Revised Handout for Grand Rounds; UCLA School of Medicine; Moss Auditorium

August 22, 1996; 5:30-7:00 PM

Revised February 5, 1999

 THEORY(S) OF AGING --

The Fable of the Blind Men Touching the Elephant

 by

 L. Stephen Coles, M.D., Ph.D., Co-Founder

Los Angeles Gerontology Research Group

Marina del Rey, California

E-mail: scoles@grg.org

URL: http://www.grg.org

 I. INTRODUCTION

 This short paper provides five definitions of aging in order of increasing refinement, surveys 25 diverse theories that have been proposed to explain aging, suggests 12 observations against which an "integrated theory of aging" must be measured, proposes nine biomarkers by which to determine the human rate of physiological aging, discusses what has been said about the prospects for physical immortality, and finally concludes with a three-step plan for research in practical interventions in the aging process itself.

II. DEFINITIONS

 Before we discuss the various theories of aging, I believe it is important to make sure that we are all talking about the same thing. Therefore, I will begin with some definitions. Here are five definitions in order of increasing refinement:

 Definition of Aging #1 (An Apologist Definition): Aging is our inability to escape from the clutches of the Grim Reaper, who is always looking over our shoulders.

 Comment: This and many other similar aphorisms "beg the question." By "begging the question" we mean a paraphrasing of the term being defined, typically using evocative or poetic language, but which sheds no light on operational mechanisms (cause-and-effect). An enormous body of apologist literature informs us that death is inevitable (so are taxes), while deliberate tampering with the aging process is either futile or doomed to fail catastrophically. According to conventional mythology, arrogant men who seek to circumvent their destiny swiftly incur the wrath of the gods; legend has it that such a man typically meets a fate worse than death.

  Definition of Aging #2 (An Intuitive Definition): Aging is whatever makes us grow old and die, and can't be explained in some other obvious manner, like acute trauma.

 Comment: Termination by conspicuous injury (homicide, suicide, execution, fatal accident, acute illness, etc.) wouldn't count as aging, since these are obvious causes of death and can, in fact, be explained. This definition suffers from the same defect and criticism as Definition #1, since you still can't get your arms around the (epi)phenomenon in question.

 Definition of Aging #3 (Ancient Greek): Aging is a disease that results from an imbalance in the "four humours" [sic] (blood, phlegm, yellow bile, and black bile).

 Comment: This definition is much more acceptable and retains the advantage that it suggests interventions through attempts to re-equilibrate our out-of-balance humors. For example, one might try blood-sucking leeches if the medical diagnosis were "excessive sanguinity." Thus the problem has now been reduced one of safety and effectiveness (but unfortunately with not very much of either). Curiously, getting rid of excess iron in the blood can be done most efficiently by bleeding the patient, so the basic idea may be correct. Menstruating women have already figured out how to do that quite well. Men, however, need help, and leaches are not adequate, since they don't really take enough. A one-pint blood donation once or twice a year is what is recommended, but more about that later.

Definition of Aging #4 (Wear-and-Tear [Claude Bernard/James Fries]): Aging is any natural biological process in sexually-reproducing, eukaryotic species following reproductive failure (menopause/andropause) that causes progressive generalized impairment of function, resulting in a measurable loss of adaptive (homeostatic) response to environmental stress and a greater risk of chronic disease, invariably and systematically leading to an increased probability of death.

Comment: A sexually-reproducing species is characterized by distinctive sets of germ line and somatic cells by means of their haploid or diploid sets of chromosomes, respectively. For humans, chronic diseases are the names that often get written on death certificates (myocardial infarction, liver cancer, Alzheimers Disease, etc.). Furthermore, this definition has the advantage that the loss of adaptive response can be measured quantitatively in the form of specific challenge-tests and their related biomarkers (e.g., flexibility of collagen in the tail of mice, forced expiratory volume in one second, greyness of the hair, elasticity of the skin, osteoporosis of the bones, etc.). This in turn gives rise to the distinction between physiological age and chronological age.

Coles's Theorem: "Human Immortality is Possible"

Proof: It is sufficient to show that a perpetual, multicellular, sexually-reproducing organism is not a violation of the laws of physics (chemistry and molecular biology). This is not to say that immortal humans must be capable of postponening death indefinitely; they may always be "hit by a truck" or devoured by a predator in the jungle, depending on where they are. On the other hand, immortal humans do not exist in nature, since, as Prof. Michael Rose of the University of California at Irvine has pointed out, mature organisms no longer matter to evolution nor to the forces of natural selection, the driving force of all vital life. Prof. Rose has demonstrated that, for drosophila, one can at least double the maximum species life span from 40 to 80 days simply by artificially manipulating selectional forces in the laboratory (systematically abandoning eggs from young mothers and preserving eggs from old mothers over a few dozen generations).

The effects of aging can be distinguished for at least seven different levels of increasing spacial resolution; they usually begin silently at the lowest levels while they propagate relentlessly upward until they achieve consciousness with specific symptomatology and finally result in catastrophic, irreversible consequences:



LEVEL AGING CHARACTERIZED BY METERS EXPONENT
1. Organismal subject unconsciousness, unresponsive to sharp pain, "cannot fog a mirror," 1 0
2. Systemic immune incompetence leads to opportunistic infection, no EEG = "brain death" 0.1 (decimeter) -1
3. Organal liver failure, heart no longer capable of contraction 0.01 (centimeter) -2
4. Tissue necrosis plus fibrosis leading to an obliteration of normal histological architecture, loss of contractility 0.001 (millimeter) -3
5. Cellular apoptosis [programmed cell death] vs. necrosis [secondary to the accumulation of passive pigments, like lipofuscin, or viral particles] 0.000001 (micron) -6
6. Organellular mitochondrial decompensation 0.00000001 -8
7. Molecular oxidative damage of bi-layer lipid membranes, protein cross-linkage, mutation of DNA secondary to cosmic radiation 0.000000001 (nanometer) -9

But not lower down than those resolutions shown in the table above, as, for example,

8. Atomic Although this is a simplification, for all practical purposes, biologically-interesting atoms [C, H, O, N, Ca, K, Na, etc.] are essentially immortal; there half-lives are measured in millions of years (Angstrom) -10
9. Electronic electrons, positrons -12
10. Quarkian pions, muons, or anything lower down in the "bestiary" of particles elaborated by our nuclear-physicist colleagues -15 (?)

Thus, the spectrum of relevant levels only ranges over 9 logs ("log 10"), from a resolution of meters down to nanometers. Frequently, professional practitioners of the subject matter at each of the nine levels don't even bother to speak to one another, due to the absence of a common vocabulary. For example, sailors measure bottom depth in fathoms, while jockeys measure race-track distances in furlongs. Yet their units frequently span the same metric dimensions. The British measure half-months in fortnights, while Americans speak of a couple of weeks. Similarly, the biologists who work with atomic-force microscopes don't often speak to the surgical pathologists who spend most of their day looking at "cases" under the light microscope. Even within Departments of Pathology gross pathologists (who do necropsies for a living) avoid speaking to surgical pathologists, since their tools and languages are so different.

The best metaphor to complete the proof of this theorem is that of the spacially nested Russian (Chinese) Dolls. The point is that the number of dolls is finite. After the seventh doll, there are no smaller ones hidden inside. There is not an infinite regression.

In summary, if one manipulate the immortal bricks of a house quickly enough on a regular basis, in a process of continual repair, then the house has effectively become immortal. This is not a constructive proof, but an existence proof. A nanoconstructor that could assemble fast enough would be needed to rebuild correct molecules out of atoms. Maybe such a device, even if it could be built, would never be able to keep up with the rate of decay instigated by the winds of entropy. However, interventions at the cellular level are more likely to be successful in our lifetimes, and therefore, understanding and manipulating doll levels 5 and 6 is where we need to put our energy.

Taking a plumbing metaphor in the context of old pipes, a common pattern of failure recapitulated across many of these levels is the "leaky faucet." Old valves can either become clogged with junk or drip due, say, to a worn out washer, or even suffer both conditions simultaneously! Borrowing some terms from cardiology, heart valves can become stenotic and/or incompetent, respectively. Hysteresis problems in either the open or the closed valvular position occurs frequently at the molecular level as well, especially at the cellular lipid-bilayer membrane level. Thus, a significant barrier for hydrogen ions (acid pH) can become compromised when membranes become either too leaky or too stiff or both. Membrane receptor structures may fail to "handshake" properly with their substrate message transmitters or, alternatively, malformed hormone molecules may get stuck and stimulate a receptor to remain in the "on" state permanently.

QED

Definition of Aging #5 (Evolutionary [Michael Rose]): Aging is the result of entropy compromising our homeostatic mechanisms, whenever the genome exhausts its biblical prime directive, "Go forth and multiply." In other words "whenever the genome runs out of specific programming telling the organism what to do next."

