Tissues 4- ECM Biology 2

Question 1

What are the characteristics of most ECM proteins

Answer 1

Most ECM proteins are very large. They have a modular architecture; ie, they are composed of characteristic protein domains of 50-200 amino acid residues. The multifunctionality of ECM proteins is a result of their modular structure. Many large modular proteins are multi-adhesive, binding various matrix components and cell-surface receptors.

Question 2

What are laminins

Answer 2

ubiquitous basement membrane glycoproteins
Consist of three chains, one each of an ,  and  chain, forming a cross-shaped molecule
Very large: each chain between 160 and 400 kDa
Multi-adhesive
Interact with cell surface receptors such as integrins and dystroglycan
Can self-associate as part of the basement membrane matrix - but can also interact with other components such as type IV collagen, nidogen and proteoglycans
Specific chain mutations associated with inherited diseases such as muscular dystrophy and epidermolysis bullosa

Question 3

Describe how laminins can bind to many different molecules

Answer 3

Different chains have different receptors complementary to different molecules

Question 4

Describe congenital MD

Answer 4

Absence of 2 in laminin 2
Symptoms evident from birth
Hypotonia (abnormally decreased muscle tension)
Generalised weakness
Deformities of the joints

Question 5

What are fibronectins

Answer 5

Family of closely related glycoproteins of ECM and body fluids
Can exist as an insoluble fibrillar matrix or soluble plasma protein
Derived from one gene - alternate splicing at the mRNA level- resulting in different isoforms
Multi-adhesive
Large multidomain molecule (dimer = 500 kDa), capable of interacting with cell surface receptors and other matrix molecules

Question 6

What are the roles of fibronectins

Answer 6

Important roles in regulating cell adhesion and migration in embryogenesis and tissue repair
Also important for wound healing (promote blood clotting)
No known mutations in humans - essential for life
Form a mechanical continuum with the actin cytoskeleton of many cell types

Question 7

What evidence is there that fibronectins are essential for life

Answer 7

There are no known mutations

Question 8

Describe the structure of fibronectin

Answer 8

A multi-domain molecule interacting with many ECM components and cell surface receptors. The basic unit is a 500kD dimer- V-shaped, dimers linked by disulphide bonds.

Question 9

Why does fibronectin form a mechanical continuum with the actin filament

Answer 9

Fibronectin binds to a transmembrane receptor, integrin on the extracellular side. The intracellular domain of integrin is bound to actin via adaptor molecules.
Fibronectin is also bound to collagen
Integrins bind to the RGD motif of fibronectin.
The actin anchors the integrin in the plasma membrane

Question 10

Describe the structure of proteoglycans

Answer 10

Core protein to which one or more glycosaminoglycan chains are covalently attached.
Glycosaminoglycans are long, unbranched sugars consisting of repeating disaccharides.
GAGs occupy a huge volume relative to their mass. These hydrated gels can be very resistant to compression.

Question 11

What are the differences in function between collagens and proteoglycans

Answer 11

Proteoglycans resist compression

Collagens resist stretching

Question 12

List some of the proteoglycan families

Answer 12

Several proteoglycan families based on structural and functional characteristics
Basement membrane: eg perlecan
Aggregating (interact with hyaluronan): eg aggrecan
Small leucine-rich, eg decorin
Cell surface: eg syndecans 1-4

Question 13

Describe the structure of GAG chains

Answer 13

One of the two sugars in repeating disaccharide is always amino sugar
Many GAGs are sulfated or carboxylated, and highly negatively charged
Small proteoglycans have a single GAG chain attached, whereas some large proteoglycans carry ~ 100 GAG chains

Question 14

What are the four main groups of GAG chains

Answer 14

Four main groups according to sugar types:

Hyaluronan
Chondroitin sulfate and dermatan sulfate
Heparan sulfate
Keratan sulfate

Question 15

What is the GAG chain in decorin

Answer 15

DS - dermatan sulfate

Question 16

What are the GAG chains in aggrecan

Answer 16

CS - chondroitin sulfate

keratan sulfate

Question 17

What is the GAG chain in syndecan

Answer 17

HS -

heparan sulfate

Question 18

What is Hyaluronan

Answer 18

Distinct features:
Hyaluronan is unique in being simply a carbohydrate chain. No core protein
Synthesized at the cell surface, not in the ER/Golgi
Unsulfated
A single long chain up to 25,000 repeated disaccharides
Not a proteoglycan

Question 19

How are GAG chains classified

Answer 19

Based on the nature of the repeating disaccharide

Question 20

How are GAGs linked to the core protein

Answer 20

Via a tetra saccharide linker protein

Question 21

Describe decorin

Answer 21

A small proteoglycan
Binds to collagen fibers
Essential for fiber formation
Mice that cannot make decorin have fragile skin with reduced tensile strength

Question 22

Describe the cartilage matrix

Answer 22

Filamentous network of proteoglycans with embedded type II collagen fibril

Question 23

Describe the hyaline cartilage

Answer 23

Most abundant type of cartilage

Found in the nose, larynx, trachea, bronchi, the ventral ends of ribs, and the articular ends of long bones

Rich in aggrecan

Functions to cushion ends of long bones

Question 24

Describe the structure of aggrecan

Answer 24

Chondroitin sulfate attachment region
Keratan sulfate attachment region
Hyaluronan binding region
Link protein
Hyaluronan

Question 25

How does aggrecan form supramolecular structures

Answer 25

Associates with hyaluronan to form aggrecan aggregates.

Question 26

Describe the role of aggrecan in cartilage function

Answer 26

The GAGs (e.g. chondroitin sulfate) of aggrecan are highly sulfated. Also present are large numbers of carboxyl groups.
These multiple negative charges attract cations such as Na+ that are osmotically active.
Large quantities of water are therefore retained by this highly negatively charged environment.
Under compressive load, water is given up, but regained once the load is reduced.
Therefore, aggrecan in the cartilage matrix is perfectly suited to resist compressive forces.

Question 27

What is osteoarthritis

Answer 27

Erosive disease - excessive ECM degradation. The cushioning properties of cartilage over the end of long bones are lost.
With age: cleavage of aggrecan by aggrecanase and metalloproteinase
Loss of aggrecan fragments to synovial fluid
Inflammation-new bones made- no cushioning- painful

Question 28

What are fibrotic disorders

Answer 28

Fibrotic disorders: excessive production of fibrous connective tissue
Liver cirrhosis
Lung fibrosis

Question 29

How are GAGs effective space fillers in the ECM

Answer 29

They occupy a huge volume relative to their mass.

Question 30

Describe how the cartilage is tough and resilient

Answer 30

GAGs are osmotically active, attracting large quantities of water into the matrix, creating a swelling pressure. This swelling pressure is balanced by tension in the interwoven collagen fibres, matrices rich in collage and GAGs have an enormous swelling pressure and counterbalancing tension.

Question 31

What are the other roles of proteoglycans

Answer 31

Form gels of varying pore size- regulate passage of molecules through ECM
Bind secreted growth factors and other proteins that serve as extracellular signals for cells.

Question 32

Describe the role of integrins in cell movement through the matrix

Answer 32

Formation and release of attachments at either end of the integrin molecule. Grabs hold matrix at front and lets go of it at its rear, catch and release method. Binding to integrin on one side can cause it to stretch out and extend to something on the other side- role in cell signalling and differentiation ( bind to protein kinases and activate them within cells)