Lecture 28 - Rheumatoid Arthritis - Bone in RA Flashcards Preview

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Flashcards in Lecture 28 - Rheumatoid Arthritis - Bone in RA Deck (45):
1

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)

2

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

3

What are the patterns of bone loss in RA?

Where does each occur?

At what stage of disease does each occur?

1. Juxta-articular / peri-articular osteopenia
• Reduced bone mineral density
• Within trabecular bone

• Occurs very early in disease


2. Focal bone erosion
• Eating away of bone
• Contributes to bone deformities
• Occurs within cortical bone

3. 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

4

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

5

In RA there are additional sources of RANKL. What are they?

• Osteoblast lineage cells

In addition, in RA:
• Synovial fibroblasts
• T cells

6

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)

7

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

8

What is the effect of RANKL-/- on inflammation?

Not protective against inflammation

9

What is the effect of OPG.Fc treatment?

• Decreased osteoclasts
• Decreased bone erosion
• Decreased systemic bone loss

However:
No effect on inflammation

10

Which cells are responsible for bone loss in RA?

Osteoclasts are the only cells responsible for bone loss in RA

11

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

12

What are the key signals that promote osteoblast differentiation?

Wnt ligands

13

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:

1. Inflammation, TNF release from synovial macrophages

2. Increased expression of Wnt ligand antagonists: DKK1 and sFRP1 (in synovial fibroblasts)

3. Blockage of Wnt signalling

4. Decreased osteoblast development

5. Inhibition of bone development

Also: Promotion of bone resorption

14

What are some Wnt ligand antagonists?

DKK-1
sFRP

15

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

16

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)

17

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

18

Describe the signal transduction pathway in Wnt signalling

1. Wnt ligand binds Frizzled receptor
2. Transduction
3. Stabilisation of B-catenin
4. B-catenin translocates to the nucleus and turns on gene transcription
5a. Differentiation into osteoblasts
5b. OPG production, osteoclasts are inhibited

19

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

2. 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

20

Describe the effect of the following on osteoclasts
• TNF
• IL-1

1. TNF
Acts more on osteoclast progenitor cells
• Increased RANK expression
• Increased n° of osteoclast progenitors

2. IL-1
Acts more on later stages of osteoclast differentiation
• Promotion of cell fusion
• Promotion of cell survival

21

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

22

What is RUNX2?

Key TF required for osteoblast differentiation

23

What is alkaline phophatase needed for?

Expressed in osteoblasts
Required for mineralisation of bone

24

What is osteocalcin?

Expressed in osteoblasts
One of the molecules that maintain calcium within the bone matrix

25

What is critical for repair of focal bone erosion in RA?

Strict control of inflammation

Through:
• TNFi
• IL-1 inhibitors
• IL-6 inhibitors

26

Is repair of focal bone erosion common?
In which patients is it seen?

No
Only seen in 10% of bone erosion foci

Seen mostly in patients who are:
• in remission or who have low disease activity
• taking a TNF inhibitor

27

What is often seen even in RA remission?

Smouldering synovitis

Hands may look normal
However, synovitis still occurring within the joints

This may compromise the bone repair

28

What is seen in mouse models of arthritis in terms of repair of focal bone erosion?

Focal bone erosion can be repaired, BUT ONLY when synovitis and local inflammation resolves

29

Which therapeutic agents can increase repair of bone erosion?

-- Targeting inflammation --

• Methotrexate
• Anti-TNF mAbs (e.g. infliximab etc.)
• Anti-IL-6R mAb (Toculizumab)

-- Targeting bone --
1. Osteoblasts
• rhPTH: recombinant Parathyroid hormone (Teriparatide)

• Anti-sclerostin mAb

• Anti-DKK1 mAb

2. Osteoclasts
• Bisphosphonates

• Anti-RANKL mAb (Denosumab)

30

What is the name of the anti-RANKL mAb?

Denosumab

31

What is Teriparatide?

Recombinant Parathyroid hormone

Quite beneficial in targeting of bone in treatment of RA

32

Describe the effect of inflammation (i.e. TNF, IL-1, IL-6) on the following:
• RANKL
• OPG
• Wnt signalling
• Wnt antagonists
• Osteoclast activity
• Osteoblast activity
• Osteocyte

RANKL: increased

OPG: decreased

Wnt signalling: decreased

Wnt antagonists: increased

Osteoclasts: increased

Osteoblasts: inhibition of maturation

Osteocyte: we still don't know

33

What are the main drivers of the following in RA:
• Increased osteoclast activity
• Decreased osteoblast activity

Increased osteoclast activity:
• TNF stimulation of synovial fibroblasts
• Synovial fibroblast production of RANKL

Decreased osteoblast activity:
• TNF stimulation of synovial fibroblasts
• Synovial fibroblasts production of Wnt antagonist

34

What can and can't targeting of osteoclasts in RA bring about?

Can:
• Reduce bone erosion

Cannot:
• Reduce inflammation of synovium
• Resolve bone erosion

35

Where does DKK-1 come from in RA?

Synovial fibroblasts

36

What is the master regulator of bone formation?

DKK-1

37

What is the drug name for rhPTH?

Teriparatide

38

Which therapeutic agents can target osteoclasts?

• Bisphosphonates
• Denosumab (Anti-RANKL)

39

What is the effect of bisphosphonates?

Inhibition of osteoclasts

40

Which therapeutic agents can / could target osteoblasts?

• Teriparatide (rhPTH)
• Anti-DKK-1
• Anti-sclerostin

41

Where is the extra DKK-1 coming from in the RA joint?

From the synovial fibroblasts

TNF causes the synovial fibroblasts to produce DKK-1

42

What do osteocytes release?

• DKK-1
• Sclerostin
• RANKL

43

What is the only approved anabolic therapy for osteoporosis?

Teriparatide

Human recombinant parathyroid hormone

44

Why are there increased Wnt antagonists in RA?

Inflammation leads to increased expression of Wnt ligand antagonists in synovial fibroblasts

45

Why is there increased RANKL in RA?

Inflammation (TNF, IL-1, IL-6) leads to increased expression of RANKL (Early-mid stage osteoblasts, T cells, synovial fibroblasts)