 Comment: This definition introduces a programmatic or "clocking" aspect to the previous definition. Entropy in this context can be thought of either in the sense of thermodynamic/metabolic processes or increasing molecular disorder in the sense of information theory. Although "net entropy" for the universe as a whole is always increasing toward maximum randomness (disorder) according to the laws of physics, from the time of the "Big Bang" until the present, there are various pockets of decreasing entropy (extropy), given a consistent, sufficiently strong local source of energy (e.g., photons from the sun), that indulge these singularities in the luxury of structural complexity, like turbulent eddy currents that can occur in flowing water which, for the most part, runs smoothly down stream (laminar flow), despite extraordinary nonlinear behavior at points of exceptional boundary conditions. The genome can be thought of here as a digital computer program (with hidden bugs) having a wide variety of temporal steps (executable phases) roughly as follows: fertilization, initial mitoses (#cells = 2,4,8,16,32,...), blastula/gastrula formation, embryogenesis, fetal development, birth, infancy, childhood, adolescence, puberty, adulthood (characterized by numerous attempts to copulate with members of the opposite sex as often as possible), reproduction of offspring with variable parity (singletons, twins, triplets,...) and gestation times (8,9,10 mo.), concluding with the successful rearing of progeny to independence. All of the "start times" for each of the 14 phases above are unbound variables in the "generic" genomic program that can be instantiated differently by the environment through successive generations (e.g., through the introduction of new predators or parasites). However, old age and death may not be explicit parts of the program (specific code that can be directly modified or tampered with). It is suggested that these epiphenomena are merely side effects. For example, no one ever saw an "old" caterpillar; there are only old butterflies or moths. The caterpillar genome still has "something to do," i.e., it must still go through chrysalis formation and metamorphosis (larval stage to pupa). What if we, too, had some new genetic programming waiting for us after we reached age 70? Using this logic, then we wouldn't age either.

By definition, a Life History for any species is a characteristic trajectory of individuals over time from birth till death (with intercepts and slopes dictated by "gerontogenes," genes whose expression determine the maximum lifespan observed for that species). For simplicity, a Life History may be thought of as being divided into three phases: (1) Childhood (plus Juvenile Development); (2) Adulthood (Reproduction); and finally (3) Senescence (Post Reproduction). Thus, according to this metaphorical paradigm, a Life History is a custom roller-coaster ride:



Actual roller coasters in the real world typically have a long rapid descent phase. But of course they serve a different purpose.

The length (duration) and topography (elevation) of each phase of the "roller coaster" is characteristic for individual species. See the Legend of the Figure for a few additional biological landmarks in this overall sequence.


Legend for Figure - "The Three Phases of Human Life History (Childhood, Adulthood, Senescence)"

The Ordinate (Y-axis) measures species vitality (extropy [= inverse entropy]) on a scale of [0-100] percent.

The Abscissa (X-axis) measures time (in years) (The -3/4 year mark (to the left of zero) represents the approximately 40 weeks of natural human gestation from fertilization to birth.)

Phase 1: Childhood [Tf-T0-T1]: (1a) fertilization [Tf](normally, but not always, preceded by intercourse, ejaculation, and sperm swimming selection; (1b) embryogenesis (initial mitotic cell divisions by powers of 2); (1b1) blastulation; (1b2) gastrulation; (1b3) implantation; (1b4) placentation; (1b5) fetal organogenesis; (1c) birth [at T0]; (1d) infancy; (1e) childhood; (1f) adolescence;

Phase 2: Adulthood [T1-T2]: (2a) puberty; (2b) reproductive competence; (2c) parenting; (2d) climacteric (menopause/andropause); and

Phase 3: Senescence [T2-T3]: (3a) grandparenting; (3b) frailty; (3c) morbidity and chronic disease [secondary to entropy unopposed by any new genetic programming]; and finally (3d) mortality (death). Note: T3 is the average life expectancy for the species, while T(lambda) is the maximum observed life span for the species [122 years for the oldest human of record, Ms. Jeanne Calment of Arles, FRANCE, born February 21, 1875 and died on August 4, 1997].


The senescent third phase of a life history is highly variable and is determined by the plasticity of the aging process itself. "Rapid Senescence" [slope a] occurs in semelparous organisms, which show a burst of reproductive activity followed by predictable, sudden contemporaneous aging in all members of the species. Examples are worms, fruit flies, and the Pacific salmon. It is not clear whether there is a single trigger or a "death switch" in the first two species, as there probably is in the salmon. "Gradual Senescence" [slope b] occurs in most mammals. "Negligible Senescence" [slope c] appears to occur in many saltwater fish, such as rock fish (Sebastes), perch, cod, and sturgeon.

By implication, Life History researchers speculate that the genes that control the timing of reproductive events (puberty and menopause) have homologies in all species. Due to the forces of natural selection, various internal clocks (both cellular and hormonal) are tightly synchronized until we pass through Step 2c. Then, these clocks seem to desynchronize, as the Darwinian forces of natural selection abandon us to entropy. On the other hand, specific gerontogenes that control the terminal slopes (a, b, and c) may be species specific. These slopes appear to mediate cellular maintenance and the repair functions needed to resist entropy or prevent environmentally-mediated and age-related disease. It seems plausible that metabolic regulation may be a general feature of Life History variation. This is consistent with the observation that caloric restriction extends longevity in various mammals.

According to Evolutionary Life History Theory, natural selection for the clock genes that dictate the onset (puberty) and duration (menopause) of the period of maximum fertility and reproductive competence directly impacts potential life span as an important side effect. Repair during the senescent phase of the Life History may be under the control of only a very few genes. A number of gerontologists have suggested that we first investigate the natural antioxidant gene-product family that synthesizes enzymes like SOD (Super Oxide Dismutase), Glutathione Peroxidase, and Catalase. It is supposed that the concentration and duration of expression of these free-radical scavengers are crucial to explaining species longevity. How else can one tell why chimps and humans, whose genomes are different by only 1.5 percent, have a maximum life span that is different by a factor of two?

The total number of cells in an adult organism is precisely determined through an ontogenic, programmatic "unfolding" process, best evidenced by embryological studies of nematodes. Numerically-coded "fate maps" (what cells derive from which predecessor cells) can be painstakingly reconstructed for all the cells in C. elegans (a free-swimming microscopic worm), by careful inspection through an optical microscope for hours-on-end, as the organism develops from fertilized egg, to embryo, to juvenile, and finally to adulthood. Randomly chosen old worms may have fewer numbers of cells, but not healthy reproductive adults. Their number is constant at 959 in the male and 1031 somatic cells in the female adult worm. Curiously, exactly 131 cells undergo apoptosis or Programmed Cell Death (PCD) during development. These cells are like the scaffolding that builders use to facilitate putting up a new building and then take down before they're done. Apoptotic cells do not undergo a slow process of necrosis, but rather a carefully-orchestrated sudden death. This occurs from animal to animal in an exquisitely-reproducible temporal sequence. The same cells always die, and each cell dies at its own appointed hour. By studying development under abnormal conditions, introducing non-lethal congenital defects by inflicting deliberate genetic mutations, nematode researchers have now identified 14 genes that play a role in the process of PCD. Some of these genes are implicated in positive control (accelerated PCD) and some in negative control (failure to complete PCD).

 Controlled experiments with guppies and fruit flies have allowed researchers to witness artificial Darwinian evolution taking place within a few dozen generations by merely introducing a new predator that favors feeding on young as opposed to adult members of the experimental population. In the absence of the predator, the maximum life span might be long; in its presence, it may shrink, perhaps as much as two fold. This way, more metabolic energy can be put into increasing the number of young per birth cycle. Bats have the luxury of postponing reproduction until its convenient. They hunt during the night when they can escape predators by flying, and they sleep during the day in a highly-protected environment (caves). However, I don't know if a bat would ever give you that sort of answer. From their point of view, providing that they speculated about their membership-function in the food chain (or web), they might have a collection of curious complaints about their "plight," like the absurdly ludicrous scarcity of edible bugs in their neighborhood, etc.

 How many genes are there that control maximum life span, and of what type are they? We don't really know yet. But it's unlikely to be the case that the structural/enzymatic protein-synthesizing genes are much different qualitatively. It's more likely that the differences lie in the few hundred control genes that dictate the degree of expression of terminal genes. For example, they might regulate the synthesis of the anti-oxidative-stress enzymes mentioned above that are needed to protect against free radicals in the environment.

Maximum longevity is dictated by the genome of a given species (for humans, The Guinness Book of Records lists Ms. Jeanne Louise Calment as having attained the documented age of 122 years, as mentioned above).

Average life expectancy was 25 years (or less) throughout most of human history. However, life expectancy has risen dramatically over the last five thousand years, as shown in the following table:
Date Average Life Expectancy Female Male Gender Delta
Prehistoric Times 25
Roman Empire (0 A.D.) 30
1870 (U.S.) 40
1915 50
1930 60
1955 70
1992 75.8 79.3 72.3 7.0 (**)
1993 75.5(*) 78.9(*) 72.1(*) 6.8 (**)
1996 75.9 79.0 72.8 6.2 (**)
1997 76.1 79.1 73.1 6.0 (**)
2000 78 (Projected) 79.8 75.2 5.8 (**)
2050 82 (Projected) 84.3 79.7 4.6 (**)
1996 75.9 (at birth) 79.0 72.8 6.2 (***)
1996 17.5 (at age 65) 18.9 15.7 3.2 (***)
1996 5.9 (at age 85) 6.4 5.4 1.0 (***)
1996 0.5 (at age 100)(****) 0.6 0.4 0.2 (***)

* This seemingly paradoxical temporary reversal was attributed by the Centers for Disease Control to two nationwide epidemics in which 82,820 people died from flu or related pneumonia.

