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Discussion 2lectures 6-8
Ryan Klimczak
February 5th, 2007
Lecture 6 - Cellular Senescence
Lecture 7 - Telomeres
Lecture 8 - Diseases of Aging
Feeding acetyl-L-carnitine and lipoic acid to old rats significantly improves metabolic function while decreasing oxidative stressTory M. Hagen*, Jiankang Liudagger ,Dagger , Jens Lykkesfeldt§, Carol M. Wehrdagger , Russell T. IngersollDagger , Vladimir Vinarskydagger , James C. Bartholomew¶, and Bruce N. Amesdagger , et al.
-ALCAR supplements improve membrane potential, caused in part by replenishment of carnitine, a betaine that shuttles fatty acids into the mitochondrion for beta-oxidation
-Increases cardiolipin levels
-Overall, improves metabolic function, improving substrate and electron flux
-However, addition of higher levels of ALCAR can actually lead to increased ROS production from increased electron flow in the electron transport chain.
-Rationale of using LA in synergy with ALCAR is its antioxidant potential.
-Also improves mitochondrial bioenergetics as a cofactor for pyruvate dehydrogenase and aplha-ketoglutarate dehydrogenase
-Shown to enhance glucose uptake as well.
-The complementation of ALCAR and LA thus has a mutually beneficial effect on metabolic function.
Cellular Senescence
What is it?
Response of normal cells to potentially cancer-causing events
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
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
Cellular Senescence
What causes it?(what causes the senescent phenotype?)
Cell proliferation (replicative senescence)= TELOMERE SHORTENING
DNA damage
Oncogene expression
Supermitogenic signals
What do inducers of the senescentphenotype have in common?
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
Cellular SenescenceAn important tumor suppressor mechanism
•Induced by potentially oncogenic events
•Most tumor cells are immortal
•Many oncogenes act by allowing cells to bypassthe senescence response
•Senescence is controlled by the two most importanttumor suppressor genes -- p53 and pRB
•Mice with cells that do not senesce die youngof cancer
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 declining forceof natural selection with age
Aging before cell phones ……
100%
Su
rviv
ors
AGE
Natural environment: predators, infections, external hazards, etcMost of
humanevolution
Modern, protectedenvironment
(very VERY recent)
Antagonistic pleiotropy:Some traits selected to optimize fitness in young
organisms can have unselected deleteriouseffects in old organisms
(what's good for you when you're young may be bad for you when you're old)
EPITHELIUMBasement Membrane
STROMA
Senescent Epithelial Cell
Senescent Fibroblast
EPITHELIUMBasement Membrane
STROMA
YOUNG TISSUE
OLD TISSUE
Degradative enzymes, Inflammatory cytokines, etc.
AGING ?
EpithelialCells
Fibroblasts
EPITHELIUMBasement Membrane
STROMA
Senescent Epithelial Cell
Senescent Fibroblast
OLD TISSUE
Degradative & inflammatory molecules, growth factors, etc
AGING ?
EPITHELIUMBasement Membrane
STROMA
YOUNG TISSUE "Initiated" Cell
Neoplastic Growth
Senescent cells can strongly alter tissue microenvironments.May contribute to age-related declines in tissue structure and function, andage related disease
P53
p53 has many anti-cancer mechanisms: ・
-It can activate DNA repair proteins when DNA has sustained damage.
-It can also hold the cell cycle at the G1/S regulation point on DNA damage recognition
-It can initiate apoptosis, the programmed cell death, if the DNA damage proves to be irreparable.
.
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
PRB
-pRb prevents the cell from replicating damaged DNA by preventing its progression through the cell cycle into its S or progressing through G1
-pRb binds and inhibits transcription factors of the E2F family. E2F transcription factors are dimers of an E2F protein and a DP protein.
-The transcription activating complexes of E2 promoter-bindingミprotein-dimerization partners (E2F-DP) can push a cell into S phase. As long as E2F-DP is inactivated, the cell remains stalled in the G1 phase.
-When pRb is bound to E2F, the complex acts as a growth suppressor and prevents progression through the cell cycle. The pRb-E2F/DP complex also attracts a protein called histone deacetylase (HDAC) to the chromatin, further suppressing DNA synthesis
Questions
1. What causes cellular senescence, what are the inducers and what do they have in common?
2. What is replicative senescence?
3. List 3 characteristics of the senescent phenotype.
4. What is the relationship between carcinogenesis, aging, and senescence?
5. Explain antagonistic pleiotropy
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)
The importance of telomeres (con’t)
•Prevent chromosome fusion by non-homologous end joining
•Provide a means for counting cell division
•They resolve the end replication problem
Ori
DNA replication is bidirectional
Polymerases move 5' to 3'
Requires a labile primer
3'
5'3'
5'5'
5' 3'3' 5'
Each round of DNAreplication leaves
50-200 bp DNA unreplicatedat the 3' end
Replicatively immortal cells bypass the restrictions telomeres impose by using the enzyme telomerase
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
HOWEVER,
CELLS THAT EXPRESS TELOMERASE
STILL UNDERGO SENESCENCE
(E.G., IN RESPONSE TO DNA DAMAGE, ONCOGENES, ETC.)
Inducers of cellular senescenceCell proliferation(short telomeres)
DNA damage OncogenesStrong mitogens/
stress
Potential Cancer Causing Events
The telomere hypothesis of aging
Telomeres shorten with each cell division and therefore with age
Short telomeres cause cell senescence andsenescent cells may contribute to aging
HYPOTHESIS:Telomere shortening causes aging and
telomerase will prevent agingTrue or False?
