Lecture 27 - Rheumatoid Arthritis - Bone Flashcards Preview

BIOM30002 - M2M > Lecture 27 - Rheumatoid Arthritis - Bone > Flashcards

Flashcards in Lecture 27 - Rheumatoid Arthritis - Bone Deck (62):
1

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

2

What is the main protein in bone?

Collagen

3

What is the composition of bone?

1. 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

3. Water - 25% A.B.M.

4

Is bone permanent?

No
5-10% of skeleton replaced each year

5

Where does bone remodelling occur?

Characterise the initiation

Occurs at 1-2 million microscopic foci

Initiation is asynchronous

6

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

7

Describe the role of bone in mineral ion homeostasis

1. Decreased Ca in blood
2. Bone 'resorbed', releasing Ca into blood
and vice versa

8

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

9

Describe the process of bone remodelling

1. Osteoclasts present on bone surface
2. Resorption of a packet of bone, leaving 'lacunae'
3. Osteoblasts come in and release collagen
4. Unmineralised bone matrix now formed: "Osteoid"
5. Minerals bind the osteoid to form bone

10

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

11

What is Osteoid?

Unmineralised bone matrix laid down initially by the osteoblasts
Composition:
• Collagen
• Bone sialoprotein
• Osteocalcin

12

What is needed to maintain healthy bone mass?

Balance between bone resorption and bone formation

i.e. between Osteoblast and Osteoclast activity

13

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

14

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

15

What is the clinical symptom of increased osteoclast activity?

Osteoporosis

• Weakened bones
• Thinner trabeculae

16

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

17

Describe the cellular features of osteoclasts

Multinucleated

Many mitochondria
• High energy needs required for bone resorption

18

Describe bone resorption by osteoclasts

1. Formation of 'sealing zone'

2. Acidification
• Forms the Ruffled border
• Dissolves the mineral component

3. Release of collagenases
• Degrades the organic component

4. Degradation products taken up by the osteoclasts

19

How do osteoclasts attach to the surface of bone?

Integrins

20

Describe the differentiation into osteoblasts

1. Mesenchymal stem cell origin

2. Differentiation down the osteoblast lineage

3. Preosteoblast

4. Mature osteoblast

3 fates:
• Osteocytes: embedded in the bone
• Lining cell (Fx unknown)
• Apoptosis

21

What do mesenchymal stem cells give rise to?

• Osteoblasts
• Chondrocytes
• Muscle cells
• Adipose cells

22

What are the functions of osteoblasts?

• Bone formation
• Support of osteoclasts

23

Describe how osteoblasts promote and inhibit osteoclasts

Promotion: RANKL
• From the early-mid stage osteoblasts

Inhibition: OPG production
• From the mid-stage-mature osteoblasts

24

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

25

Expession of which molecule is vital for rendering bone competent for mineralisation?

Alkaline phosphatase

26

Describe differentiation into osteoclasts

1. Haematopoietic stem cell

2. Myeloid precursor

-- M-CSF signalling --
• Myeloid precursor proliferation

3. Osteoclast precursor
• expressing RANK

-- RANKL expression from osteoblasts --
• Binds to RANK on osteoclast precursor

4. Mono-nucleated precursors

-- fusion --

5. Bone-resorbing osteoclast
• Multinucleated
• Attached to bone surface

27

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)

28

What is another name for RANKL?

Receptor activator of NFκB-ligand

29

An osteoclast is only an osteoclast when it is...

Sitting on the bone surface

30

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

31

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

32

What type of molecule is RANKL?

Member of TNF superfamily

33

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

34

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

35

Why is RANKL 'sufficient'?

It is the only factor required for osteoclast differentiation and function

36

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

37

What happens when there is an imbalance between RANKL and OPG?

Increased RANKL:
• Net bone loss

Increased OPG:
• Net bone gain

38

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

39

Which molecules are important for differentiation into osteoblasts?

Wnt ligands
(among others)

40

Describe Wnt signalling

1. Wnt ligand binds Frizzled receptor
Associated with LRP5/6

2. Stabilisation of β-catenin

3. β-catenin translocates into nucleus and turns on gene transcription

→→→

4. Increase in osteoblast activity:
• Increased OPG production
(→ decrease in osteoclast activity)

41

What two processes does Wnt signalling bring about?
How does it do this?

1. Increased osteoblast activity

2. Decreased osteoclast activity
• Expression of osteoprotegerin

42

Describe regulation of Wnt signalling

1. Antagonists of LRP5/6
• DKK1
• Sclerostin

2. 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

2. Increased osteoclast activity
• loss of OPG expression

43

What is sFRP-1?

Secreted frizzled related proteins

Soluble decoy receptor for Wnt ligands
Prevent Wnt from signalling
→ decreased osteoblast activity

44

What is the most abundant bone cell?

Osteocytes

45

Describe intercellular communication in osteocytes

Canaliculi:
• Dendrite-like cell processes

Enable intercellular communication with:
• Other osteocytes
• Osteoblasts
• Osteoclasts (at the bone surface)

46

Which cells are the 'mechanostat'?

Osteocytes

47

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

48

Describe how osteocytes regulate bone formation

Express DKK-1 and sclerostin

These molecules inhibit Wnt signalling → decreased osteoblast activity → inhibition of bone formation

49

What is sclerostosis?
Describe the disease pathogenesis

Mutation in sclerostin
→ Increased Wnt signalling
→ Over-expression of OPG
→ Under-active osteoclasts
→ Increased bone mass

→ Osteosclerosis

50

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)

51

Do osteocytes produce RANKL?

Still controversial:
• Some say yes, and some no

The key source of RANKL is osteoblasts

52

What is seen in bones in people with RA?

Osteoporosis

53

What are some approved therapeutics for Osteoporosis?

-- Targeting osteoclasts --

1. Bisphosphonates
• Inhibit osteoclast activity
• may lead to osteoclast apoptosis
• First port of call

2. Anti-RANKL antibody
• Injected every 6 months
• Blocks action of RANKL
• Expensive, not as widely used as bisphonates

-- Targeting osteoblasts --

3. 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

54

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

55

Describe the mode of action of rPTH

(Recombinant parathyroid hormone)

1. Stimulation of osteoblast precursors:
• Mesenchymal stem cells
• Osteoprogenitor

2. Downregulation of:
• Sclerostin
• DKK-1

→ Promotion of osteoblastic bone formation

56

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

57

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

58

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

59

What is the role of M-CSF?

Proliferation of myeloid precusors and survival of osteoclast progenitors and mature osteoclasts

60

Where do Wnt ligands come from?

They are made locally within the bone microenvironment

e.g. By Osteoblasts

61

Where do osteocytes reside?

Within the bone matrix, in the lacunae

62

What is the name of anti-sclerostin mAb?

Romosozumab