Can Your Local Molecular Geneticist Do This?

Orgel’s Second Law: “Evolution is Smarter Than You Are”

How Long Will These Organs Function?

Quiz: How old am I?

I have cataracts, lose uphill races, get really sick when I catch the flu, girls no longer whistle when I pass by, my joints ache, and if you looked closely you’d see preclinical signs of the cancer that will kill me.

So: 2, 10, 20, 35, or 75 years old?

A Defensible Overview

• Young mammals look alike, to a first approximation– Unless you are a veterinary pathologist, you cannot look at a

microscopic section of liver or kidney or lens tissue and tell me if it’s from a young mouse, young porcupine, or teen-ager.

• Old mammals look alike, to a first approximation– They have cataracts, cancer, muscle loss, poor immune defenses,

slow reflexes, poor resistance to many stresses...

• Depending on species, it takes between 2 - 70 years to turn the one into the other.

This meeting collects three species of biogerontologist:

• Comparative biologists, who compare across breeds, species, etc.

• Field biologists, who find instructive examples in nature, and might someday develop these into new lab models

• Lab biologists who work on non-mouse non-persons

What have the comparative biologists done for gerontology?

• Shown that low hazard niches produce slow aging over and over and over again

• Disproven old chestnuts– high metabolism produces short life span– high brain/body ratio implies long life span

• Shown that within a species, small breeds are typically long-lived

• Shown that some biochemical traits covary with life span across a wide range of mammalian species

• And that some (e.g.: telomere length) don’t

What have field biologists done for gerontology?

• Shown that one genome can produce bodies of vastly different life span (bees, trout)

• Shown that one order can produce species of vastly different life span (porcupines, mice)

• Shown that selective pressures can produce changes in life history traits very quickly (guppies, opossums, grasshoppers)

What have slug-and-bug biologists done for gerontology?

• Shown that single-gene mutations can dramatically increase maximal life span

• Disproven old chestnuts– Gompertz mortality plots as immutable natural law

• Suggested specific molecular hypotheses that can be tested in real mammals– Stress-resistance– IGF/insulin pathways

What is the point of bringing these three kinds of folks together?

• They have not accomplished enough and need a kick in the pants. – (Albeit a gilded kick)

• The hope that field biologists and comparative biologists can induce (or work with) lab biologists to tackle the challenge of inter-species differences in aging rate.

Case Histories: From Barn to Bench

• Wild mice: recapturing lost anti-aging genes for Science

• Size-selected mice: pleiotropic effects of genes that modulate growth trajectory

• Zoo-plots: mechanistic clues from trans-taxon regressions

• Comparative demography: which life history pieces have genetic handles?

Example #1: Wild-Derived Mice in the Lab

• Theory: lab mice are to mice as Lassie is to a wolf– domestication rapid selection for early litters– domestication cowardly, meek, slow, and oblivious to

social cues– inbreeding selection for alleles that are viable when

homozygous but may make very odd mice

• Idea: domestication and inbreeding may discard genes for slow aging

• Support: wild mice are often small and have small litters

Longevity Study Design

• Specific-pathogen free stocks from– Idaho– Majuro– Pohnpei

• Not inbred, though genetic heterogeneity unknown

• Third lab generation (avoids maternal effects)

• Control: “DC”– 25% BALB/c, B6, C3H, DBA/2– 7 generations of intercrossing

Extended Longevity in Wild-Derived Mice

Hormone Levels and Blood Chemistry in Wild-Derived Mouse stocks

90%Mortality

(days)

IGF-I

(ng/ml)

ThyroxineT4

(g/dl)

GlycatedHemoglobin

(percent of Hb)

Id 1323 302 ± 27 4.4 ± 0.3 3.8 ± 0.2

Ma 1245 464 ± 48 2.3 ± 0.1 20.2 ± 1.0

DC 1144 591 ± 25 5.0 ± 0.2 10.3 ± 1.2

B6D2F1 n/a 640 ± 68 4.7 ± 0.5 8.6 ± 0.7

B6 n/a 961 ± 40 4.9 ± 0.1 3.6 ± 0.1

Mean +/- SEM; from Miller, Harper, Dysko, Durkee, Austad, submitted

Idaho and Majuro Mice Show Delays in Female Reproductive Maturation

Wild-Derived Mice: What Next?

