Download the extracellular matrix

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THE EXTRACELLULAR MATRIX (ECM)
Cells are embedded in a jelly of proteins and
polysaccharides called the extracellular matrix or the
ground substance
In epithelial tissues, cells are tightly attached together to
form sheets; ECM is scanty and forms the basal lamina
The basal lamina forms a supporting layer underlying the
epithelia and helps prevent the cells from ripping apart
In connective tissues ECM forms a larger space and carries
the mechanical stress to which the tissue is subjected
The ECM serves as a reservoir for many extracellular
signaling molecules that control cell growth and
differentiation
In addition, it provides a network through or on which
cells can move during tissue assembly
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• The cells in animal tissues are “glued” together by cell
adhesion molecules (CAMs), proteins embedded in their
surface membranes
• CAMs mediate cell-cell and cell-ECM interactions
• CAMs fall into four major families: the cadherins,
immunoglobulin (Ig) superfamily, integrins and selectins
• CAMs mediate, through their extracellular domains,
adhesive interactions between cells of the same type
(homotypic adhesion) or between cells of different types
(heterotypic adhesion)
• A CAM on one cell can directly bind to the same kind of
CAM on an adjacent cell (homophilic binding) or to a
different class of CAM (heterophilic binding)
• Clustered CAMs form cell junctions
• The cytosolic domains of CAMs form connections with
cytoskeletal elements
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• Three types of molecules are abundant in the extracellular
matrix of all tissues:
 Highly viscous proteoglycans –a group of glycoproteins
that cushion cells and bind many extracellular molecules
 Fibrous proteins–which provide mechanical strength
 Soluble multiadhesive matrix proteins, which bind to and
cross-link cell-surface adhesion receptors and other ECM
components
The extracellular matrix of epithelial tissues
• All epithelial cells in a sheet are connected to one another
and the extracellular matrix by specialized cell junctions
consisting of dense clusters of CAMs
• Anchoring junctions and tight junctions perform the key task
of holding cells together into tissues
• Gap junctions permit the rapid diffusion of small, watersoluble molecules between the cytoplasm of adjacent cells
• The primary CAMs in the cell-cell interactions of epithelia are
cadherins; some cell–cell many cell–matrix interactions are
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mediated by integrins
• The basal lamina can be a single layer (intestine, skin,…) or it
can surround each cell (muscle, adipocytes,…)
• Most of the ECM components in the basal lamina are
synthesized by the cells that rest on it
• Four protein components are found in basal laminae:
 Type IV collagen –trimeric molecules with both rodlike and
globular domains that form a two-dimensional network
 Laminins –a family of multiadhesive proteins that form
a fibrous two-dimensional network with type IV collagen
and that also bind to integrins
 Entactin –a rodlike protein that cross-links type IV collagen
and laminin and helps incorporate other components
 Perlecan –a large proteoglycan that binds to and cross-links
many ECM components and cell-surface molecules
• Type IV collagen, is one of more than 20 types of collagen that
participate in the formation of the ECM in various tissues
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• Collagen accounts for around 25 % of the total protein content
of the body
• All collagens are fibrous proteins made from three
polypeptides called collagen α chains
• Unlike the α helix secondary structure of many proteins, the
structure of collagen is a left-handed helix
• The three chains wrap around each other to form a righthanded triple helix
• The collagen triple helix can form because of an unusual
abundance of three amino acids: glycine, proline and
hydroxyproline
They make up the characteristic repeating pattern Gly-X-Y,
where X and Y can be any amino acid but are often proline
and hydroxyproline and less often lysine and hydroxylysine
• Glycine is essential because its small side chain, a hydrogen
atom, is the only one that can fit into the crowded center of
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the three stranded helix
• Although the rigid peptidylproline and peptidyl hydroxyproline linkages are not compatible with formation of
a classic single-stranded helix, they stabilize the distinctive
three-stranded collagen helix
The hydroxyl group of hydroxyproline (by forming
hydrogen bonding with members of other chains) helps
hold its ring in a conformation that stabilizes the triple helix
• The triple helix of collagen IV is interrupted at several points
by non-helical segments that give flexibility to the structure
• Type IV collagen is a sheet/network forming type
 Triple helices associate with each other through the
globular domains at the C and N terminals
• Laminin is a heterotrimeric cross-shaped protein
• Different regions of laminin bind to cell-surface receptors and
various matrix components
e.g., collagen, proteoglycans, integrins,…
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• Proteoglycans are the products of the covalent linkage of
