MSS06 Biochemistry Of Cartilage And Bone Flashcards
Tissue homeostasis
Context - Form - Function
- Extracellular matrix components
- collagen
- proteoglycan
- glycoprotein - Cell-matrix interaction / communication
- integrins
- fibronectin - Matrix degradation / remodelling
- matrix metalloproteinase (MMP)
Extracellular matrix (ECM)
- mixture of proteins and glycoproteins
—> produced by cells and surrounding the cells - composed of
1. Collagen (insoluble fibres)
2. Proteoglycan (soluble polymers)
3. Adhesive Glycoprotein - interact specifically in an architecturally-precise manner
- take stress off movement and maintain shape e.g. bone and cartilage
Function:
- **Modulates functions and properties of tissues
1. Cornea: transparent
2. Basal lamina: thin mat-like, tough
3. Tendon: rope-like
4. Skin/artery: strength and resilience
5. Cartilage: shock absorbing
6. Teeth/bone: hard
Type and organisation of the relative amounts of matrix components
—> dictates tissue function
1. Tendon
2. Cartilage
3. Dermis (fibres orientate in different direction, allow different direction of pull)
ECM-associated degenerative disease:
- Osteoporosis
- Osteoarthritis
- Intervertebral disc degeneration
***Bone matrix
Dense matrix:
- 70% inorganic, 30% organic (90% collagen)
Inorganic:
1. ***Calcium phosphate (as hydroxyapatite crystals)
Organic:
1. Mixture of tough collagen fibre
—> mainly ***Type I Collagen arranged in regular layers
—> for mineralisation and tensile strength
- Proteoglycans + GAGs (hyaluronan, decorin, biglycan, fibromodulin)
- Glycoproteins
- Osteopontin
- Bone sialoprotein
- Osteocalcin and matrix Gla protein
Type 1 collagen molecule
**α chain: C terminal propeptide + N terminal propeptide + Gly-X-Y repeats
—> 3 α chains (2xα1, 1xα2)
—> C terminal propeptide initiates **triple helix formation in rER
—> Procollagen secreted into extracellular space
—> Procollagen peptidase: Cleave Procollagen —> Collagen
Structural organisation of fibrillar collagen
***Procollagen
—> Collagen
—> Fibril
Homogenous fibrils:
—> only type 1
—> Electron dense / NOT dense area
—> ***regular banding
Heterotypic fibrils:
—> contain other types of collagen for specialised functions
***Collagen fibres are insoluble polymers
Glycoproteins in bone
Characteristics feature: ***Anionic nature —> rich in Acidic amino acids (Asp, Glu) —> ***bind to minerals —> involvement in the ***calcification (mineralization) process
- Osteopontin:
- -ve regulator
- stretches of consecutive Aspartic residues - Bone sialoprotein:
- +ve regulator
- stretches of consecutive Glutamic residues - Osteocalcin and matrix Gla protein:
- +ve regulator
- γ-carboxyglutamic acid
Prerequisites of mineralisation of bone matrix
- ***↑ Local ion concentration
- ***↓ Mineralisation inhibitors
- Formation of ***mineral nucleators
—> sit in holes in fibrils - Collagen I fibrils support ***hydroxyapatite deposition (nucleation at hole zones)
- Continued accumulation of hydroxyapatite
- ***Regulators of mineralisation
—> e.g. Alkaline phosphatase by osteoblasts
Structure of bone
- Lamellar (compact) bone
2. Trabecular (spongy / cancellous) bone (take stress off, shock absorber)
***Cartilage matrix components
- Cellular component:
- Chondrocyte - Fibrous component:
- ***Collagen type II - Proteoglycan (Soluble polymers):
- ***Aggrecan, decorin, biglycan, lumincan, perlecan
- syndecan, glypican (membrane bound) - Glycosaminoglycans (GAG) (Soluble polymers):
- ***Hyaluronic acid
- chondroitin, heparan, keratan, dermatan sulfate - Glycoprotein:
- ***Fibronectin
Glycosaminoglycan (GAG)
- family of **linear polymers comprised of repeating **disaccharides
- (N-acetylglucosamine / N-acetylgalatosamine) + (D-glucuronidation acid / L-iduronic acid)
- > =1 **OH groups of amine sugars esterified with **sulfate
-
**highly negatively charged
—> attract cation
—> **soak up water
Hyaluronic acid
- ***Unsulphated GAG (好長)
- Not attached to a protein core
- single molecule up to 50000 repeating units with MW up to 10^7
Function:
1. **Resist compressive forces in cartilage
2. Forms clear and viscous **solutions —> serve as **Lubricants in synovial fluid of joints
3. **Space filler
—> change shape of structure
—> provide cell free space
Proteoglycan
- Core protein with >= 1 GAG covalently attached (牙刷仔)
- GAG generally much shorter than hyaluronic acid
- GAG: greater fraction, site of biological function
-
**Highly -ve charged
—> high affinity for cation —> **hydrophilic —> swelling pressure
—> hydrogen bonding + electrostatic interactions
2 types:
-
**Lecticans (much more GAG)
- Sulphated Proteoglycan
- **Aggrecan, Brevican, Neurocan, Versican - Short leucine repeat proteoglycans (SLRPs) (less GAG)
Other types:
- Small proteoglycan
e.g. ***Decorin, Biglycan, Lumican, Perlecan, Syndecan, Glypican
—> function as organising ECM, signaling etc.
