CBS - Cytoskeleton

Question 1

What is the cytoskeleton?

Answer 1

It is a network of protein filaments within a cell.

It has a range of functions:

• connection with Extracellular Matrix (ECM)
• maintaining cell shape
• intracellular transport
• cytokinesis
• chromosome separation
• cell movement

Question 2

What three structures if the cytoskeleton composed of?

Answer 2

Actin mircofilaments

• they are olymers of actin
• they are 7-9 nm in diameter

Intermediate filaments

• they are composed of tissue-specific proteins
• they are 10 nm

Microtubules

• they are polymers of tubulin
• they have a 25 nm diameter

Question 3

Describe actin microfilaments.

Answer 3

The monomer is a globular protein (called G-actin), which contains an ATP-binding domain.

G-actin polymerises to microfilaments (F-actin), and the process involves ATP hydrolysis.

The polymerisation occurs via non-covalent interactions, with the monomers in the head to tail orientation, and as a result each microfilament has a (+) and (-) end (i.e. it has polarity). Monomers can be added at both the (+) and (-) end.

It is a dynamic structure: its length depends on the relative rate of loss and gain of G-actin monomers.

The actin microfilaments are usually present as two tightly wound chains. It comprises ~5% of cellular protein.

Question 4

What are the functions of actin?

Answer 4

• muscle contraction
• mechanical support
  (the internal strength of microvilli is provided by actin filaments)
• maintaining cell shape
  (there are bands of actin that run parallel to the surface of the cell, helping it maintain its shape)
• cell movement

Question 5

What are the different actin binding proteins?

Answer 5

Actin binding proteins provide a function to the filaments.

G ACTIN BINDING PROTEINS:
- thymosin β4 : inhibits polymerisation

CROSS-LINKING PROTEINS:

• villin : crosslinks parallel bundles in microvilli
• filamin : joining the microfilaments at angles to create a mesh

SEVERING PROTEINS:
- gelsolin : cuts and binds (+) end; the other part depolymerises – “gel to sol”

Question 6

How does contraction occur in non-muscle cells?

Answer 6

The muscle myosin is also present in non-muscle cells. The interaction between myosin and actin microfilaments allows for movement, and it requires ATP hydrolysis.

It also allows movement within cells (changes shape of cell). An example would be cytokinesis : a ring of actin forms in the centre of the cell, anchors to the plasma membrane, and myosin contraction constricts the cell.

Additionally, it also allows for the movement of cells. Lamellipodia, such an example, is mediated cell movement across the extracellular matrix (ECM).

Question 7

How do lamellipodia form?

Answer 7

Extensions of cells project out, which contain an actin network. They are generated by the rapid growth of actin filaments at the cell membrane.

The tip of the lamellipodia interacts with the ECM via integrins.

The contraction involving myosin allows for cell movement.

Question 8

Describe intermediate filaments (IF).

Answer 8

They are polymers of individual IF proteins - they are 10nm in diameter.

There are different IF proteins in different cell types

• in epithelia cells we have keratin(s), which provide physical support and external structures.
• in axons, we have neurofilamin(s), which provide a structural arrangement of axons
• in universal (nuclear) cells, we have lamins A, B, and C, which support the nuclear structure

They are usually stable and not dynamic (once formed, they are stable). However, lamins are an exception as the nuclear membrane reforms during mitosis.

Question 9

How do IF polymers form?

Answer 9

We have the intermediate filament protein (which is the monomer).

It forms a helical dimer. The two dimers combine to form a tetramer (the fundamental unit of the IF).

The tetramers link in a staggered formation and end-to-end to form the filament.

Subunit exchange is slow but occurs throughout the length of the filament.

Question 10

What are the interactions between IFs and the cytoskeleton?

Answer 10

IFs can link actin mircofilaments and microtubules
e.g. plectin

There are proteins that link together these different parts of the cytoskeleton.

Question 11

What are some clinical considerations to do with intermediate filaments?

Answer 11

Mutations in keratins can result in the skin blistering disease epidermolysis bullosa simplex (EBS).

Abnormal expression of neurofilamins can result in the neurodegenerative disease amyotrophic lateral sclerosis (ALS).

