Lecture 27 - Rheumatoid Arthritis - Bone Flashcards
Describe the structure of bone
Cortical bone
• Outermost
• Dense
Cancellous / Trabecular
• Reinforces, with a cross network of struts
• Less dense
Bone marrow
• In the medullary cavity
• Production of haematopoietic cells
What is the main protein in bone?
Collagen
What is the composition of bone?
- Organic matrix - 10% adult bone mass
• 90% collagen (90% of organic matrix)
• Other proteins that help bind the minerals
2. Mineral component - 65% A.B.M. • Hydroxyapatite • Na • Mg • Bicarbonate
- Water - 25% A.B.M.
Is bone permanent?
No
5-10% of skeleton replaced each year
Where does bone remodelling occur?
Characterise the initiation
Occurs at 1-2 million microscopic foci
Initiation is asynchronous
Why is bone remodelled?
• Maintenance of metal ion homeostasis
• Adapt shape and structural organisation to alterations in biomechanics forces
- ‘Mechanostat’
• Maintain structural integrity
- repair of microdamage
Describe the role of bone in mineral ion homeostasis
- Decreased Ca in blood
- Bone ‘resorbed’, releasing Ca into blood
and vice versa
Describe the BMU
“Basic multicellular unit” in bone
Osteocytes
• Embedded within the bone matrix, in lacunae
• “Mechanosensors”
• Control the osteoblasts and -clasts
Osteoblasts
• Form bone
• Cuboidal
Osteoclasts
• Resorb bone
• Multi-nucleated
• Large
Describe the process of bone remodelling
- Osteoclasts present on bone surface
- Resorption of a packet of bone, leaving ‘lacunae’
- Osteoblasts come in and release collagen
- Unmineralised bone matrix now formed: “Osteoid”
- Minerals bind the osteoid to form bone
What are lacunae?
Resorption cavities
After osteoclasts have resorbed a packet of bone, there are cavities left over in the bone
Osteocytes reside in here
What is Osteoid?
Unmineralised bone matrix laid down initially by the osteoblasts Composition: • Collagen • Bone sialoprotein • Osteocalcin
What is needed to maintain healthy bone mass?
Balance between bone resorption and bone formation
i.e. between Osteoblast and Osteoclast activity
How long does it take for a packet of bone to be:
• Resorbed
• Formed
What is the implication of this?
Resorption: 3 weeks
Formation: 3-4 months
Implication:
If there is much resorption occurring, it is very difficult to make up for this loss, even if there are many osteoblasts present
What is the function of osteocytes?
Where are they located?
What do they differentiate from?
What is their lifespan?
Embedded within the bone matrix, within Lacunae
Function:
• “Mechanosensors”
• Control the osteoblasts and -clasts
Differentiate from osteoblasts
Lifespan: 20-25 years
What is the clinical symptom of increased osteoclast activity?
Osteoporosis
- Weakened bones
- Thinner trabeculae
What is the clinical symptom of increased osteoblast activity?
Osteosclerosis
Osteopetrosis
- High bone mass
- More opaque bones on the X ray
- Bone is much heavier
Describe the cellular features of osteoclasts
Multinucleated
Many mitochondria
• High energy needs required for bone resorption
Describe bone resorption by osteoclasts
- Formation of ‘sealing zone’
- Acidification
• Forms the Ruffled border
• Dissolves the mineral component - Release of collagenases
• Degrades the organic component - Degradation products taken up by the osteoclasts
How do osteoclasts attach to the surface of bone?
Integrins
Describe the differentiation into osteoblasts
- Mesenchymal stem cell origin
- Differentiation down the osteoblast lineage
- Preosteoblast
- Mature osteoblast
3 fates:
• Osteocytes: embedded in the bone
• Lining cell (Fx unknown)
• Apoptosis
What do mesenchymal stem cells give rise to?
- Osteoblasts
- Chondrocytes
- Muscle cells
- Adipose cells
What are the functions of osteoblasts?