** Although statistically, women live significantly longer than man on the average, this gender gap is expected to become less important in the future as women increasingly take on more masculine life styles (smoking, working outside the home, etc.).

*** For any particular year such as 1996 for which we have data, as survival increases, the Gender Gap clearly narrows with age. This is probably because postmenopausal women lose the cardiovascular protection inherent in their estrogen. It is hard to know if there was a gender gap in prehistoric times, but it may be a carry-over from the time when many more young women died in child birth, and so compensating genes were bred for female endurance (estrogen receptors in the heart?) to re-equilibrate equal numbers in each cohort during the period of maximum fertility [14-40]. Of course, recent contemporary medical interventions such as sterile deliveries and C-sections with anesthesia, obviate the need for greater endurance in women as opposed to men, and largely eliminate premature death in women due to childbirth.

**** For Centenarians (alive at age 100), when all but the healthiest and hardiest among us have already died, the odds of living another year (to reach 101) are only about 50:50. Conclusion: Who said aging wasn't a disease?


This prolific rise in life expectancy is modulated by an interaction between the genetically-determined maximum life span for our species (which hasn't changed from 122 years despite the tales of Biblical patriarchs like Methuselah who was reputed to have lived 969 years [Genesis 5:27]) and the declining lack of serious competition in our habitat (including both predators and parasites). This distinction is best seen by comparing longevity curves for a number of different species raised in zoos or on farms vs. in the "wild." The increasing rectangularization of the survival curve for Americans in the last century is largely attributable to the widespread introduction of public health measures and the corresponding reduction in communicable infectious diseases. Abating chronic disease and the "compression of morbidity" will be the major medical challenges for the next millennium.

 Corollary to Definition #5: The post-reproductive individual is on a glide path, coasting toward death from an initial position of excess physiological capacity or "spinning reserve" (borrowing a metaphor from the electric-power generating industry). Evolution selects "survival machines" by choosing organisms whose really critical organs are redundant, subject to the constraint of limited calories in the environment. Think of the similar economic tradeoffs in designing and manufacturing cars. For example, auto designers make it relatively easy for you to change tires, but not so easy to replace transmissions; nor do they invest in diamond ball bearings for the rear axle only, nor install a spare steering wheel in case the first one wears out! Likewise, there are energetic tradeoffs in dealing with predator/prey relations that determine whether birds are more fitted to an ecological environment than ground animals, even though flying is a comparatively expensive, energy-intensive activity, as are unlimited repair mechanisms (wound healing, enzymes to repair breaks in DNA, etc.) intended to prevent catastrophic failure before completing their reproductive mission. Maybe that's why some bird species loose their ability to fly when all their natural predators become extinct in their habitat. Why is it that humans lost the ability to synthesize their own Vitamin C as do other mammals? They must get it from dietary sources lest they come down with scurvy.

 III. TWENTY-FIVE SCIENTIFIC THEORIES OF AGING

 Depending on the narrowness of one's area of specialty, including the unique observational tools that were so carefully taught, one could easily fall victim to a logical trap -- thinking about "aging" in the same way as the proverbial blind men did when they attempted to describe an elephant by exclusively touching only one of its parts. Thus, we could argue about the "trunk" theory of elephanthood, the "tusk" theory, the "ear" theory, the "tail" theory, the "leg" theory, and so on. Each model has some local explanatory power, but none has a global vision. The problem is that the local model fails to appreciate the other aspects of what it means to be an elephant, let alone the dynamics of how elephants move in a herd, defend their territory, eat, sleep, breed, etc. In a table below we will list 25 modern theories of aging, all of which were fashionable at one time or another in the last twenty years, while only some of them have just recently been discredited by the latest data. One must not think of the different theories as mutually exclusive. Indeed, a few of them can clearly be thought of as secondary to other more primary causes of aging in a broader chain (or web) of causality. Of course, the relative success of any theory depends on how well it marshals data to explain the relentless process of "increasing vulnerability to environmental stress that leads to the death of the organism." (See Definitions 4 and 5 above.)

 As another metaphor, imagine that one didn't know for sure whether cigarettes cause cancer. Raw data provided by a number of researchers, however, could show that there is a significant positive correlation between the rate of death due to lung cancer and the number of matches that afflicted individuals carry around in their pockets. One could call this circumstantial evidence for the "match theory" of lung cancer! And we are still waiting for someone to come along, so to speak, and unify the data. After this is done, the expert will tell us "It's not the matches, stupid; it's the cigarettes!"

 Thus, in no particular order of importance, here is the list of my 25 theories of aging:

 1. Genetically Programmed Theory (Minot [Note: References 1-11 provide the full names of the principal researchers advocating each hypothesis])-- Hypothesis: There are deliberate (active self-destruct/suicide/death) gene(s) that appoint the hour of our demise. Examples: Pacific Salmon appear to execute such a program after they spawn. Also, one never sees old caterpillars or old tadpoles, only old butterflies or old frogs, respectively.

 2. Disposable Soma Theory (West) -- Hypothesis: The body is merely a special packaging for the

precious germ-cell line and can be abandoned following procreation. Generic Example: According to the selfish gene hypothesis, "Eggs make chickens to make more eggs;" and not conversely, "Chickens make eggs to make more chickens."

 3. Passive Wear-and-Tear Theory (Weismann)-- Hypothesis: Animals get old just like cars do.

Examples: Elephants die when they get old because they wear down their teeth so much that they can no longer eat properly. Comment: "Burning the candle at both ends," or sleeping less than usual, however, has never conspicuously shortened anyone's life, and thus increasing the "rate of living" is not the same as accelerating wear-and-tear.

 4. Rate-of-Living Theory (Weindruch)-- Hypothesis: There is a metabolic "bank account" or little

odometer in each cell that counts down as calories are consumed and finally hits zero. Examples: Caloric restriction in mice and more recently in monkeys significantly extends lifespan. Combining induced mutations in the clock genes of C. elegans, a microscopic worm, scientists have extended its maximum lifespan by an astonishing factor of five times, but at the expense of a much slower metabolic rate.

 5. Temperature Maintenance Theory (Walford)-- Hypothesis: Keeping our mammalian body

temperatures warm at 98.6oF is bad for longevity, even though it may help us escape from predators. Examples: Hibernation by bears during the winter or resetting the hypothalamic thermostat to cool the body by one or two degrees may extend the clock. On the other hand, placing testicles in an exterior (vulnerable) scrotum for the sake of maintaining spermatogenesis at a lower than normal body temperature in all mammals is an expensive solution to some sort of unknown problem, and obviously nature has deliberated about this tissue at great length before reaching the present compromise. Fevers as a defense mechanism against bacterial infection temporarily raise body temperature. Note that some poikilothermic salt water fish (rock fish) and sea turtles have relatively extended maximum life spans compared with humans without a big caloric investment in maintaining a constant body temperature. On the other hand, "no creatures die of old age in the ocean," not even the great blue whales, the largest mammals known. They are regularly preyed upon by pods of killer whales (of the "Free Willy" variety), so it's hard to know what's going on when wild-type creatures have been marked for identification by oceanographers only in this century, and aquariums have kept creatures in captivity for too short a time to provide us with good data.

 6. Redundant Message Theory (Medvedev)-- Hypothesis: Mutagenesis acts preferentially on actively

transcribed DNA. When it becomes sufficiently damaged, a fresh redundant sequence takes over. Aging occurs when preexisting redundancy is depleted.

 7. Transcriptional Event Theory (von Hahn)-- Hypothesis: There are gene products that specifically

interfere with mRNA transcription and/or protein synthesis, especially in post-mitotic cells. However, proliferative tissues can select for and eliminate damaged cells (apoptosis).

8. Error Catastrophe Theory (Orgel)-- Hypothesis: Once there are errors in the correctional

machinery itself (deformed repair enzymes), this causes an explosive cascade of errors down stream, leading to a "catastrophe" of rapid cell deterioration and death.

 9. Antagonistic Pleiotropy (Rose)-- Hypothesis: Genes that are adaptive or increase fitness at one

developmental stage may prove to be deleterious at a later stage. Example: Drosophila longevity can be manipulated by artificial selection and mating of long-lived probands. A relatively small number of genes seem to be operating in protein blots. Guppies and zebra fish can be similarly manipulated in a relatively small number of generations like 10-20. However, Prof. William R. Clark, former Chairman of Molecular, Cell, and Developmental Biology Department at UCLA, has expressed skepticism about whether the phenomenon of antagonistic pleiotropy even exists in nature.

