Adhesion of Cells and the Extracellular Matrix

Question 1

What are the components of connective tissue?

Answer 1

• Cells (scattered)
• Extracellular matrix
  
  • Fibrillar proteins
  • Hydrated gel of GAGs
    
    • The majority of GAGs are linked to specific proteins to make proteoglycans.

Question 2

Which cells are found in connective tissue?

Answer 2

• Fibroblasts
• Myofibroblasts
• Blood derived cells
  
  • Mast cells
  • Plasma cells
  • Macrophages
• Chondroblasts - cartilage
• Osteoblasts - bone
• Adipocytes

Question 3

What are the components of the connective tissue extracellular matrix?

Answer 3

• Fibrillar proteins
  
  • Collagen - strength
  • Elastin - stretch
  • Fibronectin - enables networking
  • Laminin - enables networking
• Polysaccharides glycosaminoglycans (GAGs) (in proteoglycans)

Question 4

Describe the structure and function of fibroblasts.

Answer 4

• Large cell (because it carries out protein synthesis).
• Long extensions.
• Synthesises and secretes collagen, elastin and proteoglycans.

Question 5

Decribe the production of collagen.

Answer 5

• Starts with a procollagen molecule in the RER.
• Hydroxylation of proline and lysine residues in the RER.
• Then, glycosylation of the molecule which adds either galactose or glucose to some lysine residues.
• Once this happens, the molecule becomes a triple helix.
• Then, this procollagen is secreted into the golgi, then into the EC matrix.
• Once procollagen is in the ECM, enzymes called procollagen peptidases cleave the terminal ends of the procollagen.
• Once it has cleaved the ends at the N and C terminal, the molecule is tropocollagen (highly insoluble).
• Tropocollagen molecules can aggregate to form collagen fibrils.
• Tropocollagen molecules become reinforced by hydroxy-lysine cross-links between molecules when 2 are in close contact.
  
  • These cross links are formed by the lysyl oxidase enzyme.

Question 6

Describe the structure of a collagen fibril.

Answer 6

Many staggered collagen molecules that are linked by the lysine hydroxylysine cross links which are formed by the lysyl oxidase enzyme.

Question 7

How is collagen packaged in the RER?

Answer 7

Collagen is packaged into specialised (large) vesicles.

Question 8

Where and how does collagen secretion occur?

Answer 8

Collagen secretion occurs by exocytosis at specialised sites.

Question 9

Where are collagen fibrils produced and how are they organised?

Answer 9

• In a membrane tube called a fibripositor.
• Cells organise the collagen fibrils they secrete into fibres.

Question 10

Describe the components of elastin.

Answer 10

• Fibrous protein secreted by fibroblasts and other cells.
• Elastin likes to exclude water.
  
  • Hydrophobic effect is the main driving force for recoil.
• Made in fibroblasts and also smooth muscle cells and chondroblasts.

Question 11

What are proteoglycans?

Answer 11

Assemblages of glycosaminoglycans and proteins.

Question 12

What is the function of proteoglycans?

Answer 12

• Provide:
  
  • Matrix support / cushioning / hydration
  • Glue-like function
  • Links between protein of the ECM and cell surface

Question 13

Describe GAGs

Answer 13

• Long chains of repeating disaccharide units
• Highly charged (negative) and highly hydrated

Question 14

Describe how the extracellular matrix is linked to the intracellular cytoskeleton.

Answer 14

• Collagen / proteoglycans bind fibronectin that links to integrins which themselves bind via adaptors to the actin cytoskeleton.
• Fibronectin acts as an adaptor protein on the outside.
• Integrins bridge the membrane.

Question 15

Describe the structure and function of a myofibroblast.

Answer 15

• Bi-functional
  
  • ​Fibroblast-like
    
    • Secrete collagen
  • Smooth muscle-like
    
    • Synthesise actin, myosin and desmin

Question 16

Describe the involvement of myofibroblasts following tissue damage.

Answer 16

• Proliferate.
• Secrete collagen (scaffold).
• Consolidate damaged area (fibrous scar).
• Contract (reduce size of damaged area; express focal adhesions and smooth muscle actin).

Question 17

How are myofibroblasts formed and why are they important?

Answer 17

They differentiate from fibroblasts under mechanical tension - especially important in wound healing. Also active in tissue fibrosis eg. liver cirrhosis.

Question 18

Which type of cell is shown?

Answer 18

Mast cell (granules contain heparin and histamine).

Question 19

Which type of cell is shown?

Answer 19

Plasma cell (antibody production is through the secretory pathway).

Question 20

Which type of cell is shown?

