L26 - Molecular Mechanisms of Bone Development Flashcards
Define the 2 main embryological origins for MSS system.
Lateral plate mesoderm > skeletal tissues
Somites > nerves, blood vessels, muscles
Define the 4 phases of skeletal morphogenesis?
- Migration of mesenchymal cells
- Cell interaction between mesenchymal and epithelial cells across basement membrane
- Condensation of mesenchymal cells
- Differentiation of condensed mesenchymal cells via 𝑺𝒐𝒙𝟗 into osteochondroprogenitor
Define the transcription factors that regulate differentiation from osterochondroprogenitor cells to Osteoblasts? Intramembranous ossification
RUNX2**/CBFA1: from osteochondroprogenitor to osteoblast
ATF4: from osteoblast to function osteoblasts
Form craniofacial skeleton and clavicles
Name the disease caused by RUNX2 loss of function mutation/ haploinsufficiency?
cleidocranial dysplasia (CCD)
- Delayed closure of cranial suture, fontanelles
- Dental abnormalities: late teeth change
- Hypoplasia / aplastic clavicles
Define the transcription factor that regulate differentiation from osteochondroprogenitor to chondrocytes? Endochondral ossification
Sox9**, Sox5, Sox6: osteochondroprogenitor to chondrocyte
Runx2: chondrocyte to hypertrophic chondrocyte
- expression of cartilage-specific extracellular matrix: Collagen II, IX, XI + Aggrecan
- form axial and appendicular bones
Name the disease caused by Sox9 loss of function mutation?
campomelic dysplasia
- small, abnormal cranium
- Frequent XY sex reversal, e.g. male with female genitalia
- Scapular, pelvic hypoplasia, long bone and spine abnormality
List the key signalling pathways that regulate linear bone growth?
- Fibroblast growth factors (FGFs)
2. Indian Hedgehog-PThrP loop
Which transcription factor regulating osteochondroprogenitor differentiation is sex determining?
SOX9
SRY - related = SRY must be activated for differentiation into male sex
Define the transcription factor that drives mesenchymal cell differentiation in skeletal morphogenesis?
Sox9: drives mesenchymal cell»_space; osteochondroprogenitor
Sox9 remaining turned on = form chondrocyte
Sox9 turned off + RUNX2 turned on = form osteoblast
Define the signalling pathway for Proximal-distal bone growth. Describe the process?
FGF (Fgf 8)
- Cartilage are formed as continuous rods
- Selective Apoptosis
produces bifurcations, segmentations»_space; form joints
Define the signalling pathway for Anterior- posterior patterning of digits in development?
Sonic Hedgehog (Shh) signalling
At the Zone of polarizing activity (ZPA)
Describe how sonic-hedgehog determines the ant.-post. patterning of digits?
Shh = morphogen
Morphogen concentration gradient creates positional values / different thresholds
> > determine different cell fates along the gradient
Describe how the Hedgehog protein is transported for signalling between cells?
- Hh-sending cell: efflux Hh precursor bound to cholesterol (Hn-N)
- Hh- receiving cell: Hh binds to Ptc (receptor for Hh)
- Inhibited Ptc remove tonic inhibition on Smo (smoothen) receptor
> > Smo self-activate Gli complex and trigger signalling response
Name the disease caused by abnormal anterior -posterior patterning of digits?
Greig cephalopolysyndactyly syndrome (many fingers)
What pathway determines the dorsal-central positioning of limb formation?
WNT (Wnt7a > Lmx1b):
e.g. determine back of hand vs. palm
List the sequence of events in endochondral bone formation?
- Cartilage template
- Chondrocyte hypertrophy
- Cartilage mineralization
- Vascular invasion
- Cartilage replacement with bone
- Growth plate cartilage
- Linear growth of long bones
Describe the overall action of FGF signalling on the growth of long bones?
Fibroblast growth factors (FGFs) = ligand, negative regulators/inhibitors of proliferating chondrocyte at Growth Plates**
not the usual role of FGF causing cell proliferation
Define the type of receptor of FGF-R?
receptor tyrosine kinases
Which type of FGF receptor is most important in regulating proliferation of growing long bones?
FGFR3 = most important in proliferating zone
Which protein is required to act with FGF3 on growth plates of bones?
heparan sulphate proteoglycan (HSPG)
Describe the FGF intracellular signalling events that occur in proliferative chondrochytes?
- FGF bind to FGFR (tyrosine kinase receptor)
- FGFR dimerize to activate kinase activity
- Phosphorylate STAT1 and activate transcription of p21-CIP1
- Binds CDK4, 2 complexes > inhibit cyclin D, E dependent kinases
- Inhibit cell cycle porgression from G1 to S
List the 3 different modes of FGFR3 mutation?
3 gain of function mutations:
- Transmembrane domain mutation = over-stabilize dimerization of FGFR
- Mutation forms permenant disulfide bond between 2 FGFR = Ligand independent activation
- Mutation in kinase domain = self-activate, ligand-independent
Summarize the bone developmental outcomes of FGFR3 mutation?
All gain of function mutations» increase cell cycle inhibition on proliferating chondrocyte:
- Retard bone growth
- more inhibition on hypertrophic differentiation = lower osteogenesis
Define the exact mutation in FGFR that causes Achondroplasia?
G380R (>97%) mutation in the transmembrane domain of FGFR3
List the factors that determine linear bone elongation rate in development?
Parathyroid hormone (PTH)
PTHrP (PTH-related protein)
Indian hedgehog (Ihh)
Function of Indian hegdehog? Where is it expressed in growing bones?
Stimulates chondrocyte proliferation
Prevents chondrocyte hypertrophy
Expressed in growth plate (restricted to prehypertrophic cells) **
Describe the short range effects of Indian hedgehog on bone growth?
Indian hedgehog = morphogen with concentration gradient
Short range: Highest concentration» bind to Ptc»_space; activate Smoothen (Smo)»_space; activate Gli complex for PROLIFERATION
Describe the long range effects of Indian hedgehog on bone growth?
Long range: low concentration at perichondrium»_space; activate PTHrP»_space; PTHrP diffuse back to prehypertrophic cells (express PTHrP receptors)
> > SUPPRESS hypertrophic differentiation.
Negative feedback regulation on proliferating chondrocytes in growth plate
Outcome of over-inactivation of PTHrP and PTHrP-R in prehypertrophic cells of growing bone?
Inactivate PTHrP-R, PTHrP:
Less negative feedback on hypertrophic differentiation
> > cells hypertrophy faster than rate of proliferation
> > Reduced number of proliferating cells
> > Bones become shorter
Outcome of over-Activation of PTHrP and PTHrP-R in prehypertrophic cells of growing bone?
Activation = increase inhibition on hypertrophic differentiation at prehypertrophic cells on growing bone = less osteogenesis
= Retard growth
