Last Class: Cytoskeleton 1. Cytoskeleton components: –Microtubules –Actin Filaments...
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Transcript of Last Class: Cytoskeleton 1. Cytoskeleton components: –Microtubules –Actin Filaments...
•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 epitheliumFibroblast, primary cell secreting ECM
Fibroblasts in Connective TissueWith 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 GAGSulfate, carboxyl groups, negative charges
GAGs
• Hyaluronan• Chondroitin sulfate and dermatan sulfate• Heparan sulfate• Keratan sulfate
The relative dimensions and volumes of different
moleculesVery stiff and hydrophilic,
can’t be packed
Negative charge->cation, Na+ ->water molecules->swelling
pressure->compression resistent
The repeating sequence of hyoluronanSingle chain of 25,000 sugars; No sulfate
The linkage of GAG to protein, the assembly of proteoglycanStart from a serine, 4 saccharide and GAG
Except hyalurona
Diversity of ProteoglycansMembrane-bound ribosome-> endoplasmic reticulum ->golgi apparatus
Aggrecan aggregatesNon-covalently bound to hyaluronan chainsJust like decorin decorates collagen fibrils
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
Fibrous Proteins:
Collagen, elastin, fibronectin, laminin
Collagen MoleculesGlycine, proline, hydroxyproline
Triple helix (three fibers interwined together)
Embryonic chick skinFibroblasts surrounded by collagen fibrils (high order polymers)
Degraded fragment of XVII, endostatin, inhibits angiogenesis and hence tumor growth
Crosslinking of collagen fibrilsDeaminated 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 fibrilsTwo 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 moleculesElastin is highly hydrophobic
Mainly two features: hydrophobic and cross-linked segmentsHydrophobic 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 bundlesRed: fibronectin fibrils
Green: actin filament bundlesNot only important for adhesion, but also for migration
Laminin(consisting of three polypeptides , , )
3 kinds of Basal LaminaeA 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 embryoUsually 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 DegradationMatrix metalloproteinase, serine proteaseProteases 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
IntegrinsTransmembrane heterodimers
20 nm above cell surface, dependent on Ca2+ and Mg2+
Integrins couple ECM to cytoskeleton through cytosolic
proteins, talin, -atinin, paxillin, filamin, vinculin
Inside-out Signaling for Integrins
Outside-in Signaling of integrinsFAK, a key molecule for integrin functions
FAK can bind to Talin which associates with integrin subunit, to paxillin which associates with integrin subunit
Matrix-dependent cell survival in the formation of proamniotic cavityDuring 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