Control of calcium metabolism.
Calcium and phosphorus homeostasis
Primary elements of blood tissue are calcium
(Ca) and phosphorus (P).
- up to 65 % of bone weight
- almost all Ca and P supply, half of supply
of Mg in human body
- Essential role of these elements in
physiological processes
Bone tissue
- 99 % of overall Ca, of it 99 % in mineral
component
- 1 % - quickly mobilizable and convertible
(ICF - ECF)
Extra- and intracellular calcium
Intracellular calcium
- Signaling role
- Contractility
- Excitability
- Neurosecretion
- Endocrine and exocrine secretion
- Cell differentiation and proliferation
- Cell death and its regulation
Extracellular calcium
- Cartilage and bone mineralization
- Cofactor of enzymes including
proteins of coagulation cascade
- „Source“ of intracellular calcium
- Excitable tissues
ICF
ECF
MIT ER
cca 1mM
cca 100 nM
cca 100 mMcca 25 mM
Ca2+-ATPase
Ca2+ IK
Na+/Ca2+ exchanger
VDAC/
Ca2+-uniporter
mNCX
Calcium and its intake
Stomach
- Gastric juice and role of HCl
- Signalization connected to HCl
production
Calcium absorption
- 25 – 60 %
- Age
- Dietary habits and calcium
content in diet
- Bone tissue requirements
- Vitamin D
Age-related negative calcium balance is an osteoporosis risk factor.
Small intestine
- Duodenum a jejunum – 90 %
- Adaptive intake – duodenum
and ileum
Mechanisms of calcium absorption
Paracellular
- Luminal electrochemical gradient
- Integrity of intercellular connections
- Claudins and their role in paracellular
transport
Transcellular
- TRPV6 and associated proteins
- Recyclation of TRPV6
- Alternative mechanisms?
VitaminD
Glucocorticoids Estradiol Prolactin
+ +-
Adaptation to dietary calcium levels
Calcium on blood (calcemia)
GlomerularfiltrationYES
Endocrinecontrol
GlomerularfiltrationNO
Calcium excretion
- 98 % of filtered Ca is reabsorbed
- 70 % proximal tubule
- 20 % thick ascending limb of HL
- 5 % collecting duct
- 2 % urine
- CaSR (TALH)
- Paracellin-1
- PTH
Phosphorus
Blood
- Concentration 1 mM (serum)
- Ionized form (HPO4
2-, H2PO4
-)
- 12 % protein complexes
- Intracellular concentration approximately
same as extracellular
- Cotransport with sodium
Distribution
- Bones cca 45 % - Ca10(PO4)6(OH)2
- Organic and inorganic form in ICF and ECF
- Age, sex, growth
Functions
- Structural – NA, phospholipids
- Modified saccharides, phosphoproteins,
cofactors, G proteins
- Macroergic compounds (ATP)
- Regulatory role – signaling cascade,
energetic processes
Vitamin D PTH IGF-1 FGF23
Kidneys
- Reabsorption - proximal tubule (85 %) – Npt1-3
Magnesium
Blood
- 0.7 – 1 mM
- Approx. 30 % in protein complexes
- 15 % in phosphate and low molecular
weight anion complexes
- 55 % free
Distribution
- 1 mol - bones approx. 54 %, muscles and
soft tissues approx. 56 %
- ECF – 0.5 mM
Cell
- 95 % in ATP and similar molecules
- Concentration 0.5 mM
- Ion channels?
