Extracellular matrix

Question 1

Recall the definition and function of, the ECM; and explain the relationship between the ECM and connective tissues

Answer 1

Extracellular matrix ;

definition; it is a complex network of proteins and carbohydrates (filling spaces between the cells)

function;

1. physical support (mechanical stability)
2. influences growth and adhesion
3. essential for development and tissue function

connective tissue; it is rich in ECM

• ECM + Component cells (macrophages)
• Collagen I II III IV*
• Multi-adhesive glycoproteins
  
  • Fibronectin, fibrinogen, laminins*
• Proteoglycans
  
  • Aggrecan, decorin, perlecan*

Question 2

Recall examples of human disorders resulting from ECM pathology

Answer 2

1. Excess ECM deposition can laed to
   
   • Cirhosis
   • Kidney fibrosis (Diabetic nephropathy; restriction on capillaries not allowing renal filtration)
   • Lung fibrosis
2. Loss of ECM can lead to
   
   • Osteoarthritis (cartilage properties are lost )
3. Mutation in fibrillin-1 leads to Marfan’s Syndrome
   
   • Fibrillin found in microfibrils in elastic fibres
   • integrity of elastic fibers depends on microfibrin

Osteoarthritis

1. Erosive disease resulting in excessive ECM degradation
2. Cushioning cartilage properties lost
3. Aggrecan cleaved by aggrecanases and metalloproteinases
4. Lost in synovial fluid
5. most common disease in the UK
6. strong genetic component
7. There is inflamation

Question 3

List the major components of the ECM, summarise their molecular characteristics and functional specialisation

Answer 3

Consists of;

1. Collagen
   
   • Major fibrillar proteins providing tensile strength
2. Multi-adhesive glycoproteins
3. Proteoglycans
4. Basement membrane
   
   • Collagen IV and laminins

Collagen is made up from hydroxproline

1. makes cross-linkages covalent bonds
2. requires vit.C and iron
3. provides tensile strenght and stability

Question 4

Recall the definition, function and composition of elastic fibres

Answer 4

Definition; are bundles of proteins (elastin)

Found in ; extracellular matrix of connective tissue

Produced by; fibroblasts and smooth muscle cells in arteries.

Function; Important in tissue elasticity (skin, blood vessles, lungs)

Composition; core is made up from

1. elastin; consists of two types of segments which alternate along the polypeptid chain–> hydrophobic regions and a-helical regions (rich in alanine adn lysine)
2. microfibrils which are rich in protein fibrillin

Question 5

Recall the definition of, explain the function of, and list the major components of the basement membrane

Answer 5

Definition; aka basal laminae, is a flexible thin mat of extracellular matrix underlying epithelial sheets and lamins

Component; collagen IV and lamins

Function; surround muscles, peripheral nerve and fat cells, underlie most epithelia

Question 6

Explain the biosynthesis of collagen, summarise the structural and functional post-translational modifications, explain assembly and recall functions

Answer 6

Collagen;

1. family of fibrous proteins, found inall multicellular organisms
2. most abudant protein in mammals (28 different types known and 25%of total protein mass)
3. most abudant in skin, tendons and bones
4. major component of ECM
5. fibroblasts produce collagen
6. Alignment; successive layers nearly at right angles to each other ( same in skin, cornea and bones–> resists tensile force)

molecular structure;

there are around 28 different types of collagen but 42 genes encoding them. This is because each collagen molecule consists of 3- a- chains forming a triple helix

Every third position is glycine, because it has a small size and there is no space near the helix axis for the side chain of any other residue.

Biosynthesis;

Inside the cel;

1. Synthesis of pro-a chain
2. Hydroxylation of Lysines and prolines
3. Glycosylation of Hydroxylasines ( product of step 2)
4. Self assembly of three pro-a chains
5. Procollagen triple helix formation
6. Secretion

Outside the cell;

1. peptides remove pro-peptides
2. formation of fibrilant collagens ( each alpha chain is around 1000 amino acids)
3. formation of latteral bonds to generate fibrils (covelent cross links) –> provide tensile strenght and stability

• Normal pathway of synthesizing proteins; 3 alpha chains come together to form procollagen EXCEPT
  
  • Alpha chains are synthesized as longer precursors by ribosomes on ER
  • Pro-alpha chains then undergo series of covalent modifications
  • Triple-helical pro-collagen
• THEN secreted from cell
• Extracellular peptidases remove the propeptides (N and C regions) on the procollagen
• Lysine and hydroxylysine are modified when forming covalent cross-linkages
  
  • Enzymes involved: Prolyl and lysyl hydroxylases
  • Requires VITAMIN C and IRON (lack of those results in poor collagen hydroxylation and may cause Scurvy)

The final helix produced in fibrillar collagen is left-handed

Question 7

Laminins

Answer 7

1. Basement membrane of glycoproteins
2. Very large molecules
3. Consist of three chains ( a,b,g in greeek)–> it is a cross-linked molecule
4. Interact with receptors such as integrins and dystroglycan
5. Self-associate as part of basement membrane OR with other BM components
6. Collagen IV, nidogen, proteoglycans
7. Mutations associated with inherited diseases
   
   • Muscular dystrophy (a2 of laminin 2 is missing)/epidermolysis bulosa

Question 8

Fibronectins

Answer 8

1. Large molecule
2. Exist as insoluble fibrillar matrix or soluble plasma proteins
3. They are essential for life and there are no known human mutations
4. Important roles in regulating cell adhesion, migration, embryogenesis and tissue repair (promote wound healing)
5. Forms disulfide-linked dimers
6. Form a mechanical continuum with actin cytoskeleton of multiple cell types
7. Integrin receptors to them at cell surface and actin cytoskeleton inside cell

Question 9

GAGs

Answer 9

1. Long unbranched sugars consisting of repeating disaccharide units
2. Occupy huge volume relative to mass
3. Produce hydrated gels (resistant to compression)
4. Sulfated and highly negatively charged
5. Chondroitin sulfate, dermatan sulfate, heparan sulfate, keratan sulfate
6. Most GAGs are synthesized and attached to their core protein in the ER and Golgi except for Hyaluronan is unique and has no core protein (simple carbohydrate chain)

Question 10

Aggrecan

Answer 10

1. Carries up to 100GAGs
2. Major cartilage matrix constituent
3. Associates with hyaluronan to form supramolecular complexes
4. Aggrecan aggregates
   
   • Hyaluronan (GAG)
   • Link protein
   • Aggrecan (proteoglycan)
5. Highly sulfated and negatively charged
6. Resists compressive forces in cartilage via water retention
7. Large amounts of water are retained by this molecule ( this is due to its negative charge which attracts osmotically actice Na)

Question 11

Decorin

Answer 11

1. small proteoglycan
2. essential for fiber formation
3. Single GAG attached
4. Dermatan sulfate chain
5. Binds to collagen fibers and regulates fibrillogenesis
6. Lack of results in fragile skin with reduced tensile strength (tested in mice)

Question 12

Hayaline Cartilage

Answer 12

1. Most abudant type of cartilage
2. found in; nose, larynx, trachea. bronchi, articular ends of long bones
3. It is rich in aggrecan (–> binds to hyaluronan at the binding region with the aid of a link protein)