Laboratory Investigation of Disorders of Calcium and Phosphate Metabolism Flashcards
Recap the stages of bone remodelling
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ACTIVATION
- Stimulation of osteoclast differentiation (osteoclast progenitor to osteoclast) by detecting minor stress fractures
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REABSORPTION
- Osteoclasts will dissolve old bone, digesting the collagen matrix and release minerals into ECF
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REVERSAL
- Signals to terminate osteoclast activity (osteoclast activity) and promote further osteoblast differentiation
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FORMATION
- Osteoblasts lay new bone (initially osteoid)
- Osteoid subsequently undergoes mineralisation to form new bone
What induces osteoclast differentiation?
RANK ligand binding to RANK receptor on pre-osteclasts activating transcription factor promoting differentiation of pre-osteoclasts into osteoclasts
What competes with the RANK receptor for the RANK ligand?
OPG (osteoprotogerin), a decoy receptor produced by osteocytes which prevents RANK-ligand from binding to RANK receptor, and inhibiting osteoclast differentiation to prevent excessive bone reabsorption
What is denosumab?
- Monoclonal antibody
- Prevents RANK receptor activation and inhibiting osteoclast differentiation/ activity
- Therefore prevents the reabsorption phase and excessive bone loss
What is denosumab used for?
Treatment of osteoporosis
What pathway induces osteblast differentiation?
Wnt pathway
How will osteoblasts be differentiated?
- Via the Wnt signalling pathway
- Wnt signalling protein molecule which will activate the Wnt receptor (frizzled) in presence of co-factor LRP5
- ß-catenin protein in the cytosol is released following Wnt binding and frizzled activation allowing it to act as a TF and promote specific differentiation pathways (osteoblast differentiation)
What is the negative regulation of the Wnt signalling pathway?
DKK (dickkopf) and sclerostin (SOST) proteins bind LRP5 co-receptor and prevent full activation of Wnt signalling pathway preventing osteoblast differentiation
Why is bone turnover important?
For homeostasis of calcium and phosphate
What hormones affect the homeostasis of serum calcium and phosphate?
- PTH
- Vitamin D (1,25 dihydroxy D3)
- Calcitonin
- FGF-23
What are the actions of PTH?
- Promote Ca release from bone
- Increase renal Ca reabsorption via action on DCT
- Increase renal Pi excretion from DCT
- Upregulates 1 alpha hydroxylase activity
What is the effect on PTH on bone?
PTH receptors on osteoblasts and osteoclasts, increase bone remodelling
- Increase bone formation via osteoblasts
- Increased bone reabsorption after activating osteoclasts via RANKL
What does the effect of PTH on bone depend on?
Concentration Dynamics
- Intermittent low doses are anabolic (bone formation)
- Persistent high concentration leads to excess reabsorption over formation, causing bone loss due to increased calcium release causing bone de-mineralisation and loss
What is Vitamin D?
Calcitriol
- Steroid hormone (not vitamin) synthesised in the skin in response to UV exposure
What are the actions of vitamin D (calcitriol)?
- Increase reabsorption of calcium and phosphate from GI tract
- Inhibit PTH secretion (by inhibiting transcription in chief cells in PTH gland)
- Complex effects on bone, generally in synergy with PTH
How is Vitamin D activated and converted to the active hormone calcitriol?
- 25 Hydroxylation of Vitamin D3 in liver to form 25OH D3, major circulating metabolite
- 1 alpha hydroxylase will convert 25(OH)D3 to 1,25(OH)2D3 or calcitriol, the active hormone will bind nuclear receptor and affect transcription
Where is 1 alpha hydroxylase located and what affects its activity?
Located in the kidney
- Activity is increased by
- PTH
- Low phosphate
What is the relationship betwen PTH and vitamin D
PTH will activate vitamin D which will in turn via negative feedback limit the secretion of PTH via transcriptional control
What is osteomalacia?
The loss of bone mineral component/ failure of mineralisation of the osteoid = soft bones
What is the most common cause of osteomalacia?
Vitamin D deficiency
- Usually due to a combination of low dietary intake and lack of exposure to sunlight
Who is at risk of Vitamin D deficiency?
Elderly = if in nursing home and not taking supplements
Breast fed babies kept out of sunlight
What is vitamin D dependant rickets type I?
Mutation in the 1 alpha hydroxylase enzyme
What would the hormonal levels look like with someone with vitamin D-dependant rickets type I?
What is vitamin D dependant rickets type II?
Mutation in the vitamin D receptor
- Precursor levels are normal
- Calcitriol levels will be high, however not effective because it is not adequately activating its receptor
- Calcium and phosphate will be low
- PTH will be high
What is hypophosphataemic rickets?
Rare phosphate wasting condition (excessive phosphate excretion) condition leading to bone mineralisation defects
What is the cause of hypophosphataemic rickets?
- Mutation leading to excess FGF-23 activity
- Ectopic FGF secretion (benign tumour)
What is FGF-23?
Hormone promoting renal phosphate excretion by reducing Na-Pi absorption from PCT
What is FGF-23 synthesised by?
FGF is a hormone synthesised and secreted by osteocytes
What is the structure of FGF-23?
FGF has a short half life and this half life fragment is regulated by enzymatic cleavage of the peptide into two inactive fragments
How does FGF-23 cause hypophosphataemic rickets?
- Cleavage recognition site in hypophosphataemic rickets has a single amino acid substitution (mutation) that makes it unrecognisable and the peptide isnt cleaved remaining active and promoting excessive phosphate loss
- This leads to impaired bone mineralisation and rickets
What are the actions and interactions of FGF-23?
- Increased serum phosphate will lead to an increase in FGF-23 secretion from osteocytes
- FGF-23 in turn promotes phosphate excretion (negative feedback)
- FGF-23 has an inhibitory effect on the conversion of precursor calcitriol
- Calcitriol will promote release of FGF-23
- FGF-23 inhibits PTH release
- PTH promote secretion of FGF-23
What is the effect of renal disease/ failure on bone?
Decreased renal function
- Decreased activation of calcitriol
- Decreased absorption of Ca
- Increased PTH (secondary hyperparathyroidism) which causes an excess of reabsorption over formation = bone erosion
- Decreased renal funciton also reduces H+ excretion, causing metabolic acidosis which subsequently causes bone erosion
- As renal failure progresses to the most severe stages, it can lead to renal osteodystrophy
