1. Bone Metabolism
MSS,Fall- 2015
Nabil Bashir

2. Specific objectives
Describe the biochemical structure of bone tissue, the collagen matrix and the hydroxyapatite..
List bone matrix proteins and describe their function.
Describe the formation of hydroxyapatite.
Understand the role of alkaline phosphatase, calcium and phosphate and vitamin D: 1,25-Dihydroxy-vit-D in bone formation and remodeling.
Review calcium and phosphate homeostasis
Markers of bone formation and destruction

3. Bone Inorganic (67%): Hydroxyapatite 3 Ca10(PO4)6(OH)2
Organic (33%) component is called osteoid
Type I collagen (28%)
Non-collagen structural proteins (5%)
Sialoproteins
Phosphoproteins
Bone Gla protein (BGP, osteocalcin)
matrix Gla protein (MGP) .
The synthesis of both proteins in osteoblastic cells is stimulated by 1,25(OH)2D3.
Growth factors and cytokines (Trace)

4. Calcification
Crystalization of hydroxy apetite is promoted by the availability of:
Ca2+,
PO4(3-), and
collagen,
Crystalization is retarded by naturally occurring inhibitors of mineralization such as
proteoglycans
bone-Gla protein (osteocalcin)BGP
phosphoproteins, osteonectin

5. Alkaline phosphatase
Generation of extracellular inorganic pyrophosphate
(ePPi) by nucleotide pyrophosphatase phosphodiesterase
(NPP1) from extracellular nucleoside triphosphates (eNTP) or by ankylosis (ANK) protein.
The action of TNAP decreases ePPi and increases inorganic phosphate

9. Collagen I formation
Most collagen forms in a similar manner, but the following process is typical for type I:
Inside the cell
preprocollagen
Two types of peptide chains are formed during translation on ribosomes along the rough endoplasmic reticulum (RER): alpha-1 and alpha-2 chains. These peptide chains (known as preprocollagen) have registration peptides on each end and a signal peptide.
Polypeptide chains are released into the lumen of the RER.

10. 3. Procollagen 2. pro-alpha chains.
Signal peptides are cleaved inside the RER and the chains are now known as pro-alpha chains.
3. Procollagen
Hydroxylation of lysine and proline amino acids occurs inside the lumen. This process is dependent on ascorbic acid (Vitamin C) as a cofactor.
Glycosylation of specific hydroxylysine residues occurs.
Triple helical structure is formed inside the endoplasmic reticulum from each two alpha-1 chains and one alpha-2 chain.( Procollagen)
Procollagen is shipped to the golgi apparatus, where it is packaged and secreted by exocytosis

11. Outside the cell
4. Tropocollagen
Registration peptides are cleaved and tropocollagen is formed by procollagen peptidase.
5. collagen fibrils
Multiple tropocollagen molecules form collagen fibrils, via covalent cross-linking by lysyl oxidase which links hydroxylysine and lysine residues. Multiple collagen fibrils form into collagen fibers.
Collagen may be attached to cell membranes via several types of protein, including fibronectin and integrin.

12. CALCIUM AND PHOSPHORUS HEMEOSTASIS

13. Figure 1. Control of blood Ca2+ and PO43- matrix Bioactivation
BONE
BLOOD
Ca2+
Resorption 
PO4 CT E2
PO4 
1,25 D
PTH
Mineralization
Ca2+ 
1,25 D
Absorption
CaPO4
Reabsorption
Ca2+ 
PTH
PO4
PTH
1,25 D
INTESTINE
1,25 D 
Figure 1. Control of blood
Ca2+ and PO43- matrix
PTH
Bioactivation
25D
1,25D 
KIDNEY
calcitonin (CT) can counteract the effect of PTH on bone resorption
estrogen (E2) counteracts effects of PTH and 1,25(OH)2D3 on bone resorption
1,25(OH)2D3 increases intestinal and renal absorption of phosphate to help promote bone mineralization
1,25(OH)2D3, in response to low serum Ca, increases plasma Ca by increasing intestinal absorption, bone resorption, and renal reabsorption of Ca
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)D3 to the active 1,25(OH)2D3 form

14. The parathyroid and thyroid glands function to control the level of blood calcium.

15. Bone remodeling cycle
The bone remodelling cycle begins with the recruitment of osteoclast precursor cells. These differentiate into osteoclasts when they receive signals from osteoblasts. Mature osteoclasts then synthesise and release proteolytic enzymes that digest the collagen matrix. This bone resorption is the first phase of the remodelling cycle. The length of this phase is regulated by apoptosis of osteoclasts. In the next phase of the remodelling cycle preosteoblasts are attracted from mesenchymal stem cells in the bone marrow. Mature osteoblasts synthesise the bone matrix, mainly type I collagen, and regulate the mineralisation of the newly formed bone. Some mature osteoblasts may be trapped within mineralised bone and become osteocytes

16. Figure 7. Control of bone remodeling by 1,25(OH)2D3
Osteoblast
Bone constructor
Osteoclast
Bone destructor
VDR receptor
Nucleus
IL-6
IL-6; other
cytokines 
activation
inactivation
RXR receptor
mRNA CT
PKA cAMP  Gs
Osteocalcin
ODF
1,25D3
ODF receptor
1,25D3 
osteoblast
Ca2+
Gla
Chemotaxis
osteoclast
MOP
Differentiation
and fusion
c-FMS receptor
M-CSF
Figure 7. Control of bone remodeling by 1,25(OH)2D3

17. Osteoblast Bone constructor Osteoclast Bone destructor PTH Gs
Estrogen (E2)
and Androgen
PTH Gs
 cAMP  PKA
IL-6
IL-6; other
cytokines
inactivation 
mRNA Gs CT 3
PKA cAMP 
activation
ODF
Osteocalcin
1,25D3
Estrogen (E2)
and Androgen
proliferation
collagen syn. 
osteoblast
PTH, 1,25D3  E2
ODF receptor
Gla
Ca2+
Collagen matrix
Ca2+/PO43-
 Bone mineralization
MOP
Osteoblast
osteoclast
Differentiation
and fusion
c-FMS receptor
Figure 8. Control of bone remodeling by PTH and 1,25(OH)2D3 and antagonism of their effects by estrogen and androgen
M-CSF

19. Deoxypyridinoline cross-linking in bone collagen.