MCB 135K Mid-Term IReview

February 15, 2005

GSI: Laura Epstein

General Information

• Mid-Term I – Friday– In Class Exam– 100 Points

• 50 Points Multiple Choice and True / False• 50 Points Short Answer / Essay

– Be prepared to place all personal belongings in aisle or at front of room before exam begins

Exam Material

• Intro and Demography• Comparative/Differential Aging • Mitochondrial Decay• Cellular Senescence• Functional Assessment• Epidemiology• Telomeres• Evolutionary Theory of Lifespan• Oxidants, and Anti-Oxidants• Neurodegeneration, Repair, and Plasticity• Yeast as a model for cellular lifespan

Demography• Be familiar with:

– When does aging begin– Role of genome and environment on aging– What Demography is and how it is useful– How life expectancy has changed in the last 200 years and

what has contributed the most to this change– How the population distribution has changed and what

impact this might have for the future– Centenarians and aging

• What have we learned from these studies

– How old was the oldest living documented person of each sex

Demography

• Statistical study of human populations:– Size and density distribution

• Vital Statistics:– Epidemiology: Births, deaths, diseases

Life expectancy at birth by sex, France 1806-1997

Proportion of population aged 0-14 versus 65+(In Italy)

Centenarians

• Generally good health– Escapers– Late onset of disease– Early disease that was

overcome

• SSC (Semi-Super)– 105+

• SC (Super)– 110+

• Possible role of IGF-1 Receptor

• Oldest Female– 122 years

– Jeanne Calment

• Oldest Male – 115 years

– Christian Mortensen

Women and Longevity

• Probable Causes for increased longevity– Genetic– Environmental– Lesser Life Stress– Decreased Smoking– Protective Hormones– Better Protection Against Oxidative Damage

Comparative / Differential Aging

• Be familiar with:– What are the models for study– What does physiological assessment require– What are the physiological correlates with

longevity and how do they correlate

Figure 3.1: Comparative Maximum Life Spans

**Detailed discussion of figure in the legend, pg. 26

Mitochondrial Decay

• Be familiar with:– Mitochondria function/damage– Acetyl Carnitine (ALCAR) – R-Lipoic Acid (LA) – Behavior of old rats fed ALCAR and LA – Micronutrient under-nutrition in Americans

Mitochondria

• O2 O2- H2O2 OH H2O, oxidative

metabolism• - Reaction intermediates are similar to products

formed by radiation exposure.• -We are always making lesions in our DNA and

fixing it• -With infection, our immune cells fight the

infection and release free radicals• - 1-2% leakage of reactive oxygen species (ROS)

Oxidative Damage

Products from Base Excision Repair (BER) and Nucleotide Excision Repair (NER) are secreted in the urine.

- Frequency of oxidative events increases in older individuals

Rat liver cells – Young ~24,000 oxidative

lesions/cellOld ~67,000/cell

MDA (pmol/mg

protein)MDA is a product of lipid

oxidation

Young

Old

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Brain Liver Heart Kidney Lung

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Mitochondria from old rats compared to those from young rats:

1) Lower Cardiolipin—a lipid used in the mitochondrial membrane

2) Lower Membrane Potential

3) Lower Oxygen Utilization

4) Increased Oxidant Leakage

L-Carnitine/Acetyl-L-Carnitine (ALCAR)

• Mediates the ratio of acetyl-CoA/CoA• Decreases with age in plasma and in brain• Improves cognitive function in rats• ALCAR is something beneficial!! And we lose it with age!

12

• Transports long-chain fatty acids into mitochondria

• Removes short- and medium-chain fatty acids that accumulate

• Feeding ALCAR to aging rats is able to suppress or ameliorate many age-related changes in mitochondrial function and oxidant stress.

• restores membrane potential

• although oxidant leakage remains

Effect of ALCAR Supplementation on Cardiolipin Levels—ALCAR increased cardiolipin levels. Cardiolipin is basically a marker of mitochondrial health

Young

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• R- -Lipoic Acid (LA)– Potent Anti-oxidant– Lowers oxidants in old rats– Restores ascorbic acid levels (vitamin C)– Restores glutathione levels (another anti-

oxidant)– Has been shown to be beneficial to rats in

combination with ALCAR

• Behavior of old rats fed ALCAR and LA– ALCAR/LA can reverse age-associated losses

in cognitive function – Increased ambulatory activity

Ambulatory Activity before and After Supplementation with Lipoic Acid (LA) + Acetyl-

L-Carnitine (ALCAR)

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• As organisms age…

• - mitochondrial potential decreases.

