Test 2: 38-40 Flashcards
mineral is deposited in the form of ___. Found in ___, ___ and ___
hydroxyapatite
bone, hypertrophic cartilage and teeth.
Needs permissive environment and lack of inhibitors
mineral deposition is dependent on ___ and ___
permissive environment and lack of inhibitors
Most calcium (>99%) and phosphate (~80%) ions of the body are stored in ___; ~1% exists in the serum (extracellular plasma). Total serum calcium concentration is 10 mg/dl, or 2.5 mM. About ~50% of the serum calcium is ionized or ___ ; the rest is protein-bound (~40%) or complexed with organic salts (~10%).
bone
free calcium
Most ___ (>99%) and ___(~80%) ions of the body are stored in bone.
calcium and phosphate
Serum calcium is under close ____, and is regulated on a minute-to-minute basis.
homeostatic control
Even though there is enough calcium and phosphate present, spontaneously depositing mineral doesn’t occur because ___ and ___
1) inhibitors are present that prevent mineralization
2) a permissive environment is required (such as in bone, hypertrophic cartilage, and teeth).
The mineral phase of cartilage, bone, and teeth is a calcium/phosphate precipitate in
the form of ____
hydroxyapatite.
___ particles are very small (microcrystalline), very reactive, highly insoluble, and able to bind many other atoms
hydroxyapatite
A ____ or template (provided by matrix vesicles in cartilage and hole zones of type I collagen in bone) is required for initial crystal formation. The first mineral deposited is a poorly organized apatite which gradually becomes perfected to ____.
nucleation site
hydroxyapatite
Cartilage is not vascularized, and does not mineralize. However, during ____, hypertrophic cartilage acquires the ability to mineralize and to be vascularized because the ECM components change.
endochondral ossification
During endochondral ossification, hypertrophic cartilage acquires the ability to mineralize and to be vascularized because the ECM components change from ___ to ___
collagen type 2, 9 and 11
Collagen type 10
Late hypertrophic chondrocytes release ___ (MVs) from their plasma membrane into the ECM.
matrix vesicles
matrix vesicles provide nucleation sites where ____ first occurs.
mineralization
Mineralization is accomplished by concentrating calcium and phosphate ions in the ___, and permitting mineral crystals to form within the MV membrane. As crystals grow, the MV membrane ruptures and mineral is deposited in the ECM
matrix vesicles
explain Matrix vesicles
how mineralization occurs in cartilage during EO
Late hypertrophic chondrocytes release matrix vesicles (MVs)
calcium and phosphate ions move into the MV, and mineral crystals form within the MV membrane.
As crystals grow, the MV membrane ruptures and mineral is deposited in the ECM.
4 components of Matrix Vesicles enable mineralization to take place:
- Alkaline phosphatase:
- Annexin:
- Calbindin:
- Metalloproteinases:
Alkaline phosphatase:
component of Matrix Vesicle that helps with mineralization
-
Alkaline phosphatase: increases phosphate concentration near matrix vesciles
- Ester-PO4(alkaline phosphatase)→ alcohol and Pi
Annexin
component of Matrix Vesicle that helps with mineralization
forms membrane channels for calcium
Calbindin
component of Matrix Vesicle that helps with mineralization
calcium binding protein inside matrix vesicle
Metalloproteinases
component of Matrix Vesicle that helps with mineralization
degrade ECM and inhibitors
if mineralization goes wrong
kidney stones,
artheroscerosis
tissue death- provides environment for mineralization
make up of bone ECM
Growth factor and cytokines of bone ECM
TGT beta, prostaglandins, interleukins, granulocyte and macrophage colony stimulating factor
Since bone is a structural tissue, it is subject to fatigue and needs to undergo continual
___ .
replacement or remodelling
Remodelling is involved during skeletal growth and repair, and also allows the bone to adapt to changes in ___.
mechanical pressure
Remodeling is regulated by osteoblasts and oseoclasts, and is locally controlled via ___; systemic regulation is via the ___
cytokines and growth factors
hormones
some hormones that control systemic regulation of bone remodeling are
hormones PTH, vitamin D, and calcitonin
Mechanism by which bone adapts to mechanical pressure.
