I give this lecture in the Cell Biology and Genetics course for first-year veterinary students. The core material comes from Alberts' Molecular Biology of the Cell, Fifth Edition. I have added multiple clinical examples, both human and veterinary.

1. 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
1

2. ECM has structural and signaling roles:
Organized in two main ways:
connective tissue
basal lamina (basement membranes)
Downloaded from: Robbins & Cotran Pathologic Basis of Disease (on 14 October 2008 08:50 PM)
© 2007 Elsevier
ECM consists of two main classes of molecules:
• Glycosaminoglycan polysaccharide chains
– Form hydrated gels
• Fibrous proteins
– Organize and strengthen
Figure 19-41 Molecular Biology of the Cell (© Garland Science 2008)
2

3. 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
Figure 19-55 Molecular Biology of the Cell (© Garland Science 2008)
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
Figure 19-56 Molecular Biology of the Cell (© Garland Science 2008)
3

4. 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
Figure 19-57 Molecular Biology of the Cell (© Garland Science 2008)
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
4

5. 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
The role of HA in atrioventricular canal
morphogenesis:
Itano, N. J Biochem 2008 144:131-137; doi:10.1093/jb/mvn046
Copyright restrictions may apply.
5

6. 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
Figure 19-58 Molecular Biology of the Cell (© Garland Science 2008)
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7. 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 19-59 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
Figure 19-60a Molecular Biology of the Cell (© Garland Science 2008)
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8. 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
8

9. Table 19-6 Molecular Biology of the Cell (© Garland Science 2008)
Collagens are the major proteins of
animal connective tissues:
Figure 19-63 Molecular Biology of the Cell (© Garland Science 2008)
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10. 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)
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!
Figure 9-30 Pathologic Basis of Disease (© Elsevier 1995)
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11. Intracellular and extracellular events of
collagen fibril formation:
Figure 19-66 Molecular Biology of the Cell (© Garland Science 2008)
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
11

12. 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)
Downloaded from: Robbins & Cotran Pathologic Basis of Disease (on 10
November 2008 05:31 PM)
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13. Type 1 collagen diseases
(Osteogenesis imperfecta):
Clinical signs and diagnosis of osteogenesis
imperfecta in three dogs (Ron Minor et al,1997)
Seeliger et al: Osteogenesis imperfecta in two litters of Dachsunds. Vet Pathol 40: 530-539, 2003
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
13

14. Collagen VII defects cause blistering skin diseases:
Table 19-7 Molecular Biology of the Cell (© Garland Science 2008)
14

15. Importance of collagen in wound healing:
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© 2007 Elsevier
Elastin gives tissues their elasticity:
Figures 19-70 and 19-71 Molecular Biology of the Cell (© Garland Science 2008)
15

16. Fibronectin is an extracellular protein that
helps cells attach to the matrix:
Figure 19-72 Molecular Biology of the Cell (© Garland Science 2008)
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
Figures 19-39 and 19-40 Molecular Biology of the Cell (© Garland Science 2008)
16

17. Molecular structure of the basal lamina:
• Glycosaminoglycans
– perlecan
• Fibrous proteins
– Laminin, type IV collagen, nidogen
Figure 19-43 Molecular Biology of the Cell (© Garland Science 2008)
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
Figure 19-42a Molecular Biology of the Cell (© Garland Science 2008)
17

18. 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
Downloaded from: Robbins & Cotran Pathologic Basis of Disease (on 17 November 2008 03:08 PM)
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19. 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:
Figure 19-45 Molecular Biology of the Cell (© Garland Science 2008)
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20. Change in conformation of an integrin
molecule when it binds a ligand:
Figure 19-48 Molecular Biology of the Cell (© Garland Science 2008)
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
20

21. Activation of integrins by cross-talk from
other signaling pathways:
Figure 19-49 Molecular Biology of the Cell (© Garland Science 2008)
Integrin defects are responsible for
many different genetic diseases:
Table 19-4 Molecular Biology of the Cell (© Garland Science 2008)
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22. 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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© 2007 Elsevier
22

23. Conclusion:
23

24. Integrins recruit intracellular signaling
proteins at sites of cell-substratum adhesion:
• Focal adhesion kinase (FAK)
– Cytoplasmic tyrosine kinase
24

25. Figure 19-47 Molecular Biology of the Cell (© Garland Science 2008)
25

26. Figure 19-48a Molecular Biology of the Cell (© Garland Science 2008)
26

27. Figure 19-46 Molecular Biology of the Cell (© Garland Science 2008)
27

28. 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
Figure 19-57 Molecular Biology of the Cell (© Garland Science 2008)
28