• - oxidant leakage increases.

• Do americans get enough micronutrients? NO• Iron deficiency, results in loss of complex 4 (part of

the electron transport chain in the mitochonrial membrane).– Complex 4 loss results in increased oxidative stress.– Iron deficiency mimics neurodegeneration.

• Zinc deficiency– Increased DNA damage in zinc deficient cells

• Biotin deficiency– Accelerates cell senescence

• Magnesium deficiency

Other types of oxidative damage increase with age

• Protein Carbonylation (Stadtman)• - Lipid oxidation (aldehydes "stuck" to

proteins/enzymes)• - age pigment -fluorescent lipid peroxides

accumulate with age, can see fluorescence in tissues of older individuals (humans and rats)

• Mitochondria may deteriorate with time, old or damaged mitochondria may be destroyed by lysosomes in the cytoplasm.

Senescence

• Be familiar with:– What senescence is– What the types of senescence are– What causes senescence– Why senescence is important to aging and

disease

Cellular Senescence

What is it?

Response of normal cells to potentially cancer-causing events

Cellular Senescence

What causes it?(what causes the senescent phenotype?)

Cell proliferation (replicative senescence)= TELOMERE SHORTENING

DNA damage

Oncogene expression

Supermitogenic signals

First description: the Hayflick limit

Pro

lifer

ativ

e ca

paci

ty

Number of cell divisions

FiniteReplicativeLife Span"Mortal"

InfiniteReplicativeLife Span"Immortal"

EXCEPTIONSGerm line

Early embryonic cells (stem cells)Many tumor cells

What happens when cells exhaust their replicative life span

REPLICATIVE SENESCENCE

•Irreversible arrest of cell proliferation(universal)

•Resistance to apoptosis(stem cells)

•Altered function(universal but cell type specific)

SENESCENT PHENOTYPE

Inducers of cellular senescence

Cell proliferation(short telomeres)

DNA damage

Oncogenes

Strong mitogens

PotentiallyCancerCausing

Normal cells(mortal)

Immortal cells(precancerous)Inducers

of senescence

Cell senescence Transformation Apoptosis

Tumor suppressor mechanisms

p53 and pRB proteins

• Nuclear proteins controlled by complex pathways(upstream regulators and downstream effectors)

•Control expression of other genes

•Halt cell cycle progression in response to inducersof senescence

•Crucial for allowing normal cells to sense and respondto senescence signals

Cellular SenescenceAn important tumor suppressor mechanism

What does cellular senescence have to do with aging?

•The senescent phenotype entails changes in cell function

•Aging is a consequence of the decling forceof natural selection with age

Antagonistic pleiotropyCellular senescence

Selected for tumor suppression (growth arrest)

Functional changes unselected, deleterious

FUNCTIONAL CHANGES ASSOCIATED WITH CELLULAR SENESCENCE:

Secretion of molecules that can be detrimental to tissues if not controlled

e.g., senescent fibroblasts secrete proteases, growth factors,inflammatory cytokines

Functional Assessment

• Be familiar with:– What comprises geriatric assessment– What programs are used to test assessment and

what are their parameters– Why do women have more disability– Compare/Contrast aging with physical

inactivity• Know some parameters

– Aging vs. Disease

Geriatric AssessmentInvolves a multi-dimensional diagnostic process designed

to qualify an elderly individual in terms of:

• Functional capabilities• Disabilities

• Medical & Psychological characteristics

A list of typical assessments is summarized in Table 3.3

For our discussion, we will consider particularly: • Activities of Daily Living (ADL)

• Instrumental Activities of Daily Living (IADL) **See Table 3.4**

Assessment Programs include tests that are grouped into three categories:

1. Tests examining general physical health

2. Tests measuring ability to perform basic self care (ADLs)

3. Tests measuring ability to perform more complex activities (IADLs), reflecting the ability to live independently in the community

