Bone sialoprotein plays a functional role in bone formation and osteoclastogenesis.

International audienceBone sialoprotein (BSP) and osteopontin (OPN) are both highly expressed in bone, but their functional specificities are unknown. OPN knockout ((-/-)) mice do not lose bone in a model of hindlimb disuse (tail suspension), showing the importance of OPN in bone remodeling. We report that BSP(-/-) mice are viable and breed normally, but their weight and size are lower than wild-type (WT) mice. Bone is undermineralized in fetuses and young adults, but not in older (>/=12 mo) BSP(-/-) mice. At 4 mo, BSP(-/-) mice display thinner cortical bones than WT, but greater trabecular bone volume with very low bone formation rate, which indicates reduced resorption, as confirmed by lower osteoclast surfaces. Although the frequency of total colonies and committed osteoblast colonies is the same, fewer mineralized colonies expressing decreased levels of osteoblast markers form in BSP(-/-) versus WT bone marrow stromal cultures. BSP(-/-) hematopoietic progenitors form fewer osteoclasts, but their resorptive activity on dentin is normal. Tail-suspended BSP(-/-) mice lose bone in hindlimbs, as expected. In conclusion, BSP deficiency impairs bone growth and mineralization, concomitant with dramatically reduced bone formation. It does not, however, prevent the bone loss resulting from loss of mechanical stimulation, a phenotype that is clearly different from OPN(-/-) mice

Bone Sialoprotein Deficiency Impairs Osteoclastogenesis and Mineral Resorption In Vitro

International audienceBone sialoprotein (BSP) and osteopontin (OPN) belong to the small integrin-binding ligand N-linked glycoprotein (SIBLING) family, whose members interact with bone cells and bone mineral Previously, we showed that BSP knockout (BSP(-/-)) mice have a higher bone mass than wild type (BSP(+/+)) littermates, with very low bone-formation activity and reduced osteoclast surfaces and numbers Here we report that approximately twofold fewer tartrate-resistant acid phosphatase (TRACP)-positive cells and approximately fourfold fewer osteoclasts form in BSP(-/-) compared with BSP(+/+) spleen cell cultures BSP(-/-) preosteoclast cultures display impaired proliferation and enhanced apoptosis Addition of RGD-containing proteins restores osteoclast number in BSP(-/-) cultures to BSP(+/+) levels The expression of osteoclast-associated genes is markedly altered in BSP(-/-) osteoclasts, with reduced expression of cell adhesion and migration genes (alpha V integrin chain and OPN) and increased expression of resorptive enzymes (TRACP and cathepsin K) The migration of preosteoclasts and mature osteoclasts is impaired in the absence of BSP, but resorption pit assays on dentine slices show no significant difference in pit numbers between BSP(+/+) and BSP(-/-) osteoclasts However, resorption of mineral-coated slides by BSP(-/-) osteoclasts is markedly impaired but is fully restored by coating the mineral substrate with hrBSP and partly restored by hrOPN coating In conclusion, lack of BSP affects both osteoclast formation and activity, which is in accordance with in vivo findings Our results also suggest at least some functional redundancy between BSP and OPN that remains to be clarified (C) 2010 American Society for Bone and Mineral Researc

Micrographs and quantification of total colony forming units-fibroblasts (CFU-F; B), unmineralized ALP colonies (CFU-ALP; C) and mineralized ALP colonies (CFU-O; D) in bone marrow stromal cell cultures of female mutant and WT mice

Values are the mean ± the SEM of 8–15 wells. (E) Micrographs of and female spleen cells grown with RANKL+M-CSF and stained for TRAP activity (TRAP); (F) number of TRAP cells; (G) number of osteoclasts (TRAP cells with ≥3 nuclei) formed in spleen cell and bone marrow cultures. Values are the mean ± the SEM of 12 wells. (H) Number of resorption pits on dentine slices plated with and differentiated osteoclasts; values are the mean ± the SEM of six slices. ***, P < 0.001; ****, P < 0.0001 versus matched WT.<p><b>Copyright information:</b></p><p>Taken from "Bone sialoprotein plays a functional role in bone formation and osteoclastogenesis"</p><p></p><p>The Journal of Experimental Medicine 2008;205(5):1145-1153.</p><p>Published online 12 May 2008</p><p>PMCID:PMC2373846.</p><p></p

(A) A schematic representation of the WT and mutant alleles and the targeting vector

E, EcoRI; H, HindIII; B, BamHI; X, XbaI; N, NotI; K, Kpn. (B) Southern blot analysis of tail DNA from WT (), heterozygote () and knockout () mouse littermates. The presence of the 3.2-kb wild-type fragment and the 4.8-kb recombinant fragments are seen in genomic DNA digested with HindIII. (C) Northern blot analysis of BSP mRNA expression in calvaria and femur bones of young and adult and mice. L32, housekeeping gene.<p><b>Copyright information:</b></p><p>Taken from "Bone sialoprotein plays a functional role in bone formation and osteoclastogenesis"</p><p></p><p>The Journal of Experimental Medicine 2008;205(5):1145-1153.</p><p>Published online 12 May 2008</p><p>PMCID:PMC2373846.</p><p></p

Picture (A), measurements of body length (B), body weight (C), and percentage of fat mass (D) of and mice

μCT scout images of whole femurs (E), and femur length (F) at 4 and 12 mo of age for WT () and mutant () female mice; similar results were obtained with males (not depicted). Two-dimensional images of midshaft sections in specimens from both genotypes (G) and cortical thickness (H) at 4 and 12 mo of age for and male and female mice. Values are the mean ± the SEM of 6–10 samples. **, P < 0.01; ****, P < 0.0001 versus matched WT; #, P < 0.05 versus matched females.<p><b>Copyright information:</b></p><p>Taken from "Bone sialoprotein plays a functional role in bone formation and osteoclastogenesis"</p><p></p><p>The Journal of Experimental Medicine 2008;205(5):1145-1153.</p><p>Published online 12 May 2008</p><p>PMCID:PMC2373846.</p><p></p