Tissue Osteoblasts Osteocytes Osteoclasts Flashcards
three main bone cell types
osteoclasts
osteoblasts
osteocytes
To Repair Damage Bone is Continually Being
Removed by — and Rebuilt by — (Bone Remodeling)
Osteoclasts
Osteoblasts
Osteoblasts, chondrocytes, myoblasts and adipocytes differentiate from a common — precursor
mesenchymal
osteoblasts are derived from
mesenchymal
stem cells
shape of osteoblasts
Plump, cuboidal cells located on
bone forming surfaces
osteoblasts produce large amounts of
extracellular matrix proteins
(esp. collagen type I) =
osteoid, which then mineralizes
lifespan of osteoblasts
weeks
osteoblasts marker proteins: transcription factors (2)
- Runx2
* Osterix
osteoblasts marker proteins: enzymes (1)
Alkaline phosphatase
osteoblasts marker proteins: ECM proteins (4)
- Type I collagen
- Osteopontin
- Osteocalcin
- Bone sialoprotein (BSP)
Runx2 is a — for Bone
Master Transcription
Factor
runx2 is essential for
bone and tooth development
Mice lacking RUNX2 form a
cartilaginous skeleton that
fails to
mineralize
Heterozygous mutation of RUNX2 in
humans results in
Cleidocranial Dysplasia (CCD)
Cleidocranial Dysplasia (CCD) symptoms
• Autosomal Dominant
• Haploinsuffiency of RUNX2 (due to
inactivating mutation/deletion in one allele)
• Delayed ossification of midline structures of
body (esp. membranous bone)
• Clavicles partly or completely missing
• Late closing of fontanelle
• Supernumerary teeth
• Prognathic (protruding) mandible due to
hypoplasia of maxilla
key characteristic of CCD
abnormal shoulder mobility due to hypoplastic/aplastic clavicles
— is a Key Transcription Factor for
Osteoblast Differentiation that is
Downstream of Runx2
Osterix
Runx2 induces another transcription
factor, —
Osterix
Osterix is also critical for
osteoblast differentiation
Mice lacking osterix (gene name SP7)
have
impaired osteoblast formation
Osterix controls expression of osteoblast genes: (3)
Type I collagen
Osteocalcin
Osteopontin
Human Mutations in SP7 (Osterix) - associated with
Osteogenesis Imperfecta type XII
some key signaling pathways that regulate osteoblast differentiation (8)
BMPs TGFb WNT/B catenin signaling pathway hedgehog proteins IGF-1 PTH and PTHrP FGFs Notch pathway
BMPs –
Bone Morphogenetic Proteins
TGFβs –
Transforming Growth Factor Beta
PTH and PTHrP –
Parathyroid hormone and parathyroid
hormone-related peptide
FGFs –
Fibroblast growth factors
Notch Pathway –
Notch receptors and ligands (Delta, Serrate,
Lag2)
BMPs - originally purified from bone extracts that induce
bone formation when implanted in muscle (ectopic bone assay)
BMPs are required for — — of adult bone
homeostasis
skeletal development/maintenance
BMPs promote differentiation from early — cells
osteoprogenitor
BMPs are important in
fracture healing
knockout of specific BMPs in bone leads to
skeletal defects
naturally occurring mutations in BMPs or their receptors result in
inherited skeletal disorders in humans
FOP –
Fibrodysplasia Ossificans Progressiva
Heterotopic bone formation
bone forming
in soft tissues
(3) of bone in extra-
skeletal sites - fuses joints, ribs, etc.
Ribbons, sheets, plates
Bone forms in response to — —
exacerbated by surgical intervention
tissue trauma
Mutations in BMP — — — (ACVR1
gene) - single a.a. substitution R206H
type I receptor
Mutation causes mild —
activation (i.e. in absence of ligand) and
— with BMP ligand binding.
