BIOCHEMISTRY BONE METABOLISM MSK BLOCK SYSTEM Nabil Bashir October 1 st, 2009

Bone Inorganic (67%) –Hydroxyapatite 3 Ca 10 (PO 4 ) 6 (OH) 2 –There is some amorphous calcium phosphate Organic (33%) component is called osteoid –Type I collagen (28%) –Non-collagen structural proteins (5%) Proteoglycans Sialoproteins Gla-containing proteins (gamma carboxyglutamate) Phosphoproteins Bone specific proteins: osteocalcin, osteonectin –Growth factors and cytokines (Trace) Bone undergoes continuous turnover or remodeling throughout life –About 20% of bone is undergoing remodeling at any one time

Osteoblast and Osteoclast Function Osteoblasts Bone formation Synthesis of matrix proteins –Type I collagen –Osteocalcin –Others Mineralization Activation of osteoclasts via RANKL production Osteoclasts Bone resorption –Degradation of proteins by enzymes –Acidification RANK is activated by RANKL, and this leads to cells differentiation to osteoclasts

PTH and Osteoblastogenesis

Osteoclast Mediated Bone Resorption

Osteoclastogenesis: RANK/RANKL/OPG axis RANK: Receptor activator of nuclear factor (NF)- kB RANKL: RANK ligand OPG: Osteoprotegerin (cytokine) Activating factors: M-CSF, IL-6etc. ASBMR Bone Curriculum

Osteoclastogenesis Activating factors cause the lining cells to produce RANKL, and then the RANK of the preosteoclasts binds with the RANKL and the forms multinucleated activated osteoclasts (10-20 fused cells, called polykaryons) OPG also binds with RANKL, preventing the preosteoclast RANK from binding ASBMR Bone Curriculum

Hormonal Control of Resorption: Pro-resorptive Most of the proresorptive factors upregulate mRNA expression of RANKL in osteoblasts Boyle et al, Nature 2003

Hormonal Control of Resorption: Anti-resorptive Boyle et al, Nature 2003

Genetic Mutations What would an OPG knock-out mouse look like? Boyle et al, Nature 2003

CALCIUM& PHOSPHORUS HOMEOSTASIS PTH, VIT D, CALCITONIN, ESTROGENS

Ca PO 4 1,25 D INTESTINE Absorption Ca 2+  BLOOD KIDNEY PTH Bioactivation 25D1,25D  PTH 1,25 D PO 4 BONE Mineralization PTH Figure 1. Control of blood Ca 2+ and PO 4 3- matrix 1,25 D PO 4  Reabsorption Ca 2+  PTH 1,25 D Ca 2+ Resorption  PO 4 PTH, in response to low serum Ca, increases plasma Ca by increasing bone resorption, and renal reabsorption of Ca PTH prevents hyperphosphatemia, which could be caused by the PTH effect on bone resorption, by inhibiting renal reabsorption of phosphate PTH activates the hydroxylation of 25(OH)D 3 to the active 1,25(OH) 2 D 3 form 1,25(OH) 2 D 3, in response to low serum Ca, increases plasma Ca by increasing intestinal absorption, bone resorption, and renal reabsorption of Ca 1,25(OH) 2 D 3 increases intestinal and renal absorption of phosphate to help promote bone mineralization calcitonin (CT) can counteract the effect of PTH on bone resorption estrogen (E2) counteracts effects of PTH and 1,25(OH) 2 D 3 on bone resorption  CT E2E2

Bone Remodeling Osteoclasts dissolve bone –Large multinucleated giant cells Osteoblasts produce bone –Have receptors for PTH, CT, Vitamin D, cytokines, and growth factors –Main product is collagen When osteoblasts become encased in bone, they become osteocytes

MOP ODF receptor Differentiation and fusion c-FMS receptor GsGs   cAMP   PKA Osteoblast Bone constructor ODF Osteoprotegerin receptor decoy osteoblast osteoclast Osteoclast Bone destructor Figure 3. Control of bone remodeling by PTH IL-6 IL-6; other cytokines  activation M-CSF PTH  MOP – monocytic osteoclast progenitor cells M-CSF – macrophage colony stimulating factor ODF – osteoclast differentiating factor

IL-6 GsGs   cAMP   PKA IL-6; other cytokines ODF Osteoblast Bone constructor  Osteoclast Bone destructor Figure 3. Control of bone remodeling by PTH and calcitonin activation CT   PKA   cAMP  inactivation PTH GsGs CT gene can yield calcitonin gene-related peptide (CGRP) if processed differently (alternative mRNA splicing) CGRP = a potent vasodilator Calcitonin secreted by thyroid C-cells in response to hypercalcemia

HO Vitamin D 3 Diet HO OH 25(OH) D 3 Liver 25-OHase OH HO OH 1,25(OH) 2 D 3 (active hormone form) Kidney 1-OHase HO 7 Provitamin D 3 (7-dehydrocholesterol: Intermediate in cholesterol synthesis) UV from sunlight Skin 24,25(OH) 2 D 3 (inactive form) OH HO OH 24-OHase (kidney; many other tissues) High Ca 2+ or PO 4 3- Figure 4. Photobiosynthesis of vitamin D 3 and its metabolism Specific receptors in intestine, bone, kidney Ca: Intestinal absorption Renal reabsorption Bone resorption PO 4 : Intestinal absorption Renal reabsorption

MOP Differentiation and fusion Osteoblast Bone constructor ODF receptor 1,25D 3  osteoblast osteoclast Osteoclast Bone destructor Figure 7. Control of bone remodeling by 1,25(OH) 2 D 3 inactivation c-FMS receptor M-CSF CT   PKA   cAMP  GsGs 1,25D 3 RXR receptor VDR receptor Nucleus mRNA Osteocalcin Ca 2+ Gla IL-6 IL-6; other cytokines  activation ODF Chemotaxis

MOP ODF receptor Differentiation and fusion c-FMS receptor IL-6 IL-6; other cytokines ODF Osteoblast Bone constructor  PTH, 1,25D 3  osteoblast osteoclast Osteoclast Bone destructor Figure 8. Control of bone remodeling by PTH and 1,25(OH) 2 D 3 and antagonism of their effects by estrogen and androgen activation CT   PKA   cAMP  inactivation M-CSF GsGs 1,25D 3 3 mRNA Ca 2+ Osteocalcin Gla Collagen matrix Ca 2+ /PO 4 3-  Bone mineralization GsGs   cAMP   PKA PTH Estrogen (E 2 ) and Androgen Estrogen (E 2 ) and Androgen proliferation collagen syn.   Osteoblast E2E2