Extra Cellular Matrix (ECM)

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• Extra Cellular Matrix (ECM)
• Structure, Synthesis, Function, Disease

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The Extra Cellular Matrix ECM

• Extra Cellular outside the cellMatrix
  structure made from a network of interacting
  components
• The ECM is composed of an interlocking mesh
  of fibrous proteins and glycosaminoglycans (GAGs).
  
• Components of the ECM are produced
  intracellularly by resident cells, and secreted
  into the ECM via exocytosis.

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Functions of ECM

• 1-Role in establishing and maintaining cell
  shape,
• migration, mechanical support
• 2-Anchorage for cells, segregating tissues from
  one another, and regulating intercellular
  communication
• 3-Sequesters a wide range of cellular growth
  factors, and acts as a local depot for them
• 4-Essential for processes like growth, wound
  healing etc

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What are the major proteins of the
ECM?Collagens, Proteoglycans, Elastin,
Fibronectin, Laminin, Tenascin.
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The collagens

• A family of fibrous proteins found in all
  multicellular animals
• They are secreted by connective tissue cells, as
  well as by a variety of other cell types
• They are the most abundant proteins in mammals,
  constituting 25 of the total protein mass in
  these animals

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Structural features

• Long, stiff, triple-stranded helical structure,
• Three collagen polypeptide chains, called a
  chains, are wound around one another in a
  ropelike superhelix
• A basic unit of mature collagen is called
  tropocollagen

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Composition of collagens

• Collagens are extremely rich in proline and
  glycine
• It is composed mainly of glycine (33), proline
  (13), 4-hydroxyproline (9)
• Hydroxyproline is unique for collagen and elastin

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Amino acid sequence

• Every third residue is glycine which lies in the
  center of the triple helix, with the preceding
  residue being proline or hydroxyproline in a
  repetitive fashion
• pro-Gly-X
• hydroxypro-Gly-X

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Functions of amino acids

• Proline stabilizes the helical conformation in
  each a chain
• Glycine allows the three helical a chains to pack
  tightly together to form the final collagen
  superhelix

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Hydroxylysine

• Collagen is also composed of hydroxylysine, which
  serves as attachment sites of polysaccharides
  making collagen a glycoprotein

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Lysine

• Part of the toughness of collagen is accounted by
  the cross-linking of chains via lysine residues

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How?

• Some of the lysine side chains are oxidized to
  aldehyde derivatives, which react with another
  lysine or another oxidized lysine via the action
  of lysyl oxidase

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Types of collagens

• There are about 40 collagen genes dispersed
  throughout the genome and the protein products
  combine to form more than 28 different types of
  collagen.
• The various collagens and the structures they
  form all serve the same purpose, to help tissues
  resist stretching.

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Classification of collagen
1. Fibril-forming collagens

• No interruptions in triple helix
• Regular arrangement results in characteristic
  D period of 67 nm
• Diameter 50-500 nm
• Example Types I, II, III, V, XI

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Classification of collagen
2. Network-forming collagens

• Forms network in basement (Collagen IV) and
  Descemets membrane (Collagen VIII)
• Molecular filtration
• Example Types IV, VIII, X

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Classification of collagen
3. Fibril-associated collagens with interrupted
triple helices (FACITs)

• Short collagens with interruptions
• Linked to collagen II and carries a GAG chain
• Found at the surface of fibril-forming collagens
• Example Types IX, XII, XIV

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Classification of collagen
4. Anchoring collagens

• Provides functional integrity by connecting
  epithelium to stroma
• Example Type VII

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Classification of collagen
5. Beaded-filament-forming collagens

• Form structural links with cells
• Example Type VI
• Collagen VI crosslink into tetramers that
  assemble into long molecular chains
  (microfibrils) and have beaded repeat of 105 nm

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Synthesis of collagen

• Individual collagen polypeptide chains are
  synthesized on membrane-bound ribosomes and
  injected into the lumen of the endoplasmic
  reticulum (ER) as larger precursors, called pro-?
  chains
• In the lumen of the ER, selected prolines and
  lysines are hydroxylated to form hydroxyproline
  and hydroxylysine, respectively, and some of the
  hydroxylysines are glycosylated
• Each pro-a chain then combines with two others to
  form a hydrogen-bonded, triple-stranded, helical
  molecule known as procollagen
• (Continued)

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Synthesis of collagen

• During or following exocytosis, extracellular
  enzymes, the procollagen peptidases, remove the
  N-terminal and C-terminal propeptides
• The resulting protein, often called tropocollagen
  (or simply collagen), consists almost entirely of
  a triple-stranded helix.
• Excision of both propeptides allows the collagen
  molecules to polymerize into normal fibrils in
  the extracellular space

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Collagen-related diseases

• Collagen is highly cross-linked in tissues where
  tensile strength is required such as Achilles
  tendon
• If cross-linking is inhibited, the tensile
  strength of fibers is greatly reduced,
  collagenous tissues become fragile, and
  structures tend to tear (skin, tendon, and blood
  vessels)

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Diseases associated with collagen

• Diseases caused by mutations
• Subtypes of osteogenesis imperfecta (collagen I)
  
