•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
• Cell Adhesion on
Extracellular Matrix (ECM)
Cells surrounded by ECM
(macromolecules consisting of proteins and polysacchrides)
Embryonic chick limb bud
Connective Tissue underlying an epithelium
Fibroblast, primary cell secreting ECM
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)
ECM
• Glycosaminoglycans (GAGs) covalently linked to
protein, proteoglycans forming gels
• Fibrous proteins: collagen, elastin, fibronectin,
Laminin.
The repeating disaccharide sequence of a GAG
Sulfate, carboxyl groups, negative charges
GAGs
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Hyaluronan
Chondroitin sulfate and dermatan sulfate
Heparan sulfate
Keratan sulfate
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
The repeating sequence of hyoluronan
Single chain of 25,000 sugars; No sulfate
The linkage of GAG to protein, the assembly of proteoglycan
Start from a serine, 4 saccharide and GAG
Except hyalurona
Diversity of Proteoglycans
Membrane-bound ribosome-> endoplasmic reticulum ->golgi apparatus
Aggrecan aggregates
Non-covalently bound to hyaluronan chains
Just like decorin decorates collagen fibrils
Proteoglycan functions
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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
Fibrous Proteins:
Collagen, elastin, fibronectin, laminin
Collagen Molecules
Glycine, proline, hydroxyproline
Triple helix (three fibers
interwined together)
Embryonic chick skin
Fibroblasts surrounded by collagen fibrils (high order polymers)
Degraded fragment of XVII, endostatin, inhibits angiogenesis and
hence tumor growth
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
The Life of Collagen fibril
Collagen fibrils form collagen fibers
Fibril-associated collagen helps the
organization of fibrils to resist tensile force.
Collagen fibrils in the tadpole skin. Tendon
and bone are different
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
Elastic Fibers consisting of Elastin (for tissue elasticity)
Vessel Walls: EM image of dog aorta
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
Fibronectin (crucial for angiogenesis)
A glycoprotein dimer connected by disulfite bonds at one end
Can exist in soluble or fibrillar forms
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
Laminin
(consisting of three polypeptides a, b, g)
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
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)
A molecular structure Model of basal Laminae
Function of Basal Laminae in neuromuscular junctions (besides the
supporting or filtering functions)
Muscle cells: laminin; neuronal cells: agrin (heparan sulfate
proteoglycan)
Cells communicate through
ECM to affect Cell shape
ECM affects Cell survival and proliferation
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
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
Inside-out Signaling for Integrins
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
Matrix-dependent cell survival in the formation of proamniotic cavity
During embryonic development
•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