Lecture 44: Osteoclasts Osteoporosis and Fracture Healing Flashcards
what are osteoclasts derived from
same precursors as macrophages- hematopoietic lineage
what is a characteristic of mature osteoclasts
mutlinucleateed
what are proteases in osteoclasts used for
removing ECM proteins
what do proteins that acts as proton pumps in osteoclasts do
generate H+ iions to reduce pH to dissolve mineral
where is the ruffled border and what does it do
in active osteoclasts to increase surface area in resorption compartment
what is the lifespan of osteoclasts
days (short)
what are osteoclasts responsible for
- 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
where does bone growth occur
at the epiphyseal plate
what are the main factors in regulating osteoclast differentiation
M-CSF, RANKL,OPG,
what is the master transcription factor in the regulation of osteoclast formation/function
NFATc1 and C-fod and NFkB downstream
what are the factors produced by osteoblasts/osteocytes that are essential for osteoclast differentiation
RANKL and M-CSF
what does M-CSF do
promotes proliferation/survival of osteoclast precursors
what does RANKL do
required for osteoclast fusion and differentiation
what does OPG do
natural inhibitor of RANKL
what does an osteoclast need to do
-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
what are the transcription factors for osteoclasts
NFATc1, C-fos, NFkB
what enzyme is important in osteoclasts
TRAP
what are the receptors associated with osteoclast formation/function
- RANK: receptor for RANKL
-C-fms: receptor for M-CSF
-calcitonin receptor
-integrin avB3
what generates protons/proton pump in osteoclasts
carbonic anhydrase 2 and vacuolar type ATPase
what proteases are involved with osteoclasts
Cathepsin K, MMP9 and MMP13
what do osteoclasts attach to to form the sealed zone
avB3 integrins
what generates protons in osteoclast
carbonic anhydrase 2
what do vaculoar type H+ ATPase pumps do
pumps protons into resorption lacuna to create an acidic pH
what do Cl- and HCO3- exchanger do and where is it located in osteoclasts
on basolateral surface removes excess bicarbonate
what does the chloride channel in osteoclasts do
maintain charge neutrality
what does cathepsin K in osteoclasts do
released into resorption lacuna to digest matrix proteins
what does impaired osteoclast function lead to
osteopetrosis
what can osteopetrosis be caused by
failure in osteoclast formation or osteoclasts form normally but have impaired resorptive function
what are the major clinical forms of osteopetrosis
- autosomal dominant adult type-few symptoms
- autosomal recessive infantile type - typically fatal in early childhood
describe the characteristics of bone in osteopetrosis
bones are abnormally dense and prone to fracture
what does failed osteoclastic resorption affect
bone growth, remodeling, tooth eruption
what can osteopetrosis be accompanies by
scoliosis, 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
what gene mutations are associated with osteopetrosis
TCIRG1, CLCN7, cathepsin K mutations
what mutation accounts for 50% of AR osteopetrosis
TCIRG1
what mutation accounts for 75% of AD forms of osteopetrosis
CLCN7
what are mutations in cathepsin K associated with
pycnodysostosis
what is the definition of osteoporosis
a patient with a BMD greater than 2.5 standard deviations below average for a healthy young female or male
how do bisphosphonates work
bind to hydroxyapatite, inhibit activity of osteoclasts by inhibiting mevalonate pathway important for prenylation of GTPases important for vesicular trafficking
how does hormone replacement therapy work in osteoporosis
restores hormone levels following menopause
how do SERMs work
work as partial agonist of estrogen receptor
what are bisphosphonates used for
osteoporosis and treatment of myeloma/bone metastatic cancers particularly if patients have high serum calcium
in which case is the dose of bisphosphonates are higher: cancer or osteoporosis
cancer
what is a bisphosphonate
non-hydrolyzable analog of pyrophosphate
what do bisphosphonates have a high affinity for
hydroxyapatite
what is the major side effect of bisphosphonates
osteonecrosis of the jaw (mandible or maxilla)
what is BONJ defined by
-current or previous treatment with a bisphosphonate
- exposed necrotic bone in the maxillofacial region that has been present for at least 8 weeks
- no history of radiation therapy to the jaws
what is the pathogenesis of BONJ
attributed mainly to suppression of bone turnover due to BP inhibition of osteoclast activity
how do bisphosphonates affect orthodontic tooth movement
they inhibit it
what is skeletal healing important for
- resolution of orthopedic trauma that has caused fractures
- healing of corrective surgeries where bony injuries are created intentionally to correct bone deformities
-bone regeneration in oral surgical procedures/tooth extractions
what can failed/delayed skeletal healing result from
inadequate fixation, infection, tumor, hypoxia/poor blood supply, metabolic dysfunction, chronic/inherited diseases