• Kosrae and Truk: two more island stocks

• Biochemistry: gene expression, hormone levels, stress resistance

• Genetics: segregation analysis of (Id x Lab)F2 mice– QTL for the life span genes– QTL for the genes regulating other traits– Segregation analysis for: growth, hormones, gene

expression, etc

Example #2: Pleiotropic Effects of Genes That

Regulate Growth Trajectory

Early Growth Trajectory: A Determinant of Longevity?

• Natural history: dogs, horses, people– [and wild-derived mice]

• Caloric restriction– and maybe methionine restriction

• Four mouse mutations– Snell dwarf, Ames dwarf, GHR-KO, lit/lit

American Miniature and Falabella Horses

Big Vets Die YoungTall Veterans Die YoungN = 373

Height Group (cm)

> 183 > 175 < 175 < 170

Mea

n L

ife

Sp

an (

year

s)

60

62

64

66

68

70

72

74

N = 71

N = 195

N = 178 N = 91

Ref: Samaras and Storms, Bull. WHO 70:259-267, 1992

Restricted Index Selection(Atchley Mice, Idealized)

Days Of Age

0 10 20 30 40 50 60

Wei

gh

t (g

m)

0

10

20

30

40

50ControlLS = Late smallES = Early smallLB = Late bigEB = Early big

Stock Mean Weight, 6 mon (gm)

25 30 35 40 45 50 55

Lif

e S

pan

of

Sto

ck (

day

s)

400

600

800

1000LS2

LS1

LS3

C2C1

C3

ES3

ES1

ES2

EB1

EB2

EB3

LB1

LB2

LB3

Note: symbol area is proportional to number of mice tested.

Correlation Between Stock Life SpanAnd Weight at 6 Months (Atchley Mice)

R = -0.69, p = 0.004

Example #3Zoo-Plots: When Comparative Biologists

Start Comparin’ Stuff

Zoo-Plot: Maximum Life Span vs. Five Kinds of Stress Resistance (Kapahi, Boulton, Kirkwood, 1999)

Human, cow, pig, sheep, rabbit, marmoset, rat, hamster

Zoo-Plots Under Scrutiny

• Will this hold up after adding more species?

• Will this hold up when adjusted for non-independence of selected taxons?

• Is the correlation merely an epiphenomenon of some more fundamental relationship (e.g. size/longevity)

• Is there a cause/effect relationship? If so, then:– How does the cause cause the effect?– Do the species differences reflect changes in the same

loci?

Example #4A Problem for Comparative Demographers:

What Aspects of Life History are Under Separate (Genetic) Control?

Comparative Demography:

One Provocative Formalization[Finch et al.]

SpeciesIMR

(1/year)MRDT(years)

Max(years)

C. elegans 1 – 7 0.03 0.16Drosophila 0.01 – 4 0.03 0.3 (0.5)Mouse 0.01 0.3 > 4Earthworm 0.1 > 6Pipestrelle bat 0.36 3 – 8 > 11Dog 0.02 3 20Macaque 0.02 8 > 35Horse 0.0002 4 > 45Herring gull 0.004 6 49Elephant 0.002 8 > 70Humans (POW camp) 0.007 7.7Humans (US, 1980) 0.0002 8.9 > 110Lake sturgeon 0.013 10 > 150

Some related questions:

• Do the genes that regulate IMR across species also influence MRDT (and vice versa)?

• Are the physiologic processes that mold IMR and MRDT independent?

• Is this parameterization -- IMR plus MRDT -- sufficient to account for the factors by which niches mold life history?

Are IMR and MRDT Enough?

Does the dw/dw Mutation “Slow” Aging, or “Delay” Aging?

Delayed Aging in Ames Dwarf (df/df) Mice[Brown-Borg, Bartke, et al.]

