Extracellular matrix

EXTRACELLULAR
MATRIX

Department of Natural Sciences
University of St. La Salle
Bacolod City

 Many animal cells are intrinsically linked to other
cells and to the extracellular matrix (ECM).
 Cell surface molecules bind to other cells, or to
other components of the ECM. They also play a role
in mutual recognition of similar cell types.
 Bone and cartilage are mostly ECM plus a very few
cells. Connective tissue that surrounds glands and
blood vessels, is a gelatinous matrix containing
many fibroblast cells.

The ECM contains 3 classes of molecules:
 structural proteins (collagens and elastins)
 protein-polysaccharide complexes to embed the
structural
proteins
(proteoglycans)
 adhesive
glycoproteins
to attach cells
to matrix
(fibronectins
and laminins).

PROTEOGLYCANS
1. PROTEOGLYCANS are composed of a core protein to which
glycosaminolycans (GAGs) are attached. GAGs consist of repeating
disaccharide subunits.
 One of the two sugars in the disaccharide is often an amino sugar
(N-acetyl-glucosamine or N-acetyl-galactosamine; usually with an
attached sulfate group) and the other is a sugar or sugar acid
(galactose or glucuronate).
 Chondroitin sulfate, keratan sulfate, heparan sulfate and
hyaluronate are the most common GAGs.

Each of the four classes of GAGs is
formed by polymerization of monomer
units into repeats of a particular
disaccharide and subsequent
modifications, including addition of
sulfate groups and inversion of the
carboxyl group on carbon 5 of D-
glucuronic acid to yield L-iduronic acid.

Heparin is generated by
hypersulfation of heparan
sulfate, whereas hyaluronan is
unsulfated. The squiggly lines
represent covalent bonds that are
oriented either above (D-glucuronic
acid) or below
(L-iduronic acid) the ring.

 Most GAGs in the ECM are bound to proteins
to form proteoglycans or mucoproteins.
 Numerous GAGs (1-200 per
molecule, average length of 800
monosaccharide units) are attached to a core
protein and different kinds of proteoglycans
can be made by varying the combination of
core proteins and GAGs.
 Proteoglycans (MW of~ 1 million) can be
individual or attached to long hyaluronate
molecules to form complexes (as in cartilage).
 They can be embedded in the plasma
membrane or covalently linked to membrane
phospholipids or bound to receptor proteins.

 Proteoglycans and collagen may bind to receptor
proteins (often integrins) which are reinforced by
adhesive glycoproteins, such as fibronectins and
laminins, to anchor cells to the ECM.
 GAGs in CT are highly sulfated which attracts
water of hydration. They trap water (up to 50x
their weight) to act as extracellular sponges
resistant to physical forces in cartilage and joints.
 If fluid is injected into CT, it remains
localized, walled off by a viscous ground
substance. This property acts as barrier to the
spread of bacteria that gains access to the
tissues.
 Some bacteria secrete hyaluronidase (Staphylo/
Strepto/ Pneumococci), and collagenase
(Clostridium perfringens) that breakdown matrix
components.

EDEMA is a condition characterized by
accumulation of excess tissue fluid.

 Edema accompanies pathological conditions that
cause:
 Increased hydrostatic pressure
in capillaries by obstructing
venous blood flow
(e.g. congestive heart failure)
 Decreased colloid osmotic pressure in the blood
caused by lack of blood proteins (e.g. starvation)
 Increased hydrostatic pressure in the tissue
caused by blockage of lymphatic
drainage by parasites or tumor cells
 Increased colloid osmotic pressure in
the tissue caused by excessive
accumulation of GAGs in the matrix.
 Hypothyroidism resulting from this
condition is referred to as myxedema.

 Principal producers of collagen fibers are fibroblasts;
epithelial and smooth muscle cells also secrete their
own type-IV collagen.
 Most numerous CT matrix, running in all directions in a
wavy course; dull and opaque in appearance.
 Fibers bundled together branch and anastomose;
individual fibers do not branch.
 With the EM, unit fibrils of collagen show periodic cross
striations every 67 nm of their length.