10. Hyporibosomatosis (Strehler)-- Hypothesis: Reduced ribosomal count means the loss of capacity

to utilize the mRNA to make new proteins. According to this theory, the primary defect is reduced rDNA, which means that there is less rRNA than is required to make fresh ribosomes, which in turn have a relatively short half-life and must be regularly replenished.

11. Hypomitochondriosis (Coles)-- Hypothesis: mtDNA damage means mitochondria lose efficiency

as they burn glucose over a long period of time, yielding too little ATP, the energy coin-of-the realm for cells. Ultimately, everything goes into slow motion. Also, note that the mutation rate of mtDNA in mitochondria is 17 times greater than in nuclear DNA [28]. Extrapolation of semilog plots predicts that the maximum lifespan of a colony of viable mitochondria is only 130 years.

 12. Immune Surveillance (Makinodan/Goldstein)-- Hypothesis: Decreased Thymic Hormones

leading to immunologic incompetence, increased cancer, and increased autoimmunity. Example: Thymic tissue transplants from young to old mice extends the lifespan of the old mouse.

 13. Endocrine-1 Pineal Gland (Pierpaoli/Reiter)-- Hypothesis: Decreased melatonin desynchronizes

a hierarchy of other clocks. Example: Young pineal gland transplants in old mice significantly extends lifespan.

 14. Endocrine-2 Hypothalamus/Pituitary Gland (Timiras/Finch)-- Hypothesis: Decreased growth

hormone and IGF-1 secondary to altered ratios of Hypothalamic Releasing/Inhibiting Factors (GHRH/GHIH). Example: Hypothalamic tissue transplant not yet attempted. But limited clinical trials with hGH in males over 60 years old have demonstrated reversal of aged biomarkers (Rudman). Conversely, inappropriately long-lasting amounts of cortisol (a stress hormone mediated by ACTH) is corrosive and can accelerate aging. Likewise, inappropriately excessive pulses of insulin secreted by the islet cells of the pancreas can have devastating results on all body tissues (not just the symptoms of temporary hypoglycemia). Both hormones antagonize growth hormone. Constant anxiety in teenage girls has been shown to lead to unexpected shortness in height.

15. Endocrine-3 Adrenal Glands (Schwartz/Yen)-- Hypothesis: Decreased steroid synthesis, such

as DHEA, as a precursor to sex steroids, leads to both atherosclerosis and osteoporosis. Example: DHEA-s supplementation helps extend lifespan in animal studies.

 16. Endocrine-4 Thyroid Gland (Dilman)-- Hypothesis: Decreased thyroxin slows down metabolism.

Example: Thyroid tissue transplants not done yet.

 17. Neurological (Cybernetic) Theory (Hebb/Pelletier)-- Hypothesis: Progressive loss of the Central

Nervous System's ability to control the rest of the body. In particular, the brain's EEG slows down sufficiently with time, so that our common fate will always be Senile Dementia or OBS (Organic Brain Syndrome), even if we managed to do bionic organ transplants or cosmetic surgery on everything else. Example: P300 (Positive [+] wave at 300 milliseconds) relentlessly slows with age. Note: Brain transplants are not yet in fashion.

 18. Arteriosclerosis Theory (Steinberg)-- Hypothesis: Strokes, heart attacks, kidney disease, lung

disease, etc. are leading causes of death secondary to a common etiologic pathway of slow formation of thrombotic plaque. Example: Open, flexible blood and lymph vessels slowly turn into stiff, clogged pipes. However, some cases of coronary artery disease are reversible by dietary manipulation and antioxidant supplementation.

 19. Accumulation of Trash Theory (Comfort)-- Hypothesis: The failure of metabolic waste products and/or ingested poisons to diffuse through cellular membranes can shorten our lives. Examples: (1) Lipofuscin (and/or other pigments); (2) Heavy metals (Lead, Beryllium, Mercury, Cadmium, Bismuth, Arsenic, Chromium, etc.)(However, chelation therapy with EDTA is capable of reversing some of these problems.) Comment: If iron overload is a known contributor to aging (hemochromatosis), particularly in men who obviously don't menstruate, wouldn't donating a pint of blood at one's local American Red Cross once a year make sense as a therapeutic measure (rather than just being a case of good citizenship)? Maybe the prescription of "leeches" in medieval times, under some circumstances, was actually beneficial!

 20. Protein Cross-Linkage Theory (Bjorksten)-- Hypothesis: Protein cross-linkage at the molecular

level is a primary defect and ultimately this is what does us in. Examples: Collagen cross-linkage manifests itself when our tissues wrinkle, both our external skin (clearly visible) and our internal organs (not so visible). Collagen cross-linkage also occurs in the tails of mice and serves as an excellent biomarker of rodent age.

 21. Deliberate Failure to Repair Theory (Martin/Diamond) -- Hypothesis: Repair mechanisms are

an effective way to increase the survival of individuals in the population in the presence of continual trauma, but repair is also an energy-intensive process that trades away precious calories that might well be spent on increasing fecundity. Examples: Sharks are reputed to have an indefinite supply of new teeth, while we use up our one extra set in childhood. Mammals are blessed with wound-healing mechanisms, while insects have none at all. However, our own healing is limited, since we're not able to regrow a severed limb, as certain salamanders can do with their tails. Why not? Apparently, the flow of tiny electrical currents along the tissue plays an important role. But, our rate of healing in response to trauma declines systematically over time. For example, tennis players usually retire from the pro circuit after age 35 because they cannot recover from injuries rapidly enough to stay competitive. It has been estimated that after age 50, there is a linear decline of two-thirds of one percent per year in the rate of repair of damage to DNA from a measured low dose of X-rays.

 22. Oxidative-Stress Theory (Harman/Cutler)-- Hypothesis: Our homeostatic-survival envelope

shrinks in the presence of "free radicals," created during the metabolism of glucose, which in turn leads to the breakdown of structural molecules and enzymes and the peroxidation of lipid membranes. Examples: Free (Singlet) Oxygen [O]; Ozone [O3]; Hydrogen Peroxide [H2O2] lead to the equivalent of the tanning of hides (stiff leathery skin) or the rotting/rusting-out of healthy support structures. Autopsy data indicate that carbonyls (oxygen-damaged proteins) rise exponentially in the human brain with increasing age (Carney). Certain enzymes, called free-radical scavengers, like Super Oxide Dismutase (SOD), Glutathione Peroxidase, or Catalase, and other antioxidants (like Vitamin E) are needed to hold these dangerous molecules in check. Recent work in the upregulation of the genes that make these enzymes in Drosophila have significantly extended their maximum lifespan (Orr and Sohal). "Caramelization" is the poetic name given to the process by which proteins are oxidized in the presence of excess sugar. AGES is the acronym for Advanced Glycosylation Endproducts that gum up our cells when unwanted sugars attach to proteins.

 23. Accumulation of Random Errors in DNA Theory (Szilard/Curtis)-- Hypothesis: DNA has enough

redundancy in it so that substitutional mutations at the rate of almost one in three are without consequence. Also, large blocks of non-coding repeat sequences are probably ignored. And although, on average, only six percent of the genome is active at any given time and the rest stays relatively sequestered (by being wrapped around stable histonic disks supercoiled into about 50,000 separate domains), somatic mutations in active nuclear DNA will slowly lead to irreversible deformities in structural proteins and important enzymes. Examples: Intense UV light on the skin, excessive exposure to X-rays, Chernobyl-style radiation poisoning, or mutagenic chemicals (e.g., Bleomycin) can all cause DNA damage. If the integrity of one of the domains is interrupted by radiation and this supercoil unravels randomly, genes that are normally active only during embryogenesis may again become pathologically active causing the cell to dysdifferentiate. It is speculated that an accidental demethylation of a blocking methyl group on certain nucleotides can result in the same sort of process of cellular dysdifferentiation, which of course can lead to cancer.

 24. Environmental or Occupational Exposure to Toxins Theory (Ames)-- Hypothesis: In our

modern society, exposure to air and water pollution are ultimately responsible for aging.

Examples: pesticides (DDT); air/water pollution (smog and other particulates)/infective agents (airborne [Influenza], food [Mad Cow Disease], venereal transmission [AIDS]), carcinogens (inhaled cigarette tars, secondary [passive] smoking), asbestos. Extremely small traces of pollutants [one part in a trillion] that can mimic the actions of certain hormones, like estrogens or thyroxin, have now been shown to adversely affect the brain of a developing fetus when consumed inadvertently during pregnancy. Several provocative but limited studies of children born to women eating toxin-laden fish from the Great Lakes showed that they do not perform as well as other children on tests of memory, intelligence, or verbal ability. Such hormone-mimicking pollutants are now called "EDCs" (for Endocrine Disrupting Chemicals).

 25. Replicative Senescence Theory (Fossel)-- Hypothesis: When chromosomal telomeres shorten

beyond a certain threshold with each division (Hayflick Limit), the post-mitotic cell slowly succumbs to the slings and arrows of entropy. According to the central dogma, human cells divide 60 +/- 10 times, while other shorter-lived species contain proportionally fewer doublings in their telomeric library. Curiously, single-cell paramecia in isolation reach their own Hayflick Limit at 190 +/- 10 divisions, if they are not allowed to mate with another willing paramecium. Autogamy (the transfer of genetic material between paramecia) appears to reset the clock to zero (Smith-Sonneborn). Interestingly, Werner's Syndrome patients start off with many fewer divisions than cells from normal humans (Martin). Also, freezing cultured cells in liquid nitrogen and thawing them out later on allows these cells to pick up where they left off, independently of the actual time they were sitting in the freezer.

