Prof. Ishtiaq Mahmud
ishtiaq51@yahoo.com
March 11, 2013
Lecture 10
ADVANCES IN CELL BIOLOGY
Lecture 10: The Extracellular matrix (ECM)
1. Introduction
2. Collagen, the most abundant protein
3. Post-transcriptional modification
4. Genetic diseases related to collagen synthesis
5. Elastin, Fibrillin, Laminin
6. Proteoglycans and Glycosaminoglycans
7. Functions of proteoglycans
8. Bone, a mineralized connective tissue
9. Cartilage
Introduction
1. Tissues are not made up solely of cells. A substantial part of
their volume is extracellular space which is largely filled by an
intricate network of macromolecules constituting the
extracellular matrix (ECM) (fig 10-31, P -972, Edn 3rd)
Figure 1 9-53 The connective tissue underlying an epithelium. This tissue contains a
variety of cells and extracellular matrix components. The predominant cell type is
the fibroblast, which secretes abundant extracellular matrix.
2. The ECM is composed of a variety of versatile proteins and
polysaccharides that are secreted locally and assembled into
an organized meshwork in close association with the surface
of the cell that produce them.
3. Connective tissues from the architectural framework of the
vertebrate body. In these tissues the matrix is frequently
more plentiful than the cells that it surrounds and it
determines the tissues physical properties. (Fig 19-32, P-973,
Edn 3rd)
Figure 19-54 Fibroblasts in connective tissue. This scanning electron micrograph
shows tissue from the cornea of a rat. The extracellular matrix surrounding the
fibroblasts is here composed largely of collagen fibrils. The glycoproteins,
hyaluronan, and proteoglycans, which normally form a hydrated gel filling the
interstices of the fibrous network, have been removed by enzyme and acid treatment.
4. Variations in the relative amounts of the different types of
matrix macromolecules and the way they are organized in the
ECM give rise to an amazing diversity of forms, each adapted
to the functional requirements of particular tissue. The matrix
can become calcified to form the rock-hard structures of bone
or teeth or it can form the transparent matrix of cornea. At the
interface between an epithelium and connective tissue, the
matrix forms the basal lamina, a thin but tough mat that plays
important part in controlling cell behavior.
5. (a) Until recently the vertebrate ECM was thought to serve
as inert scaffolding for stabilizing physical structure of tissues
but it is now clear that ECM plays far more active and
complex role regulating the behavior of cells that contact it,
influencing their development, migration, proliferation, shape
and function correspondingly ECM has complex molecular
composition
(b) Two main classes of extracellular macromolecules that
make up the matrix are glycosaminoglycans (GAGs) which
are linked to protein to form proteoglycans and fibrous
proteins of two functional types: Structural (collagen and
elastin) and mainly adhesive (fibronectin and laminin).
(c) The principal receptors on animal cells for binding most
ECM proteins are the integrins, a large family of homologous
transmembrane linker proteins.
Extracellular Matrix
6. The macromolecules that constitute the extracellular matrix
are mainly produced locally by cells in the matrix. In
connective tissue fibroblast secretes the macromolecules.
ECM contains three major classes of biomolecules (i) The
structual proteins, collagen, elastin and fibrillin (ii) Certain
specialized proteins such as fibrillin, Fibronectin and laminin
(iii) proteoglycans.
7. ECM has been found to be involved in many normal and
pathologic processes i.e. inflammatory states, spread of
cancer cells, rheumatoid arthritis and osteoarthritis.
Collagen
8. It is the major component of most connective tissues,
constitute approximately 25% of protein of mammals and it is
the most abundant protein in the animal world.
9. It provides extracellular framework for all animals and exists
in every animal tissues. At least 25 distinct types of collagen
made up of over 30 distinct polypeptide chains have been
identified in human tissues. (Table 47-1, P-546, Harper 27th
Edn)
10. A number of proteins that are not classified as collagens
have collagen-like domains in their structures there proteins
are sometimes referred to as ‘non-collagen collagen’.
11. Types of collagen are subdivided into a number of classes
based primarily on the structures they form. (Fig 19-41, P979, Edn 3rd)
Figure 19-63 A fibroblast surrounded by collagen fibrils in the connective tissue of
embryonic chick skin' In this electron micrograph, the fibrils are organized into
bundles that run approximately at right angles to one another. Therefore, some
bundles are oriented longitudinally, whereas others are seen in cross section. The
collagen fibrils are produced by the fibroblasts, which contain abundant
endoplasmic reticulum, where secreted proteins such as collagen are synthesized.