SUMMARY
Telomeres are essential for chromosome stability
Telomere shortening occurs owing to the biochemistry ofDNA replication
Short telomeres cause replicative senescence (other senescence causes are telomere-independent)
Telomerase prevents telomere shortening andreplicative senescence
The telomere hypothesis of aging depends on the cellular senescence hypothesis of aging
Discussion Questions:
How do the looped structures of telomeres promote chromosomal stability?
What is the correlation between aging, cellular senescence, and telomere length?
Why do cells that express telomerase still undergo senescence?
Diseases of Aging
Disease may be viewed as a process that is :
• Selective (i.e., varies with the species, tissue, organ, cell and molecule)
• Intrinsic and extrinsic (I.e., may depend on environmental and genetic factors)
• Discontinuous (may progress, regress, or be arrested)• Occasionally deleterious (damage is often variable, reversible)• Often treatable (with known etiopathology, cure may be available)
Tables 3-11 Di seases of the Elderly
Limited to aging
Osteoporosis Osteoarthritis Prostatic adenocarcinoma Polymyalgia rheumatica Temporal arteritis
Associated with aging
Known etiology Septicemia Pneumonia Cirrhosis Nephritis Cerebrovascular disease Myocardial infarction
Unknown etiology Adult-onset, Type 2 diabetes Neoplasm Hypertension Alzheimer's disease Parkinson's disease Emphysema
Causes of Death According to AgeAll Races, Both Sexes
Ages: 65-84 Ages: 85+2003 2003
1. Heart disease (28.2%) 1. Heart Disease (36.2%)2. Cancer (27.7%) 2. Cancer (11.6%)3. COPD‡ (7.1%) 3. Stroke (9.4%)
4. Stroke (6.6%) 4. Alzheimer’s Disease (5.5%)5. Diabetes Mellitus (3.6%) 5. Pneumonia/Influenza (4.6%)
6. Pneumonia/Influenza (2.4%) 6. COPD‡ (4.4%)7. Alzheimer’s Disease (2.2%) 7. Diabetes Mellitus (2.2%)
8. Nephritis (1.9%) 8. Nephritis (2%)9. Accidents (1.9%) 9. Accidents (1.9%)10.Septicemia (1.5%) 10.Septicemia (1.4%)
- All others (17%) - All others (20.9%)
Diseases as a tool for studying aging:
Syndromes in humans: having multiple characteristics of premature (early onset) of aging, or
accelerated (rapid progression) of aging
Infantile Progeria: Hutchinson-Gilford SyndromeAdult onset progeria: Werner’s syndrome
Down syndrome
Hutchinson-Gilford Progeria Syndrome:
-Hutchinson-Gilford Progeria syndrome is an extremely rare genetic condition which causes physical changes that resemble greatly accelerated aging in sufferers.
-Affects between 1 in 4 million (estimated actual) and 1 in 8 million (reported) newborns. Currently, there are approximately 40-45 known cases in the world.
-Most people with progeria die around 13 years of age
-It is caused by mutations in a LMNA (Lamin A protein) gene on chromosome 1.
-Nuclear lamina is a protein scaffold around the edge of the nucleus that helps organize nuclear processes such as DNA and RNA synthesis.
-Lamin A cannot be produced and Prelamin A builds up on the nuclear membrane, causing a characteristic nuclear blebbing. This results in the premature aging symptoms of progeria, although the mechanism connecting the misshapen nucleus to the symptoms is not known.
-The condition causes wrinkled skin, atherosclerosis and cardiovascular problems. Mental development is not affected.
-The development of symptoms is comparable to aging at a rate six to eight times faster than normal, although certain age-related conditions do not occur. Specifically, victims show no neurodegeneration or cancer predisposition. The people diagnosed with this disease usually have fragile elderly-like bodies.
Werner Syndrome
-The gene responsible for Werner syndrome (WRN) was identified (and found to be a member of the RecQ family of helicases. -The Werner protein is thought to perform several tasks in the cell, including the maintenance and repair of DNA. It also assists in making copies of DNA in preparation for cell division. Mutations in the WRN gene often lead to the production of an abnormally short Werner protein. -Some research suggests that this shortened protein is not sent to the nucleus, where it normally interacts with DNA. Evidence also suggests that the altered protein is broken down quickly in the cell, leading to a loss of Werner protein function.-Research into the biological role of the WRN protein is ongoing, but current evidence strongly suggests a role for WRN in the resolution of Holliday junctions. Roles in non-homologous end joining (NHEJ) and the restoration of stalled replication forks have also been suggested.
-Individuals with this syndrome typically grow and develop normally until they reach puberty. Following puberty, they age rapidly, so that by the time they reach age 40 they often appear as though they are several decades older.
-Overall, people affected by Werner syndrome have thin arms and legs and a thick trunk. Affected individuals typically have a characteristic facial appearance described as "bird-like" by the time they reach their thirties. Patients with Werner sydrome also exhibit genomic instability and various age-associated disorders; these include cancer, heart disease, atherosclerosis, diabetes mellitus, and cataracts. However, not all characteristics of old-age are present in Werner patients; for instance, senility is not seen in individuals with Werner syndrome. People affected by Werner syndrome usually do not live past their late forties or early fifties, succumbing to death, often resulting from cancer or heart disease.
Discussion Questions:
What are the two most common causes of death in individuals over the age of 50? What reasons underlie this trend?
How do the symptoms of Hutchinson-Gilford Progeria Syndrome and Werner Syndrome mimic the characteristics of ‘normal’ aging? How are they different?
Hypothesize how the genetic mutations responsible for these two syndromes lead to ‘accelerated aging’ phenotypes?