 It will be left as an exercise for the reader to arrange this table of 25 causes into an ordered list with the most fundamental or primary cause first. [Please send me your solution by E-mail to scoles@grg.org by July 31, 1997, and you will be eligible for a valuable prize!] As a starting point, take a look at Figure 3 (p. 197, Prehoda), which is all the more remarkable for its having been drawn 29 years ago.

   

IV. TOWARD AN INTEGRATED THEORY OF AGING

 An integrated theory of aging must not only be able to weave together all of the above attempts to explain aging and senescence by subsuming them within some sort of hierarchical framework, it must also explain a diverse number of observational data across many species. An incomplete list of 12 such observations that can serve to filter such a theory is as follows:

 1. All mammals, no matter what their lifespan, consume about 200 kcal per gram of tissue in one lifetime. Therefore, does metabolic rate really dictate the rate of aging? Perhaps instead of grams of tissue we should measure on a per cell basis or perhaps a per mitochondrion basis to see if that sharpens up the data (multivariate analysis).

 2. The brain-weight/body-weight ratio (Sacher) also appears to be a correction factor of great importance with "smarter" organisms outliving their dumber counterparts (Is this effect associated with the pedagogical utility of grandparenting?).

 3. Obviously, not all mice from the same litter (cohort) at the end of their lives drop dead on the same day. Nevertheless, the variance in lifespan is smallest (or the concordance is highest) in congenic mice reared under identical laboratory conditions as compared, say, with wild-type mice. For humans, age variance for identical twins (monozygotic) reared together is somewhat bigger than lab mice. It seems as though there is a 60:40 split with genetics contributing 60 percent and environment contributing 40 percent. We really don't have any evidence from centenarian twins yet. Identical twins reared apart is somewhat bigger. Fraternal twins (dizygotic) is next (same as siblings in general). Next would be adopted sibs (environment only). Finally, survival curves for the general population ordered by country would be next.

 4. Chimps and humans have a 98.4 percent homology in our genomes; nevertheless, we outlive chimps by a factor of two! This suggests that the number of genes that play a role in aging is relatively small, perhaps between 100 and 200.

 5. Insects have evolved extremely high variability in their maximum lifespans. The shortest known life cycle is the now famous fruit fly (Drosophila) whose wild-type members live only 21 days. Conversely, East-Coast Cicadas, with the longest known life cycle, live happily in their juvenile (larval) stage ("nymphs") in near hibernation slowly eating under ground for precisely 17 years, regardless of ground temperature, before they emerge with wings to mate and lay eggs and then die relatively quickly [Southwestern species follow a 13-year schedule]. Hellgrammites, aquatic insects with sickle-shaped mandibles, spend three years in their underwater phase. Caterpillars (voracious leaf-eating machines), as another example, transform into beautiful butterflies who then die in just a few weeks. However, some termites whiz through their adolescence and opt for a long adulthood of 10 to 12 years.

 6. There is a species of haploid (single strand of DNA) wasps and a closely-related species of diploid (regular double-stranded DNA) wasps. Furthermore, they both have the same maximum life span. In haploid strains there can be no recessive/dominant Mendelian-style inheritance. Why doesn't the diploid species live longer, since, if somatic mutations are a key to aging, the second complimentary DNA strand should confer greater protection and conversely, the single-stranded species should be much more vulnerable, right?

 7. Xeroderma Pigmentosa (failure to repair damage to DNA dimers) mimics true aging more closely than Progeria (Hutchinson Gillford Syndrome) or Werner's Syndrome (which may merely be cartoons of accelerated aging). How close to true aging is radiation poisoning (at LD50 [lethal dose for 50 percent of a sample population]) or exposure to mutagenic chemicals?

 8. Winged birds typically outlive comparably-sized land mammals. As another example, bats outlive rats by a factor of four to six. Of the 4000 odd species of mammals on the earth nearly one-quarter of them are bats. What is so special about predator/prey relationships that dictates such longevity in bats besides their ability to fly? Could it be that their principal sleeping habitat on the ceiling of caves is so secure that they're less likely to be preyed upon, especially because they're nocturnal. Also, their guano (bat dung) accumulates on the floor and further protects them by poisoning almost everything else that tries to enter the cave. In certain fish, adaptive changes in life span occur within ten generations when the predator environment is artificially manipulated by researchers. What is the trade-off between fecundity and mortality and its relation to speciation (so called "nonlinear bursts" of evolution)?

 9. Why did homo sapiens lose the ability to synthesize our own ascorbic acid, while mice retained this capability? Clearly, Vitamin C is an essential vitamin to prevent scurvy. The gene to manufacture this molecule can't be that complex, since all plants and a lot of animals have it. Didn't our hunter/gatherer ancestors get any benefit from the presence of this gene compared to the obviously plentiful dietary sources (except when taking long sea voyages on sailing ships)? It has been speculated by some that humans lost their Vitamin-C gene deliberately in a natural tradeoff when we acquired the ability to maintain a higher concentration of uric acid in our blood compared with mice.

 Also, one might ask about the natural rate of trauma to our hunter-gatherer ancestors before the invention of agriculture, as they hunted obviously unwilling prey. Today, there should be an advantage to tuning the clotting cascade to a less sensitive point more consistent with our current requirements. The down side, of course, would be a higher predilection to internal bleeding, but the problem of internal clotting is more serious when the constant risk of external bleeding is so much smaller. This may explain the protective effect of baby aspirin taken prophylactically every day (or every other day) on heart disease.

 10. With respect to the confirmation of DNA, we have learned in the last ten years that the conventional picture of a double helix that one sees everyday in the newspapers is frankly naive. In real life, this one-yard-long double-stranded molecule is packaged in an extraordinarily elaborate manner as follows: From its primary structure of sequential nucleotides {A, T, G, C}, it forms the classical secondary structure in the shape of a twisted step ladder with a ribose back-bone. But, it then goes on to form a tertiary structure of "super coils" of about 140 base pairs each. At the next level up, about 20,000 supercoils are packaged into a single "domain" (quartinary structure) that spools around a cluster of histone proteins collectively called a "nucleosome" and is further surrounded by strands of highly chromatic (color staining) connective proteins. Finally, these domains are then packaged into what are called "chromosomes" (visible at last under the light microscope with proper staining) with their own specialized architecture. For humans, there are 46 of them in the nucleus of each cell-- 22 identical pairs and an XX or an XY depending on gender; different species have widely differing numbers of chromosomes seemingly unrelated to rate of aging and only mildly correlated with animal complexity (with lots of counter examples of plants with huge numbers of chromosomes). The chromosomal architecture consists of centromeres (in the middle), p and q arms of different lengths, and tips at each end called telomeres composed of standard repeat sequences [{TTAGGG}n] where n starts out at birth at around 800 (even this is a simplification, since there are non-coding subtelomeric and peritelomeric sequences about 5,000 base pairs long that have a standard structure, but become increasingly random as you get further away from the tip).

 But this is only the beginning, since all we have described so far is the static structure of DNA conformation. We have completely neglected the dynamics. First, remember that chromosomes are paired, and only one of these pairs is active in any given cell. In other words, only one of two identical genes is capable of being transcribed into protein, while the other homologous copy is silent for many cell generations. Is this deactivation process random or is it the parental origin of the chromosome that determines which gene will be active and which will be silent? How does the cell know which of the two genes should be packaged into a heterochromatin-like state? Is it chromatin-remodeling enzymes, such as SWI/SNF or NURF, histone-modifying enzymes, such as acetyltransferase complexes, or other gene-silencing factors that establish a bad neighborhood for gene expression, such as a "heterochromatin domain"?

 Secondly, we must appreciate that these molecules actually "breathe." They unspool and respool regularly; they uncoil and recoil regularly; they unzipper and rezipper their double strands regularly. They make cruciate forms ("X" shapes) on occasion, as they unzip, depending on the natural tendency of the single strands to self-glue forming a "hairpin" shape, like tRNA does by design. None of these behaviors are random; they are all carefully orchestrated by specialized enzymes made just for that purpose in the context of external hormonal cues and/or local changes in pH. Even chromosomes indulge in bizarre behaviors (poetically called "crossing over") when they swap chromosomal arms at random points along their length during metaphase. Also, it has recently been established that under slight tension (65-70 pico newtons)(and maybe under unwinding torsion as well), the helix can stretch a bit (S-DNA is a stable conformation that is 70 percent longer), simultaneously causing its diameter to thin significantly (by 30 percent). Further stretching can even cause the helix to unravel locally into a straight step ladder with parallel rungs. Still further stretching actually nicks one of the two strands at a single point resulting in a step ladder with skewed rungs moving away from the nick point. However, it is unclear whether this DNA plasticity has any biological significance.

