Lecture 26 - Healthy Bone Flashcards
Composition of healthy bone
1)
2)
3)
1) Organic matrix
2) Mineral
3) Water
Organic matrix of healthy bone composition
1)
2)
1) 90% collagen (collagen makes up ~10% healthy bone mass)
2) Other proteins that help bind mineral
Mineral composition of healthy bone
1)
2)
1) 65% adult bone mass is hypoxyapatite
2) Smal amounts of calcium, magnesium, bicarbonate
How much of adult bone mass is water?
25%
Hypoxyapatite
Insoluble salt of calcium and phosphorus
~65% healthy adult bone mass
How much adult bone is replaced each year?
5-10%
Number of microscopic bone remodelling foci
1-2 million
How is bone remodelling carried out?
Asynchronously.
At sites that are geographically and temporally separate
Reasons for bone remodelling
1)
2)
3)
1) Maintain ion concentration in the body (EG: Ca2+)
2) Adapt shape and structural organisation to alterations in biomechanical forces (mechanostat)
3) Maintain structural integrity, repair microdamage
Proportion of body’s calcium in bone
99%
Proportion of body’s phosphorus in bone
85%
Basic multicellular unit of bone
1)
2)
3)
1) Osteocytes
2) Osteoblasts
3) Osteoclasts
Name of cavity formed by osteoclast
Resorption cavity
Lacuna
Time taken for bone resorption
3 weeks
Time taken for bone formation
3-4 months
Disease where bone density is too low
Osteoporosis
Disease of net bone gain
1)
2)
Osteopetrosis
Osteosclerosis
Osteoclast phenotype
1)
2)
1) Large, multinucleated cells (4-20 nuclei)
2) Rich in mitochondria
Process by which osteoclasts resorb bone 1) 2) 3) 4)
1) Form a sealing zone (integrin-mediated)
2) Release H+, acidify lacuna, degrade mineral component
3) Release of collagenases, other enzymes to degrade organic component
4) Bone degradation products are taken up by osteoclast, released
What do osteoblasts differentiate from?
Mesenchymal stem cells
Other cells which differentiate from mesemchymal stem cells
1)
2)
3)
1) Muscle
2) Chondrocytes
3) Adipocytes
Three fates of osteoblasts
1)
2)
3)
1) Osteocyte (encased in bone)
2) Lining cell (sits on surface of bone)
3) Apoptosis
Bone formation of osteoblasts
1)
2)
3)
1) Secretion of ECM proteins (osteoid) including collagen, bone sialoprotein, osteocalcin
2) Expression of alkaline phosphatase
3) Mineral deposition
ECM proteins secreted by osteoblast during bone formation
1)
2)
3)
1) Collagen
2) Bone sialoprotein
3) Osteocalcin
Function of alkaline phosphatase
Renders the osteoid competent for mineral deposition
Osteoid
Unmineralised bone
What is unmineralised bone called?
Osteoid
RANKL 1) 2) 3) 4) 5)
1) Receptor activator of NF-kB ligand
2) Key differentiation factor in osteoclast development
3) Binds to osteoclast precursor
4) Member of TNF superfamily
5) Mostly membrane-bound, but can be cleaved to soluble form
Osteoclast differentiation 1) 2) 3) 4)
1) Haematopoietic stem cell
2) Myeloid progenitor develops into osteoclast progenitor by binding M-CSF released by osteoblast-lineage cells
3) Osteoclast progenitor expresses RANKL receptor, RANK-L receptor binds RANKL released by osteoblast lineage cells
4) Cell binds to bone, becomes osteoclast
Factor released by osteoblast-lineage cells that causes myeloid progenitor to differentiate into osteoclast progenitor
M-CSF (macrophage colony stimulating factor)
Effect of absent RANKL/RANK
RANKL-/- mice develop severe osteopetrosis
RANK
1)
2)
3)
1) Receptor for RANKL/ODF
2) Member of TNF receptor family
3) Expressed on osteoclasts and osteoclast progenitors
OPG 1) 2) 3) 4)
1) Osteoprotegerin
2) Member of TNF receptor family
3) Binds RANKL as a decoy receptor
4) Blocks RANKL-induced osteoclast differentiation
Effect of absent osteoprotegerin
Osteoprotegerin-/- mice develop osteoporosis
Key regulator of osteoclast differentiation
Osteoprotegerin:RANKL ratio
M-CSF roles
1)
2)
1) Proliferation of myeloid precursors
2) Survival of osteoclast progenitors and mature osteoclasts
Producers of RANKL
1)
2)
1) Primary producers are early-mid stage osteoblasts
2) Can also be produced by osteocytes
Main producers of OPG
Mid- to late-stage osteoblasts
Promoters of osteoblast differentiation and function
Wnt ligands
Differentiation induced by wnt ligands
Osteoblastic progenitor to pre-osteoblast
Effect of wnt activation in osteoblasts
1)
2)
1) Promotes bone formation
2) Inhibits bone resorption
How do wnt ligands promote bone formation?
