Download Discussion 2 - Molecular and Cell Biology

Discussion 2
lectures 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 stress
Tory 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
Proliferative capacity
First description: the Hayflick limit
Finite
Replicative
Life Span
"Mortal"
Infinite
Replicative
Life Span
"Immortal"
Number of cell divisions
EXCEPTIONS
Germ 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 senescent
phenotype have in common?
Inducers of cellular senescence
Cell proliferation
Potentially
Cancer
Causing
(short telomeres)
DNA damage
Oncogenes
Strong mitogens
Normal cells
(mortal)
Cell senescence
Inducers
of
senescence
Immortal cells
(precancerous)
Transformation
Apoptosis
Tumor suppressor mechanisms
Cellular Senescence
An important tumor suppressor mechanism
•Induced by potentially oncogenic events
•Most tumor cells are immortal
•Many oncogenes act by allowing cells to bypass
the senescence response
•Senescence is controlled by the two most important
tumor suppressor genes -- p53 and pRB
•Mice with cells that do not senesce die young
of cancer
Cellular Senescence
An 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 force
of natural selection with age
Aging before cell phones ……
Modern, protected
environment
(very VERY recent)
Survivors
100%
Most of
human
evolution
Natural environment: predators,
infections, external hazards, etc
AGE
Antagonistic pleiotropy:
Some traits selected to optimize fitness in young
organisms can have unselected deleterious
effects in old organisms
(what's good for you when you're young may be
bad for you when you're old)
Senescent cells can strongly alter tissue microenvironments.May
contribute to age-related declines in tissue structure and function, and
age related disease
YOUNG TISSUE
YOUNG TISSUE
"Initiated" Cell
EPITHELIUM
EPITHELIUM
Epithelial
Cells
Basement Membrane
STROMA
Fibroblasts
Basement Membrane
STROMA
AGING ?
AGING ?
Senescent
Epithelial Cell
Senescent
Epithelial Cell
OLD TISSUE
OLD TISSUE
EPITHELIUM
EPITHELIUM
Basement Membrane
Basement Membrane
STROMA
STROMA
Degradative enzymes,
Inflammatory cytokines, etc.
Neoplastic
Growth
Senescent Fibroblast
Degradative & inflammatory
molecules, growth factors, etc
Senescent Fibroblast
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 a
TIFF (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.
2.
3.
4.
5.
What causes cellular senescence, what are the
inducers and what do they have in common?
What is replicative senescence?
List 3 characteristics of the senescent phenotype.
What is the relationship between carcinogenesis,
aging, and senescence?
Explain antagonistic pleiotropy
Why are telomeres important?
Telomeres allow cells to distinguish chromosomes
ends 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
5'
3'
3'
5'
5'
3 5'
'
DNA replication
is
5' 3
'
O
ri
bidirectional
Each round of DNA
replication leaves
Polymerases move 5' to
3'
50-200 bp DNA unreplicated
at the 3' end
Requires a labile primer
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 senescence
Cell proliferation
(short telomeres)
DNA damage
Oncogenes
Strong 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 and
senescent cells may contribute to aging
HYPOTHESIS:
Telomere shortening causes aging and
telomerase will prevent aging
True or False?
SUMMARY
Telomeres are essential for chromosome stability
Telomere shortening occurs owing to the biochemistry of
DNA replication
Short telomeres cause replicative senescence
(other senescence causes are telomere-independent)
Telomerase prevents telomere shortening and
replicative 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)
Ta ble s 3 - 11 Di sea ses of the Elderly
Limited to a ging
Osteopor osis
Osteoarthritis
Prostatic adenocarcinoma
Polym yalgia rheumati ca
Temporal arteritis
Associated with aging
Known etiology
Septicem ia
Pneumoni a
Cirrhos is
Nephritis
Cerebrovascular disease
Myocardial in farction
Unknown etiology
Adult -onset, Type 2 diabet es
Neoplasm
Hypertension
Alzhe imer's disease
Parkinson's diseas e
Emphyse ma
Causes of Death According to Age
All Races, Both Sexes
Ages: 65-84
2003
Ages: 85+
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 Syndrome
Adult 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 nonhomologous 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?