Lecture 11 - The Extracellular Matrix Flashcards
All multicellular organisms have an
Extracellular matrix
The ECM supports
Cells
Different types of ECM control (6)
Cell adhesion Signalling Differentiation Polarity Division Migration
During embryonic development, the ECM is integral to
Migration events that determine body polarity
Differentiation of stem cells
Reservoir of growth factors
How many types of matrix do animals have
Two
The interstitial matrix forms
Tissues and gives support to organs
The basal lamina
Underlies all epithelial cell layers
The two main macromolecules that form the ECM are
- Fibrous proteins (glycoproteins)
2. Polysaccharide chains (glycosaminoglycans)
What do glycosaminoglycans do
Link to core proteins to form proteoglycans
Variations in ECM composition cause
Diversity of materials to be formed
Calcified ECM forms
Bones and teeth
Soft transparent ECM forms
The cornea
The interstitial matrix is found in
Connective tissues
Which cell secretes the interstitial ECM?
Fibroblast cells
ECM molecules are synthesised
Intracellularly and then excreted by exocytosis
Once outside the cell, ECM molecules
Aggregate
to form a huge organised matrix
Chondrocytes form
Cartilage
Osteoblasts form
Bone
Collagen provides
Tensile strength in connective tissues
How many collagen genes do mammals have?
28
25% of protein mass in a mammal is
Collagen
Collagen is a very large macromolecule, so must be secreted in a precursor form called
Procollagen
Procollagen
Intracellular form of collagen that prevents aggregation
Procollagen is cleaved outside the cell which allows
Aggregation
What kind of helix is collagen?
Triple helix
Inside the cell, collagen is a
Triple helix
Outside the cell, collagen assembles into a
Fibril
Collagen fibrils assemble into collagen
Fibres
Collagen fibres lie
Criss crossed to provide strength
Collagen is
Post translationally processed
In the nucleus, collagen is
Transcribed from DNA > RNA > mRNA from the collagen gene
In the RER, collagen is
Translated into pre procollagen
Pre procollagen is labelled on the
N terminus to target it to the secretory pathway
Lysine residues on pre procollagen are
Hydroxylated
Hydroxylation of collagen is necessary for
O linked glycosylation
In the Golgi, pre pro collagen is
Hydroxylated
Glycosylated
Disulfide bonded
When the collagen leaves the Golgi, it is
Procollagen packed in secretory vesicles
Once outside the cell, procollagen is
Proteolytically cleaved
N proteinase cleaves the
N terminus of procollagen extracellularly
C proteinase cleaves the
C terminus of procollagen extracellularly
N and C proteinase cleavage allow
The higher order assembly of collagen
From fibrils to fibres
Ehlers Danlos disease is a
Genetic disease that affects procollagen or procollagen processing enzymes
In Ehlers Danlos, collagen fibrils do not
Assemble correctly (no criss crossing)
Symptoms of ED (5)
- Bruising, stretchy skin
- Dislocation of joints, hyperflexibilty
- Arterial rupture
- Scoliosis
- Thin and fragile globe of eye
ED type 6a has a defect in which gene?
Plod1
Plod genes encode for
Lysyle hydroxylases
add OH to lysine in collagen
Mutation in Plod1 results in
No hydroxylation of lysine in collagen
Therefore no O-linked glycosylation
What resists compression and fills space in the interstitial ECM?
Polysaccharide and protein gels
Proteoglycans are
Extracellular proteins linked to glycosaminoglycans (GAGs)
Proteoglycans can have
Many GAG chains attached to them
Proteoglycans and GAGs can form
Huge macromolecules resembling bottlebrushes
Proteoglycans and GAGs form gels that combine with what to form an interstitial ECM?
Collagen
The basal lamina anchors
Epithelial cells to the tissues underneath
The basal lamina separates
Different connective tissue layers
The basal lamina forms
A mechanical connection between layers of connective tissue
The basal lamina can act as a
Selective filter
In the kidney glomerulus, the basal lamina prevents
Blood and urine mixing
The basal lamina is synthesised by
The cell layers on either side of it
The basal lamina looks like
Mat like sheets
Fibrillar collagen makes up the
Intersitial ECM
Collagen type4 and laminin make up the
Basal lamina
Nidogen and Perlecan
Cross link laminin and collagen type 4 together to hold the basal lamina together
Laminin organises the
Basal sheet
Laminin forms a
Triple helix part opens into 3 separate chains to form a ‘cross’
How many laminin isoforms are there?
15
The most abundant laminin in the body is
Laminin 511
Many different protein isoforms suggest
Tissue specificity
Laminin structure
Alpha, beta and gamma chains disulfide bonded chains
Self assembles
Long arm binds to cell surface receptors
Short arms join to other laminins
Collagen 4 provides
Tensile strength
Triple helix
Procollagen
Self assembles
Why can collagen 4 form a sheet?
Primary amino acid sequence of collagen 4
GlyXY
Interrupted repeats of glycine confers bendiness to form a sheet
Epidermolysis bulleosa is a
Genetic disease that forms blisters on the skin
Caused by failure in adhesion between dermis and epidermis
During cell migration, focal adhesions form
At the cell front and disassemble at the back
Focal adhesions are
Connections between the cell cytoskeleton and the ECM
Have receptors to confer specificity
Focal adhesions provide
The force to move the cell Signalling cues (matrix molecules and growth factors)
Integrins are
Transmembrane receptors that couple the matrix to the cytoskeleton
Binding allows tension generation
Integrin structure
Heterodimer
alpha + beta subunit
Integrins link to actin via
Actin binding proteins (ABPs)
Integrins signal
Mechanically and chemically
Tensin, Talin and Vinculin
Form an adhesion complex with integrin that link the actin cytoskeleton to the ECM in a migrating cell
Mature focal adhesions are
Dynamic multi protein complexes
Integrins can signal
Bidirectionally
When integrin is folded, it is
Inactive (does not bind to ECM or cytoskeleton)
When integrin is unfolded, it is
Active (binds to ECM + cytoskeleton)
Outside-in integrin signalling activates
A protein tyrosine kinase cascade
Integrin signalling impacts (3)
Nuclear functions
RhoGTPases
Actin cytoskeleton: migration, spreading, polarity
What are neural crest cells?
Cluster of stem cells at the top of the neural tube
Downregulation of N-CAM in neural crest cells results in
A loss of adhesion which allows migration
Neural crest cells differentiate into cell type depending on
The ECM and cell environment
Laminin and Fibronectin
Cause neural crest cells to migrate