1)
2)
3)
1) Wnt ligand binds coreceptors frizzled or LRP5/6
2) Beta-catenin in cytoplasm is stabilised
3) Beta-catenin translocated to nucleus, acts as a transcription factor, induces genes that induce osteoblast differentiation, increase OPG expression
Factors that inhibit wnt signalling in osteoblasts
1)
2)
3)
1) Sclerostin binds LRP5/6
2) Dikkopf (DKK) binds LRP5/6
3) Secreted frizzled-related protein 1 (sFRP1) binds wnt ligands
Effect of increased sclerostin/DKK/secreted frizzled-related protein 1
Increased bone resorption by reducing osteoblast differentiation, increases osteoclast differentiation (increases RANKL:OPG ratio)
Cell that produces endogenous wnt antagonists
Osteoblast
Osteocyte
Terminally-differentiated osteoblast, encased in bone matrix
Osteocyte features 1) 2) 3) 4)
1) Longest-lived cell in the body (20-25 years)
2) Sit within bone matrix in lacunae
3) Dendrite-like cell processes (canaliculi) that sense mechanical loading, communicate between other osteocytes, cells on bone surface
4) Regulate osteoblasts and osteoclasts
How do osteocytes regulate osteoblasts and osteoclasts?
1) Regulate levels of DKK and sclerostin
2) Can release RANKL
What is sclerostosis?
Mutation in sclerostin, which inactivates it
Very thick bone, osteosclerotic condition
Effect of increased mechanical loading on osteocytes
Reduce expression of DKK, sclerostin
This increases bone formation, reduces resorption, because of activation of wnt signaling
Treatment strategies for osteoporosis
1) a,b
2) a
1) Target osteoclast
a) Inhibit osteoclast
b) Inhibit RANKL
2) Target osteoblast
a) Target osteoblast progenitor differentiation
Bisphosphonates
1)
2)
3)
1) Inhibit osteoclasts, lead to apoptosis
2) Used to treat osteoporosis
3) EG: Zoledronate
Denosumab
1)
2)
1) Anti-RANKL, fully-humanised MAb
2) Inhibits osteoclast differentiation and survival
Effect of bisphosphonates and anti-RANKL MAbs
1) Reduces number of fractures
2) Reduces bone loss
3) No increase in bone volume
Osteoporosis treatments that target osteoclasts
1)
2)
1) Bisphosphonates
2) Anti-RANKL MAbs
Osteoporosis treatment that targets ostoblasts
Recombinant parathyroid hormone, amino acids 1-34 (rhPTH,1-34, teriparatide)
Recombinant parathyroid hormone, amino acids 1-34
1)
2)
3)
1) Only approved anabolic therapy for osteoporosis
2) Given as an intermittent injection. Constant exposure doesn’t lead to increased bone density
3) Induces mesenchymal stem cell differentiation into osteoblasts
Emergin osteoporosis therapy that promotes bone formation
Anti-sclerostin MAb
AMG785 or Romosozumab