Delayed Aging in lit/lit Mice

Bartke Data: df Mutation Alters Inflection Point, and CR May Alter Slope of Survival Plot

New Models for Aging Research: Let Mother Nature Do The Hard Part

Long-Lived Rodents: The Official Winner

• Sumatran crested Porcupine

– Hystrix brachyura

• 7 - 9 kg

• Maximum longevity: 27¼ years

• Niche: grouchy little pincushion

Long-Lived Rodents: The Other Winner[And my personal favorite rodent]

• Naked mole rat– Heterocephalus glaber

• 25 grams (queen = 70)

• Work at 4 weeks

• Breed at 1 year

• Maximum longevity unknown: > 20 years

• Niche: underground

Another Safe Niche: Top Predators

Safe Niches In Which Longevity Pays Off

• Biggest fish in the sea (tuna)

• Up in the air (bats, birds, flying squirrels)

• Unbiteable (porcupines, turtles, elephants)

• Stay underground all the time (naked mole rat)

• Scary and mean (tigers)

• Can throw spears (us)

A Key Challenge for Comparative Gerontology:

When Nature generates a long-lived organism to exploit a low-hazard niche, does she always use the same trick?

Improved stress-resistance?

Low free radicals?

Stage-specific constraint in IGF-1?

Better DNA repair?

Anti-cancer defenses?

Other?

Construction of a Big, Long-Lived Mammal: The Cancer Problem

• Humans: one lethal cancer per 60 years for 60,000 grams of target cells

• Mice: one lethal cancer per 2 years for 30 grams

• Therefore: Human cells are ~30 x 20,000 = ~600,000 fold more resistant to cancer than mouse cells

• Whale cells: ~109-fold more resistant than mouse cells– Cetacean consultant: Steve Austad

If You Were Nature Constructing a Long-

Lived Creature To Take Advantage of a Low-

Risk Niche, How Would You Do It?

Construction of Long-Lived Mammals: Method I

• Make alleles that slow down eye aging

• Make alleles that slow down muscle aging

• Make alleles that slow down brain aging

• Make alleles that slow down tooth aging

• Make alleles that slow down immune aging

• Make alleles that slow down cancer

• Make lots of other alleles of this kind

• Do this really fast – ~ 2000 generations for opossums

Construction of Long-Lived Mammals: Method II

• Alter the genes that time the aging process*

*that is, if there is an aging process, instead of many aging processes

A Problem with Method I

• A gene for 10-year eyes might not be worth much in a mouse that’s going to get cancer, immune senescence, sarcopenia, and dementia in 2 years

• A gene for 10 years of immune function might not be worth much in a mouse that’s going to get cancer, cataracts, sarcopenia, and dementia in 2 years

• A gene for 10 years without sarcopenia might not be worth much in a mouse that’s going to get cancer, cataracts, immune senescence, and dementia in 2 years

Key Objection to Method II

• There might not be an aging process.

• Main counter-arguments:– dwarf mice (superworms, superflies, etc.)– caloric restriction– synchronicity of age-dependent decline

Synchronicity: Why Not the Widget?

• 1/3 of widgets die age 5 years of cancer

• 1/3 of widgets die age 10 years of immune deficits

• 1/3 of widgets die age 15 years of blindness

• Explanation I: fine-scale adjustments of multiple timing genes

• Explanation II: adjustments of few timing genes with multiple consequences

Summary: Three Meetings For the Price of One!

• What clues can gerontologists who work on “easy” organisms provide for those of us whose best friends are mammals?

• What can the comparative biologists infer about distribution of life history traits among related species?

• What can the field biologists do to help us find new models for exploring mechanisms in the lab?

Some Credits

• Wild mice: Steve Austad, Bob Dysko, Jim Harper

• Dwarf mice: Andrzej Bartke, Kevin Flurkey, David Harrison

• Size-Selected Mice: Bill Atchley

• Money: NIA, and its various guises (Nathan Shock Center, Claude Pepper Center)

• More money: Ann Arbor VA Medical Center