Functions
- Cofactor (glycolytic, kinase and
phosphatase systems)
- Stabilizing function (DNA, RNA,
ribosomes)
- Activator of ATP transporters
- Neuromuscular excitability
Kidneys
- 95 % of filtered amount is
reabsorbed
- 15 % PT, 70 % cortical
TAHL, 10 % DT
- Regulation – magnesemia,
calcamia, ECF volume
enterocyt
Bone tissue physiology
www.creab.org - Human Body Anatomy - Online anatomy atlas. Viktoria
Ruppel. 14. 3 2015
Trabecular (spongy) bone – cca 20 %
- High surface-to-volume ratio
- High metabolic activity
- Nutrients diffuse from ECF to trabecules
Compact (cortical) bone – approx. 80 %
- Low surface-to-volume ratio, osteocytes in resting
state
- Haversian canals with concentric layers of collagen –
osteons (Haversian systems)
- Collagen matrix impregnated with bone mineral
crystals
- 20 x 3-7 nm, mainly hydroxyapatite
Bone matrix and bone mineral
Collagen type I = most
important protein of
bone matrix
Trombospondin
Fibronectin
Matrix Gla protein
Osteocalcin
Biglykan
Decorin
Bone sialoprotein
Osteopontin
Osteoadherin
Vitamin K-dependent g
carboxylation and
phosphorylation
Ca affinity and
mineralization
Signaling+haematopoiesis
Collagen and its synthesis
Collagen synthesis
Collagen degradation
Cathepsin K
MMP
Collagenases – MMP
- MMP1 and MMP13 – osteoblasts
- Role of hormones and cytokines
Mineralization
= production of small hydroxyapatite crystals (Ca,
phosphates, carbonates, Mg, Na, K)
External mechanism – alkalic
phosphatase
Internal mechanism – phospho1
(Phosphoethanolamine/
phosphocholine phosphatase )
Cleavage of pyrophosphate
Phosphate availability for mineralization
Vesicle formation (matrix)
Collagen and its
arrangement
Ca, P, and AF availability
Diet, calcium in diet, calcium/phosphorus in ECF
Deposition of calcium
SIEBLINGs
- Osteopontin, DMP-1 (OC)
- Bone sialoprotein, MEPE
Endopeptidases, PHEX – FGF23
Bone tissue and its remodeling
osteoclasts mononuclear cells preosteoblasts osteoblasts
Bone reabsorption Building of bone tissue
Modeling
versus remodeling
of bone tissue
REMODELING UNIT - BMU
osteoblasts osteoclasts osteocytes
Bone lining cells
Bone tissue and its remodeling
Osteocytes (OC)
- Metabolic activity
- PTH receptors
- Communication with bone surface
- Mechanic sensing
- RANKL production
- Direct degradation of bone tissue
(osteocytic osteolysis)
- Adaptive remodeling
Osteoblasts (OB)
- Bone matrix production
- Production of collagen and noncollagen
peptides + their
orientation
- Regulation by hormones, local
factors and cytokines
- Differentiation and further fate –
apotosis, osteocytes, lining cells
- „recruitment“ of other cells – IGF-
1, IGF-2, TGF-b
Lining cells
- Stimulation of OB
differentiation
- OC communication
- Differentiation to OB
stimulated by PTH
Osteoclasts (OK)
- Bone tissue
reabsorption
Remodeling unit - BMU
1. Stimulatory and inhibitory signals of
osteocytes (oncostatin M - OSM,
sclerostin, PTHrP)
2. Stimulatory and inhibitory signals of
osteoclasts to osteocytes (TGF-b, IGF-1,
cardiotropin-1, Sema4D – semaforin
4D, sfingosin-1 phosphate)
3. Signalling between osteoblasts
(ephrinB2, EphB4, Sema3a, PTHrP,
OSM)
4. Stimulatory and inhibitory signals
between osteoblasts and osteoclasts
and their derivatives (RANKL, Sema3B,
Wnt5a, osteoprotegerin - OPG)
5. Signalling between haematopoietic
stem cells and osteoblasts
(macrophage-produced OSM, IL
produced by T-cells, RANKL)
Osteocyte origin
Osteocyte
- Changes in metabolic activity
- Formation of „projections“ –
communication
- Communication with other
osteocytes (syncytium – OC +
OB)
Osteoclasts
cytokines
Expression of different
receptors in time
(effect of various stimuli)
Production of mixture of
pro- and anticlastogenic
factors (differences in time)
Glucocorticoids – indirect
inhibition of bone resorption
E2 (estrogens) – inhibition
of T cell activation=
inhibition of RANKL and
TNF-a secretion
Sex hormones – regulation
of osteoblasts and
osteoclasts differentiation,
including length of their life
Key factor regulating bone
resorption is RANKL/OPG
ratio.
