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BIOM30002 > lecture 28 > Flashcards

lecture 28 Flashcards

1
Q

What are learning objectives?

A
  • describe the different forms of bone loss in RA
  • how does RA affect osteoclast differentiation and function?
  • how does RA affect osteoblast differentiation and function?
  • how is RANKL and OPG expression regulated in RA?
  • how is Wnt signalling regulated in RA?
  • how do IL-1 and TNF affect bone cells?
  • why is it important for inflammation to be controlled in RA for bone health? how would you detect if inflammation is completely resolved within in a joint?
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2
Q

What do factors produced by inflammatory cells cause?

A
  • destruction of cartilage and bone

- proinflammatory cytokines e.g.: TNFa, IL-1, IL-6, IL-17, RANKL

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3
Q

What are patterns of bone loss in RA?

A

Juxta-articular/peri-articular osteopenia
- occurs early in disease within the trabecular/cancellous bone near the affected joint

focal bone erosion
- occurs within cortical bone within the arthritic joint, initially at junction of cartilage and bone

systemic osteoporosis
- present in many of the patients: thinning of trabecular/cancellous bone and cortical bone at sites remote from affected joints e.g. hip, vertebrae

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4
Q

What is osteopenia?

A
  • reduced bone mass
  • not as severe as osteoporosis
  • loss of opacity in radiograph
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5
Q

When were osteoclasts identified as the cell responisble for bone erosion?

A

1998

  • done by staining RA tissue sections for markers identifying osteoclasts
  • RNA probe to a particular gene
  • in situ hybridisation
  • tartrate resistant acid phosphatase mRNA
  • cathepsin K mRNA
  • key osteoclast differentiation factor: calcitonin receptor mRNA (only seen in osteoclasts on the bone)
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6
Q

What are the multiple cellular sources of RANKL in RA synovium?

A
  • osteoblast-lineage cells
  • synovial fibroblasts
  • T cells
  • (B cells… not as prolific)
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7
Q

What do we see in immunohistochemistry of RA patients?

A
  • TRAP: multinucleated cells on bone surface
  • RANK: multinucleated cells expressing RANK, cells in infiltrative tissue that are rank+
  • RANKL: focal in expression pattern, at the sites where osteoclasts are resorbing bone

OPG: completely absent at bone erosion sites

RANKL expression “outweighs” OPG expression at the pannus-bone interface in RA

net bone loss

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8
Q

What is the bone phenotype of mice that have no functional RANKL?

A

Osteopetrotic

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9
Q

The RANKL ko-mice have?

A

No functional osteoclasts and thick dense bone

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10
Q

Compared to their wild type littermates what happens to bone when RA is induced in RANKL KO?

A

RANKL KO mice are protected from bone loss

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11
Q

So what is seen in mice that lack RANKL?

A
  • resistant to arthritis induced focal-erosion but have similar severity of synovial inflammation compared to Wild type (WT) mice
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12
Q

What was seen in OPG-Fc treatment?

A
  • reduces osteoclasat nmbers and focal bone erosion and systemic bone loss in animal models of RA
  • done in both hTNF.Tg mice and CIA rats
  • decreased number of osteoclasts within the inflamed joints
  • leads to decreased bone erosion and decreased systemic bone loss
  • no effect on inflammation
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13
Q

What is osteoclast differentiation and function in RA?

A
  • osteoclasts are the only cell responsible for bone erosion in RA
  • at pannus-bone interface there is increased expression of RANKL relative to OPG, promoting osteoclast differentiation and function at this site
  • in RA, synovial fibroblasts and T cells as well as osteoblast-lineage cells are ADDITIONAL sources of RANKL
  • inhibition of RANKL (either by gene deletion or blockade with OPG.Fc) protects against focal bone erosion in mouse arthritis models but has NO effect on synovial inflammation
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14
Q

Osteoblast differentiation and function in RA?

A
  • net bone loss: either osteoblasts can’t keep up or they are not functioning properly
  • effective therapy –> attenuated focal bone erosion
  • but erosive lesions often persist
  • suggests there is something wrong with the osteoblasts at that site
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15
Q

How is osteoblast maturation/bone formation impaired in RA?

A
  • at the interface of inflammation and bone

mouse models:

  • injected fluorochromes incorporated into newly formed bone shows lack of bone formation at bone surfaces adjacent inflammation
  • lack of mature osteoblast-lineage cells at bone surfaces adjacent inflammation: prevalence of Runx2+ osteoblast-lineage cells but these lack the more mature osteoblast markers osteocalcin and alkaline phosphatase expression
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16
Q

What is increased in inflammatory arthritis?

