BIOCHEMISTRYBONE METABOLISM

MSK BLOCK SYSTEM

Nabil Bashir

October 1st, 2009

Bone• Inorganic (67%)

– Hydroxyapatite 3 Ca10(PO4)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

CaPO4

1,25 D

INTESTINE

Absorption

Ca2+

BLOOD

KIDNEYPTH

Bioactivation25D 1,25D

PTH

1,25 D

PO4

BONE

Mineralization

PTH

Figure 1. Control of blood Ca2+ and PO4

3- matrix

1,25 D

PO4

Reabsorption

Ca2+

PTH1,25 D

Ca2+Resorption

PO4

PTH, in response to low serum Ca, increases plasma Ca by increasing bone resorption, and renal reabsorption of CaPTH 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 form1,25(OH)2D3, in response to low serum Ca, increases plasma Ca by increasing intestinal absorption, bone resorption, and renal reabsorption of Ca

1,25(OH)2D3 increases intestinal and renal absorption of phosphate to help promote bone mineralization

calcitonin (CT) can counteract the effect of PTH on bone resorptionestrogen (E2) counteracts effects of PTH and 1,25(OH)2D3 on bone resorption

CTE2

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

Differentiationand fusion

c-FMS receptor

Gs cAMP PKA

OsteoblastBone constructor

ODF

Osteoprotegerinreceptor decoy

osteoblast

osteoclast

OsteoclastBone destructor

Figure 3. Control of bone remodeling by PTH

IL-6IL-6; othercytokines

activation

M-CSF

PTH

PTH

MOP – monocytic osteoclast progenitor cells

M-CSF – macrophage colony stimulating factor

ODF – osteoclast differentiating factor

IL-6Gs cAMP PKA

IL-6; othercytokines

ODF

OsteoblastBone constructor

OsteoclastBone destructor

Figure 3. Control of bone remodeling by PTH and calcitonin

activationCTPKA cAMP

inactivation PTH

Gs

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 D3

Diet

HO

OH

25(OH) D3

Liver

25-OHase

OHHO

OH

1,25(OH)2 D3

(active hormone form)

Kidney

1-OHase

HO 7

Provitamin D3

(7-dehydrocholesterol: Intermediate in cholesterol synthesis)

UV fromsunlight

Skin

24,25(OH)2 D3

(inactive form)

OH

HO

OH

24-OHase(kidney; manyother tissues)

High Ca2+

or PO43-

Figure 4. Photobiosynthesis of vitamin D3 and its metabolism

Specific receptors in intestine, bone, kidney

Ca:Intestinal absorptionRenal reabsorptionBone resorption

PO4:Intestinal absorption Renal reabsorption

MOPDifferentiationand fusion

OsteoblastBone constructor

ODF receptor

1,25D3

osteoblast

osteoclast

OsteoclastBone destructor

Figure 7. Control of bone remodeling by 1,25(OH)2D3

inactivation

c-FMS receptor

M-CSF

CTPKA cAMP Gs

1,25D3

RXR receptor

VDR receptor Nucleus

mRNA

Osteocalcin

Ca2+

Gla

IL-6IL-6; othercytokines

activationODF

Chemotaxis

MOP

ODF receptor

Differentiationand fusion

c-FMS receptor

IL-6IL-6; othercytokines

ODF

OsteoblastBone constructor

PTH, 1,25D3

osteoblast

osteoclast

OsteoclastBone destructor

Figure 8. Control of bone remodeling by PTH and 1,25(OH)2D3 and antagonism of their effects by estrogen and androgen

activationCTPKA cAMP

inactivation

M-CSF

Gs

1,25D3

3 mRNA

Ca2+

Osteocalcin

Gla

Collagen matrix Ca2+/PO4

3-

Bone mineralization

Gs cAMP PKA PTH

Estrogen (E2)and Androgen

Estrogen (E2)and Androgen

proliferation collagen syn.

Osteoblast

E2