Osteoclastogenesis
(+) RANKL
(-) OPG
Bone tissue resorption by osteoclasts
Role of compartmentalization in bone
resorption - podosomes
Resorption and secretion of bone
resorption products - transcytosis
Essential role of pH for bone tissue
resorption
Factors influencing bone tissue remodeling
Osteoblasts
Lining cells
Osteoclasts
Remodeling of bone tissue
Immediate calcium need - homeostasis
Ensuring mechanical requirements
Trabecularbone
Systemic signals Local signalsResorption takes approx. 2 weeks
Mineralization and formation approx. 12 weeks
In pathophysiologic conditions is
disrupted the continuity of bone
tissue resorption and formation.
Cytokines - IL-1α, IL-1β,
TNF-α, TNF-β,
proinflammatory IL (7,
15, 17)
Cytokines - IL-4, IL-13, IL-
10, IL-18
TGF-α and EGF, FGF21,
FGF23
Prostaglandins
Prostaglandins
PDGF
VEGFA, HIF-1a (+/-)
IGF-1 (endo-/paracrine)
BMPs (OB, autocrine)
Endocrine
regulation of
bone tissue
Hormone Effect Target cells
PTH - Stimulation of resorption (long-term effect)
- Stimulation of bone formation (pulsatile effect)
- Stimulation of local secretion of IL-1 and IL-6
Osteoblasts, lining cells,
osteocytes
Vitamin D - Stimulation of resorption (higher concentration)
- Inhibition of mineralization (higher concentration)
- Stimulation of bone formation (low concentrations, with PTH)
Osteoblasts (primarily)
Calcitonin - Inhibition of resorption
- Regulation of bone tissue remodeling
Osteoclasts
Growth hormone
IGF-1
- Stimulation of bone turnover
- Stimulation of osteoblast proliferation and differentiation
- Increased synthesis of collagen and other proteins
Osteoblasts – primarily GH
Osteoblasts and osteoclasts –
IGF-1
Glucocorticoids - Decreased absorption of Ca in GIT
- Induction of osteoclastogenesis
- Increased bone resorption (+ RANKL)
- Suppressed remodeling of bone tissue
- Induction of apoptosis in osteoblasts and osteocytes
- Inhibition of IGF-1 synthesis
Osteoblasts, osteocytes,
osteoclasts
Thyroid hormones - Children – Stimulation of mineralization and epiphyseal maturation
- Adults – increased resorption
- Chondrocyte growth and proliferation (permissive effect on growth
hormone)
- Increased transcription of collagenase and gelatinase
Osteoblasts, osteoclasts (also
indirect through TSH)
Insulin - Stimulation of bone tissue formation and mineralization
- Increased collagen synthesis
- Stimulation of IGF-1 secretion
Primarily osteoblasts
Sex hormones - Epiphyseal closure (E)
- Inhibition of RANKL secretion
- Changes in speed of bone resorption and formation (stimulation of
formation and mineralization)
Primarily osteoblasts, also
other bone cells
Prolactin - Indirect effect
Insulin – osteocalcin axis
insulin
Decarboxylated OCN
(Glu13-OCN)
leptin
Bone tissue metabolism markers
Ionized calcium 8.5 – 10.5 mg/dL
Plasmatic phosphates 3 – 4.5 mg/dL
PTH 10 – 65 pg/mL
Vitamin D 30 – 100 ng/mL
Clinical relevance
• Osteogenesis imperfecta
• Osteopetrosis
• Osteomalacia
• Rachitis
• Osteopenia – T score -1 – -2.5
• Osteoporosis – T score under -2.5
Parathormone
Characteristics
- Parathyroid glands – chief cells
- Synthesis and storage of PTH
- Very quick secretion of PTH
- Ability to proliferate during
long-term stimulation
PTH
- Synthetized as pre-pro-PTH
- Several types of secretion granules (PTH; PTH+cathepsin B, H)
- Very quick metabolization (70 % liver, 20 % kidneys) – 2 min
- Presence of several types of fragments
- PTHR1, PTHR2, PTHR3 – G prot.
