Lab Investigation of Disorders of Calcium and Phosphate Metabolism

Question 1

The bone remodelling cycle

Answer 1

1. Microdamage or mechanical stress which stimulates the recruitment, differentiation and activation of osteoclasts.
2. Osteoclastic bone resorption
3. Reversal - osteoclasts die by apoptosis
4. Osteoblastic bone formation - migrate to the area of resorbed bone and replace it with unmineralised osteoid which then becomes mineralised.

Question 2

RANK

Answer 2

A receptor activator of nuclear factor Kappa-B. It has surface receptors on pre-osteoclasts, stimulates osteoclast differentiation.

Question 3

What produces RANK ligand?

Answer 3

Produced by pre-osteoblasts, osteoblasts, osteocytes which binds to RANK and stimulate osteoclast differentiation.

Question 4

OPG

Answer 4

Osteoprotogerin is produced by osteocytes and inhibits the RANK-L system

Question 5

Wnt signalling pathway

Answer 5

• This is a complex pathway, whih is highly conserved.
• This is involved in animal development.
• This is required fror osteoblast differentiatoin.
• It is negatively regulated by DKK (dickkopf) and sclerostin (SOST).
• The molecule Wnt binds to the frizzled receptor with the co-factor LRP5/6.
• In adult animals, Wnt is involved in growth, ddifferentiation and maintenance of many tissues, including bone.
• Wnt is signalling is under negative control by various proteins.

Question 6

Short term function of bone remodelling

Answer 6

Releases minerals, such as calcium into the circulation and therefore it can be controlled in the short term.

Question 7

Where is calcium located in the body?

Answer 7

99% of body calcium is in the bone
Remaining 1% is mainly intracellular
Hormonal control of tiny (<0.1%) extracellular fraction is what maintains Ca balance

Question 8

Where is phosphate located in the body?

Answer 8

85% in the bone, remainder is in the intracellular

May fluctuate more than Ca2+

Question 9

Osteomalacia

Answer 9

Vitamin D deficiency is the most common cause. Usually due to combination of low dietary intake and lack of exposure to sunlight.

Question 10

Who is at risk of osteomalacia?

Answer 10

Elderly at risk; especially if in nursing home and not taking supplements
Breast fed babies kept out of sunlight

Question 11

What is vitamin D?

Answer 11

Calcitriol which is a steroid hormone not a vitamin

Question 12

Synthesis of vitamin D

Answer 12

Synthesised in the skin in response to exposure to UV

Question 13

Activation of vitamin D

Answer 13

• 25 hydroxylation in the liver to form 25PH D3 which is a major circulating metabolite.
• 1a hydroxylation of 25OH D3 in the kidney produces calcitriol which is the active hormone

Question 14

Vitamin D deficiency

Answer 14

• Low Ca and Pi because there is no absorption of minerals
• Low 25OH D3 because it is a precursor molecule
• Low levels of Ca leading to the production of PTH
• hence there is normal calcitriol

Question 15

Renal disease

Answer 15

• In renal disease, there is low Ca because the kidney are not able to undergo it’s normal function to reabsorb Ca

Question 16

1alpha hydroxylase mutation

Answer 16

• Low Ca and Pi because there is not reabsorption
• There are normal levels of the precursor but low levels of the active product because there is no conversion to the active product.
• There is high levels of the PTH to try and compensate.

Question 17

Vit D receptor mutation

Answer 17

• Low levels of Ca and Pi again due to lack of absorption
• There are very high levels of calcitriol because of the high levels of PTH.
• The lack of absorption however is due to the receptor being non-functioning and therefore the functions and therefore not able to take place.

Question 18

Examples of Hypophosphataemia diseases

Answer 18

• X-linked hypophophataemic rickets
• Autosomal dominant hypophophataemic rickets
• Oncogenic osteomalacia

Question 19

Hypophosphataemia diseases

Answer 19

• Low/normal Ca and Low Pi
• Normal 25OH D3
• 1,25 (OH)2 D3 Low/normal
• High/normal PTH
• FGF-23 is high

Question 20

What FGF-23?

Answer 20

• It is a protein that acts on receptors on the kidneys.
• It is expressed and secreted by osteocytes.
• It is normally degraded and cleaved early because it has a short half life.
• Takes part in phosphate homeostasis and therefore when it is not degraded it leads to phosphate wasting.

Question 21

What happens if there is a mutation in FGF-23?

Answer 21

If there is a mutation in the gene coding for the FGF-23 protein, then it can lead to the protein not having a short half life anymore and therefore not degraded.

Question 22

What happens if FGF-23 is not degraded?

Answer 22

This leads to hypophosphatemic rickets which is a rare phosphate wasting condition which leads to bone mineralisation such as osteomalacia

Question 23

Increase Pi

Answer 23

Increase in Pi in the circulation leads to increase in secretion of FGF-23. This then acts in a negative feedback way to lower to Pi.

Question 24

Renal osteodystrophy - Bone erosion

Answer 24

1. Low renal function leads to low H+ excretion
2. This leads to metabolic acidosis which leads to bone erosion.
3. Furthermore, less vitamin D production will lead to low plasma Ca2+ levels in the body.
4. This will lead to high levels of PTH being produced.
5. This will lead to bone erosion.

Question 25

Renal phosphate reabsorption

Answer 25

• Sodium-phosphate co-transporter

- Requires association with Na-H exchanger regulatory factor (NHERF).