Antwoord op vraag 3.Osteocyten scheiden o.a. RANK-L af.
Vraag 4. Wat is de relatie tussenbotadaptatie en botremodellering in volwassen bot?
Poliomyelitis and bone remodeling loaded unloaded Haversian and Volkmann channels
MECHANICAL ADAPTATION relates to:• Bone MASS how much/how little bone• Bone ALIGNMENT orientation along principal loading directions IN ADULT HUMAN BONE ADAPTATION OCCURS DURING REMODELING
Antwoord op vraag 4. In volwassen humaan bot treedtbotadaptatie op tijdens het proces van botremodellering.
OSTEOCYTES, MECHANOTRANSDUCTION, AND BONE REMODELING
HypothesisREMODELING IS GUIDED BY (DAILY) LOADING
Loaded remodeling osteonFinite element model Equivalent strain distribution Smit et al. J Bone Min Res 17, 2000
Loading of remodeling bone leads toopposite strain fields in the wall of thecutting cone and the closing cone.Decreased strain occurs in front of thecutting cone, where osteoclasts areactivated.Elevated strain occurs around the closingcone, where osteoblasts are activated.
Computer simulation of bone remodeling osteoclasts in cutting cone reversal zone bone forming osteoblasts closing cone marrow new bone old boneRuimerman et al. J Biomech 38, 2005
Computer simulation model Mechanical signal10 MPa1 Hz2x2 mm2OCY density 1600 mm-2 Ruimerman et al.3 osteoclasts J Biomech 38, 2005
Vraag 5.Wat gebeurt er met de richting van de “cutting cone” als de richting van de belasting verandert?
Loading direction 30° Rotated load Ruimerman et al. J Biomech 38, 2005
Antwoord op vraag 5. De richting van de “cutting cone”verandert mee met de belastingsrichting.
Loading magnitude 20% Reduced load 20% Increased load No load Ruimerman et al. J Biomech 38, 2005
Conclusion DAILY LOADING EXPLAINS BONE TUNNELING● loading direction: orientation of the tunnel ● loading magnitude: amount of refilling
THE BONE MECHANOSENSORY SYSTEMLOADING Deformation Flow of canalicular fluid around the osteocytes Mechanosensing by the osteocytes Production of soluble factors Bone remodeling by the osteoblasts/osteoclasts OPTIMAL BONE ARCHITECTURE AND DENSITY
Fluid flow stimulates PGE2 release OCY OB PF PFF PFF PFFPGE2, pg/ µg DNA Con 1500 Con 1500 Con 1500 1000 1000 1000 500 500 500 0 0 0 0 15 30 45 60 0 15 30 45 60 0 15 30 45 60 time (min) time (min) time (min) Osteocytes (OCY) release more PGE2 than osteoblasts (OB) and fibroblasts (PF) Ajubi et al. BBRC 225, 1996
Fluid flow stimulates NO release by osteocytes osteocytes fibroblasts PFF PFFNO2 nM/103 cells NO2 nM/103 cells 240 con 240 con 120 120 0 15 30 45 0 15 30 45 minutes minutes Klein-Nulend et al. BBRC 217, 1995
Intercellular communication Fluid flow-stimulated osteocytes: inhibit osteoclastogenesis via the release of soluble factors, resulting in decreased bone resorption. produce soluble factors that modulate proliferation and differentiation of osteoblasts.
