Spindle Inhibitors

Question 1

Define microtubules

Answer 1

Key components of the cytoskeleton
essential in eukaryotic cells, long, slender, filamentous tubes comprised of alpha and beta-tubulin heterodimers connected by non-covalent bonds

Question 2

What are the functions of microtubules?

Answer 2

Cell shape, intracellular transport (transport of vesicles), cell signalling
Crucial in mitosis (spindle formation, chromosome segregation)

Question 4

What are tubulin heterodimers connected by?

Answer 4

Tubulin heterodimers are connected by non-covalent bonds.

Question 5

What characterizes the (+) end of microtubules?

Answer 5

(+) end is associated with a GTP cap.

Question 6

How do the dynamics of the (+) end compare to the (-) end?

Answer 6

(+) end dynamics are faster than (-) end, although both can add/lose new heterodimer units.

Question 7

What is the structure of microtubules?

Answer 7

Microtubules consist of 13 filaments and have a spiral structure.

Question 8

What does the dynamism of microtubules involve?

Answer 8

Microtubules are dynamic: they constantly grow/shrink via addition/loss of tubulin subunits, faster at the + end.

Question 9

What are the four factors characterizing the dynamism of microtubules?

Answer 9

1. Rate of MT growth - how fast?
2. Rate of MT shrinkage - how fast?
3. Frequency of transition from growth to shrinking - catastrophe.
4. Frequency of transition from shrinking to growth - rescue.

Question 10

What is treadmilling in microtubules?

Answer 10

Treadmilling is when units add to one end and leave from the other, allowing the MT to ‘move’ while individual heterodimers remain stationary.

Question 11

How does GTP hydrolysis affect microtubule stability?

Answer 11

GTP hydrolysis on β-tubulin regulates MT stability.

GTP cap stabilizes MT, loss of cap leads to catastrophe (shrinkage).

Question 12

What is the role of GTP/GDP in microtubule stability?

Answer 12

Both α- and β-units have a GTP/GDP binding site at the (+) end.

Question 13

What happens to GTP in heterodimers?

Answer 13

Within heterodimers, the α retains a GTP tightly bound, while the β (+) end is accessible for hydrolysis of GTP to GDP.

Question 14

What stabilizes the cap of microtubules?

Answer 14

The cap normally has either GTP or GDP/Pi and is stable.

Question 15

What occurs if the cap is lost?

Answer 15

If the cap is lost, the ends fray, resulting in catastrophe (change to depolymerization).

Question 16

What is the stabilizing role of GTP or GDP?

Answer 16

The role of GTP or GDP is to stabilize the cap.

Question 17

What happens to microtubules during interphase?

Answer 17

During interphase, microtubules are stable and turn over slowly.

Question 18

What occurs at the onset of mitosis regarding microtubules?

Answer 18

At the onset of mitosis, the entire microtubule network disassembles and is replaced by a new population of spindle microtubules that are more dynamic.

Question 19

How do microtubule dynamics change during mitosis?

Answer 19

During mitosis, microtubules become 4–100 times more dynamic.

Question 20

What is the first stage of mitosis involving microtubules?

Answer 20

Prometaphase: The nuclear envelope breaks down, and microtubules search for kinetochores.

Question 21

What happens if a microtubule misses a kinetochore during prometaphase?

Answer 21

If a microtubule misses a kinetochore, it disassembles and re-probes in a different direction until all kinetochores are located.

Question 22

What is the second stage of mitosis involving microtubules?

Answer 22

Metaphase: Chromosomes move to the equator to form the metaphase plate.

Question 23

What is the third stage of mitosis involving microtubules?

Answer 23

Anaphase: Duplicate chromosomes separate and move towards the two spindle poles.

Question 24

What is the fourth stage of mitosis involving microtubules?

Answer 24

Telophase: Duplicated chromosomes reach new spindle poles, and the cell is dividing.

Question 25

Why target microtubules (MTs) in cancer?

