Extracellular matrix

Question 1

What is the structure of connective tissue?

Answer 1

• between 2 epithelial layers
• Muscle between 2 layers of fibroblasts –>make ECM components
• In connective tissue cells are isolated and don’t tend to touch each other
  Very rich in ECM

Question 2

Define ECM?

Answer 2

ECM: complex network of carbs and proteins filling space between cells

Question 3

What are the 3 functions of ECM?

Answer 3

1. Physical support
2. Determines mechanical and physiochemical tissue properties
3. Influences growth, adhesion, differentiate status of cell with which it interacts

Question 4

What are the varied properties of connective tissue?

Answer 4

• Tendons and skin: tough and flexible
• Vitreous humor: soft and transparent
• Bone: hard and dense
• Cartilage: resilient and shock absorbing

Question 5

What are some disorders affecting matrix proteins?

Answer 5

• Osteogenesis imperfecta (Type 1 collagen)
• Marfan’s Syndrome (Fibrillin 1)
• Alport’s syndrome (Type 4 collagen)
• Epidermis bullosa (all 3 chains of laminin 5)
• Congenital muscular dystrophy (laminin 2)

Question 6

What are some characteristics of collagen?

Answer 6

• 28 different types
• Most abundant proteins in mammals
• Major proteins in bone and skin and tendons (white fibrous connective)
• Inelastic but high tensile strength

Question 7

How do collagen fibrils align?

Answer 7

• Different fibrils laid down nearly parallel to each other
• In skin and mature bone and cornea, successive layers at right angles to resist tensile forces in all directions
• ln tendons found in parallel bundles to resist force in one direction

Question 8

What are the molecular constituents of collagen?

Answer 8

• 42 genes code for
• 3 alpha polypeptide chains stiff triple helix - can be composed of 1 or different alpha chains
• Type I (most abundant) has chains from 2 different: [a1(I)]2 [a2(I)]
• Type II and III only have one chains type [a1(II)]3 and [a1(III)]3

Question 9

What is the structure collagen fibrils?

Answer 9

• Each helix wraps around the other
• Every third position must be glycine - only AA small enough to occupy helix interior so Gly-X-Y is characteristic repeat
  ○ X often proline
  ○ Y often hydroxyproline
• In fibrillar collagens each chain ca. 1000AA for LH-helix

Question 10

How is collagen biosynthesis?

Answer 10

1. Made from procollagen in ER
   
   • Procollagen have non collagenous domains at N and C terminal which are removed after secretion in fibrillar collagens but not in most other types
2. Lysine and proline hydroxylation:
   
   • Carried out by prolline and lysine hydroxylases
   • lysine and hydroxylysine modified during covalent cross linkage formations (post collagen secretion)
   • Enzyme require vitC and iron to function (needed for hydroxylation)
3. Cross linked and staggered arrangement: tensile strength and stability
   
   • Involves hydroxylysine and lysine residues
   • Type and extent of cross link in tissue changes with age

Question 11

Give two collagens that aren’t fibrils?

Answer 11

Fibril associated collagens e.g Type 9 and 12
- Associate with fibrillar collagens and regulate size and organization of collagen fibrils
Network forming collagen 4

Question 12

How do type 4 collagens assemble?

Answer 12

can associate laterally between triple-helical segments as well as head-to head and tail-to tail between the globular domains to give dimers, tetramers and higher order complexes.

Question 13

Alport syndrome:

Answer 13

• mutations in collagen 4 causing abnormally splot and laminated GBM
• progressive kidney functions and hearing loss

Question 14

What are elastic fibres?

Answer 14

• Elastic core + microfibrils rich in fibrillin
• Important for skin, blood vessel and lung elasticity
• Interwoven with collegan to limit overstretching

Question 15

What is the structure of elastin?

Answer 15

Elastin: 2 segment types that alternate along polypeptide chain

1. Hydrophobic regions
2. Alpha helical regions (riched alanine and lysine)
   
   • Like rubber bands change configuation when stressed; when stress removed returns to original configuration

Question 16

Marfan’s Syndrome:

Answer 16

• Tall, skinny, long limbs and fingers
• Some predisposed to aortic ruptures
• elastic fibre integrity depends on microfibrils containing fibrillin

Question 17

Define basement membrane:

Answer 17

• Flexible thin mats of ECM underlying epithelial sheets and tubes containing collagens, glycoproteins, proteoglycans
• Surround muscle, peripheral nerves and fat cells
• Acts as highly selective filter

Question 18

What are the characteristics of ECM proteins?

Answer 18

• ECM proteins usually very large
• Modular architecture: characteristic protein domains 50-200 AA
• Multifunctionality result of modular structure
• Many large ones are multi- adhesive - bind to various components and surface receptors
• glycosylated and are either glycoproteins or proteoglycans.

