MBC - Extracellular matrix Flashcards
What is the ECM
A complex network of macromolecules (proteins and carbohydrates) deposited by cells, and then becomes immobilised to fill up the spaces between cells.
What cells deposit the components of the ECM
Fibroblasts
What components is the ECM composed of
Fibrillar and non fibrillar components
What is the purpose of the ECM
Development, tissue function and organogenesis.
Determines mechanical and physiochemical tissue properties.
What does the ECM influence in cells due to the combination of its components
Growth, adhesion and differentiation of cells
What are the three main components of the ECM
Collagens
Multi-adhesive glycoproteins
Proteoglycans
What types of collagen are found in the ECM
I, II, III, IV
What is type I collagen used for
Bone, skin and tendons
What is type II collagen used for
Cartilage
What is type III collagen used for
Fibrillar, found in blood vessels and reticulin (type of connective tissue fibre)
What is type IV collagen used for
Basement membrane
What glycoproteins are found in the ECM
Fibrinogen
Fibronectin
Laminin (basement membrane)
What proteoglycans are found in the ECM
Aggrecans
Versicans
Perlecans (basement membrane)
Decorin
How do we get a wide variety of connective tissue in the ECM?
We get different types of collagen and different collagen orientations interacting with different components of the ECM.
What do matrix components interact with to influence the cell physiochemical behaviour?
Cell surface receptors (integrins).
What is collagen?
Collagen is a family if fibrous proteins found in all multicellular organisms
Explain the synthesis of collagen
Follows the normal pathway for secreted protein:
Pro-alpha chains are secreted by the rER ribosomes
These then undergo a series of covalent modifications, and 3 chains form a pro collagen molecule.
The pro collagen molecule then undergoes cleavage and fibril formation to become a collagen fibre, which can then cross link with other collagen fibres.
What is a homotrimer?
This is a collagen molecule made up of three identical alpha chains. For example, type II and III collagen are made up of 1 chain type.
What is a heterotrimer?
Collagen molecule made up of at least 2 different a chains. Type I collagen is made up of different chains.
What does [α1(I)]2 [α2(I)] mean in type I collagen?
This means that the collagen molecule consists of 2 alpha-1 chains, and 1 alpha-2 chain.
What arrangement to a-chains typically have?
Gly-x-y
Why do a-chains have a gly-x-y arrangement?
Glycine is required at every third position as it is the smallest amino acid (only has H as its side group) so it can face inwards when the a chains are intertwined.
What are the x-y in the a-chain arrangement?
x - typically proline
y- typically hydroxyproline
Why is the hydroxylation of proline important?
This is an important post translational modification as it contributes to the H bonds between chains
When does cross linking occur
After collagen has been secreted
What other molecular residues are involved in cross linking?
Lysine and hydroxylysine.
How does vitamin C deficiency cause scurvy?
Prolyl and lysyl hydroxyls require vitamin C as a cofactor however deficiency leads to underhydroxylated collagens and thus unstable tissue thus scurvy.
Are all collagens fibril-related?
No, for example collagen IV is associated with the basement membrane, and its N and C termini are not cleaved.
What is Ehlers-Danlos syndrome
Stretchy skin and loose joints
What is the purpose of the N and C termini on type IV collagen
Essential for communication between and regulation of fibres
What is the basement membrane?
Highly specialised, thin sheets of ECM underlying epithelia and tubes.
What does the basement membrane contain?
Distinct collagens associated with proteoglycans and glycoproteins
What collagen is characteristic off the basement membrane?
Type IV - this can associate laterally between the triple-helical segments or the globular domains in a head to head or toe to toe manner, thus forming dimers, trimers, tetramers or higher order complexes.
In the body, where is the basement membrane found?
Surrounds the muscles, peripheral nerves, fat cells and underlie most epithelia
What is the basement membrane role in the kidney?
Forms part of the filtration unit as the glomerular basement membrane
What happens to the Basement membrane in diabetic nephropathy?
GBM thickens abnormally and may cause renal failure
What happens in Alport’s syndrome?
Irregularity with type IV collagen leads to abnormal splitting of the basement membrane and irregular laminations, causes progressive loss in kidney function and hearing.
Why do we need elastic fibres?
Tissue elasticity
What limits elastic fibre elasticity?