Table 3-4 Categories of Physical Health Index MeasuringPhysical Competence

ACTIVTIES INSTRUMENTAL ACTIVITIESOF DAILY LIVING OF DAILY LIVING

Feeding CookingBathing CleaningTo ileting Using telephoneDressing WritingAmbulation ReadingTr ansfer from toilet LaundryVisual acuity Driving a carOthers Others

Figure 3. 6: % of persons 70 years & older having difficulty/inability to perform ADLs & IADLs

With advancing age, 1) disability intensity increases in men & women; 2) disability intensity is higher in women than in men at the same age (esp. at later ages); 3) females live a longer average life span but live longer with disability

Table 3-6 Physiologic Parameters in Aging, Physical Inactivity Weightlessness (In Space) Reduced Increased

Maximum oxygen consumption Systolic blood pressure and peripheral resistance

Resting and maximum cardiac output Vestibular sensitivity Stroke volume Serum total cholesterol Sense of balance Urinary nitrogen and creatinine Body water and sodium Bone calcium Blood cell mass Lean body mass Glucose tolerance test Variable Sympathetic activity and neurotransmission Endocrine changes Thermoregulation Altered EEG Immune responses Altered sleep

Changes in specific senses

Aging is associated with increased incidence of:

• Diseases

• Accidents

• Stress

Aging should be differentiated from Disease

Disease:• is selective; it varies with the species, tissue,

organ, cell molecule

• may depend on intrinsic & extrinsic factors

• is discontinuous (may progress, regress or be arrested)

• is occasionally deleterious, damage is often variable/reversible

• is often treatable with known cause(s)

Assessment of Physiological Age in Humans

Physiological age depends on

Physiologic competence: good to optimal function of all body systems

&

Health status: absence of disease

Physiological age may or may not coincide with chronological age

Table 3-1 Physiologic Correlates with Longevity

INDEX STUDIED CORRELATION

Body weight Direct

Brain/ body weight Direct

Basal metabolic rate Inverse

Stress Inverse

Reproductive function/Fe cundity Inverse

Length of growth period DirectEvolution Uncertain

Epidemiology

• Be familiar with:– What is epidemiology and why is it useful– Why is it thought that older people are at an

elevated risk for disease– What are the major age associated causes of

death– Understand why falls are a problem for the

elderly

EPIDEMIOLOGY OF AGING

• THE STUDY OF THE AGE-RELATED DISTRIBUTION AND CAUSES OF DISEASE, DISABILITY, AND MORTALITY IN HUMAN POPULATIONS.

EPIDEMIOLOGY OF AGING

• ACCUMULATION OF ENVIRONMENTAL/BEHAVIORAL INSULTS.

• REDUCED IMMUNOLOGICAL SURVEILLANCE

EPIDEMIOLOGY OF AGING

• WHY IMPORTANT?– AGING OF THE HUMAN POPULATION– HEALTH AND VITALITY OF AN AGING

POPULATION– QUALITY OF LIFE AND COST OF CARE

EPIDEMIOLOGY OF AGING

• MAJOR AGE-ASSOCIATED CAUSES OF DEATH

– CANCER

– CARDIOVASCULAR DISEASE

– CHRONIC OBSTRUCTIVE PULMONARY DISEASE

– DIABETES

EPIDEMIOLOGY OF AGING

AGE-SPECIFIC COLORECTAL CANCER INCIDENCE RATES

(Per 100,000 in population)

WM WF BM BF

<65 20.4 14.7 25.3 20.4

65+ 408.0 269.3 385.8 286.1

EPIDEMIOLOGY OF AGING

COGNITIVE FUNCTION

Moderate/Severe Memory Impairment Male Female

65-69 5.3 3.8

85+ 37.3 35.0

EPIDEMIOLOGY OF AGING

• FALLS

• 30% OF PEOPLE AGED 65+ FALL EACH YEAR.

• 10-15% OF THOSE FALLS ARE CONSIDERED “SERIOUS/NON-FATAL”

• FALLS REPRESENT THE LEADING CAUSE OF ACCIDENTAL DEATH IN PEOPLE AGED 65 AND OLDER.

• FEAR OF FALLING IS A LEADING REASON FOR NOT ENGAGING IN PHYSICAL ACTIVITY.

