Extracellular Matrix Flashcards
What is the extracellular Matrix (ECM)?
A complex network of macromolecules (carbohydrates and proteins) deposited by cells
What components does the ECM have?
Fibrillar and non-fibrillar components
What are the key functions of the ECM?
- Provides physical support
- Determines mechanical and physiochemical properties of the tissue
- Influences growth, adhesion and differentiation status of cells and tissues which it interacts with
- Essential for development, tissue function and organogenesis
What does connective tissue contain?
ECM is rich in connective tissue Contains a complete spectrum of: - collagens - multi-adhesive glycoproteins - proteoglycans with a cellular component
How do matrix compartments interact with cellular compartments?
Via cell surface receptors
What are the different collagens found in the ECM?
Type I (bone, skin and tendon) Type II (cartilage) Type III (fibrillar, blood vessels and reticulum) Type IV (basement membrane)
What are the different multi-adhesive glycoproteins found in the ECM?
Fibronectin
Fibrinogen
Laminins (basement membrane)
What are the different proteoglycans found in the ECM?
Aggrecan
Versican
Decorin
Perlecan (basement membrane)
What are collagens?
A family of fibrous proteins found in all multicellular organisms
Most abundant protein in mammals
How many collagen types are there in humans and how many genes encode for them?
At least 28
They’re encoded for by 42 different genes
How are collagen fibres aligned?
In skin, mature bone and cornea: have successive layers nearly at right angles to each other
Tissues resist force in all directions
How are collagen molecules structured?
Each collagen molecule has 3 alpha chains forming a triple helix:
can be a homotrimer or hetrotrimer
can have 1 or more different alpha chains
How is type I collagen structured?
Formed from 2 genes
Its a hetertrimer
[α1(I)]2 [α2(I)]
How is type II and III collagen structured?
have only 1 chain type
They’re homotrimers
[α1(II)]3 and [α1(III)]3
What is the primary sequence of collagens?
Primary sequence of collagen protein contains gly-x-y repeat
x is often proline and y is often hydroxyproline
Every 3rd position must be glycine to give a stiff structure- side chain is just a H so gives close packing
How long is an alpha chain in fibrillar collagen?
each alpha chain is approx. 1000 amino acids long forming a left handed helix
How are collagen fibres assembled?
One alpha chain joins 2 others to make a triple stranded collagen molecule
Many of these come together to form a collagen fibril
Lots of fibril come together to form a collagen fibre
How is collagen biosynthesised?
Collagen alpha chains are synthesised by long precursors called pro-alpha chains by ribosomes attached to the rough ER
Pro-alpha chains undergo a series of covalent modifications and fold into triple helical pro collagen molecules before their release from cells
3 pro-alpha chains collectively form pro collagen chain
This subsequently undergoes cleavage, fibril formation and cross-linkage
Why are cross-links in collagen important?
They provide tensile strength and stability
The type and extent of cross-links is tissue specific and changes with age
What is the importance of lysine and proline hydroxylation?
Protyl and lysyl hydoxylases require Fe2+ and vitamin C
Lysine and hydroxy-lysine are modified in the formation of covalent cross-linkages
This takes place only after collagen is secreted
What are the implications of vitamin C deficiency?
Vit C deficiency results in unhydroxylated collagens with dramatic consequences for stability (scurvy)
This is due to enzymes protyl hydroxylase and lysyl hydroxylase requiring vitamin C asa cofactor for functionality
What is Ehlers- Danlos Syndrome? (EDS)
EDS are a group of inherited connective tissue disorders who’s symptoms include stretchy skin and loose joints
What can mutations in collagen negatively effect?
Collagen production
Collagen structure
Collagen processing
What kind of collagens don’t form fibrils?
Type IV collagen
It’s present in all basement membranes
Collagen type IV molecules can associate laterally between triple helix segments as well as head to tail and tail to tail between the globular domains to give dimers, tetramers and higher order complexes
What is the role of fibril associated collagens?
E.g. type IX and XII
Regulate organisation of collagen fibrils
What is the basement membrane?
Aka. basal laminae
They are flexible, thin mats of ECM underlying epithelial sheets and tubes
They’re highly specialised ECMs containing a distinct repertoire of collagens, glycoproteins and proteoglycans
Where are basement membranes found?
Surround muscle, peripheral nerve and fat cells and underlie most epithelia
In the kidney they form a key part of the filtration unit in the glomerular basement membrane (GBM)
What is diabetic neuropathy?
An accumulation os ECM leading to highly thickened membrane
This results in restricted renal filtration and can lead to renal faliure
What is Alport syndrome?
