ECM Flashcards
What do aggrecans do?
cleave aggrecan
What is the extra cellular matrix?
A complex network of proteins and carbohydrates that fill the spaces between cells
What type of components does the ECM consist of?
Fibrillar and non-fibrillar components
What does fibrillar mean?
Makes fibre
What are the key functions of the ECM?
Provides physical support
Determines the mechanical and physiochemical properties of the tissue
Influences the growth, adhesion and differentiation status of the cells and tissues with which it interacts
Essential for development, tissue function and organogenesis
What are the two types of roles that the ECM plays in the cell?
an architectural role (influencing mechanical stability) and an instructional role (influences cell behaviour)
What is the connective tissue made of?
ECM and component cells
What are present in connective tissues?
collagen fibre
capillaries
elastic fibre
mast cells
glycosaminoglycans, proteoglycans, and glycoproteins
fibroblast
macrophage
fibrinectin
versican
decorin
fibrillar collagen (type I-III)
What determines the water retention of connective tissues?
glycosaminoglycans, proteoglycans, and glycoproteins
What are the 3 main components of connective tissues?
- Collagens
Type I, II, III (fibrillar)
Type IV (basement membrane) - Multi-adhesive glycoproteins
Fibronectin, Fibrinogen
Laminins (basement membrane) - Proteoglycans
Aggrecan, Versican, Decorin
Perlecan (basement membrane)
What do the matrix components interact with?
specific cell surface receptors
What is found in the basement membrane?
Type IV
Laminins
Perlecan
How can connective tissues have such varied properties?
the different types and arrangements of collagen, with the presence of different ECM components
What properties do connective tissues in the tendon and skin have?
Tough and flexible
What properties do connective tissues in bone have?
Hard and dense
What properties do connective tissues in the cartilage have?
Resilient and shock absorbing
What 4 types of human disorders result from ECM abnormalities?
Gene mutations affecting matrix proteins
Gene mutations affecting ECM catabolism
Fibrotic disorders due to excessive ECM deposition
Disorders due to excessive loss of ECM
What are examples of ‘gene mutations affecting matrix proteins’?
Osteogenesis imperfecta - Type I collagen
Marfan’s syndrome - Fibrillin 1
Alport’s syndrome - Type IV collagen (a5)
Epidermolysis Bullosa - Laminin 5 (in all 3 chains)
Congenital Muscular Dystrophy - Laminin 2 (a2 chain)
What are examples of ‘gene mutations affecting ECM catabolism’?
a.k.a. mucopolysaccharidoses (MPSs) , inability to degrade GAGs (glycoaminoglycans))
- Hurler’s syndrome
- L-a-iduronidase
What are examples of ‘fibrotic disorders due to excessive ECM deposition’?
Liver fibrosis- cirrhosis
Kidney fibrosis- diabetic nephropathy
Lung fibrosis- idiopathic pulmonary fibrosis (IPF)
What are examples of ‘disorders due to excessive loss of ECM’?
osteoarthritis
What cell produces collagen
fibroblasts
What is the most abundant protein in mammals?
collagen, 25% of total protein mass
What is connective tissue made up of?
Extracellular matrix and component cells
What is the alignment of collagen fibrils in the skin?
successive layers nearly at right angles to each other
What is the alignment of collagen fibrils in the mature bone and cornea?
same arrangement as skin (successive layers nearly at right angles to each other)
What does the skin and mature bone and cornea all have in common? (apart from collagen)
these tissues resist tensile force in all directions
What are three proteoglycans?
Aggrecan, versican and decorin
What type of protein is collagen?
Fibrous
How many different collagen types exist in humans?
28
How many genes encode collagen in humans?
42
What structure does collagen form?
Triple helix with 3 alpha chains
What is meant by a homotrimer?
When there is only one chain type
Which types of collagen are homotrimers?
Type II and III
What are the compositions of type II and type III collagen?
[a1(II)]3 and [a1(III)]3
What is meant by a heterotrimer?
When the chains arise from 2 genes
What type of collagen is a heterotrimer?
Type 1 Collagen
What is the composition of type 1 collagen?
[a1(I)]2 [a2(1)]
What is commonly the x and y in the glycine-x-y-repeat?
X = proline Y = hydroxyproline
In fibrillar collagens, each alpha chain is approximately how many AA?
1000
forming a left-handed helix
What makes a collagen fibre?
collagen fibrils
collagen fibrils are made of 3 alpha chains
Which amino acid occupies every third position in collagen proteins?
Glycine
why is glycine heavily involved in the structure of collagen?
