Lecture 17 Flashcards
What is extracellular matrix (ECM)?
Extracellular= outside of the cell
Matrix= a network of proteins, minerals, sugars and water
What does the ECM provide?
Provides:
1) Mechanical support to tissues
2) Pathways for cellular migration
3) Survival signals
4) Sequesters growth factors
What are the 5 classes of macromolecules that make up the ECM?
- Collagen
- Elastin
- Glycosaminoglycans and Proteoglucans
- Hyaluronan (a unique Glycosaminoglycan)
- Adhesive Glycoproteins
What are some facts about ECM?
All can be encoded by multiple genes
All can be alternatively spliced
Geometrical arrangments can change in different environments
This gives rise to many different types of ECMs
How is the ECM made?
Usually made by cells that are in the tissue
Eg. Fibroblasts
They secrete the fibrous part of the connective tissue and “ground substance” (the material between the fibres made of variable amounts of water and other molecules). This determines how hard or soft the matrix will be
What is collagen?
Most abundant protein in the human body and is found across the phylogenetic tree
Very strong (weight for weight they are as strong as steel)
What is the structure of collagen?
Rod shape
Triple helix that can be up to 420 nm long
What are glycine residues?
Make up the core of the collaen fibre
They are small, allowing for tight packing of the helices
What is the triple helic fibres made of in collagen?
The triple helix fibres can be made of identical chains (called homotrimers). There are also heterotrimers that use 2 or 3 different chains
How many collagens are there in the body?
Over 20 different collagens in humans
Over 100 other protein that have collagen gene triple repeating segments within them
What does a real collagen have to have?
Other proteins are also triple helixes, but to be a real collagen the protein must form fibres or other structures in the extracellular matrix
What are the 3 main types of collagen structures?
- Fibrillar Collagens
- Sheet-forming Collagens
- Anchoring/linking Collagens
What are the features of fibrillar collagens?
300nm long
Strong, but flexible
How can fibres in fibrillar collagens be seen?
Fibres can be seen in a:
Loose netowrk (eg. connective tissue in the intestine) has a higher proportion of loose ground substance [more water] than collagen
Dense network (tendon) less ground substance, or anything in between
Used to reinforce all of the tissues in the body
What kind of collagen does the eye have?
Fibrillar Collagens
Layers of orthogonal fibres (fibres at right angles).
Vitrous body (and cartilage) has glycosaminoglycans and proteoglycans trapped within the collagen
What do fibrillar collagens do in the eye?
Allows enough water to be retained in those structures to allow light to pass through and to resist compression
What are fibrillar collagens?
Usually form heteropolymers in vivo with 1 other collagen
Are conserved from sponges to vertebratesm
What are the general steps for formation of fibrillar collagen?
Fibroblasts make Collagen 1 and release it from the cells using exocytosis
How is collagen 1 made?
Very long (42 exons) collagen 1 genes encode collagen 1 chains
Preprocollagen translocates to the rough ER
What happens in the ER?
The signal sequence is removed (often referred to as pro collagen)
Sugars are added to the protein
Folding of the proteins begins
How is the protein folded?
The protein is folded into the long procollagen triple helix and secreted from the ER using COPII vesicles that are modified by accessory proteins to fit the large (>300nm) fibrils
What happens once the protein is secreted?
Once secreted from the cell, other enzymes called matrix metalloproteinases (NMPs) cleave the pro-collagen into pieces resulting in collagen
Does collagen self-assemble itself?
Collagen self-assembles into fibrils that have a 67nm stagger and are usually covalently crosslinked (at their lysines) by the enzyme lysyl oxidase to give the high tensile strength of the collagen fibrils
What are sheet-forming collagens?
Form sheets instead of fibrils that wrap around organs or tissues (basal lamina), or whole animals (Eg. cuticle of earthworms)
What are linking collagens?
These are collagens that link Fibrillar and Sheet-Forming collagens to other structures
What are elastic fibers?
Fibers that act like rubber band
Give organs the ability to recoil after being stretched
(Eg. arteries, skin, lungs have a alot of elastic fibres)
What are elastic fibers made by?
Made by embryonic and juvenile fibroblasts (so the elastic fibers that you are born with are the ones you have for life, they turn over very slowly if at all)
How long do elastic fibers survive for in your arteries?
Elastic fibers in your arteries will survive over 2.5 billion stretch-recoil cycles during your life
What are elastic fibers formed of?
Formed of Fibrillin microfibrils that are a scaffold for the protein Elastin to bind to (look at picture in PP)
How many Fibrillin genes do humans have?
Humans have 3 genes
Where is Elastin made?
Only made in vertebrates, but insects and molluscs have evolved different proteins to make elastic-type fibers
How many Elastin genes do humans have?
Humans have 1 gene, alternatively spliced
How are elastin proteins linked?