HA and aggrecan relationship
HA: long chain backbone
Aggrecan (牙刷仔): attach on HA
—> forms huge aggregate that soaks water
***General function of proteoglycan
- Provide ***hydrated space around cells
- Act as selective ***sieves regulating traffic of molecules and cells (size, charge: e.g. Kidneys)
- Binds secreted ***signaling molecules (e.g. growth factors)
- Binds and regulate ***proteases and protease inhibitors
Adhesive glycoproteins
- Large no. in ECM
- Key feature: Ability to ***interact with cells and other macromolecules in ECM
- Many have functional **peptide domains that interact with **cell surface receptors + other matrix molcules
- Some may also involve interaction with **inorganic phase e.g. Mineralized CT: bone and dentine
—> **bind to minerals
—> involvement in the ***calcification (mineralization) process - Example: Fibronectin
***Fibronectin
Functional form: ***Disulphide bonded dimer
- Promote cell ***adhesion
- Affect cell morphology, ***migration and differentiation
- Bind to **cell surface via **Integrin receptors and ***Arg-Gly-Asp motif (RGD)
- Binds ***ECM: collagen, heparin/heparan sulphate proteglycan, fibrin
- Forms fibrillar matrix: further stabilized by extensive S-S
Cartilage matrix component
Collagen and Proteoglycan:
- structural support
- shock absorption
Others:
- regulate fibril formation
- interactions between matrix molecules
Pericellular matrix: Glycoprotein (Fibronectin)
Territorial matrix:
Collagen, Proteoglycan (Decorin)
Inter-territorial matrix:
HA + Aggrecan
Composition of ECM determines Structural property of tissue
Bone:
- more collagen (mineralization: stronger)
- less proteoglycan
Cartilage:
- relatively less collagen
- more proteoglycan (soluble polymer)
Matrix receptors
Mediate interaction between cell and ECM
- ***Integrin
- Discoidin domain receptors (DDRs)
Integrin
- large family of ***transmembrane protein
- Interact with **ECM (outside cell) and **cytoskeleton (inside cell)
—> ***Transmit signals - Cell-matrix / cell-cell interactions are weak
—> however hundreds group together —> form ***Focal adhesions (within the cell)
—> Enhance specificity through multiple interactions - Matrix binding site
—> α chain + β chain (Heterodimer)
—> cysteine rich domain
—> only binds to matrix with ***specific a.a. sequence (Arg-Gly-Asp (RGD))
—> 1 single β chain interact with multiple α chains
—> forms different combinations of Intergrin heterodimers
—> diversity of integrins bind different ligands - Cytosolic side:
Interact with Cytoskeleton
ECM and cell function
Interact with each other —> Maintenance and Homeostasis —> Any of them went wrong —> influence each other in bad manner —> vicious cycle
ECM homeostasis:
- Gene expression —> Matrix assembly —> Matrix turnover —> Gene expression
- Imbalance: degenerative disease (OA, RA, IV degeneration, osteoporosis)
Enzymes involved in matrix turnover
- Matrix metalloproteinases (MMP)
- Key enzyme in **matrix degradation
- **Collagenases, **Gelatinases, Stromlysin
- Cleaves at specific site of ECM protein
- Regulation of activity
—> synthesized as inactive proenzymes
—> regulated by **TIMP (Tissue inhibitors of MMP) - Other degradative enzymes:
- ***Hyaluronidase, chondroitinase
***Overview of ECM function
- Structural
- collagen
- proteoglycan
- GAG - Signaling
- integrin - Hold / transmit cytokines, growth factors
- proteoglycan
- glycoprotein - Degradation
- MMP
- hyaluronidase
***Summary: Cartilage vs Bone
Cartilage: ***much more ECM
—> Pericellular, Territorial, Inter-territorial matrix
Bone:
Dense matrix:
Inorganic (70%):
1. ***Calcium phosphate (as hydroxyapatite crystals)
Organic:
1. Mixture of tough collagen fibre
—> mainly ***Type I Collagen arranged in regular layers
—> for mineralisation and tensile strength
- Proteoglycans + GAGs (hyaluronan, decorin, biglycan, fibromodulin)
- Glycoproteins
- Osteopontin
- Bone sialoprotein
- Osteocalcin and matrix Gla protein
Cartilage:
- Cellular component:
- Chondrocyte - Fibrous component:
- ***Collagen type II - Proteoglycan (Soluble polymers):
- ***Aggrecan, decorin, biglycan, lumincan, perlecan
- syndecan, glypican (membrane bound) - Glycosaminoglycans (GAG) (Soluble polymers):
- ***Hyaluronic acid
- chondroitin, heparan, keratan, dermatan sulfate - Glycoprotein:
- ***Fibronectin
