Extracellular Matrix lecture

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Extracellular Matrix

Antonia Jameson Jordan, D.V.M., Ph.D. November 21, 2008

Outline: •  Overview of structural and

signaling roles of ECM •  Two main classes of ECM

molecules –  Glycosaminoglycan

polysaccharide chains •  Structure and function

–  Fibrous proteins •  Structure and function

•  Basal lamina (basement membrane)

•  Integrins

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Downloaded from: Robbins & Cotran Pathologic Basis of Disease (on 14 October 2008 08:50 PM) © 2007 Elsevier

ECM has structural and signaling roles: Organized in two main ways:

connective tissue

basal lamina (basement membranes)

Figure 19-41 Molecular Biology of the Cell (© Garland Science 2008)

ECM consists of two main classes of molecules:

•  Glycosaminoglycan polysaccharide chains –  Form hydrated gels

•  Fibrous proteins –  Organize and strengthen

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Glycosaminoglycans (GAGs) are unbranched polysaccharide chains:

•  Composed of repeating disaccharide units •  Too stiff to fold into compact globular shape •  Highly negatively charged due to sulfate or carboxyl groups on the

sugars •  Highly hydrophilic

–  High density of negative charge attracts osmotically active cations


GAG chains occupy large amounts of space and form hydrated gels:

•  Turgor –  Withstand

compressive forces •  Allow rapid diffusion

of nutrients, metabolites, hormones between blood and tissues

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Hyaluronan is a major polysaccharide component of the ECM:

•  Present in all tissues and body fluids •  Simplest GAG

–  Regular repeating sequence of up to 25,000 disaccharide units

–  No sulfated sugars –  Not linked to a core protein

Functions of hyaluronan: •  Resists compressive forces in joints and tissues

–  Important constituent of joint fluid •  In osteoarthritis, decreased concentration and decreased

molecular weight of intra-articular HA

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Role of HA in morphogenesis:

•  Acts as space filler and facilitator of cell migration –  If synthesized on the basal side of an

epithelium, creates a cell-free space into which other cells can migrate

Copyright restrictions may apply. Itano, N. J Biochem 2008 144:131-137; doi:10.1093/jb/mvn046

The role of HA in atrioventricular canal morphogenesis:

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FDA approved HA for cosmetic use in humans – 2003:


Proteoglycans are composed of GAG chains covalently linked to a core protein:

•  Classified by sugar composition –  Keratan sulfate, chondroitin sulfate, dermatan sulfate,

heparan sulfate •  Modification of sugar residues allows for enormous

diversity •  Associate with each other and with other ECM

components to make complex meshworks

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Examples of proteoglycans found in ECM:

•  Decorin “decorates” collagen fibrils –  Decorin knock-out mouse has irregular collagen fibril

formation •  skin – lax and fragile •  tendons have abnormal structure

•  Aggrecan has serine-rich core protein and chondroitin sulfate and keratan sulfate chains

Figure 19-60a Molecular Biology of the Cell (© Garland Science 2008)

Aggrecan is the major glycoprotein of articular cartilage:

•  Provides hydrated gel structure that endows cartilage with load-bearing properties

•  Chondroskeletal morphogenesis during development

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Skin turgor test to assess hydration status:

Blowey & Weaver, Diseases and Disorders of Cattle (Mosby, 1997)

Signaling roles of proteoglycans:

•  Proteoglycans can regulate the activities of secreted proteins –  Gels formed by GAG chains act as “sieves” that

regulate passage of molecules by size and charge –  Bind secreted signaling molecules

•  Control diffusion, range of action, lifetime, modify signaling activity

–  e.g.,heparan sulfate immobilizes chemokines on the endothelial surface of a blood vessel at a site of inflammation

•  Cell-surface proteoglycans act as co-receptors

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Table 19-6 Molecular Biology of the Cell (© Garland Science 2008)

Collagens are the major proteins of animal connective tissues:


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Collagen chains undergo a series of post-translational modifications:

•  Hydroxylation of selected prolines and lysines •  Self-assembly of three pro-α chains

Figures 19-64 and 19-65 Molecular Biology of the Cell (© Garland Science 2008)

Figure 9-30 Pathologic Basis of Disease (© Elsevier 1995)

Vitamin C is necessary for proline hydroxlation: •  Defective pro-α chains fail to form triple helix •  Failure of collagen synthesis •  Guinea pigs need vitamin C!