Sclerostin and Van Buchem Disease (VBD)“Mineralized Tissues in Oral and Craniofacial Science: Biological Principles and Clinical Correlates”
• Genetic background: SOST gene• Its product sclerostin• The clinical features caused by SOST mutations - Van Buchem Disease• Therapeutic possibilities
Van Buchem Disease• VBD first described in 1955 and originally named hyperostosis corticalis generalisata• Extremely rare autosomal recessive sclerosing bone dysplasia (Vanhoenacker et al., 2000)• Increase in cortical bone thickness and density affecting the skull, mandible, and long bones• Classified as craniotubular hyperostosis (Beighton et al., 2007) Van Hul et al., 1998
Prevalence• Prevalence VBD is very low: in the 90’s < 30 patients, predominantly in the Dutch population (reported by Van Buchem)• 13 VBD patients in a highly inbred Dutch family with a common ancestor and living in a small ethnic isolated village (Van Hul et al., 1998)
Characteristic Features Protruding chin High forehead Thickened naseal area Facial nerve paralysis Van Hul et al., 1998
Genetic background• Mutations SOST gene chromosome 17q12-21 two similar diseases (A) SCLEROSTEOSIS (much more severe) mutations in SOST coding region (B) VAN BUCHEM DISEASE 52-kb deletion ~35 kb downstream of the SOST gene
Chronological portraits of a patient with sclerosteosis from the age of 3 years onward.She was born with syndactyly at both hands and developed facial palsy, deafness, facial distortion, and maxillary overgrowth during childhood.By the age of 30, she had developed proptosis and elevated intracranial pressure due to overgrowth of the calvaria. Craniectomy was performed, but she died nevertheless because of elevated intracranial pressure at theMoester et al., 2010 age of 54 years
Sclerostin, characteristics and expression• The SOST gene : 2 exons encoding 213-amino acid secreted sclerostin glycoprotein• Cystein-knot motif involved in dimerization and receptor binding and signaling peptide for secretion• SOST mRNA during embryogenesis expressed in many tissues• Sclerostin belongs to the evolutionary-conserved DAN (differential screening-selected gene aberrative in neuroblastoma) family of glycoproteins• Ability to affect the activity of several growth factors, including bone morphogenetic protein (BMP) and Wnts
Sclerostin in adult tissuePostnatally in osteocytes, mineralized hypertrophic chondrocytes and cementocytes Osteocytes Mineralized hypertrophic chondrocytes Cementocytes Van Bezooijen et al., 2009
Sclerostin in adult tissue• Osteocyte-derived secreted protein,• High sclerostin levels in lacunar-canalicular network Winkler et al., 2003
Expression in Van Buchem DiseaseIn VBD patients none ofthese cell types express sclerostinWinkler et al., 2003Van Bezooijen et al., 2009 Increased osteoid surface and lamellar bone Active osteoblasts Van Bezooijen et al., 2009
Sclerostin as bone inhibitor:LRP/WntSclerostin binds to Wnt co-receptors LRP5 and LRP6,thereby antagonizing Wnt/β-catenin signaling byinhibiting β-catenin nucleartranslocation andtranscription of Wnt targetgenes Nusse, 2005; Semënov et al., 2005
Sclerostin as bone inhibitor:BMP-7Inhibition of BMP/Smad signaling by blockingintracellular BMP7 secretion in osteocytes Krause et al., 2010
Mechanisms of actionBy keeping both Wnt/- catenin and BMP7/ Smad in check, sclerostin plays an important role in maintaining bone homeostasis (A)Without sclerostin, thenegative feedback onosteoblast activity is absent, like in VBD, which results in excessive bone formation (B)
Van Buchem Disease - Clinical features - Thickened skull - Thickened mandible, elongation and deformity - Diaphyseal cortex of long bones à narrowed medullary cavities - Spine - Pelvic bone
Clinical features-general• Disrupted bone contours due to subperiosteal osteophytes (exostoses), resulting in a rough surface• Hyperostosis of the skull leads to narrowing of the foramina, causing entrapment of – 7th cranial nerve, leading to facial palsy – 8th cranial nerve leading to deafness, neurological pain, visual problems, and in some cases even blindness• Annual assessment from infancy is recommended for disturbed hearing, evidence of increased intracranial pressure, and nerve entrapment
Clinical features Van Buchem Disease - general • Fractures and haematological changes are not found in VBD • Laboratory values are normal, except for several biochemical indices of bone turnover, such as elevated serum ALP levels • Serum procollagen 1 peptide, OC, and urinary type I collagen cross-linked N-telopeptide are increased (in several but not all cases)
Orofacial bone and dental aspects• No evidence for direct effects on tooth development due to loss of function of SOST• Hyperostosis and hypercementosis could result in narrowing of the periodontal space or even ankylosis - a bone-like tissue connecting root dentin and alveolar bone• Tooth extraction may be difficult and management by an orthodontic or craniofacial team is recommended (Beighton et al., 2007)
Orofacial bone and dental aspectsHowever X-ray images from VBD patients do not show clear signs ofankylosis, although the identification of periodontal gaps is not alwayspossible owing to the very dense radiopacity of the overlying bone Van Bezooijen et al., 2009