EPIDEMIOLOGY OF AGING

• CAUSES OF FALLS IN THE ELDERLY

• - DIZZINESS

• - POOR COGNITIVE FUNCTION

• - VISION PROBLEMS

• - GENERAL FRAILTY

• - ENVIRONMENTAL HAZARDS

Telomeres

• Be familiar with:– What telomeres are– Why telomeres are important– Consequence of telomere end-shortening– What telomerase is– The telomere hypothesis of aging

Telomeres

Ends of linear chromosomes

Centromere

TelomereTelomere

Repetitive DNA sequence(TTAGGG in vertebrates)

Specialized proteins

Form a 'capped' end structure

Why are telomeres important?

Telomeres allow cells to distinguish chromosomesends from broken DNA

Stop cell cycle!Repair or die!! Homologous recombination

(error free, but need nearby homologue)

Non-homologous end joining(any time, but error-prone)

Why are telomeres important?Prevent chromosome fusions by NHEJ

NHEJ

Mitosis

FUSIONBRIDGE

BREAKAGE

Fusion-bridge-breakage cycles

Genomic instability

Cell death OR neoplastic transformation

Telo

mere

Len

gth

(h

um

an

s)

Number of Doublings

20

10

Cellular (Replicative) Senescence

Normal Somatic Cells

(Telomerase Negative)

Telomere also provide a means for "counting" cell division: telomeres shorten with each cycle

Telomeres shorten from 10-15 kb(germ line) to 3-5 kb after 50-60 doublings

(average lengths of TRFs)

Cellular senescence is triggered whencells acquire one or a few critically short telomeres.

TELOMERASE:Key to replicative immortality

Enzyme (reverse transcriptase) with RNA and protein components

Adds telomeric repeat DNA directly to 3' overhang (uses its own RNA as a template)

Vertebrate repeat DNA on 3' end:TTAGGG

Telomerase RNA template:AAUCCC

The telomere hypothesis of aging

Telomeres shorten with each cell division and therefore with age

TRUE

Short telomeres cause cell senescence andsenescent cells may contribute to aging

TRUE

HYPOTHESIS:Telomere shortening causes aging and

telomerase will prevent agingTRUE OR FALSE?

The telomere hypothesis of aging

Telomere length is not related to life span(mice vs human; M musculus vs M spretus)

Telomeres contribute to aging ONLY if senescent cells contribute to aging

Telomerase protects against replicativesenescence but not senescence induce by

other causes

Evolutionary Theory of Lifespan

• Be familiar with:– Evolutionary theory of lifespan

– Aging in nature

– Disposable soma

– Antagonistic Pleiotropy

– Mutation Accumulation

– Trait that correlate with longevity

– Model systems that agree and disagree with theory

%Alive

1 2 3 4 5 6 7 8age in years

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Aging in Nature

- Most organisms do not age in a natural environment.

Life Span in the LabLife Span in Nature

Aging BeginsNatural Selection

Lifespan and extrinsic mortality:

-If mortality is high an organism will die frompredation or other hazards before it grows old.

-Therefore, if extrinsic mortality limits survival there is no reason to evolve a life spanthat is longer than an organism would

normally survive in nature.

Evolutionary Theories of Aging

Disposable Soma - Somatic cells are maintained only to ensure

continued reproductive success, following reproduction

the soma is disposable. (life span theory)

Antagonistic Pleiotropy - Genes that are beneficial at younger

ages are deleterious at older ages.(Pleiotropism = The control by a single gene of several distinct

and seemingly unrelated phenotypic effects)

Mutation Accumulation - Mutations that affect health at older

ages are not selected against (no strong evidence).

Opossums and Life Span

- ultimate prey, ~ 80% die from predation- typically reproduce once- age very rapidly

-Hypothesis: The presence of predators limits life span, naturalselection favors somatic maintenance for only as long as an average opossum can be expected to live.

Steve Austad, U. of Idaho

-How could you test this hypothesis?

Sapelo Island Opossums

- no predators (out in daytime)- longer average life span- reproduce twice (fewer offspring/litter)

-Are these changes due to a lack of predators, or a physiological change that delays the aging process?

Physiological Change - Sapelo island opossums not onlylive longer, they age slower than mainland animals.

-Sapelo Island opossums have less oxidative damage than mainland opossums.

