Aging

HOME Why do similar animals have very drastic difference sin thier lifespans? The demise of an organism to regenerate lies in its core biological machinery. It was beleived that cultivated cells could grow forever since it was observed that cancer cells can grow indefinitely. But a definite lifespan of cells do exist.

Hayflick Limit and Senescence
Hayflick noticed that human fibroblasts from embryonic tissue could only grow for several months.

Hayflick Phenomenon
The Hayflick limit is the number of times a normal cell population will divide until cell division stops. It was observed that the cell culture will divide between 40-60 times at which they will then enter a senescence phase. Senescence is the process of accumulative changes to molecular and cellular structure that disrupts metabolism with the passage of time, resulting in deterioration and eventually bringing about death.

Senescent Phenotype


There are specific phenotypes that cells show under different stages leading to and including senescence.

1) Young Cells
 * Young cells that are still a ways away from senescence are large and flat in phenotype. They are unresponsive to growth mitogens and remain viable and metabolically active.

2) Pre-senescent


 * Cells that are close to the senescent phase are observed to have an increased levels of various Cyclins such as Cyclin D1 and D2.

3) Sencesent


 * Cells that are in the sencesent phase have higher levels of SRF binding to Dna. It was observed that there are increased levels of p53 and Rb activity. Notice that the cell is trying to maintain the genome.


 * We see that senescent cells ahve DNA repair machinery becoming more active in the cell.

If we wish to immortalize the cell and avoid senescent we need to have lower lwevels of p53 and higher levels of oncogenes to promote cell division and stop cell apoptosis.

Telomeres and aging
Researchers noticed that cells could remember their place in the PDL (Population doubling level), that is, cells were able to pick up from where they left off. Cells must have a way to remember their place in the PDL- there must be some sort of 'counter'.

It was observed that telomers actually shortened during the aging of human fibroblasts. It was hypothesized that telomeres were probably the counter in senescence.

Telomeres
Telomeres are 3-20kb repeats of "TTAGGG" at each end of every chromosome. Teleomers have several functions that aid chromosomes in the cell: It was also observed that the size of telomeres actually reset in the germline regardless of their level in somatic cells.
 * Telomeres "cap" the end of chromosomes to protect against fusion with other chromosomes.
 * Aid in replications
 * and aid in chromosome positioning



DNA shortening
During DNA division there are two ways that the DNA strand can be replicated. The DNA can be opened to reveil a leading and lagging strand. With the leading strand, primers are placed at the origin of the replicaton bubble and synthesis occurs undisrupted. However, disruptions occur during the replication of the lagging strand. Multiple primers need to be added as replication occurs on the lagging strand. Disruptions occur on the lagging strands since primers cannot be added to the end of the strand.. We are left with a gap that eventually erodes the DNA strand.

To avoid early degradation of DNA, telomeres with non-coding DNA regions are added to the ends of DNA to act as a buffer. Telomeres are created with the use of a specialized  reverse-transcriptase. The reverse-transcriptase uses and RNA template to bind to the DNA strand and the enzyme rebuilds the DNA.

Telomeres can wrape around to insulate the end of chromosomes. But as they shorten they open the DNA region to repair and possibly fusion with other chromosomes.

When Telomeres are too short


When telomere ends are too short the cell detects this and becomes senescent or undergo apoptosis.

The loss of telomere activity has been shown to lead to premature aging in organisms. For instance, in a simple knockout experiment in mice, telomere ends where removed from the chromosomes of mice. It was noticed that the overall life expectancy lowered significantly and the mice seemed to undergo a rapid aging process.

Dolly the sheep and other cloned animals
Another example of the effects of telomere length on the life of an organism is in somatic nuclear transfer (or cloning). When an organism is cloned, the nucleus from the parent is transferred to a denucleated egg of host mother. Unfortunately, the transfer of the nuclear DNA does not reset the length of the telomeres as is done in the germline. This causes the transfer of nuclear DNA with telomere lengths much shorter than the germline. As such, the clone animal is susecptible to many diseases and has an overall lower lifespan. This was seen with the most famous cloned animal Dolly the Sheep.

Werner Syndrome


Werner syndome is classified as a human pathology where cells undergo early senescence. Early cell senescence causes damages to various tissues. It was observed that fibroblasts from Werner patients only divided about 20 times (remember that normal cells can divide between 40-60 times).

The cause of this disease lies in a mutation in the WRN gene which encodes for the RecQ helicase. RecQ is a helicase that is required for the unwinding of DNA at the teleomere ends. If RecQ is not active, the telomere ends are not able to be repaired with telomerease and the DNA undergoes deterioration quicker than normal.

As a check, the activity of Telomerase was upregulated in mice that had the mutation in the WRN gene. It was observed that increasing the activity of Telomerase counter-acted the effects of the WRN mutation.

Progeria
Another Human pathology that involves a rapid aging process is Progeria. Progeria is caused by point mutations in the Lamina A gene. This mutation causes senescence and destruction of tissues and organs which lead to the rapid aging phenotype.

Lamin A


The Nuclear membrane of the cell is held by a family of structural protiens called Lamins. Lamins have many different roles in the structure and function of the nucleus. Lamins have been found to coordinate proteins to the nuclear membrane and factors that recgonize DNA error.

Lamin A has a specific function when it comes to the nucleus. Lamin A is responsible for the depolymerization of the lamins in the nuclear membrane when the cell is ready to divide.

Alternative Splicing of the LMNA gene
The LMNA gene undergoes alternative splicing that creates the family of lamin proteins.

To create the Lamin A protein the LMNA gene undergoes the following modifications: #Exon 11 and Exon 12 of the LMNA gene is spliced to create the Pre-lamin A product #The pre-lamin A product is then Farnesylated at its end
 * 1) The farnesylated end is then methylated
 * 2) The metylation of the farnesylated end is recognizedby another cleavage protein
 * 3) The cleavage protein cuts away a portion of the Pre-lamin A end, which includes the methylated farnesyal group, creating the final Mature Lamin A product.

Mutation in LMNA and splicing
As stated, Progeria is caused by a sinle point mutation in the LMNA gene. This point mutation causes the process of splicing to be altered in such a way as to disrupt normal Lamin A production.

The mutation alters the cleavage site of the LMNA gene causing the removal of a 60 amino acid sequence normally found in the pre-lamin A product. This deletion disrupts the final cleavage that would normally remove the end portion (including the Farnesylated end) of the Pre-Lamin A creating a Progerin product.

Aging in C.elegans
C.elegans has a more simplistic mechanism when it comes to aging. Aging in the organism is controlled by nutrient factors such as insulin. Many of the P13K proteins are used in the regulation of aging. One of the most influential protiens in the C.elegans aging pathway is FOXO.

If you recall, FOXO is a protein that is able to sense a stressful environment in the cell and in inhibit the cell to acutally grow. Inactivation of FOXO will then lead to grow promoting factors. FOXO sense when to grow and what stress responses to use to control the lifespan of C.elegans. When FOXO is activated, the organism undergoes a hibernation-like state elongating its lifespan.

Daf2, an insulin receptor in the FOXO pathway has also been seen to effect the longevity of C.elegans. An experiment was conducted where the activity of Daf2 was blocked and lifespan measured. With the insulin receptor removed, the cell no longer has the ability to sense nutrients in the environment. With this insulin sense removed, the cell acitvates the FOXO pathway undergoing cell hibernation.