 So with all this background, any theory of aging is going to have to deal with the structural and dynamical questions of how DNA repairs mutational damage (substitutional, insertional, and deletional [The latter two types imply "frame shift" mutants which are particularly lethal, since you are much more likely to hit an inappropriate "stop codon" or punctuation mark for early termination of the protein or else get random amino acids that "flap in the breeze" instead of wrapping up into a tight ball.]) in both the somatic and germ cell lines. Furthermore, with respect to the germ cell lines, we must be able to explain the sharp differences in the rate of inherited disease with increased maternal vs. increased paternal age. Finally, DNA supercoiling appears to decline with age. A loss of supercoiling means that genes that were previously inaccessible to transcription could become accessible to transcription enzymes. For example, rogue transcription of myoglobin in old neurons but not in young (dysdifferentiation). This could also explain the increased rate of cancer with age (oncogenic transcription).

 11. Where are these local clock(s) that tick relentlessly and dictate when to start the aging process? And how are they synchronized among themselves? The clocks appear to be everywhere, possibly measuring in the hundreds. A number of clock levels are indicated below:

 Level Mechanism Cycle

1. Chromosomal Telomere Decrements toward Hayflick Limit

2. Organnellular Mitochondrial Metabolic/Temperature

3. Cellular PER/TIM24-hour cycle complementary proteins

(Mutations in these genes can stretch time to 25 hours per day or shrink time to 23 hours )

4. Tissue Nexus Contact Inhibition during embryogenesis

5. Organal Pituitary Lunar Month/Tides (LH/FSH)

Thymus adolescent involution

6. Endocrine Pineal Diurnal (Melatonin)

Pituitary diurnal (hGH, Prolactin, ACTH)

Thyroid diurnal (Thyroxin)

7. OrganismalEndocrineSeasonal Hibernation/Migration

 "Jet-lag" occurs when time zones change abruptly. SAD (Seasonal Affective Disorder) typically occurs during the winter months, when light levels are systematically lower.

 12. Caloric Restriction is defined as diminished caloric intake in an experimental animal model in the systematic presence of nutritional supplements (vitamins, minerals, essential amino acids, and essential fatty acids). Note that this sort of experiment has never been performed by nature in the wild. Starvation (or famine) is not at all the same as caloric restriction. The amount of restriction can range from 20 to 50 percent compared with controls who eat ad libitum. Congenic mouse experiments have revealed that this intervention increases maximum life span by ten to twenty percent. Similar experiments are now underway with Rhesus Monkeys. Although they are much more expensive to work with (both to acquire and maintain) and the experiments obviously take much longer than with mice, they are a much closer animal model as far as humans are concerned. So far, similar statistics are being found as compared with rodent models. Human studies with 25 percent caloric restriction in Dutch college students are now under way. Why caloric restriction works at all has not yet been explained. However, one discovery lies in the area of mitochondria (the metabolic workhorse of the cell) and free radicals, and we have yet to see whether artificial interventions in mitochondria can recreate the effect of caloric restriction. Does this work for Drosophila (fruit flies)? Maybe.

 Observation: Although the causal relation between caloric restriction and maximum life-span extension was discovered as long ago as 1932, even today, it is still the only known intervention, possibly along with pineal

cross-transplantation, shown to produce such an effect. Yet no one appears willing to use this method to prolong life. Why not? Because people are more interested in the quality of life than in its quantity, and the inconvenience factor of always being hungry makes it prohibitive. Indeed, the mice are so hungry that they must be caged separately to prevent cannibalism! Thus, we need to find out more about the mechanism of action of caloric restriction before it can yield a practical intervention for humans.

 Comment: We need to compare experimental calorically-restricted congenic rats with congenic rats in the "wild" (who regularly experience alternations between feast and famine) rather than with rats fed "ad lib" (which may be an unnatural life-shortening intervention). It may be that undernutrition doesn't really increase life span, but overfeeding does decrease it, and these experiments, as designed, are not really able to tell us that.

 Obviously, we are still quite a ways away from having a fully-integrated theory of aging (a complete picture of the elephant). But does that mean that interventions are impossible? Not necessarily. First, we need to find a way of measuring the rate of aging in long-lived species, such as ourselves. This can be done imperfectly by measuring so-called "biomarkers." 

V. BIOMARKERS OF PHYSIOLOGICAL AGE

 What are the most valuable variables (MVV) for predicting the number of years of healthy life remaining to any individual chosen at random? Why not ask the professionals-- actuaries?

 A. Life insurance companies hire actuaries (statisticians) to estimate the premiums that an individual should have to pay for a standard Term-Life policy, so that they can still run a profitable business. Inputs from the applicant typically include: (1) Chronological age [Date-of-Birth]; (2) Gender; (3) Race; (4) Countries of origin and life-long residence, if different; (5) Marital Status; (6) Educational Level; and (7) Occupation-- all data that are easy to acquire and verify at low cost.

 For persons older than 50 and not part of a standard employment or professional group, a physical exam may be required prior to eligibility either by a nurse practitioner or physician hired by the insurance company to screen out poor-risk individuals. By the way, this is where the ethics of genetic-screening comes into play in terms of whether information preserved in one's medical record is available to potential health insurers.

 B. Curiously, life insurance companies do not compile separate tables indexed by parents' age at death (mother/father). Yet, one would think that a family history of early death in first-degree relatives (siblings, parents, children, grandparents, aunts, uncles, cousins, etc.) would be a valuable low-cost predictor. Perhaps, the correlation coefficient is not that high after all.

 C. Occupational Risks

 (a) Life-Shortening Occupations:

 (1) Professional Stunt Man

(2) Soldier (in times of war)

(3) Policeman

(4) Fireman

(5) Construction Worker

(6) New York City Taxi Cab Driver

(7) Los Angeles High School Teacher

(8) Professional Athlete

 (According to the American Academy of Orthopedic Surgeons, there were more than three million sports injuries last year. However, sports form a broad spectrum of risk, listed in roughly descending order from most dangerous to safest: dueling to the death, highwire/trapeze act "without a net," matador, dare-devil motorcycle obstacle jumping, bronco-busting, horseback fence jumping, boxing, ice hockey, race car driver, bungee jumping, hang gliding, sky diving, basketball (700,000 injuries occurred on the basketball court due to the relative stiffness/springiness of the wood floor underfoot after jumping for a basket), snow skiing, scuba diving, football, soccer, lacrosse, rugby, figure skating, roller blading, skateboarding, surfing, diving, bob sledding, luge, polo, wrestling, marshal arts, weight lifting/body building, mountain biking, road cycling, water skiing, white water rafting, tennis, racquet ball, squash, baseball, fencing, archery, marathon running, jogging (knees), ballet dancing, watching professional football on television, skeet shooting, pole vaulting, javelin, shot put, ballooning, golf [greatest risk from electrocution by lightning], gymnastics, calisthenics, volley ball, hand ball, snorkeling, canoeing, kayaking, rowing, sailing, racehorse jockey, horseshoes, bowling, badminton, billiards, miniature golf, croquet, juggling, ping pong, shuffle board, pool swimming.)[As an exercise for the reader, please send me again by E-mail to scoles@grg.org a reordered list of the above sports (most dangerous first) with your reasons and win a prize.]

 (b) Life-Extending Occupations:

 (1) Symphony Orchestra Conductor

(2) Scientist/Mathematician

(3) Physician

(4) University Professor

(5) Successful Creative Artist

(6) U.S. Government Employee

 D. Religious Affiliation with a Strong Social Support System

 Church goers are more long lived.

 E. Continuing Mental Challenges After Retirement

 People who just putter around in their garden don't do as well as those who chose a new career.

 F. Lose Points for Life-Style Risk Factors (failure to act prudently as established by questionnaire):

 (1) Smoking (Lifespan reduction calculated as 8 minutes for each inhaled cigarette)

(2) Heavy Alcoholic Beverage Drinking

(dose response: light-to-moderate = palliative; excess = detrimental)

(3) Any use of recreational drugs (Heroin, Cocaine, Crack, LSD, Marijuana, Speed)

(4) Numbers of sexual partners per lifetime greater than 100 (AIDS)

(5) More than ten percent overweight

(6) No Dietary/Nutrition Plan (No supplements with vitamins and minerals)

(7) No Exercise Training Plan (30 minutes per day)

(8) No recent proximity to a birthday or major holiday, like Christmas

(9) Frequent failure to use seat belts

(10) No driver or passenger-side air bags

(11) Low Level of Education: Susceptibility to superstition (Placebo/"Voodoo" Effects)

(12) Poor Sense of Humor; Rarely laughs

(13) Type-A compulsive personality

(14) Taking more than five prescription drugs at the same time (from multiple physicians)

(15) Keeping expired prescriptions in the medicine cabinet

(16) Narrow social support system in the event of the death of a parent, child, or spouse

(17) No Financial Planning (Life Insurance/Retirement)

(19) Bouts of Severe Depression (with a History of Suicide Attempts)

(18) Poor Basic Hygiene (showering)

(20) Poor Oral Hygiene (brushing/flossing, mouthwash tid, dental cleaning every 6 months)

(21) No planning for birth-control instruction for children (both boys and girls)

(22) Failure to cook hamburgers on the barbeque to well done (no blood red showing)

(23) Males, failing to stay out of hot tubs beyond knees

(24) Failure to use a hat/suntan lotion in the hot sun (or while skiing at any time)

(25) Prone to compulsive gambling (loses greater than $5,000 per year)

(26) More than four speeding tickets in any one year

(27) Any DUI's whatsoever for drunk driving

(28) Conviction for a felony and/or time in jail

(29) Ownership of a hand gun or other lethal weapon at home

(30) More than two admissions to an ER in any year

 G. Laboratory Values and Blood Chemistry (CBC, Cholesterol, etc.)

 Small samples of blood, urine, stool, saliva, perspiration, and/or breath may be used to calculate one's net antioxidant profile, especially DNA excision products.