Question 12

Describe microtubules.

Answer 12

The tubulin monomer is a heterodimer: α-tubulin and β-tubulin. A protofilament is a long chain of alternating tubulin monomers.

The protofilament has a (-) and (+) end. Monomers can be rapidly added and removed from both (+) and (-) ends.

13 parallel protofilaments are arranged in a hollow tube. The microtubules are approx. 25nm in diameter.

Question 13

Describe Microtubule-Organising Centres (MTOC).

Answer 13

Microtubules usually have one end attached to a Microtubule-Organising Centre (MTOC). Microtubules “grow out” from MTOC until they reach their destination and then are stabilised.

One major MTOC is associated with the nucleus.

Question 14

What are the functions of microtubules?

Answer 14

They are a dynamic scaffold:
- they make up the spindle for chromatid separation during mitosis

They can move cargo to specific locations in the cell.

They are the central internal support of cilia.

They stabilise structure of cells e.g. platelets

They organise the structure of organelles
e.g. endoplasmic reticulum.

Question 15

Describe the spindle formation.

Answer 15

The spindle consists of microtubules. The spindle formation is initiated from the centrosome (which is a type of Microtubule-Organising Centre).

Centrosomes contain a pair of centrioles. Centrioles contain stable fused microtubules (so they are slightly modified microtubules).

Centrosomes form at two poles of cell. Kinetochore microtubules attach to the centromere of chromatid. Additional aster microtubules attach the centrosome to cell membrane, and thus the spindle is formed.

Question 16

How is cargo moved around the cell?

Answer 16

There are two motor proteins associated with microtubules

Kinesin moves it towards the (+) end, towards the cell periphery.
Dynein moves it towards the (–) end, near the nucleus.

ATP is hydrolysed to move cargo along the microtubule.

The vesicle is attached via kinesin or dynein on the microtubules, and this then provides movement of the vesicle along the microtubule.

Question 17

The microtubule transport provides ‘tramways’ within the cell.

Give an example of a cell where this is important.

Answer 17

This becomes relevant with neurones, as the nucleus is far from the site of neurotransmitter release, so having a structured mechanism of movement is more efficient rather than random movement.

Question 18

Describe cilia.

Answer 18

They are membrane bound, hair-like extensions. Microtubules are the central support for these structures.

Their MTOC is called the Basal Body, and is located close to the membrane.

Microtubules also facilitate the movement of components up and down within cilia.

All cells have single primary cilia. However, they are disassembled during mitosis.

There are some pecialised types of cilia:

• stereocilia in the inner ear – used for sound detection
• motile cilia in respiratory / lung ciliated epithelia – used to beat and move fluid around

Question 19

How do stereocilia in the ear detect sound (simply explained)?

Answer 19

The cells are depolarised or hyperpolarised by deflections caused by sound, and the electrical signal is detected and processed.

Actin filaments keep the stereocilia rigid.

Question 20

What additional structure in motor cilia allow it to be mobile?

Answer 20

The motor cilia have additional microtubule structures in the centre, which have a dynein component which provides ATP synthesis involved in movement of that cilia.

Question 21

Describe some clinical implications of cilia (ciliopathies).

Answer 21

Situs inversus:

• it is a congenital condition where the organs are in mirrored position
• this is as a result of a defect in cilia-mediated movement of growth factors in the embryo

Autosomal Dominate Polycystic Kidney Disease (ADPKD):

• it is one of most commonly inherited genetic disorders
• there is a formation of kidney cysts that expand and disrupt normal tissue
• the mutated proteins (polycystin-1 and -2) are associated with abnormal function of primary cilia

Question 22

What are some clinical considerations of microtubules?

Answer 22

The disruption of the mitotic spindle can interfere with cell division.

They are also implicated in anti-cancer chemotherapeutic agents:

• colchicine: works by binding tubulin monomers to prevents microtubule formation
• taxol: works by binding and stabilising microtubules

Question 23

What is the role of the cytoskeleton in cell-cell adherence?

Answer 23

The junctions between cells are connected to the cell cytoskeleton:

• desmosome
• gap junctions

The attachments to the ECM are also connected to the cell cytoskeleton

• hemidesmosomes
• focal adhesions