(collagen X-linking)

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Evolution in the Laboratory

old flies selected

young flies selected

Normal

% S

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Age in Days

- Early adult fecundity increased *antagonistic pleiotropy

Offspring of “young” flies are selected

Summary of Drosophila Selection

1) Selection at age of reproduction can alter the lifespan of Drosophila (lifespan has been doubled by this technique).

2) Increase in lifespan has a cost, reduced fecundity (reproduction). - antagonistic pleiotropy -

3) Long-lived flies are stress resistant (heat shock, oxidants).

p53 may promote aging…

p53

Cancer Aging

Why did we evolve a system thatlimits our lifespan?

-to protect against cancer! (Antagonistic Pleiotropy again!)

Life Span versus Aging

Aging - can not be selected for, results from an absenceof natural selection.

Life Span - results from selection and extrinsic mortality

Environmental Selection - predators, natural hazards

Social Selection - parental investment, sexual behavior

Main Ideas

1. Life span results from selective pressure.

2. Life span is inversely proportional to extrinsic mortality.

3. Aging results from a lack of natural selection with age.

Oxidants and Anti-Oxidants

• Be familiar with:– What is the Free Radical Theory of Aging

– What is a free radical

– What are the implications of free radicals on aging

– How oxygen can be toxic

– What the major oxidants are and what are oxidant sources

– What are the major antioxidants

– Experimental Models

Free Radicals

• Free radicals are unstableFree radicals are unstable• React quickly with other compounds, React quickly with other compounds,

doing cell and body damagedoing cell and body damage• Once produced, they multiply unless Once produced, they multiply unless

neutralized by anti-oxidants or other neutralized by anti-oxidants or other free-radical scavengers.free-radical scavengers.

• Free radicals rarely occur in natureFree radicals rarely occur in nature• Oxygen can have several unpaired Oxygen can have several unpaired

electron “pairs”electron “pairs”

The Free Radical Theory of Aging

““Aging results from the deleterious Aging results from the deleterious effects of free radicals produced in effects of free radicals produced in the course of cellular metabolism”the course of cellular metabolism”

Harman D., Aging: A theory based on free radical Harman D., Aging: A theory based on free radical and radiation chemistry, J. Gerontol. 11: 298, and radiation chemistry, J. Gerontol. 11: 298, 19561956

Free Radical Chemistry

• ReactiveReactive radicals attack indiscriminately radicals attack indiscriminately• Can add to unsaturated bondsCan add to unsaturated bonds• Can abstract electrons or hydrogen atomsCan abstract electrons or hydrogen atoms• Propagate chain reactionsPropagate chain reactions• Can cause bond scissionCan cause bond scission• Can cause crosslinkingCan cause crosslinking

– Crosslinking - Formation of bonds among polymeric chainsCrosslinking - Formation of bonds among polymeric chains

• Produce secondary toxic agentsProduce secondary toxic agents

Implications for Aging

• Some free radical-induced chemical Some free radical-induced chemical modifications may have unique impactsmodifications may have unique impacts

• Crosslinked products may not be degradableCrosslinked products may not be degradable• Scission of bonds in DNA, particularly Scission of bonds in DNA, particularly

multiple events may erase vital informationmultiple events may erase vital information

What are the Major Oxidants?

• Hydroxyl radical (OHHydroxyl radical (OH..))• Hypochlorite (HOCl)Hypochlorite (HOCl)• Singlet oxygen Singlet oxygen 11OO22

• Peroxynitrite (OONOPeroxynitrite (OONO--))• Hydrogen peroxide (HHydrogen peroxide (H22OO22))• Free or loosely-bound iron, copper or hemeFree or loosely-bound iron, copper or heme

• Superoxide radical (OSuperoxide radical (O22..--))

• Nitric oxide (NONitric oxide (NO..))