Also acquired responsiveness to activin A.
constitutive
overactivation
Most cases due to
spontaneous mutation
in gametes/early embryo (most FOP
patients can’t have children)
potential treatments for FOP (2)
Palovaratene and antibodies against activin A being investigated as potential treatments as well as kinase inhibitors selective for mutant receptor (based on animal studies)
High Bone Mass Phenotype Due to Mutations
in
LRP5 (affects Wnt/β-catenin signaling)
Wnt-β-catenin signaling pathway
important in determining
bone
mass
Activating mutations of Lrp5 lead
to — in humans
high bone mass
Inactivating mutations of Lrp5
lead to
low bone mass
Alkaline Phosphatase is an enzyme highly expressed in
osteoblasts/
odontoblasts
alkaline phosphatase hydrolyzes pyrophosphate (PPi), a natural inhibitor of mineralization,
thereby increasing local phosphate
concentration which promotes
mineralization
Mice lacking alkaline phosphatase gene
(TNAP ) have
impaired mineralization
In humans - mutations in alkaline
phosphatase gene (TNSALP) associated
with —
hypophosphatasia
Hypophosphatasia (HPP)
Rare heritable Rickets/Osteomalacia (~350
cases reported)
> 289 mutations identified in — (~80% =
missense mutations)
human alkaline phosphatase gene (TNSALP)
HPP has reduced activity of
alkaline phosphatase
symptoms of HPP
Impaired mineralization of skeleton/dentition,
leg bowing, rachitic rosary, early tooth loss,
waddling gait, muscle weakness, seizures
HPP has varying severity from
perinatal lethal to adult onset or mild forms only affecting dentition
(dependent on degree of loss of function of
alkaline phosphatase)
Expert dental care is important – (2) may be necessary
soft
foods/dentures
treatment for HPP
previously no established treatment
Infusion of — enzyme ineffective for HPP treatment
alkaline phosphatase
— — successful in two severely affected infants
— improved HPP in an adult patient (Whyte
Marrow transplantation
Teriparatide (PTH 1-34)
NEW TREATMENT RECENTLY APPROVED for HPP:
bone-targeted enzyme replacement therapy - TNSALP recombinant enzyme with a 10 amino acid bone targeting peptide sequence (deca-aspartate) (Whyte et al 2012, N. Engl. J.
Medicine, 366:904) – performing very well so far in tests with infantile HPP
osteocytes are terminally differentiated
osteoblasts
osteocytes are embedded in
bone matrix
osteocytes make up over –% of bone cells
90%
osteocytes have long
dendritic processes
osteocytes were previously thought to be
quiescent cells
osteocytes are now known to be an active cell type with key functions in
bone
is there a master transcriptional gene identified yet?
no
lifespan of osteocytes
decades
osteocyte marker proteins: transcription factor (1)
Mef2c
osteocyte marker proteins: early osteocyte markers (4)
• E11/gp38/podoplanin • Dentin matrix protein-1 (DMP1) • Matrix extracellular phosphoglycoprotein (MEPE) • Phosphate regulating endopeptidase homolog, X-Linked (PHEX)
osteocyte marker proteins: later osteocyte marker (1)
Sclerostin (SOST)
Potential Functions of Osteocytes (4)
• Mechanosensors (control responses of bone cells
to mechanical loading)
• Control bone resorption and bone formation (by
regulating osteoclast and osteoblast activity)
• Regulate mineralization
• Regulators of mineral homeostasis-both calcium
and phosphorus
Sclerostin is highly expressed in
mature
osteocytes, cementocytes,
odontoblasts
Sclerostin is a negative regulator of
bone formation - antagonizes Wnt/beta-catenin signaling pathway
sclerotin is through to act as a — to limit bone formation
brake
Sclerostin null mice show —
bone mass phenotype
high
(increased
bone formation)
Sclerostin gain of function mouse
models show — bone mass
phenotype (decreased bone
formation)
low
(decreased bone
formation)
Deletion or mutation of SOST gene
results in
Sclerosteosis or Van Buchem’s
disease in humans
Increased bone mass, especially
obvious in
craniofacial skeleton
Antibodies to sclerostin – in preclinical/
clinical trials as an anabolic treatment
for
osteoporosis
sclerotin establishes the osteocyte as a key target cell for development of
new treatments for diseases of bone loss and overgrowth
Osteocytes as Regulators of —
Homeostasis
Phosphate
Osteocytes express several genes important in
phosphate homeostasis: (3)
FGF23
DMP1
PHEX
FGF23
Fibroblast growth factor-23 (FGF23)
DMP1
Dentin matrix protein-1
PHEX
Phosphate regulating endopeptidase
homolog, X-linked
osteocytes play an endocrine role in regulation of — homeostasis
phosphate
osteoclasts are derived from same precursors as
macrophages (hematopoietic lineage)
Mature osteoclasts are
multinucleated
osteoclasts express — for removing ECM proteins (e.g. collagen)
proteases
osteoclasts express proteins that act as
proton
pumps to generate H+ ions (reduces pH
to dissolve mineral)
Active osteoclasts have specialized
“— —”, which increases
ruffled border
surface area in resorption compartment
lifespan of osteoclasts
short (days)
osteoclasts are responsible for (5)
• Bone resorption during normal bone growth and
remodeling
• Removal of alveolar bone during tooth eruption
• Resorption of tooth roots of primary teeth
• Removal of alveolar bone during orthodontic tooth
movement
• Bone loss in pathological conditions (osteoporosis,
tumor associated osteolysis, etc.)