(a) In tendons, type I
fibrils are all oriented in the Interactions of fibrous
direction of the stress applied collagens with nonfibrous
to the tendon. Proteoglycans
and type VI collagen bind
fibril-associated collagens.
noncovalently to
fibrils, coating the surface.
The microfibrils of type VI
collagen, which contain
globular and triple-helical
segments, bind to type I
fibrils and link them together
into thicker fibers. (b) In
cartilage, type IX collagen
molecules are covalently
bound at regular intervals
along type II fibrils. A
chondroitin sulfate chain,
covalently linked to the 2 type
IX chains at the flexible
kink, projects outward from
the fibril, as does the globular
N-terminal region.

1.INTRACELLULAR – free polysomes reading collagen mRNA
attach to the rER, and protocollagen or precollagen-chains
are deposited in the cisternae. Each chain has about 250
amino acids; every 3rd amino acid is glycine.
 The signal peptide is clipped off. Proline and lysine
residues within the chains are then hydroxylated in the ER
to form hydroxyproline and hydroxylysine (unusual amino
acids present in large amounts in collagen).
 Core sugars (galactose and glucose) attach to the
hydroxylysine residues in the ER.
 Each chain is synthesized with an extra length of peptides
known as registration peptides, which ensure that the
appropriate chains assemble in their correct position in the
resulting triple helical molecule called procollagen.
 Further glycosylation may occur in the Golgi
complex, where procollagen is packaged for secretion.
Golgi vesicles release procollagen into the extracellular
space by exocytosis.

2.EXTRACELLULAR- in the extracellular
space, the enzyme procollagen peptidase
cleaves the registration peptides from
procollagen, converting it to tropocollagen.
 Catalyzed by lysyl oxidase, these become
aligned in staggered fashion to form collagen
fibers, possibly under the control of adjacent
fiber-producing cells.
 The turnover of collagen is slowest in
tendons, fastest in loose CT. Macrophages and
neutrophils break down old collagen, and
replaced by fibroblasts.
 As humans age, extracellular collagen becomes
increasingly cross-linked, & turn-over slows
down in CT.

Because collagen synthesis depends on the
expression of several genes and on several post-
translation events, many human diseases are
associated with faulty collagen synthesis.
 Progressive systemic sclerosis- excessive
accumulation of collagen (fibrosis) in almost all organs
 Keloid- local swelling caused by abnormal amounts of
collagen that form in scars of skin
 Ehlers-Danlos type IV- aortic/ intestinal rupture due to
faulty transcription of collagen type III
 Ehlers-Danlos type VII- increased articular motility due
to decreased procollagen peptidase activity
 Scurvy- ulceration of gums, hemorrhages due to lack
of Vit. C, a cofactor for proline hydroxylase
 Osteogenesis imperfecta- spontaneous fractures &
cardiac insufficiency due to mutation in collagen type I

YELLOW or ELASTIC FIBERS
 Form gentle curves or spirals at their free ends
when released from tension
 Do not form bundles; individual fibers branch and
anastomose to form networks
 They can be stretched to 150% of their length
without breaking, but lose their resiliency with
advancing age.
 Appears yellowish, highly refractile, homogenous
and are not made up of fibrillar subunits that are
visible with the light microscope.
 Each fibril is made up of still smaller fibrils united
by a small amount of ground substance. These
smaller “microfibrils” have periodic cross
bandings.

Synthesis and Assembly of Elastin:
1.Intracellular- microfibrillar proteins containing
mostly hydrophilic amino acids, and proelastin
(contains large amounts of the hydrophobic amino
acids glycine, proline and valine, thus accounting for
elastin’s insolubility) are synthesized on rER and
secreted separately.
2.Extracellular- proelastin molecules polymerize
extracellularly to form elastin chains.
 Lysyl oxidases then catalyze the conversion of
certain lysine residues of elastin to aldehydes, 3 of
which condense with a 4th unaltered lysine residue
to form desmosine and isodesmosine.
 These very rare amino acids found in elastin cross-
link individual chains, which then associate with
numerous microfibrils to form a branching and
anastomosing network of elastic fibers.

