Lecture 12 - From cells to tissues Flashcards
What are the roles of the ECM and what is it composed of ?
Composed of proteins and proteoglycans
- inert scaffold to stabilise the physical structure of the tissues
- helps define the cellular phenotype of the cells residing within it
- acts as a storage compartment for cell signalling factors
What are the major componants of the ECM?
- collagens
- proteoglycans and GAGs
- elastin
What are the features of collagens?
- represent around 25% of total mammalian proteins
- composed of homo- or hetero- trimers and form a triple helical structure
- give tissues tensile strength
- amino acid sequences consists of Gly-X-Y repeats (X, Y often proline, hydroxyproline, hydroxylysine)
- vitamin C is an essential co factor in the synthesis of collagen
What is scurvy?
A vitamin C deficency,
- impaired collagen synthesis through vitamin C deiciency, lack of support
- pale skin
- teeth loss and bleeding
- sunken eyes
Why is the position of glycan important in collagen synthesis?
It is a small amino acid which enables a helical type arrangement
How is the structure of collagen formed?
different collagen molecules associate through covalent bonds to form collagen fibrils which assosiate to form fibres
What are the different types of collagens?
Fibrillar collagens
-fibre like structure, involved in mechanical and tensile strength
-Type I (dermis, bone), Type II (cartilage), Type III (dermis, muscle, blood vessels) Type V (foetal tissue)
Fibril associated collagens
-in association with fibrillar collagens, have supportive roles
-Type VI (intestinal tissues), Type IX (cartilage)
Sheet forming collagens
-in basement membrane structure, Type IV
What collagens form homotrimeric structure and which for heteromeric structures?
Homotrimeric
II, III, V
Heterotrimeric
I, VI, IX, IV
How are different types of collagen organisations formed?
Different α helical structures associated with different collagens
e.g. Type I has a more robust structural role [α1(I)][α2(I)]
than type III [α1 (III)]3
What is the process of collagen synthesis?
- Synthesised as a pro-α peptide chain or attached to ribsomes on the ER
- Undergoes various different stages of hydroylation of selected prolines and lysines
- Undergoes various stages of glycosylation of selected hydroxylysines
- Self assembly, through the formation of disulphide bonds, of 3 pro-α chains in the ER forming a procollagen triple helix
- Go through secretory pathway to the plasma membrane and secreted as procollagen
- Procollagen peptidases cleave propeptides into collagen
- Self assembly into a collagen fibrils
- Aggregation of collagen fibrils to form collagen fibre
What is Osteogenesis Imperfecta?
- brittle bone disease
- caused by mutations in α1(I) or α2(I) genes
- which encode fibrocollagen molecules
- means that the collagen is not prouced, laid down and mineralised correctly
- high propensity for fractures by minor trauma
- blueing of the white area of the eye due to thinning collagen allowing you to see veins
- shows the importance of correct collagen synthesis
What is Dermatosparaxis?
- an inherited disorder
- causes fragile and loose skin with substantial bleeding and bruising
- caused by mutations in the N-teminal propeptidase (Metalloproteinase ADAMTS-2) that removes the propeptides in type I and III collagen (skin, dermis)
- can longer spontaenously assemble into the more complex structures
- importance of the processing events outside the cell
How do glycosaminoglycans (GAGs) form proteoglycans?
GAGs are covalently attached to a core protein to form proteoglycans (except hyaluronan)
What do proteoglycans provide to the ECM?
-small hydrated, space-filling functions and compressive strength (e.g. in catilidge)
What are the 4 classes of GAGs?
- Hyaluronan
- Chondroitin sulphate
- Herparan sulphate
- Keratan Sulphate
How are the different classes of GAGs formed?
by polymerisation of specific disaccharides and modifiation (e.g. sulphation)
What are the key GAGs?
Chondroitin sulphate
Heraran sulphate
Keratan sulphate
Give some examples of proteoglycans and their features
Decorin -CS, DS (GAG chains) -connective tissue -binds type I collagen and TGF-β Aggrecan -CS, KS -cartilage -mechanical support Betagylcan -CS, DS -cell surface + ECM -binds TGF-β systems Glypican -HS -Basement membrane -structure and filtering Syndecan-1 -CS, HS -Cell surface -Adhesion, binds FGF
What are the features of Hylauronan?
