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
Why do most GAGs have a negative charge?
- the sugars are sulphated
26
What is a property of glycosaminoglycans?
- they give tissues a spongy consistency, allowing them to be compressed and recover their shape
- they are large, highly-hydrated molecules
27
How do GAGs give tissues sponginess?
- 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
How many and what size of glycosaminoglycan chains are there attached to form proteoglycans?
- the number and size of GAG chains covalently bonded to a protein is highly variable
- most GAGs are associated with proteins
29
Describe these two proteoglycans
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
Describe the feature of the GAG, hyaluronan
- Where it is found
- Its function
- Structure
- Site of synthesis
- 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
Describe the association of many Aggrecan molecules with hyaluronan
32
What are glycoproteins and what do they help do in the ECM?
- they are proteins with carbohydrate glycan chains attached
- a variety of modular, multidomain glycoproteins are key partners in organising the ECM
33
What is the difference between proteoglycans and glycoproteins?
- 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
What are modular ECM glycoproteins?
- What is their function
- How many types are there?
- What does modular mean?
- 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
In what way are modular glycoprotein ECM molecules organised in the matrix?
- 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
What are two important examples of modular ECM glycoproteins?
- fibronectin
- laminin
37
Describe fibronectins and their structure
- What forms they exist in
- How they are derived
- Size
- Binding properties
- Function
- Mutations
- 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
What are laminins?
- 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
What is the basal lamina?
- 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
What cell types possess a basal lamina?
- muscle cells
- epithelial cells
- kidney glomerulus cells
- peripheral nerve
- fat cells
41
What is this diagram showing?
- a schematic of a basal lamina
- note that collagen IV and laminin are key components