Mutation in collagen IV results in abnormally split and laminated GBM which is associated with progressive loss of kidney function and hearing loss
Why are elastic fibres important?
Important for the elasticity of tissues such as skin, blood vessels and lungs
Often collagen and elastic fibres are interwoven to limit the extent of stretching
What are elastic fibres made of?
A core made up of protein elastin and microfibrils which are rich in protein fibrillin
What is the importance of fibrillin?
Integrity of elastin fibres depends on microfibrils containing protein fibrillin
What is Marfan’s syndrome?
Associated with mutations in protein fibrillin-1
Has diverse manifestations involving primarily the skeletal, ocular and cardiovascular system
Individuals can be predisposed to aortic ruptures
What is the structure of elastin?
Elastin consists of 2 types of segments that alternate along polypeptide chain:
- hydrophobic regions
- alpha-helical regions rich in lysine and alanine
Many lysine side chains are covalently cross-linked
What is the modular architecture of ECM proteins?
Most ECM proteins are large and have a modular architecture:
compromised of characteristic protein domains of 50-200 amino acids
Multi-functionality of ECM proteins is a result of their modular structure
What are laminins?
Multi-adhesive glycoproteins which can interact with a variety of cell surface receptors including integrins and dystroglycans
They can self-associate as part of the basement membrane but all interact with other matrix components e.g. type IV collagen, nidogen and proteoglycans
How are laminins structured?
They’re heterotrimeric proteins made up of an alpha chain, beta chain and gamma chain which form cross shaped molecules
They’re very large proteins each having a molecular weight of 160-400 kDa
What is fibronectin?
Multi-adhesive glycoproteins made up of a large multi-domain molecule linked by disulphide bonds
They’re a family of closely related glycoproteins of the ECM which are also found in bodily fluids
What forms can fibronectin exist in?
Insoluble fibrillar matrix
Soluble plasma protein
How are different fibronectins made?
They’re derived from a single gene with alternate splicing of mRNAs giving rise to different types
What is the function of fibronectin?
Can interact with cell surface receptors and other matrix molecules
Play important roles in cell adhesion regulation and migration in a variety of processes, notably embryogenesis and tissue repair
Also important for wound healing, helping to promote blood clotting
How do fibronectins link to actin cytoskeleton?
Fibronectins form a mechanical continuum with actin cytoskeleton of many cell types
Integrin receptors at cell surface provide linkage between matrix and cytoskeleton
What are proteoglycans?
Core proteins which are covalently attached to one or more glycosmainoglycan (GAG) chains
What are GAG chains made of?
Repeating disaccharide units with one of the two sugars being an amino sugar
Many GAGs are sulphated or carboxylated and carry a high negative charge as a result
This attracts a cloud of cations including Na+ resulting in large amounts of water being sucked into the ECM
What type of proteoglycan families are there?
Basement membrane proteoglycans- e.g. perlecan
Aggregating proteoglycans e.g. aggrecans
Small leucine-rich proteoglycans e.g. decorin
Cell surface proteoglycans e.g. syndecans 1-4
How is cartilage structured?
Has a matrix rich in collagen with large quantities of GAGs trapped within meshwork
Balance of swelling pressure is negated by tension in collagen fibres generating great tensile strength
How many GAG chains are attached to proteoglycans?
Small proteoglycans have a single GAG chain attached
Some large proteoglycans carry up to 100 GAG chains
What are the different groups of GAG chains?
Hyaluronan
Chondroitin sulfate and dermatan sulfate
Heparan sulfate
Kertan sulfate
What is hyaluronan?
Aka. hyaluronic acid
Found in ECM of soft connective tissue
Distinct from other GAGs as its simply a carbohydrate chain without a core protein
Its unsulfated and made up of repeating disaccharides which can number up to 25000 sugars
What is aggrecan?
A major constituent of cartilage ECM
Has highly sulphated GAGs which increases their negative charge
has a large number of negatively charged carboxyl groups
Multiple negative charges attract cations such as Na+ that are osmotically active
This leads to large quantities of water being retained
What is osteoarthritis?
An erosive disease resulting in excessive ECM degredation
Cushioning properties of cartilage over end of bones is lost
What is the effect of age on aggrecan?
With increasing age aggrecan is cleaved by aggrecanases and metalloproteinases
This leads to loss of aggrecan fragments to synovial fluid
What are fibrotic diseases?
They arise as a result of excessive production of fibrous connective tissue
e.g. liver cirrhosis, fibrotic lung