Mechanicotransduction
Mechanicotransduction
how bone adapts to mechanical pressure.
Osteocytes sense changes in pressure and in calcium concentrations in the surrounding bone fluid, and respond by secreting cytokines which activate either osteoblasts, or osteoclasts.
Bone matrix is deposited in proportion to the compressional load that the bone must carry.
Bone matrix is deposited in proportion to the ___ load that the bone must carry.
compressional
Local remodeling is regulated by
TGF beta: Transforming growth factor-beta
PGE2: prostaglandin E
N2O : nitrous oxide
TGFβ : Transforming growth factor-beta
local coupling factor for bone remodeling
This protein stimulates osteoblasts to upregulate collagen, proteoglycan, and alkaline phosphatase activity.
PGE2 Prostaglandin E.
local coupling factor for bone remodeling
This lipid is derived from arachidonic acid, and can activate resorption
N2O: Nitrous oxide
local coupling factor for bone remodeling
This is a gas synthesized by osteoblasts, and serves to transiently inhibit osteoclasts.
cytokines
Local control of osteoclastogenesis
produced in marrow by hematopoietic cells and pre- osteoblasts.
These include granulocyte and macrophage colony stimulating factor (GM- CSF), interleukins (IL-1, IL-4, IL-6), and TGF Β.
These cytokines bind receptors of pre-
osteoclasts, and either promote or inhibit their differentiation to mononuclear osteoclasts.
Systemic (hormonal) factors: PTH and vitamin D
Systemic (hormonal) factors of osteoclastogenesis
activate resorption by stimulating bone lining cells and osteoblasts to secrete cytokines and thereby increase the number of pre-osteoclasts.
Calcitonin directly binds receptors on osteoclasts and inhibits their activity.
Estradiol inhibits osteoclast formation by inhibiting GM-CSF and IL-6 production; decreased estrogen secretion results in an increase in pre-osteoclasts and an elevation in bone resorption (post-menopausal osteoporosis).
___ directly binds receptors on osteoclasts and inhibits their activity.
Calcitonin
___ inhibits osteoclast formation by inhibiting GM-CSF and IL-6 production; decreased estrogen secretion results in an increase in pre-osteoclasts and an elevation in bone resorption (___).
Estradiol
post-menopausal osteoporosis
Local and systemic control of osteoclastogenesis
Local and systemic control of bone remodeling
local and systemic control of osteoclastogenesis (simple form)
myeloid precursor (bone marrow)
pre osteoclast (bone marrow)
mononuclear osteoclast (bone surface)
multinuclear osteoclast (bone surface)
bone resorption
takes 14 days
decrease in systemic calcium causes the release of ___
PTH and Vit D which inhibit Osteoblast
Osteoblasts will release cytokines which stimulate myeloid precursors to differentiate into pre osteoclast and PGE2 which will cause osteoclast to start eating bone, which releases calcium into blood supply
increase in systemic calcium causes
release TGF beta which trigger osteoblast to release N20 which stops osteoclast from eating bone
rise in calcium = increase of calcitonin which inhibits osteoclasts
summary of collagen biosynthesis
Estrogen and androgen cause
negative control of making bone
( as you get older you have less estrogen= less bone is made)
how do osteoclasts resorb bone
- Bone marrow: increase in osteoclast precursors. Pre-osteoclasts migrate to bone surface.
- Bone surface: bone lining cells and osteoblasts retract. Osteoclasts bind specific bone receptors (RGD).
