ECM Flashcards
ECM
A complex and intricate network of macromolecules that surrounds cells and is closely associated with them
Synthesised, secreted and organised by surrounding cells (i.e. no self-assembly)
Highly dynamic, constantly remodelled
Cell- and tissue-specific in terms of composition, organisation, function and amount
Functions of the ECM
- Structural role - physical support for cells, links different cells/tissues, defines tissue boundaries
- Cell behaviour role - regulates cell activities, governs motility, can activate intracellular signalling
- Adhesive role - adhesive substrate for migratory cells
- Growth factor presentation role - controls spatial distribution of growth factors, can sequester/store for presentation at a later time
ECM matrisome
The ensemble of ECM proteins and cofactors
1-1.5 % of mammalian proteins, nearly 300 proteins - but further diversity can be generated by alternative splicing
Major classes of ECM macromolecules
Collagens
Glycosaminoglycans/proteoglycans
Glycoproteins
Collagens
Largest, most abundant class of ECM protein (25 % of total protein mass in body) Fibrous
Functions of fibrillar collagens
Main structural component of ECM - provide structural stiffness and mechanical resistance to tension
Provide binding sites for other ECM proteins
Regulate cell adhesion/migration
Structure of a collagen molecule
Long, triple-stranded helix in which 3 alpha chains are wound around each other, stabilised by cross links
Alpha chains characterised by presence of Gly-X-Y repeats, where X usually Pro and Y usually hydroxy Pro
Collagen molecules are rich in Pro and Gly which are important in the formation of the triple-stranded helix
Pro = ring structure stabilises helical conformation
Gly = small size allows the 3 helical alpha chains to pack together tightly to form the superhelix
Synthesis and secretion of collagen fibrils
- Collagen first synthesised as pro-alpha chain
- Selected Pro and Lys residues are hydroxylated and selected hydroxy-Lys are glycosylated
- 3 pro-alpha chains self-assemble to form pro-collagen triple helix. This self-assembly is initiated by the C-terminal propeptide in the rough ER. Can be homo- or heterotrimers
- Pro-collagen triple helix secreted into extracellular space
- Proteolytic pro peptidase enzymes cleave the N- and C-terminal propeptides to form collagen
- Collagen molecules spontaneously assemble to form collagen fibrils. Covalent cross-links form between modified Lys residues - lysyl oxidase deaminates Lys/hydroxy-Lys residues to create highly reactive aldehydes that spontaneously react with each other/other Lys/hydroxy-Lys to form covalent links between fibrils. Most cross-links form between the short, non-helical segments at each end of the collagen fibril
Type 1 collagens
Heterotrimer
2 alpha1, 1 alpha2
Type 2 collagens
Homotrimer
3 alpha2
Fibrils composed of the same mixture of collagen molecules…
…can have different arrangements in different tissues
Arrangement of collagen fibrils in tendons
Arranged in parallel bundles
Arrangement of collagen fibrils in mature bone
Arranged in successive layers at right angles to each other
Components of the ECM that regulate collagen fibril assembly
SLRPs (small leucine-rich proteoglycans)
FACITs (fibril-associated collagens with interrupted triple helices)
Scurvy
Caused by a vitamin C deficiency
Vitamin C = cofactor for prolyl hydroxylase
No vitamin C = non-functional prolyl hydroxylase = no hydroxyproline = defective collagen synthesis
GAGs
Glycosaminoglycans
Linear, unbranched, stiff polysaccharide chains composed of repeating disaccharide units, with no associated protein
Highly negatively charged (most of their sugars are carboxylated/sulfated) so are strongly hydrophilic
Composition of disaccharide units in GAGs
Amino sugar (N-acetylglucosamine/N-acetylgalactosamine) Uronic acid (glucuronic/iduronic)
“Space filler” function of GAGs
The high density of negative charge on GAGs attracts osmotically active cations (esp. Na+) which therefore attracts water
This creates a swelling pressure/turgor
GAGs are “space fillers” (“hydrated gels”) - <10 % of the weight of proteins but fill most of the extracellular space
This property of GAGs allows us to withstand compressive forces e.g. in joints
4 main groups of GAGs
Hyaluronan
Chondroitin sulphate
Heparan sulphate
Keratan sulphate
Hyaluronan
Hyaluronic acid/hyaluronate
Simplest of the GAGs - not sulphated, generally not linked to any core protein
Linear sequence of up to 25000 identical disaccharide units
“Spun out” directly from the cell surface by an enzyme complex embedded in the plasma membrane, whereas other GAGs are synthesised intracellularly then exocytosed
Functions of hyaluronan
“Captures” space for tissue formation e.g. during development, wound healing and remodelling of the ECM
Provides resistance to compressive forces
Lubricant
Maintains and regulates tissue hydration
Organises and maintains structural integrity of ECM
Proteoglycans
= GAG chains covalently linked to a protein core
GAG chains are unbranched and sulfated - pattern of sulfation determines function
Single core protein can have GAG chains of different types
How can proteoglycans be distinguished from other glycoproteins?
