Clinical Biochemistry: Laboratory investigation of disorders of calcium and phosphate metabolism Flashcards
What are Osteoblasts?
- Cells that form new bone
- They are derived from mesenchymal stem cells
What are Osteoclasts?
- Cell that digests/breaks down tissue
- They are derived from haematopoietic stem cells
What are Osteocytes?
- They are terminally differentiated osteoblasts
- They’re encased in bone mineral matrix (lacunae)
Describe the process of bone remodelling
- Small stress fractures detected by osteocytes
- Osteocytes activate osteoclasts leading to osteoclast differentiation from osteoclast progenitors
- Osteoclasts dissolve/digest old bone - specifically digest collagen matrix of old bone by secreting enzymes.
- This leads to them releasing minerals from digested bone into extracellular fluid
- Reversal occurs when apoptosis of the osteoclasts occurs at the same time as osteoblast differentiation
- Osteoblasts will then lay down new bone (osteoid)
- Osteoblasts will mineralise osteoid to form new bone
Describe the induction of Osteoclast differentiation by the RANK ligand
- RANK (receptor activator of nuclear factor kappa-B): surface receptor on pre-osteoclasts
- RANK-ligand which is produced by pre-osteoblasts and osteocytes; binds to RANK and stimulates osteoclast differentiation
-
NOTE: There’s also OPG (osteoprotogerin) present
- OPG is a decoy receptor produced by osteocytes; binds to RANK-L, preventing activation of RANK
Describe Osteoblast differentiation
- Osteoblast differentiation controlled by Wnt signalling pathway
- Complex signal pathway, highly conserved, involved in animal development
- Wnt is a signalling protein that has a receptor called frizzled
- Wnt also has a co-receptor called LRP5/6
- Wnt binds to frizzled and LRP5/6 which activates the intracellular cascades which lead to osteoblast differentiation
How is osteoblast differentiation prevented?
- Osteoblast differentiation prevented by DKK (dickkopf) and sclerostin (SOST) binding to LRP5/6 co-receptor
What other factors can promote/inhibit Osteoclast differentiation?
- Macrophage-colony stimulating factor (M-CSF) released by osteocytes PROMOTES osteoclast differentiation
- Nitric oxide released by osteocytes INHIBITS osteoclast differentiation
What other factors can promote/inhibit Osteoblast differentiation?
- PGE2, Nitric oxide and ATP released by osteocytes PROMOTE osteoblast differentiation
- SFRP1 released by osteocytes INHIBIT osteoblast differentiation
What are some sources of calcium and phosphate?
- Both calcium and phosphate are acquired from the diet
- They are absorbed via the gut
- Both calcium and phosphate absorption requires calcitriol (vitamin D)
What is the body’s response to the extracellular calcium level going down?
- PTH will increase vitamin D activation
- This will increase calcium absorption in the gut
- PTH will cause calcium reabsorption in the renal tubules
- PTH and vitamin D will promote bone remodelling which will release minerals, including calcium, into the extracellular fluid
What effect does PTH have on phosphate levels?
- PTH causes the excretion of phosphate so causes phosphate levels to decrease
What role does FGF-23 play in phosphate regulation?
- FGF-23 is secreted from osteocytes
- It promotes excretion of phosphate
- It also inhibits the actions of calcitriol (vitamin D)
What is Osteomalacia?
- Loss of bone mineralisation
What is the most common cause of osteomalacia?
- Vitamin D deficiency
- Usually due to combination of low dietary intake and lack of exposure to sunlight
Use the following information to provide a diagnosis to this patient
A 75 year old widow was investigated for bone pain and muscle weakness. She told the GP that she was vegetarian and rarely left her home.