PTH secretion
Level of ionized calcium in blood is a key parameter for PTH secretion.
Minimal
secretion
Maximal
secretion
(reserve
capacity)
During sudden decrease of ionized calcium is PTH secretion increased.
Vitamin D decreases PTH secretion (inhibits expression and production of PTH), NOT during
chronic hypocalcemia
Phosphates stimulate production and secretion of PTH with delay.
Cell proliferation of chief cells is an important adaptive mechanism for:
- Hypocalcemia
- Low levels of vitamin D(1,25(OH)2D3)
- Hyperphosphatemia (uremia)
- Neoplastic growth
Calcium sensing receptors - CaSR - and PTH secretion
CaSR – G-protein coupled receptor
- Activation of PLC
- Inhibition of cAMP production
Various distribution in tissues – all tissues
participating in calcium homeostasis
- Parathyroid glands
- Kidneys
- Skin
- GIT epithelium, enterocytes
- G cells of stomach
- CNS
Clinical aspects
- Mutation – inactivation/activation
- familial hypocalciuric hypercalcemia (in.)
- Familial hypoparathyroidism with
hypercalciuria (ac.)
- Calcimimetics – inhibition of PTH secretion
+ amino acids, peptides, Mg
Main effects of PTH
(+) calcium resorption
- cTAHL, DT
- transcellular and paracellular transport
- TRPV5 and TRPV6 – Ca2+ inhibition
- Calbindin-D28K
- NCX1 and PMCA
(+) phosphate excretion
- PT and DT
- Inhibition of resorption
- NaPi cotransporters – internalization, degradation
(+) activity of 1a-hydroxylase - PT
(-) resorption of Na, water and bicarbonate – PT
(-) Na+/K+-APTase (basolateral membrane)
(+) gluconeogenesis – PT
(-) GFR - podocytes
Effect of PTH on osteoclasts is indirect. Pulsatile secretion stimulates osteoblasts, chronic continual osteoclasts.
PTH and bone tissue physiology
+
differentiation
of osteoblast
precursors
+ secretion of
paracrine and
autocrine
factors (IGF-1)
(-) apoptosis
(+) osteocytes
– release of
Ca, osteocytic
osteolysis
Clinical application – osteoporosis therapy
Parathyroid Hormone-Related Peptide - PTHrP
Characteristics
- First as a peptide produced by tumors – endocrine
effect – kidneys + bones
- Also paracrine – local increase of Ca concentration
- Later discovered in many tissues
Functions
- Calciotropic hormone
- Fetal development - proliferation and differentiation
- Lactation – (+) resorption of bone tissue without
possibility to affect by Ca supplementation
- Skin – proliferation and differentiation
- GIT, bladder, uterus – (+) smooth muscles relaxation
- CNS - neuroprotection
- Para-/auto-/intracrine effect
Calcitonin
Characteristics
- C cells of thyroid gland
- Family of peptides (amylin, CGRPs, adrenomedulin)
- Different distribution in various tissues
- Secretion is determined by level of ionized calcium
(CaSR)
- Stimulation of secretion:
- Glucocorticoids
- CGRP
- Glucagon
- Enteroglucagon
- Gastrin
- Pentagastrin
- Pancreozymin
- β-sympatomimetics
- Inhibition of secretion - somatostatin
Function unclear
Functions
- Bone tissue
- Inhibition of osteoclast motility and differentiation
- Inhibition of osteoclast secretion
- ATPase inhibition
- Kidneys
- Increased excretion of Ca – inhibition of resorption
(Ca2+ ion channels – LS, Na+/Ca2+ - BM)
- Skeleton development?
- Skeleton protection during pregnancy?
Clinical relevance
- Osteoporosis therapy
- Paget disease therapy
- Treatment of pain (bones metastases)
- ! Increased risk of cancer
Calcitonin gene, mRNA splicing and
posttranslational modifications
Ccells
CNSandneuronsingeneral
Russell FA, King R, Smillie SJ, Kodji X, Brain SD: CALCITONIN GENE-RELATED PEPTIDE: PHYSIOLOGY AND PATHOPHYSIOLOGY. Physiol Rev
2014, 94(4):1099-1142.
