Determination and Cellular Senescence

Question 1

What is cell determination?

Answer 1

Cell determination- a process whereby cell fate becomes stable
It is followed by cell differentiation.
When a cell chooses a particular “fate”, it is said to be determined
Implies a stable change
occurs in totipotent, embryonic stem cells
As a result of asymmetric segregation of cytoplasmic determinants

Question 2

What is asymmetric segregation and how does it occur?

Answer 2

Asymmetrical cell division due to differential distribution of cytoplasmic molecules (proteins or mRNAs) within a cell before it divides
Two daughter cells= different dates- different gene expression profile
Inductive signals from neighbouring cells is the most common cause of cell determination
One group of cells influences the development of another group of cells

Question 3

What are p-granules?

Answer 3

P-granules are a class of perinuclear granules specific to the germline

Question 4

What is the role of pioneer factors in cell determination?

Answer 4

Pioneer factors/master regulators together with co-factors are key in cell-fate decision making

Question 5

What is cell senescence?

Answer 5

Cell senescence: irreversible cell-cycle arrest mechanism in which cells cease to divide
Occurs as a response to excessive extracellular or intracellular stress
Cell senescence has the potential to influence neighbouring cells through secreted soluble factors
Strongly implicated in symptoms of ageing but also an important defence against cancer

Question 6

What is the difference between cell senescence and apoptosis?

Answer 6

Cell senescence: irreversible cell-cycle arrest mechanism in which cells cease to divide
Role is that it takes place during the process of ageing
Apoptosis: is a morphologically and biochemically form of programmed cell death that plays an essential role during the individual’s life
Role- helps to balance the cell number at a constant rate

Question 7

What is cell lifespan?

Answer 7

Cell lifespan is the total number of doublings that a cell population goes through before senescence
The length of time for which a cell exists
Normal cells have a finite lifespan as opposed to immortal cells (e.g. cancer cells), which can divide forever

Question 8

What is the Hayflick Limit?

Answer 8

The Hayflick Limit is the number of times that a normal human cell population will divide before cell division stops

Question 9

What morphological and biochemical changes does cell senescence imply?

Answer 9

A. Morphological changes:
Larger and flat cells
Prominent nucleoli
Nuclear lamina degradation
Vacuolised (vacuoles increase in number)
Chromatin reorganisation
B. Biochemical and molecular changes
Two of the best known molecular markers are lysosomal b-galactosidase and protein p16
Complex secretome involving inflammatory and proliferation products as well as changes in extracellular matrix
SASP: Senescence Associated Secretory Phenotype consists of inflammatory cytokines, growth factors and proteases.

Question 10

What are the main mechanisms associated with controlling cellular senescence?

Answer 10

P53 and retinoblastoma pathways are the main mechanisms associated with controlling cellular senescence
Telomere attrition is one of the trying factors of cell senescence

Question 11

What are telomeres?

Answer 11

Telomeres: regions at the end of the chromosomes composed of TTAGGGn DNA sequences whose function is to preserve chromosome integrity during each DNA replication thus preventing from DNA damage (part of constitutive heterochromatin)

Question 12

What can loss of telomeres result in?

Answer 12

Loss of telomere by chromosome breakage results in unstable chromosome end that can fuse with other broken chromosomes or be involved in recombination event or be degraded
It can also involve the damage of telomere-flanking genes- if telomere shorten too much the genes flaking telomeres are damaged too

Question 13

How are telomeres structured?

Answer 13

Telomeric DNA is associated with a six-member protein shelterin complex that facilitates the formation of loops which ‘cap/shield’ the chromosome end

Question 14

How does telomere shortening happen and how is the problem dealt with?

Answer 14

Progressive telomere shortening occurs in all dividing normal cells mainly due to incomplete lagging-strand DNA synthesis/replication of that area
Telomerase: ribonucleoprotein enzyme which replicates telomeric DNA by reverse transcribing DNA hexamers (TTAGGG) from RNA using its RNA subunit (TERC- telomeric RNA component) and its protein component (TERT- telomerase reverse transcriptase)
Telomerase elongates telomeric DNA by repetition of two-steps cycle: synthesis and translocation

Question 15

How do telomere length and telomerase activity vary across cells?

Answer 15

Telomere length is highly variable and telomerase activity normally absent from adult somatic cells except for highly-proliferative tissues such as blood, skin and intestine (they do not express TERT)
Replicative senescence is triggered in normal cells when telomere(s) get quite short (about 1-5 telomeres sufficient)

Question 16

What are two examples of immortal cells?

Answer 16

Germline cells (oocytes, sperm and their diploid progenitors) do express TERT so they maintain full-length telomeres.
Hence germline is immortal-cells can divide forever
Cancer cells find the way of activating telomerase leading to uncontrolled replication and survival
Nearly all cancer cell lines in culture express TERT, so they are immortal
Telomere shortening represents a major barrier to tumorigenesis, operating as a tumour suppressor pathway; however, activation of telomerase provides an escape from crisis and allows outgrowth of cells with a rearranged genome

Question 17

How do abnormalities in cell senescence lead to cancer?

Answer 17

Advanced cancer cells have usually bypassed cell senescence
Some of the commonest abnormalities found in cancer cells are those leading to defective senescence & immortality:
Expression of TERT
P53 defects
P16 defects

Question 18

How are telomeres related to ageing and where is the evidence?

Answer 18

Telomeres shorten as we age
The longer they are the healthier the person is
Evidence:
Telomere length typically very short in people aged >100
P16 and other senescence-associated proteins are expressed increasingly in ageing tissues
Telomere length at birth varies between people: genetically linked to age at death
Defective genes for telomerase subunits give syndromes with premature ageing and early death
P16 (CDKN2A) locus also genetically associated with human senile defects- cardiovascular disease, frailty, type II diabetes, neurodegeneration, cancer

Question 19

What are telomeres like in stem cells?

Answer 19

Embryonic stem cells also express TERT/telomerase, they are naturally immortal
Some adult stem cells have some telomerase activity but in general, too little to make the cells immortal
In other words, telomeres shorten less per division in somatic stem cells than in other somatic cells, but they do shorten
So most somatic stem cells do senesce gradually
Examples of connections between cell senescence (including stem cell senescence) and ageing symptoms:
Bone marrow- older people show decreased immunity, increased bone marrow failure, decreased success rate as bone marrow donors
Reduced proliferative ability of marrow stem cells
Hair greying linked to decreased melanocyte stem cell maintenance in hair follicles (data from mice)
Reduced healing ability of skin with age, increased risk of skin ulcers
Proposed to be due to senescence in dermal fibroblasts
However epidermal stem cells have very little telomere shortening and remain able to divide throughout life