core proteins with glycosaminoglycans (GAG)
• They contain more carbohydrates than glycoproteins
• GAGs are composed of repeating disaccharide units
 the first sugar of the disaccharide is either
N-acetylglucosamine or N-acetylgalactosamine
 the second sugar is usually either glucuronate, iduronate or
galactose
 the sugars are usually sulfated
• GAGs are classified based on the nature of the repeating
disaccharide unit: heparan sulfate, chondroitin sulfate,
dermatan sulfate, keratan sulfate and hyaluronan
• A hypersulfated form of heparan sulfate called heparin,
produced mostly by mast cells, plays a key role in allergic
reactions
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• Heparin is used medically as an anticlotting agent because
a pentasaccharide unit of heparin binds and activates
antithrombin III
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• The glycosylation of core proteins begins in the lumen of
the endoplasmic reticulum and continues in the Golgi
• A link trisaccharide containing xylose and two galactose
residues can be the first to attach through O-glycosidic bond
with a Serine of the core proteins and then the other
sugares are added
• Or the modified sugars could be directly be attached
through N-glycosidic bond with Asparagine residue of the
core proteins
• Once chain formation is over, sulfotransferases add sulfate
which is obtained from (PAPS)
 The abundance of negative charges increases polarity
 The proteoglycans bind large amounts of water and
fill the gaps between the other components of the
ECM in the form of a hydrated gel
• Epimerization of glucuronate to iduronate also takes place
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after the chain is synthesized
• Specific types of GAGs are abundant in certain tissues:
 Hyaluronate – especially in embryonic tissue; allows
migration of cells during morphogenesis and wound healing;
vitreous humor = 1 % hyaluronate and 98 % water
Chondroitin sulfate – along with hyaluronate, gives
compressibility to the bone and cartillage
Keratan and dermatan surface –lie between collagen fibrils
and play a role in corneal transparency
o Heparin –can dislodge LPL from capillaries
• GAGs are degraded by lysosomal enzymes: glycosidases and
sulfatases
• Various degrees of deficiency of these enzymes leads to
different types of mucopolysaccharidoses
Rare diseases inherited in an autosomal recessive manner
e.g. Hurler Syndrome - deficiency of α-L- iduronidase
Hunter Syndrome - iduronate sulfatase
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The extracellular matrix of non-epithelial tissues
Connective tissues are composed of ECM for the most
part
The main components include hyalruornate, various
proteoglycans and collagen
Rubber-like elastin fibers, which can be stretched and
relaxed, also are present in deformable sites (e.g.,
skin, tendons, heart, lungs)
Multi adhesive proteins known as fibronectins are
characteristic of connective tissue
The various components of the ECM of connective
tissues are produced largely by cells called fibroblasts
80-90% of the collagen in the body consists of types IIII –fiber forming collagen
The triple helices of type I-III collagen associate into
collagen fibrils, which in turn often aggregate into
larger bundles called collagen fibers
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• Collagen is first synthesized as preprocollagen which
contains a signal sequence
• In the lumen of the endoplasmic reticulum, preprocollagen
is modified: removal of the signal sequence, hydroxylation
of proline and lysine residues and glycosylation of some
hydroxylysine residues
 preprocollagen is changed to procollagen
• In the Golgi apparatus procollagen is associated into a triple
helix (initiated through disulfide bonds between C-terminal
residues) and released to the extracellular space
• Extracellular peptidases remove the N- and C-terminal
propeptides to give tropocollagen
• Tropocollagens associate laterally to generate fibrils
• In fibrils, adjacent collagen molecules are displaced from
one another by about one-quarter of their length
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Striation is observed in electron micrographs
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• The fibrils are strengthened through covalent cross-links
 Lysine and hydroxylysine side chains are modified by
extracellular lysyl oxidases to form aldehydes (allysine and
hydoxyallysine) in place of the amine group at the end of
the side chain
 The aldehydes form covalent crosslinks with lysine,
hydroxylysine and histidine residues in adjacent molecules
• The cross-links stabilize the side-by-side packing of collagen
molecules and generate a strong fibril
• Removal of the propeptides and covalent cross-linking take
place in the extracellular space to prevent the potentially
catastrophic assembly of fibrils within the cell
• Type I, II and III collagen fibers are characteristic of the ECM
of the (skin, bone and tendons); cartilage and arteries,
respectively
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Disorders of collagen synthesis
• Scurvy: in addition to its antioxidant properties, vitamin C plays
an essential role in collagen synthesis
 prolyl hydroxylase and lysyl hydroxylase reactions require O2,
α-ketoglutarate, ascorbic acid and the enzymes contain Fe+2
 Proline (lysine) changed to hydroxyproline (hydroxylysine);