Lipid Peroxidation

• PUFAs* contain weakly bonded hydrogen PUFAs* contain weakly bonded hydrogen atoms between double bondsatoms between double bonds

• Chain reactions are probable because of Chain reactions are probable because of high local concentrations of double bondshigh local concentrations of double bonds

*PUFAs means polyunsaturated fatty acids*PUFAs means polyunsaturated fatty acids

Oxidant SourcesTable 5.1

• Enzymes involved in Enzymes involved in cell signalingcell signaling

• Immune cellsImmune cells

• ““Leaky” electron transportLeaky” electron transport

• Damaged proteins and lipidsDamaged proteins and lipids

• Toxins (food, water)Toxins (food, water)

• SmokeSmoke

• Irradiation (UV)Irradiation (UV)

Regulated Unregulated

Major AntioxidantsTable 5.2

• Vitamins E and CVitamins E and C• Thiols, particularly glutathioneThiols, particularly glutathione• Uric acidUric acid• Superoxide dismutases (Cu/Zn or Mn SOD)Superoxide dismutases (Cu/Zn or Mn SOD)• Catalase and glutathione peroxidaseCatalase and glutathione peroxidase• Heme oxygenasesHeme oxygenases• Protein surface groups (Msr)Protein surface groups (Msr)

Glucose and Oxidants

• In cell culture models high glucose correlates In cell culture models high glucose correlates with oxidant productionwith oxidant production

• Three diabetes-linked effects can be Three diabetes-linked effects can be correlated with superoxide productioncorrelated with superoxide production

• Insulin pathway and life-span extensionInsulin pathway and life-span extension

Are Oxidants the Cause of Aging? (Table 5.8)

ProPro ConCon

•Caloric restriction may reduce Caloric restriction may reduce oxidative stressoxidative stress

•Life span extension in mutants may Life span extension in mutants may be associated with stress resistancebe associated with stress resistance

•Knockout mice lacking MnSOD Knockout mice lacking MnSOD have restricted survivalhave restricted survival

•Enzyme mimetics extend life span Enzyme mimetics extend life span in some aging models.in some aging models.

•Some drugs, probably acting as Some drugs, probably acting as antioxidants, have been claimed to antioxidants, have been claimed to extend lifespan.extend lifespan.

•Vitamin C, a superb free radical Vitamin C, a superb free radical scavenger, is not synthesized by long-lived scavenger, is not synthesized by long-lived primates.primates.

•Chronic radiation in low doses does not Chronic radiation in low doses does not shorten life span (may increase it).shorten life span (may increase it).

•Dietary supplementation with Vitamin E Dietary supplementation with Vitamin E and C does and C does notnot extend life span. extend life span.

•Tissue comparison (brain vs. muscle) Tissue comparison (brain vs. muscle) seems incompatible with seems incompatible with oxidant/antioxidant models of aging.oxidant/antioxidant models of aging.

•Exercise, that increases oxidant stress, Exercise, that increases oxidant stress, improves life span.improves life span.

Neurodegeneration, Repair, and Plasticity

Neurodegeneration, Repair, and Plasticity

• Neuroendocrine theory of aging:

Alterations in either the number or the sensitivity of various neuroendocrine receptors gives rise to homeostatic or homeodynamic changes that result in senescence.

Neuron regeneration?How is that possible?

• While until the 1990s we thought neurons couldn’t regenerate, now we’ve seen that certain neurons have the potential to regenerate under specific circumstances. – Neurons in lining of cerebral ventricles– Hippocampus– Neuroglia (astrocytes and oligodendrocytes)– Microglia (“macrophages” of nervous system)

What conditions favor regeneration?

• Whole body:– Exercise

– Nutrition

– Some stress

– Education

– Good circulation

• Neural microenvironment– Brain metabolism

– Hormonal changes

Education and death rates

• Higher education, lower death rates

• Higher education, lower disability

• Higher income, lower death rates

• Comments?

Why would education decrease death rates?

• Access to medical care

• Access to exercise

• Better nutrition

• Higher income

• Responsibility to health behaviors

• Lower rates of smoking and alcohol

Brain reserve capacity

• When these things (listed in previous slide) happen in young life, brain reserve capacity built

• This means more neuronal branches and more axonal/dendritic connections, better brain blood supply

• Evidence: a person with more education has longer dendritic branching length and more connections

• With old age there is denudation of neurons—less branching

Yeast as a Model for Lifespan

• Be familiar with:– A molecular cause of yeast aging

– SIR2

– Aging and genetic instability, in yeast and humans

Cellular senescence: finite replicative capacity of

mitotically dividing cells• Originally observed in human

diploid fibroblasts (Hayflick limit, 1965)

• Represents a limit on the number of population doublings

• Caused by telomere shortening in cells that do not express telomerase

What about simple eukaryotic cells that do express telomerase?