• Ehlers-Danlos syndrome (collagen I and V)
• Alport syndrome (collagen IV)
• Certain arterial aneurysms (collagen III)
• Ullrich muscular dystrophy (collagen VI)
• Certain chondrodysplasias (collagen IX and XI)
• Kniest dysplasia (collagen II)

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Scurvy

• The formation of hydroxyproline requires vitamin
  C
• Deficiency of vitamin C results in insufficient
  hydroxylation of proto-collagen and, hence, poor
  synthesis of collagen, formation of unstable
  triple helices preventing formation of normal
  fibrils
• Non-hydroxylated procollagen chains are then
  degraded within the cell
• This results in weakening of the collagen
  resulting in skin and gum lesions and weak blood
  vessels

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Types of OI

• At least four types of osteogenesis imperfecta
• Designated as type I through type IV
• Type I osteogenesis imperfecta is the mildest
  form of the condition
• Type II is the most severe results in death in
  utero or shortly after birth
• Milder forms generate a severe crippling disease

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Mutations of OI

• Mutations in the COL1A1 and COL1A2 genes cause OI
• These mutations typically interfere with the
  assembly of type I collagen molecules
• A defect in the structure of type I collagen
  weakens connective tissues, particularly bone,
  resulting in the characteristic features of OI
• OI types I, II, and IV have an autosomal dominant
  pattern of inheritance, which means one copy of
  the altered gene in each cell is sufficient to
  cause the condition

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Chondrodysplasias

• Mutations affecting type II collagen cause
  chondrodysplasias, characterized by abnormal
  cartilage, which leads to bone and joint
  deformities

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Ehlers-Danlos syndrome

• A heterogenous group of disorders that affect
  connective tissues, which are tissues that
  support the skin, bones, blood vessels, and other
  organs
• The signs and symptoms of Ehlers-Danlos syndrome
  vary from mildly loose joints to life-threatening
  complications

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Mutations in Ehlers-Danlos syndrome

• Ehlers-Danlos syndrome results from defects in
  synthesis of either collagen molecules type I,
  III, or V or in the synthesis of collagen
  processing enzymes like procollagen N-peptidase,
  or lysyl hydroxylase resulting in mobile joints
  and skin abnormalities

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Non-collagen component of Bone Matrix
Made up of stiffening substances to resist
bending and compression (Inorganic matter ). The
bone mineral is an analogue of crystals of
calcium phosphate hydroxyapatite
Ca10(PO4)6(OH)2, a substance that can only be
seen under electron microscopy. It is this
association of hydroxyapatite with collagen ?bres
which is responsible for the hardness of bone.
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Elastin

• The main component of elastic fibers is elastin
• A highly hydrophobic protein, which, like
  collagen, is unusually rich in proline and
  glycine
• But, unlike collagen, is not glycosylated
• Contains some hydroxyproline but no hydroxylysine

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Formation of elastic network

• Soluble tropoelastin (the biosynthetic precursor
  of elastin) is secreted into the extracellular
  space and assembled into elastic fibers close to
  the plasma membrane
• After secretion, the tropoelastin molecules
  become highly cross-linked to one another,
  generating an extensive network of elastin fibers
  and sheets
• The cross-links are formed between lysines by a
  mechanism similar to that of collagen molecules

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Elastin structure

• The elastin protein is composed largely of two
  types of short segments that alternate along the
  polypeptide chain
• hydrophobic segments, which are responsible for
  the elastic properties of the molecule and
• alanine- and lysine-rich a-helical segments,
  which form cross-links between adjacent molecules

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Function of elastic fiber

• Elastin is the dominant extracellular matrix
  protein in arteries
• Mutations in the elastin gene causing a
  deficiency of the protein result in narrowing of
  the aorta or other arteries as a result of
  excessive proliferation of smooth muscle cells in
  the arterial wall
• Apparently, the normal elasticity of an artery is
  required to restrain the proliferation of these
  cells

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Diseases of Elastic Fiber

• Cutis laxa
• Williams syndrome
• Buschke-Ollendorff syndrome 
• Menkes disease 
• Pseudoxanthoma elasticum,
• Marfan's syndrome 
• defects in copper metabolism (lysyl oxidase)

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Glycoproteins and Proteoglycans
Proteins conjugated to saccharides lacking a
serial repeat unit
Glycoproteins
Protein gtgt carbohydrate
Proteins conjugated to polysaccharides with
serial repeat units
Proteoglycans
Carbohydrate gtgt protein
Glycosaminoglycans Mucopolysaccharides
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Glycoproteins

• Proteins that contain oligosaccharide chains
  (glycans) covalently attached to polypeptide side-
  chains, in a co-translational or posttranslational
  modification.
• (N- Glycosylation), the addition of sugar chains
  can happen at the amide nitrogen on the side
  chain of the asparagine.
• (O- Glycosylation), the addition of sugar chains
  can happen on the hydroxyl oxygen on the side
  chain of hydroxy-lysine, hydroxy-proline, serine,
  or threonine.

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Functions of Glycoproteins

• Structural
• Reproduction
• Hormones
• Enzymes
• Carriers
• Inhibitors
• Immunological

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