- Osteoclasts:
a. Cytosol: carbonic anhydrase is activated—> formation of protons:
CO2+ H2O <=> H2CO3<=> H++ HCO3-
b. Ruffled border: proton pump is activated, and using ATP, H+ is pumped through the membrane
c. Anti-resorptive surface: HCO3-is exchanged for Cl-; Cl- that enters the cell passes through an anion channel coupled to the proton H+ ATPase at the resorptive membrane.
- Bone surface: H+Cl- is released into the resorptive environment (pH drops to 4.5).
Protons bind PO43- to form HPO42-; chloride binds calcium and mineral is solubilized. - Osteoclast: collagenase is released at the bone surface and cleaves collagen into two
pieces at pH 4.5. The peptides are further digested by gelatinases. Fragments are
endocytosed by the osteoclast, and transcytosed to the anti-resorptive surface. - Calcium: increase in the local calcium concentration causes the osteoclast to move across the bone.
osteoclasts make/have
cytosol
ruffled border
anti resorptive surface
cytosol
made by osteoclasts
carbonic anhydrase is activated → formation of protons
CO2+ H2O <=> H2CO3<=> H++ HCO3-
explain ruffled border of osteoclasts
proton pump is activated, and using ATP, H+ is pumped through the membrane.
explain anti resorptive surface of osteoclasts
HCO3-is exchanged for Cl-
Cl- that enters the cell passes through an anion channel coupled to the proton H+ ATPase at the resorptive membrane.
collagenase
Osteoclast release collagenase at the bone surface and cleaves collagen into two
pieces at pH 4.5.
The peptides are further digested by gelatinases.
Fragments are endocytosed by the osteoclast, and transcytosed to the anti-resorptive surface.
Osteoporosis
When resorption of bone is greater than formation of bone
osteopenia, or reduced bone mass.
Radiographic detection is not possible until ~30-40% of bone mass is lost.
With age, bone is lost from all parts of the skeleton. This is particularly evident in ~25% of postmenopausal women, and is likely caused by a sudden drop in estrogen levels.
osteoporosis happen in women when
~25% of postmenopausal women, and is likely caused by a sudden drop in estrogen levels.
how to prevent osteoporosis
Exercise to increase bone mass
Eat calcium. Your ability to take up calcium will decrease with age.
Humans need 1 gm Ca/day till 65; then ~2.5 gm/day
when is calcium demand increased
skeleton formation
egg-laying
antler formation
milk production
pregnancy
Osteopetrosis
When the rate of bone formation is greater than resorption.
osteoclast dysfunction.
Bones are dense and very brittle. Marrow is often affected, and extramedullary hematopoiesis can occur in liver and spleen. Anemia is common.
Treatment may require marrow transplants.
role of bone in calcium homeostasis
remodeling
blood-bone transfer
PTH/Calcitriol-mediated resorption
cells like to maintain a __ cytosolic calcium level
low
more calcium outside then inside
how does calcium get into blood stream
calcium in food
absorbed through small intestine
calcium bind proteins transfer calcium to organelles within cell or through cell and out to the lamina propria
blood- bone transfer of calcium
PTH/Calcitriol-mediated resorption
Calcium homeostasis is achieved through hormonal action at the level of the bone, kidney and intestine, and is mediated by 1,25- (OH)2 -vitamin D (calcitriol), parathyroid hormone (PTH), and calcitonin (CT). CT lowers
blood calcium, while PTH and Vit-D raise blood calcium.
Calcitonin ____ blood calcium, while PTH and Vit-D ___ blood calcium.
lowers
raise
serum calcium is broken into what 3 parts:
ionized calcium (50%)
protein bound calcium (40%)
calcium complexed with organic salts (10%)
serum calcium concentrations are maintained by ___, metabolites of vitamin D (“Vit-D” or calcitriol), calcitonin (CT), and the dietary intake and urinary and fecal excretion of calcium.
parathyroid hormone (PTH)
serum concentration of calcium are maintained by
PTH
vitamin D
calcitonin
dietary intake and excretion
____ concentration is in the micromolar range (1uM), compared to the millimolar range in the serum and in the ER.
cytoplasmic calcium
The cell exerts ___ of its energy maintaining this gradient to prevent calcium/phosphate crystals from forming.
