title: Embryonic ablation of osteoblast Smad4 interrupts matrix synthesis in response to canonical Wnt signaling and causes an osteogenesis-imperfecta-like phenotype.
abstract: To examine interactions between bone morphogenic protein (BMP) and canonical Wnt signaling during skeletal growth, we ablated Smad4, a key component of the TGF-β-BMP pathway, in Osx1(+) cells in mice. We show that loss of Smad4 causes stunted growth, spontaneous fractures and a combination of features seen in osteogenesis imperfecta, cleidocranial dysplasia and Wnt-deficiency syndromes. Bones of Smad4 mutant mice exhibited markers of fully differentiated osteoblasts but lacked multiple collagen-processing enzymes, including lysyl oxidase (Lox), a BMP2-responsive gene regulated by Smad4 and Runx2. Accordingly, the collagen matrix in Smad4 mutants was disorganized, but also hypomineralized. Primary osteoblasts from these mutants did not mineralize in vitro in the presence of BMP2 or Wnt3a, and Smad4 mutant mice failed to accrue new bone following systemic inhibition of the Dickkopf homolog Dkk1. Consistent with impaired biological responses to canonical Wnt, ablation of Smad4 causes cleavage of β-catenin and depletion of the low density lipoprotein receptor Lrp5, subsequent to increased caspase-3 activity and apoptosis. In summary, Smad4 regulates maturation of skeletal collagen and osteoblast survival, and is required for matrix-forming responses to both BMP2 and canonical Wnt.
authors.affiliation: Washington University School of Medicine, Division of Bone and Mineral Disease, Departments of Internal Medicine and Cell Biology and Physiology, 660 South Euclid, Campus Box 8301, Saint Louis, MO 63110, USA.
authors.forename: Valerie S
authors.initials: VS
authors.lastname: Salazar
authors.forename: Nicholas
authors.initials: N
authors.lastname: Zarkadis
authors.forename: Lisa
authors.initials: L
authors.lastname: Huang
authors.forename: Jin
authors.initials: J
authors.lastname: Norris
authors.forename: Susan K
authors.initials: SK
authors.lastname: Grimston
authors.forename: Gabriel
authors.initials: G
authors.lastname: Mbalaviele
authors.forename: Roberto
authors.initials: R
authors.lastname: Civitelli
mesh_terms: D000818:Animals; D001848:Bone Diseases, Developmental / Q000151:congenital / Q000235:genetics / Q000378:metabolism* / Q000503:physiopathology; D001857:Bone Matrix / Q000002:abnormalities / Q000196:embryology* / Q000378:metabolism*; D055396:Bone Morphogenetic Protein 2 / Q000378:metabolism; D003094:Collagen / Q000378:metabolism; D005260:Female; D006801:Humans; D008297:Male; D051379:Mice; D010006:Osteoblasts / Q000166:cytology / Q000378:metabolism*; D010012:Osteogenesis; D015398:Signal Transduction; D051901:Smad4 Protein / Q000235:genetics / Q000378:metabolism*; D016212:Transforming Growth Factor beta / Q000378:metabolism; D051153:Wnt Proteins / Q000235:genetics / Q000378:metabolism*; D051176:beta Catenin / Q000378:metabolism
chemical_list: C525719:Bmp2 protein, mouse; D055396:Bone Morphogenetic Protein 2; D051901:Smad4 Protein; D016212:Transforming Growth Factor beta; D051153:Wnt Proteins; D051176:beta Catenin; D003094:Collagen
keywords: BMP; Bone; Collagen; Osteoblast; Smad4; Wnt; β-catenin