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Intracellular and extracellular events of collagen fibril formation:

Different classes of collagen are organized in different ways:

•  Secreted fibril-associated collagens help organize the fibrils •  Fibrillar (fibril-forming)

–  Types I, II, III, V –  Long, rope-like structures –  principal collagen of bone and skin

•  Fibril-associated –  Type IX, Type XII –  link fibrils to one another and to other components of ECM

•  Network-forming –  Type IV –  major component of basal lamina

•  Anchoring –  Type VI

•  Resistance to tensile forces

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Collagen type IV forms a fine meshwork:

•  More flexible structure than fibrillar collagens

•  Interruptions of triple-helical structure

•  Not cleaved after secretion

•  Interact via uncleaved terminal domains to assemble into a flexible network

Downloaded from: Robbins & Cotran Pathologic Basis of Disease (on 17 November 2008 03:08 PM)


Type 1 collagen diseases (Osteogenesis imperfecta):

Downloaded from: Resident & Staff Physician (on 10 November 2008 05:31 PM)

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Type 1 collagen diseases (Osteogenesis imperfecta):

Seeliger et al: Osteogenesis imperfecta in two litters of Dachsunds. Vet Pathol 40: 530-539, 2003

Clinical signs and diagnosis of osteogenesis imperfecta in three dogs (Ron Minor et al,1997)

Ehlers-Danlos syndrome: •  Defect in synthesis or structure of fibrillar collagen (mutations have

been found in collagen types I, III, V) –  Skin hyperextensibility, joint laxity, fragile skin and vessels, poor

wound healing

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Collagen VII defects cause blistering skin diseases:


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Importance of collagen in wound healing:

Elastin gives tissues their elasticity:


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Fibronectin is an extracellular protein that helps cells attach to the matrix:

Basal laminae underly all epithelia and surround some nonepithelial cell types:

•  Critical role in determining the architecture of the body

•  Thin: 4-120 nanometers thick •  Synthesized by cells on each side of it


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Molecular structure of the basal lamina: •  Glycosaminoglycans

–  perlecan •  Fibrous proteins

–  Laminin, type IV collagen, nidogen


Laminin is a primary component of the basal lamina:

•  Primary organizer of the sheet structure •  Composed of 3 chains held together by disulfide bonds •  Self-assembles through interactions of the head groups

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Basal laminae have diverse functions:

•  Structural –  Critical role in the architecture of an organ –  Mechanical connection between epithelia and

underlying connective tissue •  Scaffold for tissue regeneration

Basal laminae have diverse functions: •  Selective filtration

–  Glomerulus •  Selective barrier to cell movement •  Spatial organization of the components of the

neuromuscular junction


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Interactions between cells and the ECM:

•  Cells synthesize, organize, and degrade ECM •  Matrix influences cellular behavior

–  Tissue architecture

•  Cells interact with ECM via matrix receptors –  Integrins –  Transmembrane proteoglycans


Integrins are transmembrane heterodimers that link to the cytoskeleton:

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Change in conformation of an integrin molecule when it binds a ligand:

Attachment to the ECM via integrins affects cell proliferation and survival:

•  Anchorage dependence for cellular survival and growth –  Way to ensure that cell survives and proliferates only

in the appropriate environment •  Cell spreading on matrix promotes survival and growth

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Activation of integrins by cross-talk from other signaling pathways:


Integrin defects are responsible for many different genetic diseases:

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Integrins involved in pathogenesis of other diseases:

•  Cancer – Tumor progression

•  Role in infectious diseases – Can provide a means for viral entry

•  Foot and mouth disease

•  Autoimmune diseases – Recruitment of leukocytes

•  Multiple sclerosis, Crohn’s disease


Interaction of cells with ECM via integrins leads to a variety of critical behaviors:

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Conclusion:

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Integrins recruit intracellular signaling proteins at sites of cell-substratum adhesion:

•  Focal adhesion kinase (FAK) – Cytoplasmic tyrosine kinase

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Hyaluronan acts as a space filler and facilitator of cell migration:

•  Simplest GAG –  Regular repeating sequence of up to 25,000

disaccharide units –  No sulfated sugars –  Not linked to a core protein

•  Present in all tissues and body fluids •  Resists compressive forces in joints and tissues

–  Important constituent of joint fluid •  Roles in morphogenesis

–  If synthesized on the basal side of an epithelium, creates a cell-free space into which other cells can migrate

Extracellular Matrix lecture

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Transcript of Extracellular Matrix lecture