Senescence

Senescence (senex = old man): the process of deterioration that is associated with aging.

Maximum life span: the maximum number of years that a member of a species has been known to survive. The range is tremendous. Drosophila = 3 months, humans = 120 years, some turtles and lake trout = 150 years, some trees = 1000 years or more.

Survivorship: the percentage of survivors in a population versus their age. The variation is great. Only a small percentage of wild animals enjoy maximum life span due to predation and disease. Survivorship is low for humans in underdeveloped countries, but much higher and steadily increasing in developed countries.

Jeanne Calment, the oldest confirmed human, died in 1997 at 122 years of age.

What is the oldest living organism on earth?

Celebrating its 4,645th birthday in 2002, a bristlecone pine in the White Mountains of California is the world's oldest-known living organism.

The tree clings to rocky ground at 11,000 feet in one of the driest places on Earth. This conifer took root when the Great Pyramids were going up in Egypt.

Bristlecone pines owe their longevity to their unforgiving environment. Alkaline soil, scant moisture, desiccating winds, constant freezing, and six-week growing season. Insects, fungus, and rot have little chance under such harsh conditions.

Sardinia’s mysterious male methuselahs

More men live past 100 on this Italian island, proportionally, than anywhere else in the world (Science 291:2074, 2001).

In most countries with reliable records, 5 women reach the century mark for every man. In Sardinia there is no difference between sexes. The area is very mountainous, isolated, and the population is inbred, thus, there appears to be a genetic factor that might be isolated.

The T and B cell immunity declines in these individuals, as in all older people. However, the body’s innate immunity, composed of macrophages, is ‘turned on’ and takes over many functions in these men after the age of 70.

Antonio Todde, 112oldest known man

Caloric restriction postpones senescence

One of the most reliable ways to prolong life in laboratory animals is to simply restrict their calories. When rats were maintained on a low calorie diet throughout life, they were 15% smaller but they lived 50% longer than litter mates that ate ad libitum (all they wanted).

If restriction of calories is started later in life, it still works, but lifespan is only extended 20%. Food restricted rats show less evidence of cancer, atherosclerosis, and autoimmune disease.

Why does restriction of calories delay senescence? The underlying mechanisms are being studied, but are currently unclear.

Caloric restriction induces levels of some antioxidant enzymes.

Oxidative damage hastens senescence

Oxidative phosphorylation: the process of producing energy through conversion of O2 to H20 and creation of ATP. Although this reaction is necessary for survival, it also produces hazardous by products.

Reactive oxygen species (ROS): several highly reactive radicals are produced at potentially damaging levels during oxidative phosphorylation (superoxide radical, hydroxyl radical, and hydrogen peroxide).

Detoxification enzymes: several enzymes assist in detoxification of ROS:

Antioxidants: natural molecules that reduce oxidative damage from free radicals. These include glutathione, vitamin C, vitamin E, and -carotine.

Oxidants damage DNA, lipid, and proteinDNA damage: during aging, oxidants interact with DNA to produce mutations. These inhibit DNA replication and transcription and can contribute to cancer.

Oxidants often modify protein: this occurs by inducing formation of carbonyl groups. Carbonyl groups alter or destroy the proteins normal activity. The carbonyl content of protein increases dramatically in older organisms. Greater than 40% of all proteins in older individuals are damaged.

Lipid peroxidation: modification of lipids in membranes leads to altered function. Lipid peroxidation is associated with increased risk of atherosclerosis, and is negatively associated with long lifespan. Dietary restriction strongly decreases serum levels of peroxides.

Oxidative damage causes senescence

If oxidant accumulation caused senescence one would predict that:

1. Oxidative damage should increase with aging: most types of oxidative damage do increase linearly or even exponentially with increasing lifespan.

2. Longer life span should correlate with low oxidant level or high antioxidants: levels of serum peroxides in lab animals correlate with their lifespan. Higher levels of SOD and uric acid are associated with increased lifespan in rodents.

3. Experimental increase of antioxidants should slow down senescence: injection of old rodents with the antioxidant, PNB, leads to reduced levels of oxidative damage to the brain. These animals also performed much better in tasks, such as navigation of a maze.

Senescence of C. elegans is delayed by 4 genes

C. elegans is a small worm that is studied intensively by developmental biologists. Four genes of this worm are known to postpone senescence.

Clock 1 gene (Clk-1): loss of this gene causes a slowdown of developmental processes and senescence. Clk-1 encodes a protein needed for coenzyme Q, an enzyme important for oxidative phosphorylation (Clk-1 mutants have less coenzyme Q).

Age-1, daf-2, and daf-16: these genes control entry into a dauer stage, which occurs when the worms lack food or environmental conditions are poor. If the genes are active, they can increase lifespan 2-fold.

Methusela gene mutations in Drosophila melanogaster can extend the life span of flies up to 35%. The gene encodes a G protein that influences resistance to starvation, heat and oxidative damage.

Werner’s syndrome is characterized by premature senescence

Werner’s syndrome: a rare inherited disease that results in premature aging. Individuals are normal in childhood, but stop growing in their teens. Patients are more susceptible to cancer, osteoporosis, diabetes, and cataracts., They usually die in their late 40s.

WRN: the gene responsible for Werner’s syndrome encodes a helicase, an enzyme that unwinds DNA for replication, DNA repair, or transcription. Both copies of the gene must be mutated or lost.