Elastin proteins are covalently linked to one another through the actions of the Lysyl Oxidase enzyme
How does elastin stretch?
The segments between the cross-linked elastin protein segments align with other when stretched (look at picture in PP)
What does stretched elastin mean?
Stretched, high energy, low entropy
What are glycosaminoglycans (Gags)?
Long polysaccharides
What are proteoglycans?
Proteins that are colavently post-translationally modified with Glycosaminoglycans
Where are proteoglycans made?
Made in all vertebrate cells
Where are proteoglycans secreted?
Usually secreted into the ECM
What are proteoglycans a major part of?
Cartilage
Loose connective tissue
Basement membranes
What is the length of proteoglycans?
Vary in length and sequence for the sugar groups
What is the function of proteoglycans in the ECM?
Thought to be in the CM to trap water
Can attract up to 50g of water per gram of proteoglycan
What is Hyaluronan (AKA. Hyaluronic Acid?
It is the largest glycosaminoglycan (so its a string of sugars) (20,000 disaccharide repeats) that is released extracellularly
When hydrated it has a diameter of ~200nm (huge)
Is Hyaluronan post-translationally modified?
No, it is NOT post-translationally modified
It is a crucial element of cartilage
Can block microbes in other tissues
What are some other interesting facts of Hyaluronan?
Hyaluronan and other Gags act as lubricants for joints
Are in the ey and are transparent
High levels might protecr from cancer (Eg. Naked mole rats live very long [31 years] as compaed to other rodents [Eg. mice, 4 years])
What are adhesive glycoproteins?
Are though to control interactions between proteins and sugars in the ECM
Bind to integrins within the cell
What are 2 well studied examples of adhesive glycoproteins?
- Fibronectin
- Tensactin
What is fibronectin?
Large (245 kDa) v-shaped proteins linked at their C-termini by disulfide bonds
What does fibronectin bind to?
Binds to a lot of things:
Collagen
Cell surface receptors
Proteoglycans
Other proteins
What are the 2 types of fibronectin?
2 types (alternatively spliced forms):
1) Tissue fibronectin
2) Plasma fibronectin
What is tissue fibronectin?
Makes insoluble fibrils in connective tissue
Used during wound healing
What is plasma fibronectin?
In body fluids
Used for blood clots
What is tesactin?
Found in all vertebrates
Not in invertebrates
What does tensactin do?
N-termini form 3 subunits self associate
Disulfide bonds link 2 of the 3 unit arms together to form a 6 arm protein
What is tensactin found in?
Found in:
Embryonic tissues
Wounds
Tumours
What does tensactin bind to?
Fibronectin
Integrins
Proteoglycans
Immunoglobulin-superfamily receptors (in the cell surface)
What is the basal lamina?
A 2-dimensional, thin arrangment of ECM proteins
Lies very close to the plasma membrane
What does the basal lamina form?
Forms a layer under all epithelia
Wraps around muscle cell (Sarcolemma)
What can the basal lamina do?
They can filter macromoleucles (eg. filtering blood plasma into urine in kidneys)
How does the basal lamina attach to the connective tissue by?
Collagens
Basal lamina + collagens =
Basement membrane
What is laminin?
Another protein in the basal lamina
What does lamina bind to?
Binds to integrins
How does laminin self-assemble?
Self-assembles into a 2D network by binding non-covalently to other laminin
What is laminin secreted by?
It is secreted by muscle and epithelial cells
How is laminin layed out with collagen?
The 2D laminin and collagen networks lie parallel to one another and are reinforced by other proteins:
Eg. nidogen attached laminin to collagen
Perlecan binds to itself, laminin and collagen
What do the cross links in basal lamina provide?
1) The size selectivity of the basal lamina to filter macromolecules
2) A barrier for cell migration (until enzymes [eg. matrix metalloproteinases] degrade the basal lamina… this happens during cancer tumour metastasis)
3) The mechanical support to the cells/tissues
Force transmission
Cells –> basal lamina –> connective tissue
Eg. (muscle cell contracti9on –> basal lamina –> tendons)
What are matric metalloproteinases (MMPs)?
Used to breakdown the ECM
How many members of metalloproteinases are there?
24 members
What kind of domain do metalloproteinases have?
Have a zinc protease domain
What does the autoinhibitory propeptide do in metalloproteinases?
The autoinhibitory propeptide binds to the zinc ion in the catalytic domain of the enzyme.
Where does substrate specificity come from?
Substrate specificity comes from a regulatory domain at the C-terminus
Are there also fibronectin domains in many of the MMPs?
Yes. There are 3 of them
What do metalloproteinases bind to?
They usually bind to membrane receptors.
Some dock into the plasma membrane using a C-terminal trans-membrane domain
Is metalloproteinases selective?
Yes, each MMP is selective and cleaves specific components of the ECM resulting in:
1) Weakening of the ECM
2) Release of bio-active protein fragments
