The Extracellular Matrix in health and Disease Flashcards
What are the four macromolecular components of the extracellular matrix?
- ) Structural proteins
- ) Glycosaminoglycans
- ) Proteoglycans
- ) Adhesive glycoproteins
What are the two major structural proteins?
- ) Collagen (imparts strength)
2. ) Elastin (imparts resilience)
Complex sugars that bind water and resist compression
Glycosaminoglycans (GAG)
Proteins with covalently coupled GAG chain
Proteoglycans
What are the major adhesive glycoproteins?
Fibronectin, laminin, and entactin
These macromolecules along with water and ions are termed the
Ground substance
The ground substance plus cells is termed the
Connective tissue
The basal surfce of the epithelium of most tissues is underlain with
-Sheets of exctracellulr matrix (ECM) that separate cells
Basememnt membranes
If the epithelium is specialized, the basement membrane is called
Parenchyma
Especially abundant under epithelial cells but also surround all endothelial cells and many mesenchymal cells
Basement membranes
There are three “layers” of basement membrane that can be defined at EM level that are enriched in specific ECM components. What are the three layers?
- ) Lamina rara
- ) Lamina densa
- ) Lamina reticularis
-in order of top to bottom
Contains laminin, perlecan, nidogen, and type IV collagen
Lamina Densa
Contains some fibronectin and specific laminin (type 5(, and type XVII collagen from the cell membrane
Lamina rara (lamina lucida)
Contains types I, III, and IV collagen, several proteoglycans, and fibronectin
Lamina Reticularis
Major component of adult ECM that is located outside the lamina reticularis of basement membrane that is secreted by fibroblasts and specialized mesenchymal cells
Stroma
In adults, different tissues have quite different stromal
Matrix organization
Has a dense extracellular compartment and is primarily made up of collagen
Tendon stroma
MAde up of specific collagens (type II and IX) and the proteoglycan aggrecan
Cartilage stroma
Has little matrix at all
Liver stroma
Fibronectin, collagen types I, III, VII as well as elastic fibers are in the
Stroma (Bowman’s layer)
The continuum between the cell surface and the extracellular matrix is mediated by
Syndecans and Integrins
It is important that structures are interconnected to maintain
Tissue integrity
An extracellular component in direct contact with the basal domain of epithelial cells
Basement membrane
At the EM level, the basement membrane is defined by which two layers?
- ) Basal lamina
2. Reticular lamina
In some cells, specialized fibroblasts such as chondrocytes or osteoblasts secrete the
Stroma
Has few cells, lots of matrix, and dense irregular connective tissue
Stroma
A triple-helix protein that provides tensile strength
-25% of total protein
Collagen
Colagen is resistant to
Stretching
In fibril forming collagens, many triple helical structures will assemble into
Fibrils/fibers
There is a continuum between the cell surface and ECM mediated by
Integrins and Syndecans
Cell type-specific integral membrane proteins that serve as receptors for components such as fibronectin, laminin, and several collagen types and connect ECM with intracellular signaling pathways and the cytoskeleton
Integrins
Specific heparin-sulfate containing proteoglycans that span the cell membrane and interact with actin filaments
Syndecans
Triple-stranded helical molecules containing three monomer α-chains wrapped around each other that imparts strength to the molecule
Collagens
A unique aspect of the α-chain amino acid composition of collagen is that every third amino acid is
Glycine
Between the glycines in the form G-X-Y-G, we often see Proline, lysine, and hydroxylated forms of these amino acids, which contribute to
Helix stability
The extend of post translational modification to the collagen aminos can generate
Heterogeneity
Assembled extracellularly with many collagen molecules cross-linked and aligned for maximal stability and strength
Collagen fibers
Leads to the extensive striation pattern seen at the EM level
Collagen fiber formation
90% of collagens are
Fibril forming Collagen I
A single glycine mutation in the primary structure of a collagen helix can alter the entire
Structure
The number of collagen types has expanded and now totals 20, which can be classified into what 4 major categories?
