Extracellular Matrix (ECM)

Question 1

What is the ECM?

Answer 1

Complex network of proteins and carbohydrates deposited by cells into the space between cells. It is made up of both fibrillar and non-fibrillar components

Question 2

What are the 2 broad roles of the ECM?

Answer 2

1. Mechanical stability (architectural)

2. Influence cell behaviour (instructional)

Question 3

What are the functions of the ECM?

Answer 3

Provide physical support
Determine the mechanical and physicochemical properties of the tissue
Influence the growth, adhesion and differentiation status of the cells and tissues with which it interacts

Question 4

What is connective tissue?

Answer 4

ECM + component cells (e.g. macrophages)

Question 5

What does connective tissue contain?

Answer 5

All connective tissues contain a distinct spectrum of:
Collagens - e.g. Type I, II, III (fibrillar), Type IV (basement membrane)
Multi-adhesive glycoproteins - e.g. fibronectin, fibrinogen, laminins (basement membrane)
Proteoglycans (extracellular matrix) - e.g. aggrecan, versican, decorin, perlecan (basement membrane)

Question 6

How do matrix components interact with each other?

Answer 6

Via specific cell receptors

Question 7

How does connective tissue show a variety of properties?

Answer 7

The combination / arrangement of different types of collagen coupled with different component cells
e.g. Vitreous humour (eyes) = soft and transparent but tendon = tough and flexible

Question 8

What are collagens?

Answer 8

A family of fibrous proteins found mainly in skin, tendons and bone
28 different collagen types exist in humans
Encoded by 48 different genes

Question 9

What is the molecular constitution of collagen?

Answer 9

Each collagen molecule has 3 alpha helices - forming a triple helix
Can be composed of one or more alpha chain types
Type I collagen - chains from 2 different genes [a1(I)]2 [a2(I)]
Types II and III - one chain type [a1(II)]3 and [a1(III)]3

Question 10

How is and what does the collagen triple helix form?

Answer 10

Primary sequence contains a characteristic glycine-x-y repeat
Every third position must be a glycine - this is the only amino acid small enough to occupy the interior of the triple helix (R group = H)
x is often proline and y is often hydroxyproline (hydroxylation of proline contributes to interchain hydrogen bond formation)
Lysine and hydroxylysine are similarly modified in the formation of covalent crosslinkages
3 alpha chains together forms a stiff triple helical structure

Question 11

What do crosslinks in collagen provide and how do they arise?

Answer 11

Tensile strength and stability
Involves lysine and hydroxylysine residues - Covalent crosslinking only takes place only after the collagen has been secreted

Question 12

How does Vitamin C deficiency (and iron deficiency) affect tissue stability (scurvy)?

Answer 12

Vitamin C-deficiency results in underhydroxylated collagens
Prolyl hydroxylase and lysyl hydroxylase require vitamin C as a co-factor and iron for functionality
Lack of hydroxylation results in the inability to form solid fibres

Question 13

What are the steps for collagen biosynthesis?

Answer 13

Collagen is made as procollagen in the endoplasmic reticulum
Procollagen has N and C regions which are not in triple helices
Once outside the cell - the N and C propeptides are removed
The collagen is then released and forms fibrils with cross-linkages between different collagen molecules

Question 14

How is tensile strength provided by the fibres?

Answer 14

Tropocollagen molecules form fibrils
These rearrange to form collagen fibres
Parallel bundles resist tensile force in one direction

Question 15

What is Ehlers–Danlos syndromes (EDS)?

Answer 15

A group of inherited connective tissue disorders whose symptoms include stretchy skin and loose joints
Mutations affect collagen production, collagen structure or collagen processing

Question 16

Do all collagen molecules form fibrils?

Answer 16

No

Question 17

An example of a non-fibrillar collagen?

Answer 17

Type IV (does not form fibrils) - present in all basement membranes
 Type IV collagen molecules assembling into a sheet-like network - associate laterally between triple-helical segments as well as head-to head and tail-to tail between the globular domains to give dimers, tetramers etc.

Question 18

What are basement membranes?

Answer 18

Flexible, thin mats of ECM underlying epithelial sheets and tubes
Surround muscle, peripheral nerve and fat cells and underlie most epithelia
Contain collagens, glycoproteins and proteoglycans

Question 19

In which organ is the basement membrane important and what does it form?

Answer 19

Kidneys

Glomerular basement membrane (GBM) - key part of the membrane unit

Question 20

What is the disorder diabetic nephropathy caused by and what does it lead to?

Answer 20

Accumulation of ECM leading to a highly thickened basement membrane
Restricts renal filtration and can lead to renal failure

Question 21

What is alport syndrome and what are the symptoms?

Answer 21

Mutations in collagen IV result in an abnormally split and laminated GBM which is associated with a progressive loss of kidney function and also hearing loss

Question 22

What are elastic fibres and what are they used for?

Answer 22

Elastin core and microfibrils, which are rich in the protein fibrillin
Important for the elasticity of tissues, e.g. skin, blood vessels and lungs
Often, collagen and elastic fibres are interwoven to limit the extent of stretching

Question 23

What is Marfan’s syndrome and what are the symptoms?

Answer 23

Mutations in the protein fibrillin-1
Lowers the integrity of the elastic fibres by forming abnormal microfibrils
Diverse manifestations, involving primarily the skeletal, ocular, and cardiovascular systems, arachnodactyly (spider-like fingers). Individuals can be predisposed to aortic ruptures

Question 24

What is elastin and how does it function?

Answer 24

Elastin - unusual protein with two types of segments that alternate along the polypeptide chain.
Two types of segment: Hydrophobic regions and a-helical regions (rich in alanine and lysine - many lysine side chains are covalently cross-linked)
Elastin is like a rubber band - it changes its configuration when stretched, when the stress is removed, it will return to its original configuration