• Extra Cellular Matrix (ECM)

• Structure, Synthesis, Function, Disease

The Extra Cellular Matrix: ECM• Extra Cellular: outside the cell

Matrix: 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.

Functions of ECM1-Role in establishing and maintaining cell shape, migration, mechanical support2-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

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

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

Structural features• Long, stiff, triple-stranded

helical structure,• Three collagen polypeptide

chains, called α chains, are wound around one another in a ropelike superhelix

• A basic unit of mature collagen is called tropocollagen

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

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

Functions of amino acids

• Proline stabilizes the helical conformation in each chain

• Glycine allows the three helical chains to pack tightly together to form the final collagen superhelix

Hydroxylysine

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

Lysine

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

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

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.

Classification of collagen

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

1. Fibril-forming collagens

Classification of collagen

Forms network in basement (Collagen IV) and Descemet’s membrane (Collagen VIII) Molecular filtration Example : Types IV, VIII, X

2. Network-forming collagens

Classification of collagen

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

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

Classification of collagen

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

4. Anchoring collagens

Classification of collagen

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

5. Beaded-filament-forming collagens

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)

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

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)

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)

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

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

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

Chondrodysplasias

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

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

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

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 seenunder electron microscopy.

It is this association of hydroxyapatite with collagen fibres which is responsible for the hardness of bone.

Non-collagen component of Bone Matrix

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

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

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

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

Diseases of Elastic Fiber

• Cutis laxa• Williams syndrome• Buschke-Ollendorff syndrome • Menkes disease • Pseudoxanthoma elasticum, • Marfan's syndrome 

– defects in copper metabolism (lysyl oxidase)

Glycoproteins and Proteoglycans

GlycoproteinsProteins conjugated to saccharides lacking a

serial repeat unit

ProteoglycansProteins conjugated to polysaccharides with

serial repeat units

Protein >> carbohydrate

Carbohydrate >> protein

GlycosaminoglycansMucopolysaccharides

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.

Functions of Glycoproteins• Structural• Reproduction• Hormones• Enzymes• Carriers• Inhibitors• Immunological

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