Question 19

What are the 3 main components of basement membrane?

Answer 19

1. Collagens
   ○ (1,2,3 fibrillar),
   ○ (4 basement membrane only)
2. Mutliadhesive glycoproteins
   ○ Firbonectin
   ○ fibrinogen,
   ○ laminins (BM only)

3. Proteoglycans
		○ aggrecan,
		○ versican
		○ decorin (surface of collagen fibres), 
		○ perleccan (BM only)

Question 20

What is the structure of laminins?

Answer 20

• Contain 3 chain: alhpa, beta, gamma
• Form very large cross shaped molecule
• Derived from several genes
• Multiadhesive; interact with surface receptors (integrins and dystroglycans)
• Can self associate as part of BM and interact with other ECM componenets
• At N terminus have globular regions
• Coiled coil domain: 3 chains wrapped around each othervery large

Question 21

What is the function of laminins?

Answer 21

• tissue differentiation
• cell-matrix junction formation
• cell migration.

Question 22

Congenital muscular dystrophy:

Answer 22

• absence of alpha 2 in laminin 2
• Hypotonia (abnormally decreased muslce tension)
• Generalised weakness
• Joint deformities

Question 23

What is the structure of fibronectin?

Answer 23

• V shaped
• Major connective tissue glycoprotein
• Can exist as insoluble fibrillar matrix or soluble plasma protein
• Derived from 1 gene; splicing gives rise to other forms
• Multiadhesive (interact with surface receptor and other matrix molecules)
• Large dimer with 2 subunits linkes by disulphide bridge

Question 24

What is the function of firbonectin?

Answer 24

1. Tissue repair - particularly wound healing (promotes clotting)
2. Regulating cell adhesion
3. Migration in embryogenesis

Question 25

How does fibronectin form a mechanical continuum with the actin cytoskeleton?

Answer 25

- Extracellular regions of integrin (on transmembrane receptor on cell surface) links with fibronectin and the intracellular one with actin

Question 26

What is the structure of proteoglycans?

Answer 26

- core proteins and 1 or more glucosaminoglycan (GAG) chains covalently attached - GAG chains are long, unbranched sugars consisting of repeating disaccharide ○ Occupy large volume relative to mass

Question 27

What is the function of proteoglycans?

Answer 27

○ Form hydrated gels | ○ Can be reistant to compression

Question 28

What are the categories of proteoglycans?

Answer 28

1. Basement membrane e.g perlecan 2. Small lycine rich e.g decorin 3. Aggregating (interact with hyaluron) e.g. aggregon 4. Cell surface e.g. syndecans 1-4

Question 29

What is the structure of a GAG?

Answer 29

1/2 of sugars is a disaccharide - amino sugar - Highly negatively charged - Many are sulfated/carboxylated - negatively charged - Small PG have 1 GAG chains can be large

Question 30

What are the four main groups of GAGs?

Answer 30

1. Hyaluron 2. Heparan sulfate 3. Keratan sulfate 4. Chondroitin sulfate/dermatan sulfate

Question 31

What is unique about Hyaluron?

Answer 31

- Uique because no core protein, only carb chain - technically not PG, just long dissacharide - Synthesized in cell surface (not ER/Golgi) - Unsulfated - Single long chain up to 25,000 repeat disaccharides - GAG chains and core protein linked via link tetrasaccharide

Question 32

What is the function of decorin?

Answer 32

- Regulates collagen size and arrangement | - Essential for fibre formation

Question 33

What is the structure of (Hyaline) Cartilage?

Answer 33

- filamentous network of PG with embedded collagen fibril - Most abundant cartilage type - Rich in aggrecan

Question 34

What is the function of aggrecan?

Answer 34

- core protein linked to hyaluronic | - Aggrecan aggregates: complex of aggrecan, hyaluron, link proteins

Question 35

What is the function of cartilage?

Answer 35

- negative charge attracts osmotically active cations (sodium) so environment retains a lot of water forming a gel ○ If put pressure water is squeezed out and returns when remove pressure ○ Cushions ends of long bones (b/c aggrecan)

Question 36

Why does osteoarthritis occur?

Answer 36

- excessive ECM loss - Strong genetic component but complex - Can affect any joint but often fingers and knees - Loss of cartilage, inflammation, new bone formation causing join stiffened and rubbing of bone against bone - Aggrecan is degraded/cleaved and fragments lost in synovial fluid ○ Can't form aggregan aggregates

Question 37

Why do fibrotic disorders occur?

Answer 37

- too much ECM (fibrous connective tissue) so normal tissue replaced by collagen - Scar tissue