~65%
Transient increases in calcium are required for roles in physiological functions and in signal transduction pathways; these transient releases are achieved by ___ in either the ER or plasma membrane.
calcium channels
The correct intracellular ionic milieu is maintained by a ___ pump, and
a _____, which actively pump Ca 2+ out of the cytosol to the cell exterior or into the ER lumen
calcium ATPase pump
sodium-driven calcium antiport,
Mitochondria can also pump Ca 2+ out of the cytosol, but do so efficiently only when Ca 2+
levels are high-usually as a result of ___
cell damage.
In the cell, Ca2+ is often bound by calcium-binding proteins such as ___ or ____
calsequestrin or calmodulin.
calsequestrin or calmodulin
calcium binding proteins that keep Calcium from freely floating in the cytoplasm of the cell
how does the cell keep low Calcium inside and high calcium outside
Calcium ATPase pumps and Na/Ca antiport pumps (pump Calcium out of cell)
calcium binding molecules (calsequestrin and calmodulin)
Mitrochondria can help if calcium levels are very high
dietary calcium absorption takes place in the ___
small intestine
Absorption of calcium in the small intestine is ___ , most calcium is lost via ___
50% efficient,
sweat, feces and urin
Ability to absorb calcium increases or decreases with age?
decreases
Vitamin D facilitates Calcium absorption by
activating transcription of a calcium binding protein (___) in the intestine.
calbindin
3 types of calcium deficiency
acute hypocalcemia
rickets
osteomalacia
acute hypocalcemia
Severe drop in Ca 2+ (below 5-6 mg/dl) leads to tetany, convulsions, and possibly death.
Tetany results from the requirement for Ca 2+
in nerve impulse transmission and in muscle contraction; death ensues from failure in heart muscle contraction.
Example: “milk fever” or parturient paresis syndrome in dairy cattle.
rickets
wrickken= to twist
Calcium deficiency during skeletal growth.
Defect: poor mineralization, skeletal deformities, altered chondrocyte /osteoblast differentiation.
Example: shell-less chick embryo culture
Osteomalacia
“soft bones”.
Calcium deficiency in adulthood, characterized by lameness and spontaneous fractures.
Hypercalcemia
chronic calcium over-ingestion can lead to osteopetrosis, increase in the threshold for nerve and muscle activation
brottle bones, muscle weakness, lethargy and coma
dairy bulls(male cow) subjected to prolonged high calcium diets designed for lactating dairy cows
Hyperphosphatemia
the proportion of calcium and phosphate remains constant (2:1)
A deficiency of calcium in the presence or excess of phosphate causes excessive bone resorption and has generalized skeletal defects including “ big head” disease of horses, simian bone disease of monkeys and twisted snouts in pigs
systemic regulation of calcium
Vitamin D synthesis and metabolism
produced in the skin (vitamin D3) or ingested in the diet (vitamin D2),
it requires hydroxylations (chemical transformations) in the liver and kidney to form the active metabolite 1,25 (OH)2D.
Shading indicates structural changes at each step.
Vitamin D is a ___ hormone.
steroid
Vitamin D diffuses into cells and binds a vitamin D receptor (VDR); this results in a conformation change of VDR, and enables the vitamin D/VDR complex to bind specific vitamin D regulatory elements in target DNA molecules. Gene transcription and ___ are thus stimulated or repressed.
protein synthesis
organs that have receptor for vitamin D
bone
intestine
kidney
bone and vitamin D
Vit-D inhibits bone matrix production by osteoblasts, but induces them to secrete osteoclast-stimulating factors (cytokines, PGE 2). Result is deceased bone
production by osteoblasts, and increased resorption by osteoclasts.
intestine and vit D
Increases Calcium absorption by activating transcription of calbindin.
kidney and vitamin D
Increases Calcium re-absorption by activating transcription of calbindin.