- ) Fibril forming
- ) Fibril-associated
- ) Non-fibrillar
- ) Transmembrane
Most abundant in the tendons, bones, lungs, and skin (represents 90% of all collagen)
Type I (a fibril-forming) Collagen
Mutations in type I collagen leads to
Osteogenesis imperfecta or osteoporosis
Type II collagen is found in cartilage and makes up 50% of cartilage dry weight. Mutations in type II lead to
Chondrodysplasia
Widely distributed in the skin and aorta-“reticular” collgen
-Stained with silved
Type III collgen
Mutations in type III collagen cause
Type III Ehler’s-Danlos-Type 3
Type III Ehler’s-Danlos-Type 3 is characterized by
Hyperextended skin and susceptibility to anneurism
Types I, II, and III collagen are all
Fibril-forming
Type IX collagen, found in the cartilage where it decorates the outside of type II collagen fibers is an example of a
Fibril-associate collagen w/ interrupted triple helix (FACIT)
What are the two non-fibrillar (network forming) collagens
Type IV and Type VII
Located in the basal lamina of basement membranes
Type IV collagen
Anchors fibrils at epithelia connective tissue junctions
Type VII collagens
Transmembran collagen that connects basement membrane to epithelium via anchoring filaments
Type XVII collagen
Epidermolysis bullousa is a disease associated with
Type VII collagen
In collagen synthesis, the α chains are synthesized as
Pro-α-chains on RER-attached polysomes
In collagen biosynthesis, the signal peptide is removed. Then proline and lysine are
Hydroxylated
Mutations in the collagen hydroxylases alter
Collagen assembly
Required for helix stability at physiological temperature
Hydroxylation of Proline
Required for intermolecular cross-links to form
Hydroxylation of lysine
Required as a co-factor for the collagen hydroxylases
-deficiency causes scurvy
Vitamin C
If there is a vitamin C deficiency, hydroxylation does not occur and collagen α chains are
Degraded
Once hydroxylation has occure, the hydroxylysines are
Glycosylated
Next, N- and C-terminal extension containing cys-rich globular domains (non-Gly-X-Y) guide
Helix formation
Helix formation begins in the ER and goes in the
C- to N-terminal direction
Formation of hydrogen bonds between Pro and NH2 of Gly to COOH- groups on separate chains leads to formation of
Helix
Required for stability of the helix at 37 degrees celcius
4’OH-Pro
In one form of osteogenesis imperfecta, helix formation is disrupted by a single Gly to Cys substitution that alters
Hydrogen binding
Have low Gly, Pro, OH-Pro and do not form helix. Thus their presence prevents fibril formation within cells
Extension peptides
During secretion, extension peptides of fibril-forming collagen molecules (types I, II, III) are cleaved by two procollagen peptidases specific for N- and C-ends leaving a bit of globular domain which participates in
Cross-linking
After the extension peptides are cleaved (occurs outside of the cell), the resulting collagen molecules are termed
Tropocollagen
Fibril assembly from triple helical molecules can not occur if we are missing
Procollagen peptidases
Tropocollagen first assembles into a microfibril and then into a
Fibril
Microfibrils aligned with 35 nm overlap between tropocollagen molecules, which are aligned
Head to tail
The maximum number of possible inter-chain electrostatic and hydrophobic bonds are formed
when chains are displaced by multiples of
-leads to striated pattern seen in EM
67 nm
Stain silver because they have more carbohydrate and thinner fibrils, are more branched, and have fewer bundles
Type III (“Reticular”) collagen
Covalent cross-linking of fibrils is catalyzed by
-Cu dependent
Lysyl oxidase
Type IX collagen, a fibril associated collagen, is less rigid and forms a
Hinge
The extension peptides are generally retained in
Type IX collagen
Functionally imortant for joint integrity
-Mutations cause epiphyseal dysplasia (leads to arthritis)
Type IX collagen
Major collagen of the basal lamina, more specifically the lamina densa
-has multiple regions where the triple helix is interrupted
Type IV collagen
Has little extracellular processing of procollagen chains. So, unlike fibrillar collagens, retains globular non-collagen-like extensions at both ends
Type IV collagen
The extension peptides of type IV collagen are not
Cleaved
Both the C- and N-termini of type IV collagen react to form dimers and tetramers respectively. This results in a
“Chicken-wire” array
Contain dimers that form networks of anchoring fibrils that connect lamina densa with matrix components in reticularis or stroma
Type VII collagen
In type IV collagen, the C-termini of monomers interact with eachother via disulfide bonds to form dimers, while the N-termini attach to the
Lamina densa and anchoring plaque of stroma
Absence of type IV collagen leads to
-stroma peels from basement membrane
Blistering
Attaches basement membrane to epithelial surface via anchoring filaments
-mutation leads to blistering
Type XVII collagen
If either type VII or type XVII are mutated, the result is
Blistering
If VII is mutated, the blistering occurs
Below lamina densa
If type XVII is mutated, the blistering occurs
Above lamina densa
Mutants with shortened C-termini cannot associate, which results is
Blistering
Structural component of ECM that allow stretching and recoil of connective tissues
Elastin
One tissue with high elastic fiber (elastin) content is the
Aorta
Like collagen, elastin is assembled
Extracellularly
Elastin monomers are highly cross-linked via
Unique amino acids
What are the two major molecular components in elastic fibers?