• Cells of baker’s yeast, Saccharomyces cerevisiae, express telomerase

• Microbial populations are “immortal”, can be passaged forever

• Does this mean these cells are also immortal?

What is the role of telomere length in yeast cellular

senescence?

• Telomerase is expressed throughout the lifespan

• Telomere length is maintained throughout the lifespan

• Mutating telomerase does cause cellular senescence: telomere shortening, limited population doublings, genomic instability, ALT

What causes yeast aging?

• A clue: exceptions to the rule of the resetting clock• Occasionally, daughters of old mothers are born prematurely

aged!• Their lifespan equals the mother’s remaining lifespan

• The asymmetry has broken down -- accompanied by loss of size asymmetry (“symmetric buds”)

• The daughters of symmetric buds have normal lifespan• Suggests these symmetric buds have inherited a “senescence

factor”…

What is the yeast senescence factor?

• Some clues (late 1990s):– Aging is accompanied by fragmentation of

the nucleolus– The nucleolus assembles at the site of rRNA

transcription, the rDNA– Normally, sir2 tries to keep rdna from popping out.– sir2 mutants have a short lifespan– sir2 mutants have high levels of

extrachromosomal rDNA circles (ERCs)– ERCs have the characteristics of the

senescence factor…

Extrachromosomal rDNA Circles as a cause of yeast aging

• Excised from the chromosomal array by recombination• Recombination is suppressed by Sir2, a gene that keeps

rDNA circles from “popping out”• Replicate nearly every cell cycle• Have a strong mother segregation bias at mitosis• High levels can inhibit cell division• Inherited by the daughters of old mothers

But, no ERCs in humans!(or mice, or worms, or flies…)

Why continue to study yeast aging?

• Overexpressing SIR2 homologs in flies and

worms extends lifespan

• Perhaps the regulation of lifespan is

conserved (and SIR2-dependent) while the

molecular effectors of aging vary between

organisms

• Example: calorie restriction (CR)—data is

not resolved on this

Calorie Restriction (CR) Extends Lifespan

• Decreasing caloric intake (without starvation) lengthens lifespan

• Works in yeast, flies, rats, mice, worms, …• Many reports claimed that the CR pathway is

SIR2-dependent, supporting theory of SIR2 as master aging regulator

• Heated debate over the mechanism by which SIR2 influences CR pathway

• Recent work has shown that in yeast CR is actually SIR2-independent

Genetic instability and Aging

• Frequencies of mutations and chromosomal rearrangements increase with age in various organisms

• Incidence of cancer increases dramatically with age:

• Is this due to accumulation of genetic events at a constant rate over the lifetime, or does aging itself alter the rate of new genetic events?

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An Age-induced Hyper-recombinational State

• After about 25 divisions, aging mother cells begin to produce daughters that are genetically unstable

• High rates of LOH at multiple chromosomes

• LOH is caused by recombination, not chromosome loss or deletion

• Behaves as a “switch” to a new, unstable state

• Hyper-recombinational state is eventually “diluted” in progeny of old cells

This is reminiscent of the Yeast Senescence Factor!

• Something accumulates with each cell division in mother

• Reaches a threshold, causes genetic instability• Inherited by daughters of old mothers• Eventually “reset” in distant progeny

Are ERCs the cause?

• Mutations that increase ERCs (sir2) do not accelerate onset of switch

• Mutations that decrease ERCs do not delay onset of switch

• In fact, onset of switch is unlinked to lifespan!

• Suggests an important distinction between longevity and functional senescence! This is key!! There could be different senescent factors affecting different processes of senescence in cell—ie one affecting genomic instability and one affecting division

How does Yeast Aging relate to Cellular Senescence in Humans?

• Telomere-independent

• Asymmetrically dividing cells

• For what cell type is this a model?

Stem cells in human aging and cancer

• Yeast aging may be a model for continuously dividing, asymmetrically dividing stem cells. And they express telomerase

Conclusions

• Yeast aging involves longevity regulation as well as senescence phenotypes unlinked from longevity

• Genetic instability increases with age in yeast, by an epigenetic hyper-recombinational switch (not by accumulating gene mutations but by a switch method)

• May be a good model for stem cell aging