By the number and size of chains
Proteoglycans can contain up to 95 % carbohydrate by mass, whereas glycoproteins generally contain 1-60 %
Size variation of proteoglycans
Can vary in size dramatically
e.g.
Decorin = 1 GAG chain, <100 kDa
Aggrecan = >100 GAG chains, >2500 kDa
Functions of proteoglycans
Mechanical support - can form large aggregates with hyaluronan and provide resistance to compressive forces
Regulate collagen fibrillogenesis (SLRPs e.g. decorin)
Contribute to structural integrity through binding to each other and other ECM proteins e.g. type I collagen fibrils
Bind, sequester and present signalling molecules
Glycoproteins
A large family of structurally and functionally complex macromolecules (~200 in mammalian matrisome)
Characterised by N-linked oligosaccharides (proteoglycans = O-linked)
Short, branched heteropolymers containing 2-10 sugars
(proteoglycans = long, unbranched)
1-30 % carbohydrate by mass
Most well characterised members of the glycoprotein family
Fibronectins
Laminins
Fibronectin
Dimeric glycoproteins composed of 2 large subunits
Each subunit contains multiple repeats of fibronectin domains I, II and III
The domains mediate self-assembly of fibronectins as well as their binding to ECM and cells
Fibronectins can be thought of as “bridging molecules”
Can exist in a soluble (plasma) and insoluble (ECM) form
Fibronectin type I and II domains
Folded
Stabilised by disulphide bonds
Fibronectin type III domain
Most common (>15 per monomer)
Beta-barrel structure
Flexible
Can exist in multiple conformations
Fibronectin gene
Only one fibronectin gene in human genome
Contains 50 exons - each domain encoded by a separate exon
Alternative splicing produces multiple mRNAs that each encode for a distinct fibronectin isoform - >20 isoforms have been identified in humans so far
Isoforms differ in their function and integrin-binding properties
Soluble fibronectin heterodimers
Formation occurs intracellularly
Stabilised by disulphide bonds between C-termini of monomers
Insoluble fibronectin fibrils
Assembly occurs extracellularly
Not spontaneous - will only assemble on the surface of cells expressing the appropriate integrins
Newly-synthesised dimers bind to these receptors, leading to cell-mediated fibril assembly
Functions of fibronectin
“What aren’t fibronectins involved in?”
Direct organisation of interstitial matrix
Bridging
Regulate cell adhesion, migration and differentiation
Can unfold in response to tension which exposes previously hidden binding sites, allowing further interactions
Laminins
Large family of heterotrimeric glycoproteins
Composed of 3 polypeptide monomers held together by disulfide bonds
Each polypeptide has several isoforms (5a, 4b, 3g) that can assemble in various combinations to form the 16 known family members
The heterotrimers self-assemble intracellularly through interactions between their head domains
All laminins contains coiled-coil domains in their C-terminal proximal region
Function of laminin coiled-coil domains
Direct chain assembly and the formation of the triple-helical coiled-coil structure
Distribution of laminins
Almost exclusive to the basement membrane
Fundamental BM component - bind to and regulate other BM components e.g. type IV collagen
Basement membrane
Separates epithelia, endothelia and mesothelia from the underlying stroma
Thin but tough flexible sheet
Critical for tissue morphogenesis and angiogenesis
Principal components of the basement membrane
Large, insoluble molecules that self-assemble to form a sheet-like structure
Precise molecular composition of the BM is unique in each tissue but all are characterised by the presence of:
Interlinked laminin polymer network
Collagen type IV lattice, stabilised by the glycoprotein nidogen and the HSPG perlecan
HSPG
Heparan sulphate proteoglycan
Binds to signalling molecules and can function as co-receptors/presenters of signals