Glycine is the only amino acid which is small enough to occupy the interior of the chain
What do all newly synthesised collagen chains have?
non-collagenous domains at N- and C-termini
These domains are removed after secretion in the case of fibrillar collagens but remain part of the collagen in most other types
What provides tensile strength and stability in collagen?
Intermolecular and intramolecular cross links
When does cross linking in collagen take place?
Only after the collagen has been secreted
What are the steps to fibrillar collagen biosynthesis?
- synthesis of pro-alpha chain
- hydroxylation of selected prolines and lysines
- glycosylation of selected hydroxylysines
- self-assembly of three pro-alpha chains
- pro-collagen triple-helix formation
- secretion
- cleavage of pro-peptides
- self-assembly into fibril
- aggregation of collagen to form a collagen fibre (at this point visible to light microscope)
What is an essential post- translational modification which contributes to interchain hydrogen bond formation?
The hydroxylation of proline and lysine
What are covalent cross-links in collagen and what are involved in them?
Cross linking provides tensile strength and stability
Both lysine and hydroxy-lysine residues are involved
The type and extent of cross-links is tissue specific and changes with age
What enzymes are needed for the hydroxylation of proline and lysine?
Prolyl hydroxylase and Lysyl hydroxylase
What do Prolyl hydroxylase and Lysyl hydroxylase requires as a co-factor?
Vitamin C and Fe2+
What are modified in the formation of covalent crosslinkages?
lysine and hydroxylysine
This takes place only after the collagen has been secreted
What does lysine and proline hydroxylation contribute to?
to interchain hydrogen bond formation
What happens to collagen when you have vitamin C deficiency?
The collagen is underhydroxylated, with dramatic consequences for tissue stability (scurvy)
When does the cross linking of collagen occur?
After the fibril formation
which collagens are fibril associated?
type 9 and 10
What do fibril associated collagens do?
they associate with fibrillar collagens and they regulate the organisation of collagen fibrils in the tissues
e.g., types IX and XII
What can mutations in collagen negatively effect?
collagen production, collagen structure and collagen processing
What are network forming collagen?
type IV
Present in all basement membranes, though its molecular constitution varies from tissue to tissue
Non-fibrilar, forms a network that’s key in formation of basement membranes (more flexible molecules)
Do all collagens form fibrils?
No
they form fibril associated collagens
or
Network-forming collagen
What are the symptoms of Ehlers-Danlos syndrome?
Stretchy skin and loose joints due to mutations affecting collagen production in connective tissue
What does excess collagen production lead to?
fibrotic diseases
How is the tensile strength of collegan established?
The fibres are held in parallel bundles which resist the tensile force in one direction
What are collagen alpha chains synthesized from?
Longer precursors called pro-alpha chains
what is cleaved from procollagen to make collagen?
N-terminal propetide and C-terminal propeptide
Where is type 4 collagen found?
In all basement membranes
What type of network does type 4 collagen form?
A sheet-like network
What happens to the N and C terminus in type 4 collagen?
It remains intact
What is another name for basement membranes?
Basal lamina
What is the basement membrane?
A thin, flexible mat of extra-cellular matrix which the epithelial cell sheet sits on top of
What structures are surrounded by basement membrane?
muscle, peripheral nerve and fat cells
What do basement membranes form a part of the kidney?
They form a key part of the filtration unit as the glomerular basement membrane
What is diabetes nephropathy?
Disorder where there is an accumulation of ECM leading to highly thickened basement membrane
this restricts renal filtration and can lead to renal failure
What is Alport Syndrome?
Where mutations in collage IV result in abnormally split and laminated Glomerular basement membrane which is associated with progressive loss of kidney function and hearing loss
What helps to limit the extent of elastic fibres stretching?
The elastic fibres are interwoven with collagen
What do elastic fibres do?
provide elasticity to tissues
Where are elastic fibres found?
skin, blood vessels, lungs
What do elastic fibres consist of?
A core made up of elastin protein, surrounded by microfibrils which are rich in the protein fibrillin
What is elastin?
elastin is a small protein of around 2 kDa arranged in a random coil
Alternating hydrophobic and hydrophilic domains
Hydrophilic domains contain lysine residues which are cross linked during the formation of mature elastin
What amino acid side chains are covalently cross linked in elastin?
Lysine
Describe the structure of elastin fibres
Elastin consists of two types of segments that alternate along the polypeptide chain: hydrophobic regions and a alpha-helical region that is rich in lysine and alanine - many of these lysin side chains are covalently crosslinked
Where is fibrillin found?
In the microfibrils which surround the elastin core
Mutations in fibrillin-1 are associated with what syndrome?
Marfans Syndrome
What are patients with Marfans syndrome pre-disposed to?
