Telomerase and Senescence

Quiescent Cells is a state of cellular division in which cells

“leave” the cell cycle to remain in a semi-permanent state of no longer actively dividing

cells are still _ active

metabolically

Reversible

Cells can enter and exit from G0 depending on various factors/stimuli

Primary Stimulation

lack of nutrition and growth factors

EX of Quiescence cells

Adult stem cells

capable of regenerating tissues when needed, but otherwise inactive

Hepatocytes

metabolically active liver parenchymal cells that do not divide,

but can enter the cell cycle to regenerate liver tissue when needed

Fibroblasts

active connective tissue cells that become mitotically active

during injury/inflammation to mediate repair/regeneration

Senescent Cells

State of cellular division in which cells permanently arrest somewhere within the cell cycle. Can occur in G1, S, or G2

Cells can remain _ active

metabolically

Once cells undergo senescence, they are

permanently arrested

Primary Stimulation

aging, major DNA damage

EX of Senescence

Replicative senescence

limited replicative potential of normal cells due to telomere attrition or dysfunction

Stress-induced senescence

premature senescence (before telomeres shorten) due to various stressors (e.g., oxidative damage, DNA damage, oncogene activation, etc.)

Postmitotic cellular senescence

permanent growth arrest of terminally differentiated cell types (e.g., neurons, myocytes, adipocytes, etc.)

Difference?

Quiescence is a temporary, reversible resting state where cells can re-enter the cell cycle, while senescence is an irreversible, permanent cell cycle arrest often triggered by stress.

Molecular Players

Telomere

Non-coding, repetitive sequences (TTAGGG) at the ends of linear chromosomes

Shorten with each
DNA replication due to

inability of lagging strand to synthesize (no place to put a

primer to start new Ozakai fragment)

Associated with replicative senescence

when telomeres become sufficiently shortened, stimulates

irreversible cell cycle arrest

Molecular Structure

Shelterin

protein complex bound to telomeric repeats

Telomere loops

ensure that telomere ends are not exposed and are protected from premature degradation

Telomere Shortening: shorten by _ bp with each cell division

50-200

Results of

incomplete synthesis of lagging strand

Also prone to shortening due to

oxidative destruction (despite loops and shelterin complexes)

Mitotic Clock

once reach a threshold length, stimulates senescence. Cells can only divide a finite number of times

Implications for

issue culture of non-transformed cells and aging process

It is not clear what exactly constitutes the

“critically short” telomere length

TFR2

telomere binding factor 2

POT1

protection of telomeres 1

Immortalization

Cell lines that are capable of unlimited replicative potential. Term is usually applied to cultured cells rather than in vivo cells

Process

1. Repeated cell division of cells in a tissue culture environment

2. Induction of senescence due largely to shortened telomeres

3. A subset may continue dividing despite senescent stimulation  crisis phase

4. Majority of cells that continue dividing while in crisis will undergo apoptosis

5. A small subset may survive crisis and become immortalized due to activation of telomerase

Stabilizing Telomeres

Telomerase

reverse transcriptase enzyme that can elongate chromosomes

Two main components

Telomerase reverse transcriptase (TERT)

DNA polymerase that uses internal RNA as a primer

Telomerase RNA component (TERC)

– acts as the RNA template for synthesis of new telomere DNA

Which cells express telomerase

Developmental tissues:

All/most embryonic tissues up to 20 weeks gestation

2. Variable expression in fetal tissues after 20 weeks

Adult tissues:

Lymphocytes in bone marrow and peripheral blood

2. Some epithelial cells (epithelial regenerating cells) in skin, hair, GI, uterus

3. Germ cells in testis

4. Other discreet adult stem cell populations

Majority of cancers (%)

85-95

Hallmarks - Repression of telomerase expression

and phenomenon of telomere shortening are

natural, built-in mechanisms that help prevent transformation

and oncogenesis

Transcriptional Control of TERT: where is it located

5p; complex TF regulation

Upstream promoter element is rich in

CpGs

Normal somatic cells

unmethylated

Malignant Cells

hypermethylated

Overexpression of pro-proliferative TF can activate

promoter (c-myc)

Promoter mutations observed in large percentage of certain cancers

C228T, C250T

Some oncogenic viruses express

transcriptional cofactors that promote expression

Gene amplification also observed in some

cancers