Peripheral sensory nerves Vasodilatation
Calcitonin gene-related peptide - CGRP
Neuropeptide – sensoric and integrative motoric functions
Russell FA, King R, Smillie SJ, Kodji X, Brain SD: CALCITONIN GENE-RELATED PEPTIDE: PHYSIOLOGY AND
PATHOPHYSIOLOGY. Physiol Rev 2014, 94(4):1099-1142.
Vasodilatation induced
by various mechanisms
- G prot.
- eNOS/NO
CGRP - functions
Vitamin D….hormone?...vitamin?
Characteristics
- Intake with diet or synthesized (UV)
- In blood bound to VDBP and albumin
- Very small free fraction 1,25(OH)2D – cca 0,4 %
PTH, prolactin, calcitonin, GH (+)
T3/T4, metabolic acidosis (-)
Ca, phosphates, 1,25(OH)2D,
FGF23 (-)
Ketoconazole VDBP
Estrogens (+)
1a-hydroxylase
- Expression in various tissues
- Keratinocytes
- Placenta
- Macrophages
Different 1a-hydroxylase expression = local tissue
homeostasis
Different rate of
feedback control
Cartilage,small
intestine
Physiological effects of vitamin D
VDR
- High affinity to 1,25(OH)2D
- Level of circulating 1,25(OH)2D
- Heterodimer with RXR – coactivators, corepressors
Non-genomic effects
- Rapid increase of intracellular Ca concentration
- PLC activation
- Opening of some Ca ion channels
- Required VDR presence
Vitamin D and Ca absorption/reabsorption
- (+) CBP, AP, Ca2+/Mg2+-ATPase
- (+) TRPV6 – absorption (GIT)
- (+/-) TRPV5 – reabsorption (kidneys)
- Calbindin-9K
- 1,25(OH)2D-inducible ATP-dependent Ca2+ pump
- Na+/Ca2+ exchanger
Parathyroid glands
- Gene expression regulation
- Cell proliferation regulation
- (-) PTH gene transcription
Bones and bone tissue
- (-) collagen synthesis
- (+) osteocalcin synthesis
- (+) osteoclasts differentiation – osteoclastogenesis
- (+) RANKL
- Main function – ensuring the stability of the bone
microenvironment for mineralization by the
standard intake and availability of Ca and
phosphates
Muscle tissue
- (+) uptake AAs
- (+) troponin C
- Phospholipids metabolism
Vitamin D and immune system
Macrophages
Dendritic cells
T cells
Clinical relevance
- Analogue of vitamin D without
ability to cause hypercalcemia
- Antiproliferative effect –
treatment of cancer?
- Synergy with cyclosporin B –
rejection of transplantates
- Suppression of PTH synthesis –
22-oxacalcitriol
(hyperparathyroidismus)
- Psoriasis (clinical trials)
FGF23 – fibroblast growth factor 23
Clinical relevance:
- Autosomal dominant hypophosphatemic
rickets (ADHR)
- Tumor-induced osteomalacia (TIO)
- Klotho mutation
- Prediction of chronic kidney failure prognosis
Characteristics
- New hormone?
- Overexpression = hypophosphatemia and decrease
of 1a 25(OH)D hydroxylation
Functions
- maintaining normophosphatemia and regulation of
vitamin D metabolism
- Decreased expression of IIa, IIb, and IIc (NPT) –
phosphate transport
- Increased expression of 24-hydroxylase – inactive form
- Klotho = co-receptor
Regulation
- Phosphorus availability in diet (-)
- Serum phosphorus
- 1,25(OH)2D
- iron
FGF23
Pars convoluta PT
Inhibition
Npt2aCyp271b
1, 25(OH)2D phosphate Feedback mechanism between DT and PCT endocrine, paracrine
Calcium homeostasis – still just a simplified model