- GAG that links proteoglycans
- D-glucuronic acid and N-acetyl-D-glucosamine repeats (can be around 50000)
- links together proteoglycans e.g. Aggrecan, by assosiating with N-terminal link proteins to allow linking of monomers into a larger structure
What is the process of proteoglycan synthesis and where does it occur?
- golgi
1. begin with a protein core
2. linking sugars are added Gly, Xyl
3. then chondrin sulphates repeats
How do proteoglycans have a role in cell signalling?
- they bind various secreted signalling molecules (Particularly HS)
- can enhance or inhibit the signalling activity of growth factors
e. g. Glypican and Syndecan
What is the structural organisation of Glypican and Syndecan involved in cell signalling?
Glypican
-GPI anchor with a cleavage site to release Glypican into the EC space and affect how it regulated signalling
-HS branching
-sulphide bonds maintain shape (curves back on itself)
Syndecan
- transmembrane domain and EC cleavage site
-HS branching
How does glypican regulate signalling of Wnt?
- when wnt binds its receptor it destabilises the destruction complex and gives rise to β-catenin
a) low affinity glypican mantains and presents Wnt at the cell surface, enhancing the signalling
b) the absence of glypicans or HS decreases the Wnt signal strength
c) Increased glypican can sequester Wnt, reducing signal strength
d) Notum cleavage of glypican GPI anchor reduced Wnt signal strength
How does syndecan regulate FGF-signalling?
- free FGF cannot interact with its receptor, cells cannot respond in the absence of heraran sulphate proteoglycans
- FGF binding to cell surface HS (syndecan) causes a conformational change and allows binding to FGF receptor
- Proteolytic release of extracellular HS proteoglycan fragments allows FGF to bind to its receptor
What does elastin contribute to the ECM?
- gives tissue elasticity allowing stretching
- contributes to tissue structure and function
- highly coiled structure held together by crosslinks
- -can stretch under tension
Where does Elastin function?
Skin
Arteries
-dominant ECM componant of arteries at around 50%
-enables blood flow and tensile compression to transmit and maintain blood pressure
-coiled fibres allow extension under pressure
What do mutations in Elastin lead to?
Curtis laxa
- rare inherited disorder of connective tissue
- looe hanging skin and inelasticity
- arterial complications = narrowing of the aorta, restriction of blood flow to vital organs
- due to decreased elastin gene expression and crosslinking
What is the function of the basal lamina?
allows interaction between layers
- acts as a filtering unit
- keranocytes recieve nutrients through the basal lamina
- can act as a barrier to metastasis, canercous cells mut develop mechanism to bypass the basal lamina if want to move from skin
What is the basal lamina?
A specialised structure located between epithelial cells and stoman cells e.g. in the skin
What is the basal lamina made up of?
enactin (multiadhesive matrix protein)
perecan (HS proteoglycan that regulated structural interactions)
laminin
type IV collagen (sheet forming ability)
What is the process of formation of the type IV collagen teramer?
- NCI interactions by 2[α1] and 1 [α2] globular C-terminal heads bind to form protomer as the α helical tails self assemble into a coil
- Two protomers form a dimer by interacting at the 3 α-head domain
- 7s domain of 4 dimers interact and bind to form a type IV collagen tetramer
What are the features of laminin (basal lamina)?
- multiadhesive protein with a cross like structure which allows interaction with many different componants
- made up of three covalently linked chains (α and 2Xβ chains) with globular domains
- different regions have different binding activities (base globular structure = HS binding site; β2 globular head binds to collagen and sulphated lipids; β1 chain globular head binds to type IV collagen; covalent linkage can bind to integrins and entactin)
What are the features of fibronectin (basal lamina)?
- large glycoprotein (with multiadhesive domains)
- helps matrix organisation
- homodimer
- binding motifs for proteoglycans, cells and collagen
What is anchorage dependance and what is it mediated by?
when cells need to attach to the ECM to grow and proliferate, and normally to survive
mediated by integrins and the signals they generate
How does ECM geometry regulate cell shape?
cells grown on different fibronectin shapes take up that shape
e. g.
- cells grown on teardrop shaped ECM shows a migratory phenotype
- fibronectin dots at different spacing can affect spreading
How was it shown that the ECm can affect differentiation of cells?
- cells were allowed to differentiate into either adipocytes or bone
- found that large square organisations of FN gave rise to bone cells
- small round organisations gave rise to fat cells