- Bone formation
* Support of osteoclasts
Describe how osteoblasts promote and inhibit osteoclasts
Promotion: RANKL
• From the early-mid stage osteoblasts
Inhibition: OPG production
• From the mid-stage-mature osteoblasts
Describe the process of osteoblast bone formation
Secretion of EXM proteins:
• Collagen
• Bone sialoprotein
• Osteocalcin
Expression of alkaline phosphatase
• Renders the bone competent for mineralisation
Expession of which molecule is vital for rendering bone competent for mineralisation?
Alkaline phosphatase
Describe differentiation into osteoclasts
- Haematopoietic stem cell
- Myeloid precursor
– M-CSF signalling –
• Myeloid precursor proliferation
- Osteoclast precursor
• expressing RANK
– RANKL expression from osteoblasts –
• Binds to RANK on osteoclast precursor
- Mono-nucleated precursors
– fusion –
- Bone-resorbing osteoclast
• Multinucleated
• Attached to bone surface
What are the progenitor cells for the following:
• Osteoblasts
• Osteoclasts
• Osteocytes
Osteoblasts:
• Mesenchymal stem cell
Osteoclasts:
• Haematopoietic stem cell
• Myeloid lineage
Osteocytes:
• Mesenchymal stem cell (differentiate from osteoblasts)
What is another name for RANKL?
Receptor activator of NFκB-ligand
An osteoclast is only an osteoclast when it is…
Sitting on the bone surface
Describe the role of RANKL in osteoclast differentiation
RANKL expressed by osteoblasts
RANK expressed on surface of osteoclast precursors
RANKL signalling stimulates the differentiation into osteoclasts capable of resorbing bone
Describe regulation of RANKL signalling in osteoclast differentiation
OPG: Osteoprotegerin
- OPG inhibits RANKL
- Soluble decoy receptor
- Soaks up RANKL so that there is less differentiation into osteoclasts
- Produced by early-mid stage osteoblasts
What type of molecule is RANKL?
Member of TNF superfamily
Describe the main features and functions of RANKL
What is seen in RANKL -/- mice?
Mostly membrane bound, but can be cleaved to a soluble form
Function:
• Induces osteoclast differentiation and activation
RANKL -/-:
• Severe osteopetrosis
What is RANK also called?
Describe features
Where is it expressed?
What is seen in RANK -/- mice?
Receptor activator of NFκB
- Member of TNF receptor superfamily
- Expressed on osteoclasts and their precursors
RANK -/-:
• Severe osteopetrosis
Why is RANKL ‘sufficient’?
It is the only factor required for osteoclast differentiation and function
Describe the features and function of OGP
What is seen in OPG -/- mice?
“Osteoprotegerin”
• Soluble decoy receptor for RANKL
• Member of TNF receptor superfamily
(however structurally distinct form RANK)
• Blocks differentiation into Osteoclasts
OPG -/-:
• Osteoporosis
What happens when there is an imbalance between RANKL and OPG?
Increased RANKL:
• Net bone loss
Increased OPG:
• Net bone gain
Which molecules are important for:
• Proliferation of myeloid precursors into osteoclasts
• Osteoclast differentiation and function?
Proliferation of myeloid cells into osteoclasts: M-CSF
Osteoclast differentiation and Fx: RANKL
Which molecules are important for differentiation into osteoblasts?
Wnt ligands
among others
Describe Wnt signalling
- Wnt ligand binds Frizzled receptor
Associated with LRP5/6 - Stabilisation of β-catenin
- β-catenin translocates into nucleus and turns on gene transcription
→→→
- Increase in osteoblast activity:
• Increased OPG production
(→ decrease in osteoclast activity)
What two processes does Wnt signalling bring about?
How does it do this?
- Increased osteoblast activity
- Decreased osteoclast activity
• Expression of osteoprotegerin
Describe regulation of Wnt signalling
- Antagonists of LRP5/6
• DKK1
• Sclerostin - Soluble decoy receptors for Wnt ligands
• sFRP-1
• Binds Wnt ligand, preventing it from binding the co-receptor
→ Destabilisation of β-catenin
→ Effects:
1. Decreased osteoblast activity
- Increased osteoclast activity
• loss of OPG expression
What is sFRP-1?