α-ketoglutarate is decarboxylated to succinate
 Ascorbate is not directly involved in the reaction
 The hydroxylases can also decarboxylate α-ketoglutarate
without hydroxylating proline and lysine
 During this reaction, the Fe+2 becomes oxidized, and the
oxidized form of the enzyme is unable to hydroxylate proline
or lysine
 The ascorbate consumed in the reaction presumably
functions to reduce the heme iron and restore enzyme
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activity
• If vitamin diet is deficient in vitamin C , hydrogen bonds
cannot be formed between strands in the collagen triple helix
• the melting point of collagen fibers decreases –loss of
stability
• Scurvy leads to lesions in the skin and blood vessels,
bleeding gums, swelling joints, poor wound healing, and, in
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its advanced stages, it can lead to grotesque disfiguration
• Genetic abnormalities in collagen synthesis include:
Osteogenesis imperfecta – “brittle bone syndrome”
Abnormally Short chains make up the collagen I triple
helix ; or other amino acids may take the place of glycine
Ehlers-Danlos syndrome –about ten types
 lack of peptidases; resistance to peptidases; insufficient
synthesis of collagen; rapid degradation;…
 recurrent dislocation of joints, hyperextensible skin
• In general, collagen fibers make connective tissues resistant
to stretching; the swelling caused by the water associated
with hyaluronate gives compressibility
• The predominant proteoglycan in cartilage, called aggrecan,
assembles with hyaluronan into very large aggregates
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• Elastic fibers are composed of elastin and microfibrils
• Elastin is synthesized as a soluble monomer known as
tropoelastin
• Unlike procollagen, tropoelastin does not have extension
peptides, Gly-X-Y sequences, triple helices or attached
carbohydrates; little hydroxyproline and no hydroxylysine
• Lysyl oxidase in the extracellular space oxidatively
deaminates lysine residues of tropoelastin
• Cross links are formed between elastin molecules making
use of three modified lysine residues (aldehydes) and one
unmodified lysine (amino)
 the linkage may be desmosine or isodesmosine
• The insoluble cross-linked elastin molecules are deposited
on a network of microfibrils
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• Microfibrils are polymers of the glycoprotein fibrillin
• Mutations in the gene for fibrillin lead to Marfan’s syndrome
 dislocated lens, long digits and dilated aorta
o Please read about the elastase in the lungs and its inhibitor
• Fibronectins help attach cells to the extracellular matrix by
binding to other ECM components, particularly fibrous
collagens and heparan sulfate proteoglycans, and to cell
surface adhesion receptors such as integrins
• Fibronectins are dimers of two similar polypeptides linked at
their C-termini by two disulfide bonds
• Each chain comprises six functional regions with different
ligand-binding specificities
• Integrins are usually bound by repeats of Arg-Gly-Asp (RGD)
sequences of fibronectins
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• After adhesive interactions in epithelia form during
differentiation, they often are very stable and can last for the
lifetime of the cells or until the cells become loosely-associated
• The formation and breaking of adhesive interactions is a
characteristic mainly of certain non-epithelial tissues
 e.g., during morphogenesis, inflammation,…
The movement of leukocytes into tissues
• To fight infection and clear away damaged tissue, leukocyte
must move rapidly from the blood, where they circulate as
unattached, into the underlying tissue at sites of infection
• This process is known as extravasation
• First inflammatory signals (chemokines from the endothelium
and leukocytes) cause activation of the endothelium
 The synthesis of the CAMs is part of the activation of the
endothelium
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• The lectin domain of P-selectin found on endothelial cells will
bind the oligosaccharides present in the glycoproteins and
glycolipids of leukocytes
• Because of the force of the blood flow and the rapid “on”
and “off” rates of P-selectin binding to its ligands, the
leukocytes are slowed but not stopped and roll along the
endothelium
• For tight adhesion to occur, certain integrins on leukocytes
are activated by chemokines and PAF
 the integrin is changed to a high affinity form
• Activated integrins on leukocytes then bind to each of two
distinct Ig-CAMs (ICAM 1 and 2)
• The integrin–ICAM interactions stop leukocytes from rolling;
leukocytes spread on the surface of the endothelium
• The adhered cells move between adjacent endothelial cells
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and into the underlying tissue
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o If The connection of cytoskeletal elements with the ECM is
defective, disease conditions may occur
The muscular dystrophies
•Duchenne muscular dystrophy (DMD) the most common type
• cardiac or respiratory failure in young boys
• mutations in the gene encoding dystrophin –a cytosolic adapter
protein, binding to actin filaments and to an adhesion receptor
called dystroglycan (which, in its turn, is connected to laminin)
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