1. Last Class: Cytoskeleton
1. Cytoskeleton components:
Microtubules
Actin Filaments
Intermediate Filaments
2. The regulation of microtubules and Actin
filaments
3. Molecular Motors
4. Cell behaviors related to cytoskeleton

2. Cell Adhesion on Extracellular Matrix (ECM)

3. Cells surrounded by ECM
(macromolecules consisting of proteins and polysacchrides)
Embryonic chick limb bud

4. Connective Tissue underlying an epithelium
Fibroblast, primary cell secreting ECM

5. Fibroblasts in Connective Tissue
With Collagen fibers
(SEM image of rat cornea, no elastic fiber existing, hyaluronan, proteoglycans and glycoproteins have been removed by enzymes and acids)

6. ECM
Glycosaminoglycans (GAGs) covalently linked to protein, proteoglycans forming gels
Fibrous proteins: collagen, elastin, fibronectin, Laminin.

7. The repeating disaccharide sequence of a GAG
Sulfate, carboxyl groups, negative charges

8. GAGs Hyaluronan Chondroitin sulfate and dermatan sulfate
Heparan sulfate
Keratan sulfate

9. The relative dimensions and volumes of different molecules
Very stiff and hydrophilic, can’t be packed
Negative charge->cation, Na+ ->water molecules->swelling pressure->compression resistent

10. The repeating sequence of hyoluronan
Single chain of 25,000 sugars; No sulfate

11. The linkage of GAG to protein, the assembly of proteoglycan
Start from a serine, 4 saccharide and GAG
Except hyalurona

12. Diversity of Proteoglycans
Membrane-bound ribosome-> endoplasmic reticulum ->golgi apparatus

13. Non-covalently bound to hyaluronan chains
Aggrecan aggregates
Non-covalently bound to hyaluronan chains
Just like decorin decorates collagen fibrils

14. Proteoglycan functions
Constrain the range of embedded protein actions
Affect embedded protein activities
Control embedded protein release rate
Prevent embedded protein degradation
Control the local concentration of embedded proteins
Function as co-receptors

16. Collagen, elastin, fibronectin, laminin
Fibrous Proteins:
Collagen, elastin, fibronectin, laminin

17. Glycine, proline, hydroxyproline
Collagen Molecules
Glycine, proline, hydroxyproline
Triple helix (three fibers interwined together)

18. Fibroblasts surrounded by collagen fibrils (high order polymers)
Embryonic chick skin
Fibroblasts surrounded by collagen fibrils (high order polymers)

19. Degraded fragment of XVII, endostatin, inhibits angiogenesis and hence tumor growth

20. Crosslinking of collagen fibrils
Deaminated by lysyl oxidase to yield aldehyde groups, spontaneous covelent bond between aldehyde groups (about every 67 nm)
If crosslink is broken, collagen is easy to tear

21. The Life of Collagen fibril

23. Collagen fibrils form collagen fibers

24. Collagen fibrils in the tadpole skin. Tendon and bone are different
Fibril-associated collagen helps the organization of fibrils to resist tensile force.
Collagen fibrils in the tadpole skin. Tendon and bone are different

25. Cells help the organization of collagen fibrils
Two embryonic chick hearts at the ends and collagen gel in the center
Hearts full of fibroblasts and heart muscle cells

26. Elastic Fibers consisting of Elastin (for tissue elasticity)
Vessel Walls: EM image of dog aorta

27. Stretching of a network of elastin molecules
Elastin is highly hydrophobic
Mainly two features: hydrophobic and cross-linked segments
Hydrophobic segments provide elasticity
Cross-linking provides stability

28. Fibronectin (crucial for angiogenesis)
A glycoprotein dimer connected by disulfite bonds at one end
Can exist in soluble or fibrillar forms

29. Red: fibronectin fibrils Green: actin filament bundles
Co-alignment of extracellular fibronectin fibrils and intracellular actin filament bundles
Red: fibronectin fibrils
Green: actin filament bundles
Not only important for adhesion, but also for migration

30. Laminin (consisting of three polypeptides a, b, g)

31. A thin, flexible mats underlying all epithelial cells and tubes
3 kinds of Basal Laminae
A thin, flexible mats underlying all epithelial cells and tubes
Not only for structural support and filtering, but also determine cellular functions

32. Basal Laminae in the cornea of a chick embryo
Usually synthesized by the cells seeded on it
Most laminar consists of type IV collagen, laminin and nidogen (enactin)

33. A molecular structure Model of basal Laminae

34. Function of Basal Laminae in neuromuscular junctions (besides the supporting or filtering functions)
Muscle cells: laminin; neuronal cells: agrin (heparan sulfate proteoglycan)

35. Cells communicate through ECM to affect Cell shape

36. ECM affects Cell survival and proliferation

37. Cancer Cells and ECM Degradation
Matrix metalloproteinase, serine protease
Proteases bound on cell surface receptor
Controlling of protease activity: 1. local activation; 2. surface receptor binding; 3. inhibitor secretion
tPA: tissue-type plasminogen activator; uPA: urokinase-type plasminogen activator

38. Transmembrane heterodimers
ECM Receptors
Integrins
Transmembrane heterodimers
20 nm above cell surface, dependent on Ca2+ and Mg2+
Integrins couple ECM to cytoskeleton through cytosolic proteins, talin, a-atinin, paxillin, filamin, vinculin

40. Inside-out Signaling for Integrins

41. Outside-in Signaling of integrins
FAK, a key molecule for integrin functions
FAK can bind to Talin which associates with integrin b subunit, to paxillin which associates with integrin a subunit

42. Matrix-dependent cell survival in the formation of proamniotic cavity
During embryonic development

44. Summary
1. ECM components: glycosaminoglycans (GAGs) and fibrous proteins
GAGs: Hyaluronan; Chondroitin sulfate and dermatan sulfate; Heparan sulfate; Keratan sulfate
Fibrous proteins: collagen, elastin, fibronectin, laminin
2. basal laminae
3. Cell-ECM interaction, ECM degration
4. ECM receptor, integrins