Structure of collagen
12. All collagen types have a triple helical structure. Mature
collagen type 1, containing approximately 1000 amino acids.
Each polypeptide subunit or alpha chain is twisted into a left
handed helix of three residues/ turn (Fig 47-1, P-547, Harper
27th Edn).
Structure of collagen
13. 3 of these α-chains are then wounded into a right handed
super helix forming a rod-like molecule of 1.4 nm in diameter
and about 300 nm long. A striking characteristic of collagen is
the occurrence of glycine residues at every third position of
the triple helical portion of the alpha chain.
14. Glycine is the only amino acid small enough to be
accommodated in the limited space available down the
central core of the triple helix. This repeating structure
represented as (Gly-x-y) n is an absolute requirement for the
formation of triple helix. While x and y can be any other
amino acid, about 100 of the X positions are proline and 100
of the Y positions are hydroxy proline. These two amino acids
provide rigidity on the collagen molecule
15. Collagen fibers are further stabilized by the formation of
covalent cross-links. These cross links form through the
action of lysyl oxidase, a copper dependant enzyme.
16. Type IV collagen the best characterized example of a
colagen is an important component of basement membranes
where it forms a mesh like network.
.
Collagen
undergoes
modification
extensive
post-translational
17. Collagen is synthesized on ribosome in a precursor form,
Preprocollagen, which contains a leader or signal sequence
that directs the polypeptide chain into the lumen of ER, where
the leader sequence is removed, Hydroxilation of proline and
lysine residues and glycolysation of hydroxylysine in the
procollagen molecules also take place at this site (Fig 19-43,
P-981, Edn 3rd)
Collagen
undergoes
modification
extensive
post-translational
18. The procollagen molecule contains extension peptide of 2035 kDa at both its ends. Following secretion from golgi
apparatus the extension peptides are removed and enzymes.
Then the triple helix spontaneously assembles into collagen
fibers. These are further stabilized by the formation of inter
and intrachain cross links through the action of lysyl oxidase.
19. Some cells that secrete collagen also secrete fibronectin, a
large gylcoprotein present on cell surfaces. Fibronectin binds
to aggregating procollagen fibers and alters the kinetics of
fiber formation in the matrix. Associated with fibronectin and
procollagen in this matrix are the proteoglycans heparan
sulfate and chondroitan sulfate. Once formed collagen is
relatively metabolically stable. However, breakdown is
increased during starvation and inflammation.
A number of genetic diseases result from the abnormalities
in the synthesis of collagen
20. About 30 genes encode collagen and atleast 8 enzymes
catalyzed posttranslational steps. Number of diseases occur
due to mutation in the collagen genes. (Table 47-4, P-548)
21. Elastin
It is a connective tissue protein that is responsible for
properties of extensibiity and elastic recoil in tissues.
Although not as widespread as collagen, elastin is present in
large amounts, particularly in tissues that require these
physical properties e.g. lung, blood vessels and elastic
ligaments and small amounts in ear and skin. (Fig 19-49, P985, edn 3rd)
Figure 1 9-70 Elastic fibers. These scanning electron micrographs show (A) a lowpower view of a segment of a dog's aorta and (B) a high-power view of the dense
network of longitudinally oriented elastic fibers in the outer layer of thesame blood
vessel. All the other components have been digested away with enzymes and formic
acid.
22. It is synthesized as soluble monomer of 70kDa called
tropoelastin, deletion of elastin gene have been found in 90%
of the subjects with Williams syndrome, a developmental
disorder affecting connective tissue and the central nervous
system (Table 47-5, P-549, Harper 27th edn). A number of
skin diseases are associated with accumulation of elastin.
Alternatively, a decrease of elastin is found in – conditions
such as pulmonary emphysema and aging of the skin
Fibrillin
23. A large glycoprotein (about 350 KDa) that is a structural
component of microfibrils 10-12 nm fibers found in many
tissues. It is secreted into ECM by fibroblast and becomes
incorporated into the insoluble microfibrils which appear to
provide a scaffold for deposition of elastin.
24. Marfan Syndrome, It is a relatively prevalent inherited
disease affecting connective tissues. It is inherited as an
autosomal dominant trait. It affects the eyes, the skeletal
system and hyper-extensibility of the joints and
cardiovascular system. (Abraham lincolin may have had this
condition). This syndrome is due to mutations in the gene for
fibrilin.