One possible cause of Werner’s syndrome is improper DNA repair and rapid accumulation of mutations. Another possibility is improper transcription of genes that are needed to maintain vigor or normal function.

teenage 48 years old

Progeria

Hutchinson Gilford progeria syndrome: causes children to age rapidly, undergo senescent changes, and to die as young as 12 years old. It is an extremely rare disease, and there are only 100 known cases worldwide.

It appears to be caused by a dominant mutant gene of unknown function (it is thought that the gene might suppress the aging process. Difficult to study.

Symptoms: are similar to aging in older persons. These include loss of hair, thin transparent skin with age spots, osteoporosis, and atherosclerosis.

The etiology is unclear:Infants with progeria have shorter telomeres than normal children, and this might be important in the pathogenesis of this syndrome.

The other main theory is that the gene is involved in preventing oxidative damage by free radicals.

Too few cases to allow the disease to be studied actively.

Telomerase and senescence

Somatic cells of humans show a limited capacity for proliferation in cell culture, and this corresponds to aging within the organism. Hayflick’s exp.

Phase 1: cells grow rapidly when placed in culture (30-60 pd)Phase 2: cell growth rate slows after many population doublingsPhase 3: cells stop growing and can never again enter the cell cycle. However, cells remain viable for extended periods.

The number of population doublings that occurs before cells become senescent varies in cells from different tissues.

Cells from young individuals divide many more times than cells from old individuals. This suggests that all somatic cells are capable of a limited number of divisions. Thus, cell senescence may contribute to organismal senescence.

Telomeres and senescence

One trigger for cell senescence is shortening of telomeres. Telomeres are short repeats of DNA (GGGTTA in humans) that form caps at the ends of chromosomes to protect the ends from wearing down.

Each time that a cells divides, it loses about 100 bp of telomeric DNA. Telomeres are about 10 kbp long in embryonic cells. After about 80 cell divisions, telomeres wear down to 2 kbp, which is thought to be a minimum length that triggers senescence.

Telomerase: a reverse transcriptase that restores telomeres. The enzyme is active in germ cells and stem cells, but activity is not detected in most somatic cells. Telomerase is reactivated in cancer cells or immortal cell lines.

Experimental introduction of telomerase into aging human cells prevents senescence and leads to cell immortality.

If the telomerase gene is knocked out, mice develop problems after several generations. Their telomeres become too short, they lose fertility, and they are more susceptible to several diseases.

Mouse telomeres are much longer than needed.

Why don’t all cells maintain telomerase?

A safeguard against cancer? Humans are long lived and accumulate mutations due to oxidants and environmental carcinogens. These mutations accumulate slowly, requiring about 30 cell divisions before they are significant to cause cancer.

Once a cell is ‘out of control’ (cancer has started), it needs to divide at least 30 times before it can produce a tumor of only 1 cm3. If cell senescence develops at 60-80 population doublings, it would provide a safeguard that inhibits cancer. It is hypothesized that many more tumors would arise spontaneously in humans if telomerase was expressed in all cells. Mice have active telomerase in somatic cells and mouse cells are highly sensitive to malignant transformation. Human cells are very resistant to malignant transformation.

Can telomerase be ‘switched off’ in human cancer cells as a universal type of therapy? It is an attractive idea, but there are problems:

• Some tumors maintain telomere length by poorly understood mechanisms that do not require telomerase.• Therapy would need to target every last cell. If it missed one in a million, the tumor might grow back.• If telomerase were knocked out in stem cells, there might be problems with normal tissue regeneration (skin and blood cell renewal).

Cell Death: apoptosis versus necrosis

Cell death occurs in two ways:

Necrosis occurs in response to injury. Cells are lysed and release their contents. The membranes break down causing release of organelles , DNA, and lysosomes to interact with the adjacent cells. This induces damage and inflammation (heart attack, bruise).

Apoptosis or programmed cell death is when the cell commits suicide for the good of the organism. Most cells have an intrinsic cell death program that must be constantly suppressed by survival factors (cytokines and growth factors).

Apoptosis is a genetically controlled event, it requires energy, and it allows cell death to occur in a very controlled manner. The nuclei shrinks and fragments into small pieces. These are easily phagocytized by macrophages. There is no damage to adjacent cells and no inflammation.

Physiological apoptosis is common during development.

1. Selective loss of cells between digits allows formation of fingers.

2. In the immune system, T cells that recognize self antigens undergo apoptosis.

3. After pregnancy, the breast cells undergo apoptosis as the breast tissue undergoes atrophy.

4. In the vertebrate central nervous system, up to 50% of some types of neurons undergo apoptosis if they don’t connect with other neurons and generate survival signals.

Apoptosis can also be triggered by pathological stimuli (treating cells with cytotoxic drugs causes apoptosis (cancer chemotherapy).

Apoptosis occurs normally during development

Apoptosis is controlled by a specific signal pathway

The homologs of Ced-3 in humans are called caspases. These are cysteine proteases that cleave at aspartic acids in proteins. Caspases compose a family of proteins that sequentially activate one another in a cascade that induces cell death.

There are 8-9 other downstream caspases that are activated. They amplify the signal and they digest important cell components to bring about apoptosis (DNA, structural proteins, membranes).

C. elegans is a small worm that is a good model for studying apoptosis. It only has 1090 cells and 131 die in a programmed manner. Mutations in a gene called Ced-3 alter this process.

4-17 senescence pp 770-790

4-19 EXAM #3 pp 326-790

4-22 review for final

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4-29 FINAL EXAM WEEK All inclusive final

The 3rd exam has 40 true/false and 10 short answer questions

The most important topics to focus on are included in a Power Point file on the web. Go to EXAM 3 for the link.