic differ-ences arise during the lifetime of monozygotic twins. Proc. Natl.Acad. Sci. U. S. A. 102, 10604–10609

70. Bahar, R., Hartmann, C. H., Rodriguez, K. A., Denny, A. D.,Busuttil, R. A., Dolle, M. E., Calder, R. B., Chisholm, G. B.,Pollock, B. H., Klein, C. A., and Vijg, J. (2006) Increasedcell-to-cell variation in gene expression in ageing mouse heart.Nature 441, 1011–1014

71. Martin, G. M. (2009) Epigenetic gambling and epigenetic driftas an antagonistic pleiotropic mechanism of aging. Aging Cell 8,761–764

3762 Vol. 25 November 2011 MARTINThe FASEB Journal � www.fasebj.org