MCB 135K Mid-Term I Review February 15, 2005 GSI: Laura Epstein.
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Transcript of MCB 135K Mid-Term I Review February 15, 2005 GSI: Laura Epstein.
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
160
140
120
100
80
60
40
20
0
Brain Liver Heart Kidney Lung
* *
*
**
6
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
Ca
rdio
lipin
(µ
g p
er 1
0 c
ells
)
30
20
10
0
14
+ ALCAR
Old
**
• 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)
0
200
400
600
800
+ L
A +
AL
CA
R
OldYoung
+ L
A +
AL
CA
R
*
Dis
tan
ce
Tra
vel
ed
(c
m/h
ou
r/d
ay
)
*#
#
vs. young
vs. old
*
• 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
100
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)
0
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1200 4 8
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32
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10
0
Evolution in the Laboratory
old flies selected
young flies selected
Normal
% S
urv
ivin
g
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?
0%
5%
10%
15%
20%
25%
30%
35%
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5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 95+
Age
Cu
mu
lati
ve p
rob
abil
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of
dev
elo
pin
g
can
cer
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