cell determination and cell cenescence

Question 1

define cell differentiation

Answer 1

• Cell differentiation is the process through which a cell undergoes changes in gene expression and gene activity to specialise and take on specific roles in an organism.
• The endpoint is a wide variety of specialised cell types.
• irreversible

Question 2

define cell dtermination

Answer 2

• Cell determination 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. The determination of different cell fates involves progressive restrictions in their developmental potentials.
• the fate of determined cells don’t change.

Question 3

describe assymetrical division in cell dtermination

Answer 3

• Asymmetrical cell division due to differential distribution of cytoplasmic molecules (proteins or mRNAs) within a cell before it divides.
• not a symmetrical cell division due to this differential distribution of the cytoplasmic molecules.

• Two daughter cells = different fates= different gene expression profile

Question 4

what causes cell determination

Answer 4

• Inductive signals from neighbouring cells is the most common cause
• One group of cell influences the development of another group of cells

Question 5

what are pioneer factors

Answer 5

they are Transcription factors that access silent chromatin, remodel it and initiate cell fate e.g OCT4, SOX2, NANOG. They are highly expressed in embryonic stem cells and needed to maintain their pluripotency. They are able to activate or inhibit gene expression via Histone modification or DNA methylation blockage.
• Pioneer factorsaka master regulators, together with co-factors are key to cell-fate descision making
• So pioneer factors are involved in cell differentiation and cell determination

Question 6

define cellular senescence

Answer 6

• Cell senescence: irreversible cell-cycle arrest mechanism in which cells cease to divide. Occurs as response to excessive extracellular or intracellular stress.

Question 7

define apoptosis

Answer 7

apoptosis is: a morphologically and biochemically form of programmed cell death that plays an essential role during the individual’s life. Because the process allows cells to renew and is involved in cell turnover.

Question 8

what are the basic common cellular processes

Answer 8

cell division, cell death, cell differentiation and cell metabolism.

Question 9

How are cell senescence and apoptosis interconnected ?

Answer 9

Many stimuli that lead to the DNA damage response can also induce apoptosis.
Thus both mechanisms are interconnected and share molecular signalling pathways

Question 10

cellular senescence is involved in ?

Answer 10

strongly implicated in symptoms of ageing but are also important in defence against cancer.

Question 11

compare apoptosis and cell senescence

Answer 11

apoptosis :
definition -refers to the process of programmed cell death
role-helps to balance the cell number at a constant rate
significance- chromosome condensation is the significant feature
caused by-different physiological and pathological conditions
regulation : by intracellular proteolytic mechanisms

Senescence :
definition - senescence refers to the deterioation of cells owing to age
role- takes place during the process of aging and defends against cancer
significance- irreversible arrest of cells during cell proliferation caused by - the oxidative stress, DNA damage and alternation of genetic expression
regulation - by genes involbed in ageing mechanisms.

So, in contrast to apoptosis, senescence cells are established variables, they have the potential to influence neighbouring cells through secreted soluble factors.

Question 12

recall cellular senescence

Answer 12

• When DNA is subject to an external or internal damage, eg. Telomere shortening, dna damage, oncogene activation, oxidative stress, cellular senescence is activated.
• This activation defends cells from cancer and also triggers the secretion of soluble factors. Thse factors are collectively called Senescence-associated secretory phenotype (SASP).

Question 13

describe replicative senescence or haylficks limit

Answer 13

it was detected in all types of cells in culture, except cancer cells. Therefore the only immortal cultured cells are cancer cellsnever reach Hayflick’s limit.
• Another concept linked to the Hayflick’s limit is cell life span :

• Cell lifespan is the total number of doublings that a cell population goes through before senescence. It is the length of time for which a cell exists. Can be expressed as no of days, months, years or doublings.

Question 14

define hayflicks limit

Answer 14

number of times that a normal human cell population will divide before cell division stops.

Question 15

what changes do cellular senescence cause to the cell and describe them in detail.

Answer 15

• Cellular senescence implies morphological, biochemical and chromatin changes in the cell.

• Morphological changes in senecenet cells
o Larger and flat cells
o Prominent nucleoli
o Nuclear lamina degradation
o Vacuolised ( no of vacuoles inside the cell increases)
o Chromatin reorganisation

• Cells undergo Biochemical and molecular changes during cellular senescence.
o Two of the best known molecular markers of senescence are lysosomal β-galactosidase and protein p16.
o Not all senescent cells have biochemical markers
o Complex secretome involving inflammatory and proliferation products as well as changes in extracellular matrix.
o SASP: Senescence Associated Secretory Phenotype consists of inflammatory cytokines, growth factors and proteases.

Question 16

which two pathways are central to cellular senescence control?

Answer 16

• P53 and retinoblastoma pathways are central to cellular senescence control.!!!!ie they are the main mechanisms associated with controlling cellular senescence.

Question 17

give an example of one of the main driving factors of senescence

Answer 17

telomere attrition.

Question 18

describe telomeres

Answer 18

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

Question 19

what do telomeres form at the end of chromosomes ?

Answer 19

telomeres form caps at the ends of chromosomes.

they contain a unique DNA sequence which is repeated several times.

Question 20

telomeres shorten during each cycle of cell division. what can this result in?

Answer 20

• Loss of telomere by chromosome breakage results in unstable chromosome ends that can fuse with other broken chromosomes or be involved in recombination events or be degraded. It can also be involved in the damage of telomere-flanking genes- this is because if telomeres shorten too much, the shortening will affect those genes that are flanking telomeres that might express important genes that are key for cell survival and normal biological processes.

Question 21

what is shelterin?

