Extra Cellular Matrix (ECM)

In The Name Of God

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“Half of the secrets of the cell are outside the

cell.” Dr. Mina BissellOct. 17, 2007

Erlanger Auditorium

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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The Extra Cellular Matrix: ECM

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PancreasA Compartmentalized Tissue

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Act as structural support to maintain cell organization and integrity (epithelial tubes; mucosal lining of gut; skeletal muscle fiber integrity)

Compartmentalize tissues (pancreas: islets vs. exocrine component; skin: epidermis vs. dermis)

Provide hardness to bone and teeth (collagen fibrils become mineralized)

Present information to adjacent cells:◦ Inherent signals (e.g., RGD motif in fibronectin)◦ Bound signals (BMP7, TGFβ, FGF, SHH)

Serve as a highway for cell migration during development (neural crest migration), in normal tissue maintenance (intestinal mucosa), and in injury or disease (wound healing; cancer)

Why do all multicellular animals have ECM?

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

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Types of ECMs

• Basement membrane (basal lamina)– Epithelia, endothelia, muscle, fat,

nerves

• Elastic fibers– Skin, lung, large blood vessels

• Stromal or interstitial matrix• Bone, tooth, and cartilage• Tendon and ligament

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Integrins Dystroglycan Syndecans Muscle-Specific kinase (MuSK) Others

Cells Need Receptors to Recognize and Respond to ECM

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Collagens Proteoglycans

◦ Perlecan, aggrecan, agrin, collagen XVIII Hyaluronan (no protein core) Large Glycoproteins

◦ Laminins, nidogens, fibronectin, vitronectin Fibrillins, elastin, LTBPs, MAGPs, fibulins “Matricellular” Proteins

◦ SPARC, Thrombospondins, Osteopontin, tenascins

Types of ECM Components

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Most ECM proteins are large, modular, multidomain glycosylated or glycanated proteins

Some domains recur in different ECM proteins

◦ Fibronectin type III repeats◦ Immunoglobulin repeats◦ EGF-like repeats

Generalizations

Perlecan

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Specialized layers of extracellular matrix surrounding or adjacent to all epithelia, endothelia, peripheral nerves, muscle cells, and fat cells

Originally defined by electron microscopy as ribbon-like extracellular structures beneath epithelial cells

Basement Membranes

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Fredrik Skarstedt and Carrie Phillips

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Deep-Etch Electron Microscopy

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In general, basement membranes appear very similar to each other by EM.

But all are not alike! There is a wealth of molecular and

functional heterogeneity among basement membranes, due primarily to isoform variations of basement membrane components.

Basement Membranes

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Kidney Basement Membranes

Laminin β1 Laminin β2

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Collagen IV 6 chains form α chain heterotrimers

Laminin 12 chains form several α-β-γ heterotrimers

Entactin/Nidogen 2 isoforms Sulfated proteoglycans Perlecan and Agrin

are the major ones; Collagen XVIII is another

Primary Components of All Basement Membranes

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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

The collagens

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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)

Collagen-related diseases

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Collagen I◦ Osteogenesis imperfecta◦ Ehlers-Danlos syndrome type VII

Collagen II◦ Multiple diseases of cartilage

Collagen III◦ Ehlers-Danlos syndrome type IV

Collagen IV◦ Alport syndrome, stroke, hemorrhage, porencephaly

Collagen VII◦ Dystrophic epidermolysis bullosa (skin blistering)

Some Genetic Diseases of Collagen

23Vanacore et al., Science 2009

Sulfilimine: The Bond that Crosslinks Type IV Collagen NC1 Domains

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COL4A1 mutations◦ Small vessel disease/retinal

vascular tortuosity◦ Hemorrhagic stroke◦ Porencephaly◦ HANAC syndrome

COL4A3/A4/A5 mutations◦ Alport syndrome/hereditary

glomerulonephritis

Type IV Collagen Mutations and Human Disease

Kidney Glomerular BM

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If crosslinking is inhibited (Lysyl hydroxylase mutations; vitamin C deficiency), collagenous tissues become fragile, and structures such as skin, tendons, and blood vessels tend to tear. There are also many bone manifestations of under-crosslinked collagen.

Collagen Crosslinking

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Liver spots on skin, spongy gums, bleeding from mucous membranes, depression, immobility

Vitamin C deficiency Ascorbate is required for

prolyl hydroxylase and lysyl hydroxylase activities

Acquired disease of fibrillar collagen

Scurvy

Illustration from Man-of-War by Stephen Biesty (Dorling-Kindersley, NY, 1993)

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At least four types of osteogenesis imperfecta

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

Types of OI

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Clinical: Ranges in severity from mild to perinatal lethal

bone fragility, short stature, bone deformities, teeth abnormalities, gray-blue sclerae, hearing loss

Biochemical: reduced and/or abnormal type I collagen

Molecular: mutations in either type I collagen gene, COL1A1 or COL1A2, resulting in haploinsufficiency or disruption of the triple helical domain (dominant negative: glycine substitutions most common)

Osteogenesis Imperfecta(brittle bone disease)

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• 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

Mutations of OI

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Heterotrimers are composed of one α, one β, and one γ chain.

400 to 800 kDa cruciform, Y, or rod-shaped macromolecules.

Major glycoprotein of basement membranes—it’s required!

Chains are evolutionarily related. 5 alpha, 4 beta, and 3 gamma chains

are known. They assemble with each other non-randomly.

15 heterotrimers described to date.

Laminin

LM-521

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Laminin chains assemble into trimers in the ER and are secreted as trimers into the extracellular space.

Full-sized laminin trimers can self-polymerize into a macromolecular network through short arm-short arm interactions.

The α chain LG domain is left free for interactions with cellular receptors.

Laminin Trimers Polymerize

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Receptor-mediated Assembly

Involves LG domains and receptors on the surface of cells.Results in laminin polymerization and signal transduction.

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Laminin Mutations in Mice (M) and Humans (H) Have Consequences

Lama1, Lamb1, Lamc1: Peri-implantation lethality (M)

Lama2: Congenital muscular dystrophy (M, H)

Lama3, Lamb3, Lamc2: Junctional epidermolysis bullosa (skin blistering) (M, H)

Lama4: Mild bleeding disorder, moto-nerve terminal defects (M); cardiac and endothelial defects (H)

Lama5: Neural tube closure, placenta, digit septation, lung, kidney, tooth, salivary gland defects (M)

Lamb2: Neuromuscular junction and kidney filtration defects (M); Iris muscle, neuromuscular, kidney filtration defects (H; Pierson syndrome)

Lamc3: Brain malformations, autism spectrum disorder? (H)

35Copyright ©2004 American Physiological Society

Ramirez, F. et al. Physiol. Genomics 19: 151-154 2004;

doi:10.1152/physiolgenomics.00092.2004

Major steps underlying the assembly of microfibrils and

elastic fibers

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Large glycoproteins (~350 kDa) whose primary structures are dominated by cbEGF domains that, in the presence of Ca2+, adopt a rodlike structure

Fibrillins

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Caused by dominant Fibrillin-1 (FBN1) mutations◦ Haploinsufficiency is the

culprit Skeletal, ocular, and

cardiovascular defects Deficiency of elastin-

associated microfibrils Syndrome may result

from alterations in TGFβ signaling, rather than purely structural changes in microfibrils

Marfan Syndrome

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Damage to the lung air sacs (alveoli) that affects breathing

Macrophages induced to “ingest” particles in smoke also secrete proteases that degrade elastic fibers

Loss of lung elasticity makes exhalation difficult

Increased alveolar size reduces the surface area for gas exchange

Emphysema

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Thanks for your attention