Serum investigations: reference range
Total calcium: 1.82 mmol/l (2.20 - 2.52)
Phosphate: 0.70 mmol/L (0.75 - 1.50)
Albumin: 39 g/L (35 - 48)
Alkaline phosphatase: 187 U/L (30 - 100)
Creatinine: 69 mmol/L (60 - 110)
- Serum investigations show patient has the following:
- Low calcium
- Low phosphate
- High alkaline phosphatase (sign of increased bone turnover)
- These all suggest patient has vitamin D deficiency
- Fact she’s a vegetarian and doesn’t really leave home (lack of sunlight) further suggest vitamin D deficiency
- Lack of vitamin D would cause low Calcium/phosphate as vitamin D needed for calcium/phosphate absorption
What is Vitamin D?
- Calcitriol (really a steroid hormone - binds to a nuclear receptor, so not a vitamin)
Where is vitamin D synthesised?
- Synthesised in skin in response to exposure to UV
How is vitamin D activated?
- Vitamin D3 undergoes 25 hydroxylation in liver to form 25OH D3 (major circulating metabolite)
- 25OH D3 then undergoes 1α hydroxylation in the kidney to produce 1,25(OH)2 D3, or calcitriol, the active hormone
Describe the levels of Calcium, phosphate, 25 OH D3 and 1,25(OH)2 D3 as a result of vitamin D deficiency; renal disease; 1α hydroxylase mutation and vitamin D receptor mutation
Why are levels of calcitriol (1,25(OH)2 D3) normal during vitamin D deficiency?
- Less 25 OH D3 means less initial conversion into calcitriol (1,25 (OH)2 D3)
- Less initial calcitriol means less absorption of calcium and vitamin D
- Low calcium increases PTH levels
- Increased PTH cause increased conversion of 25 OH D3 into calcitriol which is why levels remain normal
Explain why calcitriol levels are very low as a result of a 1α hydroxylase mutation
- Calcitriol levels very low because 1α hydroxylase mutation means it’s dysfunctional
- This means 25 OH D3 can’t be converted into calcitriol 1,25 (OH)2 D3
Explain why calcitriol levels are very high as a result of a vitamin D receptor mutation
- No problem in 25 OH D3 being converted into calcitriol
- Mutation in vitamin D receptor means calcitriol can’t perform its effects
- Because of this calcium and phosphate reamain low as caclitriol needed for their absorption
- This means PTH reamins high and so will increases conversion of 25 OH D3 into calcitriol
What is hypophoaphataemia?
- Low levels of phosphate
Describe the levels of Calcium, phosphate, 25 OH D3 and 1,25(OH)2 D3 as a result of: X-linked hypopho-phataemic rickets; Autosomal dominant hypopho-phataemic rickets and Oncogenic osteomalacia
What is FGF-23?
- A hormone secreted by osteocytes that ahs a central role in phosphate homeostasis
What is Hypophosphatemic rickets?
- Rare phosphate-wasting conditions leading to bone mineralization defects (osteomalacia)
Describe the structure of FGF-23
- Precusor 251 amino acids long
- Active form 226 amino acids long
- Cleavage site present in active peptide causes 2 inactive fragments to be fromed when enzymatically cleaved
How does the mutation in FGF-23 that leads to Autosomal dominant hypopho-phataemic rickets affects its half-life?
- Mutation in cleavage site of FGF-23 means it’s not recognised by enzymes
- This means active form of FGF-23 remains in circulation (increased half-life)
Describe the effects of FGF-23 on Phosphate, PTH and calcitriol (vitamin D)
- Increase in phosphate causes increase in FGF-23
- FGF-23 causes increased excretion of phosphate (negative feedback)
- FGF-23 inhibits conversion of 25 OH D3 into calcitriol (vitamin D)
- Increase in calcitriol causes increase in FGF-23
- PTH causes increase in FGF-23
- FGF-23 causes decrease in PTH
Explain how renal osteodystrophy develops
- Decrease in renal function will decrease activation of calcitriol
- This will cause a decrease in absorption of calcium from gut and in calcium reabsorption in the kidneys
- Both these effects cause a decrease in Plasma calcium level
- This causes an increase in PTH
- Maintained high level of PTH leads to bone erosion
- Decrease in renal function also causes decrease in H+ excretion which leads to metabolic acidosis
- Metabolic acidosis also leads to bone erosion
*