 H. Past Medical History (Chronic Illnesses, Past Surgeries, etc.)

Allergies, frequent infections, emotional disorders, sleep disorders, and so on could be very important in establishing risk of premature morbidity.

I. Physical Examination (Arrhythmias, Hypertension, etc.)

 The "H-SCAN" Computerized Biological Age Test System invented by Richard Hochschild of Corona del Mar, California is based on 12 carefully-selected standardized biomarkers of aging:

 (1) Reaction Time;

(2) Visual Accommodation;

(3-4) Forced Vital Capacity and Forced Expiratory Volume-- 1 second;

(5) Vibrotactile Sensitivity;

(6) Highest Audible Pitch;

(7) Memory Recall;

(8) Alternate Button Tapping Time;

(9-10) Reaction Time and Muscle Movement Time -- Visual Stimulus; and

(11-12) Decision Reaction Time -- Decision Movement Time.

 At the conclusion of a self-administered 45-minute exam each subject receives a computer printout of his or her physiological age as compared with a standard population expressed in terms of a histogram. This exam can then be taken once every six months to compute a "rate of aging" for that subject.  

VI. THE PROSPECT OF IMMORTALITY

 With respect to the question of whether evolution could ever produce immortal complex multicellular organisms, Prof. Bernard L. Strehler, of the University of Southern California and co-founder of the Los Angeles Gerontology Research Group, speculated in his now classic book Time, Cells, and Aging (p. 370, italics by author) as follows:

 "... It appears to me that there is ... no inherent property of cells or of metazoan organization which by itself precludes their organization into perpetually functioning and self-replenishing individuals."

 The observed absence of any such immortal creatures on Earth is probably due to the simple "lack of selectional or adaptive pressure" to evolve them. Translating these ideas into simpler language, there is nothing that violates the known laws of chemistry or physics to be able to construct immortal in-vivo somatic animals. Evidence comes from three independent sources: (1) Immortal in vitro cell lines do exist (e.g., cancer cells, such as HeLa cells); (2) immortal in vivo germ-cell lines also exist (e.g., mammalian sperm, consistent with the Disposable Soma Theory); (3) immortal protozoa (single-celled animals) also exist (e.g., amoeba). The problem is one of how to de-differentiate certain cells in a tissue to embryonic conditions and then redifferentiate them back to their original form while still subject to the boundary conditions of the tissue in which they participate (in other words, to preserve the structural integrity of the tissue). Tumors, by definition, crowd out their neighboring tissue by ignoring signals of contact inhibition.

 Neoteny is the name that biologists use to describe the continuation of a larval form, having many juvenile characteristics, in an otherwise sexually mature organism. Effectively, this can happen when the onset of sexual maturity is accelerated in an embryological sense. A real example of neoteny occurs in the Mexican axolotl, a type of salamander, in which the animal does not progress past the larval form, but becomes sexually mature without undergoing the usual metamorphosis, like in the transition from tadpoles to frogs, that allows such creatures to be able to breathe on land. The larval salamander is already quite advanced with well-developed limbs, but still has gills through which it breathes in the water like a fish.

 What gerontologists would like to discover is a method for forming neotenous cells within an adult organism which could provide a temporary, controlled escape from specialization. In such a case, normal post-mitotic cells, such as muscle or nerve cells, could, on command, slough off their highly specialized form, return to the lability of their embryological youth, undergo a series of mitotic divisions, and finally respecialize back to their mature form after having been rejuvenated in several important respects (e.g., the synthesis of a completely new set of mitochondria). Many salamanders, as a defense mechanism, can lose their tails while escaping from a predator with impunity. A fresh tail simply regrows outward from an embryonic "fault line." Humans retain a limited form of regeneration called "wound healing," but obviously we lose the ability to grow a new limb during embryogenesis once we pass an certain point of development. On the other hand most insects do not even have the power to heal a wound. Like a porcelain vase, once it breaks, insects stay broken.

 Research gerontologists typically follow one of two basic paradigms: the ontogenic or the Darwinian. The ontogenic paradigm measures whatever senescent changes occur in cells and tissues over a representative individual's life time, whereas the Darwinian paradigm compares species maximum life span in terms of developmental and structural organization, as they can be measured in youth, even prior to the onset of aging.

 Here's an intriguing question frequently asked by children but rarely by adults...of the form "Why do creatures look so different?" For example, How did the elephant get his long nose or the giraffe his long neck? Contemporary biologists would answer that large phenotypic differences are amplified by small differences in genotype. That is to say, elephants have somewhat different genes in so far as noses are concerned while giraffes have somewhat different genes in so far as necks are concerned. Such explanations beg the question, however. As a general principle, phenotypes diverge during embryogenesis in the differential rate of mitosis/apoptosis within specialized cell lines. In particular, a small field of precursor stem cells must have started dividing like crazy in the elephant's nose or in the giraffe's neck under instruction from some unknown growth factors. Yet basic biochemistry is universal in all mammals. The rate of DNA synthesis is identical. The rate of mRNA synthesis is identical. The rate of ribosomal ratcheting on the mRNA is identical with a fixed force of about 50 pN (pico Newtons). It is likely that standard complementary per/tim genes measure the 24-hour day in each cell in exactly the same way across all mammals. Lunar or seasonal-cycling clocks (in the pineal gland) may be customized, however, depending on adaptations for migration or hibernation. Nevertheless, almost all of the machinery is the same. So where are the differences?

Unhappily, the quantities of the growth factors that are needed by cells to cause directed differential growth during embryogenesis are tiny compared to the amount of hemoglobin or collagen proteins that all animals possess and whose structure we now know well. Therefore, finding these proteins and the genes responsible is a real challenge, but not impossible. As an initial step, we have recently identified the control molecule responsible for deleting the webbing between developing digits (fingers and toes). A congenital absence of this signal leads to a condition called syndactyly. We can now reliably grow duck feet on chickens by blocking the signal (or conversely non-functional chicken feet on ducks by introducing the signal). The problem of economically synthesizing large quantities of any given protein contained within the milk of transgenic goats is being worked on by Genzyme Transgenics, Inc. Unfortunately, however, when a gene of interest is pipetted into a fertilized goat egg, there is only a ten percent chance that it will be spliced into the correct place on the appropriate chromosome, i.e., only 3-6 kids out 60-80 goats are likely to become positive functional producers of the desired protein. On the other hand, all of their progeny should also be producers and allow for the development of specialized herds to make custom drugs on demand. Yields will rise when we better understand how to insert integrator genes along with the desired genes, so that gene insertion takes place more reliably.

 Someday, when we understand all the details, in the year 2050 or so, a standard high-school biology take-home exam might be to design and produce fabulous mythological chimera (take your choice of cyclops, unicorn, sphinx, centaur, minotaur, manticor, griffin, satyr, cerberus, roc, basilisk, pegasus [extra credit], etc.). Only then, we can really say with confidence that we truly understand what's going on with elephants and giraffes. On the other hand, without proper controls on "biohackers," the world could become a very strange and curious place indeed.

Gerontologists have always been eager to discover an evolutionary basis for senescence and death, arguing over whether aging is really adaptive or may have survival value for the next generation in the general competition for scarce resources. They frequently overlook the possibility that aging is just an epiphenomenon, a mere entropic side effect of the "flip side" of the coin, which is longevity. When we focus on the evolution of longevity for a species, we find positive, genetically-controlled mechanisms. Aging, on the other hand, is a process that the organism endures passively, and this controversy about how aging evolves is essentially meaningless. Indeed, aging does not evolve; what does evolve are the longevity-assurance systems that confer on each organism in a population the length of life and vigor requisite for its development, reproduction, and the rearing of offspring to independence.

 Up to now everything that has been reported in this article can either be found in the scientific/medical literature or is largely unsupported speculation by others. Now for some of my personal unsupported speculation-- what are the means to slow and/or reverse aging?