Aortic ruptures
What is Marfan’s syndrome?
Diverse manifestations, involving primarily the skeletal, ocular and cardiovascular systems
Aberrant thickening of the aortic with fragmentation and disarray of elastic fibres
Individuals can be predisposed to aortic ruptures
How are ECM proteins able to multi-function?
They have a modular structure
What is meant by multi-adhesive modular proteins?
The proteins can bind various matrix components and cell-surface receptors
What is the shape of the laminin molecule?
A cross shape due to the presence of three chains - alpha, beta and gamma
heterotrimer protein
Are laminins large or small proteins?
Large
Which surface cell receptors can laminins interact with?
Integrins and dystroglycans
What mutation of laminin results in muscular dystrophy?
absence of the alpha 2 chain in laminin 2/ laminin-alpha2 deficiency
What is laminin-a2 deficiency in muscular dystrophy?
LAMA2 gene encodes Laminin-α2
Forms a heterotrimer with polymerizes with laminin-β1 and laminin-γ1 (laminin-211)
Fails to interact with α7β1 integrin and α-dystroglycan that are required for adhesion and basement membrane assembly.
Lack of laminin-α2 manifests as progressive muscle weakness and degeneration.
What are some symptoms of congenital muscular dystrophy?
hypotonia, generalised muscle weakness and deformities of the joint
What is fibronectin?
The basic unit is a 500kD dimer which is disulphide linked
Fibronectins interact with cell surface receptors and other matrix molecules
Fibronectins regulate cell adhesion and migration in embryogenesis and tissue repair
Fibronectins are also important for wound healing, helping to promote blood clotting
Integrins link the ECM to what?
the cytoskeleton
What are the two forms which fibronectins can exist as?
Insoluble fibrillar matrix or a soluble plasma protein
Fibronectins bind multiple ligands and cell receptors like what?
collagen fibres
actin filaments
How are different forms of fibronectins formed?
Alternate splicing of mRNAs
What do fibronectins do?
play an important role in regulating surface adhesion and migration in a variety of processes, notably embryogenesis and tissue repair
What is a proteoglycan?
core proteins which are covalently bonded to one or more glycosaminoglycan (GAG) chains
What proteoglycan is considered small and leucine rich?
Decorin
What is the name of a cell surface proteoglycan?
Syndecans
What is a GAG chain?
Glycosaminoglycan chain
What makes up a GAG chain?
Repeating disaccharide units with one of the two sugars being an amino sugar
What is an amino sugar?
A sugar in which one of the hydroxyl group is replaced with a amine group
What has to happen to the GAG chain in order for it to carry a high negative charge?
Has to be sulfated or carboxylated
What are the main glycosaminoglycan (GAG) varieties?
Hyaluronan (hyaluronic acid)
Chondroitin sulfate/ Dermatan sulfate
Heparan sulfate
Keratan sulfate
What effect does the high negative charge of the GAG chain result in?
it attracts clouds of cations, including Na+, which therefore pulls water into the ECM through osmosis and the presence of a concentration gradient
why does cartilage have a high tensile strength?
Catilage has a ECM with alot of collagen and GAG chains trapped in the meshwork, so the balance of swelling pressure is negated by the tension in the collagen fibres
How is hyaluronan produced?
It is spun out directly from an enzyme embedded in the plasma membrane
How is hyaluronan distinct from other GAG chains?
It is simply a carbohydrate chain without a core protein
Why can hyaluranan chains occupy large volumes?
Because it can undergo a very high degree of polymerisation, which creates very large molecules
What is the role of hyaluronan in joints?
Protects the cartilaginous surfaces from damage
Where is hyaluronan found?
In highly viscous tissues e.g., the vitreous humour of the eye and in synovial fluid of joints
What is a major constituent of the cartilage ECM?
Aggrecan
What is aggrecan in the cartilage matrix perfectly suited to do?
Resist compressive forces
Under compressive load, water is given up but regained once the load is reduced.
Aggrecan is highly sulphated What effect does this have?
Increases their negative charge which attracts cations that are osmotically active - leads to large amounts of water being retained by the negatively charged environment
what is osteoarthiritis?
an erosive disease resulting in excessive ECM degradation
What is lost with osteoarthiritis?
The cushioning properties of cartilage over the end of bones
What leads to a loss of aggrecan fragments to the synovial fluid?
The cleavage of aggrecan by aggrecanases and metalloproteinases - this increases with age
How do fibrotic diseases arise?
They are as a result of excessive production of fibrous connective tissue
How is aggrecan perfectly suited to resist compressive forces in cartilage matrix?
Under compressive load, water is given up but is regained once the load is reduced