what cell types are required in fracture healing
-inflammatory cells
-chondroprogenitors/chondrocytes
-osteoprogenitors/osteocytes
-osteoclasts
-vascular cells
what is the timeline of fracture healing and how long does each phase occur
-inflammatory phase: peaks by 48h and lasts a week
-reparative phase: activated within a few days and persists for up to 2-3 months
- remodeling phase: can continue for several years
what are the 4 stages of fracture repair and what phase do they occur in
-formation of vascular hematoma (reactive phase)
- formation of (fibrocartilage) callus (reparative)
- tissue metaplasia - callus replaced by mineralized bone (reparative)
- bone remodeling and turnover (remodeling)
what happens in hematoma formation/inflammation
-formation of hematoma at injury site
- cytokines released: TNF alpha, IL-1, -6, -11, and -18
-cytokines lead to recruitment/infiltration of inflammatory cells
-inflammatory cells release more inflammatory cytokines and recruit mesenchymal stem cells/osteogenic precursors to fracture site
what happens in the formation of fibrocartilage callus
- MSC/connective tissue stem cells/blood vessels invade hematoma
-hematoma degenerates/phagocytes clear debris
-fibrous connective tissue matrix laid down by fibroblasts - granulation tissue - MSc differentiation towards chondrogenic/osteogenic lineages
- hypoxia/tissue necrosis occurs at ends of bones
- in hypoxic regions MSC differentiate into chondrocytes - initiates endochondral bone formation
-intramembranous bone may form in subperiosteal sites where vascular supply is intact - hard callus
how long does hematoma formation/inflammation last
0-2 days
how long does formation of fibrocartilagenous callus last
about 1 week
what are the cell sources of osteogenic precursors
-periosteum
-muscle
-bone marrow
-maybe circulating
what are the cell types of osteogenic precursors
-mesenchymal stem cells (MSC)
- pericyte
- muscle satellite cell
what happens in the formation of bony callus
- intramembranous bone contributed to bony callus
- cartilage undegoes endochondral ossification
- fracture considered healed when bone stability restored by bone tissue completely bridging the original fracture
-initial bone formed- woven bone
how long does formation of bony callus take
several weeks up to 2-3 months
what happens in remodeling
- initial woven bone must be remodeled
- osteoclasts resorb woven bone in fracture callous then osteoblasts lay down new lamellar bone (haversian)
-restores marrow cavity
-restores original contours of bone - biomechanical stability matches that of the original bone
how long does remodeling last
several weeks/months/years
what does fracture healing include
inflammation, endochondral bone formation, intramembranous bone formation, osteoclastic bone resorption
what do the early phases of fracture healing include
-formation of hematoma
-recruitment of MSC
- cell proliferation
-initation of chondrogenesis/osteogenesis
-vascular ingrowth/angiogensis
what are the 3 main categories of signaling molecules important in fracture healing
pro-inflammatory cytokines
-TGFbeta superfamily members
- angiogenic factors
what do TNF alpha and interleukins do
-recruit other inflammatory cells/promote MSc recruitment
-induce apoptosis of hypertrophic chondrocytes
recruit fibrogenic cells/promote formation of granulation tissue/ECM formation
- can promote osteoclast formation
what are pro-inflammatory cytokines secreted by
macrophages, mesenchymal cells, inflammatory cells
what are the pro-inflammatory cytokines
TNF alpha dn IL-1, -6, -11, -18
what are the TGF beta superfamily members
-TGF beta
-BMP2, 5 and 6
- GDF-8
what do TGF beta superfamily members do
-promote ECM synthesis and assembly/initiation of callus formation
- promote osteogenic differentiation
-GDF-8 role in cell proliferation
what are TGF beta superfamily members produced by
hematoma (platelets)/granulation tissue/differentiating MSC/periosteal callus
what are the angiogenic factors
VEGF, PDGF, and ANGPT
what do angiogenic factors do
promote vascular ingrowth from vessels in periosteum - brings pericytes
what does VEGF do
-promotes chemotaxis of osteoprogenitors
-upregulated in regions of hypoxia (under control of HIF 1 alpha)
what does vascularization do
-critical for fracture repair/bone formation
-brings in calcium and phosphate for mineralization
what could bone repair by enhanced by
-improving vascularization
-attracting progenitor cells
-acclerating bone formation
- accelerating remodelin
- BMPs, FGFs, PDGF, platelet rich plasma
what cell based therapies might enhance bone repair and formation
-autologous bone marrow collected from iliac crest and injected into non union site
-purified stem cell sources
what other approaches may be taken to enhance bone formation and reapir
-anti resopritves, bone anabolic agnets, gene therapy
what are antibodies to sclerostin being developed for
anabolic treatment for osteoporosis