Fibres are interwoven with collagen
Explain the elastic fibre structure
Elastin core surrounded by microfibrils
What protein is mainly found in microfibrils?
Fibrillin
What happens in Marfan’s syndrome?
Mutation in firbrillin-1 gene - affects the skeleton, cardiovascular and ocular system. Individuals may be predisposed to aortic rupture
What is the elastin core made up of?
Two segments: hydrophobic region in one region, and the other being a-helices with lysine and alanine which are covalently cross linked.
What are the main glycoproteins of the ECM?
Laminins (basement membrane) and fibronectins
How does protein size and modular architecture relate to the function of the glycoproteins?
Large size allows multi-functionality of the proteins
What does it mean if the proteins are multi-adhesive
They can interact with many different components of the ECM
Describe the structure of laminins
Large heterotrimeric structure, made up of alpha, beta and gamma chains to form a cross shape.
What can laminins bind to?
Cell surface receptors such as integrins and dystroglycan
what are laminins typically associated with?
Basement membrane however can also interact with other matrix components such as nidogen, proteoglycans and type IV collagen.
What can laminin mutations cause?
Inherited conditions such as epidermolysis bullosa, and muscular dystrophy
What happens in congenital muscular dystrophy?
Absence of the alpha-2 chain in laminin 2. Results in hypotonia, muscle weakness and joint deformity.
What are fibronectins?
Family of closely related glycoproteins
Where are fibronectins found?
In the ECM and also in the blood
What are the two different forms of fibronectin?
Soluble plasma protein in the blood or insoluble fibrillar component in the ECM.
How do the 2 different forms of fibronectin arise?
They both arise from the same gene however different RNA splicing gives rise to different forms.
What is the role of fibronectin in the blood?
Promotes blood clotting
What is the structure of fibronectin?
Large molecule - multi domain and linked via disulfide bonds so can bind to multiple cell surface receptors (multi-adhesive)
What is the role of fibronectin in the ECM?
Important role in regulating cell adhesion and migration, typically during embryogenesis and tissue repair.
What are proteoglycans
Core proteins attached to 1 or more glycosaminoglycan chains (GAG)
How are proteoglycans classified?
Structural and functional characteristics
What is decorin?
Small, leucine rich proteoglycans
What are syndecans 1-4?
Cell-surface proteoglycans
What is aggrecan?
Aggregating proteoglycan (interacts with hyaluronan)
What is perlecan?
basement membrane proteoglycan
Where is aggrecan mainly found?
Cartilage ECM
How and why is aggregan negatively charged?
It is highly carboxylated and sulfated to give it negative charge - therefore it attracts cations such as Na+ to draw in water
What happens when a compressive load is applied/taken away from aggrecan in cartilage?
Compressive load gets rid of water, however when the load is taken off, water is drawn back in. This makes aggrecan ideal for cartilage as it can tolerate high compressive forces.
What are GAG chains?
Chains made up of repeating disaccharide units: usually one or 2 of the sugars are converted to an amino sugar.
How is a sugar of GAG converted to amino sugar
Hyroxyl (-OH) is replaced by amino group (-NH3)
How do GAGs contribute to drawing in water to the ECM?
Carboxylated ans sulfated to give a negative charge - draw in cations such as Na+
How are GAGs grouped?
Via disaccharide unit
What are the 4 groups of GAG chain?
Hyaluronan
Heparan sulfate
Keratan sulfate
Chondroitin sulphate or derma tan sulphate
What is the difference between hyaluronan and the other GAG chains?
Hyaluronan is spun out by an enzyme on the plasma matrix, the others are synthesised and then attached onto the core proteins with the help of the ER and Golgi apparatus.
Where is hyaluronan found?
ECM of soft tissues
Is hyaluronan sulfated?
No
How would you describe the structure of hyaluronan?
Simple carbohydrate chain without a core protein
What is another name for hyaluronan?
Hyaluronic acid
What allows hyaluronan to occupy large volumes?
It can undergo a high degree of polymerisation
How would you describe the viscosity of hyaluronan?
High - typically found in the synovial fluid of joints or vitreous humour.
What happens in osteoarthritis?
With increasing age, aggrecan is exposed to aggrecanases and metalloproteinases, leading to aggregate fragments being lost in the synovial fluid, and the excessive degradation of the ECM.
What happens in fibrotic disease?
Excess production of fibrous connective tissue
