Lecture 28 - Rheumatoid Arthritis - Bone in RA Flashcards
Describe the cells that modulate bone, their function, and the factors that increase their activity
What releases these factors?
Osteoclast: bone resorbing
• RANKL
• OPG: Inhibition of RANKL
Osteoblasts:
• Wnt: activation of osteoplasts
• Inhibition: DKK-1, sclerostin, sFRP1
Osteocytes:
Regulate bone formation by increasing and decreasing production of:
• Wnt antagonists DKK-1 and Sclerostin
• RANKL (still controversial, main source is osteoblasts)
What are the key pro-inflammatory cytokines in RA?
What is their effect on bone?
TNF IL-1 IL-6 IL-17 RANKL
Destruction of bone and cartilage
What are the patterns of bone loss in RA?
Where does each occur?
At what stage of disease does each occur?
- Juxta-articular / peri-articular osteopenia
• Reduced bone mineral density
• Within trabecular bone
• Occurs very early in disease
- Focal bone erosion
• Eating away of bone
• Contributes to bone deformities
• Occurs within cortical bone - Systemic osteoporosis
• Thinning of trabecular/cancellous bone and cortical bone
• At sites remote from affected joints (e.g. hip, vertebrae)
• Present in many of the patients, but not all
When was the osteoclast recognised as the cell that resorbs bone in RA?
What were the markers that lead to its identification?
1998
Before this, it was thought to be macrophages
In situ hybridisation with an RNA probe that binds to a particular marker (that only osteoclasts express)
Markers:
• Calcitonin receptor (only expressed in osteoclasts on the bone surface)
• TRAP
• Cathepsin K
In RA there are additional sources of RANKL. What are they?
• Osteoblast lineage cells
In addition, in RA:
• Synovial fibroblasts
• T cells
Compare relative expression of RANKL and OPG at the pannus-bone interface in RA
How was this determined?
What is the net effect of this?
RANKL expression outweighs OPG
This was determined with immunohistochemistry
There are many osteoclast precursors, waiting to be stimulated
Effect:
Net resorption of bone (focal bone erosion)
What is the phenotype of RANKL-/- mice?
- No focal bone erosion
- Inflammation still present at normal levels
Osteopetrotic
No functional osteoclasts and thick, dense bones
Because:
RANKL is the key osteoclast differentiation factor
However:
No difference in inflammation between this mouse and the control
What is the effect of RANKL-/- on inflammation?
Not protective against inflammation
What is the effect of OPG.Fc treatment?
- Decreased osteoclasts
- Decreased bone erosion
- Decreased systemic bone loss
However:
No effect on inflammation
Which cells are responsible for bone loss in RA?
Osteoclasts are the only cells responsible for bone loss in RA
What happens to osteoblast activity in RA?
What is the result of this?
What brings about these changes in osteoblasts?
Impaired activity
As a result:
Focal bone erosion that has already occurred does not resolve when RA is controlled
Rationale:
RA → TNF from synovial macrophages → Synovial fibroblasts release more DKK-1 → DKK-1 inhibits osteoblast activity
What are the key signals that promote osteoblast differentiation?
Wnt ligands
Describe impaired osteoblast maturation in RA
What brings about this impairment?
At the sites of inflammation in RA there is decreased bone formation due to impairment of osteoblast maturation
Mechanism:
- Inflammation, TNF release from synovial macrophages
- Increased expression of Wnt ligand antagonists: DKK1 and sFRP1 (in synovial fibroblasts)
- Blockage of Wnt signalling
- Decreased osteoblast development
- Inhibition of bone development
Also: Promotion of bone resorption
What are some Wnt ligand antagonists?
DKK-1
sFRP
Briefly describe what happens to the following cells in RA:
• Osteoclasts
• Osteoblasts
Osteoclasts: increased activity
→ more RANKL
Osteoblasts: impaired activity
→ more Wnt ligand antagonists: DKK-1, sFRP-1
What is the effect of TNF on synovial fibroblasts?
Increased DKK-1 expression (Wnt ligand antagonist → decreased osteoblast activity)
Increased RANKL expression (→ increased osteoclast activity)
What happens to bone in TNF.Tg mouse model of RA when DKK-1 action is inhibited?
(hTNF.Tg: mouse model that over-expresses TNF)
Observation:
• Protection from bone loss (due to OPG expression)
• Active bone formation (removal of osteoblast inhibition)
Due to:
• Production of OPG (inhibition of osteoclasts)
• Differentiation into osteblasts
Describe the signal transduction pathway in Wnt signalling
- Wnt ligand binds Frizzled receptor
- Transduction
- Stabilisation of B-catenin
- B-catenin translocates to the nucleus and turns on gene transcription
5a. Differentiation into osteoblasts
5b. OPG production, osteoclasts are inhibited
Generally, what are the effects of pro-inflammatory cytokines on osteoclasts?
1. Direct: Cytokines act directly on osteoclasts to increase their activity: • RANKL • IL-1 • TNF
- Indirect:
Cytokines act on other cells
• TNF, IL-1, IL-6
• Cells: Synovial fibroblasts, osteoblast-lineage cells, T cells
• These cells then regulate RANKL production
Describe the effect of the following on osteoclasts
• TNF
• IL-1
- TNF
Acts more on osteoclast progenitor cells
• Increased RANK expression
• Increased n° of osteoclast progenitors - IL-1
Acts more on later stages of osteoclast differentiation
• Promotion of cell fusion
• Promotion of cell survival
What are the effects of pro-inflammatory cytokines on osteoblasts?
TNF:
In general: decreased osteoblast activity
- Decreased Wnt signalling
- Decreased RUNX2 protein levels (thus decreased osteoblast differentiation)
- Decreased alkaline phosphatase expression
- Decrease osteocalcin gene expression
- Increased RANKL expression
- Decreased capacity of osteoblast lineage cells to form properly mineralised bone
- Increased apoptosis
What is RUNX2?
Key TF required for osteoblast differentiation
What is alkaline phophatase needed for?
Expressed in osteoblasts
Required for mineralisation of bone
What is osteocalcin?
Expressed in osteoblasts
One of the molecules that maintain calcium within the bone matrix