Cross-linked elastin monomers and fibrillin
Participates in the cross-linking of elastin
Fibrilin
The importance of fibrillin has been seen in
-results from missense mutations in the structural gene encoding fibrillin 1
Marfan syndrom
Marfan syndrome results from missense mutations in the structural gene encoding fibrillin 1 and leads to elongated extremities and enhanced rupture of the
Aorta
At the histological level, elastin can be visualized by the orcein stain, while at EM level, mature elastin molecules are generally surrounded by
Fibrillin
Long, unbranched polysaccharide chains consisting of repeating disaccharides that are distinct in different classes but contain alternating amino sugar and uronic acid
Glycosaminoglycans
The most important glycosaminoglycans (GAGs) are
Hyaluronic acid, chondroitin sulfate, and heparan
sulfate
All classes of GAGs are sulfated on an amino sugar except
Hyaluronic acid
Are inflexible with a negative charge that binds water and osmotically active cations
GAG chains
GAG chains have a highly extended
Random coil
As a result GAG occupy huge space; ECM with high GAG content has the consistency of a hydrated gel and can resist compression and participate in
“Sieving”
There are major clinical deficits if normal catabolism of GAG is not maintained. This is seen clearly in
Lysosomal storage diseases
Lethal conditions result from accumulation in lysosome
of iduronic acid from dermatan sulfate and heparan sulfate in
Hurler’s syndrome
Most GAG classes are covalently coupled to “core” protein and thus termed
-again, hyaluronic acid is an acception
Proteoglycans
Epithelium and Basement Membrane
peeled away and type VII anchoring
fibrils are disrupted in
Epidermolys Bullosa
The prototype proteoglycan is the proteoglycan from cartilage that is 90-95% carbohydrate and is now termed
Aggrecan
Membrane associated proteoglycans
Heparin Sulfate (HS) proteoglycans
Some heparan sulfate-containing-PGs are integral
membrane proteins with membrane spanning domains in addition to extracellular attachment sites for HS chains. These include members of the
Syndecan family
These heparan sulfate
PGs can bind a wide array of molecules including both other matrix components and growth
factors.
Syndecans
The binding of one specific growth factor, fibroblast growth factor (FGF) to syndecan is required to maximize
Mitogenic activity
There are also additional heparin sulfate proteoglycans that are completely extracellular, such as
Perlecan
Located in the lamina densa of basement membranes
Perlecan
The interaction of syndecan w/ the cytoskeleton illustrates one connection between ECM and
Internal signaling pathways
What are three matrix glycoproteins?