Answer 20

Macrophage (ingesting listeria bacteria / paramecium).

Question 21

What are the functions of adipocytes?

Answer 21

• Insulation (subcutaneous)
• Packing (eg. the eye)
• Energy storage

Question 22

Describe the structure of an adipocyte.

Answer 22

• Peripheral cytoplasm
• Nucleus in the periphery

Question 23

What is leptin responsible for?

Answer 23

The satiety signal

Question 24

What is the function of cell junctions?

Answer 24

• Cell junctions / adhesion proteins link cells an their cytoskeleton to:
  
  • Other cells
  • And to the extracellular matrix

Question 25

What are the 4 types of cell junction?

Answer 25

* Tight junction * Adhesion belt * Desmosome * Gap junction

Question 26

What are the different types of cell-cell adhesion?

Answer 26

* Integrin * Selectin * CAM * Focal adhesion * Hemi-desmosome * Membrane proteoglycan

Question 27

What are the functions of tight junctions?

Answer 27

* Define polarity (fence-off membrane lipids and proteins). * Control the passage of substances between cells. * Can link to actin cytoskeleton.

Question 28

Describe adherens junctions.

Answer 28

* Plaque anchors actin filaments at the membrane (not intermediate filaments). * Not as dense as desmosomes.

Question 29

Distinct cadherins provide cell adhesion in different tissues. Give examples of this.

Answer 29

* E-cadherin - epithelia * N-cadherin - neurons and heart muscle * P-cadherins - placenta and epidermis * VE-cadherin - endothelial cells

Question 30

Decribe desmosomes.

Answer 30

* Link between strong intermediate filaments in adjacent cells. * Desmosomes are for strength. * Found in the skin.

Question 31

Describe the plaque and the filaments of desmosomes.

Answer 31

* **Plaque** * **​**Cytoplasmic dense plaque containing **desmoplakin** and **plakoglobin** proteins. * **Filaments** * **​**Keratin filaments (tonofilaments) anchored to the cytoplasmic dense plaque.

Question 32

Describe gap juctions.

Answer 32

* For communication * Hydrophilic channel * Small molecules pass * Coordination of function (important in the heart muscle).

Question 33

Describe focal adhesions.

Answer 33

* Focal adhesions link the outside of the cell (ECM) through transmembrane proteins (integrins) with the cytoskeleton (actin filaments). * Dynamic (eg. in fibroblasts). * Also act as signalling platforms. * Link to fibronectin.

Question 34

Describe hemidesmosomes.

Answer 34

* Hemidesmosomes link the outside of the cell (ECM) through transmembrane proteins (inegrins) with cytoskeleton (intermediate filaments). * More stable (eg. linking epithelial cells to the basement membrane). * Link laminin in the basement membrane.

Question 35

What are integrins?

Answer 35

A large family of proteins which bridges between cytosol and the ECM.

Question 36

Describe the molecular defect in Duchenne Muscular Dystrophy and how this leads to muscle dysfunction.

Answer 36

* Gene mutation - absence of dystrophin (adaptor) due to premature termination of translocation. * Causes: * Damage to mucle fibres due to muscle tearing * Muscle wasting * Muscle weakness * Unable to walk by 12 years

Question 37

Outline the recent experimental approach to correct the defect associated with DMD.

Answer 37

* PTC 124 (ataluren) is an experimental drug. * It is thought to override the premature stop signal mutation to produce normal dystrophin. * It also sems to work in the defective cystic fibrosis gene. * Findings: * Increasing doses of ataluren includes increased retention of dystrophin. * NICE approves ataluren for treatment of DMD.

Question 38

Describe the role of cell adhesion in cancer progression.

Answer 38

* Tumour cells accumulate (mutations disregulated cell cycle). * Cells have not breached the basement membrane. * Carcinoma is in situ. * Cells undergo epithelial to mesenchymal transition (EMT).

Question 39

Describe microinvasion of cell adhesion molecules in cancer.

Answer 39

* Cells convert to 'mesenchymal' cells and expression of cadherins is reduced. * Microinvasion starts aided by actin-based protrusions called **invadopodia**. * Secretion of metalloproteases (MMPs). * Basement membrane breached. * In invading tumours leading cells express inegrins promoting interaction with ECM and non-epithelial cells during movement.

Question 40

Describe the role of cell adhesion in cancer when progression of metastasis occurs.

Answer 40

* Autocrine motility factors from tumour (increases motility of tumour cells and decreases E-cadherin). * Angiogenesis factors (promote vascularisation). * Entry into and through lymphatics and blood vessels. * Dissemination - metastasis.