Secreted frizzled related proteins
Soluble decoy receptor for Wnt ligands
Prevent Wnt from signalling
→ decreased osteoblast activity
What is the most abundant bone cell?
Osteocytes
Describe intercellular communication in osteocytes
Canaliculi:
• Dendrite-like cell processes
Enable intercellular communication with:
• Other osteocytes
• Osteoblasts
• Osteoclasts (at the bone surface)
Which cells are the ‘mechanostat’?
Osteocytes
Are the following inhibitors or promoters of bone formation: • DKK-1 • OPG • RANKL • Sclerostin • Wnt ligands
DKK-1: inhibitor OPG: promoter RANKL: inhibitor Sclerostin: inhibitor Wnt ligands: promoter
Describe how osteocytes regulate bone formation
Express DKK-1 and sclerostin
These molecules inhibit Wnt signalling → decreased osteoblast activity → inhibition of bone formation
What is sclerostosis?
Describe the disease pathogenesis
Mutation in sclerostin → Increased Wnt signalling → Over-expression of OPG → Under-active osteoclasts → Increased bone mass
→ Osteosclerosis
Describe mechanosensation in osteocytes
→ Increased mechanical load
• Reduced expression of DKK-1 and sclerostin →
a. Increased osteoblastic bone formation
b. Decreased osteoclastic bone resorption (increased OPG)
Do osteocytes produce RANKL?
Still controversial:
• Some say yes, and some no
The key source of RANKL is osteoblasts
What is seen in bones in people with RA?
Osteoporosis
What are some approved therapeutics for Osteoporosis?
– Targeting osteoclasts –
- Bisphosphonates
• Inhibit osteoclast activity
• may lead to osteoclast apoptosis
• First port of call - Anti-RANKL antibody
• Injected every 6 months
• Blocks action of RANKL
• Expensive, not as widely used as bisphonates
– Targeting osteoblasts –
- Teriparatide
• Recombinant parathyroid hormone - rhPTH
• Increase in bone mass
• Only approved anabolic therapy
4. Anti-sclerostin mAb • "Romosozumab" • Phase II trial • Increased in bone mineral density • Better results than rhPTH
What is the effect of Bisphosphonates and Anti-RANKL Ab on
• Osteoclasts
• Bone?
- Decreased number
- Decreased function
Decreased fracture risk
Decreased bone loss
Bone volume doesn’t necessarily increase
Describe the mode of action of rPTH
(Recombinant parathyroid hormone)
- Stimulation of osteoblast precursors:
• Mesenchymal stem cells
• Osteoprogenitor - Downregulation of:
• Sclerostin
• DKK-1
→ Promotion of osteoblastic bone formation
Compare results in rhPTH and anti-sclerostin mAb
Anti-sclerostin mAb is still in trials, but is leading to better increases in bone density than rhPTH
Compare stem cell origin of the following cells:
• Osteoblast
• Osteoclast
• Osteocyte
Osteoblast: mesenchymal stem cell
Osteoclast: haematopoietic stem cell
Osteocyte: mesenchymal SC
NB Differentiates from mature osteoblast
Outline how Wnt signalling in osteoblasts can lead to inhibition of osteoclast activity
Wnt signalling leads to the intracellular stabilisation beta catenin, which turns of gene transcription for OPG
OPG ‘mops up’ RANKL, and thus osteoclast activity is inhibited
What is the role of M-CSF?
Proliferation of myeloid precusors and survival of osteoclast progenitors and mature osteoclasts
Where do Wnt ligands come from?
They are made locally within the bone microenvironment
e.g. By Osteoblasts
Where do osteocytes reside?
Within the bone matrix, in the lacunae
What is the name of anti-sclerostin mAb?
Romosozumab