VII. FUNDAMENTAL INTERVENTIONS IN THE AGING PROCESS--

A THREE STEP PLAN

 Well, what can we do to improve the hand we were dealt, if not reshuffle the whole deck? What can we do to extend our personal span significantly beyond the biblical three score and ten? Clearly, with heroic effort, some obvious interventions will reverse abnormal biomarkers or local declines in fitness (like needing to lose weight). However, it will be increasingly difficult to glue the fallen Autumn leaves back on the naked branches of a tree in Winter. Switching to scotch tape instead of glue isn't the answer. We've got to get to the primary causes. Why did the leaves fall off in the first place? Everything else is merely cosmetic.

 Thirty years ago, I used to think that the answer lay in creating cyborgs, bionics, or some other sophisticated form of biomedical engineering. For example, the naive engineering approach to irreversible heart disease is to replace the "poorly-designed" natural pump with a more efficient artificial heart. Yet, the success rate for the NIH-sponsored artificial heart program of the 1970s was so low that it was canceled in favor of a refocusing on natural heart transplants and organ rejection. However, limitations on the availability of donor hearts and histocompatibility issues makes this a risky strategy at best. Xenographic pig heart transplants are still a few years away. What's a person to do if their heart gives way, as it must sooner or later? Depending on bionics to do the job in our lifetime could be foolish.

 The ultimate insult to this organ-by-organ replacement strategy was the recent realization that even if all heart disease were cured hypothetically, the resulting increase in life expectancy would only be on the order of 14.3 years. Furthermore, if all cancer were magically cured tomorrow, the resulting increase in life expectancy would only be on the order of an additional 1.9 years. What's going on here? In fact, if we eliminated all the causes of death written on death certificates today, most people would only live to be about 100 years old. These centenarians would not be immortal; they would still die of what might be called "natural causes." In particular, they wouldn't live for 200 or 300 years, as in science fiction stories. Our disease-cure speculation has led us to a so-called full "rectangularization" of the human population longevity profile, leading to a "compression of morbidity" but not a true "right-shift in mortality."

 Is there any group in the conventional medical establishment seriously devoting themselves to this quest? No, not really. Such a goal is not even articulated by the medical profession for its students, so what do you expect? I don't have any simple answers myself, but I do know that, in outline, a strategic plan for really dramatic life extension, not the direct curing of individual diseases, will probably entail at least three steps roughly as follows:

Step 1: We need to systematically rebalance or tune-up the Neuro-Endocrine-Immune Axis, returning selected hormone levels, particularly growth hormone, to more youthful levels. This may be done by supplementation with growth hormone itself (an obviously expensive and invasive solution) or by means of one of the newly-discovered orally-active hGH secretagogues (now under development), relying on the well-endowed synthesis capabilities of the pituitary gland to make higher levels of growth hormone than it is normally asked to do by hypothalamic stimulators when a person passes the age of about 50. The same general argument may be given for levels of Melatonin, DHEA, Sex Steroids, Thyroid Hormone, and Thymosin. Successfully completing this Step might add 10-20 years. This leads to Step 2.

Step 2: The next intervention had better reset the cellular clocks that dictate replicative senescence. This will likely involve figuring out how to systematically introduce telomerase into the nucleus of cells where it's not normally found. Yet, this must be done in an exquisitely controlled manner, so as not to engender uncontrolled proliferation by any one clone of cells. Immortalization of cells that ignore the architectural constraints of the tissues they make up is doomed to fail. The methodology will likely entail the use of genetic engineering techniques with viral vectors now under development for the cure of specific genetic diseases. But even this is not likely to gain us more than another 20-50 years, since another insidious process of DNA destruction is invisibly taking place at the molecular level which is bound to do us in sooner or later. This leads us to Step 3.

Step 3: Chromosomal replacement is the only way to rejuvenate a cell whose nucleus is sufficiently damaged. The technology for how to do this can only be hinted at. Clearly, retro viruses are too small to package such a large amount of DNA as a complete chromosome. Probably some sort of artificial liposomal structure will be needed as a carrier. How to make Human Artificial Chromosomes (HACs) is an unsolved problem. Recently, researchers at Case Western Reserve University in Cleveland, Ohio have patented their technique. Once you make HACs, how to address them to just the right cells is another challenge. Conceptually interesting methods discussed in the emerging field of nanotechnology may not arrive in time to save the day. However, new techniques in molecular biology may prove to be efficacious.

With respect to the problem of liposome synthesis there are a number of companies, like Sequus Pharmaceuticals of Menlo Park, California, that are now beginning to manufacture liposomal carriers for drug delivery of antibiotics (antifungal, antibacterial, and antiviral) and chemotherapeutic agents (without the side effects of nausea, vomiting, hair loss, and bone marrow suppression). However, these are expensive, experimental treatments with multiple injections of $1000 each not being unusual. First-generation liposomes were large (200-300 nm) and unstable (half-life of 1-2 hours). They were generally swallowed up by macrophages and then quickly destroyed by the liver and the spleen. Second-generation liposomes, now in development, are smaller (less than 100 nm) with greater stability (half-life of 24-48 hours). This latter property derives from a fixed ratio of different types of phospholipids on the surface mixed with cholesterol, thereby increasing their capacity to extravasate from the blood and into the tissues where they may be needed. For their ability to evade the immune system, they are sometimes referred to as "stealth" liposomes. Third generation liposomes, now in the conceptual design stage, will not only be stealthy, they will be "smart." They will be able to target specific tissues with antibody ligands on their surface. The addressing of where they go will be by means of the equivalent of a grocery-store product "bar code" which may say "deliver this drug to the lungs" or "to the heart." These liposomes can also be nested one inside another for entrance first across the cytoplasmic cell membrane and then subsequently across the nuclear membrane. Finally, the problem of ensuring that a cell once penetrated will not be penetrated twice must be considered. This is conceptually equivalent to the problem of ensuring that only one sperm enters the egg for the purpose of fertilization, while excluding all the late comers.

 

Legend for Figure on Genosome Transfection-- A DNA-liposome complex can enter a cell by either endocytosis, fusion, or transient lipid-mediated poration. In the case of endocytosis, quick release from the endosome is essential to prevent lysosomal degradation of the DNA. Whether the DNA reaches the nucleus by diffusion or by an active "trafficking" process is not known, nor is it clear where the plasmid decondenses.

 The real problem is making sure that we're still here when these scientific/medical discoveries arrive, as they will some day. For that we need a "bridge plan" that takes us from the here-and-now to Step 1 while we seek to promote advances by government and university biology laboratories along the agenda of the other two Steps. See the Home Page listed at the beginning for a reference to a 30-page document describing such a plan. Also, this site contains a Curriculum for a "Five-Day Intensive Course in Anti-Aging Medicine." Finally, it contains our list of what we consider to be the "Top Ten Unanswered Questions in Gerontology Research," as well as a number of references to other web sites of interest to those pursuing these questions.

 As Star Trek's Mr. Spock always said, "Live long and prosper."

REFERENCES::

1. Robert W. Prehoda, Extended Youth: How Science is Reversing the Aging Process-- The Promise of Gerontology (G. P. Putnam's Sons, New York; 1968).

2. Bernard L. Strehler, Time, Cells, and Aging (2nd Edition, Academic Press, New York; 1977).

3. James F. Fries and Lawrence M. Crapo, Vitality and Aging (W. H. Freeman and Company, San Francisco, California; 1981).

 4. Kenneth R. Pelletier, Longevity: Fulfilling Our Biological Potential (A Merloyd Lawrence Book, Delacorte Press, New York; 1981).

 5. Carol Kahn, Beyond the Helix: DNA and the Quest for Longevity (Times Books, New York; 1985).

 6. Caleb E. Finch, Longevity, Senescence, and the Genome (The University of Chicago Press, Chicago, Illinois; 1990).

 7. Robert E. Ricklefs and Caleb E. Finch, Aging: A Natural History (Scientific American Library, New York; 1995).

 8. Michael R. Rose, Evolutionary Biology of Aging (Oxford University Press, Inc., New York; 1991).

 9. Lewis Wolpert, The Triumph of the Embryo (Oxford University Press, New York; 1991).

10. Thomas J. Moore, Lifespan: Who Lives Longer and Why (Simon and Schuster, New York; 1993).

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25. John J. Medina, The Clock of Ages: Why We Age, How We Age, and Winding Back the Clock (Cambridge University Press, New York; 1996).

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27. Mark K. Shigenaga, Tory M. Hagen, and Bruce N. Ames, "Oxidative Damage and Mitochondrial Decay in Aging," Proceedings of the National Academy of Sciences, Vol. 91, pp. 10771-10778 (November 1994).

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29. Waneen W. Spirduso, Physical Dimensions of Aging (Human Kinetics, Champaign, Illinois; 1995).

30. Melvin R. Werbach, Nutritional Influences on Illness, 2nd Edition (Third Line Press, Tarzana, California; 1996).

31. Ray Sahelian, DHEA: A Practical Guide (Avery Publishing Group, Garden City Park, New York; 1996).

32. Ronald M. Klatz and Robert Goldman, eds., The Science of Anti-Aging Medicine (American Academy of Anti-Aging Medicine, Colorado Springs, Colorado; 1996).

33. Ronald M. Klatz and Carol Kahn, Grow Young with HGH (Harper Collins Publishers, New York; 1997).