MCB 10: Assembly of Cells into Tissues I (Part I: The ECM) Flashcards
1
Q
What is in the spaces between cells?
A
- the extracellular matrix
- tissue fluids
2
Q
What is interstitial space?
A
- the ‘unspecialised’ matrix-containing extracellular spaces in tissues
3
Q
What is the extracellular matrix (ECM)?
A
- the ECM is a system of macromolecules secreted by cells that contributes to filling the extracellular spaces
- they are the non-living, non-cellular, insoluble component of the extracellular environment
- made up of a complex arrangement of proteins and carbohydrates
- it is present in the spaces between cells and comprises of both fibrillar and non-fibrillar components
4
Q
What are the functions of the ECM?
A
- provide physical support
- determines the mechanical and physiochemical properties of the tissue
- influences the growth, adhesion and differentiation of cells
- essential for development and tissue function
5
Q
What do these diagrams show about the presence of ECM in liver tissue?
A
- liver tissues are highly cellular, with relatively small interstitial spaces
- ECM is still present
- the dark-stained material on the right is a meshwork of ECM supporting the cells
- on the left, the light gaps are blood vessels called sinusoids
6
Q
What does this diagram tell us about the ECM presence in connective tissues e.g. tendon?
A
- connective tissues e.g. tendon, have cells forming a small proportion of their content
- most of the pink staining region is the ECM protein, collagen
7
Q
Why do you think tendons have a significantly higher proportion of ECM than liver cells?
A
- connective tissues, such as tendons, bones, ligaments, fibrous layers, often play structural, mechanical and protective roles in the body
- note: that all tissues have some ECM
8
Q
Give an example of a common tissue configuration and how the ECM plays a role in it
A
- tubes: e.g. the intestinal tube
9
Q
What does this diagram tell you about how essential ECM is?
A
- ECM is an essential component of all metazoans (multicellular animals)
- it is as ancient as multicellular life itself
10
Q
What are the three major components of the ECM?
A
- fibrils and fibres (collagen and elastin)
- proteoglycans and glycosaminoglycans
- modular adhesive glycoproteins
11
Q
Describe collagen
- type of protein
- which organisms it is found in
- where in the body is it present
- how abundant it is
A
- collagen is a fibrous protein
- found in all multicellular organisms
- found as a major protein in the skin, bones, tendon
- it is the most abundant protein in mammals: 25% of protein mass
12
Q
How many members of collagen are there?
How are they categorised?
A
- there are 28 collagen types
- they are assigned Roman numerals and categorised according to the structure they form
- see table for examples
13
Q
Which is the most common type of collagen?
A
- type I
- accounts for 90% of the collagen in our bodies
14
Q
Describe the structure of collagen
A
- it is made of three protein alpha chains that form a triple helix
- in fibrillar collagens, each alpha chain is approx 1000 amino acids
- it forms a left-handed helix (anti-clockwise)
- the amino-acid sequence is a glycine-X-Y repeat
- X and Y can be any amino acid
- X is often proline
- Y is often hydroxyproline
- a stiff triple helical structure is formed with every third amino acid being glycine
- this is because glycine is small enough to be in the interior so that a tight helix can be formed
15
Q
Describe how collagen fibres (fibrillar collagen) are synthesised
A
- the pro-alpha-chain (meaning the precursor of collagen) is first synthesised in the endoplasmic reticulum, like most secretory proteins
- it is then transported to the Golgi apparatus where it undergoes hydroxylation and glycosylation
- three pro-alpha-chains self-assemble and the triple helix forms, with propeptide chains (C-terminal and N-terminal sticking out either end)
- the procollagen triple helix is then secreted out of the cell into the ECM
- the propeptides on the molecule are cleaved by peptidases, leaving a collagen molecule (1.5nm)
- these molecules self-assemble into a fibril (10-300nm)
- collagen fibrils then aggregate in a staggered way to form a collagen fibre (0.5-3µm)
16
Q
How is vitamin C required in the formation of collagen?
A
- in the hydroxylation of lysine and proline, iron and vitamin C are needed by the hydroxylase enzymes
- hydroxylation is required for cross-linking
- there are two types of cross linking:
- within alpha-chains in a collagen molecule (intra-molecular bonding)
- between fibrils (intermolecular)
- with a vitamin C deficiency (scurvy) you see a lot of connective tissue issues
17
Q
What are two other structures that collagen can form apart from fibrils?
A
- fibril-associated collagens: help to support fibril collagen
- network-forming collagen: present in all basement membranes which is a thin, delicate membrane of protein fibres and mucopolysaccharides separating an epithelium from underlying tissue (molecular constitution varies though)
18
Q
What function does collagen have in tissues?
A
- Collagen is important for the tensile strength in tissues
- they resist stretching forces
19
Q
What are elastic fibres important for in tissues?
A
- they are important for elasticity in tissues
20
Q
Why are collagen and elastic fibres often interwoven?
A
- this is so collagen can limit the stretching of elastic fibres
21
Q
What is an elastic fibre made up of?
A
- the protein, elastin, is the core
- it is covered with microfibrils, rich in the protein fibrillin
22
Q
What makes elastin so stretchy?