Answer 25

Cancer cells divide rapidly and rely heavily on mitotic microtubules for organizing chromosomes and cleaving daughter cells at the right time.

Question 26

Why are MTs effective targets for chemotherapy?

Answer 26

MTs are effective targets for chemotherapy because many natural compounds disrupt MT function.

Question 27

What are antimitotic drugs?

Answer 27

Antimitotic drugs are agents that bind to tubulin and alter microtubule dynamics, leading to mitotic arrest and cell death.

Question 28

What are the binding sites on tubulin for antimitotic drugs?

Answer 28

There are 6 known MTA binding sites: 4 on β-tubulin and 2 on α-tubulin.

Question 29

What are the two types of antimitotic agents?

Answer 29

The two types of antimitotic agents are microtubule-stabilizing agents (MSAs) and microtubule-depolymerizing agents (MDAs).

Question 30

What are Microtubule Targeting Agents (MTAs)?

Answer 30

MTAs are agents that affect microtubule dynamics, either stabilising or destabilising them.

Question 31

What are Microtubule-Stabilising Agents (MSAs)?

Answer 31

MSAs prevent depolymerisation and stabilise microtubules.

Question 32

What are Microtubule-Destabilising Agents (MDAs)?

Answer 32

MDAs promote disassembly and destabilise microtubules.

Question 33

What is the Taxane Site?

Answer 33

The Taxane Site is located at the β-tubulin inner face and includes agents like Paclitaxel.

Question 34

What is Paclitaxel?

Answer 34

Paclitaxel is a semi-synthetic agent from Taxus brevifolia that stabilises microtubules. ## Footnote It is used in ovarian, breast, and lung cancer.

Question 35

How does Paclitaxel affect microtubules?

Answer 35

Paclitaxel strengthens lateral protofilament contacts and disrupts mitotic dynamics.

Question 36

What are the side effects of Paclitaxel?

Answer 36

The side effects include myelosuppression (reversible) and peripheral neuropathy.

Question 37

What is the Laulimalide / Peloruside Site?

Answer 37

This site is located on the outer face of microtubules and is less important in clinical use.

Question 38

What is the Vinca Site?

Answer 38

The Vinca Site is located at the β-tubulin + end and includes agents like Vincristine, Vinblastine, and Vinorelbine.

Question 39

What do Vinca alkaloids do?

Answer 39

They inhibit tubulin polymerisation and promote microtubule depolymerisation. ## Footnote Effective for haematological cancers.

Question 40

What are the side effects of Vinca alkaloids?

Answer 40

The side effects include peripheral neuropathy and reversible myelosuppression.

Question 41

What is the Maytansine Site?

Answer 41

The Maytansine Site blocks microtubule elongation by binding to the growing (+) end.

Question 42

What is Eribulin?

Answer 42

Eribulin is a synthetic analogue of halichondrin B that binds to the (+) end and blocks growth. ## Footnote It is used in metastatic breast cancer and unresectable liposarcoma.

Question 43

What are the effects of Eribulin?

Answer 43

Eribulin shows non-mitotic effects such as vascular remodelling, reducing tumour hypoxia, and decreasing metastasis and invasion.

Question 44

What is the Colchicine Site?

Answer 44

The Colchicine Site binds between α- and β-tubulin and includes agents like Colchicine and Combretastatin.

Question 45

How does Colchicine affect microtubules?

Answer 45

Colchicine stabilises tubulin in a curved conformation, inhibiting polymerisation.

Question 46

What are the resistance mechanisms for MTAs?

Answer 46

Resistance mechanisms include selective efficacy, ABC transporters, altered tubulin isotypes, and changes in checkpoint/apoptotic proteins.

Question 47

What is an example of selective efficacy in MTAs?

Answer 47

Paclitaxel is effective in ovarian, breast, and lung cancers but not in solid tumours like colon and kidney carcinomas.

Question 48

What is the role of ABC transporters in resistance?

Answer 48

ABC transporters are efflux pumps that can lead to resistance by overexpressing membrane transport proteins.