Answer 21

Telomeric DNA is associated with a six-member protein shelterin complex that facilitates the formation of loops which “cap/shield” the chromosome end. They protect the chromosomes from inappropriate DNA damage responses.

Question 22

Why does progressive telomere shortening occur ?

Answer 22

Progressive telomere shortening occurs in all dividing normal cells mainly due to incomplete lagging-strand DNA synthesis/replication of that area.

Question 23

what is a telomerase ?

Answer 23

Telomerase: ribonucleoprotein enzyme which replicates telomeric DNA by reverse transcribing DNA hexamers (TTAGGG) from RNA using its RNA subunit (TERC- telomerase RNA component) and its protein component (TERT- telomerase reverse transcriptase).

Question 24

what are the 2 componants of telomerase ?

Answer 24

Terc and TERT.

Question 25

How does telomerase elongate telomeric DNA ?

Answer 25

-Telomerase elongates telomeric DNA by repetition of two-steps cycle : synthesis and translocation

Question 26

outline the steps on how telomerase elongates telomeric DNA

Answer 26

1. On the lagging strand, the telomerase binds the first few nucleotides of the template to the last telomere sequence of the chromosome.
2. It adds a new telomere repeat complementary to the telomeres sequence ie GGTTAG.
3. It then moves and realigns the new 3’ end of the telomere to the template and it repeats the process.
4. So to summarise, telomerase reverses the telomere shortening because it uses the RNA component as a template to elongate the telomeres and thus the lagging strand.

Question 27

explain where and when telomerase activity is usually expressed and not expressed.

Answer 27

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).

• Telomere length is highly variable between individuals, between cells and between different tissues.
• Normally, somatic adult cells ( not stem cells ) don’t usually express TERT, the protein component of the telomerase.
• At some point, when the cells have divided many times, the telomeres reach a point and specific length that makes it incompatible with life.
• At that point replicative senescence is triggered.

Question 28

when is replicative senescence triggered?

Answer 28

• Replicative senescence is triggered in normal cells when telomere(s) get quite short. (About 1-5 short telomeres sufficient).
• The mechanisms of replicative senescence are activated, ie when the telomeres shorten, it signals the cells , which receive and recognise it to activate the replicative senescence.

Question 29

describe what a graph of normal cell culture would look like

Answer 29

Here there are cells in culture. Initially they grow exponentially because their telomeres are okay but shortening, but the length is okay and can continue to divide.
Then at some point, after a certain number of doublings, some cells where the telomeres are too short, arrest. They start to die and population slows down.
At some point, all the cells in the culture, their telomeres are too short because they have divided many times. the cells start to die and the cell ppopulation stops growing.

Question 30

Normally adult cells dont express tert but what are the exceptions?

Answer 30

• Germline cells (oocytes, sperm and their diploid progenitors) do express TERT so they maintain full-length telomeres. Hence germline cells are immortal-cells can divide forever.
• There are other types of immortal cells due to telomerase/TERT activation. Cancer cells.
• Cancer cells find the way of activating telomerase, activating the TERT component and thus the expression of TERT (protein component of the telomerase) leading to uncontrolled replication and survival.

Question 31

what % of cancer cell lines express tert?

Answer 31

• Nearly all cancer cell lines in culture (~90%) express TERT, so they are immortal
• This means that the telomere shortenings is needed in cells. If our cells don’t die , they could survive forever and be immortal, leading to the development of cancer.

Question 32

What is a major barrier to tumorigenesis ?

Answer 32

• 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 33

advanced cancer cells have usually bypassed what process?

Answer 33

cell senecence

Question 34

list the common abnormalities found in cancer cells that lead to defective senescence and immortality

Answer 34

• Expression of TERT
• p53 defects
• p16 defects.
• The two pathways p53 and retinoblastoma pathway, played key roles in cell seneence.

Question 35

what is the retinoblastoma pathway inhibited by that can cause cancer

Answer 35

P16 proteins

Question 36

list the evidence that telomeres shorten as we age

Answer 36

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.
• these evidence show that telomeres shorten as we age, and defects on p16, which plays a role in the senence pathway are associated with aging.

Question 37

recall telomeres in stem cells

Answer 37

• Embryonic stem cells express the protein component TERT of the telomerase. They are naturally immortal.
• Some adult stem cells have some telomerase activity, but in general it is too little to make the cells immortal.
• This means that in somatic stem cells, the telomeres still shorten, but less per division than in other somatic cells, in non-stem somatic cells,
• They do shorten and senesce gradually, but at a slower rate and so die later. They can still differentiate into different types of cells and their life spans are longer.

Question 38

give examples of connections between cell senescence(including stem cell senescence) and aging symptoms

Answer 38

o 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.
o Hair greying linked to decreased melanocyte stem cell maintenance in hair follicles (data from mice).
o Reduced healing ability of skin with age, increased risk of skin ulcers. Proposed to be due to senescence in dermal fibroblasts.
o However epidermal stem cells have very little telomere shortening and remain able to divide throughout life.
o There are also events occurring in other organs that are associated with age and are linked with cell senescence.

Question 39

Compare cell determination vs cell differentiation

Answer 39

cell determination : process by which portions of the genome are selected for expression in different embryonic cells

cell differentiation : process by which a cell becomes specialised in order to perform a specific function

cell determination : occurs in totipotent, embryonic stem cells

cell differentiation : follows cell determination

cell determination : as a result of asymmetric segregation of cytoplasmic determinants.

cell differentiation : as a result of differential gene expression

cell determination : responsible for assigning the fate of cells.

cell differentiation : responsible for the functional specialization of the cells