extracellular matrix lecture
DESCRIPTION
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.TRANSCRIPT
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
2
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
3
Figure 19-55 Molecular Biology of the Cell (© Garland Science 2008)
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 19-56 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
4
Figure 19-57 Molecular Biology of the Cell (© Garland Science 2008)
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
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
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:
6
FDA approved HA for cosmetic use in humans – 2003:
Figure 19-58 Molecular Biology of the Cell (© Garland Science 2008)
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
7
Figure 19-59 Molecular Biology of the Cell (© Garland Science 2008)
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
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
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)
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)
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!
11
Figure 19-66 Molecular Biology of the Cell (© Garland Science 2008)
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
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)
Downloaded from: Robbins & Cotran Pathologic Basis of Disease (on 10 November 2008 05:31 PM)
Type 1 collagen diseases (Osteogenesis imperfecta):
Downloaded from: Resident & Staff Physician (on 10 November 2008 05:31 PM)
13
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
14
Collagen VII defects cause blistering skin diseases:
Table 19-7 Molecular Biology of the Cell (© Garland Science 2008)
15
Downloaded from: Robbins & Cotran Pathologic Basis of Disease (on 14 October 2008 08:53 PM) © 2007 Elsevier
Importance of collagen in wound healing:
Elastin gives tissues their elasticity:
Figures 19-70 and 19-71 Molecular Biology of the Cell (© Garland Science 2008)
16
Figure 19-72 Molecular Biology of the Cell (© Garland Science 2008)
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
Figures 19-39 and 19-40 Molecular Biology of the Cell (© Garland Science 2008)
17
Figure 19-43 Molecular Biology of the Cell (© Garland Science 2008)
Molecular structure of the basal lamina: • Glycosaminoglycans
– perlecan • Fibrous proteins
– Laminin, type IV collagen, nidogen
Figure 19-42a 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
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)
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
Figure 19-45 Molecular Biology of the Cell (© Garland Science 2008)
Integrins are transmembrane heterodimers that link to the cytoskeleton:
20
Figure 19-48 Molecular Biology of the Cell (© Garland Science 2008)
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
21
Figure 19-49 Molecular Biology of the Cell (© Garland Science 2008)
Activation of integrins by cross-talk from other signaling pathways:
Table 19-4 Molecular Biology of the Cell (© Garland Science 2008)
Integrin defects are responsible for many different genetic diseases:
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
Downloaded from: Robbins & Cotran Pathologic Basis of Disease (on 14 October 2008 08:53 PM) © 2007 Elsevier
Interaction of cells with ECM via integrins leads to a variety of critical behaviors:
23
Conclusion:
24
Integrins recruit intracellular signaling proteins at sites of cell-substratum adhesion:
• Focal adhesion kinase (FAK) – Cytoplasmic tyrosine kinase
25
Figure 19-47 Molecular Biology of the Cell (© Garland Science 2008)
26
Figure 19-48a Molecular Biology of the Cell (© Garland Science 2008)
27
Figure 19-46 Molecular Biology of the Cell (© Garland Science 2008)
28
Figure 19-57 Molecular Biology of the Cell (© Garland Science 2008)
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