A
  • expression of multiple antagonists of Wnt signalling
  • inhibits osteoblast maturation and function
  • in synovial tissue isolated from mouse arthritic joints:
    • Increased DKK1/2/3
    • increased sFRP1/2/4

RA patients –> increased serum DKK1

TNF –> increased DKK1 in synovial fibroblasts

17
Q

What happens to Wnt signalling in RA?

A

inflammation –>
increased Wnt antagonists e.g. DKK1, sFRP1
decreased activation of Wnt signalling

inhibitory to bone formation:
promotes bone resorption (increased RANKL:OPG)

net bone loss

18
Q

What happens to osteoblast differentation and function in RA?

A
  • bone formation is impaired at bone surfaces adjacent inflammation (pannus) in RA
  • osteoblasts fail to mature properly leading to failure to form proper mineralised bone efficiently
  • expression of Wnt signalling antagonists (including DKK proteins and sFRP proteins) is increased in inflamed synovial tissue (including pannus)
  • -> inhibition of Wnt signalling at these sites
  • -> inhibition of osteoblast maturation and bone formation at these sites
  • DKK1 expression is increased by TNF in synovial fibroblasts
19
Q

What was seen in inhibition of DKK1 in hTNF.Tg mouse model of RA?

A
  • protection from bone loss due to active bone formation and reduced osteoclasts
  • protection from bone erosion (increase in OPG, inhibition of osteoclasts), increase in bone formation and osteophyte formation
20
Q

How do cytokines/factors produced by inflammatory infiltrat act in RA?

A
  • augment osteoclastogenesis

direct effect on osteoclasts

  • RANKL –> OCL differentiation
  • IL-1
  • TNF

indirect effect on osteoclasts

  • TNF
  • IL-1
  • IL-6
  • IL-17
  • PGE2
  • IL-11
  • oncostatin M
  • IL-7

–> osteoblast-lineage cells/synovial fibroblasts and T cells –> increase RANKL:OPG –> OCL

21
Q

What is the effect of TNF and IL-1 on osteoclast differentiation?

A
  • directly augment
  • increase RANK expression on osteoclast progenitors, increase number of osteoclast progenitors with increasing TNF
  • TNF acts more on osteoclast precursors
  • IL-1 acts more at later stages
  • promotes cell fusion and survival
22
Q

What is the direct effect of TNF on osteoblasts in vitro?

A
  • decreased Wnt signalling in the osteoblast-lineage cell
  • decreased RUNX2 protein levels (TNF signalling targets RUNX2 for degradation) (this prevents further differentiation)
  • decreased alkaline phosphatase (needed for proper mineralisation of bone matrix) and osteocalcin (helps maintain Ca++ within bone matrix) gene expression
  • increased RANKL expression (promoting osteoclastogenesis)
  • decreased capacity of osteoblast-lineage cells to form properly mineralised bone
  • increased apoptosis

role of IL-1 in bone formation in the context of RA has not been thoroughly investigated

23
Q

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

A
  • strict control of inflammation and synovitis is critical for repair of focal bone erosion in RA

in human studies of RA:

  • evidence for repair of focal bone erosions limited to ~10% erosions
  • -> most in patients who are in remission, or who have had low diease activity
  • smoldering synovitis, detected by MRI in clinically “normal” joints, is still an issue
  • -> may compromise bone repair, even when patients appear to be in remission
  • “healing of erosions” more common in patients treated with TNF inhibitor

in mouse model of arthritis:

  • focal bone erosions do repair
  • BUT ONLY when synovitis and local inflammation resolves
24
Q

Could bone repair in RA be improved by targeting bone in addition to inflammation control?

A

targeting inflammation

  • methotrexate (and other DMARDS)
  • anti-TNF (e.g. infliximab, etanercept, adalimumab)
  • anti-IL-6R (toculizumab)
  • CTLA4-UG (abatacept, T cells)
  • Rituximab (anti-CD20, B cells)

targeting bone

  • bisphosphinates
  • anti-RANKL Ab (denosumab, approved for post-menopausal osteoporosis)
  • rhPTH 1-34 (teriparatide)
  • anti-sclerostin (not yet)
  • anti-DKK1 (pre-clinical for osteoporosis)

BIOM30002 (36 decks)

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