Lamininn
25. It is a protein of about 850 kDa and 70 nm long, consists of
3 polypeptide chain linked together to form an elongated
cruciform shape. It has binding sites for type IV collagen,
heparin and integrins. It helps in the anchoring of the lamina
to the cells.
Fibronectin
26. It is a major glycoprotein of ECM, also found in soluble form
in plasma. It contains an Arg-Gly-Asp (RGD) sequence that
binds to the receptor. The protein is involved in the adhesion
of cells to ECM and cell migration. The amount of fibronectin
around many transformed cells are sharply reduced partly
explaining the faulty interaction with the ECM
Proteoglycan and Glycosaminoglycans (GAG)
27. Proteoglycans are proteins that contain covalently linked
glycosaminoglycans. At least 30 have been characterized
and given names such as syndecan, serglycin. They vary in
tissue distribution. The protein bound covalently to
glycosaminoglycan is called core proteins. The amount of
carbohydrate in a proteoglycan is usually much greater than
is found in a glycoprotein and many comprise up to 95% of
this weight.
28. There are at least seven glycosaminoglycans (GAGs),
Hyaluronic acid, chondoitin sulfate, Keratin sulfate and
chondroitan sulfate I and II, heparin, heparan sulfate and
dermatan sulfate. (Fig 19-33, P-973, Edn 3rd)
Figure I 9-55 The repeating disaccharide sequence of a heparan sulfate glycosaminoglycan (GAG) chain.
These chains can consist of as many as 200 disaccharide units, but are typically less than half that size. There
is a high density of negative charges along the chain due to the presence of both carboxyl and sulfate groups.
The proteoglycans of the basal lamina-perlecan, dystroglycan, and collagen XVlll-all carry heparan sulfate
GAGs. The molecule is shown here with its maximaf number of sulfate groups. ln vivo, the proportion of
sulfated and nonsulfated groups is variable. Heparin typically has >70o/o sulfation, while
29. A GAG is an unbranched polysaccharide made up of repeating
disaccharides. One component of which is always an amino sugar (hence
the name GAGs), either D-glucosamine or D-galactosamine. The other
component of the repeating disaccharide is an uronic acid. With the
exception of hyaluronic acid, all the GAGs contain sulfate groups. GAGs
are highly negatively charged. They occupy large amount of space and
form hydrated gels
Proteoglycans have numerous functions
30. Proteoglycans have numerous functions and found in every tissue of the
body mainly in ECM or ‘ground substance’ where they are associated with
each other and also with the other major structural components of the
matrix, collagen and elastin. These interactions are important in
determining the structural organization of the matrix. The GAGs bind
polycations and cations such as Na+ and K+, Polycations can also act as
sieves restricting the passage of large growth factors such as TGF-β,
modulating their effects on cells.
31. Some functions of specific GAGs and proteoglycans (i) Hyaluronic acid,
is in high conc. in embryonic tissues and is thought to play an important
role in permitting cell migration during morphogenesis and wound repair.
32. Chondroitin sulfate, it is located at sites of calcification in endochondral
bone and is also found in cartilage, located also in certain neurons
providing endoskeletal structure to maintain their shape. (Table 47-8, P557, Harper)
33. Keratin sulfate I and dermatan sulfate are present in the
cornea, plays important role in corneal transparency.
34. Heparin is an important anticoagulant, binds with factors IX
and XI, but it’s most important interaction is with plasma
antithrombin. It can also bind to lipoprotein lipase present in
capillary walls, causing a release of this enzyme into the
circulation. Proteoglycans are also found in intracellular
locations such as nucleus
Bone is a mineralized connective tissue
35. It contains both organic and inorganic, organic material is mainly
protein, Bone is a dynamic structure that undergoes continuous
cycle of remodeling, consisting of resorption followed by deposition
of new bone tissue. The major cell types are osteoclasts and
osteoblasts. Osteoclast is multinucleated cells derived from
pluripotent hematopoietic stem cells. Osteoblasts control
mineralization.
36. Metabolic and genetic disorder involves bone. Osteogenesis
imperfecta (brittle bones) is characterized by abnormal fragility of
bone.
37. Osteopetrosis (marble bone disease) characterized by increased
bone density is due to inability to resolve bone.
38. Osteoporosis is a generalized progressice reduction in bone
tissue mass per unit volume causing skeletal weakness (Table 4710. P-561)
Cartilage
39.Collagen is the principal protein. Elastic cartilage contains
elastin and fibroelastic cartilage contains type 1 collagen.
Cartilage contains a number of proteoglycans which play
important role in its compressibilty.