Orofacial bone vs. tubular bone• Prominent skull and mandibular bone growth in osteosclerotic and VBD patients might be related by potential differences in “bone cells” at different skeletal sites?• Osteoclasts and osteocytes from craniofacial bones differ from osteoclasts and osteocytes in the long bones regarding the expression of molecules and sensitivity for loading (Zenger et al., 2010; Vatsa et al., 2008)• Calvarial bone and long bone also differ in composition, suggesting heterogeneity between osteoblasts from both skeletal sites
Orofacial bone vs. tubular bone• Osteoblasts of craniofacial bone (intramembranous bone of different embryological origin) more sensitive to loss of sclerostin?• Osteocytes from calvarial or jaw bone produce more sclerostin than osteoblasts in long bones?• Differences in the magnitude of mechanical loading on long bone versus craniofacial bone may also play a role
Therapeutic possibilities• Surgical removal of excess bone -technically difficult, sometimes dangerous (Marmary et al., 1989; Du Plessis, 1993)• Procedure might include: – Surgical decompression of entrapped cranial nerves – Craniectomy for increased intracranial pressure – Middle ear surgery for conductive hearing loss – Reduction of mandibular overgrowth• Testing of relatives at risk is recommended: clinical appraisal, lateral skull radiography, and targeted mutation analysis for the deletion• These treatments aim to relief the symptoms, with no systemic approach to counteract the underlying hyperostosis
Glucocorticoids• Glucocorticoids attractive alternative to high risk surgical procedures (Van Lierop et al., 2010)• Glucocorticoids inhibit osteoblast proliferation and differentiation and increase apoptosis (Weinstein et al., 1998)• Van Lierop et al. (2010) suggest that sclerostin is not only involved in bone formation, but also in bone resorption (exact mechanism yet to be explored)• Glucocorticoids could serve as an additional, systemic therapy in patients with increased risk of neurological complications due to bone overgrowth like Van Buchem Disease
Glucocorticoid inhibit bone formation by stimulating sclerostin VBD • Preventing activation of bone lining cells • Inactivation of active osteoblasts
Sclerostin antibody as a bone forming agent• Pharmacologic inhibition of sclerostin promising anabolic therapy for low bone mass-related disorders like osteoporosis• Inhibition of sclerostin by injection of antibodies has already been shown to increase bone formation, bone mass, and bone strength in animal models, including primates (Li et al., 2010; Ominsky et al., 2010)• A first phase I clinical study demonstrated that a single injection of a mAb against sclerostin increases bone formation markers and bone density, decreases bone resorption, and is well tolerated (Padhi et al., 2010)
Summary• Sclerostin expressed in mineralizing cells• By keeping both Wnt/-catenin and BMP7/Smad in check, sclerostin plays an important role in maintaining bone homeostasis• Van Buchem Disease: Loss of SOST/sclerostin à abnormal bone formation skull, mandible and long bones• Intervention in sclerostin expression can stimulate or inhibit bone formation
Mechanical loading regulates sclerostin expression in osteocytes• Bone adapts mass and shape in response to mechanical loading or lack of loading.• Sclerostin is expressed in mechanosensitive osteocytes. Evidence for mechanoregulation of sclerostin expression was reported in mice and rats subjected to ulnar loading in vivo (Robling et al., 2008)• Modulation of sclerostin levels appears to be a finely tuned mechanism by which osteocytes coordinate regional and local osteogenesis in response to increased mechanical stimulation, perhaps via releasing the local inhibition of Wnt/Lrp5 signaling by sclerostin• Activation of the the Wnt/-catenin pathway in osteocytes occurs via a concerted mechanism.• Mechanical loading increases nitric oxide (NO) production as well as activates focal adhesion kinase (FAK) and the Akt signaling pathway, which results in β-catenin stabilization, followed by β-catenin translocation to the nucleus, and expression of β- catenin target genes such as CD44, connexin 43, cyclin Dd1, and c-fos (Santos et al., 2010).
Mechanical loading regulates sclerostin expression in osteocytes• Propagation of this signal occurs after induction of Wnt production by mechanical loading, which results in re-activation of the Wnt/-catenin signalling pathway (Santos et al., 2009)• Position of the osteocytes can affect production of sclerostin. Osteocytes close to the surface (probably more intense mechanical stimulation) mostly sclerostin-negative while osteocytes deeper in the tissue mostly sclerostin-positive• Osteocytes in the close proximity to an area of bone formation are also mostly sclerostin-negative (Poole et al., 2005), suggesting that not only new bone formation depends on sclerostin distribution, but also bone formation during remodeling might be dependent on the local position of sclerostin producing osteocytes
.020 .00004 .0035Relative gene expression 24 Control 23 Hip Frx 22 Hip OA 21 20 2 -1 2 -2 SOST FGF23 PHEX
Thanks to:Mel Bacabac Daisuke MizunoAstrid Bakker Peter NijweideTon Bronckers Janice OvermanElisabeth Burger Henk-Jan PrinsSteve Cowin Ronald RuimermanVanessa da Silva Ana SantosJesus Delgado-Calle Christoph SchmidtVincent Everts Cor SemeinsRik Huisvkes Theo SmitRichard Jaspers Djien TanPetra Juffer Aviral VatsaRishikesh Kulkarni Marjoleine WillemsFred MacKintosh