Parathyroid hormone
synthesized as a prehormone
cleavage converts the hormone into its active state.
It remains packaged in secretory granules until secretion into the blood stream.
Secretion is regulated by calcium serum levels, via a plasma-membrane-associated
calcium sensor receptor (CaSr) on the surface of parathyroid cells. Binding of Ca 2+ to CaSr inhibits PTH secretion; reduced occupancy of CaSr by Ca 2+ leads to release of PTH
into the blood, where it has a half-life of 2-5 minutes.
PTH effects are IMMEDIATE.
bone and PTH
PTH inhibits bone matrix deposition by osteoblasts, but induces their production of osteoclast-stimulating factors. Result = increased resorption.
kidney and PTH
- Stimulates 1-a-hydroxylase-mediated conversion of 25-OH-Vit-D to 1,25-OH-vit-D
- Increases Ca 2+ re-absorption
- Increases phosphate excretion
PARATHYROID-RELATED PEPTIDE (PTHrP)
hypercalcemia and bone resorption in malignancy:
most common paraneoplastic syndrome
associated with cancer,
caused by osteolytic cytokines produced by tumors that affect bone cell function.
Tumors that can produce hypercalcemia are lung, breast, renal, ovarian, prostate, and hematologic (especially myeloma and lymphoma).
A major mediator of hypercalcemia in malignancy is PTHrP; it’s aminoterminus has sequence homology to PTH, and thus binds PTH receptors.
Treatment: bisphosphonates = potent inhibitors of bone resorption. These are
analogues of pyrophosphates that bind tightly to calcified bone matrix preventing demineralization; they prevent osteoclast attachment to bone matrix.
bisphosphonates
treatment for parathyroid related peptide (PTHrP)
potent inhibitors of bone resorption. These are analogues of pyrophosphates that bind tightly to calcified bone matrix preventing demineralization; they prevent osteoclast attachment to bone matrix.
systemic regulation of calcium
Calcium homeostasis
is achieved through hormonal action at the level of the bone, kidney and intestine, and is mediated by 1,25-(OH) 2- vitamin D (calcitriol), parathyroid hormone (PTH), and calcitonin (CT). Calcitonin lowers blood Ca 2+, while PTH and Vit-D raise blood Ca2+. Phosphate levels are regulated by the kidney.
Vitamin D metabolism
Unfiltered sunlight is required for the synthesis of cholecalciferol in the skin. 25-OH-vitamin D is synthesized in the liver, while 1,25(OH) 2
-vitamin D is made in the kidney. Synthesis is regulated by PTH and Pi. Vitamin D is a steroid hormone, and acts by activating transcription of calcium binding proteins in the intestine and kidney, and controls osteoclast differentiation via acting on the osteoblast.
PTH
It functions as a peptide hormone, and raises cAMP levels. PTH receptors (G-protein-linked) exist on osteoblasts, but not on osteoclasts; resorption is activated by increasing osteoclast recruitment, and inhibiting bone formation. To inhibit an increase in phosphate levels during resorption, PTH stimulates phosphate excretion. An elevation in PTH also increases the hydroxylation of vitamin D by the kidney.
Calcitonin
a single peptide of 32 aa, and is secreted (as a prehormone) by C cells of the thyroid in response to hypercalcemia (via CaSr). It lowers Calcium levels by acting directly on osteoclasts and inhibiting resorption (via G-protein-linked receptor).