ARGYROPHYL or RETICULAR FIBERS
 Fibers are not branched, and are not so wavy as the
collagenous fibers when released from tension.
 They are chemically identical to collagen, hence
these fibers are considered as precursors of type I
and III collagen; however, they are thinner and form
delicate networks instead of thick bundles
 Chemical characteristics- show affinity to silver
(black) stains, hence argyrophyl; do not yield gelatin
on boiling; not easily dissolved by dilute acids and
alkali; not so easily digested by gastric juice; not so
resistant to solutions of alkaline pancreatic juice.
 Distribution- abundant in regions around blood
vessels, muscle fibers, fat cells, basement membrane
of epithelia, endoneurium, lymphoid organs and red
bone marrow.

 Glycoproteins are
globular proteins to which
shorter, branched
oligosaccharide chains
are covalently bound.
 These so-called adhesion
glycoproteins mediate
attachment of cells to
their matrix, influence the
state of differentiation of
cells, and organization of
their cytoskeleton.
 Examples are fibronectin,
laminin, thrombospondin,
chondronectin and
fibrillin.

 FIBRONECTINS, a family of closely
related glycoproteins, are soluble in
body fluids (blood), insoluble in the
ECM and partially soluble at the cell
surface.
 The fibronectins bind cells to the
matrix and guide cellular movement.
 The RGD (arginine-glycine-aspartate)
sequence binds to the integrin
fibronectin receptor.
 The fibronectins bind cells to the
ECM by bridging cell-surface
receptors to the ECM.
 The intracellular cytoskeleton will
align with the extracellular
fibronectin to detemine cell shape.
 In many kinds of cancer, cells unable
to make fibronectins loose shape and
detach from the ECM to become
malignant.

 During cell movement (as
during
embryogenesis), pathway
s of fibronectins guide
cells to their destinations.
 Soluble plasma
fibronectin promotes
blood clotting by direct
binding of fibrin.
 Fibronectins guide
immune cells to wounded
areas and thus promote
wound healing.

 LAMININS bind cells
to the basal lamina of
epithelial and
connective
tissues, and to their
surrounding muscle
cells, fat cells, and
Schwann cells.
 The basal lamina
serves as a structural
support for tissues
and as a permeability
barrier to regulate
movement of both
cell and molecules.

Laminin is a very large
protein comprised of
three proteins that
form a cross. The
domains of laminin
bind type IV
collagen, heparin, hep
arin sulfate, entactin
and laminin receptor
proteins in overlying
cells to allow bridging
between the cells and
the ECM. Progeria
(early onset of
aging), is possibly due
to a defective laminin.

 THROMBOSPONDIN - activated platelet-product.
It binds to fibrinogen, plasmalogen and its
activator; a participant in blood clotting. Its
function is poorly understood.
 CHONDRONECTIN- a component of cartilage
matrix that mediates attachment of
chondrocytes to their matrix
 FIBRILLIN- a nonsulfated glycoprotein
speculated to be essential for normal
development. It is often associated with elastic
fibers or with epithelial basal laminae.
Marfan syndrome is due to a defective fibrillin
gene on chromosome 15, characterized by
excessively long arms and legs and
progressive dilatation and fatal rupture of the
ascending aorta (Pres. Abraham Lincoln).

The DGC comprises 3 subcomplexes: Molecular Connections Between ECM
dystroglycan, sarcoglycan/
sarcospan of integral membrane and Disease: Muscular Dystrophy
proteins; and the cytosolic adapter
comprising dystrophin, other adapter
proteins, and signaling molecules.
Through its O-linked
sugars, dystroglycan binds to
components of the basal
lamina, such as laminin. Dystrophin-
the protein defective in Duchenne
muscular dystrophy, links
dystroglycan to the actin
cytoskeleton, and dystrobrevin links
dystrophin to the sarcoglycan/
sarcospan subcomplex. Nitric oxide
synthase (NOS) produces nitric
oxide, a gaseous signaling
molecule, and GRB2 is a component
of signaling pathways activated by
certain cell-surface receptors.
Mutations in dystrophin, other DGC
components, laminin, or enzymes
that add the O-linked sugars to Schematic model of the dystrophin
dystroglycan disrupt the DGC- glycoprotein complex (DGC) in skeletal
mediated link between the exterior
and the interior of muscle cells and
muscle cells.
cause muscular dystrophies.

Extracellular matrix

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