BONE MATRIX
Tim Arnett
Department of Anatomy and Developmental Biology University College London Bone is made of mineral + organic (mainly collagen) components
Treated with hydrochloric acid to dissolve mineral leaves collagen component intact
Treated with bleach (hypochlorite) to digest collagen leaves mineral component intact
Collagen shrinkage on drying Bone matrix
· bone is a composite material
· organic component is ~90% type I collagen - structural protein providing strength, flexibility
· remaining 10% of organic component is complex mixture of proteins, including growth factors, osteocalcin, osteonectin, osteopontin, glycoproteins; all produced by osteoblasts, along with type I collagen Bone Matrix - 2
· organic matrix is mineralised after secretion – unmineralised matrix = “osteoid”
· mineral component is hydroxyapatite – Ca10 (PO4)6 (OH)2 - tiny crystals impregnate & surround collagen fibres - provides rigidity, resistance to compression
· mineralisation of osteoid is dependent on the hormonally active form of vitamin D (1,25-dihydroxyvitamin D)
· vitamin D deficiency results in failure to mineralise – leads to rickets in children, osteomalacia in adults Two types of bone
Woven (immature, fracture) Lamellar (mature, adult)
large, rounded osteocytes smaller, flattened osteocytes
osteocytes irregularly spaced osteocytes regularly spaced randomly oriented collagen fibres collagen fibres show regular, “plywood” orientation – confers strength
variable collagen fibre diameter regular collagen fibre diameter
rapid matrix mineralisation delayed matrix mineralisation (few days)
forms rapidly forms slowly
rapid turnover slow turnover Collagen
• stable, structural ECM protein • collagen molecule = triple-stranded helix (3 -chains) • 25% of total body protein (mainly type I)
MAIN COLLAGEN TYPES…..
TYPE I: (90% of total): fibrils fibres bone, skin, tendon, ligaments, cornea, internal organs TYPE II: fibrils (form meshwork) not visible with light microscope cartilage, intervertebral disc, vitreous humour TYPE III: fibrils → fibres (‘reticular’ - coarse mesh) skin, blood vessels, internal organs TYPE IV: sheet-like network basal laminae Collagen biosynthesis
Alberts et al – Molecular Biology of the Cell, Ed. 2 Amino acid composition of collagen chains
…Gly-X-Y…
X or Y frequently proline 3 . d E ll, e C e h t f o y g ular Biolo olec M – l a et rts e Alb Hydroxylated amino acids in collagen
Alberts et al – Molecular Biology of the Cell, Ed. 3 Pro-collagen
Alberts et al – Molecular Biology of the Cell, Ed. 3 “¼ stagger” model of collagen molecule alignment in fibrils
Alberts et al – Molecular Biology of the Cell, Ed. 3 Extracellular assembly of individual collagen molecules into fibrils - ‘quarter stagger’ model
Alberts et al – Molecular Biology of the Cell, Ed. 3 Covalent links between and within collagen molecules formed by action of lysyl oxidase on lysine and OH-lysine residues
Alberts et al – Molecular Biology of the Cell, Ed. 3 Non-collagenous proteins in bone
• ~ 10% of organic component of bone matrix
• 4 main groups: cell-attachment proteins gamma-carboxylated (gla) proteins proteoglycans growth factors & blood proteins Non-collagenous proteins in bone
• Thrombospondin (450 kD) - influences cell attachment - strong Ca2+ binding Fibronectin (440 kD) - influences cell attachment & spreading; contains RGD sequence; binds collagen
• Osteopontin (50 kD) - phosphorylated; contains RGD sequence; influences cell attachment & spreading; strong Ca2+ binding; not bone-specific Bone sialoprotein (50 kD) - also affects cell attachment; strong Ca2+ binding; bone-specific
• Osteonectin (SPARC) (35 kD) - most abundant NCP: ~2% total protein in developing bone; binds collagen & hydroxyapatite; promotes calcium phosphate deposition in vitro
• Osteocalcin (BGP) (6kD) - contains γ-carboxy glutamyl residues (Ca2+ binding); bone-specific 2+ protein; low affinity Ca binding; synthesis stimulated by 1,25(OH)2 vitamin D. Matrix gla-protein (9kD) - not bone-specific
• Proteoglycan I (biglycan) (170 kD) - 2 chondroitin sulphate chains; mainly in developing bone, function unknown Proteoglycan II (decorin) (120 kD) - 1 chondroitin sulphate chain; present in all bone; binds collagen, may modulate fibrillogenesis
• Blood proteins (~25% of NCP) - including α2HS glycoprotein… and growth factors, including IGF-I, IGF-II, BMPs, PDGF, FGFs, TGFβ…. γ-carboxy-glutamic acid
H O
N C C
H CH2
γ-carboxylation is vit K C dependent process → OO- C Ca ++ O O - Alberts et al – Molecular Biology of the Cell, Ed. 3 Proteoglycans
Cartilage: aggrecan Bone: decorin
Alberts et al – Molecular Biology of the Cell, Ed. 3 Targeted disruption of non-collagenous protein genes ie, “knockouts”
Osteocalcin - increased osteoblast activity (bone formation) in knockout mice
Biglycan - (binds collagen + TGFβ) knockout mice show reduced growth rate + develop osteoporotic phenotype; double KO with decorin → more osteoporosis, premature osteoarthritis, abnormal collagen fibril formation
MGP - matrix γ-carboxyglutamic acid (gla) protein knockout mouse exhibits extensive, lethal calcification + cartilaginous metaplasia of media of all elastic arteries
Osteopontin - knockout mice show altered wound healing; resistance to osteoporotic effects of ovariectomy; reduced tumour metastasis; reduced inflammatory arthritis… Naturally occurring ‘knockouts’
Osteogenesis imperfecta
• diverse family of genetic disorders
• mutations of α1 and α2 chains of type I collagen
Severity varies greatly, depending on the type of mutation, from prenatal lethality through impaired stature / tooth development and deformity to minimal deformity, sometimes with hearing loss or blue sclerae