PTH-related peptide (PTHrP)
a hormone produced by tumors and by lactating breast. It functions to elevate the blood calcium. It competes with PTH for receptor binding.
carbonic anhydrase
enzyme in osetoclasts that bring produces protons and bicarbonate
Protons pumped to resorptive space by ATP pump
bicarbonate leaves cell in exchange for Cl which then is pumped into resorptive space
HCl in resorptive space= lower pH= demineralization of bone= release of calcium and phosphate and TGF beta
TGF beta tells osteoblasts to produce nitrate oxide and increase bone production
nitrate oxide tells osteoclasts to stop breaking down bone
increase calcium = calcitonin= tells osteoclasts to stop breaking down bone
collagenase produced by osteoclast cut collagen
increased calcium = ____ = ___ of osteoclasts
calcitonin = inhibition of osteoclasts
decrease calcium = ____ and ___ = ___ of osteoblasts
PTH and Vit D= inhibition
osteoblasts produce ___ cytokines and ___ PGE2 which will ___
more and more
inhibit bone production and attract osteoclasts
NO will inhibit ___ and is produced by ___ when ___
osteoclasts
osteoblasts
when TGF beta is released when bone is broken down, which tells OB to start making bone and causes NO gas to make OC go away
you are unable to defect ___ until 30% of bone is lost
osteoporisis
decrease in bone marrow density
how does vit D help calcium absorption
vit D enters nucleus and stimulates the production of calcium binding protein (calbindin) which binds to calcium and helps it move through cell
low calcium stimulates ___ and ___ which act on which organs
Vitamin D- stimulates the transcription of (calbindin)
intestine, kidney and bones
PTH: kidney and bone
High calcium stimulates ___ which acts of what ?
CT- calcitonin
bones
how is Vit D made
light (skin) or diet
liver- hydroxylated
kidney: high PTH= low Calcium: 1- alpha hydroxylase is used to hydroxylated into active form vit D (1,25)
low PTH: high calcium: hydroxylated into inactive form vit D (24,25)
1- alpha hydroxylase is activated by PTH
vit D effects osteoblasts by __
signal osteoblast to stop making bone, produce PGE2 and stimulates osteoclasts to start breaking down bone
osteocalcin increases, osteopontin increases= increase Prostaglandin PGE2
vit D effect of kidney, intestine and bone
kidney= translation of calbindin (calcium binding protein= reabsorption of calcium from the urine
intestine= translation of calbindin= absorption of calcium from small intestine into blood
Bone: stimulate OB to stop making bone, make PGE2 (prostaglandin E) which brings OC and causes break down of bone = more calcium in blood
PTH is secreted in ___ calcium levels. and binds to receptors on ___ and __
parathyroid hormone
low
kidney and osteoblasts
PTH acts on what organs and how
PTH is produced for low calcium
parathyroid hormone
Kidney→ enzyme 1- alpha hydroxylase is used to make active form of vitamin D (1,25)
bone→ inhibit OB → OB produce PGE which stimulate osteoclasts to break down bone→ increase in resorption of calcium
Calcitonin is made by ___ when there is __ calcium in the blood.
they work by ___
C cells in the thyroid
high (hypercalcemia)
stops osteoclasts and inhibit resorption of calcium via G-protein linked receptors
____ is a hormone produced by tumors and by lactating breasts. It functions to ___ blood calcium levels.
PTHrP (PTH related peptide)
elevate
competes with PTH for receptor binding on osteoblasts
PTH have receptors on ___. they inhibit bone ___. PTH stimulates ___ excretion. Increased PTH also ___ the hydroxylation of Vit D by the ___
osteoblast
formation
phosphate
increases by activating the transcription of the enzyme 1-alpha-hydroxylase
kidney
___ lowers blood calcium, while ___ and ____ raise blood calcium
calcitonin
parathyroid hormone and Vit D
active form of vit D also called
1,25 OH2D3
calcitriol
explain PTH effect on kindey
PTH stimulates transcription of 1-alpha- hydroxylase
this enzyme changes Vit D into active form (1,25 or calcitriol)
Vit D works of kidney by stimulating transcription of calbindin(calcium binding protein) that is used to resorb calcium from the urine
what happen at low Calcium
(diagram)