A
- the elastin protein consists of repeats of alternating stretches of hydrophobic and hydrophilic amino acids
- in its relaxed state, the hydrophobic parts curl up to avoid being exposed to an aqueous environment
- when physically stretched, the hydrophobic region is opened
- when released, the elastin molecules then curl again to hide their hydrophobic regions
- there is covalent cross-linking of individual elastin molecules
- this allows the formation of strong elastic fibres
23
Q
What are glycosaminoglycans (GAGs)?
A
- they are unbranched polysaccharides made of repeated disaccharides
- one is a uronic acid
- the other is an amino sugar
- amino sugar can be: N-acetylglucosamine or N-acetylgalactosamine
- uronic acid can be: glucuronic acid or iduronic acid
24
Q
What are proteoglycans?
A
- they are GAGs covalent attached to proteins
25
Q
Why do most GAGs have a negative charge?
A
- the sugars are sulphated
26
Q
What is a property of glycosaminoglycans?
A
- they give tissues a spongy consistency, allowing them to be compressed and recover their shape
- they are large, highly-hydrated molecules
27
Q
How do GAGs give tissues sponginess?
A
- the stiffness of the polysaccharide chains and their highly hydrophobic nature means that GAGs do not fold into compact structures
- the many hydroxyl groups and the fact that they are charged means that they will attract a lot of water and occupy a lot of volume
- tissues that are rich wit GAGs and proteoglycans can be squeezed and water comes out, and when pressure is released, water is taken back up again
28
Q
How many and what size of glycosaminoglycan chains are there attached to form proteoglycans?
A
- the number and size of GAG chains covalently bonded to a protein is highly variable
- most GAGs are associated with proteins
29
Q
Describe these two proteoglycans
A
Decorin:
- a relatively simple proteoglycan, with only one GAG chain linked to a moderately-sized core protein
- involved in binding to collagen and controlling fibrillogenesis
Aggregan:
- a long core protein with many GAG chains attached
- a key component of cartilage, giving it compressive properties
30
Q
Describe the feature of the GAG, hyaluronan
- Where it is found
- Its function
- Structure
- Site of synthesis
A
- also known as hyaluronic acid
- is found as a major component of the ECM
- important in space-filling in tissues, including in wound healing and is present in synovial joints a lubricant
- it is a carbohydrate chain with no core protein
- the single chain has up to 25,000 repeated disaccharides
- it is the simplest GAG, but it is very large
- it is synthesised at the cell surface, not in the ER/Golgi
31
Q
Describe the association of many Aggrecan molecules with hyaluronan
A
32
Q
What are glycoproteins and what do they help do in the ECM?
A
- they are proteins with carbohydrate glycan chains attached
- a variety of modular, multidomain glycoproteins are key partners in organising the ECM
33
Q
What is the difference between proteoglycans and glycoproteins?
A
- proteoglycans have long unbranched chains with disaccharide units as repeating structure
- its carbohydrate unit is 50-60%
- glycoproteins have short highly branched glycan chains with no repeating unit
- its carb unit is 10-15%
34
Q
What are modular ECM glycoproteins?
- What is their function
- How many types are there?
- What does modular mean?
A
- modular ECM glycoproteins are involved in matric organisation and also provide adhesive binding sites for cells
- there are estimated around 200 ECM glycoproteins
- they are modular: meaning their sequence is made of identifiable protein domains which have particular structures and functions
35
Q
In what way are modular glycoprotein ECM molecules organised in the matrix?
A
- the molecules often self-associate and form multimers
- they have binding sites for other matrix molecules and for receptors on cell surfaces (they are also matrix-cell adhesion molecules)
36
Q
What are two important examples of modular ECM glycoproteins?
A
- fibronectin
- laminin
37
Q
Describe fibronectins and their structure
- What forms they exist in
- How they are derived
- Size
- Binding properties
- Function
- Mutations
A
- they are major connective tissue glycoproteins
- they are a family of closely related glycoproteins of ECM and body fluids
- they exist as an insoluble fibrillar matrix or as a soluble plasma protein
- they are derived from one gene as a result of alternative splicing at the mRNA level
- they are large multidomain molecules, capable of interacting with cell surface receptors and other matrix molecules
- they can bind to more than one ligand (multi-adhesive)
- they regular cell adhesion and migration in embryogenesis and in tissue reapir
- they are also important in wound healing and can promote blood clotting
- there are no known functional mutation in humans so they are essential for life
38
Q
What are laminins?
A
- they are basal lamina glycoproteins
- they consist of three chains: alpha, beta, gamma, forming a cross-shaped molecule
- they are very large: each chain being between 160 and 400kDa
- they interact with cell surface receptors such as integrins
- they are multi-adhesive as they can bind to more than one protein ligand
- laminins can self-associate as part of the basement membrane matrix
- they can also interact with other basal lamina components such as type IV collagen, nidogen and proteoglycans
- specific laminin chain mutations are associated with inherited diseases, such as muscular dystrophy and epidermolysis bullosa
39
Q
What is the basal lamina?
A
- also called the basement membrane
- it is a thin, flexible mat of extracellular matrix underlying epithelial layers
- it separates the cells from the interstitial environment
40
Q
What cell types possess a basal lamina?
A
- muscle cells
- epithelial cells
- kidney glomerulus cells
- peripheral nerve
- fat cells
41
Q
What is this diagram showing?
A
- a schematic of a basal lamina
- note that collagen IV and laminin are key components
