10 - Proteins involved in regulation of the cell cycle

Question 1

Why is correct regulation of cell division important?

Answer 1

1. development
2. injury
3. adaptive responses

Question 2

1. development

Answer 2

Organs and body parts need to be the correct size / shape – not over or under developed

Question 3

2. injury

Answer 3

Cells need to divide following injury but stop when damage repaired

Question 4

3. Adaptive responses

Answer 4

Cells in bone marrow respond to low O2 – produce more red blood cells – need to stop when O2 returns to normal
Lymphocytes – division triggered in response to antigen – needs to be controlled

Question 5

Consequences of deregulated cell division

Answer 5

cancer

Question 6

Two main ways in which cell division is regulated

Answer 6

1. external signals: diffusable chemical signals produced by other cells which ‘tell the cell in question how to behave’ e.g. Growth factors (mitogens)
2. Internal signals: chemical signals produced internally by the cell itself in order to regulate its own division – present in the cytoplasm of cells e.g. Cyclin dependent kinases

Question 7

External signals regulating cell cycle progression

Answer 7

• External signals can promote or inhibit cell division
• Known as mitogens (growth factors) or anti-mitogens
• In the absence of mitogens, S phase cyclins (i.e. those cyclins which drive the cell into S phase) are not made
• Without sustained mitogen stimulation cells will not progress through the G1 checkpoint
• The cells enter G0 instead – quiet phase or quiescence

Question 8

what do mitogens (growth factors) do?

Answer 8

bind to receptors in plasma membrane and promote cell growth

Question 9

Platelet-derived growth factor (PDGF)

Answer 9

• Platelets are specialised cells present in blood
• Platelets have important roles in blood clotting and wound healing
• Release PDGF in response to injury
• PDGF binds to receptors on surface of skin cells and causes them to start dividing

Question 10

internal signals

Answer 10

Cells also have internal signalling networks that control the cell cycle
These factors are stimulated by the presence of extracellular factors, such as mitogens
The internal signals were first identified by fusing cells from different stages of the cell cycle
M+G2
M+G1 or S
G1+S
G1+G2

Question 11

How do we know that cells have internal chemical signals which can regulate cell division?

Answer 11

Cell fusion experiment (M phase + G2) = mitosis promoting factor in M phase cells
Cell fusion experiment (M phase + G1/G2/S) = Cells at any stage of the cell cycle can be stimulated by M phase cells to enter mitosis; the response is not unique to G2 cells

Question 12

Cell fusion identifies internal regulators

Answer 12

S + G1 => S : There is an S-phase promoting factor in S-phase cells
S + G2 => Cells DO NOT enter S phase: Although there is an S-phase promoting factor in S-phase cells, only G1 cells respond to this factor (not G2 cells)

Question 13

Why does the cell need internal signals to control division?

Answer 13

Uncontrolled cell division is a hallmark of cancer.
Checkpoints enable cells to stop dividing if the correct signals are not present.
Checkpoints allow cells to review current circumstances and prevent untimely exit from each cell cycle phase.
If cell proceeds from one phase of the cycle to the next inappropriately, this can cause genetic instability which can cause cancer

Question 14

Three major checkpoints

Answer 14

1. G1 checkpoint
2. G2 checkpoint
3. Metaphase checkpoint or spindle assembly checkpoint (SAC)

Question 15

G1 checkpoint

Answer 15

(START / restriction point)
Commits cell to DNA replication and cell division
Checks:
Is cell a suitable size?
Has it received appropriate external signals?
NO – cell enters non-dividing G0 phase
YES – cell proceeds to S phase

Question 16

G2 checkpoint

Answer 16

Cell makes decision whether or not to enter mitosis
Checks:
Is cell a suitable size?
Is DNA replicated?
Is environment favourable?
NO – cell does not proceed
YES – cell proceeds to M phase

Question 17

M (metaphase) checkpoint

Answer 17

Occurs in metaphase
Checks:
Are all the chromosomes attached to spindles?
NO – cell does not proceed
YES – cell enters anaphase of mitosis

Answer 18

f mitosis
Three major checkpoints
Cyclin-dependent kinases (Cdks)
AND
Cyclins
Internal molecular signals regulating G1
and G2 checkpoints

Question 19

Internal molecular signals regulating G1 and G2 checkpoints

Answer 19

Cyclin-dependent kinases (Cdks)

Cyclins

Question 20

Cyclins – discovered by Sir Tim Hunt in 1982

Answer 20

Observed that, following fertilisation, cells in sea urchin early embryos divided SYNCHRONOUSLY
Whole population of cells at same stage of cell cycle as opposed to a mixed population at different stages
Found a group of proteins levels of which increased and decreased (cycled) between INTERPHASE and MITOTIC PHASE - CYCLINS

Question 21

Basic mechanism of action of Cdk and cyclin proteins

Answer 21

Combination of the two proteins known as a ‘promoting factor’- control progression of the cell into the next phase of the cell cycle
G1 checkpoint – S phase promoting factor (SPF)
G2 checkpoint – M phase promoting factor (MPF) / Maturation promoting factor

Question 22

What does SPF regulate?

Answer 22

G1 checkpoint
Cyclin E + Cdk 2
Phosphorylates proteins required for S-phase entry
Drives cell into S phase

Question 23

what does MPF regulate?

Answer 23

G2 checkpoint
Cyclin B +Cdk 1
Drives cell into M phase

Question 24

What enables cells to pass through cell cycle checkpoints?

Answer 24

an increase in cyclin expression

Question 25

MPF and regulation of the cell cycle

Answer 25

1. MPF = maturation / (mitosis) / M phase promoting factor
   
   • controls G2 checkpoint and initiation of mitosis
2. MPF consists of a combination of Cdk1 and cyclin B
3. Cdk1 levels remain constant throughout the cell cycle
4. Peaks of MPF activity correlate with peaks of cyclin levels - threshold reached - MPF activated
   
   • cyclin expression increases sharply at various stages of the cell cycle - cyclin B particularly during G2
   • decrease sharply during M
   • maximum in early M
5. After mitosis the degradation of cyclin increases (ubiquitination and proteosomal degradation) - MPF activity decreases again

Question 26

Activation of MPF in G2 phase

Answer 26

1. Cyclin B accumulates
2. Cyclin B and Cdk1 bind to form MPF
3. MPF triggers mitosis
4. MPF activates cyclin-degrading enzyme
5. Enzyme degrades cyclin
6. Loss of cyclin inactivates enzyme
7. Cdk1 is recycled

Question 27

cyclin

Answer 27

• CDKs are present at constant levels throughout the cell cycle
• Periodic cyclin expression activates CDKs at defined points within the cell cycle
• It is cyclin which regulates G2 checkpoint progression
• Cyclin binds to CDK and forms the active MPF complex which acts directly by causing the nuclear envelope to fragment, and indirectly by stimulating other kinases
• Cyclin even initiates the process which results in its own destruction - thus switching itself off.

Question 28

The metaphase checkpoint (M checkpoint) /spindle assembly checkpoint (SAC)

Answer 28

• Checkpoint divides the metaphase and anaphase of mitosis
• Ensures that all chromosomes are attached to mitotic spindle before anaphase commences
• Internal regulator is not Cdk (as in the case of G1 and G2 checkpoints) but a complex of proteins called the ANAPHASE PROMOTING COMPLEX (APC)