- ) Fibronectin
- ) Laminin
- ) Nidogen (enactin)
A glycoprotein formed by two identical chains joined by disulfide linkages close to the C-termini
Fibronectin
Fibronectin can exist in a soluble dimeric form that circulates in serum and is involved in
Wound healing
Fibronectin can exist as oligamers attached to the
Cell surface
Fibronectin can exist as fibronectin fibrils that form an insoluble matrix in the
Stroma
Synthesized, secreted and bound to fibroblasts and other cell types
-Initially inversely correlated with tumor formation and altered growth
Insoluble form of fibronectin
Cells lacking fibronectin would lose adhesiveness, round up, and
divide
An important feature of fibronectin is its
-allows interconnection between series of cell-matrix components at the surface and in the matrix
Modular organization
Fibronectin has two binding sites for
Integrins, collagen, heparin, and fibrin
Produced by hepatocytes and secreted into the bloodstream
Plasma fibronectin
Produced by fibroblasts and forms part of the extracellular matrix
Cellular fibronectin
Can bind to both collagen and laminin
Integrins
Made up of three major subunits (α, β1, β2) arranged in pattern containing one long arm and three short arms; multiple additional forms have been identified
Laminins
Laminins generally promote
Adhesion
Located in high abundance in the lamina densa and mediates interactions between cell surface and basement membrane
-has a high affinity for type IV collagen and the HSPG perlecan
Laminin 1
Fibronectin and laminin bind to integrins through
RGD
Dumbbell shaped structure that binds to laminin 1 and type IV collagen and mediates their interaction with the lamina densa
Nidogen (enactin)
What are two major membrane-spanning molecules that serve as receptors for ECM components
- ) Syndecans
2. ) Integrins
Integrins are dimers composed of
α subunits (18 different types isolated so far) and β
subunits (8 different types)
Extracellular laminin, fibronectin, collagen and numerous other cell adhesion molecules all bind
Integrin
Important amino acid sequence for many integrin binding interactions
RGD
Found on epithelial and mesenchymal cell surfaces and also found on some circulating blood cells
Integrins
Once binding to the matrix or other cells has occurred, integrins regulate the activity of kinases such as the
-allows several intracellulr signaling pathways to assemble
Focal adhesion kinase (FAK)
Also bind cytoskeletal-associated proteins and thus serve to link
the ECM and cytoskeleton
Integrins
Links fibronectin to actin filaments probably through talin
Integrin α5/β1
Integrin α5/β1 is a co-receptor with other receptors that lead to phosphorylation of
Paxilin and FAK
Major receptor for laminin and may be altered in some forms of muscular dystrophy
Integrin α6/β1
Part of hemidesmosome and anchors intermediate (keratin), not actin filaments
Integrin α6/β4
Both integrin subunits are involved in binding substrate via
RGD sequence
Binds cytoskeleton (often actin) that can initiate formation of signaling complexes
β-subuint of integrin
Bind the actin cytoskeleton and organize signaling molecules
Integrins
After binding substrate, integrins
Cluster
A focal adhesion forms at the site of actin bound integrin via
Rho GTPase
One subfamily of integrin receptors (β3), is found on platelets and binds fibrinogen during clotting. Mutations in β3 result in
Glanzmann’s disease
Another subfamily (β2) is found on white blood cells. Deficiency of β2 results in
-prevents binding of white cells to endothelium
Leukocyte adhesion deficiency
Cell membrane proteins that both interact with ECM components
Integrins and Syndecans
Changes β2 integrin conformation to allow its binding to endothelial surface receptors
Lymphocyte binding to endothelium via selectin
Initial interaction with endothelium is through a different low affinity receptor (selectin). Intracelluar signaling alters integrin conformation which then allows specific, high affinity binding to endothelium and ultimately leukocyte infiltration from blood into the tissue. This describes
Leukocyte adhesion to endothelium
Regulation of platelet activation is critical because if platelets aggregated randomly, this would lead to
Erroneous clotting
Ensures that binding to integrin occurs only in appropriate environment when clotting is needed
Regulation of integrin affinity
Regulation of integrin activation is especially important for
lymphocyte binding to endothelium (β2) and platelet binding to fibrinogen (β3)
What do the following integrins bind and where are they found?
- ) Integrin α6/β4
- ) Integrin α2/β3
- ) Laminin, Hemidesmosomes
2. ) Fibrinogen, Platelets
Found on white blood cells and help mediate attachment to epithelial cells and subsequent movement of WBC into tissue
Integrin αL/β2