Osteoclastic Resorption is Important
for
Normal Bone Growth
growth occurs at the
epiphyseal plate
—: must occur to
maintain the bone shape
Modeling
Master transcription factor of osteoclast formation/function
NFATc1
(2) are downstream of NFATc 1 and also important
C-fos and NFƙB
2 factors produced by osteoblasts/osteocytes which are essential for OCL differentiation
– RANKL
– M-CSF
RANKL
receptor activator of NFkB ligand
M-CSF (a.k.a CSF-1)
Macrophage colony stimulating factor)
M-CSF promotes
proliferation/ survival of osteoclast precursors
RANKL (member of TNF
superfamily) is required for
osteoclast fusion and differentiation
OPG (osteoprotogerin)
natural inhibitor of RANKL decoy receptor
What Does an Osteoclast Need to Do? (5)
• Differentiate/fuse
• Adhere to the bone surface
• Produce acid to dissolve mineral
• Produce proteases to breakdown extracellular matrix
components
• Respond to factors that regulate osteoclast survival/
activity
Osteoclast Marker Proteins: transcription factors (3)
- NFATc1
- C-fos
- NFkB
Osteoclast Marker Proteins: enzyme (1)
Tartrate resistant acid Phosphatase
TRAP
Osteoclast Marker Proteins: receptor (4)
- RANK (receptor for RANKL)
- C-fms (receptor for M-CSF)
- Calcitonin receptor
- Integrin αvβ3
Osteoclast Marker Proteins: generates protons/proton pump (2)
- Carbonic anhydrase II
* Vacuolar-type ATPase
Osteoclast Marker Proteins: proteases (2)
- Cathepsin K
* MMP9, MMP13
Osteoclasts attach via — — to form sealed zone
αvβ3
integrins
generates protons
Carbonic anhydrase II (CAII)
Vacuolar-type H+ ATPase pumps
protons into resorption lacuna –
creates
acid pH (dissolves mineral)
(2) exchanger on
basolateral surface removes excess
bicarbonate
Cl- and HCO3-
Chloride channel maintains
charge
neutrality
— (and other proteases)
also released into resorption
lacuna (digests matrix proteins)
Cathepsin K
Impaired Osteoclast Function
Leads to
Osteopetrosis
Osteopetrosis can be due to
failure in osteoclast FORMATION or osteoclasts
form normally but have impaired resorptive FUNCTION
two major clinical forms of osteopetrosis
autosomal dominant adult (benign) type
autosomal recessive infantile (malignant) type
autosomal dominant adult (benign) type
relatively few
symptoms
autosomal recessive infantile (malignant) type -
typically
fatal (if untreated) in early childhood
Bones abnormally — and prone to —
dense
fracture
Failed osteoclastic resorption affects bone (3)
growth, remodeling,
tooth eruption, etc.
osteopetrosis can be accompanied by
scoliosis (spinal curvature), nerve
compression in head and face (hearing loss, blindness), impaired
marrow function (anemia), enlarged liver or spleen, dental
abnormalities, short stature, slow growth, recurrent infections,
etc
– mutations identified in gene
encoding α3 subunit of vacuolar
H+ ATPase (TCIRG1)
> 60
Accounts for about –% of AR
osteopetrosis in humans
50
Mutations also found in gene
encoding -– accounts for 75% of ADforms of osteopetrosis (OMIM#
166600)
ClC7 chloride channel
CLCN7
Cathepsin K mutations associated
with –– a specific
form of osteopetrosis
pycnodysostosis
